acta polytechnica ctu proceedings doi:10.14311/app.2017.12.0079 acta polytechnica ctu proceedings 12:79–82, 2017 © czech technical university in prague, 2017 available online at http://ojs.cvut.cz/ojs/index.php/app automatic generation of road infrastructure in 3d for vehicle simulators adam orlický faculty of transportation sciences, czech technical university in prague, konviktská 20, prague 1, czech republic correspondence: orlicada@fd.cvut.cz abstract. one of the modern methods of testing new systems and interfaces in vehicles is testing in a vehicle simulator. providing quality models of virtual scenes is one of tasks for driver-car interaction interface simulation. nowadays, there exist many programs for creating 3d models of road infrastructures, but most of these programs are very expensive or canâťt export models for the following use. therefore, a plug-in has been developed at the faculty of transportation sciences in prague. it can generate road infrastructure by czech standard for designing roads (čsn 73 6101). the uniqueness of this plug-in is that it is the first tool for generating road infrastructure in nurbs representation. this type of representation brings more exact models and allows to optimize transfer for creating quality models for vehicle simulators. the scenes created by this plug-in were tested on vehicle simulators. the results have shown that with newly created scenes drivers had a much better feeling in comparison to previous scenes. keywords: vehicle simulator, road design, 3d modeling. 1. introduction with emerging technologies, many branches of the technical sciences have appeared during the last few decades. one of such branches is computer graphics, whose one of the sections is the 3d modeling. it deals with displaying the real world in virtual threedimensional space. for this the 3-d modeling is used. in road transportation 3d modeling is used for the planning and design of newly projected or reconstructed roads and highways. generation of 3d models in special software is usually restricted by local laws, besides, the price of the software is rather high. most of these applications allow to create project documentation for the designed roadways, display the developed models in 3d and, in the case of highly sophisticated software, create visualization and animation. typically, exporting is not really possible, that is why this software can’t be used for other purposes[1]. in transportation, virtual reality is widely used in creation of interactive vehicle simulators. the real world can be displayed in virtual scenes with the help of projectors, or on monitors. one of the advantages of exploitation of driving simulators is a possibility to evaluate driver behavior in the conditions and situations that are precisely set up in driving scenarios. it is important that while driving the simulator a driver would get a feeling maximally approaching the experience of real driving. quality 3d models can contribute to the improvement of the visual perception. in most modern driver simulators, the road infrastructure isn’t generated according to these standards. all of this provides a motivation for the creation of a tool (as a plug-in for 3d modeling program) which is able to generate 3d road infrastructures according to czech standard[2], developed for a minimal price and for wide-ranging applications. for this purpose, we were looking for a software, where both design drawing and 3d modeling are possible. besides the generation of road infrastructure models, a tool (plug-in) should be able to complete a scene around roads in such a way, that the resulting virtual scenery would be as close to reality as possible. 1.1. driving simulator scene requirements there are rather specific requirements for 3d models of virtual scenes. driving simulator scenarios are most commonly created using the tin (triangulated irregular network) model[3]. with such a representation, the models are being created within an irregular network of triangles. these models should be as maximally close to reality as well as being simple (composed of the minimum possible number of triangles). another important requirement are the models, which are not visible to a driver (proband), needn’t be created. the last requirement would be the complexity of a model inversely proportional to the object’s distance from the road axis. application of these approaches makes it possible to use the less powerful hardware for creating virtual sceneries for driving simulator. optimized 3d scene models allow higher frame rates for the projected scenes, which provides a better illusion of a real world. 79 http://dx.doi.org/10.14311/app.2017.12.0079 http://ojs.cvut.cz/ojs/index.php/app adam orlický acta polytechnica ctu proceedings figure 1. tin models. 2. plug-in road creator for rhino plug-in road creator for rhino was created for 3d modeling and visualization program rhinoceros. the program is focused on creating models in nurbs (non-uniform rational basis spline) representation, where objects are mathematically described with the help of control points and their importance[4]. thus, the objects are more precise and don’t contain sharp edges. this kind of representation is also valuable for the possibility of further transformation of 3d model into tin model, for which it is possible to optimize the count of triangles of the model itself and thus optimize models for a driving simulator scene. figure 2. nurbs models. in the created plug-in it is possible to generate 3d models of road infrastructure (according to czech standard čsn 73 6101) into the rhinoceros software[2]. the standard is about the creation of designing roads and highways in the czech republic. other important elements in the infrastructure are generated per the corresponding standards and technical specifications. the plug-in was designed in a way that allows fast generating of the driver simulator scenes. that’s why databases with modelled objects was created for plugin. overall two databases have been formed. the first one is the database named "trees" that contains models of trees, bushes and grass and it is necessary for the completion of vegetation in the scene. the second database named "buildings" contains building and specific elements present around the roads. plug-in contains 54 new functions, due to which the scene creation is not as difficult as it used to be before. for user convenience, all the functions have been placed on a specially created control panel. for possible viewing of the scene model, the plug-in structures have newly created objects as layers. 2.1. plug-in functions the plug-in was created for the creation of driving scenarios. the creation of road infrastructure deploys five groups of functions of the plug-in. those include: • 2d road, • 3d road, • extras, • nature, • city. the functions can be accessed from the interface toolbar (see figure 3 for reference). figure 3. plug-in functions (interface toolbar). the process itself consists of four sequential stages. schematic and logical representation is shown in figure 11. stage 1 (model pre-processing) serves for the creation of a terrain model which is the basis of a road network. for transferring the 3d terrain model into the contour lines the function "contour lines" from the first group is applied (see figure 4). figure 4. terrain transfer to contour lines. at stage 2 basic curves for the future road communication are created using the functions from 2d group. with the help of horizontal (successive use of functions "tangential polygon", "arch with transition curve", and "vertical alignment" + "tangential polygon" from road 2d group) and vertical (functions "vertical alignment" and "parabolic arch") alignment combination a road track is created (see figure 5). 80 vol. 12/2017 automatic generation of road infrastructure in 3d by adding the cross-section type from the defined database of the cross-sections one defines the character of the road being created, i.e. number of lanes, their width etc. the database contains the crosssections corresponding to classes of the road defined in czech standard čsn 73 6101. figure 5. creation of road track. the plug-in provides automatic modification of the cross-sections according the curvatures and with respect to the location coordinates (e.g., at intersections) on the created road track (see figure 6). figure 6. basic road curves. now, when the basic curves are created, the 3d model of the road track can be created by using the function "3d road" from the "road 3d" menu section – the 3rd stage of the process. for the integration of the model into the terrain the function "slopes" is applied. see the result in figure 7. figure 7. road with slopes, 3d model. the 4th stage is the final one, it results in a complete road track model that includes such attributes as barriers, traffic signs, pillars etc. see example on figure 8. figure 8. complete road track model. with the help of the plug-in described, it is also possible to create other scenario attributes, such as nature, buildings etc. see the final model example in figure 9. figure 9. final 3d model. for the creation of the basic city infrastructure, a group of functions "city" shall be applied. these functions allow creation of different intersection types (see example in figure 10), sidewalks, pedestrian crossings etc. figure 10. city infrastructure – roundabout model. 3. conclusions the developed plug-in is nowadays unique regarding its possibility to create nurbs models of road infrastructure according to standard čsn 73 6101. unlike the tin models, the nurbs models are more precise, while there is a possibility of transformation of nurbs models into tin models with setting the quality of created tin models in rhinoceros software. the advantage is quality of tin models can be optimized for driving simulators. during the transferring 81 adam orlický acta polytechnica ctu proceedings figure 11. road infrastructure creation process. it has been accomplished that the models in simulator are not containing visible sharp transitions and are being formed with minimal possible number of triangles. the created 3d road infrastructure models can be also used for visualization, the plug-in can also be applied for schematic drawings of track situation and longitudinal profile, which can be used for the project documentation. besides, the major advantage of this plug-in is that it significantly reduces the time spent at modeling. creation of 3d scenery models with road infrastructure in this plug-in is the matter of several minutes. after processing of the created infrastructure in the 3d modeling engine the scenario for simulation is ready (see figure 12). figure 12. final simulation scenario. the plug-in is presently being used at transportation faculty at czech technical university in prague as a main tool for generation of scenes for driving simulators. the functions of these plug-ins were also used for the creation of animations and visualizations in the final theses of students studying at the faculty of the author. references [1] road design by autocad civil 3d and 3d max design. youtube [online]. 2014 [cit. 2016-04-29]. dostupné z: https://www.youtube.com/watch?v=eeo0wabu7ow. [2] čsn 73 6101 projektování silnic a dálnic. praha: český normalizační institut, 2004. [3] počítačová grafika: základy počítačové grafiky. ict kompetence [online]. olomouc: katedry technické a informační výchovy pdf up. [cit. 2016-04-29]. dostupné z: http://www.kteiv.upol.cz/frvs/ictkubricky/?page=pocitacova-grafika/pocitacova-grafika. [4] rhino developer docs [online]. robert mcneel & associate, ©1997-2016 [cit. 2016-04-30]. dostupné z: http://developer.rhino3d.com/. 82 acta polytechnica ctu proceedings 12:79–82, 2017 1 introduction 1.1 driving simulator scene requirements 2 plug-in road creator for rhino 2.1 plug-in functions 3 conclusions references acta polytechnica ctu proceedings doi:10.14311/app.2016.3.0071 acta polytechnica ctu proceedings 3:71–74, 2016 © czech technical university in prague, 2016 available online at http://ojs.cvut.cz/ojs/index.php/app deformation characteristics of chopped fibre composites subjected to quasi-static tensile loading jan šleichrta,∗, marcel adornaa, michaela neühauserováa, nela fenclováa, veronika petráňováb a czech technical university in prague, faculty of transportation sciences, konviktská 20, 120 00 prague 1, czech republic b czech academy of sciences, institute of theoretical and applied mechanics, prosecká 76, 190 00, prague 9, czech republic ∗ corresponding author: sleicjan@fd.cvut.cz abstract. this work presents deformation behaviour of cost effective chopped fiber composites. use of chopped fibre is advantageous for manufacturing however complex shape parts production technology could be challenging. batches of samples with different fibres composition were subjected to uni-axial tensile loading to obtain overall materials properties and inspection of proper manufacturing based on local deformation inhomogeneities. both crossbeam displacement and optical strain measurement were used for elastic characteristics evaluation. deformation response was derived from full-field optical strain measurements based on digital image correlation method. relatively large variation of mechanical properties testing of samples was found. keywords: chopped fibre composites, full-field measurement, digital image correlation. 1. introduction carbon fiber reinforced thermoplastic composite become a standard construction material in last decades mainly for its good strength to weight ratio, chemical resistivity and recyclable [1]. moreover operating of light weight structures is economically beneficial. because of damage-tolerance approach used for the composite material a comprehensive knowledge of the material deformation response is required [2]. the high strength composites are in the large majority manufactured as long fibres composited. this material was intensively investigated on macromezzoand micro-level to obtain the detailed information about its deformation response to the various type of the loading [3, 4] and description of the internal damage propagation [5, 6]. however there have excellent mechanical properties its production cost is relatively high and manufacturing of complex shaped parts is challenging. for less loaded complex shaped parts are an appropriate solution usage of chopped fibre composite [7]. this cost effective material allows due to the stochastic orientation of the carbon fibre pellets manufacturing of the parts with variable thickness, holes and branchings [8]. presented article is focused on deformation behaviour of newly developed chopped fibre composite subjected to quasi-static loading. prototyped material prepared by custom designed molding system have to be inspected to discover detect defects in materials and to describe its deformation behaviour. full-field optical strain field measurement method [9] was employed for assessed of reliability of the manufacturing techfigure 1. detail of damage structure after tensile test obtained by scanning electron microscope. nology based on overall material properties and local deformation inhomogeneities. the collapsed structure of the composite specimen can be seen in fig. 1. 2. experimental procedure 2.1. specimen description dog bone shaped samples (fig. 2) with width of 20 ± 0.4 mm and thickness of 2.5 ± 0.1 mm were manufactured using custom designed mould. two sets of samples were prepared from carbon pellets with dimensions 25 × 6 mm and 13 × 3.25 mm respectively bounded with a thermoplastic matrix. thickness of the pellets was 0.14 mm in both cases. density of the final material was around 1.35 g · cm−3. the orientation of the pellets was completely random because the process of filling pellets into the shape was uncontrolled. the temperature of glass transition was 90 ◦. 71 http://dx.doi.org/10.14311/app.2016.3.0071 http://ojs.cvut.cz/ojs/index.php/app j. šleichrt, m. adorna, m. neühauserová et al. acta polytechnica ctu proceedings figure 2. visualization of sample shape. figure 3. experimental devices: 1-led light source, 2-specimen, 3-camera, 4-loading device. final samples were cut from the plate using water-jet cutter. finally, the surface of sample was equipped by random pattern point for image processing. 2.2. experimental setup experimental setup consists of instron 3382 (instron, inc.) loading device for tensile measurements and digital single-lens reflex camera with accessories. displacement controlled tests were performed with 10µm · s−1 loading rate. force was measured by high-durability load cell with loading capacity 50 kn. strain was measured optically using eos 550d (canon, japan) camera equipped with macro-objective ef100mm/1:2.8l macro (canon, japan). complete equipment is depicted in fig. 3. artificial pattern on the sample surface was illuminated by laboratory led light source kl 2500 (shott, germany). images were captured in equidistant time intervals and were labeled by unique time stamp for a precise synchronization of dic and force log. 2.3. digital image correlation strain was evaluated from the acquired image sequence using custom dic software [10] based on lucas-kanade tracking algorithm [11]. evaluating script was written in matlab environment. in the first image (prior the loading) an array of tracking features (10 × 15 points) was defined. as shown in fig. 4, the displacement of each point was identified. paths of each point were used to calculation local deformation in measured object. figure 4. detail of the displacement of a point during measurement. based on shifts of points the map of displacement was formed to verification dic method. obtained data were further processed to establish full-field strain maps. 3. results 3.1. full-field strain map for representation inhomogeneities and local deformation in object full field maps were created. calculation and evaluation those processes were completed using matlab script as well. displacements and strains calculated in each correlation point using set of matlab scripts are represented by strain maps depicted in fig. 5 to 9. from the results of the displacement mapping of the sample no. 22 (see fig. 5) expected linear behaviour without significant local inhomogeneities was observed. strains maps of selected samples of both types of pellets represented by sample no. 22 consisted of 13.0 × 3.25 mm depicted in fig. 8 and by sample no. 45 consisted of 25.0 × 6.0 mm depicted in fig. 9 discovered local strain concentrations (represented by red colour) indicating material imperfections. in case of 13.0 × 3.25 mm pellets the absolute values are lower but spread on larger area compared to 25.0 × 6.0 mm pellets specimen where deformation are higher in discontinuous area. 3.2. young’s modulus calculation of young’s modulus was done by three different approaches. firstly, results were computed from crossbeam displacement and force log. another results were calculated using displacements of points acquired from dic. average displacement of first and last line of points was used to comparison with results calculated from 5th and 11th line of points. similarly, results of two ways of calculation from dic method were found. 72 vol. 3/2016 deformation characteristics of chopped fibre composites figure 5. map of the total displacements of points. figure 6. full-field strain map of the sample 22 (smaller pellets). figure 7. full-field strain map of the sample 45 (bigger pellets). • green curve optical strain measurement, calculated between first and last row of grid. • yellow curve optical strain measurement, calculated between 5th and 11th row of grid. • red curve calculated from crossbeam displacement. figure 8. stress-strain diagram of 13 × 3.25 mm pellets; sample 22. figure 9. stress-strain diagram of 25 × 6 mm pellets; sample 45. 4. conclusions deformation behaviour of chopped fibre composites represented by stress-strain curves and strain maps was described based on optical measurement of standard tensile test. similar elastic properties in comparison with long fibre samples [3] and small differences in young’s modulus values in each set were observed. in contrary significant differences in ultimate stress (80 − 150 mpa) were observed. differences caused by fabrication technology mainly given by imperfections in the material structure was measured. as expected, the values of strains was significantly lower using optical evaluation method. strains derived from cross-head displacement were affected by backlash of the grips of the loading device. with respect to the measured data, the reliability of the optical method for investigation of local strain concentrations was proven. delivered sets of tested material were considered still unsuitable for small structural parts for the mass production. the investigated composite material and requires further improvements of fabrication process. acknowledgements the research was supported by grant agency of czech technical university in prague (project no. sgs15/225/ohk2/3t/16), by technology agency from 73 j. šleichrt, m. adorna, m. neühauserová et al. acta polytechnica ctu proceedings the czech republic (grant no. ta03010209) and by institutional support rvo: 68378297. references [1] d. steenkamer, j. sullivan. on the recyclability of a cyclic thermoplastic composite material. composites part b: engineering 29(6):745–752, 1998. [2] j. gallagher, u. s. a. force. usaf damage tolerant design handbook: guidelines for the analysis and design of damage tolerant aircraft structures : final report for period september 1980 to march 1984. no. v. 1 in usaf damage tolerant design handbook: guidelines for the analysis and design of damage tolerant aircraft structures : final report for period september 1980 to march 1984. flight dynamics laboratory, air force wright aeronautical labortories, 1984. [3] j. valach, d. kytyr, t. doktor, et al. comparison of mechanical properties of cfrp laminate obtained from full-scale test and extrapolated from local measurement. chemicke listy . [4] d. kytyr, t. fila, j. sleichrt, et al. assessment of the post-impact damage propagation in a carbon-fibre composite under cyclic loading. materials and technology 48(5):25–28, 2014. [5] p. koudelka, t. fila, t. doktor, et al. inspection of post impact fatigue damage in carbon fibre composite using modulus mapping technique. key engineering materials 606(4):245–248, 2014. [6] t. doktor, j. valach, d. kytyr, et al. analysis of cross-section surface roughness evolution of carbon fibre reinforced polymer under fatigue loading. chemicke listy 106(3):399–400, 2012. [7] composites becoming the standard in aircraft seats. jec composite magazine 43(23):34, 2006. [8] r. bockstedt, j. sajna. low cost composites based on long carbon fiber thermoplastics. proceedings of sampe 2010 conference, anaheim, usa pp. 2011–2020, 1993. [9] j. dourmaxa, s. bouvier, a. aouafi, p. vacher. fullfield measurement technique and its application to the analysis of material behaviour under plane strain mode. materials science and engineering a(500):47–62, 2009. [10] i. jandejsek, j. valach, d. vavrik. optimalization and calibration of digital image correlation method. proccedings ean 2010 pp. 121–126, 2010. [11] b. lucas, t. kanade. an iterative image registration technique with application to stereo vision. proceedings of image understanding workshop pp. 121–130, 1981. 74 acta polytechnica ctu proceedings 3:71–74, 2016 1 introduction 2 experimental procedure 2.1 specimen description 2.2 experimental setup 2.3 digital image correlation 3 results 3.1 full-field strain map 3.2 young's modulus 4 conclusions acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2017.12.0094 acta polytechnica ctu proceedings 12:94–98, 2017 © czech technical university in prague, 2017 available online at http://ojs.cvut.cz/ojs/index.php/app simulation of electric and hybrid vehicles in a vehicle simulator based on a detailed physical model, for the purpose of hmi evaluation d. rozhdestvenskiya,b,∗, j. fulemb a czech technical university in prague, faculty of transportation science, horská 3, prague, 120 00, czech republic b their ricardo prague s.r.o., palác karlín, thámova 11-13, 186 00, praha 8, czech republic ∗ corresponding author: rozhddmi@fd.cvut.cz abstract. in this article, we propose a software solution to study hmi of electric and hybrid electric vehicles in vehicle simulators. we will start with the description of a development process of a physical model for hev simulation in ignite software and equation-based language modelica. a short introduction to the language, its possibilities, and explanation, why it is more suitable for the development of such models (vehicle powertrain), are presented in the first part of the article. a fusion mechanism of the physical engine with the model by means of fmu (functional mock-up interface)[1] is also described in this part the second part is dedicated to the description of the model constructed in third party software ignite. this model has a detailed calculation of energy consumption and energy flow based on the selected control strategy. the last part of the article describes a possible experiment methodology. keywords: electric vehicle, hybrid vehicle, powertrain simulation, hmi simulation. 1. introduction electric and hybrids vehicles are slowly gaining popularity not only because it can help to reduce co2 emissions and improve the environmental situation in big cities [2], but because of the rapid development in the area of electric motors batteries and fuel cells technologies. during the latest paris motor show (2016), both vw and mercedes benz claimed to have overcome so far minimal acceptable limit of their pure ev (300 km is a minimal range acceptable by the user when considering to switch from conventional cars to ev or hev [3]) almost twice. this can be achieved because of the new ev and charging architecture developed by companies[4]. additionally, the ambitions of the automotive industry to introduce semi-autonomous and, later, fully autonomous vehicles on public roads before 2020, puts an extra urgency into design and evaluation of hmi technics for such vehicles. user vehicle interaction is shifting to a new, so far, unstudied area with familiar knobs and gauges are being substituted with touchscreens, menus, and voice interactions[5]. the notion of "familiar" car is slowly vanishing, putting a new driver (user) in a position where he has to learn how to drive a vehicle all over again if he switches the brand. this become even more crucial if we take into account that car sharing is slowly substituting car ownership[6]. the processes of development, design, and evaluation of hmi is to adapt to this rapidly changing environment. an efficient way of assessment of vehicle interfaces at different stages of their development can be achieved by testing it in a simulated environment. a model of an interactive ev and hev driving simulator with a possibility of rapid adaptation to vehicle type or architecture, with an accurate simulation of the powertrain components developed in modelica language, is hereby suggested. 2. modelica programming language modelica is an open source object-oriented language for modeling of massive, complex systems. it is suited for multi-domain modeling (including electrical, physical, thermal, mechanical and other domains), additionally, it can be used for hardware-in-the-loop simulations and for embedded control systems [7] models in modelica are mathematically described by differential, algebraic and discrete equations. from a user perspective, models are described by schematic object diagrams). the internal part of a component is defined by another schematic or on a lower level, by an equation-based description of the model in modelica syntax. an example of such a model is shown in figure 1. components can be combined in a complex model by drawing connection lines between "connectors". connectors describe the interaction possibilities (electrical pin, mechanical flange, or an input signal. regarding development time for complex models, modelica language is on average, twice as fast as any other technics in proprietary code or in any other 94 http://dx.doi.org/10.14311/app.2017.12.0094 http://ojs.cvut.cz/ojs/index.php/app vol. 12/2017 simulation of electric and hybrid vehicles in a vehicle simulator figure 1. modelica model architecture (example of schematic diagram and corresponding programming code). figure 2. development time chart. block modeling languages. this is explained by several factors: • a developer does not have to manually derive relationships between inputs and outputs between subsystems models in modelica are mathematically described by differential, algebraic and discrete equations. no particular variable needs to be solved for manually • modelica code has been developed since 1996 so a lot of subsystem components have been already developed and available for public • implementation time and complexity is the same as for any block modeling language all these points are summarized in figure 2. as mentioned above this puts modelica language ahead of others, especially when dealing with simulation of such a complex and multidisciplinary system as hybrid and electric vehicle powertrain. integration of the model in the virtual world creates a new engineering task which can be solved with an application of the fmi (functional mock-up interface standard). 3. function mockup interface functional mock-up interface (fmi) is a standard to support both model exchange and co-simulation of dynamic models developed in different independent tools, its primary goal is to support the exchange of simulation models between suppliers and oems, providing a possibility to protect product knowledge, which could be recovered from these models. this standard is going to be used to perform a simulation of the ev or hev powertrain inside a physical world simulated in ode or bullet physical engine. figure 3. fmi integration concept[1]. 4. model creation and validation a lot of commercial software available on the market are using modelica as a simulation language, and either already have, or are planning to implement fmu import/export routine. one such software is ignite, developed by ricardo software. ignite is 95 d. rozhdestvenskiy, j. fulem acta polytechnica ctu proceedings a physics-based system simulation package focused at complete vehicle system modeling and simulation. it consists of set of libraries containing vehicle and thermal system components such as engine, transmission, controllers, motors and e.t.c. these components allow faster and easier modeling and parameterization of a powertrain system for any type of vehicle, and a simulation of temperature dependent systems, which will allow further possibilities to simulate hvac and battery management of hev and ev. figure 4. model hybridization. creation of a model of hev in ignite is a straightforward process that consists of several steps [8]: (1.) collect all necessary vehicle information (tires, aerodynamics, mass etc.). vehicle manufacturer usually provides this data in technical documentation. (2.) collect powertrain information (gear efficiencies, shifting maps, engine and electric machine maps and battery characteristics). this data can either be obtained from the automaker directly on request or derived from an experimental data mathematically. (3.) build a conventional vehicle model using ignite libraries: • parametrize the model with collected data. • run test cases to validate the model. • turn the conventional model into a hybrid – add hybrid components (electric motor, battery, controller). validation of the model cacan be performed using standard driver cycles, for instance, ndc or ftp72, results obtained from the simulation on the drive cycle should be compared with an experimental data or figure 5. engine map, torque and soc results. analyzed analytically. results of the simulation contain comprehensive data for every model component and provide the possibility to perform this analysis. an example of the validation for a p2 hybrid with 1.4l tsi motor and 25 kw electrical motor is shown in figure 5. the simulation showed an average fuel consumption of 4.3 l/100km, which is equivalent to 100 g/km co2 emissions on nedc drive cycle for this model. these results closely corelate with the experimental date for similar vehicles. this validation was performed during the development stage of the hybridmodelt at ricardo p.l.c.and described in details in [8]. figure 6. engine map results. 5. integration of fmi into the simulator once the model was created and validated, it has to be modified, to perform a simulation with a physical engine. as we are interested in the interactive simulation for the purpose of hmi evaluation, we need to provide the means of control for a driver to interact with the model. at least 3 inputs have to be 96 vol. 12/2017 simulation of electric and hybrid vehicles in a vehicle simulator introduced to provide a longitudinal control of the simulated vehicle to the user. these inputs are: • acceleration demand (acceleration pedal position <0,1>). • break demand (brake pedal position <0,1>). • current vehicle velocity. velocity input provides a feedback of an actual speed of the vehicle simulated in the ode (open dynamic engine) simulation environment, which is calculated there with respect to elevation grade, tire/road contact friction, collisions, bumps etc. as a direct output, an applied wheel torque should be obtained from the model, this torque is applied to the physical model of the vehicle in the ode or bullet physical engine which is simultaneously handling latitudinal behavior. all other losses (aerodynamic, acceleration, internal and rolling losses) are calculated in ignite model directly. for the purpose of hmi simulation, we have to obtain additional outputs from the model to simulate gauges and control element of hev. these inputs are: • state of battery charge soc. • engine speed. • electric machine engine speed. • battery temperature. • ice state. these outputs can then be further processed to be displayed on a virtual gauge or in any other graphical form. the architecture of this simulation environment is shown in figure 7. figure 7. simulation architecture. 6. possible experiments with detailed consumption model after the vehicle model and simulation environment has been created, an experiment itself should be designed. we can look at four possible types of experiments with evs and hevs: • experiments aimed at determination of general user acceptance of hmi and its efficiency. • experiments aimed at study of acceptance and effect of particular hmi element. • experiments aimed at comparing different hmi. • experiments aimed to determine preferred system types of notifications per different kinds of events. even though any of these types of experiments should be adapted to a specific requirement, one may distinguish ten high-level steps essential for any type of experiment. (janssen at al. 2004): (1.) to determine the goals of evaluation in terms of usability, safety, workload, acceptability, time frame. (2.) to describe a system (systems) which will be tested during the experiment. (3.) to define a high-level scenario (scenarios) in terms of road type, and conditions, visibility, traffic type and actors, tasks, and goals. (4.) define the subject testing cohort in terms of amount, age and experience group. (5.) define subjective and objective evaluation parameters and instruments for data acquisition. (6.) develop experimental instruction for both participants and experimenters. (7.) perform a pilot experiment test and data evaluation. (8.) carry out the experiment. (9.) analyze data. (10.) produce summary report and draw a conclusion. 7. conclusion in this article, we presented a methodology for development and integration of high fidelity powertrain model of ev or hev into three-dimensional virtual simulation environment. this model provides a comprehensive and precise data results to simulate next generation hmi for ev or hev including gauges and functions (such as power-in-out gauges, predictive navigation and others). these models are written in modelica programming language and coupled with ode or bullet physical world with the help of fmi. this approach allows a rapid development and validation of models, and a quick way of switching between them. references [1] t. b. at all. the functional mockup interface for tool independent exchange of simulation models. in proceedings of the 8th international modelica conference. [2] d. rozhdestvenskiy, p. bouchner, a. mashko, et al. dynamic human-machine interface for electrical vehicle design guidelines. smart cities symposium prague 2015, isbn 978-1-4673-6727-1. 97 d. rozhdestvenskiy, j. fulem acta polytechnica ctu proceedings [3] t. smith, h. vardhan, b. malet, d. mingay. are we there yet. thoughts on in-car hmi. 2014. [4] g. e. outlook2016. beyond one million electric cars. international energy agency, electric vehicle initiative. [5] s. heath, p. sant, b. allen. do you feel lucky? why current range estimation methods are holding back. ev adoption hybrid and electric vehicles conference 2013 (hevc 2013), iet year: 2013. [6] p. b. et al. professional vehicle drivers training and performance measurements with use of advanced driving simulator laboratory. 1.ed., prague: ctu in prague, faculty of transportation sciences, department of vehicles, 2013, 1st, electronic edition. isbn 978-80-01-05421-5. [7] j. tobolar, m. otter, t. bunte. modelling of vehicle powertrains with the modelica powertrain library. in: dynamisches gesamtsystemverhalten von fahrzeugantrieben. [8] j. fulem. ignite & wave-rt powertrain model connected to ipg virtual test driving platform. in: ricardo software usercon europe. 98 acta polytechnica ctu proceedings 12:94–98, 2017 1 introduction 2 modelica programming language 3 function mockup interface 4 model creation and validation 5 integration of fmi into the simulator 6 possible experiments with detailed consumption model 7 conclusion references acta polytechnica ctu proceedings doi:10.14311/app.2016.5.0022 acta polytechnica ctu proceedings 5:22–25, 2016 © czech technical university in prague, 2016 available online at http://ojs.cvut.cz/ojs/index.php/app infrastructure parameters affecting capacity of railways in ten-t vít janoš∗, milan kříž department of logistics and management of transport, faculty of transportation sciences, ctu in prague, czech republic ∗ corresponding author: janos@fd.cvut.cz abstract. the article presents possible solutions of the issue of the lost capacity on a double-track line. it is shown a solution where an inserted train on the double-track line use the lost capacity of the opposite line track using active overtaking. the feasibility of such a solution is discussed and the length of sections suitable for active overtaking is calculated. keywords: capacity, uic 406, speed ratio . 1. introduction the capacity of the railway infrastructure can be considered as a usable time space for the train paths. capacity is thus influenced not only by the parameters of the infrastructure, but also by the character of the traffic – by the designed train paths and their heterogeneity. with increasing degree of heterogeneity the capacity consumption increases too and the number of possible train paths in a defined time windows declines. this phenomenon is also called "capacity balance" [1]. in the capacity balance there are four factors on whose interdependencies the capacity is based: number of trains, average speed, stability and heterogeneity. relationships among the individual factors are shown in figure 2 [1]. principle of figure 2 is explained as follows: "in this qualitative model, an axis for each parameter is drawn from a unique origin. a chord links the points on the axes, corresponding to the value of each parameter. the length of the chord represents the capacity. capacity utilisation is defined by the positions of the chord on the four axes. increasing capacity means increasing the length of the chord". general effect associated with the modernization of the trans-european conventional rail network (typically the modernization of the transit railway corridors in the czech republic in the original routes to a maximum speed of 160 km/h) is the rising heterogeneity of the traffic on some routes in the network. it is caused by increasing the line speeds. on the one hand regional passenger train paths remains after increasing the line speed about as fast as before the modernization (journey speed of the regional trains usually does not exceed 60 km/h due to the higher number of stops). on the other hand long-distance passenger trains should logically utilize all characteristics of infrastructure for the maximum journey speed (at the fastest trains the journey speed of 120-140 km/h can be reached in some line sections). these both cases are thus usually marginal and between these limits figure 1. possible situation of active overtaking on a double-track line. the freight trains paths occur (with journey speeds in the range of 70-100 km/h). 2. heterogeneity and its measuring heterogeneity can be measured in different ways. one way is to compare running speeds of individual train (or train paths). for this purpose krueger [2] proposed and used so called speed ratio (sr). the speed ratio is the ratio of the fastest train speed to the slowest train speed: sr = max(v1, v2, ..., vn) min(v1, v2, ..., vn) so the sr on the typical line section on some transit railway corridor in the czech republic can reached value of 3 (the speed of the fastest train about 140 km/h and the speed of the slowest train about 45-50 km/h). 22 http://dx.doi.org/10.14311/app.2016.5.0022 http://ojs.cvut.cz/ojs/index.php/app vol. 5/2016 infrastructure parameters affecting capacity of railways in ten-t figure 2. capacity balance [1]. the very high sr is problematic only when it comes to the line or line section with a high number of train paths. but the lines of the trans-european conventional rail network usually show a high number of train paths. line sections with high demand for train paths and a high sr can usually be solved by segregating the various types of traffic. this entails increasing the number of line tracks (e.g. quadruple-track line sections in urban agglomerations with two tracks for long-distance passenger trains and fast freight trains and two tracks for slower regional passenger trains and slower freight trains) or construction of lines dedicated exclusively for a certain type of traffic (typically on the one hand high-speed railway lines and, on the other hand, lines dedicated only for freight train as e.g. betuwelijn). this approach, however, is not yet common in the czech republic. 3. active overtaking on double-track lines, where high degree of heterogeneity is because of the mixed traffic and high requirements for the number of the train paths, so called lost capacity arises. lost capacity is unusable time space between train paths with different train speeds, where any additional train path cannot be inserted (e.g. because there is no other station). lost capacity can be limited by reducing heterogeneity of train paths. either the slowest trains can be accelerated (e.g. these trains can pass some stops), or vice versa the fastest trains can be slowed. lost capacity may be also used for active overtaking1. an example of this solution is shown in the figure 1. active overtaking is a very interesting possibility of using a part of the lost capacity. this idea can be very helpful in cases, when the number of stations on a line section is reducing within modernization of infrastructure. in the former stations there are only crossovers for crossing between the line tracks for cases of interruptions. in such cases there is no possibility to insert one more train path into a sequence of differently 1active overtaking can be defined as an overtaking of slower train by faster train, when the slower train does not have to stop [3]. fast train paths, but they can also be used for crossing between line tracks and in regular operation. the actual possibility of using these crossovers for regular traffic, however, depends on the diverging speed that affects the train speed (and thus journey time) of the overtaking or the overtaken train and thus affects the length of the section required for overtaking. furthermore, it is obvious that the greater the heterogeneity of the train paths in the reference section is (i.e. the larger the difference of the journey speeds between the overtaken and the overtaking train), the shorter the section required for overtaking is. with regard to the above mentioned borderline cases it is evident that the overtaken slower train is always a regional passenger train. there are generally two cases possible: (1.) the slower train crosses over to the other line track and runs against the right direction, the faster train runs in the right direction. (2.) the faster train crosses over to the other line track and runs against the right direction, the slower train runs in the right direction. the cases from the first group are suitable for situations, in which the platforms of stations and stops in the line section are situated between the line tracks, or in which only stations equipped with a good visual and audible information system are. this system should provide clear information about platforms and tracks of arriving and departing trains. the cases from the second group are suitable, if the speed limit over the crossovers between the line tracks corresponds or is close to the line speed or the maximum train speed. 4. case study: the first transit railway corridor between prague and děčín the part of the first transit railway corridor between prague and děčín links the prague metropolitan area [4] with the ústí metropolitan area and with the north part of germany. the most typical situation for the active overtaking is an interaction of a fast freight train (maximum train speed about 100 km/h) and a regional passenger train (journey speed about 50 km/h). the active overtaking of the first type can be used in the line sections: • hněvice – hněvice seř.n. • roudnice nad labem – hrobce • lovosice jih – lovosice • prackovice nad labem – ústí nad labem jih – ústí nad labem hl.n. now we have to analyse these line section, whether the active overtaking leads to time saving in comparison with overtaking of the regional passenger train 23 vít janoš, milan kříž acta polytechnica ctu proceedings figure 3. relationship between speed ration and length of section for overtaking. by the fast freight train, when the regional passenger train is dwelling in some station. sections hněvice – hněvice seř.n. and lovosice jih – lovosice can be excluded immediately, because the result time savings will be negative. in both cases the regional passenger train is stopping only once in the line section and in both cases the speed limit over crossovers in the stations which bounds the line section is lower than the speed limit on passing sidings in these stations. sections roudnice nad labem – hrobce and prackovice nad labem – ústí nad labem jih – ústí nad labem hl.n. show always positive time savings. in both cases the regional passenger train is stopping twice in the line section2. in these cases, the parallel run is generally about 2 minutes more effective then overtaking of the slower train in the station. it is very important that the overtaking train is guided precisely to the desired time position. the accuracy of this guidance may have a significant effect on the resulting time savings. by the active overtaking of the second type the journey time of the overtaking train is extended because of the diverging speed of the crossovers in the bounding stations. in table 1 values for typical situation on the first transit railway corridor are shown. the values were calculated with linear margin 9%. value are graphically illustrated in figure 3. from table 1 it is possible to determine the length of the line section which is needed for active overtaking. if the actual line section is shorter, then the dwell times of the slower train have to be extended. 5. conclusions the verification of heterogeneity in relation to the partial use of the lost capacity by active overtaking of differently fast trains showed clearly that in the case of the interaction of regional passenger trains and freight trains there are solutions which can be divided into 2also the stops in bounding stations of the line section are counted, if the regional passenger is being already overtaken during the stay in the station. sum of operating intervals and time reserves [min] 5 journey speed of slower train [km/h] 50 speed ratio [1] speed of faster train [km/h] length of section for overtaking [km] 1,1 55 45,83 1,2 60 25,00 1,3 65 18,06 1,4 70 14,58 1,5 75 12,50 1,6 80 11,11 1,7 85 10,12 1,8 90 9,38 1,9 95 8,80 2 100 8,33 2,1 105 7,95 2,2 110 7,64 2,3 115 7,37 2,4 120 7,14 2,5 125 6,94 2,6 130 6,77 2,7 135 6,62 2,8 140 6,48 2,9 145 6,36 3 150 6,25 table 1. length of section for overtaking. two situations. either if the diverging speed over the crossovers in the bounding station of the line section is clearly lower than the running speed of the fast freight train, then the regional passenger train should run against the right direction on the other line track. or if the diverging speed over the crossovers in the bounding station of the line section is high enough (minimum is 80 km/h), then then the fast freight train should run against the right direction on the other line track. it is important to mention that by modernizing the infrastructure, which should also allow such use of the lost capacity, platforms should be mostly located between the main line tracks. with regard to the actual length of the line sections for active overtaking and with regard to demand for the train paths it is necessary to mention that on 24 vol. 5/2016 infrastructure parameters affecting capacity of railways in ten-t trans-european conventional rail network this solution will be usually marginal. furthermore, it is likely that the heterogeneity of demanded train paths will not decline. authorities ordering the long distance passenger trains and open access operators do not want to, on the one hand, slow down artificially their trains. on the other hand, local authorities ordering the regional passenger trains do not want to reduce the number of operated railway stops, which is a very problematical political topic. in the czech republic the construction of quadrupletrack line sections remains as a permanent challenge in cases where a high degree of heterogeneity connected with high demand for train paths occurs. only such a solution leading to the segregation of various types of traffic allows homogenization of the train paths and effective elimination of the lost capacity. references [1] uic leaflet 406 capacity. paris: international union of railways (uic). [2] h. krueger. parametric modeling in rail capacity planning. simulation conference proceedings. az, phoenix, 1999. [3] m. drábek. periodic freight train path in network. čvut, praha, 2014. [4] t. kostelecký, d. čermák. metropolitan areas in the czech republic definitions, basic characteristic, patterns of suburbanisation and their impact on political behaviour. sociologický ústav av čr, 2004. 25 acta polytechnica ctu proceedings 5:22–25, 2016 1 introduction 2 heterogeneity and its measuring 3 active overtaking 4 case study: the first transit railway corridor between prague and decín 5 conclusions references 235 acta polytechnica ctu proceedings 1(1): 235–239, 2014 235 doi: 10.14311/app.2014.01.0235 data analysis of globular cluster harris catalogue in view of the king models and their dynamical evolution. ii. observational evidences marco merafina1, daniele vitantoni1 1department of physics, university of rome la sapienza, piazzale aldo moro 2, i-00185 rome, italy corresponding author: marco.merafina@roma1.infn.it abstract we summarize some observational comparison concerning the features of globular clusters (gcs) population in connection to the evolution of king models. we also make a comparison with some extragalactic gcs systems, in order to underline the effects of the main body on the dynamical evolution. keywords: globular clusters gravothermal catastrophe king models thermodynamical stability. 1 introduction globular clusters (gcs), for their proprieties of symmetry and their high relaxation times, are important to test theories about thermodynamical stability of spherical self gravitating systems. the actual sample of is a mixture of various and not homogeneus gcs types. therefore, it is difficult to analyze properties of milky way (mw) gcs population in connection to core-collapse and gravotermal instability. the last version (2010) of harris gcs catalogue (see also harris, 1996) includes 157 objects. it was pointed out by van der bergh (2011) that harris catalogue could includes three not typical gcs, probably remnant cores of dsph galaxies: omega centauri, terzan 5 and ngc 6715 (m54). the harris catalogue also includes pcc gcs, namely gcs with collapsed cores that cannot be described by classical single mass king models profiles. zinn (1985) identifies two classes of gcs, respectively known as disk population (metal rich) and halo population (metal poor), distinct by the threshold value [fe/h] ' −0.8 (or, according to some authors, −0.75). recently bica et al. (2006) showed that the actual gcs population seems to have been contaminated by capture of smaller galaxies (and their possible gcs populations) during the milky way formation. possible evidences of extragalactic origin of some gcs are retrograde motion (compared to galactic disk motion) and unusual young absolute age. it seems that the original gcs population suffered deep and incisive processes of disruption (see aguilar et al., 1988; hut & djorgovski, 1992; gnedin & ostriker, 1997; mackey & gilmore, 2004), until almost 50% of original gcs are destroyed in the last hubble time. 2 discussion we start to consider the problem introduced by katz in the paper about thermodynamic stability in 1980. the study of the distribution of galactic gcs in terms of w0 (central gravitational potential) shows a peak value of 6.9. we should expect that the peak value coincides with the known stability critical value w0 = 7.4, due to the old age of mw gcs and the onset of the instability in the high w0 region. this problem had remained unsolved (fig.1). figure 1: distribution of galactic gcs at different k (katz, 1980). the quantity k is related to w0 (see merafina & vitantoni, part i). with the introduction of the effective potential (see merafina & vitantoni, part i) and including the additional term in the expression of the total energy, we can revise the katz study. the result is a very satisfactory coincidence of the observative peak value with the stability limit. we can also repeat the analysis on a more detailed and updated sample (using data of the harris gcs catalogue). the peak value, in the non-symmetric 235 http://dx.doi.org/10.14311/app.2014.01.0235 marco merafina, daniele vitantoni gaussian hypothesis, is exactly at w0 = 6.9 (fig.2). 0 2 4 6 8 10 12 14 0 2 4 6 8 10 12 14 figure 2: w0 distribution of pre-core-collapse mw gcs. for a better understanding of the evolution of a gcs population, we briefly analyze the role of environmental features. the effect of the distance from the galactic center (fig.3) is not clear at all. generally speaking, the more a gc lives near the galactic center, the more quickly it evolves towards the gravothermal catastrophe, being more affected by tidal forces of the galaxy. 2 4 6 8 10 12 14 1 10 100 r gc figure 3: galactocentric distance rgc in function of w0. the dashed lines represent the values at 3kpc and 30kpc. if we look at the gcs distribution in the [fe/h]-w0 plane (fig.4), we find no correlations between these two quantities. this means that the difference between halo and disk population, first introduced by zinn (1985) does not influence the dynamical features and the evolution. nevertheless, if we analyze the disk population (fig.5), this seems to be more dynamically evolved than the halo one. the w0 peak value is slightly larger for the disk population, mainly due to a lower rgc (in average) for this class of objects. on the other hand, it is well known that the tidal shocks played a more incisive role in the evolution of the disk gcs. 2 4 6 8 10 12 14 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 figure 4: total metallicity [fe/h] in function of w0. 2 4 6 8 10 12 14 0 2 4 6 8 10 12 n w0 external halo internal halo figure 5: w0 distribution of astronomical populations. the contamination of the halo gcs with the extragalactic origin ones is suggested by the bimodality in the absolute age distribution, by the rotation in the plane of galactic disk, and by the age-metallicity dependence. the situation is shown in figs.6, 7 and 8. regarding the age-metallicity dependence, we can say that few objects are located out of the main well defined sequence of clusters (and probably have a different origin with respect to all the others). 236 data analysis of globular cluster harris catalogue... part ii figure 6: absolute age distribution for mw gcs. older gcs, presumably all native in the milky way, are evidenced in dark grey. figure 7: motion over the galactic plane for mw gcs (from dinescu et al., 1999). figure 8: age-metallicity behavior, using the absolute age values extimated by dotter et al. (2010). pcc clusters are indicated by grey squares, suspected pcc ones by white triangles. if we analyze the behavior of the central relaxation time trc in function of w0 (fig.9), we can note a linear decreasing, that indicates an increasing of evolutional speed towards the collapse. below the treshold value log trc = 8.0 there is a region of coexistence of pre-core collapse gcs with post-core collapse (pcc) ones. the behavior of core collapse time tcc (that is the remaining time before the collapse of the model), changes over the stability limit w0 = 6.9 (fig.10). 2 4 6 8 10 12 14 4 5 6 7 8 9 10 11 rgc< 3 kpc 3 kpc < rgc< 30 kpc rgc> 30 kpc lo g (t rc /1 yr ) w0 ngc6717 figure 9: behavior of trc in function of w0; three classes of cluster distances are represented. vertical dashed line represent the stability limit, whereas the horizontal one represent the trc critical value (cohn & hut, 1984) distinguishing pre-core-collapsed and postcore-collapsed objects. suspected pcc clusters are indicated by filled symbols. in the harris catalogue ngc6723 has been erroneously included among the suspected pcc clusters in place of ngc6717. 2 4 6 8 10 12 14 7 8 9 10 11 12 lo g (t cc /1 yr ) figure 10: core-collapse time in function to w0. the tcc values are estimated as in quinlan (1996). we also consider a comparison among four gcs populations, shown in fig.11: the lmc is the less evolved, as well as the smc and fornax systems (mackey & gilmore, 2003b, 2003c); it presents a peak value close to w0 = 4.3. for this kind of gcs population, the presence of a low massive main body allowed to preserve 237 marco merafina, daniele vitantoni more informations about primeval distribution features. on the contrary, ngc5128, whose main body is a giant elliptical galaxy, seems to be an evolved population with a maximum value up to the threshold value w0 = 6.9. finally, the m31 system is the most similar to our gcs population, with a main peak value around the stability limit and an extended tail in low-w0 region, that is produced by the presence of low evolutionary speed objects or extragalactic origin clusters. also the effects of disk and bulge shocking, realistically, concurred to the formation of the tail. 3 conclusions in order to analyze the dynamical evolution of king single mass gcs, we have analyzed the mw gcs population. the mw clusters w0 distribution presents a peak very close to the new stability limit w0 = 6.9 and a pronounced tail in the low-w0 region. 3 6 9 12 0 5 10 0 5 10 0 3 6 3 6 9 12 0 3 6 w0 mw n m31 lmc figure 11: comparison among w0 distributions of different gcs systems. in addition to mw system, m31, ngc5128 and lmc systems are reported. m31 data come (or are deduced) from barmby et al. (2007), ngc5128 ones from gòmez et al. (2005), lmc data from mackey & gilmore (2003a). mw histogram differences from fig.2 are given by a different binning choice. we can instead exclude a direct relation between astronomical gcs populations and dynamical evolution, except for a very weak increasing of w0 peak value for disk clusters. from the time-scales we can deduce that clusters with high w0 value have an higher collapsing speed. the gravothermal catastrophe produce an alteration of the natural evolutionary sequence for clusters with w0 ≥ 6.9. from the comparison with extragalactic gcs systems we have deduced that, in the case of lmc, smc and fornax system, these clusters have a gaussian like distribution around a peak value w0 ∼ 5. mw and m31 system are very similar in their features and w0 distribution, with low-w0 tail and a peak value in corrispondence of the stability limit. we can assume this as the main product of disk shocking, as well as extragalactic capture and low speed evolution objects mentioned above. for ngc5128 there is no low w0 region tail, but only a narrow peaked distribution around the stability limit. references [1] aguilar, l., hut, p., ostriker, j.p.: 1988, apj, 335, 720 doi:10.1086/166961 [2] barmby, p., mclaughlin, d.e., harris, w.e., gretchen, l.h., forbes, d.a.: 2007, aj, 133, 2764 [3] bica, e., bonatto, c., barbuy, b., ortolani, s.: 2006, aap, 450, 105 doi:10.1051/0004-6361:20054351 [4] cohn, h., hut, p.: 1984, apj, 277, l45 doi:10.1086/184199 [5] dinescu, d.i., terrence, m.g., van altena, w.f.: 1999, aj, 117, 1792 [6] dotter, a. et al.: 2010, aj, 708, 698 [7] gnedin, o.y., ostriker, j.p.: 1997, aj, 474, 223 [8] gòmez, m., geisler, d., harris, w.e., richtler, t., harris, g.l.h., woodley, k.a.: 2006, aap, 447, 877 doi:10.1051/0004-6361:20053393 [9] harris, w.e.: 1996, aj, 112, 1487 [10] hut, p., djorgovski, s.: 1992, nat, 359, 806 doi:10.1038/359806a0 [11] katz, j.: 1980, mnras, 190, 497 doi:10.1093/mnras/190.3.497 [12] king, i.: 1966, aj, 71, 64 [13] mackey, a.d., gilmore, g.f.: 2003a, mnras, 338, 85 doi:10.1046/j.1365-8711.2003.06021.x 238 http://dx.doi.org/10.1086/166961 http://dx.doi.org/10.1051/0004-6361:20054351 http://dx.doi.org/10.1086/184199 http://dx.doi.org/10.1051/0004-6361:20053393 http://dx.doi.org/10.1038/359806a0 http://dx.doi.org/10.1093/mnras/190.3.497 http://dx.doi.org/10.1046/j.1365-8711.2003.06021.x data analysis of globular cluster harris catalogue... part ii [14] mackey, a.d., gilmore, g.f.: 2003b, mnras, 338, 120 doi:10.1046/j.1365-8711.2003.06022.x [15] mackey, a.d., gilmore, g.f.: 2003c, mnras, 340, 175 doi:10.1046/j.1365-8711.2003.06275.x [16] mackey, a.d., gilmore, g.f.: 2004, mnras, 355, 504 doi:10.1111/j.1365-2966.2004.08343.x [17] quinlan, g.d.: 1996, newa, 1, 255 [18] van den bergh, s.: 2011, pasp, 123, 1044 doi:10.1086/662132 [19] zinn, r.: 1985, apj, 293, 424 239 http://dx.doi.org/10.1046/j.1365-8711.2003.06022.x http://dx.doi.org/10.1046/j.1365-8711.2003.06275.x http://dx.doi.org/10.1111/j.1365-2966.2004.08343.x http://dx.doi.org/10.1086/662132 introduction discussion conclusions acta polytechnica ctu proceedings doi:10.14311/ap.2016.4.0022 acta polytechnica ctu proceedings 4:22–26, 2016 © czech technical university in prague, 2016 available online at http://ojs.cvut.cz/ojs/index.php/app thermal-hydraulic analysis of irt-4m in reactivity insertion accident at vr-1 reactor filip fejt faculty of nuclear sciences and physical engineering, czech technical university in prague, czech republic correspondence: filip.fejt@fjfi.cvut.cz abstract. the paper deals with thermal-hydraulic analysis during reactivity insertion accident, i.e. a step increase of nuclear system reactivity by 0.7βeff , at vr-1 reactor. the reactor utilizes irt-4m type of fuel assemblies, and even though these fuel assemblies are designed for an operation at the high-power research reactors, they might be also used for zero-power reactors. the thermal-hydraulic analyses must take into account several specific assumptions that are derived from vr-1 reactor specifications. the reactor does not require a forced water flow for a fuel cooling, the core is placed in an open vessel with atmospheric pressure, and amount of coolant water in the vessel is sufficient for providing the inlet water at room temperature for the whole event. coolant circulation is expected to be formed only by natural convection. keywords: irt-4m, ria, relap5, trace5, natural convection. 1. introduction thermal-hydraulic analysis is a necessary part of every safety analyses report. the adequate cooling of the fuel assemblies must be ensured to maintain the core integrity during normal and abnormal operational transients, and accidents. in general, the research reactors may be designed for both natural convection and forced convection. the pool type research reactor vr-1 uses a natural convection of coolant to remove a core heat. the upward flow in natural convection is caused by different fluid densities of heated and unheated volumes. important point of safety analyses is an onset of nucleate boiling (onb) [1] that is defined as a difference between the fuel surface temperature at the starting point of a nucleate boiling and the fuel surface temperature at a local cooling condition. reaching a nucleate boiling significantly limits the heat transfer leading to a dramatic increase of the fuel surface temperatures in order to maintain the total heat flux. the main purpose of thermal-hydraulic analysis is to inspect all of possible events that may lead to an accident, and verify that normal and abnormal operation has safety margins before the critical level is reached. 2. thermal-hydraulic codes due to the various research reactor designs, i.e. operational mode, fuel type and applications, each reactor has a unique thermal-hydraulic characteristic. computational codes must be capable of dealing with a broad range of different designs. since the current codes are still based on dimensionless quantities that are used to calculate flow patterns in different fluid situation, the equations are mostly derived from simplified benchmarks and loops that must be considered similar to a problem of interest. although many well-known codes, e.g. relap, trace, paret, have been designed for power reactors at first, they have been developed into general calculation tools that may be used even for research reactors. the main phenomena of vr-1 reactor analysis is natural convection, and therefore the chapter is followed by a brief description of natural convection part of the codes. 2.1. relap5 the relap5/mod3.3 code has been developed for best-estimate transient simulation of light water reactor coolant systems during postulated accidents [2]. the code includes several behaviors effecting a reactor coolant, e.g. loss-of-coolant accident, anticipated transient without scram, loss of feedwater, loss of flow etc. unlike its predecessors, relap5 series uses a two-fluid, nonequilibrium, nonhomogeneous, hydrodynamic model for transient simulation of the two-phase system behavior. convection calculations rely on evaluating forced convection, laminar convection, and natural convection and selecting the maximum of these values. the correlations are by dittus-boelter [3], kays [4], and churchill-chu [5] respectively. 2.2. trace trace has been designed to calculate best-estimate analyses of loss-of-coolant accidents, operational transients, accidents in pressurized light-water reactors and boiling light-water reactors [6]. the code includes multidimensional two-phase flow, nonequilibrium thermo-dynamics, generalized heat transfer, level tracking, and reactor kinetics. the partial differential equations that describe twophase flow model are solved using finite volume numerical methods. components are solved in one-dimension 22 http://dx.doi.org/10.14311/ap.2016.4.0022 http://ojs.cvut.cz/ojs/index.php/app vol. 4/2016 thermal-hydraulic analysis of irt-4m in ria at vr-1 or three-dimensions when necessary. timesteps are limited by courant-limit factor to ensure the computational stability of an examined system. the courantlimit depends on a phenomena and current conditions in a system. description of this factor can be found in [2]. similar to relap5, the trace follows a component-based approach to modeling a reactor system. each part of the system is inserted as a unique component. list of components in the code is designed to cover all power plant models. after that all the components are connected as the user requires. expected mode of calculation in examined ria is pre-critical heat flux region. the correlations are provided for the laminar and turbulent forced convection regimes and for natural convection. the wall heat transfer coefficient for single-phase liquid [6] convection is taken as the maximum of the values for laminar and turbulent forced convection and natural convection (1) hwl = max{hlam, hturb, hnc}, (1) where hnc is the wall heat transfer coefficient for natural convection, hlam is the wall heat transfer coefficient for laminar forced convection, and hturb is the wall heat transfer coefficient for turbulent forced convection. heat transfer coefficient for natural convection uses correlations for both the laminar and turbulent regimes and a maximum of the two values is used to ensure continuity. the heat transfer coefficient is computed from (2) hnc = kl dh ·nunc, (2) where k is thermal conductivity evaluated at the bulk fluid temperature and dh is the hydraulic diameter. the nusselt numbers are given by (3) and (4) nunc,turb = 0.1 · (gr l · pr l)1/3, (3) nunc,lam = 0.59 · (gr l · pr l)1/4. (4) both grashof and prandtl numbers are evaluated at bulk fluid temperature. 3. vr-1 reactor model the vr-1 core is placed in a vessel with large water reserve. during the transient the water reserve ensures the room temperature at the core inlet. even though the transient may last several hundred seconds, the average water temperature in the vessel will not increase because the flow rate caused by natural convection is very small. in order to maximize the first power peak, the coolant is expected to be stationary at the beginning of the simulation. this condition can be achieved when the main pump in the vessel is turned off and the reactor is operated at the very low power causing minimal flow velocity. the irt-4m fuel assemblies used the vr-1 reactor consist of 4, 6, or 8 fuel layers arranged in rounded rectangular tubes [7]. each fuel layer is surrounded by cladding layer from both inner and outer side. cladding layers are made of aluminium similar to fuel layers where the uo2 is dispersed also in aluminium. a thin layer thickness, both the fuel and the cladding, in addition to a high thermal conductivity of the aluminium leads to a very flat radial temperature profile in fuel assemblies. the core is represented by an averaged 8-tube fuel assembly (fig. 1) to comply with point kinetic equations that are integrated into the computational codes [8]. figure 1. top view of 8-tube irt-4m fuel assembly (yellow – water coolant, blue – cladding layer, green – fuel layer). 3.1. relap5 specification using the component-based structure of input files, a list of utilized components can be found in tab. 1. the bottom plenum serves as a coolant distributing element at the bottom part of fuel assembly. distributed coolant flows through the fuel assembly (each pipe represents one coolant flow area between heatstructures), and at the top of the fuel assembly the top plenum mixes the heated coolant that flows directly to the vessel water reserves. the model can be seen in fig. 2. component number pipe (fa coolant) 8x pipe (vessel) 1x heatstructure (fuel tubes) 8x plenum 2x table 1. list of relap5 components utilized in the single assembly vr-1 model. 23 filip fejt acta polytechnica ctu proceedings figure 2. relap5 model with green pipes connected to heatstructures, bottom and top plenum, and a large pipe serving as a reactor vessel. 3.2. trace specification unlike relap5 model, the trace model is not using the plenum component for distributing and mixing the coolant because it is assumed to be obsolete in current development. its role has been taken by advanced pipe settings; unfortunately, the low dimensions of fuel assembly flow surfaces and complex flow distribution made it inapplicable in this model. therefore the bottom and top plenum were replaced by independent pipes denying any interaction between coolant coming to/from flow surfaces. similar idea had to be applied also to the pipe representing the vessel; the vessel is divided into 8 pipes. the model can be seen in fig. 3. this pipe separation requires an extended usage of components (tab. 2). component number pipe (fa coolant) 8x pipe (vessel) 8x heatstructure (fuel tubes) 8x pipe (bottom plenum) 4x pipe (top plenum) 4x table 2. list of trace components utilized in the single assembly vr-1 model. 4. results for the sake of clarity, the graphical interpretation of results follows two main rules – relap results are plotted with connected points and -r is added in the legend; trace results are plotted with separate points and -t is added in the legend. each figure 3. trace model with green pipes connected to heatstructures, set of pipes serving as bottom and top plenum, and set of pipes serving as a reactor vessel. spatial value is considered to be averaged over flow surface, therefore such a value has a two digit number representing a number of outer and inner fuel layer (fuel layer number starts from the outside of the fuel assembly: 1, 2, 3, . . . , 8, v denotes central area). 4.1. reactor power output the reactor power and the reactivity of the system can be seen in fig. 4. the reactivity is increased to the value of 0.7βeff at t = 0.5 s, after that the value is decreased by feedback effects created from both the fuel and the moderator. the reactivity increase starting at t = 20 s is caused by the slowly increasing coolant velocity effecting its heat removal capabilities. after 200 seconds the reactivity reaches the zero value. 4.2. coolant velocity fig. 5 shows a fast velocity increase of coolant that causes the reactivity peak shown in fig. 4. this analysis is valid as long as the initial coolant velocity is zero, i.e. vessel pump is turned off, and the reactor power is very low. 4.3. coolant temperatures the temperatures of flow surfaces are shown in fig. 6. no coolant reaches the boiling point during the occurring ria event. it can be clearly seen that both relap5 and trace have a very good agreement in coolant temperatures between outside fuel layers. the difference between the codes is approx. 10 ◦c in the middle of the fuel assembly. 5. conclusions despite the equation differences that can be found in both codes, relap5 and trace code shows a very good agreement in both point kinetics and thermalhydraulic. even though a difference can be observed 24 vol. 4/2016 thermal-hydraulic analysis of irt-4m in ria at vr-1 figure 4. reactor power and reactivity of system during step increase of reactivity accident by 0.7βeff . figure 5. outlet coolant velocity during step increase of reactivity accident by 0.7βeff . figure 6. outlet coolant temperature during step increase of reactivity accident by 0.7 βeff . 25 filip fejt acta polytechnica ctu proceedings in the final power, it has no effect on maximal outlet coolant temperature. higher power, i.e. higher heating, is always compensated by an increase of velocity of a coolant that leads to a temperature decrease. the coolant velocity can be compared to negative reactivity feedback effects. the separate pipes can be considered as the most significant difference between both models. this effect can be seen in inner flow cross sections where the temperature difference can reach up to 10 ◦c. since the maximal temperatures are reached in outer flow cross sections and both codes compute the same coolant temperature, it can be safely assumed that the separate pipe model does not effect the maximal temperatures. references [1] j. daeseong, p. suki, p. jonghark, et al. cooling capacity of plate type reserach reactors during the natural convective cooling mode. progress in nuclear energy 56:37–42, 2012. [2] u.s. nuclear regulatory commission. relap5/mod3.3 code manual volume i: code structure, system models, and solution methods, 2010. division of systems research, office of nuclear regulatory research. [3] f. dittus, l. boelter. heat transfer in automobile radiators of the tubular type. publications in engineering 2:443–461, 1930. [4] w. kays. numerical solution for laminar flow heat transfer in circular tubes. american society of mechanical engineers 77:1265–1274, 1955. [5] s. churcill, h. chu. correlating equations for laminar and turbulent free convection from a vertical plate. international journal of heat and mass transfer 18:1323–1329, 1975. [6] u.s. nuclear regulatory commission. trace v5.840 theory manuals: field equations, solution methods, and physical models, 2013. division of systems research, office of nuclear regulatory research. [7] j. rataj, l. sklenka. calculations and measurement at the training reactor vr-2. international atomic energy agency, publications 2007. [8] j. daeseong, p. jonghark, c. heetaek. development of thermal hydraulic and margin anlysis code for steady state forced and natural convective cooling of plate type fuel research reactor. progress in nuclear energy 71:39–51, 2014. 26 acta polytechnica ctu proceedings 4:22–26, 2016 1 introduction 2 thermal-hydraulic codes 2.1 relap5 2.2 trace 3 vr-1 reactor model 3.1 relap5 specification 3.2 trace specification 4 results 4.1 reactor power output 4.2 coolant velocity 4.3 coolant temperatures 5 conclusions references acta polytechnica ctu proceedings doi:10.14311/app.2017.11.0039 acta polytechnica ctu proceedings 11:39–42, 2017 © czech technical university in prague, 2017 available online at http://ojs.cvut.cz/ojs/index.php/app its on railway martin leso czech technical university in prague, faculty of transportation sciences, praha, czech republic correspondence: leso@fd.cvut.cz abstract. development of technology management and security, transmission and information systems allows the concept of process management and security of railways and organizing the operation of a new, as yet non-phase in a non-traditional way. requirements for higher efficiency, accuracy and flexibility of the attractiveness of rail transport connection in the integrated transport systems are a challenge for the railways. these requirements can be solved just by changing the concept and technology of the presented here as an its-r. keywords: ertms/etcs, gsm-r, inteligent traffic systems on railway (its-r), traffic management systems (tms), railway control and command systems (ccs). 1. state of the art modern technologies are applied for control, command and signaling and in the area of information systems (for both control and passengers check-in) on the czech railway. electronic systems are installed (station electronic interlocking systems, crossing interlocking system and etc.) including remote control systems (central dispatching center). information systems allow to enter process and provide data to be able to control direct and on strategic level control and plan of railway transport. systems also provide information to passengers via panels and monitors. today’s approach is focused towards to install interlocking technologies on the infrastructure and the problems related to the movement of vehicles are missed out. vehicles are equipped by systems checking the driver’s watchfullness and by the lvz system transmiting signals to the driver’s cabine. but, the responsibility of reading signals and satisfy all rule remains on the driver himself. about 1500 km of the czech corridors is equipped by the system lvz. remaining parts of the infrastructure have no lzv and the driver is monitored only by periodical vigilance button pushing. this state of the art is analogical in easterneuropean countries (poland, slovakia, hungary). some vehicles (čd 471 series) are equipped by intelligent technology ato (automatic train operation), denoted avv in czech language, that makes possible to automate vehicle and driver’s work control. these systems lead to decrease operating costs but their wide application is not optimal now due to missing pieces of information from the infrastructure and absence of safety level of vehicle driving. despite the avv system developed in the czech republic reached excellent operating parameters on the world level. ertms implementation plan was ratified and published in 2014. this plan is a crucial document to define the ertms implementation strategy in the czech republic. the fundamental change (in comparison with existing plans) is a new strategy to equip vehicles by "etcs onboard part". on the continuous track sections equipped with etcs of hundreds of km length immediately after entry into service will be prohibited trains without functional compatible vehicle on-board etcs. it is planned to equip about 1000 vehicles by etcs onboard system up to 2020 and about 1500 vehicles up to 2024. it may be stated the most vehicles will be equipped by etcs. but ertms implementation plan is concerned only to ten-t track net, other track net is not supported yet. the czech rail net included core ten-t track with length of 1329 km (3670 km of rails), others tracks ten-t 1265 km (3022 km), other main tracks of the length 2430 km (3633 km kolejí) and 4409 km (5079 km kolejí) of regional tracks. european legislative tsi (technical specifications of interoperability) defines requests to adapt main and regional rails outside of ten-t including all vehicles and industrial vehicles to be interoperable, i.e. etcs compatible. the issue of dealing with these types of lines forming around 2 / 3 of railway network in czech republic, therefore remains a major challenge for the future. the ertms implementation plan prepare the implementation of its-r systems well. with regard to the locomotives complete on board etcs fulfilment, both national main line and regional lines shall be calculated to be vehicles to be equipped with ertms / etcs on-board units. the assumption of full ertms/etcs onboard fulfillment is the big opportunity for change scope and type of interlocking technologies installed on infrastructure. today’s "full" installation with railway interlocking systems (station, track and level crossing systems with signals, track circuits od axel counters and etc.) on this types of tracks aren’t full installed not yet. on focus mainly to the regional tracks, using of conventional interlocking systems is not economical justified. these types of track are therefore suitable for installation of ertms/etcs systems, mainly 39 http://dx.doi.org/10.14311/app.2017.11.0039 http://ojs.cvut.cz/ojs/index.php/app martin leso acta polytechnica ctu proceedings etcs – level 1, or etcs – ls mode. most forward-looking of all functions with regard to its systems may be etcs – l3 (currently validated as ertms / etcs regional). with these systems may also consider the use of new technologies gnss, which are not yet in place in the tsi, but there may be early implemented. on this types of tracks, on the corridors main tracks to, it is possible to minimalize investment and services costs by choosing the strategy and type of control of the railway system based on minimalizing of parts of infrastructure technologies with reduction of signals and vehicle detection systems. it is necessary to consider, having regard to its applicability of technologies, the use of radio communication between vehicle and infrastructure. they must also be addressed, in particular freight train integrity to enable the detection systems in infrastructure. in 2015 was also approved and published other strategic document of ministry of transportation of czech republic (md čr), the "action plan its", which also deals with the implementation of its to rail. action plan for ertms / etcs and its implementation in the czech republic and, in principle, only referred to the downstream transport technologies (remote control, automation driving, etc..). the overall system is the concept of its in rail. in another development, it is therefore advisable to ertms / etcs, gsm – r and its one unit, in order to implement appropriate design and integration of all systems to ensure the function of its – r. 2. european best practices western european countries, the issue of long-term facilities of vehicles already addressed since 1950 mainly due to the provision of safe movement of vehicles. for example, german railway has wholly used vehicle systems controlling train movements (lzb main lines, branch lines pzb). very high level of amenities vehicles ertms / etcs (level ls, level 2) currently have swiss railways. for example, german railways has a wholly owned equipment vehicles systems controlling train movements (main line system lzb, pzb byline). very high level equipment of vehicles ertms / etcs (level ls, step 2) currently has swiss railways. in europe and globally range, can be seen guideline facilities especially vehicle management systems for rail traffic on the principles of ertms / etcs. ertms / etcs is promoted by the european union as one of the subsystems to ensure the interoperability of the whole rail system in europe. ertms / etcs ensures compliance with the requirements of the its-r in particular parameters ensure the safety of vehicle movement. other parameters, such as reliability and operational efficiency, are more dependent on the conditions of application of the system in specific countries. a comprehensive approach to its technology is not yet defined, but it is limited to defining the figure 1. interlinkages partial rail subsystems in its-r. structure of the exchange of data on passenger and freight (taf, tap) – subsystem of telematics applications for freight and passenger transport. in the area of intelligent transport systems can be seen similar trends as in the czech republic, ie. centralizing operations management together with the strong support of information systems for traffic management, but at the european level is already increasingly applied in real operation automatic setting routes, including optimizing traffic management (ie. the swiss system rcs). apply new systems for the management and security of railway traffic. 3. system approach to railways the subject of deployment of the railway is an intelligent transport system whose architecture and elements are interconnected in order to optimize and increase the efficiency of the whole process (fig. 1). its overall concept must be focused on increasing the parameters of the rail system, particularly the safety, reliability, economic efficiency operation. assumption for the goals of its on the rail is availability of all vehicles as a main target in the system, which provides transport (goods and people). these include the installation of ertms / etcs system including automatic train control, wireless data transfer by train (gsm-r). these basic components enable implementation of the concept of intelligent vehicle concept, which ensures the function as safe motion control, as well as optimizing its movement. for proper operation of intelligent vehicles, it is necessary to equip the corresponding intelligent infrastructure that will provide adequate information, intelligent vehicles with the least economic and technical complexity. a necessary condition is also coverage of wireless twoway communication infrastructure, which can provide information not only for the management and operation, but it will also provide an information service for dispatching and public services to passengers. target status should lead to the integration and overall optimal solutions to the infrastructure, and 40 vol. 11/2017 its on railway figure 2. conceptual vision of its-r. especially on vehicles with the aim of using all the components for securing the services of its-r. to ensure sufficient functionality and quality of services must be technology-r its implemented using modern telecommunications and emerging technologies. as a basis for solving the safety features of the system (of trains) must be the movement of all vehicles within the control signaling system ertms / etcs (fig. 2. part draw black). these systems need infrastructure equipment gsm-r system. according to the chosen level of ertms / etcs infrastructure must be equipped with the appropriate infrastructure components. for etcs-l1 (ls) and etcs-l2 infrastructure must be fully equipped with conventional technology infrastructure. etcs level-l3 enables significant savings on the side of infrastructure facilities, conventional systems management and security are not necessary and are fully replaced by technology on the vehicle condition to ensure the integrity (integrity) of the train. to increase efficiency and optimize operations management, increase infrastructure capacity, reduce operating costs (fuel consumption and energy use, vehicle wear and infrastructure) is required all vehicles equipped with automatic train control system. this subsystem most of the information for their activities (information on the line, the vehicle allowed to travel etc ..) can be obtained through a signaling system ertms / etcs. to ensure greater attractiveness of traveling using modern information and check-in systems and logistics services for parent carriers and their customers is also needed to ensure coverage of railway infrastructure, high-speed data services (eg. the technology 4g / 5g; fig. 2. showing green and red). high speed data connectivity is to be ensured in all vehicles, i.e. not only driving the vehicle but also to all connection (in the case of complete sets embedded) vehicles. 4. advantages of the benefits of its deployment at railway the introduction of intelligent transport systems on the railways should provide ways to increase the efficiency of the entire transport system in the czech republic: (1.) state managers of transport infrastructures: (a) increasing the capacity of transport infrastructure (b) increasing transport reliability, minimize delays (c) reducing wear of the transport infrastructure (d) efficient rail transport with possibility of creation the integration transport systems with using railways as the transport backbone (e) optimization of investment in transport infrastructure (f) increasing the level of safety throughout the rail network (thanks to the strategy is to increase the safety of the entire network, after which the vehicle is moving under conditions appropriate infrastructure facilities) (2.) railway carrier and passangers: (a) a. reduction of vehicle operation costs (energy, operating costs) (b) reducing of traffic accidents (c) increasing the attractiveness of rail transport for passengers (d) increasing the attractiveness of rail transport for freight transporters (e) increasing the attractiveness of rail transport for carriers (passenger and freight) (f) increased profits from transport (3.) railway industry: (a) an opportunity for the development and implementation of new products and services (b) exports of modern technology with the potential to assert themselves on foreign markets 5. final conclusion this paper describes the concept of new technology for the management and security of railway traffic solutions enable intelligent transportation systems. at present day, all the key technologies are developed yet. if we want to fulfill the above-mentioned strategic action is needed as soon as possible to start with a pilot project of the its-r. the pilot project should be implemented at the appropriate regional railway track, which will be verified and mastering functionality of the various sub-components and the whole concept of traffic management. the subject of verification should be technology ertms / etcs l3 system including automatic train control, information and communication systems to ensure transmission 41 martin leso acta polytechnica ctu proceedings of data necessary for the management and operation, logistic processes and services for passengers. after this necessary phase of technology adoption may be considered for their further expansion into the real network traffic lanes regional and national railways. the main benefit of technology its-r is not only to increase safety on the regional railways, but also increase the flexibility and the possibility of significant automation and optimization of traffic on main line of railways. consequences should be in greater competitiveness and attractiveness of rail transport. 42 118 acta polytechnica ctu proceedings 1(1): 118–122, 2014 118 doi: 10.14311/app.2014.01.0118 v2282 sgr revisited roberto nesci1, corinne rossi2, antonio frasca3, ettore marilli3, paolo persi1, nicola cornero4 1inaf-iasf, via fosso del cavaliere 100, 00133 roma, italy 2university la sapienza, p.le a. moro 2, 00186 roma, italy 3inaf/oact, catania, italy 4cdso, aosta, italy corresponding author: roberto.nesci@iaps.inaf.it abstract the nature of v2282 sgr is examined on the basis of several multiband observations: a 20 years long i-band light curve of v2282 sgr obtained from archive photographic plates of the asiago and catania observatories; a ccd r-band light curve obtained at cornero observatory; jhk photometry from 2mass and ukdiss; spitzer irac and mips images; optical spectra from loiano observatory; x-ray flux from chandra. the star has a k-type spectrum with strong emission lines and is irregularly variable at all wavebands. the overall evidences suggest that v2282 sgr is a pre main sequence star with an accretion disk. keywords: variable stars pre-main-sequence stellar evolution. 1 introduction the m20 (trifid) nebula is a spectacular star forming region of the milky way: it is projected in the sky over the open star cluster ngc 6514, and its distance has been estimated by several authors, ranging from 1.4 kpc [6] up to 2.7 kpc [1]. the variable v2282 sgr was mainly studied by [5] who reported an amplitude of about 0.5 mag in the i band and classified it as a possible orion type variable. the star is positionally consistent with the soft x-ray source cxom20 180216.8-230347 detected by chandra in m20 [9]; it appeared not to be variable in x-rays, with a flux level of 0.167 c/s, and therefore was not further discussed in that paper. it is also consistent with a spitzer infrared source of the mid-infrared survey of the m20 by [10], classified as a proto-stellar source (class i/0 n.16) without comments about a possible optical counterpart. a nebular variable (either of the t tau, rw aur or t ori type) is believed to be a pre main sequence (pms) star already partially clear of its originating circumstellar envelope [2]: this seems at odds with classification as a class i/0 object, which is a still deeply embedded source. for this reason we decided to explore in more detail its nature. 2 photographic light curve we measured the magnitude of v2282 sgr on 25 plates with i-n emulsion and rg650 filter taken with the 65/90/235 schmidt telescope of the asiago observatory, and on 109 (i-n + rg650) plates of the 40/40/120 schmidt of the catania observatory. plates were digitized with an epson 1680 pro scanner in transparency mode at 1600 dpi. a sequence of comparison stars was defined selected from the gsc2.3.2 catalog, using the n band magnitudes (see fig. 1 and table 1). aperture photometry was made with iraf/apphot. the photometric accuracy of our magnitudes ranges from 0.08 mag at i = 13 to 0.18 mag at i = 14.5: fainter stars have larger dispersion, as expected. the rms deviation of the values of n magnitude for v2282 sgr was 0.25 mag in the asiago and 0.34 mag in the catania datasets, substantially larger than the faintest comparison star. the resulting overall light curve of the asiago and catania observations is shown in fig. 2. v2282 sgr shows substantial variations up to 1 magnitude, with an average level n = 14.0. 118 http://dx.doi.org/10.14311/app.2014.01.0118 v2282 sgr revisited figure 1: finding chart of v2282 sgr from a poss ii plate with iv-n emulsion. the variable is marked with l. comparison stars are labelled. the field is 12 arcmin wide in dec, north is up and east to the left. 0 2000 4000 6000 8000 10000 15.0 14.5 14.0 13.5 13.0 12.5 jd-2,440,000 v2282 sgr asiago-catania figure 2: light curve in the i photographic band of v2282 sgr from the asiago (open squares) and catania (filled squares) archives. due to its southern sky position the coverage shows a marked seasonal bias. 119 roberto nesci et al. 3 ccd observations ccd images of m20 were taken at the cornero observatory1 with a 30cm f/6.5 schmidt-cassegrain telescope and an rc filter in 19 different nights, from june to october 2009. the star showed an oscillating behaviour, with an amplitude of about one magnitude (see fig. 3.) the time interval between a minimum and a maximum may be as short as 10 days, so that the loose time sampling of historic plates may underestimate the actual variability range. furthermore, we obtained with the bfosc instrument at the loiano 1.52 m telescope a b, v and r photometric point on 2011 and an r one in 2012. the star was at b = 17.60, v = 16.31, r = 14.70 on 2011 and r = 14.40 on 2012; both r values are within the variability range of the light curve of 2009. 4 optical spectroscopy with the bfosc instrument we also secured two spectra of the star (on 2011-08-02 and 2012-07-18) at 4 å/pixel dispersion (resolution ∼12 å). the spectra were reduced using the iraf reduction package, following the standard procedures. due to the large airmass no reliable flux calibration could be obtained. the one dimensional spectrum of v2282 sgr shows a number of emission lines which are most probably formed in a region physically close to the star. the two spectra taken at a distance of one year from each other look rather similar. the only remarkable difference is that the [n ii] 6584å line is not apparent in july 2012. figure 3: the light curve of v22822 sgr from ccd images of the cornero observatory. crosses are a comparison star, filled squares are the variable. table 1: adopted comparison stars sequence ra dec id n b v f 270.591 -23.040 a 12.69 13.99 13.37 270.560 -23.068 b 13.52 15.47 14.73 270.522 -23.091 c 12.26 13.15 12.96 12.66 270.503 -23.103 d 13.02 13.72 270.497 -23.096 e 13.49 13.82 270.503 -23.070 f 13.29 14.81 14.44 13.57 270.494 -23.061 g 13.27 13.93 270.486 -23.001 h 12.63 13.15 12.96 12.82 270.633 -23.059 m 12.49 14.40 13.73 270.647 -23.114 n 12.82 13.37 270.637 -23.108 o 12.22 12.44 12.29 12.36 270.612 -23.095 p 12.52 13.71 13.15 12.87 270.692 -22.955 q 13.11 270.696 -22.980 r 13.68 15.36 14.55 270.703 -22.992 s 14.13 270.716 -22.993 t 14.21 14.38 270.685 -23.025 u 14.40 14.52 270.722 -23.067 v 13.89 15.35 14.53 270.689 -23.086 x 13.80 270.685 -23.095 y 12.85 14.42 13.72 13.24 270.696 -23.108 z 13.82 15.40 14.71 table 2: emission lines in v2282 sgr wavelength ion ew (å) 6584 [n ii] -5 6563 hα -20 6548 [n ii] -0.6:: 5007 [o iii] -10 4959 [o iii] -5 4861 hβ -16 4363 [o ii] -6:: 4340 hγ -15 the emission lines in the stellar spectrum are listed in table 2, together with their equivalent width (ew ). we remark that doing the sky subtraction from the raw spectrum of the star all the night-sky emission lines disappeared, while the [o iii] and balmer emission lines remained, so we are strongly convinced that they are real. 1http://www.cdso.it 120 v2282 sgr revisited among the absorption lines, the most prominent are the nai d doublet (6.5 å), the mgi b triplet (3.3 å), the blends of several atomic lines centered at 5269 and 6497 å, the cai 4227 å and the g band, indicating an intermediate g-k spectral type. the nai d doublet is very strong, compared to the typical value (2.5 å) of the main sequence or giants g-k stars, suggesting the presence of substantial interstellar/circumstellar matter, as expected in a pms star. figure 4: sed of v2282 sgr from photometric de-reddened (av = 1.3 mag) data. the best-fitting nextgen synthetic spectrum with t=4200 k and log g = 3.5, is over-plotted with a continuous line. to gain information about the stellar photosphere, we fitted the observed sed with low-resolution synthetic spectra calculated from the nextgen model atmospheres [3]. we fixed the star distance and extinction to the values d = 1.7 kpc and av = 1.3 mag reported by [4] and we let teff and log g free to vary, adopting a standard extinction law av = 3.1e(b − v ). this interstellar absorption is consistent with the typical value of av = 0.8 mag/kpc and a distance of 1.7 kpc. the fit was limited to the photometric bands from b to j , because afterwards the photospheric flux could be significantly contaminated by an ir excess. the result of the sed fit is shown in fig. 4 where the optical and near-ir fluxes have been corrected for the extinction and the error bars include the average variation of ± 0.5 mag observed both in r and i bands. the best-fit model is that with log g = 3.5 and teff = 4200 k. with these parameters, a radius of about 4.8rsun would be derived for the star at the adopted distance. the observed flux appears to be dominated by the stellar photosphere up to the k band, while an ir excess, likely related to a circumstellar disk, is quite evident longward of 3µm. to quantify how much the derived temperature is sensitive to the adopted av we performed several tests: two extreme cases are reported below. adopting av =0.5 gives teff = 3800 k, log g = 2.5 and a radius 5.1 rsun: such a low temperature is not compatible with the observed spectrum, which shows no molecular bands. adopting a substantially higher value (av =3.0) gives teff = 5600 k, log g = 3.5 and a radius 4.3 rsun: this is still compatible with the observed spectral features but produces somewhat larger residuals than the av = 1.3 case. the derived stellar temperature is clearly an increasing function of the assumed absorption. we remark that the actual absorption is the sum of the interstellar absorption, the circumstellar absorption and the possible contribution from the m20 nebula whose outskirts include the star. a measure of the actual absorption is not easy because the star spectrum is peculiar: in particular the ew of the mgi triplet is low for a g5 star (5600 k), and even more so for a k0 (4400 k) one: it would be typical of a main sequence f5 star, which is too hot for the observed spectral features. this suggests that a strong 121 roberto nesci et al. veiling is present in the spectrum, as indicated also by the presence of strong balmer emission lines. therefore we cannot reliably derive the amount of absorption by comparing the intrinsic b-v with the observed one. the ew of the nai d doublet is 6.5 å, so it is well in the saturated part of the relation between e.w. and e(b-v) by [8] and cannot give an accurate estimate of the color excess, save indicating that it must be substantially larger than the simple interstellar one. 5 infrared observations to better understand the nature of v2282 sgr we considered the j , h and k magnitudes taken at different epochs from 2mass, denis, ukidss catalogues and from [10] (see table 3). from this table it appears evident the near ir variability of the star. table 3: near infrared photometry date j h k note 1998/06/14 12.74 12.02 11.54 2mass 1996/04/01 12.90 ... 11.51 denis 2004/08/23 12.99 12.26 11.93 200inch 2006/07/23 15.71 14.21 12.78 ukidss spitzer/irac images between 3.6 and 8 µm were taken in three different epochs. we performed the photometry of the star using iraf/apphot with the same aperture size of [10]. the star was significantly variable, mainly at shorter wavelengths, enforcing the evidence of its pms nature. a bolometric luminosity, with an uncertainty of about one magnitude, may be computed from the infrared spitzer, jhk, and our optical data. we derived a value of 4.1 lsun in the range from b to k and 24.3 lsun in the range from b up to 24 µm, quite reasonable values for a pms star. 6 conclusions v2282 sgr can be classified as a probable class i premain sequence star associated with an accreting circumstellar disk. its observed optical spectrum is consistent with a late g type star. its long-term irregular optical variability, typical of accreting t tauri stars, can be due to a combination of stellar rotation and circumstellar dust occultation, while the observed transient near-ir excess arises from a combination of variable extinction and changes in the inner accretion disk. its x-ray luminosity (7.6 × 1032erg cm−2 s−1) is compatible with the value for pms with mass ∼ 2 msun [7]. one may wonder if v2282 sgr is actually a foreground object unrelated to the trifid nebula: we think that the strong high-excitation [o iii] and n[ii] lines in its spectrum prove the presence of hot gas in front of the star, which cannot be heated by the low temperature of the stellar photosphere and is therefore related to the m20 nebula. acknowledgement r. nesci thanks the direction of the asiago observatory for hospitality during the scanning of the archive plates. references [1] cambresy, l., rho, j., marshall, d.j., reach, w.t., 2011 a&a 527, 141 [2] glasby j.s., the nebular variables, pergamon press, 1974 [3] hauschildt, p. h., allard, f., ferguson, j., baron, e., & alexander, d. r. 1999, apj, 525, 871 doi:10.1086/307954 [4] lynds, b. t., canzian, b. j., & o’neil, e. j., jr. 1985, apj 288, 164 doi:10.1086/162775 [5] maffei, p., 1963, memsait 34, 441 [8] munari, u., zwitter, t., 1997, a&a 318, 269 [6] ogura, k., & ishida, k. pasj 1975, 27, 119 [7] preibisch t. 2007, memsait 78, 1 [9] rho, j., ramirez, s.v., corcoran, m.f., hamaguchi, k., lefloch, b., 2004, apj 607, 904 doi:10.1086/383081 [10] rho, j., reach, w.t, lefloch, b., fazio, g.g, 2006, apj 643, 965 doi:10.1086/503245 122 http://dx.doi.org/10.1086/307954 http://dx.doi.org/10.1086/162775 http://dx.doi.org/10.1086/383081 http://dx.doi.org/10.1086/503245 introduction photographic light curve ccd observations optical spectroscopy infrared observations conclusions 41 acta polytechnica ctu proceedings 2(1): 41–45, 2015 41 doi: 10.14311/app.2015.02.0041 cvs around the minimum orbital period s. zharikov1, g. tovmassian1 1observatorio astronomico nacional, instituto de astronomia, universidad nacional autonoma de mexico, ensenada, bc, mexico, 22860 corresponding author: zhar@astrosen.unam.mx abstract we discussed features of cataclysmic variables at the period minimum. in general, most of them must be wz sge-type objects. main characteristics of the prototype star (wz sge) are discussed. a part of wz sge-type objects has evolved past the period limit and formed the bounce back systems. we also explore conditions and structure of accretion disks in such systems. we show that the accretion disk in a system with extreme mass ratio grows in size reaching a 2:1 resonance radius and are relatively cool. they also become largely optically thin in the continuum, contributing to the total flux less than the stellar components of the system. in contrast, the viscosity and the temperature in spiral arms formed at the outer edge of the disk are higher and their contribution in continuum plays an increasingly important role. we model such disks and generate light curves which successfully simulate the observed double-humped light curves in the quiescence. keywords: cataclysmic variables dwarf novae period minimum. 1 introduction a widely accepted evolutionary theory of cataclysmic variables (cvs), as presented in kolb & baraffe (1999, and references therein), predicts a significant accumulation of cv systems around the orbital period minimum (paczynski 1981). it also envisions that ∼70% of the current cv’s population has evolved past the orbital period minimum and formed so-called bounce-back systems. figure 1 illustrates the current concept of cv evolution at the orbital period turn-around point on the mass-transfer rate and mass-ratio to orbital period diagrams. cataclysmic variables with orbital periods close to the 80 min orbital period minimum that undergo infrequent (years to decades) super-outbursts are called wz sge-type stars. objects with short periods that have not been observed in outburst or super-outburst but have spectral characteristics similar to wz sge are listed also as wz sge-type candidates. those systems are dominated at the period minimum. the bounce-back systems are cataclysmic variables evolved beyond the minimum period limit, which is reached when the secondary star becomes of a substellar mass (brown dwarf) and partially degenerate. bounce-back systems are expected to float within the 80 100 min orbital period range. bounceback systems are spectroscopically similar to these, but not every wz sge-type object has necessarily passed through the turning point. figure 1 also displays the expected position of the bounce-back systems. in this paper we discuss observational characteristics of cataclysmic variables at the period minimum and explore conditions and structure of its accretion disk. figure 1: schematic distribution of cv-types on the plot of the mass ratio and mass transfer rate vs. the orbital period. 2 wz sge-type stars wz sge is the prototype star of the class of short-period cataclysmic variables (named wz sge-type stars). it is a high inclination (∼77o; steeghs et al. 2007) cataclysmic variable with a late m-type dwarf secondary star (0.078m� < m2 < 0.13m�) orbiting at 81.6 min the fastest-spinning (∼ 28 sec, robinson et al 1978) 41 http://dx.doi.org/10.14311/app.2015.02.0041 s. zharikov, g. tovmassian white dwarf (0.88m� < m1 < 1.53m�). the distance to wz sge is only 43.5 pc. for most of its life it is in quiescence with a v ∼ 15, corresponding to mv ∼ 12. below the main characteristics of wz sge are summarised: • a short orbital period of 81.6 min, close to the predicted period minimum of cvs ∼ 77min with a main sequence secondary. • a spectrum in quiescence shows strong double-peaked balmer emission lines from the accretion disk surrounded by broad absorptions, formed by the primary white dwarf (see for an example howell et al. (2008)). • an infrequent ∼20-30yr and largeamplitude (∼ 8mag; 1913, 1946, 1978, 2001 yy.) super-outbursts succeeded by echo outbursts, absence of normal (as in su umatype stars) outbursts. in order to avoid an earlier occurrence of normal outburst in wz sge-type systems, it can be accepted as an extreme low viscosity parameter a ∼0.010.001 in accretion disks (smak (1993), osaki (1994)). • the light curve during a super-outburst shows long-lasting super-humps (patterson, et al. 2002). • the optical light curve are double-humped sometimes during super-outburst and in quiescence (patterson, et al. 1998; patterson, et al. 2002). • there is evidence of forming spiral arms in the disk during super-outburst (baba et al. 2002, howell et al. 2003). • in quiescence the accretion disk is asymmetric, and the bright spot region is shown to be extended along the mass transfer stream (skidmore et al. 2000, mason et al. 2000). • the outer layers of the accretion disk must be of low density and low temperature ∼3000k (howell et al. 2004). • a cavity most likely formed in the inner part of the disk during quiescence implying an annulus-shaped accretion disk (kuulkers et al. 2011). • the outer radius of the disk is about a 3:1 resonance radius (rdisk ≤ r3:1) in quiescence and it can reach to 2:1 resonance radius (rdisk ≤ r2:1) during a super-outburst. there are about ∼100 objects proposed or confirmed as wz sge-type systems. most of them were suggested based on features of spectra or the amplitude of the super-outbursts. figure 2 summarise current conception about wz sge-type systems which do not yet pass the period minimum. figure 2: the current conception about wz sgetype systems which do not yet pass yet the period minimum. 3 bounce-back systems after reaching the period minimum the cvs should evolve back toward longer periods and form a so-called bounce-back or post-period minimum systems. the increasing of orbital periods of bounce-back systems is accompanied by a decline in mass transfer rate about an order of magnitude according to the existing models of cvs evolution (kolb & barrafe 1999, sirotkin & kim 2010). the list of the bounce back candidates are given in table 2 and figure 3 (zharikov et al. 2013). the optical spectra in quiescence of proposed candidates are similar to wz sge. all those systems include a massive and relatively cool (∼ 12000k) white dwarf primary and a m2/m1 mass ratio less than 0.075. such low mass ratio implies a jupitersize secondary (a late type m-dwarf or a brown dwarf) and with a 2:1 resonance radius is within of the roche lobe of the primary. also, similar to wz sge, the bounce back candidates do not show normal outbursts which implies an extremely low α ∼ 0.01 − 0.001 viscosity parameter (smak (1993), osaki (1994)). this together with the low ∼ 10−11m�/year mass transfer rate, allows the accretion disk to expand in bounce back systems up to a 2:1 resonance radius and form a two spiral wave structure in those systems (lin & papaloizou, 1979). the large size of the disk and domination of white dwarf radiation in the optical range and the secondarys radiation in the jhk bands in sdss0804 and sdss1238 bounce back candidates (zharikov et al 2013, aviles et al 2010) lead to the conclusion that the ”standard accretion disk model” (frank et al. 2002) does not apply to bounce-back systems. 42 cvs around the minimum orbital period table 1: parameters of wz sge and of bounce-back candidates. nn/object porb v q m1 m2 twd eff i lc1 (days) (mag) (m�) (m�) (k) (o) wz sge 0.0567 ∼ 15 0.092 0.85 0.078 13500 77 +sq 1. gw lib∗ 0.0533 19.1 0.060 0.84 0.05 11 2. v455 and∗ 0.0563 16.5 0.060s >m9 11500 83 +q 3. al com∗ 0.0567 19.1 0.060 16300 +q 4. sdss1035 0.057 18.7 0.055e 0.94 0.05 10100 83 5. sdss1238 0.056 17.8 0.05 ∼ 1.0 0.05 12000 ∼70 +q 6. sdss0804∗ 0.059 17.8 0.05s ∼ 0.9 0.045 13000 ∼ 70 +q 7. eg cnc∗ 0.060 18.8 0.035s 12300 +s 8. rx1050-14 0.062 17.6 <0.055v 13000 <65 9. gd552 0.0713 16.6 <0.052v <0.08 10900 <60 10. re1255 0.083 19.0 <0.064v >0.9 <0.08 12000 < 5 1 light curve (lc) features: ”+” lc shows a double-hump during the orbital period; ”s” during super-outburst; ”q” during quiescence; ”-” absent of double-humps in lc. ∗ objects which demonstrated wz sge-type superoutburst. figure 3: the systems with known mass ratio vs. orbital period are plotted. the left axes is the mass ratio and estimated mass transfer rates are given on the right axes. the bounce-back candidates are enclosed in a dash-dotted box. cannizzo & wheeler (1984) studied the vertical structure of a steady-state, α-model thin-accretion disk for an accreting object of 1 m�. they found that, for low accretion rates, the disk structure is optically thin and can be doublevalued with high(∼5000 k) and low(∼2000 k) temperature branches. for α > 0.1 a warm solution is possible in the inner region of the accretion disk, but disk annuli at larger radii will be in a cold state with t < 2000 k. only the low-temperature solution exists for α ≈ 0.1. as α decreases with temperature, the tendency to develop cold solutions in quiescence is enhanced. therefore, accretion disks in bounce back systems are most probably cool (∼2500 k). the optical light curve of the high inclination bounce back candidates shows permanently a doublehumped light curve (marked by ”+q” in table 2). the spiral arm structures were found from doppler tomography mapping in quiescence in two well studied example of bounce back candidates sdss1238 (aviles et al. 2010) and sdss0804 (zharikov et al. 2013). also important, that, because the mass transfer rate significantly decrease after the period minimum the size of the magnetosphere of the primary with faint magnetic field will increase with the decreasing of ṁ and a relatively faint (≤ 1mgauss) magnetic field is enough to form a cavity in the inner part of the accretion disk (zharikov et al 2013). taking into account all these features a geometrical model of bounce back system was constructed to explain the observed double-humped light curve in quiescence. the model takes into account the positions of the bright structures in the doppler maps, the large size of the accretion disk, and the description of the spiral density waves in hachisu et al. (2004). figure 4 presents the geometry used in the model (central panel) and grayscale images present the height of the disk (left) and the temperature distribution (right) of the accretion disk. 43 s. zharikov, g. tovmassian figure 4: from left to right: vertical thickness of the accretion disk, model configuration used to calculate the light curves of bounce-back systems and the temperature distribution of the model. figure 5: left: examples of double-humped light curve; right: examples of the light curve generated by the model. we calculated a variety of models using the typical average parameters of bounce-back systems presented in table 1 and the double-hump-shape light curve (fig. 5, (left panels) is easily reproduced by such models, two examples of which are shown in fig. 5, (right panels). 4 conclusions the galactic age is high enough that ∼ 70% of cvs will have reached the cvs orbital period minimum. in general, most of them must be wz sge-type objects. a part of wz sge-type objects have already passed the period limit and formed the bounce back systems. the structure of the disk must be changed when the system pass through the minimum. before the minimum, mass transfer rate is higher and the accretion disk is hotter and smaller. after the period limit it is larger and reaches a 2:1 resonance radius. the disk is cool (∼ 2500k) and has a cavity in the inner part and presents permanently existing spiral arms. the spirals should be denser and hotter that the rest of the disk. based on these features we propose the double-humped light curve in quiescence state of a system together with orbital period longer than the period limits and the mass ratio of q < 0.08 as the main manifestation of a wz-sge system which already passed through the minimum orbital period. when accretion has ended the cvs can form very close short period wd+bd systems, two examples of which are found recently (wd0137-349, burleigh et al. (2006) and nltt5306, steele et al. (2013)). acknowledgement we acknowledge papiit grants in-109209/in-103912 and conacyt grants 34521-e; 151858. references [1] aviles, a., et al. 2010, apj, 711, 389 [2] baba, h. et al. 2002, pasj, 54, l7 44 cvs around the minimum orbital period [3] burleigh, m. r., et al. 2006, mnras, 373, l55 doi:10.1111/j.1745-3933.2006.00242.x [4] cannizzo, j. k., & wheeler, j. c. 1984, apjs, 55, 367 [5] frank, j., et al. 2002, accretion power in astrophysics, cambridge university press doi:10.1017/cbo9781139164245 [6] hachisu, i. et al. 2004, apjl, 606, l139 doi:10.1086/421295 [7] howell, s. b., et al. 2003, a&a, 399, 219 [8] howell, s. b., et al. 2004, apjl, 602, l49 doi:10.1086/382481 [9] howell, s. b., et al. 2008, apj, 685, 418 doi:10.1086/590491 [10] kolb, u., & baraffe, i. 1999, mnras, 309, 1034 [11] kuulkers, e., et al. 2011, a&a, 528, a152 [12] lin, d. & papaloizou, j., 1979, mnras, 186, 799 doi:10.1093/mnras/186.4.799 [13] osaki, y. 1995, pasj, 47, 47 [14] mason, e., et al. 2000, mnras, 318, 440 [15] paczynski, b. 1981, acta astron., 31, 1 [16] patterson, j., et al 1998, pasp, 110, 403 [17] patterson, j., et al. 2002, pasp, 114, 721 [18] robinson, e. l., et al. 1978, apj, 219, 168 [19] sirotkin, f. & kim, w. 2010, apj, 721, 1356 doi:10.1088/0004-637x/721/2/1356 [20] skidmore, w., et al. 2000, mnras, 318, 429 [21] smak, j. 1993, acta astronomica, 43, 101 [22] steeghs, d., et al. 2007, apj, 667, 442 doi:10.1086/520702 [23] steele, p. r., et al. 2013, mnras, 429, 3492 doi:10.1093/mnras/sts620 [24] zharikov, s., et al. 2013, a&a, 549, a77 discussion raimondo baptista: the opening angle of the spiral structure in an accretion disk depends on the ratio of the local sound speed to a keplerian speed. the disk region containing the spirals must be quite hot in order for the spiral will be wide open as in your model. is it physically consistent to get wide spiral in the disk with such low temperature such as ∼ 2000 k. sergey zharikov: our model is a geometrical model only. based on proposed geometrical shape and size of the disk in bounce back systems we can explain observed double-humped light curves in quiescence. formation such spiral arm structure and physical conditions in such cool disk is not clear yet. the first approach to describe such disks was presented in paper of cannizzo, & wheeler, (1984). 45 http://dx.doi.org/10.1111/j.1745-3933.2006.00242.x http://dx.doi.org/10.1017/cbo9781139164245 http://dx.doi.org/10.1086/421295 http://dx.doi.org/10.1086/382481 http://dx.doi.org/10.1086/590491 http://dx.doi.org/10.1093/mnras/186.4.799 http://dx.doi.org/10.1088/0004-637x/721/2/1356 http://dx.doi.org/10.1086/520702 http://dx.doi.org/10.1093/mnras/sts620 introduction wz sge-type stars bounce-back systems conclusions 288 acta polytechnica ctu proceedings 1(1): 288–292, 2014 288 doi: 10.14311/app.2014.01.0288 soft x-ray polarimeter: potential instrumentation and observations herman l. marshall1, norbert s. schulz1 1mit kavli institute, cambridge, ma, usa corresponding author: hermanm@space.mit.edu abstract we present an instrument design capable of measuring linear x-ray polarization over a broad-band using conventional spectroscopic optics. a set of multilayer-coated flats reflects the dispersed x-rays to the instrument detectors. the intensity variation with position angle is measured to determine three stokes parameters: i, q, and u – all as a function of energy. by laterally grading the multilayer optics and matching the dispersion of the gratings, one may take advantage of high multilayer reflectivities and achieve modulation factors >90% over the entire 0.2 to 0.8 kev band. this instrument could be used in a small suborbital mission or adapted for use in an orbiting satellite to complement measurements at high energies. we present progress on laboratory work to demonstrate the capabilities of key components. keywords: polarimetry x-rays. 1 introduction polarimetry is an inherently multiwavelength phenomenon. in the radio band, the detectors provide linear polarization information but for other bandpasses, special steps are required, so an observer has to choose whether to obtain the additional stokes parameters. polarization studies in the optical and radio bands have been very successful. radio polarization observations of pulsars provided “probably the most important observational inspiration for the polar-cap emission model” (taylor & stinebring1986) developed by radhakrishnan (1969), critical to modeling pulsars and still widely accepted (taylor & stinebring). tinbergen (1996) gives many examples in optical astronomy such as: revealing the geometry and dynamics of stellar winds, jets, and disks; determining binary orbit inclinations to measure stellar masses; discovering strong magnetic fields in white dwarfs and measuring the fields of normal stars; and constraining the composition and structure of interstellar grains. perhaps the most important contribution of optical polarimetry led antonucci & miller (1985) to develop the seminal “unified model” of seyfert galaxies, a subset of active galactic nuclei (agn). their paper has been cited in over 1000 papers in 30 years, over 5% of all papers ever written about agn. thus, the extra information from polarimetric observations has provided a fundamental contribution to the understanding of agn. although tens of thousands of x-ray sources are known from the rosat all-sky survey, polarization studies were carried out only in the 1970s and were limited to the brightest sources. in over 40 years, the polarization of only one source has been measured to better than 3σ: the crab nebula (novick et al. 1972; weisskopf et al. 1987). even for bright galactic sources, the polarizations were undetectable or were marginal 2 − 3σ results (silver, et al. 1979; long et al. 1980). furthermore, over the entire history of x-ray astronomy, there has never been a mission or instrument flown that was designed to measure the polarization of soft xrays. because of the lack of observations, there has been very little theoretical work to predict polarization fractions or position angles but there has been some recent progress in support of the now-cancelled mission, gems (the gravitation and extreme magnetism smex; see swank et al. 2010). the x-ray extreme universe satellite (xeus) and the international x-ray observatory (ixo) both had been planned to include a polarimeter but their probable successor, athena+, does not, so there is now no mission that is planned to include a polarimeter. here we describe a few potential scientific studies to be performed with an x-ray polarimetry mission with sensitivity in the 0.1-1.0 kev band that would complement observations with an instrument such as gems. 2 the scientific value of soft x-ray polarimetry there is significant scientific potential of an x-ray polarimeter operating below 1 kev. here, we discuss a few types of potential targets; for details see marshall et al. (2010), schnittman et al. (2013), ghosh et al. (2013). 288 http://dx.doi.org/10.14311/app.2014.01.0288 soft x-ray polarimeter: potential instrumentation and observations figure 1: a prediction of the variation of the polarization percentage (left) and its position angle (right) as a function of energy for agn with varying spin, a/m , and eddington ratio, l/ledd (schnittman & krolik, 2009). such studies were initiated with the prospect of obtaining polarization data in the 2-8 kev band using gems. the figures show that predictions depend strongly on energy. observations with a multilayer-based polarimeter would complement those by an instrument such as gems. 2.1 active galactic nuclei blazars are believed to contain parsec-scale jets with β ≡ v/c approaching 0.995. the x-ray spectra are much steeper than the optical spectra, indicating that the x-rays are produced by the highest energy electrons, accelerated closest to the base of the jet or to shock regions in the jet. brindle (1986) showed that the polarization of blazars increases from the ir to the optical band and can be as high as 25%, indicating that the x-ray polarization should be greater than 30%. the jet and shock models make different predictions regarding the directionality of the magnetic field at x-ray energies: for knots in a laminar jet flow it should lie nearly parallel to the jet axis (marscher 1980), while for shocks it should lie perpendicular (marscher 1985). see also poutanen (1994). mcnamara et al. (2009) recently suggested that x-ray polarization data could be used to deduce the primary emission mechanism at the base, discriminating between synchrotron, self-compton (ssc), and external compton models. their ssc models predict polarizations between 20% and 80%, depending on the uniformity of seed photons and the inclination angle. their spectra are very soft, brightest below 1 kev. theoretical work indicates that agn accretion disks and jets should be 10-20% polarized (mcnamara et al. 2009; schnittmann & krolik 2009, dovčiak, et al. 2011) and that the polarization angle and magnitude should change with energy in a way that depends on the system inclination. schnittmann & krolik (2009) particularly show that the variation of polarization with energy could be used as a probe of the black hole spin and that the polarization position angle would rotate through 90 deg between 1 and 2 kev in some cases, arguing that x-ray polarization measurements are needed both below and above 2 kev (fig. 1). 2.2 isolated neutron stars isolated neutron stars (pavlov & zavlin 1997) and magnetars (heyl, shaviv, & lloyd 2003, lai; & ho 2003) should be bright enough for soft x-ray polarimeters. with temperatures below 0.5 kev, they can only be detected below 1-2 kev. magnetars are thought to be powered by the decay of enormous magnetic fields (1014– 1015 g). these fields are well above the quantum critical magnetic field, where a particle’s cyclotron energy equals its rest mass; i.e. b = m2c3/eh̄ (=4.4×1013 g for electrons). measuring the polarization of the radiation from magnetars in the x-ray band will not only verify the strength of their magnetic fields, but also can provide an estimate of their radii and distances and provide the first demonstration of vacuum birefringence, a predicted but hitherto unobserved qed effect (heyl, shaviv, & lloyd 2003) . the extent of polarization increases with the strength of magnetic field and decreases as the radius increases so compact neutron stars are predicted to be highly polarized, > 80% (heyl et al. 2003) . detailed models of less strongly magnetized neutron star atmospheres show that the polarization fraction would be 10-20% at 0.25 kev averaged over the visible surface of the star (pavlov & zavlin 2000). we can constrain not only the orientation of axes, but also the 289 herman l. marshall, norbert s. schulz m/r ratio for the thermally emitting neutron stars due to gravitational light bending. constraining m/r, impossible from the radio polarization data, is extremely important for elucidating the still poorly known equation of state of the superdense matter in the neutron star interiors. we note that these isolated neutron stars do not produce significant flux above 2 kev, so polarimeters with significant effective area in the 0.1 to 1.0 kev band will be needed to test polarization predictions from neutron star atmospheres. figure 2: bottom: overview of a small telescope designed for soft x-ray spectropolarimetry, based on a design suggested by marshall (2008). a small set of nested mirror shells focusses x-rays through gratings that disperse to an array of detectors about 2 m from the entrance aperture. top left: view from the front aperture. gratings are placed behind the mirror with the grating bars oriented along the average radius to the mirror axis. the gratings are blazed in the directions shown. top right: the view from above the detectors and polarizers. spectra from the gratings are incident on multilayer-coated flats that are tilted along the dispersion axis which contains the zeroth order (dot in center). the angle of the tilt is the same for all mirrors and always redirects the x-rays to the adjacent detector in a clockwise direction. the multilayer coating spacing, d, increases linearly outward from zeroth order, just as the wavelength increases, in order to match the first order wavelength to the peak of the multilayer reflectivity. the detectors face the corresponding mirror. measuring the intensity at a given wavelength as a function of clocking angle then provides i(λ), q(λ), and u(λ). 3 a soft x-ray polarimeter we have been working on soft x-ray polarimetry concepts using multilayer-coated optics for almost 20 years, starting with a very simple design (marshall 1994). briefly, multilayer coatings consist of thin layers of contrasting materials usually one with a high index of refraction and the other with a low value. the input wave is divided at each layer into transmitted and reflected components. when many layers are placed on a surface, then the reflected components may constructively interfere, enhancing the overall reflectivity of the optic. the bragg condition must be satisfied: λ = 2d sin θ, where d = da + db is the thickness of the bilayer consisting of one layer of material a with thickness da, and one layer of material b with thickness db; λ is the wavelength of the incident radiation; and θ is the graze angle. when used at brewster’s angle, θ = 45 deg, the reflectivity of p-polarization is reduced by orders of magnitude, so that nearly 100% of the exiting beam is polarized with the e−vector perpendicular to the plane defined by the incoming and outgoing beams. designs for flight instruments based on multilayer coated optics generallly have limited bandpasses. we now have a new design that overcomes this weakness that we are prototyping in the lab. marshall (2007) 290 soft x-ray polarimeter: potential instrumentation and observations showed that it is possible to develop a laterally graded multilayer-coated mirror that combines with a dispersive optic to obtain a broad bandpass. we have studied how to develop a multilayer polarimeter as a small mission (marshall 2008; marshall et al. 2010). fig. 2 shows the results from a design suitable for a mission of opportunity or a small explorer, using one mirror assembly of a size planned for gems but with a 2 m focal length. this telescope would take 5 min to detect a polarization fraction of 8% for a source like mk 421 at its current intensity. 4 conclusions soft x-ray polarimetry has excellent prospects for scientific study of agn, blazars, and isolated neutron stars. we have designed an instrument capable of detecting a polarization fraction as low as 8% in a suborbital mission. we are prototyping components for such a mission in the lab and will have a flight-like configuration within a few months. acknowledgement this work was funded in part by nasa grant nnx12ah12g. references [1] antonucci, r. r. j., miller, j. s.: 1985, apj, 297, 621. [2] brindle, c., et al.:1986, mnras, 221, 739 doi:10.1093/mnras/221.3.739 [3] dovčiak, m., et al.:2011, apj, 731, 75. doi:10.1088/0004-637x/731/1/75 [4] ghosh, p., et al. 2013, arxiv:1301.5514 [5] heyl, j. s., shaviv, n. j., lloyd, d.: 2003, mnras, 342, 134. doi:10.1046/j.1365-8711.2003.06521.x [6] lai, d., & ho, w. c.: 2003, prl, 91, 071101. doi:10.1103/physrevlett.91.071101 [7] long, k.s., chanan, g.a., and r. novick: 1980, apj 238, 710 doi:10.1086/158027 [8] marscher, a.p.: 1980, apj, 235, 386. [9] marscher, a.p., gear, w.k.: 1985, apj, 298, 114. doi:10.1086/163592 [10] marshall, h.l.: 1994, in proceedings of the spie, s. fineschi (ed.), 2283, p. 75-84. [11] marshall, h.l.: 2007, in proceedings of the spie, s.l. o’dell, g. pareschi (eds.), 6688, p. 66880z. [12] marshall, h.l.: 2008, in proceedings of the spie, s.l. o’dell, g. pareschi (eds.), 7011 [13] marshall, h.l., et al.: 2010, in proceedings of the spie, s.l. o’dell, g. pareschi (eds.), 7732. [14] mcnamara, a.l., kuncic, z., and wu, k.: 2009, mnras, 395, 1507. doi:10.1111/j.1365-2966.2009.14608.x [15] novick, r., et al.: 1972, apjl, 174, l1. [16] pavlov, g.g., zavlin, v.e.: apjl, 490, l91. [17] pavlov, g.g., zavlin, v.e.: 2000, apj, 529, 1011. doi:10.1086/308313 [18] poutanen, j.: 1994, apjs, 92, 607. [19] radhakrishnan, v., et al. 1969, nature, 221, 443. doi:10.1038/221443a0 [20] schnittman, j.d., krolik, j.h.: 2009, apj, 701, 1175. doi:10.1088/0004-637x/701/2/1175 [21] schnittman, j.d., et al. 2013, arxiv:1301.1957. [22] silver,e.h., et al.: 1979, apj, 232, 248. [23] swank, j., et al. 2010, x-ray polarimetry: a new window in astrophysics by ronaldo bellazzini, enrico costa, giorgio matt and gianpiero tagliaferri. cambridge university press, p. 251 doi:10.1017/cbo9780511750809.038 [24] taylor, j.h., stinebring, d.r.: 1986, annreva&a, 24, 285. [25] tinbergen, j.: 1996, astronomical polarimetry, cambridge, uk: cambridge university press, pp. 174. doi:10.1017/cbo9780511525100 [26] weisskopf, m.c., et al.: 1978, apjl, 220, l117. discussion jim beall: this arrangement sounds like it’s pretty delicate. how much of a challenge dos this pose for rocket flight? herman marshall: all but one of the components have been flown on satellite missions. the remaining component, the laterally graded multilayer coated mirrors, are likely to survive based on experience with other ml coated optics, used primarily for solar astronomy. the remaining challenge is to maintain alignment and pointing, needed to match the dispersed x-rays to the ml coated mirrors. the sensitivity 291 http://dx.doi.org/10.1093/mnras/221.3.739 http://dx.doi.org/10.1088/0004-637x/731/1/75 http://dx.doi.org/10.1046/j.1365-8711.2003.06521.x http://dx.doi.org/10.1103/physrevlett.91.071101 http://dx.doi.org/10.1086/158027 http://dx.doi.org/10.1086/163592 http://dx.doi.org/10.1111/j.1365-2966.2009.14608.x http://dx.doi.org/10.1086/308313 http://dx.doi.org/10.1038/221443a0 http://dx.doi.org/10.1088/0004-637x/701/2/1175 http://dx.doi.org/10.1017/cbo9780511750809.038 http://dx.doi.org/10.1017/cbo9780511525100 herman l. marshall, norbert s. schulz to misalignments will require stiff structures while variations due to pointing jitter will require that the ml coatings have a sufficiently broad response at a target wavelength, which ultimately depends on the dispersion of the gratings used. so, mitigating jitter will depend on a tradeoff between improving pointing stability and the availability of high dispersion gratings. 292 introduction the scientific value of soft x-ray polarimetry active galactic nuclei isolated neutron stars a soft x-ray polarimeter conclusions acta polytechnica ctu proceedings doi:10.14311/ap.2016.4.0038 acta polytechnica ctu proceedings 4:38–42, 2016 © czech technical university in prague, 2016 available online at http://ojs.cvut.cz/ojs/index.php/app hydrogen charging of fuel cladding methodology jakub krejčía,∗, jitka kabátováb, jan kočíb, zuzana weishauptovác, věra vrtílkováb a department of nuclear reactors, faculty of nuclear sciences and physical engineering, czech technical university in prague, czech republic b ujp praha, a.s., prague, czech republic c department of geochemistry, institute of rock structure and mechanics, czech academy of sciences, v.v.i., prague, czech republic ∗ corresponding author: krejci.jakub@seznam.cz abstract. hydrogen content is a very important parameter for mechanical properties of fuel cladding, especially after loca transients. therefore, it is necessary to take into account the amount of hydrogen absorbed in the fuel cladding during normal operation (before a hypothetical loca). the required value of hydrogen content is possible to reach by a long-term pre-oxidation test or a much shorter hydrogen charging experiment. the methodology of hydrogen charging developed in ujp is described in this contribution. results of experiments aiming to prepare samples with uniform hydrides and samples with a rim-layer and other hydrides are shown. keywords: fuel cladding, hydridation. 1. introduction thanks to a low absorption of neutrons and corrosion resistance zirconium alloys are an indispensable material for nuclear fuel cladding tubes. however, their mechanical properties deteriorate during operation in the reactor conditions, where corrosion reactions take place leading to the formation of oxide layers and also to dissolution and precipitation of hydrogen, and the formation of hydrides [1]. zirconium hydrides are brittle, and can initiate crack and fissure formation in the alloy. hydrogen also has a considerable effect on the ductility of the cladding, after a hypothetical loca (loss of coolant accident). much attention has been focused on this topic worldwide [2, 3]. the requirement of sufficient fuel rod strength upon quench taking into account an additional mechanical load is used regarding to the brittle mode, which is associated with cladding oxidation in the presence of steam at high temperature. this requirement led to a new equivalent cladding reacted (ecr) design limit based on loca semi-integral tests developed by jaea (japan atomic energy agency). this criterion is expressed as a function of in-reactor hydrogen pickup [3, 4]. alternative k-criterion and o-beta criterion for e110 alloys were developed in ujp [5, 6] also. both are applicable for specimens with hydrogen content. the presented study shows the methodology of hydrogen charging developed in ujp. samples with uniform hydrides and specimens with rim-layer and other hydrides are shown. double-sided and singlesided hydrogen charging is presented. 2. experimental and methodology 2.1. speciment preparation all samples examined in this study were fabricated of the e110 alloy. the chemical composition is presented in table 1. wt. nb h o hf zr [%/ppm] [%] [ppm] [ppm] [ppm] zr1nb 1.0 3 <400 400 balanced table 1. the chemical composition of studied alloy. the tested tubular specimens had the following dimensions: outside diameter ∼9.1 mm and wall thickness ∼686 µm. for double-sided hydrogen charging 30 mm or 90 mm long non-irradiated segments were used, which were cleaned, degreased, and then weighted, and measured (length, diameter). for single-sided hydrogen charging 90 mm long nonirradiated segments and end-plugs made of a rod from the same material were used. the tube-part and endplugs were welded together using the electron beam welding method. these specimens were cleaned, degreased, and then weighted and measured (length, diameter). after that, the samples were filled with inert gas (argon) at required inner pressure (0–12.5 mpa), and welded in a pressure chamber. the procedure of welding was developed in ujp, and the device can be seen in fig. 1. the samples were weighted after welding, and based on the mass gain of inner gas the inner pressure was recalculated. 38 http://dx.doi.org/10.14311/ap.2016.4.0038 http://ojs.cvut.cz/ojs/index.php/app vol. 4/2016 hydrogen charging of fuel cladding methodology figure 1. device used for-welding samples. 2.2. hydrogen charging using gravimetric sorption analyzer double-sided hydrogen charging was performed in cooperation with the institute of rock structure and mechanics of the czech academy of sciences, and it was published in 2015 [2]. the hydrogen charging was carried out using a microscale of an iga 002 gravimetric sorption analyzer (hiden). a sample was placed into the reactor chamber of the instrument by hanging it vertically on the microscale beam, and the entire system was evacuated at a temperature of 200 ◦c. hydrogen charging was carried out by hydrogen of 99.9999 vol.% purity at a temperature of 405 ◦c and pressure of 0.1 mpa. the mass changes of the sample and the thermodynamic conditions of the process were observed during hydrogen charging. an example of the course of the hydrogen charging process is presented in fig. 2. after the required mass gain was reached, a hydride layer was observed on the surface of the samples with the higher hydride concentration. figure 2. sample packed in aluminum foil. 2.3. autoclave charging an alternative method for hydrogen charging was developed in ujp. the autoclave method provides the possibility to charging bigger amount of samples together. target amount of hydrogen content is up to 600 ppm for pwr conditions (zirlo ∼70 gwd/t [3]), and up to 120 ppm for e110 in vver environment (∼5 years in reactor conditions). specimens were packed in an aluminum foil, which was used as a protection against oxidation, see in fig. 2. the prepared samples were placed in the autoclave with an inner volume of about 4.5 dm3. the hydrogen charging was carried out by hydrogen of 99.9999 vol.% purity at a temperature of 350 ◦c, and pressure of about 0.1 mpa. the temperature and the pressure in the autoclave during the hydrogen charging process are shown in fig. 3. figure 3. temperature and pressure during hydridation process. after the hydrogen charging process as the aluminum foil was removed, a hydride layer and oxide layer was observed on the surface of the samples, see in fig. 4. figure 4. the typical outlook of specimen after hydrogen charging process. samples were weighted after, and based on the mass gain the hydrogen pick-up was calculated. the following form is used for calculation. it presumes, that the whole mass gain corresponds to hydrogen pick-up h = ∆m v0 ∆m v0 + m0 v0 × 106 = 1 1 + m0 ∆m × 106, (1) where h is hydrogen content in ppm, ∆m is mass gain during hydridation, m0 is initial weight and v0 is sample volume. 39 j. krejčí, j. kabátová, j. kočí et al. acta polytechnica ctu proceedings 2.4. vacuum annealing the dissolving of hydride-layer was processed in a vacuum furnace (clasic). at a pressure about 10−6 bar, and temperatures of about 575 ◦c were used for annealing. the length of the exposure time was chosen up to 6 hours of duration. an influence of the temperature and the exposure time on the hydride-layer dissolution was observed. the surface of the samples after annealing was the same as before the charging process, as can be seen in fig. 5. figure 5. the typical outlook of specimen before and after vacuum annealing process. the samples were weighted after the vacuum annealing, and based on the mass loss the hydrogen pick-up was recalculated. for calculation of hydrogen content is used the equation (1), where ∆m is mass gain after annealing. 2.5. metallographic observations parts of the samples intended for a metallographic evaluation of the hydrides were fixed in epoxy resin, were prepared using a standard polishing procedure, and were etched after. on the prepared metallographic sections, the hydrides were observed and evaluated using a nikon ephiphot 300 light microscope and a lucie image analyzer. 2.6. hydrogen content measurements parts of the samples intended for hydrogen content measurements were analyzed using the analyzer g8 galileo (bruker), which works on the inert gas fusion (igf) principle. 3. results 3.1. uniform distribution of hydrides preparation of specimens with uniform distribution of hydrides was one of the objectives of the presented work. the methodology consisting of autoclave charging and vacuum annealing was applied on the sample ch21. a predicted hydrogen content value – based on weight gain and calculation – was evaluated at 123 wppm. it was in a good agreement with the experimental value gained using the g8 galileo, which was 135 wppm. a satisfactory dissolution of the hydride-layer was observed after 90 minutes of annealing in the vacuum furnace at a temperature of 575 ◦c. figure 6. sample ch21 was prepared using methodology for uniform distribution of hydrides. 3.2. influence of vacuum annealing the gravimetric sorption method was used for doublesided hydrogen of the sample 1h001. this sample was cut into 7 segments which were used for hydrogen content measurements and for experiments focused on the influence of the annealing parameters. the predicted hydrogen content value – based on the weight gain during the hydrogen charging – was evaluated at 540 wppm. it was in a very good agreement with the experimental value gained using the g8 galileo, which was 544 wppm. vacuum annealing conditions for several segments of the sample were described in table 2. segment temperature exposure time figure [◦c] [min] 1h001-6 no annealing no annealing 7 1h001-4 350 20 8 1h001-3 575 20 9 1h001-1 575 60 10 and 11 table 2. the sample 1h001, annealing conditions. the sample 1h001-6 was processed for hydrides evaluation after the hydrogen charging process. hydride layers and a uniform distribution of hydrides in the sample were found (fig. 7). the detail of sample numbered 1h001-4 did not show any observable hydridelayer dissolution (fig. 8). an observable beginning of the hydride-layer dissolution was seen for detail the sample numbered 1h001-3 (fig. 9). the metallographic evaluation of sample numbered 1h001-1 40 vol. 4/2016 hydrogen charging of fuel cladding methodology showed the hydride-layer in an advanced state of dissolution (fig. 10), or the whole hydride-layer dissolved (fig. 11). figure 7. sample 1h001, section 6. prepared after hydrogen charging (without annealing). figure 8. sample 1h001, section 4. after hydrogen charging was used annealing at temperature 350 ◦c and 20 minutes of time duration. figure 9. sample 1h001, section 3. after hydrogen charging was used annealing at temperature 575 ◦c and 20 minutes of time duration. figure 10. sample 1h001, section 1, detail b. after hydrogen charging was used annealing at temperature 575 ◦c and 60 minutes of time duration. figure 11. sample 1h001, section 1, detail a. after hydrogen charging was used annealing at temperature 575 ◦c and 60 minutes of time duration. 3.3. influence of inner pressure the mechanical properties of the fuel cladding depend not only on the hydrogen content, but as well on the orientation of the hydrides. radial hydrides can appear in some specific cases of operation, which can initiate a crack and failure of the fuel cladding. precipitation of radial hydrides was observed during slow cooling of heated samples with inner gas pressure under the temperature of hydrogen dissolution. after hydrogen charging were samples cut into segments. two were used for hydrogen content evaluation and one for metallography. the experimental values of hydrogen content gained using g8 galileo, were 328 wppm (829 447), and 393 wppm (829 463). the number of radial hydrides is directly connected with the value of applied stress, which is caused by the inner gas pressure. figures 12 and 13 show hydrides distributions for several values of inner pressure. an outer hydride-layer was observed in all cases and 41 j. krejčí, j. kabátová, j. kočí et al. acta polytechnica ctu proceedings for samples with inner pressure, especially sample numbered 829 463, radial hydrides were observed. radial hydrides for inner pressure of 0.05 mpa were not observed. the number of radial hydrides was evaluated as 34 % for 829 447 (inner pressure at 20 ◦c – 3 mpa), and 70 % for 829 463 (inner pressure at 20 ◦c – 9 mpa). figure 12. sample 829 447. after hydrogen charging with inner pressure 3 mpa. figure 13. sample 829 463. after hydrogen charging with inner pressure 9 mpa. 4. conclusions the experiments have shown that a requested hydrogen content and a uniform distribution of hydrides can be reached using hydrogen charging, and a further vacuum annealing procedure. influence of temperature and time exposure of vacuum annealing was observed. the experiments have shown the level of hydride-layer dissolution under several conditions. radial hydrides were observed during experiments with inner pressure. a large amount of radial hydrides, about 70 %, were observed for an inner pressure value which can be possibly reached in reactor conditions also. this contribution presented the methodology used for hydrogen charging in ujp. prepared samples is possible to use for loca experiments focused on testing material deteriorated during operation in the reactor conditions. future works will be focused on a more detailed elaboration of several topics. a quantitative evaluation of hydride-layer dissolution and radial hydrides precipitation will be provided. acknowledgements the authors would like to thank to j. šustr, j. sýkora, v. rozkošný, f. manoch, and m. štukbauer for the samples, and metallographic sections preparation, and for the hydrogen content evaluation. financial support for this research through grants no. cz.2.16/3.1.00/21563 and “ta02011025, program alfa – tačr” is gratefully acknowledged. references [1] h.-h. hsu. an evaluation of hydrided zircaloy-4 cladding fracture behavior by x-specimen test. journal of alloys and compounds 426(1–2):256–262, 2006. doi:10.1016/j.jallcom.2005.12.113. [2] z. weishauptová, j. navrátilová, v. vrtílková. the influence of hydrogen on high temperature oxidation of zr1nb cladding tubes. in conference topfuel 2015, proceedings – poster, pp. 319–326. 2015. https://www.euronuclear.org/events/topfuel/ topfuel2015/transactions/ topfuel2015-transactions-poster.pdf. [3] y. yan, t. burtseva, m. billone. recent results at argonne on post-quench ductility of high-burnup cladding. in fuel safety research meeting. japan atomic energy agency, 2009. [4] s. boutin, s. graff. a new loca safety demonstration in france. in conference topfuel 2015, proceedings – part ii, pp. 440–449. 2015. https://www.euronuclear. org/events/topfuel/topfuel2015/transactions/ topfuel2015-transactions-oral-2.pdf. [5] v. vrtílková, l. novotný, r. doucha, j. veselý. an approach to the alternative loca embrittlement criterion. in proceedings of segfsm topical meeting on loca fuel issues. 2004. https://www.oecd-nea. org/nsd/docs/2004/csni-r2004-19.pdf. [6] m. négyesi, v. klouček, j. lorinčík, et al. proposal of new oβ oxidation criterion for new types of the zr1nb alloy of fuel claddings. nuclear engineering and design 261:260–268, 2013. doi:10.1016/j.nucengdes.2012.09.033. 42 http://dx.doi.org/10.1016/j.jallcom.2005.12.113 https://www.euronuclear.org/events/topfuel/topfuel2015/transactions/topfuel2015-transactions-poster.pdf https://www.euronuclear.org/events/topfuel/topfuel2015/transactions/topfuel2015-transactions-poster.pdf https://www.euronuclear.org/events/topfuel/topfuel2015/transactions/topfuel2015-transactions-poster.pdf https://www.euronuclear.org/events/topfuel/topfuel2015/transactions/topfuel2015-transactions-oral-2.pdf https://www.euronuclear.org/events/topfuel/topfuel2015/transactions/topfuel2015-transactions-oral-2.pdf https://www.euronuclear.org/events/topfuel/topfuel2015/transactions/topfuel2015-transactions-oral-2.pdf https://www.oecd-nea.org/nsd/docs/2004/csni-r2004-19.pdf https://www.oecd-nea.org/nsd/docs/2004/csni-r2004-19.pdf http://dx.doi.org/10.1016/j.nucengdes.2012.09.033 acta polytechnica ctu proceedings 4:38–42, 2016 1 introduction 2 experimental and methodology 2.1 speciment preparation 2.2 hydrogen charging using gravimetric sorption analyzer 2.3 autoclave charging 2.4 vacuum annealing 2.5 metallographic observations 2.6 hydrogen content measurements 3 results 3.1 uniform distribution of hydrides 3.2 influence of vacuum annealing 3.3 influence of inner pressure 4 conclusions acknowledgements references 298 acta polytechnica ctu proceedings 1(1): 298–302, 2014 298 doi: 10.14311/app.2014.01.0298 the world space observatory -ultraviolet (wso-uv) space telescope; status update in 2013 ana i. gómez de castro1, boris shustov2, mikhail sachkov2 1aegora research group, universidad complutense de madrid, spain 2institute of astronomy of the russian academy of sciences, russia corresponding author: aig@ucm.es abstract this is a short primer and a brief update on the status of the world space observatory-ultraviolet (wso-uv) project dated in may 2013. wso-uv is a 170m primary space telescope equipped for ultraviolet imaging and spectroscopy that will be operational in 2017 hosting an open science program for the world-wide scientific community. keywords: astronomical instrumentation space astronomy ultraviolet astronomy. 1 introduction the ultraviolet (uv) range is fundamental for astrophysical investigations, since the resonance transitions of the most abundant species in the universe occur at these wavelengths/energies. the radiation cut-off at wavelengths shorter than 2800 å by the earth’s atmosphere makes uv astronomy only accessible from space. thus uv astronomy began with space exploration. after the copernicus mission, the international ultraviolet explorer (iue) was launched in 1978, becoming the first uv space observatory operated in real time; the iue allowed to carry out spectroscopic observations from 1150å to 3200å. later on, the far ultraviolet spectroscopic explorer (fuse) mission (19992007) opened the 900å -1000å spectral range for spectroscopic studies. the galaxy evolution explorer (galex, 2003-2011) has mapped, for the first time, the uv sky. as today, the hubble space telescope (hst) is the only operational mission in the uv range. hst is expected to last for a few more years. all these missions have amply demonstrated the feasibility and relevance of uv studies (see gómez de castro et al. 2006, gómez de castro & brosch 2009, for detailed compilations). the world space observatory-ultraviolet (wsouv) is an international space mission born as a response to the growing demand for uv facilities by the astronomical community. in the horizon of the next decade, the wso-uv will be the only two-meters class mission in the post-hst epoch which will guarantee access to uv wavelenghts. the project is managed by an international consortium led by the federal space agency roscosmos (russia). in this article, we briefly describe the wso-uv project, its general objectives and its main features. special emphasis is made on the ground segment and the instrument issis, the contributions of spain to the project. figure 1: the wso-uv space observatory. 2 the wso-uv scientific objectives the wso-uv is a multipurpose observatory on a geosynchronous orbit, which will provide data of large importance to investigate several open problems in astrophysics. the science drivers of the project are: • the study of the diffuse baryonic content in the universe and its chemical evolution – the main topics will be the investigation of baryonic content in warm and hot igm, of damped lyman-α 298 http://dx.doi.org/10.14311/app.2014.01.0298 the world space observatory -ultraviolet (wso-uv) space telescope; status update in 2013 systems, the role of starburts and the formation of galaxies. • the study of the formation and evolution of the milky way – the uv plays a particularly important role in the determination of energy inputs of the gas interacting with stars, and in the investigation of magnetic fields on star formation. the milky way history could be tracked through observations complementary to those obtained by the gaia mission. • the physics of accretion and outflows: the astronomical engines – this cathegory includes stars, black holes, interacting binaries, pre-main sequence stars an, in general, all those objects where accretion plays an important role in the evolution of the system. the efficiency and time scales of the phenomena will be studied, together with the role of the radiation pressure and the disk instabilities. • the investigation of the (extra)solar planetary atmospheres and astrochemistry in presence of strong uv radiation fields – the properties of the atmospheres of t tauri stars to study the environment where protoplanets grow. (see also gómez de castro et al. 2009) 3 the wso-uv mission and instrumentation the wso-uv telescope has an f/10 ritchey-chretien mounting with a primary diameter of 170 cm. wsouv has been thought as an observatory-type mission henceforth carrying instrumentation for uv imaging and spectroscopy (shustov et al. 2009, 2011). the wso-uv imaging and slitless spectroscopy instrument (issis) is a multipurpose instrument with a mode selector wheel that permits to carry out imaging and slitless spectroscopy in the 1150-3200 å spectral range. the instrument is equipped with two mcp detectors, with csi and cste photocathods for fuv and nuv observations, respectively. the wso-uv spectrographs (wuvs) consists of a set of three instruments: • the far uv high resolution spectrograph (vuves) that will permit to carry out echelle spectroscopy with resolution r∼50,000 in the 1150-1760 å range. it will be equipped with a photon-counting, micro channel plate (mcp) detector • the near uv high resolution spectrograph (uves) to carry out echelle spectroscopy with resolution r∼55,000 in the 1740-3050 å range. it will be equipped with a ccd detector to observe in the near uv. • the long slit spectrograph (lss) that will provide low resolution (r∼ 1000), long slit spectroscopy in the 11500-3050 å range. the spatial resolution will be 1 arcsec also, the width of the slit is 0.5 arcsec. the detector is a ccd cooled to −100o c to be sensitive to the far uv. figure 2: the instruments compartment in wsouv.the numbered sections correspond to: [1] optical bench with issis and wuvs mounted, [2] spectrographs, [3] cylinder inset of the instruments compartment, [4] protective cover of the instruments compartment and [5] heat pipes. (see also, sachkov 2010) prior to final tests, after the end of the construction phase, wso-uv instrumentation is expected to provide sensitivities similar to those of the hst instrumentation. the factor of 2 difference in the collecting surface between hst and wso-uv is compensated by the, much more efficient, high earth orbit of the wsouv, a geosynchronous orbit with inclination 51o. this will also allow to carry out efficiently monitoring programs. moreover, modern ”state of the art” ccd detectors will be used in the spectrographs. wso-uv expected launch date is 2016 and will be operational for five years with a possible extension to five years more. the space telescope is planned to be operated from two sites at madrid (ucm) and moscow (inasan) that will also host also the science and mission archives. the ground segment is being designed under a shared operations scheme. 299 ana i. gómez de castro, boris shustov, mikhail sachkov 3.1 issis design and expected performance the imaging and slitless spectroscopy instrument (issis) will be a key part of the wso-uv instrumentation. issis is the first uv imager to be flown to high earth orbit, above the earth geocorona. hence the uv background will be dominated by the zodiacal contribution and the diffuse galactic background due to dust-scattered starlight (murthy et al. 2010). the instrument has been designed to make full benefit of the heritage left by the galactic evolution explorer (galex) mission. galex has surveyed about 80% of the sky at uv wavelengths, providing for the first time a nearly complete view of the uv universe (martin et al. 2003, bianchi et al. 2011). however, galex spatial resolution was ∼ 4.2 arcsec and had very moderate spectroscopic capabilities. issis resolution will be ≤ 0.1 arcsec. the fine guiding system of the wsouv telescope will guarantee a high pointing stability (better than 0.1 arcsec at 3σ). moreover, issis will be equipped with gratings for slitless spectroscopy with resolution 500, in the full 1150-3200 spectral range. in imaging mode, issis effective area is about 10 times that of the galex imagers. figure 3: the layout of the imaging and slitless spectroscopy instrument (issis). the acronyms mark the location of: detectors (mcp), filter wheels (fw), pickup mirror mechanism (rm), calibration lamp (cl), mode selector mechanism (msm) and the mirrors m1 and m2. issis is designed to be an instrument for analysis of weak uv point sources or clumpy extended sources, especially those with well defined geometry. uv imaging instruments have been often equipped with prisms or very low dispersion grisms. the rapid decay of the resolution of prisms such as the available in the solar blind channel (sbc) of the advanced camera system (acs) makes very difficult its use to map extended line emission at wavelengths above some 1350 å. as the transmittance of narrow band filters in the far uv is ≤ 3% , integral-field low resolution spectroscopy is the main mean to map nebular emission. issis gratings will make feasible to use the powerful uv diagnostic tools to determine the location of dusty blobs and measure electron densities and temperatures. the instrument is located below the primary mirror and above the optical bench. this location imposes additional constraints to the design, in terms of weight and size: the maximum weight on the optical bench is 61.5 kg, and the full instrument has to be fit within a flat cylinder of height 17 cm. issis is fed by the central part of the beam but a pick-up mirror is required to fold the beam from the telescope adding one reflection. the final design is a compromise between the scientific requirements and the telescope/platform requirements. figure 3 shows the layout of the instrument. 4 wso-uv science programs the wso-uv will run three major science programs (see malkov et al. 2011 for details): • the core program includes the key scientific programs that will carry over the scientific objectives of the mission. the core program will be run for the first two years of the mission by the consortium building and operating wso-uv. • national programs: each country or funding body contributing to the project is entitled to receive a fraction of the observing time proportional to its contribution. after the third year of the project, 60% of the observing time will be awarded to these programs. national calls are expected to be issued for the national programs though they will be synchronized with the general project calls. guaranteed time for the instruments teams should be included in the national contributions. • open program to the world wide scientific community. this program will handle a 40% of the observing time after the 3rd year of the mission. targets of opportunity observations will be managed within these programs. 5 wso-uv ground segment (gs) the wso-uv gs is comprised of all the infrastructure and facilities involved in the preparation and execution of the wso-uv mission operations, which typically encompass real-time monitoring and control of the spacecraft, telescope and instruments as well as reception, processing and storage of the scientific data. in principle, there will be two complete gs systems: the russian 300 the world space observatory -ultraviolet (wso-uv) space telescope; status update in 2013 one will be located in moscow (lavochkin association and institute of astronomy of the ras), and the spanish one will be sited at madrid. the satellite operations will be shared between both ground control centers, transferring the mission control from one center to the other on a regular basis (lozano et al 2011). figure 4: basic layout of wso-uv ground segment. the science operations system and a fraction of the mission operations system are part of the spanish contribution to the wso-uv. the remote proposal system (rps), the science data processing system (sdps), the science archive (sa) and the scheduling systems are defined by the international science team composed by spanish and russian science support teams based at the universidad complutense de madrid (ucm) and russian science institute of astronomy of the russian academy of science (inasan). the science support team (st) is part of the man power of the gs, and is responsible of laying the foundation of and supervising all the operations related to the mission primary users: the scientists. at mission level, the st constitutes the core of the future wso-uv international observatory. a sumary of the high level definition documents, approved at mission level, for the development of the main science systems for wso-uv gs can be found in gómez de castro et al. 2011. 6 conclusions wso-uv is an international observatory that will grant access to the uv range in the post-hst era. at the time these proceedings are being written, the project is evolving into the construction phase. acknowledgement the spanish participation in the wso-uv project is being funded by the ministry of industry, energy and tourism (minetur). the scientific contribution to wso-uv is being funded by the ministry of economy and competitivity through the grant aya2011-29754c03. the authors wish to thank the large industrial and scientific team in russia and spain who are involved in the project for their dedication and expertise. references [1] bianchi, l., efremova, b., herald, j. et al. 2011, ap&ss, 335, 161 doi:10.1007/s10509-010-0581-x [2] gómez de castro, a.i., wamsteker, w., barstow, m. et al. 2006 ap&ss, 303, 133 doi:10.1007/s10509-006-9057-4 [3] gómez de castro, a.i., & brosch, n. 2009 ap&ss, 320, 1 doi:10.1007/s10509-009-0005-y [4] gomez de castro a.i. et al. 2009, in: chavez et al., new quests in stellar astrophysics. ii. ultraviolet properties of evolved stellar populations. p.319 springer, berlin [5] gómez de castro, a.i., sestito, p., sanchez doreste, n., et al. 2012, space ops 2012, on-line proceedings (http://www.spaceops2012.org/proceedings/ proceedings.html) [6] gómez de castro et al. 2012, space telescopes and instrumentation 2012: ultraviolet to gamma ray. proceedings of the spie, volume 8443, article id. 84432w. [7] lozano, j.m. et al. 2010, space ops 2010, aiaa 2010-2212 [8] malkov, o. et al., 2011 ap&ss, 335, 323 doi:10.1007/s10509-010-0589-2 [9] martin, c. et al. 2003, future euv/uv and visible space astrophysics missions and instrumentation. edited by j. chris blades, oswald h. w. siegmund. proceedings of the spie, volume 4854, pp. 336-350. [10] murthy, j., henry, r. c., & sujatha, n. v. 2010, apj, 724, 1389 doi:10.1088/0004-637x/724/2/1389 [11] reutlinger a. et al. 2011, apss 335, 311 [12] sachkov m. 2010, apss 329, 261 [13] shustov, b. et al. 2009 ap&ss, 320, 187 [14] shustov, b. et al. 2011 ap&ss, 335, 282 301 http://dx.doi.org/10.1007/s10509-010-0581-x http://dx.doi.org/10.1007/s10509-006-9057-4 http://dx.doi.org/10.1007/s10509-009-0005-y http://dx.doi.org/10.1007/s10509-010-0589-2 http://dx.doi.org/10.1088/0004-637x/724/2/1389 ana i. gómez de castro, boris shustov, mikhail sachkov discussion james beall: any idea of a possible launch date?. it’s a beautiful instrument. ana i. gomez de castro: the foreseen launch date is end of 2016. nino panagia: i believe this is a very important project. i’m wondering what is its expected lifetime. ana i. gomez de castro: the mission lifetime is 5 years plus 5 additional years after review. a 302 introduction the wso-uv scientific objectives the wso-uv mission and instrumentation issis design and expected performance wso-uv science programs wso-uv ground segment (gs) conclusions 38 acta polytechnica ctu proceedings 1(1): 38–41, 2014 38 doi: 10.14311/app.2014.01.0038 fullerenes, pahs, amino acids and high energy astrophysics susana iglesias-groth1,2 1istituto de astrofisica de canarias, la laguna, via láctea sn, 38201 la laguna, spain 2departamento de astrof́ısica de universidad de la laguna, la laguna, tenerife, spain corresponding author: sigroth@iac.es abstract we present theoretical, observational and laboratory work on the spectral properties of fullerenes and hydrogenated fullerenes. fullerenes in its various forms (individual, endohedral, hydrogenated, etc.) can contribute to the uv bump in the extinction curves measured in many lines of sight of the galaxy. they can also produce a large number of absorption features in the optical and near infrared which could be associated with diffuse interstellar bands. we summarise recent laboratory work on the spectral characterisation of fullerenes and hydrogenated fullerenes (for a range of temperatures). the recent detection of mid-ir bands of fullerenes in various astrophysical environments (planetary nebulae, reflection nebulae) provide additional evidence for a link between fullerene families and diffuse interstellar bands. we describe recent observational work on near ir bands of c60 + in a protoplanetary nebula which support fullerene formation during the post-agb phase. we also report on the survival of fullerenes to irradiation by high energy particles and gamma photons and laboratory work to explore the chemical reactions that take place when fullerenes are exposed to this radiations in the presence of water, ammonia and other molecules as a potential path to form amino acids. keywords: ism molecules: fullerenes, pahs nebulae. 1 introduction the discovery of fullerenes in 1985 when trying to reproduce the chemistry of the atmospheres of red giant stars (kroto et al., 1985) led to the identification of the third allotropic form of carbon. the most abundant fullerene molecule produced in those experiments was c60, a hollow molecule with 60 carbon atoms distributed in 12 pentagons and 20 hexagons (truncated icosahedron symmetry). comparetively, we recall polycyclic aromatic hydrocarbons (pahs) are planar molecules consisting of carbon rings and hydrogen, these rings are similar to benzene. the naphthalene and anthracene are the most simple pahs with two and three benzene rings, respectively. these molecules have been postulated as potential carriers of the unidentified infrared emission bands and of the diffuse interstellar bands which are ubiquitous in the interstellar medium (see e.g. léger and puget 1984). similarly, fullerenes and their hydrogenated forms, are potential carriers of the interstellar bands. several studies have shown that carbon ring based molecular forms are very stable against uv radiation. this is remarkable because the basic structures of amino acids and in general of the molecular basis of life are essentially conformed by such carbon rings making them rather stable against possible mutations. laboratory work bychen et al. (2008) have aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa demonstrated that under strong uv radiation a mixture of naphthalene, ice and methane at low temperature, it is possible to form a large variety of amino acids. fullerenes with carbon atom number of 20 (m2 + n2 + nm) where n and m are integers have icosahedral symmetry groups ih and exhibit high stability. these molecules are very stable against uv/gamma radiation and collisions with other particles. fullerenes have been detected in carbonaceou chondrites, a type of meteorite that originated in the early solar system,with abundances of the order of 0.1 ppm (pizzarello et al. 2000). on earth, fullerenes have been detected in sedimentary layers of the cretaceous-tertiary boundary (ktb) in china and bulgaria and in the mineral shunghite of the region of karelia (russia). in the various phases of the interstellar medium it is likely the presence of both, fullerenes and hydrogenated fullerenes, the socalled fulleranes (cnhm). a review of the properties of fullerenes and fulleranes can be found in cataldo & iglesias-groth (2009, 2010). here, recent developments on the astrophysical search for ionised fullerenes and on the properties of fullerenes when exposed to high energy radiation. 38 http://dx.doi.org/10.14311/app.2014.01.0038 fullerenes, pahs, amino acids and high energy astrophysics 2 astrophysical searches for fullerenes theoretical work on the photoabsorption spectrum of icosahedral fullerenes suggests that these molecules can be responsible of the strongest feature in the interstellar extinction curve, the uv bump at 2175 å. for large fullerenes and buckyonions the experimental photoabsorption spectrum is very poorly known and the potential role of these molecules in interstellar absorption has been explored on a theoretical basis (see, for example, iglesias groth et al., 2002, 2003). in these works a theoretical approach to the photoabsorption specturm was based on a hückel and pariser-parr-pople (ppp) model. fits of the computed spectra to measurements of the interstellar extinction in the uv/optical for different lines of sight in our galaxy (fitzpatrick, 1999) were obtained (see details of the model can be found in iglesias-groth, 2004). very good fits to the extinction curves are provided by the photoabsorption cross sections computed for both individual fullerenes and buckyonions. the comparison of these models with the observed uv bump leads to an estimate of the number density of these molecules (in the range 0.2-0.08 ppm) and of the percentage of carbon locked in fullerenes and buckyonions in the ism (iglesias-groth 2004, 2007) which depending the size distribution of these molecules could reach up to 25 % of all carbon in the ism. fullerenes c60 and c70 present active vibrational bands in the near infrared (3-20 µm) which have been key for their identification in planetary nebulae, reflection nebulae and post-agb stars (see e.g. cami et al. 2010, zhang and kwok, 2011). experimental measurement of the molar absorptivity for these infrared transitions is essential for the determination of fullerene abundances in such astrophysical contexts. laboratory work is being conducted to measure the dependence of the molar absorptivity of fullerene infrared bands with temperature from approx. 100 k to 500 k to cover a variety of conditions in the interstellar medium (iglesias-groth et al. 2011). 2.1 new results on ionised fullerenes: c+ 60 recently, it was reported the detection of mid-ir vibrational transitions of the fullerene c60 in a carbon rich protoplanetry nebula, iras 01005+7910 (zhang and kwok 2010). we recorded the spectrum of this object between 5500 and 10000 å with resolving power of 57,000 and detected two bands at 9577 and 9632 å which are consistent with laboratory measurements of the c60 cation (see figure 1. iglesias-groth and esposito, 2013). if these two bands were produced by c+ 60 in the material surrounding the central post-agb star we could infer that ∼ 1 % of carbon is trapped in this ionized form of fullerenes and that the cation abundance is much higher than that of the neutral fullerene species. it appears that ionized fullerene species in this protoplanetary nebula are significantly more abundant than in neutral form, which is consistent with the predictions of ionization models of pahs (bakes and tielens 1995, salama et al. 1996) for irradiated clouds near a hot star. figure 1: the two near ir bands in the spectrum of iras01005+7910, attributed to the cation of c60 (iglesias-groth & esposito 2013) these observations provide additional evidence for the presence of fullerenes in protoplanetary nebulae and suggest that a significant production takes place in this late stage of stellar evolution. mid-ir bands of c+ 60 could be present in the 7-20 µm spectrum of iras 01005+7910 and are also likely to be detected in the spectra of planetary nebulae. accounting for the ionized species of fullerenes appears necessary in nebulae with a nearby source of ionizing photons. a caveat: the derived abundances and excitation temperatures of planetary nebulae with claimed detections of fullerenes may have to be revised if indeed there is a significant contribution of cations to the formation of the bands used in these analyses. high resolution spectroscopy will be required for a reliable determination of the relative abundance of neutral and ionized fullerenes in these objects. 2.2 the stability of c60 and c70 towards copuscular and γ radiation the stability of c60 and c70 fullerenes in the interstellar medium and embedded in meteorites and comets has been investigated with γ irradiation and with he+ ion bombardment. the radiation dose generated by radionuclides decay expected to be delivered to fullerenes buried at a depth of ≥ 20 m in comets and meteorites is about 3 mgy per million years. we have exposed fullerenes to various radiation doses and found that these molecules are by far resistant to this type of radiation. laboratory measurements on the concentra39 susana iglesias-groth tion of samples of fullerenes exposed to strong radiation doses indicate that these molecules can survive millions of years inside comets and meteorites (cataldo et al. 2009). this provides a natural explanation to their presence inside certain carbonaceous chondrites. in this laboratory experiments, fullerenes were also exposed to intense particle bombardment. fullerens adsorbed or deposited on the surface of carbon are exposed to cosmic ray bombardment with estimated doses of radiation comprised between 30 and 65 mgy per 106 years. we carried out experiments to test the survival of both c60 and c70 fullerenes and found that the complete amorphization occurs at about a radiation dose of 250 mgy. thus we infer that after 4-8 millions of years exposure to cosmic rays it is expected to producce a complete amorphization (cataldo et al. 2009). 2.3 gamma radiolysis of c60 fullerene in water and water/ammonia mixtures; relevance of fullerenes fate in ices of interstellar medium in order to explore the potential astrochemical role of fullerenes,iglesias-groth et al. (2013) have explored the radiolysis of fullerene c60 dispersed in h2o, h2o/nh3, h2o/methanol and h2o/nh3/methanol under gamma radiation doses of 250 and 500 kgy. it was found that c60 originally insoluble in the above mentioned hosting matrix became soluble as a consequence of multiple hydroxylation and oxidation reaction produced by the free radicals generated by the radiolysis of the hosting matrix. the changes undergone by c60 were studied by infrared spectroscopy (ft-ir) and by electronic absorption spectroscopy. the astrochemical consequences of the present study are that c60 ejected in the interstellar medium for instance from protoplanetary and planetary nebulae can condense together with water and other ices in dense molecular clouds. under the action of high energy radiation c60 reacts with the free radicals generated from the matrix where it is embedded it is solubilized and consequently its carbon content becomes available for further abiotic processes of synthesis of molecules of astrobiological interest. the behavior of c60 appears comparable to that of common pahs which are also hydroxylated and oxidized under similar conditions. 3 stability toward high energy radiation of non-proteinogenic amino acids: implications for the origins of life in order to investigate the response of amino acids to high energy radiation, cataldo et al. (2013) have taken a series of non-proteinogenic amino acids, most of them found frequently in carbonaceous chondrites, and exposed them to solid state radiolysis in vacuum to a total radiation dose of 3.2 mgy. this corresponds to 23% of the total dose expected to be taken by organic molecules buried in asteroids and meteorites since the beginning of the solar system in 4.6 x 109 years. the radiolyzed amino acids where studied by ft-ir spectroscopy, differential scanning calorimetry (dsc) and by polarimety and optical rotatory dispersion (ord). it is found that an important fraction of each type of amino acid is able to ”survive” to the massive dose of radiation and also the enantiomeric excess is partially preserved. based on these results it is concluded that it is not a surprise to find amino acids even in enantiomeric excess in carbonaceous chondrites. 4 conclusions the astrochemical consequences of the recent discoveries of fullerenes in various astrophysical contexts are not fully understood yet. fullerenes are far resistant to high doses of gamma radiation and bombardment by high energy particles. it is likely that fullerenes ejected in the interstellar medium,for instance from protoplanetary and planetary nebulae can condense together with water and other ices in dense molecular clouds. under the action of high energy radiation fullerenes react with the free radicals generated from the matrix where they are embedded. if c60 is trapped in water ices it is hydroxylated and oxidized by the radiolysis products of water. the oxidation of c60 makes this molecule hydrophilic and hence soluble in water. the same phenomenon occurs in water/ammonia, in water/methanol and in water/ammonia/methanol mixtures. thus, c60 which in the solid state displays a considerable radiation resistance, when embedded in radiolytic sensitive matrices like those just mentioned, it reacts swiftly, it is solubilized and consequently its carbon content becomes available for abiotic processes of synthesis of other molecules of astrobiological interest. this compares well with the behavior of pahs which are also hydroxylated and oxidized by the free radicals produced by the radiolysis of the hosting matrix (gudipati & allamandola 2006, ricca et al. 2007, ashbourn et al. 2007, cuylle et al. 2012, nuevo et al. 2012). it is known that pahs and ices exposed to strong uv radiation can lead to the formation of amino acids. fullerenes exposed to gamma radiation provides a path for a variety of astrochemical reactions which have to be explored in much more detail, which ultimately could lead to the formation of amino acids, which are likely to survive under the intense gamma radiation. further laboratory work is essential to explore the role of high energy radiation in astrophysical sources and carbon molecules as a route to prebiotic molecules. 40 fullerenes, pahs, amino acids and high energy astrophysics references [1] bakes, e. & tielens, a. g. g. m. 1995, aspc, 73, 59 [2] cami, j, bernard-salas, j, peeters, e & malek, s. e. 2010, science [3] cataldo, f., iglesias-groth, s. 2009, mnras, 400, 291 doi:10.1111/j.1365-2966.2009.15457.x [4] cataldo, strazzulla and iglesias-groth et al. 2009, mnras, 394, 615 [5] cataldo, f & iglesias-groth,s 2010, in fullerenes:the hydrogenated fullerenes, springer, berlin [6] chen et al. , 2008, mnras, 384, 60 [7] cuylle, s. h., tenenbaum, e. d., bouwman, j., linnartz, h. & allamandola, l. j. 2012, mnras, 423, 1825 doi:10.1111/j.1365-2966.2012.21006.x [8] fitzpatrick, e. l 1999, pasp, 111, 63 [9] iglesias-groth, s. et al., 2002, jchph, 116,1064810655 [10] iglesias-groth, s. et al., 2003, jchph, 118, 71037111 [11] iglesias-groth, s. 2004, ,apjl 608, l37 doi:10.1086/422216 [12] iglesias-groth, s. 2007, apjl, 661,l167 doi:10.1086/518832 [13] iglesias-groth, s., cataldo, f., ursini, o.& manchado, a. 2011, mnras, 210, 1447-1453. [14] iglesias-groth,s.,cataldo, f. & manchado, a. 2011 , mnras, 413, 213 doi:10.1111/j.1365-2966.2011.18124.x [15] iglesias-groth, s. & esposito, m. 2013,apjl ,776, l2 doi:10.1088/2041-8205/776/1/l2 [16] iglesias-groth, s., hafez, y., angelini, g. & cataldo, f. 2013, in press [17] kroto, h. w. et al. 1985, nature, 318, 162 [18] léger, a. & puget, j. 1984, a&a, 137,5 [19] nuevo, m., milam, s. n.& sandford, s. a. 2012, asbio 12, 295 [20] pizzarello, s. & cronin, j. r. 2000,gecoa,64, 329 doi:10.1016/s0016-7037(99)00280-x [21] ricca, a., bakes, e. l. o. & bauschlicher, c. w., jr. 2007, apj, 659, 858 doi:10.1086/512037 [22] salama, f., bakes, e. l. o., allamandola, l. j.& tielens, a. g. g. m. 1996, apj, 458, 621 [23] zhang, y., kwok, s. 2011,apj , 730, 126 doi:10.1088/0004-637x/730/2/126 41 http://dx.doi.org/10.1111/j.1365-2966.2009.15457.x http://dx.doi.org/10.1111/j.1365-2966.2012.21006.x http://dx.doi.org/10.1086/422216 http://dx.doi.org/10.1086/518832 http://dx.doi.org/10.1111/j.1365-2966.2011.18124.x http://dx.doi.org/10.1088/2041-8205/776/1/l2 http://dx.doi.org/10.1016/s0016-7037(99)00280-x http://dx.doi.org/10.1086/512037 http://dx.doi.org/10.1088/0004-637x/730/2/126 introduction astrophysical searches for fullerenes new results on ionised fullerenes: c60+ the stability of c60 and c70 towards copuscular and radiation gamma radiolysis of c60 fullerene in water and water/ammonia mixtures; relevance of fullerenes fate in ices of interstellar medium stability toward high energy radiation of non-proteinogenic amino acids: implications for the origins of life conclusions 269 acta polytechnica ctu proceedings 2(1): 269–272, 2015 269 doi: 10.14311/app.2015.02.0269 x-ray observations of vy scl-type nova-like binaries in the high and low state p. zemko1,2, m. orio2,3 1sternberg astronomical institute, moscow state university, 13, universitetskij ave., moscow, 119991, russia 2inaf osservatorio di padova, vicolo dell’ osservatorio 5, i-35122 padova, italy 3department of astronomy, university of wisconsin, 475 n. charter str., madison, wi 53704, usa corresponding author: polina.zemko@gmail.com abstract four vy scl-type nova-like systems were observed in x-rays both during the low and the high optical states. they are bz cam, mv lyr, tt ari, and v794 aql. using archival rosat, swift and suzaku observations we found that the x-ray flux for bz cam is higher during the low state, but there is no supersoft x-ray source (sss) that would indicate the thermonuclear burning predicted in a previous article. the x-ray flux is lower by a factor 2–10 in the low than the high state in other systems, and does not reflect the drop in ṁ inferred from optical and uv data. the best fit model for the x-ray spectra is a collisionally ionized plasma model. the x-ray flux may originate in a shocked wind or in accretion onto polar caps in intermediate polar systems that continues even during the low state. keywords: cataclysmic variables dwarf novae x-rays. 1 introduction nova-like (nls) stars are non-eruptive cvs (warner, 1995), classified into several subtypes according to their properties. here we will focus on the vy scl-type nls or “anti-dwarf novae” characterized by the presence of occasional dips on the ilight icurve, iso-called ilow states. the large optical and uv luminosity has suggested that in the high state these objects are undergoing mass transfer onto the wd at a high rate ṁ , > 10−10 m� year−1, sustaining an accretion disk in a stable hot state and preventing dwarf novae (dne) outbursts. the low states have been attributed to a sudden drop of the ṁ from the secondary, or even to a total cessation of a mass transfer (hessman, 2000). greiner et al. (1999) proposed a link between the vy scl-type stars and super soft x-ray sources based on a rosat observations of v751 cyg. these authors found an anti-correlation in the optical and x-ray intensity, and despite the very poor spectral resolution of the rosat hri, the spectrum appeared to be very soft in the low state. these authors suggested that quasi-stable thermonuclear burning occurs on the surface of the wd in the low state. in this framework, vy scl-type stars are key objects in the evolution of interacting wd binaries, in which hydrogen burning occurs periodically without outbursts. thus they may reach the chandrasekhar mass and the conditions for type ia supernovae explosion. using archival x-ray observations, in this paper we compare the high and low state x-ray data of 4 vy scl-type stars attempting to reveal evidence of nuclear burning during the low state, or seeking alternative explanations for the changes that take place during the transition from the high to low state. 2 observation and data analysis we examined the archival x-ray data of vy scl-type stars obtained with swift and rosat and chose the objects that were observed both in the high and low states: bz cam, mv lyr, tt ari and v794 aql. to determine when the low and high optical states occurred, we relied on the data of the variable star network (vsnet) collaboration (kato et al., 2004), aavso1 and asas databases. we used heasoft version 6-13 to analyze the data and xspec version 12.8.0 for spectral modeling. we also estimated the uv magnitudes of the objects using both swift/uvot and additional galex archival observations. 3 results in figure 1 we show the high and low state x-ray spectra of tt ari, bz cam, mv lyr and v794 aql and 1 http://www.aavso.org 269 http://dx.doi.org/10.14311/app.2015.02.0269 p. zemko, m. orio the fits to these spectra. the fit parameters are summarized in table 1 and 2. tt arietis (top panel of fig. 1) is one of the most optically luminous cvs, usually at mag. 10–11. sometimes it abruptly falls into an “intermediate state” around 14 mag. or even into a “low state” at about 16.5 mag. according to belyakov et al. (2010), this binary system consists of a 0.57–1.2 m� tt ari 1 2 3 4 5 6 7 0. 0 0. 1 0. 2 0. 3 bz cam 1 2 3 4 5 6 7 0. 00 0. 02 0. 04 0. 06 mv lyr no rm al iz ed c ou nt s (c nt s/ s/ ke v ) 2 4 6 8 0. 00 0. 02 0. 04 v794 aql 1 2 3 4 5 6 0. 00 0. 04 0. 08 0. 12 energy (kev) figure 1: the low and high states x-ray spectra of bz cam and mv lyr observed with swift. tt ari and v794 aql were observed with swift during the low and intermediate states. the high and intermediate state spectra are plotted in red and the low state spectra in black. the solid lines show the spectral fits, the dots with error bars are the data points. white dwarf and a 0.18–0.38 m� secondary component. the orbital period is porb = 0.13755114 d. (thorstensen, smak & hessman, 1985). the high state x-ray flux appeared to be about ten times larger than in the low state. we fitted the high state spectrum with a two components thermal plasma model (apec model – emission spectrum from collisionally ionized diffuse gas calculated using the atomdb code v2.0.1), and also with one component black body model. the low state spectrum is also well fitted with a 2 components thermal plasma model. bz cam is a nl star at a distance 830 ± 160 pc. (ringwald & naylor, 1998) with an orbital period of porb = 0.153693(7) d. (patterson et al., 1998). most of the time it shows brightness variations around 12– 13 mag., with rare occasional transitions to the 14– 14.5 mag. low states. bz cam is surrounded by a bright emission nebula with a bow-shock structure. hollis (1992) proposed that the bow shock structure must be due to the interactions of wind from bz cam with the interstellar medium. greiner et al. (2001) suggested that this nebula is photoionized by a bright central object that must be a super soft x-ray source, while the bow shock structure is due to the high proper motion of bz cam. from the second plot of fig. 1 it can be seen that the luminosity is higher in the low state, however, in the very soft spectral region, at energy ≤0.5 kev, the x-ray flux is almost twice higher in the high state, which is exactly the opposite of the idea proposed by greiner et al (1999). interestingly, the spectral fits in both states indicate that we may be observing a strong ne x lyα line at 1.02 kev. we fit the low and high state spectra of bz cam, with a two-component apec model with variable abundances. in this fit we found an underabundance (with respect to solar values) of c and n and and an overabundance of o, ne, and intermediate mass elements like s and ca. figure 2: the swift uvot image of bz cam, in which the bow-shock emission nebula is clearly detected. the uv magnitudes of bz cam in the high and low states indicate a smaller variation than observed in the other objects (albeit in different uv filters). this is explained by figure 2 in which we show the uv image of the nebula obtained with swift/uvot observations. obviously the ionized nebula also emits copious uv flux. comparing the image of bz cam in figure 2 and the one, presented in the figure 4 in (greiner et 270 x-ray observations of vy scl-type nova-like binaries in the high and low state al., 2001), one can see an additional prominent feature of the nebula in the former image. mv lyr is at about 12–13 mag. in the high state and 16–18 mag. in the low state. hoard et al. (2004) have shown that the distance to the object is 505 ± 50 pc. with their fuse (far ultraviolet spectroscopic explorer) observations these authors estimated an upper limit to ṁ during the low state ≤ 3×10−13 m� year−1. the orbital period of this system is porb = 0.1329 d. (skillman, patterson & thorstensen, 1995). our data show that the high state x-ray flux of mv lyr is higher by an order of magnitude than in the low state. the spectrum is also harder with additional component prominent above 1.7 kev. we fitted the high state spectrum of mv lyr with a two components thermal plasma model. a good fit is also obtained with a thermal plasma and a power law model (see table 1). v794 aql varies between 14 and 15 mag. in the high optical state, and in the low states it can plunge to 18 – 20 mag. the orbital period is porb = 0.1533 d. (honeycutt & schlegel, 1985). godon et al (2007) derived the following binary system parameters: mwd = 0.9 m�, high state ṁ = 10−8.5 − 10−8.0 m� year−1, inclination i = 60o, and distance to the system d = 690 pc. we fitted the intermediate state spectrum of v794 aql with two vapec components (see table 2). in both components we need high abundance of mg and ni and underabundant s and fe. 4 discussion and conclusions the first conclusion that can be derived from the presented spectra is that in all the objects both in high and low states there is no evidence of the thermonuclear burning on the surface of wd. an important motivation for this research has been the claim by greiner (1998) and greiner et al. (2001) that some of the wd table 1: fitting models and parameters for bz cam and mv lyr bz cam mv lyr high state low state high state low state satellite swift swift swift swift models vapec vapec+vapec vapec+vapec vapec+plow apec+apec apec+apec n(h)(1022) 0.14 ± 0.03 0.29 ± 0.11 0.6 ± 0.3 0.002+1.049 −0.002 0.87 ± 0.23 0.12+0.34 −0.12 photon index 0.8 ± 0.3 t1 (kev) 10.4 ± 2.7 0.57 ± 0.26 0.6 ± 0.4 0.9 ± 0.7 0.09 ± 0.03 0.7 ± 0.4 t2 (kev) 11 ± 3 63.9+0.1 −54.9 12 ± 7 4.8+42.8 −2.6 flux∗ abs 3.78 ± 0.26 3.95 ± 0.27 5.8+6.4 −5.8 5.3 ± 0.8 4.4 ± 0.6 0.9 ± 0.7 flux∗ unabs 4.24 ± 0.26 4.84 ± 0.27 8.6 5.5 ± 0.9 367 ± 50 1.6 ± 1.2 χ2 1.1 1.0 1.0 1.0 1.1 ∗the x-ray flux (×10−12erg cm−2 s−1) was calculated in the following energy range: 0.3 – 10.0 kev for swift xrt table 2: fitting models and parameters for tt ari and v794 aql tt ari v794 aql high state low state intermediate state low state satellite rosat swift swift swift models apec+apec apec+apec apec vapec+vapec apec n(h) (1022) 0.030 ± 0.002 0.056+0.106 −0.056 0.026+0.054 −0.026 0.05 ± 0.04 0.029+0.086 −0.029 t1 (kev) 0.812 ± 0.014 0.7 ± 0.4 3.6 ± 1.2 16.4+39.1 −8.7 8.8+29.1 −3.7 t2 (kev) 14 ± 9 4.4 ± 1.9 0.97 ± 0.35 flux∗ abs 5.52 ± 0.17 0.97 ± 0.17 0.94 ± 0.15 7.5 ± 1.1 2.1 ± 0.7 flux∗ unabs 6.76 ± 0.17 1.08 ± 0.17 1.02 ± 0.15 8.8 ± 1.1 2.7 ± 0.7 χ2 1.0 1.0 1.2 1.0 0.7 ∗the x-ray flux (×10−12erg cm−2 s−1) was calculated in the following energy ranges: 0.2 – 2.5 kev for rosat pspc and 0.3 – 10.0 kev for swift xrt 271 p. zemko, m. orio in vy scl-type stars must be burning hydrogen quietly in the low state, without ever triggering thermonuclear flashes because of the short duration of the burning. we find that the predicted supersoft x-ray source (manifestation of the thermonuclear burning) does not appear in the low states. we also find another unexpected result, namely that the x-ray luminosity does not follow the optical/uv drop. in one case, bz cam, the x-ray luminosity increases in the low state, and in the others, the decrease in x-ray flux in the low state is smaller than that in the optical and uv. the x-ray flux varies much less than the optical flux, by a factor of 2 to 10 during the transition from high to low state. the best-fit model for the 0.3–10.0 kev broad band spectra is two-component absorbed thermal plasma model. we do not find evidence of a boundary layer of the accretion disk emitting in the x-ray range. probably the emission is mainly in the far uv (e.g godon et al., 2007). it appears likely that some, or all the x-ray flux is produced in a wind from the system: an ongoing fast wind may be the cause of the extended bz cam nebula, initially classified as a planetary nebula. however, tt ari shows a quasi periodic oscillations in x-rays, which can be understood in the context of accretion, but is not explained by a wind. the x-ray flux may also be due to a different, and coexistent mode of accretion other than the disk, i.e. a magnetically driven stream to the polar caps of an intermediate polar. the x-rays and optical flux variations anticorrelate only in bz cam, which may be due to a wind causing additional absorption and obscuring the luminous disk. a second explanation for the lack of correlation of uv/optical versus x-ray flux variations, involving magnetic accretion, appears more plausible at least for three of the systems. the scenario we suggest is that the stream to the polar caps still continues, at decreased rate, when the accretion disks ceases to exist. references [1] belyakov k. v., suleimanov v. f., nikolaeva e. a., borisov n. v., 2010, in werner k., rauch t., eds, american institute of physics conference series vol. 1273 of american institute of physics conference series, p. 342–345. [2] greiner j., 1998, a&a, 336, 626 [3] greiner j., tovmassian g., orio m., lehmann h., chavushyan v., rau a., schwarz r., casalegno r., scholz r., 2001, a&a, 376, 1031 [4] greiner j., tovmassian g. h., di stefano r., prestwich a., gonzález-riestra r., szentasko l., chavarŕıa c., 1999, a&a, 343, 183 [5] godon p., sion e. m., barrett p., szkody p., 2007, apj, 656, 1092 doi:10.1086/510775 [6] hessman f. v., 2000, new astronomy reviews, 44, 155 doi:10.1016/s1387-6473(00)00030-0 [7] hoard d. w., linnell a. p., szkody p., fried r. e., sion e. m., hubeny i., wolfe m. a., 2004, apj, 604, 346 doi:10.1086/381777 [8] hollis j. m., oliversen r. j., wagner r. m., feibelman w. a., 1992, apj, 393, 217 doi:10.1086/171499 [9] honeycutt r. k., robertson j. w., 1998, astronomical journal, 116, 1961 doi:10.1086/300539 [10] honeycutt r. k., schlegel e. m., 1985, pasp, 97, 1189 doi:10.1086/131684 [11] kato t., uemura m., ishioka r., nogami d., kunjaya c., baba h., yamaoka h., 2004, pasj, 56, s1 [12] patterson j., raymond j. c., 1985, apj, 292, 535 doi:10.1086/163187 [13] patterson j., patino r., thorstensen j. r., harvey d., skillman d. r., ringwald f. a., 1996, astronomical journal, 111, 2422 doi:10.1086/117976 [14] ringwald f. a., naylor t., 1998, astronomical journal, 115, 286 [15] skillman d. r., patterson j., thorstensen j. r., 1995, pasp, 107, 545 [16] thorstensen j. r., smak j., hessman f. v., 1985, pasp, 97, 437 [17] warner b., 1995, cataclysmic variable stars (cambridge: cambridge university press). discussion dmitry bisikalo: could you estimate the impact of bow shock to observed x-ray flux? polina zemko: the bz cam image in hα (figure 4 of greiner et al., 2001) shows two distinct bright features in the bow shock front, one at ∼12 arcsec, the other at ∼40-45 arcsec from the central source. nevertheless, the psf of the source in x-ray image obtained with swift is quite symmetric and typical of a point source. so even with the modest spatial resolution of swift we can rule out that the bow shock contributes significantly to the x-ray flux. 272 http://dx.doi.org/10.1086/510775 http://dx.doi.org/10.1016/s1387-6473(00)00030-0 http://dx.doi.org/10.1086/381777 http://dx.doi.org/10.1086/171499 http://dx.doi.org/10.1086/300539 http://dx.doi.org/10.1086/131684 http://dx.doi.org/10.1086/163187 http://dx.doi.org/10.1086/117976 introduction observation and data analysis results discussion and conclusions 123 acta polytechnica ctu proceedings 1(1): 123–126, 2014 123 doi: 10.14311/app.2014.01.0123 a multi-wavelength view of the xmm-newton galactic plane ada nebot gómez-morán1, christian motch1, on behalf of the xmm-newton survey science centre 1observatoire astronomique de strasbourg, université de strasbourg, cnrs, umr 7550, 11 rue de l’université, 67000 strasbourg, france. corresponding author: ada.nebot@astro.unistra.fr abstract we present an x-ray survey of the galactic plane conducted by the survey science centre of the xmm-newton satellite. the survey contains more than 1300 x-ray detections at low and intermediate galactic latitudes and covering 4 deg2 well spread in galactic longitude. from a multi-wavelength analysis, using optical spectra and helped by optical and infrared photometry we identify and classify about a fourth of the sources. the observed surface density of soft x-ray (< 2 kev) sources decreases with galactic latitude and although compatible with model predictions at first glance, presents an excess of stars, likely due to giants in binary systems. in the hard band (> 2 kev) the surface density of sources presents an excess with respect to the expected extragalactic contribution. this excess highly concentrates towards the direction of the galactic centre and is compatible with previous results from chandra observations around the galactic centre. the nature of these sources is still unknown. keywords: stars binaries spectroscopy photometry ir optical x-rays. 1 introduction galactic x-ray surveys can help us to learn about the galaxy’s structure, stellar formation and evolution. since long we know that the soft x-ray emission (< 2 kev) of the galaxy is dominated by stars. but the x-ray luminosity function of hard x-ray emitting sources (>2 kev) is still not well constrained, in particular at low to intermediate x-ray luminosities (10 <lx< 1034erg s−1). cataclysmic variables and active binaries contribute at x-ray luminosities up to ∼ 1033 erg s−1(sazonov et al. 2006), but recent results show that other sources, such as massive ob stars, wolf-rayet and wind colliding binaries also contribute to this lx range (mauerhann et al. 2010, anderson et al. 2011). a census of these populations is thus needed. discrepancies between observations and model predictions exist since the first x-ray surveys of the galaxy were carried out by the einstein satellite. stellar population models predict less stars than we observe, in particular, there is an excess of stars with spectral type k, this is the so called “yellow excess”. the origin of this excess is highly debated and there are two possible explanations: either these k stars are young stars or old evolved giants, which can be either single or in binary systems (favata et al. 1988, sciortino et al. 1995). x-ray surveys can reveal concentrations of specific populations in certain regions of the galaxy: high-mass x-ray binaries (hmxb) seem to be concentrated in the norma arm (lutovinov et al. 2005); the gould belt contains a young population of late type stars (guillout et al. 1998). last but not least, different populations of theoretically predicted x-ray binaries are still to be discovered. binary population models predict a population of x-ray binaries at low luminosities, prehmxbs and pre-low-mass x-ray binaries that would be accreting matter coming from the wind of the companion star at low rates rather than from roche lobe overflow (willems & kolb 2003, pfahl et al 2002). other predicted objects are hmxbs with white dwarf as compact object instead of a classical hmxb where the compact object is a neutron star (motch et al 2007). so far, none of these objects is confirmed observationally. to learn about all these questions we need to study the x-ray content of the galaxy. 2 xmm-newton survey science centre the xmm-newton satellite can help us to give answer to these questions. thanks to it’s large field of view and it’s high sensitivity, each xmm-newton pointed observation discovers from 30 to 100 serendipitous sources. the xmm-newton survey science centre (ssc) generates catalogues of serendipitous sources and publishes them on a regular basis. the latest available catalogue, the 2xmmi-dr3 catalogue, contains about 250 000 unique sources, and the 3xmm catalogue will 123 http://dx.doi.org/10.14311/app.2014.01.0123 ada nebot gómez-morán, christian motch soon be released containing more than half a million detections corresponding to about 370 000 unique sources. one of the goals of the ssc is to characterise and identify these sources in an statistical manner. for that, specific surveys have been studied in great detail, such as a bright flux sample by della ceca et al. 2004, a medium flux sample by barcons et al. 2002 and the galactic survey from hands et al. 2004, with spectroscopic identification carried out by motch et al 2010. these surveys serve as learning samples for an statistical identification. in this paper we briefly describe the main results of a survey carried out by the xmmnewton ssc in the galactic plane. for details on the survey we refer the reader to nebot gómez-morán et al. 2013. epic-pn wr110 dss 2mass figure 1: example field studied in this survey. from left to right, x-ray (epic-pn), optical (dss) and infrared (2mass) images around the wr 110. 3 the galactic plane survey the xmm-newton ssc survey of the galactic plane contains 26 pointings, covering about 4◦ at low and intermediate galactic latitudes (|b| < 20◦) and well spread in galactic longitude. it contains about 1300 detections up to limiting fluxes of ∼ 2 × 10−15 erg cm−2s−1in the soft (kt< 2 kev) band and ∼ 10−14 erg cm−2s−1in the hard band (2< kt< 12 kev). 3.1 multi-wavelength identification in order to identify these sources in the optical and the infrared we cross-correlated the x-ray positions with large optical and infrared published catalogues, such as the sdss, usno and 2mass. we show an example of one of the fields studied in this survey in figure 1. the xmm-newton detected about 50 sources around the pointed observation of wr 110. if we look in the optical (middle panel) the field clearly contains a large number of sources, while in the infrared (right panel) the field becomes very crowded. this example reflects the difficulty of multi-wavelength identification of x-ray sources in the galactic plane. due to the relatively large x-ray positional errors, and the high surface density of sources at faint fluxes, only not too distant and relatively bright sources can be identified in the optical and the infrared. among the ∼ 1300 x-ray detected sources we could find counterparts in the optical to about 60%, number which drops to about 40% in the infrared. 3.2 source classification a spectroscopic campaign was carried out to classify the sources. about one forth of the detected x-ray sources are classified. most of them are active coronae, but we also found other interesting objects, such as young t tauri and herbig-ae stars, and old cataclysmic variables. we identify four be stars with x-ray luminosities around 1032 erg s−1 a value too high for normal be stars. these systems thus belong to the class of γ-cas analogues, which means that we have almost doubled the number of γ-cas objects known until the date (see talk by c. motch in this congress). we cross-matched our x-ray sources with other published catalogues making use of vizier and the xcat-db databases, and produced spectral energy distributions which helped us to discern the type of source. infrared data from the wise catalogue revealed one of the t tauri has a transition disk, which highlights the relevance of having a multi-wavelength view of high energy sources. 124 a multi-wavelength view of the xmm-newton galactic plane ‣ ‣ ‣ 10-15 10-14 10-13 10-12 s (0.5-2 kev) [erg cm-2s-1] 10-1 100 101 102 n (> s ) [s o u rc e s /d e g 2 ] |b| = 0o |b| = 15o |b| = 50o |b| = 60o b = 50º b = 60º b = 15º b = 0º 0.5-2.0 kev observed figure 2: logn – logs curves in the soft band (< 2 kev) and as a function of galactic latitude. we included the data from lopez-santiago et al. 2007 and barcons et al. 2007 at high galactic latitudes. using infrared colour-colour diagrams for the stellar content of our survey we could distinguish main sequence stars from evolved giants. on the one hand, due to magnetic braking, stellar rotation decreases with the age of the stars. on the other hand the x-ray luminosity is strongly related to rotation. this implies that normal old giant stars are not expected to be strong x-ray emitters. a possible explanation for the origin of the x-ray emission of these evolved stars could be if they are in synchronised binary systems, where the rotation period is equal to the orbital period, breaking thus the relation between x-ray emission and age. to test this hypothesis we calculated the x-ray colours expected for stars of different age and for binaries, in particular for rs cvn and by draconis. we found that observed xray and infrared colours for dwarfs are compatible with young to intermediate age stars, while giant stars have colours compatible with rs cvn binaries, i.e. binaries where at least one of the stars is a giant star. the giant stars we find in our survey mostly have k spectral type, i.e. they are “yellow stars”. 4 results we computed the logn – logs curves in the soft band, i.e. the surface density of sources as a function of the sensitivity of our survey, for different bins of galactic latitude. we find that the number of stars per square degree decreases with galactic latitude. although this result is not a surprise, it’s the first time that is shown in x-rays. we also see that the x-ray luminosity function has a varying slope with galactic latitude. we compared our results with the expected curves from a modified version of the galactic x-ray model from guillout et al. 1996. at first glance we obtained compatible results, a varying slope of the x-ray luminosity function and a decreasing number of sources towards higher galactic latitudes, reflecting the different scale-height of stars and the relative contribution of different populations. but for a given flux, the number of observed stars is higher than the predicted value. this result is not surprising since binaries have not been taken into account in our model. in other words, the excess of observed sources with respect to model predictions is due to the yellow giant stars that we have found in our survey, and are likely in binary systems. 10-14 10-13 10-12 10-11 s (2-12 kev) [erg cm-2s-1] 10-1 100 101 102 103 n (> s ) [s o u rc e s /d e g 2 ] galactic center motch et al 2010 1032 1033 1034 luminosity (2-12 kev) [erg s-1] 2-12 kev gc region (l,b) ~ (1º,0º) ‣ ‣ ‣ hands+2004, motch+2010 (l,b) =(20º,0º) b ~ 0º figure 3: logn – logs curves of hard sources (> 2 kev) in the direction of the galactic center. the expected extragalactic contribution from mateos et al. 2008 has been subtracted. we carried out the same exercise for hard sources, we constructed logn – logs curves for sources harder than 2 kev. since stars are not expected to contribute at this high energies, we subtracted the expected extragalactic contribution. the logn – logs curves present an excess of sources with respect to the expected extragalactic contribution close to the galactic center region studied in this survey (l ∼ 0.9◦, b ∼ 0◦). we compared this result with that obtained by motch et al. 2010 at l ∼ 20◦, b ∼ 0◦ and obtained that the number of sources per square degree observed at a given flux decreases steeply with galactic longitude in the direction of the galactic center. we obtained consistent results with hong et al. 2009 based on chandra observations of seven fields around the galactic center. this sources contribute to the galactic ridge emission, initially thought to be of diffuse origin. the nature of these sources is unknown, but if this population would be associated to the galactic center itself, the x-ray luminosities obtained are in the range 1033−1034 erg s−1, i.e. values which are comparable with pre-hmxbs and pre-lmxbs. 125 ada nebot gómez-morán, christian motch acknowledgement we would like to thank the organisers of this conference for their work and for giving us the opportunity to show our results. references [1] anderson l. d., bania t. m., jackson j. m., clemens d. p., heyer m., simon r., shah r. y., rathborne j. m., 2009, apjs, 181, 255 doi:10.1088/0067-0049/181/1/255 [2] barcons, x., carrera, f. j., watson, m. g., et al. 2002, a&a, 382, 522 [3] barcons, x., carrera, f. j., ceballos, m. t., et al. 2007, a&a, 476, 1191 [4] della ceca, r., maccacaro, t., caccianiga, a., et al. 2004, a&a, 428, 383 [5] favata, f., sciortino, s., rosner, r., & vaiana, g. s. 1988, apj, 324, 1010 doi:10.1086/165957 [6] guillout, p., haywood, m., motch, c., & robin, a. c. 1996, a&a, 316, 89 [7] guillout, p., sterzik, m. f., schmitt, j. h. m. m., motch, c., & neuhaeuser, r. 1998, a&a, 337, 113 [8] hands, a. d. p., warwick, r. s., watson, m. g., & helfand, d. j. 2004, mnras, 351, 31 doi:10.1111/j.1365-2966.2004.07777.x [9] hong, j. s., van den berg, m., grindlay, j. e., & laycock, s. 2009, apj, 706, 223 doi:10.1088/0004-637x/706/1/223 [10] lópez-santiago, j., micela, g., sciortino, s., et al. 2007, a&a, 463, 165 [11] lutovinov a., revnivtsev m., gilfanov m., shtykovskiy p., molkov s., sunyaev r., 2005, a&a, 444, 821 [12] mauerhan j. c., muno m. p., morris m. r., stolovy s. r., cotera a., 2010, apj, 710, 706 [13] mateos, s., warwick, r. s., carrera, f. j., et al. 2008, a&a, 492, 51 [14] motch, c., lopes de oliveira, r., negueruela, i., haberl, f., & janot-pacheco, e. 2007, in astronomical society of the pacific conference series, vol. 361, active ob-stars: laboratories for stellare and circumstellar physics, ed. a. t. okazaki, s. p. owocki, & s. stefl, 117 [15] motch, c., warwick, r., cropper, m. s., et al. 2010, a&a, 523, a92 [16] nebot gómez-morán a. et al., 2013, a&a, 553, a12 [17] pfahl, e., rappaport, s., & podsiadlowski, p. 2002, apj, 571, l37 doi:10.1086/341197 [18] sazonov, s., revnivtsev, m., gilfanov, m., churazov, e., & sunyaev, r. 2006, a&a, 450, 117 [19] sciortino, s., favata, f., & micela, g. 1995, a&a, 296, 370 [20] willems, b. & kolb, u. 2003, mnras, 343, 949 discussion david buckley: can you give the percentages of the different optical counterpart types of the ∼ 300 sources identified? ada nebot: among the 316 classified sources there are three t tauri stars, one herbig ae star, two cataclysmic variables, and four γ-cas analogues. a few sources are extragalactic (in the highest galactic latitude fields), and the remaining sources are stars, where about 10% are giants. classification of giants was only done for stars with the best 2mass photometry, meaning that the number of giants in our survey could be higher. 126 http://dx.doi.org/10.1088/0067-0049/181/1/255 http://dx.doi.org/10.1086/165957 http://dx.doi.org/10.1111/j.1365-2966.2004.07777.x http://dx.doi.org/10.1088/0004-637x/706/1/223 http://dx.doi.org/10.1086/341197 introduction xmm-newton survey science centre the galactic plane survey multi-wavelength identification source classification results 293 acta polytechnica ctu proceedings 1(1): 293–297, 2014 293 doi: 10.14311/app.2014.01.0293 design and tests of the hard x-ray polarimeter x-calibur m. beilicke1, r. cowsik1, p. dowkontt1, q. guo1, f. kislat1, s. barthelmy2, t. okajima2, j.w. mitchell2, j. schnittman2, b. zeiger2, g. de geronimo3, m.g. baring4, a. bodaghee5, t. miyazawa6, k.d. finkelstein7, h. krawczynski1 1department of physics and mcdonnell center for the space sciences, washington university, st. louis, mo, usa 2goddard space flight center, md, usa 3brookhaven national lab, ny 4rice university, tx, usa 5uc berkeley, ca 6nagoya university, japan 7cornell university, ny corresponding author: beilicke@physics.wustl.edu abstract x-ray polarimetry promises to give qualitatively new information about high-energy astrophysical sources, such as binary black hole systems, micro-quasars, active galactic nuclei, and gamma-ray bursts. we designed, built and tested a hard x-ray polarimeter, x-calibur, to be used in the focal plane of the infocus grazing incidence hard x-ray telescope. x-calibur combines a low-z compton scatterer with a czt detector assembly to measure the polarization of 20− 60 kev x-rays making use of the fact that polarized photons compton scatter preferentially perpendicular to the electric field orientation; in principal, a similar space-borne experiment could be operated in the 5−100 kev regime. x-calibur achieves a high detection efficiency of order unity. keywords: x-rays polarization black hole infocus x-calibur. 1 introduction motivation. spectral and morphological studies in the x-ray energy band have become established tools to study the non-thermal emission processes of various astrophysical sources. however, many of the regions of interest (black hole vicinities, jet formation zones, etc.) are too small to be spatially resolved with current and future instruments. spectro-polarimetric x-ray observations are capable of providing additional information – namely the fraction and orientation of linear polarization – and would help to constrain different emission models [1, 2] of sources with compact emission regions and high x-ray fluxes such as mass-accreting black holes (bhs) and neutron stars. so far, only a few missions have successfully measured polarization in the soft (oso-8 [3]) and hard (integral [4]) x-ray energy regime. the crab nebula is the only source for which the polarization of the x-ray emission has been established with a high level of confidence [3, 4]. the source exhibits a polarization fraction of 20% at energies of 2.6− 5.2 kev (direction angle of 30◦ with respect to the x-ray jet) [3] and 46% ± 10% above 100 kev (direction aligned with the x-ray jet observed in the nebula). integral observations of the x-ray binary cygnus x-1 indicate a high fraction of polarization in the hard x-ray/gamma-ray bands [5]. model predictions of polarization fraction for various source types lie slightly below the sensitivity of the past oso-8 mission, making future, more sensitive polarimeter missions particularly interesting. future missions. as polarimetry was not the main objective of the integral mission, the results are plagued by systematic uncertainties, and there are currently no other missions in orbit that are capable of making sensitive x-ray polarimetric observations. the situation could be changed by a mission like the gravity and extreme magnetism smex (gems) [6] – using two wolter-type x-ray mirrors to focus 2 − 10 kev photons onto photo-effect polarimeters. for higher energies e > 5 kev x-ray polarimeter designs can make use of the compton effect. the soft gamma-ray imager on astro-h [7] (launch scheduled for 2015) will have capabilities of measuring polarization, but the results may suffer from similar systematic uncertainties as the ones 293 http://dx.doi.org/10.14311/app.2014.01.0293 m. beilicke et al. figure 1: left: incoming x-rays are compton-scattered (scintillator rod, optical axis) and subsequently photoabsorbed in one of the surrounding czt detectors. middle left: ’exploded’ view of the polarimeter: four sides of detector columns surround the central scintillator rod. middle right: polarimeter with csi shield, electronic readout and azimuthal rotation bearing. right: the infocus balloon gondola. measured by integral. the czt imager on astrosat (http://astrosat.iucaa.in/) is expected to be used for polarization measurements in the 150− 250 kev band. the hard x-ray polarimeter x-calibur discussed in this paper has the potential to cover the energy range above 10 kev. furthermore, x-calibur combines a high detection efficiency with a low level of background and has well-controlled systematic errors. these features make it a particularly useful instrument for astronomical x-ray polarimetry. scientific potential. polarization measurements are of general interest as tests of non-thermal emission processes in the universe. synchrotron emission, for example, will result in linearly polarized photons with their electric fields oriented perpendicular to the magnetic field lines (projected) and the observed polarization map in the x-rays can therefore be used to trace the magnetic field structure of the source. an observed polarization fraction close to theoretical limits can be interpreted as an indication of a highly ordered magnetic field since non-uniformities in the magnetic field will reduce the fraction of polarization. the polarized synchrotron photons can be inverse-compton (ic) scattered by relativistic electrons – weakening the fraction of polarization (but not erasing it) and imprinting a scattering angle dependence [8] to the observed fraction of polarization. such ic signals usually (but not always) appear in hard gamma-rays, where polarimetry is difficult, due to multiple scattering in pair production detectors. another important mechanism for polarizing photons is thomson scattering which creates a polarization perpendicular to the scattering plane. curvature radiation is polarized, as well. for more details on the scientific prospects see for example [1] and references therein. addressing these science goals requires spectro-polarimetric observations over the broadest possible energy range. 2 x-calibur design the conceptual design of the x-calibur polarimeter is illustrated in figure 1. a low-z scintillator (ρ ≈ 1 g/cm3) is used as compton-scatterer: the cross section for photoelectric absorption and compton scattering are equal around 15 kev (0.26 cm2/g). at 20 kev the cross section of the photoelectric absorption already drops to 0.1 cm2/g and can be neglected as compared to the compton scattering for higher energies. the mean free path for the compton scattering is ≈ 4 cm so that the length of the scintillator (14 cm) covers ' 3.5 path lengths leading to a p ' 90% probability for absorption in the energy regime of 20 − 60 kev. for sufficiently energetic photons, the compton interaction produces a measurable scintillator signal which is read by a pmt. the scattered x-rays are photo-absorbed in surrounding rings of high-z cadmium zinc telluride (czt) detectors. this combination of scatterer/absorber leads to a high fraction of unambiguously detected comp294 design and tests of the hard x-ray polarimeter x-calibur ton events. linearly polarized x-rays will preferably compton-scatter perpendicular to their e field vector – resulting in a modulation of the azimuthal event distribution. the czt detectors were ordered from different companies (endicott interconnect, quikpak/redlen, creative electron). each detector (2× 2 cm2) is contacted with a 64-pixel anode grid (2.5 mm pixel pitch) and a monolithic cathode facing the scintillator rod. two detector thicknesses (2 mm and 5 mm) will be used. each czt detector is permanently bonded (anode side) to a ceramic chip carrier which is plugged into the electronic readout board. each czt detector is read out by two digitizer boards (32 channel asic developed by g. de geronimo (bnl) and e. wulf (nrl) [9] and a 12-bit adc). the readout noise is as low as 2.5 kev fwhm. 16 digitizer boards (8 czt detectors) are read out by one harvester board transmitting the data to a pc-104 computer with a rate of 6.25 mbits/s. xcalibur comprises 2048 data channels. four detector units form a ’ring’ surrounding the scintillator slab. the scintillator ej-200 is read by a hamamatsu r7600u-200 pmt. the (optional) pmt trigger information allows to effectively select scintillator/czt events from the data, which represent likely compton-scattering candidates. the polarimeter and the front-end readout electronics will be located inside an active csi(na) anticoincidence shield with 5 cm thickness and a passive tungsten shield at the top (fig. 1, middle right) to suppress charged and neutral particle backgrounds. the x-calibur polarimeter will be flown in a pressurized vessel located in the focal plane of the infocus1 x-ray telescope [10] (fig. 1, right). the total mass of the gondola and the x-ray telescope will be 1, 400 kg. a wolter grazing incidence mirror focuses the x-rays on the polarimeter. the x-calibur scintillator rod will be aligned parallel to the optical axis of the infocus x-ray telescope. the focal length is ∼ 8 m and the field of view (fwhm) is 10 arcmin. the design of infocus allows for very stable pointing of the telescopes to <1 arcmin as the focus of the x-ray telescope moves across the sky. in order to reduce the systematic uncertainties of the polarization measurements (including biases generated by the active shield, a possible pitch of the polarimeter with respect to the x-ray telescope, etc.), the polarimeter and the active shield will be rotated around the optical axis (∼ 3 rpm) using a ring bearing (see fig. 1, middle right). a counterrotating mass will be used to cancel the net-angular momentum of the polarimeter assembly during the flight. power will be provided to the polarimeter by sliding contacts and communication will be done via a wireless network. the data will be stored to solid state drives and will be down-linked to the ground. the advantages of the x-calibur/infocus design are (i) a high detection efficiency by using more than 80% of photons impinging on the polarimeter, (ii) low background due to the usage of a focusing optics instead of large detector volumes, and (iii) minimization and control of systematic effects and achievement of a corresponding quantitative estimate thereof. detailed simulations were performed using the geant4 package [11]. an x-calibur balloon flight in the focal plane of the infocus mirror assembly was assumed. for a crab-like source the simulations predict an event rate of 1.1(3.2) hz with (without) requiring a triggered scintillator coincidence detected by the pmt. the x-calibur modulation factor is µ = 0.52 for a 100% polarized beam. the mdp in the 20 − 60 kev energy range will be 4% assuming 5.6 hr of on-source observations of a crab-like source combined with a 1.4 hr background observation of an adjacent empty field. more detailed simulations are discussed in guo et al. (2010) [12]. czt energy [kev] 20 40 60 80 /s ] 2 f lu x[ 1/ cm -310 -210 -110 1 ring1 ring2 ring3 figure 2: energy spectra after compton scattering of a 40/80 kev x-ray beam. vertical lines: incoming beam energy (solid), energy after 90◦ scattering (dashed), and energy after 180◦ scattering (dotted). 3 performance measurements using funding from washington university’s mcdonnell center for the space sciences and nasa grant nnx12ad51g, a flight-ready version of the x-calibur polarimeter was assembled and tested. for energy calibration of the czt detectors a eu152 source is placed inside the individual rings. the calibration data also allow to quantify the energy resolution and threshold on a pixel-by-pixel basis. we find an average resolution of 4.2 kev fwhm at 40 kev. in turn, we completed the full assembly and studied the x-calibur respsonse to incoming x-ray beams. in order to measure the response to a polarized xray beam we operated the x-calibur polarimeter at the 1http://infocus.gsfc.nasa.gov/ 295 m. beilicke et al. cornell high energy synchrotron source (chess) [13]. using bragg reflection from a 2-bounce silicon (220) monochrometer a 40 kev beam was generated (2nd harmonic at 80 kev). the event rates were normalized by azimuthal acceptance on a pixel-by-pixel basis. the acceptance was determined from the response to an unpolarized beam (superposition of two perpendicular polarization planes). the energy spectrum of the compton scattered xray beam is shown in fig. 2. figure 3 shows the azimuthal scattering distribution of the polarized beam (analysis ongoing). the expected 180◦ modulation is clearly seen and the reconstructed orientation of the electric field agrees with the expected direction of the chess beam setup – confirming the functionality of the x-calibur polarimeter. detector x [m] #1 #2 #3 #4 #5 #6 #7 #8 x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x polarized (acpt) 0 100 200 300 0.5 1 1.5 0 100 200 300 0.5 1 1.5 0 100 200 300 0.5 1 1.5 0 100 200 300 0.5 1 1.5 0 100 200 300 0.5 1 1.5 0 100 200 300 0.5 1 1.5 0 100 200 300 0.5 1 1.5 0 100 200 300 0.5 1 1.5 0 100 200 300 0.5 1 1.5 0 100 200 300 0.5 1 1.5 0 100 200 300 0.5 1 1.5 0 100 200 300 0.5 1 1.5 0 100 200 300 0.5 1 1.5 0 100 200 300 0.5 1 1.5 0 100 200 300 0.5 1 1.5 azimuth angle [deg]0 100 200 300 0.5 11.5polarized (acpt) figure 3: x-calibur measurement of a polarized beam (energy cut: 36 − 40 kev). left: 2d map (pixel-bypixel) of counts. the (blue) boxes indicate the outline of the scintillator. right: azimuthal scattering distribution (corrected for azimuthal acceptance) for different rings. the data are fitted by a sine function. the vertical lines indicate the plane of the e field vector of the polarized beam. 4 discussion and conclusions we designed a hard x-ray polarimeter, x-calibur, and studied its performance and sensitivity. x-calibur combines a detection efficiency of close to 100% with a high modulation factor of µ ≈ 0.5, as well as a good control over systematic effects. x-calibur was successfully tested/calibrated in the laboratory and at the chess x-ray facility [13]. a 1-day x-calibur/infocus balloon flight is funded by nasa (nnx12ad51g) for fall 2013. our tentative observation program includes galactic sources (crab, her x-1, cyg x-1, grs 1915, exo 0331) and one extragalactic source (mrk 421) for which sensitive polarization measurements will be carried through. we envision follow-up longer duration balloon flights (from the southern hemisphere). successful balloon flights would motivate a satellite-borne hard x-ray polarimetry mission. acknowledgement we are grateful for nasa founding from grant nnx10aj56g & nnx12ad51g and discretionary funding from the mcdonnell center for the space sciences to build the x-calibur polarimeter. polarization measurements: this work is based upon research conducted at the cornell high energy synchrotron source (chess) which is supported by the national science foundation and the national institutes of health/national institute of general medical sciences under nsf award dmr-0936384. q.guo thanks the chinese scholarship council from china for the financial support (no.2009629064). references [1] krawczynski, h., et al.: 2011, aph. 34, 550. 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[13] http://www.chess.cornell.edu/ 296 http://dx.doi.org/10.1088/0004-637x/712/2/908 http://dx.doi.org/10.1086/182648 http://dx.doi.org/10.1126/science.1149056 http://dx.doi.org/10.1126/science.1200848 http://dx.doi.org/10.1088/0004-637x/744/1/30 http://dx.doi.org/10.1016/j.nima.2007.04.085 design and tests of the hard x-ray polarimeter x-calibur discussion roland walter’s comment: according to the current schedule the flight model of the hard x-ray polarimeter polar (http://www.isdc.unige.ch/polar/) should be delivered to china this year. herman marshall: how is the unpolarized flux generated? perhaps by rotating the polarization plane or by a radioactive source? matthias beilicke: we super-imposed data with the orientation of the polarization plane of 90◦ and 0◦ and independently confirmed the findings with a radiactive source (unpolarized). a 297 introduction x-calibur design performance measurements discussion and conclusions acta polytechnica ctu proceedings doi:10.14311/ap.2016.4.0062 acta polytechnica ctu proceedings 4:62–67, 2016 © czech technical university in prague, 2016 available online at http://ojs.cvut.cz/ojs/index.php/app numerical multigroup transient analysis of slab nuclear reactor with thermal feedback filip osuský∗, branislav vrban, peter ballo, štefan čerba, jakub lüley, gabriel farkas institute of nuclear and physical engineering, faculty of electrical engineering and information technology, slovak university of technology in bratislava, bratislava, slovakia ∗ corresponding author: filip.osusky@stuba.sk abstract. the paper describes a new numerical code for multigroup transient analyses with a thermal feedback. the code is developed at institute of nuclear and physical engineering. it is necessary to carefully investigate transient states of fast neutron reactors, due to recriticality issues after accident scenarios. the code solves numerical diffusion equation for 1d problem with possible neutron source incorporation. crank-nicholson numerical method is used for the transient states. the investigated cases are describing behavior of pwr fuel assembly inside the spent fuel pool and with the incorporated neutron source for better illustration of the thermal feedback. keywords: diffusion equation, neutronics, transient state, crank-nicholson method, thermal feedback. 1. introduction nuclear reactors are most reliable source of energy at present time. the advantage of new fast neutron reactor concepts is the possibility of the transmutation of the 238u into the fissionable 239pu, so the worldwide resources of fissionable materials could be enlarged when the fast neutron reactors will be commercialized. however, it is necessary to carefully investigate the transient states of these new concepts, because the recriticality after accident scenario may be achieved [1]. for this purpose, a new numerical code has been developed within the frame of neutronic calculation group at the institute of nuclear and physical engineering (inpe). the code has been used as the basic step in understanding of transient state and fast reactor r&d and we refer to it as transos. the transos code is able to solve a time dependent neutron flux distribution of slab systems with respect to the thermal feedback in multigroup approximation. the power method with infinity norm (manhattan norm) is used for a calculation of stationary state [2]. the transient state is calculated by the crank-nicholson numerical method [3]. it is possible to define the geometry in the form of coarse mesh intervals with material parameters assigned to each of the mesh interval. point and linear neutron sources can be incorporated to the design with different fluency for each investigated energy group of neutrons. the boundary conditions have to be specified in conjunction with design parameters such as temperature of the system and power generation rate. initial state of neutron flux should be given for successful run. the developed code can be used for academic purposes at the faculty and for investigation of simple systems. this paper follows the previous work [4] and next sections describe calculation methodology of time dependent and in-dependent solution, accuracy of the methodology and analysis of two nuclear systems with different 235u enrichment. higher impact of the thermal feedback can be observed on microscopic cross section properties for low energy neutron interaction with matter, so the thermal systems are investigated to show this effect. 2. calculation methodology 2.1. stationary state (time in-dependent solution) the code solves common neutron diffusion equation with internal sources for the stationary state given by [5] dg∆φg − σa,gφg − ∑ h σg→hφg+ + ∑ h σh→gφh + χg ∑ h νσf,hφh = 0. (1) lower index g in all equations represents the neutron energy group and d stands for the associated diffusion coefficient. the first term represents the leakage of neutrons from the system, the second term represents absorptions (radiation capture), the third represents scattering from the group to another group h, the fourth represents contributions of scattering from another group and the last term describes the contribution of fission source to specified energy group (χg is the proportion of neutrons emitted by fission in group g, ν is an average yield of neutrons from the fission). the box scheme was used for the discretization of examined area and therefore (1) can be written in a numerical form (2) for two energy groups 62 http://dx.doi.org/10.14311/ap.2016.4.0062 http://ojs.cvut.cz/ojs/index.php/app vol. 4/2016 numerical analysis of slab nuclear reactor with thermal feedback of neutrons [2, 4], −d̃k−1 g φk−1 g + ( d̃k−1 g +d̃k g +σkrghk ) φk−d̃k gφ k+1 g = = λχkghk ( 2∑ g′=1 νσkfg′φkg′ ) + hk ( 2∑ g′=1 g′ 6=g σks,g′gφ k g′ ) , (2) where σr represents the removal macroscopic cross section (3), k represents spatial node, hk is the length of the spatial interval, λ stands for the fundamental eigenvalue, σf is the fission macroscopic cross section, σs is the scattering macroscopic cross section between energy groups of neutrons, and d̃ arbitrary coupling coefficient (3) [2]. σkrg = σkag + 2∑ g′=1 g′ 6=g σks,gg′ d̃k k = 2 dk g hk dk+1 g hk+1 dk g hk + dk+1 g hk+1 (3) 2.2. transient state (time dependent solution) 2.2.1. crank-nicholson numerical method the crank-nicholson method is often applied to the diffusion problems and achieves good numerical stability for the transient states. the finite difference method is used as for the stationary state. this method is able to solve heat equation and similar partial differential equations [3] m ∂2u(x, t) ∂x2 = ∂u(x, t) ∂t , (4) where m represents material parameters and u is solution of the differential equation. applying the finite difference method, the central node of the common diffusion equation can be rewritten numerically for the transient state as − m̃k−1ht 2hk uk−1,j − m̃kht 2hk uk+1,j + ( 1 + m̃k−1ht 2hk + m̃kht 2hk ) uk,j = m̃k−1ht 2hk uk−1,j+1 + m̃kht 2hk uk+1,j+1 + ( 1− m̃k−1ht 2hk − m̃kht 2hk ) uk,j+1 + f(hk), (5) where the index k represents spatial node, index j represents step time ( u ·,j stands for initial condition of the function at present time step, u ·,j+1 is initial condition of the function at next time step), the hk length of the spatial interval, ht length of the time interval, f(hk) is increment of the function u (particular solution). the variables and unknowns from (5) can be rewritten in the form (6) for the neutron diffusion equation (1) and (2). − d̃k−1 g ht 2hk φk−1,j g − d̃k ght 2hk φk+1,j g + ( 1 + d̃k−1 g ht 2hk + d̃k ght 2hk ) φk,jg = d̃k−1 g ht 2hk φk−1,j+1 g + d̃k ght 2hk φk+1,j+1 g + ( 1− d̃k−1 g ht 2hk − d̃k ght 2hk ) φk,j+1 g + χkght ( 2∑ g′=1 νσkfg′φ k,j g′ ) − σkrghtφk,jg + ht ( 2∑ g′=1 g′ 6=q σks,g′gφ k,j g′ ) + sght, (6) where the last term s represents the external neutron source. the heat diffusion equation k ρc ∂2t (x, t) ∂x2 + q̇ = ∂t (x, t) ∂t (7) can be similarly rewritten to [6] − k̃k−1ht 2hk t k−1,j − k̃kht 2hk t k+1,j + ( 1 + k̃k−1ht 2hk + k̃kht 2hk ) t k,j = k̃k−1ht 2hk t k−1,j+ + k̃kht 2hk t k+1,j+1 + ( 1− k̃k−1ht 2hk − k̃kht 2hk ) t k,j+1 + ( 2∑ g′=1 σkfg′φ k,j g′ ) e ht ρkck e, (8) where k̃k = 2 kk kkρkck kk+1 kk+1ρk+1ck+1 kk kkρkck + kk+1 kk+1ρk+1ck+1 , and where k represents material conductivity, ρ is material density, c stands for material heat capacity, q̇ is power generation, t stands for temperature, e is an average released energy per fission in ev and parameter e is an electron charge. accuracy of the above mentioned method is discussed in the next subsection. 2.2.2. accuracy of the crank-nicholson method the accuracy of the method for the initial and boundary conditions (more in section 3) is presented in fig. 1. the analytical solution of the differential equation for a homogenous slab system is sinus, so the problem can be approximated by function f(x) f(x) = a sin(bx) + c. 63 filip osuský, branislav vrban, peter ballo et al. acta polytechnica ctu proceedings figure 1. accuracy of crank-nicholson method. in this case, the whole system is homogeneous subcritical nuclear system. it is necessary to insert the external neutron source because the neutron flux will never stabilize on non-zero value without it. the external neutron source is linear, located between 217–227mm, with fluency 1.0× 106 s−1 in thermal energy spectrum. deviations between the modeled problem and analytical solution are presented in table 1. parameter mean value dispersion rel. error a 111 935 126.1 0.1127 % b 7.076 99 × 10−3 2.015 × 10−6 0.028 47 % c −785.941 97.17 12.36 % table 1. deviation between numerical and analytical solution. amplitude (a) and shape parameter (b) are in accordance with the analytical solution (table 1). deviance in location parameter (c) is caused by the external neutron source and by zero flux boundary condition. 3. geometry of the nuclear slab system the investigated geometry represents middle cut of one heterogenic vver440 fuel assembly that is placed in the middle. one edge of the investigated geometry is represented by homogenous representation of the heterogeneous fuel assembly. control rod assembly is placed on the opposite edge. this case was introduced due to simulation of the transient state during operational conditions. geometry is also simplified into one dimension and some lengths are changed for faster convergence of numerical calculation (fig. 2). red material represents uo2 (dimension d1 is 7 mm and is the same for every red and orange material). orange material is the representation of uo2 with 3.35 % of 153gd. the two cases of the enrichment are investigated (table 2). green is b4c (dimension d5 is 142 mm), figure 2. graphical representation of investigated nuclear reactor core. enrichment material case 1 case 2 uo2 5 % 22 % uo2 + 153gd 4.4 % 21.4 % table 2. enrichment of uo2. blue stands for h2o (dimension d2 is 5 mm, d3 = 19 mm, d4 = 10 mm). purple is a mixture of the whole area between x–y points (d5 is 142 mm). all cross sections are obtained from endf/b-vii.1 library for the temperatures of 296.3 k, 600 k and 1800 k, for the discrete energies of 0.0253 ev and 2 mev [7]. isotropic scattering is assumed and the diffusion coefficient can be calculated according to (9) [8], where σt stands for the total macroscopic cross section, σtr is the transport macroscopic cross section, and µ̄0 represents unsymmetrical parameter of neutron scattering (if 64 vol. 4/2016 numerical analysis of slab nuclear reactor with thermal feedback isotropic scattering is applied µ̄0 = 0). in this case, a molecular cross section of h2o is not considered, even if the molecular cross section is higher than calculated cross sections of particular nuclides [8]. d ' λtr 1 3 = 1 3σtr = 1 3 ( σt − µ̄0σs ) ≈ 1 3σt . (9) more details about geometry input can be found in [4]. it is necessary to note that this is the only theoretical approximation and that it is mainly because the values of the macroscopic scattering cross section were simplified with certain degree of uncertainty. 4. initial and boundary conditions the zero incoming current boundary condition is used in all cases of the neutron diffusion α = j φ = 0.5, where j is the neutron current and φ is the neutron flux. this can be interpreted as that the environment behind the boundary of the nuclear system is vacuum. according to (2), the stationary state is represented by proportional neutron flux distribution that is normed to maximal value 1. it is necessary to set the initial condition in the form of total power generation p = ∫ v e ( 2∑ g′=1 σfg′φg′ ) edv. (10) the absolute value of the neutron flux distribution is calculated from the proportional neutron flux distribution (2) and from the initial power generation condition (10). the power generation rate is calculated for the case 1 from the typical power generation of vver reactor (1375 mwth) and it is approximately 1.576 kw/m per the assembly p [w/m] = pth assembly nr ·height = = 1375× 106 349 · 2.5 = 1576, where assmebly nr is the total number of fuel assemblies in the reactor and height represents height of the core. the power generation rate is the same for the case 2 with higher enrichment to lower impact of the thermal feedback to ensure the nuclear system’s subcritical state. temperature is kept at constant 296.3 k at the boundaries that correspond to dirichlet boundary condition. temperature of the whole system is also set to 296.3 k before the transient for all cases. no thermo-mechanical physics is applied (the densities of the particular materials are constant). also no fluid mechanics is applied (the heat diffusion equation solves just thermal conduction equation) and this simplification results in total temperature increase of the whole nuclear system. 5. results 5.1. case 1 keff = 0.282 601 is calculated from the stationary state of trnasos code for this scenario. this condition may be achieved in spent fuel pool. the worst case is simulated by the neutron flux corresponding to the operational state before the transient (the power generation is 1.576 kw/m). it is possible to observe (fig. 3) that the thermal energy spectrum of neutrons decreases rapidly due to deep subcritical state of the nuclear system. the same evolution can be seen in case of fast energy spectrum of neutrons in fig. 4. the temperature is set to 296.3 k at the beginning of the transient state. the power is generated by (10) according to the neutron flux and the temperature evolution is shown in fig. 5. the temperature increases up to 460 k and according to the temperature change, the microscopic cross sections are modified. the whole system is below melting point temperature of the fuel. figure 3. neutron flux distribution for energy group 0.0253 ev in case 1 (the neutron flux decreases with time evolution due to deep subcritical state). figure 4. neutron flux distribution for energy group 2 mev in case 1 (the neutron flux decreases with time evolution due to deep subcritical state). 65 filip osuský, branislav vrban, peter ballo et al. acta polytechnica ctu proceedings figure 5. temperature evolution in case 1 with keff = 0.282601 (the temperature decreases with time evolution – the nuclear system is brought into safeconditions). 5.2. case 2 the keff is equal to 0.982 693 in this case, due to higher enrichment of 235u (table 2). the neutron source is incorporated within the core and its total emission of neutrons is 1.1× 106 s−1 in the area between 217–227mm on x-axis. it corresponds to the concept of super-thermal liquid-helium (4he) source (ucn) [9] and portion of 1.0× 106 s−1 is in the thermal energy spectrum. subcritical multiplication can be derived from n∞ = s 1 1− keff = 1.1× 106 1 1− 0.982 693 ≈ 108. the result of subcritical multiplication formula is consistent with the calculated results that are shown in fig. 6 and fig. 7. the whole system starts from zero power level. the transient state of the neutron flux is stabilized approximately after 50 000 s. however, the transient of the temperature evolution is represented by higher inertia and the transient state is stabilized in the region of 140 000 s (fig. 8). the temperature does not exceed 380 k. figure 6. neutron flux distribution for energy group 0.0253 ev in case 2 (nuclear system is stabilized after subcritical multiplication for keff = 0.982 693). figure 7. neutron flux distribution for energy group 2 mev in case 2 (nuclear system is stabilized after subcritical multiplication for keff = 0.982 693). figure 8. temperature evolution in case 2 (temperature is stabilized below melting point of fuel material for keff = 0.982 693). 6. conclusions the successful application of the transos code was demonstrated. it has been found that the maximum value of the neutron flux and its temperature is located in the middle of the system within the heterogeneous region. on the other hand, the diffusion theory is more appropriate for the homogenous problems and calculation error increases with greater heterogeneity. in all cases, the temperature does not exceed melting point of fuel material. the code that was written in c++ can be used for academic purposes at the institute and for solution of simple nuclear systems. it has been confirmed that the diffusion theory is suitable for fast calculations and it is used during development of theoretical reactor design. therefore, development of simple codes increases the knowledge level of neutron physics and it can result in enhancing of nuclear safety. the contribution of 238u thermal feedback with thermo-hydraulic properties of the system has to be investigated in the future. acknowledgements this work was financially supported by grant of science and technology assistance agency no. apvv-0123-12 and stu grant scheme for support of young researchers. 66 vol. 4/2016 numerical analysis of slab nuclear reactor with thermal feedback references [1] h. ninokata, t. sawada, h. tomozoe, et al. a study on recriticality characteristics of fast reactors in pursuit of recriticality-accident-free concepts. progress in nuclear energy 29:387–393, 1995. doi:10.1016/0149-1970(95)00067-t. [2] j. han-gyu. solution of one-dimensional, one-group neutron diffusion equation, lecture note 1. seoul national university, reactor physics laboratory, 2008. [3] p. b. patil, u. p. verma. numerical computational methods. alpha science international, ltd, 2006. [4] f. osuský, b. vrban, š. čerba, et al. two-group numerical analysis of one dimensional table nuclear reactor. in proceedings of the 20th international conference on applied physics of condensed matter (apcom2014), pp. 320–323. 2014. [5] p. reuss. neutron physics. edp sciences, 2008. [6] m. kalousek, b. hučko. prenos tepla. vydavateľstvo stu, bratislava, 1996. [7] nuclear data services. point2015 data. temperature dependent endf/b-vii.1 cross section data, https://www-nds.iaea.org/point/. [8] j. r. lamarsh. introduction to nuclear reactor theory. amer nuclear society, 2002. [9] e. lychagin, v. mityukhlyaev, a. muzychka, et al. ucn sources at external beams of thermal neutrons. an example of pik reactor. nuclear instruments and methods in physics research section a: accelerators, spectrometers, detectors and associated equipment 823:47–55, 2016. doi:10.1016/j.nima.2016.04.008. 67 http://dx.doi.org/10.1016/0149-1970(95)00067-t https://www-nds.iaea.org/point/ http://dx.doi.org/10.1016/j.nima.2016.04.008 acta polytechnica ctu proceedings 4:62–67, 2016 1 introduction 2 calculation methodology 2.1 stationary state (time in-dependent solution) 2.2 transient state (time dependent solution) 2.2.1 crank-nicholson numerical method 2.2.2 accuracy of the crank-nicholson method 3 geometry of the nuclear slab system 4 initial and boundary conditions 5 results 5.1 case 1 5.2 case 2 6 conclusions acknowledgements references 194 acta polytechnica ctu proceedings 1(1): 194–199, 2014 194 doi: 10.14311/app.2014.01.0194 multi-wavelength view of supernova remnants manami sasaki1 1institute for astronomy and astrophysics, eberhard karls university tübingen, sand 1, d-72076 tübingen, germany corresponding author: sasaki@astro.uni-tuebingen.de abstract this contribution gives a very short overview on the emission of supernova remnants and the processes that are responsible for both the thermal and non-thermal origins of the emission, typically observed in radio, x-rays, and up to γ-rays. we discuss in particular the case of the galactic snr ctb 109. as detailed x-ray studies combined with observations in radio have shown, ctb 109 is interacting with a giant molecular cloud complex. the interaction of the snr shock with dense interstellar clouds is responsible for both the unusual semi-circular morphology of the snr and the bright x-ray feature inside the snr, and, as has been shown recently, seems also to play a major role in the production of γ-rays. keywords: supernova remnants radio ir optical x-rays γ-rays. 1 introduction supernova remnants (snrs) are objects that are formed by a supernova (sn) explosion at the end of the life of a star. owing to the shock waves of the sn explosion, the stellar ejecta as well as the ambient medium is heated to temperatures higher than a million kelvin. therefore, snrs are responsible for the heating of the interstellar medium (ism) and the distribution of heavier elements that were processed inside a star. in addition, both electrons and nuclei can be accelerated in the shock waves of the snrs to relativistic energies. therefore, snrs are also considered to be one source of galactic cosmic rays. by now, about 280 snrs are known in our galaxy and about 50 in the magellanic clouds. detailed studies of snrs in the galaxy and the magellanic clouds allow us to understand the nature of their shocks, the various radiation processes that are responsible for the complex emission observed over the full electromagnetic spectrum from radio to γ-rays, the interaction of the shock with the cooler, and often inhomogeneous ism, and the acceleration of particles. 2 snrs at lower energies 2.1 radio emission in the strong shock waves of snrs, electrons are accelerated and interact with the interstellar magnetic fields. these electrons emit synchrotron radiation and thus make the snr bright in radio. the distribution of the surface brightness of the electron synchrotron emission in snrs can be obtained from radio imaging observations. in general, the radio emission consists of narrow shell-like or filamentary emission and additional diffuse emission. the reason for these two kinds of synchrotron emission is that particles with higher energies lose their energy faster through synchrotron emission than those with lower energies. the long-lived lower-energy particles can diffuse further away from the shock and radiate over a much wider area, producing the more extended diffuse emission with a steeper spectrum. figure 1: radio continuum emission from snr da 530 (1420 mhz) in contour representation (left) and gray scale (right). vectors in the left image show the polarized intensity by their length and the direction of the e-field by their orientation (landecker et al., 1999). since synchrotron emission is polarized, radio observations can reveal the magnetic field structure in and around snrs. observations have shown that young remnants in general have a radial field structure. for remnants in the free expansion phase and in the early phases of the adiabatic expansion, radial motions due 194 http://dx.doi.org/10.14311/app.2014.01.0194 multi-wavelength view of supernova remnants to rayleigh-taylor instabilities at the contact discontinuity between the shocked ejecta and the shocked ism can enhance the radial magnetic fields. on the contrary, older remnants have a more tangential field structure with higher polarized fractions (milne, 1987). the polarized fraction in younger remnants are typically p = 10− 15%, while older remnants like snr da 530 can have a polarized fraction of p = 50% (fig. 1, landecker et al., 1999). 3 thermal x-ray emission the outward moving shock wave of an snr heats the interstellar medium, while the reverse shock that propagates inward heats the ejecta of the sn explosion. as a whole the ionized gas inside an snr has temperatures of ∼ 106−7 k typically. in plasmas the excitation and ionization of the atoms are mainly induced by electrons that collide with ions. snrs have very low densities and thus are optically thin. they can be considered to be out of thermal and ionization equilibrium, especially when they are still young. if the time after the plasma has been shocked is short, only a few collisions have occurred that cause ionization. the ionization states of the ions at a given temperature are thus lower than in the case of collisional ionization equilibrium (cie). instead, the plasma in snrs is characterized by non-equilibrium ionization (nei). the shape of the continuum of the thermal plasma is primarily determined by the electron temperature, while the temperature of ions might be different than that of electrons. the line emission in the x-ray band can be used to derive the ionization states of the ions as well as the element abundances. therefore, the x-ray spectrum can, on the one hand, reveal the element distribution in the ejecta, and on the other hand, the typical abundances of the ism in which the snr expands. 3.1 charge exchange newer observations of snrs in x-rays have revealed spectral features indicating charge exchange (cx) between the hot thin plasma of an snr and the cold interstellar gas. x-ray emission from cx can in principle be produced at any astrophysical site where hot plasma interacts with (partially) neutral gas, and was originally discoverd in x-ray data of comets (lisse et al., 1996, cravens, 1997). first observational evidences of cx in snrs had been obtained as a broad component of hα emission (e.g., chevalier et al., 1980). observations of the rim of the cygnus loop using suzaku and xmmnewton have revealed a significant emission feature at 0.7 kev, which was identified to be a complex of he-like o k(γ + δ + ε) and was interpreted as cx emission from interaction between he-like o and neutrals (fig. 2, katsuda et al., 2011). figure 2: non-x-ray background subtracted spectrum of a northeastern region of the cygnus loop snr taken with suzaku along with the best-fit model consisting of a bremsstrahlung plus gaussians (katsuda et al., 2011). line identifications are given on top of the upper panel. the gaussian shown in red represent the cx emission. the x-ray background components are shown in green. 3.2 overionization new suzaku observations of mixed-morphology (mm) snrs in the last few years have shown that in many cases the thermal x-ray emission can be interpreted as that from an overionized, recombining plasma in ionization non-equilibrium (e.g., yamaguchi et al., 2009, see fig. 3). mixed-morphology snrs, also called thermalcomposite remnants, are usually found in dense regions near molecular clouds and show very little or no shell emission. the x-ray emission is bright in the (projected) interior and is of thermal nature. the signature of an overionized plasma is a radiative recombination continuum (rrc) and strong ly emission of, e.g., si, s, or fe. in general, snrs are underionized and the shocked plasma slowly develops to ionization equilibrium. therefore, it is rather surprising to observe overion195 manami sasaki ization in snrs. to reach overionization in a plasma above ∼ 106 k, electrons have to cool rapidly. possible processes for such a cooling are either fast expansion, thermal conduction, or radiative cooling with enhanced abundances, which are all processes that are likely to occur in particular in snrs evolving in significantly inhomogenous ism. figure 3: suzaku xis spectrum in the 1.75 – 6.0 kev band (black: fi, red: bi). the best fit model is shown including the cxb (black) and vapec and gaussians for the source emission. rrc emisison of h-like mg, si, and s are shown in magenta (yamaguchi et al., 2009). 4 snrs as particle accelerators 4.1 non-thermal x-ray emission supernova remnants are non-thermal, extended radio sources with a flux spectrum described by a power-law with sν ∝ να with α ≈ −0.5. the synchrotron radiation is emitted by a non-thermal population of relativistic electrons, which can be described by a powerlaw. synchrotron emission in radio indicates that there are electrons with energies in the gev range in the snr shell. synchrotron emission from an snr in the x-ray band was confirmed for the first time for the remnant of sn 1006 (koyama et al., 1995) based on asca data, which showed that the non-thermal emission is dominating the emission of two outer shell regions. soon after, more snrs were also confirmed to radiate non-thermal x-ray emission: cas a (e.g., the et al., 1996, allen et al., 1997), rx j1713.7-3946 (g347.30.5, pfeffermann & aschenbach, 1996), rx j0852.04622 (g266.2-1.2, vela jr, aschenbach 1998). the xray emission of rx j1713.7-3946 and rx j0852.0-4622 is completely of non-thermal nature with no thermal emission detected so far. x-ray synchrotron emission is only observed in younger snrs and is indicative of electrons with energies of up to 1014 ev. the angular resolution of the newer x-ray telescopes, in particular that of the chandra x-ray observatory, made it possible to reveal narrow filaments of non-thermal x-ray emission close to the outer shock in several young snrs: sn 1006, rx j1713.7-3946, rx j0852.0-4622, cas a, tycho, and kepler. these filaments are very thin (∼ 1 − 2′′) and indicate relatively high magnetic fields (b = 100 − 600µg). recently, a deep chandra observation of tycho’s snr revealed bright non-thermal stripes, clearly seen in the energy band of > 4kev (fig. 4, eriksen et al., 2011). the stripes are likely caused by peaks in magnetic fields. figure 4: chandra image of the tycho’s snr in the energy band of 4.0 – 6.0 kev. the boxes show the regions with stripes or filaments of non-thermal emission (eriksen et al., 2011). 4.2 gamma-ray emission in the last decade some snrs have been detected with γ-ray observatories and are now well established astrophysical γ-sources. there are two major origins of γ-ray emission in snrs: • leptonic: interactions of relativistic electrons with low-energy photons of the cosmic microwave background cause inverse compton up-scattering of the photons. in addition, the non-thermal electrons will also emit bremsstrahlung through interactions with ions in the snr. • hadronic: accelerated nuclei can produce γray emission if they collide with atomic nuclei, thereby creating, among others, neutral π0s, which decay into γ-ray photons. this is believed to be enhanced in snrs near high-density clouds where the probability for the collision of the accelerated protons from the snr with cold protons or nuclei is higher (e.g., ic 443, w44, see fig. 5). the gev and tev emission of snrs can thus give us a direct view of the accelerated particles. 196 multi-wavelength view of supernova remnants figure 5: gamma-ray spectra of the snrs ic 443 and w44 and fitted models (ackermann et al., 2013). 5 the galactic snr ctb 109 the galactic supernova remnant ctb 109 (g109.1– 1.0) is the host of the anomalous x-ray pulsar (axp) 1e 2259+586 (fahlman & gregory, 1981) and has an interesting semi-circular morphology in both the x-ray and the radio. it is believed to be located next to a giant molecular cloud (gmc) complex and is thus one of the most striking examples of an interaction of an snr with a molecular cloud. the snr was discovered in x-rays with einstein by gregory & fahlman (1980). hughes et al. (1981) identified the radio counterpart in the galactic plane survey at 49 cm with the westerbork synthesis radio telescope. its shell is incomplete on the western side both in x-rays and in radio. the semi-circular radio and x-ray morphology of ctb 109 suggests the shock has been stopped by the gmc complex in the west. an elongated feature in co (‘co arm’) extends from the gmc complex to an x-ray minimum inside the snr in the north (tatematsu et al., 1987), indicating that a part of the dense gas of the gmc complex extends in front of the remnant. we performed observations of the entire snr ctb 109 with xmm-newton using the european photon imaging cameras (epics). a mosaic of all observations is shown in fig. 6. as mentioned before, ctb 109 seems to have expanded into a dense medium in the west, while the shock wave is expanding into a lowerdensity medium in the east. the radii of the northern and the southern periphery of ctb 109 are slightly smaller than to the east suggesting that ctb 109 is confined by denser material also in the north and the south. however, the relative difference is only ∼5 – 10%. therefore, the overall shape of the shell can be considered as a semi-sphere. there is an x-ray bright interior region known as the lobe, which is brighter than any part of the snr shell. the x-ray spectrum from the lobe obtained with xmm-newton is completely thermal (sasaki et al., 2004). we found co clouds around the lobe in new high-resolution co data from the five college radio observatory (sasaki et al., 2006). one of the co clouds overlaps with the lobe and has an additional component towards higher negative velocities in the velocity profile. we believe that the bright x-ray emission of the lobe is the result of the interaction of the snr shock with a dense molecular cloud, which was hit by the blast wave on the western side. the main part of the cloud was heated and is now visible as the lobe, while some denser parts might have survived. figure 6: mosaic rgb image of the snr ctb 109 created from the xmm-newton observations: red = 0.3 – 0.9 kev, green = 0.9 – 1.5 kev, blue = 1.5 – 4.0 kev (sasaki et al., 2004). 197 manami sasaki 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 vnei+vnei fe (0.0 5.0 x solar) 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 vnei+vnei abund. (0.0 5.0 x solar) 1 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8 3 vnei+vnei si significance (1 3 ) 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 vnei+vnei si (0.0 5.0 x solar) 1 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8 3 vnei+vnei fe significance (1 3 ) (b) (d) (e) (c)(a) figure 7: best fit abundances obtained for the emission of snr ctb 109 from the analysis of the chandra acis spectra by assuming a thermal model that consists of two vnei components for thermal non-equilibrium plasma for the shocked ism and for the shocked ejecta (sasaki et al., 2013). the shown parameters are: abundances for elements other than si, s, and fe fitted for the ejecta emission (a), si abundance fitted for the ejecta emission (b) and its significance calculated as (abundance of the si – abundance of the other elements)/error of the abundance of si (c), fe abundance and its significance (d and e, respectively). through the analysis of a deep, high-resolution data of the region around the lobe taken with chandra advanced ccd imaging spectrometer (acis), we discovered ejecta emission inside the snr west of the lobe (sasaki et al., 2013). the distriubtion of the siabundance and fe-abundance are shown in in fig. 7. these images show the parameter values obtained from the analysis of the spectra extracted in the regions indicated. the abundances measured for the ejecta emission are all comparable to or lower than solar values, except for the lobe. here, the si and fe abundances are in particular significantly higher. the enhanced abundances in the lobe suggest that there is an ejecta clump or a conglomeration of an ejecta clump and a shocked cloud. at the position of ctb 109 a gev source was detected with fermi (castro et al., 2012, see fig. 8). this γ-ray source is located southwest of the lobe where the snr appears dark in x-rays and thus is most likely highly absorbed. castro et al. (2012) performed a comprehensive modeling of the emission of ctb 109, in which they could reproduce the fluxes in radio, x-rays, and γ, as well as the x-ray spectrum taken with the xmm-newton epic in a model including a significant contribution by π0-decay. this is again consistent with the picture that there is significant interaction between the snr shock and a dense interstellar cloud. 6 summary being driven by strong shock waves formed in supernova explosions, supernova remnants are one of the main drivers of the matter cycle and dynamic evolution of galaxies. they are responsible for the chem198 multi-wavelength view of supernova remnants ical enrichment of galaxies. the shock waves create new structures in the ism by forming a cavity filled with hot thin gas and sweep the colder gas around it. particles are accelerated in the shock waves and their emission is observed over the entire range of the observable electromagnetic spectrum. in recent years observations of x-ray synchrotron radiation confirmed the existence of electrons with energies in the tev range in snr shocks. newest gev and tev obervations indicate inverse compton scattering as well as pion-decay processes in and around snr shocks. self-consistent modeling of the spectrum from radio to tev as in the case of the galactic snr ctb 109 helps us to understand both the heating and acceleration processes in snr shocks. figure 8: smoothed fermi-lat count maps in the range 2 – 200 gev (castro et al., 2012). white contours show the x-ray emission (0.5 – 5.0 kev) from xmm-newton data. acknowledgement m.s. acknowledges support by the deutsche forschungsgemeinschaft through the emmy noether research grant sa 2131/1. references [1] ackermann, m., et al. 2013, science, 339, 807 doi:10.1126/science.1231160 [2] allen, g. e., et al. 1997, apjl, 487, 97 [3] aschenbach, b. 1998, nature 396, 141 doi:10.1038/24103 [4] castro, d., slane, p., ellison, d. c., patnaude, d. j. 2012, apj, 756, 88 doi:10.1088/0004-637x/756/1/88 [5] chevalier, r. a., kirshner, r. p., raymond, j. c. 1980, apj, 235, 186 [6] cravens, t. e. 1997, geophys. res. lett., 24, 105 [7] eriksen, k. a., et al. 2011, apjl, 728, 28 doi:10.1088/2041-8205/728/2/l28 [8] fahlman, g. g. & gregory, p. c. 1981, nature, 293, 202 doi:10.1038/293202a0 [9] gregory, p. c. & fahlman, g. g. 1980, nature, 287, 805 doi:10.1038/287805a0 [10] hughes, v. a., harten, r. h., costain, c. h., van den bergh, s. 1981, apj, 246, l127 doi:10.1086/183568 [11] [12] katsuda, s., et al. 2011, apj, 730, 24 doi:10.1088/0004-637x/730/1/24 [13] koyama, k., et al. 1995, nature, 378, 255 doi:10.1038/378255a0 [14] landecker, t. l., et al. 1999, apj, 527, 866 [15] lisse, c. m., et al. 1996, science, 274, 205 doi:10.1126/science.274.5285.205 [16] milne, d. k., 1987, aujph, 40, 771 [17] pfeffermann, e., aschenbach, b. 1996, proc. of ‘röntgenstrahlung from the universe’, 267 [18] sasaki, m., kothes, r., plucinsky, p. p., gaetz, t. j., brunt, c. m. 2006, apj, 642, l149 doi:10.1086/504844 [19] sasaki, m., et al. 2004, apj, 617, 322 [20] sasaki, m., plucinsky, p. p., gaetz, t. j., bocchino, f. 2013, a&a, 552, 45 [21] tatematsu, k., et al. 1987, a&a, 184, 279 [22] the, l.-s., et al. 1996, a&as, 120, 357 [23] yamaguchi, h., et al. 2009, apjl, 705, 6 doi:10.1088/0004-637x/705/1/l6 discussion carlotta pittori’s comment: to distinguish between ic and π0 decay in the gev range, at least in some cases like w44, the new planck data combined with agile and fermi data are very much constraining. the mw picture in the case of middle-aged snrs interacting with molecular clouds seem to largely favour the hadronic scenario (π0 decay). 199 http://dx.doi.org/10.1126/science.1231160 http://dx.doi.org/10.1038/24103 http://dx.doi.org/10.1088/0004-637x/756/1/88 http://dx.doi.org/10.1088/2041-8205/728/2/l28 http://dx.doi.org/10.1038/293202a0 http://dx.doi.org/10.1038/287805a0 http://dx.doi.org/10.1086/183568 http://dx.doi.org/10.1088/0004-637x/730/1/24 http://dx.doi.org/10.1038/378255a0 http://dx.doi.org/10.1126/science.274.5285.205 http://dx.doi.org/10.1086/504844 http://dx.doi.org/10.1088/0004-637x/705/1/l6 introduction snrs at lower energies radio emission thermal x-ray emission charge exchange overionization snrs as particle accelerators non-thermal x-ray emission gamma-ray emission the galactic snr ctb 109 summary 222 acta polytechnica ctu proceedings 1(1): 222–226, 2014 222 doi: 10.14311/app.2014.01.0222 x-ray and near-infrared spectroscopy of dim x-ray point sources constituting the galactic ridge x-ray emission kumiko morihana1, masahiro tsujimoto2, ken ebisawa2 1nishiharima astronomical observatory, center for astronomy, university of hyogo, 407-2 nishigaichi, sayo-cho, sayogun, hyogo, 679-5313, japan 2japan astrospace exporation agency, institute of space and astronautical science, 3-1-1 yoshino-dai, chuo-ku, sagamihara, kanagawa 252-5210, japan corresponding author: morihana@nhao.jp abstract we present the results of x-ray and near-infrared observations of the galactic ridge x-ray emission (grxe). we extracted 2,002 x-ray point sources in the chandra bulge field (l =0◦.113, b = 1◦.424) down to ∼10−14.8 ergs cm−2 s−1 in 2–8 kev band with the longest observation (∼900 ks) of the grxe. based on x-ray brightness and hardness, we classified the x-ray point sources into three groups: a (hard), b (soft and broad spectrum), and c (soft and peaked spectrum). in order to know populations of the x-ray point sources, we carried out nir imaging and spectroscopy observation. we identified ∼11% of x-ray point sources with nir and extracted nir spectra for some of them. based on x-ray and nir properties, we concluded that non-thermal sources in the group a are mostly active galactic nuclei and the thermal sources are mostly white dwarf binaries such as cataclysmic variables (cvs) and pre-cvs. we concluded that the group b and c sources are x-ray active stars in flare and quiescence, respectively. keywords: galaxy: bulge galaxy: disk ir x-rays. 1 introduction since the dawn of the x-ray astronomy, an apparently diffuse emission of low surface brightness has been known to exist along the galactic plane (gp; |l| <45◦, |b| <1◦), which is referred to as the galactic ridge xray emission (grxe; e.g.,worral et al. 1982; warwick et al., 1985). the x-ray spectrum is characterized by hard continuum with a strong 6.7 kev fe k emission line (koyama et al., 1986a). the origin of the grxe had been a mystery for a long time. a long-standing debate had been whether it is a truly diffuse plasma (ebisawa et al., 2001, 2005) or a sum of unresolved xray point sources (revnivtsev et al. 2006). recently, revnivtsev et al (2009) showed that ∼80% of the fe k emission line was resolved into dim point sources using the deepest x-ray observations (∼900ks) made with the chandra at a slightly off-plane region of (l, b)=(0◦.113, –1◦.424) in the galactic bulge (chandra bulge field; hereafter, cbf). if the grxe is composed of the dim x-ray point sources, new questions arise. what are the populations of the dim x-ray point sources? which class of sources contribute to the fe k emission line? we do not know the population of majority of the dim point sources due to a limited number of x-ray photons. thus, we focus on near-infrared (nir), which has almost the same penetrating power as x-rays into deep interstellar extinction toward the gp. we studied the population constituting the grxe combining x-ray data with nir data in this paper. in particular, we focus on the population contribute to the fe k line of the grxe spectrum. 2 analysis and results 2.1 x-ray 2.1.1 observation and source extraction we retrieved 10 archived data of the cbf taken with the advanced ccd imaging spectrometer (acis)-i array on board chandra with a total exposure time of ∼900 ks. we merged 10 data set and extracted 2,002 valid point sources down to ∼10−14.8 ergs cm−2 s−1 in 2–8 kev (figure 1). for all the sources, we extracted source and background events. 2.1.2 spectral fittings for the bright sources (source counts>100), we constructed the background-subtracted spectra and generated instrumental response files. we carried out spectra fittings with thermal (apec; smith et al. 2001) and 222 http://dx.doi.org/10.14311/app.2014.01.0222 x-ray and near-infrared spectroscopy of dim x-ray point sources... non-thermal models (power-law) for the bright sources. as the results, 11 bright sources with more than 1000 counts have hard power-law like hard spectra with the photon index γ∼1.5. figure 1: smoothed and exposure-corrected x-ray image of the cbf (0.5–8 kev). the field of view of the sirius (nir) observations are shown by red squares. the white circle shows the region, the result of which was published in revnivtsev et al (2009). 2.1.3 grouping 0.5 1.0 1.5 2.0 2.5 3.0 -1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 1 2 3 4 5 q 2 = 3 *q 2 5 /q 7 5 q1=log10 q50/(1-q50) median energy (kev) c a b figure 2: x-ray color-color diagram of all detected xray sources. the converted median energy is shown in the upper x-axis. color difference shows groups defined in § 2.1.3. we constructed an x-ray color-color diagram (hong et al., 2004) using the quantiles (e25, e50, and e75) characterizing the spectral shape of each source (figure 2). here, ex (kev) is the energy below which x% of photons reside in the energy-sorted event list. e50 is equivalent to the median energy (the details are in morihana et al., 2013). here, the q1 value indicates the degree of photon spectrum being biased toward the higher (q1 > 0) or lower (q1 < 0) energy end (hard or soft spectra), and the q2 value indicates the degree of photon spectrum being less (q2 > 1) or more (q2 < 1) concentrated around the peak (broad or narrow spectra). based on the x-ray color-color diagram, we classified all the point sources into three groups, which are the group a (hard), b (soft and brooded spectrum), and the group c (soft and peaked spectrum). 2.1.4 global spectral fittings furthermore, we performed global spectral fittings in 0.5–8 kev for the composite spectrum for each group in a similar manner in § 2.1.2 (figure 4). for the group a spectrum, neither a power-law nor a thin-thermal plasma model reproduced the spectrum well, respectively because of the excess emission at 6.7 kev or a flatness of the continuum. so we fitted the spectrum with a combination of the two models, which was successful. the equivalent width (ew) of the fe k feature becomes larger as the flux decreases (figure3), which suggest that the thermal component becomes strong against the non-thermal component as the flux decreases. for the group b spectrum, several emission lines are seen, including the 6.7 kev emission line from fexxv and 2.5 kev from sxv. this set of emission lines indicates a multiple-temperature plasma, and indeed the spectrum was reproduced well with two thin-thermal plasma components, but not with one component. for the group c spectrum, several emission lines are also seen. unlike the group b sources, the fexxv emission at 6.7 kev is absent. we fitted the spectrum using the same model with group b. 0 200 400 e q u iv a le n t w id th ( e v ) 10-1610-1510-1410-13 fx (ergs cm-2 s-1) figure 3: equivalent width of the fe k line against the decreasing flux in 2-8 kev above which the cumulative combined spectra in the group a were constructed. the 1σ statistical uncertainty is shown for each data. 2.2 nir imaging 2.2.1 observation and source extraction to identify the x-ray point sources with nir, we carried out nir observations using simultaneous infrared for unbiased survey (sirius; nagayama et al., 1999) on the infrared-survey facility (irsf) 1.4 m telescope 223 kumiko morihana, masahiro tsujimoto, ken ebisawa c o u n ts s -1 k e v -1 0 .0 1 1 0 -3 1 0 -4 -2 0 2 χ energy (kev) 0.5 1.0 2.0 3.0 5.0 (a) a c o u n ts s -1 k e v -1 0 .0 1 1 0 -3 1 0 -4 -2 0 2 χ energy (kev) 0.5 1.0 2.0 3.0 5.0 (b) b c o u n ts s -1 k e v -1 0 .0 1 1 0 -3 1 0 -4 -2 0 2 χ energy (kev) 0.5 1.0 2.0 3.0 5.0 (c) c figure 4: composite spectra and the best-fit global model of the group (a) a, (b) b, and (c) c. the lower panel shows the residuals of the data to the fit. the best-fit parameters can be found in table 1. table 1: best-fit parameters for global spectral model in 0.5–8.0 kev group nh (1)1 kbt (1)2 nh (2) kbt (2) z3 γ4 χ2/d.o.f. (1022 cm−2) (kev) (1022 cm−2) (kev) a 1.09+0.39 −0.50 6.65+3.24 −3.03 2.46+2.35 −0.58 ... 0.97+0.36 −0.32 1.29+0.18 −0.40 205.36/504 b 0.75+0.06 −0.05 0.74+0.54 −0.45 0.80+0.22 −0.18 7.87+1.86 −4.84 0.99+0.33 −0.29 ... 98.68/103 c 0.70+0.18 −0.11 0.78+0.04 −0.03 0.04+0.05 −0.04 4.50+0.65 −0.35 0.15+0.16 −0.12 ... 85.86/102 1interstellar extinction column density for the first component (the lower temperature component for the twotemperature model. 2plasma temperature for the second component (the higher temperature component for the two-temperature model). 3metal abundance relative to the solar value for the thermal component. 4photon index for the power-law model as the second component. in south africa astronomical observatory. we covered the cbf as we show in figure 1. we extracted nir source with a 3σ level using sextractor version 2.8.6, which are 52312 (j), 61,188 (h), and 65,051 (ks) sources down to ks∼16 mag. for asymmetry and photometry correction, we rendered the two micron all sky survey (2mass) point source catalog. for asymmetry, the sirius positions are determined at an accuracy of the pixel size of sirius (0.45′′). 2.2.2 cross correlation we search for possible nir counterparts for all the xray point sources using 2mass and the sirius. we searched nir counterpart sources within 1σ error circle (x-ray-2mass source; 1.3′′, x-ray-sirius source; 1.2′′). when there are two or more sources within the 1σ circle, we assumed the closest one to be the counterpart. then, we finally recognized 222 x-ray sources to have nir counterpart within 1σ circle (∼11% of all the x-ray point sources). 2.3 nir spectroscopy we conducted nir spectroscopy observation for some selected objects in the cbf using subaru/multi-object infrared camera and spectrograph (moircs). we selected 51 sources for spectroscopy in ks-band based on x-ray hardness and source variability. figure 5 shows examples of nir spectrum. we finally obtained 33 nir spectra of the x-ray point source in the cbf. combined x-ray results, there are two type of sources in the cbf, which are (1) sources with hi (brγ) and co absorption features in nir spectra and hard x-ray spectra, (2) sources only with co absorption features in nir spectra and soft x-ray spectra. from these properties, type (1) sources are k or m spectral type stars and type (2) sources are m spectral type stars. 21000 22000 23000 24000 wavelength (angstroms) b a b a c c coco co cohi hei cainai n o rm a liz e d i n te n s it y figure 5: examples of ks-band spectra in the cbf. upper label of each spectrum shows groups in § 2.1.3 defined by x-ray properties. 224 x-ray and near-infrared spectroscopy of dim x-ray point sources... 3 discussions & conclusion we now discuss likely populations in each group based on the results presented above. the transition from one class to another is continuous along the color and flux, so the groups are naturally a mixture of sources of different classes. first, we consider that the group a sources are mostly mixture of active galactic nuclei (agns) and white dwarfs (wds) binaries, each responsible for the power-law and thermal plasma components in the composite spectrum (figure 4). in the spectral fitting, the power-law component has a spectral index of 1.29+0.18 −0.40, which is similar to the typical spectral form of agns (table 1 e.g.; rosati et al., 2002). for the thermal component of the composite spectrum of the group a, a strong fe k feature and 6.7 kev plasma temperature are seen. both of these features are observational characteristics of magnetic cvs (ezuka & ishida 1999). other classes of wd binaries, such as dwarf novae, pre-cvsmay also considered for the likely classes. precvs are poorly recognized class of sources, which are detached binaries of a wd and a late-type star, unlike conventional cvs that are semi-detached systems. some of them show strong fe k emission in the hard x-rays (matranga et al. 2012). in fact, showed nearinfrared spectra of selected x-ray sources presented in this paper, in which some thermal a sources do not exhibit the brγ emission that is typical for the conventional cvs (dhillon et al. 1997). we thus consider that pre-cvs, most of which do not show the brγ emission (howell et al. 2010; schmidt& mikoajewska 2003), also account for at least some fraction of the thermal source population in the group a. the group b and c sources are galactic sources with a soft thermal spectrum, and we consider that most of them are likely to be x-ray active stars. the composite spectra of these two groups were fitted with two plasma components. in these groups, the spectra type of the sources are kor m-type spectra, which has no brγ emission line. we speculate that the differences between b and c is that most c sources represent active binary stars in the quiescence, while most b sources represent those during flares. from these things, we consider that the group b and c sources are x-ray active stars in flare and quiescence, respectively. acknowledgement this research has made use of public data obtained from chandrax-ray center which is operated for nasa by the smithsonian astrophysical observatory. this work has also made use of software from high energy as astrophysics science archive research center (heasac) which is provided by nasa goddard space flight center. references [1] dhillon, v. s., marsh, t. r., duck, s. r., & rosen, s. r. 1997, minareas, 285, 95 [2] ebisawa, k., maeda, y., kaneda, h., & yamauchi, s. 2001, science, 293, 1633 doi:10.1126/science.1063529 [3] ebisawa, k., tsujimoto, m., paizis, a., et al. 2005, apj, 635, 214 doi:10.1086/497284 [4] ezuka, h. & ishida, m. 1999, apjs, 120, 277 [5] hong, j., schlegel, e. m., & grindlay, j. e. 2004, apj, 614, 508 doi:10.1086/423445 [6] koyama, k., makishima, k., tanaka, y., & tsunemi, h. 1986, pasj, 38, 121 [7] matranga, m., drake, j. j., kashyap, v., & steeghs, d. 2012, apj, 747, 132 doi:10.1088/0004-637x/747/2/132 [8] morihana, k., tsujimoto, m., yoshida, t., & ebisawa, k. 2013, apj, 766, 144m doi:10.1088/0004-637x/766/1/14 [9] nagayama, t., et al., 2003, in society of photooptical instrumentation engineers (spie) conference series, vol. 4841, m. iye & a. f. m. moorwood, 459–464 [10] revnivtsev, m., sazonov, s., gilfanov, m., churazov, e., & sunyaev, a&a, 452, 169r [11] revnivtsev, m., sazonov, s., et al., 2009, nature, 458,1142 doi:10.1038/nature07946 [12] rosati, p., tozzi, p.,et al. 2002, apj, 566, 667 doi:10.1086/338339 [13] schmidt, m. r., & miko lajewska, j. 2003, nearinfrared spectra of a sample of symbiotic stars [14] smith, r. k., brickhouse, n. s., liedahl, d. a., & raymond, j. c. 2001, apjl, 556, l91 doi:10.1086/322992 [15] warwick, r. s., turner, m. j. l., watson, m. g., & willingale, r. 1985, nature, 317, 218 doi:10.1038/317218a0 [16] worrall, d. m., marshall, f. e., boldt, e. a., & swank, j. h. 1982, apj, 255, 111 225 http://dx.doi.org/10.1126/science.1063529 http://dx.doi.org/10.1086/497284 http://dx.doi.org/10.1086/423445 http://dx.doi.org/10.1088/0004-637x/747/2/132 http://dx.doi.org/10.1088/0004-637x/766/1/14 http://dx.doi.org/10.1038/nature07946 http://dx.doi.org/10.1086/338339 http://dx.doi.org/10.1086/322992 http://dx.doi.org/10.1038/317218a0 kumiko morihana, masahiro tsujimoto, ken ebisawa discussion takeshi go turu’s comment: is the fe line a mixture of fe lines (6.4, 6.7 7.0 kev) or 6.7 kev line? the fe line in our grxe spectrum is a mixture of the fe lines (6.4, 6.7, and 7.0 kev). the line center of the fe line is ∼6.7 kev. guainazzi matteo’s comment: radio-quiet agns normally exhibit iron kα lines. this may suggest that your iron free agns could be orimary radio-loud. do you have radio measurements, that could validate this hypothesis? i consider that fe k line of most background agns are red-shifted, because most background agns constituting the grxe are at far distance. 226 introduction analysis and results x-ray observation and source extraction spectral fittings grouping global spectral fittings nir imaging observation and source extraction cross correlation nir spectroscopy discussions & conclusion 84 acta polytechnica ctu proceedings 1(1): 84–89, 2014 84 doi: 10.14311/app.2014.01.0084 agn/starburst connection eleonora sani1 1inaf osservatorio astrofisico di arcetri corresponding author: sani@arcetri.astro.it abstract two main physical processes characterize the activity in the nuclear region of active galaxies: an intense star formation (starburst, sb) and an active galactic nucleus (agn). while the existence of a starburst-agn connection is undisputed, still it is not clear which process dominates the energetic output in both local and high redshift universe. moreover there is no consensus on whether agn fueling is synchronous with star formation or follows it during a post-starburst phase. here i first review how to disentangle the relative sb-agn contribution, then i focus on the physical and geometrical properties of the circumnuclear environment. keywords: agn starburst merging feedback imaging spectroscopy mid-ir x-rays. 1 introduction the issue of agn – star formation (agn–sf) connection in local and distant galaxies is possibly relevant for understanding several processes: from galaxy formation and evolution, and the star formation and metal enrichment history of the universe, to the the origin of the extragalactic background at low and high energies and the origin of nuclear activity in galaxies. it is well known that sf traces the growth of a galaxy in terms of stellar mass, and that galaxies assemble their mass through sb episodes. for example, local ulirgs (ultraluminous infrared galaxies) and high-z smgs (submillimeter galaxies) harbor the large molecular-gas reservoirs that are necessary to switch on sb episodes (tacconi et al. 2010, 2013). on the other hand, agns trace the growth of supermassive black holes (smbhs, with masses mbh > 106m�) and quiescent smbhs are believed to dwell in almost all galaxy bulges as agn relics, i.e. the result of a past agn activity (soltan 1982, marconi et al. 2004, merloni & heinz 2008). 1.1 evidence of agn–sf connection a direct link between bulge formation and the growth of the central smbhs has been inferred from tight relations between bh mass mbh and the bulge structural parameters, such as velocity dispersion (ferrarese & merrit 2000, gebhardt et al. 2000, gultekin et al. 2009), luminosity (kormendy & richstone 1995, marconi & hunt 2003, sani et al.2011) and mass (magorrian et al. 1998, mh03, häring & rix 2004). since a sb is a natural consequence of dissipative gaseous processes associated with spheroid formation (e.g. barnes & hernquist 1991) and agn–sf connection dating back to the early universe is implied by these results, a mergerdriven scenario is well suited to reproduce such scaling relations. recently, attention has been drawn to the coexistence of pseudobulges and bhs (nowak et al. 2010, sani et al. 2011). indeed pseudobulges seem to follow their own relation with bhs, hosting less massive bhs than classical bulges (greene, ho, & barth 2008, hu 2009), or they are at least displaced from the scaling relations for classical bulges (sani et al. 2011, kormendy et al. 2011). this may indicate a dichotomy in the formation history of galaxies (major merger vs. secular evolution) and/or that activity in galaxy nuclei can be stochastically driven by local processes such as minor mergers, bar/disk instabilities. in the following i concentrate the discussion on the major-merger scenario, its implications and observational evidences. the reader is anyhow advised about the importance of secular evolution processes, especially in low-luminosity, local seyfert galaxies. 1.2 the major-merger scenario (ulirgs-qso path) the merger between galaxies of approximately the same size is relevant for the cosmological evolution of sbs, quasars and early type galaxies (sanders et al. 1988, silk & rees 1998, fabian 1999, granato et al. 2001, di matteo et al. 2005, hopkins et al. 2008, menci et al. 2008). a schematic view of the main steps in the merging process is given in fig. 1. 84 http://dx.doi.org/10.14311/app.2014.01.0084 agn/starburst connection sfr dmbh/dt figure 1: major-merger scenario. starting from the merger of two spiral galaxies (left side), the merger produces a violent sb and the onset of qso activity (middle) and ends with the formation of a dead spheroid (right side). from hopkins et al. 2008. during first encounters, the mergers between gasrich galaxies (i.e. spirals) drag the gas which fuels both sf and agn activity. thus a violent sb occurs (ulirg-phase) as well as heavily embedded smbh growth (observed as obscured agn). then, the peak of accretion happens when the systems coalesce. at this point the quasar is cleaning the circumnuclear environment thanks to strong winds that blow out the gas, the agn becomes x-ray and optically thin. finally, due to feedback processes both bh growth and further sf are quenched leaving a qso relic in a red galaxy (passive evolution). therefore, obscured qso and feedback processes are key ingredients in the bh– host galaxy co–evolution. in the following, after a brief description of the agn and sf emission, i review the agn-sf observations for both single sources (interesting for their physics/structure) and large samples (to infer statistical properties). 2 the agn vs sf emission an issue in agn–sf studies is to correctly identify agns that represent only a small fraction of sources compared to the population detected at optical and ir wavelengths, and their properties can be elusive. obscured agns can be elusive because their optical emission resembles that of normal galaxies and therefore they cannot be identified by the color-selection techniques used for qsos, or because their spectra do not show any agn signature. their ir emission can be dominated by pah features, typical of star-forming galaxies, instead of continuum emission from hot dust. radio observations reveal the radio-loud population of obscured agns (e.g., radio galaxies), but this comprises only a small fraction, 10%, of the total agn population. a hard x-ray selection fails to detect the luminous narrow-line agns in the numbers predicted by some models of the cosmic x-ray background. polletta and collaborators (2007) show the differences in the spectral energy distributions (seds) of agn and star forming galaxies. the average seds of the three classes show some clear differences. they become increasingly blue in the optical–near-ir (λ < 1 µm), warmer in the ir (i.e. red at λ ' 1−10 µm), and brighter in x-rays in the sequence sf→agn2→agn1. 85 eleonora sani more in detail, most of the diagnostics developed till recently fail in identifying an agn when it is intrinsically faint and/or deeply obscured. as an example of a possible case where the agn is hardly detectable, i consider in fig. 2 an intrinsically faint agn, contributing only 20% to the bolometric source luminosity, and moreover its primary emission is absorbed by gas and dust. only two spectral regions are useful for such an agn detection: the hard x-rays (above 2 kev) and the near-mid infrared between 3 and 10 µm. indeed in the near-ir and optical ranges the dust and gas extinction respectively avoid the agn detection, and at radio wavelengths the sb emission peak dilutes agn emission properties. figure 2: top: simulated seds of a sb (blue) and an agn (black) contributing to 80% and 20% respectively of the lbol. bottom: extinction of the agn emission by gas (green) and dust (red), the horizontal black line indicates an absorption factor of 5. the agn is undetectable in the range 3 µm-2 å due to dust and gas extinction, and at λ ≥ 10 µm for the sb dilution. only the hardest x-rays and 3-10 µm observations reveal the agn. it is thus clear that the diagnostic performed in the x-ray and ir bands can highlight the main agn-sf differences and represent the wavelength rages where we do have the most detailed information respectively. 3 x-ray and ir screenings: imaging and spectroscopy here i briefly describe recent results obtained thanks to x-ray – ir imaging and spectroscopy in two puzzling sources. the final goal is to constrain the physics of ongoing processes and the structure of the nuclear region. 3.1 the case of ngc 6240 thanks to imaging techniques we can simultaneously observe the agn and sf activity in the nuclei of local galaxies (e.g. ngc 1068, ngc 6240, mrk 231, arp 299, circinus...). the angular resolution represents the only limit for this analysis. ngc 6240 is a well known double-merging system 107 mpc far away. here the xray imaging and spectroscopy performed with chandra (komossa et al. 2003) revealed a compton thick double agn, that was confirmed at all wavelengths, plus a sb covering a projected ∼ 2 kpc scale (risaliti et al. 2006, medling et al. 2011, feruglio et al. 2013). hst and spitzer images (bush et al. 2008) show how the thermal emission follows the optical dust obscuration very closely. ngc 6240 is thus an active interacting remnant viewed at the point of nuclei merging where two agns are visible. the results are well consistent with a major merger scenario with the transition from disk galaxies to a spheroid. 3.2 iras 12071-0444: a ct type 2 qso mid-ir (data from irs/spitzer) and x-ray spectroscopy (data from bepposax, chandra, suzaku) allow to investigate the nature and peculiar structure of the nearby (z= 0.128) ulirg (lir = 9 × 1045erg/s) iras 12071-0444. it is one of the few ulirgs optically classified as a type 2 agn and its mid-ir spectrum (in fig. 5–right, nardini et al. 2009) shows the typical features of ongoing sb (traced by pah emission) plus a buried agn (traced by a reddened power-law). however, the marginal emission below 10 kev (fig. 5– left) is consistent with the sole sb component, the typical fe kα line is undetected, and the agn is only seen above 20kev where the much narrower suzaku fov eventually confirms an early tentative detection by bepposax. the x-ray data are well fitted by an absorbed power-law with nh = 3.5 × 1024cm−2 and l2−10 ∼ 7×1044erg/s, i.e. about 10% of the bolometric luminosity is given by the agn intrinsic emission. the result proves iras 12071-0444 as the closest comptonthick quasar 2 yet known. finally, a simple visual comparison with the prototypical ct seyfert ngc 4945 reveals that the reflection efficiency is about a factor of ten lower than for a standard geometry. a peculiar geometry of the absorber is thus required to explain the x-ray/ir data. there are basically two possibilities: (i) a geometrically thick torus, with a small opening angle (i.e. large covering factor). in this case both the direct and reflected components are absorbed. 86 agn/starburst connection agn template sb template figure 3: iras12071-0444 data. left: the spitzer/irs ∼ 5− 8µm spectrum (green dots) is reproduced (black line) by means of an agn (blue dashed line) and sb (red dot-dashed line) templates. right: suzaku data (blue) well agrees with the chandra/acis spectrum (red) in the 0.5-5 kev range, and beyond 15 kev with the bepposax/pds points (orange). the shaded blue and orange areas in the upper left corner allow to compare suzaku and bepposax/pds fields of view respectively. i also compare iras 12071-0444 with the ngc 4945 x-ray spectrum observed by xmm-newton (green). the ngc 4945 data are scaled to match iras 12071-0444 at 0.5–10 kev. alternatively, (ii) the reflection component is absent due to a small compact cloud along the line of sight (i.e. with a small covering factor). the last picture implies both photoelectric compton absorption without scattering. the resulting intrinsic luminosity exceedes the bolometric one a factor of 3 thus the compact cloud scheme is unphysical. as a probe of the iras 120710444 represents a key evolutionary step of the galaxybh growth. 4 statistical analysis of composite sources by selecting large samples of composite sb+agn sources, it is possible to link the agn–sf connection processes with the system properties. in the following i show recent results concerning ir and x-ray statistical analysis. i ) the availability of a reliable measure of the relative agn/sb contribution to ulirgs (risaliti et al. 2006, sani et al. 2008, nardini et al. 2008) allows a quantitative investigation of the relation between total luminosity and agn contribution. it is known from optical spectroscopy that the fraction of seyfert-like systems among ir galaxies grows along with luminosity (veilleux et al. 1999; goto 2005). in nardini et al. 2010 we have quantified how sf and nuclear activity are the primary engine at the opposite ends of the ulirg luminosity range. the sb component dominates at log(lir/l�) < 12.5, where the agn is a significant contributor in only ∼ 30% of the composite sources. the power supplied by bh accretion grows stronger along the luminosity scale, and ultimately it represents the trigger of the extreme ir activity. ii ) the same technique has been applied by sani et al. 2010 to a sample of local seyfert galaxies to relate their sb activity with the bh growing parameters. we found that sf activity around narrow line seyfert 1 (nls1s), i.e. the more accreting agns, is larger than around bls1 of the same agn luminosity. this result seems to hold over the entire range of distance and luminosity. moreover, the star formation rate is higher in low black hole mass (log(mbh/m�) < 7) and high eddington ratios (l/ledd > 0.1) systems indicating that black hole growth and star formation are occurring simultaneously. iii ) with ir and x-ray analysis we can also investigate the sf–agn connection as a function of obscuration. on one side we do have evidence (veilleux et al. 2009, nardini et al. 2010) that, as expected, the fraction of sb-dominated objects shows a constant decline as the radiation field grows harder with absorption features anti-correlating with the optical classification (siroki et al. 2008). concerning the individual classes, it is worth noting that a sizable number of very powerful agn actually lies among liners (low ionization narrow emitting regions, see also risaliti, imanishi & sani 2010). interestingly seyfert 2, liners and hii regions harbor a similar agn content, supporting the idea of a connection between the sf activity and nuclear obscuration. moreover, sb dominated galaxies host the most obscured agns (see also georgakakis et al. 2004, sani et al. 2008, nardini & risaliti 2011). on the other hand, no enhancement of sf is observed 87 eleonora sani in obscured agns (rovilos et al. 2012) and the specific sf rate of type 2 agn hosts is consistent with the value for normal galaxies (daddi et al. 2007, sarria et al. 2010, mainieri et al. 2011). this is thus a debated matter, but we can not relate the sf physics to the bh physics in a complete and unbiased way because we can not measure e.g. bh mass and accretion rate in type 2 objects. 5 catching feedback in action as mentioned at the beginning of this review, a key ingredient of the merger scenario is feedback. so far a direct detection of the agn winds able to quench sf in the host galaxy is lacking. indeed according to veilleux et al. 2005, ∼ 30% of qso show fast winds (100 km/s < vout < 1000 km/s) but the outflow affects the circumnuclear environment only on the broad/narrow line region scales and thus can not inhibit the sf in the host (muller-sanchez et al. 2011). only in nearby sources like mrk 231 is possible to detect strong molecular outflows (feruglio et al. 2010, fisher et al. 2010) acting on kpc scales as expected by qso-feedback models. the current challenge is to detect such molecular winds at the epoch of the agn activity peak (i.e. at about redshift 2). 6 what we know and what is missing • while the agn accretion history is well traced, we still do not know how the smbh mass function evolves. • if the sf–agn structure on large scales is well studied, we still lack in constrainig the physical processes on small scales (i.e. with extreme adaptive optics corrections for imaging and ifu observations). • we are confident that smbh growth happens simultaneously to the bulk of sf in unobscured composite sources, but the issue for type 2 objects is open. to solve it, we need to identify the entire population and to measure their mbh , l/ledd, sfr etc. • it is well known that agn powers strong winds in local galaxies. nonetheless most of them affect only the ionized gas on circumnuclear spatial scales and further observational efforts are required to identify kpc–scale molecular outflows at the peak of agn activity (z∼ 2). acknowledgement i would like prof. giovannelli and the soc for organizing this stimulating workshop. references [1] barnes, j. e., & hernquist, l. e.: 1991, apjl 370, l65. 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[51] veilleux, s., kim, d.-c., rupke, d. s. n., et al.: 2009, apj 701, 587. discussion casiana munoz-tunon: more obscured agns have larger sf, and you mention that no correlation is found between sf and obscuration. what sort of connection do you expect? eleonora sani: on the basis of hydrodinamical symulations of major mergers, we do expect the co-presence of a violent sb event and a buried agn at the time of galaxies coalescence. thus a correlation between the sfr and the agn obscuration behaviours (e.g. column density) is expected. analysis based on the cosmos survey don’t find such relation. nonetheless the issue is still a matter of debate since the lack of this correlation could be due to aperture effects. moreover deeply embeded agns are found in local ulirgs and high-redshift smgs. 89 http://dx.doi.org/10.1088/0004-637x/743/1/32 http://dx.doi.org/10.1086/518113 http://dx.doi.org/10.1086/500588 http://dx.doi.org/10.1086/523784 http://dx.doi.org/10.1038/nature08773 http://dx.doi.org/10.1088/0004-637x/768/1/74 introduction evidence of agn–sf connection the major-merger scenario (ulirgs-qso path) the agn vs sf emission x-ray and ir screenings: imaging and spectroscopy the case of ngc 6240 iras 12071-0444: a ct type 2 qso statistical analysis of composite sources catching feedback in action what we know and what is missing acta polytechnica ctu proceedings doi:10.14311/ap.2016.4.0050 acta polytechnica ctu proceedings 4:50–55, 2016 © czech technical university in prague, 2016 available online at http://ojs.cvut.cz/ojs/index.php/app ion source for a single particle accelerator tomáš matlochaa,b a faculty of nuclear sciences and physical engineering, czech technical university in prague, břehová 7, 115 19 praha 1, czech republic b nuclear physics institute of the cas, 250 68 řež u prahy, czech republic correspondence: matlocha@ujf.cas.cz abstract. this paper proposes a method to obtain very low beam current on the injection side of an particle accelerator. by using a combination of a well known ion source type and a short photo-emissive laser pulse, very low amount of ions can be created. the method could be used in the ion source of various types of the accelerators, where very low beam current is essential. the design is adapted to build a compact internal ion source and the dimensions of the device are adjusted to fit central region of the cyclotron u-120m. the functional parameters of the device are discussed and the amount of the produced ions is estimated. keywords: single particle accelerator, cyclotron ion source, low beam current, laser ion source, pulsed ion source, u-120m, proton radiography. 1. introduction this paper presents a principle of delivering a very low beam current to the injection system of a single stage or low energy accelerator. a control of the ion creation process is a complex task with effect of many parameters. one of the control parameter can be the length of the primary ionizing pulse in the ion source. this length is naturally restricted and usualy can not be arbitrary lowered. when the ionizing pulse is driven using a high voltage discharge, the pulse length with duration about tens of nanosecond can hardly be achieved. from this reason, a primary ionizing plasma in presented ion source is not made using ordinary high voltage discharge, but using a laser pulse. with the present-day laser systems, the high intensity laser pulses with the duration of few ps could be generated and used for photo-emission. electrons from photo-emission then controllably ionize gas and produce ions for the low energy proton or heavier particle accelerator. a usage of the low current beams vary accordingly with the technical field where the beam is applied. the low current beams are required in the radio-biological microbeam study of the living cells [1], in the single proton detectors tests or in the proton radiography. a benefit of using proton beams instead of classical gamma-rays in the radiography, is that the scanned sample receive significantly lower dose for similar image result [2]. with a proper magneto-optic lens system, very high-resolution images could be obtained. usual resolution even with the low energy proton beams is several tens of µm. this resolution of the proton radiography method could be apparently further improved and its limits are far beyond present ranges [3]. very good resolution of the proton radiography [4] could lead to a specific high resolution 3d imaging, where a complete 3d image of the sample could be obtained by two or three laser shots in the proposed ion source. for a very small and sensitive scanned samples, the lowest possible irradiation can be important, so precise control of the number of the created ions is essential. schematic principle of the proton radiography is shown in fig. 1. figure 1. the proton beam lens system with the particle trajectories as colored lines [3]. 2. beam current limitation the idea presented in this paper has grown from the response tests of the silicone detectors and electronics for the planed upgrade of the inner tracking system of the alice detector [5]. the tests were performed on the old cyclotron u-120m, shown in fig. 2. these tests however require very low intensity of the delivered beam current [6]. a minimal beam current which can be delivered comfortably by this cyclotron is ca. 2.5 fa/cm2, which corresponds to ca. 15 600 protons per second per cm2. the beam intensity is usually decreased only after its extraction from the ion source. it means that regardless of the output cyclotron beam current, the amount of ions at the 50 http://dx.doi.org/10.14311/ap.2016.4.0050 http://ojs.cvut.cz/ojs/index.php/app vol. 4/2016 ion source for a single particle accelerator figure 2. cyclotron u-120m. beginning of the acceleration process is almost the same. this factor limits stability of the accelerator and reduces utilization of low current beams. the laser induced photo-emission ion source presented in this paper, can significantly reduce the number of ions entering the machine. 2.1. u-120m pig ion source the ion source of the cyclotron is a movable part and its position can be fine-adjusted with respect to the puller. the extraction window of the ion source has area approx. 2 mm2. the puller has an input window with an area of about 20 mm2. an optimal position of the ion source opposite to the puller guarantees a maximal extraction to loss ratio. in normal operation this extraction ratio is kept as high as possible. when the low current beam is required, this ratio is lowered. this is achieved mainly by changing the position of the ion source extraction window. the ion source is moved slightly to one side, so the extraction window of the ion source does not overlap with the puller input window. subsequently, only a minimal number of ions are able to pass the initial extraction. the rest of the ions are lost on the puller body as can be seen in fig. 3 and 4. although the above mentioned current regulation is very easy, it has some disadvantages. the u-120m cyclotron ion source is a penning ion gauge (pig) type ion source with a cold cathode, operating in continuous regime. in fig. 5 is photo of the ion source oriented with its head on the left side, and in fig. 6 is its head detail. the ion source has a cylindrical plasma chamber parallel to the cyclotron magnetic field of approx. 1 t. the chamber is enclosed from both sides by tantalum cathodes. the cylindrical wall of the ion source is an anode and whole body is made from a tungsten-copper composite. electrons emitted from the cathodes are accelerated by the anode potential. due the strong magnetic field the electrons are trapped by the field lines and are not able to directly reach the anode. the electrons repeatedly oscillates between the cathodes and are slowly drifting to the anode figure 3. central region top view – optimal extraxion for proton regime 37 mev simulated with durycnm4 [7]. figure 4. central region top view – minimal extraxion for proton regime 37 mev simulated with durycnm4 [7]. body. the electrons ionize the gas in the cylinder and create the plasma. the amount of the produced ions can be slightly regulated by changing the plasma current and by regulating the ion source gas presure. for stable operation, the penning ion source needs some minimal plasma current to heat up cathodes and some minimal corresponding gas concentration. in the usual working regime the plasma current is about 1 a and the minimal plasma current for stable operation is several miliamperes. basically in the ion source are two concurrently 51 tomáš matlocha acta polytechnica ctu proceedings figure 5. u-120m pig ion source. figure 6. current u-120m pig ion source. processes which lead to creation of ions. the first process is based on stripping orbital electrons of neutral atoms and generates positive ions. the degree of ionization is done by temperature of the plasma electrons and by a time of interaction. our penning ion source has plasma electron temperature up to approx. 70 ev. it is enough to produce ions with low ionization potentials, e.g. h+, d+, he2+, li2+, c4+, etc. in second process, the negative ions are produced. the second process leads to production of negative ions via the surface and the volume production process. the section 5.1 describes the negative hydrogen ions formation process in more detail. after their creation, both negative and positive ions are simultaneously extracted from the ion source by an alternating rf high voltage field. negative polarity extracts positive ions and oppositely. polarity of the cyclotron magnetic field determines the type of ions which will be accelerated. the second type of ions are simply extinguished on the puller body. 2.2. pulsed pig ion source from above mentioned penning ion source operation principle it follows, that the ions are produced permanently. from that reason the lowest current of extracted ions is limited from below. there were several attempts to switch our penning ion source to a pulsed operation mode. these studies were motivated by other reasons than lowering the output current, but a deeper knowledge of the pig pulse regime capabilities were gained. an ignition process of the pig ion source from the full off state is very long and lasts couple of seconds. when the pig plasma is once ignited, the plasma current can be lowered to five miliamperes, and the ion source is in low current state. in this state changing of the plasma current can be done quickly with a reaction time of a few microseconds. then the ion source can be fast and easily modulated between low and high current states. but the ion source even in the low plasma current mode is still producing much more ions than required. there is an option to further lower the low current value by at least an order of magnitude by driving the cathodes temperature. nevertheless, continuous operation of the ion source is not suitable for the low current beams. from this reason, the possibilities of other ion production were studied and the main findings are presented in the next paragraphs. 3. inspiring ion sources 3.1. ebis/ebit ion source a principle of operation of an electron beam ion source (ebis) and an electron beam ion trap (ebit) is very similar. the electrons are extracted from a cathode by thermionic emission and accelerated in a strong solenoidal magnetic field by an anode voltage. the shape of the magnetic field forms the entering electrons to well focused beam with densities up to several ka/cm2. this beam ionizes the gas and forms the ions. because the electron beam temperature can be very high, a high ion charge state is possible. in the ebit the ions are confined in a space charge of a dense electron beam, where attractive force of electrons prevent the ions to move. an electrostatic potential of external electrodes is applied to trap the ions and hold their position. when the trapping potential is switched off, the ions can be extracted. this type of ion source is mainly used for heavy ions for which high charge states or full stripping by other techniques is complicated. temperature of electrons could be from 10 kev to 200 kev. the extracted ion current is low, typically ranging from 105 to 109 ions per second. the ebis/ebit sources exist in many designs from ultra compact devices for low current and low charge states to huge machines capable of delivering fully stripped uranium ions [8, 9]. principial schematic of the ebit is in fig. 7. figure 7. schematic design of ebit ion source [10]. 52 vol. 4/2016 ion source for a single particle accelerator 3.2. rf gun modern light sources such as the energy recovery linac (erl) or free electron lasers (fel) use as an electron beam generator pulse operated rf guns. the rf gun is a laser induced photo emission electron gun, where photo electrons are accelerated by a strong electric field generated during the rf pulse. schematic design is shown in fig. 8. the photosensitive cathode is placed near the center of a 3/2 cell rf cavity (0.5–2ghz), where a high electric field is produced (up to 120 mv/m). a very short laser pulse (10 ps) with a proper phase is applied onto the photo-cathode. a bunch of photoelectrons is emitted, accelerated to proper energy and delivered further into the system. currently exist a number of devices of this type, capable to produce electron beams with energies in the range 0.1–5 mev. figure 8. schematic design of the rf gun. photo-cathodes used in these electron sources can be from metal or semiconductor. in general, metals have lower electron conversion efficiency(qe) and need uv lasers. on the other hand they have fast response and are resistant to impurities. semiconductor cathodes are more efficient and can be operated with visible/ir laser, but have slower time response and are sensitive to contamination and photon radiation. charge emitted per bunch is approx. 1 nc. required peak rf power is up to 10 mw. mean laser power is 1 w and required frequency is few thousands pulses per second [11, 12]. 4. laser induced photo-emission ion source the above mentioned devices could probably be combined. by applying a short laser pulse on a photo sensitive cathode one could produce an electron bunch. this bunch could be then accelerated to a desired energy by an anode potential. during travel from a photo-cathode to an anode, this bunch could ionize gas present in the discharge chamber. because the electron temperature is expected only few tens ev, sufficient to ionize hydrogen and helium, and emitted charge is low, the device could be kept compact. such a compact device could be placed inside the central region of the cyclotron and in demand of very low beam currents replace the present pig ion source. dimensions of the pig ion source must be preserved to fit into the cyclotron central region. these dimensions determine the discharge chamber shape and possible arrangement of the electrodes. unlike in the pig ion source, where the cylindrical chamber is anode enclosed by cathodes from both sides as in fig. 6, here will be photo-cathode on one side and anode on the other side of the cylinder as in fig. 9. in this configuration the electron bunch will pass through the chamber only once per one laser pulse. the laser pulses with energies of few µj are driven to the photo-cathode through ion source arms enclosed by a high vacuum sealed window, from an external laser generator placed out of the cyclotron site. figure 9. proposed laser ion source. 4.1. photo-cathode in general, the photo-cathode should have work function below 3 ev to avoid a necessity of using uv lasers. considering a gamma radiation always present at the cyclotron site, the photo-cathode should be metallic. other reason for this choice is good resistance to adverse conditions such as bad vacuum and impurities in the system, where semi-conductors cathodes are very sensitive. in theory, the metal photo-cathodes should not work when irradiated by photons with energies below the work function hν < φ. in reality, an effect of multiple photon absorption were observed in high density laser pulses, and even photons energies below the threshold energy can be used for photoemission [13]. the quantum efficiency is defined by ratio between incident photons and emitted electrons. the metals have qe low, but in our case it should be helpful for limiting a parasitic emission by photons captured from de-excitation and neutralization of the gas atoms. copper, which has qe 5× 10−5 at wavelength 320 nm, could be suitable for the first try. 53 tomáš matlocha acta polytechnica ctu proceedings 5. ions production 5.1. negative hydrogen ion formation as the principle of proposed pulsed ion source is simple, many practical limitations will probably occur. the main questions are, if this device will be capable to produce any negative ions and how long this ions will live for. a creation of the negative hydrogen ion h– is a multi-step process, where several conditions should be fulfilled. there are many processes involved in h– formation. one of them, the volume negative ion production process which needs at least two different temperature of electrons. faster electron with energies near 20 ev which ro-vibrationally excites the hydrogen h*2 and a cold electron, which can than dissociate the h2 molecule and attach to one of the atoms to form an negative hydrogen ion: h2 + e− −−→ h∗ 2 + e− −−→ h0 + h−. this process is called a dissociative attachment process and the dissociative low temperature electron energy needs to be close to 1 ev. bounding energy of the second electron in the h– ion is 0.75 ev, so the once formed h– ion can be easily destroyed by collisions with other ion or on residual gas. the second important h– formation process is the surface negative ion production. in this process the previously formed h0 atom from h2 dissociated molecule collides with a metallic surface of the plasma chamber. when bouncing back from the wall, an conductive electron from the metal surface can be trapped by the atom and form an h– ion. work function of the chamber metal tungsten surface is near 4.5 ev, far above 0.75 ev electron affinity of h atom. from this reason the cross-section of h– formation from pure metal surfaces is commonly increased by covering surface with caesium layer. this thin cs layer lowers the work function of surface to approx. 1.6 ev. the cs is widely used in h– ion sources and has its significant role also in the volume h– production. in both processes significantly increases h– formation probability. in the u-120m cyclotron penning ion source cs is not used, so the volume process is therefore considered to be more important [14–16]. 5.2. positive ions formation a situation with positive ions should be more simple. ionization of a low atomic number elements is done by stripping their orbital electrons in energetic collisions. as for heavier elements the process is gradual, because the atoms are ionized in a stepwise processes where each electron collision removes one or two orbital electrons. for light atoms such as h and he, this can be done very fast. ionization energy for hydrogen is 13.6 ev and has maximal cross-section σ = 0.6× 10−16 cm2 at 50 ev. full ionization energy for helium is 54.4 ev and its maximal cross-section σ = 0.3× 10−16 cm2 is near 100 ev [17]. 5.3. ions yield estimation for the laser pulses with duration 20 ps the emitted charge 100 pc could be expected. with optimal electron temperature and for h2 concentration in range from 109 cm−3 to 1013 cm−3 [18] in a volume similar to current used pig ion source 2 cm3, from current density 18 a/cm2 there can expected from 102 to 105 positive ions. in normal operation the ratio between protons and h– ions is 10−2. very low probability of formation h– ion in the single pulse can be expected. even if the production ratio is lowered by two orders to 10−4, still up to 10 h– ions per volume could be expected. with considering a fact, than an exact h– formation process understanding is still in progress, an influence of more factors or processes, such as a wall biasing, could play its role. from this reason a formation of an h– ion in the single discharge bunch should not be rejected in advance. 5.4. discharge conditions and ions extraction after formation of the electron bunch by an irradiation of the photo-cathode, a multiple side effects can be expected. one from these effects could be the above mentioned parasitic photo-emission. other important effect will be positive ion bombardment of the cathode. parameter which can highly affect performance of the device will be a lifetime of produced ions which will probably reduce a minimal length of the ion pulse. 6. beam time distribution the low current regime of h– ions with energy 36.9 mev has its cyclotron frequency 25.8 mhz. this main carrier frequency is modulated by a 150 hz signal. particles in the cyclotron are accelerated with each half period and are caught in phase angle of about from −30° to 30° of this half period. the ions are revolving in approx. 6 ns micro-bunches with 32 ns spaces between them. with 10 % carrier modulation these micro-bunches are formed into 0.67 ms long bunches with 6 ms spaces. a simulation of the cyclotron response to a 110 ps short input pulse with 330 ions was made in the code durycnm4 [7, 19]. the ions from one input pulse are accelerated for 503–507 periods and divided into five output micro-bunches. output signal than lasts for 160 ns. energy range of the particles is from 36.75 to 36.9 mev. this cyclotron impulse response is made by a phase instabilities and space distribution of the micro-bunch, which affects energy increment difference particle gains each turn and leads to energy spread of the beam. because the extraction is made on the desired energy, each particle from the input bunch needs a different number of the accelerating turns. as a result on the output, the signal is time dispersed. this dispersion is coming from the machine nature and cannot be avoided by shortening of the input pulse. number of ions in individual micro-bunches are shown in fig. 10. 54 vol. 4/2016 ion source for a single particle accelerator figure 10. simulated u-120m impulse response. 7. conclusions this paper proposes a method which possibly could be used to produce a very small amount of the ions. considering many variable parameters that can be used to tune the ion source, it can be expected that the ions could be produced in countable amounts and delivered to the accelerating process on the request. the benefits of the presented method is mainly in the controlled production of the selected charge state and the number of the generated ions. as the demand of low current ion beams is increasing rapidly, overcoming the difficulties with generation the ions by the laser induced photo-emission 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http://www.lepp.cornell.edu/~hoff/lectures/08s_688/08s_688_080225.pdf http://www.lepp.cornell.edu/~hoff/lectures/08s_688/08s_688_080225.pdf http://dx.doi.org/10.1007/s00339-013-7600-z http://dx.doi.org/10.5170/cern-2013-007.265 http://dx.doi.org/10.1109/tps.2005.860067 http://physics.nist.gov/cgi-bin/ionization/ion_data.php?id=hi&ision=i&initial=&total=y http://physics.nist.gov/cgi-bin/ionization/ion_data.php?id=hi&ision=i&initial=&total=y http://dx.doi.org/10.1109/tps.2005.860069 http://accelconf.web.cern.ch/accelconf/c07/papers/385.pdf http://accelconf.web.cern.ch/accelconf/c07/papers/385.pdf acta polytechnica ctu proceedings 4:50–55, 2016 1 introduction 2 beam current limitation 2.1 u-120m pig ion source 2.2 pulsed pig ion source 3 inspiring ion sources 3.1 ebis/ebit ion source 3.2 rf gun 4 laser induced photo-emission ion source 4.1 photo-cathode 5 ions production 5.1 negative hydrogen ion formation 5.2 positive ions formation 5.3 ions yield estimation 5.4 discharge conditions and ions extraction 6 beam time distribution 7 conclusions references 175 acta polytechnica ctu proceedings 1(1): 175–180, 2014 175 doi: 10.14311/app.2014.01.0175 be/x-ray binaries with black holes in the galaxy and in the magellanic clouds janusz zió lkowski1 1copernicus astronomical center, ul. bartycka 18, 00-716 warsaw, poland corresponding author: jz@camk.edu.pl abstract i will start with the statistics indicating that the objects named in the title of my talk are either non-existing or very elusive to detect (not a single such object is known against 119 known be/neutron star x-ray binaries). after brief reviewing of the properties of be/x-ray binaries i discuss several objects that were proposed as the long sought for candidates for be/black hole x-ray binaries. after three unsuccessful candidates (ls i +610 303, ls 5039 and maxi j1836-194), a successful candidate (agl j2241+4454/mwc 656) was finally, very recently, announced. keywords: stars: binaries – stars: x-ray binaries – stars: be/x-ray binaries – stars: black holes – stars: be. 1 introduction the objects named in the title of my talk seem not to exist neither in our galaxy nor in the magellanic clouds. not a single such object is known today (this statement was still true during our conference). at the same time, we know (today) 184 be/x-ray binaries (74 in the galaxy and 110 in the magellanic clouds). 2 properties of be/x-ray binaries be x-ray binaries (be xrbs) are the most numerous subclass among high mass x-ray binaries. they outnumber all other high mass x-ray binaries by a factor of three (we know about 180 be xrbs vs about 60 other high mass x-ray binaries). these systems consist of a be star and a compact object (a neutron star or a black hole). the be stars are massive, generally main sequence, stars of spectral types a0-o8 with balmer emission lines (negueruela 1998). the be xrbs are rather wide systems (orbital periods in the range of ∼ 10 − 1180 days). the orbits are frequently eccentric. a compact component accretes from the wind of a be star (even massive be stars are well within their roche lobes for these wide orbits, so there is no question of roche lobe overflow). at present, 184 be xrbs are known in the galaxy and in the magellanic clouds, and in 119 of them, the compact object was confirmed to be a neutron star by the detection of the x-ray pulsations (with the pulse periods in the range of 34 ms to ∼ 1400 s). in the remaining cases, whenever we have information concerning the nature of the compact component (such as an x-ray spectrum), it also indicates a neutron star. although one cannot exclude that a few of these systems contain white dwarfs or black holes, it is safe to state that majority of them contain a neutron star as compact component. however, not a single black hole binary containing a be type component has been found so far. this disparity (119 be xrbs with neutron stars versus not a single one with a black hole) seems indeed striking (this was the situation during our conference; today the statistics is 119 to one − see the section 4.2). the x-ray emission from be xrbs (with a few exceptions) is of a distinctly transient nature with rather short (days to weeks) active phases separated by much longer (months to tens of years) quiescent intervals (a typical flaring behavior). there are two types of flares, which are classified as type i outbursts (smaller and roughly regularly repeating) and type ii outbursts (larger and irregular. this classification was defined by negueruela & okazaki 2001 and negueruela et al. 2001. type i bursts are observed in systems with highly eccentric orbits. they occur close to periastron passages of a neutron star. they are repeating at intervals ∼ porb. type ii bursts may occur at any orbital phase. they are correlated with the disruption of the excretion disc around be star (as observed in hα line). they repeat on time scale of the dynamical evolution of the excretion disc (∼ few years to few tens of years). this recurrence time scale is generally much longer than the orbital period (negueruela et al. 2001). be xrbs systems are known to contain two discs: excretion disc around be star and accretion disc around neutron star. both discs are temporary: excretion disc disperses and refills on time scales ∼ few years to few decades (dynamical evolution of the be star disc, formerly described as ”the activity of a be star” (negueru175 http://dx.doi.org/10.14311/app.2014.01.0175 janusz zió lkowski ela et al. 2001)), while the accretion disc disperses and refills on time scales ∼ weeks to months (which is related to the orbital motion of a neutron star on an eccentric orbit and, on some occasions, also to the major instabilities of the other disc). the accretion disc might be absent over a longer period of time (∼ years), if the other disc is very weak or absent. the x-ray emission of be xrbs binaries is controlled by the centrifugal gate mechanism, which, in turn, is operated both by the periastron passages (type i bursts) and by the dynamical evolution of the excretion disc (both types of bursts). this mechanism explains the transient nature of the x-ray emission ( see zió lkowski 2002 and references therein). the more detailed description of the properties of be xrbs is given, e.g. in negueruela et al. 2001, zió lkowski 2002, belczyński & zió lkowski 2009, reig 2011 and references therein. considering the numbers of different (neutron star vs black hole) be xrbs, it is important to note, that if we take all other xrbs (both high and low mass), except be xrbs, then the ratio of neutron star systems to black hole systems is about 2:1 (remillard & mcclintock 2006, zió lkowski 2008). for be xrbs this ratio is 119 to zero. the disparity is indeed striking. this disparity is referred to as a missing be – black hole x-ray binary problem. trying to understand the reasons for which we do not observe be – black hole xrbs, belczyński & zió lkowski (2009) carried out stellar population synthesis calculations aimed at estimating the ratio of neutron star to black hole be xrbs, expected on the basis of the stellar evolution theory. the results of their calculations predict that for our galaxy the expected ratio of be x-ray binaries with neutron stars to the ones with black holes fns/bh should be, most likely, equal ∼ 54. since we know 48 neutron star be systems in the galaxy, then it comes out that the expected number of black hole systems should be just one. the observed number (zero) is consistent with this prediction. zió lkowski & belczyński (2010) carried out also preliminary stellar population synthesis calculations for the magellanic clouds. this time, the result was that the expected ratio fns/bh for magellanic clouds be xrbs should be equal ∼ 9. since we know 71 neutron star systems in the magellanic , then the expected number of black hole systems should be about 8. this time, the observed number (zero) is not consistent with the prediction. the most likely reason for this discrepancy is a different history of the star formation rate in the magellanic clouds, with respect to the galaxy. the calculations mentioned above used the galactic scenario for the star formation history. the new, more realistic calculations for magellanic clouds are necessary. 3 new be/x-ray binaries zió lkowski & belczyński (2011) published a full list of the 170 be/x-ray binaries known in 2011 (72 in the galaxy and 98 in the magellanic clouds). table 1 of this paper contains the list of the 14 new objects belonging to this class which were discovered in meantime (2 of them lie in the galaxy and 12 − in the magellanic clouds). this list contains two interesting systems (added after the end of our conference). one of them is system with the longest orbital period (swift j010745.0-722740 with porb ≈ 1180 days). the second is the first long sought be/black hole x-ray binary (agl j2241+4454). 4 candidates proposed for be/black hole x-ray binaries 4.1 unsuccessful candidates 4.1.1 ls i +610 303 this binary of the orbital period 26.5 d consists of a b0ve star and a compact object the nature of which is not firmly established yet. no pulsations were detected in the x-ray emission. the radio emission also does not show pulses. this might indicate a black hole rather than a neutron star. however, in many respects, the system reminds another x-ray and radio binary − psr b1259-63, the nature of which is already well understood. psr b1259-63 belongs to the category of so called colliding winds binaries. this wide (3.4 y orbital period) binary consists of an o9.5ve star and a neutron star which is observed as a fast (pspin = 47.7 ms) radio pulsar. neutron star moves along a very eccentric (e = 0.97) orbit. the system is also a source of tev emission (in fact, it was the first binary detected in this energy range). in all three energy ranges (radio, x-ray and tev) the emission is modulated with the orbital period. the binary was most recently extensively discussed by dubus (2013). after reviewing both the observations and the numerical simulation modeling, he strongly supports the much earlier idea of tavani et al. (1994) that both x-ray and tev emission are due to collision of two winds: stellar wind from be star and pulsar wind 176 be/x-ray binaries with black holes in the galaxy and in the magellanic clouds table 1: new be x-ray binariesa porb pspin lb x,max spectral name [d] [s] [erg/s] type refc xmmu j004814.0-732204 11.866 4.2 × 1036 b1.5-2.5ve 1 xmmu j005011.2-730026 29.9 214 5.0 × 1034 be 2,3,4 igr j00569-7226 17 5.05 5.5 × 1037 b0.5e 5,6,7,8 cxou j005758.4-721620 40.03 7.918 4 × 1035 be 9,10 2xmm j010247.4-720449 490 ? 521.4 2.8 × 1035 be 11,12 swift j010745.0-722740 1180 be 13 igr j01217-7257 84 ∼ 1 × 1037 be ? 14 cxo j012745.97-733256.5 656 1062 6.3 × 1035 b0-0.5iiie 15,16,17 igr j0154-7253 36.3 11.483 2.5 × 1037 o9.5-b0iv-ve 18 igr j015712-7259 35.1 11.6 4 × 1036 be 19,20,21 swift j04558.9-702001 5.1 × 1035 be 22 swift j053041.9-665426 ∼ 415 9.7 × 1036 be 23,24,25 maxi j1932+091 be 26,27 agl j2241+4454 60.37 3.7 × 1031 b1.5-2iiie 28,29,30,31,32 asince this table is a natural supplement to tables 1 and 2 of zió lkowski & belczyński (2011), i should add following updates to these tables: swift j0513.4-6547 porb ≈ 27 d (coe et al. 2013d) rx j0520.5-6932 pspin = 8.035 s (vasilopoulos et al. 2013b), porb = 23.93 d (kuehnel et al. 2014) smc sxp707 pspin is unknown (707 s is an instrumental effect, maggi et al. 2013) bmaximum x-ray luminosity c (1) sturm et al. 2011a; (2) coe et al. 2011; (3) schmidtke & cowley 2011; (4) schmidtke et al. 2013a; (5) coe et al. 2013a; (6) kennea 2013; (7) schmidtke & cowley 2013a; (8) coe et al. 2013b; (9) israel et al. 2013; (10) schmidtke & cowley 2013b; (11) sturm et al. 2011b; (12) sturm et al. 2013; (13) maggi et al. 2014; (14) coe et al. 2014; (15) henault-brunet et al. 2012; (16) schmidtke et al. 2012a; (17) schmidtke et al. 2012b; (18) townsend et al. 2011; (19) coe et al. 2008; (20) bodaghee et al. 2009; (21) schmidtke et al. 2013b; (22) vasilopoulos et al. 2013a; (23) sturm et al. 2011c; (24) charles et al. 2011; (25) sturm et al. 2011d; (26) negoro et al. 2014; (27) itoh et al. 2014; (28) williams et al. 2010; (29) casares et al. 2012; (30) casares et al. 2014; (31) paredes & ribo 2014; (32) munar-adrover et al. 2014. from rapidly rotating neutron star. the shock formed as a result of the collision gives the rise to a non-thermal emission in a similar way as in the pulsar wind powered nebulae. the characteristic element of such model is elongated radio emission with position angle dependent on orbital phase. elongated emission is pointing from the vicinity of the pulsar in the direction opposite to the direction towards the be star. such ”comet tail” behavior of radio emission is clearly observed for psr b1259-63. at present, there are no significant doubts that the system is powered by the pulsar spindown and that the colliding winds model is, in general, a correct explanation of the properties of this binary. ls i +610 303 is, in many respects, similar to psr b1259-63. in particular, the spectral and timing properties of the emission (through all ranges of electro177 janusz zió lkowski magnetic spectrum) are very similar. also the ”comet tail” behavior of radio emission is clearly seen for ls i +610 303 (dhawan et al. 2006). as i mentioned earlier, no pulsations were detected and, therefore, the case is not so clear cut as for psr b1259-63. this system (ls i +610 303) is also discussed by dubus (2013) and he concludes that indirect evidence strongly supports the colliding winds case. the compact object is, most likely, a rapidly rotating pulsar. the lack of the observed pulsations might be explained as the result of the absorption of the radio emission by the circumstellar material. even in psr b1259-63 the pulsed radio emission disappears for about 40 days around periastron. ls i +610 303 is much more compact binary (orbital period is ∼ 50 times shorter) and the absorption is strong throughout all orbital cycle. since the compact component is, most likely, a neutron star, the system is not a good candidate for a be/black hole binary. 4.1.2 ls 5039 this binary of the orbital period 3.9 d consists of an o6.5((f)) star and a compact object the nature of which is still a subject of controversy. no pulsations were detected neither in the x-ray nor in the radio emission. both x-ray and tev emission are modulated with the orbital period. the radio emission is variable but the flux is not modulated with the orbital phase. the morphology of the radio emission (which has an elongated shape) is variable with the orbital phase but the positional angle, at first look, seems to be roughly constant (moldon et al. 2012). this would not be a typical ”comet tail” behavior observed in psr b1259-63 and in ls i +610 303. however, analyzing more subtle changes in radio morphology and modeling radio emission with colliding winds model, moldon et al. (2012) demonstrate that the observed radio emission is compatible with the presence of a young non-accreting pulsar in the system. the authors obtain the best fit for a relatively high inclination of the orbit (i ≈ 700). as they note, this value has deep implication for the estimate of the mass of the compact component. radial velocities of ls 5039 (the optical component) were measured by casares et al. (2005). assuming pseudo-synchronization at periastron, they got i ≈ 200 for the inclination of the orbit and mx = 4.0 ÷ 7.3 m� for the mass of the compact component. this value clearly indicates a black hole. however, using the same radial velocities with the inclination derived by moldon et al. (2012), one gets only mx = 1.3 ÷ 2.7 m� for the mass of the compact component. this value corresponds to a neutron star and is compatible with the observed variability of radio morphology. it seems, that at present we cannot firmly establish the nature of the compact component. the black hole still cannot be excluded but its presence became somewhat doubtful. even if the presence of a black hole in the system would be confirmed, it is clear that the stellar component is not a be star and therefore the system is not a good candidate for a be/black hole binary. 4.1.3 maxi j1836-194 this system was discovered as an x-ray source by maxi in august 2011 (negoro et al. 2011). further observations strongly suggested that the system contains an accreting black hole (spectral states, timing characteristics, ejection of a radio jet). however, to classify the system as a be/black hole binary, we need a second ingredient − namely a be component. initially, it seemed that it is indeed present in the system. cenko et al. (2011) after spectroscopical observations of the optical counterpart with the 8 m gemini south telescope concluded that it is a be star. this finding was questioned by rau et al. (2011), who noted that the archival search of kennea et al. (2011) indicates that the optical counterpart undergoes outbursts of at least 4-5 magnitudes which is very unlikely for a be star. subsequent spectroscopical observations were made by pakull & motch (2011) who used vlt ut1 telescope. they found that the optical spectrum is not that of a be star. in this way, maxi j1836-194 joined the ranks of unsuccessful candidates. 4.2 the first successful candidate 4.2.1 mwc 656 this system was first discovered by agile as a gammma-ray source agl j2241+4454 (williams et al. 2010). the discovery paper identified the optical counterpart as a be star hd 215227 (known also under the name mwc 656) and determined the orbital period as equal 60.37 d. the nature of the optical component (of spectral type b1.5-2iiie) permits no doubts − it is a be star. the nature of the compact component was not established until very recently. casares et al. (2012) measured the amplitude of the radial velocities of mwc 656 and its rotational broadening obtaining 41.7 ± 6.8 km/s and 346 ± 10 km/s respectively. next, they analyzed fe ii lines reflecting the the keplerian rotation in the excretion disc around be star and under some assumptions estimated the inclination of the orbit as i = 67÷800. finally, they estimated the mass of the compact component to be in the range 2.7 to 5.5 m�. this range suggested rather a black hole but a neutron star could not be excluded. taking into account the overall low precision of this estimate, the nature of the compact component remained undecided. authors made more observations and published their analysis in 178 be/x-ray binaries with black holes in the galaxy and in the magellanic clouds a most recent paper in nature (casares et al. 2014). this time they got two sets of radial velocities: they measured the accretion disc emission line he ii 4686 reflecting the orbital motion of the compact component and they improved the measurements of radial velocities of be star by using sharp fe ii lines of the equatorial excretion disc. as a result they could come to a firm conclusion that the compact component is a black hole of the mass 3.8 to 6.9 m�. in this way the first be/black hole binary was finally found! the authors noted that the system seems to be xray quiescent (lx < 1032 erg/s). this led them to a general comment that due to lack of a solid surface and a very low mass transfer rate (leading to extremely long outburst recurrence periods), be binaries with black hole companions might be difficult to detect by conventional x-ray surveys. after completing their work, the authors requested and obtained observing time with xmm newton. they found that the system emits xrays, after all (munar-adrover et al. 2014), but at a very low level lx ≈ 3.7 × 1031 erg/s, similar to the quiescent luminosity of black hole x-ray novae. this result does not change their conclusions. 5 summary during our conference, the statistics of be x-ray binaries in our galaxy indicated that we know 72 such systems. among them, 48 contain a confirmed neutron star and none, so far, is known to contain a black hole. the stellar population synthesis calculations (belczyński & zió lkowski 2009) indicate that the expected ratio of be x-ray binaries with neutron stars to the ones with black holes, fns/bh is equal ∼ 54. this ratio implies that, if we know 48 neutron star systems, then the expected number of black hole systems should be just one. few months after the conference this one system was just found! it is a binary agl j2241+4454/mwc 656. the problem for magellanic clouds remains unsolved (8 expected black hole be x-ray binaries and none observed). acknowledgement this work was partially supported by the polish ministry of science and higher education grant n203 581240 and by the polish national science center project 2012/04/m/st9/00780. references [1] belczyński, k., zió lkowski, j.: 2009, apj 707, 870. doi:10.1088/0004-637x/707/2/870 [2] bodaghee, a., tomsick, j.a., rodriguez, j.: 2009, atel 2252. 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[55] zió lkowski, j., belczyński, k.: 2011, in frontier objects in astrophysics and particle physics (procs. of the vulcano workshop 2010), f. giovannelli & g. mannocchi (eds.), conference proceedings, italian physical society, editrice compositori, bologna, italy, 103, 511 (arxiv:1111.2330). 180 http://dx.doi.org/10.1007/s10509-010-0575-8 http://dx.doi.org/10.1086/187542 http://dx.doi.org/10.1088/2041-8205/723/1/l93 introduction properties of be/x-ray binaries new be/x-ray binaries candidates proposed for be/black hole x-ray binaries unsuccessful candidates ls i +610 303 ls 5039 maxi j1836-194 the first successful candidate mwc 656 summary acta polytechnica ctu proceedings doi:10.14311/app.2015.1.0022 acta polytechnica ctu proceedings 2:22–28, 2015 © czech technical university in prague, 2015 available online at http://ojs.cvut.cz/ojs/index.php/app vector maps in mobile robotics ales jelinek central european institute of technology, brno university of technology, technicka 10, brno, czech republic correspondence: alesjelinek@ceitec.vutbr.cz abstract. the aim of this paper is to provide a brief overview of vector map techniques used in mobile robotics and to present current state of the research in this field at the brno university of technology. vector maps are described as a part of the simultaneous localization and mapping (slam) problem in the environment without artificial landmarks or global navigation system. the paper describes algorithms from data acquisition to map building but particular emphasis is put on segmentation, line extraction and scan matching algorithms. all significant algorithms are illustrated with experimental results. keywords: robotics, slam, vector map, point cloud, edge extraction. 1. introduction mapping in robotics is a subject of research for a number of years and large progress was already achieved. paper [1] describes theoretical way, how to build and update a consistent map, which converges to a precise image of the real environment (at certain level of detail). through the years, more improvements and new algorithms were invented for this task, good tutorial papers are for example [2] [3], or the book [4]. formulation and mathematical description of the slam problem is unified and generally accepted today, but a complete solution for 3d dynamic environment is still not known and slam as a whole is still under active research. the most of today’s work in the field is concentrated on two main bottlenecks of known algorithms: (1.) computational optimization. every map contains some data describing the real environment. computational efficiency is usually evaluated in big o notation with respect to n, reflecting the number of cells in an evidence-grid map, the number of landmarks in a geometric map, etc. non-optimized solutions have o(n2) or even exponential complexity, but several algorithms are more efficient, for example fastslam [5] with o(m log(n)) complexity (where n is the number of landmarks and m the number of particles in the rao-blackwellized filter). this algorithm can handle orders of magnitude more landmarks than o(n2) solutions. computational efficiency is a key to large maps with a lot of details. (2.) feature recognition and matching. robust and reliable feature (landmark) extracting and matching is crucial for most of slam algorithms. new data has to be associated with older measurements precisely, or at least with negligible number of mismatches. feature extraction is needed when camera or laser scanner is used to provide information about the robot surroundings. for example corners and t-junctions are usually searched in input data and then the whole pattern is correlated with an already known map. several complications may arise, because the same feature may look different from different points of view and data registration in its straightforward implementation is exponentially complex task. data structure of a map is closely related to both of these problems. operations executed on the map have to be as fast as possible especially updates with the new information and feature search/comparison are crucial. representation of obstacles, free space and unexplored areas effects feature recognition robustness. another attribute of a map is its ability to approximate the real environment. importance of this attribute depends on further usage of the map. if the robot is meant to explore unknown area and provide information about it, requirements will be probably higher than in case of a robot, which just needs to avoid obstacles and memorize a short history of its path. there are more possible ways to represent a robotic map. one of the most common is robot evidence grid, which parcels continuous world into a set of squares (cubes in 3d). size of a square is optional and may be even adaptive to better fit the reality, but this style of approximation of smooth world is still rather crude, because straight edges which are not co-linear with grid lines are jagged regardless of grid resolution. evidence grids are widely used and well established in many applications, where this downside is negligible. visually better results are achieved with vector maps, where edges are represented with line segments. good example of classical vector based mapping system is vecslam described in [6]. the aim of authors work is to create a system for high quality mapping, therefore vector maps were chosen as a good approximation of the real world. map representation is only one part of the problem. robot exploring unknown environment needs 22 http://dx.doi.org/10.14311/app.2015.1.0022 http://ojs.cvut.cz/ojs/index.php/app vol. 2/2015 vector maps in mobile robotics to update its map with new information. data are usually obtained from rangefinder sensors such as laser scanners and sonars in the form of a point cloud and need to be incorporated in the current map. probably the most used technique for point cloud registration is iterative closest point (icp). good description and historical survey of this method is available in [7]. efficient variants of this method were published in [8]. furthermore, it is possible to use icp for registration of more complex formations, such as surfaces of scanned objects. this approach is described in [9] and [10]. there are several alternatives to icp, which are trying to generalize the notion of “closeness” or “similarity” from points to lines, triangles etc. similarity of points is usually measured as their euclidean distance. an edge is often described as a segment of a line. having two such segments, we can define an area they demarcate and use it as a measure of similarity. this definition is not unified and it is possible to find several different approaches, for example sum of squared distances from point in a scan to the nearest edge in the map [11], penalizing function of “non collinearity” and spatial distance [12], or a criterion based on newly introduced matrix scalar product [13]. another interesting approach is critical ray method proposed in [14], which does not use line similarity at all. diversity of these approaches is high and none of them is considered to be remarkably better than the others, therefore the author have decided to develop his own method and test new ideas, because this field of research is definitely not fully explored. reducing point cloud to a set of line segments (in 2d) or triangles (in 3d) has very useful properties. at first, information from hundreds or thousands of points is expressed with (at most) tens of objects. data reduced in count are processed faster and therefore more complex algorithms may be used. on the other hand, mathematics becomes more complicated, because lines and faces have more degrees of freedom and constraints than points and as written above, similarity is not as easy to enumerate. the process of line extraction has generalizing ability. after interpolation by line segments (or triangles), corners become sharper and noise is suppressed, which results in better looking (more corresponding to the real world) map. in addition (depending on line extraction algorithm) edges might be estimated with higher accuracy than single point due to averaging effect. there is no restriction on using n − 1 dimensional objects in n dimensional space. a generalization of icp for lines in 3d space is described in [15]. paper [16] describes whole set of algorithms for matching sets of lines in 3d and many others are possible to find. for even better approximation of the real world, curves and curved planes could be used. these are more complicated for computation and not too much papers are related to this topic. an example of such approach is a method [17] based on graph theory. 2. point cloud processing system the overall diagram of the measurement and mapping process is depicted in fig. 1. every time a measurement is acquired, the whole process is run through. each rectangular box in the diagram means one algorithm performing operation on its input data. the whole process leads to update of the map the robot creates and the pose of the robot itself. slam itself is implemented in a very elementary way and no attempts to build converging map were made yet. the main objective in current state of development is to test data processing, edge extraction and scan registration algorithms. laser scanner used to obtain all data in this chapter is velodyne 32 hdl, which provides approximately two thousand points (per turn) in polar coordinates for each of its thirty two lasers. only horizontal set of measurements was taken into account for 2d map. at the beginning, all data from sensors need to be converted into the data format useful for following algorithms. this step also ensures usability of the system for any instrumentation with suitable properties. a disadvantage of this process is a certain enlargement of data size, because laser scanners and other sensors usually use some sort of data compression to minimize data flow. in case of velodyne 32 hdl this means conversion from 16-bit integer to 32-bit floating point number. to speed up corresponding edge selection, crude pose estimation is useful. it enables to specify part of the map, where the robot most probably is, and therefore, the search space of correspondences is reduced a lot. as is well known in robotics community, techniques such as inertial sensors or odometry cause integration of error of pose estimation. this makes them useless after a certain amount of time. in the presented algorithm, these are only used for measurement of relative pose change between two following scans. this ensures that the estimated pose error is always in a known limit. to get an absolute pose, this estimation is added to the last known pose, determined in the previous iteration of the data processing (dashed arrow leading to “pose estimation” block in fig. 1). 2.1. filtering and clustering treatment of raw point cloud is a more complex task. at first it is necessary to filter out noise and outliers and to find dense clusters of points defining edges of solid objects in the environment. raw data from one set of measurements are shown in fig. 2a. it is clear that scans are full of irrelevant points and odometry was not very accurate. some noise is always present. laser scanner measures distance with certain precision, which is stated in the datasheet. the other sake of noise is too dissected environment, where individual faces of objects 23 ales jelinek acta polytechnica ctu proceedings measured data conversion from scanner data format to cartesian and polar coordinate system point cloud segmentation and filtering edge extraction corresponding edge selection finding the best transformation for incorporation of the scan in a map map update map inertial sensors pose estimation most probable pose pose correction data flow data from previous step scanner data processing operation result conversion form scanner data format to common physical quantities figure 1. overall scheme of point cloud processing system from data acquisition to map and pose update. are so small that rangefinder cannot scan them properly. a good examples are treetops and bushes. noisy parts of point cloud are best to treat as a solid body with defined borders and density. other possibility is to filter them out, which causes loss of information, but on the other hand, with a careful filtering, it is possible to “see” through low density objects (e.g., see wall behind a bush). outliers in point cloud are usually caused by scanning too distant or too close objects, which are out of the range of the particular laser scanner. these are easy to identify and can be removed without serious impact on the quality of a scan. more interesting to deal with are outliers originating from a reflection. in a common environment, there can always be objects, which are able to reflect laser beam and corrupt the measurement. if the reflecting object is small and curved, few beams, which hit it, are scattered wide apart and appear as randomly distributed single points in a point cloud. these are generally easy to remove. another case are large, flat objects such as mirrors and windows, which uniformly reflect a large amount of rangefinder beams. this effect causes phantom objects to appear in the scan and there is probably no way to detect this without additional sensors or a set of scans from different positions. clustering is used to remove all unnecessary points from a scan and to determine dense clusters of points, which could potentially form an edge of a real object. an algorithm used to get result depicted in fig. 2b is as follows: from the laser scanner we get data sorted by an angle. algorithm proceeds from one point to another and checks if k previous points are closer or farther, than a certain distance. three alternatives may occur. a point might be too far away from all of the previous ones. this leads to establishment of a new cluster. if the working point is close enough to some previous points and all these points belong to the same cluster, the working point is added to it. third case arises, when the working point can belong to more than one cluster. in such a situation, all those clusters and the working point are joined together. in the end, clusters with the size under a certain threshold are called outliers and are removed from the scan. from a practical point of view, it is wise to make the distance comparison threshold adaptive, because the farther the detected obstacle is, the sparser the measured points are. scaling the threshold linearly with the distance between the working point and the robots position is a good strategy. the proposed algorithm safely removes noise from too dissected objects as well as random outliers, because these create large number of very small clusters (typically less than ten points), which are removed in the final stage of the process. phantom objects are not recognized. noise caused by laser scanner itself removes the following algorithm for line extraction. this theme is directly related to line simplification algorithms (because points in a scan are sorted) and both of these are discussed in a lot of fields of re24 vol. 2/2015 vector maps in mobile robotics (a) . raw data. (b) . segmentation. (c) . line extraction. (d) . complete map. figure 2. a) depiction of all measured point clouds (yellow) with estimated robot path (green). no scan fitting or other postprocessing is used, image corresponds to raw data from sensors. b) segmentation of one scan is shown on this figure. each color means a cluster, which is believed to be continuous edge in reality and which is separate from the others. noise and outliers are filtered out and not displayed. c) line extraction in segmented point cloud (black). each cluster is approximated by a line or a poly-line (red). d) a map composed using the line similarity criterion. red edges are long enough to participate in scan fitting process, blue edges are shorter and just add details to the map. green trajectory is acquired from odometry and dark blue one is an estimation of true trajectory. search. in addition to robotics, similar approaches can be found in cartography, computer vision and computer graphics. plenty of algorithms were developed during last decades, having different properties, such as speed, memory requirements and quality of approximation. good survey comparing line extraction/simplification algorithms in robotics is [18]. results of this paper clearly show that algorithms such as ransac or hough transform are not fast enough for online processing of point clouds. good performance was observed when using splitmerge, incremental and line regression algorithms. split-merge (also known as douglas-peucker algorithm [19]) was the fastest one and belongs to wide family of o(n) and o(n log(n)) complex algorithms used in cartography [20]. main drawback of these algorithms is that they are based on point elimination, and therefore, a lot of information is discarded. the edge extraction algorithm from the diagram in fig. 1 is based on line regression and works as follows: it passes through the cluster and computes the least square approximation and the variance of selected set of points. if the variance gets over predefined limit, the line is saved, and algorithm proceeds computing new line. the last line terminates at the end of the cluster. this approach is advantageous, because all points contributes to the result and contained information is better utilized. although loss of information was overcome by the least squares approximation, the algorithm is still not optimal in terms of “best fit poly-line”. all but the last line have maximal permitted variance, which means, that for too high threshold lines go over corners and an approximation is worse, than it could be. more technically, the total sum of squared distances from each point to its regression line is higher, than is achievable with given number of lines. fine tuning of the threshold is therefore necessary. complexity of the algorithm is o(n). result of line extraction is depicted in fig. 2c. 2.2. scan fitting when updating a map with a new scan, corresponding edges must be selected at first. it is responsibility of an operator or a path planing algorithm to obtain new scans frequently enough to ensure overlap of a new scan and the map. this means the robot always has to see part of the already known environment (except the first scan of course). acquiring more scans from one location leads to a more precise map (averaging effect). scan fitting is the point, where crude pose estimation comes in useful, because searching through the whole map would be very computationally expensive. knowing approximate pose and its maximal error allows us to considerably reduce search space and keep computational complexity constant, independent on the size of the whole map (assuming the number of visible details is comparable across the map). if the robot is suddenly moved far away from its previous location, it becomes lost and the algorithm is not capable of working correctly anymore. only long enough edges are used for the process, because the shorter ones are not determined with a sufficient precision. the maximal angular error of the pose estimation is directly used as the maximal angle between possibly similar edges. if the angle is inside a tolerance, shortest distances of end points of first edge to the second edge are computed. if at least 25 ales jelinek acta polytechnica ctu proceedings one is shorter than maximal permitted shift between two scans, edges are overlapping each other and can belong to one real edge. there are three possible situations after this step. for some edges from the new scan, there is no similar one in the map. these probably belong to an area which was not explored yet, or where only short edges were determined. new edges do not contribute to scan fitting process. the second group of new edges has one similar match in the map and these are directly used for scan matching. in the last group, there are edges with more than one possible match. at this situation, it is hard to say, which possible pair is the best one, or if more edges should be connected together. current practice is to remove edges already used in the second group and from the rest to select the closest one (in the sense of overlapping). in rare cases, the edges corresponding in real world are not the closest ones. this leads to mismatch and can even corrupt the map, but in most cases, correctly matched edges prevail and the mismatch is unnoticeable. at this point, pairs of corresponding edges from the new scan and the map are known and the actual fitting may take place. as well as in the previous step, the line similarity criterion will be used, but now with slightly different properties. an idea, that lines are more similar as the angle they form is smaller is still true, but overlapping criterion used before is not advantageous. instead, as already mentioned in the introduction, an area defined by both edges is used. the main idea of the criterion is depicted in fig. 3. let two edges be defined by lines p and q with the end points ab and cd. at first, an axis of angle formed by these edges is found. then, for all end points, their perpendicular projections (with respect to the axis o) on an opposite line are found. two end points and their projections form a quadrangle as shown in fig. 3 for points ab (both cases). there are six possible quadrangles and the largest one has to be used for the criterion enumeration. the square of that area is used as a similarity metric. the function for the similarity enumeration is not strictly convex, therefore for one pair of edges, there is not a single minimum defining one pose in which both edges are the most similar. instead, there is an infinite number of possible poses, in which the criterion is zero. this property is clear from the geometrical visualization of collinear edges sliding over each other and corresponds to the problem of localization in a long corridor. if a robot “sees” only collinear walls, it is not able to determine the displacement along an axis of the corridor one degree of freedom cannot be estimated from available data. for an exact solution, at least two skew pairs of edges must be used. fitting itself is an iterative process, because no closed form solution was found so far. a new scan is transformed using the standard homogeneous transformation. the goal is to find such translation and (a) . no intersection. (b) . with intersection. figure 3. line segments similarity criterion. the smaller is the gray area, the more similar are both line segments. rotation, so that the sum of the squared areas of all edge pairs is the smallest possible. nelder-mead simplex method was used to find the global minimum. for longer edges, the area they demarcate is large even if an angle they form is relatively small. this means, that longer edges have greater impact on scan fitting, which is correct, because long edges are determined by more points, and therefore, be more precisely known. 2.3. map building after the fitting process is finished, a new scan is transformed using the found parameters and edge pairs are joined to form a new set of edges in updated map. the same transformation, which was used to update the map, is applied on the path estimate to correct the odometry error. assuming static environment, the information about pose gotten from scan fitting is absolute, and therefore, an error should not have integrative characteristics, however, this is ideal state which requires a precise map corresponding to the environment. the main problem of map building process in current state of development is that the map does not converge. scans are incrementally fitted to the map, but new measurement influences only the part of the map, which is currently visible and new information is not propagated through the whole map to assure convergence. in other words, change in length of one edge should affect all connected edges (including those which are not actually seen), but current implementation is not able to handle this. integrative 26 vol. 2/2015 vector maps in mobile robotics figure 4. a larger map composed using the presented processing system. edges are colored in the same way as in fig. 2d. gray trajectory is acquired from odometry and dark blue one is an estimation of the true trajectory. characteristics of error of pose and map is therefore not suppressed. on the other hand, using edges and line similarity criterion for scan fitting is very accurate and divergence process is much slower. results of this step are maps in fig. 2d and fig. 4. in comparison with a naive scan merging in fig. 2a, new maps are much more usable for navigation as well as environment documentation. further processing for even better results can follow at this point, one example of possible operations is additional merging of shorter edges (blue edges in fig. 2d and fig. 4) to form a cleaner map. 2.4. experimental results practical testing was made in office and laboratory environment. algorithms based on edge detection are well suited for such application, because artificial objects are usually simply shaped with a lot of flat surfaces. on the contrary, a natural environment is much more complicated with lot of details and lack of flat surfaces, therefore effective line extraction would be very complicated or even impossible. the robot used for testing was orpheus x3, which is four-wheeled vehicle with differential steering. laser scanner used was velodyne 32 hdl. during the experiments, the robot was manually operated and all measurements were taken, when the robot was not moving. the first experiment used for demonstration of the processing pipeline is depicted in fig. 2a to fig. 2d. ten measurements 60 centimeters apart were taken. the odometry error was intentionally increased to demonstrate correction ability of scan fitting algorithm. outliers and noise filtration was also proved to be working, for example small cloud of points on top of fig. 2a was correctly removed. the second experiment was made in a larger scale on a corridor and resulting map is shown in fig. 4. robot moved from starting position to the end of the corridor and then returned back. total length of the traveled path was 46.710 meters and 33 measurements were taken along the way. the difference between the real and the estimated position of the last point of trajectory was only 6 mm, which is even less than the precision of used laser scanner (20 mm). the experiment was held only once, therefore repeatability is not yet determined. fairly good results are caused by two main factors. at first, the line extraction algorithm with the least squares approximation is capable of reducing noise through averaging and provide more accurate edge positions than single points in a point cloud. the second reason is that the robot moves in one room and always sees edges from the first scan, therefore an error of map and pose is not integrating. if the robot would move along a path with a lot of turns and after some time would not see its starting location, this problem would start to manifest. however, these experiments were made to proof the concept of scan fitting via edge similarity criterion and slam itself is not yet at the center of the research. 3. conclusion and future work the paper presents the set of algorithms used to process data from laser scanner and build a vector based map. the whole scheme of the process is depicted in fig. 1. most algorithms used are new and still under testing, but findings stated in literature (e.g., [10], [12], [16]) were proved to be true, because scan matching based on edge detection yields quality results and definitely can be used for solving slam problem. at least, considering results in fig. 2d and fig. 4, current state of the research is very promising. the main task for the nearest future is development of good representation of the topological-metric map, to enable a real slam to be implemented. topological information is necessary for visible (or just close) edges finding algorithm and for path planning. metric information is crucial for mapping itself, scan matching, and path planning. term “good representation” means such data structure, that would be efficient for search and computing algorithms. nearest neighbor of polygon search or finding an edge in area with certain coordinates are only a few examples of problems which are necessary to be efficient in a good mapping algorithm. the second requirement expected from a good map is a convergence. as the robot explores surrounding 27 ales jelinek acta polytechnica ctu proceedings environment, the map should adapt to be closer to reality. this is not achieved now and lot of work is focused on this task. with an authentic map and well working fitting algorithm, localization itself should not be too complicated and a solution for the slam problem using vector maps should be completed (at least in its main aspects). acknowledgements this work was supported by the project ceitec central european institute of technology (cz.1.05/1.1.00/02.0068) financed from european regional development fund. references [1] m. dissanayake, p. newman, s. clark, et al. a solution to the simultaneous localization and map building (slam) problem. ieee transactions on robotics and automation 17(3):229–241, 2001. doi:10.1109/70.938381. [2] h. durrant-whyte, t. bailey. simultaneous localization and mapping: part i. ieee robotics & automation magazine 13(2):99–110, 2006. doi:10.1109/mra.2006.1638022. [3] t. bailey, h. durrant-whyte. simultaneous localization and mapping (slam): part ii. ieee robotics & automation magazine 13(3):108–117, 2006. doi:10.1109/mra.2006.1678144. [4] s. thrun, w. burgard, d. fox. probabilistic robotics. the mit press, 2005. [5] m. montemerlo, s. thrun, d. koller, b. wegbreit. fastslam: a factored solution to the simultaneous localization and mapping problem. aaai/iaai 2002. [6] h. j. sohn, b. k. kim. vecslam: an efficient vectorbased slam algorithm for indoor environments. journal of intelligent and robotic systems 56(3):301–318, 2009. doi:10.1007/s10846-009-9313-2. [7] l. oswald. recent development of the iterative closest point algorithm, 2010. [8] s. rusinkiewicz, m. levoy. efficient variants of the icp algorithm. proceedings third international conference on 3-d digital imaging and modeling pp. 145–152. doi:10.1109/im.2001.924423. [9] p. besl, h. mckay. a method for registration of 3-d shapes. ieee transactions on pattern analysis and machine intelligence 14(2):239–256, 1992. doi:10.1109/34.121791. [10] z. zhang. iterative point matching for registration of free-form curves and surfaces. international journal of computer vision 13(2):119–152, 1994. [11] h. pottmann, s. leopoldseder, m. hofer. registration without icp. computer vision and image understanding 95(1):54–71, 2004. doi:10.1016/j.cviu.2004.04.002. [12] j. elseberg, r. t. creed, r. lakaemper. a line segment based system for 2d global mapping. 2010 ieee international conference on robotics and automation pp. 3924–3931, 2010. doi:10.1109/robot.2010.5509138. [13] q. li, j. griffiths. iterative closest geometric objects registration. computers & mathematics with applications 40(10-11):1171–1188, 2000. doi:10.1016/s0898-1221(00)00230-3. [14] e. tsardoulias, l. petrou. critical rays scan match slam. journal of intelligent & robotic systems 72(3-4):441–462, 2013. doi:10.1007/s10846-012-9811-5. [15] m. alshawa. icl : iterative closest line a novel point cloud registration algorithm based on linear features. ekscentar (10):53–59, 2007. [16] b. kamgar-parsi, b. kamgar-parsi. algorithms for matching 3d line sets. ieee transactions on pattern analysis and machine intelligence 26(5):582–93, 2004. doi:10.1109/tpami.2004.1273930. [17] k. demarsin, d. vanderstraeten, t. volodine, d. roose. detection of closed sharp edges in point clouds using normal estimation and graph theory. computer-aided design 39(4):276–283, 2007. doi:10.1016/j.cad.2006.12.005. [18] v. nguyen, s. gächter, a. martinelli, et al. a comparison of line extraction algorithms using 2d range data for indoor mobile robotics. autonomous robots 23(2):97–111, 2007. doi:10.1007/s10514-007-9034-y. [19] d. h. douglas, t. k. peucker. algorithms for the reduction of the number of points required to represent a diditized line or its caricature. cartographica: the international journal for geographic information and geovisualization 10(2):112–122, 1973. doi:10.3138/fm57-6770-u75u-7727. [20] w. shi, c. cheung. performance evaluation of line simplification algorithms for vector generalization. the cartographic journal 43(1):27–44, 2006. doi:10.1179/000870406x93490. 28 http://dx.doi.org/10.1109/70.938381 http://dx.doi.org/10.1109/mra.2006.1638022 http://dx.doi.org/10.1109/mra.2006.1678144 http://dx.doi.org/10.1007/s10846-009-9313-2 http://dx.doi.org/10.1109/im.2001.924423 http://dx.doi.org/10.1109/34.121791 http://dx.doi.org/10.1016/j.cviu.2004.04.002 http://dx.doi.org/10.1109/robot.2010.5509138 http://dx.doi.org/10.1016/s0898-1221(00)00230-3 http://dx.doi.org/10.1007/s10846-012-9811-5 http://dx.doi.org/10.1109/tpami.2004.1273930 http://dx.doi.org/10.1016/j.cad.2006.12.005 http://dx.doi.org/10.1007/s10514-007-9034-y http://dx.doi.org/10.3138/fm57-6770-u75u-7727 http://dx.doi.org/10.1179/000870406x93490 acta polytechnica ctu proceedings 2:22–28, 2015 1 introduction 2 point cloud processing system 2.1 filtering and clustering 2.2 scan fitting 2.3 map building 2.4 experimental results 3 conclusion and future work acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2017.12.0024 acta polytechnica ctu proceedings 12:24–31, 2017 © czech technical university in prague, 2017 available online at http://ojs.cvut.cz/ojs/index.php/app development of an electric bicycle for a sharing system in prague william deleenheera,∗, lukáš jánešb, akshaya jayakumarb a kuleuven, faculty of industrial engineering, gebroeders de smetstraat 1, 9000 gent, belgium b czech technical university in prague, faculty of transportation sciences, department of vehicle technology. konviktská 20, 110 00 prague ∗ corresponding author: william.deleenheer@gmail.com abstract. by means of a development of an e-bike sharing system the electromobility project wants to provide an alternative way of comfortable transportation for students and staff of the ctu, primarily to commute between different campuses. the research for this project contains at least three different fields of study, namely electric vehicle and docking station development, intelligent transport systems and management and economics of transportation and telecommunication. after briefly stating general requirements for the sharing system, this paper focuses on the development of the electric bicycle. first an ideal bike design is defined. then necessary motor power and battery capacity are calculated by estimating characteristics of cycling in prague. a prototype was developed by converting a normal bicycle to an electric bicycle. being equipped with devices for e-bike monitoring, controlling and data recording for a post trip analysis, this prototype is also intended to have an educational value for future students in the project. results consist of an electrical bicycle configuration that matches the requirements and a sketch of an ideal e-bike for this project. keywords: e-bike sharing, requirements, e-bike design, motor power, battery capacity. 1. introduction the demand and supply of electric bicycles is a fastgrowing market. a market study on e-bike purchasing has increased for this year, mainly in the netherlands, denmark and germany. it is said that the market first concentrated on the netherlands and germany due to its overall size. figure 1. e-bike unit sales per 1000 inhabitants in 2015, data from ecf advocacy group’s publication[1]. but eventually the per head market demand increase in switzerland, austria, denmark proved to be relatively stronger. customers start to have innumerable options to combine any kind of bicycle with different sorts of batteries and motors at a wide range of prices. nevertheless, to compose a suitable e-bike for a sharing system is less easy. first of all, a responsibility that comes with owning a bicycle lies not with the end user anymore and thus the user has no incentive to care for it or maintain it. and second a sharing system takes into account many bicycles thus the cost of maintenance or theft as well as an initial investment of one e-bike should be as low as possible. furthermore, there are some requirements for a successful and economically viable sharing system, mostly regarding user experience, such as safety and comfort. this paper has tried to list these requirements and develop an electric bicycle accordingly. 2. general requirements for the sharing system determining the requirements for a viable e-bike sharing system for the ctu but also for prague in general has been carried out in three ways. in the first place, there was an existing project vision that originated both in the project goal as in earlier research. then more detailed requirements were defined by a research of existing examples in the academic world and in the market. finally, the requirements as for the station and software for an initial pilot project were summarized. 24 http://dx.doi.org/10.14311/app.2017.12.0024 http://ojs.cvut.cz/ojs/index.php/app vol. 12/2017 development of an electric bicycle in prague 2.1. project vision the primary goal of this project is to provide a more efficient way of transportation for students and staff of the faculty of transportation science as a means to commute among different faculty buildings. the challenge exists in achieving this goal without losing comfort that comes with using a currently available public transport system. the concept of comfort in this context can be specified in four clear requirements: • minimal human physical effort, • protection against bad weather conditions, • possibility to transport certain cargo, • easy access to the system. a priority condition for all other requirements is obviously safety, all the more because cyclists are vulnerable road users and the bicycle infrastructure is still limited in prague. 2.2. market research market examples and previous experience with both conventional bike sharing schemes as well as a few existing e-bike sharing schemes such as go bike[2], bewegen.com[3], bonopark[4] and cycleushare[5] were researched to define components of a successful e-bike sharing system. another useful source was the experience of the institute of transportation and development policy (itdp)[6] with planning and exploitation of bike sharing systems. the following characteristics have been found repeatedly and are here briefly summarized: • integrated and theft protected e-bike components (motor, battery, chain, lights, wires, hid, automated lock); • ergonomic bicycle design (step-thru frame with adjustable saddle); • user friendly bicycle checkout and payment procedures; • high station density (10-16 stations for every square kilometre for conventional bikes); • many bicycles (10-30 bikes for every 1,000 residents for conventional bikes); • large coverage area (10 square kilometres for conventional bikes); • others: auxiliary (digital) lock, gps tracking, modular docks. discussable options are a use of high-end integrated components such as internal gears, belt drives or puncture free tires that reduce maintenance or theft costs but require higher initial investment. the university of brighton in uk conducted a similar e-bike research. the research aimed to understand user needs and to develop a system design necessary to start a fleet of e-bike program. the main outcome was a development of a "smart tracking system" which was implemented in a few e-cycles and proved successful. the system autonomously recorded and transmitted a bikes position, route, level of assistance required in real time and then sent it all in an online interface which was used for both research analysis and participant review. this proved to be an efficient solution for the fleet management and theft control. most of the users were daily commuters to office, the surveys revealed that most of them had found it easy to use and that it was something which had not disturbed or damaged their clothing much. 2.3. station and software requirements this project opted for a system in which the batteries have to be rented and plugged onto a bike by the user. this concept of battery charging and vending machine was first used in the "cycleushare project"[5]. the main advantage is that the station can have more batteries than bicycles and thus always has a fully charged battery for the user. in this concept all the batteries are stored in one charging box which protects the batteries from weather conditions and theft. this way the charging system and the bicycle docks can be made much cheaper compared to a more common way in which every dock needs a charger and the battery is integrated in the bicycle. another advantage is that the batteries, which mostly have a shorter life span than the bicycles themselves, can be maintained or replaced very easily. the biggest disadvantage is a decreased ease of use since the check in and check out procedure involves handling a battery. more experience is needed to determine an exact opportunity cost of this concept. the system is required to be an intelligent system. this includes necessity of mutual communication between the user, server, station, bicycle and battery. the tools to be used for this communication are isic student cards (rfid), gps tracking, rfid controlled locks, hid’s, system managing software and a web application. the main requirements for the software are based on the characteristics of sharing software like the one from "opensourcebikeshare"[7] and general requirements. crucial properties of the software interacting with the users via a web application are a map with real time data about station proximity and bicycle availability, instructions and help for users, feedback from users, reward system for a balanced e-bike distribution. additionally, it could have a routing tool if this is not integrated in the e-bicycle hid. the gui interacting with the administrators needs to provide data about system usage and provide a possibility to manage the fleet in real-time. this is possible via communication with rfid and gps tracking devices or if you like on docks and bicycles as well as user feedback. additionally, it will be connected to the battery charging machines which will provide data about battery state, such as number of cycles and usage patterns. 25 w. deleenheer, l. jáneš, a. jayakumar acta polytechnica ctu proceedings 3. electric bicycle research and design the above stated requirements have been applied on a development of a suitable electric bike. this involved researching an optimal bicycle design as well as a simulating power output and energy consumption during cycling to determine necessary motor and battery specifications. 3.1. bicycle design properties of bicycles in an average bike-sharing scheme are already a good approach to what a comfortable e-bike should look like. they are city-bikes with a step-thru frame, a rear rack for panniers or a basket mounted to the handlebars. with regards to cobblestone streets in prague it is desirable to have some form of suspension. e-bikes with the same purpose should have a similar design except the design also involves a decision about where to place a motor and battery. the requirement of a minimal human effort includes optimal handling and steering behaviour of the bike. the centre of a mass of the rider-bike system affects this behaviour. in fact, the act of cycling is known to be a fairly complex mechanism. in addition to the rider’s skill and gyroscopic forces, there are, acting on the wheel, the centre of gravity lowering torque and castoring forces[8]. in the context of this project we can simplify it by saying that keeping a bicycle from falling comes down to keeping the centre of mass of the rider-bike system over its wheels. at very low speeds or when coming to a halt, the bike is easier to handle with a low centre of mass because thanks to the the leverage, the rider also needs to apply relatively small forces on the handlebars to keep the bike upright. from a certain minimum forward speed, it is easier to steer a bike with a higher centre of mass. f for the same reason it is easier to balance a tall broomstick on the end than a short pencil. the broomstick is a slower inverted pendulum for which one has more time to bring it back to balance[9]. on the other hand, as it is the case for normal bicycles, the mass of the rider still has the most impact on the overall centre of mass of the coupled electric bike – rider system. thus for a placement of a battery and motor it is sufficient to look at handling behaviour at very slow speeds or standstill. furthermore, taking in consideration the idea of transporting certain cargo on the rear rack it is preferable to have the battery weight inside the frame for a better mass distribution, for a minimal inertia high in the rear and thus for a safe and comfortable ride. the decision to choose a mid-drive motor contributes to this logic but is also a consequence of reasons stated in the next paragraph. disadvantages of hub motors are an increased unsprang weight and a more difficult maintenance. apart from an extra mass added to the bicycle, the motor and battery need extra space. one idea on this matter is to use smaller wheels (24 to 18 inch) which are already often used in cargo bikes for this reason. smaller wheels also lower the overall centre of the mass and enable higher acceleration amongst other advantages[10]. one disadvantage is a decreased cushioning. a compromise could be only to have a smaller rear wheel. figure 2. e-bike design: sketch by the writer, scale 1/10. to visualize the previously outlined ideas, the above draft of a possible e-bike design was sketched, keeping in mind low cost, easy fabrication for example with standard aluminum tubes. this draft could be a base for a more advanced (e.g. with integrated components) and attractive design that can be validated with a strength analysis. a last point to look at is a protection against wind, rain and sunlight. there are some solutions on the market to solve this problem, such as detachable shields or poncho’s. in case a full integration of this feature is opted, it is recognizable that the lower the rider sits to the ground the easier it is to design a cover/shield that protects a full body of the rider from the impact of precipitation and wind owing to its forward movement. 3.2. motor the main specification of an electric bike motor is its power. on the market the motors are distinguished by an electrical power they can infinitely continuously handle, for example "a 250 watt motor". this figue is misleading because of two reasons: the input power the motor uses is determined by the voltage of the battery multiplied by the current the controller draws from the battery (e.g. 36v × 15a = 540 watt). the motor will mostly be built for a certain voltage (e.g. a 36v 250 watt motor with a 36v battery) so the maximum power will be proportional to a current limit in the controller. secondly the electrical power is not equal to an output power on the axis of the motor because of internal losses. the latter one is the power assisting the rider in giving a bicycle a forward motion. the average efficiency η of most motors on the market is around 80% at the optimal rpm so 350 watts into the controller will result in 280 watts of a driving power[11]. 26 vol. 12/2017 development of an electric bicycle in prague minimal human effort and a possibility to transport certain cargo are the two initial requirements which come in to play when determining required motor power for this project. in other words, it can be stated that cycling should not be experienced as an exercise and the user can go from a to b in normal clothing without sweating (disregarding the influence of abnormal values for humidity and ambient temperature). the question is which human output power corresponds to this requirement. professional cyclists can sustain 400 watts of leg power for an hour while for a person of an average health and fitness this would be 100 to 200 watts. these values apply for exercise till exhaustion[12]. for a comfortable commuting a continuous human power output of 50 watts is decided to be a reasonable maximum. this amount is comparable to the power needed for a person of 80kg to walk 1 m/s[13]. the last steps in defining a necessary motor power consist first of all of specifying a required driving power for cycling as a function of speed [km/h] and uphill slope [%] and last of simulating required performances for cycling in prague. at a constant speed the driver and motor combined have to overcome three forces: (1.) rolling resistance (frol), frol = mtot × g × cr where: mtot = mass of the rider-bike system, g = gravitational acceleration (9.81 m/s2) and cr = rolling resistance coefficient (±0.006 for normal tire) (2.) aerodynamic resistance (faer), faer = 1 2 × ρ× v2 × cw ×af where: ρ = air density (±1.20 kg/m3 for 20°c and 1 atm), v = bicycle speed [m/s], cw = air resistance coefficient (±1.1 for upright cyclist plus touring bike) and af = frontal surface area (0.6 m2 for upright cyclist) (3.) inclination resistance (fincl), fincl = mtot × g × sinα where: mtot = mass of the rider-bike system, g = gravitational acceleration (9.81 m/s2) and α = is the inclination grade in degrees then ftot = frol + faer + fincl pdriving,tot = ftot × v assumption for mtot = mdriver + mloadedbicycle = 75 + 35 = 110kg. all assumptions were made with a rather inefficient but still realistic set up in mind. the chosen parameter values are the result of a comparison of [12] and [14]. on figure 3: power input during acceleration => from 0 to 25km/h in 3 seconds => f = m×(v2−v1)/t. figure 3. driving power for speeds up to 50km/h and slopes up to 20%. simulating cycling in prague is a rather subjective process but as for a speed it is relevant to take 25km/h which is the maximum speed to which a motor can assist an electric bike by eu legislation. it is known that prague has uphill slopes up to 15% in the form of short streets. it is preferable these can be done with comfort and without the motor failing. with the above two values as a maximum the necessary driving power will be 1300w. eu legislation restricts a rated power to a maximum of 250w. in fact this stands for a 36v motor with a 15a controller which means this motor will be able to handle 540w although not continuously. in the latter case the output power will average around 430w which is far below a defined required driving power. as long as continuous comfort is more important for this project than a continuous speed, it is also useful to define a minimum speed. it was arbitrarily chosen to take 10km/h, which is still a double of an average walking speed, as a minimum speed to go uphill. now the maximum necessary total driving power is 472w. providing the rider takes 50w on his behalf, the required motor input power becomes (472 − 50) × 100/80 = 527.5w. it can be recognised that a motor rated as 250w is still insufficient, especially because at low speeds motor efficiency will be lower than 80% and the total mass might be higher. there is a clear preference of a mid-drive motor to a wheel hub motor or a direct drive for implementation in a hilly city like prague since a mid-drive motor can still work in (or closer to) the optimal rpm range at low speeds such as 10km/h provided a good use of the bicycle gears. present extensive e-bike testing and user experience will tell if this was a good simulation. 3.3. battery the battery is a fuel tank for an electric vehicle(ev). the required amount of energy it must store depends on energy consumed in a trip with a certain minimal range or minimal time. a range of 15 to 20km is 27 w. deleenheer, l. jáneš, a. jayakumar acta polytechnica ctu proceedings v [km/h] avgup hill [%] distance [km] #accel etot,el [wh] +human etot,el [wh] throttle mode 15 0 20 1 9.6 93.0 20 0 20 1 67.8 130.3 25 0 20 1 128.3 178.3 25 0 40 1 255.9 355.9 25 0 50 1 319.7 444.7 25 2 5 1 70.1 82.6 25 2 5 10 76.7 89.2 25 2 5 20 84.1 96.6 25 2 10 1 139.5 164.5 25 2 15 1 208.8 246.3 25 2 20 1 278.2 328.2 25 2 30 1 416.9 491.9 20 2 20 1 217.7 280.2 10 5 5 1 79.0 110.2 15 5 5 1 96.2 117.0 20 5 5 1 110.9 126.5 25 5 5 1 126.2 138.7 10 10 1 1 34.5 40.7 10 10 5 1 171.8 203.1 15 10 1 1 38.0 42.2 20 10 1 1 41.1 44.2 20 10 5 1 203.7 219.3 25 10 1 1 44.4 46.9 table 1. estimated energy consumption within 23 different trips. more than enough to cover an area that includes the center of prague and the majority of ctu buildings. the electrical energy e consumed is equal to the time integral of electrical power p(t). if p = constant for certain time interval ∆t, following expression applies: eelectric = pelect × t = η × pdriv,mot × t assuming the same values for the rider-bike system as in the previous section the energy consumption in over 230 trips was simulated by setting discrete values for the following variables: velocity [km/h] (10,15,20,25), average uphill slope [%] (0, 2, 5), distance [km] (1,3,5,10,15,20) and amount of accelerations at 1.388m/s2 (1,5,10). the table below (table ??) shows some of the more relevant possible trips including scenarios that did not fall into the scope of the simulation. the values in yellow show an effect on power and energy of changing this particular variable. the values in orange show trips with an energy consumption of more than 200w in the throttle mode. the values in green show trips which are more typical for a bike sharing system ( <5km & <0.5h). the values in red show trips that require more than 650w of an input power. taking into account that a deep discharge is to be avoided when trying to maximize a battery life span, it can be stated that 150 to 200wh of a stored energy will be more than enough in the context of this project. practically this could be a 36v × 6.6ah or a 25v × 8ah. the main specifications of a battery pack are voltage [v] and amp-hours [ah]. the electrical energy storable in the battery pack can be estimated with this theorem: eelectric = v (t) × i(t) × t where v (t) = rated voltage of the battery pack [v] and i(t) × t = current discharge over time [ah]. for the 36v motor from the previous section and 300wh, equation (7) gives a 36v × 8.3ah (theoretical) battery. in practice, a battery pack is configured by a spot welding a number of cells in series and parallel e.g. 3.6v × 2200mah cells in a 4parallel10serie configuration makes a 36v × 8.8ah battery pack. the number one chemistry for these cells is lithium-ion e.g. lifepo4 (ifr), licoo2 (icr), limn2o4(imr), linicoalo2 (inr), each with its own benefits and downsides. when optimizing this project, it will be highly relevant to compare these chemistries. for the latter examples it can be simplified by stating li-manganese are especially safe, li-iron-phosphate are the most economical (expensive but a high life span) and li-cobalt have the highest specific energy but they need a good bms to be safe[15]. another important property of a cell is its maximum continuous discharge rate. this rate is defined by its c-rate which expresses the time in which a battery can be discharged compared to its capacity in amp-hours, e.g. a 2c battery cell with 2200mah can be discharged at 4400ma in 30 minutes[16, 17]. if the motor controller draws equal or more amps from the battery pack than its configuration can handle, this will result in a decreased capacity and probably also a decreased life span. in other words: if the e-bike is equipped with a 15amp controller, a 2c rated battery will need to have at least 7.5ah but preferably 10ah. thus, it can be stated that for this e-bike sharing project it is preferable to have a cell chemistry with a high c-rate since enough power is more important than a high capacity depending on what is an average distance between the stations and the characteristics of an average trip. a standard size for these cells is 18650 (18mm diameter, 65mm long, round shape) and their weight is a little less than 50g. a charging rate is mostly between 0.5c and 1c e.g. a 8.8ah battery pack charged at 2a will take about 5-6 hours to be fully charged. note that it will take about as much time to charge a battery from 0 to 80% as from 80% to 100%. it was the case in the "cycleushare" project that the batteries were made available by the system from the moment they were 75% charged which increased efficiency but still a reasonable range[18]. 28 vol. 12/2017 development of an electric bicycle in prague 3.4. electronics just like the system itself is required to be intelligent, so much is the e-bike itself. via a gui on the handlebars the rider can be informed about multiple variables as speed, trip distance, trip time, instantaneous power input and battery capacity, etc. more advanced devices as the cycle analyst will also provide information about wh or ah consumed, estimated wh left (instead of the typical battery icon) and an amount of human power amongst other variables. if a gps tracker is built in and a logging device is installed on the bike it is possible to carry out full post trip analyses which are interesting for further research in this project. using a mid-drive motor, it is preferable to have a gear sensor that cuts off motor power momentarily when the user shifts gears, thereby preventing too much stress on the chain and the gearing system which causes rough gearing and can cause damage as well. a better but more expensive solution would be a continuously variable transmission (cvt). the motor is mostly controlled with hall sensors. there are three main ways the rider can get assistance from the motor: throttle mode (current flow is proportional to angle of (thumb) throttle), cadence based pedal assist (current flow is proportional to crank rotation speed), torque based pedal assist (current flow is proportional to amount of torque exerted on the chain or the pedals). from these three ways the latter feels the most intuitive while the cadence based pas is much cheaper and thus more common. another advantage of torque proportional assistance is an increased battery range[19]. to achieve the proclaimed 50w of human power at all speeds and slopes a torque sensor would be ideal. in practice this can be done with a chain tension sensor or a strain gauge on the cranks or pedal axles. usually the hid offers a chance to set different pas levels so the user can choose how much proportional assistance he or she requires from the motor. most battery packs on the market will have a battery management system (bms) which is a circuitry to keep individual cell voltages balanced during charging and discharging. furthermore, it protects the cells from different circumstances such as high temperatures, deep discharge and overcharging. the quality and intelligence of this bms will have a high impact on the life span of a battery pack and depending on the chosen cell chemistry the bms is essential for safety, more specifically for preventing fires or explosions. 4. e-bike building, testing and results 4.1. choosing a configuration due to a low initial budget it was decided to convert a normal bicycle to an e-bicycle with a commercially available mid-drive kit and a downtube battery. because this battery requires presence of water bottle cage mounting holes it was more convenient to buy a male framed bicycle (see figure 4). figure 4. photo of an assembled e-bike. the configuration of the research bike consisted of a 350w rated mid-drive motor, meaning a 36v motor with 18a controller, i.e. a peak input power of 648w. the battery pack has a capacity of 36v × 8.8ah which is a 4p10s configuration of samsung icr1865022p cells (3.62v × 2150mah). samsung claims these cells have a maximum continuous discharge rate of 3c which means 26.4a. in fact, discharging at 3c the battery will already put quite some stress on the cells and the capacity will be lower due to heat losses as can be seen on discharge curves of this particular cell[7]. 4.2. tests and results firstly, the balancing of the e-bike was found very easy. one indicator for this is that it was very easy to balance the bike without hands on the handlebar. the bike enabled flexible maneuvering. this proves the chosen placement of battery and motor to be correct. secondly acceleration, top speed in the throttle mode was up to 34 km/h on a flat in an upright position. the accelerations perceived a big difference with normal cycling. the ease of getting up to speed was so easy both with the pedal assistance and the throttle mode is similar to riding a scooter or a small motorcycle. the motor did tend to heat up quite quickly after riding the bike several minutes in the highest pas levels (4 and 5). this shows that the set up with 36v and 18a the limit approaches the level a motor with these 62 dimensions can handle. the same motor is rated "250w" with a 15a current limit, in this case the motor will probably not heat up as much, combined with less stresses on the plastic gears inside. it might therefore have a longer lifespan with 15a maximum. riding in prague with this e-bike also revealed possible dangers, mostly concerning the higher speeds achievable. being very vulnerable on an e-bike speeds tended to drop when riding among car traffic. 29 w. deleenheer, l. jáneš, a. jayakumar acta polytechnica ctu proceedings another inconvenience is that not being able to go to a lower gear at low speeds or standstill feels rather unhandy regarding smooth accelerations. this is not any different with normal bikes but with this e-bike there was a tendency to prevent shifting too much and staying in a high gear for higher speeds. because the brakes cut off the motor power there is no need to go down in gears when slowing down and coming to a stop. although the electric motor can handle starting in the highest gear, the acceleration at lower gears is more logical, but shifting to a lower gear at standstill causes much stress on the gearing system and is perceived unsmooth. 5. experiments 5.1. first experiment for last three years two experiments have been carried out with the e-bike for transportation among the three buildings of ctu. these two experiments have been performed by students of the project electromobility. the bikes for the experiments were borrowed with similar or lower configuration as the bicycle, which was constructed. the first experiment covered the connections between the three buildings of ctu. in this project only students of ctu were included. in every buildings there was a spot for borrowing an e-bike, where the students borrowed the e-bike and rode to another building of ctu, where they returned the e-bike. during the trips the students measured parameters of the trip by mobile application and sent data to the folder for processing. in 15 days there were made about 50 rides and every ride proved that transport by e-bike among buildings is faster than other types of transport, such as automobile or public transport[20]. 5.2. second experiment the second experiment was made in 2014, and it examined advantages and parameters of riding the e-bike to the center of prague from an outskirt of the city. for this experiment typical trips for riding to center of prague were chosen, the center was selected as the building of ctu horská. there were 10 trips and every measuring showed that every trip is faster by e-bike opposite to the car or public transport. only one trip, the trip of about 10 kilometers, was faster by metro than e-bike[21]. 5.3. experiments conclusion the e-bike constructed for this thesis and presented in this article, was tested only for the driver’s comfort and for technical parameters, but not for measuring parameters and comparison with another type of transport. the previous two experiments were only for testing e-bikes as a means of transport for students among the buildings of ctu or for transporting students from home to school. it should start as a private public transport for students in "campuses", with private spots for rental and private system of borrowing and security. in the future, it would be beneficial to add new places, such as the central station, where it can be extended to general public which will help to make the system more popular among general population and tourists. 6. conclusion it is clear that e-bike sharing is a useful technology which can be a transportation method made ready to fit to a wide range of possible users. however, the small portion of real world examples show the concept still faces some difficulties, most of which being financial or organizational ones. thus, it is important to plan implementation and exploitation thoroughly with all stakeholders involved in the debate with a strong business plan laid on the table. the research e-bike, equipped with the cycle analyst and logger can be used to acquire accurate data about energy consumption and other trip characteristics when e-cycling in prague meanwhile experimenting with different types of bicycles, installing wind and rain shields or full covers to see their effect on energy consumption and comfort. as a result, the developed e-bike can be optimized further and fabricated. specific recommendations for further successful realization of the e-bike sharing project: • set up a strong business plan by means of a user survey and investment analysis and specific action plans. • further visualize the final result by implementing a (3d) design of both station and e-bike. • cooperate with other institutions and faculties. • set up an organization which will manage this interdisciplinary project and represent it. these 4 recommendations will significantly help with the challenge of an increasing student interest and involvement and increasing interest from sponsors, advertisement companies, university management and city municipalities. according to the market studies prague manages to keep up with the trends and efficient modes of transport. e-bike sharing could be then very soon visualized in prague at ctu. references [1] "european bicycle market analysis 2015", advocacy means sales, european cyclists’ federation. edited by: alexander scholz, published on 25.08.2016. [2] "gobike", gobike. [online], available: http://gobike.com/solution/the-bike/, [geopend 10 may 2016]. [3] "bewegen", bewegen technologies inc. [online], available: http://bewegen.com/our-system/#top, [geopend 10 may 2016]. 30 http://gobike.com/solution/the-bike/ http://bewegen.com/our-system/#top vol. 12/2017 development of an electric bicycle in prague [4] "bonopark", bonopark. [online], available: http://www.bonopark.es/en, [geopend 10 may 2016]. [5] "cycleushare", university of tennessee. [online], available: http://www.cycleushare.com/, [geopend 10 may 2016]. [6] "itdp", the institution of transportation and development policy. [online], available: https://www.itdp.org/wp-content/uploads/2014/ 07/itdp-bike-share-planning-guide-1.pdf, [geopend 10 may 2016]. [7] "open source bike share". [online], available: http://opensourcebikeshare.com/, [geopend 25 may 2016]. [8] d. e. jones. the stability of the bicycle. physics today. pp. 34-40, 1970. [9] j. m. j. p. a. r., a. j. d. g. kooijman. a bicycle can be self-stable without gyroscopic or caster effects. science, no. 332, pp. 339-342, 2011. [10] t. hadland. small wheels for adult bicycles. 25 june 2012. [online], available: https://hadland.wordpress.com/2012/06/25/ small-wheels-for-adult-bicycles/, [geopend 25 may 2016]. [11] j. lemire-elmore. ebiks.ca. grin technologies. [online], available: http://www.ebikes.ca/tools/simulator.html, [geopend 28 april 2016]. [12] m. ikonen. ip-zev. 2 april 2011. [online], available: http://www.ip-zev.gr/files/teaching/t4-2_human_ power.pdf, [geopend 28 april 2016]. [13] d. j. farris, g. s. sawicki. ncbi. pmc, 25 may 2011. [online], available: http://www.ncbi.nlm.nih.gov/ pmc/articles/pmc3223624/table/rsif20110182tb1/, [geopend 28 april 2016]. [14] "coefficient of friction, rolling resistance and aerodynamics", tribology-abc. [online], available: http://www.tribology-abc.com/abc/cof.htm# rolweerstand, [geopend 30 may 2016]. [15] "battery chemistry finally explained", battery bro, 19 february 2015. [online], available: https://batterybro.com/blogs/ 18650-wholesale-battery-reviews/ 18880255-battery-chemistry-finally-explained, [geopend 26 may 2016]. [16] "batteries", grin technologies. [online], available: http://www.ebikes.ca/learn/batteries.html, [geopend 26 may 2016]. [17] "bu-402: what is c-rate?", battery university, 29 february 2016. [online], available: http://batteryuniversity.com/learn/article/ what_is_the_c_rate, [geopend 27 may 2016]. [18] c. r. cherry, s. ji, l. d. han, d. a. jordan. electric bike sharing: simulation of user demand and system. journal of cleaner production. vol. 85, pp. 250-257, 2014. [19] "torque sensors on electric bikes", ectricbike.com, 18 october 2012. [online], available: http://www.electricbike.com/torque-sensors/, [geopend 27 june 2016]. [20] m. lapáčková. bike sharing na fakultě dopravní. praha, 2014, diplomová práce. čvut fd. [21] l. jáneš. využití elektrokol v praze z hlediska bezpečnosti a členitosti terénu. praha, 2015, bakalářská práce. čvut fd. 31 http://www.bonopark.es/en http://www.cycleushare.com/ https://www.itdp.org/wp-content/uploads/2014/07/itdp-bike-share-planning-guide-1.pdf https://www.itdp.org/wp-content/uploads/2014/07/itdp-bike-share-planning-guide-1.pdf http://opensourcebikeshare.com/ https://hadland.wordpress.com/2012/06/25/small-wheels-for-adult-bicycles/ https://hadland.wordpress.com/2012/06/25/small-wheels-for-adult-bicycles/ http://www.ebikes.ca/tools/simulator.html http://www.ip-zev.gr/files/teaching/t4-2_human_power.pdf http://www.ip-zev.gr/files/teaching/t4-2_human_power.pdf http://www.ncbi.nlm.nih.gov/pmc/articles/pmc3223624/table/rsif20110182tb1/ http://www.ncbi.nlm.nih.gov/pmc/articles/pmc3223624/table/rsif20110182tb1/ http://www.tribology-abc.com/abc/cof.htm#rolweerstand http://www.tribology-abc.com/abc/cof.htm#rolweerstand https://batterybro.com/blogs/18650-wholesale-battery-reviews/18880255-battery-chemistry-finally-explained https://batterybro.com/blogs/18650-wholesale-battery-reviews/18880255-battery-chemistry-finally-explained https://batterybro.com/blogs/18650-wholesale-battery-reviews/18880255-battery-chemistry-finally-explained http://www.ebikes.ca/learn/batteries.html http://batteryuniversity.com/learn/article/what_is_the_c_rate http://batteryuniversity.com/learn/article/what_is_the_c_rate http://www.electricbike.com/torque-sensors/ acta polytechnica ctu proceedings 12:24–31, 2017 1 introduction 2 general requirements for the sharing system 2.1 project vision 2.2 market research 2.3 station and software requirements 3 electric bicycle research and design 3.1 bicycle design 3.2 motor 3.3 battery 3.4 electronics 4 e-bike building, testing and results 4.1 choosing a configuration 4.2 tests and results 5 experiments 5.1 first experiment 5.2 second experiment 5.3 experiments conclusion 6 conclusion references acta polytechnica ctu proceedings doi:10.14311/app.2017.8.0008 acta polytechnica ctu proceedings 8:8–10, 2017 © czech technical university in prague, 2017 available online at http://ojs.cvut.cz/ojs/index.php/app the influence of titanium-niobium alloys on the growth of the oxide layer jan krčil∗, vladimír mára, jana sobotová, stanislav krum department of materials engineering, faculty of mechanical engineering, czech technical university in prague, karlovo náměstí 13, 121 35 prague 2, czech republic ∗ corresponding author: jan.krcil@fs.cvut.cz abstract. this paper discusses issues of growth of a thin oxide layer formed on the surface of titanium alloys. the oxide layer on the surface introduces a corrosion resistance and a bio-compatibility, both of which are required for medical applications. the oxide layer is a result of a spontaneous passivation, but for the practical applications it is necessary to control the growth of oxides. in present work, the oxide layer was formed on the cp ti grade 2, ti39nb alloy and cp ti grade 2 with ti39nb pvd coating of two different thicknesses by a thermal oxidation at 600 ◦c for 8 hours. after the oxidation process the oxide layer was observed by sem. keywords: oxide layer, tinb, coating, biomaterials. 1. introduction nowadays, there is a need for better bio-implants and better bio-materials. it is due to several reasons: prolonging of human life, different lifestyle and activities, formerly incurable diseases and injuries. this may lead to the repairing or replacing parts of human body. the replacement should fit perfectly, act as the integral part of the body, have long lifetime, be bio-compatible, corrosion resistant and should not be expensive [1–3]. there are numerous existing materials used for body replacements. predominantly used are titanium alloys (especially ti-6al-4v and cp titanium) and stainless steel aisi 316l. but as the time progresses, these materials are slowly becoming obsolete. there is need for improved mechanical properties (ideally the same as human bones) and increased bio-compatibility. the new perspective group of materials is the titanium β alloys ti alloys with β structure, usually containing nb, zr, ta and other elements. these show lower modulus of elasticity, higher bio-compatbility but are more expensive on the other hand [2, 4, 5]. titanium alloys are not bio-compatible and corrosion resistant on their own. this effect is brought by oxides on the surface. oxide layer forms on the surface of ti alloys whenever the surface is in contact with air or moisture. resulting layer is very thin (the thickness is in the range of nanometers). the layer of such thickness would not act as the sufficient barrier and it would lead to corrosion and dissolution of the base material into the human body. the easiest way to prepare applicable oxide layer is the controlled oxidation: anodic or thermal. the composition of a layer may differ due to the chemical composition of the oxidized alloy, but the main component of the oxide layers on ti alloys is titanium dioxide (tio2). the tio2 naturally occurs in three different modifications: anatase, rutile and brookite. the rutile structure is stable up to melting temperature (unlike the other two) and in some cases is more suitable for the human body [2, 6]. the rutile structured oxide layer can be obtained by thermal oxidation above 500 ◦c [2, 7, 8]. 2. experiment the experiment was based on the comparison of the oxidation results on cp ti grade 2 and tinb alloy with 39 wt.% of nb (from now on ti39nb). bulk samples and substrates for coating were prepared by grinding and polishing to mirror sheen. afterwards, the two groups of cp ti samples were coated by ti39nb (one with 2 µm, other with 5 µm of ti39nb) using flexicoat 850, hauzer. subsequently, the samples were oxidized. the type of oxidation process and its parameters were based on the literature [7, 9, 10]: oxidation at temperature of 600 °c in the air atmosphere at atmospheric pressure with dwell of 8 hours in the lac lh 30/13 furnace equipped with mt825-a regulator. the course of the oxidation was following: heating from the room temperature with speed of 7.5 °c/min up to required 600 °c. at the end of the oxidation period, the furnace was turned off and the samples were cooled down to ambient temperature. it was expected that this process would result in formation of oxide layers with good adhesion to the substrate material. the measurement of oxide layers was carried out on the cross-section of samples using the jeol jsm 7600f scanning electron microscope. 3. results and discussion the fig. 1 represents the cross-sections of all 4 groups of oxidized samples. the substrate material, the coating and the oxide layer are labeled in every figure. the fig 1a shows oxidized sample of cp ti grade 2. the measured thickness of the oxide layer was app. 500 nm. in the case of oxidized sample of ti39nb 8 http://dx.doi.org/10.14311/app.2017.8.0008 http://ojs.cvut.cz/ojs/index.php/app vol. 8/2017 influence of titanium-niobium alloys on growth of oxide layer (a). (b). (c). (d). figure 1. cross-section of thermally oxidized (600 °c /8 hours) samples: (a) cp ti grade 2, (b) ti39nb, (c) cp ti grade 2 + 2 µm of ti39nb, (d) cp ti grade 2 + 5 µm of ti39nb. (fig. 1b), the oxide layer was considerably thinner than in case of the sample from cp ti. in average, the thickness was app. 140 nm. samples from cp ti with coating of ti39nb are in fig. 1c and 1d. on the sample with 2 µm thick coating the oxide layer with thickness ranging from 300 nm to 400 nm was found. the oxide layer thickness on the sample with 5 µm coating was ranging from 200 nm to 300 nm. from the images it is evident that the oxide layers formed on all samples are compact and with relatively uniform thickness. the adhesion of the oxide layers to the substrate/coating seems to be the same for all samples. the main difference between oxide layers was found in the resulting thickness. the thickest layer was found on cp ti grade 2, the thinnest one on ti39nb. the thickness of oxide layers on ti39nb coatings was in between. these preliminary results indicate that the tinb alloy appears to be less vulnerable to the oxidation process than cp ti grade 2 or that passivation process on tinb alloy is stronger than on cp ti grade 2. it leads to the formation of thinner oxide layer when compared to the samples of cp ti grade 2 which were oxidized in the same conditions and in the very same process. these findings contradict to the information found in the literature [6]. this phenomenon should be further investigated using different oxidizing conditions when compared with other materials and with other methods of the oxidation process. 4. conclusions • the oxide layer was created by the thermal oxidation on all four groups of samples and resulted in oxide layers of different thickness. • however against the assumption, the thickness of the oxide layers showed a different trend. the oxide layer found on the samples with thicker ti39nb coating was thinner than on the samples with thinner ti39nb coating. • the results indicate the dependence between the thickness of ti39nb coating and the thickness of the resulting oxide layer. • it will be further investigated how the results will differ under different oxidation conditions and with use of others method of oxidation. 9 j. krčil, v. mára, j. sobotová, s. krum acta polytechnica ctu proceedings acknowledgements this work was supported by the grant agency of the czech republic (grant no. 15-01558s) and by the ministry of education, youth and sport of the czech republic, program npu1, project no lo1207. references [1] u. kamachimudali, t. m. sridhar, b. raj. corrosion of bio implants. sadhana 28(3):601–637, 2003. doi:10.1007/bf02706450. [2] y. oshida. bioscience and bioengineering of titanium materials. elsevier, oxford, second edition edn., 2013. doi:http://dx.doi.org/10.1016/b978-0-44462625-7.00001-7. [3] biomedical engineering, trends in materials science, chap. novel titanium manganese alloys and their macroporous foams for biomedical applications prepared by field assisted sintering. intech, 2011. doi:10.5772/12874. [4] i. jirka, m. vandrovcová, o. frank, et al. on the role of nb-related sites of an oxidized β-tinb alloy surface in its interaction with osteoblast-like mg-63 cells. materials science and engineering: c 33(3):1636 – 1645, 2013. doi:http://doi.org/10.1016/j.msec.2012.12.073. [5] a. gutiérrez, m. f. lópez, j. a. jiménez, et al. surface characterization of the oxide layer grown on ti-nb-zr and ti-nb-al alloys. surface and interface analysis 36(8):977–980, 2004. doi:10.1002/sia.1816. [6] medical device materials v: proceedings from the 2009 materials & processes for medical devices conference, chap. titanium oxide layer on the surface of anodized dental implants. asm international. [7] g. zorn, a. lesman, i. gotman. oxide formation on low modulus ti45nb alloy by anodic versus thermal oxidation. surface and coatings technology 201(3-4):612 – 618, 2006. doi:http://doi.org/10.1016/j.surfcoat.2005.12.009. [8] z. abdolldhi, a. a. ziaee, a. afshar. investigation of titanium oxide layer in thermal-electrochemical anodizing of ti6al4v alloy. international journal of chemical & biomolecular engineering 2(1):44, 2009. [9] h. jiang, m. hirohasi, y. lu, h. imanari. effect of nb on the high temperature oxidation of ti-(0-50 at.%)al. scripta materialia 46(9):639 – 643, 2002. doi:https://doi.org/10.1016/s1359-6462(02)00042-8. [10] s. sartale, a. ansari, s.-j. rezvani. influence of ti film thickness and oxidation temperature on tio2 thin film formation via thermal oxidation of sputtered ti film. materials science in semiconductor processing 16(6):2005 – 2012, 2013. doi:https://doi.org/10.1016/j.mssp.2013.07.006. 10 http://dx.doi.org/10.1007/bf02706450 http://dx.doi.org/http://dx.doi.org/10.1016/b978-0-444-62625-7.00001-7 http://dx.doi.org/http://dx.doi.org/10.1016/b978-0-444-62625-7.00001-7 http://dx.doi.org/10.5772/12874 http://dx.doi.org/http://doi.org/10.1016/j.msec.2012.12.073 http://dx.doi.org/10.1002/sia.1816 http://dx.doi.org/http://doi.org/10.1016/j.surfcoat.2005.12.009 http://dx.doi.org/https://doi.org/10.1016/s1359-6462(02)00042-8 http://dx.doi.org/https://doi.org/10.1016/j.mssp.2013.07.006 acta polytechnica ctu proceedings 8:8–10, 2017 1 introduction 2 experiment 3 results and discussion 4 conclusions acknowledgements references 123 acta polytechnica ctu proceedings 2(1): 123–127, 2015 123 doi: 10.14311/app.2015.02.0123 photometric and spectroscopic investigation of the dwarf nova hs 0218+3229: a short review n. katysheva1, s. shugarov1,2, n. borisov3, m. gabdeev3, p. golysheva1 1sternberg astronomical institute of the moscow state university, 119991 moscow russia 2astronomical institute of the slovak academy of sciences, 059 60 tatranská lomnica, slovakia 3special astrophysical observatory of the russian academy of sciences, 369 167 nizhnij arkhyz, russia corresponding author: natkat2006@mail.ru abstract this paper is devoted to the study of the cataclysmic variable hs 0218+3229 using the photometric and spectroscopic observations. keywords: cataclysmic variables dwarf novae optical photometry spectroscopy individual: hs 0218+3229. 1 introduction cataclysmic variables (cvs) are highly evolved close binaries consisting of a white dwarf (wd) accreting matter from a red dwarf companion. this matter creates an accretion disc around the white dwarf. dwarf novae (dne) are a subclass of cataclysmic variables, whose members experience outbursts either caused by a thermal-viscous instability in the accretion disc (di) or by a sudden increase in mass transfer rate. outbursts of dne last a few days. in gcvs (samus et al., 2007-2012) dne are described as stars of the type ugss (u gem–ss cyg), with the recurrent time between outbursts lasting from days to several years. hs 0218+3229 (α2000 = 02h21m34s; δ2000 = +32◦ 43’ 24”) was discovered spectroscopically during the large-scale search for cvs in the hamburg quasar survey by gänsicke et al. (2002). rodriguez-gil et al. (2009) carried out photometric and spectroscopic observations of hs 0218+3229 from 2000 to 2005 and determined its orbital period to be 0.d297229661(1). the phase light curve (lc) in the r passband exhibits ellipsoidal modulation. the k5v secondary component gives a contribution of 80-85% to the r light. an analysis of the time-resolved optical spectroscopy and r passband photometry provided some parameters of hs 0218+3229: a mass ratio of 0.52< q <0.65, a white dwarf mass of 0.44< m1/m� <0.65, a secondary component mass of 0.23< m2/m� <0.44, the orbital inclination i = 59o ± 3o. the distance to the system was estimated to be 0.87–1.0 kpc. hs 0218+3229 has been identified as the x-ray source 1rxs j022133.6+324343 and the 2mass source j02213348+3243239. 2 photometric observations of hs 0218+3229 2.1 optical outbursts in 2006 the object was discovered independently by s. antipin using the photo plates of the photographic archive at the sternberg astronomical institute (sai) and identified as a dn. one outburst with an amplitude of about 4.5 mag in the bph passband was detected in september 1980. another outburst was detected in 2002 by wills from neat database (2002). golysheva et al. (2012, 2013) described photometric and spectroscopic observations of hs 0218+3229 from 2006 till 2010. ccd photometry in ubvrcic passbands was carried out at the crimean laboratory of sai (nauchny) and at the stará lesná observatory of the astronomical institute of the slovak academy of sciences. the magnitudes of comparison stars were determined in golysheva et al. (2012). in october 2007 they detected the outburst of hs 0218+3229 with an amplitude of 4 mag and duration of 15–16 days (jd 2454380–395). the shape of the outburst lc was more symmetric than for other dne. its asymmetry coefficient (the ratio of the duration of the ascending branch to the descending branch) was ∼0.22. a new ephemeris was calculated using the initial epoch of the minimum from rodriguezgil et al. (2009). the epoch corresponds to the inferior conjunction (the secondary is in front of the wd): hjdmin = 2453653.0286 + 0.2973559× e (1) the orbital period of the object (7.13 h) largely ex123 http://dx.doi.org/10.14311/app.2015.02.0123 n. katysheva et al. ceeds the average orbital periods of dne, which are in the range 1.5–4 hours. the maximum v magnitude of the 2007 outburst was determined as vmax=12.34±0.05 mag. the maximum magnitudes in other passbands were estimated by extrapolation as u=11.3±0.2 mag, b=12.3±0.2 mag, rj 'ij =12.2±0.2 mag. during 2011–13 we continued observations of the object. as seen from bvrcic light curves (lcs) of the object, obtained in 2006-13 and presented in fig. 1, a new outburst occurred in 2013. its v-maximum magnitude was the same as in 2007 outburst. a sudden 0.5 mag increase of brightness in the b passband was detected on january 31, 2011. the bvrcic plots in fig. 1 show long-term brightness variations during quiescence with a characteristic timescale of a few tens of days and an amplitude of ∼0.2 mag, noted in golysheva et al. (2012). figure 1: the bvrcic lcs of hs 0218+3229 in 2006–2013. the aavso data are marked by grey circles. our observations of the last outburst of hs 0218+3229, detected on september 5, 2013 (vsnet, 2013), were obtained at the stará lesná observatory. taking into account that the object exhibited outbursts also in 2002 and 2007, it is possible to estimate the recurrent time between outburst to be 5–6 years. fig. 2 shows the lcs of the outbursts in 2007 (upper panel) and 2013 (lower panel). the nightly lcs are inserted in separate panels. left lc, obtained directly before the outburst in 2007, shows half of orbital wave with an ellipsoidal effect. this effect is not visible on the other nightly lcs because of the presence of the outbursting disk. in the framework of the di model, the shape of the outburst lc of hs 0218+3229 can be explained by a low mass transfer rate in the system and ”inside-out” outburst in accordance with smak (1984). outburst starts from the inner parts of the disk and extends outward. the 2013 outburst shape is similar to the previous one in 2007. the nightly bv lcs are shown in fig. 2. figure 2: the outburst lcs in 2007 (top) and 2013 (bottom) in v (black points and circles) and b (grey points). the aavso v data are marked by grey circles. the ∆ubv rcic outburst lcs, near the maximum brightness, are presented in fig. 3. all lcs look similar. figure 3: the lcs after removal of the declining trend in brightness during the outburst, on september, 8, 2013. the lcs are displaced by arbitrary magnitudes. 124 photometric and spectroscopic investigation of the dwarf nova hs 0218+3229: a short review the quiescence lcs in the bvrcic passbands, folded with the ephemeris (1), are shown in fig. 4. double wave is clearly seen in the vri passbands, and less clearly in the b passband. it should be noted that the heights of the maxima (in phases 0.25 and 0.75) and depth of the minima (in phases 0 and 0.5) can change from night to night. the displacement of the second minimum around phase 0.5 is clearly seen. this feature was noted also by rodriguez-gil et al. (2009). golysheva et al. (2012, 2013) noted a large uvexcess in quiescence in 2010. during the 2007 outburst the position of the star in the two-colour diagram was close to that of an absolute black body with a temperature of about 15000 k. at the minimum brightness the position of the variable corresponded to a star of spectral class k5v-k6v. more detailed information about two-colour diagrams can be found in the paper of golysheva et al. (2012). figure 4: the average orbital phase bvrcic lcs in 2006–2012, folded with ephemeris (1). individual observations are marked by grey points, mean values by black points. 2.2 the analysis of the aavso data we also present data by the american association of variable star observers (aavso), whose observers were asked to observe the object to support the hubble observations (hst/cos, the aavso ”alert 471” (2012) about monitoring of hst targets). the hst/cos observations were obtained in december 2012. the main bulk of aavso observations were carried out in a clear filter. because of a very good time coverage, the orbital lcs were very well covered by observations in some nights from september 17, 2012 till january 2, 2013. we reduced the data to heliocentric values. the nightly lcs in september and october 2012 are presented in fig. 5 and folded lcs are shown in fig. 6. in spite of the error 0.02–0.03 mag, the shape of the nightly lcs is distinctive. the depth of the primary minimum (marked by 1) is smaller than the depth of the secondary minimum (marked by 2) and the hump after the primary minimum is brighter than the hump after the secondary minimum. this phenomenon is especially evident in the orbital folded phase lcs (fig. 6). figure 5: the aavso lcs obtained in september and october 2012. the primary and secondary minimum are marked by 1 and 2, respectively. figure 6: the average lcs in september–october 2012, folded with ephemeris (1). the notation is the same as in fig. 4. 125 n. katysheva et al. 3 spectroscopic observations our spectroscopic observations of hs 0218+3229 was taken in 2010 and 2012, when the object was in quiescence. golysheva et al. (2013) obtained two spectra of hs 0218+3229 on september 17/18, 2010 in the prime focus of the russian 6-m telescope bta of the special astrophysical observatory of the russian academy of sciences (sao ras) at the spectroscopic mode of the scorpio multi-mode focal reducer (afanasiev, moiseev, 2005) with the long slit and ccd-camera eev ccd 42-40 (2048×2048 pixels) with the exposures of 300 sec. our new spectroscopic observations of the objects were carried out on november 5, 2012 with the same reducer at the 6-m bta telescope. the vphg1200g grating (1200 grooves/mm) were used in both cases. the spectroscopic resolution ∆λ = 5.0 å in the wavelength interval 3950–5700 å was achieved. the spectra were reduced with sky and bias substraction and division by a flat-field frame. the discovery spectrum of the object obtained in 2000 (rodriguez-gil et al., 2009) revealed the absorption-line spectrum of the k5 v secondary, accompanying by the balmer and he i emission lines, arising in the accretion disk of the white dwarf primary. the 2010 spectrum was different. emission line of neutral helium λ5875 å became stronger, emission balmer decrement more flat and the features of the red companion became less pronounced (golysheva et al., 2012, 2013). these changes were probably caused by enhancing of a mass transfer rate and veiling of the spectrum of the secondary component. our spectrum obtained on november 5, 2012 is similar to the spectrum taken in 2000. the red star manifested itself again. 4 discussion and conclusions four outbursts of hs 0218+3229 in 1980, 2002, 2007, 2013 were detected up to now. the 2002 outburst (wills, 2002) was found in the database neat. the duration of 2007 outburst was 15–16 days and its asymmetry coefficient ∼0.22. the maximal amplitudes were 5 mag in the u, b passbands and 4 mag, 3.5 mag, 3 mag in the v, r, i passbands, respectively. it is possible to estimate the recurrence time of the outbursts to 5–6 years. we determined the orbital period to be 0.d2973559(10) and updated the ephemeris of the object. two-colour diagrams, presented in golysheva et al. (2012, 2013), showed that the object was bluer during its outburst than in quiescence. it was caused by cthe additional ontribution of a bright accretion disk to the total luminosity. outburst colour temperature was about 15 000 k. colour indices in quiescence correspond to a star k5 v. as a result, this cataclysmic variable was classified as a very rare subtype of ugsstype dn with a low accretion rate and sparse and more symmetric (”inside-out”) outbursts than it is usual in u gem-type and ss cyg-type of dne. figure 7: the spectrum of hs 0218+3229 on november 5, 2012. acknowledgement we thank to the large telescopes program committee of the sao ras for supporting our program of spectroscopic study of cvs. we would like to express our deep gratitude to dr. d. chochol for his valuable suggestions and critical reading of the manuscript. we are grateful to many amateur astronomers whose observations from the aavso international database were used in this research. nk thank to the soc for possibility to make a report. the work was supported partially by grants: nsh-1675.2014.2, rfbf-11-11-02-00258, 12-0200186, 12-02-97006 and vega grant 2/0002/13. references [1] samus, n., durlevich, o.v., kazarovets, e v. et al.: 2007-2012, general catalogue of variable stars, vizier on-line data catalog: b/gcvs. [2] gänsicke, b. et al.: 2002, asp conf. ser., 261, 190. [3] rodriguez-gil, p., torres, m.a.p., gänsicke, b.t. et al.: 2009, a&a, 496, 805. [4] wills, p.: 2002, near-earth asteroid tracking: http://neat.jpl.nasa.gov. [5] vsnet-alert.: 2013, http://ooruri.kusastro. kyoto-u.ac.jp/mailarchive/vsnet-alert/16355. 126 photometric and spectroscopic investigation of the dwarf nova hs 0218+3229: a short review [6] golysheva, p.yu., antipin, s.v., zharova, a.v. et al.: 2012, astrophysics, 55, 208. doi:10.1007/s10511-012-9229-6 [7] golysheva, p., katysheva, n., shugarov, s. et al.: 2013, ceab, 37, 345. [8] aavso-alert.: 2012, http://www.aavso.org/ aavso-alert-notice-471 [9] afanasiev, v., moiseev, a.: 2005, astron. lett., 31, 194. doi:10.1134/1.1883351 [10] smak, j.: 1984 pasp, 96, 575. doi:10.1086/131295 127 http://dx.doi.org/10.1007/s10511-012-9229-6 http://dx.doi.org/10.1134/1.1883351 http://dx.doi.org/10.1086/131295 introduction photometric observations of hs 0218+3229 optical outbursts the analysis of the aavso data spectroscopic observations discussion and conclusions acta polytechnica ctu proceedings doi:10.14311/app.2017.12.0064 acta polytechnica ctu proceedings 12:64–67, 2017 © czech technical university in prague, 2017 available online at http://ojs.cvut.cz/ojs/index.php/app subjective methods for assessment of driver drowsiness alina mashko czech technical university in prague, faculty of transportation sciences, konviktská 20, prague 1, czech republic correspondence: mashkali@fd.cvut.cz abstract. the paper deals with the issue of fatigue and sleepiness behind the wheel, which for a long time has been of vital importance for the research in the area of driver-car interaction safety. numerous experiments on car simulators with diverse measurements to observe human behavior have been performed at the laboratories of the faculty of the authors. the paper provides analysis and an overview and assessment of the subjective (self-rating and observer rating) methods for observation of driver behavior and the detection of critical behavior in sleep deprived drivers using the developed subjective rating scales. keywords: driver drowsiness, subjective methods, observer rated sleepiness. 1. introduction driving sleepy, drowsy or fatigued at a high extent affects road traffic safety. the research by american centers for disease control and prevention (cdc) reports that 7500 fatal accidents happened in years 2011-2012 due to the drowsy factor [1]. per aaa foundation for traffic safety, 21% of drowsy drivers were involved in fatal traffic accidents in the usa, which is 4,5% higher as compared to the results of the preceding years [2]. similar involvement rates (20% of accidents due to sleepiness at the wheel) have been reported by the australian transport accident commission [3]. two-thirds of the population (71%) are said to be sleeping less than 8 hours per day. the most vulnerable to sleep deprivation are so called risk group that includes shift workers, young drivers and people with children in household. driving drowsy is dangerous, mainly due to the possible consequent impairments caused by this state. according to the national sleep foundation’s poll in 2000, drowsy drivers tend to feel stressed (42%), be impatient (32%) or drive faster (12%) [4] or not use the seat belt during night driving [5]. known reasons of drowsy driving and inattention are driving at the time of day when circadian rhythms are low, or driving for a long time or sleep loss [6]. 2. definition of fatigued and drowsy state due to the individual behaviour of every person in a particular state as well as because of a variety of reasons for drowsy driving, finding common attributes or indicators in this state is especially difficult. drowsiness and fatigue, per self-reported perceptions, influence performance, physiological state, feeling. it has been found that in a state of fatigue individuals have experienced such feelings as being tired, sleepy or exhausted, not feeling inclined to continue performing the task because of a perceived reduction of efficiency, feeling tired, exhausted, lack of energy as related to impaired of both physical and cognitive functioning, need of restoration of resources [7]. individual perception of one’s drowsiness can be determined with the help of questioning objects, conducting self-evaluation tests and is related to subjective observation. according to [8], the physiological state of a fatigued individual this corresponds to the organism’s muscular, visceral and central nervous system a decrease of capacity or energy to maintain initial activity due to insufficient rest is observed. detection of physiological signs of drowsiness and fatigue is carried out via measurements of the heartbeat rate, brain activity, eye blinking and closure. eye behavior can be observed with the help of electrooculogram (eog) [9, 10], with eye tracker, such as in researches presented in [11–13], based on eye image video analysis [14], etc. a lot of research has been done for exploration of brain activity of sleepy and drowsy drivers, some of the latest are [15–19]. the measurement itself is performed with the help of special electrodes of an eeg device that are placed on the human scalp to read brain wave activity. evaluation of the quality of the task performance is another possible measure of drowsiness and fatigue. fatigue caused by sleep deprivation affects driving performance which can reflect in delayed reaction, inability to maintain constant speed, loose of lateral control [20, 21] etc. measurements can be conducted in real traffic as well as in simulated environment, which is safer. usually, the performance indicators are measured for each object under two conditions: in a state of sleep deprivation and in fresh (control or reference) state. 64 http://dx.doi.org/10.14311/app.2017.12.0064 http://ojs.cvut.cz/ojs/index.php/app vol. 12/2017 subjective methods for assessment of driver drowsiness 3. subjective drowsiness and fatigue subjective indicators of sleepiness level include sleepiness scales, such as epworth sleepiness scale (ess), karolinska sleepiness scale (kss) and stanford sleepiness scale. during these tests short questionnaires are given to subjects who are asked to rate their state according to the suggested scale. the self-rating is usually conducted repeatedly during the testing time with either a time interval, or situations (conditions). this kind of rating allows evaluation of the participants’ awareness of their alertness level. when compared to other driver drowsiness measures, it has been demonstrated that kss defined level of sleepiness in most cases has a curve-linear relation to lateral position and blink duration [22]. another method used in experimental investigation of driver fatigue is observer rated sleepiness (ors). the method quantifies observation of behavior by trained individuals or experts who are rating driver performance based on video recordings of the experiment or in real time monitoring. the method can be used for validation of other measures or as a separate measure of driver fatigue [23]. 4. subjective evaluation during experiments on driver simulator all observations described in this chapter are related to experiments conducted on a full body (skoda superb) fixed driving simulator based on a wide projection angle (210° of driver’s view is provided with 5 screens plus 2 screens for side mirrors. see figure 1 for reference). detailed description of simulator and lab is provided in [24]. figure 1. advanced driver simulator for driver drowsiness research. previously, we have conducted observations of sleep deprived drivers during a 2 hours’ driver simulator measurements [25]. different observers tracked changes in the drivers behavior based on video recordings. the target areas were head, face (facial expression and mimicking, eye behavior) and upper torso (hands, shoulders and body). observers were instructed to document all noticed behavioral patterns out of (but not limited to) the suggested list: frequency of eye blinking, change in duration of eye closures, yawning, face muscles relaxation, lip biting, head tilt, shoulders shaking, hands on the wheel position, body relaxation, change of sitting position. also from the shots of the driving scenery driving behavior (steering, correction of position in a lane, lane departure and accident) were observed. it was possible to notice that blinking frequency was observed after 40th minute of driving. blinking was then gradually substituted by longer eye closures often accompanied by worsening of steering behavior, such as weaving and loose of vehicle control up to accidents. in most drivers head rotations (neck muscles stretching) and head tilt were noticed. this, however, didn’t reflect in the alteration of driving behavior (i.e., steering, speed fluctuation etc.) and was considered as a voluntary type of behavior caused by both sleepiness and fatigue from a long drive taken by subjects with a purpose of finding more comfortable position. all observed subjects have displayed dangerous driver behavior, that in real traffic might lead to accident, mainly starting after 30 minutes of experiment. inability to maintain straight trajectory, weaving and lane departure could be observed. at the laboratory for driver sleepiness research a self-rating sleepiness scale has been developed and is being used to assess driver sleepiness rate by the subject him/herself. the scale is presented in table 1, each of its levels corresponds to change in sleepiness level as perceived by subject. state description as perceived by subject 1 i feel fine/fresh & driving does not make me any problems. 2 i feel drowsy & driving does not make me any problems. 3 i feel drowsy & i notice some problems. 4 i feel very drowsy & i need to excessively concentrate to drive correctly. 5 i experienced ’blackouts’ & losing control over the car. table 1. self-rating sleepiness scale developed at faculty of transportation sciences, ctu in prague. evaluation per this scale will be presented in further research outcomes. the ors method was applied for analysis of video recordings of sleep deprived drivers on simulator. 9 observers (7 male and 2 female) who are students of faculty of transportation sciences, czech technical university conducted evaluation. the observations combined body movements, facial expressions and driving and vehicle behavior. initially developed scale contained three basic levels: (1.) subject is sitting straight behind the wheel with eyes open and both hands on the wheel in 10/2 position, he/she is following the road situation and 65 alina mashko acta polytechnica ctu proceedings figure 2. results for ors evaluation by 9 observers of one sleep deprived subject. figure 3. difference between maximum and minimum rates provided by observers. cluster indicators, vehicle is within the lane and is following straight path, (2.) subject experiences some features of drowsiness, like more frequent blinking and episodes of prolonged blinking, vehicle is experiencing some weaving, correction movements are present in steering, subject sitting position is not straight (head inclinations), subject is performing some self-awakening actions, hands positioned in lower part of the wheel, (3.) episodes of lane departure due to the result of longer eye closures (sleep episodes) steering correction movements are more abrupt, body position is not straight (subject adjusts to be more comfortable for rest position), head inclined to the side, one hand may be off the wheel, or on the bottom of the steering wheel. during the evaluation process, it had been decided to extend it to a 5-scale grade by introducing the half-scores for so called transition states (i.e., giving 1.5 and 2.5 scorers when subjects demonstrated only some features of the state). outcome of such observation is presented in figure 2. there one may track the mean, max and min rates of subject state sampled per 5 minutes of two-hour experiment. every human behavior is individual and people display different behavior under same conditions. besides, the subjective perception of each observer influences the ors data (see the difference chart between rates – figure 3). the same type of behavior may vary per subject in severity and impact that’s why it is not possible to assign same weights to the displays of human behavior. for those reasons, precise definition of a state to correspond the rate score is sometimes compromised. this is confirmed by a study, where results have demonstrated poor correlation of ors to srs (self-reported sleepiness) [26]. it is still possible to track a tendency of an increasing sleepiness rate per suggested scale after the 40th minute of driving, which corresponds to conclusions mentioned in [25]. 5. conclusion the research of driver fatigue covers the applications of many branches of science such as sociology, physiology, neuroscience, engineering. individual human behavior is a challenge in finding a single method for detection of drowsiness. besides, validation of detection methods in real traffic conditions is more complicated because these experiments may cause serious accidents. subjective methods have proved to be reliable for rating of fatigue in laboratory experiments and can be considered as a viable method for further exploration of the problem. self-rating and expert observations have proven to provide an adequate picture of driver fatigue and the tendency of increasing sleepiness during prolonged driving and can be correlated to objective measurements. rigid definition of the states to correspond the scale rate is however crucial for evaluation results. difference in scores given 66 vol. 12/2017 subjective methods for assessment of driver drowsiness by observers to the same behavior is crucial of data validity and could make the results questionable. the current research is aimed at tailoring the suggested evaluation method for its further implementation for analyses of driver sleepiness experiments. acknowledgements this research was supported by grant sgs16/254/ohk2/3t/16 "experimental research of driver fatigue by means of observation visual behavior". references [1] a. wheaton, r. shults, d. chapman, et al. drowsy driving and risk behaviors – 10 states and puerto rico, 2011-2012. morbidity and mortality weekly report, vol. 63, no. 26. july 4, 2014. [2] aaa foundation for traffic safety. acute sleep deprivation and risk of motor vehicle crash involvement, 2016. [3] transport accident commission. fatigue statistics. http://www.tac.vic.gov.au/road-safety/ statistics/summaries/fatigue-statistics. [4] national sleep foundation. 2000 facts and stats. http://drowsydriving.org/about/facts-and-stats. [5] u. d. of transpotation. nhtsa.guidelines to observe and estimate statewide seat belt use at night. 2010. dot hs 811 288. [6] u. d. of transpotation. nhtsa.guidelines to observe and estimate statewide seat belt use at night. 1999. dot hs 808. [7] r. phillips. a review of definitions of fatigue – and a step towards a whole definition. transportation research part f 29 (2015) 48-56. [8] r. soames-job, j. dalziel. defining fatigue as a condition of the organism and distinguishing it from habituation, adaptation and boredom. in p. a. hancock & p. a. desmond (eds.), stress, workload and fatigue. ny: crc press. 2000. [9] m. villamizar, j. suarez, j. villanueva, et al. design and implementation of sleep monitoring system using electrooculographs signals. 9th pan american health care exchanges conference, pahce 2014. [10] n. r.-i. nez, p. meca-calderón, m. garcía-gonzález, et al. drowsiness detection by electrooculogram signal analysis in driving simulator conditions for gold standard signal generation. international conference on biomedical electronics and devices, biodevices 2013. barcelona, spain. [11] r. coetzer, g. hancke. eye detection for a real-time vehicle driver fatigue monitoring system. 2011 ieee intelligent vehicles symposium, iv’11; baden-baden, germany. [12] k. anjali, k. athiramol, v. athira, et al. time nonintrusive monitoring and detection of eye blinking in view of accident prevention due to drowsiness. international conference on circuit, power and computing technologies, 2016. [13] a. rahman, m. sirshar, a. khan. real time drowsiness detection using eye blink monitoring. national software engineering conference (nsec), 2015. [14] j. ahmed, l. jian-ping, k. saeed, a. riaz. eye behavior based drowsiness detection system. 12th international computer conference on wavelet active media technology and information processing (iccwamtip). 2015. [15] g. usha, b. radhika, t. shanmuganathan. eeg-based brain wave sensor to detect drowsiness with eye open. far east journal of electronics and communications. volume 16, march, 2016. [16] r. foong, k. ang, c. quek, et al. an analysis on driver drowsiness based on reaction time and eeg band power. 37th annual international conference of the ieee engineering in medicine and biology society, embc, 2015. [17] t. brown, r. johnson, g. milavetz. identifying periods of drowsy driving using eeg. 57th annual scientific conference of the association for the advancement of automotive medicine, canada, 2013. [18] m. jiřina, p. bouchner, s. novotný. identification of driver’s drowsiness using driving information and eeg. neural network world, 2010. [19] j. faber, j. pěkný, r. pieknik, et al. simultaneous recording of electric and metabolic brain activity. neural network world, 2010. [20] r. v. loon, r. brouwer, m. martens. drowsy drivers’ under-performance in lateral control: how much is too much? using an integrated measure of lateral control to quantify safe lateral driving. accident analysis and prevention. volume 84, 2015. [21] s. james, b. vila. police drowsy driving: predicting fatigue-related performance decay. policing, volume 38, issue 3, 2015. [22] m. ingre, t. akerstedt, b. peters, et al. subjective sleepiness, simulated driving performance and blink duration. journal of sleep research. volume 15, issue 1, 2006. [23] a. anund, c. fors, d. hallvig, et al. observer rated sleepiness and real road driving: an explorative study. plos one, 2013. [24] s. novotný, p. bouchner, j. pěkný, et al. hybrid mirrors for driving simulators – design, construction and experiments. in: computational intelligence, man-machine systems and cybernetics. athens: wseas press, 2007, pp. 310-315. issn 1790-5117. isbn 978-960-6766-21-3. [25] a. mashko, p. bouchner. visual investigation of driver behavior for fatigue detection – driving simulator experiments. recent advances on systems, signals, control, communications and computers. 2015. [26] c. ahlstrom, c. fors, a. anund. eur. transp. res. rev. (2015) 7:38. doi:10.1007/s12544-015-0188-y. 67 http://www.tac.vic.gov.au/road-safety/statistics/summaries/fatigue-statistics http://www.tac.vic.gov.au/road-safety/statistics/summaries/fatigue-statistics http://drowsydriving.org/about/facts-and-stats acta polytechnica ctu proceedings 12:64–67, 2017 1 introduction 2 definition of fatigued and drowsy state 3 subjective drowsiness and fatigue 4 subjective evaluation during experiments on driver simulator 5 conclusion acknowledgements references 273 acta polytechnica ctu proceedings 2(1): 273–276, 2015 273 doi: 10.14311/app.2015.02.0273 the ubv color evolution of classical and symbiotic novae i. hachisu1, m. kato2 1department of earth science and astronomy, college of arts and sciences, the university of tokyo, komaba, meguroku, tokyo 153-8902, japan 2department of astronomy, keio university, hiyoshi, kouhoku-ku, yokohama 223-8521, japan corresponding author: hachisu@ea.c.u-tokyo.ac.jp abstract we identified a general course of classical nova outbursts in the b − v vs. u − b diagram. it has been reported that novae show spectra similar to a–f supergiants near optical light maximum. however, they do not follow the supergiant sequence in the color-color diagram, neither the blackbody nor the main-sequence sequence. instead, we found that novae evolve along a new sequence in the pre-maximum and near-maximum phases, which we call the nova-giant sequence. this sequence is parallel to but ∆(u − b) ≈ −0.2 mag bluer than the supergiant sequence. after optical maximum, its color quickly evolves back blueward along the same nova-giant sequence and reaches the point of free-free emission (b − v = −0.03, u −b = −0.97) and stays there for a while, which is coincident with the intersection of the blackbody sequence and the nova-giant sequence. then the color evolves leftward (blueward in b−v but almost constant in u −b) due mainly to development of strong emission lines. this is the general course of nova outbursts in the color-color diagram, which is deduced from eight well-observed novae including various speed classes. for a nova with unknown extinction, we can determine a reliable value of the color excess by matching the observed track of the target nova with this general course. this is a new and convenient method for obtaining color excesses of classical novae. using this method, we redetermined the color excesses of nineteen well-observed novae. keywords: cataclysmic variables novae individual: fh ser, pu vul, pw vul, v1500 cyg, v723 cas. 1 introduction a classical nova is a thermonuclear runaway event on a mass-accreting white dwarf (wd) in a binary. despite of their overall similarity, optical light curves of novae have a wide variety of timescales and shapes. recently, hachisu & kato (2006) found that, in terms of free-free emission, the optical and infrared (ir) light curves of several novae follow a similar decline law. moreover, the time-normalized light curves were found to be independent of the wd mass, chemical composition of ejecta, and wavelength. they called it the universal decline law. evolution of colors is another challenging subject that attracts many researchers, who attempted to find a general behavior among various types of novae. for example, duerbeck & seitter (1979) discussed that color evolutions of six novae are remarkably similar in the intrinsic (b−v )0 versus (u −b)0 color-color diagram independently of their different nova speed classes. van den bergh & younger (1987) derived general trends of color evolutions in nova light curves, i.e., (b − v )0 = 0.23 ± 0.06 at optical maximum and (b − v )0 = −0.02 ± 0.04 at t2, where tm (m = 2 or 3) is the days during which a nova decays by mth magnitude from its optical maximum and (b − v )0 is the intrinsic b − v color of the nova. these two relations, however, often show large deviations from the values obtained by other methods. miroshnichenko (1988) found a stabilization stage of novae in the b−v and u − b color evolutions soon after optical maximum and that this stage showed a general trend of (b − v )0 = −0.11 ± 0.02. he derived extinctions, e(b−v ), of 23 novae assuming that all novae have the same intrinsic (b−v )0 color at the stabilization stage, i.e., e(b−v ) = (b−v )ss−(b−v )0 = (b−v )ss+0.11, where (b−v )ss is the observed b−v color at the stabilization stage. this method looks powerful but sometimes results in a large difference from the true value. according to hachisu & kato’s (2006) universal decline law, optical fluxes in the ubv bands could be dominated by free-free emission. if it is the case, its color is simply estimated to be (b − v )0 = 0.13 and (u − b)0 = −0.82 for the optically thin free-free emission (fν ∝ ν0), or to be (b − v )0 = −0.03 and (u −b)0 = −0.97 for the optically thick free-free emission (fν ∝ ν2/3, see, e.g., wright & barlow 1975), where fν is the flux at the frequency ν. the latter value of (b−v )0 = −0.03 is close to (b−v )0 = −0.02±0.04 273 http://dx.doi.org/10.14311/app.2015.02.0273 i. hachisu, m. kato at t2 derived by van den bergh & younger (1987). however, many novae do not keep these pivot points but further evolves blueward. these different trends of nova color evolutions may represent different sides of the true color evolution which we do not yet fully understand observationally or theoretically. the aim of this presentation is to find a general path of nova color evolutions as duerbeck & seitter (1979) tried to find about thirty years ago. first we examine the color evolutions of slow and fast novae, fh ser, pw vul, v1500 cyg, v1668 cyg, v1974 cyg, pu vul, hr del, and v723 cas. we found that the tracks of color-color evolutions of these slow/fast novae are common. then we redetermined the reddening of nineteen novae by fitting the color evolution of a target nova with the common path in the color-color diagram. thus, we propose a new method for estimating the color excess. figure 1: light and color curves of fh ser 2 nova-giant sequence in the color-color diagram the first example of our analysis is the moderately fast nova fh ser. the v , (b − v )0, and (u − b)0 light curves of fh ser are plotted in figure 1. fh ser showed a gradual optical decay with t2 = 41 and t3 = 62 days followed by a sudden drop of the brightness due to dust shell formation about 2.8 mag below the optical maximum. the (b−v )0 vs. (u −b)0 color-color evolution is plotted in figure 2 (see hachisu & kato 2014 for more detail). the reddening toward fh ser was obtained by kodaira (1970) to be e(b − v ) = 0.6 from the mmrd relation and interstellar reddening relation. della valle et al. (1997) obtained e(b − v ) = 0.82 from the color at optical maximum, i.e., e(b − v ) = (b − v )max − (b − v )0,max = 1.05 − 0.23 = 0.82, e(b − v ) = 0.61 from the line ratio of hα/hβ = 5.7, and e(b − v ) = 0.5 from the equivalent width of na i λ5890. then, della valle et al. (1997) adopted an averaged value of e(b − v ) = 0.64± 0.16. since kodaira’s e(b−v ) = 0.60 and della valle et al.’s e(b−v ) = 0.61 from the line ratio of hα/hβ are coincident with each other, we use e(b−v ) = 0.60 in the present paper and confirm below that this value is reasonable. figure 2: color-color evolution of fh ser in figure 2 we plot only the data before the dust blackout started about 70 days after the outburst. we also depict three known sequences, the blackbody, supergiant, and main-sequence sequence, the data of which are taken from allen (1976). we also added a point, i.e., optically thick free-free emission spectra (open diamond denoted by f) of fν ∝ ν2/3. we have frequently seen in literature such a statement that nova spectra near maximum are similar to those of a–f type supergiants. however, the track of fh ser in the colorcolor diagram does not follow the supergiant sequence as clearly shown in figure 2. the shape of fh ser track is very similar to that of the supergiant sequence but 274 the ubv color evolution of classical and symbiotic novae it locates about ∆(u − b) ≈ −0.2 mag bluer than the supergiant sequence. therefore, we are forced to define a new sequence based on the data of fh ser, which is designated by points a, b, c, d, and f from redder to bluer. these points correspond to epochs in figure 1. in what follows, we will see that many novae evolve along this sequence when their photospheric spectra are similar to those of a–f type supergiants. therefore, we call this track “the nova-giant sequence” after the supergiant sequence. 3 common paths of novae in the color-color diagram we have also studied color-color evolutions of the fast and slow novae, pw vul, v1500 cyg, v1668 cyg, v1974 cyg, pu vul, hr del, and v723 cas, in the (b−v )0 (u−b)0 diagram and showed that these well observed novae follow very similar evolutionary courses in the intrinsic color-color diagram as shown in figure 3. we finally specified four templates, (1) the moderately fast nova fh ser, a proto-type of the nova-giant sequence, (2) the slow nova pw vul and the very fast nova v1500 cyg, (3) the fast novae v1668 cyg and v1974 cyg, and (4) the symbiotic nova pu vul, the very slow novae v723 cas and hr del. the tracks are characterized with several specified points (a, b, c, d, f, 0, 1, ..., 5, 4’, 5’, 4”, and 5”). figure 4(a) shows templates of these eight well observed novae. figure 3: several typical tracks of novae in the colorcolor diagram since these well observed eight novae follow the templates in figure 4(a), we expect other novae also follow the same paths. in other words, if all novae follow this template, we are able to determine the color excess of a target nova by directly comparing its track in the colorcolor diagram with our general track in figure 4(a). figure 4: color-color evolution of (a) template 8 novae, (b) rs oph, (c) v446 her, and (d) v533 her. figure 5: (a) t pyx, (b) lv vul, (c) iv cep, and (d) nq vul. 275 i. hachisu, m. kato 4 estimates of extinctions toward various novae we are able to determine color excesses of various novae using our general color-evolution tracks found in this work. we collected novae as many as possible from literature that have sufficient data points (usually more than ten). we assume that all novae follow the colorcolor evolution tracks in figure 4(a). in order to obtain e(b − v ), we change e(b − v ) by steps of 0.05 to fit the observed track of a target nova with our general tracks in figure 4(a). figure 6: (a) v1370 aql, (b) gq mus, (c) qu vul, and (d) os and. figure 7: (a) qv vul, (b) v443 sct, (c) v1419 aql, and (d) v705 cas. figure 8: (a) v382 vel, (b) v2274 cyg, (c) v475 sct, and (d) v5114 sgr. figures 4, 5, 6, 7, and 8 show results of our analysis for 19 novae. corresponding e(b−v ) excess is denoted in each panel (see hachisu & kato 2014, in detail). references [1] allen, c. w. 1976, astrophysical quantities (3rd ed.; london: athlone) [2] della valle, m., gilmozzi, r., bianchini, a., & esenoglu, h. 1997, a&ap, 325, 1151 [3] duerbeck, h. w., & seitter, w. c. 1979, a&ap, 75, 297 [4] hachisu, i., & kato, m. 2006, apjs, 167, 59 doi:10.1086/508063 [5] hachisu, i., & kato, m. 2014, apj, 785, 97 doi:10.1088/0004-637x/785/2/97 [6] kodaira, k. 1970, pasj, 22, 447 [7] miroshnichenko, a. s. 1988, soviet astronomy, 32, 298 [8] van den bergh, s., & younger, p. f. 1987, a&ap suppl., 70, 125 [9] wright, a. e., & barlow, m. j. 1975, mnras, 70, 41 doi:10.1093/mnras/170.1.41 276 http://dx.doi.org/10.1086/508063 http://dx.doi.org/10.1088/0004-637x/785/2/97 http://dx.doi.org/10.1093/mnras/170.1.41 introduction nova-giant sequence in the color-color diagram common paths of novae in the color-color diagram estimates of extinctions toward various novae acta polytechnica ctu proceedings doi:10.14311/app.2017.11.0031 acta polytechnica ctu proceedings 11:31–34, 2017 © czech technical university in prague, 2017 available online at http://ojs.cvut.cz/ojs/index.php/app railway traffic optimization dušan kamenický∗, tomáš brandejský, vít fábera, valerii gopak faculty of transportation sciences, czech technical university in prague, konviktská 20, 110 00 praha 1 ∗ corresponding author: kamendus@fd.cvut.cz abstract. the article focuses on analysis of possibilities of operatic control of railway traffic and its automatization and its support by information systems. conditions, necessary pieces of input information like infrastructure description are discussed. railml format is mentioned as suitable format for description in railway area. optimization criterion and use of evolutionary techniques are suggested. the system is modeled in railway laboratory of the faculty of transportation sciences, ctu in prague. keywords: railway traffic optimization, railml, evolutionary techniques, prediction. 1. railway traffic optimization trends increasing demands to railway capacity require to build new railway lines and to apply new approaches like information technologies supporting railway traffic to use effectively given infrastructure. train running is controlled and checked by interlocking systems (and by operations over the devices) that evaluates the state of the infrastructure (mainly occupancy of track sections is detected using track circuits or axle counters), controls infrastructure elements (switches), transmits instructions how to move through signals or automatic train stopping device. there are several trends of rail operation control. firstly, the way of control is shifted from distributed system spread over stations to centralized one in all europe. the most railway lines in the czech republic should be controlled from two offices in přerov and prague. the centralization of the operation control gives spread view of the situation and more information to the operator. on the other hand, there is a disadvantage of this approach. information quantity to evaluate increases if the area is large. it is clear the information capacity of each human operator is limited and different. moreover, operator is negatively influenced by rotes. secondly, next modern trend (especially in switzerland, belgium and germany) is a separation of strategy control (route planning from daily to annual interval) from operative control (conflicts solution caused by deviations from regular time-table due to infrastructure or vehicle faults) and from direct control (safety device operation, shunting). each level of operation control architecture process specific functions and it needs it support. this article is focused especially on operative control and its it support, strategy control is mentioned only marginally. it is supposed that output of the strategy level support is a scheduled daily time table with valid and conflict-free train routes (really realizable with respect to the route capacity). operative control it support should react to deviation from planed daily time table; it monitors actual trains routes and solves potential conflicts with others trains in advance (from minutes to hours). the output is modified conflictfree time table – specific route plan transmitted to direct control level (realized by human operator or automatic system). the direct control level is freed from searching the sequence of runs if the planned time-table is disturbed and processes of the direct control level are focused on safety device operations and guarantee of rail operation safety. 2. it support of operative control it support of operative control was missed out in the czech republic for a long time. today systems only shows planned timetable which is modified based on information about actual train position entered by operators in stations. they reflect neither train properties (parameters) nor infrastructure properties. it depends only on abilities and experiences of operators (dispatchers) if they are able to predict potential conflict and to suggest appropriate solution. moreover, todays systems are not able to transfer such modified timetable to the direct level operating on the plant-interlocking devices. to increase capacity of railway traffic and effectivity by optimization of operative control, it means to solve several partial problems that are very difficult: prediction of train paths, conflicts detection and optimal solution of these conflicts. the necessary premise to be able to search automatically optimal solution is to predict train path. some problems hamper to solve this problem satisfactory. above all, it is necessary to dispose enough description of the infrastructure. the description must contain the information related to the construction of time table (position and height of platform edges, line parameters affecting train resistance – slope, curves, tunnels, ...) and related to the interlocking system (track sections and their dependencies ensuring safety 31 http://dx.doi.org/10.14311/app.2017.11.0031 http://ojs.cvut.cz/ojs/index.php/app d. kamenický, t. brandejský, v. fábera, v. gopak acta polytechnica ctu proceedings running). the prediction is also affected by actual train parameters and their correctness, especially by type of motive power and its traction characteristic, train weight, braking ability of the train (both parameters can change in dependence of weather), eventually coefficients of train resistances. the strategy of run is important as well – ratio of times of thrust/coasting/braking. if the trains are manually controlled by engine drivers the strategies can be dramatically different. the application of ato (automatic train operation) brings the improvement in this area, when strategies became more stable (but the strategies can be different within several ato systems depending on producers). very stochastic quantity is time of stop in stations (getting on and off, operations on train). one possibility to eliminate partially randomness is to collect long-term statistics. dependencies between trains are necessary to take into account – connections, locomotives, coaches and train crew turnround cycles. completeness and up-to dateness of data stored in is are necessary. train routes conflict detection is highly dependent on the interlocking systems. each infrastructure operator has different equipment and it works with different set of risks and their consequences. due to this reason, each operator defines different functional requirements for interlocking systems and procedures to guarantee safety of traffic. different requirements can affect conditions for compatible routes. conditions depend not only on infrastructure but also on equipment of rolling stocks. it is not sufficient to consider only occupation of track sections but locking of sections which are running – before running and after running during defined interval. route locking can have time and space aspect. the specific case is such running conflict when one train run through the station or it is coming to the platform and the arrival to the second platform is at the same level (there is no subway or upperbridge). if a conflict is detected it must be solved. while previous phases (train path prediction and conflict detection) can be defined, modeled and computer relatively exactly, the definition of the optimization criterion by conflicts solution is complicated and "soft". one criterion can be "accuracy of planned time timetable", i.e. minimum of total sum of train delays. afterwards, weights of several trains and stations must be defined (delay of ic train has greater impact). but, weight assignment trenches offering politics of carriers. energetic criterion can be more objective and better definable. power consumption is possible to be good computed. the final criterion function can be multicriterial and be of the weight sum form. requirement to minimize time of closing level-crossing can be included to the criterion function. the set of situation is large that can happen. modern systems are able to solve only several situations automatically and sophistically. it is always considered a human operator and automatic system cooperation. own intervention can be realized not only by time shift of train path but also/or by change of train route – only within the station (to the different line) or the new whole path. in the case of new path there are new constraints as knowledge of line by driver, ability to pass new line by the train, operation of stations, ... 3. implementation in railway laboratory railway laboratory was founded at the faculty of transportation sciences which serves as a place to simulate train traffic control and interlocking system. the laboratory is equipped by interlocking devices typically used in the czech republic – from the oldest (mechanical) ones up to modern (computer based) ones. so, the level of direct control is modeled realistically. the laboratory is extended by higher levels traffic control architecture, namely on operative level. applications are developed which reflect modern trends of rail traffic control, focusing on train path prediction, conflicts detection and their solution. the topology of model railway in railway laboratory was selected as infrastructure on which applications will be developed and tested. we analyzed current concepts of infrastructure description used in reality. the way used in the czech republic prefers time-table point of view. this way is not compatible with the format representing infrastructure from the interlocking architecture point of view. the railml.org initiative [1] tries to uniform the infrastructure description and defines railml standard. the railml standard is based on xml format which is generally used nowadays to store structured data. it is a text format consisting of elements that can be nested; element is a block bounded by tags enclosed in <>. typically, configuration files, some database files have xml structure (html file are types of xml too). the version railml 2.3 was released, the specification railml 3 is in progress – the version 3.0 is released only for internal use in initiative, the version 3.1 is supposed to be released in autumn 2017. the railml defines several subschemas to describe and store information of various railway subsystems and parts: timetable and rostering, infrastructure, rollingstock, interlocking and common. a fragment of railml code 2.3 describing track is shown bellow (the complete example is available at [2]) – fig. 1. experiences show that extraction of infrastructure and connections from railml 2.x is not so clear especially due to separate description of elements and their connections. the railtopomodel project exists concurrently to the railml and it is a logical object model to standardize the representation of railway infrastructure-related data and it uses railml 3.x; the schema railml 3.x is different from 2.x and much better. nevertheless, version 3, as mentioned above, is still in progress. therefore research team decided to utilize existing own xml format describing 32 vol. 11/2017 railway traffic optimization figure 1. description of connection using railml 2.x (source [2]). figure 2. optimization system and its inputs. infrastructure which is used for interlocking devices configuration in railway laboratory [3]. optimization of the timetable is understood dynamic change of timetable (including time shift and route changes) to solve conflicts in our conception, not creation of the new (for example, daily, annual timetable) one. let’s summarize inputs of the optimization system (fig. 2): infrastructure description, train description, timetable description. infrastructure of railway laboratory is described by own xml format, as mentioned above. the net is composed from several types of elements like: tracks sections, signals, insulated rail joints, switches. the model contains other elements which are omitted in our application due to irrelevancy: power supplies, derailers, track circuits. elements and their bindings are read from the file and an oriented graph is constructed in the computer memory. own library was implemented in c++ programming language due to its speed and effective work with pointers. the base of own xml parser to read infrastructure description is an open-source library. there are many general freeware libraries for xml format analysis. we chose xerces library which is also written in c++ and it holds dom (document object model) in its internal structures. it is supposed that railml will be widely applied in real railway in the future. similar parser of railml format can be created and it is not a big problem to program such software module (library) "railml parser" containing functions for the analysis of the railml files – just due to existence of free general xml parsers like xerces. train description has our proprietary format. parameters of specific trains are stored to compute their traction and another physical characteristics (maximal speed) allowing modeling of train move and change it: increment or decrement train speed as one possible solution of collisions. naturally, the railml format concept calculates to store rolling-stock description in xml. timetable is stored in mysql server in railway laboratory (the future is railml format as well) and software library to read the timetable was created. the database contains only stations passed through and arrival and departure times. amount of information is insufficient. it is not possible to derive uniquely train path (there exist more paths between stations very often) and times of track sections occupancy. the identification of collision is not possible without knowledge of this piece of information. the time-table database has to be extended to contain sequence of track sections passing by trains and intervals of occupancy. each track section is labeled during simulation if it is free, occupied or locked. conflicts are searched like intersections of time intervals. if conflict is detected time intervals can be changed (shifted, shortcut, stretched) by the change of the train speed with using train characteristics. conflict solution can be implemented by some heuristic (evolutionary technique). if the change of travelling time doesn’t lead to the conflict solution an alternative paths can be used (which can generate unfortunately additional conflicts). alternative path searching can be implemented by evolutionary technique too. the technique of travelling time change can be applied to alternative paths. the conflict is solved by the path change of one train in the simplest case. if it is not solved the area of changes is spread (to other trains). it is evident the task is very complicated and it must be solved in reality by parallel computation. 4. conclusion the effective railway traffic optimization and following control is very difficult task. it is necessary to have enough described infrastructure, time table and rolling stock and store the data in information systems. prepared railml format and railtopomodel appears to be very perspective. to optimize railway traffic dynamically, especially when unexpected events happen, it needs to predict train runs, search conflict and solve them, for example to shift runs in timetable or to plan alternative paths. the optimal solution can be searched by evolutionary techniques where optimization criterion can be to minimize total sum of train delays or minimize power consumption. parallel computation is supposed to be applied. references [1] railml.org. initiative [online]. [cit. 2017-03-08] "http://www.railml.org". 33 d. kamenický, t. brandejský, v. fábera, v. gopak acta polytechnica ctu proceedings [2] railtopomodel [online]. [cit. 2017-03-08] "http://www.railtopomodel.org". [3] p. koutecký. electronic signal box for ctu transport lab, diploma thesis. faculty of electrical engineering, 2011. 34 acta polytechnica ctu proceedings 11:31–34, 2017 1 railway traffic optimization trends 2 it support of operative control 3 implementation in railway laboratory 4 conclusion references 111 acta polytechnica ctu proceedings 2(1): 111–115, 2015 111 doi: 10.14311/app.2015.02.0111 flickering in cvs and accretion disc viscosity r. baptista1 1departamento de f́ısica, universidade federal de santa catarina, campus trindade, 88040-900, florianópolis, brazil corresponding author: raybap@gmail.com abstract i review observational constraints on accretion disc viscosity inferred from changes of disc structure with time and from disc flickering distributions. the radial run of the disc viscosity parameter in four cases are presented and discussed. keywords: cataclysmic variables dwarf novae accretion discs optical time-series photometry individual: v2051 ophiuchi, ht cassiopeia, v4140 sagitarii, uu aquarii. 1 context accretion discs are cosmic devices where angular momentum and gravitational energy are extracted from matter by an anomalous, still unknown viscosity mechanism, allowing it to be accreted onto a central star (frank, king & raine 2002). currently, the most promising explanation for the disc viscosity is related to magneto-hydrodynamic (mhd) turbulence in the differentially rotating disc gas (balbus & hawley 1991). from the observational standing point of view, because the properties of steady-state discs are largely independent of viscosity, one must turn to observations of timedependent disc behavior in order to obtain quantitative information about disc viscosity. here we adopt the prescription of shakura & sunyaev (1973) for the accretion disc viscosity, ν = αss csh, where αss is the nondimensional viscosity parameter, cs is the local sound speed and h is the disc scaleheight. non-magnetic1 cataclysmic variables (cvs) are excellent sites for studies of disc viscosity, because of their well constrained binary environment and because their accretion discs are usually the dominant light source at uv and optical wavelengths. in particular, the subclass of dwarf novae (dns) show recurrent outbursts in which their discs brighten by factors 20-100 as a consequence of mass and angular momentum redistribution on timescales of a few days. dn outbursts are explained in terms of either a thermal-viscous disc-instability (dim, lasota 2001) or a mass-transfer instability (mtim, bath 1975). dim predicts matter accumulates in a low viscosity disc (αcool ∼ 10−2) during quiescence, whereas in mtim the disc viscosity is always high (α ∼ 10−1). therefore, measuring α of a quiescent disc is key to infer which model is at work in a given dn. 2 flickering in cvs: a short review flickering are the intrinsic brightness fluctuations on timescales of seconds to dozens of minutes seen in light curves of t tau stars, mass-exchanging binaries and active galactic nuclei (bruch 2000 and references therein). the first step in understanding what causes flickering is to find out where is comes from. clues for the location of the flickering sources in cvs come from the analysis of eclipsing systems. in u gem, flickering is stronger at orbital hump maximum and disappears during the eclipse of the bright spot (bs) – where the mass transfer stream hits the outer edge of its accretion disc. these results led to the conclusion that flickering in this dn arises at the stream-disc impact region, because of either unsteady mass inflow (warner & nather 1971) or post-shock turbulence (shu 1976). on the other hand, the flickering in ht cas disappears during eclipse of the central source and recovers well before the egress of its bs (patterson 1981), indicating that it originates in the inner disc regions or at the wd-disc boundary layer (bl). possible explanations for this disc+bl flickering include mhd turbulence + convection (geertsema & achterberg 1992), unsteady wd accretion (bruch 1992) or events of magnetic reconnection at the disc atmosphere (kawagushi et al. 2000). a series of extensive optical flickering studies along the 90’s (bruch 1992, 1996, 2000) strengthened the idea that there are two different sources of flickering in cvs: in objects with strong anisotropic emission from the bs the bs-stream flickering component dominates, whereas the disc-bl flickering component prevails in objects where emission is mostly from the accretion disc. 1white dwarf (wd) surface magnetic fields bwd < 105 g. 111 http://dx.doi.org/10.14311/app.2015.02.0111 r. baptista the power density spectra (pds) of flickering sources show a characteristic power-law behaviour at higher frequencies (∝ f−n), with indexes ranging n = 1 − 2, and a flat slope below a cut-off frequency, fc (bruch 1992). the values of n and fc vary from one object to the other and for the same object at different brightness levels (fig. 1). figure 1: optical (b-band) power density spectra for v2051 oph (top) and ht cas (bottom) at two different brightness levels in their quiescent states (differences between bright, intermediate, and faint are 1-2 orders of magnitude lower than differences between outburst and quiescence). dashed lines show the best-fit powerlaw in each case; the corresponding power-law index n is depicted in each panel. 3 estimating the αss parameter 3.1 time changes in disc structure by measuring the (viscous) timescale tv = r/vr ' r2/ν ' r2/(αss csh) with which the disc responds to changes in mass input rate (frank et al. 2002), one might infer a spatially-averaged disc viscosity parameter αss, αss ' r cs tv [ h r ]−1 (1) where vr is the viscous radial drift speed. application of this time-lapse mapping technique to accretion discs of dn in outburst lead to αss ' 10−1 (e.g., baptista & catalán 2001). 3.2 flickering relative amplitude geertsema & achterberg (1992) investigated the effects of mhd turbulence in an accretion disc. they found that the convection of turbulent eddies lead to large fluctuations in the energy dissipation rate per unit area at the disc surface d(r), which could be a source of flickering in cvs and x-ray binaries. encouragingly, the pds of these fluctuations resemble those of flickering sources, with a power-law dependency of similar index range and the flat slope at low-frequencies. perhaps more important, their model gives a direct relation between the energy dissipation rate fluctuations and the disc viscosity parameter, providing an interesting observational way to infer the local accretion disc viscosity – by measuring the relative amplitude of the energy dissipation rate/flickering. in this model, the number of turbulent eddies that contribute to the local fluctuation is, n(r) = 4π r h ( h l )2 , (2) where l is the size of the largest turbulent eddies. the local rms value of the fluctuations σd(r) in the average energy dissipation rate per unit area 〈d(r)〉 is given by, σd(r) ' 2.5 〈d(r)〉/ √ n(r) , (3) while the disc viscosity parameter can be written as, αss = 3 νt 2 csh ' 0.9 ( l h )2 , (4) where νt is the local disc viscosity. if the disc-related flickering is caused by mhd turbulence, it is possible to infer αss from the relative flickering amplitude, σd/〈d〉, αss ' 0.23 [ r 50h ] [ σd(r) 0.05 〈d(r)〉 ]2 . (5) in the thin disc limit (h � r), disc regions with flickering amplitudes of a few percent already lead to large local αss values (≥ 10−1). in this scenario, cvs with highly viscous accretion discs are expected to show significant disc flickering component. how can we measure σd/〈d〉? from a large, uniform ensemble of light curves of a given cv it is possible to separate the steady-light component, lowand highfrequency flickering amplitudes as a function of binary phase, to derive corresponding maps of surface brightness distributions from their eclipse shapes and, thereafter, to compute the radial run of the relative amplitude of the disc flickering component (flickering mapping, see baptista & bortoletto 2004). 112 flickering in cvs and accretion disc viscosity figure 2: the radial run of the disc viscosity parameter inferred from disc flickering relative amplitude distributions, for three strong flicker dns and for the nova-like variable uu aqr. dots with solid line show average αss values, while dashed lines indicate their 1-σ limits. horizontal dashed lines depict the typical αcool = 10−2 value expected for quiescent dn discs according to dim. 4 results and discussion baptista et al. (2011) analyzed an extensive data set of optical light curves of ht cas. their observations frame a 2 d transition from a low state (largely reduced mass transfer rate) back to quiescence, allowing the application of both time-lapse and eclipse mapping techniques to estimate αss and to compare the independently derived results. they find that, in the low state, the gas stream hits the disc at the circularization radius rcirc, and the accretion disc has its smallest possible size. the disc fast viscous response to the onset of mass transfer, increasing its brightness and expanding its outer radius at a speed v ' +0.4 kms−1, implies αss ' 0.3 − 0.5. the newly added disc gas reaches the wd at disc centre soon after mass transfer recovery (∼ 1 d), also implying a large disc viscosity parameter, αss ' 0.5. flickering mapping reveal a minor bs-stream flickering component in the outer disc, and a main disc flickering component the amplitude of which rises sharply towards disc centre, leading to a radial dependency αss(r) ∝ r−2, and to large values αss > 10−1 for r < rcirc (fig. 2) – in agreement with the time-lapse results. a similar analysis was performed for the dn v4140 sgr (baptista et al. 2012). standard eclipse mapping in quiescence indicate that the steady-light is dominated by emission from an extended disc with negligible contribution from the wd, suggesting that efficient accretion through a high-viscosity disc is taking place. flickering maps show an asymmetric source at disc rim (bs-stream flickering) and an extended central source (disc flickering) several times larger in radius than the wd at disc centre. unless the thin disc approximation breaks down, the relative amplitude of the disc flickering leads to large αss’s in the inner disc regions (' 0.15 − 0.3), which decrease with increasing radius. flickering mapping of the dn v2051 oph reveals that the low-frequency flickering arises mainly in an overflowing gas stream and is associated with the mass transfer process. the high-frequency flickering originates in the accretion disc and has a relative amplitude of a few percent, independent of disc radius and brightness level, leading to large αss ' 0.1 − 0.2 at all disc radii (baptista & bortoletto 2004). in uu aqr, optical flickering arises mainly in tidallyinduced spiral shocks in its outer disc (baptista & bortoletto 2008). assuming that the turbulent disc model applies, its disc viscosity parameter increases outwards and reaches αss ∼ 0.5 at the position of the shocks, sug113 r. baptista gesting that they might be an effective source of angular momentum removal of disc gas. since αss increases by two order of magnitude with increasing radius, it is not surprizing that dobrotka et al. (2012) were not able to reproduce the observed pds of uu aqr with a turbulent disc model of constant αss. 5 summary and future steps the picture that emerges from flickering mapping experiments of three dns and of the nova-like variable uu aqr underscores earlier results, indicating that there are mainly two sources of flickering in cvs: the stream-disc impact region in the outer disc and a turbulent inner accretion disc, the relative importance of which varies from system to system. in combination with an mhd turbulent disc model, flickering mapping provides a powerful probe of accretion disc viscosity by allowing one to derive the local magnitude and the radial run of αss from the distribution of the disc flickering relative amplitude. the large αss values found for the three quiescent dns are critical for the outburst mechanism dispute. they are at odds with dim and indicate that the outbursts of a group of dn (strong flickers) are powered by mass-transfer instabilities. two questions related to this result seem to demand further theory development. the first one is why (and how) mass transfer from the donor star in cvs is unstable? the second one is what is responsible for the difference between the low-viscosity discs of quiescent dim-driven dwarf nova and the highviscosity discs of their mtim-driven counterparts? a good first step towards solving this problem might be asking the related question of what is the influence of the wd magnetic field on the radial distribution of αss (e.g., see bisikalo 2014)? an important test of the mhd turbulent disc model still to be done is to perform a flickering mapping experiment on a bona-fide dim-driven dn to check whether it has a low-viscosity accretion disc (as predicted by dim) and if αss is independent of radius (as generally assumed by dim). acknowledgement this work is supported by cnpq/brazil grant 302.443/2008-8. references [1] baptista, r., catalán, m.s.: 2001, mnras, 324, 599 doi:10.1046/j.1365-8711.2001.04320.x [2] baptista, r., bortoletto, a.: 2004, aj, 128, 411 [3] baptista, r., bortoletto, a.: 2008, apj, 676, 1240 doi:10.1086/528706 [4] baptista, r. et al.: 2011, in the physics of accreting binaries, universal academic press (arxiv 1105.1382) [5] baptista, r., borges, b., oliveira, a.: 2012, iau symposium 285, r.e.m. griffin, r.j hanish & r. seaman (eds.), cambridge univ. press, 278 [6] balbus, s.a., hawley, j.f.: 1991, apj, 376, 214 doi:10.1086/170270 [7] bath, g.t.: 1975, mnras, 171, 311 doi:10.1093/mnras/171.2.311 [8] bisikalo, d.: 2014, this proceedings [9] bobinger, a. et al.: 1997, a&a, 327, 1023 [10] bruch, a.: 1992. a&a, 266, 217 [11] bruch, a.: 1996. a&a, 312, 97 [12] bruch, a.: 2000. a&a, 359, 998 [13] dobrotka, a., mineshige, s., casares, j.: 2012, mnras, 420, 2467 doi:10.1111/j.1365-2966.2011.20210.x [14] frank, j., king, a., raine, d.: 2002, accretion power in astrophysics, cambridge univ. press doi:10.1017/cbo9781139164245 [15] geertsema, g.t., achterberg, a.: 1992, a&a, 255, 427 [16] kawagushi, t., et al.: 2000, pasj, 52, l1 [17] lasota, j.p.: 2001, new astronomy review, 45, 449 doi:10.1016/s1387-6473(01)00112-9 [18] patterson, j.: 1981, apjs, 45, 517 doi:10.1086/190723 [19] shakura, n.i., sunyaev, r.a.: 1973, a&a, 24, 337 [20] shu, f.h.: 1976, in structure and evolution of close binary systems, p. eggleton, s. mitton & j. whelan (eds.), dordrecht, 253 [21] warner, b., nather, r.e.: 1971, mnras, 152, 219 doi:10.1093/mnras/152.2.219 discussion john cannizzo: what is the theoretical expression used in determining αss from the speed of the cooling front? raymundo baptista: inferences of αss from time-lapse mapping of outbursting dns are generally 114 http://dx.doi.org/10.1046/j.1365-8711.2001.04320.x http://dx.doi.org/10.1086/528706 http://dx.doi.org/10.1086/170270 http://dx.doi.org/10.1093/mnras/171.2.311 http://dx.doi.org/10.1111/j.1365-2966.2011.20210.x http://dx.doi.org/10.1017/cbo9781139164245 http://dx.doi.org/10.1016/s1387-6473(01)00112-9 http://dx.doi.org/10.1086/190723 http://dx.doi.org/10.1093/mnras/152.2.219 flickering in cvs and accretion disc viscosity based on measurements of the speed of the heating wave during rise, instead of the cooling wave. [note added: bobinger et al. (1997) adopted the expression αcool = vcool/cs to estimate the disc viscosity parameter from their measured speed of the cooling wave vcool. this expression lead to α values smaller than derived from eq. (1) by a factor (h/r).] 115 context flickering in cvs: a short review estimating the ss parameter time changes in disc structure flickering relative amplitude results and discussion summary and future steps acta polytechnica ctu proceedings doi:10.14311/app.2017.10.0038 acta polytechnica ctu proceedings 10:38–42, 2017 © czech technical university in prague, 2017 available online at http://ojs.cvut.cz/ojs/index.php/app temperature of soil monitored at experimental sites in perm region (russia) andrey ponomaryov∗, aleksandr zakharov perm national research polytechnic university, kuibysheva street 109, perm, russia ∗ corresponding author: spstf@pstu.ru abstract. the paper presents the monitoring results for soil temperatures in the geological and climatic conditions of the city of perm (russia). the monitoring results are presented for two experimental sites. site 1 is characterized by dense urban development and related to type i soil conditions in perm (the left-bank area of perm). site 2 is located in a low-urbanised region of the city and is related to type ii soil conditions in perm (the right-bank area of perm). keywords: temperature of soil, monitoring site, energy-efficient foundations. 1. introduction the department of construction technology and geotechnics of perm national research polytechnic university has conducted studies of the temperature fields on ground bases in the city of perm since 2009. research is being carried out to quantify the thermal behavior of soil for the main types of geotechnical condition. quantitative assessment of subgrade temperature is one of the main factors allowing introduction of a process based on the use of the soil thermal energy. in particular, these are the energy-efficient foundations and underground structures of buildings. as a rule, technologies based on the use of soil thermal energy are employed for the heating and cooling of buildings, at the very least to provide electricity and hot water. these technologies have been widely used in european and neighboring countries [1–5]. their introduction in russia at this point is not yet much in evidence. 2. geological section of experimental sites research of temperature fields on ground bases are being carried out for two basic types of geotechnical conditions specific to the left-bank and right-bank areas of perm city. studies are being conducted at two sites: • site 1 the monitoring system is installed at the civil engineering faculty pnrpu in the sverdlovsky district of perm (left bank of the city). monitoring has been carried out continuously from december 2008 to the present time. site 1 is characterized by dense urban development. the development age is more than 50 years. the distance from the observation well to the nearest building is about 3 m. taking the results of a geological engineering survey, experimental site 1 is comprised of quaternary alluvialdealluvial clayey soil of a total thickness of 11.6 m, with up to 60-70 % of pebbles at the bottom. quaternary soils are overlaid by filled-up ground of thickness 6.0 m. filled-up ground is represented by clay soil of semi-solid to plastic consistency at the base with an admixture of 60-70 % of construction waste (rubble, broken bricks, glass, wood). the bedrock is argillites uncovered at a depth of 17.6 m. quaternary alluvial-dealluvial deposits are mainly represented by clay of solid to semi-solid consistency underlying the pebble with loamy sand of solid consistency (the content of gravel and pebbles being 60-70 %). the geological section is shown in fig. 1a. • site 2 the monitoring system is installed on the territory of the university campus in the leninsky district of perm (the right bank of the city). monitoring has been carried out since august 2015. currently monitoring data has been obtained up to january 2016. site 2 is located on the undeveloped area of the city. the distance to the nearest building is more than 30 m. according to the results of archival research of the surrounding area, site 1 is comprised of quaternary alluvial sand and clay of a total thickness of 15 m, with 25 % of gravel at the bottom. quaternary alluvial deposits are represented by fine sand, clay and clay soils of plastic to liquid consistency. the gravel inclusion was marked from a depth of 1.0 m along the full thickness of the alluvial soils with an increase to the base up to 25 %. according to the results of archive research the bedrock was uncovered at a depth of 17.0 m. the geological section is shown in fig. 1b. according to the results of geotechnical studies sites 38 http://dx.doi.org/10.14311/app.2017.10.0038 http://ojs.cvut.cz/ojs/index.php/app vol. 10/2017 temperature of soil monitored at experimental sites figure 1. geological section of experimental sites 1 and 2 are assigned to the i and ii types of geotechnical conditions respectively, specific to perm [6]. 3. monitoring results of the temperature fields studies of the temperature field distribution in the soil were carried out by installing a resistive temperature transducer (temperature sensors). installation of this resistive temperature transducer was made in a predrilled hole with a protective casing. to gather the data rtm 59 loggers were used for measuring, continuous recording and monitoring of temperature and other non-electrical values (frequency, pressure, flow, level, etc.), converted into electrical signals of strength, dc voltage and dc resistance [7]. the thickness of the observed soil mass on the site 1 is 19 m. the temperature sensors were installed at intervals of 1 m. monitoring of temperature fields has been held for more than 5 years on site 1. the summarized monitoring results (average monthly temperatures for the period from january 2009 to january 2016) are shown in fig. 2. as given in the diagram the temperature of the soil mass from a depth of 8-9 m is practically independent of the seasonal outdoor temperature fluctuations. the temperature is about 12 °c, falling to 10 °c to the depth of 19 m. to analyze the temperature fluctuations for several annual cycles the average monthly temperatures in september over the period 2009-2015 is shown in fig. 3. as given in the diagram the temperature of the soil mass from a depth of 8-9 m is constant (the difference is not more than 1 °c) during 6 years of monitoring. soil temperatures in september 2015 are explained by abnormally low outside temperatures in summer 2015. the thickness of the observed soil mass on the site 2 is 37 m. the temperature sensors were installed 39 andrey ponomaryov, aleksandr zakharov acta polytechnica ctu proceedings figure 2. site 1. diagram of average monthly temperatures of soil mass according the monitoring results from 2009 to 2016 figure 3. site 1. diagram of average monthly temperatures of soil mass in september for the 2009-2015 at intervals of 2 m, to a depth of 3 m with intervals equal to 0.5 m. monitoring of the temperature fields on site 2 has been carried out since august 2015. currently monitoring data of temperature distribution in the soil mass has been obtained up to january 2016. monitoring results (average monthly temperatures) are shown in fig. 4. analysis of monitoring results on site 2 displays the temperature of soil mass from a depth of 9 m as constant and equal to 6-7 °c. for further analysis the average monthly temperatures of soil mass for september 2015 are shown in table 1 and fig. 5 for both sites. 4. conclusions based on the monitoring of the temperature regime of soils, conclusions are drawn: (1.) the temperature of the ground for the second site is + 6-7 °c and does not change over a depth of 9 m. (2.) the temperature of the ground of the first site is + 10-12 °c and does not change during the year from a depth of 9 m. with the depth reading, the temperature decreases. (3.) on the temperature regime of soils, the density of development of the territory under study has a significant influence. the higher temperatures of the ground massif of the first site are due to the proximity of the building of the faculty of civil engineering with a heated basement. 40 vol. 10/2017 temperature of soil monitored at experimental sites figure 4. site 2. diagram of average monthly temperatures of soil mass according the monitoring results from august 2015 to january 2016 figure 5. diagram of average monthly temperatures in september for sites 1 and 2 table 1. average monthly temperatures of soil mass in september for sites 1 and 2 41 andrey ponomaryov, aleksandr zakharov acta polytechnica ctu proceedings references [1] h. brandl, d. adam, r. markiewicz. ground-sourced energy wells for heating and cooling of buildings. acta geotechnica slovenica 3(1):5–27, 2006. [2] r. katzenbach, d. adam, t. waberseck. on the geotechnical conditions of construction in the city of perm. problems of soil mechanics and foundation engineering in difficult ground conditions. in proc. of the international scientific conference dedicated to the 50th anniversary of bashniistroy 3-5 october. ufa: bashniistroy, 2006. [3] v. g. shapoval, b. v. morklyanyk. the temperature field in the soil base of heat pumps: monograph. dnepropetrovsk: porohy, 2011. [4] a. b. ponomaryov, a. v. zakharov. use of geothermal energy of soils for heating and air-conditioning of buildings. bulletin of volgograd state university of architecture and civil engineering 17(36):119–122, 2010. [5] a. zakharov. application of geothermal energy of the ground for heating of buildings in the climatic and engineering geological conditions of the perm region. bulletin of civil engineers 2 (23) 2(23):85–89, 2010. [6] r. katzenbach, t. waberseck, d. adam, a. ennigkeit. innovationen bei der nutzung geothermischer energie durch erdberührte bauwerke, wie z.b. pfahlgründungen mittels energiepfählen. geothermie-symposium "erdwärme energieträger der zukunft", 25.11.2002 bremerhaven / bremerhavener energiemanagement-agentur. 2002. [7] a. b. ponomaryov, a. v. zakharov. analysis of the interaction of energy foundations in the geological and climatic conditions of the city of perm. pnrpu bulletin 4:24–33, 2011. 42 acta polytechnica ctu proceedings 10:39–43, 2017 1 introduction 2 geological section of experimental sites 3 monitoring results of the temperature fields 4 conclusions references acta polytechnica ctu proceedings doi:10.14311/ap.2016.4.0056 acta polytechnica ctu proceedings 4:56–61, 2016 © czech technical university in prague, 2016 available online at http://ojs.cvut.cz/ojs/index.php/app proton radiography with the pixel detector timepix václav olšanskýa,b,∗, pavel krista, jiří bílab, carlos granjac, radek maříkd, david chvátila a nuclear physics institute of the czech academy of sciences, husinec-řež, czech republic b faculty of mechanical engineering, czech technical university in prague, czech republic c institute of experimental and applied physics, czech technical university in prague, czech republic d faculty of electrical engineering, czech technical university in prague, czech republic ∗ corresponding author: olsansky@ujf.cas.cz abstract. this article presents the processing of radiographic data acquired using the positionsensitive hybrid semiconductor pixel detector timepix. measurements have been made on thin samples at the medical ion-synchrotron hit [1] in heidelberg (germany) with a 221 mev proton beam. the image contrast is generated by the energy of charged particles imparted after passing through the sample. experimental data from the detector were processed for the establishment of the energy loss of each proton using calibration matrices. the interaction point of the proton on the detector was determined with subpixel resolution by a model fitting of the individual signals in the pixel matrix. three methods have been used for the calculation of these coordinates: hough transformation, 2d gaussian fitting and estimate of the 2d mean. parameters of the calculation accuracy and the calculation time are compared for each method. the final image was created by the method with the best parameters. keywords: radiography, charged particles, timepix, medipix, image processing. 1. introduction radiography is often associated with the x-ray radiography, where the image is formed by an attenuation of the beam after passing through the sample. the image is recorded on a radiologic film, a foil or a memory area detector [2]. the process is similar to conventional photography, where negative is created according to radiation intensity. in contrast to this principle proton radiography can get the imaging information about one pixel with only one particle that has passed through this point [3]. therefore it is necessary to know the energy of the particle. thus the principle of proton radiography is to determine the energy of each particle after passing through the analyzed sample. hence proton radiography is suitable for imaging thin samples. the advantages of proton radiography are high sensitivity, high spatial resolution imaging of thin samples and the imaged objects receiving a lower dose [4]. although the absorbed dose by a single proton is significantly higher than that of a photon, the total radiation dose is reduced due to the need for only a single proton to display a single pixel [5, 6]. the aim is to create a system of automatic image processing of proton radiography. proton radiography, unlike traditional x-ray radiography, offers new possibilities and specific imaging thin samples, which can achieve greater sensitivity and spatial resolution. 2. pixel detector timepix 2.1. hybrid semiconductor detector timepix the timepix detector [7] is a position-sensitive semiconductor pixel detector of the family of medipix detectors developed at cern [8]. timepix, based on the previous device medipix2 [9], timepix provides perpixel energy sensitivity. the detector consists of a radiation sensitive semiconductor sensor bump-bonded to the timepix asic readout chip. the sensor can be of different material (si, cdte, gaas) and thickness (e.g. 300, 700, 1000, 1500 µm). the detector provides an array of 256 × 256 pixels for a total of 65 536 independent channels. size of one pixel is 55 µm and the full sensor size is 14 × 14 mm (1.98 cm2). each pixel is connected with its analog and digital signal chain including amplifier, discriminator, counter and digital integrator [7]. 2.2. timepix principle ionizing particles produce in the sensor a cloud of charge which is collected as in a standard semiconductor diode. the collection of charges undergoes charge sharing (due to diffusion) and results in a signal containing several pixels forming a so-called cluster of pixels [10]. the distribution of charge around the impact point has the character of a gaussian. data are recorded in frames which are saved to individual file frames which are time stamped. all data of events are saved to one frame during the selected time. events 56 http://dx.doi.org/10.14311/ap.2016.4.0056 http://ojs.cvut.cz/ojs/index.php/app vol. 4/2016 proton radiography with the pixel detector timepix figure 1. one frame with cropped selection. the left figure shows one frame of radiographic data. this frame contains 9 suitable events and few unwanted values which are recorded by x-rays. three suitable events and three unwanted values are shown on the right figure, where it is displayed the cropped area from left figure. which have been recorded at next selected time are saved to next frame. the detector pixels can be independently operated in three ways depending on the mode in which the detector works [7]: • medipix mode: each event is counted in this mode. it can be used to count the event rate. • timepix mode: value which is recorded in this mode corresponds to the time of interaction during the frame. • time over threshold (tot) mode: the time registers the amount of charge which is given by the energy deposited. data which are measured by mode tot is necessary to calibrate into energy. calibration matrices are used for this calculation [11]. four calibration matrices are used a, b, c and t. the calibration coefficients are obtained by calibration procedure using discrete-energy x-rays from laboratory x-ray sources on xrf [11–13]. the energy was calculated according to equation x = ta + y − b + √ (b + ta − y )2 + 4ac 2a , where x is the matrix with pixel values in energy in [kev], y is the matrix which contains pixel values recorded in [tot] counts and matrices a, b, c and t contain calibration values. 3. experiment and data processing 3.1. description of experimental setup and radiographic data radiographic data used in this work were measured at the hit ion synchrotron [1] in heidelberg (germany). the sample was irradiated by almost monoenergetic proton beam with energy 221 mev. a sample consisting thin foils which were stacked one over the other and a fly’s wing were used a sample. the sample was irradiated in air. the timepix detector which worked at tot mode was placed behind the sample. protons were registered by the detector after passing through the sample. data from the detector were structured into 34 files. each of these files contained around 1000 frames. the duration of one frame was 0.2 seconds. the value of tot and coordinate of each pixel where the threshold had been exceeded were stored. the average number of protons events per frame was about 10 events per frame. the impact of proton on detector creates a round track with a diameter around 7 pixels as shown in fig. 1. this event has as character so called cluster (see fig. 1 and fig. 2) that can be fitted by a 2dgaussian. the highest tot values are located in the center of the track (fig. 2). the position of impact on the detector can be accurately determined based on analysis and fit of all pixels of the event recorded. sub-pixel resolution can be achieved as well. the total energy deposited to the detector by one proton corresponds to the sum of the energy values of all pixels in the cluster. 3.2. data processing methods and their comparasion because all data that were contained in the files were not suitable for processing, it was necessary to select a suitable event record, to avoid unadvisable events and prepare the data for further processing. 2d filtering was used for eliminating unwanted and false events. nonzero pixels were calibrated using energy calibration matrices. furthermore approximate coordinates of each protons impact have been found. an area of 11 × 11 pixels was cropped around these approximate coordinates. because the diameter of the event record was about 7 pixels, it was probable that each pixel corresponding to this event record was included in this cropped area. then the area was evaluated whether it would be suitable for further processing. if two or 57 v. olšanský, p. krist, j. bíla et al. acta polytechnica ctu proceedings figure 2. registration of a proton event in timepix operated in tot (energy mode). the color and height of bars represents the energy of one pixel of the detector. more event records overlapped at one cropped area, this area was not suitable for further work. it was necessary to accurately determine the location of protons impact for image reconstruction. three methods were used for calculation of the precise location of the impact of the proton: hough transform [14], 2d gaussian fitting [15] and estimation of 2d mean [16]. these methods were mutually compared and the reconstruction of image was performed from the results of the most suitable method. hough transform. the detection of the circle, described by equations (1), was used for assessing the center of the event by hough transformation. the idea was in presumption that each event record had a circular nature. a matrix with data of the one frame was converted into a binary map. all contour points of event record were found by using the edge detector. coordinates of circles points which had the same radius as a radius of event records were calculated from the coordinates of each edge point. eighty points of circle coordinates were calculated for each edge point. the value of one was added at each point, whose coordinates were calculated in this manner. maximum of added values was subsequently calculated for each event. there is the impact point. equations (1) describes the calculation of the circles coordinates with center at each edge points, x(θ) = x0 + r cos θ, y(θ) = y0 + r sin θ, (1) where x0, y0 are coordinates of edge points, θ is an angle from 0 to 2π. step of angle θ calculation was chosen up to 1/80 of the circle. variables x(θ), y(θ) are coordinates of the new circle for one angle θ, r is the radius of the circle [14]. 2d gaussian fitting. gaussian fitting was used for calculating center coordinates the impact of one proton as another way. it was performed for only cut selection around traces of impacts. data were fitted to 2d gaussian with equation f(x, y) = c1 + c2 exp ( − ( x− c3 c4 )2 − ( y − c5 c6 )2 ) , where c3 a c5 are coordinates of the center of 2d gaussian. it corresponds to the location of a proton impact. coefficient c1 corresponds to dislocation, c2 is amplitude and c4 and c6 corresponds to spread of the blob. estimation of 2d mean. estimate of 2d mean was used as the last method for determination center of events records. the energy values of events records were normalized, so that the total sum of all values was equal to one. first of all the normalized values were added up vertically. then the results of these sums were multiplied by the respective horizontal coordinates. finally these values were added up too. this result was one number only and it was the horizontal center coordinate. the same method was used for the calculation of the vertical center coordinates. 4. results 4.1. comparision of methods methods have been mutually compared. similar accuracy was reached using methods estimating 2d mean and 2d gaussian fitting. calculation of the estimate of 2d mean value was about 30 times faster than the 2d gaussian fitting. calculation using the hough transform amounted to less accuracy, and it was also significantly slower than the calculation of 2d mean. comparing of the average euclidean distance between the locations of particles impacts detected using each method and their standard deviations are shown by using each method in table 1. compared methods avg. eukl. dist. std [px] [px] gaussian fitting vs. estimate of 2d mean 0.107 0.057 hough transform vs. gaussian fitting 0.749 0.302 estimate of 2d mean vs. hough transform 0.729 0.295 table 1. comparison of euclidean distance between the location of protons impacts and its standard deviation. it is evident from table 1 that similarly accurate results were achieved using methods: 2d gaussian fitting and the estimation of 2d mean. mean euclidean distance between of these methods was caused by mutual displacement in one direction. this shift 58 vol. 4/2016 proton radiography with the pixel detector timepix was caused slight displacement of the resulting reconstructed image only. the representation of centers of events records using different methods is shown in fig. 3. histograms which are shown in fig. 4 and 5 were created on the basis of the calculation euclidean distances between the centers coordinates of events records detected using each method. figure 3. centroid fitting for one event recorded. the calculated centroid coordinates are shown in the middle of this figure. data1 shows centered calculated using the 2d gaussian fitting, data2 shows calculated center using the estimation of 2d mean and a data3 shows the detected center using hough transform. figure 4. histogram euclidean distances between centroids computed by gaussian fitting and estimation the mean 2d. 4.2. image reconstruction the final image was created from centers coordinates of each trace of impact whose calculation was described in the previous section. this image was created from the sum of the total energy. the value of the total energy which was given to the detector by a proton was calculated as the sum of all energy values of one event record. this value was assigned figure 5. histogram euclidean distances between centroids computed by gaussian fitting and hough transform. to the point whose coordinates have been detected as the place of protons impact. if one pixel of the final image was located for more events than one, the average value of these energetic records was assigned this point. points without localized event are assigned zero value. these points appeared as holes in final image. values for these points can be improved in the final image by extrapolating values from the surrounding area. just one proton can suffice for displaying a pixel. each proton passed through the sample carries the information about a point of the sample. the final image can be displayed magnified due to the subpixel resolution of located impacts coordinates. the picture created from all the 33 000 frames was refined at double zoom. localized have been around 340 000 events. this corresponds to an average of 1.3 protons per pixel. an example of such image is shown in fig. 6. 5. discussion and conclusion the resulting images, which were reconstructed from data obtained using the methods of 2d gaussian fitting and estimation of 2d median values were essentially identical. to display a single image pixel was sufficient to locate the coordinates of only one proton impact. because here we do not measure the decline of are irradiance detector, but directly the decrease of the energy particles is achieved by lower radiation load samples compared with other radiological imaging methods for example x-ray radiography. experimentally it was found that it was ideal to enlarge final image twice. in this view, it may well calculate the value of the missing pixel, because in the vicinity of missing values there are many pixels designed so as to have one missing value extrapolated. the algorithm was created for a complete data processing. the data were processed automatically starting from 59 v. olšanský, p. krist, j. bíla et al. acta polytechnica ctu proceedings figure 6. final radiogram. proton radiography of a composite sample consisting of an assembled foil array and a fly’s wing. the final image was made from coordinate locations of proton impacts using estimating the 2d mean. one pixel represents 22.5 × 22.5microns. there was located on the average 1.3 protons per pixel. the records of the detector and the resulting final image. another advantage of proton radiography in addition to lower radiation burden is the availability of application accelerators. for the future it is planned to apply radiography using charged particle accelerators van de graaff hv2500 – ieap ctu [17] and cyclotron u-120m – npi ascr [18], which would also examine pokes energy charged particles after passing the samples. 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[18] department of accelerators. isochronous cyclotron u-120m. http://accs.ujf.cas.cz/. [19] a. belov, d. chavatil, c. simane, m. vognar. electron and gamma bremsstrahlung beams of jinr and ctu microtrons. czechoslovak journal of physics 50(1):385–386, 2000. doi:10.1007/s10582-000-0079-9. [20] c. granja, p. krist, d. chvatil, et al. energy loss and online directional track visualization of fast electrons with the pixel detector timepix. radiation measurements 59:245–261, 2013. doi:http://dx.doi.org/10.1016/j.radmeas.2013.07.006. [21] f. merrill, f. harmon, a. hunt, et al. electron radiography. nuclear instruments and methods in physics research section b: beam interactions with materials and atoms 261(1–2):382–386, 2007. doi:http://dx.doi.org/10.1016/j.nimb.2007.04.127. 61 http://accs.ujf.cas.cz/ http://dx.doi.org/10.1007/s10582-000-0079-9 http://dx.doi.org/http://dx.doi.org/10.1016/j.radmeas.2013.07.006 http://dx.doi.org/http://dx.doi.org/10.1016/j.nimb.2007.04.127 acta polytechnica ctu proceedings 4:56–61, 2016 1 introduction 2 pixel detector timepix 2.1 hybrid semiconductor detector timepix 2.2 timepix principle 3 experiment and data processing 3.1 description of experimental setup and radiographic data 3.2 data processing methods and their comparasion 4 results 4.1 comparision of methods 4.2 image reconstruction 5 discussion and conclusion references acta polytechnica ctu proceedings doi:10.14311/ap.2016.4.0107 acta polytechnica ctu proceedings 4:107–112, 2016 © czech technical university in prague, 2016 available online at http://ojs.cvut.cz/ojs/index.php/app comparison of s-co2 power cycles for nuclear energy ladislav veselya,∗, vaclav dostala, slavomir entlerb a faculty of mechanical engineering, czech technical university in prague, technická 4, 166 07 prague 6, czech republic b institute of plasma physics of the cas, za slovankou 3, 182 00 prague 8, czech republic ∗ corresponding author: ladislav.vesely@fs.cvut.cz abstract. the supercritical carbon dioxide (s-co2) is a possible cooling system for the new generations of nuclear reactors and fusion reactors. the s-co2 power cycles have several advantages over other possible coolants such as water and helium. the advantages are the compression work, which is lower than in the case of helium, near the critical point and the s-co2 is more compact than water and helium. the disadvantage is so called pinch point which occurs in the regenerative heat exchanger. the pinch point can be eliminated by an arrangement of the cycle or using a mixture of co2. this paper describes the s-co2 power cycles for nuclear fission and fusion reactors. keywords: s-co2, power cycle, fusion. 1. introduction the supercritical carbon dioxide (s-co) cycles are recently very prospective power cycles for different applications. these applications are ranging from nuclear through geothermal, solar energy and waste heat recovery systems. these cycles are researched all around the world. the research of the power cycles with co2 as working medium has a long history. the first reference is dated back to 1948, when sulzer bros. patented a brayton cycle with the partial condensation of co2. the worldwide research of s-co2 power cycles is dating to the second half of the 20th century [1]. researchers began to realize benefits of co2 as working medium in power cycles at that time. among the first researchers who studied benefits of co2 power cycles belongs angelino, feher, verhivker and gokhstein [2]. the research on czech technical university in prague (ctu) is oriented on the analysis of the s-co2 power cycles with a potential for the nuclear reactors, as well as for the fusion power reactors. the design of the s-co2 power cycle is very important and it has effect on the cycle efficiency and net power. the s-co2 cycle is also suitable for utilization of heat from multiple heat sources with different temperature and heat power. this is important in a case of multiple heat sources providing high and low potential heat like fusion power reactors. this paper is focused on comparison of the s-co2 cycles for nuclear energy. benefits of the s-co2 cycle will be described for the nuclear fission and fusion reactors. the design of the s-co2 cycle will be applied for multiple heat source in fusion reactor. 2. advantages and disadvantages of s-co2 cycles the main advantage of the s-co2 cycles is the compression work which is lower than in case of helium [2]. a compressor work reduction is caused due to operation in near the critical point. the critical point of co2 occurs at the temperature 30.98 ◦c and pressure 7.32 mpa. the s-co2 cycle is more compact than water and helium because this cycle operating at high pressure and allows small size of components. another advantage is that the s-co2 cycle achieves high efficiency with low operates temperature. the s-co2 cycles also have several disadvantages. the existence of so called “pinch point” in heat exchangers significantly affecting their design is the most important and well-known disadvantage. the pinch point may be present for any type of medium, but its influence on components is especially high when co2 is employed as a working medium. the pinch point primarily occurs in recuperative heat exchangers with identical working media and mass flow on both the hot and the cold side. the pinch point is caused by the variations of heat capacity of co2 and occurs when the heat capacity of the hot and cold streams (each at a different pressure level) intersect. due to the pinch point, the heat exchangers may have a large size and low efficiency. however, this problem can be removed in several ways. one of them is an addition of the small amount of other substance into the pure co2. the substances for shift of pinch point could be ar, he, co, o2 or n2 [3]. the other ways is a change of the design of cycles and usage of different mass flows in hot and cold side of heat exchangers. 107 http://dx.doi.org/10.14311/ap.2016.4.0107 http://ojs.cvut.cz/ojs/index.php/app l. vesely, v. dostal, s. entler acta polytechnica ctu proceedings 3. description of gas cycles the s-co2 cycle is a gas cycle derived from the ericsson-brayton cycle, which offers many different layouts for solar, geothermal or nuclear power plants and waste heat recovery. each layout tries to approach the carnot cycle and its efficiency. the basic layouts considering the use of s-co2 are [2]: • simple brayton cycle, • re-compression cycle, • pre-compression cycle, • split expansion cycle, • partial cooling cycle, • partial cooling with improved regeneration. figure 1. simple brayton cycle. the simple brayton cycle is shown in figure 1. this cycle has a turbine (t), compressors (c1), recuperative heat exchanger (rh), a cooler (c) and a heater (h). figure 2. re-compression cycle. the re-compresion cycle is shown in figure 2. the difference between the simple brayton cycle and recompression cycle is twice a number of the compressors and recuperative heat exchanger. the re-compression cycle has a turbine (t), two compressors (c1 and c2), two recuperative heat exchangers (ltr and htr), cooler (c) and heater (h). in figure 3, the pre-compression cycle is shown, and figure 4 presents the split expansion cycle. the pre-compression cycle and the split expansion cycle have same components as the re-compression cycle. only two turbines has the split expansion cycle. figure 3. pre-compression cycle. figure 4. split expansion cycle. the partial cooling cycle is shown in figure 5. the partial cooling cycle contains the two coolers (ca and cb) and three compressors. figure 5. partial cooling cycle. 108 vol. 4/2016 comparison of s-co2 power cycles for nuclear energy 4. the gfr re-compressing cycle the s-co2 cycles for nuclear energy are based on the presented cycles of co2. the re-compression cycle applied to the gas-cooled fast reactors (gfr) is analyzed in this study. the re-compression cycle is used as basic concept for gfr. other type of s-co2 cycle has similar results as the re-compression cycle [4]. the cycle layout is arranged according to the figure 2. the ltr exchanger is sensitive to the pinch point due to operation near the critical point. the recompression cycle eliminates the pinch point using different mass flows of the ltr exchanger. the parameters for calculation are shown in table 1. the compressor inlet temperature is 34 ◦c. the turbine inlet temperature is 550 ◦c. the heat source is considered with the minimum inlet temperature into the heater about 600 ◦c. the thermal power is 600 mw [5]. the cycle was optimized for the best parameters with thermal power 600 mw. thermal power 600 mw compressor efficiency 68 % turbine efficiency 79 % recuperator effectiveness 90 % compressor inlet temperature 34 ◦c turbine inlet temperature 550 ◦c table 1. parameter of the gfr s-co2. 5. the s-co2 cycle for demo2 fusion reactor in the case of the fusion reactor, the re-compression cycle is used as the first. the heat sources are arranged behind [6]. however, the layout of the re-compression cycle can be designed differently. the heat sources can be situated to other streams [7]. the different layout of the s-co2 power cycle can be a benefit for the heat transfer and net power of cycles. a demonstration fusion power plant (demo) represents the first fusion power station capable of producing electricity and operating with a closed fuel-cycle. two demo design options are currently investigated, in an attempt to identify a realistic range of possibilities: a near-term demo1 and an advanced design concept demo2. demo1 is the concept based on reliable technology deliverable in the term of 20 years from now, and it is planned to work in the pulse operation mode. demo2 based on advances in the physics basis deliverable on a longer term is expected in the steady-state operation mode [8]. the demo2 power plant based on the steady-state fusion power reactor is analysed in this study. the demo2 fusion reactor has several different heat sources. the main heat sources are the blanket, first wall, and divertor. each of them operates on different temperatures and powers. the preheating s-co2 cycles may be more suitable for demo2. a layout of the preheating cycle is shown in figure 6. the preheating cycle has benefit for the pinch point mitigation as well as the layout of re-compression cycle. figure 6. preheating cycle. the table 2 brings parameters of the analyzed fusion reactor demo2 model [9]. thermal power of the fusion reactor demo2 is 4109 mw. the blanket has the thermal power of 3887 mw. the thermal power of the divertor and the first wall is 222 mw. the blanket and first wall are cooled by helium, the divertor is cooled by water. the high-grade reactor outlet temperature is projected to 500 ◦c. the low-grade reactor outlet temperature is projected to 160 ◦c. the turbine inlet temperature is 475 ◦c. consequently, the analyzed fusion reactor demo2 has two different heat source. the preheating cycle uses the heater h1 for high-grade primary heat (first wall and blanket). second heater h2 is used for lowgrade secondary heat (divertor). thermal power 4109 mw primary (high-grade) heat 3887 mw secondary (low-grade) heat 222 mw compressor efficiency 68 % turbine efficiency 79 % recuperator effectiveness 90 % compressor inlet temperature 34 ◦c turbine inlet temperature 475 ◦c table 2. parameter of s-co2 for fusion reactor demo2. 6. result of gfr reactor and demo2 fusion reactor the thermodynamic calculation was done for precompression cycle and preheating cycle. the calculation was performed using programming language python. the codes of cycles have been written in python. properties of pure co2 and mixtures are embedded into the python. source of gases and mixtures properties is nist reference fluid thermodynamic and transport properties database, version 9.1. [10]. the calculation of the gfr s-co2 cycle was performed according to the parameters included in table 2. the cycle was optimized for the best results. the results of the gfr re-compression cycle are 109 l. vesely, v. dostal, s. entler acta polytechnica ctu proceedings figure 7. t-s diagram of the gfr re-compression cycle. shown in table 3. the figure 7 shows a t-s diagram of the cycle. according to the table 3, the total net power of re-compression cycle is 201 mw. the cycle efficiency is 33.56 % and the mass flow is 2730 kg/s. cycle efficiency 33.56 % turbine power output 324.98 mw compressor no.1 in. power 89.44 mw compressor no.2 in. power 33.84 mw compressor input power 123.28 mw added heat 600.98 mw removed heat 399.28 mw regenerative heat 1249.15 mw mass flow in ltr (cold s.) 2299.89 kg/s mass flow in com. no.2 430.1 kg/s mass flow ratio 0.842 net power 201.69 mw mass flow 2730.0 kg/s pressure ratio 2.8 pressure no.2 34.0 mpa table 3. result of gfr re-compression cycle. the calculation of s-co2 cycle for demo2 was performed for two cycle. the first cycle is the recompression cycle and the second cycle is preheating cycle. the calculations were performed with the parameters from the table 2. the cycles were optimized for the best results. the result of the demo2 recompression cycle is shown in the table 4. the layout of heat sources corresponds to the figure 2. the heat source h is split into two heat sources arranged serially. according to the table 4, the total net power of the re-compression cycle is 1251 mw. the cycle efficiency is 30.45 %. however, the mass flow for this case is very high and achieves of 20 250 kg/s. cycle efficiency 30.45 % turbine power output 2013.02 mw compressor no.1 in. power 719.62 mw compressor no.2 in. power 42.41 mw compressor input power 762.03 mw added heat 4108.83 mw removed heat 2857.84 mw regenerative heat 8340.95 mw mass flow in ltr (cold s.) 19 589.33 kg/s mass flow in com. no.2 660.66 kg/s mass flow ratio 0.967 net power 1250.98 mw mass flow 20 250.0 kg/s pressure ratio 2.6 pressure no.2 34.0 mpa table 4. result of demo2 re-compression cycle. the figure 8 shows the t-s diagram of the demo2 re-compression cycle. the result of the demo2 pre110 vol. 4/2016 comparison of s-co2 power cycles for nuclear energy figure 8. t-s diagram of the demo2 re-compression cycle. figure 9. t-s diagram of the demo2 preheating cycle. heating cycle is shown in the table 5 and the t-s diagram of the cycle is shown in the figure 9. the layout of heat sources corresponds to the figure 6. the heater h1 was used for high-grade primary heat, the heater h2 was used for low-grade secondary heat. according to the table 5, the total net power of preheating cycle cycle is 1145 mw. the cycle efficiency is 27.87 %, and the mass flow for the preheating cycle is 14 040 kg/s. 111 l. vesely, v. dostal, s. entler acta polytechnica ctu proceedings cycle efficiency 27.87 % turbine power output 1727.14 mw compressor input power 582.005 mw added heat 4108.54 mw removed heat 2963.41 mw regenerative heat 4125.79 mw mass flow in hex 13 338.0 kg/s mass flow in h2 702.0 kg/s mass flow ratio 0.95 net power 1145.13 mw mass flow 14 040 kg/s pressure ratio 3.4 pressure no.2 28 mpa table 5. result of the demo2 preheating cycle. 7. conclusion the results from the table 3 represent the suitable results for the gfr designing with re-compression cycle. improvement of the parameters is possible by future research. the results of the s-co2 cycle for the fusion reactor show the effect of the use of multiple heat sources. the re-compression cycle is better than the preheating cycle according to the table 4 and table 5. however, it is obvious that this advantage is valid only for the net power. the critical disadvantage of the recompression cycle is the mass flow of about 20 000 kg/s. the mass flow of the preheating cycle is lower in comparison with the re-compression cycle. designing and optimization of the s-co2 cycles are of great importance for fusion energy. the preheating cycle is a convenient cycle for utilization of low potential heat, and such cycles are suitable for exploitation of the multiple heat sources of the fusion power reactor like analyzed demo2. further research of the s-co2 power cycles for the fusion power reactors will be focused on the detailed comparison of the s-co2 power cycle and on the development of the new modifications of the s-co2 power cycle, which will take into account the fusion reactor multiple heat source design. references [1] g. angelino. carbon dioxide condensation cycles for power production. j eng power 90(3):287–295, 1968. doi:10.1115/1.3609190. [2] v. dostal, m. driscoll, p. hejzlar. a super critical carbon dioxide cycle for next generation nuclear reactors, vol. mit-anp-tr-100 of advanced nuclear power program. mit center for advanced nuclear energy systems, 2004. [3] v. ladislav, d. vaclav, b. ondrej, n. vaclav. pinch point analysis of heat exchangers for supercritical carbon dioxide with gaseous admixtures in ccs systems. energy procedia 86:489–499, 2016. doi:http://dx.doi.org/10.1016/j.egypro.2016.01.050. [4] e. j. parma, s. a. wright, m. e. vernon, et al. supercritical co2 direct cycle gas fast reactor (sc-gfr) concept. tech. rep. sand2011-2525, sandia national laboratories, 2011. [5] e. a. harvego, m. g. mckellar. optimization and comparison of direct and indirect supercritical carbon dioxide power plant cycles for nuclear applications. in asme 2011 international mechanical engineering congress and exposition, pp. 75–81. 2011. doi:10.1115/imece2011-63073. [6] b. halimi, k. y. suh. computational analysis of supercritical {co2} brayton cycle power conversion system for fusion reactor. energy conversion and management 63:38–43, 2012. doi:http://dx.doi.org/10.1016/j.enconman.2012.01.028. [7] j. i. linares, l. e. herranz, i. fernández, et al. supercritical co2 brayton power cycles for demo fusion reactor based on helium cooled lithium lead blanket. applied thermal engineering 76:123–133, 2015. [8] g. federici, r. kemp, d. ward, et al. overview of eu demo design and r&d activities. fusion engineering and design 89(7–8):882–889, 2014. doi:http://dx.doi.org/10.1016/j.fusengdes.2014.01.070. [9] r. kemp. demo2 reference design, 2015. eurofusion idm eu_d_2lcbvu. [10] e. w. lemmon, m. l. huber, m. o. mclinden. nist reference fluid thermodynamic and transport properties – refprop. national institute of standards and technology, 2013. version 9.1. 112 http://dx.doi.org/10.1115/1.3609190 http://dx.doi.org/http://dx.doi.org/10.1016/j.egypro.2016.01.050 http://dx.doi.org/10.1115/imece2011-63073 http://dx.doi.org/http://dx.doi.org/10.1016/j.enconman.2012.01.028 http://dx.doi.org/http://dx.doi.org/10.1016/j.fusengdes.2014.01.070 acta polytechnica ctu proceedings 4:107–112, 2016 1 introduction 2 advantages and disadvantages of s-co2 cycles 3 description of gas cycles 4 the gfr re-compressing cycle 5 the s-co2 cycle for demo2 fusion reactor 6 result of gfr reactor and demo2 fusion reactor 7 conclusion references acta polytechnica ctu proceedings doi:10.14311/ap.2016.4.0013 acta polytechnica ctu proceedings 4:13–18, 2016 © czech technical university in prague, 2016 available online at http://ojs.cvut.cz/ojs/index.php/app modelling of nuclear fuel cladding tubes corrosion miroslav cecha,∗, martin seveceka,b a department of nuclear reactors, faculty of nuclear sciences and physical engineering, czech technical university in prague, v holesovickach 2, praha 8, czech republic b alvel a.s., opletalova 37, praha 1, czech republic ∗ corresponding author: miroslav.cech@post.cz abstract. this paper describes materials made of zirconium-based alloys used for nuclear fuel cladding fabrication. it is focused on corrosion problems their theoretical description and modeling in nuclear engineering. keywords: corrosion, zirconium alloys, nuclear fuel, cladding, modelling. 1. introduction tubes covering nuclear fuel in current light water reactors (lwr) are made of zirconium-based alloys since the very origin of nuclear power utilization. zirconiumbased alloys were first used in nuclear reactors of u.s. nuclear submarines. later, zr was adopted by fuel vendors as a suitable material for fuel cladding for commercial reactors around the world. zirconium has been chosen for its low cross section for neutron absorption, good corrosion resistance, and other outstanding thermomechanical characteristics. various degradation processes jeopardizing nuclear cladding integrity take place during reactor normal operation such as grid-to-rod fretting, debris-induced failures, crud-induced localized corrosion, waterside corrosion, and hydriding. this article is focused on water corrosion, its quantification, and theoretical description. corrosion reaction caused mainly by an interaction of nuclear fuel cladding and coolant takes place on an external surface of cladding tubes, less frequently reacts internal cladding surface with oxygen released from pelets. in case of lwrs, metal reacts with water and zirconium oxide arises: zr + 2 h2o −−→ zro2 + 2 h2. (1) hydrogen released from the reaction described above partly dissolve in coolant water and partly penetrates tubes causing hydriding of zirconium. the fraction of hydrogen released from the reaction that is locally absorbed by the cladding tube is called pickup fraction and in pwr conditions is found to be constant for particular zr-based alloys [1]. oxygen diffuses through the zirconium oxide layer and in an interface of metal and oxide causes additional oxidation. the density of the zro2 is smaller than zirconium alloy density. the difference in density and different thermal expansion of materilas is the primary cause of tension, internal stresses, and strains in cladding tubes. moreover, thermal conductivity λ of zro2 is much smaller than that of zirconium based alloys causing the zirconium dioxide layer to decrease the heat transfer from the fuel pellet over the cladding to the coolant and consequently increase maximal temperature in the fuel pellet. the exact physical parameters depend on temperature and models of thermal conductivity of zirconium dioxide. they are in details described in [2] and [3]. for example, the thermal conductivity of e110 alloy is about 18 w/mk at the temperature of 300 ◦c. the thermal conductivity of the zro2 for the same temperature is only about 2 w/mk. the bulk density of the zircaloy-4 alloy and zirconium dioxide at the same temperature is 6.5 and 5.6 kg/m3 respectively. when the zirconium dioxide layer thickness exceeds about 100 µm (zircaloy-4), it might crack and it is washed away by the streaming coolant which can lead to cold spots, additional oxidation, hydridation and later cladding breach [4]. oxidation takes place also on the internal surface of fuel cladding, where metal reacts with oxygen released from the fuel pellets where fission takes place. when a high burn-up is reached, a bonding layer consisting of zro2, uo2, and fission products appears. this layer might be the cause of a firm connection between the fuel cladding and pellets. 2. zirconium based alloys zirconium-based alloys has been used as the nuclear fuel cladding since 1950s [6]. there were originally two main groups of zr-based alloys developed: (1.) zirconium-tin and iron-based alloys (originally developed in the u.s.) (2.) zirconium-niobium based alloys (originally developed in former ussr) during the evolution of the nuclear fuel, fuel vendors and research organizations developed dozens of concepts of fuel cladding alloys. however, the two main groups remained as can be seen in figure 1. different cladding concepts can be based on the figure 13 http://dx.doi.org/10.14311/ap.2016.4.0013 http://ojs.cvut.cz/ojs/index.php/app miroslav cech, martin sevecek acta polytechnica ctu proceedings figure 1. nuclear fuel cladding alloys developed for usage in pwr reactors [5]. there were two main groups of cladding alloys developed: zr-sn (right branch) and zr-nb (left branch) based alloys with dozens other concepts depending on the alloying elements introduced around the world. divided into four development directions depending on their composition and development history. each alloy has a different corrosion characteristics depending on the alloying elements but also on manufacturing process and reactor type. 2.1. zirconium-tin alloys a well-known family of alloys called zircaloy was developed under a westinghouse-led program. an alloy first developed is called zircaloy-1. this alloy was replaced by the innovative zircaloy-2, which is still in use in the bwr reactors. after abandoning of the zircaloy-3 alloy development and utilization due to metallurgical processing issues, concerns with the high hydrogen pickup fraction exhibited by the zircaloy-2 alloy led to the development of the zircaloy4 alloy. nickel was substituted by iron in this alloy and it was used from the 1950s to 1990s. 2.2. zirconium-niobium alloys an alloy called e110 has been used by the russian nuclear industry for nuclear fuel cladding fabrication for vver reactors fuels. similar alloys made of zirconium-doped by niobium were used from the origin of the nuclear power utilization in the ussr. recently, westinghouse replaced zircaloy-4 alloy in most of their nuclear fuel for pwrs by the zirlo alloy. in bwr reactors, zircaloy-2 is still in use. zirlo is doped with niobium and is similar to the russian alloy called e635. french alloy called m5 is a zirconium-based alloy containing 1 % of niobium with oxygen. the m5 alloy is produced by the french company areva since 1990s. generally, alloys doped with niobium such as e110, m5, and zirlo have a higher corrosion resistance than alloys from the zircaloy family. summary of composition of currently widely employed alloys is presented in table 1 [7]. 3. corrosion models the growth and development of the cladding corrosion layer for each zirconium-based alloy is usually expressed by corrosion models. 3.1. garzarolli models models developed by garzarolli et al. [8] are adopted for describing the thickness of the corrosion layer of cladding tubes made of zircaloy-4 , m5, and zirlo alloys under pwr conditions. these models generally based on arrhenius law divide a growth of corrosion layer into two phases: (1.) the first phase continues until a transition thickness of oxide layer str is reached. the rate of corrosion layer growth is expressed by the cubic rate law – equation (2) (original units and the temperature of oxide-metal interface is used) [2, 3]. (2.) after reaching an alloy-specific transition thickness of oxide layer str, second phase quantified by a linear differential equation (3) taking into account also fast neutron flux φ is adopted. there are other transition points observed during the corrosion process. however, only the first transition is significant for the general progress of the corrosion kinetics. for modelling purposes the use of only one transition is satisfactory. model is defined for temperature range of 250–400 ◦c. ds3 dt = a s2 exp ( − q1 rt ) , (2) ds dt = (c0 + u(mφ)p ) exp ( − q2 rt ) , (3) where s . . . oxide layer thickness [µm], t . . . temperature [◦c], φ . . . neutron flux [neutrons/m2s], a = 6.3× 109 m3/day, r = 1.98 cal/molk, c0 = 8.04× 107 µm/day, m = 7.46× 10−15 cm2s/n, p = 0.24, u = 2.59× 108 µm/day. values of constants q1, q2 and transition thickness of oxide layer str are alloy-dependent and are summarized in table 2 for four widely used alloys which were subject to many studies. in literature, there were modifications of correlation models (2) and (3) defined. modified models differ by values of constants and consider other physical phenomena neglected in the presented models. these models are implemented for example in femaxi and frapcon fuel performance codes. 14 vol. 4/2016 modelling of cladding tubes corrosion alloying element zircaloy-4 zirlo m5 e-110 e-635 sn 1.2–1.7% 0.96–0.98% 1.25–1.30% fe 0.18–0.24% 0.094–0.105% < 500 ppm 0.30–0.35% cr 0.07–0.13% 79–83 ppm < 200 ppm nb 1.02–1.04% 0.8–1.2% 0.9–1.1% 1.0% ni 0.01% n < 65 ppm 22–30 ppm < 60 ppm 30–40 ppm c 150–400 ppm 60–80 ppm < 200 ppm <200 ppm o 900–1400 ppm 900–1200 ppm 0.11–0.17% < 1000 ppm 0.07% table 1. composition of zirconium-based alloys widely used around the world in pwrs as materials for cladding tubes fabrication [7]. q1[cal/mol] q2[cal/mol] str zircaloy-4 32289 27354 2µm m5 27446 29816 7 µm zirlo 32289 27080 2µm opt.zirlo 32289 27354 2 µm table 2. values of corrosion model’s constants used in corrosion models (2) and (3) as defined by [3]. figure 2. zirconium dioxide layer development from the point of reaching first transition thickness str corresponding to the transition time ttr. axis x represents time in days after the moment, when transition thickness was reached. the temperature of 320 ◦c and neutron flux of 1 × 1015 neutrons/m2s were chosen in this model situation. 3.2. three-phase model another model for describing of zircaloy-4 corrosion layer thickness in pwr conditions divides its evolution into three phases instead of two. the purpose is a faster oxide thickness growth after the second transition thickness is reached [7]. the model was developed by experimental data fitting and is more precise for higher values of oxide thickness than model developed by garzarolli et al. [8]. oxide layer growth during the first phase can be calculated by the following expression [6] ds3 dt = 2.187× 10−13 exp ( −135188 rt ) . (4) the first transition thickness is the same as in the previous model – 2 µm for zircaloy-4. afterward, different formula is used instead of equation (3) ds dt = ( 9.31× 10−4 + 2.75× 10−3 ( φ 5.24× 1018 )0.24 ) · exp ( − 114526 rt ) . (5) after reaching the second transition thickness – 35µm, following equation is used ds dt = ( 9.31× 10−4 + 2.75× 10−3 ( φ 5.24 · 1018 )0.24 ) · 1.8 exp ( − 114526 rt ) . (6) 3.3. e110 corrosion model a model describing the corrosion layer growth of e110 alloy in vver conditions was developed by fitting experimental data at the russian a.a. bochvar hightechnology scientific research institute for inorganic materials. for the e110 alloy following relation was derived [9] ds dt = 40 exp ( −5147 t ) . (7) model was derived byl data base on experiments, which took place in temperature range of 320–360 ◦c. this model considers only one phase of corrosion layer development, transition thickness is disregarded. comparison between corrosion layer growth of zircaloy-4 and e110 is plotted in figure 3. figure 3. comparison of the zirconium oxide layer thickness of zircaloy-4 and e110 alloys in conditions of 320 ◦c and neutron flux of 1e15 neutrons/m2s. the corrosion growth of e110 is considerably lower in comparison with the zircaloy-4 alloy. accelerating 15 miroslav cech, martin sevecek acta polytechnica ctu proceedings growth can be seen for the zircaloy-4 three-phase model which is not the case for the e110 alloy. for that reason, the neglecting of transitions in case of e110 is justified. if the behavior of nuclear fuel during loca, ria or other design basis accidents is calculated by fuel performance codes like fraptran or transuranus, the thickness of oxide layer is an initial condition which strongly influences a progress of the accident and its consequences. 4. corrosion in loca conditions large break loss of coolant accident (lbloca) is the maximal design basis accident of pwrs of second and third generation. during this accident, a temperature of the whole fuel rod including pellets quickly rises due to limited cooling conditions. the high temperature of fuel pellets leads to high-temperature gradients, stresses, and cracking of pellets. rapid release of fission gases from the fuel can be observed and the internal pressure of filling gas rises. the high temperature of cladding together with the high internal pressure can be a cause of a deformation, ballooning, or bursting of the cladding. this geometry changes can limit the coolant flow and further reduce the heat transfer from the fuel rods to the coolant. construction of reactor, design of the nuclear fuel and its properties must ensure that acceptance limits for loca accidents will not be violated: (1.) a peak cladding temperature can nowhere exceed 1204 ◦c (2.) sufficient fuel rod strength upon quench taking into account an additional mechanical load (maintain post-quench ductility) (3.) fraction of zirconium reacted into oxide cannot exceed 1 % (due to hydrogen production) (4.) melting temperature of fuel can not be reached in any place of the reactor core 4.1. corrosion models in loca conditions to develop corrosion models in loca conditions, it is necessary to measure experimental data in similar conditions. experiments are done at high-temperature steam (800–1200 ◦c). experiments with as-received cladding tubes and as well as with cladding tubes with corrosion layer has been performed. preoxidation of experimental samples ensures that simulation will be performed in conditions which are similar to the real loca accident conditions with operating fuel. a model of high-temperature corrosion of sponge based e110 alloy was developed at the ujp in the czech republic and is based on its experimental data [10]. experiments cover a wide range of conditions (temperature 600–1300 ◦c and 0–480 minute long exposition). these wide conditions enable to use the model in various conditions and for various scenario for e110 alloy. no. t [◦c] process 1 600–750 phases α + β transformation 2 750–950 formation of β phase 3 950–1100 delated transformation, tetragonal oxide 4 1100–1300 tetragonal oxide table 3. physical processes taking place in zirconium e110 alloy in different temperature intervals during loca accident conditions. this model describes a mass growth of oxide as defined in [10] ∆g = a exp ( e t ) tn = ktn, (8) where ∆g . . . mass growth [mg/dm2], a,e, k . . . fitting parameters, t . . . time [s], n . . . kinetic constant. figure 4. development of the parameter n with temperature as defined by [10]. figure 5. development of the parameter k with temperature as defined by [10]. for n and k parameters were derived following formulas n = 0.4 for t < 768.4 ◦c (9) n = 2.609− 4.898× 10−3(t − 273.15) + 2.633× 10−6(t − 273.15)2 for t < 960.3 ◦c (10) n = 1.202× 10−3(t − 273.15)− 0.8208 for t < 1098.9 ◦c (11) n = 0.5 for t > 1098.9 ◦c (12) 16 vol. 4/2016 modelling of cladding tubes corrosion figure 6. power history of rods bsm-25 and bk365. figure 7. oxide layer thickness of bsm-25 and bk365 rods. k = 85265.6 exp ( −9875.59 t ) for t < 934.1 ◦c (13) k = 1072.21 exp ( −4592.6 t ) for t < 1054.5 ◦c (14) k = 33.33 for t < 1098.0 ◦c (15) k = 96482.3 exp ( −10913.1 t ) for t > 1098.0 ◦c (16) temperature intervals of equations (9)–(16) approximately correspond to physical processes, which take place in the cladding material during loca accident. these processes are described in table 3. this model well describes a corrosion kinetic for all ranges of temperature covered by experiment. the value of parameter n is 1/2 for high-temperature corrosion and 1/3 for middle-temperature corrosion [7]. these values are the same as in other used models. another model for n also very well describes the n temperature reliance. constant k equals approximately equals to 0 and increase with temperature to about 90 at 1300 ◦c. dividing model into four ranges where different formulas are used brings a good agreement of the model with experimental data. a comparison between this model and data can be found in [10]. 5. model of corrosion in femaxi a calculation of oxide layer development in the femaxi-6 code has been performed for two fuel rods: bsm-25, and bk365. these rods were irradiated in the br-3 reactor and reached burn-up of 66 and 52mwd/kghm. rods were irradiated within the high burnup effect program in the br-3 reactor, cladding was made of zircaloy-4 alloy, coolant inlet temperature was 255 ◦c. a model originally developed by garzarolli et al. [8] (equations (2) and (3)) was used for the zircaloy-4 alloy corrosion modeling. the two power histories used as a model input are plotted in figure 6. corresponding oxide layer thickness growth is shown in figure 7. for both tested fuel rods, an average thickness and the maximal oxide layer thickness are shown. maximal thickness of the corrosion layer is about two times higher than the average value. maximal thickness was reached in the middle of the fuel rod which does 17 miroslav cech, martin sevecek acta polytechnica ctu proceedings not correspond to the case in commercial power reactors. the oxide layer growth is strongly dependent on the temperature, the higher the temperature is the faster the growth. therefore, the maximal thickness of the zirconium oxide layer in commercial power reactors is at the top of the fuel rod where the coolant temperature is highest. figure 7 shows, that higher linear heat rate causes faster oxide layer creation. the graph also shows that the first phase of oxide layer growth is independent of fast neutron flux. small differences are caused by higher temperature. in the later phase of fuel rod’s irradiation, there is a clear dependance of corrosion layer thickness on fast neutron flux and linear heat rate. when bsm-25 rod was operating in high heat rate condition, layer growth was much faster than in the case of smaller heat rate of the second fuel rod. higher heat rate (and corresponding temperature) causes the bigger corrosion layer, even when the burnup of the bsm-25 rod is lower. 6. conclusion this article describes models quantifying corrosion of nuclear fuel cladding tubes made of zirconium-based alloys widely used in nuclear industry in nominal conditions and loca accident conditions. models used for calculating of an oxide layer thickness in normal operation conditions for the widely used alloys zircaloy-4, zirlo, m5 and e110 are presented and compared. a model for corrosion and high-temperature oxidation in loca conditions is described and reliance of particular parameters used in models are shown in graphs. a corrosion model for nominal conditions was applied in the femaxi-6 code to calculate corrosion of fuel rods bk363 and bsm-25 tested in the br-3 reactor. the relations of burnup, linear heat rate, and corrosion layer thickness growth is illustrated in the example. results show faster oxide growth in case of bsm-25 rod after reaching the transition thickness for zircaloy-4 alloy, because this rod was operated at higher linear heat rate. acknowledgements this work was supported by the grant agency of the czech technical university in prague, grant no. sgs ohk4-008/16. references [1] k. geelhood, w. luscher. frapcon-3.5: integral assessment. progress in nuclear energy 2014. 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[10] j. krejčí. oxidace palivového pokrytí během havárie loca. bezpečnost jaderné energie 45:311–343, 2015. 18 acta polytechnica ctu proceedings 4:13–18, 2016 1 introduction 2 zirconium based alloys 2.1 zirconium-tin alloys 2.2 zirconium-niobium alloys 3 corrosion models 3.1 garzarolli models 3.2 three-phase model 3.3 e110 corrosion model 4 corrosion in loca conditions 4.1 corrosion models in loca conditions 5 model of corrosion in femaxi 6 conclusion acknowledgements references 96 acta polytechnica ctu proceedings 1(1): 96–102, 2014 96 doi: 10.14311/app.2014.01.0096 star formation in tadpole galaxies casiana muñoz-tuñón1,2, jorge sánchez almeida1,2, debra m. elmegreen3, bruce g. elmegreen4 1instituto de astrof́ısica de canarias, e-38205 la laguna, tenerife, spain 2departamento de astrof́ısica, universidad de la laguna, tenerife, spain 3department of physics and astronomy, vassar college, poughkeepsie, ny 12604, usa 4ibm research division, t.j. watson research center, yorktown heights, ny 10598, usa corresponding author: cmt@iac.es abstract tadpole galaxies look like a star forming head with a tail structure to the side. they are also named cometaries. in a series of recent works we have discovered a number of issues that lead us to consider them extremely interesting targets. first, from images, they are disks with a lopsided starburst. this result is firmly established with long slit spectroscopy in a nearby representative sample. they rotate with the head following the rotation pattern but displaced from the rotation center. moreover, in a search for extremely metal poor (xmp) galaxies, we identified tadpoles as the dominant shapes in the sample – nearly 80% of the local xmp galaxies have a tadpole morphology. in addition, the spatially resolved analysis of the metallicity shows the remarkable result that there is a metallicity drop right at the position of the head. this is contrary to what intuition would say and difficult to explain if star formation has happened from gas processed in the disk. the result could however be understood if the star formation is driven by pristine gas falling into the galaxy disk. if confirmed, we could be unveiling, for the first time, cool flows in action in our nearby world. the tadpole class is relatively frequent at high redshift – 10% of resolvable galaxies in the hubble udf but less than 1% in the local universe. they are systems that could track cool flows and test models of galaxy formation. keywords: starburst galaxies tadpoles cool flows rotation curves galaxy disks. 1 introduction elongated galaxies with bright clumps at one end are visible in deep field images taken with hst or from the ground; van der bergh et al., 1996 called them “tadpole” galaxies”. figure 1 in elmegreen et al. (2005) shows different morphologies of galaxies observed with the hubble ultra deep field (udf). the fourth row presents images of tadpoles. this asymmetric morphology is rather common at high redshift but rare in the local universe. for example, tadpoles constitute 10% of all galaxies larger than 10 pixels in the udf (elmegreen et al. 2007; elmegreen & elmegreen, 2010), and they represent 6% of the udf galaxies identified by straughn et al. (2006) and windshorst et al. (2006) using automated search algorithms. in contrast, elmegreen et al. (2012; hereafter paper i), find only 0.2% tadpoles among the uv-bright local galaxies of the kiso survey by miyauchi-isobe et al. (2010). this decrease suggests that the tadpole morphology represents a common but transition phase during the assembly of some galaxies. since local tadpole galaxies are very low mass objects compared to their high redshift analogues, this phase must be already over for the local descendants of high redshift tadpoles. the tadpole structure has inspired several explanations, such as ram pressure stripping that triggers star formation at the leading edge, to mergers (see sánchez almeida et al. 2013 for an extensive review). the explanation that we propose, based on observational evidence summarized here, is that the starburst head may result from the accretion of an external flow of pristine gas that penetrates the dark matter halo and hits and heats a pre-existing disk, which is viewed to the side as the tail. we will briefly present a summary of recent results showing, first that the local tadpoles share the properties of their higher redshift and higher mass counterparts (section 2), that they belong to the extremely metal poor (xmp) sample of the blue compact dwarfs family (section 3), they are rotating discs (section 4) and, finally, that the starbursts (heads) show a drop in the already low metallicity that can only be understood if fresh metal-poor gas is falling onto the galaxy (section 96 http://dx.doi.org/10.14311/app.2014.01.0096 star formation in tadpole galaxies 5). we finish with a brief summary and future actions. 2 local and hight z tpg share properties we used sloan digital sky survey data to determine the ages, masses, and surface densities of the heads and tails in 14 local tadpoles (shown in figure 1) selected from the kiso and michigan surveys of uv-bright galaxies, and we compared them to tadpoles previously studied in the hubble ultra deep field. the result, published in paper i is that the young stellar mass in the head scales linearly with the rest-frame galaxy luminosity, ranging from ≈ 105 m� at galaxy absolute magnitude u = −13 mag to 109 m� at u = −20 mag. the tails in the local sample look like bulge-free galaxy disks. their photometric ages decrease from several gyr to several hundred myr with increasing redshift. the far-outer intensity profiles in the local sample are symmetric and exponential. we suggest that most local tadpoles are bulge-free galaxy disks with lopsided star formation, perhaps from environmental effects such as ram pressure or disk impacts, or from in-situ gas collapse to a giant star-forming clump with a jeans-length that is comparable to half the disk size. the existence of a disk, proposed from the analysis of the luminosity profiles, has been further confirmed spectroscopically (see section 4). figure 1: tadpole galaxies from the kiso and um samples. figure from elmegreen et al., 2012. 3 belong to the bcdsxmp class blue compact galaxies are important targets for a number of reasons. they are small systems, thought to remain as fossils or debris from the formation of larger galaxies in the early epochs of the universe. their low metallicities, high ssfrs and low doubling time (less than 1gy) and active stabursts make them ideal targets to study the old universe at low redshift. their complete census, as well as the likelihood that they have a long quiescent phase between starbursts, took us to study the whole sample by making use of sdss. the properties of the quiescent and burst phases were determined from detailed studies of the bursts and host galaxies of a nearby sample (amorin et. al., 2007, 2008). these properties allowed us to identify such objects in the database. the sdss dr6 database provides ≈ 21.500 quiescent bcd candidates, a number 30 times larger to those bursting (bcds). this result implies that one out of every three dwarf galaxies in the local universe may be a quiescent bcd. the properties of the two samples are consistent with a single sequence in galactic evolution with the quiescent phase lasting 30 times longer than the burst phase (sánchez almeida et. al., 2008). in sánchez almeida et al. (their figure 9) there is a clear subsample of objects with the lowest luminosity which turn out to be also those that are most metal poor. we carried out a systematic search for extremely metal-poor (xmp) galaxies in the spectroscopic sample of sloan digital sky survey (sdss) data release 7 (dr7) (morales-luis et al., 2011). the xmp candidates are found by classifying all of the galaxies according to the form of their spectra in a region 80 å wide around hα using an automatic classification algorithm, k-means (sánchez almeida et al.,2009). our systematic search renders 32 galaxies having negligible [n ii] lines, as expected in xmp galaxy spectra. twenty-one of them were previously identified as xmp galaxies in the literature, and 11 were new. this was established after a thorough bibliographic search that yielded only some 130 galaxies known to have an oxygen metallicity 10 times smaller than the sun (explicitly, with 12 + log [o/h] ≤ 7.65). xmp galaxies are rare; they represent 0.01% of the emission lines galaxies in sdss/dr7. the xmp galaxies constitute 0.1% of the galaxies in the local volume, or ≈ 0.2% of the emission-line galaxies. all but four of our candidates are bcd galaxies, and 24 of them have either cometary shape or are formed by chained knots. note that this result, the morphology, is absolutely independent of our search criterium (spectra). 4 tadpoles are rotating disks the work in paper i was followed up by mean of high spectral resolution long slit observations at the int (2.5m) at observatorio del roque de los muchachos (orm) with ids spectrograph. in order to determine the dynamical properties and metallicities of lo97 casiana muñoz-tuñón et al. figure 2: sdss mugshots of all xmp candidates (from spectroscopy). the figure is taken from morales-luis et al., 2011. note the prevalence of tadpole shapes. cal tadpoles, we measured hα spectra along the headtail direction in a representative fraction of the original sample. further details are in sánchez almeida et al. (2013). in figure 3, we show spectral flux (the solid lines) and hα flux (the dashed lines) along the slit. note the obvious lopsidedness of the light distributions, as expected from the tadpole shape. the origin of distance on the abscissa has been set as the position of the maximum of the spectral flux distribution. the dotted line represents a gaussian fitted to the hα flux around the head. the horizontal bar in each panel gives a common length scale corresponding to 1 kpc. the thin vertical solid lines indicate the center of rotation obtained from the rotation curve fit shown in fig. 4. velocities, masses, abundances and other physical parameters were determined from the spectra. bulk velocities were measured from the displacement of hα. we computed the displacement both as the barycenter of the emission line, and as the center of a gaussian function fitted to the profile. errors were estimated from the s/n measured in the continuum and then propagated to the centroids. the fwhm of the profiles were also measured directly from the profiles and from the gaussian fits. their errors were also inferred from the noise measured in the continuum by error propagation. figure 4 shows the velocity curves of the tadpole galaxies included in the study. the abscissae represent distances along the major axes of the galaxies from the position of the tadpole head; i.e., the brightest point on the galaxy. the range of distances differs for the different targets, but the horizontal bar in each panel gives a common length scale corresponding to 1 kpc. the points with error bars show the observations whereas the thick solid line represents the best fit of the observed points to the analytic rotation curve. the part of the rotation curve shown in red indicates the portion of the velocity curve used for fitting. the thin horizon98 star formation in tadpole galaxies tal and vertical lines indicate the systemic velocity and the center of rotation obtained from the fit, respectively. the little arrows on top of each panel also indicate the centers of rotation. the zero of the velocity scale is set by velocity of the spatially integrated spectrum of the galaxy; positive velocities are redshifts. figure 3: spectral flux (the solid lines) and hα flux (the dashed lines). formal error bars for photometry are not included since they are negligibly small. the main result shown in figure 4 is that five out of seven targets show velocity gradients interpreted as rotation. another important result is that the tadpole head is not at the rotation center. figure 4: velocity curves of the seven tadpole galaxies obtained from long slit spectroscopy. figure from sánchez almeida et al. 2013 sometimes the interpretation of the velocity curve as rotation is obvious (e.g., kiso8466), but other times the curve looks more like a perturbed rotation (e.g., kiso5639). kiso3193 and kiso3867 have a rather flat velocity curve, and therefore no obvious rotation. however, one of them, kiso3867, shows a systematic line shift of the order 10–20 km s−1 between the two extremes of the galaxy (figure 4). the amplitude is of the order of the error bars, but the displacement is in the raw data as judged by inspection of the individual hα profiles. 5 drops in metallicity at the head location the metallicity along the slit was estimated using the ratio [nii]λ6583 to hα. figure 5 taken from sánchez almeida et al (2013) shows the variation across the galaxies of the oxygen abundance, including their error bars. all galaxies have sub-solar metallicity the thick horizontal line marks the solar oxygen abundance given by 12 + log(o/h)� = 8.69± 0.05 (asplund et al., 2009). the galaxies also present significant abundance gradients, with the lowest abundances tending to coincide with the largest hα emissions (e.g., kiso6669 and kiso6877) in fig. 5. remember that the vertical dotted lines mark the position of the peak hα fluxes. two targets, kiso5639 and kiso6877, have metallicities well below one-tenth the solar value, therefore, they belong to the xmp galaxies class (kunth & östlin, 2000). xmp galaxies are really rare objects: one out of a thousand galaxies in the local universe according to morales-luis et al. (2011). therefore the fact that we observe two in a sample of seven cannot be a coincidence. it is known that a significant fraction of xmp galaxies turn out to be cometary or tadpole (papaderos et al., 2008). we have found that the reverse holds too, i.e., that tadpole galaxies have a significant probability of being xmp. this fact supports the idea that the tadpole morphology is a sign of dynamical youth, as the low metallicity is a sign of being chemically young (see discussion in sánchez almeida et al., 2013). 6 summary and future tadpole galaxies have an easily identified shape, belong to the blue compact dwarf class, and are rare in the nearby universe. their appearance suggests then to be the consequence of some interaction or star formation triggered by ram pressure processes. these local tadpoles seem though to form a continuous sequence with the udf tadpoles, seen in relatively larger number at high redshift (elmegreen & elmegreen 2010). regarding their photometric properties, local tadpoles occupy the low mass end in sequences such as star formation, surface density and mass-to-light ratio. in addition, the radial intensity profiles of both samples (local and high redshift tadpoles) show an exponential decrease at large galactocentric distances, which we interpreted as an evidence for the existence of an underlying disk. from the point of view of their chemical content, extremely metal poor (xmp) galaxies are the least evolved objects in the local universe (pagel et al., 1992; 99 casiana muñoz-tuñón et al. figure 5: oxygen abundance variation across the galaxies. the vertical solid line represents the center of rotation, whereas the vertical dotted line indicates the location of the maximum in the hα flux profile. the thick horizontal solid line indicates the solar metallicity. note the existence of abundance variations, with the minima coinciding with the largest hα signals. note also that kiso5639 and kiso6877 reach very low abundances, below one-tenth of the solar value; therefore, they are members of the set of rare xmp galaxies. kunth & östlin, 2000). they represent only 0.1 % of the galaxies in an arbitrary nearby volume (moralesluis et al., 2011) and a significant fraction of these chemically primitive objects turn out to have tadpole or cometary shape (brinchmann et al. 2008). this association between low metallicity and tadpole shape suggests that both are attributes of very young systems. detailed studies of a significative sample of the local tadpole class by means of high-resolution long-slit spectroscopy put into evidence two important facts: they do rotate and they show oxygen abundances that vary along the disk. a high percentage of them are xmp galaxies and the metallicity is minimum were starformation is maximum. moreover, the rotation center does not coincide with the current starburst location (the head). the oxygen metallicity estimated from [nii]6583/hα often shows significant spatial gradients across the galaxies (∼0.5 dex), being lowest at the head and increasing in the rest of the galaxy, tail included. a similar result with the highest sfr been the lowest metallicity region has also been found in low metallicity grb host galaxies (levesque et al., 2011). the sense of the resulting metallicity gradient differs from the observation of local disk galaxies, where the gas-phase metallicity increases toward the galaxy centers (vilchez et al., 1988) or is just constant (genel et al., 2008). however, the type of variation we measure, with a minimum metallicity at the most intense star-forming region, has been observed in galaxies at redshift around 3 by cresci et al. (2010), where it is interpreted as evidence for infall of pristine gas triggering star formation. as a result of all the evidence found, we propose that local tadpole galaxies are disks still in the process of being formed in our present-day universe. moreover 100 star formation in tadpole galaxies their otherwise very low star formation is enhanced and triggered by cool-flow accretion of pristine metal-poor gas. this triggering causes an extremely metal-poor head to shows up on the side of an immature (still forming) rotating disk. there are a number of parallel actions now being undertaken to go further. we have just finished a study of the hi content of a complete sample of xmp galaxies (fihlo et al., 2013) and compared the properties of the clumps of different galaxy types at different redshifts (elmegreen et al., 2013). the recent analysis of a new sample corroborates the metallicity drop in the xmp galaxies using the so call direct method (sánchez almeida et al. 2014). cosmological simulations predict cold-flow buildup to be the main mode of galaxy formation (dekel et al., 2009). the incoming gas is expected to form giant clumps that spiral in and merge into a central spheroid. to confirm such a scenario at high redshift is neither easy nor feasible and we think we have discovered nearby cases that may allow for comparisons. we are making a comprehensive study of the disks and bursts of other bcds with xmp properties to search for extremely metal poor clumps in 2d spectroscopy. the final aim is to characterize the local xmp galaxy sample within the cool flow paradigm. acknowledgement this work has been partly funded by the spanish ministry for science, project aya 2010-21887-c04-04. results based on observations at the observatorio del roque de los muchachos (orm), operated by the iac at la palma. thanks to franco giovannelli and lola sabau-graziatti, organizers of this workshop for their kindness and dedication. thanks to the referee for useful comments. references [1] van den bergh, s., abraham, r.g., ellis, r.s., tanvir, n.r., santiago, b.x., & glazebrook, k.g. 1996, aj 112, 359 [2] emegreen, d.m., elmegreen, b.g., rubin, d.s. & schaffer, m.a. 2005, apj, 631, 85 doi:10.1086/432502 [3] elmegreen, d. m., elmegreen, b. g., ravindranath, s., & coe, d. a. 2007: apj, 658, 763 doi:10.1086/511667 [4] elmegreen, b. g. & elmegreen, d. m. 2010: apj, 722, 1895 doi:10.1088/0004-637x/722/2/1895 [5] straughn, a. n., cohen, s. h., ryan, r. e., et al. 2006, apj, 639, 724 doi:10.1086/499576 [6] windhorst, r. a., cohen, s. h., straughn, a. n., et al. 2000, newar, 50, 821. 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[26] sánchez almeida, j., morales-luis, a.b, muñoztuñón, c., et al., 2014: apj in press. 102 http://dx.doi.org/10.1038/nature07648 introduction local and hight z tpg share properties belong to the bcdsxmp class tadpoles are rotating disks drops in metallicity at the head location summary and future acta polytechnica ctu proceedings doi:10.14311/app.2017.8.0011 acta polytechnica ctu proceedings 8:11–13, 2017 © czech technical university in prague, 2017 available online at http://ojs.cvut.cz/ojs/index.php/app dlc/ti thin films properties prepared by hybrid laser technologies jan mikšovskýa,b,∗, miroslav jelíneka,b, petr písaříka,b, tomáš kocoureka,b, jan remsaa,c, karel jurekb a czech technical university in prague, faculty of biomedical engineering, nam. sitna 3105, kladno, czech republic b institute of physics ascr, na slovance 2, prague, czech republic c czech technical university in prague, faculty of nuclear sciences and physical engineering, břehová 7, prague, czech republic ∗ corresponding author: jan.miksovsky@fbmi.cvut.cz abstract. layers of diamond-like carbon are usable in many fields of industry as well as in medicine. many scientific groups have worked with different types of deposition techniques to prepare dlc layers with improved or unique properties. the dlc properties could be improved by various dopations. in this study, we focused on dlc layers doped by titanium, prepared by hybrid laser depositions. two techniques were used: dual pulse laser deposition (dualpld) and pulse laser deposition in combination with magnetron sputtering (pld/ms). preliminary tests for morphology, wettability, adhesion, hardness, corrosion, friction and wearability were examined. dlc samples were prepared on si(100) wafer and on ti6al4v alloy substrates with titanium concentration from pure up to 25 at.%. friction of the prepared layers ranged from 0.09 to 0.18. the films exhibited very low wear for loads 1 n and 2 n. the tested pin wear grew with decreasing titanium dopation. the measurements of the contact angle and determination of surface free energy were done for water, diiodomethane and ethylene glycol. hardness and young's modulus decreased with higher titanium dopation, on the other hand adhesion improved with increasing amount of titanium in film. keywords: pld, dlc, titanium, surface properties, laser deposition, magnetron sputtering. 1. introduction in this study, we focused on the changes in the mechanical and corrosion behavior with various amount of titanium in dlc/ti thin films. two techniques for dopation of titanium were used [1–3]. the first technique was based on pulsed laser deposition (excimer laser, krf, λ = 248 nm) combined with dc magnetron sputtering. the second technique was dualpld. laser fluency was 8 j/cm2 for dlc deposition and 5 j/cm2 for metal dopant. power of magnetron was from 40 w to 300 w for metal dopant. we studied behavior of layers for ti concentration from 1 at.% to 25 at.%. si(100) and ti6al4v alloy were used as substrates. prepared films were characterized with focus on their basic physical properties, such as crystallinity by xrd, morphology by afm and mechanical profilometry, chemical bonding nature by wdx, surface free energy by contact angle. hardness and young’s modulus were studied by nanoindentation. macro adhesion was studied up to 30 n. wearability and tribological properties were studied as well. the content of sp3 bonding was studied by xps. special focus was given on tribological properties. corrosion rates were measured as function of ti concentration. 2. experimental wearability test was performed by tribometer (anton paar) with linear movement. chromium steel testing ball with 6 mm diameter was used. speed of the testing ball was 5 cm/s, total testing length 10 m, used loads during tests were 1 n and 2 n. the tests were performed as dry. the friction was determined from the test records. nanoindentation tests were performed on all samples for determination of hardness and elastic modulus. all measurements were implemented on layers with thickness 500 nm and the indentation depth was less than 10 % of the layers thickness. for more details, see [4]. adhesion of the films was tested for samples on ti6al4v substrates. we tested samples by progressive load from 1 n to 30 n. three types of critical forces were evaluated. at least two scratches for each layer were tested. for more details, see [4]. wettability was measured by three liquids: distilled water, diiodomethane and ethylene glycol, by contact angle measurement system (dsa100, krüss company). surface free energy was determined by drop shape analysis software with fowkes method. the volume of droplets was (0.4 ± 0.1) ml. the measurement technique was sessile droplet and static contact angle. each sample was tested at least on three 11 http://dx.doi.org/10.14311/app.2017.8.0011 http://ojs.cvut.cz/ojs/index.php/app j. mikšovský, m. jelínek, p. písařík et al. acta polytechnica ctu proceedings figure 1. from left wearability of the ti6al4v substrate and samples ti:dlc7(pure dlc), ti:dlc10 (5 at.% ti) and ti:dlc12(25 at.% ti). the figures are captured at magnification 200x. figure 2. wearability of the testing ball after tests for ti6al4v substrate and for samples ti:dlc7(pure dlc), ti:dlc10 (5 at.% ti) and ti:dlc12(25 at.% ti). the figures are captured at magnification 200x. dual pld 0 at.% ti 1 at.% ti 3 at.% ti 5 at.% ti 10 at.% ti 25 at.% ti 0.12 0.14 0.13 0.12 0.11 0.09 pld/ms 0 at.% ti 1 at.% ti 3 at.% ti 5 at.% ti 10 at.% ti 25 at.% ti 0.13 0.15 0.14 0.18 0.15 0.16 table 1. friction on the layers of ti:dlc prepared by dualpld and pld/ms methods. different places. for each liquid, a new sample was used. from each droplet, series of 15 pictures were evaluated. laplace profile fitting was used. tested samples were on si (100) and thickness of the films was around 100 nm. we finished preliminary tests of the corrosion behavior as function of the amount of the titanium dopant in the dlc layers. we used potentiodynamic measurement and tafel plots for icorr determination and then for corrosion rate determination. 3. results contents of the titanium dopant in the dlc layers were determined by wdx and confirmed by xps. prepared samples concentrations were 0, 1, 3, 5, 10 and 25 at.% of titanium. dualpld technique was better to reach the desired dopants concentration. the xrd tests confirmed that all the samples were amorphous with detection of ti and tic phases for the highest ti concentration (10 and 25 at.% ti). thickness of the prepared layers were approximately 500 nm, for layers on ti6al4v substrates and 100 nm for layers on si (100). afm was used for morphology determination. both methods produced smooth layers around 1 nm, just for dualpld method roughness increased for 10 and 25 at.% ti up to 5 resp. 15 nm. wearability tests have been done for loads 1 n and 2 n. for these loads, there were no damage observed on samples. the wearability of the ball decreased with increased amount of titanium in layers. examples are on the figures 1 and 2. from comparison of the coated and uncoated materials, the uncoated materials were seriously damaged. the friction for pure dlc layers was 0.125 ± 0.05. for dual pld method we observed slight increase in friction for lower concentration up to 0.14 and then decrease to 0.09. for pld/ms method we observed slight increase for dopated layers but without clear trend, for more detail see table 1. the hardness for pure dlc layers starts at (31 ± 1) gpa and was decreasing with increasing of the titanium content to 12 ± 2 gpa for 25 at.% of titanium. the decrease was slightly quicker for pld/ms technique. the same behavior we observed for elastic modulus with the highest values around 250 gpa for pure dlc and decrease to 150 gpa for 25 at.% ti content of titanium practically same for both methods. the adhesion values seemed to increase with content of the titanium with exception for highest concentration of titanium for pld/ms method. critical force fc3 started at 5.6 n for pure dlc and increased to 18.4 n for dualpld technique for 25 at.% ti. for pld/ms technique critical force was increasing up to 20.2 n for 10 at.% ti but for 25 at.% ti it reduced to 9.2 n. this could be connected with lower hardness of titanium. the contact angle for water was in range from 57.7° to 71.5° for dual pld method and from 53.8° to 70.0° for pld/ms method. ca for ethylene glycol was in range from 34.5° to 42.8° resp. from 30.1° to 44.0° and for diiodomethane the range was from 12 vol. 8/2017 dlc/ti thin films properties prepared by hybrid laser technologies 34.9° to 38.3° resp. 32.0° to 36.4° for dual pld resp. pld/ms method. the results showed no trends in ca in correlation with titanium content in layers. from the free surface energy calculation we can conclude that all the tested layers are dominantly disperse. from the corrosion tests, the high concentration of the titanium dopants seemed to improve corrosion resistivity against the pure dlc. for various concentrations there were no clear trends observed and further tests should be performed before any conclusions could be made. 4. conclusion we prepared diamond-like carbon samples doped by titanium in concentration up to 25 at.% by two deposition techniques dual pld and pld/ms. we made standard testing as xrd, wdx, xps, afm, profilometry measurements etc. the main focus was put on mechanical and tribology testing. for both techniques, the hardness and elastic modulus were decreasing with higher content of the titanium, as we expected. there were no significant changes based on used methods. we observed better adhesion related to increasing titanium concentration in films. this is connected with lower compression stress due dopation. on the other hand, for the highest concentration for pld/ms there was lower adhesion observed, which could be connected with lower hardness. wearability test showed no damage to the coatings at load 1 n or 2 n (no matter on ti concentration) but wear of the testing ball was significantly higher for lower ti concentration in films. this is probably connected to higher layer hardness. tests will be continued with higher loads. for corrosion behavior, only preliminary tests are available. corrosion resistance was better for higher titanium amounts, when the highest concentration had up to three times better icorr parameter. for wettability, there are no clear trends based on titanium content. all layers are dominantly disperse. acknowledgements this work was supported by the grant agency of the czech technical university in prague, sgs16/110/ohk4/1t/17, sgs16/190/ohk4/2t/17, by meys cr grant lo1409 and by czech grant agency ga15-05864s. references [1] j. bulíř, m. novotný, m. jelínek, et al. plasma study and deposition of dlc/tic/ti multilayer structures using technique combining pulsed laser deposition and magnetron sputtering. surface and coatings technology 200(1 4):708 – 711, 2005. doi:http://dx.doi.org/10.1016/j.surfcoat.2005.01.087. [2] q. wei, r. narayan, j. narayan, et al. improvement of wear resistance of pulsed laser deposited diamond-like carbon films through incorporation of metals. materials science and engineering: b 53(3):262 – 266, 1998. doi:http://dx.doi.org/10.1016/s0921-5107(98)00150-0. [3] a. voevodin, m. capano, s. laube, et al. design of a ti/tic/dlc functionally gradient coating based on studies of structural transitions in ti-c thin films. thin solid films 298(1 2):107 – 115, 1997. doi:http://dx.doi.org/10.1016/s0040-6090(96)09145-6. [4] m. jelínek, j. zemek, t. kocourek, et al. dual laser deposition of ti:dlc composite for implants. laser physics 26(10):105605, 2016. 13 http://dx.doi.org/http://dx.doi.org/10.1016/j.surfcoat.2005.01.087 http://dx.doi.org/http://dx.doi.org/10.1016/s0921-5107(98)00150-0 http://dx.doi.org/http://dx.doi.org/10.1016/s0040-6090(96)09145-6 acta polytechnica ctu proceedings 8:11–13, 2017 1 introduction 2 experimental 3 results 4 conclusion acknowledgements references 157 acta polytechnica ctu proceedings 1(1): 157–162, 2014 157 doi: 10.14311/app.2014.01.0157 agile highlights after six years in orbit carlotta pittori1,2 on behalf of the agile collaboration 1asi science data center (asdc), via del politecnico snc, 00133 roma, italy. 2inaf-oar, via frascati 33, i00040 monte porzio catone (rm), italy. corresponding author: carlotta.pittori@asdc.asi.it abstract agile is an asi space mission in collaboration with inaf, infn and cifs, dedicated to the observation of the gammaray universe in the 30 mev – 50 gev energy range, with simultaneous x-ray imaging capability in the 18–60 kev band. the agile satellite was launched on april 23rd, 2007, and produced several important scientific results, among which the unexpected discovery of strong flares from the crab nebula. this discovery won to the agile pi and the agile team the bruno rossi prize for 2012 by the high energy astrophysics division of the american astronomical society. thanks to its sky monitoring capability and fast ground segment alert system, agile detected many galactic and extragalactic sources: among other results agile discovered gamma-ray emission from the microquasar cygnus x-3, detected many bright blazars, discovered several new gamma-ray pulsars, and discovered emission up to 100 mev from terrestrial gamma-ray flashes. we present an overview of the main agile data center activities and the agile scientific highlights after 6 years of operations. keywords: gamma-rays: observations catalogs. 1 introduction agile (astro-rivelatore gamma a immagini leggero) (tavani et al., 2009a) is a gamma-ray astrophysics mission of the italian space agency (asi), with inaf, infn and cifs participation which has been in orbit since 23 april 2007. the agile payload is composed of a pairproduction gamma ray imager (grid) sensitive in the energy range 30 mev-50 gev (barbiellini et al., 2001; prest et al., 2003), an x-ray imager (superagile) sensitive in the energy range 18-60 kev (feroci et al., 2007), and a mini-calorimeter (mcal) sensitive to gamma-rays and charged particles with energies between 300 kev and 100 mev (labanti et al., 2009). the instrument, weighting only ∼ 100 kg, is the most compact ever operational for high-energy astrophysics (approximately a cube of about 60 cm size) with excellent detection and imaging capability. agile is characterized by a very large field of view (∼ 3 sr), a good angular resolution, 0.1 0.2 degrees in gamma rays and 1-2 arcminutes in x rays, and a small dead time (100 µs), which make it a very good instrument to study persistent and transient gamma-ray sources even on very short timescales. recent calibration results of the agile-grid using in-flight data and monte carlo simulations producing instrument response functions have been presented in chen et al. (2013). 2 the ground segment and the agile data center the agile data center (adc) is part of the asi science data center (asdc), previously located in the esa establishment of esrin in frascati, and recently moved within the asi headquarters, rome (italy). satellite data are routinely collected every ∼ 96 minutes by the asi ground station in malindi (kenya), then quickly sent to the satellite operations centre in telespazio, fucino, and then transferred, preprocessed, analyzed, and stored at the adc. adc is in charge of all the scientific oriented activities related to the analysis, archiving and distribution of agile data. from july 2007 july to october 2009 (∼ 2.5 years) agile was operated in “pointing observing mode”, characterized by long observations called observation blocks (obs), typically of 2-4 weeks duration, following a predefined baseline pointing plan, mostly concentrated along the galactic plane. on november 4, 2009, agile scientific operations were reconfigured following a malfunction of the rotation wheel occurred in mid october, 2009. the satellite is currently operating regularly in “spinning observing mode”, surveying a large fraction (about 70%) of the sky each day. since december 21, 2012 due to asi’s malindi ground station technical problems, the acquisition of telemetry data from the agile satellite has been reduced. 157 http://dx.doi.org/10.14311/app.2014.01.0157 carlotta pittori figure 1: whole sky agile intensity map (ph cm−2 s−1 sr−1) in galactic coordinates and aitoff projection, for energies e > 100 mev, accumulated during the first ∼ 6 years of observations up to december, 2012 (pointing + spinning observing modes). malindi downlink efficiency for agile-grid data is currently at a daily level of about 80%. all agile payload functions are nominal, and the daily grid skymonitoring activities are fully operational, despite the telemetry gaps in the archive due to intermittent data acquisition at the ground station. the agile quicklook (ql) alert system is composed of two independent automated analysis parts: the ql scientific pipeline running at adc (pittori et al., 2013) and the agile-grid science alert system (sas) pipeline running at the inaf-iasf bologna (bulgarelli et al., 2013). data are automatically analyzed at every downlink and accumulated on different timescales. lightcurves of potential gamma-ray sources are produced using blind search techniques and crosscorrelation with a reference list of known gamma-ray emitters. as proper flux thresholds are exceeded, alerts for transient gamma-ray sources are automatically generated and notified through e-mails, sms messages, and also through a dedicated application for smartphones and tablets. these alerts are crosschecked, and a manual analysis is performed for the most interesting candidate gamma-ray sources. the daily monitoring activity of the agile ql alerts resulted in the publication of 104 atel and 40 gcn up to may, 2013, among which we mention the first detection of gamma-ray emission from microquasars above 100 mev, from cygnus x-3 and cygnus x-1 in the cygnus region, the prompt alert of many gamma-ray flares from blazars and galactic transients, and the surprising discovery of gamma-ray flares from the crab nebula. the 2012 bruno rossi international prize has been awarded to the pi, marco tavani, and the agile team for this important and unexpected discovery. 3 new agile catalogs and asdc interactive web tools the agile first catalog of high-confidence γ-ray sources detected by agile during its first year of operations (pittori et al., 2009) was recently updated using data covering the whole period of pointed observations, ∼ 2.3 years, in verrecchia et al. (2103) (“an updated list of agile bright gamma-ray sources and their variability in pointing mode”). refined analysis of complex regions of the galactic plane with improved event filter and calibrations yielded a new reference list, and a multi-source maximum likelihood analysis was performed over the timescale of the agile pointed observation blocks (ob) resulting in an updated list of 54 sources. eight 1agl sources, with significance above 4σ from the analysis over the entire observing period, were not detectable on the ob timescale either due to low ob exposures and/or to their positions within complex galactic regions. the new agile-grid variability catalog (verrecchia et al., 2103) is also available from the adc webpages as an interactive web table1. in the on-line version, light curves from fluxes in each ob are accessible within the asdc data explorer, 1http://www.asdc.asi.it/agile1rcat/ 158 agile highlights after six years in orbit in the agile-grid data products tab. the asdc data explorer is a multimission web tool which allows the user to browse both internal multimission archive catalogs (grouped by energy band) and external catalogs from other services (vizier, ned, simbad etc.), providing an easy way to explore and cross-correlate large data sets from radio to tev. moreover by using the asdc sed builder tool (v3.0) it is possible to build and handle spectral energy distributions (seds), time resolved seds, and multi-frequency lightcurves. virtual observatory (vo) tools like topcat can also be used to handle sed data in the time domain. the agile reference list of bright gamma-ray sources above 100 mev over the first 2.3 years of observations thus includes at the moment 62 sources, as compared to the 47 1agl in the first catalog. work is in progress to build new complete catalogs of agilegrid sources over the whole observing period (pointing+spinning). the mcal instrument of the agile satellite can observe the high-energy part of gamma-ray bursts (grb) from 350 kev to 100 mev with good timing capability, and the agile-mcal grb catalog, including 85 grb observed from april 2007 to october 2009, was recently published in galli et al. (2013). an online version of the new agile-mcal grb catalog is available from the adc web pages2. we adapted the asdc data explorer to provide the user with additional services dedicated to gamma-ray bursts through a grb explorer tool. the interactive web table in this case contains all the data for the published grbs, including spectral data for 21 bursts, plus some supplementary information on other 7 events. light curves and energy spectra (when available) can be accessed from the grb explorer tool, in the agile mcal data products tab. it is also possible to browse external services to look for grb-related data products from other missions, such as swift-xrt light curve from leicester repository, swift-bat product analysis, fermigbm quicklook lightcurve, gcns and blog for the chosen grb. 4 agile science highlights fig. 1 shows the total gamma-ray intensity above 100 mev as observed by agile up to december, 2012, during the first ∼ 6 years of observations. we present here a selection of the main agile science highlights and some recent updates. agile has provided the first detection of a colliding wind binary (cwb) system above 100 mev in the η-carinae region (tavani et al., 2009b). agile detected a gamma-ray source (1agl j1043-5931, now 1aglr j1044-5944) consistent with the position of the cwb massive system η-car during the time period 2007 july 2009 january. a 2-day gamma-ray flaring episode was also reported on 2008 oct. 11-13, possibly related to a transient acceleration and radiation episode of the strongly variable shock in the system. a revised gamma-ray source list in the complex carina region shown in fig. 2 has been published in verrecchia et al. (2013). figure 2: agile intensity map of the carina region above 100 mev over ∼ 2.3-year pointing mode observation period. new sources and revised (r) positions for 1agl sources are indicated in magenta, the 1agl error circles are indicated in black. the agile detection of gamma-ray emission from the pwn vela-x, described in a science paper (pellizzoni et al., 2010), has been the first experimental confirmation of gamma-ray emission (e> 100 mev) from a pulsar wind nebula. this result constrains the particle population responsible for the gev emission and establishes a class of gamma-ray emitters that could account for a fraction of the unidentified galactic gamma-ray sources. subsequently the nasa fermi satellite has confirmed the vela-x gamma-ray detection, and has also firmly identified other 4 pulsar wind nebulae plus a large number of candidates. gamma-ray flaring activity for a source positionally consistent with cygnus x-1 microquasar was reported twice by agile (bulgarelli et al., 2010; sabatini et al., 2010a and 2010b). agile extensive monitoring of cygnus x-1 in the energy range 100 mev 3 gev during the period 2007 july 2009 october confirmed the existence of a spectral cutoff between 1-100 mev during the typical hard x-ray spectral state of the source. however, even in this state, cygnus x-1 is capable of producing episodes of extreme particle acceleration on 1-day timescales, and even shorter lived flares in the tev range as detected by magic in 2006 (albert et al., 2007). gamma-ray flares of cygnus x-1 detected 2http://www.asdc.asi.it/mcalgrbcat/ 159 carlotta pittori by agile have been recently confirmed for the first time by a reanalisis of fermi-lat data (bodaghee et al., 2012). agile detected for the first time several gammaray flares from cygnus x-3 microquasar and also a weak persistent emission above 100 mev from the source (tavani et al., 2009c). gamma-ray flares occur either in coincidence with low hard-x-ray fluxes or during transitions from low to high hard-x-ray fluxes, and usually appear before major radio flares, following a clear repetitive pattern. this important agile finding has been confirmed by fermi-lat, which also detected the orbital period (4.8 hours) of the binary system in gammarays, securing unambigously the temporal signature of the microquasar (abdo et al., 2009). in the 9 days from december 2 to december 11, 2009 agile and fermi were able to answer a long-lasting question: cygnus x3 system is able to accelerate particles up to relativistic energies and to emit gamma-rays above 100 mev. figure 3: the cygnus region in gamma-rays: agile intensity map from 100 mev to 10 gev. data taken in ∼ 2-year pointing observing mode, from nov. 2007 to oct. 2009. figure adapted from g. piano presentation, 9th agile workshop, 2012. understanding the origin of cosmic rays is one of the most important issues of high-energy astrophysics, and galactic supernova remnants (snr) are considered to be an ideal laboratory to study cosmic-ray acceleration. experimental data analysis below e = 200 mev is crucial to discriminate between theoretical models, since in this energy range hadronic and leptonic emission spectra have a well distinct behavior due to a steepening of the hadronic spectrum due to the neutral pion emission, which is missing in the leptonic case. the agile gamma-ray imager reaches its optimal sensitivity just at the energies in the 50 mev a few gev range at which neutral pions produced by proton-proton interactions radiate, and it was the first to discover a pattern of gamma-ray emission from the supernova remnant w44 that, combined with the observed multifrequency properties of the source, can be unambiguously attributed to accelerated protons interacting with nearby dense gas. this important agile result was reported in (giuliani et al., 2011), and recently confirmed by new fermi-lat data (ackermann et al., 2013) before agile and fermi observations a direct identification of sites in our galaxy where proton acceleration takes place was elusive. figure 4: the w44 supernova remnant spectral energy distribution in gamma-rays. upper panel: the first direct evidence of the pion bump from agile data below 200 mev (giuliani et al., 2011). lower panel: new fermi-lat data below 200 mev confirm the observed steepening due to the neutral pion emission (ackermann et al., 2013). preliminary results from a new detailed study of the snr gamma-cygni with agile have been presented by g. piano during the 11th agile workshop3 (piano et al., paper in progress). from agile data analysis there are hints of non-thermal emission possibly related to shock-cloud interactions in the north-western part of the shell. the agile minicalorimeter is also detecting terrestrial gamma-ray flashes (tgfs), intense and brief pulses of gamma-rays originating in the earth atmosphere, and associated with thunderstorm activity. tgfs last a few thousandths of a second, and produce gamma-ray flashes up to 100 mev, on timescales as low as < 5 ms (marisaldi et al., 2010a). agile joins other satellites in detecting tgfs, but its unique capa3http://www.asdc.asi.it/11thagilemeeting/program.php 160 agile highlights after six years in orbit bility of detecting photons of the highest energies within the shortest timescales makes it an ideal instrument to study these impulsive phenomena. agile data have shown for the first time that tgf cumulative spectrum at high energy deviates from a power law with exponential cutoff model and can be better fit with a broken power law with significant counts above background up to 100 mev. the crucial agile contribution to tgf science is thus the discovery that the tgf spectrum extends well above 40 mev, and that the high energy tail of the tgf spectrum is harder than expected. combined observations of high-energy photons detected by the agile-grid detector have also provided the first direct localization of tgfs in gamma-rays, within a region clustered around the sub-satellite point (fuschino et al., 2011). thanks to its very low inclination orbit in the ±2.5 latitude band, agile provides the largest tgf surface density across the equator available up to now. as statistics grows, agile data show that the tgf sample is possibly composed by two distinct populations: a low-energy population, consistent with typical tgf characteristics, and a high-energy population, with quite distinct features (tavani et al., 2011; marisaldi et al., 2012). crab nebula variability: the surprising discovery by agile of variable gamma-ray emission above 100 mev from the crab nebula in sept. 2010 (tavani et al., 2010; tavani et al., 2011b), and the fermilat confirmation (buehler et al., 2010; abdo et al., 2011) started a new era of investigation of the crab system. the 2012 bruno rossi international prize has been awarded to the pi, marco tavani, and the agile team for this important and unexpected discovery. astronomers have long believed the crab to be an almost ideal standard candle, a nearly constant source (at a level of few percent) from optical to gamma-ray energies (meyer et al., 2010), with possible long-term nebular flux variations over a few-year timescale reported in the hard x-ray range. on september 2010 agile detected a rapid gamma-ray flare over a daily timescale, see fig.4, and thanks to its rapid alert system, made the first public announcement on september 22, 2010. this finding was confirmed the next day by the fermi observatory. agile had also previoulsy detected a giant flare from the crab in october, 2007 during the initial science verification period of the satellite, and in the first agile catalog paper (pittori et al., 2009), it was reported that anomalous episodic flux values observed from the crab in 2007 were under investigation. we know now of five major gamma-ray flares from the crab nebula detected by the agile-grid and fermi-lat, up to may 2013. gamma-ray data provide evidence for particle acceleration mechanisms in nebular shock regions more efficient than previously expected from current theoretical models. we estimate a recurrence rate for strong gamma-ray flares of ∼ (1 − 2)/year. figure 5: the crab nebula flare in september 2010, as observed by agile at energies above 100 mev, (tavani et al., 2011b). a recent study (striani et al., 2013 ) of the flux and spectral variability of the crab nebula above 100 mev on different timescales, ranging from days to weeks, both in agile-grid and in fermi-lat data, also gives evidence for week-long and less intense episodes of enhanced gamma-ray emission that we call “waves” which can occur by themselves or in association with shorter flares. both the flaring and “wave” events can be related to a population of accelerated electrons consistent with a mono-chromatic or relativistic maxwellian distribution. gamma-ray observations challenge standard magneto-hydrodynamics models of nebular emission. 5 conclusions the study of cosmic gamma-rays in the energy range from a few tens of mev to a few tens of gev is only possible from space due to atmospheric absorption. with the successful launch of the new generation gamma-ray space asi telescope agile on april 2007, followed by nasa fermi on june 2008, gamma-ray astrophysics entered a new era. agile is substantially improving our knowledge on various known gamma-rays sources, such as supernova remnants and black hole binaries, pulsars and pulsar wind nebulae, blazars and gamma ray bursts. moreover, agile has contributed to the discovery and study of new galactic gamma-ray source classes, and of galactic gamma-ray transients. the mission is also giving a crucial contribution to the study of the terrestrial gamma-ray flashes seen in the earth atmosphere. gamma-ray observations challenge standard theoretical models of particle acceleration and dynamics under extreme matter conditions. acknowledgement research partially supported through the asi grants i/089/06/2, i/042/10/0 and i/028/12/0. 161 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[32] tavani, m. et al., 2011b, science, 331, 73. doi:10.1126/science.1200083 [33] verrecchia, f., et al., 2013, a&a 558, a137. discussion luigi piro: how can you distinguish between inverse compton and π0 decay in the gev range (question to manami sasaki talk on snrs, tue 28 afternoon). carlotta pittori comment: my comment to luigi piro question is that, as i have shown in my talk, at least in some cases as w44, in which middle aged snr interacts with molecular clouds, the new infrared planck data are very much constraining, and the overall multiwaveleght picture seem to largely favour π0 decay and hadronic models over the leptonic ones. 162 http://dx.doi.org/10.1126/science.1182174 http://dx.doi.org/10.1126/science.1199705 http://dx.doi.org/10.1088/0004-637x/775/2/98 http://dx.doi.org/10.1029/2009ja014502 http://dx.doi.org/10.1126/science.1183844 http://dx.doi.org/10.1088/2041-8205/712/1/l10 http://dx.doi.org/10.1038/nature08578 http://dx.doi.org/10.1126/science.1200083 introduction the ground segment and the agile data center new agile catalogs and asdc interactive web tools agile science highlights conclusions 170 acta polytechnica ctu proceedings 1(1): 170–174, 2014 170 doi: 10.14311/app.2014.01.0170 black hole results from xmm-newton norbert schartel1 1xmm-newton science operations centre, esa, villafranca del castillo, apartado 78, e-28691 villanueva de la cañada madrid, spain corresponding author: norbert.schartel@sciops.esa.int abstract xmm-newton is one of the most successful science missions of the european space agency. since 2003 every year about 300 articles are published in refereed journals making directly use of xmm-newton data. all xmm-newton calls for observing proposals are highly oversubscribed by factors of six and more. in the following some scientific highlights of xmm-newton observations of black holes are summarized. keywords: x-ray agn black hole. 1 introduction xmm-newton ([1]) is the second cornerstone of european space agency’s (esa) horizon 2000 science programme, providing an observatory-class x-ray facility. the spacecraft was launched by an ariane 5 on 10 december 1999. the observatory provides simultaneous non-dispersive spectroscopic imaging and timing (european photon imaging camera; epic, [2] and [3]), medium resolution dispersive spectroscopy (reflection grating spectrometer; rgs, [4]) and optical/uv imaging, spectroscopy and timing from a co-aligned telescope (optical monitor; om, [5]). the three x-ray mirrors ([6]) in combination with the cameras of epic offer a large effective area over the energy range from 300 ev to 12 kev, up to 2500 cm2 at 1.5 kev and ∼1800 cm2 at 5 kev. the scientific potential of the effective area may be illustrated by the first observation of an evolving dust-scattered x-ray halo around a gamma ray burst ([7]). each of the two modules of the rgs cover the energy range from ∼0.4 kev to 2.2 kev with an effective area of 60 cm2 at 15 å. 2 scientific highlights scientific highlights resulting from the first decade of xmm-newton and chandra observations can be found in [8]. in the following i list a number of highlights from xmm-newton observations of black holes. in this paper i have focused on some of the most exciting discoveries in this field, which also received wide publicity in public relations announcements by esa. 2.1 galactic black holes and ultraluminous x-ray sources globular clusters (gc), containing thousands of stars packed within tens of light years, were considered as a possible breeding ground for black holes. a rival hypothesis suggests that black holes are ejected through close star encounters and consequently gcs are devoid of black holes. xmm-newton observations of ngc 4472 allowed the first detection of a black hole in a gc ([9]) excluding the latter hypothesis. ultraluminous x-ray sources (ulx) were proposed to harbour intermediate-mass black holes, which provides the link between stellar mass black holes and supermassive black holes (smbh) in the centres of galaxies. xmm-newton and chandra observations of cxom31 j004253.1+411422 in andromeda allowed to connect this ulx to low mass x-ray binaries ([10], implying accretion onto a stellar-mass black hole in the eddington regime. a second ulx found in andromeda, xmmu j004243.6+412519, allowed to observe the emission of the accretion disk in x-rays together with the emission form its jets in radio ([11]). the experimental key for both observations was the low absorbing column density towards andromeda whereas absorption is a major obstacle of ulx observations in our own galaxy. [12] found an intermediate-mass black hole in ngc 1313. the x-ray spectra of the ulx can be described with a power law plus an accretion disk (kt ∼= 150 ev) implying a mass of ∼= 103m�. a intermediatemass black hole with m > 500 m� could be associated with an ulx in eso 243-49 based on luminosity variations observed by xmm-newton ([13]). 170 http://dx.doi.org/10.14311/app.2014.01.0170 black hole results from xmm-newton 2.2 the strong gravitational field currently, x-ray observations are the only way to observe the strong gravitational field in the direct vicinity of black holes and neutron stars ([14]). special and general relativistic effects distort the spectra of particles orbiting black holes depending on the orbital parameters and the black hole’s spin. theoretical discussion can be found in [15], [16], [17] [18] or more recently in [19] and [20]. experimentally the iron kα line is most studied as there are no lines of abundant elements nearby. early examples are the xmm-newton observations of the galactic black hole xte j1650-500 in outburst ([21]) and the active galactic nuclei (agn) mgc-6-30-15 in low state ([22]). both spectra are explained by a fast spinning black hole and the extraction and dissipation of rotational energy from it. the simultaneous xmm-newton and nustar observation of ngc 1365 revealed that reflection from an ionized disk readily explains the spectra taken by both satellites ([23]). the xmm-newton spectra of the galactic black hole, gx 339-4 in outburst ([24]) is an example of a black holes with almost maximum spin. the observation of 1h 0706-495 is unique as it shows not only the iron kα, but also the iron lα line ([25]). in addition the light cure shows the expected characteristic variability of reflection from an ionized disk (compare also [26], [27], [28], [29]). 2.3 active galactic nuclei (agn) in low states following [30] x-ray spectra of agns are composed of a power-law continuum emitted above a black hole plus reflection from an ionized disk. during the low state the continuum emission region moves nearer to the black hole and gravitational bending affects its light path. observationally, during low states the continuum emission appears suppressed whereas the reflected emission appears constant or even enhanced, compare also [31]. the most intensive studied agn in low state with xmm-newton is pg 2112+059, where an additional layer of ionized material was used to favour the reflection interpretation versus alternative scenarios ([32], [33] and [34]). [35] used variability considerations to discriminate the reflection interpretation versus an absorption scenario for pg 0844+349 in an x-ray weak state. and [36] could demonstrate for the low state observation of 1h0707-495 reflected emission within one gravitational radius of the event horizon of the black hole. 2.4 aspects of variability near supermassive black holes (smbhs) whereas quasi-periodic-oscillations (qpo) are well established in x-ray binaries for almost 30 years, qpos remained elusive in agns. xmm-newton measured a ∼1 hour qpo for re j1034+396 ([37]). [38] found evidence for orbital motion of material close to the central black hole of mrk 766. a xmm-newton observation allowed [39] to observe a co-rotating flare at a distance of only 3.5 to 8 schwarzschild radii to the smbh of ngc 3516. 2.5 energy budget, winds and outflows [40] established for the fist time simultaneous spectral energy distributions for the majority of the [41] reverberation mapped sample of agn based on xmmnewton epic and om measurements. [42] used xmmnewton observations to show that radio-galaxies produce sufficient mechanical energy to unbind a significant fraction of the intra-group medium, an effect which is negligible in massive clusters of galaxies. combining high resolution rgs spectra with sensitive light-curves of epic, [43] demonstrated an accretion-disk origin for the two warm absorber winds in ngc 4051. 1h 0707495 shows a mildly relativistic, highly ionized outflow which changed its velocity from about 0.11c to 0.17c between 2008 january and 2010 september ([44]). ultrafast outflows are present in >35% of radio-quiet agn observed with xmm-newton, providing a significant contribution to the agn cosmological feedback ([45], [46], [47]). 2.6 flares and tidal disruption events [48] observed several peaks in the power density spectrum of the x-ray light curve of the smbh in the galactic centre during which period a bright x-ray flare was detected ([49]). theoretical studies revealed a previously unknown topological structure inherent to black holes with high spin: in a small region near the event horizon of the spinning black hole the orbital velocity decreases for decreasing orbital radius ([50]). this effect is now rightfully known as the aschenbach effect ([51]). [52] could identify a tidal disruption event based on rosat, chandra and xmm-newton observations. suzaku and xmm-newton observations taken shortly after the occurrence of the tidal disruption event swift j164449.3+573451 reveal a 200-second x-ray quasiperiodicity ([53]). this qpo might be explained with the forming of an accretion disc or precession of the jet. 171 norbert schartel 2.7 deep fields and cosmology a total of 1000 agn detections from a variety of rosat, xmm-newton and chandra surveys allowed [54] to obtain for the first time reliable space densities for low-luminosity (seyfert-type) x-ray sources at high cosmological redshifts. their evolutionary behaviour shows strong dependency on the x-ray luminosity and differs from the dependency found for high luminosity agns and quasars. xmm-newton allows the detection of quasars at highest redshift, e.g. sdss j104433012502 at z=5.80 ([55]). the spacecraft could even establish an x-ray spectrum of sdss j1030+0524 at z=6.30 ([56]). an ionized iron kα absorption edge in the x-ray spectrum of apm 08279+5255 allowed to obtain an, at the time of publication, highly interesting constrain on the age of the universe ([57]). 3 discussion and conclusions since 2003 every year about 300 articles are published in refereed journals making directly use of xmm-newton data. all xmm-newton calls for observing proposals are highly oversubscribed by factors of six and more. within esa’s mission extension scheme all missions are evaluated every 2 years and possibly extended by 4 years subjected to midterm confirmation. xmmnewton is funded up to end of 2016 subject to midterm confirmation and further extension discussion in 2014. currently, the xmm-newton mission is implementing four-reaction-wheel operation schemata, which will reduce fuel consumption significantly. the envisaged operation mode will allow technically operating the mission up to 2026. acknowledgement i thank the organizers of the workshop and especially franco giovannelli for giving him the opportunity to show highlights of xmm-newton black hole observations. and i thank the anonymous referee for many fruitful comments and suggestions. references [1] f. jansen et al. xmm-newton observatory. i. the spacecraft and operations. a a, 365:l1–l6, january 2001. 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[57] g. hasinger, n. schartel, and s. komossa. discovery of an ionized fe k edge in the z=3.91 broad absorption line quasar apm 08279+5255 with xmm-newton. apj, 573:l77–l80, july 2002. 174 http://dx.doi.org/10.1103/physrevd.71.024037 http://dx.doi.org/10.1126/science.1223940 introduction scientific highlights galactic black holes and ultraluminous x-ray sources the strong gravitational field active galactic nuclei (agn) in low states aspects of variability near supermassive black holes (smbhs) energy budget, winds and outflows flares and tidal disruption events deep fields and cosmology discussion and conclusions 20 acta polytechnica ctu proceedings 1(1): 20–26, 2014 20 doi: 10.14311/app.2014.01.0020 gravitational waves and dark energy peter l. biermann1,2,3,4,5, benjamin c. harms1 1department of physics and astronomy, the university of alabama, box 870324, tuscaloosa, al 35487-0324, usa 2mpi for radioastronomy, bonn, germany 3karlsruhe institute of technology (kit) institut für kernphysik, germany 4department of physics, university of alabama at huntsville, al, usa 5department of physics & astronomy, university of bonn, germany corresponding author: bharms@bama.ua.edu abstract the idea that dark energy is gravitational waves may explain its strength and its time-evolution. a possible concept is that dark energy is the ensemble of coherent bursts (solitons) of gravitational waves originally produced when the first generation of super-massive black holes was formed. these solitons get their initial energy as well as keep up their energy density throughout the evolution of the universe by stimulating emission from a background, a process which we model by working out this energy transfer in a boltzmann equation approach. new planck data suggest that dark energy has increased in strength over cosmic time, supporting the concept here. the transit of these gravitational wave solitons may be detectable. key tests include pulsar timing, clock jitter and the radio background. keywords: cosmology dark energy black holes gravitational waves. 1 introduction dark energy was originally detected as accelerated expansion seen in the distance scale for supernovae of type ia (schmidt et al., 1998, riess et al. 1999, perlmutter et al. 1999; for a review see frieman et al. 2008). many suggestions have been made about what dark energy is, what its strength is, what its time evolution is, and what possible further observational results are. the idea that dark energy is gravitational waves may explain its strength and its time-evolution. one possible concept is that dark energy is the ensemble of coherent bursts (solitons) of gravitational waves originally produced when the first generation of supermassive back holes was formed (caramete & biermann 2010); the energy density of such solitons would suffice within the uncertainties. these solitons get their initial energy as well as keep up their energy density throughout the evolution of the universe by stimulating emission from a background (biermann & harms 2013). our model of the background metric resembles the randall-sundrum ideas (1999a, b) but is timedependent, and describes the energy flow from the background (strong-gravity) brane to our world (weakgravity) brane. planck data suggest that dark energy has increased in strength over cosmic time (planck 2013 xvi), as predicted by our model. gravitational waves were far below today’s dark energy at the epoch of early nucleosynthesis and of the formation of the microwave background ripples (as summarized in ligo+virgo-coll. 2009), both much earlier than the likely formation epoch of the first generation of super-massive black holes. our model is also consistent with early star formation (biermann et al. 2014), as we argue below. the transit of the gravitational wave solitons postulated here may be detectable. we discuss the predictions briefly below and elsewhere. we focus on the boltzmann equation approach, working out the energy transfer from the strong gravity background in stimulated emission. 1.1 gravitational solitons from black hole mergers inspired by bekenstein’s (1973) considerations we posit: when the first generation of super-massive black holes was formed, each produced a coherent burst of solitonlike gravitational waves which combine to give a total energy of order ∼ 1 2 nbh,0mbh c 2 (1 + z?) 3 . (1) in the following we also call this an ensemble of soliton waves, or shell fronts. nbh,0 is the original comoving density of super-massive black holes. today supermassive black holes have a density of 10−1.7±0.4 mpc−3 20 http://dx.doi.org/10.14311/app.2014.01.0020 gravitational waves and dark energy above mbh = 3·106 m� (caramete & biermann 2010); assuming that they grow by merging, and allowing for statistical and systematic errors, an original comoving density of nbh,0 = 1 mpc−3 seems possible. this comoving density is the density black holes had at the beginning, so transposed to today without change in their numbers per comoving volume. the data suggest that there was a generation of first super-massive black holes with a mass between mbh ∼ 106m� and mbh ∼ 107m�. the original black hole mass may be ∼ 3 · 106m� considering (i) the black hole mass function (greene et al. 2006, caramete & biermann 2010), (ii) the instability of massive stars (appenzeller & fricke 1972a, b) in an agglomeration picture (spitzer 1969, sanders 1970), and (iii) the observed black hole in our galactic center (e.g. eckart et al. 2005). the redshift of creation z? may be large, as formation of massive stars may begin at redshift 80 (biermann & kusenko 2006). redshifts z? from about 30 to 50 allow a quantitative interpretation of the data of dark energy. at the original density of black holes adopted here redshift 50 is consistent with the mass of mbh ∼ 3 · 106m�, and redshift 30 would imply mbh ∼ 107m�, in either case to make the estimate consistent with dark energy today. what is the motivation for considering gravitational waves? bekenstein (1973) wrote about the entropy of the universe: “... we must regard black hole entropy as a genuine contribution to the entropy content of the universe”. however, entropy is also information, and information must have a carrier. a natural suggestion is that this carrier is gravitational waves, with an energy commensurate with the black hole scale. this suggestion is consistent with the fact that cosmological black holes are not in thermodynamic equilibrium, and therefore the entropy associated with such black holes should be described by statistical mechanics as advocated in harms and leblanc (1992, 1993). this speculation immediately gives s kb = ngw,0 = 4π ( mbh mpl )2 , for zero spin , (2) where ngw,0 is the number of gravitons at the formation of the black hole. for mbh = 3 · 106 m� this is ngw,0 ' 1090. mbh is the original mass of the black hole, mpl is the planck mass, c is the speed of light, and gn is newton’s constant of gravity. egw is the average graviton energy given by egw = 1 8π h̄c3 gnmbh = c2 8π m2 pl mbh . (3) this gives a graviton energy of egw ' 10−30 erg for this black hole mass. the entire energy content then is ngw,0egw = 1 2 mbh c 2 . (4) we picture this as a coherent burst of gravitational waves, or a soliton wave, ejected at formation of the black hole. it is clear from the considerations above that we are not using the weak-field approximation. multiplying with the original density of super-massive black holes reproduces our estimate above. 1.2 five-dimensional background model in our model for the background, which has some similarity to the randall-sundrum (1999a, b) ideas, we identify a possible local metric to describe a 5d world with a 4d strong gravity brane and our 4d world weakgravity brane. ds2 = −e(u/l) m t/ψ c2 dt2 + e(ul ) p t/β du2 + e(1−b(ul ) n ) 2 t/αth e−(ul ) k (1− t φ ) dxi dx i , (5) where i = 1, 2, 3, τh is the hubble time, l = lpl is the planck length, τpl = lpl/c is the planck time, u is the coordinate in the fifth dimension, and the remaining, non-coordinate quantities are arbitrary parameters. although the five-dimensional covariant divergence of the energy-momentum tensor does not vanish everywhere, it vanishes on the weak-gravity (weak) brane (u = 0), and it is approximately zero on the strong-gravity (strong) brane (u = l) for very small dimensionless ratios β/τh and ψ/τh with ψ < 0 and β > 0, with the conditions a) τh(ψ+β)/(ψβ) >> 1, b) 2(b−1)/α > τh/φ−1, and c) α2 = 3. g00 in the metric above then defines the confining potential for the strong brane. gravitons from our brane stimulate transitions between bound states on the strong brane, resulting in the emission of gravitons onto our brane. this metric describes a weak brane for our world which is expanding with time, and a strong brane which is contracting with time, albeit very slowly for the latter brane. the 5d-cosmological constant measured on the weak brane is ωweak = − ( τpl τh )2 . figure 1: the stimulated emission of gravitons in a shell in our model the gravitons on the strong-gravity brane obey a planck-like distribution with planck temperature. we adopt the point of view that the planck scales are limits: nothing can go below planck time 21 peter l. biermann, benjamin c. harms and planck length, and no single particle can go beyond planck energy, in any frame. the strong brane is stable against collapse, since given a planck spectrum for any wavelength λ the free-fall time scale τff is always either equal or longer than the pressure wave time scale τs. figure 2: the strong-gravity (planck brane) and weak-gravity (tev brane) branes τff = τpl ( λ lpl )3/2 ≥ τpl ( λ lpl ) = τs. 2 stimulate emission of energy from the strong-gravity brane in the following we use a particle-wave duality for gravitons at high energy, which thus associates a wavenumber ~k/h̄ and a corresponding length-scale λ to each spatial direction, and we assume localization is possible to about a wavelength. 2.1 rate of energy transfer the distribution function n (k, t) is the distribution of occupied allowed states on our brane for gravitons with momenta k = |~k| and p = |~p| at time t on the shell and satisfies the equation( ∂ ∂t − ṙ(t) r(t) k ∂ ∂k ) n (k, t) = 1 k ∫ d3 k′ (2π)3 2 k′ ∫ cd3 p (2π)3 2e(p) ∫ cd3 p′ (2π)3 2e(p′) ∫ d3 k′′ (2π)3 2 k′′ γ2 |m |2 (2π)4δ4(k +q−k ′ −k ′′ −q′) (2π)6 δ3(~k′ − ~k′′) δ3(~k − ~k′) [gb(p ′, t)(1 +n (k, t)) ((n (k′, t) + 1)(n (k′′, t) + 1)− 1) − n (k, t)gb(p, t)(1 +n (k′, t))(1 +n (k′′, t))] . (6) where k = (k,~k), γ = k3ref,1, and q = (e, ~p). kref,1 is a reference momentum to be determined below. the δ-functions, δ3(~k′ − ~k′′) and δ3(~k − ~k′), have been inserted to impose coherence of the outgoing gravitons. |m |2 is the matrix element squared for the quadrupole emission of a graviton of 4-momentum k′′, and has the dimensions of (momentum)−3 (time)−1. we will assume that this matrix element squared is proportional to k5. gb(p, t) is the occupation number distribution of the background particle sea. r(t) = (1 + z?)/(1 + z) is the scale factor for an expanding universe. the following analysis is done in the observer frame. the boltzmann equation for n (k, t) to lowest order in the expansion of the 4-dimensional δ-function is( ∂ ∂t − ṙ(t) r(t) k ∂ ∂k ) n (k, t) ' +κ k n (k, t) (n (k, t) + 1) , (7) where the factor κ is given, after integration over all the δ-functions, by κ = k2ref,2h(z) 24πkbh |m |2 ln{kbh+ k } . (8) in the equation above kbh+ is the maximum momentum at which stimulated emission of gravitons occurs, just above the momentum of the peak of n (k, t). since the log-term in eq.8 varies very slowly over the range of k of interest we approximate this term with a constant and set β = {ln(kbh+/k)}/(24 π). |m |2 is related to |m |2 by extracting the factors (k/kref,1)3/k3ref,1, h(z). a threshold function of 2 gbn/(gb + n ) arises from the normalized interaction between the gravitons on the background brane and our brane; this function connects to the strong brane only if n > gb, which is the condition for stimulated emission. these choices do not introduce new constraints or additional assumptions. next we redefine (k/kref,2)2 κ = κ to extract the k-dependence from |m |2. making the change of variables k = k̃/r(t), eq.7 can be written as ∂n (k̃, t) ∂t ' h(z) k̃ β kbh r(t) n (k̃, t) (n (k̃, t) + 1) . (9) in terms of the frequency of the wave at emission this equation is ∂n (ν0, t) ∂t ' h(z)h ν0 β kbh r(t) c n (ν0, t)(n (ν0, t) + 1) . (10) introducing the dimensionless variables x = h ν0 kb tg0 , and y = ∫ t 0 kbtg0h(z)β kbh cr(t) dt′ , (11) where kbhc = kbtg0 = mplc 2 mpl 8πmbh . eq.(10) becomes ∂n ∂y ' +xn (x, y) (n (x, y) + 1) . (12) the solution of this equation is n (x, y) = 1 ex(a−y)+b − 1 , (13) 22 gravitational waves and dark energy where a and b are constants, to be determined later. this distribution (eq.13) is planck-like with a timedependent normalized temperature 1/a. the rate at which energy is created can be calculated from the expression for n in eq.10. the rate at which energy is created can be calculated from the expression for n in eq.(10). the rate of energy creation per existing graviton (of the total number ngw,0r(t)4) is d < e > dt = βh(z) r(t) × (14)∫ x3 hν0n (ν0, t) (n (ν0, t) + 1) dx . the total rate of energy creation is d < et > dt = ngw,0r(t)3 kbh ch(z)β a , (15) where a = ∫ x4n (x, t) (n (x, t) + 1) dx (16) inserting all these constants into the integral for y demonstrates that y approaches a constant for the redshift z? being large, and integrating down to today or even into the future, when y approaches a constant of β << 1. a is equivalent to an inverse temperature, and should be of order unity. without loss of generality we can set b = 0. this integral strongly depends on the exact value of a − y, and is of order 30 for a − y ' 1, and b approaching zero. the matrix element |m | does not evolve with time, and scales as momentum |m | = εm ( k mplc ) . (17) above we have used εm = 1; we now generalize and allow εm to be different from unity. writing |m | in this way suggests that the interaction between the gravitons on our brane and the gravitons on the background brane comes down to a fundamental coupling constant. this behavior is consistent with the idea [22], that the gravitational coupling strongly increases with energy to approach the other three coupling constants at near planck energies. this allows the expression for d<et> dt to be consistent with the observed energy density under the condition that aβ εm = 3. for the constant a = 1 above, β of order 0.1, and εm = 1, the quantity {aβ εm} is in fact close to 3. however, if we were to require that the k-range be very large, then β would be larger, and εm would be required to be smaller than unity accordingly. inserting this parameter dependence into eq.(15) then leads back, to within the approximation that aβ εm = 3, to the result we were seeking, 3 2 mbh c 2h(z) ( 1 + z? 1 + z )3 . (18) after integrating we obtain with this redshift dependence a constant dark energy density as in eq.( 1) by multiplying by the redshift evolution of black holes nbh,0 (1 + z)3. the factor of 4 multiplying ρde in eq.22 below derives from the sum of dark energy density and pressure, and corresponds to 3 (ρde+pde/c 2). therefore the rate of change of dark energy with time is 3 (ρde + pde/c 2)h(t) and today 3ρdeh(z = 0) = 3 2 mbh c 2h(z = 0) (1 + z?) 3 . (19) this justifies the ρde h(t) term in eqs.22 and 25. this shows that dark energy remains at the level of eq.(1) throughout the evolution of the universe, in the approximation that most early super-massive black holes were formed over a short span of time. 2.2 equation of state we define ρde(t, u) as the dark energy density, ρ(t, u) as the total energy density, p (t, u) as the total pressure, and we use the equation of state pde(t, u) = ρde(t, u) c2/3 . the dark energy density ρ(t, u) is scaled to the value of the dark energy density observed on our brane today ( redshift z = 0 ). the friedmann-robertson-walker form of the einstein equations on the weak-gravity brane must be (h(t)) 2 = ( ˙r(t) r(t) )2 = 8πgn 3 ρ(t, 0) (20) and r̈(t) r(t) = −4πgn 3 ( ρ(t, 0) + 3 p (t, 0) c2 ) + 16πgn 3 ρde(t, 0) + 4πgn sinj 3h(t) , (21) where the rate of change of the energy density used is ρ(t, 0) = − 3 ( ρ(t, 0) + p (t, 0) c2 ) h(t) + 4 ρde(t, 0)h(t) + sinj . (22) we emphasize that in the second equation, eq.21, the term 16πgn ρde(t, 0)/3 corresponds to the continuous energy transfer by stimulated emission on the basis of existing dark energy; the additional term (4πgn sinj)/(3h(t)) describes new formation of dark energy. this allows a different equation of state for exactly the same cosmological observations, as now for pde = ρdec 2/3 the modified equation 21 becomes identical to the canonical version of this equation for 23 peter l. biermann, benjamin c. harms pde = −ρdec2. the corresponding set of equations for the strong-brane are hsb(t)2 = 8πgn 3 (ρde(t, lsb)− λsb) (23) where λsb is the cosmological constant on the strong brane at the beginning of the epoch of black hole formation and r̈sb(t) rsb(t) = −16πgn 3 ρde(t, 0) h(t) hsb(t) (24) + 8πgn 3 ( 3 ρde(t, lsb)− λsb)− 4πgn sinj 3hsb(t) , the rate of change of the energy density on the strongbrane is correspondingly ρ̇de(t, lsb) = 3 ( ρde(t, lsb) + 1 c2pde(t, lsb) ) hsb(t) −4 ρde(t, 0)h(t)− sinj . (25) eq.24 is derived from eq.23 using the conservation of energy-momentum equations; eqs. 22 and 25 insure that energy is conserved between the two branes, the 4-dimensional boundaries of the 5-dimensional universe described by our model. for epochs before the energy transfer started ρde(t, 0) = 0, and sinj = 0, and we can set ρde(t, lsb) = λsb for t < t(z = z?). 3 comparison to experimental limits our model can be tested by several different types of experiment. we discuss two of these below. 3.1 pulsar timing experiments figure 3: gravitational wave background limit from pulsar timing (dashed line) , and our inferred gravitational wave background from stimulated emission of gravitational waves from the background planck sea constituting dark energy (straight line). the ordinate is the fraction of closure density ω per log bin of frequency, and the abscissa is the frequency of the gravitational waves f . the gravitational waves in our model arise from the production of black holes in the early universe at a redshift of z ' 50. the observed constant dark energy density is maintained by the continuous production of gravitational waves by black-hole interactions with the planck sea background. for an idealized model in which all black holes were created at the same time, and with the same mass, the gravitational wave background peaks near fgw,max ' 10−4.5 hz m−1bh,6.5 (1+50)/(1+ z?). a soliton comes past a given point in space-time on the order of every 20 seconds. wave forms are created by uncorrelated solitons passing by a given point. ultra-precise timing experiments which test the steadiness of timing over time scales of order a few seconds to a few minutes would show these variations in the energy density if the precision is high enough. the precision to detect such a signal has to correspond to seconds in the expansion rate of the universe, which requires the precision to be of order 10−17.5, or a few 10−18. this precision is expected to be reached in the next generation of clocks [16]. t ρ 1 figure 4: the sequential passing of solitons (sharp peaks) can be approximated by a sinusoidal wave form. 4 conclusions if the validity of our model is proven by experimental tests such as the pulsar timing experiments (fig.3), the detection of time jitter (fig.4), or the detection by an observatory such as ligo or virgo of the passage of a shell front by a dedicated type of data analysis, the implications for cosmology are great. the basis of our model is that the source of dark energy is the creation of gravitational waves by the interaction of surfaces at critical density, e.g. the planck surfaces surrounding black holes, with a strong-gravity brane located a few planck lengths from our weak-gravity brane. our model is consistent with the big-bang theory after the first planck time. however, in our model the universe starts from a lemaitre-like ‘atom’ or ‘seed’ [5]. our model also has implications for quantum gravity theory. experimental evidence for the validity of 24 gravitational waves and dark energy our model would imply the existence of a strong-gravity brane and extra dimensions as well as a smallest distance and impenetrable planck surfaces rather than horizons. although our model describes several observed cosmological phenomena, it is largely heuristic. we are currently working on an exact solution of the fivedimensional space-time metric and a more formal mathematical description of the stimulated emission amplitude for the creation of the solitons. acknowledgement discussions with lou clavelli greatly contributed to the development of the paper; discussions with laurenţiu caramete (bucharest, romania), roberto casadio (bologna, italy), marco cavaglia (oxford, ms), laszlo gergely (szeged, hungary), shaoqi hou (tuscaloosa, al), pankaj joshi (mumbai, india), gopal-krishna (pune, india), octavian micu (bucharest, romania), piero nicolini (frankfurt, germany), norma sanchez (paris, france), joe silk (oxford, united kingdom), allen stern (tuscaloosa, al), dijan stoikovic (buffalo, ny), and hector de vega (paris, france) are gratefully acknowledged. helpful comments on earlier versions of the manuscript were received from r. casadio, m. cavaglia, l. gergely, a. graham, p. joshi, o. micu, p. nicolini, and d. stoikovic. this research was supported in part by the doe under grant de-fg02-10er41714. references [1] appenzeller, i., & fricke, k., a&a 18, 10 (1972a); appenzeller, i., & fricke, k., a&a 21, 285 (1972b) [2] bekenstein, j.d., phys. rev. d 7, 2333 (1973) doi:10.1103/physrevd.7.2333 [3] biermann, l., schlüter, a., phys. rev. 82, 863 (1951) doi:10.1103/physrev.82.863 [4] biermann, p.l., kusenko, a., phys. rev. lett 96, 091301 (2006) doi:10.1103/physrevlett.96.091301 [5] biermann, p.l., & harms, b.c., eprint arxiv:1205.4016 (2012); 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ade, p.a.r., et al., eprint arxiv:1303.5078 (2013) [16] predehl, k., et al., science 336, 441 (2012) [17] randall, l., sundrum, r., phys. rev. lett 83, 3370 (1999a); randall, l., sundrum, r., phys. rev. lett 83, 4690 (1999b) [18] riess, a.g., et al. aj 116, 1009 (1998) [19] sanders, r.h., apj 162, 791 (1970) [20] schmidt, b.p., et al., apj 507, 46 (1998) [21] spitzer, l., jr., apjl 158, l139 (1969) doi:10.1086/180451 [22] dimopoulos, s, raby, s.a., wilczek, f. phys. today oct., p. 25 (1991) discussion moshe elitzur: are there predictions for the effect of your model on the imprint of fluctuations on cmb and baryon acoustic oscillations? benjamin harms : dark energy in our mode is due to the merging of black holes at a ' 50, so well after the formation of the cmb. there may be some effect on the propagation of the cmb photons, but we have not yet worked out the exact nature of this effect. 25 http://dx.doi.org/10.1103/physrevd.7.2333 http://dx.doi.org/10.1103/physrev.82.863 http://dx.doi.org/10.1103/physrevlett.96.091301 http://dx.doi.org/10.1093/mnras/stu541 http://dx.doi.org/10.1146/annurev.astro.46.060407.145243 http://dx.doi.org/10.1016/j.newar.2006.06.080 http://dx.doi.org/10.1038/nature08278 http://dx.doi.org/10.1086/180451 peter l. biermann, benjamin c. harms jim beall: can you comment on the effect these ’seed’ black holes have on galaxy formation? benjamin harms: the main difference between our model and the ’big bang’ theory is that our model allows for large assemblies of stars which have never merged and do not have an agn, which have apparently been observed. 26 introduction gravitational solitons from black hole mergers five-dimensional background model stimulate emission of energy from the strong-gravity brane rate of energy transfer equation of state comparison to experimental limits pulsar timing experiments conclusions 240 acta polytechnica ctu proceedings 1(1): 240–245, 2014 240 doi: 10.14311/app.2014.01.0240 unveiling the nature of integral objects: a review pietro parisi1, on behalf of a large collaboration 1istituto di astrofisica e planetologia spaziali (inaf), via fosso del cavaliere 100, roma i-00133, italy corresponding author: p. parisi abstract since its launch in october 2002, the integral observatory has improved our knowledge of the hard x-ray sky above 20 kev, carrying out more than ten years of observations in the energy range from 5 kev to 8 mev. the most recently published integral/ibis surveys listed more than seven hundred sources in the 20-100 kev band. most of these objects are either active galaxies (agns) or x-ray binaries; a fraction of both classes is made of highly absorbed sources, often associated with dim optical counterparts. despite the big effort in the identification process, a large part of these ibis objects (∼25% of them) still remains unclassified. cross-correlation with archival catalogues and/or multiwaveband follow-up observations are of invaluable help to identify and properly classify this unknown objects, but only optical or ir spectroscopy with ground based telescopes in the northern and southern hemisphere can reveal the real nature of these objects. in this work we report on source types that we find among the unidentified objects in the most recent integral surveys. keywords: galaxies: seyfert x-ray binaries techniques: spectroscopic. 1 introduction one main objective of the integral mission is a regular survey of the entire sky in the hard x-ray band. this makes use of the unique imaging capability of the ibis instrument (ubertini et al. 2003) which allows the detection of sources at the mcrab level with a typical localization accuracy of a few arcmin. during the first 5 years of its life, the observatory concentrated mainly on a deep exposure of the galactic central radian, regular scans of the galactic plane, pointed observations of the vela region, and target of opportunity follow up observations. through the ibis imager, optimized for survey work with excellent imaging and spectroscopy capability, catalogs devoted either to the galactic plane scan (gps) or to the improvement of the extragalactic coverage were published. up to now, ibis detected more than 700 sources in the hard x-rays between 20 and 100 kev (bird et al. 2010, krivonos et al. 2010, 2012), and ∼ 30% had no obvious counterpart at other wavelengths and therefore cannot yet be associated with any known class of high-energy emitting sources. comparisons with catalogues or surveys at other frequencies (especially soft x-rays, optical, infrared and radio) are of invaluable help in reducing the localization uncertainty of these ibis sources from arcminutes down to a few (< 10) arcseconds, thus making the search for their optical or ir counterparts much easier. once this is done, then spectroscopic follow up observations provide a classification of the source, a confirmation of the proposed association and the study of that source individually or at population level. thanks to the above method, many new hard x-ray sources have been studied for the first time, including new classes of galactic objects, such as absorbed high mass x-ray binaries (hmxb see walter 2007), supergiant fast x-ray transients (sfxt, e.g., sguera et al. 2005, 2006; leyder et al. 2007), magnetic cataclysmic variables (cvs; barlow et al. 2006; bonnet-bidaud et al. 2007; landi et al. 2008) and symbiotic x-ray binaries (syxbs); in the extragalactic sky, a higher percentage of absorbed active galactic nuclei (agns) were reported compared to softer (2-10 kev) surveys, including a few new nearby compton thick objects (malizia et al. 2009), systematically pinpointing, for the first time, extragalactic sources in the so-called ‘zone of avoidance’, which hampers observations in soft x-rays and optical along the galactic plane due to the presence of gas and dust. all of these objects were classified by means of intensive optical and ir spectroscopic campaigns at various telescopes located worldwide. 2 surveys through hard x-ray surveys we can obtain all-sky maps of the celestial high-energy emission and study catalogues of sources unbiased in term of absorption and which are capable of producing non thermal emission 240 http://dx.doi.org/10.14311/app.2014.01.0240 unveiling the nature of integral objects: a review processes, or being the site of the most extreme astrophysical phenomena observed in the universe. the most recent integral/ibis surveys are that of bird et al. (2010) and krivonos et al. (2010, 2012): • the 4th ibis survey (bird et al. 2010) collected data from november 2002 to october 2008 for ∼80 ms of exposure in the 20-100 kev energy range and produced a catalogue of 723 sources (29% unidentified) ; • the ibis 7-years all-sky hard x-ray survey (krivonos et al. 2010) collected data from december 2002 to july 2009 for ∼80 ms of exposure in the 17-60 kev energy range and found 521 sources (12% unidentified); • the 9-year galactic hard x-ray survey (krivonos et al. 2012) collected data from december 2002 to january 2011 for 132 ms of exposure in the 17-60 kev energy range and reported 402 sources (9% unidentified). 3 identification method in the second semester of 2004 we started an optical spectroscopy campaign performed at ground based telescopes of the northern and southern hemisphere to identify unknown hard x-ray sources detected by integral (masetti et al. 2004, 2006a,b,c,d, 2007, 2008a,b, 2009, 2010, 2012, 2013), and we selected unidentified or unclassified hard x-ray sources that contain, within the ibis 90% confidence level error box, a single bright x-ray object detected either in the rosat all-sky surveys (voges et al. 1999, 2000), or in the slew survey (saxton et al. 2008) and/or in the serendipitous source catalog (watson et al. 2009) of xmm-netwon, or having pointed observations either from chandra, swift/xrt or xmm-netwon satellites. this approach was proven by stephen et al. (2006) to be very effective in associating, with a high degree of probability, ibis sources with a softer x-ray counterpart, in turn drastically reducing the positional error circles to better than a few arcsec in radius, making the search area smaller by a factor of 104. taking into account the hard x-ray characteristics and the global properties of the sources, such as the position in the sky, the 20-100 kev light curve and the broad band spectrum, we can have some clues about their nature. after this first selection, we chose among these objects those that had, within their refined 90% confidence level soft x-ray error boxes, a single possible optical counterpart with magnitude r < 20 in the dss-ii-red survey, so that optical or ir spectroscopy could be obtained with reasonable signal-tonoise ratio using medium-sized telescopes (i.e. with diameter up to 4 meters; masetti et al. 2004, 2006a,b,c,d, 2007, 2008a,b, 2009, 2010, 2012, 2013). 4 galactic sources many important results from the integral mission have been obtained in the observations of the galactic sources. specifically, they have shown the existence of a new class of heavily absorbed x-ray binaries and of the sfxts, doubling the number of known hmxbs and allowed the detection of a substantial number of new magnetic cvs. 4.1 cvs cvs are close binary systems consisting of a late-type (i.e. red dwarf) star trasferring material onto a white dwarf (wd) via roche lobe overflow. those which have a magnetic field are called magnetic cvs and fall into two categories: • polar cvs have a strong magnetic field (b > 107 g); their accretion does not occur via accretion disc and the accreting material is channeled by the magnetic field along its lines and falls on the magnetic poles of the wd; • intermediate polars (warner 1995), instead, have a weaker magnetic field (b ∼ 106 − 107 g) that truncates the accretion disc in the inner region close to the magnetosphere, resulting in an accretion curtain, where the accreting material follows the magnetic field lines down to the wd poles. if the wd is not magnetic the accreting material flows towards the wd through an accretion disc. up to now the cvs detected by integral are 35 (∼ 80% are magnetic, mostly intermediate polars), and 29 of these have been identified through optical or nir spectroscopy. this is an important result, if we consider that before integral very few cvs have been detected at high energies. fig. 1 shows the cvs distribution in the sky (white filled circles superimposed on an ibis 20-100 kev image). figure 1: distribution of cvs (white filled circles) in the 20-100 kev sky imaged by ibis. 241 pietro parisi 4.2 low mass x-ray binaries low mass x-ray binaries (lmxbs) are systems consisting of an accreting compact object (neutron star or black hole) and a low-mass (< 1 m�) main-sequence or slightly evolved late-type star. the low mass companion fills and overflows its roche lobe, therefore accretion of matter always occurs through the formation of an accretion disc. up to now ∼ 100 lmxbs have been detected by integral, and only 15 have been classified through optical or nir spectroscopy, this means that almost all the lmxbs have been detected and very few are found among unidentified integral sources. in fig. 2 we report the lmxbs distribution (white filled circles superimposed on an ibis 20-100 kev image): it is clear that they lie in the galaxy bulge or in globular clusters, where we the majority of old stellar populations are located. figure 2: distribution of lmxbs (white filled circles) in the 20-100 kev sky imaged by ibis. they are segregated in the galactic bulge and in globular clusters. 4.2.1 symbiotic x-ray binaries three of the lmxbs spectroscopically identified among unidentified objects are symbiotic x-ray stars (syxbs). they are part of a small subclass of lmxbs in which the compact object (generally a neutron star), receives matter from a red giant rather than from a late-type companion star on the main sequence. compared to lmxbs they show an optical continuum typical of a red giant of m spectral type, with balmer series generally in absorption (of the 7 syxbs known only one, gx 1+4, shows hα in emission, because the donor star is a red supergiant star that fills its roche lobe, accreting matter via accretion disc) and with the presence of absorption bands, and matter is accreted via stellar wind from the donor star. 4.3 hmxbs the galactic plane scans performed with integral revealed a wealth of new hmxbs. these binary systems are composed of a compact object (neutron star or black hole) orbiting and accreting matter from a luminous early spectral type ob high mass (> 10 m�) companion star. hmxbs have different ways to accrete matter: • via roche lobe overflow, but we know only very few cases of this type; • star with a circumstellar disc: the compact objects with a wide eccentric orbit crosses the decretion disc produced by a rapidly rotating be iii/iv/v star, producing accretion through stellar wind; • a massive star (supergiant i/ii star) ejects a fast and dense radially outflowing wind, and the compact object directly accretes from it. up to now ∼ 90 hmxbs have been detected by integral; of these, 45 were identified through optical or nir spectroscopy. this class of objects is distributed along the galactic plane (see fig.3). if we also consider the hmxbs distances, we note that they closely trace the underlying distribution of the massive starforming regions that are expected to produce the progenitor stars of hmxbs (bodaghee et al. 2012). figure 3: distribution of hmxbs (white filled circles) in the 20-100 kev sky imaged by ibis. they lie along the galactic plane. 4.3.1 sfxts this is a new class of transient hmxbs discovered by integral. the sfxts host a massive blue supergiant star (ob) and a compact object, mainly a neutron star. they have fast x-ray flares, from few hours to few days duration, with luminosity of ∼1036-1037 erg s−1; they also have short duty cycles (tinflare/ttotal=0.05% 3%) the accretion mechanism is not clear yet. there are different scenarios, such as the clumpy wind (negueruela et al. 2008, ducci et al. 2009) or the centrifugal/magnetic barrier (bozzo et al. 2008). the clumpy wind scenario has two possible configurations: a neutron star orbiting a supergiant star on a circular orbit, or on an eccentric orbit, accreting 242 unveiling the nature of integral objects: a review from the clumpy stellar wind of the supergiant. in the centrifugal/magnetic barrier scenario, a magnetic barrier or a centrifugal barrier can sets in, and according to the spin period and the strength of the magnetic field of the neutron star, it can cause or not the inhibition of accretion. up to now the sfxts optically or nir identified are ∼10 and other ∼10 are candidates. 5 extragalactic objects the integral satellite has been able to obtain also important results in the field of extragalactic objects. observations are giving fundamental insights into the study of agns located in the zone of avoidance along the galactic plane (see fig. 4 for the agns distribution in the sky) but also positioned across the whole sky. figure 4: distribution of agns (white filled circles) in the 20-100 kev sky imaged by ibis. this is an ibis 20-100 kev image. 5.1 seyfert galaxies the most common extragalactic objects detected by integral are seyfert galaxies; their luminosity ranges from 1042 to 1045 erg s−1, they are located in the nearby universe (z < 0.5) and have an optical spectrum characterized by emission lines. all the seyferts show narrow high ionization emission lines, such as [oiii] or [nii] forbidden emission lines. some show broad permitted lines in emission (generally balmer series), suggesting the presence of a dense and fast moving gas: we call these seyfert 1 galaxies. in these objects, according to the unified model, our line of sight intercepts both broad line regions and narrow line regions. those agns with only narrow emission lines are instead named seyfert 2 galaxies. according to the unified model, in these sources our line of sight intercepts only the narrow line region due to the presence of a torus which hides the broad line region. these gas and dusty regions are photoionized by the central engine; blrs are confined to sub-pc scales around an accretion disk, producing kinematically broadened emission lines with typical velocities of 103–104 km s−1, nlrs are characterized by narrow-lines with typical velocities of 102–103 km s−1 and can span over kpc scales, which are comparable to the size of the bulge or even the entire galaxy. up to now 148 agns detected by integral have been identified through optical and nir spectroscopy. in particular, 68 agns are seyfert 1, while 55 are seyfert 2 galaxies. 5.1.1 narrow-line seyfert 1 these galaxies are peculiar seyfert 1 agns (osterbrock & pogge 1985) with a full width at half maximum (fwhm) of the hβ emission line smaller than 2000 km s−1, with permitted lines which are only slightly broader than the forbidden ones, with a [oiii]5007/hβ ratio < 3, and finally with evident feii and other highionization emission-line complexes (e.g. see fig. 5). a few nls1 galaxies have been discovered so far by integral and recognized as such by optical spectroscopy. figure 5: optical spectrum (not corrected for the intervening galactic absorption) of a typical narrow-line seyfert 1. the main spectral features are labeled. 5.2 low ionization nuclear emission-line regions low ionization nuclear emission-line regions (liners; heckman 1980) are peculiar agns with a level of activity much smaller than that in classical agns and in which some low-ionization lines ([oii]3723, [oi]6300, and [nii]6584) are stronger than in typical seyfert 2 galaxies; the permitted emission-line luminosities are weak; and the emission-line widths are comparable with those of type 2 agns (see fig. 6). according to heckman (1980), liners have [oii]3723 > [oiii]5007 and [oi]6300 > 1/3 [oiii]5007, and often [nii]6584/hα > 0.6. 243 pietro parisi figure 6: optical spectrum (not corrected for the intervening galactic absorption) of a typical liner. the main spectral features are labeled. 5.3 x-ray bright optically normal galaxies some integral objects show a continuum typical of a normal galaxy, dominated by absorption lines due to star forming regions, they are x-ray bright, optically normal galaxies (xbongs; comastri et al. 2002), that is, x-ray bright galactic nuclei with no emission lines in their optical spectra (e.g. fig. 7). figure 7: optical spectrum (not corrected for the intervening galactic absorption) of a typical xbong. the main spectral features are labeled. 5.4 blazars using medium-sized telescopes (i.e. tng, eso), we are also able to identify and classify sources at high redshifts (> 0.6) detected by integral. they are blazars, distant and powerful agns which are oriented in such a way that a jet expelled from the central black hole is directed at small angles with respect to the observers line of sight. an example is igr j12319-0749 a powerful blazar at z = 3.12 (masetti et al. 2012), the farthest opticallyidentified object of any integral survey and the second furthest of all objects detected by integral. 6 summary and conclusions up to now 273 integral objects have been identified and classified through optical and nir spectroscopy using ground-based telescopes of the northern and southern hemisphere. of these objects, 62% are agns, 36% are x-ray binaries and the remaining 2% are chromospherically active stars. going into details, of the 62% agns, 27.8% are seyfert 1 galaxies, 22% are seyfert 2 agns, 8.5% qsos, 2.9% xbongs and 1.1% are other sources. among the galactic sources (36%), we found that 18% are hmxbs, 12.5% are cvs and 5.5% are lmxbs. the ibis surveys secured the detection of extragalactic sources in the so-called zone of avoidance, which hampers observations in soft x-rays along the galactic plane due to the presence of gas and dust. moreover, these surveys are expanding our knowledge about galactic x-ray binaries, by showing the existence of a new class of heavily absorbed supergiant massive xray binaries (first suggested by revnivtsev et al. 2003), by allowing the discovery and the study of supergiant fast x-ray transients (e.g., sguera et al. 2005, 2006; leyder et al. 2007), by doubling the number of known hmxbs (see walter 2007), and by detecting a substantial number of new magnetic cvs. indeed, the new ibis catalogue (in progress) will offer new spectral and timing information on newly detected sources and an insight on peculiar ones, giving us the unique opportunity to discover new hmxbs and understand the differences among them. references [1] barlow, e. j., knigge, c., bird, a. j., et al. 2006, mnras, 372, 224 doi:10.1111/j.1365-2966.2006.10836.x [2] bird, a. j., bazzano, a., bassani, l., et al. 2010, apjs, 186, 1 doi:10.1088/0067-0049/186/1/1 [3] bodaghee, a., tomsick, j.a., rodriguez, j., et al. 2012, apj, 744, 108 doi:10.1088/0004-637x/744/2/108 [4] bonnet-bidaud, j. m., de martino, d., falanga, m., mouchet, m., & masetti, n., 2007, a&a, 473, 185 244 http://dx.doi.org/10.1111/j.1365-2966.2006.10836.x http://dx.doi.org/10.1088/0067-0049/186/1/1 http://dx.doi.org/10.1088/0004-637x/744/2/108 unveiling the nature of integral objects: a review [5] bozzo, e., falanga, m. & stella, l., 2008, apj, 683, 1031 doi:10.1086/589990 [6] comastri, a., mignoli, m., ciliegi, p., et al. 2002, apj, 571, 771 doi:10.1086/340016 [7] ducci, l., sidoli, l., mereghetti, s., et al. 2009, mnras, 398, 2152 doi:10.1111/j.1365-2966.2009.15265.x [8] heckman, t. m. 1980, a&a, 87, 152 [9] krivonos, r., tsygankov, s., revnivtsev, m., et al. 2010, a&a, 523, a61 [10] krivonos, r., tsygankov, s., lutovinov, a., et al. 2012, a&a, 545, [11] landi, r., bassani, l., dean, a. j., et al. 2008, mnras, 392, 630 doi:10.1111/j.1365-2966.2008.14086.x [12] malizia, a., stephen, j.b., bassani, l., et al. 2009, mnras, 399, 944 doi:10.1111/j.1365-2966.2009.15330.x [13] masetti, n., palazzi, e., bassani, et al. 2004, a&a, 426, l41 [14] masetti, n., mason, e., bassani, l., et al. 2006a, a&a, 448, 547 [15] masetti, n., pretorius, m.l., palazzi, e., et al. 2006b, a&a, 449, 1139 [16] masetti, n., bassani, l., bazzano, a., et al. 2006c, a&a, 455, 11 [17] masetti, n., morelli, l., palazzi, e., et al. 2006d, a&a, 459, 21 [18] masetti, n., landi, r., pretorius, m.l., et al. 2007, a&a, 470, 331 [19] masetti, n., mason, e., morelli, l., et al. 2008a, a&a, 482, 113 [20] masetti, n., mason, e., landi, r., et al. 2008b, a&a, 480, 715 [21] masetti, n., parisi, p., palazzi, e., et al. 2009, a&a, 495, 121 [22] masetti, n., parisi, p., palazzi, e., et al. 2010, a&a, 519, a96 [23] masetti, n., parisi, p., jiménez-bailón, e., et al. 2012, a&a, 538, a123 [24] masetti, n., parisi, p., palazzi, e., et al. 2013, a&a, in press. [25] negueruela, i., torrejon, j. m., reig, p., ribo, m., & smith, d. m. 2008, in a population explosion: the nature & evolution of x-ray binaries in diverse environments. aip conference proceedings, volume 1010, 252 doi:10.1063/1.2945052 [26] osterbrock, d.e., & pogge, r.w. 1985, apj, 297, 166 doi:10.1086/163513 [27] revnivtsev, m. g., sazonov, s. y., gilfanov, m. r., & sunyaev, r. a. 2003, astron. lett., 29, 587 doi:10.1134/1.1607496 [28] saxton r.d., read, a.m., esquej, p., et al. 2008, a&a, 480, 611 [29] sguera, v., barlow, e. j., bird, a.j., et al. 2005, a&a, 444, 221 [30] sguera, v., bazzano, a., bird, a.j., et al. 2006, apj, 646, 452 doi:10.1086/504827 [31] stephen, j.b., bassani, l., malizia, a., et al. 2006, a&a, 445, 869 [32] ubertini, p., lebrun, f., di cocco, g., et al. 2003, a&a, 411, l131 [33] voges, w., aschenbach, b., boller, t., et al. 1999, a&a, 349, 389 [34] voges, w., aschenbach, b., boller, t., et al. 2000, iau circ. 7432 [35] walter, r. 2007, ap. space sci., 309, 5 doi:10.1007/s10509-007-9477-9 [36] warner, b. 1995, cataclysmic variable stars (cambridge: cambridge university press) doi:10.1017/cbo9780511586491 [37] watson, m.g., schroder, a.c., fyfe, d., et al. 2009, a&a, 493, 339 discussion pieter meintjes: the novalike cataclysmic variable ae aquarii has been detected by suzaku above 10 kev, showing evidence of non-thermal emission. has it been detected by integral as well? pietro parisi: the cataclysmic variable ae aquarii has not been observed by integral. 245 http://dx.doi.org/10.1086/589990 http://dx.doi.org/10.1086/340016 http://dx.doi.org/10.1111/j.1365-2966.2009.15265.x http://dx.doi.org/10.1111/j.1365-2966.2008.14086.x http://dx.doi.org/10.1111/j.1365-2966.2009.15330.x http://dx.doi.org/10.1063/1.2945052 http://dx.doi.org/10.1086/163513 http://dx.doi.org/10.1134/1.1607496 http://dx.doi.org/10.1086/504827 http://dx.doi.org/10.1007/s10509-007-9477-9 http://dx.doi.org/10.1017/cbo9780511586491 introduction surveys identification method galactic sources cvs low mass x-ray binaries symbiotic x-ray binaries hmxbs sfxts extragalactic objects seyfert galaxies narrow-line seyfert 1 low ionization nuclear emission-line regions x-ray bright optically normal galaxies blazars summary and conclusions 116 acta polytechnica ctu proceedings 2(1): 116–122, 2015 116 doi: 10.14311/app.2015.02.0116 inner disk structure of dwarf novae in the light of x-ray observations ş. balman1 1middle east technical university, department of physics, inonu bulvarı, ankara, 06531, turkey corresponding author: solen@astra.physics.metu.edu.tr abstract diversity of the x-ray observations of dwarf nova are still not fully understood. i review the x-ray spectral characteristics of dwarf novae during the quiescence in general explained by cooling flow models and the outburst spectra that show hard x-ray emission dominantly with few sources that reveal soft x-ray/euv blackbody emission. the nature of aperiodic time variability of brightness of dwarf novae shows band limited noise, which can be adequately described in the framework of the model of propagating fluctuations. the frequency of the break (1-6 mhz) indicates inner disk truncation of the optically thick disk with a range of radii (3.0-10.0)×109 cm. the rxte and optical (rtt150) data of ss cyg in outburst and quiescence reveal that the inner disk radius moves towards the white dwarf and receeds as the outburst declines to quiescence. a preliminary analysis of su uma indicates a similar behaviour. in addition, i find that the outburst spectra of wz sge shows two component spectrum of only hard x-ray emission, one of which may be fitted with a power law suggesting thermal comptonization occuring in the system. cross-correlations between the simultaneous uv and x-ray light curves (xmm −newton) of five dne in quiescence show time lags in the x-rays of 96-181 sec consistent with travel time of matter from a truncated inner disk to the white dwarf surface. all this suggests that dwarf novae and other plausible nonmagnetic systems have truncated accretion disks indicating that the disks may be partially evaporated and the accretion may occur through hot (coronal) flows in the disk. keywords: cataclysmic variables white dwarfs accretion, accretion dics stars: binaries x-rays: stars radiation mechanisms: thermal stars: dwarf novae. 1 introduction dwarf novae (dne) are a class of cataclysmic variables (cvs) which are interacting compact binaries in which a white dwarf (wd, the primary star) accretes matter and angular momentum from a main (or post-main) sequence star (the secondary) filling its roche-lobe. the matter is transferred by means of an accretion disk that is assumed to reach all the way to the wd surface. ongoing accretion at a low rate (quiescence) is interrupted every few weeks to months or sometimes with longer durations by intense accretion (outburst) of days to weeks where ṁ increases (see warner 1995 for a review). the material in the inner disk of nonmagnetic cvs initially moving with keplerian velocity dissipates its kinetic energy in order to accrete onto the slowly rotating wd creating a boundary layer (bl) (see mauche 1997, kuulkers et al. 2006 for an overview). standard accretion disk theory predicts half of the accretion luminosity to originate from the disk in the optical and ultraviolet (uv) wavelengths and the other half to emerge from the boundary layer as x-ray and/or extreme uv (euv)/soft x-ray emission which may be summerized as lbl∼ldisk=gmwdṁacc/2rwd=lacc/2 (lyndenbell & pringle 1974, godon et al. 1995). during low-mass accretion rates, ṁacc<10−(9−9.5)m�, the boundary layer is optically thin (narayan & popham 1993, popham 1999) emitting mostly in the hard xrays (kt∼10(7.5−8.5) k). for higher accretion rates , ṁacc≥10−(9−9.5)m�, the boundary layer is expected to be opticallly thick (popham & narayan 1995) emitting in the soft x-rays or euv (kt∼10(5−5.6) k). the transition between an optically thin and an optically thick boundary layer, also depends on the mass of the white dwarf (also rotation) and on the alpha viscosity parameter. 2 x-ray observations of dwarf novae in quiescence and outburst the quiescent x-ray spectra are mainly characterised with a multi-temperature isobaric cooling flow model of plasma emission at tmax=6-55 kev with accretion rates of 10−12-10−10 m� yr−1. the x-ray line spectroscopy indicates narrow emission lines (brightest oviii kα) and near solar abundances, with a 6.4 kev line expected to be due to reflection from the surface of the wd. the detected doppler broadening in lines during quies116 http://dx.doi.org/10.14311/app.2015.02.0116 inner disk structure of dwarf novae in the light of x-ray observations cence is <750 km s−1 at sub-keplerian velocities in the boundary layer with electron densities >1012 cm−3 (see baskill et al. 2005, kuulkers et al. 2006, rana et al. 2006, pandel et al. 2005, balman et al. 2011, balman 2012). the total x-ray luminosity during quiescence is 1029 -1032 erg s−1. lack of bl emission in the xrays have been suggested due to low lx/ldisk ratio (see kuulkers et al. 2006). it has been suggested for dn in quiescence that if the wd emission is removed and some disk truncation is allowed, this ratio is ∼1 (pandel et al. 2005). dn outbursts are brightenings of the accretion disks as a result of thermal-viscous instabilities summerized in the dim model (disc instability model; lasota 2001,2004). during the outburst stage, dn x-ray spectra differ from the quiescence since the accretion rates are higher (about two orders of magnitude), the bl is expected to be optically thick emitting euv/soft xrays (lasota 2001, see the x-ray review in kuulkers et al. 2006). on the other hand, soft x-ray/euv temperatures in a range 5-25 ev are detected from only about five systems (e.g., mauche et al. 1995, mauche & raymond 2000, long et al. 1996, mauche 2004, byckling et al. 2009). as a second and more dominantly detected emission component, dn show hard x-ray emission during the outburst stage however, at a lower flux level and x-ray temperature compared with the quiescence (e.g., ww cet & su uma: collins & wheatley 2010, fertig et al. 2011, ss cyg: wheatley et al. 2003, macgowen et al. 2004, ishida et al. 2009). on the other hand, some dn show increased level of x-ray emission (gw lib & u gem: byckling et al. 2009, guver et al. 2006). the total x-ray luminosity during outburst is 1030 -1034 erg s−1. the grating spectroscopy of the outburst data indicate large widths for lines with velocities in excess of 1000 km s−1 mostly of h and he-like emission lines (c,n,o,ne, mg, si, fe, ect.) (mauche 2004, rana et al. 2006, okada et al. 2008). a characteristic of some dn outburst light curves are the uv and x-ray delays in rise to outburst (w.r.t. optical) indicating possible disk truncation ((meyer & meyer-hofmeister 1994 and references therein) . these delays are a matter of several hours (upto a day) that need dedicated simultaneous multi-wavelength observations. during outburst no eclipses are detected in the eclipsing systems (particularly of soft x-ray emission) or no distinct orbital variations are seen. (e.g., pratt et al. 1999, byckling et al. 2009). note that both in ss cyg (mc gowan et al. 2004) and in su uma (collins & wheatley 2010) the x-ray flux in between outbursts have been found to decrease as opposed to expectations of the dim model. wz sge is a short period su uma type dwarf nova with long interoutburst interval of 20-30 yrs. the system is believed to show no normal outbursts but only superoutbursts. the most recent outburst in julyaugust 2001 (previous outbursts in 1978, 1946 and 1913) was observed from ground and space over several different wavelength bands including the x-ray wavelengths and the euv using the chandra observatory (see wheatley & mauche 2005, kuulkers et al. 2002). there has been three letg (low energy transmission grating) and three acis-s (advanced camera imaging specreometer) observations obtained in the continuousclocking (cc) mode. i report a preliminary analysis on the spectral fitting of the acis-s cc-mode data. figure 1: chandra x-ray outburst spectra of wz sge in 0.3-10.0 kev range obtained with acis-s (ccmode). the observation days after the outburst are labeled on each spectrum. the spectra and response files were prepared in a standard way (task specextract is used) by choosing the source and the background photons using a suitable extraction box (10 arcsec in size) on the data strip using ciao 4.4 and the caldb 4.4.8 (see http://cxc.harvard.edu/ciao/). for further analysis, heasoft version 6.13 is utilized. the spectra were fitted with the multi-temperature plasma emission models (e.g., cevmkl in xspec, see http://heasarc.nasa.gov/xanadu/xspec/) according to the expections from earlier work of dn analysis in quiescence and outburst. cevmkl model is a multitemperature plasma emission model (built from mekal code, mewe et al. 1986) where emission measures follow a power-law in temperature (dem = (t/tmax)α−1 dt/tmax). the reduced χ2 of the fits were above a value of 2 with either a single mekal model or cevmekl alone (the α parameter of the power law distribution of temperatures is set free) for days 6, 15, 30 after outburst. i added a second component of a power law which reduced the χ2 values to desirable levels. in general, the plasma has variable abundance of n,o,ne,fe, and s. the spectral parameters for day 6 after the outburst were ktmax=0.7-1.3 kev with a photon index of γ=0.8-1.4. the x-ray flux of the ther117 ş. balman mal cevmkl component was 1.1×10−11 erg s−1 cm−2 and lcevmkl is 2.7×1030 erg s−1 where the power law component has 5.8×10−12 erg s−1 cm−2 and lpower is 1.3×1030 erg s−1 (43.5 pc is assumed, harrison et al. 2004). the parameters for day 15 were ktmax=0.5-1.5 kev and γ=0.2-1.1. between these two dates the x-ray flux of the cevmkl component decreased by 2 and the power law component inceased by about 1.4 . for day 30 ktmax=1.2-3.0 kev and γ=1.5-2.0. therefore, the x-ray emitting region gets hotter and the photon index decrease. the x-ray fluxes recover to day 6. the final observation on day 58 which is almost quiescence after the outburst shows ktmax=26-46 kev and no power law emission is detected. the x-ray flux is the highest 4.4×10−11 erg s−1 cm−2 and lx is 1.1×1031 erg s−1 (at 43.5 pc). all ranges correspond to 90% confidence level errors. the neutral hydrogen absorption has stayed at the interstellar level in all fits within errors. no optically thick blackbody emission is detected and only x-ray suppression of the quiescent emission is observed just like some other dn. the x-ray temperatures on days 6-30 are in good agreement with the letg results as well. a model of three additive mekals yield acceptable fits on days 6-30, however allowing for very high x-ray tempartures. a partial covering absorber model improves the cevmkl fits, however the high intrinsic absorption when modeled yields inconsistent count rates for the letg observations. 3 inner disk structure using eclipse mapping techniques flickering is a fast intrinsic brightness scintilation occurring on time scales from seconds to minutes (e.g., amplitudes of 0.01 1 mag in the optical). it is observed in all accreting sources. eclipse mapping methods have been used to reproduce the spatial distribution of flickering in cvs in the optical and uv wavelengths. some eclipse mapping studies of flickering in quiescent dwarf novae indicate that mass accretion rate diminishes by a factor of 10-100 and sometimes by 1000 in the inner regions of the accretion disks as revealed by the brightness temperature calculations which do not find the expected r−3/4 radial dependence of brightness temperature in standard steady-state constant mass accretion rate disks (see e.g., z cha and oy car: wood 1990, v2051 oph: baptista & bortoletto 2004, v4140 sgr: borges & baptista 2005). biro (2000) finds that this flattening in the brightness temperature profiles may be lifted by introducing disk truncation in the quiescent state (e.g., r ∼ 0.15rl1 ∼4×109 cm; dw uma, a nova-like). a comprehensive uv modeling of accretion disks at high accretion rates in 33 cvs including many nova-likes and old novae (puebla et al. 2007) indicate an extra component from an extended optically thin region (e.g., wind, corona/chromosphere) evident from the strong emission lines and the p cygni profiles. this study also indicates that the mass accretion rate may be decreasing 1-3 orders of magnitude in the inner disk region (divergence starts r≤ afew ×1010 cm towards the wd). 4 inner disk structure of dwarf novae the truncation of the optically thick accretion disk in dne in quiescence was invoked as a possible explanation for the time lags between the optical and uv fluxes in the rise phase of the outbursts (meyer & meyerhofmeister 1994), and for some implications of the dim (see lasota 2001, 2004) or due to the unusual shape of the optical spectra or light curves of dne (linell et al. 2005, kuulkers et al. 2011). theoretical support for such a two-phase flow was given by a model of the disk evaporation of (meyer & meyer-hofmeister 1994). this model was later elaborated to show that the disk evaporation (coronal “syphon” flow) may create optically thick-optically thin transition regions at various distances from the wd (liu et al. 1997, mineshige et al. 1998). 4.1 propagating fluctuations model another diagnostic tool proposed to study the inner disk structure in accreting objects is the aperiodic variability of brightness of sources in the x-rays. while the long time-scale variability might be created in the outer parts of the accretion disk (warner & nather 1971), the relatively fast time variability (at f >few mhz) originates in the inner parts of the accretion flow (bruch 2000; baptista & borteletto 2004). properties of this noise is similar to that of the x-ray binaries with neutron stars and black holes. now, the widely accepted model of origin for this aperiodic flicker noise is a model of propagating fluctuations (lyubarskii 1997, churazov et al. 2001, uttley & mchardy 2001, revnivtsev et al. 2009,2010, uttlet et al. 2011, scaringi et al. 2012).the modulations of the light are created by variations in the instantaneous value of the mass accretion rate in the region of the energy release. these variations in the mass accretion rate, in turn, are inserted into the flow at all keplerian radii of the accretion disk due to the stochastic nature of its viscosity and then transferred toward the compact object. thus, variations are on dynamical timescales. this model predicts that the truncated accretion disk should lack some part of its variability at high fourier frequencies, i.e. on the time scales shorter than a typical time scale of variability. 118 inner disk structure of dwarf novae in the light of x-ray observations 4.2 broad-band noise of dwarf novae a recent work by balman & revnivtsev (2012) have used the broad-band noise characteristic of selected dn in quiescence (only one in outburst: ss cyg) and studied the inner disk structure and disk truncation via propagating fluctuations model. the power spectral densities (pds) expressed were calculated in terms of the fractional rms amplitude squared following from (miyamoto et al. 1991). the light curves were divided into segments using 1-5 sec binning in time and several pds were averaged to create a final pds of the sources. the white noise levels were subtracted hence leaving us with the rms fractional variability of the time series in units of (rms/mean)2/hz. next, the rms fractional variability per hertz was multiplied with the frequencies to yield νpν versus ν. for the model fitting a simple analytical model is used p (ν) ∝ ν−1 ( 1 + ( ν ν0 )4 )−1/4 , which was proposed to describe the power spectra of sources with truncated accretion disks (see revnivtsev et al. 2010,2011). the broad-band noise structure of the keplerian disks often show ∝ f−1...−1.3 dependence on frequency (churazov et al. 2001, gilfanov et al. 2005), and this noise will show a break if the optically thick disk truncates as the keplerian motion subsides. balman & revnivtsev (2012) show that for five dn systems, ss cyg, vw hyi, ru peg, ww cet and t leo, the uv and x-ray power spectra show breaks in the variability with break frequencies in a range 1-6 mhz, indicating inner disk truncation in these systems. the truncation radii for dn are calculated in a range ∼(310)×109 cm including errors (see table 2 in balman & revnivtsev 2012). the same authors used the archival rxte data of ss cyg in quiescence and outburst listed in table 1 of their paper to derive the broad-band noise of the source in different states (i.e., accretion rates). for the outburst phases, authors investigated times during the x-ray suppression (e.g., the x-ray dips; optical peak phases of the outburst) and the x-ray peak. this, in turn is expected to indicate the motion of the flow in the inner regions of the disk and the geometry of the inner disk. authors show that the disk moves towards the white dwarf during the optical peak to ∼ 1×109 cm and receeds as the outburst declines to quiescence to 5-6×109 cm. this is shown for a cv, observationally, for the first time in the x-rays (see figure 2). this is also supported with the optical data analysis of ss cyg in quiescence and outburst (revnivtsev et al. 2012) 4.3 x-ray and uv light curve cross-correlations balman & revnivtsev (2012) calculated the crosscorrelation between the two simultaneous light curves (x-ray and uv), using the archival xmm -newton epic pn and om data utilizing heasoft task crosscor. to obtain the ccfs (cross-correlation functions), datasets were divided into several pieces using 1-5 sec binning in the light curves and fitted the resulting ccfs with double lorentzians since it was necessary for adequate fitting (see the paper for details). the ccfs for all the dne show clear asymmetry indicating that some part of the uv flux is leading the x-ray flux. in addition, they detect a strong peak near zero time lag for ru peg, ww cet and t leo, suggesting a significant zero-lag correlation between the x-rays and the uv light curves. figure 2: the rxte pds of ss cyg in quiescence and during the optical peak of outburst (top). the solid lines show the fit with the propagating fluctuations model along with two lorentzians. the same authors also calculated the crosscorrelation between the simultaneous uv and x-ray light curves by subtracting the zero time lag components in the five dne pds, yielding more correct time lags consistent with delays in the x-rays of 96-181 sec (see the paper on details of the modeling). the lags occur such that the uv variations lead x-ray variations which shows that as the accreting material travels onto the wd, the variations are carried from the uv into the x-ray emitting region. therefore, the long time lags of the order of minutes can be explained by the travel time of matter (viscous flow) from a truncated inner disk to the white dwarf surface. zero time lags (∼ light travel time) indicate irradiation effects in these systems since we do not have resolution better than 1 sec. 119 ş. balman figure 3: see figure 10 in balman & revnivtsev (2012). the cross-correlation of the epic pn (x-ray) and om (uv) light curves with 1 sec time resolution. the two-component lorenzian fits are shown as solid black lines (except for ss cyg). figure 4: the rxte pds of su uma in quiescence and in optical outburst is displayed (top panel). xmm newton epic pn pds of v426 oph and ht cas in quiescence (middle and bottom panel). the solid lines are the fits with the propagating fluctuations model. an analysis on the rxte data of su uma in quiescence and outburst following six consecutive outburts reveal a similar broad-band noise structure to ss cyg in quiescence showing a break frequency ∼ 5.5-7.5 mhz with a truncated optically thick disk ∼ 3.8×109 cm. the preliminary analysis of the outburst data during the x-ray suppression episodes (optical peak of the outburst) indicates no disk truncation or a truncation around 0.1 hz (see figure 4). therefore, su uma indicates a similar behaviour of the disk during the outburst to ss cyg where the inner disk moves in almost all the way to the wd during the optical peak and moves out in decline to quiescent location further out. the lower level of broadband noise during outburst stage of su uma may be due to the high radiation pressure support of an optically thick disk flow during the peak of the outburst suppressing the variations. a similar pds analysis of the xmm -newton data of the dne, v426 oph and ht cas in quiescence reveals break frequencies 4.6-2.6 mhz and 7.2-2.8 mhz, respectively. the approximate keplerian radii where the optically thick disk truncates is ∼ 6.2×109 cm and ∼ 4.5×109 cm, respectively (see figure 4). 5 conclusions studies of dne broad-band noise characteristics in the x-rays (see also balman & revnivtsev 2012) indicate that dne have truncated accretion disks in quiescence detected in at least 8 systems in a range ∼(3.0-10.0)×109 cm including errors. the magnetic cvs (mcvs) show rather smaller truncation radii (0.9120 inner disk structure of dwarf novae in the light of x-ray observations 2.0)×109 cm (revnivtsev et al. 2010, 2011). this can also explain the uv and x-ray delays in the outburst stage and the accretion may occur through hot (coronal) flows in the disk. note that extended emission and winds are detected from dn in the outburst stage which may be an indication of the coronae/hot flows in these systems (e.g., mauche 2004). time delays detected in a range of 96-181 sec, are also consistent with matter propogation timescales onto the wd in a truncated nonmagnetic cv disk in quiescence. balman & revnivtsev (2012) approximate an α of 0.1-0.3 for the inner regions of the dne accretion disks in quiescence. in addition, the outburst spectra of wz sge shows two component spectrum of only hard x-ray emission, one of which may be fitted with a power law suggesting thermal comptonization of the optically thick disk photons or scattering from an existing wind during the outburst. the spectral evolution and disapearance of the power law component after outburst may support this issue and that the accretion disk goes back to its quiescent truncated structure and comptonization stops. a general picture of the accretion flow around a wd in quiescence thus might be somewhat similar to that of the black hole/neutron star accretors with an optically thick colder outer accretion disk and an optically thin hot flow in the inner regions where the truncation occurs (see esin et al. 1997, done et al. 2007). the appearance of a hot flow (e.g., adaf-like) in the innermost regions of the accretion disk will differ from that of ordinary rotating keplerian disk because it is no longer fully supported by rotation, but might have a significant radial velocity component with sub-keplerian speeds. it is important to monitor dne in the x-rays to measure the variability in the light curves in 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[58] wilms, j., allen, a., & mccray, r. 2000, apj, 542, 914 [59] wood, j. h.: 1990, mnras, 243, 219 122 http://dx.doi.org/10.1086/421438 http://dx.doi.org/10.1086/309489 http://dx.doi.org/10.1086/380758 http://dx.doi.org/10.1038/362820a0 http://dx.doi.org/10.1086/587161 http://dx.doi.org/10.1086/429983 http://dx.doi.org/10.1111/j.1365-2966.2012.20512.x http://dx.doi.org/10.1046/j.1365-8711.2001.04496.x http://dx.doi.org/10.1111/j.1745-3933.2011.01056.x http://dx.doi.org/10.1017/cbo9780511586491 http://dx.doi.org/10.1093/mnras/152.2.219 http://dx.doi.org/10.1111/j.1365-2966.2003.07149.x http://dx.doi.org/10.1086/317016 introduction x-ray observations of dwarf novae in quiescence and outburst inner disk structure using eclipse mapping techniques inner disk structure of dwarf novae propagating fluctuations model broad-band noise of dwarf novae x-ray and uv light curve cross-correlations conclusions 90 acta polytechnica ctu proceedings 1(1): 90–95, 2014 90 doi: 10.14311/app.2014.01.0090 cosmological evolution of the central engine in high-luminosity, high-accretion rate agn matteo guainazzi1 1european space astronomy centre (esac), p.o. box, 78, e-28691 villanueva de la cañada, madrid, spain corresponding author: matteo.guainazzi@sciops.esa.int abstract in this paper i discuss the status of observational studies aiming at probing the cosmological evolution of the central engine in high-luminosity, high-accretion rate active galactic nuclei (agn). x-ray spectroscopic surveys, supported by extensive multi-wavelength coverage, indicate a remarkable invariance of the accretion disk plus corona system, and of their coupling up to redshifts z ' 6. furthermore, hard x-ray (e ∼>10 kev) surveys show that nearby seyfert galaxies share the same central engine notwithstanding their optical classification. these results suggest that the high-luminosity, high accretion rate quasar phase of agn evolution is homogeneous over cosmological times. keywords: active galactic nuclei. 1 outline thanks to its leap in sensitivity by over two orders of magnitude, the medium sensitivity survey carried out by the einstein observatory (emss, maccacaro et al. 1981) collected for the first a statistically sizable sample of extragalactic x-ray sources. studying a sample of 190 extragalactic sources, giovannelli & polcaro (1986) concluded that the observed linear relation between xray luminosity and redshift over several orders of magnitude made evident a physical continuity between the different classes of extragalactic x-ray objects. the sample in giovannelli & polcaro (1986) was a “mixed bag” of different objects: active galactic nuclei (agn), elliptical/s0 galaxies, and spiral/irregular galaxies. while it is now accepted that most of the x-ray emission in inactive galaxies is due to a combination of diffuse inter-stellar medium, halos, and x-ray binaries (see fabbiano 1989 for an early review), there is almost undisputed consensus that the formidable energy output emerging from agn is due to accretion onto super-massive black holes (see, however, the contribution by prof. kundt for a different view). sub-arcsecond resolution imaging with chandra, as well as the unprecedented xmm-newton throughput have made possible a detailed characterisation of the high-energy spectral energy distribution (sed) in large samples of agn, covering a range in luminosity of almost ten orders of magnitudes, and cosmological distances well beyond the peak of the nuclear activity (brandt & hasinger 2005, elvis et al. 2012, pounds 2013). time is ripe to address some fundamental questions on the nature of accretion onto super-massive black holes, that the small size of the emss extragalactic sample left open. in this contribution i will focus on two of them: • do agn share the same engine at all cosmological times? • do agn share the same engine? the answer for all types of agn at all accretion rates would be clearly: “they don’t”. however, similarities in the observational properties of specific subclasses of agn allow us to give these question astrophysically useful positive answers, at least as far as the members of these sub-classes are concerned. in this review i deal with mainstream radio-quiet1, higheddington ratio (ṁ/ ˙medd ∼> 10−2)2 agn. i assume hereby the traditional wisdom separation in bolometric luminosity between seyfert galaxies (l < 1044 erg s−1) and quasars (qso), whenever relevant. the contribution by m.elitzur to these proceedings discusses a central engine unification scheme encompassing also lowluminosity, low-accretion rate systems. 1in this paper, i assume a qualitative definition of radio-quiet (as opposed to radio-loud) agn as those where the contribution of relativistic jets to the optical-to-x-ray spectral energy distribution is negligible. quantitative definitions of agn radio-loudness were introduced by kellermann et al. (1989), miller et al. (1990), and panessa et al. (2007), among others. 2we define the mass accretion rate in units of eddington as: ṁ ≡ ηledd/c 2, where ledd ≡ 4πgmbhmpc, mbh is the black hole mass, mp is the proton mass, c is the speed of light, and η ' 0.1 is the efficiency of gravitational losses into radiation conversion. 90 http://dx.doi.org/10.14311/app.2014.01.0090 cosmological evolution of the central engine in high-luminosity, high-accretion rate agn 2 cosmological evolution of the agn central engine 2.1 direct observables agn are multi-wavelength machines. they emit comparable power per decade frequency from the ir to the γ-ray band (elvis et al. 1994). more importantly for the astrophysical study of accreting super-massive black holes, the bulk of the radiation emitted in each rest-frame energy band carries information on a specific astrophysical process occurring in the nuclear region. uv and x-rays are the most appropriate bands to study spectroscopically the sub-pc scale around accreting massive black holes. in agn accreting at a rate ṁ ∼>10−2 the eddington value the uv emission is dominated by a “blue bump”, believed to be due to thermal emission from the accretion disk (czerny & elvis 1987). the high-energy band is dominated by comptonisation of disk photons by a ∼102 kev temperature corona (zdziarski et al. 1995, perola et al. 2002) with a largely unknown geometry. haardt & maraschi (1991) proposed a “sandwich” geometry embracing the accretion disk. compact coronal geometries have recently experienced a revival following the possible discovery of relativistic light bending (miniutti & fabian 2004) and disk reverberation (fabian et al. 2009) in x-ray observations of nearby seyfert galaxies. uv and x-rays are therefore the natural energy bands where to look for any cosmological evolution of the central engine in luminous agn. in practical terms, metrics accessible to ccd-resolution agn spectra are the intrinsic shape of the x-ray spectrum, parametrised through the photon index γ, which yields information on the corona; and the uv to x-ray flux ratio, parametrised through the αox ∝ log(l2kev /l 2500å ) (tananbaum et al. 1979), which yields information on the disk-corona coupling. for unsaturated comptonisation of non relativistic thermal distributions of electrons, γ ∝ y−1/2, where the comptonisation parameter y ∝ kt × max(τes, τ 2 es), kt is the electron temperature and τes the optical depth for electron scattering (rybicki & lightman 1979). the measurements of the spectral exponential cut-off at energies e ∼ kt would provide a full characterisation of the physical properties of the corona. however, such measurements are accessible only for a handful of x-ray bright nearby agn even in the nustar (harrison et al. 2013) era. other observables probe in principle the innermost regions of the accretion flow, such as the ubiquitous, and black hole mass dependent, x-ray variability (mc hardy et al. 2006, ponti et al. 2012) and the relativistic distortion of x-ray emission lines produced in a x-ray illuminated accretion disk within a few gravitational radii from the innermost circular stable orbit (fabian et al. 1989). these effects are, however, also difficult to measure at high redshift. fig. 1 shows a qualitative scheme of the intrinsic agn uv-to-x-ray sed for reference. figure 1: schematic view of an agn intrinsic uvto-x-ray spectrum. blue: thermal emission from a luminous accretion disk with a temperature at the innermost radius of 30 ev; red: unsaturated comptonisation of soft disk photons with kte=100 kev and τes=1; magenta: reflection from a plane-parallel infinite slab of cold (∼104k) matter (compton continuum plus fe and nickel kα and kβ fluorescent emission lines; after nandra et al. 2007) sizable samples of x-ray selected agn up to redshift '6 have been collected by difference catalogues and surveys: champ (green et al. 2012), chandra deep fields (steffen et al. 2006), 2xmm (watson et al. 2009, young et al. 2009), cosmos (elvis et al. 2012, lanzuisi et al. 2013). they agree in finding no evidence for cosmological evolution of either γ or αox. similar results were obtained by specific observational programs targeting the farthest x-ray selected qso (vignali et al. 2005). the constrains on αox are particularly tight: once its dependency on the luminosity is corrected for, the average of the αox distributions in different redshift bins agree within ±0.1 (steffen et al. 2006). taken as a whole, these results indicate a remarkable invariance of the corona-disk coupling over 90% of the look-back time. 2.2 indirect observables other observables can be used to probe indirectly the nuclear sed: we focus in this section on emission lines in the optical, and in the x-ray band. optical spectroscopy probes the physical condition of gas on a variety of different scales in the nuclear environment: from the broad line regions (blrs) clouds, emitting lines primarily from permitted transitions with widths ∼>2000 km s−1, to the narrow line regions 91 matteo guainazzi (nlrs), emitting lines primarily from forbidden transitions with widths ∼<1000 km s−1. these gaseous systems cover the whole range of distances from the central engine between light-days (peterson et al. 2004) to tens-hundreds of kpc (pogge 1988, 1989a, 1989b). extended nlrs are also copious sources of x-rays (young et al. 2001, bianchi et al. 2006). gas in these regions is photo-ionised by the agn. photo-ionisation models applied to the observed spectra allows one to reconstruct the ionising sed, and even in a few cases the past history of the accreting black hole (dadina et al. 2010). the farthest optically identified agn is ulasj1120+0641 at z = 7.085, a mbh = 2 × 109m� quasar (qso). its optical spectrum is almost indistinguishable from composite spectra of agn at lower redshift (mortock et al. 2011). studies of qso optical spectra at z ≥ 5 indicate no evolution of the metallicity as a function of redshift (juarez et al. 2009). these results suggest that the invariance of the central engine extends to the immediate environs of the accreting black hole up to redshifts '7. due to the combination of high fluorescent yield and abundance, iron kα fluorescent emission line is among the most common spectral features in x-ray spectra of cosmic sources. it is well isolated from transitions due to the closest elements; even at ccd resolution (e/∆e∼40) one can easily resolve fluorescence emission lines from he-like, and h-like recombination lines (statistics permitting). an unresolved component of the neutral iron kα fluorescent line has been detected almost ubiquitously in nearby agn (yaqoob & padmanabhan 2004; nandra 2006). this feature traces reprocessing of the primary nuclear emission by optically thick gas. while this feature is too weak to be detected in x-ray spectra of individual sources at high redshift, spectra stacking techniques allowed the cosmos survey to probe its presence up to redshift z ' 3. fitting these stacked spectra with simple phenomenological continua leaves residuals consistent with neutral iron kα emission (and weaker residuals consistent with recombination lines from heand h-like iron). the equivalent width of such features is independent of redshift (chaudhary et al., 2010; iwasawa et al. 2012), once corrected for the known dependence on the absorption-corrected x-ray luminosity, the so called “iwasawa-taniguchi” effect (iwasawa & taniguchi 1993; bianchi et al. 2007). this result implies a similarity of the primary x-ray sed, as well as of the geometrical configuration and ionisation stage of the reprocessing matter at different cosmological times. 2.3 however, agn do evolve ... these results shall not be interpreted as due to the fact that the agn population as a whole do not evolve! the agn population evolves in density and luminosity over cosmological times (see brandt & hasinger 2005 for a recent review). this may in principle affect the agn population being probed at different redshift by current surveys. hopkins et al. (2008) and hickox et al. (2009) present an evolutionary scenario whereby all agn go through the same evolutionary steps. merging or secular processes (gas instabilities) trigger simultaneously the quasar phase and the growth of the stellar bulge when a galaxy’s dark matter halo reaches a critical mass ∼1012−13 m�. the peak of the quasar phase accreting at high eddington ratio, when massive black holes accrete the bulk of their mass, occurs at different redshifts depending on the mass of the primeval dark mass halo. the whole agn evolutionary sequence would start from a cold gas-rich, rotation dominated galaxy, pass through a dustand optically-thick gas enshrouded phase, evolve into the qso phase, and then to a gradual decline of the agn activity until the galaxy become a “red and dead” elliptical with intermittent radio activity. all these phases would be present in an observed agn sample at any redshift. the qso phase is, however, the best one to probe the conditions of the innermost regions close to the central engine. the discovery of a ∼109m� black hole at z ∼ 7 is puzzling for models of cosmological black hole growth. it is not simple to understand how the required mass could have accreted from seed black holes at z ≥ 10. broadly speaking (see volonteri 2010 for a review) models of seed black hole formation and evolution can be classified as: a) light (mbh ∼102−3 m�) and rapidly accreting seeds forming at very early cosmic time (z '20–50); b) heavy (mbh ∼104−6 m�) and slowly accreting seeds forming at later stages (z '5– 10): c) intermediate seeds between the two cases above. none of these scenarios is free of potential shortcomings (milosavljević et al. 2009; volonteri & begelman 2010). if massive enough (∼>105 m�), electromagnetic signals from these early seeds could be detected by future large observatories. otherwise the detection of gravitational waves produced when compact objects fall onto the seed could provide constraints on a population in the mass range mbh ∼104−7 m�. these topics will be addressed by a future esa’s l-class x-ray mission in the framework of the cosmic vision program, one of whose basic science theme is: “how did the universe originate and what is it made of?” (esa 2005). 3 do really agn share the same engine? so far we have assumed the all agn in the quasar phase at a given time share the same central engine. x-rays provide the ideal wavelength to test this assumption for radio-quiet agn in the local universe. in order to 92 cosmological evolution of the central engine in high-luminosity, high-accretion rate agn understand why, we need to have a closer look at the configuration of gas and dust surrounding the agn central engine, and on how it affects the phenomenological appearance of agn. in sect. 2.2 it was shown that the ubiquitous detection of kα fluorescent emission lines corresponding to neutral or mildly ionised iron (together with the associated continuum compton-reflection “hump”; nandra & pounds 1994) is considered a clear evidence for reprocessing of the primary nuclear emission by optically thick gas in the agn environs. the geometry of this gas structure is controversial. traditional wisdom invoked a compact symmetrical molecular “torus” (antonucci & miller 1985). its azimuthal symmetry would introduce a dependence of the agn observational properties on the line-of-sight to the nucleus. these orientation effects would be primarily (but not exclusively; see nicastro 2000) responsible for the properties of their optical spectra: blrs would be visible only through lines-ofsight not intercepting the torus. for the high reddening expected when radiation passes through the torus clouds (nh > 1022 cm−2) optical broad lines would be suppressed, and only narrow emission lines would be visible (see antonucci 1993 for a review). in this “agn unified scenario” broadand narrow-line agn share the same common engine. hoverer, early simulations (pier & krolik 1992) pointed out that it is difficult to prevent such a structure from gravitationally collapsing on time-scales much shorter than the typical agn duty-cycle (∼>1 gyr). recent models invoke a ”clumpy” model as a solution to this issue (nenkova et al. 2002, 2008). in this framework, the agn classification into broadand narrow-line agn should be reinterpreted in probabilistic terms: even agn seen at very high inclination angles (i.e., for which the traditional unified scenario would predict that they could be only narrow-line agn), would have some probability of being detected as broad-line agn depending on the number, size and dynamics of the torus clouds (eliztur 2012; see also eliztur’s contribution to these proceedings for a discussion of this interpretative scenario). first direct x-ray evidence for the torus clumpiness might have been recently detected in the heavily obscured agn markarian 3 (guainazzi et al. 2012). notwithstanding the detailed structure of the torus, and the consequently corrected interpretation of the unified scenario, there is a tight correlation between optical and x-ray spectroscopic properties in nearby agn: optical “narrow-line” agn exhibit x-ray spectra seen through large column densities of cold photoelectrically absorbing gas (nh ∼> 1022 cm−2; awaki et al. 1991; risaliti 2002); optical “broad-line” agn are x-ray unobscured. exceptions to this rules are '3% in large agn x-ray spectroscopic surveys (mateos et al. 2010), and can be easily explained by variability between non-simultaneous optical and x-ray observations, x-ray obscuration by matter in the host galaxy, or even classification issues in poor-quality optical spectra. one needs to carefully correct for absorption (singh et al. 2011), or observe in energy ranges (as much as possible) unaffected by obscuration to probe the innermost region of agn belonging to different optical classes. the latter avenue have been recently opened by the unprecedented sensitivity achieved by the integral/ibis and the swift/bat instruments above 10 kev. sample of more than 100 nearby agn detected above 10 kev are discussed, among others, by beckmann et al. (2009); ricci et al. (2011), molina et al. (2013) for ibis, and burlon et al. (2012) for bat, respectively. the distributions of γ in samples of x-ray obscured and unobscured agn are consistent within ∆γ ' ±0.1. the distributions of αox remain indistinguishable also when the x-ray luminosity density is calculated in the hard x-ray band (beckman et al. 2009). interestingly enough, ricci et al. (2011) suggested that the reprocessing continuum spectra components are stronger when agn are obscured by a column density 1023 cm−2≤nh≤1024 cm−2. this ingredient does not belong to the 0th order unified scenarios. at this level of obscuration a contribution by blr clouds is likely (elvis et al. 2004; risaliti et al. 2005, 2007; bianchi et al. 2009). while this result is a wise reminder of the intrinsic complexity of gas and dust structures responsible for orientation(and time-)dependent obscuration of the primary continuum, it does not in itself invalidate the basic assumptions of the unified scenario. 4 conclusions in summary: • hard x-ray spectroscopy shows that all (local) high-accretion rate agn share the same central engine notwithstanding the optical classification • uv and x-ray spectroscopy shows that agn in the same phase of their evolution (the high accretion rate, unobscured “quasar phase”) share the same engine up to z ' 6 • optical (blr emission) and x-ray spectroscopy 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done c., smith d., et al., 1995, apj, 438, 63 95 outline cosmological evolution of the agn central engine direct observables indirect observables however, agn do evolve ... do really agn share the same engine? conclusions 94 acta polytechnica ctu proceedings 2(1): 94–98, 2015 94 doi: 10.14311/app.2015.02.0094 supersoft x-ray source cal83: a possible ae aqr-like system a. odendaal1, p. j. meintjes1, p. a. charles2, a. f. rajoelimanana3 1department of physics, university of the free state, p.o. box 339, bloemfontein, 9300, south africa 2school of physics and astronomy, southampton university, southampton so17 1bj 3department of physics, north-west university, private bag x2046, mmabatho 2735, south africa corresponding author: winka@ufs.ac.za abstract cal 83 is a close binary supersoft x-ray source in the large magellanic cloud. a ∼67 s periodicity detected in supersoft x-rays is most probably associated with the spin period of a highly spun-up white dwarf (wd). the variability in the period is ascribed to the obscuration of the wd by the hydrogen burning envelope surrounding it, rotating with a period that is close to, but not quite synchronized with, the wd rotation period. optical spectra obtained with salt exhibit accretion disc emission lines with broad wing structures and p cyg profiles, indicating mass outflows. timing analysis of photometrical observations performed at the south african astronomical observatory (saao) revealed variable signals at ≤1 mhz which are thought to be associated with quasi-periodic oscillations from an accretion disc. the short spin period inferred for cal 83 can be the result of spin-up by accretion disc torques during a long mass transfer history, placing this source on a similar evolutionary track as the cataclysmic variable ae aqr. keywords: stars: individual: cal 83 binaries: close white dwarfs stars: oscillations stars: winds, outflows stars: evolution. 1 introduction supersoft x-ray sources (sss) are highly luminous in the supersoft x-ray band, with >90% of the unabsorbed x-ray flux being below ∼0.5 kev. these sources were first discovered by the einstein x-ray observatory and rosat (see kahabka & van den heuvel (2006) and references therein). van den heuvel et al. (1992) (hereafter vdh92) showed that the low effective temperatures and high luminosities of sss can be explained by the nuclear burning of hydrogen on the surface of a white dwarf (wd) accreting from a binary companion (the close binary sss model). the accretion rate required for steady nuclear burning is ∼ 1− 4× 10−7 m� yr−1. such a high accretion rate can be sustained if the companion mass is comparable to or greater than the wd mass, as this will cause the roche lobe of the donor to shrink and drive mass transfer on the thermal time-scale of the donor. cal 83 is a close binary sss in the large magellanic cloud. however, it is not a persistent x-ray source and several x-ray off-states have been observed, with long-term optical variability anti-correlated with the long-term x-ray variability, as described by e.g. rajoelimanana et al. (2013). these authors also derived a refined orbital period of 1.047529(1) d for cal 83 from ogle-iii photometry. the inclination has been estimated to be in the i = 20 − 30◦ range (see cowley et al. (1998) and references therein, hereafter co98). lanz et al. (2005) used nlte models during a combined analysis of xmm-newton and chandra data, and their results indicate a massive wd (mwd ∼ 1.3± 0.3 m�). schmidtke & cowley (2006) found a periodic signal of 38.4-minutes in the chandra letg data of cal 83, which they ascribed to possible non-radial pulsations in the accreting wd. the optical spectrum of cal 83 is characterized by balmer and he ii accretion disc emission lines (co98 and references therein). the he ii λ4686 emission line has a broad, variable wing structure that has also been observed in some of the balmer lines. in each particular spectrum, these wing structures are either all towards the blue side of the main components of these emission lines, or all towards the red side. according to crampton et al. (1987) (hereafter cr87), this may be associated with the slow precession of an accretion disc with outflows through either wind or a weakly collimated jet. the ∼67 s x-ray periodicity is briefly summarized in §2.1 (a detailed discussion is provided in odendaal et al. 2014). preliminary results of the analysis of optical data are presented in §2.2 and §2.3. after this discussion, the possible evolutionary scenario of cal 83 is brought into context with that of the cv ae aqr in §3, followed by the conclusions in §4. 94 http://dx.doi.org/10.14311/app.2015.02.0094 supersoft x-ray source cal 83: a possible ae aqr-like system 2 cal83: observations and results 2.1 the ∼67 s x-ray pulsation the xmm-newton archive contains 23 observations of cal 83, 4 of which were during an x-ray off-state. a systematic search for short time-scale periodicities that may be associated with the wd spin was performed on the 19 on-state light curves. the starlink period1 package was used to obtain a lomb-scargle (ls) periodogram of each light curve. a power peak at a period of 66.8 ± 0.4 s was discovered at a > 99.9999% significance level in the periodogram of the epic pn light curve of observation 0506531501, and at a lower significance level in 6 of the other pn periodograms. in 3 of these observations, 1 of the mos light curves also exhibited a weak ∼67 s periodicity. the ls periodogram of observation 0506531501, as well as the light curve folded on the 66.8 s period, are shown in fig. 1. 0 10 20 30 40 0 15 30 45 60 75 90 l o m b -s ca rg le p o w er frequency (mhz) 99.73% -0.60 +0.60 7.98 0.0 0.5 1.0 1.5 2.0 c o u n ts s -1 phase figure 1: lomb-scargle periodogram (left) and folded light curve (right) of cal 83 xmm-newton dataset 0506531501 pn. both the periodogram and folded light curve display a significant 66.8 s oscillation. the overall significance of this detection, considering the pn datasets of all 19 the on-state observations, is ≥ 3σ. the detected period exhibits a spread of ±3 s around ∼67 s between different observations, and even within a single observation. this variability cannot be explained by doppler shifts due to wd orbital motion. the long-term presence of the pulsation suggests its association with the wd spin, although the variation in the detected period complicates matters. it is possible, however, that we are observing the wd spin, but through an extended envelope around the accreting wd. according to ibragimov et al. (2003), the photospheric radius in a steady nuclear burning sss may have a radius 2-3 times the radius of the wd itself. the spread in the observed period can possibly be explained by the envelope not being quite synchronized with the wd, resulting in a slippage effect and associated variation in the pulsation period. 2.2 optical spectroscopy with salt a series of optical spectra of cal 83 has been obtained with the robert stobie spectrograph (rss) on the southern african large telescope (salt), using the pg0900 grating to obtain a resolution of r ∼ 1500. these spectra show similar characteristics to those discussed in §1, with strong balmer and he ii emission lines. the velocity widths of these lines indicate emission from regions compatible with an accretion disc around the wd. these lines also exhibit blue wing structures extending to ∼2000 km s−1 from the line centres, supporting the existence of a wind or weakly collimated jet in the system. p cyg profiles are also evident in the hα and hβ lines, serving as further evidence of mass outflow. shown in fig. 2 are the hα and hβ profiles from a spectrum obtained on 2 february 2013. 1000 1250 1500 1750 2000 −5000 −2500 0 2500 c o u n ts velocity (km s −1 ) hβ λ4861 1000 2000 3000 4000 −5000 −2500 0 2500 velocity (km s −1 ) hα λ6563 figure 2: hβ and hα profiles of cal 83, obtained with salt on 2 feb 2013. for mwd ∼ 1.3 m�, the wd radius is rwd ≈ 4 × 108(mwd/1.3 m�)−0.8 cm (see eracleous & horne 1996 and references therein). the mass function can be determined if the semi-amplitude (k1) of a spectral line originating close to the wd is known. although the orbital coverage of the salt spectra was not adequate to calculate a new k1, various values have been reported in the literature. adopting k1 ∼ 35 km s−1, i ∼ 25◦ (see co98) and porb = 1.047529 d, the mass function is 4.7× 10−3 m�, and a secondary mass of m2 ∼ 0.61 m� is obtained, yielding a mass ratio q = m2/mwd ∼ 0.47. however, this value of k1 was derived from the radial velocity modulation of the he ii λ4686 disc emission line, and this line would only represent the wd orbital motion if the he ii emission was distributed symmetrically about the wd. as discussed by e.g. kaitchuck et al. (1994), there are some problems with this assumption, compromising the mass function calculation. 2.3 fast photometry with shoc in april 2013, fast photometrical observations were obtained with one of the sutherland high-speed optical 1www.starlink.rl.ac.uk/star/docs/sun167.htx/sun167.html 95 a. odendaal et al. cameras (shoc2) on the saao 1.9-m telescope at sutherland. these cameras have been commissioned recently, and are optimized for high time resolution photometry. a clear filter was used for the cal 83 observations, and with this sensitive instrument, exposure times in the 0.5-5 s range were adequate, depending on observing conditions. aperture photometry was utilized to obtain light curves, and the cal 83 light curves were corrected by performing differential photometry using 4 comparison stars. the starlink period package was used to perform a ls analysis. the ls periodograms of cal 83 did not reveal any significant pulsation at the inferred spin period of ∼67 s or at higher frequencies. however, some of the cal 83 periodograms exhibited a significant peak in the region just below 1 mhz. these peaks have broadened profiles, and their positions do not stay constant. in fig. 3, the ls periodogram of the shoc observation on 15 april 2013 is presented, showing a peak at 1168± 214 s. the light curve folded on this period is also shown. this observation had a total length of 3245 s, and the light curve was binned to 20 s for this analysis. these peaks are ascribed to quasi-periodic oscillations associated with the accretion disc around the wd. 0 3 6 9 12 15 0 2 4 6 8 10 l o m b -s ca rg le p o w er frequency (mhz) 99.73% -7.650 -7.625 -7.600 -7.575 -7.550 -7.525 0.0 0.5 1.0 1.5 2.0 d if fe re n ti al m ag n it u d e phase figure 3: shoc periodogram and light curve folded on the detected period of 1168± 214 s (obtained on 15 april 2013). these quasi-periods at ≤ 1 mhz may be the keplerian periods of blobs of material orbiting the wd in the accretion disc, or possibly beat periods between the presumed spin period of p∗ ∼ 67 s and blobs orbiting in the inner regions of the disc. assuming that the 1168±214 s periodicity is a keplerian period, the associated keplerian radius is 46rwd. on the other hand, it may be a beat period between p∗ ∼ 67 s and a keplerian period of ∼71 s associated with blobs orbiting at ∼7rwd. adopting the orbital parameters in §2.2 yields a circularization radius of ∼150rwd, supporting the compatibility of both the keplerian radii above with emission from the accretion disc. 3 the evolution of cal83: an ae aqr analogue? ae aqr is a nova-like variable with a short wd spin period (33.08 s), and a long orbital period (9.88 h). a review of the multi-wavelength properties of ae aqr is provided by meintjes, odendaal and van heerden elsewhere in this volume. it has been shown by meintjes (2002) and schenker et al. (2002) that the properties of this source indicate a high mass transfer history, during which ae aqr could have been a supersoft source. during this phase, accretion disc torques would have been able to spin-up the wd to such a short rotation period. the probable association of the ∼67 s period in cal 83 with a short wd spin may place this source on a similar evolutionary path as ae aqr. evidence indicating that cal 83 has already passed through quite a long mass transfer history does indeed exist. the high wd mass, just below the chandrasekhar limit, is one of the factors supporting an extended period of mass accretion. however, with the wd being more massive than the wd in ae aqr, it may be driven over the chandrasekhar limit before entering the cv stage. emission lines of ionized carbon, nitrogen and oxygen are present in the optical and uv spectra of cal 83, and as remarked by cr87, the low ratio of h to he ii and cno lines indicate a donor with an h-poor envelope. therefore the secondary may already have shed most of its envelope during a long period of mass transfer to the wd, with cno cycling resulting in the observed line ratios. assuming that the secondary mass is as low as ∼0.61 m� (see §2.2), the roche lobe of the secondary should be r2 ∼ 1.7 r�. as the secondary has to be in contact with its roche lobe for roche lobe overflow, this implies a donor that is too large for its mass, possibly an evolved star that has lost a large fraction of its envelope, also supporting the long mass transfer history. the thermal time-scale is of the order τth ≤ 5 × 107 (m2/0.61 m�) −2 (r2/1.7 r�) −1 yr (e.g. meintjes 2002 and references therein), while the spin-up timescale corresponding to a ∼1.3 m� wd accreting from a disc extending to its surface is τs−u ≤ 107 yr (e.g. wang 1987). these time-scales are comparable, showing that the wd in cal 83 could have been spun-up to a short spin period if it has been in the supersoft phase with its characteristic high mass transfer rates for a sufficiently long time. to sustain a τ ∼ 107 yr mass transfer, q > 5/6 is required for a conservative process, as described by the vdh92 model. however, evidence indicating significant mass and accompanied angular momentum losses is present in the optical spectra, suggesting a nonconservative mass transfer process. given the possible wd spin period of ∼67 s, the outflow mechanism 96 supersoft x-ray source cal 83: a possible ae aqr-like system may be the ejection of accreting material by a similar magnetospheric propeller mechanism as in ae aqr. for non-conservative mass transfer, the roche lobe will shrink according to the relation ṙl,2 rl,2 = −ṁ2 m2 [ 5 3 − 2 (1− α) q − 2 3 qα 1 + q − 2η m2 (gmwdrcirc) 1/2 jorb ] + 2j̇ j , (1) where α and η represent the fraction of mass and angular momentum transferred through l1 that is lost from the system (e.g. meintjes 2002). therefore, for cal 83, mass transfer that can sustain nuclear burning is perhaps only possible in the phase when the evolved donor is expanding, with significant angular momentum loss contributing substantially in keeping the roche lobe in contact with the donor. this will be explored more quantitatively in the future. 4 conclusion it has been shown that cal 83 exhibits a transient ∼67 s x-ray periodicity. this is expected to be associated with the wd spin period, which would make it one of the shortest known spin periods in the white dwarf binary population. results from optical data support the scenario of accretion through an accretion disc, creating the possibility of the wd being spun-up by disc torques during an extended period of mass accretion, similar to what happened during the evolution of ae aqr. optical spectra also present evidence of mass outflows in cal 83, which can be related to the ejection of accreting blobs from a fast rotating wd magnetosphere or accretion disc winds. however, many questions about this fascinating source still remain unanswered, and detailed follow-up studies are essential. acknowledgement the authors would like to thank the conference organisers for the invitation to present this work. the observations reported here were obtained with xmmnewton, salt (proposal 2012-2-rsa uksc-006), and the saao 1.9-m telescope. the authors would also like to thank the salt team for performing the observations, and marissa kotze for technical support and the use of the shoc pipeline. iraf was also used during the optical data analysis. the financial assistance of the south african ska project towards this research is hereby acknowledged. references [1] cowley a. p., schmidtke p. c., crampton d., hutchings j. b., 1998, apj, 504, 854 [2] crampton d., cowley a. p., hutchings j. b., schmidtke p. c., thompson i. b., liebert j., 1987, apj, 321, 745 [3] eracleous m., horne k., 1996, apj, 471, 427 [4] fender r. p., southwell k., tzioumis a. k., 1998, mnras, 298, 692 doi:10.1134/1.1562213 [5] ibragimov a. a., suleimanov v. f., vikhlinin a., sakhibullin n. a., 2003, astronomy reports, 47, 186 [6] kahabka p., van den heuvel e. p. j., 2006, super soft sources. cambridge university press, new york, pp 461–474 doi:10.1086/192065 [7] kaitchuck r. h., schlegel e. m., honeycutt r. k., horne k., marsh t. r., white ii j. c., mansperger c. s., 1994, apjs, 93, 519 doi:10.1086/426382 [8] lanz t., telis g. a., audard m., paerels f., rasmussen a. p., hubeny i., 2005, apj, 619, 517 doi:10.1046/j.1365-8711.2002.05731.x [9] meintjes p. j., 2002, mnras, 336, 265 doi:10.1093/mnras/stt2111 [10] odendaal a., meintjes p. j., charles p. a., rajoelimanana a. f., 2014, mnras, 437, 2948 doi:10.1093/mnras/stt645 [11] rajoelimanana a. f., charles p. a., meintjes p. j., odendaal a., udalski a., 2013, mnras, 432, 2886 doi:10.1046/j.1365-8711.2002.05999.x [12] schenker k., king a. r., kolb u., wynn g. a., zhang z., 2002, mnras, 337, 1105 [13] schmidtke p. c., cowley a. p., 2006, aj, 131, 600 [14] van den heuvel e. p. j., bhattacharya d., nomoto k., rappaport s. a., 1992, a&a, 262, 97 [15] wang y.-m., 1987, a&a, 183, 257 discussion mariko kato: how do you determine the secondary mass for cal 83? is it really so small? i expect q ≥ 1 to explain anti-correlation with x-ray. alida odendaal: the radial velocity semiamplitude of the he ii λ4686 line was used to calculate 97 http://dx.doi.org/10.1134/1.1562213 http://dx.doi.org/10.1086/192065 http://dx.doi.org/10.1086/426382 http://dx.doi.org/10.1046/j.1365-8711.2002.05731.x http://dx.doi.org/10.1093/mnras/stt2111 http://dx.doi.org/10.1093/mnras/stt645 http://dx.doi.org/10.1046/j.1365-8711.2002.05999.x a. odendaal et al. m2 ∼ 0.61 m�, which is only valid if the he ii emission is uniformly distributed around the wd. this assumption can be highly problematic (e.g. kaitchuck et al. 1994), therefore this approach does not yield a conclusive m2. a more reliable wd radial velocity profile may be obtained by constructing doppler tomograms for the emission lines. margaretha pretorius: is there a radio detection of cal 83? alida odendaal: no, but an upper limit of < 0.12 mjy at 3.5 and 6.3 cm was determined by fender, southwell and tzioumis (1998) from data obtained with the australia telescope compact array. 98 introduction cal83: observations and results the 67 s x-ray pulsation optical spectroscopy with salt fast photometry with shoc the evolution of cal83: an ae aqr analogue? conclusion acta polytechnica ctu proceedings doi:10.14311/ap.2016.4.0033 acta polytechnica ctu proceedings 4:33–37, 2016 © czech technical university in prague, 2016 available online at http://ojs.cvut.cz/ojs/index.php/app impact of the chemical form of in-containment source on fission product release from wwer-1000/v-320 type npp containment during loca adam kecek department of nuclear reactors, faculty of nuclear sciences and physical engineering, czech technical university in prague correspondence: kecekada@fjfi.cvut.cz abstract. nuclear power plant accidents may be followed by a release of fission products into the environment. this release is dependent on several phenomena, such as chemistry, pressure, type of the accident etc. the aim of this paper is to assess the impact of the chemical form of iodine on the fission product release into the environment. keywords: npp, loca, iodine, containment, cocosys, fission products transport, wwer1000/v-320. 1. introduction fission product behaviour and its simulation recently become more important. nuclear power plant accidents in past decades revealed the need of proper definition and calculation of fission product mass entraining the environment. to successfully calculate such phenomena, one has to specify the in-containment source correctly. several different approaches existing up to date vary in the mass, the chemical form or time characteristics. this paper is focused on assessment of the impact of the chemical form of iodine release, which is well known for its extensive range of chemical compounds and reactions within the containment. a fundamental opinion on the complexity of iodine chemistry in the containment during loss-of-coolant accidents can be obtained from fig. 1. used models, codes and brief overview on existing approaches are presented further. 2. description of cocosys the containment code system is a lumped parameter computer code developed and maintained at gesellschaft für reaktorsicherheit (grs mbh). it is used for best-estimate analysis of lightwater reactor containments during severe accidents. the code itself consists of several modules, which calculate all relevant phenomena. the thermal hydraulic (thy) module solves the pressure and temperature build-up history together with mass and energy flow. furthermore it can simulate behaviour of engineering systems like valves, fans, pumps and sprays. a feedback from aerosol and fission products is also considered. the aerosol fission product (afp) module is used to estimate the behaviour and transport of fission products and aerosols within the containment. the core concrete interaction (cci) module provides an investigation of interaction of the molten core with concrete structures during severe accidents. all menfigure 1. scheme of iodine chemistry in npp containment [1]. tioned modules are coupled to provide the desired feedback on relevant phenomena. furthermore, cocosys provides a coupling option with several codes like athlet to simulate primary circuit behaviour or a cfd code for detailed analysis [2]. cocosys has been validated against several validation experiments and is often used in safety analysis of npp concepts and existing power plants including wwer type npps [2]. 3. wwer-1000/v-320 model description the wwer-1000/v-320 type npp containment is a cylindrical shaped building made of pre-stressed 33 http://dx.doi.org/10.14311/ap.2016.4.0033 http://ojs.cvut.cz/ojs/index.php/app adam kecek acta polytechnica ctu proceedings concrete with an inner steel liner. for thermal hydraulic calculations, the inner volume has to be divided into several smaller rooms. the presented model provides an approximation based on 20 volumes, so called nodes, which simulate the real space surrounded by the building. these volumes are connected through junctions providing mass and energy transfer. the walls, floor, ceiling and inner structures are also modeled to provide realistic heat transfer. furthermore the containment spray systems are also modeled. two large nodes (environ and building) simulate the outer volume surrounding the containment. these provide boundary conditions of the problem. the nodalisation of the containment is presented in tab. 1. compartments nodes description e1 environ environment building surrounding buildings iga501 ga501 upper part of the reactor shaft iga301 ga301a lower part of the reactor shaft iga401 ga401a2 spent fuel pools iga701 ga701 reactor hall iasupp cnt-u1 upper section of the building surrounded by the containment wall and steam generator boxes cnt-u2 iasgbox1 sgbox-1 steam generator box 1 iasgbox2 sgbox-2 steam generator box 2 iasmed cnt-m1 middle section of the builcnt-m2 ding at the containment wall iaslow cnt-l1 lower section of the containment building cnt-l2 ga311 ga306-1 ga306-2 ga306-3 iga201 ga201 spray water sump iastl22 tl22p fan system table 1. containment discretization. the simulation of containment leak is maintained by atmospherical junction. this is estimated to follow the limits and conditions of operation of the wwer1000/v-320 type containment. the junction itself is placed in the upper part of the containment connecting the reactor hall and environment. the reason for choosing this position is to simulate shortest and easiest way for ejecting the material from containment into the environment. for calculation of fission product and aerosol behaviour the containment building has to be divided into volumes so-called compartments formed by nodes. thus, compartments provide coarser fragmentation of the containment building. the fragmentation of the containment volume for iodine calculations is presented in tab. 1 and fig. 2. the calculations were done using the afp and thy cocosys computational modules. figure 2. iodine compartment discretization of wwer-1000/v320 npp containment. 4. initial and boindary conditions before the accident, the nuclear power plant is operated at 104 % nominal power. the initiating event is a primary circuit hot leg guillotine rupture at t = 0 s with leak through breaks of equivalent diameter equal to 850 mm (large break loca) into one of the steam generator boxes. moreover, following assumptions are taken into account [3]: • loss of on-site power, • only 1/3 of safety systems in operation (according to conservative approach), • no action expected from operators. the entrainment of fission products into the containment is maintained through the break in the primary hot leg. the ejecting coolant brings the fission products and aerosols released from the core. in accordance with local practice the in-containment source term is based on ref. [4]. following this approach failure of all fuel assembly is expected. in addition, whole fission product mass released into the primary circuit is transferred into the containment. this means that no fission products remain in the primary circuit. the ref. [4] divides the problem of fission product release into two areas, namely normal and core melt. 34 vol. 4/2016 impact of the chemical form of in-containment source the investigated accident does not expect the molten core, so only the pellet-cladding gap inventory is taken into account. the fraction of released fission products is given in tab. 2. fission product pellet-cladding gap fuel melt release release fraction fraction (30–1800 s) (1800–4680 s) i, br 0.05 0.95 xe, kr 0.05 0.35 cs, rb 0.05 0.25 te, sb, se 0.00 0.05 ba, sr 0.00 0.02 ru, rh, pd, mo, 0.00 0.0025tc, co la, zr, nd, eu, 0.00 0.0005nb, pm, pr, sm, y, cm, am ce, pu, np 0.00 0.0002 table 2. fraction of the fission products released during an accident [4]. a research on available in-containment fission product source showed a significant difference between each approach. synchronization efforts on this problem throughout europe pointed out the diversity of the iodine species, the target medium (water, sump or aerosol) and also the fraction of the core inventory released [5]. it has to be mentioned that there is no information on time characteristics of the release. based on this it was decided to test, whether the chemical form and also the target part of the zone has any influence on iodine mass entraining the environment. the time characteristics and fraction released was adopted from ref. [4]. finally, five calculations were done in this sensitivity study, where the distribution of iodine mass varied in chemical form. in addition, the target part of the zone was changed too. overview on all calculations is given in tab. 3. calculation chemical target part iodine mass form of the zone distribution [%] c00 csi atmosphere1 95 i2 4.85 ch3i 0.15 c01 csi atmosphere 0.15 i2 95 ch3i 4.85 c02 csi sump 0.15 i2 95 ch3i 4.85 c03 csi atmosphere 4.85 i2 0.15 ch3i 95 c04 csi sump 4.85 i2 0.15 ch3i 95 table 3. overview on calculated variants. the total mass of fission products in the core is based on values provided in ref. [3]. the total amount of iodine used in this calculation is 22.7 kg. it has to be noted that not all the fission product mass is expected to be radioactive. this study calculates the behavior of the whole mass of iodine entraining the containment. 5. results and comparison the calculations were done for 14 400 s, what equals to 4 hours. this time range was chosen to be representative as the testing calculations showed that the marginal contribution to the mass ejecting the containment during the accident is within first two hours. calculation c04 was neglected due to numerical error. no. description mass [g] deviation from c00 [%] c00 base case (95 % csi, 4.85 % i2, 0.15 % ch3i) a 4.94 × 10−3 0.00 c01 modified iodine mass distribution (95 % i2, 4.85 % ch3i, 0.15 % csi) a 2.33 × 10−2 371.51 c02 modified iodine mass distribution (95 % i2, 4.85 % ch3i, 0.15 % csi) s 4.78 × 10−3 −3.39 c03 modified iodine mass distribution (95 % ch3i, 4.85 % csi, 0.15 % i2) a 1.14 × 10−1 2204.44 c04 modified iodine mass distribution (95 % ch3i, 4.85 % csi, 0.15 % i2) s err err table 4. overview on calculation and results. the trend of the release is in good agreement with all calculations except c03, where almost constant release rate can be observed. the ejected mass in calculations c00 through c02 shows a massive growth of the release within the first hour, then it slows down and in long time view it shows stabilization of the mass entraining the environment. this may be caused by several phenomena, such as sedimentation or wash down of fission products from walls into the sump. the worst case presented in this sensitivity analysis is c03, where the main mass was injected as organic iodine into the atmosphere. it has to be noted that this form is not expected to be a typical contributor to the in-containment source, thus the results seem to be unrealistic. the second worst case is c01, where the iodine in elemental form is injected into the gas part of the zone. this leads to a high release of iodine into the environment. in accordance to ref. [6] the elemental 1the csi aerosol can be injected only into the gas part of the zone. 35 adam kecek acta polytechnica ctu proceedings figure 3. distribution of iodine in the containment (c00–c03, 14 400 s). iodine is one of the main options of iodine mass release into the environment. the base case c00 provided results lower than the two calculations presented before. the mass behaves significantly different as the compound is expected to be in aerosol particle form. the lowest release was calculated for elemental iodine injected into the sump part. this may be caused by high solubility of iodine species. the results are presented in tab. 4 and fig. 4. the distribution within the containment varies for each calculation. in the case of c00 the mass is centralized in the containment sump. this may be induced by the aerosol wash down. the c01 shows significant concentration in the reactor hall. this is probably caused by the chemical form, as the injection was in form of elemental iodine into the atmosphere. furthermore, c02 provides results close to the c00 calculation. the injected iodine might be dissolved in water and then washed down into the containment sump. finally the c03 calculation shows distribution close to the c01. this is again induced by the injection of iodine as a gas. the iodine mass is mainly distributed within the reactor hall. the results of the calculations c00 through c03 at 14 400 s are presented in fig. 3. figure 4. iodine release into the environment (c00–c03, 14 400 s). 36 vol. 4/2016 impact of the chemical form of in-containment source 6. conclusion the presented calculations proved a significant impact of the chemical form of iodine on release into the environment. furthermore the injected phase (water, gas) is also an important factor significantly influencing the ejected mass. future effort should be made on the assessment of the in-containment source, e.g. the time range and the fraction of ruptured fuel assembly. references [1] irsn. the astec software package. http://www.irsn.fr/en/research/ scientific-tools/computer-codes/pages/ the-astec-software-package-2949.aspx. [2] w. klein-heßling, et al. cocosys v2.4 user’s manual. gesellschaft für anlagenund reaktorsicherheit (grs) mbh, 2010. [3] a. kecek. testovací výpočty úniku štěpných produktů z kontejnmentu je temelín v průběhu loca události programem cocosys. tech. rep. z4339, újv, 2016. [4] u.s. nuclear regulatory commission. alternative radiological source terms for evaluating design basis accidents at nuclear power reactors, 2000. regulatory guide 1.183. [5] nuclear fuel behaviour in loss-of-coolant accident (loca) conditions. tech. rep. 6846, nea oecd, 2009. [6] u.s. nuclear regulatory commission. methods and assumptions for evaluating radiological consequences of design basis accidents at light-water nuclear power reactors, 2003. regulatory guide 1.195. 37 http://www.irsn.fr/en/research/scientific-tools/computer-codes/pages/the-astec-software-package-2949.aspx http://www.irsn.fr/en/research/scientific-tools/computer-codes/pages/the-astec-software-package-2949.aspx http://www.irsn.fr/en/research/scientific-tools/computer-codes/pages/the-astec-software-package-2949.aspx acta polytechnica ctu proceedings 4:33–37, 2016 1 introduction 2 description of cocosys 3 wwer-1000/v-320 model description 4 initial and boindary conditions 5 results and comparison 6 conclusion references 113 acta polytechnica ctu proceedings 1(1): 113–117, 2014 113 doi: 10.14311/app.2014.01.0113 star formation as seen by low mass stars nino panagia1,2,3, guido de marchi4 1space telescope science institute, 3700 san martin dr, baltimore md 21218, usa 2inafosservatorio astronomico di capodimonte, salita moiariello 16, i-80131, naples, italy 3supernova ltd, oyv #131, northsound road, virgin gorda, british virgin islands vg1155 4european space agency, keplerlaan 1, 2200 ag noordwijk, netherlands corresponding author: panagia@stsci.edu abstract using the hubble space telescope (hst) we have characterised and compared the physical properties of a large sample of pre-main sequence (pms) stars spanning a wide range of masses (0.5 − 4 m�), metallicities (0.1 − 1 z�) and ages (0.5−30 myr). this is presently the largest and most homogeneous sample of pms objects with known physical properties. the main results of this ongoing study are briefly summarised here. keywords: star formation solar mass stars magellanic clouds milky way photometry. with a group of european colleagues, whose names are listed in the acknowledgments, we have undertaken a systematic study of the star formation process in the milky way and magellanic clouds (mcs), with special emphasis on moderate mass stars, say 0.5 to 4 m�. our motivations are: i) stars in this range account for most of the star formation in a galaxy, e.g. more than 65% for a salpeter initial mass function (imf); ii) low mass stars can form in small parent clouds as well as in big ones; iii) as a consequence, low mass stars are forming near massive stars as well as in isolated groups. on the other hand, when considering distant galaxies we can probe their star formation rates only by considering the phenomena induced by the presence of massive stars, such as bright hii regions, supernova explosions, unresolved, bright stellar clusters. however, using massive star diagnostics to measure the overall star formation rates is valid if and only if we are confident to be able to extrapolate correctly from what we see coming from a bunch of massive stars to the total amount of mass that is forming the bulk of stars, i.e. the low mass stars. in other words, we need to understand how the formation of moderate mass stars works as a function of the environmental conditions and how it relates to the massive star formation. ground-based spectroscopic studies of nearby young star-forming regions (e.g. taurus, auriga, ophiuchus) show that the mass accretion rate appears to decrease steadily with time, from about 10−8 m� yr−1 at ages of ∼ 1 myr to ∼ 10−9 m� yr−1 at ∼ 10 myr. at face value this is in line with the expected evolution of viscous discs, but the scatter of the data exceeds 2 dex at any given age (see also fig. 3). such a scatter may be explained in part by the wide mass range covered by the observations, but the true limitation is the paucity of available measurements. indeed, all the results so far obtained are based on the mass accretion rates of a few hundred stars, located in nearby regions, covering a limited range of ages and no appreciable range of metallicity. using the hst, we have started a study of the pms phase is a number of star forming regions in the local group. among others, these include ngc 3603 in the milky way, 30 doradus and surrounding regions in the large magellanic cloud, and ngc 346 and ngc 602 in the small magellanic cloud. thanks to a novel method that we have developed to combine broad-band (v , i) photometry with narrowband hα imaging (de marchi et al. 2010), we have identified more than 3 000 bona-fide pms stars still undergoing mass accretion (fig. 1). we have determined their physical parameters, i.e. temperature, luminosity, age, mass and mass accretion rate. this is presently the largest and most homogeneous sample of pms objects with known physical properties. 113 http://dx.doi.org/10.14311/app.2014.01.0113 nino panagia, guido de marchi figure 1: the v −hα colour as a function of v − i in fields around sn 1987a in the lmc (left; de marchi et al. 2010), in the cluster ngc 346 in the smc (middle; de marchi et al. 2011a) and in ngc 3603 in the galaxy (right; beccari et al. 2010). all colours are corrected for extinction. stars with small photometric errors (small dots in light gray) define the reference template for normal stars (i.e. with no hα emission), shown here as a dashed line. strong hα emission results in a greater than average value of v −hα. we have identified in this way ∼ 3 400 pms stars in the three galaxies. figure 2: colour–magnitude diagrams for the fields around sn 1987a (left) and around ngc 346 (right). stars with significant hα excess are highlighted. whilst some are still very young, and as such still very distant from the ms, many are older objects already approaching the ms, and as such would not be distinguishable from normal ms stars using standard broad-band photometry alone. by detecting all stars with an hα excess, we can easily identify and study multiple stellar populations in any given region. the virtue of this new method is that it derives the luminosity of the photospheric continuum of a star inside the specific hα band simply by interpolation from the average v −hα colour of stars with the same v −i index (fig. 1), the majority of which have no emission. equipped with the knowledge of the continuum level in the hα band, we can easily determine the hα luminosity, lhα, of each star. the method and its applications are fully explained in a series of papers (de marchi et al. 2010; 2011a, 2011b; spezzi et al. 2012) and the accuracy of the hα continuum and lhα derived in this way is independently confirmed by spectroscopic measurements (barentsen et al. 2011). this method allows us to identify all objects with an excess emission, including relatively “mature” pms stars, already close to the ms (see fig. 2), that broad-band photometry alone could not distinguish from normal ms stars. thus, we can (1) identify different generations of stars within the same region, (2) derive their physical propertiest through comparison to evolutionary tracks for the appropriate metallicity, and (3) study their spatial distribution to establish the relationship to massive stars and nebular gas. 114 star formation as seen by low mass stars figure 3: the left panel shows the mass accretion rate as a function of stellar age for pms stars in ngc 346 (diamonds) compared with that of galactic t tauri stars (see legend) from the work of sicilia–aguilar et al. (2006; the large cross indicates the uncertainties as quoted in that paper). the solid line shows the evolution for current models of viscous disc evolution from hartmann et al. (1998). our measurements are systematically higher than the models, and the effect remains when we consider separately stars of different masses (right panel, see legends for mass values). all four mass groups show the same decline of ṁacc with age (α ' −0.6, thick dashed lines), but ṁacc is higher for more massive stars (see value of intercept q at 1 myr). figure 4: run of ṁacc/m as a function of age for the pms objects in ngc 346 (circles) and for galactic stars in taurus (crosses) and in trumpler 37 (squares). the hatched band show the best fit (±1σ) to the ngc 346 data, the dark-shaded band is the fit to the pms stars in the 30 dor region (de marchi et al. 2011c) and the light-shaded band is the fit to the galactic stars. we note that the value of ṁacc in the galaxy is systematically lower than in the mcs at all masses and ages. all regions that we studied exhibit multiple recent episodes of star formation, indicating that star formation has proceeded over a long time, even though our age resolution cannot discriminate between an extended episode or short and frequent bursts. we also find that there is no correlation between the projected spatial distribution of young and old pms stars and that the younger population is systematically more concentrated. a fundamental parameter that we can derive with this method is the mass accretion rate, ṁacc. since the energy released by the accretion process goes towards ionising and heating the circumstellar gas, the accretion luminosity lacc can be derived from lhα. with the mass and radius of each pms star determined from the evolutionary tracks, the value of ṁacc can be obtained from the free-fall equation. we find that older pms stars have typically lower mass accretion rates (see fig. 3) and this suggests an evolutionary effect (assuming that the initial conditions are the same at all times). however, the effects of temporal evolution are partly masked by the considerable scatter around the best fit due to the wide range of masses that we cover. a multivariate linear regression fit to the distribution of ṁacc as a function of age (t) and mass (m), allowed by the size of our sample, gives log ṁacc ' −0.6 × log t + logm + c, where c is a function of the metallicity z and is approximately proportional to z−1/3 (fig. 5). 115 nino panagia, guido de marchi figure 5: the average mass accretion rates for stars of 1m� and age 2 myrs for clusters in the smc (ngc 346, ngc 602), lmc (30 dor, sn 1987a field) and the mw (ngc 3603) as a function of their average metallicities. it appears that the accretion rates are a monotonically decreasing function of the metallicity, i.e ṁ ∝ z−1/3. in terms of this relationship, the systematic offset of galactic pms stars relative to stars in the magellanic clouds is as large as factor of six (see fig. 4). while our method will inevitably miss some pms stars with weak hα excess emission, it is very unlikely that ground-based searches of nearby star-forming regions have missed pms stars with strong lacc. thus, the difference is real, showing that the accretion process depends not only on age and mass, but also on metallicity (see fig. 5). other important results of our studies are: • in each region we find evidence for several episodes of star formation, with typical time separation of 10 myrs. • younger generations are spatially more concentrated. • there are large mass function variations across star formation regions, in that we see stellar groups that are rich of young low mass stars but lack any massive stars, and, viceversa, there are isolated massive stars with no low mass stars associated to them. • these findings suggest that the processes of star formation be appreciably different for highand low-mass stars. actually it is possible that a large number of lowmass stars are forming in regions where massive stars cannot be produced and, therefore, they remain unnoticed. for certain low surface density galaxies this might be the predominant mode of star formation, which would imply that their total mass based on their luminosity can be severely underestimated and that their evolution not be correctly understood. acknowledgement we are indebted to our collaborators martino romaniello, giacomo beccari, loredana spezzi, elena sabbi, pier prada moroni, scilla degl’innocenti, francesco paresce, and morten andersen for participating in this project. np research was funded in part by stsci-ddrf grant d0001.82435. references [1] barentsen, g., vink, j., drew, j., et al. 2011, mnras, 415, 103 [2] beccari, g., spezzi, l., de marchi, g., et al. 2010, apj, 720, 1108 doi:10.1088/0004-637x/720/2/1108 [3] de marchi, g., panagia, n., romaniello, m. 2010, apj, 715, 1 doi:10.1088/0004-637x/715/1/1 [4] de marchi, g., panagia, n., romaniello, m., et al. 2011a, apj, 740, 11 doi:10.1088/0004-637x/740/1/11 [5] de marchi, g., panagia, n., sabbi, e. 2011b, apj, 740, 10 doi:10.1088/0004-637x/740/1/10 [6] de marchi, g., paresce, f., panagia, n., et al. 2011c, apj, 739, 27 doi:10.1088/0004-637x/739/1/27 [7] hartmann, l., calvet, p., gullbring, e., d’alessio, p. 1998, apj, 495, 385 doi:10.1086/305277 [8] sicilia–aguilar, a., hartmann, l., furesz, g., et al. 2006, aj, 132, 2135 [9] spezzi, l., de marchi, g., panagia, n., et al. 2012, mnras, 421, 67 discussion manami sasaki: in order to increase the statistics for metal abundance dependencies would it be possible to study stellar populations in m 31 od m 33? nino panagia: i wish it was possible but, unfortunately, at present the angular resolution of the available telescopes, even hst, is not good enough to resolve solar mass objects in star forming regions at the distance of m 31 or m 33. in the future the combination of very large telescopes, say >30 m and adaptive optics should eventually do the miracle. jim beall: would you expect the same relationship between accretion rates and abundances in massive stars? 116 http://dx.doi.org/10.1088/0004-637x/720/2/1108 http://dx.doi.org/10.1088/0004-637x/715/1/1 http://dx.doi.org/10.1088/0004-637x/740/1/11 http://dx.doi.org/10.1088/0004-637x/740/1/10 http://dx.doi.org/10.1088/0004-637x/739/1/27 http://dx.doi.org/10.1086/305277 star formation as seen by low mass stars nino panagia: the pre-main-sequence evolution of massive stars is so fast that it is hard to catch one of them while it is still accreting from their parent envelope. therefore, i don’t have an observational answer to your question. from a theoretical point of view, one would need a detailed knowledge of what determines the accretion process under conditions appropriate for massive stars. and, unfortunately, i don’t have that, either. 117 acta polytechnica ctu proceedings doi:10.14311/app.2017.12.0056 acta polytechnica ctu proceedings 12:56–63, 2017 © czech technical university in prague, 2017 available online at http://ojs.cvut.cz/ojs/index.php/app human-machine interaction in solving tasks of the planning department boris kucherov∗, vladimir artyushenko department of information technology and control systems, information and technology faculty, university of technology, gagarina 42, korolev, moskovskaya oblast, russia ∗ corresponding author: boris.ku4erov@gmail.com abstract. the paper discusses issues of human-machine interaction in solving tasks of the planning department under severe resource restrictions using information technology. the negative factors influencing specialists of the planning department in solving their tasks under the given circumstances are shown. specific features of designing the user interface in this subject area are noted. directions to increase the efficiency of reaction of the planning department’s specialists to change the current situation by visual and sound notification of various events are marked. various ways to develop user interface to generate a conflict-free plan under severe resource restrictions are considered. the variants of informative presentation of operational and statistical information to stakeholders are analyzed. these issues are discussed by the example of the planning department which solves the tasks of allocation of control facilities for spacecraft (a subset of satellite range scheduling problem). keywords: planning department, human-machine interaction, notification, conflict-free plan, spacecraft. 1. introduction the allocation of shared resources (resources of collective usage) is a current problem with a large number of different modifications [1–5]. planning departments solving such tasks are used in many industrial areas, such as transportation industry, rocket and space industry, nuclear power industry, etc. as examples, planning departments of railway traffic, planning departments of road works, department of logistics transportation, traffic control departments, etc. [4–9]. in modern world such departments often work under severe resource restrictions caused not only by scarcity of allocated resources, but also by high requirements on time for solving the tasks. these requirements could be satisfied by using modern information technology. however, interaction of specialists with an automated information system of a planning department has sufficient influence on the efficiency of solving the tasks even when there is a high degree of automation. the main negative factors influencing the specialists of a planning department in solving their tasks under current conditions may include: • a large amount of data to be processed, a significant number of factors to be analyzed; • strict requirements to time of solving tasks under conditions when specialists should be resistant to stress; • a necessity of the current situation continual monitoring for a prompt response to its changes. the usage of successful approach to human-machine interaction can significantly increase the efficiency of solving tasks of a planning department. the issues of human-machine interaction in the already mentioned subject area are considered by the example of planning department solving tasks of allocation of control facilities for spacecraft. in such case, the control facilities are considered as shared resources (resources of collective usage) and the spacecraft are considered as their consumers. the control facilities (ground stations) are used to carry out communication sessions with spacecraft to receive telemetry information, to transfer command and program data, to measure parameters of spacecraft orbit, etc. [10, 11]. herewith, a control facility can carry out only one communication session with only one spacecraft at any given time. the problem of allocation of control facilities for spacecraft can be considered as a subset of satellite range scheduling (srs) problem. most papers related to srs [4, 5, 12, 13] discuss mainly a schedule (plan) optimization. various approaches, based on an integer linear programming [12], genetic algorithms [13–17], graph color theory [14, 18], etc. [2, 6, 19, 20], are proposed. however, not enough attention is paid to issues of human-machine interaction in solving tasks of allocation control facilities for spacecraft under severe resource restrictions using information technology. 2. the increasing efficiency of response of the planning department’s specialists on changing the current situation one of the main factors, influencing the efficiency of solving tasks of a planning department, is a timeli56 http://dx.doi.org/10.14311/app.2017.12.0056 http://ojs.cvut.cz/ojs/index.php/app vol. 12/2017 human-machine interaction in solving tasks of the planning department ness of specialists’ response on changing the situation. this factor could be parried by prompt notification of specialists about changing the situation. such notification has a number of specificities. first of all, a group of persons to be informed is not so large (from few to tens). employees’ workplaces equipped with computers are located on a quite small area (one or few rooms). the data flows of notifications have address direction. generally [21, 22], specialists of a planning department are always at their workplaces during their shift (except time for endorsing originals of documents and submitting them for approval), so they can be notified about changes in the current situation by computers. receiving information into a database can be considered as a completion of one technological operation and a start of another. when receiving new information from subscribers, specialists of a planning department are notified about this fact using both visual and sound messages. its implementation can be based on principles of multimodal alerts used in various services and applications (email, social media, organizers, etc.). after receiving a new message a pop-up window with brief information about it appears and a playback of sound alert or speech comment is played [23–27]. the notification may vary depending on the properties of the received information and of the current situation. it enables the specialist to make a decision promptly about the level of urgency to process the received data even if he/she performs other tasks and does not work on the computer at that time. the specialist can interrupt the current task and switch to performing a more priority one or begin processing the received information after the completion of the current task. the visual component of notification is a pop-up window containing data about the received information. an individual icon can be assigned to each type of event. a specialist can switch to processing the received data directly from the message of visual notification. the sound notifications may vary depending on the properties of the received information. they may be repeated at specified intervals before receiving notification by the specialist. however, notification about large number of events during short time can cause cognitive overload of specialists of a planning department. such overload could be prevented by tasks reallocation [28]. for a small number of specialists on the shift (e.g., 2-3 persons) and a large number of simultaneously occurring events, notifications can be arranged by its priorities and processed consistently according to their priorities in addition to tasks reallocation. during the process of notification of specialists about changes of situation, all information is received into a database, which is a central element of planning department’s automated information system. the generalized diagram of propagation of messages about receiving new information is presented in figure 1 figure 1. the propagation of messages about receiving new data figure 2. schematic representation of the principle of increasing the efficiency of solving tasks of planning department by using the notification system about changes of situation [22]. it was implemented by a mechanism of advanced queues support in database management system oracle oracle advanced queuing. the visual component of notifications (message) was implemented by devexpress vcl (visual component library). it should be noted that the usage of notification about changes of situation usually does not influence the time of making a decision, but it enables the specialists to start making a decision earlier (see figure 2). this causes one of two consequences (or their combination): the decision making process is completed earlier or there is more time to make a decision. thus, the notification about changes of situation can not only increase the efficiency of solving tasks of a planning department by earlier start and, as a consequence, by the earlier end of the decision making process: it can also contribute to improving the decisions due to additional time for its production. when the notification is received by the specialist and all more priority tasks are completed, then the processing of received data starts. for example, after the planning department receives requests on using control facilities, developing plan of allocation of control facilities starts. another example can be receiving data about changing the state of control facilities data preparation for analysis of current situation begins after it. processing various data about changing the situation may require different time. so, the processing of 57 boris kucherov, vladimir artyushenko acta polytechnica ctu proceedings such data as requests on using control facilities that don’t lead to conflict situations (which will be discussed below), request for cancellation of communication sessions, maintenance plans received beforehand, consists of performing a sequences of typical (routine) actions. that is, the processing of such data is carried out at the rule-based level of skill-rule-knowledge framework of rasmussen [29] and does not create a large cognitive load on specialists. processing other data such as requests on using control facilities that lead to conflict situations, troubles when performing communication sessions, malfunctions of control facilities is carried out at the knowledge-based level which creates a significant cognitive load on specialists and require more time to process. 3. developing a conflict-free plan under strict resource restrictions issues of human-machine interaction during solving the task of developing conflict-free plan under strict resource restrictions have significant influence on the efficiency and the quality of solving this task. so, information capacity and ergonomic of user interface of software for developing a plan are very important. it is especially significant when the scarcity of shared resources grows due to increasing the number of consumers of shared resources under resource restrictions. such circumstances cause increasing the number of conflict situations (conflicts). the conflict is a contradiction between request on using shared resources and existing restrictions [21]. it can be caused by the need to use the same resource for different consumers at the same time, the need to use the resource at the time of service operations, etc. resolution of conflicts is rather time-consuming process. moreover, it is hard for specialists of planning department to solve successfully a large number of conflicts under severe resource restrictions without using appropriate software. as a result, growing number of conflicts causes the increase of workload of specialists of planning department. so, developing the user interface of software for planning department should include proactive steps aimed to parry the increase of workload of the specialists. for such purpose it is necessary to predict a situation in the allocation of control facilities for spacecraft. also, the results of situation prediction can be used to make decisions during developing a plan to control facilities allocation. 3.1. prediction workload of specialists of planning department to solve tasks of workload prediction of specialists it is necessary to take into account a number of factors: structure and state of shared resources (control facilities), number and properties of requests on its usage, statistical data about its usage, planned maintenance, etc. as it has been already mentioned, one figure 3. the dependence of a quantity of conflicts per day from the number of spacecrafts and the number of control facilities (calculated in 100 model experiments): 2d-plot (top) and 3d-plot (bottom) of the major factors influencing the workload of specialists is a number of conflicts, arising during the allocation of control facilities for spacecraft. the number of conflicts significantly depends on the number and properties of requests on the use of control facilities as well as structure and state of control facilities. the number and properties of requests depend on structure and state of spacecrafts constellation and their mission profiles (technological control cycles of spacecrafts). the workload of specialists of planning department can be predicted by evaluating the number of conflicts which occur when changing the structure and state of spacecraft constellation and control facilities, using series of model experiments. each series can be used to determine the dependence of each indicator (e.g., number of conflicts) from some factors (e.g., the number of spacecraft). the example of dependence of quantity of conflicts from the number of consumers for the various number of planning objects is presented in figure 3. this dependence was obtained as a result of series of 100 model experiments using the model, developed to solve the tasks of workload prediction of specialists of planning department. the developed model implemented in matlab, which allows getting numerical probability characteristics of the situation in allocation of control facilities 58 vol. 12/2017 human-machine interaction in solving tasks of the planning department for spacecrafts, contains the following units: • a unit of loading initial data; • a unit of calculation windows of radio visibility for spacecrafts by control facilities; • a unit of generation different sets of input data for series of model experiments; • a unit of generation requests on using control facilities for spacecrafts (choosing windows of radio visibility to carry out communication sessions); • a unit of detection and count conflicts; • a unit of statistical data processing. the calculation of windows of radio visibility for spacecrafts was implemented using orekit a low level space dynamics library written in java. precision of orekit was proved during its usage in real missions [30]. windows of radio visibility were calculated using input data in the form of tle (two-line elements), which are provided by norad (north american aerospace defense command) and publicly available for free in its catalog at [31]. sampling 140 spacecraft on low-earth orbits from this catalog was performed. windows of radio visibility between these spacecrafts and 4 ground points were calculated. it was mentioned that there are 3 control facilities on each ground point. time of preparation for communication sessions was set as 15 minutes. a hypothetical constellation of 35 spacecrafts was generated during each experiment by random sampling spacecrafts from the previously sampled ones. for each constellation requests on using control facilities were generated from various number of spacecrafts (from 5 to 35) for various number of control facilities (from 3 to 12 facilities on 4 ground point). the requests were generated using approach of random sampling 3 windows of radio visibility from the most preferred 10 ones (which have the largest duration). conflicts were detected and counted for each subset of requests. the model results show numerical probability values of progressive increase in number of conflicts during increasing the number of spacecrafts and, consequently, progressive increase of workload of specialists of planning department. the largest number of conflicts is observed at the least number of control facilities. note that the number of conflicts can vary significantly for the same number of spacecrafts and control facilities (e.g., figures 4 and 5). however, the trend of progressive growth of the number of conflicts during increasing the number of spacecrafts is still preserved. it should be taken into account in developing the user interface of software for planning department. this is particularly important due to the fact that increasing the number of specialists on the shift will not completely parry the increasing load. it is caused figure 4. the example of range of variation of the conflicts’ number for 5 control facilities figure 5. the example of the probability density of the number of conflicts for 35 spacecrafts and 5 control facilities by the specificities of the task of developing (changing) a plan of allocation of control facilities which do not allow to parallelize its execution between several specialists. for example, parallel solution of several conflicts at the same date by different specialists in the most cases is impractical due to the fact that new conflicts may occur. furthermore, as shown in [32], for a large number of specialists tasks reallocation between them may lead to a significant increase of resource costs of these operations. 3.2. the user interface for developing a conflict-free plan under strict resource restrictions after receiving requests to use control facilities, they are aggregated to one table. this table is a basis for developing a draft of the plan of allocation of control facilities. the draft (candidate) plan can be viewed in a tabular form (e.g., figure 6) as well as in a form of a diagram for user convenience. color indication of conflicts with the possibility to view data about their reasons was implemented. so, the user can view requests that conflict with each other and data about the state of the shared resources which cannot be used in accordance with the requirements of the received requests. 59 boris kucherov, vladimir artyushenko acta polytechnica ctu proceedings figure 6. the main form of the program for developing conflict-free plan when new requests on using resources are received, the indication about existing not-processed data is carried out by color highlighting these data and showing total number of records to be processed. colors used for highlighting not-processed data as well as conflicts are quite important for information perception by the specialists of planning department. on the one hand, colors for alarms (e.g., conflicts) should attract attention to the highlighted data. on the other hand, color indication can be shown during quite long time, so it shouldn’t be annoying or cause fatigue of the specialists. also, various colors should be used for conflicts caused by different reasons (e.g., conflict with another communication session and conflict with maintenance of control facility). as the result of practical use of the developed software, colors with not very high saturation were chosen (hues of red for alarms, hues of blue for not-processed data). the specialist can resolve conflicts in manual mode as well as in automated mode (using special software for conflicts resolution), or using both of them. he/she chooses appropriate mode after analysing conflicts and reasons of their formation. advisability of solving conflicts in manual mode in some situations could be caused by small number of conflicts or by simplicity of solution (e.g., moving a communication session to another control facility without changing its time). time costs of solving conflicts in manual and automated modes can be comparable in such case. the resolution of conflicts in typical situations where the decisions are already known and stored in the corresponding database should also be mentioned. candidate solutions of conflicts can be developed using visualization of plan in the diagram containing available times of using each resource for each consumer. such times can be considered as possible solutions of the conflicts. also, a user can view data about possible solutions of the conflicts. such view of data representation is more intuitive and can make user’s work easier. however, it makes sense to use software for automated conflicts resolution (developing conflict-free plans) in many cases. a possibility to use such external (third-party) software is provided to ensure resolving conflicts in automated mode. in such case, all necessary data, including a draft of the plan of allocation control facilities, data about existing resource restrictions and reference datais sent to software for automated development of a conflict-free plan. note that the specialist could adjust the settings of developing conflict-free plans. for example, the specialist can mark certain communication sessions as mandatory to be included in the plan. also, the specialist can resolve some conflicts in manual mode before sending a draft of the plan to the software for automated development of a conflict-free plan. in addition, the specialist can make changes in the draft of the plan generated by such software. when a conflict-free plan is developed, it is sent into the reverse direction (from software for automated developing conflict-free plan to developed software for allocation of control facilities for spacecrafts). automated conflicts resolution can be carried out using, for example, methods of solving srs discussed in [4, 5, 12–18]. when all conflicts were resolved (if they have arisen), the plan of allocation of control facilities is approved. as soon as it was done, an extract from the plan for each subscriber is generated and sent. it should be noted that specialists of planning department have to prepare a large number of documents in hard copy after developing or changing the plan. the batch printing of documents on the results of a developing plan is provided to reduce time of this operation. the list of the documents to be printed is dynamically generated according to the content of the plan or its correction. to develop a conflict-free plan of allocation of control facilities as well as to solve other tasks of planning department it is necessary to have various data about the current situation (e.g., data about state of control facilities). its perception by the specialists can influence on the efficiency and quality of their decisions. so, its representation is also one of important issues of human-machine interaction in solving tasks of the planning department. 4. the variants of informative representation of operative and statistical data specialists of planning department need compact and informative representation of data about the current situation as well as the results of analysis of using shared resources to perform prompt evaluation and analysis of it. data about current situation include such parameters as current state of control facilities and spacecrafts, requirements of spacecrafts, and can 60 vol. 12/2017 human-machine interaction in solving tasks of the planning department be considered as operative data. the results of analysis of using control facilities for spacecrafts include such data as a number of planned and performed communication sessions, a number of troubles when communication sessions take place and its effects, statistical characteristics of repairing control facilities with typical malfunctions, etc. one of the most informative and convenient for the users ways to represent operative data is to use interactive schematic form and to represent the results of analysis of using control facilities is to use statistics. 4.1. representation of operative data compact and informative representation of operative data can be reached by presenting data in an interactive schematic form with the possibility to drill down to the desired level (e.g., figure 7). in this example pictograms of shared resources (control facilities) are schematically shown on geographical map in accordance with their locations. color indication of the status of control facilities for spacecrafts is carried out both in terms of serviceability and in terms of current usage. detailed reference and operational data can be viewed for each shared resource. also, the pictograms of consumers (spacecrafts) are shown at the top part of the interactive scheme. the indication of status and existence of current communication sessions for spacecrafts is implemented. detailed reference and operational data can be viewed for each consumer. also, a set of allowed shared resources is shown for each consumer (a set of control facilities which are included in ground control complex of each spacecraft is shown). the progress of performing the plan is showed in tabular form for each control facility at the top of interactive scheme. moreover, a separated tabular form to visualize the current progress of performing the plan of control facilities allocation for spacecrafts with color indication of state of communication sessions is provided. in addition, the data about the state of spacecrafts constellation are displayed using 3d-model of spacecrafts flights on near-earth orbit (e.g., figure 8). space dynamics calculations in the developed model are performed using sgp4 model (sgp simplified general perturbations) for spacecrafts on low-earth orbit and sdp4 model (sdp simplified deep space perturbations) for other spacecrafts. the possibility to manage progress of simulated time is provided. it can be normal or accelerated. also, a progress of simulated time can be stopped (it would be mean showing the results of modelling at fixed time). it should be noted that one of the directions of further development is integration between a 3d-model and interactive geographical map. it will have positive influence on the efficiency of evaluating the current situation. figure 7. the operative presentation data about the current situation in the schematic form (cf control facility, sc spacecraft, cmp control and measurement point) figure 8. example of visualization of data about state of spacecrafts constellation it is important to note that data about current situation is obtained as a result of collecting and processing a large amount of data, such as results of communication sessions and reprimands to them, data about occurrence and parrying malfunctions of control facilities, maintenance of control facilities, etc. much of this data, e.g. reprimands to communication sessions, can occur many times. so, using a list of typical reprimands is proposed. it favors classification data and improvement of quality of processing data by standardization of definitions. moreover, it allows increasing the efficiency of input data about reprimands by choosing its description from the list instead of manual input. possibility to view collected data in summary table for selected time interval was implemented to use in creating reports and further analysis of the data. the data in such table can be filtered and grouped flexibly to concentrate user’s attention on a part of the data which should be processed at the present. 4.2. representation of statistical data as it was mentioned above, the allocation of shared resources should take into account the results of analysis of their usage. specialists of planning department need various statistical data to perform such analysis. 61 boris kucherov, vladimir artyushenko acta polytechnica ctu proceedings statistical data manipulation can be done for various periods and various data slices (e.g., consumers, resources, results of resources usage, time intervals, etc.). the flexible generation of reports in accordance with current requirements of users is provided. in addition, there should be an opportunity to calculate new statistical indicators based on the basic ones with applying to them predefined functions and groups. statistical data manipulation is implemented both on server-side using features of oracle database and on client-side. server-side implementation is done using aggregate and analytical sql (structured query language) functions. client-side implementation is done using components quantumgrid and pivotgrid from devexpress vcl. use of such components provides flexible filtering and grouping statistical information. it helps to increase efficiency of analysis of using control facilities. also, a user can view data in the standard as well as in a customizable view while working with pivot tables, which are implemented by pivotgrid component. it should be noted that this current view of pivot table can be printed. flexibility of data representation in pivot tables, achieved by changing their view through moving headers of fields into different table areas, allows to generate reports promptly according to actual requirements of the specialists of the planning department. data from these tables can also be presented in charts. it ensures increasing not only clearness of data visualization, but also clearness of reports prepared by the specialists of the planning department. 5. conclusions thus, the analysis of specific features of humanmachine interaction in solving the tasks of planning department under strict resource restrictions using advanced information technology has been performed. the proposed approaches to developing user interface provide concentrated attention of specialists of planning department on the most important data requiring priority processing. the system of visual and sound notification about important events was developed to increase the efficiency of specialists’ response on changes of the current situation. specific features of user interface for developing conflict-free plan have been noted. the variants of informative presentation of actual information about the current situation on the interactive scheme and flexible presentation of statistical data have been proposed. 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[32] t. de jong. cognitive load theory, educational research, and instructional design: some food for thought. instructional science 38(2):105–134, 2010. doi:10.1007/s11251-009-9110-0. 63 http://dx.doi.org/10.1109/taes.2009.5259176 http://dx.doi.org/10.1109/aero.2013.6496849 http://dx.doi.org/10.1007/s10951-008-0066-8 http://dx.doi.org/10.1109/estel.2012.6400125 http://dx.doi.org/10.1109/aero.2015.7119277 http://dx.doi.org/10.1145/1718918.1718926 http://dx.doi.org/http://dx.doi.org/10.1016/j.intcom.2011.05.006 http://dx.doi.org/http://dx.doi.org/10.1016/j.intcom.2008.04.004 http://dx.doi.org/http://dx.doi.org/10.1016/j.ijhcs.2015.02.001 http://dx.doi.org/10.1145/2254556.2254607 www.orekit.org http://celestrak.com/norad/elements/ http://dx.doi.org/10.1007/s11251-009-9110-0 acta polytechnica ctu proceedings 12:56–63, 2017 1 introduction 2 the increasing efficiency of response of the planning department's specialists on changing the current situation 3 developing a conflict-free plan under strict resource restrictions 3.1 prediction workload of specialists of planning department 3.2 the user interface for developing a conflict-free plan under strict resource restrictions 4 the variants of informative representation of operative and statistical data 4.1 representation of operative data 4.2 representation of statistical data 5 conclusions references acta polytechnica ctu proceedings doi:10.14311/app.2016.5.0069 acta polytechnica ctu proceedings 5:69–73, 2016 © czech technical university in prague, 2016 available online at http://ojs.cvut.cz/ojs/index.php/app deployment of ertms in czech republic jiří vopava∗, mária jánešová, radek kratochvíl ctu in prague, czech technical university in prague – faculty of transportation sciences, konviktská 20, 110 00, prague ∗ corresponding author: jiri.vopava@gmail.com abstract. the article describes current situation and expected deployment of international standard for automatic rail vehicle protection in czech republic for period 2014 – 2020. article introduces expected status in 2020 and subsequent implementation of commitments until 2026. it introduces expected result of gradual implementation for tracks and vehicles. it also displays brief deployment overview of european rail traffic management system in european union countries for each level of european train control system. keywords: ertms, etcs, gsm-r, interoperability. 1. introduction the ertms is automatic train protection which represents basic part of interoperability in european rail system. the term is abbreviation of "european rail traffic management system" and it was defined to synchronize nationals systems to the one common compatible system. urgency of the unification and cooperation between european countries caused an increase in freight which had negative impact for environment and ecology. the implementation of interoperability and automatic train protection across european countries allows competition to air and car freight. implementation of the standard is business initiative that transforms the operation of the tracks, enhance safety, capacity, performance and reliability. it also have impact on reduce of cost of operation and maintenance. ertms is divided into two basic components. the first one is etcs "european train control system" and second gsm-r "global system for mobile communication for railway". the ertms allow one compatible system for crossborder traffic what reduce cost for implementation several regional stand-alone systems. the synchronization of automatic train protection increase competitiveness of the european rail sector. gsm-r "global system for mobile communications – railway" is a radio system based on gsm standard for providing of voice and data communication between vehicle and track. etcs "european train control system", is an automatic train protection system to replace the existing national systems. the etcs replaces about twenty different automatic train protections across european track. it can be configured to operate in one of the following application levels to enable etcs equipped vehicles to operate in not etcs equipped system where the safe movement of the vehicle is controlled by the default national control system: figure 1. etcs l1 [1] • level 0 – etcs equipped vehicles operating on lines without etcs system, • level 1 – an conventional line-side signal is passed to the train via a switched transmitter on the track, repeating the indication from the conventional signalling system, which continues to be used to support safe train separation. it is the figure 1, • level 2 – movement authority is passed by the gsm-r radio network from a radio block centre to the train. conventional train detection systems are utilized in conjunction with interlocking systems used to enforce safe train separation. it is the figure 2, • level 3 – it builds on level 2, but increases safe train separation using critical data from the train. it is the figure 3. 2. deployment of ertms in czech republic the czech republic contributing to the fulfillment of legal solutions arising from following documents: • directive 2008/57/ec of the european parliament and of the council 69 http://dx.doi.org/10.14311/app.2016.5.0069 http://ojs.cvut.cz/ojs/index.php/app j. vopava, m. janesova, r. kratochvil acta polytechnica ctu proceedings figure 2. etcs l2 [1] figure 3. etcs l3 [1] of 17 june 2008 on the interoperability of the rail system within the community • decisions 2012/88/eu, commission decision of 25 january 2012 on the technical specification for interoperability relating to the control-command and signalling subsystems of the trans-european rail system (2012/696/eu a 2015/14/eu) • regulation (eu) no 1315/2013 of the european parliament and of the council of 11 december 2013, on union guidelines for the development of the transeuropean transport network and repealing decision no 661/2010/eu • regulation (eu) no 1316/2013 of the european parliament and of the council of 11 december 2013, establishing the connecting europe facility, amending regulation (eu) no 913/2010 and repealing regulations (ec) no 680/2007 and (ec) no 67/2010 the deployment is defined in document "národní implementační plán ertms" released by ministry of transport. the document defines maximum amount of payloads for each project gsm-r and etcs l2 with respect to co-financing from ertms funds. it also defines expected deployment schedule of each project. between 2005 and 2014 was equipped 1132 km of tracks across of the czech republic. it is connected to figure 4. map of gsm-r implementation [2] figure 5. map of etcs implementation [2] the gsm-r sždc, db, öbb, prorail and the home network operator. there are now mobile terminals equipped with gsm-r all čd vehicles used in regular operation. almost all čd vehicles used in regular operation are now equipped with gsm-r terminals. the first stage with fully deployed etcs system at 2021 will be track "praha – česká třebová – brno – břeclav". following tables 1, 2, 3 display figures for expected schedule of deployment of gsm-r, etcs l2 and vehicle on-board equipment for period 2014 – 2020 (– 2026). the figure 6 below visualizes data from tables above. it displays total amount of deployments for each year until 2020. figure 6. total amount of expected deployments 70 vol. 5/2016 deployment of ertms in czech republic no. track distance [km] deployment 1 uzel praha (beroun – praha – benešov u prahy – lysá nad labem) 120 2014-2015 2 české velenice st. hr. rakousko – české budějovice – horní dvořiště st. hr. rakousko 110 2015-2016 3 plzeň – české budějovice 136 2015-2016 4 kolín – havlíčkøuv brod – křižanov – brno 195 2014-2016 5 benešov – votice 20 2015-2016 6 beroun – plzeň – cheb st. hr. německo 169 2014-2016 7 cheb – vojtanov st. hr. německo 20 2015-2016 8 znojmo – šatov st. hr. rakousko 13 2015-2016 9 ústí nad orlicí – lichkov st. hr. polsko 35 2015-2016 10 votice – české budějovice 100 2016-2017 11 hranice na moravě – horní lideč st. hr. slovensko 67 2016-2017 12 ústí nad labem – oldřichov u duchcova/úpořiny – most – karlovy vary – cheb 212 2017-2018 13 pardubice – hradec králové 22 2019-2020 14 zábřeh na moravě – šumperk 13 2019-2020 15 praha – letiště v. havla ruzyně – kladno 31 after 2020 16 brno – přerov 90 after 2020 17 plzeň – domažlice st. hr. německo 72 after 2020 18 velký osek – hradec králové – choceň 96 after 2020 total 1521 table 1. expected track deployment with gms-r [3] no. track distance [km] deployment 1 kolín – břeclav st. hr. rakousko/slovensko 277 2012-2016 2 kralupy nad vltavou – praha – kolín 110 2016-2017 3 st. hr. německo – dolní žleb – kralupy 112 2017-2019 4 petrovice u karviné st. hr. polsko – přerov – břeclav 206 2015-2017 5 praha-uhřiněves – votice 54 2016-2017 6 votice – české budějovice 101 2017-2018 7 česká třebová – přerov 110 2016-2018 8 plzeň – cheb st. hr. německo 117 2016-2018 9 beroun – plzeň 70 2016-2018 10 dětmarovice – mosty u jablunkova st. hr. slovensko 53 2017-2019 11 české velenice st. hr. rakousko – české budějovice – horní dvořiště st. hr. rakousko 112 2017-2019 12 ústí nad orlicí – lichkov st. hr. polsko 37 2018-2020 13 kolín – nymburk – mělník – děčín východ – děčín-prostřední žleb 159 after 2020 14 kolín – havlíčkøuv brod – brno 200 after 2020 15 praha – lysá nad labem 35 after 2020 16 praha – letiště v. havla ruzyně – kladno 31 after 2020 17 praha – beroun 43 after 2020 18 plzeň – domažlice st. hr. německo 72 after 2020 19 pardubice – hradec králové 22 after 2020 20 plzeň – české budějovice 136 aftre 2020 21 brno – přerov 90 after 2020 22 hranice na moravě – horní lideč st. hr. slovensko 67 after 2020 23 uzel praha 40 after 2020 24 český těšín – ostrava-svinov 41 after 2020 25 velký osek – hradeck králové – choceň 96 after 2020 26 cheb – karlovy vary – chomutov 111 after 2020 27 ústí nad labem – chomutov, ústí nad labem – bílina 101 after 2020 total 2603 table 2. expected deployment of equipment upgrade ertm/etcs [3] 71 j. vopava, m. janesova, r. kratochvil acta polytechnica ctu proceedings no. track distance [km] deployment 1 kolín – břeclav st. hr. rakousko/slovensko 250 2015-2019 2 kralupy nad vltavou – praha – kolín 100 2016-2019 3 st. hr. německo – dolní žleb – kralupy 100 2017-2019 4 petrovice u karviné st. hr. polsko – přerov – břeclav 130 2015-2020 5 praha-uhřiněves – votice 40 2016-2020 6 votice – české budějovice 50 2017-2020 7 česká třebová – přerov 50 2016-2020 8 plzeň – cheb st. hr. německo 40 2016-2020 9 beroun – plzeň 40 2016-2020 10 dětmarovice – mosty u jablunkova st. hr. slovensko 20 2017-2020 11 české velenice st. hr. rakousko – české budějovice – horní dvořiště st. hr. rakousko 50 2017-2020 12 ústí nad orlicí – lichkov st. hr. polsko 20 2018-2020 13 kolín – nymburk – mělník – děčín východ – děčín-prostřední žleb 100 after 2020 14 kolín – havlíčkøuv brod – brno 100 after 2020 15 praha – lysá nad labem 30 after 2020 16 praha – letiště v. havla ruzyně – kladno 20 after 2020 17 praha – beroun 30 after 2020 18 plzeň – domažlice st. hr. německo 40 after 2020 19 pardubice – hradec králové 20 after 2020 20 plzeň – české budějovice 50 after 2020 21 brno – přerov 60 after 2020 22 hranice na moravě – horní lideč st. hr. slovensko 30 after 2020 23 uzel praha 20 after 2020 24 český těšín – ostrava-svinov 20 after 2020 25 velký osek – hradec králové – choceň 40 after 2020 26 cheb – karlovy vary – chomutov 50 after 2020 27 ústí nad labem – chomutov, ústí nad labem – bílina 50 after 2020 total 1550 table 3. expected track deployment of etcs l2 [3] after 2020 will be deployed around 250 km tracks and 150 vehicles every year until 2026. strategies transition from the current national system vz ss to european interoperable system class a – etcs consists a combination of investment in track equipment and train equipment. in the first phase will be equipped only international traffic trains. during migration will vehicles equipped with etcs parallel with vz ls that allow operation of both systems together. etcs should be fully migrated for first and second rail corridor and the track přerov – česká třebová at the latest to 2030. after the migration period will be completely decommissioned national systems vz ls and fully deployed etcs system. in some cases can be deployed new special sections equipped solely with etcs, which will be designed to operate only vehicles equipped with etcs even before end of migration period. i.e. a railway connection václav havel airport prague and prague center. 3. deployment of ertms in other european countries unification of europe requires to all transport systems operates without restrictions and time losses, especially when crossing borders. the deployment started in most of european union countries. following countries started participation on deployment of the ertms standard: austria, belgium, bulgaria, croatia, czech republic, denmark, finland, france, germany, greece, hungary, italy, luxembourg, netherlands, poland, romania, slovakia, slovenia, spain, sweden and united kingdom. the figure 7 displays amount of kilometers contracted tracks for each country. the figure 8 displays covering of the ertms divided by level of tracks. the figure 9 displays amount of deployed on-board equipment of vehicles for each country. the figure 10 displays levels of on-board equipment implemented in vehicles. 72 vol. 5/2016 deployment of ertms in czech republic figure 7. ertms contracted tracks [km] in eu figure 8. tracks in eu separated by level 4. conclusions by 2020 the gsm-r will cover major sections of the rail freight corridors in the czech republic. the system will be further deployed progressively on all 3700 km of national routes in order to create comprehensive operational tracks. etcs deployment priority by 2020 is rail freight corridor 7, partially corridors 8, 9, 5 and the track přerov – česká třebová. by 2020, it is necessary to ensure the development of the etcs about 1350 kilometers of tracks and 890 vehicles, ie. for additional six years, around 250 km of tracks and 150 vehicles every year, but this is conditional on the timely implementation of the modernization infrastructure. this requires the necessary financial resources for the trackside and on-board ertms and creating mofigure 9. global ertms vehicles contracted in europe figure 10. vehicles in eu separated by level tivational factors and efficient financial support equipping vehicles with on-board equipment of gsm-r and etcs. references [1] the european rail traffic management system, portál ertms [online]. [2016-02-12], http://www.ertms.net/?page_id=42. [2] národní implementační plán ertms 2014-2020, portál acri [online]. [2016-02-12], http://www.acri.cz/uploads/acri-akademie/15-06% 20etcs/2015_cech,%20varadinov.pdf. [3] národní implementační plán ertms, portál ministerstva dopravy [online]. [2016-02-12], http://www.mdcr.cz/nr/rdonlyras/ 7ff4faf1-1ebe-4116-bf90-808ec7306e04/0/ nipertmsnpweb.pdf. 73 http://www.ertms.net/?page_id=42 http://www.acri.cz/uploads/acri-akademie/15-06%20etcs/2015_cech,%20varadinov.pdf http://www.acri.cz/uploads/acri-akademie/15-06%20etcs/2015_cech,%20varadinov.pdf http://www.mdcr.cz/nr/rdonlyras/7ff4faf1-1ebe-4116-bf90-808ec7306e04/0/nipertmsnpweb.pdf http://www.mdcr.cz/nr/rdonlyras/7ff4faf1-1ebe-4116-bf90-808ec7306e04/0/nipertmsnpweb.pdf http://www.mdcr.cz/nr/rdonlyras/7ff4faf1-1ebe-4116-bf90-808ec7306e04/0/nipertmsnpweb.pdf acta polytechnica ctu proceedings 5:69–73, 2016 1 introduction 2 deployment of ertms in czech republic 3 deployment of ertms in other european countries 4 conclusions references acta polytechnica ctu proceedings doi:10.14311/app.2015.1.0045 acta polytechnica ctu proceedings 2:45–50, 2015 © czech technical university in prague, 2015 available online at http://ojs.cvut.cz/ojs/index.php/app impact assessment of image feature extractors on the performance of slam systems taihú pirea,∗, thomas fischera, jan faiglb a university of buenos aires, intendente güiraldes 2160, ciudad autónoma de buenos aires, argentina b department of computer science, faculty of electrical engineering, czech technical university in prague, technická 2, 166 27, prague, czech republic ∗ corresponding author: tpire@dc.uba.ar abstract. this work evaluates an impact of image feature extractors on the performance of a visual slam method in terms of pose accuracy and computational requirements. in particular, the s-ptam (stereo parallel tracking and mapping) method is considered as the visual slam framework for which both the feature detector and feature descriptor are parametrized. the evaluation was performed with a standard dataset with ground-truth information and six feature detectors and four descriptors. the presented results indicate that the combination of the gftt detector and the brief descriptor provides the best trade-off between the localization precision and computational requirements among the evaluated combinations of the detectors and descriptors. keywords: image features, visual slam, stereo vision. 1. introduction during the last decade, the simultaneous localization and mapping (slam) problem has been one of the main research interests in mobile robotics. particularly, the use of cameras as the main sensors has been given a special attention [1] [2] [3] [4] because of their benefits such as low-cost and passive sensing. in vision-based slam approaches, local image features are used to build a map and simultaneously estimate the robot pose using the environment landmarks represented as the image features. in this way, the map is represented as a sparse point cloud, where each point results from triangulating salient points (image features) matched from a pair of stereo images. currently, there exist several local image feature extractors in the literature. a feature extractor is a combination of a salient point (called keypoint) detection procedure and a computation of a unique signature (called descriptor) for each such a detected point. the most commonly used detectors are sift [5], surf [6], star [7], gftt [8], fast [9], and relatively recently proposed orb [10], while among the most used descriptors we can mention sift, surf, orb, brief [11], and brisk [12]. in visual slam systems, the feature extraction process has a huge impact on the accuracy of the whole system. on one hand, the precision of the robot localization is heavily correlated to the sparsity of features in images and the ability to track them for a long period during the robot navigation, even from different points of view. on the other hand, if the number of points in the map grows too quickly, it may slow down the whole system. to be able to keep the response of the system under real-time constraints, images have to be dropped or other parts of the system, like optimization routines, need lower computational requirements. in this work, we evaluate the impact of different state-of-the-art feature extractors on the performance of the visual slam localization method. in particular, the evaluation is based on the stereo visual slam approach s-ptam introduced in [4]. the presented results indicated that the combination of the gftt detector and brief descriptor is the most reliable choice for our slam system among the other evaluated combinations. the rest of the paper is organized as follows. section 2 presents overview of the related work while section 3 summarizes the most used feature detectors and descriptors in the visual slam literature. section 4 briefly comments the considered stereo visual slam system using for the evaluation. in section 5, we present the evaluation of the features extractors and the achieved results. section 6 is dedicated to the conclusions and future work. 2. related work several evaluations of features extractors can be found in literature. each of them is driven by the particular application or issue at the hand they are aimed to address. for example, in [13], authors evaluate several features extractors in the context of autonomous navigation in outdoor environments under seasonal changes. they came to a conclusion that the best performing method is the star–brief combination of the detector–descriptor, which outperforms sift by more than thirty percentage points. in addition, they argued that the star–brief extractor is also less computationally demanding than other extractors and thus it seems to be the most suitable feature 45 http://dx.doi.org/10.14311/app.2015.1.0045 http://ojs.cvut.cz/ojs/index.php/app t. pire, t. fischer, j. faigl acta polytechnica ctu proceedings detector–descriptor for navigational purposes. on the other hand, authors of [14] provide a performance comparison of feature extractors against illumination changes in outdoor scenes in the context of the visual navigation. they concluded that the configuration of the fast–surf is the optimal in their setup. besides, they report that this combination provides an effective computational time per image, which is favorable for the real-time vision-based navigation application. the work [15] compares contemporary point features detector and descriptor pairs in order to determine the best combination for the robot visual navigation. the authors concluded that the fast– brief combination is a good choice when processing speed is an important parameter of the system setup. they also argued that under camera movement conditions, additional computational cost—needed for the descriptors and detectors that are robust to in-plane rotation and large scaling— seems to be unjustified. however, they do not tested the method in a real slam application. regarding the aforementioned evaluation of the detectors and descriptors, the work presented in this paper is within the context of the full 6dof slam. 3. local image features an image feature extractor consists of detection and description phases. the feature detector serves to locate salient areas of the image while the feature descriptor captures information about the local neighborhood of the detected area. here, we provide a brief overview of the considered feature extractor and descriptor algorithms in this evaluation study. sift – scale invariant feature transform [5]. an established feature detector with a high precision and good robustness, which is known to be computationally demanding. surf – speeded up robust features [6] is a similar to sift, but it is computationally less demanding due to approximations. star – a modified version of the censure (center surrounded extrema) [7] detector, which is computationally less demanding at the expense of a lower precision. brief – binary robust independent elementary features [11] is a descriptor that describes an image area using a number of intensity comparisons of random pixel pairs. it is saved as a binary string, which reduces the computational complexity of the subsequent matching. fast – features from accelerated segment test [9] is a feature detector focused on lowering the computational cost. brisk – binary robust invariant scalable keypoints [12] is a scale and rotation invariant version of brief, but unlike brief, it uses a deterministic comparison pattern. orb – oriented fast and rotated brief [10] is another attempt to achieve a scale and rotation invariant brief, as a computationally efficient alternative to sift and surf. it uses the fast detector to achieve low computational requirements. gftt – a detector focused on selecting features relevant to motion tracking by analyzing the amount of information they provide for that particular task [8]. the surf and sift descriptors rely on the their own detectors, which are also considered in the presented evaluation. however, for the brief and brisk binary descriptors the considered detectors are the gftt, fast and star which results in the additional six combinations of the detector–descriptor pairs in the presented evaluation. 4. overview of s-ptam s-ptam [4] is a stereo visual slam method for a large scale map navigation based on the monocular parallel tracking and mapping (ptam) method introduced in [1]. the method consists of two processes working in parallel: 1) the tracking of the detected features and; 2) creating a map of the features (mapping). during a robot navigation, the method works as follows. s-ptam extracts features from the incoming stereo images to match and construct a virtual map of the environment. the newly extracted feature descriptors are matched against descriptors of the points stored in the map according to the estimated field of view. the matches may then be used to refine the estimated camera pose using an iterative least squares minimization method, e.g., using the levenberg-marquardt algorithm. the particular stereo matches between the features that cannot be matched to the map are triangulated and inserted as new map points, for the tracking of future frames. in parallel, a map refinement algorithm is running. it is also based on the levenberg-marquardt optimization that continuously performs the bundle adjustment on the current local portion of the map. in [4], s-ptam uses the gftt feature detector and the brief descriptor extractor. in this work, we consider other combinations of the detector–descriptor to evaluate an impact of the combination to the performance of the localization and mapping processes. 5. evaluation the kitti vision benchmark suite [16] is used to evaluate s-ptam for each type of considered detector– descriptor configuration. in particular, we present the results obtained for the sequence 00, shown in figure 1. the sequence records the stereo camera frames captured by a moving car in an urban scenario for almost 4 km long path. the particular parameters of 46 vol. 2/2015 impact assessment of image feature extractors on slam the evaluated feature extractors have been selected in such a way that allows s-ptam to run without ever loosing localization. they have been tuned from a strong restrictive value and then relaxed until the method completes the whole sequence. the parameters are listed in table 1. detector / parameter valuedescriptor sift noctavelayers 1 l2normthreshold 100 surf hessianthreshold 1000 noctaves 1 l2normthreshold 0.2 star responsethreshold 20 brief bytes 32 hammingthreshold 25 fast threshold 60 brisk hammingthreshold 100 orb nfeatures 2000 nlevels 1 hammingthreshold 50 gftt nfeatures 2000 mindistance 15.0 table 1. parameters used for feature detectors and descriptors. the parameters which do not appear in the list use the default value in the opencv implementation. in the case of the binary descriptors, the hamming distance is used to compute the valid matches while the l2 norm is used for the surf and sift descriptors. the evaluation has been performed using an intel core i7 processor with 4 cores running at 2.2 ghz. although s-ptam strongly exploits parallelism, the experiments were run in a sequential fashion that allow us to simulate ideal conditions and abstract from the limitations of the available computational power. this ensures that no frames are dropped and that the iterative optimization routines always converge or reach a maximum threshold of iterations. nevertheless, the tracking process performs pose optimization using an iterative algorithm; so, the less time is used in the features extraction, the more iterations the method can compute. figure 2 shows a characterization of the total tracking time for each pair of frames, as achieved by using the evaluated extractors. moreover, the iterative least-squares optimization, which is utilized in the mapping and tracking processes, depends linearly on the number of tracked points (the density of the map). thus, regarding the computational burden, the map should be as small as possible while the map points should contain strong enough features to support a robust tracking of the figure 1. path tracked by every method run under different extractors, against the ground truth. the path is nearly 4 km long. the shown distances at the axes are in meters. figure 2. total tracking time achieved by each configuration frames. table 2 shows the final number of points contained in the map after finishing each trial for a particular combination of feature detector and descriptor. in figure 3, we can see how the map size impacts directly on the temporal performance of the tracking process. combinations of the detector–descriptor that build the most dense maps also take the longest time to compute. differences in the map size for the evaluated descriptor in the feature extractors with the same detector can have two reasons. the first reason is that new 47 t. pire, t. fischer, j. faigl acta polytechnica ctu proceedings figure 3. tracking time without taking into account feature extraction points are created from the stereo features only if these features are not matched to the map. the second reason is that the points marked as outliers during the refinement processes are discarded. in the first case, this can be caused by descriptors that are not robust enough to be matched to the map for a long time. in the second case, the descriptor matching may be too permissive and it allows bad matches that are later discarded as outliers. extractor final map size gftt / brief 990 455 gftt / brisk 1 314 356 sift / sift 1 581 876 star / brief 1 893 372 surf / surf 2 059 879 fast / brief 2 420 652 star / brisk 2 447 418 fast / brisk 3 207 003 orb / orb 5 192 885 table 2. the number of points contained in the map after completing the sequence for each evaluated extractor, in ascending order. since the goal of this work is to assess the impact of the feature extractor choice also on the accuracy of the slam method, the achieved performance is presented as two independent relative errors for each estimated pose: εt for the translation error and; εθ for the orientation. let xk be the estimated pose at the frame k, which can be decomposed as the translation tk and the rotation rk. let x∗k be the reference pose, which can be decomposed in the same fashion. the aforementioned errors are computed as εt,k+1 = ‖(tk tk+1) ( t∗k t∗k+1 ) ‖, εθ,k+1 = angle ( (rk rk+1) ( r∗k r∗k+1 )) , where is the inverse of the standard motion composition operator. for pure translations, we can rewrite t1 t2 = t2− t1, and for the pure rotations as r1 r2 = rt1r2. ‖x‖ stands for the euclidean norm and angle (r) extracts the magnitude of the rotation. the computed errors are shown in figure 4 and figure 5, respectively. although the angular deviation to the ground truth, shown in figure 5, seems to be similar in all methods, the same is not true for the translation error, as it can be seen in figure 4. the brisk descriptor seems to be a more reliable with the fast detector, while the same holds for the brief descriptor with the gftt detector. figure 4. relative translation error figure 5. relative orientation error for completion, the absolute errors ε′t,k = ‖tk t∗k‖ ε′θ,k = angle (rk r∗k) are shown in figure 6 and figure 7. 48 vol. 2/2015 impact assessment of image feature extractors on slam figure 6. absolute translation error figure 7. absolute orientation error 6. conclusions in this paper, we present an evaluation of the impact of different state-of-the-art image feature extractors on the performance of the slam method proposed in [4]. the kitti benchmark suite dataset with a ground truth is used to evaluate the achievable precision of the method for different feature extractors. based on the presented results, the main conclusion is that the gftt detector is the most suitable choice for the best performance in the evaluated dataset. the gftt (accompanied with the brief or brisk descriptors) outperforms the other methods in terms of the required computational time and the map quality. although the map density is far smaller, the computed translation error is similar, even slightly better, than the one achieved by other extractors. this insight can be interpreted as the most useful features (regarding the navigation) are extracted while the descriptor also support efficient matching resulting in a more precise localization. recently, a novel stereo feature extractors have been proposed, e.g., [17], which motivates us to consider them in s-ptam. an evaluation of the novel extractors is a subject of our future work. acknowledgements this work is a direct result of the bilateral cooperation program between the czech and argentinian republics support by the argentinian project arc/14/06 and travel support of the czech ministry of education under the project no. 7amb15ar029. the work of j. faigl is supported by the czech science foundation (gačr) under the research project no. gj15-09600y. references [1] g. klein, d. murray. parallel tracking and mapping for small ar workspaces. 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[14] dzulfahmi, n. ohta. performance evaluation of image feature detectors and descriptors for outdoor-scene visual navigation. in acpr, pp. 872–876. 2013. doi:10.1109/acpr.2013.159. [15] a. schmidt, m. kraft, m. fularz, z. domagala. comparative assessment of point feature detectors in the context of robot navigation. journal of automation, mobile robotics and intelligent systems 7(1):11–20, 2013. [16] a. geiger, p. lenz, c. stiller, r. urtasun. vision meets robotics: the kitti dataset. ijrr 32(11):1231– 1237, 2013. doi:10.1177/0278364913491297. [17] r. arroyo, p. alcantarilla, l. bergasa, et al. fast and effective visual place recognition using binary codes and disparity information. in iros, pp. 3089–3094. 2014. doi:10.1109/iros.2014.6942989. 50 http://dx.doi.org/10.1109/acpr.2013.159 http://dx.doi.org/10.1177/0278364913491297 http://dx.doi.org/10.1109/iros.2014.6942989 acta polytechnica ctu proceedings 2:45–50, 2015 1 introduction 2 related work 3 local image features 4 overview of s-ptam 5 evaluation 6 conclusions acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2016.5.0017 acta polytechnica ctu proceedings 5:17–21, 2016 © czech technical university in prague, 2016 available online at http://ojs.cvut.cz/ojs/index.php/app design parameters of buffer stops petr guziur brno university of technology, faculty of civil engineering, veveří 331/95, 602 00 brno correspondence: guziur.p@fce.vutbr.cz abstract. paper discuses reasons of building buffer stops and situations that may occur in railway station leading to build safety tracks. also discusses parameters of buffer stops that enter its design, such as collision speed and kinetic energy absorbing capacity. furthermore, presents categories of buffer stops depending on principles of absorbing the kinetic energy and points pros and cons of each structure. keywords: buffer stop, kinetic energy, braking force, dead-end track. 1. introduction buffer stop is a device at the end of dead-end track or closed track with a purpose to stop the rolling stock. in czech republic it is allowed to use three types of buffer stops (according to national regulation čd ž9 [1]). all three types are fixed (rigid) construction, yet there are numerous designs of buffer stops used abroad. we can divide buffer stops in categories depending on the principles of absorbing the kinetic energy, which is the most important parameter of buffer stops. as basic categories we can name fixed buffer stops, hydraulic buffer stops and friction buffer stops. every type has its pros and cons and suitable place to be installed (depending on circumstances). 1.1. flank protection one of the places, where buffer stop has to be installed, is the end of safety track which represents direct flank protection of train paths. there are two types of flank protection of train path: • direct flank protection – protection of rolling stock from non-permitted ride of rolling stock from side approaching track – flank protection switch or derail is used, • indirect flank protection – protection of rolling stock from non-permitted ride of rolling stock from side approaching track – signals with prohibiting signal is used. and there are different conditions for flank protection of train path with speed up to 120 km.h−1 and above this speed. direct flank protection of train path with speed higher 120 km.h−1 is required: • from all sidings, • from all service tracks. in this case, rolling stock arriving from siding or service track should head to the safety track and probably has low speed at its end (by the buffer stop). another situation with possibility of higher impact speeds may occur on safety tracks from running tracks (see chapter 1.1.1). 1.1.1. mutually excluded train path amongst other conditions, safety and signalling plant of second or higher category has to forbid the simultaneous setting of train path for which the train path with higher speed than 120 km.h−1 meets, crosses or overlaps with continuation of another train path. this condition is crucial for direct flank protection. if there is no direct flank protection of the train path with speed higher than 120 km.h−1, one of train paths is not allowed to be set or the speed is restricted up to 120 km.h−1. speed restriction depends on possibilities of main signals (what speed can be signalled). in railway station, where restricted speed is used, the restriction brings speed to lower level, than in railway station, where no restriction is needed (with speed of 120 km.h−1). this issue causes regular delay of trains on daily bases, since not even system like kango is able to cope with this situation. the only result is to build flank protection switches not only from sidings and service tracks but even from running tracks where threat of the train path with speed higher than 120 km.h−1 occurs. 2. buffer stop’s design parameters as a crucial parameter for designing buffer stop is its kinetic energy absorbing capacity, thus stopping a rolling stock of certain mass with a certain collision speed. there are various situations of buffer stop’s location, therefore it is needed to approach the design individually. whether the buffer stop is at the end of a dead-end track in station by platform, or ends a safety track (either from sidings, service track or even running tracks). each situation gives us different speed and different type of train (its mass) that can run into a buffer stop. as mentioned above, amount of kinetic energy that buffer stop is able to absorb is the crucial parameter for its design. kinetic energy of moving rolling stock can be calculated as sum of the kinetic energy of transitional motion and kinetic energy of rotating parts of the rolling stock (1.). 17 http://dx.doi.org/10.14311/app.2016.5.0017 http://ojs.cvut.cz/ojs/index.php/app petr guziur acta polytechnica ctu proceedings vehicle groups vehycle types ρ[-] trains regular passengers or freight trains 0,06 electric motor unit 0,15 0,20 cars cars with mechanical traction transition 0,12 0,15 cars with traction motors 0,20 0,025 passengers cars 0,04 0,06 full freight cars 0,04 0,05 empty freight cars 0,1 0,12 locomotives steam 0,08 0,10 electric 0,20 0,30 motor 0,15 0,30 table 1. coefficient of rotating parts [2] speed [km.h−1] speed [m.s−1] impact mass [t] 100 200 300 400 500 600 700 800 900 1000 10 2,78 386 773 1159 1546 1932 2319 2705 3091 3478 3864 15 4,17 869 1739 2608 3478 4347 5217 6086 6956 7825 8694 table 2. examples of kinetic energy [kj] (1.) ekin,c = ekin,p + ekin,r (2.) ekin,p = 1 2mv 2 (3.) ekin,r = 1 2iω 2 with substitution equation (2.) and (3.) into equation (1.) and following modification it is possible to calculate kinetic energy of moving rolling stock, using equation (4.). (4.) ekin,c = 1 2m(1 + ρ)v2 • ekin,c ... total kinetic energy [j]; • ekin,p ... kinetic energy of transitional motion [j]; • ekin,r ... kinetic energy of rotating parts [j]; • m ... weight of rolling stock [kg]; • v ... collision speed [m.s−1]; • i ... moment if inertia [kg.m2]; • ω ... angular speed [s−1]; • ρ ... coefficient of rotating parts [-]. examples of kinetic energy are given in table 2 for buffer stop, as a device (structure) that has to work properly and must ensure high level of safety and reliability, we need to involve a safety coefficient in calculations. therefore, the buffer stop’s kinetic energy absorbing capacity has to be determined as follows: r >= ekin,ck • r ... buffer stop’s kinetic energy absorbing capacity [j]; • ekin,c ... total kinetic energy [j]; • k ... safety coefficient [-]. in table 3 are shown examples of safety coefficient graded by the level of protection and type of train according to austrian standards [3]. 2.1. collision speed the collision speed is defined as the maximum permissible speed in which trains may travel when colliding with buffer stop. there are various approaches, how to determine collision speed. german standard ds 800 01 [4] use the collision speed based on train type: • passengers trains: 15 km.h−1 • freight trains: 10 km.h−1 austrian standard dv b 53 [3] use the collision speed based on track type: • main line trains: 15 km.h−1 • empty passengers trains or shunting: 10 km.h−1 take an example how the collision speed can be calculated. driver is responsible to make a proper braking according to signals. by error he overlooks a presignal and has no information about decelerating the train. he starts to decelerate at the point of signal with prohibiting signal using the emergency brake. table 4 displays braking distances (using operating brakes and emergency brake) and speeds of trains at the end of dead-end track (model situation: assuming the distance between signal with prohibiting signal and buffer stop is 100 m). parameters for table 4: • * train will stop before the buffer stop, • os deceleration – 0,50 m.s−2 (2x unit 451/452) [5], • r, ex, ic deceleration – 0,45 m.s−2 (locomotive 363 + 8 cars type y) [5], 18 vol. 5/2016 design parameters of buffer stops type of train and security level k [-] passengers trains 1,5 freight trains and shunting 1,2 freight trains and shunting, when it is necessary to protect various systems which are located behand or nearby buffer stop 1,5 freight trains and shunting, in cases where there are traffic zones, structures or residential houses located behind or nearby buffer stop 1,8 preventing the fall of any train or rolling stock into abyss 2,0 table 3. safety coefficient [3] speed [km.h−1] braking distance [m] speed at the end of dead-end track [km.h−1] os r, ex, ic emergency braking os r, ex, ic emergency brake 50 192,90 214,33 40,19 35 37 * 60 277,78 308,64 57,87 48 49 * 70 378,09 420,10 78,77 60 61 * 80 493,83 548,70 102,88 71 72 13 90 625,00 694,44 130,21 82 83 43 100 771 857,34 160,75 93 94 61 table 4. deceleration of trains buffer stop type 6 cars of 15 t 1 car of 80 t rail buffer stop 1,0 km.h−1 1,6 km.h−1 concrete type "sudop" 0,7 km.h−1 1,1 km.h−1 concrete type "dsb" 1,0 km.h−1 1,6 km.h−1 table 5. resistances of buffer stops used in czech republic [1] • emergency brake deceleration – 2,40 m.s−2. 3. types of buffer stops 3.1. fixed buffer stops with mechanical bumpers fixed (rigid) buffer stop is one of the most used types of buffer stops considering its history as an oldest one. construction of fixed buffer stop consists of a block or frame fixed rigidly to the rails or the ground. as other constructions, fixed buffer stop has its pros and cons. one of advantages is that it can be placed at the end of the dead-end track, thus it does not reduce the usable length of the track. however, in this case cons prevail, such as low resistance and manner of deceleration. resistances (as a state of usability) of fixed buffer stops used in czech republic are shown in table 5. if the mass or speed is higher, either the buffer stop or the train is destroyed. assuming both buffer stop and rolling stock frame will be reinforced significantly, and therefore cannot deform, the deceleration figure 1. example of hydraulic buffer stop (oleo comp.) [7] is unacceptable (see table 6). 3.2. hydraulic buffer stop (fixed buffer stop with hydraulic bumpers) hydraulic buffer stops are similar in construction to fixed. consist of a block or frame fixed rigidly to the rails or the ground. hydraulic buffer stops absorb kinetic energy in gradual manner (depends on the type of the hydraulic bumpers). if the energy is higher than the bumper is able to absorb, buffer stop is destroyed. parameters of the buffer stop on figure 1: • kinetic energy absorbing capacity: 2688 kj, • bumpers stroke: 2400 mm. 3.3. friction buffer stop friction buffer stop is the most effective way to stop moving rolling stock. the way of absorbing the kinetic energy, thus decelerating the rolling stock is the most efficient and safe. not only the train decelerates in 19 petr guziur acta polytechnica ctu proceedings impact speed [km.h−1] impact speed [m.s−1] stroke of mechanical bumpers [mm] [6] 75 105 decelartion [m.s−2] overload decelartion [m.s−2] overload 5 1,39 6,43 0,66g 4,59 0,47g 10 2,78 25,72 2,62g 18,37 1,87g 15 4,17 57,87 5,90g 41,34 4,21g table 6. deceleration of train and overload on passengers on impact the fixed buffer stop figure 2. length needed for buffer stop placement and to ensure braking distance [8] gradual manner over a longer distance (time period), but the kinetic energy absorbing capacity could be very high. friction buffer stop generally consist of rigid steel frame with buffers, connected to rails using arresting devices (friction jaws). in case of collision, kinetic energy is transformed into heat by means of friction. therefore, energy absorbing capacity of friction buffer stop depends on the number of friction jaws, friction coefficient resp. braking force of jaws and length of braking. nevertheless, there is a disadvantage. friction buffer stop needs a braking distance, therefore cannot be placed at the end of the deadend track and shortens its usable length. moreover the track behind the buffer stop needs to be horizontally straight and contain no welds and joints or other obstacles over rails. notes for figure 2: • lv ... length needed for buffer stop placement and to ensure braking distance; • lb ... buffer stop length; • lw ... maximum braking distance. three types of friction buffer stops can be named: • friction buffer stop (without additional brakes), see figure 3; • friction buffer stop with additional brakes, see figure 4; • friction buffer stop with hydraulic bumpers (with/without additional brakes). notes for figure 3: (1.) collision triangle; (2.) breaking devices; (3.) buffers; (4.) device for returning the buffer stop to working position after collision; (5.) reinforcement; figure 3. friction buffer stop without additional brakes [8] figure 4. friction buffer stop with additional brakes [8] (6.) reinforcement. notes for figure 4: (1.) additional arresting devices for added braking; (2.) steel profile below rails for reinforcement of the track; (3.) jointed connection belts between arresting devices; (4.) lateral connection between arresting devices. 3.3.1. braking force braking force is the determining factor of friction buffer stop. as mentioned above, buffer stop must absorb high amount of kinetic energy. calculation of breaking force in case of friction buffer stops depends on numbers of arresting devices, braking force of each arresting device and maximum braking distance. r = nbfblw • r ... buffer stop’s kinetic energy absorbing capacity [j]; 20 vol. 5/2016 design parameters of buffer stops figure 5. rawie 16 zeb, stopping distance/impact speed/impactmass diagram, safety coefficient 1,0 and 1,5 [9] • nb ... number of arresting devices [-]; • fb ... braking force of a single arresting device [n]; • lw ... maximum braking distance [m]. braking force of friction buffer stop with additional brakes comes from formula up there. formula is modified considering braking force of additional brakes while braking distance of each additional arresting device is included separately. r = nz∑ i=1 2fbilw i • r ... buffer stop’s kinetic energy absorbing capacity [j]; • nz ... number of a pair of additional arresting devices [-]; • fbi ... braking force of a single arresting device, based on a braking distance [n]; • lw i ... length of braking distance of a pair of arresting devices [m]. an example of friction buffer stop’s braking effectiveness is shown in figure 5 (friction buffer stop without additional brakes with absorbing capacity of 640 kj.m−1). 4. conclusion many aspects have to be taken in account while designing buffer stop. one has to consider its location – ending of dead-end track in railway station, ending safety track from sidings, service track or running track. every scenario brings different requirements such as type of train, collision speed, impact mass etc. in general, three types of buffer stops are used, depending of the principle of absorbing the energy. fixed buffer stops are more than useless for higher collision speeds considering its manner of deceleration and its resistances. however, using fixed buffer stops is justified e.g. in shunting yards, where are low speeds and no passengers on board. as more appropriate ending of dead-end track is usage of hydraulic or friction buffer stop. resistances of those types are much higher than fixed buffer stops. both this construction decelerates train in gradual manner and no harm to the rolling stock or buffer stop itself is done if the design was precise. acknowledgements the paper was created with support of the project no. lo1408 "admas up advanced materials, structures and technologies" supported by the ministry of education, youth and sports within the targeted support of program "national program for sustainability i". references [1] čd ž9 železniční spodek, vzorový list železničního spodku, zarážedla. české dráhy, s.o., divize dopravní cesty, o. z. praha, 2001. in effect from: 2002-04-01. [2] j. široký. mechanika v dopravě i âăş kolejová vozidla [online]. updated 2003, [2016-01-13], http://homen.vsb.cz/~s1i95/mvd/skr_mvd.pdf. [3] ’́obb: dv b 53. die gestaltung von oberbauanlagen. [4] db: ds 800 01. bahnanlagen entwerfen allgemeine entwurfsrichtlinien. [5] l. fiala. provozní dopady aplikace ochranných vzdáleností podle tnž 34 2620 master thesis. university of pardubice, pardubice. supervisor pavel drda, 2010. [6] tnž 28 2605 kolejová vozidla – železniční. trubkové nárazníky s korýtkovým vedením. typy, základní parametry, technické požadavky, zkoušení. nymburk: čsd, 1991. in effect from: 1991-07-01. [7] oleo end stops. oleo international [online]. updated 2015-07-09, [2015-11-07], http://www.oleo.co.uk/products/end-stops. [8] israel railways ltd. railway buffer stops planning guidlines [online], 2009. updated 2013, [201511-10], http://www.iroads.co.il/sites/default/ files/imce/ir_buffer_stops_guidelines.doc. [9] rewie bahntechnik strassenbahn. rawie gmbh & co. kg [online]. updated 2015-08-20, [2015-11-07], http://www.rawie.de/index.php/de/bahntechnik/ strassenbahn. 21 http://homen.vsb.cz/~s1i95/mvd/skr_mvd.pdf http://www.oleo.co.uk/products/end-stops http://www.iroads.co.il/sites/default/files/imce/ir_buffer_stops_guidelines.doc http://www.iroads.co.il/sites/default/files/imce/ir_buffer_stops_guidelines.doc http://www.rawie.de/index.php/de/bahntechnik/strassenbahn http://www.rawie.de/index.php/de/bahntechnik/strassenbahn acta polytechnica ctu proceedings 5:17–21, 2016 1 introduction 1.1 flank protection 1.1.1 mutually excluded train path 2 buffer stop's design parameters 2.1 collision speed 3 types of buffer stops 3.1 fixed buffer stops with mechanical bumpers 3.2 hydraulic buffer stop (fixed buffer stop with hydraulic bumpers) 3.3 friction buffer stop 3.3.1 braking force 4 conclusion acknowledgements references 128 acta polytechnica ctu proceedings 2(1): 128–132, 2015 128 doi: 10.14311/app.2015.02.0128 investigating long-term behaviour of x-ray binaries using archival data m. m. kotze1,2, p. a. charles2,3 1south african astronomical observatory, p.o. box 9, observatory 7935, south africa (sa) 2astrophysics, cosmology and gravity centre (acgc), astronomy department, university of cape town, rondebosch 7701, sa 3school of physics & astronomy, university of southampton, southampton so17 1bj, uk corresponding author: marissa@saao.ac.za abstract long term modulations have been detected in a wide variety of both low and high-mass x-ray binaries. the all sky monitor on board the rossi x-ray timing explorer provides the most extensive (∼15 years) and sensitive x-ray archive for studying such behaviour. since those variations were often intermittent and/or aperiodic, we used a time-dependent dynamic power spectrum method to examine how the modulations themselves vary with time in a systematic way. some were found to be remarkably stable, while others show a range of properties, from even longer variability time-scales to quite chaotic behaviour. keywords: accretion discs x-ray binaries. 1 introduction kotze & charles (2012) contains the results of our timedependent period analysis on 25 x-ray binaries (xrbs) with reported long term variability. for the purpose of these conference proceedings, some of those results are reproduced herein to illustrate the importance of such an approach when dealing with varying periodic and/or aperiodic signals. we also use this opportunity to present some of the highlights contained in our previously published results. 2 data analysis 2.1 rxte/asm archival data the massachusetts institute of technology (mit) operated an all sky monitor (asm) on board the rossi x-ray timing explorer (rxte) from 1996 to 2012. the asm observed the x-ray sky with 3 rotating scanning shadow cameras (ssc) which scanned ∼80% of the sky during ∼90 min orbits. data were reduced and compiled weekly by the asm team and made publicly available 1 as dwell-by-dwell or one-day-averages in four energy bands, which includes a sum-band (1.5-12 kev). one-day-average data 2 are the dwell-by-dwell data binned into 1-day bins. archival asm datasets contain the lightcurves of all xray sources in the rxte catalogue. for full details see levine et al. (1996). 2.2 time-dependent period analysis periods are typically found by variability analysis of time-series/lightcurve datasets. while this approach may allow distinction between periodic and quasiperiodic behaviour, it contains no information regarding the variation or consistency of aperiodic/quasi-periodic signals. clarkson et al. (2003) presented results from their time-dependent periodic analysis on 4 x-ray binaries in the form of dynamic power spectra (dps). the method requires the datasets to be split into windows that are of sufficient length to allow detection of the maximum period considered. transition from one independent window to the next is smoothed (i.e. increased resolution) by adding sliding windows between the positions of adjacent independent data windows. lomb-scargle (l-s; lomb 1976 & scargle 1982) periodograms were produced for every data window and the results of all the periodograms for a source were plotted together in a density map, using the l-s power for each frequency plotted at every window’s mid-point along the time-axis. the frequency domain allowed appropri1http://xte.mit.edu/ and http://xte.mit.edu/asmlc.html 2i have made all their plots available on: http://www.saao.ac.za/∼marissa/lc/lc.html 128 http://dx.doi.org/10.14311/app.2015.02.0128 investigating long-term behaviour of x-ray binaries using archival data ate coverage over frequencies associated with previously reported periodic behaviour (table 1). since the dps method requires a significant number of computations, its application has been limited until recent advances in desktop technology allowed it to be employed more readily. figure 1: accretion disc stability to radiation-driven warping in xrbs, as functions of q and rb ( [ gm1 c2 ] ), adapted from ogilvie & dubus (2001) to include only xrbs with known super-orbital periods and updated with their latest system parameters (see table 1). squares indicate low-mass xrbs (lmxbs) and circles high-mass xrbs (hmxbs). tidally-induced disc precession (sec 3.4) may occur left of the dashed line. 3 results several mechanisms have been proposed to account for the super-orbital variations (psup) observed in xrbs. we list our results according to the mechanisms that have been considered most likely candidates for the cases included here. please refer to kotze & charles (2012) for more details. 3.1 radiation-induced disc warp/tilting ogilvie & dubus (2001) provided stability predictions for xrb accretion discs against radiation-driven warping/tilting. therein the binary separation (rb) and mass ratio (q = m2/m1) determine the location of a source on their predictive diagram, which is divided into zones where stable warps (between the dashed and solid line), chaotic warps (above the dashed line) or no warps (below lower solid line) are expected. a source near the transition boundary between the stable warp zone and the chaotic warp zone, may experience instability that could result in variable warps. table 1: system parameters for included xrbs source psup porb q rb/106 [days] [days] [m2 m1 ] [gm1 c2 ] her x-1 33-37 [1] 1.700 [2] 1.56 3.1 smc x-1 50-70 [3] 3.89 [4] 11.0 8.9 lmc x-4 30 [5] 1.41 [6] 10.6 4.5 cyg x-2 60-90 [7] 9.844 [8] 0.34 8.0 x1820-303 171 [9] 0.008 [10] [0.1] 0.1 ss433 162 [11] 13.10 [12] [1.0] [3.0] x1916-053 5 [13] 0.035 [14],[15] [0.1] 0.2 199 [16] [1] leahy & igna (2010), [2] tananbaum et al. (1972), [3] clarkson et al. (2003), [4] schreier et al. (1972), [5] lang et al. (1981), [6] chevalier & ilovaisky 1977, [7] clarkson et al. (2003), [8] cowley et al. (1979), [9] chou & grindlay (2001), [10] stella et al. (1987), [11] margon (1984), [12] crampton et al. (1980), [13] homer et al. (2001), [14] walter et al. (1982), [15] white & swank (1982), [16] priedhorsky & terrell (1984) figure 2: her x-1: this steady super-orbital period have been associated with a stable radiation-driven warp. 129 m. m. kotze, p. a. charles figure 3: smc x-1: the super-orbital period that is evolving dramatically over an even longer time-scale, may be associated with a radiation-driven warp that is variable. figure 4: lmc x-4: the steady long-term period may be produced by a stable radiation-driven warp. figure 5: cyg x-2: the unstable/chaotic behaviour may be associated with chaotic warping. 3.2 third body zdziarski et al. (2007) performed a comprehensive analysis of the triple system scenario and how binary eccentricity oscillations could modulate mass transfer through l1, finding agreement between theoretical and observed lightcurves for x1820-303. figure 6: x1820-303. 3.3 precessing relativistic jets ss433 is the prototypical microquasar. its precessing relativistic jets and their resulting ∼ 162 day superorbital period was obtained by hjellming et al. (1981) and margon (1984) from variations in the radial velocity measurements from balmer and he i emission lines. figure 7: ss433. 3.4 tidally-induced disc precession whitehurst & king (1991) described how tidal interactions with the donor may excite resonances in the accretion disc, causing it to precess and produce quasi130 investigating long-term behaviour of x-ray binaries using archival data periodic variations in the lightcurve. essentially the disc becomes elliptical, expanding beyond its critical radius while remaining within its roche-lobe radius, so that disc precession is effectively the result of changes in the orientation of this elliptical accretion disc with respect to the donor. figure 8: x1916-053. tidal disc precession depends on the mass ratio (q = m2 m1 ) and will only occur if q < 0.25 − 0.33, producing so-called “superhumps”, which have been detected in the su uma sub-class of cataclysmic variables (cvs). the vertical dashed line on fig. 1 indicates this boundary, to the left of which xrbs are expected to be susceptible to tidal disc precession (e.g. x1916-053 and x1820-303). 4 discussion the composite figure for each source contains a wealth of information to assist interpretation. the lightcurves are shown in the top panels and density maps with the dps results form the main panels, for which the scales are indicated to the right. the 99.9%-confidence white noise levels (green lines), together with the l-s for the entire datasets, are shown in the left panels. all included sources show significant (exceeding the white noise levels) periodic signals in their dps. previously reported periodicities (listed in table 1), indicated by red bars, are clearly present in the dps panels and often also in the l-s (left-hand) panels along with additional variability. 5 conclusions quasi-periodic or aperiodic signals should be investigated using time-dependent period analysis techniques. while these examples focussed on long-term superorbital behaviour of xbs, it should be immediately apparent how useful such a technique would be for investigating short-term quasi-periodic oscillations in cvs. acknowledgement asm results were provided by the asm/rxte teams at mit and at the rxte science operations facility and guest observer facility at nasa’s goddard space flight center (gsfc). this work formed part of my phd thesis, supervised by prof. phil charles and co-supervised by prof. brian warner, at the university of cape town (uct) and was funded by the national research foundation (nrf) of south africa. references [1] chevalier c., ilovaisky s. a. : 1977, a&a, 59, l9 [2] chou y., grindlay j. e. : 2001, apj, 563, 934 doi:10.1086/324038 [3] clarkson, w. i., charles, p. a., coe, m. j., laycock, s. : 2003, mnras, 343, 1213 doi:10.1046/j.1365-8711.2003.06761.x [4] cowley a. p., crampton d., hutchings j. b. : 1979, apj, 231, 539 doi:10.1086/157216 [5] crampton d., cowley a. p., hutchings j. b. : 1980, apj, 235, l131 doi:10.1086/183176 [6] hjellming, r. m., & johnston, k. j. : 1981, nature, 290, 100 doi:10.1038/290100a0 [7] homer l., charles p. a., hakala p., muhli p., shih i.-c., smale a. p., ramsay g. : 2001, mnras, 322, 827 [8] kotze, m. m., charles, p. a. : 2012, mnras, 420, 1575 doi:10.1111/j.1365-2966.2011.20146.x [9] lang f. l., et al. : 1981, apj, 246, l21 [10] leahy d. a., igna c. d. : 2010, apj, 713, 318 doi:10.1088/0004-637x/713/1/318 [11] levine, a. m., bradt, h., cui, w., et al. : 1996, apjl, 469, l33 doi:10.1086/310260 [12] lomb, n. r. : 1976, apss, 39, 447 [13] margon, b., anderson, s. f., aller, l. h., downes, r. a., & keyes, c. d. : 1984, apj, 281, 313 [14] ogilvie, g. i., & dubus, g. :2001, mnras, 320, 485 131 http://dx.doi.org/10.1086/324038 http://dx.doi.org/10.1046/j.1365-8711.2003.06761.x http://dx.doi.org/10.1086/157216 http://dx.doi.org/10.1086/183176 http://dx.doi.org/10.1038/290100a0 http://dx.doi.org/10.1111/j.1365-2966.2011.20146.x http://dx.doi.org/10.1088/0004-637x/713/1/318 http://dx.doi.org/10.1086/310260 m. m. kotze, p. a. charles [15] priedhorsky w. c., terrell j. : 1984, apj, 280, 661 [16] scargle, j. d. : 1982, apj, 263, 835 [17] schreier e., giacconi r., gursky h., kellogg e., tananbaum h. : 1972, apj, 178, l71 doi:10.1086/181086 [18] stella l., priedhorsky w., white n. e. : 1987, apj, 312, l17 doi:10.1086/184811 [19] tananbaum h., gursky h., kellogg e. m., levinson r., schreier e., giacconi r. : 1972, apj, 174, l143 [20] walter f. m., mason k. o., clarke j. t., halpern j., grindlay j. e., bowyer s., henry j. p. : 1982, apj, 253, l67 [21] whitehurst, r., & king, a. : 1991, mnras, 249, 25 [22] white n. e., swank j. h. : 1982, apj, 253, l61 doi:10.1086/183737 [23] zdziarski, a. a., pooley, g. g., & skinner, g. k.: 2011, mnras, 412, 1985 discussion koji mukai: can you clarify what you saw in x1916? it is a low q system, but i thought the disc precessesion period was about 5 days, while your plot showed only longer periods. marissa kotze: our dps analysis for this source show no significant periodic behaviour below 10 days. but this is not unexpected, since the 5 day period associated with tidal disc precession in x1916-053 was not determined from x-ray flux variations, but rather from optical variability and changes in the structure of the x-ray dips, which our dps analysis of rxte asm data is unable to detect. 132 http://dx.doi.org/10.1086/181086 http://dx.doi.org/10.1086/184811 http://dx.doi.org/10.1086/183737 introduction data analysis rxte/asm archival data time-dependent period analysis results radiation-induced disc warp/tilting third body precessing relativistic jets tidally-induced disc precession discussion conclusions acta polytechnica ctu proceedings doi:10.14311/app.2017.7.0069 acta polytechnica ctu proceedings 7:69–71, 2017 © czech technical university in prague, 2017 available online at http://ojs.cvut.cz/ojs/index.php/app performance of steel fiber reinforced concrete – comparability of tests according to dafstb-guideline "stahlfaserbeton" and en 14651 steffen anders∗, melanie schovenberg institute of structural engineering, bergische universität wuppertal, pauluskirchstr. 11, 42285, wuppertal, germany ∗ corresponding author: baustoff@uni-wuppertal.de abstract. for the determination of the performance of steel-fiber reinforced concrete (sfrc), the post-peak flexural strengths are used. in different national and european standards, different test-setups are defined, resulting in double efforts for testing for the manufacturers. in addition, the german national guideline "stahlfaserbeton (dafstb)" on sfrc is well established european-wide, but the test standard is specifically national, demanding a four-point-bending tests using unnotched beams. contrarily, the european standard en 14651 as well as the model code 2010 [1] demand three-point bending tests using notched specimens. applying the national guideline is obligatory in germany for structural use of sfrc. therefore, it is essential to standardize the performance evaluation of sfrc based on commonly applied international guidelines. in the following, an approach is presented especially dealing with the problem of random occurrence of cracks in the four-point-bending tests. it is shown, that neglecting the point of crack can systematically under-estimate the performance of sfrc especially at deformations. keywords: steel-fiber reinforced concrete, performance classes, post-peak flexural strength. 1. introduction in order to get permission to distribute and sell steelfibers in europe, a ce-certificate is obligatory. therefore, it is necessary to determine the performance of the steel-fibers in concrete, depending on the type of steel fibers as well as the fiber contents. in this case, the en 14651 [2] is the relevant standard, demanding a test using third-point loading on notched specimens. in order to get the permission to sell steel-fiber reinforced concrete e.g. as ready mixed concrete the performance of sfrc has to be evaluated according to the german national guideline "stahlfaserbeton (dafstb)" [3]. this guideline demands tests on unnotched specimens in a 4-point bending set-up. in both guidelines, performance of sfrc is measured in terms of post-peak flexural strengths. seen from the mechanical point of view the evaluation of post-peak flexural strengths is a fracture mechanical problem, demanding determination of strengths and load bearing capacities directly in the crack. this precondition is fulfilled only by en 14651 but not by the german guideline, because the crack can occur anywhere in the mid third of the beam due to the four-point-loading scheme. in order to overcome the problem of mechanical incorrectness as well as achieving international standardization, establishing a connection between the two test methods would be advantageous to facilitate the conversion of results from one method to another. one important aspect in this context is to include the point of crack into the evaluation according to the guideline "stahlfaserbeton". this aspect will be dealt with in the following. 2. test methods and evaluation of post-peak flexural strengths the most important difference between the national guideline "stahlfaserbeton (dafstb)" and the european standard en 14651 is the test set-up as shown in fig. 1. in the four-point loading test, which is demanded by the guideline "stahlfaserbeton (dafstb)", the crack should occur randomly in the mid third between the two loads due to lacking of a notch. the deflection of the beam is measured using transducers at mid-span. as described, due to this distance the testing method is not consistent from a fracture mechanical point of view. the method given in en 14651 uses a notched beam and just one load exactly above the notch at mid-span. thus, deflection and cracking are in-line. the evaluation of the post-peak flexural strength in the guideline "stahlfaserbeton (dafstb)" uses the mid-span deflection δ. in this guideline, the post-peak flexural strength is calculated from the load at two defined deflections of δ = 0.5 mm and δ = 3.5 mm at mid-span (see fig. 2-up). seen from point of view of fracture mechanics, the deflection at the point of crack is always higher than the deflection at mid-span, resulting in a systematic underestimation of the postpeak flexural strength, provided a strain softening behavior of the steel-fiber reinforced concrete. 69 http://dx.doi.org/10.14311/app.2017.7.0069 http://ojs.cvut.cz/ojs/index.php/app steffen anders, melanie schovenberg acta polytechnica ctu proceedings figure 1. comparison of test set-ups on the other hand, the en 14651 standard uses the crack mouth opening displacement (cmod) being directly correlated to the mid-span deflection [4]. to specify the post-peak flexural behavior, strength values at four different points are defined in terms of cmod (see fig. 2-down). the deflection at δ = 0.47 mm at cmod1 correlates well to the german guideline. cmod4 at a deflection of δ = 3.02 mm shows a difference of about 0.5 mm for the evaluation point. additionally, the cross-sections differs either, for the notch reduces the height of the cross section (both beams are produced with a cross-section of 150 × 150 mm). the approach of the en 14651 is consistent with respect to fracture mechanics, because the deflection is measured directly in the cracked crosssection. the shown differences in testing and calculating make it nearly impossible to compare the results of the guidelines. a special problem being the random cracking with german test set-up. 3. calculation approach to adjust the results taken during the tests according to the guideline "stahlfaserbeton (dafstb)" the point of the first crack has to be measured. if this is done the deformations measured at mid-span, can be transferred to the cracked cross-section by simply using theorem of intersecting lines applied on the larger part of the beam, assuming both parts of beam are not deformed themselves. fig. 3 shows that the deflection in the cracked cross-section is always larger than deflection at mid-span. the consideration of the point of the crack results in an increased stretching of the force-deflection curve. the correction of the curve increases with an increasing deflection during the test as well as with an infigure 2. evaluation of post-peak flexural strengths at different points. up dafstb-guideline "stahlfaserbeton", down en 14651 figure 3. calculation approach for taking the point of crack into account using the theorem of intersecting lines creasing distance of the crack from the middle of the beam. in fig. 4, the increased stretching can be easily seen especially at higher deflections. figure 4. change of mid-span deflection in case of strain hardening material at low deflections and strain softening at high deflections 70 vol. 7/2017 performance of steel fiber reinforced concrete depending on the effectiveness of the fibers (and on the fiber content), generally two different material behaviors are distinguished. either the material is strain hardening, meaning that the transferred load increases with an increasing deflection or the material is strain softening, showing decreasing transferable loads with increasing deflections. in fig. 4 the postpeak flexural strength for low deflections of about δ = 0.5 mm shows strain hardening. in case of strain softening, as can be seen at a deflection higher than about 1 mm a correction of the load-deflection curve leads to increasing post-peak flexural strengths. 4. results and discussion after measuring the point of crack of about 360 fourpoint bending tests, the resulting distribution of the point of crack is given in fig. 5. the displayed distance of the crack from mid-span does not differ whether the crack occurred left or right from the mid of the beam. assuming a perfectly homogeneous concrete, the crack should theoretically occur between the loads. an analysis of the point of the first crack shows, that only 50 % of all cracks occur in a distance less than 4 cm around the mid of the beam. at distances of less than 2 cm from mid-span, less than 25 % of all cracks occur. figure 5. distance of point of crack from mid-span applying the correction approach for the point of crack on several beams, the post-peak flexural strength values are affected as well. the larger the distance of the crack from mid-span the higher the correction, strain softening assumed. most of the beams have shown force-deflection curves comparable to fig. 4. as mentioned, these beams often show little strain hardening at low deflections. because the correction at δ = 0.5 mm is small, the post-peak flexural strength values are unaffected for a large number of beams. due to the slight strain hardening, the post-peak flexural strength values decrease for a number of beams. at larger deflections of δ = 3.5 mm the correction is higher, because of the higher deflection in combination with the strain softening. the mean increase ranges at about 0.15 n · mm−2, which is half a performance class. figure 6. effect of correction of post-peak flexural strength due to random cracking of beams tested according to dafstb guideline "stahlfaserbeton" 5. conclusions for the determination of performance of sfrc, different testing and evaluation approaches are used in germany and in europe. from a point of view of fracture mechanics, the german approach is incorrect, due to a random occurrence of the cracks predominantly in the mid-third. this leads to a systematic underestimation of the deflection in the cracked cross-section. therefore, it is essential to take the point of the first crack into account, leading to a stretching of the forcedeflection curve. two different modes of behavior can occur. for strain hardening, which is often found at low deflections and at higher fiber contents, the postpeak flexural strength can decrease, and for strain softening behavior, usually found for concrete having lower fiber contents as well as at higher deflections, the post peak flexural strength increases. taking the point of crack into account leads to an increase of about half a performance class on an average. in further steps, the results should directly be related to results from tests according to en 14651. additionally, the different geometries caused by the notches have to be taken into account. references [1] din en 14651: prüfverfahren für beton mit metallischen fasern bestimmung der biegezuggfestigkeit (proportionalitätsgrenze, residuelle biegezugfestigkeit) 2004. [2] deutscher ausschuss für stahlbeton (dafstb): dafstb-richtlinie stahlfaserbeton 2012. [3] fédération international du béton (fib): modelcode for concrete structures 2010. [4] b. barr, et al. round-robin analysis of the rilem tc 162-tdf beam-bending test: part 2 application of delta from the cmod response. materials and structures 36(263):621–630, 2003. doi:10.1617/13954. 71 http://dx.doi.org/10.1617/13954 acta polytechnica ctu proceedings 7:69–71, 2017 1 introduction 2 test methods and evaluation of post-peak flexural strengths 3 calculation approach 4 results and discussion 5 conclusions references acta polytechnica ctu proceedings doi:10.14311/ap.2016.4.0097 acta polytechnica ctu proceedings 4:97–101, 2016 © czech technical university in prague, 2016 available online at http://ojs.cvut.cz/ojs/index.php/app quench front propagation in the annular channel jan štěpánek∗, vaclav bláha, vaclav dostál faculty of mechanical engineering, czech technical university in prague, technická 4, prague, czech republic ∗ corresponding author: j.stepanek@fs.cvut.cz abstract. understanding the quench front propagation during bottom core reflooding is crucial for the effective cooling during the loca accident. the results presented in this paper were obtained on an experimental loop with an annular test section. the test section consists of a vertical electrically heated stainless steel tube with outer diameter 9 mm and length of 1.7 m. the heated tube is placed inside a glass tube with the inner diameter 14.5 mm. water mass flux during the reflooding is in the range from 100 kg m−2 s−1 up to 140 kg m−2 s−1 and the initial wall temperature of the stainless steel tube is in the range from 250 ◦c up to 800 ◦c. the presented results show the influence of the initial conditions on the quench front propagation and the complexity of the phenomenon. keywords: quench front, rewetting, loca. 1. introduction cooling of very hot surfaces is still not satisfactorily described area. the main difficulty of this kind of cooling is that coolant can’t reach the hot surface because of a thin vapor layer generated between the hot surface and the coolant. this vapor layer acts as a thermal insulating gap and it prevents effective cooling. heat transfer in this gap is much lower than in rewetted area. as the surface cools down, the vapor layer collapses and coolant rewetts the surface. a high heat transfer rate takes place at this moment. the location where coolant, for example water, rewetts the surface is called the quench front. because of rapid cooling at the quench front, a heat accumulated in front of the quench front is conducted towards the rewetted area. this heat conduction accelerates the cooling of dry surface and it helps the quench front to advance forward [1]. the speed of this rewetting advance is called the quench front velocity. quenching takes place in many technical applications such as steel hardening, cryogenic technology and many others. one of the most crucial applications is a nuclear reactor safety. this phenomenon is dominant for safety during core reflooding phase of loca (loss-of-coolant accident). the temperature of the fuel cladding rapidly rises up (over 1200 ◦c [2]) after blow-down phase. then emergency core cooling system (eccs) starts to pump relatively cold water into the core. as the water reaches the hot fuel rods quenching appears [3]. this complicates effective core cooling. the quenching phenomenon has been solved analytically in many studies based on experimental data [4]. in these studies cooled geometry is usually divided into several regions. the most common approach divides the geometry into two regions. one region is dry region in front of the quench front. in this region a heat transfer coefficient is assumed as zero, i.e. adiabatic boundary condition. the second region is wet or rewetted region, where a heat transfer coefficient is taken as constant number [5]. this problem is then usually solved using wiener-hopf technique or heat balance integral method (hbim). there were many experimental efforts in the second half of 20th century. but many of these experiments were done for short geometries with limited range of initial wall temperatures [6]. the main goal of upcoming experiments is to cover wide range of initial wall temperatures, water mass flow rates and to describe effects of accumulated heat (different wall thickness and uneven tube heating) and geometrical elements such as spacer grids. in this paper a first set of experimental data is presented. 2. experimental loop for the purpose of quench front phenomenon investigation an experimental loop has been built. the loop consists of test section, dc power source, data acquisition system and variable hydraulic circuit. the test section is assembled from inner electrically heated steel tube and from outer glass tube. the steel tube is 1.7 m high and it is equipped with set of k-thermocouples (see tc1, tc2 and tc3 in fig. 1). these thermocouples are in contact with inner surface of the heated tube. the thermocouples in the wall are not calibrated and its precision is then ±2.5 ◦c but the precision of these thermocouples is not important because temperature time gradient is measured. the spacing between thermocouples is 0.5 m. the outer diameter of the steel tube is 9 mm, its wall thickness is 0.5 mm and the tube is made of x6crniti18 stainless steel. the outer glass tube has inner diameter of 14.5 mm and its wall thickness is 1.75 mm. it gives annular flow cross-section of 100 mm2. the test section is situated between lower and upper chamber. in these chambers are situated pressure sensors with accuracy of 0.5 % fso and calibrated class 2 k-thermocouples. initial wall temperature, water mass flow rate, lower chamber 97 http://dx.doi.org/10.14311/ap.2016.4.0097 http://ojs.cvut.cz/ojs/index.php/app j. štěpánek, v. bláha, v. dostál acta polytechnica ctu proceedings pressure, inlet temperature, power input and outer temperature of the glass tube (tcgl in fig. 1) are scanned during the experiment. the water mass flow rate is continuously measured by turbine flow meter. the precision of the flow meter is ±3 %. p tin tc1 tc3 tc2 q + tcgl figure 1. simplified experimental loop scheme. 3. setup the initial wall temperature of the experiment was in range from 250 ◦c up to 800 ◦c with step of 50 ◦c, the initial flow rate was set to 130 kg m−2 s−1 and its actual values are dependent on hydraulic characteristics of the test section during the experiment. this initial flow rate is adjusted in short circuit of the loop. firstly the steel tube is heated up to desired temperature. the thermocouple on the glass tube is used as an indicator of the stationary state. when the temperature of this thermocouple is constant, the reflooding begins. once the water reaches upper chamber, the experiment is finished. collected information are then evaluated on the pc. 4. quench front velocity in experimental studies quench front velocity is usually identified in several ways. one of these approaches is usage of time dependence of measured temperature on the cooled geometry. another approach for example uses optical methods such as high speed cameras. the quench front velocity was obtained as the distance between thermocouples (0.5 m) overcame in measured time. time, when a thermocouple has been rewetted is identified via maximum value of the first derivative of the measured temperature. this derivative value also represents position of the quenching temperature. when all rewetting times for each thermocouple are obtained, the quench front velocity can be calculated. 5. results each experiment was repeated five times. the measured temperatures and other experimental data were interpolated using piecewise cubic hermite interpolation polynominal (pchip). the quenching temperature has been calculated as an average from valid experimental data. in fig. 2 we can see the quenching temperatures for each initial wall temperature level and for each thermocouple. as can be seen this dependence is almost linear. the rewetting temperature of the first thermocouple behind the inlet is highest and for the last thermocouple is this temperature the lowest of all. this temperature drop is caused by two-phase mixture propelled by steam generated at the quench front. this mixture precools dry region of the heated tube and this precooled surface is then rewetted at lower temperature. initial wall temperature also strongly influences the value of the quench front velocity. higher wall temperature generates more steam at the quench front. on the one hand this steam helps to cool down the dry region but on the other hand this steam generates higher pressure drop and this leads to lower quench front velocity. but the most significant slowdown is caused by evaporated fraction of the inlet water mass flux. this dependence can be seen in fig. 3. from fig. 3 we can see, that quench front velocity drop is very significant between initial wall temperature levels 250 ◦c and 300 ◦c. from 300 ◦c this dependence is almost linear. for 300 ◦c the quench front velocity is 7.6 cm s−1 and for 800 ◦c 2.6 cm s−1. this velocity difference represents about 70 % velocity drop in this temperature range. with higher wall temperatures a significant pressure bump can be observed when the water meets the hot surface in the test section. this prompt pressure increase is caused by rapid steam generation at the quench front. the steam moves at high speed along the hot surface and the steam is then slightly overheated. these conditions lead to high pressure drop along the test section and it slows the quench front down. the pressure drop can be observed from initial wall temperature of 400 ◦c and it grows from 2 kpa up to 17 kpa at 800 ◦c wall temperature level. the value of the pressure bump was calculated as a difference between maximum pressure and uninfluenced pressure at lower wall temperatures in the lower chamber. this dependence is plotted in fig. 4. the maximum value of pressure bump at 800 ◦c represents nearly 90 % of pressure drop of the fully reflooded test section. also heat transfer rate at the quench front has been calculated. this heat transfer rate is calculated using first derivative of the wall temperature and material characteristics of the heated tube such as 98 vol. 4/2016 quench front propagation in the annular channel figure 2. rewetting temperature on initial wall temperature. figure 3. quench front velocity on initial wall temperature. heat capacity and electric resistance. using known difference of accumulated heat and known value of electric current a heat transfer rate at the quench front can be enumerated. when the heat transfer rate is known, the heat transfer coefficient can be also obtained from this heat transfer rate and from known quenching temperature and temperature of saturated water. resulting values of heat transfer coefficient are plotted in fig. 5. 6. experiment and real nuclear reactor the heated tube diameter corresponds to dimension of fuel rod of pressurized water reactor and its length is approximately halved. the results from the experiment give us the basis for estimating the quench front velocity as a local process. in a real nuclear reactor this process goes into complex spatial process. on the other hand a material used for the heated tube 99 j. štěpánek, v. bláha, v. dostál acta polytechnica ctu proceedings figure 4. pressure bump on initial wall temperature. figure 5. heat transfer coefficient at the quench front on initial wall temperature. also affects the quench front propagation. the use of zirconium alloy will probably significantly slow down the quench front propagation along the heated surface due to exothermic reaction of the zirconium and steam vapour approximately above 800 ◦c [7]. inclusion of this phenomenon is necessary for experimental efforts at very high temperatures in the future. the heat generation along the tube is constant in this effort but also an uneven heat generation will be investigated in the upcoming experimental work. this unevenness will affect the quench front velocity as well. 7. conclusions the presented results in this paper show the complexity of the quenching phenomenon for one coolant mass flux level and for range of initial wall temperature from 250 ◦c up to 800 ◦c in the annular channel. from these results can be concluded, that quenching velocities and rewetting temperatures are strongly dependent on the initial wall temperature. with increasing initial wall temperature the quench front velocity rapidly decreases. moreover increasing initial 100 vol. 4/2016 quench front propagation in the annular channel wall temperature goes with significant pressure bumps. these pressure bumps strongly influence the hydraulic characteristic of the heated channel and this leads to a big pressure drop in the channel. these effects can lead to unevenness of water flow through the core during loca. the unevenness can lead to cooling water flowing around the central area of the core, where the surface temperature is lower, and the central area of the core can be cooled down distinctly later. on the other hand, geometrical elements, such as spacer grids, create secondary quench fronts moving along the fuel rod [8]. due to these secondary quench fronts, the last rewetted point can’t be clearly determined. in other words, the water presence above the core doesn’t mean that the rest of the core is rewetted and cooled down. all these weaknesses of the quenching phenomenon should be uncovered for better handling of nuclear reactor accident. more experimental efforts will be done in the future. in these efforts a wide range of water mass flux will be covered. references [1] e. elias, g. yadigaroglu. a general one dimensionalmodel for conduction-controlled rewetting of a surface. nuclear engineering and design 42 (1977), 185–194. [2] m. billone, y. yan, t. burtseva, r. daum. cladding embrittlement during postulated loss-of-coolant accidents. u.s. nuclear rerulatory commision nureg/cr-6967, anl-07/04, 2008. http://www.nrc.gov/docs/ml0821/ml082130389.pdf [3] h.c. yeh. analysis of rewetting of a nuclear fuel rod in water reactor emergency core cooling. nuclear engineering and design 34 (1975), 317–322. [4] s.k. sahu, p.k. das, s. bhattacharyya. analytical and semi-analytical models of conduction controlled rewetting: a state of the art review. thermal science 19 (2015), 1479–1496. doi:10.2298/tsci121231125s. [5] s. olek. on the two region model with a step change in the heat transfer coefficient. nuclear engineering and design 108 (1988), 315–322. [6] a.k. saxena, v. venkat raj, v. govardhana rao. experimental studies on rewetting of hot vertical annular channel. nuclear engineering and design 208 (2001), 283–303. doi:10.1016/s0029-5493(01)00356-9. [7] j. belle, m.w. mallett. kinetics of the high temperature oxidation of zirconium. j. electrochem. soc. 101 (1954), 339–342. doi:10.1149/1.2781278. [8] j. stepanek, v. blaha, v. dostal, p. burda. the effect of spacer grid’s elements on the rewetting velocity. in proceedings of international conference on nuclear engineering (icone23). new york: asme, 2015. isbn 978-4-88898-256-6. 101 http://www.nrc.gov/docs/ml0821/ml082130389.pdf http://dx.doi.org/10.2298/tsci121231125s http://dx.doi.org/10.1016/s0029-5493(01)00356-9 http://dx.doi.org/10.1149/1.2781278 acta polytechnica ctu proceedings 4:97–101, 2016 1 introduction 2 experimental loop 3 setup 4 quench front velocity 5 results 6 experiment and real nuclear reactor 7 conclusions references acta polytechnica ctu proceedings doi:10.14311/app.2017.11.0070 acta polytechnica ctu proceedings 11:70–73, 2017 © czech technical university in prague, 2017 available online at http://ojs.cvut.cz/ojs/index.php/app the detailed priority of international freight expresses in the overlapping section of rfc 7 and rfc 9 kolín – choceň přemysl šrámek∗, tatiana molková university of pardubice, jan perner transport faculty, department of transport technology and control, studentská 95, 532 10 pardubice, czech republic ∗ corresponding author: premysl.sramek@student.upce.cz abstract. the main goal of this article is to research the optimal priority of international freight expresses, especially on european freight corridors. this article shows this problem in the context in the overlapping section of rfc 7 and rfc 9 kolín – choceň. there is solved changing priority of freight expresses in detail, whereas other train priorities are the same. keywords: capacity, freight expresses, freight corridors, timetable stability. 1. introduction one of the aims of european transport policy is to redirect capacity of road freight traffic to other modes of transport, where rail transport is an interesting and environmentally friendly alternative, especially in terms of speed, availability and amount of transported cargo. on the other hand, by customers is required delivery time is guaranteed by carriers, what is unfortunately not always possible, especially due to high utilization of european rail infrastructure. the solution is the construction of new lines, increasing the capacity of existing lines through construction and reconstruction measures or through operational measures. among the operational measures may be included alternative routing of trains as well as changing priorities of a particular type of train, e.g. train category fex (freight express) [1]. the aim of this article is to find the optimal priority of international freight expresses to satisfy the customers required delivery times due to optimization of train priority. 2. materials and methods there are mentioned some information about european freight corridors rfc and the simulation method with using of simulation tool simut (average delay increment (adi) calculation). 2.1. rail freight corridors (rfc) on the basis of the european parliament and the eu council no. 913/2010 for competitive freight and no. 1316/2013, which is created the connecting europe facility (cef), is the gradual establishment of rail freight corridors rfc (rail freight corridors). the main aim of the operation of these corridors is to strengthen the competitiveness of railways synergies between rail systems and harmonization of allocation interstate freight routes by national infrastructure managers [2]. czech republic, respectively czech railway infrastructure administration (cria), as the infrastructure manager and allocator of capacity, is a member of rfc 5 (baltic – adriatic), rfc 7 (orient / eastmediterranean), rfc 8 (north sea – baltic) and rfc 9 (czechoslovak corridor). each corridor is operated on the basis of the corridor information document (cid); each corridor has a single point of contact (c oss) and allows carriers to apply international prearranged train paths facilitated freight via c-oss. the request must be filed by information system rne pcs and must include the cross-border section. this paper deals with the model study of common section of rfc 7 and rfc 9 kolín choceň. 2.1.1. rfc 7 rfc 7 – the orient corridor runs from central europe to easternand southern-europe connecting 7 member states – czech republic, austria, slovak republic, hungary, romania, bulgaria and greece. the total length of main lines is approx. 3900 km and the length of alternative and connecting lines is almost 2500 km altogether. most limiting factors of rfc 7 are low capacity, speed limit, limited length of trains, limited axle load, not electrified sections and lack of adequate safety equipment (signalling track circuits with 25 hz frequency, etcs, gsm-r, etc.). the railway infrastructure managers and capacity allocation companies responsible for establishing and running rfc7 are committed to offer reliable, high-quality, competitive transport services in order to increase the market demand, to operate the infrastructure costeffectively on the long run through harmonization of technical and procedural conditions and to facilitate the environmentally sustainable development of the european economy and the achievement of a better quality of life for its people (3). in capacity analysis, published in implementation plan of rfc 7 [3], there are found lines with capacity utilization higher than 90 %. the longest sections with this high capacity 70 http://dx.doi.org/10.14311/app.2017.11.0070 http://ojs.cvut.cz/ojs/index.php/app vol. 11/2017 the detailed priority of international freight expresses utilization are situated in the czech republic, concretely sections poříčany – pardubice (65 km) and choceň – česká třebová (25 km). 2.1.2. frc 9 rfc 9 – the czech-slovak rail freight corridor (cs corridor) runs from prague to čierna nad tisou (slovak-ukrainian border) connecting 2 member states – czech republic and slovak republic. the total length of main lines is 972 km and the length of alternative and connecting lines is 276 km altogether. most limiting factors of rfc 9 are the same as in rfc 7. in capacity analysis, published in implementation plan of rfc9 [4], there are found lines with capacity utilization higher than 90 %. 2.2. the simulation method in this part is said something about the simulation method in context of the transit of international freight express trains (fex) in the overlapping section of rfc 7 and rfc 9 kolín – choceň. in this section there are a total amount of 10 railway stations, in which it is possible overtaking trains (double track line). interstation sections are divided by an automatic block into track sections; all crossing safety devices are equipped with gates. for simulation was used simulation tool simut, developed by pavel krýže, phd. from cria. the simulation tool simut is developed in visual basic, therefore it can run on every pc with microsoft office. it has to be input the option of a simulation, then railway stations and their shortcuts, station tracks for each railway station, line tracks, connection of station and line tracks, length of interstation departments and amount of line departments, type of train for priority, type of train for each number of a train and the path of each train. the simulation program simut can put new paths in a daily timetable with the solution for arisen path conflicts [1]. it was used daily timetable 2016, which included the amount of 384 trains. the similar topic was solved in (1), but not so exactly, in the evaluation there were used for fex only the importance values 0.5; 1.7; 1.9 and there was created periodic timetable. in the table 1 are displayed the importance values of trains, which remain the same, and the importance value of fex is increasing from 0.5 to 2.0 through step 0.1. therefore, it was solved 16 simulations, every simulation had 365 simulation runs (whole timetable period). about train types in table 1 – en means euronight train, sc is supercity train, ec – eurocity train, ic – intercity train, r is fast train, sp – speed-up passenger train, os – stop passenger train. the remaining train types are types of railway cargo transport – nex is here fex and its importance value is variable (the research object). pn is normal cargo train and mn means slow cargo trains servicing e. g. sidings. within the simulation program was established the average delay increment (adi). the average delay figure 1. adil in the relation with fex importance value. increment was calculated by dividing the difference between total output and total input delay and the total number of trains. this indicator was calculated as an ongoing basis for each simulation run, so the total for the entire graph (all simulation runs). the indicator was also calculated for different types of transport, i.e. for long-distance passenger transport (adil), regional passenger transport (adir) and freight transport (adif). as part of the simulation was set for all simulation runs random entry delay based on the exponential probability distribution. there were solved conflicts of station tracks, freight trains were allowed to ride before their schedule time (in the case of free capacity) [1]. 3. results and discussion in the table 2 there are displayed timetable stability indicators (adi, adil, adir, adif) in the relation with changing importance value of fex. all timetable stability indicators have unit min/train, it means every train of the category should get this delay in minutes. the timetable stability indicators are in the relation with fex importance value compared on the figure 1 (adil), figure 2 (adir), figure 3 (adif) and figure 4 (adi). adil is with increasing fex importance value just always increasing and it is logical – cargo transport has more and more preference. in the case, when fex importance value is the same as the ex importance value, it means 2.0, is adil strongly rising to 9.31 min/train. against fex important value 1.9 (4.39 min/train) is twofold value – the result is fex can’t have higher or the same priority as ex. on the figure 1 there is created the linking curve with the equation and reliability. for adil has the highest reliability exponential linking curve. on the figure 2 there is displayed adir – in this case is evaluation very difficult. the linking curve reliability is the best for polynomial linking curve. over 1.5 fex importance value is adir affirmative, the best compromise is for fex importance value 1.3 – adir is – 0.27 min/train. the result is fex should 71 přemysl šrámek, tatiana molková acta polytechnica ctu proceedings train type en sc ec ic ex r sp os sv nex pn mn lv importance value 2.5 1.8 3.0 2.8 2.0 1.8 1.5 1.0 0.3 var. 0.1 0.03 0.08 table 1. the train importance values. fex importance value adil adir adif adi 0.5 1.04 -0.54 1.19 0.84 0.6 1.19 -0.61 1.63 1.07 0.7 1.36 -0.37 1.21 1.02 0.8 1.42 -0.30 1.12 1.02 0.9 1.47 -0.51 1.31 1.09 1.0 1.88 -0.31 0.46 0.97 1.1 2.02 -0.08 0.31 1.01 1.2 2.22 -0.12 0.39 1.12 1.3 2.60 -0.27 -0.38 0.96 1.4 2.50 -0.06 0.79 1.42 1.5 3.05 0.05 0.25 1.47 1.6 3.08 0.01 0.57 1.60 1.7 3.00 0.06 -0.17 1.28 1.8 4.46 0.31 -0.36 1.89 1.9 4.39 0.01 -0.72 1.67 2.0 9.31 0.53 -1.44 3.66 table 2. timetable stability indicators in the relation of changing fex importance value. figure 2. adir in the relation with fex importance value. have higher priority than regional passenger trains, especially than stop passenger trains. the linking curve reliability for adif is the best for polynomial linking curve. from 1.7 fex importance value is adif always negative and decreasing, but negative is for 1.3 fex importance value, too. the result is here not so clear. on the figure 4 there is displayed average delay increment for whole timetable. in every case is the timetable unstable, adi is affirmative, the output delay is higher than the input delay. the linking curve has really high reliability for polynomial. for 2.0 fex importance value is adi getting really high due to increasing adil. less than 1 min/train is adi only in 3 cases – for fex importance values 0.5, 1.0 and 1.3. the best adi is for 0.5 (0.84 min/train), but figure 3. adif in the relation with fex importance value. figure 4. adi in the relation with fex importance value. it is in contrast with higher fex priority than regional passenger trains priority. the second best adi is for 1.3 (0.96 min/train), adil is 2.60 min/train, adir is – 0.27 min/train and adif is – 0.38 min/train. according to previous results the total result for best possible timetable stability is to set the fex importance value for chosen timetable to 1.3. and it corresponds the rules of prescription cria d1 [5], where the fex trains have importance between stop and speed-up passenger trains (1.0 – 1.5). 4. conclusions it was checked in detail the relation between the importance of fex trains and timetable stability. there were made 16 simulation variants in simulation program simut and then compared results. the research result is clear – for timetable 2016 in the overlapping section rfc 7 and rfc 9 kolín – choceň is necessary for best possible timetable stability to set the fex importance value (priority) to 1.3, it means between stop passenger trains priority and speed-up passenger trains priority. it corresponds in the czech republic 72 vol. 11/2017 the detailed priority of international freight expresses the contemporary prescription statement – it suffices only to keep it. acknowledgements this paper is supported by sgs. references [1] p.šrámek, t.molková. the priority of international freight expresses in the overlapping section of rfc 7 and rfc 9 kolín – česká třebová. studentská vědecká konference interoperabilita v železniční dopravě iricon 2016 – sborník, 2016. isbn 978-80-01-05939-5. [2] r.čech, m.šlachtová. železniční nákladní koridory rfc. železniční magazín czech raildays, m-presse plus, s. r. o., 2015. issn 1212-1851. [3] rfc 7 orient corridor, implementation plan of rail freight corridor 7 orient corridor [online], 2015. [cit. 2015-20-11]. [4] rfc 9 cs corridor, implementation plan of rail freight corridor 9 [online], 2013. [cit. 2015-23-11]. [5] sždc. d1 dopravní a návěstní předpis. sždc, s. o., 2013. 73 acta polytechnica ctu proceedings 11:70–73, 2017 1 introduction 2 materials and methods 2.1 rail freight corridors (rfc) 2.1.1 rfc 7 2.1.2 frc 9 2.2 the simulation method 3 results and discussion 4 conclusions acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2017.12.0013 acta polytechnica ctu proceedings 12:13–23, 2017 © czech technical university in prague, 2017 available online at http://ojs.cvut.cz/ojs/index.php/app affective computing and augmented reality for car driving simulators dragoş datcua,∗, leon rothkrantzb,c a twnkls, groothandelsgebouw, stationsplein 45, rotterdam, the netherlands b intelligent interaction, delft university of technology, mekelweg 4, delft, the netherlands c faculty of transportation sciences, czech technical university in prague, konviktská 20, prague 1, czech republic ∗ corresponding author: email@dragosdatcu.eu abstract. car simulators are essential for training and for analyzing the behavior, the responses and the performance of the driver. augmented reality (ar) is the technology that enables virtual images to be overlaid on views of the real world. affective computing (ac) is the technology that helps reading emotions by means of computer systems [1][2][3], by analyzing body gestures, facial expressions, speech and physiological signals. the key aspect of the research relies on investigating novel interfaces that help building situational awareness and emotional awareness, to enable affect-driven remote collaboration in ar for car driving simulators. the problem addressed relates to the question about how to build situational awareness (using ar technology) and emotional awareness (by ac technology), and how to integrate these two distinct technologies [4], into a unique affective framework for training, in a car driving simulator. keywords: affective computing, augmented reality, serious gaming, car driving simulator. 1. introduction due to its capability to improve perception of reality, to support collaboration, a visual display of virtual objects, and to enable transitions between real and virtual environments, augmented reality (ar) technology can be used to create novel interfaces for face-to-face and remote collaboration for training [5][6]. affective computing-based technologies for car driving simulators have capability to lower the cost of accessing the expertise (by reducing a need to move trainers to where their expertise is needed), and to increase availability of expertise. this applies primarily to multi station driving simulators. ar technology can support new types of visualization and can help to develop new learning experience for the trainee. the whole driving simulation can be rendered using ar, using ar glasses, screens or windshield projections. by using augmented reality, the trainee can be presented various stimuli taking the form of virtual representations. in addition, visual notifications can be presented, as generated automatically by the system or as specifically instructed by expert. in addition to augmented reality, reading driver’s affect by affective computing technology has capability to adapt the simulation given the affect state of the trainee. 1.1. augmented reality augmented reality [7] is a technology that enables virtual images to be overlaid on views of the real world. due to its capability to improve the perception of reality, to support teamwork, visual display of virtual objects, and to enable transitions between real and virtual environments, the augmented reality can be used to create novel interfaces for face-to-face and remote collaboration. for example, ar enables users to see virtual representations of remote people in front of them and have spatial interactions with them, as if being there in person. wearable computers and cameras can be combined with augmented reality information display to support remote collaboration and to significantly improve performance on physical tasks. however, new research on interaction paradigms, presence and situational awareness needs to be conducted to create an augmented reality system that naturally enhances collaboration and establishes virtual co-location. in this case, situational awareness is defined as perception of given situation, its comprehension and prediction of its future state. 1.2. affective computing affective computing (ac) is a technology that "relates to, arises from, or deliberately influences emotions" [8]. emotions guide cognition to enable adaptive responses to the environment, and can have a major impact on the perception, attention, memory and decisionmaking. also, affect can have a significant impact on the driving behavior [9]. the affect reading by computer systems [1][2][10][3] is realized through the analysis of body gestures, facial expressions, speech and physiological signals. 13 http://dx.doi.org/10.14311/app.2017.12.0013 http://ojs.cvut.cz/ojs/index.php/app dragoş datcu, leon rothkrantz acta polytechnica ctu proceedings figure 1. ar trainee using the car driving simulator figure 2. training session including one trainer and two trainees. 1.3. hybrid ar-ac for car driving simulations the research aims to find solutions to specific challenges regarding integration of the affective computing technology with the augmented reality technology, with application in car driving simulations. the motivation of this approach comes from lack of understanding on emotional awareness models in arbased interactions between trainees and trainers, in a computer supported collaborative setup. previous studies [4], [11] indicate advantages of combined ar/vr and ac for different scenarios. the research aims to address the following questions: • what are the best means for sensing and collecting affect data from the car drivers engaged in driving training sessions? three sensing hardware will be investigated first, namely e-health sensor [12], cortrium [13] and empatica e4 wristband [14]. multimodal approaches for affect recognition will be studied. • which models and (contact and non-contact) sensors are best suited for affect reading in car driving simulators? • how can existent models be adapted in a data-fusion approach?, considering: . physiological sensors (i.e. heart rate, galvanic skin response, respiration rate, temperature, etc.), . facial expressions (from environmental cameras), with occlusion of the eye region by ar headset, . emotion in speech (prosody). • how to build emotional awareness among trainees and the expert/the trainer? how to integrate and maintain emotional awareness? • how to integrate and maintain emotional awareness and operation-specific situational awareness? • what performance oriented model can be built to automatically adapt the ar system support (such as user interface, etc.) for a human-to-human interaction in virtual co-location, given the emotional awareness? • in which way can (free-hands) hci for ar/vr headsets benefit from emotional awareness, targeting the individual performance during collaboration? • how to adapt existent interaction models? • which is the appropriate affect multimodal framework to support remote collaboration in ar/vr? • how to adapt shared memory spaces to support affect-driven virtual environments? • how to store, transfer and represent audio-video data and affect data, in a multi-user secure environment, with robustness to network breakdowns? • what data-fusion models are to integrate multiple (marker-less) tracking (slam) independent systems for ar? • what driving performance model can be built to automatically adapt the ar system support (such as the user interface, etc.) given the emotional awareness? using this, for instance the ar-based simulation can be dynamically adapted according to the trainee’s affect state, so that to increase drive learning performance. • how to improve the (marker-less) tracking in the shared virtual environment to cope with rather high operational pace of security teams, and with large illumination variation? . rgb-d tracking models section 2 presents a previous research on remote collaboration by augmented reality. section 3 presents the model that integrates affective computing technology and an augmented reality technology. section 4 details on aware, an affect-driven collaboration framework to support car driving simulations in augmented reality. section 5 presents hardware and software components for affect reading during car driving simulation. the last section emphasizes on conclusions and future work for the research on combined augmented reality and affective computing technologies. 14 vol. 12/2017 affective computing and augmented reality for car driving simulators 2. related work there have been car driving simulator-related approaches for rule-based systems that predicts in real-time the driver’s intent [15] or for driver vigilance monitoring [16][17] applied to enhance safe driving experience. moeslund et al. [18] propose arthur, an ar meeting system that permits multiple users wearing hmds at a round table, to interact with objects specific to architecture and an urban planning domain. the interaction with augmented world is in two ways, using physical objects placeholder objects and a wand, and by hand gestures. dong et al. [19] propose arvita, an advanced collaborative ar tool with problem solving capabilities to be applied in classroom and in professional practice. in these scenarios, multiple users wearing hmds and sitting around a table are able to perform interaction and to visualize dynamic simulations of engineering processes overlaid on the surface of the table. the work of wang and dunston [17] advances two ar based systems for remote collaboration and a face-to-face co-located collaboration in the scenario of detecting design errors. jailly et al. [20] presents an ar system for enhancing the comprehension of the manipulated remote devices in distance learning domain that allow for communication between both students and teachers. ferrise et al. [21] tackle the domain of maintenance operations of industrial product. in addition to using vr technology to support an operator to learn performing maintenance operations by combining traditional instruction manuals with simulation, the ar technology is employed to extend the scenario to tele-assistance. a vr-based skilled operator guides from the distance a trainee that is equipped with ar technology already displaying instructions on top of real product. nilsson et al. [22] propose an ar tool to improve collaboration between actors from different organizations such as the rescue services, the police and military personnel in a crisis management scenario while the same time sustaining individual needs. yabuki et al. [23] present a system in the early phase of development, aiming at supporting the cooperation between people working outdoor on environmental issues. the information provided to the users wearing hmds relates to 3d representations of temperature distribution and wind distribution, velocity and direction. alem et al. [24] propose remote, a remote guiding system that integrates non-mediated hand gesture communication in the mining industry. the work scenario of the system implies the expert remotely assisting a worker using the hands to point to certain locations and to show specific manual procedures. testing and validation of four early user interface design iterations aimed at maintenance tasks for repairing a photocopy machine, removing a card from a computer mother board and assembling a lego toy. wichert [25] describes a mobile collaborative ar environment that uses web technologies. the collaborative environment allows a 3d game like tetris to be played in real time by several users wearing hmds. the players can be located in the same room, with possibility for extending the collaboration with a remote player. the game setup provides support for studying the two types of ar based collaboration: the co-located cooperative interaction with skilled workers, each having a different view of the ar world and the indirect interaction with remote expert that has the same view as the skilled worker. in a similar way, datcu et al. [4][5] propose an ar based scenario of playing a game collaboratively, to study complex problem solving by physically co-located and virtually co-located participants. within the game, the goal of jointly building a tower of coloured blocks represents an approximation of a shared task. individual expertise is modelled as possibility to move blocks of a distinct colour and shared expertise is modelled by possibility of all players to move blocks of same colour. by scaling down real-life, more complex problems, the study compares presence, workload and situational awareness in real world and ar collaboration scenarios. additionally, datcu et al. [6][7] developed a platform for tele-collaboration by ar for supporting teams in the security domain. schnier et al. [26] focus on studying the issues around establishing joint attention toward the same object or referent, in a physically co-located collaboration ar environment. gu et al. [27] conduct a study on the impact of 3d virtual representations and use of tangible user interfaces as support for synchronous design collaboration using the ar technology. the results indicate that the change from a physically co-located working environment to the virtual co-located scenario encourages the ar users to smoothly move between working on the same tasks and working on different tasks or different aspects of the design process. the current state of the art on collaboration in ar provides relevant examples for ar based models that support a synchronous collaboration among users either being physically or virtually co-located, using free-hands or tangible interaction, static or mobile, or either using hmds or other display devices. these research outcomes, however, have to be still investigated in car driving simulation domain, especially with regard to remote visualization, spatial interaction and remote authoring in training scenarios. 3. model augmented reality systems are not limited to use of head-mounted devices and mainly have to combine real and virtual objects be interactive in real-time 15 dragoş datcu, leon rothkrantz acta polytechnica ctu proceedings and to register objects within the 3d. due to the capability: • to improve perception of reality, • to support teamwork process, • to support manual annotation by virtual objects, • to support an interaction between virtual and augmented environments, the technical solutions based on augmented reality have the potential to enhance novel interfaces for a computer-aided collaborative process, in face-to-face and remote collaboration scenarios. augmented reality systems can be used to establish the experience of being practically co-located by means of simulated presence. augmented reality systems have been used to allow experts to spatially collaborate with others at any other place in the world without traveling and thereby creating the experience of being virtually co-located, e.g. in the field of a crime scene investigation [6],[28]. the affective computing technology makes use of measurements from (contact and non-contact) physiological body sensors to automatically recognize in real-time, the affect of the field personnel. the affect data is further on used to automatically: • improve interaction in augmented reality (for the field personnel), • increase immersion and situational awareness for spatially distributed users in the virtual reality [29] and augmented reality [30]. from the hardware point of view, a solution is to use a microsoft hololens ar head mounted device (hmd) (see figure 3) that has a depth sensor already integrated. figure 3. standalone hololens ar device [31]. the investigation first considers a closed-loop arac model for the virtual reality, proposed by wu et al. [11]. next, the closed-loop model is adapted for augmented reality. 3.1. closed-loop architecture according to the yerkes-dodson law, the performance in mental tasks is dependent on arousal in the form of a non-monotonic function (figure 4). the performance increases with arousal, given arousal is at low levels, reaches the peak at a given arousal level and decreases after that optimal level. figure 4. yerkes-dodson-law the closed-loop system consists of three components namely affect recognition component, affect-modeling component and affect control component. these components are displayed in figure 5. the affective computer-aided collaboration approach based on augmented reality for car driving simulators, consists of three components namely: • the affect recognition component, • the affect-modeling component, and • the affect control component. figure 5. closed-loop affective computing architecture for augmented reality. 3.1.1. affect recognition the affect recognition component performs the assessment of the user’s affect by analyzing different body signals such as psychophysiological signals and facial expressions, emotion in speech, etc. ideally, these techniques to sense the trainee’s affect state should be working in real-time, should be automatic, noncontact, non-invasive and should have a high accuracy. multi-modal approaches aggregate data from different types of sensors to decrease the error over estimated affect states. 16 vol. 12/2017 affective computing and augmented reality for car driving simulators 3.1.2. affect modeling the second component, the affect modeling creates a relationship between the trainee’s affect and the features of the user’s environment. this component determines how the trainee’s affect should be changed, given her/his profile, the known arousal level for the optimal learning performance. this mapping is further used to identify car driving simulation parameters including the ar settings and the scenario and training session stimuli. 3.1.3. affect control the third component, the affect control provides the means for adapting the environment in such a way to get the trainee to the target affect state. this component can be semi-automatic, especially in scenarios including a human expert, such as in car driving simulation when the trainer can change the course of the training session with new stimuli that fit better the learning procedure. given the car driving simulation parameters previously determined, this component applies selected changes. the aim is by applying these changes, the trainee’s affect is changed towards the arousal state that is associated with optimal learning performance. in order to optimize the car driving performance, the three components of sensing, modeling and control adjust the functionality of the car driving simulator according to the trainee’s affect state. 4. affective framework for augmented reality virtual co-location relies on the augmented reality technology [32][33] to create spaces in which the trainer, the trainees and the objects are either virtually or physically present: it allows people to engage in spatial remote collaboration. an affect-driven collaboration framework supports the virtual co-location technique for the trainee and for the trainer, the collection and processing (especially the data fusion) from the low/sensory level to the high semantic levels. in the following text, the trainee is referred to as the local user and the trainer is referred to as the remote expert. the framework is called affective framework for augmented reality aware. the aware framework has been developed with the goal to support computational demands and multi-modal data streams among running applications and data processing modules. aware is a highly scalable, modular and parallel environment for a distributed collaboration using ar (the diagram in figure 6). it is developed to support virtual co-location of multiple users playing different roles in well-defined scenarios. the following text describes aware in detail. figure 6. aware framework for adaptive ar training. 4.1. local and remote user applications aware is based on a centralized architecture. it contains different applications for local and remote users. the applications have different user interfaces which are created using the unity game engine. the application for the local user is adapted for hmds (standalone hololens optical see-through hmd in figure 3). the application for the remote user runs on a desktop computer or laptop. thus, the remote user gets a screen-based visualization and can interact with the system by using the keyboard and a standard mouse device. 4.2. directions by the trainer aware supports the collaboration process by providing the trainer (remote user) with tools to give directions in form of spatial annotations. the annotations appear in the view of the trainee (local user). the remote user can augment the view of the local user with the following elements: • 3d-aligned objects (arrow/cube/sphere), • 2d-screen aligned content (text, photo and video), • dialog boxes for introducing text for the graphical objects and also for colours of some visual elements, • screen aligned counter and text, • screen aligned (flashing) panic button in the form of a text with the frame, • screen aligned image and colour border (to show info about a person), and 17 dragoş datcu, leon rothkrantz acta polytechnica ctu proceedings • virtual stickers (indicating scanning area at the crime scene). all annotations are encoded as distinct data messages and events that pass through shared memory system. updates are automatically sent to each software module or application (such as the software application of the local user or the software application of the remote user) by using a notification and a push system of events and data. 4.3. consistency of actions the consistency of actions is a critical aspect for establishing a virtual co-location. for that purpose, local updates are processed only when data and events are received as a feedback from the aware shared memory space module. consequently, the application for the trainer (remote user) does not execute user input updates in the graphical user interface immediately. such updates are applied only after the data and events of the user input are available in the shared memory space. the flow of data and event notifications are illustrated in the diagram in figure 7. figure 7. diagram of data and event notifications for user actions between aware modules and user applications. 4.4. data and event notifications through aware the data communication is established via the shared memory mechanism of aware. the shared memory mechanism handles parallel connections from different users located in different physical environments, and allows data sharing across different types of devices, including mobile ar hmd systems. in aware, the network communication is implemented using both tcp/ip standard for the data transfers between the server and software modules running on hardware devices linked via network cables, and udp standard for data transfers between software modules connected via wireless links. in case of the udp-based network communication, each frame from video sequence is encoded as a compressed image (using vga resolution and jpeg encoding) into a udp packet. 4.5. shared visualization virtual co-location is enabled by sharing a video channel from the hmd of the trainee (local user) with the trainer (remote user). the shared video stream, thereby, forms basis for the shared work environment. the shared memory mechanism provides functionality for synchronizing the video frames with the updates from the hmd camera tracking and with the manual annotations made by the remote user. via a graphical user interface, the remote user can modify the ar content, which is visible on the screen. the ar system has also capability to record, store and display a photo and video content. the aware framework provides functionality to store data remotely, not only locally. such data are persistent as long as the aware server is up and running. especially for videos, this mechanism ensures synchronization of the display: at a time moment, the same video frame is played for both the local and the remote user. the photos and videos are stored in separate files using standard formats, such as .png, .jpg and .avi. the transmission of video streams makes use of the motion jpeg mjpeg video compression format. each frame of the video sequence is compressed and transferred as a jpeg image. the automatic process of event and data notification takes place only for the software modules and applications that register for the specific type of data. in this way, for instance the application for the local user does not have to receive, via the network, video from its own camera (it has a direct access to this video). 4.6. communication decoupling aware achieves decoupling of communication by supporting both local user’s and remote user’s applications to access updates through a shared memory space. this functionality is tightly coupled to the virtual co-location paradigm that enables multiple users at different physical locations to work collaboratively according to their roles in a well-defined scenario. collaboration at a distance is possible as long as both users have a network connectivity to the aware server. 4.7. local user tracking in the physical environment the local user’s motion in the physical environment is automatically tracked in an augmented reality by using a slam system. a slam system (simultaneous localization and mapping) generates and updates a map of an unknown physical environment while simultaneously keeping track of a user’s location within it. 18 vol. 12/2017 affective computing and augmented reality for car driving simulators aware can run different slam systems. one of them is rdslam [34], a real-time marker-less monovision slam more suited for indoor environments developed at zhejiang university in hangzhou, china. another slam is lsd-slam [35], a real-time marker-less slam running on semi-dense depth maps. a third slam system is orb-slam2 [36], a real-time system for monocular, stereo and rgb-d cameras that computes camera trajectory and a sparse 3d reconstruction. the tracking module is integrated as a module of the aware framework and can run on computer of the trainer (local user) or on a separate hardware system. in addition to tracking the hmd position and orientation, slam performs a mapping of the physical environment (of the trainer) and generates an internal 3d representation of the physical world. the physical world is represented in terms of data structures related to key points discovered during the tracking, and to recordings of a camera position and orientation with respect to the key-points. such data are mainly used internally by the slam during the tracking process. the tracking procedure identifies a set of key points (as natural visual features) from each frame of the video sequence. estimation of the camera parameters (location and orientation) keeps track of all the key points during the time. this means newly discovered key points are stored in the system memory and already-known key-points are tracked when detected. as a part of the aware functionalities, the tracking module stores all detected key-points in the current frame of the digital video sequence, the camera location and orientation data in the shared memory space. once these data are stored on the shared memory space, all the system modules are notified on the updates so that the data can be further read, if necessary. this mechanism is illustrated in figure 7. the result of a markerless tracking provides the hmd camera location and orientation while the mapping result provides a representation of the physical world in form of a sparse cloud of 3d points. 4.8. selection and positioning of 3d virtual objects the sparse cloud of 3d points represents visual key-points, which connect the augmented world to the physical world and further act as virtual anchors supporting annotation by ar markers. the remote user can attach a virtual 3d object by using the user interface. after choosing a desired type of augmentation (by selecting a specific icon) from the bar of icons on the top-left side of the user interface, the remote user proceeds by directly mouse-clicking on the screen showing the current frame of the video sequence. the mouse click event associates the 2d coordinates of the mouse cursor on the laptop screen. in order to attach the selected 3d virtual object, the 2d mouse cursor location has to be converted to 3d coordinates, related to the reference system of the slam tracking. this conversion is done by requesting the slam module via the shared memory space implemented in aware, to map the 2d coordinate in the current video frame to the 3d coordinate. the mapping implies searching for the closest key point from the equivalent 3d location of the 2d point of the mouse click event generated through the user interface of the remote expert. once computed, the closest 3d point in the coordinate reference system of slam is sent back to the expert system application via the shared memory space. this is illustrated in the diagram in figure 8. figure 8. diagram of data and event notification for user actions within aware modules and applications. once attached to key point, a 3d-aligned virtual object is correctly rendered in the next video frames on the user interfaces for both local and remote conditions during run-time, the generation of graphical content being consistent with the hmd camera motion (figure 4, the upper part of the diagram). 5. setup 5.1. hardware the affect adaptive aware framework for training supports hardware equipment for the augmented reality and physiological sensors. such equipment includes microsoft hololens headset (figure 3), and physiological sensors such as: physiological ehealth kit (figure 9), heart rate detector cortrium (figure 10) and empatica e4 (figure 11). the e-health sensor shield v2.0 [12] (figure 9) can perform a real-time body monitoring for biometric and medical applications by using 10 different sensors including: pulse, oxygen in blood (spo2), airflow (breathing), body temperature, elec19 dragoş datcu, leon rothkrantz acta polytechnica ctu proceedings trocardiogram (ecg), glucometer, galvanic skin response (gsr sweating), blood pressure (sphygmomanometer), patient position (accelerometer) and muscle/eletromyography sensor (emg). figure 9. ehealth platform [12]. cortrium [13] (figure 10) can assess body surface temperature, activity, and respiration rate and it also contains a high-performance three-channel ecg for screening and diagnostics of cardiological diseases. figure 10. cortrium heart rate monitor [13]. empatica e4 [14] (figure 11) is a real-time wristband monitor of the physiological signals. the device incorporates a photoplethysmography sensor to measure blood volume pulse (providing estimations on the heart rate, heart rate variability and other cardiovascular features). in addition, it has a 3-axis accelerometer, and an infrared thermopile that reads peripheral skin temperature. an built-in electrodermal activity sensor measures the sympathetic nervous system arousal and derives features related to stress, engagement, and excitement. 5.2. affect recognition 5.2.1. non-contact heart rate detection non-contact analysis of physiological indicators has a major impact on several application domains making use of live monitoring of the people’s faces, in realistic scenarios involving working, playing and resting. photoplethysmography represents a noncontact, noninvasive and low-cost method that makes use of variations of transmitted or reflected light to determine figure 11. empatica e4 wristband physiological signal monitoring [14]. cardiovascular blood volume. considering normal ambient light as illumination source, allows for using video cameras and even regular webcams as heart rate detection sensors. according to the previous study [37], specific face skin areas provide enough information to estimate the heart rate, by using computer vision techniques only, including the viola&jones object detector and active appearance models. the results of such a non-contact face analysis method for the heart rate detection are depicted in figure 12. the tests have proved that the whole face area does not necessarily provide optimal region of interest for the pulse detection, the best being the cheek and forehead regions. the findings can be used to optimize the scanning of the face region which in turn leads to shorter computation times and more accurate results for the heart rate detection. in addition, lower face features can be used to sample color information for the analysis while the trainee in the car driving simulator wears an augmented reality hmd such as hololens. 5.2.2. bi-modal emotion recognition the assessment of emotional levels from speech can be naturally done by identifying patterns in the audio data and by using them in a classfication setup. the features we extract are the energy component and 12 mel-frequency cepstral coeffcients together with their delta and the acceleration terms. hmms models makes use of gaussian mixtures with different number of components. the evaluation indicates the most effcient hmm model makes use of 4 states and 40 gaussians per mixture. the accuracy of this hmmbased classifier is 55.90% [38] (table 1). making facial expression recognizers with hidden markov models and local binary pattern features (lbps) implies the identification of the optimal model parameters. finding the best number of states, the best number of gaussian mixture components and the best set of local binary patterns, is not a trivial task. we start from the results of the adaboost.m2 classifiers. for evaluation of the facial expression recognition, we have generated hmm models for each 20 vol. 12/2017 affective computing and augmented reality for car driving simulators figure 12. box plots of heart rate median errors together with the 25th and 75th percentiles, for each face region. emotion category. the best facial expression recognition model uses 268 distinct features that correspond to a selection of 45 features from each facial expression category. the accuracy of this classifier is 37.71% [38] (table 2). the recognition of emotions based on a decision level fusion implies the combination of final classification results obtained by each modality separately. for this, we take into consideration four sets of unimodal classification results namely from the speech-oriented analysis and from the separate lbp-oriented analysis which use visual features from the whole face image from specific face regions. we use these sets together with a weight function that allows for setting different importance levels for each set of unimodal results. this weight-based semantic fusion models asynchronously the emotion in visual and auditory channels. the best model obtained in this way has the accuracy of 56.27% [38]. 6. conclusions this is the first research to study a role of the affect in an augmented reality for the car driving simulation. the augmented reality technology for a remote collaboration by virtual co-location, is used to support the trainer and the trainee during the car driving training session. in order to improve learning performance during training, the affective computing technology is used to sense the trainee’s affect state and to further update semi-automatically car driving simulator parameters. the affect recognition consists of a multimodal technique that processes inputs from contactbased physiological sensors, from facial expressions readings and from the emotion in speech. the way of using the affective computing technology to further control 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[38] d. datcu. multimodal recognition of emotions. phd. thesis. delft. delft university of technology, 2009. 23 http://dx.doi.org/10.1186/s13388-015-0025-9 http://dx.doi.org/10.1007/s10606-013-9190-x http://dx.doi.org/10.1145/2702613.2732839 http://dx.doi.org/10.1109/38.963459 http://dx.doi.org/10.1162/pres.1997.6.4.355 http://dx.doi.org/10.1109/ismar.2013.6671781 http://dx.doi.org/10.1007/978-3-319-10605-2_54 http://dx.doi.org/10.1109/tro.2015.2463671 http://dx.doi.org/10.1145/2516775.2516805 acta polytechnica ctu proceedings 12:13–23, 2017 1 introduction 1.1 augmented reality 1.2 affective computing 1.3 hybrid ar-ac for car driving simulations 2 related work 3 model 3.1 closed-loop architecture 3.1.1 affect recognition 3.1.2 affect modeling 3.1.3 affect control 4 affective framework for augmented reality 4.1 local and remote user applications 4.2 directions by the trainer 4.3 consistency of actions 4.4 data and event notifications through aware 4.5 shared visualization 4.6 communication decoupling 4.7 local user tracking in the physical environment 4.8 selection and positioning of 3d virtual objects 5 setup 5.1 hardware 5.2 affect recognition 5.2.1 non-contact heart rate detection 5.2.2 bi-modal emotion recognition 6 conclusions references 261 acta polytechnica ctu proceedings 2(1): 261–263, 2015 261 doi: 10.14311/app.2015.02.0261 properties of recurrent nova t pyxidis based on 2011 outburst k. tanabe1 1faculty of bioshere-geoshere science,okayama university of science,okayama 700-0005 japan corresponding author: tanabe@big.ous.ac.jp abstract we reexamine the properties of the recurrent nova t pyxidis based on our own spectroscopic data accompanying with the photometric ones by vsolj (variable star observers league in japan) during 2011 outburst. one of the purpose of this paper is whether a missing outburst could be happen around 1988-1989. comparing the 2011 outburst data with previous ones, we may conclude that any essential difference can not be found. accordingly it is difficult to deny a small possibility of a ”missing” outburst from 1988 to 1989, taking into account the seasonal gap in its observation for northern hemisphere observers . the problem whether im normae belongs to be a member of t pyx subclass or not is to be postponed by its next outburst taking into account of t pyx’s peculiar spectral behavior. keywords: cataclysmic variables recurrnt novae t pyxidis optical spectroscopy photometry. 1 introduction recurrent novae (rne hereafter), whose definite total number is just 10 (see warner (2008) for example), are divided into three distinct subclass, namely t pyx subclass, u sco subclass and t crb subclass. among these three, t pyx subclass has only one member, t pyxdis itself and has a quite unique property of very short orbital period (1.8 h). t pyxidis is the only galactic nova belonging to the southern constellation pyxis. outburst of this object was discovered by h. leavitt in 1902 photographically. this rn has experienced 5 outbursts (1902, 1920, 1944, 1966) including 1890 detected on the harvard archival plate, before 2011 outburst. while detailed photometric observations were performed before 2011 outburst (see schaefer (2010)), spectroscopic observations performed from early time were rather fragmentary. moreover this rn accompanies nebulosity. the 2011 outburst of the rn t pyxidis was detected after 45 year of absence (twice as long as recurrence period) by linnolt on april 14.2931(ut), 2011(waagen(2011)). im mediately after discovery we started low-resolution spectroscopy and obtained detailed feature of this rn before maximum light. the obtained results is reported in the paper by imamura and tanabe (2012). on the other hand the results of the photometric observations are based on the results performed by the members of vsolj (variable star observers league in japan). in section 2, we summarize the spectroscopic observational results by imamura and tanabe (the present author), including both the basic properties of t pyx obtained before 2011 outburst and the essentials of socalled tololo classification scheme. in section 3, we try to compare the photometric observations by vsolj during 2011 outburst with the template light curve by schaefer (2010) based on its previous outbursts. then we discuss a possibility of ”missing outburst” to be expected in 1988-1989 by comparing 2011 outburst with the previous ones by using the schaefer’s template light curve (schaefer (2010)). also we mention about the problem whether im normae can be another member of t pyx subclass. at the end of this section we summarize our results. 2 summary of low-resolution optical spectroscopy 2.1 basic properties of t pyxidis based on pre-2011 outburst as it is mentioned above, t pyx is a quite unique rn for its very short orbital period (1.83 hour) compared with other rne. photometric behavior is characterized by slow rise and slower decline. its position is (α,δ)=(9h04m41.47,-32deg22.0min) and (λ,β)=(150deg,-46deg). this latter ecliptic coordinate makes its seasonal gap (almost half a year) the for northern hemisphere observers. the observed spread of magnitude is b=6.4-15.4. in addition the inclination is about 10 degree (for example, utas et al (2010)). the distance had not been known; recent value by sokoloski(2013) is 4.8±0.5kpc. 261 http://dx.doi.org/10.14311/app.2015.02.0261 k. tanabe 2.2 spectral classification of novae modern and phenomenological spectral classification of classical novae(we denote cne hereafter) is established by williams (so-called tololo classification system). according to this system, spectra of cne are divided into two classes, namely fe ii class and he/n class. the former corresponds to slow evolution novae and the latter to fast ones. as this phenomenological scheme is thought to be a reflection of the physical stage of nova ejecta, it seems to be applicable not only to cne but also to rne. 2.3 spectral evolution of t pyxidis in its early stage of 2011 outburst we started our low-resolution (r∼400) spectroscopy, using dss-7 spectrograph with st-402 ccd camera (both of them of sbig (santa barbara instrumental group)) production attached to a 28 cm schmidt cassegrain telescope (celestron production) at tanabe’s personal observatory(tpo) located in the north west edge of okayama city in japan (many photometric data obtained at tpo on dwarf novae are published by vsnet collaboration team). this site is quite suitable for astronomical observation because of its fine weather and good seeing. the okayama astrophysical observatory (oao) of national observatory in japan (naoj) is located at 40 km west from tpo. figure 1: spectral evolution of t pyxidis. upper panel denotes earlier stage,which shows he/n feature. on the contrary lower panel is later stage that shows typical feii spectral phase. the date after maximum light (minus means before maximum) are denoted on the right shoulder on each spectrum. spectral observations started at two days after discovery (april 16th and ended at 14th of may due to its position close to the sun. total observation are 13 nights, among which 9 nights are before the maximum light. details of the results are reported in imamura and tanabe (2011). here we only summarize the peculiarity about the pre-maximum behavior that the spectral evolution is not only hybrid but opposite direction from he/n phase to fe ii one. this feature is quite exotic (see figure 1). according to the obtained p cygniprofle, the expansion velocity turns from decreasing to increasing. such a transition seems to cause this extraordinary evolution from he/n to fe ii phase. the relation between this evolutional feature and the temporal change of physical state as a binary system will be discussed in a separate paper. 3 discussion 3.1 ”missing” outburst? as is shown in figure 2, the photometric behavior of t pyx during its 2011 outburst seems to be essentially the same as the template curve combined by previous outbursts (schaefer (2010)). taking into account that 45 year interval is twice as averaged interval from those 5 outburst records, we feel temptation to find out evidence of outburst around 1988-1989. in addition we can find no incompatible feature on our spectra with past fragmentary spectral data. hence it is plausible that the structure of the t pyx system does not change in binary structure after 45 years absence from outburst. therefore it is worth seeking the possibility of ”missing” outburst around 1988-1899. we tried to search the photometric data in vsolj database and found several records from the end of 1988 to 1989 with visual magnitude of 13 mag. however, unfortunately, these records were by a single observer and no distinct light variation could be seen. so this cannot be a strong evidence for missing outburst. figure 2: light curve obtained by vsolj members during 2011 outburst. smooth curve denotes a template light curve by schaefer (2010) based on previous photometric (b magnitude) data. we can see coincidence between two and conclude that no essential difference exist. 262 properties of recurrent nova t pyxidis based on 2011 outburst 3.2 is im normae a real member of t pyx subclass? recurrent nova im normae is a possible member of t pyx subclass from the point of view of light curve and its orbital period (schaefer (2010)). however taking into account of the spectroscopic properties based on 2011 outburst, spectral evolution of t pyx subclass is to posses such a property as the inverse change of normal transition from fe ii phase to he/n one. to confirm that im nor really belongs to t pyx subclass, it is necessary for us to wait the next outburst (some 80 years) for obtaining a new data on its spectral evolution. 3.3 conclusions 1) during the early stage of outburst, t pyx shows the hybrid spectral transition from he/n phase to fe ii. 2) light curve obtained by vsolj which seems to be identical to the schaefer’s template curve suggests the missing 1988-1989 outburst. but no strong evidence exists during these years. 3) without spectroscopic data, the problem whether im normae really a member of t pyx subclass is not settled. acknowledgement the author thanks vsolj members (leader seiichiro kiyota) for permitting us to make use of photometric data. minako ogi, my graduate student, helped me for her assistance of making a figure 2. the author also thanks naoko tanabe for her preparation of the instruments during our spectroscopic observations. references [1] leavitt,h.1913,harvard college obs.cir.179,1 [2] imamura,k and tanabe,k.: 2012 pasj 64,l9 [3] schaefer,b. : 2010, apjs, 187, 275 doi:10.1088/0067-0049/187/2/275 [4] utas et al. 2010,mnras, 409,237 doi:10.1111/j.1365-2966.2010.17046.x [5] sokoloski et al. 2013,apjl 770,l33 doi:10.1088/2041-8205/770/2/l33 [6] waagen,e.et al. 2011,cent.bur.electron.telegram,2700 [7] warner, b. 1995, in cataclysmic variable stars (new york: cambridge university press) doi:10.1017/cbo9780511586491 [8] warner, b. 2008, in classical novae, ed. bode, m. f. & evans, a. (new york: cambridge university press) [9] williams, r. e. 1992, aj.104, 725 [10] williams, r. e. 2012,aj 144, 98 discussion ashley pagnotta: there was no missed eruption in the year of 1980s.too many observers were working.brad schaefer has worked at all possibble archival amateuer’s data looking for trails hasn’t forward anything. kenji tanabe: i asked recently the observer t.kato for his 1988-1989 vsolj data whether the value of 13 magnitudes data indicating this rn’s high state is reliable or not.his answer was of little confidence. he said that the telescope used was 20cm and visual observation at his early day of vs observations. at first i thought this was performed using 60cm telescope of kyoto university. 263 http://dx.doi.org/10.1088/0067-0049/187/2/275 http://dx.doi.org/10.1111/j.1365-2966.2010.17046.x http://dx.doi.org/10.1088/2041-8205/770/2/l33 http://dx.doi.org/10.1017/cbo9780511586491 introduction summary of low-resolution optical spectroscopy basic properties of t pyxidis based on pre-2011 outburst spectral classification of novae spectral evolution of t pyxidis in its early stage of 2011 outburst discussion "missing" outburst? is im normae a real member of t pyx subclass? conclusions acta polytechnica ctu proceedings doi:10.14311/app.2017.11.0016 acta polytechnica ctu proceedings 11:16–21, 2017 © czech technical university in prague, 2017 available online at http://ojs.cvut.cz/ojs/index.php/app railtopomodel and railml 3 in overall context adam hlubuček ctu in prague, faculty of transportation sciences, konviktská 20, prague, czech republic correspondence: hlubuada@fd.cvut.cz abstract. this paper aims to provide a brief insight into the uic railtopomodel and railml initiative. concerning the railml exchange format, mainly the current form of forthcoming third edition, involving especially the infrastructure schema, based on the railtopomodel modelling principles, is taken into account. when rewarding, the comparison between railml 3 and the previous railml 2.3 version is given. at the end, the author focuses on selected issues of possible applicability of these tools for the needs of such systems as the automatic train control, automatic train operation, prediction of train etc. if essential, some extensions of the structures are proposed. keywords: railtopomodel, railml, railway infrastructure, information systems. 1. introduction railml [1] is an open source exchange format used in the railway branch to ensure a data transfer among different software applications. the railway markup langue, based on xml, was originally designed to enable data exchange in the scope of timetabling. later on, the specifications [2] were extended to the subsystems of rolling stocks and infrastructure. in these days, also the schema of interlocking is being developed in relation to oncoming railml 3 release. another milestone, as regards the railml 3, is a significant reedition of the infrastructure concept. whereas the principles of modelling of the railway topology were rather casually defined within the previous versions, railml 3 is supposed to be strongly founded on the uic railtopomodel [3] which provides the rules how to describe the infrastructure in predefined way, nevertheless, allowing all kinds of user-specific extensions. the railtopomodel initiative was introduced in 2013 [4] in order to create a common standard in railway infrastructure modelling. the generic railway topology model aims to meet multiple needs in the field of railway data exchange, ensured by the means of the railml 3 format. in 2016, the railtopomodel was released in the form of uic international railway standard 30100 [5]. 2. the railtopomodel overview the uic railtopomodel [3] is a topological model of railway infrastructure, based on a „connexity graph“. describing a railway topology, two basic types of elements can be taken into account: the linear elements (e.g. tracks and lines) and nonlinear elements (e.g. operational points). these elements allow describing the infrastructure at different levels of detail, in order to be suitable for different specific purposes. regarding the “connexity graph”, the elements of both these categories are supposed to be modelled as the nodes, whereas the edges in the node-edge diagram represent the physical connections between these particular interconnected net elements. the linear elements shall have their orientation defined. therefore, when concerning the (again orientated) relation between two of them, it should be in both cases apparent, whether the beginning or the end of the net element is taken into account. this generic principle is illustrated in the figure 1 considering the tracks to be the nodes. in order to express the fact, whether a train is able to pass from one net element to another, additional information is necessary to be given. for this reason, the relations between net elements shall be described also by the navigability attribute, besides, concerning the direction of allowable train movement. [5] in this way, a feasible passage through a switch from one track to another can be modelled as well as the line change when passing through an operation control point. currently, the model enables to express the topology including positioning and location of net entities to be projected to the topological layer taking into account different views. there are several predefined levels of detail (macro, meso, micro) at which a railway network can be described. a user may utilize (or start with) any of them, depending to his specific use case. furthermore, he is authorised to create own levels, to be appropriate for his needs (e.g. nano or corridor level). these levels of detail should be vertically interconnectable by the means of aggregation and disaggregation principles. therefore, when passing from one level to another, sometimes an extra transition level is needed. except the different views of the topology, the particular level enables to deal with particular entities in different ways. for instance, it allows displaying a certain entity as a section at a detailed level, as a spot at a less detailed level and not to include it to the least detailed level at all. [5–7] 16 http://dx.doi.org/10.14311/app.2017.11.0016 http://ojs.cvut.cz/ojs/index.php/app vol. 11/2017 railtopomodel and railml 3 in overall context figure 1. an application of the railtopomodel principles on an example of a railway station with five simple turnouts and one single slip modelled by the means of positioned net relations. later on, the model together with railml 3 should be extended so as to enable digital continuity for infrastructure life cycle and operation supporting bim development and using railml 3 for web services as well. it should be able to provide a unique continuous network description, not only taking into account past, current and future states, but also involving the project management aspect of the lifecycle dimension. [6] concerning the objects being modelled, expressing their validities (active / non active), variants (alternative current states) and versions (states evolving over time) should be possible. [4] 3. railtopomodel packages 3.1. base currently, the railtopomodel consists of four packages. concerning the uml class diagram designed to express its structure, there is a base network domain consisting of the base object, network, level network and network resource data classes, defining the general overview of the network being modelled, including the modelling principles used. the classes of the other subsystems are mostly some kind of extensions of the network resource class, eventually of the base object class itself. the base object class specifies the fundamental attributes to be common for all the derived xml elements (in order to express its unique id, name and validity). as regards the id, especially the uuid should be used. [5, 8] 3.2. topology the topology package is especially based on the net elements and relations which are modelled by the means of node-edge diagram. the aggregation principle (of net elements) is managed within the topology domain as well. particular net elements can be grouped to larger units, creating new net elements, which are supposed to be used to express the topology at less detailed level. for instance, several tracks (linear net elements in the scope of micro-topology) can be aggregated in order to form a station area, further regarded to be an operational point (a nonlinear element in the scope of macro-topology). nevertheless, the railtopomodel defines the data classes using the most general approach (not yet specifying the particular elements of infrastructure). [5, 8] 3.3. positioning the positioning package enables to define several positioning systems (linear as well as geometric – both the geographical and these ones used to display particular entities on a monitor) as well as the corresponding coordinates. to a particular net element, several of these positioning systems can be associated, nevertheless every positioning net element involves own intrinsic positioning system, whose coordinates takes values in the range of <0;1> referring to a relative position within the element. [5, 8] in order to be able to express the intrinsic position in absolute terms, the attribute of length related to the oriented net element would have to be specified in addition. 3.4. net entities the last subsystem deals with net entities. the net entities represent these object and characteristics of the infrastructure, which can be connected to the undelaying topology layer using positioning systems. differently from the previous railml versions, enabling only to link these object to a one certain track forming its data substructure, the railtopomodel (as designed for railml 3) allows them to be handled separately without relation to the topology (for instance to describe an asset, which has not been fitted to the infrastructure yet). closer specification of these net entities is provided by the railml 3 infrastructure schema. together with the entities, their location in relation to net elements and positioning systems is managed. an entity location can be a spot location, linear location or area location, while an entity is allowed to have several locations (including both different and the same types). [5, 8] 17 adam hlubuček acta polytechnica ctu proceedings figure 2. illustration of different approaches in locating of entities in the scope of railml 2 and railml 3. 4. railml evolving over time currently, railml 3 is available for testing purposes only. in 2016, the railml 3.0.is4 alpha version was released containing xsd files (defining allowable structure of particular xml documents) especially regarding the railtopomodel schema (introduced with the railml 3 release) and the infrastructure schema. [8] the railml 3.0 releases are designed for possible railml 3 users, who are asked to compare it to their needs and to formulate a “use case” defining what kind of data they intend to exchange. these “use cases” [9] serve as the basis for further development of the railml 3 infrastructure schema. by means of that approach, the creators want to avoid accumulation of such data items which no one needs. the railml 3 infrastructure schema, as regards the 3.0.is4 release, consists of following subsections: the topology, the coordinates, the operation, the geometry, the assets and the infrastructure parameters (while the last one was removed with the 3.0.5 version). [8] 4.1. topology concerning the previous railml versions, [10] topology information is stored within the particular track elements using mutual connectors situated at the track ends and track begins or within switch points. the points defining a switch are allowed to be situated both at the track begin and at a certain distance from the track begin. such approach makes the model ambiguous, as it is not clear how to segment the tracks, not to mention confusing manner in which railml 2 describes the branching tracks of a switch. [7, 10] starting from the third version, the topology is considered separately and based on the abovementioned principles of railtopomodel. concerning the micro level, a connection between tracks is allowed to be situated at the track begin or at the track end only, which admits the values (expressed in relative units) of either 0 or 1. [8] among others, lengths of tracks are not necessary to be given in order to describe “the pure topology” properly. the same approach may be used for any other level of detail. nevertheless, there are still some questions under discussion within the railml community. railml 3 should support precious description of the track geometry. it has not been decided yet, whether the geometry will be considered separately or if it should depend on the topology. [6] 4.2. positioning systems and coordintes in railml 3, the positioning systems are managed in the scope of the coordinates subsection. previous versions are only able to express position from the beginning of a track completed with information of absolute position and geographical coordinates. these are expressed as the attributes or subelements of the elements representing the object being placed. [10] the absolute position should represent traditional mileage, which indicated values are not guaranteed to correspond to real distances. it happens for historical reasons (due to reconstructions of railway lines) and it is caused by different lengths of parallel tracks in horizontal curves and in the rail branching areas. in railml 2, there is a mileage change element enabling to express mileage jumps and overlaps being a part of the track topology assigned to a particular track. within railml 3, the issue is addressed in more complex way, following the railtopomodel principles again. a user is allowed to define different linear and geometric positioning systems using line, geometric and screen coordinates. the mileage irregularities are resolved by the means of binding different linear positioning systems. regarding the intrinsic positioning system, inseparably associated to given net element, as already mentioned, the position is measured in relative values from 0 to 1). [6, 8, 10] 18 vol. 11/2017 railtopomodel and railml 3 in overall context 4.3. net entities: operational entities, geometry entities and assets the most of particular subsections of the railml 3 infrastructure schema deal with located net entities, to be fit to the topological layer. these are the operational entities, geometry entities and assets (in railml 3.0.is4 mostly representing technical facilities of railway and with 3.0.5 version renamed to functional assets in order to be distinguished from physical assets). as regards railml 3.0.is4, there are following entities included, while users can add any others (not to be a part of the common basis) to meet their needs. [6, 8] in terms of further releases, the lists are expected to be extended, based on the “use cases”. [9] • operational entities . line . operational point . border . speed profile . speed restriction . track • geometry entities . horizontal curve . gradient curve . superelevation curve . track geometry point • assets . bridge . buffer stop . derailer . isolation rail joint . level crossing . platform . platform edge . service section . signal . stop post . switch . train detection element . train protection element . tunnel for comparison, there are two major groups of entities (the track elements group and the ocs elements group) in railml 2 creating data substructures of each track element. their location within their parent track shall be given by means of the position attribute, expressing the distance from the beginning of a track in metres. in the case of linear character of the entity being placed, railml 2 uses two different approaches how to represent it; either to determine the spots where the data change (e.g. the radius changes, speed changes. . . ), or to determine the spot location of the object itself, possibly completed with the attribute of length (e.g. the platform edges, bridges, level crossings. . . ). in some cases, it is essential to determine the considered direction. [10] the railml 2 way of description, indeed, sometimes implies data inconsistency. the data could be misunderstood. the fact that all entities are strictly allocated to one specific track may cause some difficulties concerning the infrastructure description. for instance, how to define and describe track circuits when only their borders can be marked out? structural object as bridges, tunnels and level crossings often belong to more than one track. in the scope of railml 2, every affected track needs to involve an extra instance of related data classes, although the original real world object is always the same (as seen in the figure 2). [7, 10] railml 3, building on the railtopomodel principles, offers solutions of these issues. not only that it enables spot, linear and aggregated location of net entities, but it also allows them to be multiply located (see again the figure 2). locating of all the net entities is managed in the same way, no matter what kind of entity it is. [8] every xml element representing a net entity can contain unlimited amount of spot, linear and aggregated location elements involving location spots and linear sections expressing different locations of the entity. regarding the aggregated location, it can be expressed both by spots and linear sections defining an area within the railyard. the location can be also represented by point and linear types of coordinates. their definition is taken from the gml geometry schema. it means that the net entities are already not necessarily assigned to one specific track. [7, 8] the xml elements describing the entities of the same type are included in common container elements within relevant subsection. in some cases, more complex substructure is used (especially as regards the signals, nevertheless, some of their items could be possibly managed within the interlocking schema). concerning the assets, their operating time, constriction time and blocked time is possible to be expressed, in addition. prospectively, a link of these functional assets with physical assets to be a part of inventory management is intended to be ensured. [6, 8] 4.4. infrastructure paramteres the last category, also enabling to describe characteristics of railway infrastructure, used in railml 3.0.is4 is the subsection of infrastructure parameters. the following (again extendable) list of the parameter classes is given, as regards the 3.0.is4 release. [8] • infrastructure parametres . track gauge . speed electrification . weight limit . train protection . clearance gauge . train radio . track alignment 19 adam hlubuček acta polytechnica ctu proceedings in railml 2, the most of these parameters can be expressed in two different ways: through infrastructure attribute groups (to be assigned to a certain track) or as track elements (changes within a track). [10] the railml 3.0.is4 approach is closer to the first attitude mentioned. the infrastructure parameters are managed in rather different way than the net entities. every net element (as be a part of the topologic layer) can refer to unbounded number of infrastructure parameters representing specific track conditions. besides, it means that the parameters are valid for the relevant net entity (e.g. a track or line section) as a whole. except they cannot be positioned, the related xml elements are handled in the same manner as the net entities. [8] apparently, there was an effort to remove the redundancy. unfortunately, it would mean that these global parameters cannot be changed within a particular net element. indeed, in some cases it is not essential; however, for example, as regards the electrification, it could cause serious difficulties. for that reason, based on discussions within the railml community, [11] the infrastructure parameters subsection was cancelled with railml 3.0.5 (released in march 2017) and the entities included was moved to the functional assets group (thereby becoming net entities). at the same time, the network location was introduced in order to represent the intended function of the cancelled subsection. [8] 5. selected application issues the railml 3 exchange format is supposed to find application in interconnection of such it systems as infrastructure registers, maintenance systems, evidences of assets, construction projects, visualisation of railway infrastructure (georeferenced documents, gis applications, schematic track plans, longitudinal sections), simulations of railway operation, calculations of energy consumption, capacity planning, information systems for operative control, predictions of train running, automatic routing, automatic train control, automatic train operation and many others. apparently, it is big challenge for developers to meet all the multiple needs. certainly, the alpha release does not cover all the desired aspects, as surely as it will be gradually complemented reflecting particular use cases. [9] 5.1. railml for atc? as the infrastructure description used by railml is essentially based on real track lengths, not only to use absolute mileage which can be misleading, railml seems to be very useful tool to provide data for such applications that operate with actual distance travelled by a train. among other applications, it is essential for the ertms / etcs [12] as an automatic train control system using balises to be reference points for measuring the distance travelled. the balises and balise groups are included in the railml 2 specifications, nevertheless, railml 3 have not been saturated with such entities yet. perhaps, it is caused by the unresolved issue in which way to perform referencing between the balises and balise groups elements. [8, 10] except the balise locations, other additional data are needed, including locations of important objects, speed restrictions, slopes, track conditions (change of traction systems, powerless sections, non stopping areas, sound horn commands...) road suitability parameters (loading gauge, traction system, axle load category...). many of them have already been included into the railml 3 infrastructure schema, nevertheless, some of them should be added or revised. [8, 11] 5.2. transfer times planning concerning predictions of train running (and in some cases capacity planning as well), not only the railway infrastructure itself must be taken into account, but also some related facts are important. for instance, when ensuring correspondences among trains, transfer times are often critical. among others, they depend on parameters of the paths connecting the platform edges. when there are platforms with level access, the transferring passengers are influenced by passing trains (and conversely). this fact shall be taken into account when assigning the platform edges and tracks to the trains as well (for instance, not to allow a passing train run over the foot level crossing being used by passengers). not to mention the vital importance of the foot level crossing location register for such application as the automatic train operation because of the need to ensure human safety. to cope with these cases, the model and the exchange format would need to be significantly extended. 6. proposal of a rtm based passenger movement model the abovementioned issue is more complex, as the passenger routes (connecting particular platforms together and with station building and other important locations in a station) consist of different elements (e.g. platform ramps, stairways, lifts, underpasses, overpasses, foot level crossings, footpaths). all these facilities could be expressed using railml 3 as net entities (although the railml 3 alpha release, for instance, enables to express the existence of a lift on a platform by the means of its attribute only). these platform accessories and related facilities should be interconnectable and describable by such attributes expressing their usability by persons with reduced mobility and other important parameters so as to enable searching suitable routes connecting particular “passenger access nodes” (in the sense of significant spots assigned to platforms, station building and other net entities). even such situations as a lift being out of service should be taken into account. 20 vol. 11/2017 railtopomodel and railml 3 in overall context figure 3. an author’s proposal of railtopomodel application in the scope of passenger routing issue exemplified for the case of a railway station equipped with platforms with level access. there are several possible solutions how to meet these needs. one of them is to apply railtopomodel again, this time not to connect pieces of railway infrastructure but the abovementioned “passenger access nodes”, henceforth to be considered as a new sort of the net elements (see the figure 3). this approach, in general, would also enable to model emergency access roads to railway tunnels, for instance, and to cope with other related issues. 7. conclusions railtopomodel and the corresponding data exchange format railml 3 have a great opportunity to become very useful tools in describing railway infrastructure and ensuring the rail data sharing. involving also the schemas of timetable, rolling stocks and prospectively interlocking, the railml language covers almost the whole range of the railway branch. although the railway system is based on relatively similar principles across the whole europe, there are many nationally specific details that should be addressed in a coordinated manner in order to make the constantly evolving railml system universal enough to enable different software application to consistently communicate with each other. to make the system as usable as possible, the potential users should formulate their “use cases” [9] and be in contact with the railml community creating the specifications. then they should be able to influence the final form of the railml 3 first public release in order to be suitable for their needs. nevertheless, the very special matters can be solved individually by the means of user-specific extensions. within this document, some of possible extensions were outlined as well. references [1] railml. railml [online]. https://www.railml.org/. [2] railml. the railml subschemas [online]. https://www.railml.org/en/user/subschemes.html. [3] railtopomodel. railtopomodel [online]. http://www.railtopomodel.org//. [4] uic. feasibility study uic railtopomodel and data exchange format [pdf], 2013. http://www.railtopomodel.org/files/download/ railtopomodel/270913_trafit_ finalreportfeasibilitystudyrailtopomodel.pdf. [5] uic. uic international railway standard [pdf], 2016. http://www.railtopomodel.org/files/download/ railtopomodel/270913_trafit_ finalreportfeasibilitystudyrailtopomodel.pdf. [6] railml, uic. 7th railtopomodel and 30th railml conference, paris, 2016. [7] v. p. kolmorgen, c. rahmig. railml/rtm explanation session, prague, 2017. [8] railml. railml schema version 3.0.is4 [xsd], 2016. [9] railml. use cases [online]. https://www.railml.org/en/user/use-cases.html. [10] railml. railml schema version 2.3 [online], 2016. https://www.railml.org/files/download/schemas/ 2016/railml-2.3/documentation/railml.html. [11] m. karlsson, c. rahmig. [railml3|alpha] modelling of track conditions [online], 2016. http://www.railml. org/forum/index.php?t=msg&th=474&start=0&. [12] ertms. the european traffic management system [online]. http://www.ertms.net. 21 https://www.railml.org/ https://www.railml.org/en/user/subschemes.html http://www.railtopomodel.org// http://www.railtopomodel.org/files/download/railtopomodel/270913_trafit_finalreportfeasibilitystudyrailtopomodel.pdf http://www.railtopomodel.org/files/download/railtopomodel/270913_trafit_finalreportfeasibilitystudyrailtopomodel.pdf http://www.railtopomodel.org/files/download/railtopomodel/270913_trafit_finalreportfeasibilitystudyrailtopomodel.pdf http://www.railtopomodel.org/files/download/railtopomodel/270913_trafit_finalreportfeasibilitystudyrailtopomodel.pdf http://www.railtopomodel.org/files/download/railtopomodel/270913_trafit_finalreportfeasibilitystudyrailtopomodel.pdf http://www.railtopomodel.org/files/download/railtopomodel/270913_trafit_finalreportfeasibilitystudyrailtopomodel.pdf https://www.railml.org/en/user/use-cases.html https://www.railml.org/files/download/schemas/2016/railml-2.3/documentation/railml.html https://www.railml.org/files/download/schemas/2016/railml-2.3/documentation/railml.html http://www.railml.org/forum/index.php?t=msg&th=474&start=0& http://www.railml.org/forum/index.php?t=msg&th=474&start=0& http://www.ertms.net acta polytechnica ctu proceedings 11:16–21, 2017 1 introduction 2 the railtopomodel overview 3 railtopomodel packages 3.1 base 3.2 topology 3.3 positioning 3.4 net entities 4 railml evolving over time 4.1 topology 4.2 positioning systems and coordintes 4.3 net entities: operational entities, geometry entities and assets 4.4 infrastructure paramteres 5 selected application issues 5.1 railml for atc? 5.2 transfer times planning 6 proposal of a rtm based passenger movement model 7 conclusions references 90 acta polytechnica ctu proceedings 2(1): 90–93, 2015 90 doi: 10.14311/app.2015.02.0090 simultaneous ubvri observations of the ae aquarii blobs g. latev, r. zamanov, s. boeva, k. stoyanov 1institute of astronomy and national astronomical observatory, bulgarian academy of sciences, 72 tsarighradsko shousse blvd., 1784 sofia, bulgaria corresponding author: glatev@astro.bas.bg abstract we summarize the results of our study of the cataclysmic variable ae aqr on the basis of simultaneous ubv ri observations. for the flares, we estimated the average risetime of about 300 sec, and colours (u −b)0 ∼ 1.1 and (b − v )0 ∼ 0.1. we also calculated temperatures, sizes, masses and expansion velocities of a few individual fireballs. in a single night (16.08.2012), we detected ∼ 8 min quasi-periodic modulation. keywords: stars: individual: ae aqr novae, cataclysmic variables binaries: close stars: flare. 1 introduction ae aquarii is a fascinating magnetic cataclysmic variable (cv) with orbital period of 9.88 h. like the typical cvs, it consists of a k4 iv/v star which transfers material toward a magnetic fast spinning white dwarf (wd). ae aqr has a relatively long orbital period and is one of the largest cvs having semimajor axis a = 2.33±0.02 r�, wd mass mwd = 0.63±0.05 m�, secondary mass m2 = 0.37 ± 0.04 m� (the quantities obtained with high-dispersion time-resolved absorption line spectroscopy by echevarŕıa et al. 2008). a review of ae aqr was presented by p. meintjes (this volume). ae aqr has radio and millimeter synchrotron emission (bastian, dulk & chanmugam 1988), and could be source of tev γ-rays (oruru & meintjes 2012 and references therein). however, magic does not detect γ−ray emission from ae aqr (aleksić et al. 2014). wynn, king & horne (1997) demonstrated that the wd is acting as magnetic propeller and is ejecting most of the matter transferred through the inner lagrangian point in the form of blobs (‘fireballs’). the spitzer infrared spectrum above 12.5 µm can be interpreted as synchrotron emission from electrons accelerated to a power-law distribution in expanding clouds (dubus et al. 2007). ae aqr hosts a rapidly rotating white dwarf. its spin period is p = 33.08 s and spin down rate – 5.64 × 10−14 s s−1 (patterson 1979, mauche et al. 2011), corresponding to a spin-down luminosity of 6×1033 erg s−1. a part of this spin-down power goes for ejection of the blobs. the magnetospheric propeller is effective and only a small fraction (∼3%) of the trasferred mateial eventually accretes on to wd (oruru & meintjes 2012). 2 observations our observations of ae aqr were started in 1998 with an electophotometer in v-band and are now extended to simultaneous multicolour (ubv ri) observations with the telescopes of the national astronomical observatory rozhen (2.0m rcc, 50/70 cm schmidt and 60 cm telescope) and the belogradchick astronomical observatory (60 cm telescope). the observational sessions can be summarized as follow: (i) 1998 1999: v-band electrophotometric observations. (ii) aug 2010 aug 2011: simultaneous multicolor (ubv ri) ccd observations. (iii) sep 2011 aug 2013: new simultaneous multicolor ccd observations. observational methods and data reduction are described in zamanov et al. (2012). 3 photometric behavior strong flickering and flaring activity was noted by henize (1949). on time-scales of about 10 minutes, the light curve of ae aqr displays flares with an amplitude up to ≈ 1 mag (see fig.1 and fig.2). multicolour optical photometry was performed by chincarini & walker (1981) in ubv bands. later van paradijs, van amerongen, & kraakman (1989), reported five-colour (in walraven bands) observations and showed that the flares have rise time ∼ 100 − 200 s and occur throughout the whole orbit. in fig.2 we plot the orbital light curve of ae aqr using the orbital period of 0.411655530 d and the zeroorbital phase jd0 = 2449281.4222200 (casares et al. 1996). in this figure a part of our data obtained in august 2013 are shown. the quiescent flux curve is recognizable as smooth orbital variation with two maxima and two minima per orbital cycle. 90 http://dx.doi.org/10.14311/app.2015.02.0090 simultaneous ubvri observations of the ae aquarii blobs figure 1: optical light curve of ae aqr obtained on september 27, 2011 in the bv ri bands. figure 2: the b band light curve of ae aqr (august 2013) folded with the orbital period (9.8797 h). 3.1 the flares as expanding fireballs pearson, horne & skidmore (2003, 2005) formulated analytic expressions for the spectral evolution and continuum light curves of flickering and flaring variability that occur over a wide range of astrophysical objects. they applyed these expressions to the observations of the cataclysmic variables ae aqr and ss cygni, and of the supernova sn 1987a, deriving physical parameters for the material involved. they have shown that the observed flare spectrum and evolution of ae aqr is reproducible with an isothermal fireball with population ii abundances. the interested reader is directed to their papers for a full discussion. the basic assumptions are: (i) the flares of ae aqr are due to the appearance and expansion of fireballs (blobs); (ii) the blobs are isothermal; (iii) they represent spherically symmetric expansion of a gaussian density profile with radial velocity proportional to the distance from the center of the expansion. figure 3: upper panel: dereddened fluxes of a fireball at the maximum of the flare. the solid line is a black body fit. lower panel: time evolution of the flare in the v band. the (red) solid line represents the isothermal fireball model. following pearson et al. (2005), the dimensionless time β is defined as β = 1 + h(t − tpk), where tpk is the time of the peak of the flare, h is an “expansion constant” setting the speed of the expansion. the dimensionless time β is also the expansion factor being the constant of proportionality between the current and peak scale length apk: β ≡ a/apk. the central density of the fireball is ρ = m (πa2)3/2 , (1) where m is the total mass of the material involved in the expansion (the fireball mass). the speed of expansion at a is v = ha. the optical depth parallel to the 91 g. latev et al. observer’s line of sight is τ(y) = − ∫ −∞ ∞ κ dx = τ0 e −2( y a )2 (2) where y is the impact parameter (the distance from the fireball center perpendicular to the line of sight), κ1 is the linear absorption coefficient, ε is the correction for stimulated emission, ε = 1 − e(hν/kt ). here t is the fireball temperature, a is the length scale (which we call the fireball size). the optical depth on the line of sight through the center of the fireball (y = 0) is τ0 = κ1 ε m 2 21/2 t 1/2 0 ν3 π5/2 a5 . (3) the emission of the fireball is: fν = π a2 bν(t ) 2 d2 s(τ0), (4) where s(τ0) is the ”saturation function” (see fig. 1 of pearson et al. 2005). to calculate the fireball parameters t,m, a,h, ρ and v, we performed the following: 1. we compute the peak flux of the fireball, fpk, in the five optical bands (ubv ri). an example is given in fig.3, where the calculated peak fluxes (corrected for the reddening) are plotted. 2. we derive the temperature of the fireball with a black body approximation applying iraf nfit1d routine. 3. we evaluate the size of the blob at the peak, apk, using τ0 = 6.8202 and eq.4. 4. we calculate the mass of the fireball (eq.3). the calculations are done adopting population ii abundances (κ1 = 1.27 × 1052 m−1). 5. fitting the v band light curves, we derive the expansion constant h. an example is given in fig.3 lower panel) . 6. we derive the speed of expansion and the central density. part of the calculated parameters are given in table 1. we reach lovely agreement between between the model and the light curves of the optical flares using fireballs with a temperature t ∼ 15000 k and mass m ∼ 1020 g. blobs are detected in two other close binaries containing white dwarfs (the recurrent nova rs oph and the symbiotic star ch cyg). in future it will be interting to understand is it the same mechanism (magnetic propeller), which generates the blobs. figure 4: a) light curve (in johnson b band) of ae aqr obtained on august 16, 2012. quasi-periodic oscillations are visible. b) power spectrum of the light curve. the maximum indicates a period of about 8 min. 4 unusual behaviour on august 16, 2012 usually, ae aqr has light curve as shown in fig.1. however, in our observation obtained on august 16, 2012 (see fig.4) a clear periodicity is visible. the power spectrum has a maximum corresponding to t = 7.5 ± 0.2 min. possible explanations of this unusual behavior are: (1) beat modulation between the orbital period and the wd spin period (e.g. as supposed for bg cmi, fig.7.3 of warner 1995). the equation 1/pbeat = 1/pspin − 1/porb (5) gives pbeat = 33.03 s, which is too short in comparison with the observed value. (2) blob rotating with keplerian velocity at the border of the magnetosphere: rblob = (2π)−2/3 (gmwd)1/3 t 2/3, (6) the period t = 7.5 min corresponds to keplerian rotation at distance rblob ≈ 0.11 r�. the magnetospheric radius is estimated as rm ≈ 4−8rwd ≈ 0.05−0.10 r� (zamanov et al. 2012 and references therein). in our opinion, we had the chance to observe a blob rotating at the magnetosphere boundary. 92 simultaneous ubvri observations of the ae aquarii blobs table 1: the computed parameters of the fireballs. the time is in format yyyymmdd hh:mm. in the table are given as follows: rise time in seconds, colours of the peak emission of the fireball (corrected for the interstellar reddening), temperature of the fireball, its mass and size (at the peak of the flare). quantity 20100813 23:40 20100814 19:20 20100814 19:48 20100814 20:10 20110831 21:43 rise time [sec] 260±20 230±25 290±30 440±20 260±30 (u −b)0 -1.36±0.06 -1.43±0.03 -0.93±0.04 -0.80±0.05 -1.02±0.07 (b − v )0 0.24±0.03 0.23±0.03 0.17±0.03 0.19±0.03 0.03±0.06 temperature t [k] 14 545±1000 27 292±1500 10 856±150 9 527±100 13 395±200 mass m [1019 g] 9.6±1.5 6.8±1.5 39 ±6 78±12 97±15 size apk [109 cm] 3.0±0.3 2.5±0.3 5.3±0.3 7.1±0.4 7.7±0.4 5 conclusions using 4 telescopes, we performed simultaneous observations in 5 bands (ubv ri) of the flare activity of the cataclysmic variable ae aqr. adopting the model of an isothermically expanding ball of gas, we calculated parameters (temperature, size, mass) of a few individual fireballs. in a single night, we detected ∼ 8 min quasiperiodic modulation, which might be due to a blob rotating at the magnetosphere boundary. in future we intent to measure more blobs and to search for correlations between their physical parameters. also, we will try to do a follow-up study of the qpos phenomenon. acknowledgement the authors are very grateful to the anonymous referee for useful notes and comments. this work was supported in part by the op ”hrd”, esf and the bulgarian ministry of education and science (bg051po0013.3.06-0047). references [1] aleksić, j., ansoldi, s., antonelli, l. a., et al. 2014, a&a, 568, aa109 [2] bastian, t. s., dulk, g. a., & chanmugam, g. 1988, apj, 324, 431 doi:10.1093/mnras/282.1.182 [3] casares, j., mouchet, m., martinez-pais, i. g., & harlaftis, e. t. 1996, mnras, 282, 182 [4] chincarini, g., & walker, m. f. 1981, a&a, 104, 24 doi:10.1111/j.1365-2966.2008.13248.x [5] dubus, g., taam, r. e., hull, c., watson, d. m., & mauerhan, j. c. 2007, apj, 663, 516 [6] echevarŕıa, j., smith, r. c., costero, r., zharikov, s., & michel, r. 2008, mnras, 387, 1563 doi:10.1111/j.1365-2966.2012.20410.x [7] henize, k. g. 1949, aj, 54, 89 [8] oruru, b., & meintjes, p. j. 2012, mnras, 421, 1557 doi:10.1046/j.1365-8711.2003.06079.x [9] patterson, j. 1979, apj, 234, 978 doi:10.1086/426582 [10] pearson, k. j., horne, k., & skidmore, w. 2003, mnras, 338, 1067 [11] pearson, k. j., horne, k., & skidmore, w. 2005, apj, 619, 999 [12] van paradijs, j., van amerongen, s., & kraakman, h. 1989, a&as, 79, 205 [13] warner, b. 1995, cambridge astrophysics series, 28 doi:10.1002/asna.201211718 [14] wynn, g. a., king, a. r., & horne, k. 1997, mnras, 286, 436 [15] zamanov , r. k., latev, g. y., stoyanov, k. a., et al. 2012, astronomische nachrichten, 333, 736 discussion ashley pagnotta: what is the timescale of the short unexplained flares superimposed on the orbital modulations? georgi latev: sampling every 10 seconds, average time from peak to peak is about 8 minutes simone scaringi: given the simultaneous multi-color photometry can anything be said about color evolution of ae aqr? g. latev: we will try to do this in our future work. 93 http://dx.doi.org/10.1093/mnras/282.1.182 http://dx.doi.org/10.1111/j.1365-2966.2008.13248.x http://dx.doi.org/10.1111/j.1365-2966.2012.20410.x http://dx.doi.org/10.1046/j.1365-8711.2003.06079.x http://dx.doi.org/10.1086/426582 http://dx.doi.org/10.1002/asna.201211718 introduction observations photometric behavior the flares as expanding fireballs unusual behaviour on august 16, 2012 conclusions acta polytechnica ctu proceedings doi:10.14311/app.2017.11.0006 acta polytechnica ctu proceedings 11:6–11, 2017 © czech technical university in prague, 2017 available online at http://ojs.cvut.cz/ojs/index.php/app development of the municipal noise-absorbing screen and test section construction technology ondřej bret∗, petra čížková czech technical university in prague, faculty of civil engineering ∗ corresponding author: ondrej.bret@fsv.cvut.cz abstract. the article presents the development and construction technology of the municipal noise-absorbing screen (mnas) – in czech "městská protihluková clona" (mphc). the municipal noise-absorbing screen is a new acoustical element developed in cooperation of the department of railway structures, faculty of civil engineering, ctu in prague and the montstav cz s.r.o. company. it is characterised by very small dimensions – its height is only ca 0.3 metre above the top of rail. the element serves to attenuate the noise induced by rail traffic arising at the wheel-rail contact. the municipal noise-absorbing screen consists of individual, mutually interlocked units, placed one after another, made of bound recycled rubber. the article describes the major limiting conditions for the placement, shape and execution of the municipal noise-absorbing screen. it also describes the construction of the test section situated on a tram track in prague and sums up the experience obtained after the first year of operation. keywords: municipal noise-absorbing screen, noise pollution, tram traffic, tram cars, recycling, rubber granulate. 1. introduction the present-day requirements for admissible noise levels induced by both road and railway traffic in builtup residential areas have been increasingly growing. this issue is a subject of long-term discussions not only among professionals, but more and more frequently also within the public media space. the problem arising during the approval of reconstructed tram tracks in 2017 can serve as an example [1]. traffic induced noise can be limited by active or passive noise reduction measures. active noise reduction measures on the railway track in construction terms include, above all, a high-quality and well-maintained roadway, appropriate track construction design, rail fastening, application of rail absorbers and elimination of rail corrugation [2]. active noise reduction measures also include measures performed on vehicles. these are, above all, the wheel profile maintenance, bogie-mounted plates and other modifications of bogies. even the application of suitable combinations of active measures, however, comes up against the ceiling of its potential and passive measures are necessary. passive noise reduction measures include, for example, classic noise barriers, applied not only in rail transport, but also in road transport. the latest efforts are aimed at developing smaller-size elements, placed closed to the source of noise taking the form of various low noise-absorbing screens [3]. low noise-absorbing screens are elements or building units placed along tram tracks, railway tracks or other rail guided transport systems whose application results in the reduction of undesirable effects of noise arising during the passage of rail traffic [2]. low noiseabsorbing screens are placed as close as possible to the structure gauge set by a technical standard. thanks to their dimensions, they can be installed much closer to the source of noise as compared to classic noise barriers being thus able to attenuate the dominant noise arising at the wheel-rail contact (rolling noise) and the bogie noise [4, 5]. depending on the material from which they are produced, they can have absorption or reflection properties. 2. municipal noise-absorbing screen the municipal noise-absorbing screen (hereinafter also referred to as "mnas") is a noise-absorbing element developed in cooperation of the department of railway structures, faculty of civil engineering, ctu in prague and the montstav cz s.r.o. company, engaged in the recycling and production of rubber elements. the operational verification runs in cooperation with the prague public transit company (pptc). mnas serves for the attenuation of the noise induced by rail traffic, arising at the wheel-rail contact. the characteristic height of the screen is ca 30 cm above the top of rail (tr). the screen is placed in the immediate vicinity of the vehicle gauge and thus it effectively reduces the arising noise. the municipal noise-absorbing screen is characterised by the material used – a composite based on recycled rubber granulate, bound by a polyurethane binder. the rubber granulate is a secondary raw material that can be used in railway construction in different applications [6], either in a bound [7, 8] or unbound state [9]. unlike other similar elements, mnas is easily demountable, which is a great advantage in the case of e.g. a nec6 http://dx.doi.org/10.14311/app.2017.11.0006 http://ojs.cvut.cz/ojs/index.php/app vol. 11/2017 development of the municipal noise-absorbing screen essary intervention of the integrated rescue system for which concrete noise barriers create a significant obstacle to a fast and smooth intervention [10]. the first-generation mnas, which is the subject of this article, is presently being verified under operating conditions and its durability is monitored – due to favourable thermal and technical properties of the rubber granulate [9] mnas is presumed to resist winter conditions and degradation quite well [11]. it is made up of units (segments) manufactured of bound recycled rubber with dimensions of 250 x 600 x 750 mm (width x height x length), which are partially embedded below the ballast bed so that the height of the screen is ca 30 cm above the top of rail depending on the execution. 3. limiting conditions for mnas placement, shape and execution there were many limiting factors for the placement, shape and execution of the municipal noise-absorbing screen emerging already in its design phase. the major limitations are the structure gauge, or the tramway vehicle gauge respectively. the design of the mnas shape and placement was initially based on the vehicle gauge under čsn 28 0318 – "structure gauges of tramway tracks" [12]. according to this standard, the vehicle gauge width in a straight track section is 1300 mm from the rail axis. the reference vehicle considered in the above standard is a three-section tram of the kt8d5n type with a vehicle width of 2.5 m and a length of 30.3 m. in prague, this type is represented by a bi-directional tram of the kt8d5.rn2p type, which originated by the reconstruction of the kt8d5 tram [13, 14]. due to the fact that the vehicle gauge is very significantly extended in directional curves of small radii [12], the installation of mnas is feasible mainly in straight sections and curves with radii from ca 200 m. in straight sections and curves with large radii, it must be placed respecting the vehicle gauge pursuant to čsn 28 0318 [12], or other limitations specific for the respective tram operation. other parameters for the mnas placement on a test section within the prague public transit company (pptc) network included potential passage of the psp-01 bogie snow plough (figure 1). the psp01 snow plough was developed to meet the growing need for clearing snow from the rail space for the use of new low-floor vehicles. due to the low-floor part, these vehicles have a greater width in the lower parts of the body casing than older (high-floor) trams, being, in some cases,equipped with other components in the part close to the top of rail which may cause problems in contact with water (snow). to allow smooth snow removal even in directional curves, the share of the psp-01 bogie plough has a greater width than standard operated trams. its total width is 2.6 m – i.e. 1.3 metres from the rail axis. based on the need to preserve the possibility of the plough passage, this parameter must also be taken into account in figure 1. psp-01 snow plough [15]. prague and the screen must be placed with some reserve slightly further away from the rail axis than would be necessitated by the tram vehicle gauge. based on consultations with pptc representatives over designs taking into account the above limitations, a question arose concerning a potential passenger escape exit from the tram in case of emergencies (accidents, vehicle failures, etc.,...) with respect to the screen placement. while older trams and their retrofits usually have four-section folding doors (opening in the inside direction) and modern trams are, as a rule, fitted with ejecting doors (opening in the outside direction, but only to a distance of several cm from the body casing), the possibility of opening tilting doors in the t6a5 type trams appeared as potentially problematic interfering with the screen placement. as seen in figure 2, while opening the door the door leaves move along a circular trajectory getting to a distance of up to 360 mm from the body casing (1610 mm from the rail axis). therefore, when placing mnas to the right in the riding direction, the opening of the t6a5 tram door must be allowed, which can be achieved in two ways: 1) move the municipal noise-absorbing screen further way from the rail axis, to a distance exceeding 1610 mm – which is a solution totally inappropriate it terms of the acoustic effectiveness of the mnas element, or 2) select such a height of the screen above the top of rail allowing the tilting of the door – due to the height of the door at the maximum admissible wheel wear this level is at a height of 28 cm above tr – which is a solution feasible without problems. in placing the screen in the test section, this issue did not need to be solved as mnas is placed to the left of the rail axis there, i.e. on the side without a door where it does not affect a potential evacuation of the passengers. it should be added that t6a5 trams are gradually discarded from traffic operation in prague and replaced with new vehicles in which the door opening is solved by the above mentioned ejection of leaves, so that this limitation in placing mnas on the door side will no longer apply in the future. based on the above conditions and limitations, a 7 ondřej bret, petra čížková acta polytechnica ctu proceedings figure 2. t6a5 tram – cross section, including the tilting door movement – plotting in a type drawing [16]. figure 3. mounting of mnas prototypes – vandal resistance test. design of the mnas placement in characteristic sections was developed and verified by the production and mounting of three prototype segments. 3.1. verification of prototypes the trial placement of the prototypes was carried out on 23. 3. 2016 on a tram track close to the pptc central workshops, in a section between ústřední dílny dopravního podniku – na homoli stops. both the team of the department of railway structures fce ctu and the representatives of pptc, tt (tram track) administration section and dct (tram traffic route) special structures participated in the test. apart from the construction / installation test, the verification included the compatibility and interaction of individual units and their vandal-resistance was tested – resistance to the displacement of individual segments of the screen (figure 3). based on local investigation, the final design of the placement and shape was developed and the connection of mnas units was optimized. 3.2. selection of a section for trial installation the test section for the placement and verification of the acoustic properties of the municipal noiseabsorbing screen (mnas) was selected in cooperation with the prague public transit company (pptc) again. tracks with an open track structure were considered (even though mnas can also be installed in a section with vegetation cover) of which the final choice fell on a tram track in prague 4, direction sídliště modřany, the section pobřežní cesta – přístaviště in the prague district of braník. the double track tram line in the respective section is led parallel to modřanská street, on a separate body with an open ballast bed. grooved rails of the nt1 shape on tb93 concrete sleepers with w14 elastic baseplateless fastening are used there. the rail is executed as continuous welded rail. the screen was installed on the centre-bound track, to the left of the riding direction (i.e. in the inter-track space). the mnas position was selected with regard to the subsequent technical noise level measurement aimed at identifying the acoustic effectiveness of the designed noise reduction measure. the sound level meter microphone was located at a distance of 7.5 metres from the monitored track, on the side more distant from the road to eliminate the effect of passing car traffic. the characteristic section at the point of the noise-absorbing screen placement is shown in figure 4. the selected distance of the screen face from the rail axis was 1420 mm, and its height was 300 mm above the top of rail. 3.3. test section construction the test section was established on 9. 4. 2016 in coordination with a one-day tram operation lockout in the dvorce – sídliště modřany section, planned for the overhaul and maintenance of traction wires. in the morning before the lockout, accurate measurement and marking of the site for the placement and installation of the municipal noise-absorbing screen plus other preparatory works had been performed. immediately after the start of the lockout, the mhs two-way excavator made a groove in the ballast bed for the installation of mnas units (figure 5). successive work was done manually, without machinery. the site construction camp and vehicle parking were established near pobřežní cesta stop, by the exit of the link road from barrandov bridge onto modřanská street, ca 200 m from the established test section. from there, the mnas segments, tools and other material were transported by a track trolley. the width of the previously excavated groove necessary for mounting the mnas segments and the depth related to the top of rail (tr) were checked and further extended as necessary. before the installation of units, the gravel bearing surface was compacted with a manual petrol powered vibrating plate (figure 6). individual mnas units were gradually mounted onto the compacted surface, proceeding step by step from the start of the section towards přístaviště stop. the position of installed units in relation to the rail was continuously checked – their distance from the rail axis (structure gauge) and their height above tr (figure 7). mounted units were gradually backfilled 8 vol. 11/2017 development of the municipal noise-absorbing screen figure 4. characteristic section at the point of the noise-absorbing screen placement. figure 5. groove excavation for mounting mnas using the mhs two-way excavator. figure 6. compaction of groove surface prior to the screen placement. with gravel, which was compacted, both manually and using machinery. individual units were mutually interlocked by wooden laths placed on the upper side of the screen (figure 8) – this ensures the interaction of the whole screen (necessary e.g. to resist vandalism) and also secures its permanent position. figure 7. checking the screen distance and height from the rail axis and tr. an atypically chamfered unit preventing the crash of tram units into the front of the screen in case of emergency (e.g. displacement of screen units due to vandalism) was mounted onto the front of the municipal noise-absorbing screen in the riding direction (i.e. in the direction from pobřežní cesta stop) (figures 89). after the final check of the position of all units, final modifications of the ballast bed profile were made and the gravel around the installed mnas was compacted with a manual petrol powered vibrating plate. a total of 32 mnas units were installed, the test section being 24 metres in length. 3.4. screen modification following noise measurement monitoring the state after the mnas construction, the screen was modified to improve the acoustic effectiveness of the noise reduction measure used. this modification consisted in adding boards of recycled rubber, 25 mm in thickness, to the upper screen wall, which created an overlap, 9 ondřej bret, petra čížková acta polytechnica ctu proceedings figure 8. mnas seen from the start of the section – chamfered first unit is evident in the front. figure 9. detail of modified mnas execution. 70 mm in length, at the screen face to further reduce the arising noise. thanks to the board overlap, the screen face was shifted towards the track, from the original distance of 1.42 m from the rail axis to 1.35 m after the modification. the modification is evident from figure 9. the screen was modified under traffic operation on 26.4.2016. the modification was performed using boards of recycled rubber, pre-cut to the necessary dimensions. the boards were fastened to individual mnas segments with 0.5 m spacing by means of dowels into polystyrene – this solution proved to be the most convenient based on tests. after the installation of additional boards providing the overlap of the screen towards the track, the measurement of noise was carried out to verify the effectiveness of modified mnas. the solved section with completed mnas, including the modification, is presented in figure 10. 4. conclusion the designed and implemented noise reduction measure proved to be an effective alternative for the attenuation of noise induced by tram traffic. the measured data served to establish the average energy value for all measured passages of tram units. the resulting reduction due to the municipal noise-absorbing screen (mnas) application after its modification identified figure 10. municipal noise-absorbing screen after modification. was 3.9 db. some types of trams (in relation to the body casing shape at the bogie level) even reached higher noise reduction levels. more detailed information about the mnas effectiveness will be published in the professional peer-reviewed journal, akustika, in autumn 2017 (www.akustikad.com). the test section is currently subjected to continuous monitoring and different parameters are surveyed on it on a long-term basis. after the first year of operation and the first winter season, the municipal noise-absorbing screen does not show any signs of mechanical damage, degradation or other defects. in 2016, the industrial property protection agency awarded the municipal noise-absorbing screen the industrial and utility model certificate. in the following phases of research and development, further monitoring of the first-generation mnas test section, described in this article, and the construction and verification of the second-generation mnas test section or sections is presumed where structural improvements, designed on the basis of experience obtained from the construction and monitoring of the first-generation test section, will be applied. references [1] k. frouzová. praha musí řešit hlučné tramvaje na opravených tratích. hrozí omezení nebo zrušení desítky spojů / prague must solve noisy trams on repaired tracks. there is a threat of limitation or cancellation of ten lines, in: www.ihned.cz: hospodářské noviny [online]. [cit. 2017-03-10] http://domaci.ihned.cz/c1-65569370-praha-musi-resithlucne-tramvaje-na-opravenych-tratich-hrozi-omezeninebo-zruseni-desitky-spoju. [2] p. čížková. the acoustic effectiveness of low height noise barrier. in: kropp, w, ed. proceedings of the 45th international congress on noise control engineering. inter-noise 2016, hamburg, 2016-08-21/2016-08-24. berlin: german acoustical society (dega), 2016. isbn 978-3-939296-11-9. [3] j. smutný, l. pazdera, v. tomandl. assessment of the effectiveness od low noise barrier brens barrier in tetčice village. in: eisereich, j.e., ed. rychlost s tichostí. rychlost s tichostí 2015, plzeň. praha: powerprint, 2015. isbn 978-80-87994-28-3. 10 vol. 11/2017 development of the municipal noise-absorbing screen [4] p. čížková, l. lomoz. the effectiveness of a low height noise barrier. in: 14th geoconference on ecology, economics, education and legislation. 14th international multidisciplinary scientific geoconference sgem 2014, albena, 2014-06-17/2014-06-26. sofia: stef92 technology ltd., 2014. issn 1314-2704, isbn 978-619-7105-18-6, doi 10.5593/sgem2014/b52/s20.094. [5] p. čížková, l. lomoz. comparison of efficacy of low and hight acoustic screens. akustika, 2014. issn 1801-9064. [6] m. lidmila. nové perspektivy využití recyklovaných materiálů v železničním stavitelství / new perspectives of using recycled materials in railway construction, in: udržitelná výstavba. čvut v praze, 2011. isbn 978-80-01-04961-7. [7] l. horníček, h. krejčíková, m. lidmila, et al. antivibrační rohože v železničním stavitelství v čr / antivibration mats in railway construction in the czech republic. české vysoké učení technické v praze, fakulta stavební, 2011. isbn 978-80-01-04963-1. [8] j. makuch. maty wibroizolacyjne we wrocławskich torach tramvajowych. in: xiv konferencja naukowo-techniczna "drogi kolejowe 2007" [online], 2007. [cit. 2017-03-7] http://www.zits.pwr.wroc.pl/makuch/art38.pdf. [9] m. lidmila, v. lojda, o. bret. laboratory models of a railway track with the subgrade layer based on rubber granulate in: advances and trends in engineering sciences and technologies ii: proceedings of the 2nd international conference on engineering sciences and technologies, 29 june – 1 july 2016, high tatras mountains, tatranské matliare, slovak republic. boca raton: crc press, 2016. isbn 9781138032248. [10] protihlukové stěny hluku nezabrání a jsou bezpečnostní komplikací / noise-absorbing walls do not prevent noise being an obstacle to safety, želpage [online]. [cit. 2017-03-10] http://www.zelpage.cz/zpravy/8188?lang=cs. [11] o. bret. determination of rubber granulate frost protection. in: juniorstav 2017, 19. odborná konference doktorského studia. brno: vysoké učení technické v brně, fakulta stavební, 2017. isbn 978-80-214-5473-6. [12] čsn 28 0318. průjezdné průřezy tramvajových tratí a obrysy pro vozidla provozovaná na tramvajových dráhách / structure gauges and vehicle gauges for vehicles operated on tram tracks. praha: český normalizační institut, 2015. [13] katalogový list tramvajového vozu kt8d5. skd trade, a.s. / catalogue sheet of t6a5. tram car of skd trade a.s. [online]. [cit. 2017-03-18] http://www.skd.cz/tramvaje/kt8d5.htm. [14] r. mara. pražské tramvaje. praha: pavel malkus dopravní vydavatelství, 9/2012. isbn 978-80-87047-28-6. [15] archives of the prague public transit company. [16] katalogový list tramvajového vozu t6a5. skd trade a.s. / catalogue sheet of t6a5. tram car of skd trade a.s. [online]. [cit. 2017-03-18] http://www.skd.cz/tramvaje/t6a5.htm. 11 acta polytechnica ctu proceedings 11:6–11, 2017 1 introduction 2 municipal noise-absorbing screen 3 limiting conditions for mnas placement, shape and execution 3.1 verification of prototypes 3.2 selection of a section for trial installation 3.3 test section construction 3.4 screen modification 4 conclusion references acta polytechnica ctu proceedings doi:10.14311/ap.2016.4.0102 acta polytechnica ctu proceedings 4:102–106, 2016 © czech technical university in prague, 2016 available online at http://ojs.cvut.cz/ojs/index.php/app activation analysis of historical samples and neutron spectrum determination at vr-1 training reactor jan šturma∗, milan štefánik department of nuclear reactors, faculty of nuclear sciences and physical engineering, czech technical university in prague, břehová 7, 115 19 praha, czech republic ∗ corresponding author: sturmja2@fjfi.cvut.cz abstract. vertical irradiation channel of the vr-1 training reactor of the department of nuclear reactors ctu was used for activation analysis of historical samples from the 14th to the 19th century. for determination of mass fractions of materials such as copper, gold and silver in irradiated samples, the relative method of activation analysis was used. further, a set of 24 activation detectors of 12 various materials served for the determination of neutron spectrum of the vr-1 reactor using analytical method; moreover the analytical solution was compared to unfolded spectrum obtained from sand-ii deconvolution program. keywords: activation analysis, reaction rate, neutron spectroscopy, historical samples, westcott formalism. 1. introduction research reactors can be used as a significant source of radiation, especially neutrons. there are several ways of neutron applications from medical purposes such as the boron capture therapy to material testing, or transmutation technologies et cetera. one possible application is also a radioanalytical method called the neutron activation analysis. this method is based on intentional production of radionuclides in studied materials through the neutron interactions; it is an exact tool that can be used to qualitatively and quantitatively determine composition of various samples. the pool-type light water training reactor vr-1 of the department of nuclear reactors ctu is frequently used for purposes of neutron activation analysis especially due to its experimental equipment such as vertical and horizontal irradiation channels. presented project aims to determine the composition of historical samples and neutron spectrum of vr-1 training reactor by using the neutron activation technique. 1.1. neutron activaton analysis measuring the characteristic radiation of the sample, usually gamma or beta particles, produced in radionuclide decays, the saturated activity of irradiated sample can be determined as asat = λtreal s(eγ) tlive εfep(eγ) iγ(eγ)( 1 − e(−λta) ) e(−λtc) ( 1 − e(−λtreal) ) ∏ i ci, (1) where λ is the decay constant of measured radionuclide, treal is the real time of the gamma spectrum measurement, tlive is the live time of the measurement, s(eγ) is number of counts in the full energy peak at energy eγ , εfep is the detector efficiency for full energy peak, iγ is the probability of gamma emission, ta is the irradiation time and tc is the cooling time. possible corrections, e.g. the correction for reactor power fluctuation, self-shielding of gamma radiation or the correction for gamma coincidences, are represented by factors ci. for determination of sample composition, the relative method of the activation analysis is standardly used. studied samples are irradiated together with the activation standards of known composition. the amount of investigated material in the sample can be determined as follows ast mst = asp msp , (2) where ast and asp are the saturated activities of produced radionuclide in standard or in sample, and mst and msp are the masses of investigated material in standard foil or in sample. other technique of neutron activation analysis is the absolute method, where the amount of material in sample is defined as msp = aspm naφσa , where m is the molar mass of examined material, na is the avogadro’s number, φ is the integral neutron flux and σa is the spectrum averaged microscopic activation cross-section. 1.2. neutron spectrum determination by activation detectors methods of neutron activation analysis can be also used for measurement of neutron flux using activation detectors of various materials. the response of the 102 http://dx.doi.org/10.14311/ap.2016.4.0102 http://ojs.cvut.cz/ojs/index.php/app vol. 4/2016 activation analysis and neutron spectrum determination at vr-1 reactor detector to the neutron field per one nuclei, called reaction rate, can be calculated as rr = asat n0 , (3) where n0 is the number of target nuclei in the activation detector. reaction rates obtained by irradiating the set of various detectors together with the cadmium cover as well as some threshold interactions make possible to reconstruct the neutron field using analytical methods. the analytical expression of neutron flux is φ(en) = φt(en) + φe(en)∆1∆2 + φf(en). (4) neutron flux in thermal energy region, φt(en), is for well moderated reactors defined according to the westcott formalism as [1] φt(en) = en (ktn)2 e− en ktn rt r g(tn)σ0 , where en is the energy of neutrons, k is the boltzmann constant, tn is the thermodynamic temperature of neutrons, rt r is the response of detector to thermal neutrons, g(tn) is the deviation of the activation cross section from the law 1/v in thermal energy region and σ0 is the activation cross-section for energy en = 0.0253 ev. the activation detector in cadmium cover is activated mostly by neutrons of energies associated with the energy of most significant resonance in crosssection, and therefore it is suitable to determine the epithermal neutron flux. discrete values of neutron flux for resonance energies eres of activation materials are defined as [2] φt(eres) = repi r k∫ e2 e1 σ(en) den , where repi r is the response of the detector in cadmium cover to the neutron field, k is the proportional representation of the most significant resonance in the cross-section in epithermal energy region, and e1 or e2 represents the lower or upper boundary of resonance. experimentally obtained discrete values of neutron flux can be fitted using the expected shape of epithermal reactor spectrum in the form of a/exn , and consequently it can be smoothly connected to the thermal and fast analytical solution of neutron spectra using the joining functions ∆1 and ∆2 defined as ∆1 = 1 1 + ( 4.95ktn en )7 , ∆2 = exp ( − en 3 · 105 ) . threshold activation detectors are serving to express the reaction rates rf r of activation detectors to the fast neutron region. in fast region, the differential neutron flux is described as φf(en) = rf r σt s(en), where σt is the spectrum averaged threshold crosssection and the s(en) is the fission neutron spectrum. 2. materials and methods 2.1. activation experiments of historical samples the vertical irradiation channel of vr-1 training reactor was used to perform the activation experiments of historical coins; the main purpose was to determine the qualitative and quantitative composition by employing the relative method of neutron activation analysis. in 2014, six historical coins were irradiated together with the reference standards of expected materials. in particular, kurush-1923, pfennig-1745, kreutzer-1843, parvus-14.century, centessimo-1852, and heller-1854 were studied, and copper, gold, silver, and nickel foils were used as activation standards in two sets. in 2015, another three coins (korona-1893, kreutzer-1861, and pfennig-1776) together with the silver and copper standards in the form of foils were irradiated. figure 1. first set of historical samples with reference materials. figure 2. third set of historical coins analyzed using activation analysis. figure 3. hpge detector in pb shielding. each set of samples was irradiated twice. the first irradiation at low reactor power was performed to disprove the presence of inappropriate isotopes, such as 59co. because after irradiation of such isotope, the long term radionuclide is produced, and it is inconvenient. gamma lines of 116min were observed in the spectrum of one sample, and due to this fact, the 103 jan šturma, milan štefánik acta polytechnica ctu proceedings indium reference activation standard was added to the set before main irradiation at the nominal power. during the second irradiation, the reactor was operated at power-level of 1 kw, and the irradiating time was set to 15–30min. after the main irradiation, the gamma-ray spectra of irradiated samples were measured by the semiconductor hpge detector. for all radionuclides observed in spectra, the saturated activity was determined by using the equation (1). 2.2. irradiationg of activaction detectors altogether 24 activation detectors made of 12 various materials (al, ag, au, cu, fe, in, lual, mn, mo, nacl, u and w) in the form of thin foils (nacl detector had a form of tablet) were irradiated in the vertical irradiation channel at mid-height of reactor core of vr-1 reactor. irradiating time was 15–45min at the reactor-power of 1 kw. activation materials were chosen with respect to the activation cross-sections in order to cover all energy regions of reactor spectrum; therefore the cadmium cover and some threshold detectors were used. after irradiation, the gamma spectra of detectors were measured. for each observed reaction, the reaction rates were determined using the expression (3). figure 4. set of activation detectors. reaction t1/2 rr [s−1] dev [s−1] 197au(n,γ)198au 2.70 days 1.77 · 10−13 4.04 · 10−15 197au(n,γ)198au cd 2.70 days 3.70 · 10−14 2.61 · 10−16 63cu(n,γ)64cu 12.70 hours 7.89 · 10−15 1.62 · 10−16 63cu(n,γ)64cu cd 12.70 hours 3.51 · 10−16 5.50 · 10−18 186w(n,γ)187w 23.72 hours 9.05 · 10−14 7.77 · 10−16 186w(n,γ)187w cd 23.72 hours 1.56 · 10−14 1.20 · 10−16 98mo(n,γ)99mo 65.94 hours 8.46 · 10−16 2.12 · 10−17 98mo(n,γ)99mo cd 65.94 hours 5.61 · 10−16 1.46 · 10−17 115in(n,γ)116min cd 54.29min 4.06 · 10−14 2.13 · 10−16 115in(n,n’)115min 4.47 hours 1.44 · 10−16 3.76 · 10−18 115in(n,γ)116min 54.29min 2.19 · 10−13 1.79 · 10−15 238u(n,γ)239u 23.45min 8.70 · 10−15 3.23 · 10−17 238u(n,γ)239u cd 23.45min 4.99 · 10−15 1.45 · 10−17 23na(n,γ)24na cd 14.96 hours 2.64 · 10−17 4.01 · 10−19 27al(n,p)27mg 9.46min 7.77 · 10−18 3.12 · 10−19 27al(n,α)24na 14.96 hours 5.29 · 10−19 4.12 · 10−20 56fe(n,p)56mn 2.58 hours 1.10 · 10−18 2.72 · 10−20 109ag(n,γ)110mag cd 249.79 days 1.79 · 10−15 7.59 · 10−18 55mn(n,γ)56mn cd 2.58 hours 9.25 · 10−16 6.14 · 10−18 55mn(n,γ)56mn 2.58 hours 2.37 · 10−14 5.28 · 10−16 176lu(n,γ)177lu cd 6.73 days 6.89 · 10−14 1.85 · 10−15 176lu(n,γ)177lu 6.73 days 5.70 · 10−12 5.05 · 10−14 23na(n,γ)24na 14.96 hours 7.65 · 10−16 7.82 · 10−18 37cl(n,γ)38cl 37.24min 7.01 · 10−16 9.07 · 10−18 table 1. reaction rates of irradiated activation detectors. 3. results 3.1. analysis of historical coins composition the relative method of neutron activation analysis was used to determine the quantitative composition of five materials in all irradiated historical coins using equation (2). measured mass and mass fractions wi are listed in table 2. sample material m [mg] wi [%] kurush au 1 052.64± 12.54 58.44± 0.70 pfennig ag 17.97± 0.16 5.62± 0.05 (1745) cu 265.72± 5.49 83.17± 1.72 kreutzer cu 3 682.40± 96.71 94.95± 2.49 (1843) parvus ag 49.11± 0.43 13.67± 0.12 cu 194.81± 4.65 54.22± 1.29 centessimo cu 1 312.61± 11.46 99.95± 0.87 in (12.34± 0.58)·10−3 (76.10± 4.42)·10−5 heller ag 2.01± 0.31 0.04± 0.01 cu 4 797.12± 77.15 93.83± 1.51 pfennig ag 37.48± 0.99 12.82± 0.34 (1776) cu 265.88± 6.71 90.96± 2.29 korona ag 2 674.09± 3.097 54.98± 0.64 cu 448.64± 11.9 9.22± 0.42 kreutzer cu 2 889.20± 71.95 92.89± 2.31 (1861) table 2. determined composition of irradiated coins [3]. parameters of radioactive decays from [4], such as the half-life period, energy and intensity of gamma radiation, were used to assign the observed gamma lines in measured spectra to specific radionuclides. then according to the neutron energies reached in thermal nuclear reactors, cross-sections, and natural isotopic composition, the observed radionuclides were assigned to considered materials. qualitative analysis of composition was carried out in this way for the coin heller. except the contribution of natural background, the gamma spectrum of heller (see fig. 5) contains the gamma lines of 64cu, 110mag marked in blue color, and then 122sb, 124sb, 76as and 24na as well. observed radionuclides referred to presence of antimony, arsenic, mercury, silver and sodium in irradiated samples. figure 5. analyzed gamma spectrum of heller. 104 vol. 4/2016 activation analysis and neutron spectrum determination at vr-1 reactor 3.2. neutron spectrum reconstruction reaction rates of interaction observed in used activation detectors through produced radionuclides covered all the energy regions of neutron spectra for typical thermal reactor. the use of cadmium cover enabled to separate the contributions of thermal and epithermal neutrons. threshold interactions served to determine the response of detector to the fast neutron. using nuclear data from endf/b-vii.1 [5], the neutron spectrum was determined according to equation (4). the irradiation position in the fuel cell dummy with the overmoderated neutron spectrum allowed to utilize the westcott formalism. separately described energy regions of neutron spectrum together with the discrete points of neutron flux, associated with the most significant resonances in activation cross-sections, and total neutron flux, determined using analytical methods, are depicted in fig. 6 and fig. 7 respecitvely. figure 6. analytic description of neutron flux regions. figure 7. total neutron flux determined by analytical methods. results obtained by analytical methods were compared to numerical solution using the unfolding procedure of sand-ii code [6]. as the input (guess) neutron spectrum requested for the deconvolution process, the analytical spectrum was used. in the 5th iteration and using nuclear data from the endf/bvii.1 data library and experimentally determined reaction rates, the neutron spectrum in 36 energy bins from 10−12 mev to 101 mev was successfully unfolded. comparison of determined numerical and analytical neutron spectrum in the mid-height of reactor core of vr-1 training reactor is in fig. 8. calculated ratios c/e, representing calculated over experimental reaction rates rations, as a parameter of spectrum unfolding, are listed in table 3. figure 8. comparison of analytical and unfolded spectrum. reaction c/e [–] reaction c/e [–] 197au(n,γ)198au 0.92 56fe(n,p)56mn 0.99 63cu(n,γ)64cu 0.87 55mn(n,γ)56mn 0.84 115in(n,γ)116min 1.18 176lu(n,γ)177lu 1.06 23na(n,γ)24na 1.03 37cl(n,γ)38cl 0.92 27al(n,α)24na 1.06 238u(n,γ)239u 1.01 table 3. c/e ratios of reactions used for the spectrum unfolding. 4. conclusions according to the experiences and results of activation experiments, the vr-1 reactor is suitable for analyzing of composition of historical samples, especially coins. table 2 clearly shows the possibilities of determination of all materials commonly used in coin production, such as gold, silver, or copper. main advantages of vr-1 training reactor is the possibility of easy way of inserting samples into irradiating position in vertical channel and relatively low reactor power, comparing to others research reactors in czech republic, so it is hard to produce emitters of very high activities. the first time used method of analytical spectrum description in full energy range showed the new possibility of neutron spectrum determination using the activation detectors at vr-1 reactor. moreover, the analytical spectrum corresponds to expectations, but compared to results of numerical solution using the sand-ii unfolding code appears slightly undervalued in the thermal energy region. this brings a new way of potential further specifying of the analytical spectrum description. results illustrated in this paper are a summary of outcome achieved within the bachelor’s thesis in 2013– 2014 and the master’s degree project in 2015–2016. references [1] c. westcott, w. walker, t. alexander. effective cross sections and cadmium ratios for the neutron spectra of thermal reactors. tech. rep., atomic energy of canada ltd., 1959. conference: 2. united nations international conference on the peaceful uses of atomic energy, geneva (switzerland), 1958. http://www.osti.gov/scitech/biblio/4279663. [2] h.-l. pai, p.-y. ma, s.-c. wang, w. s. lee. measurement of epithermal neutron spectra by 105 http://www.osti.gov/scitech/biblio/4279663 jan šturma, milan štefánik acta polytechnica ctu proceedings resonance detectors. nuclear science and engineering 9(4):519–520, 1961. [3] j. šturma. neutronová aktivační analýza historických předmětů na školním reaktoru vr-1. bachelor’s thesis, kjr fjfi, české vysoké učení technické v praze, 2014. [4] s. chu, l. ekström, r. firestone. the lund/lbnl nuclear data search, version 2.0, february 1999. http://nucleardata.nuclear.lu.se/toi/. [5] national nucelar data center. endf/b-vii.1 evaluated nucelar data library. http://www.nndc.bnl.gov/endf/b7.1/. [6] p. griffin, j. kelly, j. vandenburg. user’s manual for snl-sand-ii code. tech. rep. sand93-3957, sandia national laboratories, 1994. http://prod.sandia. gov/techlib/access-control.cgi/1993/933957.pdf. 106 http://nucleardata.nuclear.lu.se/toi/ http://www.nndc.bnl.gov/endf/b7.1/ http://prod.sandia.gov/techlib/access-control.cgi/1993/933957.pdf http://prod.sandia.gov/techlib/access-control.cgi/1993/933957.pdf acta polytechnica ctu proceedings 4:102–106, 2016 1 introduction 1.1 neutron activaton analysis 1.2 neutron spectrum determination by activation detectors 2 materials and methods 2.1 activation experiments of historical samples 2.2 irradiationg of activaction detectors 3 results 3.1 analysis of historical coins composition 3.2 neutron spectrum reconstruction 4 conclusions references acta polytechnica ctu proceedings doi:10.14311/app.2017.7.0007 acta polytechnica ctu proceedings 7:7–11, 2017 © czech technical university in prague, 2017 available online at http://ojs.cvut.cz/ojs/index.php/app comparison of virtual fields method, parallel network material model and finite element updating for material parameter determination florian dirisamer∗, umut d. çakmak, imre kállai, martín machado, zoltán major institute of polymer product engineering, johannes kepler university linz, altenbergstrasse 69, 4040 linz, austria ∗ corresponding author: florian.dirisamer@jku.at abstract. extracting material parameters from test specimens is very intensive in terms of cost and time, especially for viscoelastic material models, where the parameters are dependent of time (frequency), temperature and environmental conditions. therefore, three different methods for extracting these parameters were tested. firstly, digital image correlation combined with virtual fields method, secondly, a parallel network material model and thirdly, finite element updating. these three methods are shown and the results are compared in terms of accuracy and experimental effort. keywords: dic, finite element updating, parallel network model, vfm. 1. introduction elastomers are often used as damping elements and undergo, depending on their constraints, uniand multiaxial loadings in tension, compression and shearing [1–4]. the nonlinear inherent viscoelastic material behaviour of the epdm shore a 50 with a maximum elongation of several hundred percent makes it further complicated to extract material parameters for the whole range of time (frequency), temperature and environmental conditions relevant for the application [5, 6]. therefore a large number of tests at different temperatures and loading velocities have to be performed (75 tests [7] for the standard method (std)). to circumvent these difficulties and to extract the material parameters sufficiently, three different and well-known methods were used here for comparison purposes. the motivation is the biaxial characterization of elastomers for different applications (sealing, damping, etc.). firstly, a combined methodology based on digital image correlation (dic) [8] and virtual fields method (vfm) [9]. in this method, a heterogeneous strain field is introduced in order to acquire more than one parameter in a single test. this leads to drastically reduced time and costs. secondly, a parallel network material model (pnmm) with the procedure of bergström [10] (mcalibration and polyumod), where a test procedure including hysteretical and relaxation behaviour at certain temperatures, velocities, as well as test conditions leading to the modeling of the viscoelastic behaviour. thirdly, a finite element updating (feu) procedure using the abaqussolver [11] is utilized. measured force-displacement signals are compared to the simulated ones and the material parameters are updated until these two signals fit sufficiently within a defined tolerance. finally, these three methods are compared in terms of accuracy 36 80 120 r22 (a) . specimen 0 100 200 300 400 500 6000 5 10 15 20 25 30 35 40 45 50 55 time in s d isp la ce m en t in m m 0.1 mm s−1 1 mm s−1 10 mm s−1 hyst. & res. (b) . displacement-time diagram. figure 1. biaxial test specimen with dimensions in mm and displacement-time diagram for 0.1 mm s−1, 1 mm s−1, 10 mm s−1 and with the procedure including hystereses and relaxation for the experiments. and experimental effort and the advantages as well as disadvantages are summarized for each method. 2. description of the methods the biaxial test specimens (figure 1a) were tested with a biaxial test frame [2] at 20 °c in a temperature chamber (cts gmbh, jennersdorf, aut), loaded by an 1.5 kn actuator and measured with a 4.5 kn load cell (bose electroforce systems group, mn, usa). the optical strain measurement was performed with a dic-camera system (aramis, gom mbh, braunschweig, d) and a cold light source (dedocool, dedotec usa inc., ma, usa) and a speckle pattern was manually applied to the biaxial test specimen. the further workflow for each of the three methods, dic combined with vfm (figure 4), pnmm with the procedure of bergström (mcalibration and polyumod) (figure 7) and feu (figure 10), is explained in detail in the subsections 2.12.3. 7 http://dx.doi.org/10.14311/app.2017.7.0007 http://ojs.cvut.cz/ojs/index.php/app f. dirisamer, u.d. çakmak, i. kállai et al. acta polytechnica ctu proceedings (a) (b) (c) (d) (e) (f) εxx εyy εxy εmajor εminor εres = εminor εmajor legend εxx legend εyy legend εxy legend εmajor legend εminor legend εres -0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 -0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 -0.2500 -0.1875 -0.1250 -0.0625 0.0000 0.0625 0.1250 0.1875 0.2500 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.000 0.025 0.050 0.075 0.100 0.125 0.150 0.5 0.6 0.7 0.8 0.9 1.0 figure 2. digital image correlation results at max strains in εxx (a), εyy (b) and εxy (c) and calculated results for εmajor (d), εminor (e) and εminor/εmajor (f). 0 10 20 30 40 500 1 2 3 strain in % st re ss in m pa 0.1 mm s−1 1 mm s−1 10 mm s−1 figure 3. stress-strain diagram for 0.1 mm s−1, 1 mm s−1 and 10 mm s−1. 2.1. digital image correlation (dic) and virtual fields method (vfm) the workflow for the vfm is shown in figure 4. displacement controlled tests at velocities of 0.1 mm s−1, 1 mm s−1 and 10 mm s−1 (figure 1b) were performed with optical strain measurement and the loaddisplacement data for each of the three velocities, as well as the optical images were recorded. these image series are evaluated via dic (ncorr, georgia institute of technology, ga, usa) to generate the strain fields εxx, εyy and the shear field εxy (figure 2a-c). afterwards εmajor (figure 2d) and εminor (figure 2e) were calculated according to equation 1. εmajor/minor = εxx + εyy 2 ± √( εxx + εyy 2 )2 − ( εxx · εyy − ε2 xy ) (1) the equibiaxial area is the region, where εmajor and εminor are “equal”, therefore, εres = εmajor/εminor (figure 2f) was calculated and the equibiaxial area was extracted in the range of εres ≥ 0.5. that area perform test capture images image series matlab (dic) strain field matlab major/minor strain matlab stress-strain data load-displacement data figure 4. workflow for the virtual fields method (vfm). was afterwards converted to an equibiaxial diameter and the biaxiality ratio φ, the biaxial stress σ was calculated by dividing the measured force with the equibiaxial diameter and the thickness and the strain ε was calculated for each image series (figure 3). 8 vol. 7/2017 comparison of vfm, pnmm and feu 0 10 20 30 40 50 600 1 2 3 strain in % st re ss in m pa 0 20 40 601. 45 1. 75 time in s s tr es s in m p a 0 20 40 601. 95 2. 3 time in s s tr es s in m p a 0 20 40 602. 2 2. 55 time in s s tr es s in m p a 0 20 40 602. 35 2. 75 time in s s tr es s in m p a figure 5. stress-strain diagram for load with hystereses and relaxation with marked relaxation areas. parameter value parameter value bulk modulus 106.59 lock stretch 4.0841 pressure dependence of flow −0.129 temp. exp. flow resistance 0 temp. factor for stiffness 0 therm. expansion coefficient 0 thermal expansion ref. temp. 293 flow resistance a 0.1743 shear modulus a 7.9898 stress exponential a 11.499 final shear modulus b 0.9803 flow resistance b 1.4018 initial shear modulus b 2.0318 norm. evolution rate of µb 10.212 stress exponential b 10.156 relative contribution of i2 c 9.3e-05 shear modulus c 1.3068 table 1. parameter table for three network material model. 2.2. parallel network material model (pnmm) the workflow for the pnmm is shown in figure 7. for the procedure of bergström (mcalibration and polyumod), tests which include hysteresis and relaxation domains (figure 1b) was performed. the hysteresis and relaxation domains are necessary to model the viscoelastic behaviour with one material test at a certain temperature and velocity. as a material model, the three parallel network model [12] was chosen. it consists of: 1st network: temperature-dependent 8-chain model [13, 14], power-law flow rate, hydrostatic pressure dependence, power-law temperature dependence strain in % 0 10 20 30 40 500.0 0.5 1.0 1.5 2.0 2.5 3.0 st re ss in m pa experimental predicted figure 6. experimental and predicted stress-strain diagram. perform test load-displacement data abaqus stress-strain data mcalibration & polyumod material model material model parameter figure 7. workflow for the parallel network material model (pnmm). 2nd network: temperature-dependent 8-chain model, shear modulus evolution with plastic strain, power-law flow rate, hydrostatic pressure dependence, power-law temperature dependence 3rd network: 8-chain model with first order i2 (second invariant of deformation tensor) dependence. with a simulation, the stress-strain data (figure 5) was calculated, which is imported and processed in mcalibration. the material parameters are calculated analytically by comparing the predicted results to the experimental ones and updating these parameters until the results fit. in that way, the material parameters are generated (table 1) and the experimental as well as predicted strain-stress curves are shown in figure 6. 9 f. dirisamer, u.d. çakmak, i. kállai et al. acta polytechnica ctu proceedings 0 10 20 30 40 500 50 100 150 200 250 displacement in mm lo ad in n 0.1 mm s−1 1 mm s−1 10 mm s−1 experimental predicted figure 8. experimental and predicted forcedisplacement diagram with the mooney-rivlin model. velocity c10 c01 d[ mm s−1] [mpa] [mpa] 0.1 0.5849 0.0856 0 1 0.5924 0.1016 0 10 0.5979 0.1506 0 table 2. parameter table of the received mooneyrivlin model-parameters. 2.3. finite element updating (feu) the workflow for the feu is shown in figure 10. tests with velocities of 0.1 mm s−1, 1 mm s−1 and 10 mm s−1 (figure 1b) were performed and the load-displacement data were recorded (figure 8, solid lines). these data were imported separately to matlab [15] and a custom-made feu-script is executed. this prepares the input file for abaqus automatically, executes the solver, performs a numerical simulation and extracts the simulated results. it compares the simulated (predicted) results to the experimental ones, updates the material parameters in the input file and repeats that procedure until the load-displacement curve fit within a defined tolerance. the gained material parameters are liste in table 2 and the compared load-displacement data are shown in figure 8. furthermore, the evolution of the parameters c10 and c01 are visualized in figure 9. 3. results and discussion in order to analyze the difference of the three terms of accuracy and experimental effort, the advantages as well as the disadvantages are summarized individually. 10−1 100 101 0.58 0.59 0.6 velocity in mm s−1 c 10 in m pa c10 0.06 0.11 0.16 c 01 in m pa c10 c01 figure 9. evolution of c10 and c01 over the velocity. perform test load-displacement data matlab abaqus material model parameter figure 10. workflow for the finite element updating (feu). • virtual fields method (vfm) the stress-strain data were calculated by vfm, in which the actual full-field strain data is considered. this full-field data includes some information about the sliding of the clamping system, elastic extension of the machine due to the force and some other small effects, which cause deviation between the measured and applied length. therefore, these effects can be taken into account and excluded. furthermore, more than one material parameter can be analyzed by generating inhomogeneous strain fields (different loading states) in one test and specimen. however, there are difficulties in finding the right equations for the virtual strain field of each loading condition and its specific (geometric) constraints to receive the right parameters and dic data is needed, which cause higher effort for acquisition and a huge amount of computational power. • parallel network material model (pnmm) the result of pnmm are the nonlinear material model parameter which include here (but is not limited to) viscoelastic, relaxation and cyclic behaviour, which gives the whole range of velocity dependence at a certain temperature and loading configuration with one test. stress-strain data must be evaluated by a simulation and the analytical calculation of the material model parameters is time consuming. furthermore, different parameter combinations of the same material model and the same test data can be received by choosing different initial parameters. therefore, the results have to be examined carefully. the selection of the material model is important for the characteristics and the accuracy. 10 vol. 7/2017 comparison of vfm, pnmm and feu std vfm pnmm feu 0 0.5 1 1.5 2 2.5 1 0.33 0.2 11 2 1.5 0.5 eff or t manual experimental computational figure 11. manual experimental and computational effort of virtual fields method (vmf), parallel network material model (pnmm) and finite element updating (feu) normalized to the standard method (std). • finite element updating (feu) the result of feu are the nonlinear material model parameters. therefore, results from specimen, as well as component tests can be taken to create a simulation. to receive the appropriate results, it is necessary to create a detailed simulation with the proper boundary conditions. furthermore, the selection of the material model is essential for the results. however, there are different parameter combinations for the same material model and simulation just by choosing different initial parameters. careful analyses of the results at various loading rates are required. the comparison of the manual and computational experimental effort of the three methods vfm, pnmm and feu, normalized to the standard method (std) (cf. section 1) is shown in figure 11. the manual experimental effort for the standard test consists of three test procedures (uniaxial, pure shear, equibiaxial), five temperatures and five test velocities, which leads to 75 tests (section 1). through the inhomogeneous strainfield of the vfm, the three test procedures can be minimized to one, which gives 25 tests, however, because of dic, the computational effort is more than doubled compared to std. for the pnmm, the whole range of velocity data for a certain temperature is included in one test, this cause 15 tests altogether, but the computational effort is by 50% greater than std. this is caused by the desired stress-strain data and the analytical calculations. the feu has no benefit on the reduction of the number of tests, nevertheless, the computational effort for small fem-simulations can be minimized, as the force-displacement data is selected directly. acknowledgements the research for this paper was performed within the framework of ffg 7th call of “produktion der zukunft” (project number: 848620). references [1] l. treloar. the elasticity and related properties of rubbers. reports on progress in physics 36(7):755, 1973. [2] u. d. çakmak, z. major. experimental thermomechanical analysis of elastomers under uniand biaxial tensile stress state. experimental mechanics 54(4):653–663, 2014. [3] u. d. çakmak, i. kallaí, z. major. temperature dependent bulge test for elastomers. mechanics research communications 60:27–32, 2014. [4] e. leiss-holzinger, u. d. çakmak, b. heise, et al. evaluation of structural change and local strain distribution in polymers comparatively imaged by ffsa and oct techniques. express polym lett 6:249–256, 2012. [5] n. w. tschoegl, w. g. knauss, i. emri. the effect of temperature and pressure on the mechanical properties of thermoand/or piezorheologically simple polymeric materials in thermodynamic equilibrium–a critical review. mechanics of time-dependent materials 6(1):53–99, 2002. [6] u. d. çakmak. experimental studies on time-temperature-dependent bulk and surface behavior of soft composite materials. ph.d. thesis, johannes kepler university, 2014. [7] a. grambow. bestimmung der materialparameter gefüllter elastomere in abhängigkeit von zeit, temperatur und beanspruchungszustand. ph.d. thesis, bibliothek der rwth aachen, 2002. [8] ncorr. ncorr. http://www.ncorr.com, 2016. [9] f. pierron, m. grédiac. the virtual fields method: extracting constitutive mechanical parameters from full-field deformation measurements. springer science & business media, 2012. [10] j. bergström. polyumod user’s manual. veryst engineering, llc, needham, ma 2009. [11] hibbett, karlsson, sorensen. abaqus/standard: user’s manual, vol. 1. hibbitt, karlsson & sorensen, 1998. [12] j. bergström, j. bischoff. an advanced thermomechanical constitutive model for uhmwpe. the international journal of structural changes in solids 2(1):31–39, 2010. [13] e. m. arruda, m. c. boyce. a three-dimensional constitutive model for the large stretch behavior of rubber elastic materials. journal of the mechanics and physics of solids 41(2):389–412, 1993. [14] j. bergström, m. boyce. large strain time-dependent behavior of filled elastomers. mechanics of materials 32(11):627–644, 2000. [15] c. moler, et al. matlab user’s guide. university of new mexico, 1982. 11 http://www.ncorr.com acta polytechnica ctu proceedings 7:7–11, 2017 1 introduction 2 description of the methods 2.1 digital image correlation (dic) and virtual fields method (vfm) 2.2 parallel network material model (pnmm) 2.3 finite element updating (feu) 3 results and discussion acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2015.1.0040 acta polytechnica ctu proceedings 2:40–44, 2015 © czech technical university in prague, 2015 available online at http://ojs.cvut.cz/ojs/index.php/app planning and coordination in driving simulation martin schaefer department of computer science, fee, ctu in prague, czech republic correspondence: martin.schaefer@fel.cvut.cz abstract. we present an overview of coordination and planning tasks that we face with during the development of the agentdrive simulation platform. we particularly describe an integration of the agentdrive with a driving simulator opends. we demonstrate how the planning and coordination mechanisms can be applied in a driving simulator for automated driving applications or realistic traffic generation. we emphasize particular planning and/or coordination methods that were already developed using agentdrive platform. keywords: autonomous driving, driving simulation, automated driving, adas, automotive, multiagent systems, agent-based simulation, multi-agent coordination. 1. introduction safety and efficiency of the road traffic transport systems are common goals in the related research areas. automotive industry keeps producing safer and more efficient cars to reach these goals. there is a boom of intelligent solutions to make driving safer, efficient and comfortable. there are passive safety systems followed by active ones in modern cars. active safety systems are represented by a scale of driver assistance systems as emergency braking systems, lane-assist system, automated parking features, etc. these, so called advanced driver assistance systems (adass), are possibly just a step to a fully automated driving. the concept of the fully autonomous car, mostly advertised by google, is well known. the increasing automation in driving should bring the desired safety and efficiency of the road traffic. the road traffic efficiency from traffic perspective is often optimized by controlling traffic flows centrally in the road network. differently, the safety, apart the high-level precautions, is mostly assured individually by on-board systems of each single vehicle. the distribution of the responsibility over vehicles is natural with regards to the distributed physical manner of the domain. the road traffic is a multi-agent system. the distributed agents have usually several common desires, e.g, the safety. on the other hand, there is a competition for the limited resources, i.e., the road network capacity. the private properties of the individual cars and also their heterogeneity motivate multi-agent paradigm to be suitable. verification of such a complex and critical multiagent system is difficult. coordination mechanisms are not directly applicable in the real world traffic. the cost of the experiments with a real hardware is high. the simulation is a tool to be used to evaluate the concepts before the real world deployment. we will describe several particular coordination and planning tasks in this domain on the particular example of the integration of agentdrive and opends. figure 1. opends simulator view, highway scenario the agentdrive platform is being developed to allow to design, prototype and evaluate agent-based coordination mechanisms [1]. opends is an open sourced driving simulator (see figure 1). the agentdrive platform is introduced in [1]. the driver assistance system developed on the top of the integration of agentdrive and opends is described in [2]. in this paper, we will refer to the integration of agentdrive and opends as the simulation platform. 2. simulation platform the integration of agentdrive with a driving simulator is motivated by the need to involve the human factor into the simulation of autonomous road traffic. the driving simulation brings the capability to let a human driver to be a part of the simulation, i.e., human-in-the-loop simulation. also, the driving simulation works as a physics simulator for the agentdrive simulation. on the other hand the agentdrive brings the functionality of a traffic simulation (map data based scenarios, demand modelling, road network structures, etc.). and mainly, it allows to develop agent-based coordination mechanisms on the top of the traffic and the driving simulation. the figure 2 shows the components of the simulation platform. in the following sections, we present the main task 40 http://dx.doi.org/10.14311/app.2015.1.0040 http://ojs.cvut.cz/ojs/index.php/app vol. 2/2015 planning and coordination in driving simulation driving simulation physical sim. hmi 3d vis. human driver sumo osm scenario traffic analysis2d visualization road network multi-agent coordination coordination module srtm 3d model driving simulation figure 2. simulation platform architecture, the driving simulation module represents opends part and the module on right is the agentdrive platform. note, we use openstreetmap data for the road traffic generation. that simulation platform is ready to perform. the tasks are overlapping in several aspects. moreover, combinations of the tasks are also possible to be done. we present the categories for better organization and readability. 2.1. traffic studies the simulation is reasonable if it is able to reflect the important features of the studied real world situation. in the context of the road traffic we aim to instantiate the traffic so it corresponds to the real one. the defining aspects are: amount of vehicles on the road, distribution of the origin and the destination locations and the local behaviour of the vehicles/drivers. this can be achieved by a static routing and scheduling of the traffic. having such a traffic generated, we can study, for example, the effects of different local behaviour of the vehicles/drivers on the global traffic flow. as the local behaviour can have significant effect on the traffic, the original static routing and road utilization can become irrelevant. this usually leads to the usage of a mobility demand model instead of the static traffic instantiation. the mobility demand model describes the origin and the destination locations of trips in time. in this case, online routing and driver models are needed to perform the simulation. 2.2. human factors studies it is important to consider the human factor when a new technology is about to be deployed. nowadays, drivers are fully responsible for controlling the cars. all the technologies that are about to penetrate the road traffic need to be acceptable for the human drivers. moreover, the technologies should help drivers, not to distract them, nor to make them be less aware of the driving. the driving simulators are used to study human factors. also, in our simulation platform the opends is used to incorporate the human into the simulation 3. the opends also allows the researcher to analyse the experiment in the analyser part of the simulator. the external devices to track driver’s behaviour, e.g, stress, attention, can be used. the opends provides the researchers with a possibility to export the experiment outputs directly into the reports. our main interest is particularly in the human factors in automated driving or driving with the advanced driver assistance systems. such studies require (apart the mentioned driving simulator features) the automated driving or adas support respectively. we dedicate a separate section to both of these wide areas. 2.3. driver assistance system studies assistance systems are one of the ways to bring automation into the traffic. driver assistance systems should help drivers to observe the traffic situation by sensors and eventually warn the driver of the danger. active systems, e.g., emergency braking systems, can even intervene in case of emergency. there are several aspects of adas to research. first, it is the development or prototyping of such a system. later on, it is important to study the effects of such a system on the driver’s behaviour and in consequence on the safety. also, we can refer to the studies of the effects on traffic discussed in 2.1. there is a significant effect on the traffic flow even with only few vehicles equipped with adas (published in [3]). 41 martin schaefer acta polytechnica ctu proceedings figure 3. the basic human-in-the-loop simulation setting for human factor studies the simulation platform needs to provide a researcher with a human-in-the-loop simulation and support for adas functionality support. the humanin-the simulation features are similar to the ones described in the human factors section. on the top of it, the external dashboard availability is desired for the implementation of the adas interface. the adas functionality support means that the simulation platform has to offer a way to implement the particular functionality of the system. usually, it means to provide sensory data and an interface for the presentation of the outputs of the system to the driver. the interface can be an already mentioned output to the external dashboard or a human-machine interface (hmi) directly in the simulation visualization. in case of active adas, it is needed to provide a mechanism how the adas can intervene into the controlling of the vehicle. the logic of the assistance system is often in the scope of sensor processing, sensor fusion, reasoning and eventually the control theory. the first two phases related to sensors can be replaced by a model of a sensor in the simulated environment. if there are no real sensory data, it is possible to present a sensor model with a defined uncertainty instead of applying sensor processing on the artificial data. the mentioned reasoning stands for motion planning, trajectory prediction, pathfinding, etc.. the reasoning is then in the focus of our interest, it is where the coordination and planning is applicable. 2.4. connected car studies connected cars term stands for vehicles that are equipped with communication devices that allow them to communicate with other vehicles (v2v), infrastructure (v2i) or with manufacturer, emergency services and others (v2x). the communication device can be approached as a sensor. then, the communication sensor can be used for collective localization or world mapping. the communication sensor is providing the reasoning unit with an additional information that can be used for the reasoning. connectivity is well utilized in the cooperative setting. here, the communication channel is used as an infrastructure for cooperative protocols. more advanced cooperative techniques can be beneficial but the high demands on the connection is often a drawback of such approaches. 2.5. autonomous driving studies autonomous or driver-less vehicles is a wide research field. it offers also many cooperative and planning solutions to be applied. the agentdrive platform is primary designed to support a development of the multi-agent coordination and planning algorithms. driving simulation is not explicitly needed for autonomous driving as there is no driver any more. nevertheless, the driving simulation is useful for many purposes in the autonomous driving related studies. it is not realistic that all the vehicles become driver-less overnight. so, there will be a period of time when there are human-driven vehicles and driver-less cars on the road at the same time. also, the autonomous driving could be firstly applied to only controlled areas or situations (e.g., highways), so the driver will need to share the responsibility with the computer. completely different application of autonomous driving is in automated traffic generation for a driving simulation. mostly, if the traffic is present in the driving simulation, it is scripted to perform just a really basic model of a reactive traffic. then, the algorithms to control an autonomous vehicle can be used to generate a more realistic and autonomous traffic for a better driving simulation experience. 3. planning and coordination applications the integration of opends and agentdrive is built on the idea to perform studies described in the previous section. it shows, that the planning and coordination applications play a key role in our simulation platform. this section presents the particular already developed applications of coordination and planning. such applications form a toolbox for the previously mentioned studies. 3.1. routing and navigation routing is the shortest path planning problem on the road network graph. the problem is well known and a simple dijkstra’s or a* algorithm are applicable on the problem. the graph edges can be weighted according to the travelling speed and the occupation of the roads. this allows the vehicles to optimize the routing according the actual traffic as it is commonly done nowadays in navigation application. also, the online rerouting is applicable if it is desired. the routing algorithm from the current location is used in this case. 42 vol. 2/2015 planning and coordination in driving simulation the navigation is a routing on the finer graph. the routing is performed on the level of lanes in this case. it is used for proper lane navigation, so that the vehicle is able to get into the desired lane changing lanes. 3.2. driver assistance systems we have already developed several solutions related to the adas. the applications are all related to the highway driving. highways are the simpler domain for assistance systems like an adaptive cruise control (acc) or lane change assists (lca). we can consider only longitudinal dimension to be continuous, the lanes are considered to be discrete in the highway domain. in [2] we demonstrate how the agentdrive is suitable for adas development. the integrated cooperative adaptive cruise control and cooperative lane change assist was developed and presented in [4] and [5]. we employed a safe-distance method originally designed in [6] to provide a driver with an assistance. the driver with the assistance was advised to change speed or to change lane by the assistance system. the results show that the drivers claimed the decreased cognitive load with the activated assistance system. another interesting extension of the acc is presented in [7]. the author claims that the acc functionality for linear space (i.e., vehicles in line) could be extended for in junction coordination. it is possible if we are able to project the junction situation (i.e., vehicles approaching from different directions) into the linear space. 3.3. local coordination algorithms the purpose of such algorithms is to control vehicles so that the vehicles follow their desired paths while avoiding collisions with each other. the algorithms can be applied to the fully controlled vehicles, i.e., autonomous driving, or to feed the advanced driver assistance systems. there are several approaches to the road traffic coordination. the basic approach is a reactive mechanism, where an agent representing a vehicle observes the neighbourhood and reacts to it. the safe-distance method presented in [6] is an example of a such approach. the main idea of the method is in the maintaining a safe distance between the vehicles. an action that an agent performs must result into a safe state. a vehicle is in a safe state when the vehicle can perform a sequence of actions that leads the vehicle to stop without any collision. the method is described in detail in [6] or its generalization is in [8]. the generalization allows to use the method on the general road network with junctions, not only on the highway without crossings as it was constrained in the original work. the inspiration for the generalization is in the lane merging when the vehicles must merge into a single lane. the vehicles must create a gap for a vehicle in the next lane. the similar concept can be applied in the junctions, we can imagine a junction as the single lane we need to figure 4. visualization of junction passing using the orca algorithm merge into. the gaps are virtually created when approaching the junction and the junction is then passed smoothly. the same core algorithm is also mentioned in section 3.2 in the context of the generalized acc. another reactive coordination method is based on the optimized reciprocal collision avoidance (orca) [9]. we applied the state-of-the art algorithm for the multi-robot collision avoidance in the road traffic domain (see 4. in contrast to the previous safe-distance method, the orca is a free ride algorithm based on the velocity obstacles. the advantage of that is that it is independent of the lanes on the road, but at the same time it can be its disadvantage. the application into the driving simulation shows that the orca could perform better in utilization of the road capacity. on the other hand it is not compatible with the current traffic as it does not respect the lanes and behaves unpredictably for the human drivers. junctions and coordination on the junction is often a separate problem in the road traffic coordination. the setting with the traffic lights is simple, the vehicles must follow the instructions of the central authority through the traffic lights. the optimization of the timing and similar studies are in scope of pure traffic simulations. the junctions without the centralized coordination are in practice controlled by the rules defining who gives a way to who. we already presented the generalized safe-distance method that can serve as a coordination method in junctions in our simulations. there is no give a way sign or concept of main street, the priority comes from the position to the junction. also, the multi-agent planning is able to maintain collision free junction passing. the asynchronous decentralized prioritized planning (adpp) method for road vehicles coordination was proposed and applied 43 martin schaefer acta polytechnica ctu proceedings in [10] and in detail in [11]. the prioritized planning is a good approach to reduce computation complexity of the multi-agent planning, however, the priority assignment is an open problem. the static priority assignment is a simple but not convenient and compatible solution with the current traffic rules, the dynamic priority assignment is a challenge for the future research. the cited work on adpp for the road traffic already presented several adjustments of the algorithm so that it is applicable in the highly dynamic traffic domain. as an example, we can mention dynamic constraints introduced to model dynamism of the road vehicles. another cooperative technique based on the peer-topeer algorithm was researched in [12]. the approached suffered from the excessive peer-to-peer communication in case of a conflict in plans. the cooperation in sense of plan sharing was adopted also in works related to the platooning. the platooning is a concept of vehicle trains. the advantages of the platoons are fuel reduction, road utilization efficiency and reduction of driver load. platooning is mostly spelled in context of the commercial truck transportation. the control theory aspects with the presence of the communication was studied in [13]. the focus on formations of the vehicles and the effects of the platoons on the traffic flow was described in [14]. the coordination algorithms form the core of the simulation platform. the simulation platform is designed to allow a wide range of in terms of computation resources usage, empirical solution quality or subjective user experience. 4. conclusions we presented the integration of agentdrive and opends. we presented the main areas that the simulation platform can be used in. we emphasize the requirements of each of the areas. we identify that the integrated simulation platform contains several planning and coordination applications. the applications were listed, categorized and presented to show the work done using the simulation platform. also, we mention open issues in the related research fields. the content gives a view on the application possibilities of the simulation platform. acknowledgements this work was supported by the grant agency of the czech technical university in prague, grant no. sgs15/209/ohk3/3t/13. references [1] m. schaefer, j. vokrinek. agentdrive: towards an agent-based coordination of intelligent cars. in advances in practical applications of agents, multi-agent systems, and sustainability: the paams collection, pp. 211–224. springer international publishing, 2015. doi:10.1007/978-3-319-18944-4_18. [2] m. schaefer, j. vokrinek. developing agent-based driver assistance systems using agentdrive. in advances in practical applications of agents, multi-agent systems, and sustainability: the paams collection, pp. 312–315. springer, 2015. doi:10.1007/978-3-319-18944-4_35. [3] a. kesting, m. treiber, m. schönhof, et al. jam-avoiding adaptive cruise control (acc) and its impact on traffic dynamics. in traffic and granular flow 05, pp. 633–643. springer, 2007. doi:10.1007/978-3-540-47641-2_62. [4] j. vokrinek, m. schaefer, d. pinotti. multi-agent traffic simulation for human-in-the-loop cooperative drive systems testing. in proceedings of the 2014 international conference on autonomous agents and multi-agent systems, pp. 1691–1692. international foundation for autonomous agents and multiagent systems, 2014. [5] m. schaefer, j. vokrinek, d. pinotti, f. tango. multiagent traffic simulation for development and validation of autonomic car-to-car systems. in autonomic road transport support systems. birkhauser basel, in print. [6] m. schaefer. noncooperative collision avoidance of road vehicles, 2011. [7] d. kubeša. autonomous cruise control for general collision avoidance of road vehicles. in proceedings of the 19th international scientific student conferenece poster 2015. czech technical university in prague, 2015. [8] d. kubeša. collision avoidance on general road network, 2015. bachelor’s thesis. [9] m. schaefer. collision avoidance of highway traffic. master’s thesis, czech technical university in prague, 2014. [10] m. vavřinec, m. schaefer. prioritized planning for road vehicles coordination. in proceedings of the 19th international scientific student conferenece poster 2015. czech technical university in prague, 2015. [11] m. vavřinec. asynchronous decentralized prioritized planning for cooperative vehicles, 2015. bachelor’s thesis. [12] p. janovský. cooperative collision avoidance of road vehicles, 2011. [13] m. švandrlík. convoy platooning using vehicle to vehicle communication. master’s thesis, české vysoké učení technické v praze. vypočetní a informační centrum., 2015. [14] o. borovec. cooperative formations on highway, 2015. bachelor’s thesis. 44 http://dx.doi.org/10.1007/978-3-319-18944-4_18 http://dx.doi.org/10.1007/978-3-319-18944-4_35 http://dx.doi.org/10.1007/978-3-540-47641-2_62 acta polytechnica ctu proceedings 2:40–44, 2015 1 introduction 2 simulation platform 2.1 traffic studies 2.2 human factors studies 2.3 driver assistance system studies 2.4 connected car studies 2.5 autonomous driving studies 3 planning and coordination applications 3.1 routing and navigation 3.2 driver assistance systems 3.3 local coordination algorithms 4 conclusions acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2017.11.0001 acta polytechnica ctu proceedings 11:1–5, 2017 © czech technical university in prague, 2017 available online at http://ojs.cvut.cz/ojs/index.php/app comparison of language subset and language extension by safe related information approach to safe system development tomáš brandejský∗, vít fábera department of applied informatics in transportation, faculty of transportation sciences, ctu in prague ∗ corresponding author: brandejsky@fd.cvut.cz abstract. presented contribution is dedicated to discussion of two different approaches into increase of programming language safety. they are language subset and extension of original safety mechanisms. as examples we used misra c/c++ subset and spark language builded on the base of ada language. in the last chapters we discuss novel approaches based on application of programmable hardware which is described in vhdl language, which is also modification of ada language. especially spark and vhdl languages represents novel approaches to safe system development which are now discussed in relation to new railway sw projects. keywords: misra, ada, spark, vhdl, programmable hardware, software safety. 1. introduction nowadays we can observe two opposite approaches to modification of programming languages to be more suitable to safe system development. the first is the reduction of chosen programming language expressions to safe subset, like misra c/c++ [1, 2] and this approach is also supported by today standards of safe software development like iso61508 [3] or en50128 [4]. in the last few years, novel and partially opposite development is expanded. it is based on extension of original language by additional explanations containing description of such aspects of software, that are not described enough by original language. typical example is spark [5] language precisiating original ada one [6] even though it was originally designed as language specialized to safe control system development. 2. misra c/c++ misra c (motor industry software reliability association) c language subset was developed in 1998, latter it was updated and also there was developed analogical support for c++ (misra c++). misra c/c++ defines itself as guidelines. it defines language subset in the form of rules partially suggesting good practice solutions, frequently warning programmers away of wrong application of the language. especially valuable of misra guideline are parts explaining why was each rule formed, explaining what is the subject of possible future problem. applying of this rules allows to write robust code with decreased probability of malfunction. now it is possible to apply compilers and source code checkers implementing misra guidelines to allow automatic static analysis of code. even, there exists free software tool clang-misracpp2008 [7] but the development of this tool probably stopped before two years. from the practical viewpoint, misra guidelines checker implemented in compiler is also very interesting. it is even better than standalone checker because checking in compiler runs in each code making and does not require additional checking operation step. many companies producing compilers for development of embedded systems based especially on single chip micro-controllers, for example keil software company which is widely used by railway control system developers community in our country. there exists effort to implement misra rules into gnu c/c++ compilers because these compilers are free, proven in use and supports many processors and micro-controllers. one of the first works in this areas was [8], where authors describe its checker for gnu c which is written in ciao prolog language into which misra rules they were transformed. ciao prolog is similar to gnu prolog and is also free and compatible with gnu one but it seems to be more advanced than gnu prolog in its design. this project probably is not active now. there was also european project within the frame of itea3 call for development of gcc fork called ggcc [9] with aim to implement misra rule checking into gcc compiler. this project probably newer started or produced no reasonable result. thus it is possible to conclude that now there is no applicable support of misra for gnu c compilers and that gnu compilers development effort now focuses rather to support of parallel (super-)computers, like implementation of acc extension of standard c/c++ languages. misra c specification is not public domain and this fact might also cause small interest of gnu community to implement this subset. there exist also similar subset called high integrity c++ [10] by programming research, but also this specification is not public domain as well as checking tools. 1 http://dx.doi.org/10.14311/app.2017.11.0001 http://ojs.cvut.cz/ojs/index.php/app tomáš brandejský, vít fábera acta polytechnica ctu proceedings 3. ada language origin of ada language falls into falls into 80’s of previous century. this language was developed for us army applications as safe, strong typing programming language supporting parallelism, inter-process safe communication and defensive programming style preventing programmers errors and decreasing the need of testing. this pascal (or algol) like language was adopted in avionics and nuclear industry and it is referred by safe software system development standards [3, 4]. because the language was designed with respect to extremely large scale control system design operated by us ministry of defence, it was designed to eliminate maximum of programmers errors, ability to detect large percentage of them during compilation of code and with ability to operate in distributed environment with respect to possible communication errors and system component faults. the language was equipped by specific typing mechanism supporting defensive programming to support ability of error detection by compiler. defensive programming is related to strong typing, which means, that magnitudes between variables of different types cannot be shared without explicit conversion. because mainstream programming languages like c offer limited list of types (integer, real, character), the use of type control is able to discover only limited number of programmer errors when such languages are used. on the opposite side, ada language offers ways how to define new data types e.g. by constraining of definition domain of sample type (e.g. type secondfloornumbers is range 200..253; defines data type which magnitudes are constrained to really existing scale of numbers and allows to compiler to identify each impossible magnitude caused by programmer or communication error, e.g. 0). because strong typing might be exhausting, when it is required in situations when it brings to benefit, in ada it is possible to define subtypes, which magnitudes are transformed automatically without explicit type conversions between original type and all its subtypes. all data structures use closing clause unambiguously defining its end to prevent mistakes caused by wrong understanding of nested structures in the time of development, testing and maintenance. on the end of function and procedure definition it is supported its name stating. for many years, in mainstream programming languages the support of concurrent programming was extraordinary. ada language contains it from its origin in 70’s. ada allows not only define processes, but it contains strong support of safe inter-process communication and process synchronization called rendezvous. each process has defined so called entries to accept messages sent by other processes and is able to send messages to entries of other ones. but not only. it is possible to choose, which messages will be accepted in given state of process and it is possible to define reactions to unexpected situations, especially to fact, that given message was not received for given time. each process might define its local variables and separate itself from others. due to its real time orientation, ada language supports exception handling to solve unexpected situation without program fail. there is present concept of modular programming which applied templates (modules without precisely defined types, operators, procedures and functions which might be precisiated later in the time of final program linking. it allows e.g. to write sorting algorithm library without knowledge of sorted data type. such library si then applicable to any data type implementing assignment and comparison operators. latter ada adopted object oriented programming concept but it is not widely used as e.g. c/c++, java and some other languages, especially due to high price of commercial compilers and need of specific education of programmers. similarly like for c/c++, also for ada language there was effort to develop safe subset to increase its (high) safety. also standards [3, 4] refers this subset but in reality nothing such exists. the improving of ada language was moving different way and tends to spark language, especially due to impossibility do make such simple programmers errors as they are solved by misra for c/c++. significant part of such errors can not occur in standard ada and thus its improvement is much difficult, because even original (full scale) ada operates on different (much higher) level of safety than e.g. c. there exist support of ada language based on gnu compiler collection, but it means that there are supported especially versatile operating systems like linux and common purpose processors. there is also interesting, that compiler is not named ada compiler, but by acronym gnat. it is caused by licensing, when ada compiler term can be used only for compiler satisfying large (and thus expensive) verification, which is out of scope of gnu community. spark language is not pure subset. it also extends ada code by additional information to eliminate potential ambiguities and insecurities hidden in ada source codes. the main changes to standard ada language are the following: • the use of access types and allocators is not permitted (term access in ada denotes pointer – more frequent term known from the other programming languages, its misuse might cause significant errors hard to discover by testing). • all expressions (including function calls) are free of side-effects. this improvement was reached by incorporating of additional description of use of variables and symbols visible from neighborhood to prevent e.g. mistaking of global and local variable of the same name. • aliasing of names is not permitted in general but the renaming of entities is permitted as there is a static 2 vol. 11/2017 comparison of language subset and language extension relationship between the two names. in analysis all names introduced by a renaming declaration are replaced by the name of the renamed entity. this replacement is applied recursively when there are multiple renames of an entity. • the goto statement is not permitted. • the use of controlled types is not currently permitted. • tasks and protected objects are permitted only if the ravenscar profile (or the extended ravenscar profile) is specified. ravenscar profile denotes set of pragmas influencing compiler work to prevent some possibly unsafe construction in ada code. ravenscar profile is available in standard ada too, in spark is is not option but he standard. • raising and handling of exceptions is not currently permitted (exceptions can be included in a program but proof must be used to show that they cannot be raised). misra c/c++ especially eliminates confusing structures which might be understand differently by compiler, programmer and structures which are differently understand by different compilers, especially due to vague language definition (it is especially problem of c language, c++ in its last version was significantly improved, precisiated). for example if there is used condition ((x==a)&&(y==b)), there is not defined in language specification if the sub-expression (x==a) is evaluated as the first or the second. it is no problem until we do modification of this condition to form ((x()==a)&&(y()==b)). in this new condition x and y are not static objects (variables), but they are function calls and functions might contain so-called side effects, in this case they might operate the same global variable and thus the results of calls of functions x() and y() might be influenced by call order. this subset has also many rules eliminating confusing structures causing misunderstandings by programmers and testers. misra subsets contain tens such confusing structures. e.g. it is possible to mention constants starting with zero (they are understood by compilers as octal numbers by definition), problem of nested structures if-else which might not contain else branch and no terminal symbol stopping this structure is defined in the language and so on. 4. spark spark language introduces explicit description of use of global objects (and commonly objects defined on higher level of visibility) called dependencies. the use of these additional descriptions is known from design by contract methodology, which was on the origin of such languages as eiffel [11]. this methodology prescribes that software designers should define formal, precise and verifiable interface specifications for software components, which extend the ordinary definition of abstract data types with preconditions, postconditions and invariants. preconditions are conditions which must be satisfied to enter the following section. opposite postcondition must be satisfied on the end of the block and the invariant must be valid inside the block. 5. vhdl vhdl (vhsic hardware description language) [12] is now frequently used in programming of field-programmable gate arrays. it describes structure and/or behaviour of digital hardware. vhdl is similar to ada language both in concept and syntax (parallel processes, subtypes, ...). it was primarily proposed for simulation, now the vhdl description is an input for synthesis tool instead of schematic diagram to produce final design. the interesting question is, if the similarity of vhdl and ada languages cannot be used to description of both sw a fpga implementation of systems with high requirements to reliability, applicability, maintenance and safety. simulation of system described by vhdl language is not straightforward due to need to respect signal and not process oriented nature of fpga. this nature caused presence of sensitivity list in vhdl code. in vhdl, the proces statement has an part called sensitivity list. the syntax is as follows: [process_label:]process[(sensitivity_list)] declarative_part begin statement_part end process[process_label]; example: andgate1: process (a,b) begin y <= a and b after 5 ns; end process; the sensitivity list consists of input signal identifiers separated by commas. sensitivity list is important at the moment of the simulation. during the initialization phase of simulation, the process runs and it assigned initial values of signals and variables (signal y in our case). then the process is suspended. if the any signal listed in the sensitivity list is changed, the process is resumed and executed. the sensivity list after the process keyword is syntactically optional, but we can really say that is mandatory. the sensitivity list placed after the process keyword can moved at the end ot the process section and written like the wait on statement: andgate1: process begin y <= a and b after 5 ns; wait on a, b; end process; 3 tomáš brandejský, vít fábera acta polytechnica ctu proceedings figure 1. corresponding schematic diagrams to vhdl code related to simulation. the process having sensitivity list after process keyword must contain no wait on statement. if some signal is missing in the list, it affects naturaly the simulation. for example, signal b is missing: andgate1: process (a) begin y <= a and b after 5 ns; end process; let consider the set of simulus: both signals are set to 0 at time 0ns, a signal is changed to 1 at 10ns, b signal is changed to 1 at 20 ns. the result of simulation in isim (a part of xilinx ise design pack) is constant zero signal y, because the process in not activated at the time 20 ns due to missing signal b in the sensitivity list. while process andgate1 really describes combinational and, andgate2 process has sequential behavior. we show one more example of code and corresponding schematic diagrams to simulation (fig. 1). multiplexor1: process (s, i0, i1) begin if s = ’0’ then y <= i0; else y <= i1; end if; end process; multiplexor2: process (s) begin if s = ’0’ then y <= i0; else y <= i1; end if; end process; in simulation, the output y of multiplexor2 process changes only if event on s occurs. it means that output y follows the value of i0 at the moment of falling edge on s and value of i1 at the moment of rising edge on s. 6. synthesis processes andgate2 and multiplexor2 have sequential behaviour during the simulation with isim. how process these codes synthesis tools xilinx ise 12.4? if a signal is missing in the sensitivity list, tools writes a warning like "i0 should be on the sensitivity list of the process". if signal y is listed in the sensivity and it y an output of the entity, the vhdl compiler writes an error "y with mode ’out’ cannot be read". if y is an internal signal, it can state in the sensitivity list without warning or error. but all variants of code lead to the same rtl (register transfer level) – combinational logic – and gate and multiplexor. syntesis tool decides if it uses combinational logic or flip-flop according to typical vhdl code structures (if clk’event and clk = ’1’ then for flip-flops). so, if we forget to put a signal in the sensitivity list and if we ignore warnings, the simulation of the code and the behavioral of the target design can be different. the reason is: to put all input signal in the sensitivity list and or use a variant of the sensitivity list with keyword all, which was added into new specification of the language. the gcc compiler has -wall switch, that cause to stop compiling also if a warning occurs. switch -wall is recomended in c language if we create a safety software. synthesis tools (xilinx ise) have usually no switch like -wall, vivaldo design suite from xilinx makes possible to change severity from warning to errors. as seen, vhdl, although declared as language with safety properties, has also some weak points. a question is: wouldn’t it be suitable to extend vdhl compilers in the sense of -wall switch and to add a variable list like in sparc, espetially, if vhdl is speculated as a language for railway systems? by the way, rules for safety design using vhdl was presented in [13]. 7. discussion vhdl language uses control structures and programming methodology analogical to original ada language. it is the subject of future research, if it is possible to modify it in the way analogical to spark to increase its safety and analysis if such extension could bring significant increase to safety due to influence of hardware (gate array) to it. 8. conclusion the presented paper brings analysis of impact of two basic methods to increase language safety to cover safety requirements of systems working on high safety integrity levels, e.g. on railways. it is not only focused to comparison of ada and spark languages but it is illustrated on problems bring by implementation of vhdl simulator designed with respect to describe safe system by single language without need to distinguish if this system will be implement by hw or sw. references [1] guidelines for the use of the c language in critical systems, march 2013. isbn 978-1-906400-10-1 (paperback), isbn 978-1-906400-11-8 (pdf). 4 vol. 11/2017 comparison of language subset and language extension [2] guidelines for the use of the c++ language in critical systems, june 2008. isbn 978-906400-03-3 (paperback), isbn 978-906400-04-0 (pdf). [3] iec 61508 – electronic functional safety package. [4] en50128: railway applications – communications, signaling and processing systems – software for railway control and protection systems, 2011. [5] iso/iec 8652:2012(e). ada reference manual. copyright ©1992, 1993, 1994, 1995. [6] spark 2014 reference manual. copyright ©2013-2016. [7] "https://github.com/rettichschnidi/clangmisracpp2008". [8] g. marpons. a coding rule conformance checker integrated into gcc [online]. "babel.ls.fi.upm.es/ gmarpons/pubs/prole08codingrules.pdf". [9] ggcc reseny mandrivou v ramci itea3. [10] "http://www.codingstandard.com/section/index/". [11] eiffel language [online]. "http://www.ecmainternational.org/publications/standards/ecma367.htm". [12] ieee standard vhdl language reference manual, 2000. [13] t.musil. návrh metodiky pro vývoj a verifikaci bezpečných algoritmů implementovaných v dynamicky rekonfigurovatelných fpga (methodology for design and verification of algorithms with demands for safety on dynamically reconfigurable fpga), doctoral thesis. czech technical university, 2015. 5 acta polytechnica ctu proceedings 11:1–5, 2017 1 introduction 2 misra c/c++ 3 ada language 4 spark 5 vhdl 6 synthesis 7 discussion 8 conclusion references acta polytechnica ctu proceedings doi:10.14311/app.2017.7.0017 acta polytechnica ctu proceedings 7:17–21, 2017 © czech technical university in prague, 2017 available online at http://ojs.cvut.cz/ojs/index.php/app numerical discretisation of built structures assumption and reality m. gutermann∗, c. schröder university of applied sciences bremen, institute for experimental static, neustadtswall 30, bremen, germany ∗ corresponding author: marc.gutermann@hs-bremen.de abstract. the second half of the 20th century was affected by the introduction of electronic data processing. the numerical methods appeared to be capable of solving every engineering problem. however, dealing with real problems, an appropriate numerical discretization of built structures is in many cases a difficult task, since fundamental input data concerning the actual properties of the structure, material and mechanical boundary condition couldn’t always be met. typical difficulties are an incomplete documentation, unknown effects of structural faults and uncertain material properties. in this paper our experience in experimental assessment of structures is described and the outcome of experimental versus numerical results is compared in several examples. this paper summarizes finally both prospects and limitations of numerical simulations of built structures and shall show that the identified load bearing capacity is strongly bound to the quality of the numerical structural model. keywords: numerical discretisation, hybrid static, load tests, bridge, foundation, carousel. 1. introduction “a mathematical or mechanical model may be almost perfect it stays a model”. this citation from prof. opitz (tu dresden) points to the fact that many students and even some engineers believe in computational results without reflection since a computers calculation seems always correct. the truth is that the results are as good as their assumptions which lay hopefully always on the safe side. more and more existing structures have to be assessed according their serviceability and load carrying safety. the common procedure and solution strategies are shown in fig. 1. but even numerical assessment requires input data concerning actual material and structural properties as well as mechanical boundary conditions. these preconditions could not always be met so that not any calculated proof is successful. possible reasons are an incomplete documentation, unknown effects of structural faults and uncertainties in the modelling of the structural system with appropriate boundary conditions. another problem occurs if the structures have a historic design: the current technical recommendations are made for building of (new) structures and does not meet the requirements to assess historic designs. figure 1. solution strategies to asses load carrying capacity of built structures a parallel application of computational and experimental methods allows improving the numerical model and leads mostly to a much higher assessment of load carrying capacity, since the calibrated discretisation predicts the structural behaviour in a more realistic way (so called hybrid static fig. 1). on the basis of the experimental observation, the input parameters required for the analysis are obtained, e.g. to assess a railway bridge (see fig. 2). the modelling of boundary conditions as well as material parameters were calibrated using experimental results and led to a more realistic folded-plate-discretisation (see fig. 3). all loadings, recommended by guidelines, could be calculated using this model and sufficient load bearing capacity was proved. the advantage of subsequent extrapolating analysis is its totally non-destructive character. however, the disadvantage is the remaining uncertainty concerning the correctness of a numerical extrapolation. parametric studies or the consideration of large safety margins are means for handling these uncertainties. 17 http://dx.doi.org/10.14311/app.2017.7.0017 http://ojs.cvut.cz/ojs/index.php/app m. gutermann, c. schröder acta polytechnica ctu proceedings figure 2. measurements during operation figure 3. fe-model, parameters calibrated by measurements the following chapters give some examples how experimental data supplement numerical assessment and which advantages result hence. 2. case studies 2.1. masonry arch bridge “clevertorbrücke”, hanover the stone masonry arch bridge crosses the river leine in the center of hanover to transfer a multi-lane principal road. a recurring inspection revealed several open joints, where the mortar has been deeply washed out. an engineering office measured the actual shape of the three-center arch as well material properties. first numerical discretisation, taking only the arch into account as load-carrying member, showed that the bridge should have collapsed under its self-weight already. it seemed obvious that the chosen assumptions did not fit the actual load transfer so that a second calculation took backfill into account [1, 2]. this showed a higher load carrying capacity, but revealed two aspects: first, the proven load-carrying capacity was still too low for high frequented roads; second, the load carrying transfer was a theoretical assumption and could not be proven without experimental confirmation. since load carrying behaviour of backfill may be non-linear, we chose test loading with a special vehicle [3] (see fig. 4). figure 4. belfa test loading of stone masonry arch bridge, hanover the results showed, that there was an effective load transfer in the backing and that this influence stays constant even at higher load levels including partial safety factors (see tab. 1). with the experimental results it was possible to calibrate finite elements models afterwards, even simple beam-spring-models, since it was possible to choose all unknown parameters as spring stiffness or boundary conditions. in summary many influences (e.g. back filling, effective constraints, ...) supports in fact the load carrying system [4], but cannot be considered in calculations without experimental evaluation, especially if their load carrying behaviour may be non-linear at higher stress levels. 2.2. reinforced concrete foundations of wind energy converters in the first decade of this century several reinforced concrete foundations of onshore wind energy converters showed defects as cracks and spalling at concrete surface. some defects had simple causes as shrinkage or creeping but some defects were more severe and led to shut-down of sites (see fig. 5). one incorrect approach had been, that the foundation was preliminary constructed as a foundation of an immobile structure. in fact a wind energy converter is more likely a machine than an immobile construction. an experimental assessment of several affected construction showed under operation movements of several millimetres (f ≤ 2.3 mm) between foundation surface and steel built-in-part. a simple post-calculation by hand confirmed the assessed dimension, although they contain only deformation caused by elongation of concrete or steel as well as materials effects (shrinkage or creeping). poor workmanship may cause even higher deformations. our investigations showed clearly that in case of machine foundations, not only forces but also deformations should be taken into account, especially constructing connections between different materials. 18 vol. 7/2017 numerical discretisation of built structures results calculation measurement (fe-model with backing) strain deflection strain deflection action ε[µm/m] f[mm] ε[µm/m] f[mm] drive through of crane 19 0.33 5 0.35 (g = 48 t) test load bk 16/16 56 0.92 35 0.7 (incl. γ and crane) test load bk 30/30 97 1.57 85 1.2 (incl. γ and crane) table 1. comparison of results at arch crown (fe-model with backing vs. measurement) figure 5. double flange built-in part and observed defect mechanism 2.3. carousel, wuhan (china) after erection of a carousel the local authorities surveyed the construction by measuring strain at one of the main beams and compared these results with the submitted structural analysis of the construction. unfortunately these values differ more than 200 %. we were invited to conduct additional measurements to investigate this deviation. we measured strains at the main beam at several positions (see tab. 2) and compared the results with both simple bar model and 3d-fe-model (see fig. 6). the results showed that the upper sensors close to the weld (d1tk d3tk) measured much higher strains, caused to the influence of the nearby joint. sensor d4tk was situated out of the joints influence, removed more than half of the beam diameter (see fig. 6). as a result this sensor showed appropriate values compared to both bar and 3d-fe-model. figure 6. barand 3d-fe-model model incl. sensor positions in summary an experimental evaluation of calculations has to take into account which assumption had been made for the structural analysis. otherwise the comparison will fail without being well founded. 2.4. steel truss bridge, lomé (togo) the state togo built a steel truss bridge in the year 1960 to transport phosphates from inland to seaport (see fig. 7). in the last decades more and more steel girders showed increasing deterioration by corrosion, obviously caused by the rough environmental conditions (high humidity, salt water, phosphates). it was decided to rehabilitate the bridge by replacing top and bottom chord during train operation. fatigue had to be proven for the remaining girders, e.g. diagonals and support grid. a calculation had not given satisfactory results, so that additional information should be collected by on-site experiments and measurements including the verification of rehabilitation success. figure 7. steel truss bridge crossing the lac togo and tested fields the results were diverging: individual parts showed even higher values, but most stress levels were lower than calculated. the maximum reduction of more than 50 % was reached at the grid beams. the following proof of fatigue showed that almost all members had a sufficient remaining live time of 20 years. 19 m. gutermann, c. schröder acta polytechnica ctu proceedings sensor average measured strain interference cal. strain min [µm/m] max [µm/m] [µm/m] [µm/m] d1tk -147 129 ±40 ∼190 d2tk -196 89 ±50 ∼143 d3tk -259 68 ±50 – d4tk -95 128 ±30 ∼100 d5tk -62 56 ±25 – vb1 -30 21 ±1 26–47 table 2. relative deformation between foundation surface and steel built-in part (double flange system) figure 8. influence of discretisation: a) simple spatial framework b) detailed dispersed truss the experimental results were used afterwards to investigate the influence of calculational assumption on the stress level of truss members and consequentially on their remaining lifetime [5]. in the process the main emphasis was led on the discretisation of static system (see fig. 8) and load application. this showed that the chosen loading (single load, partial distributed load) varied the results of grid beams for several ten percent, as measured and analysed. this was actually an exceptional outcome, all other investigated parameters, as • connection of truss members as hinge or rigid joints • consideration of sheet plates between truss members • realistic discretisation of boundary conditions • joint stiffness between longitudinal and cross beams showed much lower influence, so that they can be generally neglected. in summary the investigations showed, that a more detailed spatial discretisation and especially a realistic load application can lead to much better results, even without experimental assessment. a simple analysis and discretisation of built structure is sufficient as a first approximation. 3. conclusions if we want to describe the physical reality using numerical discretisation we need additional information that may be gained by experiments, e.g. non-destructive testing, monitoring or load-tests. this may be cost and time consuming, but sometimes a little effort may have a great benefit. our experiences, documented in this paper, are summarized with the following statements: • a numerical model stays model, even if it seems perfect. concomitant experimental analysis helps to improve significantly both model and results. • experimental evaluation should meet calculation assumptions. otherwise they could not be compared. • many influences (e. g. back filling, effective constraints, ...) support in fact the load carrying system, but cannot be considered in calculations without experimental evaluation, especially if their load carrying behaviour may be non-linear at higher stress levels. • the calculated result is dependent on both system and boundary assumptions (especially in cases of arches and fatigue proof). change of system parameters may vary the results by 100 % or more. • discretisation of machine foundation should take (tolerable) deformations into account. an analysis as static and stiff system may not be sufficient and may lead to severe defects. acknowledgements we want to send special thanks to the university of applied sciences bremen and all involved parties, which enables us to have done more than 300 successful investigations in the field of analysing load carrying safety of built structures. references [1] d. proske, p. lieberwirth, p. h. van gelder. sicherheitsbeurteilung historischer steinbogenbrücken. mauerwerk 11(4):186–189, 2007. doi:10.1002/dama.200700327. [2] z. orbán, m. gutermann. assessment of masonry arch railway bridges using non-destructive in-situ testing methods. engineering structures 31(10):2287–2298, 2009. doi:dx.doi.org/10.1016/j.engstruct.2009.04.008. [3] m. gutermann, c. schröder. loading vehicle belfa development and experience gained in 10 years of practice. bridge structures 11(1,2):19–31, 2015. doi:10.3233/brs-150081. [4] m. gutermann. an article on experimental assessment of structural safety of solid bridges. proceedings of icem12-12th international conference on experimental mechanics 28, 2004. 20 http://dx.doi.org/10.1002/dama.200700327 http://dx.doi.org/dx.doi.org/10.1016/j.engstruct.2009.04.008 http://dx.doi.org/10.3233/brs-150081 vol. 7/2017 numerical discretisation of built structures [5] w. malgut, k. ammermann, t. sander, m. gutermann. pont lagunaire, togo experimentelle tragwerksanalyse einer stahlfachwerkbrücke zum nachweis der restnutzungszeit. 8 symposium "experimentelle untersuchungen von baukonstruktionen" 2015. 21 acta polytechnica ctu proceedings 7:17–21, 2017 1 introduction 2 case studies 2.1 masonry arch bridge "clevertorbrücke", hanover 2.2 reinforced concrete foundations of wind energy converters 2.3 carousel, wuhan (china) 2.4 steel truss bridge, lomé (togo) 3 conclusions acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2015.1.0034 acta polytechnica ctu proceedings 2:34–39, 2015 © czech technical university in prague, 2015 available online at http://ojs.cvut.cz/ojs/index.php/app on traversability cost evaluation from proprioceptive sensing for a crawling robot jakub mrva∗, martin stejskal, jan faigl dept. of computer science, czech technical university in prague, technická 2, 166 27 prague, czech republic ∗ corresponding author: jakub.mrva@fel.cvut.cz abstract. traversability characteristics of the robot working environment are crucial in planning an efficient path for a robot operating in rough unstructured areas. in the literature, approaches to wheeled or tracked robots can be found, but a relatively little attention is given to walking multi-legged robots. moreover, the existing approaches for terrain traversability assessment seem to be focused on gathering key features from a terrain model acquired from range data or camera image and only occasionally supplemented with proprioceptive sensing that expresses the interaction of the robot with the terrain. this paper addresses the problem of traversability cost evaluation based on proprioceptive sensing for a hexapod walking robot while optimizing different criteria. we present several methods of evaluating the robot-terrain interaction that can be used as a cost function for an assessment of the robot motion that can be utilized in high-level path-planning algorithms. keywords: terrain traversability, proprioceptive sensing, walking robot. 1. introduction exteroceptive sensing is widely analyzed and summarized in the survey [1], where authors conclude that the most favored method is to assess the traversability characteristics before actually driving the robot into the respective region. such an assessment is based on a terrain model created from exteroceptive sensors like laser range finder or stereo camera. however, the difficulties a robot has while traversing rough terrain (e.g., slippages, softness of the ground, energy consumption, etc.) cannot be foreseen in advance. one has to actually walk through the terrain to feel and measure the interaction between the robot and the ground using proprioceptive sensors like accelerometers, force and torque sensors, etc. according to the survey that provides a study of unmanned ground vehicles (ugv), the generic capability of a robot to negotiate the terrain is the most commonly called as the traversability (along with occasional terms like terrainability, trafficability, mobility, etc.). based on that, we follow this established term and refer to its value as to the traversability cost. although the cost is a common name, its meaning varies among researchers. regarding the mission the robot is requested to accomplish, we need to balance between the time or the distance traveled, the energy consumption, the stability, the danger along the path, etc. having a terrain model, a common method of optimizing the traversability cost is to prefer a smooth and obstacle-free path, but a usage of proprioceptive sensors as indicators of the expected cost does not have such a common direction. in this paper, we report on existing approaches for traversability assessment based not only on exteroceptive sensors but we rather focus on proprioceptive sensors. moreover, instead of wheeled robot, we consider legged robots. the rest of the paper is organized as follows. we identify the main components of the terrain traversability analysis in section 2. first, we provide a brief overview of the exteroceptive sensing based methods for a comparison. next, we follow with an overview of known approaches using proprioceptive sensing. we formulate the main objectives in the view of the terrain traversability evaluation in section 3 and propose traversability evaluation methods for walking robots while focused on utilizing proprioceptive sensors in section 4. finally, a discussion about possible future approaches is given in section 5. 2. related work a great progress in the field of autonomous, perception-based, off-road navigation in robotic unmanned ground vehicles (ugv) was influenced by the darpa learning applied to ground vehicles (lagr) program [2], which ran from 2004 until 2008. the challenge evoked several solutions [3–5] with different approaches of robot sensing which can be divided into two groups as: 1) exteroceptive; 2) and proprioceptive sensing. 2.1. exteroceptive sensing it is reasonable to have information about the terrain prior to traversing it. laser scanner and camera are examples of exteroceptive sensors mostly used in the case of acquiring terrain characteristics. the terrain scan can provide an elevation map for a proper foothold planning [6] and estimation of its traversability [7] using a laser scanner. characteristic features can be obtained from a far-field scan (color image, stereo camera, etc.) and classified based on a model learned from near-field [3, 4, 8] where more data are 34 http://dx.doi.org/10.14311/app.2015.1.0034 http://ojs.cvut.cz/ojs/index.php/app vol. 2/2015 available. with data source getting closer to the robot, we get more precise models of the terrain, but usage of only exteroceptive sensors does not provide much information about the real interaction between the robot and the terrain being traversed and thus the robot can hardly learn from its experience. 2.2. proprioceptive sensing on the other hand, the robot needs to receive a feedback from the terrain to utilize learning from experience. proprioceptive sensors measure the modalities of the terrain that affect the robot motion directly. in the path planning task, the robot plans according to estimated traversability costs. however, the estimation can change when the robot actually encounters the terrain, as it has been applied in the previous approaches [3, 4, 8] using wheeled robot with bumpers, wheel encoders for a slip measurement or imu with gyros and accelerometers for measuring the roughness of the terrain. for example, a tall-grass area can be estimated as a non-traversable using only a range measurement, but it turns out that the robot can go through it without a significant difficulty. the traversability cost assessment is closely related to the terrain classification (or discrimination) assuming that the terrains of the same kind provide the same conditions for traversing. based only on proprioceptive sensors, the classification can be done using accelerometers [9] (the faster the speed, the better the accuracy) or yaw-angle variations [10] on a wheeled robot, or using servo drives feedback on a hexapod walking robot [11, 12]. focusing now mainly on legged robots, an early analysis of terrain traversability for legged locomotion using active perception was proposed by krotkov back in 1990 [13] studying the terrain stiffness and surface friction upon foot contact on a planetary rover – the ambler robot. however, the final path of the ambler robot is computed on a grid terrain elevation map avoiding occluded cells [14], i.e., without the possible knowledge gained from the proprioceptive sensors. a more matured solution [15] utilizes a biologically inspired gait on a hexapod crawler. carrying a lot of proprioceptive sensors, the robot was able to negotiate small obstacles using few hard-coded reflexes which ensured quality footholds during the motion. however, the estimation of the traversability cost relies only on exteroceptive sensors and the shape of the terrain. regarding our focus, we consider the work of hoffmann et al. as the most related to our approach. they presented how different sensory modalities affect the accuracy of the terrain discrimination using a quadruped puppy robot [16]. they also studied the relationship between the gait used and the classifier accuracy [17] including using sequences of different gaits to get better results. 3. problem statement the terrain sensing and the evaluation of the traversability cost of the robot motion fits into the scope of path planning. assuming we have a map of the environment in terms of positions of untraversable obstacles or regions, we can build a weighted graph on top of the map. although we consider evaluation of the cost for path planning, the planning itself is out of the scope of this paper. therefore, we consider the planning problem can be solved using optimal planners like a* [18] or d* [19] on a graph with known weights, i.e., the costs on the edges. the path consists of a sequence of actions (the robot moves along an edge using a particular gait) that have some cost—the traversability cost—which can be computed from various sources based on different criteria and different tasks the robot is performing. in general, we can distinguish sources of information the robot receives (terrain-sensor modalities) and the outcomes of the resulting robot motion (cost modalities) as can be seen in fig. 1. proprioceptive exteroceptive sensors terrain gait control f(s) evaluation c cost terrain-sensor modalities cost modalities figure 1. schema of the robot perception module. the input modalities represent different sensor measurements of the terrain which are used in a function f to evaluate the traversability cost c. the cost is multimodal in general but usually only one modality is used for planning (e.g., the robot speed). we can imagine that a terrain can be described by several characteristic features: shape, consistency, temperature, friction, etc. which can be measured by various sensors. each such a feature can be viewed as a modality that is either directly measured or remains hidden. each sensor refers to different modalities and combined together, we get a set of observable modalities on the input side of the robot perception module generating a multidimensional vector of sensor data. in general, we can say that more modalities— and hence more information—about the terrain we have, the better the estimation of its traversability cost we can get. notice that some of the modalities may provide more useful information (regarding the cost evaluation) than others. the output side of the module is also multimodal although it is not as obvious because usually only one modality is used for the traversability cost estimation (e.g., the robot speed while traversing the terrain). 35 j. mrva, m. stejskal, j. faigl acta polytechnica ctu proceedings the main problem is the middle part (of the schema in fig. 1) that consists of an evaluation function f which transforms the sensory measurement into a single (albeit multimodal) cost. the methodology how to extract a single cost from a lot of data in time, frequency or other domain is not a straightforward task and hence results vary among researchers from the simple and obvious findings to more and more sophisticated methods. 3.1. terrain-sensor modalities the input modalities refer to the robot-terrain interaction from which we can gather the sensory data. however, we can gather such data only from the observable modalities. for example, without a camera image, we can hardly determine the color of the terrain. a (certainly not complete) list of terrain-sensor modalities should include following terrain features: • shape (surface) – the 3d shape of the terrain or obstacles is useful for precise motion planning and it is usually scanned by exteroceptive sensors but can also be inferred from an advanced touch sensor. • color – a color camera image of the terrain area can enhance the terrain discrimination (e.g., flat sand vs. concrete). • consistency – a gravel, snow or sand is not a solid terrain and its surface is usually changed during and after the robot motion while leaving trace or marks on such terrain. this can be measured by sensible touch sensors or by comparing the surface shape before and after the motion referring to the terrain changeability or granularity. • softness – differences between solid (e.g., concrete) and soft (e.g., carpet) terrains can affect greatly the robot performance. this modality is measurable only by proprioceptive (e.g., force) sensors. • compliance – a terrain can look stiff (e.g., grass or small branch) but does not resist to the robot motion and usually regains its shape afterwards; so, a compliant terrain is penetrable. this is also measurable only by proprioceptive sensors. • adhesion – an adhesive terrain offers better friction and hence better traction which is crucial to ensure stable footholds and permit high accelerations. a force sensor or slippage analysis is needed to measure the adhesion with respect to the robot leg shape and material. • temperature – in some cases, e.g., on a volcano, a robot might need to avoid hot surfaces in order not to damage itself. the temperature (scanned by a thermal camera) can also be used for a better estimate of terrain traversability by analysing the relations between the terrain granularity (compactness) and thermal transients [20]. as it is shown in fig. 1, the values of input modalities measured by exteroceptive sensors are (in general) dependent on the terrain being scanned and also on the gait that drives the robot. the sensor readings can be affected when using a different gait— e.g., a fast tripod gait causes shaky motion of the robot and the images taken can be blurred. similarly, when measuring by proprioceptive sensors, some of the terrain-sensor modalities can change its value depending on the robot motion. for example, the terrain can seem solid and adhesive while traversing slowly, and crumbling or slippery when accelerating (on gravel or ice). another example for a wheeled vehicle is a sand (desert) that can be traversable until a small change in the control is applied and then it become completely untraversable. focusing on proprioceptive sensors and legged robots, the most important terrain-sensor modalities are those which directly affect the robot motion and hence its performance: shape, consistency, softness, compliance, and adhesion. 3.2. cost modalities in general, the outcome (or the cost) of the robot motion does not have a single scalar value. for example, answering a question: “how was the robot going?” with “5” looks like some information has been lost. instead, for example, we would like to know that it went fast but spent a lot of energy and hit several obstacles along the way. the crucial part is then to balance the trade-off between all of the possible outcomes—the cost modalities— of our interest. the outcomes (cost modalities) can be the following (again, we do not claim it is a complete list): • average speed (time) – the overall time and speed has to be evaluated relatively to the robot capabilities. either a reliable odometry or external motion capture system is needed for a proper evaluation. • energy consumption – a robot might have a limited energy capacity to fulfill its task, and therefore, it needs to care about the energy consumption (e.g., to switch to a more energy-efficient gait [21]). • maximal torque – in very rough terrains, the balance in exploiting all motors the same can be impaired and very high torque values of some motors can cause overheating or damage of servos and the robot itself. • uncertainty of localization – continuous robot body motion—if not smooth enough—can negatively affect the reliability of range sensors or camera-based visual localization. • stability risk – the robot posture can be close to its stability margins during the motion, which increases the risk of falling and should be counted in further path planning. • damage risk – a precise terrain analysis can unveil risky areas where the robot can be damaged (e.g., after a small slip), and therefore, these areas should be either avoided or at least considered. 36 vol. 2/2015 notice that the aforementioned cost modalities are dependent not only on the terrain the robot is traversing but also on the gait that is used to control the robot (except the risk of damage from a prior terrain analysis to avoid such a terrain). the traversability cost is usually evaluated using only a single modality when a robot is performing a simple task. however, if the robot has to perform a long-term mission and meanwhile learn from its own experience to achieve better results, a single modality is not enough and the other modalities have to be considered. a robot can then optimize its decisions in order to keep a high performance in a long period. 4. use case the proposed discussion of multiple modalities on both the terrain-sensor interface and the evaluation of the traversability cost can be applied on almost every mobile robot. however, each robot has different set of sensors and thus it can measure different terrain features and thus measure different cost modalities. here, we present an example—a use case—for a hexapod walking robot operating in a rough terrain with limited proprioceptive sensory data. the platform, used methodology, and testing scenarios are described in the following parts. in section 4.4, achieved results are presented. 4.1. used hexapod platform we used an affordable platform phantomx hexapod mark ii with an adaptive gait [22] that enables this robot to traverse uneven terrains and negotiate small obstacles. the gait is a periodic-based (in terms of alternating the legs in a given order) but alters and reacts on the underlying terrain surface. the robot utilizes its servo drives feedback for the tactile sensing of the ground and servo drives are also the only sensory information the robot has (i.e., the robot is technically blind). 4.2. methodology of cost evaluation following the set of possible traversability cost modalities listed in section 3.2 and the sensors available, we consider only the speed, energy (power) consumption, and maximal torque. the speed is given from the running time of the experiments and known distance of the traversed path. the energy consumption is estimated very roughly using several approximations. firstly, the instantaneous energy consumed (the power) is proportional to the drawn current assuming a constant voltage (p ∝ i). secondly, the current is proportional to the torque of the servomotor (i ∝ τ). thirdly, the torque is (according to the servo manufacturer) proportional to the servo drive controller position error e, (τ ∝ e). therefore, the power consumption (in a small discrete time step) can be inferred from the sum of absolute values of servo position errors p ∝ 18∑ i=1 |ei|. (1) for simplicity, we leave the units and scale because we only need to know the relative changes in the energy consumption under different conditions. finally, the maximal torque is computed similarly as τmax ∝ max e. (2) 4.3. testing scenarios the traversability cost evaluation was tested on three different terrains shown in fig. 2. the office floor is perfectly flat while the wooden blocks include obstacles with height about 5 cm (for comparison, the leg femur-tibia and tibia-foot links are 7 cm, resp. 13 cm long). the third terrain contains free wooden obstacles (2 cm high) that are not fixed to the floor and thus can be shifted during the robot traversal. the trajectory during experiments was equally long on all of the terrains and the robot was driven by adaptive gait [22] under 3 different configurations (pentapod, tetrapod and tripod). office floor movable obstacles wooden blocks figure 2. terrains traversable by the adaptive gait 4.4. results the measured experimental results are shown in table 1. as can be seen from the speed comparison, the robot is not slowed by obstacles and hence the used gait (in each of the three configurations) is very adaptive. nevertheless, its speed is slow even on the flat office floor and there can surely be found a faster gait for flat terrains which, however, may not be able to traverse other terrains. terrain office movable blocks gait 5/4/3 5/4/3 5/4/3 speed [mm/s] 9/15/22 9/15/22 9/15/22 work [e/mm] 45/28/19 52/32/23 47/28/21 power [e/s] 39/42/43 45/48/51 42/43/48 max torque 47/45/44 53/48/65 62/60/66 table 1. different cost modalities experimentally evaluated on different terrains for the same traveled distance using adaptive gait with different number of legs in support phase. three values in each cell stand for pentapod / tetrapod / tripod gait. 37 j. mrva, m. stejskal, j. faigl acta polytechnica ctu proceedings if we look at the traversability cost regarding different modality—the energy consumption, we can see that traversing the flat terrain is the least energy consuming. however, the other terrains have no more than about 10% increase in power consumption. traversing movable obstacles causes the highest energy consumption, which can be explained considering friction and leg configurations. when a leg slide sideways on a floating obstacle, a more momentum is created on the joints and thus more torque is needed to counteract this behavior. naturally, it is easier to walk with legs under the body than with legs straddled. another perspective of the evaluation is by comparing the maximal torque in the servo drives measured during the robot motion. we can see that the highest torque was measured when traversing the blocks. this indicates that such a terrain causes legs to be occasionally more loaded than others (e.g., after a small slip on the edge of a block), which is projected also into the average energy consumption (which is based on torque values). regarding the maximal torque values measured, we can also see that the tetrapod gait suffers less from high torque values (caused mainly by slippages), such that the impact after a slippage is not as big as for the pentapod or tripod gait. 5. conclusion we have shown that the evaluation of traversability cost, which is an important part needed for pathplanning, depends on the modality of sensory data as well as on the modality of the cost itself. each cost modality represents a different perspective of evaluating the robot performance and we show how sensory data can be transformed into the traversability cost estimation. while different situations need different cost modalities to be considered, in general, we need to find a trade-off between them to assess the cost more appropriately. we present a use case of the proposed idea in realexperimental evaluation with a hexapod walking robot traversing terrains with various difficulty. using only a single gait for all terrains might look appropriate according to the measured speed, but considering another perspective can unveil the potential risk of servo damage and switching to another gait (slower, but not the slowest) would be a more suitable solution. getting more into the problem of traversability cost estimation in the future work, we would like to take into account another modality—a gait modality. combined all modalities together, we can better model the perception of the robot which is a key factor in assessing the traversability costs to different terrain areas. moreover, the perception can be connected to learning and mapping between the terrain features and corresponding costs can be found automatically. acknowledgements the presented work has been supported by the czech science foundation (gačr) under research project no. 1509600y. the work of jakub mrva has been also supported by the grant agency of the czech technical university in prague, grant no. sgs15/208/ohk3/3t/13 references [1] p. papadakis. terrain traversability analysis methods for unmanned ground vehicles: a survey. engineering applications of artificial intelligence 26(4):1373–1385, 2013. doi:10.1016/j.engappai.2013.01.006. 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[22] j. mrva, j. faigl. tactile sensing with servo drives feedback only for blind hexapod walking robot. in proceedings of the 10th international workshop on robot motion and control (romoco), pp. 240–245. 2015. doi:10.1109/romoco.2015.7219742. 39 http://dx.doi.org/10.1177/027836499601500204 http://dx.doi.org/10.1016/j.robot.2014.07.006 http://dx.doi.org/10.1109/tssc.1968.300136 http://dx.doi.org/10.1109/icra.2015.7139750 http://dx.doi.org/10.1109/icra.2015.7139915 http://dx.doi.org/10.1109/romoco.2015.7219742 acta polytechnica ctu proceedings 2:34–39, 2015 1 introduction 2 related work 2.1 exteroceptive sensing 2.2 proprioceptive sensing 3 problem statement 3.1 terrain-sensor modalities 3.2 cost modalities 4 use case 4.1 used hexapod platform 4.2 methodology of cost evaluation 4.3 testing scenarios 4.4 results 5 conclusion acknowledgements references 99 acta polytechnica ctu proceedings 2(1): 99–102, 2015 99 doi: 10.14311/app.2015.02.0099 suzaku view of magnetic cataclysmic variables t. yuasa1 1institute of space and astronautical science (isas), japan aerospace exploration agency (jaxa), 3-1-1 yoshinodai, chuo, sagamihara, kanagawa 252-5210, japan corresponding author: yuasa@astro.isas.jaxa.jp abstract results of magnetic cataclysmic variable studies performed with the suzaku satellite are reviewed in this article. particular emphasis is placed on the recent update of x-ray spectral model of intermediate polars, possible kinematically redshifted fluorescent fe k emission lines, and the magnetic cv contribution to the galactic ridge x-ray emission. keywords: cataclysmic variables intermediate polars x-rays. 1 introduction magnetic cataclysmic variables emit hard x-rays from hot plasma with temperatures of 108−9 k contained in the post-shock accretion column on the magnetic pole(s) of a white dwarf. japan’s x-ray observatory satellite suzaku , providing x-ray ccd coverage below 10 kev and hard x-ray response up to 50 kev with the si-pin diode, has been actively observing magnetic cvs, particularly, intermediate polars through its initial observation phase to following ao phases. as of 2013, more than 20 intermediate polars have been observed, and main results have been reported in [11], [15], [13], and [3]. in the following sections, we review major results reported in these articles. 2 x-ray spectral model of intermediate polars in [15], we have constructed, based on the previous study by [10], an x-ray spectral model of the postshock plasma of intermediate polars, by numerically solving the three conservation equations (mass, momentum, and energy) with a newly reported plasma cooling function [9]. since the shock temperature depends on the gravitational potential depth, this model contains a mwd/rwd ratio as a free parameter, and by combining the mwd/rwd ratio with a theoretical mwd − rwd ratio (e.g. by [5]), one can estimate mwd and rwd from observed spectra. figure 1 shows an example of spectral fitting with the multi-temperature model. a single-temperature model apparently fails to reproduce the observed spectrum, especially in the fe k emission line region (1 upper panels). this can be understood as follows; the hard x-ray continuum requires hotter temperature (due to emission from hot plasma close to the shock) although the apparent he-like and h-like fe kα emission line ratios require much cooler plasma temperatures (the model underestimates the he-like fe kα line and overestimate the h-like fe kα line). figure 1: suzaku spectra of tv col fitted with a single-temperature model (upper panels) and our model [15] with emissivity gradient (lower panels). crosses and red curves are observed source count rates and response-convolved model count rates. right panels present blow-up view of the fe emission line region. note that in both cases, 6.4 kev fe k fluorescent line was modeled with a single gaussian of which normalization was allowed to freely vary. our multi-temperature model, on the other hand, successfully reproduces the fe k emission lines as well as the over-all spectral shape. the best fit parameters give acceptable fit statistics, and a wd mass estimate of 0.91(0.83−1.00) m�. figure 2 summarizes estimated wd masses of 17 ips compared with the average wd masses of isolated and non-magnetic accreting systems. 99 http://dx.doi.org/10.14311/app.2015.02.0099 t. yuasa 2.1 recent updates [2] has recently reported an updated version of the intermediate polar spectral model that takes into account several aspects in the model geometry and the numerical calculation; (i) the new model can have a dipolar accretion column geometry as well as a cylindrical column, (ii) conservation equations are set for ions and electrons separately, and (iii) the accretion rate can be variable. figure 3 illustrates the accretion geometry which was considered in their paper. the two-fluid treatment is to account for the delayed increase of the electron temperature after the shock where ions are first heated, and electrons are collisionally heated relatively slowly. figure 2: distribution of estimated masses of wds in 17 nearby intermediate polars [15]. average wd masses are shown for isolated wds [4] and non-magnetic cvs [8]. broad-band spectra calculated by [2] are shown in figure 4 together with ratios between different specific accretion rates at the shock. major differences are expected, in particular, in lower accretion rates with dipolar geometry. based on this, the authors expect that the estimated wd mass could have been artificially reduced in systems like ex hya which has a low accretion rate. 3 spin modulation of centroid energy of fluorescent fe kα emission line [3] analyzed suzaku data of v1223 sgr and found a spin modulation of the centroid energy of the fluorescent fe kα emission line as shown in the third panel of figure 5. the maximum energy shift in terms of centroid energy of a single gaussian was ∼ 20 ev. when the fluorescent line is fitted with two gaussians, one at 6400 ev and the other having freely varying centroid energy, the minimum of the centroid energy of the second component is 6.29+0.10 0.11 kev. figure 3: dipolar and cylindrical geometry of the post-shock accretion region that was assumed in [2]. figure 4: upper panels: model spectra calculated for mwd = 0.7 m� with various specific accretion rates a and (a) cylindrical and (b) dipolar accretion geometry. lower panels: ratio spectra normalized by that for a = 100 g cm−2 s−1. the authors attributed this red-ward shift to the kinematic doppler shift caused by reflection from the rapidly falling pre-shock accretion matter (vff ∼ a few×1000 km s−1), and interpreted that the spin modulation is caused by the changing viewing angle (timedependent radial velocity of the accreting pre-shock material). although the detection of the second (redshifted) component is marginal in suzaku data, the near future astro-h observatory will definitively detect this with splendid energy resolution of ∼ 5 ev 100 suzaku view of magnetic cataclysmic variables (fwhm), and will provide a new technique to estimate the gravitational potential depth of a wd from the magnitude of the energy shift. figure 5: spin-modulation of count rates, and best-fit parameters of absorption column density, centroid energy of fluorescent fe kα emission line, and its equivalent width. adopted from [3] with additional labels. 4 serendipitous discoveries although suzaku x-ray telescope has a tightly collimated field of view of ∼ 18 arcmin× 18 arcmin, several new magnetic cvs are identified with their intense three fe kα emission lines as presented in figure 6. figure 6: suzaku/xis spectra of serendipitous sources that are thought to be magnetic cvs. adopted from [6], [12], and [1]. as claimed and spectroscopically confirmed in [7] and [14], respectively, magnetic cvs (more precisely intermediate polars) are the major contributors of the galactic ridge x-ray emission (grxe), so there should be enormous number of magnetic cvs in the field. these findings partially support this idea of omnipresence of hard x-ray emitting magnetic cvs in the galaxy. figure 7 shows a fit result of the broad-band grxe spectrum with the ip model and an additional lower temperature component (which is attributed to superposition of a number of dim coronal sources). figure 7: broad-band spectral fitting of the grxe using the intermediate polar spectral model [14]. 5 conclusion suzaku will continue to observe several magnetic cvs, for example, in 2013 (ao-8), v1223 sgr will be reobserved for 180 ks to reconfirm the spin modulation of the centroid energy of the fluorescent fe kα emission line. the forthcoming astro-h mission will provide a great step forward in understanding the accretion geometry, post-shock region, reflection, and many other physical processes taking place in magnetic cvs. references [1] bernardini, f. et al., 2012, a&a, 542, a22 [2] hayashi, t., & ishida, m. 2013, arxiv e-prints [3] hayashi et al., 2011, pasj, 63, 739 [4] kepler, s. o. et al., 2007, mnras, 375, 1315 doi:10.1111/j.1365-2966.2006.11388.x [5] nauenberg, m. 1972, apj, 175, 417 doi:10.1086/151568 [6] nobukawa, m., koyama, k., matsumoto, h., & tsuru, t. g. 2009, pasj, 61, 93 [7] revnivtsev, m., sazonov, s., gilfanov, m., churazov, e., & sunyaev, r. 2006, a&a, 452, 169 [8] ritter, h., & kolb, u. 2003, a&a, 404, 301 101 http://dx.doi.org/10.1111/j.1365-2966.2006.11388.x http://dx.doi.org/10.1086/151568 t. yuasa [9] schure, k. m., kosenko, d., kaastra, j. s., keppens, r., & vink, j. 2009, a&a, 508, 751 [10] suleimanov, v., revnivtsev, m., & ritter, h. 2005, a&a, 435, 191 [11] terada, y. et al. 2008, pasj, 60, 387 [12] uchiyama, h. et al., 2011, pasj, 63, 865 [13] yuasa, t. 2013, suzaku studies of white dwarf stars and the galactic x-ray background emission, springer theses ser. (springer) [14] yuasa, t., makishima, k., & nakazawa, k. 2012, apj, 753, 129 doi:10.1088/0004-637x/753/2/129 [15] yuasa, t. et al., 2010, a&a, 520, a25 discussion s. balman: have you checked any correlation between m�, temperature, and luminosity in your paper on the suzaku sample? t. yuasa: there was no correlation for any parameter such as m�, temperature, or luminosity. s. scaringi: does the geometry of the post-shock region affect the wd mass estimation? t. yuasa: yes, it affects it as explained in [2]. quantitatively, the dipolar case poses geometrical compression which leads to increased emissivity and therefore lower apparent plasma temperature compared to the cylindrical case. 102 http://dx.doi.org/10.1088/0004-637x/753/2/129 introduction x-ray spectral model of intermediate polars recent updates spin modulation of centroid energy of fluorescent fe k emission line serendipitous discoveries conclusion 1 acta polytechnica ctu proceedings 1(1): 1–12, 2014 1 doi: 10.14311/app.2014.01.0001 old and new from multifrequency astrophysics franco giovannelli1, lola sabau-graziati2 1inaf istituto di astrofisica e planetologia spaziali, area di ricerca di tor vergata, via fosso del cavaliere, 100 i00177 roma, italy 2intadpt. cargas utiles y ciencias del espacio, c/ra de ajalvir, km 4 e28850 torrejón de ardoz, madrid, spain corresponding author: franco.giovannelli@iaps.inaf.it abstract in this short review paper we comment on some the most important steps that have been made in the past decades for a better understanding of the physics governing our universe. the results we discuss come from the many groundand-space-based experiments developed for measuring astrophysical sources in various energy bands. these experimental results are discussed within the framework of current theoretical models. because of the limited length of this paper, we have selected only a few topics that, in our opinion, have been crucial for the progress of our understanding of the physics of cosmic sources. keywords: multifrequency astrophysics. 1 introduction with the advent of space-based experiments, it has been demonstrated that cosmic sources emit energy practically across the entire electromagnetic spectrum, albeit from different physical processes. several observations stand as witness to these processes. since the observed fluxes from cosmic sources can be highly variable in time and frequency, it follows that the physical processes from which they originate are in themselves highly variable. therefore simultaneous multifrequency observations are strictly necessary in order to understand the actual behaviour of cosmic sources. indeed, space experiments have opened practically all of the electromagnetic “windows” on the universe. a discussion of the most important results coming from multifrequency photonic astrophysics experiments will provide new inputs for the advance of our knowledge of physics, very often in the most extreme physical conditions. we remark on the sheer magnitude of the high quality data across practically the whole electromagnetic spectrum that has become available to the scientific community since the beginning of the space era. with these data, we are attempting to explain the physics governing the universe, and, moreover, its origin, which has been and still is a matter of the greatest curiosity for humanity. we know for certain that the universe has an absolute power limit lmax ∼ εpl/tpl ∼ c5/g ∼ 3.6×1059 erg s−1, where εpl and tpl are the planck energy and planck time, respectively, c the light velocity and g the gravitational constant. this amount of power is produced in different kind of cosmic sources, namely, the early universe (eu), quasars (qsos) and active galactic nuclei (agns), supernovae (sne), neutron stars (nss), and black holes (bhs), all types of galaxies with their stars and interstellar medium (ism), and intergalactic medium (igm) (e.g., lipunov, 1995). they radiate particles and photons at different levels of energy across the entire electromagnetic spectrum from their origins. however, the cosmic particle radiation arriving near earth, with energies from ∼ 106 to ∼ 1020 ev, is nearly isotropic, because of the galactic magnetic field, which cancels any particular directionality in the galaxy. such cosmic particle radiation apparently includes the nuclei of all known elements, as well as electrons, positrons, and antiprotons. so in spite of the cosmic rays being carriers of rich astrophysical information, it is very difficult to understand their message clearly. indeed, although it is evident that they must originate in different sources, it is at the same time extremely difficult to separate the different contributions. for more complete discussions, the reader can note the substantive books on this topic, containing excellent reviews, published by the italian physical society (giovannelli & mannocchi, 1989, 1991, 1993, 1995, 1997, 1999, 2001, 2003, 2005, 2007, 2009, 2011), by the italian astronomical society (giovannelli & sabau-graziati, 1996; 1999, 2002a, 2002b, 2010a, 2012a), by the chinese journal of astronomy and astrophysics (giovannelli & sabau-graziati, 2003, 2006, 2008), and by acta poly1 http://dx.doi.org/10.14311/app.2014.01.0001 franco giovannelli, lola sabau-graziati technica (giovannelli & mannocchi, 2013; giovannelli & sabau-graziati, 2014). with “passive” physics experiments (.i.e., observations) we view our universe, while with active physics experiments we try to reproduce some of the physical conditions and processes occurring somewhere in the universe. both kinds of experiments converge to the knowledge of the physics governing the universe. in this paper we will discuss what seem to us some of the most relevant results obtained in the recent past that significantly improve our knowledge of the physics governing our universe. deeper discussions about astroparticle physics can be found in the review papers by giovannelli (2007, 2009, 2011, 2013). giovannelli & sabau-graziati (2010b, 2012b), and de angelis, mansutti & persic (2008) discussed in their review papers the multifrequency behaviour of high energy cosmic sources, and very high energy (vhe) γ-ray astrophysicsal sources. 1.1 astroparticle physics development the subject of high energy astrophysics is generally approached through the study of cosmic rays. the reason for this is historical in nature. since the discovery of this extraterrestrial radiation by victor hess (1912), the scientific research involved in trying to discover the nature nature of these sources has been extensive. as a result, many separate research fields have been developed. before particle accelerators came into operation, high energy cosmic rays were the laboratory tools for investigations of elementary particle production, and to date they are still the only source of particles with energies greater than 1012 ev. the research into the composition of the radiation led to the developing study of the astrophysical environment using the information in the charge, mass, and energy spectra; this field is also known as particle astrophysics. now, the large hadronic collider (lhc), described by straessner et al. (2011), is able to reach tev energies for p-p interactions, and has attained energyies of 7 tev, in order to search for the higgs’ boson with the atlas detector (aad et al., 2012). no significant excess of events over the expected background is observed and limits on the higgs boson production cross section are derived for a higgs boson mass in the range 110 gev < mh < 300 gev. the observations exclude the presence of a standard model higgs boson with a mass 145 < mh < 206 gev at 95% confidence level. of great importance was the discovery of high energy photons near the top of the earth’s atmosphere. this originated the development of new astronomical fields such as the x-ray or γ-ray astronomy. but many of these high energy photons have their origin in the interactions of the high energy charged particles with cosmic matter, light, or magnetic fields. the particle astrophysics and the astronomical research fields have found in this fact a bond to join their efforts in trying to understand the high energy processes which occur in astrophysical systems. a summary on the status of the search for the origin of the highest energy cosmic rays has been published by biermann (1999). he mentioned several competing proposals, such as the supersymmetric particles. biermann notes that gamma ray bursts must also give rise to energetic protons, interacting high energy neutrinos and cosmological defects. in his paper, biermann discussd the propagation of these particles, assuming that they are charged, and concluding that the distribution of arrival directions of the highest energy particles on the sky ought to reflect the source distribution as well as the propagation history. he remarked that the present status of our observations can be summarized as inconclusive. however, he concluded as follows: if we can identify the origin of the events at the highest energies, beyond 5× 1019 ev, the greisen–zatsepin–kuzmin cutoff due to the microwave background, near to 1021 ev, and if we can establish the nature of their propagation through the universe to us, then we will obtain a tool to do physics at eev energies. the arrival directions of ≥ 60 eev ultra-high-energy cosmic rays (uhecrs) cluster along the supergalactic plane and correlate with active galactic nuclei (agn) within ≈ 100 mpc (abraham et al., 2007, 2008). the association of several events with the nearby radio galaxy cen a supports the paradigm that uhecrs are powered by supermassive black-hole engines and accelerated to ultra-high energies in the shocks formed by variable plasma winds in the inner jets of radio galaxies. the gzk horizon length of 75 eev uhecr protons is ≈ 100 mpc, so that the auger results are consistent with an assumed proton composition of the uhecrs. in this scenario, the sources of uhecrs are fr ii radio galaxies and fr i galaxies like cen a with scattered radiation fields that enhance uhecr neutral–beam production. radio galaxies with jets pointed away from us (i.e., more toward the plane of the sky) can still be observed as uhecr sources due to deflection of uhecrs by magnetic fields in the radio lobes of these galaxies. a broadband ∼ 1 mev–10 eev radiation component in the spectra of blazar agn is formed by uhecr–induced cascade radiation in the extragalactic background light. this emission is too faint to be seen from cen a, but could be detected from more luminous blazars (dermer et al., 2009). recent evidence from the pierre auger observatory suggests a transition, at 5 eev-10eev in the composition of ultra high energy cosmic rays (uhecrs), from protons to heavier nuclei such as iron (abraham et al., 2010). piran (2010) considered the implications of 2 old and new from multifrequency astrophysics the heavier composition on the sources of uhecrs. he concluded that with typical reasonable parameters of a few ng for the extra–galactic magnetic field (egmf) intensity and a coherence distance of a mpc the distance that nuclei uhecr above the gzk energy traverses before photodisintegrating is only a few mpc. in spite of the significantly weaker limits on the luminosity, cen a is the only currently active potential source of nuclei uhecrs within this distance. the large deflections erases the directional anisotropy expected from a single source. if indeed the composition of above gzk– uhecrs is iron and if the egmf is not too small then cen a is the dominant source of observed nuclei uhecrs above the gzk limit. in summary, charged cosmic rays are influenced in their propagation through space by the magnetic fields in the galaxy, and for the lowest energy particles also in the solar system. the result is that the distribution of arrival directions as the radiation enters the earth’s atmosphere is nearly isotropic. it is not possible to identify the sources of the cosmic rays by detecting them. however, in the high energy interactions produced at the source, electrically neutral particles such as photons, neutrons, and neutrinos are also produced and their trajectories are not deviated, being directed from their point of origin to the observer. owing to their short lifetime, neutrons cannot survive the path length to the earth (decay length ∼ 9 pc at 1 pev) and neutrinos do not interact efficiently in the atmosphere. it is in this context that the gamma ray astronomy has demonstrated itself to be a powerful tool. the observations made to date have detected γ-rays from many astronomical objects such as neutron stars, interstellar clouds, the center of our galaxy and the nuclei of active galaxies (agns). one might expect very important implications for high energy astrophysics from the observations at energies greater than 1011 ev of extragalactic sources (e.g., hillas & johnson, 1990). the fluxes of γ-rays at these energies are attenuated because of their interactions with the cosmic radio, microwave, infrared and optical radiation fields. measurements of the flux attenuation can then provide important information on the distribution of such fields. for instance, the threshold energy for pair production in reactions of photons with the 2.7 k background radiation is reached at 1014 ev and the absorption length is of the order of ∼ 7 kpc. for the infrared background the maximum absorption is reached at energies greater than 1012 ev. the qualitative problem of the origin of cosmic rays is practically solved, while the quantitative problem in determining the fraction of them coming from the different possible sources is still open. 2 very high energy sources the most exciting results of the last decade have been obtained in the field of vhe astrophysics from different experiments (e.g. cgro/egret, wipple, hegra, cangaroo, celeste, stacee, tibet, hess, veritas, milagro, magic) that detected many vhe cosmic sources. the high energy sky. with the exception of crab nebula, vela x, and 3c 273, was empty until middle nineties. “fast forward” to 19th april 2012, the vhe sky (e > 100 gev) is populated by 107 cosmic sources: 46 out of 107 extragalactic and 61 galactic (http://www.mppmu.mpg.de/∼rwagner/sources/ or http://tevcat.uchicago.edu). one of the most interesting results has been the determination of the spectral energy distribution (sed) of the crab nebula, thanks to many measurements obtained by different he–vhe experiments (albert et al., 2008b). another exciting result has been the detection of the first variable galactic tev source, namely the binary pulsar psr b1259-63 (aharonian et al., 2005). they found that the radio silence occurs during the time in which the pulsar is occulted by the excretion disk of the be star. the many detected sources, representing different galactic and extragalactic source populations, are supernova remnants (snrs), pulsar wind nebulae (pwne), giant molecular clouds (gmcs), star formation regions (sfrs), compact binary systems (cbss), and active galactic nuclei (agns). paredes & persic (2010) reviewed the results from magic cherenkov telescope for most of the former class of sources. models of tev agns have been discussed by lenain (2010). 3 diffuse extragalactic background radiation after the big bang, the universe started to expand with a fast cooling. the cosmic radiation observed now is probably an admixture of different components which had their origin in different stages of the evolution as the results of different processes. this is the diffuse extragalactic background radiation (debra), which, if observed in different energy ranges, allows the study of many astrophysical, cosmological, and particle physics phenomena (ressell & turner, 1990. debra is the witness of the whole history of the universe from the big bang to present time. such history is marked by three main experimental witnesses supporting the big bang theory (e.g. giovannelli & sabau-graziati, 2008): the light element abundances (burles, nollett & turner, 2001); the cmbr temperature at various redshifts as determined by srianand, petitjean & ledoux (2000), and 3 franco giovannelli, lola sabau-graziati the references therein; the cmb at z = 0 as result of cobe (tcmbr(0) = 2.726 ± 0.010 k), which is well fitted by a black body spectrum (mather et al., 1994). at z' 2.34, the cmbr temperature is: 6.0 k < tcmbr(2.34) < 14.0 k. the prediction from the hot big bang: tcmbr = tcmbr(0) × (1 + z) gives tcmbr(2.34) = 9.1 k, which is consistent with the measurement (srianand, petitjean & ledoux, 2000). 4 reionization of the universe after the epoch of recombination (last scattering) between ≈ 3.8 × 105− ≈ 2 × 108 yr (z ≈ 1000 − 20), the universe experienced the so–called dark ages, where the dark matter halos collapsed and merged until the appearance of the first sources of light. this ended the dark ages. the ultraviolet light from the first sources of light also changed the physical state of the gas (hydrogen and helium) that fills the universe from a neutral state to a nearly fully ionized one. this was the reionization era where the population iii stars formed, and consequently, the first sne and grbs. this occurred between ≈ (2− 5)× 108 yr (z ≈ 20− 10). soon after population ii stars started to form and probably the second wave of reionization occurred and stopped at ≈ 9 × 108 yr (z ≈ 6) after the big bang, and then the evolution of galaxies started (e.g. djorgovski, 2004, 2005). quasars – the brightest and most distant objects known – offer a window on the reionization era, because neutral hydrogen gas absorbs their ultraviolet light. reionization drastically changes the environment for galaxy formation and evolution and in a hierarchical clustering scenario, the galaxies responsible for reionization may be the seeds of the most massive galaxies in the local universe. reionization is the last global phase transition in the universe. the reionization era is thus a cosmological milestone, marking the appearance of the first stars, galaxies and quasars. recent results obtained by ouchi et al. (2010) give an important contribution for solving such a problem. indeed, from the the lyα luminosity function (lf), clustering measurements, and lyα line profiles based on the largest sample to date of 207 lyα emitters at z = 6.6 on the 1 deg2 sky of subaru/xmm-newton deep survey field, ouchi et al. (2010) found that the combination of various reionization models and observational results about the lf, clustering, and line profile indicates that there would exist a small decrease of the intergalactic medium’s (igm’s) lyα transmission owing to reionization, but that the hydrogen igm is not highly neutral at z = 6.6. their neutral-hydrogen fraction constraint implies that the major reionization process took place at z >∼ 7. the w. m. keck 10-m telescope has shown the quasar sdss j1148+5251 at a redshift of 6.41 (≈ 12.6× 109 yr ago). this is currently the most distant quasar known (djorkovski, 2004). this measurement does not contradict the result found for the epoch of reionization. however, the search of the epoch of reionization is still one of the most important open problems for understanding the formation of the first stars, galaxies and quasars. 5 clusters of galaxies the problems of the production and transport of heavy elements seems to have been resolved. indeed, thermally driven galactic winds, such as from m82, have shown that only active galaxies with an ongoing starburst can enrich the icm with metals. the amounts of metals in the icm is at least as high as the sum of the metals in all galaxies of the cluster (e.g. tozzi et al., 2003). several clusters of galaxies, having strong radio emission, have been associated with egret sources. this is an important step in clarifying the nature of many unknown egret sources (colafrancesco, 2002). however, in the first 11 months of operations of the fermi lat monitoring program of cgs no γ-ray emission from any of the monitored cgs has been detected (ackermann et al., 2010b). in spite of many important results coming from satellites of the last decade, the hierarchical distribution of the dark matter, and the role of the intergalactic magnetic fields in cgs are still open. simultaneous multifrequency measurements with higher sensitivity instruments, in particular those in hard x-ray and radio energy regions and optical-to-near infrared (nir) could solve such problems. 6 dark energy and dark matter by using different methods to determine the mass of galaxies it has been found a discrepancy that suggests ∼ 95% of the universe is in a form that cannot be seen. this form of unknown content of the universe is the sum of dark energy (de) and dark matter (dm). colafrancesco (2003) deeply discussed about new cosmology. the discovery of the nature of the dark energy may provide an invaluable clue for understanding the nature and the dynamics of our universe. however, there is ∼ 30% of the matter content of the universe which is dark and still requires a detailed explanation. baryonic dm consisting of machos (massive astrophysical compact halo objects) can yield only some fraction of the total amount of dark matter required by cmb observations. wimps (weakly interacting massive particles) (non-baryonic dm) can yield the needed cosmological amount of dm and its large scale distribution provided that it is “cold” enough. several options have been proposed so far like: i) light neutrinos with mass in 4 old and new from multifrequency astrophysics the range mν ∼ 10−30 ev, ii) light exotic particles like axions with mass in the range maxion ∼ 10−5−10−2 ev or weakly interacting massive particles like neutralinos with mass in the range mχ ∼ 10− 1000 gev, this last option being favored at present (see, e.g., ellis 2002). eros and macho, two experiments based on the gravitational microlensing, were developed. two lines of sight have been probed intensively: the large (lmc) and the small (smc) magellanic clouds, located 52 kpc and 63 kpc respectively from the sun (palanque-delabrouille, 2003). with 6 years of data towards the lmc, the macho experiment published a most probable halo fraction between 8 and 50% in the form of 0.2 m� objects (alcock et al., 2000). most of this range is excluded by the eros exclusion limit, and in particular the macho preferred value of 20% of the halo. among experiments for searching wimps as dark matter candidates, there is pamela, an experiment devoted to a search for dark matter annihilation, antihelium (primordial antimatter), new matter in the universe (strangelets?), the study of cosmic-ray propagation (light nuclei and isotopes), electron spectrum (local sources?), solar physics and solar modulation, and terrestrial magnetosphere. a comparison of the expected pamela results with many other experiments has been discussed by morselli (2007). bruno (2011) discussed some results from pamela. the search for dm is one of the main open problems of today’s astroparticle physics. 7 the galactic center the galactic center (gc) is one of the most interesting places for testing theories in which frontier physics plays a fundamental role. there is an excellent review of mezger, duschl & zylka (1996), which discusses the physical state of stars and interstellar matter in the galactic bulge (r ∼ 0.3–3 kpc from the dynamic center of the galaxy), in the nuclear bulge (r < 0.3 kpc) and in the sgr a radio and gmc complex (the central∼ 50 pc of the milky way). this review reports also a list of review papers and conference proceedings related to the galactic center with bibliographic details. in the review paper by giovannelli & sabau-graziati (2004, and the references therein) the multifrequency behaviour of the galactic center has been also discussed. larosa et al. (2000) presented a wide-field, high dynamic range, high-resolution, long-wavelength (λ = 90 cm) vla image of the galactic center region. this is the most accurate image of the gc. while highly obscured in optical and soft x-rays; it shows a central compact object (a black hole candidate) with m ∼ 3.6× 106 m� (genzel et al., 2003a), which coincides with the compact radio source sgr a∗ [r.a. 17 45 41.3 (hh mm ss); dec.: -29 00 22 (dd mm ss)]. sgr a∗ in x-rays/infrared is highly variable (genzel et al., 2003b). the gc is also a good candidate for indirect dark matter observations. moreover, the detected excess of he γ-rays at gc would be produced by neutralino annihilation in the dark matter halo. such an excess could be better measured by the fermi observatory. 8 gamma-ray bursts the many theoretical descriptions of gamma-ray bursts (grbs) show that the origin of these sources is still an open and strongly controversial topic. fireball (fb) model (meszaros & rees, 1992; piran, 1999), cannon ball (cb) model (dar & de rújula, 2004), spinninprecessing jet (spj) model (fargion, 2003a,b; fargion & grossi, 2006), fireshell (izzo et al., 2010) model — directly coming from electromagnetic black hole (embh) model (e.g. ruffini et al. 2003 and the references therein) — are the most popular, but each one against the others. important implications on the origin of the highest redshift grbs are coming from the detection of the grb 080913 at z =6.7 (greiner et al., 2009), grb 090423 at z ∼ 8.2 (tanvir et al., 2009), and grb 090429b (cucchiara et al., 2011). this means that really we are approaching to the possibility of detecting grbs at the end of dark era, where the first pop iii stars appeared. izzo et al. (2010) discussed successfully a theoretical interpretation of the grb 090423 within their fireshell model. wang & dai (2009) studied the high-redshift star formation rate (sfr) up to z ' 8.3 considering the swift grbs tracing the star formation history and the cosmic metallicity evolution in different background cosmological models including λcdm, quintessence, quintessence with a time-varying equation of state and brane-world models. λcdm model is the preferred which is however compared with other results. although great progress has been obtained in the last few years, grbs theory needs further investigation in the light of the experimental data coming from old and new satellites, often coordinated, such as bepposax or batse/rxte or asm/rxte or ipn or hete or integral or swift or agile or fermi or maxi. 9 extragalactic background light space is filled with diffuse extragalactic background light (ebl) which is the sum of starlight emitted by galaxies through the history of the universe. high energy γ-rays traversing cosmological distances are expected to be absorbed through their interactions with the ebl by: γvhe + γebl −→ e+ e−. then the γ-ray flux φ is suppressed while travelling from the emission 5 franco giovannelli, lola sabau-graziati point to the detection point, as φ = φ0e−τ(e,z), where τ(e,z) is the opacity. the e–fold reduction [τ(e,z) = 1] is the gamma ray horizon (grh) (e.g. blanch & martinez, 2005; martinez, 2007). the direct measurement of the ebl is difficult at optical to infrared wavelengths because of the strong foreground radiation originating in the solar system. however, the measurement of the ebl is important for vhe gamma-ray astronomy, as well as for astronomers modelling star formation and galaxy evolution. second only in intensity to the cosmic microwave background (cmb), the optical and infrared (ir) ebl contains the imprint of galaxy evolution since the big bang. this includes the light produced during formation and reprocessing of stars. current measurements of the ebl are reported in the paper by schroedter (2005, and references therein). he used the available vhe spectra from six blazars. more recently, the redshift region over which the gamma reaction history (grh) can be constrained by observations has been extended up to z = 0.536. upper ebl limit based on 3c 279 data have been obtained (albert et al., 2008a). the universe is more transparent to vhe gamma rays than expected. thus many more agns could be seen at these energies. indeed, abdo et al. (2009a) observed a number of tev-selected agns during the first 5.5 months of observations with the large area telescope (lat) on–board the fermi gamma-ray space telescope. redshift– dependent evolution is detected in the spectra of objects detected at gev and tev energies. the most reasonable explanation for this is absorption on the ebl, and as such, it would represent the first model–independent evidence for absorption of γ-rays on the ebl. abdo et al. (2010b) by using a sample of γ-ray blazars with redshift up to z ∼ 3, and grbs with redshift up to z ∼ 4.3, measured by fermi/lat placed upper limits on the γ-ray opacity of the universe at various energies and redshifts and compared this with predictions from well–known ebl models. they found that an ebl intensity in the optical-ultraviolet wavelengths as great as predicted by the ”baseline” model of stecker, malkan & scully (2006) that can be ruled out with high confidence. 10 relativistic jets relativistic jets have been found in numerous galactic and extragalactic cosmic sources at different energy bands. the emitted spectra of jets from cosmic sources of different nature are strongly dependent on the angle formed by the beam axis and the line of sight, and obviously by the lorentz factor of the particles (e.g. bednarek et al., 1990 and the references therein; beall, guillory & rose, 1999, 2009; beall, 2002, 2003, 2008, 2009; beall et al., 2006, 2007). so, observations of jet sources at different frequencies can provide new inputs for the comprehension of such extremely efficient carriers of energy, like for the cosmological grbs. the discovered analogy among µ–qsos, qsos, and grbs is fundamental for studying the common physics governing these different classes of objects via µ–qsos, which are galactic, and then apparently brighter and with all processes occurring in time scales accessible by our experiments (e.g. chaty, 1998). chaty (2007) remarked the importance of multifrequency observations of jet sources by means of the measurements of grs 1915+105. dermer et al. (2009) suggest that ultra-high energy cosmic rays (uhecrs) could come from black hole jets of radio galaxies. spectral signatures associated with uhecr hadron acceleration in studies of radio galaxies and blazars with fermi observatory and ground–based γ-ray observatories can provide evidence for cosmic-ray particle acceleration in black hole plasma jets. also in this case, γ-ray multifrequency observations (mev–gev–tev) together with observations of pev neutrinos could confirm whether black-hole jets in radio galaxies accelerate the uhecrs. despite their frequent outburst activity, microquasars have never been unambiguously detected emitting high-energy gamma rays. the fermi/lat has detected a variable high-energy source coinciding with the position of the x-ray binary and microquasar cygnus x-3. its identification with cygnus x-3 is secured by the detection of its orbital period in gamma rays, as well as the correlation of the lat flux with radio emission from the relativistic jets of cygnus x-3. the γ-ray emission probably originates from within the binary system (abdo et al., 2009b). also the microquasar ls 5039 has been unambiguously detected by fermi/lat being its emission modulated with a period of 3.9 days. analyzing the spectrum, variable with the orbital phase, and having a cutoff, abdo et al. (2009c) conclude that the γ-ray emission of ls 5039 is magnetospheric in origin, like that of pulsars detected by fermi. this experimental evidence of emission in the gev region from microquasars opens an interesting window about the formation of relativistic jets. 11 cataclysmic variables the detection of cvs with the integral observatory (barlow et al., 2006) have recently renewed the interest of high energy astrophysicists for such systems, and subsequently involving once more the low–energy astrophysical community. the detection of cvs having orbital periods inside the so-called period gap between 2 and 3 hours, which separates polars (apparently generating gravitational radiation) from intermediate polars (which suffer magnetic braking) renders attractive 6 old and new from multifrequency astrophysics the idea of the physical continuity between these two classes. further investigations are necessary for solving this important problem. for a recent review on cvs see the paper by giovannelli & sabau-graziati (2012c). 12 high mass x-ray binaries for general reviews see e.g. giovannelli & sabaugraziati (2001, 2004) and van den heuvel (2009) and references therein. hmxbs are young systems, with age ≤ 107 yr, mainly located in the galactic plane (e.g., van paradijs, 1998). a compact object — the secondary star —, mostly a magnetized neutron star (x-ray pulsar) is orbiting around an early type star (o, b, be) — the primary — with m ≥ 10 m�. the optical luminosity of the system is dominated by the early type star. such systems are the best laboratory for the study of accreting processes, thanks to their relatively high luminosity in a large part of the electromagnetic spectrum. because of the strong interactions between the optical companion and collapsed object, low and high energy processes are strictly related. in x-ray/be binaries the mass loss processes are due to the rapid rotation of the be star, the stellar wind and, sporadically, to the expulsion of casual quantity of matter essentially triggered by gravitational effects close to the periastron passage of the neutron star. the long orbital period (> 10 days) and a large eccentricity of the orbit (> 0.2) together with transient hard x-ray behavior are the main characteristics of these systems. among the whole sample of galactic systems containing 114 x-ray pulsars (johnstone, 2005), only few of them have been extensively studied. among these, the system a 0535+26/hde 245770 is the best known thanks to concomitant favorable causes, which rendered possible thirty eight years of coordinated multifrequency observations, most of them discussed by e.g. giovannelli & sabau-graziati (1992, 2008), burger et al. (1996). accretion powered x-ray pulsars usually capture material from the optical companion via stellar wind, since this primary star generally does not fill its roche lobe. however, in some specific conditions (e.g. the passage at the periastron of the neutron star) and in particular systems (e.g. a 0535+26/hde 245770), it is possible the formation of a temporary accretion disk around the neutron star behind the shock front of the stellar wind. this enhances the efficiency of the process of mass transfer from the primary star onto the secondary collapsed star, as discussed by giovannelli & ziolkowski (1990) and by giovannelli et al. (2007) in the case of a 0535+26. giovannelli & sabau-graziati (2011) discussed the history of the discovery of optical indicators of high energy emission in the prototype system a0535+26/hde 245770≡ flavia’ star, updated to the march–april 2010 event when a strong optical activity occurred roughly 8 days before the x-ray outburst (caballero et al., 2010) that was predicted by giovannelli, gualandi & sabaugraziati (2010). this event together with others occurred in the past allowed to giovannelli & sabaugraziati (2011) to conclude that x-ray outbursts occur ∼ 8 days after the periastron passage. giovannelli, bisnovatyi-kogan & klepnev (2013) developed a model for explaining such a delay by the time of radial motion of the matter in a non–stationary accretion disk around the neutron star, after an increase of the mass flux in the vicinity of a periastral point in the binary. this time is determined by the turbulent viscosity, with the parameter α = 0.1− 0.3. however how x-ray outbursts are triggered in xray pulsars constitute one important still open problem giving rise to controversy within astrophysicists. important news are coming also from gev observations of hmxbs. indeed, abdo et al. (2009e) present the first results from the observations of lsi + 61◦303 using fermi/lat data obtained between 2008 august and 2009 march. their results indicate variability that is consistent with the binary period, with the emission being modulated at 26.6 days. this constitutes the first detection of orbital periodicity in high–energy γ-rays (20 mev-100 gev). the light curve is characterized by a broad peak after periastron, as well as a smaller peak just before apastron. the spectrum is best represented by a power law with an exponential cutoff, yielding an overall flux above 100 mev of ' 0.82×10−6 ph cm−2 s−1, with a cutoff at ∼ 6.3 gev and photon index γ ∼ 2.21. there is no significant spectral change with orbital phase. the phase of maximum emission, close to periastron, hints at inverse compton scattering as the main radiation mechanism. however, previous very high-energy gamma ray (> 100 gev) obser vations by magic and veritas show peak emission close to apastron. this and the energy cutoff seen with fermi suggest that the link between he and vhe gamma rays is nontrivial. this is one open problem to be solved in future. 12.1 obscured sources and supergiant fast x-ray transients relevant are integral results about a new population of obscured sources and supergiant fast x-ray transients (sfxts) (chaty & filliatre, 2005; chaty, 2007; rahoui et al., 2008; chaty, 2008). the importance of the discovery of this new population is based on the constraints on the formation and evolution of hmxbs: does dominant population of short-living systems – born with two very massive components – oc7 franco giovannelli, lola sabau-graziati cur in rich star-forming region? what will happen when the supergiant star dies? are primary progenitors of ns/ns or ns/bh mergers good candidates of gravitational waves emitters? can we find a link with short/hard γ-ray bursts? 13 ultra–compact double–degenerated binaries ultra-compact, double-degenerated binaries (ucd) consist of two compact stars, which can be black holes, neutron stars or white dwarfs. in the case of two white dwarfs revolving around each other with an orbital period porb ≤ 20 min. the separation of the two components for a ucd with porb ≈ 10 min or shorter is smaller than jupiter’s diameter. these ucd are evolutionary remnants of low–mass binaries, and they are numerous in the milky way. the discovery of ucd is foreboding interesting hints for gravitational–wave possible detection with lisa observatory. 14 magnetars the discovery of magnetars (anomalous x-ray pulsars – axps – and soft gamma-ray repeaters – sgrs) is also one of the most exciting results of the last years (mereghetti & stella, 1995; van paradijs, taam & van den heuvel, 1995; and e.g. review by giovannelli & sabau-graziati, 2004 and the references therein). indeed, with the magnetic field intensity of order 1014 − 1015 g a question naturally arises: what kind of sn produces such axps and sgrs? are really the collapsed objects in axps and sgrs neutron stars? (e.g. hurley, 2008). with such high magnetic field intensity an almost ‘obvious’ consequence can be derived: the correspondent dimension of the source must be of ∼ 10 m (giovannelli & sabau-graziati, 2006). this could be the dimension of the acceleration zone in supercompact stars. could they be quark stars? ghosh (2009) discussed some of the recent developments in the quark star physics along with the consequences of possible hadron to quark phase transition at high density scenario of neutron stars and their implications on the astroparticle physics. important consequences could be derived by considering the continuity among rotation-powered pulsars, magnetars, and millisecond pulsars. such continuity has been experimentally demonstrated (kuiper, 2007). however, the physics underlying that observational continuity is not yet clear. 15 neutrino astronomy for a short discussion about neutrino astronomy, see for instance the paper by giovannelli (2007 and the references therein), as well as all the papers of the session neutrino astronomy, which appeared in the proceedings of the vulcano workshops 2006, 2008, and 2010 (giovannelli & mannocchi, 2007, 2009, 2011). however, it is important to remark that several papers have appeared about: i) the sources of he neutrinos (aharonian, 2007) and diffuse neutrinos in the galaxy (evoli, grasso & maccione, 2007); ii) potential neutrino signals from galactic γ-ray sources (kappes et al., 2007); iii) galactic cosmic-ray pevatrons with multi-tev γ-rays and neutrinos (gabici & aharonian, 2007); iv) results achieved with amanda: 32 galactic and extragalactic sources have been detected (xu & the icecube collaboration, 2008); diffuse neutrino flux from the inner galaxy (taylor et al., 2008); discussion about vhe neutrino astronomic experiments (cao, 2008). important news and references can be found in the proceedings of the les rencontres de physique de la vallée d’aoste (greco, 2009, 2010). news about the neutrino oscillations have been reported by mezzetto (2011). the angle θ13 is different than zero: sin2 θ13 = 0.013. this result opens the door to cp violation searches in the neutrino sector, with profound implications for our understanding of the matter–antimatter asymmetry in the universe. 16 conclusions and reflections it is becoming increasingly clear that the energy régime covered by vhe γ-ray astronomy will be able to address a number of significant scientific questions, which include: i) what parameters determine the cut-off energy for pulsed γ-rays from pulsars? ii) what is the role of shell-type supernovae in the production of cosmic rays? iii) at what energies do agn blazar spectra cut-off? iv) are gamma blazar spectral cut-offs intrinsic to the source or due to intergalactic absorption? v) is the dominant particle species in agn jets leptonic or hadronic? vi) can intergalactic absorption of the vhe emission of agn’s be a tool to calibrate the epoch of galaxy formation, the hubble parameter, and the distance to γ-ray bursts? vii) are there sources of γ-rays which are ‘loud’ at vhes, but ‘quiet’ at other wavelengths? it appears evident the importance of multifrequency astrophysics. there are many problems in performing simultaneous multifrequency, multienergy multisite, multiinstrument, multiplatform measurements due to: i) objective technological difficulties; ii) sharing common scientific objectives; iii) problems of scheduling and budgets; iv) politic management of science. in spite of the many ground-based and space-based experiments providing an impressive quantity of excellent data in different energy regions, many open problems still exist. we believe that only by drastically 8 old and new from multifrequency astrophysics changing the philosophy of the experiments will it be possible to solve most of the present open problems. for instance, in the case of space–based experiments, small satellites, dedicated to specific missions and problems, and having the possibility of scheduling very long time observations, must be supported because of their relative faster preparation, easier management and lower costs with respect to medium and large satellites. we strongly believe that in the next decades “passive” physics experiments in space, as well as groundbased, and perhaps lunar-based observatories will be the most suitable probes in sounding the physics of the universe. probably the active physics experiments have already reached the maximum dimensions compatible with a reasonable cost/benefit ratio, with the obvious exception of the neutrino astronomy experiments. acknowledgement we wish to thank the soc of the karlovy vary 10th integral/bart workshop for inviting us to discuss this review during the workshop, and the loc for logistical support for one of us (fg). this research has made use of nasa’s astrophysics data system. references [1] aad, g. et al. 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(icecube collaboration): 2008, n. phys. b 175-176, 401. 12 http://dx.doi.org/10.1063/1.1587092 http://dx.doi.org/10.1086/431173 http://dx.doi.org/10.1038/35050020 http://dx.doi.org/10.1038/nature08459 http://dx.doi.org/10.1063/1.3076687 http://dx.doi.org/10.1086/376731 introduction astroparticle physics development very high energy sources diffuse extragalactic background radiation reionization of the universe clusters of galaxies dark energy and dark matter the galactic center gamma-ray bursts extragalactic background light relativistic jets cataclysmic variables high mass x-ray binaries obscured sources and supergiant fast x-ray transients ultra–compact double–degenerated binaries magnetars neutrino astronomy conclusions and reflections 161 acta polytechnica ctu proceedings 2(1): 161–164, 2015 161 doi: 10.14311/app.2015.02.0161 study of photometric variability of selected su uma dwarf novae i. voloshina1, v. metlov1 1sternberg astronomical institute, lomonosov moscow state university, moscow, russia corresponding author: vib@sai.msu.ru abstract in this work we present time-resolved photometry of several poorly-studied dwarf novae during recent superoutbursts. observations were made with a ccd, mounted on 50and 60-cm telescopes of the sternberg astronomical institute in crimea in april may and october 2012 and june july 2013. superhumps were detected in light curves of all the dwarf novae. the amplitudes and periods of detected light variations were calculated. superhumps evolution was also followed up for all systems and classification is improved. keywords: cataclysmic variables dwarf novae su uma stars outbursts photometry light curves periods. 1 introduction su uma stars represent one of the three sub-types of dwarf novae. besides the frequent normal outbursts, they could show outbursts of larger amplitudes and much longer duration than the normal ones,superoutbursts. during superoutbursts they exhibit socalled superhumps,an increase of system brightness on the small part of the orbital light curve, that repeats with a period a few percents longer than the orbital one. the amplitudes of superhumps are around 0m.3. the orbital periods of su uma stars are about 80-180 min. there are two small subtypes of dwarf novae inside of this group: wz sge stars with short superhumps period and very long time of outburst recurrence and er uma stars with very short intervals between superoutbursts. it was found from observations that evolution of the superhumps period consists of 3 stages: early evolutionary stage with a longer period of superhumps (stage a), middle stage with systematically varying superhumps period (stage b) and the last stage with a shorter, stable period of superhumps (stage c). all stages are most distinct in wz sge systems. the superhumps, seen during the early stages of wz sge dwarf novae, are double-wave humps and their periods are close to the orbital ones (kato, 2002), the ordinary superhumps are one-wave humps. they have the largest amplitude of periodicity and period slightly longer than the orbital period. according to the tidal-thermal model of osaki (1996) superhumps arise as a result of accretion disk precession triggered by gravitational disturbances from the secondary component. the period of superhumps is beat period between the precession period pprec and orbital period porb. the present paper is aimed at investigating of superhumps phenomenon in selected su uma stars. 2 program stars the stars for our study were chosen from the vsnet list of dwarf novae undergoing superoutbursts during our observational sets. the criteria for our choice were, first of all, the brightness of an object limited by telescope power and relatively rare superoutbursts (and therefore poorly studied). they are: css 121004: 205146-035827 (j2051), v844 her, pnv j19150199+0719471 (j1915), sdss j150240.98+3334239 (j1502). 3 observations the main part of our observations was obtained at the 60-cm telescope with ccd apogee 47 (528 × 512 pxl, pixel size 12×12µm) in v,r and rc bands and partly at the 50-cm telescope with a new ccd apogee alta-8300 (3326×2504 pxl, pixel size 5.4 × 5.4µm) in v band in crimea. the duration of observational sets varied from 3 to 6 h. the accuracy is 2 − 3% for r band, but is less for v band 4−5%. the reference stars were taken mostly from aavso list. if we used the local standard star from the vicinity of a program star special high precision observations with ubv photometer at the 60-cm telescope were conducted during the nights with good weather conditions for the standard calibration. ccd data were reduced using the maxim dl standard package, ubv data with software developed by v. lyuty. 161 http://dx.doi.org/10.14311/app.2015.02.0161 i. voloshina, v. metlov 3.1 v844 her the dwarf nova v844 her underwent a superoutburst in may, 2012 (vsnet-alert 14525). this object showed frequent outbursts during 2009–2011, but was relatively inactive next year. the superoutburst that we followed up occurred 370 days after the last superoutburst of 2011. our first observation of v844 her was conducted on april 22 when this nova was in quiescence (∼ 18m) but restarted later after receiving information about the outburst and lasted practically until the end of the outburst when v844 her returned to its pre-outburst brightness. figure 1 show the evolution of superhumps in v844 her. 6052.40 6052.45 6052.50 6052.55 13.1 12.9 12.7 6053.40 6053.45 6053.50 6053.55 13.2 13.0 12.8 6054.40 6054.45 6054.50 6054.55 13.4 13.2 13.0 jd 2450000+ 04.05.2012 05.05.2012 06.05.2012 v v v figure 1: v844 her light curves obtained at the beginning of the outburst. decreasing of superhumps which is clearly seen on the light curve at the bottom indicates that a new stage is approaching. 3.2 sdss j150240.98+3334239 the discovery of outburst of this dwarf nova was reported by vsnet on april 2, 2012. its optical magnitude ranges from 13m.7 to 19m.6 during quiescent and outburst phases, respectively. usno photometry reveals a deep (2.5 mag) eclipse occurring at a period of 84.24 minutes (szkody et al., 2006) which evidence that it is a high inclination system. our observations of j1502 cover 6 nights from april 23 to june 2, 2012. observational light curves for three of them are presented in figure 2. they show deep eclipses besides superhumps. our analysis was restricted by consideration of superhumps phenomenon in this system. figure 2: light curves of high inclination system j1502 showing both superhumps and eclipses. 3.3 css 121004: 205146-035827 the object was discovered by crts on october 4, 2012. observations provided by dr. maehara on october 8 (vsnet-outburst 14685) demonstrate superhumps with figure 3: stage b superhumps in the light curves of dwarf nova j2051. an amplitude of 0m.2 and with a short period of 0d.056 in the light curves of this object. we started our observations later, on october 10 because of the bad sky conditions before this date. at this moment the brightness of j2051 was 14m.9. the object started to fade rapidly on october 20 with rate 0.37m · d−1 and became too faint for observations on october 22 (∼ 18m). our observations of dwarf nova j2051 in v band are shown in figure 3. 162 study of photometric variability of selected su uma dwarf novae 3.4 pnv j19150199+0719471 on june 1, 2013 e.de miguel (vsnet-outburst 15466) reported discovery of an outburst of this transient with magnitude ∼ 10m.8. the next day he reported about detection of early superhumps in the light curve of this object with a possible period of 0d.0641(2) (probably stage a). on june 9 the double-peaked superhumps with an average amplitude of 0m.1 and psh ∼ 0d.0569, were detected by several observers. spectroscopic observations provided by echevarria et al.(2013) suggested the high inclination and low massive component,brown dwarf. our observations of this nova started on june 4, 2013 and lasted until the middle of july 2013. during our observations the brightness of j1915 dropped from 11m.47 at the beginning to 18m (v) at the end of the superoutburst. the light curves of j1915 obtained at the different moments of 2013 superoutburst as an example are shown in figure 4. 6449.3 6449.4 6449.5 11.8 11.7 11.6 11.5 11.4 r c a 6457.3 6457.4 6457.5 12.5 12.4 12.3 12.2 r c b 6467.3 6467.4 6467.5 13.0 12.9 12.8 12.7 r c c 6482.3 6482.4 6482.5 16.0 15.9 15.8 15.7 r c jd2450000 + figure 4: evolution of superhumps in dwarf nova j1915: a beginning of stage a superhumps; b stage b superhumps; c transition from b to c stage; d post-fading stage superhumps. scale is the same for all panels. 4 results for data analysis and search of periodicities we used a special code period98 (sperl, 1998) and period04 (lenz & breger, 2004). the analysis was firstly done for every observational night and later the mean values for different stages of each star were calculated. for v844 her although stages a-c were observed, the period of stage a superhumps was not determined, due to limited observations during this stage. pdot for stage b was positive, as seen in other superoutbursts of this object. in the light curves of dwarf nova j2051 the stages b and c can be distinguished. the superhumps period variations suggests that the object more resemble ordinary su uma-dwarf novae than wz sge-stars. the numerous observations obtained for j1915 (about 6000 images at all) permitted us to follow up superhumps evolution in detail beginning from the first night of our observations, june 4. no superhumps were detected in the light curve of j1915 on this particular night. subsequent observations provided on the next night, june 5, recorded a periodic variation in the light curve of j1915. we suggest that it could be beginning of the stage a. the superhumps period for different stages was determined. the absence of recorded outburst in the past, large amplitude of present outburst, values of parameters (like ε, pdot and other) and the pattern of superhumps period variation are typical for wz sge-stars. so we conclude that the recently discovered dwarf nova j1915 belongs to the wz sge-subtype. some of the power spectra calculated for each star as an example are shown in figures 5–8. figure 5: one of the v844 her power spectra as an example. figure 6: the power spectrum of j1502, calculated for all 6 nights of our observations. 163 i. voloshina, v. metlov figure 7: the power spectrum of j2051. figure 8: the power spectrum of j1915. the periods which were determined in frame of this study are collected in table 1. the orbital periods were taken from the literature. table 1: determined periods. object psh(stage b) psh(stage c) porb v844 her 0.05590 ± 2 0.05587 ± 3 0.054643 j2051 0.05715 ± 5 0.05680 ± 6 — j1502 0.05891 ± 2 — 0.06944 j1915 0.05833 ± 2 0.05800 ± 3∗ 0.06164 * for j1915 the period in the second column represent the post-fading superhumps period. 5 summary the main results of our study of su uma dwarf nova photometric variability can be summarized as follows: • the time-resolved photometry in v and r bands was obtained for several su uma-type dwarf novae whose superoutbursts were observed during the 2012–2013 season: – for v844 her 10 nights, ∼ 1400 images in v and rc bands; – for css 120004: 205146-03582 8 nights, ∼ 1000 images in v and r bands; – for pnvj19150199+0719471 13 nights, about 6000 images both in v and rc bands; – for sdss j150240.98+3334239 6 nights, ∼ 750 images in v band. • the individual light curves of all program stars were constructed on the base of these numerous observations. superhumps were detected in the light curves all of them and their evolution during the outbursts was followed up. • the superhumps periods and amplitudes were determined from our data and mean superhumps periods were obtained for each nova (see table 1). • the amplitude of outburst, phase-averaged profile of superhumps and superhumps period variations for css 121004: 205146-035827 suggest that this object more resemble ordinary su uma-stars than wz sge-dwarf nova. • on the base of its outburst properties, the recently discovered nv j19150199+0719471 was classified as a dwarf nova of wz sge subtype. the obtained results are preliminary and reflect the work in progress. they may be slightly improved by using more sophisticated methods and thorough subsequent analysis. acknowledgement i. voloshina gratefully acknowledges professor f. giovannelli for the invitation to take part in this conference as well as his kind hospitality in palermo. the authors thank a. bykov for helping with observations in crimea in 2012. this work was supported in part by the program of state support of russian federation leading scientific schools through grant 2374.2012.2. references [1] lenz, p., breger, m.: 2005, coast 146, 53l. [2] sperl, m.: 1998, coast 111, 1. [3] szkody, p., et al.: 2006, astron. j. 131, 973. doi:10.1086/499308 [4] osaki, y.: 1996, pasp 108, 39. doi:10.1086/133689 [5] . kato, t., et al.: 2012, pasj 64, 21. [6] kato et al.: 2013, pasj 65, 23. (survey iv) 164 http://dx.doi.org/10.1086/499308 http://dx.doi.org/10.1086/133689 introduction program stars observations v844 her sdss j150240.98+3334239 css 121004: 205146-035827 pnv j19150199+0719471 results summary acta polytechnica ctu proceedings doi:10.14311/ap.2016.4.0008 acta polytechnica ctu proceedings 4:8–12, 2016 © czech technical university in prague, 2016 available online at http://ojs.cvut.cz/ojs/index.php/app post critical heat transfer and fuel cladding oxidation vojtěch caha∗, jakub krejčí department of nuclear reactors, faculty of nuclear sciences and physical engineering, czech technical university in prague, czech republic ∗ corresponding author: vojtech.caha@fjfi.cvut.cz abstract. the knowledge of heat transfer coefficient in the post critical heat flux region in nuclear reactor safety is very important. although the nuclear reactors normally operate at conditions where critical heat flux (chf) is not reached, accidents where dryout occur are possible. most serious postulated accidents are a loss of coolant accident or reactivity initiated accident which can lead to chf or post chf conditions and possible disruption of core integrity. moreover, this is also influenced by an oxide layer on the cladding surface. the paper deals with the study of mathematical models and correlations used for heat transfer calculation, especially in post dryout region, and fuel cladding oxidation kinetics of currently operated nuclear reactors. the study is focused on increasing of accuracy and reliability of safety limit calculations (e.g. dnbr or fuel cladding temperature). the paper presents coupled code which was developed for the solution of forced convection flow in heated channel and oxidation of fuel cladding. the code is capable of calculating temperature distribution in the coolant, cladding and fuel and also the thickness of an oxide layer. keywords: heat transfer, post dryout, cladding oxidation, isolated channel. 1. introduction the post critical heat flux heat transfer is encountered when the surface temperature becomes too high to maintain a continuous liquid contact, and the surface becomes covered by a continuous vapour blanket. it results in coverage of the heated surface by a continuous vapour film in the case of film boiling regime, or an intermittent vapour film in the case of transition boiling regime. the boundary between these post dryout heat transfer boiling regimes is the minimum of film boiling temperature, or leidenfrost temperature. the typical boiling curve is depicted in figure 1. post dryout heat transfer is initiated as soon as the critical heat flux condition is reached and it remains until rewetting or quenching of the surface takes place. the post critical heat flux heat transfer regimes in boiling flow can be divided as: (1.) transition boiling regime, (2.) flow film boiling regime, (a) inverted annular film boiling, (b) slug flow film boiling, (c) dispersed flow film boiling. the transition boiling is a combination of unstable film boiling and unstable nucleate boiling consecutively existing at a given location on a heating surface. whereas, film boiling is generally defined as a boiling where only the vapour phase is in contact with the heated surface. inverted annular film boiling is characterized by a vapour layer separating the continuous liquid core from the heated surface and usually encountered at void fractions below 40 %. while dispersed flow film boiling is characterized by the existence of figure 1. typical boiling curve [1]. discrete liquid drops entrained in a continuous vapour flow; normally encountered at void fractions above 80 %. the transition between these two cases is the slug flow film boiling. the existence of film boiling depends on flow conditions and surface temperature. the main parameters influenced heat transfer in post dryout region are pressure, equilibrium quality and mass flux. severe non-equilibrium conditions between liquid and vapour phase can occur and must be taken into account in calculation during low coolant mass flux. it is important to point out that post dryout transition behaviour is reliant on independent boundary condition. if heat flux is the independent boundary condition, the transition boiling regime does not occur. in contrary, when wall temperature is an independent variable, each post dryout flow regime can succeed. as a consequence, the nucleate boiling regime is immediately followed by film boiling regime in the nuclear reactor core [1]. 8 http://dx.doi.org/10.14311/ap.2016.4.0008 http://ojs.cvut.cz/ojs/index.php/app vol. 4/2016 post critical heat transfer and fuel cladding oxidation 2. basic equations first, the main objective of the project was to develop a program for the thermo-hydraulic calculation of nuclear reactor core or experimental electrically heated tubes and rod bundles cooled by light water. the temperature profile of the fuel cladding and fuel with the influence of oxide layer was also included. eventually, the program was used for a comparative assessment of different heat transfer coefficient correlations in the post dryout region and also for evaluation of fuel cladding oxidation correlations. as a conservative approach for the solution of this calculation is used a method of isolated channel model. the calculation can be conducted for stationary states as well as for transients (pseudo-stationary model). it is possible to input nonuniform power distribution and also local resistance (spacer and mixing grids). the program considers homogeneous equilibrium model (hem) which respects thermodynamic imbalance between phases. the calculation can be run at constant pressure or with respect to pressure drops. there are included pressure drops caused by friction, spacer and mixing grids, change of elevation and change of coolant density. pressure drops related to friction between coolant and wall are determined by fiolenko correlation [2]. the wall temperature correction is set by protopov correlation [2] and two phase region correction is calculated by armand’s model [2]. the calculation is based on the solution of equation h = ∫ l 0 q(z) dz. (1) the thermo-physical properties of light water are evaluated by a library using iapws-if97 definition. the program also contains equations and correlations for real quality, slip ratio, void fraction etc. during the development, the code was aimed to a simplicity of input data entering and an ability of easy addition of new models and correlations. 3. heat transfer the fuel cladding surface temperature is very important parameter in operation and safety of nuclear reactors. that is influenced by a local boiling regime and by a suitable correlation for heat transfer from wall to the coolant. in the subcooled liquid region the calculation of nusselt number is conducted by dittus-boelter equation [2] nu = 0.023re0.8pr0.4. (2) whereas nucleate boiling occurs when wall temperature exceeds coolant saturation temperature by jenslottes difference [2] ∆tjl = 0.791 exp ( − p 6.201× 106 ) q0.25. (3) the beginning of bulk boiling is defined by saturation temperature of coolant and then thom correlation is used [2] tw = tsat + 22.52 √ q exp ( p 8.6875 ) . (4) the wall temperature in the transitional region is linearly approximated. the post critical heat flux heat transfer correlation is used immediately after dnbr decreases below one. the user can choose pg-t [3] or bezrukov [2] critical heat flux correlation. eventually, if equilibrium quality is greater or equal one, convection to superheated steam region is set – sieder-tate correlation nu = 0.021re0.8pr0.4 ( pr prw )0.25 . (5) 3.1. post critical heat flux heat transfer due to a large number of post critical heat flux heat transfer correlations, it was necessary to carefully select suitable one for use in pressurized light water reactors. the main criterion for selection was validity range of correlation. finally, it was chosen seven correlations for the post critical heat flux heat transfer. a list of chosen post chf heat transfer correlations and their validity ranges are shown in table 1. it is obvious that all correlations are valid in wide ranges and covered most of operational and emergency states of pwr. most of them consider thermo-dynamic equilibrium between the phases, i.e. it assumes that the temperature of vapour phase is equal to the temperature of saturation. in general, these correlations have similar form to dittus-boelter correlation. correlations which respects thermo-dynamic imbalance between the phases, consider superheated steam which coexists with liquid drops. the difference between them is the heat flux calculation using wall temperature and superheated steam temperature. the typical correlation which consider thermo-dynamic imbalance is groeneveld-delorme correlation. correlation p g x [mpa] [kg m−2 s−1] [–] bishop 4.08–21.9 700–3140 0.07–1 groeneveld 5.7 0,07–21,5 130–4000 −0,12–3,09 groeneveld 5.9 3,4–21,5 700–5300 0,1–0,9 miropolskiy 3,9–21,6 800–4550 0,06–1 groe.-delorme 0,7–21,5 130–5200 −0,12–3,09 condie-beng. 0,42–21,5 16,5–5234 −0,2–1,73 pdo tables 0,1–20,0 0–7000 −0,2–2,0 table 1. validity range of used post chf correlations. the first correlation considering thermo-dynamic equilibrium is bishop correlation (6), [1]. the calculation of heat transfer coefficient is applied to the temperature of vapour blanket on the heated surface which can be obtained as the average of wall and 9 vojtěch caha, jakub krejčí acta polytechnica ctu proceedings saturation temperature. this correlation is used for example in vipre or fraptran codes. nu = 0.0193re0.8 vf pr1.23 vf ( %v %l )0.068( x+ %v %l (1− x) )−0.68 (6) the next usually used equation is groeneveld correlation (7), [4]. the user can find to versions (5.7 and 5.9) in the program. the versions differ in coefficients and validity ranges. this correlation is used in cobra-flx and fraptran codes. the temperature of vapour and liquid are related to the saturation temperature. nu = c1 ( gxdh αλv )aprbw( 1− 0.1 (( %l %v − 1 ) (1− x) )0.4 )−c (7) the program also contains miropolskiy correlation (8), [5]. it is also correlation which respects thermodynamic equilibrium between the phases. but it is valid only for 0.23 ≤ q ≤ 1.16mw/m2 and 8 ≤ dh ≤ 24 mm. nu = 0.023 ( gdh λv )0.8pr0.8 w ( x+ %v %l (1− x) )0.8( 1− 0.1 (( %l %v − 1 ) (1− x) )0.4 )−1 (8) groeneveld-delorme correlation (9), [6] is a typical example of correlation which considers thermodynamic imbalance. it is used for example in fraptran code. nu = 0.008348pr0.6112 vf( gdh µvf ( xa + %v %l (1− xa) ))−0.8774 (9) the last correlation which was used in the program is condie-bengston correlation (10), [6]. it is an empiric correlation that respects thermo-dynamic equilibrium between the phases. α′ = 5.345× 10−5pr2.2598 w re(0.6249+0.2043 ln(x+1)) v (103λv)−0.4593d0.8095 h (x+ 1)2.0514 . (10) moreover, groeneveld’s look-up post dryout tables [7] are included in the program. its advantage is in the wide range of validity due to the large experimental database which is used and simply searching of heat transfer according to pressure, mass flux equilibrium quality and wall and saturation temperature difference. besides, basic table model includes corrections for hydraulic diameter, cold wall, narrow gap or local resistances. these tables are used for example in assert-pv code. 4. temperature profile and cladding oxidation the module solving fuel temperature profile and cladding oxidation is also included. first, the coolant and wall temperature calculation is performed. an increase of cladding oxide layer and its influence on cladding temperature calculation is followed. then, a conduction in the cladding is solved. heat transfer in fuel-cladding gap calculation is the next step. finally, the temperature profile in fuel is found. all used models consider a cylindrical geometry. the temperature increase caused by lower thermal conductivity of zro2 compared to cladding depends on oxide thickness and its thermal conductivity (function of temperature) ∆tox = qdox λox . (11) the growth of the oxide layer greatly depends on the temperature, cladding material and chemical composition of coolant. the current version of the program contains correlation for low temperature oxidation [8] from matpro library, but is prepared for addition another formulae ∆dox = 0.175 exp ( − 14080 t ) t. (12) heat conduction in the cladding is provided with standard equation of fourier’s law in cylindrical geometry. the calculation of the heat transfer coefficient in fuel-cladding gap is the most complicated part in the temperature profile determination. state of gap depends on many parameters. the program solves it as a sum of three parts – heat conduction in gas, in fuel-cladding contact and thermal radiation. all parts contain several models and many constants. a detailed description can be found in [9]. the fourier’s law is used also for calculation of temperature profile in the fuel [9]. the heat source is considered radially uniform. thermal conductivity in fuel respects dependency on temperature, porosity, burnup, cracking, composition etc. 5. post chf experiments there were carried out a whole range of experiments in post dryout region in tubes as well as in rod bundles which most of them are unavailable due to their commercial utilization. available experiments were carried out at different input parameters (pressure, mass flux, power) and geometries. a comprehensive research was done in [1]. a comparative analysis of post dryout heat transfer coefficient correlations described in section 3.1 were performed. owing to wide range of experimental parameters were chosen experiments measured at the royal institute of technology in stockholm [3]. in these experiments which were carried out over 500 were gained more than 15000 points in post chf region. the experimental equipment was connected to light water loop and was in the shape of a vertical round tube that was electrically heated. there were 10 vol. 4/2016 post critical heat transfer and fuel cladding oxidation also placed 49 thermocouples on the outer wall at different elevation. the axial power distribution was uniform. ranges of experimental parameters are given in table 2. heated length [m] 7.0 inner diameter [m] 0.0100, 0.0149, 0.0247 inlet coolant subc. [◦c] 5 ± 1, 10 ± 3 pressure [mpa] 3, 5, 7, 10, 12, 14, 16, 18, 20 mass flux [kg m−2 s−1] 500, 1000, 1500, 2000, 2500, 3000 heat flux [mw m−2] 0.10–1.25 quality (pdo) [-] 0.03–1.60 table 2. range of experimental parameters and geometry. 6. post chf experiments results the comparison of all described post chf heat transfer coefficient correlation was performed on the group of 120 experiments which were chosen carefully to satisfy validity ranges of all correlations (table 1). the calculation found wall temperatures which were compared with experimentally obtained values in the post chf region. an example of one experiment results is given in figure 2. the figure shows a dependence of wall temperature on the axial length. immediately after chf conditions are reached the rapid increase in wall temperature is observed. then, the wall temperatures differ due to used correlation. figure 2. dependence of calculated and experimental wall temperature on axial length. another capability of developed program is shown in figure 3. the graphical module is able to draw any important calculated parameter in time (transient calculations) and also temperature profiles of coolant, cladding and fuel in each time step. the best results were obtained with groeneveld’s look-up pdo tables. statistical evaluation is given in table 3 where ∆i = tw,calc,i − tw,exp,i tw,exp,i , (13) figure 3. coolant, cladding and fuel temperature profile. ∆̄ = 1 n n∑ i=1 ∆i, (14) σ̄ = √√√√ 1 n − 1 n∑ i=1 (∆i − ∆̄)2. (15) correlation ∆̄ [%] σ̄ [%] bishop 2.22 8.69 groeneveld 5.7 0.19 8.3 groeneveld 5.9 −4.93 7.37 miropolskiy 12.58 22.37 groe.-delorme 92.01 12.08 condie-beng. 13.73 13.69 pdo tables 2.82 5.67 table 3. statistical evaluation of post chf correlations. figure 4 shows a comparison of experimental and calculated wall temperatures achieved with groeneveld’s look-up pdo tables. figure 4. groeneveld’s look-up pdo tables results. 11 vojtěch caha, jakub krejčí acta polytechnica ctu proceedings 7. conclusion a thermo-hydraulic program based on isolated channel model was developed. it includes the wide range of heat transfer correlations for different one phase and two phase flow regimes. great attention was paid to the post chf boiling regime. the program also includes the module for evaluation of temperature profile in fuel and cladding which contains influence of oxide layer. the program is prepared for the addition of new oxidation model correlations which are being developed for different cladding tubes materials and temperature ranges. all calculated results can be shown in 3d graphs created by post-processing module using python mayavi library. a comparative analysis of different heat transfer correlations in the post chf region was done. the analysis included a comparison of seven post chf heat transfer correlations with experimental data which were obtained on electrically heated tube. the best results were achieved with groeneveld’s look-up pdo tables. the continue of this work should aim to verify this results on different experimental data set with axially nonuniform power distribution, on transient experiment with the return from post chf region and on rod bundle experiment. list of symbols g mass flux [kg/m2s] h enthalpy [k/kg] l length [m] q heat flux [w/m2] q linear power [w/m] nu nusselt number [–] p pressure [pa] pr prandtl number [–] re reynolds number [–] t time [s] t temperature [k] x quality [–] α void fraction [–] λ heat transfer coefficient [w/m2k] µ dynamic viscosity [pa s] % density [kg/m3] acknowledgements this work was supported by the grant agency of the czech technical university in prague, grant no. sgs14/156/ohk4/2t/14. references [1] thermohydraulic relationships for advanced water cooled reactors. tech. rep. iaea-tecdoc-1203, international atomic energy agency, 2001. http://www.iaea.org/inis/collection/ nclcollectionstore/_public/32/024/32024154.pdf. [2] n. e. todreas, m. kazimi. nuclear systems volume i: thermal hydraulic fundamentals. crc press, 1989. [3] k. m. becker, c. h. ling, s. hedberg, g. strand. an experimental investigation of post dryout heat transfer. tech. rep. kth-nel-33, royal institute of technology, stockholm, sweden, 1983. http://www.iaea.org/inis/collection/ nclcollectionstore/_public/15/049/15049472.pdf. [4] d. c. groeneveld. post-dryout heat transfer at reactor operating conditions. in national topical meeting on water reactor safety. 1973. http://www.iaea.org/inis/collection/ nclcollectionstore/_public/04/089/4089010.pdf. [5] n. hammouda. subcooled film boiling in non-aqueous fluids. ph.d. thesis, university of ottawa, 1996. http://hdl.handle.net/10393/9949. [6] s. k. moon, s. y. chun, s. cho, et al. an experimental study on post-chf heat transfer for low flow of water in a 3x3 rod bundle. nuclear engineering and technology 37(5):457–468, 2005. [7] d. c. groeneveld. look-up table for fully developed film-boiling heat-transfer coefficients for light water. university of ottawa, 2001. [8] j. krejčí, v. vrtílková, d. gajdoš, d. rada. proposal of new oxidation kinetics for sponge base e110 cladding tubes material. in topfuel2015 – conference proceedings, pp. 466–473. 2015. http://www.euronuclear.org/ events/topfuel/topfuel2015/transactions/ topfuel2015-transactions-poster.pdf. [9] j. krejčí, v. caha. ube-postchf. tech. rep., čvut, praha, 2015. 12 http://www.iaea.org/inis/collection/nclcollectionstore/_public/32/024/32024154.pdf http://www.iaea.org/inis/collection/nclcollectionstore/_public/32/024/32024154.pdf http://www.iaea.org/inis/collection/nclcollectionstore/_public/15/049/15049472.pdf http://www.iaea.org/inis/collection/nclcollectionstore/_public/15/049/15049472.pdf http://www.iaea.org/inis/collection/nclcollectionstore/_public/04/089/4089010.pdf http://www.iaea.org/inis/collection/nclcollectionstore/_public/04/089/4089010.pdf http://hdl.handle.net/10393/9949 http://www.euronuclear.org/events/topfuel/topfuel2015/transactions/topfuel2015-transactions-poster.pdf http://www.euronuclear.org/events/topfuel/topfuel2015/transactions/topfuel2015-transactions-poster.pdf http://www.euronuclear.org/events/topfuel/topfuel2015/transactions/topfuel2015-transactions-poster.pdf acta polytechnica ctu proceedings 4:8–12, 2016 1 introduction 2 basic equations 3 heat transfer 3.1 post critical heat flux heat transfer 4 temperature profile and cladding oxidation 5 post chf experiments 6 post chf experiments results 7 conclusion list of symbols acknowledgements references 246 acta polytechnica ctu proceedings 1(1): 246–250, 2014 246 doi: 10.14311/app.2014.01.0246 the x-ray binary ks 1731–260: possible analogy with her x-1 vojtěch šimon1,2 1astronomical institute, academy of sciences of the czech republic, 25165 ondřejov, czech republic 2czech technical university in prague, fel, prague, czech republic corresponding author: simon@asu.cas.cz abstract the x-ray binary with the neutron star (ns), ks 1731–260, displays superorbital cycle similar to that in her x-1. the accretion disk had the memory of the cycle-length even when this modulation sometimes disappeared in the main outburst of ks 1731–260, and during anomalous low state in her x-1. the disk still existed during such seasons. although irradiation of the disk by x-rays is a viable explanation for the disk precession and warping (see model of foulkes et al.), the mechanisms which give rise to the observed x-ray modulation are quite different for each of these systems. variable absorption can explain this cycle only in her x-1. we propose a variable mass accretion rate onto the ns in ks 1731–260 due to a highly variable impact of the inflowing mass stream with the changing phase of the cycle. keywords: radiation mechanisms accretion, accretion disks x-rays: binaries optical x-rays. 1 introduction ks 1731–260 (e.g. [2, 32]) and her x-1 (e.g. [10]) are mass exchanging x-ray binary systems, with the neutron star (ns) accreting matter from a low (or medium)-mass companion. they can be considered specific type of transient systems as regards their activity (on/off transients in the notation of [13] (rapid rise, very long (years) lasting state of activity, rapid return to the inactive state)). their active x-ray states were accompanied by intense brightenings by more than a magnitude in the optical [12] and near infrared bands [21, 32]. both these systems display cyclic modulation of xray intensity during the active state (her x-1: the cyclelength pc ≈ 35 d [28], ks 1731–260 pc ≈ 38 d [22]). this modulation is superorbital (about 20 times longer than the orbital period porb) in her x-1 because its porb is only 1.7 d [28]. since the observations of ks 1731– 260 in quiescence showed that the donor is of type g5v (with a contribution of a synchrotron jet) [32], it is possible that the cycle is superorbital, too, if this star fills its roche lobe. 2 observations the data from asm/rxte [15] (one-day means) were used. they contain the intensities isum (1.5–12 kev) and ia (1.5–3 kev), ib (3–5 kev), ic (5–12 kev). the hardness ratios are hr1 = ib/ia and hr2 = ic/ib. the optical data of her x-1 come from the aavso international database (massachusetts, usa) [11]. 3 active state in ks 1731–260 in ks 1731–260, the x-ray active state with a complicated evolution lasted for about 12.5 years [4]. after return to quiescence, the luminosity is very similar to that of the other quiescent ns systems [31]. the end of the active state was accompanied by a decrease of brightness by about 1.7 mag(h) in the infrared [21, 32]. asm/rxte monitored part of the active state. it consisted of two segments of a largely different activity. in the scenario of [24] using the model of [6], the whole disk was thermally stable in the hot (ionized) state during the main outburst. the rapid end of the main outburst was followed by a series of the echo outbursts, ascribed to a thermally unstable disk with a series of recurrently propagating cooling and heating fronts. the x-ray light curve observed by asm/rxte is shown in fig. 1a. moving averages (ma) of isum with the filter half-width q = 5 d enhance the visibility of the modulation on a timescale of days. to determine the long-term evolution of isum averaged over these fluctuations, the data were fitted with the code hec13. it was written by prof. p. harmanec using the method of [30]. the changes of isum are not accompanied by the variation of hr1 in the sense expected if absorption is the cause of the modulation (using the coefficients of [18]) (fig. 1b). the same is true for hr2. search for the cycle-length in ma from fig. 1a was carried out with the weighted wavelet z-transform (wwz) developed by [7]. this method enables one to determine the period and amplitude of unevenly sam246 http://dx.doi.org/10.14311/app.2014.01.0246 the x-ray binary ks 1731–260: possible analogy with her x-1 pled time series. wwz indicates whether or not there is a periodic fluctuation at a given time at a given frequency f (fig. 2a). a similar result was obtained for the one-day means of isum. it emerges that neither the period of the modulation, pc, nor its amplitude are stable. pc is not conclusively detected in some segments. this is caused by a real, abrupt change of the light curve. figure 1: (a) x-ray light curve of ks 1731–260 during the main outburst. ma of isum (q = 5 d) enhance the visibility of the modulation. the smooth curve shows the main trend (the hec13 fit). the time interval around the conjunction with the sun is marked. (b) time variation of hr1. the smooth line represents ma for q = 8 d. the errors of ma are the standard errors. the big error bar and arrow in panel b mark the influence of absorption. three prominent extrema of isum and their hr1 are marked. see sect. 3 for details. adapted from [25]. fig. 2b shows the best pc, determined from f that has the biggest value of wwz at a given time (only the segments in which the amplitude is larger than 50 percent of its peak value). see [25] for details. the recurrence time tc of the echo outbursts is about 51 days [24], which is quite different from pc in the main outburst (see also [22]). absorption of x-rays would influence mostly ia. however, the variations of soft x-ray intensity during the cycle suggest intrinsic changes of emission at unchanged absorption (fig. 1b). the echo outbursts after the main outbursts in some transients are helpful in investigating the nature of the above-mentioned cycle in ks 1731–260. the recurrence time tc of the echo outbursts is significantly longer for the systems with longer porb [24]. in this regard, we present arguments in favor of the superorbital nature of the cycle in the main outburst of ks 1731–260. if pc of 38 d were close to porb, the expected tc of the echo outbursts would be at least 800 days, much longer than observed. figure 2: (a) wwz-transform of the moving averages of ks 1731–260 from fig. 1a. (b) the best cycle-length. see sect. 3 for details. adapted from [25]. 4 activity in her x-1 the x-ray light curve of her x-1 displays only two segments (discrete spikes) of the on state (main-on and short-on) during the 35 d cycle (e.g. [28, 14]). on and off-states do not form a clear profile of the long-term light curve in the asm/rxte data; only two anomalous low states als1 and als2 in which isum was below the detection limit can be resolved. instead of the light curve, the activity was therefore studied using the fluence of each main-on spike (fig. 3a). this fluence was determined by an integration of isum over the profile of the main-on state. the scatter of fluences of the individual spikes is real. their time evolution is emphasized by the hec13 fits. they reveal a cycle of about 400 d. evolution the 35 d cycle, measured by the turn-on times of the main-on state, is correlated with that of the spin period of the ns. this suggests the evolution of the transfer of angular momentum from the disk to the ns [27]. the 35 d cycle-length is always unstable, but the largest changes coincide with als. als thus represents an episode of the largest change of the mass flow through the disk onto the ns. the disk remembers the conditions for the 35 d cycle-length and its phase 247 vojtěch šimon (mainly after als2). figure 3: (a) time evolution of fluence of main-on states (1.5–12 kev) in her x-1. each episode of this state represents a point in the curve. the smooth lines represent the fits to the time evolution. (b) smoothed evolution of the optical orbital modulation of her x-1 with time. see sect. 4 for details. the profile of the unique optical emission plays an important role in distinguishing between the character of the active and inactive states in her x-1. dominant part of the optical emission in the active state is due to reprocessing off x-rays from the close vicinity of the ns on the part of the photosphere of the donor facing the ns (e.g. [10]). occasional inactive states in which the orbital modulation remained flat and the optical emission faint occurred only before the x-ray astronomy era (e.g. [12]). they were caused by temporary decreases of this irradiation. a heavy smoothing of the aavso optical data enabled to study the time evolution of the orbital modulation, hence of the x-ray irradiation of the donor (fig. 3b). this revealed a relation between the optical and x-ray long-term activity. in spite of a decrease of the optical brightness accompanying the fall of x-ray flux in als1, the x-ray irradiation remained. this implies that accretion continued even during als1. this event was therefore not a real low state like those in the pre-x-ray astronomy era. a similar situation was the als in 1983 when the optical modulation of her x-1 remained comparable to that in the active state [5]. however, the observations by the astron satellite revealed very large differences between the x-ray spectra obtained in this als and after recovery from this low state (1984) [26]. intense cyclotron emission of the ns dominated during normal main-on state of the 35 d cycle (aug 25, 1984). on the contrary, the x-ray spectrum obtained in als in the phase when main-on state was expected (june 30, 1983) showed a combination of a faint cyclotron emission of the ns and reflection of the donor. both the orbital modulation in the optical band and the reflection of the donor in the x-ray band confirmed that the mass accretion continued during als. als1 in fig. 3a appears to be extreme in the sense that both the change of the 35 d cycle of the disk and the increase of the spin period of the ns had the largest amplitude among the episodes of als [27]. the decrease of the optical brightness in als1 (fig. 3b) then suggests that these changes of the disk were also accompanied by a decrease of the x-ray irradiation of the donor. 5 conclusions the modulation of x-ray emission in ks 1731–260 was first attributed to the precession of the accretion disk by [22]. its pc ≈ 38 d in the main outburst is quite different from tc ≈ 51 d of the echo outbursts. the series of the echo outbursts in ks 1731–260 [24] can be used as an independent method for determining its porb to be about 0.5 − 1 d [24]. since the length of porb is by more than an order of magnitude shorter than pc, ks 1731–260 became a new member of the family of x-ray binary systems with superorbital cycles. the superorbital cycle of ks 1731–260 cannot be caused by transitions of the disk between the hot and the cool state because it occurred only during the phase of the hot (ionized) state of the disk (see also [25]). the disk had the memory of the cycle as regards the length of pc in ks 1731–260 even when this modulation sometimes disappeared in the main outburst, and in her x-1 during als. nevertheless, the disk still existed even during these phases, which is suggested by the intense x-ray emission in ks 1731–260 and the reprocessing off x-rays in the optical band in her x-1. intense irradiation of the disk by x-rays is a viable explanation for the disk precession and warping (see models of [8, 9]). indeed, ks 1731–260 and her x-1 are strong emitters (lx ≈ 1.8 × 1037 erg s−1 in the main outburst of ks 1731–260 [20], lx ≈ 2× 1037 erg s−1 in the main-on state of her x-1 [17]). this is about 0.1 of the eddington luminosity. however, it is necessary to find how this precession gives rise to the modulation of the observed x-ray intensity. in her x-1, absorption causes x-ray variations during the cycle [23]. however, 248 the x-ray binary ks 1731–260: possible analogy with her x-1 this explanation is not applicable to ks 1731–260. we propose an alternative explanation for the modulation in ks 1731–260. the models of [8, 9] show that the stream of matter inflowing from the donor reaches quite different impact regions of the tilted and warped disk during the cycle. this impact can occur deep in the inner disk region in some phases. this therefore affects the mass flow in the inner disk during the individual phases. in this scenario, the true cycle-length can be twice as long if the profile is double-wave. the almost constant hr1 and hr2 during the cycle can be explained if the spectral components [4] and their intensities vary with a variable mass accretion rate onto the ns. the number of mechanisms for the modulation during superorbital cycle therefore increases in this family of systems. in ks 1731–260, the atoll shifts on a timescale of less than a day [3] are superimposed on the superorbital cycle. according to [1], atoll type shifts are caused by variations of the mass accretion rate onto the ns. this suggests that the accretion is due to the contributions of various simultaneous flows of matter. magnetic flux density b of the ns is important for the emission mechanism in the x-ray band. is there any relation between this mechanism and the superorbital cycle? her x-1 contains a pulsar with the spin period of 1.24 s (0.806 hz) [28] and b ≈ 5 × 1012 gauss [29]. it produces cyclotron emission (e.g. [16]). on the other hand, ks 1731–260 is a millisecond pulsar with the spin period of 524 khz [19]. in the main outburst, its emission is a combination of an optically thick comptonization component and a blackbody component [20]. this suggests b of the ns much smaller than in her x-1. all this implies that the superorbital cycle can operate even for such largely different mechanisms of x-ray emission, hence for the largely different b and the accretion flows near the ns. acknowledgement this research has made use of the observations provided by the asm/rxte team. i used the aavso international database (massachusetts, usa). this study was supported by the grants 102/09/0997 and 13-33324s provided by the grant agency of the czech republic. i made use of the code developed by dr. g. foster and available at http://www.aavso.org/data/software/wwz.shtml. i thank prof. petr harmanec for providing me with the code hec13. the fortran source version, compiled version and brief instructions how to use the program can be obtained via http:// astro.troja.mff.cuni.cz/ftp/hec/hec13/. references [1] bloser, p.f., et al.: 2000, apj, 542, 1000 doi:10.1086/317019 [2] barret, d., et al.: 1998, a&a, 329, 965 [3] barret, d., et al.: 2000, apj, 533, 329 doi:10.1086/308651 [4] chelovekov, i. v., et al.: 2006, astl, 32, 166 [5] delgado, a. j., et al.: 1983, a&a, 127, l15 [6] dubus, g., et al.: 2001, a&a, 373, 251 [7] foster, g.: 1996, aj, 112, 1709 [8] foulkes, s.b. et al.: 2006, mnras, 366, 1399 doi:10.1111/j.1365-2966.2005.09910.x [9] foulkes, s.b., et al.: 2010, mnras, 401, 1275 doi:10.1111/j.1365-2966.2009.15721.x [10] gerend, d., boynton, p. e.: 1976, apj, 209, 562 [11] henden, a.: 2011, aavso database [12] hudec, r., wenzel, w.: 1976, baicz, 27, 325 [13] king, a.r.: 2006, in compact stellar x-ray sources, cambridge univ. press [14] leahy, d. a.: 2002, mnras, 334, 847 doi:10.1046/j.1365-8711.2002.05547.x [15] levine, a. m., et al.: 1996, apj, 469, l33 [16] lewin, w. h. g., et al.: 1995, x-ray binaries, camb. astr. s., vol.26 [17] mccray, r. a., et al.: 1982, apj, 262, 301 [18] morrison, r., mccammon, d.: 1983, apj, 270, 119 [19] muno, m. p., et al.: 2000, apj, 542, 1016 doi:10.1086/317031 [20] narita, t., et al.: 2001, apj, 547, 420 doi:10.1086/318326 [21] orosz, j.a., et al.: 2001, atel 75 [22] revnivtsev, m., sunyaev, r.: 2003, a&a, 399, 699 [23] roberts, w.j.: 1974, apj, 187, 575 doi:10.1086/152667 [24] šimon, v.: 2010, a&a, 513, a71 [25] šimon, v.: 2012, newa, 17, 697 doi:10.1016/j.newast.2012.04.006 [26] sheffer, e. k., et al.: 1992, sva, 36, 41 249 http://dx.doi.org/10.1086/317019 http://dx.doi.org/10.1086/308651 http://dx.doi.org/10.1111/j.1365-2966.2005.09910.x http://dx.doi.org/10.1111/j.1365-2966.2009.15721.x http://dx.doi.org/10.1046/j.1365-8711.2002.05547.x http://dx.doi.org/10.1086/317031 http://dx.doi.org/10.1086/318326 http://dx.doi.org/10.1086/152667 http://dx.doi.org/10.1016/j.newast.2012.04.006 vojtěch šimon [27] staubert, r., et al.: 2009, a&a, 494, 1025 [28] tananbaum, h., et al.: 1972, apj, 174, l143 doi:10.1086/180968 [29] truemper, j., et al.: 1978, apj, 219 l105 doi:10.1086/182617 [30] vondrák, j.: 1969, baicz, vol.20, 349 [31] wijnands, r., et al.: 2001, apj, 560, l159 doi:10.1086/324378 [32] zurita, c., et al.: 2010, a&a, 512, a26 250 http://dx.doi.org/10.1086/180968 http://dx.doi.org/10.1086/182617 http://dx.doi.org/10.1086/324378 introduction observations active state in ks 1731–260 activity in her x-1 conclusions acta polytechnica ctu proceedings doi:10.14311/app.2017.11.0097 acta polytechnica ctu proceedings 11:97–102, 2017 © czech technical university in prague, 2017 available online at http://ojs.cvut.cz/ojs/index.php/app analysis of dynamic effects acting on railway crossings daniela vukušičová institute of railway structures and constructions, faculty of civil engineering, brno university of technology, brno, czech republic correspondence: vukusicova.d@fce.vutbr.cz abstract. the paper is focused on measurement and analysis of dynamic effects on railway turnouts. two same-type turnouts with different fastening elasticity were chosen. attention was focused mainly on the crossing part of the turnout, where the highest dynamic impacts occur. the point of the paper is the assessment of influence of soft rail pads on spread of dynamic energy through the construction. keywords: turnout, crossing part, dynamic effects, vibrations. 1. introduction switches and crossings are the key part of a railway track. in terms of dynamic effects they belong to the most loaded constructions on the railway infrastructure especially crossing part with common crossing. they are not only the places where wheel moves from wing rail to frog (or vice versa), but also places where the stiffness changes. however the length of track with switches and crossings constitutes just a small part of the railway network, maintenance of the crossings generates relatively high cost. [1] it is primarily caused by complex forces action which is induced by passage of the train through the crossing and also by the need of maintenance of many turnout components. the study of dynamic effects will help us to clarify some regularities which may be used for future optimisation of the construction and it should lead to its service life extension. 2. aims the main aim is a comparison of dynamic behaviour of two constructions which different fastening system elasticity. the comparison is based on evaluation of transmission of vibration through these constructions. it is required to repeat measurements several times in reasonable time periods to compare the constructions in terms of increasing dynamic effects in time, as well. one of important things to do is also a choice of suitable mathematic apparatus for evaluation of dynamic effects in time, frequency and time-frequency domain. 3. stress of a crossing part the most important place of a crossing part in terms of dynamic effects is the area of transition from a wing rail to crossing nose, where the dynamic impact occurs. this impact is transmitted through the bearers to ballast which is then extremely stressed and it leads to abrasion of gravel particles on contact with a bearer. due to this stress, degradation of ballast shape under the bearer happens and it causes inadequate support of the turnout. if a turnout is not supported appropriately, the geometry of transmission forms a wing rail to crossing nose collapses and the whole degradation process accelerates. the crossing part is stressed more and more which can cause damage of a frog. 4. mechanism of wheel transition from the wing rail to the crossing nose mechanism of wheel transition from the wing rail to the crossing nose is a very complex spatial problem. the wheel passes three phases during the transition of the frog. at first, it goes on the wing rail, then it goes through a transition zone on both wing rail and crossing nose and at last it goes on crossing nose and related rail. this mechanism is shown in fig. 1 in ideal case of geometry of transition a wheel passes from the wing rail to a crossing nose continuously without dynamic impacts. this complex problem is often simplified. the simplification consists in the fact that a wheel runs on the wing rail, which deviates from the axis of the track to create a space for a crossing nose. due to the geometry of the wheel and the wing rail the wheel often descends slightly at an angle α1. after impact on the crossing nose a wheel rises again at an angle of a crossing nose α2, as long as it does not reach the starting position at the end of the transition zone. this could be seen on the simplified model of the wheel movement through the crossing part shown in fig. 2. from fig. 2 it is evident, that the total angle, at which wheels impact the crossing nose is α = α1 +α2, in this angle the influence of transition geometry on dynamic effects is simplified described. 97 http://dx.doi.org/10.14311/app.2017.11.0097 http://ojs.cvut.cz/ojs/index.php/app daniela vukušičová acta polytechnica ctu proceedings figure 1. schematic representation of the wheel set crossing over the frog – view from above [2]. figure 2. simplified model of wheel movement through the crossing part [3]. 5. measuring location and description of measured turnouts the measurements were carried out at the railway station usti nad orlici on main lines turnouts no. 3 and 4 (fig. 3). maximum speed on main line is 130 km.h−1 (160 km.h−1) for tilting vehicles, such as pendolino). both chosen constructions are the same except elasticity of the fastening system. description of the turnouts: • part of single crossover • rails profile uic 60 • fastening system (ribbed baseplate with vossloh tension clamp skl 24), turnout no. 3 has new ribbed baseplates with different rail pads – reduce overall stiffness of the track. • monoblock crossing from manganese steel • concrete sleepers • ballasted track • crossing angle 1:12 and radius 500 m • used almost exclusively in trailing direction three measuring campaigns were made on both above figure 3. turnout no. 4 and single crossover with turnout no. 3. mentioned turnouts. 6. methodology of the measurement the certified methodology of the measurement was used [4]. measurement methodology is designed for in-situ measurement in condition of full operation and was certified by the ministry of transport of the czech republic. the certified methodology consists of bearers displacements measurements, vibration acceleration measurements (on crossing, bearers and into the ballast), and pressure measurements (in two levels into railway substructure). for the purposes of this paper only small part of vibration acceleration measurement is described, only the part which is used below in evaluation part of the paper. the focus is on vibration transition from the crossing trough bearer to the ballast. the three-axis accelerometer was placed on flange of the wing rail (fig. 4) and one-axis sensor was placed on bearer nearest to the crossing nose. piezoelectric vibration acceleration sensors on wing rail and on bearer were attached to plastic handles and glued to the cleaned surface of the measured structure. 98 vol. 11/2017 analysis of dynamic effects acting on railway crossings figure 4. the three-axis accelerometer placed on flange of the wing rail. for measurement into the ballast the measuring stone was developed. the three-axis accelerometer was inserted into the measuring stone which was placed into the ballast directly under the crossing nose. 7. mathematical apparatus when dynamic action from railway traffic is measured in-situ it is necessary to describe and evaluate stochastic signal. this is rather difficult. in order to get necessary information and compare the dynamic action on each construction it is possible to evaluate stochastic signal from three different perspectives. first perspective is time domain. the calculated value is area under the curve of moving rms (rms stands for root mean square). thanks to this time perspective it’s possible to determine total dynamic action on construction. for detailed analysis of the dynamic actions above described method is not sufficient because frequency composition is unknown. for this reason it is useful to transform signal from time domain to frequency domain. for this transformation we use fourier transformation method. in frequency domain it is possible to determine which parts of signal are the strongest ones. for detailed analysis it is useful to use third perspective, which evaluate signal from time-frequency domain [5]. with this perspective we can see not only frequency composition but also its occurrence in time. 8. the welch method the welch method is certain modification of the fast fourier transformation. digital signal x[n] (n = 0, 1, 2, ..., n − 1) is divided into k segments each of them with length m (xi[m], i = 0, 1, ..., k − 1,m = 0, 1, ...,m − 1). segments are either placed in row one by one then n = k∗m or they overlap. each segment is weight by function w[m]. after transformation and following calculations periodograms components sj[k] are created. these components put together represent approximate spectral density s[k]. this estimation is described by following formulas. component of periodograms is determined by formula 1 [6]. sj [k] = 1 u ∗m ∗ [m−1∑ m=0 x[m+i∗m ]∗w[m]∗ε( −j2πmk m ) ]2 (1) where u = 1 m ∗ m−1∑ m=0 w2[m] (2) is vector’s standard of window function, w[m] is window function. resultant estimation is done by averaged component periodogram. s = 1 k ∗ k−1∑ i=0 sj [k] (3) 9. short time fourier transform (stft) stft provides compromise between time and frequency signal representation [7]. its integral definition is stft (ω) x (t′, f) = ∫ ∞ −∞ [ x(t)∗g(t−t′) ] ∗ε−i2πf(t−t′)∗dt (4) where g is window function, ’*’ complex conjunction, t′ is time displacement of window, x(t) its time representation of signal and stft (ω) x (t′, f) is its timefrequency representation [7]. 10. evaluation of vibration acceleration evaluation of vibration acceleration is divided to three sections: time domain, frequency domain and timefrequency domain. for time analysis comparable trains that passed over both turnouts were chosen. for detailed frequency analysis were chosen locomotive type 380 and two multiple units (leo express and pendolino) because they passed over both turnouts the maximum line speed. the most interesting is transmission of vibration from wing rail (a4z) through bearer (a3z) to the ballast (a33z). the evaluated signals are identified as follows: first letter a indicates vibration acceleration, number in the middle is a measuring channel number and the last letter indicates direction (z is vertical direction). 11. evaluation in time domain from many graphs and tables i chose the one which best represents results of my analysis. table 1 represents areas under the curve of moving rms on both turnouts calculated from vibration signals measured from selected trains and their comparison in all measurements campaigns. red colour typifies percentage loss of dynamic energy by transmission from rail to bearer and to ballast. value of the area under the 99 daniela vukušičová acta polytechnica ctu proceedings curve of moving rms on rail (a4z) is thought as 100% and it is calculated how many percent of this value is transmitted to bearer (a3z) and to ballast (a33z). it is obvious that values on wing rail (a4z) are increasing especially from third campaign on turnout no. 4. this fact is apparently caused by degradation of transition geometry, as the turnout no. 4 has bigger bearer displacement. on the bearer (a3z) there are also increasing values from third measuring campaign, but the increase is not significant in either of turnouts. on the turnout no. 4 there is slight increase of the percentage of dynamic energy transfer to the bearer, while on turnout no. 3 not, which is interesting. into the ballast both turnouts are comparable, but from overall point of view more dynamic energy is transferred from turnout no. 4. if we put into account all measurement campaigns on turnout no. 3, approximately 24% of dynamic energy is transmitted to the bearer, while on turnout no. 4 it is about 35%. into the ballast almost 5% of dynamic energy is transmitted from turnout no. 3 and 6,6% from turnout no. 4. the positive contribution of new fastening system on turnout no. 3 is shown in these results. 12. evaluation in frequency domain very important information can be obtained from frequency spectrum of the evaluated signal. for better comparison frequency curves were converted to numbers by calculating the areas under the curve. the area under frequency curve was divided into two frequency bands. first frequency band (150 – 600 hz) is the band where high frequency contact impact force occurs. this high frequency impact cases especially plastic deformation on the crossing nose and that is the reason why these high frequencies are not transferred into the bearer. second frequency band (0 – 150 hz) is the band where low frequency bending process occurs. these low frequencies are responsible for bearer displacements and ballast bed bending. based on the above mentioned on the wing rail both frequency bands were evaluated while on the bearer and in into the ballast only second frequency band (0 – 150 hz) was evaluated [8]. two multiple units (pendolino and leo express) and locomotive type 380 were selected for comparison of the turnouts (table 22). it is obvious that between second and third measurement campaign all values except that into the ballast are increased. lower values into the ballast can be explained by the fact that space between the bearer and ballast occurred. the bearer has worse support which resulted in bigger displacements. bigger bearer displacements are on the turnout no. 4, this is reflected in the values from the bearer (a3z). on the wing rail (a4z) especially on the frequency band 150 – 600 hz bigger values are on turnout no. 3, bud the difference between turnouts decreases. this fact can be caused by bigger wear of the turnout no. 3. difference decreases because of displacement on turnout no. 4 bearers are more significant than on turnout no. 3. based on above mentioned evaluation it is obvious that it is possible to valorise the wear of the crossing or bearer displacement only on the basis of vibration acceleration measurements. this is an important fact. 13. evaluation in time-frequency domain time-frequency method can be used for confirmation of conclusions from frequency and time analysis. the advantage of this method is that it can display time and value of frequency action in one chart. as an example i present time-frequency graph calculated for signal detected on bearer when pendolino train passed the crossing. as we can see on figure 5 there is time on horizontal axis and frequency on vertical axis. figure 5 shows the fact that the densest frequencies were detected in time when train wheel sets were passing the measuring point. maximum frequency range is from 70 to 80 hz and about 50 hz. 14. conclusion and recommendation within three measurement campaigns two turnouts with different fastening elasticity were compared. the comparison was based on measurement of dynamic effects in-situ. according to evaluation it is possible to declare that the new fastening system on turnout no. 3 appears to be perspective. it has a positive influence in reduction of dynamic effects on bearer and ballast. i would like to recommend observation of both measured turnouts and realize more measurements on them with use of the certified methodology at least once a year. if the positive influence of new fastening system confirmed, i would recommend its installation on more turnouts. references [1] a. m. zarembski. factors involved in turnout maintenance. railway track & structures, 1995. issn 0339016. [2] r. müller, b. nelain, j. nielsen, et al. description of the vibration generation mechanism of turnouts and the development of cost effective mitigation measures. 2013. report from project rivas, code: rivas_uic_wp3-3_d3_6_v02, version 2. [3] fischer, oberaigner, daves, et al. the impact of a wheel on a crossing. die stosswirkung eines rades auf das herzstück einer weiche. zev rail glasers annalen 129, 2005. issn 1618-8330. [4] j. smutný, l. pazdera, i. vukušič. hodnocení dynamických účinků na výhybky. certifikovaná metodika. ministerstvo dopravy – odbor kosmických aktivit a its, 2014. 124/2014-710-vv/1. 100 vol. 11/2017 analysis of dynamic effects acting on railway crossings area under curve of moving [m/s] measurement train a4z on crossing no. a3z on crossing no. a33z on crossing no. 3 4 3 4 3 4 1 loko 380 49 43 12 13 3 3 24 31 6 6 2 loko 380 45 34 8 10 3 3 18 29 7 10 3 loko 380 64 40 11 14 4 3 17 35 6 8 1 leo express 123 93 31 34 4 4 26 36 3 5 2 leo express 121 89 27 30 5 6 22 33 4 6 3 leo express 152 105 32 37 4 4 21 35 3 4 1 pendolino 217 308 74 116 11 13 34 38 5 4 2 pendolino 264 229 74 76 15 14 28 33 6 6 3 pendolino 312 239 85 104 16 23 27 44 5 10 average 149,7 131,1 39,4 48,1 7,1 8,1 24,1 34,8 4,9 6,6 table 1. area under curve of moving rms and the percentage transfer of dynamic energy from rail to sleeper and to ballast. area under curve of frequency spectrum hz * m/s2 measurement train a4z on crossing no. a3z on crossing no. a33z on crossing no. 3 4 3 4 3 4 0150hz 150600hz 0150hz 150600hz 0150hz 0150hz 0150hz 0150hz 1 loko 380 1735 6413 1340 4555 988 986 328 344 2 loko 380 1206 5854 1438 4688 621 855 314 391 3 loko 380 1620 6428 1747 4635 864 1311 199 153 1 leo express 675 3461 500 2463 502 448 72 75 2 leo express 449 2402 489 1932 293 379 66 90 3 leo express 604 3463 616 2750 421 604 47 32 1 pendolino 1018 3731 737 2718 634 816 96 135 2 pendolino 1000 3615 853 3074 642 626 132 126 3 pendolino 1085 3985 991 4008 794 1175 83 72 average 1044 4372 968 3425 640 800 149 158 table 2. area under frequency curve divided in to two frequency bands. 101 daniela vukušičová acta polytechnica ctu proceedings figure 5. time-frequency spectrogram calculated by stft method, turnout no. 3, pendolino train, third measurement campaign. [5] j. smutný, l. pazdera, m. bajer, i. moll. the analysis of dynamic effects in turnout structures by the method of quadratic time and frequency invariant transformations. proceedings of the sixth international symposium "computatuonal civil engineering 2008" iasi. societatea academica matei – teiu botez, iasi, romania, 2008. isbn 978-973-8955-41-7. [6] j. smutný. the modern method of the noise and vibration analysis applied on railway traffic. thesis of the doctoral dissertation. vut brno. isbn 80-214-0988-6. [7] j. smutný, l. pazdera. time-frequency analyses of real signals. 2003. isbn 80-86433-23-4. [8] i. vukušič. analýza dynamických účinků ve výhybkách. disertační práce. vysoké učení technické v brně, fakulta stavební, ústav železničních konstrukcí a staveb, 2005. vedoucí práce prof. ing. jaroslav smutný, ph.d. 102 acta polytechnica ctu proceedings 11:97–102, 2017 1 introduction 2 aims 3 stress of a crossing part 4 mechanism of wheel transition from the wing rail to the crossing nose 5 measuring location and description of measured turnouts 6 methodology of the measurement 7 mathematical apparatus 8 the welch method 9 short time fourier transform (stft) 10 evaluation of vibration acceleration 11 evaluation in time domain 12 evaluation in frequency domain 13 evaluation in time-frequency domain 14 conclusion and recommendation references acta polytechnica ctu proceedings doi:10.14311/app.2016.5.0041 acta polytechnica ctu proceedings 5:41–43, 2016 © czech technical university in prague, 2016 available online at http://ojs.cvut.cz/ojs/index.php/app modernization of railway stations česká lípa and karlovy vary with the expected co-financing from european funds form opd2 radek kratochvíl∗, maria janešová, jiří vopava ctu in prague, czech technical university in prague faculty of transportation sciences, konviktská 20, 110 00, prague 1, czech republic ∗ corresponding author: kratochvil.radek@seznam.cz abstract. the article describes the planned modernization of railway stations in česká lípa and karlovy vary, their scope and financing. the emphasis is laid on the expected co-financing of these projects from european structural and investment funds planned for the years 2016-2020. it is case of the operational program transport opd2. the study compares the two projects from many aspects, in particular from the economic and structural-urban planning points of view. keywords: modernization, railway station, opd2, construction costs. 1. european structural and investment funds opd2 on 12 may, 2015 the european commission approved operační program doprava pro období 2014-2020 – the newly designated as opd2. in the past, the transport operational program designed for the years 2007 – 2013 was used the czech republic. this opd2 program will provide the czech republic with the possibility to draw from the eu funds the amount of almost eur 4.7 billion, in particular for the development of road and rail infrastructure. general allocation of opd2: 5 524 434 631eur, including the european share: 695 769 435eur programming document opd2 (2014cz16m1op001) contains detailed information about co-financing projects in various areas of transportation. we will within this article deal with the issue of rail transport. here, there are some important and interesting facts contained in this document relating to the modernization of rail and multimodal transport: currently approximately 9,500 km of railway tracks are operated in the czech republic, which makes it considering the total area of the czech republic one of the densest rail networks in the world. the big problem of the railway network in the czech republic is its low technical level (insufficient track speed and frequent speed drops, low capacity, lack of interoperability, insufficient parameters for freight transportation, particularly the length of tracks in stations and terminals for multimodal transport), poor condition and lack of amenities of transport terminals, railway stations and stops and the related low comfort for passengers, resulting in low competitiveness in comparison with the road transport in most important directions and aspects. co-financing of the rail transport within opd2 is possible under the priority axis 1, where inter alia, the following objectives are determined: • point 1.1 improving the infrastructure for increased competitiveness and greater use of rail transport for this priority axis is the support of the eu is set in the amount of eur 2,395,964,680. the share of overall eu support in the operational program is 51.02% it is clear from table 2 (overview of investment strategy of the operational program) [1] 2. modernization of the railway station in česká lípa the main railway station česká lípa [2] is located in the liberec region. the railway station is located in the south-western part of the town of česká lípa. this is the crossroads of four railway lines, see the specification below. the railway junction česká lípa is the second largest rail hub in the liberec region (just behind liberec). specifications: ids: idol (liberec) list of numbers of tracks: 080, 081, 086, 087 [2] gps of the railway station: n 50◦40′31, 302; e 14◦31′59, 46 the railway station should undergo a complete modernization. the railway infrastructure administration [3], hereinafter referred to as sždc (as the main investor) and skanska a.s. (as the contractor / builder) have officially concluded the contract for this modernization. funds will be provided by the state fund for transport infrastructure and the construction is designed for co-financing from the european structural and investment funds – 41 http://dx.doi.org/10.14311/app.2016.5.0041 http://ojs.cvut.cz/ojs/index.php/app r. kratochvil, m. janesova, j. vopava acta polytechnica ctu proceedings figure 1. modernization of the česká lípa railway station the architectural design of the construction project (14-334-201) [5] operational program transport – opd2. at this railway station in česká lípa a new terminal building will be built for passengers and at the same time the existing station building is to be reconstructed so that it can accommodate the new safety equipment technology. a new underpass subway and three new platforms with steel roofing and elevators will be built at the station. furthermore, two bridges are to be remodelled and several other alterations shall be made within this construction project. basic information on the construction project: name: modernizace žst. česká lípa investor: správa železniční dopravní cesty, státní organizace (sždc) contractor/builder: skanska a.s. [4] designer: sudop praha a.s. construction period: november 2015 – may 2017 construction costs: 1 006 416 679 czk not including vat according to the statement of så¡dc the eu usually contribute up to 80% of the total eligible costs. the project documentation prepared by sudop praha a.s. indicates that the construction project of the modernization of the česká lípa railway station will fundamentally affect the transport in the whole česká lípa county. in addition to rebuilding the railway station the project also involves the revitalization of some connecting sections, on which the speed of trains will be increased consequently. the aim of the changes is a new configuration of the track so that it can accommodate multilevel platforms, increasing the speed from 40-60 to 65-100 km/h on most tracks for passenger trains and elimination of non-essential parts of the track. the railway station of česká lípa has been negatively affected by the division into two disjointed parts. as a result of this the station is confusing for passengers, too large and not very comfortable for a train change. the construction adjustments will unite the two parts of the railway figure 2. modernization of the česká lípa railway station the architectural design of the construction project (14-334-201) [5] figure 3. modernization of the česká lípa railway station the architectural design of the construction project (14-334-201) [5] station and the passenger area will move closer to the town center by about 250 m. 3. modernization of the railway station in karlovy vary the railway station of karlovy vary [2] is located in the karlovy vary region. the railway station (unofficially called the ”upper station”) is a transport hub of the city karlovy varyboth for the passenger and freight rail transport. the station is located on a major route cheb chomutov, track 140. specifications: ids: duk (ústí nad labem), idok (karlovy vary) list of numbers of tracks: 140, 141, 142 [2] gps of the railway station: n 50◦14′7, 737; e 12◦52′, 647 the railway station should undergo a complete modernization. sždc (as the main investor) and skanska (as the contractor/builder) have agreed on this modernization. the construction project has been designed to be co-financed from the european structural and investment funds – operational program transport – opd2. the new railway station with optimum passenger handling capacity will be 42 vol. 5/2016 modernization of railway stations česká lípa and karlovy vary figure 4. visualization of the railway station of karlovy vary, the project by sudop praha a.s. [5] built by skanska a.s. without interrupting the train service. to ensure the functionality of the station the eastern part of the existing building will be retained, which will perform the basic functions of the railway station during the construction period. after the construction project completion also this eastern part of the station building will be demolished. consequently, platform adjustments will be implemented. the new building will have one above ground and one underground floor. it will be dominated by the hall with a glass roof, which passes through both floors and divides the building into two halves. basic information on the construction project: name: modernizace žst. karlovy vary investor: správa železniční dopravní cesty, státní organizace (sždc) contractor/builder: skanska a.s., division pozemní stavitelství západ [4] designer: sudop praha a.s. construction beginning: 22.4.2015 construction costs: 75 800 000 czk not including vat 4. conclusion this article has compared two different projects on modernization of railway stations. the two projects vary greatly in terms of their scope of works and, of course, in terms of the related budgets. the planned modernization of the railway station of česká lípa is much broader and includes, for example reconstruction figure 5. the modernization of the railway station of karlovy vary station building, a cross-section b-b, project (14-217.240) [5] žel. st. česká lípa 1 006,4 žel. st. karlovy vary 75,8 table 1. construction costs of projects (czk million excluding vat) of two bridges, construction of 19 railway crossings and 32 switches on the 33 km long track. both construction projects have the same investors, designers and construction contractors and both have been designed to be co-financed from european structural and investment funds operational program transport opd2. this is a very important parameter of the project, because it would be very difficult to finance the projects of this size without the eu support. references [1] portál sždc [online]. evropské strukturální a investiční fondy, operační program doprava. [2016-02-10], http://www.opd.cz. [2] portál čd [online]. české dráhy a.s. informace o nádražích. [2016-02-10], http://www.cd.cz/cd-online/stanice.php. [3] portál opd [online]. správa železniční dopravní cesty, s.o. spolufinancování z eu. [2016-02-10], http://www.szdc.cz/modernizace-drahy/ spolufinancovani-z-eu/op-doprava.html. [4] portál společnosti skanska a.s. [online]. skanska. [2016-02-10], http://www.skanska.cz. [5] portál sudop praha [online]. sudop praha a.s. [2016-02-10], http://www.sudop.cz. 43 http://www.opd.cz http://www.cd.cz/cd-online/stanice.php http://www.szdc.cz/modernizace-drahy/spolufinancovani-z-eu/op-doprava.html http://www.szdc.cz/modernizace-drahy/spolufinancovani-z-eu/op-doprava.html http://www.skanska.cz http://www.sudop.cz acta polytechnica ctu proceedings 5:41–43, 2016 1 european structural and investment funds opd2 2 modernization of the railway station in ceská lípa 3 modernization of the railway station in karlovy vary 4 conclusion references acta polytechnica ctu proceedings doi:10.14311/ap.2016.4.0001 acta polytechnica ctu proceedings 4:1–7, 2016 © czech technical university in prague, 2016 available online at http://ojs.cvut.cz/ojs/index.php/app study of identification of two-phase flow parameters by pressure fluctuation analysis ondrej burian∗, vaclav dostal, ladislav vesely faculty of mechanical engineering, czech technical university in prague, technická 4, praha 6, czech republic ∗ corresponding author: ondrej.burian@fs.cvut.cz abstract. this paper deals with an identification of a parameters of simple a pool boiling in a vertical rectangular channel by analysis of pressure fluctuation. a small experimental facility about 9 kw power, which was used for simulation of apool boiling phenomena and creation of steam-water volume in this work is introduced. a several pressure fluctuations measurements and differential pressure fluctuations measurements at various were carried out. main changed parameters were a power of heaters and a hydraulics resistance of a channel internals. a measured pressure data were statistically analysed and compared with goal to find dependencies between parameters of a two-phase flow and statistical properties of pressure fluctuation. at the end of this paper are summarized final results and applicability of this method for parameters determination of two phase flow for pool boiling conditions at ambient pressure is discused. keywords: two-phase flow, pressure fluctuation measurement, void fraction. 1. introduction boiling of liquids is a high dynamic phenomena, which is characterized as a phase change of a liquid phase to a gaseous phase. the most common example of that is boiling of water to water steam, which occurs in many facilities in our life. it may exist in wide range of parameters and regiments, which includes pressure, temperature, heat flux, bulk temperature etc. the one of the key parameters is a void fraction which represents a share of steam fraction in overall volume. the knowledge of this parameter is key for identification other parameters of boiling and operational parameters of facilities which operate with boiling phenomena like steam boilers, boiling water reactors or steam generators. one of a perspective method for identification a key parameters of boiling and two-phase flow phenomena, which is focused on analysis of measured differential pressure and its a time fluctuation, is described in this work. this method is based on fact that differential pressure between two measured locations in a two-phase mixture located in some distance at above is dependent on the void fraction. void fraction in volumetric form is defined as a ratio of a steam fraction volume to overall a two-phase volume, see (1). the density of a two-phase mixture can be expressed by (2), where ρl is density of liquid fraction and ρg is density of steam fraction. α = vg v (1) ρm = ρl(1− 〈α〉) + ρg〈α〉 (2) second assumption is that the value of this differential pressure is not stable in stable state, but it fluctuated around some mean value. this dependency is a result of influence of the void fraction on a hydrostatic pressure at measured locations – a measurement points. hydrostatic pressure of a two-phase volume about some void fraction is value between hydrostatic pressure of a liquid volume and a steam volume, which decreases with increasing a void fraction. a pressure fluctuation is a result of a process of a bubble generation and its flow throw the measured section. if we find dependency between void fraction and measured differential pressure, which is measured between two measurement points located at above in some distance, we can use it like fundamental of a method for determination of the void fraction. however more interesting may be an information about boiling and two-phase flow parameters which we can get from statistical analysis of fluctuation of measured differential pressure. research of this method, identification advantages and disadvantages of this method is subject of this research work which will be introduced in this paper. due to a boiling and a two-phase flow are complicated phenomenas, is impossible to determinate an analytical solution with physical model of dependency between void fraction and differential pressure. they are complex phenomenas which are affected many external and internal influences. from that reasons as described at above was chose experimental way which should give empirical view of scope to this problem. using differential pressure as a parameter is more useful than point pressure, because we do not know position of a level of a two-phase mixture up the measured points. the beginning phase of this research project is focused on an experimental research at an ambient pressure conditions. the aim of this phase is a verification an initial assumptions, assessment a general values of 1 http://dx.doi.org/10.14311/ap.2016.4.0001 http://ojs.cvut.cz/ojs/index.php/app title:workshop on the golden age of cataclysmic variables and related objects � ii. editors: franco giovannelli, inaf istituto di astro�sica e planetologia spaziali, area di ricerca di tor vergata, roma, italy lola sabau-graziati, inta dpt. cargas utiles y ciencias del espacio, madrid, spain co-published by: czech technical university in prague (ctu), zikova 4, 166 36 prague 6, czech republic, and astronomical institute of the czech academy of sciences, fri£ova 298, 251 65 ond°ejov, czech republic. editorial o�ce: ctu � central library, technická 6, 160 80 praha 6, czech republic. acta@cvut.cz i�: 68407700. head of the editorial board: v. petrá£ek. editorial board � internal members: robin arthur healey, rectorate of ctu, department of external a�airs, ctu, czech republic ta´ána jaro²íková, faculty of biomedical engineering, department of natural sciences, ctu, czech republic milan jirásek, faculty of civil engineering, department of mechanics, ctu, czech republic jitka jírová, faculty of transportation sciences, department of mechanics and materials, ctu, czech republic petr jizba, faculty of nuclear sciences and physical engineering, department of physics, ctu, czech republic pavel kalina, faculty of architecture, department of theory and history of architecture, ctu, czech republic zuzana masáková, faculty of nuclear sciences and physical engineering, department of mathematics, ctu, czech republic jan píchal, faculty of electrical engineering, department of physics, ctu miroslav sýkora, klokner institute, department of structural reliability, ctu, czech republic radek �ulc, ctu, faculty of mechanical engineering, department of process engineering, ctu, czech republic editor-in-chief: i. adlerová. graphic design and typesetting: v. neºerka, m. somr. cover: m. r·ºi£ková, j. jílek. print: ctu publishing house, zikova 4, 166 36 prague 6, czech republic. [date of issue] april 20, 2015 in: acta polytechnica ctu proceedings(2),1, http://ojs.cvut.cz/ojs/index.php/app isbn 978-80-01-05688-2 (�vut v praze) isbn 978-80-905129-3-1 (astronomický ústav av �r, v.v.i.) issn 2336�5382 the publication of these proceedings was �nancially supported by the czech academy of sciences. c© czech technical university in prague and astronomical institute of the czech academy of sciences, 2015. reprinting is allowed only with the agreement of the editorial o�ce and with citation of the source. palermo workshop 2013 the second international workshop on the golden age of cataclysmic variables and related objects ii palermo, italy, september 9 – 14 edited by franco giovannelli & lola sabau-graziati 1 doi: 10.14311/app.2015.02.9001 organizing institutions inaf – istituto di astrofisica e planetologia spaziali, roma – inaf-iaps, roma, italy inta – dpt. de cargas utiles y ciencias del espacio, inta-dcuce, madrid, spain czech republic academy of sciences – astronomical institute, ascr-ai, ondřejov, czech republic czech technical university, ctu in prague, czech republic department of physics, technion, haifa, israel russian academy of sciences – institute of astronomy, ras-ia, moscow, russia inaf osservatorio astronomico di capodimonte, inaf-oac, napoli, italy department of astronomy, university of washington, seattle, wa, usa scientific organizing committee dmitry bisikalo, ras-ia, moscow (russia) massimo della valle, inaf-oac, napoli (italy) franco giovannelli, inaf-iaps, roma (italy) (chairperson) rené hudec, ascr-ai., ondřejov (czech republic) lola sabau-graziati, inta-dcuce, madrid (spain) giora shaviv, department of physics, technion, haifa (israel) paula szkody, department of astronomy, university of washington, seattle, wa, usa local organizing committee franco giovannelli, inaf-iaps, roma, italy (chairperson) daniela giovannini, cnr-ibcn, roma, italy francesco reale, cnr-isc, roma, italy iv foreword new results coming from the new generation satellites, especially in the hard x–ray and γ–ray regions, renewed the interest of scientific community about cvs. this, together with the new developments in searching for the progenitors of type ia supernovae was the main reason for organizing this workshop on the golden age of cataclysmic variables and related objects. after the great success attained with the first 2011 edition, we decided to organize, in agreement with the scientific community, this second meeting of a series with biennial cadence in palermo in the beautiful mondello bay. we are completely convinced that a deep discussion about cvs and related objects, mostly through the physical processes occurring inside those systems, is the most powerful way for a better and faster development of our knowledge just of the cataclysmic processes, rather usual in the universe. for this purpose, we invited distinguished colleagues representative the most important space– and ground–based experiments of the present generation. they painted the whole panorama on the experimental results and on their interpretation by using current models and/or developing new ideas for a better comprehension of the detected phenomena. during the discussion of a lot of new results, obtained along most of the electromagnetic spectrum, we have learned that the astrophysics of galactic sources is still extremely interesting and unavoidable. indeed it provides crucial information on the same physical processes occurring also in the extra–galactic sources, with obvious different scale factors. many thanks to the authors, which accepted our reminders recalled at the beginning of the workshop, for keeping to schedule. however, in spite of the accurate respect of the dead line of a number of authors, we still suffer a delay in publishing these proceedings because of the laziness of a considerable part of other authors. in spite of this, these proceedings will appear within one year from the workshop date. this is the second edition of the workshop, whose proceedings are published by the ‘acta polytechnica ctu proceedings’, an international refereed journal of the czech technical university in prague (czech republic). we would like to thank all participants and especially the speakers for their active contributions in rendering this workshop updated with their talks, alive with their discussions, and friendly with their attitudes. particular thanks to mr francesco reale for his excellent assistance for informatics in roma, during the preparation of the workshop, and for his faultless assistance in palermo. a very special thank to dr daniela giovannini for her precious help in managing the workshop. we are grateful to the pianist maestra valentina usai, to the violinist maestro alessandro perpich and to the actresses lisa colosimo and flavia giovannelli for their wonderful performance ”hic habitat felicitas: mozarterie from the kingdom behind” staged in the suggestive atmosphere of the splendid la torre hotel. v many thanks to: i. the director of the istituto di astrofisica e planetologia spaziali, roma of the istituto nazionale di astrofisica (iaps-inaf) (dr pietro ubertini); ii. the caen s.p.a. (viareggio) who partially supported the organization of this workshop. particular thanks to the plan nacional de i+d+i of spain for partially supporting the publication of this book. finally, we would like to thank mr daniele inzerillo, the chef of the splendid la torre hotel, who delighted us with his creations during our stay and to the whole staff of the hotel for their professionalism and kindness. the editors franco giovannelli & lola sabau-graziati vi list of participants arai akira, nishi-harima astronomical observatory, university of hyogo, japan arai@nhao.jp balman solen, middle east technical university, turkey solen@astroa.physics.metu.edu.tr baptista raymundo, universidade federal de santa catarina (ufsc), brazil raybap@gmail.com bisikalo dmitry, institute of astronomy ras, russia bisikalo@inasan.ru blondin stéphane, service d’astrophysique, cea saclay, france stephane.blondin@oamp.fr breytenbach hannes, university of cape town, south africa johbenbrey@gmail.com buckley david, southern african large telescope, south africa dibnob@saao.ac.za cannizzo john, umbc/nasa/gsfc, usa john.k.cannizzo@nasa.gov carter philip, department of physics, university of warwick, uk philip.carter@warwick.ac.uk chochol drahomir, astronomical institute of the slovak academy of sciences, slovakia chochol@ta3.sk de bianchi silvia, tu dortmund, germany silvia.debianchi@tu-dortmund.de della valle massimo, capodimonte astronomical observatory, inaf-napoli & icranet, pescara, italy dellavalle@na.astro.it ederoclite alessandro, centro de estudios de f́ısica del cosmos de aragón, spain aederocl@cefca.es gilmozzi roberto, european southern observatory, garching, germany rgilmozz@eso.org giovannelli franco, inaf – iaps, roma, italy franco.giovannelli@iaps.inaf.it vii gonzalez-riestra rosario, xmm–newton science operations ctr, esac, villafranca del castillo, villanueva de la cañada, madrid, spain rosario.gonzalez@sciops.esa.int hachisu izumi, university of tokyo, japan hachisu@ea.c.u-tokyo.ac.jp henze martin, xmm–newton science operations ctr, esac, villafranca del castillo, villanueva de la cañada, madrid, spain martin.henze@sciops.esa.int hernandez santisteban juan venancio, university of southampton, uk j.v.hernandez@soton.ac.uk hudec rené, astronomical institute as–ondřejov & ctu–prague, czech republic rene.hudec@gmail.com idan irit, technion israel institute of technology, israel idan@technion.ac.il isern jordi, institut for space sciences ice-csic/ieec, barcelona, spain isern@ieec.cat ishida manabu, the institute of space and astronautical science/jaxa, sagamihara, japan ishida@astro.isas.jaxa.jp izzo luca, sapienza university of rome and icranet, roma, italy luca.izzo@gmail.com kajikawa tomoyo, kyoto sangyo university, kyoto, japan kajikawa@cc.kyoto-su.ac.jp kato mariko, keio university, yokohama, japan mariko@educ.cc.keio.ac.jp katysheva nataly, sternberg state astronomical institute, moscow, russia natkat2006@mail.ru kawakita hideyo, koyama astronomical observatory kyoto, sangyo university, kyoto, japan kawakthd@cc.kyoto-su.ac.jp knigge christian, university of southampton, uk c.knigge@soton.ac.uk kononov dmitriy, institute of astronomy ras, moscow, russia dkononov@inasan.ru kotze marissa, south african astronomical observatory, cape town, south africa marissa@saao.ac.za viii kotze enrico, south african astronomical observatory, cape town, south africa ejk@saao.ac.za latev georgi, institute of astronomy and national astronomical observatory, sofia, bulgaria glatev@astro.bas.bg martino bruno luigi, associated iaps-inaf, roma, italy brunolmartino@gmail.com mason paul, university of texas at el paso, usa pmason@nmsu.edu meintjes pieter, university of the free state, bloemfontein, south africa meintjpj@ufs.ac.za mikolajewska joanna, n. copernicus astronomical center–pan, warsaw, poland mikolaj@camk.edu.pl mukai koji, nasa/gsfc/cresst and umbc, greenbelt, md, usa koji.mukai@umbc.edu nagashima masayoshi, kyoto sangyo university, kyoto, japan g0837714@gmail.com naito hiroyuki, nagoya university, nagoya, japan naito@phi.phys.nagoya-u.ac.jp nebot ada, observatoire astronomique de strasbourg, france ada.nebot@astro.unistra.fr ness jan-uwe, xmm–newton science operations ctr, esac, villafranca del castillo, villanueva de la cañada, madrid, spain juness@sciops.esa.int neustroev vitaly, university of oulu, finland vitaly.neustroev@oulu.fi odendaal alida, university of the free state, bloemfontein, south africa winka@ufs.ac.za orio marina, inaf–osservatorio astronomico di padova, italy marina.orio@oapd.inaf.it pagnotta ashley, american museum of natural history, new york, ny, usa pagnotta@amnh.org parisi pietro, inaf – iaps, roma, italy pietro.parisi@iaps.inaf.it ix patterson joseph, columbia university, new york, ny, usa jop@astro.columbia.edu poggiani rosa, dipartimento di fisica, università di pisa, italy rosa.poggiani@df.unipi.it potter stephen, south african astronomical observatory, cape town, south africa sbp@saao.ac.za pretorius margaretha, university of southampton, uk m.pretorius@soton.ac.uk qian shengbang, yunnan astronomical observatory, kunming, china qsb@ynao.ac.cn reale francesco, cnr – isc, italy francesco.reale@sic.rm.cnr.it reinsch klaus, universität göttingen, institut für astrophysik, germany reinsch@astro.physik.uni-goettingen.de ritter hans, max-planck-institut für astrophysik, germany hsr@mpa-garching.mpg.de sabau-graziati lola, instituto nacional de técnica aeroespacial (inta) — departamento de cargas utiles y ciencias del espacio (dcuce), torrejón de ardóz, madrid, spain sabaumd@inta.es scaringi simone, k.u. leuven, the netherlands simone.scaringi@ster.kuleuven.be schmidtobreick linda, eso, santiago, chile lschmidt@eso.org schreiber matthias r., universidad de valparaiso, chile matthias.schreiber@uv.cl sekeras matej, astronomical institute of slovak academy of sciences, tatranska lomnica, slovakia msekeras@ta3.sk semena andry, space research institute (iki)–ras, moscow, russia san@iki.rssi.ru shaviv giora, technion – israel institute of technology, haifa, israel giora@shaviv.org šimon vojtěch, astronomical institute, academy of sciences, ondřejov, czech republic vojtech.simon@gmail.com x sion edward, villanova university, pa usa edward.sion@villanova.edu skopal augustin, astronomical institute of slovak academy of sciences, tatranska lomnica, slovakia skopal@ta3.sk smith robert, department of physics & astronomy, university of sussex, brighton, uk r.c.smith@sussex.ac.uk stoyanov kiril, institute of astronomy and national astronomical observatory, sofia, bulgaria kstoyanov@astro.bas.bg suleimanov valery, institute for astronomy and astrophysics, university of tübingen, germany suleimanov@astro.uni-tuebingen.de szkody paula, university of washington, wa, usa szkody@astro.washington.edu tanabe kenji, okayama university of science, okayama-shi, japan tanabe@big.ous.ac.jp tappert claus, dpto de f́ısica y astronomı́a, facultad de ciencias, universidad de valparaiso, chile claus.tappert@uv.cl ubertini pietro, inaf – iaps, roma, italy pietro.ubertini@iaps.inaf.it uemura makoto, hiroshima university, higashi-hiroshima, japan uemuram@hiroshima-u.ac.jp voloshina irina, sternberg astronomical institute, moscow state university, moscow, russia vib@sai.msu.ru yuasa takayuki, jaxa, sagamihara, japan yuasa@astro.isas.jaxa.jp zemko polina, sternberg astronomical institute, moscow state university, moscow, russia polina.zemko@gmail.com zharikov sergey, instituto de astronomı́a, unam, ensenada, mexico zhar@astrosen.unam.mx zhu liying, yunnan astronomical observatory, kunming, china zhuly@ynao.ac.cn xi xii 329 acta polytechnica ctu proceedings 1(1): 329–331, 2014 329 doi: 10.14311/app.2014.01.0329 concluding address franco giovannelli1 1inaf istituto di astrofisica e planetologia spaziali, area di ricerca di tor vergata, via del fosso del cavaliere, 100 i00177 roma, italy corresponding author: franco.giovannelli@iaps.inaf.it abstract before i officially conclude this workshop — far be it from me to attempt to compete with some concluding remarks already delivered at the meeting with various levels of passion by gennady bisnovatyi-kogan, giulio auriemma and sergio colafrancesco — i would like to comment on some of the highlights emerging from our fruitful week of discussions about multifrequency behaviour of high energy cosmic sources, without any pretension of completeness. keywords: photonic astrophysics particle astrophysics neutrino astrophysics. undoubtedly the advent of spacecraft gave a strong impulse to astronomy. starting roughly in the mid 1970s, almost the whole electromagnetic spectrum has been continuously surveyed in a large number of space experiments. astroparticle physics, a new field of physics, was born roughly twenty years ago by joining the efforts of the high energy astrophysics community and the particle physics community. during this relatively short period of time, astroparticle physics has developed strongly through studies of cosmic sources that are emitters of photons, charged particles, and neutrinos. some of the sources could produce gravitational waves that will probably be detectable in the near future with the new generation of ground–based gravitational experiments, such as ligo, and space– based experiments, such as lisa. meanwhile the large hadron collider (lhc) has been producing excellent results on the higgs boson, and in general about the study of pp collisions at tev energies, never obtained before in ground–based laboratories. these results, together with the results coming from the hubble space telescope (hst), with its deep survey of faraway objects of the universe, and with the vhe emission detected from a number of cosmic sources, both galactic and extragalactic, are witnesses of the validity of the big bang theory, described by the standard model. among many experimental and theoretical results discussed during this workshop, i would like to mention a couple of them that, in my opinion, will underpin the future of astroparticle physics. the first is the vhe sky at energies eγ > 100 gev. about 150 tev sources have been detected (see fig. 1, http://tevcat.uchicago.edu/). only 20 years ago, this sky was practically empty. it is expected that over the next decade the ongoing operation of fermi will be accompanied by observations with the current ground– based h.e.s.s., magic, veritas experiments and the planned cta and hawc experiments. data obtained in the very wide energy range from 100 mev to 1 pev will provide very deep insights into a number of problems of high-energy astrophysics and fundamental physics (see the review paper about tev astronomy by rieger, de oña-wilhelmi & aharonian, 2013). figure 1: tev sources catalog (http://tevcat.uchicago.edu/) the second is connected with the reionization epoch of the universe at z ≈ 20 − 30, when the first stars appeared (e.g. lamb & reichart, 2000; ciardi & loeb, 2000; bromm & loeb, 2002). therefore, quasars and grbs could be detectable up to roughly that epoch. type–ia supernovae (sne ia) have been detected up to z ≈ 1.7; the future jwst will be able to detect sn ia in the range 1.7< z < 3 (aldering et al., 2007). quasars have been detected at z = 6.419, 6.43 (fan et al., 2003), and z = 7.085 (mortlock et al., 2011), and grbs up to z = 9.4 (cucchiara et al., 2011). these detections support the theory fixing the reionization epoch to z ≈ 20 − 30 (see fig. 2). 329 http://dx.doi.org/10.14311/app.2014.01.0329 franco giovannelli after this workshop, the importance of multifrequency astrophysics and multienergy astro-particle physics once more appears evident. however, there are many problems in performing simultaneous multifrequency, multisite, multiinstrument, multiplatform and multienergy measurements due to: i) objective technological difficulties; ii) sharing common scientific objectives; iii) problems of scheduling and budgets; iv) political management of science. figure 2: sketch of the evolution of the universe from the epoch of recombination up to the present time. the epoch of reionization is also marked (after dai zhigao, nanjing university). the positions of the highest z quasars and grb are superimposed. during this fruitful workshop, we hope to have demonstrated once more the “vulcano theorem” enunciated in my concluding address in 1984: it is possible to develop science seriously even if smiling. and finally, i would like to conclude with a few wonderful words of dr daisaku ikeda (2001)–president of soka gakkai international (sgi)–reported in the booklet for today and tomorrow the thought for may 30th: “whoever has many friends has greater opportunities for growth. in this way, one both makes society a better place, and lives happier and more satisfied. in all cases, human relations, interpersonal interaction and communication are of vital importance. we must establish and nurture friendship and contacts with many people, both in our environment, and in society in general. in this manner our life will open up and will flourish”. we could go back to early childhood when we were like the little prince. one sees clearly only with the heart. what is essential is invisible to the eye (from the little prince, by antoine de saint exupéry). acknowledgement it is my pleasure to thank: the scientific organizing committee (j.h. beall, th. boller, r. hudec, l. sabau-graziati, a. santangelo); the informatics operator (francesco reale alias figaro, the factotum) for helping with informatics and in solving all related problems during the preparation and development of the workshop, and at the registration desk in palermo; the executive secretary, daniela giovannini, who smoothed any inconveniences that occurred at the registration desk with professionalism and moreover with her nice smile; the directors of inaf-iaps, dcu&ce-inta, mpe, e-o. hulburt ctr-nrl, st. john’s college, iaa/university sand 1, ascr/ai ondřejov & the czech technical university in prague; the actresses lisa colosimo and flavia giovannelli; the violinist alessandro perpich and pianist valentina usai; the staff of the splendid hotel la torre. on behalf of the soc and loc, i am pleased to thank all participants and especially the speakers for their active contributions in updating this workshop with their talks, alive with their discussions, and friendly with their attitudes. my special thanks to three colleagues and friends (gennady bisnovatyikogan, giulio auriemma and sergio colafrancesco), who kindly took on the challenging task of making the concluding remarks of the workshop. finally, on behalf of all participants, i would like to express my warm thanks to the chef, mr daniele inzerillo, who prepared a large number of delicacies for us. i hope to meet all of you once again during our next palermo workshop. references [1] aldering, g. et al.: 2007, astropart. phys. 27, 213. doi:10.1016/j.astropartphys.2006.11.001 [2] bromm, v., loeb, a.: 2000, apj 575, 111. doi:10.1086/341189 [3] ciardi, b., loeb, a.: 2000, apj 540, 687. doi:10.1086/309384 [4] cucchiara, a. et al.: 2011, apj 736, 7. doi:10.1088/0004-637x/736/1/7 [5] fan, x. et al.: 2003, aj 125, 1649. 330 http://dx.doi.org/10.1016/j.astropartphys.2006.11.001 http://dx.doi.org/10.1086/341189 http://dx.doi.org/10.1086/309384 http://dx.doi.org/10.1088/0004-637x/736/1/7 concluding address [6] ikeda, d.: 2001, for today and tomorrow the thought of 30th may, edizioni esperia. [7] lamb, d.q., reichart, d.e.: 2000, aip conf. proc. 526, 658. doi:10.1063/1.1361618 [8] mortlock, d.j. et al.: 2011, nature 474,616. doi:10.1038/nature10159 [9] rieger, f.m., de oña-wilhelmi, e., aharonian, f.a.: 2013, front. phys. 8(6), 714. doi:10.1007/s11467-013-0344-6 331 http://dx.doi.org/10.1063/1.1361618 http://dx.doi.org/10.1038/nature10159 http://dx.doi.org/10.1007/s11467-013-0344-6 acta polytechnica ctu proceedings doi:10.14311/app.2017.13.0035 acta polytechnica ctu proceedings 13:35–38, 2017 © czech technical university in prague, 2017 available online at http://ojs.cvut.cz/ojs/index.php/app foamed cement composites: detection of the modulus of elasticity using dic analysis and comparison with other methods jakub ďurejea,∗, zdeněk prošeka,b a czech technical university in prague, faculty of civil engineering, thákurova 7, 166 29 prague 6, czech republic b university centre for energy efficient buildings, czech technical university in prague, třinecká 1024, 273 43 buštěhrad, czech republic ∗ corresponding author: jakub.dureje@fsv.cvut.cz abstract. a modulus of elasticity was determined for eight differently foamed cement paste samples. samples were loaded in the laboratory by a hydraulic press. the force acting on the sample was read directly from the laboratory press. digital image correlation (dic) analysis were used to draw deformations. before loading pressure test was applied a random contrast pattern to the samples. samples were captured by the camera in a one-second interval during the loading pressure test. the images were edited in the adobe photoshop lightroom and then evaluated using ncorr software. the result is a vertical and horizontal shift field. on the basis of the results obtained, it was possible to calculate the modulus of elasticity of each sample. keywords: dic analysis, foamed cement paste, field of displacement, static modulus of elasticity, dynamic modulus of elasticity. 1. introduction there are many different methods to detect deformation of the sample during the loading pressure test. the displacement of the crosspiece of the hydraulic press does not correspond to the deformation of the sample in the direction of the force. this is due to the deformation of the exponed parts of the test device. to measure deformations, it is possible to use some type of extensometer and place it directly between the jaws of the press in which the sample is tested and measure the displacement directly between the jaws. thanks to the ever-improving technologies, we can obtain the entire field of displacement by dic analysis by capturing a series of photos during sample testing and subsequent evaluation. the advantage is that, compared to the measurement with an extensometer, we obtain deformations in all directions on the measured surface of the sample [1, 2]. dic analysis is used in many industries, by many companies, including airbus, awe, stresscraft and british energy. it is used, for example, for measuring the heat dissipation and curvature of electronic components, for measuring the 3d shape of the air bags, for monitoring the components of the power plants to determine their status and to reduce their emissions, or for the behavior of building materials during the loading tests [2]. dic analysis is an optical method, to get the most accurate results it is necessary to avoid optical defects. for each lens, the image is distorted towards the edges. depending on the deformation of the square, we distinguish distortions to barrel distortions, pincushion distortions, or a combination of both – wave distortions. lens manufacturers are trying to eliminate these phenomena with the appropriate lens system. the lenses are constructed at certain distances where the distortion occurs least. for this reason, it is necessary to select a suitable lens and capture distance according to the size of the subject to be sensed [3, 4]. 2. materials and samples the sample properties were measured at 28 days of age and the day after manufacture the samples were removed from formwork and stored freely in a laboratory environment at a temperature of 22 ± 1 ◦c and a relative humidity of 50 ± 2%. test beams of dimensions 40×40×160 mm were made, which were broken in half during the previous bend test. the reference mixtures for testing were made of portland cement cem i 42.5r radotin (refc), a combination of portland cement and finely ground concrete recyclate in a ratio of 0.66 : 0.34 (refr). fine-grained recycled concrete was produced by high-speed grinding, from waste generated from drainage concrete blocks. subsequently, the reference mixtures were lightened using foaming additives such as industrial additive from the mapei (m) company or the additive used by firefighters (h). the composition of the individual samples and the bulk density are given in table 1 [5]. 3. experimental methods dic analysis was performed on a total of 10 samples, on two references and eight differently foamed cement pastes. samples were loaded in the laboratory by a 35 http://dx.doi.org/10.14311/app.2017.13.0035 http://ojs.cvut.cz/ojs/index.php/app jakub ďureje, zdeněk prošek acta polytechnica ctu proceedings sample nr. designation cement (c) recyclate (r) weight (c+r) water to mix cement putty (v1) water added to the foam (v2) water (v1+v2) w/c foam bulk density [g] [g] [g] [g] [ml] [ml] [-] [ml] [kg.m−3] 45 c7.5h 750.0 0.0 750 225 75 300 0.40 3.75 603.83 40 c7.5m 750.0 0.0 750 225 75 300 0.40 3.75 677.84 29 c10h 1000.0 0.0 1000 300 50 350 0.35 2.50 1333.10 21 c10m 1000.0 0.0 1000 300 50 350 0.35 2.50 1394.40 24 r7.5h 487.5 262.5 750 225 75 300 0.40 3.75 1177.08 34 r7.5m 487.5 262.5 750 225 75 300 0.40 3.75 895.14 15 r10h 650.0 350.0 1000 300 50 350 0.35 2.50 1603.99 12 r10m 650.0 350.0 1000 300 50 350 0.35 2.50 1457.30 5 refc 1000.0 0.0 1000 350 0 350 0.35 0.00 1646.10 9 refr 650.0 350.0 1000 385 0 385 0.39 0.00 1641.70 table 1. composition of the samples and the bulk density [5]. figure 1. random pattern on a sample in grayscale. hydraulic press heckert model fp100, at a speed of 1 mm per minute. subsequently, a static modulus of elasticity was specified. after that, static modulus of elasticity obtained on the basis of dic analysis, static modulus of elasticity obtained on the basis of the displacement of the hydraulic press and the dynamic elastic modulus obtained by the ultrasonic and resonance method were compared. nondestructive resonance method was used to measure dynamic modules. the brüel & kjær measuring set consists of the figure 2. field of vertical displacement from the ncorr post software. brüel & kjær type 3560-b-120 measuring switchboard, the brüel & kjær type 4519-003 acceleration sensor, the brüel & kjær type 8206 impact hammer and the control laptop. modulus of elasticity obtained from data from crosspiece of the hydraulic press does not correspond to the reality, because the deformation of the test instrument itself is included in the data. also, the values of the dynamic modulus of elasticity are rather inaccurate due to the inappropriateness of the resonant and ultrasonic method on porous systems. dynamic modulus of elasticity should achieve higher values than static modulus of elasticity, because by non-destructive methods we get the tangent modulus 36 vol. 13/2017 foamed cement composites: detection of .... figure 3. results of static and dynamic moduli of elasticity. of elasticity and the secant modulus of elasticity we get from the loading test [6]. 4. princip of dic analysis dic (digital image correlation) is an optical method used to measure displacement on the sample surface. compares the grayscale values of individual groups of points called subsets on the surface of the sample between the pictures taken during the loading test. it is necessary to have a sufficiently contrasting random pattern on the surface of the sample. some materials have this pattern naturally. for other samples, the pattern should be applied appropriately. the software compares shades of gray and their displacement between individual photos (figure 1). the result is the field of displacement between the reference and each picture. the reference picture is usually taken before the start of the test [7, 8]. 5. advantages of dic analysis dic analysis is a cost-effective optic method that can be used in laboratory and outdoor environment. it is used to inspect large important structures (bridges, industrial buildings, power plants) that degrade due to external influences. the capture of large objects can be done directly from the air using drones thanks to advanced technologies. this solution is relatively simple, accurate and financially advantageous [1, 2]. during the measurements in the laboratory, we obtain a field of displacement in the horizontal and vertical directions. we can also measure cracks occurring. during the loading tests, the exact course of cracks can be determined by dic analysis, even those that are not observable by the naked eye. these data are very beneficial for the presumption of the durability of the tested material [2]. 6. sample measurement dic analysis was used on foamed cement paste samples. half of cement paste beams were tested according to csn en 12390-3 test of hardened concrete part 3: compressive strength of test samples [9]. a random contrast pattern of black and white spray was applied to the samples. during the testing, images were taken with canon eos 70d on a tripod from a distance of approximately 300 millimeters. the images were taken in a one-second sequence during loading. for each sample, 50 − 100 photos were taken. the photos were edited in adobe lightroom for higher contrast, and then the dic analysis was performed in ncorr version 1.2.1. the results were scaled and the virtual extensometer was placed in the field of displacement in the ncor_post software (figure 2). the results of the individual displacement were assigned to the respective forces using the extractforces software. subsequently, a static modulus of elasticity in microsoft excel was calculated from the results. 7. results and discussions samples ref c and ref r were references. the other eight samples were differently foamed. samples 37 jakub ďureje, zdeněk prošek acta polytechnica ctu proceedings c 7.5 h, c 7.5 m, and r 7.5 m were foamed too much therefore it was not possible to measure the dynamic modulus of elasticity by the resonance method because it was not possible to capture the response from the impact hammer. the results of the other modulus of elasticity in these three samples, because of their high foaming, are not very accurate. in the case of the compression test, these three samples were observed (especially for samples c 7.5 h and c 7.5 m), as the material collapsed successively on the floors. it was not about the flexible behavior that is assumed to acquire the modulus of elasticity. therefore, the results of the measurements of these samples should be considered inaccurate, and due to their poor mechanical properties, do not use this highly foaming cement paste for practical use in the building industry. the static modulus of elasticity for all samples obtained from the displacement of the hydraulic press crosspiece are very low. this is due to the deformation of the test rig itself together with the sample deformation. for this reason, it makes no sense to further consider the static modulus of elasticity thus obtained. measurement of c 10 m, r 10 h and r 10 m samples were expected, static moduli of elasticity are slightly less than dynamic. for samples r 7.5 h, ref c, and ref r, the static moduli of elasticity are approximately half that of the dynamic moduli of elasticity. on the other hand, the static modulus of elasticity is higher than the dynamic for c 10 h sample (figure 3). this is due to the inaccuracy of measuring the dynamic modulus of elasticity because of the inappropriateness of the resonant and ultrasonic method on porous systems. 8. conclusion for this type of samples, the most accurate method is dic analysis. measurement by ultrasonic method or resonance method is not accurate in porous systems. measurement of deformation directly from the hydraulic device is very inaccurate as it involves deformation of the hydraulic device itself. the results show that the less or no foamed samples without recyclate have a higher modulus of elasticity than samples with recyclate, but with more foamed samples, it is the opposite. furthermore, it is noticeable that the less foamed samples with a foaming additive used by firefighters have a higher modulus of elasticity than samples with a foaming additive for industrial use, but with more foamed samples, this is the opposite. acknowledgements this outcome was supported by the czech technical university in prague under no. sgs16/201/ohk1/3t/11 and no. sgs17/060/ohk1/1t/11 and by the ministry of education, youth and sports within national sustainability programme i, project no. lo1605. the authors also thank the center for nanotechnology in civil engineering at the faculty of civil engineering, czech technical university in prague, and the joint laboratory of polymer nanofiber technologies of the institute of physics, academy of science of czech republic, and the faculty of civil engineering, czech technical university in prague. references [1] p. k. rastogi, e. hack. optical methods for solid mechanics: a full-field approach. john wiley & sons, 2012. [2] n. mccormick, j. lord. digital image correlation. materials today 13(12):52–54, 2010. [3] jasar. optické vady a vlastnosti objektivu. http: //photo.mysteria.cz/clanky/objekt2.html#aberace, 2002. [4] l. jelinek. darktable: korekce objektivu a odstranění skvrn. https://www.linuxexpres.cz/praxe/ darktable-korekce-objektivu-a-odstraneni-skvrn, 2015. [5] v. růžička. mechanické vlastnosti pěnových cementových pas. master’s thesis, czech technical university in prague, prague, 2017. [6] m. ballon. porovnaní modulů pružnosti stavebních materialů. master’s thesis, brno university of technology. faculty of civil engineering, brno, 2014. [7] m. a. sutton, j. j. orteu, h. schreier. image correlation for shape, motion and deformation measurements: basic concepts, theory and applications. springer science & business media, 2009. [8] s. yoneyama, g. murasawa. digital image correlation. experimental mechanics 2009. [9] čsn en 12390-3–zkoušení ztvrdlého betonu–část 3: pevnost v tlaku zkušebních těles. úřad pro technickou normalizaci, metrologii a zkušebnictví 10, 2009. 38 http://photo.mysteria.cz/clanky/objekt2.html#aberace http://photo.mysteria.cz/clanky/objekt2.html#aberace https://www.linuxexpres.cz/praxe/darktable-korekce-objektivu-a-odstraneni-skvrn https://www.linuxexpres.cz/praxe/darktable-korekce-objektivu-a-odstraneni-skvrn acta polytechnica ctu proceedings 13:36–39, 2017 1 introduction 2 materials and samples 3 experimental methods 4 princip of dic analysis 5 advantages of dic analysis 6 sample measurement 7 results and discussions 8 conclusion acknowledgements references 174 acta polytechnica ctu proceedings 2(1): 174–177, 2015 174 doi: 10.14311/app.2015.02.0174 application of total variation minimization to doppler tomography m. uemura1, t. kato2, d. nogami3, r. mennickent4 1hiroshima astrophysical science center, hiroshima university, kagamiyama 1-3-1, higashi-hiroshima, hiroshima 739-8526, japan 2department of astronomy, kyoto university, kitashirakawa-oiwake-cho, sakyo-ku, kyoto 606-8502, japan 3kwasan and hida observatories, kyoto university, yamashina-ku, kyoto 607-8471, japan 4universidad de concepción, departamento de astronomı́a, casilla 160-c, concepción, chile corresponding author: uemuram@hiroshima-u.ac.jp abstract we have developed a new model of the doppler tomography using total variation minimization (dttvm). we demonstrated that this method can reconstruct localized and non-axisymmetric profiles possibly having sharp edges in the doppler map. we apply this model to the real data of the dwarf nova, wz sge in superoutburst. dttvm can reproduce the observed spectra with a high precision, while the previous models fail to reproduce localized sources. keywords: cataclysmic variables dwarf novae optical spectroscopy individual: wz sge. 1 introduction doppler tomography (dt) is a widely-used method in the field of cataclysmic variables (cvs) [1]. dt is an ill-posed inverse problem in most cases. a filtered backprojection method provides a way to solve the problem. it masks high frequency components in the doppler maps in order to uniquely determine the solution. the other way is to introduce regularization terms in addition to the likelihood term. the maximum entropy method (mem) has been used to regularize the dt problem. the dt with mem determines the solution by maximizing the information entropy over the default image. the comparison between the back-projection method and mem is reported in [2]. those dt methods tend to smeared out the localized and sharp-edge profiles in the doppler map. on the other hand, such profiles are expected to appear in accretion disks of cvs, such as spiral shocks and hot spot. it has been reported that there are significant residuals between the observed and model spectra that are obtained from dt (e.g. [3]). it is, however, unclear if the residuals indicate real emission components that are not reproduced by the model. in this paper, we introduce a new dt by total variation minimization (tvm), which is described in [4]. tvm has received attention in the field of image reconstruction because it can reconstruct sharp-edge features by making image sparse in its gradient domain. 2 doppler tomography by total variation minimization 2.1 model suppose there are m data given by d = {d0, d1, · · · , dm}. the i-th element of d, di, is the flux at a radial velocity vr,i and phase φi. also suppose n × n = n points in the doppler map given by s = {s0,0, s0,1, · · · , sn,n}. the j-th element in the map has the coordinate of (vx,j , vy,j) and intensity of sj . the goal of dt is to estimate ŝ from the observation d according to ŝ = arg min s { 1 m ‖d−as‖22 + λ n φ(x) } . (1) the first term in the right side is a least-square term, and the second term is a regularization term which consists of a hyperparameter λ and a function φ. a is a m × n matrix whose element depends on the phase, radial velocity, and instrumental response. for the response function, we assumed a gaussian profile with a variance σ2, where σ corresponds to the velocity resolution of the observed spectra. an element of a, aij is then given by aij = 1√ 2πσ2 exp { − (vr,i − vr,ij) 2 2σ2 } . (2) tvm provides a method to solve ill-posed problems. 174 http://dx.doi.org/10.14311/app.2015.02.0174 application of total variation minimization to doppler tomography the isotropic total variation φtvm,iso is defined as φtvm,iso(s) = ∑ i,j √ (si+1,j − si,j)2 + (si,j+1 − si,j)2. (3) φtvm,iso is used as φ in equation (1) in our model. we use the twist algorithm to estimate ŝ in equation (1) [5, 6].1 the calculation code of dttvm can be available at our web site.2 2.2 demonstrations using artificial data we show an example of the results of dttvm using artificial data sets in figure 1. first, we made the assumed doppler maps, then simulated observations of line-profile variations, and reconstructed the maps using dttvm. we calculated three doppler maps from the simulated spectra with different values of λ in equation (1). we assumed the original doppler map containing three spot structures, as shown in the upper leftmost panel of figure 1. the upper two spots have sharp-edge and flat-top profiles of different size and intensity. the lower left spot has a gaussian profile. dttvm reproduced the original map in all three cases. in case (a), the edges of the flat-top spots are slightly smeared out compared to cases (b) and (c). in addition, the peak intensity of the gaussian spot is underestimated in case (a). these smearing effects are less prominent for smaller values of λ. the model spectra and residuals are also shown in figure 1. the residuals are smaller for smaller λ. these results demonstrate that dttvm can reconstruct both sharp-edge and smooth profiles in a doppler map. 2.3 application to the data of wz sge we apply dttvm to the data of the dwarf nova, wz sge taken on the 10th day of the superoutburst in 2001 [7]. figure 2 presents the doppler maps and model spectra. the results were computed using log10 λ of (a) −2.2, (b) −3.2, and (c) −5.2. all doppler maps exhibit a disk feature having non-axisymmetric distribution of intensity. the peak intensity of the localized bright regions is lower in case (a) than the others. as a result, the residuals are large in case (a), compared with the other cases. the model spectra reproduce the observed one without any prominent residuals in cases (b) and (c). figure ?? shows the result of the doppler tomography using the same data of wz sge calculated with mem. we used the calculation code presented in [8].3 the calculation code defines α as a hyperparameter of the model. cases (a), (b), and (c) in figure ?? were calculated with logα = −1.0, −2.0, and −3.0, respectively. in cases (a) and (b), smooth two-armed structures appear on a weak sign of a disk feature. in case (c), patchy structures appear, but they are evidently artifacts. in all cases of (a), (b), and (c), there are significant residuals of the spectral data at −500 km s−1 for phase 1.75 and at +500 km s−1 for phase 1.60. both features have counterparts at phases shifted by 0.5, which indicate they are rotating components with the orbital period. these rotating residuals definitely arise from localized structures in the doppler map, which can be successfully reconstructed by dttvm: the former one corresponds to the feature at (vx, vy) ∼ (0, 500) and the latter one to the lower right region in figure 2. 3 summary as demonstrated above, dttvm can reconstruct both sharp-edge and smooth profiles in a doppler map. hence, this model is advantageous for the reconstruction of highly localized or edge structures in the doppler map, such as the emission from the secondary star, hot spot, or shock wavefront. the doppler map of wz sge obtained by dttvm is surprisingly different from that built with the mem method. the difference in the regularization term could cause the different maps especially when the data sample is small. dttvm can reproduce the observed spectra with a high accuracy, even in the case that the mem method generate significant residuals in spectra. this indicates that tvm is probably a suitable regularization method for real doppler map, compared with mem. acknowledgement this work was supported by jsps kakenhi grant number 22540252 and 25120007. we appreciate comments and suggestions from dr. shiro ikeda. rem acknowledges support by fondecyt grant 1110347 and the basal centro de astrofisica y tecnologias afines (cata) pfb–06/2007. 1〈http://www.lx.it.pt/˜bioucas/twist/twist.htm〉 2〈http://home.hiroshima-u.ac.jp/uemuram/dttvm/〉 3〈http://www.mpa-garching.mpg.de/˜henk/〉 175 m. uemura et al. references [1] marsh, t. r. & horne, k. 1988, mnras, 235, 269 doi:10.1093/mnras/235.1.269 [2] marsh, t. r. 2001, in astrotomography, indirect imaging methods in observational astronomy, ed. h. m. j. boffin, d. steeghs, & j. cuypers (springer-verlag), 1 [3] tappert, c., mennickent, r. e., arenas, j., matsumoto, k., & hanuschik, r. w. 2003, a&a, 408, 651 [4] uemura, m., kato, t., nogami, d., & mennickent, r. e. 2013, pasj, submitted [5] bioucas-dias, j. m. & figueiredo, m. a. 2007b, in image processing, 2007. icip 2007. ieee international conference on vol. 1 (ieee), pp i–105 [6] bioucas-dias, j. m. & figueiredo, m. a. 2007a, image processing, ieee transactions on, 16, 2992 [7] nogami, d. & iijima, t. 2004, pasj, 56, 163 [8] spruit, h. c. 1998, arxiv:astro-ph/9806141 discussion dmitry kononov: 1. it’s nice that your method can restore flat features. 2. it’s in a sense incorrect to compare two results of inverse-problem solving methods if you have no physical basics under the structures you are trying to explain. makoto uemura: as you pointed out, both tvm and mem have no physical meaning. but, i think the comparison still makes a sense. we can select a better model based on residuals between observations and model. in the case of wz sge, the residuals obtained with our method are much smaller than those with mem. the mem method fails to reproduce the localized structures of the line profile, while our method can reproduce them. dmitry bisikalo: is it possible to use tvm in 3d doppler tomography? makoto uemura: yes, it is. the 3d doppler tomography is also a linear problem, so it is easy to apply the method to it. vx (km s−1) v y (k m s −1 ) −1000 0 +1000 − 10 00 0 + 10 00 original − 1 5 10 −1000 0 +1000 − 10 00 0 + 10 00 (a)logλ=−1.0 −1000 0 +1000 (b) logλ=−2.0 −1000 0 +1000 (c) logλ=−3.0 rv (km s−1) o rb ita l p ha se −2000 0 +2000 0. 0 0. 2 0. 4 0. 6 0. 8 1. 0 −2000 0 +2000 0. 0 0. 2 0. 4 0. 6 0. 8 1. 0 0 +2000 0 +2000 0 +2000 0 +2000 0 +2000 figure 1: results of dttvm using an artificial map having three spots. the artificial map is depicted in the upper left-most panel. the simulated spectra from the map is shown in the lower left-most panel. three sets of results are shown with different λ, which is indicated by the top of the figure. for each set, the estimated doppler map is shown in the upper panel. the lower left and right panels are the model spectra and residuals between the original and model spectra, respectively. 176 http://dx.doi.org/10.1093/mnras/235.1.269 application of total variation minimization to doppler tomography vx (km s−1) v y (k m s −1 ) −1000 0 +1000 − 10 00 0 + 10 00 (a) logλ=−2.2 vx (km s−1) v y (k m s −1 ) −1000 0 +1000 − 10 00 0 + 10 00 (b) logλ=−3.2 vx (km s−1) v y (k m s −1 ) −1000 0 +1000 − 10 00 0 + 10 00 (c) logλ=−5.2 rv (km s−1) o rb ita l p ha se −2000 0 +2000 1. 0 1. 5 2. 0 2. 5 rv (km s−1) −2000 0 +2000 1. 0 1. 5 2. 0 2. 5 rv (km s−1) −2000 0 +2000 1. 0 1. 5 2. 0 2. 5 rv (km s−1) −2000 0 +2000 1. 0 1. 5 2. 0 2. 5 rv (km s−1) −2000 0 +2000 1. 0 1. 5 2. 0 2. 5 rv (km s−1) −2000 0 +2000 1. 0 1. 5 2. 0 2. 5 figure 2: results of dttvm for the data of wz sge. upper panels: the doppler maps. lower left and right panels: the model spectra and residuals between the model and observed spectra, respectively. the three sets of the panels were calculated with different λ, which are indicated in the figure. vx (km s−1) v y (k m s −1 ) −1000 0 +1000 − 10 00 0 + 10 00 (a) logα=−1.0 vx (km s−1) v y (k m s −1 ) −1000 0 +1000 − 10 00 0 + 10 00 (b) logα=−2.0 vx (km s−1) v y (k m s −1 ) −1000 0 +1000 − 10 00 0 + 10 00 (c) logα=−3.0 rv (km s−1) o rb ita l p ha se −2000 0 +2000 1. 0 1. 5 2. 0 2. 5 rv (km s−1) −2000 0 +2000 1. 0 1. 5 2. 0 2. 5 rv (km s−1) −2000 0 +2000 1. 0 1. 5 2. 0 2. 5 rv (km s−1) −2000 0 +2000 1. 0 1. 5 2. 0 2. 5 rv (km s−1) −2000 0 +2000 1. 0 1. 5 2. 0 2. 5 rv (km s−1) −2000 0 +2000 1. 0 1. 5 2. 0 2. 5 figure 3: as in figure 2, but for the mem case. 177 introduction doppler tomography by total variation minimization model demonstrations using artificial data application to the data of wz sge summary 143 acta polytechnica ctu proceedings 2(1): 143–147, 2015 143 doi: 10.14311/app.2015.02.0143 model atmosphere spectrum fit to the soft x-ray outburst spectrum of ss cyg v. f. suleimanov1,2, c. w. mauche3, r. ya. zhuchkov2, k. werner1 1institute for astronomy and astrophysics, kepler center for astro and particle physics, eberhard karls university, sand 1, 72076 tübingen, germany 2kazan (volga region) federal university, kremlevskaya str. 18, 42008 kazan, russia 3lawrence livermore national laboratory, l-473, 7000 east ave., livermore, ca 94550, usa corresponding author: suleimanov@astro.uni-tuebingen.de abstract the x-ray spectrum of ss cyg in outburst has a very soft component that can be interpreted as the fast-rotating optically thick boundary layer on the white dwarf surface. this component was carefully investigated by mauche (2004) using the chandra letg spectrum of this object in outburst. the spectrum shows broad (≈ 5 å) spectral features that have been interpreted as a large number of absorption lines on a blackbody continuum with a temperature of ≈ 250 kk. because the spectrum resembles the photospheric spectra of super-soft x-ray sources, we tried to fit it with high gravity hot lte stellar model atmospheres with solar chemical composition, specially computed for this purpose. we obtained a reasonably good fit to the 60–125 å spectrum with the following parameters: teff = 190 kk, log g = 6.2, and nh = 8 · 1019 cm−2, although at shorter wavelengths the observed spectrum has a much higher flux. the reasons for this are discussed. the hypothesis of a fast rotating boundary layer is supported by the derived low surface gravity. keywords: cataclysmic variables dwarf novae radiation transfer x-rays individual: ss cyg ≡ bd+42◦ 4189a. 1 introduction ss cyg is one of the brightest cataclysmic variables (cvs), one of the best-studied dwarf nova stars (warner 1995), and was the first cv discovered in x-ray radiation (rappaport et al. 1974). the properties of the x-ray radiation of this close binary system have been extensively investigated, and are observed to be dramatically different in quiescence and in outburst. in quiescence, the x-ray spectrum is hard and can be described by an optically thin hot (kt ≈ 20 kev) plasma with an observed flux ≈ 2 · 10−10 erg s−1 cm−2. in outburst, this hard component decreases by a factor of ten, the plasma temperature is reduced to ∼ 6–8 kev, and an additional soft component appears with a blackbody temperature ≈ 200–300 kk (córdova et al. 1980; mcgowan et al. 2004; ishida et al. 2009). it is commonly accepted that the x-ray radiation of non-magnetic cvs arises in the boundary layer (bl) between the white dwarf (wd) and the accretion disc (pringle & savonije 1979; tylenda 1981; patterson & raymond 1985a, b; kley 1991), which are optically thick at high accretion rates (ṁ > 1016 g s−1) and optically thin at lower accretion rates. the soft x-ray spectrum of ss cyg in outburst was carefully investigated by mauche (2004) using a highresolution spectrum obtained with the chandra letg. he phenomenologically described the observed 40–130 å spectrum by a blackbody with temperature t ≈ 250 kk and numerous broad absorption features of ions of cosmically abundant o, ne, mg, si, s, and fe. the bl luminosity and wd spin were also evaluated in this work. on the other hand, this spectrum looks like the photospheric spectra of super-soft x-ray sources (lanz et al. 2005; rauch et al. 2010; van rossum 2012), so it probably could be described using the spectra of hot stellar model atmospheres. boundary layers possibly rotate with almost keplerian velocities and could have reduced (in comparison with wd) surface gravities close to the local eddington limit. therefore, we consider close to eddington limit models in the present work. here we present our attempt to fit the chandra letg spectrum of ss cyg using such model spectra. we also make estimates of the bl parameters in the context of our model fits. 2 model atmospheres the version of the lte computer code atlas (kurucz 1970; 1993), modified by us to deal with high temperatures (ibragimov et al. 2003; suleimanov & werner 2007), was used to model high temperature at143 http://dx.doi.org/10.14311/app.2015.02.0143 v. f. suleimanov et al. mospheres. in this code, local thermodynamic equilibrium (lte) is assumed and the pressure ionization effects using the occupation probability formalism (hummer & mihalas 1988) as described by hubeny et al. (1994) are taken into account. coherent electron scattering together with the free-free and bound-free transitions of all ions of the 15 most abundant elements using cross-sections from verner & yakovlev (1995) were adopted for the continuum opacity. line blanketing is also included using ∼ 25000 spectral lines from the chianti, version 3.0, atomic database (dere et al. 1997). 140 160 180 200 220 240 260 5.6 5.8 6.0 6.2 6.4 6.6 6.8 g < g edd lo g g t eff , kk 10-5 10-4 10-3 10-2 10-1 1 10 102 0 1 2 3 4 5 6 7 8 9 10 11 12 g ra d / g column density, g cm-2 t eff = 200 kk, log g = log g edd + 0.2 = 6.28 t eff = 200 kk, log g = log g edd + 0.4 = 6.48 t eff = 250 kk, log g = log g edd + 0.2 = 6.67 figure 1: top panel: positions of the computed model atmospheres in the teff–log g plane. the dashed curve demarcates the eddington limit (log g = log gedd). bottom panel: the relative radiation force vs. depth for various model atmospheres. twenty-two model atmospheres with solar chemical composition were computed using the described code. the effective temperatures of the models range between 150 kk and 250 kk with a step of 10 kk. two values of the surface gravity for each effective temperature, namely log g = log gedd +0.2 and log g = log gedd +0.4, were used. here log gedd = log(σe σsbt 4 eff/c) = 4.88 + 4 log(teff/105k) is the surface gravity that has an equal radiation pressure force for a given teff , and σe ≈ 0.34 cm2 g−1 is the electron scattering opacity for the assumed solar chemical composition. the positions of the computed models on the teff–log g plane are shown in fig. 1 (top panel). for the considered model atmospheres, a radiation pressure force grad due to spectral lines becomes larger than the surface gravity at the upper atmospheric layers (see fig. 1, bottom panel). to enforce hydrostatic equilibrium, we took a gas pressure equal to 10% of the total pressure (pgas = 0.1ptot) at all atmospheric layers where grad > g. as shown in fig. 2, the computed emergent spectra are dominated by a forest of absorption lines, and have to be convolved with the letg spectral resolution and the interstellar gas transmission to be compared with the observed spectrum of ss cyg (see fig. 3). the spectra of models with different surface gravities are sufficiently different to discriminate them from a comparison with the observed spectrum (see fig. 2, bottom panel). 40 60 80 100 1011 1012 1013 1014 1015 1016 1011 1012 1013 1014 1015 1016 log g = log g edd + 0.2 = 6.28 log g = log g edd + 0.4 = 6.48 t eff = 200 kk h , er g cm -2 s-1 a -1 wavelength, a t eff = 150, 200, 250 kk log g = log g edd + 0.2 h , er g cm -2 s-1 a -1 figure 2: top panel: emergent spectra of three model atmospheres with the same log g = log gedd + 0.2 and various effective temperatures: 150 kk (solid curves), 200 kk (dashed curves), and 250 kk (dotted curves). bottom panel: emergent spectra of two model atmospheres with the same effective temperature (200 kk) and different log g. 3 results the model spectra convolved with the chandra letg spectral resolution ∆λ = 0.05 å were used to fit the observed soft x-ray spectrum of ss cyg. the interstellar absorption (with the hydrogen column number den144 model atmosphere spectrum fit to the soft x-ray outburst spectrum of ss cyg sity nh as a fitting parameter) was also taken into account. the observed spectrum was fitted in the 60–125 å wavelength range because at the shorter wavelengths our model spectra could be incorrect (see next section). the best-fit model parameters are teff = 190 kk, nh = 8·1019 cm−2, and normalization k = 7.82·10−26, and correspond to the models with the lower surface gravity log g = log gedd + 0.2. the reduced χ2 = 3.9 is relatively large, hence the formal parameter errors are large, too, and we have not attempted to determine them. the best-fit model spectrum together with the observed spectrum is shown in fig. 3. the contours of χ2 on the teff–lognh parameter plane are shown in fig. 4. the normalization can be expressed as k = fr2 wd/d 2 where d is the distance to ss cyg and f is the wd fractional area occupied by the bl, which can be expressed as the relative bl extension along the wd surface f ≈ (2πrwd 2hbl)/(4πr2 wd) = hbl/rwd. the basic properties of the bl can be derived from the obtained fit parameters. using the same system and wd parameters as used by mauche (2004) — mwd = 1 m�, rwd = 5.5 · 108 cm (therefore, model log g = 8.46), d = 160 pc, and an accretion disk bolometric luminosity in outburst ldisk = 1035 erg s−1 — we obtain the fractional area of the bl f = 6.3 · 10−2 (5 · 10−3), the bolometric bl luminosity lbl = 1.8 · 1034 (5 · 1033) erg s−1, and the relative bl luminosity lbl/ldisk = 0.18 (0.05), where the best-fit parameter values obtained by mauche (2004) are shown in parentheses for comparison. the spin period of the wd in ss cyg is 12 (9) s as inferred using the relation between the bl and accretion disk luminosities (kluźniak 1987; kley1991) lbl/ldisk = [1 − ωwd/ωk(rwd)]2, (1) where ωk(rwd) is the kepler angular velocity at the wd radius. the accepted model gravity on the wd surface in ss cyg (log gwd = 8.46) is more than two orders of magnitude higher than the obtained bl effective surface gravity log geff = 6.2. the surface gravity of the bl can be reduced only by fast rotation of the accreting matter geff = gwd − ω2 blrwd (2) = gwd (1 − [ωbl/ωk(rwd)]2). therefore, a relative bl angular velocity ωbl/ωk(rwd) ≈ 0.98 was obtained using this relation. figure 3: the chandra letg spectrum of ss cyg in outburst (thick black curve) and the best-fit model atmosphere spectrum with teff = 190 kk, log g = 6.2, and lognh = 19.9 (thin red curve). the fitting was performed in the 60–125 å wavelength range. the model spectrum at shorter wavelengths is shown by the dashed red curve. figure 4: position of the best-fit model in the teff–lognh parameter plane and contours of χ2 = [1.5, 3, 6]χ2 min. 4 discussion and conclusion we present here the results of fitting the ss cyg chandra letg spectrum in outburst with model atmosphere spectra. the obtained best fit model spectrum does not describe the observed spectrum at short wavelengths (< 60 å) and in the 82–90 å wavelength region and, therefore, it is not statistically acceptable (χ2/dof = 3.9). these deficiencies can be connected with model shortcomings: the most important ignored effect is atmosphere expansion due to a spectral linedriven stellar wind. this expansion can be significant 145 v. f. suleimanov et al. because grad > g at the outer layers of our model atmospheres (see also van rossum 2012). in addition, non-lte effects could be important (see, e.g., rauch et al. 2010), the chemical composition may differ from solar, and, finally, the atomic data are almost certainly neither complete nor entirely accurate. the second important shortcoming is connected with a likely complicated bl structure with a distribution of effective temperatures and surface gravities over its surface. therefore, a simple one-zone bl model presented here is most probably insufficient and more sophisticated bl models have to be considered. nevertheless, the spectral modeling presented here supports a bl in ss cyg that is, to first approximation, a hot (≈ 190 kk), fast rotating [ωbl ≈ 0.98 ωk(rwd)], narrow (hbl ≈ 0.063rwd) belt on the wd surface. acknowledgement this work is supported by the dfg sfb / transregio 7 “gravitational wave astronomy” (v.s.) and the russian foundation for basic research (grant 12-02-97006r-povolzhe-a) (r.zh.). c.w.m.’s contribution to this work was performed under the auspices of the u.s. department of energy by lawrence livermore national laboratory under contract de-ac52-07na27344. references [1] córdova, f. a., chester, t. j., tuohy, i. r., & garmire, g. p. 1980, apj, 235, 163 [2] dere, k. p., landi, e., mason, h. e., monsignori fossi, b. c., & young, p. r. 1997, a&ass, 125, 149 [3] hubeny, i., hummer, d. g., & lanz, t. 1994, a&a, 282, 151 [4] hummer, d. g., & mihalas, d. 1988, apj, 331, 794 doi:10.1086/166600 [5] ibragimov, a. a., suleimanov, v. f., vikhlinin, a., & sakhibullin, n. a. 2003, astronomy reports, 47, 186 doi:10.1134/1.1562213 [6] ishida, m., okada, s., hayashi, t., nakamura, r., terada, y., mukai, k., & hamaguchi, k. 2009, pasj, 61, 77 [7] kley, w. 1991, a&a, 247, 95 [8] kluźniak, w. 1987, ph.d. thesis, stanford univ. [9] kurucz, r. l. 1970, sao special report, 309 [10] mauche, c. w. 2004, apj, 610, 422 [11] mcgowan, k. e., priedhorsky, w. c., & trudolyubov, s. p. 2004, apj, 601, 1100 doi:10.1086/380758 [12] patterson, j., & raymond, j. c. 1985a, apj, 292, 535 doi:10.1086/163187 [13] patterson, j., & raymond, j. c. 1985b, apj, 292, 550 doi:10.1086/163188 [14] pringle, j. e., & savonije, g. j. 1979, mnras, 187, 777 doi:10.1093/mnras/187.4.777 [15] rauch, t., orio, m., gonzales-riestra, r., nelson, t., still, m., werner, k., & wilms, j. 2010, apj, 717, 363 doi:10.1088/0004-637x/717/1/363 [16] rappaport, s., cash, w., doxsey, r., mcclintock, j., & moore, g. 1974, apj, 187, l5 doi:10.1086/181378 [17] suleimanov, v., & werner, k. 2007, a&a, 466, 661 [18] tylenda, r. 1981, acta astr., 31, 267 [19] van rossum, d. r. 2012, apj, 756, 43 [20] verner, d. a., & yakovlev, d. g. 1995, a&ass, 109, 125 [21] warner, b. 1995, cambridge astrophysics series, 28 discussion dmitry kononov: what is a reason to initially use the solar chemical composition instead of chemical compositions for stars of later spectral type? valery suleimanov: unfortunately, we do not know the chemical composition of the secondary in ss cyg. therefore, we started from the most conservative case, which is the solar chemical composition. jan-uwe ness: the assumed wd mass in the model (1 m�) seems discrepant from the mass given in the introduction (0.55 m�), a factor 2. this suggest that all parameters of the model may be discrepant from reality, so what do we learn? valery suleimanov: sorry, you mixed up the white dwarf mass with the mass of the secondary (0.55 m�). dmitry bisikalo: how can you discriminate between a boundary layer and a pseudo-photosphere or a hot disk halo? 146 http://dx.doi.org/10.1086/166600 http://dx.doi.org/10.1134/1.1562213 http://dx.doi.org/10.1086/380758 http://dx.doi.org/10.1086/163187 http://dx.doi.org/10.1086/163188 http://dx.doi.org/10.1093/mnras/187.4.777 http://dx.doi.org/10.1088/0004-637x/717/1/363 http://dx.doi.org/10.1086/181378 model atmosphere spectrum fit to the soft x-ray outburst spectrum of ss cyg valery suleimanov: the observed soft xray luminosity is a discrimination factor. any pseudophotosphere or an optically thick hot disk halo with this low surface gravity (log g ≈ 6.2) and the derived effective temperature (≈ 190 kk) would have a radius that is tens of times larger than the white dwarf radius, and the luminosity would be hundreds of times larger than the observed luminosity. 147 introduction model atmospheres results discussion and conclusion acta polytechnica ctu proceedings doi:10.14311/app.2017.10.0065 acta polytechnica ctu proceedings 10:65–68, 2017 © czech technical university in prague, 2017 available online at http://ojs.cvut.cz/ojs/index.php/app landslide reclamation in šárka valley martin vaníček geosyntetika s.r.o., nikoly tesly 3, prague 6, czech republic correspondence: mvanicek@geosyntetika.cz abstract. housing development in šárka’s valley is situated in former brickworks borrow pit for brick-clay. on its north side the borrow pit cutting is up to 8m high with main road on top. this slope was suffering from progressive landslide mainly due to weathering and surface erosion. the reclamation works proposed and afterwards executed consisted of stabilizing the slope by combination of soil nailing and connecting reinforced slope with surcharge bench at its toe. the final reinforced soil structure nicely merged into the surrounding environment. keywords: landslide reclamation, muralex stone facing system, reinforced soil structure. 1. introduction main task of this project was to design and later to execute the reclamation of landslide below the edge of horoměřická street. the slope is part of housing development project top residence šárecké údolí. this development project is situated in the area of former brickworks. the slope that has been progressively collapsing was up to 8m height with the slope as steep as 1:1. the progressive collapse was initiated by climatic degradation of brick clay and surface erosion on the steep slope. 2. geotechnical conditions in the area of the whole development project the geotechnical investigation has been performed. several boreholes and trial pits have been used to determine the geological structure of the area. as is typical for such projects the soil properties have been suggested from the index tests performed on few disturbed samples taken and from the description of soils encountered during exploratory works. in the area of landslide additional gi was undertaken, however again without determination of shear strength of encountered soils. based on the above information simple geotechnical model was determined. on site bedrock from weathered clayey shales was found, categorised as r5 based on czech code of practise. the depth to weathered bedrock was indicated between 1 and 4 m with respect to the borehole location within the slope. the shales were overlaid by clayey sandy silt (brick clay), based on classification cs sandy clay firm to stiff consistency. near the surface a layer with organic content was present. shear strength properties of soils have been selected with respect to eurocode 7 guidelines as very cautious value from tabled values of soil properties in discontinued czech code of practise čsn 731001. for the construction of new earth structure body, it was in the design assumed to use local soils from basement excavation for houses. also for recompacted soils from excavations the tests for determination of shear strength haven’t been performed, therefore their characteristic properties have been selected as very cautious values from tabled soil properties in čsn 752410, czech code of practise for earth dams. selected characteristic soil properties for reinforced soil structures are presented in table 1 below. based on above mentioned investigation typical cross section through land slide has been created and assumed location of slip surface together with individual soil layers has been indicated in fig. 1. ground water level has been found under the slope relatively shallow, about 1m below ground level. in the borehole 5.5 m deep from top of the slope, ground water hasn’t been found. this indicates that ground water level does not copy the ground surface and is rather naturally gradual. 3. proposed solution in accordance with proposed development project the bottom line of reclamation is defined by edge of unbound cycleway. in order to guarantee required stability of the reclaimed slope, it’s inclination is proposed at 1:1.75 1:2.00 with reinforcement in 1 m spaced layers. due to limited space, the slope has to be ended by retaining wall with max. height of 1.6 m. based on main architect requirement the retaining wall should have gabion facing. at the top edge of the slope, it was not possible to do any large excavations, as there is situated highly trafficked street horoměřická. in order to cut through the potential slip surface, driven soil nails in 1m vertical and 1.5 m horizontal spacing inclined by 60° from horizontal have been proposed. these soil nails from 50 × 6.3 mm steel pipe with 1. 5m length have been located into the steps in existing slope excavations created for better connection with new slope. stability of the new slope is maintained by reinforcing geogrids fortrac® mpt, with vertical spacing of 0.5 m or 1 m depending on the position in the slope. 65 http://dx.doi.org/10.14311/app.2017.10.0065 http://ojs.cvut.cz/ojs/index.php/app martin vaníček acta polytechnica ctu proceedings table 1. characteristic soil properties assumed. figure 1. cross section of collapsed slope with indicated assumed geotechnical layers and slipped surface. the length of geogrids is given between slope face and steps in existing ground, where they are connected to the soil nails by nail heads. the base retaining wall with vertical gabion type facing was designed as muralex® stone system. above the base retaining wall drainage channel was created to drain the surface water from the slope. on eastern side the reclamation is framed by the underpass under the horoměřická street with brick cladding of the steep slope above the underpass portal (see visualization on fig. 2). slope above portal wing wall is again retained by max. 3 m high retaining wall with muralex® stone system. the western side of slope reclamation is framed by newly planned staircase. the level difference between existing slope and reclaimed slope towards the planned staircase is proposed as reinforced steep slope (4:1) with muralex® green system, which provides vegetated surface. figure 2. visualization of reclaimed slope integration into the existing terrain design solution has been developed in accordance to eurocode 7, namely based on experience. typical cross section through reclaimed slope is shown in fig. 3. 66 vol. 10/2017 landslide reclamation in šárka valley figure 3. typical cross section through the reclaimed collapsed slope 4. reclamation execution next to design solution the developer and main contractor of the whole housing development asked us to also take care about the execution of reclamation works. the actual conditions have proven the design assumptions, except the shallower depth of weathered shales close to slope base, where we have some problems with nails driving. this issue was easily solved by shortening of the nails, which does not influence the overall stability. in the upper part the soil nails have been driven well behind the assumed slip surface, along which the slope most probably earlier collapsed. another emerged issue in lack of locally available soil was again easily solved by importing of the suitable soil. this imported soil was tested in lab for shear strength and compaction efficiency by proctor standard test including the optimal moisture content for compaction. reclamation was executed according to plan also due to highly permitting weather. wall from reinforced soil was build using wrap around technique, when individual layers have been compacted against mobile formwork (see fig. 4). at each geogrid layer, anchoring elements have been installed for future connection of muralex® facing elements. at these layers were also installed driven figure 4. formwork for proper compaction of reinforced soil structure with wrap-around facing in the facing area. soil nails and connected to the geogrids strips with help of nail heads (see fig. 5). above the base retaining wall new slope was built with the inclination in between 1:1.75 and 1:2.00 based on space availability. the soil was compacted by plate and ditch roller and compaction quality was tested by light dynamic plate test. once the slope was built and ramp for machinery access removed, attached front facing muralex® system was installed (see fig. 6). in locations with vertical facing, the system was filled with stones 63/125 mm and in locations with 4:1 slope facing, sieved organic soil filling was used. lastly the slope 67 martin vaníček acta polytechnica ctu proceedings figure 5. connection of reinforcing geogrids fortrac® mpt and driven soil nails figure 6. attached front facing system muralex® before 63/125 mm stone filling was covered with topsoil and temporary erosion protection mesh from coconut fibres. afterwards the whole slope surface was covered by grass seeds. already figure 7. overall view on the finished reclaimed slope, where grassing can be seen after couple of days the slope started to be nicely grassed (see fig. 7). 5. summary finally it can be concluded that even at confined conditions it is possible to reclaim collapsed slope by appropriate combination of soil reinforcement techniques soil nailing and soil reinforcement by fortrac® mpt geogrids. the final visual perception nicely implemented the reclaimed slope into high standard housing development. the system of attached front facing muralex® creates aesthetic element, which can be during the lifetime of reinforced soil structure repaired or modified from whatever reasons (e.g. accident, change of aesthetic feeling, . . . ) without compromising the main retaining soil structure. 68 acta polytechnica ctu proceedings 10:65–68, 2017 1 introduction 2 geotechnical conditions 3 proposed solution 4 reclamation execution 5 summary 152 acta polytechnica ctu proceedings 2(1): 152–155, 2015 152 doi: 10.14311/app.2015.02.0152 evolution and outbursts of cataclysmic variables s.-b. qian1,2,3, l.-y. zhu1,2,3, e.-g. zhao1,2, e. fernández lajús4,5, j. zhang1,2, g. shi1,2, z.-t. han1,2 1yunnan observatories, chinese academy of science, p. o. box 110, 650011 kunming, china 2key laboratory for the structure and evolution of celestial objects, chinese academy of sciences, 650011 kunming, china 3university of the chinese academy of sciences, yuquan road 19#, sijingshang block, 100049 beijing, china 4facultad de ciencias astronómicas y geof́ısicas, universidad nacional de la plata, 1900 la plata, buenos aires, argentina 5instituto de astrofisica de la plata (cct la plata conicet/unlp), argentina corresponding author: qsb@ynao.ac.cn abstract mass transfer and accretion are very important to understand the evolution and observational properties of cataclysmic variables (cvs). due to the lack of an accretion disk, eclipsing profiles of polars are the best source to study the character of mass transfer in cvs. by analyzing long-term photometric variations in the eclipsing polar hu aqr, the property of mass transfer and accretion are investigated. the correlation between the brightness state change and the variation of the ingress profile suggests that both the accretion hot spot and the accretion stream are produced instantaneously. the observations clearly show that it is the variation of mass transfer causing the brightness state changes that is a direct evidence of variable mass transfer in a cv. it is shown that it is the local dark-spot activity near the l1 point to cause the change of the mass transfer rather than the activity cycles of the cool secondary star. our results suggest that the evolution of cvs is more complex than that predicted by the standard model and we should consider the effect of variable mass accretion in nova and dwarf nova outbursts. keywords: cataclysmic variables polars photometry individual: hu aqr, dp leo, mn hya, and v2301 oph. 1 introduction cataclysmic variables (cvs) are semi-detached binaries where a low-mass (spectral types of k and m) component star transfers mass to a white dwarf (e.g., warner 1995). according to the standard theory of cvs, the evolution of long-period (p > 3 hours) cvs are driven by angular momentum loss (aml) via magnetic braking (mb), and the ”period gap” (the apparent scarcity of cvs in the period range from 3 to 2 hours) was explained as the cessation of the mb at the period of 3 hours when the secondary component becomes completely convective (e.g., rappaport et al. 1983; spruit & ritter 1983). cvs below the ”period gap” are governed by gravitational radiation. cvs as a mass transferring binary, the aml should keep the secondary filling the critical roche lobe and transferring mass to the white dwarf continuously. in this way, cvs will evolve from a higher mass secondary with longer period to a lower mass secondary with shorter period. mass transfer and accretion in cvs are very important to understand their evolution and observational properties (e.g., the novae and dwarf-novae outbursts). most cvs (e.g., polars and some dwarf novae and novalike cvs) usually show brightness changes in high state, intermediate and low states. the most possible reason to cause these changes is the variation of mass transfer. however, direct evidence is lacking because of the influence of an accretion disk in a normal (non-magnetic) cv. in a polar (magnetic cv), the magnetic field of the white dwarf is very strong to prevent the formation of an accretion disk (e.g., giovannelli & sabau-graziati 2012). the transferred material from the secondary is quickly and directly accreted onto the white dwarf. during the eclipses, light components from the accretion stream and spot on the white dwarf can be isolated. therefore, eclipsing polars can provide invaluable information on the character of mass transfer. the changes of eclipsing profiles directly reflect the variations in the mass-transfer rate. the results will shed light on our understanding of several key astrophysical problems such as the high and low brightness state 152 http://dx.doi.org/10.14311/app.2015.02.0152 evolution and outbursts of cataclysmic variables changes and dwarf nova outbursts. however, only a few polars were monitored photometrically including the brightest polar am her (e.g., hessman et al. 2000; kafka & honeycutt 2003, 2005; wu & kiss 2008), no eclipsing polars were monitored for a long time (e.g., up to several years) before. on the other hand, the eclipse times of eclipsing polars can be determined with a high precision because the accretion stream and the hot spot on the white dwarf can be isolated during the eclipses. very smallamplitude cyclic changes in the observed-calculated (oc) diagram can be detected. therefore, they are also good targets to search for planets and brown dwarfs orbiting cvs by analyzing the light travel time effect. some substellar objects were recently discovered to be orbiting the polars dp leo and hu aqr (qian et al. 2010a, 2011). by using the same method, planets orbiting other evolved binaries were discovered. some examples discovered by our group are those orbiting the detached white-dwarf binaries qs vir and rr cae (qian et al. 2010b, 2012a) and the subdwarf b-type binary ny vir (qian et al. 2012b). 2 targets and observations we have selected ten eclipsing polars, dp leo, ep dra, v2301 oph, ek uma, hu aqr, uz for, v1432 aql, mn hya, v1309 ori, and sdss j015543.40+002807.2, for monitoring photometrically. we monitored those sample stars since 2009 by using the 2.4-m and 1.0m telescopes at yunnan observatories in china and the 2.15-m jorge sahade telescope at complejo astronomico el leoncito (casleo), san juan, argentina. more than 100 eclipse profiles of the selected polars were observed. the eclipse profile for the eclipsing polar, v2301 ophshown in fig. 1. it was obtained on feb. 15, 2012 by using the 2.4-m telescope in lijiang observational station of yunnan observatories. during the observation, no filters were used. among the selected eclipsing polars, we pay more attention to hu aqr. it is one of the brightest eclipsing polars at both optical and x-ray wavelengths. this polar is a total eclipsing binary with an orbital inclination of i = 87◦ and has only one accretion pole (e.g., schwope et al. 2001). all these make it the most ideal target to investigate the properties of mass transfer through the long-term photometry. we have monitored it since 2009 and nearly 70 eclipse profiles were obtained. 3 direct evidence for mass transfer and accretion two of white-light eclipse profiles of hu aqr obtained at high and low brightness states are shown in fig. 2. as shown in the figure, the eclipses start with the limb of the red-dwarf component eclipsing the hot spot (accretion region), and the primary is also eclipsed nearly at the same time. then the accretion stream is the dominant source of the brightness with a small contribution of the secondary, and the distortion in the ingress profile is visible. finally, only the red dwarf is visible and provides a constant contribution at the bottom of the eclipse. 73.440 73.443 73.446 73.449 73.452 73.455 73.458 3.0 2.5 2.0 1.5 1.0 0.5 0.0 -0.5 -1.0 figure 1: white-light photometric observations of v2301 oph obtained on feb. 15, 2012 by using the 2.4m telescope in lijiang observational station of yunnan observatories. solid dots refer to the magnitude differences between v2301 oph and the comparison star, while open circles to those between the comparison and the check stars. -0.10 -0.05 0.00 0.05 0.10 0.15 0.20 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 figure 2: comparison of two eclipse profiles for hu aqr at high and low brightness states, respectively. the two eclipse profiles were obtained in 2011 by using the 2.4-m telescope in yunnan observatories. the sequence of the egress process is approximately reversed. as displayed in fig. 2, the egress profile of hu aqr is rather stable. therefore, the mid-egress times were used to detect two planetary objects orbiting this binary star by analyzing the light travel time 153 s.-b. qian et al. effect (qian et al. 2011) and the new eclipse times will be used to revise the parameters of the circumbinary planets. as shown in fig. 2, when hu aqr is at a high brightness state, the distortion in the ingress profile is visible. these properties suggest that there is an accretion stream from the secondary to the white-dwarf component and at the same time we see a hot spot on the white dwarf. both the existences of the accretion stream and the hot spot indicate a high mass accretion rate at high brightness state. on the other hand, at a low brightness state, the distortion in the ingress profile disappeared and the eclipse profiles are symmetric. these can be explained as the results that no hot spots on the white dwarf and no accretion streams are seen between the component indicating that no mass transfers and accretions between both components. the brightness variation is correlated with the change of the eclipse profile suggesting that both the hot spot and the accretion stream are produced instantaneously. this is a direct evidence of variable mass transfer and accretion in the cv. two eclipse profiles at high brightness states are plotted in fig. 3. the two eclipse profiles were observed on may 29 and 31,2011 with the 2.15-m jorge sahade telescope in argentina. as displayed in the figure, both the brightness before the eclipse and the ingress profile are variable in two days. other two eclipse profiles obtained in 2010 by using the 1.0-m telescope in yunnan observatories are displayed in fig. 4. it is shown in the figure that hu aqr is on the way from a high state to a low state indicating that the rate of mass transfer is decreasing. 4 discussion and conclusions the observations in previous section give a direct evidence for variable mass transfer and accretion in cvs. the orbital period of hu aqr is about 2.08 hours, just below the cv period gap. non-magnetic cvs with orbital periods in this range are usually dwarf novae (dn). two dwarf novae, ir com and dv uma, have the nearly same orbital period as that of the eclipsing polar hu aqr. they show dn outbursts (a sudden increase of brightness up to 2-7 magnitudes) at usually irregular intervals (nogami et al., 2001). their dn outbursts are explained as the disc instability (cannizzo 1993). however, it is not clear that the outbursts are pure disc phenomena that occur without some contribution of variable mass transfer from the secondary (livio & verbunt, 1988; duschl & livio, 1989). our conclusions suggest that we should consider the contribution of mass transfer in the model of cv outbursts (e.g., cannizzo 1993). the evolution of cvs may be more complex than that predicted by the standard model. -0.20 -0.15 -0.10 -0.05 0.00 0.05 0.10 0.15 0.20 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 oct 22, 2010 figure 3: white-light eclipse profiles observed with the 1.0-m telescope in yunnan observatories. open circles refer to the data obtained on oct. 22, 2010, while solid dots to those obtained on oct. 30. some physical mechanisms, such as irradiationdriven mass transfer and star-spots cause (e.g., wu et al., 1995; livio & pringle 1994), have been proposed to explain the changes of mass transfer in cvs. to solve this question, we analyzed the brightness variation around phase 0.5. it is discovered that the brightness from phase 0.3 to 0.65 usually shows a large dip at low states when the mass transfer rate is very low. two of the light curves obtained at low states are shown in fig. 5 where a large dip is seen around phase 0.5. these brightness dips can be explained by the presence of dark spots near the l1 point on the cool secondary star. this conclusion indicates that it is the coverage of dark spots near l1 point that produce the decrease of the mass transfer rate. 3.52 3.53 3.54 3.55 3.56 3.57 3.58 3.59 3.60 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 4.51 4.52 4.53 4.54 4.55 4.56 4.57 4.58 4.59 4 3 2 1 figure 4: two light curves of hu aqr observed at low brightness states with the 2.15-m jorge sahade telescope in argentina. it is shown that the brightness around phases 0.5 shows a large dip. the light curve in the left was obtained on nov. 11, 2011, while that in the right was obtained on nov. 12. at the bottom of the eclipse, all of the hot spot, the white dwarf, and the accretion stream are eclipsed by 154 evolution and outbursts of cataclysmic variables the secondary. only the red-dwarf component is visible and provides a constant contribution. therefore, it is the only chance to measure the brightness change of a mass donor in a cv. a cyclic variation in the brightness of the secondary is discovered that can be explained as the dark-spot activity cycle. it is the first time to detect an activity cycle for a fast-rotating mass donor in a cv. our results reveal that the variable mass accretion is not caused by magnetic activity cycles of the fast-rotating red dwarf. acknowledgement this work is supported by chinese natural science foundation through a key project (no.11133007). new ccd photometric observations of those eclipsing polars were obtained with the 2.4-m and 1.0-m telescopes at yunnan observatories (yno) in china and the 2.15-m ”jorge sahade” telescope in argentina. references [1] cannizzo, j. k., 1993, apj 419, 318 [2] duschl, w. j. & livio, m., 1989, a&a 209, 183 [3] giovannelli, f. & sabau-graziati, l., 2012, mmsai 83, 446 [4] hessman, f. v., gänsicke, b. t., mattei, j. a., 2000, a&a 361, 952 [5] kafka, s. & honeycutt, r. k., 2003, aj 125, 2188 [6] kafka, s. & honeycutt, r. k., 2005, aj 130, 742 [7] livio, m. & pringle, j. e., 1994, apj 427, 956 doi:10.1086/174202 [8] livio, m. & verbunt, f., 1988, mnras 232, 1 doi:10.1093/mnras/232.1.1p [9] nogami, d., kato, t., baba, h., novk, r., lockley, j. j., somers, m., 2001, mnras 322, 79 doi:10.1046/j.1365-8711.2001.04114.x [10] qian, s.-b., liao, w.-p., zhu, l.-y., dai, z.-b., 2010a, apj 708, l66 doi:10.1088/2041-8205/708/1/l66 [11] qian, s.-b., liao, w.-p., zhu, l.-y., dai, z.-b., liu, l., he, j.-j., zhao, e.-g., and li, l.-j., 2010b, mnras 401, l34 doi:10.1111/j.1745-3933.2009.00780.x [12] qian s.-b., liu, l., liao, w.-p., li, l.-j., zhu, l.-y., dai, z.-b., he, j.-j., zhao, e.-g., zhang, j., and li, k., 2011, mnras 414, l16 doi:10.1111/j.1745-3933.2011.01045.x [13] qian, s.-b., liu, l., zhu, l.-y., dai, z.-b., fernandez lajus, e., baume, g. l., 2012a, mnras 422, l24 doi:10.1111/j.1745-3933.2012.01228.x [14] qian s.-b., zhu l.-y., dai z.-b., fernández lajús, e., xiang f.-y., and he, j.-j., 2012b, apj 745, l23 doi:10.1088/2041-8205/745/2/l23 [15] rappaport, s., verbunt, f., joss, p. c., 1983, apj 275, [16] schwope, a. d., schwarz, r., sirk, m., howell, s. b., 2001, a&a 375, 419 [17] spruit, h. c., & ritter, h., 1983, a&a 124, 267 [18] warner, b., 1995, cataclysmic variable stars, cambridge univ. press, cambridge [19] wu, k. & kiss, l. l., 2008, a&a 481, 433 [20] wu, k., wickramasinghe, d. t., warner, b., 1995, pasa 12, 60 discussion k. reinsch: which filters did you use for your photometry? could you see a colour dependence in the orbital variation as would be expected for a light curve contribution originating from the secondary? qian shengbang: since we need high time resolution for our data, the exposure time is only a few seconds. therefore, no filters were used during the observations. 155 http://dx.doi.org/10.1086/174202 http://dx.doi.org/10.1093/mnras/232.1.1p http://dx.doi.org/10.1046/j.1365-8711.2001.04114.x http://dx.doi.org/10.1088/2041-8205/708/1/l66 http://dx.doi.org/10.1111/j.1745-3933.2009.00780.x http://dx.doi.org/10.1111/j.1745-3933.2011.01045.x http://dx.doi.org/10.1111/j.1745-3933.2012.01228.x http://dx.doi.org/10.1088/2041-8205/745/2/l23 introduction targets and observations direct evidence for mass transfer and accretion discussion and conclusions 139 acta polytechnica ctu proceedings 1(1): 139–145, 2014 139 doi: 10.14311/app.2014.01.0139 latest results from the fermi gamma-ray telescope aldo morselli1 1infn roma tor vergata corresponding author: aldo.morselli@roma2.infn.it abstract can we learn about new physics with astronomical and astro-particle data? since its launch in 2008, the large area telescope, onboard of the fermi gamma-ray space telescope, has detected the largest amount of gamma rays in the 20 mev 300 gev energy range and electrons + positrons in the 7 gev1 tev range, opening a new observational window on a wide variety of astrophysical objects. keywords: gamma ray gamma ray detectors dark matter. 1 introduction the fermi observatory carries two instruments onboard: the gamma-ray burst monitor (gbm) [1] and the large area telescope (lat) [2]. the lat is a pair conversion telescope for photons above 20 mev up to a few hundreds of gev. the field of view is ∼2.4 sr and lat observes the entire sky every ∼ 3 hours (2 orbits). these features make the lat a great instrument for dark matter (dm) searches. the operation of the instrument through the first three years of the mission was smooth at a level which is probably beyond the more optimistic prelaunch expectations. the lat has been collecting science data for more than 99% of the time spent outside the south atlantic anomaly (saa). the remaining tiny fractional down-time accounts for both hardware issues and detector calibrations [4], [5]. more than 650 million gamma-ray candidates (i.e. events passing the background rejection selection) were made public and distributed to the community through the fermi science support center (fssc) 1. over the first three years of mission the lat collaboration has put a considerable effort toward a better understanding of the instrument and of the environment in which it operates. in addition to that, a continuous effort was made to in order to make the advances public as soon as possible. in august 2011 the first new event classification (pass 7) since launch was released, along with the corresponding instrument response functions (and a release of a new event class ’pass 7 reprocessed’ is planned for the near future). compared with the prelaunch (pass 6) classification, it features a greater and more uniform exposure, with a significance enhancement in acceptance below 100 mev. the fermi lat results on the extragalactic sky will be covered by benoit lott [3]. here we will present the main results regarding the indirect dark matter searches and the origin of cosmic rays. 2 indirect dark matter searches one of the major open issues in our understanding of the universe is the existence of an extremely-weakly interacting form of matter, the dark matter, supported by a wide range of observations including large scale structures, the cosmic microwave background and the isotopic abundances resulting from the primordial nucleosynthesis. complementary to direct searches being carried out in underground facilities and at accelerators, the indirect search for dm is one of the main items in the broad fermi science menu. the word indirect denotes here the search for signatures of weakly interactive massive particle (wimp) annihilation or decay processes through the final products (gammarays, electrons and positrons, antiprotons) of such processes. among many other ground-based and spaceborne instruments, the lat plays a prominent role in this search through a variety of distinct search targets: gamma-ray lines, galactic and isotropic diffuse gamma-ray emission, dwarf satellites, cr electrons and positrons. 2.1 galactic center the galactic center (gc) is expected to be the strongest source of γ-rays from dm annihilation, due to its coincidence with the cusped part of the dm halo density profile [10], [11], [12]. a preliminary analysis of the data, taken during the first 11 months of the fermi satellite operations is presented in [13], [14]. 1the fssc is available at http://fermi.gsfc.nasa.gov/ssc 139 http://dx.doi.org/10.14311/app.2014.01.0139 aldo morselli figure 1: derived 95% c.l. upper limits on wimp annihilation cross sections in the milky way halo, for the muon (left) and tau (right) annihilation channels. the purple and blue contours show pamela and fermi positron excess dm interpretation constraint regions. the diffuse gamma-ray backgrounds and discrete sources, as we know them today, can account for the large majority of the detected gamma-ray emission from the galactic center. nevertheless a residual emission is left, not accounted for by the above models [13], [14]. improved modeling of the galactic diffuse model as well as the potential contribution from other astrophysical sources (for instance unresolved point sources) could provide a better description of the data. analyses are underway to investigate these possibilities. 2.2 galactic halo in order to minimize uncertainties connected with the region of the galactic center, analysis [15] considered a region of interest consisting of two off-plane rectangles (5◦ ≤ |b| ≤ 15◦ and |l| ≤ 80◦) and searched for continuum emission from dark matter annihilation or decay in the smooth galactic dark matter halo. they considered two approaches: a more conservative one in which limits were set on dm models assuming that all gamma ray emission in that region might come from dark matter (i.e. no astrophysical signal is modeled and subtracted). in a second approach, dark matter source and astrophysical emission was fit simultaneously to the data, marginalizing over several relevant parameters of the astrophysical emission. as no robust signal of dm emission is found, dm limits are set. these limits are particularly strong on leptonic dm channels, which are hard to constrain in most other probes (notably in the analysis of the dwarf galaxies, described below). this analysis strongly challenges dm interpretation [16] of the positron rise, observed by pamela [17] and fermi lat [18, 19] (see figure 1). 2.3 dwarf galaxies dwarf satellites of the milky way are among the cleanest targets for indirect dark matter searches in gamma-rays. they are systems with a very large mass/luminosity ratio (i.e. systems which are largely dm dominated). the lat detected no significant emission from any of such systems and the upper limits on the γ-ray flux allowed us to put very stringent constraints on the parameter space of well motivated wimp models [20]. a combined likelihood analysis of the 10 most promising dwarf galaxies, based on 24 months of data and pushing the limits below the thermal wimp cross section for low dm masses (below a few tens of gev), has been recently performed [21]. the main advantages of the combined likelihood are that the analysis can be individually optimized and that combined limits are more robust under individual background fluctuations and under individual astrophysical modelling uncertainties than individual limits. the derived 95% c.l. upper limits on wimp annihilation cross sections for different channels are shown in figure 2 (top). the most generic cross section (∼ 3 · 10−26cm3s−1 for a purely s-wave cross section) is plotted as a reference. these results are obtained for nfw profiles [22] but for cored dark matter profiles the j-factors for most of the dsphs would either increase or not change much so these results includes j-factor uncertainties [21]. with the present data we are able to rule out large parts of the parameter space where the thermal relic density is below the observed cosmological dark matter density and wimps are dominantly produced non-thermally, e.g. in models where supersymmetry breaking occurs via anomaly mediation for the mssm model [20]. future improvements (apart from increased amount of data) will include an improved event selection with a larger effective area and photon energy range, and the inclusion of more satellite galaxies. in figure 2 (bottom) are shown the predicted upper limits in the hypothesis of 10 years of data instead of 2; 30 dsphs in140 latest results from the fermi gamma-ray telescope stead of ten (supposing that the new optical surveys will find new dsph); spatial extension analysis (source extension increases the signal region at high energy e ≥ 10 gev,m ≥ 200 gev). figure 2: derived 95% c.l. upper limits on wimp annihilation cross sections for different channels. down: predicted 95% c.l. upper limits on wimp annihilation cross sections in 10 years for bbar channel. other complementary limits were obtained with the search of possible anisotropies generated by the dm halo substructures [23], the search for dark matter satellites [24] and a search for high-energy cosmic-ray electrons from the sun [25]. 2.4 gamma-ray lines a line at the wimp mass, due to the 2γ production channel, could be observed as a feature in the astrophysical source spectrum [12]. such an observation would be a “smoking gun” for wimp dm as it is difficult to explain by a process other than wimp annihilation or decay and the presence of a feature due to annihilation into γz in addition would be even more convincing. no significant evidence of gamma-ray line(s) has been found in the first two years of data from 7 to 200 gev [26] (see also [27]). recently, the claim of an indication of line emission in fermi-lat data [28, 29] has drawn considerable attention. using an analysis technique similar to [27], but doubling the amount of data as well as optimizing the region of interest for signal over square-root of background, [28] found a (trial corrected) 3.2 σ significant excess at a mass of ∼ 130 gev that, if interpreted as a signal would amount to a cross-section of about < σv >∼ 10−27cm3s−1. figure 3: dark matter annihilation 95% cl cross section upper limits into γγ for the einasto profile for a circular region of interest (roi) with a radius rgc = 16◦ centered on the gc with |b| < 5◦ and |l| > 6◦ masked. the signal is found to be concentrated on the galactic centre with a spatial distribution consistent with an einasto profile [30]. this is marginally compatible with the upper limit presented in [26]. in the analysis of the 4 year data the fermi lat team has improved over the two year paper in three important aspects: i) the search was performed in five regions of interest optimized for dm search under five different assumptions on the morphology of the dm signal, ii) new improved data set (pass 7 reprocessed) was used, as it corrects for loss in calorimeter light yield due to radiation damage during the four years of the fermi mission and iii) the energy dispersion was improved by adding a 2nd dimension to the previously used triple gaussian probability distribution function (pdf) model, leading to a so called ’2d’ pdf (such procedure is shown to increase the sensitivity to a line detection by 15%).. in that analysis [31] no globally significant lines have been fond and new limits to this dm annihilation channel were set (see figure 3). in a close inspection of the 130 gev feature it was found that indeed there exist a 135 gev signal at 4.01σ local significance, when a ’1d’ point spread 141 aldo morselli function (psf) and old data sets were used (consistently with what [28, 29] have found). however, the significance drops to 3.35σ (local, or ≤ 2σ global significance once trials factors are taken into account). in addition, a weaker signal is found at the same energy in the control sample (in the earth limb), which might point to a systematics effectpresent in this data set. in order to examine this possibility weekly observations of the limb are scheduled, and a better understanding of a nature of the excess in the control sample should be available soon. a new version of the event-level reconstruction and analysis framework (called pass 8 ) is foreseen soon from the fermi lat collaboration. with this new analysis software we should increase the efficiency of the instrument at high energy and have a data set based on independent event analysis thus gaining a better control of the systematic effects. 2.5 the cosmic ray electron spectrum the experimental information available on the cosmic ray electron (cre) spectrum has been dramatically expanded with a high precision measurement of the electron spectrum from 7 gev to 1 tev by the fermi lat [18], [19]. the spectrum shows no prominent spectral features and it is significantly harder than that inferred from several previous experiments. recently the fermi-lat collaboration performed a direct measurement of the absolute e+ and e− spectra, and of their fraction [35]. as the fermi-lat does not carry a magnet, analysis took advantage of the fact that due to its magnetic field, the earth casts a shadow in electron or positron fluxes in precisely determined regions. as a result, this measurement confirmed a rise of the positron fraction observed by pamela, between 20 and 100 gev and determined for the first time that it continues to rise between 100 and 200 gev (see figure 4). these measurements show that a new component of e+ and e− are needed with a peak at ∼ 1 tev. the temptation to claim the discovery of dark matter from detection of electrons and positrons from annihilation of dark matter particles is strong but there are competing astrophysical sources, such as pulsars, that can give a strong flux of primary positrons and electrons (see [16] and references therein). at energies between 100 gev and 1 tev the electron flux reaching the earth may be the sum of an almost homogeneous and isotropic component produced by galactic supernova remnants and the local contribution of a few pulsars with the latter expected to contribute more and more significantly as the energy increases. if a single nearby pulsar gives the dominant contribution to the extra component a large anisotropy and a small bumpiness should be expected; if several pulsars contribute the opposite scenario is expected. so far no positive detection of cre anisotropy was reported by the fermi-lat collaboration, but some stringent upper limits were published [34] and the pulsar scenario is still compatible with these upper limits. energy (gev) 1 10 210 p os itr on f ra ct io n −110 fermi 2011 pamela 2009 ams 2007 heat 2004 figure 4: positron fraction measured by the fermi lat and by other experiments [32, 33, 17]. the fermi statistical uncertainty is shown with error bars and the total (statistical plus systematic uncertainty) is shown as a shaded band. after the conference the ams-02 collaboration presented the result on the positron fraction [36] that confim the positron ratio rise observed by pamela and fermi and extend it up to 350 gev. forthcoming measurements from ams-02 and calet are expected to reduce drastically the uncertainties on the propagation parameters by providing more accurate measurements of the spectra of the nuclear components of cr. fermi-lat and those experiments are also expected to provide more accurate measurements of the cre spectrum and anisotropy looking for features which may give a clue of the nature of the extra component. 3 origin of cosmic rays cosmic rays are particles (mostly protons) accelerated to relativistic speeds. despite wide agreement that supernova remnants (snrs) are the sources of galactic cosmic rays, unequivocal evidence for the acceleration of protons in these objects is still lacking. when accelerated protons encounter interstellar material they produce neutral pions, which in turn decay into gamma rays. this offers a compelling way to detect the acceleration sites of protons. the identification of pion-decay gamma rays has been difficult because high-energy electrons also produce gamma rays via bremsstrahlung and inverse compton scattering. we detected the characteristic pion-decay feature in the gamma-ray spectra of two snrs, ic 443 and w44, with the fermi large area 142 latest results from the fermi gamma-ray telescope telescope. this detection provides direct evidence that cosmic-ray protons are accelerated in snrs. energy (ev) 810 910 1010 1110 1210 ) -1 s -2 d n /d e ( er g cm 2 e -1210 -1110 -1010 best-fit broken power law fermi-lat veritas (acciari et al. 2009) magic (albert et al. 2008) agile (tavani et al. 2010) -decay0π bremsstrahlung bremsstrahlung with break ic 443 energy (ev) 810 910 1010 1110 1210 ) -1 s -2 d n /d e ( er g cm 2 e -1210 -1110 -1010 best-fit broken power law fermi-lat agile (giuliani et al. 2011) -decay0π bremsstrahlung bremsstrahlung with break w44 figure 5: (a and b) gamma-ray spectra of ic 443 (a) and w44 (b) as measured with the fermi -lat [37] . color-shaded areas bound by dashed lines denote the best-fit broadband smooth broken power law (60 mev to 2 gev), gray-shaded bands show systematic errors below 2 gev due mainly to imperfect modeling of the galactic diffuse emission. at the high-energy end, tev spectral data points for ic 443 from magic [38] and veritas [39] are shown. solid lines denote the best-fit pion-decay gamma-ray spectra, dashed lines denote the best-fit bremsstrahlung spectra, and dash-dotted lines denote the best-fit bremsstrahlung spectra when including an ad hoc low-energy break at 300 mev c−1 in the electron spectrum. these fits were done to the fermi lat data alone (not taking the tev data points into account). magenta stars denote measurements from the agile satellite for these two snrs, taken from [40] and [41], respectively. figure 5 shows the spectral energy distribution obtained for ic 443 and w44 through maximum likelihood estimation. the normalizations of the fluxes of ic 443 and w44 and those of neighboring sources and of the galactic diffuse model, were left free in the fit for each bin. to determine whether the spectral shape could indeed be modeled with accelerated protons, we fit the lat spectral points with a π0-decay spectral model, which was numerically calculated from a parameterized energy distribution of relativistic protons. the measured gamma-ray spectra, in particular the low-energy parts, matched the π0-decay model (fig.5) [37]. the π0-decay gamma rays are likely emitted through interactions between “crushed cloud” gas and relativistic protons, both of which are highly compressed by radiative shocks driven into molecular clouds that are overtaken by the blast wave of the snr unless we introduce in an ad hoc way an additional abrupt break in the electron spectrum at 300 mev c−1 (fig.5 dash-dotted lines), the bremsstrahlung models do not fit the observed gamma-ray spectra. if we assume that the same electrons are responsible for the observed synchrotron radiation in the radio band, a lowenergy break is not expected to be very strong in the radio spectrum and thus the existing data do not rule out this scenario. the introduction of the low-energy break introduces additional complexity and therefore a bremsstrahlung origin is not preferred. finding evidence for the acceleration of protons has long been a key issue in attempts to elucidate the origin of cosmic rays. our spectral measurements down to 60 mev enable the identification of the π0-decay feature, thus providing direct evidence for the acceleration of protons in snrs. the proton momentum distributions, well-constrained by the observed gamma-ray spectra, are yet to be understood in terms of acceleration and escape processes of high-energy particles. 4 conclusions fermi turned four years in orbit on june, 2012, and it is definitely living up to its expectations in terms of 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[40] m. tavani, et al., [agile coll.], apj 710, l151 (2010). doi:10.1103/physrevlett.110.141102 [41] a. giuliani, et al., [agile coll.], apj 742, l30 (2011). doi:10.1126/science.1231160 145 http://dx.doi.org/10.1103/physrevd.82.092003 http://dx.doi.org/10.1103/physrevlett.110.141102 http://dx.doi.org/10.1126/science.1231160 introduction indirect dark matter searches galactic center galactic halo dwarf galaxies gamma-ray lines the cosmic ray electron spectrum origin of cosmic rays conclusions 42 acta polytechnica ctu proceedings 1(1): 42–48, 2014 42 doi: 10.14311/app.2014.01.0042 lhc, astrophysics and cosmology giulio auriemma1,2 1università degli studi della basilicata, potenza,italy 2infn sezione di roma, rome, italy corresponding author: giulio.auriemma@cern.ch abstract in this paper we discuss the impact on cosmology of recent results obtained by the lhc (large hadron collider) experiments in the 2011-2012 runs, respectively at √ s = 7 and 8 tev. the capital achievement of lhc in this period has been the discovery of a spin-0 particle with mass 126 gev/c2, very similar to the higgs boson of the standard model of particle physics. less exciting, but not less important, negative results of searches for supersymmetric particles or other exotica in direct production or rare decays are discussed in connection with particles and v.h.e. astronomy searches for dark matter. keywords: hep physics cosmology dark matter dark energy. 1 introduction on july 4th 2012 the atlas[1] and cms[2] collaboration has announced the discovery of a new massive boson, which subsequentely was shown to look like the standard model higgs bosonn. this was possible only after the 2012 runs of lhc at √ s = 8 tev. the two central experiment atlas & cms have collected during 2011 and 2012 an integrated luminosity of ≈ 40 fb−1. several reason support the general belief that this boson could be the long expected higgs boson [3], in particular • it is definitively a boson with spin 6= 1 because decays in γ-γ channel (h → γγ). quantum numbers jp = 0+,predicted by sm are strongly favored [4, 5] ; • the pdg averaged mass is [6] : mh = 125.9 ± 0.4(stat) ± 0.4(syst) gev/c2 consistent with ew precision measurements that requires mh = 102−24 −20 gev/c2 [7]; • production cross section σpp→h agrees well inside the errors to the prediction of sm, that are affected by the uncertainty of 15%. • branching ratios to leptons, hadrons and gauge bosons are close enough to the sm predictions, with some tension in the channels h → γγ (µ ' 2) and h → bb (µ < 1) for atlas data. • angular distribution is slightly in favor of spin 1, but only with more data this could be confirmed, analyzing the decay h →ww → 2`2ν. at present, from february 2013 to november 2014, lhc is engaged in the first long shutdown aimed at the consolidation of the accelerator for running at the full design c.m.s. energy √ s = 14 tev. after this point, hopefully, supersymmetry (susy) hunting will be open. 2 supersymmetric higgs from the supersymmetry theory is expected a relation between the half-integer spin fermions to the integer spin bosons, introduced initially on a purely mathematical ground [8]. the real appeal of this theoretical framework is that it incorporates not only the three gauge fields of the sm but also gravity [9, 10]. figure 1: mssm model higgs masses, the hatched area indicates the decoupling susy parameters space ma > 2mz (adapted from ref. [11]). 42 http://dx.doi.org/10.14311/app.2014.01.0042 lhc, astrophysics and cosmology the particle spectrum of susy is extremely more crowded then the particle spectrum of sm. the present universe is constituted by matter fermions and force fields mediated by bosons, no susy particles has been identified. it means that at a time m−1 pl ≤ tsusy ≤ tew the supersymmetry has been broken, in the sense that masses of susy partners became m̃susy � msm . for this reason the lightest neutral supersymmetric particles are excellent candidates for dark matter [12]. the higgs sector of susy theories is more complicated then in the sm. in the minimal supersymmetric extension of the sm, the higgs sector is schematically:( h0 u h− ) ( h+ h0 d ) a0 (1) namely a cp-odd pseudo scalar field a0 and two doublets of scalar cp-even fields. whose neutral components have v.e.v., assumed to be 〈 h0 u 〉 = vu and〈 h0 d 〉 = vd normalized to the value of fermi constant v2 u + v2 d = √ 2gf , with a ratio parameterized as vu/vd = tanβ. the physical fields detectable at accelerator h and h are mixed states of the neutral components of the doublets with masses mh < mh , although it was never discarded the possibility to detect also the other states, especially the charged ones. if the mass of the pseudo-scalar ma and tanβ are taken as free parameters, the masses of the other four are fixed by the equations [13]: m2 h± = m2 a +m2 z + ∆m2 h± m2 h,h = 1 2 ( m2 a +m2 z ) + ∓ √ m2 a +m2 z − (2mamz cos 2β) 2 + ∆m2 h,h (2) where ∆m2 h±and ∆m2 h,h are the appropriated radiative corrections. fig. 1 shows the predictions of eq. (2) in which is clear that the light susy higgs mass saturates in the limit of decoupling ma � mz to mh → mz |cos 2β| + ∆m2 h. the actual value of this limit, that depends strongly from the radiative corrections, is estimated to be in the range 130-150 gev/c2 [15, 16]. fig. (2) from cms [14] shows the result of the search of higgs like particles decaying to pairs of gauge bosons, giving evidence that a susy high mass higgs with mass lower than 700 gev/c2 is excluded. in facts the total production cross sections for the neutral susy higgs is essentially the same of that expected in sm, but the dominant decay channels are h → (w±, z) with vector bosons in final state [11]. fig. 2 shows the results of the cms search of higgs with mh > 200 gev/c2 [14], that clearly excludes any mass mh ≤ 600 gev/c2 at 95% cl. figure 2: observed (solid line) and expected (dashed line) 95% cl upper limit on the ratio of the product of production cross section and branching ratio to the sm expectation for the higgs boson (from ref. [14]). 3 higgs boson in cosmology inflation is nowadays a well accepted paradigm in cosmology. the only problems is what is physically the “inflaton”. the original field considered as inflaton by guth was actually the higgs field [17], whose potential v (φ) = 1 4λφ 4 can naturally produce inflation if λ is small [18]. but this model predicts that the amplitude of density perturbation is δρ/ρ ∼ √ λ. therefore, in order to explain the observed δρ/ρ ∼ 10−4, it should be λ ∼ 10−10, absolutely irreconcilable with an higgs mass mh = √ 2λv, with v = (√ 2gf )− 1 2 given by the strengths of the weak interactions. a solution could be a coupling of the higgs field with the gravity [19], with a lagrangian density lgr+sm = ( 1 2 m2 pl + ξφ2 ) r+ 1 2 |∂µφ|2 − vsm (φ) (3) where mpl = (8πgn ) −1 is the reduced planck mass, r the ricci scalar, vsm (φ) = 1 4λ ( φ2 − v2 )2 is the potential of the sm higgs field and ξ 6= 0 its coupling to the gravity. applying a rescaling of the metric g̃µν = ( 1 + 2ξφ2/m2 pl )−1 gµν [20] the potential of the field becomes veff (φ) = λ 4 ( φ2 − v2 )2( 1 + 2ξφ2 m2 pl )2 (4) for the present small value of the higgs field φ � mpl/ √ 2ξ we have ṽ (φ) 'v(φ), while for φ & mpl/ √ 2ξ the effective potential of eq. (4), has a 43 giulio auriemma plateau that allows a successful slow-roll inflation. a first constraint on the coupling ξ is obtained from the amount of expansion of the universe reached at the end of the inflationary phase. the number of e-folding, defined from a(tf ) = a(ti) exp(−n) is predicted to be, using the slow-roll approximation [21], the higgs field that varies in the expansion from an initial value φi to a final one φf ≈ mpl/ √ 2ξ. the ratio is φi/φf ≈ √ 8n/3 for ξ � 1. the amplitude of anisotropy depends on the mass of the higgs boson, which fixes the value of the quartic coupling λ. the predicted scalar amplitude is δρ/ρ ≈ n π √ 18 √ λ/ξ. in order to justify the observed δρ/ρ ≈ 10−4 a strong coupling ξ & 104 is required. more refined calculations, including the non negligible radiative corrections to the higgs potential due the coupling with heavy quarks [22, 23], show that mh =126 gev/c2 is compatible with the spectral index of the power law for scalar perturbations ns = 0.962± 0.002 and the upper limit for the tensor-to-scalar ratio r < 0.11 (95% c.l.) measured by planck [24]. theoretical studies have confirmed that susy inflation is possible, in many different scenarios [25] either with minimal [26] or non-minimal coupling [27]. as noted by linde [28] we can expect that due to the rich structure of the higgs sector in susy theories, the effective potential of the superfield will show several minima, interleaved by maxima where v ′(φ)� v (φ), suitable for slow-roll inflation in the minimal coupling case ξ = 0. the case of a non minimal coupling ξ > 0, is shown in ref. [27]. in a recent paper nakayama & takahashi [29] examined a susy model in which the lightest neutral higgs boson could be identified with the sm-like one. figure 3: planck best-fit value of the spectral index and mh = 126 gev/c2 from lhc fits very well mssm inflaton model with small β (adapted from ref. [29]). 4 dark matter cosmological non-baryonic dark matter with ωdmh 2 = 0.120 ± 0.003(stat) ± 0.03(syst) has been estimated by planck [30]. figure 4: status of experimental direct search for wimps. the 68% cl region of possible positive detection are shown as filled area, while solid line represents 95% cl upper limits. the type of relics that can supply this density at “freeze out” should have a thermally averaged rate of annihilation at “freeze-out” (tf ' mdm/20) : 〈σav〉tf ' 2× 10−26 ( 0.12 ωdmh2 ) cm3s−1 (5) for mdm > 10 gev/c2 [31]. it is worth noticing that the dm particles will be non-relativistic at freeze-out with β ≈ 4 × 10−2, that is dubbed “cold dark matter” (cdm). in addition the annihilation cross section that produces a rate comparable to the one of eq. (5) is close to the weak scale σ ≈ g2 fm 2/16π for an hypothetical weakly interacting massive particle (wimp), not existing in the sm. the density of wimp’s in the solar neighborhood is estimated to be ρdmlocal = 0.39± 0.03 gev/cm3 and is flowing with a velocity ≈ 200 km/s. wimp of mass 10 gev/c2, scattering against nucleons with a cross section σsi ≈ 10−40 cm2, give 0.5 interactions/100 kgday in a suitable detector[32]. the present controversial status of the direct searches is summarized in fig. 4. this figure shows that the claims for positive detections [33, 34, 35] seem to cluster around a “low” mass value mwimp ≈ 10 gev/c2 and a “high” elastic scattering cross section σsi ≈ 0.5× 10−40 cm2/nucleon. particularly tantalizing is the claimed observation of a yearly modulation of the detected signal [36], that could be a clear signature of the association of the detected particles with the galactic halo. but unfortunately fig. 4 shows also that all but dama/libra experiments do not show evidence of a similar signal, even if it should be well inside their sensitivity [37]. 44 lhc, astrophysics and cosmology indirect searches for dm are based on the detection of radiation produced in the annihilation and decay of relic wimp’s. neutrino, gamma and antimatter astronomy are the basic tools of this search. from the eq. 5 it is possible to derive an order of magnitude of the annihilation rate. it is worth to stress however that this estimate is an average of σav over the thermal velocity distribution at tf � 1 gev, while annihilations in a galactic environment will take place at much lower temperature. moreover the source function of the astrophysical radiation will be qk = 〈σav〉 bkρ2/m2 in photons cm−3s−1 being bk the inclusive branching ratio to the sm particle k, ρ and m respectively the local energy in gev/c2 cm−3 and the mass of the wimp in gev/c2. the spectrum of the γ-rays (or neutrinos) produced is composed by a continuum, extending up to the kynematical limit eγ . m and several monochromatic lines, each corresponding to a two body final states. the latter very attractive signature was proposed since 1988 by the compton gamma ray obsevatory [38]. in facts the first indication of a possible dm component in the diffuse galactic γ-rays from the galactic plane was given by the egret spark-chamber calorimeter [39], that found a significant excess on the galactic plane for eγ ≥ 1 gev. the poor energy resolution of the egret calorimeter for hard γ-rays did not allowed any search for lines, but the intensity and distribution in the galactic frame of this radiation was found to be close to what expected for annihilation or decay of particles with mass in the range 50-100 gev/c2. in the case of wimp’s the γ inclusive annihilation rate is σγv ≈ 10−26 cm3s−1 [40]. the diffuse γ−rays emission from selected regions on the galactic plane has been measured with high energy resolution (∼ 8% in the range 1-300 gev) by the csi scintillator trackercalorimeter of the fermi-lat instrument [41, 42]. the observed γ-rays spectrum is fitted with a single power law e−αγ for eγ ≥ 12.6 gev, with α = 2.44±0.01. narrow lines of width compatible with the instrumental resolution have been searched using background+signal max-likelihood method. fermilat has not detected any statistically significant γ-ray line in the range from 5 to 300 gev. an upper limit of the rates σ2γv ≤ 0.02 − 3.6 × 10−27 cm3s−1, for wimp’s masses 5 ≤ m ≤ 200 gev/c2, can be estimated from the flux upper limit at 95% c.l., applying the most optimistic galactic halo wimp density profile from n-body simulations [43]. if a simple isothermal profile is assumed, the limit increases by 50%. this limit starts to be close to the predicted rate for susy dm candidates [44]. unfortunately with this new data the claimed line at ≈130 gev [45, 46] corresponding to σ2γv ≈ (1.1± 0.5) × 10−27 cm3s−1 has not gained statistical significance (≈ 3.3σ). an excess of positrons in the energy range 4 ≤ ee ≤ 50 gev in cosmic rays, was discovered by a balloonborne instrument launched in 1974 from palestine, tx, in the heroic age of astroparticle physics [47]1. it is remarkable that after about 40 years we have a high precision mesurement of the positron flux covering the range 0.5 ≤ ee ≤ 350 gev from the 8.5 tons particle’s spectrometer ams-02 installed on the international space station [48]. the ams-2 positron fraction vs. energy is first decreasing from 8.42% at 1 gev to a minimum of 5.1% at 7 gev then increasing up to ' 15% at 260 gev. an excellent fit to this behaviour can be obtained either from pair emission by pulsar[49] or wimps annihilation [50]. in the latter case the mass of the wimp should be in the range 750 gev ≤ m ≤ 1.5 tev and inclusive annihilation rate 10−23 ≤ σe+v ≤ 10−22 cm3s−1[52] significantly larger then the one of eq. 5 ref. [51]. the discriminating observation is definitively the detection of an excess in the antiproton flux, because in the annihilation to lepton pair and quarks one is strongly correlated. up to now the pamela data [53] do not support any deviation from cosmic rays secondary production. alternatively, by assuming that the excess of positrons is all due to astrophysical sources, ref. [54] finds an upper limit to the annihilation rate that varies from 10−26 cm3s−1 at m = 10 gev/c2 to 10−23 cm3s−1 at m = 1 tev/c2, assuming that the dominant annihilation channel is µ+µ−. 5 lhc search for dark matter candidates the lower mass state of the four spin 1 2 neutralinos χ0 1, χ 0 2, χ 0 3, χ 0 4, predicted in r-parity conserving susy models, is a good candidate for the role of wimp. other candidates do exists, as for example the superpartner of the graviton, the spin 3 2 gravitino, that would be a superwimp because it couples with ordinary matter only via the gravitational interaction, making direct dm detection practically impossible [55]. at hadronic colliders such as lhc only wimps that couple with protons, such as the neutralino, can be directly produced. the production of neutralino candidate can be tested, quasi-model independently, using the process: pp→ ( χ0 1χ̄ 0 1 ) +x (6) wherex can be one (or more) hadronic jet, hard leptons or photons, while the neutralino pair (if r-parity is conserved) do not interact with the detector, but uppears in the kinematics of the event as missing trans1note that the young member of this slac team will be the nobel laureate of 2006 for cobe. 45 giulio auriemma verse energy emisst [56]. the dominant sm physical background for the reaction (6) is pp → w±/z0 → ``, where the leptons are neutrinos or are not detected, that can be subtracted and/or reduced by optimized kinematical cuts. figure 5: upper limits on σsi (left) and σ2γv (right) inferred from monojet+emisst atlas data [57]. considering the simpler case that x is a single hadronic jet the cross section for neutralino pairs production, that contributes to process (6), will be σqq→χχ̄ ≈ αsg 2 χg 2 qp jet t /m4 ∗ where αs = 0.64 is the qcd constant, gχ, gq the susy couplings being q = q, q̄ or g and finally m∗ � pt is suppressing mass scale. the detection of a number of events significantly larger the the calculated sm background would be strong indication of a wimp candidate, whose mass could be inferred from the missing energy distribution. in addition a comparison with direct dm search experiments, discussed in the previous section, would be possible because the scattering cross section, from the same coupling, is predicted to be σsi ≈ g2 χg 2 qµ 2/m4 ∗ being µ the reduced mass of the wimp-nucleon system [58]. moreover the same suppression factor m−4 ∗ enters, together with phase space factor depending only from the masses, in the annihilation rate of the neutralino (see for example eq. (10) and (11) of ref. [59]) that allows comparison with the dm relic density and the indirect searches. fig. (5) shows the potentiality of this type of searches at lhc [57, 60]. figure 6: lhcb signal for b0 s → µ+µ− [62] (left) and presently cmssm allowed region (green) on the plane (m0,m1/2) [55] (right). important constraints on the supersymmetries, beyond the effective field theories discussed before, that can also set limits to the wimp candidates properties, are given by the search for rare decays of heavy neutral mesons like the b0 d and b0 s decays, copiously produced at lhc [61], performed by the lhcb experiment as well as the cms experiment. lhcb is the lhc experiment devoted to flavor physics. both lhcb and cms have measured the br(bs → µµ) which is an important test bench for new physics, because it strongly suppressed in the minimal smit is enhanced by susy. limit at 95% cl is br(bs → µµ)susy < br(bs → µµ)sm . 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3college of sciences, george mason university, fairfax, va; 4voss scientific, albuquerque, nm; corresponding author: beall@sjc.edu abstract we are developing the capability for a multi-scale code to model the energy deposition rate and momentum transfer rate of an astrophysical jet which generates strong plasma turbulence in its interaction with the ambient medium through which it propagates. we start with a highly parallelized version of the vh-1 hydrodynamics code (coella and wood 1984, and saxton, et al., 2005). we are also considering the pluto code (mignone et al. 2007) to model the jet in the magnetohydrodynamic (mhd) and relativistic, magnetohydrodynamic (rmhd) regimes. particle-in-cell approaches are also being used to benchmark a wave-population models of the two-stream instability and associated plasma processes in order to determine energy deposition and momentum transfer rates for these modes of jet-ambient medium interactions. we show some elements of the modeling of these jets in this paper, including energy loss and heating via plasma processes, and large scale hydrodynamic and relativistic hydrodynamic simulations. a preliminary simulation of a jet from the galactic center region is used to lend credence to the jet as the source of the so-called the fermi bubble (see, e.g., su, m., & finkbeiner, d. p., 2012) *it is with great sorrow that we acknowledge the loss of our colleague and friend of more than thirty years, dr. john ural guillory, to his battle with cancer. keywords: jets active galaxies blazars intracluster medium non-linear dynamics plasma astrophysics. 1 introduction recent high-resolution (vlba) observations of astrophysical jets (see, e.g., lister et al. 2009) reveal complex structures apparently caused by ejecta from the central engine as they interacts with both surrounding interstellar material such as broad-line region (blr) and narrow-line region (nrl) clouds, and ejecta from prior episodes of activity. a particularly trenchant example of these complex interactions is also shown by the galactic microquasar, sco x-1 (fomalhaut, geldzahler, and bradshaw, 2001). such observations can be used to inform models of the jet-ambient-medium interactions. based on an analysis of these data, we posit that a significant part of the observed phenomena come from the interaction of the ejecta with prior ejecta as well as interstellar material. 2 scales of jet interactions with the ambient medium large scale hydrodynamic simulations of the interaction of astrophysical jets with the ambient medium through which they propagate can be used to illuminate a number of interesting consequences of the jets’ presence. these include acceleration and entrainment of the ambient medium, the effects of shock structures on star formation rates, and other effects originating from ram pressure and turbulence generated by the jet (see, e.g., basson and alexander, 2002; zanni et al. 2005; and krause and camenzind 2003; perucho, et al. 2012). we have presented results for large scale hydrodynamic simulations and initial relativistic hydrodynamic simulations in previous works (beall et al., 1999, 2003, 2006). as noted in those papers, magneto-hydrodynamic (mhd) and (rmhd) simulations neglect important species of physics: the microscopic interactions that occur because of the effects of particle-particle interactions and the interactions of particles with the collective effects that accompany a fully or partially ionized ambient medium (i.e. a plasma). while the physical processes (including plasma processes) in the ambient medium can be modeled in small regions by pic (particle-in-cell) codes for some parameter ranges, simulations of the larger astrophysical jet structure with such pic codes are not possible with current or foresee274 http://dx.doi.org/10.14311/app.2014.01.0274 multiscale modeling of astrophysical jets able computer systems. for this reason, we have modeled these plasma processes in the astrophysical regime by means of a system of coupled differential equations which give the wave populations generated by the interaction of the astrophysical jet with the ambient medium through which it propagates. a detailed discussion of these efforts can be found, variously, in scott et al. (1980), rose et al. (1984), rose et al. (1987), beall (1990), and beall et al. (2003). the scales of these interactions range from kilometers to kiloparsecs. 2.1 energy loss, energy deposition rate, and momentum transfer from plasma processes the system of equations used to determine the normalized wave energy densities is very stiff. scott et al. (1980) estimated the equilibrium solution of this system of equations for heating of clusters of galaxies, and rose et al. (1984) and beall (1990) showed dynamical solutions that confirmed the stability of the equilibrium solutions. solving the system of equations yields a time-dependent set of normalized wave energies (i.e., the ratio of the wave energy divided by the thermal energy of the plasma) that are generated as a result of jets interaction with the ambient medium. these solutions can yield an energy deposition rate (de/dt), an energy deposition length (de/dx), and ultimately, a momentum transfer rate (dp/dt (1/vb) ∗ (de/dt) that can be used to estimate the effects of plasma processes on the hydrodynamic evolution of the jet. for this part of the analysis, we suppose that a portion of the jet is composed relativistic particles of either e±, p − e−, or more generally, a charge-neutral, hadron-e− jet, with a significantly lower density than the ambient medium. the primary energy loss mechanism for the electron-positron jet is via plasma processes, as beall (1990) notes. kundt (1987, 1999) also discusses the propagation of electron-positron jets. beall et al., (2006) illustrate two possible solutions for the system of coupled differential equations that model this mode of the jet-ambient medium interaction: a damped oscillatory and an oscillatory solution. the landau damping rate for the two-temperature thermal distribution of the ambient medium is used for these solutions. the average energy deposition rate, < d(αε1)/dt >, of the jet energy into the ambient medium via plasma processes can be calculated as < d(αε1)/dt >= npkt < w > (γ1/ωp)ωpergs cm −3s−1, where k is boltzmann’s constant, t is the plasma temperature, < w > is the average (or equilibrium) normalized wave energy density obtained from the wave population code, γ1 is the initial growth rate of the two-stream instability, and α is a factor that corrects for the simultaneous transfer of resonant wave energy into nonresonant and ion-acoustic waves. the energy loss scale length, deplasma/dx = −(1/nbvb)(dαε1/dt), can be obtained by determining the change in γ of a factor of 2 with the integration ∫ dγ = − ∫ [d(αε1)/dt]dl/(vbnbm ′ c2) as shown in rose et al., 1978 and beall 1990, where m ′ is the mass of the beam particle. thus, lp = ((1/2)γcnbmc 2)/(dαε1/dt) cm is the characteristic propagation length for collisionless losses for an electron or electron-positron jet, where dαε1/dt is the normalized energy deposition rate (in units of thermal energy) from the plasma waves into the ambient plasma. in many astrophysical cases, this is the dominant energy loss mechanism. we can therefore model the energy deposition rate (de/dt) and the energy loss per unit length (de/dx), and ultimately the momentum loss per unit length (dp/dx) due to plasma processes. figure 1: simulation using a highly-parallelized version of the vh-1 hydrodynamics code. the figure shows the transverse cross-section (figure 1a) and the detail of the jet head in the y-z cross-section (figure 1b) for a fully 3-dimensional hydrodynamic simulation of jet with v = 1.5x109 cm/sec. the cross section at z = 300 for a jet with v = 1.0x1010 cm/s. the cross section represents approximately 2 kiloparsecs from the jet origin. we have used the 5123 simulation in this representation for ease of presentation, given the size and difficulty of displaying the 20483 data. note the well-developed rayleigh-taylor instabilities at the jet-ambient medium boundary in figure 1a. beall, guillory, and rose (2009) have compared the results of a pic code simulation of an electronpositron jet propagating through an ambient medium of an electron-proton plasma with the solutions obtained by the wave population model code, and have found good agreement between the two results (see figure 1 from that paper). at the same time, that paper demonstrates that the ambient medium is heated and entrained into the jet. that analysis also shows that a relativistic, low-density jet can interpenetrate an ambient gas or plasma. initially, and for a significant fraction of its propagation length, the principal energy loss mechanisms for such a jet interacting with the ambient medium is via plasma processes (rose et al. 1984, beall 1990). 275 james h. beall et al. as part of our research into the micro-physics of the interaction of jets with an ambient medium, we continue to investigate the transfer of momentum from the jet, and expect to present these results shortly. in order to proceed to a more detailed analysis of the issue of momentum transfer, we have used modern pic code simulations to study the dynamics of caviton formation, and have confirmed the work of robinson and newman (1990) in terms of the cavitons’ formation, evolution, and collapse. 2.2 results of hydrodynamical calculations figure 1 shows two views of a simulation for an astrophysical jet for parameters appropriate to a seyfert galaxy, using a highly-parallelized version of the vh1 hydrodynamics code. the figure shows the transverse cross-section (figure 1a) and the detail of the jet head in the y-z cross-section (figure 1b) for a fully 3-dimensional hydrodynamic simulation of jet with v = 1.5x109 cm/sec. the cross section at z = 300 for a jet with v = 1.0x1010 cm/s. the cross section occurs at approximately 2 kiloparsecs from the jet origin. figure 2: simulation using the vh-1 code of a supersonic jet with parameters appropriate to the galactic center region. in this simulation, also, we use the vh-1 code for a supersonic jet in the x-y pressure cross section (1a) and with an x-y density cross section (1b) for a supersonic jet with v = 2.0x108 cm/s. such a jet originating from the supermassive black hole at the center of the milky way could account for the production of the gamma-ray structure known as the galactic center bubble found by the fermi satellite. the vertical scale of the jet represents approximately 2 kiloparsecs from the jet origin. we have used the 5123 simulation in this representation for ease of presentation, given the size and difficulty of displaying the 20483 data figure 2 shows a simulation of a supersonic jet for parameters appropriate to the galactic centre region. in this simulation, also, we use the vh-1 code for a supersonic jet in the x-y pressure cross section (1a) and with an x-y density cross section (1b) for a supersonic jet with v = 2.0x108 cm/s. such a jet originating from the supermassive black hole at the center of the milky way could account for the production of the gamma-ray structure known as the galactic center bubble found by the fermi satellite. the vertical scale of the jet in figure 2a represents approximately 2 kiloparsecs from the jet origin. we have used the 5123 simulation in this representation for ease of presentation. the detail of the jet-head structure (figure 2b) shows complex, transverse shock structures at the leading edge of the jet. the simulations we have run using 20483 cells confirm these features. 3 concluding remarks the effects of collective and particle processes, including plasma effects, can have observational consequences. beall (1990) has noted that plasma processes can slow the jets rapidly, and beall and bednarek (1999) have shown that these effects can truncate the low-energy portion of the γ-rays spectrum (see their figure 3), a similar effect will occur for particle-particle productions of neutrinos, pions, and (perhaps) neutrons. this could also reduce the expected neutrino flux from agn. the presence of plasma processes in jets can also greatly enhance line radiation species by generating high-energy tails on the maxwell-boltzmann distribution of the ambient medium, thus abrogating the assumption of thermal equilibrium. an analytical calculation of the boost in energy of the electrons in the ambient medium to produce such a high energy tail, with ehet ∼ 30 − 100kt, is confirmed by pic-code simulations. aside from altering the landau damping rate, such a high-energy tail can greatly enhance line radiation over that expected for a thermal equilibrium calculation (see beall et al. 2006, and beall, guillory, and rose (1999) for a detailed discussion). we are in the process of setting up runs for the pluto code (mignone et al. 2007) in order to benchmark its results with our results of the vh-1 code in appropriate parameter ranges. acknowledgement jhb and mtw gratefully acknowledge the support of the office of naval research for this project. thanks also to colleagues at various institutions for their continued interest and collaboration, including kinwah wu, curtis saxton, s. schindler and w. kapferer, and s. colafrancesco. references [1] basson, j. f., and alexander, p., 2002, the longterm effect of radio sources on the intracluster medium, mnras, 339, 353. 276 multiscale modeling of astrophysical jets [2] beall, j. h. et al., 1978, radio and x-ray variability of the nucleus of centaurus a (ngc 5128), ap.j., 219, 836. doi:10.1086/155845 [3] beall, j. h. and rose, w. k., 1981, on the physical environment in 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[15] lister, m. l. et al., 2009, astronom. j., 137, 3718. doi:10.1088/0004-6256/137/3/3718 [16] mignone, a. et al., 2007, ap.j. suppl., 170, 228. [17] newman, d. l., winglee, r. m., robinson, p. a., glanz, j., and goldman, m. v., 1990, simulation of the collapse and dissipation of langmuir wave packets, phys. fluids b, 2, 2600. doi:10.1063/1.859385 [18] perucho, m., 2012, jets in high-mass microquasars, mem. s. a. it., 83, 297. [19] rose, w. k., guillory, j., beall, j. h., and kainer, s., 1984, the interaction of relativistic chargedparticle beams with interstellar clouds, ap.j., 280, 550. [20] rose, w. k., beall, j. h., guillory, j., and kainer, s., 1987, radiation from relativistic beams interacting with interstellar gas, ap.j., 314, 95. doi:10.1086/165042 [21] saxton, c. j., bicknell, g. v., sutherland, r. s., and midgley s., 2005, interactions of jets with inhomogeneous cloudy media, mnras, 359, 781. doi:10.1111/j.1365-2966.2005.08962.x [22] scott, j. h., holman, g. d., ionson, j. a., and papadopoulos, k., 1980, the heating of gas in clusters of galaxies by relativistic electrons collective effects, ap.j., 239, 769. [23] su, m., and finkbeiner, d. p., 2012, evidence for gamma-ray jets in the milky way, ap.j., 753, 61. doi:10.1088/0004-637x/753/1/61 [24] zanni, c. et al., 2005, heating groups and clusters of galaxies: the role of agn jets, astron. astrophys., 429, 399. discussion manel perucho could you comment on the viscosity of the bow shock in your 3-d simulations? what is causing it? jim beall the viscosity is likely due to turbulence generated by the jet-ambient medium interaction and from internal shock structures within the jet and in the jet interface with the interstellar medium. gennady bisnovatyi-kogan was a magnetic field included in your numerical simulations? jim beall the vh-1 code we are using is a hydrodynamic code with no capability for the inclusion of magnetic fields. we can estimate the magnetic field strength by assuming equipartition with the gas pressure, but this is not entirely satisfactory. we want to explore the pluto code, which has an mhd capability, in the near future. 277 http://dx.doi.org/10.1086/155845 http://dx.doi.org/10.1007/978-94-009-0545-0_20 http://dx.doi.org/10.1016/0021-9991(84)90143-8 http://dx.doi.org/10.1086/322479 http://dx.doi.org/10.1016/s1387-6473(03)00096-4 http://dx.doi.org/10.1088/0004-6256/137/3/3718 http://dx.doi.org/10.1063/1.859385 http://dx.doi.org/10.1086/165042 http://dx.doi.org/10.1111/j.1365-2966.2005.08962.x http://dx.doi.org/10.1088/0004-637x/753/1/61 introduction scales of jet interactions with the ambient medium energy loss, energy deposition rate, and momentum transfer from plasma processes results of hydrodynamical calculations concluding remarks acta polytechnica ctu proceedings doi:10.14311/app.2017.13.0039 acta polytechnica ctu proceedings 13:39–42, 2017 © czech technical university in prague, 2017 available online at http://ojs.cvut.cz/ojs/index.php/app on development of the interactive design tool based on isogeometric analysis edita dvořáková∗, bořek patzák czech technical university in prague, faculty of civil engineering, thákurova 7, 166 29 prague 6, czech republic ∗ corresponding author: edita.dvorakova@fsv.cvut.cz abstract. isogeometric analysis is a new concept of finite element method which has been proposed to bridge the gap between the cad systems and the fem solvers. in isogeometric analysis, the same basis functions, typically splines or nurbs, are used for geometry description as well as for approximation of unknowns and thus the same model can be shared between cad and iga systems. this results in a higher accuracy and overall efficiency of the analysis. many isogeometric elements have been already proposed and implemented into the existing finite element solvers, but the automatic connection with a cad system is usually missing. our goal is to develop such a connection and to provide a tool which would interactively run the analysis when the model changes. this approach can enormously enhance the design process as it can provide the basic knowledge about the structural behavior already in conceptual design phase. keywords: curved beam element, discrete shear gap, interactive design, isogeometric analysis. 1. introduction finite element solvers are the integral part of a nowadays structural design. once the architectural model is developed in cad (computer-aided design) system, the structure is further analyzed in fem (finite element method) solver. there are two independent models (cad model and fem model) and thus when one of them changes during the design process the other has to be correspondingly (and usually manually) adjusted, leading to the higher time-consumption. moreover, the inaccuracies in geometry description are usually introduced as the cad and fem models use different functions for geometry representation. the standard finite element method usually uses polynomial basis functions, while the cad systems are based on splines. the use of spline functions for geometry and unknown approximations in finite element analysis enables to share one model between architectural design and structural analysis. this idea has been proposed by huges et al. [1] and it is called isogeometric analysis (iga). since 2005, when isogeometric analysis has been introduced, many researchers have been focusing on its development and the results proved the advantages of iga over standard fem in many fields. our focus is placed on its application for the analysis of curved beams as the curved geometries can especially benefit from the isogeometric approach. we use the formulation of curved beam element presented by bouclier et al. [2] with the locking treatment based on discrete shear gap (dsg) method [3, 4]. the element has been implemented into oofem finite element solver [5] and our main goal is to develop a connection between cad system and the solver to provide a tool encapsulating the analysis within the architectural design environment. 2. curved beam element the presented two-dimensional curved beam element is based on timoshenko beam theory. it has three independent unknowns, axial displacement ut, transverse displacement un and rotation θ. a strain-displacement matrix b defined as ε = br, (1) where ε = {εm, γs, χb}t and r = {ut, un, θ}t , is derived from the relations for membrane strain εm, transverse shear strain γs, and bending strain χb εm = u′t − un r , γs = ut r + u′n − θ, χb = θ′, (2) where s ∈ 〈0, l〉 runs along the midline of a beam and prime indicates a derivative with respect to s. by omitting the terms which are divided by radius of curvature r the straight beam formulation is obtained. a material matrix d results from n = eaεm, q = kgaγs, m = eiχb, (3) where n,q and m are axial force, transverse shear force and bending moment, respectively. young’s modulus e, shear modulus g, shear coefficient k, area a and moment of inertia i are the material and 39 http://dx.doi.org/10.14311/app.2017.13.0039 http://ojs.cvut.cz/ojs/index.php/app edita dvořáková, bořek patzák acta polytechnica ctu proceedings cross-section characteristics. a stiffness matrix is then calculated as k = ∫ l 0 bt db ds. (4) to evaluate the stiffness matrix the gaussian quadrature involving p+ 1 gauss points over each knotspan is used, where p is a degree of approximation. 2.1. nurbs-based formulation in case of iga, the langrange polynomial basis functions used in fem are replaced by splines. the presented element is formulated using non-uniform rational b-splines (nurbs) [6], which is the most widespread and developed technology in cad industry. a nurbs curve is defined by its degree, control points with weights, and knotvector. nurbs basis functions are generated by weighting the b-splines functions, which are piecewise polynomial functions and are a special subset of nurbs functions. nurbs curve is given as the linear combination of basis functions and control points coordinates. the knotvector is a set of non-decreasing parametric coordinates (knots) which influence the mapping. the knots divide a parametric space into knotspans (usually referred to as “elements” in iga) and each knotspan is influenced only by limited number of control points. therefore the knotspan defines which control points affect particular part of the domain. see figure 1 for an example of a nurbs curve geometry. very important aspect of isogeometric analysis is a non-interpolatory nature of the basis functions, as the nurbs do not satisfy kronecker-delta property. another distinct feature of the nurbs basis functions is the higher inter-element continuity. while only c0 continuity is provided by traditional polynomial functions, in case of iga up to cp−1 continuity can be achieved. by repeating knots in a knotvector, the continuity at that knot can be artificially reduced. 2.2. locking treatment shear locking is a well-known problem of standard timoshenko beam elements and this phenomena persists also in isogeometric formulation. this problem occurs because the displacements and rotation are treated independently and approximated by the functions of the same order. from eq. 2 for shear and bending strains it can be seen that formula for bending strain results in lower order term than formula for shear strain, but actually this should be vice-versa. also the field inconsistency within the term for shear strain causes that zero shear strain cannot be satisfied along entire patch when the same order interpolation of unknowns is used. several locking removal techniques have been proposed to unlock isogeometric beam element including reduced integration, b̄-method, dsg method, see [2, 3] for the references. in this study, the dsg-based formulation is used. while this method can increase computational cost (as well as b̄-method), it has a big advantage over reduced integration: for the reduced integration the recovered strains can be evaluated only at gauss points, dsg method enables to evaluate them along the entire patch. discrete shear gap method has been originally developed for the standard finite elements [3] but echter and bischoff [4] have extended its use also for isogeometric analysis. the approach can be divided into several steps yielding the modified b matrix. the idea is not to satisfy the equation γs = ut r + u′n − θ (5) pointwise, but in integral sense. the shear contribution uγhi n (called “shear gaps”) to the deflection is obtained by integration of eq. 5 as uγ hi n = ∫ si 0 γhs ds = ∫ si 0 ut r + u′n − θ ds = bdsgr, (6) at collocation points si calculated as greville abscissa of the control points [6]. modified transverse displacements are interpolated using nurbs basis functions ni uγ modh n = n∑ i=1 niũ γhi n . (7) please note, that discrete shear gaps ũγhi n are noninterpolatory in isogeometric analysis. to expressed the discrete shear gaps by values at the control points the transformation matrix a is derived{ uγ h n } = a { ũγ h n } , aij = nj(si), (8) where { uγ h n } are interpolatory values of shear gaps at control points and nj(si) is the jth-basis function evaluated at ith-collocation point. the modified shear strain is then given as γmod h s = n∑ i=1 n ′i ũ γhi n . (9) combining eqs. 6-9 results in the modified straindisplacement matrix b b = n ′a−1bdsg, (10) which is used to evaluate stiffness matrix k (eq. 4). 3. interactive design tool thanks to the isogeometric formulation, the presented element can be easily connected to cad system. the possibility of seamless connection between cad and fea is the major benefit of iga. this goal can be achieved even without use of isogeometric approach (see e.g. [7]), nevertheless the costly transformations 40 vol. 13/2017 on development of the iga based interactive design tool knots control points s = l s = 0 s ξ ξ1 = 0 ξ2 = 0 .5 ξ3 = 1 knot span patch y x figure 1. description of nurbs geometry. between cad and fem are necessary in such a workflow. firstly, the geometrical model is completed by analysis data, such as loads, boundary conditions and material characteristics, then the model is transformed into suitable computational model and the analysis mesh is generated, and finally the analysis is performed in fem solver. to visualize the results within the cad system the model has to be transformed back to the original model. with the use of isogeometric analysis, once the model is completed with analysis data, the analysis can be performed directly [8] and no transformation is needed for both the analysis and the visualization. for our purposes, the use of cad system rhinoceros (rhino) [9] has been chosen. the geometry representation of rhino is based on nurbs, moreover rhino enables plug-in development and use of built-in tool grasshopper [10]. grasshopper is a visual programming interface within rhino which can directly access nurbs geometry. in addition, visual programming is intelligible even to a user with no programming background, so the user can define additional analysis data at this level. the workflow of the tool is illustrated in figure 2. firstly, the nurbs geometry specified by a user in rhino is passed to the grasshopper, where the model is completed with boundary conditions and material and crosssection characteristics. currently, these data are provided in the text format similar to one used in oofem, but in the next phase of the development, the special plug-ins can be developed. once all the analysis data are collected, the python script within grasshopper together with other available grasshopper tools directly passes these data to oofem and run the analysis. when the analysis is finished, the output file is uploaded back to the grasshoper, where the visualization of the results is carried out. schematic illustration of grasshopper environment with results visualized in rhino can be seen in figure 3. very important feature of the developed tool is, that the grasshopper (and in turn also oofem) recomputes the results immediately when something changes. no matter whether the user adjusts the geometry model results analysis data geometry material, crosssection boundary conditions analysis results visualisation rhinoceros 3d grasshopper grasshopper oofem figure 2. interactive design tool. geometry or changes the boundary conditions, the results are interactively updated. this provides the immediate knowledge about behaviour of the designed structure and eases the process of finding optimal design. in the future work the designer will receive graphical indication of sections where load-bearing capacity or stability limits are achieved. such an approach would allow to establish viable conceptual design from both architectural and structural point of view without the necessity of understanding details of structure behaviour. 4. conclusions the formulation of the curved beam element with locking treatment has been presented and the element has been implemented into oofem finite element code. thanks to the isogeometric formulation, the automatic connection of cad system and the analysis has been provided or modified. the developed tool enables to display the results in rhino immediately 41 edita dvořáková, bořek patzák acta polytechnica ctu proceedings figure 3. schematic illustration of grasshopper environment with results visualized in rhino. the example corresponds to a cantilever circular arc beam (black) subjected to horizontal and vertical load at its tip. the deformed geometry is in red and the bending moment is in blue. after the geometry and analysis data are provided. this gives the user instant knowledge about behaviour of the designed structure and enables to see differences of alternative designs. acknowledgements the financial support of this research by the grant agency of the czech technical university in prague (sgs project no. sgs17/043/ohk1/1t/11) is gratefully acknowledged. references [1] t. j. r. hughes, j. a. cottrell, y. bazilevs. isogeometric analysis: cad, finite elements, nurbs, exact geometry and mesh refinement. comput methods appl mech engrg 194:4135–4195, 2005. [2] r. bouclier, t. elguedj. locking free isogeometric formulations of curved thick beams. comput methods appl mech engrg 245-246:144–162, 2012. [3] k. bletzinger, m. bischoff, e. ramm. a unified approach for shear-locking-free triangular and rectangular shell finite elements. computers and structures 75:321–334, 2000. [4] r. echter, m. bischoff. numerical effiency, locking and unlocking of nurbs finite elements. comput methods appl mech engrg 199:374–382, 2010. [5] b. patzák. oofem project home page, 2017. http://www.oofem.org. [6] l. piegl, w. tiller. the nurbs book. springer-verlag berlin heidelberg, new york, 1997. [7] l. svoboda, j. novák, l. kurilla, j. zeman. a framework for integrated design of algorithmic architectural forms. advances in engineering software 72:109–118, 2014. [8] m. hsu, c. wang, a. herrema, et al. an interactive geometry modeling and parametric desigh platform for isogeometric analysis. computers and mathematics with applications 70:1481–1500, 2015. [9] rhinoceros, 2017. http://www.rhino3d.com/. [10] grasshopper, 2017. http://www.grasshopper3d.com/. 42 http://www.oofem.org http://www.rhino3d.com/ http://www.grasshopper3d.com/ acta polytechnica ctu proceedings 13:40–43, 2017 1 introduction 2 curved beam element 2.1 nurbs-based formulation 2.2 locking treatment 3 interactive design tool 4 conclusions acknowledgements references 13 acta polytechnica ctu proceedings 1(1): 13–19, 2014 13 doi: 10.14311/app.2014.01.0013 γ-rays, ν’s and particle astronomy as messengers of the universe maurizio spurio1 1dipartimento di fisica e astronomia università di bologna and infn corresponding author: spurio@bo.infn.it abstract observations of charged cosmic-rays, γ-rays and neutrino are possible thanks to the availability of new detectors coming from technologies typical of experimental particle physics. in conjunction with more traditional techniques used by astronomers, these multimessenger correlations of experimental data are opening a new scenario in astrophysics. we review some of the more recent developments in the field. keywords: cosmic-rays γ-rays neutrinos satellites ground-based experiments neutrino telescopes. 1 introduction the multimessenger inter-correlation between cosmic rays (crs), γ-rays and neutrinos is of fundamental importance for a deeper comprehension of high energy (he) processes in astrophysical sources. one of the main questions in astrophysics is the origin and nature of crs (§2), whose energy spectrum extends up to above 1020 ev. there are many indications of the galactic origin of the cr bulk (protons and other nuclei up to ∼ 1016 ev). due to the influence of galactic magnetic fields, charged particles detected on earth do not point to the sources. gamma-rays and neutrinos do not suffer the effect of magnetic fields: they represent the decay products of accelerated charged particles but cannot be directly accelerated. the possibility of particle astronomy rely on the detection of these neutral probes, or to the detection of the highest component of charged crs (see castellina, these proceedings). recent advances on gev and tev γ-ray astronomy by satellites and ground-based experiments (§3) have led to the discovery of different classes of galactic and extragalactic objects that can be shed light on the cr acceleration mechanisms. most objects observed in the γ-rays (see fig. 1) are known sources from measurements of their electromagnetic emission at different wavelengths, in particular in the radio and in the x-rays. accelerated protons will interact in the surroundings of the crs emitter with photons predominantly via the ∆+ resonance: p+ γ → ∆+ → πo + p or → π+ + n (1) protons will interact also with ambient matter (protons, neutrons and nuclei), giving rise to the production of charged and neutral mesons. the relationship between sources of γ-ray and neutrinos is the meson-decay channel. neutral mesons decay in πo → γγ while charged mesons decay in neutrinos: π+ → νµ + µ+ ↪→ µ+ → νµ + νe + e+ π− → νµ + µ− ↪→ µ− → νµ + νe + e− (2) in particular circumstances, the energy escaping from the source is distributed between crs, γ-rays and neutrinos. only the coincident measurement of neutrinos (§4) from the source would give a uncontroversial proof of the discovery of the galactic acceleration sites of protons and nuclei. figure 1: sketch of the production of crs, neutrinos, γ-rays and lower energy photons from an astrophysical jet. the highest energy crs are probably originated from extragalactic sources. if they are protons with e > 5 × 1019 ev, they could interact with the cosmic microwave background radiation. this process restricts the origin of highest energy protons seen on earth to a small fraction of the universe, a sphere of the order of 13 http://dx.doi.org/10.14311/app.2014.01.0013 maurizio spurio 100 mpc (the greisen-zatsepin-kuzmin suppression). the prediction of a diffuse flux of he neutrino of extra-galactic origin is a direct consequence of the ultra he cr observations. this connection between crs, neutrinos and γ-rays was used to put upper bounds on the expected neutrino flux from extragalactic sources, since the neutrino energy generation rate will never exceed the generation rate of high energy protons. in this paper, we will review some of the recent updates in these fields. 2 crs and antimatter figure 2: cr intensity as a function of the energy from 1010 to 1020 ev. below few gev, the contribution of particles coming from the sun is not negligible. the energy range of the flux measured by direct experiments is reported as blue line; that measured by indirect experiments as red line crs are mainly protons (fig. 2) and heavier nuclei which are constantly hitting the upper shells of the earth’s atmosphere (blümer et al, 2009). up to energies of ∼ 1014 ev, the cr spectrum is directly measured above the atmosphere. stratospheric balloons and satellites have provided the most relevant information about the composition of crs. above ∼ 1014 ev, cr measurements are only accessible from groundbased large area arrays. recent observations of tev photons seem to confirm the diffusive shock acceleration (dsa) model, where the shocks powered by supernova explosions in our galaxy provide the iterative scattering mechanism for crs acceleration up to ∼ 1016 ev (see hillas, 2005). the key feature of this process is that an energy powerlaw spectrum of the type ∼ e−2 is produced. the dsa model is consistent with the balance between the energy transferred to the accelerated particles and the energy loss due to the escape of crs out of the galaxy. the e−2 emission at sources, when corrected for energydependent leakage from the galaxy, is in agreement with the observed cr energy spectrum. the details of acceleration mechanism and propagation of cosmic rays at higher energies are not completely understood. an important open item is the presence of antimatter in the radiation. equal amounts of matter and antimatter should have been produced at the beginning of the universe. the fact that there seems to be only matter around us is one of the major open problems in cosmology and in particle physics. the antiprotons, as well as the positrons, are a component of the cosmic radiation being produced in the interaction between crs and the interstellar matter. being exactly the same as particles except for their opposite charge sign, antiparticles are readily distinguished as they bend in opposite directions in magnetic field. magnetic spectrometers provide a clear and simple particle/antiparticle separation and probe the existence of antimatter in our galaxy. figure 3: the positron ratio measured by the pamela, fermi and ams-02 as presented in the first public presentation of ams-02 results. the expectation from the production of positrons as secondary particles is shown as dashed line. the discrepancy below 10 gev is explained by different phases of solar activity when data were collected. from http://ams.cern.ch/ an important feature in the e+ spectrum was observed by the pamela satellite and confirmed with the high precision measurement of ams-02. launched in 2011, the ams-02 experiment started immediately to take date on the international space station (iss). it is the largest particle physics detector never carried outside the earth atmosphere. the first results were presented in april, 2013 and refer to the measurement of positron in the crs. the e+ fraction in the e− + e+ flux found in the cosmic radiation increases steadily be14 γ-rays, ν’s and particle astronomy as messengers of the universe tween 10 gev to ∼ 250 gev, see fig. 3. at energies above 250 gev, the spectrum appears to flatten but to study the behavior at higher energy more statistics is required. the positron fraction spectrum exhibits no spatial anisotropy, structures or time dependence. these measurements have stimulated a large debate, as they cannot in fact be understood by models describing the production of secondary crs during propagation in the galaxy (see dashed line in fig. 3). several explanations have been proposed to interpret the observed excess: an astrophysical origin, such as nearby pulsars or microquasars, or exotic sources, as for instance the annihilation of dark matter particles in our galaxy. the p/p ratio (∼ 10−5 ÷ 10−4) shows that the antiproton flux is in overall agreement with a pure secondary component: antiparticles are produced during the interaction of crs with the interstellar matter. on the contrary, the ratio e+/e− ∼ 0.1 seems indicate that most of the detected electrons are of primary origin. due to energy loss processes, the majority of he electrons must be originated by sources closer than a few hundred pc. he electrons really probe cr production and propagation in the nearby region of our galaxy. in the next years, the ams-02 experiment will undoubtedly be the leading experiment for a systematic study of the crs through direct measurements, for the searches of antimatter in space, and for the searches for particles originated by dark matter annihilations. 3 high energy γ-rays he astrophysical processes producing relativistic particles are in most cases associated with the production of γ-rays in a wide range of energies. starting from the egret satellites in the ’90th, a new window has been opened in the observation of photons above ∼ 100 mev 1. the egret catalog consists of about 270 galactic and extragalactic objects. the present generation of space-based gamma ray telescopes, agile and lat, has opened a new era with thousands of galactic and extragalactic sources. the large number of sources and the high data quality are producing a deeper insight into the understanding of the processes of acceleration and radiation of non-thermal particles in the universe at sub-tev energies. as the flux of γ-rays decreases with increasing energy, space-based observations of most sources are limited to less than 100 gev. gamma-rays above 100 gev 2 are detected on ground, using extensive air shower (eas) particle detectors or the imaging atmospheric-cherenkov technique (iact). tev γ-rays are absorbed when reaching the earth atmosphere, and the absorption process proceeds by creation of a cascade of secondary particles. these emit cherenkov radiation, at a characteristic angle in the visible and uv range, which passes through the atmosphere. as a result of cherenkov light collection by a suitable mirror in a camera, the showers can be observed on the surface of the earth. the comparisons between the main features of space-based, iacts, eas and neutrino telescopes (§4) are presented in table 1. table 1: characteristic of space-based, iacts, eas and neutrino telescopes. from the top raw: energy range; effective area; the background rejection power; the angular and energy resolutions; the characteristic aperture of the telescopes; the duty cycle. space iact eas ν-tel energy (tev) 10−4-0.3 0.5-100 0.5-50 1-1000 area (m2) 1 104 104 0.1-300 bck rejec. > 99% > 99% 95% 99% angular resol. 0.5◦ 0.05◦ 0.7◦ 0.5◦ ∆e/e 10% 15% 50% 40% aperture (sr) 2.7 0.003 1.8 2π duty cycle 85% 10% 95% 95% 3.1 he γ-rays on satellites the large area telescope (lat) on the fermi satellite (launched in june 2008) is a γ-ray detector designed to distinguish photons in the energy range 20 mev to more than 300 gev from the high background of energetic charged particles. the γ-ray sky (unlike at other wavelengths) is strongly dominated by diffuse radiation originating in our galaxy by cr interactions with the interstellar gas and photon fields through the processes of inelastic nucleon scattering, bremsstrahlung, and inverse compton scattering. the lat mapping of the galactic γ-rays offers a way to derive information about the spatial distribution of crs and matter. the second catalog of high-energy γ-ray sources (2fgl) detected by the lat (nolan p.l. et al., 2012) derives from data taken during the first 24 months of the science phase of the mission, which began on 2008 august 4. the 2fgl catalog contains 1873 sources detected and characterized in the 100 mev to 100 gev. it includes source location regions, energy dependence of the flux in terms of a power-law with an exponentially cutoff and light curves on monthly intervals for each source. among the 1873 sources, 127 are firmly identified and 1171 reliably associated with counter1we indicate with he γ-rays the photons between 0.02-100 gev 2we indicate with vhe γ-rays the photons between 100 gev-100 tev 15 maurizio spurio parts of known or likely γ-ray producing source classes, see fig. 4. agn, and in particular blazars, are the most prominent class of associated sources: 917 sources are associated, of which 894 are blazars, 9 are radio galaxies, 5 seyfert galaxies. normal galaxies are now established as a class of γ-ray emitters and 7 2fgl sources are associated with such objects. pulsars have been traditionally studied through radio astronomy methods, with about 1800 pulsars found beaming radio waves. however, most of their radiation (a few percent of their spin-down power) is emitted at high-energies. in the last few years, the number of pulsars detected in the γ-ray has increased from half a dozen to more than 150 thanks to the agile satellite and the fermi-lat. among the pulsar, the crab plays a fundamental role, as it is the strongest γ-ray source in our field of view. the crab was since recently thought to be a steady source of radiation, from optical to tev energies. agile unexpected has discovered in 2010 (tavani et al., 2011) strong and rapid γ-ray flares from the crab nebula over daily timescales. this observation have changed the understanding of this cosmic object, and challenged emission models of pulsar wind and particle acceleration processes. pulsed γ-ray emission above 100 gev and up to 400 gev was recently detected from the crab with the veritas and magic iacts. figure 4: fermi-lat 2fgl full sky map (top) and blowup of the inner galactic region (bottom) showing sources by source class. identified sources are shown with a red symbol, associated sources in blue. about 70 sources of the 2fgl catalog are associated with pulsar wind nebulae. supernova remnants are a special class because a substantial number of the known objects are sufficiently extended to be potentially resolved with the lat. these objects are particularly interesting because they can represent regions of cr acceleration. in total, twelve sources in the catalog are modeled as spatially extended. 3.2 vhe γ-rays at ground one of the most recent and remarkable achievements in astrophysics is the discovery of more than 110 galactic and extragalactic sources of vhe radiation (see for a review rieger et al., 2013). at present, seven shell-type snrs have been firmly identify as vhe γ-ray emitters the pioneering ground based γ-ray experiment was built by the whipple collaboration. during the last decade, several ground-based γ-ray detectors were developed, both in the north and south earth hemisphere. at present, the new generation apparatus are the h.e.s.s. and veritas telescope arrays and the magic telescopes. these iacts have produced a catalogue of tev γ-ray sources which is continuously updated and available at http://tevcat.uchicago.edu/. electron can produce he γ-rays through the socalled leptonic model. synchrotron radiation from radio to the x-ray band is originated by accelerated electrons moving in the source magnetic fields. these particles can also produce gev-tev γ-rays through inverse compton scattering on the produced radiation field, or on external radiation fields. therefore, measurements of the synchrotron x-ray flux from a source can constrain the predictions on the accompanying γ-rays produced in leptonic processes. accelerated hadrons could either produce γ-rays via interaction with ambient matter or photon fields with sufficient high density. in this case, γ-rays are produced by the decay of neutral mesons while ν’s are produced by the decay of charged mesons. this γ-rays and neutrino production refer to a so-called astrophysical hadronic model. in this framework, the energy spectrum of secondary γ and ν particles follows the same power law of the progenitor crs. both the leptonic and the hadronic models, or a combination of them, could provide an adequate description of the present experimental situation. if high energy photons are produced in the hadronic models, high energy neutrinos will be produced as well. most of observed tev γ-ray galactic sources have a power law energy spectrum e−αγ , where αγ ∼ 2.0÷ 2.5. the values of the spectral index are very close to the expected spectral index of cr sources, αcr. this lead to the conclusion that sources of tev γ-rays can also be the sources of galactic crs. 16 γ-rays, ν’s and particle astronomy as messengers of the universe 4 neutrino astrophysics the main detection signatures for neutrinos in a neutrino telescope (see chiarusi & spurio, 2010) are long, straight tracks and approximately spherical cascades. the former are created by neutrino-induced muons while the latter are produced by neutrino-induced electromagnetic and/or hadronic showers. charged particles emit cherenkov light. from the measured arrival time of the cherenkov light, the direction of the neutrino can be derived. the accurate measurement of the νµ direction (up to 0.3o in water) could allows the association with (known) sources. because the mechanisms that produce crs can produce also neutrinos and γray, potential neutrino sources are in general also γ-ray emitters. due to the production mechanism (eq. 2), the flavor ratio at sources is νe : νµ : ντ = 1 : 2 : 0, which is changed by the neutrino oscillations to 1 : 1 : 1 on earth. figure 5: neutrino telescopes searches for an excess of events over the irreducible background of atmospheric neutrinos. the points represent the measurements of atmospheric νµ spectrum by three telescopes. the red curve represent the still unmeasured contribution of prompt neutrinos from charmed mesons decay. the green lines are the waxman& bahcall (1999) upper bound from diffuse flux of neutrinos from extragalactic sources. the blue line the possible contribution of ν from the gzk effect. some of the most promising candidate neutrino sources in our galaxy are extremely interesting, due to the recent results from tev γ-ray detectors. a neutrino telescope in the mediterranean sea is looking at the same southern field-of-view as the h.e.s.s. including the galactic center. the small interaction cross section of neutrinos allows them to come from far away, but it is also a drawback, as their detection requires a large target mass. assuming the present boundaries arising from γ-rays sources, the challenge to detect galactic neutrinos is open for a multi kilometer-scale apparatus, see fig. 5. also the extragalactic crs-neutrinos connection sets the scale of the detectors to 1 km3. icecube (http://icecube.wisc.edu/) is a 1 km3-scale neutrino detector buried in the antarctic ice. it comprise 86 strings, with 5160 photomultiplier tubes (pmt). each string includes 60 digital optical modules. the deepcore infill array to icecube reduces the energy threshold of icecube to energies as low as 10 gev. in water the antares collaboration (ageron et al, 2011) has completed in 2008 the construction of the largest neutrino telescope (∼ 0.1 km2) in the northern hemisphere. galactic sources. although much smaller than icecube, antares is advantaged by its geographical location for the study of galactic sources. to give a figure of merit, the tev photon flux from a possible neutrino candidate source as rx j1713.7-3946 is e2 γφγ ' 10−11 tev cm−2 s−1. assuming the same flux from neutrinos and no he cutoff, few events/year are expected in a 1 km3 telescope over the background of atmospheric neutrinos in a ∼ 1◦ search cone. the sensitivity of antares for 1000 days of livetime is ∼ e2 νµφsensνµ ' (3 − 8) × 10−11 tev cm−2 s−1 in the declination range from δ = −90◦ to +43◦. in total 51 possible neutrino sources have been studied. none of them shows a significantly excess of tev events over the expected background. figure 6: expected neutrino spectra from full numerical neucosma (red) and simply analytic guetta (blue) models. limits on these predictions are shown in the energy ranges where 90% of the flux (dashed lines) is expected. the icecube limit on the neutrino emission (black dashed) is based from 300 grbs. the antares limit on 296 grbs (red dashed) takes into account the neutrino oscillation effect. ν from grbs. gamma-ray bursts (grbs) are short and very intense flashes of he γ-rays, which occur unpredictably and isotropically over the sky. in model describing the γ-ray emission, protons can also be shockaccelerated, yielding secondary emission of he ν’s accompanying the electromagnetic signal. the detection of neutrinos in coincidence with a grb would be unambiguous proof for hadronic acceleration in cosmic sources. 17 maurizio spurio fig. 6 shows the result from these searches. the stringent icecube limit (black dashed line, abbasi et al., 2012) was a factor of 3.7 below the predictions made using the guetta et al. (2004) model (blue line in fig. 6), creating tension between the nonobservation of a signal and the prevailing models for neutrino emission from grbs. this could either indicate the need for rejection of these models or for more detailed modeling of the neutrino emission. the ν prediction from an advanced numerical calculation of grb (neucosmahümmer et al., 2012), including the full photohadronic interaction cross-section, independent losses of secondary particles, and flavor mixing, is shown as read line. the new predicted flux is about an order of magnitude below than the previous analytic approach, and thus it is still compatible with the limit as published by icecube. figure 7: declination vs reconstructed energy of the he cosmic neutrino candidates in the icecube detector (whitehorn at wipac 2013) diffuse he neutrinos. recently icecube reported on the observation of two he particle shower events discovered in a search for pev neutrinos (aartsen et al., 2013). these events (reconstructed as downward going) are of much higher energy than expected from the background of atmospheric neutrinos, fig. 5. stimulated by these two events, almost during the same time of this frascati workshop, the icecube announced the results of a dedicated search for downward going neutrino-induced events in a restricted fiducial volume. here, 28 events (7 muon-like and 21 shower-like) were discovered, with an expected background of 12.1 events (from atmospheric and prompt neutrinos, and atmospheric muons). the distribution of the declination and deposited energy for these events is shown in fig. 7. the collaboration asserts that the events seems to be neutrinos, with flavor ratios consistent with the expected 1 : 1 : 1 and compatible with an isotropic flux. 5 conclusions multimessenger astrophysics becomes more and more important to obtain a complete picture of non-thermal processes in the universe. this program can be achieved by combining pieces of information from all three messengers: photons, charged crs and ν’s. the implications extend from the origin of crs to the origin of dark matter, from processes of acceleration of particles by strong shock waves to the magnetohydrodynamics of relativistic jets, from distribution of matter in the interstellar medium to the intergalactic radiation and magnetic field distributions. the strong impact of he and vhe γ-rays discoveries on several topical areas of modern astrophysics and cosmology are recognized and of fundamental importance for the astronomical communities. in the near future, the role of ams-02 is of overwhelming importance for direct measurement of crs. finally, the recent multi-tev excess of neutrino events may be a first hint of an astrophysical he neutrino flux, opening the field of neutrino astronomy for the next decade. references [1] aartsen m.g. et al., 2013. accepted by prl [2] abbasi, r. et al.: 2012, nat, 484, 351 doi:10.1038/nature11068 [3] ageron, m. et al.: 2011, nim a, 656, 11 doi:10.1016/j.nima.2011.06.103 [4] blümer j. et al: 2009, prog.part.nucl.p. 63, 293 doi:10.1016/j.ppnp.2009.05.002 [5] chiarusi t, spurio m.: 2010, epj. c65, 649 [6] guetta, d. et al.: 2004, astr. phys., 20, 429 doi:10.1016/s0927-6505(03)00211-1 [7] hillas a. m.: 2005, j. phys. g 31, 39. doi:10.1088/0954-3899/31/5/r02 [8] hümmer, s. et al.: 2012, prl 108, 231101 doi:10.1103/physrevlett.108.231101 [9] nolan p.l. et al.: 2012 apjs 199 31 doi:10.1088/0067-0049/199/2/31 [10] rieger f.m. et al.:arxiv:1302.5603 [11] tavani m. et al.: 2011, science vol. 331 doi:10.1126/science.1200083 [12] waxman e., bahcall j.: 1999, prd59:023002 18 http://dx.doi.org/10.1038/nature11068 http://dx.doi.org/10.1016/j.nima.2011.06.103 http://dx.doi.org/10.1016/j.ppnp.2009.05.002 http://dx.doi.org/10.1016/s0927-6505(03)00211-1 http://dx.doi.org/10.1088/0954-3899/31/5/r02 http://dx.doi.org/10.1103/physrevlett.108.231101 http://dx.doi.org/10.1088/0067-0049/199/2/31 http://dx.doi.org/10.1126/science.1200083 γ-rays, ν’s and particle astronomy as messengers of the universe discussion arnon dar: concerning the icecube events, the quoted error on the atmospheric neutrino background seems to be very small. maurizio spurio: in my opinion the main uncertainty could arise not from atmospheric neutrinos, but from the vetoing efficiency estimate for surviving atmospheric muons. there are some peculiarity of the events which are intriguing and only a detailed publication can clarify. for instance, the fact that (after background subtraction) the muon neutrinos are fewer than the expected from the quoted 1 : 1 : 1 ratio; that a cutoff energy at ∼1 pev should be present; that the upgoing signal (4 events) is compatible with the background (3 events). for these reasons, if there is no underestimated contamination from atmospheric muons, the signal seems to be more likely of galactic origin than diffuse extragalactic. 19 introduction crs and antimatter high energy -rays he -rays on satellites vhe -rays at ground neutrino astrophysics conclusions 107 acta polytechnica ctu proceedings 2(1): 107–110, 2015 107 doi: 10.14311/app.2015.02.0107 broad-band variability in accreting compact objects s. scaringi1,2 1instituut voor sterrenkunde, k.u. leuven, celesijlaan 200d, leuven, belgium 2max planck institute fur extraterrestriche physik, d-85748 garching, germany corresponding author: simo@mpe.mpg.de abstract cataclysmic variable stars are in many ways similar to x-ray binaries. both types of systems possess an accretion disk, which in most cases can reach the surface (or event horizon) of the central compact object. the main difference is that the embedded gravitational potential well in x-ray binaries is much deeper than those found in cataclysmic variables. as a result, x-ray binaries emit most of their radiation at x-ray wavelengths, as opposed to cataclysmic variables which emit mostly at optical/ultraviolet wavelengths. both types of systems display aperiodic broad-band variability which can be associated to the accretion disk. here, the properties of the observed x-ray variability in xrbs are compared to those observed at optical wavelengths in cvs. in most cases the variability properties of both types of systems are qualitatively similar once the relevant timescales associated with the inner accretion disk regions have been taken into account. the similarities include the observed power spectral density shapes, the rms-flux relation as well as fourier-dependant time lags. here a brief overview on these similarities is given, placing them in the context of the fluctuating accretion disk model which seeks to reproduce the observed variability. keywords: cataclysmic variables x-ray binaries optical timing photometry individual: mv lyrae individual: lu cam. 1 introduction cataclysmic variables (cvs) are close interacting binary systems where a late-type star transfers material to a white dwarf (wd) companion via roche lobe overflow. with an orbital period ranging from hours to minutes, the transferred material from the secondary star forms an accretion disc surrounding the wd. as angular momentum is transported outwards in the disc, material will approach the innermost regions close to the wd in the absence of strong magnetic fields, and eventually accrete on to the compact object. x-ray binaries (xrbs) are also compact interacting binaries which are similar to cvs in many ways, but where the accreting compact object is either a black hole (bh) or a neutron star (ns). both cvs and xrbs, as well as active galactic nuclei (agn; accreting supermassive bhs), have been shown to display strong aperiodic variability on a broad range of time-scales as well as in different wavelength ranges. xrbs have shown variability ranging from milliseconds to hours, whilst for cvs this ranges from seconds to days. this difference can be mainly attributed to the fact that the innermost edges of the accretion discs in cvs sit at a few thousand gravitational radii, whilst for xrbs it can reach down to just a few gravitational radii. the fact that material can get deeper within the gravitational potential of xrbs, as compared to cvs, also explains why they are more luminous and emit predominantly in x-rays, compared to cvs, which emit predominantly at optical/uv wavelengths. aperiodic broad-band variability (also referred to as flickering) has extensively been studied in x-rays for xrbs over several decades in temporal frequency (see for example terrell 1972; van der klis 1995; belloni et al. 2000; homan et al. 2001; belloni, psaltis & van der klis 2002). as cvs emit mostly at optical/uv wavelengths, timing studies of these objects had to rely on optical observing campaigns from earth, which are inevitably hindered by large interruptions, poor cadence, and in many cases poor signal-to-noise ratios. furthermore, the key time-scales to probe in cvs are much longer than in xrbs, requiring long, uninterrupted observations. recently, cv timing studies have been facilitated thanks to the advent of the kepler satellite (gilliland et al. 2010; jenkins et al. 2010), which is able to provide long, uninterrupted and high-precision light curves in the optical light from space. thanks to these capabilities it is now possible to probe over four orders of magnitude in temporal frequency in cvs. more importantly, it is now possible to compare the aperiodic variability properties observed in xrbs to those 107 http://dx.doi.org/10.14311/app.2015.02.0107 s. scaringi observed in cvs after taking into account the relevant timescale and wavelength “translations”. 2 broad-band aperiodic variability in accreting white dwarfs one important discovery relating the flickering properties observed in cvs to those observed in xrbs has come from studying the kepler lightcurve of the nova-like cv mv lyrae displaying typical flickering behaviour as observed in other cvs. specifically, scaringi et al. 2012a have reported the discovery of the so-called rms(root-mean-square)-flux relation within the kepler lightcurve of mv lyrae, where the rms-variability power linearly correlates with the mean cont rate of the source. this relation has been previously observed in a number of xrbs and agn (uttley & mchardy 2001; uttley et al. 2005), and, together with the observed log-normal flux distributions, rules out simple additive processes as the source of flicker noise (e.g. superposition of many independent shots), and instead strongly favours multiplicative processes (e.g. coupling of masstransfer variations travelling from the outer to inner disc for the latter) as the source of variability. more importantly, the fact that very similar rms-flux relations are found within all different types of accreting compact objects (bhs, nss, wds) on all scales strongly suggests that the driving mechanism responsible for the observed aperiodic variability is the same in all systems, irrespective of mass, size or type. additionally to displaying the rms-flux relation, scaringi et al. 2012b have reported that the power spectral density (psd) of mv lyrae is also qualitatively similar to those observed at x-ray wavelengths in xrbs. specifically, all psds display single or multiple quasi-periodic osscillations (qpos) as well as a high frequency break. both the psds of xrbs and cvs can be qualitatively modelled with a combination of lorentzian shaped functions, with the main difference arising from the characteristic frequencies involved. for example, xrbs display aperiodic variability on a wide range of timescales with high-frequency breaks at ≈ 100 − 101 hz (belloni et al. 2005), whilst cvs display very similar psd, but scaled to lower frequencies such that the high frequency break occurs at ≈ 10−3 hz. the difference between the psds in xrbs and cvs can be mainly attributed to the fact that material within the accretion disk can fall deeper within the embedded gravitational potential well of xrbs as opposed to cvs. this will result in xrbs displaying variability on shorter timescales than in cvs, and furthermore, will result in xrbs emitting most of their radiation at x-ray wavelengths as opposed to cvs emitting mostly at optical/uv wavelengths. 3 fourier-dependent time-lags in cvs similarities between the flickering properties of xrbs and cvs are not only limited to single-band observations. it has been known for over a decade that xrbs display high levels of coherence between two simultaneously observed x-ray lightcurves in different energy bands (vaughan & nowak 1997; nowak et al. 1999). associated to this, fourier-dependent time-lags are also observed at x-ray wavelengths, where hard xray photons are delayed with respect to the soft photons, with larger delays at the lowest temporal frequencies (known as hard lags). a natural explanation for hard lags in xrbs comes from the fluctuating accretion disk model (see arevalo & uttley 2006 and references therin), where the coupling of fluctuations in the mass-transfer rate at different accretion disk radii is responsible for the observed variability. as matter propagates from the cooler outer regions of the disk towards the hotter inner ones, the observed variability in two different energy bands will be delayed with respect to each other as a result of the temperature gradient in the accretion disk and fluctuations moving inwards. using the ultracam instrument (dhillon et al. 2007) mounted on the 4.2 meter william herschel telescope, scaringi et al. 2013 have obtained simultaneous optical lightcurves in the u′, g′ and r′ bands on the two nova-like systems mv lyrae and lu cam. the results from this observing campaign show that both systems display soft lags (where blue photons are observed before the red ones) at the lowest observed frequencies, with larger lags at low frequencies. soft-lags have also been observed at x-ray wavelengths in xrbs, as well as agn, and have been explained as photoionising reflection of hard-x-ray photons originating from the inner regions of the disk (possibly the corona) onto the outer regions. in this case, the lag delay would be the lighttravel time from the emitting region to the reflecting region. the soft lags observed in scaringi et al. 2013 are however too large (on the order of ≈ 10 seconds for lu cam) to be explained by light-travel time delay. nevertheless, it might still be that photons from the hot inner regions are being reprocessed by the cooler outer disk, possibly on the thermal timescale. 4 a physical model for the flickering variability the observed flickering properties in xrbs have been modelled in the context of the fluctuating accretion disk model (see arevalo & uttley 2006 and references therin). ingram & done (2011,2012) have shown how this model can be successfully applied to reproduce the observed psd shapes in xrbs. in the most recent at108 broad-band variability in accreting compact objects tempt by ingram & van der klis (2013), the authors have demonstrated how the model can be implemented analytically (rather than numerically), thus reducing the computation time of specific models from hours to seconds. in scaringi 2013 a simplification of the ingram & van der klis (2013) model has been applied to the kepler lightcurve of the cv mv lyrae by removing general relativistic effects which are negligible in accreting wds. the model prescription employed allows to fit the high-frequency psd of mv lyrae by associating the observed flickering to the viscous timescale at specific disk radii. assuming the accretion disk extends all the way to the wd surface, the model allows to fit for the disk outer edge, as well as for α(h/r)2, where α is the viscosity parameter and h/r the disk scale height (shakura & sunyaev 1973). a qualitatively good fit to the data is achieved (with reduced χ2 ≈ 1.2). the model parameters resulting from the fit are displayed in table 1, and suggest that the observed high-frequency flickering is driven by a geometrically thick disk extending from r ≈ 0.12r� all the way to the wd surface. table 1: best-fit parameters with associated (1σ) errors for the fluctuating accretion disk model applied to the kepler lightcurve of mv lyrae (scaringi 2013). mwd(m�) ≡ 0.73 rin(r�) ≡ 0.0105 rout(r�) 0.117+0.029 −0.020 γ 0.853+0.047 −0.041 α(h/r)2 0.705+0.289 −0.182 fvar 0.220+0.001 −0.001 although the observed emission at optical wavelengths in cvs is though to originate from a cold, geometrically thin outer disk, the results from this analysis seem to suggest that the flickering is driven by a geometrically thick inner disk. it is thus possible that the geometrically thin outer disk is reprocessing photons from the geometrically thick inner one, allowing to explain the inferred results. similar geometric configurations have also been inferred in xrbs. both x-ray timing and spectral analysis suggest that geometrically thick disk exists close to the compact object (optically thin, referred to as the corona), possibly sandwiching the geometrically thin, optically thick disk. if a similar configuration is confirmed in cvs, then it is possible that the physics responsible for generating the inner geometrically thick disks in cvs might be the same as that in xrbs. 5 conclusion both cvs and xrbs, as well as agn, are observed to display aperiodic broad-band variability. this variability can be associated to the accretion disks in these systems. as the accretion disks in xrbs can fall much deeper within the embedded gravitational potential well as opposed to cvs, most of their emission will occur at x-ray wavelengths as opposed to optical/ultraviolet. furthermore, the timescales associated with the disk inner edge are a few orders of magnitude higher in temporal frequency than those of cvs. here, a brief overview of the broad-band variability properties observed in cvs has been presented. in particular, the aperiodic properties discussed (psd shapes, rms-flux relations and fourier-dependent time-lags) are in many ways similar to those observed at x-ray wavelengths in xrbs once the relevant temporal scaling has been taken into account. the fluctuating accretion disk model which seeks to explain the observed variability in xrbs has also been briefly discussed in the context of cvs. this model associates the observed variability to the viscous timescale at specific disk radii. in this respect, applying this model to the kepler lightcurve of the nova-like cv mv lyrae suggests the existence of a geometrically thick disk close to the wd responsible for driving the observed high-frequency flickering. acknowledgement the author wishes to acknowledge funding from the fwo pegasus marie-curie fellowship program, as well as useful and insightful discussions with christian knigge, elmar koerding, phil uttley and tom maccarone. references [1] terrell 1972: apj, 174, l35. [2] van der klis 1995: x-ray binaries. cambridge: cambridge univ. press; 1995. p. 252. [3] belloni et al. 2000: a&a, 355, 271. [4] homan et al. 2001: apjs, 132, 377. doi:10.1086/318954 [5] belloni et al. 2002: apj, 572, 392. [6] gilliand et al. 2010: apj, 713, l160. [7] jenkins et al. 2010: apj, 713, l87. doi:10.1088/2041-8205/713/2/l87 [8] scaringi et al. 2012a: mnras, 427, 3396. doi:10.1111/j.1365-2966.2012.22022.x [9] uttley & mchardy 2001: mnras, 323, l26. [10] uttley et al. 2005: mnras, 359, 345. doi:10.1111/j.1365-2966.2005.08886.x 109 http://dx.doi.org/10.1086/318954 http://dx.doi.org/10.1088/2041-8205/713/2/l87 http://dx.doi.org/10.1111/j.1365-2966.2012.22022.x http://dx.doi.org/10.1111/j.1365-2966.2005.08886.x s. scaringi [11] scaringi et al. 2012b: mnras, 421, 2854. doi:10.1111/j.1365-2966.2012.20512.x [12] belloni et al. 2005: a&a, 440, 207 [13] vaughan & nowak 1997: apj, 474, l43. [14] nowak et al. 1999: apj, 510, 874. [15] lyubarskii 1997: mnras, 292, 679. [16] kotov 2001: mnras, 327, 799. [17] arevalo & uttley 2006: mnras, 367, 801. [18] dhillon et al. 2007: mnras, 378, 825. [19] caastella et al. 2012a: mnras, 422, 2407. [20] cassatella et al. 2012b: mnras, 427, 2985. doi:10.1111/j.1365-2966.2012.22021.x [21] de marco et al. 2013: mnras, 431, 2441. doi:10.1093/mnras/stt339 [22] scaringi et al. 2013: mnras, 431, 2535. doi:10.1093/mnras/stt347 [23] ingram & done 2011: mnras, 415, 2323. [24] ingram & done 2012: mnras, 419, 2369 doi:10.1111/j.1365-2966.2011.19885.x [25] ingram & van der klis 2013: mnras, 434, 1476. doi:10.1093/mnras/stt1107 [26] scaringi 2013: mnras, 438, 1233 [27] shakura & sunyaev 1973: apj, 24, 337. [28] narayan & yi 1994: apj, 428, 13. [29] fender et al. 2004: mnras, 355, 1105. discussion linda schmidtobreick: for nls we see a difference in the spectra between low and high inclination systems. low inclination systems show balmer absorption, possibly from an inner optically thick accretion disk which might coincide with your geometrically thick inner disk. does one see a similar relation of the presence of high frequency flickering with inclination? simone scaringi: the current model has only been applied on one system at the moment (mv lyrae, low inclination). future applications of the model on other systems will be able to determine whether the high frequency flickering displays different properties as a function of inclination. solen balman: how do you get an optically thick disk out of α(h/r)2 = 0.7? if α = 0.1 the disk is hot and it is no longer optically thick! simone scaringi: the inference of α(h/r)2 = 0.7 from the modelling of the psd in mv lyrae does not suggest that the disk is optically thick. it might well be that the inner region of the disk inferred here are optically thin as suggested. raymundo baptista: is you α constant with radius? simone scaringi: in the current model prescription α is kept as a constant with radius. however, it is more physical and realistic to allow α to have a radial dependence. this will be explored with the current model in the future. 110 http://dx.doi.org/10.1111/j.1365-2966.2012.20512.x http://dx.doi.org/10.1111/j.1365-2966.2012.22021.x http://dx.doi.org/10.1093/mnras/stt339 http://dx.doi.org/10.1093/mnras/stt347 http://dx.doi.org/10.1111/j.1365-2966.2011.19885.x http://dx.doi.org/10.1093/mnras/stt1107 introduction broad-band aperiodic variability in accreting white dwarfs fourier-dependent time-lags in cvs a physical model for the flickering variability conclusion acta polytechnica ctu proceedings doi:10.14311/app.2017.11.0035 acta polytechnica ctu proceedings 11:35–38, 2017 © czech technical university in prague, 2017 available online at http://ojs.cvut.cz/ojs/index.php/app level crossing safety in the czech republic martin kobosil∗, jan novák ctu in prague, department of forensic experts in transportation, horská 3, 128 03 prague 2, czech republic ∗ corresponding author: kobosmar@fd.cvut.cz abstract. this paper describes safety threats at level crossings according to the different type of security equipment. initially were explored statistical data, which consist of numbers of level crossings classified by crossing type and numbers of traffic accidents on level crossings. afterwards paper describes specific problems of each individual level crossing type. specifically their causation and consequences, which are being discussed between professional and general public. in conclusion paper describes possible solutions to these problems leading to reducing risks at level crossings. at first is mentioned the statistics number of crossing and number of accidents on the crossing. then paper describes specific problems, which are discussed in professional and general public. they are also solutions of this problems, which brings another point of view and search the actual causes. keywords: railway, level crossing, active safety, visibility splays . 1. level crossings safety in numbers we can classify railway level crossing according to the different type of security equipment. on the 31th december 2016 was in the czech republic total number of 7,961 level crossings. 3,938 of them was equipped only with warning cross (w. cross). on these 49.5 percent of all level crossings it depends only on driver’s decision whether he will enter crossing or not. this decision is based on the level of an attention, driver’s experiences and especially on visibility splays. remaining 4,023 level crossings was secured by on type of an active security equipment. 2,356 of which was equipped with warning lights (w. lights), 1,310 level crossings was equipped with warning lights accompanied by automatic barriers (w. lights with a.b.) and 326 level crossings was equipped with mechanically operated barriers (mech. o.b.). the last 31 level crossings had some other form of protection [1]. 1.1. level crossings accidents rules about how to behave on level crossings are well known. they have been established by the law, taught in driving schools and presented in mass media and in primary schools. theoretically, if these rules would be obeyed, there should be no extraordinary incidents. therefore from legal point of view are traffic accidents on level crossings in majority of cases caused by driver. however in terms of prevention it is essential to examine other consequences and to investigate what factors led to the breaking the law. number of traffic accidents on level crossings according to type of security equipment is shown in table 1. 1.2. specific issues of each type of security equipment each type of above mentioned security equipment is different and has typical safety problems. the security equipment accidents killed injur. w. cross 67 4 27 w. lights 85 25 49 w. lights with a.b. 27 14 3 mech. o.b. 1 0 1 total 180 43 80 table 1. number of traffic accidents on level crossings according to the type of security equipment (2014) [2]. vast majority of these issues is linked to active safety. when we will solve them, we will significantly decrease the number of traffic accidents on level crossings and decrease the economic loss from traffic accidents. according to the rail safety inspection in 2013 there was a loss of 37.5 million czech crowns on railway vehicles, 3.7 million crowns on infrastructure and 19.0 million crowns on other participants involved in traffic accidents. to these figures have to be also added society-wide losses from injuries and deaths [2]. 2. level crossings equipped with warning cross main cause of problems on level crossings equipped only by warning cross are undoubtedly visibility splays, which are very often insufficient. owner of the railway has legal obligation to carry out regular maintenance of vegetation and to remove any other obstructions, which negatively impacts on visibility splays. in particular circumstances, where this cannot be achieved, i.e. due to the difficult ground conditions or due to land ownership disputes, it should be installed signalling equipment ideally in combination with crossing barriers. 35 http://dx.doi.org/10.14311/app.2017.11.0035 http://ojs.cvut.cz/ojs/index.php/app martin kobosil, jan novák acta polytechnica ctu proceedings figure 1. vegetation intruding visibility splays on level crossing (source police czech republic). visibility splays are defined in design standard csn 73 6380, which is legally binding on the basis of regulation no. 177/1995 sb. this standard explains how to correctly determine visibility splays, however it has one issue. according to the paragraph no. 88 of this regulation, above mentioned standard csn 73 6380 is not binding for level crossings, which project documentation was approved before this standard came into effect. vast majority of railway was designed and built in the end of the 19th century and in the first half of the 20th century. visibility splays on these older railways are defined by internal regulation of a railway operator szdc s 4/3. calculations are in the principle similar to methods used in design standards and are based on physical principles, however in the end of the regulation s 4/3 it is allowed to reduce visibility splays by half. this statement is based on the theory of probability and enables to reduce visibility splays in the case of a low probability of a collision, which implies on level crossings with low operational speed on the line along with low traffic volumes. however with this statement cannot be agreed either from the scientific view or from the human view. it is essential to respect physical laws and give driver enough time to decision, reaction and for safe crossing of a level crossing [3, 4]. another problem might be complete overlooking of a level crossing, which comprise of two inconspicuous rail tracks across the road. drivers are notified about presence of a level crossing by three traffic signs a31 complemented by warning cross sign a32, but these are only traffic signs, which is nothing unusual in road traffic and can be overlooked. other vulnerable group are drivers, which are familiar with level crossing, but can simply forget about its presence as well as they forget they are daily driving around memorial. solution to this problem has to be something, which is not as common as traffic signs and should be on the road, where driver is concentrating his attention. road markings could be applied as an optical psychological brake v18. in technical specifications tp 65 ”traffic signs manual” is about traffic sign v5 quoted ”additionally it is used on intersection to mark out where vehicle has to stop to give way, in front of the controlled pedestrian crossing and alternatively also in front of the level crossing” [5]. figure 2. level crossing highlighted by optical psychological brake v18 (source mr sachl). 3. level crossings equipped with warning lights severity of traffic accidents on these level crossings is highest from all of the level crossing types due to the higher operational speed on the line. reasons for disrespecting rules on these level crossings can be overlooking of red warning lights or driver’s risky behaviour. visibility splays on these level crossings does not have to be large, because they are used only when security equipment is out of service. basic rule of this security equipment says: ”do not enter the level crossing, when the red light is flashing.” but what is the reason for disobeying this rule? in mass media the reason is often attributed to the aggressive and undisciplined drivers, but the truth could be somewhere else. hardly will anybody drive in front of the moving train because of the curiosity ’if i will make it’ or because of the desire for adrenaline. first problem could be visibility of the warning lights, which does not comply with the selfexplanatory principle. especially dangerous are level crossings in urban areas, where buildings, foliage, traffic signs or other structures are forming longitudinal lines, which may cause overlooking of a warning lights. in this case it is appropriate to mark and highlight transverse lines by using road markings or ’lights bar’, which is placed on the road as can be seen on figure 3. however it is important to keep in mind that every level crossing has to be assessed individually and that solution of this problem requires cooperation of the railway operator and road maintenance provider. second problem with higher level of risk are drivers, which deliberately enters level crossing despite flashing warning lights. this phenomenon can be seen especially at small cities and villages, where local residents 36 vol. 11/2017 level crossing safety in the czech republic figure 3. level crossing highlighted by ’light bar’ and optical psychological brake (source mr skladany). knows, that it usually takes a long time before the train approaches level crossing after the warning light came on. this situation is particularly dangerous for the next people in a row, because human psychology works like ’he made it, so will i’. these people enter the level crossing following the example they have seen and tragedy is near. another problem, which is typical on level crossings oriented to the west or to the east is also sun and its glare. the intensity of warning lights on these crossings should be increased, e.g. by the use of a led technology. 4. level crossings equipped with warning lights accompanied by automatic barriers this is the safest and the most advanced type of level crossing safety equipment. the barriers are lowered across the road, which makes it clear, that it is forbidden to enter the crossing. on these level crossings is caused only a small number of traffic accidents, but when an accident happens it has usually greater consequences. typical and almost only threat on this type of security equipment is called ’s-manoeuvre’ between the half-barriers as shown on figure 4. these drivers are the biggest gamblers with their lifes. from this reason are some level crossings being equipped with enforcement video cameras, which can record and penalise hazardous drivers [6]. second cause of dangerous situation is when driver is closed between the safety barriers. but this mistake does not have to be fatal. first option is that second passenger can raise the barrier or the barrier can be lifted up by the bonnet of a car. barriers are intentionally balanced, that only a minimal strength is required to raise them. second option is breaking the barrier with a car. every barrier is designed to be easily broken by impacting on it in horizontal direction. it is necessary to raise public awareness about figure 4. s-manoeuvre [6]. this subject and to spread these information between road users. this pack of information is necessary to get to the subconscious mind of general public by enlightenment. the another solution consists of installing detection devices, which can detect trapped vehicle on level crossing. this device send information about trapped car and can give appropriate command to warn or to stop approaching train. the main technical problem of this solution is reliability of the vehicle recognition. low reliability can cause many false alarms and many delays of trains. these detecting systems are already successfully used abroad. figure 5. level crossing with vehicular detectors [7]. 5. how to reduce risk on level crossings we can hear from mass media and between professional and general public that it is drivers who makes mistakes on level crossings. this statements is generally correct, because majority of collisions between motor vehicles and railway vehicles has origin in driver errors, but often as an originator of traffic accidents is considered aggressive behaviour of drivers. it is true that many drivers are speeding, tailgating or making other traffic offences, however on level crossings it is usually simple human errors. driver is concentrating on the road in front of him, but on the road he sees 37 martin kobosil, jan novák acta polytechnica ctu proceedings only two inconspicuous lines across the road. just after double swing of a vehicle he realizes it was rail tracks. by lucky chance it ends up with wiping sweat from a forehead, sometimes it ends up tragically. for effective reducing traffic accident numbers on level crossings it is necessary to change an approach in assessing the whole issue and to focus on genuine causes of traffic accidents. it is essential to make every inconspicuous or hazardous level crossing visible. police has to focus on dangerous evading crossing barriers. additionally it is necessary to shut down duplicate level crossings and level crossings with low usage, to build grade separated railway crossings and to take care of visibility splays by maintaining vegetation and removing other obstructions. equally important is raising of public awareness in driving schools and through mass media and various campaigns of safety organisations. references [1] zeleznicni prejezdy v cislech, cit 2017-02-10. operator of the track szdc [online], http://www.szdc.cz/web/ prejezdy/prejezdy-v-cislech.html. [2] statistika nehodovosti na zeleznicnich prejezdech, 2015. the rail safety inspection office. [3] csn 73 6380 zeleznicni prejezdy a prechody, 2014. czech technical standad. [4] s 4/3 internal regulation, 2014. operator of the track szdc. [5] tp 65 "traffic signs manual", 2013. technical manual. [6] zeleznicni prejezdy budou hlidat kamery, cit 2017-02-10. czech television [online], http://www. ceskatelevize.cz/zpravodajstvi-ostrava/zpravy/ 204475-zeleznicni-prejezdy-budou-hlidat-kamery/ l. [7] i. r. p. .-. [online]. ieee railway projects 2015-2016, cit 2017-02-10. http: //ieeerailwayprojects2015-2016.blogspot.cz/. 38 http://www.szdc.cz/web/prejezdy/prejezdy-v-cislech.html http://www.szdc.cz/web/prejezdy/prejezdy-v-cislech.html http://www.ceskatelevize.cz/zpravodajstvi-ostrava/zpravy/204475-zeleznicni-prejezdy-budou-hlidat-kamery/l http://www.ceskatelevize.cz/zpravodajstvi-ostrava/zpravy/204475-zeleznicni-prejezdy-budou-hlidat-kamery/l http://www.ceskatelevize.cz/zpravodajstvi-ostrava/zpravy/204475-zeleznicni-prejezdy-budou-hlidat-kamery/l http://www.ceskatelevize.cz/zpravodajstvi-ostrava/zpravy/204475-zeleznicni-prejezdy-budou-hlidat-kamery/l http://ieeerailwayprojects2015-2016.blogspot.cz/ http://ieeerailwayprojects2015-2016.blogspot.cz/ acta polytechnica ctu proceedings 11:35–38, 2017 1 level crossings safety in numbers 1.1 level crossings accidents 1.2 specific issues of each type of security equipment 2 level crossings equipped with warning cross 3 level crossings equipped with warning lights 4 level crossings equipped with warning lights accompanied by automatic barriers 5 how to reduce risk on level crossings references acta polytechnica ctu proceedings doi:10.14311/app.2017.10.0056 acta polytechnica ctu proceedings 10:56–60, 2017 © czech technical university in prague, 2017 available online at http://ojs.cvut.cz/ojs/index.php/app design of subsoil improvement below hall floors peter turček∗, monika súľovská stu bratislava, faculty of civil engineering, radlinského 11, bratislava ∗ corresponding author: peter.turcek@stuba.sk abstract. the construction of an industrial park is now being prepared near the town of nitra. the investor fixed very strict conditions for the bearing capacity and, above all, the settlement of halls and their floors. the geological conditions at the construction site are difficult: there are soft clay soils with high compressibility and low bearing capacity. a detailed analysis of soil improvement was made. stone columns were prepared to be fitted into an approximately 5 m thick layer of soft clay. the paper shows the main steps used in the design of the stone columns keywords: engineering geological investigation, soil improvement, gravel columns, decreasing the settlement, consolidation. 1. introduction the construction of an industrial park was prepared near the town of nitra in 2015. difficult geological conditions, created by a layer of soft to firm quaternary clays approximately 5 m in thickness resulted in unfavourable soil characteristics: low bearing capacity and high settlement. for this reason, it was decided to improve the soil properties. following the evaluation of a preliminary engineering geological investigation, an initial design of stone columns was made during detailed investigation works. before the final design, the efficiency of the stone columns was verified by field tests. three test fields for three various improvement technologies were prepared. penetration tests were applied before and 10 days after the stone columns completion. in test field no. 1, stone columns with a diameter of 500 to 600 mm were prepared using the technology of deep vibrating compaction, and filled with gravel of grain size 4/15 mm, 8/32 mm and 4/32 mm. the stone columns were arranged to form a square and triangular grid. in test field no. 2, dynamic compaction combined with stone columns preparation using a nontraditional technology was used for soil improvement. first, a borehole with a diameter of 600 mm was made and filled with gravel of grain size 0/63 mm or 0/90 mm containing low contents of fine particles. soil particles smaller than 0.063 mm formed less than 5 %. the boreholes were not protected by casing pipes; they had to be filled by gravel as soon as possible to avoid the collapse of boreholes. the next step was compaction of the stone column in two phases by 50 hits. the stone columns were spaced within a square and triangular grid with varied distances from each other. in field test no. 3, dynamic consolidation with a weight of 20 t falling in free fall from a height of 10 m was performed. using this technology created craters approximately 2 m in diameter. this squishy space was filled by stones with grain size 0-400 mm. the content of fine soil was very low. the space of compacted columns formed a square and triangular grid with a spacing of more than 5 m. after the evaluation of data from field experiments [1], dynamic consolidation was rejected, because the results had proved this technology improper for application in local geological conditions. also, the settlement of columns produced by dynamic compaction was higher than by the other technologies. as the depth reached by dynamic compaction was not sufficient for the project requirements, this technology was also rejected. field experiments helped to prepare recommendations which became input data for soil improvement. more halls with different floor loading will be constructed in the industrial park in the near future. the paper focuses on the design of soil improvement for uniform floor load intensity of 60 kpa. 2. design of stone columns various connections of columns to other structural layers were analysed before the design of the stone columns. after removing the topsoil, the separation geotextile was put on the ground level. then, a 500 mm thick gravel layer was laid as a base below the embankment. the 500 mm layer was made up of two layers of crushed stone with grain size 0/125 mm compacted to min. id = 0.85. from this working plane, the stone columns were assembled. after they had been completed, the ground surface was levelled and compacted, as needed, to min. id = 0.85 again. the embankment was built on this base layer. its first three layers levelled the non-horizontal terrain. the main part of the embankment was designed from these layers: 200 mm of the bottom layer of crushed stone 0/125 mm, 300 mm of pit-run gravel with small volumes of fine soil (grain size max. 100 mm), then 56 http://dx.doi.org/10.14311/app.2017.10.0056 http://ojs.cvut.cz/ojs/index.php/app vol. 10/2017 design of subsoil improvement below hall floors again 200 mm of crushed stone 0/125 mm and 300 mm of pit-run gravel with small volumes of fine soil (grain size max. 100 mm). the last 800 mm of the embankment were prepared from a 400 mm layer made up of crushed stone 0/63 mm and two 200 mm layers of crushed stone of grain size 4/32 mm. the main reason for using different types of material was the possibility of their delivery to the site. the length of the construction site was 1380 m. a schematic sectional view of improved subsoil is in fig. 1, while fig. 2 presents an overall view of making the stone columns. one part of the construction site will contain halls whose floors will be loaded by 60 kpa. the investor’s requirement for the settlement of the floors was 10 mm. the thickness of the embankment situated on improved subsoil was 1.5 m. based on a parametric study, a raster of stone columns (sc) 1.8 · 1.8 m in a square grid was selected. the average length of the stone columns was 5.5 m. this length resulted from the requirement that the bottom of the columns would extend into the quarternary gravel layer. it was necessary to verify the bearing capacity, settlement and consolidation of the stone columns for the prepared project. the bearing capacity validation manifested great reserve and it will be not be dealt with this paper. a traditional analytical calculation method was applied for the calculation of settlement and consolidation, partly complemented by priebe’s theory for vertical gravel stone columns. 2.1. traditional calculation of settlement and consolidation after the withdrawal of a humus layer, a 0.5 m thick gravel layer would be prepared on the ground. the stone columns would be built from this level. calculations were made for a simulated model area of 20 · 20 m, as there must a reserve left for transport corridors and, if necessary, a free space for technology in the halls. when a stone column with a diameter of 600 mm is made, the cross-sectional area equal to one stone column is a1 = π r2 = π 0.32 = 0.283 m2 (1) using the distribution of the stone columns in a square grid with a distance between the columns of 1.9 · 1.9 m, 27.67 columns will be placed in an area of 10 · 10 m (5.26 columns fit in a line up to 10 m in length, the distance between the rows is 1.9 m and the 10 m section holds 5.26 columns). therefore, the total area of the columns will be ac = a1 · 27.67 = 0.283 · 27.67 = 7.83 m2 (2) the original soil will occupy the area as = 100− 7.83 = 92.17 m2 (3) the following bedrock model was compiled for the area where a hall with a floor loaded by 100 kpa will be placed: • 3.3 m layer of clay of high plasticity with mainly soft consistency; where edef = 3 mpa; eoed = edef/β = 3/0.37 = 8.1 mpa (4) • 1.2 m thick layer of clayey sand with mainly firm consistency; where edef = 5.95 mpa; eoed = edef/β = 5.95/0.62 = 9.6 mpa (5) • 1.0 m thick layer of silty gravel with medium relative density; where edef = 35 mpa; eoed = edef/β = 35/0.74 = 47.3 mpa (6) • tertiary clay with high plasticity and firm consistency was found below silty gravel; where edef = 4 mpa; eoed = edef/β = 4/0.37 = 10.8 mpa (7) the average oedometric modulus value in a 3.3 m thick clay layer was determined using the equation: eoed = e1 ·a1 + e2 ·a2 a1 +a2 = = 166 · 7.83 + 8.1 · 92.17 100 = 20.46 mpa (8) similarly, for the average value of clayey sand the oedometric modulus determined was eoed = 21.85 mpa, for silty gravel eoed = 56.59 mpa and for tertiary clay eoed = 22.95 mpa. the calculation of settlement was made (as described above) for an area of 20 · 20 m. the embankment layer thickness of 1.5 m acts on the surface of the original ground by a stress with an intensity of 1.5 · 20 = 30 kpa. it was anticipated that a soft clay layer 3.3 m thick with clayey sand and tertiary clay would be compressed after improvement by 5.5 long stone columns in a raster of 1.9·1.9 m (fixing the stone columns 1.0 m into the gravel layer was considered). the calculation is processed in tab. 1. in the same way, the settlement due to a uniform surcharge of the floor by 60 kpa was calculated. the expected settlement under this load is s2 = 9.012 mm. the total estimated settlement (embankment + surcharge of the floor) will reach stot = 4.886+9.012 = 13.898 mm. but the magnitude of this settlement needs to be corrected taking into account consolidation, because part of the settlement will occur during the construction time. the calculation of consolidation in a drainage system consists of two parts: consolidation into vertical elements (stone columns) and consolidation into the horizontal permeable subbase. then, using terzaghi’s theory the average degree of consolidation can be expressed as 57 peter turček, monika súľovská acta polytechnica ctu proceedings figure 1. schematic sectional view of improved subsoil. figure 2. overall view on making stone columns. point no. h z z/b i2 σ2 σor m2 · σor eoed s (m) (m) (kpa) (kpa) (kpa) (mpa) (m) 1 1.6 0.80 0.040 0.98 29.4 16 1.6 20.46 0.002174 2 1.7 2.45 0.122 0.85 25.5 49 4.9 20.46 0.001712 3 1.2 3.90 0.195 0.71 21.3 78 7.8 21.85 0.000741 4 1.0 5.00 0.250 0.62 18.6 100 10.0 56.59 0.000152 5 2.8 6.90 0.345 0.52 15.6 138 13.8 47.30 0.000107 6 2.0 9.30 0.465 0.43 12.9 186 18.6 10.80 7 2.0 11.30 0.565 0.38 11.4 226 22.6 10.80 -∑ s1 = 0.004886 table 1. settlement of the embankment (σ = 30 kpa) after improvement by sc in a raster of 1.9 · 1.9 m. explanatory notes: h thickness of the soil layer; z distance between the footing bottom and the point at which vertical stress is calculated; b width of the foundation; i2 coefficient of stress spreading influence; σ2 vertical stress from the foundation at a depth z; σor original vertical stress at depth z; m2 coefficient of structural strength; eoed oedometric modulus of soil; s settlement of the soil layer with a thickness h. 58 vol. 10/2017 design of subsoil improvement below hall floors u = 1− (1− uh) · (1− uv) (9) where: uh degree of consolidation for horizontal dewatering, uv degree of consolidation for vertical dewatering. firstly, the average degree of subgrade consolidation was determined: eoed = ∑ ei · hi∑ hh = 8.1 · 3.3 + 9.6 · 1.2 3.3 · 1.2 = 8.5 mpa (10) in the calculation of consolidation in the vertical direction, the time factor tv was determined: tv = tvkveoed γwh2 = 1.0368 · 107 · 1 · 10−9 · 8.5 0.01 · 4.52 = 0.435 (11) where: tv duration of vertical consolidation 4 months (4 · 30 · 86400 = 1.0368 · 107 s); kv filtration coefficient in the vertical direction (1 · 10−9 m/s); eoed oedometric modulus of soil (8.5 mpa); γw specific gravity of water (0, 01 mnm−3); h thickness of the drainage layer of soil (4.5 m). from terzaghi’s consolidation graph, we obtained the degree of consolidation uv = 83 %. these boundary conditions have been taken into account for the calculation of consolidation in the horizontal direction: de = 1.13 · d = 1.13 · 1.9 = 2.147 (where d is the axial distance of drains). the time factor th for determining the degree of consolidation uh is: th = thkheoed γwd2 e = 1.0368 · 107 · 1 · 10−9 · 8.5 0.01 · 2.1472 = 1.91 (12) where: th duration of horizontal consolidation 4 months (4 · 30 · 86400 = 1.0368 · 107 s); kv filtration coefficient in the horizontal direction (1 · 10−9 m/s); for n = de d1 = 2.147 0.60 = 3.58 ⇒ in this case, auxiliary graphs yielded uh = 99 %. the resulting average degree of consolidation is then u = 1−(1−uh)·(1−uv) = 1−(1−0.83)·(1−0.99) = = 0.9983 ≈ 100 % (13) thus, it can be concluded that as long as the floor is not loaded earlier than 4 months after the completion of embankment bodies, the consolidation below the embankment will be finished. the measurable settlement of the hall structure, therefore, should only be the component induced by the operating load in the hall. the value of this part of settlement is 9.012 mm. 2.2. calculation of settlement using partially priebe’s theory the solution comes from the definition of the equivalent diameter of the cell de. auxiliary magnitudes: equivalent diameter: de = 1.13 · 1.9 = 2.147 m cross sectional area of the stone column: ac = πr2 = π · 0.322 = 0.283 m2 area of a cell with a diameter de: as = πr2 = π · 1.07352 = 3.62 m2 ratio of the column area to the cell area: ac = ac/as = 0.283/3.62 = 0.078 stress in the foundation base of the floor: σ = 60 kpa the basic design parameter is the so-called concentration ratio of stresses n = ec es = 180 3 = 60 (14) where: ec is young’s modulus of the stone column, es is young’s modulus of soil around the stone column. the settlement of subsoil with reinforcement by stone columns is expressed by the parameter β this parameter represents the ratio of the reinforced soil to original soil settlement. the parameter β can be approximately determined using the equation β = 1 1 + (n− 1) · ac = = 1 1 + (60− 1) · 0.078 = 0.1785 (15) the calculation of settlement caused by a 1.5 m high embankment (σ = 30 kpa) without improvement and by the floor followed. the expected settlement of the embankment was s3 = 11.854 mm and the floor settlement due to loading by 60 kpa was s4 = 20.150 mm. then, the total settlement without improvement is stot = 11.854 + 20.150 = 32.004 mm. using priebe’s theory the assumed settlement of the stone columns is reduced to the value s = sn · β = 32.004 · 0.1785 = 5.713 mm (16) comparing both theoretical methods we obtained the difference: 9.012 mm against 5.713 mm. this 59 peter turček, monika súľovská acta polytechnica ctu proceedings difference can be explained in this way: in classical theory medium stiffness of subsoil was assumed, while in priebe’s theory the load is carried by the stone columns to a greater extent. according to both methods, the investor’s requirement was fulfilled: the settlement was less than 10 mm. 3. conclusion the extremely exposed construction of an industrial park near the town of nitra required improving an approximately 5 m thick layer of soft and firm quaternary clays. in the first step, the suitable technology of soil improvement was selected. to this end, a field experiment in a 1:1 scale was carried out. deep vibrating compaction was evaluated as the most suitable technology. in the second step, the design of stone columns spacing was made. the paper shows the procedure of the design of soil improvement below the floors exposed to uniform loading by 60 kpa. after the accomplishment of subsoil improvements of the whole area of the industrial park, the construction of halls started. 4. acknowledgements the paper presents part of the results from a research project supported by the slovak vega agency no. 1/0882/16 “the influence of boundary conditions on limit states of geotechnical structures”. [2] [3] references [1] k. ströhle, a. tomozei. execution of soil improvement in the trial area. construction project jaguar land rower, nitra, slovakia. zt-ströhle, wien, 2015. [2] j. soták, et al. project darwin. final report of detailed engineering-geological investigation (in slovak). tech. rep., tpa, s.r.o. bratislava, 2015. [3] p. turčekm, m. súľovská. rough landscaping and design of subsoil improvement using stone columns in an industrial park near the town of nitra (in slovak). t-g., bratislava, 2015. 60 acta polytechnica ctu proceedings 10:57–61, 2017 1 introduction 2 design of stone columns 2.1 traditional calculation of settlement and consolidation 2.2 calculation of settlement using partially priebe's theory 3 conclusion 4 acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2016.5.0012 acta polytechnica ctu proceedings 5:12–16, 2016 © czech technical university in prague, 2016 available online at http://ojs.cvut.cz/ojs/index.php/app on operation of 740 metre long freight trains on czech ten-t railway network michal drábek∗, vít janoš, zdeněk michl department of logistics and management of transport, faculty of transportation sciences, ctu in prague, czech republic ∗corresponding author: xdrabek@fd.cvut.cz abstract. regulation (eu) no 1315/2013 defines actual scope of core and comprehensive ten-t network, including both networks for railway freight transport. for the core network, possibility to operate 740 m long freight trains is required. the aim of this paper is to analyse availability of appropriate overtaking tracks for 740 m long freight trains. due to etcs braking curves and odometry, such trains, after etcs implementation, will require 780-800 m long overtaking tracks. for practical reasons (e.g. bypass lines), whole czech railway ten-t network is analysed. the overtaking track, whose occupation means influence on scheduled traffic or threat to boarding passengers, are excluded. the data was collected from station schemes from collection of official requisites for 2015/16 timetable, issued by sždc, czech state infrastructure manager. most of appropriate tracks are over 800 m long, but their density in the network and in particular directions varies considerably. for freight traffic, gradient of the line is important, so in the resulting figure, there are marked significant peaks for particular lines as well. czech ten-t lines are further segmented on the basis of number of tracks and their traffic character. then, specific issues on overtaking or crossing of 740 m long freight trains are discussed. as a conclusion, for long-term development of czech ten-t lines, targeted investment is recommended not only for passenger railway, but also for freight railway. an attractive capacity offer for railway undertakings, which can stimulate freight traffic on european rail freight corridors, can be represented by network-bound periodic freight train paths with suitable long overtaking tracks outside bottlenecks. after the overtaking by passenger trains, a freight train should run without stop through large node station or a bottleneck area. before the sections with high gradients, coupling of additional locomotives should be connected with the overtaking process. next suitable overtaking tracks should be available behind every significant peak of the line. keywords: european freight corridor, long freight train, overtaking track, freight train path, peak . 1. introduction directive 2008/57/ec of the european parliament and of the council of 17 june 2008 on the interoperability of the rail system within the community [1] defines in annex ii infrastructure as a structural subsystem of the rail system, and traffic operation and management as a functional subsystem of the rail system. in the traffic operation and management subsystem, traffic planning is also included. regulation (eu) no 1315/2013 of the european parliament and of the council [2] defines actual scope of core and comprehensive ten-t network, including both networks for railway freight transport. for the core freight railway network, infrastructure requirements are defined. one of them is possibility to operate 740 m long freight trains. regulation (eu) no 913/2010 of the european parliament and of the council of 22 september 2010 concerning a european rail network for competitive freight [3] defined 9 initial rail freight corridors (rfcs) across the eu. according to the regulation, the corridors should provide guaranteed capacity for freight trains – "under good conditions in terms of commercial speed and journey times". the exact scope of each rfc are subject to negotiation by particular member states. three rfcs run directly through the czech republic: rfc 5 baltic-adriatic, rfc 7 orient and rfc 9 czech-slovak. moreover, negotiations about a branch of rfc 8 north seabaltic from germany to ústí nad labem and praha are proceeding. present, as well as planned or negotiated rfcs are displayed in the official presentation of czech ministry of transport by kušnír and ilík [4]. as a result, only few czech ten-t lines of comprehensive network are not included in any present rfc (or negotiated about being included in it). because of possible future changes of rfcs layout, and because of practical need for bypass lines, the whole czech railway ten-t network was included in the analysis. under the term "overtaking track" it is further understood any station track, which does not proceed any railway line in direct direction, i.e., it is not a main station track. for the sake of simplicity, the "not main" station tracks also in single-track lines, which serve mainly for crossing of trains, are referred to as "overtaking tracks". usable length of an overtaking track (further referred to only as "track length") is defined as a distance between last departure main signals, each one for the opposite direction. in the 12 http://dx.doi.org/10.14311/app.2016.5.0012 http://ojs.cvut.cz/ojs/index.php/app vol. 5/2016 on operation of 740 metre long freight trains case of group departure main signals (common for more parallel tracks), or absence of them, distance between last spots is considered, where rolling stock can be located, so that using of neighbouring track by another train is possible. the term "long trains" will be used for freight trains up to 740 m long. the term "long overtaking tracks" will be used for overtaking tracks at least 780 m long. for collection of data on length of overtaking tracks, railway station schemes from sždc (czech state infrastructure manager) collection of official requisites for 2015/16 timetable [5] were used. 2. methods 2.1. rules for determination of available overtaking tracks for 740 m long trains the basic question is, how long overtaking track is sufficient for a 740 m long train. for present czech signalling system and actual sždc regulations on traffic management, a track with additional 12 m length is sufficient. a 10 m distance is required between the locomotive and the departure main signal (in the train direction). the 2 m distance between an insulated gap and the departure main signal for the opposite direction is calculated. brejcha and čech [6] and binko [7] [8], however, state that for the future etcs implementation, a supplementary length of 40 to 60 m to the train length must be added, due to braking curve and inaccuracy of the etcs odometry. on the basis of the facts stated above, the authors have decided to analyse two categories of available long overtaking tracks: • 780 to 799 m long (in figure 1, separate presence of such track or tracks in particular direction is marked in yellow colour) • 800 m long and longer (in figure 1, presence of at least one such track in particular direction is marked in orange colour) presence of the tracks of both categories is marked in orange colour. presence of at least one long overtaking track in particular direction (on the train’s right, which is valid also for single-track lines or sections) is marked by obtuse-angled triangle of particular colour. its obtuse angle lays in direction, which is this track preferably used for. on czech railway network, there is only right-hand operation nowadays. in some recently modernized stations, an amended czech technical standard tnž 34 2620 railway station and line signalling and interlocking systems [9] was implemented. for the through run routes with allowed speed over 120 km/h, if there is no flank protection from the overtaking track and this track is occupied, only "restricted run route" (vco) with maximum speed 100 km/h is allowed. this mandatory function of interlocking systems obviously leads to longer runtime for the fast train than scheduled. so, overtaking of freight trains on vco-affected tracks is not desirable, but not critical. vco-affected long overtaking tracks are marked by pink edge in figure 1. for possible overtaking (or eventual crossing on single-track lines), only such overtaking tracks were considered, that did not lay between any main station track and waiting room (or entrance/exit of the station) in the case of at-grade access to platform (neither footbridge nor underpass). also such overtaking tracks were not considered that were (or were supposed to be, based on actual timetable [5]) occupied by passenger trains to be overtaken, or turned around. in stations with sidings, an expert estimation of need of station tracks for the freight trains that start or terminate here, was carried out. however, even such "occupied" or "reserved" long overtaking tracks were recorded (separately), if they were long enough. but they are neither displayed if figure 1 nor further discussed in this paper. in the case of a station with more yards (even a passenger and a freight yard), or with station tracks divided in the middle by switch region, an individual approach was chosen. if two subsequent tracks can be used by a long freight train without any impact on regular traffic, overall usable length of the resulting track was recorded. the czech ten-t lines will be further segmented into double-track lines, partially double-track lines and single-track lines. a triple-track line poříčany – praha-libeň and rather short triple-track sections bílina – odb české zlatníky and hranice na moravě – drahotuše will be added to double-track lines. the double-track group comprises vast majority of czech ten-t railway network. these groups can be further divided into subgroups on the basis of their traffic character. the presence of absence of appropriate overtaking tracks will be discussed by particular segments of the lines. 2.2. where to stop for overtaking – a soft decision process it is evident that any stop of a freight train in a bottleneck area leads to lower capacity utilization. so, overtakings should preferably be planned outside the bottlenecks. the following attributes of overtaking stations influence the quality of overtaking: • gradient on following line section (acceleration up the hill lengthens runtime of a freight train significantly and increases consumption of traction energy) • maximum allowed arrival speed on overtaking track (from the infrastructure viewpoint – it usually depends on arrival turnouts) • gradient on previous line section (braking downhill increases wear of brakes) • maximum allowable departure speed from overtaking track (usually depends on departure turnouts) • usable length of the longest overtaking track in particular direction 13 m. drabek, v. janos, z. michl acta polytechnica ctu proceedings in the authors’ opinion, it is very difficult (if not impossible) to set an exact order of priorities of the attributes listed above. each line, each passenger timetable and each set of freight trains together make a unique combination of specific conditions. so, a choice of the most suitable station for overtaking is always a soft decision process [10]. because of importance of gradients for freight railway, significant line peaks are also considered and marked in figure 1. 3. results and discussion czech ten-t lines are further segmented on the basis of number of tracks and their traffic character. presence or absence of long overtaking tracks is commented for each line group. for přerov and bohumín node stations, data on track length and on position of main departure signals were not available. some interesting findings occurred. firstly, during the analysis there emerged few theoretically suitable overtaking tracks which were approximately 770 m long. more precise calculations can determine the exact minimum practically usable track length for the long trains after implementation of etcs. secondly, most of the suitable overtaking tracks are over 800 m long. unfortunately, their density in the network varies considerably. 3.1. double-track (or triple-track) lines busy national mainlines with mixed traffic: • praha – česká třebová – olomouc – přerov – ostrava – bohumín (– chalupki) – dětmarovice/petrovice u karviné (– katowice) – třinec – bystřice busy mainlines with significant (or prevailing) freight traffic: • praha – děčín (– dresden), • lysá nad labem – děčín prostřední žleb, • ústí nad labem – úpořiny – bílina, • přerov – břeclav (– vienna/bratislava), • bystřice – mosty u jablunkova (– žilina). busy suburban sections with mixed traffic: • praha – kralupy nad vltavou, • praha – lysá nad labem – nymburk – kolín, • praha – benešov u prahy, • praha – beroun, • brno – tišnov, • brno – skalice nad svitavou, • brno – břeclav. middle-busy mainlines: • ústí nad labem – teplice – bílina – most – chomutov – karlovy vary – cheb (– marktredwitz), • plzeň – beroun, • kolín – kutná hora (station with both ac and dc traction power supply systems) – havlíčkův brod – tišnov, • česká třebová – skalice nad svitavou, • brno – blažovice, • hranice na moravě – horní lideč (– žilina). due to practically exhausted capacity of the line praha – olomouc – ostrava – bystřice, freight trains should be able to run homogeneous train paths with a long-distance passenger train, so their maximum speed should be 120 km/h, in the worst case 100 km/h. the locomotives should be enough powered to accelerate the train to such speed before it has to be overtaken by next fast train. even if such conditions are fulfilled, there are hardly any available train paths in morning and afternoon peaks between praha and česká třebová. because of busy traffic, the actual number of long overtaking tracks is not sufficient at all. moreover, some ones are located in kolín and pardubice, the busiest nodes within the section mentioned above. for a freight train it is extremely complicate to find a train path that enables conflict-free departure from such busy nodes with level crossing of trains in various directions (e.g. fast train praha – havlíčkův brod – brno with non-stop express train ostrava – pardubice – praha). the section kolín – lysá nad labem is busy by suburban traffic, but enables homogeneous train paths even to slower freight trains (e.g. 90 km/h). however, any stop of a freight train in this section is highly undesirable, because the trains run in 5-8 min wide time slots after each other. behind lysá nad labem, passenger traffic is considerably lower, but freight trains mostly do not need to stop because of train path homogeneity. in any case, the long trains should be scheduled early enough before a fast train, so that they could not delay them. most freight trains need rear-end or head-end assistance in line sections with high gradients, e.g. praha – strančice, český těšín – čadca, kutná hora – žďár nad sázavou, brno – žďár nad sázavou, hranice na moravě – horní lideč etc. for the long trains, 800 m long or longer overtaking tracks are desirable, because of length of the additional locomotive. it is appropriate to combine coupling and uncoupling of this locomotive with the overtaking process. due to speed profile of the line kralupy – děčín – dresden, even 100 km/h fast freight trains can run in homogeneous train paths with non-stop eurocity trains. there is lack of long overtaking tracks in the direction from praha. practically, a non-stop train path from praha-holešovice to děčín is required. on another lines of this sub-category, there is mostly almost no reason for the overtaking of freight trains, if the gradients are low. in busy suburban sections with mixed traffic, best capacity utilization means no stop of a freight train, which requires speed bundling (homogeneous train paths) either with suburban or with fast trains. 14 vol. 5/2016 on operation of 740 metre long freight trains figure 1. presence of available long overtaking tracks in czech ten-t railway network. map source: [11] 3.2. partially double-track lines (middle-busy mainlines) • plzeň – české budějovice (with double-track sections nepomuk – horažďovice předměstí and číčenice – zliv), • benešov u prahy – tábor – české budějovice (mostly double-track line, with remaining several single-track sections). it is desirable to avoid crossing of the freight trains with trains of the opposite direction in single-track sections, because of necessity of stop. the line benešov – české budějovice has not much passenger traffic, but three significant peaks. thus, double or triple traction for running through the whole line is worth consideration. 3.3. single-track lines busy national mainlines with mixed traffic: • domažlice – plzeň jižní předměstí, • brno – holubice – nezamyslice – přerov. mainlines with significant freight traffic: • české budějovice – horní dvořiště (– linz). middle-busy mainlines: • cheb – plzeň (there are double-track sections cheb – lipová u chebu and pňovany – plzeň only in ends of the line, so there is a character of a single-track line), • (furth im wald) – domažlice, • ústí nad orlicí – lichkov (– wroclaw). mainlines with little traffic: • české budějovice – české velenice (– gmünd nö). on single-track lines, longer overtaking tracks in almost every station are necessary, because of crossing of a freight train with any train which runs in the opposite direction. the alternative, which may be more efficient, is targeted partial doubling of the line. in the czech republic, most of passenger trains operate in (more or less) periodic timetable. if there are two systems of periodic passenger services with different section runtimes (and hence, with different crossing stations), there usually remains very little usable capacity left for freight trains. so, if there is a demand after a freight train paths approximately every 2 hours or more often, targeted partial doubling of the line is desirable. "targeted" means in the sections where trains from opposite direction cross each other. for the passenger trains in the czech republic, it usually corresponds to "zero symmetry" time, i.e. the minute 00. freight train paths should be, in the authors’ opinion, if possible, designed as periodic with identic symmetry time, and meeting of the freight 15 m. drabek, v. janos, z. michl acta polytechnica ctu proceedings trains of the opposite direction should be designed into the planned double-track section. if the freight traffic is irregular, at least crossing of a freight and a passenger train should be designed there. 3.4. sections and nodes with lack of long overtaking tracks there are listed lines, sections or nodes where additional long overtaking tracks are desirable: • cheb – plzeň, • furth im wald – domažlice – plzeň, • plzeň – české budějovice (a bypass line), • plzeň – beroun – praha (establishment of flank protection for long overtaking tracks can mostly solve the problem), • české budějovice – horní dvořiště (– linz) (a bypass line), • praha – české budějovice (a bypass line. after completed modernization, overtaking by fast trains is likely inevitable), • praha – ústí nad labem – děčín (this direction only), • děčín prostřední žleb – lysá nad labem, • kolín – havlíčkův brod – brno, • brno – přerov (modernization of the whole line is planned), • praha – olomouc (lack of long overtaking tracks outside large nodes), • přerov – ostrava (this direction only), • brno – břeclav (in the middle of the line and břeclav freight yard). 4. conclusion besides sections with high gradients, overtaking of a freight train is sometimes necessary because of fitting into spare time slot in a bottleneck area, or on a neighbouring line with a different timetable pattern. so, even on a line with sufficient capacity and train path homogeneity, a pair of long overtaking tracks is desirable for this reason. another reason is preparedness for traffic disruptions. in the case of higher gradients, such track should lay behind the peak of the line and before beginning of higher gradient uphill section, but neither in busy node nor in suburban or another bottleneck area. the discussion above has shown that for fulfilling eu requirements for enabling operation of 740 m long freight trains, it is not necessary to lengthen overtaking tracks in every station on double-track lines. but such tracks are desirable outside bottleneck areas and, if need be, behind a peak of the line. for the freight railway undertakings, the probably best capacity offer would be system of periodic catalogue freight train paths without unnecessary stops, as proposed by drábek [10], but with overtaking tracks long enough. this is the strategy that is recommended to sždc for long-term infrastructure improvement, as a conclusion of this paper. acknowledgements the authors would hereby like to propose a vote of thanks to sždc for enabling them to use railway station schemes [5] and especially to propose a vote of thanks to its employee dr. pavel krýže, who is the author of clearly arranged official maps of czech railway network [11]. the authors were glad to use the mentioned sources for the research presented in this paper. references [1] european commission. directive 2008/57/ec of the european parliament and of the council of 17 june, 2008 on the interoperability of the rail system within the community [online], 2011. http://eur-lex.europa.eu/legal-content/en/txt/ pdf/?uri=celex:02008l0057-20110322&from=en. [2] european commission. regulation (eu) no. 1315/2013 of the european parliament and of the council of 11 december 2013 on union guidelines for the development of the trans-european transport network and repealing decision no. 661/2010/eu [online], 2013. http://eur-lex.europa.eu/legal-content/en/all/ ?uri=uriserv:oj.l_.2013.348.01.0001.01.eng. [3] european commission. regulation (eu) no.913/2010 of the european parliament and of the council of 22 september 2010 concerning a european rail network for competitive freight [online]. european union, brussels, 2010. http://eur-lex.europa.eu/legal-content/en/ all/?uri=celex:32010r0913. [4] j. kušnír, j. ilík. railway in 2030, keynote speech, in: future of railway passenger transport in the czech republic, czech raildays [online], 2013. ostrava: czech ministry of transport, in czech, http://www.railvolution.net/czechraildays/2013/ seminare/konference-kusnir.pdf. [5] sždc: collection of official requisites for 2015/16 timetable, unpublished, praha: sždc, 2015. [6] r. brejcha, r. čech. operation of freight trains long up to 740 m. in: scientific-technical proceedings of czech railways. vol. 40/2015 [online], 2015. praha: czech railways, in czech, http://vtsb.cd.cz/vts/clanky/vts40/4008.pdf. [7] m. binko. railway infrastructure for freight transport, in czech raildays: adaptation of railway to both actual and future transport needs of passengers and goods [online], 2015. praha: m-presse plus s.r.o., in czech., http://www.railvolution.net/ czechraildays/2015/buletin2015.pdf. [8] m. binko. railway infrastructure for freight transport, keynote speech, in czech raildays conference [online], 2015. ostrava: sždc s.o., [2015-06-16], in czech, http://binko.webzdarma.cz/2015-6b.pdf. [9] czech technical standard tnž 34 2620 railway station and line signalling and interlocking systems, amended, effective since july 1. 2002, unpublished, olomouc: české dráhy, 2002. [10] m. drábek. periodic freight train paths in network, doctoral thesis. 2014. praha: ctu in prague, http://takt.fd.cvut.cz/cargo/drabek_thesis.pdf. [11] p. krýže. maps of railway network [online], 2015. praha: sždc, http://provoz.szdc.cz/portal/ viewarticle.aspx?oid=594598. 16 http://eur-lex.europa.eu/legal-content/en/txt/pdf/?uri=celex:02008l0057-20110322&from=en http://eur-lex.europa.eu/legal-content/en/txt/pdf/?uri=celex:02008l0057-20110322&from=en http://eur-lex.europa.eu/legal-content/en/all/?uri=uriserv:oj.l_.2013.348.01.0001.01.eng http://eur-lex.europa.eu/legal-content/en/all/?uri=uriserv:oj.l_.2013.348.01.0001.01.eng http://eur-lex.europa.eu/legal-content/en/all/?uri=celex:32010r0913 http://eur-lex.europa.eu/legal-content/en/all/?uri=celex:32010r0913 http://www.railvolution.net/czechraildays/2013/seminare/konference-kusnir.pdf http://www.railvolution.net/czechraildays/2013/seminare/konference-kusnir.pdf http://vtsb.cd.cz/vts/clanky/vts40/4008.pdf http://www.railvolution.net/czechraildays/2015/buletin2015.pdf http://www.railvolution.net/czechraildays/2015/buletin2015.pdf http://binko.webzdarma.cz/2015-6b.pdf http://takt.fd.cvut.cz/cargo/drabek_thesis.pdf http://provoz.szdc.cz/portal/viewarticle.aspx?oid=594598 http://provoz.szdc.cz/portal/viewarticle.aspx?oid=594598 acta polytechnica ctu proceedings 5:12–16, 2016 1 introduction 2 methods 2.1 rules for determination of available overtaking tracks for 740 m long trains 2.2 where to stop for overtaking – a soft decision process 3 results and discussion 3.1 double-track (or triple-track) lines 3.2 partially double-track lines (middle-busy mainlines) 3.3 single-track lines 3.4 sections and nodes with lack of long overtaking tracks 4 conclusion acknowledgements references 242 acta polytechnica ctu proceedings 2(1): 242–245, 2015 242 doi: 10.14311/app.2015.02.0242 line evolution of the nova v5587 sgr from early to nebula phase t. kajikawa1, a. arai1,2, m. nagashima1, h. kawakita1, m. yamanaka3,4,5, k. kawabata3, s. kiyota6 1kyoto sangyo university, koyama astronomical observatory 2university of hyogo, nishi-harima astronomical observatory 3hiroshima university 4kyoto university 5konan university 6vsolj corresponding author: kajikawat8@gmail.com abstract the spectral evolution of the nova v5587 sgr has been monitored at koyama astronomical observatory and higashihiroshima observatory, japan, from the early to nebula phase. the nova rebrightened several times. the spectra during the early phase showed emission lines of hα, hβ, o i, he i, he ii, n ii, fe ii. nova v5587 sgr is classified into the fe ii type. the helium abundance of the nova is estimated as n(he)/n(h) = 0.134 ± 0.09. the light curve, the spectral evolution, and the helium abundance in v5587 sgr are similar to those of the nova pw vul. keywords: cataclysmic variables classical novae optical spectroscopy photometry individual: v5587 sgr. 1 introduction photometric observations of novae are usually performed by many observers worldwide. the light curves of the novae have been revealed in detail, and they can be classified into several classes (strope et al. 2010). in contrast, spectroscopic observations of novae are not performed as frequently as photometric observations because (i) the number of available spectrographs (and observers) are limited; (ii) some fraction of novae are too faint to observe by spectrographs in their later phase (or even at the maximum for some novae). as novae show a variety of spectral evolutions, spectroscopic monitoring can provide new insights into the physics and chemistry of novae. for example, recently nagashima et al. (2013) detected molecular absorption bands of c2 and cn in nova v2676 oph. (this is the first detection of c2 in novae and the second for cn.) this detection was performed as a part of long-term spectroscopic observations of this nova. the light curves of novae in some classes (strope et al. 2010) can be successfully reproduced by models (as reviewed by prof. kato at this meeting). however, the light curves in the “jitter”and “oscillation”classes (strope et al. 2010) could not be reproduced well by the models. in this paper, we focus on the “jitter”class and introduce our recent spectroscopic observations of v5587 sgr classified in this class. we discuss the nature of this nova as it is likely that there are differrent types of oscillations (i.e., different physical conditions) of novae in the “jitter”class. different types of novae might 6 8 10 12 14 16 18 0 100 200 300 400 500 v ( m ag ni tu de s) days after discovery (t) v5587 sgr pw vul figure 1: v band magnitudes of v5587 sgr and pw vul. the light curve of v5587 sgr is from higashihiroshima observatory, vsnet, and aavso observations. the light curve of pw vul was refer to the online data of strope et al.(2010). 242 http://dx.doi.org/10.14311/app.2015.02.0242 line evolution of the nova v5587 sgr from early to nebula phase be mixed together in this class. even though there are high-quality photometric and spectroscopic observations for some jitter class novae, the mechanism for their rebrightening is still unclear (tanaka et al. 2011). by frequent spectroscopic observations, we can observe phenomena useful to reveal the physics of the nova. in order to investigate the mechanism for the rebrightening in the “jitter”class novae, we have conducted spectroscopic observations of v5587 sgr in 2011 – 2012. the nova v5587 sgr (= nova sgr 2011 no.1) was discovered on ut 2011 january 25.86 by nishimura (nakano et al. 2011). the first low-dispersion spectrum was obtained on ut 2011 january 28 at koyama astronomical observatory. the spectrum shows prominent emission lines of hα, hβ and o i. these features suggest that this object is a classical nova (arai 2011; imamura 2011). 2 observations our photometric and spectroscopic observations were performed at two sites. one site is koyama astronomical observatory (kyoto sangyo university, kyoto, japan). we used the 1.3m araki telescope with the losa/f2 spectrograph (arai et al., in prep.) for low-dispersion spectroscopy. the wavelength coverage is 400 – 800 nm and the spectral resolving power is r = λ/∆λ ∼ 580 at 600 nm. the other site is higashi-hiroshima observatory (hiroshima university, hiroshima, japan). we used the 1.5m kanata telescope with trispec (triple range imager and spectrograph with polarimeter, shutdown in 2012) for photometry and with howpol (hiroshima one-shot wide-field polarimeter) for spectroscopy. its wavelength coverrage is 400 – 1000 nm, and spectral resolving power is r ∼ 400 at 600 nm. 450 500 550 600 650 700 750 800 850 n or m al iz ed fl ux + c on st . wavelength (nm) +596d +229d +213d +185d +167d +154d +147d +129d +100d +93d +81d +69d +68d +61d +60d +45d +43d +39d +35d +27d +11d +9d +8d h e ii (4 68 .6 ) h β f e ii( 42 ) (4 92 .4 ) f e ii( 42 ) (5 01 .8 ) f e ii( 42 ) (5 16 .9 ) [o ii i] (4 95 .9 ) [o ii i] (5 00 .6 ) n ii ( 56 8. 0) [n ii ] ( 57 5. 5) h e i ( 58 7. 4) [o i] ( 63 0. 0) [o i] ( 63 6. 3) h α h e i ( 66 8. 1) h e i ( 70 6. 5) [o ii ] ( 73 3. 0) o i (7 77 .4 ) figure 2: the normalized spectra of v5587 sgr observed from +8d (= ut 2011 february 2) to +596d (= ut 2012 september 12) after the discovery. these spectra were obtained at koyama astronomical observatory except for the data on +35d and +45d (= ut 2001 march 3 and 12), taken at higashi-hiroshima observatory. 243 t. kajikawa et al. 3 results figure 1 shows light curves of v5587 sgr. since v5587 sgr showed an erratic variation in the light curve, we consider that the nova is classified in the “jitter”class. particularly, v5587 sgr seems to be a member of the sub-class prototyped by pw vul as shown in strope et al. (2010). based on the light curves, we determined the date of the visual maximum as 9 days after discovery and also determined t2 = 12 days and t3 = 108 days. from (b v) at maximum magnitude and at t2, we estimate the color excess by intersterllar extinction; e(b v) = 0.85 – 1.26. the light curve of v5587 sgr is quite similar to that of the “jitter”class nova pw vul as shown figure 1 (schwarz et al. 1997, and references therein). pw vul was discovered on 1984 july 27 (= jd 2445917) by wakuda (kosai 1984). this nova reached its maximum light of 6.3 magnitude in the v band on 1984 aug 4th. we compare v5587 sgr with pw vul from various viewpoints in the next section. 500 550 n or m al iz ed fl ux wavelength (nm) +45d +43d +39d +35d +27d +11d +9d +8d hβ fe ii (42) figure 3: the spectra of v5587 sgr in early phase. the nova never showed any regrowth of a p-cygni profile. figure 2 shows the growth of hα, hβ, he i, he ii, n ii, [n ii], o i, [o i], [o ii], and fe ii emission lines of v5587 sgr. novae that exhibit rebrightening in the early phase often show the regrowth of a p-cygni profile (tanaka et al. 2011, csak et al. 2005) as in v4745 sgr. however, v5587 sgr has never shown this behavior (figure 3). it is considered that the nova entered into the nebula phase between july 11 to july 29 because the [o iii] forbidden emission lines dominated hβ at that time (figure 4). we assume that the electron density in the nebula phase was ne = 106 ∼ 108 cm−3 according to iijima & esenoglu (2003) and iijima (2006), and we estimate the helium abundance of the ejecta based on the data between july 11 and july 29 ; n(he)/n(h) = 0.134 ± 0.09 (for ne = 106 cm−3), n(he)/n(h) = 0.139 ± 0.09 (for ne = 108 cm−3). 450 500 550 600 650 700 750 800 n or m al iz ed fl ux wavelength (nm) +147d +129d h e ii (4 68 .6 ) h β [o ii i] + f e ii( 42 ) n ii ( 56 8. 0) h e i ( 58 7. 4) [o i] ( 63 0. 0) [o i] ( 63 6. 3) h e i ( 66 8. 1) h e i ( 70 6. 5) [o ii ] ( 73 3. 0) o i (7 77 .4 ) figure 4: the spectra of v5587 sgr in nebular phase(+129 days and +147 days after its maximum). an emission line of he ii was recognized in these spectra. 4 discussion & conclusion the decline rates, amplitudes, and intervals of rebrightening observed in v5587 sgr are similar to those of pw vul. this fact suggests that the wd mass and physical parameters related to the explosion of v5587 sgr would be similar to those of pw vul. comparison between the spectra of v5587 sgr and pw vul also lead us to the same conclusion, namely, they are similar to each other, e.g., the late-phase spectra of v5587 sgr (shown in figure 4 in rosino and iijima 1987). figure 5 also shows helium abundances vs t3 in 20 various novae. helium abundance of v5587 sgr seems to be similar to that of pw vul. this figure indicates that helium abundance is not correlated with t3. in summary, we have performed photometric and low-dispersion spectroscopic observations of v5587 sgr from early to nebula phase. the photometry showed erratic variations of the light curve. the spectra during the early phase showed emission lines of hα, hβ, and fe ii (i.e., fe ii type) and v5587 sgr never showed regrowth of a p-cygni profile. the nova entered the nebula phase between july 11 to july 29 (t = 158 to 178days). we estimated that the helium abundance of v5587 sgr is n(he)/n(h) = 0.134 ± 0.09. the nova is very similar to pw vul considering decline rates, spectral features, and helium abundance. we obtained a new sample of the “jitter”class novae. 244 line evolution of the nova v5587 sgr from early to nebula phase -1 -0.5 0 0.5 0.5 1 1.5 2 2.5 3 3.5 lo g 1 0( n (h e) /n (h )) log10t3 v1494 aql(1) v723 cas(2) v2468 cyg(3) cl aql(4) v407 cyg(5) v1500 cyg(6) qu vul(7) v351 pup(8) u sco(9) pw vul(10),(11) v842 cen(11) v827 her(11) qv vul(11) v2214 oph(11) v977 sco(11) v443 sct(11) v1668 cyg(11) v693 cra(11) v1370 aql(11) v5587 sgr figure 5: the helium abundance of 20 novae. references: (1) iijima & esenoglu (2003); (2) iijima (2006); (3) iijima & naito (2011); (4) iijima (2012a); (5) iijima (2012b); (6) ferland (1978); (7) schwarz (2002); (8) saizar (1996); (9) iijima (2002); (10) schwarz et al. (1997); (11) andreä et al. (1994). references [1] andreä, j., drechsel, h., starrfield, s.: 1994, a&a 291, 869. [2] arai, a.: 2011, iauc 9196, 2. [3] chochol, d., et al.: 2012, mmsai 83, 767. [4] ferland, g.j.: 1978, apj 219, 589. doi:10.1086/155818 [5] iijima, t.: 2002, a&a 387,1013. [6] iijima, t., esenoglu, h.h.: 2003, a&a 404, 997. [7] iijima, t.: 2006, a&a 451, 563. [8] iijima, t., naito, h.: 2011, a&a 526a, 73. [9] iijima, t.: 2012a, mmsai 83, 835. [10] iijima, t.: 2012b, a&a 544a, 26. [11] imamura, k.: 2011, iauc 9196, 2. [12] nagashima, m., et al.: 2014, journal?, in press. [13] nakano, s., nishimura, h., kiyota, s., yusa, t.: 2011, iauc 9196, 1. [14] rosino, l., iijima, t.: 1987, ap&ss 130, 157. doi:10.1007/bf00654989 [15] saizar, p., et al.: 1991, apj 367,310. doi:10.1086/169630 [16] saizar, p., et al.: 1996, mnras 279, 280. doi:10.1093/mnras/279.1.280 [17] schwarz, g.j., et al.: 1997, mnras 290, 75. doi:10.1093/mnras/290.1.75 [18] schwarz, g.j.: 2002, apj 577, 940. doi:10.1086/342234 [19] strope, r.j., schaefer, b.e., henden, a.a.: 2010, aj 140, 34. [20] tanaka, j., et al.: 2011, pasj 63, 159. [21] tanaka, j., et al.: 2011, pasj 63, 911. 245 http://dx.doi.org/10.1086/155818 http://dx.doi.org/10.1007/bf00654989 http://dx.doi.org/10.1086/169630 http://dx.doi.org/10.1093/mnras/279.1.280 http://dx.doi.org/10.1093/mnras/290.1.75 http://dx.doi.org/10.1086/342234 introduction observations results discussion & conclusion acta polytechnica ctu proceedings doi:10.14311/app.2017.13.0029 acta polytechnica ctu proceedings 13:29–34, 2017 © czech technical university in prague, 2017 available online at http://ojs.cvut.cz/ojs/index.php/app synthesized enrichment functions for extended finite element analyses with fully resolved microstructure martin doškářa,∗, jan nováka, jan zemana,b a faculty of civil engineering, czech technical university in prague, thákurova 7/2077, 166 29 prague 6, czech republic b the institute of information theory and automation, academy of sciences of the czech republic, pod vodárenskou věží 4, 182 08 prague 8, czech republic ∗ corresponding author: martin.doskar@fsv.cvut.cz abstract. inspired by the first order numerical homogenization, we present a method for extracting continuous fluctuation fields from the wang tile based compression of a material microstructure. the fluctuation fields are then used as enrichment basis in extended finite element method (xfem) to reduce number of unknowns in problems with fully resolved microstructural geometry synthesized by means of the tiling concept. in addition, the xfem basis functions are taken as reduced modes of a detailed discretization in order to circumvent the need for non-standard numerical quadratures. the methodology is illustrated with a scalar steady-state problem. keywords: wang tiling, microstructure synthesis, microstructure-informed enrichment functions, extended finite element method. 1. introduction all materials exhibit heterogeneous microstructure at certain level of detail. besides the physical properties of microstructural constituents, the overall response is also dictated by their spatial arrangement. with the advance of numerical methods and computational power, there is an emphasis on devising approaches that take the microstructural characteristics into account and propagate knowledge of material composition into upper-scale models. in our contribution, we focus on problems in which the underlying microstructural geometry is explicitly known. we consider problems where the characteristic length of the microstructure is comparable to the dimension of an upper-scale model; hence, the nested numerical homogenization, such as the fe2 method [1], is not applicable because the underlying assumption of the separation of scales is violated [2, and references therein]. this work supplements our previous results regarding efficient modelling of materials with stochastic heterogeneous microstructure, e.g., [3, 4]. the microstructural geometry of a macro-scale model—as well as its finite element discretization—is synthesized using the wang tiling concept, recalled in section 2. here, we present a method to extract dominant responses of a compressed microstructure to prescribed macroscopic loading. we reuse the extracted responses to synthesize a microstructure-informed enrichment basis in order to accelerate the upper scale calculations. the numerical scheme, outlined in section 4, combines extended finite element framework (xfem) [5, 6, and references therein], used for definition of local basis functions, with the reduced order modelling approach [7], allowing for utilization of standard finite element procedures. finally, section 5 illustrates the methodology with a two-dimensional problem of steady-state thermal conduction in an l-shaped domain. 2. wang tiling concept microstructure of random heterogeneous materials is usually represented by means of a periodic finitesize model—appearing in the literature under various names such as statistically optimal representative unit cell, statistically similar representative volume element, or statistically equivalent periodic unit cell (sepuc)—that resembles the material in terms of various spatial statistics. this setting is particularly suitable for the family of fe2 simulations,e.g., [1], in which it is often accompanied with periodic boundary conditions imposed at the micro-scale level. however, when used for microstructure synthesis, this representation introduces unrealistic, spurious periodic correlations. the formalism of wang tiles represents a compromise between the amount of induced correlation and complexity of a microstructure representation. instead of a single cell, microstructural information is compressed into a set of equi-sized wang tiles with pre-defined requirements on mutual compatibility. in the abstract setting, the compatibility constraints are represented by codes assigned to tile edges, illustrated with distinct colours in figure 1. a realization of the compressed microstructure is then synthesized following a simple stochastic algorithm: an empty grid of 29 http://dx.doi.org/10.14311/app.2017.13.0029 http://ojs.cvut.cz/ojs/index.php/app m. doškář, j. novák, j. zeman acta polytechnica ctu proceedings a desired size is sequentially filled with instances of wang tiles such that the adjacent tile edges are compatible. at each position, a tile is randomly chosen from a subset of tiles compliant with previously placed tiles; hence, the algorithm generates stochastic realizations, providing that set of tiles is rich enough such that multiple tiles are admissible at each assembly step; see figure 1 for an illustration. figure 1. the compressed microstructural geometry represented with the set of wang tiles (top) and an illustration of a step in the tiling algorithm with partially assembled microstructure (bottom). procedures designed to compress a given microstructure into a sepuc can be straightforwardly extended to take into account generalized periodicity occurring in the tiling concept, e.g., the standard optimization procedure based on minimizing discrepancy in the two-point probability function was used in [3]. we also adapted a sample-based approach originated in computer graphics in order to address the high computational cost of the optimization-based design [4]. currently, a variety of material microstructures, ranging from particulate media to complex foam-like microstructures, can be represented with the framework of wang tiling. individual realizations of the microstructure are then generated in linear time with respect to the required sample size, which makes the tiling concept appealing for applications where multiple microstructure samples are required, e.g., investigation of the rve size [8]. 3. synthesized enrichments fields the idea of compressing fluctuation fields using the wang tiling concept has been introduced in novák et al. [9], where the microstructural geometry of individual tiles was optimized with respect to (i) meeting prescribed spatial statistics and (ii) minimizing traction discrepancies among congruent edges in an auxiliary tiling. however, the resulting synthesized stress fields exhibited unavoidable discontinuities and the objective function required careful tuning of weight parameters because the two requirements turned to be hard to achieve simultaneously. in contrast, the present method generates continuous fields of the primal unknowns for any existing microstructure compression. we illustrate the methodology with the steady-state heat conduction problem, governed by ∇ · (−k(x) · ∇θ(x)) = 0 , x ∈ ω , (1) where θ denotes the unknown temperature field, k is a local heat conductivity tensor and no heat sources are considered. first, we assume the decomposition of θ into a fluctuation part θ̃, caused by the presence of heterogeneities, and a macroscopic part controlled with a macroscopic temperature gradient h, θ(x) = θ̃(x) + h · x , ∀x ∈ t(i),∀t(i) ∈ s, (2) where t(i) denotes the domain of the i-th tile and s ={ t(i), i = 1 . . . nt } represent the set of nt available tiles. without loss of generality, each tile is assumed centred, i.e., t(i) = [−α, α]2, where 2α is the size of the tile edge. in order to enforce continuity of the extracted fields, all tiles are loaded with the prescribed macroscopic gradient h and solved simultaneously. providing that the tile geometries are discretized with a compatible triangulation—meaning that discretization of the corresponding tile edges is the same across all tiles—the continuity is ensured by associating the corresponding edge unknowns during the localization of an element contribution into the conductivity matrix of the whole set. the resulting system takes the standard form ks θ̃ = f (h) , (3) where ks = at(i)∈s kt(i) is the conductivity matrix and f (h) is a h-dependent loading vector arising from the weak form after plugging eq. (2) into eq. (1). ks has block-diagonal structure similar to primal domain decomposition approaches. in addition, two types of constraints are imposed on the set system. first, we eliminate zero-energy modes of ks by requirement 1 |s| ∫ s θ̃(x) dx = 0 . (4) the second constraint prevents the fluctuation field θ̃ from compensating for the loading induced through h . 30 vol. 13/2017 synthesized microstructure-informed enrichments for xfem analyses inspired in numerical homogenization, e.g. [2], we introduce the second constraint enforcing 1 |ω| ∫ ω ∇θ̃(x) dx = 0 (5) in a chosen domain ω loaded analogously to eq. (2). using green’s first identity, eq. (5) can be recast into a boundary integral∫ ∂ω θ̃(x)n(x) ds = 0 , (6) where n(x) denotes the outer normal of the boundary. in particular, we consider three mutually exclusive types of the second constraint: (k) the first type prescribes zero temperature fluctuations at tile boundaries, θ̃|∂t(i) = 0 , ∀i = 1 . . . nt , (7) where ∂t(i) stands for the boundary of the i-th tile. (p) in the second type, the requirement (6) is posed tile-wise, i.e.,∫ ∂t(i) θ̃(x)n(x) ds = 0 , ∀i = 1 . . . nt . (8) (s) the last type imposes eq. (6) over the whole set,∫ ∂s θ̃(x)n(x) ds = 0 , (9) where ∂s = { ∂t(i), i = 1 . . . nt } . note that (k) corresponds directly to kinematic uniform boundary conditions used in numerical homogenization, while (p) and (s) mimic periodic and statically uniform boundary conditions, respectively. instead of eq. (3), the final fluctuations θ̃ follows from ks ct i ct ii ci 0 0 cii 0 0  θ̃ λi λii  = f(h) 0 0  , (10) where ci and cii matrices represent the discretized versions of eq. (4) and one of eqs. (7), (8), or (9), respectively, and λ• stands for the corresponding vector of lagrange multipliers. while cii straightforwardly eliminates edge unknowns for the constraint type (k), constructing contributions of individual tile separately and localizing them into cii leads to an ill-posed problem for types (p) and (s) from eq. (6), it clearly follows that if two opposite sides of a tile are required to be compatible— hence they feature the same trace of θ̃—the corresponding component of ∇θ̃ average vanishes and so does the tile contribution to the relevant row in cii. in addition, the ill-posedness also stems from the repetition of edge code pairs within the tile set (swapping the order of codes changes only the sign of the edge contribution to eq. (5)). in certain cases, this can even result in zero matrix cii for the least restrictive constraint (s), in which tile contributions are summed together. one way to treat the singularity of cii is to perform singular value decomposition on the constraint and take into account only the non-zero singular values; however, such a procedure is sensitive to numerical precision. thanks to the simple structure of the constraint, the regular part of the constraint can be directly established as a projection of the original matrix cii. first, all nc combinations of edge codes pertinent to either horizontal or vertical edges are identified and enumerated. next, an empty matrix p ∈ r2nt ×nc , for (p) type of the constraint, or p ∈ r2×nc , for (s) type, is created and populated with 1 (or −1) if the tile contains an enumerated pair of codes (or a pair in the inverse order). odd and even rows of the matrix correspond to vertical and horizontal edges, respectively. if p remains empty for (s), cii constraint is discarded entirely. otherwise, the non-singular constraint matrix cii, replacing cii in eq. (10), is obtained from a projection cii = ptcii. the outlined strategy allows to define one enrichment field θ̃ for a prescribed h and the selected type of the second constraint. for the linear problem considered in this work, two mutually orthonormal loading cases h = { 1, 0 }t and h = { 0, 1 }t cover all possible load cases. combined with three types of cii, we can generate up to six enrichment fields. note that unlike the nested numerical homogenization, where the choice of boundary conditions is based on their suitability in capturing effects of a surrounding medium, we combine different types of cii to control cardinality of the set of compressed fluctuations. figure 2. a compressed fluctuation field obtained by prescribing (s) variant of the second constraint and setting h = {1, 0}t. 4. numerical strategy the fields introduced above are employed as enrichments in a upper-scale model whose microstructural geometry is assembled by means of tiling from the set depicted in figure 1. assume a upper-scale problem governed by the same differential equation as in eq. (1), valid in a upperscale domain ω and accompanied by boundary conditions θ(x) = θ̂(x) x ∈ γθ 6= ∅ , (11) (−k(x)∇θ(x)) · n(x) = q̂(x) x ∈ γq , (12) 31 m. doškář, j. novák, j. zeman acta polytechnica ctu proceedings where θ̂ and q̂ are the given temperature and normal heat flux profiles, respectively, prescribed at parts γθ and γq of the domain boundary ∂ω. an approximate numerical solution arises from the weak form of the problem find θ ∈ v such that a(θ, ϑ) = b(ϑ), ∀ϑ ∈ v0 . (13) and substitutes for the analytical solution that is usually intractable. in eq. (13), a(θ, ϑ) and b(ϑ) denote the bilinear and linear form pertinent to eq. (1); v and v0 stand for the space of admissible temperature fields. quality of the approximate solution is governed by suitability of a finite-dimensional subspace vc ⊂ v in which the solution is sought for. in particular, domain discretization must accurately resolve all geometrical details in the standard finite element (fe) setting. this requirement leads to very fine meshes in applications where microstructural geometry of the domain is involved. besides local mesh refinements, the approximation space can be enhanced by incorporating prior knowledge of local character of the solution. concretely, in the extended finite element framework [5, 6], the approximate solution θc ∈ vc takes the form θc(x) = nc∑ i=1 ni(x) θi + nc∑ i=1 ne∑ j=1 ni(x)ψj(x) θji , (14) where the first sum contains the standard finite element shape functions ni(x) and the second term adds ne enrichment functions ψj for each discretization node; θi denotes the regular degree of freedom (dof) associated with finite-element mesh node xi, θji are the additional dofs. in traditional xfem, only patches of the domain are usually enriched with ψj , suited for one geometrical feature. typically, the enrichment provides asymptotic solution near a crack tip or introduces strong/weak discontinuities [6]. here, we add global enrichments that capture collective response of a material microstructure. the original shape functions ni(x) model the macroscopic response of the domain, while products ni(x)ψj(x) cover the fluctuations caused by material heterogeneity. a similar idea was introduced by strouboulis et al. [10], who used numerical “handbook” functions for assemblies of closely packed inclusions, or plews and duarte [11]. however, both approaches solve local boundary value problems defined on subdomains first and subsequently run the analysis of the whole domain. in contrast, we precompute the fluctuations purely “off-line” at the level of microstructure compression. i.e., without any information of the domain geometry or loading, and construct the enrichments as an assembly of the tiledefined fluctuation fields. in our approach, the upper scale discretization is constructed irrespectively of the underlying microstructural geometry of ω. hence, the approximation basis functions in the form of eq. (14) together with fluctuations in material parameters due to the presence of microstructural geometry preclude efficient use of standard numerical procedures for evaluation of the bi-linear form in eq. (13). in order to circumvent this drawback, we construct another, finer domain discretization space vf , assembled from the tile discretization used in the off-line phase for extracting fluctuation fields. the shape functions ni(x), defined at the coarser discretization, are projected onto the fine mesh; enrichments ψj(x) are already defined within the fine discretization by construction. consequently, eq. (14) can be understood as a definition of reduced modes for the fine discretization. let matrix φ comprises individual reduced modes as its columns, computed as an element-wise product of the projectedni(x) and ψj(x) following eq. (14). the range of φ thus defines a subspace of vf corresponding to vc. instead of solving the large, fine-discretization system kfθf = ff , we restrict the fine-discretization unknowns θf via θf = φ a , (15) where a denotes the vector of unknown coefficients pertinent to the reduced modes. the final algebraic system then takes the form φtkfφa = φtff . (16) 5. results we considered the material microstructure of monodisperse elliptical inclusions, shown in figure 1. both phases were assumed isotropic, with the inclusion material being more conductive (k(x) = 100 i) than the matrix phase (k(x) = 1 i). six fluctuation fields in total, discretized with linear triangular elements, were extracted, see figure 2. the microstructure was superimposed to an upperscale problem represented by an l-shaped domain with uniform temperature profiles prescribed at the bottom and right-hand side edges. the initial coarse discretization of the domain is depicted in figure 3. the linear lagrange basis functions were assumed at the coarse level, whilst the fine space comprised the quadratic lagrange approximation arising from the product of linear coarse shape functions and the enrichments. the solution θxfem of eq. (16) was compared against direct numerical simulation (dns) θdns, which was taken as the reference solution. a sequence of five uniformly refined coarse discretizations, the first one shown in figure 3, was investigated. the proximity of the reference (dns) and xfem solutions 32 vol. 13/2017 synthesized microstructure-informed enrichments for xfem analyses θ̂ = 20 10 tx 10 ty θ̂ = 0 10 tx 10 ty figure 3. a scheme of the problem definition. the edge lengths tx and ty of the tiles were set as tx = ty = 6. all quantities are in consistent units. was quantified via relative errors εl2 = ∥∥θdns − θxfem ∥∥ l2 ‖θdns‖l2 and (17) εh1 = ∥∥θdns − θxfem ∥∥ h1 ‖θdns‖h1 , (18) where ‖u‖2l2 = ∫ ω u2(x) dx and (19) ‖u‖2h1 = ∫ ω u2(x) +∇u(x) · ∇u(x) dx . (20) mean and standard deviations of the errors computed for five coarse discretizations and six different tiling realizations are given in figure 4. 10!4 10!3 10!2 xfem/dns dofs 10!2 10!1 0 0 l2 h1 figure 4. convergence of l2 and h1 errors with increasing number of dofs in the coarse discretization. 6. conclusions we have presented a method for extracting continuous fluctuation fields from a microstructure compressed by means of wang tiles. we have also demonstrated utilization of these fields as enrichments in xfem. in order to avoid non-standard quadratures, xfem basis functions were projected onto a discretization arising from assembly of the tile discretization used for computing the fluctuation fields. the proposed methodology was applied to a steadystate heat conduction problem defined in an l-shaped domain with microstructural geometry provided by the tiling concept. only 1.6 % of the original degrees of freedom were sufficient to obtain 2 % relative error compared to dns. on the other hand, the reduction in dofs accelerates only the solution of eq. (16). for more significant time savings, a low-rank additional approximation of kf , e.g. using the idea of hyper-reduction [12], has to be introduced and is the focus of our current work. we have also restricted ourself to the first-order decomposition, eq. (2); higher order expansion of the macroscopic loading can be considered to further enrich the approximation space. acknowledgements m. doškář and j. novák acknowledge the endowment of the ministry of industry and trade of the czech republic under the project no. fv10202. in addition, m. doškář was supported by the grant agency of the czech technical university in prague, project no. sgs17/042/ohk1/1t/11 “numerical methods for modeling uncertainties in civil engineering”. references [1] f. feyel, j.-l. chaboche. fe2 multiscale approach for modelling the elastoviscoplastic behaviour of long fibre sic/ti composite materials. computer methods in applied mechanics and engineering 183(3-4):309–330, 2000. doi:10.1016/s0045-7825(99)00224-8. [2] m. g. d. geers, v. g. kouznetsova, k. matouš, j. yvonnet. homogenization methods and multiscale modeling: nonlinear problems. in e. stein, r. de borst, t. j. r. hughes (eds.), encyclopedia of computational mechanics second edition, pp. 1–34. john wiley & sons, ltd, chichester, uk, 2017. doi: 10.1002/9781119176817.ecm107. [3] j. novák, a. kučerová, j. zeman. compressing random microstructures via stochastic wang tilings. physical review e 86(4):040104(r), 2012. doi:10.1103/physreve.86.040104. [4] m. doškář, j. novák, j. zeman. aperiodic compression and reconstruction of real-world material systems based on wang tiles. physical review e 90(6):062118, 2014. doi:10.1103/physreve.90.062118. [5] t.-p. fries, t. belytschko. the extended/generalized finite element method: an overview of the method and its applications. international journal for numerical methods in engineering 84(3):253–304, 2010. doi:10.1002/nme.2914. [6] t. strouboulis, k. copps, i. babuška. the generalized finite element method. computer methods in applied mechanics and engineering 190(32-33):4081–4193, 2001. doi:10.1016/s0045-7825(01)00188-8. 33 http://dx.doi.org/10.1016/s0045-7825(99)00224-8 http://dx.doi.org/10.1103/physreve.86.040104 http://dx.doi.org/10.1103/physreve.90.062118 http://dx.doi.org/10.1002/nme.2914 http://dx.doi.org/10.1016/s0045-7825(01)00188-8 m. doškář, j. novák, j. zeman acta polytechnica ctu proceedings [7] j. yvonnet, q.-c. he. the reduced model multiscale method (r3m) for the non-linear homogenization of hyperelastic media at finite strains. journal of computational physics 223(1):341–368, 2007. doi:10.1016/j.jcp.2006.09.019. [8] m. doškář, j. novák. a jigsaw puzzle framework for homogenization of high porosity foams. computers & structures 166:33–41, 2016. doi:10.1016/j.compstruc.2016.01.003. [9] j. novák, a. kučerová, j. zeman. microstructural enrichment functions based on stochastic wang tilings. modelling and simulation in materials science and engineering 21(2):025014, 2013. doi:10.1088/0965-0393/21/2/025014. [10] t. strouboulis, l. zhang, i. babuška. generalized finite element method using mesh-based handbooks: application to problems in domains with many voids. computer methods in applied mechanics and engineering 192(28-30):3109–3161, 2003. doi:10.1016/s0045-7825(03)00347-5. [11] j. plews, c. duarte. bridging multiple structural scales with a generalized finite element method. international journal for numerical methods in engineering 102(3-4):180–201, 2015. doi:10.1002/nme.4703. [12] d. ryckelynck. hyper-reduction of mechanical models involving internal variables. international journal for numerical methods in engineering 77(1):75–89, 2009. doi:10.1002/nme.2406. 34 http://dx.doi.org/10.1016/j.jcp.2006.09.019 http://dx.doi.org/10.1016/j.compstruc.2016.01.003 http://dx.doi.org/10.1088/0965-0393/21/2/025014 http://dx.doi.org/10.1016/s0045-7825(03)00347-5 http://dx.doi.org/10.1002/nme.4703 http://dx.doi.org/10.1002/nme.2406 acta polytechnica ctu proceedings 13:29–34, 2017 1 introduction 2 wang tiling concept 3 synthesized enrichments fields 4 numerical strategy 5 results 6 conclusions acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2017.12.0108 acta polytechnica ctu proceedings 12:108–111, 2017 © czech technical university in prague, 2017 available online at http://ojs.cvut.cz/ojs/index.php/app interactive motion platforms and virtual reality for vehicle simulators evžen thöndel pragolet, s.r.o., director and czech technical university in prague, assistant professor at the department of electric drives and traction correspondence: thoendel.jr@pragolet.cz abstract. interactive motion platforms are intended for vehicle simulators, where the direct interaction of the human body is used for controlling the simulated vehicle (e.g. bicycle, motorbike or other sports vehicles). the second use of interactive motion platforms is for entertainment purposes or fitness. the development of interactive motion platforms reacts to recent calls in the simulation industry to provide a device, which further enhances the virtual reality experience, especially with connection to the new and very fast growing business in virtual reality glasses. the paper looks at the design and control of an interactive motion platform with two degrees of freedom to be used in virtual reality applications. the paper provides the description of the control methods and new problems related to the virtual reality sickness are discussed here. keywords: motion platform, motion cueing, simulator, simulation, virtual reality, vr sickness. 1. introduction interactive motion platforms are intended for a simulation where direct interaction with a human body is required. this type of simulators is e.g. a motorbike or bicycle simulator or other types of vehicles where direct interaction with the human body is used for controlling the vehicle. a simulator with an interactive motion platform is a closed loop system, where a human being is a part of the system. in contrast to standard humanin-the-loop systems[1], the human does not interact with the system through control elements (such as a steering wheel or pedals) but creates forces and torques which are directly measured and used as input to a mathematical model. the structure of the closed loop system is shown in the next picture. the structure and principle of individual blocks are further discussed. 2. motion platform the purpose of a motion platform in simulators is to reproduce motion effects, such as acceleration, and position towards gravity. different types and principles are discussed in the literature[2]. the important parameter of each motion platform is the number of degrees of freedom (dof) and the range of motion in all directions. the most widely used motion platforms today have 2, 3 or 6 dof. in the case of simulators of land vehicles, the dominant motion is rotation around the horizontal axis. this motion can also be used for reproducing linear accelerations (by using a motion cueing algorithm[3]). the second aspect which must be considered, when designing a new motion platform, is the final price which significantly grows with the number of degrees of freedom. a motion platform with two degrees of freedom is the first design and implementation of an interactive motion platform. an extension with a horizontal rotation of 360 degrees is considered in the future, to avoid or to solve a problem connected with virtual reality sickness. the design of the 2dof interactive motion platform is shown in the picture below. figure 1. design of the motion platform. all kinematic and dynamic parameters are summarized in the following table: maximum payload 150 kg tilt angle 16 degrees degrees of freedom 2 (pitch and roll) maximum tilt rate 100 degrees/s maximum tilt acceleration 500 degrees/s2 power supply requirements 230 v/50 hz, 16 a weight 80 kg table 1. kinematic and dynamic properties. 108 http://dx.doi.org/10.14311/app.2017.12.0108 http://ojs.cvut.cz/ojs/index.php/app vol. 12/2017 interactive motion platforms and virtual reality for vehicle simulators figure 2. structure of the system with an interactive motion platform. 3. mathematical model of the dynamic response a mathematical model of the dynamic response reads information about the acting forces and torques and calculates the response of the motion platform. besides the input from the human, the mathematical model of the dynamic response can also read signals from a superior mathematical model where the dynamic response of the simulated vehicle is implemented. an implementation of the mathematical model of the dynamic response is shown in the next picture. this structure applies a motion cueing algorithm as well as a model of direct dynamic response (called stiffness in the following structure). the principle of a motion cueing algorithm is described in detail in the previous author’s publications[4], and will not be discussed here. a model of the direct dynamic response can be any dynamic system which reacts directly to forces measured on the motion platform. one possible use of the interactive motion platform is rehabilitation or fitness where the model can implement a system of the load. one example is presented in the next picture where a simple model of a rubber ball is shown. all mathematical models have been implemented in matlab/simulink and directly compiled to the control hardware. this method of programming is very effective, and it allows the motion platform to also be used in research projects[5]. 4. load sensors as discussed above, the main innovation of the new motion platform is the possibility to measure forces and torques which are created by a person standing or sitting on the motion platform. four load cells, weighing 100 kg each, are used to measure and further calculate the position (center of gravity) and the intensity of the loading force. figure 3. load sensors attachment. 5. the use in virtual reality there has been fast growth in virtual reality over the last two years, especially connected to the use of virtual glasses. new applications and technologies are continuously being researched. the presented motion platform is ideally suited for vehicle simulators where virtual glasses are used to generate visual perception. several designs of applications have been created, and the first real application of a kayak simulator has been finished in 2016[6]. the first test and the response from many users show a promising potential for business success and further technological enhancement of the use of vr glasses because it significantly reduces the occurrence of virtual reality sickness. 6. virtual reality sickness virtual reality sickness is a common problem which is often discussed. any mismatch between what is seen and what is felt can lead to virtual reality sickness. the motion platform can reproduce a wide range of motion effects, and therefore it contributes to the reduction of the probability of occurrence of virtual reality sickness. however, two degrees of freedom do not cover all motion directions. the horizontal rotation especially is often mentioned in relation to this phenomenon, and it is recommended not to rotate the camera in the vr world at all[7] or to use a 109 evžen thöndel acta polytechnica ctu proceedings figure 4. mathematical model of the dynamic response. figure 5. mathematical model of the direct dynamic response – rubber ball. figure 6. application of the motion platform. 110 vol. 12/2017 interactive motion platforms and virtual reality for vehicle simulators motion platform with a rotary axis[8]. the vertical linear acceleration can partially be compensated for by rotating the motion platform. therefore, it is planned to extend the construction with a rotation of 360 degrees. this new design will be an optimal construction for simulators using vr headsets with respect to the range of reproduction of motion effects and the financial costs. 7. conclusion a new interactive motion platform has been presented here. the possibility to measure acting forces and torques makes the motion platform suitable for the use in vehicle simulators where direct interaction with a human body is required (bicycle or motorbike). the main advantage is the enhancement of the experience and the contribution to the reduction of the virtual sickness phenomenon. we plan to extend the motion platform with a rotary axis which further improves the solution of the problem with vr sickness. acknowledgements the research and development of the new interactive motion platform is supported by the company pragolet, s.r.o. member of the center of intelligent drives and advanced machine control (cidam) supported by the technology agency of the czech republic (tacr). references [1] human-in-the-loop. wikipedia, the free encyclopedia. wikipedia, the free encyclopedia, 10 dec. 2016. web. 10 dec. 2016. [2] e. thöndel. design and optimal control of a linear electromechanical actuator for motion platforms with six degrees of freedom. 2011, intelligent automation and systems engineering, springer, 65-77. [3] e. thöndel. design and optimization of a motion cueing algorithm for a truck simulator. 2012, in proceeding of the 2012 european simulation and modeling conference, essen, germany, 165-170. [4] e. thöndel. model predictive motion cueing algorithm for a truck simulator. 2013, in proceeding of the 2013 european simulation and modeling conference, lancaster, united kingdom, 188-192. [5] p. members. vývoj zpětnovazebního rehabilitačního přístroje založenãľho na principu pohybové terapie. research project, https://www.rvvi.cz/cep?ss=detail&h=ta03010313, 2015. [6] p. contributors. 4d vr experience modul. the global association for marketing at retail, popai awards 2016, http://www.popai.cz/files/tinymce/files/ popai%20awards%202016/vyher/20.pdf, 2016. [7] o. contributors. oculus rift vr motion sickness. oculus http: //riftinfo.com/oculus-rift-motion-sickness/ -11-techniques-to-prevent-it, 2016. [8] r. contributors. yaw motion sickness. http://www.rotovr.com, 2016. 111 https://www.rvvi.cz/cep?ss=detail&h=ta03010313 http://www.popai.cz/files/tinymce/files/popai%20awards%202016/vyher/20.pdf http://www.popai.cz/files/tinymce/files/popai%20awards%202016/vyher/20.pdf http://riftinfo.com/oculus-rift-motion-sickness/-11-techniques-to-prevent-it http://riftinfo.com/oculus-rift-motion-sickness/-11-techniques-to-prevent-it http://riftinfo.com/oculus-rift-motion-sickness/-11-techniques-to-prevent-it http://www.rotovr.com acta polytechnica ctu proceedings 12:108–111, 2017 1 introduction 2 motion platform 3 mathematical model of the dynamic response 4 load sensors 5 the use in virtual reality 6 virtual reality sickness 7 conclusion acknowledgements references 71 acta polytechnica ctu proceedings 1(1): 71–78, 2014 71 doi: 10.14311/app.2014.01.0071 clustering measurements of broad-line agns: review and future mirko krumpe1,3, takamitsu miyaji2,3, and alison l. coil3 1european southern observatory, karl-schwarzschild-straße 2, 85748 garching bei münchen, germany 2instituto de astronomı́a, unam, apdo. postal 106, ensenada, bc, méxico 3university of california san diego, cass, 9500 gilman drive, la jolla, ca 92093-0424, usa corresponding author: mkrumpe@eso.org abstract despite substantial effort, the precise physical processes that lead to the growth of super-massive black holes in the centers of galaxies are still not well understood. these phases of black hole growth are thought to be of key importance in understanding galaxy evolution. forthcoming missions such as erosita, hetdex, eboss, bigboss, lsst, and panstarrs will compile by far the largest ever active galactic nuclei (agns) catalogs which will allow us to measure the spatial distribution of agns in the universe with unprecedented accuracy. for the first time, agn clustering measurements will reach a level of precision that will not only allow for an alternative approach to answering open questions in agn and galaxy co-evolution but will open a new frontier, allowing us to precisely determine cosmological parameters. this paper reviews large-scale clustering measurements of broad line agns. we summarize how clustering is measured and which constraints can be derived from agn clustering measurements, we discuss recent developments, and we briefly describe future projects that will deliver extremely large agn samples which will enable agn clustering measurements of unprecedented accuracy. in order to maximize the scientific return on the research fields of agn and galaxy evolution and cosmology, we advise that the community develops a full understanding of the systematic uncertainties which will, in contrast to today’s measurement, be the dominant source of uncertainty. keywords: large-scale structure of universe galaxies: active. 1 introduction large area surveys such as the two degree field galaxy redshift survey (2dfgrs; colless et al. 2001) and the sloan digital sky survey (sdss; abazajian et al. 2009) have measured positions and redshifts of millions of galaxies. these measurements allow us to map the 3d structure of the nearby universe1. galaxies are not randomly distributed in space. they form a complex cosmic network of galaxy clusters, groups, filaments, isolated field galaxies, and voids, which are large regions of space that are almost devoid of galaxies. the current understanding of the distribution of galaxies and structure formation in the universe is based on the theory of gravitational instability. very early density fluctuations became the “seeds” of cosmic structure. these have been observed as small temperature fluctuations (δt/t ∼ 5 × 10−5) in the cosmic microwave background with the cosmic background explorer (smoot et al. 1992). the small primordial matter density enhancements have progressively grown through gravitational collapse and created the complex network seen in the distribution of matter in the later universe. during a galaxy’s lifetime different physical processes, which are still not well understood, can trigger a mass flow onto the central super-massive black hole (smbh). in this phase of galaxy evolution, the galaxy is observed as an active galactic nucleus (agn). after several million years, when the smbh has consumed its accretion reservoir, the central engine shuts down, and the object is again observed as a normal galaxy. the agn phase is thought to be a repeating special epoch in the process of galaxy evolution. in recent years it has become evident that both fundamental galaxy and agn parameters change significantly between low (z < 0.3) and intermediate redshifts (z ∼ 1 − 2), e.g., global star formation density (hopkins & beacom 2006) and accretion rate onto smbhs. for example, the contribution to black hole growth has shifted from high luminosity objects at high redshifts to low luminosity objects at low redshifts (agn “downsizing”; e.g., hasinger et al. 2005). it has also become clear that smbh masses follow a tight relation with the mass or velocity dispersion of the stars in galactic bulges (magorrian et al. 1998; gebhardt et al. 2000; ferrarese & merritt 2000). these observational correlations moti1a visual impression is given in this video: http://vimeo.com/4169279 71 http://dx.doi.org/10.14311/app.2014.01.0071 mirko krumpe, takamitsu miyaji, alison l. coil vate a co-evolution scenario for galaxies and agns and provide evidence of a possible interaction or feedback mechanism between the smbh and the host galaxy. the interpretation of this correlation, i.e., whether and to what extent the agn influences its host galaxy, remains controversial (e.g., jahnke & macció 2011). since agns are generally much brighter than (inactive) galaxies, one major advantage of agn large-scale (i.e., larger than the size of a galaxy) clustering measurements over galaxy clustering measurements is that they allow the study of the matter distribution in the universe out to higher redshifts. at these redshifts, it becomes challenging and observationally expensive to detect galaxies in sufficient numbers. furthermore, as the distribution of agns and galaxies in the universe depends on galaxy evolution physics, large-scale clustering measurements are an independent method to identify and constrain the physical processes that turn an inactive galaxy into an agn and are responsible for agn and galaxy co-evolution. in the last decade the scientific interest in agn large-scale clustering measurements has increased significantly. as only a very small fraction of galaxies contain an agn (∼1%), the remaining and dominating challenge in deriving physical constraints based on agn clustering measurements is the relative small sample size compared to galaxy clustering measurements. however, this situation will change entirely in the next decade when several different surveys come online that are expected to identify millions of agn over ∼80% of cosmic time. we therefore review broad-line agn clustering measurements. a general introduction to clustering measurements is given in sections 2 & 3. in section 4 we briefly summarize how agn clustering measurements have evolved and discuss recent developments. in section 5 we discuss the outlook for agn clustering measurements in future upcoming projects. 2 understanding observed clustering properties in our current understanding, the observed galaxy and agn spatial distribution in the universe – i.e., largescale clustering – is caused by the interplay between cosmology and the physics of galaxy evolution. in the commonly assumed standard cosmological model, lambda-cdm, the universe is currently composed of ∼70% dark energy, ∼25% dark matter (dm), and ∼5% baryonic matter (larsen et al. 2011). dark matter plays a key role in structure formation as it is the dominant form of matter in the universe. baryonic matter settles in the deep gravitational potentials created by dark matter, the so-called dark matter halos (dmhs). the term “halo” commonly refers to a bound, gravitationally collapsed dark matter structure which is approximately in dynamical equilibrium. the parameters of the cosmological model determine how the dmhs are distributed in space (fig. 1, left panel, a-branch) as a function of the dmh mass and cosmic time. different cosmological models lead to different properties of the dmh population. figure 1: current conceptual model of the physical processes involved in large-scale galaxy and agn clustering. left: the two branches (a and b) in the diagram show the primary causes of clustering: (a) the properties of the dark matter halo population, which are based on the cosmological model, and (b) the physics of complex processes in galaxy formation and evolution, which lead to a distinct baryonic population within collapsed dark matter halos. figure adapted from weinberg 2002. right: illustration of the spatial distribution of galaxies within a dark matter halo. the picture maps the galaxy cluster abell 1689, where an optical image showing the galaxy cluster members is superimposed with the distribution of dark matter shown in purple. credit: nasa, esa, e. jullo, p. natarajan, and j-p. kneib. inside dmhs, or within halos inside another dmh, called sub-halos, the baryonic gas will radiatively cool. if the gas reservoir is large enough, star and galaxy formation will be initiated. the gas can also be accreted onto the smbh in the center of the galaxy. on scales comparable to the size of the galaxy, the agn can heat and/or eject the surrounding gas, preventing star formation, and eventually removing the gas fueling the agn itself. all the galaxy evolution processes described here determine how galaxies and agns are distributed within dmhs (fig. 1, left panel, b-branch). this distribution of agn and galaxies within dmhs (fig. 1, right panel) is described by the halo occupation distribution (hod; peacock & smith 2000). in addition to the spatial distribution of agn and galaxies in dmhs, the hod describes the probability distributions 72 clustering measurements of broad-line agns: review and future of the number of agns and galaxies per dmh of a certain mass and the velocity distribution of agns and galaxies within a dmh. the interplay between cosmology and galaxy evolution causes the observed large-scale clustering of galaxies and agns. the goal of agn and galaxy clustering measurements is to reverse the causal arrows in the fig. 1 (left panel), working backwards from the data to the galaxy & agn halo occupation distribution and dmh population properties, in order to finally draw conclusions about galaxy and agn physics, as well as to constrain fundamental cosmological parameters. 3 clustering measurements the most common statistical estimator for large-scale clustering is the two-point correlation function (2pcf; peebles 1980) ξ(r). this quantity measures the spatial clustering of a class of object in excess of a poisson distribution. in practice, ξ(r) is obtained by counting pairs of objects with a given separation and comparing them to the number of pairs in a random sample with the same separation. different correlation estimators are described in the literature (e.g., davis & peebles 1983; landy & szalay 1993). the large-scale clustering of a given class of objects can be quantified by computing the angular (2d) correlation function, which is the projection onto the plane of the sky, or with the spatial (3d) correlation function, which requires redshift information for each object. obtaining spectra to measure the 3d correlation function is observationally expensive, which is the main reason why some studies have had to rely on angular correlation functions. however, 3d correlation function measurements are by far preferable, since the deprojection (limber 1954) of the angular correlation function introduces large systematic uncertainties. despite these large caveats and the already moderately low uncertainties of current 3d correlation measurements, the use of angular correlation functions might still be justified when exploring a new parameter space. however, the next generation multi-object spectrographs (e.g., 4most (de jong et al. 2012), bigboss (schlegel et al. 2011), and weave (dalton et al. 2012)), will make it far easier to simultaneously obtain thousands of spectra over wide fields. hence, measurements of the 3d correlation function will soon become ubiquitous. as one measures line-of-sight distances for 3d correlation functions from redshifts, measurements of ξ(r) are affected by redshift-space distortions due to peculiar velocities of the objects within dmhs. to remove this effect, ξ(r) is commonly extracted by counting pairs on a 2d grid of separations where rp is perpendicular to the line of sight and π is along the line of sight. then, integrating along the π-direction leads to the projected correlation function, wp(rp), which is free of redshift distortions. the 3d correlation function ξ(r) can be recovered from the projected correlation function (davis & peebles 1983). the resulting signal can be approximated by a power law where the largest clustering strength is found at small scales. at large separations of >50 mpc h−1 the distribution of objects in the universe becomes nearly indistinguishable from a randomly-distributed sample. only on comoving scales of ∼100 mpc h−1 can a weak positive signal be detected (e.g., eisenstein et al. 2005; cole et al. 2005) which is caused by baryonic acoustic oscillations (bao) in the early universe. the spatial clustering of observable objects does not precisely mirror the clustering of matter in the universe. in general, the large-scale density distribution of an object class is a function of the underlying dark matter density. this relation of how an object class traces the underlying dark matter density is quantified using the linear bias parameter b. this contrast enhancement factor is the ratio of the mean overdensity of the observable object class, the so-called tracer set, to the mean overdensity of the dark matter field, defined as b = (δρ/〈ρ〉)tracer/(δρ/〈ρ〉)dm, where δρ = ρ− 〈ρ〉, ρ is the local mass density, and 〈ρ〉 is the mean mass density on that scale. in terms of the correlation function, the bias parameter is defined as the square root of the 2pcf ratio of the tracer set to the dark matter field: b = √ ξtracer/ξdm. rare objects which form only in the highest density peaks of the mass distribution have a large bias parameter and consequently a large clustering strength. theoretical studies of dmhs (e.g., mo & white 1996; sheth et al. 2001) have established a solid understanding of the bias parameter of dmhs with respect to various parameters. comparing the bias parameter of an object class with that of dmhs in a certain mass range at the same cosmological epoch allows one to determine the dmh mass which hosts the object class of interest. a halo may contain substructures, but the dmh mass inferred from the linear bias parameter refers to the single, largest (parent) halo in the context of hod models. 3.1 why are we interested in agn clustering? agn clustering measurements explore different physics on different scales. at scales up to the typical size of a dmh (∼ 1 − 2 mpc), clustering measurements are sensitive to the physics of galaxy and agn formation and evolution. constraints on the galaxy and agn merger rate and the radial distribution of these objects within dmhs can be derived. on scales larger than the size of dmhs, the large-scale clustering is sensi73 mirko krumpe, takamitsu miyaji, alison l. coil tive to the underlying dm density field, which essentially depends only on cosmological parameters. consequently, with only one measurement both galaxy and agn co-evolution as well as cosmological parameters can be studied. future high precision agn clustering measurements have the potential to accurately establish missing fundamental parameters that describe when agn activity and feedback occur as a function of luminosity and redshift. since they will precisely determine how dmhs are populated by agn host galaxies, these measurements will also improve our theoretical understanding of galaxy and agn evolution by enabling comparisons to galaxy measurements and cosmological simulations. here, we elaborate on some (though not all) of the critical observational constraints which are provided by agn clustering measurements: • agn host galaxy – agn cannot be more clustered than the type of galaxies they reside in. thus, agn clustering measurements determine the host galaxy type in a statistical sense for the entire agn sample, regardless of the agn’s luminosity. comparing the observed agn clustering to very accurate galaxy clustering measurements, which depend on different galaxy subclasses (morphological, spectral type, luminosity), constrains the agn host galaxy type. • external (mergers) vs. internal triggering – different theoretical models (e.g., fry 1996; sheth et al. 2001; shen 2009) of how agns are triggered predict very different large-scale clustering properties with agn parameters such as luminosity and redshift. moderately precise agn clustering measurements allow us to distinguish between these different models (allevato et al. 2011). furthermore, the validity of different models can be tested for different luminosities and cosmological epochs. • fundamental galaxy/agn physics – agn largescale clustering dependences with various agn properties could potentially be a key source of independent constraints on galaxy/agn physics. comparing the observed agn clustering properties with results from simulations with different inputs for galaxy/agn physics could identify the physics that links the evolution of agns and galaxies. • agn lifetimes – agn clustering measurements allow us to estimate the agn lifetime at different cosmological epochs (martini & weinberg 2001). the underlying idea is that rare, massive dmhs are highly biased tracers of the underlying mass distribution, while more common objects are less strongly biased (kaiser 1984). therefore, if agns are heavily biased they must be in rare, massive dmhs. the ratio of the agn number density to the host halo number density is a measure of the “duty cycle”, i.e., the fraction of the time that the object spends in the agn phase. • cosmological parameters – as agn clustering measurements extend to much higher redshifts than galaxy clustering measurements, they can be used to derive constraints on cosmological parameters (e.g., basilakos & plionis 2009) back to the time of the formation of the first agns. currently, the detection of the bao imprint on clustering measurements at different cosmological epochs is of great interest to constrain the equation of state of dark energy. agn large-scale clustering measurements with very large agn samples can detect the bao signal in a redshift range that is not accessible with galaxy clustering measurements. 4 agn clustering measurements: past and present until the 1980s, studies had to primarily rely on small, optically-selected, very luminous agn samples for clustering measurements. then the main question was whether agns are randomly distributed in the universe (e.g., bolton et al. 1976; setti & woltjer 1977). the extremely small sample sizes did not allow clustering measurements at scales below ∼50 mpc, where a significant deviation from a random distribution is present. thanks to the launch of major x-ray missions in the 1980s and 1990s such as einstein (giacconi et al. 1979) and rosat (truemper 1993), much larger agn samples enabled successful detections of the agn large-scale clustering signal. a detailed review on the history of x-ray agn clustering measurements is given in cappelluti et al. (2012). although agn clustering measurements are far from being as precise as galaxy clustering measurements, some general findings have emerged in recent years. interestingly, over all of studied cosmic time (z ∼ 0 − 3), broad-line agns occupy dmh masses of log (mdmh/[h −1m�]) ∼ 12.0−13.5 and therefore cluster like groups of galaxies. more detailed information about the current picture of broad-line agn clustering is presented in section 6.6 of krumpe et al. (2012). some puzzling questions remain. for example, at z < 0.5 a weak x-ray luminosity dependence on the clustering strength is found (in that luminous x-ray agns cluster more strongly than their low luminosity counterparts, e.g., krumpe et al. 2010; cappelluti et al. 2010; shen et al. 2013). however, at high redshift it seems that high luminosity, optically-selected agns cluster less strongly than moderately-luminous 74 clustering measurements of broad-line agns: review and future x-ray selected agns. whether this finding is due to differences in the agn populations, an intrinsic luminosity dependence to the clustering amplitude, or an observational bias is yet not understood. we note that different studies have used different relations to translate the measured linear bias parameter to dmh mass, as well as different σ8 values. therefore, instead of blindly comparing the derived dmh mass, re-calculating the masses based on the same linear bias to dmh mass relation and the same σ8 is essential when comparing measurements in the literature. 4.1 recent developments in the last few years several new approaches have been used to improve the precision of agn clustering measurements or their interpretation. we summarize these developments below. cross-correlation measurements: auto-correlation function (acf) measurements of broad-line agns often have large uncertainties due to the low number of objects. especially at low redshifts, large galaxy samples with spectroscopic redshifts are frequently available. in such cases, the statistical uncertainties of agn clustering measurements can be reduced significantly by computing the cross-correlation function (ccf). the ccf measures the clustering of objects between two different object classes (e.g., broadline agns and galaxies), while the acf measures the spatial clustering of objects in the same sample (e.g., galaxies or agns). ccfs have been used before to study the dependence of the agn clustering signal with different agn parameters. however, these values could not be compared to other studies as the ccfs also depend on the galaxy populations used and their clustering strength. only recently has an alternative approach (coil et al. 2009) allowed the comparison of the results from different studies by inferring the agn acf from the measured ccf and acf of the galaxy tracer set. the basic idea of this method, which is now frequently used (e.g., krumpe et al. 2010, 2012; mountrichas & georgakakis 2012; shen et al. 2013), is that both populations trace the same underlying dm density field. photometric redshift samples: large galaxy tracer sets with spectroscopic redshifts are not available at all redshifts. some studies therefore rely on photometric redshifts. the impact of the large uncertainties and catastrophic outliers when using photometric redshifts is commonly not considered but it can be essential. the use of the full probability density function (pdf) of the photometric redshift fit, instead of a single value for the photometric redshift, has been used in some studies (e.g., mountrichas et al. 2013). here, photometric galaxies samples are used as tracer sets to derive the ccf between agn and galaxies. each object is given a weight for the probability that it is actually located at a certain redshift based on the fit to the photometric data. figure 2: in the conceptual model of the hod approach, there are two contributions to the pairs that account for the measured correlation function. pairs of objects (black stars) can either be located within the same dmh (pink filled circles), such that their measured separation contributes to the 1-halo term (red solid line in the large dmh), or can reside in different dmhs, such that their separations (green dotted line) contribute to the 2-halo term. agn halo occupation distribution modeling: instead of deriving only mean dmh masses from the linear bias parameter, hod modeling of the correlation function allows the determination of the full distribution of agn as a function of dmh mass. the derived distribution also connects observations and simulations as it provides recipes for how to populate dmhs with observable objects. in the hod approach, the measured 2pcf is modeled as the sum of contributions from pairs within individual dmhs (fig. 2; 1-halo term) and in different dmhs (2-halo term). the superposition of both components describes the shape of the observed 2pcf better than a simple power law. in the hod description, a dmh can be populated by one central agn or galaxy and by additional objects in the same dmh, so-called satellite agn and galaxies. applying the hod approach to the 2pcf allows one to determine, e.g., the minimum dmh needed to host the object class of interest, the fraction of objects in satellites, and the number of satellites as a function of dmh mass. instead of using the derived agn acf from ccf measurements, miyaji et al. (2011) utilize the hod model directly on high precision agn vs. galaxy ccf and achieve additional constraints on the agn and galaxy co-evolution and agn physics. 75 mirko krumpe, takamitsu miyaji, alison l. coil theoretical predictions: only recently have several different theoretical models been published which try to explain the observed agn clustering with different physical approaches (e.g, fanidakis et al. 2013; hütsi et al. 2014). the key to observationally distinguish between these models are their different predictions for the clustering dependences of different agn parameters. in addition to theoretical models of the observed clustering, other very recently developed models predict the halo occupation distribution of agns at different redshifts, e.g., chatterjee et al. (2012). the major challenge presented by all of these models is the urgent need for observational constraints with higher precision than can be provided with current agn samples. in the future, progress in agn physics and agn and galaxy evolution will be achieved through a close interaction between state-ofthe-art cosmological simulations and observational constraints from high precision clustering measurements. simulations which incorporate different physical processes will lead to different predictions of the agn and galaxy large-scale clustering trends and their halo occupation distributions. observational studies will then identify the correct model and consequently the actual underlying physical processes. 5 the future of agn clustering measurements agn clustering measurements from several upcoming projects will significantly extend our knowledge of the growth of cosmic structure and will also provide a promising avenue towards new discoveries in the fields of galaxy and agn co-evolution, agn triggering, and cosmology. for example, erosita (predehl et al. 2010; launch 2015/2016) will perform several all-sky x-ray surveys. after four years the combined survey is expected to contain approximately three million agns. hetdex (hill et al. 2008; start 2015) will use an array of integral-field spectrographs to provide a total sample of ∼20,000 agns without any pre-selection over an area of∼ 450 deg2. the sdss-iv/eboss and bigboss builds upon the sdss-iii/boss project and will use a fiber-fed spectrograph. over an area of 14,000 deg2, it will observe roughly one million qsos at 1.8 < z < 3.5. in addition to these projects, there will be other major enterprises such as lsst (lsst collaboration 2009) and pan-starrs (kaiser et al. 2002) which will detect several million agns but currently lack dedicated spectroscopic follow-up programs. in the following we will focus on erosita, as this mission will compile the largest agn sample ever observed. figure 3 shows that erosita agn detections will outnumber current galaxy samples with spectroscopic redshifts at z > 0.4. using a large number of agns that continuously cover the redshift space, will allow us (in contrast to galaxy samples) to measure the distribution of matter with high precision in the last ∼11 gyr of cosmic time. to fully exploit the erosita potential for agn clustering measurements, a massive spectroscopic follow-up program is needed. several spectroscopic multi-object programs and instruments are currently planned or are in an early construction phase (e.g., sdss-iv/spiders and 4most). figure 3: number of expected erosita agns (red) and currently available galaxies with spectroscopic redshifts (black solid line at z < 0.4 – sdss data release 7; black dotted line – primus (coil et al. 2011); black solid line at z ∼ 1 – deep2 (davis et al. 2003) and vvds (le fèvre et al. 2005)). instead of the full sky area, we consider only the expected number of erosita agns with spectroscopic redshifts from 4most over 14,000 deg2. erosita agn clustering measurements at z ∼ 0.8 − 1 will even allow for the detection of the bao signal. the feasibility of such a measurement can be estimated using the bao detection found with ∼46,000 sdss lrgs (〈z〉 = 0.35) over 3,816 square degrees of sky (0.72 h−3 gpc3) as a standard for comparison (eisenstein et al. 2005). the observed agn x-ray luminosity function (gilli et al. 2007) and the erosita sensitivity determine the number density of erosita agns. in the abovementioned redshift range, the erosita agn area density will be comparable to that of sdss lrgs at lower redshifts. therefore, the comoving volume number density of erosita agns will be five times lower than that of sdss lrgs. since erosita will conduct an all-sky survey, the increased sky area will counterbalance the lower volume density. given the signal-to-noise ratio (s/n) of the bao detection of eisenstein et al. (2005) and an assumed spectroscopic area of 14,000 deg2, we expect a ∼3σ bao detection using erosita agns only in the redshift range of z ∼ 0.8 − 1. this is consistent with 76 clustering measurements of broad-line agns: review and future kolodzig et al. (2013), who use a different approach based on the angular power spectrum for estimating the significance of a bao detection with erosita agns. with the much larger agn datasets that will exist in the future, the statistical uncertainties in clustering measurements will be significantly decreased. systematic uncertainties will then be the dominant source of uncertainty. the impact and level of different systematic uncertainties can only be carefully explored and quantified through simulations. thus far, there has not been a need for such studies because the agn samples to date are i) drawn from surveys that (with exceptions) cover a rather moderate sky area and are therefore likely to suffer from the problem of cosmic variance2 and/or ii) comprised of up to several thousand objects and are consequently poisson noise dominated. both limitations will be removed in future agn clustering measurements with the upcoming extensive agn samples covering extremely large sky areas. however, to derive reliable constraints on agn physics and cosmology, as well as to avoid any possible misinterpretations of future unprecedented high precision agn clustering measurements, we have to fully understand and be able to correctly model the impact of the systematic uncertainties. only then can we maximize the scientific return of future agn clustering measurements and have a major impact in the field of cosmology and galaxy and agn evolution. acknowledgement mk received funding from the european community’s seventh framework programme (/fp7/20072013/) under grant agreement no 229517. tm is supported by unam/papiit in104113 and conacyt 179662. alc acknowledges support from nsf career award ast-1055081. references [1] abazajian et al. 2009, apjs, 182, 543. doi:10.1088/0067-0049/182/2/543 [2] allevato et al. 2011, apj, 736, 99. doi:10.1088/0004-637x/736/2/99 [3] bolton et al. 1976, apj, 210, l1. 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[54] weinberg 2002, aspc, 283, 3. 78 http://dx.doi.org/10.1088/0067-0049/192/2/16 http://dx.doi.org/10.1086/318331 http://dx.doi.org/10.1088/0004-637x/726/2/83 http://dx.doi.org/10.1111/j.1365-2966.2011.20059.x http://dx.doi.org/10.1093/mnras/sts666 http://dx.doi.org/10.1046/j.1365-8711.2000.03779.x http://dx.doi.org/10.1046/j.1365-8711.2001.04006.x introduction understanding observed clustering properties clustering measurements why are we interested in agn clustering? agn clustering measurements: past and present recent developments the future of agn clustering measurements 163 acta polytechnica ctu proceedings 1(1): 163–169, 2014 163 doi: 10.14311/app.2014.01.0163 modelling the multifrequency sed of agn candidates among the unidentified egret and fermi gamma-ray sources pieter j. meintjes1, pheneas nkundabakura2,1, brian van soelen1, alida odendaal1 1department of physics, university of the free state, p.o. box 339, bloemfontein, 9300, south-africa 2kigali institute of education, p.o. box 5039, kigali, rwanda corresponding author: meintjpj@ufs.ac.za abstract of the 271 sources in the 3rd egret catalogue, 131 were reported as unidentified, i.e. not associated with any particular class of point source in the sky. since the largest fraction of the egret sources were extragalactic, a sample of 13 extragalactic unidentified sources have been selected for multi-wavelength follow-up studies. five of the selected egret sources coincide with gamma-ray flux enhancements seen in the fermi-lat data after one year of operation. in this article, we report the multi-wavelength properties of, among others, the 5 sources detected by fermi-lat from our sample of high galactic latitude unidentified egret sources. recent spectroscopic observations with the southern african large telescope (salt) confirmed one of the unidentified egret sources as a possible seyfert 2 galaxy, or alternatively, a narrow line radio galaxy. the detected gamma-ray emission (eγ > 30 mev) of the 5 coinciding egret/fermi-lat sources are fitted with external compton and synchrotron self compton (ssc) models to investigate the energetics required to produce the egret/fermi gamma-ray flux. in all the models the inclination angle of the jet with respect to the observer is θjet ≈ 60◦, between those of seyfert 1 and seyfert 2/radio galaxies. these results confirm the possibility of seyfert and radio galaxies sources are constituting a new class of γ-ray source in the energy range eγ > 30 mev. keywords: radiation mechanisms: non-thermal line: identification techniques: spectroscopic galaxies: jets bl lacertae objects. 1 introduction the energetic gamma ray telescope experiment egret (30 mev 10 gev) provided the highest gamma-ray window on board the compton gammaray observatory (cgro). egret detected 271 gamma-ray sources above 100 mev, 92 % of which were blazars. of the 271 sources detected, 131 remained unidentified, i.e. could not be associated with any specific point source of gamma-ray emission (hartman et al., 1999). the large number of unidentified egret sources above and below the galactic plane inspired a search for possible extra-galactic radio loud active galactic nuclei (agn) counterparts that could possibly be associated with these unidentified sources. to avoid confusion with possible galactic sources, especially molecular cloud distributions, the search for counterparts was restricted to those unidentified sources at galactic latitudes | b | > 10◦. source selection criteria: the candidate counterparts should be inside the error box associated with the egret detection (hartman et al., 1999), confirmed as extragalactic in the nasa extragalactic database (ned), possess a radio brightness above 100 mjy at 8.4 ghz (sowards-emmerd, romani & michelson, 2003), exhibit hard spectra with spectral indices | α | < 0.7 (sowards-emmerd, romani & michelson, 2003) and display variability (e.g. fan, 2005) that may be associated with an inner accretion disc or jet. based upon these criteria, 13 sources have been selected for further follow-up study (meintjes & nkundababura, 2012; nkundabakura & meintjes, 2012). of the 13 candidate sources selected, 5 have confirmed gamma-ray excesses in the fermi-lat catalogue containing the first year’s observations (abdo et al., 2010a). the fermi large area telescope (fermilat) is a pair conversion γ-ray telescope sensitive to photon energies between 20 mev and 300 gev. launched on 11 june 2008, fermi-lat started to collect data in august 2008. the data are made available on a daily basis and can be accessed online at the official website of the fermi science support centre (http://fermi.gsfc.nasa.gov/cgi-bin/ssc /lat/latdataquery.cgi). the results of detections made within the first 24 months of operation were released in june 2011, in the form of the second fermi-lat catalog 163 http://dx.doi.org/10.14311/app.2014.01.0163 pieter j. meintjes et al. (2fgl; see nolan et al., 2012) 1. five sources from our previous sample of unidentified egret sources have been detected with fermilat, namely 2fgl j0727.0-4726, 2fgl j1304.3-4353, 2fgl j1703.2-6217, 2fgl j1709.0-0821 and 2fgl j1815.6-6407, which are counterparts of 3eg j07244713, 3eg j1300-4406, 3eg j1659-6251, 3eg j17090828 and 3eg j1813-6419 respectively. the fermi-lat sources that coincided with the egret sources are presented as circled crosses in fig. 1. the egret (30 mev-10 gev) gamma-ray spectra of our chosen sample of sources that were observed between april 1991 october 1995 (cycles 1, 2 3 and 4 of the mission) have been determined. the photon spectral index distribution is displayed in fig. 2. the spectral distribution of these unidentified sources corresponds remarkably well with the gamma-ray blazar photon spectral index distribution observed by fermilat (abdo et al., 2010a). b=−90° b=+90° l=0° b=+10° b=−10° b=0°,l=+360° b=+30° b=+60° b=−30° b=−60° figure 1: galactic distribution of the unidentified egret sources (circles) and their fermi-lat counterparts (crosses). 1 1.5 2 2.5 3 3.5 4 3eg j0159-3603 3eg j0500+2529 3eg j0702-6212 3eg j0706-3837 3eg j0724-4713 3eg j0821-5814 3eg j1300-4406 3eg j1659-6251 3eg j1709-0828 3eg j1800-0146 3eg j1813-6419 3eg j1822+1641 3eg j1824+3441 γra y ph ot on in de x hsp-bllacs l and isp-bllacs fsrqs γ=2.03 γ=2.38 figure 2: gamma-ray photon spectral indices of the egret sources between 30 mev 10 gev. the gamma-ray spectra of the sample of egret sources with fermi-lat counterparts were determined. the spectra are presented in fig. 3. noticeable is the apparent change in the spectral index between the egret and the fermi-lat gamma-ray data, which may point to a transition in the gamma-ray production process. -100 0 100 10 100 1000 10000 100000r es id ua ls ( % ) e (mev) 1e-15 1e-14 1e-13 e 2 (d n /d e )( w .m -2 ) 3eg j0724-4713 dn(e)=const e-2.6de a.) 3eg j0724-4713 -100 0 100 10 100 1000 10000 100000 1e+06r es id ua ls ( % ) e (mev) 1e-15 1e-14 1e-13 e 2 (d n /d e )( w .m -2 ) 3eg j1300-4606 dn(e)=const e-3.07de c.) 3eg j1300-4406 -100 0 100 10 100 1000 10000 100000r es id ua ls ( % ) e (mev) 1e-15 1e-14 1e-13 e 2 (d n /d e )( w .m -2 ) 3eg j1659-6251 dn(e)=const e-2.83de 1e-15 1e-14 1e-13 e 2 (d n /d e )( w .m -2 ) 3eg j1659-6251 dn(e)=const e-2.83de b.) 3eg j1659-6251 -100 0 100 10 100 1000 10000 100000r es id ua ls ( % ) e (mev) 1e-15 1e-14 1e-13 e 2 (d n /d e )( w .m -2 ) 3eg j1709-0828 dn(e)=const e-3de d.) 3eg j1709-0828 -100 0 100 10 100 1000 10000 100000r es id ua ls ( % ) e (mev) 1e-15 1e-14 1e-13 e 2 (d n /d e )( w .m -2 ) 3eg j1813-6419 dn(e)=const e-2.85de e.) 3eg j1813-6419 figure 3: the gamma-ray spectra of the egret (30 mev 10 gev) and coinciding fermi-lat (20 mev 300 gev) sources. 2 gamma-ray variability the multi-wavelength emission of accretion driven sources like agn is characterized by very high luminosity, assuming isotropic emission, and variability over several time scales (e.g. fan, 2005). this is reconciled with the fact that the bulk of the spectral energy distribution (sed) of these sources is produced in a non-homogeneous and variable jet. gamma-ray flux 1http://fermi.gsfc.nasa.gov/ssc/data/access/lat/2yr catalog/ 164 modelling the multifrequency sed of agn candidates among the unidentified egret... variability has also been confirmed from agn-blazars (mattox et al., 1997). aperture photometry of the fermi-lat data over a time span of 4.6 years (from august 2008 to march 2013) has been performed to investigate possible long term variability. photons in the energy interval 100 mev to 200 gev were counted in an area with a radius of 1 degree centered on the source and monthly averages were determined. two of the sources, 2fgl j1304.3-4353 and 2fgl j1703.2-6217, showed definite signs of variability, quantified in terms of a variability index v (nolan et al., 2012), which implies that for v > 41 there is a < 1% probability of the source being steady. although variability was detected in 2fgl j1304.3-4353 (v = 47) and 2fgl j1703.2-6217 (v = 167) respectively, no periodicity could be detected (see fig. 4). 1e-07 1.5e-07 2e-07 2.5e-07 3e-07 3.5e-07 54500 55000 55500 56000 56500 ph to n fl ux ( ph ot on s. cm -2 .s -1 ) time (mjd) 2fgl j1304.3-4353 (3eg j1300-4406) average a.) 2fgl j1304.3-4353 1e-07 1.5e-07 2e-07 2.5e-07 3e-07 3.5e-07 4e-07 4.5e-07 5e-07 5.5e-07 54500 55000 55500 56000 56500 ph ot on f lu x (p ho to ns .c m -2 .s -1 ) time (mjd) 2fgl j1703.2-6217(3eg j1659-6251) average b.) 2fgl j1703.2-6217 figure 4: the gamma-ray lightcurves of the two fermi-lat sources, 2fgl j1304.3-4353 and 2fgl j1703.2-6217, which displayed variability over a period of 4.6 years. the vertical error bars indicate the 68% confidence level. 3 optical follow-up studies the optical counterparts of two unidentified egret sources have already been identified (meintjes & nkundabakura, 2012; nkundabakura & meintjes, 2012). the spectrum of pks j0820-5705 (3eg j08215814) resembles that of a fsrq at redshift z = 0.06, while the spectrum of pmn j0710-3850 (3eg j0706-3837) shows broad and narrow lines resembling the spectrum of a liner or seyfert i galaxy at redshift z = 0.129. what distinguishes the spectrum of pks j0820-5705 from that of a normal radio galaxy is the shallow k4000 depression of only 8.8 ± 2.5 %, indicating substantial non-thermal activity, while the corresponding value for pmn j0710-3850 is 80 ± 1 % (meintjes & nkundabakura, 2012; nkundabakura & meintjes, 2012), in agreement with the value expected for a liner-seyfert 1 galaxy (e.g. caccianiga et al., 1999). spectroscopic observations of other sources from our sample have been performed with the southern african large telescope (salt) (see fig. 5), equipped with the robert stobie spectrograph (rss), during 2012, in order to determine the redshift and to identify the class of agn. the spectrum of one of the unidentified egret sources, 3eg j 0159-3603, could be determined (see fig. 6), showing distinct narrow emission lines of o ii, o iii and he ii redshifted by z = 0.35. the spectrum resembles that of a typical seyfert 2 galaxy, or alternatively, a narrow line radio galaxy. this implies the possible association of two seyfert galaxies with the unidentified sources 3eg j0706-3837 and 3eg j 0159-3603 respectively. figure 5: the southern african large telescope (salt). −5e−16 0 5e−16 1e−15 1.5e−15 2e−15 4500 5000 5500 6000 6500 7000 7500 f lu x (e rg s− 1 c m − 2 å − 1 ) wavelength (å) [n ei ii] + h ε [o ii] [n ei ii] [o iii ] h γ pmn j0156−3616 (z=0.35) [o iii ] * * * * h ei i figure 6: salt spectrum of the optical counterpart of the unidentified egret source 3eg j0159-3603, showing emission lines redshifted by z=0.35 4 the spectral energy distributions (sed) the spectral energy distributions (seds) of all the counterparts corresponding to the unidentified egret 165 pieter j. meintjes et al. sources have been pieced together through multiwavelength archival data as well as multi-wavelength observations from radio to optical, using the hartebeesthoek radio astronomy observatory (hartrao) 26 m telescope, as well as various optical telescopes at the south african astronomical observatory (saao) in south africa. infrared data were obtained from the two micron all sky survey (2mass) conducted between june 1997 and february 2001. the seds of the sample of egret sources and the associated fermilat counterparts are presented in fig. 7. 1e-18 1e-17 1e-16 1e-15 1e-14 1e-13 1e-12 1e-11 1e+10 1e+15 1e+20 1e+25 νf ν (w m -2 ) ν (hz) radio opt. fermi egret 3eg j0724-4713 a.) 3eg j0724-4713 1e-18 1e-17 1e-16 1e-15 1e-14 1e-13 1e-12 1e-11 1e+10 1e+15 1e+20 1e+25 νf ν (w m -2 ) ν (hz) radio opt. rosat egret 3eg j1300-4406 fermi c.) 3eg j1300-4406 1e-18 1e-17 1e-16 1e-15 1e-14 1e-13 1e-12 1e-11 1e+10 1e+15 1e+20 1e+25 νf ν (w m -2 ) ν (hz) radio opt. nir egret 3eg j1659-6251 fermi iras b.) 3eg j1659-6251 1e-18 1e-17 1e-16 1e-15 1e-14 1e-13 1e-12 1e-11 1e+10 1e+15 1e+20 1e+25 νf ν (w m -2 ) ν (hz) radio opt. egret 3eg j1709-0828 fermi d.) 3eg j1709-0828 1e-18 1e-17 1e-16 1e-15 1e-14 1e-13 1e-12 1e-11 1e+10 1e+15 1e+20 1e+25 νf ν (w m -2 ) ν (hz) radio opt. nir egret 3eg j1813-6419 fermi e.) 3eg j1813-6419 figure 7: the multifrequency seds for the sample of egret sources. the association of some of the unidentified egret sources with seyfert galaxies/liners poses a possibility of non-aligned agn and radio galaxies constituting a new class of gamma-ray sources. before discussing the modeling of the seds presented above, a brief discussion of the gamma-ray properties of some radio galaxies are presented to illustrate that these sources possess the required energetics to produce measurable γ-ray emission in the egret and fermi-lat energy domain 5 non-aligned agn: a new class of gamma-ray source at high energies (he; eγ > 100 mev) fermi-lat reported about ten misaligned radio galaxies (abdo et al., 2010b,c; rieger, 2012a). at very high energies (vhe; eγ > 100 gev), four radio galaxies have been detected, cen a (d ≈ 3.8 mpc), m87 (d ≈ 16.7 mpc) and the perseus cluster (d ≈ 77 mpc; z ≈ 0.018) sources ngc 1275 and ic 310 (e.g. rieger, 2012a). cen a was the only non-blazar detected at mev to gev energies by cgro (see steinle, 2010 for a review). the he emission from cen a reported by fermi-lat seems to come from both the extended radio lobes and the core region (abdo et al., 2010b,c). the detected he gamma-rays from the extended lobe regions suggests that particle acceleration up to vhes occurs in the disrupted jet region (bordas, 2012). the reported tev emission from cen a (aharonian et al., 2009) provided further evidence of a very effective particle accelerator in cen a. the spectrum up to 5 tev is consistent with a power-law with a photon index ∼ 2.7 ± 0.5, with no apparent variability. the he (eγ > 0.2 gev) emission in cen a is explained in terms of inverse compton (ic) upscattering of the cosmic microwave background (cmb) photons (eph = 8 × 10−4 ev) and infrared (ir) background photons by relativistic electrons with lorentz factors γe = 6 × 105, in a jet with bulk flow velocities between βγ ∼ 0.1-0.5. another peculiar aspect of cen a is a rather low, sub-eddington inferred accretion rate, ṁ ∼ 10−3ṁedd, resulting in a rather low bolometric luminosity lb ∼ 1043 erg s−1 (whysong & antonucci, 2004). the inferred equipartition magnetic field in the radio lobes is b ≈ 9 µg (rieger, 2012b), with the equipartition magnetic field near the black hole (bh) ranging between b ∼ 103 − 104 g (rieger, 2012b). the detection of non-aligned agn in the he and vhe regime poses interesting theoretical challenges regarding particle acceleration and associated gammaray emission in the jets of agn. for example, cen a shows that the radio lobes of radio galaxies may possess the required energetics to accelerate electrons to vhes producing the he gamma-rays through ic upscattering the cmb photons, even though the bulk flow lorentz factor of the jet is fairly low. the detection of tev gamma-rays is explained in terms of γe ∼ 107 electrons up-scattering disc photons to the tev domain (rieger & aharonian, 2009). the nuclear sed of cen a, based on non-simultaneous data, shows two peaks, one around∼ 1013 hz and another around 0.1 mev (e.g. chiaberge et al., 2001; abdo et al., 2010c). the sed below a few gev seems to be satisfactorily explained by a one-zone synchrotron self-compton (ssc) model (chiaberge et al., 2001) but the same approach fail to account for the tev emission observed by h.e.s.s. (abdo et al., 2010c). 166 modelling the multifrequency sed of agn candidates among the unidentified egret... the discussion presented above underlines the fact that normal, non-aligned agn do possess the required energetics to accelerate leptons to vhe energies. the production of sub-gev gamma-rays through a ssc process in the nuclear region, combined with the he emission in the radio lobes and tev emission in the inner disc region close to the black hole (bh), presents a new paradigm in particle acceleration and gamma-ray production in agn. with this in mind, the sed of the fermi-lat counterparts of the unidentified egret sources were modelled. this is a first attempt to explain the sed of these sources within the framework of a synchrotron self-compton (ssc) and external compton (ec) model. these sources are all treated as misaligned agn, with a fixed inclination of θjet ≈ 60◦ between jet and observer. 6 sed modelling the multi-wavelength data from radio to gamma-rays have been combined to create the sed over more than 15 decades in energy (fig. 7). the egret and fermilat gamma-ray data are evaluated with the theoretical framework of a single zone synchrotron self-compton (ssc) (e.g. katarczynski, sol & kus, 2001) and external compton (ec) model (e.g. moderski et al., 2003; sikora, begelman & rees, 1994), i.e. where relativistic jet electrons up-scatter infrared (ir) photons from the disc torus and optical photons from the broad emission line (bel) regions to high energies. the model parameters for the ssc and ec models are presented in table 1 (ssc) and table 2 (ec). the corresponding sed model fits are presented in fig. 8. table 1: parameters related to ssc models for the respective sources. source 3eg j0724-4713 3eg j1659-6251 3eg j1709-0828 3eg j1813-6419 parameter units 1 radius (m) 1.5e+14 1.0e+14 1.0e+14 1.0e+14 2 b (t) 7.0e-04 7.5e-04 2.5e-04 2.5e-04 3 p1 -2 -2 -2 -2 4 p2 -3 -2.6 -2.6 -2.6 5 γmax 1.6e+03 6.2e+02 1.6e+03 2.0e+03 table 2: parameters related to the ec models for the respective sources. source r θobs γbr ke νbel αbel νir (1) (2) (3) (4) (5) (6) (7) 3eg j0724-4713 1.0e+17 1 2.0e+03 4.0e+55 10 -1 0.1 3eg j1300-4406 (bel) 1.0e+17 1 2.0e+04 6.0e+53 1 -1 0.1 3eg j1300-4406 (ir) 1.0e+17 1 3.0e+03 4.0e+54 1 -1 0.1 3eg j1659-6251 (bel) 1.0e+17 1 2.0e+03 6.0e+54 10 -1 0.1 3eg j1659-6251 (ir) 1.0e+17 1 2.0e+03 3.0e+55 10 -0.5 0.1 3eg j1709-0828 1.0e+17 1 2.0e+03 3.0e+55 10 -1 0.1 3eg j1800-0146 2.5e+16 1 5.0e+02 3.0e+55 1 -1 0.1 3eg j1813-6419 1.0e+17 1 2.4e+03 1.3e+55 10 -1 0.1 (1): radius of the emitting region (in cm), (2): viewing angle (in radians), (3): electron lorentz factor at spectral break, (4): electron normalization constant, (5): characteristic frequency of bel (in ev), (6): bel distribution photon index, (7): radiation characteristic frequency of ir (in ev). 167 pieter j. meintjes et al. 1e-18 1e-17 1e-16 1e-15 1e-14 1e-13 1e-12 1e-11 1e+10 1e+15 1e+20 1e+25 νf ν (w m -2 ) ν (hz) radio opt. fermi rosat egret 3eg j0724-4713 ec (ir) scc sync scc ec (bel)ic (tot) a.) 3eg j0724-4713 1e-18 1e-17 1e-16 1e-15 1e-14 1e-13 1e-12 1e-11 1e+10 1e+15 1e+20 1e+25 νf ν (w m -2 ) ν (hz) radio opt. rosat egret 3eg j1300-4406 fermi sync scc ic (tot) c.) 3eg j1300-4406 1e-18 1e-17 1e-16 1e-15 1e-14 1e-13 1e-12 1e-11 1e+10 1e+15 1e+20 1e+25 νf ν (w m -2 ) ν (hz) radio opt. nir egret 3eg j1659-6251 fermi iras ec (ir) sync ssc ec (bel) b.) 3eg j1659-625 1e-18 1e-17 1e-16 1e-15 1e-14 1e-13 1e-12 1e-11 1e+10 1e+15 1e+20 1e+25 νf ν (w m -2 ) ν (hz) radio opt. egret 3eg j1709-0828 fermi ec (ir) ssc sync ec (bel) ic (tot) d.) 3eg j1709-0828 1e-18 1e-17 1e-16 1e-15 1e-14 1e-13 1e-12 1e-11 1e+10 1e+15 1e+20 1e+25 νf ν (w m -2 ) ν (hz) radio opt. nir egret 3eg j1813-6419 fermi ec (ir) ssc sync ic (tot) ec (bel) e.) 3eg j1813-6419 figure 8: the modeled seds for the various egret/fermi-lat sources. from these results (see fig. 8) it can be seen that the egret and fermi-lat data are consistent with an ec model, i.e. the ic upscattering of ir photons from the disc and uv/optical photons from the line emitting clouds (broad line regions) to the egret and fermi-lat energy domain. in all the models the electron lorentz factors were γe ∼ few × 103, rather modest in comparison with the lorentz factors required to explain the γ-ray emission from, for example, cen a. 7 conclusions we report the discovery of 13 flat spectrum extragalactic sources within the error boxes of some high galactic latitude, unidentified egret sources. five of these egret sources have been detected with fermi-lat within the first 11 months of operation. in all cases the egret and fermi-lat gamma-ray emission could be successfully explained in terms of the ic upscattering of bel photons, as well as ir photons from the disc, to egret and fermi-lat energies. the adopted electron energy is of the order of γe ∼ few × 103, which is rather moderate compared to the electron energies of γe ∼ 106 − 107, inferred from the he and tev emission from cen a. these preliminary results definitely confirm that seyfert and radio galaxies could be associated with a significant fraction of the still unidentified extragalactic egret and fermi-lat sources. acknowledgement the authors thank the organisers for the invitation to present this work at this conference. some of the observations reported in this paper were performed by the southern african large telescope (salt). the salt observations were performed under the proposal: 20121-rsa-003 (pi: b van soelen). the authors thank dr. petri vaisanen from the salt science team for his guidance with the data reduction. references [1] abdo, a.a., et al.(fermi): 2010a, apj, 710, 1271 doi:10.1088/0004-637x/710/2/1271 [2] abdo, a.a., et al. 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distributions (sed) non-aligned agn: a new class of gamma-ray source sed modelling conclusions acta polytechnica ctu proceedings doi:10.14311/app.2017.11.0012 acta polytechnica ctu proceedings 11:12–15, 2017 © czech technical university in prague, 2017 available online at http://ojs.cvut.cz/ojs/index.php/app friction buffer stop design petr guziur institute of railway structures and construction, faculty of civil engineering, brno university of technology correspondence: guziur.p@fce.vutbr.cz abstract. friction buffer stops are the favoured construction of buffer stop, mainly due to its high resistance and variety of layout. last but not least is its manner of deceleration induced upon impact and during the braking what makes it smart solution in railway transport safety. the general approach of designing buffer stops is via usage of the kinetic energy and its conversion into work. paper describes input parameters such as train velocity or buffer stop vicinity which is expressed by the safety coefficient implanted within the calculation. furthermore, the paper shows the principle of calculation the friction buffer stop work, or to be more precise, the work of its braking jaws and optionally the work of additional braking jaws located behind the buffer stop. last section of the paper is focused on the examples of designing friction buffer stops, points out the main complications and shows the charts of relation amongst braking distance, kinetic energy and braking force and the charts of relation between deceleration rate and braking distance. keywords: buffer stop, kinetic energy, braking force, braking jaw. 1. introduction with increasing demands on safety in transportation, every threat needs to be considered. this means to keep safe onboard passengers and anyone who can be endangered in case of an accident. this applies even for buffer stops – especially for buffer stops at the dead-end tracks in stations by platforms with pedestrians. the use of fixed buffer stops has become less suitable due to their low resistance and manner of deceleration, which arises in an order of tens of m.s−2 [1] a clearly unacceptable state. according to the czech standard cd z9 [2], it is allowed to design fixed buffer stop (table 1). using those constructions with very limited resistance is justified in particular cases such as deadend tracks in shunting yard, rolling stock depots etc. recently the motivation is to use more reliable constructions to increase the safety in railway transportation. the friction buffer stop with its high resistance and manner of deceleration is more suitable one. buffer stop type 6 cars of 15 t 1 car of 80 t rail buffer stop 1.0 km.h−1 1.6 km.h−1 concrete type "sudop" 0.7 km.h−1 1.1 km.h−1 concrete type "dsb" 1.0 km.h−1 1.6 km.h−1 table 1. resistances of buffer stops used in czech republic [2]. 2. friction buffer stop friction buffer stops have become the preferred constructions at the end of a dead-end track in railway stations abroad (e.g. switzerland, germany [3, 4]). resistance of a friction buffer stop (work of the buffer stop) could be much higher than a fixed buffer stop, and can be customized for various train mass, train speeds and acceptable deceleration rates. disadvantage of a friction buffer stop is obviously its space requirement due to its braking distance (see figure 1). kinetic energy of moving train and its reduction along the braking distance is the criteria to calculate and design the buffer stop work. to be more precise the objective is to design the number of braking jaws and their braking distances. there are two possibilities how to design the friction buffer stop: • friction buffer stop without additional braking jaws (see figure 1); • friction buffer stop with additional braking jaws (see figure 2). figure 1. friction buffer stop [5]. lv – track length required for friction buffer stop lw – braking distance lb – buffer stop length 12 http://dx.doi.org/10.14311/app.2017.11.0012 http://ojs.cvut.cz/ojs/index.php/app vol. 11/2017 friction buffer stop design 3. buffer stop design 3.1. train kinetic energy defining the precise value of kinetic energy of moving rolling stock is to include rotating parts into calculation. to simplify the calculation of train kinetic energy one can use formula 1 for calculation the kinetic energy of non-rotating object of certain mass and velocity. ekin = 1 2mv2 (1) ekin – train kinetic energy [j] m – train mass [kg] v – train velocity by impact [m.s−1] 3.2. collision speed collision speed is probably the most questionable parameter entering the calculation. in tracks with train control system, which can interfere with driver inappropriate driving (e.g. etcs) – in this case an insufficient braking at the end of dead end track, it is possible to define the collision speed. nevertheless, there are numerous dead-end tracks without any speed controlling system and therefore the driver is responsible for proper braking. collision speeds that could be found in literature are v = 10 km.h−1 or v = 15 km.h−1 (depends on train type or track type) [4–6]. 3.3. safety coefficient for including an inaccuracy entering the calculation of kinetic energy and to ensure the reliability of the design, safety coefficient k has to be counted in calculation. the value of the coefficient varies and depends on type of train, location of dead end track etc. various infrastructure managers approach towards the assessment of the safety coefficient differently. values of safety coefficient usually varies between 1,0 – 2,0 [3, 4, 6]. type of train k [-] freight trains and shunting 1.2 passenger trains, freight trains and shunting, when it is necessary to protect various systems which are located behind or nearby buffer stop 1.5 all trains, where there are traffic zones, structures or residential buildings located behind or nearby buffer stop 1.8 preventing the fall of any train or rolling stock into an abyss 2.0 table 2. safety coefficient [6]. 3.4. buffer stop work in general, the resistance of buffer stop is described as its energy absorbing capacity, therefore the buffer stop work required to stop the train. in case of friction buffer stop, it is the work of braking jaws w with braking forces fb,i,j on braking distances li,j . work of the braking jaws can be obtained using formula 2 and 3. wi = ni ∗ n∑ j=1 fb,i,j ∗ li,j (2) w = s∑ i=1 wi (3) wi – work of i-group of braking jaws [j] ni – number of braking jaws in i-group [-] fb,i,j – braking force of a single braking jaw, based on a braking distance [n] li,j – length of braking distance of i-group of braking jaws, based on lw [m] braking force of each braking jaw decreases along the braking distance [4]. length of braking distance li,j [m] braking force fb,i,j [kn] 0 – 5 40 5 – 8 36 8 – 12 32 12 – 20 28 table 3. braking force of a braking jaw along the braking distance [4]. the design of friction buffer stop (number of braking jaws and their braking distances) is correct if the buffer stop work is equal or higher than the train kinetic energy increased by the safety coefficient: w ≥ ekin ∗ k (4) 3.5. deceleration rate deceleration rate could be the limiting parameter when designing number of braking jaws and their braking distances. the most challenging design comes with dead-end track where parameters of trains vary (mostly the train mass). deceleration rate has to be monitored both upon impact and along the braking distance where additional braking jaws could be designed. fortunately, restriction in deceleration rate applies only to the passenger trains. maximal deceleration rate is set as a = 2.5 m.s−2 [7]. deceleration rate is calculated as follows: amax = fmax m (5) 13 petr guziur acta polytechnica ctu proceedings figure 2. buffer stop with additional braking jaws. amax – maximal deceleration rate [m.s−1] fmax – maximal braking force along the braking distance [n] m – train mass [kg] 3.6. design procedure the design of buffer stop is customized for two types of trains, the lightest one and the heaviest one running on the dead-end track. this approach covers both deceleration rate restriction (usually the limiting factor for light trains) and the highest value of kinetic energy of heavy train (sufficient work of the buffer stop). braking distance for braking jaws on buffer stop is considered as the main braking distance. the tricky part comes with additional jaws behind the buffer stop thus in their braking distances where each pair of additional braking jaws has different braking distance. therefore it is necessary to consider the variable braking force for each pair of additional braking jaws along the braking distance according to table 3. for better understanding, see following examples. 3.6.1. example 1 as it was described in section 3.6, two types of trains were selected for calculation. a light train with mass of 170 t and a heavy train with mass of 300 t. collision speed of trains v = 15 km.h−1, is the same for both of them, and safety coefficient used to increase the kinetic energy k = 1, 8 (see section 3.3). in this situation, the main braking distance is set to 11,3 m (lw – buffer stop) with five pairs of braking jaws and one pair of additional braking jaws with braking distance of 4,0 m (lw – 1. row of additional braking jaws). under those circumstances, the mass of the light train is high enough to design the large number of braking jaws on buffer stop and the limit of deceleration rate is not exceeded. nevertheless, the offset for the first row of additional braking jaws behind the buffer stop must be set. if the light train reaches the additional braking jaws, braking force will increase and therefore the deceleration rate will increase over the limit. for better understanding, see figure 3 and figure 4. figure 3. kinetic energy and braking force along the braking distance (example 1). figure 4. deceleration rate along the braking distance (example 1). 3.6.2. example 2 in this example, the light train with mass of 80 t and the heavy one with mass of 300 t were selected to point the difficulty and variety of design. collision speed of trains v = 15 km.h−1 is the same for both of them and the safety coefficient used to increase the kinetic energy k = 1, 8 was chosen (see section 3.3). in this case, the main braking distance is set to 15,5 m (lw – buffer stop) with just 2 pairs of braking jaws and three rows of additional braking jaws. each row of additional braking jaws consists of 2 pairs of braking jaws. braking distances are designed as follows: 7,0 m (lw – 1. row of additional braking jaws), 5,5 m (lw – 2. row of additional braking jaws) and 4,0 m (lw – 3. row of additional braking jaws). the mass of light train is significantly lower and one has to design fewer braking jaws on buffer stop in order not to exceed the limit of deceleration rate. the offset of first row of additional braking jaws has to be set similarly to example one. 14 vol. 11/2017 friction buffer stop design figure 5. kinetic energy and braking force along the braking distance (example 2). figure 6. deceleration rate along the braking distance (example 2). 4. conclusion the usage of the fixed buffer stops in the railway stations is slowly becoming an anachronism. one can still come across many of those constructions in present railway stations but the latest progression, is to design buffer stops with high resistance and high level of safety. friction buffer stops meet those criteria in every point. the possibilities variety of designing the friction buffer stop is the huge advantage. the range of options varies form light trains with low collision speed, up to heavy trains with high collision speed. the paper discusses the parameters of the buffer stop and shows some examples of design for certain situations. as described above, design buffer stop for some combinations of trains on dead-end track could be challenging while all criteria, such as buffer stop kinetic energy absorbing capacity, restriction of deceleration rate and space options, must be met. acknowledgements the paper was created with support of the project no. lo1408 "admas up – advanced materials, structures and technologies" supported by the ministry of education, youth and sports within the targeted support of program "national program for sustainability i" and student’s grant tender of the faculty of civil engineering, brno university of technology (fast-j-16-3776). references [1] m. bugarín, j.-m. díaz-de-villageas. conception of buffer stop blocks [online]. 2001. [cit. 2016-11-10] "http://www.railway-research.org/img/pdf/329.pdf". [2] c. z9. železniční spodek, vzorový list železničního spodku, zarážedla. české dráhy, s.o., divize dopravní cesty, o.z., praha, 2001. in effect from 1.4.2002. [3] ausführungsbestimmungen zur eisenbahnverordnung / ab-ebv, 2014. artikel: gleisabschluss, bern. [4] db:ds 800 01. bahnanlagen entwerfen – allgemeine entwurfsrichtlinien. [5] i. r. ltd. railway buffer stops planning guidlines [online]. 2009. [cit. 2016-11-05] "http://www.iroads.co.il/sites/default/files/imce/ ir_buffer_stops_guidelines.doc". [6] rve 05.05.31 oberbau, oberbaukonstruktionen, gleisabschlüsse. wien. 2014. [7] guide for the application of tsi loc&pas. version 2.0, 2015. european railway agency, france. 15 acta polytechnica ctu proceedings 11:12–15, 2017 1 introduction 2 friction buffer stop 3 buffer stop design 3.1 train kinetic energy 3.2 collision speed 3.3 safety coefficient 3.4 buffer stop work 3.5 deceleration rate 3.6 design procedure 3.6.1 example 1 3.6.2 example 2 4 conclusion acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2017.7.0076 acta polytechnica ctu proceedings 7:76–78, 2017 © czech technical university in prague, 2017 available online at http://ojs.cvut.cz/ojs/index.php/app evaluation of the fatigue response of polyester yarns after the application of abrupt tension loads emilio luiz vieira louzadaa,∗, carlos eduardo marcos guilhermea,b, felipe tempel stumpfa,b a programa de pos graduacao em engenharia mecanica, av. italia, km. 8, rio grande, brazil b policab, av. italia, km. 8, rio grande, brazil ∗ corresponding author: emilioluizlouzada@gmail.com abstract. the discovery of oil fields in deeper waters through out the last decades has led the oil industry to the necessity of replacing the mooring systems of offshore platforms from steel to synthetic cables. consequently, both the industry and the academy started to join forces in order to better understand the mechanical behavior of such materials when subjected to different service conditions. this work aims to assess the change in the fatigue life of polyester (pet) yarns if the material is submitted to an abrupt tension load prior to the application of the fatigue cycling. it was found that the fatigue life of the yarns tested are substantially reduced if the specimen is subjected to this kind of abrupt load in comparison to virgin samples. keywords: mooring lines, synthetic cables, offshore platforms. 1. introduction in the course of the last decades, offshore oil fields had been discovered along the brazilian shore in increasingly deeper waters throughout the years, which forced the oil industry to look for mooring systems to substitute the traditional systems based on steel cables. with the increase of the water depth in which the platform will operate, these systems tend to become potentially heavy, to the point where the mooring system weight itself could overcome the platform floating forces, leading to the sinking of the structure [1, 2]. in order to overcome this situation, the oil industry started to look for lighter mooring systems, which meant lighter materials, and the obvious choice was to invest in the development of mooring cables based on polymeric materials. however, there was a lack of understanding regarding these materials’ mechanical response when submitted to the service conditions of an offshore platform, which basically forced the engineers and academics to develop new research in the field [1–3]. in the present days, polyester (pet) is found to be a widely used material in applications such as anchoring cables. its good performance when submitted to cyclic loadings and the fact that its fatigue life is not affected by its submersion in salt water was the main reason it was the first material to be considered by the industry to such application [4]. however, if the maximum tension force is considerably high, the fatigue life of pet is dramastically reduced [5]. it was found that when operating between cyclic tension loads of 10% and 70% of the material’s tension resistance, samples failed after approximately 100,000 cycles [6]. if the maximum load is reduced to 60% of the material’s yarn break load (ybl), fatigue life increases to 1,000,000 cycles [5]. cyclic tension loads, however the main loads to which the cables are submitted during service, are not the only ones. during the installation of the anchoring cables, the launch of the torpedo anchor might eventually be submitted to a sudden stop, caused either by a failure of the launching system or even due to a stop because of heavy weather conditions, which would lead to abrupt axial tension loads in the synthetic cable being launched/installed. it is expected that this kind of sudden (high) tensile load affects the future mechanical behavior of the polymeric material, specially its fatigue life. this paper aims to correlate the change in the behavior of pet under cyclic loadings before and after it was submitted to abrupt axial loads. such study will be based on the material’s diagram relating number of cycles up to failure (log n) and the minimum tension load throughout the cyclic loading. 2. materials and methods the material used was polyester (pet). yarn samples were of 500 mm length with sandwich-type terminals (figure 1) [7]. prior to the application of the impact load, and throughout all the tests, the samples were left 24 hours at controlled temperature of 20±2°c and relative humidity of 65±4%. tension tests were carried out in an emic dl-2000 electromechanic universal testing machine in order to obtain the material’s ybl of virgin samples. using a 500mm/min displacement ramp, and ten specimen, it was found that this pet has an ybl of 165,44n. 76 http://dx.doi.org/10.14311/app.2017.7.0076 http://ojs.cvut.cz/ojs/index.php/app vol. 7/2017 evaluation of the fatigue response of polyester yarns after the application of abrupt tension loads figure 1. sandwich-type terminals. figure 2. impact testing device. a set of virgin samples were taken to the impact testing device (figure 2) and submitted to an axial impact load corresponding to 10% of the material’s ybl. this load is achieved releasing a 1.7 kg dead weight from a 300mm height, which transfers to the sample a potential energy of 5 j. the maximum elongation of the samples were measured using a marker attached to the bottom end of the yarns running along a ruler. after the impact test, each specimen is taken to an instron 8801 servohydraulic unversal testing machine to be submitted to force-controlled cyclic tension loads. a total of eight ranges of tension loads were used during the cyclic tests: 10-90%ybl, 2090%ybl, 30-90%ybl, 40-90%ybl, 50-90%ybl, 6090%ybl, 70-90%ybl, 80-90%ybl. the frequency used was 0.1hz. the same fatigue procedure was applied to virgin samples, in order to compare their behavior with those damaged by the impact. each load range was applied to ten different samples (both virgin and damaged by the impact test), which means that a total of 160 yarn samples were used throughout the fatigue assessment. figure 3. cycles up to failure versus load ranges: superposed results. 3. results 3.1. effect of impact load on the fatigue life of pet figure 3 shows the number of cycles up to failure versus the minimum load of each load range in the case of virgin samples and damaged samples. regardless of the minimum load applied, the virgin samples have a longer fatigue life compared to the ones that were previously submitted to the abrupt tension load. table 1 sumarises the number of cycles up to failure for both cases for each load range. 3.2. effect of impact load on the yarn elongation together with the tests to determine the material’s ybl, data regarding the samples elongation in rupture were collected. that was made in order to compare the elongation in rupture of the samples submitted to the quasi-static tests performed on the universal testing machine with those obtained when the yarns fail due to impact only. it was found that a potential energy of 6.5 j, which corresponds to a tension load of 13% of the material’s ybl, is required to take the yarns to fail by impact only. a set of ten samples were submitted to this condition and a scale was used to get the yarn’s elongation in rupture. results comparing the elongation of the yarns during quasi-static test and dynamic test are shown in table 2. it was found that the samples tend to elongate considerably less in rupture when the load applied is abrupt (51.6mm versus 57.5mm of the quasi-static tests). 4. conclusions the present paper showed a preliminary assessment of the change in the fatigue life of polyester yarns when they are submitted to an abrupt axial load in comparison to the virgin material, and it was found that a sudden load of approximately 10% of the yarn break load is enought to lead the material to a considerable drop on its fatigue resistance. in terms of the yarn’s elongation in rupture, it was also found that quasi-static tests lead to a larger elongation, and it 77 e. l. v. louzada, c. e. m. guilherme, f. t. stumpf acta polytechnica ctu proceedings minimum load [% ybl] 10 20 30 40 50 60 70 80 virgin samples 50 73 108 127 127 141 153 75 after impact 30 34 41 48 57 71 68 44 table 1. number of cycles up to fatigue rupture. elongation in rupture quasi-static test 57.52mm impact test 51.6mm table 2. elongation in rupture for quasi-static and dynamic tests. can be explained basically by the viscoelastic behavior of the material, because during the impact load the material has no time to dissipate the potential energy in terms of heat energy, for example. it is also interesting to notice that a sudden load of only 13% of the material’s yarn break load is sufficient to lead to its rupture by impact, which is considerably low and could be used to warn the industry to take that into consideration when operating with mooring ropes made of polyester. references [1] c. j. m. del vecchio. light weight materials for deep water moorings. phd thesis 1992. [2] m. m. salama. lightweight materials for mooring lines of deepwater tension leg platforms. marine technology 21(3):234–241, 1984. [3] f. e. g. chimisso. the past, the present and the future of policab: the challenge of synthetic mooring ropes anchorages at pre-salt petroleum basin, in brazil. proceedings of the 10th youth symposium on experimental solid mechanics 2011. [4] r. r. rossi. cabos de poliester para ancoragem de plataformas de petroleo em aguas ultraprofundas, in portuguese. msc thesis 2002. [5] f. v. de carmago, c. e. m. guilherme, c. fragassa, a. pavlovic. cyclic stress analysis of polyester, aramid, polyethilene and liquid crystal polymer yarns. acta polytechnica 56(5):402–408, 2016. doi:10.14311/ap.2016.56.0402. [6] r. l. bosman. on the origin of heat build-up in polyester ropes. oceans96 mts/ieee prospects for the 21st century conference proceedings 1996. [7] d. pfarrius, e. duarte, f. e. g. chimisso. theoretical and experimental modeling of a socket sandwich for use in tension tests of synthetic ropes. proceedings of the vith youth symposium on experimental solid mechanics 2007. 78 http://dx.doi.org/10.14311/ap.2016.56.0402 acta polytechnica ctu proceedings 7:76–78, 2017 1 introduction 2 materials and methods 3 results 3.1 effect of impact load on the fatigue life of pet 3.2 effect of impact load on the yarn elongation 4 conclusions references acta polytechnica ctu proceedings doi:10.14311/app.2016.5.0004 acta polytechnica ctu proceedings 5:4–11, 2016 © czech technical university in prague, 2016 available online at http://ojs.cvut.cz/ojs/index.php/app on efficient operational concept of future high-speed railway in the czech republic michal drábek department of logistics and management of transport, faculty of transportation sciences, ctu in prague, czech republic correspondence: xdrabek@fd.cvut.cz abstract. the aim of this paper is to elaborate a layout of the first operational concept of rapid services with 1 hour system travel time between praha and brno. two basic methods are used – integrated periodic timetable (periodic rendezvous of all services in ipt-nodes) and operational concept economy approach, as defined below by the author. in this paper, three recent high-speed railway concepts for the future so-called rapid services network of the czech republic are followed-up. the first one is an operational traffic planning study by kalčík, janoš et al. on behalf of czech ministry of transport from 2010. the second one is the high-speed railway promoting book high speed rail even in the czech republic by šlegr et al. from 2012, with likely the most detailed concept of rapid services network. the third one is a paper on progress of the official spatial-technical studies for some future czech high-speed lines by šulc from 2014. the importance of achievement of 1 hour travel time between the largest agglomerations is briefly presented. the presented methodological approach, although soft and manager-oriented, comprises some firm principles: segmentation of high-speed train offer, so that more expensive rolling stock is not wasted by operation on long conventional line sections, consideration of system travel times for efficient rolling stock circuit, restriction of need for links from high-speed to conventional lines, and utilization of high-speed lines as a "rail highway". this approach is intended to be particularized iteratively, with every application. so, in this paper, first version of operational concept economy approach is introduced. the key idea is that passengers should be offered such travel times and service intervals (headways) and such number of direct services, which are adequate to their potential demand, but as much synergistic effect as possible should be strived to be achieved for every proposed construction (new or modernized one). such approach goes towards economic efficiency, which is crucial indicator for political decision necessary for building, let alone eu co-funding of the construction. experience shows that in many czech feasibility studies, achievement of sufficient economic efficiency was the most complicated part of the study. results show that an efficient operational concept can be designed not at the expense of runtimes between the largest cities. keywords: high-speed railway, high-speed line, rapid services, integrated periodic timetable . 1. introduction the aim of this paper is to elaborate a layout of the first operational concept of rapid services with 1 hour system travel time between praha and brno. two basic principles are used – integrated periodic timetable, and operational concept economy approach, as defined below by the author. directive 2008/57/ec of the european parliament and of the council of 17 june 2008 on the interoperability of the rail system within the community [1] defines in annex ii infrastructure and rolling stock as structural subsystems of the rail system, and traffic operation and management as a functional subsystem of the rail system. in traffic operation and management subsystem, traffic planning is also included. this paper is organized as follows. after list of abbreviations and list of largest czech cities, recent concepts of high-speed lines network for rapid services, in the context of recent ten-t regulation [2] are introduced. then, significance of 1 hour travel time is summarized, based mostly on revealedpreference data. introduction of used methods follows: integrated periodic timetable and newly developed soft operational concept economy approach. further, scheme of the resulting operational concept is presented, commented and discussed. the paper is closed by conclusion, including recommendation for further studies in this field. for the sake of brevity and clarity, following abbreviations will be used in this paper: • emu – electric multiple unit • hs – high-speed (railway) • hsl – high-speed line • ipt – integrated periodic timetable • put line – public transport line (not to be mistaken with railway line as a built structure) – a group of services which operate all day in regular period, and serve particular sequence of stations and stops • rs – rapid services (czech system of hsls and follow-up uprgraded or new lines, in czech: rychlá spojení) 4 http://dx.doi.org/10.14311/app.2016.5.0004 http://ojs.cvut.cz/ojs/index.php/app vol. 5/2016 on efficient operational concept • sždc – railway infrastructure administration, state organization – czech state railway infrastructure manager ( in czech: správa železniční dopravní cesty ) • ten-t – trans-european transport networks • tsi – technical specification of interoperability • uic – international union of railways 2. list of largest czech cities table 1 displays list of all czech cities over 40,000 inhabitants. these cities can be divided into four subgroups based on their population, as of 1st january 2015. regional capitals are emphasized. note that population of ostrava agglomeration is comparable with praha. these cities build up five largest agglomerations: praha, ostrava region, brno, northwestern bohemia (from děčín to chomutov) and central moravia. 3. ten-t context and recent concepts of hsls for rapid services most of the intended czech hsls are part of ten-t network, but only three of them are involved into the core ten-t network: hsls praha – ústí nad labem and brno – břeclav, and modernization of the line brno – přerov. for the hsl brno – břeclav, there are several alternative directions southward of vranovice. for the hsl praha – wroclaw (in comprehensive network), there are two alternative directions: via liberec or via hradec králové. the author has followed up from three main sources. each one has been carried out for different purpose. the study carried out by kalčík, janoš et al. (including the author) [4] contains, to the author’s knowledge, the only model timetable for the whole rs network up to now. approximate routes for hsls were defined, and, based on their parameters and on available hs rolling stock parameters, model timetables were constructed using timetabling software fbs. one of the results is a netgraph (scheme of put lines with arrival and departure minutes in node stations). šlegr et al. [5] have written and published the book high speed rail even in the czech republic with many hsl drawings in the map. the book introduces european and world best practice in hs railway and, in a detailed manner, presents potential benefits of the rs system for every czech region. as a result, the most detailed concept of rs network (although not officially approved), with qualified estimation of section runtimes, is introduced there. šulc [6] has presented recent progress in five spatial-technical studies of hsls, carried out on behalf on sždc. the aim of these studies is to find suitable hsl routes, so that necessary space for their future construction can be legally protected. for this purpose, simplified timetable layout and cost estimation were carried out. other hsls were already subject of more detailed feasibility studies: praha – liberec and brno – přerov. the latter study recommended the doubling and modernization of present single-track line up to 200 km/h as the best one, although the assessed line is located directly in the corridor brno – ostrava. uic [7] records that hsls for the maximum speed of 350 km/h are in operation in china (1,028 km), and they are planned in portugal (206 km). in the united kingdom, hsls for the maximum speed of 360 km/h (543 km) are planned. thus, such maximum speed is nothing uncommon for the recently built and planned hsls. figure 1 shows the maximalist future czech hsl network as a backbone of rapid services. it does not include possible local new lines or line modernizations. the scheme was created as follows. for each hsl, always the higher maximum speed from šlegr et al. [5] and šulc [6] was chosen. inclusion of each hsl (or its section) into ten-t core, or comprehensive network based on [2] was marked. the actual phase of project preparation (based on šulc [6] and to the author’s knowledge) was also marked. from the possible links to conventional railway network, only the most significant ones were displayed. the stations or stops, which are intended to be built directly on hsls were adopted from šulc [6]. international hsls are strongly dependent on the will of both neighbouring states: the hsl from ústí nad labem to dresden is supported by czech ministry of transport, as well as the government of saxony (although it requires a long tunnel under the mountains), but the hsl from plzeň to bavaria is not quite a priority for the government of bavaria, despite its indisputable european significance. 4. one hour as a breakthrough in travel time the practice has proven that, when the public transport travel time changes to 1 hour or less, and if this travel time is competitive to car, public transport ridership considerably increases, and some people can even commute daily. if the present travel time by cars between two largest czech cities is approximately 2 hours, only 1 hour is fully competitive, because the travel time to and from the station must be added. in switzerland, new line for 200 km/h was built to achieve travel time of 56 min between zurich and berne (swiss largest city and capital). this line was not an isolated project, but a part of targeted investments for implementing bahn 2000 concept. its main aim was to achieve necessary system travel times for the ipt. this new line, as a kind of swiss ”rail highway”, serves also for many other long-distance put lines. the modal-split of railway for the journeys zurich – berne has achieved 88%, for the journeys zurich – st. gallen (1:02 h travel time) it was 73% (voev utp, 2010 [8]). janoš and baudyš [9] state that improvements in 5 michal drábek acta polytechnica ctu proceedings city population city population praha 1,259,079 zlín 75,112 brno 377,440 havířov 75,049 ostrava 294,200 kladno 68,552 most 67,089 opava 57,772 frýdek místek 56,945 karviná 55,985 city population city population plzeň 169,033 jihlava 50,521 liberec 102,562 teplice 50,079 olomouc 99,809 děčín 49,833 ústí nad labem 93,409 karlovy vary 49,781 české budějovice 93,285 chomutov 48,913 hradec králové 92,808 jablonec nad nisou 45,594 pardubice 89,693 mladá boleslav 44,318 přerov 44,278 prostějov 44,094 table 1. list of largest czech cities. data from czech statistical office [3]. figure 1. maximalist czech hsl network 6 vol. 5/2016 on efficient operational concept timetable offer (more frequent and more periodic service) in long-distance railway in the czech republic have led to increase of number of passengers. ”between 2003 and 2005, incomes increased by 7.86%, and number of passengers by 3.63%. however, in some relations, thanks to new supply, the increase of passengers was 40 to 50%. ipt is being further developed. thanks optimal train connections in junctions and synergic effect of linked supply number of passengers raises by 2-10% per year even on lines with no supply change.” the raise of service for timetable 2004/05 was 7%, compared to timetable 2003/04, when new offer (supply) concept was introduced. vichta [10] has also shown figures that justified timetable offer improvement for large enough cities (ústí nad labem and teplice). since 2003/04 timetable, two segments of periodic service between ústí and praha were introduced: a non-stop fast train with travel time slightly above 1 hour and a 120 min interval, and fast train with several stops hourly. since 2008, a non-stop segment has been operated hourly, and every second service connected teplice directly with praha. compared with train ridership from 2002 (100%), the train ridership between ústí and praha grew to 152% in 2006, and to 325% in 2008. compared to the same year, the train ridership between teplice and praha grew to 228% in 2006 and to 369% in 2008. in both cases, only relative changes can be published, to avoid publishing of czech railways’ business sensitive data. kordis jmk, an integrated transport system coordinator of south moravia region, has carried out a stated-preference study of potential ridership of hsl between praha and brno [11]. the study has proved that about 30,000 passengers daily would use the new hsl, majority of them instead of cars. the potential ridership varied depending on train travel time and ticket price. the results for 45 min travel time were only slightly better than the ones for 60 min. the acceptable ticket price was up to 299 czk. for 300 czk and more, the potential ridership fell to approximately 18,000 passengers. a travel time between praha and brno up to 60 min is also supported officially, at least by some departments of czech ministry of transport and sždc – see šulc, page 14 [6] and kušnír and ilík, page 13 [12]. the graph in the latter presentation shows clear intention to ensure better connection not only for the largest agglomerations, but also for the regions, so that their development can be stimulated. however, a more detailed study on potential rs ridership, which should include a transport demand forecasting model and more detailed economic calculations, has to be carried out. sždc has called a bid for so called ”rapid services – study of opportunities”, that had to be cancelled, because only one eligible applicant remained. recently, a new bid for the study is being in progress. 5. methods 5.1. principle of ipt ipt is a special case of periodic timetable, which ensures connections between various put lines in the whole network. for this effect, the following requirements must be fulfilled. all services operate in put lines, in unified interval which is equal to 2k-multiple of basic period (60 min as a rule), where k is an integer. in every put line, services from opposite directions meet each other at the same time (symmetry time). this time repeats after half an interval. should services of two put lines enable mutual connections with an equal changing time for each direction, these two put lines must have equal symmetry time. in european long-distance railways, symmetry time slightly before the minute 00 is common. this is called zero symmetry axis. in practice, symmetry times in the minute 57 to 01 are used, which, in the case of hourly interval, implies another symmetry time in the minute 30. for the 30 min interval, additional symmetry times in the minutes 15 and 45 occur. an ipt-node is a railway station where connections with services from other put lines can be ensured, because the symmetry time is reached there. to achieve connections in every ipt-node, system travel time between them must be ensured. it is an integer multiple of half period. it consists of sum of regular runtimes between two iptnodes, sum of dwell times and waiting times between them, and of proportional part of dwell, changing or waiting times in mentioned ipt-nodes (it depends which time is limiting in each particular case) [13]. it is obvious that two put lines with equal runtimes, which operate on long enough common line section and do not need to be connected for the passenger transfer, can be interposed into a half interval. this interposition is used for more attractive timetable offer. on the other hand, if the transfer is desirable, the trains of two put lines run as close as possible after each other (so that they reach the same ipt-node, e.g., in berne). 5.2. operational concept economy approach the presented methodological approach, although soft and manager-oriented, comprises some firm principles, which will be explained as follows. this approach is intended to be particularized iteratively, with every application. so, in this paper, first version of operational concept economy approach is introduced. the key idea is that passengers should be offered such travel times and service intervals (headways) and such number of direct services, which are adequate to their potential demand, but as much synergistic effect as possible should be strived to be achieved for every proposed construction (new or modernized one). such approach goes towards economic efficiency, which is a crucial indicator for political decision necessary for building, let alone eu co-funding of the construction. 7 michal drábek acta polytechnica ctu proceedings experience shows that in many czech feasibility studies, achievement of sufficient economic efficiency was the most complicated part of the study. travel times should correspond to system travel times according to ipt rules. of course, system travel time is always considerably larger than a corresponding runtime, because of times for changing between services in the ipt nodes, and necessary time supplement for the sake of timetable stability. because of high-speed railway dynamics, such supplement should be larger here than by the conventional railway. runtime calculations are not subject of this paper, but it is obvious, that for approx. 200 km distant cities praha and brno and for system travel time 60 min between them, maximum speed of 200 km/h is totally insufficient. so, higher speed, e.g. 350 km/h, is considered. moreover, in city centres, only lower speed is possible due to densely built-up areas and probably one more stop in praha before the centre (zahradní město) [6]. if a new hsl is justified by the potential demand, it should be at the same time used by as many trains as possible, to make faster as many connections as possible. thus, at least central parts of the hs network should serve as ”rail highways” for fast enough long-distance, or even fast regional, put lines. this idea leads to requirements for junctions to connect a hsl with conventional rail network. a recent concept of rs of czech ministry of transport [12] corresponds to such idea, because of ensuring fast railway service also for czech regions. but, because of economy, there should be less junctions with as much put lines using one junction as reasonable. as a rule of thumb, every junction from hs line should be used by minimum 1 pair of trains hourly. the design of put lines themselves should also be in compliance with economy. for high-speed railway, a hierarchy in the form of following segments of offer can be defined: • fastest put lines (hs-a) for the speed up to 350 km/h, with dedicated emus. in figure 2, such put lines are marked in red colour. • long-distance put lines (hs-b1) which operate on long sections of fast high-speed lines, so the rolling stock should be able to run 230 to 250 km/h to avoid frequent overtaking by trains of higher segment. rolling stock can vary from emu to classical trainset, or push-pull units hauled by a locomotive (e.g. railjet). in figure 2, such put lines are marked in violet colour, and in pink colour, in the case of tilting trains. • long-distance put lines (hs-b2) which operate on short sections of fast high-speed lines, or they are overtaken by hs-a trains. thus, for the rolling stock the maximum speed of 200 km/h is sufficient. rolling stock can vary from emu to classical trainset. in figure 2, such put lines are marked in blue colour. • fast regional put lines (hs-c), which can achieve significantly lower travel time to the agglomeration centre by using of hsl section. the rolling stock should run at least 160 km/h and be interoperable with tsis required for high-speed railway (for the other segments, this is evident). for maintaining reasonable complexity, this segment is neglected in this paper. for the sake of economy, the higher segment, the less trainsets should be required. this aim can be achieved by two means: as little number of put lines as reasonable, and as little number of trainsets per put line as reasonable. but, on the other hand, appropriate service interval, and some reserve trainsets should be ensured. the basic service interval for each put line is considered 60 min. the only exception is international put line warsaw – vienna, which is of international more than national significance. the basic service interval of 30 min (of a single put line or an interposition of them) is considered for connection of all regional centres above 90 000 inhabitants with praha, brno or ostrava, if the system travel time to the particular agglomeration is 1 hour or lower. the hs-a segment should end in cities, which are large enough to justify the most expensive and maintenance sensitive rolling stock. for the hs-a segment between praha and brno, a basic interval 15 min is considered. in some studies, even an interval 10 min is discussed, but it would complicate operation of hs-b trains on the same hsl. so, the longest possible trains (400 m), and double-decker trains (such as tgv duplex) are considered as mostly desirable. the number of four basic hs-a put lines (each one with 60 min interval) should not be exceeded. thus, every hs-a train is economically justified because of service between praha and brno. because of ”biggest possible” trains, a 30 min period of hs-a segment between praha and ostrava is likely to be sufficient. it is obvious that vast majority of the demand would be national, mostly between large cities. so, for the sake of economy, a standard hs-a trainset consisting of two half-trains (such as ice 3, but double-decker) is further proposed. this measure would enable eventual disconnection of one trainset in the station, where many passengers leave and only little number of passengers board. such disconnection enables also direct service to two various destinations. as a rule of thumb for the czech republic, middle-sized cities with 30,000 or more inhabitants can be considered as appropriate ends of hs-a put lines. it is, however, important that a runtime from regional centre to such city must not exceed approximately 25 minutes. thus, service for such middle-sized city in 60 min interval would cost only 1 trainset extra (a half-train only). for the hs-b segment, the choice of rolling stock depends on timetable of hsl section used (required maximum speed), and on justification of tilting technology (a long enough conventional line with many curves, and potential time savings related to the population served). the put lines should be composed with respect to equal rolling stock requirements on both sides of a metropolis (for 8 vol. 5/2016 on efficient operational concept the czech republic, this means in practice praha and brno). for the hs-c segment, runtime savings should justify extra costs for ”high-speed-compatible” technical equipment of the rolling stock. thus, such trains should use high-speed line sections both before and after stop in the agglomeration centre. for the feasibility of the proposed operational concept, electrification and upgrading of several mainlines is supposed. further, a new line from třebíč to moravské budějovice is required, to enable direct connection of jihlava, třebíč and znojmo. for the sake of reasonable scope, timetables are not constructed. it is supposed, that in every ipt-node, slower trains leave after faster trains, if they run on the same line. thanks to the symmetry of ipt, arrival sequence is symmetric (slower trains arrive before faster ones). the system travel times are estimated on the basis of kalčík, janoš et al. [4], and lengthened due to additional stops if needed (by qualified estimation). 6. results and discussion the scheme of put lines, ipt-nodes and system travel times between large cities is displayed in figure 2. an interposition into half interval (30 min) is marked by two parallel lines close together. for the hs-a segment, the interpositions into 30 min and 15 min intervals have resulted in the fact that either plzeň or ústí nad labem, but not both cities, can be connected directly with ostrava. considering importance of direct connection of berlin and vienna via praha for the czech republic, ostrava should be connected directly with plzeň rather than ústí. thus, two bundles of hs-a put lines emerged: • berlin – dresden – praha – brno – břeclav – vienna/bratislava – budapest and litvínov/děčín – ústí nad labem – praha – brno – zlín • munich (or nuremberg) – plzeň/plzeň – praha – brno – ostrava – petrovice u karviné – warsaw/opava and plzeň – praha – brno – ostrava – havířov – žilina the sign ”/” stands for coupling and decoupling of half-trains. for international directions, full trains were only considered for berlin (because of national demand between dresden and berlin) and žilina, because of traditional ”czechoslovak” passenger demand (e.g. students, workers or tourists). from plzeň, only a half-train proceeds to bavaria. in břeclav, decoupling enables direct service from praha both to vienna, and bratislava (and budapest). the put line, which ends in ústí nad labem, can proceed both to děčín and teplice, thanks to decoupling. if the line to litvínov would be upgraded, a half-train can end there. so, service of both branches would cost only 1 half-train extra per branch. from ostrava, only a half-train would proceed to warsaw. thus, the second half-train can proceed to opava, and even with present runtime it would cost only 1 half-train extra. hs-b1 segment is proposed mostly in moravia for the connection of central moravia between brno and ostrava. the reason for faster trains is the absence of stops directly on the hsl brno – ostrava. the absence of clear centre resulted in necessity of more put lines. olomouc is connected with brno by a non-stop service every 30 min. every second train proceeds north-westward to connect mohelnice, zábřeh and šumperk directly with brno. due to longer runtime, two extra trains are required. olomouc is also connected with ostrava every 30 min. one put line proceeds as an express segment to pardubice and hradec králové, the second one as a slower inter-regional train to pardubice. from pardubice to praha, the same put line changes to a non-stop segment. between praha and olomouc, an interposition into 30 min interval is proposed. the interposing put line proceeds to vsetín. because the last put line uses hsl only between praha and pardubice, a trainset for 200 km/h could be appropriate. přerov is connected hourly both with brno and ostrava. behind přerov, both put lines proceed as a conventional fast train. hradec králové is connected with praha only hourly. the supplementary connection can be ensured by transfer in pardubice. the put line from hradec králové is proposed to be composed of two emus, because of further decoupling, so that both trutnov and náchod can be connected directly with praha. liberec is connected with praha every 30 min, with only intermediate stop in mladá boleslav. jablonec nad nisou is connected with liberec (and hence with praha) by suburban trains. the same case is kladno with praha (but there are supposed fast regional trains, running directly to praha main station). north-western bohemia is connected with praha every 30 min, with likely stop in louny (for the connection of this peripheral region) between most and praha. one of two put lines proceeds to karlovy vary and cheb, and, because of the terrain, tilting train operation can save considerable amount of runtime. both mentioned put lines proceed from praha to jihlava, but with various stopping. the tilting train as hs-b1 segment runs nonstop, and proceeds to třebíč and znojmo (possible proceeding to vienna is worth consideration), where tilting is supposed to save the runtime. another put line stops in benešov, and on the hsl in vlašim and vysočina (a stop between pelhřimov and humpolec, linked to bus and park+ride). overtaking by hs-a trains in some of these stations is very likely – that is why hs-b2 segment is sufficient. from jihlava, this put line proceeds to havlíčkův brod and žďár nad sázavou. instead of separate link from hsl to havlíčkův brod, a thorough modernization and doubling of the line jihlava – havlíčkův brod is proposed here. jihlava is connected with brno every 30 min, with intermediate stop in velké meziříčí rs, most likely with overtaking by hs-a segment at the same time. thus, hs-b2 segment seems to be sufficient. from jihlava, one put line would proceed to havlíčkův brod, and the second one to české budějovice. modernization of 9 michal drábek acta polytechnica ctu proceedings figure 2. proposed operational concept for hsl network of rs. the line jihlava – veselí nad lužnicí and a new tunnel ševětín – nemanice are supposed to be comlpeted. the last put line of hs-b2 segment is děčín – ústí nad labem – litoměřice – roudnice nad labem rs – praha – tábor – české budějovice. the corridor to české budějovice is supposed to be finished. one of the most important results is a reduction of links from hsls to conventional lines outside large nodes. for long-distance put lines (hs-b1 and hs-b2) there have remained: litoměřice, benešov u prahy (twice), jihlava (twice), olomouc (twice) and přerov (twice) – in total 9 links. it is desirable to try to design links near olomouc and přerov together as well as to design single-track links, if they would be used by only one pair of trains hourly. of course, the proposed operational concept implies plenty of alternatives. for instance, a 30 min interval praha – hradec králové, or another put line layout for the connection of jihlava. if the put line interval is 60 min, the timetable should be constructed so that additional peak trains (which leads to interposition into 30 min interval) can be added without any conflict. the probably biggest disadvantage of the rs concept used by the author is that pardubice cannot be connected with brno within 1 hour system travel time. the author is aware of alternative concepts of hsl praha – brno via pardubice. but, given that system travel time 1 hour between praha and brno is not quite easy to achieve even via jihlava, the longer hsl via pardubice would hardly enable such time, even with a non-stop train running up to 350 km/h. 7. conclusion the presented operational concept is only a brief layout, which served as a test of operational concept economy approach, and a feedback for future making this approach more accurate. the author believes than his layout would also contribute to constructive discussion on future czech rs system, and on the role of hsls in it. the author considers as an important contribution in this paper, that there were shown close economic interdependencies between operational concept, rolling stock and infrastructure, and the operational concept was designed on the basis of such interdependencies, towards economic efficiency, but not at the expense of runtimes between the largest cities. the czech rs network always will be an integral part of both european and national transport system. thus, its layout and construction will be always subject of political decisions. the presented operational concept has clearly demonstrated many possible alternative solutions. although detailed studies will calculate which alternative brings higher benefits for lower costs, the most likely alternative to be constructed will be, in the 10 vol. 5/2016 on efficient operational concept author’s opinion, a result of political compromise or trade-off. but, even by choosing any such sub-optimal alternative the decision makers should keep in mind than it should enable efficient allocation of resources. there is always a high risk not only of construction of excessive infrastructure, but also of inefficient operational concept (too many put lines for too low demand). thus, every alternative, before its detailed economic assessment, has to be designed cautiously to minimize such risk. the compromises are inevitable, but they should not generate disproportionate extra costs. acknowledgements the author would hereby like to propose a vote of thanks to mr. petr šlegr for his long-standing promotion of highspeed railway in the czech republic, which he has crowned by editing and publishing book high speed rail even in the czech republic [5]. the author would hereby like to propose a vote of thanks to dr. vít janoš, a supervisor of his doctoral thesis, for initial layout of an operational concept for rs [4], that the author was glad to follow up. the author would hereby like to propose a vote of thanks to mr. jiří kalčík, a freelance road and railway designer, for searching for efficient high-speed lines, both in official studies (e.g. [4]), and in his spare time [5]. so, he has contributed significantly to the concept mentioned above. all engineers enabled the author to follow their ideas from timetabling viewpoint – for instance, in this paper. the author would hereby also like to propose a vote of thanks to sždc as a former author’s employer, for the occasion to assess five spatial-technical studies of hsls simultaneously from timetabling viewpoint (in summer 2013). this task has made the author to create his own view on efficient operational concept for the future czech rs network. his interest in this field has endured up to now, and has resulted in writing this paper. the author hopes it will not remain his last and the most detailed work in this field. references [1] european commission. directive 2008/57/ec of the european parliament and of the council of 17 june 2008 on the interoperability of the rail system within the community [online], 2011. european union ,brussels, http://eur-lex.europa.eu/legal-content/en/txt/ pdf/?uri=celex:02008l0057-20110322&from=en. [2] european commission. regulation (eu) no 1315/2013 of the european parliament and of the council of 11 december 2013 on union guidelines for the development of the trans-european transport network and repealing decision no 661/2010/eu [online]. 2013. european union ,brussels, http://eur-lex.europa.eu/legal-content/en/all/ ?uri=uriserv:oj.l_.2013.348.01.0001.01.eng. [3] czech statistical office. populations of municipalities [online], 2015. [2016-04-30], https://www.czso.cz/ csu/czso/pocet-obyvatel-v-obcich-k-112015. [4] j. kalčík, v. janoš. operational traffic planning solution for backbone network of high-speed lines, study on behalf of czech ministry of transport. chrást u plzně: projektové středisko kalčík, czech republic, 2010. unpublished. [5] p. šlegr et al. high speed rail even in the czech republic, 2012. editors: p. šlegr, m. robeš, m. drábek, m. stach; isbn 978-80-905005-0-1. [6] j. šulc. actual progress of preparation of rapid services in sždc. in: czech raildays: building of high-speed rail system in the czech republic. separate article [online]. praha: m-presse plus s.r.o., 2014. in czech, http://www.railvolution.net/ czechraildays/2014/buletin_szdc.pdf. [7] uic. high speed lines in the world [online], 2014. paris: uic, http://old.uic.org/img/pdf/20140901_ high_speed_lines_in_the_world.pdf. [8] voev utp: fact and arguments in favour of swiss public transport. p. 46. bern: voev utp, 2010. in german. [9] v. janoš, k. baudyš. railway timetable in czech republic, in systemy transportowe teoria i praktyka, 2009. isbn 978-83-926923-1-7. [10] f. vichta. requirements for public transport system in the czech republic, keynote speech, in future of railway passenger transport in the czech republic, czech raildays [online], 2011. ostrava: railvolution, [2011-06-14], in czech, http://www.railvolution.net/ czechraildays/2011/seminare/1.3_md_vichta.pdf. [11] railhuc project. final results [online], 2014. brno: kordis jmk, isbn 978-80-260-7787-9, http: //www.railhuc.eu/download/category/126-book2. [12] j. kušnír, j. ilík. railway in 2030, keynote speech, in future of railway passenger transport in the czech republic, czech raildays [online], 2013. ostrava: czech ministry of transport, in czech., http://www.railvolution.net/czechraildays/2013/ seminare/konference-kusnir.pdf. [13] m. drábek. periodic freight train paths in network, doctoral thesis, 2014. ctu in prague, http://takt.fd.cvut.cz/cargo/drabek_thesis.pdf. 11 http://eur-lex.europa.eu/legal-content/en/txt/pdf/?uri=celex:02008l0057-20110322&from=en http://eur-lex.europa.eu/legal-content/en/txt/pdf/?uri=celex:02008l0057-20110322&from=en http://eur-lex.europa.eu/legal-content/en/all/?uri=uriserv:oj.l_.2013.348.01.0001.01.eng http://eur-lex.europa.eu/legal-content/en/all/?uri=uriserv:oj.l_.2013.348.01.0001.01.eng https://www.czso.cz/csu/czso/pocet-obyvatel-v-obcich-k-112015 https://www.czso.cz/csu/czso/pocet-obyvatel-v-obcich-k-112015 http://www.railvolution.net/czechraildays/2014/buletin_szdc.pdf http://www.railvolution.net/czechraildays/2014/buletin_szdc.pdf http://old.uic.org/img/pdf/20140901_high_speed_lines_in_the_world.pdf http://old.uic.org/img/pdf/20140901_high_speed_lines_in_the_world.pdf http://www.railvolution.net/czechraildays/2011/seminare/1.3_md_vichta.pdf http://www.railvolution.net/czechraildays/2011/seminare/1.3_md_vichta.pdf http://www.railhuc.eu/download/category/126-book2 http://www.railhuc.eu/download/category/126-book2 http://www.railvolution.net/czechraildays/2013/seminare/konference-kusnir.pdf http://www.railvolution.net/czechraildays/2013/seminare/konference-kusnir.pdf http://takt.fd.cvut.cz/cargo/drabek_thesis.pdf acta polytechnica ctu proceedings 5:4–11, 2016 1 introduction 2 list of largest czech cities 3 ten-t context and recent concepts of hsls for rapid services 4 one hour as a breakthrough in travel time 5 methods 5.1 principle of ipt 5.2 operational concept economy approach 6 results and discussion 7 conclusion acknowledgements references 170 acta polytechnica ctu proceedings 2(1): 170–173, 2015 170 doi: 10.14311/app.2015.02.0170 new insights from inside-out doppler tomography e. j. kotze1,2, s. b. potter1 1south african astronomical observatory, po box 9, observatory 7935, cape town, south africa 2astrophysics, cosmology and gravity centre (acgc), department of astronomy, university of cape town, private bag x3, rondebosch 7701, south africa corresponding author: ejk@saao.ac.za abstract we present preliminary results from our investigation into using an “inside-out” velocity space for creating a doppler tomogram. the aim is to transpose the inverted appearance of the cartesian velocity space used in normal doppler tomography. in a comparison between normal and inside-out doppler tomograms of cataclysmic variables, we show that the inside-out velocity space has the potential to produce new insights into the accretion dynamics in these systems. keywords: accretion accretion discs methods: spectroscopic binaries: close dwarf novae polars cataclysmic variables. 1 introduction cataclysmic variables (cvs) are quintessential stellar objects for the study of mass transfer, accretion flows and accretion discs. the typical interacting system consists of a secondary low-mass, red dwarf star which is filling its roche lobe and is transferring mass via the inner lagrangian point (l1) onto the primary white dwarf star (see warner 1995 for a comprehensive review). doppler tomography, as introduced by marsh & horne (1988), is aimed at rendering the information locked-up in phased-resolved spectra of a cv into a twodimensional map of the binary components in velocity space (doppler tomogram). 2 doppler tomography 2.1 spatial coordinates fig. 1 shows a model cv with an accretion disc in a cartesian spatial coordinate frame that co-rotates with the system. as described by marsh & horne (1988), this two-dimesional frame has its origin at the centre of mass of the system [marked with a plus (+)], the x-axis along the line connecting the centres of mass of the primary and secondary [marked with crosses (×)], and the y-axis parallel to the velocity vector of the secondary. the orbital motion is assumed to be counter-clockwise around the centre of mass of the system. the model cv assumes the following system parameters: inclination i = 87◦; mass of the primary m1 = 0.8; mass ratio q = m2/m1 = 0.2 and orbital period porb = 0.083333 days (120 minutes). the inner disc radius is derived using these parameters and assuming a keplerian velocity of ∼ 2.37×103 km s−1. the 3:1 resonance radius is taken to be the outer disc radius. secondary accretion stream inner edge of disc outer edge of disc roche lobe of primary a(0.02,-0.12) b(-0.62,0.0) x y figure 1: co-rotating cartesian spatial coordinate frame for a model cv. 2.2 velocity coordinates the left and middle panels of fig. 2 show, respectively, the two-dimensional cartesian and polar velocity coordinate frames which correspond to the co-rotating spatial frame shown in fig. 1, with overlays of the velocity profiles of all the main components of the model cv. the polar velocity coordinate frame is obtained by either transforming the cartesian spatial frame to a polar spatial frame which is then projected into a polar velocity frame or by transforming the cartesian velocity frame into a polar velocity frame. since we found that a polar frame allows for easier transformation of the circularly symmetric velocity profile of a doppler tomogram, this is the first step in establishing an inside-out layout. 170 http://dx.doi.org/10.14311/app.2015.02.0170 new insights from inside-out doppler tomography v(r′,θ) [10 3 km s -1 , degrees] secondary accretion stream inner edge of disc outer edge of disc roche lobe of primary a(0.86,55.2°) b(0.73,270°) 0.0 1.0 2.0 r′ θ v(r′,θ) v(r,θ) [10 3 km s -1 , degrees] secondary accretion stream inner edge of disc outer edge of disc roche lobe of primary a(0.86,55.2°) b(0.73,270°) 1.0 2.0r θ v(r,θ) -2.0 -1.0 0.0 1.0 2.0 -2.0 -1.0 0.0 1.0 2.0 v y [ 1 0 3 k m s -1 ] v x [10 3 km s -1 ] secondary accretion stream inner edge of disc outer edge of disc roche lobe of primary a(0.49,0.70) b(0.0,-0.73) figure 2: cartesian (left), polar (middle) and inside-out polar (right) velocity space. 3 inside-out doppler tomography the right panel of fig. 2 shows the same model cv presented in the left and middle panels, but with the zero velocity origin and the maximum velocities transposed, creating an inside-out polar velocity space. the most notable aspects of the inside-out frame are the inner and outer edges of the accretion disc appearing the “right” way around while the secondary and ballistic stream are outside the disc. the secondary appears upside down because it is orbiting as a solid body, i.e., the outside is moving faster than the inside. the ballistic stream also “curves” inwards as it accelerates towards the disc and primary. 4 examples the example doppler tomograms have been created in polar and inside-out polar velocity space using a modified version of the fast maximum entropy doppler mapping code presented by spruit (1998). 4.1 spiral shocks in the accretion disc of the dwarf nova ip peg fig. 3 shows comparative non-axisymmetric normal and inside-out tomograms for the heii 4686å emission line from phase-resolved spectroscopy of ip peg. the model velocity overlays were calculated using porb = 0.158206 days (∼ 228 minutes), q = 0.48, m1 = 1.16 and i = 83.8◦ (copperwheat et al. 2010). the inner and outer edges of the disc (solid lines), the roche lobes of the primary (dashed line) and secondary (solid line) as well as a single particle ballistic trajectory (solid line) from l1 to 20◦ in azimuth around the primary, are shown. in the normal tomogram the secondary appears as a bright spot inside the disc, whereas it becomes a diffuse patch (spread over more pixels) outside the disc in the inside-out tomogram. however, in the inside-out tomogram the disc appears the “right” way around as the two spiral shocks appear to be spiralling “inward” towards higher velocities. 4.2 ballistic and magnetic accretion flow in the polar hu aqr comparative normal and inside-out tomograms for the heii 4686å emission line from phase-resolved spectroscopy of hu aqr are shown in fig. 4. porb = 0.086820 days (∼ 125 minutes), q = 0.4, m1 = 0.875 and i = 84◦ (one of the models from schwope et al. 1997) were used to calculate the model velocity overlays. the roche lobes of the primary (dashed line) and secondary (solid line) as well as a single particle ballistic trajectory (solid line) from l1 to 65◦ in azimuth around the primary, are shown. a dipolar axis azimuth and co-latitude of ∼ 38◦ and ∼ 12◦ (heerlein et al. 1999) respectively, were used to calculate magnetic dipole trajectories (thin dotted lines) at 10◦ intervals from 15◦ to 65◦ in azimuth around the primary. the secondary appears as a bright spot in both the normal and the inside-out tomograms. in the normal tomogram the ballistic part of the accretion flow appears as a prominent ridge with an apparent consistent brightness from l1 to 1.0×103 km s−1, but with almost no discernible detail at higher velocities. in the insideout tomogram the ballistic flow is more exposed and varying in brightness, but retaining discernible detail to at least 1.5× 103 km s−1. low-velocity (0.0− 0.5× 103 km s−1) emission associated with the magnetic coupling region is seen as a diffuse patch in the lower left quadrant of both tomograms. there is no high-velocity (> 1.5 × 103 km s−1) emission discernible in the lower left quadrant of the normal tomogram, whereas in the inside-out tomogram there is a small patch of emission between 1.5 − 2.0 × 103 km s−1 that can be linked to the magnetically confined accretion flow close to the primary. 171 e. j. kotze, s. b. potter 5 conclusions in a normal tomogram the lower-velocity features tend to dominate the brightness scale since they are concentrated over fewer pixels compared to higher-velocity features. in an inside-out tomogram the converse is true with teneous higher-velocity features being enhanced while prominent lower-velocity features are more spread out and exposed. teneous lower-velocity features, however, may be diluted to the point of becoming indiscernible similar to teneous higher-velocity features in a normal tomogram. we have shown that inside-out doppler tomography projects the accretion disc of a cv the “right” way around with the ballistic stream and the secondary outside the disc. furthermore, the accretion flow of a polar appears more intuitive in an inside-out tomogram with the ballistic stream curving “inwards” and the magnetic flows being more exposed. therefore, we conclude that our new technique of inside-out doppler tomography is complementary to the existing technique. acknowledgement the authors would like to thank axel schwope and danny steeghs for sharing their data of hu aqr and ip peg, respectively. references [1] copperwheat c.m., et al.: 2010, mnras, 402, 1824. doi:10.1111/j.1365-2966.2009.16010.x [2] heerlein c., horne k., schwope a.d.: 1999, mnras, 304, 145. doi:10.1046/j.1365-8711.1999.02311.x [3] marsh, t.r., horne, k.: 1988, mnras, 235, 269. doi:10.1093/mnras/235.1.269 [4] schwope a.d., mantel k.-h., horne k.: 1997, a&a, 319, 894. [5] spruit h.c.: 1998, arxiv:astro-ph/9806141. [6] warner b.: 1995, cambridge astrophysics series 28, cataclysmic variable stars. cambridge univ. press, cambridge figure 3: doppler tomography of ip peg. normal (top) and inside-out (bottom) tomograms are shown for comparison. the input and reconstructed trailed spectra are shown in the left and right panels, respectively. 172 http://dx.doi.org/10.1111/j.1365-2966.2009.16010.x http://dx.doi.org/10.1046/j.1365-8711.1999.02311.x http://dx.doi.org/10.1093/mnras/235.1.269 new insights from inside-out doppler tomography figure 4: doppler tomography of hu aqr. normal (top) and inside-out (bottom) tomograms are shown for comparison. the input and reconstructed trailed spectra are shown in the left and right panels, respectively. discussion dmitry kononov: how does your new technique redistributes brightness and are there some physical suppositions behind this redistribution? enrico kotze: as we see in the example of ip peg the upper bright spiral shock in the normal tomogram appears less bright in the inside-out tomogram since it is projected over a larger area of the image. similar to the normal technique the brightness distribution in the new technique is purely a function of the projection of the spectra into the velocity space frame and we have no claims that this is a representation of the physical brightness distribution of the system. linda schmidtobreick: do you encounter problems with the resolution as the low velocities are now spread over a large circle? enrico kotze: yes, there can be a loss of resolution at low velocities. this is the reverse of what happens in the normal tomograms where high velocities are spread over a larger area and we can encounter a loss of resolution in the high-velocity features. that is why we feel the inside-out technique is complementary to the normal technique. where the normal technique tends to enhance low-velocity features with some loss in the resolution of high-velocity features, the inside-out technique tends to enhance high-velocity features with some loss in the resolution of low-velocity features. david buckley: is the magnetic longitude of the accreting pole(s) a parameter in the inside-out maps? enrico kotze: yes, for the model velocity profile overlays in the inside-out tomograms of polars we take both the azimuthal and longitudinal inclination of the assumed magnetic dipole into account. 173 introduction doppler tomography spatial coordinates velocity coordinates inside-out doppler tomography examples spiral shocks in the accretion disc of the dwarf nova ip peg ballistic and magnetic accretion flow in the polar hu aqr conclusions acta polytechnica ctu proceedings doi:10.14311/app.2016.3.0082 acta polytechnica ctu proceedings 3:82–85, 2016 © czech technical university in prague, 2016 available online at http://ojs.cvut.cz/ojs/index.php/app thermo-viscoelastic characterization of polymer laminate films eliza truszkiewicza,∗, daniel tscharnutera, gerald pilzb a polymer competence center leoben gmbh, rossegerstrasse 12, 8700 leoben, austria b materials science and testing of polymers, montanuniversität leoben, otto glöckel-straße, 8700 leoben, austria ∗ corresponding author: eliza.truszkiewicz@pccl.at abstract. the investigated material laminate is intended as a substrate for small electronic components, electrodes and printed circuits, which are processed onto the laminate prior to thermoforming. the placement of the electronic components and the connecting circuits must be carefully designed to prevent damage during the thermoforming. the thermo-viscoelastic behavior of a polymer laminate film was characterized by mechanical measurements to obtain data for material modeling. the strain was measured using digital image correlation. the film is anisotropic and is able to deform to strains up to 60 %. keywords: thermoviscoelasticity, polycarbonate, polyethylene terephthalate, dynamic mechanical analysis, tensile test, stress relaxation. 1. introduction the material investigated in this study is laminate film composed of a blend of polycarbonate and acrylonitrile butadiene styrene (pc-abs) and polyethylene terephthalate (pet). the thermo-mechanical behavior of neat polycarbonate has been studied extensively, e.g. [1, 2]. they have unique combination of the high processability of abs and very good mechanical properties and heat resistance of pc. blends of pc and abs have also been the subjected of some scientific studies [3–5]. e.g. greco et al. [4] have shown that the impact strength can be improved by adding 20 % to 40 % abs and that tensile strength, young’s modulus and hardness decrease with increasing abs content. due the large variation in properties of different grades of a particular polymer or blend these data are not necessarily applicable for the present material. additionally, the pc-abs and pet films are stretched during manufacture, which introduces anisotropy in the material. the laminate is intended as a substrate for small electronic components, electrodes and printed circuits, which are processed onto the laminate prior to thermoforming. the placement of the electronic components and the connecting circuits must be carefully designed to prevent damage during the thermoforming. the polymer laminate may be stretched substantially, but for e.g. copper, the fracture strain is approx. 10 to 12 % and the ampacity may be compromised already at smaller deformation. the major task is to conduct a thermo-viscoelastic characterization of the laminate to obtain data for a material model that may be used in finite element simulations of the thermoforming process. the parameters will be determined by measurements like uniaxial and biaxial tension testing, dynamic mechanical analysis (dma) and thermomechanical analysis (tma). in this part of the work the first measurements were conducted using digital image correlation on a sample of the thermoformed laminate film (figure 1) to determine the strain field for subsequent uniaxial tensile and relaxation tests. dma was also conducted. 2. the thermoforming process the thermoforming process is a non-isothermal process of converting a sheet into a three-dimensional shape. in the first step the laminate film is heated to the forming temperature, which may be slightly above or much larger than the glass transition temperature depending on the forming pressure. in next step the film is formed using pressurized air or vacuum. the mold temperature is chosen below the glass transition temperature. due to the contact with the mold the film is cooled and the polymer chains are locked in their deformed state. when the desired shape is ready the film is removed from the mold. the thermo-viscoelastic behavior of the film is studied in a non-isothermal tensile test to imitate the heating, deforming and cooling steps of the thermoforming process. 3. experimental the thermo-viscoelastic behavior was characterized by dynamic mechanical analysis, monotonic tensile and relaxation tests for several temperatures and orientations. the dynamic mechanical analysis were carried out using a perkinelmer dma 8000. other mechanical tests were performed using a zwick/roell z250 testing machine equipped with a temperature 82 http://dx.doi.org/10.14311/app.2016.3.0082 http://ojs.cvut.cz/ojs/index.php/app vol. 3/2016 thermo-viscoelastic characterization of polymer laminate films chamber and using digital image correlation (dic; aramis, gom) for strain measurement [6]. a sample of the laminate film has been thermoformed using a generic mold. a millimeter grid has been printed on the film by the supplier. the millimeter grid may be used for strain measurement by dic, however it lacks a unique feature for the tracking of motion in subsequent images. a traceable pattern was therefore created by filling points on the grid using a permanent marker. the same pattern was applied on undeformed grid paper, which was used as a reference sample, and on deformed film to measure the strain field after the thermoforming (figure 1). figure 1. coated laminate film. figure 2. strain field on the thermoformed laminate film. figure 2 shows the major strain field as measured described above. there is almost no strain on the top, where the laminate film comes into contact with the mold first. the slopes lead to tensile strains of around 40 %. on the shoulders, additional shearing leads to major strains of up to 60 %. for the mechanical characterization, testing temperature varied between room temperature and 145°c. specimens were cut at angles of 0°, 45°and 90°and spray-coated with a pattern for dic. the monotonic tensile tests were performed with a nominal strain rate of 1/50 mm/s. the same rate was also used for the initial loading in the relaxation tests. in a non-isothermal tensile test, the film is heated to 145°c without applied stress. in the next stage the specimen is stretched to a nominal strain of 25 % and kept at this strain. the film is quickly cooled using cooling packs. the polymer chain mobility at the low temperature is strongly reduced and deformation of film is therfore locked. when the film is removed from the temperature chamber it retains this stretch. this part of the non-isothermal test is shown in figure 3. the whole process takes about 15 minutes after the removal of the stretched and cooled specimen from the testing machine for the temperature to stabilize at 145°c again. in last step the specimen is placed again in the temperature chamber, but only the upper specimen grip is closed and in this specimen position the continuation of dic strain measurement in again possible. the specimen is heated without external stress, which allows the viscoelastic part of the locked deformation to recover with time. 4. results figure 3 shows the time-dependent recovery from nonisothermal tensile test. the effect of recovery behavior of the two materials is curling up the laminate the pc layer of the film laminate recovers quickly, but it is constrainted because of the pet layer. the rolled-up shape does not permit a dic evaluation. when the laminate unfurls (the stresses have relaxed sufficiently) the strain measurement can configure 3. non-isothermal tensile test with recovery to mimic the thermoforming process. figure 4. dma results. 83 eliza truszkiewicz, daniel tscharnuter, gerald pilz acta polytechnica ctu proceedings figure 5. relation between logarithmic strain and cauchy stress – results for different orientation of specimen. figure 6. relation between the cauchy stress and time depends on temperature for 0°and 90°. tinue (figure 3, starting at stage 381). we observe that 80 % of the deformation is recovered in a short time by reheating the specimen. this test confirms the initial assumption that this thermoforming process creates predominantly viscoelastic deformation. the glass-transition temperature was determined from the dma measurements. the glass-transition of the laminate is defined by the glass-transition of the pc-abs blend and is 122°c (for separte pc amounts around 145°c and 105°c for abs [5]). a small peak at 80°c was observed (figure 4), which is characteristic for the pet film [7]. due to the high crystallinity of the pet film, the effect of this transition is small. the tensile tests showed the expected strong temperature dependence (figure 5), where the stress at a given strain decreases by a factor of 3 to 4 when raising the test temperature from 23°c to 145°c. the effect of orientations is pronounced for strains larger than 10 %. the ultimate stress at 145°c in the transverse direction is for example twice as high as in the extrusion direction, which indicates substantial stretching following the extrusion process. figure 6 shows the stress relaxation in 0°and 90°at approximately 40 % strain for temperatures of 80°c, 120°c and 145°c. for all temperatures, we observe a significant amount of stress relaxation in the first seconds after the initial loading. the relaxation rate then decreases substantially, but the viscoelastic effects are still not negligible after 1000 s of relaxation. 5. conclusions a polymer laminate film was characterized to provide data for thermo-viscoelastic material modeling. the measurements covered temperatures from room temperature to 145°c, which is approximately 20°c above the glass-transition temperature of the laminate. significant anisotropy of the laminate was observed. additional stress relaxation tests were conducted to measure the viscoelastic response of the material. acknowledgements the financial support by the austrian research promotion agency (ffg project no. 843685) is gratefully acknowledged. 84 vol. 3/2016 thermo-viscoelastic characterization of polymer laminate films references [1] s. sarva, m. boyce. mechanics of polycarbonate during high-rate tension. journal of mechanics and structures, mathematical sciences publisher 2(10):1853–1880, 2007. [2] h. azman, w. yeanjwu. mechanical properties of high impact abs/pc blends effect of blend ratio. 2005. [3] r. krache, i. debbah. some mechanical and thermal properties of pc/abs blends. materials sciences and applications 2(5):404–410, 2011. [4] r. greco, m. astarita, l. dong, a.sorrentino. polycarbonate/abs blends: processability, thermal properties and mechanical and impact behavior. polymer engineering and science 27(9):632–639, 1987. [5] w. chiang, d. hwung. properties of polycarbonate/acrylonitrile-butadiene-styrene blends. advances in polymer technology 13(4):259–274, 1994. [6] m. jerabek, z. major, r. lang. strain determination of polymeric materials using digital image correlation. polymer testing 29(3):407–416, 2010. [7] b. demırel, a. yaraş, h. elçıçek. crystallization behavior of pet materials. baü fen bil enst dergisi cilt 13(1):26–35, 2011. 85 acta polytechnica ctu proceedings 3:82–85, 2016 1 introduction 2 the thermoforming process 3 experimental 4 results 5 conclusions acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2016.5.0001 acta polytechnica ctu proceedings 5:1–3, 2016 © czech technical university in prague, 2016 available online at http://ojs.cvut.cz/ojs/index.php/app problems of en 50 128:2011 railway standard tomáš brandejský czech technical university in prague, department of applied informatics in transportation, faculty of transportation, praha, czech republic correspondence: brandejsky@fd.cvut.cz abstract. the second version of railway standard en50128:2011 published by cenelec is used for five years and thus the cenelec reasons about preparation of new version of this standard are coming, because the cenelec defined 10-year cycle of its standards innovation. the paper discusses some observations of problems of this standard application from the position of safety assessor which gives us the contact with developers applying this standard into the real railway products. keywords: railway software applications, assessment, safety, reliability, accessibility, maintainability, development. 1. introduction since the origin, the group of cenelec’s railway stands [1] [2] [3] was declared as standard of all safety related railway applications including not only interlocking systems, but rolling stock and related communication too. the standard en50126 defines its applicability to "the specification and demonstration of rams for all railway applications and at all levels of such an application, as appropriate, from complete railway systems to major systems and to individual and combined subsystems and components within these major systems, including those containing software". the standard en50128 defines its application domain as: "this european standard specifies procedures and technical requirements for the development of programmable electronic systems for use in railway control and protection applications. it is aimed at use in any area where there are safety implications. these may range from the very critical, such as safety signalling to the non-critical, such as management information systems. these systems may be implemented using dedicated microprocessors, programmable logic controllers, multiprocessor distributed systems, larger scale central processor systems or other architectures". and the standard en50129 defines its scope as electronic systems related to safety in application to railway interlocking systems. it means that at least standards en50126 and en50128 must be reasoned in any railway system related to safety. on the beginning of application of these standards safety assessors required application of these standards especially in the domain of interlocking systems and the rest was frequently omitted. after the year 2011, when the second generation of these standards has come into the operation the situation changed and the conformity with these standards is required also by assessors of rolling stock and other railway systems. it brings some problems described below due to different requirements on system functions in situation, when the system malfunction is detected. the main aim of the paper is the discussion of problems in nowadays application of the standard en50128 and offering of ways how to improve it. 2. problems of en50128 application the cenelec’s standard en 50128 "railway applications – communications, signalling and processing systems – software for railway control and protection systems" was defined as part of three standards group [1] [2] [3] applying standard iec 61508 "functional safety of electrical/electronic/programmable electronic safety-related systems" into the specific area of railway systems. the main difference is in different safety assessment scheme, where not only developer, validator and assessor are reasoned but also railway safety authority as next independent body. in the next chapter, particular problems will be discussed: (1.) the last version of the standard is less transparent than original one. the change of the standard structure was asked by cenelec due to unification of all cenelec standard structure. whilst the original standard was divided into 17 chapters and 2 annexes, the 2011 version is organized into 9 chapters and 4 annexes (2 are new). this change causes not only that there are no separate chapters e.g. for software validation and assessment, but software assurance chapter describing testing, verification, validation and assessment is before chapter 7 describing development process! furthermore, the chapter 7 is named "generic software development" now not looking that the term "generic software" in computer science doscon mean "common sense software", as this term is used in the standard, but "a class of software that can be used for a number of different purposes without requiring modification", as it is defined e.g. in [4] [5]. what is worse, the 1 http://dx.doi.org/10.14311/app.2016.5.0001 http://ojs.cvut.cz/ojs/index.php/app tomáš brandejský acta polytechnica ctu proceedings chapter 3.14 of 50128:2011 standard defines generic software meaning some way. thus the topic of the chapter 7 does not give any sense. similarly in the chapter v 3.1.4 of original en50128 introduces the term "component" which is defined extremely vague (and in the en50128:2011 even it is not defined), but especially in the area on object oriented programming the meaning of this term if defined precisely and totally different. because the standard en50128 is used especially by programmers, such drifting of frequent term sense might cause many misunderstandings. because en50128:2011 allows to understand term component also as object, module or even function, the meaning of e.g. chapter 7.5 might be understand totally different by particular people in particular situations. (2.) there is not explicitly declared applicability of the standard to railway vehicles and especially differences in requirements to behaviours of vehicle control systems in the case of malfunction, which is opposite to required behaviours of interlocking control systems in the analogical situation. typically, in the case of control system error vehicle must keep moving but interlocking system can signalize stop to all tracks and perform e.g. restart of the control system. (3.) there is not solved defence against targeted attack (terrorism). defence of mission critical systems against targeted attacks e.g. requires ability of the system to isolate attacked parts and thus to reconfigure system. it is a big problem, because dynamic reconfiguration is listed in table a.3 as technique 14 and it is not recommended non respecting fact that this technique is frequently adopted e.g. in military, cosmic or avionics systems. it is not possible to constraint this problem only to subjects of safe communication and physical barriers. on the other hand, standards of the information security like iso 27001 are rather oriented to managing sensitive company information than to control systems located in open space and communicating across it. (4.) the standard does not solve design of programmable hw, where on interface of sw/hw occur many specific problems which solution is not defined in the actual standard. many of these problems were discussed in [6], like specific problems of design, timing and application structure but there also another problems like problem of dedication of safety related functions to development tools which is related to problems discusses herein in chapter (6.) and which is not solved. design tools transforming e.g. vhdl language into configuration bit stream of the chip does not work deterministically and this fact is hard to accept in safe system design. (5.) the standard en50128 contains extensive appendix b "key software roles and responsibilities" on ten pages and small appendix c "documents control summary", which only recapitulate textual part figure 1. the example of lifecycle hierarchy of complex system. of the standard and do not bring any additional information. it is good to delete such chapters because they only decrease clarity of the standard. (6.) chapter 8 "development of application data or algorithms: systems configured by application data or algorithms" should explain better requirements of systems with different level of configurability. it is hard to compare simple system which is configured by one dimensional table of parameters and system where safety-related functions are described in configuration language, which is interpreted in real time and where part on safety checks is dedicated to development tools. the figure 1 draws possible configuration of different life-cycles cooperating on development of final (sometimes also generic) application based on configurable generic kernel where e.g. life cycle of generic system determines requirement on lc of final application and final application development tools life cycle. (7.) in comparison to standard iec 61508-3 this railway standard does not contain equivalent of iec 61508-3 appendix c explaining which sw property is achieved by which technique. such explanation is useful both for designers and assessors because it simplifies, objectivises and formalizes selection of design techniques in atypical, non-standard situations where groups of design techniques predefined in the table a.3 are inapplicable. typical example of this situation is in this moment application of programmable hw, which is not solved in the standards and which requires a little bit different approach than standard hw and sw. (8.) there is not solved problem of design of systems with safety integrity level greater than 4. growing complexity of railway systems, especially of such systems as ertms, tends to requirement of such sil. it is true, that many calls for higher sils are given by misunderstandings of sil meaning. sil is related to tolerable hazard rate, as it is introduced in the standard en 50129, table a.1. tolerable hazard rate is related to one hour and one function, but many designers of railway systems missrelates it to number of invocations of the function and means 2 vol. 5/2016 problems of en 50 128:2011 railway standard that systems working on higher operation frequency must have higher sil. it is mistake. (9.) there is not well defined that sils are determined to top-level functions, not to whole system and not to any sub-functions. the standard en50129 uses formulation single function without precision of the complexity of the function. many top-level functions are performed by groups of lower level functions. this lack of precise definition tends to many useless discussions between manufactures and assessors. (10.) the standard defines five different sil levels but de facto it specifies only three different cases: sil0, sil1 and 2; and sil 3 and 4. in addition, requirements to sil 1 and 2 systems are inappropriate strict as well as there are requirements to documentation complexity of sil0 systems, especially in comparison to cots systems of the equal sil level. but in fact it would be just opposite – e.g. it is need to do precise testing developed by other company, which design process is not well described, where the documentation is missing, than in the case of well documented one. 3. discussion ten year cycle of cenelec standard upgrade is difficult especially to rolling stock manufacturers. locomotives, carriages and other vehicles have extremely long life-cycle taking any tens of years. there is long design phase including real research, own assessment phase sometimes takes about ten years and also operation is reasoned in horizon of thirty years or more. thus the idea that between start of design and start of operation the required standard will be once or even two times changes is not acceptable. because cenelec does not accept this fact, rolling stock making companies provide "diverse activity" in related cenelec work-groups and prevents significant changes of these standards. on the opposite side, above mentioned problems does not represent significant changes of the standards and their acceptance in the new generation of them might increase their applicability. 4. conclusions the above presented paper summarises problems on the today railway standard en50128:2011, which were observed in certification body of czech technical university in prague – cov fd čvut v praze during certifications of many railway systems serving especially (but not only) in czech republic. references [1] en50126 railway applications – the specification and demonstration of reliability, availability, maintainability and safety (rams). [2] en50128 railway applications – communication, signalling and processing systems – software for railway control and protection systems. [3] en50129 railway applications – communication, signalling and processing systems – safety-related electronic systems for signalling. [4] http://www.teach-ict.com/glossary/g/ genericsoftware.htm. [5] http://www.computingstudents.com/dictionary/ ?word=generic%20software. [6] t. musil. návrh metodiky pro vývoj a verifikaci bezpečných algoritmøu implementovaných v dynamicky rekonfigurovatelných fpga, phd. thesis, 2015. 3 http://www.teach-ict.com/glossary/g/genericsoftware.htm http://www.teach-ict.com/glossary/g/genericsoftware.htm http://www.computingstudents.com/dictionary/?word=generic%20software http://www.computingstudents.com/dictionary/?word=generic%20software acta polytechnica ctu proceedings 5:1–3, 2016 1 introduction 2 problems of en50128 application 3 discussion 4 conclusions references 165 acta polytechnica ctu proceedings 2(1): 165–169, 2015 165 doi: 10.14311/app.2015.02.0165 superoutburst of a new sub-period-minimum dwarf nova css130418 in hercules d. chochol1, s. shugarov1,2, n. katysheva2, i. volkov2, a. zharova2, e. pavlenko3, n. pit3, r. zvagelsky3, o. antoniuk3, k. antoniuk3, a. baklanov3, n. borisov4, m. gabdeev4 1astronomical institute of the slovak academy of sciences, 059 60 tatranská lomnica, slovakia 2sternberg astronomical institute of the moscow state university, russia 3crimean astrophysical observatory, nauchny, ukraine 4special astrophysical observatory of the russian academy of sciences, nizhnij arkhyz corresponding author: chochol@ta3.sk abstract multicolour photometry of a new dwarf nova css130418 in hercules, which underwent superoutburst on april 18, 2013, allow to classified it as a wz sge-type dwarf nova. the phase light curves for different stages of superoutburst are presented. the early superhumps were used to determine the orbital period porb = 64.84(1) minutes, which is shorter than the period minimum ∼ 78 minutes for normal hydrogen-rich cataclysmic variables. we found the mean period of ordinary superhumps psh = 65.559(1) minutes. the quiescent spectrum is rich in helium, showing double peaked emission lines of h i and he i from accretion disk, so the dwarf nova is in a late stage of stellar evolution. keywords: cataclysmic variables dwarf novae su uma-type wz sge-type photometry spectroscopy individual: css 130418. 1 introduction cataclysmic variables (cvs) are close binary systems with the white dwarf (wd) accreting matter from a companion which fills its roche lobe. due to non-zero momentum the matter creates accretion disk around the primary component. su uma-type dwarf novae (dne) are subclass of cvs which exhibits outbursts and less frequent brighter superoutbursts. the superhumps are periodic tooth shaped light variations observed during superoutbursts. superhumps periods are a few percent longer than the orbital period. their amplitudes at superoutburst maximum are 0.2−0.7 mag and decrease afterwards. according to the model proposed by whitehurst (1988) and hirose & osaki (1990), superhumps are caused by tidal 3:1 resonance in the outer parts of accretion disk, leading to the formation of an eccentric outer ring undergoing apsidal motion. smak (2009) proposed alternative interpretation of superhumps as being due to modulated mass transfer rate resulting in periodically enhanced dissipation of the kinetic energy of the stream. the wz sge-type objects are the most extreme subgroup of su uma-type dne with the typical orbital periods 80−90 minutes that exhibits only superoutbursts with amplitude 6−8 mag and a long (several years) recurrence time. one of the most remarkable signatures of wz sge-type objects is a presence of early superhumps, observed in the first days of superoutburst. early superhumps have a period very close to a binary period and commonly show a double humped profile. osaki & meyer (2002) suggested, that this phenomenon is manifestation of the tidal 2:1 resonance in accretion disks of binary systems with extremely low mass ratio. resonance forms a two-armed spiral pattern of tidal dissipation, which leads to a double-peaked feature in the orbital light curve. very few cvs have orbital periods below the so called period minimum ∼78 minutes. 2 css130418 2.1 discovery on 2013 april 18.47, the catalina real-time transient survey (crts) detected an outburst of css130418: 174033.5+414756 in hercules (drake et al., 2009). the brightness of the object at the time of discovery was 14.04 mag. according to crts circular, the object had an 7.5 mag outburst in 2007, that lasted two month. prieto et al. (2013) discovered the transient independently with the all sky automated survey for supernovae (asas-sn) on 2013, april 19.53, when it 165 http://dx.doi.org/10.14311/app.2015.02.0165 d. chochol et al. brightness was v = 12.7 mag. they found a precursor with b = 19.2 and r = 18.2 mag in usno-a2 catalog and previous outburst in galex uv source in the galex dr5 catalog with fuv = 13.62 and nuv = 14.17 mag on 2007 may 27. an optical spectrum of the transient, taken by the osmos imagine spectrograph of the mdm 2.4m telescope on 2013, april 21.41, shows a blue continuum (black-body teff = 16 000 k) with balmer and he i lines in absorption and weak he ii in emission. 2.2 light curve of the superoutburst 56410 56420 56430 56440 56450 19 18 17 16 15 14 13 our data dubovsky aavso v jd 2400000+ figure 1: v light curve of superoutburst. 56410 56420 56430 56440 56450 18 16 14 0,2 0,0 -0,2 -0,4 -0,6 v u-b b-v v-r r-i v jd 2400000+ c.i. figure 2: mean v light curve and colour indices curves of superoutburst. our ubv (ri)c ccd observations were carried out with the 0.6m telescope at the stará lesná observatory, 0.4, 1m and 1.25m telescopes of the crimean observatory and 0.6m telescope at the crimean laboratory of the sternberg astronomical institute. we added the ccd observations of p. dubovský taken by 1m telescope at kolonica saddle (slovakia) available at http : //var2.astro.cz/slovakia and aavso v data. the superoutburst v light curve is presented in fig. 1. -0,2 0,0 0,2 0,4 0,6 0,8 0,6 0,4 0,2 0,0 -0,2 -0,4 -0,6 -0,8 u-b b-v f0 g0 b3 a0 12000 10000 8000 6000 5000 2 5 10-15 20 25 k0 figure 3: two colour diagram. the average night magnitudes of v observations from fig. 1 and our colour indices data are shown in fig. 2. the evolution of the nova in a two colour (u−b,b− v ) diagram is displayed in fig. 3. the main and black body sequences are plotted with solid and dashed lines, respectively. the positions of field stars are marked by stars. filled circles correspond to the positions of the dwarf nova at the days after maximum, which occurred at jd=2456403. 2.3 early superhumps early superhumps are periodic modulations of the brightness of wz sge-type dwarf novae seen during superoutburst with a period identical to the orbital period 0,0 0,2 0,4 0,6 0,8 1,0 1,2 1,4 0,06 0,04 0,02 0,00 -0,02 -0,04 phase jd 2456403-406m figure 4: phase light curve of early superhumps. (ishioka et al., 2002). therefore, they are known as orbital humps. after the declining trend removal from v and b light curves of all available data, taken in the first 4 days after maximum, we found the ephemeris 166 superoutburst of a new sub-period-minimum dwarf nova css130418 in hercules for brightness maxima of early superhumps by fourier period analysis as follows: jdmax = 2456402.076 + 0.045026 × e. (1) the phase light curve of early superhumps including the mean light curve is shown in fig. 4. 2.4 ordinary superhumps ordinary superhumps are periodic modulation seen during superoutburst after disappearance of early superhumps. our v observations of superhumps of the object, obtained during the night run 15 days after maximum (am), are presented in fig. 5. 56417,35 56417,40 56417,45 56417,50 56417,55 14,24 14,20 14,16 14,12 v jd 2400000+ figure 5: v light curve of superhumps. after the declining trend removal from v and b light curves of all available data taken on days 5-22 am we found the mean ephemeris for brightness maxima of ordinary superhumps by fourier period analysis as follows: jdmax = 2456402.092 + 0.0455269 × e. (2) the evolution of ordinary superhumps during the superoutburst are presented in figs. 6-10 in different days am. the ephemeris (2) was used for all data in phase light curves. 0,0 0,2 0,4 0,6 0,8 1,0 1,2 1,4 0,06 0,04 0,02 0,00 -0,02 -0,04 phase jd 2456407m figure 6: ordinary superhumps on day 5 am. 0,0 0,2 0,4 0,6 0,8 1,0 1,2 1,4 0,10 0,05 0,00 -0,05 -0,10 phase jd 2456409-410m figure 7: ordinary superhumps on days 7-8 am. 0,0 0,2 0,4 0,6 0,8 1,0 1,2 1,4 0,05 0,00 -0,05 -0,10 phase jd 2456415-416m figure 8: ordinary superhumps on days 13-14 am. 0,0 0,2 0,4 0,6 0,8 1,0 1,2 1,4 0,06 0,04 0,02 0,00 -0,02 -0,04 -0,06 -0,08 phase jd 2456419-420 m figure 9: ordinary superhumps on days 17-18 am. 0,0 0,2 0,4 0,6 0,8 1,0 1,2 1,4 0,08 0,06 0,04 0,02 0,00 -0,02 -0,04 -0,06 -0,08 phase jd 2456423-424 m figure 10: ordinary superhumps on days 21-22 am. 167 d. chochol et al. 3 our spectroscopy our spectra were taken with the scorpio spectrograph of the 6m telescope in sao during the superoutburst and quiescence of the dn in the range 40505100å. they are presented in fig. 11. the spectrum during superoutburst shows h i and he i lines in absorption and he ii in emission. quiescent spectrum is rich in helium, showing double peaked emission lines of h i and he i from accretion disk. the spectrum is similar to the spectrum of the helium-rich cataclysmic variable sbss 1108+574, which has the orbital period 55.3 minutes (carter et al. 2013). figure 11: spectrum of the object in outburst (may 4/5, 2013) and quiescence (august 2/3, 2013). 4 discussion and conclusions we found a new sub-period-minimum cv with the orbital period porb = 64.84(1) minutes and classified it as a wz sge-type dwarf nova. period of ordinary superhumps p = 65.559(1) minutes allowed to determine the fractional superhumps excess ε = [psh/porb 1] = 0.0111(4) and also the mass ratio q = 0.0565(20), applying the relation of patterson et al. (2005): ε = 0.18q + 0.29q2. presented object css130418 enlarged the group of dne under the period minimum and can help to understand its evolutionary status. hydrogen lines in the spectrum exclude the possibility that the object belongs to am cvn stars. the existence of early and ordinary superhumps shows that css130418 is evolved wz sge-type dn. the most probable scenario of evolution is that the mass transfer started near the end of a main-sequence lifetime of the donor star. after the donor transferred its hydrogen envelope, the mass transfer continue from hydrogen deficient interior of the star. the presence of strong he i lines in the spectrum of the object supports this view. acknowledgement we acknowledge with thanks the variable star observations from the aavso international database contributed by observers worldwide and used in this research. this study was supported by the vega grants 2/0094/11, 2/0002/13 and rfbr research projects no. 11-02-01213a, 14-02-00825 and nsh 1675.2014.2. references [1] carter, p.j., steeghs, d., de miguel e., goff, w., et al.: 2013, mnras 431, 372. doi:10.1093/mnras/stt169 [2] drake, a.j., djorgovski, s.g.,mahabal, a., beshore, e., et al.: 2009, apj 696, 870. doi:10.1088/0004-637x/696/1/870 [3] hirose, m., osaki, y. : 1990, publ. astron. soc. japan 42, 135. [4] ishioka, r., uemura, m., matsumoto, k., ohashi, h., et al.: 2002, a&a 381, l41. [5] osaki, y., meyer, f. : 2002, a&a 383, 574. [6] patterson, j., kemp, j., harvey, d.a., fried, r.e., et al.: 2005, pasp 117, 1204. doi:10.1086/447771 [7] prieto, j.l., hainline, k., hickox, r., goulding, a., et al. : 2013, atel 4999. [8] smak, j. : 2009, aca 59, 121. [9] whitehurst, r. : 1988, mnras 232, 35. doi:10.1093/mnras/232.1.35 discussion christian knigge: i think there are 3 possible interpretation for your system: 1) it could move started contact, when the donor was already nuclear evolved (like ei psc). 2) it could be a low metallicity halo object (like j1507 = patterson/uthas). 3) it could be a system in which the secondary was already a substellar object before contact (as had been suggested for j1507 by littlefair et al. (2007). dmitry kononov: how deeply evolved cvs can go beyond the period minimum? drahomír chochol: the shortest orbital period from already known cvs, excluding am cvn stars 168 http://dx.doi.org/10.1093/mnras/stt169 http://dx.doi.org/10.1088/0004-637x/696/1/870 http://dx.doi.org/10.1086/447771 http://dx.doi.org/10.1093/mnras/232.1.35 superoutburst of a new sub-period-minimum dwarf nova css130418 in hercules (orbital periods 5 60 min), has a dwarf nova css102808: 52.1 minutes (woudt et al., 2012). vojtech šimon: a group of cvs with unexpected orbital periods of ≈60 min emerging from the observations. what about the mutual similarities of the spectroscopic properties of these systems? do they show similarities for example in the hydrogen/helium ratio? drahomír chochol: the heiλ6678/hα ratios of css 100603 (p = 65.23 min) and sbss 1108+574 (p = 55.3 min) are 1.49 and 0.81, respectively (breedt et al. 2012; carter et al. 2013), so they are helium rich systems. the ratios in ei psc and v485 cen are higher than those typically seen in su uma systems. spectrum of css130418 is similar to the spectra of previous objects. on the other hand in j1507 heiλ6678/hα ratio is only 0.07. paula szkody: what criteria are used for calling it he-rich? many other cvs show he i strong. unusual short period systems have evolved donors. drahomír chochol: we just simply compare our spectrum with the spectrum of helium rich cv sbss 1108+574 (carter et al. 2013). 169 introduction css130418 discovery light curve of the superoutburst early superhumps ordinary superhumps our spectroscopy discussion and conclusions 139 acta polytechnica ctu proceedings 2(1): 139–142, 2015 139 doi: 10.14311/app.2015.02.0139 high-speed photo-polarimetry of magnetic cataclysmic variables s. b. potter1 1south african astronomical observatory, po box 9, observatory 7935, cape town, south africa corresponding author: sbp@saao.ac.za abstract i review recent highlights of the saao high-speed photo-polarimeter (hippo) on the study of magnetic cataclysmic variables. its high-speed capabilities are demonstrated with example observations made of the intermediate polar ny lup and the polar igrj14536-5522. keywords: cataclysmic variables polars intermediate polars optical polarimetry photometry x-rays individual: ny lup, igrj14536-5522. 1 introduction saaos hippo was designed and built in order to replace its highly successful but aging single channel equivalent, namely the uct (university of cape town) photo-polarimeter (cropper 1985). its purpose is to obtain simultaneous all-stokes parameters, of unresolved astronomical sources. in addition, it is capable of high speed, simultaneous 2 filtered, photo-polarimetry in order to permit investigations of rapidly varying polarized astronomical sources. of particular interest are magnetic cataclysmic variables (mcvs). in the following two sections we demonstrate its capabilities. 2 photo-polarimetry the intermediate polar ny lup figure 1: i-band photo-polarimetric observations of the intermediate polar ny lup. bottom and top panels are the phase-spin-folded photometry and circular polarization respectively. left and right panels were made with the 1.9m of the saao and the vlt respectively. figure reproduced from potter et al. (2012). 139 http://dx.doi.org/10.14311/app.2015.02.0139 s. b. potter fig. 1 shows two sets of i-band observations of the intermediate polar ny lup, phase-folded on its white dwarf spin period of 693s. the data in the left hand panels were made with the 1.9m telescope of the saao (taken from potter et al 2012) and the right hand panels were made with the 27 times larger vlt of the eso (taken from katajainen et al 2010). the saao polarimeter (hippo; potter et al 2010) is equiped with photo-multiplier tubes whereas the vlt polarimeter used a superior, 4 times more quantum efficient, ccd camera. both data sets show a spin modulation in the photometry and agree on the average value of circular polarization of ∼ -1.5 percent. however, contrary to expectations, the hippo observations are by far the superior dataset for the following reasons: the vlt photometry appears to have a residual repeating pattern of about 8 cycles per spin period, probably arising as a result of the different waveplate position angles. the hippo circular polarization shows a clear spin modulation whereas the vlt polarimetry displays a more random scatter about the mean. in addition the hippo also obtained simulatenously b-band observations. these observations (not shown) clearly show the presence of a circularly polarised spin modulation. the vlt b-band were not only not simultaneous, but also did not detect a spin modulation. furthermore the hippo simultaneously measured linear polarization (not shown) albiet a non-detection. the superior performance of the hippo is because it is optimised to measure polarization variability. specifically the exposure readout times of the photomultiplier tubes are very fast (sub-second) thus enabling polarization measurements to be made on a timescale much shorter than the intrinsic variability of the polarization. the ccd readout times on the vlt were too slow leading to the smearing of the polarimetric variability and systematic residuals in the photometry. 3 photo-polarimetry the polar igrj14536-5522 igrj14536-5522 (=swift j453.4-5524) was discovered as a hard x-ray source by integral (kuiper, keek, hermsen, jonker & steeghs 2006) and by swift/bat (mukai et al. 2006). a pointed swift/xrt observation led to the identification with a rosat all-sky survey (rass) source 1 rxs j145341.1-552146, and hence to its optical identification (masetti et al. 2006). based on these observations and the presence of short period optical and x-ray periodic modulations, revnivtsev et al. (2008) classified it as an ip. followup photo-polarimetric observations with the hippo (potter et al. 2010) however clearly shows orbitaly modulated photometry and circularly polarization from ∼ 0 to ∼ 18 per cent, unambiguously identifying igrj14536-5522 as a polar (fig.2, left and right upper panels respectively: from potter et al. 2010). in addition to the orbital modulation, a close inspection of the photometry and polarimetry reveals short period modulations throughout the orbit which, in addition to being a hard integral source, contributed to its initial mis-identification as an ip. the short period modulations are mostly consistent with being due to noise or flickering. however, detailed fourier analysis of all of the data reveals that some of the data sets show significant singularly persistent peaks that are consistent with qpos. the results are shown as a trailed amplitude spectrum in the second row of fig.2. between phases 0.2-1.0 the amplitude spectra do not show any significant peaks which indicates that the variations are mostly flickering or noise. however, there is a significant dominating signal centered on 0.0032(1) hz (5.2 minutes, indicated by the dashed line) between phases 1.0 and 1.3 in both the photometry and the polarimetry. this is clear evidence of a qpo. in the third row of fig. 2 we show the normalised photometry and polarimetry during the phase range that is dominated by the qpo. over plotted are the least squares fit of the qpo frequencies. as one can see, the qpo is very well described by the single dominant frequency as found in the trailed amplitude spectra. the plots in the bottom row of fig. 2 show the corresponding amplitude spectra for the qpo dominated phase range. our polarimetric results unambigously demonstrate that the qpo emissions (photometric and polarimetric) originate from the accretion shock. ultimately, these are caused by variations in the accretion flow. the most natural place to modulate the accretion flow would be at the threading region, perhaps caused by instabilities as a result of the interaction of the accretion flow with the magnetic field. we are now using mhd modelling to investigate this possibility. 140 high-speed photo-polarimetry of magnetic cataclysmic variables figure 2: left plots, a-d: the photometry, the corresponding trailed amplitude spectra, the normalised photometry for the phase range 1.0-1.3 and its corresponding amplitude spectra respectively. the solid curve is the least squares fit using the frequency derived from the trailed spectra (dashed line). right plots: as in the left plots but for the circular polarization. figure reproduced from potter et al. (2010). 4 conclusions we have shown that the saao high-speed photopolarimeter (hippo) is capable of high-speed, multifiltered, simultaneous all-stokes observations. it is therefore ideal for investigating rapidly varying astronomical sources such as magnetic cataclysmic variables. in particular white dwarf spin modulations and quasi-periodic-oscillations (qpos). acknowledgement i thank the organsers for giving me the opportunity to present some of my recent work and for a great conference. references [1] cropper, m. s.: 1985, mnras 212, 709. doi:10.1093/mnras/212.3.709 [2] kuiper, l., et al: 2006, atel 684. [3] katajainen, s., et al.: 2010, apj 724, 165. doi:10.1088/0004-637x/724/1/165 [4] masetti, n. et al: 2006, atel 783. [5] mukai, k. et al: 2006, atel 686. [6] potter, s.b., et al: 2010, mnras 402, 1161. doi:10.1111/j.1365-2966.2009.15944.x [7] potter, s.b., et al: 2012, mnras 420, 2596. doi:10.1111/j.1365-2966.2011.20232.x [8] revnivtsev, m., et al: 2008, a&a 489, 1121. discussion christian knigge: do you have any speculative ideas already for the origin of the ∼ 6 minute qpos? 141 http://dx.doi.org/10.1093/mnras/212.3.709 http://dx.doi.org/10.1088/0004-637x/724/1/165 http://dx.doi.org/10.1111/j.1365-2966.2009.15944.x http://dx.doi.org/10.1111/j.1365-2966.2011.20232.x s. b. potter stephen potter: since the qpo is seen in polarized light then it must be associated with the accretion shock region. so best guess is that the qpo may represent an instability in the threading region causing quasi-modulated accretion onto the white dwarf. pieter meintjes: in one of your slides you showed distinct qpo features with periods of ∼ few seconds. any idea what may have caused that and is that associated with high or low states in accretion? in other words, are they sometimes more prominent than other times (depending on accretion) and is the polarization level also fluctuating depending on accretion? stephen potter: photometric qpos of the order of a few seconds are thought to arise from oscillations in the shock itself. there is not yet a sufficient amount of observational data to see if there is any correlation with e.g. accretion rate and/or magnetic field strengths etc. we detect polarized qpos at ∼ minute timescales but not ∼ second timescales. 142 introduction photo-polarimetry the intermediate polar ny lup photo-polarimetry the polar igrj14536-5522 conclusions 183 acta polytechnica ctu proceedings 2(1): 183–187, 2015 183 doi: 10.14311/app.2015.02.0183 close-in substellar companions and the formation of sdb-type close binary stars l. y. zhu1,2,3, s. b. qian1,2,3, e.-g. zhao1,2, e. fernández lajús4,5, z.-t. han1,2,3 1yunnan observatories, chinese academy of sciences, p.o. box 110, 650011 kunming, p.r. china 2key laboratory for the structure and evolution of celestial bodies, chinese academy of sciences, p.o. box 110, 650011 kunming, p. r. china. 3physical sciences department, graduate university of the chinese academy of sciences, yuquan road 19, sijingshang block, 100049 beijing, china. 4facultad de ciencias astronómicasy geof́ısicas, universidad nacional de la plata, 1900 la plata, buenos aires, argentina. 5instituto de astrofisica de la plata (cct la plata conicet/unlp), argentina. corresponding author: zhuly@ynao.ac.cn abstract the sdb-type close binaries are believed to have experienced a common-envelope phase and may evolve into cataclysmic binaries (cvs). about 10 % of all known sdb binaries are eclipsing binaries consisting of very hot subdwarf primaries and low-mass companions with short orbital periods. the eclipse profiles of these systems are very narrow and deep, which benefits the determination of high precise eclipsing times and makes the detection of small and close-in tertiary bodies possible. since 2006 we have monitored some sdb-type eclipsing binaries to search for the close-in substellar companions by analyzing the light travel time effect. here some progresses of the program are reviewed and the formation of sdb-type binary is discussed. keywords: hot subdwarf binary photometry individual: ny vir, hs 0705+6700, nsvs 07826147. 1 introduction hot subdwarf stars (sdb) lie in the extreme horizontal branch (ehb). they are burning helium in their cores and have very thin hydrogen envelopes. after leaving the main sequence, the progenitors of sdbs would evolve to red giant, ignite helium and settle down on the ehb (heber 2009). they have high temperatures and gravities. to explain these characters, their progenitors must have experienced enhanced mass loss at the tip of red giant branch (castellani and castellani, 1993; d’cruz et al., 1996). however, what causes this extreme mass loss remains an open question. many authors suggested that the mass loss is triggered by the presence of a companion. in 2007, silvotti et al., found a giant planet orbiting around a sdb star, v391 pegasi, at a distance of 1.7 au. after that, an even more extreme case was published by charpinet et al., (2011). they reported the presence of two nearly earth-sized bodies orbiting the sdb star kic 05807616 at distances of 0.0060 au and 0.0076 au, with orbital periods of 5.7625 and 8.2293 hours, respectively. these detections support the way to form single sdb stars that these bodies were originally giant planets immersed in the red-giant envelope and massive enough to survive engulfment and triggered the enhanced mass loss necessary for the formation of an sdb star. so the close-in substellar companions paly an important role to form single sdb stars. actually, about half (or may be more) of the sdbs reside in close binaries (morales-rueda et al., 2006). are there any substellar objects around sdb close binaries just like the substellars founded around the single sdb stars? recently, some circumbinary substellars orbiting around sdb eclipsing binaries were indeed detected, such as hw vir (lee et al., 2009), hs 0705+6700 (qian et al, 2009), ny vir (qian et al., 2012) and sdssj0820+0008 (geier et al, 2012). the sdb close binaries are believed to be formed from binary systems through common envelope(ce) ejection (han et al., 2002, 2003). because the separation of the two components in sdb close binaries is much less than the size of the subdwarf progenitor in its red-giant phase, these systems must have experienced a ce phase. after spiral in and envelope ejection, the present binary system consisting of a sdb star and a low-mass companion star was formed with short periods. the presence of substellar tertiary, especially the close-in objects, orbiting sdb close binaries provides some clues to the inter183 http://dx.doi.org/10.14311/app.2015.02.0183 l. y. zhu et al. active effect between them, and may give evidences of the formation of the planets and the sdb close binaries. 2 targets and method for searching for close-in substellar companions of the sdb close binaries, we chose eclipsing sdb close binaries (i.e. hw vir binaries) as our targets to monitor because their eclipsing profiles provide us with very useful information about their orbital period variations, which may be caused by the presence of third bodies. hw vir binaries have been known for more than four decades. they are detached eclipsing binaries composed of a sdb star and a low-mass companion star. till now, not more than fifteen of such objects were discovered. their well-detached configuration indicates that their eclipses might not be influenced by the accretion-driven radiations from other parts (e.g., the accretion discs and hot spots etc.) observed in cvs. and the compact structures of both components make the eclipses in the light curves very narrow and deep. these characteristics favour a highly precise determination of their times of light minimum, generally with errors less than 0.0001days. thus very small amplitude orbital period variations due to the substellar tertiaries can be detected by analyzing the observed-calculated (o-c) diagrams. therefore they are the most hopeful targets for detecting substellar objects around them with light time effect. using 2.4-m, 1-m and 60-cm telescopes in yunnan observatories, 2.16-m and 85-cm telescopes in xinglong station of national observatories and 2.15-m jorge sahade telescope in argentina, we began to monitor hw vir binaries since 2006. our targets are listed in table 1. table 1: summary of our targets name coordinate period mag. (j2000) h (mag) hw vir 12 44 20.24 -08 40 16.8 2.801 v=11.2 ny vir 13 38 48.14 -02 01 49.2 2.426 v=14.2 hs 0705+6700 07 10 42.09 +66 55 44.0 2.296 b=14.1 hs 2231+2441 22 34 20.89 +24 57 00.4 2.654 v=14.1 nsvs 14256825 20 20 00.51 +04 37 55.6 2.649 v=14.3 bulsc16-335 14 45 20.21 -06 44 03.2 3.001 v=16.3 nsvs 07826147 15 33 49.40 +37 59 28.6 3.883 v=13.6 sdssj 0820+0008 08 20 53.5 +00 08 53.4 2.304 g=14.9 2m1938+4603 19 38 32.61 +46 03 59.1 3.024 g=11.9 ec10246-2707 10 26 56.47 -27 22 57.1 2.844 b=14.2 asas 102322-3737 10 23 21.89 -37 36 59.9 3.343 v=11.6 supposing that there is a third body existing in this system, under the mutual gravitation force, the components of eclipsing pair would rotate around the barycenter of this triple system. seen by a distant observer, the light-travel time of this system will change because of the change in the orbital movement caused by the additional body, which can result in the observed cyclic change in the o-c diagram constructed by the eclipsing timings. this light time effect is very useful to detect tertiaries within au distance level in evolved eclipsing binaries compared with the radial velocity and transit methods, which have been extensively used to search for planets around solar-type main-sequence stars. that is because the high surface gravity of the evolved stars prevents us to achieve the high radial velocity precision required to detect planets, and the small size of the components (for example, r1 = 0.166 and r2 = 0.152 for nsvs 07826147 derived by for et al., 2010.) make the low probability of transits, see silvotti (2009) for more information. 3 new results here we present new results of our three targets (hs 0705+6700, ny vir and nsvs 07826147) from our ongoing search for circumbinary substellars orbiting around sdb eclipsing binaries. for hs 0705+6700 and ny vir, new observations confirmed our previous study (qian et al., 2009, 2012). for nsvs 07826147, preliminary analysis of the o-c diagram constructed by our five years data implied the presence of a close-in planet orbiting around it at a distance about 0.64au. 3.1 hs 0705+6700 hs0705+6700 ( = gsc 4123-265) was listed as a dwarf candidate from the hamburg schmidt survey (hagen et al. 1995 ). the observations indicated that it is an eclipsing binary. a detailed photometric and spectroscopic investigation was carried out by drechsel et al. (2001). absolute parameters of both components were determined suggesting that the primary is an sdb star, while the secondary is a cool stellar object. a cyclic change in the o-c curve of hs 0705+6700 was discovered by qian et al. (2009), which indicated the presence of a possible brown dwarf companion. it was later confirmed by camurdan et al. (2012), beuermann et al. (2012), and qian et al. (2013) who revised the parameters of the brown dwarf. the (o − c)1 diagram of the sdb-type binary hs 0705+6700 with respect to the following linear ephemeris, min.i = bjd 2451822.760549 + 0.09564665×e, (1) was displayed in fig. 1, where dots refer to our new data obtained after qian et al. (2009). it is shown that, apart from the cyclic change reported by previous authors, there is an upward parabolic variation in the (o−c)1 curve. this reveals a period increase at a rate of ṗ = +9.8×10−9 days/year. the angular momentum loss via gravitational radiation or/and magnetic braking should cause a decrease in the orbital period rather than increase. moreover, the period increase cannot be 184 close-in substellar companions and the formation of sdb-type close binary stars 0 10000 20000 30000 40000 50000 -0.003 -0.002 -0.001 0.000 0.001 0.002 0.003 e figure 1: o-c diagram of hs 0705+6700. dots refer to our new data obtained after qian et al. (2009), and open circles to the other data. dashed line refers to the continuous increase in the period, while the solid line to a combination of the period increase and a cyclic change. explained by mass transfer between the components because of the detached configuration of the system. therefore, the observed upward parabolic variation may be only a part of another cyclic change with very longperiod that is caused by the light-travel time effect due to the presence of another substellar object in a wider orbit. 3.2 ny vir ny vir (pg 1336-018) was discovered as a hw vir binary by kilkenny et al., (1998) which is composed of 2.580 2.585 2.590 2.595 2.600 2.605 0.4 0.2 0.0 -0.2 -0.4 6.750 6.755 6.760 6.765 0.4 0.2 0.0 -0.2 -0.4 figure 2: eclipse profiles of ny vir observed in november 2012 and april 2013 with the 2.15-m ”jorge sahade” telescope. dots represent the magnitude differences between the ny vir and the comparison star. a rapid pulsator and an m5-type star. kilkenny et al., (2011) found the orbital period of the binary is decreasing at a rate of ṗ = −11.2 × 10−13 d per orbit. a cyclic change was discovered to be superimposed on the long-term decrease by qian et al. (2012), which was explained by the presence of a circumbinary planet. qian et al. (2012) pointed out that the long-term period decrease can not be explained by angular momentum loss via gravitational radiation or/and magnetic braking and proposed that there is another substellar object in a wider orbit as in the case of hs 0705+6700. new observations of the sdb binary were obtained. the eclipse profiles obtained by using the 2.15-m ”jorge sahade” telescope in argentina were shown in fig. 2. the new determined eclipse times confirm the presence of the circumbinary planet and the parameters will be revised by including more times of light minimum. 3.3 nsvs 07826147 nsvs07826147 (2m 1533+3759) has been suspected as a possible eclipsing sdb binary system by kelley and shaw (2007) using the data from the northern sky variability survey and the two micron all sky survey. they derived the period of this system as 0.16177 days. zhu and qian (2009) improved its period as 0.16177046 days based on all available times of light minimum until that time. the detailed absolute parameters derived by the combination of photometric and spectroscopic observations were published by for et al., (2010). till now, we have monitored this target for more than 5 years. new light curves obtained with the 85-cm telescope and the 2.4m telescope are displayed in fig. 3. the constructed o-c diagram shows the trend of the periodic variation, which implies the presence of a closein circumbinary planet in nsvs 07826147 at a distance about 0.64au. the detailed analysis is in progress. 72.34 72.35 72.36 72.37 0.9 0.0 -0.9 m hjd2455900+ 74.256 74.260 74.264 74.268 74.272 74.276 2 1 0 -1 figure 3: light curves of nsvs07826147. left panel: eclipsing profiles of nsvs07826147 obtained with 85cm telescope on may 26, 2012 in r band. right panel: obtained with 2.4-m telescope on feb. 14, 2012 without filters. dots refer to the magnitude differences between the target and the comparison star, and open circles to the magnitude differences between the comparison and check star. 185 l. y. zhu et al. 4 summary one formation method of sdb-type binaries in theory is through a common envelope (ce) phase after the more massive star in the initially detached system evolves into a red giant. the ejection of the ce results in a large amount of angular momentum loss, and then a shortperiod sdb-type binary is formed. in previous section, new results of the three sdb-type eclipsing binaries are introduced. the presence of substellar companions especially the one orbiting nsvs 07826147 with the closest distance will give some constrains on the formation of this type of objects as well as on the interaction between red giants and their companions. to improve the results, more observations are required in the future. acknowledgement this work is supported by chinese natural science foundation (no.11133007 and 11325315) and yunnan natural science foundation (no. 2013fb084). new ccd photometric observations were obtained with the 2.16m and 85-cm telescope at xinglong station, the 2.4m, 1.0-m, and 60-cm telescopes at yunnan observatories in china, and the 2.15-m ”jorge sahade” telescope in argentina. references [1] beuermann, k., breitenstein, p., debski, b., diese, j., dubovsky, p. a., dreizler, s., et al., 2012, a&a 540, 8 [2] castellani, m., castellani, v., 1993, apj, 407, 649 [3] camurdan, c. m., zengin camurdan, d., ibanoǧlu, c., 2012, newa 17, 325 doi:10.1016/j.newast.2011.08.004 [4] charpinet, s., fontaine, g., brassard, p., et al., 2011, natur, 480, 496 doi:10.1038/nature10631 [5] d’cruz, n. l., dorman, b., rood, robert, t., o’connell, r. w., 1996, apj, 466, 359 doi:10.1086/177515 [6] drechsel, h., heber, u., napiwotzki, r., ostensen, r., solheim, j.-e., johannessen, f., schuh, s. l., deetjen, j., zola, s., 2001, a&a 379, 893 [7] for, b. q., et al., 2010, apj, 708, 253 doi:10.1088/0004-637x/708/1/253 [8] hagen, h-j., groote, d., engels, d., & reimers, d. 1995, a&as, 111, 195 [9] han, z. et al., 2002, mnras, 336, 449 [10] han, z. et al., 2003, mnras, 341, 669 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et al., 2007, natur, 449, 189 [23] zhu, l. y., qian, s. b., 2010, ap&ss, 329, 107 discussion raymundo baptista: light time travel effect is not the only interpretation for physical period changes in binary stars. an alternative is the magnetic activity cycle of the late-type star modulating the binary period. it is hard to believe a planet at 1 au orbit would survive commom envelope phase of a binary which was at comparable separation before. liying zhu: firstly, the cyclic period change caused by light time effect is strictly periodic change in o-c diagram not only quasi-periodic variation aroused 186 http://dx.doi.org/10.1016/j.newast.2011.08.004 http://dx.doi.org/10.1038/nature10631 http://dx.doi.org/10.1086/177515 http://dx.doi.org/10.1088/0004-637x/708/1/253 http://dx.doi.org/10.1046/j.1365-8711.2003.06451.x http://dx.doi.org/10.1146/annurev-astro-082708-101836 http://dx.doi.org/10.1111/j.1365-2966.2010.17919.x http://dx.doi.org/10.1046/j.1365-8711.1998.01432.x close-in substellar companions and the formation of sdb-type close binary stars by the magnetic activity cycle. secondly, in our fiveyears monitor, no asymmetric light curves caused by the magnetic activity for these hw vir binaries were found. their light curves are very stable. thirdly, one of our target sdss j0820+0008 is composed of an sdb star and a brown dwarf, the cyclic change can not be explained by magnetic activity cycles. the detected planet at 1 au orbit may be protected by the engulfment of the low mass stellar companion during the ce phase. and the other possibility is that the planet is the second-generation one. 187 introduction targets and method new results hs0705+6700 ny vir nsvs07826147 summary 178 acta polytechnica ctu proceedings 2(1): 178–182, 2015 178 doi: 10.14311/app.2015.02.0178 the hidden population of am cvn binaries in the sloan digital sky survey p. j. carter1, t. r. marsh1, d. steeghs1, e. breedt1, c. m. copperwheat2, b. t. gänsicke1, p. j. groot3, g. nelemans3,4 1department of physics, university of warwick, coventry cv4 7al 2astrophysics research institute, liverpool john moores university, ic2, liverpool science park, 146 brownlow hill, liverpool l3 5rf 3department of astrophysics/imapp, radboud university nijmegen, po box 9010, 6500 gl nijmegen, the netherlands 4institute for astronomy, ku leuven, celestijnenlaan 200d, 3001 leuven, belgium corresponding author: philip.carter@warwick.ac.uk abstract we present results from a spectroscopic survey designed to uncover am canum venaticorum (am cvn) binaries hidden in the photometric database of the sloan digital sky survey (sdss). the discovery of only 7 new am cvns in the observed part of our sample suggests a lower space density than previously predicted. based on the complete g ≤ 19 sample, we calculate an observed space density for am cvns of (5 ± 3) × 10−7 pc−3. we also compare the cataclysmic variables (cvs) discovered via this survey to those found in the sdss spectroscopy, and we discuss sbss 1108+574, an unusually helium-rich cv that has a spectroscopically confirmed orbital period of 55 minutes, well below the cv period minimum (∼80 min). sbss 1108+574 may represent an am cvn forming via the ‘evolved cv’ formation channel. keywords: cataclysmic variables dwarf novae binaries: close stars: individual: sbss 1108+574. 1 introduction the am canum venaticorum (am cvn) binaries are a rare group of hydrogen-deficient, ultra-compact, masstransferring white dwarf binaries. they have orbital periods in the range 5 – 65 minutes, well below the observed period minimum of hydrogen accreting cataclysmic variables (∼80 min, gänsicke et al. 10). three formation channels have been proposed for the am cvn binaries, each characterised by the donor. the donor can be (1) a second, lower mass, white dwarf – these systems have the shortest minimum periods, beginning mass-transfer at periods as short as a few minutes [9, 22]; (2) a semi-degenerate helium star [12, 31]; or (3) an evolved main sequence star [23, 32, 34, 36]. the third channel is known as the ‘evolved cv’ channel, and has generally been considered to be unimportant in comparison to the double white dwarf and helium star channels. the evolution of these systems is thought to be governed by gravitational wave radiation [e.g. 22], causing the mass accretion rate to be a steeply decreasing function of orbital separation. this gives rise to a strong dependence of their observational properties on the orbital period. the shortest period systems are expected to undergo direct impact accretion, and no disc forms [19, 28]. for systems with 10 . porb . 20 min, the accretion disc is in a stable high-state, with spectra showing helium absorption from the optically thick disc [e.g. 21]. am cvns with periods greater than ∼40 min, are thought to be in a stable low-state, the spectra of these systems are characterised by emission lines from the accretion disc [e.g. 30]. in the intermediate period systems (20 . porb . 40 min), an instability occurs in the accretion disc, and their appearance varies between that of the high-state and the low-state systems, similarly to the hydrogen-rich dwarf novae [14, 24]. in the past 10 years, the known population of am cvns has quadrupled, largely as a result of dedicated surveys using the sloan digital sky survey (sdss; york et al. 37), and the palomar transient factory [16, 17]. here we discuss estimates of the am cvn space density, and results from our spectroscopic survey designed to uncover new am cvn binaries amongst colour-selected objects from the sdss photometric database [6, 25, 29]. 2 the serendipitous sdss am cvns the first am cvn binaries were discovered in a variety of different ways, and only 10 were known prior to 178 http://dx.doi.org/10.14311/app.2015.02.0178 the hidden population of am cvn binaries in the sloan digital sky survey the burst of discoveries over the last decade. roelofs et al. [26] and anderson et al. [1, 2] discovered a total of six new am cvn systems in the sdss spectroscopic database via their helium emission dominated spectra. this provided the first sufficiently complete and homogeneous sample of am cvn systems that a study of the population became possible. roelofs et al. [27] used these ‘serendipitous sdss am cvns’ to estimate the observed space density of am cvn binaries – a crucial quantity for calibration of predictions from binary evolution theory. they calculate the completeness of the sdss spectroscopy as a function of colour and galactic latitude, and using an assumed magnitude distribution for am cvns, determine the number and distribution of systems that would be expected from nelemans et al. [20]’s population synthesis. these numbers are then compared to the number of systems found in the sdss, giving a value for the am cvn space density of 1–3 × 10−6 pc−3, an order of magnitude lower than the expected value at the time (2 × 10−5 pc−3, nelemans et al. 20). roelofs et al. [27]’s study also indicated that there should be at least 50 am cvns in total in the sdss photometry, most of them ‘hidden’ due to the absence of spectroscopic data. 3 the search for the hidden population since the completeness of the sdss spectroscopy in the area of colour space occupied by the am cvns is low, and this area is sparsely populated (see fig. 1), roelofs et al. [29] began a dedicated spectroscopic survey of objects in this region, intended to uncover the ∼40 am cvns that were expected [27]. fig. 1 shows the low density of sources in the sdss photometry in the colour region occupied by the am cvn binaries. also shown are the ∼2000 candidates selected from the sdss dr7 database by applying the colour cuts given in roelofs et al. [29]. our targeted region of colour space selects those am cvns with emission line spectra – the longer period systems. as am cvns should spend only a few percent of their lifetime as mass-transferring systems at orbital periods below 30 minutes, our selection should include the vast majority of am cvns in the sdss footprint [6]. this region of colour space is also occupied by db white dwarfs, which are the most significant contaminant in our survey. to date we have taken low-resolution, low signal-tonoise ratio spectra of ∼70% of these candidates, uncovering 30 cvs and 7 new am cvns (see carter et al. 6 for more details). −0.5 −0.4 −0.3 −0.2 −0.1 0.0 0.1 0.2 g− r −0.4 −0.2 0.0 0.2 0.4 u − g figure 1: the greyscale represents the density of sources in the sdss photometric database as a function of colour, to a limiting magnitude g = 20.5 (dereddened). the long period sdss am cvn binaries are indicated by star symbols. the solid line marks the blackbody cooling track, the dotted and dot-dashed lines indicate model cooling sequences for da and db white dwarfs. the dashed lines indicate the colour cuts given by roelofs et al. [29], and the dots indicate the candidates selected. 4 the cv population spectra of the cvs discovered in our survey were presented in carter et al. [6]. we compare this sample to the sdss cv population [33, these proceedings] by plotting the equivalent widths (ews) of their hα and he i 5875 emission lines, see fig. 2. the distributions of the two populations appear similar, with the strongest hα emitters falling outside our survey colour box. there are two obvious outliers visible in fig. 2, css 1122-1110 (sdss j1122-1110; breedt et al. 5) from the sloan sample, and sbss 1108+574 (sdss j1111+5712; carter et al. 7) from our sample, both having much stronger helium emission compared to hydrogen than the majority of cvs. the porb = 59 min cv, v485 cen [3] is also shown in fig. 2 for comparison. followup phase-resolved spectroscopy of sbss 1108+574 reveals an orbital period of 55.3± 0.8 minutes [7], consistent with independent photometric and spectroscopically determined periods [13, 18]. this is clear evidence for the evolved nature of the donor, which has been stripped of most of its hydrogen by, or prior to the onset of, mass-transfer. sbss 1108+574 and the similar systems, css 1122179 p. j. carter et al. 1110 [5] and css 1740+4147 (chochol et al., these proceedings), may be evolving along the ‘evolved cv’ evolutionary pathway toward am cvn binaries. 1:1 2:1 3:1 −100 −80 −60 −40 −20 0 ew he i 5875(å) −350 −300 −250 −200 −150 −100 −50 0 e w h α (å ) css1122 sdssj0804 sbss1108 v485cen figure 2: ew of he i 5875 versus hα for our cv sample (triangles), and the cv population from the sdss (crosses). the star symbols represent the am cvns from our sample, which fall on the ew(hα) = 0 line. also plotted are lines showing hα to he i 5875 ew ratios; cvs with unusually low ew ratios are labelled, v485 cen is shown for comparison. the position of the am cvn sdss j0804+1616 is due to strong he ii 6559 emission. 5 the am cvn space density roelofs et al. [27]’s observed space density, 1–3 × 10−6 pc−3, corresponds to ∼40 am cvns in our survey sample. we have found 7 new am cvns in the ∼70% of the sample that has been observed, suggesting that either there is some bias that we have not considered, or the space density is lower than previously estimated. biases in the survey are discussed in detail by roelofs et al. [29] and carter et al. [6]. they show that due to the distribution in colour space of the spectroscopic observations from the sdss, it is unlikely that a large number of am cvns lie outside the selected colour box. there is a slight bias in our survey observations towards the easier to observe brighter objects, but there is no significant colour bias, and the edges of our colour box are well explored. we therefore consider it likely that a lower space density is required to explain our results. since our survey is essentially complete down to a gband magnitude of 19, we use this smaller sample to recalibrate the population synthesis results. following the method described in roelofs et al. [27] we calculate the expected magnitude distribution of am cvn binaries in the sdss photometric database. the roelofs et al. [27] space density corresponds to 11 am cvns with g ≤ 19. in our essentially complete to g = 19 sample there are 4 known am cvns, scaling the space density to match this number, we obtain a value of (5 ± 3) × 10−7 pc−3 [6]. 6 summary the sdss has significantly increased both the numbers of am cvn stars known, and our understanding of the population. our spectroscopic survey of colourselected objects from the sdss photometric database has uncovered a further seven new am cvns. we have also identified a helium-rich cv, sbss 1108+574, follow-up observations of which reveal an orbital period of 55 minutes, well below the cv period minimum. sbss 1108+574 and similar helium-rich systems may be examples of am cvns forming via the ‘evolved cv’ pathway. discovering only seven new am cvns in the observed sample likely indicates a significantly lower space density than previously predicted. we use the essentially complete, brighter part of our sample to estimate the observed space density to be (5 ± 3) × 10−7 pc−3. acknowledgement pjc is grateful to f. giovannelli and the organisers for the invitation to attend this workshop. funding for sdss-iii has been provided by the alfred p. sloan foundation, the participating institutions, the national science foundation, and the u.s. department of energy office of science. the sdssiii web site is http://www.sdss3.org/. fig. 1 makes use of p. bergeron’s synthetic white dwarf colours [4, 11, 15, 35], kindly made available by the authors at http://www.astro.umontreal.ca/∼bergeron/cooling models. references [1] anderson, s. f., et al., 2005, aj, 130, 2230 [2] anderson, s. f., et al., 2008, aj, 135, 2108 [3] augusteijn, t., van kerkwijk, m. h., van paradijs, j., 1993, a&a, 267, l55 [4] bergeron, p., et al., 2011, apj, 737, 28 [5] breedt, e., et al., 2012, mnras, 425, 2548 [6] carter, p. j., et al., 2013, mnras, 429, 2143 180 the hidden population of am cvn binaries in the sloan digital sky survey [7] carter, p. j., et al., 2013, mnras, 431, 372 doi:10.1093/mnras/stt169 [8] espaillat, c., et al., 2005, pasp, 117, 189 doi:10.1086/427959 [9] faulkner, j., flannery, b. p., warner, b., 1972, apj, 175, l79+ [10] gänsicke, b. t., et al., 2009, mnras, 397, 2170 doi:10.1111/j.1365-2966.2009.15126.x [11] holberg, j. b., bergeron, p., 2006, aj, 132, 1221 [12] iben, jr., i., tutukov, a. v., 1987, apj, 313, 727 [13] kato, t., et al., 2013, pasj, 65, 23 [14] kotko, i., et al., 2012, a&a, 544, a13 [15] kowalski, p. m., saumon, d., 2006, apj, 651, l137 doi:10.1086/509723 [16] levitan, d., et al., 2011, apj, 739, 68 doi:10.1088/0004-637x/739/2/68 [17] levitan, d., et al., 2013, mnras, 430, 996 doi:10.1093/mnras/sts672 [18] littlefield, c., et al., 2013, aj, 145, 145 [19] marsh, t. r., steeghs, d., 2002, mnras, 331, l7 doi:10.1046/j.1365-8711.2002.05346.x [20] nelemans, g., et al., 2001, a&a, 368, 939 [21] o’donoghue, d., et al., 1994, mnras, 271, 910 doi:10.1093/mnras/271.4.910 [22] paczyński, b., 1967, acta astron., 17, 287 [23] podsiadlowski, p., han, z., rappaport, s., 2003, mnras, 340, 1214 doi:10.1046/j.1365-8711.2003.06380.x [24] ramsay, g., et al., 2012, mnras, 419, 2836 doi:10.1111/j.1365-2966.2011.19924.x [25] rau, a., et al., 2010, apj, 708, 456 doi:10.1088/0004-637x/708/1/456 [26] roelofs, g. h. a., et al., 2005, mnras, 361, 487 doi:10.1111/j.1365-2966.2005.09186.x [27] roelofs, g. h. a., nelemans, g., groot, p. j., 2007, mnras, 382, 685 doi:10.1111/j.1365-2966.2007.12451.x [28] roelofs, g. h. a., et al., 2010, apj, 711, l138 doi:10.1088/2041-8205/711/2/l138 [29] roelofs, g. h. a., et al., 2009, mnras, 394, 367 doi:10.1111/j.1365-2966.2008.14288.x [30] ruiz, m. t., et al., 2001, apj, 552, 679 doi:10.1086/320578 [31] savonije, g. j., de kool, m., van den heuvel, e. p. j., 1986, a&a, 155, 51 [32] sienkiewicz, r., 1984, acta astron., 34, 325 [33] szkody, p., et al., 2011, aj, 142, 181 [34] thorstensen, j. r., et al., 2002, apj, 567, l49 [35] tremblay, p.-e., bergeron, p., gianninas, a., 2011, apj, 730, 128 [36] tutukov, a. v., et al., 1985, soviet astronomy letters, 11, 52 [37] york, d. g., et al., 2000, aj, 120, 1579 discussion nataly katysheva: do you classify sbss 1108+574 as an am cvn or an su uma star? philip carter: we do not classify sbss 1108+574 as an am cvn because the am cvn classification is classically applied only to objects in which no hydrogen is seen; we decided to refer to it simply as a helium-rich cv. since this system is clearly below the cv period minimum it does not seem entirely appropriate to group it with normal cvs either. whilst it is not certain whether it will become sufficiently hydrogen-depleted to become a ‘true’ am cvn, it is clear that sbss 1108+574 and similar systems lie between the classical definitions of these two classes. koji mukai: is your survey sensitive to am cvn systems with high-state discs, which would have absorption lines, or to direct impact accretors? philip carter: es ceti is a border-line direct impact accretor, showing emission lines [8], and does fall within our colour box. we would have been able to detect systems similar to es cet. those systems showing absorption lines from a high-state disc would appear indistinguishable from db white dwarfs at the low s/n employed in our survey, and so we are not sensitive to them. however, such systems are expected to be a small minority of the total population, and our conclusion about the space density takes this into account. christian knigge: i know the numbers are still small, but have you started to look at the period and magnitude distribution of your systems and checked how they compare to the theoretical predictions? 181 http://dx.doi.org/10.1093/mnras/stt169 http://dx.doi.org/10.1086/427959 http://dx.doi.org/10.1111/j.1365-2966.2009.15126.x http://dx.doi.org/10.1086/509723 http://dx.doi.org/10.1088/0004-637x/739/2/68 http://dx.doi.org/10.1093/mnras/sts672 http://dx.doi.org/10.1046/j.1365-8711.2002.05346.x http://dx.doi.org/10.1093/mnras/271.4.910 http://dx.doi.org/10.1046/j.1365-8711.2003.06380.x http://dx.doi.org/10.1111/j.1365-2966.2011.19924.x http://dx.doi.org/10.1088/0004-637x/708/1/456 http://dx.doi.org/10.1111/j.1365-2966.2005.09186.x http://dx.doi.org/10.1111/j.1365-2966.2007.12451.x http://dx.doi.org/10.1088/2041-8205/711/2/l138 http://dx.doi.org/10.1111/j.1365-2966.2008.14288.x http://dx.doi.org/10.1086/320578 p. j. carter et al. philip carter: we have not yet investigated this in detail, and there are still a few systems for which we do not yet have an orbital period. in a magnitude limited sample, the expected period distribution peaks at ∼50 min, with approximately similar numbers above and below this peak [27]. we have found more systems in the sdss with periods below 50 min, than above, but the numbers are still too small to draw any strong conclusion from this. 182 introduction the serendipitous sdss am cvns the search for the hidden population the cv population the am cvn space density summary acta polytechnica ctu proceedings doi:10.14311/app.2017.13.0043 acta polytechnica ctu proceedings 13:43–48, 2017 © czech technical university in prague, 2017 available online at http://ojs.cvut.cz/ojs/index.php/app non-invasive inverse problem in civil engineering jan havelka∗, jan sýkora, anna kučerová czech technical university in prague, faculty of civil engineering, thákurova 2077/7, 166 29 prague 6, czech republic ∗ corresponding author: jan.havelka.1@fsv.cvut.cz abstract. in this contribution we focus on recovery of spatial distribution of material parameters utilizing only non-invasive boundary measurements. such methods has gained its importance as imaging techniques in medicine, geophysics or archaeology. we apply similar principles for non-stationary heat transfer in civil engineering. in oppose to standard technique which rely on external loading devices, we assume the natural fluctuation of temperature throughout day and night can provide sufficient information to recover the underlying material parameters. the inverse problem was solved by a modified regularised gauss-newton iterative scheme and the underlying forward problem is solved with a finite element space-time discretisation. we show a successful reconstruction of material parameters on a synthetic example with real measurements. the virtual experiment also reveals the insensitivity to practical precision of sensor measurements. keywords: calderón problem, finite element method, boundary inverse method, neumann-todirichlet map, heat transfer. 1. introduction non-invasive methods are gaining increasing interest in various disciplines spanning from medicine to geophysics. the main advantage of such methods is that they preserve the observed sample intact while getting information about its inner properties. such tasks can therefore play fundamental role in analysis of existing structures, improving future design, minimising the gap between model prediction and real observations etc. moreover today trend is in favour of complex models described by multiple parameters which have to be properly identified from real measurements, often requiring a specially designed set destructive tests with a high precision laboratory equipment. such process often requires a set of samples which do not have to be available and features a rather limited description of the studied material. in this contribution we propose a non-invasive parameter identification technique based merely on boundary observations of some structure. this idea was inspired by a medical imaging technique named electric impedance tomography (eit). the mathematical procedure was first rigorously described by an argentinian mathematician alberto calderón in his foundational paper [1] in 1980, although the idea was studied earlier in 1930 in geophysics [2, 3]. the goal of eit is to recover an electric conductivity field inside some domain using only boundary measurements. basic idea of this method lies in the difference of surface measurements due to variations in the subsurface conductivity distribution. meanwhile a single set of surface measurements for a given loading conditions might result in a number of possible conductivity fields, calderón surpassed this problem by sequentially implying multiple loading conditions each followed by a measurement, which in essence has the potential to contain the complete information about the underlying conductivity distribution. in this contribution we will employ similar techniques of calderón inverse problem paradigm to a parabolic partial differential equation with two independent parameters to describe thermal properties of some structure. our method is designed to be external loading-free, i.e. it completely relies on the environmental factors as a source of changes which are necessary in order to identify the material inside a domain. 2. inverse problem parameters recovery of a given model is often regarded to as ill-posed problem [4, 5]. for such task it is typical that even a small perturbation in the input data can result into a large variation in the solution. this can be due to several reasons e.g. uniqueness issues stemming from information shortage, i.e. data insufficiency or it can be an intrinsic property of the system itself. for this reason needs a stabilization provided by additional assumptions, e.g. some prior information, enforcing smoothness, preferring solution with the smallest norm, provide bounds to the unknown entity, etc. procedures for determining the constrains for solving such problems are generally called regularisation methods and within this contribution we shall employ a deterministic approach. an inverse problem is usually an optimization task solved by minimising some functional which is often stated as follows, with the first term representing the cost function and the second term being the regularisation penalty θ̃ = arg min θ ‖f (θ)− um‖p +g(θ), (1) 43 http://dx.doi.org/10.14311/app.2017.13.0043 http://ojs.cvut.cz/ojs/index.php/app j. havelka, j. sýkora, a. kučerová acta polytechnica ctu proceedings where f (θ) is output of the forward operator, um is the vector of measured quantity, p indicates `p-norm, g(θ) is the regularisation functional introducing the additional constrains to the solution and θ̃ are the parameter fields being reconstructed. a typical choice of the regularisation penalty term in (1) is of following form [4] g(θ) = λ2‖l(θ − θr)‖2`2 , (2) where the hyper-parameter, λ controls the trade-off between solution stability, given constrains and a distance from the actual solution. additional parameters θr represent some a priori known and possibly non-smooth behaviour of the unknown parameters [6]. a straightforward way of explaining the regularisation operator l is such that it draws the solution towards its null space, i.e. ker(l). in our case it takes a form of a discrete approximation to the laplacian. in this contribution, both terms in (1) were chosen to be `2-norm due to the convenience for computational purposes and preference of smooth solutions [7]. in such case, by combining the minimisation scheme (1) for p = 2, penalty term (2) and a gauss-newton approximation of newton-raphson multi-variable method, one can obtain a following iterative formula [6] θk+1 = θk + δθk, (3) δθk = ( jtk jk + λ2r )−1 (jtk δu− λ2r (θk − θr) ) , (4) where jk is the jacobian evaluated at θk, δu is a vector containing the difference between the model output and real measurements, θr is a conductivity field containing its prior assumptions and r = ltl is regularisation operator composed from a pre-calculated laplacian l of a piece-wise constant functions on a finite element mesh. δu = ur − f (θk) , (5) where f (θ) ∈ rvw represents a discrete neumann-todirichlet operator of a forward model with v being the number of measurement points, i.e. a number of nodes of fe mesh on a subset γv of boundary ∂ω that is being observed and w is the number of experiments. the a priori measured quantity is stored in vector ur ∈ rvw. in all cases the reference field θr(x) = θ0(x) = 1 for x ∈ ω. from our experience, the most stable and simple choice of hyper-parameter λk is the one utilized by levenberg-marquardt regularisation (lmr) [8], which is gradually decreasing the parameter during iteration and takes following form λk = max ( max ( jtk jk )) . (6) jacobian jk is updated in each iteration and was calculated numerically in a following way j (jkl) i = ∂ujk ∂θli , (7) where ji is the third-order tensor in i-th iteration, indexes jk are representing measurement nodes in a finite element mesh and individual measurements respectively. index l denotes a conductivity change on l-th fe element. the tensor is calculated numerically and matricised along indexes jk for convenience of calculation purposes. in general θ can represent multiple parameter fields and in such case equations (3) to (7) are evaluated for each parameter individually. additional numerical treatment the iteration scheme (3) does not include any supplementary constrains which provide the solver robustness. therefore one have to adopt further numerical treatment to maintain the assumptions 1 and 2 valid within the iteration. introduction of the following two operators increased the stability and robustness of results substantially. the constrain for strict positivity of conductivity field during the iteration is treated with an ad-hoc algorithm which is not allowing the occurrence of negative numbers in the solution and gives rise to a positivity operator, i.e. tp in a following way tp(θ(x)) = { θ(x) > 0→ x+, θ(x+) = θ(x+), θ(x) < 0→ x−, θ(x−) = min(θ(x+)). (8) since the problem is inherently non-linear and the iteration scheme proceeds with non-restricted linear steps we introduce a cap operator tc limiting the resulting step size to a chosen value k in a following way tc(θ(x)) = { θ(x) > k, θ(x) = sign(θ(x)) · k, θ(x) < k, θ(x) = θ(x). (9) the final modified gn iteration scheme after applying (8) and (9) operators to (3) then takes following form θk+1 = tp (θk + tc (δθk)) . (10) 3. transient model since the numerical model is repeatedly used in each iteration of (10) it plays a fundamental role in the entire inverse procedure. in our case it will to some extent also substitute an actual experiment. numerical solution is obtained using finite element method space-time discretisation [9–12]. the main idea of employing the calderón’s technique is created upon foundational research conducted by k. s. chen [13] in 1989, who proposed and experimentally validated so called complete electrode model (cem) which allows to consider electrodes actual physical size and introduces a contact impedance layer 44 vol. 13/2017 non-invasive inverse problem in civil engineering between the skin and the electrode. further proof for cem solution uniqueness was given by e. somersalo, et al. in 1992 [14]. significant contributions to the theory were papers [15, 16] proving the solution is uniquely determined in dimension n ≥ 3 for complete dirichlet-to-neumann or neumann-to-dirichlet data. unlike in n ≥ 3, in dimension n = 2 the problem is undetermined and the solution was proved to be unique up to a change of coordinates [17, 18]. specific choice of this model is motivated by real application on historical and other structures, where the standard procedure of taking samples is too difficult or the structure should not be weakened or damaged. also in real conditions it is not an easy task to sustain a stable and steady state conditions. not only the surrounding temperature will fluctuate even in a laboratory environment, but for standard building materials like bricks, concrete, wood, etc. the steady state, after changing the loading conditions, can be reached after several hours or days depending on the volumetric capacity, heat conductivity and material thickness. therefore, we intend to apply the identical principles used in a calderón problem for transient models. to capture a time dependent heat transfer, one can adopt following set of equations ρs(x)cp(x)∂u ∂t (x, t)−∇ · (λs(x)∇u(x, t)) = f̃(x, t), n(x) · (λs(x)∇u(x, t)) = f̃n (x, t), α(i)(u(x, t)− u0(x, t)) = f̃t (x, t), u(x, t) = f̃d(x, t), t > 0, ∂ω(i) = ∂ω(i) n ∪ ∂ω(i) t ∪ ∂ω(i) d , i = 1, . . . , i, (11) where ρs is volumetric mass density, cp is specific heat capacity and ∂ω(i) (n,t,d) are disjoint subsets of boundary ∂ω(i) in i-th loading condition with corresponding environmental factors u0, α(i) and f̃n,t,d. in order to maintain neumann-to-dirichlet sensing and uniqueness of the inverse problem, the set of equations in (11) is subjected to following constrain assumption 1. let γm be a subset of boundary ∂ω that is subjected to measurements. then ψ = ( ∂ω(i) n ∪ ∂ω(i) t ) : (ψ ∩ γ) /∈ ∅, ∀i, must hold, i.e. the boundary subjected to measurements must contain at least some neumann conditions. another set of constrains similar to the ones in [14] is represented by following assumptions assumption 2. the material parameters λs, ρs, cp and transfer coefficients α(i) satisfy following (i) λs ∈ l∞(ω;r), inf x∈ω λs(x) = λ+ s > 0, (ii) 0 < ρ−s ≤ ρs ≤ ρ+ s <∞, (iii) 0 < c−p ≤ cp ≤ c+p <∞, (iv) 0 < α (i) − ≤ α(i) ≤ α(i) + <∞, ∀i. in a definition of this model (11), one can notice that there is no mention of electrodes, thermocouples or measurements indicating our intention not to consciously intervene in the system itself, but only to rely on external influences and natural fluctuation of temperature throughout the day and night. for systems where their boundaries ∂ω are not exposed to different external influences, one can adapt a similar techniques to complete electrode model, i.e. to attach a device that can excite a different boundary conditions, or control the ambient temperature in the second or third condition in equation (11). a special feature of this model lies in its independence on excitation device, i.e. stimulation electrodes or thermocouples, and therefore has the least requirements on equipment and the full measurement setup consists of arrays of thermometers and/or thermal cameras. numerical solution of the forward model the forward operator f consists of two independent parameters and solves the system for all time steps nt at once, giving rise to a following formulation f (λs, cv) = um, um ∈ rm ·nt , (12) where the inputs are heat conductivity λs and volumetric capacity cv = ρs · cp. the measurement matrix um ∈ rm ·nt , where m is number of measurements within single time step and nt is number of time steps, is obtained as a subset of complete solution vectors ui, i = 1, . . . nt from a following set of linear equations kcui+1 = ( kc −∆tk̃ ) ui−1 + . . . ∆t ( (1− τ)f̃n,i−1 + τ f̃n,i ) + . . . ∆t ( (1− τ)f̃t,i−1 + τ f̃t,i ) + . . . ∆t ( (1− τ)f̃d,i−1 + τ f̃d,i ) , (13) k̃ = k + kt, (14) kc = c + τ∆tk̃, (15) where ∆t = ti+1− ti is a time step, τ ∈ 〈0; 1〉 is a time integration parameter1 and the system matrices and right hand side vectors are computed in a following way kkl = ∫ ω λs∇φk · ∇φl da k, l = 1 . . . n, (16) 1the foregoing computations were calculated using cranknicolson integration method, i.e. τ = 0.5 45 j. havelka, j. sýkora, a. kučerová acta polytechnica ctu proceedings ckl = ∫ ω ρscpφkφl da k, l = 1 . . . n, (17) kt,kl = ∫ ∂ωt αφkφl ds k, l = 1 . . . n, (18) f̃t,li = ∫ ∂ωt αu0,iφl ds i = 1 . . . nt, l = 1 . . . n, (19) f̃n,li = − ∫ ∂ωn f̃n (ti)φl ds i = 1 . . . nt, l = 1 . . . n, (20) f̃d,i = − (k + kt) f̃d(ti) i = 1 . . . nt. (21) for detailed information and derivation of individual terms, the interested reader is referred to [9–12] and literature therein. 4. results we consider two finite element meshes for each physical domain, i.e. one is used to simulate real measurements and the second is used for the reconstruction algorithm. both finite element meshes are shown on figure 1. further improvements were made by integrating measurement errors in the calculation, specifically we assumed only the direct measured quantity is subjected to errors in a following way ume = um + ε, (22) where ume is some measured quantity with the error included, um is its true value and ε ∼ n ( µ, σ2) is the error. we assume the sensor is calibrated and its deviation is known to fall within 3·σ region, i.e. µ = 0 and deviation dm = 0.25 oc given by manufacturer implies σ = dm/3. all the sensor errors are considered to be mutually independent and identically distributed random variables, although using thermal camera would rather impose spatially correlated random variables forming a random field. figure 1. finite elements meshes: coarse (top) and fine (bottom). observed boundary γm depicted in red colour, where m = 122 nodes of finite element mesh being measured. we assume changes in temperature on the observed boundary over several days can provide sufficient amount of data for the algorithm to recover material fields θ = [λs; cv], i.e thermal conductivity and volumetric capacity respectively. in case the observed structure is located in an environment without sufficient changes in loading conditions, one may use a device that excites a different boundary conditions, e.g. a heater or air conditioning system. the measure of reconstruction algorithm performance is represented by an error on the reconstructed material field in comparison to the original field in a following way εθi = ‖θi,r − θi,o‖2 ‖θi,o‖2 , (23) where θi,r is the i-th recovered material field obtained from (10) and θi,o is the i-th original material field. in order to simulate a natural environment conditions and capture temperature fluctuations in the interior and exterior, we use a realistic measurements shown on figure 2. the environment was in total monitored for eleven consecutive days at minute intervals. for the calculation purposes data were further sparsified to one hour intervals which satisfy both sufficient precision in describing the temperature curve and reasonable computational load. boundary conditions were set accordingly to figure 2 as follows 46 vol. 13/2017 non-invasive inverse problem in civil engineering figure 2. interior and exterior temperatures. preceding temperature in grey, actual observation period in colour. λ ∂u ∂n (x, t)|∂ω1 = 10 · (u(x, t)− t1(t)) , λ ∂u ∂n (x, t)|∂ω2 = 10 · (u(x, t)− t2(t)) . (24) the magnitude of reference fields were set according to regular building materials, i.e. λr = 1 [ w m−1 k−1 ], cv,r = 1× 106 [ j m−3 k−1 ]. (25) in figure 2, the measurement starts from day two and the observation period lasts 9 day. measurement is taken every hour which yields into 216 measurements of boundary γ122 = ⋃2 i=1 ∂ωi. for purpose of this contribution a set of 500 realisations of measured temperatures was given to the reconstruction algorithm. resulting mean and standard deviation of reconstructed fields are displayed on following figures 5. conclusions in oppose to eit where the crucial part of successful material field recovery is a precise placement of measurement electrodes together with boundary shape and knowledge of loading conditions2, in our application the problems might arise from insufficient environmental factors variability leading to imperfections and unwanted artefacts in the reconstructed fields. this can be identified although it can be remedied by a use of an external devices providing different boundary conditions. in a simple case shown on figures 3 and 4 one can observe the possibility of recovering material parameters of such system in the first place. the difference between figures 3a and 4a shows that the variation of temperature in time was not sufficient to fully reconstruct the volumetric capacity field cv resulting into higher reconstruction error of e[εcv ] = 0.2641 in oppose to e[ελs ] = 0.1414. looking closer at deviations of errors std[εcv ] = 0.0014 and std[ελs ] = 0.0017 2especially in eit, where the electrodes are placed on a human body, the boundary shape and electrodes are in continuous motion as the patient breath and move. (a) . e[ελs ] = 0.1414 [-] (b) . std[ελs ] = 0.0017 [-] figure 3. mean (a) and standard deviation (b) of reconstructed conductivity field. in grey: original field, in colour: reconstructed field. one can notice the insensitivity of reconstruction algorithm to induced error on measurements which can be also seen by comparing the magnitudes of plots 47 j. havelka, j. sýkora, a. kučerová acta polytechnica ctu proceedings (a) . e[εcv ] = 0.2641 [-] (b) . std[εcv ] = 0.0014 [-] figure 4. mean (a) and standard deviation (b) of reconstructed capacity field. in grey: original field, in colour: reconstructed field. in figures 3a and 3b or 4a and 4b, i.e. the standard deviation is in both cases approximately two orders of magnitude lower than the mean. our intention is then to extend the existing model with imperfections or errors on all the other levels, i.e. uncertainty in ambient temperature, transfer coefficient or a geometrical level inaccuracies. acknowledgements the financial support of this research by the ga15-07299s and sgs17/042/ohk1/1t/11 is gratefully acknowledged. references [1] a. p. calderón. on an inverse boundary value problem. computational & applied mathematics 25:133 – 138, 2006. [2] l. b. slichter. the interpretation of the resistivity prospecting method for horizontal structures. physics 4(9):307–322, 1933. doi:10.1063/1.1745198. [3] r. e. langer. an inverse problem in differential equations. bulletin of the american mathematical society 39(10):814–820, 1933. [4] m. vauhkonen. electrical impedance tomography and prior information. phd thesis, kuopio university, 1997. [5] a. kirsch. an introduction to the mathematical theory of inverse problems. applied mathematical sciences 120. springer-verlag new york, 2nd edn., 2011. [6] d. s. holder. electrical impedance tomography: methods, history and applications. series in medical physics and biomedical engineering. taylor & francis, 1st edn., 2004. [7] y. mamatjan, a. borsic, d. grsoy, a. adler. experimental/clinical evaluation of eit image reconstruction with `1 data and image norms. journal of physics: conference series 434(1):012078, 2013. [8] h. niu, p. guo, l. ji, et al. improving image quality of diffuse optical tomography with a projection-error-based adaptive regularization method. optics express 16(17):12423–12434, 2008. [9] k. j. bathe. finite element procedures. klaus-jurgen bathe, 2006. [10] t. j. r. hughes. the finite element method: linear static and dynamic finite element analysis. courier corporation, 2012. [11] g. strang, g. j. fix. an analysis of the finite element method, vol. 212. prentice-hall englewood cliffs, nj, 1973. [12] o. c. zienkiewicz, r. l. taylor. the finite element method, vol. 3. mcgraw-hill london, 1977. [13] k. s. cheng, d. isaacson, j. c. newell, d. g. gisser. electrode models for electric current computed tomography. ieee transactions on biomedical engineering 36(9):918–924, 1989. doi:10.1109/10.35300. [14] e. somersalo, m. cheney, d. isaacson. existence and uniqueness for electrode models for electric current computed tomography. siam journal on applied mathematics 52(4):1023–1040, 1992. doi:10.1137/0152060. [15] r. m. brown, g. a. uhlmann. uniqueness in the inverse conductivity problem for nonsmooth conductivities in two dimensions. communications in partial differential equations 22(5-6):1009–1027, 1997. doi:10.1080/03605309708821292. [16] j. sylvester, g. uhlmann. a global uniqueness theorem for an inverse boundary value problem. annals of mathematics 125(1):153–169, 1987. [17] j. sylvester, g. uhlmann. a uniqueness theorem for an inverse boundary value problem in electrical prospection. communications on pure and applied mathematics 39(1):91–112, 1986. doi:10.1002/cpa.3160390106. [18] a. i. nachman. global uniqueness for a two-dimensional inverse boundary value problem. annals of mathematics 143(1):71–96, 1996. 48 http://dx.doi.org/10.1063/1.1745198 http://dx.doi.org/10.1109/10.35300 http://dx.doi.org/10.1137/0152060 http://dx.doi.org/10.1080/03605309708821292 http://dx.doi.org/10.1002/cpa.3160390106 acta polytechnica ctu proceedings 13:44–49, 2017 1 introduction 2 inverse problem 3 transient model 4 results 5 conclusions acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2017.8.0014 acta polytechnica ctu proceedings 8:14–16, 2017 © czech technical university in prague, 2017 available online at http://ojs.cvut.cz/ojs/index.php/app batio3 films deposited onto tinb and ti substrates amount and stability of barium kamila moriováa, zdeněk toldeb, přemysl vaněkc, vladimír starýb, václav nehasila,∗ a faculty of mathematics and physics, charles university, v holešovičkách 2, 180 00 prague 8, czech republic b department of materials engineering, faculty of mechanical engineering, czech technical university in prague, karlovo náměstí 13, 121 35 prague 2, czech republic c institute of physics of the czech academy of sciences, na slovance 1999/2, 182 00 prague 8, czech republic ∗ corresponding author: nehasil@mbox.troja.mff.cuni.cz abstract. batio3 films deposited onto tinb and ti substrates using hydrothermal synthesis method were studied in the presented work. these films are supposed to improve properties of bone implants due to their ferroelectric behaviour, because ferroelectrics induce improved bone formation. a great question is the chemical stability of the used material. it can be crucial for its biocompatibility and possible in vivo application. we studied chemical composition of prepared samples, especially concentration of ba and ti and trends of these concentrations stimulated by a solution saline action. the ba and ti concentrations were determined by xps under ultra high vacuum condition. the batio3 films were investigated as received after the preparation procedure as well as after a long time treatment in solution saline. every sample was introduced to the solution saline at first for 1 and later for 3 weeks. ti concentration almost does not change during our experiments while a meaningful ba decrease is observed. nevertheless, barium release seems to slow down with respect to the time of solution saline action. stability of barium titanate films in a period of several months and an absolute amount of the released barium will be a subject of the next research. keywords: barium titanate, xps, concentration, stability. 1. introduction barium titanate could be a promising material for the development of bone implants. this material exhibits ferroelectric properties. if it is polarised, it can improve the bone adhesion to the implant [1–3]. chemical elements included in barium titanate are biocompatible except for barium which is toxic. we studied chemical composition of thin barium titanate film deposited onto titanium and titanium-niobium alloy substrates using xps method. we have determined the chemical composition of as prepared samples and after their treatment in solution saline. barium release is observed and discussed. 2. experimental barium titanate films deposited onto tinb or ti substrates were prepared by hydrothermal synthesis. samples were in the form of discs of 10-14 mm diameter and thickness 0.5-1.4 mm. using calotest, thickness of deposited films was estimated to be about 1-2 µm . before the preparation of barium titanate films, the substrates were cleaned (ultrasonic cleaning in acetone, etching in 6m hcl) and annealed in oxygen at 500 ◦c for 24 hours. after this procedure, the substrate surface is covered by a tio2 rutile layer. barium titanate films were prepared in an autoclave with a teflon inlet by the hydrothermal reaction of tio2 and ba(oh)2 at 250 ◦c for 6 weeks. using this procedure, two sets of batio3 films deposited onto tinb and one set deposited onto ti substrates were prepared. these sets are marked batio3/tinb 1st and 2nd series and batio3/ti, respectively. because the batio3 films within a given set were prepared in the same run under completely identical conditions, they are identical so only one sample from every set was analysed by xps. before xps measurement, the samples were not specially cleaned except brief rinsing in ethyl alcohol. xps measurements were performed in an ultrahigh vacuum chamber with a base pressure better than 1 × 10−9 torr. system is evacuated by titanium ion getter pump and equipped by a quick sample load lock. working pressure inside of the vacuum system is monitored by a bayard-alpert manometer (varian) and remained about 3 × 10−9 torr during xps measurements. dual anode (al and mg) xray source (vg) and hemispherical electron energy analyser (omicron ea -125) equipped with a 5 channel multiplier detector were used to acquire xp spectra. as a primary radiation, al-kα line of 1486.6 ev energy was used. distance between the x-ray source and the samples was about 1 cm. composition of the barium titanate films was determined using the method of relative sensitivity factors. the obtained data describe the surface film of the depth of about 2 − 4 × 10−9 m, 14 http://dx.doi.org/10.14311/app.2017.8.0014 http://ojs.cvut.cz/ojs/index.php/app vol. 8/2017 batio3 films deposited onto tinb and ti substrates figure 1. photoelectron spectrum batio3 on tinb substrate after preparation of sample. visible peaks (photoelectron as well as auger) are marked, those used to calculation of element concentration are in bold. the relative uncertainty of calculated concentrations should not exceed 20 % of the particular value. the as prepared samples were characterised by means of xps and consequently introduced to the solution saline (0.9 mg of nacl in 100 ml of solution) to check the stability of the batio3 films in a physiological-like environment. the solution saline was temperature-stabilized at 38 ◦c, kept under ambient atmosphere and protected against dust and other impurities. the main goal of this treatment was to observe a potential release of ba. the samples remained in the solution saline for 7 days. after this period, their composition was checked by xps to determine any possible changes. subsequently they were re-introduced to a fresh solution saline for next 3 weeks and measured by xps once more. after every treatment procedure in the solution saline, the samples were rinsed in distilled water. 3. results and discussion typical electron spectrum of batio3 is presented in figure 1. this particular spectrum was obtained for the barium titanate deposited onto tinb within the first series. we can recognise peaks of ba, ti, o, and c. nb peaks are also visible in the spectrum. they originate from the tinb substrate. taking into account the thickness of the barium titanate films, their origin is not clear and is subject of further investigation. a segregation of niobium on the surface or non-completely continuous batio3 films would explain this observation. moreover, different samples contain various amounts of carbon. we assume its quantity is influenced by the transfer time between the preparation of the samples and the xps measurements, by storage conditions etc. it is, however, out of the scope of this contribution to discuss the chemical forms and concentrations of all the involved elements, we will restrict our discussion figure 2. decrease of barium concentration due to an action of solution saline. only to the barium content with respect to titanium and oxygen. oxygen is present in more chemical forms and some of them are obviously included in carbon species. therefore we only used the so-called lattice oxygen for our calculation of the barium content. it is the oxygen chemically bonded within batio3. after fitting o 1s peak, it can be recognised at the binding energy of about 529.5 ev, according to ref. [4], where electron spectra of batio3 powders exhibit similar features as ours. the change in the amount of barium is crucial indicator in this study. in figure 2, relative amount of barium with respect to the total amount of ba, ti, and lattice o is presented. it can be seen that the content of barium on the batio3/tinb sample (2nd series) exhibits much lower values. we are currently not able to explain this effect. the most essential, however, is the general trend of barium content decay with the time of sample treatment in the solution saline, which is clearly observed on all the samples. this decrease is more considerable after the 1st week than after the following three weeks. the slowdown of the barium release into the solution saline is the most obvious on the 1st series of batio3/tinb samples. the barium release was independently confirmed by an xps measurement of nacl obtained from the used solution saline. along with na and cl peaks, we were able to recognise traces of barium in the xp spectra. contrary to barium, the change of titanium concentration is much weaker as can be seen in figure 3. generally, it slightly increases except for the 1st series of batio3/tinb where a decrease is observed after the 1st week. extremely high value of titanium concentration on the batio3/ti sample is probably given by the contribution of the base substrate material. all these results can be identically interpreted as a release of barium to the solution saline. this process could also occur in a human body if barium titanate was used as a material for bone implants. in the follow-up research, we will further investigate if this process fades after some prolonged period of time resulting 15 k. moriová, z. tolde, p. vaněk et al. acta polytechnica ctu proceedings figure 3. titanium concentration due to an action of solution saline. in a stable barium titanate thin film. another topic to be studied on these materials is their ferroelectric properties after the treatment in a solution saline or in a physiological-like environment. 4. conclusion a depletion of barium concentration in batio3 thin films under the impact of solution saline was studied. barium releases into the solution saline, but it can be expected that this effect will become weaker over an extended period of time and the chemical stability of the material will be achieved. such long-time experiment are currently under the progress. acknowledgements this work was supported by grant agency of the czech republic, project number 15-01558s. xps system used to get the experimental results was supported by czech ministry of education (contract number lm2015057). references [1] n. setter, d. damjanovic, l. eng, et al. ferroelectric thin films: review of materials, properties, and applications. journal of applied physics 100(5):051606, 2006. [2] f. r. baxter, c. r. bowen, i. g. turner, a. c. e. dent. electrically active bioceramics: a review of interfacial responses. annals of biomedical engineering 38(6):2079–2092, 2010. doi:10.1007/s10439-010-9977-6. [3] p. vaněk, z. kolská, t. luxbacher, et al. electrical activity of ferroelectric biomaterials and its effects on the adhesion, growth and enzymatic activity of human osteoblast-like cells. journal of physics d: applied physics 49(17):175403, 2016. [4] m. wegmann, l. watson, a. hendry. xps analysis of submicrometer barium titanate powder. journal of the american ceramic society 87(3):371–377, 2004. doi:10.1111/j.1551-2916.2004.00371.x. 16 http://dx.doi.org/10.1007/s10439-010-9977-6 http://dx.doi.org/10.1111/j.1551-2916.2004.00371.x acta polytechnica ctu proceedings 8:14–16, 2017 1 introduction 2 experimental 3 results and discussion 4 conclusion acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2016.3.0043 acta polytechnica ctu proceedings 3:43–46, 2016 © czech technical university in prague, 2016 available online at http://ojs.cvut.cz/ojs/index.php/app surface treatment and examination of grade 2 and grade 5 titanium peter nagya,∗, miklos weszlb, krisztian laszlo tothb, imre kientzla, gyorgy ringa, peter szabaditsa, eszter bognara,c a budapest university of technology and economics, department of materials science and engineering, budapest, hungary b nanoti limited, birmingham, uk c mta–bme research group for composite science and technology, budapest, hungary ∗ corresponding author: npeter@eik.bme.hu abstract. surface characteristics play an important role in the implant-bone integration that is required for the long-term reliability of dental and orthopedic implants. in this paper, we investigate the effect of acid etching on the mass reduction and roughness of grade 2 and grade 5 ti under controlled experimental conditions. three different etching compounds were investigated: 30% hcl, 85% h3po4 and the compound of 30% (cooh)2 × 2h2o and 30% h2o2 in various treatment intervals under controlled temperature. stereo microscopy, scanning electron microscopy, roughness and weight measurements were carried out on the samples. we found that neither 85% h3po4 nor the compound of 30% (cooh)2 × 2h2o and 30% h2o2 were able to remove the machining marks from the surface of ti discs in our experimental setting. on the other hand, etching in 30% hcl yielded even surfaces both on ti grade 2 and 5 discs. we also found that etching at higher temperatures in 30% hcl resulted in significant mass loss. keywords: titanium, chemical etching, mass reduction, roughness. 1. introduction pure titanium and titanium alloys are well-established standard materials of dental and orthopedic implants because of their favorable mechanical strength, chemical stability and biocompatibility [1]. nowadays, the most frequently used pure titanium is identified as “commercially pure” (cp) grade 2 and 4. however, a titanium-vanadium-aluminum alloy, reported as grade 5 or labeled as ti6al4v, is also commonly used. the integration of titanium dental implants with the surrounding bone tissue (osseointegration) is critical to ensure the long-term survival and functionality of the implants [2]. the concept of osseointegration was discovered by brånemark and his co-workers. their research has had a dramatic influence on the design of implants and consequently the clinical outcome of dental implant surgeries [3]. the first generation of successfully applied titanium implants had simple machined surfaces. since then, implant surfaces have long been recognized to play an important role in molecular interactions, cellular response and osseointegration. therefore, tremendous scientific efforts have been focused on the development of implants with surfaces that have the capability of accelerating the osseointegration and improving the long-term survival of the implants. for that purpose, various methods have been developed during the last 20 years, such as sandblasting, acid etching, bioactive coatings, anodization and, more recently, laser modification of surfaces [4–6]. these implants have been extensively investigated in in vivo experiments, including longterm clinical studies and experimental histological and biomechanical evaluation in animal models [7]. titanium and titanium alloys are prone to develop a superficial oxide layer upon contact with oxidants [8]. the quality of the oxide layer may profoundly affect the biological behavior of titanium implants. the mechanical properties of the oxide layer, such as the thickness, texture and roughness are known as the most important parameters that may affect the rate of osseointegration and the biomechanical properties of the devices [9]. several types of surface treatment methods have been proposed in the scientific literature, including mechanical and chemical procedures to improve the osseointegration ability of dental implants. however, usually these publications do not disclose details about the effect of treatment parameters on the mechanical properties of titanium, but rather focus on the optimization of biological properties of the oxide layer [7–10]. 2. aim of experiment in this paper, we focus our attention on the effect of acid etching on the mass reduction, surface texture and roughness of ti grade 2 and ti grade 5 samples under controlled experimental conditions. 3. experiment titanium discs of 2 mm thickness and 14 mm in diameter were cut from grade 2 and grade 5 rods with a turning machine. the weight, roughness and surface texture of the discs were recorded after machining. 43 http://dx.doi.org/10.14311/app.2016.3.0043 http://ojs.cvut.cz/ojs/index.php/app p. nagy, m. weszl, k. l. toth et al. acta polytechnica ctu proceedings figure 1. stereo microscopic images of titanium discs before and after etching in h3po4, in the compound of (cooh)2 × 2h2o and h2o2 and in 30% hcl. 3.1. chemical etching after turning, 54 pieces of ti grade 2 and 54 pieces of ti grade 5 discs were subjected to acid etching. we investigated the effect of three different acids: 30% hydrochloric acid (hcl), 85% phosphoric acid (h3po4), and the compound of 30% oxalic acid ((cooh)2 × 2h2o) and 30% hydrogen peroxide (h2o2). the etching intervals were 10, 600, 1800, 3600, 5400 and 7200 seconds at three temperatures: 20°c, 40°c and 60°c. 3.2. weight measurement the weight of the samples was measured (denver instrument apx-200, new york, united states) after each etching interval. the differences in the weight of the titanium discs measured before and after chemical etching were calculated and plotted. 3.3. microscopy stereo microscopic (olympus szx16, pennsylvania, united states) images were taken of the titanium discs after each etching interval in each experimental group in order to record the surface texture of the samples (figure 1). only those samples were further investigated by scanning electron microscopy (philips xl 30, zagreb, croatia) where the machining marks disappeared from the surface of the discs (see hcl row on figure 1). 3.4. roughness measurement five discs were selected from each experimental group and three measurements were performed on each hcl, 40°c, 1800 s hcl, 60°c, 1800 s 0.00 0.25 0.50 0.75 1.00 ti grade 2 ti grade 5 ti grade 2 ti grade 5 p ro fil e ro ug hn es s (r a) ± 9 5% c i before etching after etching figure 2. mean profile roughness values and their 95% cis for ti grade 2 and ti grade 5 discs before and after etching in 30% hcl at 40°c for 1800 seconds and at 60°c for the same duration. disc. profile roughness (ra) measurements (mitutoyo surftest 211) were performed along 3 radial lines in 120°increments from the outer edge towards the center of each disc. 4. results and discussion the stereo microscopic images showed that machining marks were visible on the surface of titanium discs with width of 15–20 µm (figure 1). after etching either in 85% of h3po4 or in the compound of 30% of (cooh)2 x 2h2o and 30% h2o2 the surface morphology of ti grade 2 and ti grade 5 discs was similar to the untreated surface meaning that the machining marks remained visible even after the 7200 second treatment (top and middle rows of figure 1). on the other hand, the etching in 30% hcl markedly reduced the unevenness of the surfaces both on the ti grade 2 and the ti grade 5 discs (bottom row on figure 1). thus, only hcl etched titanium discs were subjected to further investigations. concerning the treatment parameters, such as time and temperature, the ti grade 2 discs that were treated in hcl at 40°c for 1800 seconds and those that were treated in hcl at 60°c for 1800 seconds showed even surface patterns (figure 3). however, we found a considerable difference in the dissolution profiles of the ti discs treated at 40°c and 60°c. our findings show 44 vol. 3/2016 surface treatment and examination of titanium figure 3. sem images of grade 2 and grade 5 titanium discs before etching and after etching in hcl at 40°c and 60°c for 1800 seconds. ti grade 2 ti grade 5 0 2.5× 10−9 5 × 10−9 7.5× 10−9 1 × 10−8 20°c 40°c 60°c 20°c 40°c 60°c temperature of etching compound m as s di ss ol ve d pe r un it su rf ac e pe r un it tim e [g /m m 2/ s] etching compound hcl cooh h3po4 figure 4. mass dissolution per unit time of ti grade 2 and ti grade 5 in each etching compound. that increasing the temperature has a more pronounced effect on the mass reduction of ti grade 2 than the prolonged treatment time (figure 5). the same treatment parameters were found to be favorable to create even surfaces on ti grade 5 discs as on ti grade 2 discs. the etching of ti grade 5 in hcl at 40°c for 1800 seconds completely removed the machining marks from the surface of the discs (see figure 3). increasing the temperature from 40°c to 60°c resulted in densely pitted surfaces within the same (1800 seconds) treatment period (see figure 3). concerning the dissolution profiles of ti grade 5, increasing the temperature of the 30% hcl etchant resulted in more intense mass reduction of the ti grade 5 than ti grade 2 discs (figure 5). interestingly, higher temperatures increased the dissolution rate of both the ti grade 2 and ti grade 5 samples irrespective to the composition of the etchant at 60°c. ti grade 5 showed higher dissolution rates than ti grade 2 discs and the effect of increased temperature was higher this case than on pure titanium (figure 4). the profile roughness measurements did not unambiguously reflect what was observed visually. the roughness of both ti grade 2 and ti grade 5 showed slight tendencies to decrease when etched in 30% of hcl at 40°c for 1800 seconds and increase when etched in 30% of hcl at 60°c for the same duration, however neither changes were statistically significant. (figure 2) the direction of these changes can be explained by the removal of burr in the case of the lower temperature and by the appearance of pits in case of the higher, however we expected a more distinct effect such as those apparent in the optical tests. 5. conclusion our results show that the optical characterization of an etched titanium surface can be more informative concerning biological performance than the mechanical measurement of the surface roughness. the production of an oxide layer with pitted surface texture on a titanium implant in order to support its osseointegration may be achieved at the cost of considerable material loss depending on the chemical composition of the titanium. ti grade 5 shows higher dissolution rates 45 p. nagy, m. weszl, k. l. toth et al. acta polytechnica ctu proceedings hcl cooh h3po4 ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● 0.0e+00 2.5e−05 5.0e−05 7.5e−05 0.0e+00 2.5e−05 5.0e−05 7.5e−05 t i g rade 2 t i g rade 5 0 2000 4000 6000 0 2000 4000 6000 0 2000 4000 6000 etching time [s] d is so lv ed m as s pe r un it su rf ac e [g /m m 2] temperature ● 20°c 40°c 60°c figure 5. dissolution profiles of ti grade 2 and ti grade 5 discs in 85% h3po4, in the compound of 30% (cooh)2 × 2h2o and 30% h2o2 and 30% hcl at temperatures of 20°c, 40°c and 60°c. than ti grade 2 that might be attributed to the potential difference of its alloying compounds creating an electrochemical cell where the etchant also acts as an electrolyte. from a practical point of view, the uncontrolled etching may adversely affect the fine geometrical structures of dental implants, like the self-tapping part of the thread. blunted threads may cause undue mechanical stress to the bone tissue during insertion that may detrimentally affect osseointegration. our team plans to investigate the optimal combination of etching time and temperature in terms of material loss and surface roughness for various etchants and their effect on the fine geometry of dental implant screws. acknowledgements the present work was funded by the rea under the fp7 grant agreement no. 606624 (nanoti). references [1] m. long, h. rack. titanium alloys in total joint replacement a materials science perspective. biomaterials 19(18):1621 – 1639, 1998. doi:http://dx.doi.org/10.1016/s0142-9612(97)00146-4. [2] r. vayron, e. soffer, f. anagnostou, g. haiat. ultrasonic evaluation of dental implant osseointegration. journal of biomechanics 47(14):3562 – 3568, 2014. doi:http://dx.doi.org/10.1016/j.jbiomech.2014.07.011. [3] r. adell, u. lekholm, b. rockler, p.-i. branemark. a 15-year study of osseointegrated implants in the treatment of the edentulous jaw. international journal of oral surgery 10(6):387 – 416, 1981. doi:http://dx.doi.org/10.1016/s0300-9785(81)80077-4. [4] l. l. guehennec, a. soueidan, p. layrolle, y. amouriq. surface treatments of titanium dental implants for rapid osseointegration. dental materials 23(7):844 – 854, 2007. doi:http://dx.doi.org/10.1016/j.dental.2006.06.025. [5] y. shibata, y. tanimoto. a review of improved fixation methods for dental implants. part i: surface optimization for rapid osseointegration. journal of prosthodontic research 59(1):20 – 33, 2015. doi:http://dx.doi.org/10.1016/j.jpor.2014.11.007. [6] m. kulkarni, a. mazare, e. gongadze, et al. titanium nanostructures for biomedical applications. nanotechnology 26(6):062002, 2015. [7] p. g. coelho, r. jimbo, n. tovar, e. a. bonfante. osseointegration: hierarchical designing encompassing the macrometer, micrometer, and nanometer length scales. dental materials 31(1):37 – 52, 2015. doi:http://dx.doi.org/10.1016/j.dental.2014.10.007. [8] u. diebold. the surface science of titanium dioxide. surface science reports 48(5-8):53 – 229, 2003. doi:http://dx.doi.org/10.1016/s0167-5729(02)00100-0. [9] j. w. choi, s. j. heo, j. y. koak, et al. biological responses of anodized titanium implants under different current voltages. journal of oral rehabilitation 33(12):889–897, 2006. doi:10.1111/j.1365-2842.2006.01669.x. [10] y. iwaya, m. machigashira, k. kanbara, et al. surface properties and biocompatibility of acid-etched titanium. dental materials journal 27(3):415–421, 2008. doi:10.4012/dmj.27.415. 46 http://dx.doi.org/http://dx.doi.org/10.1016/s0142-9612(97)00146-4 http://dx.doi.org/http://dx.doi.org/10.1016/j.jbiomech.2014.07.011 http://dx.doi.org/http://dx.doi.org/10.1016/s0300-9785(81)80077-4 http://dx.doi.org/http://dx.doi.org/10.1016/j.dental.2006.06.025 http://dx.doi.org/http://dx.doi.org/10.1016/j.jpor.2014.11.007 http://dx.doi.org/http://dx.doi.org/10.1016/j.dental.2014.10.007 http://dx.doi.org/http://dx.doi.org/10.1016/s0167-5729(02)00100-0 http://dx.doi.org/10.1111/j.1365-2842.2006.01669.x http://dx.doi.org/10.4012/dmj.27.415 acta polytechnica ctu proceedings 3:43–46, 2016 1 introduction 2 aim of experiment 3 experiment 3.1 chemical etching 3.2 weight measurement 3.3 microscopy 3.4 roughness measurement 4 results and discussion 5 conclusion acknowledgements references 148 acta polytechnica ctu proceedings 2(1): 148–151, 2015 148 doi: 10.14311/app.2015.02.0148 ss cygni revisited r. c. smith1, j. echevarŕıa2, j. v. hernandez2,3, p. szkody4 1astronomy centre, university of sussex, uk 2unam, mexico city, mexico 3physics & astronomy department, university of southampton, uk 4astronomy department, university of washington, usa corresponding author: r.c.smith@sussex.ac.uk abstract new spectroscopic and photometric observations of ss cygni, the brightest dwarf nova system, have been obtained, with the aim of mapping starspots on the surface of the secondary star. four nights of echelle spectroscopy in quiescence have been obtained using the 2.2-m telescope at san pedro martir (mexico) in august 2012 and another two nights at the 3.5-m telescope at apache point observatory, usa, in september 2012, but these data are still being reduced. simultaneous ccd photometry was also obtained at the two sites, and the mexican photometry was extended into the subsequent long outburst. this presentation reveals some interesting photometric behaviour in that outburst, but further data will be necessary before the nature of the behaviour can be determined. keywords: cataclysmic variables dwarf novae optical spectroscopy photometry individual: ss cyg. 1 introduction the canonical model of cataclysmic variable (cv) evolution requires secondaries to be magnetic, to allow magnetic braking to keep the star in contact with its roche lobe. magnetic activity should produce starspots, as shown by observations of rapidly rotating single stars. detection of starspots on cv secondaries provides evidence for magnetic fields, and roche tomography has been used to reveal spots on four systems: ae aqr (watson et al. 2006), bv cen, v426 oph (watson et al. 2007a, 2007b) and ru peg (dunford et al. 2012). the unusual nova-like system ae aqr is the brightest of these, and the others are at least two magnitudes fainter. ss cygni is the brightest dwarf nova, and in quiescence is only half a magnitude fainter than ae aqr. it is therefore an obvious candidate for study and highresolution echelle data were obtained in 2012 august (in mexico) and september (in the usa), with simultaneous photometry. the mexican echelle data were taken with the 2.2-m telescope at the san pedro martir (spm) observatory in baja california; the v photometry was taken with the spm 0.84-m telescope. the us echelle spectra were taken with the 3.5-m telescope at the apache point observatory (apo); simultaneous photometry was also obtained, using the nmsu 1-m telescope at apo (v ) and the uw 0.76-m at manastash ridge observatory (g, r, i). 2 spectra the us spectra were taken on two nights with the apo 3.5-m and have been fully reduced but not yet analysed. in figure 1 we show part of the average spectrum from the first night; the region shortward of 6750 å makes it clear that there are many absorption lines that can be used for roche tomography. four nights of data were taken with the spm 2.2-m; these spectra are still being reduced. figure 1: a portion of the average of 29 spectra of ss cygni taken with the echelle spectrograph on the 3.5-m apo on 20/21 september 2012. in addition to strong emission lines, there are many absorption lines suitable for roche tomography of the secondary. 148 http://dx.doi.org/10.14311/app.2015.02.0148 ss cygni revisited 3 photometry all the photometry has been fully reduced; here we just discuss the analysis of the mexican data. at the san pedro martir (spm) observatory in mexico, simultaneous v photometry with the 0.84-m telescope was taken to enable flux calibration of the echelle spectra being taken with the 2.2-m telescope. by good fortune, ss cygni went into outburst one day after the spectra had been taken, and our mexican colleagues kindly allowed us to continue photometric observing; the complete coverage is shown in figure 2. unfortunately, no useful data were obtained on the fifth night in outburst, which was clouded out; however, the mean level on that night does appear to be higher than on the previous nights, possibly lending support to the prediction of cannizzo (2013; see also cannizzo 2012) that long outbursts should have a precursor outburst at the beginning. subsequent aavso data confirmed that this was a long outburst. figure 2: v photometry of ss cygni in quiescence, rise and outburst, august 2012. we first analysed the quiescent data, looking for the dominant period. as expected, the only significant period to be found was the orbital period; a light curve folded on that period showed the characteristic double hump arising from ellipsoidal variation. there were four nights of useable outburst data. the data were first detrended, by removing linear trends from each individual night and then adding or subtracting suitable constants to bring all the nights to the same average magnitude. the starlink package period was used to search for the dominant period, using five different methods: string-length, minimum chisquared, lomb-scargle, fourier transform and clean. some of these methods produced no very useful results, and the clearest and most consistent results were obtained with the minimum chi-squared and lombscargle methods. the results below quote only the results from those two methods. figure 3: night 3 outburst data folded on a period of 0.35273 days and binned into 400 bins. note that the magnitude scale has brightness increasing downwards. analysing the entire data-set gave a best-fit period that was about 1.5% smaller than the orbital period, but the signal was not strong and this result is not thought to be significant. however, when the different nights were analysed separately a very different pattern emerged: each night had a different dominant period. the clearest result was for the 3rd night in outburst, where the two methods agreed on a period of 0.353 days. this is approaching the length of the data stream on that night (0.384 days), but is significantly shorter, and there is no sign of the data length in the period analysis. this period is also significantly longer than the orbital period of 0.27513 days. figure 3 shows the night 3 data binned and folded on a period of 0.35273 d. the best period on night 2 was close to the orbital period, with minimum chi-squared giving 0.2715 d and lomb-scargle giving 0.2769 d. however, the other two nights both gave significantly shorter best periods: 0.1433 d on night 1 and 0.1963 d on night 4. the lombscargle plots for all the nights are shown in figure 4 – note that the maximum power varies from night to night. thus, apart from night 2, the outburst data are not consistent with the orbital period, with two shorter periods and one longer one. there is a temptation to consider the night 3 data as evidence for a positive superhump – boneva et al. (2009) suggested that in outburst ss cyg has an elliptical disc. if so, ss cyg is not impossibly far off the standard psh, porb relation, and the orbital period of 6.6 h would be the longest on that relation. however, with a well-determined mass ratio q of 0.683 (bitner at al. 2007), it strongly violates the normal resonance criterion of q < 0.33. 149 r. c. smith et al. figure 4: lomb-scargle power plots for the four outburst nights. by far the strongest signal is on night 3, with a maximum power of ∼350. the maximum power on the other nights are: ∼50 (night 1), ∼200 (night 2) and ∼100 (night 4). the dominant periods on nights 1 to 4 are 0.1433 d, 0.27-0.28 d, 0.35273 d and 0.1963 d respectively. the horizontal axis is frequency, in cycles/day. furthermore, the amplitude is low (∼0.08 mag), the superhump excess (28%) is rather large, and two of the other nights suggest negative superhumps (although these have even larger differences from the orbital period, at 48% and 29% smaller, and have similarly small amplitudes). is there a better explanation? one possibility was raised by bisikalo (2013; see also zhilkin & bisikalo 2010), who showed models where fluctuations in the accretion rate onto the white dwarf, caused by variations in the generation of magnetic field in the disc, produced brightness variations on various timescales. perhaps we are seeing evidence in our data for similar brightness variations, which might be stochastic in nature. 4 conclusions the spectral data we have obtained for ss cygni appear to be good enough for us to be able to map starspots on the surface of the secondary component. the photometric data in quiescence appear to show ellipsoidal variations on the orbital period. however, the photometric data in early outburst are generally not consistent with the orbital period. the data from the 1st and 4th nights have shorter periods (negative superhumps?) while the data from the 3rd night show a longer period (a positive superhump in the longest-period system so far?). alternatively, we may have found evidence for variations in the accretion rate onto the white dwarf, caused by magnetic effects in the disc. whatever the explanation, it would be worth monitoring ss cygni intensively during its long outbursts, to see whether this peculiar behaviour is repeated. it turns out that the kind of photometric data we obtained during the long outburst in august 2012 is rare: not many people have carried out time-resolved observations of the long outbursts in dwarf novae and so the nature of the behaviour during these outbursts is still 150 ss cygni revisited quite uncertain (cf. cannizzo 2012, 2013). in order to test whether any changes are periodic or stochastic, it will be necessary in future to monitor as many of these long outbursts as possible, in ss cygni and in similar dwarf novae. it is hoped to do this by organising an international campaign involving both the amateur community and the many robotic telescopes scattered around the world. by the time these data are available, it is hoped that predictions of the magnetic effects in the disc, by bisikalo and others (e.g. bisikalo 2013), will have reached the stage where they may be compared in detail with the data to discover whether the model is compatible with observations. acknowledgement we thank leslie hebb for her painstaking reduction of the apo spectra, our mexican colleagues for allowing us to observe ss cyg in outburst, phil charles for comments on the outburst photometry and tom marsh for help with software. we acknowledge the use of the period program from the starlink package, supported at the joint astronomy centre in hawaii, and of the aavso light curve generator. references [1] bisikalo, d.: 2013, these proceedings. [2] bitner, m.a., robinson, e.l., behr, b.b.: 2007, apj 662, 564. doi:10.1086/517496 [3] boneva, d. et al.: 2009, astronomy reports 53, 1004. doi:10.1134/s1063772909110055 [4] cannizzo, j.k.: 2012, apj 757, 174. doi:10.1088/0004-637x/757/2/174 [5] cannizzo, j.k.: 2013, these proceedings. [6] dunford, a., watson, c.a., smith, r.c.: 2012, mnras 422, 3444. [7] watson, c.a., dhillon, v.s., shahbaz, t.: 2006, mnras 368,637. [8] watson, c.a., et al.: 2007a, an 328, 813. [9] watson, c.a., et al.: 2007b, mnras 382, 1105. doi:10.1111/j.1365-2966.2007.12173.x [10] zhilkin, a.g., bisikalo, d.v.: 2010, astronomy reports 54, 840. doi:10.1134/s1063772910090088 discussion david buckley: i am puzzled by the superhump excess ε for tx col. is there a reference for the superhump period? robert smith: i took the superhump period and excess for tx col from montgomery (2009, apj, 705, 603), who quoted retter et al. (2005, assl, 332, 251). the retter et al. conference paper is suggestive but not definitive. christian knigge: since you typically see only about one cycle of your periods in each epoch, is it not possible that the variability is stochastic rather than periodic? robert smith: in some cases we have two cycles. but more generally, yes, to call them periodic changes is a bit speculative. however, there is a clear single peak in the lomb-scargle power spectrum on each night, and they are certainly at different frequencies on different nights. the night-to-night changes may be stochastic, as you suggest, but there is definitely something interesting happening. 151 http://dx.doi.org/10.1086/517496 http://dx.doi.org/10.1134/s1063772909110055 http://dx.doi.org/10.1088/0004-637x/757/2/174 http://dx.doi.org/10.1111/j.1365-2966.2007.12173.x http://dx.doi.org/10.1134/s1063772910090088 introduction spectra photometry conclusions 326 acta polytechnica ctu proceedings 1(1): 326–328, 2014 326 doi: 10.14311/app.2014.01.0326 some personal conclusions sergio colafrancesco1 1university of the witwatersrand, johannesburg (south africa) corresponding author: sergio colafrancesco. email: sergio.colafrancesco@wits.ac.za abstract i present here some personal considerations on the main theoretical, observational and technological challenges offered to the discussion during the meeting ”multifrequency behaviour of high energy cosmic sources”. 1 the theme this meeting presented a wide collection of excellent results on a wide range of subjects in modern multifrequency astrophysics. i am not able to remember a single talk that did not produce a vibrant discussion in and out of the audience. beyond any attempt to summarize the outcomes of the meeting, it seems more appropriate for me to briefly present here some personal considerations on the main challenges offered to the discussion during this meeting. 2 the challenges one of the main aspects of this meeting is its capacity to induce global discussions on some of the most important challenges in astrophysics and cosmology. these challenges address the nature of fundamental questions in the physics of the universe, the frontier of exa-scale data mining and analysis and the technological challenges related to the construction of the largest astronomical facilities in the next decade. 2.1 theoretical challenges: fundamental questions big theoretical challenges regard two important aspects of the structure and evolution of our universe: the cosmic origins and the cosmic extremes. origins. beyond the successes of the standard cosmological scenario, we are still facing some crucial questions: what happened at the beginning of the universe? inflation and precision cosmology. there’s been incredible progress recently in finding the traces that inflation left behind and upcoming experiments on cmb polarization promise to provide even more evidence of what happened during the universe’s infancy. through its sensitivity to gravitational waves, the study of the cmb provides a glimpse into the state of the universe just ∼ 10−35 seconds after the beginning and of physics on grand-unification-theory (gut) energy scales around ∼ 1016 gev, some ≈ 13 orders of magnitude above the energies achievable by current terrestrial particle accelerators. a gravitational-wave background in the early universe would leave a unique, oddparity pattern of polarization in the cmb (b-modes), the magnitude of which is characterized by the tensorto-scalar ratio r. a gwb is generically predicted to exist by inflationary theories, and the current generation of cmb polarization experiments will probe the interesting parameter space of r ∼< 0.05 corresponding to single-field inflationary models at gut scales. the challenges here seem to be a statistically confident measurements of b-modes and the imperative study of the systematics that are present in such a difficult measurement. a multi-frequency study of b-modes with extremely high sensitivity, over a wide range of frequencies and large sky areas would likely be able to measure in the not-so-distant future the value of r with a ∼ 10% precision and will open the field to a deep theoretical exploration of inflationary cosmologies. what is the dark matter? this is a particularly exciting time for dark matter study because there are some intriguing clues pointing to where dark matter particles might be hiding. these clues are helping researchers develop a variety of searches. the three major strategies are direct detection, collider production and indirect detection. while there is a mounting scientific frustration in the non conclusive evidence for the detection of dm particles with direct and collider detection techniques, the sensitivity and the specific strategy of the next coming astronomical observatories (mainly the cta and the ska) will probably allow in the next decade to shed a conclusive word on the nature of dm, or close a large fraction of the dm parameter space and thus open the way to a more detailed exploration of realistic alternatives of the theory of gravity. 326 http://dx.doi.org/10.14311/app.2014.01.0326 some personal conclusions what makes up the rest of the universe? dark energy. in the past 15 years or so, scientists have realized that the ”stuff” making up all the atoms in all the galaxies, stars, planets, and humans we have ever observed only constitutes about 5% of the universe. while we might be close to pinning down the nature of part (≈ 27%) of the missing stuff (e.g., dark matter), what we know about the dominant (≈ 68%) component of the universe, (named historically dark energy for a vague analogy with dm) is still almost nothing. observational efforts are planned (e.g., euclid, ska, des, lsst, etc.) and theoretical exercises to narrow down the available de parameter space are the subject of a restless activity, but a detailed physical characterization of the ”dark energy” is still missing and probably this theoretical activity will be very relevant in pointing at the true nature of the physical mechanism providing the late-stage cosmic acceleration. extremes. where did that come from? cosmic rays and intergalactic particle accelerators. after a century of study, researchers still struggle to understand the origin of cosmic rays, and especially those of ultra high energy. we believe these extreme high-energy oddities play a key role influencing the physics and chemistry that form stars and planets, and even influence life on earth by occasionally causing mutations in dna. and yet, the exact ways in which cosmic rays are accelerated remains a major open question. we’ve discovered where many come from within our galaxy, but the most extreme cosmic rays continue to confound us. the need of a very large collecting area experiment for the extremely rare uhecr events poses several observational challenges, but its delivery will probably open the way to a physical clarification of their origin and of the production sites. challenges to particle acceleration theories in cosmic structures are stronger and stronger and the scientific qualification of the possible product sites is reducing more and more towards regions of compact objects with extremely strong gravitational fields and magnetic fields. what can compact objects teach us? black holes and neutron stars, extreme physics in small packages. enormously powerful gravitational fields that warp the local fabric of space and time. incomparably strong magnetic fields that can stretch atoms themselves into long spindles. materials so dense a teaspoonful would weigh billions of tons. these are just some of the exotic properties of compact objects, a catch-all term for several types of unbelievably dense and remarkable objects, like white dwarfs, neutron stars and black holes. compact objects are known to possess some of the most extreme physical properties ever observed. scattered throughout our galaxy and agns, these objects serve as astrophysical laboratories that test the very limits of physics as we know it. 2.2 observational: big data science it is now becoming clear that the answers to big questions in astrophysics and cosmology requires to address the challenge of dealing with big data quantities, analysis and transport. how will we make sense of it all? astronomically big data. astrophysics and cosmology deal with big everything: big datasets, big simulations and big collaborations. we have information on billions of astronomical objects, and expect to make measurements of many billions more in the next decade. yet the challenge with such a large dataset is not so much in handling its size, but in its complexity. the struggle in trying to find a single rare star in a haystack of billions of nearidentical stars, or understanding the relationships between every single galaxy in the universe, goes beyond simply the enormous number of gigabytes. as more and more data piles up, the teams who are most innovative about analyzing and combining those datasets will be the ones who will likely make the biggest discoveries. instruments like the ska will fully live into the era of big data complexity and will have to address not just the complex techniques of data reduction and analysis, but also the challenges of data distribution and transport over inter-continental distances and yet with high computing capacity on our everyday desktop computer. the next generation of astronomical surveys (and even single observations) will contain information that spans multiple frequencies, and many epochs in time. however there is not yet a next generation of catalog than can describe these surveys. a more meaningful description of catalog data will ultimately lead to better comparisons of astronomical surveys and more robust scientific output. it is not unreasonable that a stratification of various computational -tasks and a possible -re-definition -of -the -data -analysis -objectives and -techniques -will -be -(re-)considered in the light of the experimental and theoretical challenges of the next decade big astronomy science. in this context, small or mid-size multifrequency data centers will probably play a major role in developing innovative applications for (astronomical) data science and producing cutting edge scientific exploitation of the data achieved by large astronomical facilities. 2.3 experimental: big facilities the future large astronomical projects will live inherently in the era of multifrequency astronomy. examples of this synergy are provided by the ska, the cta, the lsst, and the largest future space missions like e.g. 327 sergio colafrancesco euclid, millimetron. the wealth of planed astronomy projects is such that it seems that the future of astronomy can only be limited by the financial constraints of international funding available. both ground-based and space-borne experiments are facing challenges specific to their different nature. some of these are technical and will give rise to extensive technological developments and innovations. some other are programmatic and are related to the complex and ambitious nature of future projects. it is not unlikely that the complementarity between large-scale projects and pathfinders will provide the scientific community the clarity on the technological routes to be followed for the construction of the most successful and challenging astronomical facilities of the next decade, as well as the directions in the optimal integration of these facilities in the most successful multifrequency strategy. this seems to be mandatory in order to answer the remaining big questions on the nature and the evolution of our universe. 3 predictions based on the decennial successes of this meeting, as also testified by this specific one, it is not difficult to predict that the next meeting of this series will provide an even more extended discussion on the major challenges of multifrequency astrophysics, cosmology and astro-particle physics in the next future. acknowledgments s.c. acknowledges support by the south african research chairs initiative of the department of science and technology and national research foundation and by the square kilometre array (ska). 328 the theme the challenges theoretical challenges: fundamental questions observational: big data science experimental: big facilities predictions acta polytechnica ctu proceedings doi:10.14311/app.2017.10.0016 acta polytechnica ctu proceedings 10:16–33, 2017 © czech technical university in prague, 2017 available online at http://ojs.cvut.cz/ojs/index.php/app testing methods for mechanically treated soils: reliability and validity ana petkovšek∗, matej maček, jasna smolar university of ljubljana, faculty of civil and geodetic engineering, jamova 2, ljubljana, slovenia ∗ corresponding author: ana.petkovsek@fgg.uni-lj.si abstract. a possibility of in-situ mechanical improvement for reducing the liquefaction potential of silty sands was investigated by using three different techniques: vibratory roller compaction, rapid impact compaction (ric) and soil mixing. material properties at all test sites were investigated before and after improvement with the laboratory and the in situ tests (cpt, sdmt, dpsh b, static and dynamic load plate test, geohydraulic tests). correlation between the results obtained by different test methods gave inconclusive answers. keywords: liquefaction, experimental site, rapid impact compaction, soil mixing. 1. introduction the term ground improvement refers to any technique or process that improves the engineering properties of the treated soil mass in order to fulfil the purpose intended. usually, the modified properties are strength, stiffness, permeability, workability (willingness for compaction), resistance to frost, etc. [1–4]. the type of ground improvement/soil treatment depends on the demands of the structure to be built, the soil type, the available materials and tools, and the local tradition and experience. the efficiency of a specific improvement technique may be measured by using different testings and instrumentation. whenever a laboratory or field test/measuring device is used as part of the data collection process, the validity and the reliability of the test/device is important. just as we would not use a test for the determination of particle size distribution to assess the activity of clay, we would not want to use any measuring device not capable of measuring properties which are of the design or qa – qc interest. this paper focuses on the validity and sensitivity of the laboratory and field tests used to assess the efficiency of the mechanical improvement of siltysandy soils. special attention is given to the use of kozeny-carman equation for the estimation of the soil permeability. analysis presented in the paper refers to the investigation performed at the location of hydropower plant (hpp) brežice, se slovenia, where the top layer of the foundation ground indicates properties of liquefiable soil. the main goal of the ground improvement is to increase the soil liquefaction resistance and to assure the required permeability of the fill material to be used for app. 14 km long dikes along the hpp reservoir. 2. background 2.1. site description the reservoir of the hpp brežice requires the construction of app. 7 km long and up to 10 m high dikes on both sides of the river sava. the top layer of the foundation ground consists of loose silty sands, prone to liquefaction. hpp brežice is the 5th in the chain of the lower sava river hpps (figure 1) and is situated in the influential area of nucler power plant krško (npp). tectonically, the wider area belongs to the tectonic unit named “krško basin”, which is considered a potentially active structure [5]. a well-documented earthquake with a magnitude of 5.7 and intensity of viii ems occurred in january 1917 [6]. the so called “zagreb earthquake” in 1880 triggered liquefaction along the river bank between brežice and zagreb (croatia) [7, 8]. the design ground accelerations in the krško basin for different earthquake return periods are presented in table 1. 2.2. geological profile the geological profile is quite uniform over the entire area of the hpp (figure 2). the top layer is up to 5 m thick, made of very loose sandy silts and sands (ml, sm, sp), deposited by recent floods. the top soil layer contains fine coal particles that origin from the flotation processes from the area around 60 km upstream where the coal mines have operated for the last 300 years. coal particles confirm the very young origin of the sediments. poplar tree roots appear up to 4 m deep and also prove the development of the “paleo” ground between the larger flood events. beneath the top layer the medium dense to dense quaternary gravels are deposited. the gravel is classified as silty sandy gravel (gm) to poorly graded gravel (gp) and consists mainly of limestone grains. 16 http://dx.doi.org/10.14311/app.2017.10.0016 http://ojs.cvut.cz/ojs/index.php/app vol. 10/2017 testing methods for mechanically treated soils table 1. design ground accelerations for different earthquake return periods [9–11] figure 1. location of the hpp brežice. the thickness of gravel deposits varies between 8 and 12 m. the pre-quaternary bedrock below the gravel consist of miocene sediments, represented by overconsolidated, not cemented silts and marls, with some inclusions of the weak, soft limestones. the ground water (gw) level is controlled by the sava river. during most of the year, the gw level appears app. 4 – 6 m below the ground surface, inside the gravel layer. during the floods, the gw may rise up to 2 m above the ground and may stay there for days. when the hpp brežice is put into operation, the existing ground surface and silty/sandy layers will be permanently flooded in the area between the dikes on the left and the right river bank. the permanent rise of the gw level is forecast also at the influential area from brežice to krško, on the dry sides of the dikes. 2.3. soil properties 2.3.1. index properties the grain size distribution of the quaternary soils is not uniform. in the top layer they may vary from sandy silt to poorly graded sand (figure 3). according to empirical data [12, 13], the top layer soils belong to the groups of most liquefiable and potentially liquefiable soils. index properties of typical soils from the top layer are given in table 2. laboratory proctor compaction tests show that due to the presence of coal particles, unfavourable grading and uneven water content distribution, the top layer soils willingness to compaction is poor. different compaction curves (figure 4) confirm heterogeneity of different strata, deposited inside the top layer. 2.3.2. liquefaction potential the spt testsperformed in top soil layer shown the modified blow count (n1)60 between 2 and 8. seed and idriss [14] proposed an empirical method for assessing 17 a. petkovšek, m. maček, j. smolar acta polytechnica ctu proceedings figure 2. schematic geological profile (left) and schematic cross section of the valley after the hpp construction (right). not in scale. figure 3. relationship between grading and liquefaction potential [13], with the boundaries of the grading of top layer and quaternary gravel at the hpp brežice. grading curves *nova loza and *moj dvor, reported by veinović et al. [7] belong to the area, where liquefaction appeared in 1880. figure 4. proctor curves for three typical groups of soils from the top layer. 18 vol. 10/2017 testing methods for mechanically treated soils table 2. properties of three typical groups of soils from the top layer. the likelihood of liquefaction of level sites (figure 5, left). the cyclic stress ratio (csr) is calculated from eq.1 [9]. (csr)m=7,5 = csr msf = β ( σv0 · amax σ′v0 ) · rd msf (1) msf = 6.9 exp (−m 4 ) − 0.058 msf ≤ 1.8 (2) where β is a ratio between real maximum and equivalent seismic stresses (0.65) [15], amax is the maximum ground surface acceleration on the foundation ground, rd is stress reduction factor that drops from 1 at the surface to about 0.9 at 10 m depth. msf is a factor to account for the varying number of cycles associated with different magnitude events and has values shown in table 1. the seismic dilatometer tests (sdmt) in top layers gave the shear wave velocities (vs) of 160-300 m/s, while the seismic refraction test indicates the vs of 90-110 m/s. seed and idris [14], youd et al. [16] and andrus [17] proposed an empirical relation for assessing the likelihood of liquefaction of level sites, based on relationship between the csr and vs (figure 5, right). the cyclic simple shear test (csst) was used to study the influence of the density on the liquefaction potential (figures 6,7). the tests were carried out on soil samples with properties given in table 2. specimens were prepared at different initial void ratios, saturated, consolidated at 75 kpa of effective stress and then loaded with horizontal sinusoidal load at desired stress ratio in undrained condition. the beginning of liquefaction was defined when the increase of pore water pressure was equal to 95 % of effective vertical stress before cyclic loading. the reference number of cycles was defined after kramer [18], for an earthquake of magnitude 7.5. results of the laboratory tests are given in figure 7 and confirm that the top layer soil are prone to liquefaction. the compacted soil samples at 95 % of maximum proctor density (dpr) will very probably resist the 475 year return period earthquakes and will very probably liquefy at stronger earthquakes. 2.4. preliminary conclusions the grain size distribution, the spt, the seismic refraction as well as csst indicate the same ranges of liquefaction potential and can be estimated as valid and reliable. the test results can also be confirmed by the historical data from the 1880’ zagreb earthquake. the sdmt test gave the shear wave velocities (vs) in a range of 160 – 300 m/s. comparing the sdmt test results with other tests as well as with calculation, using the empirical equation 3 and 4, explained in chapter 4, we may conclude, that the sdmt test values are too high for the top soil layer probably affected by field site heterogeneity or by the influence of other men activities. based on other tests the conclusions are clear: the top layer represents a very unfavourable foundation ground and must be improved or removed from the influential area of the dikes. 3. discussion on improvement technique several options were discussed to improve the liquefaction resistance of the foundation ground, as follows: • loose material from the top layer shall be removed and replaced with compacted gravel material. the idea was recognized as the most promising and reliable choice as large quantities of good quality gravel material are available at the location. however, due to the large volumes of top soil to be replaced (length of app. 14 km, depth up to 5 m, width that locally exceeded 50 m) the decision was made to investigate other options too. 19 a. petkovšek, m. maček, j. smolar acta polytechnica ctu proceedings figure 5. relationship between cyclic stress ratios causing liquefaction and (n1)60 (after seed and idriss [14]) and the position of the top soil in it (left). relationship between cyclic stress ratios causing liquefaction and shear wave velocity values (after seed and idris [14], youd et al. [16] and andrus [17]) and the position of the top soil in it (right). bc – before compaction, fc – fines content, ts – top soil layer. figure 6. the seiken cyclic simple shear apparatus, used for the laboratory tests (left) and the cell (right). figure 7. the relationship between the number of cycles needed for liquefaction and the cyclic stress ratio (csr) for top soils at different void ratios. 20 vol. 10/2017 testing methods for mechanically treated soils • option 1 (es 1): static and vibratory roller compaction. the main attention was paid to the following questions: (a) is the 95 % of max. proctor density attained on site, (b) what is the effective depth of the roller compaction and (c) what are the potentially harmful effects of coal particles and roots to the soil compaction behaviour? the four test fields were made to study the compaction efficiency of the foundation ground and up to 2.5 m high test embankment was constructed to study the roller compaction efficiency above the soft subgrade, the vertical and horizontal movements, the internal erosion and to analyse the sensitivity of different test methods, like cone penetration test (cpt), flat dilatometer test (dmt, sdmt), dynamic probing super heavy (dpsh b), etc. the test field was made in september and october 2009. • option 2 (es 2): rapid impact compaction (ric). to avoid the shortcomings of the shallow effects of roller compaction, the ric technique was proposed. two test sites were made in february 2011. a comprehensive investigation was made on site before and after compaction [19, 20]. • option 3 (es 3): soil mixing on site to achieve liquefaction resistance, low permeability and better willingness to compaction. the idea was to excavate top soil and mix it with quaternary gravel in such proportion that it would fulfil three criteria: (a) resistance against liquefaction, (b) the designed permeability to eliminate the need for additional sealing of the dike and (c) improved willingness to compaction with reduced influence of seasonal water content changes on stiffness. the trial tests were made in april and may 2014, and the roller with the roller integrated continuous compaction control (ccc) was used for compaction. as the trial fields showed promising results, the temporary, app. 1.2 km long dike was constructed in order to protect the deep excavation for the powerhouse against the floods, as well as to get more data about the homogeneity, permeability and the resistance against liquefaction of the dike. figure 8 shows the locations of the field trial tests es 1 to es 3 and the temporary dike. 4. results from the experimental sites 4.1. experimental site es 1: roller compaction trial field 4.1.1. site description the soil profile and the soil properties were the same as presented in figure 2 and table 2. the thickness of the top layer was 4 m, the roots appeared in dense network to the depth up to 3.5 m. figure 9 shows the depth of the roots (left) and the size of the first of four test fields prepared to study the roller compaction efficiency of the foundation ground – subgrade (right). figure figure 8. location of the field trial tests: es 1: roller compaction; es 2: ric; es 3 and temporary dike: soil mixing. 10 shows the test embankment (left), constructed by using the silty sands from the top layer and the amount of coal particles (right) that floated up after the layer wetting. during compaction different modes of roller operation, layer thickness, drying and wetting of compacted layers were studied. the efficiency of the roller compaction was measured by using the nuclear gauge and by the calibrated cylinders to measure the water content and the density. stiffness of compacted layers was measured by static and dynamic plate load test. undisturbed samples were taken to check the liquefaction potential in the csst. the spt, cpt and the sdmt tests, planned to be realised at the test sites, were not possible, as the flood destroyed the trial sites soon after construction. 4.1.2. results and discussion at the foundation ground, the roller compaction was found not effective due to the influence of soft sublayers, coal particles and roots. the average achieved dp r, expressed as ratio ρd/ρd,max, was 88 % and the average stiffness, measured by the dynamic load plate test (zorn) was evd < 7 mpa. the efficiency of compaction on embankment layers strongly depended on the layer position above the ground level, as the influence of the soft subgrade decreases with the embankment height. at the level of 2 m above the subgrade, the average compaction of dp r = 94 % was achieved (figure 11), while the average stiffness of the embankment layers remained at the same average ranges as measured on the subgrade, evd = 5 − 10 mpa. figure 12 compares the results of dry densities (left) and water content (right), achieved by using the nuclear gauge and the calibrated cylinders. the nuclear gauge gives significantly higher values of water content, which can be explained by the presence of coal particles and the nuclear gauge water content mea21 a. petkovšek, m. maček, j. smolar acta polytechnica ctu proceedings figure 9. depth of the roots (left) and the size of the first of four test fields prepared to study the roller compaction efficiency (right). figure 10. finished test embankment (left) and the amount of coal particles (right). suring principles. figure 7 (red diamonds) shows the csst results on the undisturbed samples from the test field. the data conform well to the preliminary forecast. 4.1.3. experimental site es 1 conclusions with the use of roller compaction, it is not possible to improve the top soil layer in order to reduce its liquefaction potential. the dynamic and static load plate test results strongly depend on saturation/water content and therefore do not measure parameters, which are decisive for the intended purpose. 4.2. experimental site es 2: rapid impact compaction (ric) test fields 4.2.1. background the hpp brežice test sites represented the first attempt to introduce the ric in slovenia. the detailed description of the ric test site is given in vukadin [20]. to highlight the importance of the test method validity and reliability, we will focus our discussion on the results of the dmt and sdmt tests, not reported in vukadin [20], as well as on the results of the cpt and dpsh b test, interpreted from a standpoint differing from that of vukadin [20]. 4.2.2. ric test site description the locations of the test sites are shown in figure 8, while a view to the ric during operation and the test site layout is given on figure 13. rough dimensions of each test field were 18 × 25 m. before the ric was put in operation, the vegetation layer of app. 1 m had been removed from the test site. the thickness of the top layer was from 3.8 to 4.0 m, with the fines content between 21 and 66 %, water content between 16 and 21 % and the dry densities of 1.2 – 1.6 t/m3 [20]. the gw table was in the quaternary gravel. at each test field, a comprehensive program of field tests (cpt, dmt, sdmt, dpsh b and dynamic cone penetrometer (dcp) panda) was performed before and after compaction. the filed measurements were complemented by samplings from trial pits and laboratory tests. each test field was divided in 4 sections (p1 to p4, figure 13) in order to study the influence of different compaction modes on the compaction efficiency. the compaction point raster density increased from section p1 to section p4 and the compaction was executed in one, two or three passes. after each compaction pass the ground was levelled by filling the holes created during compaction with the locally available sand. the decision about the end of the compaction at each point was made considering two criteria: (a) the accumulated total settlement per pas of 800 mm, or (b) the minimal depth of settlement per blow less than 10 mm. the average energy per unit cell area was 800 (p1), 1160 (p2), 1000 (p3), and 1230 kj/m2 (p4). 22 vol. 10/2017 testing methods for mechanically treated soils figure 11. comparison of standard proctor test curve (spp) and dry density achieved during trial embankment compaction. figure 12. comparison between nuclear gauge and laboratory measurements of water content and dry density. 4.2.3. results and discussion the efficiency of ric was controlled by using different field test methods. this paper will focus primarily on the interpretation of the cpt, sdmt and dpsh b results. cpt test figure 14 shows the soil behaviour type (sbtn), the normalized cone resistance (qtn) and the sleeve friction resistance (fs) evaluated from the cpt test. the qtn is a non-dimensional parameter taking into account the in-situ vertical effective stress and the soil type and is not depth dependent. the fs depends on friction between soil and metal sleeve and horizontal effective stress. the horizontal stress increases with vertical effective stress (σ′v) and could be calculated by multiplying σ′v with horizontal stress coefficient during the cpt testing. the influence of compaction on the fs is given in two diagrams, as the relationship fs – depth and fs/σ′v depth. the fs/σ′v diagram presents the increase of horizontal stress coefficient during the cpt testing due to compaction. due to the impact of levelling of ground surface, the uppermost 1 m of the test field is excluded from the evaluation of the results. figure 14 shows that classification based on sbtn was not changed due to compaction (5: sand mixtures – silty sand to sandy silt and 6: sands – clean sand to silty sand). the qtn increases from the initial 40 to 60-160 after the ric. in natural ground the fs is increasing with the depth. after compaction the fs increases slightly to the depth of 1.5 m. at the depth of 1.5 m the sharp increase of fs is detected towards the value of 100 kpa. the fs/σ′v is constant with the depth before ric, thus indicating homogeneous ground conditions. after the ric the highest increase of fs/σ′v is observed at the depth of 1.6 m and then decreases with the depth towards the initial value at the depth of 4 m. the cpt test did not recognize effects of different compaction energies on the increase of qtn and the fs. figure 15 compares the qtn and the fs before and after ric. after compaction the qtn is 2 to 4 times higher and the fs is 2 to 6 times higher than before compaction. below the depth of 3 m the effect of ric is decreased. no influence of different compaction energies used for the compaction can be recognized. (s)dmt test the results of the dmt test are presented as intermediate parameters: material index (id), horizontal 23 a. petkovšek, m. maček, j. smolar acta polytechnica ctu proceedings figure 13. ric during compaction (left) and the test site es 2 layout (right – adapted from vukadin, [20]). figure 14. results of cpt tests. cpt 1 and cpt 2 before ric, cpt p1 to cpt p4 after ric performed on sub-fields p1 to p4 (tests performed by irgo, [19]). figure 15. results of cpt after ric compared to those before ric (tests performed by irgo, [19]). 24 vol. 10/2017 testing methods for mechanically treated soils stress index (kd), and dilatometer modulus (ed) (figure 16). the id and kd are effective vertical stress independent. the value of ed is presented also in relationship with the vertical effective stress (ed/σ′v) [21]. results show that the classification based on id changed a little due to compaction (mostly sandy silt and silty sand). the general soil behaviour type remains the same. below the depth of 1 m, thekd was around 5 before compaction. this value was higher than expected for normally consolidated soils [22]. after compaction the kd first increases and then, below the depth of 2 m, decreases; however, it remains slightly higher than recorded before compaction. the kd is related to fs [23], as the first one gives a ratio between horizontal and vertical effective stress during the driving of dmt, and the second one reflects the friction between soil and metal sleeve induced by horizontal effective stress during the driving of the cpt probe. therefore, the development trend of the kd with the depth should be of the same range as the developing of fs/σ′v. the ed increases with the depth before compaction, as expected, and results in higher values after compaction. the ed/σ′v exhibits nearly constant values with the depth before compaction and clearly increases after compaction. to estimate the ric efficiency with the depth, the values of kd and ed before and after compaction are compared (figure 17). the kd increases by 2 to 3 times and the ed increases by 2 to 4 times. the dmt tests after compaction exhibit almost no increase of kd and ed at the depth of 2 to 2.5 m. at the depth of 3 m both parameters start to decrease towards the values measured before compaction; however, the decrease is not as clear as in case of cpt. contrary to the cpt results, the dmt shows clear influence of compaction energy on the increase of the measured values. the sdmt enables measurements of shear wave velocity (vs). shear wave velocity depends on void ratio, effective vertical stress, stress history, cementation and over-consolidation (ocr) ratio [24]. the increase of all parameters with the exception of void ratio increases the vs. during compaction the void ratio decreases and the ocr increases. thus, the increase of vs is expected. figure 18 compares the constrained moduli (mv /cpt and md/dmt ) and the vs before and after ric. the right graph gives also the lines of vs, calculated by using the empirical equation (3) given by hardin and drnevich [25]. gmax = 1230 · (2.973 − e)2 (1 + e) · (ocr)k · p′ 1 2 (3) gmax = ρ · v2 s (4) where gmax is shear modulus at small deformations, ρ is density, e is void ratio, ocr overconsolidation ratio, k is empirical material factor (depends on plasticity index) and p′ is mean effective stress. figure 18 shows that before compaction, the vs, calculated after hardin and drnevich [25], is lower than measured and after compaction the calculated vs is close to the measured. when comparing the measured values of vs before and after compaction, no improvement and no influence of the compaction energy can be detected. cpt and dmt show the increase of constrained modulus (mv and md) after compaction. the range of measured values after compaction is similar. this is not in accordance with the findings reported by schmertmann [26], lee et al. [27], among others, who found the increase of dmt constrained modulus (md) higher than that of cpt (mv ). dynamic probing super heavy (dpsh b) dpsh b raw results are presented in figure 19 (upper line). before compaction the number of blows (n20sb) increases slightly with the depth. the n20sb values are lower than minimum required (5) according to en iso 22476-2. after compaction the n20sb values are higher and nearly constant with the depth, but the scatter of the result is high. from the n20sb the equivalent spt blow-count (n1)60 was calculated using the empirical equation (5) adapted after macrobert et al. [28] and using corrections given by ec 7.2. equivalent nsp t = n20sb 0.02 ·n20sb + 0.53 (5) dpsh b clearly indicates the efficiency of ric and the influence of compaction energy on the increase of the measured values. from this point of view, the sensitivity of the dpsh b test could be estimated as more indicative for the intended purpose than the sensitivity of cpt, dmt and sdmt methods. contrary to cpt and sdmt, the dpsh b confirms the compaction efficiency along the whole depth of the top layer, which was not observed by using the cpt and sdmt. the question for the discussion is: which method could be estimated as decisive? can we estimate the efficiency of the improvement by preferring optimistic test results (dpsh b) and neglecting less optimistic (cpt, sdmt) results? experimental site es 2, conclusions the analysis of the test results confirm that the ric technique is an effective tool for improving the geotechnical properties of silty and sandy soils, as it was already reported by adam and paulmichl [29]. however, the decision about the use of the ric technique to reduce the liquefaction potential must rely on the test results which are consistent and confirm without any doubts the efficiency of the improvement. as it was shown in the graphs, the scattering of the collected data raises serious questions about the validity, sensitivity and reliability of the test methods used to confirm the ric efficiency as well as questions about the homogeneity of the improvement. analysis of the results shows: 25 a. petkovšek, m. maček, j. smolar acta polytechnica ctu proceedings figure 16. intermediate dmt results. dmt 1 before ric, dmt p1 and dmt p4 after ric performed on sub-fields p1 and p4. figure 17. results of sdmt after ric compared to those before ric. figure 18. constrained modulus (mv-cpt and md-dmt)and measured/calculated vs (sdmt) 26 vol. 10/2017 testing methods for mechanically treated soils figure 19. comparison of n20sb and calculated (n1)60 values before and after compaction (tests performed by irgo, [19]). • the cpt and the dmt do not confirm the compaction efficiency along the whole depth of the top layer, while the dpsh b does • the cpt does not recognize the effect of compaction energy on the interpreted values, while the dmt and the dpsh b do • the sdmt (vs) does not recognize any improvement after compaction • the cpt and the dmt clearly recognize the less effective compaction at the depth of 2 m, while the dpsh b does not. although the dpsh b tests clearly show the efficiency of ric, the achieved improvement does not satisfy the criteria needed to reduce the liquefaction potential (figure 19). due to lack of previous experiences with the ric technique in slovenia and inconsistency of the test results, the ric technique was not recognized as a promising method for reducing the liquefaction potential. however, the authors of the paper believe that by preparing more test fields and by using slightly modified execution of ric and field testing, the results would be different and very probably more promising, too. 4.3. experimental site es 3: soil mixing 4.3.1. background the idea of soil mixing arose by joining three demands: (a) to reduce the liquefaction potential of silty-sandy soils, (b) to assure low enough permeability to avoid any additional sealing of the dikes, and (c) to improve the mixture willingness to compaction. 4.3.2. soil mixing test site description figure 8 shows the location of the test site and figure 20 gives a view to one of the test sites during test compaction and calibration of roller with ccc. as the results from the test fields were promising, an app. 1.2 km long temporary dike was constructed to protect the up to 70 m deep excavation and to gain additional data necessary to support the decision for the construction of 14 km long dikes (figure 21). 4.3.3. soil mixture demands before the soil mixing tests started, the designer prepared the requirement for the soil mixture, as follows: 27 a. petkovšek, m. maček, j. smolar acta polytechnica ctu proceedings • hydraulic conductivity lower than 1 × 10−6 m/s • shear strength must assure stable slopes at 1:1.75 (β = 30°) • homogeneity and good willingness to compaction • resistance to liquefaction, as already explained in previous chapters. 4.3.4. test program the test program was divided in five steps, i.e.: • mixture design, using the kozeny-carman equation • laboratory mixture preparation and testing • test fields on site to define the mixing and compaction procedures and to correct the laboratory mixture design, if necessary • construction of temporary dike with the qa-qc control tests during construction, using the ccc and conventional field test methods: the nuclear gauge, the dynamic and static load plate test and the double ring infiltrometer test • field testing on the temporary dike by using the sdmt, dpsh b and geohydraulic tests in boreholes. 4.3.5. the laboratory mixture design the mixture was proposed by using the kozeny–carman semi-empirical equation [30] [31] [32]. the equation relates the hydraulic conductivity to specific surface area, porosity and tortuosity. the main obstacle to wider use of the kozeny–carman equation is the specific surface area (ssa) of soil particles [33]. carrier [34], among others, proposed the estimation of ssa based on grain size distribution. the derived hydraulic conductivity based on the kozeny–carman equation and estimation of the ssa area based on grain size distribution is: k = 1.99×104  100 %∑ fi d0.404 i−1 ·d0.595 i  · ( 1 sf )2 · ( e3 1 + e ) (6) where k is hydraulic conductivity at 20 ◦c (cm/s), sf shape factor (6 for rounded grains), di particle size at sieve size i (cm), fi fraction between sieve i and i-1, e void ratio (%). the problem of the above equation is the determination of the finest fraction. based on chapuis in legare [35], 1/3 of the smallest measured grain size could be used for the estimation of an equivalent grain size of this fraction. fi d0.404 i−1 ·d0.595 i = fi 1 3di−1 (7) there are some limitations to the use of the kozeny–carman equation: • not valid for clays • valid for laminar flow of water (darcy law) and therefore not valid for clean, coarse gravel with pore size greater than 3 mm and large hydraulic gradients • the grains should be rounded or cubic (blocky) • soils with a long tail of small fractions (soils which are subject to internal erosion and segregation) can make problems. particles from the tail of small fractions are not in soil matrix but rather on larger grains. in this case the grain size distribution could be corrected by using 20 % of the increase of passing by 10 times of the increase of the grain size [33]. • anisotropy of hydraulic conductivity. it is assumed that the calculated value relates to the vertical hydraulic conductivity. the grading ranges of the available soils are given in figure 3. based on the kozeny–carman equation the mixture of sandy and gravely soil was proposed in mass ratio of 20:80 to 25:75 (figure 22). 4.3.6. laboratory testing the hydraulic conductivity of sands was measured in the triaxial cell and in variable head permeameters, while gravel and mixtures were tested in a large constant head permeameter. the diameter (d) of the large constant head permeameter was 304 mm and its height (h) was approximately 600 mm (figure 23). there were 9 piezometers on 3 levels to measure the local hydraulic head (total potential). the disadvantage of large constant head permeameter is poor compaction due to thin plastic walls. the smaller constant head permeameter (d = 150 mm h = 125 mm) enabled proper compaction of soils with maximum grain size of 31.5 mm. during the gravel tests in large permeameter a non-linear decline of hydraulic head in piezometers was observed. three hydraulic conductivities were calculated (average, maximum and minimum). this problem indicates possible segregation and anisotropy of compacted gravels. the results of the tests are presented in figure 24. the calculated hydraulic conductivity using the kozeny-carman equation are in agreement with the laboratory test results. the measured value on different mixtures was up to 10 times different than calculated. the reason for this is very probably in material segregation and imperfect saturation. in general, the laboratory tests confirmed predictions of the kozeny-carman equation and the suitability of proposed mixture. 4.3.7. test fields the dimension of the test field was app. 50 m × 10 m. each test field was made of two 55 cm thick layers of mixture. at the beginning, due to the rainy weather, it was difficult to assure the homogeneity due to the high 28 vol. 10/2017 testing methods for mechanically treated soils figure 20. vibratory roller during compaction on one of the test fields (left) and a view to the control pit in one of the test layers (right). figure 21. a view to the temporary dike, constructed from soil mixture and used as part of the large scale test embankment. figure 22. calculated ranges of soil mixture to achieve the required hydraulic conductivity (k < 10−6m/s) and stiffness. water content of the silty sandy soils. the achieved dry density and stiffness of the mixture were in direct relationship with the water content and the fines content, detected at each controlling point (figure 25). on mixtures with higher content of fines the achieved layer stiffness after compaction was lower. the difference in stiffness on different test tracks was clearly observed by ccc (figure 26). later on, the weather improved and the mixing processes and the compaction ran according to expectations. the decision was accepted to construct a temporary dike by using the soil mixture. 4.3.8. temporary dike construction control during the construction of the temporary dike, a comprehensive program of control test was realised, including the 61 sieve analyses and water content tests, 13 double ring infiltrometer tests, 325 spot tests – nuclear gauge. the ccc was used to control the stiffness and homogeneity. 53 gradation curves were inside the required area and 8 samples contained app. 1% lower content of fines than required. results of all hydraulic conductivity tests were inside the design requirements (figure 27). the average achieved density was 98 % of the modified proctor (mpt) maximum dry density. 4.3.9. temporary dike post construction control two years after the construction control tests were made from the dike crown. in three bore holes field water permeability (variable head) tests were performed at different depths. two tests (at depths of 2-4 m and 4-6 m) were performed in the dike and one test (at a depth of 6-8 m) in the foundation ground (figure 27). four tests in the dike met the design requirements (1 × 10−6 m/s) and two tests indicated k higher than required. results from four dpsh b probing are presented in figure 28. as the crown has been used as a temporary unsealed road for the needs of construction site for 2 years, higher n20sb values were observed at the depths up to 2 m below the crown. at greater depths 29 a. petkovšek, m. maček, j. smolar acta polytechnica ctu proceedings figure 23. a view to the permeameter cell (left) and a view to the surface of gravel and mixture (right). figure 24. comparison of calculated (kc) and measured hydraulic conductivity (k). the n20sb values were higher than recorded at the ric test sites. three sdmt probings were performed. before the test started, the boreholes were filled with dry uniform sand. therefore, a greater variation in result can be expected due to the effects of backfill. the third probe (sdmt3b) was repeated to check the repeatability of the results due to the difficulties during the test in probe sdmt3. the vs results in all probes at the dike were around 200 to 450 m/s and were significantly higher than those measured in the top layer before compaction and after the ric compaction. the measured shear wave velocities are close to the values calculated by using hardin and drnevich equation [25] (figure 28). 4.3.10. experimental site es 3 conclusions the kozeny-carman equation was found to be an effective tool for the design of the mixture with the given hydraulic conductivity considering the equation limitations. the laboratory test and the field double ring infiltrometer test results were in good agreement with the calculation. however, two of six tests in the borehole using open systems gave by a decade higher value of k. the ccc test results show that the compaction behaviour and layer stiffness of sandy gravel mixtures containing app. 10 20 % of fines are mainly controlled by the water content of fines. this means that even though the mixture is classified as silty gravel, its compaction behaviour is typical for less permeable soil mixtures. the achieved dry density after compaction, the sdmt (vs) and the dpsh b tests show that the mixture has properties of gravel. figure 29 shows that for a given csr the mixing of locally available soil is a promising method to reduce the liquefaction potential. however, the decision was made not to use the soil mixing for the construction of the dikes. all the soils from the top layer were removed and replaced by clean gravel. gravel was also used as fill material for all dikes and the sealing of the dikes was achieved by the geosynthetic bentonite barriers. 30 vol. 10/2017 testing methods for mechanically treated soils figure 25. results from the first test section. high water content in silty part of the mixture control the achieved stiffness and density. evd – dynamic deformation modulus measured by dynamic load plate test; evib – dynamic elasticity modulus of soil beneath drum (terrameter ccc system). figure 26. ccc record from one of the first test sections showing heterogeneity of the compaction. figure 27. comparison of hydraulic conductivity predicted after kozeny-carman equation (upper limit, lower limit) and measured values in laboratory and on site. 5. conclusion mechanical stabilisation for ground improvement has become a widely accepted system to support foundations for a wide variety of structures as well as for the construction of embankments and embankment dams. the use of mechanically stabilized locally available soils is also one of the top priorities of the philosophy generally understood as “sustainable development” [36]. the paper focuses on the interpretation of the results of the tests used for the control of the efficiency of some of the stabilisation techniques. the analysis of the tests shows inconclusive results. in foundation ground, where the requirements are different than those given for regular earthworks and the safety demands are higher, the use of different test methods is necessary. due to the lack of experiences, the use of different methods may lead to inconsistent results and often raises more questions than gives answers. understanding the local conditions and mechanisms that may influence the test results in different soil types/stabilisation techniques is important for engineering judgement and leads to the ability to make tough decision. however, past experiences are also important and sometimes a lot of time and energy are needed before a new method/ technique becomes widely accepted. 6. acknowledgement this work was funded by the hess and realised in close cooperation with irgo ljubljana. authors would like to express special thanks to mr. b.sc. andrej unetič (hse invest) for his help at realising the investigation program and to many engineers and technicians for their essential field work and data collection. the advice of prof. dietmar adam from tu vienna is highly appreciated. 31 a. petkovšek, m. maček, j. smolar acta polytechnica ctu proceedings figure 28. temporary dike. shear wave velocity, hydraulic conductivity, and results of dpsh b (from left to right). figure 29. diagrams for evaluation of liquefaction potential based on (n1)60 (upper) and vs1 (lower). bc – before compaction, fc – fines content, mix – soil mixture, ric – rapid impact compaction, ts – top soil layer. references [1] w. f. v. impe. soil improvement techniques and their evolution. aa balkema, rotterdam, netherlands., 1989. [2] h. brandl. geotechnics for roads, rail tracks and earth structures, chap. the importance of optimum compaction of soil and other granular material, pp. 47–68. crc press, 2001. [3] i. vaníček, m. vaníček. earth structures in transport, water and environmental engineering. springer netherlands, 2008. [4] v. r. raju, j. daramalinggam. ground improvement: principles and applications in asia 165(2). [5] t. verbič. quaternary stratigraphy and neotectonics of the eastern krško basin part 2: neotectonics. razprave iv. razreda sazu, xlvi-1, 2005. [6] v. ribarič. seizmičnost slovenije – katalog potresov. seizmološki zavod sr slovenije, ljubljana, 1982. [7] ž. veinović, d. domitrovič, t. lovrić. historical occurrence of liquefaction in zagreb area and estimation of reoccurrence in case of another strong earthquake. rudarsko-geološko-naftni zbornik 19:111–120, 2007. [8] ž. veinović, p. kvasnička, v. jurak. geohazard characteristics of the sava river plain near zagreb. konferencija razvitak zagreba 2008. acessed 15.9.2016. [9] i. idriss, r. boulanger. semi-empirical procedures for evaluating liquefaction potential during earthquakes. soil dynamics and earthquake engineering 26(2):115– 130, 2006. 11th international conference on soil dynamics and earthquake engineering (icsdee): part ii, doi:https://doi.org/10.1016/j.soildyn.2004.11.023. [10] d. aleksovski, g. mirakovski, d. dojčinovski, et al. he brežice geofizikalne raziskave in seizmična mikrorajonizacija,. geofizikalne raziskave in seizmična mikrorajonizacija, iziis univerze sv. kiril i metodij, skopje., 2008. [11] arso, slovenian environment agency. agencija republike slovenie za okolje. 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[16] t. l. youd, s. k. noble. liquefaction criteria based on statistical and probabilistic analyses p. 201–215, 1997. [17] r. d. andrus, k. h. stokoe. liquefaction resistance of soils from shear-wave velocity. journal of geotechnical and geoenvironmental engineering 126(11):1015–1025, 2000. doi:10.1061/(asce)1090-0241(2000)126:11(1015). [18] s. l. kramer. geotechnical earthquake engineering. prentice-hall international series in civil engineering and engineering mechanics. upper saddle river, new jersey., 1996. [19] irgo. he brežice, poročilo o impulznem globinskem zgoščevanju na testnih poljih. faza: pgd. report nr. ic161/11. ljubljana, slovenia., 2011. [20] v. vukadin. the improvement of the loosely deposited sands and silts with the rapid impact compaction technique on brežice test sites. engineering geology 160(supplement c):69–80, 2013. doi:https://doi.org/10.1016/j.enggeo.2013.03.025. [21] p. k. robertson. cpt-dmt correlations. journal of geotechnical and geoenvironmental engineering 135(11):1762–1771, 2009. doi:10.1061/(asce)gt.1943-5606.0000119. [22] s. marchetti. in situ tests by flat dilatometer. journal of the geotechnical engineering division 106(3):299–321, 1980. [23] j. m. o. hughes, p. k. robertson. full-displacement pressuremeter testing in sand. canadian geotechnical journal 22(3):298–307, 1985. doi:10.1139/t85-043. [24] r. dobry, m. vucetic. state of the art report: dynamic properties and seismic response of soft clay deposits. in proceedings of the intl. symposium on geotechnical eng. of soft soils, vol. 2, pp. 51–87. mexico city, 1987. [25] b. hardin, v. p. drnevich. shear modulus and damping in soils: design equations and curves 98:667–692, 1972. [26] j. h. schmertmann. comparing dmt with cpt in nc/oc sand bucket test. dmt digest (4), 1984. [27] m. lee, s. choi, m. kim, w. lee. effect of stress history on cpt and dmt results in sand. engineering geology 117(3-4):259–265, 2011. doi:10.1016/j.enggeo.2010.11.005. [28] c. macrobert, d. kalumba, p. beales. correlating standard penetration test and dynamic probe super heavy penetration resistance values in sandy soils. journal of the south african institution of civil engineering 53(1):46–54, 2011. [29] d. adam, i. paulmichl. rapid impact compactor an innovative dynamic compacttion device for soil improvement. in proc. 8th international geotechnical conference, pp. 183–192. slovak university of technology, bratislava, 2007. [30] j. kozeny. über kapillare leitung des wassers im boden, 136 (2a). hölder-pichler-tempsky, a.-g. akad. d. wiss., wien., 1927. [31] p. c. carman. the determination of the specific surface of powders. journal of the society of chemical industry 57:225–238, 1938. [32] p. c. carman. flow of gases through porous media. new york: academic press, 1956. [33] r. chapuis, m. aubertin. predicting the coefficient of permeability of soils using the kozeny-carman equation. école polytechnique de montréal 2003. [34] w. d. carrier. goodbye, hazen; hello, kozeny-carman. journal of geotechnical and geoenvironmental engineering 129(11):1054–1056, 2003. doi:10.1061/(asce)1090-0241(2003)129:11(1054). [35] r. chapuis, p. p. légaré. a simple method for determining the surface area of fine aggregates and fillers in bituminous mixtures. effects of aggregates and mineral fillers on asphalt mixture performance pp. 177–177–10, 1992. doi:10.1520/stp24217s. [36] i. vaníček. sustainable construction. ed. 1st. czech technical university in prague, prague, 2011. 33 http://dx.doi.org/10.3208/sandf1972.20.4_127 http://dx.doi.org/10.1061/(asce)1090-0241(2000)126:11(1015) http://dx.doi.org/https://doi.org/10.1016/j.enggeo.2013.03.025 http://dx.doi.org/10.1061/(asce)gt.1943-5606.0000119 http://dx.doi.org/10.1139/t85-043 http://dx.doi.org/10.1016/j.enggeo.2010.11.005 http://dx.doi.org/10.1061/(asce)1090-0241(2003)129:11(1054) http://dx.doi.org/10.1520/stp24217s acta polytechnica ctu proceedings 10:16–33, 2017 1 introduction 2 background 2.1 site description 2.2 geological profile 2.3 soil properties 2.3.1 index properties 2.3.2 liquefaction potential 2.4 preliminary conclusions 3 discussion on improvement technique 4 results from the experimental sites 4.1 experimental site es 1: roller compaction trial field 4.1.1 site description 4.1.2 results and discussion 4.1.3 experimental site es 1 conclusions 4.2 experimental site es 2: rapid impact compaction (ric) test fields 4.2.1 background 4.2.2 ric test site description 4.2.3 results and discussion 4.3 experimental site es 3: soil mixing 4.3.1 background 4.3.2 soil mixing test site description 4.3.3 soil mixture demands 4.3.4 test program 4.3.5 the laboratory mixture design 4.3.6 laboratory testing 4.3.7 test fields 4.3.8 temporary dike construction control 4.3.9 temporary dike post construction control 4.3.10 experimental site es 3 conclusions 5 conclusion 6 acknowledgement references acta polytechnica ctu proceedings doi:10.14311/app.2017.11.0053 acta polytechnica ctu proceedings 11:53–62, 2017 © czech technical university in prague, 2017 available online at http://ojs.cvut.cz/ojs/index.php/app domains of railway traffic in the czech republic, which need the safety improvement dana procházková∗, jan procházka, tomáš kertis czech technical university in prague, faculty of transportation sciences, praha, czech republic ∗ corresponding author: prochdana7@seznam.cz abstract. the paper analyses the railway accidents sources in the czech republic on the basis of knowledge on complex system behaviour. it derives seven categories of sources of railway accidents. the individual categories include the accident sources from the same field domain. these domains are: technical related to rail traffic vehicles; technical related to rail infrastructure and railway station; railway operation control – organizational causes; railway operation control – cyber causes; control of rail traffic vehicles; attack on the train; domain legislative and other. the results show that for railway traffic safety improvement, it is necessary to pay attention to all categories, and especially to these that lead to organizational accidents origination. keywords: railway, railway accident, safety, security, criticality, railway accident causes. 1. introduction humans need infrastructures for their life quality and development. especially, at critical conditions the infrastructures’ operations support the human survival. the traffic infrastructure in present dynamic age has three basic tasks at critical conditions caused by disasters occurrences, namely to ensure: the human evacuation from afflicted region; the transposition of response units to afflicted region; and interconnections between afflicted region and its vicinity. therefore, its safety (the set of anthropogenic measures and activities that improve the system security) is the goal of human management and behaviour. from the general view the railway system plays very important role in terms of transportation in the european area. the transportation mode is widely used not only for middle and long-distances, but it has also very important role in short distances between small agglomerations and for the public transport in cities. therefore, the safety of railway system has long-term tradition as well as railway operating itself since twenties 19th century. it is very followed in the czech republic that has very dense railway network with 9458 km of rail track, including 1329 km of rail track for the international transportation within the european railway system. although the railway safety has the long tradition, the number of requirements on safety and their depths are increasing because the increase of population density and vulnerability of environ, in which the railway system operates. especially, in the european railway system the requirements on interoperability are very important [1]. because the social phenomena based on human intent were found as the significant threat, the security requirements have been step by step introduced into practice. from the system viewpoint there are other influencing aspects which affect the railway safety, e.g. the interconnection of cyber and physical systems, the human-machine interfaces and overall perceiving and handling with such complex systems, i.e. sociotechnological (technical) systems [2]. the practice shows that their violation is often the cause of serious railway accidents, and therefore, we need to protect the railway system against them. the monitoring of railway system and the critical assessment of previous railway accidents are necessary for understanding the complicated nature of railway system safety, for which reach there are necessary to implement the protective, mitigating, response and renovation measures. owing to complex structure of problem, it is also needed to understand the railway system in terms of integral safety that deals with systems of systems (sos) safety, because we also need to reduce losses caused by the cascades effects or by phenomena that are the consequences of interfaces between systems of completely different nature, i.e. technological, cyber, social, environmental etc. the results of research described in [3] show the causes of traffic accidents on railway and road that belong to the technical domain and the human factor domain, which is connected with drivers error. the present work is only directed to railway system. its goal is to obtain the detail findings that can help to improve the safety in the railway traffic, which is the important part of critical infrastructure. the next given results are based on the concept of integral safety of complex system with type "system of systems (sos)" [2]. the main target is to reveal real causes of railway accidents. the authors’ effort is especially focused on the recognition of risks that are connected with interdependences in the railway traffic system. they are mainly concentrated to those interdependences that are originators of organizing accidents, i.e. accidents caused due to bad human 53 http://dx.doi.org/10.14311/app.2017.11.0053 http://ojs.cvut.cz/ojs/index.php/app d. procházková, j. procházka, t. kertis acta polytechnica ctu proceedings decision or due to bad human management. 2. risk and safety if we want to ensure the safety of any system, we need to determine the sources of losses and damages (generally denoted by term "disasters") and to determine the sizes of their destructive potentials. it means that incident, accident and other similar phenomena are the subcategories of category "disaster" [4]. in the risk engineering we work with three fundamental terms: • danger marks the conditions of human system at which the origin of harms on protected assets has the high probability; it is almost sure that the harm will origin [5], i.e. the term marks the rate of conditions. it means that it goes on mark of possibility of origin of harm, loss or damage of one or more assets. the danger is predetermined by substance properties that are in facility, object or territory and by properties of processes that are running in facility, object or territory. it is immediate, if the course uncontrollably goes to the disaster origin that causes the emergency situation; and it is creeping, if the course goes to disaster origin inconspicuously and without clear-cut precursors [5]. the danger for human means both, the big phenomena (e.g. natural disasters, industrial accidents, environmental or social disasters) and the seemingly small phenomena from daily life as slump of snow, icicle or roofing from roof, rough pavement etc. [5]. • hazard marks the disaster potential to cause the harms, losses and damages on protected assets in a given site that is prescriptively determined. it goes on prescriptive measure of danger that is connected with the given disaster. for the strategic planning needs, the centennial disaster is often considered, i.e. the hazard is size of disaster that occurs once in hundred years, or professionally exactly, the disaster size that has return period 100 years; at special buildings and facilities it is considered from safety reasons the hazard determined by connected with thousand years’ disaster or ten thousand years’ disaster [5]. • risk connected with a given disaster is the probable size of damages, harms or losses on protected assets that originate in given place at origin of disaster with size of normatively determined hazard, which is normalized to the certain territory unit or number of individuals and the time unit [5]. the difference between risk and danger is the following: the danger is specific (it denotes the topical conditions) and the risk is only expected opportunity. the humans ensure the protection of human society and populated territory against risks by the way that for each disaster they determine the certain size (so called design disaster). they only perform the preventive measures so the possible risk size may be acceptable. the problem arises if disasters with size greater than design disaster occur, because great damages, harms and losses origin as the consequence of failure of man-made technological systems [2, 5–7]. from the practical reasons it is necessary to consider that the entity risk connected with the given disaster does not represent only the direct losses on assets but also the indirect ones; the indirect losses are caused by: delays or errors in response, cascades of failures caused by synergic and cumulative effects, which are caused by linkages and couplings among the assets; and by domino effects. due to the entity structure its risk is the integral risk that is expressed by following formula r(h) = [ ∑n i=1 ai(h)zi(h) +∑n i=1 ∫ t 0 ∫ f (h,ai, pi, o, t)dsdt ] ∗ τ−1 where: h is the hazard connected with the considered disaster; ai are the values of assets, i = 1, 2, ..., n that are considered in connection with complex technological facility safety, where n is the number of monitored assets; zi are the vulnerabilities of assets taken under account, i = 1, 2, ..., n; f is the loss function; pi is the occurrence probability of i-th asset damage – conditional probability; o is the vulnerability of safeguard measures; s is the size of followed territory/facility; t is the time that is measured from the origin of harmful phenomenon in facility; t is the time for which losses arise; and τ is the return period for the given disaster [2]. because the loss function f form is not known, we use for determination of total risk (i.e. the integral risk) the scheme given in figure 1 [2]. the problem is complicated by reality that the world is in dynamic development, i.e. both, the entity conditions and the risk sources are changing in time. moreover, there is necessary to respect that the risk and safety are not complementary quantities – it holds that the risk reduction leads to safety increase but at the same risk value the safety can increase if humans perform special measures or at their behaviour use special manners following from correct safety culture [2]. therefore, at solution of practical tasks connected with both, the entity safety and the entity risk, it is necessary to consider that risks are normal and for the entity safety it is necessary to apply not only the risk prevention measures and activities determined on the basis of correct intent and correct data and methods, but also: the safety culture by which the human behaviour in the entity and its vicinity is targeted to safety; and the tools that reduced losses and damages if some important disasters occur. therefore, it is necessary to prepare the qualified response for important risks realizations, such as: the risk management plans for both, the entity and the entity vicinity for all relevant risks; the continuity 54 vol. 11/2017 domains of railway traffic in the czech republic figure 1. flowchart for determining the risks for the strategic management of safety; a – assets and z losses, damages and harms to the assets; description: 1 – the human lives and health, 2 – human security, 3 – property, 4 – the public welfare, 5 – the environment, 6 – infrastructures and technologies, p – private. plans for survive of important complex technological objects and facilities; and the operational crisis plans for both, the complex technological objects and facilities and their vicinities. according to knowledge concentrated in [2], the risk engineering uses the following principles: • the risk is followed and considered during the given system whole life cycle, i.e. at sitting, designing, building, operation and putting out of operation, and eventually at territory bringing in original condition, • the risk determination is directed to user’s demands and to the level of provided services, • the risk is determined according to the criticality of impacts on facility processes, provided services and on assets that are determined by public interest, • the unacceptable risks are mitigated by tools according to technical and organisational proposals, by standardisation of operating procedures or by automatable check-up. the advanced risk engineering directed to human system safety respects the co-existence of systems with different nature (sos), and so fulfils present demands of humans [6]. to prepare groundwork it is necessary to combine analytical methods with expert judgement by which we remove vagueness in data. the problems that we need to solve in this consequence consist in acquisition of knowledge and in assignment "who is expert"; the last mentioned problem was broadly discussed in world conference esrel2011 [8]. for the first problem solution we need systematically to monitor the human system and obtained data to process by qualified methods [9]. the process model for work with risks is shown in figure 2 [5]. the criterions determine the condifigure 2. process model of work with risks targeted to safe entity. tions at which the risk is acceptable, conditionally acceptable or unacceptable. the aims in real case are selected from further given possibilities: to reduce risk to certain level; to secure the system, i.e. to ensure its security; to ensure safe system, i.e. to ensure security for system and its vicinity. the feedbacks are used in case if the monitoring shows that the risk is unacceptable; firstly, it is used the cheapest feedback 1; in case of its failure the feedback 2 etc.; at huge harms immediately it is used the feedback 4 that means the change of concept of work with risks. 3. criticality and safety management in followed context the criticality is directed to failures and hazards [10]. the infrastructure criticality needs to be determined on the basis of analysis of relevant and dangerous incidents and failures, losses and damages caused by functionality loss, outer disasters, mitigation measures, reactions and substances in a given facility, releases, leakages or discharges of substances (products’ pipelines) etc. the criticality determines the condition at which the system does not ensure expected functions in a required time, a site and in a required quality. to criticality of each partial infrastructure we can approach from two viewpoints, teleological and systemic [8]. from the teleological viewpoint it follows that the criticality is a result of role and function of partial infrastructure in the society. this concept enables to work with non-network and non-technical objects and processes. partial infrastructure criticality from the systemic viewpoint is a result of its position in the system or of its link to another partial infrastructure. from both approaches it follows that the partial infrastructure criticality also influences the system that is a social partial infrastructure created by public administration, business subjects, educational and research institutions and civic clubs. 55 d. procházková, j. procházka, t. kertis acta polytechnica ctu proceedings adopting the findings from the systems system safety management [10] the set of partial infrastructures in a region is critical one, if it is only capable to ensure activities at which there is only ensured human surviving in a given region. for this purpose, there have been in the world performed the analyses of sectors to which individual partial infrastructure belong and they have been followed the dependences among sectors, and the safety management then respect both, the conditions for functionality of individual partial infrastructures and the conditions for functionality of set of infrastructures; aggregated (critical) infrastructure. the term "criticality" was firstly used in a connection with the nuclear reaction where it denoted the threshold after which the spontaneous chain reaction followed. in connection with partial infrastructures and with critical infrastructure (set of infrastructures), the criticality is the most competently expressed by phrases: (1.) criticality is a relative measure of impacts of frequently occurred defects and failures. (2.) criticality is expressed by conditions that describe a transition between quality differ conditions. (3.) criticality is a condition of extensive urgency. from the given criticality definitions, it is possible to derive that the criticality is a threshold value that may be designed and that can relate to event, process/function parameter, type of defects and resistance. the determination of criticality is consistently related to a size of impacts caused by loss of functionality of each infrastructure on the society [10]. from this reason, at the criticality determination, it is necessary to consider: (1.) concentration of humans and assets (especially protected ones). (2.) sectors of economy (sector analysis). (3.) types of mutual dependences among the partial infrastructures/sectors: (a) on which item the assets of given sector are dependent? (b) what is the mutual dependence of assets among sectors? (4.) types of services for public: (a) how long has been taken the renovation of services furnishing? (b) which compensations/substitutes can be accessible and available? (5.) public confidence in the public administration institutions: (a) can defect of assets/public services result in a fall of moral of citizens, a loss of national prestige, panic, rebellion or civic disorders? (b) can defect of assets induce some impacts/changes in the environment? the determination of criticality in service of territory goes from the hazard assessment for disasters possible in a given region, considering the vulnerability of partial infrastructures in a given region, the mutual interconnections of partial infrastructures in a given region, i.e. theoretically the same principle as in analysis and assessment of risks in a region, at which several protected interests is considered. therefore, the criticality determination process is the following: (1.) characteristics of assets (there are considered physical, cyber and human assets). (2.) determination of criticality (hazard analysis and consideration of assets vulnerabilities in a given site). (3.) assessment of impacts on assets (concentration of humans and assets, economic impacts, mutual dependences, reliability). (4.) assessment of consequences of losses, victims, damages and harms of assets. (5.) determination of priorities according to the given rules. (6.) the criticality is mostly determined by scoring, i.e. by decision making matrix. the criticality matrix of infrastructure shows scaling the infrastructure vulnerability vs. infrastructure importance (i.e. its damage causes the infrastructure failure). for safe infrastructure operation there are used the tools: for middle criticality assets the continual monitoring and risk management plans for important risks; and for high criticality assets the operational crisis plans and continuity plans [2]. it is true that above described procedure shows that assessment of infrastructure/technology/set of infrastructures according to two criteria, namely measure of vulnerability and measure of importance of service in a region is not a result of objective computation of process analysis but rather result of subjective estimations, that is only tolerable in case of determination of basic frame. more complex it is the determination of criticality for some process. at scoring the vulnerability and importance of service it is necessary according to [2] to consider following items: duration of renovation of infrastructures and technologies; impact of failure of infrastructures and technologies on human lives and security; caused detriment, harms and loses; impacts on environment; and induced adverse situation. in the frame of ensuring the human system security and sustainable development it is necessary permanently to perform the measures that reduce an infrastructure criticality in a region. by building the new infrastructure it is necessary to ensure suitable number and regional distribution of objects of important infrastructure that are sufficiently resistant to expected disasters in a given region, and by that systematically to reduce infrastructure criticality. 56 vol. 11/2017 domains of railway traffic in the czech republic expenses for critical infrastructure are not only costs for its design and building but they also include costs for its operation, maintenance, repair and modernisation. therefore, the risks connected with each infrastructure must also include the risks from just given domains and the region management must know how to deal with them. it is necessary to assess the risks from disasters that can be denoted as financial market failure because with them it is connected failure of finances for maintenance, operation, repair and modernisation of objects of critical infrastructure. it is caused by the fact that infrastructure criticality increases if not good maintenance and good repair are performed (which cause the vulnerability increases). because nothing is out of defects, it must be prepared the plan for renovation of each infrastructure, namely critical one. this plan needs to be proactive, properly assessed; it needs to contain transparently managed risks and answers to questions as: (1.) what to do? (2.) how to do? (3.) in which time interval? (4.) do not risks for other protected interests in crease? etc. risk reduction in the context of safety management covers several topics: safety of processes; the protection of workers’ health and safety (safety of work); and reducing the impacts on the environment. therefore, in practice, it was introduced that the analysis of the impacts of management on the safety of the facility shall be carried out according to the reason model of organizational accident [11]. the causes of organizational accidents are in three basic aspects: organizational processes; the conditions which cause the origin of errors or infringements of rules; and no solved problems, which permit the errors and/or violation of regulations. safety management is increasingly focused on resilience due to increasing complexities and connectivity of real technological facilities. this is based on the concept that it is increasingly difficult to identify all the hazards and unwanted accidents, and therefore, to avert and reduce the consequences of various disorders it is important the resilience. investigation of failures and accidents of complex technological systems indicate that these are often caused by a combination of several improbable accidents and the capability of their predictions means to avoid combinations that have the potential to cause accidents, accompanied by large losses, which should enhance the safety. the procedures and tools are described in [2]. 4. data on railway system criticality and accidents the railway system including the trans-european transport network corridors and also the local specific rails creates the complex transportation infrastructure. in many cases it goes on the critical infrastructure, because the railway system ensures the state basic functions for human survival in the territory, state and may be also on the continent level at critical conditions. the elements of critical infrastructure, therefore, need the special attention, study and approaches for increasing their resilience, i.e. it is necessary to ensure their robustness, durability and adaptability with regard to the origin of non-acceptable events. in other words, it means to secure the systems under consideration in which the individual elements are located. individual elements shall also to have inherent safety so that they and their surrounding (i.e. public assets in their vicinity) might not be endangered by their imperfection and failure. certain level of safety shall be also provided the less important local elements of systems with less importance, the location and nature of which can cause at their failure the significant problems to the whole system, and at huge failures they could cause the losses on public assets (including the human lives and health) in immediate or far off surrounding [9]. by introducing the new technologies which are not well verified in practice, and by increasing the system interdependences caused by interface of cyber, material, energetic, economic, social etc. technologies, the systems are becoming more complex. it is the fact that the complexity influences the system safety, because it leads to increasing number of possible systems’ conditions, and also to the origin of non-assessable emergent phenomena, i.e. phenomena which occur at certain conditions suddenly and unpredictably [2]. in work [12] the criticality rates for individual types of transportation infrastructure in the czech republic were judged by experts from the areas: transportation; transportation management in the territory; supply chains; public administration; and the integrated rescue system. the experts assessed 14 factors and the result was determined by using the multi-attribute utility theory. the result showed that the rail transport has very high criticality rate. regarding to just describe research results, it is necessary to trade-off with risks very carefully and to use the advanced tools of risk engineering. for investigation of railway accidents, two databases were compiled [13]. the first one was created for the whole world by help of internet [14] with using the passwords "rail disasters", "railway disasters", "rail accidents" and "railway accidents", especially from the sources [15–18]. the second one was created on the basis of national data that are given in the database of the rail safety inspection from the period 2006 up to 2015 [19]; it contains 204 special reports on railway accidents in the czech republic and in some reports the description of similar accidents that happen in another sites since 2006 and were not often the object of investigation of inspection. 57 d. procházková, j. procházka, t. kertis acta polytechnica ctu proceedings 5. methods used at data processing the data on railway traffic accidents in database [13] are judged in the context of integral safety of railway system, i.e. not only from the viewpoint of railway system, but from the human security and development. the data are processed by current statistic methods and by special risk engineering procedures as the cba, separation of accidents into seven accident sources’ categories, determination of logic interconnections among the accident sources and their expression by fish-bone diagram [20]. 6. items important for safety improvement of railway traffic in the czech republic data summarizes in [13] show that the railway accidents have been reported since 1650. the first described accident originated in whickham, county durham, uk two boys die when they are run down by a wagon on a wooden coal tramway. the continuous reports are after the accident in philadelphia, county durham, uk on july 15, 1815 that was described by words thirteen or sixteen people, mainly spectators, are killed and 40 are injured by the boiler explosion of the experimental locomotive. comparison of data in [13] shows that the worst train disaster in the rail history was caused by tsunami on sri lanka at december 26, 2004 – the death of over 1700 people. from the same source it follows that the railway accidents often occur in india; the biggest accident was in the state of bihar in june 6, 1981, it died more than 800 people when passenger train derailed when it had crossing the bridge over the bagmati river. the huge consequences have the railway accidents of freight trains shipping the hazardous substances. the huge railway accidents with presence of hazardous substances was on the january 6, 2005 in the us, graniteville, south carolina at which 9 people died, more than 250 people were injured when the freight train collided head-on with a parked local freight train near the avondale mills plant in graniteville. 16 wagons (including a tank car that ruptured 90 tons of chlorine gas into the air) derailed in the accident. the us ntsb determined that the cause of the accident was the failure of the local freight’s crew members to realign the switch for mainline operations. very great accident of freight train was on the july 6, 2013 in lac-mãľganic, quebec, canada (figure 3). the freight train containing 72 tank cars of crude oil runs away unattended and derails. several wagons explode, destroying over 30 buildings in the town’s centre, roughly half of the downtown area, and requiring the demolition of all but three of the remainder of the buildings in the downtown area due to contamination by petroleum from the train; these combine to require the evacuation of 2,000 people, a third of figure 3. consequences of freight train accident in lac-méganic, quebec, canada on july 6, 2013 [19]. the town’s population. 42 are confirmed killed, along with 5 missing and presumed dead, making this the fourth-deadliest rail accident in canadian history. the results of detail study of railway accidents with presence of hazardous substances are in [3]. the analysis of consequences of railway accidents in the czech republic given in [13] shows that there is only considered the direct loses on humans and rail traffic components. to the accidents’ costs there are not included the costs on responses performed by the integrated response system, the costs on renovation and the costs for alternative traffic serviceability. the sites of rail accidents in the czech republic in the period 2006 up to 2015 are shown in figure 4. from this figure it follows that rail accidents occur on all railways; mostly on the railways with high frequency shipping. the main causes of accidents and near-misses in the czech republic supplemented by the accident causes from the world that are possible in our country due to present situation (e.g. cyberattacks) were revealed in domains [21]: • technical – related to rail traffic vehicles, • technical – related to rail infrastructure and railway station, • railway operation control – organizational causes, • railway operation control – cyber causes, • control of rail traffic vehicles, • attack on the train, • legislative and other. founded technical causes related to rail vehicles (as locomotive, coach, car, wagon, freight car) are the following: • errors in design or at construction of traffic vehicle (wrong wagon or locomotive construction from the stability viewpoint, inappropriate location of fuel tank or power conductor at a terminal block in the locomotive – possibility of short circuit etc.), • wrong maintenance of locomotive or wagons, 58 vol. 11/2017 domains of railway traffic in the czech republic figure 4. places of railway incidents in the czech republic. • wrongly performed technical overhaul of traffic vehicle, • wrongly performed the repair of rail traffic vehicle (e.g. brake systems, mechanical parts of brake systems, mainly in the case of car hand brake), • wrongly loaded wagons, • wrongly closed wagon doors, • sudden technical fault of locomotive or some of wagons (damage of axle bearing, engine broken, traction interruption, direction controller out of operation, or failure of another crucial device, outage of air-conditioning etc.), • fuel shortages or power outage, • failure of technical equipment of locomotive control system (broking the speedometer, outage of radio connection with control centre (dispatching) etc.), • malfunction of redundant system when it is needed. founded technical causes related to rail infrastructure and railway station are the following: • siting the railroad in the territory (steeper gradient, insufficient load capacity of rail track foundation, sharp rail swerve, a lot of unprotected crossings with roads or field roads, tall and lush vegetation caused the decrease of visibility etc.), • construction errors at the railway station design and building (too short operating space, sitting the rails in the direction in which the tall buildings are, which decreases the view of engine-driver and switchmen (shunter operator) in the case of change of position of rail vehicle, headwind etc.), • rail track condition (error in construction, disorder in the railway station area, wrong maintenance – ruggedness, ice, snow, rail track drift, cracked rail switch, unanchored rail track to sleepers etc.), • missing the periodic overhauls of rail tracks, • wrongly performed technical overhaul of rail tracks (no detection of cracked rail switch), • failure of early repair execution of identified relevant faults on the rail track or on signalling equipment, • missing the signalling equipment or it has insufficient power, • sudden technical failure of operation control devices (insufficient maintenance, fault or failure of technical equipment of control system in operation control centre, etc.), • equipment layout for service of rail vehicles (refuelling, loading and unloading of goods, entrance and exit of passengers etc.), • physical damaging the railway station or rail tracks (war, robbery attack, terrorist attack, vandalisms etc.), • siting the train to the wrong rail, • obstacles on the rail tracks, • insufficient radio equipment of railway station, • deficit of knowledge and experience of railway station service (e.g. worker who operates the movement of rail vehicles on rail tracks in railway area etc.), • out of function of warning system at railway station signalling the minimal safe distance between two trains that are on one rail track. founded causes related to railway operation control the – organizational causes are the following: • wrong setting up the train route, • failure of start-up of operation of crossbars, lights or sound signals before train arrival to crossing the rail tracks with road or field road, • keeping the obstacles on the rail tracks, • insufficient identification of rail track, • insufficient identification of crossing the rail track with road or field road, • guiding the train approaching to railway station on the incorrect rail track (train collisions, derailment etc.), • bad decision of train dispatcher (wrong evaluation of report from police and stopping the train operation at another rail track than at this on which the obstacles were occurred, • non-passing the information on fire to the enginedrivers of relevant trains, • wrong consideration of meteorological conditions (wrong information to engine-driver), • dispatching the wrong instructions to trains owing to the control system failure in dispatcher workplace (e.g. as a consequence of electricity outage, the pc outage etc.), • dispatching the wrong instructions to trains owing to the dispatcher (controller) error or ignorance, • chaos at operation control/dispatcher workplace/room (wrong information to engine-drivers, information delay etc.), 59 d. procházková, j. procházka, t. kertis acta polytechnica ctu proceedings • deficiency in railway station staff (train collisions etc.), • wrong maintenance or wrong platform lighting, • wrong communication between train dispatchers at setting up the train routes, • insecurity of movement of shunted vehicle, • shunters are not furnished by a red signal lighting, • insufficient training the shunters, • insecurity of control at crossing the rail track with road or field road at train shunting, • error of railway station staff (at train guidance, cleaning the railway station and rail tracks, rail station and rail tracks maintenance etc.), • wrong communication between the operation control centre and companies which repair the rail tracks, • wrongly distributed responsibilities at the operation control centre, • insufficient communication among the enginedrivers in service area, • insufficient knowledge and experiences of staff in the operation control centre, • absence of instructions for support to engine-drivers who occur in unexpected emergency up to critical conditions. founded causes related to railway operation control – cyber causes are the following: • distortion of data from monitoring – the operation data acquisition system (false information to the engine-drivers and from engine-drivers, chaos at operation control centre etc.), • false software (it does not consider all possible variants of possible operational conditions, from which it follows the false instructions to engine-drivers and other staff), • insufficient hardware (wrong data processing and evaluation, dispatching the false instruction to engine-drivers in operation owing to pc failure, delay of reports etc.), • hacking attack to control centre of dispatch workplace. founded causes related to control of rail traffic vehicles are the following: • error of engine-driver at train control – e.g. forbiddance of train drive behind the semaphore with signal forbidding the train drive (e.g. signal at danger), non-keeping the view conditions at bad visibility (owing to health conditions, tiredness, stress, wrong information from the rail operation control centre, failure of critical equipment of locomotive or other vehicle due to wrong maintenance, incorrect evaluation of situation as reduced speed signalling owing to effort to correct the disharmony with the time table, bump to obstacle on the rail track, turn off the functional device instead of faulty one – exit and entrance to the station, derailment, non-use of wedge at train stop at train shunting etc.), • engine-driver fault at assessment of meteorological conditions (hoar, snow drifts, obstacles on the rail track etc.), • engine-river fault at unexpected conditions occurrence (owing to insufficient preparation for coping with emergency conditions – storm, reduced visibility etc.), • engine-drive fault at locomotive preparation for drive (bad instruction studying before the drive – e. g. regarding to freight, wrong setting the speedometer, false adjusted input data for the drive, e. g. at expensive goods shipping etc.), • engine-driver fault at radio control, • wrong co-operation of engine-driver, conductor and other train staff, • fault of engine-driver at reporting (using the false train code – short distance between trains), • fire or smoke in locomotive, passenger cars, in cargo wagons, or the engine fire, • bad intent of engine-driver (change of speed, nonresponding to instructions from the control – dispatcher – centre or from surrounding trains, etc.), • engine-driver ignorance of procedures of train control (at unexpected emergency up to critical situations – e.g. obstacle on the rail track etc.). founded attacks on the train are the following: • rocket/missile from another train or buildings lying outside the track (throwing the stones and other heavy objects from the bridge above the rail track on the train etc.) • intent damage of rail track or track foundation, • illegal act in the train, • bad dispatcher intent, • bad intent of railway station service staff (worker guiding the movement of train at the railway station), • train collision with aircraft or another flying object. founded legislative and other causes are the following: • lack of regulations which prevent the wrong track setting at railway station, • missing the precise instructions for performance of maintenance of train body, rail tracks, track foundation and rail track vicinity, • missing the texts of intelligible and precise instructions for communication between engine-driver and control operation centre, • the absence of a uniform system of marking the railway crossings with roads and field roads serving 60 vol. 11/2017 domains of railway traffic in the czech republic figure 5. causes of railway accidents in the czech republic. to their identification from the viewpoint of rail and road topologies, enabling the direct designation and informing, • inappropriate behaviour of passengers at boarding, train drive or exit from train (non-observing the instructions, indiscipline, poor care on children behaviour at the train), • soggy or else damaged rail track foundation, • behaviour of road vehicle drivers on the road and rail level crossing (non-respecting the traffic symbols, sound signals or obstructions). the fish-bone diagram showing the main categories of railway accident causes is shown in figure 5. it only contains six main categories of causes. the diagram helps the specialists and the paper readers to in-depth insight into the problem of railway accidents. we see the causes arranged according to affinity, which enables to look up the possible actions, i.e. the measures for railway system safety improvement, for the whole groups of accident causes. because some accident causes repeat, it is necessary to prepare quality technical and organisational measures. sometimes, the relative simple measures can avert the accidents; the examples belonging to this category are for example the following: • the many railway accidents are on the crossing the rail track with road or field road where safety is only ensured by the symbol on the road "cautionary cross", • the huge material losses are at accidents of freight trains with expensive goods or with hazardous substances, • all rail track crossings with field routes are not denoted by warning symbols, • rail track crossings with routes are not clearly identified, • many works do not respect simple rule "the responsible person for order is the person who gives it, and the responsible person for order implementation is the person, who has to carry out it". therefore, it is necessary to pay attention to freight trains, the check-up from the side of rail workers at train loading seems very necessary – for this purpose the special check lists are necessary to prepare. for communication at critical situations it is necessary to prepare the texts of reports that are clear, concise and comprehensible. there are necessary rules for maintenance, regular technical overhauls of rail tracks etc. generally, it is necessary to take into account that accidents can occur and for these cases to prepare both, the train personnel, especially the engine-drivers and the railway stations personal, especially those who set up the train rotes. because each train accident means the reduction of traffic service and also costs for responses that are performed by the integrated rescue system that is paid from the public sources, the state administration needs to prepare legislative for improvement the safety in rail sector. 7. conclusion the railway accidents in the czech republic were divided into seven categories according to the accident sources. individual categories have the same domain of accident sources – technical – related to rail traffic vehicles; technical – related to rail infrastructure and railway station; railway operation control – organizational causes; railway operation control – cyber causes; control of rail traffic vehicles; attack on the train; legislative and other. for railway traffic safety improvement, it is necessary to pay attention to all categories and especially to this that lead to organizational accidents. the critical analysis of railway accidents revealed that some of accident causes often repeat, e.g. the insufficient maintenance, low-class overhauls and renovation. they have immediate cause that is not the root cause of such accident type; the root cause is in poor safety culture in the sector and in deficits in training. our research will continue in preparation of real tools for individual sectors of followed domain such as check lists, risk management plans and operational crisis plans for great railway stations, especially those in which the hazardous substances in great amount are present. acknowledgements authors thanks to czech technical university in prague for support (grant sgs2015-17). references [1] eu. directive 2004/49/ec of the european parliament and of the council of 29 april 2004 on safety on the community’s railways and amending council directive 95/18/ec on the licensing of railway undertakings and directive 2001/14/ec on the allocation of railway infrastructure capacity and the 61 d. procházková, j. procházka, t. kertis acta polytechnica ctu proceedings levying of charges for the use of railway infrastructure and safety certification (railway safety directive). [2] d.procházková. safety of complex technological facilities. saarbruecken: lambert academic publishing, 2015. isbn: 978-3-659-74632-1. [3] d. procházková, j.procházka, h.patáková, et al. kritické vyhodnocení přepravy nebezpečných látek po pozemních komunikacích v čr. čvut v praze, 2014. isbn 978-80-01-05599-1. [4] g.giannopoulos, r.filippini, m. schimmer. risk assessment methodologies for critical infrastructure protection. part i: a state of the art. european commission, joint research centre, institute for the protection and security of the citizen, eur 25286 en. luxembourg: publications office of the european union, 2012. isbn:978-92-79-23839-0. [5] d.procházková. analýza a řízení rizik. čvut v praze, 2011. isbn 978-80-01-04841-2. [6] d.procházková. strategie řízení bezpečnosti a udržitelného rozvoje území. čvut v praze, 2011. isbn 978-80-7251-243-0. [7] d.procházková. rizika spojená s pohromami a inženýrské postupy pro jejich zvládání. čvut v praze, 2014. isbn 978-80-01-05479-6. [8] ch.bérenguer, a. grall, c. g. s. (eds). advances in safety, reliability and risk management. london: taylor & francis group, 2012. isbn 978-0-415-68379-1. [9] d.procházková. základy řízení bezpečnosti kritické infrastruktury. čvut v praze, 2013. isbn 978-80-01-05245-7. [10] d.procházková. challenges connected with critical infrastructure safety. saarbruecken: lambert academic publishing, 2014. isbn: 978-3-659-54930-4. [11] j.reason. human error. cambridge: cambridge university press, 1990. [12] j. procházka, d.procházková. criticality of transportation infrastructure in czech republic, iricon 2016. acta polytechnica ctu proceedings, 2016. isbn: 978-80-01-06022-3, issn 2336-5382. 5. [13] archiv poznatků o pohromách a haváriích. čvut v praze, fakulta dopravní. [14] google. "www.google.com". [15] railway-technology. "www.railway-technology.com". [16] revolvy. "www.revolvy.com". [17] isdo. "www.isdo.org". [18] u. dot. railway accident reports. "http://specialcollection.dot.library.dot.gov/home". [19] d. inspekce. archiv. "http://www.dicr.cz". [20] d.procházková. metody, nástroje a techniky pro rizikové iženýrství. čvut v praze, 2011. isbn 978-80-01-04842-9. [21] t.kertis, d.procházková, j.procházka. railway accidents in the czech republic, causes of risks and their mitigation, in: esrel 2017 proceedings. london: crc pres / balkema. in print. 62 acta polytechnica ctu proceedings 11:53–62, 2017 1 introduction 2 risk and safety 3 criticality and safety management 4 data on railway system criticality and accidents 5 methods used at data processing 6 items important for safety improvement of railway traffic in the czech republic 7 conclusion acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2017.11.0063 acta polytechnica ctu proceedings 11:63–69, 2017 © czech technical university in prague, 2017 available online at http://ojs.cvut.cz/ojs/index.php/app possible ways of improving railway operation and infrastructure in area between děčín and rumburk pavel purkarta,∗, david vodáka, tomáš hořeníb, tomáš javoříka a department of transportation systems, ctu in prague faculty of transportation sciences, prague, czech republic b metroprojekt praha a.s., prague, czech republic, graduate of ctu in prague faculty of transportation sciences, prague, czech republic ∗ corresponding author: purkapav@fd.cvut.cz abstract. this paper is dealing with improving the railway transport on axis děčín – rumburk. this journey can be undertaken in two possible ways (as an interstate one or an international one through germany), however the connections between trains are limited (especially problematic is the rumburk junction, where the connection would be desired). the aim of this paper is to improve the connections in the whole area, in order to reach the maximal effectiveness of a railway passenger transport. keywords: rumburk, děčín, ústecký region, railway transport, periodic timetable, node, service conception, travel times reduction. 1. introduction and goals 1.1. researched area the researched area is located in ústecký region near borders with federal republic of germany, particularly in the area between towns děčín and rumburk (mainly north and south from děčín). the most significant towns in the area are děčín, česká kamenice, rumburk, varndorf and šluknov. whole area is also very attractive for tourists – some railway lines are located very close to the borders of national park české švýcarsko. following railway lines can be included into the researched area (numbers according to the timetable 2016/2017 [1], see fig. 1): • railway line 080 in the section česká lípa – jedlová, • railway line 081 děčín – benešov nad ploučnicí – jedlová – rumburk • railway line 083 děčín – bad schandau – dolní poustevna – šluknov – rumburk, • railway line 084 rumburk – panský – mikulášovice with bypass panský – krásná lípa (– rumburk), • railway line 086 in the section benešov nad ploučnicí – česká lípa, • railway line 089 liberec – zittau – rybniště, • railway line rumburk – jiříkov – ebersbach without regular passenger transport. 1.2. reasons behind selecting the researched area main reason for selection of this area was restoration of operation on border crossing dolní pustevna sebnitz in the year 2014. this brought certain changes in service conception in the area. unfortunately, those changes also caused some connection between trains in the station rumburk to disconnect. despite the fact, that some conceptual changes were made after the year 2014, current situation is still not optimal and that is why the problem is still topical. after the opening of border crossing dolní pustevna – sebnitz, most of the trains in rumburk were concentrated to the time period around minute x.30 [1], but it did not created periodic timetable node, because many trains remained without mutual connection due to infrastructural issues. it is also hardly possible to consider it partial periodic timetable node, because connections were actually interrupted, rather than made functional. change in passenger service conception came with the activation of railway station markvartice, where regular crossing of the passenger trains was moved. paradoxically, this made the creation of connections in the railway station rumburk even more matter of distant future. this step creates really close sequence of trains děčín – jedlová – rumburk and děčcín – česká lípa in the section děčín hl.n. – benešov nad ploučnicí that disrupts periodic sequence of trains in this section. therefore, effect of implemented conceptual changes is questionable. if the goal was to disconnect links to ec trains in the station děčín hl. n. (due to their limited reliability – more frequent and higher delays, compared with average values), it is possible to understand those changes. on the other hand, it is also possible to see those actions as a form of resignation to solve problems where they appear. formulation of those reasons made the team of authors to reflect on this issue and they attempted to find a solution. this solution should solve all the problems and also offer attractive pass through the railway 63 http://dx.doi.org/10.14311/app.2017.11.0063 http://ojs.cvut.cz/ojs/index.php/app p. purkart, d. vodák, t. hoření, t. javořík acta polytechnica ctu proceedings figure 1. diagram of railway network in the researched area. junction rumburk for the passengers. importance of this pass through is regional, but still important. 2. understanding and solving the issue solving of the issue was performed in several steps, as it is usual in transport planning: • detailed analysis of the commuting in the researched area in the relation to the railway transport and its possibilities, • specifying the principles of new service conception and important construction points of the periodic timetable • detailed design of service conception, • specifying necessary infrastructural changes to reach service conception 3. solution of the issue 3.1. summary of facts about commuting in the researched area to asses meaningfulness of transit through the railway junction rumburk it is necessary to evaluate regular commuting in the researched area first. to do so, data from the national counting of inhabitants, houses and flats 2011 [2], has been used. the evaluation of data shows following: • from the regional point of view, both commuting to děčín and rumburk are important. the closer to děčín or rumburk a populated area is, the more people are commuting, • part of inhabitants also commutes to smaller cities such as česká kamenice, benešov nad ploučnicí, šluknov and etc. • very significant is commuting to the regional capital ústí nad labem, but commuting to another regional capital – liberec, has also been found in a part of the area (railway line 038), • many inhabitants from the rumburk area are commuting to varnsdorf and we also cannot omit the influence of česká lípa, • it is appropriate to ensure availability of prague for whole area, • breaking branches of railway lines 081, 083 in rumburk does not influence major numbers of passengers (breaking somewhere else can have worse consequences), but their connection may bring significant improvement of transport in the whole area (direction to/from varnsdorf) of transport to larger cities – that makes this idea meaningful. it is crucial to identify transport relations, which would pass through this junction, to defend creation of connections in rumburk. as it has been already 64 vol. 11/2017 possible ways of improving railway operation and infrastructure mentioned – good connections and fast pass through station rumburk may attract many local inhabitants. significant interactions in relations šluknov – varnsdorf, velký šenov – varnsdorf, mikulášovice – varnsdorf, were identified in regular commuting. another relations, which can be considered as long distance commuting, were also identified. improving of pass through rumburk could have positive effect also on these relations. there is a potential of 250 – 300 regular commuters in villages located only near railway lines (with acceptable walking distance from village to railway line), this number is sufficient enough to take this into consideration. however it is necessary to prefer more important transport relations in order to not create a negative impact on them. 3.2. design of the new service conception design of the new service conception was made according to all the periodical timetable rules. the most important passenger line, which should eliminate all problems in the station rumburk, is new passenger line děčín – benešov nad ploučnicí – jedlová – rumburk – dolní poustevna – sebnitz – bad schandau – děčín. it is proposed to connect branches of both lines děčín – rumburk (an interstate one and an international one through germany) and create pass through model in the station rumburk. basic interval is 120 minutes. interval is shortened to 60 minutes in the section děčín – česká kamenice (– jedlová). another section with interval 60 minutes is děčín – schöna. combination of the new line and long-distance trains in the section jedlová – rumburk – šluknov – dolní poustevna also creates interval of 60 minutes (long-distance trains in the section šluknov – dolní poustevna are operated only on work days). it is crucial to establish important line construction points for periodical timetable. key connection in the station děčín is connection to ec trains from/to prague (node in the minute x.00). extra trains should be extended to/from ústí nad labem in the time of afternoon rush hours, as a fast connection to the regional capital (ec trains are operated only in 120 minutes interval). these trains should be operated with minimum stoppings (in the section děčín – ústí nad labem). other important construction point is in the station jedlová in minute x.00 and in the station rumburk in minute x.30 (because of important connections). second line branch from děčín, in direction bad schandau, arrives to děčín in the minute x.00. there are important connections to long-distance trains děčín – ústí nad labem – lovosice – praha and regional trains děčín – most. both railway line branches, which are entering the junction have the connection to/from prague ensured. in one case the new connection is faster than the one in current timetable. there are 4 vehicles necessary to provide base interval of 120 minutes (without reserve). those trains are also capable of ensuring extra trains děčín – schöna. another two vehicles are necessary for extra rush hour trains ústí nad labem – děčín – česká kamenice – jedlová (without reserve). together with designing new circular line, service conception of the lines in the area was also modified. this includes long-distance trains ústí nad labem – liberec and kolín – rumburk – šluknov and regional trains děčín – česká lípa, jedlová – česká lípa or rybniště – varnsdorf – liberec. also new routes of mikulášovice dolní nádraží – panský – rumburk – ebersbach trains were designed, in case of shortening intervals (mikulášovice dolní nádraží – panský – rumburk) or restoring the regular passenger operation (rumburk – ebersbach). and finally routes for a pair of seasonal tourist trains děčín – jedlová – krásná lípa – panský – mikulášovice dolní nádraží has also been found. complex design of whole system is summarized in the network diagram (fig. 2). 3.3. infrastructural changes necessary for the new service conception following infrastructure modifications are necessary for the application of designed timetable: • reconstruction of the railway station rumburk, so it can handle the operation of periodic timetable node at minute x.30, • reducing the travel times at the benešov nad ploučnicí – rumburk branch (reducing time by 2 or 3 minutes, in some parts of the branch reduction could be reached by using modern vehicles) • reducing the travel times at the rumburk – panský – mikulášovice dolní nádraží and rumburk – šluknov – mikulášovice dolní nádraží branches (reducing time by 5 minutes), • reducing the travel times in the section jedlová – česká lípa to the systematical travel time 30 minutes, • reducing the travel times of long distance trains in the section bakov nad jizerou – česká lípa by 10 minutes, • consider necessary modifications of the stations on railway lines 081 and 083, • in case of implementing a pair of seasonal tourist trains, it is necessary to divide section panský – mikulášovice into two blocks. 3.4. reconstruction of the railway station rumburk new timetable brings higher requirements for continuity and stability of train operations in the station rumburk. 65 p. purkart, d. vodák, t. hoření, t. javořík acta polytechnica ctu proceedings figure 2. network diagram summarizing proposal of service conception in the researched area. 66 vol. 11/2017 possible ways of improving railway operation and infrastructure railway station rumburk is junction for four railway lines from four directions with four neighboring railway stations: • krásná lípa (railway line 081), • šluknov (railway line 083), • panský (railway line 084 – currently with regular passenger transport only on weekends), • jiříkov (currently without regular passenger transport – not numbered). designed timetable assumes regular passenger transport on all four railway lines. new timetable is periodical with base intervals 60 or 120 minutes. in addition, full periodical timetable node in the minute x.30 is established in the station rumburk. first 3 railway lines above are connected to the western part of the station. railway line from jiříkov is connected to the eastern part of the station. operation on station head in the western part of the station, where three railway lines are meeting, must be organized very strictly. main reason is implementing periodical timetable, which causes all arrivals and departures to take place in 10 minutes (6 train routes in 10 minutes). due to pass through model many routes are colliding in the area of station head. to ensure the stability of new timetable arrivals of trains from šluknov, krásná lípa a panský must be performed without collision of routes. based on all mentioned conditions reconstruction of the railway station rumburk seems to be the best solution. reconstruction should include at least tracks configuration, platforms and interlocking [3, 4]. fig. 3 shows ideal variant of reconstruction. western station head allows simultaneous arrival from all three directions at the same time. longer platform edges are primary located by tracks, where trains to krásná lípa and šluknov are expected to stop. these trains have potential to be longer than trains on other branches. signal should be placed to the longer edge of platform 3, in order to coupling train sets. platform 1 is extra. this configuration of the railway station is crucial for implementation of new service conception. 3.5. infrastructure modifications for reducing the travel times and possible new stops from infrastructural point of view, this paper offers solution for benešov nad ploučnicí – jedlová – rumburk – šluknov – mikulášovice dolní nádraží section. upgrading this section is crucial for possible new circular line operation. all modifications are mainly undertaken in the sections between railway stations, railway stations are skipped as a kind of "black box" – cutting the travel times is considered, but modifications were not proposed in detail. main infrastructure modifications are changes in geometrical alignment and establishing speed profile for cant deficiency 130 mm. the longest section with new geometrical alignment has length of 9 km. steel sleepers with shape of "y" are used when there is a new geometrical alignment in a vertical curve, in order to avoid extending subgrade. limit values of the parameters of geometrical alignment are usually reached while increasing the maximal speed. maximal/minimal values are reached on exceptionally [4, 5]. also the reconstruction of signalling and control systems is designed in order to establish device with the third safety level. safety level is increased on some level crossing as well. section rumburk – šluknov is not considered in interlocking reconstructions, because current interlocking has already been ungraded to the third safety level. section šluknov – mikulášovice d.n. (– dolní poustevna) is also skipped, because all railway operation is currently organized according to rule sždc d3. table 1 shows reached travel times reduction on sections. travel times has been calculated in software "dynamika 1.7.1.", with railcar 843 as a reference vehicle. some proposed infrastructure modifications are not necessary, because travel times has already been reached in timetable 2016/17, but their introduction may increase the timetable stability. in order to improve railway transport availability some new stops are proposed – some are designed to replace inappropriate ones and some as completely new. for example: village horní habartice is located really close to the railway, so the new stop could be made there. new stop can replace current stop in veselá pod rabštejnem, which is located too far away from inhabited area. establishing new stop rumburkdolní křečany (fig. 4) on the railway line 083 could be also interesting, because this stop would be located in the center of inhabited area. stop rumburk-dolní křečany already exists on the 084 railway line, but second stop could attract more passengers. 4. conclusion during the problem solving process was managed to prove that the attractive connections between trains in the station rumburk can be reached without major infrastructure changes and without disconnecting any other important connections. this allowed formation of service conception not only for děčín and rumburk areas, but also for parts of česká lípa and liberec areas with overlap to germany. the result is the paper, which presents a possible way how to increase the effectiveness of a railway passenger transport in researched area. acknowledgements all the facts presented in the article are based on the results of work on project 12x2zs railway network of the czech republic and europe, which is undertaken on ctu faculty of transportation sciences, department of transport systems. 67 p. purkart, d. vodák, t. hoření, t. javořík acta polytechnica ctu proceedings figure 3. optimal variant of modifications of the railway station rumburk. section length travel time reduction [min] os r section benešov nad ploučnicí – jedlová benešov n. pl. – markvartice 5,4 -0,5 – markvartice – česká kamenice 7,9 -1 – česká kamenice – jedlová 15,4 -2 – section jedlová – rumburk jedlová – rybniště 9,3 -1,5 -1,5 rybniště – krásná lípa 4,8 -0,5 -0,5 krásná lípa – rumburk 6 -0,5 -0,5 section rumburk – mikulášovice dolní nádraží rumburk – šluknov 10 -2,75 -3 šluknov – mikulášovice d.n. 10,6 -1,75 – table 1. travel time reduction on individual sections. figure 4. proposed location of the new stop rumburk-dolní křečany. 68 vol. 11/2017 possible ways of improving railway operation and infrastructure references [1] sždc. aids of theoretical graph of train running. (for researched area between years 2014 – 2016). [2] commuting to work and school according to summation of inhabitants, houses and flats 2011 – ústecký region [online], 2013. [cit. 2015-02-20], https://www.czso.cz/csu/czso/dojizdka-do-zamestnania-skol-podle-scitani-lidu-domu-a-bytu-2011-usteckykraj-2011-b9vyzff32a. [3] čsn 73 4959. platforms and roofs of platforms of state, regional and industrial railways. únmz praha, 2009. [4] čsn 73 6360-1. geometrical characteristics of railway tracks – part 1: layout. únmz praha, 2008. [5] sždc s.o. drawing tables of superstructure. (for researched area). 69 acta polytechnica ctu proceedings 11:63–69, 2017 1 introduction and goals 1.1 researched area 1.2 reasons behind selecting the researched area 2 understanding and solving the issue 3 solution of the issue 3.1 summary of facts about commuting in the researched area 3.2 design of the new service conception 3.3 infrastructural changes necessary for the new service conception 3.4 reconstruction of the railway station rumburk 3.5 infrastructure modifications for reducing the travel times and possible new stops 4 conclusion acknowledgements references 3 acta polytechnica ctu proceedings 2(1): 3–20, 2015 3 doi: 10.14311/app.2015.02.0003 the golden age of cataclysmic variables and related objects (old and news) f. giovannelli1, l. sabau-graziati2 1inaf istituto di astrofisica e planetologia spaziali, area di ricerca di tor vergata, via fosso del cavaliere, 100 i00177 roma, italy 2intadpt. cargas utiles y ciencias del espacio, c/ra de ajalvir, km 4 e28850 torrejón de ardoz, madrid, spain corresponding author: franco.giovannelli@iaps.inaf.it abstract in this paper we review cataclysmic variables (cvs) discussing several hot points about the renewing interest of today astrophysics about these sources. we will briefly discuss also about classical and recurrent novae, as well as the intriguing problem of progenitors of the type ia supernovae. this paper is an extended and updated version of the review by giovannelli (2008). because of limited length of the paper and our knowledge, this review does not pretend to be complete. however, we would like to demonstrate that the improvement on knowledge of the physics of our universe is strictly related also with the multifrequency behaviour of cvs, which apparently in the recent past lost to have a leading position in modern astrophysics. keywords: cataclysmic variables dwarf novae intermediate polars polars novae optical spectroscopy photometry sub-mm ir radio uv x-rays multifrequency astrophysics x-ray binary systems high energy astrophysics. 1 introduction in the 1950s it was recognized that the various phenomena displayed by the cvs are all the consequence of accretion of matter onto a white dwarf (wd) from a low mass donor star (e.g., warner, 1976; 1995a). cvs are binary systems in which the primary component is a wd (mwd ∼ 1 m�) and the secondary is a late type main sequence star (ms ≤ 1 m�) (e.g., smak, 1985a). mass transfer is strongly depending, besides the orbital parameter of the system, on the magnetic field intensity at the surface of the primary. such process produces a large fan of behaviour that are detectable in different energy ranges: from radio to x-rays, and even in γ-rays. the orbital periods of cvs are ranging from ∼ 80 m to ∼ 12 h with a distribution showing a gap between 2 and 3 hours, in which few systems have been detected. in the past this gap was empty and this was the reason because was nicknamed ’period gap’. more than 600 cvs are known, most of them discovered through optical observations, and some, especially those in which the magnetic field of the wd is strong, discovered through x-ray observations, but with the detectors of the second and further generations, since cvs are in general not very bright in x-ray energy range. those with known or suspected orbital period are listed by ritter & kolb (1998). the first cv detected in the x-ray range, with rocket experiments, was the dwarf nova ss cyg (rappaport et al., 1974; heise et al., 1978). the uhuru satellite detected two cvs, which were not recognized as such. warner (1976) proposed the identification of 4u 1249-28 with ex hya, and the variable am her, which on further optical studies was recognized as a cv (forman et al., 1978). the magnetic field in these two systems is strong (≈ 107–108 g). a few dozen cvs were detected in x-rays with heao-1 satellite, with exosat, and with the einstein satellite (e.g., reviews of cordova & mason, 1983; cordova, 1995). later verbunt et al. (1997) recognized 91 cvs from a sample of 162 systems with known or suspected binary periods by using data of the rosat xrt-pspc all sky survey. historically, because cvs were observed photometrically and without seeming to follow any regular pattern, they were named with the term cataclysmic (from the greek word kataklysmos = flood, storm; hack & la dous, 1993). as collecting of observational data progressed it became apparent that these objects were regular binary systems which for some reason changed in brightness; some of them also regularly (recurrent novae and dwarf novae) while some others only once (classical novae). therefore the classification of cvs was based on the optical outburst properties, by which one may distinguish four groups of cvs: (i) classical novae; (ii) recurrent novae; (iii) dwarf novae; (iv) nova3 http://dx.doi.org/10.14311/app.2015.02.0003 f. giovannelli, l. sabau-graziati like objects (e.g., giovannelli & martinez-pais, 1991 and references therein; ritter, 1992; giovannelli, 2008). this classification, however, is neither self-consistent nor adequate and it is much better to consider primarily the observed accretion behaviour (smak 1985b). one obvious advantage of such an approach is connected with the time scales of various accretion phenomena, which are sufficiently short to avoid any major observational bias: the mass accretion rates in cvs usually range from 10−11 to 10−8 m� yr−1 (patterson, 1984); the time scales are from tens of seconds (oscillations in dwarf novae at outbursts) to years (super-outbursts of su uma stars or long term variations in vy scl stars). however, in the class of nova-like objects there are two sub-classes: the dq her stars and the am her stars. in these sub-classes of cvs the wds possess magnetic fields with intensity enough high for dominating the accretion disk and all the phenomena related to it. these classes of magnetic cvs, whose names are coming from the prototypes dq her and am her took later the names of intermediate polars and polars, respectively. a short history of their discovery has been discussed by warner (1995b). fundamental papers about these subclasses are those by patterson (1994), warner (1996). the class of ips has been split into two subclasses with relatively large and relatively weak magnetic field (norton et al., 1999). one example of a system belonging to the latter subclass is do dra (previously registered as yy dra) (andronov et al., 2008). there is another class of cvs, the rare am canum venaticorum (am cvn) star systems. they have extremely short orbital periods between ∼ 10 − 65 minutes. their spectra do not show evidence for hydrogen. they appear to be helium–rich versions of cvs (e.g. warner, 1995c; nelemans, 2005). there is an old suggestion, that in these systems the mass transfer is driven by gravitational wave radiation losses, proposed by paczyński (1967), after the discovery of the prototype with an orbital period of ∼ 17 minutes (smak, 1967). depending on the magnetic field intensity at the wd, the accretion of matter from the secondary star onto the primary can occur either via an accretion disc (in the so-called non-magnetic cvs: nmcvs) or a channelling through the magnetic poles (in the case of polars: pcvs) or in an intermediate way (in the case of intermediate polars: ipcvs). cvs in a time scale of order between weeks and years flare up almost periodically, about few magnitudes in optical wavelengths; the duration of the outbursts is much shorter than the recurrence time. typical light curves for classical novae and dwarf novae of the u gem, z cam, and su uma types can be seen in ritter (1992) and e.g. in giovannelli (2008). the recurrence time-scale of outbursts in dwarf novae is correlated with their amplitude and the outburst duration is depending on the orbital period (warner, 1987). in pcvs the wd magnetic field is strong enough to make the alfvén radius greater than the circularization radius, so no accretion disc is formed and the accretion structure is fully governed by the magnetic field, which canalize the accreting matter across the field lines. owing to the intense magnetic field (∼ 10–200 mg), the wd rotation is synchronized with the binary orbital period (a few hours). however, there are few systems (v1432 aql, by cam, v1500 cyg, v4633 sgr, and cd ind) in which pspin and porb differ by around 2% or less. these are assumed to be polars that have been disturbed from synchronism by a recent nova explosion (norton, sommerscales & wynn, 2004, and the references therein). ipcv wds have moderate magnetic fields (order of a few mg); the alfvén radius is smaller than the circularization radius but it is greater than the wd radius. therefore an accretion disc is formed in these systems but being disrupted at its inner region. in ipcvs matter follows again the magnetic field lines but just inside the alfvén radius. the rotating wd is asynchronous with the binary orbital period (pspin � porb). however, there are few systems that may be best described as nearly synchronous intermediate polars (v381 vel, rxj0524+42, hs0922+1333, and v697 sco) (norton, sommerscales & wynn, 2004, and the references therein). two of these systems lie in the ‘period gap‘. probably all four systems are ips in the process of attaining synchronism and evolving into polars. the last group defined by the accretion structure criterion, nmcvs, includes those systems whose wd magnetic fields are not relevant in governing the accretion structure. in these systems the accretion disc extends down to the wd surface and a boundary layer is formed. this family shows a great diversity of observational behaviour; for this reason the historical criterion of classification is, in this case, more appropriate for distinguishing their sub-classes. however, it is simply an attempt of classification for lack of a more general physical classification (e.g., giovannelli, 1991 and references therein). indeed, in general, we can consider the wd of a cv as a gravimagnetic rotator, characterized by a mass m — accreting matter at a rate ṁ from the optical companion (the secondary star) — rotating with a velocity ~ω and having a magnetic moment ~µ, not necessarily coaxial with the rotational axis (lipunov, 1987; 1991). then the accreting system is completely characterized by the following physical parameters: mass m, accretion rate ṁ, rotational velocity ~ω, and magnetic moment ~µ. in the plane spin period of the wd (and in general of the compact object) – gravimagnetic param4 the golden age of cataclysmic variables and related objects (old and news) eter y = ṁ/µ2 it is possible to find any sort of physical conditions of gravimagnetic rotators, as discussed by e.g. giovannelli (1991). figure 1 shows the positions of few cvs in the diagram spin period of the wd – gravimagnetic parameter (in unit -42): am her (terada et al., 2010 and references therein), ae aqr (patterson, 1979; de jager et al., 1994; wynn, king & horne, 1997), dq her (patterson, 1994; zhang et al., 1995), ei uma (reimer et al., 2008), ss cyg (giovannelli & sabau-graziati 2012a). it appears evident the power of this diagram obtained by lipunov (1987) using 1 m� white dwarf. the polars am her and ae aqr lie in the zone of propeller, how they must stay, while the ipcv dq her (the prototype of this class), ei uma (a very well known ipcv), and ss cyg (whose nature as ipcv is claimed by giovannelli’s group on the base of many circumstantial proofs and also because of a cogent similarity with ei uma, e.g. giovannelli & sabau-graziati 2012a,c) lie just in the zone predicted by lipunov for such objects. figure 1: the positions of several cvs in lipunov’s diagram calculated for 1 m� white dwarf (after lipunov, 1987). as recalled by giovannelli & sabau-graziati (1999), it is evident that the properties of an outburst in cvs depend crucially on the accretion rate, the mass of the wd, and the chemical composition of its hydrogen rich envelope in which the thermonuclear runaway occurs. and the accretion process onto the wd is strongly influenced by its magnetic field intensity. indeed, the three kind of cvs (non-magnetic, polars, and intermediate polars) obey to relationships between the orbital period of the system and the spin period of the wd (warner & wickramasinghe, 1991), where the magnetic field intensity plays a fundamental role. the orbital evolution of cvs, and hence the mass-transfer rate (ṁ) from the secondary to the white dwarf is driven by magnetic braking of the secondary for long-period systems (porb > 3 hr) and gravitational radiation for shortperiod systems (porb < 2 hr). however, such a gap — which was believed true for long time — is now partially filled by the sw sex systems (e.g. rodriguez-gil, 2003; rodriguez-gil et al., 2007). the apparent ’period gap’ was due to a smaller number of systems having orbital periods in such an interval, which were escaping from the observations. therefore the investigation on the magnetic field intensities in wds is crucial in understanding the evolution of cvs systems. the fundamental parameters to be searched are the magnetic moment, the mass accretion rate and the orbital parameters of the systems. in this way it will be possible to fulfill the plane log pspin– log porb, where a priori there are not restricted ranges of magnetic moment | ~µ |, or special correlations between pspin and porb and | ~µ |. the distribution of objects in that diagram is owed to the interaction of braking torques and accretion torques, with the superposition of the observed or implied variations of the accretion rate on long time scale (> 102 yr), acting on a continuum of magnetic moments. in this way each system is completely described by those physical parameters. davis et al. (2008) applied population synthesis techniques to calculate the present day number of two types of wd-main sequence star (wdms) binaries within the ’period gap’. the first are post-common envelope binaries with secondary stars that have masses 0.17 ≤ ms/m� ≤ 0.36 (gpcebs), such that they will commence mass transfer within the period gap. the second type are systems that were cvs at some point in their past, but detached once they evolved down in orbital period to ≈ 3 h as a consequence of disrupted magnetic braking, and are crossing the ’period gap’ via gravitational radiation (dcvs). they predicted an excess of dcvs over gpcebs within the ’period gap’ of ∼ 4 to∼ 13. this excess is revealed as a prominent peak at the location of the ’period gap’ in the orbital period distribution of the combined gpceb and dcv population. they suggest that if such a feature is observed in the orbital period distribution of an observed sample of short orbital period wdms binaries, this would strongly corroborate the disruption of magnetic braking. willems et al. (2005) and willems et al. (2007) by using population synthesis tools studied the population of nmcvs with orbital periods 1) < 2.75 h, and 2) > 2.75 h, respectively. 1) a grid of detailed binary evolutionary sequences was calculated and included in the simulations to take account of additional angular momentum losses beyond that associated with gravitational radiation and mass loss, due to nova outbursts, from the system. as a specific example, willems et al. (2005) considered the 5 f. giovannelli, l. sabau-graziati effect of a circumbinary disc to gain insight into the ingredients necessary to reproduce the observed orbital period distribution. the resulting distributions showed that the period minimum lies at about 80 minutes, with the number of systems monotonically increasing with increasing orbital period to a maximum near 90 minutes. there is no evidence for an accumulation of systems at the period minimum, which is a common feature of simulations in which only gravitational radiation losses are considered. the shift of the peak to about 90 minutes is a direct result of the inclusion of systems formed within the period gap. 2) the population of nmcvs with unevolved main– sequence–like donors at orbital periods greater than 2.75 h was investigated. in addition to the angular momentum losses associated with gravitational radiation, magnetic braking, and mass loss from the system, willems et al. (2007) also included the effects of circumbinary discs on the evolution. for a fractional mass input rate into the disc, corresponding to 3×10−4 of the mass transfer rate, the model systems exhibit a bounce at orbital periods greater than 2.75 hr. the simulations revealed that: i) some systems can exist as dwarf novae throughout their lifetime, ii) dwarf novae can evolve into novalike systems, and iii) novalike systems can evolve back into dwarf novae during their postbounce evolution to longer orbital periods. among these subclasses, novalike cataclysmic variables would be the best candidates to search for circumbinary discs at wavelengths ≥ 10 µm. the theoretical orbital period distribution is in reasonable accord with the combined population of dwarf novae and novalike systems above the period gap, suggesting the possibility that systems with unevolved donors need not detach and evolve below the period gap as in the disrupted magnetic braking model. experimental data are necessary for checking the validity of theoretical predictions. the field strength distribution of mcvs differs from that of single wds, although both cluster around 30 mg. the distribution of isolated wds extend on a wide range of magnetic field strength (∼ 105–109 g), whilst in accreting wds of cvs, as far as is presently known, there is a lack of systems at both high and low field strengths. however, the apparent absence of low field mcvs might be explained by the ips, which generally have unknown field strengths, and the lack of high field systems is still not understood (e.g., beuermann, 1998). wynn (2000) discussed the problem of accretion flows in mcvs. on the base of the ratio pspin/porb he divided the mcvs in three classes: class 1, class 2 and class 3 if such a ratio is � 0.1, ∼ 0.1, and � 0.1, respectively. for the systems in class 1 the disc equilibrium condition is clearly satisfied. those in class 2 are very unlikely to possess accretion discs. the systems in class 3 are ex hya-like systems which lie below the ’period gap’ and cannot possibly contain accretion discs. these are ex hya, ht cam, rxj1039.7-0507, and v1025 cen. these all have pspin/porb > 0.1 and porb < 2 hr. dd cir and v795 her lie within the ’period gap’ with pspin/porb ∼ 0.1 and may be included in the class 2 (norton, somerscales & wynn, 2004, and the references therein). wynn (2000) crudely classified the mcvs according to the magnetic moment and orbital period. ex hya systems have magnetic moment similar to ips above the ’period gap’ and comparable to the weakest field am her-like systems. this indicates that mcvs above the ’period gap’ will evolve to long spin periods below it. norton, wynn & somerscales (2004) investigate the rotational equilibria of mcvs. they predict that ipcvs with µ ≥ 5× 1033 g cm3 and porb > 3 hr will evolve into pcvs, whilst those with µ ≤ 5 × 1033 g cm3 and porb > 3 hr will either evolve into low field strength polars that are presumably unobservable, and possibly euv emitters, or into pcvs when their fields, buried by high accretion rate, revive when the mass accretion rate reduces. warner (1996) deeply discussed torques and instabilities in ips on the base of measured spin periods of the primaries and found several important relationships between fundamental parameters of these systems, such as log ṁ vs log porb, log µ33 vs log ṁ17, log lx vs log ṁ, as shown in fig. 2 in the left, central, and right panels, respectively. there is a range of magnetic moments µ and mass transfer rates in which synchronized rotation of the primary can occur even though it possesses an accretion disc. ak et al. (2010), using available astrometric and radial velocity data, computed the space velocities of cvs with respect to the sun and investigated kinematical properties of various sub-groups of cvs. the orbital period distribution of cvs in the refined sample of 159 systems resembles that of the whole sample of cvs (e.g. connon smith, 2007). ak et al. (2010) found that the mean kinematical age (mka) of the 159 systems is mka159 = 5 ± 1 gyr. in the sample, 134 of 159 systems are non magnetic (nmcv) having mkanmcv = 4.0± 1.0 gyr. in the sub–sample of nmcvs, 53 of 134 have porb < 2.62 h and their mka is 5.0±1.5 gyr, whilst 81 of 134 systems have porb > 2.62 h and their mka is 3.6±1.3 gyr. this means that cvs below the ’period gap’ are older than systems above the gap. this result in agreement with the standard evolution theory of cvs. the selection of 2.62 h as the border between the two groups of systems lies roughly in the middle of the ’period gap’, where systems have been detected. this means that the ’period gap’ does not exist anymore and the systems inside this ’gap’ are just frontier objects between systems experiencing gravitational radiation and those experiencing magnetic braking. the reason because they are not so numerous as 6 the golden age of cataclysmic variables and related objects (old and news) figure 2: left panel: intermediate polars in the plane ṁ-porb. the two lines are lines of disc stability: stable above and dwarf nova outburst below. they were computed for f=0.2 and 0.3, with rout = f × a, being a the separation of the two stars in the system (by courtesy of warner, 1996). central panel: magnetic moment in units of 1033 g cm3 versus mass accretion rate in units of 1017 g s−1. boundaries for white dwarf mass m1 = 1 m� and m1 = 0.6 m� have been computed for porb = 4 h (by courtesy of warner, 1996). right panel: mass transfer rate onto white dwarf versus (2-10 kev) x-ray luminosity (by courtesy of warner, 1996). those placed at sides could be the relative shorter time of permanence in the ’gap’, and then difficult to be detected. our opinion is that a more appropriate investigation of the class of the so-called ipcvs is necessary. indeed, such systems could show surprises if deeply studied, as for instance occurred for ss cyg. this system, usually considered a nmcv because of a classification once made by bath & van paradijs (1983), whilst since 1984 it has been claimed as an ipcv by giovannelli et al. (1985) and later confirmed several times (e.g., giovannelli & martinez-pais, 1991; giovannelli, 1996; giovannelli & sabau-graziati, 1999, gaudenzi et al., 2002). giovannelli & sabau-graziati (2012a) discussed all the circumstantial proofs in favor of the magnetic nature of ss cyg, as well as those adverse, concluding with reasonable certainty that its nature is magnetic, being the magnetic field intensity b = 1.7 ± 0.8 mg, in agreement with the value (b ≤ 1.9 mg) derived by fabbiano et al. (1981) by using x-ray, uv and optical coordinated measurements. this would teach a lesson: it is mandatory to observe cvs for long time in order to follow at least a whole period of the binary system between two successive outbursts. this is, of course, possible only for systems like dwarf novae where the almost periodical outbursts occur in time scales of weeks-months. networks of robotic telescopes can help in this matter (giovannelli & sabau-graziati, 2012b) however, we can say that cvs form a broad stellar family of highly variable and dynamical members. when it comes to explaining particulars about, e.g., the detailed interaction between the transferred matter and the wd’s atmosphere; irregularities within regular photometric behaviour; turbulent transport in the disc; or the final fate of these objects, more is missing than what is known, rendering their study ever more challenging. at least, cvs are natural multi–wavelength laboratories offering us the possibility of studying in detail the behaviour of plasma and radiation under extreme physical conditions. the understanding of stellar evolution, electromagnetism and polarization, mass and radiation transfer or 3-d geometrical effects, in a broad spectral range from hard x-rays to radio, is mandatory for improving the knowledge of the nature of cvs. variability, from milliseconds to hundreds of years, follows from different physical processes taking place in these systems and can be studied by means of several astronomical techniques. as our skills in developing further these techniques grow our understanding of the cvs insights also grows; and the more we learn about cvs the further techniques and theory develop. on the other hand, it is well known that conclusions obtained in the field of cvs have been extrapolated, upwards or downwards in scale, to other fields such as agns or lmxrbs, and vice versa. from such exchanges of information and results astrophysical research in general always benefits. rapid oscillations in cvs are particularly interesting. as reviewed by warner (2004), the rich phenomenology of dwarf nova oscillations (dnos) and quasi–periodic oscillations (qpos) observed in cvs favour the interpretation that these rapid brightness modulations (3 to 11,000 s timescales) are magnetic in nature — magnetically channelled accretion from the inner accretion disc for dnos and possible magnetically excited traveling waves in the disc for qpos. there is increasing evidence for the magnetic aspects, which extend to lower fields the well–known properties of strong field (pcvs) and intermediate strength field (ipcvs) cvs. the result is that almost all cvs show the presence of magnetic fields on their wd primaries, although for many the intrinsic field may be locally enhanced by the accretion process itself. there are many behaviour that parallel the qpos seen in x-ray binaries, with 7 f. giovannelli, l. sabau-graziati high– and low–frequency x–ray qpos resembling, respectively, the dnos and qpos in cvs. other papers about rapid oscillations in cvs are those by warner & woudt (2005) and pretorius, warner & woudt (2006). the current estimate of the space density of cvs is of ∼ 3 × 10−6 pc−3 (warner, 2001). this may be a significant underestimate of cvs space density, as discussed by patterson (1984). although densities from the most comprehensive optical palomar–green survey raises the estimate at (3−6)×10−6 pc−3, x-ray all-sky surveys give densities of ∼ 1 × 10−5 pc−3 for detected systems of low ṁ in hard x-rays (patterson, 1998). then from observational point of view, it is necessary an intensive search for the faint cvs predicted by population synthesis with orbital periods at ∼ 80 − 100 min that have passed through the orbital period minimum at ∼ 78 min and have increasing orbital periods. this research must be done among the low ṁ systems detected by x-ray surveys. thanks to its high sensitivity, integral is very useful for this purpose. up to now, it discovered several new faint cvs, with porb > 3 hr, and only one with porb < 3 hr (e.g. šimon et al., 2006; hudec et al., 2008). high speed photometry of faint cvs have shown that: i) 1 of 10, tv crv has porb = 1.509 hr (woudt & warner, 2003); ii) 5 of 13 have porb < 2 hr (woudt, warner & pretorius, 2004); iii) 1 (cal 86) of 12 has porb = 1.587 hr (woudt, warner & spark, 2005); iv) 3 of 11 have porb > 3 hr (witham et al., 2007). for reviews about cvs see the fundamental papers by robinson (1976), patterson (1984, 1994), hack & la dous (1993), and the books of warner (1995a) and hellier (2001). more recent reviews are those by connon smith (2007), and giovannelli (2008). the long review the impact of space experiments on our knowledge of the physics of the universe by giovannelli & sabaugraziati (2004) contains also a part devoted to cvs. 2 multifrequency emissions in cvs there are several components that are responsible for the total emission. deep discussions about these components can be found in the literature, but there is a recent review by giovannelli (2008, and the references therein) that exhaustively summarize their contributions to the total multifrequency emission of cvs. briefly, such components are: a) the secondary stars are cool main sequence stars with spectral type ranging from g8 to m6, corresponding to temperatures from 5,000 to 3,000 k. their contribution is mainly in red and ir regions of the electromagnetic spectrum. b) the primary stars. the temperatures of wds are known only in few cases: when they belong to high inclination systems, or when they accrete matter with a very low mass transfer rate. however, the wd temperatures range between 10,000 and 50,000 k (sion, 1986; 1991). urban & sion (2006) found that the wds in cvs above the period gap are hotter and more accretion heated (teff = 25, 793 k) than those below the gap (teff = 18, 368 k). therefore wds are expected to radiate essentially in the uv, but they can be visible also in the optical range if they are not too hot. c) the accretion disc: it does not have a homogeneous temperature, but spans a large range. since the temperature distribution in discs is poorly known, in order to obtain a rough evaluation of their contribution to the total emission it is necessary to evaluate the contributions at different frequencies of a synthetic disc constituted of black bodies at different temperatures, the temperature distribution being that of a stationary accretion disc (e.g., la dous, 1994). it then appears evident that the contribution of such an accretion disc is important in the whole range between euv and ir, depending on the choice of the disc parameters. furthermore, the uv radiation can be supplied from a zone in the vicinity of the wd (some ten stellar radii), which could contain any optically thick material left there. however, the argument of accretion discs deserves an important comment. it appeared evident that the viscosity of matter inside the accretion discs plays a fundamental role in the description of physical processes occurring there. in spite of numerous attempts in determining such a viscosity, the physical nature of that still remains largely indeterminate. the best training for study the viscosity is the subclass of dwarf novae, showing quasi-periodic outbursts which occur on a time scale from weeks to months (or even years) and are due to non-stationary accretion. meyer & meyer-hofmeister (1981, 1982, 1983) firstly discussed the physical mechanism responsible for dwarf nova outbursts which is connected with the thermal instability of the disc which occurs in the temperature range corresponding to the ionization of hydrogen. soon after smak (1984a,b) extended the study of such a mechanism. the details are summarized by smak (2002). it is important to point out that the shapes of dwarf nova light curves, which depend on a number of relevant parameters, depend also on viscosity. in particular, the characteristic time-scales observed during outbursts depend on the viscous time-scale. this provides an important and almost unique opportunity of obtaining some constraints on viscosity or – within the α disc approach (shakura & sunyaev, 1973) – of an empirical determination of α. for a more complete and detailed discussion of dwarf novae and models of their outbursts – see reviews by cannizzo (1993), osaki (1996), and lasota (2001). 8 the golden age of cataclysmic variables and related objects (old and news) d) the boundary layer: a very important zone for the emission is that of the transition between the accretion disc and the wd surface, namely the boundary layer. it is possible that all, or at least a significant fraction, of the kinetic energy of the material contained in the accretion disc must be radiated away within the geometrically very small boundary layer in order to have the possibility of the material accreting onto the wd’s surface. then, whatever the situation, one can assume the presence of a strong x-ray source at the boundary layer, which will be visible also in the euv and shortwavelength uv according to the choice of disposable theoretical parameters. most of the radiation then comes from the accretion disc and boundary layer, which contribute roughly 50% each. from the accretion disc the radiation is essentially emitted in the optical and uv, whilst from the boundary layer — optically thick (which occurs at high accretion rates) — the radiation is emitted in the soft x-ray range; when the accretion is at low rates the boundary layer is optically thin and appears as a hard thermal bremsstrahlung source. these predictions have been tested experimentally, comparing the observations of cvs in optical, uv and soft x-ray ranges (e.g. wood et al., 1989; horne et al., 1994). e) the gas stream: it is definitively optically thin and cool and contains rather little material; so, probably, its contribution to the total emission of cvs is negligible at all frequencies as source of continuum, whilst it could contribute to the formation of lines in the red and ir regions. f) the hot line: the energy excess zone in the place where the stream comes to the disc is a shock wave. this zone was previously known as ‘the hot spot‘. its structure and radiation characteristics are still an open problem; it is visible in many systems in optical photometry (less in the ir and never in the uv) as a periodically recurring hump in the orbital light curve. its temperature must be ≤ 10, 000 k. g) hot corona or chromosphere: is a shell of optically thin and rather hot gas, below and above the accretion disc. x-ray and uv line radiation are tentatively attributed to it, whilst it probably does not contribute to the uv, optical or ir continuum emission. usually no radio emission from cvs has been measured. only upper limits for individual systems of order of a few mjy are available (e.g ≤ 10 mjy in ss cyg — cordova, mason & hjellming 1983). however, recently, körding et al. (2008) detected a radio flare from ss cyg peaked at 1.1 mjy with a duration of order 20 days, above the upper limit of 0.08 mjy. this radio flare was simultaneous with the optical long outburst peaked at about 1 jy. pringle & wade (1985) computed the contribution functions of the most important components of a cataclysmic system, previously discussed. the plot can be found also in fig. 6 of giovannelli’s paper (2008). during quiescence dwarf novae emit essentially hard x-rays (∼ 0.1–4.5 kev) and the flux distribution is rather well approximated by a thermal bremsstrahlung with ktbrems ≈ 10 kev (cordova & mason, 1983). a direct correlation between the hard x-ray/optical fluxes ratio and hβ equivalent width has been found by patterson & raymond (1985). during outburst dwarf novae emit soft x-rays (0.18– 0.5 kev) with an increase of the flux of the order of 100 or more, although most of the radiation is hidden in the euv range (cordova & mason, 1984). the soft x-ray spectra can be fitted either with black bodies at ktbb ≈ 25–30 ev or, alternatively, with bremsstrahlung spectra at ktbrems ≈ 30–40 ev. the most important features are the anti-correlation between the hard and soft x-ray emissions during the outburst cycle and the correlation between soft x-ray and optical emissions, as measured for ss cyg (watson, king & heise, 1985), or — what is the same — anti– correlation between the hard x-ray and optical emission (ricketts, king & raine, 1979). during an outburst of ss cyg there is also the correlation of optical and euv emissions, that are anti–correlated with the hard x-ray emission detected by rxte (wheatley et al., 2003). what does that mean? the uv flux and the bulk of optical flux in dwarf novae and nova-like stars originate in the accretion disc. the ir flux observed during quiescence and possibly some of the optical flux come from the secondary late-type star. the rise to an outburst either occurs simultaneously at all wavelengths when it is slow, or progressively starts later with decreasing wavelengths when it is fast, since ever more central hotter parts of the disc become involved. indeed, several dwarf novae have been observed in the uv and optical during the rise to maximum outburst brightness and their behaviour are quite similar: the uv rise lags the optical rise by up to a day (e.g., vw hyi: hassall et al. (1983); cn ori and rx and: cordova, ladd & mason (1986); wx hyi: hassall, pringle & verbunt (1985). with respect to the optical band, this lag is similar also in the euv region covered by the voyager (50–1200 å) for ss cyg (polidan & holberg, 1984) and vw hyi (polidan & holberg, 1987). this fact strongly supports the origin of the outburst being in the cooler outer part of the disc rather than in the hotter parts near the wd; therefore mininova models for the outbursts are probably excluded (cordova & howarth, 1987). the two models for triggering the outbursts, compatible with the lag observations, are then: 9 f. giovannelli, l. sabau-graziati • an instability in the secondary star which allows the transfer of more mass to the disc; • a thermal instability in the outer disc, which results in material stored there being suddenly transported through the disc. during the decline, the whole disc cools simultaneously. the contribution to the total emission from the boundary layer between the disc and the wd surface is in uv and x-ray ranges: the boundary layer is optically thin during quiescence and then emits hard x-rays, but it is optically thick during outburst and then emits soft x-rays since the radiation is thermalized before escape (la dous, 1993). from the euve (extreme ultraviolet explorer) craig et al. (1997) have shown the euv spectra of three npcvs in outburst, namely vw hyi, u gem and ss cyg. vw hyi shows the softest euv spectrum peaked at ∼ 250 å. however, its boundary layer/disc luminosity ratio — lbl/ldisc ∼ 0.2 (mauche et al., 1991) — is in contrast with the boundary layer models. in u gem the total size of the euv emitting region is comparable to that of the wd itself, which indicates that the outburst in mainly confined to the inner disc/boundary layer region (long et al., 1996). its lbl/ldisc ∼ 1. ss cyg is the hardest of the three euv nmcvs. its euv spectrum is rather complex and changed by a factor 100 during the outburst with an almost constant spectral energy distribution. quasi-coherent oscillations (∼ 7−9 s) have been detected in the euv emission (mauche, 1996). ss cyg shows no euv emission longward ∼ 130 å. mauche, raymond & mattei (1995) found that for ss cyg the relation lbl/ldisc ∼ 1, valid for the boundary layer models is strongly violated, being this ratio lbl/ldisc ∼ 0.07. and this is one more proof that ss cyg is not a nmcv — as already remarked — as claimed by giovannelli’s group. iue satellite deserves special comments since it was fundamental in improving the knowledge of cvs. a detailed review can be found in giovannelli (2008). briefly, the iue gave significant contributions on: i) knowledge of disc accreting and magnetic cvs, as extensively discussed by cordova (1995), and references therein. ii) nature of the high velocity winds. during outburst of ss cyg the spectral emission features disappear or go into absorption, some of them showing p cygni profile (e.g. civ), which clearly indicate the presence of high velocity wind from the system. the emission features appear again when the system is going into quiescence (giovannelli et al., 1990). iii) boundary layer emission. multifrequency observations show that x-ray luminosity at all outburst phases is much lower (about at least a factor 10) than the uv/optical luminosity from the disc, as expected from the models (e.g., mauche, 1998). this simply means that the boundary layer models are not correct. iv) underlying wd and its photosphere. iue provided the first evidence that the wd is heated by the dwarf nova outburst and subsequently cooled. a list of nine such systems has been reported by szkody (1998). these measurements are very difficult because of the long quiescence–outburst–quiescence cycles (from weeks to years). the short outburst period dwarf nova vw hyi cooled to 18,000 k (from 20,500 k) in the 14 days before the next outburst began (verbunt et al., 1987). v) magnetic field of the wd. indirect evaluations of magnetic field intensities in cv wds have been obtained through multifrequency observations (e.g., fabbiano et al., 1981). our feeling is that the problem of magnetic fields in wds has been underestimated in the studies of cvs. too many simplified models of disc accreting and magnetic cvs have been developed under the hypothesis that cvs can be sharply divided into three classes: polar, ip, non-magnetic. magnetic fields are smoothly varying in their intensities from one class to another. the discovery in some ips of a circularly polarized optical emission suggests that these intermediate polars will evolve into polar systems (e.g., mouchet, bonnetbidaud & de martino et al., 1998). some evidence of the continuity between the ipcvs and pcvs is coming from the detection of the sw sex systems. they have orbital periods just inside the so-called ’period gap’, which separates the two classes of ipcvs and pcvs (e.g. rodriguez-gil, 2003 and references therein; rodriguez-gil et al., 2007). looking at the homogeneous set of data coming from the iue for pcvs and ipcvs, it has been possible to obtain important information on common properties and peculiarities of these binaries (de martino, 1999 and references therein), which render the two classes rather similar in some of their uv behaviour. mouchet, bonnet-bidaud & de martino (1998), and de martino (1998) made the hypothesis that the two classes are evolutionary related. the far uv vs near uv colour–colour diagram for mcvs was constructed by de martino (1999). such a diagram was constructed measuring broad band continua in the iue short wavelength range (1420–1520 å and 1730–1830 å) and in the long wavelength range (2500–2600 å and 2850–2900 å). clearly the uv continua cannot be simply described by a single component but possess different contributions, as discussed by de martino (1999) and already noted in the past, since 1984, by giovannelli et al. (1985). araujo-betancor et al. (2005) obtained hubble space telescope (hst) stis data for a total of 11 pcvs 10 the golden age of cataclysmic variables and related objects (old and news) as part of a program aimed at compiling a homogeneous database of high–quality fuv spectra for a large number of cvs. comparing the wd temperatures of pcvs with those of nmcvs, they find that at any given orbital period the wds in pcvs are colder than those in nmcvs. the temperatures of wds in pcvs below the period gap are consistent with gravitational radiation as the only active angular momentum loss mechanism. the differences in wd effective temperatures between pcvs and nmcvs are significantly larger above the period gap, suggesting that magnetic braking in pcvs might be reduced by the strong field of the primary. araujo-betancor et al. (2005) derive a lower limit on the space density of pcvs of 1.3× 10−6 pc−3. 3 renewed interest for cataclysmic variables before the advent of rosat x-ray satellite, mcvs were relegated to a subsection of conferences about cvs that were mainly concentrated on nmcvs. the rosat satellite discovered many mcvs that even menaced to overthrow our understanding of the secular evolution of ‘normal‘ cvs by appearing – apparently inexplicable – in the so–called ‘period gap‘ in the orbital–distribution of cvs (e.g. vrielmann & cropper, 2004). but in spite of this, cvs were not considered, in general, for many years as principal targets of high energy x-ray experiments. at the beginning of the nineties of the last century, acceleration of particles by the rotating magnetic field of the wd in intermediate polars in the propeller regime — ae aqr -– detected by ground-based cherenkov telescopes in the tev passband (e.g. meintjes et al. 1992), and tev emission from the polar am her detected by ground-based cherenkov telescopes (bhat et al. 1991) — measurements never confirmed — were the main reasons of renewed interest for cvs in the high energy astrophysicists community. the integral observatory, until the beginning of 2007, had observed over 70 percent of the sky, with a total exposure time of 40 million seconds. bird et al. (2007) published the third integral catalogue of gamma-ray sources. it contains a total of 421 gammaray objects. most have been identified as either binary stars in our galaxy containing exotic objects such as black holes and neutron stars, or active galaxies, far away in space. but a puzzling quarter of sources remain unidentified so far. they could be either star systems enshrouded in dust and gas, or cvs. integral observes in the gamma-ray band so it can see through the intervening material. it has demonstrated that it can discover sources obscured at other wavelengths. one surprise has been the efficiency with which integral has detected just one minor subclass cvs, the socalled ipcvs. initially astronomers were not sure that cvs would emit gamma rays. indeed, integral has already shown that only about one percent of them do. this fact overbearingly renewed the interest for cvs, apparently fallen into disgrace in favour of binary systems containing either neutron stars or black holes. the fourth ibis/isgri catalog reports 331 additional sources when compared to the third catalog. of these, 120 are associated with extragalactic sources, while only 25 are associated with known galactic sources, and the remainder are so far unidentied (bird et al. 2010). cvs constitute ∼ 5% of the total sources. moreover, since the cvs measured by the integral observatory are magnetic in nature, the interest for such class of objects has been addressed to evolutionary problems. the long–standing fundamental predictions of evolution theory are finally being tested observationally. all facets of the accretion process in cvs, including variability, disc winds and jets, are universal with accreting wds, neutron stars, and black holes (knigge, 2010, 2011). knigge, baraffe & patterson (2011) extensively discussed the reconstruction of the complete evolutionary path followed by cvs, based on the observed mass–radius relationship of their donor stars, following knigge (2006) that discussed the observational and theoretical constraints on the global properties of secondary stars in cvs using the semi–empirical cv donor sequence, and concluded that most cvs follow a unique evolutionary track. in the standard model of cv evolution, angular– momentum–loss (aml) below the period gap are assumed to be driven solely by gravitational radiation (gr), while amls above the gap are usually described by a magnetic bracking (mb) (rappaport, verbunt & joss (1983). knigge, baraffe & patterson (2011) with their revised model, found the optimal scale factors fgr = 2.47 below the gap and fmb = 0.66 above, whilst the standard model gives fgr = fmb = 1. this revised model describes the mass–radius data much better than the standard model. the sub-class of cvs, named classical novae (cne), which are the third more powerful stellar explosions in a galaxy, have been observed as close as a kpc and as far as galaxies in fornax cluster. the time to report on the recent renaissance in studies on cne thanks to observations with 8-10m class telescopes, high resolution spectroscopy, in synergy with observations from space carried out with swift, xmm, chandra, hst, and spitzer, coupled with recent advances in the theory of the outburst, seems now in order. moreover, the possible connection among some cv-types and sne-ia will definitively justify the renewed interest about cvs. 11 f. giovannelli, l. sabau-graziati 4 classical and recurrent novae classical novae are expected to recur on timescales from 100,000 years to just a few decades. the most important physical parameters controlling this recurrence timescale are the wd mass, and the mass accretion rate from the secondary (e.g. yaron et al. 2005). once classical nova (cn) is recorded more than once, it can be designated as “recurrent” (rn). since the wd and the binary system remain intact after an outburst, it is possible that classical novae may actually be the same as recurrent novae if observed over a long enough time period. while the interval between outbursts of recurrent novae range from 10 to 100 years, it has been estimated that the time interval for classical novae would range from about 30,000 years for a 1.3 m� wd to 100,000 years for a 0.6 m� wd. given long enough it is expected that all classical novae will be observed as recurrent novae. the long term behaviour of classical old novae, and the optical behaviour of cne in outburst were discussed by bianchini (1990), and seitter (1990), respectively. the books by cassatella & viotti (1990) and by bode & evans (2008) are very useful for studying the physics of classical novae. recurrent novae are a rare sub-class of cataclysmic variable stars; wds accreting material from a binary companion in which more than one classical nova-type outburst has been observed (see the book of hellier, 2001 for a comprehensive review of cvs). nova outbursts are suspected to be due to a thermonuclear runaway on the surface of the wd, which releases huge amounts of thermal energy once a critical pressure is reached at the base of the shell of accreted material. one of the most interesting rne is rs ophiuchi (rs oph). it is an amazingly prolific recurrent nova, with recorded outbursts in 1898, 1907, 1933, 1945, 1958, 1967, 1985 and 2006 (schaefer 2010). the short time between outbursts (∼ 20 yrs) suggests that rs oph hosts a massive wd accreting material at a significantly high rate. in the latter paper schaefer discussed not only rs oph, but also the photometric histories of all known galactic rne. classical and recurrent nova outbursts have been recently discussed by bode (2011a,b) and evans (2011). the proceedings of a conference about rs oph and recurrent phenomenon can be very useful for details (evans et al., 2008). general properties of quiescent novae have been discussed by warner (2002). the very useful book of bode & evans (2008) about classical novae examines thermonuclear processes, the evolution of nova systems, nova atmospheres and winds, the evolution of dust and molecules in novae, nova remnants, and observations of novae in other galaxies. it includes observations across the electromagnetic spectrum, from radio to gamma rays, and discusses some of the most important outstanding problems in classical nova research. of the ∼ 400 known galactic classical novae, only 10 of them are recurrent. eight of them harbour evolved secondary stars, contrary to classical novae that contain main sequence stars (darnley et al., 2011). they propose a new nova classification based on the evolutionary state of the secondary star, contrary the current schemes based on the properties of outbursts. such classification contains three groups of novae: i) main sequence nova (ms–nova); ii) sub–giant nova (sg– nova); and iii) red giant branch nova (rg–nova). rne play an important role in the studies of sn ia progenitors (surina et al., 2011). rne are likely progenitors of type–ia supernovae. in order to brave this important problem the use of archival data is the only way to answer the big question. now, huge and comprehensive set of archival rn data go back to 1890. 5 progenitors of sn ia it is well accepted by the community that type–ia sne are the result of the explosion of a carbon–oxygen wd that grows to near chandrasekhar’s limit in a close binary system (hoyle & fowler, 1960). but the debate is focussed around the different kinds of progenitors. indeed, in the past, two families of progenitor models have been proposed. they differ in the mode of wd mass increase. the first family is the so–called single degenerate (sd) model (whelan & iben, 1973), in which the wd accretes and burns hydrogen–rich material from the companion. the second family is the so–called double degenerate (dd) model, in which the merging of two wds in a close binary triggers the explosion (webbing, 1984; iben & tutukov, 1984). the two scenarios produce different delay times for the birth of the binary system to explosion. thus it is hopefully possible to discover the progenitors of type–ia sne by studying their delay time distribution (ddt). the ddt can be determined empirically from the lag between the cosmic star formation rate and type–ia sn birthrate. the energy released through runaway thermonuclear process ejects the majority of the unburnt hydrogen from the surface of the star in a shell of material moving at speeds of up to 1.5 ×103 km s−1. this produces a bright but short-lived burst of light the nova. although type–ia supernova appear to have similar origin to classical novae, there are key differences. the most important is that in a classical nova, the thermonuclear runaway occurs only on the surface of the star, allowing the wd and the binary system to remain intact (e.g. townsley & bildsten, 2005). in a type–ia 12 the golden age of cataclysmic variables and related objects (old and news) supernova, the thermonuclear runaway occurs within wd itself, completely disrupting the progenitor. this is reflected in the amount of energy released in the explosions, with classical novae releasing ∼ 1044 erg, and type–ia supernovae ∼ 1051 erg. the possible progenitors of sn ia are: i) recurrent novae; ii) symbiotic stars; iii) super-soft sources; iv) double wd binaries; and v) wds accreting material from red–giant companions. i) recurrent novae are just a subset of ordinary novae that happen to go off more than once per century. as such, they are binary systems with matter flowing off a companion star onto a wd, accumulating on its surface until the pressure gets high enough to trigger a thermonuclear runaway that is the nova. only 10 rne are known in our milky way galaxy, including: u sco (1863, 1907, 1917, 1936, 1945, 1969, 1979, 1987, 1999); t pyx (1890, 1902, 1920, 1944, 1967); t crb (1866, 1946); rs oph (1898, 1907, 1933, 1945, 1958, 1967, 1985, 2006). to recur with τrec < 100 years, rne must have: high wd mass (1.2m� < mwd < mchandra), and high accretion rate (ṁ ∼ 10−7 m� yr−1). sn ia occurs if: i) the mass ejected for each eruption is less than the mass accreted onto the wd (mejected < ṁ τrec); ii) the rate of death rne must be enough to produce the sn ia rate (rrndeath = rsnia), being rrndeath = nrn ×(0.2m�ṁ). in order to solve the problems we need to know τrec (recurrence time scale) from archive plates, nrn (number of rne in the milky way) from archive plates and aavso, ṁ (mass accretion rate onto wd) from the average in the last century, mejected (mass ejected in eruption) from pre–eruption eclipse timing. some results have been obtained for becoming optimists in solving the problem of sn ia production. indeed schaefer (2011) obtained for ci aql and u sco mejected << ṁ τrec). thus, wds are gaining mass and the latter rne will collapse as sn ia. moreover, for the milky way, m31, and lmc rrndeath ∼ nrn. then there are enough rne to supply the type–ia sn events. ii) symbiotic stars contain wds efficiently accreting material from the secondary star. in most cases they steadily burn h–rich material allowing them to grow in mass. some of these systems can produce high mass wds. in symbiotic rne (syrne) the wd mass is already very close to chandrasekhar’s limit. for instance in v 407 cyg a very massive wd is accreting material at a rate of ∼ 10−7 m� yr−1 from a mira– type companion (miko lajewska, 2011). iii) super–soft sources are probably wds that accrete material and burn hydrogen. voss & nielemans (2008) discovered an object at the position of the type– ia sn2007on in the elliptical galaxy ngc1404 on pre– supernova archival x–ray images. this result favours the accretion model (sd) for this supernova, although the host galaxy is older than the age at which the explosions are predicted in sd models. however, the dd model cannot be ruled out by this event because a hot accretion disc is probably the intermediate configuration of the system, between first wd–wd roche–lobe contact and explosion (yoon, podsiadlowski & rosswog, 2007). greggio, renzini & daddi (2008) starting from the fact that type–ia sn events occur over an extended period of time, following a distribution of delay times (ddt), discussed theoretical ddt functions that accommodate both ‘prompt‘ and ‘tardy‘ sn events derived by empirically–based ddt functions. moreover such theoretical ddt functions can account for all available observational constraints. the result is that sd/dd mix of snia’s is predicted to vary in a systematic fashion as function of cosmic time (redshift). iv) double wds binaries are systems containing two wds that can merge and giving rise to sn explosion. yoon, podsiadlowski & rosswog (2007) explored the evolution of the merger of two carbon–oxygen (co) wds. their results imply that at least some products of double co wds merger may be considered good candidates for the progenitors of type–ia sne. brown et al. (2011) and kilic et al. (2011) studied a complete colour–selected sample of double–degenerate binary systems containing extremely low mass (elm) (≤ 0.25 m�) wds. milky way disc elm wds have a merger rate of ≈ 4 × 10−5 yr−1 due to gravitational wave radiation. the elm wd systems that undergo stable mass transfer can account for about 3% of am cvn stars. the most important fact is that the elm wd systems that may detonate merge at a rate comparable to the estimate rate of underluminous sne. these sne are rare explosions estimated to produce only ∼ 0.2 m� worth of ejecta. at least 25% of elm wd sample belong to the old tick disc and halo components of our galaxy. thus, if merging elm wd systems are the progenitors of underluminous sne, transient surveys must find them in both elliptical and spiral galaxies. v) wds accreting material from red–giant companions. observations carried out by patat et al. (2008) with vlt–uves allowed to detect circumstellar material in a normal type–ia sn. the expansion velocities, densities and dimensions of the circumstellar envelope indicate that this material was ejected from the system prior to the explosion. the relatively low expansion velocities favour a progenitor system where a wd accretes material from a companion star, which is in the red–giant phase at the time of explosion. bianco et al. (2011) searched for a signature of a non–degenerate companion in three years of supernova 13 f. giovannelli, l. sabau-graziati legacy survey data. they found that a contribution from wd/red–giant binary system to type–ia sn explosions greater than 10% at 2σ, and than 20% at 3σ level is ruled out. type–ia sne are used as primary distance indicators in cosmology (e.g. phillips, 2005). phillips (2011) reviewed the near–infrared (nir) of type–ia sne concluding that such sne are essentially perfect standard candles in the nir, displaying only a slight dependence of peak luminosity on decline rate and colour. lira (1995) first noted that b–v evolution during the period from 30 to 90 days after v maximum is remarkably similar for all sn ia events, regardless of light–curve shape. this fact was used by phillips et al. (1999) to calibrate the dependence of the bmax–vmax and vmax– imax colours on the light curve parameter ∆m15 (b) which can, in turn, be used to separately evaluate the host galaxy extinction. using these methods for eliminating the effect of the reddening, they reanalyzed the functional form of the decline rate versus luminosity relationship and gave a value of the hubble constant of h0 = 63.3± 2.2± 3.3 km s−1 mpc−1. the use of type–ia sne is also fundamental for determining some cosmological constraints, such as ωm and ωλ that fit a λcdm models with values of 0.211±0.034 (stat) ±0.069 (sys) using a set of 252 high– redshift sne (guy et al., 2010) and 0.713+0.027 −0.029 (stat) +0.036 −0.039 (sys) using a set of low–redshift nearby–hubble– flow sne (kowalski et al., 2008), respectively. in order to explore the difficult topic of the expansion of the universe it is necessary to know the evolution of metallicity in old universe that changes the hubble diagram shape. the proposed space observatory super nova acceleration probe (snap) is designed to measure the expansion of the universe and to determine the nature of the mysterious dark energy that is accelerating this expansion. snap is being proposed as part of the joint dark energy mission (jdem) (stril et al., 2010), which is a cooperative venture between nasa and the u.s. department of energy. if selected it will be launched before 2020. snap cannot achieve its main goal without progenitor/evolution solution. for comments and prospects about type–ia sn science in the decade 2010–2020 see the paper by howell et al. (2009). 6 some open questions several fundamental questions concerning cvs still remain waiting for a proper answer. we will present briefly only some of them here. one of them is the lack of a coherent classification, especially for nls. on the other hand, in gross features and in most respects, dn and nls, as well as quiescent novae, are almost indistinguishable, although, in addition to their different outbursts’ behaviour, there appear to be some further minor differences which are not yet understood (see hack & la dous 1993). the question arises of whether the outburst behaviour, the current basis of almost all classification is really a suitable criterion for sorting cvs in physically related groups. there are also too many exceptions, either systems that do not fit in any particular group or that can be included in several of them, to be able to render the observational behaviour, at least as it is used at the present, suitable. could cvs be considered simply gravimagnetic rotators? this should be the most suitable approach for studying them from a physical point of view. studies of rotational equilibria of mcvs predict that ipcvs will evolve either into pcvs or into low field strength polars — presumably unobservable, and possibly euv emitters — depending on their magnetic moments and orbital periods. indeed, there are systems, like ex hya-type, having magnetic moment similar to ipcvs above the ’period gap’ and comparable to the weakest field am her-like systems. moreover, the detection of several sw sex systems having orbital periods inside the so-called ’period gap’ opens a new interesting problem about the continuity in the evolution of cvs. the rare am canum venaticorum (am cvn) stars have extremely short orbital periods, between 10 and 65 minutes, and their spectra show no evidence for hydrogen. they appear to be helium-rich versions of cvs. they are still waiting for a general model. they are probably binary systems of two white dwarfs, but even this is still controversial. despite all the work developed during the last decades, the problem of modeling accretion discs in cvs is by no means closed, especially in quiescence. closely related is the problem of the cause of outbursts. we really do not know which of the present two families of models (disc instability models or secondary instability models) is responsible for the cvs outburst phenomenon, or in which system is each model valid, although martinez-pais et al. (1996) gave a contribution in solving this problem at least in the case of ss cygni; they found some evidence for an increase of the mass transfer rate from the secondary star as the mechanism responsible for symmetric outbursts. something similar can be said about the super-outburst phenomenon in su uma systems. gaudenzi et al. (1990), analyzing iue spectra of ss cygni, discussed about the outburst production as due to the destruction of the accretion disc. the matter, passing through the boundary layer, slowly accretes onto the wd. long and short outbursts correspond to 14 the golden age of cataclysmic variables and related objects (old and news) total or partial destruction of the disc, respectively. alternatively, could nuclear burning be responsible of the production of outbursts in cvs? indeed, nuclear burning onto white dwarf’ surface was proposed by mitrofanov (1978, 1980) as a mechanism suitable to generate x-rays in cvs. in spite of this shrewd suggestion, the community of theoreticians did not consider such a mechanism — certainly possible — worthy of taking up a part of their time. however, we believe that this alternative solution in explaining the generation of outbursts in cvs would deserve theoretician community’s care. for instance, the white dwarf surface interested in the accretion in the system ss cygni has been evaluated as 24% of the total (gaudenzi et al., 2002). there, nuclear burning could occur. accretional heating by periodic dn events increases substantially the surface temperature of the wd in cvs (godon & sion, 2002). then, the envelope thermal structure resulting from compression and irradiation should be a crucial component in understanding the envelope structure of a pre–nova wd. another problem still open is connected with the classification of cvs in three kinds, namely nmcvs, pcvs and ipcvs. this is, in our opinion, another convenient classification, although artificial, probably not necessary if cvs are studied as gravimagnetic rotators. in this way a smooth evolution of the systems could be responsible of the variations of the gravimagnetic parameters. are the ipcvs and pcvs smoothly connected via the sw sex-like systems placed just in between? sw sex systems have indeed orbital periods belong to the so-called ’period gap’, and then their presence there sure cancel that gap. could some systems behave in different ways depending on their instantaneous physical conditions? for this reason they could apparently behave sometimes as pcvs and sometimes as npcvs. an example very clear is that of ss cygni, usually classified as a non-magnetic dwarf nova. it has been detected by the integral observatory in a region of the spectrum (up to ∼ 100 kev). this emission is very hard to be explained without the presence of polar caps in the wd of the system. several proofs have been shown and discussed many times by giovannelli’s group in order to demonstrate the intermediate polar nature of it (e.g., giovannelli, 1996, and references therein; giovannelli & sabau-graziati, 1998; 2012a); indeed, ss cygni shows characteristics of a nmcv, as well as those of ip and sometimes even those of polars, although its position in the log pspin–log porb plane is very close to the line where ips lie. important results are coming from the spitzer space telescope with the detection of an excess (3-8) µm emission from magnetic cvs, due to dust (howell et al., 2006; brinkworth et al., 2007). gaudenzi et al. (2011) discussed about the reasons of the variable reddening in ss cyg and demonstrated that this reddening is formed by two components: the first is interstellar in origin, and the second (intrinsic to the system itself) is variable and changes during the evolution of a quiescent phase. moreover, an orbital modulation also exists. the physical and chemical parameters of the system are consistent with the possibility of formation of fullerenes. the spitzer space telescope detected the presence of fullerenes in a young planetary nebula (cami et al., 2010). fullerenes are the first bricks for the emergence of the life. therefore, the possible presence of fullerenes in cvs opens a new line of investigation, foreboding of new interesting surprises. 7 conclusions at the end of this review it appears evident that the most suitable approach for studying cvs from a physical point of view is to consider them as gravimagnetic rotators. the detection of several sw sex systems having orbital periods inside the so-called ’period gap’ opens a new interesting problem about the continuity in the evolution of cvs. are the ipcvs and pcvs smoothly connected via the sw sex-like systems placed just in between? in order to fully understand the emission properties and evolution of cvs, the mass–transfer process needs to be clearly understood, especially magnetic mass transfer, as well as the properties of magnetic viscosity in the accretion discs around compact objects. consequently, the investigation on the magnetic field intensities in wds appears crucial in understanding the evolution of cvs systems, by which it is possible to generate classical novae (e.g., isern et al., 1997) and type-ia supernovae (e.g., isern et al., 1993). in those catastrophic processes the production of light and heavy elements, and then the knowledge of their abundances provides strong direct inputs for cosmological models and cosmic ray generation problems. acknowledgments this research has made use of nasa’s astrophysics data system. references [1] andronov, i.l., chinarova, l.l., han, w., kim, y., yoon, j.-n.: 2008, a&a, 486, 855. 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[172] zhang, e., robinson, e.l., stiening, r.f., horne, k.: 1995, apj, 454, 447. 20 http://dx.doi.org/10.1038/nature06602 http://dx.doi.org/10.1093/mnras/227.1.23 http://dx.doi.org/10.1017/cbo9780511586491 http://dx.doi.org/10.1007/bf00613240 http://dx.doi.org/10.1007/bf00645229 http://dx.doi.org/10.1063/1.1518170 http://dx.doi.org/10.1086/381742 introduction multifrequency emissions renewed interest for cataclysmic variables classical and recurrent novae progenitors of sn ia some open questions conclusions 86 acta polytechnica ctu proceedings 2(1): 86–89, 2015 86 doi: 10.14311/app.2015.02.0086 ae aquarii: a short review p. j. meintjes1, a. odendaal1, h. van heerden1 1department of physics, university of the free state, p.o. box 339, bloemfontein, 9300, south-africa corresponding author: meintjpj@ufs.ac.za abstract the nova-like variable ae aquarii has been continuously studied since its discovery on photographic plates in 1934. in this short review the peculiar multi-wavelength properties of ae aquarii will be reviewed and explained in context of its evolution from a high mass transfer phase, during which period it could have been a supersoft x-ray source (sss). keywords: cataclysmic variable: ae aquarii radiation mechanisms: non-thermal line: identification techniques: spectroscopic. 1 introduction since its discovery on photographic plates in optical wavelengths (zinner 1938), the rapid variable star ae aquarii has been a source of continuous observational and theoretical study involving a wide range of frequencies from radio to tev gamma-rays. optical photometric studies showed that the system is highly variable, with ∆mv ≤ 2 magnitude outbursts occurring nearly continuously (e.g. henize 1949, patterson 1979, van paradijs, kraakman & van amerongen 1989). distance estimates place the source at ∼ 100 pc (welsh, horne & oke 1993). 2 optical and x-ray pulsations the first photometric pulse-timing study of ae aquarii in the optical band (patterson 1979) revealed steady coherent pulsations in the power spectra at p◦ ≈ 33 s and p1 ≈ 16 s, which were interpreted as the spin period of an obliquely rotating white dwarf and its associated first harmonic, caused by the second pole reflecting from the inner edge of the accretion disc (patterson 1979). this led to its initial classification as a dq herculis type system (patterson 1979). fast optical spectroscopy performed later with the mount wilson 2.5 m coudé spectrograph in the wavelength band 636 682 nm, combined with simultaneous photometry in the same wavelength band (welsh, horne & gomer 1993), revealed that the pulsations originate from the white dwarf. however, it has been pointed out (patterson 1979) that the low pulsed fraction (≤ 1%) of the 33 s oscillation of ae aquarii is peculiarly low for a disc accreting white dwarf. in order to place ae aquarii with its rapidly rotating white dwarf in the dq herculis sub-class of the cataclysmic variables, accreting in the slow rotator limit from a well developed accretion disc, a magnetic surface field of b∗ ≤ 100 kg is required (e.g. patterson 1994). this is significantly lower than the inferred surface field of a disc accreting dq herculistype system, which is b∗ ∼ 106 − 107 gauss (patterson 1979). optical polarimetry (cropper 1986, stockman et al. 1992; beskrovnaya et al. 1995) revealed circular polarization ∼ 0.05− 0.1 % in optical light, which confirmed that the dwarf surface magnetic field is at least of the order b∗ ≥ 106 g (e.g. chanmugam & frank 1987). x-ray observations made by einstein (0.1 4 kev) showed the 33 s oscillation, with no indication of the associated 16 s first harmonic (patterson et al. 1980). this is confirmed by recent chandra x-ray observations (e.g oruru & meintjes 2012), which shows a single coherent pulse at 33 s. in optical wavelengths the 33 s period often disappears during outbursts (e.g patterson 1979; meintjes et al. 1992;1994), while the x-ray pulsed fraction determined with the einstein data seems to show some correlation with increasing count rate according to the relation pf(%) = 45(%/s−1)cr(s−1) in the observed count rate range 0.15-0.5 s−1 (e.g. meintjes & de jager 2000). no noticeable changes in the hardness ratio of the x-ray data has been observed during periods of enhanced activity (e.g. meintjes & de jager 2000; oruru & meintjes 2012). the weak correlation between the optical pulsed fraction of the coherent 33 s period and outbursts, together with the radial velocity measurements of uv lines observed during time-resolved spectroscopic observations made with the hubble space telescope hst, suggest a very effective propeller driven mass outflow in ae aquarii (e.g. patterson 1994; eracleous & horne 1996). 86 http://dx.doi.org/10.14311/app.2015.02.0086 ae aquarii: a short review 3 the propeller phase doppler tomography profiles of the ae aquarii system (e.g. wynn, king & horne 1997; ikhsanov, neustroev & beskrovnaya 2004) show tell-tale signatures of a propeller driven mass outflow from the system. the magnetospheric propeller process in ae aquarii was first explained within the framework of large diamagnetic blobs being propelled by a magnetospheric drag (wynn & king 1995; wynn, king & horne 1997). roche tomography of the secondary star (dunford & smith 2005; watson et al. 2006; smith, dunford & watson 2012) shows that ∼ 20% of the surface of the secondary star is covered with starspots, which may suggest a magnetically active secondary star. the effect of secondary star magnetic fields on the mass transfer process has been investigated (meintjes 2004). it has been shown that magnetic prominences can fragment the mass transfer flow into a blob-like stream, which upon interacting with the fast rotating white dwarf field, can be propelled from the system on time-scales that are short compared to the keplerian periods at the distance of closest approach of the stream (meintjes & venter 2005). recent spectroscopy performed with the 1.9 m telescope at sutherland revealed broad emission lines, which is most probably an indicator of high velocity gas in the system. the full-width half maximum width of these lines all implies velocities in excess of vesc ≥ 1000 km s−1 (meintjes, oruru & odendaal 2012), which is of the order of the escape velocity from the radius of closest approach of the stream. it has been shown (venter & meintjes 2006) that the interaction between the fast rotating magnetosphere and the stream of material from the secondary star may result in the development of kelvin-helmholtz instabilities, resulting in effective turbulence driven mixing of magnetic field with plasma. this results in a very effective transfer of angular momentum to the stream, propelling it from the system (venter & meintjes 2006). this has been confirmed by numerical simulations of the interaction between the fast rotating magnetic field of the white dwarf in ae aquarii and the mass flow from the secondary star (bisikalo & zhilkin 2012). the low accretion rate of the white dwarf in ae aquarii (ṁ∗ ∼ 1014 g s−1) compared to the mass transfer rate deduced from the uv emission line spectra (ṁ2 ∼ 1017 g s−1) implies that the white dwarf in ae aquarii is in a super-propeller (ejector) state (bisikalo & zhilkin 2012; ikhsanov & beskrovnaya 2012). the brightening of the source observed in optical on a regular basis, can therefore not readily be explained in terms of enhanced mass accretion onto the white dwarf. it has been shown (beardmore & osborne 1997) that the flaring activity in ae aquarii can be explained satisfactorily within the framework of colliding blobs propelled form the system by the magnetospheric propeller. these authors showed that collisions between faster and slower blobs will result in heating and resultant radiative cooling which can account for the outbursts in ae aquarii. 4 spin-down and particle acceleration the dissipated mhd power due to the magnetospheric propeller process in ae aquarii (e.g. meintjes & de jager 2000; meintjes & venter 2005) is of the order of pmag ∼ 1034 erg s−1, and comes at the expense of the rotational kinetic energy of the rapidly rotating white dwarf. a detailed pulse timing study of the white dwarf spin period, using a 14 year baseline (de jager et al. 1994) revealed that the white dwarf is spinning down at a rate of ṗ∗ ∼ 5.64 × 10−14 s s−1. this translates to a spin-down luminosity of the order of ls−d = iωω̇ ∼ 1034 erg s−1. a more recent study (mauche 2006) revealed that the spin-down rate of the white dwarf has sped-up. this can possibly be explained within the framework of mass transfer variations from the secondary star. the low accretion in comparison to the mass transfer α = ṁ∗/ṁ2 ∼ 0.1% may indicate that the white dwarf in ae aquarii is currently in a super-propeller (bisikalo & zhilkin 2012) or ejector phase (ikhsanov & beskrovnaya 2012) and that the white dwarf in ae aquarii may exhibit the same properties as a spun-up radio pulsar (e.g ikhsanov & bierman 2006). this implies that the low mass accretion and rapid rotation of the highly magnetized white dwarf may provide a mechanism for pulsar-like particle acceleration and nonthermal emission. recent results from suzaku x-ray satellite (terada et al. 2008) showed a non-thermal spectrum at energies εx ≥ 10 kev. it has been showed recently (e.g. oruru & meintjes 2012) that particles can be accelerated in the magnetosphere of the white dwarf to energies in excess of ε ≥ 100 gev, which may radiate synchrotron radiation at energies of the order of εx ∼ 10 kev in the weak magnetic field outside the corotation radius. 5 non-thermal emission radio observations performed in the late 1980’s showed that ae aquarii is a non-thermal emitter (bookbinder & lamb 1987, bastian, dulk & chanmugam 1988), showing continuous radio outbursts with maximum flux smjy ≤ 15 at frequencies ν ≤ 22.5 ghz. the high brightness temperature measured tb ≥ 1010 k definitely implies non-thermal emission, with the spectral slope smjy ∝ να, with α ∼ 0.3 − 0.5 (bastian, dulk & chanmugam 1988). the spectral properties of the radio emission in ae aquarii can be explained in 87 p. j. meintjes, a. odendaal, h. van heerden terms of a superposition of synchrotron emitting plasmoids, which expand and cool radiatively through synchrotron radiation (van der laan 1963; 1966). subsequent studies in infrared with iso (abada-simon et al. 2005) and spitzer (dubus et al. 2007) showed that the smjy ∝ να (α ∼ 0.5) non-thermal spectrum extends to a frequency ν ∼ 2000 ghz (dubus et al. 2007). the non-thermal radio-ir spectrum has been modeled successfully (meintjes & venter 2003; venter & meintjes 2006) in terms of synchrotron emission from relativistic electrons in expanding magnetized plasmoids in the propeller outflow. it has been shown that the frequency where the spectrum turns optically thin to radio emission is of the order of νt ≤ 3000 ghz. reports of pulsed burst-like vhe and tev gammaray emission in the 1990’s (bowden et al. 1992; meintjes et al. 1992;1994) sparked interest in ae aquarii as a non-thermal source of high energy emission. subsequent studies with various modern cerenkov detectors could unfortunately not confirm the earlier reports, leaving the vhe-tev status of ae aquarii still in doubt (e.g. lang et al. 1998; sidro et al. 2008; mauche et al. 2012). 6 the evolution the current properties of ae aquarii is consistent with a high mass accretion history (meintjes 2002; schenker et al. 2002). if one consider initial parameters porb,i ∼ 15h, m1,i ∼ 0.6m�, m2,i ∼ 1.6m� (i.e. qi ∼ 2.7) and r2,i ∼ 1.6r�, it can be shown that the initial thermal time-scale (τth ∼ 6.3 × 106(m2/1.6m�)−1 yr) mass transfer could have been of the order ṁ2,i ∼ 2×1019(m2,i/1.6m�)(τ/τth)−1 g s−1, still below the eddington value of ṁedd ≤ 7 × 1020(m1,i/0.6m�)−0.8 g s −1. it has been shown that this could have lasted until a critical q-ratio was reached, i.e. qcrit = 0.73 (meintjes 2002) during which time accretion disc torques could have spun-up the white dwarf to periods around ∼ 30 s over a time scale τsu ∼ 3 × 106 yr, which is similar to the thermal mass transfer time scale, i.e. τṁ ∼ few × 106 yr (meintjes 2002). the high mass accretion in this initial phase could have resulted in ae aquarii being a supersoft xray source (sss) (meintjes 2002; schenker et al. 2002). 7 conclusions the multi-frequency properties of ae aquarii have been summarized. the current asynchronicity of the spin and orbital period of ae aquarri can be explained in terms of a high mass accretion history during which period the white dwarf could have been spun-up by accretion disc torques to a short period. in this phase the high accretion rate could have resulted in the system being a sss. a subsequent decrease in the mass transfer rate from the secondary drove the system into the propeller state, which may be the driving mechanism behind the multi-frequency emission from the system. acknowledgement the authors thank the organisers for the invitation to present this work at this conference. references [1] abada-simon, m. et al.: 2005, a&a, 433, 1063 [2] bastian, t.s., dulk, g.a. & chanmugam, g.: 1988, apj, 324, 431 doi:10.1086/165906 [3] beardmore, a.p. & osborne, j.p.: 1997, mnras, 290, 145 doi:10.1093/mnras/290.1.145 [4] bisikalo, d.v. & zhilkin, a.g.: 2012 in golden age of cataclysmic variables and related objects, palermo (september 12-17, 2011), memorie s.a.it, vol 83 (2), p. 562 (eds. f. giovanelli & l. sabau-graziati) [5] beskrovnaya, n.g., ikhsanov, n.r., bruch, a. & shakovskoy, n.m.: 1995, in proc. of the cape workshop on magnetic cataclysmic variables, cape town (1995), asp conf. ser. vol. 85, p. 364, astron. soc. pacific, san francisco (eds. d.a.h. buckley & b. warner) [6] bookbinder j.a. & lamb, d.q.: 1987, apj, 323, l131 doi:10.1086/185072 [7] bowden, c.c.g. et al.: 1992, astropart. phys., 1, 47 doi:10.1016/0927-6505(92)90008-n [8] chanmugam, g. & frank, j.: 1987, apj, 320, 746 doi:10.1086/165592 [9] cropper, m.: 1986, mnras, 222, 225 doi:10.1093/mnras/222.2.225 [10] de jager, o.c., meintjes, p.j., o’donoghue, d. & robinson, e.l.: 1994, mnras, 267, 577 doi:10.1093/mnras/267.3.577 [11] dubus, g., taam, r.e., hull, c., watson, d.m. & mauerhan, j.c.: 2007, apj, 663, 516 doi:10.1086/518407 [12] dunford a. & smith r.c.: 2005, proc. of the astrophysics of cataclysmic variables and related objects workshop, strasbourg (france), asp conference series, vol. 330, p. 399, astron. soc. pacific, san francisco (eds. j.-m hameury & j.-p lasota) 88 http://dx.doi.org/10.1086/165906 http://dx.doi.org/10.1093/mnras/290.1.145 http://dx.doi.org/10.1086/185072 http://dx.doi.org/10.1016/0927-6505(92)90008-n http://dx.doi.org/10.1086/165592 http://dx.doi.org/10.1093/mnras/222.2.225 http://dx.doi.org/10.1093/mnras/267.3.577 http://dx.doi.org/10.1086/518407 ae aquarii: a short review [13] eracleous, m, & horne, k.: 1996, apj, 471, 427 doi:10.1086/177979 [14] henize, k.g.: 1949, aj, 54, 89 [15] ikhsanov, n.r., neustroev, v.v. & beskrovnaya, n.g.: 2004, a&a, 421, 1131 [16] ikhsanov, n.r. & biermann, p.l.: 2006, a&a, 445, 305 [17] ikhsanov, n.r. & beskrovnaya, n.g.: 2012, arxiv: 1205.4330v1 [astro-ph.he] (19 may 2012) [18] lang, m.j. et al.: 1998, astropart. phys., 9, 203 doi:10.1016/s0927-6505(98)00020-6 [19] mauche, c.w.: 2006, mnras, 369, 1983 doi:10.1111/j.1365-2966.2006.10447.x [20] mauche, c.w. et al.: 2012, in golden age of cataclysmic variables and related objects, palermo (september 12-17, 2011), memorie s.a.it, vol 83 (2), p. 651 (eds. f. giovanelli & l. sabau-graziati) [21] meintjes, p.j. et al.: 1992, apj, 401, 325 [22] meintjes, p.j. et al.:1994, apj, 434, 292 [23] meintjes, p.j., oruru, b. & odendaal, a.: 2012, in golden age of cataclysmic variables and related objects, palermo (september 12-17, 2011), memorie s.a.it, vol 83 (2), p. 643 (eds. f. giovanelli & l. sabau-graziati) [24] meintjes, p.j. & de jager, o.c.: 2000, mnras, 311, 611 [25] meintjes, p.j.: 2002, mnras, 336, 265 [26] meintjes, p.j.: 2004, mnras, 352, 416 [27] meintjes, p.j. & venter l.a.: 2003, mnras, 341, 891 [28] meintjes p.j. & venter, l.a.: 2005, mnras, 360, 573 [29] oruru, b. & meintjes p.j.: 2012, mnras, 421, 1557 doi:10.1111/j.1365-2966.2012.20410.x [30] patterson, j.: 1979, apj, 234, 978 doi:10.1086/157582 [31] patterson, j., branch, d., chincarini, g. & robinson e.l.: 1980, apj, 240, l133 doi:10.1086/183339 [32] patterson, j.: 1994, pasp, 106, 209 doi:10.1086/133375 [33] schenker, k. et al.: 2002, mnras, 337, 1105 doi:10.1046/j.1365-8711.2002.05999.x [34] sidro, n. et al.: 2008, international cosmic ray conference (icrc), 2, 715 [35] smith, r.c., dunford, a. & watson, c.a.: 2012, in golden age of cataclysmic variables and related objects, palermo (september 12-17, 2011), memorie s.a.it, vol 83 (2), p. 708 (eds. f. giovanelli & l. sabau-graziati) [36] stockman, h.s., schmidt, g.d., berriman g., liebert, j., moore, r.l. & wickramasinghe, d.t.: 1992, apj, 401, 628 [37] terada, y. et al.: 2008, publ. astron. soc. japan, 60, 387 [38] van der laan, h.: 1963, mnras, 126, 535 [39] van der laan, h.: 1966, nature, 211, 1131 [40] van paradijs, j., kraakman, h. & van amerongen, s.: 1989, a&a, 79, 205 [41] venter, l.a. & meintjes, p.j.: 2006, mnras. 366, 557 [42] watson, c.a., dhillon, v.s. & shahbaz, t.: 2006, mnras, 368, 637 [43] welsh, w.f., horne, k., & oke,j.b.: 1993, apj, 406, 229 [44] welsh, w.f., horne, k., & gomer, r.: 1993, apj, 410, l39 [45] wynn, g.a. & king, a.r. 1995:, mnras, 275, 9 [46] wynn, g.a., king, a.r. & horne, k. 1997: mnras, 286, 436 [47] zinner, e.: 1938, astron. nach., 265, 345 discussion dimitri bisikalo: is there any observational evidence of mass flow surrounding the white dwarf? pieter meintjes: there is observational evidence of mass being ejected from the system. the propeller is extremely effective, only ∼ 0.1% of the mass transfer flow gets accreted onto the surface. the rest is ejected from the system. optical and uv spectroscopy indicate high velocity flows. there may be a circumbinary ring of ejected matter present surrounding the system. 89 http://dx.doi.org/10.1086/177979 http://dx.doi.org/10.1016/s0927-6505(98)00020-6 http://dx.doi.org/10.1111/j.1365-2966.2006.10447.x http://dx.doi.org/10.1111/j.1365-2966.2012.20410.x http://dx.doi.org/10.1086/157582 http://dx.doi.org/10.1086/183339 http://dx.doi.org/10.1086/133375 http://dx.doi.org/10.1046/j.1365-8711.2002.05999.x introduction optical and x-ray pulsations the propeller phase spin-down and particle acceleration non-thermal emission the evolution conclusions acta polytechnica ctu proceedings doi:10.14311/app.2017.9.0008 acta polytechnica ctu proceedings 9:8–12, 2017 © czech technical university in prague, 2017 available online at http://ojs.cvut.cz/ojs/index.php/app phase analysis of niti wire based on synchrotron 2d x-ray diffraction pattern in matlab martin dudra,∗, daniel šimekb, jan drahokoupila,b a department of solid state engineerign faculty of nuclear sciences and physical engineering czech technical university in prague, trojanova 13 120 00 prague 2, czech republic b institute of physics , na slovance 2 182 21 prague, czech republic ∗ corresponding author: dudrmart@fjfi.cvut.cz abstract. this paper deals with simplified phase analysis of 2d diffraction patterns obtained using microfocused x-ray at various places of superelastic niti wire under combined load. for this purpose, matlab routines and functions were created, which carry out integration of the patterns in various sectors of debye ring, fitting of the patterns by a mixture of austenite and martensite (having neglected the r-phase) and successive evaluation of the quantitative phase content. the most crucial simplificating assumptions were: the neglection of the r-phase and the assumption of a) fibre texture or b) no texture. keywords: nitinol spring, combined load, x-ray diffraction, phase analysis, synchrotron. 1. introduction a superelastic nitinol wire under combined load was subjected to xrd measurements using microfocused beam at european synchrotron radiation facility (esrf) in grenoble. quantitative phase analysis at different points on the wire was expected to be carried out. as the number of outcoming images was large (12 000), there was need of their automated processing. moreover the sample was strongly textured and therefore the integration of debye rings was preferred to be done "per partes", that means separately in various sectors ("pieces of cake") in order to implement some texture corrections later. some matlab routines were created that fulfilled these specific demands. this paper briefly explains basic principles of the approach in use and presents results obtained from the routines. 2. material and methods 2.1. nitinol nitinol is a well-known shape memory alloy of a near equiatomic stoichiometric composition ni-ti. the shape memory in niti is based on so called martensitic transformation (mt) between cubic austenite (high-temperature phase) and monoclinic twinning martensite (low-temperature phase). mt can be induced by lowering the temperature below transition point or by applying sufficient mechanical stress onto austenite. there is a third phase called r-phase, which under certain conditions stands in between the former two. it is unstable and hardly preparable solely. [1] r-phase is trigonal. [2] 2.2. quantitative phase analysis quantitative phase analysis (qpa) responds to both questions "what?" and "how much?" when talking about a mixture of phases. the diffraction pattern outcoming from measurement on a polycrystalline sample in transmission geometry recorded on 2d detector looks like a set of concentric circles. each of the circles corresponds to one set of equivalent crystallographical planes of the sample. when the pattern is integrated in tangential direction 1d patterns are obtained for the integrated interval. position of the peak in 1d pattern (i.e. the radius of the corresponding circle) is important for qualitative phase analysis (i.e. "what?"), the integrated intensity of the peak determines the amount of phase in a mixture ("how much?"), hence it is used by quantitative phase analysis. there are many factors determining the real value of integrated intensity. let ihkl be an integrated intensity of crystallographical planes (hkl), then we can write [3] ihkl = k ·phkl ·lθ ·pθ ·aθ ·thkl ·ehkl · |fhkl|2 . (1) k is a scale factor, which is proportional to diffracting volume of the phase and therefore it is of great importance in qpa. other factors are multiplicity of planes phkl, lorentz factor lθ, polarization factor pθ, absorption factor aθ , texture correction thkl which will be discussed further on, extinction factor ehkl and important structure factor squared |fhkl|2. the principle of qpa is very simple: to isolate the scale factors k for the phases in mixture from the equation 1. nevertheless, some of the other factors and corrections need not to be easily determinable. especially texture correction may be troublesome as texture (also called preferred orientation) description usually requires specific measurement and nontrivial data analysis. in this paper the authors tried to approximate the real texture by two simplificating cases separately: 8 http://dx.doi.org/10.14311/app.2017.9.0008 http://ojs.cvut.cz/ojs/index.php/app vol. 9/2017 phase analysis of niti wire based on synchrotron 2d xrd pattern mm 0 1 2 3 4 5 6 7 8 9 10 m m 0 0.5 1 figure 1. visualization of gridpoints showing diffraction on the sample. one pixel corresponds to one image, axes have equal scale. a) radially symmetric fibre texture and b) no texture at all. 2.3. experimental the sample was a coiled round wire of 0.2 mm diameter from superelastic alloy niti#1 [4]. this helical spring was elongated in uniaxial tension, which induced a complex stress state consisting of torsion and tension/compression due to bending of the spring. the tensile load was of 1.6 n. measurements were carried out at esrf, grenoble, at beamline 15a [5]. the wavelength of used radiation was λ = 0.0268128 nm. microfocused x-ray beam of rectangular cross-section 20 µm× 5 µm was used to scan in a rectangular grid of 12 000 points of which approximately 2 200 covered the sample. more precisely speaking it was a grid of 100× 120 points and of measures 10 mm× 1.1 mm. so the step was 100 µm in the x direction and cca 9.1 µm in the y direction. in figure 1 the scanned area is to be seen. each pixel corresponds to one gridpoint (one measurement, one image). blue pixels represent measurements where only background was captured (low average intensity in the image) and the yellow pixels correspond to images capturing the sample diffraction pattern (average intensity was larger). the wire profile is clearly visible. 2.4. data processing before the data processing itself the wavelength and sample-detector distance were determined using two calibration images taken on calibrant ceo2 in different sample-detector distances. then the integration in polar angle was carried out. due to strong texture of the sample and as a macrodeformation is expected, the integration was carried out in nine sectors separately, as indicated in figure 2. as the structure of austenite and martensite is known [6] it is possible to estimate the positions of diffraction lines of these phases. there was designed such an algorithm, that tries to fit an analytical function pearson vii to place where we expect austenitic or martensitic line. the whole pattern is then superposition of these pearsons which are all fitted at once, hence it is a whole profile fitting approach, more specifically le bail fitting the latter means that the intensities of lines are fitted parameters (see parameter a1 below). it is possible to write pearson vii in figure 2. indication of nine sectors in which the integration was carried out. each sector has angular width of 20°. a following form [7] pvii(x) = a1 1 [1 + (x−a2)2a3]a4 . (2) parameters a1, a2, a3 and a4 define the height of the function, its position in x, width and shape respectively. in the work other form was used, but the above mentioned one is more clear. the parameter a2 was a function of lattice parameters and macrodeformation (voigt model of strain distribution was assumed). the parameter a3 was a function of crystallite size and microdeformation (accordingly to williamson-hall equation for width of a diffraction line). parameters a1 (for all lines) and a4 (for each phase austenite and martensite) were fitted directly. the area under pearson vii function is possible to express analytically from the parameters a1, a2, a3 and a4. this area represents the integral intensity of a given line. the fitting was performed using nonlinear least squares, by so called levenberg-marquardt method. the fitting was carried out in all the sectors at once. some parameters were related only to a specific sector (e.g. a height a1 of a particular line in a sector), others were fitted on basis of all sectors (e.g. 3 macrodeformation-describing parameters per phase). there were 60 linear parameters in each sector (height a1 of 50 lines of martensite and 7 lines of austenite plus 3 parameters describing background) and cca 10 other nonlinear parameters which was possible to exclude from fitting (these were related to a2, a3 and a4). altogether it makes cca 550 parameters to be fitted in one image. once the integral intensity of all lines (of austenite as well as martensite) is assumed to be known from fitted parameters, the left side of equation 1 is known and thus it is possible to start isolating scale factors k of both phases from the equation (if we know all other terms). in order to finish, the texture evaluation (in this case approximation) has to be carried out. the main difference between the fibre-texture approximation and no-texture approximation was especially in the weights, which were given to integral intensities 9 martin dudr, daniel šimek, jan drahokoupil acta polytechnica ctu proceedings mm 0 2 4 6 8 10 m m 0 0.2 0.4 0.6 0.8 1 aust mart mm 0 2 4 6 8 10 m m 0 0.2 0.4 0.6 0.8 1 aust mart figure 3. relative volume fractions of austenite and martensite with respect to the whole diffracting volume. in the left-hand figure the fibre-texture correction was applied, while on the right it was the no-texture approximation. the martensite is being represented by red channel, the austenite by the blue one. sin3/6 [angst-1] 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 in te ns ity [c ts ] #104 0.5 1 1.5 2 2.5 3 3.5 4 4.5 data fit figure 4. demonstration of fit in one sector. of lines from different sectors and in the number of lines which were taken into account. it is important to notice here that the r-phase was not taken into account in fitting. 3. results and discussion an example showing fit in one sector of image is in figure 4. each of the images taken on the sample (yellow pixels in figure 1) was processed individually and the ratios a/(a+m) and m/(a+m) were determined, where a resp. m stand for the volumes of austenite resp. martensite which contribute to diffraction, i.e. the volume fractions of the phases. the resulting volume fractions of both phases are in the figure 3. the strong prevalence of martensite is easily to be seen. four scans across the wire were chosen to demonstrate the results in detail, concretely scans 11, 36, 66 and 96 with no-texture correction. they have cca a quarter of spring coil period spacing in between, as mm 0 1 2 3 4 5 6 7 8 9 10 m m 0 0.2 0.4 0.6 0.8 1.1 11 966636 figure 5. indication of four scans which are presented in more detail. there were 100 scans in total, each of the indicated scans has its number denoted nearby. indicated in figure 5. each scan consists of cca 22 images. for the results themselves see figure 6. besides the diffracting volume of austenite and martensite there are two more informations in each graph. firstly a sum of austenite and martensite, which should follow an ellipse-like arch (the wire has more mass in the center than at the borders) and secondly illustrative semi-ellipse chosen so that its maximum value is slightly higher than maximum point of the sum of the phases. this is because the absorption correction was applied only partly during image integration, but it is still expected that the resulting intensity will be affected by absorption. the illustrative wire profile (semi-ellipse) serves only to easier comparison of the sum of volumes and the expectations. scans 36 and 96 mutually seem to be a mirror image of the other (with the mirror being placed vertically in the middle of each figure) in both we can see a decrease of sum of diffracting volumes close to the middle of the wire but shifted more to the right in scan 36 and to the left in scan 96. below this effect is explained by presence of r-phase. in figure 7 there is to be seen a central group of lines in the measured angle interval, where one line of austenite (110) and four lines of martensite are expected. obviously, there are two other lines which lay in between the austenitic line (110) and martensitic lines (1-11) and (002). the positions of austenitic and 10 vol. 9/2017 phase analysis of niti wire based on synchrotron 2d xrd pattern pattern no. 5 10 15 20 di ffr ac in g vo lu m e [a .u .] 0 0.05 0.1 0.15 scan no. 11 pattern no. 5 10 15 20 di ffr ac in g vo lu m e [a .u .] 0 0.05 0.1 0.15 scan no. 36 pattern no. 5 10 15 20 di ffr ac in g vo lu m e [a .u .] 0 0.05 0.1 0.15 scan no. 66 pattern no. 5 10 15 20 di ffr ac in g vo lu m e [a .u .] 0 0.05 0.1 0.15 scan no. 96 aust mart sum aust+mart wire profile figure 6. results of phase analysis for four scans (no-texture approximation). the y axis shows diffracting volumes, x shows the order number of the images in the given scan. blue color stands for the diffracting volume of austenite, the red one for the diffracting volume of martensite and the green one is a summation of the two. the black line indicates the theoretical wire mass profile. martensitic lines in the group are well defined the pattern was fitted at once, hence all the well-isolated lines in the pattern fix positions of lines in the group. they are all bound together by the lattice parameters. but then the two excessive lines must belong to other phase. it is concluded that this phase is r-phase. as the algorithm could not move the lines, it could minimize the sum of squares only using height, width and shapes of lines. the height is most significant factor then, and its adjustment subsequently influences the resulting austenite or martensite volume fraction. but this compensation is not perfect, which implies the loss of overall sum of the fractions. r-phase is usually found somewhere in between the austenite and martensite. if the interface is more or less of a planar aspect then from some angle the microfocused beam could be passing through the r-phase region rather perpendicularly (scans 11 and 66 in figure 6) and so the irradiated r-phase volume would be low. on the other hand it would be significantly higher if the beam was passing along the r-phase region (scans 36 and 96). in that case the sum of diffracting volumes evaluated from the presented fit would be most deficient (as explained above). when the overall sum of diffracting volumes was subtracted from virtual wire profile, pictures in figsin( 3)/6 [nm -1 ] 0.42 0.44 0.46 0.48 0.5 0.52 in te ns ity a .u . #10 4 0.5 1 1.5 2 2.5 3 3.5 4 4.5 data fit aust+mart fit aust fit mart lines mart lines aust (1-11) (110) (021)(111) (002) figure 7. central group of peaks proving the presence of r-phase. positions of austenitic, resp. martensitic lines are denoted by crosses of the same color as the fit of the given phase (austenite is denoted by blue, martensite by red color). green line stands for the sum of the former two fits, black dots are measured values. ure 8 were obtained. these pictures depict this difference relative to the virtual wire profile, i.e. (vvirt − vcalc)/vvirt. because the absorption correc11 martin dudr, daniel šimek, jan drahokoupil acta polytechnica ctu proceedings mm 0 2 4 6 8 10 m m 0 0.2 0.4 0.6 0.8 1 -1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1 mm 0 2 4 6 8 10 m m 0 0.2 0.4 0.6 0.8 1 -1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1 figure 8. relative difference of calculated diffracting volume from virtual wire profile with respect to wire profile (symbolically (vvirt −vcalc)/vvirt). on the left with fibre-texture correction, on the right with no-texture correction. tion was not carried out thoroughly, some excess of calculated diffracting volume was observed on the borders and a systematic decrease in the central part. still, some deviation from this behavior is well noticeable. notice that extreme values (±1) are nearly only on the wire border. notice also that the virtual wire profile is only illustrative and serves for visualization of "shape" of the sum of diffracting volumes. it is also possible to see that the regions of greatest decline (orange-red regions in figure 8, where r-phase is expected, especially in the figure on the left) are the same as the regions where the most austenite was observed (compare with figure 3). from this we conclude that in this case we are unable to distinguish between austenite and r-phase. 4. conclusion a simplified quantitative analysis of 2d diffraction patterns was carried out from transmission diffraction measurements of niti spring at european synchrotron radiation facility (esrf), grenoble. routines were created, which enabled automatic integration of 2d diffraction patterns and successive fitting of 1d patterns regarded as a two-phase mixture (whole-profile fitting based on analytical approach and levenbergmarquardt method of nonlinear least squares). with simplifying assumptions about texture it was possible to roughly estimate the ratio of the phases (austenite and martensite). martensite appears to be strongly prevalent in the sample. as the r-phase (which was present) was not taken into account the fit had troubles with convergence in those images where the r-phase occured significantly. these images show a loss of calculated overall diffracting volume. regions of higher austenite amount seem to be correlated with regions showing this loss, i.e. with regions where r-phase is expected too. on basis of presented results these two are not distinguishable. acknowledgements this work was supported by the grant agency of the czech technical university in prague, grant no. sgs16/245/ohk4/3t/14. references [1] šittner, landa, lukáš, novák. r-phase transformation phenomena in thermomechanically loaded niti polycrystals. mechanics of materials 38:475–492, 2006. [2] t. goryczka, h. morawiec. structure studies of the r-phase using x-ray diffraction methods. journal of alloys and compounds 367:137–141, 2004. [3] v.k. pecharsky, p.y. zavalij. fundamentals of powder diffraction and structural characterization of materials, 2nd edition, springer, usa, e-isbn: 978-0-387-09579-0, 2009. [4] fort wayne metals specification of nitinol#1 alloy http://www.fwmetals.com/services/ resource-library/nitinol-1/ [5] esrf official web page: id15a materials chemistry and materials engineering. [2016-10-14] , http://www.esrf.eu/usersandscience/experiments/ structmaterials/id15a. [6] h. p. stuwe,y. shimomura. gitterkonstanten der kubisch raumzentrierten phasen feti, coti, niti. zeitschrift fur metallkunde 51:180-181, 1960. [7] r. kužel. informace v práškovém difraktogramu a jeho zpracování. materials structure 10(1a): 18-19, 2003. 12 http://www.fwmetals.com/services/resource-library/nitinol-1/ http://www.fwmetals.com/services/resource-library/nitinol-1/ http://www.esrf.eu/usersandscience/experiments/structmaterials/id15a http://www.esrf.eu/usersandscience/experiments/structmaterials/id15a acta polytechnica ctu proceedings 9:1–5, 2017 1 introduction 2 material and methods 2.1 nitinol 2.2 quantitative phase analysis 2.3 experimental 2.4 data processing 3 results and discussion 4 conclusion acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2016.3.0075 acta polytechnica ctu proceedings 3:75–77, 2016 © czech technical university in prague, 2016 available online at http://ojs.cvut.cz/ojs/index.php/app preliminary assessment of the change in the mechanical behavior of synthetic yarns submitted to consecutive stiffness tests felipe tempel stumpf∗, carlos eduardo marcos guilherme, fulvio enrico giacomo chimisso policab stress analysis laboratory, av. itália km.8 , rio grande, brazil ∗ corresponding author: felipe.stumpf@furg.br abstract. anchoring systems of offshore platforms are designed to work uninterruptedly for 20 to 30 years, and despite the increasing number of studies concerning these systems’ mechanical behavior in working conditions, most of them are conducted in speeding environments, due to the necessity of results in the short term. there are standardized stiffness tests, such as iso 18692, based on quantities like dynamic stiffness and quasi-static stiffness, through which one can compare, quantitatively, two different ropes. however, there is a lack of research aiming to assess the change in the mechanical behavior of these ropes after certain number of consecutive loading cycles (or stiffness tests). this study aims to assess, at the yarn level, the change in the quasi-static and dynamic stiffness parameters of two materials when submitted to repeated stiffness tests. results in terms of qualitative assessment of the mechanical behavior, as well as quantitative changes in the parameters are presented. keywords: synthetic mooring ropes, ultra-deepwater, experimental mechanics. 1. introduction mooring lines used as anchoring systems of oil platforms must be designed to work uninterruptedly over more than 20 years, which means that, from a mechanical design point of view, their material’s mechanical behaviour must be well known (or, at least, accurately predictable) at each moment over the system’s life span. in the case of steel wire cables, for quite a long time that was not a major concern of engineers, since the mechanical behaviour of such metallic materials tends not to suffer considering changes in that amount of time. however, over the course of the last couple of decades, new oil fields have been discovered in ultradeepwater (5000 m to 7000 m water depth), what has led the oil companies to use light-weight materials as mooring lines of their platforms’ anchoring system [1]. initially, polyester (pet) was considered the most suitable material for such applications [1], especially due to its low cost and its reliability in terms of small changes in the mechanical behaviour over time (very low creep rate, for instance). one of the main disadvantages of pet, however, is that it tends to show considerable elongation (aprox. 20 % in rupture) in tension, what might present a problem for long mooring lines in terms of station-keeping [1, 2]. aiming to find a more suitable option for these systems, the oil companies are testing several other synthetic materials, such as aramid, polyamide, high modulus polyethylene (hmpe), liquid crystal polyethylene, etc. all of those show a considerable lower elongation in comparison to pet [3, 4], but sometimes the price to be paid is a significant increase of their creep rate, or even higher production costs [5, 6]. high modulus polyethylene, for example, has the advantage to have a lower-than-water density [2], which means that it does not add weight to the platform when installed. there are some standardized stiffness tests, such as iso 18692:2013 [7], based on quantities such as static stiffness, dynamic stiffness (ds) and quasistatic stiffness (qss), through which one can compare, quantitatively, the mechanical behavior of two different ropes after some pre-defined tension/creep loads. however, there is a lack of research aiming to assess the change in the mechanical behavior of these materials after a certain number of consecutive loading cycles (or stiffness tests). the present study aims to promote a preliminary assessment, at the yarn level, of the change in the quasi-static and dynamic stiffness parameters of these materials when submitted to repeated stiffness tests. yarns of two different hmpe were submitted to a series of four consecutive stiffness experiments, with a period of ten hours between tests to promote possible relaxation of the samples in controlled environment. 2. quasi-static stiffness and dynamic stiffness the industry, both oil companies and rope makers, make use of [7] to measure qss and ds of their products in order to compare the stiffness of different materials or different rope configurations. 2.1. quasi-static stiffness according to the standard, the qss consists in "the static stiffness, which is reduced to account for the 75 http://dx.doi.org/10.14311/app.2016.3.0075 http://ojs.cvut.cz/ojs/index.php/app stumpf, guilherme, chimisso acta polytechnica ctu proceedings figure 1. test routine according to iso 18692:2013 [7]. rope creep under an environmental event". during its life in service, a mooring rope is submitted not only to dynamic loadings, but also to constant loads at moderate to high intensity. therefore, one must consider the creep behavior of these systems when designing an anchoring system, as well as the influence that this kind of loading has over the mechanical behavior of the rope. the quasi-static stiffness attempts to evaluate the loss in stiffness of the rope due to these creep regimes to which it is submitted during its lifetime. 2.2. dynamic stiffness the definition of ds follows as "the ratio of change in load to change in strain in a rope under cyclic loading". it is a quite straighforward definition, since instead of the classic stiffness which is measured during quasistatic tension tests, it is obtained at the hundredth load cycle, as depicted in figure 1. 3. materials and methods for a specific rope, subrope, strand, or yarn, the test routine depicted in figure 1 together with equation 1 are used to calculate qss and ds. qss, ds = fy − fx y bl ly − lx l0 + ly (1) the test routine (tr) showed in figure 1 was performed four consecutive times (tr#1, tr#2, tr#3 and tr#4) for each sample and for each type of hmpe (hmpe#1 and hmpe#2), with an interval of 10 hours between experiments. at the end of each routine, the quasi-static (qss#1, qss#2, qss#3, total qss) and the dynamic stiffness (ds#1, ds#2, ds#3) were calculated. the yarn samples were 500 mm long and were attached to the clamps of the machine using sandwich type endings. figure 2 shows one of the samples. all the tests have been run at room temperature of 20 ± 2 °c and 65 ± 4 % of air relative humidity. the tests were performed on an instron 8801 servohydraullic fatigue testing machine using a 10kn dynacell load cell. figure 2. typical yarns sample with sandwich type endings. 4. results figures 3–6 and tables 1–4 resume the results obtained for the evolution of qss and ds of the two types of hmpe over each test routine. figure 3. evolution of qss over each tr for hmpe#1. qss#1 qss#2 qss#3 total qss tr#1 18,9 33,2 32,4 15,7 tr#2 19,8 30,1 34,0 17,1 tr#3 18,7 33,9 34,6 17,3 tr#4 17,8 30,0 31,7 15,6 table 1. results for qss: hmpe#1. 5. conlcusions it can be seen that both quasi-static and dynamic stiffness of both materials show, in general, a stabilizing tendency along calculations #1, #2 and #3. the first stiffness (qss#1 and ds#1) is always the lowest one, followed by a considerable increase at moment #2 (qss#2 and ds#2) and, finally, a smaller increase 76 vol. 3/2016 mechanical behavior of synthetic yarns figure 4. evolution of ds over each tr for hmpe#1. ds#1 ds#2 ds#3 tr#1 58,1 59,5 60,3 tr#2 57,3 58,4 59,0 tr#3 57,4 60,0 59,2 tr#4 57,1 60,1 59,6 table 2. results for ds: hmpe#1. figure 5. evolution of qss over each tr for hmpe#2. qss#1 qss#2 qss#3 total qss tr#1 18,4 30,9 32,4 16,4 tr#2 19,4 31,7 34,7 20,0 tr#3 19,9 31,3 32,2 18,1 tr#4 19,2 32,2 33,3 19,0 table 3. results for qss: hmpe#2. figure 6. evolution of ds over each tr for hmpe#2. up to calculation #3 (qss#3 and ds#3). in other words, the more you cycle these materials, the more stiff they get, until they reach what appears to be a limiting stiffness. ds#1 ds#2 ds#3 tr#1 54,4 57,4 57,5 tr#2 54,0 55,5 56,9 tr#3 52,6 57,1 57,0 tr#4 54,3 57,1 57,7 table 4. results for ds: hmpe#2. in terms of the quasi-static stiffness, the total qss (dashed lines in figures 3–6) tends to be similar to qss#1, leading to the conclusion that both can be used for a conservative approach. comparing results between tr#1, tr#2, tr#3 and tr#4, there is no evidence, so far, that the consecutive loading of the same specimen leads to a trending change of these stiffness properties for hmpe. perhaps, if the routines were repeated a few more times, we could see a drop in qss and ds, what would mean that the material is becoming less stiff with time, what could be a problem in terms of designing a mooring line. list of symbols f force [n] y bl yarn break load [n] l elongation [m] acknowledgements policab team would like to thank petrobras brazilian petroleum for sponsoring this work. references [1] c. j. m. d. vecchio. light weight materials for deep water moorings. phd thesis. university of reading, 1992. [2] p. davies, y. reaud, l. dussud, p. woerther. mechanical behaviour of hmpe and aramid fibre ropes for deep sea handling operations. ocean engineering 38:2208–2214, 2011. doi:10.1016/j.oceaneng.2011.10.010. [3] p. davies, s. d. weller, l. johanning, s. j. banfield. a review of synthetic fiber moorings for marine energy applications. 5th international conference on ocean energy (icoe 2014) 2014. [4] s. leite, j. boesten. hmpe mooring lines for deepwater modus. offshore technology conference (otc) 2011. [5] w. huang, h. liu, y. lian, l. li. modeling nonlinear creep and recovery behaviors of synthetic fiber ropes for deepwater moorings. applied ocean research 39:113–120, 2012. doi:10.1016/j.apor.2012.10.004. [6] h. s. d. c. mattos, f. e. g. chimisso. modelling creep tests in hmpe fibres used in ultra-deep-sea mooring ropes. int j sol struct 48:144–152, 2011. doi:10.1016/j.ijsolstr.2010.09.015. [7] international organization for standardization. iso 18692:2013: fibre ropes for offshore stationkeeping — polyester, 2013. 77 http://dx.doi.org/10.1016/j.oceaneng.2011.10.010 http://dx.doi.org/10.1016/j.apor.2012.10.004 http://dx.doi.org/10.1016/j.ijsolstr.2010.09.015 acta polytechnica ctu proceedings 3:75–77, 2016 1 introduction 2 quasi-static stiffness and dynamic stiffness 2.1 quasi-static stiffness 2.2 dynamic stiffness 3 materials and methods 4 results 5 conlcusions list of symbols acknowledgements references 34 acta polytechnica ctu proceedings 1(1): 34–37, 2014 34 doi: 10.14311/app.2014.01.0034 a novel approach in the wimp quest: cross-correlation of gamma-ray anisotropies and cosmic shear marco regis1 1dipartimento di fisica, università di torino and infn, torino, italy corresponding author: regis@to.infn.it abstract we present the cross-correlation angular power spectrum of cosmic shear and gamma-rays produced by the annihilation/decay of weakly interacting massive particle (wimp) dark matter (dm), and by astrophysical sources. we show that this observable can provide novel information on the composition of the extra-galactic gamma-ray background (egb), since the amplitude and shape of the cross-correlation signal depend on which class of sources is responsible for the gamma-ray emission. if the dm contribution to the egb is significant (at least in a definite energy range), although compatible with current observational bounds, its strong correlation with the cosmic shear (since both signals peak at large halo masses) makes such signature potentially detectable by combining fermi-lat data with forthcoming galaxy surveys, like dark energy survey and euclid. keywords: dark matter extragalactic gamma rays background weak gravitational lensing. 1 introduction the presence of gravitational anomalies at different scales (galactic, cluster, large scale structure, and cosmological scales) is observationally well-established. however, it is not fully understood yet whether such anomalies are due to a new form of matter (i.e., dark matter (dm)) or to a new form of interaction (e.g., a modification of the laws of gravity). currently, the dm solution represents the mainstream, mainly because there is no theory of gravity which can account for all the anomalies at different scales simultaneously. for example, mond (modified newtonian dynamics) is a theory stating that acceleration is not linearly proportional to gravitational force at small accelerations, and seems to be quite successful in explaining galactic rotation curves. however, it cannot explain cluster dynamics without the addition of some dm components. on the other hand, it is possible that we just haven’t been good enough to build the proper extension to newtonian and einstein gravity. moreover, if dm interacts only gravitationally with ordinary matter, the two classes of solution are hardly distinguishable. one of the best ways to settle the issue would be to detect a non-gravitational signal of dm coming from the regions with gravitational anomalies. in this talk, we will present a novel strategy in this direction. in particular, if dm is made by weakly interacting massive particles (wimps), they have weak but non-negligible interactions with ordinary matter, and one of the predictions of the model is sizable fluxes of γ-rays from dm halos. both cosmic shear and cosmological gamma-ray emission stem from the presence of dm in the universe: dm structures are responsible for the bending of light in the weak lensing regime and while γ-rays can be produced by astrophysical sources hosted by dm halos (i.e. star-forming galaxies (sfg) or active galactic nuclei (agn)), dm itself may be a source of γ-rays, through its self annihilation or decay, depending on the properties of the dm particle. those γ-rays emitted by dm should therefore have the potential to exhibit strong correlation with the gravitational lensing signal. the most recent measurement of the egb was performed by the fermi-lat telescope in abdo et al. (2010)a, covering a range between 200 mev and 100 gev: the emission is obtained by subtracting the contribution of resolved sources (both point-like and extended) and the galactic foreground (due to cosmic rays interaction with the interstellar medium) from the whole fermi-lat data. unresolved astrophysical sources like blazars or radio galaxies contribute to the egb but the exact amount of their contribution is still unknown. the γ-rays produced by dm annihilation or decay can also contribute to egb (e.g., ullio et al. (2002)). however, the fact that the egb energy spectrum is compatible with a power-law, without any evident spectral feature, suggests that dm cannot play a leading role in the whole energy range, see abdo et al. (2010)b. 34 http://dx.doi.org/10.14311/app.2014.01.0034 a novel approach in the wimp quest: cross-correlation of gamma-ray anisotropies and cosmic shear in the angular anisotropies of the egb emission, the dm also plays a subdominant role: indeed, a detection of a significant auto-correlation angular ps has been recently reported in ackermann et al. (2012) (for multipoles ` > 100, which is the range of interest for our analysis, since there the contamination of the galactic foreground can be neglected), but the features of such a signal (in particular its independence on multipole and energy) seem to indicate an interpretation in terms of blazars, harding & abazajian (2012). both cosmic shear and γ-ray emission depend on the large scale structure of the universe: because this is what generates the lensing effect and because those same structures can produce γ-rays. here we will show how to use the lensing signal in order to possibly disentangle a dm signal in the egb. a detection of such cross-correlation would demonstrate that the weak lensing observables are indeed due to particle dm matter and not to possible modifications of general relativity. this presentation is based on camera et al. (2012). figure 1: left: egb emission as a function of observed energy for the four extragalactic components described in the text. data are from abdo et al. (2010)a. right: γ-ray angular ps at e > 1 gev for the same models of the left panel. the observed angular ps is summarized by the black band from ackermann et al. (2012). this figure is taken from camera et al. (2012). figure 2: left: window functions vs. redshift. for γ-ray sources we consider the flux above 1 gev normalized to the total egb intensity measured by fermilat. right: three-dimensional ps of cross-correlation shear/γ-rays at z = 0. this figure is taken from camera et al. (2012). 2 theoretical modeling the source intensity along a given direction ~n can be written as: ig(~n) = ∫ dχ g(χ,~n) w̃ (χ) , (1) where χ(z) is the radial comoving distance, g is the density field of the source, and w̃ is the window function (which does not depend on ~n). we then define a normalized versionw = 〈g〉 w̃ , so 〈ig〉 = ∫ dχ w (χ). expanding the intensity fluctuations of two source populations i and j in spherical harmonics, one can compute the cross-correlation angular ps (here in the dimensionless form): c (ij) ` = 1 〈ii〉〈ij〉 ∫ dχ χ2 wi(χ)wj(χ)pij(k = `/χ, χ) . (2) the definition of the 3-dimensional ps pij is 〈f̂gi(χ,~k)f̂∗gj (χ′,~k′)〉 = (2π)3δ3(~k − ~k′)pij(k, χ, χ ′) , (3) where fg ≡ [g(~x|m, z)/ḡ(z) − 1] (f̂g is its fourier transform) and the limber approximation (k = `/χ) is assumed to hold. we consider the sources to be characterized by a parameter m (typically the mass), and g(~x|m) is the density field of an object associated to m, while ḡ(z) = 〈g(~n, z)〉. pij can be computed following the so-called halo-model approach. the two-point correlation is given by the sum of two components, the 1-halo and 2-halo terms, i.e. pij = p 1h ij +p 2h ij , (scherrer & bertschinger (1991), ando & komatsu (2006)): p 1h ij (k) = ∫ dm dn dm f̂∗i (k|m) f̂j(k|m) (4) p 2h ij (k) = [∫ dm1 dn dm1 bi(m1)f̂∗i (k|m1) ] × [∫ dm2 dn dm2 bj(m2)f̂j(k|m2) ] p lin(k) ,(5) where dn/dm is the number density distribution of sources, p lin is the linear matter ps, and bi(m) is the linear bias between the object i and matter. note that the average of 〈g〉 is given by: ḡ(z) = 〈g(~n, z)〉 = ∫ dm dn dm ∫ d3~x g(~x|m, z) , (6) which implies that at small k (where f̂ ∼∫ d3~x g(~x|m)/ḡ) the terms in the square-brackets in eq. (5) are ∼ 1 (except in the case of a significant bias). 35 marco regis the 2-halo term is thus normalized to the standard linear matter ps at small k, which motivates the normalization of the window function introduced above. we aim at cross-correlating the shear signal (source i in eqs. (2–5)) with γ-rays emitted by dm, sfgs, and blazars (source j in eqs. (2–5)). 3 results the above formalism can be applied to all the mentioned components, by considering the specific g, w , and dn/dm of each case. for the sake of clearness we will focus on a benchmark model for each component. they are described in camera et al. (2012). in particular, we note that, although we take dm to provide a significant contribution of the egb at e ≥ 1 gev in fig. 1a, it is basically impossible to obtain an evidence for dm from the angular ps of γ-rays alone because the latter is dominated by the blazar contribution. the contributions to the egb and to the γ-ray autocorrelation aps of each components are shown in fig. 1. in fig. 2 we show the ingredients of eq. (2) for the computation of the shear/γ-ray cross-correlation angular ps: the window functions and 3d power spectra. the window function for the cosmic shear signal nicely overlaps with the dm window function, both for annihilating and decaying dm, while this happens only at intermediate redshifts for the sfg window function and only at high redshifts for the case of blazars. this suggests that a tomographic approach could be a powerful strategy to further disentangle different contributions in the angular ps (this will be explored in a forthcoming work, camera et al. (in preparation)). the shear signal is stronger for larger dm masses. the same is true also for the γ-ray signal from dm and this fact gives a large 1-halo contribution which dominates in the range k = 1−10h/mpc in fig. 2b. those wavenumbers correspond to ` ' 100 − 1000. galaxies have masses ≤ 1014m�, thus they correlate with the shear signal of lower-mass halos and the 1-halo contribution becomes important at smaller scale k � 1h/mpc. thus, an important result is that, since both the shear and dm-induced γ-ray signals are stronger for larger halos, their cross-correlation is more effective with respect to the case of astrophysical sources at intermediate wavenumbers. this, together with the sizable overlapping of the dm γ-ray and shear window functions at low redshift, leads to the expectation of a sizable dm signal in the angular ps, which is indeed what we find in fig. 3. the observational forecasts for the cross-correlation between dark energy survey (des), abbott et al. (2005), or euclid, laureijs et al. (2011), and fermilat are shown for the benchmark models considered in this work (for error estimates, we take observational performances from atwood et al. (2009), laureijs et al. (2011), and abbott et al. (2005). fig. 3 shows that a dm signal can be disentangled in the angular ps at ` ≤ 103. the same conclusion can be derived for dm models with different mass and annihilation/decay channels, provided the dm is a significant component of the total γ-ray egb (at least in one energy bin) as in our assumptions. figure 3: left: cross-correlation between cosmic shear and γ-ray emission, for the different classes of γray emitters described in the text (with a γ-ray threshold expected for fermi-lat after 5 years of exposure). each contribution is normalized by multiplying eq. (2) by 〈ij〉/〈iegb〉 to make them additive. des is taken as the reference galaxy survey. error bars are estimated for the total signal (in black). right: same as in the left panel but for annihilating dm, with euclid as the reference galaxy survey. this figure is taken from camera et al. (2012). 4 conclusions in this talk, we discussed the cross-correlation angular power spectrum of weak lensing cosmic shear and γrays produced by wimp annihilations/decays and astrophysical sources. we showed that this method can provide novel information on the composition of the egb. since the shear signal is stronger for structures of larger masses and most of the γ-ray emission from decaying and annihilating dm is also produced in large mass halos, their cross-correlation is typically stronger than the case of astrophysical sources (which are associated to galactic-mass halos) for ` ' 100 − 1000. the combination of fermi-lat with forthcoming surveys like des and euclid can thus potentially provide evidence for wimps. acknowledgement we would like to thank the organizers for the invitation to the workshop and the participants for interesting discussions. we acknowledge infn grant fa51. 36 a novel approach in the wimp quest: cross-correlation of gamma-ray anisotropies and cosmic shear references [1] t. abbott et al. [des collab.], astro-ph/0510346. [2] a. abdo et al. [fermi-lat collab.], phys. rev. lett. 104 (2010) 101101. doi:10.1103/physrevlett.104.101101 [3] a. abdo et al. [fermi-lat collab.], jcap 1004 (2010) 014. [4] m. ackermann et al. [fermi lat collab.], phys. rev. d 85 (2012) 083007. doi:10.1103/physrevd.85.083007 [5] s.’i. ando, e. komatsu, phys. rev. d73 (2006) 023521. [6] w. atwood et al. [lat collab.], ap. j. 697 (2009) 1071. doi:10.1088/0004-637x/697/2/1071 [7] s. camera, m. fornasa, n. fornengo and m. regis, astrophys. j. 771 (2013) l5 [arxiv:1212.5018 [astro-ph.co]]. [8] s. camera, m. fornasa, n. fornengo and m. regis, in preparation. [9] j. harding and k abazajian, jcap 1211 (2012) 026. [10] r. laureijs et al., arxiv:1110.3193; l. amendola et al. [euclid theory working group collab.], arxiv:1206.1225; http://www.euclid-ec.org. [11] r. scherrer and e. bertschinger, ap. j. 381, 349 (1991). [12] p. ullio et al. phys. rev. d 66 (2002) 123502. doi:10.1103/physrevd.66.123502 discussion sergio colafrancesco: what are the effects of dm-density profiles and of the cross-correlation of dm halos with gamma-ray emitting galaxies on the results you have shown? marco regis: dm density profiles in the inner part are model dependent; however this affects the cross-correlation angular power spectrum only at multipoles ` > 103−104, so above the range of our interest. since the bulk of anisotropies from dm comes from very large halos, the galactic mass halos are not crucial in the estimate of the total dm signal. jim beall: how much of an improvement in the error bars do you need to better delimit the model? marco regis: this changes with the overall normalization of the wimp cross-correlation signal which depends on the (unknown) interaction rate. however, the bottom-line is that, provided the dm is one of the most relevant components of the egb in at least a energy band, current and next future experiments, like fermi-lat and des, would suffice. 37 http://dx.doi.org/10.1103/physrevlett.104.101101 http://dx.doi.org/10.1103/physrevd.85.083007 http://dx.doi.org/10.1088/0004-637x/697/2/1071 http://dx.doi.org/10.1103/physrevd.66.123502 introduction theoretical modeling results conclusions 103 acta polytechnica ctu proceedings 2(1): 103–106, 2015 103 doi: 10.14311/app.2015.02.0103 why a new code for novae evolution and mass transfer in binaries? g. shaviv1, i. idan1, n. j. shaviv2 1department of physics, israel institute of technology, haifa, israel 2racah institute of physics, the hebrew university, jerusalem, israel corresponding author: gioras@physics.technion.ac.il abstract one of the most interesting problems in cataclysmic variables is the long time scale evolution. this problem appears in long time evolution, which is also very important in the search for the progenitor of sn ia. the classical approach to overcome this problem in the simulation of novae evolution is to assume: (1) a constant in time, rate of mass transfer. (2) the mass transfer rate that does not vary throughout the life time of the nova, even when many eruptions are considered. here we show that these assumptions are valid only for a single thermonuclear flash and such a calculation cannot be the basis for extrapolation of the behavior over many flashes. in particular, such calculation cannot be used to predict under what conditions an accreting wd may reach the chandrasekhar mass and collapse. we report on a new code to attack this problem. the basic idea is to create two parallel processes, one calculating the mass losing star and the other the accreting white dwarf. the two processes communicate continuously with each other and follow the time depended mass loss. keywords: nova modeling thermonuclear runaways. 1 introduction the classical prediction or identification of a sn ia progenitor as a wd in a compact binary system is usually based on the calculation of a single thermonuclear flash or at most few and how much mass the wd gains or loses in such a flash. extrapolations of the behavior of binary systems based on single or few thermonuclear flashes are not expected to be reliable. prialnik and kovetz (1995) were the first to simulate numerically a rather long series of thermonuclear runaway, up to a 1000. these authors solved in this way the question of the initial conditions assuming that after so many flashes, the wd converges to periodic behavior. idan et all (2013) carried a similar calculation for a high accretion rate and the results were not similar, nor were they strictly periodic. in both calculations the rate of accretion was constant in time and many flashes were calculated. however, the behavior of the mass losing star and its response to mass loss are not uniform in time so that the assumption of constant mass loss (and accretion at a constant rate on the wd) is not justified. for a star of radius r undergoing mass loss dr dt = ( ∂r ∂t ) ev + ∂r ∂m ṁ. (1) the index ev means change of r due to normal secular stelar evolution. one usually assumes that ( ∂r ∂t ) ev � ∂r ∂m ṁ. (2) but then r − rroche 6= constant and clearly, the expression for mass loss due to roche lobe overflow yields a non constant rate of mass loss, as this expression depends on r(t). note that if we assume that dr/dt = 0, namely an equilibrium or a steady state and we neglect the time variability of rroche, then ṁ = − ( ∂r ∂t ) ev / ∂r ∂m � 1, (3) in units of solar radius per solar mass. hence this expression does not yield observable values and does not imply that ṁ(t) = const. hence, this assumption is unacceptable. webbink (1977) evaluated the mass derivative of the roche radius and obtained:( ∂ lnrroche ∂ lnm ) m̄,j = f(q) (4) where q = ln(m+0.005m∗)−0.5 lnp − ln(p +1016) has to do with the mass shell division of the calculation. webbink also assumed that a = ( d lnr d lnm ) t ≈ d lnr d lnm � d lnrroche d lnm (5) 103 http://dx.doi.org/10.14311/app.2015.02.0103 g. shaviv, i. idan, n. j. shaviv where a is the adiabatic constant and the inequality holds for all ṁ. 2 mass loss rate there are several empirical expressions for the mass loss. webbink (1977) for example, assumed that ṁ = −λ ( r−rroche r )2 with λ = const (6) provided r − rroche > 0, while ritter (1988) wrote that ṁ = ṁ0 exp (r−rroche) hp for r−rroche > 0. (7) here m0 is a constant to be evaluated from the geometry of the roche lobe while hp is the pressure scale height near the l3 point. it is clear that if these expressions are valid, then a change caused by a change in the radius of the star affects the accretion rate (and the nova long time evolution). 3 time scale involved the reaction of the donor star depends on three time scales. (a) the kelvin-helmholtz-ritter time scale given by: τkhr = gm2 donor rldonor ( ∆m mdonor ) , (8) where ∆m is the mass affected by the mass loss perturbation. (b) the accretion time scale τacc = ∆mflash ṁ (9) where ∆mflash is the accreted mass at which the thermonuclear runaway takes place (of the order of 10−5m�) and (c) the dynamic time scale is given by: τdyn = √ 3 4πgρ̄ . (10) here ρ̄ is the mean density of the star. only mflash, the mass at which the nuclear flash occurs, depends (slightly) on the mass of the wd. the dynamic time depends on the entire star and τkhr depends on the outermost mass-shell involved. the interplay between these three time scales controls the phenomenon and it varies with the rate of mass loss. consequently, we calculated the hydrodynamic and thermodynamic evolution of the star under the condition that the accretion rate is given (by the parameters of the binary system). as the present calculation is carried out irrespective of the mass of the accretor, we cannot evaluate the accretion rate but have to impose it as given. 4 the dynamic behavior of the donor our goal is to investigate the dynamic response of the donor star to mass loss. we assume spherical symmetry and solve the full hydrodynamic equations of the donor and evaluated the requested derivatives. in figure 1 we see how dr/dm behaves in the case of low accretion rate (10−10m�/yr). the donor is a main sequence star of 1.25m�. 0 2000 4000 6000 time(years) 2x105 4x105 6x105 8x105 106 0 – 2 – where index ev means change of radius due to stellar evolution and @r/@m means change of radius due to change in the total mass due to mass loss from the surface. this equation does not expose the full processes taking place in the star and assumes that @r/@m) does not depends on ṁ. moreover. the derivative @r/@m) is usually calculated assuming the star is in thermal equilibrium and in strict hydrostatic equilibrium, namely the effect of ṁ on the process is ignored. if we do neglect these effect then we may ask what happens when dr/dt were to vanish. in this case we could write that ṁ = � � @r @t � ev @r @m . (2) this however, yields extremely small ṁ and implies that the assumption of ’steady state’, namely dr/dt = 0, is untenable. the thermal relaxation of the donor star in response to the mass loss of mass �m is given by ⌧khr = gm2 d rld ✓ �m md ◆ (3) where ld is the luminosity of the donor and md its mass. the accretion time scale is given by ⌧acc = mflash ṁ (4) where mflash is the accreted mass which leads to a thermonuclear runaway and ṁ is the accretion rate generated by the donor envelope. we assume that of the mass lost by the donor a fraction 1 � ⌘ is lost and does not reach the accreting star. we first ask when the thermal time scale of the donor is of the same order and the accretion time, namely gm2 rld �m md = ⌘�mmwd ṁ (5) or: ṁ mwd = rld gm2 d (6) and hence the critical accretion rate is: (7) 2. references 10�6m�/yr 10�7m�/yr 10�9m�/yr 10�10m�/yr @r @m ⇣ r� m� ⌘ – 2 – where index ev means change of radius due to stellar evolution and @r/@m means change of radius due to change in the total mass due to mass loss from the surface. this equation does not expose the full processes taking place in the star and assumes that @r/@m) does not depends on ṁ. moreover. the derivative @r/@m) is usually calculated assuming the star is in thermal equilibrium and in strict hydrostatic equilibrium, namely the effect of ṁ on the process is ignored. if we do neglect these effect then we may ask what happens when dr/dt were to vanish. in this case we could write that ṁ = � � @r @t � ev @r @m . (2) this however, yields extremely small ṁ and implies that the assumption of ’steady state’, namely dr/dt = 0, is untenable. the thermal relaxation of the donor star in response to the mass loss of mass �m is given by ⌧khr = gm2 d rld ✓ �m md ◆ (3) where ld is the luminosity of the donor and md its mass. the accretion time scale is given by ⌧acc = mflash ṁ (4) where mflash is the accreted mass which leads to a thermonuclear runaway and ṁ is the accretion rate generated by the donor envelope. we assume that of the mass lost by the donor a fraction 1 � ⌘ is lost and does not reach the accreting star. we first ask when the thermal time scale of the donor is of the same order and the accretion time, namely gm2 rld �m md = ⌘�mmwd ṁ (5) or: ṁ mwd = rld gm2 d (6) and hence the critical accretion rate is: (7) 2. references 10�6m�/yr 10�7m�/yr 10�9m�/yr 10�10m�/yr @r @m ⇣ r� m� ⌘ figure 1: the derivative dr dm in units of m�/τkhr for an imposed mass loss of 10−10m�/yr. in this case τflash ≈ 105yr. we realize that neither dr/dt nor (∂r/∂m)ṁ are constant and one should not expect the accretion rate to be constant (in time) either. even during the period of mass building for a single flash, the accretion rate is not constant. – 2 – where index ev means change of radius due to stellar evolution and @r/@m means change of radius due to change in the total mass due to mass loss from the surface. this equation does not expose the full processes taking place in the star and assumes that @r/@m) does not depends on ṁ. moreover. the derivative @r/@m) is usually calculated assuming the star is in thermal equilibrium and in strict hydrostatic equilibrium, namely the effect of ṁ on the process is ignored. if we do neglect these effect then we may ask what happens when dr/dt were to vanish. in this case we could write that ṁ = � � @r @t � ev @r @m . (2) this however, yields extremely small ṁ and implies that the assumption of ’steady state’, namely dr/dt = 0, is untenable. the thermal relaxation of the donor star in response to the mass loss of mass �m is given by ⌧khr = gm2 d rld ✓ �m md ◆ (3) where ld is the luminosity of the donor and md its mass. the accretion time scale is given by ⌧acc = mflash ṁ (4) where mflash is the accreted mass which leads to a thermonuclear runaway and ṁ is the accretion rate generated by the donor envelope. we assume that of the mass lost by the donor a fraction 1 � ⌘ is lost and does not reach the accreting star. we first ask when the thermal time scale of the donor is of the same order and the accretion time, namely gm2 rld �m md = ⌘�mmwd ṁ (5) or: ṁ mwd = rld gm2 d (6) and hence the critical accretion rate is: (7) 2. references 10�6m�/yr 10�7m�/yr 10�9m�/yr 10�10m�/yr @r @m ⇣ r� m� ⌘ – 2 – where index ev means change of radius due to stellar evolution and @r/@m means change of radius due to change in the total mass due to mass loss from the surface. this equation does not expose the full processes taking place in the star and assumes that @r/@m) does not depends on ṁ. moreover. the derivative @r/@m) is usually calculated assuming the star is in thermal equilibrium and in strict hydrostatic equilibrium, namely the effect of ṁ on the process is ignored. if we do neglect these effect then we may ask what happens when dr/dt were to vanish. in this case we could write that ṁ = � � @r @t � ev @r @m . (2) this however, yields extremely small ṁ and implies that the assumption of ’steady state’, namely dr/dt = 0, is untenable. the thermal relaxation of the donor star in response to the mass loss of mass �m is given by ⌧khr = gm2 d rld ✓ �m md ◆ (3) where ld is the luminosity of the donor and md its mass. the accretion time scale is given by ⌧acc = mflash ṁ (4) where mflash is the accreted mass which leads to a thermonuclear runaway and ṁ is the accretion rate generated by the donor envelope. we assume that of the mass lost by the donor a fraction 1 � ⌘ is lost and does not reach the accreting star. we first ask when the thermal time scale of the donor is of the same order and the accretion time, namely gm2 rld �m md = ⌘�mmwd ṁ (5) or: ṁ mwd = rld gm2 d (6) and hence the critical accretion rate is: (7) 2. references 10�6m�/yr 10�7m�/yr 10�9m�/yr 10�10m�/yr @r @m ⇣ r� m� ⌘ 0 0.5 1 1.5 2 2.5 3 1013 1012 1011 1010 109 108 time(years) figure 2: the derivative dr/dm in units of m�/τkhr for an imposed mass loss of 10−6m�/yr. in this case τflash ≈ 10yr. the results for all accretion rates are collected and summarized in figure 3. we see that in all cases, irrespective of the accretion rate, the time dependence of the derivative is given by: dr dm = 14.07 t2.35 + ε(ṁ) for all ṁ, (11) where ε(ṁ) is a constant in time which depends on ṁ. 104 why a new code for novae evolution and mass transfer in binaries? 10-6 10-5 10-4 10-3 10-2 10-1 100 101 102 103 1014 1013 1012 1011 1010 109 108 107 106 105 104 103 102 – 2 – where index ev means change of radius due to stellar evolution and @r/@m means change of radius due to change in the total mass due to mass loss from the surface. this equation does not expose the full processes taking place in the star and assumes that @r/@m) does not depends on ṁ. moreover. the derivative @r/@m) is usually calculated assuming the star is in thermal equilibrium and in strict hydrostatic equilibrium, namely the effect of ṁ on the process is ignored. if we do neglect these effect then we may ask what happens when dr/dt were to vanish. in this case we could write that ṁ = � � @r @t � ev @r @m . (2) this however, yields extremely small ṁ and implies that the assumption of ’steady state’, namely dr/dt = 0, is untenable. the thermal relaxation of the donor star in response to the mass loss of mass �m is given by ⌧khr = gm2 d rld ✓ �m md ◆ (3) where ld is the luminosity of the donor and md its mass. the accretion time scale is given by ⌧acc = mflash ṁ (4) where mflash is the accreted mass which leads to a thermonuclear runaway and ṁ is the accretion rate generated by the donor envelope. we assume that of the mass lost by the donor a fraction 1 � ⌘ is lost and does not reach the accreting star. we first ask when the thermal time scale of the donor is of the same order and the accretion time, namely gm2 rld �m md = ⌘�mmwd ṁ (5) or: ṁ mwd = rld gm2 d (6) and hence the critical accretion rate is: (7) 2. references 10�6m�/yr 10�7m�/yr 10�9m�/yr 10�10m�/yr @r @m ⇣ r� m� ⌘ – 2 – where index ev means change of radius due to stellar evolution and @r/@m means change of radius due to change in the total mass due to mass loss from the surface. this equation does not expose the full processes taking place in the star and assumes that @r/@m) does not depends on ṁ. moreover. the derivative @r/@m) is usually calculated assuming the star is in thermal equilibrium and in strict hydrostatic equilibrium, namely the effect of ṁ on the process is ignored. if we do neglect these effect then we may ask what happens when dr/dt were to vanish. in this case we could write that ṁ = � � @r @t � ev @r @m . (2) this however, yields extremely small ṁ and implies that the assumption of ’steady state’, namely dr/dt = 0, is untenable. the thermal relaxation of the donor star in response to the mass loss of mass �m is given by ⌧khr = gm2 d rld ✓ �m md ◆ (3) where ld is the luminosity of the donor and md its mass. the accretion time scale is given by ⌧acc = mflash ṁ (4) where mflash is the accreted mass which leads to a thermonuclear runaway and ṁ is the accretion rate generated by the donor envelope. we assume that of the mass lost by the donor a fraction 1 � ⌘ is lost and does not reach the accreting star. we first ask when the thermal time scale of the donor is of the same order and the accretion time, namely gm2 rld �m md = ⌘�mmwd ṁ (5) or: ṁ mwd = rld gm2 d (6) and hence the critical accretion rate is: (7) 2. references 10�6m�/yr 10�7m�/yr 10�9m�/yr 10�10m�/yr @r @m ⇣ r� m� ⌘ – 2 – where index ev means change of radius due to stellar evolution and @r/@m means change of radius due to change in the total mass due to mass loss from the surface. this equation does not expose the full processes taking place in the star and assumes that @r/@m) does not depends on ṁ. moreover. the derivative @r/@m) is usually calculated assuming the star is in thermal equilibrium and in strict hydrostatic equilibrium, namely the effect of ṁ on the process is ignored. if we do neglect these effect then we may ask what happens when dr/dt were to vanish. in this case we could write that ṁ = � � @r @t � ev @r @m . (2) this however, yields extremely small ṁ and implies that the assumption of ’steady state’, namely dr/dt = 0, is untenable. the thermal relaxation of the donor star in response to the mass loss of mass �m is given by ⌧khr = gm2 d rld ✓ �m md ◆ (3) where ld is the luminosity of the donor and md its mass. the accretion time scale is given by ⌧acc = mflash ṁ (4) where mflash is the accreted mass which leads to a thermonuclear runaway and ṁ is the accretion rate generated by the donor envelope. we assume that of the mass lost by the donor a fraction 1 � ⌘ is lost and does not reach the accreting star. we first ask when the thermal time scale of the donor is of the same order and the accretion time, namely gm2 rld �m md = ⌘�mmwd ṁ (5) or: ṁ mwd = rld gm2 d (6) and hence the critical accretion rate is: (7) 2. references 10�6m�/yr 10�7m�/yr 10�9m�/yr 10�10m�/yr @r @m ⇣ r� m� ⌘ – 2 – where index ev means change of radius due to stellar evolution and @r/@m means change of radius due to change in the total mass due to mass loss from the surface. this equation does not expose the full processes taking place in the star and assumes that @r/@m) does not depends on ṁ. moreover. the derivative @r/@m) is usually calculated assuming the star is in thermal equilibrium and in strict hydrostatic equilibrium, namely the effect of ṁ on the process is ignored. if we do neglect these effect then we may ask what happens when dr/dt were to vanish. in this case we could write that ṁ = � � @r @t � ev @r @m . (2) this however, yields extremely small ṁ and implies that the assumption of ’steady state’, namely dr/dt = 0, is untenable. the thermal relaxation of the donor star in response to the mass loss of mass �m is given by ⌧khr = gm2 d rld ✓ �m md ◆ (3) where ld is the luminosity of the donor and md its mass. the accretion time scale is given by ⌧acc = mflash ṁ (4) where mflash is the accreted mass which leads to a thermonuclear runaway and ṁ is the accretion rate generated by the donor envelope. we assume that of the mass lost by the donor a fraction 1 � ⌘ is lost and does not reach the accreting star. we first ask when the thermal time scale of the donor is of the same order and the accretion time, namely gm2 rld �m md = ⌘�mmwd ṁ (5) or: ṁ mwd = rld gm2 d (6) and hence the critical accretion rate is: (7) 2. references 10�6m�/yr 10�7m�/yr 10�9m�/yr 10�10m�/yr @r @m ⇣ r� m� ⌘ – 2 – where index ev means change of radius due to stellar evolution and @r/@m means change of radius due to change in the total mass due to mass loss from the surface. this equation does not expose the full processes taking place in the star and assumes that @r/@m) does not depends on ṁ. moreover. the derivative @r/@m) is usually calculated assuming the star is in thermal equilibrium and in strict hydrostatic equilibrium, namely the effect of ṁ on the process is ignored. if we do neglect these effect then we may ask what happens when dr/dt were to vanish. in this case we could write that ṁ = � � @r @t � ev @r @m . (2) this however, yields extremely small ṁ and implies that the assumption of ’steady state’, namely dr/dt = 0, is untenable. the thermal relaxation of the donor star in response to the mass loss of mass �m is given by ⌧khr = gm2 d rld ✓ �m md ◆ (3) where ld is the luminosity of the donor and md its mass. the accretion time scale is given by ⌧acc = mflash ṁ (4) where mflash is the accreted mass which leads to a thermonuclear runaway and ṁ is the accretion rate generated by the donor envelope. we assume that of the mass lost by the donor a fraction 1 � ⌘ is lost and does not reach the accreting star. we first ask when the thermal time scale of the donor is of the same order and the accretion time, namely gm2 rld �m md = ⌘�mmwd ṁ (5) or: ṁ mwd = rld gm2 d (6) and hence the critical accretion rate is: (7) 2. references 10�6m�/yr 10�7m�/yr 10�9m�/yr 10�10m�/yr @r @m ⇣ r� m� ⌘ time (years) m*=1.25msun figure 3: the derivative dr/dm in units of m�/τkhr for all accretion rates calculated here (for a ms star of mass 1m�). moreover, time dependence of the derivatives tend for sufficiently long times, to an asymptote. we find that: for t� ε(ṁ) dr dm → 14.07 t2.35 , (12) namely, all results (for a given mass of the donor) converge for long times to an asymptote. we do not know at the moment how this asymptote changes with the mass of the donor. at the same time we can write for the roche lobe radius (eggelton, 1984) that: rroche ≈ 0.49q2/3 0.6q2/3 + ln(1 + q1/3) where q = mwd + ṁt md − ṁt (13) assuming conservative mass loss. hence for sufficiently small t (at the beginning) we have that: ∣∣∣∣ drroche dm ∣∣∣∣� ∣∣∣∣ drdonor dm ∣∣∣∣ (14) the thermodynamic state is shown in figure 4 time(years) – 2 – where index ev means change of radius due to stellar evolution and @r/@m means change of radius due to change in the total mass due to mass loss from the surface. this equation does not expose the full processes taking place in the star and assumes that @r/@m) does not depends on ṁ. moreover. the derivative @r/@m) is usually calculated assuming the star is in thermal equilibrium and in strict hydrostatic equilibrium, namely the effect of ṁ on the process is ignored. if we do neglect these effect then we may ask what happens when dr/dt were to vanish. in this case we could write that ṁ = � � @r @t � ev @r @m . (2) this however, yields extremely small ṁ and implies that the assumption of ’steady state’, namely dr/dt = 0, is untenable. the thermal relaxation of the donor star in response to the mass loss of mass �m is given by ⌧khr = gm2 d rld ✓ �m md ◆ (3) where ld is the luminosity of the donor and md its mass. the accretion time scale is given by ⌧acc = mflash ṁ (4) where mflash is the accreted mass which leads to a thermonuclear runaway and ṁ is the accretion rate generated by the donor envelope. we assume that of the mass lost by the donor a fraction 1 � ⌘ is lost and does not reach the accreting star. we first ask when the thermal time scale of the donor is of the same order and the accretion time, namely gm2 rld �m md = ⌘�mmwd ṁ (5) or: ṁ mwd = rld gm2 d (6) and hence the critical accretion rate is: (7) 2. references 10�6m�/yr 10�7m�/yr 10�9m�/yr 10�10m�/yr @r @m ⇣ r� m� ⌘ – 2 – where index ev means change of radius due to stellar evolution and @r/@m means change of radius due to change in the total mass due to mass loss from the surface. this equation does not expose the full processes taking place in the star and assumes that @r/@m) does not depends on ṁ. moreover. the derivative @r/@m) is usually calculated assuming the star is in thermal equilibrium and in strict hydrostatic equilibrium, namely the effect of ṁ on the process is ignored. if we do neglect these effect then we may ask what happens when dr/dt were to vanish. in this case we could write that ṁ = � � @r @t � ev @r @m . (2) this however, yields extremely small ṁ and implies that the assumption of ’steady state’, namely dr/dt = 0, is untenable. the thermal relaxation of the donor star in response to the mass loss of mass �m is given by ⌧khr = gm2 d rld ✓ �m md ◆ (3) where ld is the luminosity of the donor and md its mass. the accretion time scale is given by ⌧acc = mflash ṁ (4) where mflash is the accreted mass which leads to a thermonuclear runaway and ṁ is the accretion rate generated by the donor envelope. we assume that of the mass lost by the donor a fraction 1 � ⌘ is lost and does not reach the accreting star. we first ask when the thermal time scale of the donor is of the same order and the accretion time, namely gm2 rld �m md = ⌘�mmwd ṁ (5) or: ṁ mwd = rld gm2 d (6) and hence the critical accretion rate is: (7) 2. references 10�6m�/yr 10�7m�/yr 10�9m�/yr 10�10m�/yr @r @m ⇣ r� m� ⌘ – 2 – where index ev means change of radius due to stellar evolution and @r/@m means change of radius due to change in the total mass due to mass loss from the surface. this equation does not expose the full processes taking place in the star and assumes that @r/@m) does not depends on ṁ. moreover. the derivative @r/@m) is usually calculated assuming the star is in thermal equilibrium and in strict hydrostatic equilibrium, namely the effect of ṁ on the process is ignored. if we do neglect these effect then we may ask what happens when dr/dt were to vanish. in this case we could write that ṁ = � � @r @t � ev @r @m . (2) this however, yields extremely small ṁ and implies that the assumption of ’steady state’, namely dr/dt = 0, is untenable. the thermal relaxation of the donor star in response to the mass loss of mass �m is given by ⌧khr = gm2 d rld ✓ �m md ◆ (3) where ld is the luminosity of the donor and md its mass. the accretion time scale is given by ⌧acc = mflash ṁ (4) where mflash is the accreted mass which leads to a thermonuclear runaway and ṁ is the accretion rate generated by the donor envelope. we assume that of the mass lost by the donor a fraction 1 � ⌘ is lost and does not reach the accreting star. we first ask when the thermal time scale of the donor is of the same order and the accretion time, namely gm2 rld �m md = ⌘�mmwd ṁ (5) or: ṁ mwd = rld gm2 d (6) and hence the critical accretion rate is: (7) 2. references 10�6m�/yr 10�7m�/yr 10�9m�/yr 10�10m�/yr @r @m ⇣ r� m� ⌘ – 2 – where index ev means change of radius due to stellar evolution and @r/@m means change of radius due to change in the total mass due to mass loss from the surface. this equation does not expose the full processes taking place in the star and assumes that @r/@m) does not depends on ṁ. moreover. the derivative @r/@m) is usually calculated assuming the star is in thermal equilibrium and in strict hydrostatic equilibrium, namely the effect of ṁ on the process is ignored. if we do neglect these effect then we may ask what happens when dr/dt were to vanish. in this case we could write that ṁ = � � @r @t � ev @r @m . (2) this however, yields extremely small ṁ and implies that the assumption of ’steady state’, namely dr/dt = 0, is untenable. the thermal relaxation of the donor star in response to the mass loss of mass �m is given by ⌧khr = gm2 d rld ✓ �m md ◆ (3) where ld is the luminosity of the donor and md its mass. the accretion time scale is given by ⌧acc = mflash ṁ (4) where mflash is the accreted mass which leads to a thermonuclear runaway and ṁ is the accretion rate generated by the donor envelope. we assume that of the mass lost by the donor a fraction 1 � ⌘ is lost and does not reach the accreting star. we first ask when the thermal time scale of the donor is of the same order and the accretion time, namely gm2 rld �m md = ⌘�mmwd ṁ (5) or: ṁ mwd = rld gm2 d (6) and hence the critical accretion rate is: (7) 2. references 10�6m�/yr 10�7m�/yr 10�9m�/yr 10�10m�/yr @r @m ⇣ r� m� ⌘ – 3 – 10�11m�/yr (lph�lnuc) lnuc @r @m ⇣ r� m� ⌘ – 2 – where index ev means change of radius due to stellar evolution and @r/@m means change of radius due to change in the total mass due to mass loss from the surface. this equation does not expose the full processes taking place in the star and assumes that @r/@m) does not depends on ṁ. moreover. the derivative @r/@m) is usually calculated assuming the star is in thermal equilibrium and in strict hydrostatic equilibrium, namely the effect of ṁ on the process is ignored. if we do neglect these effect then we may ask what happens when dr/dt were to vanish. in this case we could write that ṁ = � � @r @t � ev @r @m . (2) this however, yields extremely small ṁ and implies that the assumption of ’steady state’, namely dr/dt = 0, is untenable. the thermal relaxation of the donor star in response to the mass loss of mass �m is given by ⌧khr = gm2 d rld ✓ �m md ◆ (3) where ld is the luminosity of the donor and md its mass. the accretion time scale is given by ⌧acc = mflash ṁ (4) where mflash is the accreted mass which leads to a thermonuclear runaway and ṁ is the accretion rate generated by the donor envelope. we assume that of the mass lost by the donor a fraction 1 � ⌘ is lost and does not reach the accreting star. we first ask when the thermal time scale of the donor is of the same order and the accretion time, namely gm2 rld �m md = ⌘�mmwd ṁ (5) or: ṁ mwd = rld gm2 d (6) and hence the critical accretion rate is: (7) 2. references 10�6m�/yr 10�7m�/yr 10�8m�/yr 10�9m�/yr 10�10m�/yr 102 101 100 10-1 10-2 10-3 10-4 10-7 10-6 10-5 10-4 10-3 10-2 10-1 100 101 102 103 – 3 – 10�11m�/yr (lph�lnuc) lnuc @r @m ⇣ r� m� ⌘ figure 4: the time to reach thermodynamic equilibrium. we see that stars with mass loss rate smaller than 10−9m� tend to thermal equilibrium. the timescales to reach the thermal equilibrium vary. accretion rates higher than 10−9m� diverge, namely they become unstable and runaway. the rate of accretion drives the star out of thermal equilibrium to be never restored. this fact should be taken into account in evaluating the mass loss rate from the donor. 5 conclusions first conclusion: it is not justified to assume that τkhr is negligible. second, polytropic estimates are nice and simple, but wrong (motl et al.2002) the mass loss is not constant in time. the mass loss does not start suddenly and reaches the assumed value gradually. there is no fixed period between eruptions. the system can have n eruptions with an almost constant time interval and then pause and let the donor recover on a kelvin-helmholtz-ritter time scale of its envelope. during this time the wd may relax to a new state. the accretion rate changes in time and affects the evolution of the nova. a nova calculation must include the evolution of the donor and the accretion rate. an important element in the evolution of nova is the time variability of the accretion rate. 0 2x1013 4x1013 6x1013 8x1013 1x1014 -0.0014 -0.0012 -0.001 -0.0008 -0.0006 -0.0004 -0.0002 0 time (seconds) m as s l os t f ro m d on or (s ol ar m as se s) figure 5: the time depended mass loss from the donor. 6 discussion and conclusions a significant part of present day interest in nova includes the cases that may become progenitors of sn ia. in this case the behavior of the binary system is followed through a single or few thermonuclear runaways and then the result is extrapolated over 6-7 orders of magnitudes. the mass accreted or lost is of the order of 10−7m�. the initial mass of the wd is of the order of 1m� and if a conservative mass transfer is assumed this means a huge extrapolation in the behavior of the accretor and the the donor. we conclude that the evolution of the two stars must be followed simultaneously. we developed a code which 105 g. shaviv, i. idan, n. j. shaviv does just that. two processors are created, each devoted to a star. thus the donor star is calculated on one cpu and the accretor on a second cpu. the two processes, which may run on different computers or on a computer with more than one cpu, communicate with one another via an open gate. the communication can take place at fixed time intervals or whenever the conditions on one star deviate significantly and an update is due. 0 2x1013 4x1013 6x1013 8x1013 1x1014 0 5x107 1x108 1.5x108 2x108 2.5x108 time (seconds) ln uc / ls un ln uc /l su n figure 6: the flashes of nuclear energy as a function of time. the code is in the debugging phase but the results seem to justify the claims present here. we find that the accretion rate is not constant in time and it stops when the radius of the donor shrinks below the roche lobe radius. in figure 5 we show one such example. the periods of no accretion appear as horizontal section (no change in mass lost). the period of mass loss appear as decreasing lines. note that these parts of the curve are straight lines at the beginning but not later. hence the rate of mass loss changes even during accretion. the recovery time depends on the mass loss rate and the τkhr of the donor. the resulting nuclear flushes are shown in figure 6. we see that the picture of flashes at a constant rate is correct only of a couple of flashes and the flashes come in groups. the time between the groups, the relaxation time of the donor increases gradually. acknowledgement this research has been supported by the israel science foundation, grant 1589/10. shaviv giora is delighted to thank dr. ilan shaviv in setting the code. references [1] eggleton,p.p.1983,apj.,268,368. doi:10.1086/160960 [2] idan, i., shaviv, n.j. & shaviv, g. , 2013, mnras, 433, 2884 doi:10.1093/mnras/stt908 [3] kolb, u. & ritter, h. 1990. a&a, 236, 385. [4] motl, p.m., tohline, j.e. & frank, j., 2002, apjs, 138,121. doi:10.1086/324159 [5] prialnik, d. & kovetz, a., 1995, apj, 445, 789. [6] ritter, h. 1988, a&a, 202,93. [7] webbink, r. f. , 1977, apj, 211, 881. 106 http://dx.doi.org/10.1086/160960 http://dx.doi.org/10.1093/mnras/stt908 http://dx.doi.org/10.1086/324159 introduction mass loss rate time scale involved the dynamic behavior of the donor conclusions discussion and conclusions 27 acta polytechnica ctu proceedings 1(1): 27–33, 2014 27 doi: 10.14311/app.2014.01.0027 astrophysics without black holes, and without extragalactic gamma-ray bursts wolfgang kundt1 1argelander-institut für astronomie der universität bonn, auf dem hügel 71, d-53121 bonn corresponding author: wkundt@astro.uni-bonn.de abstract contrary to common belief, it is argued that our universe contains neither black holes, nor extracalactic gamma-ray bursts. keywords: black holes, gamma-ray bursts galactic center burning disks magnetars. 1 introduction modern cosmology makes confident statements about processes which have happened throughout the universe, all the way back to its beginnings, even though no convincing cosmological initial data have ever been reliably proposed [hoyle et al 2000]. such confidence is somehow based on the undoubted successes of einstein’s general theory of relativity in our immediate cosmic surroundings – the inner solar system – with or without a λ-term which has been termed ”dark energy” (by mike turner) when it has the sign opposite to that of energy, whose necessity is still debated. in these years, almost generally accepted as cosmic building blocks are (i) supermassive black holes (smbhs) in the centers of (almost all?) galaxies, without any physical justification, (ii) sources of almost all the gammaray bursts (grbs) at cosmic distances, viz. of the bursts with line redshifts z & 1, and (iii) non-baryonic dark matter, even though we cannot even reliably measure the amount of ionised hydrogen between us and the (neighbouring) andromeda galaxy [braun 2013].. in this presentation, i will restrict myself to the first two listed source classes, and start with the smbhs. not only have all the well-observed galactic central engines (ces) defied a plausible bh interpretation, by their (large) luminosities, (high) temperatures, (strong) outflows, (relativistic) twin-jets, nonpointlike central potential, and halo-scale high-energy (.102gev) plumes, but even their theoretical existence is no longer justified because rigorous calculations have (recently) shown that the expectations of the 1970s have been hasty, that roger penrose’s hypothesis of ‘cosmic censorship’ has been premature. we now know that complete gravitational collapse leads to naked singularities – not to bhs – except for a highest-symmetry subset of initial data, of measure zero. on top of not having been verified throughout more than 35 years, therefore, bhs are now no longer expected by theorists [joshi 2009, quevedo 2011, joshi & malafarina 2013]. in particular, our galactic center (sgr a*) deviates from containing a smbh for at least 15 reasons [kundt 2012], the 15th being the happy survival, so far, of cloudlet g2 during its ongoing approach of sgr a*. the second goal of this presentation is to refine my earlier considerations – dating back to the 1980s, when galactic grbs used to be common wisdom – that all detected grbs come from local-galactic neutron stars, rather than from ill-understood supersources at cosmic distances. not only have there five quantitative distance estimates been published in the past which must not be forgotten, based on well-measured properties, which place their sources nearer than a few kpc from us, but meanwhile, a number of bursters have even been directly identified, and their tentative distances lowered, with the convergent result that we deal with the spectacular class of nearby magnetars in our galaxy [kundt 2008, 2010-2013, ghisellini 2010, archibald et al 2013, rea et al 2013, ho 2013]. the grbs are a local-galactic phenomenon, is my iterated message. 2 bh physics reconsidered black holes have been baptised in princeton, in 1971, by john wheeler and remo ruffini. their name stuck immediately, because years of hard international research had been invested, mainly in the united states and in europe, until the complete family o f spinning bh spacetimes was finally published by roy kerr in 1963, and until the holes’ hairlessness had been (essentially) proven, during the decade to follow, with brandon carter among the cheerleaders. mass, spin, and 27 http://dx.doi.org/10.14311/app.2014.01.0027 wolfgang kundt electric charge are a bh’s (three) only free parameters [heusler 1996]. but how to detect bhs in the universe? could we ever hope to identify a bh in the sky? wouldn’t they hide forever, on grounds of their expected dimness, their not being able to radiate? to my knowledge, this question was first asked seriously in 1972, during international meetings between theorists and observers, with a distinct emphasis on candidates of stellar mass. a first, promising suggestion was the x-ray emitting stellar binary system cyg x-1 – the brightest stellar x-ray source in the cygnus region – whose optically invisible component had to be more massive than a neutron star: bingo! slowly, please: cannot the dark component be a neutron star surrounded by a self-gravitating disk [kundt 1979]? aren’t self-gravitating disks stable? disks are thought to form stars, their planets, and probably likewise all their moons; from what other instabilities could a disk suffer? probably none. and how to blow the jets seen to spring forth from the dark component of the cyg x-1-system? binary neutron stars are observed to blow jets, whereas bhs cannot do that because they lack an inclined, co-rotating magnetosphere, for generating the jets’ pair plasma [kundt 2011]. in 1989, daniel fischer and i could not find a single bh in the whole class of (stellar-mass) bhcandidates [kundt & fischer 1989]. in order to still detect a bh in the sky, should we perhaps search the hole in a higher weight class, as a supermassive bh, at the center of some galaxy, perhaps even our own? but how should it form there in the first place, or how else should it get to a galaxy’s center? are not self-gravitating disks stabilised radially by centrifugal forces, and vertically by pressure forces, if necessary via nuclear burning, like stars? i received “honorable mention” for this idea in 1978, by grg, and described its structure more in detail at bad honnef in 1995 [kundt 1996], in vital contact with peter scheuer, (see his contribution to that same book). from suzy collinsouffrin i had learned that mass spiral-in rates through galactic disks – determined indirectly by the central disk’s luminosity – tend to balance mass-ejection rates through the broad line region (blr), determined by its emission-line intensities, whereby the ejection energy is supplied by nuclear burning, like in stellar explosions. instead of like a bh, the stable galactic disk would act like a burning disk, a ”bd”. mass would spiral in through the disk, at a rate of .m�/yr for a galaxy like ours, and would subsequently be ejected explosively through its blr, nlr, elr out to the eer beyond the galaxy’s halo, in the form of a galactic-scale fountain [kundt 1987, 1990, 1996, 2005], cf. fig. 1, and [kormendy 2011]. the case of our milky-way galaxy was my topic last year at vulcano [kundt 2012]; it will not be repeated here. no bh is needed for this fountain to function; on the contrary, a bh near the center would have prevented its formation, via irreversible accretion. it would have swallowed the incoming nuclear fuel, and thereby grown in mass. instead, the sdss plot of galacticcenter masses has shown a (statistical) monotonic decrease of ce masses from some initial 109.3m� down to the present-day rather low values, of order 10−3 that much, in the redshift range from z = 4.5 down to (the present) z = 0. galactic ces have monotonicly lost mass during cosmic evolution, defying a bh interpretation, see fig. 2. figure 1: most impressive evidence against a bh at our galactic center: the fermi bubbles (or plumes), at photon energies .102gev, probably emitted by buoyantly rising relativistic pair plasma from the near vicinity of sgr a*, throughout the milky-way halo, to heights well above 20 kpc, taken from [su et al 2010, 2011]. these same halo structures had already been detected and mapped decades earlier by [sofue 2000], from radio and x-ray data. at this point, my reasoning departs drastically from that of most of my colleagues, who use words like ”downsizing” and ”co-evolution” to describe the average monotonic shrinkage of ce masses plotted in fig. 2. they speak of a high bh ‘efficiency’, without indicating how a bh should do that, because as already stated in the introduction, galactic centers are often observed to be quite luminous, stormy, jet-blowing, and pluming (at .102gev), from their center all the way out into their halo. how is this central activity powered? my answer reads: by nuclear burning of the central disk, combined with magnetic reconnections in its (very) fast and differentially rotating corona [kundt 1996]. a bd avails of abundant rotational, infall, and nuclear energy, for both non-thermal and thermal ejections: of radiation, jets, winds, and plumes. a smbh would suppress all this. instead, a few of my decades-long friends call a smbh the ‘only thinkable’ central engine (ce) of galactic centers without telling the reader how such a smbh could have formed and grown, how it could e.g. emit the observed hard spectra [aharonian et al 2004], and 28 astrophysics without black holes, and without extragalactic gamma-ray bursts how it could perform the multiple hard ejections [su et al 2010]. i think in particular of max camenzind, of harald lesch, and of andreas müller. they have published books, or contributed to books in which they claim, already on the book’s cover, that mankind would not have formed without bhs [müller 2010]. these publications have appeared fairly recently so that i have not gotten a chance to sort myself out with their authors, in friendly, animated discussions. how did my friends reach their present opinions? have they found flaws in my 1996 model? why did they never tell me? figure 2: the sdss plot of the ce masses m(z) of all the (&104) sampled galaxies by the sloan digital sky survey. as functions of cosmicredshift z (≥ 0.2), [vestergaard et al, 2008]. small squares denote (vertical) averages at fixed redshift bin. the plot shows that m(z) decreases monotonicly, from 109.3m� at z = 4.5 down to .108m� at the smallest plotted redshift, z = 0.2. the scientific situation is, in fact, more inconsistent than described so far. exact analytical calculations have meanwhile shown that mass concentrations collapsing under their own weight will no longer form bhs, rather naked singularities, except for configurations of highest symmetry which are, however, of measure zero among all initial data [joshi 2009, quevedo 2011, joshi & malafarina 2013]. penrose’s hypothesis of “cosmic censorship” ought to be strengthened into the hypothesis of an “avoidance of unhalted gravitational collapse”, auc, unless we do not mind contenting ourselves with a piecewise singular theory, or unless we are ready to search for a replacement of gr by some new, nonsingular spacetime theory. my own preference is for auc, as i have never failed to find hurdles to any proposed unhalted gravitational collapse. 3 gamma-ray bursts from galactic magnetars now to item (ii) of this pamphlet: what are the sources of the grbs? isn’t it clear since some 20 years that those sources ‘must be’ at cosmological distances? to my mind, it has never been: true: the bursts reach us from all directions, three per day on average from outer space, in an almost isotropic fashion, but small anisotropies of well-defined subclasses of bursts have repeatedly been published; the isotropy is not perfect, and a nearby population of sources (.0.3 kpc) can certainly not be excluded, in particular when individual bursters have anisotropic radiation patterns, oriented with respect to their (low-mass) accretion disks whose normals prefer to lie in the galactic plane. (such a preference is expected for disks assembled by neutron stars during their oscillatory motion through the galactic disk). next think of the redshifts z of the bursts’ afterglows, and host galaxies, z lying between 0 and 9, corresponding to lorentz-factors γ between 1 and 5, according to the kinematic law z + 1 = γ(1 + β). such redshifts may well be kinematic redshifts of matter ejected centrifugally from near the speed-of-light cylinder of a sufficiently magnetised, slowly spinning neutron star, hence need not mean a (cosmologically) large distance. note that a few bursts have even had z = 0, including those from the sgrs, so that a distal interpretation of z would deal with a range of distances through a factor of order 108, corresponding to a range of powers through a factor of 1016, for essentially the same type of sources, viz. accreting neutron stars. this reason alone has never encouraged me to take a cosmic interpretation of the grbs seriously: the distant sources would have to radiate almost infinitely times brighter than their cousins in the milky way, even if you try to save a factor of γ2 .106 by postulated beaming (of both the prompt and the afterglow emission!). to me, the redshifts mean velocities, not distances (of the sources). note that there is no problem with missing blueshifts from spherical explosions: only the first flash from a distant, spherical explosion is blueshifted, all later radiation (from a small angular neighbourhood of the explosion site, due to scattering) is received redshifted, mainly from the receding polar cap of the expanding sphere of ejecta. and what about host galaxies, at the same redshift? only a quarter of all bursts have had reliable-looking hosts, whereby no host galaxy has ever been intrinsicly brighter than the lmc! fancy the gigantic intrinsic energy (&1053erg) of a grb interpreted cosmologically, illuminating the csm of its host galaxy: the host would have to flare, rather than be just about detectable. no ‘ordinary’ galaxy (like ours) has ever hosted a grb! .my preferred explanation: what observers call ‘host’ are the flaring ejecta plus dragged-along csm of the (nearby) bursting magnetar, before the hadronic ejecta start losing a significant fraction of their kinetic energy received at launch. we receive cooling radiation from 29 wolfgang kundt thermalised (kinetic) burst energy, around the magnetar, part of which is still moving transrelativistically away from us, with the same redshift as the early ejecta. all bursts come from inside our milky way; the published ‘hosts’ are all fake. now let us look again at the five published distance estimates for grbs, all of which imply source distances of less than a few kpc, depending on the strength s of the recorded burst [kundt 2010]. the first three of them take similar shapes: d / kpc < {1/ √ s−4, γ √ l38, 1/ √ s−11} , (1) in which s stands respectively for the peak incoming energy-flux density of the {prompt at & mev, early optical afterglow} emission in the {first, third} entry, where s−4 := s/10−4erg/cm2s is evaluated in cgs units (as always), and γ stands for the (assumed) bulk lorentz factor of the neutron-star system w.r.t. which the power l is emitted. more in detail, the first estimate in (1) uses photon crowding at the source, which would soften a grb’s spectrum above the pair-formation threshold (of ≈mev) [schmidt 1978, baring 1992] unless it were emitted in a highly beamed way. it rules out a distance of 50 kpc (of lmc) for the sgr 0526-66 – which flared on 5 march 1979 – and it likewise rules out a distance of 8.7 kpc for sgr 1806-20; in the main table of mcgill sgr/axp online catalog. in the meantime, the fermi mission has measured energetic hard tails to many grbs reaching up in energy towards &10 gev, which begin to reach us right after the prompt emission, see [ghisellini 2010], and eleonora troja’s contribution to this workshop. even so, their roof-like spectral peak (near 1 mev), at the top of the band spectrum, did not look grossly distorted in the new spectra. for all of them, therefore, distances larger than one kpc are inconsistent with their (hard) spectra unless their power was emitted in a highly beamed fashion, whose detailed mechanism has never been explained convincingly at conferences, or anywhere in the literature. required would be hadronic jets formed on the timescale of seconds, but lasting long enough (months) in order to allow likewise for beamed afterglows. such postulated jets would have to function very differently from the hundreds of well-studied quasi-stationary pair-plasma jets observed to spring forth from the various well-known jet sources, all powered by heavy rotating magnets [kundt 2011]. the second inequality in (1) holds under the assumption that the source of the grb is a neutron star, whose small surface area restricts its maximal possible luminosity l, via the eddington constraint. it was always satisfied during the 1980s, when scientists trusted the galactic-neutron-star model for the grbs. finally, the third estimate in (1) assumes that the afterglows are emitted incoherently, from the near surroundings of the burst sites which are hit by the explosion. bright afterglows tend to have poynting fluxes .10−10erg/cm2s, for which (1) yields distances d . 0.3 kpc., in accord with the estimated magnetar densities in the milky way. again, this third estimate would not hold for jetlike emission; but how, or why would such be expected? two even much smaller distance estimates d have been derived for the nearest among all grbs, for which we even see their much more frequent but much fainter repetitions, for the soft gamma-ray repeaters (sgrs), of which 13 are presently listed in the mcgill catalog. (only 8 sgrs were listed in [kundt 2010]). so far, none of them has flared more than once at their large, original intensity, but their intense flares were much brighter than all those of the non-repeaters. for this reason, i feel encouraged to use again the second estimate of (1), but this time not with the classical eddington luminosity l = 1038erg/s of a steadily accreting neutron star, but instead with the maximal possible luminosity expected for an isolated, clumpy impact onto a neutron star, of order l = 1042erg/s. another, independent estimate involves the typical galactic peculiar velocity cβ of a magnetar (blowing radio bubbles, with measured angular velocities), of order β .10−3. the two estimates read [kundt 2010]: d / 30pc . { √ l41.9, β−2.9} , (2) in agreement with the nearest expected magnetar distances, and also with our additional knowledge about the sgrs. all these distance estimates are some 108 times smaller than their cosmological counterparts, given by d ≈ z c / h0. so where have we arrived? in the first three paragraphs of this section, we have argued that neither quasi-isotropic arrivals, nor relativistic redshifts (z . 9), nor frequent (seeming) host galaxies needed imply cosmological source distances for plausible source properties. then, in the following five paragraphs, we have repeated earlier inequalities, all of which rule out extragalactic distances for magnetar models. in the future, it will certainly help to get more reliable distance estimates for the small subset of already identified burst sources. but let us look once more at the evidences: to begin with, all the soft repeaters have been directly identified, as grbers, during their first, large bursts, with measured spin rates during their early afterglow oscillations. unfortunately, their distances are highly controversial, with grb 050379 as their worst case, projecting onto n49 in the lmc, though having been some 106-times too bright during outburst for that (large) distance, in30 astrophysics without black holes, and without extragalactic gamma-ray bursts consistent with its band spectrum [schmidt 1978, baring 1992]. next the axps: they share the (long) spin periods of the sgrs, qualifying them as dead pulsars, after 106.4yr of active pulsar spindown [kundt 2008]. their ‘anomaly’ as permanent x-ray emitters can be understood as their being surrounded by low-mass fallback disks, from the collapsed cavity blown into their csm during their past pulsar activity. their (comparatively large) number agrees with that of the dying pulsars, and with their expected small distances from us, inconsistent with certain estimates in the literature. figure 3: schematic drawing of the expected ejection geometry of a grb by a magnetar: an ultramassive clump from the inner edge of its (low-mass) accretion disk hits the surface of the central neutron star, heats up, expands, and gets seized and centrifugally re-ejected by the magnetar’s strong, corotating magnetosphere, near its speed-of-light distance, with lorentz factors γ . 5. as a result, a hard electromagnetic flash is emitted (mostly peaking near 1 mev, but ranging from radio frequencies up through ir, optical, x-ray, and γ-ray frequencies all the way towards and beyond 102gev photon energies), closely followed by a centrifugally ejected transrelativistic hadronic shell. this couple of two more or less radially emitted and/or boosted transrelativistic shells of ejecta impact on the grb’s csm, they both heat up, and give rise to the burst’s prompt and afterglow emissions, as well as to the evolving appearance of a ‘host galaxy’. this model can be shown to be flexible enough to account for all the observed grbs, including their intrinsic differences, and occasional anomalies, such as short and long bursts (due to repeated accretions), pre-cursors and post-cursors (for the same reason), redshifts, early afterglow oscillations (by the output of the spinning central neutron stars), strong flaring of the x-ray afterglows (during collisions with the csm), linear polarisations (for scattered radiation), and so on and so forth. independently of the distance problem, the hard spectra of the grbs – reaching up to and beyond 102gev – tell me that their sources involve neutron stars. these neutron stars must not be isolated, or else they would be radio pulsars. which means that we deal with strongly magnetised neutron stars, though not in excess of 1014g surface strength, a neutron star’s extreme field [kundt 2008]. the bursters’ occasional strong magnetic torques can be achieved via confinement by a surrounding low-mass accretion disk, which explains their name ‘magnetars’. their (invisible) disk was first indicated by glitches of the sgrs 1900+14 and 1806-20, during which their spindown rates increased, i.e. required a stronger torque after the glitch, and has been recently fostered by the anti-glitch of the axp 1e 2259+586 [archibald et al 2013], whose sudden spindown cannot be understood (to my mind) without a strong centrifugal mass ejection [kundt 2010]. and how do the magnetars emit their bright, hard bursts? fig. 3 sketches my understanding of how they are thought to do it. every now and then, at large temporal separations (of &50 yr), a supermassive clump (&1016g) accretes onto a magnetar’s surface, from the inner edge of its surrounding accretion disk, via rayleigh-taylor instabilities. preand post-cursors occur when even several successive such clumps fall down. the freefall potential of a heavy clump, crashing onto a neutron star’s surface, creates transient temperatures near 1012k on impact, so that matter gets ionised, and raised to scale heights kt / mg & 106cm.. this lifted, ionised matter is seized by the n*’s corotating magnetosphere, and centrifugally ejected at speed-of-light lever arms, with lorentz factors γ . 5. this hot re-ejected matter emits a bright photonic flash, closely followed by an intense, transrelativistic hadronic flash. when these two successive outgoing flashes hit the burster’s csm, they cause its afterglow emission, from radio frequencies up to 102gev energies, and eventually also the burst’s seeming ‘host galaxy’. 4 summary old and new arguments have been compiled that bhs are no longer even plausible, and that all grbs are emitted by the galactic magnetars. acknowledgement my warm thanks go to ole marggraf for help with the proceedings, and to thomas tauris for acquainting me with up-to-date literature on magnetars. references [1] aharonian, f., et al (100 authors): astron. astrophys. 425, l13-l17, 2004. [2] archibald, r.f., kaspi. v.m., et al (9 authors): an anti-glitch in a magnetar, nature 497, 591-593, 2013. doi:10.1038/nature12159 31 http://dx.doi.org/10.1038/nature12159 wolfgang kundt [3] baring, m.g.: spectral breaks for theorists, nature 358, 624, 1992. [4] braun, r.: the cosmic web in focus, nature 497, 191-192, 2013. doi:10.1038/497191a [5] ghisellini, g., ghirlanda, g., nava, l., celotti, a.: gev emission from grbs: a radiative fireball?, mnras 403, 926-937, 2010. doi:10.1111/j.1365-2966.2009.16171.x [6] heusler, m.: black hole uniqueness theorems, cambridge lecture notes in physics 6, 1-249,1996. [7] ho, wynn c.g.: more than meets the eye: magnetars in disguise, mnras 429, 113-118, 2013. [8] hoyle, f., burbidge, g., narlikar, j.: a different approach to cosmology, cambridge univ. press, 2000. [9] joshi, p.s.: sci. am., february, 26-33, 2009. [10] joshi, p.s., malafarina, d., instability of bh formation under small pressure perturbations, in: gen. rel. grav. 45, 305-317, 2013. [11] kormendy, j., bender, r. : nature 469, 374-379, 2011. [12] kundt, w.: cygnus x-1 – a neutron star surrounded by a massive disk? astron. astrophys. 80, l7-l10, 1979. [13] kundt, w.: our galaxy a former seyfert?, astrophys. & space sci. 129, 195-201, 1987. [14] kundt, w.: in: jets from stars and galactic nuclei, lecture notes in physics 471, 265-270, 1996. doi:10.1007/bfb0102617 [15] kundt, w.: astrophysics, a new approach, springer, 147-152, 2005. [16] kundt, w.: pulsar physics without magnetars, chin. j. astron. astrophys. vol. 8, supplement, 213-218 (2008). [17] kundt, w.: the local-galactic interpretation of the gamma-ray bursts, in: multifrequency behaviour of high-energy cosmic sources, memsait vol. 81, n.1, 449-459, 2010. 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[25] schmidt; w.: distance limit for a class of model grb sources, nature 271, 525-527, 1978. [26] sofue, y.: bipolar hpershell galactic center starburst model: further evidence from rosat data and x-ray simulations, apj 540, 224-235, 2000. [27] su, m., slatyer, t.r., finkbeiner, d.p.: giant gamma-ray bubbles from fermi-lat: agn activity or bipolar galactic wind?, apj 724, 10441082, 2010. [28] su, m., slatyer, t.r., finkbeiner, d.p.: fermi bubbles: a 10 kpc shock from the galactic center, aas 43, 2011. [29] vestergaard, m., et al (5 authors): mass functions of the active bhs in distant quasars from the sloan digital sky survey data release 3, apj 674, l1l4, 2008. discussion pieter meintjes: you propose an extended burning disc, instead of a supermassive bh in the center of the mw galaxy. what is the size of that disc in relation to the orbits of the innermost stars that orbit that region? also, what influence will such an extended disc have on the orbits of these stars compared to a bh, for example? wolfgang kundt: thank you, pieter, for asking me these constructive questions. in my approximation, the bd inside sgr a* – the ce of the mw galaxy – has a radius of ≈1014cm, whilst the peri-astron distance of the innermost monitored star, 32 http://dx.doi.org/10.1038/497191a http://dx.doi.org/10.1111/j.1365-2966.2009.16171.x http://dx.doi.org/10.1007/bfb0102617 astrophysics without black holes, and without extragalactic gamma-ray bursts s2, measures &1015cm. as already mentioned in my talk of last year [kundt 2012], the orbit of s2 showed a peri-astron advance of 3 deg after its first revolution, consistent with a maclaurin mass distribution (instead of pointlike potential), and six successive measured positions near peri-astron were offset by .10 marcsec towards ne, which i interpret as a fata morgana in the infrared (caused by the high plasma density in the inner disk). at this same time, the star s2 flared by half a magnitude, plausibly due to friction on the disk. 33 introduction bh physics reconsidered gamma-ray bursts from galactic magnetars summary acta polytechnica ctu proceedings doi:10.14311/app.2017.13.0049 acta polytechnica ctu proceedings 13:49–54, 2017 © czech technical university in prague, 2017 available online at http://ojs.cvut.cz/ojs/index.php/app plasma treatment impact on physical and chemical properties of polymeric fibers radim hlůžeka,∗, zdeněk prošeka,b, jan trejbala, josef fládra, štěpán potockýc a czech technical university in prague, faculty of civil engineering, thákurova 7, 166 29 prague 6, czech republic b university centre for energy efficient buildings of technical university in prague, třinecká 1024, 273 43 buštěhrad, czech republic c czech academy of science, institute of physics, na slovance 1999/2, 182 21 prague 8, czech republic ∗ corresponding author: radim.hluzek@fsv.cvut.cz abstract. presented work focuses on chemical and physical properties of plasma modified polymeric macro-fibers. polyethylene terephthalate (pet) and polypropylene (pp) fibers having approx. 300 µm in diameter were modified using cold oxygen plasma in order to achieve their surface changes needed for durable bond and adhesion with cement matrixes. a duration of plasma modification differed between 5 to 480 seconds, where an effect of the treatment was examined. fiber surfaces chemical changes were researched via wettability measurement with demineralized water (the measurement was repeated immediately and after 1, 7 and 30 days to find out the changes stability). physical changes were studied by means of weight balance (determination of weight loss) and tensile strength tests. it was found that wettability was enhanced significantly – up to two times, while mechanical properties of treated fibers decreased only slightly. keywords: polymer macro-fibers, plasma treatment, wettability, tensile strength. 1. introduction usage of man-made macro fibers as reinforcement of any composites become a commonality in majority of industry fields, civil engineering including. in the final consequence, a small bulk property improvement has strong impact in civil engineering due to mass production [1]. fiber-reinforced materials exhibit good mechanical properties, e.g. high mechanical resistance (abrasion, impact resistance), ductility, water resistance etc. [2, 3]. the main task of the macro fiber addition is the distribution of shrinkage into several small cracks in the case of materials based on shrinking binders (mainly lime and cement), and to prevent the single crack openings after linear elastic response in the case of loaded samples [1]. in the civil engineering, the macro fiber reinforcement is the most often used for a) production of watertight concretes and concretes exposed to risk of steel reinforcement corrosion (absence of cracks disallows water penetration), and b) for production of large-scale construction exposed to temperature or moisture changes, dynamic loads, point loads (the fiber reinforcement provides a compactness – hence usability – of materials after the crossing the material loading capacity) [4, 5]. the fiber reinforcement can be classified by fibers material, diameter, tensile strength and their modulus of elasticity. as material, polymeric fibers can be used. both polymeric (in particular pet and pp) fibers have high tensile strength equal to about hundred or even thousands mpa. their diameter is equal to tens or hundreds micrometers. other characteristic property is low ratio of diameter to length (and related high specific surface enabling better stress transferring from matrix to fibers) and favorable cost [6, 7]. polymeric fibers reveal low surface wettability (hydrophobicity). on the other hand, fiber reinforcement requires good adhesion between the fiber surface and matrix. to improve the mechanical strength of reinforced materials, adhesion between the fiber surfaces and matrix must be unambiguously ensured [5, 8]. the required adhesion can be modified by fiber surface treatment by chemical and physical methods [7, 9– 13]. currently, the newly introduced plasma treatment becomes popular as a progressive physico-chemical method. the low-pressure plasma treatment represents a universal, efficient and eco-friendly alternative for surface modifications. plasma can be defined as ionized gas (composed of electrons, ions, and neutral species). the mechanism for plasma surface modification relays on surface atoms replacement by oxygen atoms and formation of polar groups. the presence of polar or functional chemical groups enhances the reactivity with the matrix based on cement or lime binder (both contain water) [5]. in the present work, we report on the modification of polyethylene terephthalate (pet) and polypropylene (pp) fibers by oxygen plasma treatment. the influence of the plasma treatment on the contact angle, the weight and the tensile strength of the fibers is studied. 49 http://dx.doi.org/10.14311/app.2017.13.0049 http://ojs.cvut.cz/ojs/index.php/app r. hlůžek, z. prošek, j. trejbal et al. acta polytechnica ctu proceedings figure 1. digital camera photo of pet (left), pp fibers (right). 2. materials 2.1. polymer macro-fibers two different types of fibers were used: pet and pp. both of them were made by spokar manufacturer (czech republic). they were primary made for the production of most types of brushes (tooth, paint etc.) and brooms [14]. the fibers had diameter equal to approx. 300 µm (measured using mechanical micrometer), original length was 1200 mm (subsequently chopped according to need). an industrial water flushable sizing is standardly applied onto fiber surfaces as an integral part of their production. fibers are shown in figure 1. 3. experimental methods 3.1. plasma treatment to improve the wettability and related adhesion between fiber surfaces and surrounded matrix, oxygen treatment in inductively coupled plasma was realized using device tesla vt 214 (13.56 mhz). parameters of the whole process differing only in exposition time from 5 to 480 seconds, the others were constant; total power of rf source 100 w, total gas pressure 20 pa, oxygen flow 50 sccm. 3.2. contact angle measurement according to reference [15], using horizontal direct optical method, contact angles were measured between meniscus of distilled water (adhering on fiber surfaces) and fiber perpendicularly submerged into the water. thus obtained results were averaged from 6 independent measurements. 3.3. weight ratio analysis due to ion bombardments and polymer oxidation and surface structuring (etching) during the plasma treatment, the fibers can be damaged. weight ratio observation carried out before and after treatment enables to assess the fiber weight loss, which is connected to tensile strength capacity loss. in order to quantify the fiber weight loss, fibers were weighed before and immediately after plasma treatment on weighing-machine kern alj 120-4 (d = 0.1 mg). 3.4. tensile strength tests physical and chemical processes mentioned earlier can influence the fibers tensile strength. to determine the extent of fibers mechanical properties damage, tensile strength test was carried out using loading frame web tiv ravestein fp100. the test was displacement controlled at a constant rate equal to 6 mm/min in the case of pet and 3 mm/min in the case of pp fibers, respectively. the results were averaged from 6 independents experiments for each plasma treatment time of both fiber types. 4. results and discussions 4.1. contact angle measurement contact angle measurement revealed 84.7±2.2◦ on untreated pet fibers and 88.3±0.6◦ on untreated pp fibers. the results provided on the treated fibers showed the unexceptionable positive plasma impact of the fibers wettability. all treated fibers independently on plasma exposition time showed significant decrease of contact angle. the angle measured immediately after plasma treatment on both types of fibers was moving between approx. 25–30◦. after 1, 7 and 30 days from the treatment, contact angle increased up to 65–70◦ for pet fibers and for the pp fibers up to 55–70◦. besides, the results showed that it is not effective to do any treatment longer duration than several seconds. another finding is the fact that storage of fibers, even for one day, is very inefficient, and the contact angle increase rapidly towards the reference values. dependence between contact angle, duration of plasma treatment and time of storage in atmospheric conditions is shown in figure 2 and figure 3. based on these finding, it was decided that plasma treatment duration realized within other experiments throughout the study were shortened to 120 seconds only. demonstration of the fibers wettability captured during contact angle measurement is shown in figure 4. 4.2. weight ratio analysis weight ratio observation performed on fibers before and after plasma treatment showed that the weight 50 vol. 13/2017 physical and chemical properties of polymeric macro-fibers figure 2. pet fibers – dependence between contact angle, duration of plasma treatment and time of exposure to atmospheric conditions. figure 3. pp fibers – dependence between contact angle, duration of plasma treatment and time of exposure to atmospheric conditions loss is negligible. fibers treated by 120 seconds exhibited practically no weight loss, just less than 0.25 %. comparing both types of fibers, it can be said that the weight loss is the same until 30 seconds of plasma treatment. after 30 seconds of treatment, there are some small difference between pet and pp fibers. after the longest time of treatment (120 seconds), the maximum averaged weight loss was 0.06 % and 0.21 % for pet and pp fibers, respectively. based on these findings, it can be assumed that mechanical properties of treated fiber were not affected by plasma treatment, which is a positive result. graphic relation between weight loss and duration of plasma treatment is shown in figure 5. 51 r. hlůžek, z. prošek, j. trejbal et al. acta polytechnica ctu proceedings figure 4. contact angles captured during the wettability measurement figure 5. relation between fibers weight loss and duration of plasma treatment obtained before and after plasma treatment. figure 6. dependence between fibers tensile strength and duration of plasma treatment. 52 vol. 13/2017 physical and chemical properties of polymeric macro-fibers 4.3. tensile strength tests tensile strength (more precisely load carrying capacity) of reference and plasma treated fibers was approximately constant for all plasma treatment times (measured on reference, 10, 60 and 120 seconds treated fibers). pet fibers reached on approx. 20 n, while pp fibers on approx. 32 n. as it was already shown by weight loss ratio mentioned earlier, fiber tensile strength test confirmed that the plasma treatment had no significant effect on fiber mechanical performance. the relation between fibers tensile strength and duration of plasma treatment is shown in figure 6. 5. conclusions polypropylene (pp) and polyethylene terephthalate (pet) commercial macro fibers spokar having approx. 300 µm in diameter were surface treated by mean of low-pressure cool oxygen plasma treatment in order to achieve their chemical and physical surface changes. the aim of the executed treatment was to improve fiber surface wettability (from hydrophobic to hydrophilic) and morphology (from smooth to roughened), both standardly required for strong interphase interaction with cement matrix, when fibers used as reinforcement. the plasma modification differed in duration (from 5 to 480 seconds). an effect on physico-chemical changes on fiber surfaces was examined by wettability measurement, wight loss ratio (fibers were weighed before and after plasma treatment) and finally by fiber tensile strength tests, when the main focus of the research was targeted to impact of plasma treatment on fiber mechanical performance. besides, time-stability of chemical changes on modified fibers was researched – wettability between fiber surfaces and distilled water was examined using direct optical set for contact angle measurement immediately, 1, 7 and 30 days after plasma modifications. it was found out that: • the wettability of plasma treated (regardless to duration of treatment time) both pp and pet fibers increased almost three times, if compared to reference samples. • the modified wettaility was very short-term. it was degraded almost to reference values already after one day, when fibers exposed to standard atmospheric conditions. • impact of realized treatment did not have no obvious effect on fiber mechanical performance, as demonstrated by no weight loss and no tensile strength changes for much as treated fibers. acknowledgements this work was financially supported by czech technical university in prague – sgs project sgs16/201/ohk1/3t/11, by the czech science foundation research project 15-12420s and by the ministry of education, youth and sports within national sustainability programme i, project no. lo1605. the work occurred in frame of the lnsm infrastructure (no. lm2015087). references [1] p. k. mallick. materials. in fiber-reinforced composites, materials, manufacturing and design, chap. 2. crc press, 2007. [2] j. trejbal, l. kopecký, s. potocký, j. fládr. the effect of glass fiber reinforcement on flexural strength of lime-based mortars. in 53rd conference on experimental stress analysis, pp. 450–453. czech technical university in prague, 2015. [3] k. šeps, j. vodička. fibre reinforced concrete with recycled concrete and insulation material stered. advanced materials research 1054:262–266, 2014. doi:10.4028/www.scientific.net/amr.1054.262. [4] z. lin, v. li. crack bridging in fiber reinforced cementitious composites with slip-hardening interfaces. journal of the mechanics and physics of solids 45(5):763–787, 1991. [5] j. trejbal, v. šmilauer, a. kromka, et al. wettability enhancement of polymeric and glass micro fiber reinforcement by plasma treatment. in nanocon 2015 7th international conference on nanomaterials – research and aplication – conference proceedings, pp. 315–320. tanger, 2015. [6] j. bartoš. modification of pet fiber surfaces to increase their cohesion with cement matrix. diploma thesis. czech technical university in prague, 2014. [7] j. trejbal. modification of glass fibers surface and their application in mortars used for reconstructions of historical buildings. diploma thesis. czech technical university in prague, 2014. [8] p. chen. influence of fiber wettability on the interfacial adhesion of continuous fiber-reinforced ppesk composite. journal of applied polymer science 102:2544–2551, 2006. doi:10.1002/app.24681. [9] s. elsaka. influence of chemical surface treatments on adhesion of fiber posts to composite resin core materials. dental materials 29:550–558, 2013. doi:10.1016/j.dental.2013.03.004. [10] d. cökeliler, s. erkut, j. zemek, et al. modification of glass fibers to improve reinforcement: a plasma polymerization technique. dental materials 23(3):335–342, 2007. doi:10.1016/j.dental.2006.01.023. [11] m. kim, m. kim, k. rhee, s. park. study on an oxygen plasma treatment of a basalt fiber and its effect on the interlaminar fracture property of basalt/epoxy woven composites. composites part b: engineering 42(3):499– 504, 2011. doi:10.1016/j.compositesb.2010.12.001. [12] h. wu, v. li. fiber/cement interface tailoring with plasma treatment. cement and concrete composites 21(3):205–212, 1999. doi:10.1016/s0958-9465(98)00053-5. [13] j. trejbal, j. fládr, l. kopecký. utilization of plasma treated polymeric macro-fibers as reinforcement in concrete constructions. advanced materials research 1144:70–75, 2017. doi:10.4028/www.scientific.net/amr.1144.70. 53 http://dx.doi.org/10.4028/www.scientific.net/amr.1054.262 http://dx.doi.org/10.1002/app.24681 http://dx.doi.org/10.1016/j.dental.2013.03.004 http://dx.doi.org/10.1016/j.dental.2006.01.023 http://dx.doi.org/10.1016/j.compositesb.2010.12.001 http://dx.doi.org/10.1016/s0958-9465(98)00053-5 http://dx.doi.org/10.4028/www.scientific.net/amr.1144.70 r. hlůžek, z. prošek, j. trejbal et al. acta polytechnica ctu proceedings [14] spokar a.s. fiber production. http: //www.spokar.com/cz/technologie/vyroba-vlaken. [15] j. trejbal, l. kopecký, s. potocký, z. remeš. direct optical method for contact angle measurement on micro fibers. in nanomaterials and nanotechnology in civil engineering 2015 – conference proceedings, pp. 89–90. czech technical university in prague, 2015. 54 http://www.spokar.com/cz/technologie/vyroba-vlaken http://www.spokar.com/cz/technologie/vyroba-vlaken acta polytechnica ctu proceedings 13:50–55, 2017 1 introduction 2 materials 2.1 polymer macro-fibers 3 experimental methods 3.1 plasma treatment 3.2 contact angle measurement 3.3 weight ratio analysis 3.4 tensile strength tests 4 results and discussions 4.1 contact angle measurement 4.2 weight ratio analysis 4.3 tensile strength tests 5 conclusions acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2017.7.0043 acta polytechnica ctu proceedings 7:43–47, 2017 © czech technical university in prague, 2017 available online at http://ojs.cvut.cz/ojs/index.php/app numerical predictions and mechanical testing of braided composite structures utilising digital image correlation emil pitz∗, matei-constantin miron, imre kállai, zoltán major institute of polymer product engineering, johannes kepler universität linz, altenberger straße 69, linz, austria, 4040 ∗ corresponding author: emil.pitz@jku.at abstract. the current paper is describing the implementation of a multiscale numerical model for prediction of stiffness and strength in braided composites. the model is validated by experimental testing of single-layer braided tubes under torsional loading utilising digital image correlation (dic). for the numerical model the entire braided structure is modelled at yarn detail level, taking into account the yarn behaviour as well as individual yarn-to-yarn interactions by using cohesive contact definitions. by means of hashin’s failure criteria and cohesive contact damage, failure of the yarns and failure of the yarn-to-yarn interface is being accounted for. thereby the material failure behaviour can be predicted. for validation of the model, torsion tests of biaxially braided single-layer composite tubes were performed. the strain distribution at the specimen surface was studied using the dic system aramis in 3d mode. keywords: braided composites, finite element analysis, failure prediction, torsion testing, digital image correlation. 1. introduction aim of this research is the implementation and validation of a numerical tool providing reliable predictions of the mechanical properties of braided composite materials. modelling the structure at yarn detail level (meso-level), the undulation of the yarns is taken into account as well as individual yarn-to-yarn interactions, utilising cohesive contact interactions in the finite element software abaqus. by implementation of failure models, the approach can predict occurring failure mechanisms, distinguishing damage at the yarn level (intra-yarn) and yarn-interface level (inter-yarn). material properties required as input parameters are obtained by mean-field homogenization (mfh) from the individual constituents behaviour (matrix and fibres). for validation of the model, single layer braided tubes are tested under torsional loading utilising dic for full-field strain measurement on the surface of the specimen. the global loading response predicted by the numerical model as well as the local strain distribution and occurring instability behaviour is compared to the experimentally obtained results. 2. numerical model generally three structural levels can be distinguished in textile composites: the macro-level, defining the overall 3d geometry of the composite structure, the meso-level, defining the internal structure of the reinforcement, consisting of the impregnated undulating yarns and resin pockets corresponding to the empty space between the yarns, and the micro-level, defining the arrangement of the fibre filaments in the impregnated yarn [1]. a schematic representation of the multiscale problem can be seen in figure 1. to describe this structure in the numerical model, a multiscale modelling approach was utilised, describing the entire structure at yarn-detail level and distinguishing between intra-yarn (fibre-dominated) and inter-yarn (matrix-dominated) behaviour [3, 4]. 2.1. 3d-model of the braided composite in order to achieve the mentioned degree of accuracy, a 3d-model of the braiding geometry was developed, modelling both the yarn undulation path and the yarn cross section. the path of each undulating yarn was analytically approximated by a tapered cosine function: r (x) = r0 − t 2 sin (π 2 cos (x− δ) ) . (1) the phase shift δ is changing periodically for each yarn path, depending on the modelled weave pattern (plan or twill), leading to interwoven and repeating undulation paths (see figure 2). the yarn cross section shape is linked to the shape of the yarn path. in this approach, it is taken from projection of the undulation path onto a plane orthogonal to the braiding direction. for simulation, mid-plane shell bodies are extracted from the solid geometry and the local thickness is mapped on the shell mesh. 43 http://dx.doi.org/10.14311/app.2017.7.0043 http://ojs.cvut.cz/ojs/index.php/app e. pitz, m. miron, i. kállai, z. major acta polytechnica ctu proceedings figure 1. structural levels of a textile composite. (adapted from [2]) figure 2. undulation paths of the braided yarns modelled with a tapered cosine function following an arbitrary spline. it should be noted that, due to the analytical modelling of the yarn undulation paths and cross sections, the numerical model assumes perfect in phase arrangement of the yarns. hence there is total absence of any nesting or imperfections in the alignment of the yarns in the model. 2.2. material model to take into account the meso-structure of the composite in the presented modelling approach it is distinguished between yarn behaviour (intra-yarn behaviour) and yarn-to-yarn interactions (inter-yarn behaviour). the yarns (intra-yarn behaviour) are modelled utilising a transversely isotropic elastic material, taking into account the change of orientation along the yarn due to the yarn undulation. the material properties are determined from the constituents behaviour (matrix and fibres) using the mfh approach by mori and tanaka [5]. furthermore, for modelling failure of the yarns (intra-yarn failure) hashin’s composite failure criteria [6] are implemented. the determined yarn stiffness and strength properties can be found in table 1. inter-yarn behaviour (yarn-to-yarn interactions) is being modelled using cohesive contact behaviour to prevent the yarns from separating during loading. by implementing a quadratic traction damage initiation e‖ (gpa) 183 xt (mpa) 1810 e⊥ (gpa) 20 xc (mpa) 669 ν‖⊥ (-) 0.21 y t (mpa) 64 ν⊥⊥ (-) 0.46 y c (mpa) 174 g‖⊥ (gpa) 8.1 sl (mpa) 105 g⊥⊥ (gpa) 6.9 st (mpa) 105 α (-) 1 table 1. yarn stiffness and strength properties. criterion and energy based damage evolution, failure of the yarn-to-yarn interface is being accounted for. the used cohesive properties are specified in table 2. normal stiffness (gpa) 20 tangential stiffness (gpa) 6.9 damage initiation (mpa) 3 damage evolution (mj/mm2) 3 · 10−3 table 2. cohesive contact surface properties. 3. experimental torsion tests were performed on thin walled single layer carbon fibre braided tubes using an instron 8850 (250 kn / 2400nm) testing machine. the specimens had a total length of 300mm, an inner diameter of 70mm and a wall thickness of 0.4mm. for gripping, glass fibre tabs with a thickness of 5mm were glued to the specimens giving them an active length of 150mm. fibre volume ratio (fvr) of the specimens was determined by burnt residue analysis to be approximately 53%. an overview of the specimen geometry can be seen in table 3. the torsion tests were conducted in rotation controlled mode with axial displacement restricted. full-field strain measurement was conducted using the dic system aramis 6.3 in 3d mode. to provide the necessary contrast for the aramis measurement a speckle pattern was sprayed onto the specimens. during testing initial failure was caused by torsional buckling of the structure (see figure 3) followed by failure of the matrix. upon further loading, buckling 44 vol. 7/2017 braid fvr (%) 53 yarn thickness (mm) 0.2 yarn width (mm) 4 yarn count (yarns/cm) 2.15 layer thickness (mm) 0.4 braiding angle (◦) 45 table 3. yarn and braiding geometry of the test specimens. of the structure developed but failure of the yarns was not observed. a more detailed overview of the experimental results will be given in the following section. figure 3. torsion test of a tubular specimen with applied speckle pattern for full-field strain measurement and visible torsional buckling of the specimen. 4. results and comparison the presented numerical model accurately predicts stiffness and strength of the braided tubular structure under torsional loading (a comparison is given in table 4) and correlates well with the experimentally obtained data (see figure 4). experiment simulation torsional stiffness (nm/◦) 137 140 maximum torque (nm) 48 48 table 4. comparison of experimental results and predicted values. 4.1. instability behaviour in the numerical model, damage is initiated, during loading, at the yarn-to-yarn interface (inter-yarn level), leading to torsional buckling of the structure upon 0 0.1 0.2 0.3 0.4 0.50 10 20 30 40 50 60 rotation (◦) to rq ue (n m ) specimen 1 specimen 2 specimen 3 simulation figure 4. comparison of the simulated and tested torque-rotation response. further loading (see figure 5(a)). failure at the intrayarn level does not develop in the simulated rotation range. similar instability and failure behaviour was observed during the experimental torsion tests (see figure 5(b)). comparing the displacement magnitude of the simulated structure and determined by dic (figure 5), the computed displacement generally follows the qualitative trend of the measured values while the maximum displacement of the buckles varies. variations of the maximum displacement magnitude were also observed between the tested specimens though. as buckling under torsional load is generally prone to imperfections, this leads to the assumption that the differences originate mainly from imperfections in the specimens that were not taken into account in the numerical model. (a) (b) figure 5. buckling of the simulated structure (a) and of the tested specimen with an overlay of the dic results (b), at a rotation angle of 0.65◦. 45 e. pitz, m. miron, i. kállai, z. major acta polytechnica ctu proceedings 4.2. strain distribution as the strains obtained by digital image correlation are measured on the surface of the specimen while the simulated strains are received at the yarn level, with the orientation following the undulation paths of the yarns, numerically and experimentally determined strains cannot be directly compared. therefore a thin, low stiffness, and isotropic membrane layer was tied to the yarn surfaces at the outside of the tubular specimen in the numerical model. this does not influence the overall behaviour of the model but the strain distribution of the membrane layer now correlates to the surface strains of the specimen and hence it enables comparison with the digital image correlation results. the strain in circumferential direction (εx) on the surface of the sample examined for the numerical model and the dic measurements (see figure 6), shows a pattern of periodically repeating tensile and compressive strains as a consequence of the periodically crossing yarns. the measured strain generally follows the trend of the numerically predicted strain, with the simulation showing a more repetitive distribution. x y (a) x y (b) figure 6. strain distribution in the simulated structure (a) and experimentally obtained strain distribution (b) at a rotation angle of 0.25◦, with εx being the strain in circumferential direction of the cylinder. to further compare the distribution, the measured and simulated strain profiles were traced along a circumferential line on the surface of the specimen, as can be seen in figure 7. the period length of the computed strain oscillations is approximately 13.7mm, which corresponds to the circumferential distance between two recurring yarn crossings. compared to the calculated strain profile, amplitude and period length of the measured strain profile show variations along the path. it is again assumed that these variations arise from irregular spacing of the yarn crossings in the specimen, which causes variations of the local specimen wall thickness and the local stiffness. such irregularities are not taken into account in the numerical model and hence cause the more repetitive strain profile of the numerical model. 0 5 10 15 20 25 30 −1 −0.5 0 0.5 1 ·10−3 circumferential distance between two recurrent yarn crossings. x y line length (mm) ε x () digital image correlation simulation figure 7. comparison of the circumferential strain distribution (εx) along a circumferential path (y-axis being parallel to the braiding direction). 5. conclusions the proposed modelling approach accurately reproduces the macroscopic response to loading and exhibits similar instability behaviour compared to experimental torsion tests and accurately predicts overall stiffness and strength of the component. further validation of the model was accomplished by utilising dic, enabling full-field strain measurement on the surface of tubular braided samples. comparing the numerically and experimentally obtained local displacement and strain profiles, generally similar trends can be observed. arising differences are assumed to be caused by imperfections in the tested samples that were not regarded in the numerical model. this states the necessity to further develop the numerical model to realistically represent imperfections in the structure and predict their influence on the performance of the component. list of symbols fv r fibre volume ratio [%] e‖ longitudinal elasticity modulus [gpa] e⊥ transversal elasticity modulus [gpa] g‖⊥ longitudinal shear modulus [gpa] g⊥⊥ transversal shear modulus [gpa] ν‖⊥ longitudinal poisson’s ratio [–] ν⊥⊥ transversal poisson’s ratio [–] xt longitudinal tensile strength [mpa] xc longitudinal compressive strength [mpa] y t transversal tensile strength [mpa] y c transversal compressive strength [mpa] sl longitudinal shear strength [mpa] st transversal shear strength [mpa] 46 vol. 7/2017 α contribution of shear stress to fibre failure [–] r current radius of the braiding layer [mm] r0 mean radius of the braiding layer [mm] t braiding layer thickness [mm] δ phase shift [mm] εx circumferential strain [–] acknowledgements the current research is performed within the project number 5320343 / 849425 (andisia) supported by the austrian research promotion agency, in cooperation with teufelberger gmbh austria and the university of applied sciences upper austria. references [1] s. v. lomov, d. s. ivanov, i. verpoest, et al. full-field strain measurements for validation of meso-fe analysis of textile composites. composites part a: applied science and manufacturing 39(8):1218 – 1231, 2008. full-field measurements in composites testing and analysis, doi:http://dx.doi.org/10.1016/j.compositesa.2007.09.011. [2] s. neudorfer. modelling and simulation of braided and filament-wound composite torsion-shaft, 2015. master’s thesis, johannes kepler universität. [3] m.-c. miron, h. katzinger, s. neudorfer, et al. mechanical testing and numerical predictions of the behavior of braided carbon fiber components. procedia structural integrity 2:3593 – 3600, 2016. 21st european conference on fracture, ecf21, 20-24 june 2016, catania, italy, doi:http://dx.doi.org/10.1016/j.prostr.2016.06.448. [4] e. pitz, m.-c. miron, z. major. failure analyis of braided composite structures in torsional loading. submitted for publication. [5] t. mori, k. tanaka. average stress in matrix and average elastic energy of materials with misfitting inclusions. acta metallurgica 21(5):571 – 574, 1973. doi:http://dx.doi.org/10.1016/0001-6160(73)90064-3. [6] z. hashin. failure criteria for unidirectional fiber composites. journal of applied mechanics 47(2):329– 334, 1980. doi:http://dx.doi.org/10.1115/1.3153664. 47 http://dx.doi.org/http://dx.doi.org/10.1016/j.compositesa.2007.09.011 http://dx.doi.org/http://dx.doi.org/10.1016/j.prostr.2016.06.448 http://dx.doi.org/http://dx.doi.org/10.1016/0001-6160(73)90064-3 http://dx.doi.org/http://dx.doi.org/10.1115/1.3153664 acta polytechnica ctu proceedings 7:43–47, 2017 1 introduction 2 numerical model 2.1 3d-model of the braided composite 2.2 material model 3 experimental 4 results and comparison 4.1 instability behaviour 4.2 strain distribution 5 conclusions list of symbols acknowledgements references acta polytechnica ctu proceedings doi:10.14311/ap.2016.4.0068 acta polytechnica ctu proceedings 4:68–72, 2016 © czech technical university in prague, 2016 available online at http://ojs.cvut.cz/ojs/index.php/app measurement of neutron spatial distribution of the bnct epithermal beam at the reactor lvr-15 michaela rabochováa,∗, miroslav vinšb, jaroslav šoltésb, božena michalcovác a faculty of electrical engineering, czech technical university in prague, czech republic b department of neutron physics, research centre řež, husinec-řež, czech republic c department of neutronography processing, nuclear physics institute of the cas, husinec-řež, czech republic ∗ corresponding author: michaela.rabochova@seznam.cz abstract. in this study, a measurements of neutron field using a special positioning device with a 6li + si detector and image plate is described. the measurements were provided for boron neutron capture therapy (bnct) channel of the lvr-15 reactor in the research centre rez ltd., czech republic. mapping of neutron field represents an essential and crucial part of planning bnct treatment (especially for patients suffering from brain tumor glioblastoma multiforme). keywords: bnct, map of neutron field, 6li + si detector, positioning device. 1. introduction glioblastoma multiforme [1–4] is the most common malignant radio-resistant and chemoresistant tumor of the central nervous system which has been incurable for decades and even today, unfortunately, the prognosis is not favorable [5]. this type of tumor is characterized by very fast growth and aggressive invasion to the surrounding normal brain tissue. it is very difficult to remove all of the affected tumor cells without serious damage of the brain. despite the advanced diagnostics (especially in neuroimaging), and various multimodal therapies that include surgical resection followed by radiation therapy (and alternatively chemotherapy) still remains a considerable problem with finding efficient and gentle treatment of the sensitive area of brain tissue. in this regard, boron neutron capture therapy (bnct) is unique selective radiotherapy based on capture of non-radioactive nuclide 10b by cancer cells and the subsequent capture of thermal neutrons resulting in the nuclear reaction 10b(n, α)7li. these products of reaction selectively damage cancer cells while healthy tissue is spared of radiation load. an absolutely essential part of the treatment planning is determination of parameters and proper setting of correct compensation of neutron beam. the purpose of this project was to measure the spatial distribution of neutrons inside the epithermal horizontal channel of the research reactor lvr-15 (the research centre rez ltd., czech republic) [6], which is used for the method of boron neutron capture therapy. 2. research background 2.1. the special positioning device the mapping of neutron beam was done by using a special positioning device which fixed a si semiconductor detector with 6li converter. the positioning device can moves the detector in three axis and it basically consists of a specially modified support frame with three engines. the special positioning device is shown in fig. 1. figure 1. the special positioning device. 2.2. 6li + si detector for the spatial mapping of neutron beam was used a detector which was consisted of 6li converter and si semiconductor detector [7]. the 6li converter provided a production of 3h (after its insertion into neutron flux) via 6li(n, α)3h reaction. the total energy of the reaction is 4.78 mev (2.73 mev for 3h and 2.05 mev for α particle). the tritium 3h is then detected in a si detector. the active diameter of 6li 68 http://dx.doi.org/10.14311/ap.2016.4.0068 http://ojs.cvut.cz/ojs/index.php/app vol. 4/2016 measurement of neutron spatial distribution of the bnct beam converter is 3 mm; the case of si detector has diameter 25 mm. the distance between converter and detector is approximately 8 mm [8]. the scheme of the detector is shown in fig. 2. figure 2. the scheme of 6li + si detector [8]. 2.3. the image plate another method that has been performed is the experimental determination of the intensity distribution excitations caused by neutron beam by means of the image plates (fig. 3). displaying the neutron flux through the imaging plate is mainly used for neutron radiography method. the principle is based on passing the neutron beam through an object and its subsequent detection on the imagine plate. the result is a 2d (or 3d) image of the different intensities of the neutron flux which is determined by the absorption properties of the object material. in the case of the experiment mentioned in this work was used this method by the same way, but not for displaying a particular subject, but purely for displaying the parameters of the neutron beam intensities. figure 3. the image plate. 3. method of measurement 3.1. the mesaurement with the positioning device the measurement was carried out from march 31 to april 1, 2016 in front of the horizontal channel bnct of the research nuclear reactor lvr-15 in the research centre rez ltd. the acquisition of the data was based on communication between genie-2000 program (detector) and special positioning device utility program. the getting device to work was accomplished by sequential modifications of batch codes. the codes control position of the positioning device and also provide a record of the data of measurements. the main aim of this measurement was to determine the spatial distribution of neutron beam, which is important to verify its homogeneity for subsequent applications of bnct. the map of the route of detector was created and loaded into the control position program of positioning device. the movement of positioning device with 6li + si detector was determined with fixed increments of 1 cm along the horizontal axis y and along the vertical axis z. the time of measurement was ∼ 4 minutes for every position of detector. the scheme of route of the detector is shown in fig. 4. figure 4. the scheme of the route of 6li + si detector. the entire measurement lasted 9 and 1 2 hours (240 second for one position) and data from detector were analyzed with the software genie-2000 by canberra company [9]. the resulting peak areas of 3h have to be corrected for the change in reactor power as the power was not stable during whole measurement. the power of reactor was higher at the beginning of measurement. for the time of the correction was used the data from ionization chambers which monitoring the bnct channel (see fig. 5). figure 5. the development of reactor power. 69 m. rabochová, m. vinš, j. šoltés, b. michalcová acta polytechnica ctu proceedings the resulting peak areas after correction were calculated based on the following relationship spk = m xi sp, where spk denotes the peak area after correction, m is a median of the numbers of pulses during irradiation, xi is an arithmetic average of the numbers of pulses during the i-th interval of irradiation in a given position and sp is the peak area. 3.2. the measurement with image plate the second experiment of mapping neutron field was performed on april 21, 2016. the main aim was to take a radiography image of thermal neutron beam and its evaluation using neutron imaging plate. the plate was placed under the outlet of the neutron beam. all manipulation were done by hand, the image plate was moved to the beam exit after the beam was opened. this system was provided for pulling the plate upward for start measuring so that the enter of the channel was subsequently overlapped and after the end of irradiation the plate was pull down to the start position. this principle was chosen in order to avoid an unwanted exposure during opening and closing the bnct horizontal channel. start experimental position of image plate is shown in fig. 6. overall exposure time was 2 minutes. figure 6. the neutron image plate (in the red frame) located in the default position below the enter of the bnct horizontal channel (green frame). after the irradiation, the plate was evaluated at the institute of nuclear physics institute of the cas in rez. subsequently, the measured data was processed by the device fujifilm bas-1800 (bas – bioimaging analyzer system), which evaluated the measured intensity. 4. result of measurement 4.1. the mesaurement with the positioning device based on neutron beam radiation of 6li + si detector the map of thermal neutron flux distribution was obtained (fig. 7). values within each field represents the value of the peak area. figure 7. the map of neutron flux distribution. the graphical representation of the measured data is on the fig. 8. the axis x represents the values of the peak areas of 3h and the axes y and z are the positions. the measurement uncertainty was around 1 %. figure 8. 3d graph of neutron flux distribution. 4.2. the measurement with image plate the measured neutrogram of the neutron beam of reactor lvr-15 was then rendered fig. 9. 70 vol. 4/2016 measurement of neutron spatial distribution of the bnct beam figure 9. the neutrongram of neutron beam. the neutron beam is seen here as a red-edge circle. the dark blue color represents the lowest level of excitation (the largest decline), while the white color represents the high intensity. on the neutrongram are partially visible muffled two horizontal bars in the upper and lower half, which are dependent on the properties of a neutron plate and therefore do not reflect the actual intensity values at those locations. unfortunately it is not possible to suppress this phenomenon. small local changes in intensity are the result of small surface scratch of the plate. on the picture is evident symmetrical circular beam (red-purple-white colored intensity) with the lowest attenuation values in the centre, as one might expect. however, the upper half of the image shows a higher level of exposure than the lower half. possible in this case, this could be the influence of gamma radiation to which the neutron plate can be also sensitive. this impact would be affected by the opening bnct channel. another additional measurements intended to clarify these results are planned. it is possible to plot a 3d graph (fig. 10). figure 10. 3d neutrogram of neutron beam. the values on the axes x, y represent the pixel, resp. one unit corresponds to one pixel, which is equivalent to 50 micrometers. thus e.g. 500 px = 2.5 cm, 1000 px = 5 cm, 2000 px = 10 cm etc. the z axis is then plotted excitation intensities in individual pixels. the values are relative and they do not represent actual neutron flux density. the point [x, y] = [0, 0] represents the upper-left corner of the neutron plate placed looking towards the enter of the horizontal channel. again, this graph shows the slope, which is probably caused again by the opening of the channel. 5. conclusions the measurement results show that the profile of the beam is quite homogeneous in its whole crosssection without any significant peaks. in the future, other measurements with other types of converter are planned. also properly functionality of special positioning system which fixed the 6li + si detector was confirmed. the measurement using neutron image plates showed that the neutron beam is symmetrical with the highest intensity in the centre of the beam. it was detected intensity changes in the upper half of the image compared to the lower half of the image. this situation may be caused by the influence of unwanted exposure during opening horizontal channel or by unwanted exposure of gamma radiation at the opening horizontal channel. both methods were sensitive to thermal neutrons (i.e. 6li converter and gd layer of neutron image plate). both experiments confirmed the symmetry of the neutron beam of bnct horizontal channel of lvr-15 reactor. acknowledgements the presented work was financially supported by the ministry of education, youth and sport czech republic project lq1603 (research for susen). references [1] k. urbánek. nádory mozku: gliomy – astrocytom, glioblastoma, příznaky, léčba, diagnostika, 2008. http://www.zbynekmlcoch.cz/informace/medicina/ neurologie-nemoci-vysetreni/nadory-mozkugliomy-astrocytom-glioblastom-priznaky-lecbadiagnostika-prognoza. [2] h. biliková, p. buzrla, j. dvořáčková. problematika mozkových nádorů astrogliální řady. european journal for biomedical informatic 6(1):cs3–cs7, 2010. http://www.ejbi.org/en/ejbi/article/9-csproblematika-mozkovych-nadoru-astroglialnirady.html. [3] m. m. abd-el-barr, e. a. chiocca. how much is enough? the question of extent of resection in glioblastoma multiforme. world neurosurgery 82:e109–e110, 2014. doi:10.1016/j.wneu.2014.05.006. 71 http://www.zbynekmlcoch.cz/informace/medicina/neurologie-nemoci-vysetreni/nadory-mozku-gliomy-astrocytom-glioblastom-priznaky-lecba-diagnostika-prognoza http://www.zbynekmlcoch.cz/informace/medicina/neurologie-nemoci-vysetreni/nadory-mozku-gliomy-astrocytom-glioblastom-priznaky-lecba-diagnostika-prognoza http://www.zbynekmlcoch.cz/informace/medicina/neurologie-nemoci-vysetreni/nadory-mozku-gliomy-astrocytom-glioblastom-priznaky-lecba-diagnostika-prognoza http://www.zbynekmlcoch.cz/informace/medicina/neurologie-nemoci-vysetreni/nadory-mozku-gliomy-astrocytom-glioblastom-priznaky-lecba-diagnostika-prognoza http://www.ejbi.org/en/ejbi/article/9-cs-problematika-mozkovych-nadoru-astroglialni-rady.html http://www.ejbi.org/en/ejbi/article/9-cs-problematika-mozkovych-nadoru-astroglialni-rady.html http://www.ejbi.org/en/ejbi/article/9-cs-problematika-mozkovych-nadoru-astroglialni-rady.html http://dx.doi.org/10.1016/j.wneu.2014.05.006 m. rabochová, m. vinš, j. šoltés, b. michalcová acta polytechnica ctu proceedings [4] a. phuphanich. glioblastoma y astrocitoma maligno. american brain tumor association, 2012. http://www.abta.org/resources/spanish-languagepublications/glioblastoma-y-astrocitomamaligno.pdf. [5] s. s. stylli, a. h. kaye, l. macgregor, et al. photodynamic therapy of high grade glioma – long term survival. j clin neurosci 12(4):389–398, 2005. doi:10.1016/j.jocn.2005.01.006. [6] j. burian. lvr-15 reactor – applications of neutron beam in medicine, biology, dosimetry. iaea technical meeting lecture, vienna, austria, 2011. [7] l. viererbl, j. šoltés, m. vinš, et al. measurement of thermal neutron beam parameters in the lvr-15 research reactor. in transaction of igorr 2013 conference. 2013. http://www.igorr.com/home/ liblocal/docs/igorr2013/07_1001.pdf. [8] l. viererbl, j. burian, s. hladky, et al. si diode with converter used for measurement of epithermal neutron beam of lvr-15 reactor. nuclear instruments and methods in physics research section a: accelerators, spectrometers, detectors and associated equipment 580(1):366–368, 2007. doi:10.1016/j.nima.2007.05.180. [9] canberra industries, inc. genie 2000 basic spectroscopy software, 2016. http://www.canberra. com/products/radiochemistry_lab/pdf/g2kbasicspect-ss-c40220.pdf. 72 http://www.abta.org/resources/spanish-language-publications/glioblastoma-y-astrocitoma-maligno.pdf http://www.abta.org/resources/spanish-language-publications/glioblastoma-y-astrocitoma-maligno.pdf http://www.abta.org/resources/spanish-language-publications/glioblastoma-y-astrocitoma-maligno.pdf http://dx.doi.org/10.1016/j.jocn.2005.01.006 http://www.igorr.com/home/liblocal/docs/igorr2013/07_1001.pdf http://www.igorr.com/home/liblocal/docs/igorr2013/07_1001.pdf http://dx.doi.org/10.1016/j.nima.2007.05.180 http://www.canberra.com/products/radiochemistry_lab/pdf/g2k-basicspect-ss-c40220.pdf http://www.canberra.com/products/radiochemistry_lab/pdf/g2k-basicspect-ss-c40220.pdf http://www.canberra.com/products/radiochemistry_lab/pdf/g2k-basicspect-ss-c40220.pdf acta polytechnica ctu proceedings 4:68–72, 2016 1 introduction 2 research background 2.1 the special positioning device 2.2 6li + si detector 2.3 the image plate 3 method of measurement 3.1 the mesaurement with the positioning device 3.2 the measurement with image plate 4 result of measurement 4.1 the mesaurement with the positioning device 4.2 the measurement with image plate 5 conclusions acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2017.10.0034 acta polytechnica ctu proceedings 10:34–37, 2017 © czech technical university in prague, 2017 available online at http://ojs.cvut.cz/ojs/index.php/app fiber reinforced sand backfill for underground pedestrian crossing andrey b. ponomarev∗, v.i. kleveko, o.v. moiseeva, k.r. kashapova perm national research polytechnic university, kuibysheva street 109, perm, russia ∗ corresponding author: spstf@pstu.ru abstract. crosswalk is an essential element of the transport system of any city. a priority assignment in any such crossing is to ensure the safety of pedestrians. underground pedestrian crossings are much safer than crossings at another level, but they lose in attraction when it comes to cost. the cost of the construction of an underground pedestrian crossing often leads to project abandonment. the cost of construction can be reduced through the use of flexible corrugated metal structures instead of ordinary concrete structures. the stress-strain state of the structural shells need to be known to facilitate their rational design. the stress-strain state of flexible corrugated metal shells largely depends on the strength and deformation characteristics of the surrounding soil. therefore, improving the characteristics of backfill soil is an urgent task in reducing the cost of construction for such tunnels. one way to improve the strength and deformation characteristics of soils is the use of reinforcement. currently, there are a large number of reinforcement schemes and also associated reinforcing materials. one of the most prospective methods of soil reinforcement is the use of fibre filaments. fibre reinforced soils have significantly higher strength and deformation characteristics in comparison with unreinforced soils. numerical modelling of a tunnel shell made out of a corrugated metal structure was undertaken to evaluate the effectiveness of using fibre reinforced sand. ordinary sand and sand reinforced with polypropylene fibres have been used as soil backfill. the calculation results for a pedestrian tunnel structure involving different strength and deformation characteristics of the backfill soil are presented in this article. keywords: fibre reinforced soil, fem, tunnel lining. 1. introduction crosswalk is an integral element of the transportation system of any city. the first and foremost function of a pedestrian crossing is user safety. annually, in russia, a large number of people travelling on foot are killed, which indicates some lack of effectiveness in the transfer and transport system as a whole [1, 2]. the most commonly used and the most projected type is a pedestrian crossing with a light regulation of type “zebra”. this type of transit actually maximises the probability of pedestrian-car contact. underground pedestrian crossings offer a significantly safer transit to light regulation, but lose on the cost criterion. this cost of construction often leads to abandonment of the concept. however, the cost of construction can be reduced through the use of corrugated metal structures instead of the standard concrete shell [3–6]. to reduce the earthworks and consequently the cost of construction, the pedestrian tunnel is laid at the ground-level for traffic vehicles and in this case is carried out using the device of the mound (fig. 1). earlier studies showed that the optimal cross section is a round pipe [6]. the stress-strain state of flexible metal corrugated shells largely depends on the strength and deformation characteristics of the surrounding soil [7, 8]. therefore, the characteristics of the soil backfill gives an actual opportunity to reduce the cost of construction of pedestrian tunnels. one way to improve the strength and deformation characteristics of soils is the use of reinforcement. currently, there are a large number of schemes for reinforcement and their related reinforcing materials. 2. fibre reinforced sandy backfill one of the most promising ways of reinforcement of soils is using fibre. fibre reinforced soils have a significantly higher strength characteristics compared to unreinforced soils [9–12]. fig. 1 shows the structure of the pedestrian crossing at ground level. this design of pedestrian crossing does not need light regulation, and thus saves time for drivers and pedestrians, requires less effort compared to aboveground and underground pedestrian crossings and generally serves to enhance the safety of all road users. 34 http://dx.doi.org/10.14311/app.2017.10.0034 http://ojs.cvut.cz/ojs/index.php/app vol. 10/2017 fiber reinforced sand backfill figure 1. the construction of a pedestrian crossing at ground level figure 2. a round pipe figure 3. specifications of corrugated sheet gl 55 to identify the optimal soil backfill there was calculation by the finite element method, implemented in the software package plaxis 2d. the construction of a pedestrian crossing at ground level was made for specific geological conditions. at the construction site there were identified the following primers: • loam, layer thickness h = 7.2 m, intercept cohesion c = 20.5 kpa, angle of internal friction ϕ = 18°, modulus of deformation e = 11 mpa; • sand, h = 1.0 m, c = 2 kpa, ϕ = 18°, e = 11 mpa; • gravel, h = 1.8 m, c = 1 kpa, ϕ = 40°, e = 30 mpa; • mudstone, h = 2.1 m, e = 20 mpa. fig. 2 shows the geometrical dimensions of a metal corrugated pipe. in the pipe there was used corrugated sheet gl55 (fig. 3), which had the following characteristics: • thickness 4 mm; • the moment of inertia of the cross section i 181.92 cm4/m; figure 4. design scheme • the moment of resistance of section w 61.67 cm3/m; • the plasticity moment of resistance of section wp 79.72 cm3/m. the most common soil backfill for underground structures is sand. therefore, to evaluate the influence of soil characteristics on the stress-strain state of the shirred casing, two calculations were done. the first calculation was derived from a filling with sand with the following characteristics: e = 30 mpa; ϕ = 40°; c = 1 kpa. the second calculation was made for the filling out of fibrocartilagenous sand with the following characteristics: e = 30 mpa; ϕ = 45°; c = 67.0 kpa. the characteristics of sand reinforced with polypropylene fibres to an amount of 0.5 % is taken from test results given in [9–12]. according to the current design standard sp 35.13330.2011 “bridges and pipes”, the cargo load was a four-axle truck n14 with an axle load of 252 (kn), and the distance between the axles 1.2 m. the design scheme is shown in fig. 4. the results of calculating the stress-stain state of the tunnel lining with soil backfill are in figs. 5, 6, 7 and 8. analysis of the graphs showed that in the tunnel lining there occurs compressive strength with a maximum value of nmax = 147.87 kn and a considerable alternating bending moment with a maximum value 35 a. b. ponomarev, v.i. kleveko, o.v. moiseeva, k.r. kashapova acta polytechnica ctu proceedings figure 5. deformed scheme figure 6. the diagram of shell displacements for sand backfill (umax = 29.32 mm) of mmax = 6.5 knm. the maximum displacement of the tunnel lining was observed in the upper zone and amounted to 29.32 mm. the results from calculating the stress-strain state of the tunnel lining and soil backfill from fibroareolar sand is shown in figs. 9, 10 and 11. based on the calculation results the following data was obtained: • max. travel: imax = 20.71 mm; • max axial force nmax = 164.28 kn; • max bending moment mmax = 3.78 knm. the analysis of the above plots shows that using fibre reinforced soil instead of unreinforced soil leads to a decrease of the deformations of the tunnel shell by 8.6 %, a bending moment at 42 % and to an increase in longitudinal force of 9.7 %. based on the obtained calculation results there were calculated the normal stresses in the metal corrugated shell. the calculation results are presented in figs. 12 and 13. the analysis of these plots shows that the normal stresses in a metal enclosure when using the filling out fibre reinforced soil is much less than for soil that is unreinforced. figure 7. the diagram of axial force for sand backfill (nmax = 147.87 kn) figure 8. the diagram of bending moment for sand backfill (mmax = 6.5 knm) figure 9. the diagram of shell displacements for fibre reinforced sand backfill (umax = 20.71 mm) figure 10. the diagram of axial force for fibre reinforced sand backfill (nmax = 164.28 kn) 36 vol. 10/2017 fiber reinforced sand backfill figure 11. bending moment diagram for fibre reinforced sand backfill (mmax = 3.78 knm) figure 12. stress diagram for sand backfill (σmax = 105.39 mpa) 3. conclusions (1.) using fibre reinforced soil can improve the strength and deformation characteristics of the soil backfill corrugated metal tunnel lining, which leads to a significant reduction of stresses and strains in it. stress was reduced by 42 %, and the deformation by 8.6 %. (2.) the results of the calculations showed that improvement of the mechanical characteristics of the soil backfill for the tunnel allows the use of thinner corrugated sheets, which reduces the cost of construction. references [1] o. moiseeva, v. kleveko. analysis of accident cases involving pedestrians in perm. pnrpu construction and architecture bulletin (4):134–143, 2015. [2] p. a. tretyakova, v. kleveko. modern methods of increasing the efficiency of urban transport systems. pnrpu construction and architecture bulletin (1):101–108, 2012. figure 13. stress diagram for fibre reinforced sand backfill (σmax = 61.19 mpa) [3] o. v. moiseeva, v. i. kleveko. choice of a rational type of pedestrian crossing with safety for pedestrians and construction costs. world science and innovation 8(2):90–93, 2015. [4] a. e. polovnikova, v. i. kleveko. choice of a rational type of pedestrian crossings with regard to the safety of pedestrian traffic. modernization and research in the transport sector 2:356–361, 2015. [5] o. v. moiseeva, v. i. kleveko. choice of rational type of crosswalk with regard to the safety for pedestrians and the cost of construction. sworldjournal j21504.(2(9)):3–5, 2015. [6] o. v. moiseeva, v. i. kleveko. using prefabricated corrugated metal structures for the construction of a pedestrian tunnel. modern scientific researches and innovations 12(56):409–411, 2015. [7] v. v. kovalchuk. the effect of corrugated elements thickness on the deflected mode of corrugated metal structures. nauka ta progres transportu 3(57):199–207, 2015. [8] a. l. novodzinsky, v. i. kleveko. the account influence of a thickness of the goffered element on durability and stability of a metal water throughput pipe. pnrpu construction and architecture bulletin (1):81–94, 2012. [9] v. i. kleveko, y. m. shangina. the impact of the use of fibre reinforced sand as backfilling on the value of act. sworldjournal 04(116 (10)):16–21, 2016. [10] a. b. ponomarev, a. s. kuznetsova, v. g. ofrikhter. application of fibroreinforced sand as the foundation of buildings and constructions. bulletin of volgograd state university of architecture and civil engineering series: construction and architecture (30):101–107, 2013. [11] a. s. kuznetsova, v. g. ofrikhter, a. ponomarev. strength research of sand reinforced by discrete polypropylene fibres. pnrpu construction and architecture bulletin (1):44–55, 2012. [12] a. b. ponomarev, a. s. kuznetsova. laboratory testing of the strength characteristics of fibre reinforced sand with various water. bulletin of civil engineers 6(47):127–132, 2014. 37 acta polytechnica ctu proceedings 10:34–37, 2017 1 introduction 2 fibre reinforced sandy backfill 3 conclusions references 322 acta polytechnica ctu proceedings 1(1): 322–325, 2014 322 doi: 10.14311/app.2014.01.0322 final remarks giulio auriemma1,2 1universiẗı¿œ degli studi della basilicata, potenza,italy 2infn sezione di roma, rome, italy corresponding author: giulio.auriemma@cern.ch this year we have to celebrate the 50th anniversary of the book “the structure of the scientific revolutions” by thomas kuhn that has been published in 1963.. in this book, the more influential science philosopher of the last century, changed the old view of the development of the natural sciences as a linear process accumulation of knowledge into a substantially discontinuous passage from one paradigm to a new one. two drastic changes of paradigm occurred near the beginning of last centuries with the shift from newton’s absolute time to relativistic space-time and from laplace determinism to quantum mechanics. around the middle of the same century another crucial shift of paradigm was the introduction of gauge symmetries, that led to the formulation of the standard model of electromagnetic, weak and strong interactions (sm for brief) [1], whose complete experimental verification has been given recently by lhc [2, 3]. the carriers of those three forces are spin-1 particles, namely the photon for the electromagnetic, the gluon for the strong and the w±,z for the weak one. it is well known that the first two particles are massless, while the others, that were discovered at lep. have large masses (∼ 80− 90 times the mass of the proton), which can explain the weakness of the corresponding interactions. since the sm does not include gravity, there is no hint in it for the mass of the particles, that are put “by hand” in the lagrangian. what is now usually called the “higgs mechanism” [4, 5] is the possibility that the mass of particles, in particular that of the weak bosons, could be originated by the coupling with a universal scalar field, which carriers would be a weakly interacting spin-0 particles very similar to the one recently observed at lhc. nevertheless even a confirmed discovery of the higgs would not be the end of the story, because thanks to astronomy and cosmology, we have strong observational evidences for phenomena like inflation dark matter dark energy baryon asymmetry that are not understood in the framework of the minimal sm. this justify the diffuse opinion (hope?) that a lot of new physics remains to be discovered in the sky and laboratory. 1 cosmological parameters the results presented by rubiño-martin [6] on behalf of the planck collaboration is one of the highlights of this conference. the new measurement of the hubble constant from the fit of the cmbr obtained from the planck satellite 2013 data is h0 = (67.3± 1.2) km s−1 mpc−1 is fully compatible with the previous value of 70 ± 2.2 published by wmap collaboration. however the central value of h0 is changed by non-negligible factor (-4%) that indicates that the age of the universe is effectively t0 = (13.82± 0.12) gy, about 100 million longer then the previous estimate from wmap. incidentally that caused a funny “communication” problem with the media, because it was public ally announced on being the “universe older then the big bang”. figure 1: reproduced from ref. [6] the tension of the h0 value between hst key project (cepheid+sne ia) and the cmbr anisotropy fit, shown in fig. 1, that was marginal (−1.2σ) with 322 http://dx.doi.org/10.14311/app.2014.01.0322 final remarks wmap, is now at a level of −2.5σ with the new planck data. not a suspicious level yet, but close to become interesting. table 1: universe composition from cmbr before planck after planck dark energy 72.8% 68.3%(−4.5% ⇓) dark matter 22.7% 26.8%(+4.1% ⇑) ordinary matter 4.5% 4.9%(+0.4% ') total 100% 100% a similar situation is observed in the determination of the λcdm composition breakdown as shown in table 1. the uncertainty on the fractions is estimated by the planck collaboration to be about ±3% [7]. but it should be also stressed that this fraction are obtained with a multivariate fit over a constrained domain, in which correlations play an essential role. therefore the fit error given independently for each of the variables could be strongly underestimated. generally speaking a reliable error on the determination of each parameter should be obtained using the full covariance matrix of the fit, if definite positive. 2 dark energy nino panagia [8], presenting the most recent results of the hubble supernovae cosmology project (scp), strongly stressed that the λcdm model with ωλ = 0.729 ± 0.014 indicating a present universe dominated by the mysterious dark energy, are all based on the assumption that the explosive properties of the sne ia do not depend from redshift, up to z ' 1.4. both the scp and ground based telescope surveys of light curves and spectroscopic distributions of hundreds of supernovae, consistently found that the supernovae around z ≈ 0.5 appear to be ≈ 0.3 mag dimmer then expected from a flat universe with ωλ = 0. what if a cosmical conspiracy made the sne at redshift ≈ 0.5 intrinsically dimmer? as predictable this pebble thrown in the placid pool (near stockholm) started a heated discussion. many in the audience supported the argument that the good fit of distance modulus vs. redshift curve [9] and of the tt spectrum of cmb fluctuations. in the discussion following the presentation of his paper, nino questioned from a philosophical point of view that a successful fit is not to be taken as a definitive proof of validity for a theory. his point is absolutely correct, but popper would object that a scientific theory can be falsified, not affirmed. therefore we can exclude theories that do not fit the data, but not viceversa. on the same ground i think that would be very hard to fit the planck data with ωλ = 0. harmes [10] has presented an interesting paper on a possible identification of the dark energy with solitonic primordial gravitational waves in the framework of quantum gravity with one warped extradimension, similar to the russel saunders one. beside the specific model presented in that talk, that may explain both strength and time evolution observed by planck [11], it has been speculated that dark energy could be the present manifestation of long lasting relics, with lifetime much larger then the hubble time, produced in the planck era (tu . 5 × 10−44 s) by quantum gravity effects. 3 dark matter astrophysical evidence for dark matter is now overwhelming. in a recent paper bahcall and kuilier [12] tracing the mass-to-light ratio with dynamical and lensing methods, show that this ratio m/l � 1 is nearly constant for scales from 350h.1 kpc up to 22h−1 mpc, while the fraction of the stellar mass over the total mass remains of the order of few percent over all scales and environments. it worth noticing that quantitatively the observed mass-to-light ratio on large scales gives an estimate of the mass density of the universe ωm = 0.26 ± 0.02, slightly smaller (≈ 1.5σ) then the planck value [6]. after the discovery of the higgs particle, dark matter is considered to be the most compelling evidence for new physics [13, 14]. particle theory offers several type of objects that could play the role of observed dark matter, as for example axions and majorana neutrinos, but excellent theoretical candidates are the supersymmetric neutral partners of the sm particles. lhc has definitively the possibility of discovering these particles if their mass is in the tev range. however, as i said in my talk, there is a fruitful confrontation among astronomical observatories and accelerators but not a competition, because astronomy cannot demonstrate that wimp’s are supersymmetrical particles and accelerators cannot prove that a certain kind of particle really is the dark matter. several talks have described the status of the search for wimp annihilation signal in γ-rays survey. in particular morselli [15] presented on behalf of the fermilat collaboration several interesting results both on the diffuse component and of the gc region. at present we have some indications that the allowed mass scale for wimp as well as the one of the susy neutralino at accelerators is to be found above several hundreds of gevs. 4 uhe neutrinos (?) the detection of neutral particles in the km3 detector icecube [16] in antarctica with an energy release ≥ 50 323 giulio auriemma tev is something waited since a long time. the study of a deep underwater muon and neutrino detector (dumand) in the pacific ocean, off the shore of the island of hawaii, usa, started in the late ’70s [17]. but unfortunately after many years of r&d the project was interrupted in 1996, after the decision of the doe science committee of not funding its deployment. in the meanwhile many other projects were initiated in many parts of the world, and often the collaborations have presented progress reports to the vulcano conferences. the icecube collaboration has announced last april the detection of two contained showers with an energy estimated from the total number of photo-electrons & 1 pev. it is worth noticing that this energy estimate is based upon the assumption that all the light is emitted in the electromagnetic cascade. the total number of events (28) detected until now by the detector [18] seems to give the first clear indication of nongeophysical and extra-solar origin neutrinos. 1. fargion[19] has proposed in his talk some intriguing possible mechanism originating these events, that could be tested in the future, when a larger number of events will be hopefully detected. it is however to be stressed that these events are really exceptional under many aspects. if they are true neutrinos the c.m.s energy of their interaction: √ s = √ 2mneν = 316− 1400 gev�mz is order of magnitudes larger than any neutrino-nucleon interaction observed in laboratory until now. it has been proposed in the past [20, 21, 22] that different kind of new physics could enhance substantially in this energy range. moreover the detected showers could have been originated not by neutrinos but by new physics messenger, such as for example susy particles. acknowledgments i am sure to interpret the feelings of all the participants to this very successful meeting thanking the scientific committee for having assembled such an interesting program; all the speakers for their efforts in communicating results and ideas; daniela and francesco for the precious help given to run smoothly the meeting; valentina, lisa, flavia and alessandro for the good music and readings, that gave us a moment of true ... “felicitas” the weather in mondello not making us to regret of being segregated inside the lecture room; and finally franco ... everybody knows why! references [1] g. altarelli, standard model of particle physics, in: j.-p. francoise, g. l. naber, , t. s. tsun (eds.), encyclopedia of mathematical physics, academic press, oxford, 2006, pp. 32-38. arxiv:hep-ph/ 0510281. [2] g. aad, et al., observation of a new particle in the search for the standard model higgs boson with the atlas detector at the lhc, phys.lett. b716 (2012) 1-29. arxiv:1207.7214 doi:10.1016/j.physletb.2012.08.020 [3] s. chatrchyan, et al., observation of a new boson at a mass of 125 gev with the cms experiment at the lhc, phys.lett. b716 (2012) 30–61. arxiv:1207.7235. doi:10.1016/j.physletb.2012.08.021 [4] p. w. higgs, broken symmetries, massless particles and gauge fields, phys. lett. 12 (1964) 132– 133. doi:10.1016/0031-9163(64)91136-9 [5] f. englert, r. brout, broken symmetry and the mass of gauge vector mesons, phys. rev. lett. 13 (9) (1964) 321–323. doi:10.1103/physrevlett.13.321 [6] j. rubiño-martin, planck results : a review, in: these proceedings, 2013. [7] p. ade, et al., planck 2013 results. xvi. cosmological parameters, arxiv:1303.5076. [8] n. panagia, the hubble space telescope cluster supernova survey, in: these proceedings, 2013. [9] n. suzuki, et al., the hubble space telescope cluster supernova survey. v., astroph. j. 746 (2012) 85. arxiv:1105.3470, [10] b. harmes, gravitational waves and dark energy, in: theese proceedings, 2013. [11] p. l. biermann, b. c. harms, can dark energy be gravitational waves? arxiv:1305.0498. [12] n. a. bahcall, a. kulier, tracing mass and light in the universe: where is the dark matter?, mon. note r. astron. soc. in press. arxiv:1310.0022. [13] g. altarelli, the higgs: so simple yet so unnatural, in: talk given at the nobel symposium on lhc results, krusenberg, sweden, 3-17 may,, 2013. arxiv:1308.0545. 324 http://arxiv.org/abs/hep-ph/0510281 http://arxiv.org/abs/hep-ph/0510281 http://arxiv.org/abs/1207.7214 http://dx.doi.org/10.1016/j.physletb.2012.08.020 http://arxiv.org/abs/1207.7235 http://dx.doi.org/10.1016/j.physletb.2012.08.021 http://dx.doi.org/10.1016/0031-9163(64)91136-9 http://dx.doi.org/10.1103/physrevlett.13.321 http://arxiv.org/abs/1303.5076 http://arxiv.org/abs/1105.3470 http://arxiv.org/abs/1305.0498 http://arxiv.org/abs/1310.0022 http://arxiv.org/abs/1308.0545 final remarks [14] l. bergstrom, cosmology and the dark matter frontier, in: invited talk at the nobel symposium on lhc physics, krusenberg, sweden, may 13-17,, 2013. arxiv:1309.7267. [15] a. morselli, last results from the fermi-lat gamma-ray telescope, in: these proceedings, 2013. [16] m. aartsen, et al., first observation of pev-energy neutrinos with icecube, physical review letters 111 (2) (2013) 021103. doi:10.1103/physrevlett.111.021103 [17] a. roberts, r. donaldson (eds.), 1976 dumand summer workshop, 1977. [18] m. spurio, photonic, neutrino and particle astronomy as messenger of the universe, in: these proceedings, 2013. [19] d. fargion, uhecr correlated tev anisotropy connection with pev neutrino shower events, in: this proceedings, 2013. [20] m. s. carena, d. choudhury, s. lola, c. quigg, manifestations of r-parity violation in ultrahighenergy neutrino interactions, phys.rev. d58 (1998) 095003. arxiv:hep-ph/9804380, doi:10.1103/physrevd.58.095003 [21] m. kachelriess, m. plumacher, ultrahighenergy neutrino interactions and weak scale string theories, phys.rev. d62 (2000) 103006. arxiv:astro-ph/0005309. doi:10.1103/physrevd.62.103006 [22] r. gandhi, ultrahigh-energy neutrinos: a review of theoretical and phenomenological issues, nucl.phys.proc.suppl. 91 (2001) 453–461. arxiv:hep-ph/0011176. doi:10.1016/s0920-5632(00)00975-0 hope to meet you all at the next frascati workshop 325 http://arxiv.org/abs/1309.7267 http://dx.doi.org/10.1103/physrevlett.111.021103 http://arxiv.org/abs/hep-ph/9804380 http://dx.doi.org/10.1103/physrevd.58.095003 http://arxiv.org/abs/astro-ph/0005309 http://dx.doi.org/10.1103/physrevd.62.103006 http://arxiv.org/abs/hep-ph/0011176 http://dx.doi.org/10.1016/s0920-5632(00)00975-0 cosmological parameters dark energy dark matter uhe neutrinos (?) 320 acta polytechnica ctu proceedings 1(1): 320–321, 2014 320 doi: 10.14311/app.2014.01.0320 concluding remarks gennady s. bisnovatyi-kogan1 1space research institute, profsoyuznaya str. 84/32, moscow 117997, russia, and national research nuclear university ”mephi”, kashirskoye shosse, 31, moscow 115409, russia corresponding author: gkogan@iki.rssi.ru abstract short comments to the conference talks. keywords: cosmology x-ray sources sne star formation agn jets grb. 1 cosmology sz effect (s. colafrancesco, m. arnaud): does it help investigation of cmb fluctuation? sz influence on the cmb spectrum in clusters interferes with primordial cmb perturbations. so we have: *contamination with cluster scale cmb perturbations *clumpiness of a gas inside cluster does not permit to extract cosmological parameters from the comparison of radio and x ray observations. b. harms: gravitational waves as a dark energy strong gravity branes support gw density constant (seems like vacuum eos), but gw produce gravity, not antigravity, like de. n. panagia: sn1a, wmap, planck are the cosmological parameters coincide inside error bars (hubble constant and de density) ? 1 2 magnetic field in astrophysics *core-collapse sn explosions *cr acceleration in snr *accretion disk structure and coronae formation *jet formation and collimation *magnetic accretion and cyclotron lines *magnetars 2.1 x-ray sources heating of accretion disk corona by magnetic reconnection (s. orlando). about 35 years ago this problem was considered in papers of g. s. bisnovatyi-kogan and s. i. blinnikov ”a hot corona around a black-hole accretion disk as a model for cygnus x-1.” sov.astron.lett. 2, 191-193 (sep.oct. 1976) and a.a. galeev, r. rosner, g.s. vaiana ”structured coronae of accretion disks”. apj, 229, 318-326 (april 1979). in the first paper a convective instability of the radiation dominated region in accretion disks was established, and its connection with formation of a hot corona was considered. ”heat transfer in the region of maximum energy release of an accretion disk will take place mainly by convection, serving to enhance the turbulence and to generate a powerful acoustic flux. the hard x-rays emitted by cyg x-1 (e ≤ 200 kev ) might result from comptonization of soft photons in a corona formed around the disk through this heating.” in the second paper the magnetic mechanism of the corona heating over the convective region of the accretion disk was studied. ”a model for the fluctuating hard component of intense cosmic x-ray sources (such as cyg x-i) is developed, based upon the amplification of magnetic fields by convective motions and differential rotation within a hot (t > 106 k) accretion disk. field reconnection within the inner portion of the disk is shown to be ineffective in limiting field amplification; magnetic fields may therefore attain strengths comparable to the equipartition value, leading to their emergence via buoyancy in the form of looplike structures and resulting in a very hot (t > 108k) magnetically confined, structured corona analogous to the observed structure of the solar corona. the energy balance of these loop structures is examined, and it is shown that the disk soft x-ray luminosity determines the predominant energy loss mechanism in loops: at low disk luminosities, thermal bremsstrahlung from these loops domi1resent discovery (arxiv:1312.3313) of an error in the planck detector at 217 ghz make the differences even less convincing. 320 http://dx.doi.org/10.14311/app.2014.01.0320 concluding remarks nates and contributes a steady, shot-noise-like hard xray component. at high disk luminosities the emerging loops are compton-cooled; the soft x-ray flux from the disk is comptonized by the emerged loops, forming a transient, flarelike hard x-ray component.” schematic drawing of the inner accretion disc corona, heated by magnetic reconnection in the emergent magnetic loops, (from the second paper) is given in fig.1. v. simon: analogy between lmxb ks1731-260 (ms pulsar), and her x-1 (p ≈ 1.24 sec) does not seems to be realistic. in particular, these objects have very different magnetic fields, with 2-4 orders of magnitude difference in strength 2.2 jets, grbs and sgrs a long time before the magnetar concept for sgrs came out, a related object called magnetoid was considered by l.m. ozernoi in the paper ”a theory for the formation and structure of quasistellar radio sources quasar model as supermassive star.” astron. zh. 43, 300-312 (1966). ”a ”magnetoid” is a quasistationary configuration whose equilibrium is governed by a magnetic field, provides an idealized representation of many effects occurring in galactic nuclei and especially in the central parts of quasars. the gravitational energy released through secular contraction of the nucleus is the ultimate source of the intense quasar radiation. the magnetic field is an important intermediary. ... local jets and streams of matter connected with active regions may occur when quasar nuclei reach global quasiequilibrium. although the magnetoid approach furnishes a unified explanation for the basic property of quasistellar radio sources an intense and variable flux emitted over a fairly long period many aspects necessarily remain speculative.” now there is a general opinion that quasars and agn are connected with an accretion into supemassive black holes, may be, besides a minority, represented by w. kundt, who gave a talk ”astrophysics without black holes and extragalactic grbs”. there are some observational evidences which do not support a magnetar model for sgrs, so later it could join its magnetoid predecessor. j. beall: numerical simulations, jets of different scale. heating of the surrounding matter by jet propagation. m. della valle: grb and sne connection questions remain. 2.3 sn 1987a b ashenbach: x ray development of sn 1987a 3 fundamental problems of physics chemistry in astrophysics. susana iglesias groth the talk ”the impact of fullerenos, pahs and amino acides in high energy astrophysics”. thermodynamics and gravity. marco merafina & daniele vitantoni the talk ”data analysis of globular cluster harris catalog in view of the king’s models and evolution” gravitational theory. mariafelicia de laurentis the talk ”testing f(r) theories using the first time derivative of the orbital period of the binary pulsars” leopoldo milano the talk ”eccentric eclipsing detached binaries: a toll for testing gravitational theories” 4 conclusion many topics, covering almost all sides of the modern astrophysics were presented. number of participants and number of reports is increasing. duration of talks is decreasing proportionally from 35 min maximal duration talk (1999), to 25 min (2013) in the conference of japan astronomical society (1978) the duration of each talk was 5 minutes, and some participants managed to show more than 50 transparances during this time (competition). let us hope, that franco giovannelli will find a better solution! thank you franco, and collaborators! 321 cosmology magnetic field in astrophysics x-ray sources jets, grbs and sgrs sn 1987a fundamental problems of physics conclusion 278 acta polytechnica ctu proceedings 1(1): 278–282, 2014 278 doi: 10.14311/app.2014.01.0278 havms: highly available virtual machine computer system fault tolerant with automatic failback and close to zero downtime memmo federici1, carlo gaibisso2, bruno l. martino3 1istituto di astrofisica e planetologia spaziali, inaf iaps via fosso del cavaliere 100, 00133 roma, italy 2istituto di analisi dei sistemi ed informatica ”antonio ruberti”, iasi-cnr viale manzoni 30, 00185 roma , italy 3associated inaf iaps corresponding author: memmo.federici@iaps.inaf.it abstract in scientific computing, systems often manage computations that require continuous acquisition of of satellite data and the management of large databases, as well as the execution of analysis software and simulation models (e.g. monte carlo or molecular dynamics cell simulations) which may require several weeks of continuous run. these systems, consequently, should ensure the continuity of operation even in case of serious faults. havms (high availability virtual machine system) is a highly available, ”fault tolerant” system with zero downtime in case of fault. it is based on the use of virtual machines and implemented by two servers with similar characteristics. havms, thanks to the developed software solutions, is unique in its kind since it automatically failbacks once faults have been fixed. the system has been designed to be used both with professional or inexpensive hardware and supports the simultaneous execution of multiple services such as: web, mail, computing and administrative services, uninterrupted computing, data base management. finally the system is cost effective adopting exclusively open source solutions, is easily manageable and for general use. keywords: havms high availability fault tolerant open source multitask. 1 introduction havms effectively solves the problems related to the ”robustness” of computer systems wholly embracing the concept of high availability. a system in high availability, ha in what follows, must ensure the continuity over time of the provided services, which, in case of fault, must be restored in the shortest possible time. havms, through an accurate design of the hw and sw, significantly reduces the faults and their negative effects on the provide services. havms has been designed mainly keeping in mind the following requirements: cost effectiveness, ease of management and, above all, the ability to automatically implement restoring strategies of the provided services without any interruption. all the above requirements have been met by an accurate choice of the available open source solutions meeting our targets and their integration with the sw specifically conceived by the authors. in particular, the last of them makes the system unique in its kind and competitive with analogous commercial solutions. havms is made by two servers, one active and the other dormant, but from the point of view of users, as well as from that of applications, the system is seen as a single server. the fault tolerance of the system is ensured by a continuous synchronization of the two servers. this synchronization keep the data and the states of all the virtual machines, vms in what follows, perfectly aligned. every service runs on a different vm each of which has its own ram, storage and some computing resources. each vm is independent of the others. when a fault occurs, the sleeper server is awakened and automatically takes the place of the broken server. once the fault is fixed, it will be enough to reconnect the repaired server and automatically the failback procedure restores the proper functioning of the system including its ha capabilities. 1.1 the context havms is a general purpose system, this is one of its strengths. among the potential areas in which the system proved its effectiveness, there is the acquisition and processing of data from space missions. the main abilities required to havms by this particular context are: • managing of processes that require uninterrupted data analysis and data acquisition; 278 http://dx.doi.org/10.14311/app.2014.01.0278 havms: highly available virtual machine computer system fault... • storing the results of the analysis for long periods of time; • making these results available to the scientific community, in an effective, continuos and reliable way. in fact, scientists in this area are interested in performing their analysis and tests on spatial observations as soon and on as much data as possible. furthermore, missions such as integral (international gamma-ray astrophysics laboratory) winkler, et al. (2003), have to maintain for a long period of time the results of the scientific analysis performed on the whole data set and usually released, as surveys, about every year. finally a further important requirement that has been met is making the management of the whole system, as far as possible, easy and automatic, thus minimizing the costs related to human resources devoted to the management itself. 1.2 the alternatives in designing, havms we deeply analyze and consider as points of reference the more widespread alternatives currently implementing the concept of ha. all these alternatives, in general, suffers from the problem of the adoption of often expensive sw and hw architectures. more in details these are, from our point of view and with respect to the particular operation context we are considering, the main drawbacks of the considered solutions: • windows server failover clustering (wsfc): requires the purchase of a quite expensive sw license (when this paper is written, about 900 euros per processor). all servers in the cluster must be absolutely identical. ha is guaranteed by a mirroring system, replicating the status of the severs every 5-10 minutes: if there is a fault during a replica, the replica itself may fail and the running jobs are aborted with a high probability. • vmware vsphere: requires the purchase of a quite expensive enterprise license (when this paper is written, about 4000 $). requires professional hw and an additional server to handle the nodes in the system. the setup is not simple. • red hat cluster manager (red hat enterprise linux server): requires the purchase of an annual license (when this paper is written, about 500 $). in case of fault the system must be restarted, as a consequence all runs in progress are stopped. does not support an automatic failback mechanism. 2 our solution figure 1: havms block diagram fig.1 shows the general architecture of havms. continuity of service is provided through an automatic failover (activates the dormant server in case of fault of the active one) and failback (restores the initial system configuration and operation) mechanisms. the software components implementing these mechanisms are: • xen vms manager, a manager of virtual machines; • drbd (protocol d), a synchronizer of block devices; • remus, provides transparent, comprehensive high availability to ordinary vms running on xen; • some components, designed and developed by the authors especially conceived, among the others, to implement the automatic failback and system startup. the system is based on the services provided by xen vms manager. each vm has its own ip address by which it is accessible from the outside. as previously stated, one server is active, the other one is dormant. the two servers are connected through a lan at 1 gb/sec. remus (in green) and drbd (in red) seamlessly (once every 40 msec) replicates the state of the active server on the dormant one. in particular, remus replicates the states of vms and simultaneously sends a trigger to drbd, which, in turns, synchronizes 279 memmo federici, carlo gaibisso, bruno l. martino the block devices (hard disks) of the two servers. as a consequence the two servers are aligned once every 40 msec. users connect to the vms through their unique ip address. vms makes it possible to install different operating systems, on which different softwares and services can be run, such as: matlab, idl and web servers, compilers and so on. 3 servers software and hardware architecture both servers have the same software architecture, already introduced in the previous section. the whole architecture leans on linux ubuntu server 10.04 64 (with a kernel customized to support xen). in what follows the single components of the architecture are briefly described. 3.1 xen xen hypervisor is a virtualization platform licensed under the gpl developed at the computer laboratory of the university of cambridge. xen is included in all major linux distributions and increasingly adopted by commercial solutions. one of the its most interesting feature is the ability to effectively control the requests of access to physical resources coming from vms through a paravirtualization mechanism. this mechanism guarantees a minimum decay of performance due to virtualization, since requests of access coming from vms are mainly executed on the physical computing resources. paravirtualization requires a customized version of the ubuntu kernel. 3.2 drbd drbd (distributed replicated block device) is a distributed storage system for the gnu/linux platform, usually adopted by ha clusters. drbd is responsible for the synchronization of data between the servers: one of them is identified as primary, the other one as secondary. when the primary server fails, a management process promotes the secondary one to the role of primary. when the fault is fixed, the system may reestablish the roles initially assigned to the servers, after a resynchronization of the data storage devices. this synchronization is particularly effective since only those blocks that were changed during the outage are resynchronized. 3.3 remus remus is part of xen and implements the ha concept by replicating on the dormant server the state of all the vms machine running on the active server. this snapshot occurs once every 40 msec (this value can be changed at setup time); at the same time remus sends a trigger to drbd for the synchronization of the hard disks. in this way the active server is constantly aligned, both for what concerns data and computational aspects, with the dormant server. in case of fault of the active server, the dormant one is awakened and becomes immediately active, thus avoiding any interruption in the provisioning of services. moreover, the tcp/ip protocol guarantees the correct transmission of data packets, even when the connectivity is temporary interrupted. the architecture required to effectively support our ha solution, does not expect the use of professional hw. in fact, as will be evident from the following description of our experiences in the field, an entry level solution with the following characteristics turns out to be absolutely appropriate to the achievement of our goals: • i7 8 cores intel processor • 8/16 gb ram ddr3 • 2 tb hard disk • 2 network interfaces 3.4 failover the typical faults of a operating environment such as the one here considered here, can be attributed mainly to two categories: lack of connection, malfunctioning of some vms. in case of a fault, the failover process is automatically triggered, which: 1. stops the drbd synchronization between servers; 2. if running, stops the execution of remus on the active server; 3. awakens the dormant server. the execution of vms is consequently resumed from the last committed checkpoint (at most 40 msec before). 3.5 failback unfortunately, remus does not support any kind of automatic failback. the sw developed for havms compensates for this deficiency and gives it its unicity. this process, which is automatically triggered and implemented, restores the initial configuration and operational capabilities of the system once the fault has been fixed, included its ha functionality. 280 havms: highly available virtual machine computer system fault... 4 practical applications the development of havms was determined by some practical needs. below we briefly describe the two applications that currently rely on the services offered by the system. 4.1 the ha data storage system of integral the laboratory of distributed computing at iaps (inaf) is in charge of aves federici et al. (2012), the cluster devoted to the analysis of data collected by integral. aves is connected to data storage subsystem (dss) a dedicated storage system martino and federici (2011) adopting havms. dss automatically downloads from integral science data center (isdc) courvoisier et al. (2003) the data collected by the satellite, backups this data and makes them available to aves (16 tb to rise). as a consequence of a fault of dss the download is interrupted and aves as no longer access to the shared data. this might have severe consequences on the ongoing activities since, in turn, may cause a data misalignment and a crash of the running analysis. havms avoid these risks and their unwanted consequences. 4.2 continuity of service at iasi havms guarantees the continuity of the service provided by iasi it division, among them: attendance control, centralized computing, storage and backup, printing services management. a different vm is allocated to each service. 5 further potential applications in the following we describe two potential fields of application of havms we are investigating. these fields differ substantially from one another. each of them is representative of a significant and large family of applications. 5.1 high end solution large structures, such as hospitals or national administrative offices should provide a relevant number of services that are usually hosted on individual servers. these servers, to ensure an acceptable level of reliability, should be equipped by systems for the backup of the status of the provided services and the data they deal with. this is therefore a privileged context within which to exploit the characteristics of havms. a quick market survey has shown that our solution can meet the above mentioned reliability requirements with not negligible costs but still low if compared with those of similar commercial solutions. in fact, one of the adoptable hw solutions could be the following: motherboard supermicro xeon mp series x9qxxx, equipped with 4-socket xeon processors (32 cores), 120 gb of ram, 4 ethernet interfaces etc. this configuration may provide up to 30 services, each with its own vms and ip address, with a cost of approximately 5000 euros per server. 5.2 a little gem in this section we describe a solution adopting havms that fundamentally, both in terms of computing power and cost, differs from the high end solution, which is a good example of the great versatility of our ha solution. this solution has been designed to deal with the monitoring of atmospherical and geological events by the use of sensors in hard to reach sites. the resulting system, which has a cost of about 300$ when this paper is written, is composed by three raspberry pi collecting analog and digital signals from sensors and transferring them by a radio link to a remote control station. thanks to the low power consumption, the system can be powered by a small photovoltaic panels. two of the three raspberry’s are allocated to havms; the third one, which is initially turned off, is switched on in case of failure of any one of the first two. this is obtained by a hw signal sent through lan (wakeup on lan). the awakened raspberry takes the place of the broken one thus guaranteeing the continuity of service, included those offered by havms. this solution substantially limits the number of maintenance interventions, thereby reducing the relative costs. figure 2: the raspberry pi three node havms 6 further work we plan to improve the ha capabilities of havms by adding to the current configuration a third server in 281 memmo federici, carlo gaibisso, bruno l. martino wake on lan. in case of fault this third server is automatically awakened by an hw signal sent through the lan and takes the place of the broken one. this automatic mechanism should not require any intervention by the system manager. with this solution, the system maintains its characteristics of ha also in case of fault of one server. to improve the effectiveness of havms in relation to the use of network resources, multiple network connections can be combined in parallel (bonding) to increase throughput beyond what a single connection could sustain, and to provide redundancy in case one of the links fails. 7 conclusions and discussion havms is a highly available fault tolerant general purpose, recyclable system based on the use of vms, assuring continuity of operation and no interruption in services providing in case of fault. havms is cost effective since it only adopts open source solutions. it is also extremely versatile, providing all operating systems supported by xen. paravirtualization dramatically reduces the negative impact of virtualization on the performance of the whole system. the system supports automatic failover and failback within times close to zero. automatic failback is an exclusive feature of havms. acknowledgement the authors thanks to: giuliano sabatino, fabio guglietta. references [1] winkler, c., et al.: the integral mission. astron. astrophys. 411, l1l6 (2003) [2] m. federici, et al. 2012, aves: a high performance computer cluster array. exp astron (2012) 34:105121 [3] b.l. martino and m. federici ”an high availability data storage subsystem for the integral data analysis”, mem. s.a.it. vol. 83, 377 2011 [4] courvoisier, t.j.-l., et al.: the integral science data centre (isdc) for the integral satellite scientific data analysis. astron. astrophys. 411, l53l57 (2003) doi:10.1051/0004-6361:20031172 discussion beall james’s question: do you plan to use the raspberry pi machine for computation? memmo federici’s answer: not in the traditional sense because rb is equipped with small amount of ram (only 256 mb) and little computing power. there are some applications that see this hw used in small clusters with low consumption. we plan to use rb to make some services such as web servers and print servers. this small device is very versatile and lends itself also to the process control. – 282 http://dx.doi.org/10.1051/0004-6361:20031172 introduction the context the alternatives our solution servers software and hardware architecture xen drbd remus failover failback practical applications the ha data storage system of integral continuity of service at iasi further potential applications high end solution a little gem further work conclusions and discussion 269 acta polytechnica ctu proceedings 1(1): 269–273, 2014 269 doi: 10.14311/app.2014.01.0269 active galactic nuclei: jets as the source of hadrons and neutrinos athina meli1,2,3, paolo ciarcelluti3 1department of physics and astronomy, university of gent, belgium 2ifpa, department of astrophysics, geophysics and oceanography, university of liege, belgium 3web institute of physics, www.wiph.org corresponding author: athina.meli@ugent.be abstract active galactic nuclei are extragalactic sources, and their relativistic hot-plasma jets are believed to be the main candidates of the cosmic-ray origin, above the so-called knee region of the cosmic-ray spectrum. relativistic shocks, either single or multiple, have been observed or been theorized to be forming within relativistic jet channels in almost all active galactic nuclei sources. the acceleration of non-thermal particles (e.g. electrons, protons) via the shock fermi acceleration mechanism, is believed to be mainly responsible for the power-law energy distribution of the observed cosmic-rays, which in very high energies can consequently radiate high energy gamma-rays and neutrinos, through related radiation channels. here, we will focus on the primary particle (hadronic) shock acceleration mechanism, and we will present a comparative simulation study of the properties of single and multiple relativistic shocks, which occur in agn jets. we will show that the role of relativistic (quasi-parallel either quasi-perpendicular) shocks, is quite important since it can dramatically alter the primary cr spectral indices and acceleration efficiencies. these properties being carried onto gamma-ray and neutrino radiation characteristics, makes the combination of them a quite appealing theme for relativistic plasma and shock acceleration physics, as well as observational cosmic-ray, gamma-ray and neutrino astronomy. keywords: active galactic nuclei jets relativistic shocks cosmic rays acceleration gamma-rays neutrinos. 1 introduction cosmic rays (crs) are subatomic particles (e.g. protons) and radiation of extra-terrestrial origin. their energies range between a few ev to hundreds of pev. these particles register a power-law spectrum, and the low-energy ones are believed to originate from galactic, while the crs with higher energies originate from extra-galactic sources. specifically, crs above energies of ∼ 1017ev, almost certainly originate from extragalactic sources such as active galactic nuclei (agn) and their relativistic jets. it is the particle shock acceleration in the superalfvenic jet plasmas, which is believed to be the main mechanism responsible for the production of the nonthermal crs (e.g. krymskii, 1977). especially, the importance of the very high energy cr acceleration is a favourable theme of study, as it is also the tev photons and neutrinos theme, which are produced either by the primary proton-photon or primary proton-nucleon interactions, offering further insights about the environment of a considered astrophysical source. agn sources are actually a compact region at the centre of a galaxy, which is believed to be a supermassive black hole. the radiation from an agn is believed to be mainly a result by the accretion of mass around the black hole. the accretion of matter around the spinning core of an agn produces twin, highly collimated fast outflows of hot gas, called jets, extending to distances of a few parsecs to thousands of parsecs. it is of importance to mention that the physics of jet mechanism and the jet composition on very small scales, are not perfectly understood at present. we know thought, that the agn black holes and their jets radiate in all wavebands, from the radio up to the gamma-ray regime via mostly the synchrotron and the inverse-compton scattering processes. moreover with a typical agn luminosity of l ∼ 1042 − 1047 erg/s, it is possible to explain the origin of the very high energy cr flux, under the assumption that these objects follow the star-formation-rate. for example, moran et al. (2001) find an average luminosity from an estimated 95 x-ray active agn within 60 mpc of 4.8 × 1044 erg/s, this distance being the cr absorption horizon at about 2×1020 ev. within the regions of local space accessible to cr diffusion, the energy supply over a hubble time is 6.6× 10−16 erg/cm3. by comparison, the gamma ray bursts supply is 6.7×10−20 erg/cm3. thus, agn as the most luminous permanent sources in the sky can stand as the most likely candidate for the acceleration of very 269 http://dx.doi.org/10.14311/app.2014.01.0269 athina meli, paolo ciarcelluti high energy crs. interpretation of data on the electron synchrotron radiation observed in the radio regime, suggests the presence of mildly-relativistic shocks with mean boost factors of γ = 10 − 30 in the jets of agn, see biermann and strittmatter (1987). moreover, assuming that hadrons (as we do in this work) are also accelerated along with the electrons in the jet, justify agn to be as good candidates as to provide at least a significant fraction of the extragalactic component of the hadronic cr flux observed. here we will discuss the primary particle shock acceleration mechanism in agn jets, in the context of comparative test-particle simulation studies, for relativistic single and multiple quasi-parallel and quasiperpendicular shocks. we will also briefly highlight the possible consequent tev-photon and neutrino emission. 2 single-relativistic shocks here we will review shock acceleration simulations, based on the relativistic single-shock, test-particle monte carlo code of meli and quenby (2003a,b), as a comparative tool to the multiple-shock acceleration study of the following section. in our simulations we inject a large number of particles n = 104 upstream towards a single planar shock, with an initial particle energy γ = γsh + 5. we use shock lorentz factors of γ = 10 and 30, and we numerically let the particles’ guiding-centers to scatter with the assumed background magnetic field irregularities, around a shock region of a length d = 2 pc. for details on the code see meli and quenby (2003a,b) and meli et al. (2008). briefly, our exemplary simulations show that : the relativistic quasi-parallel (e.g. ψ = 25o) and quasiperpendicular (e.g. ψ = 75o) single-shock acceleration mechanism manifests into a clear deviation of the cr spectral index (compared to standard, unaffected non-relativistic spectral index of ∼ 2.0) with an absent ’universal’ power-law spectrum, which depends on three parameters: i) speed of the shock, ii) shock inclination, and iii) different scattering modes, see table 1. we clearly see a high acceleration efficiency (in terms of emax) for the cases of quasi-parallel shocks. albeit, quasi-perpendicular shocks result into more ’conservative’ acceleration efficiencies and lower attained energies. it is important to note that it is been reported that the lower the turbulence, in terms of the mean-free-path η = λ/rg, the steeper the particle spectrum (meli et al. 2008). near-perpendicular shocks strongly appear to give the steepest spectra compared to quasi-parallel shock cases, and are the less efficient accelerators, see also e.g. niemec and ostrowski (2004), stecker et al. (2007), etc. summarizing, it is widely accepted that (with a few exceptions) as soon as an astrophysical shock speed becomes comparable to the speed of light, the power-law index of the accelerated cr spectra, depends strongly not only on the speed of the shock but also on the shock obliquity (scattering mode is also important, see meli, 2011), identifying clearly flatter spectra for small shock inclinations, with greater acceleration efficiencies. table 1: examples of simulations for single shocks with different shock lorentz factors (γ) and shock inclination angles (ψ), and the attained spectral indices (σ), and maximal particle energies e, respectively. γ ψ = 25o ψ = 75o 10 σ = 2.0, e = 109.3gev σ = 2.4, e = 105.6gev 30 σ = 1.9, e = 109.8gev σ = 2.3, e = 106.0gev 3 multiple-relativistic shocks after the brief insights on the relativistic single-shock acceleration above, we will now discuss a case study of particle acceleration in multiple-shocks, based on the fully relativistic multiple-shock, test-particle monte carlo code by meli and biermann (2013). in the past it was shown that for an infinite number of subsequent non-relativistic shocks with injection at each shock, the flattening of the spectrum (compared to a single shock) extends even to the highest energy particles with a momentum dependence of f(p) ∝ p−3 (e.g. white 1985). here we will investigate the acceleration mechanism in relativistic, multiple, quasi-parallel and quasiperpendicular shocks. we can envisage blobs of hot plasma ejected by the black hole of an agn that can travel along the jet and pass through a sequence of shocks, as it has been also observed in blac pks 1510089 (marscher et al. 2010) and other sources. here we numerically calculate the scattering of the particles’ guiding-center with pre-determined background magnetic field irregularities, around multipleshock regions and within a distance d = 5 pc. at the beginning of the acceleration we allow a single initial injection of particles of a fixed number n = 104, with an initial γ = γsh + 5, and shock lorentz factor γ = 30 (for the first two shocks) and γ = 10 (for the second pair), justified by the decompression-compression conditions of the downstream plasma (melrose and pope 1993) of the precedent shock pattern (i.e., a velocity compression ratio r=3). that is, during the acceleration we allow a probability of escape pesc = 0.1 between shocks due to decompression effects. the escape probability gives the fraction of particles of the downstream distribution of each shock, that will not be accelerated 270 active galactic nuclei: jets as the source of hadrons and neutrinos more. these particles remain in the system and contribute directly to the downstream distribution of the 4-th shock. this factor would naturally result in steeper spectral slopes comparing to a single shock acceleration spectrum as the relativistic conditions render the spectra flatter. assuming also a field compression ratio rbc equal to , and an assumed magnetic field strength of the order of a few mgauss, the distance between each shock is safely set as d = 1pc. we study two cases of shock patterns: i) a set four quasi-parallel shocks (with the same apex to the assumed jet axis), and inclination angles ψ1, ψ2, ψ3, ψ4 = 25o, and ii) a set of four quasi-perpendicular shocks and inclination angles ψ1, ψ2, ψ3, ψ4 = 75o. table 2: aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa γ ψ σ e[gev ] 30 25o 2.6 105.9 30 25o 2.2 106.9 10 25o 2.1 108.6 10 25o 1.5 109.5 table 3: in the tables above we see indicative simulation numbers for two sets of four-shocks, their maximal attained energies e at each shock downstream, and their spectral indices σ. shock inclination angles are denoted as ψ, and γ is the shock lorentz factor. γ ψ σ e[gev] 30 75o 2.7 104.6 30 75o 2.4 105.2 10 75o 2.3 105.9 10 75o 2.2 106.4 briefly, from the tables above, one notices that (i) the spectra by each shock in every shock-set become gradually flatter, following on one hand the findings of single relativistic shocks discussed in the previous section, and on the other hand the trends reported in e.g. melrose and pope (1993) and gieseler and jones (2000) for multiple shocks. (ii) the flatness of the spectra is more evident for the consecutive quasi-parallel shocks than for the quasi-perpendicular ones, following, as expected, the individual shock effects we discussed in the previous paragraph. (iii) the acceleration in the case of quasi-parallel shocks seems very efficient and comparable to single shock acceleration. 4 neutrino and gamma-ray astronomy over the years many models have been created (e.g. stecker, 2005), assuming protons are accelerated at the shocks of agn jets and interact with different photon fields to produce high energy gamma-rays and neutrinos. the cr acceleration cases shown here, have implications for neutrino and gamma-ray astronomy, and especially for the tev sources. we know that only from e.g. the p−γ interaction (where the optical depth must be equal or larger than 1), the consequent decay of the π0 results into a high energy photon (in 0.33 of the cases). also π± particles produce neutrinos. specifically, the probability for π+ production is 0.66. around 0.20 of the proton energy goes into pion, and the products carry equal energy, so that a fraction of 0.25 (0.50) is transferred to each neutrino (photon) and furthermore, one must also consider that neutrinos oscillate. therefore the high-energy photon and neutrino spectra can be connected as dnν deν = 1 8 · dnγ deγ . additionally it can be assumed that the neutrino carries about 1/20th of the proton’s energy (see halzen, 2008). all these can be very helpful in understanding crs and related consequent emissions. nevertheless, in many agn the photon signal at tev energies is not unique, since leptonic processes like inverse compton scattering could also contribute at the same energies. but there is a way of identifying hadronic interactions in astrophysical shocks by estimating the flux of neutrinos. not to let unmentioned, one can also assume that the spectral indices for the tev photon and neutrino emission at a break threshold energy, generally at around 102gev, can be considered to be about the same. it is accepted that not all agn sources can be seen or inferred emitting crs, and this due to either the high galactic background absorptions, or particle’s random diffusion in magnetic fields, or even insufficient power to accelerate very high crs. nevertheless, neutrino signals, could reveal the possible hadronic nature of a tev source. we saw in this work that relativistic quasiperpendicular shocks in agn, single or multiple, cannot accelerate crs to very high energies. we know also that a photon field in the jet of an agn can range from low energies to many tev. we also know (e.g. becker et al. 2011) that the proton energy necessary to produce a delta resonance in the observer’s frame, can be given as, ep ≥ γ2 (1+z)2 m2 ∆−m2 p 4 · e−1 γ , where m∆ is the mass of the delta resonance, mp is the proton-mass, eγ is the characteristic photon energy and z is the redshift of the source (e.g. cen a, z = 0.001). therefore, simplistically, one can assume a photon field (i.e. but dense enough for photo-hadronic interactions) to make 271 athina meli, paolo ciarcelluti an estimation of the maximal energy of the proton (as shown in this work) necessary to produce tev-photons and gev-tev neutrinos, or vice-versa, and make further estimations on the correlations of different wavelengths from the same source. as an example here for the case of the favourable tev emitter, cen a, with z = 0.001 and an assumed shock(s) lorentz factor of γ = 30, the proton energy sufficient to produce a delta resonance and with these, tev-photos and neutrinos, can be given as ep ≥ 105 · (1 + z)−2 · ( γ 30 )2 · ( eγ 1 mev )−1 gev . thus, for quasi-perpendicular shocks, the proton spectra found in our simulations reach low energies ep ∼ 106 gev, which could not be directly observed. in order though to achieve tev photons and along with them neutrinos, one would require photon field energies at the jet of equal or greater than eγ ≥ 10 mev. of course these energies can be produced via the inverse compton scattering of synchrotron or external photons with the accelerated electrons certainly present at the jet. therefore one would expect a high energy signal of neutrinos, if the source is of a hadronic nature. what one can in generally conclude from the above brief discussion, is that the extragalactic tev sources, which can not be observed as candidates of ultra-high energy crs, may nevertheless emit high energy neutrinos implying the hadronic nature of the cosmic rays. this fact can be used as a supplementary tool to filterout neutrino source candidates. calculations on flux and further details on radiative consequences are under away. 5 conclusions the relativistic shocks in the jets of agn are a favourable accelerator of high-energy as well as lowenergy crs, via the shock acceleration mechanism. here, we performed comparative monte-carlo simulation studies for single and multiple relativistic shocks, assuming protons as the primary accelerated particle and we showed that relativistic (quasi-parallel either quasi-perpendicular) shocks are very important since they can alter dramatically the cr spectral indices and acceleration efficiency. these properties are carried onto gamma-ray and neutrino emission from the same sources. thus, assuming a hadronic primary accelerated scenario, one could have an additional understanding regarding expected neutrino fluxes from tev sources. specifically, agn with tev gamma-ray emission, and non-observable or low-energy crs, could have the potential to carry an additional high energy neutrino flux signal, mimicking accordingly the primary accelerated particle spectra in the jet, separating it from the inverse compton leptonic or other related processes. accurate flux expectancies of gamma-rays and mostly neutrinos within earth neutrino-observatory potentials (e.g. icecube, km3net), is the next step for deeper understanding of agn jets. acknowledgement a.m. would like to thank the organisers for the invitation to give an advanced talk on the topic of shock acceleration and cosmic rays. she also likes to thank p. l. biermann, j. j. quenby and j. k. becker for contributing their valuable expertise to the cited studies above; she acknowledges fruitful discussions with antares and icecube/icetop collaborations; and she is constantly indebted to a. mastichiadis, s. dimitrakoudis, r. protheroe, a. donea, t. kneiske, m. dieckmann, a. reimer, m. ostrowski, i. brancus, j. wentz, m. giller, a. wolfendale, t. stanev, k.nishikawa for offering, in different occasions over the last years, valuable advice and insights on related studies. references [1] becker, j., meli, a. & biermann p. l., 2011 nucl. ist. meth., 630 269 doi:10.1016/j.nima.2010.06.082 [2] biermann, p. l. & strittmatter, p. a., 1987 apj, 322, 643 [3] gieseler, j. & jones, t. w., 2000, aa 357, 1133 [4] halzen, f., j., 2008 phys. conf. ser., 120 (6) 062004 doi:10.1088/1742-6596/120/6/062004 [5] krymskii, g. f., 1977 akademiia nauk sssr, 234, 1306 [6] marscher, a. p., jorstad, s. g., larionov, v. m., et al., 2010 apj, 710l, 126 doi:10.1088/2041-8205/710/2/l126 [7] meli, a. & quenby, j. j., 2003b aph 19, 649 [8] meli, a., becker, j. k., j., quenby, j. j., 2008 a&a 492, 323 [9] meli, a., 2011, astrspsc 7, 287 [10] meli, a & p. . biermann, 2013 a&a in press [11] melrose, d. b. & pope, m. h., 1993 pasau, 10, 222 [12] moran, e. c., kay, l. e., davis. m., filippenko, a. v., barth., 2001 ast.j., 566, l75 [13] niemiec, j. & ostrowski, m., 2004, apj 610, 851 doi:10.1086/421730 [14] stecker f. w., 2005 phrvd, 72j 7301 [15] stecker f. w., baring j., summerlin e. j., 2007 apj, 667l, 29 doi:10.1086/522005 [16] white r. l., 1985 apj 289, 698 discussion pieter meintjes: you mentioned that strong relativistic shocks will be accompanied by turbulence. can you comment on the role that mhd can possibly play in the 272 http://dx.doi.org/10.1016/j.nima.2010.06.082 http://dx.doi.org/10.1088/1742-6596/120/6/062004 http://dx.doi.org/10.1088/2041-8205/710/2/l126 http://dx.doi.org/10.1086/421730 http://dx.doi.org/10.1086/522005 active galactic nuclei: jets as the source of hadrons and neutrinos acceleration process? is it possible that strong mhd turbulence can distort a power-law spectrum into a kind of relativistic maxwellian as a result of acceleration through mhd waves? if this happens, under what conditions? athina meli: the numerical approach of the model presented here is that of the test-particle approach, investigating a collision-less plasma (kinetic theory) and not a collisional plasma (fluid theory). i note that shocks do exist in collisionless media, i.e. without collisional dissipation (resistivity/viscosity). let me also add though that an mhd description of a collisionless plasma it is not entirely wrong to be assumed, because all fluid equations (except closure) are exact conservation laws (and closure is not too sensitive to microphysics at large scales), but there are many other exceptions mainly when large scales are not independent of the small ones with critical limits. for the present case, the mhd turbulence effect connection can be attained at the context of assuming the strength of scattering modes directly in the code (i.e. strong or weak scattering, as large angle diffusion or pitch angle scattering), by relating it to the diffusion coefficient (κ) which is dependent on the level of media turbulence, denoted by the ratio λ/rg. please see detailed descriptions of the particle kinematics in meli and quenby (2003b), meli et al.(2008) and meli (2011). as i discussed here, the type of scattering hence the level of mhd turbulence in the media can alter the produced spectra in the relativistic shock acceleration cases. jim beall: does the spectra you get from these shocks depend on the specific shock structure? athina meli: the shock structure can alter the steepness of the shock, and i deal with this matter in another recent work (meli, 2013) (not presented here, planar shocks only). maurizion spurlo: cosmic rays with energies above 1019 ev, following the auger measurements, seem to be mainly heavy nuclei. can the acceleration model from agn jets you presented, accelerate heavy nuclei instead protons maintaining the same features? athina meli: the relativistic shocks presented here produce spectra mainly below 1019ev , favoured for protons. nevertheless, in some ideal cases one can obtain easily spectra that reach energies of 1020−21ev, as i showed in a related work. this acceleration model can treat the acceleration of heavy nuclei, but the consideration of added losses must be taken into account in the code, which will mostly steepen the spectrum by an order of magnitude or more, as some of my preliminary test-runs have shown. i note here though an important point: the spectra even above energies of 1019ev seem to be a (unknown) mix of protons and heavy nuclei, since pure heavy nuclei population seems highly improbable. 273 introduction single-relativistic shocks multiple-relativistic shocks neutrino and gamma-ray astronomy conclusions acta polytechnica ctu proceedings doi:10.14311/app.2017.9.0026 acta polytechnica ctu proceedings 9:26–31, 2017 © czech technical university in prague, 2017 available online at http://ojs.cvut.cz/ojs/index.php/app removal of diquaternary ammonium cations from as-synthesized ssz-16 zeolite tatana supinkovaa,c,∗, ivan jirkaa, jan drahokoupilb,d, jan langmaiera, vlastimil filac, libor brabeca, milan kocirika a j. heyrovsky institute of physical chemistry,czech academy of sciences, dolejskova 2155/3, prague 8, czech republic b faculty of nuclear sciences and physical engineering, czech technical university, trojanova 13, prague 2, czech republic c faculty of inorganic technologies, university of chemistry and technology in prague, technicka 5, prague 6, czech republic d institute of physics, czech academy of sciences, na slovance 1999/2, prague 8, czech republic ∗ corresponding author: tatana.supinkova@jh-inst.cas.cz abstract. zeolites are stable microporous aluminosilicates with numerous applications in chemical technology such as separation of species and catalytic transformations. our study is focused on a weakly explored zeolite ssz-16 with pore constrictions defined by 8-membered oxygen rings. key results are the preparation of et6-diquat-5 dication used as a structure directing agent (sda) and finding the optimum synthesis conditions with respect to zeolite phase purity. stability of sda was examined in conditions similar to those of autoclave synthesis (concentration, ph, temperature). moreover, the content and location of sda species in zeolite phase and conditions of sda decomposition were investigated. keywords: zeolite, hydrothermal synthesis, ssz-16, afx, et6-diquat-52+. 1. introduction zeolites and zeolite-like materials consist of tetraedral building units to4 where t are central atoms most frequently si and al+. tetraedra are linked to each other through oxygen atoms. the spaceous arrangement of tetrahedra form as a rule microporous structure where micropores take the shape of regular voids (channels and/or cavities) which are repeated through the crystals with the regularity of the crystalline structure. in zeolites, t-atoms can be replaced by isomorphous substitution, for example ge, ti, p, fe, ga, b and thus form zeolite analogues. zeolites and their analogues are crystalline materials with regular microporous structure (with pore width < 2 nm). the intracrystalline pores are thus of molecular dimensions. the surface of intracrystalline pores is formed by oxygen atoms. the critical pore width is for atoms and molecules as a rule determined by oxygen rings containing 8, 10 or 12 oxygen atoms. small cations can pass even through 6-membered oxygen rings. micropores can accommodate spherical molecules typically with the diameter of 0.3-1.0 nm or cylindrical molecules with cylinder diameter in the above range. zeolites are also called molecular sieves due to possible exclusion of molecules with size exceeding some critical value. zeolites are perspective materials for industrial applications due to their molecular sieve effect, high adsorption capacity, and frequently to the presence of catalytic sites or to their ion exchange capability. a relatively novel zeolite ssz-16 belongs to the class of small pore zeolites characterized by 8membered oxygen rings which may exhibit molecular sieve effect for gases with small molecules. the main field of its application is believed to be in gas separation of various hydrocarbons and mixtures containing molecules such as co2, h2, ch2 and other gases [1–3]. furthermore, zeolite ssz-16 can be used as microporous layers and particles to develop composite and mixed matrix materials [4–6]. zeolite ssz-16 was also tested as a catalyst. for example, cu-ssz-16 [7, 8] seems to be promising as a selective catalytic reduction catalyst for the direct conversion of no into n2 and o2. zeolite h-ssz-16 was found to be highly selective to dimethylamine in the catalytic reaction of ammonia and methanol [9]. the principal aim of this work is to optimize zeolite ssz-16 hydrothermal synthesis and contribute to answering questions concerning limits to phase purity, yield and posibility to control morphology and size of crystals. moreover, the research could clarify the role of diquaternary ammonium species in the synthesis. the principal aim of this work is to contribute to the knowledge of structure-directing agent (diquaternary ammonium dications) chemistry in relation to its (i) synthesis and purity, (ii) role in the synthesis of ssz-16 zeolite, (iii) stability in synthesis batches and (iv) its thermal removal from the zeolite framework. 26 http://dx.doi.org/10.14311/app.2017.9.0026 http://ojs.cvut.cz/ojs/index.php/app vol. 9/2017 removal of diquaternary ammonium cations from as-synthesized ssz-16 zeolite figure 1. left afx framework type representation using cbu and their arrangement (adapted from www.iza-online.org), right afx framework type representation using 6-6 sbu (adapted from ilyushin et al. 2015). figure 2. view of the afx structure along the caxis; yellow spheres t-atoms, red spheres oxygen atoms, blue patterns visualization of electron density distribution. 2. theory 2.1. zeolite ssz-16 and its structure zeolite ssz-16 is isotypic with its zeolite analogue sapo-56 [7, 10, 11]. both these microporous materials exhibit the afx framework type. this framework shows the hexagonal symmetry, the corresponding space group of symmetry is p63/mmc and the unit cell idealized parameters are a = b = 13.7 å, c = 19.7 å. fundamental articles on ssz-16 present the afx framework type in the terms of composite building units (cbu): gmelinite (gme) and aft cavities and d6r (double six-ring) cages [10, 12, 13], see figure 1 (left). this method of afx framework type representation is widely used to visualize spaces capable of accommodating sorbing species. however, this method is less suitable for the analysis of various zeolite framework topology features due to the existence of common t-atoms for the neighbouring cbu. an alternative way of afx framework representation is by secondary building units (sbu) using either an array of 6-membered rings (6 code) or double 6-membered rings (6-6 code), see figure 1 (right). figure 2 represents the view of the afx structure along the c-axis using the visualization of electron density distribution (computed by dft calculation implemented in castep program). zeolites ssz-16 and sapo-56 belong to small-pore zeolites with 3d pore system. the narrowest passages through these pores (also called bottlenecks or configure 3. 8-membered oxygen ring in sapo-56 viewed normal to [001], red spheres oxygen atoms, yellow spheres t-atoms figure 4. structure of sda hexaethylpentane diammonium dication (blue spheres nitrogen n+, grey carbon, white hydrogen). strictions) are formed by 8-membered oxygen rings. dimensions of these rings were published for sapo56 [11], see figure 3. 2.2. role of sda species in synthesis of ssz-16 essential components for zeolite synthesis are silica, alumina, source of alkali, sda and water. theoretical approaches offer a great number of hypothetical zeolite structures. however, only a small fraction of them can be synthesized. besides, a considerable number of zeolite phases can be formed only in the presence of particular organic species termed as structure-directing agents (sda). a mechanism of sda action in synthesis mixtures is still an open question. attempts were made to explain sda effect on the basis of sda species size and shape, its rigidity, hydrophobicity/hydrophilicity and sda species interactions with solution components. diquaternary ammonium species were proposed in the nineties as a novel group of sda species for the synthesis of zeolites. the research in our laboratory showed that hexaethylpentane diammonium dication (c2h5)3n+(ch2)5n+(c2h5)3 (also called as et6-diquat-52+), appears to be, among diquaternary ammonium species, the most efficient sda for the synthesis of zeolite ssz-16 [14]. the model of this sda dication is shown in figure 4. 27 t. supinkova, i. jirka, j. drahokoupil et al. acta polytechnica ctu proceedings figure 5. computer simulated location of sda molecules in uc of ssz-16 a theoretical picture of sda species in ssz-16 structure was derived using the following assumptions: • a non-decomposed sda dication can be accommodated only in the aft cavity and thus oriented with its maximum dimension along the c-axis (see figure 5). • any aft cavity accomodates just a single sda dication. • there are two aft cavities per uc. therefore, the uc contains two sda dications non-decomposed during the synthesis procedure. • in principle, the gme cavities can accomodate only some products of sda decomposition; this phenomenon is not considered in the theoretical estimate of sda content. based on these assumptions, the following formula for sda dication mass fraction (gsda)th in molar mass of uc for non-calcined zeolite ssz-16 can be formulated as (gsda)th = nsda msda (muc)0 +nsda msda , where nsda stands for the number of sda per uc and is equal to 2, msda denotes the molar mass of sda and equals to 272.32 gmol−1. (muc)0 represents the molar mass of zeolite ssz-16 free of sda and can be expressed depending on the atomic ratio si/al = z as (muc)0 = 48 mna 1 + z + 48 msi z 1 + z + 48 mal 1 + z + 96 mo, for z = 6 and (muc)0 = 3029.23 gmol−1. based on these equations, the sda dication mass fraction (gsda)th can be calculated and it equals to 0.152. the experimentally measured sda content (gsda)th in the zeolite synthesis product will be compared to this value. 3. experimental 3.1. synthesis of sda in view of the fact that the price of species containing et6-diquat-52+ dication appeared to be prohibitive, a protocol for its synthesis was developed. the reactants used for this purpose were 1,5-dibromopentane and triethylamine. the reaction proceeded in ethanol solvent. the crystallization was performed in diethyl ether solvent. because the synthesis proceeds in twostages, the purity of the synthesis product were examined by x-ray diffraction and by thermogravimetry. the heating program for sda thermogravimetry was the same as for zeolite samples calcination and zeolite thermogravimetry, see below. sda with significant content of the first stage synthesis product can thus be rejected or recycled. 3.2. synthesis of ssz-16 particles the research was directed to the development of a static in-situ hydrothermal crystallization procedure of ssz-16 zeolite [14, 15]. the synthesis was performed in three steps: preparation of starting synthesis mixture, ageing of synthesis mixture (formation of viable nuclei), and crystallization of aged mixture. the most suitable silica source appeared to be colloidal silica. the starting synthesis mixture was composed of colloidal silica ludox as 30 (30% sigma-aldrich), aluminum nitrate nonahydrate, sodium hydroxide, organic template et6-diquat-5 br2 and deionized water [14, 16]. the synthesis process was adapted by changing the synthesis solution composition, ageing period, temperature and duration of crystallization [15, 16]. the composition of synthesis solution was varied and optimized to obtain pure phase of ssz-16 particles. the ageing period was performed for seven days to promote nucleation of precursor particles (formation of viable nuclei). the ageing process of ssz-16 was performed at elevated temperature with the synthesis solutions heated up to 80 °c inside an oil bath, continuous stirring was ensured by a magnetic stirrer inside teflon vessels. the crystallization process was performed for seven days, temperature adjusted to 160 °c. the hydrothermal in-situ synthesis of ssz-16 crystalline particles performed inside teflon-lined stainless steel stationary autoclaves from the pre-aged solutions under autogenous pressure. the crystalline ssz-16 products were purified inside an ultrasonic bath, washed and dried in a pre-heated oven overnight. after the synthesis, the sda was removed by the two-cycle thermal calcination process under nitrogen/air atmosphere to open pores inside the aft cage. the temperature program for both the cycles was as follows: heating with the rate 0.5 °c/min to 120 °c, keeping at 120 °c for two hours, heating to the temperature 550 °c with the rate 0.5 °c/min, then keeping at 550 °c for 8 hours and cooling to the room temperature with the rate 0.5 °c/min. 28 vol. 9/2017 removal of diquaternary ammonium cations from as-synthesized ssz-16 zeolite 3.3. characterization zeolite ssz-16 particles were characterized by scanning electron microscopy (sem) technique using jeol jsm 5500lv to examine crystal morphology and particle size. powder x-ray diffraction (xrd) technique was used to determine crystallinity and phase purity of zeolite ssz-16 particles. x-ray diffraction was performed on panalytical x’pert pro diffractometer with co anode (wavelength λ = 0.1789 nm) in bragg-brentano geometry. the fixed divergent slits in primary beam and 1d detector x’celerator in diffracted beam were used. the spectra were collected in the 2θ range of 5-30°. the measurements and analyses of collected spectra were performed in the institute of physics, cas. the micropore volume of calcined zeolite ssz-16 particles was evaluated from nitrogen adsorption-desorption isotherms. the isotherms were measured on asap 2020 (micromeritics, usa) volumetric instrument at -196 °c. the sample was degassed under a vacuum at 350 °c for 8 h prior to the analysis. the micropore volume was evaluated by the t-plot method. particular attention was paid to sda. it concerned sda purity (xrd analysis), thermal stability and its decomposition both in synthesis solutions and products. thermal stability of sda in alkaline solutions were analyzed after the heat treatment in an autoclave. sda concentration was measured using cyclic voltammetry (cv) [17] with the home-made apparatus in j. heyrovsky institute of physical chemistry, cas. sda content in zeolite samples and sda residua after their calcination were measured using elemental analysis performed on elementar vario el iii in chns operation mode, and further on with x-ray photoelectron spectroscopy xps using esca 3 mk ii spectrometer (vg) in fixed transmission mode. the kinetics of sda removal/decomposition and related heat effects were investigated with thermogravimetry and differential scanning calorimetry using setsys evolution tga, setaram. 4. results and discussion among silica sources for synthesis mixtures, ludoxbased synthesis mixtures had the best properties both from the point of view of hydrogel density and capacity of crystallization. optimum synthesis batch composition to eliminate unwanted phases in product was determined as 3 sda: 15 na2o: 0.5 al2o3: 30 sio2: 1200 h2o. deviation from this composition causes considerable changes in phase composition and formation of different zeolitic phases. the introduction of ludox as 30 as silica source had two consequences: (i) reducing the size of amorphous core of zeolite particles which was observed in the early stages of synthesis procedure development (see figure 6a) and (ii) dramatic reducing the size of particles. the size of particles decreased to approximately 2 µm and well-developed polycrystalline particles formed (see figure 6b). this favourable development of product quality manifests itself also (i) in x-ray spectra (see figure 6. a amorphous core and crystalline shell of a particle (2 000×), b well developed polycrystalline particles (10 000×). figure 7. xrd spectra of selected samples and theoretical spectrum of ssz-16 figure 7) and (ii) in sda content measured gravimetrically from mass difference between non-calcined and calcined samples, showing on selected zeolite samples the development of (gsda)exp. in contrast to earlier preparations where amorphous cores were visible on disrupted particles and the values of (gsda)exp ranged between 0.160 and 0.180, in more recent experiments the amorphous cores were not observed and (gsda)exp ranged between between 0.140 and 0.150 approximately. it is suspected that amorphous cores exhibited a higher content of sda as compared with zeolite phase. figure 7 represents an example of the x-ray diffraction spectra of selected calcined samples compared to the theoretical diffraction spectrum for the zeolite ssz-16. the samples were confirmed to be crystalline and pure phase ssz-16 particles. regarding the content of organic species in the synthesized zeolite, a question arises whether some decomposition products of sda cannot be located in gme cavities. the evaluation of sda thermal stability in alkaline solutions performed by cyclic voltammetry showed a relatively rapid sda decomposition. the time dependence of sda concetration during its thermal treatment in alkaline solution at temperature, ph, and sda concentration close to those in synthesis mixtures is shown in figure 8. it is obvious that during the reaction time 6 to 8 hours the sda concentration decreases to a value lower than 50 % of its initial value. this may generate a considerable number of sda fragments and enhance the mass of organics incorporated in ssz-16 crystals. nevertheless, for optimized synthesis procedure the values of (gsda)exp remain in the range 0.140 to 0.150 which cast doubt on additional places of organics accommodation in ssz-16 crystals. accessibility of microporous crystal cavities after 29 t. supinkova, i. jirka, j. drahokoupil et al. acta polytechnica ctu proceedings figure 8. kinetics of et6-diquat-52+ decomposition at 160 °c. initial concentration of et6diquat-5 br2 c0 sda = 0.168mol dm−3, initial concentration of naoh c0 sda = 1.66mol dm−3, solvent deionized water. figure 9. n2 adsorption-desorption isotherm. full circles adsorption, empty circles desorption. template removal was confirmed by measurements of adsorption and desorption isotherms of n2 at -196 °c. this result is exemplified by isotherms measured on calcined sample (figure 9). the isotherm shows a sharp rise in n2 adsorption at low relative pressure and negligible hysteresis loop. its shape shows that the material is microporous with negligible effect of mesopores. the sample showed the micropore volume of 0.24 cm3 g−1. at the same time, the elemental analysis excluded presence of organic residua in crystal bulk after ssz-16 calcination. this is a contrast to the situation in crystal subsurface region where carbonaceous deposits were detected after zeolite calcination by xps. the thermogravimetric curves monitored during sda removal from zeolite samples using the above temperature program are exemplified on figure 10. temperature calibration was performed using cuso4.5h2o and sn. on the picture is the comparfigure 10. examples of tg curves percentage of the sample mass based on initial mass vs. temperature; red silicalite-1 templated with tpa+; blue ssz-16 templated with et6-diquat-52+. ison of sda removal from silicalite-1 (tpa+) with that from ssz-16 (et6-diquat-52+). there is bigger amount of organics in ssz-16 and steeper decay in the first portion of the tg curve. we suspect that it may be related to more rapid thermal decomposition of et6-diquat-52+ in synthesis batch as compared with tpa+ in silicalite-1 which is considerably more stable. we also suspect that sda fragments may be incorporated into gme cavities. 5. conclusions the principal contribution of this work is the feasibility of zeolite ssz-16 synthesis. the selected sda hexaethylpentane diammonium cation turned out to be appropriate and the synthesis route to this species was elaborated. by variation of synthesis parameters of aging and crystallization, the particle size was reduced from cca 50 µm to approximately 2 µm. moreover, the conditions to attain purity of ssz-16 higher than 98 % were found and polycrystalline samples with well developed morphology were obtained. using n2 adsorption and desorption at -196 °c, volume of micropores vmicro = 0.24 cm3 g−1 was evaluated. an insight into the chemistry of sda was presented. in particular, in its synthesis and conditions of its purity. further on, a novel technique was introduced to measure concentration variation of cationic sdas under conditions close to those at synthesis conditions. particular attention was also paid to the investigation of template removal kinetics. acknowledgements the financial support by czech science foundation via grant ga16-02681s is gratefully acknowledged. references [1] m. pera-titus et al. porous inorganic membranes for co2 capture: present and prospects. chem rev 114(2):1413–1492, 2014. doi:10.1021/cr400237k. 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[17] j. langmaier et al. extreme basicity of biguanide drugs in aqueous solutions: ion transfer voltammetry and dft calculations. j phys chem a 120:7334–7350, 2016. doi:10.1021/acs.jpca.6b04786. 31 http://dx.doi.org/10.1021/cm4015095 http://dx.doi.org/10.1016/j.seppur.2007.09.008 http://dx.doi.org/10.1039/c4ta02744b http://dx.doi.org/10.1016/j.apcatb.2010.12.022 http://dx.doi.org/10.1039/c4cc09645b http://dx.doi.org/10.1016/j.apcata.2006.02.044 http://dx.doi.org/10.1021/cm960289c http://dx.doi.org/10.1016/s1387-1811(98)00293-5 http://dx.doi.org/10.1021/cs300558x http://dx.doi.org/10.1016/s1387-1811(01)00441-3 http://dx.doi.org/10.1016/s1387-1811(03)00381-0 http://dx.doi.org/10.1016/j.micromeso.2016.03.033 http://dx.doi.org/10.1016/j.micromeso.2013.03.007 http://dx.doi.org/10.1021/acs.jpca.6b04786 acta polytechnica ctu proceedings 9:26–31, 2017 1 introduction 2 theory 2.1 zeolite ssz-16 and its structure 2.2 role of sda species in synthesis of ssz-16 3 experimental 3.1 synthesis of sda 3.2 synthesis of ssz-16 particles 3.3 characterization 4 results and discussion 5 conclusions acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2015.1.0008 acta polytechnica ctu proceedings 2:8–14, 2015 © czech technical university in prague, 2015 available online at http://ojs.cvut.cz/ojs/index.php/app on fpga based acceleration of image processing in mobile robotics petr čížek∗, jan faigl department of computer science, faculty of electrical engineering, ctu in prague, technická 2, 166 27 prague, czech republic ∗ corresponding author: petr.cizek@fel.cvut.cz abstract. in visual navigation tasks, a lack of the computational resources is one of the main limitations of micro robotic platforms to be deployed in autonomous missions. it is because the most of nowadays techniques of visual navigation relies on a detection of salient points that is computationally very demanding. in this paper, an fpga assisted acceleration of image processing is considered to overcome limitations of computational resources available on-board and to enable high processing speeds while it may lower the power consumption of the system. the paper reports on performance evaluation of the cpu–based and fpga–based implementations of a visual teach-and-repeat navigation system based on detection and tracking of the fast image salient points. the results indicate that even a computationally efficient fast algorithm can benefit from a parallel (low–cost) fpga–based implementation that has a competitive processing time but more importantly it is a more power efficient. keywords: fpga, system–on–chip, image processing, fast feature detector, visual navigation. 1. introduction one of the key indicators of the intelligent behaviour of a mobile robot is its ability to react promptly in complex situations and make fast and correct decisions regarding the robot goals. on micro-robotic platforms, this is a very comprehensive task due to limited computational resources. a typical example of micro-robotic platforms are micro aerial vehicles (mavs) [1], small legged robots [2] or robots used to study swarm intelligence [3]. they can be characterized by constrained dimensions and payload, which is directly related to the limited battery capacity. altogether, these parameters determine an available computational power on-board of the mobile robot. a direct trade-off between the computational capabilities and power consumption can be identified, characterizing the maximum time for which the robot can perform its mission. this trade-off between computationally demanding and power efficient decision-making techniques is especially recognizable in navigation algorithms that rely on computer vision methods based on processing of a large amount of visual data. salient point extractors are popular techniques of machine perception in the mobile robot navigation tasks; however, not all available approaches are currently suitable for micro-robotic platforms due to the limited on-board computational resources. moreover, a practical deployment of a mobile robot always demands a real-time performance of decision-making algorithms, which imposes further restrictions on the applicable approaches regarding the available resources. two fundamental approaches how to deal with limited computational resources on micro-robotic platforms can be considered. the first approach is a simplification of the computationally demanding processing, e.g., by introducing approximations of the demanding method and simplifying the whole principle, which can be a daunting task as it may not be always possible. the second approach is a utilization of the optimized implementations for newly available features of the modern processors, e.g., a dedicated optimization for special (typically simd– single instruction multiple data) instructions of the new cpus. besides, dedicated co-processors can be utilized to speedup the computationally demanding calculations by massively parallel processing that is available using conventional graphics cards, e.g., using cuda or opencl. although modern graphics cards are able to provide superior peak performance, they also have high power consumption requirements (in order of hundreds of watts) and thus they are not suitable for micro-robotic platforms. therefore, as an alternative solution for parallel based and power-efficient computations, it is a more suitable to consider field-programmable gate array (fpga) technology to develop a custom architecture specifically designed for the particular computational task, which, in the end, can be very power and computationally efficient while also small in dimensions and cheap in development. however, fpga-based solutions need a significantly different approach for an efficient implementation of the particular algorithms than a conventional cpu. thus, the deployment time can be high compared to a gain from the implementation of a custom design computational architecture for the fpga. in this paper, we report our results on a comparison 8 http://dx.doi.org/10.14311/app.2015.1.0008 http://ojs.cvut.cz/ojs/index.php/app vol. 2/2015 on fpga based acceleration of image processing in mobile robotics of the selected image processing techniques suitable for embedded computation of the visual navigation task implemented on a conventional on-board platform and a dedicated solution based on fpga processing of the image data. in particular, we consider a detection of the salient points in the image. the salient points are image patterns which differs from their local neighbourhood and are expected to be reliably and repeatably detectable, preferably invariant to camera viewpoint changes. such features provide a mobile robot with a limited set of environmental anchor points which are then utilized for a vision-based navigation. feature based methods consist of three stages. the first stage is a detection of features to identify salient points in the image. then, for each detected feature its descriptor is calculated to describe the local image surrounding of the feature in order to distinguish individual features. the third stage is a process of establishing feature correspondences which is called feature matching. the matching is based on a comparison of the feature descriptors to determine whether the features correspond to the same salient object already detected in the environment. regarding the computational complexity of the feature detection, we can classify the features to artificial, like blobs or patterns, and naturally occurring in the environment. for the field robotics, natural landmarks are more important; however, artificial landmarks are much easier to detect. one of the foundational feature detectors is the scale-invariant feature transform (sift) algorithm [4], which is unfortunately computationally very demanding, and therefore, a speeded-up robust features (surf) algorithm has been proposed [5] to provide salient points with less computational requirements. although surf is less demanding than sift, it can be still computationally restrictive on small platforms. that is why researchers investigate other methods how to detect and describe salient objects in the environment by a computationally efficient algorithm such as the fast feature detector [6]. fast is widely adopted by robotic community for its low computational complexity. it has been deployed in several visual navigation methods, e.g., [7, 8], and it also provides a base for different feature extractors [9, 10]. following our intention to have a power and computationally efficient image processing available on a mobile robot, we consider fast as a suitable algorithm for visual navigation task computed on-board. therefore, we compare a performance of the fast feature extraction implemented on a conventional embedded platform with a cpu based on an arm architecture and a custom implementation of the fast detector on an fpga-based dedicated co-processor. the comparison indicates that even a computationally efficient feature extraction based on the fast algorithm can benefit from parallel fpga-based implementation that has a competitive processing time figure 1. hexapod walking robot platform and it is a more power efficient. the paper is organized as follows. an overview of the most related reports on enhanced computationally and power efficient vision-based navigation approaches based on fpga co-processor is presented in section 2. the proposed evaluation of the fpga-based image processing is considered in the context of a monocular vision-based teach-and-repeat autonomous navigation [11] that is briefly described in section 4 together with the main idea of the fast feature detection and the selected brief feature descriptor [12]. results on the evaluation of the computational burden reduction using an embedded platform of the hexapod walking robot (see figure 1) and fpga–based solution are presented in section 5. concluding remarks and future work is dedicated to conclusion in section 6. 2. related work this paper is focused on fpga-based implementations of computer vision techniques that are utilized in mobile robot navigation tasks. several approaches have been published and their authors reported improvements of the computational and power efficiency in vision-based navigation tasks by a dedicated implementation on fpga. probably one of the first fpga based implementation of the surf feature extraction has been introduced in [13], where an embedded module capable of image processing suitable for mobile robotics is presented. the module implements a surf feature extraction [14] from images with the resolution of 1024×768 pixels. the reported frame rate is 15 fps, which can be considered low in a comparison to the 24 fps gpu-based implementation of surf; however, the power consumption is only 6 watts. notice, the fpga implementation outperforms a pure cpu implementation running on an intel atom dual-core processor, which is able to process only a single frame per second. another fpga-based module for stereo image processing has been recently introduced by the eth 9 petr čížek, jan faigl acta polytechnica ctu proceedings computer vision and geometry group [15]. it is based on the fpga and arm cortex a9 quad-core cpu and calculates dense disparity images with the resolution of 752×480 pixels at the 60 frames per second. the disparity estimation relies on the semi global matching approach, which is computationally a very intensive task. the reported power consumption of the module is 5 watts. in [16], the same authors extend their module by implementing a reactive based collision avoidance method for mavs and tested it in an outdoor environment. authors of [17] presented a miniature module with a low-cost fpga and mcu for visual navigation of mavs. they utilize a reduced version of the ptam algorithm [8] (mapping maximum of 200 features due to memory limitations) for an estimation of the visual odometry. their approach is based on the fpga implementation of the fast features detection and brief [12] feature description. the mcu is utilized for visual odometry calculation and the whole system operates at 30 frames per second on images with the resolution of 160×120 pixels. in [18], authors consider the fpga to synchronize data from the imu and fast feature extraction from a camera stereo pair for a robust inertial assisted realtime visual slam. the reported update rate is about 20 frames per second for images with the resolution of 752×480 pixels. similar results with a slightly different approach are presented in [19]. an embedded fpga–based computer vision module has been presented in [20]. the proposed fpga architecture uses an efficient pipelining for the fast feature detection in a video stream with the resolution of 752×480 pixels at the frame rate of 60 frames per second, which is utilized in the visual teach–and– repeat navigation method. regarding the physical dimensions of the aforementioned modules, they are all based on system–on–chip (soc) solutions on fpga development boards which (in all cases) do not exceed 12 cm × 8 cm. the power consumption of these modules is less than 10 watts while all of them exhibit a real-time performance in the particular robotic navigation task. the soc architecture combines the advantages of the fpga with a regular cpu in order to reduce deployment costs and improve performance of the system. its main aspects are described in section 3. all of the discussed approaches seem to be a suitable choice for deployment on a micro-robotic platform. however, the most promising are the approaches ([17– 20]) based on the fast feature detector [6] because this detector is widely adopted by the robotic community due to its low computational complexity. hence, it represents a base for further methods. for example, it is a source of the parallel tracking and mapping (ptam) method [8] that can be considered as a visual monocular odometry (or slam – simultaneous localization and mapping) technique, also suitable for mavs, albeit the original ptam method was designed to be used in small augmented reality workspaces. the method relies on tracking and mapping of fast features for the 6dof position estimation in the environment. fast has been recently utilized in the semi-direct monocular visual odometry [7] in which fast features are tracked and used to build a map that is further used for a visual odometry estimation. the algorithm was tested on mav equipped with the odroid-u2 arm quad-core computer achieving computational speed of 50 frames per second. despite of the cpu implementation, the reported power consumption of the system is 10 watts, which is a competitive to the one of the first fpga module for the surf detection introduced in [13]. therefore, we consider the fast features detection on the arm–based computer and fpga–based implementation [20] in this evaluation study of the potential benefits of the parallel (fpga–based) implementation of the image processing for visual navigation. 3. processor centric design a combination of the fpga and cpu allows to accelerate a computer vision application and also allows to carry out more complex tasks by an improved system. the main idea of combining these two types of computational architectures is the processor centric system–on–chip co-design, which allows to exploit benefits of both architectures. generally, a cpu is a more suitable for general purpose computations and it is also a more easily programmable. however, a parallel nature of the fpga fabric makes it well suitable for accelerating demanding or repetitive computational operations performed by the cpu. besides, it is also suitable for online signal processing and sensory interfacing [21]. there are two principal ways how to incorporate both the fpga and cpu in a single embedded design. it is possible to have both of them soldered on the printed circuit board and utilize a communication channel like spi for a fast data transmission between them. however, much more efficient is to utilize the system-on-chip (soc) solution, where the cpu is a part of the fpga chip. the cpu can be either softcore or hardcore. the fpga fabric is extremely versatile, and therefore, it can implement even a cpu in its fabric, which is called a softcore cpu. on the other hand, manufacturers started to incorporate the cpu in the design of the chip to further exploit the fpga–cpu co-design possibilities. in that case, the cpu is etched into the silicon of the chip together with the fpga fabric and such a solution is then referenced as the hardcore cpu. nowadays hardcore processors are usually based on multi-core arm architecture and they are running at the units of ghz. in the case of the softcore processors, their speed is limited to around 200 mhz due to the fpga fabric limitations. the main advantage of the both hardcore and softcore processors is that they 10 vol. 2/2015 on fpga based acceleration of image processing in mobile robotics can be directly programmed in the c programming language. in addition, they can host a real-time operating system, which makes their programming for time critical applications even simpler. from the processor point of view, the fpga fabric can act either as a memory mapped slave device or it can be directly connected to the cpu core and accelerates the processor instructions. in that case, the instruction set of the cpu is extended by custom designed instructions. the main advantage of fpga–based soc systems is their versatility and reconfigurability which may be of a particular use in the field of mobile robotics. the programmer can choose components which assemble the system according to the needs of the particular application. the fpga resources allow to build both internal computational units and io communication interfaces, which can be directly connected to the main cpu. moreover, fpga always posses a lot of general purpose io pins which can be used for any communication not exceeding maximum frequency of the given fpga chip (usually about 300∼350 mhz) regardless the communication, which can be serial like spi and i2c or parallel. in addition, specialized hardcore bus endpoints for a high-speed communication are common integral parts of nowadays fpgas, e.g., gigahertz serial endpoints like pcie, sata or parallel dram interfaces. 4. bearing-only visual navigation the navigation algorithm used for the evaluation of implementations of the fast feature detector on the cpu and fpga is based on the teach-and-repeat navigation algorithm proposed in [11]. the navigation is based on tracking previously mapped visual features that are used for a bearing-only navigation. the approach relies on the relative localization provided by the odometry, which would eventually integrate unbounded position error over time. however, the approach considers the odometry only locally for traversing a short straight line segment along which detected visual landmarks are utilized to correct the robot heading and thus suppresses the odometric error. the corrections of the heading are based on a modus of the horizontal displacements of the tentative correspondences between previously mapped and the currently perceived visual features. the modus is found by a histogram voting as it is visualized in figure 2. the tentative correspondences are established using the fast feature detector and the brief feature descriptor [12] which are briefly described in the following subsections. 4.1. features from accelerated segment test the fast feature detector [6] belongs to the family of the so called appearance based detectors because it figure 2. surfnav navigation algorithm. matched features from the current and previously learned image. the corresponding navigation histogram is depicted at the bottom. searches the image for corner-like structures. the detector is optimized with respect to the computational complexity. the algorithm uses a set of comparisons between the center pixel p and pixels defined on the 7×7 neighbourhood using the circle in the image determined by bresenham’s circle algorithm, which is shown in figure 3. figure 3. fast feature detection. courtesy of [6]. at first, the pixels on the circle are labelled dark or bright depending on their relative brightness to the central pixel. the candidate pixel is considered as a feature if there are n contiguous bright or dark pixels in the circle. for n ≥ 12 it is possible to use a rapid rejection method for a faster outlier rejection which leads to a common setting of n = 12. however, the original paper [6] approved that the best repeatability is exhibited for the setting of n = 9. the corner score, which is necessary for the non-maxima suppression, is calculated as the sum of the absolute differences between the pixels in the contiguous arc and the central pixel. the fpga architecture presented in [20] uses efficient pipelining for the fast feature detection acceleration. the image data are processed as an online stream and the architecture performs all the steps of the algorithm, i.e., the corner detection, corner score calculation, and non-maxima suppression, in parallel as data are transmitted from the image sensor. the architecture also allows to use different values of n without an impact on the computational speed or used 11 petr čížek, jan faigl acta polytechnica ctu proceedings resources. 4.2. binary robust independent elementary features once features are detected, individual salient points are characterized by descriptors. the purpose of the descriptor is to describe the image neighbourhood of the point and thus characterize the salient object of the environment. in the proposed evaluation, the selected descriptor is the brief descriptor which stands to the binary feature descriptor [12]. it is based on pairwise intensity comparisons of pixels inside an image patch surrounding the located feature. these comparisons form a set of unique binary tests which are subsequently stored into a q-dimensional bit vector. the pairwise comparisons can be chosen either randomly or evolutionary, e.g., they can be trained by methods of reinforcement learning [22], which optimize the descriptor for the particular environment. typically used values of q are 64, 128, and 256 bits that correspond to the brief8, brief16, and brief32 variants of the feature descriptor, respectively. the descriptor similarity is evaluated using the hamming distance that computes how many bits of two given feature vectors are different. 5. evaluation the main motivation behind the utilisation of fpga– based systems in visual navigation tasks is a latency reduction in the robotic navigation system which is especially recognizable in experiments performed in dynamic or confined environments, which impose imminent threats to the mobile robotic platform [1, 23] and the reduction of the computational load thus power consumption of the mobile robot platforms. this section presents the results of the experimental evaluation of the impact of the fpga–based coprocessor utilization on the reduction of the computational burden of the feature extraction process in the monocular vision–based teach–and–repeat autonomous navigation. the particular parts of the whole navigation system under the evaluation are the feature extraction chain and the computationally most demanding parts of the navigation algorithm: the feature detection and description; features matching to the map previously created in the teach mode; construction of the navigational histogram; and determination of the robot heading correction based on the maximum peak in the histogram voting method. for the feature extraction the fast feature detector [6] is used with the setting of n = 12, while 256 bit long brief [12] (brief32) is utilized as the feature descriptor. the number of features is set to approx. 200 features per image and the pre-learned map in the navigation pipeline evaluation contains 200 features. the pure cpu based implementation running on the odroid u3 board [24] was compared to the (a) . outdoor urban env. (b) . indoor lab env. figure 4. testing environments. fpga–based soc implementation on terasic de0nano board [25]. in particular, the hardware and software used in the evaluation are as follows: odroid u3 – a small embedded micro-computer suitable for robotic applications. it features 1.7 ghz arm cortex a9 quad-core microprocessor (samsung exynos4412 prime) running the ubuntu 12.04 linux operating system. the attached camera is the mobius actioncam with the resolution of 640×480 with 60 fps. the implementation of the navigation algorithm was based on the open computer vision library [26] (opencv 2.4.9) implementations of the fast feature detector and brief32 descriptor. de0-nano board – an embedded module with the altera cyclone iv ep4ce22 fpga equipped with the aptina mt9v034 grayscale camera sensor with the global shutter and the resolution of 752×480 providing images with 60 fps. the implementation of the navigation algorithm follows the approach described in [20]. it utilizes the bare-metal programmed nios iie softcore processor clocked at 150 mhz for the feature description, matching, and histogram voting and a dedicated fpga–based parallel co-processor for the fast feature detection. the proposed evaluation consists of the focused examination of the feature detector and descriptor and examination of the performance of the whole navigation system. the tests were performed in field conditions both in outdoor urban (fig. 4a) and indoor lab (fig. 4b) environments. first, we evaluated the performance of the fpga– based and cpu–based implementations of the fast feature detector and computation of the brief descriptor. two criteria are considered in this evaluation: correctness of the fpga–based implementation and computational requirements. regarding the correctness of the fpga–based implementation, it provides identical results to the cpu– based counterpart for the detected features and also the same descriptors. this test was performed on artificially generated data in order to ensure the correctness of the results. the data consists of a checkerboard pattern with grayscale gradient which allows to detect various number of corners depending on the predefined threshold. concerning the computational requirements of the 12 vol. 2/2015 on fpga based acceleration of image processing in mobile robotics feature extractor, the odroid implementation needs (in average) 17.07 ms for approximately 200 fast feature detections with brief32 descriptors in an image with the resolution of 640×480 pixels. the de0nano implementation exhibits similar performance with the processing time of 16.91 ms for 200 feature detections and descriptions. the navigation stack performance benchmarking consists of the feature extraction, feature matching against the pre-learned map, and the histogram voting method to establish the heading correction of the robot. the achieved results are summarized in table 1. platform odroid u3 de0-nano cpu cores 4 1 cpu clock 1.7 ghz 150 mhz fpga 22320∗∗ fpga usage 28% feature extraction 17.07 ms 16.91 ms navigation pipeline 35.89 ms 24.38 ms power consumption 8.13 w 1.82 w ∗∗ the number of available logic elements. table 1. navigation pipeline processing results of odroid u3 and de0-nano soc platforms 5.1. discussion the results presented in table 1 indicate that the fpga–based soc approach does not significantly reduce the required time for feature extraction; however, the power consumption is reduced by 4.5 times magnitude in comparison to the purely cpu–based implementation. regarding the update rate of individual methods the fpga–based implementation is capable to process each other image in the 60 fps camera stream, which gives about 30 processed images per second while the cpu–based implementation can process 20 frames per second if we consider a uniform image retrieval; otherwise the update rate of the cpu–based implementation is 27 fps but images are dropped at non-deterministic way which leads to uneven distribution of information in time. the fast feature detector of the de0-nano implementation performs feature detection online on the stream of camera data, which implies that the image coordinates of all salient points in the image are known during the readout of a single image from the camera sensor. the feature description and matching to the pre-learned map is performed in an instant when the feature is detected in the camera stream. however, 256 bit long brief32 descriptor calculation together with matching to approx. 200 features from pre-learned map on the 150 mhz single core cpu makes it impossible to finish all the computations during the readout of one single image. therefore, each other frame in a 60 hz camera stream is skipped, which allows the algorithm to finish all the calculations in less than 32 ms during the readout of the next image. 6. conclusion in this paper, we consider a problem of decreasing the computational load of the embedded computational resources utilized in the vision-based navigation tasks. the presented results show that although the processing times of both the fpga–based and cpu– based implementations are almost similar most-likely due to the overall low complexity of the used feature extraction, the fpga–based system–on–chip design can significantly reduce the power consumption of the embedded processor in comparison to a purely cpu–based solution. in order to further verify and quantify the results, we plan to thoroughly benchmark each part of the visual navigation algorithm stack and incorporate the fpga-based image processing in a more complex navigation task of a visual slam, where the computational power of the fpga platform can be of potential benefit. acknowledgements the presented work has been supported by the czech science foundation (gačr) under research project no. 13-18316p. references [1] n. michael, d. mellinger, q. lindsey, v. kumar. the grasp multiple micro-uav testbed. robotics automation magazine, ieee 17(3):56–65, 2010. doi:10.1109/mra.2010.937855. 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evaluation 5.1 discussion 6 conclusion acknowledgements references frascati workshop 2013 the tenth international workshop on multifrequency behaviour of high energy cosmic sources palermo, sicily, italy, may 27 – june 1, 2013 edited by franco giovannelli & lola sabau-graziati workshop on multifrequency behaviour of high energy cosmic sources palermo, sicily, italy may 27 june 1, 2013 organizing institutions istituto di astrofisica e planetologia spaziali iaps-inaf, roma, italy dpt de cargas utiles y ciencias del espacio, dcuce-inta madrid, spain e.o. hulburt center for space research, hca-nrl, washington d.c., usa max-planck institut fr extraterrestrische physik, mpe, garching, germany st. john’s college, sjc, annapolis md, usa institute für astronomie und astrophysik, karls eberhard, universität sand 1, tübingen, germany ascr astronomical institute, ondřejov, czech republic ctu czech technical university, prague, czech republic scientific organizing committee james howarth beall, hca-nrl & sjc franco giovannelli, inaf-iaps (chairperson) thomas boller, mpe lola sabau-graziati, inta-dcuce andrea santangelo, iaa, universitt tübingen rené hudec, ascr & ctu, prague, czech republic local organizing committee franco giovannelli, inaf-iaps (istituto di astrofisica e planetologia spaziali) (chairperson) daniela giovannini, cnr-ibcn (istituto di biologia cellulare e neurobiologia) paolo persi, inaf-iaps (istituto di astrofisica e planetologia spaziali) francesco reale, cnrisc (istituto dei sistemi complessi) ii foreword the workshop on multifrequency behaviour of high energy cosmic sources is a classical biennial meeting held in the same historical place for our workshops: the beautiful and wild island of vulcano. unfortunately, because of the eolian hotel is closed for repairs, this tenth edition of our workshop has been developed in palermo at the splendid hotel la torre. we are completely convinced that a deep discussion on the multifrequency behaviour of high energy cosmic sources, defined mostly by the physical processes rather than the types of objects, is the most powerful way for a better and faster development of our knowledge of the physics governing our universe. for this purpose, we invited distinguished colleagues representative the most important space– and ground–based experiments of the present generation. they painted the whole panorama on the experimental results and on their interpretation by using current models and/or developing new ideas for a better comprehension of the detected phenomena. during the discussion of a lot of new results, obtained along most of the electromagnetic spectrum, we have learned that the astrophysics of galactic sources is still extremely interesting and unavoidable. indeed it provides crucial information on the same physical processes occurring also in the extra– galactic sources, with obvious different scale factors. moreover, new results coming from very high energy γ-ray experiments have shown the richness — and its potential increase — of the sky at those energies having detected hundreds cosmic sources until few years ago completely unknown, but hopefully to be detected since the cosmic ray spectrum extends up to energies as high as ≥ 1020 ev. the fluxes of very high energy γ-rays are attenuated because of their interactions with the cosmic radio, microwave, infrared and optical radiation fields. measurements of the flux attenuation can then provide important information on the distribution of such fields. then, very high energy γ-ray astronomy is the most powerful tool for solving the problem about the origin of cosmic rays. many thanks to the authors, that accepted our reminders recalled at the beginning of the workshop, for keeping to schedule. however, in spite of the accurate respect of the dead line of a number of authors, we still suffer a delay in publishing these proceedings because of the laziness of a considerable part of other authors. in spite of this, these proceedings will appear roughly within one year from the workshop date. this is the tenth edition of the workshop (tenth if we consider the first historical one, held in vulcano in 1984), whose proceedings are published in the international czech journal ‘acta polytechnica’. we would like to thank all participants and especially the speakers for their active contributions in rendering this workshop updated with their talks, alive with their discussions, and friendly with their attitudes. particular thanks to mr francesco reale for his excellent assistance for informatics in roma, during the preparation of the workshop, and for his faultless assistance in palermo. a special thank to daniela giovannini for her help during the critical moments of the registration of participants. we are grateful to the pianist maèstra valentina usai, to the violinist maèstro alessandro perpich and to the actresses lisa colosimo and flavia giovannelli for their wonderful performance ”hic habitat felicitas: mozarterie from the kingdom behind”. iii many thanks to: the director of the istituto di astrofisica e planetologia spaziali of the istituto nazionale di astrofisica (iaps-inaf) (dr pietro ubertini); the caen s.p.a. (viareggio) who partially supported the organization of this workshop. particular thanks to the plan nacional de i+d+i of spain for partially supporting the publication of this book. finally, we would like to thank mr daniele inzerillo, the chef of the splendid hotel la torre, who delighted us with his creations during our stay and to the whole staff of the hotel for their professionalism and kindness. the editors franco giovannelli & lola sabau-graziati iv list of participants amati lorenzo , inaf–iasf, bologna, italy amati@iasfbo.inaf.it ambrosino filippo , la sapeinza university, roma, italy filippo.ambrosino@roma1.infn.it anderson gemma , university of southampton, uk gemma.anderson@hotmail.com arnaud monique , cea service d’astrophysique, gif sur yvette, saclay, france monique.arnaud@cea.fr aschenbach bernd , prv vaterstetten, germany bernd.aschenbach@t-online.de auriemma giulio , universit degli studi della basilicata, italy giulio.auriemma@cern.ch barret didier , irap, toulouse, france didier.barret@irap.omp.eu barrière nicolas , space sciences lab uc berkeley, ca, usa barriere@ssl.berkeley.edu beall james h. , ssd-nrl, washington dc, usa beall@sjca.edu behar ehud , technion. haifa, israel behar@physics.technion.ac.il beilicke matthias , washington university in st.louis, la, usa beilicke@physics.wustl.edu bisnovatyi-kogan gennady , space research institute, iki, moscow, russia gkogan@iki.rssi.ru v blay pere , gace–lpi–universidad de valencia, spain pere.blay@uv.es boller thomas , mpe, garching, germany bol@mpe.mpg.de bordas pol , iaat & isdc, tübingen, germany pol.bordas@uni-tuebingen.de buckley david , southern african large telescope, cape town, south africa dibnob@saao.ac.za castellina antonella , inaf–oato and infn, torino, italy castelli@to.infn.it castro cerón josé maŕıa , villanueva de la cañada, madrid, spain josemari@alumni.nd.edu colafrancesco sergio , school of physics wits university, johannesburg, south africa sergio.colafrancesco@wits.ac.za costamante luigi , dipartimento di fisica, università di perugia, italy luigic2011@gmail.com dar arnon , technion, israel institute of technology, haifa, israel arnon@physics.technion.ac.il de laurentis mariafelicia , dpt scienze fisiche, università ”federico ii”, napoli, italy felicia@na.infn.it de ugarte postigo antonio , iaa–csic, granada, spain adeugarte@gmai.com della valle massimo , inaf–osservatorio astronomico di capodimonte, napoli, italy dellavalle@na.astro.it di sciascio giuseppe , infn–sezione di roma tor vergata, italy disciascio@roma2.infn.it vi ebisawa ken , jaxa/isas, sagamihara, japan ebisawa@isas.jaxa.jp moshe elitzur , university of kentucky, ky, usa moshe@pa.uky.edu fargion daniele , dipartimento di fisica, università ”la sapienza” di roma, italy daniele.fargion@roma1.infn,it federici memmo , inaf–iaps, roma, italy memmo.federici@iaps.inaf.it georganopoulos markos , university of maryland, baltimore county, nasa–gsfc, baltimore, md, usa georgano@umbc.edu giovannelli franco , inaf–iaps, roma, italy franco.giovannelli@iaps.inaf.it gómez de castro ana inés , universidad complutense de madrid, spain aig@mat.ucm.es guainazzi matteo , european space agency, villanueva de la cañada, spain matteo.guainazzi@sciops.esa.int harms benjamin , university of alabama, al, usa bharms@bama.ua.edu hudec rené , astronomical institute academy of science, ondřejov & ctu prague, czech republic rene.hudec@gmail.com iglesias-groth susana , instituto de astrof́ısica de canarias, la laguna, spain sigroth@iac.es katsuda satoru , riken, wako, saitama, japan satoru.katsuda@riken.jp vii kreykenbohm ingo , dr karl remeis observatory & ecap, bamberg, germany ingo.kreykenbohm@sternwarte.uni-erlangen.de krumpe mirko , eso, garching, germany mkrumpe@ucsd.edu kundt wolfgang , argelander institute of bonn university, germany wkundt@astro.uni-bonn.de lott benoit , cenbg, gradignan, france lott@cenbg.in2p3.fr maeda keiichi , kavli ipmu, university of tokyo, japan keiichi.maeda@ipmu.jp marshall herman , mit, cambridge, ma, usa hermanm@space.mit.edu martino bruno luigi , cnr–iasi (istituto analisi dei sistemi ed informatica), roma, italy brunolmartino@gmail.com meintjes pieter , university of the free state, bloemfontein, south africa meintjpj@ufs.ac.za meli athina , university of gent, dept. of physics and astronomy, gent, belgium ameli@phys.uoa.gr merafina marco , dipartimento di fisica università ”la sapienza” di roma, italy marco.merafina@roma1.infn.it milano leopoldo , dipartimento di scienze fisiche, università ”federico ii”, napoli, italy milano@na.infn.it morihana kumiko , institute of physical and chemical reserch (riken), wako, saitama, japan morihana@crab.riken.jp viii morselli aldo , infn roma tor vergata, roma, italy aldo.morselli@roma2.infn.it motch christian , cnrs–observatoire astronomique de strasbourg, france christian.motch@unistra.fr muñoz-tuñon casiana , instituto de astrof́ısica de canarias, la laguna, spain cmt@iac.es nebot ada , observatoire astronomique de strasbourg, france ada.nebot@astro.unistra.fr nozawa takaya , kavli ipmu, university of tokyo, kashiwa, japan takaya.nozawa@ipmu.jp orlando salvatore , inaf–osservatorio astronomico di palermo, italy orlando@astropa.inaf.it panagia nino , space telescope science institute, baltimore, maryland, usa panagia@stsci.edu parisi pietro , inaf–iaps, roma, italy pietro.parisi@iaps.inaf.it persi paolo , inaf–iaps, roma, italy paolo.persi@iaps.inaf.it perucho manel , dpt d’astronomı́a i astrof́ısica, universitat de valència, spain manel.perucho@uv.es piro luigi , inaf–iaps, roma, italy luigi.piro@iaps.inaf.it pittori carlotta , inaf–oar and asdc, frascati, italy pittori@asdc.asi.it reale francesco , cnr–isc, roma, italy francesco.reale@sic.rm.cnr.it ix rebolo rafael , instituto de astrof́ısica de canarias, la laguna, spain rrl@iac.es regis marco , università di torino and infn–torino, italy regis.mrc@gmail.com reimer anita , universität innsbruck, austria anita.reimer@uibk.ac.at ribó marc , universitat de barcelona, spain mribo@am.ub.es rodŕıguez fŕıas , space and astroparticle group, uah–madrid, alcal de henares, spain dolores.frias@gmail.com rosinska dorota , kepler institute of astronomy, university of zielona gora, poland dorota@astro.ia.uz.zgora.pl rubino-martin josé alberto , instituto de astrof́ısica de canarias, la laguna, spain jalberto@iac.es sabau-graziati maŕıa dolores , inta–división de cargas utiles y ciencias del espacio, torrejón de ardoz, madrid, spain sabaumd@inta.es sanchez-portal miguel , herschel science centre, esac/european space agency, villanueva de la cañada, spain miguel.sanchez@sciops.esa.int sani eleonora , inaf-oa arcetri, firenze, italy sani@arcetri.astro.it santangelo andrea , institute of astronomy and astrophysics, kepler center, university of tübingen, germany andrea.santangelo@uni-tuebingen.de sasaki manami , institute of astronomy and astrophysics, kepler center, university of tübingen, germany sasaki@astro.uni-tuebingen.de x schartel norbert , esa/xmm–newton soc, villanueva de la cañada, madrid, spain norbert.schartel@sciops.esa.int sikora marek , pan–nicolaus copernicus astronomical center, warszawa, poland sikora@camk.edu.pl šimon vojtěch , astronomical institute academy of science, ondřejov, czech republic vojtech.simon@gmail.com van soelen brian , university of the free state, bloemfontein, south africa vansoelenb@ufs.ac.za spurio maurizio , university of bologna and infn–bologna, italy spurio@bo.infn.it stanev todor , university of delaware, newark, de, usa stanev@bartol.udel.edu tluczykont martin , university of hamburg, germany tluczykont@gmail.com tornambé amedeo , inaf–roma observatory, monteprozio catone, italy tornambe@oa-teramo.inaf.it troja eleonora , nasa–gsfc/cresst, greenbelt, md, usa eleonora.troja@nasa.gov tsuru takeshi , physics dpt, university of kyoto, japan tsuru@cr.scphys.kyoto-u.ac.jp ubertini pietro , inaf–iaps, roma, italy pietro.ubertini@iaps.inaf.it vitantoni daniele , dipartimento di fisica università ”la sapienza” di roma, italy daniele.vitantoni@roma1.infn.it walter roland , isdc, university of geneva, versoix, switzerland roland.walter@unige.ch xi yoon sung-chul , university of bonn, germany astroyoon@gmail.com zió lkowski janusz , pan–nicolaus copernicus astronomical center, warszawa, poland jz@camk.edu.pl xii acta polytechnica ctu proceedings doi:10.14311/app.2017.7.0058 acta polytechnica ctu proceedings 7:58–63, 2017 © czech technical university in prague, 2017 available online at http://ojs.cvut.cz/ojs/index.php/app experimental, numerical and analytical investigations of wind-induced net pressures for industrial buildings with envelope porosities vanessa saubke∗, rüdiger höffer ruhr-universität bochum, universitätsstraße 150, 44801 bochum, germany ∗ corresponding author: vanessa.saubke@rub.de abstract. the magnitude and the spatial distribution of wind-induced net pressures (external and internal) on buildings are frequently discussed among research communities and construction industries. this paper deals with this topic based on a case study about an industrial building in denmark, which was damaged due to the wind impact during a storm when a large part of the roof covering was blown off. in order to detect the reason for the damage the wind-induced loads were studied by i) wind tunnel experiments on the external pressures due to different wind directions, ii) analytical investigations of internal pressure due to envelope porosities and planned openings and iii) numerical analyses for the internal and the external pressure. the reynolds averaged navier-stokes (rans) method is employed to build a numerical model. the experimental, analytical and numerical results are compared with the indicated characteristic loads from the eurocode [1]. keywords: net pressure, internal pressure, industrial building, eurocode. 1. introduction the storm "christian" caused a lot of significant damages in northern europe in october 2013, including also failure of supporting structures of buildings. especially in denmark high wind velocities were measured. according to the danish meteorological service local wind peaks of 53.5 m/s were listed, which were the highest values ever measured in denmark. the wind velocity distribution for the maximum values for this day is given in figure 1 (b). the diagram in figure 1 (c) shows the data of the two measurement stations nearby the building which is considered here. the peaks of the wind velocity at 1 p.m. and at 3 p.m. are clearly visible. figure 1. (a) cardinal points (google earth [2], modified), (b) distribution of the peak wind velocity in denmark during hurricane "christian" in october 2013 (danmark meteorologiske institut [3], modified) and (c) measurements of the wind velocities at stations near to the building. at 3.15 p.m. the industrial building was damaged due to the wind impact. a large part of the roof structure breaks off and leads to an extended hole in the roof construction (see fig. 3 (a)). the debris were blown over the building and damaged also the leeward building (see fig. 2). the wind direction was west-southwest (for the cardinal points see fig. 1 (a)). figure 2. pictures from a security camera taken during the storm in 2013, the roof failure is visible. the damage started at the edge of the roof where the foil peeled off. this led to a deformation of the parapet wall (see fig. 3 (d)). approximately 7.5 % of the roof construction was completely blown off, additional 6 % of the roof foil was torn off and the trapezoidal sheets were folded and buckled (see fig. 3 (b)). in total more than 10 % of the roof construction was destroyed. futhermore this caused a damage at the sprinkler system and the leaking water destroyed a big part of the stored goods. the impact of the wind load was huge so that also the girders of the supporting roof construction were deformed (see fig. 3 (c)). 58 http://dx.doi.org/10.14311/app.2017.7.0058 http://ojs.cvut.cz/ojs/index.php/app vol. 7/2017 exp., num. and anal. investigations of wind-induced net pressures figure 3. qualitative description of typical damages: (a) partically opened roof, (b) buckled sheets, (c) bent girder and (d) deformed parapet. the considered building is located in the east part of denmark. it is situated at the west of a complex of three storage buildings of the same type. each building is about 135 m long, 55 m wide and 38 m tall. the shelfs for the stored goods are part of the supporting steel construction which consists of a regular framework of stiffening frames (see fig. 4). this main structure is covered by a sandwich-element facade. figure 4. example of a warehouse building under construction (google maps [2]). the roof consists of supporting girders, trapezoidal sheets, mineral insulating panels and a roof foil. the sheets are connected to the supporting girders by screws and the foil is fixed by spikes. for the sandwichelement facade a connection type with unsealed joints was used. after lange and rädel [4] this specific joint construction can lead to a leakage of the building walls. for the consideration of this leakage an effective joint was estimated as 3.5 mm (for investigations on the leakage of joint see kuhnhenne [5]). with this information the general porosity φ, the relation of the area of the openings to the area of the surfaces, was calculated: φ = ∑ a′∑ a = 0.0026 = 0.26%. (1) at the southern facade the gates for the delivery of the goods are located behind an antecedent flat hall. if the gates are not closed the total area of openings is 61.43 m2. 2. materials and methods 2.1. wind loads after 1991-1-4 after the eurocode external and internal pressures due to wind impact are taken into account [1]. a superposition of both leads to the net pressure. the definition of the signs of this values are given in figure 5. figure 5. three typical cases for the internal pressure (cook [6], modified). the code defines the wind load we,i as multiplication of the peak velocity pressure and a pressure coefficient: we = qp · cpe and wi = qp · cpi. (2) the peak velocity pressure qp is influenced by different factors like the terrain category, the building height and the wind zone. the external pressure coefficient cpe is influenced by the shape of a building and the internal pressure coefficient cpi by the distribution of the openings over the four faces of the building. in the following investigations pressure coefficients are considered and compared. for this purpose the external pressure coefficients for the roof of this building are determined in table 2. figure 6. distribution of the pressure coefficients for a flat roof with (a) wind direction on the short side and (b) wind direction on the long side. regarding the flow around a rectangular cube of a certain height (see hucho [7]) the code estimation of 59 vanessa saubke, rüdiger höffer acta polytechnica ctu proceedings the distribution of the pressure coefficients is given in figure 6. the distribution depends on the dimensions of the building and has to be estimated for every building and every wind direction individually due to the regulations. the cpe,1 values should be used for loaded areas of less than 1 m2 and the cpe,10 values for loaded areas of more than 10 m2 with the possibility of a logarithmic interpolatin for intermediate values. the internal pressure can be calculated according to two different regulations: (1.) for homogeneously distributed openings the pressure coefficient can be determined from a diagram by calculating the opening ratio µ, which is the relation from the area of the openings in the suction region divided by the area of all openings: µ = ∑ area of openings with cpe ≤ 0∑ area of all openings (3) (2.) a face with an area of openings that is more than twice the area of the sum of the openings in the remaining faces is considerd as a dominant face. • the internal pressure coefficient can be calculated with the following equation: cpi = 0.75 · cpe. (4) • if the area is more than three times the remaining openings then it is calculated with the following equation: cpi = 0.9 · cpe. (5) the considered building has firstly a general porosity because of the leakage due to the facade-elements. secondly it has large openings for the delivery of the stored goods at the short face where the open flat building is located. it is not known yet if openings were open or closed during the storm. therefore, three different situations for two wind directions are considered. these different cases are explained in table 1. the results for the internal pressure coefficients for these six cases are listed in the table 3. case windward face porosity delivery gates 1 x 2 short x x 3 x 4 x 5 long x x 6 x table 1. description of the six load cases. 2.2. experimental tests wind tunnel tests were performed at the boundary layer wind tunnel at ruhr-universität bochum to build up a reference data-base of wind-induced loads on the structure for specifically chosen spots of the building envelope, especially on the parts damaged during the storm event. the focus of the measurements was set on the roof construction which failed (see fig. 7 (a)). the effect of the parapet wall was neglected due to the small size in relation to the building. this leads to results on the safe side. figure 7. (a) positions of the pressure measurement points and (b) the principal set-up of the model. the tests were performed in a geometrical scale of 1:300 for different wind directions from southwest to northwest. the building was investigated as a (i) single standing, isolated building and (ii) in a group arrangement with the neighboring buildings (see fig. 7 (b) for the set-up). the evaluated cpe values represent the 78 % fractiles of the analyzed measurement data. the pressures are measured over a point-like averaging area which represents an area smaller than 1 m2 in full scale. the minimum values from the flow direction on the long face and of all measured directions for every zone are listed in table 2. figure 8. (a) the minimal cpe values of the flow direction west (on long face) and (b) of all measured flow directions. 2.3. numerical simulations for the numerical investigations a cfd method applying finite volumes and the rans method with the standard k-ε-model is used. the applied inlet velocity profile was introduced by richards and hoxey [8] in 1993 and is commonly used for this method. effects of building interferences were neglected by running all simulations with a single building. figure 9. (a) idealization of the joints and (b) idealization of the openings. 60 vol. 7/2017 exp., num. and anal. investigations of wind-induced net pressures because a simulation of all explicit openings and joints would lead to an extremely high number of volume elements the openings were idealized. the general porosity of the joints was considered with four explicit idealized joints distributed over the height (see fig. 9, left). the openings of the delivery gates were also idealized as one large opening with the same area which is located in the center of gravity of all gates (see fig. 9, right). figure 10. numerical results for the internal pressure coefficients cpi: (a) case 1, (b) case 2, (c) case 3, (d) case 4, (e) case 5 and (f) case 6. figure 11. numerical results of the external pressure coefficients of the roof for (a) flow on the short side and (b) flow on the long side. geometrical models and meshes were generated for all six cases (see table 1) with approximately three million volume elements per case. the distribution of the external pressure coefficients is pictured in figure 11. the maximum values for every zone are listed in table 2. the internal pressure coefficients for the six different cases are plotted in figure 10 and numbered in table 3. 2.4. analytical calculations an analytical investigation is performed for the estimation of the internal pressure. as cook [6] 1990 already described, the internal pressure pi depends on the external pressure pe at the openings a′. it is defined as the balance between the inflow and outflow of the building (see also fig. 5 (c)): inflow∑ ( a′ (pe − pi) 1 2 ) = outflow∑ ( a′ (pi − pe) 1 2 ) . (6) the equation cannot be solved directly but iteratively with an estimated value for the internal pressure. the left-hand and right-hand side of equation 6 are not fixed during the iteration process, because an outflow can become an inflow and vice versa. figure 12. distribution of the external pressure coefficients from a wind tunnel test for (a) the wind direction on the short face and (b) the wind direction on the long face (tpu aerodynamic database [9], modified). to calculate the internal pressure analytically a distribution of the external pressure is needed. because the performed wind tunnel tests are putting the focus on the damaged area of the roof construction, complete roof and wall distributions of external pressures have not been measured and are missing for analytical calculations of internal pressures. therefore mean values of external pressure coefficients from wind tunnel tests of a building with similar proportional dimensions from the tpu aerodynamic database [9] were used (see fig. 12). because the building from the database was smaller than the original one, the area of the openings was scaled down whereas the opening ratio µ stayed unchanged. in this scale the distributions of the pressure coefficients were evaluated and the area of the different pressure coefficients calculated. in order to maintain the comparability the same idealized opening of the delivery gates like those for the numerical investigations in section 2.3 were used. for the joints a general porosity of 0.26 % was considered. the internal pressure was calculated for all six cases as given in table 1 after cook’s method, [6]. the results are listed in table 3. 61 vanessa saubke, rüdiger höffer acta polytechnica ctu proceedings 3. results the results of the wind tunnel investigations correspond relatively good to the required values of the eurocode, for example the eurocode requires a value of cpe = 2.2 for the zone f and the wind tunnel results leads to the highest value of cpe = 2.14 considering all investigated directions. the results are compared to the cpe,1 values although the area of the measurement points in the experiments correspond to an area of smaller 1 m2 in full scale. the normative values of zone f and g suit to the minimum values of the testing results for all directions while the normative value of zone h fits better to the averaged value. but as one can see in figure 8 the values of this zone have a certain variation, they decrease in direction to the leeward side of the building. therefore, it is obvious that a design for the maximum value would not be economic, but one should be aware of the possibility that higher values can occur. zone f g h eurocode1) -2.2 -1.8 -1.2 windtunnel test (long face) -1.64 -1.34 -1.43 (-1.192)) windtunnel test (all directions) -2.14 -1.84 -1.63 (-1.292)) numerical simulation (case 1-3) -1.19 -1.11 -1.01 numercial simulation (case 4-6) -1.18 -1.13 -1.03 table 2. comparison of the external pressure coefficients cpe for the roof construction. 1)cpe,1 values in order to maintain comparability 2)averaged value of zone h the cpe values of the wind tunnel tests for the wind on the long face can be compared to the numerical results of cases 4 to 6. it should also be taken into account that the rans method does not lead to peak values and deliver average value only. the distribution of the external pressure coefficient shows a satisfactory similarity to the experimentally determined distribution from the windtunnel tests. but rans underestimates the external pressure in the separation area because the turbulence is modelled mathematically and local instationary vortices are neglected. the values of the numerical results are between 72 % to 84 % of the values of the wind tunnel test. the investigations of the internal pressure coefficients lead to different results for the six cases. the magnitude of the values are similar, in most of the cases, like case 1, 3 and 6, the eurocode overestimates the values and is on the safe side, but in case 5 it underestimates the internal pressure in comparison to the analytical solution. case 1 2 3 4 5 6 windward face short long porosity x x x x delivery gates x x x x eurocode -0.30 0.34 0.64 -0.03 -0.33 -0.83 numerical simulation -0.21 -0.12 0.52 -0.24 -0.31 -0.63 analytical calculation -0.20 0.34 0.59 -0.23 -0.48 -0.62 table 3. comparison of the internal pressure coefficients cpi for the six considered load cases. in case 2 the numerical analysis lead to a complete different value than the results of the analytical solution, which seems to fit perfectly to the value of the eurocode. in a further investigation, e.g. with the les-method, the numerical results should be checked, but it is sure, that this is connected to the position of the opening. if the position of the opening in the analytical calculation is changed to a higher position in the middle of the face (see fig. 13), the value increases from cpi = 34 to cpi = 0.45. figure 13. changed position of the opening at the windward face (tpu aerodynamic database [9], modified). also in some additional numerical investigations on a 1m x 1m x 1m cube, where the position of the openings over the height was changed, the same behaviour was determined (see fig. 14). this shows that the internal pressure reacts quite sensitive to the position of the openings, especially for openings in the windward side. figure 14. investigation on a 1m x 1m x 1m cube due to the position of the opening: (a) opening at the bottom, (b) opening in the middle and (c) opening at the top. in case 4 the eurocode diagram leads to a value of nearly zero (with µ = 0.63 in fig. 15) in contrast to the numerical and analytical solution, which leads to good corresponding results of -0.24 and -0.23. in 62 vol. 7/2017 exp., num. and anal. investigations of wind-induced net pressures the old german code near to zero values were avoided. in the range of the diagram where the value leads to zero one has to consider two values (0.2 and -0.3) on the safe side (see the red square in fig. 15 (b)). figure 15. diagram for the cpi values: (a) eurocode [1] and (b) the previous code din 1055 [10]. 4. conclusions the results show, that the regulation of the internal pressure for a dominant face is conservative for dimensioning a building (see regulation (2.) in section 2.1). the regulation for homogeneously distributed openings however seems to falsify the internal pressure (see regulation (1.) in section 2.1). as well for very homogeneously distributed openings, like a general porosity, the regulation (1.) does not work properly, like in case 4. also in case 5 the value is underestimated in comparison to the analytical value. the normative din en 1991-1-4 [1] offers the opportunity to use the fixed values of +0.2 and -0.3, if it is not possible or not justifiable to estimate a µ. these values seems to represent an underestimation of the effects of the internal pressures as one can understand from the results of the investigations for cpi ranging from +0.59 to -0.63. this apparent load underestimation through the quoted simplified code regulation does not necessarily lead to damages of the structure as it can be assumed to be based on experiences and sound practice requirements which are included into code regulations. however, such configuration can be insecure in connection with a certain systematical underestimation of external pressures after specific design regulation for trapezoidal sheets. for example, the german din 18807-3 [11] considers the higher pressures at the edge of the roof (zone f and g) just for the design of the screw connections; for the design of the sheets the high suction in these zones is neglected, which leads to an underestimation of the external pressure at the edge of the roof. together with the simplified eurocode regulation as described above this can result into a safety relevant lack of net design pressures. the research on the topic of the internal pressure should be continued to validate the results. to perform some additional wind tunnel tests on buildings with different proportions in combination with numerical simulations would be useful. for the numerical simulations some further methods should be used to improve the results. the aim would be to find a stable model that leads to good results for the different proportions so that it can be used as a tool to check and improve the regulation of the code. list of symbols a area of the faces [m2] a′ area of the openings [m2] cpe external pressure coefficient cpi internal pressure coefficient pe external pressure [n/mm2] pi internal pressure [n/mm2] we external pressure (eurocode) [n/mm2] wi internal pressure (eurocode) [n/mm2] µ opening ratio φ general porosity references [1] e. c. for standardization. en 1991-1-4 eurocode 1: actions on structures part 1-4: general actions wind actions. 2010. [2] google maps. panoramio, 2007. [3] danmark meteorologiske institut (dmi). orkan satte danmarksrekord i vind se kortene, 2013. [4] j. lange, f. rädel. die fugendichtigkeit von sandwichelementkonstruktionen wasserund luftdichtigkeit in längsfugen und fensteranschlussfugen. in festschrift zum 60. geburtstag von gerhard hanswille, heft 20. institut für konstruktiven ingenieurbau, bergische universität wuppertal, 2011. [5] m. kuhnhenne. energetische qualität von gebäudehüllen in stahl-sandwichbauweise. dissertation, rheinisch-westfälische technische hochschule aachen, fakultät für bauingenieurwesen, 2009. [6] n. j. cook. the designer’s guide to wind loading of building structures part 2: static structures. butterworths, 1990. [7] w.-h. hucho. aerodynamik der stumpfen körper. vieweg+teubner, 2nd edn., 2011. [8] p. richards, r. hoxey. appropriate boundary conditions for computational wind engineering models using the k-ε turbulence model. journal of wind engineering and industrial aerodynamics 46-47:145–153, 1993. [9] t. p. u. aerodynamic database. database of isolated low-rise building without eaves. http://www.wind.arch.t-kougei.ac.jp/info_ center/windpressure/lowrise/mainpage.html. [10] din 1055-4 action on structures part 4: wind loads. din deutsches institut für normung e.v., 2005. [11] din 18807-3 trapezoidal sheeting in buildings; steel trapezoidal sheeting; strength analysis, structural design. din deutsches institut für normung e.v., 1987. 63 http://www.wind.arch.t-kougei.ac.jp/info_center/windpressure/lowrise/mainpage.html http://www.wind.arch.t-kougei.ac.jp/info_center/windpressure/lowrise/mainpage.html acta polytechnica ctu proceedings 7:58–63, 2017 1 introduction 2 materials and methods 2.1 wind loads after 1991-1-4 2.2 experimental tests 2.3 numerical simulations 2.4 analytical calculations 3 results 4 conclusions list of symbols references acta polytechnica ctu proceedings doi:10.14311/app.2016.6.0011 acta polytechnica ctu proceedings 6:11–17, 2016 © czech technical university in prague, 2016 available online at http://ojs.cvut.cz/ojs/index.php/app using robot operating system for autonomous control of robots in eurobot, erc and robotour competitions grzegorz granosik∗, kacper andrzejczak, mateusz kujawinski, rafal bonecki, lukasz chlebowicz, błażej krysztofiak, konrad mirecki, marek gawryszewski lodz university of technology, stefanowskiego 18/22, lodz, poland ∗ corresponding author: granosik@p.lodz.pl abstract. this paper presents application of the navigation stack available in robot operating system as a basis for the autonomous control of the mobile robots developed for a few different robot competitions. we present three case studies. keywords: ros, navigation, robot, erc, eurobot. 1. introduction students from the scientific association for robotic research skaner [1] at the lodz university of technology (tul) have been competing in various international robotic contests (e.g. eurobot, robot challenge, robotour, sumo challenge, european rover challenge) for several years. they have been working in separate teams focused on robot construction, electronics and control specific for the contest. this year we have decided to join forces and use robot operating system (ros) as a common software platform to build autonomous mobile robots for various competitions. ros has been introduced in automatic control and robotics curricula at tul a few years ago in a very smooth way [2]: (1) by modeling various robots in our web-based application mymodelrobot in order to learn xml syntax and robot structure used in gazebo and ros, and (2) by utilizing rosserial and rosserial_arduino to gather sensory data and control small mobile robots. that way the interest in ros started to grow and we have learned how to use all advantages of this system effectively, and specifically: • a multitude of tools and libraries that were created around ros to rapidly prototype the high and middle level of control system for mobile robots. • its modern software architecture to easily connect different applications and devices. we could build systems where most of processes work in parallel and on different machines without building multithreading or networking procedures by ourselves. • its “thin“ ideology — in some cases programs did not need to be highly integrated with ros. • we could reuse many tools created outside this system by adding only small parts bridging them to the ros. documentation and support provided by books [3], [4], tutorials online, and ros forum [5]. • ease of debugging, visualization and logging by using several tools: rxgraph to see system‘s state graph, rxplot to plot different variables online, rviz to visualize the whole robot and its sensors readings, or to archive and replay all data. • free and open access based on straightforward licenses [6]. we could use many libraries, adapt them to our needs and compile by ourselves. the most complex problem all skaner teams have been facing is an autonomous navigation of a robot. in the further part of this paper we will present our approach to use ros to solve this problem in the case of three robots built for different contests: eurobot (robot husarz), european rover challenge (raptors rover) and robotour (quadron and raptors rover). these robots have different constructions, different kinematics and control systems. further description starts with application of navigation stack at the husarz robot then the specific adaptations to other robots are presented. 2. ros navigation stack the husarz system (composed of two robots, shown in figure 1) was developed by our skaner team for eurobot competitions. each edition of eurobot. has different main topic, in year 2016 it was beachbot and the tasks symbolized enjoying time on the beach. • the flags — robots must close doors of a shed to rise the flag. • the sea fishing — robots must catch plastic fishes from a aquarium with water and put them in a net. • the sand castle — robots must build a sand castle from several elements (cubes, cylinders, cones). • the sea shell — shells are symbolized by round discs, and they are in five configurations on the playing area. robot must take shells to the starting place. 11 http://dx.doi.org/10.14311/app.2016.6.0011 http://ojs.cvut.cz/ojs/index.php/app g. granosik, k. andrzejczak, m. kujawinski et al. acta polytechnica ctu proceedings figure 1. husarz robots at the eurobot competitions • hide in the shade — this is the funny action, after 90 seconds (the end of a match) robot must rise a beach umbrella to get extra points. each team is allowed to have up to two robots on the playing area. robots must be fully autonomous during a match (no interference with human operator) and avoid crashes with opponents (referee could give negative points for a crash). we have built two differential drive robots, the bigger one has wheels while the smaller uses tracks. they share the control system schematically shown in figure 2. we can clearly see that this control system is organized according to the ros navigation stack presented in figure 3 and in [7]. we used this approach to take advantage of the provided programs (or nodes in ros nomenclature, see figure 3): planner/navigator (move_base) and the adaptive monte carlo localization (amcl). on the other hand we had to provide exact structure of the control system and format of all topics (messages being sent between nodes). the high-level controller knows all tasks (required by competitions) and can choose which robot will perform the specific one. these tasks are provided in the script (in the form of sentences). every sentence is composed of several stages. the first one always uses the move_base node to set the robot in an appropriate position to run the task. next steps depend on the type of the task (for example: sending order with a linear velocity directly to the robot to move something on the playing area, or sending order to the manipulators to pick up something, or some combination of such orders). in the application prepared for eurobot 2016 contest we have specified four global tasks: move pads (1), close doors (2), pick up and bring the sand castle to point area (3) and catch the fish (4). tasks 1 and 2 could be performed by either robot, while 3 and 4 could be made only by the bigger one. at the beginning of a match, the big robot has got three tasks to do, the small one has one regular task and an extra task to be performed after the end of the match (opening an umbrella after 90 seconds). figure 2. control sytem of the husarz robot let‘s look closer on the sea fishing task. always in a first step robot uses move_base planner to get to the point nearby the task execution (close to the aquarium in this case). after the robot gets to the right place (with the right orientation) the main controller sends order to the low-level controller to deploy a manipulator (fishing boom). in the next step the main controller sends the value of linear velocity directly to the low-level controller (which control drivers). the main controller continuously analyzes position read from /odom topic. when the robot reaches the other end of aquarium the main controller sends order to stop, then to return (sends negative value of linear velocity). after that, the controller repeats the above operation one more to go along the wall. on the next run robot goes further to reach a net and leave fishes (appropriate order sent to the manipulator) to score points. each step has got a time limit. after reaching a time limit before completing a step, the program has to choose either to repeat the step one more time or tag this task failed and proceed to the next task. the main controller counts how many times it sent specific orders. if this value reaches the limit, the program will move to another task. the high-level controller node publishes the /move_base_simple/goal topic containing information about position and orientation of the target point in the standard format of geometry_msgs/posestamped message. this is one of the inputs of the planner — which do all hard work of finding safe path on the playing area and avoiding obstacles — and further publishes /cmd_vel topic containing reference linear and angular velocities for the mobile robot in the format: geometry_msgs/twist. this topic is subscribed by the node which realizes the low level controller and which is located on the arduino board controlling motor drivers. the low-level controller also publishes an odometry information. planer and navigator hidden in the move_base node has to be configured before use, we have to set robot‘s size, maximum and minimum angular and linear velocities, and accelerations, planner‘s frequency, map 12 vol. 6/2016 using robot operating system for autonomous control of robots figure 3. standard ros navigation stack [7]. figure 4. building the navigation map (middle) based on the geometry of playing board (up) of eurobot competitions, or based on several scans taken on the real board (down) parameters etc. if the robot has different kinematics one can use another ros planer leaving the operating principles of the navigation stack unchanged. the most important parameters in case of eurobot contest were pdist/_scale and gdist_scale: pdist_scale — the relative importance of sticking to the global path as opposed to getting to the goal. gdist_scale — the relative importance of getting to the goal rather than sticking to the global path. by trying different values we have tuned these values pdist_scale = 0.75, and gdist_scale = 1. our robot do not stick to global path but tries to get to goals in a smooth way. when pdist_scale is larger than gdist_scale the robot drives only trough global path (often stops and sets up orientation) and wastes more time (match lasts only 90 seconds). the other important information is the map which can be created directly using external sensors (e.g. laser scanner) or prepared in any graphics software. the format of /map topic is also standardized by map.pgm (portable greymap) file. in case of the eurobot competitions we had to prepare the map based on geometric data describing playing area and a few criterions. the elements on the playing board are divided into three groups: • fixed (walls), • movable untouchable (opponent robots), • movable touchable (playing elements). only fixed elements are included in the initial map, as shown in figure 4 (middle). this map is further used by simultaneous localization and mapping (slam) method. robot localizes itself and creates cost maps in every step of the control. necessary sensor data are delivered by hokuyo laser scanner in message sensor_msgs/laserscan — visible as white dots in figure 4 (down). these scans together with a laser-based map and transform messages are used by amcl node to produce pose estimates alternative to dead reckoning. laser scans are also used for obstacle avoidance in the navigator — /local_costmap. based on the map, current scans and the next target point (provided by the high-level controller), the planner is generating reference path for the robot, as shown in figure 5. the high-level controller takes care of the husarz behavior, it tracks the time of match, monitors communication between nodes and the status of the task. 13 g. granosik, k. andrzejczak, m. kujawinski et al. acta polytechnica ctu proceedings figure 5. rviz screens presenting two consecutive steps of planning robot‘s movements (red line), red arrow shows orientation of the robot required in the step we have assumed that robot can have four attempts to fulfil each task. these attempts are limited in time, because the match lasts only 90 seconds. after 4 unsuccessful tries the status of the task is switched to “fail“ and the robot moves to the next task. program selects the next task on the basis of distance to move and the remaining time. if one of the robots cannot finish the task it can be sent to the other robot if that robot would be able to run the task. in case of problems with communication the controller will wait until communication is reestablished. the main controller monitors condition of connections between ros and robots. we are using a part of standard ros messages called header. header includes sequence id (uint32 format), time stamp (time format) and frame_id (string format). for connection monitoring the most important is time stamp indicating ros time at the moment when the message was sent. the monitor compares time stamps of incoming messages. when the time stamp does not increase with real time the main controller knows that the connection with robot it lost. in this case main controller goes to stop mode. while the controller is in the stop mode it will not send messages to lower level to avoid situation when the robot loses some orders. in the stop mode the controller continue to check time stamps of incoming messages in order to detect recovery of communication. 2.1. bridging ros with labview another mobile robot requiring mobility planning and autonomous control is raptors rover (see figure 6) figure 6. the raptors rover and its control system being prepared for european rover challenge (erc). one of the tasks in this contest is navigation — intended to demonstrate rovers and teams ability of approaching locations on the field with limited or without supervision. we have to develop a system which will allow operator to navigate rover without access to visual data such as video streaming, photos and other sensor sources placed on the rover that are presenting visual information. the raptors rover was built entirely in house — it is a six wheels platform with well-known rockerbogie suspension system, wireless communication and several cameras. the robot is also equipped with exchangeable tools: 5dof manipulator with gripper and drilling sampler. the control system is organized into 4 levels (see figure 6): the operator‘s console working on a pc computer and developed in labview, the onboard main controller based on sbrio embedded system and developed also in labview, low-level controllers based on arm processors with our own software written in c++, and motor drivers. the communication system employs mainly wifi and can networks with local connections based on rs232 and spi. application of the navigation stack in this case requires the following modifications the original control system: • providing coordinates of goal points based on the 14 vol. 6/2016 using robot operating system for autonomous control of robots figure 7. raptors rover navigation system (up) nodes and topics of the ros structure (down) topological 3d map of erc field, • extending slam to 3d or providing alternative source of pose estimation insensitive to traveling on the rough terrain, • connecting ros nodes to labview based control system to seamlessly exchange control and feedback data. the first requirement can be fulfilled by changing script inside the high-level controller described in the previous chapter. to address the second requirement we have built an independent navigation system based on ros, as shown in figure 7. our system runs on multiple machines with different hardware architectures, file systems and oss but it is not a problem for ros. inertial data is provided by high quality imu with six degrees of freedom, containing gyroscope and accelerometer. the sensor‘s output is three axis angular velocity and linear acceleration. using specialized filters system computes very accurate orientation. gps module outputs position data with one meter accuracy. these data pass through the raspberry pi3 (rpi3) microcontroller and ethernet to the computer in base station that is ros master and process the most demanding calculations (this computer can be finally placed on the robot for full autonomy). wheel odometry is read from the sbrio board via wifi connection. to address the third modification correctly we need to understand that ros requires all nodes to be created in certain way and in specific programming language (usually python or c++). however, there are appliances which requires specific programming language or hardware platform, and this leads to problem with incompatibility of such ros and non — ros system. this problem is common for various robotics projects, and two bridging methods between ros and external applications were proposed: • rosserial — simple wrapper for ros messages, defigure 8. ros bridge functionality signed to allow communication between microcontrollers and ros nodes, • ros_bridge — dedicated node, which translate ros messages into standard json format, and route them to the right node. non-ros application is required to provide support for websockets and serialization of json messages, as shown in figure 7. in our application, sbrio board runs the labview application, which is not able to talk with ros nodes. however, labview includes functions allowing serialization and deserialization of messages in json format and is capable to communicate over the network. ros_bridge package was chosen as a method of communication, because of its flexibility and full support of ros message format. additionally, format of the control data in the raptors rover is different than required by navigation stack. robot has 6 independently driven wheels, the steer maneuver is obtained by differential control of the wheel speeds of the left and right side of the robot and these speeds are scaled in per cents. linear velocity is in the range 0-255 (0-100%), the angular velocity is also 0-255 (0-100%), but we do not steer directly the angle, instead we use a difference of control between the left and right side drives of the robot. due to the further coding of these control signals on the can bus (in order to send them to stm modules), we have used the following ranges of values of a linear speed: 127-128 (50%) means stop, 0-127 is reverse, and 128-255 is forward. similarly, the angular velocity of the robot or the difference between the left and the right side is zero for values of 127-128 (50%), the range 0-127 means turning left, and 128-255 means turning right. to obtain a uniform data format for input to the ros module, before sending data through the ros bridge we have to scale it in an additional vi (section of main program in labview) which produces a reference linear velocity and angular velocity — see figure 8 during tests we were using crossbow imu800ca which was directly publishing raw data over rs232. in this case we were forced to use a specialized filter, 15 g. granosik, k. andrzejczak, m. kujawinski et al. acta polytechnica ctu proceedings as mentioned, to parse the data from the raw digital sensor outputs to proper quaternion based format. we have tried two filters from the navigation stack: madgwick‘s algorithm and complementary filter. they can be found in imu_tools package. these filters are responsible for inertial data fusion and noise reduction. we have also tested the new imu unit that is xsens mti 1. it has both accelerometer and gyroscope and additional magnetometer on board. usually we would have used our node that reads data from serial port but more often ros comes with packages supporting particular hardware eq. xsens. the package we use is called xsens_driver and contains nodes for gathering data and launch file that even automatically searches for a right serial port and baudrate. for getting gps data from our omnistar 3200lr12 we use the excellent nmea_navsat_driver that plugs to the serial port directly and parses nmea sentences from gps unit into gps location form on-the-fly. the final data in a form of digital latitude and longtitude is being sent in sensor_msgs/ navsatfix via /fix topic. the only thing we had to do here was to create the launch file that indicates which sub-nodes to launch and which serial port and baudrate to choose. wheel odometry data has longer way to go. it is published from wheel encoders via can bus to the sbrio computer and then using rosbridge and json format to ros. in a specialized node it is used to calculate position in respect of rover‘s kinematics. having these three data lines, we could use it all over the distributed ros system eq. plug it to a kalman filter available in robot_localization node and connect the output topic of estimated odometry to planning structure made by our colleagues from eurobot team. 2.2. quadron robot we have built quadron robot (shown in figure 9) to compete in robotour contest. it has a carlike kinematic configuration, the base of this vehicle is an electric quad. we have introduced a number of electrical and mechanical modifications, especially in the supporting frame, the drive systems and steering mechanisms, that now can be done with electric motors. for the main drive we have used 500w dc motor, while for the steering the smaller dc gear motor and a coupler system developed in house. each engine is equipped with a driver, encoders and security features the same as used in raptors rover. the robot is further equipped with a number of sensors and modules responsible for security, localization and navigation in the field: navigation module with gps and imu (described earlier), sick lms291 lidar and ultrasonic sensors facing forward. the concept of a multilayer control required the use of computers at different levels: arm microcontroller (stm32 nucleo) at the lower, and mini size pc at the higher layer, as shown schematically in figure 10. entire system is based on ros. the low level node with the software written in c++ is interfacing motors, figure 9. quadron robot for robotour contest encoders, sensors and gamepad. it also receives a higher level commands from the pc, sends appropriate signals to the motors and data from sensors to pc. the board and the pc communicate using rosserial. basically, nucleo is connected to the computer using a serial connection and data is transmitted using usb port. the rosserial protocol can convert the standard ros messages to embedded device equivalent data types, it also implements multi-topic support. a package which enables us to run a ros node at the embedded devices such as nucleo is called the rosserial_client. on the pc side, we need some other packages to decode the serial message and convert to exact topics from the rosserial_client libraries. the recommended pc side node for handling serial data from a device is rosserial_python. for high-level layer is realized by mini pc located on the robot. it is connected to nucleo, laser scaner, imu and gps to receive sensor values which are essential for map building process using slam, obstacle avoidance and also the odometry as mandatory input to the navigational stack. we use standard ros messages inside i.e. sensor_msgs/laserscan, sensor_msgs/range, nav_msgs/odometry. we have applied the same navigation stack (as in husarz robot) and used teb_local_planner node. it supports car-like kinematics, minimizes the trajectory execution time (time-optimal objective), provides separation from obstacles and compliance with kinodynamic constraints (i.e. maximum velocities and accelerations). the base control system of quadron robot is very similar to husarz, and is shown in figure 11. most of the components are the same, the main differences are described below. we have added a node called base_controller which communicates directly with the electronics of the robot. this node finally generates the motor speed 16 vol. 6/2016 using robot operating system for autonomous control of robots figure 10. controller of the quadron robot commands for each motor (pwm). base_controller node subscribes to a topic cmd_vel, which is being published by the local planner and it converts standard message type geometry_msgs/twist (which publishes linear v and angular omega velocities) to the other type called ackermann_msgs/ackermanndrive according to the kinematic structure of the robot (see figure 12) and using classic equations for ackerman steering robots. the base_controller node incudes also an implementation of pid controller, that is why it subscribes odomerty. what is more, in the quadron robot we used additional odometry sources i.e. imu and gps to obtain precise localization, and ultrasonic sensors in order to improve obstacle avoidance. in case of any problems with high-level controller we are still able to control robot manually through gamepad which is connected directly to nucleo board over spi interface. 3. conclusions the great advantage of ros is that we can supervise and visualize our data in real time. however, plotting is available only on x86 or x64 architecture because arm processors (raspberry pi 3) do not support ros graphic libraries. in our system the external computer is just a client that has an access to topics and can run its own nodes. the only requirement is to share the same network and configure the /.bashrc file to define the master‘s and client‘s ip addresses. basically, it is the vital that we use appropriate forms of messages — specific message types and topic names to meet the nodes requirements. it is possible to remap topic names in launch files but it is somehow against ros‘s philosophy that assumes focusing on preestablished design rules. this design concept makes ros systems extremely extendable. easy and logical file figure 11. navigation structure of the quadron robot figure 12. kinematics of the quadron robot structure gives the opportunity to modify source code and add new nodes quickly. the growing ros society is still supplying the repositories with a new software that can be used immediately after committing it to github. acknowledgements research and participation in robotic contests supported by the polish ministry of science and higher education under grant no. mnisw/2016/dir/199/nn references [1] an official webpage of skaner. http://skaner.p.lodz.pl. [2] i. zubrycki, g. granosik. introducing modern robotics with ros and arduino, including case studies. journal of automation, mobile robotics & intelligent systems jamris 8(1):69–75, 2014. doi:10.14313/jamris_1-2014/9. [3] p. goebel. ros by example. lulu, 2013. [4] a. martinez, e. fernaandez. learning ros for robotics programming. packt publishing, 2013. [5] ros answers forum. http://answers.ros.org. [6] ros developers guide. http://wiki.ros.org/developersguide. [7] setup and configuration of the navigation stack on a robot. http: //wiki.ros.org/navigation/tutorials/robotsetup. 17 http://skaner.p.lodz.pl http://dx.doi.org/10.14313/jamris_1-2014/9 http://answers.ros.org http://wiki.ros.org/developersguide http://wiki.ros.org/navigation/tutorials/robotsetup http://wiki.ros.org/navigation/tutorials/robotsetup acta polytechnica ctu proceedings 6:11–17, 2016 1 introduction 2 ros navigation stack 2.1 bridging ros with labview 2.2 quadron robot 3 conclusions acknowledgements references acta polytechnica ctu proceedings doi:10.14311/ap.2016.4.0043 acta polytechnica ctu proceedings 4:43–49, 2016 © czech technical university in prague, 2016 available online at http://ojs.cvut.cz/ojs/index.php/app assessment of burnable absorber fuel design by uwb1 depletion code martin loveckýa,∗, jana jiřičkováa, radek škodaa,b a regional innovation centre for electrical engineering, university of west bohemia, univerzitní 8, 306 14 plzeň, czech republic b faculty of mechanical engineering, czech technical university, technická 4, 160 07 praha 6, czech republic ∗ corresponding author: lovecky@rice.zcu.cz abstract. uwb1 depletion code is being developed as a fast computational tool for the study of burnable absorbers in university of west bohemia in pilsen, czech republic. the research of fuel depletion aims at a development and introduction of advanced types of burnable absorbers in nuclear fuel. burnable absorbers compensate for the initial excess reactivity and consequently allow for lower power peaking factors and longer fuel cycles with higher fuel enrichments. the paper describes the depletion calculations of candu, pwr and sfr nuclear fuel doped with rare earth oxides as burnable absorber. uniform distribution of burnable absorber in the fuel is assumed. based on performed depletion calculations, rare earth oxides are divided into two groups, suitable burnable absorbers and poisoning absorbers. moreover, basic economic comparison is performed based on actual stock prices. keywords: burnable absorber, fuel depletion, monte carlo. 1. introduction uwb1 nuclear fuel depletion code [1, 4] is being developed as a fast fuel depletion code to conduct burnable absorber research. the goal of the research is to optimize new materials as burnable absorbers (ba) in nuclear fuel. bas compensate for the initial excess reactivity and consequently allow for lower power peaking factors and longer fuel cycles with higher fuel enrichments. research of advanced types of burnable absorbers (ba) in nuclear fuel requires fast depletion code that would be able to calculate broad range of elements, nuclides or their combination. state-of-art depletion codes require large amount of computational time, therefore, decision to develop fast depletion code was made. currently developed uwb1 depletion code comprises of transport and burnup solver that works in 2spc depletion scheme in order to perform fast calculation of nuclear fuel depletion. in order to achieve high precision in uwb1 code, monte carlo solver for lattice cell calculations is used to solve transport equation. bateman equations are solved in special depletion scheme that relies on nuclide-based predictorcorrector method in both transport and burnup part of fuel depletion. the paper describes the depletion calculations of candu, pwr and sfr nuclear fuel doped with rare earth oxides as burnable absorber. uniform distribution of burnable absorber in the fuel is assumed. based on performed depletion calculations, rare earth oxides are divided into two groups, suitable burnable absorbers and poisoning absorbers. moreover, basic economic comparison is performed based on current stock prices. 2. uwb1 depletion code first version of the newly developed uwb1 fast nuclear fuel depletion code [1] significantly reduced calculation time by omitting the solution step for the boltzmann transport equation. however, estimation of multiplication factor during depletion was not sufficiently calculated [2]. moreover, 1-group effective cross sections for strong absorber models like gadolinium showed disagreement between the uwb1 tested code and the serpent reference code [3]. monte carlo transport solver for uwb1 code was introduced [4] in order to improve code accuracy and remove pre-calculated case-dependent data libraries, therefore, eliminate constant effective cross section assumption. nuclear data from endf/b-vii.1 library is used for uwb1 monte carlo solver. standard raytracing algorithm for neutron random walk is a part of the solver. two-dimensional fuel pin models geometry, where fuel lattice model is assumed. the speed of the solver is higher than for the mcnp6 reference code. for light water reactor models, uwb1 monte carlo solver is on average 10 times faster. two-step predictor-corrector method (2spc) was developed for uwb1 code. the idea is to change the coupling of transport and burnup solvers by omitting major fraction of transport solver callings, because transport solver is, by orders of magnitude, slower than burnup solver. both transport and burnup variables are calculated for predicted states and corrected with more precise values as the two parts of fuel depletion are coupled. only three transport solver solutions are used, the initial fuel state and predicted and corrected states for final burnup state. effective cross sections are evaluated during fuel depletion by 43 http://dx.doi.org/10.14311/ap.2016.4.0043 http://ojs.cvut.cz/ojs/index.php/app m. lovecký, j. jiřičková, r. škoda acta polytechnica ctu proceedings assuming nuclide-based non-linear dependency. multiplication factors in the depletion steps other than the first and the last one are estimated by neutron production to absorption ratio that is calculated without the need to call the transport solver. employing 2spc depletion scheme leads to a significant reduction of calculation time by a factor of 10. with the monte carlo speed-up against the reference code, fuel depletion with uwb1 code is around 100 times faster than with mcnp6 reference code. 3. burnable absorbers in order to mark a material as a good burnable absorber, two properties are desired. these are a high neutron absorption cross section with low daughter nuclide’s neutron absorption cross section. high neutron capture cross section causes neutrons to be absorbed, therefore, multiplication factor of the fuel is decreased and initial excess of reactivity is compensated. the absorber is burnable if the nuclide resulting from neutron absorption has a lower neutron absorption cross section, therefore, reactivity worth of inserting burnable absorber is decreasing during fuel depletion. the higher the differences between cross sections, the faster the burnable absorber will be burned. typical absorption reaction for burnable absorbers is (n,γ), although another reactions are possible, e.g. (n,α) for light elements like boron. present nuclear fuels use mainly gadolinium, europium and erbium oxides. gadolinium oxide is burned faster than the two others and because of the very high absorption cross section of gd-157 nuclide, initial reactivity compensation for gadolinium higher than for other materials. gadolinium nuclides with interesting burnable properties are gd-155 and gd-157 with natural abundances 14.80 at% and 15.65 at%. europium has natural abundance of 47.81 at% eu-151 and 52.19 at% eu-153. both europium nuclides behaves as a good burnable absorber nuclide, both nuclides have have high neutron capture cross sections with end-product nuclide characterized by low capture cross section. erbium nuclides with interesting burnable properties are er-166 and er-167 with natural abundances 33.50 at% and 22.87 at%. total cross section for thermal energy 0.0253 ev are: 60 801 barns for gd-155, 253 929 barns for gd-157, 9190 barns for eu-151, 367 barns for eu153, 31 barns for er-166 and 652 barns for er-167. 4. rare earth oxides burnable absorber research will be conducted in two phases. the first phase will be represented by parametric study of various types of ba materials. the uwb1 depletion code will be used in order to calculate a large number of cases, employing its fast calculation scheme. promising materials will be used in the second phase for a deeper neutronic analysis with state-of-art codes, possible discrepancies or inaccuracies of the uwb1 code will be removed. the output of the research is to sort materials by their suitability as ba from neutronics point of view. after that, another types of analyses, like chemical compatibility, thermal conductivity, economic evaluation and other relevant areas will be performed. the paper focuses only on rare earth oxides as burnable absorbers. these are often used or planned to use as ba in pwr nuclear fuel. the most common is gadolinium oxide, followed by europium and erbium oxides. rare earth oxides were briefly analyzed in [5] for vver nuclear fuel, in this paper, pwr, candu and sfr nuclear fuels are compared. rare earth elements are available as metals or oxides. thermal reactors and some of proposed fast reactors use uranium oxide fuel, therefore, rare earth oxides were evaluated. seventeen rare earth elements are abundant in nature (scandium, yttrium and 15 lanthanides from lanthanum to lutetium). most of them are available in oxides with x2o3 stechiometry, in order to introduce the same normalization for all elements, masses of all other rare earth cases were recalculated to have x2o3 stechiometry. 5. calculation cases candu, pwr and sfr nuclear fuel depletion was calculated. of seventeen available rare earth oxides, 16 were calculated. promethium, having only radioactive nuclides, was excluded from the study. it the first part of the study, criticality calculations with uwb1 code were performed in order to determine rare earth oxides content in the fuel (wt% x2o3). the target multiplication factor was selected to be around 0.05 lower than for the fresh fuel without ba, namely 1.10 for candu fuel, 1.25 for pwr fuel and 1.15 for sfr fuel (fresh fuel multiplication factors are 1.15, 1.32, 1.20). in the second part, fuel depletion with uwb1 code were calculated and multiplication factor dependency on fuel burnup was analyzed. finally, economic comparison was performed. uwb1 code requires simple fuel cell geometry. recently, the option to include arbitrary number of concentric cylinders with either square or triangular lattice was included. candu fuel was homogenized and modeled as 18 concentric cylinders that represents four fuel rings of the total number of 37 pins. natural uranium oxide fuel with final burnup 10 000mwd/mtu was assumed. fuel depletion was divided into 31 steps with constant power 32.4364mw/mtu. standard candu materials were used in the model – zircaloy-4 for fuel cladding, zr2.5nb alloy for pressure tube that is divided by co2 gap from zircaloy-2 calandria tube, heavy water with density 0.8179 g/cm3 and purity 99.11 wt% as coolant in the pressure tube and heavy water with density 1.086 99 g/cm3 and purity 99.97 wt% as moderator between calandria tube. pwr nuclear fuel was modeled as 5.0 wt% u-235 enriched uranium oxide with zircaloy-4 cladding and light water moderator with 600 ppm boric acid. fuel radius 0.4025 cm and cladding radius 0.4750 cm was 44 vol. 4/2016 assessment of burnable absorber fuel design by uwb1 depletion code used, lattice half pitch 0.6295 cm was assumed as typical pwr dimensions. the fuel was depleted with power 40.0mw/mtu in 34 steps up to final burnup 50 000mwd/mtu. sfr nuclear fuel was modeled as 15.0 wt% u-235 enriched uranium oxide with 7.9 g/cm3 iron cladding and 0.88 g/cm3 sodium coolant. fuel radius 0.28 cm and cladding radius 0.33 cm was used, lattice half pitch 0.42 cm were used as typical sfr dimensions (bn-800 reactor was assumed as typical sfr reactor). the fuel was depleted with power 130.0mw/mtu in 74 steps up to final burnup 150 000mwd/mtu, three times higher than of pwr fuel. 6. criticality results candu, pwr and sfr nuclear fuel criticality calculation results from uwb1 code are presented in fig. 1 to fig. 3. the figures show dependency of multiplication factor for fresh fuel with various ba content (mass fraction) on logarithm scale. 1 e 0 5 1 e 0 4 1 e 0 3 1 e 0 2 1 e 0 1 1 e + 0 0 0 . 0 0 . 2 0 . 4 0 . 6 0 . 8 1 . 0 1 . 2 mu ltip lica tio n f ac tor (-) b a w e i g h t c o n t e n t ( ) 2 1 s c 3 9 y 5 7 l a 5 8 c e 5 9 p r 6 0 n d 6 2 s m 6 3 e u 6 4 g d 6 5 t b 6 6 d y 6 7 h o 6 8 e r 6 9 t m 7 0 y b 7 1 l u c a n d u : m u l t i p l i c a t i o n f a c t o r v s b a c o n t e n t figure 1. candu multiplication factor vs ba content. 1 e 0 5 1 e 0 4 1 e 0 3 1 e 0 2 1 e 0 1 1 e + 0 0 0 . 0 0 . 2 0 . 4 0 . 6 0 . 8 1 . 0 1 . 2 1 . 4 mu ltip lica tio n f ac tor (-) b a w e i g h t c o n t e n t ( ) 2 1 s c 3 9 y 5 7 l a 5 8 c e 5 9 p r 6 0 n d 6 2 s m 6 3 e u 6 4 g d 6 5 t b 6 6 d y 6 7 h o 6 8 e r 6 9 t m 7 0 y b 7 1 l u p w r : m u l t i p l i c a t i o n f a c t o r v s b a c o n t e n t figure 2. pwr multiplication factor vs ba content. multiplication factor reduction from ba insertion into the fuel is higher for candu fuel that needs only a small ba content to achieve target multiplication 1 e 0 5 1 e 0 4 1 e 0 3 1 e 0 2 1 e 0 1 1 e + 0 0 0 . 2 0 . 4 0 . 6 0 . 8 1 . 0 1 . 2 mu ltip lica tio n f ac tor (-) b a w e i g h t c o n t e n t ( ) 2 1 s c 3 9 y 5 7 l a 5 8 c e 5 9 p r 6 0 n d 6 2 s m 6 3 e u 6 4 g d 6 5 t b 6 6 d y 6 7 h o 6 8 e r 6 9 t m 7 0 y b 7 1 l u s f r : m u l t i p l i c a t i o n f a c t o r v s b a c o n t e n t figure 3. sfr multiplication factor vs ba content. factor. pwr fuel is similar to candu, however, due to higher fuel enrichment, more ba content is needed. lastly, sfr with fast neutron spectra and high fuel enrichment requires considerably higher ba content than previous fuel types in order to achieve desirable initial reactivity compensation. based on performed criticality calculations and selected target multiplication factor values 1.10 for candu fuel, 1.25 for pwr fuel and 1.15 for sfr fuel, ba weight fraction was calculated. linear interpolation between selected ba contents (3 values for each logarithm order) was used. calculated ba content is shown in fig. 4 to fig. 6. 2 1 s c 3 9 y 5 7 l a 5 8 c e 5 9 p r 6 0 n d 6 2 s m 6 3 e u 6 4 g d 6 5 t b 6 6 d y 6 7 h o 6 8 e r 6 9 t m 7 0 y b 7 1 l u 1 e 0 6 1 e 0 5 1 e 0 4 1 e 0 3 1 e 0 2 1 e 0 1 1 e + 0 0 ba ox ide we igh t fr ac tio n ( -) b u r n a b l e a b s o r b e r t y p e ( ) c a n d u : b u r n a b l e a b s o r b e r w e i g h t f r a c t i o n figure 4. candu burnable absorber weight fraction. in the case of both candu and pwr fuel, even maximum considered ba content (50 wt% x2o3) was not enough for dropping the multiplication factor to the target one. for pwr fuel, 58-ce content was also too high. for sfr fuel, it was possible to achieve desired target multiplication factor for all rare earth oxides ba. 45 m. lovecký, j. jiřičková, r. škoda acta polytechnica ctu proceedings 2 1 s c 3 9 y 5 7 l a 5 8 c e 5 9 p r 6 0 n d 6 2 s m 6 3 e u 6 4 g d 6 5 t b 6 6 d y 6 7 h o 6 8 e r 6 9 t m 7 0 y b 7 1 l u 1 e 0 5 1 e 0 4 1 e 0 3 1 e 0 2 1 e 0 1 1 e + 0 0 ba ox ide we igh t fr ac tio n ( -) b u r n a b l e a b s o r b e r t y p e ( ) p w r : b u r n a b l e a b s o r b e r w e i g h t f r a c t i o n figure 5. pwr burnable absorber weight fraction. 2 1 s c 3 9 y 5 7 l a 5 8 c e 5 9 p r 6 0 n d 6 2 s m 6 3 e u 6 4 g d 6 5 t b 6 6 d y 6 7 h o 6 8 e r 6 9 t m 7 0 y b 7 1 l u 1 e 0 3 1 e 0 2 1 e 0 1 1 e + 0 0 ba ox ide we igh t fr ac tio n ( -) b u r n a b l e a b s o r b e r t y p e ( ) s f r : b u r n a b l e a b s o r b e r w e i g h t f r a c t i o n figure 6. sfr burnable absorber weight fraction. minimum ba content was calculated for gadolinium oxide for candu and pwr fuel (7.52 × 10−6 and 1.02 × 10−4 weight fraction) and for europium oxide for sfr fuel (8.67 × 10−3 weight fraction). for most of rare earth oxides, candu and pwr fuel requires ba content from 0.1 wt% x2o3 to 1.0 wt% x2o3. for sfr fuel, required ba content mostly varies from 1.0 wt% x2o3 to 10.0 wt% x2o3, i.e. 10 times higher than of thermal spectra fuels. maximum ba content was calculated for 58-ce. for candu fuel, 26.8 wt% ce2o3 was determined. slightly lower conten of 20.4 wt% ce2o3 was calculated for sfr fuel. pwr fuel required more than 50.0 wt% ce2o3 and cerium oxide was excluded, making 39-y the case of maximum ba content for pwr fuel (40.5 wt% y2o3). 7. depletion results candu, pwr and sfr nuclear fuel criticality calculation results from uwb1 code are presented in fig. 7 to fig. 9. 0 2 0 0 0 4 0 0 0 6 0 0 0 8 0 0 0 1 0 0 0 0 0 . 9 0 0 . 9 5 1 . 0 0 1 . 0 5 1 . 1 0 1 . 1 5 mu ltip lica tio n f ac tor (-) b u r n u p ( m w d / m t u ) n o _ b a 6 0 n d 6 7 h o 2 1 s c 6 2 s m 6 8 e r 3 9 y 6 3 e u 6 9 t m 5 7 l a 6 4 g d 7 0 y b 5 8 c e 6 5 t b 7 1 l u 5 9 p r 6 6 d y c a n d u : f u e l d e p l e t i o n w i t h b a figure 7. candu multiplication factor during fuel depletion. candu figure shows that from macroscopic view, only 63-eu and 64-gd compensate initial reactivity in the way that initial reactivity transient due to fission product build-up almost disappears. for other ba, initial reactivity is compensated, however, initial reactivity transient is still presented. for good burnable absorbers, residual poisoning (decrease of multiplication factor compared to no ba case) is negligible, other ba have residual poisoning that, for some cases, is even higher than initial reactivity compensation. 0 1 0 0 0 0 2 0 0 0 0 3 0 0 0 0 4 0 0 0 0 5 0 0 0 0 0 . 8 5 0 . 9 0 0 . 9 5 1 . 0 0 1 . 0 5 1 . 1 0 1 . 1 5 1 . 2 0 1 . 2 5 1 . 3 0 1 . 3 5 mu ltip lica tio n f ac tor (-) b u r n u p ( m w d / m t u ) n o _ b a 6 0 n d 6 7 h o 2 1 s c 6 2 s m 6 8 e r 3 9 y 6 3 e u 6 9 t m 5 7 l a 6 4 g d 7 0 y b 5 8 c e 6 5 t b 7 1 l u 5 9 p r 6 6 d y p w r : f u e l d e p l e t i o n w i t h b a figure 8. pwr multiplication factor during fuel depletion. for pwr fuel, similar conclusions as for candu fuel can be deduced. only 63-eu and 64-gd oxides are able to change the shape of multiplication factor dependency on burnup, i.e. remove initial reactivity transient. however, for pwr fuel, initial reactivity transient is not only removed, but the effect is reversed, 46 vol. 4/2016 assessment of burnable absorber fuel design by uwb1 depletion code multiplication factor increases in the first days of fuel operation. for sfr fuel, all rare earth oxides show similar behavior, burning of the oxides is very slow and reactivity compensation remains relatively constant, therefore, rare earth oxides behaves mostly as non-burnable absorbers. 0 2 5 0 0 0 5 0 0 0 0 7 5 0 0 0 1 0 0 0 0 0 1 2 5 0 0 0 1 5 0 0 0 0 0 . 9 0 0 . 9 5 1 . 0 0 1 . 0 5 1 . 1 0 1 . 1 5 1 . 2 0 mu ltip lica tio n f ac tor (-) b u r n u p ( m w d / m t u ) n o _ b a 6 0 n d 6 7 h o 2 1 s c 6 2 s m 6 8 e r 3 9 y 6 3 e u 6 9 t m 5 7 l a 6 4 g d 7 0 y b 5 8 c e 6 5 t b 7 1 l u 5 9 p r 6 6 d y s f r : f u e l d e p l e t i o n w i t h b a figure 9. sfr multiplication factor during fuel depletion. based on performed depletion calculations and multiplication factor progress during fuel depletion, ba worth was calculated. ba worth is defined as reactivity differences between two cases, the one with ba in the fuel and the one without ba. results are depicted in fig. 10 to fig. 12. 0 2 0 0 0 4 0 0 0 6 0 0 0 8 0 0 0 1 0 0 0 0 8 0 0 0 6 0 0 0 4 0 0 0 2 0 0 0 0 ba wo rth (p cm ) b u r n u p ( m w d / m t u ) n o _ b a 6 0 n d 6 7 h o 2 1 s c 6 2 s m 6 8 e r 3 9 y 6 3 e u 6 9 t m 5 7 l a 6 4 g d 7 0 y b 5 8 c e 6 5 t b 7 1 l u 5 9 p r 6 6 d y c a n d u : b a w o r t h figure 10. candu burnable absorber worth during fuel depletion. from ba worth curve variation during fuel depletion, it can be stated which rare earth oxides behaves like more or less suitable burnable absorbers and which behaves like poisoning absorbers. the latter group can be divided into constant poisoning absorbers (ba worth is constant function of burnup) and increasingly poisoning absorbers (ba worth is decrasing function of decrasing function of burnupburnup, i.e. diverges 0 1 0 0 0 0 2 0 0 0 0 3 0 0 0 0 4 0 0 0 0 5 0 0 0 0 1 2 0 0 0 1 0 0 0 0 8 0 0 0 6 0 0 0 4 0 0 0 2 0 0 0 0 ba wo rth (p cm ) b u r n u p ( m w d / m t u ) n o _ b a 6 0 n d 6 7 h o 2 1 s c 6 2 s m 6 8 e r 3 9 y 6 3 e u 6 9 t m 5 7 l a 6 4 g d 7 0 y b 5 8 c e 6 5 t b 7 1 l u 5 9 p r 6 6 d y p w r : b a w o r t h figure 11. pwr burnable absorber worth during fuel depletion. 0 1 0 0 0 0 2 0 0 0 0 3 0 0 0 0 4 0 0 0 0 5 0 0 0 0 5 0 0 0 4 0 0 0 3 0 0 0 2 0 0 0 1 0 0 0 0 ba wo rth (p cm ) b u r n u p ( m w d / m t u ) n o _ b a 6 0 n d 6 7 h o 2 1 s c 6 2 s m 6 8 e r 3 9 y 6 3 e u 6 9 t m 5 7 l a 6 4 g d 7 0 y b 5 8 c e 6 5 t b 7 1 l u 5 9 p r 6 6 d y s f r : b a w o r t h figure 12. sfr burnable absorber worth during fuel depletion. from zero reactivity worth, rather than converging behaviour of burnable absorbers). because of the fact that ba worth curve does not change its derivation sign (constant, increasing or decreasing function), it is possible to define ratio of eoc to boc reactivity worth and compare it to unity, see final results in fig. 13. candu and pwr fuel behaves similarly, burnable absorbers are more depleted for final burnup in the case of candu fuel, therefore, residual poisoning is lower for candu and burnable absorbers are more suited for candu fuel. total number of 9 out of 16 rare earth oxides are marked as burnable absorbers for both candu and pwr fuels, namely 21-sc, 60-nd, 62-sm, 63-eu, 64gd, 66-dy, 67-ho, 68-er and 69-tm. total number of 4 out of 16 rare earth oxides are marked as burnable absorbers only for candu fuel, but behaves as poisoning absorbers for pwr fuel. these are 39-y, 57-la, 59-pr and 71-lu. the rest 3 out of 16 rare earth oxides, 65-tb, 58-ce and 70-yb, are poisoning absorbers for both candu and pwr fuel. 47 m. lovecký, j. jiřičková, r. škoda acta polytechnica ctu proceedings 2 1 s c 3 9 y 5 7 l a 5 9 p r 6 0 n d 6 2 s m 6 3 e u 6 4 g d 6 5 t b 6 6 d y 6 7 h o 6 8 e r 6 9 t m 7 1 l u 0 . 0 0 . 5 1 . 0 1 . 5 2 . 0 ba wo rth eo c / ba wo rth bo c ( -) b a e l e m e n t ( ) c a n d u p w r s f r b a w o r t h r a t i o b e t w e e n e o c a n d b o c figure 13. rare earth oxides as ba eoc/boc worth ratio. situation for sfr fuel is simpler than for both thermal spectra fuels. only 2 out of 16 rare earth oxides, 66-dy and 69-tm, are suitable as burnable absorbers. thulium oxide is a far better ba for sfr than dysprosium oxide that burns very slowly. however, significant fraction of thulium oxide remains in the fuel at eoc. 8. economic comparison economic comparison of rare earth oxides as burnable absorbers is based on prices from early 2016 [6], however, rare earth oxides are mined mainly in one country and prices are volatile. table 1 (or fig. 14) summarizes the results. input oxides prices (rmb/mt) were converted to match x2o3 stechiometry and multiplied by target ba content (weight fraction) to evaluate oxides prices for unit uranium mass (rmb/mtu). total fuel price comprises of ba price, uranium price and other factors, only ba price is compared in this study. element price (rmb/mt) 39-y 34 000 57-la 13 250 58-ce 13 900 59-pr 429 000 60-nd 322 500 62-sm 17 000 63-eu 1900 64-gd 87 500 65-tb 4300 66-dy 1915 68-er 265 000 table 1. rare earth oxides as ba – economic comparison. 3 9 y 5 7 l a 5 9 p r 6 0 n d 6 2 s m 6 3 e u 6 4 g d 6 5 t b 6 6 d y 6 8 e r 1 e 0 1 1 e + 0 0 1 e + 0 1 1 e + 0 2 1 e + 0 3 1 e + 0 4 1 e + 0 5 ba pr ice (r mb /m tu ) b a e l e m e n t ( ) c a n d u p w r s f r r a r e e a r t h o x i d e p r i c e s a s b u r n a b l e a b s o r b e r s figure 14. rare earth oxides as ba – economic comparison. for the best burnable absorbers, the economic comparison showed the price of rare earth oxide in mtu vary from 1rmb to 1000rmb for candu and pwr fuels and approximately 100 times higher prices for sfr fuels. 9. conclusions uwb1 depletion code was used to assess burnable absorber design in the nuclear fuel. candu, pwr and sfr fuel lattice criticality and depletion calculations were performed for rare earth oxides as burnable absorbers uniformly distributed in the fuel. rare earth oxides content needed to achieve target initial reactivity compensation was evaluated. based on multiplication factor behavior during fuel depletion and ba reactivity worth ratio for final burnup and inital state, rare earth oxides were divided into two groups, burnable absorbers and poisoning absorbers. candu and pwr fuel behaves similarly. total number of 9 out of 16 rare earth oxides are marked as burnable absorbers for both candu and pwr fuels, namely 21-sc, 60-nd, 62-sm, 63-eu, 64-gd, 66-dy, 67ho, 68-er and 69-tm. total number of 4 out of 16 rare earth oxides are marked as burnable absorbers only for candu fuel, but behaves as poisoning absorbers for pwr fuel. these are 39-y, 57-la, 59-pr and 71-lu. the rest 3 out of 16 rare earth oxides, 65-tb, 58-ce and 70-yb, are poisoning absorbers for both candu and pwr fuel. situation for sfr fuel is simpler than for both thermal spectra fuels. only 2 out of 16 rare earth oxides, 66-dy and 69-tm, are suitable as burnable absorbers. thulium oxide is a far better ba for sfr than dysprosium oxide that burns very slowly. for the best burnable absorbers, the economic comparison showed the price of rare earth oxide in mtu 48 vol. 4/2016 assessment of burnable absorber fuel design by uwb1 depletion code vary from 1rmb to 1000rmb for candu and pwr fuels and approximately 100 times higher prices for sfr fuels. further analysis of possible burnable absorbers with broad range of elements, nuclides or their combination is in progress. the research aims to a quantitative comparison of materials that could be used as burnable absorbers. the resulting data are planned to be utilized in subsequent fuel design analyses including thermal conductivity, chemical compatibility, irradiation stability, economic evaluation etc. list of symbols boc beginning of cycle (irradiation) eoc end of cycle (irradiation) mt metric ton mtu metric tons of uranium rmb renminbi (chinese currency system) acknowledgements research and development has been funded by tacr, project no. te01020455 – centre for advanced nuclear technologies (canut). references [1] m. lovecky, l. piterka, j. prehradny, r. skoda. uwb1 – fast nuclear fuel depletion code. annals of nuclear energy 71:333–339, 2014. doi:10.1016/j.anucene.2014.04.021. [2] j. prehradny, m. lovecky, r. skoda. burnable absorber comparison between vver, pwr and sfr with uwb1 and serpent codes. in proceedings of the 2014 22nd international conference on nuclear engineering. 2014. doi:10.1115/icone22-30894. [3] j. leppanen. development of a new monte carlo reactor physics bode. ph.d. thesis, helsinki university of technology, 2007. [4] m. lovecky, j. jirickova, r. skoda. monte carlo solver for uwb1 nuclear fuel depletion code. annals of nuclear energy 85:778–7879, 2015. doi:10.1016/j.anucene.2015.06.035. [5] j. prehradny, m. lovecky, r. skoda. rare earth oxides as burnable absorber for vver nuclear fuel. in proceedings of the 2015 23rd international conference on nuclear engineering. 2015. [6] shanghai metals market, prices of rare earth oxide, http://www.metal.com/metals/rare-earth/prices. 49 http://dx.doi.org/10.1016/j.anucene.2014.04.021 http://dx.doi.org/10.1115/icone22-30894 http://dx.doi.org/10.1016/j.anucene.2015.06.035 http://www.metal.com/metals/rare-earth/prices acta polytechnica ctu proceedings 4:43–49, 2016 1 introduction 2 uwb1 depletion code 3 burnable absorbers 4 rare earth oxides 5 calculation cases 6 criticality results 7 depletion results 8 economic comparison 9 conclusions list of symbols acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2016.6.0006 acta polytechnica ctu proceedings 6:6–10, 2016 © czech technical university in prague, 2016 available online at http://ojs.cvut.cz/ojs/index.php/app on evaluation of motion gaits energy efficiency with a hexapod crawling robot lukáš černý∗, petr čížek, jan faigl czech technical university, faculty of electrical engineering, technicka 2, prague, czech republic ∗ corresponding author: cernylu6@fel.cvut.cz abstract. in this work, we are concerning the problem of energy efficient locomotion of a hexapod crawling robot. we are emphasizing a practical verification and deployment on a real walking robot to evaluate relations between the energy consumption, motion speed, and terrain type with a particular motion gait. the tripod, tetrapod, and pentapod motion gaits are considered in the presented evaluation report. keywords: rgb-d slam, localization, ground-truth construction, legged robot, rough terrain traversal, adaptive motion gait. 1. introduction energy efficient and fast locomotion is a key ability for the mobile robot to extent its operation capabilities. as legged robots offer the potential to traverse various types of terrains, it is desirable to adjust their locomotion to be as energy efficient as possible. in general, there are two principal ways of locomotion control for legged robots operating in rough terrains. either deliberative control where individual foot-tip positions of each leg are planned in advance (e.g., [1]) or reactive control based on a repetitive motion gait with prescribed leg trajectories with a certain amount of adaptability such as [2]. the later approach builds on the fact that legged robots often utilize collections of gait patterns to locomote over a variety of surfaces with each gait tuned for a specific surface and set of operational conditions. several commonly used gait patterns such as tripod, tetrapod, and pentapod motion gaits have been developed for hexapod crawling robots. during traversing a rough terrain, it is possible to select a particular gait pattern and stick with it for the whole process of the navigation; however, it may be beneficial to utilize knowledge of the terrain currently being traversed to select the most appropriate gait to optimize a particular criterion, e.g., speed, stability, or energy consumption. the gait selection can be based on both exteroceptive and proprioceptive sensing. in this paper, we consider proprioceptive sensory data only, specifically the current sensory data to evaluate the energy efficiency of individual gaits on different terrain types. we use the information about the overall current drawn from the robot energy source (the used battery pack) to compute instantaneous power and consequently the consumed energy during the robot operation. we performed a series of experiments to collate information about the energy efficiency of different gaits on different terrain types to establish a gait selection strategy that would let us alter gaits according to the terrain that is being traversed. for this purpose, we expect the terrain to be foreknown, i.e., classified in advance. we distinguish following terrain types: office floor, grass, inclined plane, and wooden blocks, see figure 1. online terrain classification can be done using exteroceptive data; however, this topic is out of the scope of this paper, therefore in the herein presented evaluation of the energy consumption, we rather assume the current terrain type is available. an evaluation of the energy efficiency applied for the hexapod crawling robot has been shown in [3]. the authors propose a method of energetic cost evaluation and they applied it in a gait transition algorithm that alters between two different gaits according to the power consumption and thus, according to the particular terrain type. several methods of traversability cost evaluation based on proprioceptive sensors are proposed in [4]. the authors also consider a hexapod walking robot and utilize information about the current torque at the leg joints to estimate the power consumption. contrary to this approach, we consider a direct current measurement in this paper, which provides a more precise estimation of the overall power consumption during crawling. this paper is organized as follows. section 2 describes the utilized hexapod robot platform, the considered motion gait patterns, and used evaluation methodology for measuring the energy efficiency of the individual motion gaits. section 3 is dedicated to the experimental results. finally, in section 4, we discuss the experimental results and propose further research directions. 2. problem statement 2.1. hexapod platform the idea of optimizing the energy consumption by altering gaits according to the terrain type can be applied to any legged robot. in this work, we utilize a robot based on an off-the-shelf phantom x mark 6 http://dx.doi.org/10.14311/app.2016.6.0006 http://ojs.cvut.cz/ojs/index.php/app vol. 6/2016 on evaluation of motion gaits energy efficiency (a) . office floor (b) . grass (c) . inclined plane (d) . wooden blocks figure 1. terrain types. ii platform with six legs. each leg consists of three actuated joints driven by the dynamixel ax-12 intelligent servos. this makes the robot easy to use and relatively cheap with surprisingly large locomotion capabilities in rough environments. 2.2. motion gaits a straightforward approach to a hexapod robot locomotion is to predefine and fix a particular motion gait pattern and simply repeat such a pattern over and over. based on the number of legs moving simultaneously and on the order in which the legs are altering between the support phase and the swing phase, different motion gaits can be recognized. this include regular tripod, tetrapod, pentapod, and ripple gaits. further details on these regular gaits can be found in [5]. a basic locomotion controller assume crawling on a flat terrain by default; however, it is necessary to negotiate the landscape using the robot sensory feedback to surpass a rough terrain. in this report, we consider the default tripod, tetrapod, pentapod, and ripple gaits which assume crawling on a flat terrain and their adaptive variants for tripod, tetrapod, and pentapod gaits [2]. the adaptive gaits exploit the sensory feedback from the intelligent servo drives to detect the contact point of the leg with the supporting surface [6]. in particular, a position error of the servo joint, which is proportional to the applied torque, is utilized to stop the motion of the leg during the swing phase, and thus to detect the contact point of the leg with the ground. 2.3. energetics the fundamental sensory input for the proposed evaluation of the energy efficiency of the individual gaits is based on measuring of the current i drawn from the robot main battery. from the measured current i we can compute the instantaneous power as pin = v i, (1) where i is the instantaneous current (in units of amperes), v is voltage of the battery (in units of volts), and pin is the instantaneous power (in units of watts). as shown in [3], pin is the total power consumed by the robot that consists of the power generating mechanical forces and the power representing the energy loss. however, we consider the total power only because we aim to minimize the overall energy consumed. from the gathered information about the instantaneous power (1), we can estimate the energy consumed during traversing a particular terrain. based on the relationship between the consumed energy e and the instantaneous power pin(t) = de(t) dt (2) we can write e(t) = ∫ t 0 pin(τ) dτ , (3) where e(t) is the energy consumed (in units of joules) from the beginning of the experiment up until the specified time t. from (3) we can estimate the overall energy consumption by integrating the instantaneous power numerically, e.g., using the trapezoidal rule. 3. evaluation 3.1. experimental set-up the following experimental setup has been designed to evaluate and compare particular abilities of each considered gait to traverse a particular terrain type. the robot has been requested to walk a fixed distance of one meter straight ahead and traverse a particular terrain type. considered terrain types are the office floor, artificial grass, inclined plane, and wooden blocks of irregular heights (see figure 1). the office floor and artificial grass represent terrain types with different adhesion, and thus with a different slippage. the inclined plane and wooden blocks represent an environment with elevated demands on the power consumption. the locomotion of the robot on the wooden blocks is also often subjected to slippage and abrupt motions which apply further load on the servo drives, leading to a higher energy consumption. the experiment has been repeated three times for each gait on each terrain type except the default gaits and wooden blocks because the default gaits are not capable of traversing such a rough terrain. the distance traveled by the robot has been estimated based on the legged odometry only. 7 l. černý, p. čížek, j. faigl acta polytechnica ctu proceedings time [s] 0 5 10 15 20 25 c o n su m ed en er g y [j ] 0 50 100 150 200 250 300 energy consumption of default gaits on o/ce .oor default tripod gait default tetrapod gait default pentapod gait default ripple gait (a) . default gaits on office floor time [s] 0 5 10 15 20 25 c o n su m ed en er g y [j ] 0 50 100 150 200 250 300 350 energy consumption of default gaits on grass default tripod gait default tetrapod gait default pentapod gait default ripple gait (b) . default gaits on grass time [s] 0 5 10 15 20 25 c o n su m ed en er g y [j ] 0 50 100 150 200 250 300 350 400 450 energy consumption of default gaits on slope default tripod gait default tetrapod gait default pentapod gait default ripple gait (c) . default gaits on inclined plane time [s] 0 20 40 60 80 100 120 c o n su m ed en er g y [j ] 0 200 400 600 800 1000 1200 energy consumption of adaptive gaits on o/ce .oor adaptive tripod gait adaptive tetrapod gait adaptive pentapod gait (d) . adaptive gaits on office floor time [s] 0 20 40 60 80 100 120 c o n su m ed en er g y [j ] 0 200 400 600 800 1000 1200 1400 energy consumption of adaptive gaits on grass adaptive tripod gait adaptive tetrapod gait adaptive pentapod gait (e) . adaptive gaits on grass time [s] 0 20 40 60 80 100 120 c o n su m ed en er g y [j ] 0 200 400 600 800 1000 1200 1400 1600 energy consumption of adaptive gaits on slope adaptive tripod gait adaptive tetrapod gait adaptive pentapod gait (f) . adaptive gaits on inclined plane time [s] 0 20 40 60 80 100 120 c o n su m ed en er g y [j ] 0 500 1000 1500 energy consumption of adaptive gaits on wooden blocks adaptive tripod gait adaptive tetrapod gait adaptive pentapod gait (g) . adaptive gaits on wooden blocks figure 2. energy consumption over time. the overall current drawn from the battery has been measured using the acs712 hall effect-based linear current sensor with the measuring range of ±5 a. each current data sample was provided with a timestamp. the reading frequency was not completely invariant due to the time consumed by the motion control and other factors. in spite of that the reading frequency was approximately 100 hz. before each experiment, we measured the battery voltage and we assumed it is constant during the robot movement because the drop of the voltage was insignificant during the experiment. thereby, we have acquired all the information needed to estimate the energy consumed by the robot motion. 3.2. results having done all the experiments described above, the instantaneous power for each current sample using (1) has been computed. consequently, the power has been numerically integrated according to (3). the plotted energy consumption over time is presented in figure 2, where the particular plots are as follows. figure 2a and figure 2d show the results of the experiments performed on the office floor. figure 2e and 2e show the results for the artificial grass. experimental results performed on the inclined plane are shown in figure 2c and figure 2f. finally, the results from traversing the wooden blocks are presented in figure 2g. the experiments have been run three times for each terrain type and gait. therefore, results from the particular trials have been grouped and marked by a specific color in the plots. each color represents one gait pattern, and thus each color group consists of three lines that correspond to the three experiments performed for every gait and terrain type. 3.3. discussion in figure 2, we can clearly see that a gait selection can significantly influence the energy efficiency of the robot motion. tables 1 and 2 and figure 3 together with figure 4 summarize the overall energy consumption and the time needed for traversing the terrain (a distance of 1 m) by each motion gait. the results were computed as the mean values of the data resulting from each triplet of the performed experiments. according to table 1, we can establish a strategy for gait selection to prefer the default tripod gait whenever the particular terrain type can be traversed by it. in rough terrains, the adaptive tripod gait is preferred. considering the time in table 1, the switching strategy that minimizes the time consumption (the speed of the robot) yields the same result. however, such a gait selection rule optimizes only the energy and time consumption. different optimization criteria might be induced by the application scenario, e.g., the tripod gait is the least safe gait in terms of the robot stability, and thus it is not capable of traversing such challenging terrains as the pentapod gait, which is the most stable gait. the results presented above indicate the influence of the energy consumption is proportional to the time by a multiplication factor. the factor can be utilized as a single parameter describing the energy efficiency of the gait pattern on a particular terrain type. the multiplication factor actually coincides with the average power consumed by the robot. moreover, the average power might be used as a characteristic for the terrain classification. however, to get relevant information about an average power, we need to collect data over a longer time interval because the instantaneous power changes signif8 vol. 6/2016 on evaluation of motion gaits energy efficiency gait consumed energy [j] floor grass plane blocks def. tripod 130.9 144.0 148.3 — def. tetrapod 168.7 183.0 200.9 — def. pentapod 289.7 327.9 403.2 — def. ripple 171.9 183.7 218.4 — ad. tripod 386.6 482.5 485.6 507.6 ad. tetrapod 543.1 617.0 699.0 700.0 ad. pentapod 1019.1 1248.7 1543.4 1270.3 table 1. energy consumption. office floor artificial grass inclined plane wooden blocks 0 200 400 600 800 1000 1200 1400 1600 c on su m ed e ne rg y [j ] consumed energy adaptive pentapod adaptive tetrapod adaptive tripod regular pentapod regular tetrapod regular tripod figure 3. energy consumption on different terrain types. icantly during a single gait cycle. this can be observed for the terrain of wooden blocks as fluctuations of otherwise relatively straight lines in figure 2. based on the presented results, we can deduce that the time interval of few seconds (or few gait cycles) should be sufficient to provide the relevant data. notice, the wooden blocks are not, unlike the other terrain types, uniform, and therefore, fluctuations in the estimated energy consumption may be observed in the presented results. the energetic cost together with the traversability of the terrain vary from place to place and they both depend on exact location of the robot in the environment. this observation motivate us that a terrain classification might divide the area into smaller parts and evaluate traversability in these local areas. notice that all the gait parameters such as speed, stride length, etc. were fixed for each gait individually for all experiments reported in this paper. however, the energy efficiency can be used in the optimization of the gait and it can be also improved by a mechanical construction of the robot. a further experimental evaluation can also be enhanced by a precise measurement of the distance traveled by the robot. in this manner, factors such as traction and leg slippages can be taken into account, which may provide additional information about suitability of particular gaits for specific terrain types. gait time [s] floor grass plane blocks def. tripod 8.1 8.1 8.1 — def. tetrapod 12.2 12.2 12.2 — def. pentapod 24.4 24.4 24.3 — def. ripple 12.3 12.2 12.2 — ad. tripod 32.6 35.8 35.7 35.7 ad. tetrapod 56.5 56.6 57.3 56.1 ad. pentapod 110.6 112.5 114.6 95.8 table 2. traversing time. office floor artificial grass inclined plane wooden blocks 0 20 40 60 80 100 120 140 c on su m ed ti m e [s ] consumed time adaptive pentapod adaptive tetrapod adaptive tripod regular pentapod regular tetrapod regular tripod figure 4. traversing time on different terrain types. 4. conclusion in this work, we study the problem of energetic characteristics of the hexapod legged robot locomotion. we have evaluated the consumed energy for the most common motion gait types and different terrain types. from the presented experimental results, we can observe that the default tripod gait is the fastest and the most energy efficient gait for flat terrains. this observation yields the main strategy for the gait switching rule that minimizes the overall energy consumption. the gait switching rule can be formulated such as whenever the default gait is not capable of traverse a terrain due to the terrain roughness, its adaptive version should be preferred. the results indicate the proportional relation between the operational time and the energy consumption, which, in fact, is not a surprising result. however, the multiplication factor can be used as a characteristic for the terrain classification. all the proposed applications and implications of presented results are intended as a motivation for further research. a deployment of the gait switching decision rule and further investigation of the gait optimal parametrization using the energy characteristics evaluation is of our particular interest. acknowledgements the presented work was supported by the czech science foundation (gačr) under research project no. 15-09600y. the support of grant no. sgs16/235/ohk3/3t/13 to petr čížek is also gratefully acknowledged. 9 l. černý, p. čížek, j. faigl acta polytechnica ctu proceedings references [1] d. belter, p. skrzypczynski. rough terrain mapping and classification for foothold selection in a walking robot. journal of field robotics 28(4):497–528, 2011. doi:10.1002/rob.20397. [2] j. mrva, j. faigl. tactile sensing with servo drives feedback only for blind hexapod walking robot. in robot motion and control (romoco), 2015 10th international workshop on, pp. 240–245. 2015. doi:10.1109/romoco.2015.7219742. [3] n. kottege, c. parkinson, p. moghadam, et al. energetics-informed hexapod gait transitions across terrains. in ieee international conference on robotics and automation (icra), pp. 5140–5147. 2015. doi:10.1109/icra.2015.7139915. [4] j. mrva, m. stejskal, j. faigl. on traversability cost evaluation from proprioceptive sensing for a crawling robot. in acta polytechnica ctu proceedings, pp. 34–39. 2015. doi:10.14311/app.2015.1.0034. [5] g. dudek, m. jenkin. computational principles of mobile robotics. cambridge university press, new york, ny, usa, 2000. [6] j. mrva, j. faigl. tactile sensing with servo drives feedback only for blind hexapod walking robot. in 10th international workshop on robot motion and control, romoco, pp. 240–245. 2015. doi:10.1109/romoco.2015.7219742. 10 http://dx.doi.org/10.1002/rob.20397 http://dx.doi.org/10.1109/romoco.2015.7219742 http://dx.doi.org/10.1109/icra.2015.7139915 http://dx.doi.org/10.14311/app.2015.1.0034 http://dx.doi.org/10.1109/romoco.2015.7219742 acta polytechnica ctu proceedings 6:6–10, 2016 1 introduction 2 problem statement 2.1 hexapod platform 2.2 motion gaits 2.3 energetics 3 evaluation 3.1 experimental set-up 3.2 results 3.3 discussion 4 conclusion acknowledgements references 238 acta polytechnica ctu proceedings 2(1): 238–241, 2015 238 doi: 10.14311/app.2015.02.0238 spectroscopic monitoring observations of nova v1724 aql in 2012 t. kajikawa1, m. nagashima1, h. kawakita1, a. arai1,2, y. ikeda1,3, m. isogai1,4, m. fujii5, k. ayani6 1kyoto sangyo univearsity / koyama astronomical observatory 2university of hyogo / nishi-harima astronomical observatory 3photocoding 4national astronomical observatory of japan 5fujii-kurosaki observatory 6bisei astronomical observatory corresponding author: kawakthd@cc.kyoto-su.ac.jp abstract spectroscopic and photometric monitoring observations of nova apl 2012 (v1724 apl) were conducted at koyama astronomical observatory, fujii-kurosaki observatory and bisei astronomical observatory. the nova was initially considered as an outbursting pre-main-sequence young stellar object. our monitoring observations have revealed the nova to be a fe ii type classical nova. the temporal evolution of spectra and light curves of the nova were similar to those of a slow nova (e.g., v1280 sco and v5558 sgr). we observed no evidence of molecule formation in v1724 aql in contrast with v2676 oph in which dust formation occurred after the molecular formation in the nova outflow. keywords: cataclysmic variables classical novae optical spectroscopy photometry individual: v1724 aql. 1 introduction nova aql 2012 (later named as v1724 aql) was discovered on 2012 october 20.4 ut (nishiyama & kabashima 2012). the brightness of the nova during the discovery was reported as 12.6 mag. just after the discovery, we conducted our spectroscopic monitoring observations at the koyama astronomical observatory (kao), fujii-kurosaki observatory (fko) and bisei astronomical observatory (bao) in japan. the first optical spectrum taken on october 21.4 ut was reported by fujii (2012) at fko. the object showed a sharp hα emission ( probably with a p cygni profile) on a very red continuum. this object was considered a classical nova affected by a severe interstellar extinction (strong absorption of the na d line was observed). munari (2012) also reported that the spectrum of this object on october 21.8 ut showed a weak and sharp hα emission on a very red continuum, with strong na i and ba ii absorption lines. this spectrum was basically consistent with that reported by fujii (2012). however, it was considered that the spectrum is more similar to that of an outbursting premain-sequence young stellar object than that of a nova before maximum brightness. on 2012 october 23.5 ut, ayani (2012) also reported the optical spectrum of this object taken at bao, with fe ii emission and o i emission with a p cygni profile. this object was finally identified as a fe ii type classical nova according to the classification by williams (1992). here we report the spectroscopic and photometric observations of v1724 aql performed in a collaboration among three observatories (kao, fko and bao) in japan. 2 spectroscopic and photometric observations the observations conducted in our collaboration were follows. spectroscopic observations were performed at three sites: 1. a 0.4-m telescope with a low-dispersion spectrograph (λ = 4500 — 9000 å with r ∼ 500) at fujii-kurosaki observatory, 2. a 1.01-m telescope with a lowand moderatedispersion spectrograph (λ = 3600 — 9000 å with r ∼ 1500 for a low-resolution mode) at bisei astronomical observatory, and 3. a 1.3-m araki telescope with a low-dispersion 238 http://dx.doi.org/10.14311/app.2015.02.0238 spectroscopic monitoring observations of nova v1724 aql in 2012 spectrograph losa/f2 (λ = 3800 — 8000 å with r ∼ 600; shinnaka et al. 2013) at koyama astronomical observatory. in addition to the spectroscopic observations, photometric observations were performed at kao by the imaging camera adler (araki dual-band imager; nakagawa et al. 2013) with intermediate and broadband filters (y-, i′-, and z′-band filters). figure 1 shows the spectral evolution of v1724 aql. figure 2 shows the light curves (with the multi-band light curves taken from aavso database). we also show the close-up views for hα and o i emission lines to make clear the evolution of their line shapes. clearly, the line widths increased for later epochs in the case of hα while the temporal change in the p cygni profile of o i emission indicated faster expansion velocities at later dates. 0 5 10 15 20 25 4000 5000 6000 7000 8000 9000 wavelength [a] +35d:k +26d:k +25d:f +24d:f +22d:f +22d:k +17d:k +16d:f +16d:k +10d:k +8d:k +5d:f +4d:k +4d:b +4d:f +3d:f +2d:f +2d:b +1d:k +0d:f h β f e ii (7 4) f e ii (7 4) f e ii (7 4) o i ( 77 74 ) n a i( 58 89 ) m gi i+ o i o i( 84 46 ) f e ii (4 2) h α[o i] (6 30 0) c a ii k: kao f: fko b: bao lo g ( f lu x) + c on st . 10 figure 1: low-dispersion spectra of v1724 aql obtained at three sites from 2012 oct 21 to nov 25 ut. all spectra are shifted by different offsets for readability. emission lines were on the highly red continuum. the line width of hα emission became broader and stronger compared with the nearby continuum, and the ca ii triplet emission lines bacame stronger later. note that the bluish continuum might overlap with the spectra of the nova in the shorter wavelength region (λ < 4500 å ), probably due to a forground (or background) star. 239 t. kajikawa et al. 3 discussion and conclusion from the viewpoint of the spectral evolution, the spectra of v1724 aql slowly changed during the period of our observations. emission lines (hα, hβ, fe ii, na i, o i and ca ii) were on the reddened continuum. we estimated e(b − v ) based on the measurements of balmer decrement by the method described in helton et al. (2010), e(b − v )∼3.1, which indicated severe interstellar extinction for the nova. this value is slightly higher than that reported by rudy et al. (2012) but both values indicate severe interstellar reddening. note that the spectra of the nova showed enhancement in a shorter wavelength region (λ < 4500å ) that appeared similar to a bluish continuum component but was noisier. the nova may be overlapped with another foreground (or background) stellar object that had a high temperature. although one may consider that we should estimate e(b − v ) based on the (b − v ) of the nova around the optical brightness maximum, we had to be careful of the contamination with the b-band brightness of the nova. 8 9 10 11 12 13 14 15 16 17 18 19 0 5 10 15 20 25 30 35 40 45 m ag ni tu de s days from 2012 oct 20 ut v r i b y i’ z’ figure 2: optical light curves of v1724 aql obtained at kao with the multi-band light curves of the nova taken from aavso database. the vertical tick-marks at the top of panel indicate the dates of our spectroscopic observations. the line width of hα emission had clearly increased and became stronger compared with the nearby continuum, and o i and ca ii triplet emission lines grew at later dates. the ejection velocity was ∼400 km/s based on the p cygni profile of hα at t=1d after the outburst (figure 3). it then increased; 630 km/s (t=2d), 640 and 750 km/s (t=5d), and 1150 km/s (t=9d) based on the p cygni profiles of o i as shown in figure 4. finally, the ejection velocity reached ∼2000 km/s based on the fwzi of hα (figure 3) after the brightness maximum at t=17d. the ejection velocities were slower during the earlier phase, and faster during later phase. such temporal changes in ejection velocity in v1724 aql was similar to those of slow novae, e.g., v1280 sco (naito et al. 2012) and v5558 sgr (tanaka et al. 2011). v1724 aql was similar to a slow nova from the viewpoint of spectral evolution. 0 0.2 0.4 0.6 0.8 1 -4000 -2000 0 2000 4000 r el at iv e flu x velocity [km/s] +10d:k +8d:k +5d:f +4d:k +3d:f +2d:f +1d:k 0d:f 0 0.2 0.4 0.6 0.8 1.0 -4000 -2000 0 2000 4000 r el at iv e flu x velocity [km/s] +35d:k +26d:k +25d:f +24d:f +22d:k +17d:k +16d:k +10d:k figure 3: temporal change of hα emission line before and after the brightness maximum at t=17d after the outburst (upper and lower panels, respectively). the line widths became broader at later dates. the black arrow in the lower panel shows the change in blue-shift component that could have been caused by the change in the velocity structure of the ejected materials. based on the light curves shown in figure 2, nova v1724 aql showed very slow evolution in brightness after the outburst until its brightness maximum in the v band (t=17d, v=13.6±0.2 mag). the visual brightness was almost unchanged (but oscillating with small amplitude) until the brightness maximum since the outburst (14.4±0.4 mag in v -band). the evolution in brightness was slow for v1724 aql in earlier phase. however, after the brightness maximum (t=17d), its brightness in v -band become fainter with a decline rate of +0.14 mag/day (t2 was 14.3 days). although this decline rate was indicative of a fast nova rather than a slow nova (payne-gaposchkin 1957), the light curves after the brightness maximum might be affected by dust formation as suggested by the molecular formation observed by rudy et al. (2012). 240 spectroscopic monitoring observations of nova v1724 aql in 2012 -1 -0.5 0 0.5 1 -4000 -3000 -2000 -1000 0 1000 2000 3000 4000 r el at iv e flu x velocity [km/s] +10d:k +8d:k +4d:k +1d:k figure 4: temporal change of the p cygni profile for o i (7774 å at rest) emission line before the brightness maximum. the measured ejection velocites based on the p cygni profiles were 630 km/s (at t=2d), 640 and 750 km/s (at t=5d), and 1150 km/s (at t=9d). rudy et al. (2012) reported the detection of co molecular emission in the near-infrared spectra taken on 2012 oct 27 and 28 ut (before the brightness maximum), indicating that molecule formation was ongoing, with dust formation likely to follow. nova v1724 aql was probably a slow nova that showed co emission during the pre-maximum halt (at least, before the maximum). however, there were no hints for c2 and cn absorption bands in optical spectra of the nova (c.f. v2676 oph, nagashima et al. 2014). those molecular absorption bands were expected to appear during several days (as in the cases of v2676 oph and dq her). our spectroscopic observations were performed frequently enough to detect them if a considerable amount of molecules formed in the outflow of the nova. we propose the hypothesis that the ejecta of v1724 aql was oxygen-rich and the atomic carbon might not be over-abundant with respect to the atomic oxygen (c < o) in the ejecta. as demonstrated by the model calculations (pontefract & rawlings 2004), formation of c2 (and also cn) is more difficult than co in an oxygen-rich atmosphere. v1724 aql might have oxygen-rich ejecta. the spectroscopic observations of v1724 aql in the nebular phase (if reported in the future) will be helpful to test this hypothesis. references [1] ayani, k. 2012, electronic telegram no. 3273, central bureau for astronomical telegrams, international astronomical union (ed., green, d.). [2] fujii, m. 2012, electronic telegram no. 3273, central bureau for astronomical telegrams, international astronomical union (ed., green, d.). [3] helton, l.a., woodward, c.e., walter, f.m., vanlandingham, k., schwarz, g.j., evans, a., ness, j.-u., geballe, t.r., gehrz, r.d., greenhouse, m., krautter, j., liller, w., lynch, d.k., rudy, r.j., shore, s.n., starrfield, s., & truran, j. 2010, aj, 140, 1347. [4] munari, u. 2012, electronic telegram no. 3273, central bureau for astronomical telegrams, international astronomical union (ed., green, d.). [5] nagashima, m., arai, a., kajikawa, t., kawakita, h., ktao, e., arasaki, t., taguchi, g., & ikeda, y. 2014, apjl, 780, l26. doi:10.1088/2041-8205/780/2/l26 [6] naito, h., mizoguchi, s., arai, a., tajitsu, a., narusawa, s., yamanaka, m., fujii, m., iijima, t., kinugasa, k., kurita, m., nagayama, t., yamaoka, h., & sadakane, k. 2012, a&a, 543, 86. [7] nakagawa, s., noguchi, r., iino, e., ogura, k., matsumoto, k., arai, a., isogai, m., & uemura, m. 2013, publ. of astron. soc. japan, 65, 7. doi:10.1093/pasj/65.3.70 [8] nishiyama & kabashima 2012, electronic telegram no. 3273, central bureau for astronomical telegrams, international astronomical union (ed., green, d.). [9] payne-gaposchkin, c. 1957, the galactic novae. amsterdam:north-holland. [10] pontefract, m., & rawlings, j.m.c. 2004, mnras , 347, 1294. doi:10.1111/j.1365-2966.2004.07330.x [11] rudy, r.j., laag, e.a., crawford, k.b., russell, r.w., puetter, r.c., & perry, r.b. 2012, electronic telegram no. 3287, central bureau for astronomical telegrams, international astronomical union (ed., green, d.). [12] shinnaka, y., kawakita, h., kobayashi, h., naka, c., arai, a., arasaki, t, kitao, e., taguchi, g., & ikeda, y. 2013, icarus, 222, 734. doi:10.1016/j.icarus.2012.08.001 [13] tanaka, j., nogami, d., fujii, m., ayani, k., kato, t., maehara, h., kiyota, s., & nakajima, k. 2011, publ. of astron. soc. of japan, 63, 911. [14] williams, r. e. 1992, aj, 104, 725. 241 http://dx.doi.org/10.1088/2041-8205/780/2/l26 http://dx.doi.org/10.1093/pasj/65.3.70 http://dx.doi.org/10.1111/j.1365-2966.2004.07330.x http://dx.doi.org/10.1016/j.icarus.2012.08.001 introduction spectroscopic and photometric observations discussion and conclusion acta polytechnica ctu proceedings doi:10.14311/app.2017.9.0039 acta polytechnica ctu proceedings 9:39–42, 2017 © czech technical university in prague, 2017 available online at http://ojs.cvut.cz/ojs/index.php/app surface characterization of titanium alloys for nitrogen ion implantation petr vertata,∗, jan drahokoupilb, petr vlcakc a department of solid state engineering, fnspe ctu in prague, trojanova 13, 120 00, prague 2, czech republic b department of advanced structural materials, institute of physics as cr, na slovance 2, 182 21, prague 8, czech republic c department of physics, fme ctu in prague, technicka 4, 166 07, prague 6, czech republic ∗ corresponding author: vertatp@gmail.com abstract. ion implantation is one of the modern methods of the surface modification of various materials. industrially used ti–6al–4v titanium alloy and commercially pure ti grade 2 were characterized using the x–ray diffraction methods. texture of the material and dependence of the microstructural properties on the method of the surface modification were examined in order to determine suitable conditions for application of the process in the industry. the structure of ti–6al–4v alloy before and after the nitrogen ion implantation process is discussed and observed surface hardening is explained. keywords: ti–6al–4v, ti grade 2, ion implantation, x–ray diffraction. 1. introduction ti–6al–4v titanium alloy (also called ti grade 5) and commercially pure ti grade 2 are materials widely used in the industry and the biomedicine for their low density, high tensile strength, biocompatibility and resistance to corrosion in some common environments. disadvantages of the titanium alloys for their applications are their poor performance in sliding, insufficient surface hardness and insufficient resistance to some chemical environments. for these reasons is the surface of common titanium alloys often modified. nowadays, one of the most suitable surface modification methods seems to be the ion implantation of nitrogen ions into the surface of the sample. it has been shown that mechanical qualities and wear and corrosion resistance of many materials can be improved using this method. concerning the titanium and its alloys, creation of hard tinx phases [1] or stabilization of the α–ti phase [2] leading to the increase of the surface hardness is expected. compared to the other commonly used surface modification methods, the advantages of the nitrogen ion implantation are the ability to control the process precisely (esp. the fluence of the implanted ions, their energy and thus the depth distribution) and so the ability of automating the process and the possibility to implant at the low temperatures. 2. experimental details for purpose of characterizing the sample preparation method and the surface modification using the nitrogen ion implantation the ti–6al–4v (ti grade 5) alloy with a microstructure consisting of the α + β ti phases with a chemical composition of (wt% 0.1% c, 5.50–6.75% al, 3.50–4.50% v, 0.2% o, 0.05% n, 0.0125% h, 0.3% fe and ti balance) and the ti grade 2 consisting mainly of the α-ti phase, chemical composition of (max. wt% 0.08% c, 0.25% o, 0.03% n, 0.015% h, 0.3% fe and ti balance), were examined. the disc samples were cut from commercially available bars, ground and polished with a diamond paste in order to obtain a flat smooth surface suitable for the ion implantation. phase composition of the polycrystalline samples, microstrain and texture were determined using the x–ray diffraction. data were measured on the panalytical x’pert pro horizontal powder diffractometer with a cobalt anode (λkα1= 0.1789 nm) using the divergent bragg-brentano geometry and parallel beam geometry. for pole figures measurements the atc–3 texture cradle was used allowing rotation of the sample in the φ angle, change of the inclination angle χ and horizontal movement of the sample. for temperature dependences the htk 2000 heating chamber was used. measured data were processed by rietveld analysis using the topas 3 software. in this method the measured diffraction pattern is compared with the calculated one and the input parameters are varied on the basis of differences between them. for the calculation of diffraction pattern, this program uses so called "fundamental parameter approach" [3], i.e. that the calculated diffraction profile is based on convolution of particular contributions to the total profile. three components of diffraction peak broadening are taken into account: an instrumental broadening, an average crystallite size and microstrains. thus 39 http://dx.doi.org/10.14311/app.2017.9.0039 http://ojs.cvut.cz/ojs/index.php/app p. vertat, j. drahokoupil, p. vlcak acta polytechnica ctu proceedings figure 1. measured diffraction patterns of the ti–6al–4v and ti grade 2 samples. figure 2. detailed diffraction diagram of the ti– 6al–4v sample. both present titanium phases: α–ti and β–ti are marked using different colors and their reflections are indexed. rietveld fit of the measured data is marked with red color. precise determination of average crystallite sizes and microstrains is possible. the pole figures for reflections 100, 002, 110 and 101 were measured using the reflection geometry. the inability of this method to measure the whole pole figures was solved by calculation of the odf function followed by recalculation of desired pole figures. the background and defocusing corrections were applied using the homemade software. calculation of the odf and recalculation of the pole figures were made in the x’pert texture program. in order to verify the effect of the nitrogen ion implantation on the surface properties of the ti– 6al–4v alloy and in order to explain such possible effect, samples were implanted using the ion implanter tecvac 221 in conventional setup the fluency of figure 3. detailed diffraction diagram of the ti grade 2 sample, only the α–ti phase (blue line, corresponding miller indices are shown) is present. rietveld fit of the measured data is marked with red color. 6·1017 cm−2 and the 90 kev accelerating voltage at the temperatures lower than 80 ◦c. 3. results and discussion 3.1. samples characterization before the nitrogen ion implantation ti–6al–4v and ti grade 2 samples were examined using the methods of the x-ray diffraction analysis, measured diffraction patterns are shown in figure 1. differences in the peak positions are caused by the different chemical compositions of the alloys. measurements of the ti–6al–4v sample (detailed view in figure 2) showed the presence of two titanium structures: hexagonal α–ti (76%) phase and cubic β–ti (24%). in the ti grade 2 sample only α–ti phase 40 vol. 9/2017 surface characterization of ti alloys for ni ion implantation figure 4. recalculated direct pole figures of the 002, 010, 011 and 110 reflections of the ti grade 2 sample. figure 5. detail of the diffraction profile of the ti grade 2 sample (reflection 211 of the α–ti phase) after different preparation treatment. was present. diffraction diagram of ti grade 2 sample after the electro-polishing might be seen in figure 3. the texture analysis shows the samples are textured (figure 4). the crystallographic direction 001 has low rate in the direction that is normal to the surface of the sample. although the samples physicaly have the rotational symmetry, the pole figures reveal only the mirror symmetry of the inner microstructure. this has to be taken into account when doing other xrd measurements, namely measurements with the inclined sample or with fixed incident angle. influence of the sample preparation on the microstructure of ti grade 2 has also been studied. detailed look on the diffraction profiles of the sample treated only by grinding and polishing using the diamond paste reveals broadening of the diffraction lines indicating lower crystallite size and higher microstrain in the structure. considerable asymmetry indicates the depth gradient of the macroscopic residual stress (figure 5). in an effort to eliminate such effects that have arisen due to the preparation of the sample, influence of the electrolytic polishing and annealing on the microstructure was examined. figure 6. time dependence of the microstrain during the annealing of the ti grade 2 sample. annealing temperatures are distinguished using different colors. initial value at the room temperature obtained after grinding and polishing the sample using a diamond paste is also shown. absolute error < 0.003 %. diffraction profiles measured after the mentioned procedures are shown in figure 5. measured influence of the annealing at different temperatures (300 ◦c, 400 ◦c and 500 ◦c) on the microstrain is shown in figure 6 representing the time dependence of this quantity. the dominant effect of removing the residual microstresses is reached during the first minutes after reaching particular temperature. for the temperature of 300 ◦c there is no visible progress after several minutes at all. for higher temperatures there is a jump followed by a continuous decreasing of microstrains. the results show sufficient decrease of the microstrain value when compared with the initial state of the sample after its necessary treatment by grinding and polishing using the diamond paste. however, the influence of the sample preparation is not eliminated completely. state closest to the initial microstructure of the bulk sample is revealed (according to figure 5) by the electro-polishing. under the common circumstances 41 p. vertat, j. drahokoupil, p. vlcak acta polytechnica ctu proceedings figure 7. depth profile of the indentation hardness measured on ti–6al–4v sample before and after the nitrogen ion implantation. [4] values influenced by the relative error originating from the calibration ∼ 10 %. this method of the surface treatment can be considered as the most suitable one. regarding the ti–6al–4v alloy, analogous behavior can be expected. 3.2. samples characterization after the nitrogen ion implantation after the nitrogen ion implantation into ti–6al–4v samples using the suitable implantation parameters, desired surface hardening is observed as proven by the typical depth profile of the nanoindentation hardness (figure 7) [4]. phase composition of the ti–6al–4v samples after the implantation with parameters described in section 2 was characterized. in addition to hexagonal α–ti and cubic β–ti phases observed before the implantation, two new phases were detected: smaller amount of the solid solution α–ti(+n) and majority of the tin solid phase. formation of these two phases describes the mechanism of observed surface hardening in our alloy. dependences of different implantation parameters on concrete phase composition and mechanical properties of mentioned titanium alloys are now the object of further research. 4. conclusions samples of ti–6al–4v (ti grade 5) and ti grade 2 alloys were characterized by the x–ray diffraction methods. texture in the samples of such commonly available materials and improvement of their microstructural properties by annealing at different temperatures and by electro-polishing were discussed. best results were obtained using the last method. presented results create the precondition for a further research in the field of functionalizing the surface of ti–6al–4v and ti grade 2 alloys aiming mainly at characterizing the influence of various ion implantation parameters on the phase composition and physical properties of these materials for their future application in the industry and biomedicine. acknowledgements authors thank dr. j. sepitka for the nanoindentation measurements. this work was supported by the grant agency of the czech technical university in prague, grant no. sgs16/245/ohk4/3t/14, and czech science foundation, grant no. 16-22276s. references [1] y. z. liu, x. t. zu, s. y. qiu, j. cao, c. x. li, x. q. huang, c. f. wei: phase formation and modification of corrosion property of nitrogen implanted tialv alloy. vacuum 81 (1):71–76, 2006. doi: 10.1016/j.vacuum.2006.02.013 [2] h. schmidt., a. schminke, m. schmiedgen, b. baretzky: compound formation and abrasion resistance of ionimplanted ti6al4v. acta mater. 49 (3):487–495, 2001. doi: 10.1016/s1359-6454(00)00326-8 [3] r. w. cheary, a. a. coelho, j. p. cline: fundamental parameters line profile fitting in laboratory diffractometers. j. res. natl. inst. stand. technol. 109:1–25, 2004. doi: 10.6028/jres.109.002 [4] p. vlcak, f. cerny, j. drahokoupil, j. sepitka, z. tolde: the microstructure and surface hardness of ti6al4v alloy implanted with nitrogen ions at an elevated temperature. j. alloys compd. 620:48–54, 2015. doi: 10.1016/j.jallcom.2014.09.125 42 acta polytechnica ctu proceedings 9:39–42, 2017 1 introduction 2 experimental details 3 results and discussion 3.1 samples characterization before the nitrogen ion implantation 3.2 samples characterization after the nitrogen ion implantation 4 conclusions acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2016.6.0001 acta polytechnica ctu proceedings 6:1–5, 2016 © czech technical university in prague, 2016 available online at http://ojs.cvut.cz/ojs/index.php/app on construction of a reliable ground truth for evaluation of visual slam algorithms jan bayer∗, petr čížek, jan faigl czech technical university, faculty of electrical engineering, technicka 2, prague, czech republic ∗ corresponding author: bayerja1@fel.cvut.cz abstract. in this work we are concerning the problem of localization accuracy evaluation of visual-based simultaneous localization and mapping (slam) techniques. quantitative evaluation of the slam algorithm performance is usually done using the established metrics of relative pose error and absolute trajectory error which require a precise and reliable ground truth. such a ground truth is usually hard to obtain, while it requires an expensive external localization system. in this work we are proposing to use the slam algorithm itself to construct a reliable ground truth by offline frame-by-frame processing. the generated ground truth is suitable for evaluation of different slam systems, as well as for tuning the parametrization of the on-line slam. the presented practical experimental results indicate the feasibility of the proposed approach. keywords: rgb-d slam, localization, ground truth construction, legged robot, rough terrain traversal, adaptive motion gait. 1. introduction reliable localization is an essential prerequisite in many practical robotic scenarios, thus its accuracy is the most important parameter in the evaluation of newly developed localization techniques. there are numerous methods based on various principles that can localize the robot in its operational environment. however, when benchmarking such localization methods it is necessary to use a reliable ground truth together with some established metrics (e.g. [1, 2]). in this work, we address the problem of localization accuracy assessment in a visual simultaneous localization and mapping (slam) task [3]. in principle, it is possible to use publicly available datasets, e.g., the freiburg uni dataset [1], the rgb-d slam dataset [2], or the kitty dataset [4], to evaluate the accuracy of slam method. however, our particular environment, setup, and sensory equipment might not exhibit similar properties as the ones used in these standardized datasets. therefore we need to construct our own dataset and acquire a ground truth to assess the localization accuracy. all the above-mentioned datasets are supplied with a centimeter-level precision ground truth provided by an external localization system like a differential gps [4], which is based on a standard gps enhanced with ground reference stations with known geospatial positions. other types of external localization systems used in publicly available datasets are motion capture system [2] or a total station [5]. the former utilizes the data captured by camera sensors to localize the robot marked with special patterns well-distinguishable in the environment. the later measures distance between the observed object and the static ground station using time of flight principle. a 3-dof position of the robot in a global reference frame is obtained from figure 1. experimental environment the distance and the angles measured by the total station. however, acquisition of a reliable ground truth using any of the mentioned localization techniques might be a considerable problem while all of the aforementioned principles are very expensive. thus we are proposing to use the dataset itself for the ground truth construction by offline frame-by-frame processing of the dataset using the slam algorithm. in such a case, we are not interested in an on-line performance of the algorithm rather than the reliability and accuracy of the constructed trajectory estimate to be used as a ground truth. such a processing is assumed to provide the best trajectory estimate for the given dataset and localization technique. moreover, by taking into account certain factors in the dataset construction., which are discussed further in this paper, it is possible to improve the reliability of the generated ground truth. 1 http://dx.doi.org/10.14311/app.2016.6.0001 http://ojs.cvut.cz/ojs/index.php/app j. bayer, p. čížek, j. faigl acta polytechnica ctu proceedings to verify our hypothesis, we are reporting on the results of such a ground truth construction evaluation based on the rgb-d slam method [6] compared to the ground truth provided by the whycon motion capture system [7]. we evaluated the proposed approach on a challenging dataset obtained with the rgb-d camera mounted on a hexapod walking robot crawling in rough terrain (see fig. 1). such a scenario requires full 6-dof position estimation, while the camera is subjected to unpredictable motion and the resulting dataset is very different in comparison to all the standardized datasets obtained mainly by wheel platforms. the presented approach to ground truth construction is somehow similar to the task of structure-frommotion studied in computer vision community. in [8], the authors use color images from the rgb-d microsoft kinect camera to recover its pose, but for the scene and object recognition purposes. however, in our work we are focusing solely on the localization accuracy rather than the quality of the model (map) of the environment. the paper is structured as follows. section 2 explains our method of getting the ground truth for the evaluation and the used evaluation metrics. the set-up of a practical experiment together with the evaluation results are described in section 3. conclusion and remarks for future work are dedicated to section 4. 2. proposed solution the main motivation of our work is to provide a reliable ground truth estimate whenever a precise external localization is not available to be used as the ground truth. for this reason, we are proposing to use the captured dataset and process it with a slam system offline frame-by-frame with special settings of parameters to minimize the trajectory error. afterward, the resulting trajectory estimate can be used as a ground truth to compare to the online generated trajectories, while in deployment scenarios we are interested in the online performance of slam methods. such an estimated ground truth can be used to compare different slam methods or to find the best parametrization for the online slam trajectory estimation. 2.1. evaluation metrics the slam algorithms can be compared either by the quality of the generated map or the accuracy of the estimated trajectory. as the evaluation metrics, we use the established metrics of ate (absolute trajectory error) and rpe (relative pose error) presented in [2]. ate is given as: fi = q−1 i spi, (1) where q−1 i is transformation matrix, which maps (i + 1)th point of ground truth to ith point of ground truth. pi is a similar matrix, which maps ith point of the estimated trajectory to the (i + 1)th point of the trajectory. s is a transformation matrix, which aligns the estimated trajectory to the ground truth. ate measures the absolute error of all estimated 6-dof positions, which means that ate can be represented by atet and ateφ, where atet are euclidean distances between estimated poses and corresponding ground truth positions and ateφ is the absolute error of the estimated orientations. rpe measures the drift of the estimated trajectory (error in shape of the trajectory estimate) and it is given by the equation: ei = (q−1 i qi+∆)−1(p−1 i pi+∆), (2) where ∆ defines a fixed frame interval. we used ∆ = 1 for the evaluation, which means that rpe measures a drift of visual odometry. rpe can be also represented by rpet (the relative translational error) and rpeφ (the relative rotational error). 3. evaluation to test our hypothesis and prove its feasibility we designed a practical experiment where we evaluated the localization accuracy provided by an offline frame-byframe processing of a visual slam dataset captured by a hexapod robot traversing rough terrain with a ground truth provided by an external localization system. in particular, we focused on evaluation with the structured light camera and rgb-d slam algorithm based on [6]. we choose to use the structured light sensor because it is inexpensive and provide the slam method with a metric information unlike the stereo and monocular slam methods which can estimate the trajectory up to scale. for the establishment of a reliable ground truth, we used motion capture system whycon [7] which tracks the circular markers attached to the robot using two logitech hd pro webcam c920 cameras. used external localization provided centimeter-level precision ground truth. we used the rgb-d camera asus xtion pro for the visual slam, which acquires a color image and also depth measurements. the experimental dataset captured using the rgb-d camera mounted on a hexapod walking platform contains sequences of rgb and depth images in full asus xtion pro resolution 640 × 480 px at 10 frames per second. the whole dataset for the evaluation was captured using the ros framework [9]. 3.1. rgb-d slam the considered rgb-d slam system is based on [6] and it operates as follows. first positions of the salient image features are extracted from the rgb image supplemented by their depth provided by the rgb-d sensor. then, a rigid transformation is found 2 vol. 6/2016 on construction of a reliable ground truth frame-by-frame processing online processing trial atet ateφ rpet rpeφ end dist. atet ateφ rpet rpeφ end dist. [cm] [rad] [cm] [rad] [cm] [cm] [rad] [cm] [rad] [cm] no. 1 4.00 – 0.92 – 7.11 9.60 – 1.13 – 3.5 no. 2 4.32 – 0.54 – 12.54 8.13 – 0.56 – 8.93 no. 3 3.68 – 0.40 – 5.21 22.02 – 0.87 – 33.15 no. 4 9.97 0.08 1.21 0.01 9.13 12.50 0.09 1.31 0.01 7.12 no. 5 11.44 0.06 0.80 0.01 15.96 15.64 0.06 0.94 0.02 27.55 table 1. trajectory estimation results y [m] ground-truth and rgb-d slam trajectories 1 0.5 0 -0.5 -12 x [m] 1 0 1.5 -1 -0.5 0 0.5 1 z [ m ] ground-truth rgb-d slam (a) . no.3 y [m] ground-truth and rgb-d slam trajectories 1 0.5 0 -0.5 -12 x [m] 1 0 0.5 -0.5 0 1 z [ m ] ground-truth rgb-d slam (b) . no.4 y [m] ground-truth and rgb-d slam trajectories 1 0 -1 2 x [m] 1 0 1.5 1 -1 0.5 0 -0.5 z [ m ] ground-truth rgb-d slam (c) . no.5 figure 2. trajectory estimation results. using the ransac [10] algorithm between the currently processed frame and a subset of already mapped frames. this subset consists of np directly preceding frames, ng graph neighboring frames and nr random frames from the whole map which serves to detect large loop closures. the estimated pose is then added to the map (pose graph) and refined using the g2o graph optimizing algorithm [11]. the optimization is especially beneficial in case of large loop closures when the whole trajectory estimate is improved. 3.2. experimental setup experimental environment (see fig. 1) that has been used for capturing of the dataset consists of a plane, low stairs, ramp, and square filled with wooden blocks of different heights. the dataset has been captured by a hexapod walking robot equipped with adaptive motion gait [12] which enables the robot to traverse difficult obstacles. the robot was guided by an operator along the squared path of approx. 9 m length. our experimental environment has been designed to represent some basic problems, which can slam method face. these include high speed of rotation at the corners of the trajectory, which affects the precision of rotation estimation. the stairs and wooden blocks parts of the trajectory force angular rotations along the geometrical axis and the descending ramp force the slam method to deal with the limited number of image features. moreover, the locomotion of the hexapod robot induces unpredictable and abrupt camera motions during the whole trajectory. 3.3. results altogether 5 trajectories were captured and analyzed in order to test our hypothesis. ground truth for the first three trajectories; however, does not contain information about the heading of the robot, and thus only translational component of the ate and rpe can be verified. the considered rgb-d slam parametrization for the frame-by-frame dataset processing has been the surf [13] feature extractor detecting 600 features and keeping 400 best matches with a very long comparison horizon of np = 16, ng = 4 and nr = 1. ransac stopping criteria have been set to 85% of inliers and 1000 iterations considering point an inlier if it is not outside the range of 3 pixels. optimization precision has been set to the best values optimizing each individual frame. such a processing took more than 4 hours per one trajectory on intel core i5 machine. to show the feasibility of our approach we processed the dataset online as well. the parametrization for the online processing has been the surf feature extractor detecting 400 features and keeping 250 of them, np = 3, ng = 3 and nr = 1 with 500 ransac iterations and optimization after every 10 frames. such a processing achieves an online performance of average 5.2 frames per second on the same evaluation hardware. the results are summarized in table 1. resulting trajectories for trials no. 3, no. 4 and no. 5 are depicted in figure 2. 3 j. bayer, p. čížek, j. faigl acta polytechnica ctu proceedings 3.4. discussion table 1 indicates that an average atet, when using frame-by-frame processing, is about 1% of the trajectory length and the average rpet is under 1 cm. notice the atet and rpet values are always higher for the online processing results. on the other hand, differences in the ateφ and rpeφ are neglectable although the slight imperfections in the orientation estimation are the most influencing factor in the accuracy of the localization [14]. the results also indicate that the frame-by-frame processing provides the best result obtainable by a given slam method. thus, it is possible to use the estimated ground truth for the evaluation of the online versions of the same algorithm to find the best parametrization which is deployable online onboard of the mobile robot. note, the resulting localization accuracy is also greatly influenced by the loop closure in case the robot visits previously mapped location. if there is no loop closure, then the trajectory estimate is given as the visual odometry only, which error is asymptotically unbound. in case the loop closure is detected, the optimization process corrects the pose estimates along the whole trajectory. the phenomenon is illustrated in figure 2c, where the loop did not close; thus, the trajectory estimate exhibits higher error in the end part of the trajectory, and figure 3 which presents the results of the frame-by-frame processing of the same trajectory with open-loop enabled and disabled. the results for the closed-loop trajectory are atet = 4.19 cm, rpet = 0.94 rad·10−2 and end dist = 7.33 cm, while for the open-loop trajectory are atet = 8.72 cm, rpet = 0.96 rad·10−2 and end dist = 13.73 cm. although the drift of the visual odometry represented by the rpet is similar in both cases, the resulting end distance and absolute trajectory error are higher when the open-loop scenario is considered. thus, we recommend designing the experimental trajectory to exploit the loop closures to obtain a better ground truth estimate. note the prerequisite of our method is that there are no systematic errors, which can significantly decrease the precision of ground truth estimate. to avoid systematic errors we recommend to use a special shape of the robot’s path, e.g., square. 4. conclusion in this paper, we have studied the problem of the localization accuracy assessment in a slam task whenever a reliable ground truth provided by an external localization system is not available. we have proposed to use frame-by-frame processing of the captured dataset to obtain a sufficiently reliable ground truth for the quantitative evaluation of the online slam systems. the estimated ground truth is then suitable for the comparison of different slam methods or determination of the best parametrization of the online slam. note, the important thing for the dataset creation is ground-truth and rgb-d slam trajectories -2 -1.5 x [m] -1 -0.5 0 1.5 1 0.5 y [m] 0 -0.5 0 0.6 0.4 0.2 z [ m ] ground-truth closed-loop open-loop figure 3. comparison of closed-loop and open-loop frame-by-frame processed trajectories the loop closing, which can significantly improve the robot localization and it is also important to design the shape of the path in order to avoid systematic errors. we tested the presented method with a hexapod crawling robot and rgb-d slam system, but we suppose that the proposed method is applicable to different slam methods and different platforms too. presented results support the feasibility of the proposed approach and can be utilized in cases when expensive and hard to setup external localization systems are not available or suitable. acknowledgements the presented work was supported by the czech science foundation (gačr) under research project no. 15-09600y. the support of grant no. sgs16/235/ohk3/3t/13 to petr čížek is also gratefully acknowledged. references [1] r. kümmerle, b. steder, c. dornhege, et al. on measuring the accuracy of slam algorithms. autonomous robots 27(4):387–407, 2009. doi:10.1007/s10514-009-9155-6. [2] j. sturm, n. engelhard, f. endres, et al. a benchmark for the evaluation of rgb-d slam systems. in ieee international conference on intelligent robots and systems, pp. 573–580. 2012. doi:10.1109/iros.2012.6385773. [3] s. thrun, w. burgard, d. fox. probabilistic robotics. mit press, 2005. doi:10.1162/artl.2008.14.2.227. [4] m. menze, a. geiger. object scene flow for autonomous vehicles. in conference on computer vision and pattern recognition, pp. 3061–3070. 2015. doi:10.1109/cvpr.2015.7298925. [5] n. sünderhauf, k. konolige, s. lacroix, p. protzel. visual odometry using sparse bundle adjustment on an autonomous outdoor vehicle, pp. 157–163. springer berlin heidelberg, 2006. doi:10.1007/3-540-30292-1_20. [6] f. endres, j. hess, n. engelhard, et al. an evaluation of the rgb-d slam system. in ieee international conference on robotics and automation, pp. 1691–1696. 2012. doi:10.1109/icra.2012.6225199. 4 http://dx.doi.org/10.1007/s10514-009-9155-6 http://dx.doi.org/10.1109/iros.2012.6385773 http://dx.doi.org/10.1162/artl.2008.14.2.227 http://dx.doi.org/10.1109/cvpr.2015.7298925 http://dx.doi.org/10.1007/3-540-30292-1_20 http://dx.doi.org/10.1109/icra.2012.6225199 vol. 6/2016 on construction of a reliable ground truth [7] t. krajník, m. nitsche, j. faigl, et al. a practical multirobot localization system. journal of intelligent & robotic systems 76(3-4), 2014. doi:10.1007/s10846-014-0041-x. [8] s. bao. semantic structure from motion. in intl. conf. on computer vision and pattern recognition (cvpr), pp. 2025–2032. 2011. doi:10.1109/cvpr.2011.5995462. [9] m. quigley, k. conley, b. p. gerkey, et al. ros: an open-source robot operating system. in ieee international conference on robotics and automation (icra) – workshop on open source robotics. 2009. [10] m. a. fischler, r. c. bolles. random sample consensus: a paradigm for model fitting with applications to image analysis and automated cartography. communications of the acm 24(6):381–395, 1981. [11] r. kümmerle, g. grisetti, h. strasdat, et al. g2o: a general framework for graph optimization. in ieee international conference on robotics and automation, pp. 3607–3613. 2011. doi:10.1109/icra.2011.5979949. [12] j. mrva, j. faigl. tactile sensing with servo drives feedback only for blind hexapod walking robot. in 10th international workshop on robot motion and control, romoco, pp. 240–245. 2015. doi:10.1109/romoco.2015.7219742. [13] h. bay, a. ess, t. tuytelaars, l. van gool. speeded-up robust features (surf). computer vision and image understanding 110(3):346–359, 2008. doi:10.1016/j.cviu.2007.09.014. [14] p. čížek, j. faigl. on localization and mapping with rgb-d sensor and hexapod walking robot in rough terrains. in ieee international conference on systems, man, and cybernetics, pp. 2273–2278. 2016. 5 http://dx.doi.org/10.1007/s10846-014-0041-x http://dx.doi.org/10.1109/cvpr.2011.5995462 http://dx.doi.org/10.1109/icra.2011.5979949 http://dx.doi.org/10.1109/romoco.2015.7219742 http://dx.doi.org/10.1016/j.cviu.2007.09.014 acta polytechnica ctu proceedings 6:1–5, 2016 1 introduction 2 proposed solution 2.1 evaluation metrics 3 evaluation 3.1 rgb-d slam 3.2 experimental setup 3.3 results 3.4 discussion 4 conclusion acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2015.1.0015 acta polytechnica ctu proceedings 2:15–21, 2015 © czech technical university in prague, 2015 available online at http://ojs.cvut.cz/ojs/index.php/app reconnaissance micro uav system petr gabrlika,∗, vlastimil kriza, ludek zaludb a ceitec central european institute of technology, brno university of technology, technicka 3082/12, 616 00 brno, czech republic b faculty of electrical engineering and communication, brno university of technology, technicka 3082/12, 616 00 brno, czech republic ∗ corresponding author: petr.gabrlik@ceitec.vutbr.cz abstract. this paper describes the design and implementation of the uranus uav. this quad-rotor flying robot was created to extend the abilities of the hitherto developed with airborne missions. the first part deals with the mathematical model of the robot. next, the control system is designed, and the proposed hardware as well as the implemented software solution are presented. for integration into the robotic system, a new communication protocol was created and is described here too. keywords: uav, multirotor, robotics, mathematical model, state-space controller. 1. introduction in the research laboratories of ceitec and but, the cassandra robotic system has been developed [1]. this is a heterogeneous group of robots controlled mainly by telepresence from the operator base station. within the described concept, each robot is assigned a specific task, such as reconnaissance, terrain mapping or contamination measurement. the intended purpose of the entire group is to perform these tasks in locations which are dangerous or inaccessible for humans. until recently, this group included only ground robots. in order to enable the group to pursue the assigned tasks from the air or to reach locations unapproachable for ground robots, it was decided to extend the group with a flying robot. to facilitate operation in a closed environment where airplanes can manoeuvre only with difficulty, an aircraft with the ability to hover was required. because airships provide a poor payload/dimensions ratio, helicopters were taken into consideration; the first attempts were carried out with classical machines. however, such helicopters were not robust enough for the intended use in a difficult environment, mainly due to the fact that they contain lot of small moving parts which get easily damaged. quadrotors (and multi-rotor copters in general) have proved to be the best platform for our purposes. the only moving parts on these devices are their rotors rigidly connected to the engines. everything is controlled just by changing their rotor speeds. yet this solution also exhibits a disadvantage, namely internal instability; thus, a control system needs to be implemented to provide the necessary operational ability [2]. currently, quadrotors are being developed within a large number of projects, from hobby to professional ones. after investigating some of these projects, we considered the given status and decided to create our own solution. this approach offers a significant advantage: as the applied software is not license-restricted, figure 1. uranus uav during a flight. we will have full control over the solution and will be able to modify our device if necessary. in this context, a specific airframe and control system were created. to design the control system, the behavior of the designed airframe with rotors was described and mathematical model assembled. 2. quadrotor model the mathematical model of the uranus uav comprises two parts. the first part handles the relation between the speed of the rotors and the forces and torques affecting the quadrotor rigid body. the second portion defines the dynamics of the rigid body, including transformation between frames and effect of gravity force. this separation allows us to design a controller with better dynamics, because the first part of the model can be easily linearized in the controller. the forces and torques caused by propellers of the rotors and affecting the quadrotor rigid body are denoted as u1 to u4. the meaning is as follows: u1 torque around the x axis caused by different thrust of rotors 2 and 4; u2 torque around the y axis caused by 15 http://dx.doi.org/10.14311/app.2015.1.0015 http://ojs.cvut.cz/ojs/index.php/app p. gabrlik, v. kriz, l. zalud acta polytechnica ctu proceedings different thrust of rotors 1 and 3; u3 torque around the z axis caused by different reaction torques from rotors rotating in opposite directions; u4 force in the z axis caused by common thrust of all the rotors. the relation between the thrust and propeller speed of a rotor is ft = kt · n2, (1) and the relation between the reaction torque and propeller speed of a rotor is mr = km · n2. (2) here, kt and km are constants determined by the measurement. the first part of the model then could be described as u1 = (−n2 2 + n2 4)kt l u2 = (n2 1 − n2 3)kt l u3 = (−n2 1 + n2 2 − n2 3 + n2 4)km u4 = (n2 1 + n2 2 + n2 3 + n2 4)kt (3) l denotes the distance between the rotors and the center of gravity of a quadrotor. the second part of the model can be further divided into four parts: equations that describe rotational movement; equations of linear movement; equations describing transformation between frames; and equations expressing the effect of gravity force. all variables are defined in 2 frames: robot frame and ground frame. the robot frame is bound to the uav while the x axis faces forward, y faces right, and z faces down. the ground frame is bound with the ground and oriented north-east-down. rotational movement (robot frame): ω̇x = (iyy − izz)ωyωz + u1 ixx ω̇y = (izz − ixx)ωxωz + u2 iyy ω̇z = (ixx − iyy)ωzωy + u3 izz (4) ixx, iyy and izz represent moments of inertia around a given axis. translation movement (robot frame): v̇x = mωyvz +mωzvy +gx m v̇y = mωxvz +mωzvx +gy m v̇z = mωxvy +mωyvx +gz − u4 m (5) gx, gy and gz are the gravity forces component in a given axis: gx = −mg sin θ gy = mg cos θ sinφ gz = mg cos θ cosφ (6) transformation between the robot frame and ground frame [3]: φ̇ = ωx + ωy sinφ tan θ + ωz cos θ tan θ θ̇ = ωy cos θ − ωz sinφ ψ̇ = ωy sinφ cos θ + ωz cosφ cos θ ẋ = vz(sinφ sinψ + cosφ cosψ sin θ) − −vy(cosφ sinψ − cosψ sinφ sin θ) + +vx cos θ cosψ ẏ = vy(cosφ cosψ + sinφ sin θ sinψ) − −vz(cosψ sinφ− cosφ sin θ sinψ) + +vx cos θ cosψ ż = vz cosφ cos θ − vx sin θ + vy cos θ sinφ (7) the parameters m, l, kt , km of the models was measured and ixx, iyy, izz were computed. the values are as follows: m = 1.05 kg l = 0.35 m kt = 0.14 · 10−6 n ·rpm−2 km = 4.6 · 10−9 nm ·rpm−2 ixx = 8.6 · 10−3 kg ·m2 iyy = 8.6 · 10−3 kg ·m2 izz = 12.0 · 10−3 kg ·m2 (8) 3. controller design to stabilize the uranus uav, a controller based on state space representation of the quadrotor was designed. as shown in sect. 2, the model of the quadrotor comprised 2 parts. the first part handles the non-linear relation between the rotation speed of the propellers and the introduced variables u1 u4 denoting the forces and torques affecting the rigid body. because this part of the model does not contain any dynamics, this relation can be expressed by purely static equations (3). to handle this, a mathematical block with 16 vol. 2/2015 reconnaissance micro uav system inverse function (9) is implemented in the controller. n1 = √ 2kmu2 − kt lu3 + km lu4 4kmkt l n2 = √ −2kmu1 + kt lu3 + km lu4 4kmkt l n3 = √ −2kmu2 − kt lu3 + km lu4 4kmkt l n4 = √ 2kmu1 + kt lu3 + km lu4 4kmkt l (9) this block, together with the first part of the model, will act as a linear section with unitary transfer function (while operating in the working range of the rotor drivers). this allows direct use of the variables u1 u4 as inputs to the system and enables us to treat these parts of the system as linear. the schema of the system with the controller is shown in fig. 2; here, a classical state space controller is extended by few enhancements. the first one is a bias (u4−0) added to the input u4, whose function is to ensure the thrust needed to keep the quadrotor in hover flight. the next one consists in adding a new state iz , which is an integral of the altitude error, z. the purpose of this is to achieve zero steady-state error while hovering, because it is almost impossible to set up a thrust that exactly compensates the gravity force. the design of the state space controller is based on a linearized model of the rigid part of the quadrotor. equations (6) and (7) are linearized around the working point, which is hovering. for the equilibrium point, we have φ = θ = ωx = ωy = 0. (10) to reach this state, the thrust of the rotors must compensate the gravity force acting in z axis at horizontal flight. thus u4−0 = mg. (11) the system matrices a and b required for the controller design are derived from linearized equations (4), (5), (6) and (7). the matrix c is an identity matrix, and the matrix d is a zero matrix. the matrix k, which determines the control law, was obtained experimentally by the pole placement method. matrix k: ωx ωy ωz vx vy vz φ θ ψ x y z iz u1 1.05 0 0 0 0.29 0 2.66 0 0 0 0.12 0 0 u2 0 1.05 0 −0.29 0 0 0 2.66 0 −0.12 0 0 0 u3 0 0 0.66 0 0 0 0 0 0.44 0 0 0 0 u4 0 0 0 0 0 −4.08 0 0 0 0 0 −6.32−3.06 it can be observed from this matrix that the linearized state space description decoupled the system into independent subsystems for each axis.; the controller therefore handles it in this manner. in fact, from the perspective of attitude control, the state space controller can be understood as a set of separated pi or pd controllers. the results then prove that the approach involving a space controller and the approach employing pid controllers provide comparable results in this mode of flight. first, the real behavior of the designed controller was tested in a laboratory. figs. 3 and 4 show the step responses for the angular rotations pitch and yaw, which were both measured independently on a special tool. 4. implementation 4.1. construction the uranus uav has a well-known, four-rotor structure often referred to as quadrotor or quadrocopter. the construction consists of two orthogonal aluminum beams, which form a symmetric cross. all the electronic devices and printed circuit boards (pcb) are situated in the middle of the cross to ensure that the center of gravity is also in the middle. at the end of each beam there is one brushless dc (bldc) axi 2212/34 electromotor equipped with a graupner 11"x 5" carbon propeller. to suppress the mechanical vibrations, the motors are mounted on the rigid quadrotor frame through rubber pads. the uav overall dimensions are 701x701x188 mm, and the weight is 1.05 kg. the maximal combined motor thrust is about 24 n, so it is possible to carry a 150 g payload without reducing the maneuverability. the construction is mechanically simple and also allows additional modifications. 4.2. hardware the uav is equipped with several pcbs. the central one is the control board, which contains a control microcontroller (mcu), a power supply, and various sensors. the other boards are the communication module, bldc controllers, and ultrasound sensor. the sensor unit and gps receiver are encased in their own packages, and they are connected to the control board. the block scheme of the uranus uav system is shown in fig. 5. all the electronic devices on the uav are powered by a li-pol accumulator. in this construction, a 3-cell accumulator with the nominal voltage of 11.1 v, and the capacity of 2200 mah is used. this voltage is fed to all the engines, while the remaining electronics are supplied with 3.3 v and 5 v. the voltage level reduction is performed by switching and ldo (lowdropout regulator) stabilizers. the control board was designed specifically for this application. the main part of the board consists in an mcu lm3s8962, which is based on the 32-bit arm cortex m3 architecture and is able to operate at frequencies up to 50 mhz. it provides sufficient 17 p. gabrlik, v. kriz, l. zalud acta polytechnica ctu proceedings figure 2. designed controller. 0 5 10 15 −5 0 5 10 15 time [sec] θ [° ] figure 3. pitch angle step response. 0 5 10 15 0 20 40 60 80 100 time [sec] ψ [° ] figure 4. yaw angle step response. computing power [4], all the required communication buses (spi, uart, i2c), and internal a/d converters. an xsens mti-g is applied as the main sensor unit; this component is an inertial measurement unit (imu) and an attitude and heading reference system (ahrs). firstly, instead of this imu, a vectornav vn-100 was used. with this imu, however, we obtained worse results in stabilization, since the linear movement also cause a change in the measured angles. thus, it was replaced with the xsens mti-g, a component widely used in other uav projects, e.g. [5], [6]. the unit contains triaxial digital devices, namely an accelerometer, a gyroscope, and a magnetometer; further, the unit also comprises a gps receiver in one package with an external gps antenna. an internal kalman filter provides precise orientation estimation with 1 deg rms roll/pitch and 2 deg rms heading dynamic accuracy and the accuracy of 2.5 meters cep in position. the control board is also equipped with a bmp085 barometric sensor for the purpose of altitude measurement. an ultrasound sensor, which is used at low altitudes (during take-off and landing), is situated on the special pcb. the battery level is determined on the basis of the voltage and current measurement. regularly available bldc controllers bl-ctrl v2.0 provide the continuous current of 35 a for each motor and a very compact design. the controllers communicate with the control board via an i2c bus. it is also possible to mount an ip wireless camera on the construction and broadcast the image to the base station. 4.3. microcontroller software the software for the mcu was written in the embedded c language due to high requirements for the real-time behavior and reliability. the software does not use a commercial rtos but a simple and nonpreemptive os written specifically for this application. to achieve the real-time behavior, a system timer with the period of 100 ns was used; this is the shortest possible period to handle any application task. the application also uses mcu hardware interrupts to handle various asynchronous events. the interval for the computing of the main quadrotor stabilization (angular rotations stabilization) is 10 ms. at the same period, the data from the imu/ahrs are read and the actuator speeds updated. the sample period in altitude measurement depends on the used sensor; for example, we thus have the period of 25 ms for the ultrasonic sensor. the implemented control structures for the stabilization are constant, but their parameters can be tuned according to the current uav configuration. the application operates in several modes: the configuration/stop mode, applied when the uav is switched on but the actuators are disabled; the flight remote mode, used for standard remotely controlled operation; the signal lost mode, activated in the flight mode when the signal is lost and the uav is landing automatically; and the flight auto mode, which is ready for future autonomous flying using gps. 18 vol. 2/2015 reconnaissance micro uav system mcu arm cortex m3 ultrasound board 3-axis accelerometer xsens mti-g 3-axis gyroscope 3-axis magnetometer gps receiver 4x bldc electromotor 4x bldc controller zigbee wireless module zigbee wireless module notebook + cassandra hid / gamepad uranus uav base station uart1 uart2 i c 2 usb uart usb pressure sensor i c 2 figure 5. the block scheme of uranus uav system. figure 6. uav control board and the other electronics. 4.4. communication the communication between the robot and the base station is wireless. it is provided by two paired xbee pro s2b rf modules transmitting within the band of 2.4 ghz; the modules use the zigbee protocol and operate in a transparent mode. in this mode, the 2 devices work as a virtual uart wire. thus, every uart frame send to the input of one module appears on the output of the other (if the rf channel is established). the baud rate of this line is 57.6 kbps. above this layer of communication, the uranuslink protocol was created. it was designed as a packetoriented, non-reliable protocol. this protocol defines the packet structure as well as the transmitted data representation. as shown in table 1, every packet in uranuslink consists of 6 fields: the preamble (pre); the sequence number (sqn); the message identification (mid); the data length (len); the data as such; and the checksum (cs). all the data are in the big endian format. the preamble has the constant value of 0xfd. it is used to determine the beginning of the packet. the sqn is intended for the numbering of the packets; while every sent packet increases this number by 2, the sqn can only take even values. this property is utilized in one of the packet validation mechanisms. another important application of the sqn is in packet loss detecting. the mid determines the interpretation of bytes in the data part of the message, while the len specifies the length of this part. there are 24 different packet types distinguished by the mid. the most important ones are: compounded joystick command, robot mode, and acknowledge in the direction towards the robot; and all data stream, battery voltage, and acknowledge in the direction towards the base station. the computed joystick command holds the required roll, pitch, yaw, and thrust. the robot mode is used to switch between the robot modes, as described in sect. 4.3. this packet is specific, because it must be confirmed by the acknowledge packet from the robot. the acknowledge packet carries, in its data section, the sqn number of the packet which is confirmed by it. towards the base station, the all data stream carries important telemetric data such as the robot attitude and altitude, gps position, and status (e.g., if the gps data are valid or the inertial measurement unit was initialized successfully). while these data are sent every 10 ms, the battery voltage, for example, is sent once per second. in the direction towards the robot, the compounded joystick command is sent approximately every 100 ms and the robot mode only when it is required to switch the mode. as stated above, this packet must be confirmed by the acknowledge packet. if it is not, the station begins retransmitting it and continues until confirmation is obtained. this is carried out to ensure that the robot mode will be switched even when the packet is lost. the other packets remain unconfirmed, because the system overheads will be more significant than the advantage of ensuring that all flight data will be transmitted ok. if there is a packet outage lasting longer than 0.5 s, the robot will handle this as signal loss and take an action. it will set the required roll, pitch and yaw to zero; also, the system performs slow descent and can proceed to eventual landing. 4.5. base station one of the requirements for the uranus was its integration in the cassandra system. this system has been developed at ceitec and the but for several 19 p. gabrlik, v. kriz, l. zalud acta polytechnica ctu proceedings pre sqn mid len data ... cs 1b 2b 1b 1b 1-252 b 1b table 1. uranuslink packet format. years. this system comprises a heterogeneous group of robots and a base station. the core of the base station consists of software written in c#, communication hw, and human-device interface. this interface includes a joypad and a virtual reality helmet, or, in the simpler version, a monitor screen and an optional keyboard. the base station software can be run on a classical pc, a laptop, or an embedded pc; thus, the base station can be a mobile or even wearable system. a new communication hw was built to operate the uranus uav. this portable device consists of x-bee modules (as described in section 4.4), a high-gain antenna, and an uart-over-usb converter, all placed in box attached to a foldable tripod stand to ensure a higher position of the communication system above the ground (and, thus, better coverage). from the base station, the operator can control all the powered robots by switching among them. the display provides the operator with a first person view from the robot. the robot movements are controlled via the joypad. if the camera on a given robot is capable of rotating, its motion can by controlled via movement of the operator’s head (in case that the virtual reality helmet is used) or by means of the joypad. the cassandra base station display provides, besides a view from the onboard camera, important telemetric and other mission-related data, as shown in fig. 8. the bottom part of the display presents attitude information, namely the actual roll and pitch angle in the artificial horizon in the left circular indicator, and the yaw angle in the compass in the right one. the upper left corner includes the robot status indicator. this contains the name of the active robot, its battery voltage, signal outage indicator, actuator state, robot mode, and other information if required. the lower part of this indicator can show important text information and warnings. all the indicators can be hidden, plus there are a couple of indicators not opened by default. such indicators display, for example, the status of the other robots, the actual position of the robot on the map, or the actual camera orientation. 5. operation for flying with the uranus uav, we use the following configuration: a laptop with the cassandra system installed; a joypad; and a designed communication device. the uranus is controlled by a joypad with 2 joysticks, of which the left one regulates the pitch and roll and the right one modifies the thrust and yaw. before the flight, the following steps need to be carried out: after switching the power on, the uranus figure 7. base station and uranus uav. figure 8. the user interface of the cassandra system. uav is in the configuration/stop mode. this mode has to be switched to the run mode from the joypad. subsequently, all the rotors start and operate at a very low speed. after that, the right joystick, which controls the thrust, needs to be pushed all the way down. then the rotors will gradually achieve the speed corresponding to the actual joystick position. this solution was implemented to prevent the rotors from starting at very high rpms if the mode is switched with the joystick in default position. from this moment on, the uranus uav is ready to flight. if the thrust joystick is released, the robot will take off and hover at about 1 meter above ground. to precisely suppress the gravity force by the thrust, the output levels sent to the motors from the controller are modified according to the battery voltage. this occurs because the thrust of the motors varies in accordance with the battery voltage. the battery voltage level is also reported to the base station to warn the pilot if the voltage drops too low. a warning is also produced by signal loss. if this happens, the uranus uav will automatically land. during the test flights, it was proven that uranus is stable, and while the control joysticks are released, the quadrotor hovers in position. but because no position stabilization has been implemented to date, the uranus could slowly drift from the given position. this feedback will be integrated only in the next versions. controlling the uranus with the joypad was easy, and the operator was able to operate the uav 20 vol. 2/2015 reconnaissance micro uav system effectively after a short while. 6. conclusion regarding the successful test flights, it can be concluded that the assembled mathematical model clearly expresses the quadrotor behavior; thus, the designed control system was able to stabilize the robot suitably. although the controller was designed as a state-space device, it was proven that the resulting controller can be interpreted as a set of separate pi or pd controllers in each axis. this is due to the model linearization around the working point, namely hovering. for intended use, the described approach provides good results: the quadrotor was able to hover and to maneuver. to facilitate integration in the cassandra system, a new communication protocol was designed, and the cassandra system was modified to be capable of handling the flying robots. as the purpose of the developed uranus uav was to enable the cassandra robotic system to perform air missions, it can be stated that the given goal was achieved. for the future, we plan to navigate the uranus uav by the gps, to design our own imu, and to create a bigger airframe with a higher payload capacity. list of symbols θ angle of rotation around the lateral axis [◦] φ angle of rotation around the longitudinal axis [◦] ψ angle of rotation around the vertical axis [◦] ω angular speed [◦ s−1] ft thrust [n] g gravitational acceleration [m s−2] g gravity force [n] i mass of inertia [kg m2] km rotational speed to reaction torque ratio constant [nm rpm−2] kt rotational speed to thrust ratio constant [n rpm−2] l length [m] m weight [kg] mr reaction torque [nm] n rotations per minute [rpm] u1 torque in the longitudinal axis caused by motors [nm] u2 torque in the lateral axis caused by motors [nm] u3 torque in the vertical axis caused by motors [nm] u4 force in the vertical axis caused by motors [n] u4−0 force in the vertical axis required to compensate gravity force [n] v linear speed [m s−1] x longitudinal axis; position in this axis [–; m] y lateral axis; position in this axis [–; m] z vertical axis; position in this axis [–; m] acknowledgements this work was supported by the project ceitec central european institute of technology (cz.1.05/1.1.00/02.0068) financed from european regional development fund and by the project fekt-s-142429 “the research of new control methods, measurement procedures and intelligent instruments in automation” financed from internal grant system but. references [1] l. zalud, l. kopecny, f. burian, t. florian. cassandra heterogeneous reconnaissance robotic system for dangerous environments. in 2011 ieee/sice international symposium on system integration (sii), pp. 1275–1280. 2011. doi:10.1109/sii.2011.6147632. [2] s. bouabdallah. design and control of quadrotors with application to autonomous flying. ph.d. thesis, ãľcole polytechnique fãľdãľrale de lausanne, 2007. doi:10.5075/epfl-thesis-3727. [3] f. solc. modelling and control of a quadrocopter. advances in military technology 5(2):29–38, 2010. [4] h. lim, j. park, d. lee, h. kim. build your own quadrotor: open-source projects on unmanned aerial vehicles. ieee robotics automation magazine 19(3):33–45, 2012. doi:10.1109/mra.2012.2205629. [5] s. grzonka, g. grisetti, w. burgard. a fully autonomous indoor quadrotor. ieee transactions on robotics 28(1):90–100, 2012. doi:10.1109/tro.2011.2162999. [6] k. alexis, c. papachristos, g. nikolakopoulos, a. tzes. model predictive quadrotor indoor position control. in 2011 19th mediterranean conference on control automation (med), pp. 1247–1252. 2011. doi:10.1109/med.2011.5983144. 21 http://dx.doi.org/10.1109/sii.2011.6147632 http://dx.doi.org/10.5075/epfl-thesis-3727 http://dx.doi.org/10.1109/mra.2012.2205629 http://dx.doi.org/10.1109/tro.2011.2162999 http://dx.doi.org/10.1109/med.2011.5983144 acta polytechnica ctu proceedings 2:15–21, 2015 1 introduction 2 quadrotor model 3 controller design 4 implementation 4.1 construction 4.2 hardware 4.3 microcontroller software 4.4 communication 4.5 base station 5 operation 6 conclusion list of symbols acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2017.12.0112 acta polytechnica ctu proceedings 12:112–115, 2017 © czech technical university in prague, 2017 available online at http://ojs.cvut.cz/ojs/index.php/app dangerous driver’s behavior libor topol∗, ivo drahotský univerzita pardubice, dopravní fakulta jana pernera, studentská 95, 532 10 pardubice ∗ corresponding author: libortopol@seznam.cz abstract. different types of dangerous driver’s behavior are discussed in this paper. case study analysis of young drivers’ hazardous behavior is also provided from authors’ practices. the practical research is collaborated with the statistical data that aggressive driving induces the number of fatalities on the roads in the czech republic by 25%. the improvement to driver’s training and other countermeasures is then suggested. keywords: dangerous driving behavior, driver training, road safety. year number of accidents number of fatalities number of serve injuries number of light injuries 2010 75522 753 2823 21610 2011 75137 707 3092 22519 2012 81404 681 2986 22590 2013 84398 583 2782 22577 2014 85859 571 2409 18074 2015 93067 660 2540 24426 table 1. evolution of basic indicators of accidents in 2010-2015. 1. introduction road transport occupies a significant segment in transportation in general and due to its high accident rates and hence economical losses, considering the affecting factors is rather crucial for safety improvement. road traffic safety may be characterized by direct and indirect safety indicators. direct indicators are the number of road accidents, their consequences and economical losses. indirect indicators are phenomena or circumstances that project safety assessment into experimental plane of relations between human behavior and transportation safety (following the speed limits, application of safety systems, maintaining safe distances between vehicles, estimation and anticipation of errors in real traffic). high traffic intensity on roads is a reason of high workload for car drivers. this overload is related to the number of accidents in the czech republic, which, despite various measures for its reduction in comparison with other european countries, is still very high. traffic accidents are a major social issue. the table tracks a noticeable increase of fatalities in traffic accidents. number of light and heavy injuries increases as well. one may notice a constant increase in total number of accidents. in 2015 total of 93,067 accidents was figure 1. total number of recorded accidents in the czech republic from 2011-2015. reported and investigated. for example, compared to 2014, there were about 7,208 more accidents in 2015. every day the czech police has conduct around 255 accident investigations on average. this is equal to approximately 14.901.712,00 czk of property losses every day[1]. 2. dangerous driver’s behavior among other factors, aggressive driving contributes to traffic accidents to a high extent. per úamk, 91% of drivers have been exposed to aggressive driving to a certain extent. under the term of dangerous driver’s behavior, we may assume the one that is in contradiction to official and so-called unofficial or nonwritten rules, an unexpected, aggressive or unusual and non-customary behavior on the road[2]. human behavior is diverse. driver’s behavior differs from every person. it is possible to categorize dangerous behavior from different perspectives. inattentive driving can be related to dangerous behavior as it is, besides speeding, the most common cause of traffic accidents. another category is driving while being involved in some secondary activities that are not related to vehicle control. these are smoking, talking 112 http://dx.doi.org/10.14311/app.2017.12.0112 http://ojs.cvut.cz/ojs/index.php/app vol. 12/2017 dangerous driver’s behavior accident #1 accident #2 time of accident: 5:15 am time of accident: 6:54 road class: ii road class: ii road section type: roundabout road section type: two-way road weather conditions 8°c, clear weather conditions 12°c, clear driver’s age: 25 y.o. driver’s age: 28 y.o. vehicle vehicle car make: audi a6 car make: suzuki swift year of issue: 2001 year of issue: 2004 outcomes outcomes # of fatalities: 3 – immediate (including driver), 1 – in hospital # of fatalities: 3 – immediate (including driver) # of severely injured: 2 # of severely injured: 3 description: driver approached and entered the roundabout without any or with low speed reduction and thus couldn’t manage the turning but ran across the safety island in the middle of roundabout and broke through the roundabout fence ended up on a roof in retention tank. accident sketch can be seen in the image bellow. description: driver exit the lane and with right vehicle side hit front of vehicle in an opposing traffic. accident sketch can be seen in the image bellow. table 2. figure 2. damaged vehicles from accident 1 (left) and accident 2 (right). 113 libor topol, ivo drahotský acta polytechnica ctu proceedings on the phone while driving, eating, drinking, involving into conversations with other passengers and so on. a driver’s personality, psychological and medical condition, lack of knowledge of traffic rules can be the risk itself[3]. these and many features of driver’s behavior that can be gathered under one term of aggressive driving. committing traffic violations can occur both because of negligence, and unintentionally[4]. driver’s age is also one of the factors that influences road safety. lack of experience of novice drivers can cause a potential danger in traffic situations as well as deteriorating physical state of senior drivers[5]. hence, we can categorize dangerous behavior by age or driving experience. young drivers (age group under 24 years old) are said to be the highest risk group of drivers in agewise[6]. they lack the capacity to adequately evaluate dangerous situations, they tend to overestimate their own abilities, have little driving experience and, therefore, fewer skills in vehicle control. beginner drivers are often subject to influence of opinions of other car occupants, they often drive vehicles in poor technical condition and tend to drive high-speed powerful vehicles and thus are exposed to a false feeling of safety. statistical research confirms that young drivers are referred as the most dangerous group. they, more often than others, exceed the speed limit, overtake in sections with low visibility road sections or when not allowed and tend to hit the red traffic light signal. in this group there are also individuals who tend to drive more often under the influence of alcohol and other drugs. young male drivers tend to take more risky maneuvers, rather than young women who tend to avoid dangerous situations, by extent and frequency. young males are also less attentive than females of the same age category. it is, of course, worth noticing that percentage of female drivers in general is still lower than that of male, women drive less in general (kilometers-wise)[7, 8]. driving school instructors in many european countries believe that nowadays, young drivers behave more hazardously on the road, not realizing the potential threat of accidents and their consequences. speeding is associated with masculinity. nowadays, young drivers’ attention, compared to the one in the past, is affected by many disturbing factors such as advertising and bill-boards, loud car acoustic systems, mobile phones etc. driving instructors believe that it may be necessary to reconsider approach to young driver training in terms of study program duration and extension of training programs on driving simulators[9]. 3. traffic accidents with participation of young drivers – case study information for the following study is based on authors’ personal practice of forensic expertise. the expert evaluation of vehicles has been performed, the technical information is given in table 2. the two accidents have some typical features in common. both drivers were from the dangerous group – young drivers. in each case, the accident took place at early morning hours, which makes experts relate to so called "after party" accidents. both were older cars, which may have affected the severity of consequences. the number of passengers at the time of the accident was higher than allowed (6 car occupants in both cases). 4. conclusion dangerous behavior of younger drivers is a reason of many fatal accidents on the roads. one of the important reasons is lack of training for new drivers, which should include both psychological preparation of novice drivers and extensive practical training. also lack of experience is one of the reasons younger drivers overestimate their ability to handle vehicle at high speeds or while turning, neglect the car technical state (old cars) or its power (new cars). this aspect shall be addressed along with other valid methods of accidents rate reduction[10]. references [1] directorate of traffic police 2016. statistics of traffic accidents. online: policie.cz/clanek/statistika-nehodovosti. [2] m. šucha. dopravní psychologie pro praxi: výběr, výcvik a rehabilitace řidičů. praha: grada publishing, 2013. isbn 978-80-247-4113-0. [3] m. šucha. agresivita na cestách. 1. vyd. olomouc: univerzita palackého v olomouci, 2009. isbn 9788024423753. [4] p. bouchner, m. novák, s. novotný, et al. human behavior while driving the road vehicle. prague: faculty of transportation sciences, ctu in prague, department of vehicle technology, 2015. 1st electronic. isbn 978-80-01-05872-5. [5] m. novák, p. bouchner, j. faber, et al. senioři za volantem. praha: department of informatics academy of sciences in czech republic, v.v.i., 2008. isbn 978-80-87136-02-7. [6] s. novotný, p. bouchner, m. novák. agresivní a nebezpečné chování řidičů motorových vozidel v čr – analýza dat z plovoucích vozidel. in: dopravní nehodovost a rizikové chování řidičů motorových vozidel. praha: vyšší policejní škola mv v praze, 2013, pp. 49-59. isbn 978-80-260-5466-5. [7] z. šejdová. riskantní jednání mladých řidičů v české republice. master thesis. palacký university olomouc, 2015. [8] mladí řidiči a dopravní nehodovost. 2007. source: czrso.cz. available on 8.5.2017. [9] goodyear europe, middle east and africa. a report by goodyear europe, middle east and africa (emea). november 2013. 114 policie.cz/clanek/statistika-nehodovosti czrso.cz vol. 12/2017 dangerous driver’s behavior [10] p. bouchner, j. faber, p. hrubeš, et al. road accident reduction. roma: aracne editrice s.r.i., 2010. isbn 978-88-548-3550-4. 115 acta polytechnica ctu proceedings 12:112–115, 2017 1 introduction 2 dangerous driver's behavior 3 traffic accidents with participation of young drivers – case study 4 conclusion references 311 acta polytechnica ctu proceedings 2(1): 311–320, 2015 311 doi: 10.14311/app.2015.02.0311 who discovered the hoyle level? g. shaviv1 1department of physics, israel institute of technology, haifa, israel 32,000 corresponding author: gioras@physics.technion.ac.il abstract the prediction of hoyle that the nucleus of 12c must have a resonance at 7.62mev was the trigger to the anthropic principle. we review the history of the discovery of this level and investigate to what extent this was a genuine prediction. keywords: anthropic principle nuclear physics stellar evolution. 1 who is who three dominant personalities were involved in the present story: sir fred hoyle frs (1915-2001), who was one of the greatest astrophysicists in the second half of the twentieth century with major contributions to stellar structure (nuclear astrophysics synthesis of the elements) and to cosmology (steady state theory dubbed the name big bang) as well as planetary formation. william a. fowler (1911-1995), who can be considered as the father of nuclear astrophysics and edwin salpeter (1924-2008) top theoretical astrophysicist, who has numerous seminal contributions in many astrophysical fields as well as in physics. fred hoylewilly fowlered salpeter edwin e, salpeter william a. fowler fred hoyle figure 1: the dominant personalities involved in this story: the discovery of the triple α process. 2 what is the hoyle level the synthesis of helium into carbon in stars proceeds via resonant reaction, namely the three α particles fuse into an excited energy level in the 12c nucleus. the rate of the reaction was calculated before the existence of this level was known and was found to be very low compared to the rate of destruction of 12c by collisions with α particles. as a consequence, it was impossible to predict the evolution off the main sequence towards the red-giant branch and the calculation implied that all the 12c is converted into 16o. but stars do evolve and we know that somehow carbon is synthesized. in view of the impass, hoyle predicted therefore, that 12c has an energy level just at the right place and the reaction of carbon synthesis proceeds via this resonance level. the level was then discovered in the laboratory. this chain of events: prediction the existence of a nuclear level from astrophysical constraints, wa s considered as a big victory for astrophysics and the level was named the hoyle level. 3 what is the anthropic principle the anthropic principle is a philosophical hypothesis that measures of the physical universe must be compatible with the existence of conscious life that observes it. the phrase ”anthropic principle” appeared first in brandon carter’s contribution to the 1973 krakow symposium honoring copernicus’s 500th birthday. carter argued as well, that humans do not occupy a privileged position in the universe. the trigger to the idea that life as we know it, and the cosmos around us, ”are tuned”, emerged from hoyle’s prediction of the existence of a special energy level in the nucleus of 12c. if such a level did not exist, argued hoyle, life could not have develop in the cosmos, more accurately, 12c based life could not emerge. was it really so amazing? was it a full prediction? the basic astrophysical problem emerged when hoyle and schwarzschild calculated, in the early nineteen-fifties, the evolution of stars off the main sequence into the red-giant using reaction rates known at the beginning of the nineteen fifties and got that as soon as 12c is synthesized from helium, it absorbs another α particle and becomes 16o leaving no carbon. the reaction forming 12c was much slower than the reaction that destroys it. if so, argued hoyle, life should not exist! alternatively, as 12c does exist in our uni311 http://dx.doi.org/10.14311/app.2015.02.0311 g. shaviv verse, there must be a resonance in this nucleus that accelerates the formation of carbon by many orders of magnitude. this was ’reverse engineering’ at its best. you know what should happen and find out how can it be. could 12c be synthesized elsewhere? the only known alternative was the big-bang. but hoyle argued that all elements were synthesized in stars. on the other hand, gamow argued that all elements were synthesized in the big bang. however, the various models of big-bang nucleosynthesis failed to produce elements like 12c and higher (in atomic weight), and left stars as the only cosmic site for synthesis of 12c and heavier nuclei. 4 the nuclear barrier two nuclear barriers exist in the synthesis of the elements from hydrogen up (see fig. 2). the barriers are the non existence of a stable a=5 and a=8 nuclei as shown in the figure. this means that the synthesis of the elements, if started from hydrogen, must jump over these nuclei.236 chapter 8. how nature overcomes its own barriers figure 8.1: the light elements in the n, z plane. stable nuclei are painted brown. unstable elements with long lifetime are painted yellow. unstable nuclei with very short lifetime are painted red. the a=z+n=5 and a=z+n=8 nuclear barriers are shown as straight lines. the nuclear forces do not yield a stable nucleus for these combinations of protons and neutrons. there is only one nucleus with a=4 stable or unstable. although it is very stable and has a large binding energy, it has no excited state, only the ground state. 1h z n 1 2 3 4 1 2 3 4 5 6 7 8 5 6 7 8 14c 15n 16o 17o a=8 barrier 18o 9 10 7be 3.10-22s h he li be b c n o 5.10-21s 4.10-22s 8.10-19s10-17s 0.84s 177ms 8.7ms 8.10-22s 807ms 3.10-21s 119ms 12.3 yr 20.3m 5730yr 2.45s 0.75s 20ms 174ms 14ms 9ms 19.29s 11ms 9.97m 0 finally, bethe examined the triple body reaction, namely, the three ↵ particles interact simultaneously with one another to produce the carbon nucleus. since collisions of an ↵-particle with one other particle, proton or alpha, do not lead to stable nuclei the alternative was to assume a triple collision of which three types were plausible: (a) 4he + 2p ! 6be, (b) 24he + p ! 9b, and (c) 34he ! 12c. the first two reactions were rejected because they do not lead to a stable nucleus. the third reaction leads directly to 12c. however, bethe knew from eddington’s models that the temperatures in main sequence stars are around 2 ⇥ 107k, while the triple collision of ↵-particles requires according to bethe’s estimates, temperatures of the order of 109k, which are way beyond the range of temperatures according to eddington, in main sequence stars. hence, bethe dropped the idea of a three body reaction. in 1951 von weizsacker1 discussed the problem and claimed that the energy source can still be gravitation if only the condensation is extremely large2, which is essentially a milne type model. weizsacker noted that there were giants among the old population of stars as defined by baade. namely, these stars must have a very long life time. consequently, these stars must be low mass stars. if giants are main sequence stars which have exhausted their hydrogen fuel in the core and energy is not produced in the core any longer, then there need not be any temperature gradient in the core. weizsacker thought that the fusion of h into he is the only process that could operate in stars because he believed that there is no way to overcome the a=5, 8 barriers. consequently, nuclear energy was not an option for these stars because of the assumption that they exhausted their hydrogen and no other fusion can take place. from this point of view, weizsacker put the giants and the white dwarfs in the same category of stars. still, weizsacker added a reservation that in view of results known at his time, there is a possibility for hydrogen burning outside the isothermal core3. however, the idea weizsacker put forward was that the di↵erence between white dwarfs and giants should be that giants have masses above the chandrasekhar 1von weizsacker, c.f., apj, 114, 165, (1951) 2jordan, p., die herkunft der sterne, 1947, p 38. 3richardson, r.s. & schwarzschild, m. apj, 108, 373, (1948) gamow, g. & teller, e., phys. rev. , 55, 791, (1939) & gamow, g. & keller, g., rev. mod. phys. ,17, 125, (1945) & gamow, g. phys. rev. 67, 120, (1945) figure 2: the nuclear barriers at a=5 and a=8, namely the non existence of stable nuclei with these number of protons and neutrons. many attempts and suggestions to overcome the barriers were suggested. we mention here only bethe’s attempt, namely a three body reactions. bethe knew from eddington’s stellar models what are the central temperatures and densities in stars and soon realized that a 3-body reaction is much too rare under such conditions. we note that eddington’s results were derived without reference to what is the energy source of stars (which he did not know but hypothesized, already in 1919, that it must be the fusion of hydrogen into helium). in fig. 3 we show the structure of all nuclei with n+z=8. the instability of the a=8 nucleus implied that any fusion reaction leading to this nucleus will not create it. the so formed nucleus may live a short time but eventually it decays.8.1. unsuccessful attempts to overcome the a=8 barrier 237 12b5 12c6 12n7 17.42mev 13.37mev β+ βenergy the n+z=12 system z=5 z=6 z=7 τ1/2=11.00msτ1/2=20.20ms figure 8.2: the a=12 nuclei system. the diagram explains why of all nuclei with 12 protons and neutrons, the 12c is the most stable one. the other nuclei with a=12 are all unstable and decay via �+ or �� into 12c. the energies above the ground state of carbon are given in mev. also given the half-lifetime of the unstable nuclei. after ajzenberg-selove and lauritsen 2008. 8he 8li 8be 8b 8c 4he+4he -0.0918 3.03 11.35 16.9217.02 16.36 26.3228.09 reference level energy z2 3 4 5 6 0 figure 8.3: the energy levels of the nuclear system with a=8 for di↵erent atomic numbers z. the nucleus 8b can decay into an excited state of 8be, which may decay eventually to the 8be ground state. but the ground state of 8be is above the state of two free ↵ particles (colored green) and hence, the fast breakdown of 8be is unavoidable. consequently, 8be does not exist in nature. limiting mass4 and hence, have enough gravitational energy in the contraction towards extremely high densities, even as high as nuclear densities5. however, no detailed calculations were carried out, just speculation. weizsacker attempts and ideas to explain the giants were typical to the hopeless situation in front of the insurmountable a=5, 8 barriers. in 1949 gamow published a paper on relativistic cosmology6 in which he brought up an idea wigner communicated to him in private. another ingenious method of crossing the mass 5 crevasse was proposed by e. wigner. it is known as the method of the ’nuclear chain bridge’. wigner’s plan is that all that is required for building a chain bridge is an assumption that there was originally one single nucleus on the right-hand side of the crevasse. such an assumption can easily be granted argued gamow, since some building up is still maintained across the crevasse by the reaction 4he +3 t !7 li + � in spite of the low probability of its occurrence. gamow published his rejection of wigner’s idea in a book7 just when the salpeter and hoyle started the work on the triple ↵ process which ended few years later with the detailed nuclear physics. gamow gave no reference to where wigner published his suggestion. 4chandrasekhar, s., mnras, 95, 207, 226, 676, (1935) 5nuclear densities are densities at which the nuclei touch each other. the radius of the nucleus is about 10�5 the radius of the atom, namely, the radius of the outer occupied electronic shells. the atom has an infinite number of electronic shells with radius increasing to infinity. but as the number of electrons in each atom is finite, they occupy only the lowest in energy levels and for this reason the actual atoms has a finite radius, the radius of the last occupied level. normal matter densities on the earth are ⇠ 1g/cm3. if the matter is compressed to densities as high as 1015gm/cm3 the nuclei touch each other and form one gigantic nucleus. this is nuclear matter. 6gamow. g., rev. mod. phys., 21, 367, (1949) 7gamow. g., the creation of the universe, viking press, new york, 1952. figure 3: the relative energies of the ground states of the n+z=8 nuclei. the numbers are the energies in mev relative to the ground state of 8be, which has the lowest ground state energy of all a=8 nuclei. however, the state of two free α’s has still lower energy. hence, there is no stable nucleus with a=8. 5 why nuclear physicists were interested in the problem? the fundamental problem in nuclear physics at the beginning of the nineteen thirties was the nuclear structure. it was already known that the α particle is the most bound nucleus as it has the highest binding energy per nucleon. similarly, nuclei like 12c,16 o,20 ne etc have a higher binding energy than their neighboring nuclei. the question was therefore, are these so called α nuclei composed of α particles 238 chapter 8. how nature overcomes its own barriers 8.2 the pursuit after the structure of the carbon nucleus figure 8.4: two possible structures of the carbon nucleus. on the left, the nucleons move randomly inside the nucleus. on the right, there are three clusters of ↵ particles and the ↵ particles move randomly inside the nucleus. figure 8.5: the structure of 14n according to the revised ↵-particle nuclear model. the story starts as early as 1917, when harkins8 observed that the binding energies of the so called ↵ nuclei, (12c,16o,20ne,24mg, . . . ) are particularly large. consequently he suggested a periodic table for nuclei (cf. fig.??). the basis for this periodic table was harkin’s idea that the basic building block of nuclei is the ↵-particle. harkin’s original idea was put forward before the neutron was discovered. after the discovery of the neutral particle the model was modified to: the nucleus is made of a core of ↵ particles and up to 3 nucleons moving around it (cf. fig.??) consequently, the following fundamental question was raised: to what extent a nucleus of type 4n+2 would yield a nucleus of type 4n, where n = 1, 2, 3, . . . and an ↵ particle, if bombarded with nucleus with atomic weight 2, namely a deuterium. in symbols the reaction is: 4n+2x2n+1 + 2d1 ! 4ny2n + ↵, where x is any nucleus and y is the element the precedes the element x in the periodic table. the di↵erence between the two models is fundamental. the ↵ particle is fully symmetric and has zero spin, hence, it must follow the bose-einstein statistics. on the other hand, the protons and the neutrons both have spin 1/2 and hence, follow the fermi-dirac statistics. obviously the behavior of the particles in the potential well and how they fill up the levels is completely di↵erent in two the cases. the di↵erence should express itself in the binding energy as well as in the energy levels. if there is a potential well and the particles moving inside obey the fermi-dirac statistics, we can expect periodicities in the properties. on the other hand, if the ↵ particles preserve their spin while they move in the nucleus, then they do not behave like bosons and can all stay in the same nuclear level. obviously, the binding energy in such a case is larger than in the case of individual protons and neutrons (because in the latter case the nucleons are spread over many energy levels while in the first case they are all in the lowest most bound level.) the idea that nuclei are composed of ↵ particles emerges from the fact that the ↵ particles is extremely stable and we do not know that it has any excited levels. like deuterium which is composed of a neutron and a proton does not have any excited state, only a ground state. hence the helium nucleus is not unique from this point of view. the binding energy of a deuterium is 2.225mev while that of an ↵ particle amounts to 28.3mev, namely, significantly more than just two deuterium nuclei put together. it seems that the potential well inside which the ↵ particle resides is very deep and still does not posses any excited state. lauritsen and crane9 investigated this point in 1934 and indeed, did not discover any 8harkins, w.d., j. amer. chem. soc., 39, 856, (1917), phys. rev. 15, 85, (1920) 9lauritsen c.c. & crane, h.r. prl, 46, 537, (1934) figure 4: the two possible options for the structure of the carbon nucleus. on the left we see 12 nucleons moving independently of each other and on the right we see groups of 4 nucleons moving as a bound unit. or of just an equal number of protons and neutrons, as shown in fig. 4. in particular, is the excited state of 12c a three body state? consequently, attempts to find the energy level structure of the excited 12c were 312 who discovered the hoyle level? carried out long before any astrophysical interest in the problem arose. actually, even before bethe discovered how the cn cycle powers the sun (today we know that the cno cycle contributes about 4% of the total energy produced by the sun and the pp chain contributes the rest.) 6 the history of the discovery of the 12c nuclear levels already in 1933 lewis et et al. experimented with the reaction:14n+2d → 12c+4he. the nucleus of 12c so formed has significantly more energy than the ground state as it forms in an excited state. the decay of the excited state to the ground is performed by one or more emissions of γ photons. in principle, if you measure the energy of the γ′s you can easily figure out the energies of the energy levels in the newly formed nucleus. lewis et et al. did not measure any γ′s as they did not have the equipment, they however, discovered the emitted α particles. as they did not detect all particles emitted in the reaction they found that the emitted α particles have about half the energy difference and it was not known where the other half was lost. lawrence et al (1935) repeated the experiment, again without any device to measure γ′s. however, they measured the energies of the α′s and found two groups of α particles having different energies, and so were able to infer that 12c has two energy levels: at 3.8mev and 4.7mev. actually, the interpretation of the experiment was not complete. as shown in fig. 5, there are two possibilities to interpret this experiment. 0 0 3.8 3.8 8.5 4.7 ground state 00 figure 5: the interpretation of lawrence et al. result. one alternative can be two levels at 3.8 and 8.5mev with allows transitions between these two levels and forbidden transition directly from the 8.5mev level to the ground state. the other alternative is two levels with allowed direct transition to the ground state and forbidden transition between the levels. lawrence et al assumed the latter case but as we will see that in reality it was the first alternative. crane and lauritsen observed γ’s from the reaction: 11b+2d → 12c+p. these experiments which were carried out in the early 1930th made it clear that 12c has excited states but their energy was not known to better than ±1mev . 8.5. back to new developments in the structure of 12c 241 table 8.1: bethe’s estimate for the lifetime of the 8be nucleus as a function of the energy excess above the state of two free ↵ particles (in seconds). the second row gives the crossing time. energy excess in kev 50 100 200 300 400 r = 2.5 ⇥ 10�13cm 4 ⇥ 10�13 3 ⇥ 10�14 2 ⇥ 10�15 2 ⇥ 10�19 5 ⇥ 10�20 crossing time (sec) 4 ⇥ 10�22 2.8 ⇥ 10�22 2.0 ⇥ 10�22 1.6 ⇥ 10�22 1.4 ⇥ 10�22 r = 2.5 ⇥ 10�13cm 7 ⇥ 10�14 4 ⇥ 10�17 3 ⇥ 10�19 3 ⇥ 10�20 1 ⇥ 10�20 crossing time (sec) 8 ⇥ 10�22 5.6 ⇥ 10�22 4 ⇥ 10�22 3.2 ⇥ 10�22 2.8 ⇥ 10�22 8.5 back to new developments in the structure of 12c αα αα3 +8 bebe + 7.366 7.275 7.68 4.43 12c 7.1 7.8 figure 8.6: the energy levels of the 12c nucleus relative to the rest mass energy of 8be + ↵ and 3↵. the level at 7.68mev can decay into a 8be + ↵ or 3↵. the green color span the range where the various experiments gave an indication of an energy level. all energies are in mev. the correct explanation was put forward five years later, in 1940, and by gaerttner and pardue25, then in the kellogg radiation laboratory in cal-tech. using the same nuclear reaction, 14n+2d ! 12c+↵, gaerttner and pardue discovered that on top of the emerging ↵, also � rays at 1.9, 3.1, 4.0, 5.3 and 7.0mev were produced and explained their origin as due to excited levels in 12c at energies of 4.32 and 7.2 ± 0.4mev above the ground state. the authors even stated that the level at 7.2mev is weakly coupled with the ground state, namely, the probability for a transition from the 7.2mev state to the ground state is very small and was not observed. here we note that the energy of 8be + ↵ is 7.367mev above the ground state of 12c. hence only a level above this rest mass energy plus the kinetic energy can be directly reached by the 8be+↵. at stellar temperatures of the order of 108 � 109k the nuclei have kinetic energy of about 0.01 � 0.1mev, so that the kinetic energy is not the dominant one.26 we conclude that for the colliding 8be and ↵-particle to enter into an energy level of 12c, the energy must be above 7.366mev but not by very much. the knowledge of the structure of the 12c nucleus at that time is shown in fig.??. the green zone spans the uncertainty in the location of the energy level just above the energy of 8be+↵ or 3↵. since the reaction rate is very sensitive to the exact value of the energy level, accurate positions are mandatory. most nuclear reactions at low energy have two quite independent steps: formation and decay. • the formation: so far we discussed the probability of formation. due to the strong interaction between the particles and the relatively long time the particles stay together, the 0nucleus forgets how the particular level was reached0. the newly formed excited 12c lives for a while, exactly like the 8be nucleus lives for a while, before it decays. nuclear physicists call the new nucleus the 0compound nucleus0. • the decay: the new nucleus may have several modes of decay. it can disintegrate back into the original constituents or lose energy in one way or another and descend into a bound state, provided there is one, or even the ground state. no bound nucleus can be formed without any loss of energy. the most common way of losing energy is by emitting a �, however, other ways, though with lower probabilities, exist. 25gaerttner, e.r., & pardue, l.a. , phys. rev., 57, 386, (1940) 26the relevant particles are those with energies equal to the gamow peak and not with the average energy. figure 6: the structure of the 12c nucleus as known in 1940. the level at 7.68mev can decay into a 8be+α or 3α. the green color spans the range where the various experiments gave an indication of an energy level. all energies are in mev. in 1940 gaerttner & pardue of the kellogg laboratory at caltech investigated the reaction 14n+2d → 12c+α and discovered that on top of the emitted α particles also γs with energies 1.9,3.1,4.0,5.3 & 7.0 mev were emitted. it became clear already at this epoch that there exists a level at ∼ 7.2mev but it is weakly coupled to the ground state, namely the transition to the ground state was too weak to be observed! hence, only an upper limit to the rate of transition could be found. the implication was that indeed, if the reaction product goes to excite this level in 12c, then extremely few 12c nuclei would decay from this state to the ground state. it appeared therefore, that from stellar nucleosynthesis this is not likely to be the way 12c forms. but this question of transition probability was not yet raised. in the same year holloway and moore (1940) repeated the experiment 14n+2d → 12c+4he and confirmed the existence of the levels at 4.37 & 7.62mev and suggested that in most cases the (excited) 12c∗ disintegrates by emitting an α particle. they wrote that: the corresponding excited state of 12c would be unstable against α emission, but it is still easily conceivable that such a state could not actually emit an α because 313 g. shaviv of selection rules. clearly, in the first case this is not the way to form carbon while if carbon is formed in this way we can claim that astrophysical situation implies that the channel in which 12c∗ disintegrates by emitting an α may exist but along with the decay to the ground state. as for our particular reaction, what holloway and moore argued was that in the reaction 8be+α↔12 c+γ the compound nucleus disintegrates mostly into the incoming channel. terrell (1950) examined the 9be+α → 12c+n and found no evidence for excited states in 12c. in the same year johnson (1952) investigated the 11b+2d→ 12c+n reaction and found the 4.4 & 9.6mev levels but not the one around 7mev. it should be realized that these experiments are very difficult and tricky and hence no wonder that it took such a long time to find the accurate structure of the nucleus of 12c. in 1950 hornyak et al. summarized the known data of the 12c nuclear structure as depicted in fig. 6. the 7.86mev level is shown but very weak, namely only rare transitions into it, and hence a problem to experimentalists. lauritsen and fowler were co-authors on this paper. in 1951 miller & cameron followed the motion of 8be nuclei in nuclear emulsion and observed their decay. they found a lifetime of 5 ± 1 × 10−14sec. this lifetime is 106 times longer than the two α’s mutual crossing time. this was the paper that put an end to a long line of papers that claimed alternatively that 8be is unstable and unstable. miller & cameron succeeded to watch the motion of the 8be nucleus in emulsion and see its decay. we mentioned above that α-like nuclei are expected to be more bound than their neighbors. here we have a nucleus composed of just two α nuclei and it is unstable! 7 the wrong solution some people claim that salpeter, who discovered the basis for the triple alpha process, must share the credit with öpik. öpik, according to this claim, solved the problem of the helium fusion to carbon already in 1951. the trouble was, so goes the claim, that öpik published his paper in the proc. royal irish acad. 1951. a seldom visited by astrophysicists journal. but öpik’s paper is wrong and not identical with that of salpeter. öpik, who had many important and very original contributions, apparently did not read the nuclear literature or ignored it, because he ignored the fact the 8be is unstable, a fact known already in 1951. so öpik assumed a 3 body collision. the α + α penetration takes about 0.8 × 10−20sec and the third α must collide within this time. clearly, the lifetime of 8be as assumed by öpik, is off by a factor of 106. this factor enters into the rate. moreover, despite the fact that the energy level in the 8be continuum was already known, öpik overlooked it. öpik assumed that he burning lasts 1014sec. the question then was at what temperature would helium burning last this time and with his wrong reaction rate öpik derived that helium burning takes place at 6 × 108k. this is known today as far off and it implies a discrepancy between the formation of 8be via two α′s and the formation of 12c, as we will shortly see. öpik’s paper did not attract the astrophysical community and from its publication in 1951 till 2009 öpik’s paper was cited just once, and it was by salpeter... in 1972 marshal wrubel wrote a review on ernst öpik’s contributions to red giants and did not mention his contribution to the 3α process. the uncertainty in the energy levels prevailed in 1950. guier and roberts looked at: 9be+α → 12c +n and claimed the level is at 7.8mev. the experiment was repeated with bertini joining the team, and no level was observed. in 1952 azjenberg & lauritsen prepared a compilation of all the experiments and provided the following summary in fig. 7. our particular level is well marked. 246 chapter 8. how nature overcomes its own barriers in a figure which provided the structure of energy levels in 12c. as can be seen from fig.??, a level at 4.43mev and a level at 7.5mev appeared as well as the quantum numbers j = 2+ and j = 0, respectively49. no parity is assigned to the upper level. the only reference given for this information is 0cloud-chamber investigation0 without the details of the paper. however, the only researchers to apply a cloud chambers on a close problem were crane, delsasso, fowler and lauritsen50 back in 1935. figure 8.12: the structure of 12c as was known in 1952. for the ease of reading this heavy with data figure, we marked energy level at 7.5mev with red and the level of 8be +↵ is marked with green. note that the spin of the level is given, indicating that the 8be+↵ can create the excited 12c nucleus. it is this coincidence which led later to philosophical hypothesis. from ajzenberg and lauritsen 1952. the data on the quantum numbers of the energy levels is important. it was well known that the ground state of 12c is j = 0+. it is also well known that a transition from a j = 0+ to j = 0+ is strictly forbidden. the spin of the photon is 1. hence a zero spin to a zero spin transition does not leave room for the photon, so that angular momentum can be conserved. hence, if a bound carbon nucleus is to be the end product, it is not su�cient to have this energy level because no direct transition from the 7.5mev state to the ground state is possible. another energy level above the ground state and below the level at 7.5mev must exist and must have the right quantum numbers so that the transition to the ground state can go through an intermediate state. 8.9 salpeter the solution is two plus one in the early nineteen fifties, willy fowler, who was already then the insatiable force behind the kellogg laboratory, in caltech, consulted extensively hans bethe, who was at cornell. after a while, bethe agreed to send to fowler one of his best young men and edwin salpeter (19242008) was his choice. salpeter spent the summer of 1951 in the kellogg laboratory. it is during this and subsequent stays that salpeter carried out his research on how helium can be synthesized into carbon. on october 1952 salpeter submitted his first short paper to the astrophysical journal51. in the same month of october, the physical review published a paper by ajzenberg and lauritsen, who worked in the same kellogg laboratory, in which the resonance level in 12c was marked at 7.5mev and with quantum number assignment of j = 0. no parity was given. salpeter submitted his paper after returning 49in 1950 azjenberg and lauritsen had the level at 7.mev and it is not clear why the level 0moved0 half an mev upward. 50crane, h.r., delsasso, l.a., fowler, w.a., & lauritsen, c.c., phys. rev. 48, 100, (1935) 51salpeter, e.e. apj, 115, 326, (1952) figure 7: the structure of the 12c nucleus as summarized in 1952. the energy level under discussion is marked in red and is at 7.5mev. note that the authors assigned the level j = 0, which is identical with that of the ground state. but 0→ 0 transitions are forbidden. 314 who discovered the hoyle level? 8 salpeter enters the game salpeter entered the game in 1951 just after the publication of the bethe-salpeter equation. it was upon fowler’s request for help in the theory, that bethe decided to send his distinguished young physicist salpeter to the kellogg laboratory at caltech. salpeter spent two summers in caltech. on october 1951 salpeter submitted his first paper to the apj: 12c had no level around 7.6mev. on october 1951 ajzenberg & lauritsen submitted a paper to physical review in which 12c had a level at 7.5mev with j=0 (no known parity). two of the most important results for the triple alpha process were published at the same time by people from the same laboratory in two different journals and no citation was given to one another. they simply did not know of each other. when gamow considered in 1938 the possible energy source of ms stars he assumed, in an attempt to overcome the a=5 barrier, that: 24he ↔ 8be, namely the reaction is in a dynamic equilibrium and assumed 8be to be stable. salpeter was unaware of this publication of gamow but knew already that 8be is unstable. so salpeter, facing the same dilemma as gamow, assumed that the two α′s go into the 95kev level (which was known already to salpeter) in the continuum and the so formed nucleus lives long enough (just 10−14sec which were inferred from the width of the level) for a third α to collide and create a 12c nucleus. salpeter realized that 10−14sec is orderes of magnitude longer than the two α′s self-crossing time and hence his treatment was justified because 8be lives a long time before it decays and the assumption of equilibrium is fully justified. if so, there is no need for a cross section! next, salpeter assumed that the 12c nucleus ”somehow decays” in flight (no level in 12c was known to certainly exists) into the ground state of 12c. salpeter stressed that he assumed no resonances in 12c,16 o,20 ne,24 mg . . .. salpeter felt uneasy about it and wrote that: the nuclear γ-ray width for the formation of 12c (but not the one for 8be) is required. this width has not yet been measured, and the position of resonance levels was not known accurately enough and an estimate of 0.1ev was used for this width. hence the correct production rate could be smaller by a factor of as much as 10 or larger by as much as 1000. thus the reaction rate known in the literature was estimated ignoring the possible existence of unknown resonances. two comments: in 1954 öpik wrote a paper about wd and the 3α. he wrote that: the lifetime of the temporary nucleus 8be formed is assumed equal to ∼ 8 × 10−21sec being an estimate of the duration of penetration. the lifetime of true 8be is probably much shorter, about 10−22sec . . . no resonance capture is assumed in this case. moreover, öpik complained that salpeter did not cite him and added: his method of calculation is not quite clear from his brief note. it seems that the reaction 2α → 8be∗ he has treated in a manner similar to ours, where as in 8be+4he → 12c +γ he has postulated a resonance process. the outcome is a formula yielding 1.4× 1013 times higher an energy generation with practically similar temperature as our formula. how many errors can be written in a single sentence? 8.12. the new experiment 249 7.59 4.43 0+ 2+ 0+ gγ gγ4.43 3.16 12c figure 8.13: the status of the level structure of 12c after beghian et al. 1953. figure 8.14: the clear evidence for the two groups of � rays as discovered by beghian et al. 1953. known and documented by researchers from kellogg, as described before. however, many of the required details were still missing and above all fowler did not trust the experimental results. the abstract was presented in the american physical society in albuquerque, new mexico, september 2-7, 1953. the sensational paper was the last one in the last session. the session itself was on nuclear physics not astrophysics. it was a victory to astrophysics and hoyle, in particular in front of skeptical fowler63, that the location of a nuclear level that wandered so much from one experiment to the other, could be determined from observing the stars. shortly after, two papers were published, one by the experimentalists in the physical review and one by the theoretician in the astrophysical journal supplement. this triumph forged the fowler-hoyle collaboration that contributed immensely to nuclear astrophysics for many years to come. the exact reference is an abstract by hoyle, dunbar et al.64 with the title: a state in 12c predicted from astrophysical evidence. the idea was that: the observed 0cosmic0 ratio of he:c:o can be made to fit the yield calculated for these reactions if the reaction 8be + ↵ ! 12c + � has a resonance near 0.31mev, corresponding to a level at 7.68mev in 12c. the abstract also claimed that: a level had previously been reported at 7.5mev, based on ajzenberg and lauritsen 1952. in the same abstract they report about the experiment which led to the discovery of the level at 7.68 ± 0.03mev. the mode of decay of the level is not discussed. 8.12 the new experiment dunbar et al.65 repeated the 14n + 2h ! 12c + ↵ experiment once more using a new special double focusing magnetic spectrometer and discovered that the excited level is at 7.68 ± 0.03mev as can be seen from fig.??. unfortunately, they did not give the quantum numbers of the levels, and it was impossible to determine whether the reaction a stable 12c could be formed. this sophisticated and accurate experiment settled for good the question where is the level but could not answer the crucial question: can a stable 12c be formed? 63salpeter, e.e., publ. astron. soc. australia, 25, 1, (2008) 64hoyle, f., dunbar, d.n.f., wenzel, w.a. & whaling, w., phys. rev, 92, 1095, (1953) 65dunbar,d.n.f., pixley, r.e., wenzel, w.a., & whaling,w., phys. rev, 92, 649, (1953) figure 8: the very nice discovery of the energy levels by beghian et al (1953). in 1953 beghian et al investigated the 9be+α → 12c+n reaction. they detected two groups of γ rays: at 3.16mev and at 4.43mev and concluded that: 12c has two levels: at 4.43 & 7.59mev (the first alternative is shown in fig. 5) . they did not find any γ rays with energies close to 7.5mev. hence they concluded that the probability of the 7.59mev level to decay to the ground state is < 1/2500. willy fowler did not trust any of the previous measurements of the carbon level and searched for a way to carry out a trustful experiment. 8.12. the new experiment 249 7.59 4.43 0+ 2+ 0+ gγ gγ4.43 3.16 12c figure 8.13: the status of the level structure of 12c after beghian et al. 1953. figure 8.14: the clear evidence for the two groups of � rays as discovered by beghian et al. 1953. known and documented by researchers from kellogg, as described before. however, many of the required details were still missing and above all fowler did not trust the experimental results. the abstract was presented in the american physical society in albuquerque, new mexico, september 2-7, 1953. the sensational paper was the last one in the last session. the session itself was on nuclear physics not astrophysics. it was a victory to astrophysics and hoyle, in particular in front of skeptical fowler63, that the location of a nuclear level that wandered so much from one experiment to the other, could be determined from observing the stars. shortly after, two papers were published, one by the experimentalists in the physical review and one by the theoretician in the astrophysical journal supplement. this triumph forged the fowler-hoyle collaboration that contributed immensely to nuclear astrophysics for many years to come. the exact reference is an abstract by hoyle, dunbar et al.64 with the title: a state in 12c predicted from astrophysical evidence. the idea was that: the observed 0cosmic0 ratio of he:c:o can be made to fit the yield calculated for these reactions if the reaction 8be + ↵ ! 12c + � has a resonance near 0.31mev, corresponding to a level at 7.68mev in 12c. the abstract also claimed that: a level had previously been reported at 7.5mev, based on ajzenberg and lauritsen 1952. in the same abstract they report about the experiment which led to the discovery of the level at 7.68 ± 0.03mev. the mode of decay of the level is not discussed. 8.12 the new experiment dunbar et al.65 repeated the 14n + 2h ! 12c + ↵ experiment once more using a new special double focusing magnetic spectrometer and discovered that the excited level is at 7.68 ± 0.03mev as can be seen from fig.??. unfortunately, they did not give the quantum numbers of the levels, and it was impossible to determine whether the reaction a stable 12c could be formed. this sophisticated and accurate experiment settled for good the question where is the level but could not answer the crucial question: can a stable 12c be formed? 63salpeter, e.e., publ. astron. soc. australia, 25, 1, (2008) 64hoyle, f., dunbar, d.n.f., wenzel, w.a. & whaling, w., phys. rev, 92, 1095, (1953) 65dunbar,d.n.f., pixley, r.e., wenzel, w.a., & whaling,w., phys. rev, 92, 649, (1953) does not exist! figure 9: the structure of 12c after beghian et al discovery. the direct transition from the 7.59mev to the ground state is forbidden. 315 g. shaviv the symmetry of the 12c ground state leads to a j = 0+ state. the state at question appears to have also j = 0+. angular momentum conservation do not allow the 0+ state to decay into the ground state which is also 0+. hence, if eventually the decay of this state leads to the ground state, there must be another intermediate level below the 7.59mev one with the proper spin, to which this level can decay. hoyle never mentioned these requirements nor allowed and forbidden transitions. he tacitly assumed that the 7.69mev level ’somehow’ decays to the ground state. 9 1953 kellogg phase i when martin schwarzschild and fred hoyle tried to evolve main sequence stars off the main sequence and derive the red giant, the modeling failed and they could not get the red giant branch. beside the failure to derive the branch, there was a problem with the composition. the rate of 12c + α (carbon conversion into 16o) was much greater than the rate of carbon formation and hence no carbon was left after helium burning. the model moved from helium to oxygen leaving no carbon. hence, hoyle argued that there should be a level at about 7.6mev which accelerates the reaction and leads to the formation of carbon (relative to its destruction). hoyle spoke only on the energy of the level and he knew at what energy in 12c it should be because he assumed it takes place via 8be + α. this is exactly the level we discuss here. 250 chapter 8. how nature overcomes its own barriers 8.13 hoyle: we must have carbon in our universe! in 1954 hoyle published an extensive research on the synthesis of the heavy elements66 and discussed the synthesis of carbon as the starting point. this is the same paper in which hoyle proposed how the synthesized elements spread in the galaxy, see sec. ??. hoyle followed salpeter and assumed the dynamic equilibrium for ↵ + ↵ *) 8be and applied statistical mechanics to find the abundance of the beryllium. as salpeter did not provide the equation and only quoted the result, hoyle cited himself for the equation of equilibrium.67 in that paper hoyle discussed the formation of the heavy elements under statistical equilibrium. no mention of gamow’s 1938 paper and his equilibrium 8be. figure 8.15: dunbar’s et al. 1953 result. a beautiful manifestation of the predicted 7.68mev level. once carbon is synthesized it can absorb an additional ↵ particle and become oxygen, which in turn can again absorb an ↵ particle and become neon: 12c + ↵ ! 16o + � followed by 16o + ↵ ! 20ne + �. in principle, the 20ne can capture another ↵ particle to form 24mg, but the increasing coulomb barrier reduces the rate so much that it is permissible to neglect the ↵ capture past oxygen. the crucial point is therefore, that carbon and oxygen are synthesized at the same time and not one after the other. thus, on one hand the carbon is synthesized and on the other it is destroyed. by the end of the day, the final amount of carbon depends on the ratio between the rate of formation and the rate of destruction. hence, hoyle defined a parameter:  = a ⇥ rate(12c + ↵ ! 16o) rate(3↵ ! 12c) where a is a numerical factor, so that relevant values of  will be of the order of unity. clearly,  depends on the temperature as well as on the properties of the energy levels in carbon and oxygen. of all the data which enters into the expression for , it is most sensitive to the eres = e � e(8be + ↵) because  ⇠ e �eres kbt = e � (e�e(8be+↵)) kbt , where e is the energy of the level in the 12c nucleus. all energies are above the ground state of 12c. the energy dependence is exponential. assuming the salpeter’s rate for the triple ↵ reaction and the problematic (see later) subsequent reaction 12c + ↵ !16 o, hoyle solved for the abundances of carbon and the results are shown in fig. ??. it can be seen from the figure that when  = 1/9 the amount of carbon produced by the time the helium is exhausted, is 1/3 which corresponds to an abundance ratio of c/o=1/2, the value hoyle took as the observed cosmic value. also, one can see that a small variation in  from 1/3 to 1/15 spans a carbon abundance which covers the observed value. on the other hand, if there would be no resonance at all in 12c, the value of  would have been orders of magnitude smaller and no carbon would be left at the 66hoyle, f., apjs, 1, 121, (1954) 67hoyle, f., mnras, 106, 343, (1946) figure 10: the structure of 12c after dunbar et al discovery. when hoyle came to kellogg in the first time he convinced ward whaling to look for this level and indeed whaling went to the lab and found the level exactly where hoyle claimed it must be. the level was rediscovered! hoyle spoke about the energy of the missing in calculation level and did not discuss any spin, selection rules etc. the abstract was presented in the american physical society meeting in albuquerque, nm, sep 2-7, 1953. the paper was presented in the session on nuclear physics not astrophysics. it was a victory for hoyle in particular in front of skeptical fowler. 10 how did hoyle predict the nuclear level in 12c hoyle assumed the that κ defined as: κ = a rate(12c + α→16 o + γ) rate(3α→ γ) = destruction formation is given and calculated the resulting mass fraction of 12c after the burning of helium into carbon. hoyle obtained in this way the results shown in fig. 11. it is easily seen that κ � 1 (the destruction is much faster than formation) yields no carbon at the end of helium burning. 8.14. the kellogg laboratory group phase ii 251 end of the helium burning.if the formation of 12c is much slower than its destruction, the end product if almost pure 16o. figure 8.16: the formation of carbon from helium as a function of hoyle’s parameter . this is the figure which convinced hoyle that 12c should have a resonance. in addition, hoyle carried out the inverse calculation in a style which is known today as 0reverse engineering0, namely what must t and eres be so as to get the observed abundance, and he found t = 1.4 ⇥ 108k and eres = 0.33mev (above the rest mass of 8be + ↵) which corresponds to e = 7.705mev in the 12c nucleus. in summary, hoyle was able to apply astrophysical arguments to solve the problem of the carbon energy level, the exact location of which wandered from one experiment to the other. shortly after the idea that such a resonance must exist or else, the cosmos will have no carbon to support life the way we are familiar with, was born. the fantastic story about how astrophysical consideration led to the prediction of a nuclear level triggered the invention of the anthropic principle68. 8.14 the kellogg laboratory group phase ii hoyle’s victory was not complete. cook et al.69 found it necessary to carry out a new experiment again because the experimental evidence on the character of the 7.7mev 12c state is not entirely clear. it seems well established that the state does not radiate directly to the ground state but rather cascades via the 4.43mev state. furthemore, cook et al added that one must (a) make sure that the 12c level can be formed by 8be+↵. the prerequisite for that are the right spin and parity and that there is a non vanishing probability that it decays by emitting these particles. (b) it must have a finite probability to decay to the lower excited state or the ground state. these questions should be best explored by bombarding 8be with ↵ particles, but since 8be is unstable, recourse must be made to study the two possible modes of decay of the excited state of 12c, namely 12c⇤ ! 8be + ↵ and 12c⇤ ! 12c + �. then one has to rely on the principle of reversibility of nuclear reactions. indeed, hoyle did not predict the quantum numbers of the level and without these the prediction is not terribly powerful the experiment cook et al. was innovative and di↵ered substantially from all previous ones. they created the radioactive isotope 12b, which has an energy of 13.378mev above the ground state of 12c and decays via a � decay (in contrast to a � decay) into all excited levels of 12c with lower energy and in particular into the two relevant levels for our discussion here, cf. fig.??. special arrangement was prepared to detect the emitted ↵’s using a strong focusing magnetic spectrometer. the basic problem cook et al. faced with the 7.68mev level was the following: it was established before that there is no direct transition to the ground state. on the other hand, no one observed that this level disintegrates into 8be + ↵70. there were merely conflicting estimates of the probability to emit 68the anthropic principle is a philosophical claim that the universe must be compatible with the existence of life as is observed. 69cook, c.w., fowler, w.a., lauritsen, c.c. & lauritsen, t., phys. rev., 107, 508, (1957) 70recall, nuclear reactions under astrophysical conditions are usually very slow because the energies are low. so the 8be+↵ yielding 12c and decaying back to the products was hopelessly slow and hence, not observed figure 11: the argument by hoyle that the resonance must exist. the paper by hoyle, dunbar et al had the title: a state in 12c predicted from astrophysical evidence. the abstract also admitted that: a level had previously been reported at 7.5mev, based on ajzenberg and lauritsen 1952. . on the other hand, the decay mode of the level a crucial point was not discussed at all. beghian discovery of the 7.59mev level was overlooked. but hoyles victory was not complete his friends did not accept the idea. could hoyle invert the process and learn nuclear physics from the stars? cook, fowler, t. lauritsen and c.c. lauritsen 1957 argued that:experimental evidence on the character of the 7.7mev state is not entirely clear. it seems well established that the state does not radiate directly to the ground state but rather cascades via the 4.43mev state. the authors added that: one must (a) make sure that the level can be formed by 8be + α. the spins should 316 who discovered the hoyle level? agree. (b) it must have a finite probability to decay to the ground state. note: the direct reaction cannot be investigated in the lab because the rate is very slow. consequently, the 12c level must be populated indirectly. cooks et al. had a sever technical problems: on one hand: it was established that there is no direct way to the ground state. on the other hand no one observed that the compound 12c nucleus disintegrates into 8be∗ + α. there were conflicting estimates of the probability to emit an α. uebergang and independently steffen, got that the excited 12c nucleus emits α at about << 50% of the cases while bent et al and independently hornyak got that it emits an α in >> 97% of the cases. 252 chapter 8. how nature overcomes its own barriers an ↵ particle. on one hand, uebergang71 and ste↵en et al.72 estimated the probability to be ⌧ 50% and on the other hand, bent et al.73 and hornyak74 estimated it to be � 97%. figure 8.17: cook et al. 1957 created the radioactive nucleus 12b which decays into 12c. most of the decays go into the ground state and only 1.3% go into the investigated level. cook et al. managed to get only an upper limit to the probability of decay into 8be + ↵. since this was by far the most probably mode of decay of this level, the exact value was crucial. so here they were forced to rely on russell, phillips and reich75 and heydenburg and temmer76 who found an upper limit which was about 250 times smaller than predicted. with the improved (lower) upper limit they were able to reach their most important result, namely, that the probability for � decay to the 4.43mev level is: 0.0010.0028. very small but not zero. next they found that the probability to decay through the emission of an electron-positron pair is about 10�5 and hence negligible. 8.15 salpeter-1957: completing the job salpeter’s second and updated paper77 was submitted together with the cook et al. paper and the two papers were published back to back. at the time, beginning of 1957, the spin of the 7.68mev level was not yet certain though very recent experiments seemed to provide the final proof of the quantum numbers of this level. fregeau and hofstadter78 were interested in using high energy electron beams to investigate the surface of nuclei and they considered that the 12c would be an ideal case. so they bombarded 12c with high energy electrons. this and subsequent works of this type won for hofstadter the 1961 nobel prize.79 fregeau and hofstadter treated already in 1955(!) the 4.43mev and 7.68mev levels of carbon as so well established, that they did not give any reference to prior experiments. as for the spin of the 7.68mev level, they state that: j = 0+ is not inconsistent with the experimental results that they found. the levels emerged very nicely in their electron scattering experiments. on the other hand, salpeter argued that the only possibilities were 0+ or 2+. if the 2+ was assumed then the probability for electron-positron decay (symbolized in nuclear physics as �e±) could be theoretically calculated and it was found to be 1000 smaller than the one obtained if 0+ was assumed and this 71uebergang, r.g., australian j. phys.,7, 279, (1954) 72ste↵en, k.g., hinrichs, o., & neuert, h., zeit. f. phys., 145, 156, (1956) 73bent, r. d., bonner, t. w., mccrary, j. h., ranken, w. a., phys. rev., 100, 771, (1955) 74hornyak, w.,f., bull. am. phys. soc. ser, ii, 1, 197, (1956) 75russell, j. l., phillips, g. c., & reich, c. w., phys. rev. 104, 135, 143, (1956) 76heydenburg, n.p. & temmer, g.m., phys. rev., 104, 123, (1956) 77salpeter, e.e., phys. rev., 107, 516, (1957) 78fregeau, j.h., & hofstadter, r., phys. rev., 99, 1503, (1955). ibid. 104, 225, (1956) 79the citation of the prize read for his pioneering studies of electron scattering in atomic nuclei and for his thereby achieved discoveries concerning the structure of the nucleons . figure 12: the final experiment by cook et al. 1957: creates the nucleus 12b and follow its decay into the 7.653mev level (only 1.3%). to overcome the problems cook et al 1957 created the radioactive nucleus 12b which decays into 12c. most of the decay goes into the ground state and only 1.3% decays into the investigated level. 11 1957 salpeter finishes the job remaining problems: what is the spin of the 7.68mev state? fregeau & hofstadter (1955) using electrons scattering from the nucleus (an experiment for which hofstadter got the nobel prize 1961), claimed that j=0+ is not inconsistent with experiment. so salpeter assumed j = 0+ or 2+. the latter yields a result which is about 1000 smaller than the first one. 8.15. salpeter-1957: completing the job 253 result contradicted the result of the cal tech experiment80. now came the critical question, what is the probability to decay back into the incoming channel, 8be + ↵. salpeter resorted to the wigner theory81 and calculated an upper limit for the case 0+ which was 40% higher than the one adopted by cook et al. the other possibility was proven by salpeter to contradict the experimental results. to further provide support to his estimates of the probabilities, salpeter derived the probabilities by assuming the ↵ model for the 8be nucleus. the numbers agreed. 3α 8 be+α q1=0.278 q2=0.094 j=0+ j=2+ j=0+ 7.654 4.43 0 γα γe+e-γγ figure 8.18: the structure of 12c assumed by salpeter in 1957. �↵ is the probability of decay emitting an ↵ particle, �� is the probability of decay emitting a � photon and �e± is the probability to decay by emitting an electron positron pair. this is after salpeter obtained the improved results of cook et al. 1957. finally came the question what is the probability �� to decay into the bound 4.43mev level. after all, this was the dominant mode to form a stable carbon nucleus. again, the estimate depended on the model for the carbon nucleus. if one assumed that the carbon nucleus was made of three ↵ particles, one got a very small probability to decay into the first excited state. so after some lengthy theoretical discussion salpeter adopted that the probability to decay into the ground state plus the probability to decay into excited state, namely the total probability to decay into the ground state, was 0.00014, quite small. salpeter made an additional crucial assumption. since the probability to decay back into the incoming channel (8be+↵) is by far the largest, or in other words, the leakage to modes of decay other than the mode of formation is extremely small, it is permitted to assume an equilibrium 8be + ↵ *) 12c ⇤ . the unbound 2 ↵’s are reluctant to absorb a third ↵ and become bound. once this is done and statistical mechanics can be used to calculate the abundance of the 12c nuclei, the details of the nuclear reaction became irrelevant! the probability to form carbon in the ground state is then the number of carbon nuclei in the excited state (as obtained form the equilibrium condition) times the probability that they decay to the ground state of carbon. at the end of january 1955 the american physical society had its annual meeting in new york. in a session on reactions of transmutation and nuclear energy level, salpeter presented preliminary results on the e↵ect of the 7.68mev82. by now, salpeter became aware of the ajzenberg and lauritsen coming up summary (it appeared in his paper as 0to be published0) of the energy levels of 12c and derived for the first time the full reaction rate. hayakawa et al.83 got the information from salpeter, who also informed them about the probability that the 7.68mev excited state decays to the 4.43mev level, and submitted their paper to the progress in theoretical physics japan, on july 1956 to be published november 1956. salpeter’s paper, on the other hand, was submitted march 1957 to be published july 1957, that is to say, over a year later. so before salpeter managed to publish his results in a journal, hayakawa et al. published the calculation of the rate including the excited state of 12c and wrote that: this rate is about 106 times larger than that given by salpeter. they meant salpeter 1952. they continued to state that: such a great di↵erence is due mainly to the fact that he did not take the e↵ect of the recently discovered resonance level in 12c into account. this is one of the most important conclusion obtained in our work. it may have been the most important 80most recent result: chemykh, m. and 5 authors, proc.13th int. symposium capture gamma-ray spect., aip conf. proc. 2009 is �± e = 6.2 ⇥ 10�5ev . 81blatt, j., & weisskopf, v., theoretical nuclear physics, john wiley & sons, inc. ny, 1952. 82salpeter, e.e., phys. rev. 98, 1183, (1955) 83hayakawa, s., hayashi, c., imoto, m., & kikuchi, k. prog. theor. physics, 16, 507, (1956) figure 13: the part of the energy schemes of the 12c nucleus relevant to the 3α reactions as was finally assumed by salpeter in his final paper in 1957 the structure of 12c as assumed by salpeter in 1957, is given in fig. 13. γα is the probability of decay emitting an α particle, γγ is the probability of decay emitting a γ photon and γe± is the probability to decay by emitting an electron-positron pair. this is a rare mode of decay and of no importance to our story here. this was the structure of 12c after salpeter obtained the improved results of cook et al 1957. salpeter made some critical assumptions: the disintegration channel 12c∗ decaying back into 8be + α is the dominant channel and all other possibilities are very small leakages. consequently, salpeter assumed a dynamic equilibrium, namely 12c∗ ↔8 be + α. if so, the concentration of 12c is determined from the equilibrium (small sensitivity to the energy of the level) and all the uncertainty is in the small leakage to the other channels. in particular, the is no need for a cross section,to evaluate the rate. the reaction is in a statistical equilibrium. on january 1955 salpeter presented the results in the new york meeting of the physical society. by now he became aware of the ajzenberg & lauritsen old result and cited it. fowler won the 1983 nobel prize (with chandrasekhar) for contributions to nuclear astrophysics, but fowler’s nobel speech was one long discussion about . . . hoyles contributions. as for salpeter, fowler commented that he ignored the 7.68mev state which is a very wrong claim. he ignored it when it was unknown to him. fowler was a great experimental nuclear physicists but without knowledge in statistical mechanics and he did not like that ’the nuclear physics’ was eliminated from the problem. 317 g. shaviv hoyle and salpeter shared the 1997 crafoord prize1 in particular for the physics of the triple α reaction. the process is called today the salpeter process and the energy level is called the hoyle level. fight for priority: salpeter informed hayakawa et al. about the probability that the 7.68mev state decays into the 4.43mev state. hayakawa et al hurried and submitted their paper to progress of theoretical physics japan in july 1956 in which they made just this claim. the journal published on november 1956. salpeter published his work only in 1957. surprisingly the japanese authors claimed that: this (meaning theirs) rate is 106 times greater than given by salpeter 1952. such a great difference is due mainly due to the fact that he did not take into account the 12c resonance. they did not mention that it was salpeter who told them about his work and the 12c level..... 12 is this the entire story? is this the entire story? no! what about the structure of 16o? following hoyle, we treated the reaction 12c + α16 → o + γ as well established. however, this is not the case. in 1974 dyer and barnes from caltech attempted to measure the 12c+α reaction. the next attempt was carried out in 1982 when kettner et al. from münster measured the reaction and discovered that it is 3 to 5 times faster then what dyer and barnes from caltech found. as a consequence, the most abundant specie at the end of helium burning was found to be 16o and not 20ne as the formation of 16o is faster than its destruction. langanke and koonin (1983) criticized the analysis and the conclusions of the experimenters and repeated the very long theoretical analysis of the experimental data. however, they could not resolve the discrepancy between the caltech data (dryer and barnes) and the münster data (kettner et al.) and fitted each experiment separately. thus, the münster data was 1.5 time higher than that of caltech, which in turn was 3 times higher than what stellar modelers used. as lankange and koonin wrote, such higher values , as those found in münster and caltech, lead to 16o rather than 12c, as the final product of helium burning, and in this way cast a shadow on hoyles argument. in fig. 15 we show a comparison between two recent experimental result and a theoretical fit. the red arrow shows the energy range in stars where this reaction takes place. so far this experimental discrepancy is not sett led. 8.18. carbon structure is half the story: the 12c + ↵ !16 o + � reaction 255 very high. due to the fact that these levels are rather wide86, they all contribute to the rate of the reaction. consequently, the locations of the levels up to about 10mev are important for the rate of 8be + ↵ at t < 109k. the higher in energy levels corresponds to such a high energy range that by the time the colliding species have enough energy to reach these levels, the temperature is so high that all nuclei disintegrate and hence, cannot fuse any further. in 1994 freer at el87 investigated whether the 12c 7.65mev level could decay directly into 3 ↵ in contrast of a decay to 8be + ↵ first and then the 8be would break into 2 ↵s. the results for this direct decay is less than 4%. this was an additional experimental proof that the triple alpha is not a three body reaction. the problems in getting accurate value for the probability for the 7.68mev level transition to the 4.43mev one continued to annoy physicists. the recent values are: alburger88 (3.3±0.9)⇥10�4, hall and tanner89 (3.5±1.2)⇥10�4, seeger and kavanagh90 (2.8±0.3)⇥10�4, and mak et al.91 (4.30±0.2)⇥10�4. it is di�cult to achieve high accuracy! 8.18 carbon structure is half the story: the 12c+↵ !16 o+� reaction αα12c+ 6.049 6.130 6.917 7.1157.162 j=1j=2+ j=3j=0+ j=0+ o16 figure 8.19: the structure of the oxygen nucleus 1952. based on ajzenberg and lauritsen 1952. we come to the destruction of carbon. the existence of excited levels in 16o was discovered by burcham and freeman92, who used the reaction 19f + p ! 16o + ↵ and found the 6.94mev and the 7.15mev levels. they however, did not determine the quantum numbers of the level. in 1950 millar, bartholomew and kinsey93 investigated the � rays from the decay of 16n ! 16o + e�. this radioactive isotope is produced in the cooling water of nuclear reactors via the reaction 16o + n ! 16n + p. the half lifetime of the isotope is 7.35 sec. the energy of 16n is so high that it can decay into the first two levels of 16o by emission of an electron. thus, they discovered � rays which corresponded to the level at 6.133 ± 0.011mev and 7.10 ± 0.02mev. these results were in good agreement with the values found by chao, tollestrup, fowler and lauritsen94 of 6.136 ± 0.030mev and 7.111 ± 0.03mev. to complicate the issue, they did not discover � rays which would correspond to the level at 6.91mev. they were however, able to determine the quantum numbers of the level as j = 1�. the situation became complicated once more. the 12c + ↵ have a restmass energy of 7.162mev, while the closest energy level in the 16o nucleus is at 7.115mev, that is to say, just below the threshold of the colliding particles. because all energy levels save the ground state, are not stable they have a certain width in energy. the width �e is related to the lifetime �t through the relation �e�t ⇠ h̄. the 7.115mev level in 16o is su�ciently wide to extend into energies slightly above the 86according to the uncertainty principle, the width of the level is connected to the life time of the system in that level. from a nuclear reaction point of view it means that if the energy of the colliding particles is within the wide range of the level, and not just equal to the prescribed energy, the reaction can proceed. 87freer, m., wuosmaa, a.h., betts, r.r., henderson, d.j., wilt, p., zurmuhle, r.w., balamuth, d.p., barrow, s., benton, d., li, q., liu, z., & miao, y., phys. rev. c, 49, 1751, (1994) 88alburger, d.e., phys. rev., 124, 193, (1961), who used the reaction 10b + 3he ! 12c + p. 89hall & tanner , nuclear phys.,53, 673, (1964), who used the reaction 10b + 3he ! 12c + p. 90seeger, p.a. & kavanagh, r.w., nucl. phys., 46, 577, (1963), who used the reaction 14n + 2d ! 12c + ↵ 91mak, h.-b., and 4 co-authors, phys. rev. c,12, 1158, (1975) 92freeman, j.m. & baxter, a.s., nature,162, 696, (1948). burcham, w.e., & freeman, j.m., phys. rev. 75, 1756, (1949) 93millar, c. h. , bartholomew, g. a. & kinsey, b. b., prl, 81, 150, (1951) 94chao, c. y. ,tollestrup, a. v. , fowler, w. a. & lauritsen, c. c. , phys. rev. 79, 108, (1950) figure 14: the structure of 16o based on ajzenberg & lauritsen 1952. the partial width of the relevant level at 7.115mev is not known because it is a very difficult measurement. the uncertainty in the level is marked by the grey zone around the level. the difficulty stems from the fact that the level lies just below the continuum and hence extremely difficult to feed in. the next question is where carbon is synthesized? or what happens if the carbon level were at another energy? would carbon still be formed? the answer is 256 chapter 8. how nature overcomes its own barriers 12c + ↵ threshold. thus the carbon plus the alpha combine to form the excited oxygen nucleus via the upper tail of the level. on the other hand, as the width of the level is large, the probability to breakup into 12c + ↵ is very small and as a matter of fact, was never observed. in 1974 dyer and barnes95 from caltech attempted to measure the 12c + ↵. the next attempt was in 1982 when kettner et al.96 from münster measured the reaction and discovered that it is 3 to 5 times faster then what dyer and barnes from caltech found. as a consequence, the most abundant specie at the end of helium burning was found to be 16o and not 20ne langanke and koonin97 criticized the analysis and the conclusions of the experimenters and repeated the very long theoretical analysis of the experimental data. they could not resolve the discrepancy between the caltech data (dryer and barnes) and the münster data (kettner et al.) and fitted each experiment separately. thus, the münster data98 was 1.5 time higher than that of caltech, which in turn was 3 times higher than what stellar modelers use. as lankange and koonin wrote such higher values , as those found in münster and caltech, lead to 16o rather than 12c, as the final product of helium burning, and in this way cast a shadow on hoyle’s argument. figure 8.20: the measured capture probability (in units of nano-barns) by the munster (1982) and the kellogg (1974) laboratories. the arrow marks the energy range at which the reaction take place in stars. the continuous lines are the theoretical fit by langanke and koonin 1983 and it includes the e↵ect of the resonance. 0 2 4 6 8 10 0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4 temperature in 109k 12c+α 16o+γ ra tio of pr ed ic te d re ac tio n ra te s kunz et al(2002) caughlan & fowler (1988) figure 8.21: the ratio between the newest rate calculated by kunz et al (2002) and the standard rate compiled by caughlan and fowler (1988) used in many calculation of stellar evolution. the ratio is given as a function of the temperature. 95dyer, p. & barnes, c. a., nuc. phys. a233, (1974) 96kettner, k.u. and 8 co-authors, z. phys. a, 308, 73, (1982) 97langanke, k. & koonin, s.e., nuc. phys. a410, 334, (1983). ibid. nuc. phys., a439, 384, (1985). the first paper was published when langanke was still in munster. the second paper was published after he joined koonin in caltech 98the probability for this reaction to take place at thermal energies (t = 2 ⇥ 108 or 300kev) was given by s(300kev ) = 350 kev barn (münster), 240 kev barn (caltech) and 80 kev barn in standard nucleosynthesis calculations at that time. figure 15: the measured capture probability (in units of nano-barns) by the munster (1982) and the kellogg(1974) laboratories. the arrow marks the energy range at which the reaction takes place in stars. the continuous lines are the theoretical fit by langanke & koonin 1983 and it includes the effect of the resonance. 1 the crafoord prize in astronomy and mathematics, biosciences, geosciences or polyarthritis research is awarded by the royal swedish academy of sciences annually according to a rotating scheme. the prize sum of sek 4 million makes the crafoord one of the worlds largest scientific prizes. 318 who discovered the hoyle level? given in fig. 16 for low mass stars and fig. 17 for high mass stars. we see that ’moving’ the level quite significantly changes the dominant stellar mass at which carbon is formed but not in a way that would required a revision of our ideas.258 chapter 8. how nature overcomes its own barriers -150 -100 -50 0 50 100 150 0.0001 0.001 0.01 0.1 1 10 100 1000 10000 c/ o δe-kev m=1.3msun m=5msun figure 8.22: the c/o ratio and obtained in the evolution of low mass stars (1.3m� and 5.0m�) upon a hypothetical change in the location of the resonance in 12c. -100 -50 0 50 100 2 4 6 8 10 δe-kev c/ o m=15msun m=25msun figure 8.23: the c/o ratio and obtained in the evolution of high mass stars (15m� and 25m�) upon a hypothetical change in the location of the resonance in 12c. 8.20 the c/o ratio today observations and what they imply not a single star without oxygen or without carbon was discovered. detailed observations have shown that the abundance ratio c/o varies from star to star. gradually it became clear that there is no universal value for the c/o abundance ratio which was the basis for hoyle’s analysis and di↵erent stars expose di↵erent ratios. for example, stars with extra high abundance of carbon do not show extra high abundances of oxygen107. examination of the gases ejected from low mass stars shows that when the metals are more abundance by about 50% the c/o ratio is lower by about 50%.108 the c/o ratio appears to be higher in stars with low abundance of heavy elements.109 finally, the c/o appears to vary in low mass stars from 0.2 to 13110. in short, the picture is more complicated and the last word has not yet been said. 8.21 the structure of 12c and the ↵ model: retrospect in 1971 brink111 concluded from the nuclear cluster model that 0our0 state forms a linear chain of three ↵-particles. further indication (from bombardment by electrons), imply that this state has an unusually large radius112. these properties enhance the probability of decay into three ↵ particles. this conclusion was supported in 1997 when pichler, oberhummer, csoto and moskowski113 who concluded that the 0+ level is a genuine three alpha resonance. if so, one would expect that the level would decay directly into three ↵’s and then salpeter’s equation for the nuclear statistical equilibrium may need a change. but the probability of decay directly into three alpha appears to be only about 4%. the reason for this is purely statistical, the phase space of 8be+↵ is that much bigger than the 3↵ mode, in layman terminology: all possible modes of decay have equal probabilities and the number of ways the system can decay into 8be + ↵ is much larger that the number of di↵erent ways it can decay into 3↵s. 107bujarrabal, v. & cernicharo, j., a&a, 288, 551, (1994) 108wang, w. & liu, x.-w., mnras, 381, 669, (2007) 109wahlin, r., and 6 co-authors, memorie della societa astronomica italiana, 77, 955, (2006) 110cohen, m. & barlow, m. j., mnras, 362, 1199, (2005) 111brink, in the alpha particle model of light nuclei, proc. ”enrico fermi” school course xxxvii, varenna, 1966. 112takigawa, n., & arima, a., nucl. phys., a168, 593, (1971) 113pichler, r., oberhummer,h., csoto, a., and moskowski, s.a., nucl. phys. a618, 55, (1997) figure 16: the c/o ratio obtained in the evolution of low mass stars (1.3m� and 5.0m�) upon a hypothetical change in the location of the resonance in 12c. we did not discuss the possible decay of the excited nucleus 12c∗ to 3α. we can, however, repeat hoyle’s argument and argue that the observations imply that this decay, though possible energetically and should take place, is very rare and hence neglected.258 chapter 8. how nature overcomes its own barriers -150 -100 -50 0 50 100 150 0.0001 0.001 0.01 0.1 1 10 100 1000 10000 c/ o δe-kev m=1.3msun m=5msun figure 8.22: the c/o ratio and obtained in the evolution of low mass stars (1.3m� and 5.0m�) upon a hypothetical change in the location of the resonance in 12c. -100 -50 0 50 100 2 4 6 8 10 δe-kev c/ o m=15msun m=25msun figure 8.23: the c/o ratio and obtained in the evolution of high mass stars (15m� and 25m�) upon a hypothetical change in the location of the resonance in 12c. 8.20 the c/o ratio today observations and what they imply not a single star without oxygen or without carbon was discovered. detailed observations have shown that the abundance ratio c/o varies from star to star. gradually it became clear that there is no universal value for the c/o abundance ratio which was the basis for hoyle’s analysis and di↵erent stars expose di↵erent ratios. for example, stars with extra high abundance of carbon do not show extra high abundances of oxygen107. examination of the gases ejected from low mass stars shows that when the metals are more abundance by about 50% the c/o ratio is lower by about 50%.108 the c/o ratio appears to be higher in stars with low abundance of heavy elements.109 finally, the c/o appears to vary in low mass stars from 0.2 to 13110. in short, the picture is more complicated and the last word has not yet been said. 8.21 the structure of 12c and the ↵ model: retrospect in 1971 brink111 concluded from the nuclear cluster model that 0our0 state forms a linear chain of three ↵-particles. further indication (from bombardment by electrons), imply that this state has an unusually large radius112. these properties enhance the probability of decay into three ↵ particles. this conclusion was supported in 1997 when pichler, oberhummer, csoto and moskowski113 who concluded that the 0+ level is a genuine three alpha resonance. if so, one would expect that the level would decay directly into three ↵’s and then salpeter’s equation for the nuclear statistical equilibrium may need a change. but the probability of decay directly into three alpha appears to be only about 4%. the reason for this is purely statistical, the phase space of 8be+↵ is that much bigger than the 3↵ mode, in layman terminology: all possible modes of decay have equal probabilities and the number of ways the system can decay into 8be + ↵ is much larger that the number of di↵erent ways it can decay into 3↵s. 107bujarrabal, v. & cernicharo, j., a&a, 288, 551, (1994) 108wang, w. & liu, x.-w., mnras, 381, 669, (2007) 109wahlin, r., and 6 co-authors, memorie della societa astronomica italiana, 77, 955, (2006) 110cohen, m. & barlow, m. j., mnras, 362, 1199, (2005) 111brink, in the alpha particle model of light nuclei, proc. ”enrico fermi” school course xxxvii, varenna, 1966. 112takigawa, n., & arima, a., nucl. phys., a168, 593, (1971) 113pichler, r., oberhummer,h., csoto, a., and moskowski, s.a., nucl. phys. a618, 55, (1997) figure 17: the c/o ratio obtained in the evolution of high mass stars (15m� and 25m�) upon a hypothetical change in the location of the resonance in 12c. 13 in retrospect in 1985 arnet and thielemann questioned the logic of hoyles original argument as the synthesized carbon may be fused into heavier nuclei or locked in wds. hence, we should re-think how carbon is synthesized and it is not clear that any remnant of the anthropic principle will prevail. however, hoyle’s reverse engineering methodology is right: if we see carbon in nature, there should be a way to synthesize it! today, the principle has its own life and the origin which triggered it is mostly forgotten. in any case the old justification that lead to its inception is not that valid. the story is told in more detail in shaviv (2011). references [1] ajzenberg, f. & lauritsen, t., rev. mod. phys.24, 321, (1952) in 1950 azjenberg and lauritsen had the level at 7.mev and it is not clear why the level ?moved? half an mev upward. [2] beghian, l.e., halban, h.h., husain, t., & sanders, l.g., prl, 90, 1129, (1953) submitted april, 1953 and published june 1953. [3] bethe, h.a. , rev. mod. phys., 9, 167, (1937). this extensive review of nuclear physics earned justifiably the title the bethes bible. doi:10.1103/revmodphys.9.69 [4] bothe, w., & becker, h., zeits. f. physik, 76, 421, (1932) doi:10.1007/bf01336726 [5] cook, c.w., fowler, w.a., lauritsen, c.c. & lauritsen, t., phys. rev., 107, 508, (1957) doi:10.1103/physrev.107.508 [6] crane,h.r., delsasso, l.a., fowler & lauritsen, c.c., phys. rev.,46, 1109, (1934) doi:10.1103/physrev.46.1109.2 [7] crane,h.r., & lauritsen, c.c., phys. rev.,45, 497, (1934) [8] dyer, p. & barnes, c. a., nuc. phys. a233, 495, (1974) doi:10.1016/0375-9474(74)90470-9 [9] gaerttner, e.r., & pardue, l.a. , phys. rev., 57, 386, (1940) doi:10.1103/physrev.57.386 [10] gamow, g., phys. rev., 53, 595, (1938) doi:10.1103/physrev.53.595 [11] guier, w.h., & roberts, j.h. , phys. rev. 79, 719, (1950) doi:10.1103/physrev.79.719 [12] hayakawa, s., hayashi, c., imoto, m., & kikuchi, k. prog. theor. physics, 16, 507, (1956) doi:10.1143/ptp.16.507 [13] holloway, m.g., & moore, b.l., phys. rev., 58, 847, (1940) doi:10.1103/physrev.58.847 [14] hornyak, w.f., lauritsen, t., morrison, p. & fowler, w.a., rev. mod. phys., 22, 291, (1950) doi:10.1103/revmodphys.22.291 [15] kettner, k.u. and 8 co-authors, z. phys. a, 308, 73, (1982) 319 http://dx.doi.org/10.1103/revmodphys.9.69 http://dx.doi.org/10.1007/bf01336726 http://dx.doi.org/10.1103/physrev.107.508 http://dx.doi.org/10.1103/physrev.46.1109.2 http://dx.doi.org/10.1016/0375-9474(74)90470-9 http://dx.doi.org/10.1103/physrev.57.386 http://dx.doi.org/10.1103/physrev.53.595 http://dx.doi.org/10.1103/physrev.79.719 http://dx.doi.org/10.1143/ptp.16.507 http://dx.doi.org/10.1103/physrev.58.847 http://dx.doi.org/10.1103/revmodphys.22.291 g. shaviv [16] kunz, r. and 7 co-authors, apj, 567, 643, (2002) doi:10.1086/338384 [17] johnson, v.r., phys. rev., 86, 302, (1952) doi:10.1103/physrev.86.302 [18] langanke, k. & koonin, s.e., nuc. phys. a410, 334, (1983). ibid. nuc. phys., a439, 384, (1985). [19] lawrence, e.o., mcmillan, e., & henderson, m.c., phys. rev, 47, 273, (1935) [20] lewis, g.n., livingston,m.s., & lawrence, e.o., prl, 44, 55, (1933) [21] miller, c., & cameron, a.g.w., phys. rev., 81, 316, (1951) [22] öpik e. proc. roy. irish acad. a54, 49, (1951). the paper was published over a year after it was read before the irish academy. [23] öpik, e.j., msrsl, 1, 131, (1954), les processus nuclaires dans les astres, communications prsentes au cinquime colloque international dastrophysique tenu liege les 10-12 septembre. [24] shaviv,g. the synthesis of the elements: the astrophysical quest for nucleosynthesis and what it can tell us about the universe (astrophysics and space science library) , springer, 2011. [25] terrell, j., phys. rev., 80, 1076, (1950) [26] wrubel, m.h., irish ajs, 10, 77, (1972) discussion tzumi hachisu: is the carbon-burning c/o ratio still uncertain? giora shaviv: the reduced width of the 16o level is still unknown and there are only guess on its value. sp any c/o ratio between 1/4 to -3/4 is to my mind plausible. in 2002 kunz et al. carried out an extensive theoretical analysis based primarily on kunzs experimental phd thesis. the result of kunz et differs significantly from caughlan & fowler known tables at low temperature which are the relevant temperatures for quiet helium burning. however, it is not the last word on the subject. 320 http://dx.doi.org/10.1086/338384 http://dx.doi.org/10.1103/physrev.86.302 who is who what is the hoyle level what is the anthropic principle the nuclear barrier why nuclear physicists were interested in the problem? the history of the discovery of the 12c nuclear levels the wrong solution salpeter enters the game 1953 kellogg phase i how did hoyle predict the nuclear level in 12c 1957 salpeter finishes the job is this the entire story? in retrospect acta polytechnica ctu proceedings doi:10.14311/ap.2016.4.0027 acta polytechnica ctu proceedings 4:27–32, 2016 © czech technical university in prague, 2016 available online at http://ojs.cvut.cz/ojs/index.php/app self-absorption corrections based on monte carlo simulations kamila johnová faculty of nuclear science and physical engineering, czech technical university in prague, břehová 7, 115 19 prague, czech republic correspondence: kamila.johnova@fjfi.cvut.cz abstract. the main aim of this article is to demonstrate how monte carlo simulations are implemented in our gamma spectrometry laboratory at the department of dosimetry and application of ionizing radiation in order to calculate the self-absorption within the samples. a model of real hpge detector created for mcnp simulations is presented in this paper. all of the possible parameters, which may influence the self-absorption, are at first discussed theoretically and lately described using the calculated results. keywords: mcnp, self-absorption, gamma spectrometry. 1. introduction gamma spectrometry is powerful non-destructive method useful for samples containing gamma ray emitting radionuclides (spontaneous or induced gamma ray activity). due to the principles of gamma ray emission, energy of photons emitted during radioactive decay is specific for each radionuclide. this means that with a proper detection technique it is possible to identify what radionuclide is present in the sample. in addition, with a suitable efficiency calibration the amount of radionuclide in the sample can be estimated. gamma spectrometry can be applied in many scientific fields, like contamination studies, natural radioactivity, activation analysis, astroand high energy physics or geology studies. different applications require a different detection techniques and data analyses. this paper discuss a laboratory gamma spectrometry for natural samples and samples of natural origin. 1.1. interactions of gamma radiation with matter for the issue discussed in this paper three type of photon interactions are the most important: photoelectric absorption, compton scattering and pair production. the probability that a certain photon of a given energy will interact in the matter is expressed by its cross section. the cross section can sometimes be expressed in special unit barn = 10−28 m2. the sum of cross sections for all possible interaction is denoted as total cross section σt . the cross section is closely connected to the attenuation of photons in the matter. mono-energetic photon beam is along a path of length d attenuated exponentially, which is expressed by equation i = i0e −µd, where i0 is a number of photons in non-attenuated beam, i is a number of photons at the end of path d and µ is a linear attenuation coefficient. µ can be expressed in the terms of cross sections for individual interactions (τ -photoelectric absorption, σ-compton scattering, κ-pair production, σrs -elastic scattering) or in the term of total cross section σt µt ρ = na a (τ + σ + κ+ σrs) = na a σt , where µt means total attenuation coefficient (over all interactions), ρ is the density of material, na the avogadro constant and a is an average atomic mass. the expression µt ρ is called total mass attenuation coefficient. 1.2. self-absorption coefficient the previous text suggested that there has to be a certain level of self-absorption within the sample itself. based of the previous informations one can correctly assume that this effect depends on composition of the sample, measurement geometry, density of the material and energy of photons. in general, there are 3 ways how to estimate the self-absorption correction. estimation based on mass attenuation coefficients. several spectrum analysis programs give the user this option, where the library of mass attenuation coefficients is provided. this method is useful over the energy range, where the compton scattering is the dominant interaction. according to the fact, that mass attenuation due to compton scattering is almost independent of an atomic number, reasonable self-absorption correction can be made even for the sample of barely known composition. empirical estimation. this procedure can sometimes be useful for laboratories providing routine measurements. it is specially useful, when the sample of unknown composition and containing lowenergy emitting radionuclide need to be measured. 27 http://dx.doi.org/10.14311/ap.2016.4.0027 http://ojs.cvut.cz/ojs/index.php/app kamila johnová acta polytechnica ctu proceedings in this method three spectra have to be acquired: the sample itself, the sample with a point source place on its top and the point source without the sample. the method with its limitations is in detail described in [1]. using mathematical tools. with the information technology rapid development the mathematical methods, monte carlo in the first place, will probably soon replace all the other ways of estimating the self-absorption corrections. monte carlo algorithm generally is based on repeated random sampling. in the case of particle transport, the algorithm generates a particle and according to cross section database computes its whole path through the material until it leaves tracked volume or is fully absorbed. with a high number of repetition the final result of this simulation approximates very well the real situation. as the title of this article suggests, in this paper the last item will be further discussed. monte carlo method was previously applied to solve the problem of self-absorption successfully. one of the first works on this topic was [3]. many other articles folowed [4–6], etc. testing different geometries and different monte carlo codes. the following text will describe how this verified method was implemented at our department. 2. experimental equipment many different samples are measured in our laboratory, the most common are: environmental samples (possible contamination monitoring), building materials (radiation protection), soil and rock (geological studies). our equipment includes two hpge detectors designed and produced by canberra company. both of them are classical coaxial germanium detectors with a useful energy range 50 kev to 10 mev. the set up in our laboratory however is suitable for measurement in reduced energy range 50 kev to 3 mev since the laboratory usually works only with natural radioactivity or 137cs contamination. the main part of the detector is ge crystal with ntype and p-type contacts on its surfaces. application of certain potential to the contacts will cause the charge carriers (electrons and holes) move towards the electrodes. due to the very high purity of the ge crystal, only moderate bias is sufficient to “empty” the whole volume between the electrodes, creating so called depleted volume. this region becomes than an active area, where charge carriers, created by photon interactions, are swept by electric field and collected by electrodes. the pre-amplifier, which is incorporated in the cryostat, converts collected charges into voltage pulses. the height of this impulse is proportional to the energy deposited in the volume of the detector by photon interaction. one of the advantages of hpge compared to lithium-drifted detectors (second most common type of semiconductor detector) is that they do not need to be cooled at all time. the detector is stored without cooling, however during the measurement it is cooled in order to prevent thermally generated leakage current. in our laboratory the liquid nitrogen cooling system is used. the contact of ge crystal with the nitrogen (stored in dewar flask) is provided by so called cooling finger made of cu and cover with vacuum. the whole cooling system is called cryostat. the measurement itself can be realized in several geometries of the sample. our laboratory can provide a certified measurement in two geometries. first of them is so called marinelli beaker with a volume 600 ml. this geometry will be denoted as m600 in this article and it is commonly used geometry. the shape of this special container provides very good efficiency of measurement since it surrounds almost whole detector, see figure 1. the samples with volume smaller than 600 ml can be measured using less efficient geometry: a small container (volume 280 ml), which will be denoted as m280 in the rest of this paper. the smaller container can be seen as well in the figure 1. the calibrations of our gamma spectrometry systems are made using certified standards of activity produced by eurostandard cz (czech metrology institute). those standards are the same containers (both m600 and m280) filled with silicon rubber with dispersed known activity of known radionuclide. the spectra are acquired and processed by genietm 2000 analysis software, which is also a product of canberra. 3. monte carlo simulations the monte carlo simulation were implemented using mcnp code. mcnp (monte carlo n-particle) is a general-purpose code developed by los alamos national laboratory. its wide area of application includes radiation protection and dosimetry, shielding against radiation, nuclear criticality safety, detection technique, etc. in discussed problem an mcnp6, the latest release, was used. all geometry plots presented in this article were made using vised – a visual editor for mcnp [2]. 3.1. model of the detector the primer of mc (monte carlo) simulations was model of the detector itself. a rough model was created first using the data provided by the producer of the detector (canberra) in the technical documentation. for more details an x-ray scan of the detector was made. unfortunately some details like thickness of the dead layer or very thin layers generally could not be recognized from the x-ray picture very well. since the model did not still correspond to the reality (calculated and measured efficiencies differed with more than 10 % of relative error), the geometry had to be adapted experimentally. based on the previous experiences the thickness of the dead layer was alternated as was the volume of the vacuum 28 vol. 4/2016 self-absorption corrections based on monte carlo simulations figure 1. 3d visualization of detector model. yellow inner part is the crystal, the top green part represents the m600 (right) and m280 (left) container, blue part is the cooling finger. filled cell. after every change a new set of simulations was made and the results were compared to the measured efficiencies. the agreement of simulated and measured data was checked for m600 and m280 geometry in order to provide the most accurate model of the real detector. the final geometry can be seen in figure 1. 3.2. source and tally the source of radiation in the model was previously described marinelli beaker filled with different materials. both geometries: m600 and m280 were simulated. 3d visualization of the detector with m600 and m280 containers is in figure 2. the simulated photons were generated homogeneously in the whole volume of the container’s filling as it is shown in figure 2. the materials used as a filling of the containers were chosen in order to represent the whole spectrum of samples which are processed in our laboratory. selected materials are: silicon rubber (standards), water, blueberry, wood, soil, rock and brick. the energies of simulated photons were set based on the commonly analysed radionuclides, which are 238u and 232th decay chains, 40k and 137cs. the concentrations of these radionuclides, mainly 238u and 232th, are determined using large number of energy lines originating from different daughter products. all the necessary energy lines were simulated and are part of our database. for the needs of this paper only seven energies were chosen in order to demonstrate the behaviour of correction factor as a function of density and composition. the list of simulated energies is in table 1. in the case of need any other photon energy can be simulated. figure 2. the photons (blue dots) being emitted in the filling of m600 marinelli beaker. choosing tally (i.e. “what should be calculated”) was a simple task, since the objective was to compare simulated data with measured photopeak efficiencies. the best choice for this purpose is f8 type tally, which scores the number of impulses in certain geometry cell. f8 tally (as the other tallies) can be further specified by defining so called energy bins. energy bins provide an information about energy range in which the impulses should be counted. in this case the bin was set so it fully covers the full absorption peak. 29 kamila johnová acta polytechnica ctu proceedings energy [kev] radionuclide decay serie 186.21 226ra 238u 351.92 214pb 238u 661.66 137cs 137cs 727.17 212bi 232th 1460.81 40k 40k 1764.49 214bi 238u 2614.53 208tl 232th table 1. the example photon energies simulated for the needs of this paper. 3.3. data evaluation our approach to the problem allows two different ways of presenting the data. since the previous steps included creating a model of a real detector, complete efficiency curve can be calculated directly for each specific sample. the measured spectra would then be evaluated using this curve. this method is useful for very specific samples or samples that require high precision of measurement. for routine measurements the following way is preferred. the spectra are evaluated using one all-purpose calibration curve estimated by measurement of silicon rubber standards, which were described in the previous text. this process results in the activity a defined for fixed energy e by a = n t · y · ηstandard , where n is number of impulses in the full absorption peak, t is time of measurement, y is the yield of the energy line and ηstandard represents the photo-peak efficiency of the detector for silicon rubber standard. our objective is to estimate correction factor c = c(e, ρ, composition) so the activity of radionuclide in the sample asample can be calculated as asample = a · c = n t · y · ηsample , while ηsample means efficiency of the detector for sample. naturally ηsample = ηsample(e, ρ, composition). it is self-evident that at this point the required correction factor c equals c = ηstandard ηsample . (1) it was previously mentioned, that for fixed geometry the factor c is a function of energy e, density of sample ρ and composition of the material. the importance of those three variables can be forecast using cross section plot. it is one of mcnp option, which allows the user to plot the cross sections for any material and type of particle used in the model. the total cross sections (all interactions included) σt are plotted in figure 3 for all materials discussed in this paper. the detection system in our laboratory can detect photons within the energy range: 50–3000 kev, however energy lines below 100 kev are rarely processed. according to this fact and the information provided in figure 3, it can be assumed that considering the material composition there are only two groups of samples. first group contains “biological materials” or “biomass”, and is represented by wood, blueberries, water and silicon rubber. these materials consists of light elements only (h, o, c) and usually contains a significant amount of water. in can be expected that in the region 100–3000 kev, those samples will not need correction for composition, however the correction for density is still necessary. in another words only one correction curve (the correction factor as a function of density) will be sufficient for all the samples belonging into this group. the other group with similar cross sections contains again natural material but with a certain content of heavier elements (al, si, ca, fe, etc.). in this model this group is represented by brick, soil and rock. again in the region 100–3000 kev only one correction curve will be sufficient for all of these samples. only for lower energies and accurate measurements every material would need a special corrections. 4. results and discussion since this work uses the model of a real detector, it is appropriate at first comment on the comparison of simulated response of the detector with the measured one. it was already mentioned that the model of the detector had been experimentally modified in order to agree to the reality. the comparison of measured and simulated efficiency curves is in figure 4. this plot, made for m600 geometry, indicates a good agreement of measured and simulated data. taking into account the uncertainty of measured data caused mainly by errors in fitting and processing the spectra the model represent very well the real detector. the 5 % uncertainty of measured data is plotted in figure 4 by error bars. similar agreement of measured and simulated data was achieved for m280 geometry. as it was mentioned before the correction factor (1) is a function of composition, material density and energy. in section 3.3 some presumptions about the relation of c with those three parameters were made. in this part these assumptions, based on cross section data, will be confirmed using calculated results. at first the influence of composition of material will be discussed. the cross section plot in figure 3 denoted that the corrections for “biomass” materials as blueberry, wood or any other sample containing mainly h, o and c will be practically the same. the same statement should be valid for group of samples of natural origin containing heavier elements like (al, si, ca, fe, etc.). this group is represented by soil, rock and brick. figure 5 confirms those hypotheses. the correction factors within these two groups were practically the same, with less than 2 % difference. 30 vol. 4/2016 self-absorption corrections based on monte carlo simulations figure 3. the example of cross sections for materials used in the model. figure 4. comparison of measured and simulated efficiency (m600) curve for simulated detector. as a result of this, there are only two curves in this plot, each of them representing one of the groups mentioned above. it is also evident that the differences between the two correction curves are not so significant and for the most of the samples with relatively high uncertainty of measurement (around 10 %) one “mean” correction curve is more than sufficient. the cross section data in the figure 3 also suggested that the photon energy will be affecting the correction factors. the probability of photon interaction (total cross section) was dramatically dropping for low energies (10–70 kev) and continued decreasing slowly for the rest of energy region. this behaviour of cross section for all materials is reflected in figure 6, which plots the correction factor as a function of photon energy. while for the soil-rock types of samples the figure 5. correction factor for m600 as a function of density. composition influence. c is decreasing with energy, for the “biomass” group it is slightly increasing. the dependence of correction factor is logarithmic denoting that for higher energies (above 1500 kev) one correction curve (correction factor as a function of density) will be sufficient. the last parameter which will be commented in this article is a material density. as it was already shown in figure 5 this function is linear. figure 7 presents all correction factors for energies (radionuclides) listed in table 1. these corrections were calculated for m600 filled with brick. however as it was expected based on cross section plot and also proven in the results discussion above, the same curve can be used for soil and rock samples as well. in most of the cases the same corrections can be applied to other samples (wood, water solutions, etc.). 31 kamila johnová acta polytechnica ctu proceedings figure 6. correction factor for m600 as a function of photon energy. figure 7. correction factor for m600 geometry filled with brick. all the previous plots were based on the data calculated for m600 geometry. figure 8 shows the correction factors for smaller m280 geometry as s function of density. even though the behaviour of c = c(ρ) is the same (linear), the corrections itself are higher compared to the m600 geometry. 5. conclusions this article presents a way how the self-absorption problem is dealt with in our gamma spectrometry laboratory. the resulting correction factors c, defined by equation (1), were presented in figure 7, where c was plotted as a function of material density. presented simulations also proved that unless the high precision of measurement is required, the correction factor is independent of material composition. figure 8. correction factor for m280 geometry filled with brick. all the simulations were made using very accurate model of hpge detector. the comparison of measured and simulated efficiencies was plotted in figure 4. this model can be used further more for example for calculations of efficiency for non-standard geometry (geometry which is not calibrated using real certified standard of activity). references [1] g. gilmore. practical gamma-ray spectrometry. 2nd revised edition edition. wiley-blackwell, 2008. [2] r.a. schwarz, et al. graphical user input interface for mcnp. trans. am. nucl. soc. 69:401, 1993. [3] t. nakamura, t. suzuki. monte carlo calculation of peak efficiencies of ge(li) and pure ge detectors to voluminal sources and comparison with environmental radioactivity measurement. nucl. inst. and meth. in phys. res. 205(1):211–218, 1983. doi:10.1016/0167-5087(83)90191-6. [4] o. sima. monte carlo simulation versus semiempirical calculation of autoabsorption factors for semiconductor detector calibration in complex geometries. progress in nuclear energy 24:327–336, 1990. [5] f. sanchez, et. al. a monte carlo based method of including gamma slf-attenuation for the analysis of environmental samples. nuc. ins. and met. in phys. res. 61(4):535–540, 1991. doi:10.1016/0168-583x(91)95334-a. [6] r.m.w. overwater, et al. gamma-ray spectroscopy of voluminous sources corrections for source geometry and self-absorption. nuc. inst. and met. in phys. res. 324(1):209–218, 1993. doi:10.1016/0168-9002(93)90978-q. 32 http://dx.doi.org/10.1016/0167-5087(83)90191-6 http://dx.doi.org/10.1016/0168-583x(91)95334-a http://dx.doi.org/10.1016/0168-9002(93)90978-q acta polytechnica ctu proceedings 4:27–32, 2016 1 introduction 1.1 interactions of gamma radiation with matter 1.2 self-absorption coefficient 2 experimental equipment 3 monte carlo simulations 3.1 model of the detector 3.2 source and tally 3.3 data evaluation 4 results and discussion 5 conclusions references acta polytechnica ctu proceedings doi:10.14311/app.2017.9.0032 acta polytechnica ctu proceedings 9:32–38, 2017 © czech technical university in prague, 2017 available online at http://ojs.cvut.cz/ojs/index.php/app real structure and residual stresses in advanced welds determined by x-ray and neutron diffraction karel trojana,∗, charles hervochesb, kamil kolaříka, nikolaj ganeva, pavol mikulab, jiří čapeka a ctu in prague, department of solid state engineering, faculty of nuclear sciences and physical engineering, trojanova 13, 120 00 prague 2, czech republic b nuclear physics institute, ascr, v.v.i., department of neutron physics, 25068 řež, czech republic ∗ corresponding author: karel.trojan@fjfi.cvut.cz abstract. the paper outlines the capability of x-ray diffraction (xrd) for evaluation of real structure changes and residual stresses (rs) on cross-section of advanced thick welds due to the welding of ferromagnetic plates. the results of neutron diffraction describe a three-dimensional state of rs and also verify previous assumptions of rs redistribution as a result of the surface preparation for determination 2d maps measured by xrd. keywords: laser and mag welding, residual stresses, x-ray diffraction, neutron diffraction. 1. introduction high local residual stresses existing in components could have vital influence on its properties. when a welded part with high local rs is machined the significant distortion may occur, due to the disruption of the equilibrium state of rs. on the other hand high tensile rs due to welding have a strong negative effect on the strength properties, especially under fatigue loading. therefore, the determination of the rs distribution in complex welded components is accentuated. recent practical approaches distinguish between local and global welding rs [1]. the local rs are resulting from the local heating and cooling processes in the weld metal and adjacent heat affected zone (haz). global rs exist in the entire component and they are resulting from shrinkage processes which occur in the whole component [1]. these global rs are affected by the stiffness of the component and the local shrinkage processes in the direction of black arrows in fig. 1. tensile rs around a single pass weld in a plane sample are expected due to the restraint which is generated by the cool adjacent zones in the base material of the plate [1]. therefore, the main direction of tensile rs due to shrinkage is applied along the weld. in the perpendicular direction, the tensile rs should reach lower values according to this model. for a complete description of the austenite phase, the transformation associated with a change in volume must be taken into consideration as a significant source of rs especially in high strength steels. the local compressive rs, which arise in the weld zone as a consequence of the restrained volume expansion during the transformation of austenite in martensite, bainite or ferrite, are superimposed to the global tensile rs figure 1. shrinkage tendency and constraint conditions in a single pass weld (schematically) [1]. explained according to the model described above (fig. 1) [1]. the increase in the cooling rate of the welded plates will lead to quenching and formation of local compressive rs. laser welding supplies into the weld lower heat input so that cooling of the weld is faster compared to the mag technology. the resulting distribution of residual stresses is a combined effect of hindered shrinkage and phase transformations. developed laser welding methods using high power diode lasers (hpdl) took over the capability to fill groove with cold or hot wire metal from metal active gas (mag) welding and thus to change mechanical properties of welds reducing their hardness due to the quenching [2]. profitable changes of real crystallographic structure and rs in comparison with conventional laser welds improve the results during impact and tensile test and mainly enhance fatigue life as a result of a favourable distribution of rs and its relaxation during welding in the weld zone and haz [3]. 32 http://dx.doi.org/10.14311/app.2017.9.0032 http://ojs.cvut.cz/ojs/index.php/app vol. 9/2017 real structure and residual stresses in advanced welds to verify the results obtained by xrd, where the samples were cut in two pieces, it is preferable to use the neutron diffraction. it has distinguished benefits such as the unique deep-penetration, threedimensional mapping capability, and volume-averaged bulk measurements characteristic of the scattering neutron beam for steel sample thickness up to 50 mm without cutting [4]. 2. experimental methods the analysed samples were prepared by a hpdl laser with cold wire and mag welding (metal active gas (mag) welding subtype of gas metal arc welding) from two 20 mm thick sheets made of s355j2 steel. the xrd measurements were performed by proto ixrd combo diffractometer with ω-goniometer and {211} diffraction line of α-fe was measured by crkα radiation. deformation of interplanar distances of variously rotated planes was converted to rs using the generalized hooke’s law according to the sin2ψ method. the corresponding neutron measurements were carried out on the diffractometer spn-100 of the nuclear physics institute which is installed at the channel hc-4 of the research reactor lvr-15 and operates at the neutron wavelength of 0.235 nm. the steca software [5] was used to extract and analyse diffraction patterns from the recorded area detector data. the diffraction peak {110} of α-fe was measured. the strain was calculated using the hooke’s law based on the angular deviation of the diffraction profile position from the value related to the stress-free sample. full widths of the measured diffraction lines at half of the maximum (fwhm ) were also evaluated for both methods. fwhm parameter depends on microstrains, crystallite size, density of dislocations and weight proportion of the hard phases. sample orientations during xrd and neutron diffraction are shown in figs. 2a and 2b. the direction of the arrows always indicates the direction of the measured rs. the rotation of measured planes during sin2ψ measurements is also shown in fig. 2a for xrd. equivalent direction are therefore l’ for xrd and t for neutron diffracton. the measurements were performed in three lines perpendicular to the welds. one line passes through the centre of the weld and the other two are located three millimetres below each surface, see red lines in fig. 2c. upper side of the plate is the site which was firstly welded. to determine the real structure and rs by xrd on the cross section, it was necessary to cut the plate in two parts (solid lines and dashed lines in fig. 2c) and the affected surface layer electrochemically etched. 3. experiment gradients of rs measured by xrd in two directions on the surface of welded plates with a depth of penetration of the used radiation into the sample approx. (a) . sample orientations during xrd. (b) . sample orientations during neutron diffraction. (c) . measured lines during neutron diffraction. figure 2. sample orientations during experimental methods. 4 µm are plotted in fig. 3. dependences of threedimensional state of rs determined by neutron diffraction technique related to the middle line and two lines with depth of three millimetres below both surfaces of laser and mag samples can be seen in figs. 4 and 5. the comparison of rs obtained by xrd in direction l’ and neutron diffraction in direction t is plotted in figs. 6 and 7. the comparison of fwhm obtained 33 k. trojan, ch. hervoches, k. kolařík et al. acta polytechnica ctu proceedings figure 3. surface residual stresses measured by xrd. by xrd in direction l’ and neutron diffraction in direction t is shown in figs. 8 and 9. the parameter x is the perpendicular distance from the axis of the weld in all graphs. the most significant tensile rs on the surface are in the direction s according to the predictions (see fig. 1 and 3) for both samples. tensile rs of the laser weld in the direction s exceed the yield strength of the used steel and for upper side even the tensile strength. this would suggest that the rapid cooling of laser weld created non-equilibrium hard phases with higher strength [6]. in l direction differently wide haz could be well observed. according to figs. 4 and 5 tensile rs within all the three measured lines obtained by neutron diffraction have the maximum in the longitudinal direction for both samples. rs in the laser weld at middle line exceeding 300 mpa and for mag in bottom line only 230 mpa. in the middle of the laser and mag weld, the distribution of rs in both remaining directions n and t is the same. shrinkage and other effects caused that rs were created in both directions homogeneously. in the middle of the laser weld according to figs. 6 and 7, rs obtained by neutron diffraction in the perpendicular direction to the weld reported more tension character than by xrd. this is most probably due to a significant contraction in the direction parallel to the weld. on the other hand in the case of the mag weld, there are compressive stresses. this could suggest a predominance of phase transformation during the formation of rs rather than shrinkage. for both welds, rs obtained by xrd reported higher compressive results than obtained by neutron diffraction. the only exception is the upper line for the mag weld, see fig 7. laser weld (according to the fig. 4) has a higher tensile residual stresses along the weld, therefore, redistribution after cutting the sample probably produced the greatest compressive stresses measured by xrd. after cutting the plate, the weld probably slightly dropped, and thus it caused compressive rs in the plane perpendicular to the weld. the comparison of the parameter fwhm using both methods exhibits in nearly all cases the same pattern, see figs. 8 and 9. change in the parameter fwhm for mag sample, which can be correlated with a width of haz, is recorded at a greater distance than for the laser weld. fwhm is in all cases for both methods higher for the laser sample, this would indicate a higher microstrain, density of dislocations and a greater amount of hard phases. 4. conclusions it was found from the residual stresses measured by neutron diffraction in three different sample orientations and in three different depths under the surface that the greatest gradient of residual stresses is in the direction parallel to the weld. according to the literature study and our results, greater tensile residual stresses exhibits the laser weld in almost all measured lines. furthermore, the results describing haz width (changes in values of rs and fwhm ) are very similar for both analytical methods (xrd and neutron diffraction). both diffraction methods are able to complement each other. furthermore, the resulting residual stresses gradients obtained using both methods with a different approach are in certain correlation. each method (xrd and neutron diffraction) describes the outcome of residual stresses in different context. the redistribution of rs during the cutting has not yet been accurately described, so further measurements and verification using mathematical modelling will be needed. we are currently performing the validation of measurement on the second mag sheet. acknowledgements measurements were carried out at the canam infrastructure of the npi ascr řež supported through mšmt project no. lm2011019 and czech science foundation gačr through the project no. 14-36566g entitled as multidisciplinary research centre for advanced materials. this work was supported by the student grant competition ctu in prague grant no. sgs16/245/ohk4/3t/14. the authors thank to matex pm for supplying the laser weld. 34 vol. 9/2017 real structure and residual stresses in advanced welds (a) . laser — upper line. (b) . laser — middle line. (c) . laser — bottom line. figure 4. rs for laser sample along three measured lines obtained by neutron diffraction. (a) . mag — upper line. (b) . mag — middle line. (c) . mag — bottom line. figure 5. rs for mag sample along three measured lines obtained by neutron diffraction. 35 k. trojan, ch. hervoches, k. kolařík et al. acta polytechnica ctu proceedings (a) . laser — upper line. (b) . laser — middle line. (c) . laser — bottom line. figure 6. comparison of rs obtained by xrd in direction l’ and neutron diffraction in direction t along three measured lines for laser sample. (a) . mag — upper line. (b) . mag — middle line. (c) . mag — bottom line. figure 7. comparison of rs obtained by xrd in direction l’ and neutron diffraction in direction t along three measured lines for mag sample. 36 vol. 9/2017 real structure and residual stresses in advanced welds (a) . laser — upper line. (b) . laser — middle line. (c) . laser — bottom line. figure 8. comparison of fwhm obtained by xrd in direction l’ and neutron diffraction in direction t along three measured lines for laser sample. (a) . mag — upper line. (b) . mag — middle line. (c) . mag — bottom line. figure 9. comparison of fwhm obtained by xrd in direction l’ and neutron diffraction in direction t along three measured lines for mag sample. 37 k. trojan, ch. hervoches, k. kolařík et al. acta polytechnica ctu proceedings references [1] t. nitschke-pagel, k. digler. sources and consequences of residual stresses due to welding. mater sci forum pp. 2777–2785, 2014. doi:10.4028/www.scientific.net/msf.783-786.2777. [2] m. sokolov, a. salminen, m. kuznetsov. laser welding and weld hardness analysis of thick section s355 structural steel. mater design 32(10):5127–5131, 2011. doi:10.1016/j.matdes.2011.05.053. [3] c. lachmann, t. h. nitschke-pagel, h. wohlfahrt. nondestructive characterization of residual stress relaxation and fatigue processes in cyclically loaded welded joints. pp. 1400–1409. oxford, uk, 2000. [4] w. woo, et al. neutron diffraction measurements of residual stresses in a 50 mm thick weld. mater sci eng a 528(12):4120–4124, 2011. doi:10.1016/j.msea.2011.02.009. [5] c. randau, u. garbe, h. g. brokmeier. stresstexturecalculator: a software tool to extract texture, strain and microstructure information from area-detector measurements. j appl cryst 44(3):641–646, 2011. doi:10.1107/s0021889811012064. [6] n. jia, et al. an in situ high-energy x-ray diffraction study of micromechanical behavior of multiple phases in advanced high-strength steels. acta mater 57(13):3965– 3977, 2009. doi:10.1016/j.actamat.2009.05.002. 38 http://dx.doi.org/10.4028/www.scientific.net/msf.783-786.2777 http://dx.doi.org/10.1016/j.matdes.2011.05.053 http://dx.doi.org/10.1016/j.msea.2011.02.009 http://dx.doi.org/10.1107/s0021889811012064 http://dx.doi.org/10.1016/j.actamat.2009.05.002 acta polytechnica ctu proceedings 9:32–38, 2017 1 introduction 2 experimental methods 3 experiment 4 conclusions acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2017.9.0013 acta polytechnica ctu proceedings 9:13–18, 2017 © czech technical university in prague, 2017 available online at http://ojs.cvut.cz/ojs/index.php/app influence of machining technologies on values of residual stresses of oxide cutting ceramics jakub němeček∗, kamil kolařík, jiří čapek, nikolaj ganev department of solid state engineering, faculty of nuclear science and physical engineering, czech technical university in prague ∗ corresponding author: j.nemecek91@gmail.com abstract. currently, the intensive development of engineering ceramic and effort to replace sintered carbides as cutting materials are in progress. with the development of the sintering technology it is now possible to produce compact ceramic cutting samples with very good mechanical properties. the advantage of these materials is their easy accessibility and low purchase price. in this work, the influence of the finishing machine technology on the values of surface residual stresses of cutting ceramic samples al2o3+tic were studying. the samples were supplied by moscow state university of technology stankin. measurements made in the x-ray diffraction laboratory at the department of solid state engineering were performed for both the phases. the influence of the parameters of machining to residual stresses was studied and the resulting values were compared with each other. keywords: cutting ceramic; machining technology; x-ray diffraction; residual stresses. 1. introduction ceramic tools have been used since the stone age, when sandstone containing large amounts of silica blades was used, for example for the sharpening of knives, razor blades or other tools. this material was used henceforward as abrasive wheels during the ages of cutting weapons for sharpening of swords, knives and other blades. only recently these grinding tools have been replaced by other modern materials for example by sic or diamond. at the beginning of the 20th century, the first use of al2o3 ceramics as a cutting tool occurred. over the years, its development had moved to a level at which we were able to replace, for example, sintered carbides using those ceramic materials. this effort has been caused by the mechanical properties of aluminium oxide and the easy availability of the starting materials [1, 2]. the aim of this paper is to study the real structure of al2o3+tic oxide cutting ceramic. the microstrains and a size of crystallites was measured and influence of machining technologies on the macroscopic residual stresses was studied. 1.1. preparation of samples the starting materials and their properties have been known for a long time. the problem was how to make compact samples with the desired mechanical properties and dimensions. the current production process consists of three parts: first, grinding the starting material to a powder with grain size of the order of 100 nm, second, sintering the powders into a compact form and third, the surface treatment of the samples to the desired dimensions. an important technological aspect of the properties of cutting ceramics is the grain size of the starting powders. if the individual grain size is approximately the same (if they have ideal spherical shape), the uniform distribution and the pore size are gained more easily during forming and thereby for uniform compaction throughout the volume and easier reorganization of the particles during sintering. preference is also to achieve the smallest grain size of the sample, because it leads to faster material transport [3]. the sintering process is very similar to that used for sintered carbides. the powder is pressed below the melting point into a compact body, see figure 1. but the main difference is the absence of a binder which during sintering formed a liquid phase, because it would cause degradation of mechanical properties. today, ceramic is most commonly sintered into rods of cross section of finite samples and then cut to the desired dimensions. individual plates are then variously surface treated and shaped [4]. 2. x-ray diffraction analysis of macroscopic residual stresses in cooperation with the moscow state technological university stankin that provides manufacturing technology, the influence of machining technologies on the value of residual stresses in samples of al2o3+tic oxide cutting ceramics were investigated in x-ray diffraction laboratory of the department of solid state engineering of fnspe ctu in prague. all experimental errors were around 10 % of absolute value of measurement. 2.1. grinding by diamond disc after sintering, all samples were ground using a diamond wheel. an effect of displacement of the workpiece on the lift at constant cutting speed, feed rate 13 http://dx.doi.org/10.14311/app.2017.9.0013 http://ojs.cvut.cz/ojs/index.php/app j. nemecek, k. kolarik, j. capek, n. ganev acta polytechnica ctu proceedings figure 1. scheme of creation technology of cutting ceramic sample: refinement powders (wet grinding), evaporation, pressing, sintering [5]. sample fd, displacement on the lift [mm/displ.] ap, depth of cut [mm] 1 0.5 0.04 2 1.0 0.04 3 1.5 0.04 table 1. parameters of grinding by diamond disc. figure 2. scheme of grinding by diamond disc [5]. of the workpiece in the cutting direction and depth of cut was observed. grinding parameters are given in table 1, where cutting speed vc = 30 m/s and feed rate f = 12 m/min. values of residual stresses were determined in two orthogonal directions l (direction of the grinding wheel) and t (direction of displacement on the lift). because of unidirectional grinding anisotropy of values was observed in both directions. different values are due to the different structure of al2o3 (trigonal) and tic (cubic) and the different number of slip planes (see table 2). 2.2. thermal annealing of the samples after grinding by diamond disc next samples were ground by diamond disc with the same machining parameters and then thermally anfigure 3. structure of al2o3 (up) and tic (down). nealed at 800 ◦c. it is evident from table 3 that the values of the macroscopic residual stresses decreased considerably. in contrast, the value of microstrains and grain size given by the parameter fwhm remained almost unchanged. 2.3. air abrasive machining in this method of machining, air of high pressure with added cutting microparticles is jetted on a rotating sample. the effect of air pressure changes on the residual stress in the sample was studied (table 4) with constant time of blasting t = 60 s and rotates per minute rpm = 50. the obtained values (table 5) show that when a pressure increases from 2 to 2.5 bars a decrease of compressive residual stresses (stress relaxation) is ob14 vol. 9/2017 influence of machining technologies on values of residual stresses al2o3 tic l direction rs mnkα (0210) l direction rs crkα (311) sample σn [mpa] σs [mpa] fwhm [2θ] sample σn [mpa] σs [mpa] fwhm [2θ] 1 -216 33 0.787 1 -518 77 0.759 2 -182 39 0.693 2 -536 77 0.741 3 -165 37 0.859 3 -530 74 0.708 t direction rs mnkα (0210) t direction rs crkα (311) sample σn [mpa] σs [mpa] fwhm [2θ] sample σn [mpa] σs [mpa] fwhm [2θ] 1 -436 36 0.804 1 -776 67 0.668 2 -470 30 0.709 2 -805 93 0.759 3 -580 37 0.772 3 -938 89 0.679 table 2. the values of normal (σn ) and shear (σs) residual stresses after grinding by a diamond disc. al2o3 tic l direction rs mnkα (0210) l direction rs crkα (311) sample σn [mpa] σs [mpa] fwhm [2θ] sample σn [mpa] σs [mpa] fwhm [2θ] 12 -185 19 0.666 12 -516 91 0.965 13 -203 16 0.888 13 -290 40 0.609 14 -199 30 0.889 14 -289 112 0.847 t direction rs mnkα (0210) t direction rs crkα (311) sample σn [mpa] σs [mpa] fwhm [2θ] sample σn [mpa] σs [mpa] fwhm [2θ] 12 -303 20 0.781 12 -599 71 0.782 13 -359 6 0.802 13 -474 57 0.613 14 -342 27 0.769 14 -473 104 0.908 table 3. the values of normal (σn ) and shear (σs) residual stresses after thermal annealing. sample p, air pressure [mpa] 4 1.5 5 2.0 6 2.5 table 4. parameters of air abrasive machining. figure 4. scheme of air abrasive machining and a picture of machining chamber [5]. served. it is probably a result of overcoming the yield strength of the material. perhaps machining process with higher pressure values influences deeper sub-surface layers of material. the energy required for crack propagation is therefore higher for these seemingly lower pressure residual stresses. to confirm this assertion, a depth-profile analysis of residual stresses by sequential force-free etching of surface layers is needed. rotational movement of the specimen during the machining leads to an isotropic stress distribution i.e., the values in both the directions l and t are similar. 2.4. water jet machining water jet machining (wjm) is based on the same principle as air abrasive machining, when a focused beams of water is jetted out by varying the pressure on the rotating sample (see figure 6 and table 6). other parameters: time of blasting t = 60 s, distance of samples l = 2 mm, number of nozzles n = 50. it is evident, that for applied values of pressure jetting the yield strength value have not been overcome. the values of residual compressive residual stresses increase steadily with increasing pressure in contrast to the samples after air abrasive machining. due to the rotating of the samples an isotropic residual stress distribution is achieved again (see table 7). 15 j. nemecek, k. kolarik, j. capek, n. ganev acta polytechnica ctu proceedings al2o3 tic l direction rs mnkα (0210) l direction rs crkα (311) sample σn [mpa] σs [mpa] fwhm [2θ] sample σn [mpa] σs [mpa] fwhm [2θ] 4 -352 34 0.787 4 -799 117 0.965 5 -367 45 0.694 5 -928 111 0.609 6 -242 32 0.827 6 -278 83 0.847 t direction rs mnkα (0210) t direction rs crkα (311) sample σn [mpa] σs [mpa] fwhm [2θ] sample σn [mpa] σs [mpa] fwhm [2θ] 4 -295 42 0.781 4 -737 119 0.641 5 -389 44 0.802 5 -899 123 0.890 6 -284 36 0.769 6 -283 69 0.611 table 5. the values of normal (σn ) and shear (σs) residual stresses after air abrasive machining. sample p, pressure [mpa] 7 1.5 8 2.0 9 2.5 table 6. parameters of water jet machining. figure 5. scheme of water jet machining. 2.5. laser shock peening during machining by laser the samples are heated and it results in stress relaxation. increased power (see table 9) will lead to higher temperatures, and thus to higher stress relaxation. furthermore, it is possible to observe the influence of the laying of individual spots (overlapping) on the anisotropy of distribution of residual stresses. 2.6. stress tests — the effect of friction after grinding of samples by diamond disc their surface was loaded by rotating steel disk at different time intervals. the influence of time of friction on values of residual stresses was investigated (see table 10). figure 6. scheme of linear and surface laying of spots for laser shock peening [5]. 3. microstrains and crystallite size simultaneously with the measurement of the macroscopic residual stresses, determination of crystallite size and microstrains was provided using the rietveld analysis. all results are demonstrated in [6]. from the measured values is evident that crystallite size after thermal annealing and after laser machining increased, which is probably due to recrystallization during these treatments. a decrease of microstrains is observed after air abrasive machining when the yield point was overcome and the grain sizes increased. after stress tests by steel disc the largest crystallite size al2o3 are observed, that could be again due to thermal relaxation and recrystallization of the surface. 4. conclusions values of residual stresses of cutting ceramics al2o3+tic were analysed in two perpendicular directions. the influence of machining technologies to residual stresses was studied in both the phases. anisotropy and different values for the two phases during grinding by diamond disc were obtained, and 16 vol. 9/2017 influence of machining technologies on values of residual stresses al2o3 tic l direction rs mnkα (0210) l direction rs crkα (311) sample σn [mpa] σs [mpa] fwhm [2θ] sample σn [mpa] σs [mpa] fwhm [2θ] 7 -99 38 0.712 7 -406 118 0.644 8 -149 42 0.733 8 -363 74 0.691 9 -255 47 0.732 9 -568 118 0.765 t direction rs mnkα (0210) t direction rs crkα (311) sample σn [mpa] σs [mpa] fwhm [2θ] sample σn [mpa] σs [mpa] fwhm [2θ] 7 -85 43 0.790 7 -374 116 0.644 8 -166 50 0.752 8 -364 88 0.689 9 -238 45 0.668 9 -484 108 0.669 table 7. the values of normal (σn ) and shear (σs) residual stresses after water jet machining. al2o3 tic l direction rs mnkα (0210) l direction rs crkα (311) sample σn [mpa] σs [mpa] fwhm [2θ] sample σn [mpa] σs [mpa] fwhm [2θ] 10.1 -53 31 0.731 10.1 -318 111 0.594 10.2 31 31 0.717 10.2 -259 96 0.657 10.3 -29 23 0.757 10.3 -200 106 0.637 10.4 -62 17 0.751 10.4 -220 95 0.568 t direction rs mnkα (0210) t direction rs crkα (311) sample σn [mpa] σs [mpa] fwhm [2θ] sample σn [mpa] σs [mpa] fwhm [2θ] 10.1 -155 9 0.856 10.1 -509 148 0.619 10.2 -125 9 0.868 10.2 -448 135 0.545 10.3 -71 42 0.737 10.3 -367 135 0.543 10.4 -48 39 0.704 10.4 -288 123 0.618 table 8. the values of normal (σn ) and shear (σs) residual stresses after laser shock peening. sample p, power [w] 10.1 2 10.2 5 10.3 10 10.4 15 table 9. parameters of laser shock peening. pulse frequency f = 30 khz, displacement sx = sy = 40 µm. the thermal relaxation of macroscopic residual stresses after thermal annealing was also measured. for air abrasive cutting overcoming of the yield stress was found out with increasing the pressure to 2.5 mpa. due to rotational movement during air abrasive machining and water jet machining the values of surface residual stresses are isotropic. samples machined by laser exhibit thermal relaxsample t, time of loading [s] 11.1 2 11.2 6 11.3 12 table 10. parameters of stress tests. ation of residual stresses. at the same time, the found anisotropy of state of residual stresses is due to overlapping of laser-processed areas. on the samples after the loading tests thermal relaxation of residual stresses was observed. it was shown that titancarbide is more sensitive to load than aluminium oxide. for more detailed information see [6]. acknowledgements this work was supported by grant student grant competition ctu no. sgs16/246/ohk4/3t/14 and grant agency of the czech republic (no. 14-36566g). 17 j. nemecek, k. kolarik, j. capek, n. ganev acta polytechnica ctu proceedings al2o3 tic l direction rs mnkα (0210) l direction rs crkα (311) sample σn [mpa] σs [mpa] fwhm [2θ] sample σn [mpa] σs [mpa] fwhm [2θ] 11.1 -42 27 0.693 11.1 -453 64 0.655 11.2 3 21 0.606 11.2 -129 76 0.530 11.3 -78 40 0.596 11.3 -242 49 0.577 t direction rs mnkα (0210) t direction rs crkα (311) sample σn [mpa] σs [mpa] fwhm [2θ] sample σn [mpa] σs [mpa] fwhm [2θ] 11.1 -145 17 0.612 11.1 -348 95 0.580 11.2 -15 19 0.610 11.2 -190 59 0.563 11.3 47 24 0.649 11.3 -240 66 0.446 table 11. the values of normal (σn ) and shear (σs) residual stresses after stress testing. references [1] r. kumar, et al. synthesis and characterization of al2o3+tic nano-composite by spark plasma sintering. international journal of refractory metals and hard materials 577:9–15, 2016. [2] z. yin, et al. friction and wear behaviors of al2o3+tic micro-nano-composite ceramic sliding against metals and hard materials. international journal of refractory metals and hard materials 577:9–15, 2016. [3] a. humár. materiály pro řezné nástroje. first printing. mm publishing, 2008. [4] c. b. carter, m. g. norton. ceramic materials: science and engineering. second edition. springer science and business media, 2013. [5] m. university stankin. materials for experimental measurement. [6] j. němeček. studium reálné struktury oxidových řezných keramik. 18 acta polytechnica ctu proceedings 9:13–18, 2017 1 introduction 1.1 preparation of samples 2 x-ray diffraction analysis of macroscopic residual stresses 2.1 grinding by diamond disc 2.2 thermal annealing of the samples after grinding by diamond disc 2.3 air abrasive machining 2.4 water jet machining 2.5 laser shock peening 2.6 stress tests — the effect of friction 3 microstrains and crystallite size 4 conclusions acknowledgements references 234 acta polytechnica ctu proceedings 2(1): 234–237, 2015 234 doi: 10.14311/app.2015.02.0234 the nova v5584 sgr: a short review r. poggiani 1 1department of physics, università di pisa corresponding author: rosa.poggiani@df.unipi.it abstract the nova v5584 sgr was discovered during 2009 october. it has been monitored in different domains of the electromagnetic spectrum: optical, infrared and x-rays. the optical and infrared observations suggest that v5584 sgr is a fe ii nova that formed dust. no x-ray emission was observed around the time of maximum. keywords: cataclysmic variables -novae optical spectroscopy individual: v5584 sgr. 1 introduction nova sagittarii no. 4 was discovered by nishiyama and kabashima (2009) on 2009 october 26 and later designated v5584 sgr (samus, 2009). v5584 sgr is a classical example of multi-wavelength astrophysics, since it has been observed in different parts of the electromagnetic spectrum: optical, infrared and x-rays. the very early optical spectroscopic observations secured by kinugasa et al. (2009), maehara et al. (2009), fujii (maehara et al., 2009), munari et al. (2009) in the optical and by raj et al. (2009) in the infrared showed that v5584 sg is a fe ii nova, in the context of the classification by williams et al. (1991), williams (1992). spectra secured by russell et al. (2010) during 2010 february showed that dust formation had occurred. poggiani (2011) has monitored v5584 sgr during the late decline. v5584 has been investigated in the optical domain by the stony brook/smarts consortium (walters et al., 2012). the present paper reviews the history of v5584 sgr observations. 2 light curve the photometric evolution and the main parameters of v5584 sgr has been reported by poggiani (2011) and will be briefly summarized below for completeness. the v band light curve is reported in fig. 1. the epoch of maximum is mjd=55134.208 (2009 october 29), while the decline time by two magnitudes is 27±2 days, making v5584 sgr a moderately fast nova, according to the classification by payne-gaposchkin (1957). the reddening of v5584 sgr is 0.82±0.12. the estimated distance of v5584 sgr is in the interval 5.8-7.1 kpc. the absolute magnitude at maximum is in the range -7.2....7.7, while the white dwarf mass is in the range 0.8-0.9 m�. all parameters extracted by the analysis of the light curve suggest that v5584 sgr is a fe ii nova, according to the classification by della valle & livio (1998). the spectroscopic observations described below will provide further evidence for the classification. 55100 55150 55200 55250 55300 55350 55400 55450 55500 time (mjd) 8 9 10 11 12 13 14 15 16 17 v m a g n it u d e figure 1: v band photometry of v5584 sgr 3 optical spectroscopy spectroscopic observations of v5584 sgr around maximum have been secured by different authors. kinugasa et al. (2009) observed hα and fe ii lines in emission with p cyg profiles on 2009 october 27. the presence of hα in emission was confirmed by maehara et al. (2009) on the same day. munari et al. (2009) observed a highly reddened absorption continuum on october 28, with hα and fe ii showing an emission component and the same continuum with faint emission from balmer ad fe ii multiplets on october 29. it was suggested that v5584 sgr was a fe ii nova caught around maximum, according to the classification by williams et al. (1991), williams (1992). v5584 sgr has also been observed at the higashi-hiroshima observatory with the 234 http://dx.doi.org/10.14311/app.2015.02.0234 the nova v5584 sgr: a short review kanata telescope (uemura, 2009) on october 31 and november 3, 5 1. the intensity of emission lines progressively strengthened, while the continuum decayed. v5584 sgr is part of the stony brook smarts spectral atlas of southern novae that includes data of more than sixty novae (walters et al., 2012). their observations of v5584 sgr on november 11 showed balmer lines, fe ii multiplets and o i. poggiani (2011) has observed v5584 sgr during the late decline (after the seasonal gap) at the loiano observatory, italy, using the 152 cm telescope and the bfosc imager and spectrograph. the observation of june 4 is reported in fig. 2. the spectrum has been secured using grism #4 that has a range 4000 to 8500 å and a resolution of 3.97 å/pixel, reduced with bias and flat frames, extracted using the optimal extraction method by horne (1986), calibrated in wavelength with an hear lamp and corrected with the instrumental response. the spectrum shows a prominent hα transition, intense forbidden lines of [o iii], [n ii] and the high ionization line [fe vii] 6087, typical of a nova in the nebular stage a0 (williams et al., 1991; williams 1992). the same pattern of lines has been observed by the smarts consortium on october 17. figure 2: optical spectrum of v5584 sgr during the nebular stage (poggiani, 2011) the line profiles are a signature of the ejecta distribution. optically thin winds and optically thin shells show rounded and rectangular line profiles, respectively. the profile of hα at two epochs (labelled with the number of days elapsed from maximum) is reported in fig. 3 (poggiani (2011)). the profiles show a red wing, probably explained by the emerging of [n ii] 6584. the rest wavelength is coincident with the 0 km/s label. the observations of v5584 sgr during the nebular stage are part of a monitoring program (poggiani, 2012) active since 2005 at the loiano observatory, italy, using the 152 cm telescope and the bfosc imager and spectrograph. the most part of the spectra are secured with grism #4 to build an homogeneous data set of spectra over the range 4000 to 8500 å. the monitoring aims to build a spectral atlas and includes most novae observable at the telescope latitude that underwent outburst after 2005. other sources are v1663 aql, v1722 aql, v2362 cyg, v2467 cyg, v2468 cyg, v2491 cyg, v407 cyg, kt eri, v959 mon, v2670 oph, v496 sct, v5558 sgr, v458 vul, v459 vul. the novae are observed at several epochs, from the early decline to the nebular stage and on the way to quiescence. figure 3: line profiles of hα during the nebular stage (poggiani, 2011) the importance of monitoring of novae over long time intervals has been addressed by several contributors at this conference (pagnotta, 2013), (ederoclite, 2013), (tappert, 2013). at the moment of writing v5584 sgr is very far from quiescence. at the moment of discovery, corelli (2009) showed that nothing was visible at the nova position on a palomar plate with limiting magnitude 21. 4 infrared observations the relevance of observations of novae in the infrared has been discussed by banerjee and ashok (2012) and by chesneau and banerjee (2012). banerjee and ashok (2012) suggest that the fe ii and he/n classes share a set of common spectral lines in the near infrared, namely hydrogen, helium, nitrogen and oxygen. on the other hand, there is a clear signature belonging only to novae of fe ii class, the presence of strong carbon lines: thus the near infrared observations can provide an additional suggestion of the spectroscopic class of a nova. several novae, mostly novae belonging to fe ii class, produce dust during the decline. the dust presence is marked by a rise of infrared emission and can become apparent also as a dip in the optical light curve. the last signature is not necessarily observed, since dust can be arranged as an optically thin shell. 1http://f.hatena.ne.jp/kanataobslog/20091106162703 235 r. poggiani infrared observations of v5584 sgr have been secured by raj et al. (2009) on october 29 at mt. abu telescope showed paschen, brackett, o i, c i, n i lines with p cyg profiles, whose components wew separated by about 550-650 km/s. the emission components strengthened, while the absorption components faded by november 3. the transitions observed in spectra are typical of the fe ii class novae, providing an independent confirmation of the classification of v5584 sgr. the following observations in the infrared were secured after the seasonal gap. on 2010 february 10 russell et al. (2010) observed v5584 sgr in the region 3-14 µm with aeos telescope, discovering that the nova had formed dust since the previous observations. the infrared continuum was dominated by the thermal emission of the dust at a temperature of 880±50 k; no details of the dust composition or the derivation of the temperature are reported. the above infrared observatons provide a clue that was not available due to the lack of observations during the seasonal gap. 5 x-ray observations the importance of x-ray observations of novae has been addressed by several authors, also at this conference (ness, 2013), (orio, 2013). v5584 sgr has been observed with the monitor of all-sky x-ray image (maxi) (shimanoe et al., 2010). the system is on board of iss and uses two high sensitivity x-ray detectors, the gas slit camera (32 ccd chips) and the solid state slit camera (a xenon filled proportional conunter). maxi scans all sky every 92 minutes. shimanoe et al. (2010) have searched the prompt x-ray emission at the ignition of the thermonuclear runaway. the authors suggested that classical novae could emit x-rays at the outburst in analogy to the type i x-ray bursts of x-ray binaries. the observations included several peculiar novae: v1723 aql, v407 cyg, v2673 and v2674 oph, v1722 aql, kt eri, v496 sct. the authors have focused on the archive data in the energy band 1.5-4 kev, the lowest energy available, assuming that nova outbursts preferentially emit soft x-rays. they also investigated the 4-10 kev and 10-20 kev bands. no prompt emission was detected in the three bands for any nova. there are no reported x-ray observations of v5584 sgr at later stages, when supersoft emission could have occurred. 6 conclusions v5584 sgr is a standard fe ii nova. the sinergy of observations in different domains allowed a classification of the spectral class and the discovery of dust production during the seasonal gap. acknowledgement the author thanks the organizers for the invitation to this very stimulating workshop. thanks to the tac committee of the loiano observatoy for the allotted time and to i. bruni for his precious assistance. the author is grateful to the anonymous referee for the careful comments. references [1] banerjee, d. p. k. and ashok, n. m.: 2012, basi 40, 243. [2] chesneau, o. and banerjee, d. p. k.: 2012, basi 40, 267. [3] corelli, p.: 2009, cbet 1994, 1. [4] della valle, m. & livio, m.: 1998, apj 506, 818. doi:10.1086/306275 [5] ederoclite, a.: these proceedings [6] horne, k.: 1986, pasp 98, 609. doi:10.1086/131801 [7] kinugasa, k. et al.: 2009, iauc 9089, 2. [8] maheara, h. et al.: 2009, iauc 9089, 3. [9] munari, u. et al.: 2009, cbet 1999, 1. [10] ness, j. u.: these proceedings [11] nishiyama, k. & kabashima, f.: 2009, iauc 9089, 1. [12] orio, m.: these proceedings [13] pagnotta, a.: these proceedings [14] payne-gaposchkin, c.: 1957, the galactic novae, north holland, amsterdam. [15] poggiani, r.: 2011, astrophy. space sci. 333, 115. doi:10.1007/s10509-011-0595-z [16] poggiani, r.: 2012, mem. sait 83, 753 [17] raj, a. et al., 2009: cbet 2002, 1. [18] russell, r. w. et al.: 2010, iauc 9118, 2. [19] samus, n. & kazarovets, e. v.: 2009, iauc 9089, 4. [20] shimanoe, j. et al.: 2010, the first year of maxi: monitoring variable x-ray sources, 4th international maxi workshop held november 30 december 2, tokyo, japan [21] tappert, k.: these proceedings 236 http://dx.doi.org/10.1086/306275 http://dx.doi.org/10.1086/131801 http://dx.doi.org/10.1007/s10509-011-0595-z the nova v5584 sgr: a short review [22] uemura, m. on behalf of the kanata team: 2009, asp conf. ser. 404, 69 [23] walters, f. m. et al.: 2012, pasp 124, 1057. doi:10.1086/668404 [24] williams, r. e. et al.: 1991, apj 376, 721. doi:10.1086/170319 [25] williams, r. e.: 1992, aj 104, 725. discussion martin henze: you described x-ray observations and mentioned soft band of 1.5-4 kev in which soft x-rays would be expected. i just wanted to comment that in this band we would not expect to see supersoft x-ray emission which normally does not go above 1 kev. rosa poggiani: the maxi astronomers stated that they were looking for soft x-ray emission in the above energy range. i agree that to detect the x-ray emission you are mentioning you should use another instrument jan-uwe ness: the maxi collaboration was looking for prompt emission. 237 http://dx.doi.org/10.1086/668404 http://dx.doi.org/10.1086/170319 introduction light curve optical spectroscopy infrared observations x-ray observations conclusions acta polytechnica ctu proceedings doi:10.14311/app.2017.13.0061 acta polytechnica ctu proceedings 13:61–65, 2017 © czech technical university in prague, 2017 available online at http://ojs.cvut.cz/ojs/index.php/app the effect of plasticizer on mechanical properties of the cement paste with fine ground recycled concrete jaromír hrůza∗, zdeněk prošek czech technical university in prague, faculty of civil engineering, thákurova 7, 166 29 prague 6, czech republic ∗ corresponding author: jaromir.hruza@fsv.cvut.cz abstract. this article deals with the usage of recycled concrete, which arises from the demolition of concrete structures. the work is focused on the development of mechanical properties (young's modulus, compressive and flexural strength) depending amount of plasticizer in the mixture. in the experiment were prepared three sets of samples with different amounts of plasticizer (0, 0.5 and 1.0 wt. % of cement). each pair always contained reference samples (only cement) and 35 wt. % of fine ground recycled concrete. one of the main reasons for the use of finely ground recycled concrete was a certain substitution of cement in the mixture, which is the most expensive component. development of young's modulus was measured by the nondestructive method. the aim of the experiment was to determine the effect of plasticizer on the resulting physical and mechanical properties of cement pastes with fine ground recycled concrete. keywords: recycled concrete, young's modulus, plasticizer, mechanical properties, cement replacement. 1. introduction already since the first production of concrete, which has become the most widely used composite material, attempts have been made to improve the properties of this material. for adjustment of various concrete properties are used plasticizers, delayers or accelerators of solidification and hardening, aerating additives, hydrophobic additives, waterproofing and anti-frosting additives, etc. however, it is often the case that the use of the additives or admixtures to improve the desired properties cause the deterioration of others properties, and it is, therefore, necessary to evaluate the positive and also negative aspects of the used additives. the use of plasticizers for the adjustment of cement-based composites is currently relatively widespread due to the reduction of the water coefficient and the achievement of greater strengths, for example in high-performance concrete. plasticizers are organic macromolecular substances adsorbing on the surface of cement grains, allowing them to change their electric charge, disperse particle agglomerates and thereby modify the consistency of fresh concrete [1]. with recycled concrete and plasticizing additives, we meet in the area of high-performance concrete. a relatively large number of studies are focused on the use of recycled concrete aggregates in highperformance concrete, where recycled concrete is mostly a natural stone replacement [2–5]. this is the use of recycled concrete in the form of gravel of different fractions. the use of recycled fine concrete aggregates is not so widespread because it is work with fine ground recycled concrete which is overshadowed by the use of coarse recycled aggregate and brings more problems in designing the mixture in terms of water absorption. this is the reason why much less experts deal with this topic [6–8]. the use of fine ground recycled concrete in cement paste mixtures with a plasticizer and without the use of aggregate is practically not found. 2. materials and methods in the experiment was made the design of cement mixtures with fine ground recycled concrete, plasticizer and water. the aim was to propose basic mixtures with identical or similar workability. this means that the water-cement ratio has varied with each mixture (w/c = 0.23 to 0.38) due to the substitution of 35 wt. % of fine ground recycled concrete (fgrc) as the absorption requirements of fgrc for water is far greater than for cement. the water-cement ratio adjustment to achieve the desired processability of the mixture was also influenced by the added amount of plasticizer (0, 0.5 and 1.0 wt. % of cement). the polycarboxylate-based plasticizer has been used to reduce the water-cement ratio to achieve greater strengths and less pore formation in the matrix, which is also affected by the amount of mixing water. the plasticizer was added to the fresh mixture which was prepared by hand mixing. fine ground recycled concrete was used as a filler with certain bonding properties of unhydrated cement grains in fgrc. as a binder was used the portland cement cem i 42.5 r from radotín (prague, cz) 61 http://dx.doi.org/10.14311/app.2017.13.0061 http://ojs.cvut.cz/ojs/index.php/app jaromír hrůza, zdeněk prošek acta polytechnica ctu proceedings set weight of weight of w/c weight of plasticizer plasticizer cement fgrc water [wt. %,of [g] [g] [-] [g] [g] cement] a (cem) 1000 0 0.30 300 0.00 0.0 b (cem + fgrc) 650 350 0.38 380 0.00 0.0 c (cem + 0.5plast) 1000 0 0.25 250 5.00 0.5 d (cem+fgrc+0.5plast) 650 350 0.35 350 3.25 0.5 e (cem + 1.0plast) 1000 0 0.23 230 10.00 1.0 f (cem+fgrc+1.0plast) 650 350 0.32 320 6.50 1.0 table 1. overview of the mixtures composition at a weight of 1000 g mixture. figure 1. bulk density of the produced samples, standard deviation. was used. fine ground recycled concrete was obtained from road gutters. the road gutters were coarsely crushed to particle-size fraction 0–63 mm. for the intended use as a filler, or partial replacement of cement, the fraction of 0–16 mm was separated by sieving and further was modified by high-speed grinding. the composition of the proposed mixtures and the weight of the individual components for the sample production are shown in table 1. the reference set labeled as a is made only of cement and water. the pair of sets a + b, c + d and e + f differ by the amount of plasticizer contained, and each pair contains a mixture without and with 35 wt. % of fgrc (table 1). the water-cement ratio was calculated from the dry mixture. the prepared mixture was subjected to the spillage test and then placed into molds of dimension 160×40×40 mm. samples, which were made for this experiment, had been stored for 28 days in air at room temperature. the resonance method was used for measuring of the young's modulus of elasticity of the samples. it is a method of nondestructive character, which is together with measurement speed its advantage. the method is based on calculating of the dynamic modulus and dynamic shear modulus from the measured basic resonance frequencies of longitudinal, lateral (flexural) and torsional oscillation. the resonance frequencies were measured in the laboratory using a measuring assembly brüel&kjær, which consisted of an acceleration sensor brüel&kjær type 4519-003, impact hammer brüel&kjær type 8206, measuring station brüel&kjær fron-end 3560b-120 and the control notebook [9]. the flexural strength of cement mortar samples was tested by a destructive method a three-point bend. for bending tensile testing was used apparatus the heckert type fp100 with a maximum possible loading force of 100 kn with a press speed of 6 mm/min. the distance of the support supports of the test beams was 100 mm. the resulting flexural strength of a given set is an arithmetic mean of 3 measurements. compressive strength was determined on fractured samples (halves) after the flexural strength test. the compression strength test load over the previous test was 20 mm/min. mea62 vol. 13/2017 the effect of plasticizer on mechanical properties ... figure 2. the dependence of the dynamic young's modulus on the amount of plasticizer (plasticizer in wt. % of cement) and fine ground recycled concrete in time (longitudinal oscillation), standard deviation. figure 3. the dependence of the dynamic young's modulus on the amount of plasticizer (plasticizer in wt. % of cement) and fine ground recycled concrete in time (flexural oscillation), standard deviation. surement of the compressive strength was performed on five samples from each set. 3. experimental results from the bulk density overview in figure 1, can be observed a clearly visible trend in each pair of samples (a + b, c + d, e + f). we see that samples without finely ground recycled concrete have a higher bulk density than those in which 35 wt. % cement is replaced by recycled concrete. further, we can notice the increase in the values of bulk density with the increasing amount of added plasticizer. from these results, it can be assumed that the plasticizer affects the increase of bulk density. the following graphs (figure 2, figure 3) show the development values of the dynamic modulus of elasticity of each set depending on the age of the concrete (from 1 to 28 days) with 0 and 35 wt. % recycled concrete (fgrc) and various amounts of plasticizer (0, 0.5 and 1.0 % by weight of cement). as first the values for longitudinal oscillation were evaluated for each set (figure 2). to check the accuracy of the results the values for flexural oscillation were also evaluated (figure 3). because the values of both graphs (figure 2 and figure 3) are almost identical, we can say that the measurement was done properly. from the presented results of the development of the dynamic modulus of elasticity (figure 2, figure 3) we can see that the dynamic moduli of elasticity of all mixtures are approximately the same, only the set e exceeds 20 gpa. this also confirmed that 35 wt. % fgrc will not adversely affect the resulting values. an interesting finding is the higher values of set d with the addition of recycled concrete (fgrc) and 0.5 wt. % of the plasticizer compared to the reference set (a). we can only assume that the presence of plasticizer has caused higher values of dynamic young's modulus. this means that the mixture (set d) with 35 wt. % of the cement replacement with fine ground recycled concrete achieves a higher modulus of elasticity due to the reduced water-cement ratio. we can also notice a sharp increase in values after the first day of age for the sets e and f, i.e. the mixtures with 1.0 wt. % of plasticizer. the phenomenon of this kind is only visible for mixtures containing the largest amount of plasticizer. it is likely that the plasticizer caused a robust jump of values of dynamic young's modulus between first and seventh day. if we compare the blends with the added plasticizer, we see a slight improvement in the case of a larger amount of plasti63 jaromír hrůza, zdeněk prošek acta polytechnica ctu proceedings figure 4. flexural strength of 28-day old samples, standard deviation. figure 5. compressive strength of 28-day old samples, standard deviation. cizer in the mixture. when comparing mixtures b, d and f, we conclude that the plasticizer affects the resulting values of dynamic young's modulus relatively little (max. about 1.2 mpa). the flexural strength (figure 4) was improved with samples without recycled concrete (a, c, e) with increasing amounts of plasticizer. on the other hand, we can’t see virtually any difference or increasing trend of samples with fine ground recycled concrete (b, d, f). it has been found that the plasticizer does not affect the flexural strength of the cement pastes with recycled concrete. from the compression strength graph (figure 5), is visible the decrease of compression strength of the samples with the added plasticizer without recycled concrete. thus, we can observe the adverse effect of the plasticizer on the compressive strength of the samples only with cement. if we focus on samples with fine ground recycled concrete and plasticizer, we will see a slight decrease in compressive strength of set d with 0.5 wt. % plasticizer. on the other hand, the set f with the largest amount of plasticizer shows an overall improvement and an increase of strength about 5.1 mpa compared with the samples without plasticizer (set b). 64 vol. 13/2017 the effect of plasticizer on mechanical properties ... 4. conclusions the purpose and objective of the experiment were to design cement mixtures with an acceptable amount of fine ground recycled concrete and plasticizer to determine the effect of the plasticizer on the resulting mechanical and physical properties. mixtures of similar consistency and workability were designed by adjusting the water-cement ratio for each mixture. based on the results of the tests presented in this experiment were drawn the following conclusions: • the bulk densities of mixtures with the fgrc are smaller than the bulk densities of the reference mixtures. • adding the plasticizer increases the bulk density of all mixtures. • the added plasticizer practically does not affect the dynamic modulus of elasticity of samples with fgrc. • improved compressive strength after addition of 1.0 wt. % plasticizer. • the greater amount of plasticizer (1.0 wt. %) affects compressive strengths more favorably than smaller amount of plasticizer (0.5 wt. %). acknowledgements this was financially supported by the czech science foundation research projects gacr 17-06771s and by the czech technical university in prague project sgs16/201/ohk1/3t/11. the authors also thank the center for nanotechnology in civil engineering at the faculty of civil engineering, czech technical university in prague and the joint laboratory of polymer nanofiber technologies of the institute of physics, academy of science of czech republic, and the faculty of civil engineering, czech technical university in prague. references [1] m. zajíček. chemické přísady do betonu a malt. 2008 [cit 2016-11-17] available from: http://www asb-portal cz/stavebnictvi/materialy-a-vyrobky/cihlytvarnice/chemickeprisady-do-betonu-amalt 2008. [2] a. ajdukiewicz, a. kliszczewicz. influence of recycled aggregates on mechanical properties of hs/hpc. cement and concrete composites 24(2):269–279, 2002. [3] t.-y. tu, y.-y. chen, c.-l. hwang. properties of hpc with recycled aggregates. cement and concrete research 36(5):943–950, 2006. [4] b. v. kumar, h. ananthan, k. balaji. experimental studies on utilization of recycled coarse and fine aggregates in high performance concrete mixes. alexandria engineering journal 2017. [5] p. pereira, l. evangelista, j. de brito. the effect of superplasticizers on the mechanical performance of concrete made with fine recycled concrete aggregates. cement and concrete composites 34(9):1044–1052, 2012. [6] e. ledesma, j. jiménez, j. fernández, et al. properties of masonry mortars manufactured with fine recycled concrete aggregates. construction and building materials 71:289–298, 2014. [7] z. zhao, s. remond, d. damidot, w. xu. influence of fine recycled concrete aggregates on the properties of mortars. construction and building materials 81:179–186, 2015. [8] c. neno, j. d. brito, r. veiga. using fine recycled concrete aggregate for mortar production. materials research 17(1):168–177, 2014. [9] j. hrůza. design of masonry elements with use of recycled concrete. prague 2016. 65 acta polytechnica ctu proceedings 13:62–66, 2017 1 introduction 2 materials and methods 3 experimental results 4 conclusions acknowledgements references vol. 3/2016 characterization of ultrasonic assisted adhesive joints rainer pauskaa,∗, umut cakmaka, rainer lottesb, zoltan majora a johannes kepler university, faculty of engineering and natural sciences, institute of polymer product engineering, altenbergerstraße 69, 4040 linz, austria b magna cosma engineering europe ag, r&d department, weikersdorf, austria ∗ corresponding author: rainer.pauska@jku.at abstract. joining experiments using different adhesives were carried out. in addition to the adhesive, the specimens were also treated with ultrasonic waves to improve the load carrying capacity of the joined parts. lap joint shear tests have been conducted to quantify this improvement. keywords: ultrasonic, adhesion, adhesives, epoxy. 1. introduction products are becoming more complex and sophisticated in order to meet consumer needs while the environmental impact has to be reduced. focusing on automotive industry components, the challenge is to exploit lightweight design concepts and provide the expected comfort standards. a consequence of this is the multi-material (hybrid) design approach, even for non-crucial sub-components. a particular important process in the assembly of these parts is the joining. for thermoplastics the joining techniques are divided into mechanical joining, adhesive bonding and welding. joining with adhesives is becoming more popular and proper automated bonding processes without any further time consuming surface preparations are needed [1]. this work presents an advanced ultrasonic assisted joining technique. an external ultrasonic field applied to the bonding area has been shown to improve the adhesive dispersing as well as the curing kinetics [2, 3]. here, we have adopted the ultrasonic-assisted bonding technology and investigated the influence of the process parameter on the load-carrying capacity. for this purpose, we have selected a polyamide (pa) reinforced with glass fibre and optimized the ultrasonic joining parameters (joining force, time and ultrasound wave amplitude) in order to bond two samples with each other. it should be mentioned, that the selected pa shows no significant bonding strength when it is bonded without ultrasonic-treatment. this observation highlights the capability of the bonding technique. the objective of this work is to gain fundamental insights of the relationship between the ultrasonic joining parameters and the final joint’s load-carrying capacity. this relationship is crucial for the understanding of the potential and limitation of ultrasound assistance during bonding. consequently, process parameters can be adjusted in order to achieve desired mechanical behavior of the interface. 2. materials the studied polymers were polyamide 12 reinforced with 50 vol. % short glass fibre (pa12gf50). they were provided as multipurpose specimen according to iso 20753-a1. adhesives based on cyanacrylat, polyurethane and epoxy were tested due to their compatibility with the ultrasonic treatment. 3. ultrasonic-assisted bonding this section provides a brief introduction to an ultrasonic welding machine and introduces the concept used for the optimization and characterization of the ultrasonic assisted adhesive joints. 3.1. ultrasonic-welding device all the specimens were manufactured on an ultrasonic welding device stapla k35 xt (schunk sonosystems, heuchelheim, germany). this device was adopted and modified in order to perform adhesive bonding. figure 1 shows the machine, which was used to treat the prepared specimen with ultrasonic waves. in this figure the main components of the ultrasonic joining machine are shown. the ultrasonic joining parameters are adjustable on the control unit. a brief description of the parameters which were assumed to be most important for this study is included as follows: • amplitude: defines the amount of energy transferred into the joint during the process and is adjustable in % of possible maximum. • joining force: defines the pressure applied on the joining parts during the process and can be adjusted in newton. • joining time: defines how long the ultrasound and the joining force are active. has to be adjusted in seconds. the actual us welding device consists of a converter which transforms the high voltage output of the control unit to a mechanical wave, a booster and a pneu51 r. pauska, u. cakmak, r. lottes, z. major acta polytechnica ctu proceedings matic cylinder. the sonotrode is the actual "tool" of the machine. during processing the sonotrode is moved towards the upper surface of the specimen till it establishes contact with the part. after the preselected joining force is applied the ultrasonic generator is activated and the process starts. figure 2 shows the sonotrode and the clamping system in detail. figure 2. specimen clamping system. one specimen is placed in the clamps and an adhesive layer is applied on its surface. before the start of the joining process another specimen is placed on top of the clamped specimen with the adhesive layer between them. the two parts are now fixed and the joining process can take place. 3.1.1. optimization of joint properties a large number of process as well as environmental factors influences the final bonding quality. figure 3 illustrates the approach to optimize the joint properties. here, we mainly focused on the parameters of the external induced ultrasonic-treatment in order to enhance the interface between the adhesive and the pa12gf50. the ultrasonic joining parameters were assumed to be the most important factor for the optimization of the bonding quality. furthermore, a suitable geometry for the joining process and a proper selected adhesive were also considered for the optimization of the joint properties. figure 3. approach for the optimization of the joint properties. the three factors shown in figure 3 are systematically studied hereafter: geometry joining with an ultrasonic device is hardly possible without an optimization of the part geometry. multipurpose specimens have plane surfaces and hence are hard to process with the ultrasonic joining (welding) technique. in addition, plane surfaces consequently slip during the ultrasonic-treatment. this has to be avoided and an appropriate geometry has to be developed. only the upper part of the multipurpose specimen is used for this investigation, since preparation cost of the part should be as low as possible. the idea is to find a geometry which prevents slip and offers compatibility with the ultrasonic joining process. a good approach to a suitable ultrasonic joining geometry is to mill the surface and leave a chamfer (ideally triangle shaped with a sharp tip) which prevents slipping and can be molten up easier compared to the plain surface during the joining process. figure 4 shows one of four (a-d) options for ultrasonic appropriate geometry which had been tested. as already mentioned the surface was prepared with a milling tool and on the edges a chamfer was left over. the left side of the figure shows the specimen after milling. on the right side a partial cut through the chamfer can be seen. as already mentioned slipping of the parts during the ultrasonic joining process is problematic. the chamfer prevents the slipping and contributes to properly conduct the ultrasonic joining process. selection of a proper adhesive to evaluate which of the studied adhesives (cf. "materials") are suitable for the treatment with ultrasonic waves, optical assessments after ultrasonic-treatment were conducted. for the purpose of studying the influence of ultrasonic-treatment on the adhesive, the polymeric specimens were not bonded. so, the final mechanical properties of the adhesives were not determined and remain unknown. a thin film of each adhesive was applied on the surface of a specimen and treated with ultrasonic waves. the sonotrode established contact with the surface of the adhesive and ultrasound was induced directly into it. a reference sample was also prepared under the same environmental conditions, but without ultrasonic-treatment. after the adhesives were cured, the ultrasonic-treated specimen were compared to the reference sample. the main criteria for this optical comparison (assessment) were: • appearance of the adhesive after curing (colour) • crack formation through adhesive layer • compatibility with material. influence of ultrasonic joining device parameters as already mentioned the joining time, amplitude and joining force were assumed to be the most important parameters for the investigation. the joining force defines the pressure applied by the sonotrode 52 vol. 3/2016 characterization of ultrasonic assisted adhesive joints figure 1. ultrasonic joining device. pdfill p df editor w ith free write r and tools figure 4. geometry option a. onto the surface of the part and using a higher joining force usually results in a greater mechanical strength of the bonding. here, we bonded two specimens of same material and realized a lap shear joining. the displacement resulting from the melting of the specimens in contact is defined by either the force or the time parameters (depending in which mode the process is operated). the specimen created in this step had an adhesive layer between them. the ultrasonic stimulus should also have a positive influence on the interface quality between the adhesive and the pa12 surface. we conducted a pre-study to determine the optimized ultrasonic joining parameter in order to achieve joints with enhanced load-carrying capacity. by focusing on these three main factors during the conducted experimental studies, the process sensitivity regarding changes of these factors is analysed. consequently, the impact of these factors (process parameters) on the final mechanical behaviour of the bonding is determined. 3.2. testing the lap joint shear test was done on a modular test setup (testbench, bose corp., electroforce systems group, mn, us). figure 5 shows the lap joint shear test setup in detail. in this illustration the lap joint shear test setup can be seen with a mounted specimen in the clamp. for all the experiments a testing speed of 5mm/min was used according to astm d1002-01. figure 5. specimens bonded by the adopted ultrasonic joining device. to meet all the prerequisites for the lap joint shear test the lower side of the clamping unit, where the load cell was mounted, was aligned in order to avoid (minimize) bending loading at the bonded area. 4. results 4.1. first ultrasonic joining parameter optimization after the pre-study was done specimen with the found parameters were created. the adhesive best suited for the ultrasonic treatment was a two component epoxy type which was used for all the conducted experiments. a part of the results is shown in figure 6. this figure illustrates some of the results generated from the first ultrasonic welding parameter optimization. the influence different parameter settings have on the load-carrying capacity is shown. different joining forces n were applied while ultrasound amplitude a and joining time t remain unchanged. higher joining force resulted into higher load-carrying capacity. the spikes in the slope of the green line are caused by slip of the clamp during measuring. experiments with optimized geometry have not been conducted in this 53 r. pauska, u. cakmak, r. lottes, z. major acta polytechnica ctu proceedings figure 6. comparison of the load-carrying capacity of specimens bonded with varying ultrasonic parameters. first experimental series. 4.2. second ultrasonic parameter optimization and first geometry experiments in this second experimental series the potential of this joining technique is explored. figure 7 shows a comparison of specimen created with untreated adhesive, adhesive with ultrasonic treatment and different geometry possibilities without the use of an adhesive between the joining parts. the geometry specimen are bonded by the material melting up during the joining process. this was done to explore how much the geometry can contribute to the total load carrying capacity of a combined sample (ultrasonic treated adhesive, geometry). figure 7. normalized load-carrying capacity improvement comparison. the column on the left side represents the specimens which have been joined just by the use of an adhesive. their average load carrying capacity value was used as reference to normalize the values of the other specimen. comparison shows a significant raise in the load-carrying capacity between the three left columns. a more detailed optimization of the ultrasonic joining parameters improved the load-carrying capacity by a large amount compared to untreated and specimen belonging to the first optimization attempt. the first geometry specimen created (a-d) did not provide a load-carrying capacity as high as the optimized samples created with ultrasonic treated adhesive did. results of latest unfinished experiments showed, geometry also has a high potential to increase the load carrying capacity of the joining parts more than what can be seen in figure 7. if parametrization of the ultrasonic joining device is done correctly and the geometry matches the application higher load carrying capacities can be achieved. to further increase the specimen’s load-carrying capacity a combination of ultrasonic assisted adhesion and an application matching geometry joined with the adopted ultrasonic joining parameters will be examined. this combination is assumed to offer even higher load carrying capacity than the specimens tested till now. 5. conclusions based on a systematic parameter study, the influence of three main factors (geometry, adhesive, ultrasonictreatment parameters) on the final bonding quality was examined. moreover, the potential of ultrasonicassisted bonding was highlighted throughout experimental studies. in figure 7 the significant improvement of load-carrying capacity can be seen. comparing the treated specimen of the second optimization with the untreated reference samples load carrying capacity was improved approximately five times. basically, the ultrasonic-treatment parameters have the highest impact on the final bonding quality for 54 vol. 3/2016 characterization of ultrasonic assisted adhesive joints both the geometry and ultrasonic treated adhesive. here, the load-carrying capacity was determined from lap shear measurements and was used as a measure of the bonding quality. as the utilized geometry was not according to the respective lap shear test standard (astm d1002-01), the bond strength was not evaluated. the performed mechanical experiments were only for relative comparisons within the here studied specimens. bond strength has to be determined in future works and compared to data achieved by other bonding technologies. list of symbols a amplitude n joining force t joining time references [1] j. czarnecki, m. hayek-boelingen, h.-j. gudladt, h. schenkel. kontaminationstolerantes kleben aktueller stand der entwicklung. adhäsion kleben & dichten 48(4):36–39, 2004. doi:10.1007/bf03255632. [2] m. hayek-boelingen. boelingen, m. (2004). wege zum kontaminationstoleranten kleben. ph.d. thesis, universität der bundeswehr münchen, neubiberg, germany, 2004. [3] j. holtmannspötte, m. wetzel, j. czarnecki, h.-j. gudladt. how acoustic cavitation can improve adhesion. ultrasonics 52(7):905–911, 2012. doi:10.1016/j.ultras.2012.02.013. 55 acta polytechnica ctu proceedings doi:10.14311/app.2016.3.0035 acta polytechnica ctu proceedings 3:35–38, 2016 © czech technical university in prague, 2016 available online at http://ojs.cvut.cz/ojs/index.php/app identification of damaged zones in c/pps specimens subjected to fatigue loading based on high-resolution thermography petr koudelkaa,∗, daniel kytýřa,b, nela fenclováa,b, martin šperlb a czech technical university in prague, faculty of transportation sciences, konviktská 20, 120 00 prague 1, czech republic b czech academy of sciences, institute of theoretical and applied mechanics, prosecká 76, 190 00, prague 9, czech republic ∗ corresponding author: koudelka@fd.cvut.cz abstract. in this study high resolution thermography is used for identification of damaged zones in carbon fiber/polyphenylene sulfide (c/pps) long fiber composite specimens with induced impact damage subjected to tensile fatigue loading. image processing techniques were applied on thermographs from all loading cases to obtain segmented images of the damaged location that were then used for calculation of the heated area. results show that the considered method can be used to identify heated area in the vicinity of damage with high confidence at low number of cycles where no significant fatigue effect is present in the material. keywords: fatigue, damage, long fiber composite, c/pps, thermography. 1. introduction composites with different matrix materials reinforced by short or long fibers have been widely used as structural materials thanks to their favorable mechanical properties (particularly relative stiffness and strength) for several decades. one of very popular composite compounds widely used in aviation industry consists of polyphenylene sulphide (pps) matrix and carbon fiber reinforcement. the material is commonly used for production of both external and internal aircraft components because of its excellent mechanical properties, chemical resistance to aerospace fluids, low density and good resistance to heat and flames [1]. micrograph of damaged composite structure after material failure is depicted in figure 1. figure 1. detail of damaged structure obtained by scanning electron microscope, the plies are consisted of yarn with individual 5µm fibres. still, application of this material in airframes is limited by its relatively low impact resistance and fragile behavior that have to be thoroughly investigated before its operational deployment [2]. in the life-cycle of an airframe, damage from impacts may be inflicted during flight or ground maintenance. this in combination with unique nature of composites and high cycle fatigue may lead to unexpected failure due to damage accumulation in the material’s microstructure [3]. in this study high-resolution thermography has been used to observe dynamically loaded damaged specimens in order to quantify thermal characteristics of the damaged area and to evaluate suitability of considered approach for ndt inspection/damage tolerance analysis [4–6]. thus, primary goal of this study was to identify lowest possible stress level for reliable assessment of the damage in low number of loading cycles. 2. materials and methods 2.1. specimen description the c/pps material used in this study consists of 8-ply of carbon fabric with volume fraction of fibres approximately 60 %, overall density 1.35 g · cm−3 and quasi-isotropically distributed fibres. surface of the samples was covered by thin glass fibre cloth protecting the core against uv light, surface damage and chemical influences. set of eight dog bone shaped specimens were prepared using numerical controlled water jet cutter from two plates of c/pps material (dimension 160×160 mm) with thickness 2.5±0.1 mm. overall length of specimens was 160 mm. distal part with width 25 mm was used for best possible fixation in mechanical grips whereas central part with length 105 mm and width 20 mm was subjected to the initial loading and thermographically investigated. 35 http://dx.doi.org/10.14311/app.2016.3.0035 http://ojs.cvut.cz/ojs/index.php/app p. koudelka, d. kytýř, n. fenclová, m. šperl acta polytechnica ctu proceedings 2.2. initial damage before dynamic loading the specimens were subjected to impact loading to impose initial damage under controlled conditions using instrumented drop tower. experimental setup with maximal impact energy 50 j was used in configuration with steel spherical impactor having diameter 15 mm. impact energy of 15 j was set to inflict initial damage in the central part of the sample leading to imprint diameter in range of millimeters and depth in range tens of micrometers. the geometrical characteristics of imprints were identified using laser profilometry method [7]. 2.3. experimental setup for the investigation of thermal response to the mechanical loading mikrotron (russenberger prüfmaschinen, germany) resonant testing machine was employed. loading was performed in tensile mode at four stress levels with mean load 4.5, 5.0, 5.5, 6.5, 8.0 kn and amplitude 2.0, 3.0, 4.0, 4.5, 5.0, 6.0 kn in sinusoidal cyclic force. testing frequency was approximately 75 hz. highest stress level did not exceed 35 % of tensile strength of the material and fully elastic behaviour during loading cycles was assumed. one of damage degradation parameter obtained from resonant excitation is cycle dependent decrease of natural frequency. according to our previous study eigenfrequency decrease lower than 0.05 % indicates no damage inflicted in the material’s microstructure. over the whole testing campaign this level was not exceeded as can be seen from graph of natural frequencies plotted against number of cycles (figure 2). −0.02 −0.01 0 0.01 0.02 0.03 0.04 0.05 0 10000 20000 30000 40000 50000 ∆ na tu ra l f re qu en cy [h z] loading cycles 6.5 ± 6.0 kn 8.0 ± 6.0 kn 5.5 ± 5.0 kn 5.0 ± 4.5 kn 4.5 ± 4.0 kn 8.0 ± 4.0 kn 5.0 ± 3.0 kn 8.0 ± 2.0 kn figure 2. variation of natural frequency as a function of number of cycles. the test was performed at constant room temperature 22 ◦c. loading scene was observed using scientific-grade thermal imaging camera sc7650 (flir systems, usa). the device is equipped with insb cooled sensor with pixel pitch 15µm and resolution 640 × 512 px. the device allows acquisition of images in temperature range −20 to 300 ◦c with full frame sampling frequency 100 hz. in this work full frame resolution and 0.5 hz acquisition frequency was used. control of the camera and acquisition of figure 3. experimental setup for thermographical observation of dynamic loading. specimen initial damage 1 1 1 1 figure 4. distribution of rois in thermograph for segmentation and area calculation: 1) background temperature, 2) specimen temperature, 3) heated area. thermographs was performed using gige interface. experimental setup is depicted in figure 3. 2.4. image processing procedure thermographs were subjected to image processing procedures in order to identify the damaged zone and to calculate the heated area in its vicinity. adaptive identification technique based on the median value of the background (blue areas in figure 4) was used for differentiation of the sample and background. median of specimen’s temperature was then found and area with temperature higher than 105 % of the median temperature was subjected to investigation (red area in figure 4). standard morphological opening and closing procedures with circular structuring element were applied on the identified heated areas to improve accuracy of impacted area calculation. in result all holes in the identified area smaller than 3 px2 were morphologically closed and areas smaller than 10 px2 were neglected from the calculation. visualisation of a series of thermographs subjected to investigation using image processing techniques is shown in figure 5. 3. results during the thermographically observed experiments it has been found out that normalized peak temperature (temperature of specimen relative to ambient temperature) converges to constant value in less than 50 000 36 vol. 3/2016 identification of damaged zones in c/pps based on thermography loading cycles. in figure 6 dominant influence of the amplitude compared to mean value of applied load to the maximal temperature is clearly visible. in case of the highest loading level temperature of the specimen reached absolute value of 45 ◦c corresponding to less than 50 % of the material’s glass transition temperature. therefore negative thermal effects influencing the material properties (particularly the assumed elastic behavior) during the test were avoided. figure 5. example of a series of acquired thermographs showing heating of the damaged zone during cyclic loading at 1 s (left), 32 s (middle) and 200 s (right); temperature scale is in ◦c. 1 1.2 1.4 1.6 1.8 2 2.2 0 10000 20000 30000 40000 50000 m a x im u m t e m p e ra tu re t o b a c k ro u n d r a ti o loading cycles 6.5 ± 6.0 kn 8.0 ± 6.0 kn 5.5 ± 5.0 kn 5.0 ± 4.5 kn 4.5 ± 4.0 kn 8.0 ± 4.0 kn 5.0 ± 3.0 kn 8.0 ± 2.0 kn figure 6. ratio of specimen to ambient temperature as a function of number of cycles. threshold temperature higher than 105 % of average specimen temperature (calculated from the yellow area in figure 4) was chosen for identification of the local damaged zone. according to acquired results significant increase of the temperature in damaged zone became observable after only a few hundred of loading cycles. after that peak temperature in the damaged zone rose sharply together with average temperature in the entire volume of the samples. this average temperature reached its maximum equal to temperature after approximately 10 000 loading cycles yielding the thermal footprint of the samples homogeneous. hence higher number of loading cycles was unnecessary for our study. identified damaged zones for all loading cases are depicted in figure 7. it can be seen that minimal loading level 5 ± 4.5 kn was sufficient for reliable identification of the dam 0 20 40 60 80 100 120 140 0 10000 20000 30000 40000 50000 id e n ti fi e d a re a r a ti o 5 x 1 0 − 5 loading cycles 6.5 ± 6.0 kn 8.0 ± 6.0 kn 5.5 ± 5.0 kn 5.0 ± 4.5 kn 4.5 ± 4.0 kn 8.0 ± 4.0 kn 5.0 ± 3.0 kn 8.0 ± 2.0 kn figure 7. variation of identified heated area in the vicinity of damage as a function of number of cycles. aged zone in the vicinity of impact due to its apparent heating. detailed plot of identified heated area in the vicinity of damage and median temperature of specimens for two representative loading cases is shown in figures 8 and 9. here increase of the loading to 6.5 ± 6 kn shows significant increase of the relative heated area (ratio of identified heated area in the vicinity of damage to total area of specimen) of magnitude more than 30 times. this together with the fact that in all cases number of loading cycles lower than 10 000 was sufficient for identification of the influenced zone indicates that the proposed method is suitable for non-destructive inspection of c/pps long-fiber composites. 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 0 10000 20000 30000 40000 50000 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 id e n ti fi e d a re a r a ti o 5 x 1 0 -5 m a x im u m t e m p e ra tu re t o b a c k ro u n d r a ti o loading cycles area temperature figure 8. evolution of identified heated area during mean temperature of specimen during 5 ± 4.5 kn loading. 4. conclusions it can be seen that in majority of loading cases the material exhibits consolidation and hardening in the first 50 000 cycles. for the two cases where combination of mean force and amplitude causes damage accumulation image processing methods have been applied on thermographs to obtain segmented images of the damaged location. from the segmented 37 p. koudelka, d. kytýř, n. fenclová, m. šperl acta polytechnica ctu proceedings 0 20 40 60 80 100 120 140 0 10000 20000 30000 40000 50000 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2 2.1 id e n ti fi e d a re a r a ti o 5 x 1 0 -5 m a x im u m t e m p e ra tu re t o b a c k ro u n d r a ti o loading cycles area temperature figure 9. evolution of identified heated area during mean temperature of specimen during 6.5 ± 6 kn loading. images heated areas were calculated together with specimen/background temperature ratio and plotted against number of loading cycles. it is apparent that heated area in the vicinity of damage can be identified with high confidence at low number of cycles (10 000) where no significant fatigue effect is present in the material. the considered inspection method is thus suitable for ndt inspections or damage tolerance analysis. acknowledgements the research was supported by grant agency of czech technical university in prague (projects no. sgs15/225/ohk2/3t/16, 1051505f014 rpmt 2015 43e), by technology agency of the czech republic (project no. ta03010209) and by rvo: 68378297. references [1] m. favarolo: proceedings of sampe 2010 conference, 2010. [2] p. koudelka, t. fíla, t. doktor, d. kytýř, j. valach, et al.: inspection of post impact fatigue damage in carbon fibre composite using modulus mapping technique, key engineering materials, 606:245-248, 2014. doi:10.4028/www.scientific.net/kem.606.245 [3] r. aoki, j. heyduck, developments in the science and technology of composite materials, 633-642, 1993. [4] j. p. gallagher, usaf damage tolerant design handbook, flight dynamics laboratory, air force wright aeronautical laboratories, 1984. [5] m. naderi, a. kahirdeh, m.m. khonsari: dissipated thermal energy and damage evolution of glass/epoxy using infrared thermography and acoustic emission, composites part b: engineering, 43(3):1613-1620, 2011. doi:10.1016/j.compositesb.2011.12.013 [6] e.z. kordatos, d.g. aggelis, t.e. matikas: monitoring mechanical damage in structural materials using complimentary nde techniques based on thermography and acoustic emission, composites part b: engineering, 43(6):2676-2686, 2012. [7] kytýř, d., fíla, t., šleichrt, j., doktor, t., šperl, m. assessment of post impact damage propagation in carbon-fibre composite under cyclic loading. materials and technology, 48(5):777-780, 2014. 38 http://dx.doi.org/10.4028/www.scientific.net/kem.606.245 http://dx.doi.org/10.1016/j.compositesb.2011.12.013 acta polytechnica ctu proceedings 3:35–38, 2016 1 introduction 2 materials and methods 2.1 specimen description 2.2 initial damage 2.3 experimental setup 2.4 image processing procedure 3 results 4 conclusions acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2016.3.0011 acta polytechnica ctu proceedings 3:11–14, 2016 © czech technical university in prague, 2016 available online at http://ojs.cvut.cz/ojs/index.php/app analysis of welded polypropylene structures: combination of experiments and simulations theresa distlbacher∗, martin reiter, zoltan major johannes kepler university, faculty of engineering and natural sciences, institute of polymer product engineering, altenbergerstraße 69, 4040 linz, austria ∗ corresponding author: theresa.distlbacher@jku.at abstract. the effect of joining by extrusion welding on the tensional stiffness and strength of a polypropylene copolymer was analysed. short-term and creep tests with laboratory specimens were conducted. welded joint sub-components were simulated with the finite element method and the results were validated by experiments. keywords: thermoplastic welding, extrusion-welding, fem, welded structure. 1. introduction extrusion welding is a joining technology suitable for manufacturing of large and complex polymeric structures. filler material is plasticised by a small extruder and the welding partners are pre-heated locally to soften the parent material in the joining area. the molten filler then is deposited in the weld gap between the joining partners. consolidation and solidification of the melt gives the joint its strength. [1] this technology is well established in the industry but there is hardly any information available about the impacts of this process on the material properties and the mechanical behaviour of welded components. the presented work examined conceivable altering of the material in terms of true stress true strain relation. also possible implication to the joint geometry was determined by analysing stiffness and strength of subcomponents based on finite element method (fem) simulations combined with experimental tests. 2. material under examination was a semi-crystalline polypropylene copolymer with enhanced low temperatures impact resistance. mechanical short-term tension tests were performed and elastic-plastic material models for the fem based on the experimental results were developed. 2.1. material testing two types of cylindrically laboratory specimens with a diameter of 8mm and a gauge length of 30mm were prepared. figure 2-a shows drawings of the laboratory specimen types, the reference specimen type (a.1), without any weld and the weld specimen (a.2) containing an extrusion k-weld. short-term uni-axial tensile tests at ambient temperature were performed with a tension rate of 1mm/s. figure 2-b illustrates the longitudinal strain on the surface of the specimen measured with a digital image correlation system during testing. a concentration of strain can be observed at the location of the k-weld. evaluations of strains in longitudinal and transversal direction were done locally in those areas highlighted in figure 2b by a rectangle. in further data processing steps a logarithmic measure for true strain was used. ε(t) = ln(1 + εl(t)) (1) true stress is related to the actual minimum cross section of the specimen and was calculated according to the following equation. σ(t) = f (t) a(t) = f (t) εt(t)2a0 (2) current values for the cross section can be derived by multiplying the initial cross section with the squared current transversal strain. 2.2. material modelling from the true stress-strain relations, the constitutive parameters for an isotropic multi-linear elastic-plastic material model were determined. figure 1. experimental results from tensile tests with parent and weld materials and the resulting material models. 11 http://dx.doi.org/10.14311/app.2016.3.0011 http://ojs.cvut.cz/ojs/index.php/app t. distlbacher, m. reiter, z. major acta polytechnica ctu proceedings figure 2. drawing of laboratory specimens: without weld (a.1), with k-weld (a.2); longitudinal strain measured during testing (b). an illustration of the experimentally determined true stress over true strain and the material model curves for parent material and extrusion k-weld can be seen in figure 1. this graph outlines a reduction in stiffness as well as strength of the welded material compared to the parent material. the elastic-plastic material model is an appropriate representation of the polymeric material’s behaviour, even though the model is primarily applied for a large variety of metals. polymeric materials exhibit not only elastic and plastic but also viscous behaviour ([2],[3]). viscoelasticity is not included in this simple model. therefore, an arbitrary chosen apparent young’s modulus and a yield point were defined for the fe software nx nastran (siemens plm software inc./usa). these apparent values should not be confused with the actual, physical ones resulting from tension tests. table 1 lists the physical and apparent mechanical behaviours of the parent material. physical apparent e [mpa] 1534 1530 σy [mpa] 29.9 15.3 εy [-] 0.04 0.01 table 1. physical and apparent parent material properties. basic models like this are convenient in use and could be capable also for representing some polymeric materials in special cases. in this work it was used because no unloading or dynamic effects were simulated. the application furthermore requires the analysis of time dependent behaviour (relaxation, creep, rate dependency), this simple model is not applicable and advanced models have to be utilized. 3. subcomponent the extrusion welding technology is not only used for butt-joints but more often to manufacture more complex joint-geometries like t-joints. therefore, weld samples were manufactured according to the standard of the german welding society (dvs [4]). a t-joint figure 3. t-joint geometry. geometry as shown in figure 3 was analysed. one meter long weld samples were made by joining a 20mm and a 15mm thick extruded polypropylene copolymer sheet via extrusion welding technology. then, subcomponents with a length of 60mm were cut off these weld samples. 3.1. subcomponent testing the experiments were conducted with the servohydraulic testing system mts 852 damper test system (mts systems corporation, eden prairie, usa). figure 4. subcomponent test setup. 12 vol. 3/2016 analysis of welded polypropylene structures figure 5. fe stress results of the 2-material (left) and the reference model (right). figure 6. fe model of the t-joint. figure 4 depicts the experimental set-up. the subcomponents were locked into position by the fixing frame, which is mounted on the non-moving side of the testing machine. the 15mm thick plate was fixed by a grip connected to the actuator, moving downwards in displacement controlled mode with a speed of 1mm/s. the force was measured with a load cell located above the fixing frame. 3.2. subcomponent fe modelling to simulate the above described experiment a standard 3d structural fem procedure was applied. geometrically modelling of the t-joint was done by 3d cad. preand postprocessing of the simulation were performed in the commercial fem software nx nastran (siemens plm software inc./usa). from figure 6 we can see the meshed model. sheet and weld material were modelled as separate parts and were connected by the surface-to-surface-gluing condition. the plunger was modelled to move with -1mm/s (enforced displacement constraint), while the subcomponent was fixed by a boundary condition locking all figure 7. experimental and fe analysis results. degrees of freedom. the parameters of the material models described in section 2.1 were assigned to the mesh. two different simulation sub-cases were created: • 2-material model: the k-weld material model was assigned to the weld geometries and the parent material model was assigned to the sheets. • reference model: the parent material model was assigned to the sheet and the weld geometries. the sub-cases are identically in terms of mesh, boundary conditions and applied load. also the same solver, the nx nastran’s solution type sol106, a basic nonlinear implicit solver was used. 3.3. results and validation in figure 7 the measured force is plotted against displacement of the plunger for the experiment, the 2-material fe model and the fe reference model. one can see a nearly linear increase in force until a displacement of 1.5mm. the simulation fits the experiment very well in this region. above a force 13 t. distlbacher, m. reiter, z. major acta polytechnica ctu proceedings figure 8. local validation of the subcomponent reference model fe analysis at an actuator displacement of 2mm. of 15 kn the curves level off until they reach another nearly linear region. the experimentally evaluated curve shows a negative slope and the simulated ones still exhibit a slight upwards trend. difference between experiment and simulation in the force-displacement graph might be caused by the ideal representation of the fixing in the simulation. the fe results of the 2-material model and the reference model are nearly equal. evaluation of the stress distribution on the subcomponent’s surface was done to find out possible local effects provoked by the second material model. in figure 5 the stress von mises at a displacement of 2mm of the 2-material model is compared to those of the reference model. elements where the stress has reached the physical yield stress of 29.9mpa are visualized in grey. yield is already reached in those regions where the fixing constraint starts. in general one cannot see a significant differences between 2-material model and reference model in the stress distribution. only a slightly reduced stress in the lower weld toe can be observed for the 2-material model, which is a result of the more flexible weld material. locally the simulation results were validated by comparing the measured major strain at the surface of the subcomponent with the simulated one, see figure 8. the experimentally measured and the simulated maximum strain was found at the same location, where the clamping of the 15mm thick sheet starts. a maximum strain of 0.17 was measured and the simulated strain was 30% smaller. 4. conclusions this work has shown that with validated data a material model for extrusion welded structures can be easily generated, to analyse larger welded structures by fem. the elaborated material models take a nonlinear stress-strain behaviour of parent and weld material into account. a significant change in stiffness and strength of the examined material resulting from the extrusion welding process was found. due to the nature of extrusion welding process imperfections in the weld seam and interface are difficult to avoid. these irregularities and voids are crucial for the structural integrity. considering such effects in fe analysis would allow further structural improvements and reliable lifetime predictions. in future works the focus will be given to fracture mechanical analyses, including the crack propagation in the extrusion weld. list of symbols a cross section a0 initial cross section e young’s modulus f force ε true strain εl longitudinal strain εt transversal strain εy strain at yield σ true stress σy yield stress acknowledgements this work is part of a master thesis carried out at the institute of polymer product engineering (johannes kepler university, linz-austria) in cooperation with the rosenbauer ag (leonding-austria). the author gratefully thanks the institute and the company for initiation of this research topic and for the scientific as well as the financial support. references [1] d. a. grewell, a. benatar, j. b. park. plastics and composites welding handbook. carl hanser verlag, munich, 2003. [2] r. s. lakes. viscoelastic materials. cambridge university press, 2009. [3] y.-h. lin. polymer viscoelasticity: basics, molecular theories, experiments and simulations. world scientific publishing company, new jersey, 2nd edn., 2010. [4] fügen von kunststoffen taschenbuch dvs merkblätter und richtlinien. dvs-verlag, düsseldorf, 13th edn., 2012. 14 acta polytechnica ctu proceedings 3:11–14, 2016 1 introduction 2 material 2.1 material testing 2.2 material modelling 3 subcomponent 3.1 subcomponent testing 3.2 subcomponent fe modelling 3.3 results and validation 4 conclusions list of symbols acknowledgements references 133 acta polytechnica ctu proceedings 2(1): 133–138, 2015 133 doi: 10.14311/app.2015.02.0133 observing cvs and lmxbs with salt: updates and recent results david a. h. buckley1 1southern african large telescope, c/o south african astronomical observatory, po box 9, observatory 7935, cape town, south africa corresponding author: dibnob@salt.ac.za abstract i present an overview of the ongoing observational programs utilizing the southern african large telescope (salt), focussing on magnetic cataclysmic variables and low mass x-ray binaries. salt’s instruments and capabilities are well suited to time resolved studies of the accretion phenomena exhibited in these systems. initial observations, using salticam, have been used to derive high time resolution (∼100ms) eclipse light curves, with high signal-to-noise, of polars. recently this work has been extended to time resolved spectroscopic studies, utilizing the salt robert stobie spectrograph (rss), allowing an opportunity to probe how the emission lines change during eclipse. a program to search for and characterize quasi-periodic oscillations (qpos) in magnetic cvs using the saao 1.9-m telescope, begun in 2012, has been expanded to include observations with a photon counting camera (bvit) on salt. a multi-longitude campaign involving salt, other saao facilities plus the eso ntt, was carried out in march/april 2012 on the enigmatic gamma ray source, xss j12270–4859, which has revealed the that system exhibits spectral line variations, from absorption to emission, seemingly over timescales of < 1 h. keywords: cataclysmic variables polars intermediate polars optical spectroscopy photometry x-rays individual: fl cet, ex hya, hu aqr, xss j12270-4859. 1 introduction the southern african large telescope (salt) is one of five 10-m class segmented mirror telescopes and the only one situated in the southern hemisphere. at the last palermo cv meeting in 2011, i summarized the capabilities and the status at that time (buckley 2012) and presented some early science results. since then salt has entered full time science operations (in november 2011) and has begun to steadily increase its scientific productivity (up to ∼60 refereed publications). there are still some remaining issues to resolve, however, notably implementation of a mirror edge sensing system to allow active control of the primary mirror array. this is currently under development and the first batch of new inductive edge sensors will be installed early in 2014, while the final ones will be installed late in 2015. this will lead to much improved image quality and, more importantly its retention during an observation. similarly there are improvements being made to the throughput of the main work-horse salt science instrument, the robert stobie spectrograph (rss), which should greatly improve its performance, particularly in the blue (320 400nm). this is being achieved by fabrication of new collimator optics and opto-mechanics, designed to avoid the optical fluid contamination problems that have led to the current throughput under-performance. these new optics are expected to be installed by mid-2014. salt is a 100% queue-scheduled telescope, where all observations are undertaken in a service observing mode by salt staff. this modus operandi lends itself to target-of-opportunity observations and synoptic monitoring campaigns, both of which are often appropriate for observations of cataclysmic variables and related objects. for example, to cover specific phase intervals, to coordinate with x-ray or other satellite observations, to monitor a system through an outburst or accretion state change, etc. just such observations have already been carried out with salt and some examples are discussed in this paper. 1.1 high speed photometry with salticam salticam employs frame transfer (ft) ccds, which allows for fast acquisition and imaging. it also has a moving occulting mask which allows for 3 imaging modes: full field imaging, frame transfer mode (half of the field is imaged) and a specialized high-speed mode, 133 http://dx.doi.org/10.14311/app.2015.02.0133 david a. h. buckley where a narrow slot is imaged. the latter, known as ”slot mode”, allows for high-speed (up to ∼12.5hz) photometry and has been used extensively for observing cvs, x-ray binaries and related systems. figure 1: the 11-m southern african large telescope (salt), consisting of 91 × 1.2 m (corner-to-corner) hexagonal mirror segments. this mode allows for just 144 rows of the ccd mosaic to be illuminated, equivalent to ∼20 arcsec × 10 arcmin rectangular region on the sky. the object of interest, plus ideally a nearby (< a few arcmin) constant source (i.e. a comparison star), are placed in the centre of this slot, which can be achieved by rotating the instrument to the desired position angle. the illuminated region of the ccd is then exposed (typically with exposure times ranging from 80 – 1000 ms) and then quickly (within a few ms) row-shifted into the storage array, while another exposure begins. thus exposed slot regions are sequentially moved through the storage array to the serial readout register. during the next exposure, the latest exposed region to reach the bottom of the chip is readout in a time < the exposure time, which is dependent on the on-chip binning factor (usually 4 × 4 pixels). 1.2 time resolved spectroscopy with rss like for all current salt instruments, the robert stobie spectrograph (rss) resides at salt’s prime focus (f/4.2). the advantages of this include maximizing the performance at short wavelengths (< 400 nm), by avoiding use of optical fibres, and ability to conduct fabryperot imaging spectroscopy. rss was designed to have a range of capabilities and observing modes, each one remotely and rapidly reconfigurable. figure 2: the narrow occulting slot that can be moved just in front of the salticam ccds to allow for high time resolution (down to ∼80 ms) photometry. in keeping with the overall philosophy of exploiting those areas where salt has a competitive edge, the instrument has a range of capabilities, many well suited to cv and lmxb studies, including: � wavelength coverage from 320 to 900 nm for the initial uv-vis configuration (a second near ir arm, extending to 1.7 µm, is under construction). low to medium resolution spectroscopy (up to r ∼6000 with 1 arcsec slits) using efficient and tuneable vph gratings. � narrow band, tuneable filter and fabry-perot imaging. � high time resolution (∼0.1 sec) spectroscopic and polarimetric modes. � imaging and spectropolarimetry (linear, circular and all stokes modes), time resolved modes plus low resolution (r ∼50) wide-field imaging spectropolarimetry. high time resolution spectroscopy is achieved using a similar slot-mode option, as for salticam. however, rather than the mask being just above the detector, it is placed at the telescope’s focal plane, coplanar with the spectrograph’s slit. this allows for the spectrum of the variable target to be imaged in a narrow rectangular region and then shuffled into the storage array at the end of the exposure. three 2k × 4k ccds are mosaiced to allow spectra to be obtained over the entire 8 arcmin fov (e.g. for mos). figure 3 shows the arrangement of 134 observing cvs and lmxbs with salt:updates and recent results the slot with respect to the ccd mosaic, where the slot is placed just above the imaging / storage boundary. figure 3: schematic showing the region on the ccd where time resolved rss spectra are recorded using a focal plane mask together with the spectrograph slit. spectra of the target and sky are obtained in the rectangular region, which shuffled down across the frame transfer boundary at the end of a short exposure. 2 time-resolved studies of polars the first high-speed observations conducted with salt were mostly of eclipsing magnetic cataclysmic variable stars (mcvs), specifically polars. these systems emit the bulk of their luminosity (from x-rays to the optical / ir) from small accretion regions near the magnetic pole(s) of a strongly magnetized (∼101 to 102 mg) white dwarf. the dominant emission components in such systems includes hard (> 10 kev) thermal bremsstrahlung x-rays from an accretion shock just above the white dwarf surface. a fraction of this radiation is reprocessed in the white dwarfs photosphere into softer x-rays, uv and optical radiation. in some systems, ballistic ”blobs” of accreting material bury into the white dwarf’s photosphere, thermalize and contribute to the soft x-ray/extreme uv component. shock cooling occurs through polarized optical/ir cyclotron emission. polars therefore present a wealth of multi-wavelength observational opportunities, which are being exploiting with salt. 2.1 light curves of eclipsing polars in eclipsing polars, the size, structure and location of accretion regions can be determined from high time resolution (sub-second) photometry. since 2005 observations have been carried out with salticam to obtain continuum eclipse curves with unprecedented time resolution and signal to noise. an example is for the eclipsing system fl cet, with a 1.45 h orbital period (o’donoghue et al. 2006). an eclipse egress light curve, taken with salticam in ”slotmode” with 112 ms data sampling, is shown in fig. 4. the intensity steps are due to the progressive re-appearance of two accretion hot-spots near the magnetic poles, which take ∼1.5 s to be uncovered. these data have been fitted with a model based on the likely masses of the component stars, as derived from the eclipse parameters and orbital period, plus the orbital inclination of the system (i) and the co-latitudes of the accretion spots (β1 and β2). figure 4: salticam eclipse egress light curve of fl cet, showing the re-appearance of two accretion spots. other polars are also being systematically observed in order to derive similar quality light curves from which to determine the accretion geometries. an example is shown below for the system hy eri, which has been observed in both high and low accretion states. figure 5: eclipse light curves of hy eri, at high (upper curve) and low (lower curve) accretion states. 2.2 spectra of eclipsing polars since 2011 the eclipse light curve program has been extended to spectroscopic investigations of the emission lines of eclipsing polars, using rss in time-resolved spectroscopy mode. the aim of these observations is to probe the mass transfer flow from the secondary star to the white dwarf, specifically where it changes from a 135 david a. h. buckley ballistic to a magnetically controlled trajectory. the geometry of this so-called ”threading region” varies from system to system and is dependent on the accretion rate and magnetic field topology. previous attempts at probing the nature of the threading region (e.g. hakala et al. 2002, bridge et al. 2004) have often been frustrated by the problem of unambiguously determining the accretion stream structures from the 1-d data derived from broad band eclipse light curves. these new salt observations are aimed to address this issue and resolve details of the mass transfer stream through high time resolution emission line spectroscopy. such data will lead to stream eclipse light curves as a function of wavelength and hence radial velocity, enabling the structure of the mass transfer stream to be determined. doppler tomographic techniques will be applied to these data in order to attempt to map the positions of the line emitting regions. recently a new ”inside-out” doppler tomography technique has been devised (e.g. see kotze & potter; these proceedings). eventually we plan to undertake phase resolved spectropolarimetric observations of these systems, using rss (when the polarimetric modes are re-activated in 2014). these observations will probe the phase dependent cyclotron emission, giving us an independent view of the polarized emission arising from the accretion spots. during low states it may also be possible to study the polarization of the zeeman lines from the white dwarf and the accretion halo. figure 6: an example of time resolved (10s) trailed spectroscopy of the eclipsing polar hu aqr. 2.3 qpos in magnetic cvs in 2012 a program to study quasi-periodic oscillations (qpos) in magnetic cvs was instigated at saao, mostly using the new electron multiplication ccd (emccd) cameras (shoc) on the saao 1.9-m telescope. the aim is to search for and characterise the nature of the optical qpos, which have typical periods of a few seconds. the commonly held understanding is that they are due to plasma oscillations in the magnetically confined accretion columns which constrain the accreted plasma before it cools and settles onto the surface of the magnetic white dwarf. the qpo frequency is thought to be related to the physical parameters inside the column (or sub-columns), particularly the shock height, accretion rate and magnetic field strength. this work will hopefully lead to a better understanding of the physics underlying the qpos, taking the study from being purely phenomenological to one based on quantitative physical parameters. in conjunction with this observational program, we expect that the experimental results from a parallel program of simulating accreting columns in the laboratory will yield further relevant insights (e.g. michaut et al. 2012). the project involves new technology detectors, namely emccds, which have both good qe and fast timing capability (e.g. ∼10 ms). these detectors are superior to those used in the first studies of polar qpos (e.g. larsson 1989, and references therein), which relied on pmt (photomultiplier tube)-based instruments. high time resolution photometry of mcvs have recently been carried out at salt, utilizing the newly installed (in 2012) berkeley visible image tube (bvit), a photon counting instrument using a microchannel plate (mcp) detectors (buckley et al., 2010, welsh et al. 2011). while not of particularly high qe, compared to a ccd, bvit is capable of time tagging photons to ∼50 ns, and is ideal for studying extremely fast variability (e.g. pulsars). short (typically ∼1 h) bvit observations have been made of several mcvs, but the only positive detection to date has been for the intermediate polar, ex hya, where a ∼100 s qpo (and possibly its harmonic), were detected (fig. 7). figure 7: a salt observations of ex hya using bvit which revealed a ∼100/50 s qpo. 136 observing cvs and lmxbs with salt:updates and recent results 3 salt spectroscopy of xss j12270-4859 figure 8: salt spectra of xss j12270-4859, averaged over ∼1 h on 4 different nights. the enigmatic fermi gamma ray source, xss j122704859 (e.g. bonnet-bidaud et al. 2012, and references therein), was the subject of a saao-eso observing campaign in march/april 2012, utilizing the ntt, salt and several smaller saao telescopes. spectroscopy was carried out with salt over 6 nights, contemporaneous with ntt spectroscopy. in addition, high speed u-band photometry and jhk imaging was also undertaken. xss j12270-4859 was first suggested to be an intermediate polar (butters et al. 2008), but this was not confirmed from follow-up suzaku and xmm observations. the system exhibits x-ray flares and dips, somewhat reminiscent of exo0748-676, but without any phase dependency and no spectral changes. the x-ray spectrum is also characterized by a weakly absorbed power law, with no cut-off up to 100 kev and no fe lines. the system has been suggested to be similar to a microquasar (e.g. cyg x-3, but it is not as variable in x-rays) or possibly a millisecond pulsar (e.g. psr 1023+0038), but no spin period has been detected to date. the salt spectra were obtained during ∼1 h tracks, with 300 s repeat exposures. they show significant changes, often over the timescale of an exposure, sometimes exhibiting weak emission lines of the balmer series, hei and heii, and other time showing balmer and other absorption features, as can be seen in fig. 8. radial velocity variations of the combined spectra indicate a possible ∼7 h orbital period, which is under investigation. the high speed photometry also shows variations on a similar timescale, but with periods of enhanced flickering possibly occurring at the same orbital phase. 4 future plans in 2014 it is planned to extend the salt observational work on cvs and related objects to include time resolved polarimetric imaging and spectropolarimetry. polarized continuum flux variations will be used to probe the cyclotron emission, for example using the stokes imaging technique (see potter et al. 2004). this will be combined with doppler and roche tomography and eclipse mapping to assist in building detailed models of mcvs, and may include using relevant particle or mhd codes. following the launch of the indian x-ray satellite, astrosat (in 2014), it is planned to begin coordinated observations using salt to undertake contemporaneous optical observations of mcvs, lmxbs and related objects. astrosat will have both, hard and soft x-ray instruments, the former with excellent sensitivity and energy range, plus a uv imaging telescope. adding such data will allow for a comprehensive multiwavelength analysis of these systems. 137 david a. h. buckley acknowledgement the work summarized in this paper represents a number of collaborative salt projects with various colleagues, including jean marc bonnet-bidaud, hannes breytenbach, domitilla de martino, marissa kotze, darragh o’donoghue and stephen potter, to name a few. the research of the author is supported by nrf rated researcher and incentive funding awards. the observations presented were carried out using the southern african large telescope. references [1] bonnet-bidaud, j.m., et al.: 2012, mem. s.a. it., 83, 742. [2] bridge, c.m., et al., 2004, mnras, 351, 1423. doi:10.1111/j.1365-2966.2004.07884.x [3] buckley, d.a.h., swart, g.p & meiring, j.g., 2006, proc. spie, 6267, 62670z. doi:10.1117/12.673750 [4] buckley, d.a.h., et al., 2010, poprc. spie, 7735, 77359-1. [5] butters, o.w., et al., 2008, a&a, 487, 271. [6] kotze, e. & potter, s.b., 2013, these proceedings. [7] larsson, s., 1989, a&a, 217, 146. [8] michaud, c., et al., 2012, mem. s.a. it., vol. 83, 665. [9] o’donoghue, d., et al., 2006, mnras, 372, 151. doi:10.1111/j.1365-2966.2006.10834.x [10] potter, s.b., et al., 2004, mnras, 348, 316. doi:10.1111/j.1365-2966.2004.07379.x [11] welsh, b., et al., 2011, iau symp. 285, 99. discussion paula szkody: can you give more information on the qpo in ex hya? is it seen in the lines or continuum? david buckley: the qpo was detected using salt and bvit with a broad-band r-filter, so includes both continuum and hα. the characteristic period was ∼100 s, or more likely, its harmonic. 138 http://dx.doi.org/10.1111/j.1365-2966.2004.07884.x http://dx.doi.org/10.1117/12.673750 http://dx.doi.org/10.1111/j.1365-2966.2006.10834.x http://dx.doi.org/10.1111/j.1365-2966.2004.07379.x introduction high speed photometry with salticam time resolved spectroscopy with rss time-resolved studies of polars light curves of eclipsing polars spectra of eclipsing polars qpos in magnetic cvs salt spectroscopy of xss j12270-4859 future plans acta polytechnica ctu proceedings doi:10.14311/app.2016.3.0056 acta polytechnica ctu proceedings 3:56–59, 2016 © czech technical university in prague, 2016 available online at http://ojs.cvut.cz/ojs/index.php/app determination of thermal response of carrara and sneznikovsky marble used as building material veronika petráňováa,∗, jaroslav valacha, alberto vianib, marta peréz estébanezb a institute of theoretical and applied mechanics as cr, v.v.i., prosecka 76, prague, czech republic b institute of theoretical and applied mechanics as cr, v.v.i., centre of excellence teläŋ, batelovska 485, teläŋ, czech republic ∗ corresponding author: petranova@itam.cas.cz abstract. physical weathering of marble, widely used as a cladding material on buildings, is one of the most common damaging mechanism caused by anisotropic thermal expansion of calcite grains. the extent of marble deterioration depends mainly on stone fabric and texture. dry cuboids of carrara marble and marble from dolni morava quarry were subjected to microscopic analysis and thermal cycling, to determine the thermal expansion related to stone fabric and predominant lattice orientation of grains (i.e. texture). keywords: marble, anisotropic thermal expansion, thermal dilatation. 1. introduction marble has been predetermined as building and decorative natural stone since antiquity because of its good workability and unique visual appearance. these properties are related to the composition of carbonate sediments and to the nature and intensity of the recrystallization process under the regional metamorphosis conditions. marble is considerably vulnerable to both chemical and physical weathering by reason that this type of stone predominantly consists of carbonate minerals calcite caco3 and dolomite camg(co3)2. periodic temperature variations, together with changing in moisture can cause serious damage to the stone structure, finally leading to the complete stone decay. the enormous sensitivity of marble to variations in temperature originates in the thermal response of calcite crystals. while the dolomite crystals subjected to thermal cycling expand only, the calcite crystals show anisotropic thermal dilatation introduced by thermal cycles, i.e., it expands along c axis during heating and contracts in other directions. however, cooling of calcite crystals leads to the opposite effect, i.e., contraction along the c axis and expansion in the other directions as is illustrated in figure 1. both mechanisms are considered as the major cause of creation and propagation of cracks along the grain boundaries in the stone. thermal expansion can be expressed using the thermal expansion coefficient α, describing the length change per unit of temperature. the coefficient value depends on the considered temperature interval. to quantify the extent of stone elongation, the residual strain ε is more suitable [1]. the preferred lattice orientation of calcite grains i.e. texture is one of the most important stone characteristics, essentially influencing thermal response in different spatial directions. marble with strong texture is generally more vulnerable to thermal weathering, which can be enhanced by significant differences between the α values along the cand a-axis of individual calcite grains [2]. thermal expansion of marble is measurable at low temperatures enabling simulation the climate conditions in the central europe. nevertheless, experimentally obtained thermal dilatation values often deviated from calculation based on theoretical models considering only the texture variations in a marble. another important parameters are grain shapes, grain sizes and the type of grain boundaries because of the strong influence on the rate of stone decay. a certain directional relationship between the shape preferred orientation (spo) of calcite grains and their lattice preferred orientation (lpo) can extensively intensify the thermal response of marble. for example, marble with lpo parallel to spo exhibits a strong thermomechanical response because of the large concentration of maximum principal stress in the direction of shape elongation. thermal stresses during heating can initiate the formation of new cracks or widening of preexisting cracks or pores. microcracking formation can also be influenced by grain size since the level of marble decay is also connected with the number of adjacent grains and with the grain boundaries [3–5]. however, thermal response and consequently the rate of physical deterioration of marble depends also on other stone characteristics, including the ratio of calcite and dolomite, moisture content, porosity and pore size distribution of the investigated marble [6]. depending on all the mentioned stone characteristics, the thermal response of marble after cooling down to room temperature can be determined as isotropic or anisotropic, with or without residual strain [1]. 56 http://dx.doi.org/10.14311/app.2016.3.0056 http://ojs.cvut.cz/ojs/index.php/app vol. 3/2016 thermal response of carrara and sneznikovsky marbles figure 1. anisotropic thermal expansion of calcite crystals. adopted from [1]. 2. materials and methods testing of the thermal response was performed on two structurally different marble types. the first one from the carrara area in northern italy was chosen for its simple fabric. the second one sneznikovsky marble was taken from dolni morava quarry in the czech republic. the texture as well as the shape and size of grains was investigated by optical microscopy and electron back-scattered diffraction (ebsd) method in a scanning electron microscope (sem, fei quanta 450, fei corp., usa). for the sem-ebsd investigations, slide specimens of marble were coated with a thin layer of carbon to protect against specimen charging. to evaluate the dependence of dilatation on the sample’s microstructure, two pairs of perpendicular cuboids a-a, b-a, c-1 and c-4 (6 x 6 x 20 mm) were prepared from the original marble blocks. the orientation of the cuboid samples is depicted in figure 2. thermal expansion measurements were performed using a vertical dilatometer (l75 pt linseis corp., germany). the specimen elongation was measured after each 1 °c step. the cuboid samples were subjected to heating and cooling cycles, ranging in temperature from -40 to 100 °c. the cycles were repeated 7 times for specimens a-a, b-a and c-1 and 5 times for specimen c-4. the temperature interval was gradually increased for the a-a, b-a and c-1 specimens. the interval for specimen c-4 remained stable in all 5 cycles (from -40 to 100 °c). the values for the thermal expansion coefficient α values were calculated using equation (1). the length change in 4l was determined within a temperature interval 4t , from 18 °c to the maximum temperature in the considered cycle, i.e. for cycle 1, the maximum temperature was 33 °c, whereas for cycle 7, it was 100 °c. to determine α, only a positive temperature range was selected, because the results from the range below the freezing point were not reliable. α = 4l l4 t (1) figure 2. orientation of cuboid specimens, a-a and b-a is sneznikovsky marble and c-1 and c-4 is carrara marble. the symbols and their physical meanings are listed at the end of the paper. 3. experimental results based on optical and sem-ebsd observations, the carrara marble exhibits a weak texture in both directions (x-y and x-z). specimens c-1 and c-4 contain calcite grains with a nearly uniform size of around 0.3 mm with straight polygonal grain boundaries and no shape preferred orientation. contrary to carrara, sneznikovsky marble exhibits cracking, predominantly on planes parallel to the x-y plane. the microfabric of 57 v. petráňová, j. valach, a. viani, m. peréz estébanez acta polytechnica ctu proceedings figure 3. a microfabric of investigated samples, b orientation of calcite grains in b-a sample. figure 4. dependence of relative extension on the temperature. the investigated marble types is illustrated in figure 3. the size of the calcite grains in the a-a and b-a specimens is not uniform; grains in sizes up to 4 mm with sutured grain boundaries can be found. as in the carrara specimen, both perpendicular planes of sneznikovsky marble contain grains with no shape preferred orientation. the elongation of the cuboid specimens was expressed as relative extension, i.e. percentage difference between 4t for each temperature step and original length of the specimen. the highest relative extension of 0.18% was observed in c-4 at 100 °c in the first cycle. in the other cycles (2-5) the value was equal to 0.16%. specimen c-1 showed an increasing trend over the temperature and reached its maximum relative extension of 0.14% at 100 °c. regarding the coefficient α, specimen c-4 also showed the highest average value of 12.5 · 10−6k−1. specimen c-1 achieved the most significant difference in α, varying from 3.9 · 10−6k−1 to 11.3 · 10−6k−1. the results are illustrated in the figures 4 and 5. the first cycle of the measurements performed on the specimen c-1 was excluded from the results. values obtained from the first cycle performed in c-1 specimen weren’t included to the results. during the 7 cycles in specimens a-a and b-a relative elongation also showed an increasing trend, but the maximum values were more different than in the carrara marble. at 100 °c in cycle 7, the relative figure 5. thermal expansion coefficient α of investigated specimens, loaded in the 18-100 °c temperature interval. the value marked "a" represents an average value of α determined by [7] on large set of marble specimens. extension in specimen a-a was equal to 0.15% while in b-a it was just 0.10%. the α coefficients determined for the sneznikovsky marble were not as variable as in the carrara type. in the a-a sample, α reached an average value of 10 · 10−6k−1 and in the b-a sample of 5 · 10−6k−1. based on the relative extension difference between corresponding cycles, an α variation of 5 · 10−6k−1 was determined, which can be attributed to the observed anisotropy in the sneznikovsky mar58 vol. 3/2016 thermal response of carrara and sneznikovsky marbles ble, i.e. the x-y orientated plane in specimen a-a was more fissured. 4. conclusions it can be concluded that simultaneous microscopic and dilatometry investigations can help in elucidation of thermally induced degradation of marbles. based on the results, the thermal response and generation of damage caused by accumulation of residual strains related to stone fabric and texture in investigated marbles will be determined on a larger set of perpendicular specimens, heated and cooled in the positive range of temperatures (i.e. from 20 to 100 °c). investigations carried out on both specimen types reveal an increasing rate of thermal expansion with increasing temperatures. in presence of a temperature gradient, this characteristic can contribute to the well known bowing effect on marble slabs. list of symbols α thermal expansion coefficient [ k−1] 4l difference between the actual specimen length and the length at the room temperature [m] l length of the specimen at the room temperature [m] 4t temperature difference between the actual and the room temperature [k] acknowledgements the research has been supported by czech science foundation (project no. p105/12/g059). references [1] m. steiger, a. charola. weathering and deterioration. in the stone in architecture, chap. 4., springer-verlag, 2011. doi:10.1007/978-3-642-14475-2_4 [2] s. siegesmund, k. ullemeyer, t. weiss, e.k. tschegg. physical weathering of marbles caused by anisotropic thermal expansion. inter j earth sci 89(1):170-182, 2000. [3] v. shushakova, e.r. fuller jr, s. siegesmund. influence of shape fabric and crystal texture on marble degradation phenomena:simulations. env earth sci 63(7-8):1587-1601, 2011. doi:10.1007/s12665-010-0744-7 [4] b. leiss, t. weiss. fabric anisotropy and its influence on physical weathering of different tyes of carrara marbles. j struct geol 22(11-12):1737-1745, 2000. [5] u. akesson, j.e. lindqvist, b. schouenborg, b. grelk. relationship between microstructure and bowing properties of calcite marble claddings. bull eng geol env 65(1):73-79, 2006. [6] e. cantisani, e. pecchioni, f. fratini, c.a. garzonio, p. malesani, g. molli. thermal stress in the apuan marbles: relationship between microstructure and petrophysical properties characteristics. inter j rock mech min sci 46(1):128-137, 2009. doi:10.1016/j.ijrmms.2008.06.005 [7] a. zeisig, s. siegesmund, t. weiss. thermal expansion and its control on the durability of marbles. in the natural stone, weathering phenomena, conservation strategies and case studies, chap. 2., the geological society of london, 2002. 59 http://dx.doi.org/10.1007/978-3-642-14475-2_4 http://dx.doi.org/10.1007/s12665-010-0744-7 http://dx.doi.org/10.1016/j.ijrmms.2008.06.005 acta polytechnica ctu proceedings 3:56–59, 2016 1 introduction 2 materials and methods 3 experimental results 4 conclusions list of symbols acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2017.13.0016 acta polytechnica ctu proceedings 13:16–19, 2017 © czech technical university in prague, 2017 available online at http://ojs.cvut.cz/ojs/index.php/app parallel approach to solve of the direct solution of large sparse systems of linear equations michal bošanský∗, bořek patzák czech technical university in prague, faculty of civil engineering, thákurova 7, 166 29 prague 6, czech republic ∗ corresponding author: michal.bosansky@fsv.cvut.cz abstract. the paper deals with parallel approach for the numerical solution of large, sparse, non-symmetric systems of linear equations, that can be part of any finite element software. in this contribution, the differences between the sequential and parallel solution are highlighted and the approach to efficiently interface with distributed memory version of superlu solver is described. keywords: linear equation solver, domain, parallelization, threads, distributed memory, communication. 1. introduction many engineering problems lead to large-scale problems. the solution of these problems is being limited by available resources. in many cases, the parallel and distributed computing paradigms can be successfully used to enable solution of large and complex problems, which are not possible to solve on single machine with one processing unit [1]. the traditional serial computers permit to run simulation codes only sequentially and often have limited memory. parallelization can significantly reduce computational time by more efficient use of modern hardware. any parallel algorithm is based on idea of the partitioning the work into the set of smaller task which can be solved concurrently by the simultaneous use of multiple computing resources. parallel computers can be classified, for example, by type of memory architecture. the shared, distributed, and hybrid memory systems [2] exist. in the shared memory system, the main memory with a global address space is shared between all processing units which can directly address and access the same logical memory. the global memory significantly facilitates the design of parallel program, but the memory bus performance is limiting factor for scalability with increasing number of processing units. in distributed memory systems, the memory is physically distributed to individual processing units and there is no global address space. when a processor needs to access data on another processor, it is usually the task of the programmer to explicitly define how and when data is communicated. the cost of communication compared to local memory access can be very high, on the other hand, the advantage is that the overall memory is scalable with increasing number of processors. hybrid systems combine the features of shared and distributed memory system, providing global, shared memory for reasonably small number of processing units that are combined into the distributed memory system. the design of parallel algorithms requires the partitioning of the problem into a set of tasks, that can be processed concurrently. the partitioning of the problem can be either fixed (static load balancing) or can change during the solution (dynamic load balancing) [3]. the latter option is often necessary to achieve a good load balancing of individual processors and consequently also reasonable scalability. the scalability of computation is often regarded as one of the most important goals of parallel computing. the parallel algorithm is considered as scalable, when by using increasing number of processing threads, the execution time is monotonically decreasing, ideally in a linear trend. however, when individual computing threads are mutually dependent, the ideal scalability is difficult to obtain due to the overhead of parallel algorithm (necessary synchronization and communication between threads that is not present in serial code) and due to the fact that some parts of the overall algorithm are essentially serial. in addition to the achieved speedup, the parallel computing allows to solve large, complex problems that do not fit into a single, even well equipped machine. the superlu library provides parallel implementation of the direct solver [4]. it provides serial, multithreaded (shared-memory), and distributed memory version. the implementation and performance of the multithreaded version has been reported by the authors in [5, 6]. in this papers, the focus is on distributed version of superlu based on message passing interface (mpi) [7]. numerical modeling using high performance computers brings in new opportunities. many engineering problems lead to solution of sparse linear system of equations. this is also the case of the finite element method (fem) which is used here as an representative example. in fem the differential equations are converted to the algebraic system of equations by using variational methods with the help of decomposition of the problem domain into sub-domains called elements 16 http://dx.doi.org/10.14311/app.2017.13.0016 http://ojs.cvut.cz/ojs/index.php/app vol. 13/2017 direct solution of large sparse systems of linear equations 68 0 2 11 00 0 3 5 09 2 6 0 04 0 0 13 17 0 0 3 k = k_nzval 8 6 2 11 3 5 9 2 6 4 13 7 1 3 k_colind 1 2 4 5 4 5 1 3 4 1 5 1 2 3 k_row_ptr 1 5 7 10 12 21 3 4 5 1 2 3 4 5 figure 1. the compressed row format of matrix schema. and smart choice of interpolation functions. for linear elasticity problem, considered here, the resulting algebraic equations, represent the discrete equilibrium equations at nodes of the computational mesh k ∗ r = f, (1) where k is the global stiffness matrix, f is global vector of external forces, and r is vector of unknown displacements. one of the key feature of the fem is that the stiffness matrix is typically positive definite, symmetric matrix with sparse structure, which is a consequence of using interpolation and test functions with limited nonzero support. the efficient algorithm for solving linear system must exploit the symmetry and sparse structure, by saving considerable memory and cpu resources. there exist number of different storage schemes for sparse matrices. the most widely used are skyline, compressed row, and compress column formats. in this contribution, the compressed row format will be used. this choice comes from superlu library, which requires the sparse matrix in this format on input. in the compressed row format, only nonzero entries of the sparse matrix are stored in one dimensional array. additional integer array is needed to store column indices of the stored values. in practical implementation, the one dimensional arrays storing nonzero values are merged together into a single array containing all nonzero entries, row by row. the same applies for arrays of column indices. an additional array with size equal to number of rows is needed to point to the beginning of each row record, see fig. 1. 2. implementation superlu interface as noted already in the previous section, the distributed memory programming model does not provide global address space and global memory accessible by all processing nodes, but the memory is distributed among the processing units. for large problems, it is therefore essential to establish a distributed representation of the sparse matrix. this feature is also supported by superlu library, allowing the user to split the global sparse matrix stored in comprow format into blocks, representing compressed row storage for consequent, non-overlapping groups of rows, which are distributed across computing nodes. we assume, that the decomposition of the discretized problem domain has been established and individual, non-overlapping sub domains (partitions) are assigned to and stored locally on individual computing nodes. the so called node-cut strategy is assumed, where the cut dividing the problem domain runs through the nodes. nodes on mutual partition boundaries are called shared nodes, the nodes inside individual partitions are so-called local nodes. each processing node is responsible for assembling its contribution to the global system matrix by summing up the contributions from individual elements. to minimize the communication, it is natural that each processing node will assemble and maintain in its local memory the block of global stiffness matrix rows corresponding to unknowns on its partition. this is uniquely defined for local nodes, which are exclusively shared by local elements on individual partitions. for shared nodes, which are shared by elements from multiple partitions, the ownership has to be defined. in this work, the partition with lowest rank sharing the shared node is the owner of shared node and thus responsible of maintaining the corresponding row entry of the global stiffness matrix. it is clear that for shared nodes, the contributions to the corresponding row have to be received from partitions sharing particular node. the assembly process requires to establish global, unique numbering of equations across computing nodes in addition to the local numbering on individual partitions. the assembly process of the global, distributed system matrix requires basically two steps: set up of data structures required to store the block of compressed row records on individual partitions and the assembly process itself based on localization of individual element contributions into global equilibrium equations according to their connectivity. 2.1. initialization of data structures for distributed matrix this phase should determine the required size of arrays used to store all nonzero entries of the global matrix. the nonzero contributions of each element can be identified from element code numbers. the code numbers represent simultaneously the numbering of equilibrium equations and numbering of corresponding unknowns. the individual values in the stiffness matrix represent nodal forces caused by corresponding unit displacement. the entry in element stiffness matrix representing the nodal force contributing to k-th equilibrium equation and being multiplied by displacement located in global displacement vector at l-th position, should be added to position (k,l) in the global matrix. by assuming that element contributions are full matrices, one can initialize the nonzero structure of the global stiffness matrix using element 17 michal bošanský, bořek patzák acta polytechnica ctu proceedings 1 (0 ,1 ) 1 2 (2 ,3 ) 5 (8 ,9 ) 3 (4 ,5 ) 4 (6 ,7 ) 6 (1 0, 11 ) 7 (1 2, 13 ) 8 (1 4, 15 ) 0 2 3 4 5 6 7 8 9 10 11 12 13 14 15 1 0 2 3 4 5 6 7 8 9 10 11 12 13 14 15 g # 1 #2 g g # 3 2 5 3 4 6 7 8 # 1 # 2 # 3 4 6 42 # 2 6 k# 1 k# 2 k# 3 -n od e -s ha re d no de # 3 # 3 # 3 # 2 # 2 4 1 f1 f2 10 2 3 4 5 6 7 1 0 2 3 4 5 6 7 32 8 9 6 7 10 11 3 2 8 9 6 7 10 11 76 10 11 12 13 14 15 7 6 10 11 12 13 14 15 { { p1 p2 p3 figure 2. example of simple problem consisting of three elements distributed into different computing nodes, illustrating the global sparse matrix structure and its distribution. 18 vol. 13/2017 direct solution of large sparse systems of linear equations code numbers. in distributed case, each partition is responsible for initializing and prealocating its block structure of compressed rows records. in case of the compressed row storage, the number of nonzero entries in each row has to be determined together with corresponding nonzero column positions of the individual entries. each processing node is responsible for initialization and assembly of the assigned block of rows. this block is composed from local contributions, as well as from shared node contributions from neighboring partitions. therefore, the overall initialization can be divided into (i) local stage, where the local contributions to nonzero pattern are identified (from the partition elements), and (ii) communication stage, where contributions from neighboring partitions are received for locally maintained rows, corresponding to equilibrium equations for shared nodes. any partition has to therefore keep its locally maintained block of compressed rows and also block compressed rows corresponding to shared nodes not being locally maintained. these two blocks can be stored separately. in the initial stage, the block structure is initialized on every partition using only the contributions from local elements. the row entries, that correspond to the shared nodes which are not locally maintained are then communicated (send) to the corresponding partition maintaining corresponding row. after the sending the data, the contributions from neighboring partitions are received containing the contributions to the locally maintained shared equations. non-blocking communication [8] is used to send/receive the contributions. non-blocking communication allows to utilize the potential overlap of communication and computation, which is utilized in the actual implementation. after finishing the communication stage, each partition has fully initialized data structure for locally maintained block of rows. 2.2. assembly of the global distributed matrix after finishing the initialization step, in which memory has been allocated for every non zero entry of the global distributed stiffness matrix, one can proceed with the actual assembly of the matrix from the element contributions. this is done in a similar fashion to the previous step. on each partition the contributions from local elements are assembled. then, the rows corresponding to shared nodes not being locally maintained have to be sent to the partition maintaining corresponding row. finally, each partition has to receive remote contribution to the locally maintained rows of shared nodes. this process is illustrated on fig. 2. in this example, the problem is composed of three elements (corresponding to sub domains as well) and eight nodes, from which five are local on individual partitions and three nodes are shared. 2.3. assembly of the global right hand side the process of assembling the global right hand side vector is very similar to the process of assembling the stiffness matrix, but in many aspects is simpler, do to the assembly of local vector contributions. the global vector is again distributed among partitions, the distribution corresponds to the distribution of matrix rows. the contributions from local elements are assembled first, then those corresponding to rows maintained on remote partitions are communicated and contributions from neighbors are received and entries updated. by finishing the steps described above, the distributed sparse matrix, represented by locally maintained blocks of row entries on individual partitions, can be directly passed into superlu routines. 3. conclusions in this work the algorithm of assembling the distributed sparse matrix in row-column format is described, in order to develop the interface to the distributed superlu solver. the evaluation of the performance and scalability is the subject of ongoing work. the authors also aim to compare the efficiency of the distributed solver to the efficiency of the multithreaded superlu version. acknowledgements this work was supported by the grant agency of the czech technical university in prague, grant no. sgs16/038/ohk1/1t/11 – advanced algorithms for numerical modeling in mechanics of structures and materials. references [1] b. patzak. parallel computations in structural mechanics. české vysoké učení technické v praze, 2010. [2] c. hufhes, t. hughes. parallel and distributed programming using c++. pearson education, 2003. [3] b. patzak, d. rypl. parallel adaptive finite element computations with dynamic load balancing. proceedings of the first international conference on parallel, distributed and grid computing for engineering 2009. [4] x. s. li, j. w. demmel, j. r. gilbert, et al. superlu users‘ guide program interface. http://crd.lbl.gov/ xiaoye/superlu/, september 1999. [5] m. bosansky, b. patzak. evaluation of different approaches to solution of the direct solution of large, sparse systems of linear equations. advanced materials research 1144(1144):97–101, 2016. [6] m. bosansky, b. patzak. on parallelization of linear system equation solver in finite element software. engineering mechanics 2017 1(1):206–209, 2017. [7] p. s. pacheco. a user‘s guide to mpi. univ. of san francisco, 1997. [8] t. saif, m. parashar. unferstanding the behavior and performance of non-blocking communications in mpi. department of electrical and computer engineering, rutgers university, 2002. 19 acta polytechnica ctu proceedings 13:16–19, 2017 1 introduction 2 implementation superlu interface 2.1 initialization of data structures for distributed matrix 2.2 assembly of the global distributed matrix 2.3 assembly of the global right hand side 3 conclusions acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2017.7.0064 acta polytechnica ctu proceedings 7:64–68, 2017 © czech technical university in prague, 2017 available online at http://ojs.cvut.cz/ojs/index.php/app experimental identification of an elasto-mechanical multi-degree-of-freedom-system using stochastic signals simon schleiter∗, okyay altay, sven klinkel rwth aachen university, faculty of civil engineering, chair of structural analysis and dynamics, mies-van-der-rohe-str. 1, 52074 aachen,germany ∗ corresponding author: schleiter@lbb.rwth-aachen.de abstract. the determination of dynamic parameters are the central points of the system identification of civil engineering structures under dynamic loading. this paper first gives a brief summary of the recent developments of the system identification methods in civil engineering and describes mathematical models, which enable the identification of the necessary parameters using only stochastic input signals. relevant methods for this identification use frequency domain decomposition (fdd), autoregressive moving average models (arma) and the autoregressive models with exogenous input (arx). in a first step an elasto-mechanical mdof-system is numerically modeled using fem and afterwards tested numerically by above mentioned identification methods using stochastic signals. during the second campaign, dynamic measurements are conducted experimentally on a real 7-story rc-building with ambient signal input using sensors. the results are successfully for the relevant system identification methods. keywords: system identification, civil engineering structures, stochastic signal, ambient signal, fdd, arma, arx. 1. introduction starting with the 19th century, aesthetic and economic challenges have induced an impulsive interest on the design of the dynamic behavior of civil engineering structures, as the new constructions are getting higher and slenderer. hence the calculation of relevant system parameters, such as natural frequencies, damping ratios and modal shapes, become more important. in many cases it is necessary to identify system parameters after construction, so that the use of experimental methods becomes essential to get the current behavior. generally, all experimental methods can be distinguished between either time or frequency domain. the investigated models in this paper are the frequency domain decomposition (fdd) in frequency domain, autoregressive moving average models (arma) and the autoregressive models with exogenous input (arx), both in time domain. to describe the system response accurately, the output data should be known as well as the excitation data. but in most cases the determination of the excitation data is very costly for civil engineering structures. for instance, the detection of the wind induced excitation or microseism requires a simultaneous measurement using several sensors. whereas the arx-model is using excitation data in the calculation, the arma-model, as well as the fdd are neglecting any excitation signal. in this case an assumption must be made. the excitation may only be a broad band excitation, otherwise there is no interpretation of the results possible. especially for this case the fdd has been developed by brincker [1]. for complex structures the use of arma-model and fdd are recommendable and mostly unavoidable. a numerical implementation of all presented models is possible and is executed in this paper. results are validated on the basis of two examples: one on a numerically simulated 4-dof system and the other one on an experimentally analyzed 7-story real structure. aim of these applications is to show upcoming problems for the presented system identification methods and potential for further research on this topic. 2. system identification methods: fdd, arma, arx the frequency domain method (fdd) is based on the spectral power density, which can be used to describe signals in frequency domain. it is approved that the spectral density of the excitation signal (gxx) and of the response signal (gyy) can be decomposed by the frequency response function (h), all terms depend on the circular natural frequency and the exponent ∗ illustrates the complex conjugate tensor as t represents the transpose, as follows: gyy(ω) = h∗(ω)gxx(ω)ht (ω) (1) using some mathematical transformations and the assumption of broad band excited systems a single value decomposition of this formulation yields the modal shapes and the natural frequencies of the system. for further information the lecture of brincker [1] is recommended. damping ratios can be calculated 64 http://dx.doi.org/10.14311/app.2017.7.0064 http://ojs.cvut.cz/ojs/index.php/app vol. 7/2017 example of an article with a long title figure 1. structure of the 4-dof system. by extensions of this system, such as enhanced fdd (efdd) or curve fitting fdd (cfdd) explained in brincker et al. [2] and jacobsen et al. [3]. based on the time domain, the aim of armaand arx-models is to recreate the time signal by correlation coefficients. this correlation coefficients or parameters can describe the complete system behavior with the assumption of periodic behavior. additionally, it is possible to create the frequency response function (frf) for each signal by knowing these parameters. using this frf, working from now on in the frequency domain, the modal shapes can be calculated by an algorithm as well as the natural frequencies. damping ratios are calculated by the so called half power method, mentioned in he et al. [4], also using the above-mentioned frf. a detailed overview of these and more models based on time domain is given in ljung [5] and a closer explanation of the functionality of the frf in he et al. [4]. 3. application examples to validate these methods, a numerical application of a 4-dof system (see figure 1) was conducted. for this a cantilever beam was simulated in a fe-program with four similar masses distributed in the same distance to each other. to simulate a measurement, ambient input signals were generated and afterwards used as excitation signals for the structure. by using the numerical response signals, the methods were applied. as a validation of the methods, an identification in the fe-program itself using modal analysis gave the natural frequencies and the damping ratio shown in table 1 and 2, illustrated as fe-values. the results of the fe-calculation (modal analysis) are in good agreement to the used identification methods (arma, arx, fdd). the modal shapes are shown in figure 2, where the -1 -0.5 0 0.5 1 0 1 2 3 4 d eg re e of f re ed om 1. eigenmode arma fe, arx, fdd -1 -0.5 0 0.5 1 0 1 2 3 4 d eg re e of f re ed om 2. eigenmode arma fe, arx, fdd -1 -0.5 0 0.5 1 0 1 2 3 4 d eg re e of f re ed om 3. eigenmode arma fe, arx, fdd -1 -0.5 0 0.5 1 0 1 2 3 4 d eg re e of f re ed om 4. eigenmode arma fe, arx, fdd figure 2. comparison of the modal shapes of the 4-dof system. reference modal shapes can be compared with the modal shapes of the different applications. except the arma-model the applications give exactly the same results compared to the fe-reference values. in contrast a certain percentile difference for the armamodel can be realized. it only can describe the modal shapes in a rough shape and could only be accepted for preliminary design purposes. in a second application a first plausibility check of fdd has been conducted experimentally, based on real measurement data. in this example a 7-story rc-building (see figure 3) is investigated. the measurement data is acquired by using three identical bi-directional geophones, measuring the velocity (sensitivity of 100 v/m/s).the measurement goal was the estimation of natural frequencies of the bending mode shapes. therefore the sensors were located in the sixth 65 s. schleiter, o. altay, s. klinkel acta polytechnica ctu proceedings natural frequency [hz] fe fdd arx arma 1. eigenmode 1,2 1,3 1,2 1,2 2. eigenmode 7,7 8,0 7,7 7,7 3. eigenmode 19,3 20,1 19,3 19,3 4. eigenmode 28,4 29,7 28,4 28,5 table 1. comparison of the natural frequencies of the 4-dof system. damping ratio [%] fe fdd arx arma 1. eigenmode 1,0 1,0 0,9 2. eigenmode 1,1 1,5 1,1 1,1 3. eigenmode 1,9 1,9 2,2 2,1 4. eigenmode 2,5 2,5 2,7 2,7 note: the used algorithm according to the fdd makes a determination of the damping ratio for low frequencies impossible and thus for the first eigenmode. table 2. comparison of the damping ratios of the 4-dof system. figure 3. experimentally investigated 7-story rcbuilding. [6] story on one axis and a distribution over the short building side was not necessary (see figure 4). a detailed system identification only would be possible by using more measurement points. in this application the fdd method only investigates the applicability for complex civil structures and possible upcoming problems. load-bearing walls of the structure are illustrated in red in figure 4. an interaction of the x-direction with the ydirection is expectable for some modes, but for the sake of simplicity the modes are investigated separate for each direction. table 3 shows the results of natural frequencies and damping ratios for the x-direction and table 4 according to the y-direction. the results show that there is an interaction caused by non-symmetry and varied stiffness contribution between the second modes of xand y-direction. the significant variation of the damping ratio down to 0.5 % for the second eigenmode can be interpreted as that it is probably not a bending mode shape like the first and the third eigenmodes. the identified damping ratios are obviously lower than the standard values for rc-buildings indicating the drawbacks of the presented methods for the estimation of the strucnatural frequency [hz] damping ratio [%] 1. eigenmode 1,9 1,2 2. eigenmode 2,8 1,3 table 3. natural frequencies and damping ratios of the x-direction of the rc-building. natural frequency [hz] damping ratio [%] 1. eigenmode 2,1 1,2 2. eigenmode 2,6 0,5 3. eigenmode 4,3 2,0 table 4. natural frequencies and damping ratios of the y-direction of the rc-building. tural damping. since these eigenmodes are decoupled the damping ratio can be completely different compared to the bending eigenmodes. at this stage using the measurement results of the sixth floor, only a preliminary general identification of the eigenmodes in y-direction can be concluded, as in x-direction all measurement points lie on the same axis. a more sophisticated system identification can be done by including additional measurement points, also in vertical direction. the results of the modal shapes by application of the fdd are shown in figure 57. it can be seen, that the first and the third modes both can be interpreted as the bending modes and thus the second mode shows a torsional behavior. 66 vol. 7/2017 example of an article with a long title y staircase corridor offices lecture hall staircaseoffices offices offices officesrestrooms x y mp 1 xmp 2 mp 3 x y 9,00m29,60m44,70m 10,00m 7,60m figure 4. floor plan of the story 6 of the rc-building. direction of motion figure 5. 1st modal shapes of 7-story rc-building identified by using fdd. direction of motion figure 6. 2nd modal shapes of 7-story rc-building identified by using fdd. 4. conclusion in this paper different system identification methods for civil engineering are introduced and verified both on numerical and experimental examples. the three presented models are the frequency domain decomposition (fdd), the autoregressive moving average models (arma) and the autoregressive models with exogenous input (arx), which either use the time domain or the frequency domain. special attention is paid to the type and existence of the excitation signal, which has an influence on the models and the accudirection of motion figure 7. 3rd modal shapes of 7-story rc-building identified by using fdd. racy of the results. it is found that the arma-model achieves less exact results than other methods, so that they should be preferred, if they are applicable. the first validation example presents the system identification of a numerical 4-dof system model, yielding good agreement with the fe-values. second example shows the results of an experimental investigation carried out on the 6th floor of a 7-story rc-building, which allows a preliminary identification of its behavior. 5. future work in order to increase the applicability of the system identification methods for civil engineering structures some further research goals need to be accomplished. the algorithms used so far are valid for simple structures without considering the interaction of multiple directions. for real structure applications these methods should be transferred to complex models by modifying the algorithms. a further innovative approach would be the stepwise investigation of a building, e.g. ambient system responses are measured for several points on separate time windows. the time dependence of the measured data can be corrected by using signal processing methods. hence compared to other methods the "incremental identification method" (iim) is a low-cost method, which needs a less extensive measurement preparation by using only one sensor with a numerically affordable data evaluation algorithm. 67 s. schleiter, o. altay, s. klinkel acta polytechnica ctu proceedings acknowledgements the authors gratefully acknowledge the financial support of the daimler and benz foundation. references [1] r. brincker, l. zhang, p. andersen. modal identification from ambient responses using frequency domain decomposition. proc of the int modal analysis conf imac xviii 2000. [2] r. brincker, c. e. ventura, p. andersen. damping estimation by frequency domain decomposition. proc of the int modal analysis conf imac xviv 2001. [3] n.-j. jacobsen, p. andersen, r. brincker. applications of frequency domain curve-fitting in the efdd technique. proc of the int modal analysis conf imac xxvi 2008. [4] j. he, z.-f. fu. modal analysis. butterworth heinemann, oxford, 2001. [5] l. ljung. system identification. prentice hall, new jersey, 1987. [6] rwth aachen university. 68 acta polytechnica ctu proceedings 7:64–68, 2017 1 introduction 2 system identification methods: fdd, arma, arx 3 application examples 4 conclusion 5 future work acknowledgements references acta polytechnica ctu proceedings doi:10.14311/ap.2016.4.0089 acta polytechnica ctu proceedings 4:89–96, 2016 © czech technical university in prague, 2016 available online at http://ojs.cvut.cz/ojs/index.php/app evaluation metrics applied to accident tolerant fuel cladding concepts for vver reactors martin ševečeka,b,∗, mojmír valachc a department of nuclear reactors, faculty of nuclear sciences and physical engineering, czech technical university in prague, v holešovičkách 2, praha 8, czech republic b alvel a.s., opletalova 37, praha 1, czech republic c ujv řež, a.s., department of severe accidents and thermomechanics, nuclear power and safety division, hlavní 130, řež, 250 68 husinec, czech republic ∗ corresponding author: martin.sevecek@fjfi.cvut.cz abstract. enhancing the accident tolerance of lwrs became a topic of high interest in many countries after the accidents at fukushima-daiichi. fuel systems that can tolerate a severe accident for a longer time period are referred as accident tolerant fuels (atf). development of a new atf fuel system requires evaluation, characterization and prioritization since many concepts have been investigated during the first development phase. for that reason, evaluation metrics have to be defined, constraints and attributes of each atf concept have to be studied and finally rating of concepts presented. this paper summarizes evaluation metrics for atf cladding with a focus on vver reactor types. fundamental attributes and evaluation baseline was defined together with illustrative scenarios of severe accidents for modeling purposes and differences between pwr design and vver design. keywords: nuclear fuel, accident tolerant fuel, evaluation metrics, lwr, vver. 1. introduction after the events at the fukushima daiichi npp, enhancing the accident tolerance of lwrs became a topic of high interest in many countries. the goal of accident tolerant fuel (atf) development is to develop alternative fuels to further enhance the safety, competitiveness, and economics of nuclear power. the world’s leader in atf development are the u.s., where in 2012, the congress directed to doe to: • develop enhanced fuels and cladding for lwrs to improve safety in the event of accidents in the reactor or spent fuel pools. • emphasize and fund activities aimed at the development and near-term qualification of meltdownresistant atf that would enhance the safety of present and future generations of lwrs. • create a plan for development of meltdown-resistant fuels leading to reactor testing and utilization by 2020. other countries (e.g. france, south korea, china) have been conducting their own atf research with similar goals. recently, russia introduced its plan for atf development with a focus on vver reactors. activities related to the atf development are also coordinated by international organizations as iaea or oecd/nea. the initial effort is focused on applications in operating lwr reactors (pwr, bwr, and vver). the goal set by the u.s. or south korea is to insert a lead test assembly (lta) into a commercial reactor by 2022. other countries including russia or the czech republic have not defined their specific goals, yet. this paper summarizes an evaluation methodology, proposed set of metrics, related tests and illustrative accident scenarios for the evaluation of vver atf cladding concepts. the metrics are based on the detailed evaluation approach proposed in the [1], with an emphasis given on the fuel cladding, vver reactors, and their departures from standard pwr reactor design. the evaluation methodology will be applied to assess each atf cladding concept relative to the current vver fuel system and will be described in details in future technical report. 1.1. atf definition atf fuels are according to the oecd/nea [2] defined as fuels that can tolerate a severe accident in the reactor core for a considerably longer time period than the current uo2 – zr alloy fuel system, while maintaining or improving the fuel performance during normal operations and operational transients. 1.2. coping time each concept will be evaluated besides other characteristics based on its ability to increase the “coping time” under severe accident conditions. where coping time is according to the oecd/nea defined as [2]: the time to significant loss of geometry such that the fuel can no longer be cooled or cannot be removed from the reactor. for each concept, there will be an analysis of failure modes and effects (fmea) performed to determine the onset of particular initiating event that leads to severe accident. however, the common baseline has to be set and general illustrative scenarios defined. 89 http://dx.doi.org/10.14311/ap.2016.4.0089 http://ojs.cvut.cz/ojs/index.php/app martin ševeček, mojmír valach acta polytechnica ctu proceedings 1.3. development strategy the strategy for atf development includes three phases: (1.) feasibility assessment and down-selection (laboratory experiments; code updates; assessment of economical, operational, safety, fuel cycle, and environmental impacts using evaluation metrics). (2.) development and qualification (atf fabrication; irradiation; safety basis testing; qualification and licensing). (3.) commercialization (technology transfer to industry). any new atf concept must comply with current safety and performance constraints, fuel cycle impacts or additional lwr design constraints. moreover, it’s attributes should be better in comparison with the current fuel system to achieve comparable or better performance in normal and accident conditions. for that reason, the quantitative metrics have to be developed and described in details taking into account also the requirements of utilities, fuel vendors, and regulatory bodies. 1.3.1. challenges in developing atf one of the main concerns is a definition of testing and qualification requirements. individual attributes are not equal, which requires concepts prioritization and determination which attributes to test and what metrics are needed to evaluate attribute compliance. if new atf concept should be accepted by utilities and vendors, it must be capable of integration into the current nuclear fuel cycle system (figure 1). the challenge is to get the best performance at each step of nuclear fuel cycle, and to understand how it affects other parts of the system. figure 1. atf shall not negatively (economically or technically) impact fuel cycle technologies. for that reason, integrated evaluation approach has to be adopted. 2. attributes of atf the attributes for mitigation of fuel failure during severe accidents provide fundamental guidance for atf evaluation. it may not be necessary to improve fuel system in all attributes. some attributes may provide substantial enhancement in accident tolerance, while others may provide only marginal benefits. the desired attributes highlight the performance of the fuel under normal and accident conditions. key attributes for a fuel system demonstrating enhanced accident tolerance include reduced steam reaction kinetics, lower hydrogen generation rate, and reduction of the initial stored energy in the core. the desired behaviors should be accomplished while maintaining or improving cladding thermo-mechanical properties, fuel-cladding interactions, and fission-product retention. in some cases, the described consequences of accident conditions may be concept-specific (e.g. hydrogen generation in the current zr alloy-uo2 fuel system). other atf concepts could present additional effects not expressed here (e.g. generation of co in case of sic) that must be considered in evaluation of the proposed system. a brief description of the desired attributes is provided in this section and summarized in figure 2. figure 2. major issues that need to be addressed in establishing accident tolerant fuel attributes. candidate fuel systems must first not harm, which means that the fuels must perform as well as or better than the current fuel system. moreover, the fuel system must additionally provide a comparable or improved response to aoos (anticipational operational occurences), dbas (design basis accidents) and bdbas (beyond design basis accident). 2.1. hydrogen generation rate hydrogen generation in the reactor core can lead to energetic explosions similar as in fukushima-daiichi accident. under a high-temperature steam environment, it is not possible to avoid hydrogen generation with standard zr-alloys. rapid oxidation of cladding results in free hydrogen generation. this exothermic reaction increases the cladding temperature, which further accelerates free hydrogen generation. a related issue is the diffusion of free hydrogen into the unoxidized portion of the cladding, resulting in enhanced embrittlement and potential cladding failure. the desired alternative is a cladding material that resists oxidation or reduces the rate of oxidation resulting in a slower hydrogen generation rate. materials with lower heat of oxidation are important due to the limitation of temperatures during an accident. materials that are less susceptible to hydrogen diffusion 90 vol. 4/2016 evaluation metrics for vver atf fuels may address the rapid embrittlement issue typical for standard zr alloys. 2.2. fission product retention cladding provides an important barrier between fission products and primary circuit. the potential release of fission products to the environment has to be avoided, therefore, retention within the fuel is of the highest importance. while total retention may not be possible, higher partial retention would be a substantial improvement. the desired improvement is to prevent melting or dispersion of the fuel by utilization of hightemperature/strength cladding materials that would retain cladding integrity beyond the current limitations of zr-alloy cladding. 2.3. cladding reaction with steam when cladding is exposed to steam at high temperature, multiple issues need to be considered: hightemperature steam interaction with zr-alloy cladding, an exothermic oxidation reaction, hydrogen generation, and degeneration of the structural integrity of the cladding. atf materials should demonstrate enhanced tolerance to radiation and oxidation under hightemperature exposure while specifically considering mechanical strength and structural integrity at the end of life and when exposed to high-temperature steam for an extended duration. 2.4. fuel-cladding interactions pellet cladding chemical interactions (pcci), pellet cladding mechanical interactions (pcmi) and fuel heating are important properties that must be understood during normal operation and accident conditions for all new fuel concepts. the desired design option is to develop fuels with reduced pcci and pcmi, with lower operating temperatures relative to the zr alloy-uo2 system, with higher melting point, and with structural integrity at high temperatures. chemical and physical compatibility of cladding and fuel for all proposed concepts must be ensured. 3. constraints on development of enhanced atf except for a few rare events, the current uo2-zr fuel system meets all performance and safety requirements while keeping nuclear energy economically competitive. any new atf concept should be compliant with and evaluated against current design, operational, economic, and safety requirements. fuel cycle considerations must also be considered, especially for concepts that represent a significant departure from the current technology. a brief summary of the constraints is illustrated in figure 3. figure 3. constraints on new fuel designs. the main constraints include: • potential atf geometry deviations and compatibility with co-resident fuel. • evaluation of all potential accident condition parameters, including: . fmea completion to ensure that any potential atf operating vulnerabilities are recognized and mitigated as possible. . completion of a § 50.59-like process (changes, tests and experiments section from the 10cfr50 regulation) to illuminate any necessary licensing or logistical preparations for operation of the atf in a commercial lwr. the process and its guidance is described in [3]. • quantifying the recommended minimum additional coping time to be provided by the atf. • the need to include a metric to address those concepts that require enrichment greater than 5 wt%. • the mission of the utility is not to test new fuels. any atf operation must necessarily address and minimize impacts on the utility by ensuring full compatibility with co-resident fuel and reactor components, and by limiting perturbations of the normal operation of the plant. • nuclear industry has optimized the current zr-uo2 which represents a large financial investment. the additional cost of atf must be relatively low. at present, it is not feasible to determine quantitative results. 3.1. backward compatibility atf concepts must be suitable for use in existing lwrs. longer term concepts may be considered in conjunction with the near-term focus. proposed fuel concepts should not require plant modifications to the current reactors. they should be compatible with existing fuel handling equipment, fuel rod or assembly geometry, and co-resident fuel in existing lwrs. 3.2. impact on operations a concept must maintain or extend plant operating cycles, reactor power output, and reactor control. re91 martin ševeček, mojmír valach acta polytechnica ctu proceedings figure 4. a diagram that describes an approach to assessing enhanced accident attributes of a proposed fuel design. ducing the availability or power output would be unacceptable to utilities. to maintain current operation, some of the fuel system concepts would require higher fuel enrichment. while the impact of higher enrichment is fairly well understood from a technical perspective, regulatory and safeguard issues have to be addressed. 3.3. economic impacts any atf concept is likely to be more expensive than the current uo2-zr system, at least initially. fuels that require higher enrichment are especially likely to cost more. increased enrichment could additionally require modifications to fuel fabrication facilities. therefore, it is important to carefully assess the economic impact of the new technology and to determine how much additional fuel cost the utilities will accept. on the other hand, increasing burnup and power densities (power upgrades) could reduce or mitigate the negative economical impacts. 3.4. safety impact performance of a new fuel system will be compared to the performance of the uo2-zr alloy system which defines a baseline for the atf evaluation. the performance must be shown to be similar to or better than of the current system. 3.5. fuel cycle impact atf must adhere to regulations and policies, for both the fuel fabrication facility and the operating plant, with respect to technical, regulatory, equipment, and fuel performance considerations. a new atf system could also have an impact on the back-end of the fuel cycle. the storage, repository performance of the fuel or possibility of reprocessing must not be degraded. 4. metrics and related testing the metrics determine a clear technical methodology for evaluation that can be used to rank two or more concepts. the approach has to be simplified due to the complex multiphysics behavior of nuclear fuel and the large set of performance requirements that must be met. dozens types of atf cladding ranging from metallic to fully ceramic cladding each with very different material properties and behavior has been studied. each concept has to be evaluated and compared to determine the concept viability, and prioritize resources to obtain the cladding with the best compromise in terms of properties and behavior in both nominal and accidental conditions. 4.1. desired cladding properties, behavior and performance the cladding of a nuclear fuel has three fundamental functions: (1.) confinement of fissile material and fission products within the fuel rod. (2.) not affecting chain reaction (low absorption cross section). (3.) efficient heat transfer from the fuel to the coolant. to fulfill these three functions a new ideal cladding material needs to have certain properties and exhibit specific behaviors as follows: fabricability: • compatibility with fabrication facilities; material availability; welding/sealing behavior. thermo-mechanical behavior: • high thermal conductivity, strength, and ductility; leak-tightness throughout plant operation (impermeable to fission gas and fission products). corrosion behavior: • high corrosion resistance in vver reactor environment; high corrosion resistance in high temperature steam; low h2 production and low hydrogen embrittlement. chemical compatibility: • compatibility with fuel, coolant, and the fuel assembly components; no eutectic. irradiation behavior: • stable or predictable thermal, mechanical, and corrosion behavior under irradiation; dimensional stability; low irradiation embrittlement; low thermal neutron absorption cross-section; low activation. back-end behavior: • low tritium permeation; no impact on the reprocessing; satisfactory storage behavior. new atf cladding material will not exhibit all presented ideal properties. for that reason, a baseline for evaluation and assessment of various concepts has 92 vol. 4/2016 evaluation metrics for vver atf fuels to be established. this corresponds to the metrics described further. 4.2. metrics definition the metrics are divided into three sections: (1.) material properties inherent to the material. (2.) material behavior observed through standardized tests or experiments for normal and accident conditions. (3.) material behavior observed through standardized tests for accident conditions. note that, material behavior can depend also on particular reactor type and its performance. 4.2.1. inherent material properties the inherent material properties that should be compared are mainly thermal and neutronic properties: high melting/sublimation temperature; high thermal conductivity; optimal heat capacity/enthalpy; optimal thermal expansion coefficient to ensure dimensional stability; thermal neutron absorption crosssection. there are other concept-specific inherent properties. for example in case of multi-layer coating, compatibility between different materials has to be ensured. additionally, eutectic temperature might decrease the melting temperature of the cladding so it should be avoided. 4.2.2. normal operation behavior in contrast with the inherent physical properties described above, the behavior of the materials has to be assessed through tests with standardized conditions to enable direct comparison and evaluation of the various concepts. long-term corrosion tests are proposed to evaluate the corrosion behavior in normal operating conditions in a different chemical environment. an acceleration of kinetics is likely under irradiation which has to be also investigated. the important data to assess are: visual inspection – potential delamination of coatings; corrosion kinetics – weight change, oxide thickness; water chemistry analyzes; hydrogen pick-up. mechanical tests suggested for mechanical behavior evaluation are: tensile tests; internal pressure creep tests; internal pressure burst tests; leaktightness behavior; fatigue tests – thermal cycling, internal pressure load cycling. 4.2.3. accident conditions behavior first, tests simulating normal operating conditions will be performed. later, the material performance under selected accident conditions will be assessed. specific tests selected to determine accident performance of candidate cladding materials are series of standard tests simulating ria and loca conditions. for example: high temperature oxidation, isothermal internal pressure burst tests at multiple temperature set points, dynamic internal pressure burst tests with increasing temperature until cladding failure, rapid burst test with increasing temperature until cladding failure, tensile tests at elevated temperature, internal burst tests, tensile test at elevated temperature etc. 5. evaluation baseline at the beginning of the evaluation, it is recommended to establish a common understanding of the current state-of-the-art cladding. key characteristics and performance criteria were identified, with quantitative values assigned where possible. all of the parameters are zr alloy specific, some are reactor design and reactor performance (chemistry regime, power changes) specific. however, there can be common baseline for most of lwrs defined with only small uncertainties. 5.1. baseline for normal operating conditions generally accepted characteristics of zr-based alloys during normal operations are [4]: • fission product retention: 10−6 pin failure rate, radiation tolerant: up to 10 dpa, 1022 n/cm2 fast (>1 mev), dimensional stability: max 1 % axial elongation. • thermal conductivity λ: ∼17.4 w/m k, heat capacity cp: 293 j/kg k, heat transfer coefficients: coolant/cladding coupling (reactor design dependent). • corrosion performance: oxide growth ≤100 µm, hydrogen content ≤800 wppm, maximum limit on corrosion product buildup (linked to water chemistry specifications), galvanic corrosion limits, requirement for all corrosion products to be harmless to other system components. • tritium release into coolant: limit for tritium permeation not established, but “low” permeation is desirable, <10 % of the total tritium generated permeating into the coolant could be reasonable. vendors assume that less than 2 % of this tritium permeates through zr-based alloy cladding to the coolant (e.g. for ss-316, the permeation could be as high as 90 %). • hydrogen pick-up: typical for zr-based alloys, depends on corrosion level and varies with alloy from 100 ppm to 800 ppm (600 ppm should be acceptable under normal operating conditions). • mechanical criteria – ductility: currently 1 % minimum of total elongation; modern alloys show uniform elongation up to 5 % and total elongation up to 20 %; yield strength: current as-fabricated alloys, ∼150–400 mpa at 340 ◦c, ultimate tensile strength: 437 mpa at room temperature (zry-4), creep strain limit: 0.66 % (zry-4); resistance to crack propagation (fracture toughness). 93 martin ševeček, mojmír valach acta polytechnica ctu proceedings • fretting wear: <10 % of wall thickness; debris wear resistance; fabricable: weldable, thin-walled tube that maintains hermeticity. • pci interaction behavior: includes consideration of fission product interactions (possible scc), oxide layer on the inside of the clad (∼10 micron). • burnup limit: peak rod average: 62 mwd/kg u. 5.2. baseline for accident conditions the accident performance of zr-based cladding differs from the normal operation behavior. the main current metric for fuel system performance is that the cladding should maintain post-quench ductility by limiting the peak cladding temperature to 1204 ◦c and the maximum oxidation level to 17 % of the cladding wall thickness. it has been shown that the temperature and oxidation limits result in maintaining ductility for as-fabricated and very low burnup cladding. changes underway nrc regulatory guides and iaea guidelines will limit the oxidation level for dbas based on pre-dba hydrogen content (e.g. 4 % for 600 wppm hydrogen content) [5]. under beyond bdba loca conditions, cladding will be subjected to higher temperatures and oxidation levels. zr-based cladding alloys would experience longer times at increasing temperatures, higher oxidation rates, higher hydrogen release rates, and higher internal heat-of-oxidation rates. key performance measures for fuel behavior during accident conditions include coping time, behavior under elevated temperature conditions over long periods of time, and material oxidation [6]. the relevant temperatures for lwr severe accidents which result in the formation of liquid phases due to melting or chemical interactions are summarized in figure 5. • coping time under dba and bdba – lbloca conditions: from 50 to 300 sec at 1204 ◦c, sbloca conditions: ∼1 hr at ≤1050 ◦c. • elevated temperature issues – melting temperature: ∼1760 ◦c, temperature for eutectic formation (decrease of melting point, effects on other components), runaway oxidation – relates to heat of oxidation and acceleration of the oxidation process. • steam oxidation kinetics: oxidation rate and associated heat of oxidation, performance considered for time at elevated temperature and maximum temperature, breakaway oxidation may occur at elevated temperature (e.g. time at ∼1000 ◦c before breakaway oxidation occurs should be defined). • hydrogen production and release: <1 % zr-metal conversion. • maintain ductility following dba: most important accident performance criterion under licensing standards, regulation specifies at least 1 % ductility post-quench. • high temperature mechanical properties: balloon and burst are not currently defined as a fuel failure and are not limited in the licensing criteria; while only a fraction of rods burst, those that do typically burst at ∼800 ◦c; burst may occur at lower temperature with higher burnup due to higher internal pressure; burst is a ductile failure; size of ballooning and burst opening can be important to the results of the failure. • severing and/or shattering the cladding with subsequent fuel release are considered as a failure; creep performance must be also considered. • flow blockage under accident scenarios: required calculation for licensing, related to maintaining coolable geometry, flow blockage may result from swelling / ballooning of cladding. • thermal shock resistance: current cladding alloys do not shatter until they have reached high oxidation levels (>17 % of the cladding wall thickness consumed by oxidation), standard test applied to determine thermal shock performance. figure 5. lwr severe accident-relevant melting and chemical interaction temperatures which result in the formation of liquid phases [7]. the goals of evaluation are associated with defining requirements for down-selection among options during the feasibility assessment. it includes identification of important attributes for atf, understanding the common baseline for evaluation and map the merit of the attributes against potential operational or safety envelope benefits which are presented in the previous sections. it is also required to define safety analyzes including accident scenarios to quantify the target 94 vol. 4/2016 evaluation metrics for vver atf fuels values of particular atf fuel characteristics which are presented in the next section. 6. illustrative scenarios for atf evaluation two “bounding scenarios” for bdba evaluation are proposed. they cover wide range of severe accidents and at the same time are not too prescriptive and specific to a particular npp design. modeling of these scenarios will utilize one of the initiating events and will continue until a defined point of failure as defined in by the coping time definition (see section 1.2) for particular atf concept. the two scenarios applicable to vver reactors are: • long-term station blackout: high-pressure scenario; it will be calculated to the point of reactor pressure vessel failure. • large-break loca: low-pressure scenario with high decay power and limited availability of emergency cooling systems. the proposed scenarios are intended to provide bounding cases for fuel performance. each atf concept will be evaluated by fuel performance and system analysis codes in regard to the two illustrative scenarios. all atf evaluations should be allowed to progress to the point defined by the coping time definition with considering failure modes and effects for particular atf concept. the goal of the evaluation is to estimate the potential increase in coping time that is offered by the atf concept and to assess potential outcomes (e.g. fuel failure, coolability, cause of failure). following completion of bounding (most severe) analyzes, more detailed studies for these illustrative scenarios should be performed to develop a better understanding of the impact of additional variables as burnup, time after scram when core cooling is lost etc. 6.1. high pressure scenario – sbo the high-pressure sbo (station blackout) scenario for atf evaluation purposes for vver-1200 reactor is defined as: • loss of all sources of ac supply. • feed water supply is unavailable. • turbine isolation valve is activated, pressure in the steam generators (sgs) increases. • after exceeding the pressure limit the quick-acting pressure reducing system opens. • a failure of the quick-acting pressure reducing system is postulated – system stays open after pressure decreases. • loss of coolant in sgs leads to decay heat removal passive system failure. 6.2. low pressure scenario – lbloca + sbo the low-pressure scenario for atf evaluation purposes for vver reactors is defined as: • guillotine rupture of the primary circuit’s cold leg near the reactor inlet with sbo. • decay heat removal passive system (dhrs) and emergency core cooling system are in operation. operation of the hydro accumulators is assumed. the decay heat removal passive system and emergency core cooling system operate according to their design characteristics: eccs – up to water volume exhaustion, dhrs – up to the end of nitrogen absorption due to eccs operation. total operation time of these systems is approximately 24 h from the accident start. 7. vver fuel specifics the russian design pressurized water reactors called vver have specific differences in comparison with western pwrs. it is recommended to adopt all evaluation metrics with both illustrative scenarios presented above and define additional vver specific metrics. the main differences of the vver fuel and primary circuit include: • different water chemistry, • hexagonal geometry and arrangement of the fuel assemblies, • various cladding and fuel materials . e110 alloy, e635 alloy, . annular pellet, different dopants, only gd-based burnable absorbers, • assembly shroud – in case of vver-440 fuel assemblies, • different control rod materials. the main difference between vvers and pwrs can be observed in nominal operation due to different chemistry of primary circuit and different material composition. while pwrs use lioh for chemistry control, vvers use koh. this difference in chemistry regime leads to slightly different test conditions for normal operation corrosion tests. however, in accident conditions the performance of vvers is similar to that of pwrs. therefore it is not necessary to define additional vver-specific metrics for accident conditions. 8. conclusion to develop new accident tolerant fuel systems an evaluation and prioritization of atf concepts has to be performed at the end of the first development phase. the prioritization will allow researchers and decision makers to focus resources on most promising concepts. due to the complicated complex multiphysics behavior of nuclear fuel it is not possible to test all the required characteristics for each concept. for that 95 martin ševeček, mojmír valach acta polytechnica ctu proceedings reason, evaluation metrics have to be defined. a good starting point for atf evaluation establishment is a common understanding of the current state-of-the-art zr-cladding and its performance in bdba conditions. it is a baseline for evaluation and assessment of various concepts. evaluation of atf cladding must consider the complete fuel system (including fuel cycle) and should encompass all performance regimes for the fuel, including: fabrication, normal operation and aoos, dbas, bdbas, and used fuel storage, transportation, and disposition. there are numerous attributes within each regime that must be considered in evaluating the fuel system performance. key attributes for the cladding were discussed in sections 4 and 5, along with a summary of standard tests recommended for measuring specific properties and characterizing performance under the specified conditions. it may not be possible to improve the current state-of-the-art fuel system in all attributes and regimes, significant improvement in some of the key attributes may outweigh modest performance gains or modest vulnerabilities in other attributes. a detailed list of proposed attributes for evaluation is provided together with metrics and standardized tests. the attributes summarized in this document, can be used as a qualitative guide to assess the performance of candidate materials relative to the current state-of-the-art materials and relative to one another. evaluation baseline for common understanding of the current cladding was presented. to further determine the common understanding of atf performance two severe accidents were defined and described. based on the illustrative scenarios the coping time can be calculated and atf concepts evaluated. most of the metrics and attributes are generally applicable for all lwr reactors but some attributes as chemistry regime are reactor-specific and for that reason additional metrics and standardized test were defined for vver types of reactors. detailed more prescriptive approach for evaluation of atf cladding for vvers will be described in future technical report. acknowledgements this work was supported by the grant agency of the czech technical university in prague, grant no. sgs ohk4-008/16. references [1] s. m. bragg-sitton, j. carmack, f. goldner. evaluation metrics applied to accident tolerant fuels. tech. rep., idaho national laboratory (inl), 2014. [2] egatfl. light water reactor accident tolerant fuel: evaluation metrics and technology readiness level definition. tech. rep., oecd/nea, 2016. [3] usnrc. 1.187, regulatory guidance for implementation of 10cfr50.59. tech. rep., 2000. [4] m. billone, y. yan, t. burtseva, et al. cladding embrittlement during postulated loss-of-coolant accidents. tech. rep., argonne national laboratory (anl), 2008. [5] g. youinou, r. s. sen. enhanced accident tolerant fuels for lwrs–a preliminary systems analysis. tech rep 2013. [6] l. braase. enhanced accident tolerant lwr fuels national metrics workshop report. tech. rep., idaho national laboratory (inl), 2013. [7] p. hofmann. current knowledge on core degradation phenomena, a review. journal of nuclear materials 270(1):194–211, 1999. 96 acta polytechnica ctu proceedings 4:89–96, 2016 1 introduction 1.1 atf definition 1.2 coping time 1.3 development strategy 1.3.1 challenges in developing atf 2 attributes of atf 2.1 hydrogen generation rate 2.2 fission product retention 2.3 cladding reaction with steam 2.4 fuel-cladding interactions 3 constraints on development of enhanced atf 3.1 backward compatibility 3.2 impact on operations 3.3 economic impacts 3.4 safety impact 3.5 fuel cycle impact 4 metrics and related testing 4.1 desired cladding properties, behavior and performance 4.2 metrics definition 4.2.1 inherent material properties 4.2.2 normal operation behavior 4.2.3 accident conditions behavior 5 evaluation baseline 5.1 baseline for normal operating conditions 5.2 baseline for accident conditions 6 illustrative scenarios for atf evaluation 6.1 high pressure scenario – sbo 6.2 low pressure scenario – lbloca + sbo 7 vver fuel specifics 8 conclusion acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2016.3.0065 acta polytechnica ctu proceedings 3:65–70, 2016 © czech technical university in prague, 2016 available online at http://ojs.cvut.cz/ojs/index.php/app load bearing capacity of the glass railing element elizabeta šamec∗, domagoj damjanović, joško krolo university of zagreb, faculty of civil engineering, fra a. k. miošića 26, zagreb, croatia ∗ corresponding author: elizabeta.samec@gmail.com abstract. in this paper some basic physical and mechanical properties of glass as structural material are presented. this research is about specifically manufactured glass railing element that will be a part of a pedestrian bridge construction in zagreb, croatia. load bearing capacity test of the glass railing element is conducted within the faculty of civil engineering in zagreb and obtained experimental results are discussed and compared to the ones provided by the numerical model. taking into account the behaviour of laminated glass and results of experimental and numerical testing, glass railing element can be regarded as safe. keywords: glass railing element, load bearing capacity test, wind load, modulus of elasticity, numerical model. 1. introduction when we have glass elements that are specifically manufactured for construction projects it is highly important to perform laboratory testing in order to confirm their load capacity. necessity for laboratory testing follows from potential unexpected breakdown under the load due to special properties of the glass as a construction material. unlike steel, plastic and wood, glass has no plastic properties and consequently no yield point [1]. because of the property of pure elasticity with brittleness the glass cannot be permanently deformed by load and it fails without warning as shown on a stress-strain curve (fig. 1). failure always happens in tension because it has high compressive strength [2]. the cause of the glass failure is not necessarily concentrated impact; fracture may occur due to bending stress, thermal stress or deformation. a fracture which occurs will depend on the number of surface defects, the duration of the load, the size of the tensed surface and the stress level. by increasing the dimensions of the glass elements the number of imperfections will increase as well [3]. this kind of behaviour is undesirable for a construction material. despite possible problems with brittle behaviour and unexpected breakdown, application of structural glass has undergone remarkable development in the last twenty years. the opportunities and attractiveness that this type of material provides have been recognized and glass has become more than “window material”. the subject of this research is the glass railing element designed for the pedestrian bridge construction planned to be built in zagreb, croatia. usually for tempered glass guardrails, live loads caused by use and wind loads need to be considered. other loads such as a permanent load, snow load, and seismic load are insignificant because of their low range compared to the overcharge caused by use or wind [4]. in this case, the wind is also the main load on the glass element, figure 1. stress-strain diagram for steel, glass and plastic [2]. so it is necessary to test the bearing capacity of the glass railing element under the influence of the wind. in most cases glass will behave entirely elastic until the moment of fracture and this kind of behaviour is expected to be confirmed by experimental analysis. the literature review in [1] shows that earlier research has been conducted on behaviour, strength, analytical and numerical modelling of laminated glass with pvb but still the knowledge required to use laminated glass as a load bearing element in every-day construction, is not sufficient. the aim of this paper is to gain a deeper knowledge of the behaviour of laminated glass by experimental investigations and by numerical model simulation. to pursue the proposed study, load bearing capacity test was performed on the glass railing element in the scale 1:1. corresponding finite element numerical models were created in the software sap 2000 to discuss and compare displacements in the points most affected by the wind influence. the main question that needs to be answered is whether the glass railing element can be regarded as safe to install on the pedestrian bridge. therefore, safety factor is calculated and expected 65 http://dx.doi.org/10.14311/app.2016.3.0065 http://ojs.cvut.cz/ojs/index.php/app e. šamec, d. damjanović, j. krolo acta polytechnica ctu proceedings figure 2. bridge plan. figure 3. glass railing element with positions of attachment details. behaviour under the wind load is tested on a life-size sample of the glass railing element. 2. bridge project details 2.1. bridge construction the future pedestrian bridge is planned to be built in miramarska street in zagreb. the railing elements are going to be put along the bridge to protect the pedestrians from the influence of wind and to ensure their safety. the plan of the future bridge is shown in fig 2. 2.2. railing element the glass element consists of two 10 mm thick tempered glass panes bonded by interlayer (polyvinyl butyral-pvb). the stiffness of laminated glass depends on the shear strength of the panes and the pvb foil. factors that influence the behaviour of these connections are load duration, intermediate layer thickness, temperature and position of the intermediate layer with respect to the center of gravity. the behaviour of laminated glass is different under the long-term and short-term load. under the influence of short-term load, laminated glass acts as a composite carrier. under the influence of long-term load, the load is distributed on two panes due to their rigidity because of the deformation of the interlayer. in the case of a breakdown, glass fragments remain linked to the pvb layer. therefore, laminated tampered glass figure 4. detail 1: spider fitting (up), detail 2: inox clamps connecting the element to the main steel profile (down). is chosen for the railing element of the pedestrian bridge. the dimensions of the railing element are 213.4 x 107.5 cm. the element is attached to the bridge construction at four points. as you can see in the fig. 3 and fig. 4, the element is put in the inox glass clamps on the bottom, and on the handlebar section it is fixed to the handlebar with two spider fittings. 2.3. wind load as mentioned in the introduction, the wind load is authoritative load and calculation of the design value used in laboratory and numerical analysis is further described below. the design value is calculated according to croatian valid norm for wind load hrn en 1991-1-4:2012/ na2012. depending on the sensitivity of construction on the dynamic impulse, two procedures exist to calculate wind load: • simplified procedure: applies to structures that are insensitive or moderately sensitive to dynamic impulse, • detailed procedure: applies to structures sensitive to dynamic impulse which have the value of the dynamic coefficient greater than 1.2. 66 vol. 3/2016 load bearing capacity of the glass railing element figure 5. load bearing capacity test. figure 6. positions of measurement points (up) and load distribution (down). for the construction of the pedestrian bridge in zagreb the simplified method is applied since the construction is not sensitive to dynamic impulse. simplified calculation implies that the wind action is taken as a replacing static load. for the bridges, the wind pressure is calculated from the forces in all horizontal directions. to determine the wind load on the glass railing element according to hrn en 1991-1-4:2012/ na2012 the following parameters are required: • basic wind speed: vb = 20 m/s • height above the ground: h = 5 m • field category: iv. the design value of the wind load determined by the project designer is 1 kn/m2. 3. load bearing capacity test the load bearing capacity of the element (in the scale 1:1) was tested on one sample until breakdown in the structural testing laboratory operating at the figure 7. breakdown of the glass railing element. faculty of civil engineering in zagreb. the testing was conducted on the universal testing machine zwick z600. during the test, displacements and strains were measured using lvdt sensors (p) and strain gauges (t) as shown in fig. 6 (up). the sample was tested in the horizontal position and the wind load was simulated with two types of loading. the first type is a continuous load (1 kn/m2) on the surface between the supports (107.5×120.3 cm) which was applied with 12 weights during the whole test. the second one is a linear load (qpok) applied on the console part with the help of the previously mentioned machine zwick z600. the two types of loading can be seen on the fig. 5. the bending moment at the handlebar section provoked by the design value of wind load (f=1 kn/m2) on the surface of the console part of the glass element (fig. 6) is calculated as follows: m = f ∗ a2 2 2 = 1 ∗ 0.9312 2 = 0.433 knm/m. (1) the linear load (qpok) applied in the experiment must cause bending moment at the support (handlebar section) equivalent to the one caused by the assumed wind force calculated above: m = qpok ∗ a2/2, (2) qpok = m a2/2 = 0.433 0.466 = 0.93 kn/m. (3) knowing the required value of liner load (qpok) concentrated force (fpok) on the steel element is according to fig. 6 (down): fpok = qpok ∗ b = 0.93 ∗ 1.075 = 1.0 kn. (4) in this way continuous load on the whole surface of the railing element that would be provoked by wind 67 e. šamec, d. damjanović, j. krolo acta polytechnica ctu proceedings figure 8. bending test for glass canopy specimens [5]. sample strain [‰] e [mpa] 1 0.60 38 833 2 0.58 40 167 3 0.56 41 607 table 1. results of the bending test at the glass canopy specimens [11]. is simulated. by using a machine to apply fpok we were able to continually increase the force until the breakdown of the sample. the linear load is applied by increments of 0.5 kn until the force value of 2.0 kn, after which it is applied continuously until breakdown. after each load stage, the element is unloaded to determine the existence of any permanent displacements and strains. when the force reached the value of 6.55 kn, breakdown started rapidly expanding through the sample (fig. 7). the safety factor of the glass element can be calculated as the ration between failure load and design load. sf = 6.55 kn 1.00 kn = 6.55 . (5) 4. simulation of load bearing capacity test in sap 2000 as well as the laboratory test, a numerical model was made in sap 2000. a numerical modelling of laminated glass by finite element method is complex, mainly because of the very thin interlayer in comparison with other dimension and because of the large difference in modulus of elasticity of glass ply and interlayer material, especially for pvb [1]. the failure of a laminated glass sheet can be subdivided in five phases [5]: (1.) elastic behavior of the glass plies. (2.) the first glass ply is broken, the other is still intact; the interlayer is not damaged. (3.) the second glass ply fails; the interlayer behaves elastically. (4.) the interlayer behaves plastically; the splinters are kept together by the interlayer. (5.) the interlayer fails by reaching its failure strength or by cutting from the splinters. while phase (1.) can be modelled with analytical or numerical methods, phases (2.) to (5.) are more figure 9. numerical model in sap2000 for fpok = 1.00 kn (up) and fpok = 6.55 kn (down). (u-displacement, r-rotation, 1-x axis, 2-y axis, 3-z axis) complex to simulate. as shown in [5] several material models based on finite elements can be found in the literature to simulate the failure of the glass as well as of the interlayer. models with one shell element through the thickness use layered materials with integration points over the thickness. in [6] a material model for the glass which allows a two dimensional failure is presented. a combination of two shells and one solid element through the thickness can be found in [7]. authors [8], [9] and [10] present 3d models with solid elements which allow using a detailed material law for the interlayer. the aim of the paper is just to compare displacement results in points most affected by wind, so it is concluded that a single thin plate model describes the behaviour of laminated glass for the current application well enough, and it is not expensive in terms of time and problem size. since the test to determine mechanical properties was not performed for this specific railing element, modulus of elasticity was chosen based on the previously known test results. these results are obtained in the same laboratory during the test conducted for the construction of glass canopy [11]. for the glass canopy analysis a bending test was performed on three laminated tempered glass specimens consisting of two 10 mm thick tempered glass panes as shown in fig 8. during the test, strains were measured using strain gauge in the middle of the sample. from the obtained data, modulus of elasticity was calculated using hooke's law. results of this testing are summarized in the table 1, and since the railing element also consists of two 10 mm thick tempered glass panes, the chosen value of modulus of elasticity for this numerical model is e= 40 000 mpa. supports are defined in reliance to fig. 4 and they have prevented displacements in all three directions (x, y and z) while the rotation angle is free around all three axes [12]. the first numerical analysis was made for the load of 1 kn/m2 on the surface between the supports and linear load qpok corresponding fpok= 1.00 kn as it is 68 vol. 3/2016 load bearing capacity of the glass railing element loading experiment sap 2000 force displacement u3 fpok = 1.00 kn 8.00 mm 6.70 mm fpok = 6.55 kn 58.54 mm 60.80 mm table 2. comparison of experimental and numerical displacement results. shown in the fig. 6 (down) and described earlier. the second numerical analysis was conducted in the same way for the maximum load at breakdown fpok= 6.55 kn obtained during the experimental testing. models and given results are shown in fig. 9. 5. results results obtained by the load bearing capacity test are processed and shown in force-strain (fig. 10) and force-displacement diagram (fig. 11). from the force-strain diagram obtained during the experimental analysis, we can confirm that the glass behaved linearly elastic without any damages until breakdown. after all phases of loading, the residual strains were within the boundaries of 4% and the residual displacements within the limits of 10%. even though the displacements were measured during the test at 6 points (p1-p6), the most interesting points to compare obtained experimental and numerical results are points p1 and p2. in those points wind will cause the biggest displacement since they are the top points of the free console part (fig. 6 (up)). in the numerical model there will be no difference between the displacement of the point p1 and p2 so the point p2 is chosen due to an easier display of numerical results. the comparison of the displacement results obtained by numerical and experimental analysis at the measurement point p2 is given in table 2. in fig. 8 displacement gained from numerical model is marked with u3. from the table we can see that the obtained experimental and numerical results differ by less than 15%. as already discussed in the previous section, the simple single plate model is chosen. the thin plate model does not take into account a fact that laminated glass is three layer composite, instead the modulus of elasticity is defined for whole element. the test to determine the mechanical parameters was not performed, so the chosen value of modulus of elasticity based on the comparison of previously known test results can differ for two main reasons. the first reason is that even though the thickness of the glass panes was the same in both cases, different glass is used in manufacturing canopy and railing elements. secondly, we do not know exactly what impact interlayer has on the modulus of elasticity of composed element. figure 10. force-strain diagram. figure 11. force-displacement diagram. 6. conclusion the combination of hard glass with a soft interlayer limits the use of laminated glass as structural load carrying element due to difficult behaviour prediction. glass has a modulus of elasticity several thousand times larger than the interlayer material, which makes the behaviour and the modelling complex. with the simple single thin plate model used in this study, where modulus of elasticity for the whole laminated glass sheet was chosen based on the previous similar testing, gained results for displacement differ from experimental ones by less then 15%. the results show that for studies which include stress-failure models, taking into account the interaction behaviour of glass with interlayer is compulsory. it is also important to perform the test to determine mechanical parameters for each composed glass element because their accuracy can considerably influence the quality of the numerical model. in future studies it is necessary to observe the influence that thickness of interlayer has on the modulus of elasticity of laminated glass elements. the proper understanding of the interaction behaviour will enable wider usage of laminated glass as load carrying element. the safety factor of tested railing element reached in regard to the design wind load is k = 6.55. the glass railing element can endure six times higher load than anticipated so we can conclude that there are high 69 e. šamec, d. damjanović, j. krolo acta polytechnica ctu proceedings reserves in the bearing capacity of tested glass element. since the test was performed at the room temperature and pvb is highly temperature dependent material, by taking into account the weather conditions on the bridge we can expect the safety factor to be lower. considering the behaviour of laminated glass during the test and results of experimental testing compared to numerical ones, glass railing element can be regarded as safe. list of symbols vb basic wind speed [m/s] fpok concentrated force in the experiment [kn] f design wind load [knm/m2] h height above the ground [m] qpok linear load in the experiment [kn/m] m moment on the handlebar section [knm/m] sf safety factor [/] references [1] akter s. t., khani m. s. characterisation of laminated glass for structural applications. master thesis, 2013 [2] “saint gobain”. updated mar. 2015, accessed 13 apr. 2015. http://uk.saint-gobain-glass.com [3] neugebauer j. design of glass structures, scientific symposium: future trends in civil engineering, zagreb, croatia, 2014. [4] “manual for design of guardrails”. accessed 05 jun. 2015. http://www.allium.com/pdf/manuel-en.pdf [5] larcher m., solomos g., casadei f., gebbeken n. experimental and numerical investigations of laminated glass subjected to blast loading. international journal of impact engineering 2012; 39:42-5 doi:10.1016/j.ijimpeng.2011.09.006 [6] müller r., wagner m. berechnung sprengwirkungshemmender fenster und fassadenkonstruktionen. bauingenieur 2008;81(11):475-87 [7] sun d., andrieux f., ockewitz a., klamser h., hogenmüller j. modelling of the failure behaviour of windscreens and component tests. 5th european lsdyna users conference, 25-26 may, 2005. [8] morison c., zobec m., frenceschet a. the measurement of pvb properties at high strain rates, and their application in the design of laminated glass under bomb blast. isiems 2007, international symposium on interaction of the effects of munitions with structures, 17-21 september,orlando, us, 2007. [9] bennison s.j., jagota a., smith c.a. fracture of glass/poly(vinyl butyral)(butacite)laminates in biaxial flexure. journal of the american ceramic society 1999; 82(7):1761-70. [10] duser a.v., jagota a., bennison s.j. analysis of glass/polyvinyl butral laminates subjected to uniform pressure. journal of engineering mechanics 1999; 125(4):435-42. doi:10.1061/(asce)0733-9399(1999)125:4(435) [11] rak m., damjanović d., krolo j.: report on testing the glass beam, universitiy of zagreb, faculty of ce., 2013. [12] “sap 2000” 16 eval, computers and structures, inc., 2013. 70 http://dx.doi.org/10.1016/j.ijimpeng.2011.09.006 http://dx.doi.org/10.1061/(asce)0733-9399(1999)125:4(435) acta polytechnica ctu proceedings 3:65–70, 2016 1 introduction 2 bridge project details 2.1 bridge construction 2.2 railing element 2.3 wind load 3 load bearing capacity test 4 simulation of load bearing capacity test in sap 2000 5 results 6 conclusion list of symbols references 265 acta polytechnica ctu proceedings 1(1): 265–268, 2014 265 doi: 10.14311/app.2014.01.0265 first hints of pressure waves in a helical extragalactic jet: s5 0836+710 manel perucho1 1departament d’astronomia i astrof́ısica. universitat de valència. c/dr. moliner 50, 46100, burjassot, valencian country, spain. corresponding author: manel.perucho@uv.es abstract one of the open questions in extragalactic jet astrophysics is related to the nature of the observed radio jet, namely whether it traces a pattern or the flow structure itself. in this paper i summarize the evidence collected for the presence of waves in extragalactic jets. the evidence points towards the peak of emission in helical jets corresponding to pressuremaxima of a wave that is generated within the core region and propagates downstream. making use of a number of very long baseline interferometry (vlbi) observations of the radio jet in the quasar s5 0836+710 at different frequencies and epochs, perucho et al. (2012a) were able to observe wave-like behavior within the observed radio-jet. the ridge-line of the emission in the jet coincides within the errors at all frequencies. moreover, small differences between epochs at 15 ghz reveal wave-like motion of the ridge-line transversal to the jet propagation axis. the authors conclude that the helicity is a real, physical structure. i report here on those results and discuss them in the light of new results recently announced by other authors that confirm the presence of waves in the close-by object bl lac (cohen et al., in preparation). keywords: galaxies: jets hydrodynamics instabilities quasars: individual: s5 0836+710. 1 introduction the parsec-scale structure of jets in active galactic nuclei (agn) is mainly observed in the radio band, using the vlbi technique. the nature and properties of the emitting region as related to the flow are still scarcely known. jets can be interpreted as flows because the larmor radius of particles is very small when compared to the spatial scales of the problem studied (blandford & rees 1974). this interpretation has different implications: such systems, which are composed of magnetic fields and particles propagating along the jet channel are expected to host the growth of different hydrodynamical and/or magnetic instabilities. the instabilities have been claimed to be the cause of many of the structures observed (knots, bendings, helices; see e.g., reviews in hardee 2006, 2011, perucho 2012). the instabilities are waves triggered by any external or internal perturbation that grow in amplitude with distance, as they are advected with the flow. thus, if instabilities do really grow in jets, we would expect to observe wave-like structures and motions in extragalactic jets. perucho et al. (2012a) used observations of the jet in s5 0836+710 at different frequencies and epochs and showed that theridge line of this jet behaves as expected if it is interpreted as a pressure wave. aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa the luminous quasar s5 0836+710 is at a redshift z = 2.16 and hosts a powerful radio jet extending up to kiloparsec scales (hummel et al. 1992). at this redshift, 1 mas ' 8.4 pc (see, e.g., mojave database). vlbi monitoring of the source showed kink structures (krichbaum et al. 1990) and yielded estimates of the bulk lorentz factor γj = 12 and the viewing angle αj = 3◦ of the flow at milliarcsecond scales (otterbein et al. 1998). the jet was observed at 1.6 and 5 ghz with vsop (vlbi space observatory program, a japanese-led space vlbi mission), and oscillations of the ridge-line were also observed (lobanov et al. 1998, 2006). these oscillations were interpreted as kelvin-helmholtz (kh) instabilities (lobanov et al. 2006, perucho & lobanov 2006). the source has also been monitored by the mojave program (lister et al. 2009). finally, it has been suggested that the lack of a collimated jet structure and hot-spot at the arcsecond scales is due to jet disruption, as indicated by the growth in the amplitude of the helical structure (perucho et al. 2012b). 265 http://dx.doi.org/10.14311/app.2014.01.0265 manel perucho 2 evidence for waves in the jet of s5 0836+710 here i summarize the evidence that points to the presence and development of waves in the jet of s5 0836+710, as given by perucho et al. (2012a). the different observations of the jet at different frequencies ranging from 1.6 ghz to 43 ghz, for a given epoch, give the same ridge-line positions within the errors. it needs to be determined whether the small scale ripples in the ridge-line are caused by uv-coverage effects at the different epochs/frequencies, which is plausible. however, the large scale motion coincides for all frequencies and this can only happen if the observed structure corresponds to a physical one. in addition, high resolution images at 15 ghz confirm that the ridgr-line position does not necessarily coincide with the centre of emission of the radio jet, as expected from an asymmetric pressure maximum triggering a helical instability in the jet. within the first milliarcsecond, high resolution images of the jet allowed the measurement of transversal displacements, which show a clear wave-like oscillation pattern with distance. the obtained ridge-line velocities at these scales are superluminal, but this is due, according to the authors, to: 1) relativistic wave velocities, 2) a small-scale oscillation of the core, very difficult to detect in this distant source, but observed in others (e.g., m 81, see mart́ı-vidal et al. 2011), and 3) displacements of the ridge-line being smaller than the errors in the determination of the position. the amplitude of the oscillation grows along the jet propagation direction, which can be due to the coupling to an unstable mode of the kh or current-driven (cd) instabilities. the evidence collected in the previous paragraph led the authors to conclude that the ridge-line of the jet is tracing a pressure-maximum of a wave within the jet, and that even the radio-core is subject to the oscillation produced by the wave. recently, cohen et al. (2013) presented new results of the nearby source bl lac (in the conference the innermost regions of relativistic jets and their magnetic fields, held in granada 10th-14th june, 2013), using long-term monitoring of the source at 15 ghz (cohen et al. in preparation), which confirm that the ridge-line of emission within the jet shows a wave-like behavior. in this case, the authors claim that the oscillation of the ridge-line is caused by a strong magnetic field. in addition, grossberger et al. (2012, and private communication) have followed the ejection and evolution of components in the broad-band radiogalaxy 3c 111 and shown that the position angles of the injected components vary with time and fill a wide channel after several years. this channel is wider than the observed jet at each epoch. this result also points to an oscillation of the bright regions of the jet (necessarily the same as that identified with the ridge-line in other works) within a wide channel in which the observed radio jet is embedded. 3 discussion 3.1 on the nature of the waves given the evidence found for the presence of waves in jets, we can now ask which kind of waves these are and what is their origin. regarding the latter, it is still unclear whether the waves show any regular periodical pattern or not, but the long-term oscillations could be attributed to periodical or quasi-periodical processes in the injection region, such as precession. shorter-term oscillations can be attributed to perturbations triggered by asymmetries in the jet itself or in the ambient medium, lateral winds or entrainment of clouds that rotate around the galactic centre from a side of the jet, for instance. however, all these processes are extremely difficult to detect and isolate. as far as the nature of the waves, there is (already) open debate as to whether they are magnetic or purely hydrodynamical waves. the claim that the waves correspond to magnetic instabilities is based on the fact that the observed radio components seem to follow the ridgeline as they propagate (cohen et al., in preparation): if the jet ridge line is identified with a kink instability in the magnetic field of a magnetically dominated jet, the flow would be forced to follow this kink. however, let us consider the possibility that the ridge-line corresponds to the pressure maximum produced by the kink (e.g., mizuno et al. 2011). this implies that within a given jet cross-section we can find a high-pressure region and a low-pressure one. in this case, the passage of a ballistic component following a straight path along the jet, could enhance the emission of the whole jet, but the effect would be more visible on the ridge-line (high-pressure region). as a result, any radio-component fitted to the region would be centered there. perucho et al. (2006) showed, via numerical simulations, that an asymmetric perturbation at the base of a relativistic jet propagates downstream and naturally creates a pressure maximum helix, which can eventually distort the jet flow and force it into a helical path. in this view, injected components associated with perturbations in flow density at injection that produce shock waves, end up following the helical path when they have dissipated most of their kinetic energy. previous to this, they propagate ballistically. a long-standing cross-debate about jet physics is related to the ballistic nature of the flow in apparently helical radio jets. this point again introduces a difference in behavior, but still not a way to distinguish, 266 first hints of pressure waves in a helical extragalactic jet: s5 0836+710 between a kinetically dominated flow and a magnetically dominated one. if the jet is strongly magnetized, the flow will always follow the field lines and will thus have a twisted path if the field is helical, whereas in the kinetically dominated case, the flow could follow a straight stream-line even if the radio-jet shows a helical structure (because it corresponds to the bright high pressure region within a wide channel). this is true while the amplitude of the instability is not large, but when it grows to nonlinear values, the jet flow starts to deviate from its original trajectory by the pressure gradients within the jet (see, e.g., nakamura & meier 2004, mizuno et al. 2011, perucho et al. 2005, 2006 for the different cases). thus, a purely hydrodynamical jet could produce observational patterns similar to a strongly magnetized jet, and further work is needed to understand the physics behind the ridge-line dynamics. moreover, even if no perturbations are injected in the jet, i.e., the flow is close to stationary, then the fitted components in the radio jet would all be associated with the ridge-line (see, e.g., hardee 2003), just because it corresponds to the brightest regions within the jet. if the ridge-line corresponds to a propagating wave, then the fitted-components would also move along the jet and, of course, on top of the ridge-line. it is thus crucial to disentangle whether the components correspond to waves propagating along the jet and whether or not they follow the ridge-line for understanding its nature. 3.2 the case of s5 0836+710 the wave associated with the ridge-line could couple to a growing instability, as indicated by the growth in amplitude of the helix with distance in the case of s5 0836+710. it is difficult to distinguish between current-driven (cd) instability and kh instability modes, both being solutions to the linearized relativistic, magnetized flow equations and both being possible sources of helical patterns. recent work on cd instability by mizuno et al. (2011) shows that a helical kink propagates with the jet flow if the velocity shear surface is outside the characteristic radius of the magnetic field, i.e., approximately the radius at which the toroidal magnetic field is a maximum. if the observed pattern corresponds to a cd kink instability, the observed transversal oscillation in the jet of 0836+710 requires that the kink be moving with the flow and implies that the transversal velocity profile is broader than the magnetic field profile, i.e., the velocity shear surface lies outside the characteristic radius of the magnetic field. thus, in this case the jet would have a magnetized spine surrounded by a particle dominated outer region and a cd kink in the spine would move with the flow.. the measurement of the opening angle reveals a very similar value at all frequencies for which we have significant measurements, which would favor the radio-jet tracing the whole channel, because the coincidence in the opening angles is less probable in the case of the radio jet corresponding to different regions across the jet. thus, it seems that the radio emission is generated in the same region across the jet at all frequencies and that the jet flow indeed follows a helical path. as stated above, non-ballistic motion is compatible with a magnetically dominated flow, but also with the development of kh-instability modes to large amplitude, because they can dissipate kinetic energy into internal energy and decelerate the flow. it has been reported that the development of the helical kh instability can force the jet flow into a helical path, albeit different from the helicity of the pressure maximum, the latter showing larger amplitude (hardee 2000). the larger the lorentz factor of the flow and the shorter the wavelength of the mode, the more different is the helicity of the flow as compared to that of the pressure maximum. this can be understood in terms of the larger inertia of the jet flow with increasing lorentz factor (hardee 2000, perucho et al. 2005). in summary, the fact that the flow is non-ballistic when the amplitude of the perturbation is large does not necessarily imply that the flow is magnetically dominated and that the waves are magneto-sonic waves. 3.3 the core one other important consequence of the work presented in perucho et al. (2012a) is the possible small-scale oscillation of the radio-core around an equilibrium position. this effect has been reported by mart́ı-vidal et al. (2011) for m 81, using the phase-referencing technique, and it could have important implications for the nature of the radio-core. 4 conclusions it is possible to associate the ridge-line of emission of helical jets to wave patterns tracing a (thermal or magnetic) pressure maximum produced by the growth of instabilities. the core of the jet could well also be oscillating and this motion should be tested and corrected in order to study the parsec-scale kinematics of the radio-components in jets. further combined observational and theoretical studies like this one are required to get more information on the nature of the growing instability and on the properties of the jet flow. acknowledgement i acknowledge financial support by the spanish “ministerio de ciencia e innovación” (micinn) grants aya2010-21322-c03-01 and aya2010-21097-c03-01. i 267 manel perucho also acknowledge p.e. hardee, j.m. mart́ı, m. cohen, and d.l. meier for interesting discussions on the topic. references [1] blandford, r.d., rees, m.j.: 1974, mnras 169, 395. doi:10.1093/mnras/169.3.395 [2] cohen, m.h., et al.: 2013, in the innermost regions of relativistic jets and their magnetic fields. http://jets2013.iaa.es/sites/jets2013.iaa. es/files/imagecache/granada may2013 v3.ppt [3] grossberger, c., kadler, m., wilms, j., et al.: 2012, acpol 52, 18. [4] hardee, p.e.: 2000, apj 533, 176. doi:10.1086/308656 [5] hardee, p.e.: 2003, apj 597, 798. doi:10.1086/381223 [6] hardee, p.e.: 2006, in proceedings of relativistic jets: the common physics of agn, microquasars and gamma-ray bursts, eds.: p.a. hughes and j.n. bregman, aip conference proceedings 856, 57. [7] hardee, p.e.: 2011, in proceedings of iau symposium 275: jets at all scale, eds.: g. romero, r. sunyaev and t. belloni, iau conference series 275, 41. [8] hummel, c.a., muxlow, t.w.b., krichbaum, t.p., et al.: 1992, a&a 266, 93. [9] krichbaum, t.p., hummel, c.a., quirrenbach, a., et al.: 1990, a&a 230, 271. [10] lister, m., aller, h.d., aller. m.f., et al.: 2009, aj 137, 3718 [11] lobanov, a.p., krichbaum, t.p., witzel, a., et al.: 1998, a&a 340, 60. [12] lobanov, a.p., krichbaum, t.p., witzel, a., zensus, j.a.: 2006, pasj 58, 253. [13] mart́ı-vidal, i., marcaide, j.m., alberdi, a.: 2011, a&a 533, a111. [14] mizuno, y., hardee, p.e., nishikawa, k.-i.: 2011, apj 734, 19. doi:10.1088/0004-637x/734/1/19 [15] nakamura, m., meier, d.l.: 2004, apj 617, 123. doi:10.1086/425337 [16] otterbein, k., krichbaum, t.p., kraus, a., et al.: 1998, a&a 334, 489. [17] perucho, m.. mart́ı, j.m., hanasz, m.: 2005, a&a 443, 863. [18] perucho, m., lobanov, a.p., mart́ı, j.m., hardee, p.e.: 2006, a&a 456, 493. [19] perucho, m., lobanov, a.p.: 2007, a&a 469, l23. [20] perucho, m.: 2012, in proceedings of high energy phenomena in relativistic outflows iii (heproiii), eds. j.m. paredes, m. ribó, f.a. aharonian and g.e. romero, ijmps 8, 241. [21] perucho, m., kovalev, y.y., lobanov, a.p., hardee, p.e., agudo, i.: 2012a, apj 749, 55. doi:10.1088/0004-637x/749/1/55 [22] perucho, m., mart́ı-vidal, i., lobanov, a.p., hardee, p.e.: 2012b, a&a 545, a65. discussion sergio colafrancesco: is there any radio polarization measurement that can be used to better describe the helical structure of the jet and, if so, what are the constraints that can be set on the acceleration mechanisms at the base of the jet? manel perucho: krichbaum et al. (1990) reported polarization measurements and claim that the field is aligned with the jet axis. if their interpretation is correct, it would be difficult to attach the helical morphology of the jet to the magnetic field. nevertheless, new polarization measurements should be performed to check whether the magnetic field plays any crucial role in the generation of the helical structure. regarding your second question, it is difficult to observe the acceleration region in this jet, but we have evidences of component acceleration in other agn jets, included in the mojave sample. herman marshall: if the perturbations are unstable and disrupt the jet at kpc scales, how do you explain that most jets seem to propagate to tens or hundreds of kpc? manel perucho: in that sense, this is a unique object, because it is classified as an frii in terms of its radio luminosity, but its morphology denies (see perucho et al. 2012b). this is one of the reasons why we think that, in this sole case, the jet could be destroyed by the growth of the instability, unlike all other known frii jets for which we can observe the kiloparsec scale structure. 268 http://dx.doi.org/10.1093/mnras/169.3.395 http://dx.doi.org/10.1086/308656 http://dx.doi.org/10.1086/381223 http://dx.doi.org/10.1088/0004-637x/734/1/19 http://dx.doi.org/10.1086/425337 http://dx.doi.org/10.1088/0004-637x/749/1/55 introduction evidence for waves in the jet of s5 0836+710 discussion on the nature of the waves the case of s5 0836+710 the core conclusions 230 acta polytechnica ctu proceedings 2(1): 230–233, 2015 230 doi: 10.14311/app.2015.02.0230 life after eruption: best of 2009–2013 c. tappert1, a. ederoclite2, l. schmidtobreick3, n. vogt1 1departamento de f́ısica y astronomı́a, universidad de valparáıso, chile 2centro de estudios de f́ısica del cosmos de aragón, teruel, spain 3european southern observatory, santiago, chile corresponding author: claus.tappert@uv.cl abstract from our ongoing survey to study the post-nova population we present details on the four objects v728 sco, ar cir, v972 oph and x cir. keywords: cataclysmic variables novae individual: v728 sco ar cir v972 oph x cir. 1 introduction in 2009 we have begun a survey on post-novae that had erupted before 1980, in order to address the lack of identified post-nova systems. scope, methods and the current state of the survey are presented by ederoclite et al. elsewhere in this volume. in the present article we take a closer look at four objects from our survey with peculiar properties that deserve further investigations. a detailed analysis of three of these systems, v728 sco, ar cir and v972 oph, can be found in tappert et al. (2013a, b), and we here only briefly summarize the results. 2 v728 sco the cv corresponding to nova sco 1862 was recovered as a v = 18.5 mag object roughly 2 arcmin north-west of its suspected position (tappert et al. 2012). its spectrum presents – for an old nova – unusually strong emission lines. photometric time-series revealed the object to be eclipsing with an orbital period porb = 3.32 h. this places v728 sco within the regime of the sw sex type systems, cvs that are characterized by very high mass-transfer rates ṁ (e.g., rodŕıguez-gil et al. 2007a, b; see also schmidtobreick & tappert, this volume). furthermore, old novae in general are expected to run high ṁ due to irradiation of the secondary star by the eruption-heated white dwarf (kovetz et al. 1988), and in general this is confirmed by observations (iben et al. 1992). for example, the old nova rr pic, with porb = 3.48 h, does indeed share the characteristics of an sw sex star (schmidtobreick et al. 2003a, 2008). v728 sco, on the other hand, appears to have a lower ṁ than expected. evidence for this does not only stem from the spectroscopic appearance, but also from the long-term behaviour that shows the system to vary between a low and a ∼1.5 mag brighter state. while the data coverage is still very sparse, it looks like this brighter state is in general short-lived and bears resemblance to the outbursts of dwarf novae. analysis of one eclipse that was observed in the low state provided an estimate of the radius of the eclipsed source to r = 0.09 r�, which is significantly larger than any feasible solution for a white dwarf. we interpret this result as the presence of a hot inner disc, such as has been invoked by schreiber et al. (2000) to explain the low-amplitude high-frequency outbursts that are observed in some post-novae and nova-like stars (e.g., honeycutt et al. 1998), and would also fit the longterm behaviour of v728 sco. our eclipse data provide the first direct evidence for the presence of such disc. 3 ar cir ar cir was a comparatively slow nova that erupted in 1906. it was recovered by duerbeck & grebel (1993). time-series spectroscopy taken by us on three consecutive nights in 2012 yielded porb = 5.14 h. the spectrum shows a moderately strong hα emission line on a red continuum, the latter being very probably entirely caused by interstellar reddening rather than due to the contribution of the secondary star. the r-band brightness shows a clear modulation with porb and an amplitude of ∼0.3 mag, indicating that the inclination is comparatively high. the hα line profile shows an intriguing variability in that at certain orbital phases, and strongest at phase 0.5 (corresponding to the maximum of the r-band light curve), a p cyg component becomes visible in the form of a high-velocity blue absorption and a red emission component. we interpret 230 http://dx.doi.org/10.14311/app.2015.02.0230 life after eruption: best of 2009–2013 this as an optically thick outflow and determine a rough estimate of the projected outflow velocity to 700 ± 100 km/s. 4 v972 oph time-series spectroscopic observations of this post-nova (nova oph 1957) yield an orbital period porb = 6.75 h thus making it a member of the minority of novae with comparatively long orbital periods (see ederoclite et al., this volume). examination of the profile of the hα emission line reveals an additional emission at ∼6580 å. this component moves roughly parallel to the main hα component. this practically excludes scenarios of an additional hα component from the secondary star or from the hot spot, as well as a nebular line like [nii] λ6584. the absence of an absorption line blueward of hα lets an origin in an outflow, such as assumed for ar cir, also appear unlikely. instead we tentatively identify that line with the cii λλ6578/6583 doublet. to our knowledge, this is the first time that this line is observed in a cv in this form (it had also be identified as originating in the outflowing material of the helium nova v445 pup; iijima & nakanashi 2008). while carbon emission has been observed before in some post-novae (e.g., schmidtobreick et al. 2003b, bianchini et al. 2012), this concerns mostly the blue part of the spectrum. unfortunately the available spectra of v972 oph by zwitter & munari (1996) and ringwald et al. (1996) that cover that range have too low s/n to confirm the presence of such lines in v972 oph. the weakness of the hα emission line (which has an equivalent width of 2 å) certainly helped in the detection of the cii line. this raises the question whether this line is not more frequently present in cvs, but hidden away in the wings of a stronger hα emission. a quick examination of seven novae finds three more systems with likely excess emission at ∼6580 å. as a caveat we note that this list includes ar cir, where we find an outflow as a more likely explanation. however, the other systems lack an accompanying blue emission or absorption component that would suggest such phenomenon as the origin. 5 x cir the eruption of nova cir 1927 was reported by becker (1929). the aftermath of the nova is little studied. duerbeck (1987) classifies it as a slow nova with t3 = 170 d. woudt & warner (2002) identified a possible candidate for the post-nova due to short-term variability, but a spectrum taken by mason & howell (2003) revealed that object to have a reddish continuum without emission lines. our ubvr photometry from may 2009 (fig. 1, top) allows for a number of candidates based on colour and distance from the reported coordinates. in 2012 we obtained spectroscopic data for five of them with the mos mode of fors2 at the eso-vlt (ut1). the post-nova was confirmed to be an object roughly 1.2 arcmin north-west of its suspected position. the spectrum shows comparatively strong emission lines (equivalent width of hα ∼30 å) of the balmer and hei series (fig. 1, bottom). 0.0 0.5 1.0 1.5 v −r [mag] −0.5 0.0 0.5 1.0 1.5 u − b [m ag ] 4000 5000 6000 7000 8000 wavelength [å] 5 10 fl ux [1 0− 17 er g cm −1 s− 1 å −1 ] figure 1: top: colour-colour diagram of the field (4.5×4.5 arcmin) of x cir. the confirmed post-nova is marked by a circle. bottom: low-resolution spectrum of the post-nova. while being situated in a promising place in the colour-colour diagram, the later confirmed post-nova was not our best candidate, but in fact rather represented the very bottom of our list. the reason for this is that our photometric data showed a larger point-spread function (psf) than for the other stars in the field, indicating an extended object. this was later confirmed by the 2d spectroscopic data. an examination of the latter showed that the emission lines appear to be confined to the one part of the spectroscopic psf that corresponds to the eastern part of the photometric psf. our preliminary conclusion is therefore that the extension is not due to a shell, but that this is an unresolved (< 0.5 arcsec) visual binary. the spectrum of x cir presents a prominent heii 4686 line, and even, albeit much weaker, heii 5411 (fig. 2). such strong presence of heii is often a signature of a magnetic system. on the other hand, the hei lines are distinctively double-peaked with a deep central valley, indicating the presence of an accretion disc. a possible explanation is that x cir is an intermediate polar. the heii 5411 line is also a transient feature. in our three spectra, taken with a time separation of ∼3 h between the first and the second, and ∼24 h between the second and the third, the line is strongest in the first (which is the one in fig. 2), not detected in 231 c. tappert et al. the second, and weakly present in the third spectrum (fig. 1). since the individual integration times amount to 15 min, this could be an orbital effect. the heii lines and the balmer lines are single-peaked, suggesting that at least a major part of their profile has an origin different from that of hei. 5400 5600 5800 wavelength [å] 1.0 1.2 n or m al is ed fl ux heii hei? ? 7000 7200 7400 wavelength [å] hei ? figure 2: close-up on the spectrum of x cir. the label mark the line identifications; a question mark is used for the three unidentified lines. note the deep central valleys in the double-peaked profile of the two present hei lines. we furthermore find three emission lines that we have not yet been able to identify (fig. 2). taking into account the observed shift of the identified lines, we estimate their corresponding rest wavelengths to ≥5678, ≥5805, ≥7233, respectively. examining lists of spectral lines (e.g., coluzzi 1999) does not present obvious suspects for an identification. we note that the profiles are also single-peaked, so that these probably do not originate exclusively in the accretion disc. we derive the ’quiescent’ brightness of x cir in the v filter to 19.3 mag. the reported photographic value of the eruption maximum is 6.5 mag (duerbeck 1987). assuming that the effect of the difference between the filters is < 0.5 mag, this yields an eruption amplitude > 12 mag. this places it a bit off the fit to the rateof-decline / eruption-amplitude relation (ederoclite et al., this volume; note that x cir is not yet included in their figure), but still within the larger scatter. 6 summary we have presented four post-novae with apparently peculiar properties. v728 sco is one of the few objects that defies the general trend that post-novae are cvs with very high mass-transfer rates (ṁ). finding such objects will be very important to answer the question whether post-novae in general have high ṁ as a consequence of the nova eruption, or if high ṁ cvs are just more likely to erupt as a nova. v728 sco is furthermore deeply eclipsing, which presents an excellent and rarely found opportunity to determine accurate system parameters of a nova. ar cir, on the other hand, while with more than 100 yr having passed since its eruption can already be counted among the ’old’ old novae, apparently still drives such high ṁ that it causes an outflow of material that is visible in the optical spectrum. since our data only covers the spectral range next to hα it would be interesting to examine other spectral ranges and lines for this phenomenon to gain more information on the outflow. in v972 oph we detect an unusual emission line that we attribute to carbon. we argue that such line could in principle be more frequent in cvs because it can be easily hidden in the wings of the usually strong hα emission that is exceptionally weak in v972 oph. the currently available data on the blue spectral range has too low s/n to examine it for other tracers of the presence of carbon in v972 oph, thus a higher quality spectrum of that region would be most welcome. the carbon line in v972 oph moves parallel to hα and thus likely originates in the accretion disk or the bright spot, or on the secondary star itself. in any case it represents material from the secondary. it was either accreted from the expelled material during the nova eruption or produced in the secondary, which then could not have formed as a late-type main-sequence star, but must have undergone some nuclear evolution prior to its semi-detached cv phase. thus, the study of carbon in (not only) post-novae bears relevance for our understanding on cv evolution. the results on x cir are still somewhat preliminary. the strong balmer lines argue for a comparatively low ṁ , the prominent presence of heii could point to a magnetic white dwarf, but at least is evidence for a hot region somewhere in the system, while the double-peaked hei profiles indicate the presence of an accretion disc. a number of emission features yet remain to be identified which means that these lines are at least very rarely observed in cvs. finally, the spectral appearance, the transient nature of heii 5411 and the double-peaked hei lines suggest a high inclination. there is therefore a good chance that the orbital period will be accessible comparatively easily via time-series photometry. we hope that this presentation motivates further studies of those systems. while their properties may appear peculiar, they might not be exclusive to those objects. because the number of detailed studies on post-novae is still fairly small, we cannot yet be certain if they are more representative of the rule or the exception. acknowledgement this research was supported by fondecyt regular grant 1120338 (ct and nv). ae acknowledges support by the spanish plan nacional de astrononomı́a y astrof́ısica under grant aya2011-29517-c03-01. 232 life after eruption: best of 2009–2013 references [1] becker, f.: 1929, astron. nachr., 237, 71. [2] bianchini, a., saygac, t., orio, m., et al.: 2012, a&a, 539, a94. [3] coluzzi, r.: 1999, vizier online data catalog, 6071. [4] duerbeck, h.w.: 1987, space sci. rev., 45, 1. doi:10.1007/bf00187826 [5] duerbeck, h.w., grebel, e.k.: 1993, mnras, 265, l9. doi:10.1093/mnras/265.1.l9 [6] honeycutt, r.k., robertson, j.w., turner, g.w.: 1998, aj, 115, 2527. [7] iben, jr., i., fujimoto, m.y., macdonald, j.: 1992, apj, 384, 580. doi:10.1086/170900 [8] iijima, t., nakanashi, h.: 2008, a&a, 482, 865. [9] kovetz, a., prialnik, d., shara, m.m.: 1988, apj, 325, 828. [10] mason, e., howell, s.b.: 2003, a&a, 403, 699. [11] ringwald, f.a., naylor, t., mukai, k.: 1996, mnras, 281, 192. [12] rodŕıguez-gil, p., schmidtobreick, l., gänsicke, b.t.: 2007a, mnras, 374, 1359. doi:10.1111/j.1365-2966.2006.11245.x [13] rodŕıguez-gil, p., gänsicke, b.t., hagen, h.-j., et al.: 2007b, mnras, 377, 1747. doi:10.1111/j.1365-2966.2007.11743.x [14] schmidtobreick, l., tappert, c., saviane, i.: 2003a, mnras, 342, 145. doi:10.1046/j.1365-8711.2003.06523.x [15] schmidtobreick, l., tappert, c., bianchini, a., mennickent, r.e.: 2003b, a&a, 410, 943. [16] schmidtobreick, l., papadaki, c., tappert, c., ederoclite, a.: 2008, mnras, 389, 1345. doi:10.1111/j.1365-2966.2008.13641.x [17] schreiber, m.r. and gänsicke, b.t. and cannizzo, j.k.: 2000, a&a, 362, 268. [18] tappert, c., ederoclite, a., mennickent, r.e., et al.: 2012, mnras, 423, 2476. [19] tappert, c., vogt, n., schmidtobreick, l., et al.: 2013a, mnras, 431, 92. [20] tappert, c., schmidtobreick, l., vogt. n., ederoclite, a.: 2013b, mnras, in press (arxiv 1302.5570). [21] woudt, p.a., warner, b.: 2002, in: classical nova explosions, hernanz, m. & josé, j., eds., aip conf. proc., vol. 637, p.532. [22] zwitter, t., munari, u.: 1996, a&as, 117, 449. discussion christian knigge: a comment on v728 sco: i think this is totally consistent with being a ”normal” sw sex star. it has the same v-shaped eclipse in the high state (and may be self-occulting), and several sw sex stars, like dw uma, also show high-state / lowstate behaviour. claus tappert: it is true that the high-state eclipse very much resembles that of sw sex stars. but i would argue that the other properties do not. we do not see any high-velocity wings (this might arguably be due to the quality of the spectra), and the emission lines are distinctively double-peaked. we need more long-term data to see if the system spends more time in low or high state, but so far my best bet would be that v728 sco is in a somewhat transitory phase, not quite sw sex, not quite dwarf nova. 233 http://dx.doi.org/10.1007/bf00187826 http://dx.doi.org/10.1093/mnras/265.1.l9 http://dx.doi.org/10.1086/170900 http://dx.doi.org/10.1111/j.1365-2966.2006.11245.x http://dx.doi.org/10.1111/j.1365-2966.2007.11743.x http://dx.doi.org/10.1046/j.1365-8711.2003.06523.x http://dx.doi.org/10.1111/j.1365-2966.2008.13641.x introduction v728 sco ar cir v972 oph x cir summary acta polytechnica ctu proceedings doi:10.14311/app.2017.7.0072 acta polytechnica ctu proceedings 7:72–75, 2017 © czech technical university in prague, 2017 available online at http://ojs.cvut.cz/ojs/index.php/app semi–automated assessment of micromechanical properties of the metal foams on the cell-wall level nela krčmářováa,b,∗, jan šleichrta, tomáš doktora, daniel kytýřa,b, ondřej jiroušeka a czech technical university in prague, faculty of transportation sciences, konviktská 20, prague 1, 155 00, czech republic b the institute of theoretical and applied mechanics as cr, prosecká 76, prague 9, 190 00, czech republic ∗ corresponding author: krcmarova@fd.cvut.cz abstract. metal foams are innovative porous material used for wide range of application such as deformation energy or sound absorption, filter material, or microbiological incubation carrier. to predict mechanical properties of the metal foam is necessary to precisely describe elasto–plastic properties of the foam on cell–wall level. indentation with low load is suitable tool for this purpose. in this paper custom designed instrumented microindentation device was used for measurement of cell-wall characteristics of two different aluminium foams (alporas and alcoras). to demonstrate the possibility of automated statistical estimation of measured characteristics the device had been enhanced by semi-automatic indent positioning and evaluation procedures based on user-defined grid. vickers hardness was measured on two samples made from alporas aluminium foam and one sample from alcoras aluminium foam. average vickers hardness of alporas foam was 24.465 hv1.019 and average vickers hardness of alcoras was 36.585 hv1.019. keywords: metal foam, vickers hardness, cell-wall, indentation under low loads. 1. introduction metal foams are biomimetic porous materials with cellular inner structure that find wide range of applications from deformation energy absorption to noise attenuation, where their very high specific stiffness greatly improves overall effectiveness of constructions [1, 2]. homogenization approach has been proposed as a method for prediction of their mechanical properties on both cell-wall level and at macroscale [3– 6]. however for calculation of macroscopic (effective) mechanical properties by homogenization, mechanical characteristics at the lower level of the foam’s hierarchical microstructure (i.e. cell-wall level) have to be assessed with high precision and reliability. here microindentation is a suitable tool for assessment of required elasto-plastic material properties (for cellwall thicknesses from few hundreds of microns) with the possibility for extension to statistical estimation when automated indents’ positioning and evaluation procedures are introduced. 2. materials and methods 2.1. specimen description and preparation closed-cell aluminium foams with similar compound al 97.0 %, ca 1.5 %, ti 1.5 % (measured using energydispersive x-ray spectroscopy) pore size 2 − 4 mm and wall thickness 100 − 200µm sales denominated as alporas® (shinko wire co., ltd., japan) and sample 1 sample 2 sample 3 material alporas alcoras alporas width 48 mm 41 mm 44 mm height 21 mm 21 mm 23 mm thickness 10 mm 11 mm 13 mm table 1. dimensions of the specimen alcoras® (alcarbon, germany) were subjected for the testing [7]. inner structure of aluminium foam is depicted in fig. 1. figure was obtained using scanning electron microscopy mira ii (tescan, czech republic). region of interest was chosen according to avoid large pores and structural defect. the structural defects were mainly connected with alcoras sample. from the delivered slabs cuboids with minimal thickness of 12 mm (to ensure sample integrity) were sectioned using water cooled oscillating diamond saw (isomet 1000, buehler gmbh, germany). dimensions of each specimen are listed in tab. 1. low cutting speed 3 mm · min−1 minimised surface damage. samples were embedded into mounting compound (varikleer, buehler gmbh, germany). grinding and polishing procedure employing silicon carbide grinding discs (320, 800, 1200, 4000 grains per square inch) and diamond suspension (1µm) was performed to remove 1 mm surface layer which could be influenced by sectioning and to obtain plan-parallel faces 72 http://dx.doi.org/10.14311/app.2017.7.0072 http://ojs.cvut.cz/ojs/index.php/app vol. 7/2017 assessment of micromechanical properties of metal foams figure 1. inner structure of aluminium foam obtained by scanning electron microscopy. with minimal surface roughness necessary for proper indentation. 2.2. indentation testing 2.2.1. indentation device description indentation testing was performed using custom designed indentation device developed and constructed on department of mechanics and material fts ctu. indentation device is suitable for hardness measurement by low loads from 10 n up to 100 n. limitation of the device by applying lower loads are: i) inaccuracy of the load recorded by load–cell and ii) insufficient accuracy of the control of the indentation axis. device consists of three independent motorised axes. two of them are designed for precise positioning of the specimen with accuracy 10µm. third axis is indentation axis equipped by load–cell with positioning accuracy 1.5µm. device is controlled by load or displacement using gnu/linux software system linuxcnc with custom made graphical user interface. device is equipped with ccd camera (manta g–504b, avt, germany) with a resolution of 2452 × 2056 px attached to a light microscope (navitar imaging, inc., usa) that provided a magnification of up to 24 ×. the acquisition of the projections was controlled by in-house-built opencv based plug-in integrated to control software [8]. indentation device is depicted on fig. 2. this camera is used for the indent place estimation as well as to make a photo of the indent that is than used for hardness measurement. due to high resolution ccd camera, calibrated indentation tip alignment and precise positioning of the specimen, indentation of the selected location of the specimen’s surface is allowed with high accuracy. for verification of the device accuracy of hardness measurement indentation in calibration hardness plate was performed. device overall error estimated by calibration meafigure 2. experimental setup. surement was 1.3 %. maximal measurement error for indentation devices is according to standard set to 3 %. it can be advantageously used for measuring hardness of metal foams because indentation is possible only in places with a sufficient width and depth of the specimen, which means joints of the walls. 2.2.2. experiment procedure description suitable places for indentation on the sample surface were identified using ccd camera and subsequently indentation was automatically performed at these selected locations. after the indentation image data of each indent were captured. the detail of an indent is depicted on fig. 3. indentation load was set to 10 n and this value was reached in 10 s. first indent was used as a testing indent to find indentation speed to respect the condition of reaching maximal force value in 10 s. this indent was removed from data set and hardness value of this indent was not evaluated. all other indents were displacement controlled with given speed identified by the testing indentation. course of the indentation was therefore: i) 10 s loading up to 10 n, ii) 10 s holding on the indentation load and iii) unloading with the same speed as the loading phase. on each specimen was created a series of about 50 indents on the interconnections of the walls. the foam microarchitecture with poresize 2 − 4 mm ensure avoiding that the plastic zones of the indents don’t affect each other. because of size of interconnection and limited minimal indentation force normative 2.5× diameter distance from sample edge can’t be meet during experimental procedure. size of thus generated imprints of the indenter was about 270µm and their 73 n. krčmářová, j. šleichrt, t. doktor et al. acta polytechnica ctu proceedings figure 3. right: marked area of the imprint obtained by semi–automatic evaluation method. depth was about 39µm. 2.2.3. evaluation of the experimental data positioning accuracy of the indentation axis does not allow automated evaluation of the hardness from the full position-load curve using oliver–pharr method [9]. therefore hardness was assessed using traditional method by measuring the imprint dimensions and evaluated as vickers hardness: hv = f a ≈ 0.1819f d2 , (1) where f is indentation load in n, a is resulting indentation area of the imprint of the indenter in mm2 and d is average length of the diagonal of the imprint of the indenter in mm. as indentation load is taken the maximal force value registered by the load– cell. length of the diagonals in pixels was measured from the image of each imprint of the indenter using semi–automatic method in matlab software. image of the calibration pattern was used for conversion of the diagonal length in pixels to millimetres. highlighted imprint of the indenter is depicted on fig. 3 3. results elasto-plastic material properties of two types of aluminium foam was measured by vickers hardness. indentation was performed on cell walls and their connections to ensure sufficient place for indentation. in order to get precise information about micromechanical properties of the foam about 50 indents were carried out at each of the three specimens. due to foam nature of specimens the under surface area can be formed by cavity which is not visible on the surface and thus some indents were deformed and these vickers hardness values are omitted from the data set. those indents are usually easily recognised by highly deformed shape of indents. success of the indention was about 60 %. hardness values calculated using equation 1 are listed in tab. 2. sample 1 sample 2 sample 3 material alporas alcoras alporas indent 1 24.630 41.507 18.149 indent 2 33.008 35.490 29.839 indent 3 25.204 36.360 24.817 indent 4 23.027 32.990 23.672 indent 5 21.665 31.514 20.633 indent 6 22.818 34.447 22.967 indent 7 22.211 30.213 23.779 indent 8 24.547 30.254 16.826 indent 9 35.845 49.110 15.233 indent 10 33.856 47.130 13.301 indent 11 27.067 45.138 15.093 indent 12 33.404 28.928 24.319 indent 13 25.699 38.507 33.361 indent 14 21.559 24.547 13.418 indent 15 25.510 41.085 25.044 indent 16 27.996 37.927 16.167 indent 17 32.651 42.484 18.410 indent 18 29.958 30.323 19.584 indent 19 31.472 32.506 24.322 indent 20 21.663 43.721 21.050 indent 21 23.802 34.975 24.192 indent 22 30.054 32.557 31.487 indent 23 34.450 28.754 15.765 indent 24 29.011 45.065 26.474 indent 25 21.114 33.645 15.346 indent 26 40.693 24.999 indent 27 39.462 26.053 indent 28 35.017 indent 29 39.386 indent 30 33.801 avarage 21.641 36.585 27.289 table 2. values of measured vickers hardness by indentation load 10 n as can be seen from the hardness values summarised in tab. 2 vickers hardness of alporas aluminium foam is about 30 % lower than vickers hardness of alcoras aluminium foam. 3.1. comparative measurement to ensure reliability of the indentation process with non-standarded indents size digital microscope vhx74 vol. 7/2017 assessment of micromechanical properties of metal foams figure 4. indent surface inspection and indent profile reconstruction equipped by digital microscope. 5000 (keyence, japan) was employed for indented surface inspection and indent profile reconstruction (see fig. 4). indentation depth ≈ 40µm was obtained from indent profile using keyence proprietary software. this value corresponds to results of the analysis presented in 2.2.2. the profile is not significantly effected by surrounding embedding resin. 4. conclusions vickers hardness measurement was performed on three samples of aluminium foam on cell-wall level. one sample was aluminium foam alcoras and two samples were made of aluminium foam alporas. about 50 indents were created on each sample on the cell– walls or their interconnections with indentation load 10 n. success of the indentation was about 60 % other indents were deformed due to wrong position on the sample surface what was not possible to avoid prior the indentation. indentation was carried out using custom designed indentation device equipped with load–cell and ccd camera. camera was used for identification of appropriate place for indentation and for acquisition of image data of each imprint. vickers hardness was evaluated from image data using semi–automatic procedure in software matlab. average vickers hardness of the sample 1 (alporas foam) was 21.641 ± 5.496hv1.019, where hv is denotation for vickers hardness and value 1.019 indicates indentation load in kgf. average vickers hardness for sample 3, which was also made from alporas, was 27.289±4.731hv1.019. average vickers hardness of the third sample, which was made from alcoras foam, was 36.585±6.061hv1.019. vickers hardness of alcoras aluminium foam is about 30 % higher than vickers hardness of alporas aluminium foam. information about elasto–plastic properties of the aluminium foams alporas and alcoras on cell–wall level can be used for calculation of macroscopic mechanical properties by homogenization. acknowledgements the research was supported by the czech science foundation (research project no. 15-15480s), by czech technical university in prague (research project no. sgs15/225/ohk2/3t/16) and by institutional support rvo: 68378297. keyence is kindly acknowledged for providing of digital microscope vhx-5000. references [1] t. miyoshi, m. itoh, s. akiyama, a. kitahara. alporas aluminum foam: production process, properties, and applications. advanced engineering materials 2(4):179–183, 2000. doi:10.1002/(sici)15272648(200004)2:4<179::aid-adem179>3.0.co;2-g. [2] m. idris, t. vodenitcharova, m. hoffman. mechanical behaviour and energy absorption of closed-cell aluminium foam panels in uniaxial compression. materials science and engineering: a 517(1–2):37 – 45, 2009. doi:http://dx.doi.org/10.1016/j.msea.2009.03.067. [3] p. koudelka, t. doktor, j. valach, et al. effective elastic moduli of closed-cell aluminium foams homogenization method. 11th imeko tc15 youth symposium on experimental solid mechanics 2012 pp. 244–250, 2012. [4] s. li, d. qian, m. l. and. multiscale mechanics and mechanics of biological materials. [5] v. králík, j. němeček. micromechanical properties of porous material based on metal foam. chemicke listy 105(17):s672–s675, 2011. [6] m. geißendörfer, a. liebscher, c. proppe, et al. stochastic multiscale modeling of metal foams. probabilistic engineering mechanics 37:132–137, 2014. doi:10.1016/j.probengmech.2014.06.006. [7] t. fíla, d. kytýř, p. koudelka, et al. micro-mechanical testing of metal foam cell walls using miniature threepoint bending. key engineering materials 586:120–124, 2014. doi:10.4028/www.scientific.net/kem.586.120. [8] g. bradski. the opencv library. dr dobb’s: the world of software development 2000. http://www.drdobbs.com/open-source/the-opencvlibrary/184404319. [9] w. oliver, g. pharr. an improved technique for determining hardness and elastic modulus using load and displacement sensing indentation experiments. journal of materials research 7(06):1564–1583, 1992. 75 http://dx.doi.org/10.1002/(sici)1527-2648(200004)2:4<179::aid-adem179>3.0.co;2-g http://dx.doi.org/10.1002/(sici)1527-2648(200004)2:4<179::aid-adem179>3.0.co;2-g http://dx.doi.org/http://dx.doi.org/10.1016/j.msea.2009.03.067 http://dx.doi.org/10.1016/j.probengmech.2014.06.006 http://dx.doi.org/10.4028/www.scientific.net/kem.586.120 acta polytechnica ctu proceedings 7:70–73, 2017 1 introduction 2 materials and methods 2.1 specimen description and preparation 2.2 indentation testing 2.2.1 indentation device description 2.2.2 experiment procedure description 2.2.3 evaluation of the experimental data 3 results 3.1 comparative measurement 4 conclusions acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2016.6.0028 acta polytechnica ctu proceedings 6:28–33, 2016 © czech technical university in prague, 2016 available online at http://ojs.cvut.cz/ojs/index.php/app mapping the surroundings as a requirement for autonomous driving martin steininger∗, christoph stephan, christian böhm, fabian sauer, roland zink faculty of electrical, media and computer engineering, deggendorf institute of technology, 94469 deggendorf, germany ∗ corresponding author: martin.steininger@stud.th-deg.de abstract. motivated by the hype around driverless cars and the challenges of the sensor integration and data processing, this paper presents a model for using a xbox one microsoft kinect stereo camera as sensor for mapping the surroundings. today, the recognition of the environment of the car is mostly done by a mix of sensors like lidar, radar and cameras. in the case of the outdoor delivery challenge robotour 2016 with model cars in scale 1 : 5, it is our goal to solve the task with one camera only. to this end, a three-stage approach was developed. the test results show that our approach can detect and locate objects at a range of up to eight meters in order to incorporate them as barriers in the navigation process. keywords: microsoft kinect, mapping, 3d mapping, robotour, autonomous driving. 1. introduction in addition to the extensive launch of full electric vehicle (fev) autonomous driving is deemed to be a key research and development trend in the automotive industry. the entire industry is working on the topic and has already developed first solutions for automatic braking, line keeping (i.e. audi “active lane assist” or bmw “active assist”) or even autopilots for driverless cars like tesla motors’ “autopilot” or google’s “driverless car” [1]. the used navigation and self-driving algorithms run on maps [2] and require very accurate digital (geo-)data of the environment [3]. therefore, the spatial coverage of the surroundings as a basis for autonomous navigation of the vehicle is common to all applications. consequently, autonomous driving – regardless of the respective task such as automatic braking, adaptive cruise control, lane keeping or autonomous driving – requires detection and imaging of the space which it will pass through. photo cameras, lidar-, radaror other sensors continuously detect and measure the surrounding of the car, creating a digital spatial image of the area (map). combined with positioning services like gps or galileo, the location of the vehicle can be determined and set in relation to surrounding objects such as other vehicles or obstacles. however, it is not an ordinary cartographic map resulting from the permanent mapping process but digital images that are stored temporarily as server frames for the navigation. the navigation of the car finally relies on mathematical functions (algorithms) to pass through the digital and real space on the shortest (spatial) or fastest (time) way. this paper addresses the issue of the space detection which is considered as an essential basis of autonomous driving and robot navigation (see for example [4]) and describes the use of a microsoft x-box one kinect stereo camera for autonomous driving for the outdoor delivery challenge robotour 2016 (robotika.cz). we present an approach for mapping the surroundings including the three steps (1) 3d capturing, (2) projection and filtering and (3) object detection. the hardware and the approach are implemented on a four wheel drive off-road buggy, scale 1 : 5. the main hardware consists of a congatec ts180 com express module. for the main processing, it communicates with the x-box one kinect and a raspberry pi and displays the crucial information on a krämer v-800 touchscreen. the raspberry pi reads the sensor values from the gps and the bosch bno055 and transmits them to the congatec module. it receives the steering and speed values from the congatec module, which are transmitted to the freescale kl25z microcontroller, which in turn controls the rc-car model via pwm signals. 2. approach the development path of the approach to map the surroundings for an autonomous driving vehicle should be well-defined to avoid inconsistencies. the first step is answering the question which hardware will be used to detect objects. with the goal to use only a kinect camera as sensor, the approach relies on the processing of the depth, and standard images. both data represent a 2d respectively 3d model of the environment. the approach is characterised in three development steps: • 3d capturing, • projection, • object recognition. 28 http://dx.doi.org/10.14311/app.2016.6.0028 http://ojs.cvut.cz/ojs/index.php/app vol. 6/2016 mapping the surroundings as a requirement for autonomous driving 3d capturing obstacle detec on projec on and ltering kinect camera figure 1. spatial mapping of the current surroundings. it is developed to simplify complex processes, describe them in a proper way and visualize all of the main development steps in a manner that is easy to understand. according to the development steps, the approach is divided into three processes, see figure 1. at first, the kinect camera films the space in front of the vehicle in order to gather potential obstacles. the second step contains the projection of the depth images on a 2d ground plane. at last, the obstacles or object will be recognized during the third step. the three main steps will be explained below. 3. related work in 2002 sebastian thrun (see [5]) showed that the problem of terrain mapping is to generate a spatial model of a robot’s or vehicle’s environment using sensors and cameras. concerning this point there are several more subproblems: • measurement noise, • robot must choose its way during mapping process, • environment changes over time, • possibly the hardest problem — the so called data association problem: correspond sensor measurements to the same physical object in the real world. “mapping is largely considered the most difficult perceptual problem in robotics.” [5, p. 6]. in 1989 herbert et al. already presented the so called locus algorithm to build terrain maps based on a single active range sensor, by “computing the intersection of the surface observed by the sensor with the vertical line passing through (x, y)” [6, p. 998]. frankhauser et al. [4, p. 2] introduced an elevation mapping approach from a “robot-centric perspective” with relative distances to the robot and a permanent update process of the entire elevation map with information about the motion of the robot. kleiner and dornhege presented their autonomous driving vehicle for urban search and rescue (usar) with two different methods. first the rapid mapping of a large-scale environment by wheeled robots, and second the mapping of rough terrain by tracked robots [7]. hornung et al. [6] developed an open source library to represent the environment in memory using octrees. the cells of the discretized space can have one of the following states: unknown, free or occupied. to map the sensor data into the octree they use a probabilistic sensor model. thrun et al. [8] describes stanley, the winner of the 2005 darpa grand challenge. the robot was built by stanford university researchers and uses laser figure 2. overview of the kinect camera. range finders as well as radar sensors for sensing the environment. the evaluation of the sensor data is supported by artificial intelligence and machine learning algorithms. 4. 3d capturing as a 3d sensor is a rather expensive component, the kinect is a cheap alternative compared to the functions it provides. the camera is doing most of the calculations on its own soc, thus reducing the processing effort for the main unit. the first version of microsoft kinect camera was developed by prime sense, an israeli 3d sensing company, which was bought by apple in 2013. after that, microsoft created its own system based the existing prime sense technology [9]. 4.1. overview the kinect camera has an array of sensors, which can be divided into three categories: a depth sensor (green), a colour sensor (red) and an audio sensor (blue), as seen in figure 2. the audio sensor is not used for the 3d reconstruction. the colour sensor consists of a cmos camera with an infrared light filter. the main use of the colour sensor is the track detection. the colour image can be mapped with the depth information, although the resolution of the colour sensor is higher than that of the depth sensor. that is the reason why there are multiple colour pixels per one depth pixel. if there are more depth pixel, than colour pixels, the colour pixels need to be split into multiple smaller pixel, with the same information. the depth sensor consists of a cmos camera and an infrared led blaster. the cmos camera is filtered so that it just sees near-infrared light which is emitted by the infrared led blaster. all the information is processed in a specially designed soc (figure 3). this allows the soc to process a depth map containing all the depths for every pixel and map the colour pixel to it. the result of the processing is a data stream containing an audio, a video, and a depth stream. 29 m steininger, c. stephan, c. böhm et al. acta polytechnica ctu proceedings figure 3. 3d reconstruction of the kinect [11, 12]. 4.2. 3d reconstruction there are multiple ways of reconstructing the depth information from an image. the classic way needs two calibrated cameras at a defined distance. this allows for calculating the distance to the object with trigonometry formulas. the difficulty will be in differencing the objects resulting in the so called correspondence problem which “refers to the problem of ascertaining which parts of one image correspond to which parts of another image, where differences are due to movement of the camera, the elapse of time, and/or movement of objects in the photos.” [10]. the kinect overcomes this problem by using a method called “time of flight”. in this method the travel time of infrared light, which is emitted by a led, is measured for each pixel, as seen in 3. “a strobed infrared light illuminates the scene, the light is reflected by obstacles, and the time of flight for each pixel is registered by the infrared camera. internally, wave modulation and phase detection is used to estimate the distance to obstacles (indirect tof)” [13]. the problem is that the infrared light can be washed out by other near-infrared light sources like sunlight, resulting in the kinect not being able to detect the infrared light and losing the depth information. 5. projection and filtering the kinect camera is mounted on the chassis in an arbitrary fixed angle, tilted to the cars chassis. the exact angle in which the camera is mounted to the ground is not known. it can be measured, but to be independent from that setting, the ground plane coefficients, which are returned from the kinect, are used. the coordinate system of the returned 3d point cloud (as camera coordinate system in the following; indicated with a car subscript) does not match with the coordinate system of the car (as car coordinate system in the following; indicated with a camera subscript). in figure 4 both coordinate systems are shown. the body-frame which can be requested from the camera contains a four-dimensional vector representing the floor coefficients by ~pcoeff = (a;b;c;d). figure 4. the camera coordinate system (green) and the car coordinate system (blue) [11, 12]. these coefficients are combined in (1) to receive the plane equation for further mathematical operations: 0 = ax+by + cz +d (1) in a first step, the distance of every point to the ground is calculated. a point is considered to be a part of the ground if a certain limit is not exceeded. therefore they are removed from the point collection if the distance to the ground is smaller than a certain distance. the remaining points are processed in the following steps. in the next step, each frame of the kinect has a resolution of 512 by 424 and can thereby result in a large amount of 3d-pixels. a pixel is returned, if it is in the specified range of the camera. the nearest distance the camera returns a pixel for is about half a meter. the maximum distance is about eight meters. so there are quite a large number of points waiting to be computed. the rather small computing system with limited processing power on the car must handle this in real-time. for the presented project, a real-time reaction is defined with a latency of a few hundred milliseconds. to reach this goal, one issue is making the algorithm faster. this could be realised by: • removing as many points as possible to save energy and processing effort converting the points from one coordinate system to another, • adjusting processes how the data is processed step by step, • using a programming language with small processing overhead. to reduce the further processing effort the points get filtered next. a new parameter named mindistance is introduced in the gui (graphical user interface) where the points must have a greater distance from 30 vol. 6/2016 mapping the surroundings as a requirement for autonomous driving the camera than that value, to prevent recognising parts of the car as obstacles. this is needed due to the current setting of the car. there may be points above the height of the car so that the car can drive beneath. these points get removed if they are above a maximum height from the ground plane which is defined as a parameter named maxheight. obstacles far away are also not interesting for the process at the moment. in the same way points far away are also not interesting and get filtered with an allowed maximum distance named maxdistance. in this section some functions are introduced to simplify the explanation of the coordinate transformation. the first three equations deal with projections. project a vector v onto another vector vonto (2), also a vector v onto a plane e : 0 = (~x− ~r) · ~n (3) and a point with position vector ~x onto a plane (4): fv2v(~vorig, ~vonto) : ~vproj = ( ~vorig � ~vonto |~vonto| ) (2) fv2p(~vorig) : ~vproj = ~vorig − fv2v (~vorig, ~n) · ~n (3) fp2p(~xorig) : ~xproj = ~xorig − fv2v (~xorig − ~r, ~n) (4) next the normalisation is defined in (5): fn(~vorig) : ~vnorm = ~vorig |~vorig| (5) since the source coordinate system of the kinect is in three dimensional space and the destination system is only two dimensional, it is necessary to project every detected 3d-pixel onto the ground plane. in the three dimensional coordinate space of the kinect we define a two dimensional coordinate space for the new coordinate system. the new coordinate system can be seen as a plane in the original space. therefore an origin and also two vectors for the two axes of that coordinate system are needed. the two dimensional coordinate system for the obstacle detection is implemented within the three dimensional coordinate system of the kinect camera. the camera position perpendicular above the ground is origin: ccamera = fp2p(o) with o as the origin of coordinate system. the axes are defined as ex = fn (fv2p ((0; 0;−1))) and ey = fn (fv2p ((−1; 0; 0))). in figure 4 you see the orientation of the axis. now, (6) can be used to project all points into the new car coordinate system: pcar = ( fv2v (pcamera − ccamera, ex) fv2v (pcamera − ccamera, ey) ) (6) next, the obstacle detection will use these projected points and create a barrier along the nearest edge of the recognized obstacles. 6. obstacle detection when implementing algorithms for autonomous vehicles it is mandatory to detect and tag obstacles in front of the craft. as described, a depth image is 0 0.5 1 1.5 2 2.5 3 3.5 4 0 1 2 3 4 5 6 7 8 l o g a ri th m ic s c a lin g o f d is ta n c e distance (m) figure 5. logarithmic distance scaling for image size reduction purposes. provided by the kinect camera from where each single depth pixel is projected to a ground plane with each having the correct distance and position to the camera’s point of view during the first stage of image processing. as a first step, the depth points contributed by the kinect camera and the projection to the ground plane are transformed into a 2d image. the next three steps described in this section include: • distance scaling, • gaussian functions, • and the actual object and obstacle detection. 6.1. distance scaling the project team faced problems concerning computing power: the large number of pixels coming from a depth frame provided by the kinect camera exceeded the available and limited hardware. for the purpose of reducing the computing effort for processing the gaussian function (see subsection 6.2) the project team decided to scale the projected image’s size by its height logarithmically. as shown in figure 5, the scaling of distances allows to have more detailed image information in a closer range to the camera’s origin than in a larger distance to this specific point. the distance we get from the kinect camera is a floating point number, the index we need to reference a pixel within and on the image is an integer. if the distance is only rounded to the next integer value there is one pixel per meter. because of this a parameter is introduced which represents the resolution of the image’s distances. the scaled value is divided by this value to get the index as an integer, after rounding of this new integer we get the correct index value for the image. for example, 0.5m in reality are mapped to an index value of 0.585 after the scaling, while 8m in the real world, which is the kinect camera’s maximum recognition distance, correspond to an index value 3.171 logarithmically scaled. 31 m steininger, c. stephan, c. böhm et al. acta polytechnica ctu proceedings g(x,y) x y g(x,y) figure 6. gaussian distribution in 2d. as can be seen in figure 7 the opening angle of the camera is segmented with a specific resolution, which can be set as a parameter. every segment is mapped to a single rectangular column into the projected image. the advantage of this approach is that near distance pixels are richer in detail than more distant pixels. 6.2. gaussian function the next step is to smoothen the depth image to reduce single error pixels or noise and smoothen pixel clouds for further and more precise computations and calculations. although there are several options for smoothing images, the chosen solution is the gaussian filter g(x, y) = 1 2πσ2 e − x2+y2 2σ2 (7) in (7), the gaussian function for two dimensions is shown. where x is the distance from the origin on the horizontal axis and y is the distance from the origin on the vertical axis, and σ is the standard deviation of the gaussian distribution. the distribution can be seen in figure 6 for two-dimensional gaussian functions. this formula produces a surface of concentric circles with a gaussian distribution from the center point. the new value of each pixel is set to a weighted average of that specific pixel’s neighborhood. the original pixel’s value receives the highest gaussian value and neighborhood pixels receive smaller weights as their distance to the original pixel increases. this results in a blur effect [14, 15]. figure 7 shows three identical depth frames which are already projected to a ground plane (you see the frames in a top view) and transformed into a gray image. on the left-hand side, the single frame is displayed without any filtering or blurring. there are many false pixels, noise and distortion. a gaussian filter is already used in the middle frame, nevertheless quite a lot noise and other false pixels can be seen. the approach works in practice but still needs a little fine tuning. therefore a smoother configuration of the gaussian settings was used. the third frame on the right-hand side displays very little distortion, noise and false pixels, but provides clearly identifiable pixel clouds. these pixel clouds mostly represent objects or other obstacles in the range of the kinect camera. 1 2 3 figure 7. three identical frames with no gaussian filtering in frame 1, minimal gaussian filtering in frame 2 and for the project’s obstacle detection best gaussian filtering settings in frame 3. 6.3. detect obstacles the last step before actually being able to detect obstacles is removing unnecessary pixels which still are in the image after applying a gaussian filter. in the scope of the project a parameter was added to set a tolerance for still existing false or error pixels: threshold. it’s inevitable to do so, due to the fact that there are not only black and white pixels anymore (values 0 and 255) because of the gaussian filtering but also values in between. the final step is to find objects in the distancescaled and gaussian filtered image. after preparation of the given depth image with gaussian filtering and logarithmic distance scaling, the final step for finding and detecting objects in range is to find pixel clouds with a defined minimal distance (parameter in algorithm: mindistancebetweenobstacles in meters) to other clouds and therefore to other obstacles. when a pixel cloud (respectively an obstacle) is detected with no other obstacle in range of mindistancebetweenobstacles, it is marked as a detected obstacle. 7. conclusions the autonomous driving requires the continuous mapping of the surroundings. the presented approach and the hardware setting show that the mapping can be done with a kinect camera in the case of model cars in a scale up to 1 : 5. with the 3d capturing of the surroundings, the projection and filtering and obstacle detection, it is possible to get a temporary map of the car’s environment as a basis for the navigation process. the navigation itself relies on the a*-algorithm and uses the projected map including the detected 32 vol. 6/2016 mapping the surroundings as a requirement for autonomous driving obstacles as the digital weighted spatial information. in a next step, we are planning to test the whole configuration and the navigation process under different conditions, i.e. different surfaces or different weather conditions. the aim is to obtain feedback on the use and the performance of the stereo camera and the navigation algorithm. beyond the self-driving car, the presented approach can be implemented in other vehicles, like unmanned aerial vehicles (uav), autonomous underwater vehicles (auv) or even in robots for different services. references [1] a. m. kessler. elon musk says self-driving tesla cars will be in the u.s. by summer the new york times, 2015. [2] a. fisher. google’s road map to global domination the new york times, 2013. [3] a. geiger, p. lenz, r. urtasun. are we ready for autonomous driving? the kitti vision benchmark suite. in ieee conference on computer vision and pattern recognition (cvpr), pp. 3354–3361. 2012. doi:10.1109/cvpr.2012.6248074. [4] p. fankhauser, m. bloesch, c. gehring, et al. robot-centric elevation mapping with uncertainty estimates. in international conference on climbing and walking robots (clawar), epfl-conf-198746. 2014. doi:10.1142/9789814623353_0051. [5] s. thrun, et al. exploring artificial intelligence in the new millennium. vol. 1, chap. robotic mapping: a survey, pp. 1–35. 2002. [6] m. herbert, c. caillas, e. krotkov, et al. terrain mapping for a roving planetary explorer. in ieee international conference on robotics and automation, pp. 997–1002. 1989. doi:10.1109/robot.1989.100111. [7] a. kleiner, c. dornhege. real-time localization and elevation mapping within urban search and rescue scenarios. journal of field robotics 24(8-9):723–745, 2007. doi:10.1002/rob.20208. [8] s. thrun, m. montemerlo, h. dahlkamp, et al. stanley: the robot that won the darpa grand challenge. journal of field robotics 23(9):661–692, 2006. doi:10.1002/rob.20147. [9] primesense, 2016. page version id: 710008223. [10] correspondence problem, 2015. page version id: 661209488. [11] s. design. rc-xd (call of duty: black ops) 3d warehouse. [12] t. plus. kinect from microsoft 3d warehouse. [13] p. fankhauser, m. bloesch, d. rodriguez, et al. kinect v2 for mobile robot navigation: evaluation and modeling. in international conference on advanced robotics (icar), pp. 388–394. 2015. doi:10.1109/icar.2015.7251485. [14] m. s. nixon, a. s. aguado. feature extraction and image processing. newnes, oxford ; boston, 1st edn., 2002. oclc: ocm49045100. [15] l. g. shapiro, g. c. stockman. computer vision. prentice hall, upper saddle river, nj, 2001. 33 http://dx.doi.org/10.1109/cvpr.2012.6248074 http://dx.doi.org/10.1142/9789814623353_0051 http://dx.doi.org/10.1109/robot.1989.100111 http://dx.doi.org/10.1002/rob.20208 http://dx.doi.org/10.1002/rob.20147 http://dx.doi.org/10.1109/icar.2015.7251485 acta polytechnica ctu proceedings 6:28–33, 2016 1 introduction 2 approach 3 related work 4 3d capturing 4.1 overview 4.2 3d reconstruction 5 projection and filtering 6 obstacle detection 6.1 distance scaling 6.2 gaussian function 6.3 detect obstacles 7 conclusions references acta polytechnica ctu proceedings doi:10.14311/app.2017.8.0001 acta polytechnica ctu proceedings 8:1–4, 2017 © czech technical university in prague, 2017 available online at http://ojs.cvut.cz/ojs/index.php/app the interaction of human osteoblast-like saos-2 cells with stainless steel and si(100) coated by silicalite 1 films ivan jirkaa,∗, marta vandrovcováb, jan plšeka, lucie bačákováb a heyrovský institute of physical chemistry, ascr, v.v.i. dolejškova 3, 182 23 prague 8, czech republic b institute of physiology, ascr, v.v.i., vídeňská 1083, 142 20 prague 4, czech republic ∗ corresponding author: ivan.jirka@jh-inst.cas.cz abstract. interaction of osteoblast-like saos-2 cells with polished stainless steel coupons and si(100) wafers covered by a film of densely intergrown a, b oriented silicalite-1 crystals is investigated. due to their unique chemical, mechanical and biological properties these films are promising new anti-corrosive coating for bone replacement. biocompatibility of silicalite 1 film is affected by its morphology and wettability. the surface properties of silicalite 1 film is controlled by synthesis conditions and postsynthetic modifications. the morphology of the film (number of a oriented crystals) is tuned by time of synthesis. wettability of the film is tuned by heat treatment in a stream of air at 300 ◦c. the silicalite 1 film is proved to be biocompatible. the number of adhered saos-2 cells increases with the number of a oriented crystals observed for the film grown on stainless steel and with increased wettability of heat treated samples. obtained results demonstrate possible ways of preparation of anti-corrosive silicalite-1 coatings with optimized cytocompatibility. keywords: biocompatibility, silicalite-1 film, outer zeolite surface, wettability. 1. introduction metals and metal alloys are nowadays the most frequently used implant materials [1]. their substantial drawback is the evolution of metal species into the body. great attention has thus been paid to developing suitable surface modifications, which do not lower the biocompatibility of the material and simultaneously suppress evolution of the metal species. silicalite-1 coating (sc ) has been found to be prospective in this area [2]. various physical-chemical parameters, including wettability and morphology, influence the biocompatibility of prosthetic materials [3]. possibility the optimization of these parameters by the mode of synthesis and post-synthetic treatments of sc to get its maximal biocompatibility is demonstrated in this paper. our preliminary data of tuning the wettability and morphology of sc by selection of support and the mode of synthesis are presented. 2. experimental the scs were synthesized in situ on the polished surface of stainless steel foil and si(100) wafer from reaction mixture of tetrapropyl-ammonium hydroxide (tpa, 1m solution in h2o, sigma aldrich), tetraethylorthosilicate (teos, ≥ 99.0 %, aldrich) and deionized water as described in [4]. the reaction mixture was aged for 2 hours. the synthesis proceeded 3 hours (sc on si(100), referred as to sc-si(100)) and 6 hours (sc on stainless steel, sc-ss) at 165 °c. the supports and synthesis conditions used enabled the preparation of the samples with substantially different surface morphologies. a portion of the synthesized samples were heated at 300 °c in a stream of dry air. the heattreated samples were abbreviated as sc-si(100)-300 and sc-ss-300. static water drop contact angle θ measurements using the see system (brno, masaryk university, czech republic) were utilized to characterize the wettability of sc. the morphology of the samples was characterized by near-field scanning electron microscopy (fesem, s 4800-1, hitachi). the human osteoblast-like saos-2 cells were seeded on the sample surfaces as described in our earlier study [5]. briefly, the samples were sterilized in a hot air sterilizer (2 hours, 120 °c), inserted into 24well cell culture plates, and seeded with the cells at a density of 15,000 cells/cm2 and in 1 ml of mccoy's 5a medium with 15 % fetal bovine serum and 40 µg/ml of gentamicin. 3. results and discussion sem images of the samples sc-si(100) and sc-ss are depicted in figure 1. while the sc-si(100) was mostly composed from a compact layer of intergrown b oriented crystals covered by a small number of a oriented crystals, a high concentration of a oriented crystals on compact layer of intergrown b oriented crystals was observed on the sc-ss. no morphological changes were induced upon heating. typical results of water drop contact angle measurements are summarized in figure 2. the θ values of sc-ss and sc-si(100) were comparable; i.e. 84°± 3°and 66°± 2°, respectively. heat treatment induced 1 http://dx.doi.org/10.14311/app.2017.8.0001 http://ojs.cvut.cz/ojs/index.php/app i. jirka, m. vandrovcová, j. plšek, l. bačáková acta polytechnica ctu proceedings (a). (b). (c). (d). figure 1. sem image of sc-si(100) (top) and sc-ss (bottom). left: a centre of the films: left, right: a periphery of the films. (a). (b). (c). (d). figure 2. water drop contact angle of sc-si(100) (top) and sc-ss (bottom) before (left) and after treatment at 300 °c. 2 vol. 8/2017 the interaction of human osteoblast-like saos-2 cells figure 3. immunofluorescence staining of vinculin, a protein of focal adhesion plaques (green) in human osteoblastlike saos-2 cells on day 3 after seeding on sc-si(100) (a) or sc-ss (b). the actin cytoskeleton is stained in red with phalloidin-tritc, and the cell nuclei are stained in blue with dapi. leica tcs spe dh 2500 confocal microscope. substantial lowering of the θ value, which was similar for sc-ss and sc-si(100), i.e., 32°± 3°and 43°± 3°, respectively. the morphology of the saos-2 cells on the sample sc-ss and sc-si(100) is shown in figure 3. the cells on both surfaces are of similar morphology, i.e. they are well-spread and polygonal, with welldeveloped vinculin-containing focal adhesion plaques and f-actin cytoskeletons. the number of saos-2 cells (n(saos-2)) on the surfaces of sc-ss and sc-si(100) on day 1 and 3 after seeding is depicted in figure 4. the n(saos-2) on the zeolite samples are compared with the cell numbers on polystyrene dish (ps). on day 1 after seeding, the number on both sc samples is equal (sc-si(100) or higher (sc-ss) than on ps ; i.e. the zeolite samples are bioactive. comparable values of n(saos-2) are observed on the sc-ss and sc-si(100) before and after heat treatment of the samples on day 1, after seeding (figure 4). however, a substantial increase of n(saos-2) on sc-si(100)300 and particularly sc-ss-300 in comparison with non-heated samples and ps was evident in day-3 after seeding. 4. conclusion silicalite-1 films are biocompatible materials. their interaction with saos-2 cells is influenced by surface wettability and topography. the number of adhered saos-2 cells depends on synthesis conditions of zeolite coating and on its post-synthetic modification. the cell number increases with increasing numbers of a oriented zeolite crystals on the surface of sc and increasing surface wettability. obtained results are prospective in the optimization of sc outer surface to reach its maximal cytocompatibility. acknowledgements supported by grant agency of the czech republic (grant no.16-02681s). references [1] x. liu, p. k. chu, c. ding. surface modification of titanium, titanium alloys, and related materials for biomedical applications. materials science and figure 4. comparison of the n(saos-2) on days 1 and 3 after seeding on the surfaces of the samples sc-ss, sc-si(100) and ps before (a) and after heat treatment (b) of the samples at 300 °c. engineering: r: reports 47(3-4):49 – 121, 2004. doi:http://dx.doi.org/10.1016/j.mser.2004.11.001. [2] r. s. bedi, d. e. beving, l. p. zanello, y. yan. biocompatibility of corrosion-resistant zeolite coatings for titanium alloy biomedical implants. acta biomaterialia 5(8):3265 – 3271, 2009. doi:http://dx.doi.org/10.1016/j.actbio.2009.04.019. [3] l. bacakova, e. filova, m. parizek, et al. modulation of cell adhesion, proliferation and differentiation on materials designed for body implants. biotechnology advances 29(6):739 – 767, 2011. doi:http://dx.doi.org/10.1016/j.biotechadv.2011.06.004. [4] z. wang, y. yan. oriented zeolite {mfi} monolayer films on metal substrates by in situ crystallization. microporous and mesoporous materials 48(1-3):229 – 238, 2001. doi:http://dx.doi.org/10.1016/s1387-1811(01)00357-2. 3 http://dx.doi.org/http://dx.doi.org/10.1016/j.mser.2004.11.001 http://dx.doi.org/http://dx.doi.org/10.1016/j.actbio.2009.04.019 http://dx.doi.org/http://dx.doi.org/10.1016/j.biotechadv.2011.06.004 http://dx.doi.org/http://dx.doi.org/10.1016/s1387-1811(01)00357-2 i. jirka, m. vandrovcová, j. plšek, l. bačáková acta polytechnica ctu proceedings [5] m. vandrovcova, i. jirka, k. novotna, et al. interaction of human osteoblast-like saos-2 and mg-63 cells with thermally oxidized surfaces of a titanium-niobium alloy. plos one 9(6):1–15, 2014. doi:10.1371/journal.pone.0100475. 4 http://dx.doi.org/10.1371/journal.pone.0100475 acta polytechnica ctu proceedings 8(0):1–4, 2017 1 introduction 2 experimental 3 results and discussion 4 conclusion acknowledgements references 222 acta polytechnica ctu proceedings 2(1): 222–225, 2015 222 doi: 10.14311/app.2015.02.0222 early super soft source spectra in rsoph j.-u. ness1 1xmm-newton science operations centre, esa, po box 78, 28691 villanueva de la cañada, madrid, spain corresponding author: juness@sciops.esa.int abstract recent swift x-ray monitoring campaigns of novae have revealed extreme levels of variability during the early super-softsource (sss) phase. the first time this was observed was during the 2006 outburst of the recurrent nova rs oph which was also extensively covered by grating observations with xmm-newton and chandra. i focus here on an xmm-newton observation taken on day 26.1, just before swift confirmed the start of the sss phase, and a chandra observation taken on day 39.7. the first observation probes the evolution of the shock emission produced by the collision of the nova ejecta with the stellar wind of the companion. the second observation contains bright sss emission longwards of 15 å while at short wavelengths, the shock component can be seen to have hardly changed. on top of the sss continuum, additional emission lines are clearly seen, and i show that they are much stronger than those seen on day 26.1, indicating line pumping caused by the sss emission. the lightcurve on day 39.7 is highly variable on short time scales while the long-term swift light curve was still variable. in 2007, we have shown that brightness variations are followed by hardness variations, lagging behind 1000 seconds. i show now that the hardness variations are owed to variations in the depth of the neutral hydrogen column density of order 25%, particularly affecting the oxygen k-shell ionization edge at 0.5 kev. keywords: cataclysmic variables recurrent novae spectroscopy x-rays individual: rs oph. 1 introduction the 2006 outburst of the recurrent symbiotic nova rs oph has attracted a large number of observers to study the outburst in unprecedented detail and many wavelength bands. in particular the coverage in x-rays has considerably improved since the previous outburst in 1985 by initiating the first high-density x-ray monitoring of a nova with the x-ray telescope (xrt) on board swift, starting on day 3.38 after initial explosion (bode et al. 2006a). early shock emission originated from kinetic energy from the nova ejecta that were dissipated in the slow, dense stellar wind of the giant companion. the expected x-ray spectrum is that of a collisional plasma which was confirmed by spectral models to the low-resolution exosat spectra taken in 1985 (figure 3 in (o'brien et al. 1992)) and the swift/xrt spectra (bode et al. 2006b). the x-ray grating spectrometers on board xmm-newton (reflection grating spectrometer rgs) and chandra (lowand high energy transmission grating spectrometers letgs and hetgs) allow spectral lines to be resolved, and in simultaneous rgs/hetgs spectra taken on day 13.8 after the initial explosion (2006 february 12.83), bremsstrahlung continuum, h-like and he-like emission lines, and numerous fe lines could be identified. a detailed analysis exploring the information from the emission lines yielded the distribution of electron temperatures and abundances (ness et al. 2009). further grating observations were taken, e.g., on days 26.1, 39.7, 54.0, 66.9, and 111.7 that were guided by continued dense swift monitoring (see table 1 in ness et al. 2009 and top of figure 2 in osborne et al. 2011). during the xmm-newton observation on day 26.1, a jump in count rate was discovered by (nelson et al. 2008) that was exclusively attributed to additional soft emission, possibly indicating the start of the sss phase. the bremsstrahlung continuum has faded and become softer, and also the ratios of h-like to he-like emission line strengths have shifted in favor of the he-like lines, indicating a cooling trend in the shocked plasma (ness et al. 2009). the swift observations of the ∼ 60 day sss phase are described in (osborne et al. 2011). between days ∼ 30 and 45, the x-ray count rate was highly variable between ∼ 10 counts per second (cps) and 200 cps, and then stabilized at ∼ 300 cps (figure 2 in osborne et al. 2011). this was a new discovery and was later also observed in other novae and might be a general phenomenon. during this high-amplitude phase, a chandra observation was taken on day 39.7, without yet knowing about the risks of observing during times of fainter emission. while this was the case, the soft emission was still bright enough for a well-exposed grating spectrum that was first presented by (ness et al. 2007). the blackbody-like continuum contained deep absorp222 http://dx.doi.org/10.14311/app.2015.02.0222 early super soft source spectra in rs oph tion lines from highly ionized species such as o viii and n vii that were blue shifted by ∼ 1200 km s−1. the line profiles contained clear signs of emission lines in the red wings (figure 5 of ness et al. 2007) which could either come from residual shock emission or are part of p cyg profiles. during this observation, the nova was also highly variably on shorter time scales, and (ness et al. 2007) reported that the hardness variations followed the same upand down trends but lagged 1000 seconds behind (see fig. 1). figure 1: reproduction of figure 8 from (ness et al. 2007). the chandra light curve on day 39.7, taken during the early high-amplitude variability phase, was also highly variable on shorter time scales. the hardness ratio, shown in the bottom, evolved with the same variability patterns but lagged 1000 seconds behind the brightness variations. reproduced by permission of the aas. i focus here on two new aspects: 1) the emission line components on top of the sss continuum (shown in figure 5 of ness et al. 2007), compared to the same lines before the sss phase started, are much stronger than expected from a cooling plasma (sect. 2.1). 2) closer inspection of the brightness/hardness changes during the day 39.7 observation show that the hardness changes are due to changes in the depth of the o i absorption edge. modeling shows that the overall column density increases with decreasing brightness (sect. 2.2). 2 observations and analysis a full log of all xmm-newton and chandra grating observation that were taken of the 2006 outburst of rs oph is given in table 1 in (ness et al. 2009) out of which i focus on the ones taken with xmm-newton between 2006 march 10, 23:04 and march 11, 02:21 (obsid 0410180201) and with chandra 2006 march 24, 12:25 15:38 (obsid 7296). the calibrated spectra are shown in direct comparison in fig. 2 using the same flux units without rescaling. figure 2: comparison of a pre-sss spectrum of rs oph taken on day 26.1 and an sss spectrum taken on day 39.7. in the top panel the entire wavelength range is shown in logarithmic units, illustrating that at short wavelengths, below ∼ 15 å, the spectra are similar, only showing a decline the bremsstrahlung continuum component from day 26.1 to day 39.5. in the panels below, narrow wavelength regions are shown where the pre-sss spectrum is added to the median of the day 39.7 spectrum in the same flux units. while short-wavelength lines have hardly changed, the excess emission lines on top of the sss continuum are much stronger than before the start of the sss phase. 2.1 contributions of shock emission to sss continuum spectrum in the top panel of fig 2, the full spectra are shown in logarithmic units to overcome the great contrast in brightness between the two observations. at wavelengths <∼ 15 å, the two spectra are similar in nature, mainly differing in the brightness of the continuum. the weaker continuum on day 39.7 can be explained by the continuation of the fading trend of the shocks that has already been established from the observations between days 13.8 and 26.1. the strengths of the emission lines have hardly changed. at wavelengths >∼ 15 å, the emission lines seen on day 26.1 were outshone by the bright continuum on day 39.7, and it is not intuitively clear how they have evolved. in the panels below, four lines are shown in more detail in linear units. the open histograms in the panels below represent the day 39.7 spectrum while the colored shades are the day 26.1 spectrum, added to the median flux of the day 39.7 spectrum, taken over the narrow wavelength range selected 223 j.-u. ness for each panel. the mg xii/xi lines have faded from day 26.1 and the ne x line at 12.1 å is about equally strong at both times while these lines have become somewhat narrower. the panels further below show that the lines on top of the sss continuum have become stronger from day 26.1 to 39.7, depending on the strength of the continuum. the fe xvii line at 15 å, in the wien tail of the sss continuum, is only slightly stronger while the o viii and n vii lines, near the peak of the sss continuum, are much stronger, defying the general trend of fading emission from the shocks. a similar result has been found by (schönrich & ness 2008) who show in their figure 1 the evolution of the volume emission measure for various emission lines as a function of their peak formation temperature assuming collisional equilibrium. the line fluxes of o vii and o viii from the observation taken on day 39.7 yield clearly discrepant values, orders of magnitude brighter than in observations without sss emission, while those emission lines that arise at shorter wavelengths are consistent with the other observations. 2.2 spectral changes with variability during early sss phase the high-amplitude variations during the early sss phase are still not understood, leaving all options open such as changes in absorption, in intrinsic brightness, temperature, or in the rate of mass loss. a first approach to narrow down the options, i present the spectra and how they changed with variability. during the early variability phase of rs oph, a short chandra observation taken (day 39.7), and the spectral evolution is illustrated in fig. 3. the light curve, already shown in fig. 1, is shown in the right, turned around by 90o clockwise to follow the vertical time axis in downward direction. the blue dotted line is the hardness light curve from the bottom panel of fig. 1 that was rescaled to fit in the same graph. along the same vertical time axis, a series of adjacent letgs spectra are shown in the central panel with wavelength along the horizontal axis and flux encoded in a color scheme, where increasing flux is represented by colors light green, yellow, orange, red, dark blue to light blue. in the top panel, two spectra are shown that have been integrated over the time intervals marked with horizontal dashed lines in the central panel shaded areas in the right panel. the colors of the dashed lines and shades correspond to the plot style of the spectra in the top, thus light blue and orange shadings in the right correspond to the light blue shade and orange thick line in the top, respectively. the two spectra have been normalized to coincide in the wavelength range 15 − 22 å. ness et al. (2007) had extracted spectra from time intervals of bright and faint episodes, roughly corresponding to the time intervals 0.3−1.0 hours of elapsed time (bright) and 1.1− 2 hours, respectively, and found differences in the spectral shape. i have now chosen time intervals that are shifted by 1000 sec in order to probe the hardness light curve rather than the intensity light curve. the two normalized spectra in the top clearly show that the variability in hardness is purely owed to changes in the depth of the o i edge at 22.8 å, yielding the steeper edge 1000 sec after a lowflux episode. ness et al. (2007) had found changes in o i on longer time scales between the grating observations taken on days 39.7, 54.1, and 66.9 and explained these by changes in the degree of ionization of oxygen. only neutral oxygen produces the deep edge at 22.8 å while ionized oxygen is more transparent to x-rays, leading to a flatter edge. the intense soft continuum x-ray source can ionize the neutral elements in the surroundings and thus make them more transparent. on the other hand, if the continuum source fades, the surrounding material recombines, leading to an increase in the o i edge. figure 3: dynamic spectrum for the chandra observation of rs oph taken on day 39.7 after the 2006 outburst. the zero-th order light curve is shown to the right with three colored shades marking time intervals from which the spectra in the top have been extracted. line labels that belong to strong transitions are included. the central image is a brightness spectral/time map that uses a color code from light green to light blue representing increasing flux values. the changes in the depth of the o i absorption edge could be caused by changes in the degree of ionization of circumstellar oxygen. ness et al. (2007) argued that the observed time lag of 1000 seconds is consistent with the time scale for ionization/recombination in a plasma with density ∼ 1011 cm−3. since all circumstellar material would experience the same changes in their degree of ionization, this would lead to an overall change in nh. another possibility is a non-uniform radial distribution of the oxygen abundance within the ejecta that could be caused by beta decay of oxygen isotopes produced during cno burning. if during times of brigher and fainter emission, different regions of the photosphere are visi224 early super soft source spectra in rs oph ble, the difference in the oxygen abundance would only affect the depth of the o i absorption edge. in fig. 4, i test these two scenarios in the top and bottom panels, respectively. shown is the normalized faint/soft spectrum in orange shadings (orange line in the top panel of fig. 3) while the black curve represents a modification of the brighter/harder spectrum, trying to reproduce the faint/soft spectrum. in the top panel of fig. 4, the black curve was computed by dividing the bright/hard spectrum itself by the transmission coefficients calculated from the warm absorption model developed by [?], and then normalized. the faint/soft spectrum is qualitatively well reproduced assuming a value of nh = 6 × 1020 cm−2, indicating that the change in hardness can be explained by an increase of nh by this amount. the full amount of the hydrogen column was found by [?] as nh = nh,ism + nh,cs = 4.7 × 1021 cm−2, consisting of the well-known interstellar component nh,ism = 2.4 × 1021 cm−2 and an additional circumstellar component nh,cs. an increase by 6 × 1020 cm−2 thus corresponds to ∼ 26% of nh,cs. figure 4: testing two hypotheses to explain the spectral changes between hard/bright (blue shades in top panel of fig. 3) and soft/faint (orange thick line in fig. 3) spectra by the black thin lines. the normalized soft/faint spectrum is shown as orange shadings while the black line corresponds to two types of modifications of the bright/faint spectrum, also normalized (see text). top: overall increase of nh by 26% of nh,cs. bottom: increase only of o i column by changes in o abundance of 15%. in the bottom panel of fig. 4, the black curve was computed by first dividing the bright-hard spectrum by the transmission coefficients of an absorber with nh = 4.7 × 1021 cm−2, assuming cosmic abundances and then multiplying by the coefficients resulting from the same absorption model assuming a modified oxygen abundance. the faint/soft spectrum is well reproduced up to ∼ 27 å, but the black curve drops well below the faint/soft spectrum at longer wavelengths. the better agreement between the black line and the faint/soft spectrum in the upper panel demonstrates that an overall increase of nh has occurred rather than a change in the oxygen abundance alone. 3 summary and conclusions a comparison of grating x-ray spectra before and after the start of the sss phase shows that the emission lines that were seen on top of the sss continuum are not simply a continuation of the early shock emission. only emission lines that arise at wavelengths were strong continuum emission is present are amplified compared to the pre-sss level, strongly suggesting photoexcitation effects. the line profiles in the early sss spectrum of rs oph can thus be understood as p cyg profiles. the early high-amplitude variability in rs oph and other novae still awaits an explanation. the chandra observation during a minimum of these variations revealed that variability also occurs on shorter time scales. the comparisons in this article show that the hardness changes following brightness changes are consistent with variations in nh of ∼ 26% which can be explained by variations in the overall degree of ionization caused by the changes in intensity and thus variations in the effectiveness of photoionization. references [1] bode, m. f., obrien, t. j., davis, r. j., et al. 2006a, iaucirc, 8675, 2 [2] bode, m. f., obrien, t. j., osborne, j. p., et al. 2006b, apj, 652, 629 doi:10.1086/507980 [3] nelson, t., orio, m., cassinelli, j. p., et al. 2008, apj, 673, 1067 doi:10.1086/524054 [4] ness, j.-u., starrfield, s., beardmore, a., et al. 2007, apj, 665, 1334 doi:10.1086/519676 [5] ness, j.-u., drake, j. j., starrfield, s., et al. 2009, aj, 137, 3414 [6] o'brien, t. j., bode, m. f., & kahn, f. d. 1992, mnras, 255, 683 doi:10.1093/mnras/255.4.683 [7] osborne, j. p., page, k. l., beardmore, a. p., et al. 2011, apj, 727, 124 doi:10.1088/0004-637x/727/2/124 [8] schönrich, r. a. & ness, j.-u. 2008, in asp conf. series, 401, 291; rs ophiuchi (2006) and the recurrent nova phenomenon, ed. a. evans, m. f. bode, t. j. obrien, & m. j. darnley [9] wilms, j., allen, a., & mccray, r. 2000, apj, 542, 914 225 http://dx.doi.org/10.1086/507980 http://dx.doi.org/10.1086/524054 http://dx.doi.org/10.1086/519676 http://dx.doi.org/10.1093/mnras/255.4.683 http://dx.doi.org/10.1088/0004-637x/727/2/124 introduction observations and analysis contributions of shock emission to sss continuum spectrum spectral changes with variability during early sss phase summary and conclusions 283 acta polytechnica ctu proceedings 1(1): 283–287, 2014 283 doi: 10.14311/app.2014.01.0283 the hiscore project m. tluczykont2, m. brückner1, n. budnev3, o. chvalaev3, a. dyachok3, s. epimakhov2, o. gress3, d. hampf2, d. horns2, a. ivanova3, n. kalmykov4, e. konstantinov3, e. korosteleva4, v. kozin4, m. kunnas2, l. kuzmichev4, b. lubsandorzhiev5, r. mirgazov3, r. monkhoev3, r. nachtigall2, m. panasyuk4, a. pakhorukov3, v. poleschuk3, a. porelli6, v. prosin4, v. ptuskin7, g.i. rubtsov5, p.s. satunin5, yu. semeney3, d. spitschan2, a. skukhin4, l. sveshnikova4, m. tluczykont2, r. wischnewski6, a. zagorodnikov3, v. zirakashvili7 1institute for computer science, humboldt-university berlin, rudower chaussee 25, 12489 berlin, germany 2institute for experimental physics, university of hamburg, luruper chaussee 149, 22761 hamburg, germany 3institute of applied physics isu, irkutsk, russia 4skobeltsyn institute for nuclear physics, lomonosov moscow state university, 1 leninskie gory, 119991 moscow, russia 5institute for nuclear research of the russian academy of sciences 60th october anniversary st., 7a, 117312, moscow, russia 6desy, platanenallee 6, 15738 zeuthen, germany 7izmiran, moscow, russia corresponding author: martin.tluczykont@physik.uni-hamburg.de abstract a central question of astroparticle physics, the origin of cosmic rays, still remains unsolved. hiscore (hundred*i square-km cosmic origin explorer) is a concept for a large-area wide-angle non-imaging air shower detector, addressing this question by searching for cosmic ray pevatrons in the energy range from 10 tev to few pev and cosmic rays in the energy range above 100 tev. in the framework of the tunka-hiscore project, first prototypes have been deployed on the site of the tunka-133 experiment, where we plan to install an engineering array covering an area of the order of 1 km2. on the same site, also imaging and particle detectors are planned, potentially allowing a future hybrid detector system. here we present the hiscore detector principle, its potential for cosmic ray origin search and the status of ongoing activities in the framework of the tunka-hiscore experiment. keywords: cosmic rays gamma rays instrumentation pevatrons. 1 introduction ever since their discovery, cosmic rays have triggered numerous experiments aiming at answering the questions about cosmic ray composition, spectral distribution, and about their origin. ground-based experiments measure cosmic ray or gamma ray induced air showers via particle or radiation detectors on the ground. the hiscore (hundred*i square-km cosmic origin explorer) concept (tluczykont et al. 2011), is based on the non-imaging air cherenkov technique, sampling the cherenkov radiation emitted by charged air shower particles. planned as a distributed array of sensitive light collecting stations (0.5 m2), with a wide field of view (0.60-0.84 sr) and a very large instrumented area (up to 100 km2), hiscore will cover gamma ray energies above 10 tev and spectral and composition measurements of cosmic rays from 100 tev up to the eev range. the main goal of hiscore is to find the cosmic ray pevatrons, that accelerate cosmic rays up to pev energies, and that are expected to emit gamma rays with a hard spectrum up to several 100 tev. air shower measurements at these energies open up several other research possibilities, (tluczykont et al. 2011). in the present paper, we describe the hiscore detector concept, its potential for a search for cosmic ray pevatrons, and the status of tunka-hiscore, currently (april 2013) in its deployment phase on the site of the tunka-133 experiment (berezhnev et al. 2012). 2 detector concept the hiscore detector is conceived as a distributed array of non-imaging light collecting stations. the array geometry planned for the tunka-hiscore engineering 283 http://dx.doi.org/10.14311/app.2014.01.0283 martin tluczykont et al. array is shown in figure 1. 150 m 106 m figure 1: array layout planned for the tunkahiscore engineering array. in the standard layout (simulated configuration) four 8 inch pmts are used per station (red squares). additional stations with larger pmts (10 or 11 inch) are envisaged for the engineering array (blue squares). first results of tests with different array layouts have shown that a graded array with a dense core and decreasing density toward the array edge provides significantly better angular resolution and maximizes the effective area. at the heart of each station, four 8 inch photomultipliers (pmts) measure the cherenkov light from secondary air shower particles. each pmt is equipped with light concentrators (winston cones) with a half opening angle of 30◦, covering a field of view of 0.84 sr. at reconstruction, a quality acceptance cut of 25◦ results in an effective field of view of 0.6 sr. the total light collection area of one station is 0.5 m2. after preamplification, the pmt signals are clipped and summed before passing a comparator. this clipped sum trigger concept prevents false triggers caused by large signal fluctuations from night sky background light or afterpulses. the signal amplitudes are digitized in time using a readout system based on the drs 4 (domino ring sampler) chip, operated at a sampling frequency of 1 ghz and a depth of 1µs. signal amplitude, timing, rise time, and width of the arrival time distribution are used for the reconstruction of the direction, energy and nature of the primary particle. for a good reconstruction quality, the relative time-synchronization is required to be better than 1 ns between all stations of the distributed array. during one year, the 0.6 sr viewcone of the detector covers a total area of π sr for more than 200 h observation time. while this area is much larger than areas typically covered in scans performed by imaging cherenkov telescope arrays, it is fixed by the location of the experiment. in order to overcome this deficit, we plan to reorient the detector stations after a few years of operation, tilting the detector axis in north-south direction, as illustrated in figure 2, extending the sky coverage and partly deepening the exposure. figure 2: schematical view of a tunka-hiscore detector station. the tilting mechanism will allow to access a larger fraction of the sky as compared to observations pointing at the zenith alone. detail (right): each of the four optical channels consists of a pmt with a light concentrator (winston cone). 3 sensitivity the sensitivity of the hiscore standard layout was computed on the basis of a full simulation (tluczykont et al. 2011) and reconstruction algorithms (hampf, tluczykont & horns 2013). we require a detection at the level of 5 standard deviations and at least 50 gamma rays, leading to the point-source survey sensitivity shown in figure 3. the blue area corresponds to the sensitivity of the hiscore non-imaging array in its standard configuration (8 inch pmts, 150 m interstation spacing). the upper edge of the sensitivity range was obtained using conservative assumptions, assuming a ratio α of the solid angles of signal to background regions of unity. the gamma ray survival probabilities are 0.68 for the angular point-source cut, and 0.6 for the hadron rejection cut. more optimistic assumptions were made for the lower bound of the dark shaded area. here, α << 1 and the angular cut was relaxed to an efficiency of 1 in the background free regime, above 2 pev. for direct comparison, the survey sensitivity of cta (dubus et al. 2013) is shown as a dashed area, and the lhaaso sensitivity adapted from cao et al. 2010 with an additional requirement of at least 50 gamma rays is shown as dash-dotted curve. since the gamma-hadron rejection is poor when using the nonimaging technique alone, the impact of an improvement of the hadron rejection efficiency is significant. for illustration, the cyan area shows the sensitivity range when including an improvement of the hadron rejection by a factor of 10. such an improvement is realistic when combinining the hiscore concept with imaging or muon detectors. imaging detectors have proven 284 the hiscore project their gamma-hadron separation power up to several 10s of tev. at higher energies with a rising average number of muons per air shower, muon detectors become more and more efficient for hadron rejection. hiscore will open up the pevatron energy range and allow detailed spectral measurements of the continuation of known gamma-ray sources in the multi-tev to pev energy regime. at these energies, the absorption of gamma rays (e+e− pair production) in the interstellar radiation field (isrf) around 100 tev and the cosmic microwave background (cmb) around 3 pev are significant. the level of gamma ray attenuation depends on the density of the radiation fields and the distance of the objects, and can be as low as few percent. moreover, the density of the cmb being known, the absorption expected at pev energies from the cmb might open up a new method for distance estimation using gamma ray observations. energy / tev -110 1 10 210 310 410 -1 s -2 in te gr al fl ux / er g cm -1410 -1310 -1210 -1110 -1010 hiscore cta survey lhaaso point-source, 1 year )νicecube milagro sources, 5 years ( kascade u.l. mgro j1908+06 hess j1908+06 hiscore figure 3: hiscore sensitivity as compared to data on mgro j1908+06 from milagro (abdo et al. 2007) and h.e.s.s. (aharonian et al. 2009), and the sensitivity of icecube to neutrinos from milagro sources (gonzalez-garcia et al. 2009). also given are an upper limit from kascade (antoni et al. 2004), and sensitivities for experiments cta (dubus et al. 2013) and lhaaso (cao et al. 2010). recently, the icecube collaboration reported the detection of 28 neutrinos (e.g. whitehorn 2013), in excess of the atmospheric and prompt neutrino expectations, but compatible with a pevatron like spectrum. these findings are a strong additional motivation for our search for galactic cosmic ray pevatrons. 4 status of tunka-hiscore 4.1 detector components different solutions are available for the individual detector components. the photomultipliers (pmts) are required to sustain a high level of noise and to operate at a gain level of 104 105. currently, 8 inch pmts fulfilling these requirements are available from emi, electrontubes and hamamatsu (including a modified 6-stage prototype). the winston cones are assembled similarly to the construction of a wine barrel in stripes along the optical axis, using reflective sheets of alanod 4300up material. in the currently operating prototypes (april 2013), standard tunka-133 preamplifiers are in use. an analog summator for the station trigger system (see section 2) was tested in laboratory and in the field. in its final implementation, the trigger board includes clipping and triggering. further three trigger solutions are available: fadc trigger onboard the tunka-133 readout, fpga based trigger on-board the white rabbit board, and amplitude discrimination on-board the drs 4 evaluation board. all three alternative triggers use the analog sum as input. two readout systems are in operation: the standard fadc readout (200 mhz) used by tunka-133, and the drs 4 evaluation board (1 ghz). 4.2 prototype results the first tunka-hiscore prototype station (2 pmt channels) was deployed in the tunka valley in april 2012. two additional detector stations were deployed in october 2012. joint operation with tunka since then provided first cross-calibration data and experience with components. the tunka-133 fadc and the drs 4 evaluation board readout were tested in parallel in spring 2013. runs with varying thresholds were performed in order to measure the integral trigger rate as a function of the trigger threshold (epimakhov et al. 2013). as can be seen in figure 4, the expectations for the cosmic ray branch and the noise wall can be reproduced. threshold [mv] 210 t rig ge r ra te [h z] 1 10 210 310 410 510 610 710 figure 4: the trigger rate (station 4) as a function of the trigger threshold, measured with the drs 4 evaluation board in spring 2013 (epimakhov et al. 2013). red squares: dead-time corrected data, red dashed line: cosmic ray triggers, black points: estimation of nsb triggers from baseline fluctuations, black dashed curve: the nsb noise wall. 285 martin tluczykont et al. similar measurements using the fadc readout yield consistent results (wischnewski et al. 2013). an effective quantum efficiency of 0.06 could be derived from the latter measurement. this translates into a threshold for cherenkov light detection of 0.6 photons/m2, which corresponds to an energy threshold of slightly less than 100 tev when using the fadc trigger (25 ns discriminator response). the final system will have a faster response. we also plan to use faster preamplifiers, larger pmts, and geo-magnetic shielding for the pmts, ultimately lowering the energy threshold of the hiscore stations to the 10–30 tev energy range. tests of the white rabbit (wr) system show that the time precision and stability at the 200 picos level measured in the laboratory can be reproduced at the tunka site (brückner et al. 2013). the observed time difference distribution of wr-tagged events is consistent with the observation of air showers with the given viewing angle of the tunka-hiscore stations (wischnewski et al. 2013). 5 conclusion and outlook using the new large-area wide-angle non-imaging hiscore detector concept, we plan to access the pevatron gamma ray energy range (10 tev – several 100 tev) and also address cosmic ray physics around the knee region. the tunka-hiscore experiment has started the deployment of detector stations in the tunka valley in siberia. 3 prototype stations are in operation, and further 6 stations will be deployed by the end of 2013. the construction of an engineering array that will cover an area of 1 km2 is planned for the near future. different alternatives for the station design (8 inch or 10 inch pmts) and for the array layout (graded array) are considered. a combination with small imaging telescopes (2-3 m2 mirrors, 10◦ field of view) is planned to complement the tunka-hiscore array. furthermore, the installation of muon detectors and fluorescence telescopes at the tunka site is considered for the future, improving gamma hadron separation and cosmic ray composition, and leading to a multi component extended air shower array in the tunka valley in siberia. acknowledgement we acknowledge the support of the russian federation ministry of education and science (g/c 14.518.11.7046, agreements n 14.b25.31.0010, n 14.b37.21.0785, and n 14.37.21.1294, g/c 14.740.11.0890, p681, contract n 14.518.11.7046), the russian foundation for basic research (grants 11-0200409, 13-02-00214, 13-02-12095, 13-02-10001, 12-0210001, 12-02-91323), the president of the russian federation (grant mk-1170.2013.2), the helmholtz association (grant hrjrg-303), and the deutsche forschungsgemeinschaft (grant tl 51-3). we thank gavin rowell for valuable discussions. references [1] abdo, a. a., allen, b., berley, d., et al.: 2007, apj l., 664, l91 doi:10.1086/520717 [2] aharonian, f., akhperjanian, a. g., anton, g., et al.: 2009, a&a, 499, 723 [3] antoni, t., apel, w. d., badea, a. f., et al.: 2004, apj, 608, 865 doi:10.1086/420736 [4] berezhnev, s. f., et al. (tunka collaboration): 2012, nima, 692, 98 [5] brückner, m., et al.: 2013, to appear in proc. of icrc, rio de janeiro, brazil, 1158 [6] cao, z., bi, x. j., cao, z., et al.: 2010, 38th cospar scientific assembly, 38, 2322 [7] dubus, g., contreras, j. l., funk, s., et al.: 2013, aph, 43, 317 [8] epimakhov, s., brückner, m., budnev, n. et al.: 2013, to appear in proc. of icrc, rio de janeiro, brazil [9] gonzalez-garcia, m. c., halzen, f., & mohapatra, s.: 2009, aph, 31, 437 [10] hampf, d., tluczykont, m., & horns, d.: 2013, nimpa, 712, 137 doi:10.1016/j.nima.2013.02.016 [11] moskalenko, i.v., porter, t.a., strong, a.w.: 2006, apjl, 640 l155 doi:10.1086/503524 [12] tluczykont, m., hampf, d., horns, d., et al.: 2011, adv. sp. res., 48, 1935 doi:10.1016/j.asr.2011.08.004 [13] whitehorn, n. h.: 2013, fermilab astro seminar [14] wischnewski, r., berezhnev, s., brückner, m., et al.: 2013, to appear in proc. of icrc, rio de janeiro, brazil, 1164 discussion (from memory) questioner: at these high energies, gamma rays are absorbed in low energy radiation fields. how can you expect sources in the 100 tev energy range? martin tluczykont: the strength of the attenuation depends on the location of the object within our galaxy. an object located at the far side of 286 http://dx.doi.org/10.1086/520717 http://dx.doi.org/10.1086/420736 http://dx.doi.org/10.1016/j.nima.2013.02.016 http://dx.doi.org/10.1086/503524 http://dx.doi.org/10.1016/j.asr.2011.08.004 the hiscore project the galaxy behind the galactic center was shown (moskalenko et al. 2006) to be absorbed at the 50 % level. any other object located closer to the sun, or off-center the galactic disk, will be less absorbed, down to only few %. 287 introduction detector concept sensitivity status of tunka-hiscore detector components prototype results conclusion and outlook 226 acta polytechnica ctu proceedings 2(1): 226–229, 2015 226 doi: 10.14311/app.2015.02.0226 when a nova becomes old a. ederoclite1, c. tappert2, l. schmidtobreick3, n. vogt2 1centro de estudios de f́ısica del cósmos de aragón (cefca), plaza san juan 1, planta 2, 44001, teruel, spain 2instituto de f́ısica y astronomı́a, universidad de valparaiso, av. gran bretaña 1111, valparaiso, chile 3european southern observatory, alonso de cordova, santiago, chile corresponding author: aederocl@cefca.es abstract here we present the preliminary results of a project aimed at unveiling the nature of classical novae decades after their eruption. the ultimate goal of this project is to describe the population of cataclysmic variables which give rise to nova explosions. so far, in four years of observations, we have concentrated on novae in the southern hemisphere, where we increased by 100% the amount of objects spectroscopically confirmed and increased by 1/5 the amount of objects with known orbital period. keywords: cataclysmic variables classical novae optical spectroscopy photometry. 1 introduction classical novae (cne) are the result of a thermonuclear runaway on the surface of a white dwarf which is accreting mass from a less evolved companion (for a review, see bode & evans, 2008). it is common to define cne as a “subclass of cataclysmic variables” which is a fair statement in both historical and observational terms but it is not quite physically appropriate: a cnexplosion is a phase during the life time of a cataclysmic variable (cv). according to the “hibernation scenario” (shara et al., 1986), post-nova cvs are expected to go through a phase of high mass transfer (triggered by the irradiation of the hot wd) and, eventually, the system gets to separate and spends 90-99% of the inter-explosion time in a quiescent “detached” state (prialnik 1986). it is clear that the use of the term “quiescence” for an oldnova system is very vague and it is the reason why it is used in quotes throughout this work. for a review on old novae, see pagnotta (2013). this project started four years ago trying to answer some fundamental questions: • how does the population of old novae look like? • how does the population of old novae compare to cvs? • do short-period old novae exist in a significant number? • what is the role of magnetic fields on the cnoutburst? despite their brightness at maximum light, the study of novae in “quiescence” is still largely incomplete (again, see pagnotta, 2013). it is important to note that, before this project started, only 39 old novae (out of the 204 novae which exploded before 1980) had a measured orbital period. so far, our project has focused mostly on the southern hemisphere (i.e. targets with δ < 20◦), where, before the start of our project, out of 153 reported novae, 34 lacked identification of the target in “quiescence”, 9 were confirmed but their period was unknown and 24 had a known orbital period. the remaining 86 did not even have a candidate. here we describe the methodology and some early results related with the population which has been recovered so far. for the description of selected targets, see schmidtobreick & tappert (2013) and tappert et al. (2013c) . 2 the methodology our project consists of three phases: the photometric identification of the nova candidate, the spectroscopic confirmation and the period determination through time-series spectroscopy. the target list has been derived mainly from the downes et al. (2005) catalogue. we selected those novae which had exploded before 1980. the selection is made to allow some time after the nova explosion for the characteristics of the underlying cv to become accessible. 226 http://dx.doi.org/10.14311/app.2015.02.0226 when a nova becomes old 1 2 3 4 5 10 20 50 porb(h) 0 2 4 6 8 10 12 14 16 18 this project, n=8 periods of novae before 1980, n=39 1 2 3 4 5 10 20 50 porb(h) 0 5 10 15 20 25 all periods of novae n=78 figure 1: left: period histogram of novae which exploded before 1980. the black histogram is the addition from this project. right: period histogram of all novae (i.e. including those which exploded after 1980). the dotted vertical lines refer to the period gap and the dashed vertical line to the period minimum. we do not include in our sample recurrent novae (rne). these represent thermonuclear explosions analogous to cne but which occur on much shorter recurrence times. the large majority of those rare stars (only a dozen is known in our galaxy) harbour well-evolved secondary stars which sets them somewhat apart from other cvs. as our project primarily originated with the idea of testing the predictions of the hibernation scenario, rne are not included in our analysis. we obtain ubvr photometry of the region of sky where a nova has been reported. here we are taking advantage of the fact that an old nova, as any cv, has a spectrum which is the sum of three components (the white dwarf, the main sequence companion and the accretion disk). this is the same discovery strategy which is being used in large photometric surveys, like sdss (see szkody, 2013). we obtain spectroscopy of those targets whose location in the colour-colour diagram, does not follow the one of the stars. this is “weighted” by the distance from the reported position (i.e. not necessarily the bluest object in the field is the first candidate for follow up spectroscopy, in case there is another object which is closer to the reported position). low-resolution spectroscopy (r = λ ∆λ ' 500) is enough to resolve cvs emission lines and to cover a spectral range (∼4000–9000å) which includes the most relevant lines for cv classification. time-series is obtained once the target has been spectroscopically confirmed. in some cases, time-series photometry is carried out (e.g. tappert et al., 2013b) but, in general, we perform time-series spectroscopy with r = λ ∆λ ' 2500 in the hα region. in order to keep a log of the observations and share the results within a group which is geographically spread over two continents, an internal webpage is being maintained. here we keep a list of our targets, observing logs, and description of the results for each target. 3 results 3.1 the period distribution the left panel of fig.1 shows the histogram of the pre1980 novae. in grey, the periods that can be obtained in the literature and, in black, the addition due to this project. the right panel of fig.1 shows the histogram of all the novae (also the ones which exploded after 1980). the periods of post-1980 novae are from ritter & kolb (2003). qualitatively, the two histograms are very similar: there are few sources with periods within the period gap and a handful of sources below the period gap. there is a “clustering” of sources right above the period gap (just in the period region where sw sex stars belong, see schmidtobreick & tappert, 2013) and less sources at longer periods. this is an interesting result by itself, since our sample is selected on the basis that the targets are back in “quiescence”. the only significant difference is the number of objects found in the 5 – 10 hours period range, which can be easily ascribed to small-number statistics. as we already mentioned, very few objects are within (or below) the period gap. this is consistent with old novae being in a high-mass transfer phase, as predicted by the “hibernation” scenario. it is also worth noting that the overall shape is consistent with the modelling by townsley & bildsten (2005), which assumes that novae occur mostly in high mass accretion rate systems. 227 a. ederoclite et al. 3.2 the role of magnetic fields as we mentioned in the previous section, there are very few objects with periods<3 hours, and, as it can be seen from fig.2, these objects are mostly magnetic. among the 47 pre-1980 old novae with a measured period, only v1500 cyg (1975) and cp pup (1942) are catalogued as am her stars and have periods of 3.36 hours and 1.68 hours, respectively. 0 5 10 15 non magnetic systems am her 1 10 porb(h) 0 5 10 15 non magnetic systems ip 0 5 10 15 non magnetic systems dq her figure 2: the histograms of the magnetic systems which are observed to have harboured a nova explosion. in white, for comparison, the histogram of nonmagnetic systems. in the upper panel, the am her, in the middle panel the dq her systems and, in the lower panel the intermediate polars. the classification is either from ritter & kolb (2003) or from tappert et al. (2012,2013a). although the number of magnetic systems is still small (hence we cannot draw firm conclusions) it is promising that polars and intermediate polars are located in a similar region of the period histogram as in the “classical” cv period histogram. moreover, as mentioned by tappert et al. (2013b), 7 out of 10 novae in the gap may be magnetic. this is in agreement with the population model by townsley & bildstein (2005), who suggest that novae within the period gap are mostly magnetic. araujo-betancor et al. (2005) derive that the total ratio of known magnetic to nonmagnetic cvs is ∼22% (at all periods). the high number of magnetic cvs within the gap, if confirmed, may imply a different evolution through the gap between magnetic and nonmagnetic systems. 3.3 the explosion amplitude from downes et al. (2005), one can derive that the average explosion amplitude is 12.57±2.47 mag. combining our data with data from downes et al. (2005), ritter & kolb (2003), duerbeck (1987), one can try to compare the explosion amplitude (i.e. the difference between the magnitude at maximum and the magnitude in “quiescence”) with the rate of decline (see fig.3). this has already been done by vogt (1990). in fig.3, we show the data from vogt (1990) plus data from our project and post-1980 novae from ritter & kolb (2003). it is interesting to note that our data points (the red squares) fall in the same region as the others. the straight line is the fit from vogt (1990) to the points in his paper and is still valid with the inclusion of the new points. −0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 log t3 4 6 8 10 12 14 16 18 20 d e lt a m figure 3: rate of decline (as measured by the t3 parameter) vs. explosion amplitude (difference between the magnitude at minimum and the magnitude at maximum). the red squares are from this project. the line is not a fit to the present data-set but to the data presented in vogt (1990). the two objects which seem to deviate from the relation are v1500 cyg (1975) and v458 vul (2007). the first one, as mentioned in the previous section, is an am her star. the second one showed a very irregular light curve, with three repeated peaks. these two examples suggest that the scatter in this relation is likely due to two independent causes: on one hand, the magnitude at “quiescence” depends on the structure of the accretion disk (if any) and the magnitude at maximum light (and the rate of decline) may be misestimated due to a series of factors (e.g. bad sampling or dust formation) which are quite typical in novae. warner (1987) has already 228 when a nova becomes old suggested that, for a better comparison of the explosion amplitude, one should take into account, at least, the orbit inclination of the system. another important caveat is that some old novae are observed to decrease their “quiescent” brightness (see e.g. vogt 1990, and references therein, or johnson et al. 2013) and, therefore, fig.3 should be considered in a more dynamical context. 4 summary and future perspectives we have presented a project aimed at the recovery, classification and characterisation of classical nova systems decades after the explosion. during this project, which started four years ago, we have already significantly increased the number of identified and spectroscopically confirmed classical novae (novae without an identified progenitor decreased by 24% and the number of spectroscopically confirmed increased by 100%). finally, we have also increased by 1/5 the number of pre-1980 oldnovae with known orbital period. we plan to finalise the southern part of our project during the next two years and to focus on targets in the northern hemisphere. once the program will be completed, this sample will be crucial in the study of the evolution of classical novae towards quiescence, the study of cn-progenitors as a subclass of the cnpopulation and the development of of the “nova populations” framework (della valle 2002 and references therein). for better sharing the results of our project, we are planning on providing access through the virtual observatory to the reduced data, thus sharing a database with basic parameters (position, brightness, type,. . . ), the spectra used for characterisation and finding charts. this data release will represent a significant legacy for the cv community. acknowledgement the cefca is funded by the fondo de inversiones de teruel, supported by both the government of spain (50%) and the regional government of aragón (50%). this work has been partially funded by the spanish ministerio de ciencia e innovación through the pnaya, under grants aya2006-14056 and through the icts 2009-14. ct and nv acknowledge financial support by fondecyt regular grant 1120338. references [1] araujo-betancor, s. et al.: 2005, apj, 622, 589 [2] bode, m. & evans, n.: 2008, “classical novae”, 2nd edition, cambridge astrophysics series, no 43, cambridge: cambridge university press, 2008 [3] della valle: 2002, aipc, 637, 443 [4] downes, r. et al.: 2005, jad, 11, 2 [5] duerbeck, h.: 1987, space sci. rev. 45, 1 doi:10.1007/bf00187826 [6] johnson, c. et al.: 2013, apj, 780l, 25 [7] pagnotta, a.: 2013, this workshop [8] prialnik, : 1986, apj, 310, 222 [9] ritter h., kolb u. 2003, a&a, 404, 301 doi:10.1086/164762 doi:10.1111/j.1365-2966.2012.21054.x [10] schmidtobreick, l. & tappert, c.: this workshop doi:10.1093/mnras/stt139 [11] shara, m. et al. 1986 apj, 311, 163 doi:10.1093/mnras/stt1747 [12] szkody, p.: 2013, this workshop [13] tappert, c. et al.: 2012, mnras, 423, 2476 doi:10.1086/430594 [14] tappert, c. et al.: 2013a, mnras, 431, 92 doi:10.1086/168866 [15] tappert, c. et al.: 2013b, mnras, 436, 2412 [16] tappert, c. et al.: 2013c, this workshop [17] townsley, d. & bildsten, l.: 2005, apj, 628, 395 [18] vogt, n.: 1990, apj, 356, 609 [19] warner, b.: 1987, mnras, 227, 23 229 http://dx.doi.org/10.1007/bf00187826 http://dx.doi.org/10.1086/164762 http://dx.doi.org/10.1111/j.1365-2966.2012.21054.x http://dx.doi.org/10.1093/mnras/stt139 http://dx.doi.org/10.1093/mnras/stt1747 http://dx.doi.org/10.1086/430594 http://dx.doi.org/10.1086/168866 introduction the methodology results the period distribution the role of magnetic fields the explosion amplitude summary and future perspectives acta polytechnica ctu proceedings doi:10.14311/app.2017.10.0012 acta polytechnica ctu proceedings 10:12–15, 2017 © czech technical university in prague, 2017 available online at http://ojs.cvut.cz/ojs/index.php/app features regarding interaction of anchor piles such as “titan” and “geoizol” with soils mikhail. i. nikitenkoa,∗, igor p. dubatovkab a stroitechnorm, stroikompleks, 220037, minsk, ul. filimonov, 8-105. belarus b monorakurs, 220116, minsk, pr. dzerzhynsky, 104b. belarus ∗ corresponding author: michnikit@gmail.com abstract. the article describes the experience of using anchors at a deep excavation for the construction of a multifunctional complex in minsk. the authors describe the features of the device and the testing of the pressure grouted anchoring structure of “geoizol” (according to the “titan” type system). keywords: deep excavation pit; underground retaining wall; anchor piles titan, geoizol.. 1. introduction cut-off diaphragm wall was used for the protection of the deep excavation in the construction of a multifunctional complex in minsk. these walls were anchored with anchors at one, two or three levels. [1] “geoizol” anchors according to “titan” type were used for anchoring of the diaphragm walls. these piles were installed with the help of pressure grouted technology [2–8] . as these anchor piles differ from conventional continuous pressure grouted anchors the nature of these differences will be described. at the end the advantages and disadvantages of the described anchor piles during their installation and application are specified. 2. construction site multifunctional complex (ifc) is situated in the northeast of minsk. the geotechnical site conditions were described as rather complicated, composed from different horizons, as holocene, mogilev, slavgorodskiy, dniprovskiy, which contains mostly moraine sediments of different compositions. starting from boulders up to fine sandy loam, arranged not only in layers, but sometimes in the shape of closed lenses. top aquitard are moraine loam mogilev sub horizon, lower moraines slavgorod sub horizon. between them is artesian water, non-aggressive to concrete. the amount of pressure was 9.0 – 13.6 m. 3. constructive solution of anchor “titan” piles and “geoizol” the design of the anchor rod comprises a steel tubular rod with a helical knurled outer surface consisting of joined screw couplings units, and on the bottom of which at the end there is screwed a drill bit with outlet holes, and at the top there is a lock nut with a spherical contact surface which rests on the construction plate (fig. 1). in this design concept the hollow anchor rod figure 1. structural elements of anchor piles “titan” and “geoizol” combines the functions of a drilling rod when drilling wells through a progressive rotational movement of the rod with a drill bit, and an injector for injecting into the cavity a cement slurry, which, under pressure erodes at the bottom soil and under agitation then plugging the borehole, creating a hardening of shaft piles. subsequently the rod reinforces the pile and on pulling out it provides a recognizable tensile force. 12 http://dx.doi.org/10.14311/app.2017.10.0012 http://ojs.cvut.cz/ojs/index.php/app vol. 10/2017 features regarding interaction of anchor piles with soils 4. sequence of operations when constructing the type of anchor piles “titan” and “geoizol” the technological operations of the device anchor piles in the soil include borehole drilling to the projected depth of the tubular rod in rotation together managed with drilling crown and soil displacement. this is pumped through the rod chamber plugging cement slurry which erodes soil and facilitates drilling of the downhole, fills it and prevents soil collapse. when the cement slurry strength attains at least 70 % of that designated (in sand 14 days during solution without plasticizer c-2 and 7 days with plasticizer), all are subject to tension anchor piles. in clayey soils a drainage solution is needed. figure 2. excavation pit overview and installation of anchoring piles. after the excavation of the construction site on the level of the first row of anchors, horizontal i beam was applied together with large steel plates, fig. 2. after that the anchoring piles were constructed. the lengths of anchor piles with diameter 0.25 m, was at first level 15 m and 12 m for the second level. the lengths of the workers (root) segments were 8.4 m and 6.9 m. they were located in the strata of sandy loam. 5. anchoring piles testing anchors testing should evaluate the shear strength of sandy loam along the trunks and roots of the anchoring piles and help a confirmation of shear strength used during the stability of retaining wall design. first three anchors are designed to identify the role of the injected grout composition to set them quality strength after 7 days, and also for definition of the actual barrel shape and diameter. to prove this the anchors were excavated (see fig. 3). the excavation was carried out by inspection and measurements. the control showed that their trunks are roughly cylindrical in shape with a diameter of 0.25 m without separation of free and working root segments. it was established that the strength of cement grout in all samples was similar, reaching over 70 % of the calculated value. the arrangement for the pull-out tests is shown on fig. 4. maximum tensile force was 549 kn, indicating the average shear strength along perimeter and length of anchoring part equal to 140 kpa. two other testing anchors were left in ground for 28 days. for one of them the hardening accelerator was used and the second anchor was constructed without hardening accelerator. no significant differences were observed. 6. advantages and disadvantages of the anchor of the “titan” piles or “geoizol” testing of 5 anchoring piles and their excavation allows some statement about the application of these anchors. advantages are as follows: • structural design ensures that the high strength hollow rod with knurling screw on the outer surface and with cross-cut bit at the bottom allows insertion of the anchor into ground, followed by injection of grouting cement slurry under high pressure. slurry primarily wash away soil at the bottom, supports borehole stability and after hardening guarantee good contact with rod surface; • minimal elongation of high strength anchor rod during loading; • low risk of corrosion. however there are also some shortcoming as: . the creation of a local broadening on the lower end of the pile, i.e., working in the root zone is problematic, since the liquid cement slurry further pumping of denser high-pressure pressure leads approximately to the same stem diameter across the length of the borehole, with the exception of more intensive compressible soil layers; . the huge over expenditure of cement due outpouring of wellhead in the process of drilling or during injection; . hardening cement slurry is excluded because it is difficult to rotate the rod with the drill bit, while increasing their water-cement ratio (w / c) sedimentation processes are amplified. up to now experiences in low permeable ground leads to the recommendation to use drainage to drain excess of water from the slurry injected with a great value of w/c in accordance with the inventions as are no 1281635, 1527378 and 139 2203 see [3],[6]. application of geotextile sleeve can fulfil this drainage role. 13 mikhail. i. nikitenko, igor p. dubatovka acta polytechnica ctu proceedings figure 3. removing from the ground (a) the form of excavated three anchor piles (b). fragment of anchor piles body remaining in the ground (c), measurement of the diameter on the contact with the stem (d). figure 4. assembling panels of support system for two experienced anchor piles and sampling testing for one of them. 7. conclusions testing of the anchoring piles of the type “titan” and “geoizol” on the construction site in minsk, make it possible to specify not only advantages of this system, but also some shortcomings. to partly eliminate the shortcomings some recommendations were proposed namely draining geotextile sleeve with local tire rings along the length of the roots and packers. references [1] p. posobie. designing and organizing the pile and wall in trench. 14-01 for snb 5.01.01-99. architecture and construction ministry of belarus, minsk, 2002. [2] p. posobie. design and construction of bored piles. 13-01 for snb 5.01.01-99. architecture and construction ministry of belarus, minsk, 2002. [3] p. posobie. designing and construction of injection anchor and piles. 18-04 for snb 5.01.01-99. architecture and construction ministry of belarus, minsk, 2005. 14 vol. 10/2017 features regarding interaction of anchor piles with soils [4] tkp. foundations and underground structures, constructed with the use of inkjet technology. terms of design and construction. 45-5.01-45-2006 (02250). architecture and construction ministry of belarus, minsk, 2006. [5] m. i. nikitenko. continuous pressure grouted piles and anchors in the construction and reconstruction of buildings and structures. belarusian national technical university, minsk, 2007. [6] a. g. malinin. application of anchor “atlant” piles in underground construction. earth science 2:248–254. [7] c. a. ritov. effective modern device technology of continuous pressure grouted piles and injecting anchors. information bulletin 16(1):7–14. [8] c. zolotov. about the bases of the device anchor piles titan. construction and real estate pp. 1–3. 15 acta polytechnica ctu proceedings 10:12–15, 2017 1 introduction 2 construction site 3 constructive solution of anchor ``titan'' piles and ``geoizol'' 4 sequence of operations when constructing the type of anchor piles ``titan'' and ``geoizol'' 5 anchoring piles testing 6 advantages and disadvantages of the anchor of the ``titan'' piles or ``geoizol'' 7 conclusions references 81 acta polytechnica ctu proceedings 2(1): 81–85, 2015 81 doi: 10.14311/app.2015.02.0081 activity of the polar am her (rx j1816.2+4952): a short review v. šimon1,2, a. henden3 1astronomical institute, academy of sciences of the czech republic, 25165 ondřejov, czech republic 2czech technical university in prague, fel, prague, czech republic 3aavso, 49 bay state road, cambridge, ma 02138, usa corresponding author: simon@asu.cas.cz abstract we show that am her displays the transitions between the high and low states with an intermittently existing dominant cycle with length between 400 and 800 days. moreover, these transitions accumulate in clusters, which produces an additional long cycle after smoothing; a single isolated short episode of the low state does not suggest a break of this cycle. the seasons of existence of the cycle can be controlled by the lifetime of the active regions (e.g. prominences, spots) on the donor. in some high-state episodes, a higher luminosity of the bremsstrahlung emission is not accompanied by a higher optical (cyclotron+stream) emission. part of the bremsstrahlung emission can be buried in some episodes. changes of the structure of the accretion region(s) are necessary to explain the variations of the optical and x-ray activity in the high-state episodes of am her. keywords: cataclysmic variables polars activity emission mechanisms optical x-rays individual: am her. 1 introduction am her is the prototype of polars (e.g. [19]). its activity is dominated by the large-amplitude long-term variations: alternating high and low states of the optical brightness usually on the timescales of months and years (e.g. [8, 21, 10]). a small accreting region (about 0.1 of the radius of the white dwarf (wd)) near the magnetic pole of this object is the source of radiation and polarization of polar via several processes: cyclotron (accretion column – mainly optical and ir emission), thermal (soft x-rays – surface of the wd heated by the accreting region), bremsstrahlung (accretion column – hard x-rays) (e.g. [19, 15]). the conditions in this region and the mass accretion rate play an important role in governing the observed activity. 2 long-term activity in the optical band fig. 1a shows the long-term activity of am her in the optical band spanning about 32.7 years. the data come from the aavso international database (massachusetts, usa) [6]. the highly variable orbital modulation has a large amplitude mainly in the high state (e.g. [9, 10]), so the character of the long-term activity is less discernible. fig. 1a therefore shows the hec13 fit running through the orbital modulation (each point is a smoothed value of brightness in the night of observing).the division between the high and low states is 14.3 mag(v ). the properties of the statistical distribution of brightness of the high states strongly evolve with time (fig. 1cd). the typical high-state brightness is near the middle of the distribution on the magnitude scale in most time segments. the low states often show skewness of brightness toward the brighter magnitudes. the fuzzy bright edge of the high-state brightness, smoothed over the orbital modulation, in the histogram suggests that the high state is not a uniquely defined level of luminosity of am her (fig. 1cd). it is questionable whether any “unperturbed” state of the system exists et all. a typical strength of the active regions (large loops of magnetically confined gas near the donor due to its magnetic activity [9] and star spots [13, 7]) during the high state is therefore needed (the times of the quite quiescent donor are thus very rare). in the scenario proposed by [21], this statistical distribution also suggests a typical configuration of the magnetic field of the whole system (donor+wd), around which the system balances during the high state of activity. the brightness of the low states in am her often stabilizes at the baseline level. the mass transfer via roche lobe overflow almost ceases, only clumpy accretion onto the wd from the donor’s wind remains [3, 9]. the strong skewness of brightness in fig. 1cd can emerge if the hec13 fit runs through the variably recurring brightenings. 81 http://dx.doi.org/10.14311/app.2015.02.0081 v. šimon, a. henden moving averages (ma) of brightness with various filter half-widths q enable to search for the evolution of the optical activity of am her on timescales of several years, significantly longer than the duration of the individual episodes of the high or low states (fig. 1). the transitions between these states tend to occur in clusters in am her. smoothing then reveals a long cycle because of the variable clustering of the episodes of the low states. since the borders of these clusters are fuzzy, a smooth profile of the cycle results when ma are applied. a single isolated short episode of the high or the low state does not suggest a break of this cycle since the cycle is caused by the repeating accumulations of these transitions in clusters. some long cycles thus may not be easily discernible without investigating the variations of the parameters of activity, which ma enable (fig. 1ab). figure 1: (a) the long-term activity of am her in the optical band (aavso data [6]). hec13 fit runs through the orbital modulation. the mapping is divided into segments. the smooth lines represent the moving averages (ma). (b) residuals of the ma. (c, d) examples of the strikingly different statistical distributions of brightness. see sect. 2 for details. search for the cycle-length in the transitions between the high and low states from fig. 1a was carried out also with the weighted wavelet z-transform (wwz) developed by [1]. this method enables one to determine the period and amplitude of unevenly sampled time series. wwz indicates whether or not there is a periodic fluctuation at a given time at a given frequency f (fig. 2a). the wwz-transform really finds that the transitions between the states are not quite chaotic. a dominant cycle in the transitions really exists. however, the cycle-length of the transitions is not stable and this cycle is not conclusively detected in some time segments. fig. 2b shows the best cycle-length, determined from f that has the biggest value of wwz at a given time (only the segments in which the amplitude is larger than 60 percent of its peak value in the investigated range of f). in summary, this cycle is characteristic only for a limited time segment. it is intermittent and unstable in length. moreover, the wwz-method shows also another temporarily existing shorter cycles in fig. 2a. for comparison, the cycles of 178 d, 1836 d, 830 d and 1520 d were found by fourier analysis by [11]. the differential rotation of the lobe-filling donor (modeled by [18]) can influence the position of the above-mentioned active regions on the donor with respect to the l1 point. the breaks of the cycle of the transitions between the high and the low states in am her can be caused by a limited lifetime of the above-mentioned active regions connected with the activity of the donor. figure 2: (a) wwz-transform of the light curve of am her from fig. 1a. (b) the best cycle-length. see sect. 2 for details. 3 optical and hard x-ray intensities in the high states in am her, increase of the mass transfer rate from the donor (giving rise to a high-state episode) also establishes specific division of the emission into various spectral regions during the accretion process (fig. 3) (see our analysis in [17] for details). the properties of the emitting region(s) on the wd are established in the 82 activity of the polar am her (rx j1816.2+4952): a short review early phase of the high-state episode (but not reproduced for every episode) of am her. a higher luminosity of the bremsstrahlung emission may not be always accompanied by a higher optical (cyclotron+stream) emission in a given episode of the high state (the intensities in both of these bands were averaged over the orbital period) [17]. this can significantly differ even for two consecutive episodes (fig. 3bc). this relation of intensities is representative for the whole such episode. purely geometric effects (asynchronous rotation of the wd, precession of the wd spin axis) cannot explain the observations – gradual evolution of the x-ray and optical intensities with time would be needed (see [17] for details). we propose the following scenario. the source of the bremsstrahlung emission is confined to a smaller region than that of the cyclotron emission. part of the bremsstrahlung emission is buried in some high-state episodes. since there is no smooth transition of the ratio of the optical and hard x-ray intensities [17], it suggests that the role of several modes of accretion is worth considering. figure 3: (a) part of the asm/rxte (1.5–12 kev) [12] light curve of am her (15-d means). the mean levels and their standard errors are marked. (b, c) intensities of the optical (afoev data, the intensity i = 1 for 14.3 mag) and hard x-ray (1.5–12 kev) (asm/rxte data) emissions in two consecutive episodes of the high state. a higher luminosity of the hard x-ray emission may not be always accompanied by a higher optical emission in a given episode. 4 role of the accretion modes the orbital modulation of am her displays significant changes in both the optical and x-ray spectral regions. unexpected mode of the x-ray (soft: 0.1–0.31 kev, hard: 1.9–8.5 kev) orbital modulation was observed in two observing runs of the same optical high state by [4]. it suggested a two-pole accretion mode in which the primary pole was the dominant emitter of the hard x-rays while the secondary pole was dominant in the soft x-rays. the accretion mode is representative only for a given high-state episode of am her [14]. a short episode of the high state (but less bright than most other highstate episodes) displayed a single-pole accretion mode with a self-occultation of the primary pole, resulting in a deep primary minimum in the bands between e = 0.6 and 10 kev, but no sign of the secondary minimum. the subsequent brighter high state displayed a two-pole accretion mode with a shallow primary minimum in the 2– 30 kev band, suggesting the big dimensions of the accretion region. the secondary minimum occurred only in the soft band (1–2 kev), suggesting that the secondary pole is the dominant emitter of the soft x-rays. the same situation as those observed in one of the previous high states by [4], that is the primary pole dominant in the hard x-rays and the secondary one dominant in soft x-rays, thus repeated. also cyclotron emission undergoes peculiar changes with the accretion mode. in the above-mentioned bright high state observed by [14], cyclotron emission from a region at the primary accreting pole dominated in the v -band [2]. since a two-pole accretion was determined from polarization of the optical emission for another high-state episode by [20], it suggests that the accretion modes are very unstable in am her. changes of the structure of the accretion region(s) are necessary to explain the variations of the optical and x-ray orbital modulation in the individual episodes of the high state in am her. the accretion modes (singlepole accretion, two-pole accretion) vary with time; they may be characteristic for a given high-state episode, but not for another one. since the properties of the accretion region at the primary pole largely differ from those at the secondary pole, these changes of the accretion modes contribute to the complicated long-term activity. the transition from the high to the low state is accompanied by dramatic changes of the size and structure of the cyclotron-emitting accretion region in am her (fig. 4). a decrease of the dimensions of the accretion region(s) during the transition emerges from these facts: the rises and declines between the extremes of brightness (especially of the branches of the primary minimum) become progressively steeper, the amplitude 83 v. šimon, a. henden of the modulation increases, the eclipse of the primary pole becomes very prominent. during the final phase of the high-state episode, the properties of the cyclotronemitting region do not scale exactly with the optical luminosity. am her appears to have the primary accreting region close to the position which is close to the grazing eclipse; a slight increase of its vertical and/or horizontal dimensions (and variable position) can give rise to a highly variable orbital modulation. this is visible not only for the site of the cyclotron emission (fig. 4), but also for the site of the bremsstrahlung emission which can increase so much that it even becomes temporarily uneclipsed in some high states [16]. figure 4: dramatic changes of the orbital modulation during transition from the high to the low state of am her (ccd v -band aavso data). time in jd– 2 400 000 is given in the legend. the orbital ephemeris of [5] is used. 5 conclusions in am her, the optical emission displays cycles in the transitions between the high and low states, which are detectable only for the limited time segments. in addition to the intermittent cycle of the transitions, accumulation of episodes of the low states in clusters produces an additional, long cycle of activity. a single isolated short episode of the low state does not suggest a break of this cycle. this has a consequence for the structures and their lifetimes on the donor and/or the changes of the configuration of the magnetic field in the whole system. the configuration of the accreting matter influences the emission of am her in various spectral bands. the accretion modes (single-pole accretion, two-pole accretion) vary with time (from one high-state episode to another). also the changes of the accretion region(s) and/or accretion modes are necessary to explain the relation between the optical and x-ray emission in the high states. acknowledgement this study was supported by grants 13-33324s and 13394643 provided by the grant agency of the czech republic. this research has made use of the observations provided by the asm/rxte team, the swift/bat team, the aavso international database (massachusetts, usa) and the afoev database (strasbourg, france). we thank the variable star observers worldwide whose observations contributed to this analysis. we used the code developed by dr. g. foster and available at http:// www.aavso.org/winwwz. we thank prof. petr harmanec for providing us with the code hec13. the fortran source version, the compiled version and brief instructions on how to use the program can be obtained via http:// astro.troja.mff.cuni.cz/ftp/hec/hec13/. references [1] foster, g.: 1996, aj, 112, 1709 [2] gänsicke, b. t., et al.: 2001, a&a, 372, 557 [3] gänsicke, b. t., et al.: 2006, apj, 639, 1039 doi:10.1086/499358 [4] heise, j., et al.: 1985, a&a, 148, l14 [5] heise, j., verbunt, f.: 1988, a&a, 189, 112 [6] henden, a.: 2011, aavso database [7] hessman, f. v., et al.: 2000, a&a, 361, 952 [8] hudec, r., meinunger, l.: 1976, ibvs, 1184, 1 [9] kafka, s., et al.: 2008, apj, 688, 1302 doi:10.1086/592186 [10] kafka, s., hoard, d. w.: 2009, pasp, 121, 1352 doi:10.1086/648579 [11] kalomeni, b.: 2012, mnras, 422, 1601 doi:10.1111/j.1365-2966.2012.20736.x [12] levine, a. m., et al.: 1996, apj, 469, l33 [13] livio, m., pringle, j. e.: 1994, apj, 427, 956 doi:10.1086/174202 [14] matt, g., et al.: 2000, a&a, 358, 177 84 http://dx.doi.org/10.1086/499358 http://dx.doi.org/10.1086/592186 http://dx.doi.org/10.1086/648579 http://dx.doi.org/10.1111/j.1365-2966.2012.20736.x http://dx.doi.org/10.1086/174202 activity of the polar am her (rx j1816.2+4952): a short review [15] michaut, c., et al.: 2012, mmsai, 83, 665 [16] priedhorsky, w., et al.: 1987, a&a, 173, 95 [17] šimon, v.: 2011, newa, 16, 405 doi:10.1016/j.newast.2011.03.001 [18] scharlemann, e. t.: 1982, apj, 253, 298 doi:10.1017/cbo9780511586491 [19] warner, b.: 1995, cataclysmic variable stars, cambridge univ. press, cambridge [20] wickramasinghe, d. t., et al.: 1991, mnras, 251, 28 [21] wu, k., kiss, l. l.: 2008, a&a, 481, 433 85 http://dx.doi.org/10.1016/j.newast.2011.03.001 http://dx.doi.org/10.1017/cbo9780511586491 introduction long-term activity in the optical band optical and hard x-ray intensities in the high states role of the accretion modes conclusions acta polytechnica ctu proceedings doi:10.14311/app.2016.5.0038 acta polytechnica ctu proceedings 5:38–40, 2016 © czech technical university in prague, 2016 available online at http://ojs.cvut.cz/ojs/index.php/app selected aspects of etcs-l3 deployment petr koutecký czech technical university in prague, faculty of transportation sciences, department of transport telematics, konviktská 20, prague, czech republic correspondence: koutepe1@fd.cvut.cz abstract. this paper focuses on selected problem areas in the implementation of etcs l3, analyz-es their impacts and offers a number of possible solutions as a basis for pilot testing. it compares the suitability of each solution and summarizes their advantages and disadvantages. keywords: ertms, etcs l3, odometry, its-r. 1. introduction control and command systems on the railways have undergone a relatively long development. on the one hand, the development is driven by increasing demands on safety, reliability, smoothness and fuel economy, on the other hand, the development was mainly limited by technical capabilities of the time in the past. the development possibilities of electronics and especially microprocessor technology and computing power have allowed in recent years to increasingly address the question of optimizing the control process quality and minimizing the necessity of the human factor’s intervention, which in earlier times was not possible to such an extent. this technical progress, pressure on cost cutting and the european union requirements for the interoperability of the railway networks have created the need for addressing the management and security of railway services in new ways. introducing new systems brings the need of identifying and addressing the potential risks and technical challenges associated with the transition to the new system. the aim of this paper is to point out possible problems in the implementation and deployment of etcs l3 on the conventional railway network and suggest possible solutions to these problems with the specification of advantages and disadvantages of the various options. 2. requirements for the full implementation of intelligent transport systems on railways (hereinafter its-r) and for the purposes of this article, we consider the target state to be a state in which the following conditions are met: (1.) there is a bidirectional data connection to the train (between mobile and stationary parts), (2.) the integrity check of the train using technical means on the vehicle is ensured, (3.) all locomotives are equipped with an on-board unit (obu) etcs. when projecting these demands while considering the current trend and the commitments of the european union member states in the field of the railway network interoperability, the ertms system on application level 3 (l3) is a suitable candidate for addressing the command and control system (ccs). 2.1. substantiation of the requirements requirements specified in the previous section are based primarily on the overall concept of its-r. the need for a data connection to the train is given mainly by the necessity of the existence of such a data transfer connection between the stationary and mobile parts of the etcs l3 system. however, the data connection can also be used with other subsystems in the its-r concept, such as the transmission of data for the ato system, transmission of diagnostic data, etc. another requirement – to ensure the train integrity – is another necessary condition for the etcs l3 system’s activity, where the existence of detection devices already disregarded. the third requirement requiring full equipment of the cabin section in all vehicles is again given by application level 3 of the etcs system, where mixed traffic (such as with l2) is operationally unacceptable after removing the signals. only the connection and fulfillment of all three requirements above at the same time gives us the opportunity to fully start taking advantage of etcs l3 âăş primarily to use the opportunity to minimize the infrastructure elements (signals, track circuits, axle counters). such minimization has several far-reaching impacts – reducing the investment demand compared to the situation where application level 2 (or even one) of the etcs system is installed to achieve the desired safety parameters; according to [1] by approximately 25% compared with l2. it is necessary to emphasize that etcs l1 and l2 are "only" an extension of conventional interlocking equipment and block signaling system and track security equipment. in practice this leads to duplication of some functions in rbc and ixl. this delivers increased investment and operating costs for the construction and operation of two parallel systems and also reduces the possibilities of 38 http://dx.doi.org/10.14311/app.2016.5.0038 http://ojs.cvut.cz/ojs/index.php/app vol. 5/2016 selected aspects of etcs-l3 deployment figure 1. required odometry accuracy [2] the system – especially if a link rbc -> ixl is not implemented for l2. only etcs l3 is not an extension of the current ixl – it is thus possible to leave out the riding principle in a fixed block and train ride control using the signals. 3. definition of problem areas 3.1. the accuracy of the odometer a key part of the function of determining train location in the etcs system is an odometer and doppler radar. since this is a safety-relevant function, it is necessary to consider a possible error in the given position in a safer direction. according to the specification [2], the required accuracy is expressed by the formula according to the traveled distance s: sdiff = 5 + 0.05xs[m] the position is specified (error is null) always after reading a balise group and the error refers to the distance traveled from that point. for the operation of the etcs system in its current form it is therefore necessary to count with this error in determining the usable length of each track. in addition, the position of the end of the train must be added to this inaccuracy in the case of etcs l3, e.g. for track length of 600 m it is necessary to consider an error of up to 35 m, and vacate the space behind the railroad switch signal at least by this distance in order for the railroad switch stretch to be regarded as vacant. this again reduces the usable length of the track. 3.2. train handling the method for determining train location and sections occupancy in the stretch of etcs l3 makes it necessary to address potential problems arising from the train handling, such as splitting, joining and shunting of train cars. the simplest situation occurs when joining and separating sets of coherent units, where with regard to the assumed equipment of all driving vehicles it will be possible to locate both parts and detect their integrity after division. however, this does not apply to the movement of parts of the train set and keeping a part of the train on the station track or to the train profiling into multiple parts. in such a situation, the vacancy of the concerned part of the infrastructure cannot be safely determined only with the use of the etcs l3 system’s technical means. 4. suggested solution 4.1. the accuracy of the odometer to increase the usable length of the transport rails, it may be necessary to increase the accuracy of the positioning system. with current specifications, this problem can be solved by placing one or more locators inside the balises of a long track section. this will split it into several smaller sections, where the odometer’s inaccuracy is proportionately smaller. another possibility is the gradual tightening of the requirement for maximum permissible error in the etcs odometer (now 5m + 5%). similar devices used in the cbtc systems achieved accuracy of about 1% according to [3] already (using e.g. a doppler radar with multiple antennas). 4.2. train handling the problem with train division, shutting down of train cars, and shifts can be solved in principle several ways, which will be discussed in the following text. the advantages and disadvantages of various options and a comment on the possible applicability are also mentioned. 4.2.1. confirmation of occupancy by human operators the occupancy of selected parts of the infrastructure can be confirmed with the participation of the human factor. this principle is used according to [4], e.g. in ertms regional in sweden. after confirming the section’s occupancy with the human factor, the ride through the stretch is possible only in the staff responsible mode with speed restrictions. after the first train passes, the stretch is already considered vacant from the point of view of previous manipulation, and the ride of the following trains is no longer restricted. the disadvantages of this solution are obvious – the need for participation of the human factor, the risk of failure is limited by limiting the speed for the next train. this thereby decreases the throughput performance parameter. however, this solution may be suitable for regional routes with limited speed and sporadic shifts. 4.2.2. partial equipment with detection resources in case of frequent handling, using conventional detection devices (track circuits, axle counters) at this point is offered as a solution to the question of detecting the occupancy of the infrastructure’s section. the disadvantage regarding the involvement of the human factor on detecting the occupancy of the section is thus eliminated, the disadvantage being the need to install detection devices, albeit to a very limited extent. 39 petr koutecký acta polytechnica ctu proceedings 5. evaluation, conclusion the article pointed out several questions that will be necessary to address within the preparation of the possible pilot deployment of etcs l3. the solution’s suitability in some cases depends on the local situation and the nature of the track on which the system will be deployed; all possible situations cannot generally always be covered. in the near future it is also possible to expect verification of the related technologies that could help contribute as another alternative to solving open points (e.g. use of satellite positioning systems instead of fixed balises, use of modern communication technology instead of gsm-r, etc.) in the event of a positive outcome. 6. abbreviation • ato ... automatic train operation • cbtc ... communication based train control • ccs ... command and control system • ertms ... european rail traffic management system • etcs ... european train control system • gsm-r ... global system for mobile communications-railway • its-r ... intelligent transport system-railway • ixl ... interlocking • obu ... on-board unit • rbc ... radio block centre references [1] w. coenraad. level 3 : from high speed vision to rural implementation [online], 2011. http://www.irse.org/knowledge/publicdocuments/ 2011_09_29_ertms_level_3_from_high_speed_ vision_to_rural_implementation.pdf. [2] ertms/etcs performance requirements for interoperability, subset-041, issue 3.1.0, 2012, unisig [online]. http://www.era.europa.eu/document-register/ documents/set-2-index014-subset-041%20v310.pdf. [3] ertms level 3 risks and benefits to uk railways, final report; transport research la-borator [online], 2010. http: //www.trl.co.uk/media/773946/cpr798_-_ertms_ level_3_risks_and_benefits_to_uk_railways.pdf. [4] ertms regional functional requirements specification, version 03.08, uic, 2009. 40 http://www.irse.org/knowledge/publicdocuments/ 2011_09_29_ertms_level_3_from_high_speed_vision_to_rural_implementation.pdf http://www.irse.org/knowledge/publicdocuments/ 2011_09_29_ertms_level_3_from_high_speed_vision_to_rural_implementation.pdf http://www.irse.org/knowledge/publicdocuments/ 2011_09_29_ertms_level_3_from_high_speed_vision_to_rural_implementation.pdf http://www.era.europa.eu/document-register/documents/set-2-index014-subset-041%20v310.pdf http://www.era.europa.eu/document-register/documents/set-2-index014-subset-041%20v310.pdf http://www.trl.co.uk/media/773946/cpr798_-_ertms_level_3_risks_and_benefits_to_uk_railways.pdf http://www.trl.co.uk/media/773946/cpr798_-_ertms_level_3_risks_and_benefits_to_uk_railways.pdf http://www.trl.co.uk/media/773946/cpr798_-_ertms_level_3_risks_and_benefits_to_uk_railways.pdf acta polytechnica ctu proceedings 5:38–40, 2016 1 introduction 2 requirements 2.1 substantiation of the requirements 3 definition of problem areas 3.1 the accuracy of the odometer 3.2 train handling 4 suggested solution 4.1 the accuracy of the odometer 4.2 train handling 4.2.1 confirmation of occupancy by human operators 4.2.2 partial equipment with detection resources 5 evaluation, conclusion 6 abbreviation references acta polytechnica ctu proceedings doi:10.14311/app.2016.3.0060 acta polytechnica ctu proceedings 3:60–64, 2016 © czech technical university in prague, 2016 available online at http://ojs.cvut.cz/ojs/index.php/app numerical method for estimation of tensile load in tie-rods kseniia riabova∗, luca collini, rinaldo garziera university of parma, parco area delle scienze, 181/a, parma, italy ∗ corresponding author: kseniia.riabova@studenti.unipr.it abstract. the work introduces a two-phase method for determination of axial loads in tie-rods. the method described here consists of an experimental activity and an automated numerical calculation. the influence of considering an elastic winkler-type bed to model the tie-rod constraint inside the wall has been investigated. the algorithm used for calculation involves a solution of a functional minimization problem, where the tensile load and the stiffness of elastic foundation at the edges are used as optimization parameters and the error function, which describes the deviation between the frequencies measured and those calculated using finite element method, is minimized. qualitative analysis of the results showed a significant reduction of the error compared to models with different boundary conditions. the method showed to be conservative for the strength evaluation of the rods, because the optimal values of tensile loads appeared to be higher than the load in perfect encastre conditions. keywords: tie-rods, non-destructive technique, numerical method. 1. introduction tie-rods are structural elements used in a wide range of civil constructions. one of purposes they serve is to provide support for masonry arches and vaults in ancient buildings (e.g. churches, castles, etc.), which are known to lurch and founder course of time. tierods are subjected to axial tension and, thus, help the building resist occurring lateral loads exerted by structural elements, walls and facades. over the years deformation of masonry walls and some displacements in the building may cause significant changes in the axial loads of tie-rods. in the extremes this can lead to either failures in tie-rods structural integrity, or to laziness of tie-rods that stop carrying out their duty when they loose loads. that is why regular monitoring of tie-rods condition is of a great importance. the goal of the presented work is to determine axial loads acting on the tie-rods of the fifteenth century casa romei located in ferrara, emilia-romagna, italy, and thus to evaluate their functionality and reliability. an approach based on an experimental investigation and a numerical method is introduced to serve this purpose. 2. previous studies some attempts have been made throughout the years to develop an appropriate non-destructive technique for and indirect estimation of the tensile load in tierods. previous studies [1–3] report a method that combined static and dynamic force identification. tierods were modeled as simply supported euler beams with rotational springs of similar stiffness added on each edge. the stiffness of the spring and the force were the two unknowns obtained from the system of equations, built with a static equation for deflection and a dynamic equation for natural frequencies. another study [4] introduced a static approach for force identification. the experiment consisted of measuring three vertical displacements and strains variations at three sections of the tie-rod under a concentrated load; in [5] an algorithm to identify the axial tensile force in ancient tie-rods by using the first three natural frequencies is developed. the tie-rod was modelled as an euler beam of uniform cross-section, neglecting the shear deformation and rotary inertia, and was assumed to be simply supported at the ends with additional rotational springs. recently, maes et al. [6] introduced a method that enables definition of axial loads in slender beams with unknown boundary conditions, taking into account affects of rotational inertia of the beam and masses of sensors. however, it requires data from five or more sensors along the length of the beam to determine all the introduced unknown of the inverted problem. a similar technique of the axial force identification was developed by li et al. [7] focusing on studies of euler-bernoulli beams and takes into account bending stiffness effects. rebecchi et al. [8] established an analytical method of processing experimental data from five instrumented sections of a prismatic slender beam, which showed excellent results in estimation of the axial load in tie-rods. the method does not require any exact value of effective length of the beam, but neglects both rotary inertia and shear deformations effects in the solution for beam vibrations. for cases of similar beams, tullini et al. [9–11] proposed a static method of axial force identification. the analytical algorithm makes use of any set of experimental data 60 http://dx.doi.org/10.14311/app.2016.3.0060 http://ojs.cvut.cz/ojs/index.php/app vol. 3/2016 estimation of tensile load in tie-rods represented by flexural displacements or curvatures measured at five cross-sections of the beam subjected to an additional concentrated lateral load. again, gentilini et al. [12] developed a procedure that combines dynamical testing with fem simulations using added masses. the method was tested out for tie-rods of various length and load intensity, showing reliable results. livingston et al. [13] identified the tensile force in prismatic beams of uniform section by using modal data and assuming rotational and vertical springs at each end of the beam. shear deformation and rotary inertia were neglected (according to the euler beam model). 3. estimation method in the present study authors intend to introduce a method of health monitoring of tie-rods inserted in ancient monumental masonry buildings. the method consists of an in-situ experimental activity and an automated two-parameter optimization algorithm which allows to evaluate the axial load in tie-rods with a good precision. the technique is basically a simply executed frequency-based identification method that allows to minimize the estimation error. about six natural frequencies can be easily identified with a simple test by experimentally measuring the frequency response functions (frfs) of the tierods with instrumented hammer excitation. further a parametric finite element model of a tie-rod is developed in the fem analysis software abaqus. the part of a tie-rod hidden inside the masonry wall is assumed to have a length of 0.2 m and the constraints given to this part are modelled as elastic foundations (winkler bed). the tensile force and the stiffness of the foundation are chosen as the unknown parameters. in some cases the length of the rod inserted inside the masonry wall can be also assumed as unknown. the sought axial load, as it is explained subsequently, is obtained through an optimization routine that minimizes the deviation between corresponding experimentally determined eigenfrequencies and those calculated by fem. the authors have previously tested the present technique with some variations over different buildings and situations [14–16]. the novelty introduced in the present work is the optimization with respect to the stiffness of the winkler bed type boundary conditions. 3.1. experimental part the tie-rods investigated during the case study have been installed at different points along the lifespan of casa romei (see fig. 1), therefore they differ in dimensions, cross-section shapes and material condition. measurements of geometrical characteristics and of natural frequencies were performed for each reachable tie-rod of the building. the natural frequencies were obtained from the analysis of response to dynamical excitation applied to tie-rods in horizontal plane for a more accurate estimation of the axial load. since figure 1. inner yard of casa romei. figure 2. a typical frf plot (acceleration amplitude vs. frequency). the studied rods are much stiffer in the vertical plane, tests in vertical direction were not performed. the used instrumentation was composed by tools listed in tab. 1. the signal from accelerometers was handled by the dynamic signal acquisition module national instrument (ni) 9234 and acquisition software developed in labview and further elaborated in matlab resulting in the frequency response functions (frf) for each tierod. frf for a ground floor tie-rod pt4 is presented in fig. 2. another important part of the experimental activity was to measure accurately the geometric characteristics of tie-rods, i.e. cross-section dimensions and lengths. for the further study it was assumed that the cross-section remains the same along all the length of a rod. the tested rods are crafted in iron for which the characteristics vary insignificantly, thus the material data was assumed as follows: e = 2 × 1011 n/m2, ρ = 7850 kg/m3, υ = 0.3. the main parameters e and υ appear in the frequencies under the square root, so even some variation in these values can only have a minor influence on the natural frequencies. for the further analysis first six natural frequencies were identified for each tie-rod by means of peakpicking from frfs. six eigenmodes are providing 61 k. riabova, l. collini, r. garziera acta polytechnica ctu proceedings tool manufacturer model sensitivity impact hammer brel&kjaer 8202 4pc/n accelerometer pcb 356a01 0,01 v/(m/s2) accelerometer pcb 356a01 0,01 v/(m/s2) table 1. list of experimental tools. an over constrained problem, thus the higher modes were not taken under consideration in this research. besides that, identification of higher modes might appear inaccurate due to larger possible measurement errors. 3.2. numerical part since the extremities of tie-rods are built into the masonry walls in a way that hardly leaves a possibility to be certain about the end constraints, as a first iteration, boundary conditions for the edges were modeled as plain fixed and/or simply supported. in both cases for any range of optimization parameters (length and force) and weight coefficients the error function reaches minimum every time for the minimal length in the range, i.e. for the measured length. this means that the minimum is forced. investigation of the function in symbolic mathematics package maple 13.0 showed good correspondence of the results obtained from fem with analytical models of a string and a beam under axial load. also the error function has an absolute minimum that lies however below the length measured. this kind of behavior proves that the bcs are irrelevant. the real conditions on the edges of tie-rods are unknown and may vary from one rod to another, because they are fixed inside masonry walls in a way that is hard to determine using non-destructive techniques. however, certain attempts can be made in order to develop a generalized parametric model suitable for numerical determination of loads in tie-rods. in order to determine the loads, tie-rods are modeled in fem software abaqus using 3d beam elements that incorporate timoshenko beam theory (b31), which allows to take into account shear deformation and rotational inertia effects. the cross section is assumed to be constant along each rod. as a first step a pretension load n is applied to the beam and as a second step modal analysis is performed. since the value of the pretension load is the subject of search, it has to be tuned to make the results of modal analysis in fem reach a good agreement with experimentally defined frequencies. however, manual tuning of the load is not considered to be an option in this case, because there is no certainty about the boundary conditions at the beam edges. this problem is overcome by means of a parametric fem model and an optimization tool for computing. in fem model the measured length of the tie-rod is represented by 50 beam elements; boundary conditions are assumed to be an elastic bed and, thus, are represented by 5 spring elements on each edge of the beam, acting in the direction of considered vibrations. also the displacement in direction x (tie-rod axis) is restrained. the sketch of the assumed model is displayed in fig. 3. stiffness of each spring element together with the sought load are chosen as optimization parameters in the problem stated. the optimization program requires a set of experimentally obtained frequencies, ranges of stiffness and force with sizes of steps for each and also a set of weight coefficients for frequencies, that define importance of the corresponding frequency for the analysis. combinations of parameters form a grid and the program launches abaqus input file for each nod of this grid and then extracts and filters natural frequencies. vibrations in z direction only are of our particular interest due to testing that was carried out in the horizontal plane. optimization criteria for the problem is represented by the residual error between experimentally defined frequencies and those obtained from fem analysis. the error e is calculated according to the following formula: e = √√√√ n∑ k=1 p2 k(fexp k − ff em k )2 (1) where pk is a weight arbitrary given to each frequency. the optimal values of stiffness of the elastic bed and of axial force deliver minimum value of the error function. the optimization process starts from a reasonable value of the load which can be predicted as an optimal load for a simplified model with fixed ends, i.e. for infinite stiffness of the elastic bed at the constraints. the second step to be done is now to vary the stiffness of the spring bed, and optimize their value to minimize again the error. this implies a refinement process of the parameters along what we call zooming technique [15]. operatively, the algorithm defines a matrix of values and refines grid of parameters to be solved just where local minima of the error as defined in eq. 1 are found. a typical graphic of the surfaces representing the error distribution is presented in eq.4. here the error values are plotted with respect to stiffness and axial load, for a specific tie-rod. the free length l, and length of lf (in fig. 3) are kept constant. the clear minima of the error functional can be observed. 62 vol. 3/2016 estimation of tensile load in tie-rods figure 3. sketch of the computational model. figure 4. error as function of bed stiffness and axial load for different sets of weight coefficients. the error which represents the difference between numerical and experimental frequencies is calculated from the eq. 1. each of the differences (in hz) between couples of frequencies is multiplied by a weight pk in order to allow us to assign higher (or lower) importance to some frequencies rather than the others. the set of weights is then arbitrary chosen, but generally, the first frequencies, or just the fundamental one, have greater importance. a sensitivity analysis showed that the minima of the error functional eq. 1 with respect to the sought tensile load is not influenced by the weights set (see fig. 4). the variation of the error is depicted in fig. 5, wherefrom it can be observed that the minimum value of the error is reached for the optimal elastic bed stiffness and is reached for the up to higher load compared to the encastre boundary conditions, thus, the method reveals to be conservative for the strength evaluation of the rods. the values of error vary from one tie-rod to another, this shows that the model with winkler bed boundary conditions fits better for some of the rods, however these errors were less than those calculated for ordinary fixed or simply supported boundary conditions. for a couple of tie-rods though a perfect encastre boundary condition at the walls appeared to be the best approximation which was demonstrated by an improvement in the error value compared to the case when a set of springs was modelled at their extremities. figure 5. influence of the elastic bed stiffness on the minimal error. 4. conclusions in this work an optimization process is shown and discussed in determining the axial load in structural iron tie-rods. the influence of considering an elastic winkler-type bed to model the tie-rod constraint into the wall has been investigated. this constitutes the major novelty of this work, with respect to the previous studies. structural assessment of the overall building stability is enabled by the described method throughout the identification of axial loads in tie-rods via experimental and numerical activity. in particular, the introduced computational model is able to take into account even unknown boundary conditions that are typical for tie-rods installed in ancient buildings. furthermore, from the simulations it was concluded that the variation of tensile load shifts the set of frequencies (higher the load, higher the frequencies), while the change in elastic bed stiffness changes the distance between natural frequencies. and finally, the conclusion about the method convergence can also be made based on the fact that different sets of weight coefficients bring to minor changes in the sought axial load in tie-rods. references [1] c. blasi. sulla determinazione del “tiro” nelle catene mediante prove statiche e dinamiche. atti iii congresso nazionale assirco 1988. [2] c. blasi. determining the axial force in metallic rods. structural engineering international (iabse) 4:241–246, 1994. 63 k. riabova, l. collini, r. garziera acta polytechnica ctu proceedings [3] s. sorace. parameter models for estimating in-situ tensile force in tie-rods. journal of engineering mechanics 122:818–825, 1996. [4] s. briccoli bati. experimental method for estimating in situ tensile force in tie-rods. journal of engineering mechanics 127:1275–1283, 2001. [5] s. lagomarsino. the dynamical identification of the tensile force in ancient tie-rods. engineering structures 27:846–856, 2005. [6] k. maes. identification of axial forces in beam members by local vibration measurements. journal of sound and vibration 332:5417–5432, 2013. doi:10.1016/j.ijrmms.2008.06.005. [7] s. li. vibration-based estimation of axial force for a beam member with uncertain boundary conditions. journal of sound and vibration 332:795–806, 2013. [8] g. rebecchi. estimate of the axial force in slender beams with unknown boundary conditions using one flexural mode shape. journal of sound and vibration 332:4122–4135, 2013. doi:10.1016/j.jsv.2013.03.018. [9] n. tullini. dynamic identification of beam axial loads using one flexural mode shape. journal of sound and vibration 318:131–147, 2008. [10] n. tullini. bending tests to estimate the axial force in tie-rods. mechanics research communications 44:57–64, 2012. [11] n. tullini. bending tests to estimate the axial force in slender beams with unknown boundary conditions. mechanics research communications 53:15–23, 2013. [12] c. gentilini. nondestructive characterization of tie-rods by means of dynamic testing, added masses and genetic algorithms. journal of sound and vibration 332:76–101, 2013. doi:10.1016/j.jsv.2012.08.009. [13] t. livingston. estimation of axial load in prismatic members using flexural vibrations. journal of sound and vibration 179:899–908, 1995. doi:10.1016/j.jsv.2012.10.019. [14] m. amabili. estimation of tensile force in tie-rods using a frequency-based identification method. journal of sound and vibration 329:2057–2067, 2010. [15] m. amabili. a hybrid method for the nondestructive evaluation of the axial load in structural tie-rods. nondestructive testing and evaluation 26(2):197–208, 2011. [16] l. collini. load and effectiveness of the tie-rods of an ancient dome: technical and historical aspects. journal of cultural heritage 16:597–601, 2015. 64 http://dx.doi.org/10.1016/j.ijrmms.2008.06.005 http://dx.doi.org/10.1016/j.jsv.2013.03.018 http://dx.doi.org/10.1016/j.jsv.2012.08.009 http://dx.doi.org/10.1016/j.jsv.2012.10.019 acta polytechnica ctu proceedings 3:60–64, 2016 1 introduction 2 previous studies 3 estimation method 3.1 experimental part 3.2 numerical part 4 conclusions references acta polytechnica ctu proceedings doi:10.14311/app.2017.10.0007 acta polytechnica ctu proceedings 10:7–11, 2017 © czech technical university in prague, 2017 available online at http://ojs.cvut.cz/ojs/index.php/app correlation relationships for the hypoplastic model for fine grained soils tomáš kadlíček∗, tomáš janda, michal šejnoha department of mechanics, faculty of civil engineering, czech technical university in prague, thákurova 7/2077, 166 29 prague, czech republic ∗ corresponding author: tomas.kadlicek@fsv.cvut.cz abstract. the paper is concerned with our ongoing research effort devoted to the development of reliable computational tools for the calibration of advanced constitutive models of soils. at present, such software is available for the hypoplastic model of clays applicable to soft soils. this software provides a stepping stone for the determination of potential links between individual model parameters and fundamental characteristics of soils. identifying such links would allow for tuning the model without performing time consuming experiments, particularly in the case of an initial design. some preliminary results are presented in the paper. keywords: hypoplasticity, fine grained soils, constitutive modeling, correlation. 1. introduction the theory of hypoplasticity has been receiving considerable attention in recent years, particularly for its capability of well simulating the basic characteristics of soils resulting in reliable predictions at a structural level. these characteristics include not only the dependency of stiffness on the void ratio, but also the stiffness dependency on strain and the loading history. such relationships are hardly available with frequently used elastoplastic models of the mohr-coulomb (mc). these are generally adopted in fem simulations because of their simplicity and straightforward links to basic strength parameters such cohesion and the angle of internal friction, which are included in regular laboratory test protocols or available in common material databases. to provide a similar comfort or at least considerably simplify the use of hypoplastic models is the principal objective of our current research focusing particularly on the masin hypoplastic model for fine grained soils (hc) [1] and the von wolffersdorff hypoplastic model for coarse grained soils (vw) [2]. in particular, two specific directions are investigated. the first aims at creating easy to use calibration software built upon standard oedometric and triaxial laboratory tests. the second option assumes a calibration procedure in the form of tables or graphs enabling the determination of model parameters on the basis of specific input data. knowing the link between the model parameters and more standard soil parameters creates the basis for the development of such a tool. this objective is examined in this paper and some preliminary results are presented. 2. hypoplastic model for fine grained soils the theory of hypoplasticity can be considered a possible alternative to the theory of elastoplasticity. unlike elastoplasticity, it does not separate an elastic and plastic deformation, but its deformation is controlled by the evolution of state variables, namely pressure and the void ratio. consequently, hypoplasticity exhibits a distinct nonlinear deformation both in primary loading and elastic unloading. the state variables are bound in the e×q×p (e âăş void ratio, q deviatoric stress, p mean stress) space, where all admissible states are enclosed by the so called state boundary surface (sbs). the hc model defines five material parameters: λ∗, κ∗, n, ϕc and ν. these parameters can be calibrated on the basis of simple oedometric and triaxial laboratory tests. when properly tuned the model provides reliable and accurate predictions of the soil response in the solution of real engineering tasks. the first three parameters: λ∗, κ∗ and n are closely related to the oedometric test. as seen in fig. 1, the parameter λ∗ controls the slope of the normal consolidation line (ncl). the position of ncl in the ln p× ln(e+ 1) space is driven by the parameter n. clearly, recall fig. 1, its value corresponds to the mean pressure p = 1 kpa. the third parameter κ∗ represents the slope of the swelling line followed upon unloading as also evident from fig. 1. for more details, the interested reader is referred to [1] and [2]. the remaining two parameters ϕc and î¡ are typically determined on the basis of the triaxial test. in the case of fine grained soils, an undrained triaxial test accompanied by measuring the pore pressure is preferred. the critical state friction angle ϕc is dis7 http://dx.doi.org/10.14311/app.2017.10.0007 http://ojs.cvut.cz/ojs/index.php/app t. kadlíček, t. janda, m. šejnoha acta polytechnica ctu proceedings figure 1. parameters λ∗, κ∗ and n in the ln p × ln(e+ 1) space figure 2. critical friction angle ϕc in the t′ × s′ space played in fig. 2. the effective stresses t′ and s′ in fig. 2 are written as s′ = σ′1 + σ′3 2 t′ = σ′1 − σ′3 3 (1) the last parameter î¡ is related to the ki/gi ratio, where ki is the bulk modulus in isotropic compression and gi is the shear modulus in undrained shear for a test starting from the isotropic normally compressed state. obtaining a reliable prediction of this parameter requires a parametric study. the result of such a parametric study in terms of an equivalent deviatoric stress plotted as a function of the vertical strain is shown in fig. 3. additional details are available in [1] and [2]. further details on the proposed calibration procedure for both hc and vw models, developed as well as validated on the basis of a large ensemble of laboratory tests, are provided in [3]. in the next section, limiting our attention to the hc model, the model parameters determined for a representative set of soil samples will be correlated with a selected set of other, easily determined, soil characteristics. 3. hc parameters’ correlations soils can be characterized and classified according to the particle-size distribution and atterberg limits. in order to determine the size distribution of particles in a specimen the sedimentation test (for grain sizes < 0.063 mm) and measurements of the amount of particles passing a particular sieve size can be performed to derive the grain size distribution curve. the atterberg limits are determined as the values of mass moisture content of a soft soil and are defined as follows: • shrinkage limit ws is the water content where an ongoing loss of moisture is not followed by any additional volume reduction • plastic limit wp is the water content where cylindrical samples of soil being 3mm in diameter fall apart in pieces 8-10mm long • liquid limit wl is the water content where a groove made in soil placed in the casagrande device merges at a length of 10mm after 20 drops. in the present study, the results from oedometric and triaxial measurements and index tests carried out on eight samples of soft soils were exploited. while the first set of experiments was employed in the calibration procedure to assess the values of individual model parameters, the actual soil classification was based on the index tests. the resulting values are listed in tab. 1. apart from model parameters and the atterberg limits wl and wp, the table also offers the percentage content of fine soil particles fwith the diameter d < 0.063mm. the particle-size distribution is also plotted in fig. 4 for the sake of clarity. when searching for possible correlations between the model parameters and the selected soil characteristics it has been proven convenient to adopt parameters calibrated on the basis of reconstituted soil samples. these samples were prepared from the original undisturbed soil by changing the sample paste consistency up to a water content of 1.5 ×wl while completely disrupting their original internal structure and as such losing any information about the prior loading history. the values of model parameters are nevertheless the same as those which would have been found when adopting the original undisturbed samples. the relationships between the values of hc parameters and the index characteristics, see fig. 5 and 6, can be conveniently specified in terms of the coefficient of correlation r ∈ (−1; 1). recall that the correlation reflects the noisiness and direction of a linear relationship of two random variables, but not the slope of that relationship. when there is no obvious relationship 8 vol. 10/2017 correlation relationships for the hypoplastic model figure 3. evolution of the equivalent deviatoric stress q as a function of the vertical strain εa for various values of ν table 1. parameters of the hc model and selected parameters from index tests for reconstituted soil samples figure 4. particle-size distribution for eight samples of sampled soils 9 t. kadlíček, t. janda, m. šejnoha acta polytechnica ctu proceedings figure 5. correlations between hc parameters and index characteristics (correlation), the coefficient of correlation is equal to 0. in the case of a perfect direct (increasing) correlation, r approaches 1 and in the case of a perfect inverse (decreasing) correlation, r approaches -1. thus, the closer the coefficient is either to -1 or 1, the stronger the correlation between the two variables. tab. 2 shows the coefficient of correlation for all five hc model parameters and the selected index characteristicswl andwp, and the percentage content of fine soil particles f . a relatively strong (direct) correlation is observed for the model parameters λ∗ and n and the content of fine soil particles f . this is also confirmed by the plots in fig. 5(a,b) showing no apparent scattering. remember that the parameters λ∗ and n control the position of ncl in the ln p × ln(e+1) space. both tab. 2 and the plots in fig. 5(c,d) suggest the parameter κ∗ be more dependent on the plasticity index wp and the liquid limit wl. the same conclusion, although not directly visible from tab. 2, can be drawn for the parameter v, see fig. 5(e,f). for this parameter, the value of the coefficient of correlation is polluted by the results derived from the soil sample labeled as “praha a”, which falls off the more or less linear relationship. this specimen 10 vol. 10/2017 correlation relationships for the hypoplastic model table 2. correlation values r figure 6. correlations between hc parameters and index characteristics shows the lowest values of the atterberg limits and the lowest content of fine soil particles f . unlike the first four parameters associated more or less with the compressive response of soil, the critical state friction angle ϕcv does not seem to show any correlation with the selected index characteristics, see both tab. 2 and fig. 6(g). however, its determination is rather simple from a standard triaxial test with no need for additional optimization. 4. conclusion although drawing a solid conclusion as to the direct link of the hc model parameters to other, more standard, soil parameters would require a considerably larger set of experiments, the potential of this goal has been clearly confirmed by this study. in particular, a relatively strong correlation of the hc model parameters λ∗, κ∗, n and ν and the selected index parameters or the particle size distribution data can be found. while limiting our attention to the hc model we expect that a similar study performed for the vw model can lead to similar conclusions. obviously, this must be supported by a much larger set of data to be examined. we expect such information to grow with the help of a web application which not only provides calibration software, but opens a way to collected results from laboratory measurements all over the world. [4] 5. acknowledgement the financial support from projects ta04031603 and sgs16/038/ohk1/1t/11 is gratefully acknowledged. references [1] d. mašín. clay hypoplasticity with explicitly defined asymptotic states. acta geotechnica 8(5):481–496, 2013. doi:10.1007/s11440-012-0199-y. [2] d. mašín. a hypoplastic constitutive model for clays. international journal for numerical and analytical methods in geomechanics 29(4):311–336, 2005. doi:10.1002/nag.416. [3] m. šejnoha, t. kadlíček, t. janda. calibration of hypoplastic models for soils. in engineering mechanics 2015, vol. 821 of applied mechanics and materials, pp. 503–511. trans tech publications, 2016. doi:10.4028/www.scientific.net/amm.821.503. [4] p. a. von wolffersdorff. a hypoplastic relation for granular materials with a predefined limit state surface. mechanics of cohesive-frictional materials 1(3):251–271, 1996. doi:10.1002/(sici)10991484(199607)1:3<251::aid-cfm13>3.0.co;2-3. 11 http://dx.doi.org/10.1007/s11440-012-0199-y http://dx.doi.org/10.1002/nag.416 http://dx.doi.org/10.4028/www.scientific.net/amm.821.503 http://dx.doi.org/10.1002/(sici)1099-1484(199607)1:3<251::aid-cfm13>3.0.co;2-3 http://dx.doi.org/10.1002/(sici)1099-1484(199607)1:3<251::aid-cfm13>3.0.co;2-3 acta polytechnica ctu proceedings 10:7–11, 2017 1 introduction 2 hypoplastic model for fine grained soils 3 hc parameters' correlations 4 conclusion 5 acknowledgement references acta polytechnica ctu proceedings doi:10.14311/app.2017.7.0085 acta polytechnica ctu proceedings 7:85–90, 2017 © czech technical university in prague, 2017 available online at http://ojs.cvut.cz/ojs/index.php/app modeling of anisotropic growth and residual stresses in arterial walls anna zahna, daniel balzania,b,∗ a technische universität dresden, institute of mechanics and shell structures, 01062 dresden, germany b dresden center for computational materials science (dcms), 01062 dresden, germany ∗ corresponding author: daniel.balzani@tu-dresden.de abstract. based on the multiplicative decomposition of the deformation gradient, a local formulation for anisotropic growth in soft biological tissues is formulated by connecting the growth tensor to the main anisotropy directions. in combination with an anisotropic driving force, the model enables an effective stress reduction due to growth-induced residual stresses. a method for the imitation of opening angle experiments in numerically simulated arterial segments, visualizing the deformations related to residual stresses, is presented and illustrated in a numerical example. keywords: biomechanics, multiplicative anisotropic growth, residual stresses, opening angle. 1. introduction as living biological medium, arterial tissue is known to adapt to changes in its mechanobiological environment in order to improve the load-bearing capacities. such adaptation processes originate from growth and remodeling and may be indicated by the existence of residual stresses under externally load-free conditions. those residual stresses can be uncovered by extracting an arterial segment from the body and slicing it in radial direction. typically, the segment contracts in axial direction and springs open to a degree which depends on various factors, for example the height of the blood pressure [1]. being associated with a reduction of stress gradients in loaded states, residual stresses need to be included in numerical simulations to obtain a predictive stress distribution. computational methods to reduce stress gradients by incorporating residual stresses are proposed in the literature [2–5]. these methods are however rather engineering approaches, as no biological processes are included to obtain the residual stresses. focusing on growth as part of the adaptation processes, the addition of material, resulting in increasing wall thicknesses and diameters, is related to a reallocation of the stresses which may therefore allow for an optimized load bearing behavior. for the purpose of modeling these effects, an anisotropic growth model based on the multiplicative decomposition of the deformation gradient into a growth tensor and a remaining elastic part is presented. in this context, different approaches for anisotropic growth in soft biological tissues have been published, see e. g. [6, 7], where the anisotropy of the growth mechanism is based on the definition of macrostructural growth directions as for instance the radial direction. although the anisotropy of the material is taken into account, isotropic strainor stress-based quantities are used as driving forces in the evolution equations characterizing the growth tensor. decomposing the growth tensor itself into two parts related to the main material anisotropy directions, the presented growth model establishes a relation between the growth tensor and the fibrous tissue structure, which yields a local formulation and therewith supersedes a geometry-specific definition of the growth directions. aiming at rendering the growth and stress reduction mechanism more effective, an anisotropic stress-based driving force is formulated. in addition, a procedure for the calculation of the deformations related to growth-induced residual stresses in numerically simulated arterial walls is developed. the entire framework is illustrated with the aid of a numerical example on an idealized arterial segment. 2. computational framework having been introduced in the context of biomechanics by rodriguez et al. [8] in 1994, the multiplicative decomposition f = fefg of the deformation gradient f into a growth part fg and an elastic part fe is nowadays a common approach for the description of growth in soft biological tissues. the growth deformation is assumed to be stress-free, such that stresses arise only from the elastic part ce = f−t g cf−1 g of the strains. utilizing constitutive equations for hyperelastic materials, the stress tensor se in the intermediate configuration can be computed from se = 2 ∂ψ ∂ce . here, the formulation for fiber-reinforced soft biological tissues by balzani et al. [9] is applied, whereby polyconvexity of the strain energy function and thus ellipticity and existence of minimizers are ensured. the growth behavior is governed by the specific form of the growth tensor, which involves one or several local variables referred to as growth factors ϑi. the evolution equations for these growth factors ϑ̇i = kϑ(ϑi)φi(zi), are formulated as function of the growth-driving quantity φ(zi) and the growth function kϑ including the time-dependency. 85 http://dx.doi.org/10.14311/app.2017.7.0085 http://ojs.cvut.cz/ojs/index.php/app anna zahn, daniel balzani acta polytechnica ctu proceedings in this section, a model for anisotropic growth, relating the growth tensor fg to the anisotropy directions of the fibrous tissue material, is presented in subsection 2.1. applying this framework to idealized arterial segments under internal pressure, residual stress distributions that qualitatively reflect experimental observations can be generated. in subsection 2.2, the focus is directed on the numerical calculation of the deformations which arise from resecting and slicing previously simulated, residually stressed arterial segments. 2.1. model of anisotropic growth related to two fiber families an effective approach to reduce stresses is the enlargement of the cross-sectional area. accounting for this, the basic form fg = ϑi + (1− ϑ) a⊗a, describing a deformation with factor ϑ in all directions perpendicular to the direction given by the vector a, is chosen among different existing approaches for the growth tensor, see e. g. [10]. in constitutive material models for arterial tissues, two main fiber directionsa(a) with a = 1, 2 are usually considered to approximate the complex layer-wise arrangement of collagen fibers. assuming the anisotropy of growth to be linked with the fibrous tissue composition, a multiplicative decomposition of the growth tensor fg = f (ii) g f (i) g (1) into two separated parts f (a) g = ϑ(a)i + ( 1− ϑ(a) ) m (a) with (2) m (a) = a(a) g ⊗a(a) g , a = i, ii (3) is considered in order to account for two different anisotropy directions defined by the vectors a(i) g and a (ii) g . this formulation is based on the assumption that in arterial walls, even in patient-specific ones, a stress state exists with one negative principle stress showing in radial direction and two positive principle stresses in a plane of tensile loading (which coincides with the axial-circumferential plane in perfect-tube arteries). this can be supposed to be the reason for the observation that mainly two collagen fiber families, oriented in directions within this plane, occur in arterial walls. the resulting tissue structure might enable an optimized load-bearing mechanism, cf. [11, 12]. in more detail, these two fiber families may favorably be oriented symmetrically with respect to the direction of the highest principal stresses and thus, growth perpendicular to these directions would be desired by the tissue to reduce the stresses. consequently, the growth orientation vectors a(a) g are defined as angle bisectors of the fiber direction vectors according to a(i) g = a(1) +a(2) ||a(1) +a(2)|| and (4) a(ii) g = a(1) −a(2) ||a(1) −a(2)|| . (5) for the determination of the growth factors ϑ(a), two evolution equations ϑ̇(a) = kϑ(ϑ(a))φ(a)(z(a)), a = i, ii (6) have to be considered. therein, the function kϑ(ϑ(a)) =  k+ ϑ [ ϑ+ − ϑ(a) ϑ+ − 1 ]m+ ϑ , φ(a) > 0 k− ϑ [ ϑ(a) − ϑ− 1− ϑ− ]m− ϑ , φ(a) < 0, (7) which has been adopted from lubarda & hoger [13], scales the growth velocity by a factor k+/− ϑ and restricts the range of the growth factors to [ϑ−, ϑ+]. the function φ(a)(z(a)) defines the quantity which induces growth. recent publications examining stressdriven growth [6, 7, 14] propagate the utilization of the trace of the elastic part of the mandel stress given by φ(cese) = cese : i as measure for the pressure acting on the arterial wall. since this quantity is isotropic, it appears not to be adequate in combination with the anisotropic growth tensor and is therefore modified as follows. using the projections of the elastic part of the mandel stress into the anisotropy directions a(a) g , φ(a)(cese) = cese : m (a) e (8) with m (a) e = a(a) g ⊗a(a) g , (9) it can be ensured that growth in the plane perpendicular to a(a) g is only emerging if the stress in this direction is nonzero. the index “e” indicates, that the structural tensors m (a) e refer to the intermediate configuration. however, being eigenvectors of the growth tensor, the growth orientation vectors a(a) g do not change their direction during the transformation to the intermediate configuration, thus m (a) e = m (a). involving strain and stress quantites in the intermediate configuration, i. e. ce = f−t g cf−1 g and se = 2 ∂ψ ∂ce , the functions φ(a) depend on both growth factors ϑ(a), yielding a coupled system of evolution equations (6). using an euler backward scheme as for instance done in [7], the approximations ϑ̇(a) ≈ ϑ (a) n+1 − ϑ (a) n ∆t , a = i, ii (10) are adopted in order to compute the growth factors belonging to the actual time step n + 1. note that the index n + 1 is skipped from now on and only 86 vol. 7/2017 modeling of anisotropic growth and residual stresses in arterial walls quantities at the last time step are denoted by the index n. consequently, the local residuals r(i) := −ϑ(i) + ϑ(i) n + kϑ(ϑ(i))φ(i)(ϑ(i), ϑ(ii)) ∆ t, r(ii) := −ϑ(ii) + ϑ(ii) n + kϑ(ϑ(ii))φ(ii)(ϑ(i), ϑ(ii)) ∆ t have to fulfill r(i) = r(ii) = 0. to solve these nonlinear equations, a local newton iteration, characterized by index k, is performed. the linearized residuals linr(i) = r(i)∣∣ ϑ(a),k + ∂r(i) ∂ϑ(i) ∣∣∣∣∣ ϑ(a),k ∆ϑ(i),k+1 + . . . . . . ∂r(i) ∂ϑ(ii) ∣∣∣∣∣ ϑ(a),k ∆ϑ(ii),k+1 = 0, linr(ii) = r(ii)∣∣ ϑ(a),k + ∂r(ii) ∂ϑ(i) ∣∣∣∣∣ ϑ(a),k ∆ϑ(i),k+1 + . . . . . . ∂r(ii) ∂ϑ(ii) ∣∣∣∣∣ ϑ(a),k ∆ϑ(ii),k+1 = 0 are solved for the incremental updates ∆ϑ(i) and ∆ϑ(ii), and ϑ(i) and ϑ(ii) are updated until an appropriate abort criterion is fulfilled. during this iteration, the incremental updates are calculated by ∆ϑ(i),k+1 = r(i) a (i,i) ϑ ∣∣∣∣∣ ϑ(a),k − a (i,ii) ϑ a (i,i) ϑ ∣∣∣∣∣ ϑ(a),k ∆ϑ(ii),k+1, (11) ∆ϑ(ii),k+1 = a (i,i) ϑ r(ii) −a(ii,i) ϑ r(i) a (i,i) ϑ a (ii,ii) ϑ −a(i,ii) ϑ a (ii,i) ϑ ∣∣∣∣∣ ϑ(a),k , (12) where the abbreviations a (i,i) ϑ = −∂r (i) ∂ϑ(i) , a (i,ii) ϑ = − ∂r (i) ∂ϑ(ii) , a (ii,i) ϑ = −∂r (ii) ∂ϑ(i) , a (ii,ii) ϑ = −∂r (ii) ∂ϑ(ii) are used. thus, the derivatives of the functions kϑ and φ(a) with respect to the growth factors have to be provided. differentiating eqs. (7) and (8) leads to ∂kϑ(ϑ(a)) ∂ϑ(a) =  m+ ϑ ϑ(a) − ϑ+ kϑ(ϑ(a)), φ(a) > 0 m− ϑ ϑ(a) − ϑ− kϑ(ϑ(a)), φ(a) < 0 (13) and ∂φ(i) ∂ϑ(j) = ∂cese ∂ϑ(j) : m (i) e = ( ∂ce ∂ϑ(j)se +ce ∂se ∂ϑ(j) ) : m (i) e (14) = ( ∂ce ∂ϑ(j)se +ce ( 1 2 ĉ e : ∂ce ∂ϑ(j) )) : m (i) e with i = i, ii and j = i, ii. herein, ĉe = 2 ∂se ∂ce is the elastic part of the tangent moduli in the intermediate configuration. the remaining unknown derivative of the strain ce with respect to the growth factor ϑ(a) has to be computed according to the chain rule by ∂ce ∂ϑ(a) = ∂ce ∂fg : ∂fg ∂ϑ(a) = ∂ce ∂fg : ∂fg ∂f (a) g : ∂f (a) g ∂ϑ(a) . (15) the first part is given through ∂ce ∂fg = −f−t g �ce −ce � f −t g (16) and the relations ∂fg ∂f (i) g = f (ii) g � i, (17) ∂fg ∂f (ii) g = i � (f (i) g )t , (18) ∂f (a) g ∂ϑ(a) = i −m (a), a = i, ii (19) can be derived for the other parts.1 combining eqs. (17)/(18) and (19), the results ∂fg ∂ϑ(i) = f (ii) g − f (ii) g m (i), (21) ∂fg ∂ϑ(ii) = f (i) g −m (ii)f (i) g (22) are obtained and all required parts for the incremental updates of the growth factors in eqs. (11)–(12) are determined. given the final values of ϑi and ϑii, the 2nd piolakirchhoff stress s = f−1 g sef −t g can be identified performing a pull back of se to the reference configuration. 2.2. visualization of residual stresses anticipating the results of the numerical example in section 3, where an idealized arterial segment under constant internal pressure is examined using finite element calculations, the question arises, how to determine the deformations which are related to residual stress states. in order to permit these deformations, a change of the boundary conditions is required, for instance the release of the right edge of a residually stressed arterial segment as shown in figure 1, which enables the opening of the segment by the angle ϕ. dealing with highly nonlinear material behavior and large deformations, this final deformation state can not be determined in one calculation step, thus a special procedure has to be applied. here, this procedure is proposed as follows. 1the symbol � is defined such that the relation (a �b)︸ ︷︷ ︸ =:d : (a⊗ b) = aa⊗bb, dijkl = aikbjl (20) is valid, which for instance results in the calculation rule abc = ( a �ct ) : b. 87 anna zahn, daniel balzani acta polytechnica ctu proceedings ϕ ϕ 2 figure 1. definition of the opening angle ϕ in an idealized, symmetric arterial geometry. (1.) define the initial boundary value problem including the discretization of the unstressed configuration, apply the loads stepwise and hold them constant until a growth equilibrium state is reached. (2.) deactivate growth and remove the loads stepwise to determine the residually stressed state. due to growth, the obtained geometry differs from the initial reference configuration. (3.) save this grown, unloaded configuration by storing the nodal coordinates as well as the fiber orientation vectors and the residual cauchy stress σres := (detf )−1fsf t at the integration points. (4.) use the saved nodal coordinates and fiber orientation vectors to define a new boundary value problem with boundary conditions that allow for the expected deformation. (5.) restore σres at each integration point and keep it throughout the following calculation. (6.) apply the residual stresses internally by updating the stress tensor according to s ← s + floadsres, where fload is a load factor ranging from 0 to 1, while growth is deactivated. in each step, the stress sres = det[f ]f−1σresf −t has to be computed corresponding to the actual deformation state. following this procedure, the wanted deformation state will arise in absence of external loads. 3. numerical example in a previous comparative study [15], an arterial segment with internal pressure and an axial displacement of 15% of the initial length was considered for a comparison of two different forms of the growth tensor and two different driving forces. each of the approaches was able to provoke growth-induced circumferential stress smoothening, but the approach presented in subsection 2.1 turned out to be the most effective, generating the most beneficial stress distributions at the lowest growth requirement, measured by the determinant of the growth tensor. therefore, a similar boundary value problem making use of this approach will be analyzed here in more detail, focussing in particular on the simulation of the opening artery. the half of a one-layered idealized arterial segment, reinforced by two fiber families arranged with angles of ± 30◦ with respect to the circumferential direction, is exposed to an internal pressure of 16 kpa (120mmhg). as an approximation of the natural conditions in the c1 [kpa] ε1 [kpa] ε2 [-] α1 [kpa] α2 [-] 17.5 499.8 2.4 30 001.9 5.1 table 1. material parameters for arterial tissue, adjusted by brands et al. [16] to the model from [9]. ϑ+ [-] k+ ϑ [-] m+ ϑ [-] ϑ− [-] k− ϑ [-] m− ϑ [-] 1.1 1.0 3.0 0.9 1.0 3.0 table 2. growth parameters used for the simulation of an idealized arterial segment. body, an axial displacement is additionally applied and three different values, namely 10%, 15% and 20% of the initial length, are compared. the material and growth parameters are chosen as given in tables 1 and 2, respectively. the load is applied within 1 s and subsequently held constant until t = 12 s, letting the arterial segment grow. in figure 2, the resulting stress distributions (solid lines) are shown and compared with the stress distributions obtained without including growth (dashed lines). for each value of axial strain, the circumferential stress distribution is significantly smoothened, which reduces the stress peak at the inner surface. the axial stress distributions are smoothened as well and moreover, a distinct stress reduction can be observed which is a consequence of the axial component of growth, compensating the axial deformation. the final axial stress level differs strongly in dependence on the axial displacement: the higher the axial displacement, the higher the axial stresses. examining the growth deformation in terms of the determinant of the growth tensor, depicted in the contour plots in figure 2, it can be observed that the volume change due to growth increases slightly with increasing axial displacement, being nevertheless in the same range. removing the internal pressure while maintaining the axial displacement, the stress distributions given in figure 3 are obtained. being thus not free of external loads in the axial direction, there are non-zero axial stresses in the non-growing reference segments in this state. in the growing segments, non-zero circumferential and axial stresses occur, which are – despite the existence of the axial load – referred to as residual stresses in this contribution. showing circumferential tensile stresses at the outer surface, each of the considered arterial segments is expected to open after a radial cut. the axial stresses in the segments are positive as well, provoking an axial contraction when releasing the axial constraint. the deformations related to these operations are depicted in figure 4. depending on the axial displacement, different opening angles and maximal values of axial strain are obtained. with increasing axial displacement, the opening angle reduces and the axial contraction rises. in the opened segments, the circumferential stresses have vanished. 88 vol. 7/2017 modeling of anisotropic growth and residual stresses in arterial walls -20 0 20 40 60 80 100 120 140 160 180 radial position [mm] 6.5 7 7.5 (a) -20 0 20 40 60 80 100 120 140 160 180 radial position [mm] 6.5 7 7.5 (b) -20 0 20 40 60 80 100 120 140 160 180 radial position [mm] 6.5 7 7.5 (c) 1.34 1.36 1.38 det(fg) 1.33 1.4 radial stress [kpa] circumf. stress [kpa] axial stress [kpa] radial stress, no growth circumf. stress, no growth axial stress, no growth figure 2. radial, circumferential and axial stress distributions as well as growth deformation at t = 12 s in an arterial segment exposed to an internal pressure of 16 kpa and different axial displacements of (a) 10 %, (b) 15 % and (c) 20 % of the initial length. the dashed lines indicate the stresses of a non-growing arterial segment. ra d ia l s tr e s s [ k p a ] -5 -4 -3 -2 -1 0 1 2 3 4 5 radial position [mm] 6.5 7 7.5 (a) c ir c u m f. s tr e s s [ k p a ] -2.5 -2 -1.5 -1 -0.5 0 0.5 1 1.5 2 2.5 radial position [mm] 6.5 7 7.5 (b) a x ia l s tr e s s [ k p a ] -10 -5 0 5 10 15 20 25 30 35 radial position [mm] 6.5 7 7.5 (c) uz = 0.10 l uz = 0.15 l uz = 0.20 l uz = 0.10 l, no growth uz = 0.15 l, no growth uz = 0.20 l, no growth ra di al ci rc um f. ax ia l figure 3. residual stresses in (a) radial and (b) circumferential direction, as well as (c) stresses in axial direction at t = 12 s after removing the internal pressure while keeping up the axial displacement. the stress distributions provoke the axial contraction and opening of a resected, radially sliced arterial segment as it is observed in experiments. (a) (b) (c) -0.5 -0.2 0 0.2 0.5 circumf. stress [kpa] -0.59 0.61 ϕ = 11.06◦ max εz = −1.24 % ϕ = 9.77◦ max εz = −4.63 % ϕ = 8.36◦ max εz = −8.00 % figure 4. deformation states associated to the residual stresses in arterial segments loaded by an internal pressure and axial displacements of (a) 10 %, (b) 15 % and (c) 20 % of the initial length. the wireframe shows the undeformed reference configuration, whereas the second chart form the inside shows the grown state after removal of the pressure. the indicated values of axial strain have been computed with respect to that deformed configuration. 89 anna zahn, daniel balzani acta polytechnica ctu proceedings 4. discussion and conclusion generating favorable results in specific cases as seen in section 3, the performance of the presented model is still strongly dependent on the boundary value problem, especially on the choice of the fiber orientation vectors and on the boundary conditions. the approximation of realistic conditions is difficult for both issues. subjected to remodeling processes, fibers in arterial tissue reorient according to the requirements of their mechanobiological environment. this effect of improving the load-bearing behavior is not included in the current framework so far, such that remodeling caused changes of the growth-inducing stresses and their consequences on growth can not be considered. therefore, the interaction between growth and remodeling processes is a topic which should be taken into account to further improve the model. the axial boundary conditions assumed in this contribution are a vague approach to approximate the natural state of arteries in the body, where axial strains and stresses are existent. thus, one could also have decided to apply axial stresses instead, unknowing which is the best way to mimic the unmeasurable, real conditions. another point with respect to the axial direction shall be mentioned. in contrast to the circumferential direction, where unloading and slicing are two different operations, the compution af axial residual stresses is tricky and demands further meditation. to be able to simulate the contraction after resecting the artery, the applied axial displacement has not been removed in the presented numerical example when computing the “residually” stressed state. doing so, the requested release of the axial constraint could be performed in the modified boundary value problem. if indeed real residual stresses are the matter of interest, the axial loading has to be removed together with the internal pressure, which is only possible when applying an axial stress instead of a displacement. then, the arterial segment can already deform in axial direction during the unloading process, such that no further cutting, revealing the deformation associated to axial residual stresses, is possible afterwards. a third topic for further examinations is the development of a procedure to adjust the growth parameters, which have been chosen arbitrarily until now. acknowledgements the authors appreciate funding from dfg through the institutional strategy “the synergetic university” of tu dresden. references [1] t. matsumoto, k. hayashi. response of arterial wall to hypertension and residual stress. in k. hayashi, a. kamiya, k. ono (eds.), biomechanics. springer, 1996. [2] d. balzani, j. schröder, d. gross. simulation of discontinuous damage incorporating residual stresses in circumferentially overstretched atherosclerotic arteries. acta biomaterialia 2:609–618, 2006. doi:10.1016/j.actbio.2006.06.005. [3] h. weisbecker, d. m. pierce, g. a. holzapfel. a generalized prestressing algorithm for finite element simulations of preloaded geometries with application to the aorta. int j numer method biomed eng 30:857–872, 2014. doi:10.1002/cnm.2632. [4] j. schröder, s. brinkhues. a novel scheme for the approximation of residual stresses in arterial walls. archive of applied mechanics 84:881–898, 2014. doi:10.1007/s00419-014-0838-x. [5] j. schröder, m. von hoegen. an engineering tool to estimate eigenstresses in three-dimensional patientspecific arteries. comput methods appl mech engrg 306:364–381, 2016. doi:10.1016/j.cma.2016.03.020. [6] s. göktepe, o. j. abilez, e. kuhl. a generic approach towards finite growth with examples of athlete’s heart, cardiac dilation, and cardiac wall thickening. journal of the mechanics and physics of solids 58:1661–1680, 2010. doi:10.1016/j.jmps.2010.07.003. [7] p. sáez, e. peña, m. a. martínez, e. kuhl. computational modeling of hypertensive growth in the human carotid artery. computational mechanics 53:1183–1196, 2014. doi:10.1007/s00466-013-0959-z. [8] e. k. rodriguez, a. hoger, a. d. mcculloch. stress-dependent finite growth in soft elastic tissues. journal of biomechanics 27(4):455–467, 1994. doi:10.1016/0021-9290(94)90021-3. [9] d. balzani, p. neff, j. schröder, g. a. holzapfel. a polyconvex framework for soft biological tissues. adjustment to experimental data. int j solids struct 43:6052–6070, 2006. doi:10.1016/j.ijsolstr.2005.07.048. [10] e. kuhl. growing matter: a review of growth in living systems. j mech behav biomed mater 29:529–543, 2014. doi:10.1016/j.jmbbm.2013.10.009. [11] i. hariton, g. debotton, t. c. gasser, g. a. holzapfel. stress-driven collagen fiber remodeling in arterial walls. biomechanics and modeling in mechanobiology 6:163–175, 2007. doi:10.1007/s10237-006-0049-7. [12] s. fausten, d. balzani, j. schröder. an algorithmic scheme for the automated calculation of fiber orientations in arterial walls. computational mechanics 2016. doi:10.1007/s00466-016-1321-z. [13] v. a. lubarda, a. hoger. on the mechanics of solids with a growing mass. int j solids struct 39:4627–4664, 2002. doi:10.1016/s0020-7683(02)00352-9. [14] e. kuhl, r. maas, g. himpel, a. menzel. computational modeling of arterial wall growth. biomechanics and modeling in mechanobiology 6:321–331, 2007. doi:10.1007/s10237-006-0062-x. [15] a. zahn, d. balzani. modeling residual stresses in arterial walls based on anisotropic growth. proceedings in applied mathematics and mechanics 16:115–116, 2016. doi:10.1002/pamm.201610046. [16] d. brands, a. klawonn, o. rheinbach, j. schröder. modelling and convergence in arterial wall simulations using a parallel feti solution strategy. computer methods in biomechanics and biomedical engineering 11:569–583, 2008. doi:10.1080/10255840801949801. 90 http://dx.doi.org/10.1016/j.actbio.2006.06.005 http://dx.doi.org/10.1002/cnm.2632 http://dx.doi.org/10.1007/s00419-014-0838-x http://dx.doi.org/10.1016/j.cma.2016.03.020 http://dx.doi.org/10.1016/j.jmps.2010.07.003 http://dx.doi.org/10.1007/s00466-013-0959-z http://dx.doi.org/10.1016/0021-9290(94)90021-3 http://dx.doi.org/10.1016/j.ijsolstr.2005.07.048 http://dx.doi.org/10.1016/j.jmbbm.2013.10.009 http://dx.doi.org/10.1007/s10237-006-0049-7 http://dx.doi.org/10.1007/s00466-016-1321-z http://dx.doi.org/10.1016/s0020-7683(02)00352-9 http://dx.doi.org/10.1007/s10237-006-0062-x http://dx.doi.org/10.1002/pamm.201610046 http://dx.doi.org/10.1080/10255840801949801 acta polytechnica ctu proceedings 7:85–90, 2017 1 introduction 2 computational framework 2.1 model of anisotropic growth related to two fiber families 2.2 visualization of residual stresses 3 numerical example 4 discussion and conclusion acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2017.12.0001 acta polytechnica ctu proceedings 12:1–4, 2017 © czech technical university in prague, 2017 available online at http://ojs.cvut.cz/ojs/index.php/app an issue of intelligent road transport in kazakhstan kairatolla k. abishev∗, ruslan b. mukanov, asylbek zh. kasenov, almagul n. baltabekova s.toraighyrov pavlodar state university, pavlodar city, republic of kazakhstan ∗ corresponding author: kairatolla76@gmail.com abstract. the article reviewed the road transport sector of the republic of kazakhstan and found a significant role in ensuring road transport of freight and passengers. shown needs to improve the work of transport-communication complex of the republic of kazakhstan on the basis of using the latest information and communication technologies. described advantages of realization of several intellectual transport technologies. presented modern level of using satellite navigational systems on the automobiles, in addition written some automation and informational aspects of controlling transport processes. also presented measures to solve technological rearmament problem of the republic of kazakhstan. keywords: the republic of kazakhstan, automobile transport, intelligent transport systems, dispatching system. 1. introduction the republic of kazakhstan is entering a new phase in the reform of its economy, defining the purpose a significant increase in the efficiency of the national development system. message from the president of the republic of kazakhstan n.a. nazarbayev to the people has become a new strategic project on the path of accelerated development of the national economy, in close collaboration with the world of the global economy. presenting in his letter deeply thoughtful and detail-calculated economic and social strategy for the further successful movement of the people of kazakhstan to the prosperity, peace and the achievement of higher goals, the president gave her the name of "nurly zhol – path to the future". this program is intended to be the growth engine of the national economy in the upcoming years to improve the welfare and quality of life of every citizen in kazakhstan. according to the president, the most important area for the development of kazakhstan’s is the transport and logistics infrastructure. improving the transport sector is more than urgent for kazakhstan in conditions when there is a problem of intensification of trade and economic cooperation with europe and east asia. the introduction of intelligent transport systems will facilitate the organization of international transport, cargo tracking, and reduce the time of delivery, provide high performance and security of transport and logistics processes. 2. main part transport plays the most important role in the republic of kazakhstan life. on one hand, it is a very big territory (2725 thousand. km2), low density of its population (an average of about 6 people / km2), very big distances between sources of raw materials and production resources. in this situation, transport and communication complex provide not only the economic but also the political integrity of the country. on the other hand, the geographical location in the heart of the eurasian continent, far from the main labor and capital markets result in the inevitable growth of the transport component in the export-import operations.it consequently leads to a weak integration into the global economic system. the geopolitical location of kazakhstan between the capacious and dynamically developing markets of europe, east and south-east asia provides a chance to compensate this, by bringing transcontinental transit on the territory of the country[1]. 2.1. the road transport role in the common transport complex of the republic of kazakhstan as we can see the world and now the domestic experience, road transport is maximally adapted to function in today’s world. automobile transport is an inseparable and a very important component of transport significant role in transport and communication complex of the republic of kazakhstan. among its most important, the following characteristics can be outlined: a. popularity and wide availability, b. high maneuverability and speed of passenger transportation and goods delivery, c. possibility of door-to-door delivery without the necessity of any additional interim operations with goods, 1 http://dx.doi.org/10.14311/app.2017.12.0001 http://ojs.cvut.cz/ojs/index.php/app k.k.abishev, r.b.mukanov, a.zh.kasenov, a.n.baltabekova acta polytechnica ctu proceedings d. providing of near to zero-option short-distance service (delivery within the city and to the suburbs, and also those in rural areas), e. high degree of adaptation to different technological processes – both in industry and service sector, f. relatively low capital capacity. given the conditions mentioned above, the automobile transport is considered an inseparable component of all modern transport technologies including the integrated and multimodal transportation. the goods transportation activity in the republic of kazakhstan follow increasing trends in terms of capacity of delivered goods. according to rok statistics agency [2], 3 627,9 million tons of goods have been shipped via transport networks of the republic of kazakhstan from january to december in 2014, which is 3.7% more than within the same period in 2013. the cargo turnover for the given period was evaluated as equal to 487.4 billion t/km, 21293,00 billion pf passengers have been transported (which is 6.5% more than during january-december 2013), passenger turnover was 249,6 billion t/km (increase per 6.8% compared to 2013). according to the agency data, the proportion of automobile transport in relation to both transportation volumes and goods turnover has been increasing each year. currently, the automobile segment occupies 86,23% from total amount of delivered goods, 7,54% belong to railway transport and 6.09% – to pipelines, and 0.14% – to the other means of transport (air, internal water transport) – see figure1. figure 1. distribution of total transportation volumes among the means of transportation. as shown by international and domestic experience, automobile transport is ultimately adapted to be functioning in the conditions of market demand and supply. it can be proved by the fact that transport and communication complex was the primary choice for realization of privatization[3]. 2.2. necessary improvement for transport and communication complex of the republic of kazakhstan at this level of social development, there is a need for technological improvement of the transport process caused by a number of additional factors. first of all, the increased density of vehicular traffic as a result of development of economic and cultural ties and the accompanying increase in the process of trade flows and mobility. in these conditions, for this time the intensification of all areas of human activity is becoming increasingly important factor of time and therefore introduces the task to improve traffic speed. meanwhile, the transport infrastructure capabilities tend to lag behind the need to provide quality transportation in the face of increased intensity and speed. this often leads to the occurrence of accidents involving property damage, injuries and deaths. the traffic growth creates many problems for instance, traffic control, associated with the need to make responsible decisions in conditions of high traffic density of vehicles that often becomes one of the causes of accidents caused by human error. from this perspective, the technological improvement of the transport process is essential and urgent in order to achieve goals for the society improving the safety of vehicles[4]. thus, the introduction of modern information and communication technologies in transport significantly reduces the influence of the human factor[5][6]. 2.3. intelligent transport systems in the republic of kazakhstan intelligent transport systems – this is the system integration of modern information and communication technology and automation with transport infrastructure, vehicles and users, focused on safety improvment and efficiency of the transport process, comfort for operators, drivers and transport users. the introduction of intelligent technologies in the transport sector of kazakhstan made in the individual modes of transport and aimed at solving corporate problems. realization of certain intelligent transport technologies, and finally the formation of intelligent transport systems both have social and economic objectives, they are: a. traffic safety improvement and, as a consequence, the reduction of social wastes in the transport sector b. reduction in transport costs in the economy and other spheres of transport services, c. improvement of the environment and saving human and material (f.e. energy) resources. the strategic aim of the creation of intelligent system of road transport is the creation in the city: republics of interconnected intelligent systems that control traffic of transport vehicles and special emergency services, housing management. the strategic goal can also determine the formation of the republic competitive "road corridors", equipped with priority its technologies: movement controlling systems, traffic management systems that provide 2 vol. 12/2017 an issue of intelligent road transport in kazakhstan figure 2. ansdu work scheme, based on the satellite navigation. driver information about road surface, traffic conditions, the recommended routes of driving in the real situations on the roads, the availability and location road service areas, roadside services and other[5]. one of the main directions of development of transport technologies in the passenger transport sector of the republic of kazakhstan is the introduction of computer-based navigation systems of dispatching management. these systems use the location of vehicles with the signals of global navigation systems gps and glonass. dispatching system based on satellite navigation systems allow operational management of transportation, fixing transport work by the transmission and processing of information about the location of vehicles, access to that information for all interested participants of the transport process. schematic diagram of the operation of the automated dispatch control navigation systems based on satellite navigation is shown in figure2. installation of navigation systems on public passenger transport are focused not so much on the movement of the vehicle itself, but rather on comfort and safety of passengers[7]. 3. conclusion the need of the introduction of modern innovative technologies in all parts of the economy is one of the circumstances to improve its efficiency in the republic of kazakhstan. it is fully recognized by the government at the highest level and has repeatedly emphasized on public statements of the president of republic of kazakhstan. as for technological modernization of the transport industry of the republic of kazakhstan, the following tasks should be solved: (1.) formation of a single information space, (2.) training of qualified personnel to work in the new technological environment, (3.) the formation of regulatory and legal framework, standardization and harmonization of requirements for technical solutions in the field of information support of transport activities. solving these problems is very important to improve the operation of transport. references [1] m. bekmagambetov, r. zhumagulov. the transport system of kazakhstan in modern conditions. almaty, 2008. [2] http://www.stat.gov.kz, last accessed may 2016. [3] k. k. abishev, r. b. mukanov. improving road traffic safety in the republic of kazakhstan (rok). driver-car interaction and safety 2014, pg. 43-46. [4] m. novák, z. votruba, m. svítek, p. bouchner. improvement of bus and truck driving safety. (2007) conference proceedings – ieee international conference on systems, man and cybernetics, 1, art. no. 4273847, pp. 310-315. 3 http://www.stat.gov.kz k.k.abishev, r.b.mukanov, a.zh.kasenov, a.n.baltabekova acta polytechnica ctu proceedings [5] m. m. bekmagambetov. intelligent transport systems in the republic of kazakhstan. almaty, 2013. [6] p. bouchner, j. faber, m. novák, et al. human decision aspects in interactions with artificial systems. (2009) proceedings of the 2009 euro american conference on telematics and information systems: new opportunities to increase digital citizenship, eatis ’09, art. no. 1551749. [7] v. m. vlasov. transport telematics in the road sector. moscow-madi, 2013. 4 acta polytechnica ctu proceedings 12:1–4, 2017 1 introduction 2 main part 2.1 the road transport role in the common transport complex of the republic of kazakhstan 2.2 necessary improvement for transport and communication complex of the republic of kazakhstan 2.3 intelligent transport systems in the republic of kazakhstan 3 conclusion references acta polytechnica ctu proceedings doi:10.14311/app.2017.11.0022 acta polytechnica ctu proceedings 11:22–30, 2017 © czech technical university in prague, 2017 available online at http://ojs.cvut.cz/ojs/index.php/app comparative analysis of bearing capacity of reinforced and non-reinforced railway embankment josef jíra∗, jitka jírová, michal micka, jitka řezníčková department of mechanics and materials, faculty of transportation sciences, ctu in prague ∗ corresponding author: jira@fd.cvut.cz abstract. a comparative analysis using the programme ansys (fem) deals with the generation of the computation model and with the assessment of the influence of the geogrids and the vertical lime-cement columns on the bearing capacity and deformation of the railway embankment on the less load-bearing subsoil. the graphic outputs give an overview of the field layout of equivalent stresses and deformation in the construction of the railway substructure. keywords: computation model, geogrid, lime-cement columns, embankment, subsoil, bearing capacity, deformation, equivalent (von misses) stress. figure 1. reinforced geogrid tensar ss. 1. introduction in the case of railway earthworks is a measure of the reliability and safety of the bearing capacity of the substructure and permanent way with a guaranteed track geometry, there should not be an unexpected track depression or the settlement of an embankment etc. modernization of the existing railway routes to bear higher axle loads requires an increase in bearing capacity of the subgrade system. one possibility to increase the bearing capacity of a railway body is to introduce geogrids, see figure 1 and 2. vertical or inclined lime-cement columns are also used for increasing bearing capacity of the subsoil [1, 2]. by using the geogrid we can avoid the problems connected with the conventional way of extracting earth in subsoil and its substitute with loose earth of good quality. the geogrids are typical of open structures where the surface of the holes covers more than 40% of the total area. the reinforcing function of the geogrids lies in using the membrane effect, i.e. the forces in the area of the reinforcement, which have a favourable effect on tensile stresses in the soil construction. tensar company’s extensive experience with the modernization of the european railway corridors are figure 2. reinforced geogrid tensar ss. fully and very effectively used in the railway construction in the czech republic. the geogrids tensar ss are widely used in the reconstruction of the existing rail network. effectiveness of tensar ss geogrids was confirmed by a number of independent surveys, which conditions of non-reinforced and reinforced layer were simulated. structural benefit of tensar geogrids ss is to reinforce the unbound layers of railroad tracks. granular material piled and compacted over the geogrid openings penetrating through the geogrid, so grains of soil and the geogrid fit together and creates the effect of mutual gearing between grains of soil and the geogrid. the geogrid tensar ss creates composite together with a material bed foundation, which is effective even on the underlying ground consisting of peat or soft silt and restricts horizontal movement of the material in the ballast and minimizing the penetration of fine particles from the subsoil. the lime-cement columns are applied in the case of building with less bearable subsoil, see figure 3 and 4. their task is to help in the absorption of loads, to improve the overall mechanical properties and thus to prevent future unwanted displacements of foundation and other building complications. there 22 http://dx.doi.org/10.14311/app.2017.11.0022 http://ojs.cvut.cz/ojs/index.php/app vol. 11/2017 comparative analysis of bearing capacity figure 3. vertical lime-cement columns – technology colmix [3]. are vertical or inclined columns which they are oblique to the horizontal plane at an acute angle. length the columns prevails over other dimensions. the manufacturing process of columns can be of two types – wet and dry process. the wet process is mixing soil with cement suspension – suspension is prepared in a central mixing pump is conveyed to the mixingdrilling tools. lime, ash or gypsum is used in the dry method. jet grouting is another suitable method for increasing the bearing capacity of foundation soil. the method enables also besides the creation of larger diameter columns takes small diameter drilling, which is suitable for use on construction sites are cramped conditions. the application range of lime-cement columns is large, covers an area of clay with very small grains after gravels with relatively large grains. colmix patented technology belongs to the group deep soil mixing (dsm). it is suitable for the stabilization of road and railway embankments and removing unwanted traffic settlement structures in low stringency (clay) soils. on the site, which is intended to stabilize, it is necessary to perform in situ analysis of soil (analysis in the field) and the result of the strength requirements and the pilots to determine the composition of the binder mixture. in case colmix may be used the mortar with possibility to change the cement components and calcium hydroxide (or these components may be used separately). to embed the pile to land need special equipment drill soil that is mixed with the binder mixture and form it after finishing a compact mass in the shape of a column. interaction figure 4. vertical lime-cement columns – technology colmix [3]. of the columns and the surrounding soil occurs when loading to improve geotechnical properties. examination of the bearing capacity or stability of such adjusted constructions is usually verified in an experimental way. experience from abroad shows that it is necessary to monitor the railway embankment settlement in the case of a non-bearing subsoil [4, 5], even if the embankment is reinforced. therefore various experimental models are also used to study the influence of ballast bed construction on the bearing capacity and to test the reinforcing effect of the geogrids and columns. it can be also solved by numerical methods [3, 6], mostly by the fem (finite element method). the aim of this work was to verify the numerical modelling of behaviour of a railway embankment reinforced by geogrids and lime-cement columns when using the ansys programme. on the model of a simple ground body the process of model creation, choice of definite elements, definition of material features, and application of contact elements, definition of marginal conditions and load and setting the parameters of computation were verified. the computation models of the reinforced and non-reinforced railway embankment were used for the comparative analysis of the bearing capacity and deformation. 23 j. jíra, j. jírová, m. micka, j. řezníčková acta polytechnica ctu proceedings figure 5. the scheme of layers in the model of reinforced earthworks. 2. computation model of reinforced embankment 2.1. geometric model the finite-element method (fem) has been successfully used to assess the performance of geosynthetics and lime-cement columns in the strengthening of soft soils. geometric models were created for all cases in the workbench design modeller. a suitable model railway body was a need to establish for numerical calculations. the railway body in the embankment was selected for representing computational model; the scheme is compiled in figure 5. creating this body carried out in accordance with czech regulations and railway literature [7]. as part of the superstructure we modelled the concrete sleepers and a gravel ballast bed. the substructure includes the embankment of impervious soil, a geogrid and columns (both for the cases where the calculated mechanical phenomena were analysed in the reinforced rail track in the case of the non-reinforced railway body is not in it). two layers of cohesive soils having unfavourable properties were used in the base of the computational model in figures 5 and 6. they also interfere with columns, in the case of the reinforced model in figure 6. the aim of this research was to develop a spatial model. it is therefore necessary to attach the crosssection a third dimension so as to be solved the problem in 3d. for all layers of reinforced and unreinforced models (i.e. a ballast bed, an embankment, a geogrid, the first layer and the second layer of subgrade soil) was chosen length of the computational model 5, 139 m. this value is needed for entering symmetric boundary conditions of the program ansys – the front and rear wall of the space model of the railway body. the axial distance between sleepers is 0, 600 m in the ballast bed. spatial spacing columns is a parameter chosen as a variable, depending on the analysis performed. figure 6. the model of the embankment with a geogrid and lime-cement columns. 2.2. modelling soil for modelling the properties of the embankment and subsoil material the classic dp model – drucker prager for geological materials is used. the material inputs are: modulus of elasticity e, poisson’s ratio µ, density ρ, angel of internal friction φ and cohesion c. therefore the computation is materially non-linear. this model is used most often and provides good results. the disadvantage of it is that it does not involve the possibility to express hysteresis curve for repeated load. on the contrary the advantage is that we can use tabular material standard values according to specification of soils used. in ansys two more material models for geological material of plasticity can be used. the edp model – extended drucker prager – takes into account the reinforcement of the material within the loading process. the edp cap model supplements the edp model with the conditions of plasticity for volume pressure and tension. the models are sensitive to proper specification of input parameters and can only be gained from laboratory tests for given material. if the material constants are for the study purposes used to convey behaviour of the soil close to the dp model, i.e. for consolidated soil, the results in transformation of the soil construction are very similar. in all cases under investigation, we use of concrete sleepers, the ballast bed was with well grained gravel, the embankment contained gravel mixed with finegrained soils and subsoil contained clays. material composition of the bottom structure (figure 5 and table 1) was considered with different material properties for the model type 1 and for the model type 2 in the calculation, i.e., that the material substructure was calculated with lower or higher modulus of elasticity (see the value in the bracket) for clay f4 and f6. also the lime-cement columns were considered for lower quality soil substructure with the value of the modulus of elasticity of 26 mpa or 60 mpa (model type 2) for higher quality of soil. 24 vol. 11/2017 comparative analysis of bearing capacity material modulus of elasticity e [mpa] poisson’s ratio µ [ ] density ρ [kg/m3] angle of internal friction φ [ °] cohesion c [mpa] geogrid 450 gravel 0/32 55 0.3 1950 30 0 gravel 32/50 100 0.35 2000 36 0 clay f4 80 (90) 0.35 1850 25 0.015 clay f6 50 (60) 0.4 1750 19 0.016 columns 26 (60) 0.3 1750 25 0 table 1. material properties of each layer of the model type 1 and (type 2). figure 7. the concrete sleeper [mm]. material characteristics of the concrete sleeper in figure 7 are: density ρ = 2500 kg/m3, e = 20000 mpa, poisson’s ratio µ = 0, 2. 2.3. fem model the model of the railway body and performed calculations were carried out using the finite element method (fem) – ansys software in version 12.1. finite element network was created automated methods in workbench. parametric studies were performed in the module ansys – workbench 14 too. all structures in the rail track in the ansys program were modelled as volumes. those will then assign material properties. it is a convenient way of considering the relatively fine particles in soils. geometrically soils are made up of prismatic volumes; volume sleepers have a specific shape and the columns are introduced as cylindrical volumes. the reinforcement of the geogrid tensar ss is modelled as a thin plate of constant thickness which is mostly stressed by normal tensile forces and the bending stiffness is insignificant. the geogrid tensar ss30 type (figure 8 and table 2) was used for the comparative analysis of bearing capacity. contact surfaces were formed at the interface between the volumes. their number depends on the structural complexity of the model. other values have e.g. the model with the geogrid and columns and other are for models without one of these reinforcing elements. contact elements of the "bonded" were used figure 8. shape and sizes of geogrid tensar ss30. size value [mm] size value [mm] al 39 tj 5.0 at 39 tlr 2.2 wlr 2.3 tt r 1.3 wt r 2.8 table 2. sizes of geogrid tensar ss30. in all cases. using these contact elements prevents shearing at the contact surfaces and cannot lead to separation of two volumes. 3. analysis of behaviour of reinforced and non-reinforced embankment the verification of the computational model was done by parametric studies for four configuration model: 25 j. jíra, j. jírová, m. micka, j. řezníčková acta polytechnica ctu proceedings (i) the embankment without reinforcement, (ii) the embankment with the geogrid, (iii) the embankment with the geogrid and the lime-cement columns and (iv) the embankment with the geogrid and the lime-cement columns sheathed the geogrid. the parametric study was conducted for the columns of diameter d = 0, 6 m which had longitudinal and transverse distance of 1, 65 m. the load was applied to the spatial computational model as the pressure on the area sleeper. this area has sizes 0, 204 m x 0, 401 m and settles on it the bottom rail. the axle load of the standard load train is 250 kn and the wheel force is equal to half of this value – 125 kn, which was taken to work. the position of the wheel between the two sleepers is considered, which results in half of the wheel force per sleeper. the wheel force once repeated over a distance 2, 284 m – there are considered two axles. the graphic outputs were carried out for above mentioned unreinforced and reinforced embankments and two types of material composition of the subgrade. for the total comparative analysis there are chosen graphic outputs: (i) equivalent (von misses) stress in the subgrade, (ii) tensile stress of geogrid in the embankment, (iii) total deformation of the embankment and the lime-cement columns. a number of parametric studies of non-reinforced and reinforced embankments were performed with different parameters, e.g. the equivalent stress (von mises) and vertical and horizontal stresses were analysed. from all figures it is possible to see that the reinforcement of the railway embankment has a favourable effect on the bearing capacity of the subgrade. the graphic outputs give a clear overview of the field layout of equivalent stresses and deformation in the construction of the railway substructure. the maximal equivalent stress (von misses) of the subgrade is 20% lower if the embankment is only reinforced by the geogrid, but it is 30% larger if the embankment is reinforced by the geogrid and columns. reinforcing embankment with vertical columns has a very positive effect on the tensile stresses in the membrane of a geogrid, because heads of the vertical columns serve as a distribution platform for loaded geogrid. this arrangement allows uniform load transfer from the upper to the subsoil. the maximal tensile stresses are in the geogrid 8% smaller and they are spread over a much smaller area. the total deformation of the superstructure (practically settlement) is lower about 2 mm if the embankment is reinforced by the geogrid and the columns sheathed a geogrid than for the non-reinforced embankment. a comparative analysis shows that the reinforcement of the embankment, especially columns is important mainly for poor soils in the railway embankment, i.e. for soils with poorer material characteristics and low bearing capacity. when the more qualitative soil is used for the construction of the embankment then the reinforcement of the geogrid is sufficient to improve the stiffness of the railway embankment too. the geogrid is able to be prestressed because of activation of the membrane effect. that is usually made manually and in the tensed position the geogrid is fixed with individual clasps or with a grid with spikes infixed into the subsoil. 4. conclusion the possibility of using the ansys programme for modelling the behaviour of earthworks with geogrids and vertical lime-cement columns was demonstrated. the bearing capacity of the railway earthworks has great importance for railway practise. the calculation using drucker-prager model of soil enables through a gradual increasing in loading in load steps attain the state of stress and deformation close to the limit state at which sliding areas and big displacements. the stiffness of the soil construction, which can be derived from the values of load and vertical displacements, is an important value. there are expected to improve useful properties of the embankment when the rail body is only reinforced by the geosynthetic grid. consideration when evaluating the results is focused on all variants unbearable subsoil. as for sleepers, so at ballast and embankment is taken for granted that they are able to periodically transmit the resulting rail loads. four basic configurations of the railway body were taken into account: • the embankment without reinforcement (labelled in figure 14 as e), • the embankment with the geogrid (e +g), • the embankment with the geogrid and the limecement columns (e +g+ c), • the embankment with the geogrid and the columns sheathed a geogrid (e +g+ sc). computational modelling has shown the usefulness of reinforcement of the embankment by geogrids and vertical lime-cement columns, which leads to a reduction of settlement and stress state of the soil layers within the rail body. it is assumed that a rail body reinforced columns exhibits greater resistance to loading. using vertical lime-cement columns is very significant for increasing bearing capacity of subsoil. the geogrids and the columns can be also used in the case of building a new railway. comparison of total deformation of the embankment for types with lower or higher values of the modulus of elasticity of the lime-cement columns showed much lower settlement values of the embankment reinforced by the geogrid and the columns with higher values of modulus of elasticity, if we compare two types of different moduli of elasticity of columns. decreasing the values of settlement is from 15% to 19% according to type of reinforcing of the embankment. 26 vol. 11/2017 comparative analysis of bearing capacity figure 9. total deformation of the non-reinforced embankment. figure 10. total deformation of the embankment reinforced by the geogrid. 27 j. jíra, j. jírová, m. micka, j. řezníčková acta polytechnica ctu proceedings figure 11. total deformation of the embankment reinforced by the geogrid and the columns. figure 12. total deformation of the embankment with the geogrid andthe columns sheathed a geogrid. 28 vol. 11/2017 comparative analysis of bearing capacity figure 13. stress field σx of the geogrid of the embankment reinforced by the geogrid and the columns sheathed a geogrid. figure 14. total deformation of the embankment for all four types of structure and for two types of subgrade with various modulus of elasticity. 29 j. jíra, j. jírová, m. micka, j. řezníčková acta polytechnica ctu proceedings acknowledgements this work has been supported by research plan msm6840770043. references [1] g. femandes, e. palmeira, r. gomes. performance of geosynthetic-reinforced alternative sub-ballast material in a railway track. geosynthetics international 15, 2008. [2] j. zheng, b. cheng, y. lu, et al. the performance of an embankment on soft ground reinforced with geosynthetics and pile walls. geosynthetics international 16, 2009. [3] j. pajkrt. space model of the part of the railway track with lime-cement columns. faculty of transportation sciences ctu, prague, 2011. [4] l.briancon, h.girard, d.poulain. slope stability of lining systems – experimental modeling of friction at geosynthetic interfaces. geotextiles and geomembranes 20, 2002. [5] r. holý, m. kalika, j. kaliková, et al. system for simultaneous object identification and sector indoor localization oidsil in. proceedings of 2014 international conference on intelligent green building and smart grid. piscataway: ieee operations center, 2014. isbn 978-1-4673-6123-1. [6] o. jiroušek, j. jíra, o. hrdlička, et al. numerical modelling of the reinforcing effect of geosynthetic material used in ballasted railway tracks. proc. imeche, part f: journal rail and rapid transit, 224 (f4), 2010. doi10.1243/09544097jrrt319. [7] track substructure s4, technical regulations. railway infrastructure administration, state organisation. 30 acta polytechnica ctu proceedings 11:22–30, 2017 1 introduction 2 computation model of reinforced embankment 2.1 geometric model 2.2 modelling soil 2.3 fem model 3 analysis of behaviour of reinforced and non-reinforced embankment 4 conclusion acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2015.1.0057 acta polytechnica ctu proceedings 2:57–61, 2015 © czech technical university in prague, 2015 available online at http://ojs.cvut.cz/ojs/index.php/app on sampling based methods for the dubins traveling salesman problem with neighborhoods petr váňa∗, jan faigl dept. of computer science, czech technical university in prague, technická 2, 166 27 prague, czech republic ∗ corresponding author: vanapet1@fel.cvut.cz abstract. in this paper, we address the problem of path planning to visit a set of regions by dubins vehicle, which is also known as the dubins traveling salesman problem neighborhoods (dtspn). we propose a modification of the existing sampling-based approach to determine increasing number of samples per goal region and thus improve the solution quality if a more computational time is available. the proposed modification of the sampling-based algorithm has been compared with performance of existing approaches for the dtspn and results of the quality of the found solutions and the required computational time are presented in the paper. keywords: dubins vehicle, dubins maneuver, dtsp, dtspn. 1. introduction path planning for a non-holonomic vehicle is a fundamental problem of surveillance mission where an unmanned aerial vehicle (such as a fixed-wing) is requested to visit a given set of locations. the basic model of such a vehicle with the limited turning radius is called the dubins vehicle [1] for which the combinatorial problem of finding optimal sequence of visits to the locations is known as the dubins traveling salesman problem (dtsp) [2]. in this paper, we consider a generalized problem of the dtsp where the particular waypoints to be visited can be selected from a set of possible locations. due to the similarity of this problem with the so-called traveling salesman problem with neighborhoods [3], the problem is called as the dubins traveling salesman problem with neighborhoods (dtspn) [4]. the dtspn is a suitable problem formulation to address surveillance missions with unmanned aerial vehicles, where it is required to take a camera snapshot (or other type of measurement) of each target location. such a snapshot can be acquired from some distance of the target location and thus it is not necessary to visit the location exactly. it is rather a more suitable to select the waypoints in such a way that all locations are covered while the total cost of the final path is minimal. there are several approaches to address the dtsp and also the dtspn in the literature [2, 4–6] including our work [7]. therefore, in this paper, we provide an overview of the existing approaches to address the dtspn and compare their performance according to the trade-off between the solution quality and computational requirements. in particular, we focus on the sampling-based algorithm [4] which is able to provide high quality solutions for very high number of samples. however, more samples increase the computational burden, and therefore, the algorithm is not directly suitable for real-time applications. on the other hand, we can get a quick estimation of the solution quality using only few samples. this has motivated us to modify the original algorithm [4] to provide a first solution quickly that is then improved if a computational time is left. the paper is organized as follows. a brief introduction to the problem background is presented in the next section. the addressed problem is formally introduced in section 3. a brief overview of the existing approaches to solve the dtspn is provided in section 4. the proposed modification of the samplingbased approach is presented in section 5 together with the analysis of its computational complexity. evaluation results of the comparison of the existing approaches with the proposed modification are reported in section 6. finally concluding remarks are denoted to section 7. 2. related work the problem of curvature-constrained path planning has been studied years ago and the fundamental work has been published in 1957 by dubins. in [1], he proved that the optimal path between two configurations q1, q2 ∈ se(2) consists of only straight line segments and segments with the minimum turning radius. he also showed that only 6 maneuvers can be optimal, which can be further divided into two main types: • ccc type: lrl, rlr; • csc type: lsl, lsr, rsl, rsr; where c stands for a circle turn (r – right, l – left) and s for a straight segment. even though the optimal path for dubins vehicle between two configurations is known and it is straightforward to compute, this is not sufficient to directly solve the dtsp. it is because the orientation of the vehicle at the waypoints is not known and thus it must be determined together with the optimal sequence of 57 http://dx.doi.org/10.14311/app.2015.1.0057 http://ojs.cvut.cz/ojs/index.php/app petr váňa, jan faigl acta polytechnica ctu proceedings visits to the waypoints. moreover, the optimal orientation depends on the sequence and vice-versa, which make the problem difficult to address. probably the simplest approach to address the dtsp is the alternating algorithm (aa) proposed in [2]. in this approach, headings are established in the way that even edges are connected by straight line segments and odd edges are connected by dubins maneuvers. in addition, the authors show that the length of the optimal solution of the dtsp can be bounded by ltspκdn/2eπρ, where ρ is minimum turning radius, ltsp is the length of the optimal solution of the euclidean tsp, n is the number of the goals, and κ < 2.658. based on the similar idea, authors of [5] proposed a receding horizon algorithm called the look-ahead (la) approach. the heading is determined with respect to the next point in the sequence. this algorithm investigates each point only once, and therefore, the la approach is very fast and suitable for real-time application while the authors reported better results than the aa. the optimal solution of the dubins planning to visit a given sequence of waypoints that are at the distance longer than 4ρ is presented in [8]. the approach is based on the convex optimization; however, the optimization needs to be solved several times because of possible alternation of the maneuvers directions. the authors bound the number of possible combinations to 2n−2 for n waypoints. the dtspn is a generalization of the dtsp, where each goal is extended by a neighborhood (goal region). as the dtsp is known to be np-hard [6], also its generalization the dtspn is np-hard. 3. problem definition the addressed problem is motivated by surveillance missions with fixed-wing aerial vehicles, which are nonholonomic vehicles due to their kinodynamic constraints. these vehicles are often modeled as the dubins vehicle [1], which can go only forward at a constant speed and has a minimum turning radius ρ. the configuration space of such a vehicle can be represented as se(2), where each configuration q ∈ se(2) is a triplet (x, y, θ), where (x, y) is the vehicle position in a plane and θ ∈ s1 is the orientation of the vehicle. the mathematical model of the dubins vehicle can be formulated as follows: ẋ ẏ θ̇  = v  cos θ sin θ u ρ  , |u| ≤ 1, (1) where v is the vehicle forward velocity, ρ is the minimum turning radius, and u is the control input, u > 0. now, we can introduce the dtspn [9] a more formally. let r = {r1, . . . rn} be a set of n regions ri ⊂ r2 that are requested to be visited by the dubins vehicle and let σ = (σ1, . . . , σn) be an ordered permutation of {1, . . . , n}. we define a projection from se(2) to r2, i.e., p(q) = (x, y), and let qi be an element of se(2) whose projection lies in ri. the dtspn is path planning problem where the dubins vehicle has to visits each region ri while satisfying the kinodynamic constraints of (1). every optimal path has to intersect each goal region ri in at least one configuration qi. hence, we can treat the dtspn as an optimization problem over all possible permutations σ and configurations (q1, . . . , qn) as follows: problem 3.1 (dtspn). minimize σ,q l(qσn , qσ1) + n−1∑ i=1 l(qσi , qσi+1) subject to p(qi) ∈ ri, i = 1, . . . , n where l(qi, qj) is the dubins distance between qi and qj . in this paper, we are focused on the problem with regions ri that are mutually exclusive. hence, we can define the minimum distance constraint dk such that for all i, j ∈ {1, 2, . . . , n}, i 6= j,∀pi ∈ ri,∀pj ∈ rj : ||pi − pj || > kρ. (2) in particular, we are focused on the problem for which the minimum distance dk is equal to 4ρ, which is denoted as the d4 constraint in the rest of this paper. problem instances of the dtspn with the d4 constraint have special properties that are shown and used in [7] to find high quality solutions. in this paper, we also consider d4 problems and compare the existing solutions with the sampling based methods. an overview of the existing methods is provided in the next section. 4. existing methods in literature, we can find several different approaches to address the dtspn. the existing approaches can be divided into three main classes: 1) the decoupled methods; 2) sampling-based methods; 3) and evolutionary methods. representatives of each particular class are briefly described in the following subsections. 4.1. decoupled methods a decoupled method addresses the dtspn by decomposition of the problem into two sub-problems. first, the permutation of the visits to the requested areas (locations) is found independently on the dubins path planning. the second sub-problem is to find a particular visiting configuration of each location. in this way, the original dtspn is transformed to the dtsp with point locations to visit. in [10], the authors address the dtspn by solving the euclidean tsp where cities represents centers of the regions to be visited. once such a permutation is found, the final solution is constructed using the aa [2]. 58 vol. 2/2015 the dubins traveling salesman problem with neighborhoods authors of [7] also consider a solution the etsp to estimate the permutation of the visits. but instead of simple aa, the proposed local iterative optimization (lio) is used to find a significantly better solution. 4.2. sampling-based methods sampling-based methods [4, 11] are another class of existing approaches to address the dtspn. similarly to the decoupled methods, also sampling-based methods are based on existing approaches for the tsp variants; however, in this case a single representative of each region is not considered as in [10] but each region is rather sampled by a particular number of random samples (including the orientation of the vehicle). the samples can be determined in the whole region or only on its boundary. then, the samples of different regions are connected by the dubins maneuver to form a roadmap. such a roadmap is considered as an instance of the generalized asymmetric tsp (gatsp). if the gatsp is solved to the optimum, the sampling-based methods are resolution complete [11]. since the optimal solution of the gatsp can be computationally very demanding, existing heuristic for the tsp can be a more suitable option. the generated gatsp can be solved by its transformation to the asymmetric tsp (atsp) using the noon-bean transformation [12]. then, the transformed atsp can be solved by existing algorithms, e.g., the linkernighan heuristic algorithm [13]. 4.3. evolutionary methods the last class of existing approaches are genetic programming based algorithms. the general idea of these methods is to encode a solution by a chromosome in which the goal permutation and the configurations of the visits are stored. similarly to another approaches, only configurations on the boundary of the regions are considered in these evolutionary methods. probably the first evolutionary approach to the dtspn was published in [14]. the authors adapted the ordered crossover operator (ox) and added two new mutation operators (orientation shift and position shift) to optimize visiting configurations. relatively recently, the genetic approach from [14] was modified into a memetic algorithm in [15]. the authors used similar operators and added a local improvement of individuals in the population by optimizing the position of visiting points. the authors used aa as heuristic algorithm to speed up the algorithm. 5. modification of existing sampling-based algorithm the main issue of sampling based algorithms for the dtspn is that they require a fixed number of samples, which need to be established in advance. we propose an iterative schema to avoid this issue and set the number of samples progressively, which provides first solutions very quickly. moreover, if there is some additional computational time, the algorithm can iteratively improve the solution by adding more samples, which results to obtain a first solution quickly that is further improved. in the basic sampling-based algorithm, a roadmap with (n ·m)2 edges is created where n is the number of regions to be visited and m is the number of samples per region. for each edge one dubins maneuver is constructed. since the noon-been transformation does not change the number of vertices, the overall time complexity of generating an instance of the atsp can be bounded by o((n ·m)2). a solution of the generated atsp can be found by the available lkh solver. according to the author of the lkh, the time complexity of the lkh is approximately o(n2.2 atsp ) [16]. hence, the expected total time complexity is o((n ·m)2.2). the time complexity to solve the atsp is greater than a roadmap generation, and therefore, we do not need to explicitly consider the construction of the roadmap in the estimation of the required computational time based on the number of samples. based on the relation of the required number of operations on the number of samples, we can establish a sequence of the increasing numbers of the samples needed to find an initial solution quickly and improve its quality later. the sampling-based algorithm works with the constant number of samples given a priory, we can run the sampling based algorithm repeatedly with an increasing number of samples. since the time complexity of the algorithm is nearly quadratic, an inverse function for the number of samples mk according to the number k of the particular iteration can be defined as: mk ≈ √ 2k. (3) after rounding the mk to an integer value, it gives us the following series of the numbers: m = {1, 2, 3, 4, 6, 8, 11, 16, 23, 32, 45, 64, . . .}. (4) figure 1 provides influence of the required computanumber of samples per region 0 100 200 300 400 500 600 700 800 t im e [s ] 10 -2 10 -1 10 0 10 1 10 2 10 3 etsp + lio sample + atsp figure 1. the average required computational time (from 20 trials) for the instance of the dtspn with 4 circle regions and d4 constraint. tional time on the number of samples per each region 59 petr váňa, jan faigl acta polytechnica ctu proceedings to visit. we plot the required computational time on the number of samples per each region to visit. in this case, a simple problem with 4 regions and d4 constraint is considered and the computational time is compared with the etsp+lio algorithm proposed in [7]. 6. results the performance of the proposed modification of the sampling based algorithm has been evaluated in a series of scenarios. the evaluated instances of the dtspn were generated randomly for convex regions satisfying the d4 constraint. several shapes of the regions have been considered: points, disks with the radius equal to ρ, ellipses with the semi-axis 2ρ and 0.5ρ, and convex polygons with up to 6 vertices created from a circle with the radius ρ. a relatively high and uniform density of the regions for the d4 constraint has been considered by generating centers of the regions inside a bounding box with the side 6 √ nρ, where n is the number of the regions to be visited by the dubins vehicles. an example of the examined problems is depicted in figure 2. (a) . d4 convex regions (b) . d1 convex regions figure 2. examples of the randomly generated instances of the dtspn. the examined algorithms are denoted as follows. the newly proposed modification of the samplingbased algorithm is denoted sample+atsp. the algorithm is compared it with the evolutionary approaches [14] and [15] denoted as genetic and memetic, respectively. in addition, we implemented three variants of the decoupled approach based on the solution of the etsp utilized as the heuristic estimation of the permutation of the visits to the regions. the first decoupled approach is denoted as etspn+aa and it is based on the alternating algorithm (aa) [2]. the second method is denoted as the etsp+lio [7] and is based on local optimization of position and heading of the waypoints. the last method is derived from the etsp+lio, but only local iterative optimization of the heading is considered, this method is denoted as the etspn+holio. the quality of found solutions has been evaluated regarding a dedicated computational time for problems with n = 20 and n = 40 regions with the d4 constraint. the quality is measured as the ratio of the path length found by the particular algorithm to the solution provided by the etsp+lio algorithm, similarly as in [7]. all instances were generated randomly, and therefore, 50 trials have been solved for each problem and the algorithm variants. the achieved results are depicted in figure 3 for the 20 regions and in figure 4 for the 40 goal regions. time [s] 10 -2 10 -1 10 0 10 1 10 2 r el at iv e le n g th o f th e so lu ti o n 1 1.1 1.2 1.3 1.4 1.5 etsp + lio etspn + holio etspn + aa memetic genetic sample + atsp figure 3. average ratio of the tour length (from 50 trials) according to the etsp+lio solution for the dtspn with n=20 convex regions. plots start from the time when the first solution is available. time [s] 10 -2 10 -1 10 0 10 1 10 2 r el at iv e le n g th o f th e so lu ti o n 1 1.1 1.2 1.3 1.4 1.5 etsp + lio etspn + holio etspn + aa memetic genetic sample + atsp figure 4. average ratio of the tour length (from 50 trials) according to the etsp+lio solution for the dtspn with n=40 convex regions. plots start from the time when the first solution is available. all the algorithms have been implemented in c++ and tested on a single core of the intel core i5-m480 cpu running at 2.67 ghz. the processor was accompanied with 4 gb ram. 6.1. discussion the presented results show that the proposed modification of the sampling-based algorithm can be considered as a meaningful alternative to the evolutionary based algorithms. in the case of the high number of regions to visit (n = 40), the modified samplingbased algorithm has even superior results to the both genetic and memetic algorithms, while it is less computationally demanding. in the results depicted in figures 3 and 4, the etsp+lio algorithm achieved the best results among 60 vol. 2/2015 the dubins traveling salesman problem with neighborhoods the evaluated algorithms. this is caused by the fact, that all instances of the dtspn were generated with non-overlapping regions with the d4 constraint and the etsp+lio has been designed on top of the properties of the optimal solution of such a problem and thus it directly searches for good solutions [7]. 7. conclusions in this paper, we proposed a modification of the sampling-based algorithm for the dtspn. the proposed algorithm repeatably executes the original sampling-based algorithm with an increasing number of the samples per each region. by this modification, the newly developed algorithm provides the first solution very quickly that can be further improve if more time is available. this makes the algorithm suitable in situations where a solution has to be found quickly and where the maximum time that can be dedicated for the computation is not known in advance. we also compared the modified algorithm with other existing approaches on the randomly generated instances of the dtspn with the d4 constraint. the modified algorithm provides better results than the implemented evolutionary algorithms while it is less computationally demanding. a further comparison of the algorithms’ performance in the instances of the dtspn where regions to visit are closer or overlapping is a subject of our future work. acknowledgements the presented work has been supported by the czech science foundation (gačr) under research project no. gp13-18316p. references [1] l. e. dubins. on curves of minimal length with a constraint on average curvature, and with prescribed initial and terminal positions and tangents. american journal of mathematics pp. 497–516, 1957. doi:10.2307/2372560. [2] k. savla, e. frazzoli, f. bullo. on the point-to-point and traveling salesperson problems for dubins’ vehicle. in proceedings of the american control conference, pp. 786–791. ieee, 2005. doi:10.1109/acc.2005.1470055. [3] a. dumitrescu, j. mitchell. approximation algorithms for tsp with neighborhoods in the plane. j algorithms 48(1):135–159, 2003. [4] j. t. isaacs, d. j. klein, j. p. hespanha. algorithms for the traveling salesman problem with neighborhoods involving a dubins vehicle. in american control conference (acc), 2011, pp. 1704–1709. ieee, 2011. doi:10.1109/acc.2011.5991501. [5] x. ma, d. a. castanon. receding horizon planning for dubins traveling salesman problems. in 45th ieee conference on decision and control, pp. 5453–5458. 2006. doi:10.1109/cdc.2006.376928. [6] j. le ny, e. feron, e. frazzoli. on the dubins traveling salesman problem. ieee transactions on automatic control 57(1):265–270, 2012. doi:10.1109/tac.2011.2166311. [7] p. váňa, j. faigl. on the dubins traveling salesman problem with neighborhoods. international conference on intelligent robots and systems (iros) pp. 4029–4034, 2015. [8] x. goaoc, h.-s. kim, s. lazard. bounded-curvature shortest paths through a sequence of points using convex optimization. siam journal on computing 42(2):662–684, 2013. doi:10.1137/100816079. [9] j. t. isaacs, j. p. hespanha. dubins traveling salesman problem with neighborhoods: a graph-based approach. algorithms 6(1):84–99, 2013. doi:10.3390/a6010084. [10] x. yu, j. hung. optimal path planning for an autonomous robot-trailer system. in 38th annual conference on ieee industrial electronics society (iecon), pp. 2762–2767. 2012. doi:10.1109/iecon.2012.6389140. [11] k. j. obermeyer, p. oberlin, s. darbha. samplingbased path planning for a visual reconnaissance unmanned air vehicle. journal of guidance, control, and dynamics 35(2):619–631, 2012. doi:10.2514/1.48949. [12] c. e. noon, j. c. bean. a lagrangian based approach for the asymmetric generalized traveling salesman problem. operations research 39(4):623–632, 1991. doi:10.1287/opre.39.4.623. [13] s. lin, b. w. kernighan. an effective heuristic algorithm for the traveling-salesman problem. operations research 21(2):498–516, 1973. doi:10.1287/opre.21.2.498. [14] k. j. obermeyer. path planning for a uav performing reconnaissance of static ground targets in terrain. in aiaa guidance, navigation, and control conference, pp. 10–13. 2009. doi:10.2514/6.2009-5888. [15] x. zhang, j. chen, b. xin, z. peng. a memetic algorithm for path planning of curvature-constrained uavs performing surveillance of multiple ground targets. chinese journal of aeronautics 27(3):622–633, 2014. doi:10.1016/j.cja.2014.04.024. [16] k. helsgaun. lkh solver 2.0.7. http://www.akira.ruc.dk/~keld/research/lkh. [cited 31 mar 2015]. 61 http://dx.doi.org/10.2307/2372560 http://dx.doi.org/10.1109/acc.2005.1470055 http://dx.doi.org/10.1109/acc.2011.5991501 http://dx.doi.org/10.1109/cdc.2006.376928 http://dx.doi.org/10.1109/tac.2011.2166311 http://dx.doi.org/10.1137/100816079 http://dx.doi.org/10.3390/a6010084 http://dx.doi.org/10.1109/iecon.2012.6389140 http://dx.doi.org/10.2514/1.48949 http://dx.doi.org/10.1287/opre.39.4.623 http://dx.doi.org/10.1287/opre.21.2.498 http://dx.doi.org/10.2514/6.2009-5888 http://dx.doi.org/10.1016/j.cja.2014.04.024 http://www.akira.ruc.dk/~keld/research/lkh acta polytechnica ctu proceedings 2:57–61, 2015 1 introduction 2 related work 3 problem definition 4 existing methods 4.1 decoupled methods 4.2 sampling-based methods 4.3 evolutionary methods 5 modification of existing sampling-based algorithm 6 results 6.1 discussion 7 conclusions acknowledgements references 21 acta polytechnica ctu proceedings 2(1): 21–25, 2015 21 doi: 10.14311/app.2015.02.0021 the ritter-kolb catalogue and its impact on research into cvs, lmxbs and related objects h. ritter1, u. kolb2 1max–planck–institut für astrophysik, d–85741 garching, germany 2department of physical sciences, the open university, milton keynes mk7 6aa, united kingdom corresponding author: hsr@mpa-garching.mpg.de abstract in the first part of this paper, the ritter-kolb catalogue (rkcat for short), its history and a few examples of its application to research in the field of cataclysmic variables are briefly described. in a second part we look forward to possible future applications of rkcat for the study of cataclysmic variables, low-mass x-ray binaries and related objects. last but not least we also briefly comment on the future of the rkcat service itself. keywords: cataclysmic variables cataclysmic binaries low-mass x-ray binaries post-common envelope binaries catalogues. 1 introduction thirty years after the first public circulation of the catalogue of cataclysmic binaries, low-mass x-ray binaries and related objects, the so-called ritter catalogue or rcat for short (ritter 1982) it is perhaps a good time to look back at its history and some of its applications to research in the field of cataclysmic variables (cvs), lowmass x-ray binaries (lmxbs) and related objects, and at the same time to also look ahead to possible future applications of what later became to be known as the ritter-kolb catalogue (ritter & kolb 2003), hereafter rkcat for short. but before discussing the history and the future of rkcat let us briefly summarize for those who are not familiar with rkcat what rkcat actually is, i.e. its purpose, which information it does provide and how it can be accessed. 1.1 rkcat: what is it? ever since it was conceived rcat (up to 1998) and later rkcat (from 1998 onwards) has provided a comprehensive compilation of detached and semi-detached postcommon envelope binaries with known (or suspected) orbital periods. the restriction to systems with “known orbital period” is important for two reasons: first, knowing the orbital period of a binary not only provides important information about the object in question, but it is usually also a sure sign for the object having undergone serious examination to some degree, and that more is known about it than just its position on the sky, its apparent magnitude, and that it may be variable. second, restriction to systems with known orbital period also considerably limits the number of objects to be dealt with which is anyway growing at an alarming rate (see below). among the semi-detached systems we then have cvs (with at least one white dwarf (wd) component) and lmxbs (with either a neutron star or a black hole primary and usually a non-degenerate companion, but low-mass wds do also occur). among the detached systems, in rcat/rkcat referred to as “related objects”, we have “white dwarf”-red dwarf binaries and “white dwarf”-”white dwarf” binaries (double degenerates), where a “white dwarf” is either a genuine wd or an immediate precursor of a wd such as a sdbor sdostar. not included in rcat/rkcat among the detached systems are binary radio pulsars although some of them do in fact match the criteria for inclusion among the “related objects”. the reason for excluding binary radio pulsars is because they are well documented elsewhere (manchester et al. 2005).1 1.2 rkcat: what does it contain? rkcat provides information about each object in tabular form, a list of references to the most relevant recent literature, a list of references to published finding charts and a comprehensive cross reference list of alias object designations. in the tables (separate for cvs, lmxbs and related objects) we provide object designations, the position on the sky (α, δ), information about the object 1the data are available electronically at http://www.atnf.csiro.au/research/pulsar/psrcat. 21 http://dx.doi.org/10.14311/app.2015.02.0021 h. ritter, u. kolb type/subtype, apparent magnitudes, periods (above all the orbital period, but also the superhump period and the spin period of the wd (in cvs), or of the neutron star (in lmxbs)), the spectral types of the components as well as other orbital and stellar parameters (orbital inclination, mass ratio, masses, radii) to the extent that these quantities are known. the purpose of the cross reference list of alias object designations (appropriately called “whoswho”) is to help avoid confusion about object idendity. many objects have a long history during which they have been given many different names by different authors. thus confusion about an object’s idendity can simply arise because earlier authors can of course not refer to later object designations. but also going backwards in the literature does not always reveal even the more common alias names. the “whoswho” list is an attempt to remedy that situation and to nail down an object’s idendity in addition to its identification on a finding chart. the most recent release of rkcat at the time of writing (ritter & kolb 2003, update rkcat7.20 as of 30 june 2013) contains information about 1093 cvs, 104 lmxbs and 483 related objects, i.e. of 1680 objects in total. 1.3 rkcat: how can it be accessed? whereas earlier editions of rcat/rkcat appeared in print (ritter, 1982, 1983, 1984, 1987, 1990; ritter & kolb 1995, 1998) the current (7th) edition (ritter & kolb 2003) is available in electronic form only from websites at the open university (ou) at http://physics.open.ac.uk/rkcat and at the max-planck-institut für astrophysik (mpa) at http://www.mpa-garching.mpg.de/rkcat, and in slightly different form but with the same content from the strasbourg astronomical data center (cds) at http://cdsarc.u-strasbg.fr/viz-bin/cat?b/cb. the main difference between the versions provided by the ou/mpa websites and the cds website is that the former provide the tables in a two lines per object format with delimeters which allows for easy visual inspection of the tables on a computer screen, whereas the cds tables come in a one line per object format which (even without the delimiters) is much too wide for that purpose. rather they are designed to be processed electronically. many more copies of rkcat are available from various other websites. but because the contents of these copies is beyond our control we do not take responsibility for the content of any other than the abovementioned sources. 2 a brief history of rcat/rkcat compiling data on cvs for what was later to become rcat actually began already in 1973, i.e. 40 years ago, as part of the first authors’s (hereafter hr) phd project (ritter 1977). although at that time there was no intention whatsoever to produce a catalogue, compiling data on cvs, lmxbs, and related objects continued also beyond the end of the phd project, and it was around 1978/79 that rumors about the existence of this compilation began to spread among colleagues, some of whom did not hesitate to ask for copies. dutifully, hand-written copies of the compilation were produced and circulated among a few priviliged individuals. in that context, one must realize that at that time neither the internet nor email did exist, and electronic storage of data on computers was unavailable the storage media of choice were either hand-written files or punched cards and consequently, there was no easy way of editing and updating such a data base. bogdan paczynski was among the first to whom a copy was sent upon his request. and based on the distribution of the orbital periods of the 43 cvs known at that time he must have realized the sharp drop of the number of cvs shortward of an orbital period of ∼ 80min, and not long after that went on explaining the physical significance of the minimum orbital period of cvs in the context of cv evolution (paczynski 1981; paczynski & sienkiewicz 1981, 1983). this was probably the first direct application of the immediate precursor of rcat to cv research. around 1981 personal storage accounts and screen editors on computers became finally available. therefore, the data of the cv compilation were migrated to permanent electronic storage and editing. from that time on it not only became much easier to update the compilation but also to produce printed copies of it. as a consequence, so to speak as a test, soon after having gone electronic, the first and second edition of rcat were produced and circulated in preprint form among a relatively large number (∼ 200) of colleagues and libraries (ritter 1982, 1983). the positive response to the catalogue then lead to the publication of the 3rd edition printed in a refereed journal (ritter 1984). with data on cvs thus becoming widely available it took not long before another of the remarkable properties of the cv period distribution, i.e. the period gap, was addressed and an explanation for it in terms of cv evolution was given (spruit & ritter 1983; rappaport, vebunt & joss 1983). around 1987, when the 4th edition of rcat (ritter 1987) was published it was also realized, not least thanks to x-ray observations with exosat, that the distribution of the orbital periods of the synchronized and non-synchronized magnetic cvs, i.e. of the am her stars and the intermediate polars, are significantly dif22 the ritter-kolb catalogue and its impact on research into cvs, lmxbs and related objects ferent and that this could be accounted for in terms of standard cv evolution by the fact that on average the mass transfer rate in cvs decreases with orbital period (hameury et al. 1987). with the publication of the 5th edition of rcat (ritter 1990) another interesting property of the period distribution of cvs became apparent, namely the peculiar distribution of the orbital periods of am her stars below the period gap with 6 am her stars piling up within a very narrow period interval of only 2 min, the so-called period spike (see fig. 1 in ritter & kolb 1992). in that paper we had also given a detailed analysis of the period spike and explained its position and width as a natural consequence of cv evolution in the context of the disrupted magnetic braking hypothesis. although at that time the am her period spike was considered to be statistically highly significant and thus worth studying, now, more that 20 years later and with many more systems known, we must admit that there is no such spike and that we were just fooled by a statistical fluke. with the 6th edition of the catalogue (ritter & kolb 1998) two major changes occurred. first, because the amount of work connected with updating the catalogue had increased so much that additional support, in particular for servicing the newly established catalogue websites, was needed, ulrich kolb (uk) stepped in, and so rcat became rkcat. since then uk has been mainly responsible for the maintenance of our websites. second, beginning with the 6th edition the data became available in electronic form only, and no more than a brief introduction to the catalogue has appeared in print. finally since 2003, the 7th edition of rkcat (ritter & kolb 2003) has become available, the data again in electronic form only. the catalogue websites of the 7th edition are now updated regularly, currently every 6 months, and at the time of writing the 20th update (release 7.20) is available. the development of rcat/rkcat over the past 35 years is perhaps best illustrated by plotting the number of objects contained in the catalogue as a function of time, as is shown in fig. 1. as is readily seen, the number of objects has been steadily and exponentially increasing with a growth rate of ∼ 9%/year over the past 30 years! 3 some examples of possible future applications of rkcat before going to discuss a few specific possibilities for future applications of rkcat, let us briefly make two rather general comments concerning rkcat and its future use. • the first comment is a warning, namely that based on the criteria according to which objects enter the catalogue, no subset of the objects listed in rkcat represents a statistically well-defined sample. one has to keep this in mind when e.g. comparing results of population synthesis calculations with distribution functions extracted from rkcat. • the second general comment is that by now the number of objects contained in rkcat is larger by a factor 10 to 20 than it was 20 30 years ago, i.e. when the first applications of rcat to problems of cv evolution were made. what this means is that now the contents of rkcat allows for statistically meaningful studies of the properties of various subsets of cvs, lmxbs or related objects, which because of too small numbers of objects had not been possible a decade or two ago. figure 1: number of cvs (open squares), lmxbs (open triangles), and related objects (filled diamonds), as well as the total number of objects (filled circles) contained in rcat/rkcat and its precursors as a function of time. let us now turn to just a few specific possible future applications of rkcat. 1. it has long been known that the average orbital period of age zero post common envelope wd-rd binaries, i.e. of the binary central stars of planetary nebulae and systems where the wd is still so hot that it gives rise to a noticeable reflection effect on the companion, is systematically longer (by about a factor of 2) than the average orbital period of the remaining detached post common envelope wd-rd binaries. the qualitative explanation for this is that post common envelope binaries undergo orbital evolution as a consequence of 23 h. ritter, u. kolb loss of orbital angular momentum. in that context it is important that now rkcat contains a relatively large number of detached wd-rd post common envelope binaries with reasonably accurately known binary parameters. based on these data it should be possible to investigate the angular momentum loss of post common envelope wd-rd binaries in more detail, e.g. its dependence on properties of the rd, i.e. whether it is evolved or fully convective or not, and maybe even quantify it. 2. the period distribution of dwarf novae shows an undisputable dearth of systems in the period range 3h . porb . 4h which is not predicted by the standard cv evolutionary paradigm, i.e. the disrupted magnetic braking hypothesis, in combination with the disc instability model for dwarf nova outbursts. a satisfactory explanation for this phenomenon is so far lacking. in the framework of the disc instability model this could be accounted for only if the mass transfer rate is decreasing less strongly with orbital period than the critical mass transfer rate for dwarf nova outbursts as a cv evolves from longer to shorter orbital periods. this, in turn, implies that the angular momentum loss time scale must decrease with the orbital period, contrary to what standard prescriptions of magnetic braking, e.g. verbunt & zwaan (1981) or mestel & spruit (1983) predict. the ratio of the number of dwarf novae to novalike systems as a function of orbital period, on the other hand, should allow for an empirical calibration of the angular momentum loss rate as a function of orbital period. 3. as the absence of a clear period gap in the period distribution of am her systems suggests, the strong magnetic field of the wd interferes with the loss of mass and angular momentum via a stellar wind from the companion star with the result that magnetic braking in magnetic cvs is significantly weaker than in non-magnetic cvs. therefore, if below the period gap angular momentum loss via magnetic braking is comparable to that caused by gravitational radiation, as has been argued e.g. by knigge, patterson & baraffe (2011), and if magnetic braking is also weaker in am her stars below the gap, one would expect that also the minimum period of am her stars is systematically shorter than that of non-magnetic systems. therefore, by a careful examination of the period distributions of magnetic and non-magnetic cvs below the gap it should be possible to get more insight into the evolution of cvs below the gap. clearly, the above list of possible future applications of rkcat is not exhaustive. what we hope for is that our colleagues working with rkcat will come up with many more and even more ingenious applications of rkcat for the research in the field of compact binaries than we have just given. 4 the future of rkcat apart from discussing future appliactions of rkcat we think that writing this paper provides us also with an exellent opportunity to briefly address the possible future of rkcat itself. in that context it is important to be aware of the following facts: • the number of objects in rkcat has been growing exponentially at a steady rate of ∼ 9%/yr over the past 30 years (see fig. 1), and so has the amount of work that has been going into rkcat. and there is no reason to expect that this will be much different in the near future. • already now, servicing the catalogue data base is practically a full-time job for hr. • hr is already several years beyond formal retirement age. taken these three points together it is obvious that hr will definitely not be able to provide this service for many more years. if rkact or something akin to it should be continued beyond hr’s definitive retirement from that job then it is clear that in view of the first two points above some changes in how this service is and in what should be provided are unavoidable. the all decisive question to the cv community then is: who is prepared to continue with rkcat and in which form? anybody out there? acknowledgement hr would like to thank franco giovanelli for inviting him to attend this workshop and thus for providing him an opportunity to talk about the past and future of rkcat. references [1] hameury, j.m., king, a.r., lasota, j.p., ritter, h.: 1987, mnras 231, 535. [2] knigge, c., baraffe, i., patterson, j.: 2011, apjs 194, 28. 24 the ritter-kolb catalogue and its impact on research into cvs, lmxbs and related objects [3] manchester, r.n., hobbs, g.b., teoh a., hobbs, m.: 2005, aj 129, 1993. doi:10.1111/j.1745-3933.2006.00242.x [4] mestel, l., spruit, h.c.: 1987, mnras 226, 57. [5] paczynski, b.: 1981, aca 31, 1. doi:10.1017/cbo9781139164245 [6] paczynski, b., sienkiewicz, r.: 1981, apj 248, l27. doi:10.1086/421295 [7] paczynski, b., sienkiewicz, r.: 1983, apj 268, 825. [8] rappaport, s., verbunt, f., joss, p.c.: 1983, apj 275, 713. doi:10.1086/382481 [9] ritter, h.: 1977, phd thesis, univ. hamburg. doi:10.1086/590491 [10] ritter, h.: 1982, catalogue of cataclysmic binaries, low-mass x-ray binaries and related objects, first edition, preprint mpa 22. doi:10.1046/j.1365-8711.1999.02926.x [11] ritter, h.: 1983, catalogue of cataclysmic binaries, low-mass x-ray binaries and related objects, second edition, preprint mpa 51. [12] ritter, h.: 1984, a&as 57, 385. [13] ritter, h.: 1987, a&as 70, 335. [14] ritter, h.: 1990, a&as 85, 1179. doi:10.1046/j.1365-8711.2000.03780.x [15] ritter, h., kolb, u.: 1992, a&a 259, 159. [16] ritter, h., kolb, u.: 1995, in x-ray binaries, w.h.g. lewin, j. van paradijs, e.p.j. van den heuvel (eds.), cambridge astrophysics series 26, cambridge university press, p. 578. [17] ritter, h., kolb, u.: 1998, a&as 129, 83. doi:10.1086/341696 [18] ritter, h., kolb, u.: 2003, a&a 404, 301. [19] spruit, h.c., ritter, h.: 1983, a&a 124, 267. doi:10.1088/0004-637x/721/2/1356 [20] verbunt, f., zwaan, c.: 1981, a&a 100, l7. discussion ashley pagnotta: regarding the future of rkcat, have you considered working with the aavso? their vsx is not nicely curated like rkcat but is a possible future home. hans ritter: up to now i had not approached anybody or any organisation regarding the future of rkcat. this is actually the first time that i have spoken in public about that problem. christian knigge: just a comment on the topics you suggested for further study, which i think are really interesting. regarding pmin for magnetic cvs, boris gaensicke, retha pretorius and i have all looked at this a bit and have not seen the expected difference to non-magnetic cvs. [note added: i think this is mostly unpublished – the only relevant publication i am aware of is pretorius, knigge & schwope 2013, mnras, 432, 570, where fig. 1 shows that the period minimum for magnetic cvs is consistent with that for all cvs, which are dominated by non-magnetic systems. but we don’t really discuss that in the text.] also, in fig. 18 of knigge, baraffe & patterson (2011) we plot the dwarf nova fraction versus porb and find that not only is it zero just above the gap, but going from shorter to longer orbital periods it declines monotonically through the gap, which is very surprising. — 25 http://dx.doi.org/10.1111/j.1745-3933.2006.00242.x http://dx.doi.org/10.1017/cbo9781139164245 http://dx.doi.org/10.1086/421295 http://dx.doi.org/10.1086/382481 http://dx.doi.org/10.1086/590491 http://dx.doi.org/10.1046/j.1365-8711.1999.02926.x http://dx.doi.org/10.1046/j.1365-8711.2000.03780.x http://dx.doi.org/10.1086/341696 http://dx.doi.org/10.1088/0004-637x/721/2/1356 introduction rkcat: what is it? rkcat: what does it contain? rkcat: how can it be accessed? a brief history of rcat/rkcat some examples of possible future applications of rkcat the future of rkcat 311 acta polytechnica ctu proceedings 1(1): 311–315, 2014 311 doi: 10.14311/app.2014.01.0311 sifap: a new fast astronomical photometer filippo ambrosino1, franco meddi1, roberto nesci 2, corinne rossi1, silvia sclavi1, ivan bruni3 1dipartimento di fisica università la sapienza, p.le a. moro 5, 00185 roma, italy, 2inaf-iaps, tor vergata, roma, italy, 3istituto nazionale di astrofisica, osservatorio astronomico, via ranzani 1, 40127 bologna, italy. corresponding author: filippo.ambrosino@roma1.infn.it abstract a fast photometer based on sipm technology was developed and tested at the university of rome “la sapienza” and at the bologna observatory. in this paper we present the improvements applied to our instrument, concerning new cooled sensors, a new version of the electronics and an upgraded control timing software. keywords: instrumentation: photometer stars: pulsars. 1 instrument description the general description of our 3-channel photometer sifap (silicon fast astronomical photometer) is presented in meddi et al. (2011; 2012). we only remind here that each channel is dedicated to a different target: variable source (channel 0), nearby sky (channel 1) and reference star (channel 2). a gps receiver sends a pulse per second (pps) to a micro processor (µp) which in turn produces a signal that we call gated pps. this last is used both to drive two leds in order to have an optical temporal marker and to synchronize our custom electronics. during the last year we replaced the old hamamatsu multi pixel photon counter (mppc) modules with new ones, having the sensor cooled by a built-in peltier cell 1; we modified our electronics and to increase the uninterrupted acquisition time and to reduce the sampling gate duration from 0.55 ms down to 0.1 ms. the new block diagram of the instrument is shown in figure 1. the thermo-electric cooled system allows to reach a fixed working temperature of -10 ◦c, with a large reduction of the mean dark count. the s/n ratio is improved by a factor 3 with respect to the previous one. the modules have limited geometrical dimensions and low weight so they can be directly located at the exit pupil of the telescope. other characteristics of the new sensors are similar to the old ones: pixel size of 50µm for a total active area of 1 mm2 and photon detection efficiency with a maximum value of about 50% at wavelength of 440 nm2. the built-in electronics of each hamamatsu module can generate three types of output: pulse count via usb interface already processed in time windows of 1 ms, analog and discriminated. we use the last one to feed our electronics which uses a new protocol for data exchange between a field programmable gate array and a µp and a new data storage support. the integration time windows for this output are now 0.1 ms. we called this electronics p3e, which stands for pulsar pulse period extractor. the pointing and the signal maximization procedures and the fast pre-analysis to check the pulsating behavior of the pointed source are described in meddi et al. (2012). 1 http://www.hamamatsu.com/resources/pdf/ssd/c11208 series kacc1176e03.pdf 2http://www.hamamatsu.com/resources/pdf/ssd/s11028 series kapd1026e04.pdf 311 http://dx.doi.org/10.14311/app.2014.01.0311 filippo ambrosino et al. figure 1: general block diagram of sifap mounted at the telescope. the gps antenna is located outside the dome. hamamatsu mppc sensors are integrated in their modules, p3e units are fed by the discriminated output. each couple of mppc sensor and p3e unit is dedicated to a different target: variable source (mppc0 and p3e0), nearby sky (mppc1 and p3e1) and a reference star (mppc2 and p3e2). figure 2: fft applied to the whole raw data acquired by p3e0 on 2012, december 19. 312 sifap: a new fast astronomical photometer figure 3: fundamental frequency and period values of the crab pulsar signal obtained by our data with applied the heliocentric corrections compared with those computed from jbo ephemeris. the uncertainties on the jbo values are due to a numerical interpolation procedure. the quoted errors on our data are statistical only. figure 4: crab pulsar light curve folded by the xronos task efold for p3e0 (2012, december 19) corrected data. 2 data analysis and results on 2012 december 19 we observed the crab pulsar with the loiano telescope, for one hour. to optimize the pointing of the target, we performed a quick pre-processing analysis on a short acquisition of the data (∼ 100 s), based on autocorrelation and fft techniques. we then analyzed the whole data set with a fft analysis to estimate the spin period of the pulsar. in figure 2 we show the amplitude spectrum of the crab pulsar signal obtained from data collected 313 filippo ambrosino et al. by p3e0. we used the xronos package of the high energy astrophysics science archive research center (heasarc) to apply the heliocentric correction (task earth2sun) and to determine the best fitting period for the crab pulsar signal (task efsearch). in figure 3 we compare the jodrell bank observatory (jbo) ephemeris3 prediction for the fundamental frequency and period of the crab pulsar signal with the estimates of the same quantities obtained by our barycentered data. the agreement between the two results is within 300 µhz for the fundamental frequency and 300 ns for the spin period. such discrepancy can not be justified without taking into account systematic effects, which are explained in terms of the clock drift which depends on the temperature. this drift is estimated by measuring the time interval between the two optical markers generated by the gated pps signal, the first at the beginning and the second one at the end of the acquisition. taking into account such effect, the differences for the spin period are reduced below 15 ns. systematic uncertainties are due to i) the propagation of the gated pps signal trough the cables and ii) the rising edge of leds. such systematic uncertainties produce a total delay of about 320 ns. finally, the accuracy on the pulse output time of the pps signal which is ±0.001 ms at the rising edge of the pulse itself. all these effects are not included in the numerical computation of the uncertainty on the period obtained by applying the barycentric correction. the crab light curves were obtained by using the task efold on the barycentered data; the result for p3e0 is shown in figure 4. the shape of the primary and the second peak, and their flux ratio, are in good agreement with the literature data (e.g. golden et al., 2000; zampieri et al., 2011). the sky and reference star measurements were processed in the same way as the ones belonging to the target to look for spurious signals which might interfere with the astronomical signal from the crab nebula: no evidence of periodicity was detected. 3 conclusions we built a fast photometer able to collect data of periodic signals with high time accuracy integrating in time windows down to 0.1 ms. with the loiano 1.5 m telescope we derived the period and a high s/n light curve of the crab pulsar with a measurement duration of about 1 hour. in these conditions we obtained a fair agreement with the fundamental frequency and the spin period calculated from the jbo database. we intend to upgrade our custom electronics aiming at reaching shorter time sampling keeping a good s/n ratio. our final goal would be to compare fast optical measurements with the γ-ray, x-ray, ir and radio ones to explore more deeply the structure and the phenomena occurring in the emitting regions of pulsars. in particular, we want to study how the spin period slows down (dissipative process) and the amount of the eventual phase delay among the peaks in the various band (phase shifting process). we need to collect high quality data with a 0.001 ms time windows to be able to perform these kind of investigations. to this purpose we modified our gps system to reach a more accurate determination of the absolute time scale (within 100 ns) by using a burst of n gated pps signals instead of the single one presently used. we started with theoretical computation of the minimum possible time sampling for a v ∼ 16 mag object as a function of the telescope diameter. the results indicate that it could be possible to measure objects of the same magnitude as the crab pulsar with similar s/n ratio in time windows shorter than 0.1 ms. for instance, with a 5 m telescope the integration time could be reduced down to 0.01 ms. moreover it is mandatory to have larger telescopes in order to have enough detectable photons and higher resolution on the absolute timing by adopting a military class gps antenna which would be able to reach at least 1 ns accuracy. in this case it would be necessary to upgrade the optical time marker by substituting standard leds with laser ones. acknowledgement the authors thank the bologna observatory for the logistic support and the technical assistance during the observations. the universita’ di roma “la sapienza” supported the project. this research has made use of the simbad database operated at cds, strasbourg, france. references [1] golden a., shearer a., redfern r. m., beskin g. m., neizvestny s. i. et al., 2000, a&a, 363, 617 [2] meddi f., ambrosino f., rossi c., nesci r., sclavi s. et al., 2011, acta polytechnica, 51, n.6, 42. [3] meddi f., ambrosino f., rossi c., nesci r., sclavi s. et al., 2012, pasp, 124, 448. doi:10.1086/665925 [4] zampieri l., germana c., barbieri c., naletto g., cade a. et al., 2011, adv. space res., 47, 365. doi:10.1016/j.asr.2010.07.016 3http://www.jb.man.ac.uk/pulsar/crab.html 314 http://dx.doi.org/10.1086/665925 http://dx.doi.org/10.1016/j.asr.2010.07.016 sifap: a new fast astronomical photometer discussion b. aschenbach: how do you explain that the second harmonic in the amplitude spectrum is higher than the first one? f. ambrosino: the amplitude of the single harmonic depends on the shape of the light curve; in the case of the crab pulsar in one period there are two emission peaks of different intensity and with a phase shift of about 0.45. 315 instrument description data analysis and results conclusions acta polytechnica ctu proceedings doi:10.14311/app.2016.3.0019 acta polytechnica ctu proceedings 3:19–24, 2016 © czech technical university in prague, 2016 available online at http://ojs.cvut.cz/ojs/index.php/app the effect of support plate on drilling-induced delamination navid zarif karimia,∗, hossein heidaryb, parnian kianfarc, mahmud hasanic, giangiacomo minaka a university of bologna, via fontanelle 40, forli, italy b university of tafresh, tehran road 1, tafresh, iran c amirkabir university of technology, hafez 424, tehran, iran ∗ corresponding author: navid.zarif@unibo.it abstract. delamination is considered as a major problem in drilling of composite materials, which degrades the mechanical properties of these materials. the thrust force exerted by the drill is considered as the major cause of delamination; and one practical approach to reduce delamination is to use a back-up plate under the specimen. in this paper, the effect of exit support plate on delamination in twist drilling of glass fiber reinforced composites is studied. firstly, two analytical models based on linear fracture mechanics and elastic bending theory of plates are described to find critical thrust forces at the beginning of crack growth for drilling with and without back-up plate. secondly, two series of experiments are carried out on glass fiber reinforced composites to determine quantitatively the effect of drilling parameters on the amount of delamination. experimental findings verify a large reduction in the amount of delaminated area when a back-up plate is placed under the specimen. keywords: drilling, composite materials, delamination, support plate. 1. introduction glass fiber reinforced plastics (gfrps) have been extensively employed in many engineering applications because of their outstanding advantages over other materials. the machining and specially drilling of gprps have become very important due to the need for assembling of subcomponents made from these materials. but, composite materials are difficult to machine due to some particular characteristics of them like non-homogeneous, anisotropic and abrasive fibers. this causes significant damages in drilling process such as matrix cracking, fiber breakage, fuzzing and thermal degradation. among the damages caused by drilling, delamination is considered as one the most crucial, which results in lowering of strength against fatigue and subsequently reducing the longterm performance of composite structures [1, 2]. several techniques have been suggested to reduce delamination in drilling process that each has its own advantages and limitations. one of the frequently used methods is to place a support plate under the specimen. during supported drilling, in contrast to unsupported drilling, the specimen is not free to bend due to constraints imposed by back-up plate. in drilling with back-up plate, the thrust force increases suddenly and severely as the chisel edge begins to penetrate into the specimen and it continues until the full penetration of cutting lips. at this point actual material removal takes place and the force reaches its steady state value, until delamination occurs. in this stage, delamination causes the force to decrease stepwise, each step associates to a crack opening. unlike supported drilling, during drilling without back-up plate, the force increases gradually at the beginning of process. this is mainly because the real feed rate is much lower than the adjusted feed due to relative movement of drill bit and workpiece. as the drill bit approaches the last uncut plies, the stress due to the thrust force exceeds its critical value and all the uncut material is burst open [3]. until now, many researchers have attempted to reduce drilling induced delamination. hocheng and dharan used linear elastic fracture mechanics to determine the critical thrust force for twist drilling as push-out delamination begins to propagate [4]. hocheng and tsao proposed some analytical models for different drill bits including candle stick drill, saw drill, core drill and step drill to determine the thrust force at the beginning of delamination growth [5, 6]. based on these models, they realized the delamination can be reduced by lowering the thrust force or distributing the force outward from the center [7]. in a similar work, they studied the effect of exit back-up plate on drilling-induced delamination when using a slot drill bit and a core drill bit [8]. they showed that the critical thrust force is increased when using a back-up plate, hence delamination is less likely to happen. many efforts have been made to reduce the thrust force exerted by drill bit on the specimen, while few researchers studied the effect of support plate on thrust force and associated delamination. this paper investigates the effect of back-up plate on drilling-induced delamination both analytically and experimentally. 19 http://dx.doi.org/10.14311/app.2016.3.0019 http://ojs.cvut.cz/ojs/index.php/app n. zarif karimi, h. heidary, p. kianfar et al. acta polytechnica ctu proceedings 2. experimental procedure 2.1. delamination measurement chen presented an index to evaluate the amount of delamination called conventional delamination factor [9]. however, this factor is not proper because the crack size does not represent the damage magnitude appropriately and also this procedure does not indicate the damage area. davim et al. suggested a superior approach to determine the delamination factor called the adjusted delamination factor fda which is expressed as eq. (1) [10]. the first part of eq. (1) shows the size of the crack contribution and the second part shows the damage area contribution. fda = α dmax d0 + β amax a0 (1) where the coefficients α and β can be calculated as below: β = ad a0 −amax α = 1 − β (2) where d0 is the nominal diameter of the hole, dmax is the maximum diameter of the damage hole, a0 is the area related to the nominal hole, amax is the area related to the maximum diameter of the delamination zone and ad is the delaminated area. 2.2. materials and tools the composite plates were produced by hand lay-up method with araldite ly556 epoxy resin reinforced with 60 % e-glass unidirectional fiber. the density of epoxy resin and glass fibers were 1.12 g/cm3 and 292 g/m2, respectively. tensile strength and tensile modules of resin were 80 mpa and 2.7 gpa, respectively, and for glass fibers were 2150 mpa and 74 gpa, respectively. the composite laminates had 16 plies and a thickness of 5 mm. the holes were drilled at the center of plates by standard hss twist drills 10 mm. during unsupported drilling, a back-up plate of 5 mm thick was positioned under the specimens. an appropriate clamping system was used to fix the specimens and back-up plate in the drilling machine, shown in figure 1. the specimens and back-up plate were fixed in position by tightening four screws at the corners. 2.3. plan of experiments in the present study, taguchi method was used to design drilling experiments. three parameters, feed rate, cutting speed and drill point angle were selected and three levels for each parameter were suggested based on our preliminary researches [11–14]. after determination of parameters and correspond levels, a proper orthogonal array need to be selected. the orthogonal array chosen was the l9. after conducting experiments, the signal to noise ratio (sn) needs to be calculated for each experiment to determine the effect of each parameter. according figure 1. experimental setup. to this method, the optimization is done by using three signal to noise ratios; smaller is better, larger is better and nominal is better. notice that, regardless of what approach is taken, always higher value of the signal to noise ratio is better. in this investigation, the delamination factor needs to be minimized; hence, smaller is better definition of the signal to noise ratio was used. sni = −10 log( ni∑ i=1 y2 i ni ) (3) where i is the experiment number and ni is the number of trials for experiment i [15]. 3. results and discussion 3.1. theoretical analysis of critical thrust force the energy balance equation at the onset of delamination propagation is: dud = dw − du (4) in which du is the infinitesimal strain energy, dw is the infinitesimal work done by the thrust force fth and drill displacement of dx and dud is the infinitesimal strain energy absorbed by crack growth which are as following: dw = fth · dx dud = gic · da (5) where da is the change in the delamination area and gic is the critical strain energy release rate in mode i, which is assumed to be constant according to saghizadeh and dharan [16]. 3.1.1. critical thrust force in drilling without support plate figure 2 depicts the schematic of delamination in unsupported drilling. in figure 2, fth is the thrust force exerted by a twist drill at the center of plate, x is the displacement of drill, h is the thickness of 20 vol. 3/2016 the effect of support plate on drilling-induced delamination figure 2. the schematic of delamination for unsupported drilling. specimen, h is the uncut depth under tool, and a is the radius of crack (delamination). in this model, two assumption are considered; the isotropic behavior and pure bending of the laminate. according to the classical plate theory, for a circular plate with clamped edges and concentrated force the amount of deflection can be expressed as [17]: w(r) = fth 16πm [2r2 ln r a + (a2 − r2)] (6) which is written in polar coordinate system (r is radius). m = eh3/(12(1 − ϑ2)) is flexural rigidity of the plate, e is modulus of young, and ϑ is ratio of poisson. the stored strain energy, the work done and the strain energy absorbed by crack growth are expressed as below equations, respectively. u = f 2 tha 2 32πm du = f 2 tha 16πm da (7) w = fthw(0) = f 2 tha 2 16πm dw = f 2 tha 8πm da (8) ud = gic ·a = gicπa 2 dud = gic2πada (9) now it is possible to calculate the critical thrust force at the onset of crack propagation by replacing eqs. (7), (8) and (9) in the energy balance eq. (4) as shown below: fth = π √ 32gicm (10) in order to prevent delamination, the thrust force exerted by the drill bit on the specimen which is related to the material properties and the uncut thickness should not go over this value. 3.1.2. critical thrust force in drilling with support plate figure 3 depicts the schematic of delamination in drilling with back-up plate. in figure 3, fb is the thrust force with back-up, r is the upward reaction force from the back-up plate, b is the radius of the applied ring force of back-up and c is the drill radius, x is the displacement. for an edge clamped circular plate under the action of a concentrated force in center and upward circular force, the amount of deflection can be expressed as: w1(r) = fb 16πm [2r2 ln r a + (a2 − r2)]− − r 8πm [(r2 + b2) ln b a + 1 2(1 − b2 a2 )(a2 + r2)] 0 6 r 6 b (11) w2(r) = fb 16πm [2r2 ln r a + (a2 − r2)]− − r 8πm [(r2 + b2) ln r a + 1 2(1 + b2 a2 )(a2 − r2)] b 6 r 6 a (12) using boundary conditions, w(b) = 0, we get: r = fb [b2 ln b a + ( b2−a2 2 )] [2b2 ln b a + ( a4−b4 2a2 )] (13) by replacing r into the plate deflection equations, eq. (11) and eq. (12), the total stored strain energy and work done by external forces can be derived as follow: u = π[ ∫ b 0 [m(∂ 2w1 ∂r2 + 1 r ∂w1 ∂r )2]rdr+ + ∫ a b [m(∂ 2w2 ∂r2 + 1 r ∂w2 ∂r )2]rdr] du = ∂u ∂a da (14) w = fbw(0) = fb [ fba 2 16πm − r 8πm (b2 ln b a + + (a 2 − b2 2 ))] dw = ( f 2 ba 8πm − rfb 8πm a2 − b2 a )da (15) finally, by calculating dud from eq. (14) and replacing dud and dw in the energy balance equation, eq. (4), we get: fb = π √ 32gicm b a (16) where a and b are defined as follow: 21 n. zarif karimi, h. heidary, p. kianfar et al. acta polytechnica ctu proceedings figure 3. the schematic of delamination for supported drilling. b = [a4 − b4 − 8a2b2 ln b a ]× × [64b4a4 ln(a2b2) − 128a4b4 ln b ln a+ 16a6b2 ln b a + + 16a2b6 ln a b + a8 + b8 − 2a4b4] (17) a = 3(a12 − b12) + 72a10b2 ln b a + 192a4b8 ln a ln b+ + 3072a6b6 ln a 2 + b b2 + a + 768a4b8 ln a 2 + b b2 + a − − 288a8b4 ln a ln b− 704a6b6 ln b+ 192a2b10 ln a ln b+ + 73a4b4(a4 − b4) + 32a2b2(a8 − b8) + 64b4a8 ln b a + + 144a6b6 ln b a + 56a2b10 ln b a + 352a6b6 ln a2b2− − 96a2b10 ln a2b2 + 144a8b4 ln a2b2 + 1024a6b6 ln b3 a3 + + 256a4b8 ln b3 a3 + 768a4b8 ln a ln b− 16b12 ln a2b2+ + 32b12 ln a ln b− 384a4b8 ln a2b2 (18) 3.2. experimental results the graphs of thrust force and the scanned images of delamination in drilling with and without support plate at a feed rate of 0.025 mm/rev, spindle speed of 1600 rpm and drill point angle of 130 ◦ are shown in figure 4. it is believed that the thrust force applied by the drill on the specimen causes delamination and it occurs when the thrust force goes beyond its critical value. however, the graphs of the force in figure 4 clearly indicate this is only true for drilling with a support plate. during unsupported drilling the force is lower than in supported drilling, whereas the delamination is more extensive, which suggests that in unsupported drilling a different mechanism is in play, and other factors must be considered. these factors can be the dynamics of the workpiece, and the way in which the force is applied. the measured adjusted delamination factor (fda) and the corresponding signal to noise ratio (sn) determined using taguchi method smaller is better for figure 4. the graphs of thrust force and the scanned images of delamination in drilling with and without support plate. figure 5. comparison of fda in drilling with and without back-up. the two cases of drilling, supported and unsupported, are reported in table 1.symbols i and ii in table 1 represent two measured values for fda. the t-test on the mean values of adjusted delamination factor demonstrates significant differences between the means in these two cases (t0 = 8.00 > t0.005,34 = 2.73). to assist in the practical interpretation of this experiment, it is helpful to construct a graph of average responses at each experiment condition described in table 1. this graph is shown in figure 5. from figure 5, a significant difference in adjusted delamination factor for supported drilling and unsupported drilling can be observed so that in all drilling tests, the average value of delamination is reduced when applying a back-up plate. this reduction is in the range of 8-27 % for different drilling conditions. this reduction is mainly due to the reduction in matrix crack growth as a result of upward reaction of back-up plate. the values of sn data for the adjusted delamination factor for unsupported and supported drilling are given in table 1 and shown in figure 6. for unsupported drilling, the most important parame22 vol. 3/2016 the effect of support plate on drilling-induced delamination test no. parameters unsupported supported feed rate(mm/rev) speed (rpm) angle (◦) fda(1) fda(2) s/n fda(1) fda(2) s/n 1 0.025 800 90 1.41 1.37 -2.86 1.04 1.05 -0.34 2 0.025 1250 118 1.25 1.28 -2.04 1.07 1.06 -0.55 3 0.025 1600 130 1.30 1.26 -2.14 1.11 1.07 -0.75 4 0.05 800 118 1.49 1.52 -3.55 1.09 1.09 -0.87 5 0.05 1250 130 1.20 1.21 -1.62 1.10 1.11 -1.25 6 0.05 1600 90 1.33 1.28 -2.31 1.16 1.15 -0.75 7 0.1 800 130 1.49 1.58 -3.73 1.15 1.14 -1.40 8 0.1 1250 90 1.32 1.30 -2.35 1.18 1.17 -1.17 9 0.1 1600 118 1.36 1.33 -2.57 1.22 1.25 -1.83 table 1. l9 orthogonal array of taguchi and experimental results. figure 6. effect of process parameters for (a) unsupported drilling, (b) supported drilling. ters affecting the delamination factor are the spindle speed followed by feed rate. the optimum process parameters on the delamination are obtained as feed rate at level 1 (0.025 mm/rev), spindle speed at level 2 (1250 rpm) and drill point angle at level 3 (130 ◦) when drilling without back-up plate. however, when the back-up plate is applied, the effect of feed rate on delamination increases so that it becomes the most important parameter followed by spindle speed, figure 6 (b). the cause of this phenomenon can be attributed to the cutting mechanism and the relative movement of the tool and specimen. it is observed that the optimal value of the feed rate should be kept at low level in order to minimize the delamination factor. the optimum process parameters on the delamination are obtained as feed rate at level 1 (0.025 mm/rev), spindle speed at level 1 (800 rpm) and drill point angle at level 3 (130 ◦) for supported drilling. 4. conclusions in this paper, the effect of exit support plate on delamination in drilling of glass fiber reinforced composites is studied. a comprehensive analysis of the critical thrust force in drilling with and without back-up plate is presented based on three theories i.e. energy conservation theory, elastic bending theory and linear elastic fracture mechanics. the experimental results confirm the effects of delamination reduction when using a back-up plate under the specimen. this reduction is in the range of 8-27 % for different drilling conditions. this reduction is attributed to the crack growth suppression by upward reaction of back-up plate. references [1] r. m. jones. mechanics of composite materials. crc press, 1998. 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[7] h. hocheng, c. c. tsao. the path towards delamination-free drilling of composite materials. journal of materials processing technology 167(1):1251– 64, 2005. doi:10.1016/j.ijmachtools.2005.10.004. [8] h. hocheng, c. c. tsao. effects of exit back-up on delamination in drilling composite materials using a saw drill and a core drill. international journal of machine tools and manufacture 45(1):1261–70, 2005. 23 http://dx.doi.org/10.1016/j.ijmachtools.2005.10.004 n. zarif karimi, h. heidary, p. kianfar et al. acta polytechnica ctu proceedings [9] w.-c. chen. some experimental investigations in the drilling of carbon fiber-reinforced plastic (cfrp) composite laminates. international journal of machine tools and manufacture 37(1):1097–108, 1997. [10] j. p. davim, j. c. rubio, a. m. abrao. a novel approach based on digital image analysis to evaluate the delamination factor after drilling composite laminates. composites science and technology 67(1):1939–45, 2007. 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[16] h. saghizadeh, c. k. h. dharan. delamination fracture toughness of graphite and aramid epoxy composites. journal of engineering materials and technology 108(1):290–5, 1986. [17] j. n. reddy. mechanics of laminated composite plates and shells: theory and analysis. crc press, 2004. 24 http://dx.doi.org/10.1016/j.compstruct.2012.10.044 http://dx.doi.org/10.1177/0021998314521258 http://dx.doi.org/10.1016/j.compstruct.2015.04.025 acta polytechnica ctu proceedings 3:19–24, 2016 1 introduction 2 experimental procedure 2.1 delamination measurement 2.2 materials and tools 2.3 plan of experiments 3 results and discussion 3.1 theoretical analysis of critical thrust force 3.1.1 critical thrust force in drilling without support plate 3.1.2 critical thrust force in drilling with support plate 3.2 experimental results 4 conclusions references acta polytechnica ctu proceedings doi:10.14311/app.2016.6.0018 acta polytechnica ctu proceedings 6:18–27, 2016 © czech technical university in prague, 2016 available online at http://ojs.cvut.cz/ojs/index.php/app framework for ad hoc network communication in multi-robot systems khilda slyusar∗, miroslav kulich czech technical university in prague, zikova 1903/4, 166 36 prague, czech republic ∗ corresponding author: slyuskhi@fel.cvut.cz abstract. assume a team of mobile robots operating in environments where no communication infrastructure like routers or access points is available. the robots have to create a mobile ad hoc network, in that case, it provides communication on peer-to-peer basis. the paper gives an overview of existing solutions how to route messages in such ad hoc networks between robots that are not directly connected and introduces a design of a software framework for realization of such communication. feasibility of the proposed framework is shown on the example of distributed multi-robot exploration of an a priori unknown environment. testing of developed functionality in an exploration scenario is based on results of several experiments with various input conditions of the exploration process and various sizes of a team and is described herein. keywords: multi-robot systems, distributed exploration, ad hoc network, limited communication, cooperation. 1. introduction one of the key issues when designing a robotic system involving more than one entity is how these entities communicate. the majority of current multi-robot solutions are centralized (i.e., they have a central element through which all messages between robots are passed) or assume an unlimited communication range allowing to send messages directly between two arbitrary robots. this setup has many advantages, but also several drawbacks. it can be hard, costly or even impossible to prepare external communication infrastructure assuring one-to-one communication accessibility in hostile, unexplored or large environments. moreover, robustness of centralized systems which are highly dependent on a single node is low. finally, it might be difficult to exchange information in large systems consisting of many robots as their communication capacities are limited and a number of messages is high. some other arguments as well as real-world applications where ad hoc networks improved or might improve performance, scalability and robustness of robotic teams can be found in [1]. elkady et al. [2] give a nice survey of robotic middlewares. besides centralized client-server architecture employed, e.g. in player/stage/gazebo [3], ace/tao library [4], implementation of corba (common object request broker architecture) standard [5], is used for peer-to-peer communication in middlewares like claraty [6], miro [7], orocos [8]. alternatively, the ice framework by zeroc [9] for remote procedure calling (rpc) can be used as it is already done in the orca project [10]. both zeroc ice and corba do not provide communication between directly inaccessible nodes in a decentralized fashion and although extensions for ad hoc networks exist [11] they were not used in the mentioned robotic middlewares. communication in robot operating system (ros) [12], the mostly popular robotic framework, is peer-to-peer, but this communication is mediated by so called rosmaster. this node provides naming and registration services and enables individual ros nodes to locate one another. once the nodes locate each other, they communicate directly. also ros does not provide routing of messages between inaccessible nodes. on the other side, several works were presented which describe application of mobile ad hoc networks with routing in robotics. witkowski et al. [13] present a cell-based network with master nodes in each cell and routing between distant nodes is provided through these master nodes which are distributed uniformly in the environment. the authors validate the approach on two scenarios. while measuring of the signal quality of wlan and bluetooth communication nodes in indoor environments with different kind of wall materials is the aims of the first one, the second experiment deals with radio-based positioning of robots in a dynamic environment. corell et al. [14] study a wireless coverage problem with minimal requirements to hardware of robots. as a communication middleware they use optimized link state routing (olsr) [15] which provides tcp/ip routing for the resulting mesh network. a suite of algorithms that cover a defined area by a fully distributed robotic team, detect network disconnections, perform network repair, and repair area coverage is presented in [16]. finally, agrawal et. al [17] experimentally evaluate two routing protocols for a robotic swarm. the swarm aims to find a source of heat and a particle swarm optimization algorithm is employed as a search strategy. the aim of this paper is to give an overview of routing protocols in ad hoc networks and choose the most 18 http://dx.doi.org/10.14311/app.2016.6.0018 http://ojs.cvut.cz/ojs/index.php/app vol. 6/2016 framework for ad hoc network communication in multi-robot systems suitable for application in the multi-robot domain. the chosen protocol is than used in a design and realization of the framework for exploration of an a priori unknown environment which thus serves as a testbed for experimental evaluation of the protocol implementation. the rest of the paper is structured as follows. section 2 contains a structured list of requirements for a routing protocol, a classification of protocols, and describes the main representatives of the categories. then a comparative analysis of routing protocols is performed. analysis of three message-oriented middlewares (mom) describes in section 3. the general application architecture and using of selected measures is presented in section 4. section 5 contains results of the experiments, which allow to verify the implementation performance of the proposed architecture. the final evaluation of the paper is in conclusion. 2. ad hoc routing protocols a robot team forms an ad hoc network as it does not rely on a pre-existing infrastructure, while interaction between team members is done by a routing protocol. operation efficient of a distributed robotic system requests a routing protocol to meet certain requirements. the key assumption is fully distributed operations and the protocol should not depend on a central control node. due to high mobility of the nodes, the protocol should easily adapt to changes in network topology caused by movement of nodes. it also needs to be localized, since global exchange of routing information will require large overheads [18]. an important requirement is the absence of routing loops. a routing loop occurs when the packets continue to be routed in an endless circle of the nodes. the protocol should not contain them, otherwise the overall efficiency is reduced, the routing loop will spend more bandwidth and more computing power of each node in this loop. the protocol should take care about saving resources of network members. in addition, the protocol needs to be scalable, this means that its effectiveness should not depend on the number of members in the network. increase or decrease of the network should not affect its performance. it also needs to be adapted to any speed of nodes and any type of their movement. in conditions of high mobility the protocol must be able to quickly recalculate cost and build the path to the destination which will include the lowest number of other nodes. while building the path and forwarding of messages, it is necessary to effectively avoid outdated routes [19]. the rest of the section contains a brief description of few most commonly used routing protocols and their comparison based on the above requirements. the main purpose of this comparison is to select the routing protocol that is the most suitable for teams of mobile robots which establish a wireless mobile ad hoc network (manet) without any centralized structure. the routing protocols can be divided into proactive (table-driven), reactive (on-demand) and hybrid, which combines advantages of the first two categories. 2.1. proactive routing protocols table-driven protocols constantly maintain the relevance of the routes between sources and destinations. to maintain a correct view of the network topology a protocol of this type responds to every change in the structure by sending changes across the entire network. moreover, the protocol periodically sends route update messages to its neighbours. these protocols require to store routing information in one or more tables in each node [20]. these protocols do not scale well and control overheads are proportional to the number of nodes in the network. destination sequenced distance vector (dsdv) [21] is one of table-driven protocols and it is a loop-free modification of the bellmanford routing algorithm [22]. each node maintains a routing table that contains entries for the next hop on the shortest path to all reachable destinations. the protocol uses a sequence number attribute to ensure the freedom of cycles and to distinguish stale routes. dsdv uses two types of dumps with updates of the routing table which sends to all immediate neighbours. the first type is periodically sent full dump which contains all available information about the routing information. the second type is an immediately sent incremental dump which sends on route changes and contains only changes that have occurred since the last full dump. another proactive protocol is optimized link state routing (olsr) [15] protocol, which is based on basic link-state algorithm. every node maintains a network topology graph. each node has thus shorter routes to every destination immediately, when data transmission begins. to keep the relevance of the topology a node periodically floods hello messages about its available links to the others. olsr uses also topology control (tc) messages that contain information about one-hop neighbours. the main optimization lies in using of multipoint relays (mprs). it reduces message overhead, because only nodes which were chosen as mprs forward broadcast messages during the flooding process. 2.2. reactive routing protocols reactive protocols perform the process of route discovery only on request. the source floods the network with route query requests when a packet needs to be routed using distance vector routing or source routing. the problem of reactive protocols is the delay of packet transmission during the route discovery. on the other side, the route is discovered only when needed, i.e., it is generally less memory demanding compared to 19 khilda slyusar, miroslav kulich acta polytechnica ctu proceedings proactive protocols and requires relatively little control traffic overhead. dynamic source routing (dsr) [23] is an example of on-demand routing protocols. dsr supports two types of operations: route discovery, when the node is required to transfer the data to the destination with an unknown route, and route maintenance, that allows to determine that the network topology is changed. after the work of these mechanisms the sender knows the complete hop-by-hop route to the destination. all transmitted routes stored in a route cache decrease time of route discovery. the protocol allows multiple alternative routes to the same destination and allows each node to perform load balancing. another example of reactive protocols is temporally ordered routing algorithm (tora) [24] based on the link-reversal algorithm. tora performs three functions: route creation, when a node uses a height metric to establish a destination oriented directed acyclic graph, route maintenance, when a node reestablishes routes due to topology changes and route erasure, when it is necessary to erase invalid routes. the protocol builds loop-free routes and provides multiple routes to alleviate congestion. however, tora may produce temporary invalid routes, as well as the light-weight mobile routing protocol [25] which it is based on. 2.3. hybrid routing protocols hybrid protocols typically try to reduce delay of route discovery from reactive systems by creating some form of routing tables and reduce control traffic overhead from proactive systems [20]. zone routing protocol (zrp) [26] is a hybrid protocol, which uses a proactive approach to a close neighborhood and a reactive approach to remotely lying nodes of the network. in other words, each node divides the network into intrazone and interzone. the node immediately knows routes to all nodes within the intrazone. the discovery of such paths is produced by the algorithm similar to dsdv. if an initiator needs to send data to a recipient which is in the interzone the sender proactively maintains a route to the destination. this approach reduces latency in route discovery and decreases the number of control messages. ad hoc on-demand distance vector routing (aodv) [27] is also a hybrid protocol based on the principles of the dsdv and dsr protocols. the protocol sends a periodic hello message, maintains the route table with a maximal single route for each destination and uses the principle of sequence numbering, as dsdv routing protocol does. on the other side, aodv provides a similar route discovery procedure as in dsr. a route between nodes is established only if it is necessary to send the data and there is no active route to the destination, i.e., the routing protocol reduces network overhead as compared with the dsdv algorithm. moreover, the route discovery packets do not contain a complete sequence of nodes, that is an advantage compared to dsr. 2.4. comparison of routing protocols there are several criteria [28] for evaluating the performance of a routing protocol: • packet delivery fraction is calculated as the ratio between the number of received messages by the destination to the number of messages sent by the sender; • end-to-end delay is the average time that elapses between the first packet which was transmitted by the sender before the first data packet received by the destination. the metric includes the time of route discovery, transmission delay, queuing delay and propagation delay; • routing overhead is calculated as the ratio between the routing packets to the total number of packets transmitted by the sender; • throughput is the amount of successfully delivered data via a communication link per time. it is usually measured in bits per second. the article [28] discusses three routing protocols dsdv, aodv and dsr. testing is carried out on teams of robots, whose size ranges from 10 to 80 pieces. robots perform a task similar to the task of terrain exploration. each node chooses some point in the terrain and moves toward it. the article considers all four criteria. dsr and aodv show better value of packet delivery fraction than dsdv. the value is about 95% for a team of 10 robots and it tends to 100% with an increase in the team size. aodv shows the smallest value of end-to-end delay. dsdv is a proactive routing protocol, so it shows better results than dsr. the value of routing overhead for aodv is better than dsdv. however, dsr has the smallest routing overhead in comparison with the other two. throughput has the lowest value for proactive dsdv. both dsr and aodv show approximately the same results, but this metric for dsr is slightly higher. a performance comparison of aodv, olsr and zrp is described in [29]. the simulation is conducted for teams of robots with sizes of 25, 50, 75, 100, and provides measurement of all metrics, except the routing overhead. the packet delivery fraction for aodv is approximately 90%, while olsr and zrp have the values of 20-40%. moreover, aodv has the highest throughput, as compared to two other routing protocols. however, aodv shows a significantly larger end-to-end delay than the reactive olsr and the hybrid zrp. the publication [30] compares the performance of dsdv, tora, dsr and aodv. the maximum number of robots in the team in the simulation is 50. the article considers several metrics, which include packet delivery fraction. both dsr and aodv show excellent results for the packet delivery ratio, which 20 vol. 6/2016 framework for ad hoc network communication in multi-robot systems activemq apollo zeromq nanomsg open-source yes yes yes has detailed documentation yes yes only manual pages has new updates yes yes yes point-to-point message passing yes yes yes publish/subscribe message passing yes yes yes written in java c++ c transport protocol tcp, udp tcp, in-proc tcp, in-proc support brokerless model no yes yes complexity of use requires broker without without configuration pre configuration pre configuration table 1. a brief comparison of message oriented middlewares. ranged from 95 to 100% depending on the network load. aodv is selected for implementation of the exploration framework on the basis of the above mentioned comparisons. simulations use robot teams of various sizes and display measurement results that are close to the conditions of the real world. the main advantage of aodv is a high percentage of packet delivery to the destination. the routing protocol has a good throughput compared to the rest. the designed application does not send many unicast messages, so the inflated end-to-end delay and routing overhead are not important. that is, these values are acceptable for the terrain exploration problem. 3. message oriented middleware the exploration application uses message passing to communicate between team members. it is a technique for invoking behavior, which uses incoming messages from other processes to run a code. there are two types of message passing: synchronous and asynchronous. the synchronous approach implies that message exchange takes place at a time when applications are running simultaneously. it uses temporary sender blocking before receiving a response from a recipient and infinitely blocking in case of recipient unavailability. the robots move in the environment during the exploration problem solving and can get out of the communication range. in addition, the robot can be turned off, for example, due to the battery discharge. in such cases, the message sender is blocked. that is, synchronous approaches are not suitable for use in distributed robotic systems that solve exploration problem. the asynchronous approach does not require simultaneous operation of a sender and a recipient. the intermediate level of software provides all the operations for receiving, storing and sending of messages. the most known type of such software is messageoriented middleware (mom). table 1 provides a brief comparison of main parameters of these three widely used asynchronous message-oriented middlewares: activemq apollo [31], zeromq [32] and nanomsg [33]. as can be seen from the table, all three libraries are open-source and release new updates. the first two libraries are well documented and have a large community of users. the latter is improved and rewritten zeromq library, which currently has only the manual pages, but the number of nanomsg users is constantly growing. all considered libraries support binding for commonly used programming languages as java, c, c# and c++. zeromq and nanomsg have also bindings for ruby, go, haskell, perl, php and others. there are two main functionalities of moms: message queuing and publish/subscribe. the first approach is a point-to-point messaging model, that is, a message is sent from a sender to a recipient. publish/subscribe approach is a many-to-many messaging model. all three libraries include both types of message passing. the libraries support a wide range of transport protocols, but transmission control protocol (tcp), user datagram protocol (udp) and in-proc are relevant for the purposes of the application. all considered libraries support transfer of data over tcp. activemq apollo additionally supports udp, while the other two do not support it. zeromq and nanomsg can use in-proc transport protocol. it is an inter-thread transport, which is much faster than tcp and can transfer data between threads of a single application. the most important difference is that the basic principle of activemq apollo is to create and configure a broker. a distributed multi-robot system has no central node, which could be a broker. a possible solution is creation of a broker at each node of the network. however, this solution is cumbersome and requires excessive pre-configuration of brokers. contrariwise, 21 khilda slyusar, miroslav kulich acta polytechnica ctu proceedings zeromq and nanomsg support brokerless model. nanomsg library was developed by one of the key creators of the zeromq library and it fixes most of the zeromq drawbacks. the main one is the problem with fault tolerance. nanomsg provides a new pattern of communication that does not require redundant confirmation from a recipient in the model requestrouter that resembles the work of procedure calls. the library allows to set a timeout for an operation of sending messages. it allows to refuse the message sending, if it is not sent for some predetermined amount of time. on the contrary, zeromq does not provide the possibility to obtain information about the current status of the recipient before sending a message. moreover, the library does not allow to set a timeout on an attempt to send a message. thus, in case of a fall, the whole system of robots gradually stops because of waiting a response from each other. the nanomsg library was chosen for use in the application on the basis of the above comparison of the characteristics of the libraries. because the library doesn’t provide an implementation of udp, the transport protocol is proposed to implement using standard sockets. 4. distributed exploration exploration as a process of autonomous navigation of a mobile robot in order to build a model of an a priori unknown working environment in a shortest possible time is one of the fundamental problems in mobile robotics. in general, exploration is an iterative process that performs the following operations at each step: the robot receives information from its sensors, updates an internal model of the surrounding space on the basis of these data, selects the next goal for the navigation on the basis of the current knowledge, and gradually moves to the selected destination. the process continues until there are no more available goals, that is, all areas of the environment will be explored [34, 35]. the main source of optimization is preparation of a set of possible goals and selection of a next goal from them, which is the most favorable for exploration purposes. besides the tasks mentioned above, the problem of distributed terrain exploration by a multi-robot team requires solving the problem of communication between team members and the problem of coordination their actions. this is why we selected this task as a testbed for evaluation of the aodv routing protocol. information about the terrain which is exchanged between team members and the method of determining the set of possible navigation goals depends on an exploration approach that is selected for implementation. frontier-based exploration [34] is chosen as one of the most common and widely used approaches which also has the extension for the case of multi-robot exploration [36]. the approach is based on the concept of a frontier, which is a boundary between the already known part of the terrain and still an unknown part. the internal representation of the map is an occupancy grid, which uses bayesian based updating technique. the main advantage of the environment exploration by a multi-robot team is the ability to easily share knowledge about the terrain between its members. exploration of non-overlapping areas by different robots allows to raise efficiency of teamwork and reduce the total exploration time. each robot in the team conducts the current goal selection independently on the basis of its current knowledge of the environment and the goals of the other team members. a greedy algorithm is chosen as the most primitive and native approach. it means, that the closest frontier to the current position of the robot is selected as the goal. in addition, the selected goal should not lie in the areas of the current goals of the other team members, which have active communication link to the robot. after the goal selection the robot informs the others about the selected goal and reports the priority for this goal. if an other team member has the goal in the same area with a higher priority, it sends information about its current goal and forces the robot with a lower priority to choose another goal and generate a new plan. description of the software architecture of a single robot performing distributed exploration and communicating with other robots making use of aodv follows in the next paragraphs. while designing the application structure for a single robot it was assumed that the application can operate in two roles: robot and display. the application for a robot role controls a robot as it carries out reading of data from its sensors, produces the robot movement in the terrain, performs path planning and communication with other team members. the application for a display role is used to monitor and display the status of the multi-robot exploration process and can vary from the specific implementation. the application of this type is also a part of the mesh network, but it does not directly participate in the exploration. figure 1. proposed application structure. it is recommended to use aodv routing protocol based on the analysis result of routing protocols 22 vol. 6/2016 framework for ad hoc network communication in multi-robot systems from section 2. the application should have the tools to unicast and broadcast messages for aodv implementation. tcp is the most appropriate protocol for the purposes of sending messages between robots. flooding and sending hello messages can be realised with using less reliable udp, which however supports broadcasting. according to the results of a comparative analysis in section 3 nanomsg recommended for implementation as a mom. thus, the nanomsg library provides the transfer of messages between robots via tcp and uses in-proc for transfer of information between different parts within the application itself. because this library does not support the udp, broadcasting should be implemented by means of language in which the application will be realized. fig. 1 shows the proposed general structure of the application, and describes the relationship between it and the external environment. application operations have different frequency of launching and some of them can be called asynchronously. e.g., plan generation usually runs once per two seconds or on the request from the other robot. a command to control robot movement can be sent every 100 milliseconds. the optimal frequency of sending the heartbeat for the purposes of aodv is 1 second. moreover, it is desirable to receive messages from the other robots instantly. message sending is also necessary to make immediately to reduce the overall time of the terrain exploration. this means that the designed application could not be structured in a single loop. for these reasons, the structure is divided into 5 logical parts, which can be implemented as threads: motion control/visualization, path planning, timer, message processing and inbox messages part. these parts of the application share three data structures: map, current plan and scans archive. map keeps an internal representation of the application’s view about the environment and depends on the selected type of the exploration. in the case of frontier-based exploration, the map will be in the form of the occupancy grid. current plan is the path to a goal selected at this iteration of the exploration process in the form of a polyline. scans archive is a data structure that contains scans received from sensors of any robot of the team. that is, the structure stores local maps created by the robot and local maps created by other robots and transmitted via flooding. the principle of the transfer of local maps between robots during the terrain exploration of a multi-robot team is described in [36]. the application for a robot role is determined by using the motion control thread. it is responsible for receiving the current plan, sending signals to the robot motors for movement to the next selected goal, using the robot sensors to obtain scans, which are then stored in the scans archive and receiving new scans from the scans archive received from other robots. finally, the thread conducts a map update on the basis of these data. if the application is launched for the purpose of visualization of the exploration process, it uses a visualization thread. the application of this type does not create its own scans, so the scans archive contains only local maps received from the robots. this thread uses them to update the map, which is displayed on the screen after that. the path planning thread is the main component of each iteration of the terrain exploration process. this thread selects the next goal for the motion and makes the path to it. this plan is written to the shared structure. the thread uses the map to plan. the application uses a timer to generate the plan after a certain predetermined period. the timer is located in the timer thread. the message processing thread is the main connection between the robot and the rest of the team. it is responsible for sending messages to other robots about a new generated path or a new laser scan. it also receives messages from the inbox messages thread. the inbox messages thread deals with receiving messages from other team members and sending them in the messages processing thread. 5. experimental results for functional testing of the proposed framework, the application of a multi-robot team member was implemented. it is implemented in c++ and simulates the work of an exploration robot for experimental verification of the effectiveness of the proposed application structure. it imitates operations of the real robot equipped with a laser range-finder. the robots are controlled by a simple controller which guides the robot exactly along a planned path with a constant speed. ray casting was used to simulated a laser range-finder with no noise incorporated. behavior and properties of the implemented application were experimentally evaluated and its characteristics were compared with a single-robot exploration. the experiments were conducted using a device which has 32 central processing units (cpu) and has 8 gb of memory. all tests were performed in simulation on the map of an empty terrain that has a size of 20 by 20 meters. each cell of the occupancy grid has a size of 5 by 5 centimeters. it means that the occupancy grid has a size of 400 by 400 cells. various numbers of robots in the team are used to test the loading of the network. the team size is varied from one to ten. the application makes the terrain exploration as a single robot, if the team consists only of one robot. various ranges of communication links are used to test the effectiveness of teamwork. the communication range takes one of the values: 5, 10, 15 or 20 meters. the laser has a range of 1.5 or 2 meters. it allows to verify the functionalities performance with a larger number of steps of the exploration process on the same map. the experiments were performed three times for each system configuration. 23 khilda slyusar, miroslav kulich acta polytechnica ctu proceedings (a). (b). figure 2. dependence of the steps on the team size. the value of the laser range is: (a) 2 meters; (b) 1.5 meters. the time measurement of the exploration is made in steps, i.e the number of calls of the function that changes the robot position. this function is triggered periodically after a certain period of time. thus, a step is a convenient alternative to time. in addition, the usage of steps metric allows to maintain usable statistics of events that pass between sequential steps. 5.1. time of the exploration fig. 2 shows the dependence of the average exploration time for a team member on the number of robots in the team. the graph shows that any team work helps to reduce exploration time. in the beginning, both graphs show a reduction of the number of steps, then there is an increase in steps quantity. this is due to the fact that the robots have to excessively agree on the choice of a goal in bigger teams. as can be seen from fig. 3, a small team disperses at the beginning of the exploration and explores non-overlapping areas of the terrain. however, an excessive number of robots in the team causes the problem of determination of the non-overlapping areas. so robots are required to negotiate the rescheduling of their goals. the graph also shows that the decrease in the communication range increases the number of steps that are required to complete the terrain exploration. some scans from the other robots could not reach the robot because of lack of communication link due to the small value of the communication range. that is, the robot is forced to explore the given area by itself. it increases the overall time of the exploration. comparison of fig. 2(a) and fig. 2(b) shows that the ratio of the steps number and the team size remains constant regardless of the laser range. however, reducing the value of laser range increases the total number of steps. if the laser has a shorter range, then each scan provides less information about the environment. thus, it is necessary to make larger movements (a). (b). (c). (d). (e). (f). figure 3. the resulting paths after the exploration. the laser range of the first column is 2 meters, the second column is 1.5 meters. rows are separated by team size: (a)+(b) 1 robot; (c)+(d) 2 robots; (e)+(f) 5 robots. 24 vol. 6/2016 framework for ad hoc network communication in multi-robot systems and get more scans to obtain a complete map of the terrain. 5.2. plan generations fig. 4 shows the frequency with which the application is launching a new request for a new generation of the plan. a plan generation runs periodically by a timer part of the application, as described in section 4. in addition, it can be run by a request from an another team member. in this implementation, if two robots have the goal at the same area, the robot with a higher priority of the goal sends a request to the robot with a lower priority to select a new navigation goal and generate a plan to the selected goal. the launching frequency of generation of the plan is 20 steps in the application. the graphs shown on fig. 4 contain all plan generations, which had one member of the robot team during complete exploration of the environment. thus, all function values under the value of 20 steps means that the corresponding plan generation is launched on request from an another robot. fig. 4(a) shows that a single robot starts a process of path generation by approximately the same intervals. the increase of the team size leads to the fact that robots can compete for the goals in a particular region of the terrain. this can have both positive and negative effects. let’s consider the situation of the terrain exploration by a team that consists of ten robots and the communication range is 20 meters. this means that all robots agree on the choice of goals with the other robots. the first low part of the graph shown in fig. 4(d) corresponds to fig. 5(a). the robots start the goal selection. however, the current implementation of the goal choice requires to select the goal that does not lie close to the goals of other team members. this means that the distance between the selected goal and goal of another robot should be greater than the laser range. an example of such area is depicted in fig. 5(a) as the yellow circle. if each robot chooses a goal which does not overlap with the goals of the other robots, the plan generation is invoked periodically by the timer thread. that is, the robot does not encounter other robots and it made the right goal choice. the straight part of the plot with a step value, which is equal to 20, corresponds to fig. 5(b). then, most of the terrain is explored and the robots are in a small unexplored part of the environment (see fig. 5(c)). the weak point of the implemented simple prioritizing between the robots appears. at this point, the frequency of plan generation begins to increase. moreover, increasing the number of plan generations leads to increase in the number of steps that are necessary to complete the terrain exploration. it then leads to increase of the total exploration time. 6. conclusion this paper addresses the problem of communication in distributed multi-robot teams forming a mobile ad hoc network. a list of requirements for a routing protocol was drawn up. the comparative analysis of routing protocols has shown that aodv routing protocol is most suitable. in addition, the analysis of three moms was conducted and the nanomsg library was chosen as the most suitable for implementation. multi-robot exploration has been selected as a proofof-concept task. the general structure of the robot application which carries out exploration of an unknown environment in a multi-robot team and which communicates with other robots employing aodv has been proposed. to verify the proposed architecture and offered tools the application has been implemented and has been tested by series of experiments. thus, the paper describes the architecture of the application and possible means of implementation in a generalized form. the specific implementation can be done for different systems and written on a wide range of languages. in future, this application will be used for testing on the group of real robots that solves the problem of the exploration of an unknown environment. goal allocation was not addressed in the paper as it was out of scope of it. analysis and comparison of a variety of sophisticated goal allocation mechanisms will be another stream we would like to go in future. acknowledgements this work has been supported by the technology agency of the czech republic under the project no. te01020197 “centre for applied cybernetics”. access to computing and storage facilities owned by parties and projects contributing to the national grid infrastructure metacentrum, provided under the programme “projects of large infrastructure for research, development, and innovations” (lm2010005), is greatly appreciated. references [1] w. zhigang, l. liu, m. zhou. protocols and applications of ad-hoc robot 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2016 1 introduction 2 ad hoc routing protocols 2.1 proactive routing protocols 2.2 reactive routing protocols 2.3 hybrid routing protocols 2.4 comparison of routing protocols 3 message oriented middleware 4 distributed exploration 5 experimental results 5.1 time of the exploration 5.2 plan generations 6 conclusion acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2016.5.0063 acta polytechnica ctu proceedings 5:63–68, 2016 © czech technical university in prague, 2016 available online at http://ojs.cvut.cz/ojs/index.php/app monitoring of track sections with long-pitch corrugation jan valehrach∗, jaroslav šmíd brno university of technology, faculty of civil engineering, institute of railway structures and constructions, veveří 331/95, 60200 brno, czech republic ∗ corresponding author: valehrach@fce.vutbr.cz abstract. the focus of this paper lies on monitoring of the track sections with rail corrugations caused by wheel sliding. short waves on the running surface of the rail head on low rail are typical for this defect. long-pitch corrugation is a significant cause of vibration and noise pollution in the railway infrastructure. therefore it is very important to understand the formation and development of this imperfection of the rails. measurements were carried out in curves of small radii on several tracks in the czech republic. in addition to the measurement of the surface of the rail head a number of supplementary parameters was evaluated for each section, such as curve radius, superelevation, track gauge etc. and the speed of passing trains as well. this paper describes rail corrugation defect and its development. our early results are presented in the conclusions. keywords: long-pitch corrugation, rail, track, measurement, speed . 1. introduction during the past recent time the railway infrastructure administrators are pushed to change their approach to the maintenance of the sections affected by the long pitch corrugation. these defects were not as frequent as they are in these days and also some of them have not been considered as a serious problem for the operability of the track. there are more factors to blame, such as increases of the track speeds, higher track loads or also (predominantly in the close vicinity of the cities) higher frequency of the passenger trains. in addition, today’s customers are not only satisfied with just the travel from point a to b or with the shortest time of transfer. they also require more comfortable travelling. it is not very difficult to notice the imperfections of the track or the increased noise levels. and there is also the second aspect of the problem, which some say is even more important – that is the influence of the imperfections for the adjacent neighborhood, such as vibrations, noise and quakes, all significantly reducing the quality of life of the people living near tracks. there is no doubt about the negative effects of the long-pitch corrugation. everyone is able to recognize that the train is much louder when passing through the curve degraded by corrugations in comparison with the passage of the same curve after the replacement of the rails. the tracks with the long-pitch corrugation require more frequent and also more costly maintenance (such as rail grinding, replacement of the rails etc.). in order to fulfil interoperable function within europe the railway network has ”to ensure a safe and uninterrupted operation of trains achieving set levels of performance” [1]. accordingly, it is necessary to guarantee required quality of the railway infrastructure and to reduce maintenance cost to minimum. with the above mentioned reasons in mind we should endavour to understand the formation and development of this defect. 2. theory of formation of long-pitch corrugation the long-pitch corrugation, sometimes called also as ”short waves”, is a sinuous inequality of the running surface of the rail. distance between wave peaks is usually between 8 and 30 cm and the depth ranges between 0.1 and 1.2 mm. a typical location, where long-pitch corrugation could be found, is the lower rail in curves of radius less than 600 m, sometimes even 700 m [2, 3]. the cause of these corrugations is the difference in length that each wheel of the wheelset rolls along the curve on the low and high rail. this difference also causes the wheel slip, occurring mostly on the low rail. slip happens not only in the longitudinal direction, but also in the lateral direction. in most cases, long-pitch corrugation does not occur alone. such defects, which are accompanying the long-pitch corrugation, can be sorted to: • those supporting the development of long-pitch corrugation (such as side wear of the outer rail), • those emerging as a consequence of long-pitch corrugation (cracked clips). since the processes surrounding the long-pitch corrugation are not documented in their entirety it is often complicated to distinguish which defect is the cause and which is the consequence. therefore, the consecuting list of defects does not take into account the above mentioned division: damaged baseplates, sleepers with transverse cracks, missing parts of fastening, cracked sleeper screws, cracked clamps in rail 63 http://dx.doi.org/10.14311/app.2016.5.0063 http://ojs.cvut.cz/ojs/index.php/app jan valehrach, jaroslav šmíd acta polytechnica ctu proceedings figure 1. long-pitch corrugation in havlíčkův brod fastening, abraded ballast behind sleeper heads etc. 3. selection of locations and measurement appropriate sections for the monitoring of long-pitch corrugation, where typical signs of this defect occur, were chosen in association with the railway infrastructure administration of the czech republic (sždc). further criteria which have influenced the selection of sections originated from the identical or at least similar characteristics and features of curves, such as: • curve radius, • track cant, • traffic load and others. subsequently, an assessment of the available data for all the individual sections was conducted. the frequency of measuring was determined with consideration to previous maintenance work, rail traffic present and planned intentions of the sždc as well. the majority of sections are double tracks thus it is possible to compare two monitored locations on one track or one location on each track, for example in a measured curve. to be able to understand more thoroughly the development of the long-pitch corrugation it is also necessary to observe other factors, which influence the defect. one of these factors is the matter of variety composition of the trains (meaning the split up between passenger trains and freight trains). variety composition of the trains complicates the whole matter, since it adds more other unknown influences. that is the reason, why we try to rule this factor out if possible. it is less difficult to succeed when considering the case with one track; hence it is more likely that the track would be passed in both directions equally. this also allows us to compare two sections within one location between themselves. on the other hand, in the case of the double track there is no certainty that both tracks would be passed with the same amount of trains or even with the same rate between the passenger and freight traffic. as an example, in some cases we can distinguish the acceleration of trains in one track and deceleration or even stopping of trains in the second track (like when approaching an entry signal of railway station). that is why it is not possible to perform comparison between the sections in different tracks within one measuring location. in both cases the monitored sections are chosen by the presume of the influence on formation and development of long-pitch corrugation, by the track alignment, by the permanent way design itself (composition of the superstructure including the effects of tested design elements such as more elastic fastening to the sleepers, clips with increased fatigue limits, under sleeper pads) and by the substructure design (e.g. the influence of the bedrock, bridge structure etc.). the sections are monitored in regular intervals depending on the present state of development of defects, traffic volume, maintenance and repair works etc. the sections with recently replaced rails allow us to observe the formation and development of long-pitch corrugation in its initial stages. measurements were repeated after 3 months. in sections, where the defects are more developed, the time between the measurements was almost doubled, since the corrugations are more noticeable (deeper and significantly better visible). that allows us to measure each wave separately. on the other hand, the measurements in sections, where defects are not yet fully developed should be performed with increased frequency, so it is easier to cover the whole development process. as you can see on figure 2, the long-pitch corrugation forms very interesting shapes which are changing fluently throughout the curve. apart from the visual inspection (which confirmed the suitability of our selected locations) all of the sections are monitored and documented using the following methods: • visual inspection and photo documentation, • measurement of track geometry by the trolley krab, • measurement of wavy deformations of the rail head by the device salamander, 64 vol. 5/2016 monitoring of track sections with long-pitch corrugation figure 2. highlighted long-pitch corrugation by contaminant in havlíčkův brod • monitoring of the speed of trains and train compositions. the track geometry is measured to describe the current state of the track, which unfortunately in many cases does not correspond to the data found in the railway documentation. another reason for these measurements is the need for additional information such as track gauge, gauge widening etc. the microgeometry of the rail head, specifically wavy deformations, was measured using the device salamander. unfortunately, in some cases it was not technically possible to measure a curve as a whole, which is the reason, why only few selected sections were measured within these locations. measured data are evaluated by the device in accordance with en 13231-3 [4] in wave lengths: • 10 ÷ 30 mm, • 30 ÷ 100 mm, • 100 ÷ 300 mm, • 300 ÷ 1000 mm. this categorization is not fully appropriate for our purposes since long-pitch corrugation can appear with a wave length shorter than 100 mm [3], particularly in small radius curves. based on the theory of formation of long-pitch corrugation [6] , our own measurements and data comparison, it can be said that the two categories for 30 ÷ 100 mm and 100 ÷ 300 mm wave lengths can be merged together for the purposes of figure 3. trolley krab (track geometry) [5] and salamander (corrugations) monitoring long-pitch corrugation. in both zones, the same or very similar wave amplitudes are achieved. apart from the monitoring of the long-pitch corrugation of the rail head, other critical elements were evaluated as important and thus were observed: • compliance with technology of construction and maintenance works, • compliance with technology of works in stations, • composition of passing trains, • operation of rail vehicles with respect to the track speed. during the measurements, the influence of the difference between the track allowed speed and actual speed of the vehicles proved to be imperative. that is the reason for closer observation of the traffic with respect to the measured speed of trains. in some cases the track allowed speed, or sometimes cogitated as a designed speed, differs significantly from the actual speed of the vehicles. this phenomenon occurs more often these days, since there is an effort of the track designers to maximize the speed limits as much as possible (they tend to ”squeeze out” the most of the infrastructure) and mostly could be found on tracks with very heterogeneous traffic flow, consisting of the various ratios of the passenger and freight trains. the more freight trains pass throughout the section of the track, the bigger is usually the difference. generally, the difference between the allowed and actual speed produces a cant excess or cant deficiency. analogically, these values grow parallelly to the difference, so when the difference increases, the values increase accordingly. therefore, a weighted average of all speed measurements in a set section was calculated, when the mass of each train was taken into consideration. the masses of the trains for the purposes of the weighted averages have been obtained by two ways. the weight of passenger trains was taken from the passenger carriages charts and from the database of the train compositions. the mass of the freight trains could be found out from the mass normatives (which is the maximum allowable mass of the whole train). the problem is that the actual load on the train is not known and thus it is not possible to know the right mass of the train. therefore the masses of the freight trains were estimated with only one value around 1000 tonnes. this approach brings the significant amount 65 jan valehrach, jaroslav šmíd acta polytechnica ctu proceedings figure 4. the processed record from the device salamander shows a track, where in one part the rails were replaced and in the other part the rails with long-pitch corrugation were left. of uncertainty to the result, but on the other hand, according to higher order masses of the freight trains in comparison to the passenger trains, the sensitivity of the result to the changes of the masses is quite small. then for the calculation of the cant excess and the cant deficiency the following scheme was used. assuming a formula for speed of trains v = √ d ∗ r 11, 8 the cant excess then applies e = d − 11, 8 ∗ v 2 r and analogically for the cant deficiency i = 11, 8 ∗ v 2 r − d ideally, the effects of cant excess and deficiency should be balanced, especially with respect to train masses. however, in some sections cant excess markedly dominates. that happens for example in the vicinity of stations, where the trains are accelerating and have not yet reached the track speed or where the greatest amount of the track load consists of slow moving trains (usually freight trains). in such locations this leads to the overload of the railway superstructure and consequently results in increased occurrence probability of the defects. figure 5. cant in track no. 1 and in track no. 2 in curve a. 4. outputs 4.1. measuring location havlíčkův brod one of the monitored track locations is a part of the track no. 324 havlíčkův brod – kutná hora in between the km 224.394 and km 225.150 in approach to the station havlickuv brod from the direction kutna hora. the location consists of a double track with consecuting reverse compound curves. first curve is for the purposes of this work named a and is closer to the station. the second curve is named b and is located further from the station in direction kutná hora. the cant of the track was measured, using the device krab, which showed a different value of the cant for each track. the results of the train speed measurements were the differing speeds of trains in both tracks and both curves. after the calculation of the weighted averages, the data revealed the presence of cant excess in both tracks and both curves. the train speed in the curve a in the track no. 1 is affected by braking/accelerating near havlíčkův brod station. some freight trains heading from kutná hora stopped at the station head in such a way that the end of the train was standing in the curve (e = 2 mm). unfortunately, this situation is a common practice in havlickuv brod. it facilitates the document handover by the stopping of the locomotive just in front of the station building. the data proved that both rails are nearly under the same load, since the weighted average e/i was close to zero. the record of the wavy deformations for wave lengths 100 ÷ 300 mm is shown on figure 6 on the first line. for the curve a on track no. 2 the weighted average equals e = 27 mm. when comparing with the track no. 1 the wavy deformations reach the amplitudes twice as high (figure 6, second line). on the contrary the cant excess markedly dominates in the curve b in comparison to the curve a. the weighted average the curve b equals e = 57 mm. the state of microgeometry as shown on figure 6 (fourth line) is practically identical to the curve a of this track. the track no. 1 of the curve b is passed with almost all trains with an cant excess (e = 72 mm) since they have to reduce the speed according to a change of allowed speed between the curves. in the past, the railway infrastructure administrator tried to improve the situation by reducing the allowable 66 vol. 5/2016 monitoring of track sections with long-pitch corrugation figure 6. category of wavelengths 100 ãů 300 mm havlickuv brod (from the top: curve a of the first track, curve a of the second track, curve b of the first track and curve b of the second track). speed to 55 kph, nevertheless this action had an adverse effect. instead of going at 70 kph, the trains are decelerating to 55 kph already during the passage through the curve b. additionally; the freight trains which are approaching the station and intended to pass through the first turnout in the side direction have to be slowed down just before the pass of the entry signal at 40 kph, which is located in the middle of the curve b. this forces such trains to travel with the reduced speed for more than 500 metres before the station. the designed cant in the curve b of 142 mm is from that reason unnecessarily large and corresponds to the recommended cant dn1 for speed 75 kph (recommended cant is the value of the cant prescribed by the standard csn 73 6360-1 [7]). the allowable speed in this curve is on the other hand only 70 kph and moreover the speed, which is achievable realistically is even less – just 55 kph. surprisingly, the wavy deformation of microgeometry is less developed in this section in comparison to other measured sections (as it could be seen on the figure 6 (third line)). one of the possible explanations of this phenomenon may be the usage of the under sleeper pads, which were installed in this curve and only in the track no. 1. according to [8], it is possible to install under sleeper pads to affect the behaviour of sleepers in a desired zone of the frequency spectrum. this is the way how it is possible to suppress the generation of vibrations and noise emissions in these frequencies. part of the energy of dynamic processes is emitted from vibrating sleepers as noise. for newly built track with under sleeper pads an increased level of noise was measured (in just units db). such a track shows over time slower development of imperfections of the rail heads. as a consequence, the noise levels for these tracks are lower than for comparable tracks without under sleeper pads. for this reason, not only the noise pollution, but also other parameters, such as vibration parameters, sleeper and rail movements and development of irregularities of the rail head are monitored on the test sections in the czech republic. the railway superstructure in the location havlickuv brod consists of the rails 49 e1, fastenings e40 with clamps e14 in the curve a and skl40 in the curve b, then sleepers b 91s/1 on both tracks. the difference between used fastening systems could not be considered as a cause of the change in the defects, since the replacement of the fastening system was the result of the development of the defects and should have been the resolution of the problem. the long-pitch corrugation was found on both tracks. additionally, massive cracking of clamps skl14 occurs in the curve b. it is surmised that high value of the weighted average of cant excess contributes to defects of the clamps. despite of the fact that the above mentioned data indicated that higher values of cant excess have the most adverse effect on microgeometry, the slowest development of the long-pitch corrugation occurred in the curve b in track no. 1. 4.2. measuring location hady the monitored location hady is the opposite to the location havlíčkův brod in the means of the unequal cant excess. in this location the cant deficiency plays the main role. this section could be found on the track from brno to česká třebová between km 161.685 and km 164.485. during the year 2015 the rails of the track no. 1 have been replaced and the development of the long-pitch corrugation thus could be observed and measured since the time zero i. e. from the perfect condition. the measuring location consists of two sections, first is located in front of the tunnel a (curve a), the second is located in front of the tunnel b (curve b). both monitored sections lay only in the track no. 1, but the line is double track. the usual direction of the passing trains in the track no. 1 is from the tunnel b to the tunnel a, which is the heading from česká třebová to brno. figure 7 shows the comparison of development speeds of wavy deformations (here represented as a percentage of the boundary overrun of the rms limit value, rms = root mean square) in both curves in location hady. initial analyses of the data show significant differences in development of the long-pitch corrugation in the curves with small radii. considering the common structure and geometrical parameters of the track (curve radii 283 m and 261 m, cant 123 mm in both curves, fastening w14, sleepers b 91s etc.) and the railway superstructure they do not exhibit significant differences. the curve b (in front of the tunnel b) embodies the weighted cant deficiency i = 67 mm. the curve a (in front of the 67 jan valehrach, jaroslav šmíd acta polytechnica ctu proceedings figure 7. comparison of rms values [%] (to the left the measuring point in front of the tunnel a and to the right the measuring point in front of the tunnel b in location hady. tunnel a) shows the weighted cant deficiency i = 47 mm. unfortunately, these values have to be considered as tentative, since the train speeds were affected by the day time in which the measuring took place. the majority of the recorded trains were passenger trains; the freight trains pass this location mostly at nights due to very strong suburban traffic. also it has to be pointed out, that the track configuration is similar to that in the location havlíčkův brod (namely the curve a of the track no. 1). there is a turnout located just behind the measuring location and freight trains approach the station through the side direction of the turnout at 60 kph at maximum. passenger trains pass the turnout in straight direction at speeds up to 70 kph ( or up to 75 kph for trains using the cant deficiency 130 mm). when we take into consideration that the measured train speeds were just tentative, the calculation offers results, in which the values for cant deficiency are logically lower (for the section in front of the first tunnel). this partial conclusion shall be confirmed or refused as soon as possible. 5. conclusions various parameters were monitored on the two measuring locations. in the every curve there could be found a different combination of factors affecting the development speed of the long-pitch corrugation and in some curves a different track parameter can be the dominant factor. nevertheless, it can be declared at the present time and based on our measurements and observations, that the speed of passing trains undoubtedly plays the significant role on the development of defects. the adverse effect of the speed occurs in two forms: • the higher the value of the speed, the more significant the development of the defect, • the higher the value of the cant inequity occurs, the more significant is the defect. at the current state of the research it might be said that the higher the speed or the bigger the difference between the actual speed of the vehicle and the designed track speed, the faster and greater the development of the defects. the curve radius has to be considered as a fixed parameter at all times, since its change in order to reduce the development of the long-pitch corrugation is in most cases nearly impossible, we have to focus on another parameters. the composition of the railway superstructure and setting of the appropriate speeds of trains and also appropriate cant values seem to be promising factors. whether these parameters alone would be sufficient the following research will show. acknowledgements the presented results were obtained with the support of centre for effective and sustainable transport infrastructure (te01020168), student’s grant tender of the faculty of the civil engineering of the brno university of technology (fast-j-15-2860) and railway infrastructure administration, state organization (szdc s. o.). references [1] r. čech. interoperabilita a rozvoj ždc [online], 2015. [2016-02-11], https://www.fd.cvut.cz/personal/ tyfal/str/predmety/ikod-pr/ikod10.pdf. [2] c. esveld. modern railway track. 2nd ed. zaltbommel: mrt-productions, 2001. doi:isbn 90-800-3243-3. [3] h. funke. broušení kolejnic, 1992. [4] en 13231-3. railway applications track acceptance part 3: acceptance of reprofilling rails in track, 2012. [5] s. s. r. kžv. krab system [online]. [2015-12-26], http://kzv.cz/english/krab.php. [6] j. valehrach. skluzové vlny teorie a praxe, 2015. doi:isbn 978-80-214-5091-2. [7] čsn 73 6360-1. konstrukční a geometrické uspořádání koleje železničních drah a její prostorová poloha část 1: projektování, 2008. [8] plášek o., hruzíková m. přínos podpražcových podložek ke zvyšování kvality geometrických parametrů koleje, in zborník prednášok z konferencie s mezinárodnou úcastou pozemné komunikácie a dráhy, environmentálna kapacita pozemných komunikacií a dráh. košice, technická univerzita v košiciach, 2006. isbn 80-8073-602-2. 68 https://www.fd.cvut.cz/personal/tyfal/str/predmety/ikod-pr/ikod10.pdf https://www.fd.cvut.cz/personal/tyfal/str/predmety/ikod-pr/ikod10.pdf http://dx.doi.org/{isbn} 90-800-3243-3 http://kzv.cz/english/krab.php http://dx.doi.org/{isbn} 978-80-214-5091-2 acta polytechnica ctu proceedings 5:63–68, 2016 1 introduction 2 theory of formation of long-pitch corrugation 3 selection of locations and measurement 4 outputs 4.1 measuring location havlíckuv brod 4.2 measuring location hady 5 conclusions acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2017.7.0079 acta polytechnica ctu proceedings 7:79–84, 2017 © czech technical university in prague, 2017 available online at http://ojs.cvut.cz/ojs/index.php/app design of concrete chimneys via beam theory and nonlinear shell analysis franziska wehr∗, reinhard harte university of wuppertal, pauluskirchstr. 7, 42285 wuppertal, germany ∗ corresponding author: fwehr@uni-wuppertal.de abstract. this paper compares the load-bearing capacity of chimneys calculated via beam and shell theory. it becomes apparent that the design via beam theory is on the safe side for the vertical reinforcement of the chosen examples for h/d ratios larger than 30. for non-slender chimneys the design via beam theory overestimates the load distribution around the circumference and yields to wrong results. on the other hand a linear elastic shell calculation underestimates the load-bearing capacity of the chimney. however a realistic distribution of stresses in the cross section of a chimney can still be calculated using shell theory with nonlinear material properties. keywords: nonlinear, shell, reinforced concrete, chimney, material nonlinear. 1. introduction chimneys are one of the main components of thermal power plants. in design of industrial concrete chimneys the structural integrity is characterized by the ultimate limit resistance against wind action and the serviceability limit resistance against constraints caused by thermal effects. detailed investigations on damaged chimneys [1] have shown that mechanical actions like wind load are not the only reason for the observed crack patterns. these actions have to be accompanied by constraints due to thermal effects. 2. design of concrete chimneys via beam theory currently the design of concrete chimneys is regulated in codes such as the international cicind model code for concrete chimneys [2], the en 13084 – freestanding chimneys [3, 4] for europe and the aci 307-08 – code requirements for reinforced concrete chimneys [5] for the usa. a comprehensive comparison of the three codes can be found in [6]. all codes have in common that the design is performed via beam theory. a model of the whole chimney with a vertical beam and a circular cross section is used for the design of the vertical reinforcement and another model with a circular beam and a rectangular cross section for the design of the hoop reinforcement (fig. 1). this paper focuses on the ultimate limit state (uls) design according to cicind model code and the european standard en 13084 for the vertical reinforcement. for the vertical reinforcement only mechanical impacts have to be considered. this means that usually only dead loads g, wind w , seismic actions e, rotations of the foundation and second order effects need to be applied. according to [2] a temperature drop in the wall may be neglected if it is less than 60 k. table 1 shows the load combinations for uls design according to [2] and [4]. figure 1. design of concrete chimneys via two beam models (source: [2], modified) typical material properties for concrete (for example c30/37) and reinforcement (for example b500b) for the uls design are shown in fig. 2. the material properties listed in en 13084 [4] are referring to the eurocode [7, 8]. the diagrammed stresses in fig. 2 are already reduced by a safety factor γ and have to be looked upon as design values. the stress-strain-curve for concrete is identical for both considered standards, but the strain limit according to [2] is reduced to 3 ‰ instead of 3.5 ‰ according to [4]. both standards allow only compressive stresses for concrete in uls design. the stress-strain-curve for steel is defined to be symmetric for the compressive and tensile stresses in both codes. the steel-curves are identical up to a strain of 10 ‰. 79 http://dx.doi.org/10.14311/app.2017.7.0079 http://ojs.cvut.cz/ojs/index.php/app franziska wehr, reinhard harte acta polytechnica ctu proceedings verification leading combination of actions combination of actions action acc. to cicind [2] acc. to en 13084 [4] vertical strength inline 1.0g+ 1.6w + 1.0t 1.0g+ 1.5w + 1.0t wind combined vertical cross 1.0g+ 1.5wcross + 1.2w + 1.0t 1.0g+ 1.5wcross + 1.5w + 1.0t strength wind shear and flexural earth1.0g+ 1.4e + 1.0t 1.0g+ 1.0e + 1.0t strength quake table 1. combination of actions at uls for chimneys acc. to [6]. a) b) figure 2. stress-strain-curves of concrete (a) and reinforcement (b) acc. to [4], with reduced strain limits acc. to [2]. the internal forces of the chimney are calculated using a beam model with linear-elastic material properties according to both codes. the stress resultants are combined as shown in tab. 1. the design for the meridional reinforcement is performed by using the stress-strain-curves from fig. 2. but it is not only a matter how the meridional stress resultants are calculated, different approaches for the design of the cross section and reinforcement need to be considered as well. 3. comparison of beam theory and linear elastic shell theory further studies by [6] and [9] have revealed that for chimneys with large diameters the shell-like behavior becomes more important. it is apparent that the linear elastically calculated meridional forces of a chimney vary from a linear distribution for small diameters (fig. 3 a) to a nonlinear distribution for larger diameters (fig. 3 b). fig. 3 shows the load-bearing behavior figure 3. meridional internal forces at the bottom for slim (a) and wide (b) cylindrical shells exposed to wind load. of a slim and a wide cylindrical shell exposed to wind load, both calculated by shell theory. the side view of the meridional internal forces at the bottom of the slim cylinder (fig. 3 a) shows the linear behavior of the stress resultants, as we expect it from linear elastic stress resultant calculation via beam theory, whereas for the wide cylinder (fig. 3 b) the distribution of the meridional forces around the circumference differs significantly. it corresponds to the ovalization of 80 vol. 7/2017 design of concrete chimneys via nonlinear shell analysis figure 4. design via beam theory for a specific wind load. the circular cross section due to the reduced bending stiffness, resulting in tensile stress resultants (uplift) in both the windward and leeward side of the shell, whereas its side flanks are subjected to compressive stress resultants. the procedure of the design according to beam theory [10] is shown in fig. 4: a linear strain distribution is estimated for the calculated outer stress resultants according to the bernoulli hypothesis over the full diameter d of the chimney (fig. 4 a). this strain gradient (fig. 4 b) is varied until the respective sum of stresses in the concrete (fig. 4 c) figure 5. design via linear elastic shell theory for a specific wind load. and the chosen reinforcement (fig. 4 d) result in the same stress resultants as calculated with the beam model for the considered section. so the design according to beam theory includes an implicit nonlinearity when the stress-strain-curves for concrete and steel are applied. therefore the stress distribution for a cross section is nonlinear for outer stress resultants calculated by linear elastic beam theory (fig. 4). the design according to shell theory can be conducted with a linear internal stress resultant distribution over the cross section for chimneys with small diameters d (fig. 5 a). here t defines the wall thick81 franziska wehr, reinhard harte acta polytechnica ctu proceedings ness. the defining points for the design of the reinforcement lie in the area with maximum normal forces (fig. 5 a) as well as in the area with minimal normal forces for the compressive strength of concrete. the decisive tensile force causes a constant strain over the wall thickness t (fig. 5 b). therefore the concrete is cracked over the entire thickness (fig. 5 c) and the load can only be taken by the reinforcement (fig. 5 d). whereas the decisive compressive force can be taken up by concrete and reinforcement (fig. 5 c+d). the differences in the design between beam and shell theory are now demonstrated on the basis of an example chimney. the diameter d is varied while height h, wall thickness t and reinforcement ratio ρ remain constant. three different reinforcement ratios for the vertical reinforcement are considered. the chimney is stressed by dead load g and the wind load w is increased gradually until either the existing reinforcement is not able to carry the load anymore or the compressive strength of the concrete is exceeded. the maximum load factor λmax on wind load for the case of failure is plotted for each reviewed chimney with fed = 1.0 ·g+ λmax ·w = red. (1) the results are presented in fig. 7 (dotted and dashed lines). for both theories in case of a slender chimney the maximal load factor is bigger for smaller h/d ratios. but according to linear elastic shell theory for h/d-ratios smaller than about 15 the behavior changes. the reason for the smaller load bearing capacity of those chimneys is on the one hand the small reinforcement ratio for the hoop direction and on the other hand the shell-like behavior as mentioned before. according to beam theory the maximal wind load increases also for smaller h/d ratios, as well for the small h/d ratios not shown in fig. 7. the great dissimilarities can be explained by the different design concepts for beam and shell theory which are described in fig. 4 and 5. according to beam theory, the load is considered to act globally on the entire circular cross section whereas in shell theory the load is distributed around the circumference before performing the design for each point of the circular cross section separately. this results in a nonlinear stress distribution over the cross section according to beam theory and a linear stress distribution according to shell theory (fig. 8, black and yellow lines). for small h/d ratios the beam theory leads to wrong results, because of the consideration of a linear strain distribution. this causes a to stiff behavior of the cross section. since in reality stresses relocate from high stressed areas to parts with higher load-bearing capacities the solution of the beam theory seems to be more appropriate for small diameters. nevertheless it should be verified by nonlinear shell calculations whether the load distribution really occurs the way it is assumed by beam theory. a) b) figure 6. stress-strain-curves for concrete (a) and reinforcement (b) according to [8]. 4. design concept for concrete structures with nonlinear material behavior it is also possible to perform a cross section design using nonlinearly calculated stresses, or by performing a nonlinear ultimate load analysis [11]. by calculating the stress resultants with nonlinear material behavior internal forces that exceed the limit strength of concrete or reinforcement are eliminated and instead relocated to less stressed areas of the cross section. this can increase the calculated load-bearing capacity of a structure compared to linear elastic shell theory. the eurocode [8] gives some rules for a nonlinear calculation. the material properties shown in fig. 6 have to be used. they differ, especially for concrete, from the material properties for the design of a cross section. the displayed stress-strain-curves are already reduced by the global security factor for a nonlinear analysis in uls. for the nonlinear calculation according to [8] the same load combinations as listed in tab. 1 can be used. as it is not possible to do a superposition of the stress resultants calculated for the separate load cases, the internal forces have to be calculated for the decisive load combinations to include the nonlinear material properties. 82 vol. 7/2017 design of concrete chimneys via nonlinear shell analysis figure 7. load bearing capacity for predefined reinforcement acc. to beam theory, linear and nonlinear shell theory. 5. comparison of design via beam theory and nonlinear shell theory to compare the design results for a chimney obtained from beam theory with a nonlinear shell model the same procedure as described in chapter 3 is applied. the calculation is executed for different diameters while height and thickness remain constant. three different reinforcement ratios are considered. the chimney is at first loaded by dead load. in the next step the wind load is added incrementally until the failure of the chimney occurs at one or several points of the shell. the maximal load factor on wind load λmax is plotted for the failure case for each h/d ratio and results in the continuous lines shown in fig. 7. the figure clearly demonstrates that the calculated load-bearing behavior according to nonlinear shell theory is higher than according to beam theory for h/d ratios larger than 30. for smaller h/d ratios the beam theory allows higher load factors λmax than the nonlinear shell theory and thus will not be on the safe figure 8. comparison of meridional forces acc. to beam and shell design with respect to different ratios h/d. side. to compare the load-bearing behavior of beam and shell theory, fig. 8 shows the vertical stress resultants over the standardized cross section in case of maximum utilization for a vertical reinforcement ratio of 13.40 cm2/m. according to beam theory and linear shell theory the distribution of the normal forces is the same for all h/d ratios for the given geometry, whereas according to nonlinear shell theory the distribution of the internal forces differs subjected to the h/d ratios. the progression of the internal forces lies between the ones calculated by beam theory and linear elastic shell theory. for larger h/d ratios the nonlinear shell curves are similar to the beam solution. the beam theory distributes the stresses in the way that the maximum tensile strength of steel and the maximum compressive strength of concrete are utilized. it can be seen that for smaller h/d ratios the redistribution of the stresses around the circumference is overestimated in beam theory. then the nonlinear shell theory is more appropriate to describe the distribution of the 83 franziska wehr, reinhard harte acta polytechnica ctu proceedings stresses. thus for chimneys with h/d ratios smaller than 30, the calculated load-bearing capacity of the beam theory is not correct and on the unsafe side. 6. outlook the next step will be to find out whether the results for other geometries are comparable to the presented ones. the correctness of the results according to beam theory for the ring reinforcement also needs to be checked. if the results of further studies lead to similar results, the aim is to define a correction factor γcorr on wind loading as a function of slenderness ratio h/d and reinforcement ratio ρ. the factor shall then be applied to produce correct results for concrete chimneys by using beam theory even for non-slender geometries. acknowledgements the paper has presented intermediate results from the research project “validation of design procedure and safety concept of concrete chimneys via non-linear shell analysis”, financed by cicind. the support by cicind is gratefully acknowledged. references [1] p. noakowski, a. harling, m. breddermann, m. rost. rehabilitation of chimneys. cicind chimney book pp. 323–344, 2005. [2] cicind 2001. model code for concrete chimneys – part a: the shell, 2011. [3] en 13084-1:2007. free-standing chimneys – part 1: general requirements, 2007. [4] en 13084-2:2007. free-standing chimneys – part 2: concrete chimneys, 2007. [5] aci 307 2008. code requirements for reinforced concrete chimneys (aci 307-08) and commentary, 2008. [6] r. harte, o. pouran. interaction and intersection in research and development of cooling towers and concrete chimneys. cicind research project rp-2013-4c. bergische universität wuppertal, 2014. [7] en 1990:2002. eurocode – basis of structural design, 2002. [8] en 1992-1-1:2004. eurocode 2: design of concrete structures – part 1-1: general rules and rules for buildings, 2004. [9] f. lupi, h.-j. niemann. international standardization of wind actions on chimneys. cicind research project rp-2011-4. ruhr-universität bochum and niemann + partner bochum, 2013. [10] p. noakowski, m. breddermann, a. harling, m. rost. turmartige industriebauwerke – bemessungsgrundlagen der cicind, din en 13084 und din v 1056 mit beispielen und anwendungen. beton-kalender 2006 turmbauwerke, industriebauten pp. 225–318, 2006. [11] r. harte, w. krätzig, s.-y. noh, y. petryna. on progressive damage phenomena of structures. computational mechanics 25:404–412, 2000. 84 acta polytechnica ctu proceedings 7:79–84, 2017 1 introduction 2 design of concrete chimneys via beam theory 3 comparison of beam theory and linear elastic shell theory 4 design concept for concrete structures with nonlinear material behavior 5 comparison of design via beam theory and nonlinear shell theory 6 outlook acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2017.13.0001 acta polytechnica ctu proceedings 13:1–10, 2017 © czech technical university in prague, 2017 available online at http://ojs.cvut.cz/ojs/index.php/app assessment of 2d-dic stochastic patterns jakub antoš∗, václav nežerka, michael somr czech technical university in prague, faculty of civil engineering, thákurova 7, 166 29 prague 6, czech republic ∗ corresponding author: jakub.antos@fsv.cvut.cz abstract. cost of experimental testing of materials and structures subjected to mechanical loading often constitutes a significant portion of a project budget. therefore a collection of data in a maximum possible amount is desirable. extensometers and strain-gauges attached to the specimen surface often fail and cannot provide full-field information about the development of displacements and strains. digital image correlation (dic) is capable of providing such information. unsuitable texture or artificially applied pattern, essential for dic analysis, can spoil the dic outcomes completely. an additional investment into preparation of new experimental testing can be avoided with the use of tools employing algorithms for stochastic pattern assessment. the development of such algorithms and their implementation into an open-source dic software is the goal of the presented research. keywords: digital image correlation, stochastic pattern, full-field displacement. 1. introduction the measurement of strains and displacements is utilized when investigating behavior of materials or specimens in laboratories, but also for assessment of existing structures. traditionally, contact measurement techniques have been used, but these introduce challenges in terms of accuracy, require sufficiently flat surface, and their attachment requires a lot of effort. the group of contact methods includes strain gauges, extensometers, or linear variable differential transformers (lvdts) [1]. contact methods techniques have a few drawbacks compared to optical full field monitoring: the strain must be within a certain range and is averaged over the strain-gauge length, the measurement is accomplished at discrete locations, and the investigated surface must be smooth enough to attach the gauge. quite recently alternatives to non-contact measurement have been developed. these include e.g., laser vibrometry, laser range finders, optical correlation methods, interferometry, moiré, or photo/videogrammetry [2, 3]. optical measurements, such as digital image correlation (dic), are capable of capturing even the most complex deformation until the ultimate material failure [3, 4]. because of that reason, optical methods became accepted in the field of experimental solid mechanics. the availability of compact and sensitive light sources, detectors, optical components, and powerful computers have been promoting the use of dic in both commercial and academic research. the full-field data can be easily processed and provides data valuable for engineers who are used to employing finite element programs in their calculations. 2. digital image correlation dic became indispensable to monitoring and analyzing a development of displacement or strain fields over time by matching stochastic pattern of deformed images with respect to a reference one [5]. the method is capable of capturing strain localization on a surface of a loaded structure (such as plasticity or cracking) in any direction. as dic tools for 3d analysis emerge [6], the method is becoming more versatile and widely used in the industry and research. dic is not only limited to the relative measurement of discrete points as conventional contact methods and is frequently more accurate than monitoring by means of extensometers or strain-gauges that often suffer from imperfect attachment to the measured surface. moreover, the use of dic can significantly reduce the cost of the experiment. dic reached its maturity during last decade owing it to the fast and successful development and elaboration of computers, digital cameras and specialized software for image processing. dic has even been characterized as “a major development in the art of experimental mechanics” [7]. the boom of dic has been reflected in the emergence of new commercial and open source software to perform dic with differently optimized correlation functions and calculation methods, most recently summarized by pan et al. [8]. 2.1. dic algorithms dic utilizes a correlation algorithm to obtain displacements and subset deformations by identifying areas of matching grey-scale values between the speckle pattern in each subset of deformed and underformed (reference) images (figure 1). to facilitate the correlation, a stochastic speckle pattern must be applied (if not present naturally) to a specimen surface in order to provide a random grey-level variations at 1 http://dx.doi.org/10.14311/app.2017.13.0001 http://ojs.cvut.cz/ojs/index.php/app j. antoš, v. nežerka, m. somr acta polytechnica ctu proceedings the sufficient quality of which is fundamental to the precision of the measured displacement data. it is well established that the trade-off between the data resolution and precision is affected by the quality of a pattern. to perform dic, each image must be divided into a grid of interrogation cells, or subsets, containing a finite number of pixels. resolution of the data is maximized by reducing the size of the subsets, but as the interrogation cell size decreases, the uncertainty in the strain measurement increases due to a reduction in the number of features to track within the subset [9]. therefore, the resolution and accuracy of the displacement and deformation fields are limited by the total number of pixels within the images. the center position of each target subset is obtained through searching the peak position of the correlation coefficient field and its deformation is calculated using an iterative approach [10, 11]. dic has traditionally utilized subset-based local approach [5] that has been very well developed and widely used in commercial or academic dic packages. however, there are certain drawbacks that make the method useless for some applications, such as experiments producing high strain gradients [12]. the reason is non-continuity of the displacement field, as the displacements of the subsets are sought separately. therefore smoothing of the noisy displacement field is required [8, 9, 11]. a local polynomial smoothing over a rectangular area (strain window) is usually employed [8]. the amount of smoothing is controlled by the subset size, step size, and strain window size. the results are then highly dependent on setting these parameters by a user [13]. an alternative global approach has been developed quite recently based on the procedures used in the finite element method (fem) [14, 15]. using the global dic approach, a complete element mesh is tracked on images, ensuring c0-continuity1, resulting in elimination of noise in strain fields. this method was initially proposed by cheng et al. [16] and later besnard et al. developed the q4-dic [17] with implementation of a bilinear (q4) or quadratic (q8) mesh. the global algorithms may be refined using user experience, or using adaptive mesh refinement [18]. 2.2. dic accuracy the accuracy of dic is an issue frequently discussed by both, theoreticians and experimentalists [19, 20]. although originally restricted to the size of one-pixel, various sub-pixel techniques have been proposed and used to improve the method accuracy. the most popular techniques include curve-fitting, gradient-based and newton-raphson algorithms. the last two cases are based on the identification of the parameters that define the mapping of a subset of pixels in order to 1ci differentiability class corresponds to the existence of i derivatives of a function. the class c0 consists of all continuous functions. figure 1. example of the subset tracking procedure using dic. maximize the image correlation. the results show that the newton-raphson approach is considered as the most accurate and stable. other interesting approaches such as genetic algorithms, finite element interpolations and b-splines are reported in [8] but they generally appear to have a lower performance. bing et al. [21] compared performance of three most used sub-pixel techniques by simulating image distortions, demonstrating the suitability of newton-rapson approach. besides the choice of the most suitable dic algorithm, other factors substantially influence the dic accuracy. the measurement errors may be classified into two categories: (i) systematic experimental errors such as bad calibration and setting of image acquisition system and (ii) errors produced by correlation algorithm. the experimental errors are related to variations in illumination and the quality of the acquisition system, i.e., the noise during the acquisition and digitalization, image distortion, or position of the camera with respect to the monitored surface. the errors related to the correlation algorithms arise from inappropriate selection of correlation methods or input parameters, such as subset size, correlation function, sub-pixel algorithm, shape function, or interpolation scheme. 3. stochastic pattern assessment a majority of research into the accuracy of dic is focused on correlation algorithms and processing parameters (recall section 2.2), such as subset size [22], shape function selection [23], or methods of obtaining sub-pixel accuracy [24]. less attention has been paid to the effect of the quality of the speckle pattern. it is important for the artificial pattern or natural texture to be adjusted in accordance with the expected displacement field in order to maximize measurement 2 vol. 13/2017 assessment of 2d-dic stochastic patterns accuracy, as speckles can be both too large and too small for the particular measurement [25]. quantitative error analysis [26–29] shows that the measurement error depends crucially on the presence of large intensity gradients within the pattern. therefore, the technique in most cases requires a specimen preparation procedure to enhance the surface texture, since the natural one does not provide a required quality in terms of intensity gradients and speckle-size distribution. pattern assessment has been discussed by a few authors [5, 30, 31], offering a number of methods to evaluate the quality of applied patterns. pan et al. [32] presented a number of assessment criteria based upon the local subset intensity gradients [20] and the global mean intensity gradient (mig) throughout the image. the latter criteria showed a good agreement to results obtained numerically. the benefit of applying the global criteria is that these are astraightforward. high mig values typically result in smaller bias and less dispersion in dic measurements. on the other hand, crammonde et al. [33] objected that global parameters, such as mig, are not sufficient to evaluate strain accuracy. based on their findings, simple means of a randomness assessment is to employ the outcomes of the shannon entropy theory [34]. cintrón et al. [35] demonstrated that the speckle size should range from 2×4 px for a high accuracy in the strain measurement. the speckle size should have a lower limit according to the study by sutton et al. [5] of 3×3 px. in the presented study, various patterns produced by the spraying white / black dots on a contrast background were investigated. in addition, natural textures of various materials were studied as well, to demonstrate their performance. the knowledge of a texture suitability is essential when monitoring existing structures where application of artificial patterns is not possible. to eliminate the impact caused by image acquisition system (such as the distortion of the camera lens, lighting variation, etc.) during the experiment, numerical approach was utilized to investigate the relationship between shannon entropy, mean intensity gradient, values of correlation coefficient, and error in dic measurements. the explanation of the mentioned parameters is provided next. 3.1. normalized cross-correlation correlation between two signals (also reffered to as cross-correlation) is a standard approach to feature detection and image registration [36–38]). the normalized form of correlation takes into account intensities relative to the highest one, thus eliminating the sensitivity to changes in illumination intensity. however, it does not have a correspondingly simple and efficient frequency domain expression. for this reason, normalized cross-correlation must be computed in a spatial domain and fast spatial domain matching methods had to be developed. detailed description of cross-correlation equation is beyond the scope of this article. reader can find it in barnea et al. [39]. 3.2. mean intensity gradient (mig) mig was proposed by pan et al. [32] as a global parameter for evaluating the quality of a speckle pattern over the entire domain. both, a mean bias error and standard deviation of measured displacements are influenced by the mig of the speckle pattern. the speckle pattern with a large mig is supposed to produce small displacement measurement errors. mig is defined as δf = w∑ i=1 h∑ j=1 |of (xij)| 1 w ×h , (1) where w and h are the image width and height in pixels, |of (xij)| = √ fi (xij)2 + fj (xij)2 is the modulus of the local intensity gradient. fi (xij) and fj (xij) are the iand j-directional intensity derivatives at pixel (xij) respectively, which can be computed using a central difference algorithm. 3.3. shannon entropy a high shannon entropy value indicates a high level of texturing, or broadness in the greyscale distribution of the image, beneficial for maximizing the correlation function peak when a correct match has been found. the shannon entropy parameter is defined as [40] ψ = w∑ i=1 h∑ i=j f(xij)log (f(xij)) . (2) 4. assessment of patterns and textures randomly distributed speckles must not exhibit any preferential directions. high contrasts are required to provide sharp peaks in the correlation function. to test this requirement, initial synthetic tests on samples of 400×400 px were performed on six model patterns (figure 2). the patterns / textures were tested with respect to four quality indicators: value of cross-correlation coefficient for two different subset sizes (10×10 px and 30×30 px), shannon entropy, and mig. the summary of the investigated patterns and textures is provided in table 1. based on the values of normalized cross-correlation functions presented in the form of surface plots in figures 3 and 4, it can be concluded that the correlation function peaks can be highlighted by setting bigger subset size since these contain more distinct features. especially in the case of small subset size, 10×10 px, the correlation peaks on coarse textures could not be clearly identified. this could lead to huge errors of dic measurements in the case of significantly distorted images. however, the findings cannot be generalized, since in some patterns the cross-correlation correlation function does not exhibit any distinct peak 3 j. antoš, v. nežerka, m. somr acta polytechnica ctu proceedings (a) . fine spray pattern (b) . coarse air-brush pattern (c) . white sprayed dots (d) . natural texture of concrete (e) . masonry wall (f) . texture of wood figure 2. investigated artificial patterns (top) and natural textures (bottom). in for small subsets, while correlation of the larger one results in unique match and low value of mean correlation coefficient (cf. figures 3(b,f), 4(b,f), and table 1). therefore, the pattern assessment based on the the mean value of the cross-correlation function must be carried out for any subset size independently. moreover, a statistical information about number of peaks in the form of a decay in the power law distribution would provide more accurate description. based on observation of distribution of correlation coefficient a power law distribution can be assumed (figure 5, 6). the power law function is defined by equation yp = x−α p , where α is the decay parameter which provides information about the distribution of correlation coefficient over the investigated domain. larger α indicates smaller amount of unwanted correlation peaks which is desirable. it is also clear that neither shannon entropy nor mig are capable of providing clear indication of the pattern quality since these two are not in agreement — once the pattern is, relative to other ones, regarded as superior to others based on shannon entropy, mig provides completely different results, see table 1. therefore, new more universal criteria taking into account size, contrast, and size-distribution of speckles should be proposed. 5. virtual stretching of patterns and textures the artificial patterns and natural textures were tested in the proof-of-concept experiments carried out virtually on a computer by prescribing a constant deformation. special attention was paid to the ability of the pattern to provide high-accuracy measurements at deformations reaching 5%, at which poor patterns fail [41]. the evaluation and testing of patterns was performed using own matlab scripts by placing virtual extensometers at the edges of the tested samples. in this initial study the images containing 8-bit speckle patterns were virtually stretched up to 5% in 10 steps. image sequences were then evaluated in open-source 2d-dic software package ncorr [10] with the same setting of parameters. subset sizes of 10 px and 30 px were chosen to demonstrate the correlation between the mean correlation coefficient mentioned in the section 4 and measurement errors. as the number of speckles within the patterns increased, the measurement errors decreased. moreover, larger sized speckles provided lower error than patterns with smaller speckles. global pattern quality parameters were discussed and shannon entropy was used as an example to demonstrate that the global measures are not sufficient to assess the quality and 4 vol. 13/2017 assessment of 2d-dic stochastic patterns (a) . fine spray pattern (b) . coarse air-brush pattern (c) . white sprayed dots (d) . natural texture of concrete (e) . masonry wall (f) . texture of wood figure 3. correlation function values for 10×10 px subset size. 5 j. antoš, v. nežerka, m. somr acta polytechnica ctu proceedings (a) . fine spray pattern (b) . coarse air-brush pattern (c) . white sprayed dots (d) . natural texture of concrete (e) . masonry wall (f) . texture of wood figure 4. correlation function values for 30×30 px subset size. 6 vol. 13/2017 assessment of 2d-dic stochastic patterns γ(xij) α ψ δf sample 10×10 30×30 10×10 30×30 fine spray pattern (a) 0.152 0.054 0.847 0.959 5.524 121.182 coarse air-brush pattern (b) 0.190 0.107 1.009 1.046 5.413 127.614 white spray dots (c) 0.089 0.138 1.148 0.997 4.972 82.198 natural texture of concrete (d) 0.090 0.039 1.027 0.975 6.292 75.849 masonry wall (e) 0.200 0.187 0.879 0.939 4.250 50.914 texture of wood (f) 0.208 0.097 0.878 0.981 5.012 29.027 table 1. mean correlation coefficient, γ(xij), power law distribution decay parameter α, ψ, and δf for tested patterns / textures and different size of subsets. 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 correlation coefficient (in absolute value) [-] 0 0.5 1 1.5 2 2.5 3 f re q u e n c y [ -] ×104 (a) . subset 10×10 px (α = 0.847) 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 correlation coefficient (in absolute value) [-] 0 1 2 3 4 5 6 7 8 f re q u e n c y [ -] ×104 (b) . subset 30×30 px (α = 0.959) figure 5. distribution of correlation coefficient, fine spray pattern. 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 correlation coefficient (in absolute value) [-] 0 0.5 1 1.5 2 2.5 3 f re q u e n c y [ -] ×104 (a) . subset 10×10 px (α = 0.879) 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 correlation coefficient (in absolute value) [-] 0 0.5 1 1.5 2 2.5 3 3.5 f re q u e n c y [ -] ×104 (b) . subset 30×30 px (α = 0.939) figure 6. distribution of correlation coefficient, masonry wall. 7 j. antoš, v. nežerka, m. somr acta polytechnica ctu proceedings (a) (b) (c) (d) (e) (f) pattern / texture 0 0.5 1 1.5 2 2.5 3 3.5 d is p la c e m e n t e rr o r [% ] (a) . 1% deformation (a) (b) (c) (d) (e) (f) pattern / texture 0 0.2 0.4 0.6 0.8 1 1.2 d is p la c e m e n t e rr o r [% ] (b) . 5% deformation figure 7. displacement error for 10×10 px subset size. (a) (b) (c) (d) (e) (f) pattern / texture 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 d is p la c e m e n t e rr o r [% ] (a) . 1% deformation (a) (b) (c) (d) (e) (f) pattern / texture 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 d is p la c e m e n t e rr o r [% ] (b) . 5% deformation figure 8. displacement error for 30×30 px subset size. properties of the patterns. this is illustrated by comparing errors at different stretching magnitude for different speckle sizes (figure 7 and 8). 6. conclusions based on the research of crammond et al. 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[41] g. stoilov, v. kavardzhikov, d. pashkouleva. a comparative study of random patterns for digital image correlation. journal of theoretical and applied mechanics 42:55–66, 2012. 10 http://dx.doi.org/10.1016/0004-3702(73)90004-0 http://dx.doi.org/10.1117/1.3115362 http://dx.doi.org/10.1109/34.254061 http://dx.doi.org/10.1109/tc.1972.5008923 acta polytechnica ctu proceedings 13:1–10, 2017 1 introduction 2 digital image correlation 2.1 dic algorithms 2.2 dic accuracy 3 stochastic pattern assessment 3.1 normalized cross-correlation 3.2 mean intensity gradient (mig) 3.3 shannon entropy 4 assessment of patterns and textures 5 virtual stretching of patterns and textures 6 conclusions acknowledgements references 286 acta polytechnica ctu proceedings 2(1): 286–290, 2015 286 doi: 10.14311/app.2015.02.0286 rotation of the mass donors in high-mass x-ray binaries and symbiotic stars k. stoyanov1, r. zamanov1 1institute of astronomy and national astronomical observatory, bulgarian academy of sciences, 72 tsarigradsko shousse blvd., 1784 sofia, bulgaria corresponding author: kstoyanov@astro.bas.bg abstract our aim is to investigate the tidal interaction in high-mass x-ray binaries and symbiotic stars in order to determine in which objects the rotation of the mass donors is synchronized or pseudosynchronized with the orbital motion of the compact companion. we find that the be/x-ray binaries are not synchronized and the orbital periods of the systems are greater than the rotational periods of the mass donors. the giant and supergiant high-mass x-ray binaries and symbiotic stars are close to synchronization. we compare the rotation of mass donors in symbiotics with the projected rotational velocities of field giants and find that the m giants in s-type symbiotics rotate on average 1.5 times faster than the field m giants. we find that the projected rotational velocity of the red giant in symbiotic star mwc 560 is v sin i= 8.2± 1.5 km s−1, and estimate its rotational period to be prot = 144 306 days. using the theoretical predictions of tidal interaction and pseudosynchronization, we estimate the orbital eccentricity e = 0.68 − 0.82. keywords: stars: binaries: symbiotic stars: rotation stars: late type stars: binaries: close x-rays: binaries. 1 introduction a high-mass x-ray binary system consists of a compact object (a neutron star or a black hole) accreting material from an o or b companion star. they are divided into be/x-ray binaries (main-sequence star as a companion) and giant/supergiant x-ray binaries (giant or supergiant star as a companion). accretion of matter is different for both types of x-ray binaries. in the be/x-ray binaries, the compact object crosses the circumstellar disc and accretes matter from that disk. in the giant/supergiant x-ray binaries, the mass donor ejects a slow and dense wind radially outflowing from the equator and the compact object directly accretes the stellar wind through bondi-hoyle-lyttleton accretion. symbiotic stars are interacting binaries, consisting of an evolved giant (either a normal red giant in s-types symbiotics or a mira-type variable in d-types symbiotics) transferring mass to a hot and luminous white dwarf or neutron star. the symbiotic stars are surrounded by a rich and luminous nebula resulting from the presence of both an evolved giant with a heavy mass-loss and of a hot companion abundant in ionizing photons and often emanating its own wind. 2 synchronization and pseudosynchronization in a binary with a circular orbit the rotational period of the primary, prot, reaches an equilibrium value at the orbital period, porb = prot. in other words the synchronous rotation (synchronization) means that the rotational period is equal to the orbital period. in a binary with an eccentric orbit, the corresponding equilibrium is reached at a value of prot which is less than porb, the amount less being a function solely of the orbital eccentricity e. in practice, in a binary with an eccentric orbit the tidal force acts to synchronize the rotation of the mass donor with the motion of the compact object at the periastron pseudosynchronous rotation. to calculate the period of pseudosynchronization, pps, we use (hut 1981): pps = (1 + 3e2 + 3 8e 4)(1 − e2) 3 2 1 + 15 2 e 2 + 45 8 e 4 + 5 16e 6 porb. (1) 2.1 stars with radiative envelopes following hurley, tout & pols (2002) the circularization timescale for stars with radiative envelopes is: 1 τcirc = 21 2 ( gm1 r3 1 ) 1 2 q2 (1 + q2) 11 6 e2 ( r1 a ) 21 2 , (2) 286 http://dx.doi.org/10.14311/app.2015.02.0286 rotation of the mass donors in high-mass x-ray binaries and symbiotic stars where m1 and r1 are the mass and the radius of the primary respectively, q2 is the mass ratio m2/m1, and a is the semi-major axis. the second-order tidal coefficient e2 = 1.592 ×10−9m2.84 1 . the synchronization time scale is given as, τsync = kτcirc, (3) where k is: k ≈ 0.015 rg 1 + q2 q2 ( r1 a )2 . (4) for the gyration radius of the primary rg we adopt rg ≈ 0.16 for giants, and rg ≈ 0.25 for main sequence stars (claret & gimenez, 1989). 2.2 stars with convective envelopes following hurley, tout & pols (2002) the synchronization timescale for stars with convective envelopes is: τsyn ≈ 800 ( m1r1 l1 )1/3 m2 1 (m1 m2 + 1)2 r6 1 p 4 orb yr, (5) where l1 is the luminosity of the giant. the circularization time scale is: 1 τcirc = 21 2 ( k t ) q2(1 + q2) ( r1 a )8 . (6) in eq. 6, (k/t ) is:( k t ) = 2 21 fconv τconv menv m1 yr−1, (7) where renv is the depth of the convective envelope, menv is the envelope’s mass, and τconv = 0.4311 ( menvrenv(r1 − 1 2renv) 3l1 ) 1 3 yr (8) is the eddy turnover time scale (the time scale on which the largest convective cells turnover). the numerical factor fconv is fconv = min [ 1, ( ptid 2τconv )2 ] , (9) where ptid is the tidal pumping time scale given by 1 ptid = ∣∣∣∣ 1 porb − 1 prot ∣∣∣∣ . (10) the pseudosynchronization timescale is τps = (7/3(α 3)) τcirc, where α is a dimensionless quantity, representing the ratio of the orbital and rotational angular momentum: α = q2 1 + q2 1 r2g ( a r1 )2 . (11) for a red giant we adopt rg ≈ 0.3 (claret, 2007). in all equations, the masses, the radii and the lumunosities are in solar units. 3 high-mass x-ray binaries the orbital and stellar parameters of 13 high-mass xray binaries are given in table 1 and table 2 in stoyanov & zamanov (2009). we add 2 more objects 4u 2206+54 and mwc 148. the orbital and stellar parameters are taken from ribó et. al. (2006) and casares et al. (2012) respectively. using eq.2 and eq.3 we estimate the circularization and synchronization timescales. the results are given in table 1. the lifetime of a star on the main sequence can be estimated as τms = 1010(m�/m)2.5 years (hansen & kawaler, 1994). comparing these lifetimes with τsync from table 1, we see that among the be/x-ray binaries only for lsi+610303 is τsync ∼ τms . this is the only be/x-ray binary for which we can expect considerable changes of the rotation of the primary during the lifetime of the be star. the lifetime of the giant is comparable or longer then τcirc and τsync for the giant/supergiant systems with short orbital periods. the exceptions are v725 tau and bp cru, for which τsync and τcirc are longer than the lifetime of the giant/supergiant stage. on fig.1 in stoyanov & zamanov (2009) is plotted prot versus pps. the giant/supergiant systems are located close to the line pps = prot, while those with mass donors from spectral class v are far away from the equilibrium. in the be/x-ray systems bq cam, v635 cas, v725 tau and 4u 2206+54, the tidal force spinning down the donor star. for the system lsi+610303, the rotation of the mass donor is close to pseudosynchronization. this is the only be/x-ray binary in which τsync is comparable with the life-time of the binary. in the binaries x per and mwc 148, the neutron star is far away from the be star and the tidal force is weak. giant and supergiant systems are close to (pseudo)synchronization. in these binaries the rotation of the mass donors is influenced by the presence of the compact object. in lmc x-4 and cen x-3, the mass donors are synchronized and the orbits are circularized. with respect to the rotation of the mass donor, v725 tau is similar to the be/x-ray binaries. cyg x-1 is synchronized and almost circularized. v830 cen is pseudosynchronized but not circularized yet. the systems lsi+650010 and vela x-1 are close to pseudosynchronization and the tidal force accelerates the rotation of the mass donors. in the case of smc x-1, the tidal force acts as a decelerator of the rotation of the mass donor. in bp cru, a gas stream from the mass donor 287 k. stoyanov, r. zamanov exists, probably resulting from the strong tidal force and spin-up of the mass donor (leahy & kostka, 2008). table 1: calculated time scales. given here are the name of the object, synchronization time scale, circularization time scale, and the lifetime. object τsync [yr] τcirc [yr] lifetime [yr] be/x-ray binaries lsi+610303* 2.8×107 2.4×108 5.6×107 x per 6.2×1017 1.8×1021 1.1×107 bq cam 3.5×1011 7.6×1013 3.9×106 v635 cas 1.4×1011 9.5×1012 7.3×106 4u 2206+54 4.9×109 3.7×1011 7.3×107 mwc 148 1.2×1017 5.1×1020 9.8×107 giant systems v725 tau 2.8×1012 8.0×1014 4×105 lmc x-4 4.5×102 7.7×102 1×106 cen x-3 2.3×103 4.2×103 5×105 supergiant systems v830 cen 7.5×103 1.4×104 1×106 lsi+650010 1.3×104 3.9×104 1×106 vela x-1 1.0×104 2.8×104 3.9×105 smc x-1 3.3×104 8.2×104 8.8×105 bp cru 1.8×106 8.8×106 8×104 cyg x-1 < 1 < 1 1×105 *assuming neutron star as a secondary component. 4 s-type symbiotic stars 43 symbiotic stars have been observed with feros spectrograph at the 2.2m eso telescope of the la silla observatory (zamanov et al. 2007). the data for the rotation of 55 field red giants are taken from the literature. m giants in s-type symbiotics rotate faster than the field m giants. histograms of the available v sin i data for the red giants are plotted in fig.2 in zamanov & stoyanov (2012). for the field m0iii-m6iii giants we calculate a mean v sin i=5.0 km s−1, median v sin i=4.3 km s−1, and standard deviation of the mean σ = 4.0 km s−1. for the m0iii-m6iii giants in symbiotics, we get a mean v sin i= 7.8 km s−1, median v sin i=8.0 km s−1, and standard deviation of the mean σ =2.1 km s−1. there are 5 objects in our sample that deviate from the synchronization. these objects are rs oph, mwc 560, ch cyg, cd-43◦14304 and z and. in three of them collimated jets are detected: z and (skopal et al. 2009); ch cyg (crocker et al. 2002), mwc 560 (tomov et al. 1990). additionally to the jets, ejection of blobs are detected from rs oph and ch cyg (iijima et al. 1994). this confirms the suggestions that in the jet-ejecting symbiotics the mass donors rotate faster than the orbital periods. probably there is a link between the jets and the mass donor rotation. on fig.1 are plotted together the high-mass x-ray binaries and the s-type symbiotic stars. it shows that none of the objects in our sample is above the line of synchronization. figure 1: prot versus porb on a logarithmic scale. the blue circles indicates the high-mass x-ray binaries filled symbols for giant/supergiant systems and open symbols for be/x-ray binaries. the red triangles indicates the s-type symbiotic stars. 5 orbital eccentricity of mwc 560 mwc 560 is a symbiotic star, which consists of a red giant and a white dwarf (tomov et al. 1990). the most spectacular features of this object are the collimated ejections of matter with velocities of up to ∼ 6000 km s−1 (tomov et al. 1992) and the resemblance of its emission line spectrum to that of the low-redshift quasars (zamanov & marziani, 2002). the jet ejections are along the line of sight and the system is seen almost pole-on (i < 16◦). this makes it difficult to obtain the orbital eccentricity of the system in a conventional way. for the system we adopt rg = 140 ± 7 r�, lg ∼ 2400 l�, mg = 1.7 m�, mwd = 0.65 m�, porb = 1931± 162 day (gromadzki et al. 2007), renv = 0.9 rg 288 rotation of the mass donors in high-mass x-ray binaries and symbiotic stars and menv = 1.0 m� (herwig 2005). with the above values of the parameters assumed, we derive the semimajor axis of the orbit to be a ≈ 860 r�. using these parameters, we calculate from eq. 5 and eq. 6 the synchronization and circularization time scales: τsync = 2.6 × 104 yr and τcirc = 3.1 × 106 yr. the typical lifetime of a symbiotic star is τss ∼ 105 yr (yungelson et al., 1995). from the rate of accretion on the white dwarf, ṁacc ≈ 5× 10−7 m� (schmid et al. 2001), we can estimate, that it will take 106 yr to accrete ∼ 0.5 m� from the envelope of the red giant companion. because the giant also losses mass via stellar wind, we find that the lifetime of the symbiotic phase of mwc 560 should be τss ≤ 106 yr. for mwc 560 we have therefore the situation in which τps < τsyn < τss < τcirc. this means that the symbiotic phase is long enough that the tidal forces can (pseudo)synchronize the rotation of the red giant. on the other hand, the value of τcirc demonstrates that the symbiotic lifetime of mwc 560 is shorter than the circularization time, and therefore the orbit can be eccentric. this is in agreement with the observational evidences found by fekel et al. (2007) that the symbiotic stars with porb > 800 days tend to have eccentric orbits. the above implies that in mwc 560, the red giant is probably synchronized, but the orbit is not circularized. to determine the orbital eccentricity of mwc 560, we need to calculate prot for the mass donor. we analyzed 21 high resolution spectra of mwc 560 and obtained value for v sin i= 8.2 ± 1.5 km s−1. using rg = 140 ± 7 r� and i = 12◦ − 16◦, we calculate prot = 144 − 306 days. this value is less than the orbital period. mwc 560 should be close to synchronization or pseudosynchronization, and prot = pps. using eq. 1 we can estimate the orbital eccentricity to be e = 0.68 − 0.82. 6 conclusions using rotational velocity measurements and the theory of synchronization/pseudosynchronization we: (1) find that the be/x-ray binaries are far away from (pseudo)synchronization. the tidal force in the be/x-ray binaries acts as a decelerator of the rotation of the mass donors. the only be/x-ray binary which is close to pseudosynchronization is the lsi+610303. the objects containing mass donors of spectral class i and iii typically have prot ∼ pps and are close to (pseudo)synchronization; (2) demonstrate that the m giants in symbiotic stars rotate faster than the field giants. most symbiotics with orbital period less than 1000 d are synchronized; (3) show that the high-mass x-ray binaries and the s-type symbiotic stars are either on the line of synchronization or they are under the line. none of the objects in our sample is above the line of synchronization. (4) calculate that the orbit of the symbiotic star mwc 560 should be highly eccentric, with e ∼ 0.7. acknowledgement we thank the anonymous referee for constructive comments. this work was supported by the op “hrd“, esf and bulgarian ministry of education, youth and science under the contract bg051po001-3.3.06-0047. references [1] casares, j., ribó, m., ribas, i., paredes, j. m., vilardell, f., & negueruela, i.: 2012, mnras, 421, 1103 doi:10.1111/j.1365-2966.2011.20368.x [2] claret, a.: 2007, a&a 467, 1389 [3] claret, a., & gimenez, a.: 1989, a&as, 81, 37 [4] crocker, m. m., davis, r. j., spencer, r. e., et al.: 2002, mnras, 335, 1100 doi:10.1046/j.1365-8711.2002.05705.x [5] fekel, f. c., hinkle, k. h., joyce, r. r., wood, p. r., lebzelter, t.: 2007, aj 133, 17 [6] gromadzki, m., miko lajewska, j., whitelock, p. a., marang, f.: 2007, a&a 463, 703 [7] hansen, c. j., & kawaler, s. d.: 1994, stellar interiors. physical principles, structure, and evolution, springer-verlag [8] herwig, f.: 2005, ara&a 43, 435 doi:10.1146/annurev.astro.43.072103.150600 [9] hurley, j. r., tout, c. a., & pols, o. r.: 2002, mnras, 329, 897 doi:10.1046/j.1365-8711.2002.05038.x [10] hut, p.: 1981, a&a, 99, 126 [11] iijima, t., strafella, f., sabbadin, f., & bianchini, a.: 1994, a&a, 283, 919 [12] leahy, d. a., & kostka, m.: 2008, mnras, 384, 747 doi:10.1111/j.1365-2966.2007.12754.x [13] ribó, m., negueruela, i., blay, p., torrejón, j. m., & reig, p.: 2006, a&a, 449, 687 [14] schmid, h. m., kaufer, a., camenzind, m., rivinius, t., stahl, o., szeifert, t., tubbesing, s., wolf, b.: 2001, a&a 377, 206 [15] skopal, a., pribulla, t., budaj, j., et al.: 2009, apj, 690, 1222 doi:10.1088/0004-637x/690/2/1222 [16] stoyanov, k. a., & zamanov, r. k.: 2009, astronomische nachrichten, 330, 727 289 http://dx.doi.org/10.1111/j.1365-2966.2011.20368.x http://dx.doi.org/10.1046/j.1365-8711.2002.05705.x http://dx.doi.org/10.1146/annurev.astro.43.072103.150600 http://dx.doi.org/10.1046/j.1365-8711.2002.05038.x http://dx.doi.org/10.1111/j.1365-2966.2007.12754.x http://dx.doi.org/10.1088/0004-637x/690/2/1222 k. stoyanov, r. zamanov [17] tomov, t., zamanov, r., kolev, d., georgiev, l., antov, a., mikolajewski, m., esipov, v.: 1992, mnras 258, 23 doi:10.1093/mnras/258.1.23 [18] tomov, t., kolev, d., zamanov, r., georgiev, l., antov, a.: 1990, nature 346, 637 doi:10.1038/346637a0 [19] yungelson, l., livio, m., tutukov, a., & kenyon, s. j.: 1995, apj 447, 656 doi:10.1086/175908 [20] zamanov, r. k., bode, m. f., melo, c. h. f., et al.: 2007, mnras, 380, 1053 doi:10.1111/j.1365-2966.2007.12150.x [21] zamanov, r., marziani, p.: 2002, apjl 571, l77 doi:10.1086/341367 [22] zamanov, r. k., & stoyanov, k. a.: 2012, bulgarian astronomical journal, 18, 41 290 http://dx.doi.org/10.1093/mnras/258.1.23 http://dx.doi.org/10.1038/346637a0 http://dx.doi.org/10.1086/175908 http://dx.doi.org/10.1111/j.1365-2966.2007.12150.x http://dx.doi.org/10.1086/341367 introduction synchronization and pseudosynchronization stars with radiative envelopes stars with convective envelopes high-mass x-ray binaries s-type symbiotic stars orbital eccentricity of mwc 560 conclusions acta polytechnica ctu proceedings doi:10.14311/app.2017.7.0033 acta polytechnica ctu proceedings 7:33–37, 2017 © czech technical university in prague, 2017 available online at http://ojs.cvut.cz/ojs/index.php/app 1d hermite elements for c1-continuous solutions in second gradient elasticity christian liebold∗, wolfgang h. müller berlin university of technology, chair of continuum mechanics and materials theory, einsteinufer 5, 10587 berlin, germany ∗ corresponding author: christian.liebold@tu-berlin.de abstract. we present a modified strain gradient theory of elasticity for linear isotropic materials in order to account for the so-called size effect. additional material length scale parameters are introduced and the problem of static beam bending is analyzed. a numerical solution is derived by means of a finite element approach. a global c1-continuous displacement field is applied in finite element solutions because the higher-order strain energy density additionally depends on second gradients of displacements. so-called hermite finite elements are used that allow for merging gradients between elements. the element stiffness matrix as well as the global stiffness matrix of the problem is developed. convergence, c1-continuity and the size effect in the numerical solution is shown. experiments on bending stiffnesses of different sized micro beams made of the polymer su-8 are performed by using an atomic force microscope and the results are compared to the numerical solution. keywords: second gradient continuum, size effect, hermite finite elements. 1. introduction materials with intrinsic microor nano-structure can show size-dependent material behavior, which is reflected, e.g., by a stiffer elastic response to external forces when the size of the material body is reduced. a quantitative understanding of a possible size effect in microand submicrostructures is of great importance when modeling microand nano-electro-mechanical systems (mems/nems). driven by miniaturization, the need for saving materials and improving the performance, the requirement of reliability in simulation technologies increases. a size effect is reflected in a different elastic response to external loads if the size of the material body is reduced. experimental validation therefore is given in, e.g., [1–6]. second gradient (sg) continuum theories are used if, in the absence of strain gradients (in uniaxial tensile tests), the elastic behavior of the material is independent of the thickness of the sample [3]. they are applicable to so-called non-simple materials of the gradient type, for example, polymers at a small scale. since conventional continuum theories based on the cauchy continuum are not able to predict size effects, the present work deals with the modified strain gradient theory (msg) developed in, e.g., [7–9]. by employing the macroscopic rotation vector and second order derivatives of the displacement vector, msg-theory is able to account for quantities like rotation and curvature. a generalization of strain gradient continua has been developed by eringen, who proposed “nonsimple materials of gradient type [10]” in order to derive the corresponding higher-order material dependencies in a rational manner. the application of conventional finite element (fe) strategies may lead to inaccurate results, if finite element formulations are used which only fullfill global c0–continuity. the scope of this work is to use fe formulations based on hermite polynomials in order to fulfill global c1–continuity for solutions in the modified strain gradient theory. section 2 presents the strain energy function of the modified strain gradient theory and a one dimensional differential equation is derived by making use of the euler-bernoulli assumptions for slender beams. the finite element formulation based on hermite polynomials is presented in sect. 3 as well as the structure of the element stiffnessand system matrix. a numerical example for a loaded cantilever beam is then given in sect. 4. 2. modified strain gradient theory 2.1. strain energy function the present study starts with one of the three reduced forms of the strain energy densities for small deformations, usg, as postulated by mindlin (1962) [8]. the einstein summation convention on repeated indices is used and spatial partial derivatives in the cartesian coordinate system are denoted by commaseparated indices. mindlin’s second form of linear isotropic strain energy density reads: usg = α1εij εij + α2εkk εmm + β1ηijk ηijk + β2ηiik ηjjk + + β3ηiik ηkjj + β4ηijj ηikk + β5ηijk ηkji , (1) where εij = 1/2(ui,j + uj,i ) denotes the small strain tensor and ηijk = 1/2(uk,ij + uj,ki ) = εkj,i the gradient 33 http://dx.doi.org/10.14311/app.2017.7.0033 http://ojs.cvut.cz/ojs/index.php/app christian liebold, wolfgang h. müller acta polytechnica ctu proceedings of strain. α1 and α2 lead to lamé’s constants and β1,...,5 are five additional higher-order material constants [11]. σij and µijk denote the cauchy stress tensor and the higher-order stress tensor, respectively: σij = ∂usg ∂εij , µijk = ∂usg ∂ηijk . (2) the inclusion of the macroscopic rotation vector ϕi = 1/2εijk uk,j (εijk being the levi-civita symbol) leads to a reduction of the additional material constants from five to three. based on fleck & hutchinson (1997) [12] we introduce independent expressions of ηijk and decompose the second order displacement gradient into a symmetric part ηs ijk and a remaining part ηr ijk (which is not necessarily anti-symmetric): hj hj hj z~hj �� �� �� �� ηijk ηs ijk ηr ijk η̄ij χs ijχa ij η (0) ijk η (1) ijkχa ijεmm,i figure 1. scheme of decomposition ηijk = ηs ijk + ηr ijk , ηs ijk = 1 3 (uk,ij + ui,jk + uj,ki ) , ηr ijk = 2 3 (εikl η̄lj + εjkl η̄li ) + εkjl η̄li , (3) where η̄ij = ϕj,i is the gradient of rotation, which we decompose into its symmetric and anti-symmetric parts, χs ij and χa ij , respectively: χa ij = 1 2 (ϕi,j − ϕj,i ) , χs ij = 1 2 (ϕi,j + ϕj,i ) . (4) the tensor ηs ijk is further decomposed into its spherical and deviatoric part, η(0) ijk and η(1) ijk , cf., fig. 1 . the quantity η(0) ijk is related to χa ij and the dilatation gradient εmm,i in the following manner: η (0) ijk = 1 5 ( δij η s mmk + δjk η s mmi + δki η s mmj ) ηs mmi = εmm,i + 2 3εilnχ a ln , η (1) ijk = ηs ijk − η (0) ijk . (5) as shown in [13, 14] χa ij does not influence the strain energy if symmetry of the couple stress tensor µij is assumed. as a consequence the linear isotropic strain energy density for non-simple materials of gradient type reads: umsg = 2gεij εij + λεkk εii + 2g`2 0εmm,i εkk,i + + 2g`2 1η (1) ijk η (1) ijk + 2g`2 2χ s ijχ s ij , (6) where λ and g are lamé’s constants, whereas `0, `1 and `2 denote additional material length scale parameters. these parameters are chosen to be of the dimension of a length and squared in order to guarantee a positive definite problem in the energy minimization. the normalization of the higher-order terms by g is arbitrary. 2.2. governing euler-bernoulli differential equation by using the displacement field of an eulerbernoulli beam of length l, thickness t, crosssection a, young’s modulus e, and second moment of inertia i cf., fig. 2, ux = −zdw(x) dx , uy = 0 , uz = w(x), (7) l, ei, a w(x) dw(x) dx f xy z figure 2. bending line of a cantilever where w(x) characterizes the bending line and f denotes the force at the free end, eqs. (3)–(6) lead to the strain energy density of the body. by applying the principle of virtual work to the variation of the function w(x), a triple integration by parts leads to the ordinary differential equation (ode) of the problem [15]: s d4w dx4 −k d6w dx6 = 0 , ∀ x ∈ [0, l] . (8) the resulting ode is of rank six, where s = ei(1 + 4308 225 `2 t2 ) and k = 7 5ei` 2 (9) are given constants, if a rectangular cross-section and `0 = `1 = `2 = ` is assumed. 3. finite element implementation 3.1. weak form and discretization a multiplication of eq. (8) with a test function v(x) and triple integration by parts yields in the following weak integral form of the ode: l∫ 0 ( s d2w dx2 d2v dx2 +k d3w dx3 d3v dx3 ) dx = 0 . (10) now thegalerkinmethod is applied to discretize the interval [0, l] by using linear combinations of independent base functions φα for each element e = 1, . . . , n , which are uniformly defined on the element’s coordinate 0 ≤ ζ ≤ 1: ve(ζ) = ∞∑ β=1 φβ , we(ζ) = ∞∑ δ=1 ce δφδ . (11) the general element function of this method is derived by inserting eq. (11) in eq. (10), evaluated on the domain ζ: 1∫ 0 ∞∑ δ=1 ∞∑ β=1 ( sce δ d2φδ dζ2 d2φβ dζ2 +kce δ d3φδ dζ3 d3φβ dζ3 ) dζ =0 . (12) 34 vol. 7/2017 c1-continuous solutions in second gradient elasticity 3.2. hermite elements for global c1–continuity in order to solve the present weak form of the problem, the base functions need to be differentiable three times at least. in order to guarantee that information of second gradients of the trial function w(x) is included, we ask for global continuity of the first derivative of w(x), which is called c1–continuity of the solution. therefore, we choose a linear combination of four different hermite polynomials as follows: h1 = 2ζ3 − 3ζ2 + 1 , h2 = ζ3 − 2ζ2 + ζ h3 = −2ζ3 + 3ζ2 , h4 = ζ3 − ζ2, (13) h1 h2 h3 h4 0 ≤ ζ ≤ 1 figure 3. four hermite polynoms per element which allow to influence the first derivative at each boundary of the element independently. the unit element now is a linear combination of the four hermite polynomials: ve(ζ) = 4∑ β=1 hβ , we(ζ) = 4∑ δ=1 ce δhδ (14) where ce δ are four constants per element. 3.3. structure of the element stiffnessand system matrix by using hermite elements as base functions eq. (12) can be written in the following manner: 1∫ 0 4∑ δ=1 4∑ β=1 ( sce δ d2hδ dζ2 d2hβ dζ2 +kce δ d3hδ dζ3 d3hβ dζ3 ) dζ=0. (15) the element stiffness matrix ke reads: ke δβ = 1∫ 0 ( s d2hδ dζ2 d2hβ dζ2 +k d3hδ dζ3 d3hβ dζ3 ) dζ =  ke 11 ke 12 ke 13 ke 14 ke 22 ke 23 ke 24 sym. ke 33 ke 34 ke 44  = const. (16) and is a symmetric 4× 4 matrix with constant components that depend on the material data, beam geometry, and on the length of the element. in this approach we choose equidistantly distributed elements. by special assembling of the system matrix it is now possible to connect the boundary values of an element to its neighbors and, in addition, to bring together the first derivatives in between. this will be achieved by the following structure of the system matrix k: k =  k1 δβ + k2 δβ + k3 δβ + k ···δβ + kn δβ  . (17) due to the summation of the last entries, the specific components of neighboring elements are treated as equivalent. this refers to both, the components that represent direct node values as well as to the corresponding derivatives. the solution of the entire beam bending problem is now approximated by a linear system of equations: kc = f ⇒ k\f = c , (18) where c is the vector of element coefficients and f is a right hand side, in which the external force f is implemented (here, situated at the very last node n). the system is solved using the ”backslash” operator of the commercial program matlab (lu decomposition, etc.). 4. numerical example as an example we study bending of a cantilever beam, fixed at the left hand side and loaded with a single force at the free end, as presented in fig. 2 . material and geometry data is given in tab. 1 . the material length scale parameter ` is chosen according to a literature value [3] and to measurements performed by the authors with an atomic force microscope [16, 17]. t w l e `0 = `1 = `2 f 100µm 2t 20t 1.44gpa 17.6µm 1µn table 1. dimensions, loads and material parameters the boundary condition for the fixation has been implemented by canceling the first and second row and column of the system matrix eq. (17), since, regarding the first node of the first element, the current values of deflection and the derivative of the deflection line are equal to zero. 35 christian liebold, wolfgang h. müller acta polytechnica ctu proceedings er r ×10−5 0 5 10 15 20 0 40 80 120 160 number of elements n figure 4. convergence the test for convergence with respect to higher numbers of elements n yield positive results (cf. fig. 4). the relative error of tip deflections err= wn/w1−1 per increase of elements achieves values below 0.001h. lkmberlin university of technology faculty v – mechanical engineering and transport systems institute of mechanics chair of continuum mechanics and materials theory prof. dr. rer. nat. w. h. müller table: dimensions, loads and material parameters 𝑡 𝑊 𝐿 𝐸 ℓ0 = ℓ1 = ℓ2 𝐹 100µm 2𝑡 20𝑡 1.44 gpa 17.6µm 1µn 𝑤 [m m ] 𝑥/𝐿 d𝑤 d𝑥 𝑥/𝐿 d2𝑤 d𝑥2 𝑥/𝐿 d3𝑤 d𝑥3 𝑥/𝐿 dr.-ing. c. liebold | exnum 2016, liblice | 1d example 9/10 figure 5. deflection line the deflection line of the beam solution is presented in fig. 5 . five elements are shown so that a closer look at the solution of each element becomes possible, which consists of reconstructed trial functions, cf. eq. (14)2. figure 6 presents a plot of the first derivative of the reconstructed trial functions per element and clearly demonstrates continuity in between neighboring elements and therefore c1–continuity. lkmberlin university of technology faculty v – mechanical engineering and transport systems institute of mechanics chair of continuum mechanics and materials theory prof. dr. rer. nat. w. h. müller table: dimensions, loads and material parameters 𝑡 𝑊 𝐿 𝐸 ℓ0 = ℓ1 = ℓ2 𝐹 100µm 2𝑡 20𝑡 1.44 gpa 17.6µm 1µn 𝑤 [m m ] 𝑥/𝐿 d𝑤 d𝑥 𝑥/𝐿 d2𝑤 d𝑥2 𝑥/𝐿 d3𝑤 d𝑥3 𝑥/𝐿 dr.-ing. c. liebold | exnum 2016, liblice | 1d example 9/10figure 6. first derivative of the solution as expected the global solution is not c2–continuous. this is confirmed by the jumps between the elements in their second derivative in fig. 7. such jumps are also present in the third derivative in fig. 8, but invisibly small. lkmberlin university of technology faculty v – mechanical engineering and transport systems institute of mechanics chair of continuum mechanics and materials theory prof. dr. rer. nat. w. h. müller table: dimensions, loads and material parameters 𝑡 𝑊 𝐿 𝐸 ℓ0 = ℓ1 = ℓ2 𝐹 100µm 2𝑡 20𝑡 1.44 gpa 17.6µm 1µn 𝑤 [m m ] 𝑥/𝐿 d𝑤 d𝑥 𝑥/𝐿 d2𝑤 d𝑥2 𝑥/𝐿 d3𝑤 d𝑥3 𝑥/𝐿 dr.-ing. c. liebold | exnum 2016, liblice | 1d example 9/10 figure 7. second derivative of the solution lkmberlin university of technology faculty v – mechanical engineering and transport systems institute of mechanics chair of continuum mechanics and materials theory prof. dr. rer. nat. w. h. müller table: dimensions, loads and material parameters 𝑡 𝑊 𝐿 𝐸 ℓ0 = ℓ1 = ℓ2 𝐹 100µm 2𝑡 20𝑡 1.44 gpa 17.6µm 1µn 𝑤 [m m ] 𝑥/𝐿 d𝑤 d𝑥 𝑥/𝐿 d2𝑤 d𝑥2 𝑥/𝐿 d3𝑤 d𝑥3 𝑥/𝐿 dr.-ing. c. liebold | exnum 2016, liblice | 1d example 9/10figure 8. third derivative of the solution e ∗ [g pa ] thickness t [mm] figure 9. size effect for decreasing thicknesses the sensitivity of the implemented numerical model of the modified second gradient continuum was tested for incorporation of size effect by decreasing the thickness values, while all other dimensions were scaled in parallel according to tab. 1 . the solution of the tip deflection was used in a calculation of an equivalent young’s modulus, e∗, with a euler-bernoulli analytical beam model of the conventional cauchy continuum. as plotted in fig. 9, a size effect is clearly present. for small thicknesses the equivalent young’s modulus is calculated to be about three times larger than the usual one. 36 vol. 7/2017 c1-continuous solutions in second gradient elasticity 5. conclusions based on a modified strain gradient theory for linear isotropic materials we have evaluated the eulerbernoulli beam assumptions to derive the ordinary differential equation of the problem. the weak form was derived using test functions. third order hermite polynomials have been summed up for an element formulation: these are able to access and influence first derivatives between the elements. the element stiffness and system matrix have been formulated for the problem of a cantilever beam. the resulting system of linear equations was solved numerically to determine the coefficients of the trial functions for each element. the global solution was reconstructed element-wise and first, second and third order derivatives have been calculated. it was demonstrated, that this procedure guarantees c1–continuity of the solution. this is considered to be an important quality in second gradient theories. furthermore, the size effect, measured by the authors in previous work, could be reproduced by using similar material as well as geometry data. this provides the impetus for the development of a three dimensional finite element formulation of a strain gradient continuum on the basis of hermite elements. references [1] w. j. poole, m. f. ashby, n. a. fleck. micro-hardness of annealed and work-hardened copper polycrystals. scripta materialia 34(4):559–564, 1996. [2] x. h. guo, d. n. fang, x. d. li. measurement of deformation of pure ni foils by speckle pattern interferometry. mechanical engineering 27(2):21–25, 2005. [3] d. c. c. lam, f. yang, c. m. chong, et al. experiments and theory in strain gradient elasticity. j mech phys sol 51(8):1477–1508, 2003. [4] a. w. mcfarland, j. s. colton. role of material microstructure in plate stiffness with relevance to microcantilever sensors. journal of micromechanics and microengineering 15(5):1060–1067, 2005. [5] s. cuenot, c. fretigny, s. demoustier-champagne, b. nysten. surface tension effect on the mechanical properties of nanomaterials measured by atomic force microscopy. physical review b 69:01–05, 2004. [6] x.-f. li, b.-l. wang, k. y. lee. size effect in the mechanical response of nanobeams. j of adv research in mech eng 1(1):4–16, 2010. [7] r. toupin. elastic materials with couple-stresses. arma 11:385–414, 1962. [8] r. mindlin. effects of couple-stresses in linear elasticity. arma 11:415–448, 1962. [9] r. d. mindlin, n. n. eshel. on first strain-gradient theories in linear elasticity. international journal of solids and structures 4:109–124, 1968. [10] a. eringen. nonlocal continuum field theories. springer-verlag, new york 2010. [11] m. lazar. irreducible decomposition of strain gradient tensor in isotropic strain gradient elasticity. zamm z angew math mech 2016. doi:10.1002/zamm.201500278. [12] n. a. fleck, j. w. hutchinson. strain gradient plasticity. advances in applied mechanics, academic press, new york 33:295–361, 1997. [13] c. liebold, w. h. müller. measuring material coefficients of higher gradient elasticity by using afm techniques and raman-spectroscopy. advanced structured materials, springer berlin heidelberg 22:255–271, 2013. [14] f. yang, c. m. chong, d. c. c. lam, p. tong. couple stress based strain gradient theory for elasticity. international journal of solids and structures 39(10):2731–2743, 2010. [15] s. kong, s. zhou, z. nie, k. wang. static and dynamic analysis of micro beams based on strain gradient elasticity theory. international journal of engineering science 47:487–498, 2009. [16] c. liebold, w. h. müller. applications of strain gradient theories to the size effect in submicrostructures incl. experimental analysis of elastic material parameters. bulletin of ticmi 19(1):45–55, 2015. [17] c. liebold, w. h. müller. comparison of gradient elasticity models for the bending of micromaterials. elsevier, computational materials science 116:52–61, 2016. 37 http://dx.doi.org/10.1002/zamm.201500278 acta polytechnica ctu proceedings 7:33–37, 2017 1 introduction 2 modified strain gradient theory 2.1 strain energy function 2.2 governing euler-bernoulli differential equation 3 finite element implementation 3.1 weak form and discretization 3.2 hermite elements for global c1–continuity 3.3 structure of the element stiffnessand system matrix 4 numerical example 5 conclusions references acta polytechnica ctu proceedings doi:10.14311/app.2016.5.0029 acta polytechnica ctu proceedings 5:29–37, 2016 © czech technical university in prague, 2016 available online at http://ojs.cvut.cz/ojs/index.php/app assets of model metro station and their criticality tomáš kertis∗, dana procházková czech technical university in prague, faculty of transportation sciences, praha, czech republic ∗ corresponding author: kertitom@fd.cvut.cz abstract. the paper deals with the identification of protected assets in the model metro station, which is a part of critical infrastructure in praha; and with the identification of their criticality. a metro station is used by a lot of people and it involves many expensive components and devices with interdependences which form a system of systems. the system, its components and fittings are very vulnerable to damages from disasters of many kinds. the first step of ensuring metro station safety is the identification of basic metro assets which are important for safe metro operation and for human (employee and passengers) security and environment safety; and the other is determination of criticalities of considered assets with taking into account all relevant sources of hazards. these facts are input data for further processes which are important for safe metro operation under the different conditions. keywords: safety, security, metro station, assets, disasters, criticality. 1. introduction a metro is very important means of transportation in cities. therefore, to its safety it is pursued special attention, and it is part of critical infrastructure, to which it is now concentrated special attention because especially at critical situation its status predetermines the capability of human society to ensure human survival [1]. metro is composed from fix and movable parts; the fix ones are structure element, stations and tunnels with many fittings; the movable ones are trains [2]. the paper takes into account the metro stations. each metro station is equipped by many technical devices and fittings that ensure metro operation and comfort space for passengers. from safety reasons it also involves support elements for safety operation of station; they are for instance construction design, operation devices and systems, control systems, protection systems, electronic devices like lifts, escalators, air conditioning, trains and systems for train control, communication systems, staff etc. mentioned elements including the humans and personnel staff of station are in interaction each other and they create complex system prone to harm [2]. moreover, the system is mostly part of whole transportation infrastructure connections with track and next stations, material, energetic and informatics flows, and number of served persons. it is reality that devices and fittings that are very vulnerable to damages from disasters of many kinds [3]. from the protection reasons in the practice it is necessary to identify the protected assets and to establish their criticalities taking into account all relevant disasters, i.e. the sources of hazards, and to consider possible risks. this procedure is a common part of formation of safety management systems in all systems, i.e. also in railway domain. the first step is the identification of basic metro assets that are important for safe metro operation and for human (employee and passengers) security; and the other is the determination of criticalities of these assets. 2. critical assets and critical infrastructure protection for ensuring the humans security and development, there is necessary the safe human system [3]. basic assets of human system called basic public assets are: human lives and health, property and welfare, environment, and also critical infrastructures and technologies. it means that the critical infrastructure is one of important asset of human system. therefore, its protection is specially ensured in the eu, usa, germany, canada etc. [1]. in the czech republic critical infrastructure it includes nine infrastructures [1] [4], i.e. infrastructure for: • energy (electricity; gas; heat energy; oil and petroleum products) supplies, • handling with water (drinking and industrial water; secure and manage the surface water and underground water resources; waste water system), • food supplies and agriculture operation (food production; care for food; agricultural production), • health care (pre-hospital emergency care; hospital care; the protection of public health; the production, storage and distribution of pharmaceuticals and medical devices), • transportation system (road; rail; air; water), • cyber-communication and information systems (fixed telecommunication network services; services of mobile telecommunication networks, radio communication and navigation; television and satellite 29 http://dx.doi.org/10.14311/app.2016.5.0029 http://ojs.cvut.cz/ojs/index.php/app tomáš kertis, dana procházková acta polytechnica ctu proceedings communications radio; postal and courier services; internet and data services), • the banking and financial sector (management of public finance; banking; insurance; the capital market), • emergency services (fire rescue brigade cr; units of fire protection for area coverage; police of czech republic; army of czech republic; radiation monitoring; forecasting, warning and mouthpiece for the service etc.), • public administration (state administration and self-administration; social protection and employment; the state social support and social assistance; performance of judiciary and prison system). the transportation infrastructure includes road, rail, air and water infrastructure. each of mentioned infrastructures is composed from elements and line structures. the protection of transportation infrastructure in the czech republic is only concentrated to the elements according to the crisis act (act no. 240/2000 col.). it means that metro station is key asset, and therefore, we deal with its safety. transportation system provides the transportation of persons and goods. it includes all modes of transport, which, under the coordination of the various transport systems work together and creates a logistical network. the transportation network, together with energy network can be considered as the foundation of economic prosperity of the state. the transportation network consists of separate parts that make up self-reliant closed units. each unit is a unit, the basis of which is the type of transport, accompanied by complex operational and administrative parts, which forms subsystems. the characteristic feature of technological units is their specialized ability to separate traffic operation. it is necessary to recognize that the individual parts are bound by the rules of their transport logistics, which coordinates the activities. in the czech republic the transport network consists of: rail transport system; the road transport system; the air transport system; and water transport system. transport system and its subsystems are vulnerable to natural disasters, the traffic accidents, technological accidents of facilities and buildings being in their vicinity, crimes, terroristic attacks, organizing accidents, war etc. [1] [3] [4]. figure 1 shows the processes that are sources of disasters [3]. to ensure safety of each entity, i.e. each infrastructure and their components it is necessary to manage the safety by the way as is expressed by process model in figure 2 [1] [5]. the safety needs to be formed on all levels of governance [3] [4] [5], as it is depicted in figure 3. the ways of coping with risks are shown in figure 4 [3] [4] [5]. each real system is composed from various heterogeneous subsystems, links and flows among them, so-called inter and intra-dependences. subsystems figure 1. sources of disasters in the human system [3]. figure 2. process model for formation of safety of entity under account [1] [5]. figure 3. levels of safety management [3] [4] [5]. figure 4. measures used for get over risks connected with disasters [3] [4] [5]. 30 vol. 5/2016 assets of model metro station and their criticality is possible categorized as controlled and control systems in the view of control. for improving system parameters (dependability, security, safety, etc.) the support systems are implemented, for instance a protection system reducing the risks. a subsystem can be classified as safety related, the safety related system carries out very important functions which failures or malfunctions could lead to improving the risks and event to accidents [6]. from the protection reasons on the concept of integral safety of complex technological facilities in the practice it is necessary to identify the protected assets and to establish their criticalities taking into account all relevant sources of hazards. 3. data on metro in praha and metro station model stations of metro in the praha capital are not the same; they have differences due to their locations in different site conditions and demands in the time of their building. all are threatened by disasters that threaten the praha capital, i.e. its buildings, structures and facilities, and locally also by specific phenomena that are associated with local conditions, which are connected with local vulnerabilities. therefore, for research needs we developed a model station that has characteristics that are common to all the stations of reference system taking into account the praha land-use plan [7] and the praha metro documentation [2] [8] [9] [10]. the structural parts of praha metro were constructed according to the building law (to 2006 act. no. 50/1976 col., from 2006 act no. 183/2006 col.), the engineering parts and cyber technique were harmonised with demand of building law so they might obtain permission for operation. the metro operation control is performed according to the standards [11] [12] and it satisfies requirements given in standards [13] [14] [15]. the model metro station contains all elements, links and flows that are same in all metro stations because they are determined by the requirements of mentioned building law; i.e. we follow general features and do not study the details in structure, devices, fittings of both, the technical and the cyber domains. the detail studies of individual stations reveal real vulnerabilities that cannot be open to the public from the safety reasons. such detail studies have been performed for the metro operator, and they are used for metro safety upgrade. the sources of phenomena that can damage the metro system will be discussed in the next chapter in which we use the advanced principle based on all-hazard-approach [16]. considering the principle defence-in-depth described for complex technological facilities in [17] we shall mostly concentrates to the management units that keep the sufficient level of safety at normal, abnormal and critical conditions. safety management system of metro (sms) has central control system and controlled systems in each metro model station, figure 5. it is very complex distributed system that involves controlled, control and protection subsystems, their interdependences, operation states and conditions. figure 5. control and controlled system in metro model station [6]. metro station has subsystems stationary, track and on-board. on the basis of results in [6] obtained by application of principles given in [17] we select the main metro station assets in individual main parts. technological parts and some support subsystems of controlled system include assets: (1.) energetic devices: transformer substations and distribution transformers, (2.) communication equipment: communication cables, vhf connection with trains; passengers information systems including automatic check-in; communication systems for passengers and staff; cctv, telephone, public address; clocks and fire alarms; protection systems and signalling, (3.) machinery: escalators; pump stations in nodes in stations and between them; elevators; workshops and warehouses maintenance stations, (4.) air conditioning, (5.) mobile machines and devices: rolling-stock; devices and substances for cleaning, including the containers, cleaning machines, ladders, scaffolding for cleaning the lighting systems; fire protection equipment, (6.) next important equipment: security keys and alarm buttons; equipment for fire alarm start; traction devices and lighting; track devices, the main water shut; moving stairs, plates, signal panel for machinery devices; closing equipment (shutters). praha metro control centre has denotation operation control centre occ. it has supervisory control and data acquisition (scada) system that monitors technological facilities and processes and enables their control. its variant used in the czech republic has denotation asdř-d (automatic dispatcher system for transport control). the scada has several parts that are responsible, for: transport (ugtms urban guided transport management and command / control system); energetic tasks; technologies; communication and lighting. the control system involves following assets: • train dispatcher (surveillance and control of trains); and occ nodes, station nodes, nodes for automatic route setting system – asdř-d, 31 tomáš kertis, dana procházková acta polytechnica ctu proceedings • energetic dispatcher – asdř-e, • technological dispatcher – asdř-t, • lighting system dispatcher – adrř-o, • communiation and protection systems dispatching, • fire dispatching, • depot dispatching for train services and maintenance. for a model metro station, the control system involves station that are also assets: • station nodes of control system, • station node for automatic route setting, • station nodes for connection of energetic and technological dispatching, • station nodes for central lighting control system, • station interacting with dispatchers (communication, protection, operation, fire men). in the railway practice context a protection system is primary understand as system needed to mitigation of risks connected with train operation. it is also asset. they are: • station protection systems (esa 11 m), • wayside protection systems (ažd 71, esa 11 m), • train protection systems (matra and on-board units). the metro stations also ready on flows that are also assets, it goes on: • energetic flow, • information flow, • central dispatching and station nodes of control system, • central dispatching and station communication units, • station nodes of control system and protection systems, • station nodes of control system and communication system, • protection systems and technologies (on the track and on-board)-telephone connection between central dispatching (occ) and station, • telephone connection between central dispatching and trains. controlled, control and support (protection) systems are not just electronical and mechanical devices, they can be also organization processes, other communication and information parts of the system, human resources and place which shall be taken into account in the assets identification context. in the czech practice the european directives and standards are respected for safety management of railway systems; there are taken in account: common safety methods [18] [19], en 50126-1 [13], clc/tr 50126-2 [20], en 62290 [14] [15], en 92267 [21]. railway domain is regulated by european directive 2004/49/ec [18] that establishes duties for national authorities. the mentioned directive refers also to railway interoperability and safety. according to the directive regulation 402/2013 on the common safety methods for risk assessment [19], there take into account only risks connected with human harm. reason is an effort to ensuring the safety of passengers, employees, level-crossing users, unauthorised persons on the track and to ensuring the total safety on the basis of the above categories. weakness of mentioned directive is that it does not consider other assets which affect to people lives and healthy, it means public assets, important technological parts, facilities, any processes and infrastructure functions. the directives are insufficient for critical infrastructure needs and it does not provide assets protection. moreover, mentioned directives are not apply to special railway systems like metro. railway standard en 50126 [13] defines the specification and demonstration of reliability, availability, maintainability and security (rams) for a railway system including the metro. the standard defines whole life cycle including the concept and design of the system, through operation, maintenance, reconstruction, and up to decommissioning of the system. risk management is included there, but it considers only risks affect to human, it means risk r = rate (of accidents) x degree of severity (of harm); according to clc/tr 50126-2:2007 [20] in this context harm may imply: human harm (causing injures, fatalities); and also environmental harm (damage to property, spread toxic substances, other impacts, etc.). most railway safety studies tend to concentrate on human harm. (clc/tr 50126-2:2007). security requirements to integral safety improving are not still covered. standard en 62290 [14] [15] defines requirement for urban guided transportation management and control / command system which is split into 5 degrees of automation. each degree of automation (goa) has specific demands and set of functional requirements. although a standardisation commission has been established, no standard with safety requirements has been prepared [21]. better approach is included in en 62267 [21] for fully automated guided transport (augt). the standard refer to en 50126 [13] about risk management but it imposes demand on assets protection. identified protected assets by the standard are: persons (passengers, staff, external emergency services, public); and property, shall be defined by railway authority and operator (infrastructure of whole system, trains, equipment, etc.) [22]. the european standard is intended to very short group of railway system and it does not establish methods for determining the protected assets. the characteristic pattern of model metro 32 vol. 5/2016 assets of model metro station and their criticality station is composed from data mentioned above and the facts that are summarised in [11] [12] [13] [14] [15] [18] [19] [20] [21] [22]. 4. disasters considered in research since in the czech republic there is held long-term use of building codes and high quality engineering practice there are metro stations relatively well protected against design disasters of natural and technological origin that belong to the term all-hazardapproach [16], which ensures fixed safety level in these cases. the problems are connected with further disasters that belong to the term all-hazard-approach [16] but they were been introduced into practice later. it involves the following types of disasters: • so called organisational accidents [3] [4] [5] [17] because these disaster sources have been considered since the end of 80s of last century, • having the origin in cyber domain or in interdependences of different nature in system [3] [4] [5] [17] because these disaster sources have been considered since the end 90s of last century, • damaging phenomena caused by intentional human activities as crime or terroristic attack [3] [4] [5] [17] because these disaster sources come to size after 2000. from above reasons last designated disasters were not considered in original designing the metro stations, and therefore, we will pay special attention to them. data for the research we obtained from the sources on the praha metro [2] [8] [10]. all above mentioned metro station assets are threatened by disasters. for praha capital and its facilities according to [5] [7] the following disasters are relevant: (1.) the results of the processes inside and outside the earth: flood; tornado; earthquake; liquefaction of the ground; landslide, (2.) the results of the human body, human behaviour and processes in human society: epidemics; pandemics; failure of the stability of human society (welfare disruption, criminality); assault; terrorist attack; attack using the chemical, nuclear, radiological and biological (cbrne) weapons; armed conflict; war, (3.) results of the processes and activities installed by people: industrial accident; accident during the transport or storage of hazardous substances; accident at transportation; failure of critical infrastructure; failure in economy; failure in cyber infrastructure; failure of supply chains; loss of serviceability, (4.) interaction of earth and environment to activities of people: disruption of bedrock stability due to vibration; air pollution; water pollution; rapid climate changes; migration of large groups of people, (5.) internal dependences in the human system (natural or man-made): organizational accident; failure of flows of raw materials and products; disturbance in the flow of energy; failure in the information flow. 5. method of determination of model netro assets criticality to ensure the safety of model metro station it is necessary to know the criticality of whole metro system and its components, i.e. in followed case it is the model metro station [17]. as it was said above, each metro station is threatened by disasters, i.e. its safety depends on the quality of the work with the risks [5] [17]. safety and risk are together in some way in relation, but they are not complementary variables (complementary value to the safety is the criticality [17]). the criticality is a result of the exposure to risk factors. there are analysed the different types of factors: factors of human activity, decision-making and management, factors in the nondistinguishability of reality (because there are random and knowledge uncertainties). according to [17] the criticality of technological system indicates a threshold value that means it the values are below this threshold, so the status is demanded (subcritical) and vice versa. the assessment of criticality of real system is not a trivial matter, because the elements and whole have different roles for different conditions of active, reactive, amplifying or dampening (but not additive), and among the sub systems of systems of system, which is a real entity, and each model metro station. therefore, in accordance with [17] for its determination we use the method of risk engineering the checklist [23], which for complex technological systems is given in table 1. because each system of systems (in our case metro station) is a very complex system, the behaviour of which cannot be easily predicted, so the assessments from the perspective of the different sectors are often different, and therefore, it is necessary to choose the method that conflicts does not override, but it is trying to reasonably consider them [23]. therefore, it was used the method, based on the expert opinion of five experts who knew enough the metro and its operation, have been independent and were from the areas: • safety and protection of human health at work, • protection of property, • economy of operation, • protection of passengers, • technical and functional safety of operation. the evaluation was carried out separately for each significant disaster using the value scale listed in table 2. 33 tomáš kertis, dana procházková acta polytechnica ctu proceedings i question asst note 1 1. has the technological facility to incorporate the principles of inherent safety? 2. has the control system of a technological facility (sms) set the basic control functions, alarms and the response of the operator set up so that the technological facility in normal (steady) state? 3. has management system (sms) instrumentation (built-in safety instructions) and relevant physical barriers, which at derogate from the normal state to keep technological system in a good condition, i.e. they prevent the occurrence of unwanted phenomenon? the operation is successful, when, after the occurrence of the abnormal state the technological facility will return to normal as a result of resilience or after the application of corrective measures (clean-up, repair, replacement of parts). 4. has management system (sms) for the case of loss of control, i.e. critical conditions measures for emergency response that mitigate impacts on technological facility system and ensure the capability to return to a normal state? operation of a technological object is successful, if it is a good continuity plan, which ensures that the technological facility shall ensure all the necessary tasks. 5. does management system (sms) for the case of loss of control, i.e. supercritical (beyond design, extreme) conditions the measures for: maintaining the operability of the technological system following its repair and maintenance; item and measures to ensure the protection of public assets (people, the environment and other assets) in the surroundings of technological facility? . ............................................................................................................................. ........ ..... n 1. has the technological facility to incorporate the principles of inherent safety? 2. has the control system of a technological facility (sms) set the basic control functions, alarms and the response of the operator set up so that the technological facility in normal (steady) state? 3. has management system (sms) instrumentation (built-in safety instructions) and relevant physical barriers, which at derogate from the normal state to keep technological system in a good condition, i.e. they prevent the occurrence of unwanted phenomenon? the operation is successful, when, after the occurrence of the abnormal state the technological facility will return to normal as a result of resilience or after the application of corrective measures (clean-up, repair, replacement of parts). 4. has management system (sms) for the case of loss of control, i.e. critical conditions measures for emergency response that mitigate impacts on technological facility system and ensure the capability to return to a normal state? operation of a technological object is successful, if it is a good continuity plan, which ensures that the technological facility shall ensure all the necessary tasks. 5. does management system (sms) for the case of loss of control, i.e. supercritical (beyond design, extreme) conditions the measures for: item maintaining the operability of the technological system following its repair and maintenance; item and measures to ensure the protection of public assets (people, the environment and other assets) in the surroundings of technological facility? table 1. identification of deficiencies for specific disasters in a given territory, i = 1, 2, ..., n, i.e. assessment of the criticality of the viewpoint of the application of access the defence-in-depth. 34 vol. 5/2016 assets of model metro station and their criticality criticality rate values [%] extremly high – 5 more than 95 very high – 4 70-95 high – 3 45-70 middle – 2 25-45 low – 1 5-25 negligible – 0 less than 5 table 2. value scale according to [17]. the resulting rates of criticality for each item and disaster are set as median [23] from particulars determined by individual experts. 6. criticality of assets in model metro station on the basis of the procedure described in the previous chapter it was obtained a set of values that are listed in table 3. the numbers in table 3 indicate the following rates of criticality: • 0 – given disaster does not have a direct impact on the metro station and its operation, • 1 – given disaster has low impact on the metro station and its operation, • 2 – given disaster has middle impact on the metro station and its operation, • 3 – given disaster has high impact on the metro station and its operation. from table 3, it follows that the rate of criticality is high in many cases. while it is moderate, i.e. it is not very high or extreme, it is necessary in the frame of public interest to supplement the measures in all cases where the criticality rate is high. on the basis of the data referred to such cases according to [17] [24] it is necessary to perform in all cases where the criticality rate is 3 the following: • the analysis of the ability/competence of operator of metro to handle with emergency situations in the station and in the infrastructure, • the inventory of available resources for quick and proper response to the emergency situation in a metro station in the subway, • the assessment of current vulnerability of items of metro stations and the entire metro subway, • the specification of the coordination of activities to support the continuity of subway stations and underground infrastructure, • the division of responsibilities for actions to promote the continuity of the subway stations and underground infrastructure, • the setting up the organisational instructions and features to support the renovation of subway stations and underground infrastructure, • the establishment of criteria for the selection of main features of elements of subway stations and underground infrastructure, • the determination of support processes that support the main features of subway infrastructure, • identification of key personnel, to ensure the continuity of the subway stations and underground infrastructure, • prioritizing the main functions on the basis of the criticality of time, sequence of key recovery processes and personnel capacity. 7. conclusion from the protection reasons on the concept of integral safety of complex technological facilities in the practice it is necessary to identify the protected assets and to establish their criticalities taking into account all sources of hazards. the results of performed research show that metro stations have a certain level of safety (according to [17] it holds that level (rate) of safety = 1 – criticality rate). however, it is necessary to carry out the measures referred to in the previous chapter, because there are items for which in the occurrence of large (i.e. beyond design extreme) disasters there are get to significant impacts of disasters, which have the ability to disrupt seriously the metro station and the entire infrastructure. it means that present standards and legislative do not provide sufficient range for integral safety, including the safety of important assets and systems that predispose sufficient human security and human society development. references [1] d. procházková. bezpečnost kritické infrastruktury, 2012. isbn 978-80-01-05103-0. [2] dopravní podnik v praze. dp kontakt: časopis zaměstnancøu dopravního podniku hl. m. prahy, akciové společnosti, 1999-2005. isbn 1212-6349. [3] d. procházková. strategické řízení bezpečnosti a udržitelného rozvoje území, 2011. isbn 978-80-7251-243-0. [4] d. procházková. krizové řízení pro technické obory, 2013. isbn 978-80-01-05292-1. [5] d. procházková. analýza a řízení rizik, 2011. isbn 978-80-01-04841-2. [6] t. kertis. bezpečnostní plán vybrané stanice metra, diploma thesis. praha 2015. [7] praha, hl. m. archives for strategis and land-use planning. [8] metroweb [online], 2015. [2015-03-15], http://www.metroweb.cz/. [9] kolektiv pracovníkøu metroprojektu praha a.s. publikace iv. [online], 2008. http://www.praha.eu/ public/0/a3/da/186337_4_publikace_iv._c2.pdf. [10] unicontrols a.s. case study: metro praha, 2014. 35 http://www.metroweb.cz/ http://www.praha.eu/public/0/a3/da/186337_4_publikace_iv._c2.pdf http://www.praha.eu/public/0/a3/da/186337_4_publikace_iv._c2.pdf tomáš kertis, dana procházková acta polytechnica ctu proceedings assets/disasters fl oo d ea rt hq ua ke li qu ef ac tio n of gr ou nd g as re le as e ep id em ic pa nd em ic w el fa re di sr up tio n c rim e in te nt di sr up tio n te rr or ist ic at ta ck a tt ac k w ith c b r n e a rm ed co nfl ic t w ar in du st ria la cc id en t le ak ag e of ha za rd ou s tr affi c ac ci de nt b la ck ou t d isr up tio n of ec on om y human lives and healthy 1 1 1 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 1 1 1 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 1 1 1 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 property and places 1 1 1 2 0 0 0 2 3 3 3 3 3 1 3 2 2 0 1 1 1 2 0 0 0 2 3 3 3 3 3 2 3 3 2 0 2 1 1 2 0 0 0 2 3 3 3 3 3 2 3 3 3 0 energetic devices 1 1 1 2 0 0 0 2 3 3 3 3 3 1 3 2 2 0 1 1 1 2 0 0 0 2 3 3 3 3 3 2 3 3 2 0 2 1 1 2 0 0 0 2 3 3 3 3 3 2 3 3 2 0 communication equipment 2 2 2 2 0 0 0 2 3 3 3 3 3 1 3 2 3 0 2 2 2 2 0 0 0 2 3 3 3 3 3 2 3 3 2 0 2 2 2 2 0 0 0 2 3 3 3 3 3 2 3 3 2 0 machinery 1 1 1 2 0 0 0 2 3 3 3 3 3 1 3 2 2 0 2 2 2 2 0 0 0 2 3 3 3 3 3 2 3 3 2 0 3 3 3 3 0 0 0 3 3 3 3 3 3 3 3 3 2 0 air conditioning 1 1 1 2 0 0 0 2 3 3 3 3 3 2 3 2 2 0 2 2 2 2 0 0 0 2 3 3 3 3 3 2 3 3 2 0 3 3 3 3 0 0 0 3 3 3 3 3 3 3 3 3 3 0 mobile machines and devices 1 1 1 1 0 0 0 3 3 3 3 3 3 3 3 3 3 0 1 1 1 2 0 0 0 2 3 3 3 3 3 2 3 3 2 0 2 1 1 3 0 0 0 3 3 3 3 3 3 3 3 3 0 0 next important equipment 1 1 1 1 0 0 0 3 3 3 3 3 3 3 3 3 3 0 1 1 1 2 0 0 0 2 3 3 3 3 3 2 3 3 2 0 2 1 1 3 0 0 0 3 3 3 3 3 3 3 3 3 0 0 station nodes of control systems of metro system 1 1 1 0 0 0 1 1 1 1 1 1 1 1 3 1 1 0 1 1 1 0 0 0 2 1 1 2 1 1 2 1 3 1 1 0 3 2 2 0 0 0 3 2 2 3 3 2 2 2 3 2 2 0 protection systems 1 1 1 0 0 0 1 1 1 1 1 1 1 1 3 1 1 0 1 1 1 0 0 0 2 1 1 2 1 1 2 2 3 1 1 0 3 2 2 0 0 0 3 2 2 3 3 2 3 3 3 2 2 0 linkages and flows 1 1 1 0 0 0 1 1 1 3 3 3 3 3 3 3 3 3 1 1 1 0 0 0 2 1 1 3 3 3 3 3 3 3 3 3 3 2 2 0 0 0 3 2 2 3 3 3 3 3 3 3 3 3 table 3. criticality of assets in metro station. [11] čr. čsn en 13816 (269389) doprava – logistika a služby – veřejná přeprava osob – definice jakosti služby, cíle a měření, 2003. [12] čr. čsn en iso 9001:2009 (01 0321) systémy managementu kvality – požadavky, 2009. [13] čr. čsn en 50126-1 (333502) drážní zařízení – stanovení a prokázání bezporuchovosti, pohotovosti, udržovatelnosti a bezpečnosti (rams): část 1: základní požadavky a generický proces, 2001. [14] čr. čsn en 62290-1:2006 (33 3530) drážní zavrízení – systémy řízení městské dopravy s vyhrazenou vodicí dráhou: část 1: systémové principy a základní pojmy, 2007. [15] čr. čsn en 62290-2:2011 (33 3530) drážní zařízení – systémy řízení městské dopravy s vyhrazenou vodicí dráhou: část 2: specifikace funkčních požadavkøu, 2009. [16] fema. guide for all-hazard emergency operations planning [online], 1996. http://www.fema.gov/pdf/plan/slg101.pdf. [17] d. procházková. bezpečnost složitých technologických systémøu, 2015. isbn 978-8001-05771-1. [18] eu. directive 2002/49/ec of the european parliament and of the council of 25 june 2002 relating to the assessment and management of environmental noise – declaration by the commission in the conciliation committee on the directive relating to the assessment and management of environmental noise, 2002. 36 http://www.fema.gov/pdf/plan/ slg101.pdf vol. 5/2016 assets of model metro station and their criticality [19] eu. regulation 402/2013 on the csm for risk assessment and repealing regulation 352/2009, 2002. [20] celenec. clc/tr 50126-2:2007 railway applications. the specification and demonstration of reliability, availability, maintainability and safety (rams). guide to the application of en 50126-1 for safety, 2013. [21] iec. project iec 62290-3 ed. 1.0: railway applications – urban guided transport management and command/control systems – part 3: system requirements specifications, 2015. [22] čr. čsn en 62267 (33 3532) drážní zařízení – automatizovaná městská doprava s vyhrazenou vodící dráhou (augt) – bezpečnostní požadavky, 2009. [23] d. procházková. metody, nástroje a techniky pro rizikové inženýrství, 2011. isbn 97880-01-04842-9. [24] d. procházková. základy řízení bezpečnosti kritické infrastruktury, 2013. isbn 978-80-01-05245-7. 37 acta polytechnica ctu proceedings 5:29–37, 2016 1 introduction 2 critical assets and critical infrastructure protection 3 data on metro in praha and metro station model 4 disasters considered in research 5 method of determination of model netro assets criticality 6 criticality of assets in model metro station 7 conclusion references 215 acta polytechnica ctu proceedings 1(1): 215–221, 2014 215 doi: 10.14311/app.2014.01.0215 highly magnetized accreting pulsars: are there accreting magnetars? pere blay1, pablo reig2, vı́ctor reglero1 1ipl, university of valencia, spain 2university of crete, greece corresponding author: pere.blay@uv.es abstract 2s 0114+650, gx 301-2, igr j16358-4726, x per, 4u 2206+54, sxp 1062, and 3a 1954+319 are thought to possess high magnetic fields. they have recently been named accreting magnetars, or highly magnetized accreting pulsars. in this work their properties are reviewed. within the context of their observational properties (mainly from integral data), and the recent models of accretion onto highly magnetized neutron stars, their similarities and differences are analyzed. the aim is to find a common framework to understand the evolution (in terms of past and present history) of these sources, and to establish the basis of a possible new kind of accreting sources. two of these sources, namely x per and 4u 2206+54, contain a massive main-sequence companion, while the rest are supergiant x-ray binaries or symbiotic systems. the variety of astrophysical scenarios represented by this set is wide, therefore the study of these systems is also important in order to establish commonalities between the different types of accreting x-ray pulsars and to study the possible evolutionary links between them. keywords: stars: binaries: close stars: magnetars stars: neutron x-rays: binaries. 1 introduction magnetars are neutron stars with bright persistent xray emission (on the order of lx ∼ 1033−36ergs−1) and spin periods on the order of a few seconds. they show a very drastic spin period evolution, with large spin period derivatives and a long-term spin down tendency. the fact that they are isolated objects, together with the difficulties in considering their rotational energy as the source of the observed bright x-ray emission, are key issues in order to postulate their highlymagnetized nature (with extreme magnetic fields, on the order or greater than ∼ 1014gauss). magnetars are usually grouped into two subclasses, namely anomalous x-ray pulsars (axps) and soft gamma-ray repeaters (sgrs). for a review of magnetars see, for example, panchapakesan (2003) and rea & esposito (2011). high mass x-ray binary systems (hmxrbs) contain a massive companion (≥10m�) and a neutron star or a black hole. the evolution of neutron stars in hmxrbs is affected directly by the wind plasma in which they are immersed (originating from the massive companion). the ambient plasma interacts with the magnetic field of the neutron star modifying its spin period evolution. neutron stars are born as rapid rotators. the ambient plasma removes rotational energy from the neutron star and transports it outwards and the neutron star suffers an spin-down process, which goes on until the ambient matter becomes radiative, cools rapidly, and accretion can take place. the time spent in this initial spin-down epoch will determine the final x-ray pulsar rotational period which will be an equilibrium period which resulting from the balance of two forces: the magnetic field pressure (which stops and drags the in-falling matter), and the inward pressure exerted by the ambient plasma. initial modelings (see, for example, illarionov and sunyaev, 1975) predicted that the slowest reachable pulse period of a neutron star in a binary system is on the order of ∼500 s. revisions of these initial models (see, for example, davies and pringle, 1981) relaxed this limit, allowing longer spin periods but only associated with higher neutron star magnetic fields. the possible presence of magnetars in some accreting hmxrbs, in which a slowly rotating neutron star has been found, has been proposed recently by a number of authors (see, for example, li & van den heuvel 1999, finger et al. 2010, reig et al. 2012, ikhsanov & beskrovnaya 2010, doroshenko et al. 2010, patel et al. 2007, popov & turolla 2012, bozzo et al. 2008). our goal is to compare the observational properties of these systems and determine which observational parameters are more suitable to derive the strength of the magnetic field. we also want to stress the difficulties in determining some of the properties of these systems when 215 http://dx.doi.org/10.14311/app.2014.01.0215 pere blay, pablo reig, vı́ctor reglero considering only the long-term analysis (i.e., comparing pulse periods from different epochs), or the temptation to identify spectral features as cyclotron resonant scattering features (crsf) and derive magnetic field values from their central energy. despite the small number of systems, the variety of astrophysical scenarios represented by this set is wide, from main sequence accreting binary systems (x per, sxp 1062 and 4u 2206+54) to supergiants (2s 0114+65 and gx 301-2), and even symbiotic systems (igr j16358-4726 and 3a 1954+319). in this sense, the study of these systems is important also to establish commonalities between the different types of hmxrbs and to study the possible evolutionary links between them. in this work we will review the observational properties of some of these systems and we will try to establish the scientific rational to develop a deeper analysis of their common properties and evolutionary status. due to the large inhomogeneity in the energy ranges and sets of data available in the literature, we have started an analysis of these systems by using integral/ibis/isgri1 data, setting up common energy ranges, spectral resolutions, and performing a homogeneous timing analysis. 2 the candidates a number of hmxrbs have already been proposed to contain highly magnetized neutron stars, namely 2s 0114+65 (li & van den heuvel 1999), 4u 2206+54 (reig et al. 2012, finger et al. 2010), x per (doroshenko et al. 2012), gx 301-2 (ikhsanov & finger 2012), igr j16358-476 (patel et al. 2007). there are also suggestions that sxp 1062 (fu & li 2012) and 3a 1954+319 (marcu et al. 2011) contain a highly magnetized neutron star. attempts have also been made to try to explain the properties of sfxts with the presence of a highly-magnetized neutron star with some degree of success (see bozzo et al. 2008 ) . a complete list of sources has been compiled in table 1. despite the fact that there are peculiarities in each of these systems which make difficult to group them as a kind (see, for example, reig et al. 2012, blay and reglero 2013), we can summarize their overall commonalities: • slowly rotating neutron stars (pspin slower than 500 s), with • high magnetic fields (larger than 1013 gauss) • most of them show a long-term spin-down tendency the neutron star properties in these systems are different from those of magnetars, for this reason the terms magnetar-like, or magnetar-descendant (making reference to a possible evolution from a neutron star born as a magnetar), have also been used to identify these sources (reig et al. 2012, blay and reglero 2013). table 1: list of sources. source pspin (s) porb (d) type 4u 2206+54 5560 19.2 ms donor x per 864 250 bexrb 2s 0114+650 9700 11.6 sxrb gx 301-2 680 41.498 sxrb igr j16358-4726 5880 −− symbiotic 3a 1954+319 19080 −− symbiotic sxp 1062 1062 ∼300 bexrb after 10 years of operations of the integral satellite, a very complete and homogeneous long-term database of observations of all these sources has been compiled. we have analysed all public observations in which 4u 2206+54, x per, 2s 0114+65, igr j16358476, and gx 301-2 were in the field of view of integral/ibis/isgri. the behavior of these source is similar in long time scales, with transient emission in the form of bursts or peaks and more or less continuous detection. they all show the typical behavior of wind accreting systems (although x per is a bex, it has been proposed that its behavior resembles that of wind-fed systems because of the large orbital distance, which will keep the neutron star away from the be disk, see doroshenko et al. 2012). the light curves, in every case, are complete enough to develop both short and long-term analysis of the spectral and timing properties of these systems. the magnetic properties of the neutron stars in these systems have been investigated/explained mainly in these terms: • spin period (neutron star evolutionary considerations) • spectral features (cyclotron resonant scattering features, crsf) 1integral (international gamma-ray laboratory) is an esa mission with contributions from usa and russia, with two high-energy imagers (ibis, working in the energy range 15 kev−10 mev , and jem-x sensitive in the 3−35 kev energy range) and one spectrometer (spi, sensitive in the range 20 kev−8 mev). ibis, in turn, has two detector layers, isgri, for the lower energy band,15 kev−1 mev, and picsit working in the 175 kev−20.4 mev energy range. a detailed description of the mission can be found in winkler et al. (2003) 216 highly magnetized accreting pulsars: are there accreting magnetars? with regard to the spin-period history, according to ikhsanov (2007), the maximum spin period reachable by a neutron star can be related to the intensity of its magnetic field and the quantity of accreted matter (i.e., mass accretion rate). the latter quantity is considered to be directly proportional to the x-ray luminosity of the source (see, for example, finger et al. 2010). the relationship between these quantities is: pmax = 15000µ 16/21 32 m −4/21 ns ( ṁ 1015gs−1 )−5/7s (1) where µ32 is the dipole magnetic moment (in units of 1032gcm−3) and mns the mass of the neutron star (in units 1.5m�), and ṁ is the mass accretion rate. a more recent calculation of quasi-spherical accretion by popov and turolla 2012, yields this relationship, directly in terms of the magnetic field intensity: b12 ∼ 8.1ṁ 1/3 16 v −11/3 300 ( p1000 porb,300 )11/2 (2) where b12 denotes the magnetic field in units of 1012 gauss, ṁ16 is the mass accretion rate in units of 1016gs−1, v300 is the wind velocity in units of 300kms−1, p1000 is the spin period in units of 1000s, and porb,300 is the orbital period of the system in units of 300d. on the other hand, a direct measurement of the magnetic field of the neutron star is possible when its high-energy spectrum shows crsfs. crsfs are due to the splitting of energy levels of the electron in the presence of a magnetic field. therefore the scattering of x-ray photons by these electrons is produced at quantized landau levels producing the absorption-like features seen in the x-ray spectra of many hmxrbs (see schönherr et al. 2007). the magnetic field strength can be calculated from the position of the crsf by the formula: ecrsf = 11.6 b12 (1 + z) (3) where z is the gravitational redshift. it should be noted that this redshift, and consequently the measured magnetic field, will depend on the height over the neutron star surface where the crsf is formed. in the next sections, we will review these methods when applied mainly to some of sources listed in table 1. 3 magnetic field determination via spin period evolution fig. 1 shows examples of spin period histories of 4u 2206+54, 2s 0114+650, igr 16358-4726, and x per. 53600 53700 53800 53900 54000 54100 54200 54300 mjd 5300 5400 5500 5600 5700 5800 p sp in ( s) 4u 2206+54 53000 53500 54000 54500 55000 55500 56000 mjd 2.00 2.25 2.50 2.75 3.00 p sp in (h ) 2s 0114+650 54000 54200 54400 54600 54800 55000 55200 55400 mjd 830 831 832 833 834 835 836 837 838 839 840 841 842 843 p sp in ( s) x per 53000 53500 54000 54500 55000 55500 56000 mjd 5780 5800 5820 5840 5860 5880 5900 p sp in ( s) igr j16358-4726 figure 1: spin period evolution of 4u 2206+54 (top panel), 2s 0114+650 (second panel from the top), x per (third panel from the top), and igr 16358-4726 (lower panel) as obtained from integral/ibis/isgri data in the 20-40 kev energy range. 217 pere blay, pablo reig, vı́ctor reglero 0.0001 0.001 0.01 0.1 1 fl ux ( co un t s -1 k ev -1 ) 20 40 60 80 100 120 140 160 180200 energy (kev) -0.04 -0.02 0 0.02 0.04 re si du al s 4u 2206+54 0.01 0.1 1 fl ux ( co un ts s -1 k ev -1 ) 20 40 60 80 100 120 140 160 180200 energy (kev) -0.4 -0.2 0 0.2 0.4 re si du al s 2s 0114+650 0.0001 0.001 0.01 0.1 1 fl ux ( co un t s -1 k ev 1 ) 20 30 40 50 60 70 80 90 100 200120 140 160 180 energy (kev) -0.02 -0.01 0 0.01 0.02 re si du al s x per figure 2: the 20-200 kev spectra of 4u 2206+54 (top panel), 2s 0114+650 (middle panel) and x per (bottom panel) extracted from integral/ibis/isgri data. the spectrum of 4u 2206+54 has been fitted with a bulk comptonization model with a χ2 red ∼ 1.4, no evidence of a feature around ∼30 kev can be seen, neither in the spectrum nor in the residuals. for 2s 0114+650 a bulk comptonization model has been used to fit the data with a χ2 red ∼ 1.2, again no evidence of a crsf at ∼30 kev is evident. for the case of x per, a comptonization model and two absorption features at 31.2 kev and 82.3 kev (indicated by arrows) have been used to fit the observed spectrum, with a χ2 red ∼ 1.1. in the case of 4u 2206+54, frequency derivative values derived by finger et al. (2010) and reig et al. (2012) are 1.5 × 10−14hz s−1 and 1.7 × 10−14hz s−1 respectively. finger et al. 2010 included an older bepposax measurement. from these spin-down measurements, finger et al. (2010) estimated a magnetic field of b∼ 1014 g. by using the relationship of ikhsanov (2007) they also derived a magnetic field for 4u 2206+54 on the order of 1014 g. reig et al. (2012) calculated a magnetic field strength on the same order of magnitude by considering the relationship of popov and turolla (2012). the upper panel of fig. 1 shows the spin period measurements from integral/isgri data of 4u 2206+54 in the period 53600 − 54300 mjd. during this epoch (see blay and reglero 2013) the data is compatible with no changes in the spin period of the system, as the pulse period measurements show large errors. the same thing happens for x per and 2s 0114+650 (second and third panels from the top, respectively), in which large errors in the spin period determination make the measurements compatible with no pulse period change. therefore we cannot report on magnetic field measurements from spin period changes for these three systems. the luminosity of 2s 0114+65 in the epoch shown in fig. 1 is 4× 1037ergs−1 (estimated from the model fitted to the spectrum shown in fig. 2), considering a distance to the source of 7.2 kpc (hall et al. 2000). therefore, the ikhsanov (2007) relationship yields a magnetic moment on the order of 1033 g cm−3, which yields a magnetic field strength of b ∼ 1015 g. the relationship from popov and turolla (2012), however, results in an unreasonable number when applied to this source. doroshenko et al. (2012) argue that, for the case of x per, according to the torques applied to the neutron star magnetosphere in the case of wind accretion, and taking into account the theoretical approaches of doroshenko et al. (2010), davidson and ostriker (1973), davies et al. (1979), and bisnovatyi-kogan (1991), the magnetic field in x per is expected to be on the order of b∼ 1014. we show in the lower panel of fig. 1 the spin period measurements made with integral/isgri data. for this source we can estimate the luminosity to be lx ∼ 1.09 × 1035ergs−1 in this epoch (estimated from the model fitted to the spectrum shown in fig. 2 ). as the luminosity of the source is closely related to the accretion rate, we can estimate the field strength of x per by using the relationship of popov and turolla (2012), yielding b∼ 1012 g, which falls within the range of normal neutron star magnetic fields. the relationship from ikhsanov (2007) results in a magnetic moment of µ ∼ 1030 g cm−3, which implies a magnetic field on the order of b ∼ 1012 g. we find, therefore, a discrepancy with the results of doroshenko et al. (2012). we cannot reproduce their calculations, however, because of the large errors in the spin period determination, which prevent the determination of a reliable frequency derivative, needed for the determination of the spin torques. for igr j16358-4726 we see in the lower panel of 218 highly magnetized accreting pulsars: are there accreting magnetars? fig. 1 that the source experiences large pulse period changes. it shows a spin-down between mjd∼53250 and mjd∼54000 of ν̇ ∼ 2.4× 10−14 hz s−1. according to the equation (9) from finger et al. (2010), which relates the magnetic moment of the neutron star with the measured spin-down, we find in igr j16358-4726 a minimum magnetic moment of µ ∼ 2 × 1032 g cm−3, which implies a magnetic field on the order of or larger than b∼ 1014 g. these calculations rely on a good measurement of the luminosity (or mass accretion rate) or of the changes in the spin period. the major uncertainty comes from the determination of the luminosity of the source, calculated from the source flux in a given energy range and the estimated distance to the source, which is usually subject to large uncertainties. with respect to the use of spin period changes, the estimates always rely on long-term or average behavior. on the one hand this is due to the poor knowledge of the short-term or detailed pulse period evolution in many cases, and, on the other hand, to the lack of a detailed and complete theoretical description of the wind accretion scenario. the efforts of postnov et al. (2013), however, try to fill this lack of good theoretical approximations. we will comment on that in the last section. 4 magnetic field determination via cyclotron resonant scatering features a more direct measurement of the magnetic field strength of the neutron star in these systems can be obtained by determining the position of the fundamental crsf seen in their x-ray spectra. crsf have been reported in gx 301-02 (la barbera et al. 2005, makishima and mihara 1992), 2s 0114+65 (wang 2010, bonning and falanga 2005), 4u 2206+54 (blay et al. 2005, , wang 2009, torrejón et al. 2004, masseti et al. 2004), and x per (this work, see fig. 2, doroshenko et al. 2010), see table 2. table 2 summarizes the crsf reported in these systems and the magnetic field strength measurement derived. table 2: crsfs and b determination. see references in the text. source ecrsf (kev) b (1012 gauss) 4u 2206+54 30 2.6 x per 31.2 2.7 2s 0114+650 22 1.9 gx 301-2 37 3.2 5 2s 0114+650 22(c) 1.9 we see in all cases that the magnetic field strength derived by this method is on the order of those found in other neutron stars in hmxrbs, i.e., 100 to 1000 times lower than those of the magnetars. we find, therefore, a discrepancy in magnetic field strength determination with respect to the methods shown in the previous section. one could think that the magnetic fields derived from spin period evolution are not properly calculated, in view of the more direct determination from crsf measurements. however the location of crsf is not free of ambiguities. in the case of 4u 2206+54 (see top panel of fig. 2), for example, this feature has been observed only marginally, and only at one epoch (see blay et al. 2005 and references there in). it has not been observed again (see blay and reglero 2013). therefore, although it has been assumed that the absorption observed was a crsf, this may not be the case. a similar situation applies to 2s 0114+65 (see middle panel of fig. 2), in which the detection of the possible crsf has only been marginal (see wang 2010) and not confirmed again. in the case of x per, doroshenko et al. (2010) show how instead of a crsf fundamental and its first harmonic (see bottom panel of fig. 2), the x per spectra can be very well described by a combination of two comptonization models (comptt+comptt, in xspec notation) demonstrating the possibility that the features observed may not be crsfs. therefore, at least in some cases, the possibility of finding magnetar-like magnetic fields in these systems is not ruled out by the reported observations of crsfs. 5 discussion and conclusion we have seen that in most cases the determination of the magnetic field strength of the neutron star is ambiguous, or even contradictory, when the result from different approaches are compared. we want to emphasize how different approaches may seem to result in different magnetic field strength estimates. an homogeneous and coherent approach is needed in order to determine if there are links between these systems (in terms of their spin period and binary evolution) and to support or discard the different theories trying to explain the puzzling presence of long spin periods in hmxrbs. there have been recent theoretical efforts in order to provide better modeling of the wind-accretion scenario. ikshanov and beskrovnaya (2013), for example, report on a likely explanation of the large spin-down shown in 4u 2206+54 by taking into account a higher than usual magnetic field of the companion star and, consequently, that the accreting material can be magnetized. the possibility of an optical companion with a large magnetic field had been already proposed by blay and reglero (2011). 219 pere blay, pablo reig, vı́ctor reglero the efforts of postnov et al. (2013) in modeling more accurately the wind accretion mechanism in hmxrbs also result in magnetic field estimations for 4u 2206+54 and sxp 1062 on the order of typical magnetic fields found in accreting x-ray pulsars (∼ 1012g). they explain the behavior of sources like gx 301-2 with the hypothesis of being older systems which have already reached their equilibrium period. together with an improvement in the modeling of wind accretion in hmxrbs, a better observational approach is needed in order to understand the behavior of these sources. multiwavelength campaigns (ir, optical, uv, x-rays) on these objects are needed (as simultaneous as possible) in order to determine the various parameters involved: obtain more accurate spectral classification and orbital solutions, measure accurate mass loss rates and wind velocity laws, etc. acknowledgement p. blay acknowledges funding from the spanish ministerio de economia y competitividad through project aya-2011-29936-c05. references [1] bisnovatyi-kogan, g. s. 1991, a&a, 245, 528 [2] blay p., ribó m., negueruela i., torrejn j. m., reig p., camero a., mirabel, i.f., & reglero v. 2005, a&a 438, 936 [3] blay p. and reglero v. 2011, bsrsl, 80, 634 [4] blay p. and reglero v. 2013, pos(integral 2012)012 [5] bonning e. w., falanga m. 2005, a&a, 436, l31 [6] bozzo e., falanga m., stella l. 2008, apj, 683, 1031 doi:10.1086/589990 [7] davidson, k., and ostriker, j. p. 1973, apj, 179, 585 [8] davies, r. e., fabian, a. c., and pringle, j. e. 1979, mnras, 186, 779 [9] davies r.e., and pringle j.e. 1981, mnras, 196, 209 [10] doroshenko v., santangelo a., suleimanov v., kreykenbohm i., staubert r., ferrigno c., klochkov d. 2010, a&a 515, a10 [11] doroshenko v., santangelo a., kreykenbohm i., and doroshenko r. 2012, a&a 540, l1 [12] finger m.h., ikhsanov n.r., wilson-hodge c.a., & patel s.k. 2010., apj, 709, 1249 doi:10.1088/0004-637x/709/2/1249 [13] fu l., li x.d. 2012, apj, 757,171 doi:10.1088/0004-637x/757/2/171 [14] fürst f., marcu d.m., pottschmidt k., grinberg v., wilms j., and cadolle bel m. 2011, pos(integrall 2010)017 [15] hall t. a., finley j. p., corbet r. h. d., thomas r. c., 2000, apj, 536, 450 doi:10.1086/308924 [16] harbel, f., sturm, r., & filipović, m. d. et al. 2012, a&a, 537, l1 [17] ikhsanov, n. r. 2007, mnras, 375, 698 doi:10.1111/j.1365-2966.2006.11331.x [18] ikhsanov n.r., finger m.h. 2012, apj, 753,1 doi:10.1088/0004-637x/753/1/1 [19] ikhsanov n.r., beskrovnaya n.g. 2010, astrophysics, 53-2, 237 [20] ikhsanov n. r. and beskrovnaya n. g.2013, arep, 57, 287 [21] illarionov a.f., and sunyaev r.a. 1975, a&a, 39,185 [22] koh d.t., bildsten l., chakrabarty d., et al. 1997, apj, 479, 933 [23] li x. d., van den heuvel e. p. j., 1999, apj, 513, l45 [24] la barbera, a., segreto, a., santangelo, a., kreykenbohm, i., & orlandini, m. 2005, a&a, 438, 617 [25] makishima k., mihara t. 1992, in magnetic fields of neutron stars, ed. t. tanaka & k. koyama (tokyo: universal academy press), 23 [26] marcu d. m., fürst f., pottschmidt k., et al. 2011, apj, 742, l11 doi:10.1088/2041-8205/742/1/l11 [27] masetti n., dal fiume d., amati l., del sordo s., frontera f., orlandini m., and palazzi e. 2004, a&a, 423, 311 [28] panchapakesan n. 2003, bull. astr. soc. india, 31, 19 [29] patel s.k., zurita j., del santo m. et al. 2007, apj, 657, 994 doi:10.1086/510374 [30] popov s.b., turolla r. 2012, mnras, 421, l127 doi:10.1111/j.1745-3933.2012.01220.x 220 http://dx.doi.org/10.1086/589990 http://dx.doi.org/10.1088/0004-637x/709/2/1249 http://dx.doi.org/10.1088/0004-637x/757/2/171 http://dx.doi.org/10.1086/308924 http://dx.doi.org/10.1111/j.1365-2966.2006.11331.x http://dx.doi.org/10.1088/0004-637x/753/1/1 http://dx.doi.org/10.1088/2041-8205/742/1/l11 http://dx.doi.org/10.1086/510374 http://dx.doi.org/10.1111/j.1745-3933.2012.01220.x highly magnetized accreting pulsars: are there accreting magnetars? [31] postnov k.a., shakura n.i., kochetkova a.yu., and hjalmarsdotter l., 2013, arxiv:1307.3032 [32] rea n., esposito p. 2011, in high-energy emission from pulsars and their systems, ed. d. f. torres & n. rea, astrophysics and space science proceedings (springer berl in heidelberg), 247 [33] reig p., torrejón j.m., & blay p. 2012, mnras, 425, 595 doi:10.1111/j.1365-2966.2012.21509.x [34] schönherr g., wilms j., kretschmar p., kreykenbohm i., santangelo a., rothschild r.e., coburn w., staubert r. 2007, a&a 472, 353 [35] torrejn j. m., kreykenbohm i., orr a., titarchuk l., and negueruela i. 2004, a&a, 423, 301 [36] wang, w. 2009, mnras, 398, 1428 doi:10.1111/j.1365-2966.2009.15200.x [37] wang w. 2010, a&a 516, a15 [38] winkler c., courvoisier t.j.l., di cocco g., et al. 2003, a&a, 411, l1 221 http://dx.doi.org/10.1111/j.1365-2966.2012.21509.x http://dx.doi.org/10.1111/j.1365-2966.2009.15200.x introduction the candidates magnetic field determination via spin period evolution magnetic field determination via cyclotron resonant scatering features discussion and conclusion acta polytechnica ctu proceedings doi:10.14311/app.2017.7.0029 acta polytechnica ctu proceedings 7:29–32, 2017 © czech technical university in prague, 2017 available online at http://ojs.cvut.cz/ojs/index.php/app deformation behaviour of gellan gum based artificial bone structures under simulated physiological conditions nela krčmářováa,∗, jan šleichrta, tomáš fílab, petr koudelkab, daniel kytýřb a czech technical university in prague, faculty of transportation sciences, department of mechanics and materials, konviktská 20, 120 00 prague 1, czech republic b institute of theoretical and applied mechanics as cr, v.v.i., prosecká 76, 190 00 prague 9, czech republic ∗ corresponding author: krcmarova@fd.cvut.cz abstract. the paper deals with investigation of deformation behaviour of gellan gum (gg) based structures prepared for regenerative medicine purposes. investigated material was synthesized as porous spongy-like scaffold reinforced by bioactive glass (bag) nano-particles in different concentrations. deformation behavior was obtained employing custom designed experimental setup. this device equipped with bioreactor chamber allows to test the delivered samples under simulated physiological conditions with controlled flow and temperature. cylindrical samples were subjected to uniaxial quasi-static loading in tension and compression. material properties of plain gg scaffold and reinforced scaffold buffered by 50 wt% and 70 wt% bag were derived from a set of tensile and compression tests. the results are represented in form of stress-strain curves calculated from the acquired force and displacement data. keywords: gellan gum scaffold, reinforcement, uni-axial loading, simulated physiological conditions. 1. introduction the worldwide incidence of bone disorders and conditions have trended steeply upward. especially high income regions are expected twofold increase between 2010 and 2020 [1]. this is the tribute for populations aging coupled with improper nutrient consumption and poor physical activity. globally more than 40 % of women and 30 % of men are at increased risk of emergence of bone disorders [2]. annually in the usa only, more than half a million bone defects are reported. worldwide the treatment cost reaches more than $2.5 billion. the bone disorders treatment using engineered bone tissue has been viewed promising and yet not fully exploited potential alternative to conventional use of autografts and allografts. artificial tissue is overcoming problems with donor site morbidity, loss of bone inductive factors and/or resorption during healing [3]. in general, several essential demands are placed on artificial structure: i) chemical biocompatibility without toxic effect ii) reduction of the stress shielding effect iii) successful diffusion of nutrients and oxygen iv) controlled degradation and resorption [4]. presented paper deals with uni-axial quasi-static testing of artificial spongy-like structure [5] proposed for bone tissue engineering purposes as a bone scaffold. the investigated gellan gum bioactive glass (ggbag) material combines organic (polysaccharitic) component with inorganic (silicon-calcium based) nanoparticles. this approach effectively enables for adaptation of physical and mechanical properties of the synthesized material according to the desired application [6]. the studied material was subjected to quasi-static loading in both tension and compression to evaluate its expected deformation response in interaction with human body. therefore the experiment was carried out under simulated physiological conditions using bioreactor with circulating synthetic plasma. 2. material bioactive-glass-reinforced gellan-gum is a promising material for wide use in bone tissue engineering [7]. originally the gellan-gum (microbial extracted polysaccharide) was used in food and pharmaceutical industry [8]. gg is composed of repeating units consisting of two d-glucose and one of each lrhamnose and d-glucuronic acid [9]. its main advantage is in ability to form highly porous 3d structures when properly cross-linked and fabricated [10]. material investigated in this study was synthesized at jozef stefan institute (slovenia) as porous spongylike structure buffered by bioactive glass (bag) nanoparticles [11]. during the production process gellan gum was dissolved in ultra-pure water by heating the solution for 30 minutes at 90 ◦c. to the hot gg solution a dispersion of bag was admixed and 0.18 wt% cacl2 was added. kept at high temperatures this mixture was then poured into required mould and let there to spontaneously jellify. finally the samples were frozen at −80 ◦c and freeze-dried. 29 http://dx.doi.org/10.14311/app.2017.7.0029 http://ojs.cvut.cz/ojs/index.php/app n. krčmářová, j. šleichrt, t. fíla et al. acta polytechnica ctu proceedings 3. methods for initial awareness about deformation characteristics of synthesized material set of quasi-static experiments was performed. the first goal was to demonstrate possibilities of in house developed experimental infrastructure for this purpose. expected collapse forces was in range of single newtons and precise loading plate positioning was required as well. to obtain more relevant results some modifications of the experimental devices in detail presented in 3.2 was carried out. using this adapted devices basic material properties and stress–strain response were obtained. 3.1. experimental procedure cylindrical samples with height h = 8.6 ± 0.4 mm, diameter d = 5.0 ± 0.1 mm and weight m ≈ 11 mg, ≈ 16 mg and ≈ 24 mg for plain gg scaffold, ggbag reinforced scaffold with 50 wt% and 70 wt% bag respectively were subjected to tensile and compressive loading under wet condition. for wet condition simulating physiological environment of human body infusion solution plasmalyte (bartex, czech republic) was used. loading plate displacement was set typically for 1000µm corresponding to deformation approx. 11 − 12 % sufficient for significant sample damage. loading rate was set to 2µms−1. force and position was read-out with sampling frequency 50 sps. 3.2. instrumentation in house developed indentation device for low-force indentation was adapted for tensile and compression test. originally the device was designed using modular aluminum profile (30 × 30 mm) frame bearing i) x and y motorized axis kk40 (hiwin, japan) for sample positioning with repeatability 10µm ii) indentation axis based on linear stage mgw12 (hiwin, japan) and linear actuator 43 series (haydon kerk, usa) with position accuracy 3µm and mounting for u9b/c series (hbm, germany) load-cell. this axis was upgraded using the linear actuator with position accuracy 1.5µm, encoder with resolution 0.5µm and load cell with nominal force 50 n (the most precise force transducer in u9b/c series). for testing under wet condition testing device was equipped with bioreactor with controlled flow and temperature of circulating fluid. from the heated reservoir is the fluid pumped to the basin surrounding the samples and loading plates. 3.3. strain calculation the investigated material exhibits very low stiffness, which, coupled with high porosity and suboptimal geometry of the samples, induces high potential for significant boundary effects. unfortunately full-field optical strain measurement of the wet samples placed in fluid basin was not possible using the available setup. therefore comparative measurements for contact and contactless strain evaluation methods was performed in previous studies [12, 13]. however production process of the gg-bag samples does not allow to reliably produce cylindrical samples, the diameter of specimens varied in average ±100µm, and the loaded faces were rough and not plan-parallel strain-stress curve derived directly from force transducer and encoder indicate relevant results. 3.4. stress calculation the stress σ in all experimental analysis was considered as engineering stress obtained using σ = f ac (1) where ac is cross-sectional area of the specimen calculated from minimal sample diameter measured before deformation. force f was acquired by the load-cell. for the purpose of stress calculations, samples were considered ideally cylindrical, neglecting all geometrical irregularities. 4. results material properties and deformation behaviour of plain gg scaffold and gg-bag reinforced scaffold with 50 wt% and 70 wt% bag content were studied in tensile and compression tests under dry and wet conditions. five experiments for each type of material and loading mode were performed. young’s modulus was calculated using linear regression applied on the elastic part of stress–strain diagrams. the calculated elastic properties and yield stresses are listed in tabs. 2–6. sample e [kpa] gg00 1 88.588 ± 0.147 gg00 2 119.507 ± 0.156 gg00 3 126.159 ± 0.142 gg00 4 93.571 ± 0.144 gg00 5 141.089 ± 0.146 gg00 mean 113.783 ± 22.219 table 1. elastic properties of plain gg samples for compression test sample e [kpa] gg00 1 56.343 ± 0.843 gg00 2 33.403 ± 0.209 gg00 3 36.383 ± 0.412 gg00 4 69.077 ± 0.945 gg00 5 70.436 ± 0.951 gg00 mean 53.129 ± 17.562 table 2. elastic properties of plain gg samples for tensile test 30 vol. 7/2017 deformation behaviour of wet gellan gum scaffold sample e [kpa] gg50 1 81.4518 ± 0.145 gg50 2 131.882 ± 0.137 gg50 3 70.8462 ± 0.141 gg50 4 113.892 ± 0.281 gg50 5 140.221 ± 0.201 gg50 mean 107.659 ± 30.528 table 3. elastic properties of gg samples with 50 wt% bag for compression test sample e [kpa] gg50 1 24.406 ± 0.090 gg50 2 27.156 ± 0.085 gg50 3 24.287 ± 0.080 gg50 4 23.719 ± 0.081 gg50 5 21.376 ± 0.089 gg50 mean 24.18 ± 2.061 table 4. elastic properties of gg samples with 50 wt% bag for tensile test sample e [kpa] gg70 1 134.424 ± 0.120 gg70 2 63.566 ± 0.077 gg70 3 79.592 ± 0.078 gg70 4 67.376 ± 0.083 gg70 5 65.131 ± 0.077 gg70 mean 82.018 ± 29.967 table 5. elastic properties of gg samples with 70 wt% bag for compression test sample e [kpa] gg70 1 22.7629 ± 0.080 gg70 2 23.4423 ± 0.082 gg70 3 20.9057 ± 0.081 gg70 4 21.1495 ± 0.101 gg70 5 22.7019 ± 0.079 gg70 mean 22.192 ± 1.105 table 6. elastic properties of gg samples with 70 wt% bag for tensile test all obtained results in form of enveloped stress– strain curves are plotted in figs. 1, 2. the stress– strain area for each type of the scaffold represents minimum and maximum stress for each strain value. 5. conclusion gg-bag samples with 0, 50 and 70 wt% fraction of reinforcing bag particles were subjected to tensile and compressive loading test to evaluate deformation response in simulated physiological condition. it was found out, that the ambient environment has significant influence on mechanical response of the material as the measured properties. the scaffolds wetted by 0 0.002 0.004 0.006 0.008 0.01 0.012 0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 c o m p re s s iv e s tr e s s [ m p a ] compressive strain [−] egg00 = 0.113±0.022 mpa egg70 = 0.082±0.029 mpa egg50 = 0.107±0.030 mpa figure 1. stress–strain curves envelope of scaffolds under compressive loading 0 0.005 0.01 0.015 0.02 0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 t e n s ile s tr e s s [ m p a ] tensile strain [−] egg00 = 0.053±0.017 mpa egg50 = 0.024±0.002 mpa egg70 = 0.022±0.001 mpa figure 2. stress–strain curves envelope of scaffolds under tensile loading the synthetic plasma solution exhibited radical loss of stiffness, where the elastic modulae decreased more than ten times [12]. no significant reinforcement effect of bag particles was observed during compression test. in case of tensile loading bag buffered samples unexpectedly exhibited lower elastic modulae and ultimate stresses compared to the plain gg samples. that could be given by scaffold cell-wall heterogenity and disintegrity caused by rigid nature of glass particles. it can be concluded that presented analysis proved use of the considered experimental methods together with available experimental infrastructure for the testing of gg based scaffolds. the most limiting part of the experimental setup is load-cell signal-to-noise ratio at desired loading level and generally suboptimal geometrical characteristics of the samples inducing shear stresses during loading. acknowledgements the research was supported by grant agency of the czech technical university in prague (grant no. sgs15/225/ohk2/3t/16), by interreg project com3dxct (atcz38) and by institutional support rvo: 31 n. krčmářová, j. šleichrt, t. fíla et al. acta polytechnica ctu proceedings 68378297. we would like to express our special thanks to ana grantar for sample synthesization. references [1] w. h. organization. world health statistics. who press, 2015. [2] w. h. organization. global recommendations on physical activity for health. who press, switzerland, 2010. [3] a. r. vaccaro, k. chiba, j. g. heller, et al. bone grafting alternatives in spinal surgery. the spine journal 2(3):206 – 215, 2002. doi:10.1016/s1529-9430(02)00180-8. [4] a. r. amini, c. t. laurencin, s. p. nukavarapu. bone tissue engineering: recent advances and challenges. critical reviews in biomedical engineering 40(5):363– 408, 2012. doi:10.1615/critrevbiomedeng.v40.i5.10. [5] l. polo-corrales, m. latorre-esteves, j. e. ramirezvick. scaffold design for bone regeneration. journal of nanoscience and nanotechnology 14(1):15–56, 2014. [6] e. r. morris, k. nishinari, m. rinaudo. gelation of gellan – a review. food hydrocolloids 28(2):373 – 411, 2012. doi:10.1016/j.foodhyd.2012.01.004. [7] m. bououdina. emerging research on bioinspired materials engineering. igi global, 2016. doi:10.4018/978-1-4666-9811-6. 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[13] j. šleichrt, m. adorna, m. neuhäuserová, et al. deformation characteristics of chopped fibre composites subjected to quasi–static tensile loading. acta polytechnica ctu proceedings 3:71–74, 2016. doi:10.14311/app.2016.3.0071. 32 http://dx.doi.org/10.1016/s1529-9430(02)00180-8 http://dx.doi.org/10.1615/critrevbiomedeng.v40.i5.10 http://dx.doi.org/10.1016/j.foodhyd.2012.01.004 http://dx.doi.org/10.4018/978-1-4666-9811-6 http://dx.doi.org/10.1002/jbm.a.32574 http://dx.doi.org/10.1007/s00264-012-1520-y http://dx.doi.org/10.1016/j.msec.2014.06.045 http://dx.doi.org/10.17222/mit.2014.190 http://dx.doi.org/10.14311/app.2016.3.0071 acta polytechnica ctu proceedings 7:29–32, 2017 1 introduction 2 material 3 methods 3.1 experimental procedure 3.2 instrumentation 3.3 strain calculation 3.4 stress calculation 4 results 5 conclusion acknowledgements references acta polytechnica ctu proceedings doi:10.14311/ap.2016.4.0080 acta polytechnica ctu proceedings 4:63–71, 2016 © czech technical university in prague, 2016 available online at http://ojs.cvut.cz/ojs/index.php/app parcs/trace coupling methodology for rod ejection on vver 1000 reactor marek ruščáka,b,∗, guido mazzinib a polytechnic university of catalonia b research centre řež, hlavní 130, husinec-řež, czech republic ∗ corresponding author: marek.ruscak@cvrez.cz abstract. the rod ejection (re) is a design basis accident in accordance with nureg-0800 and usually studied using point kinetics. in this paper a methodology and a 3d kinetic model is prepared (parcs), coupled with a thermal hydraulic system code (trace) for simulating this accident scenario for general vver 1000 technology. keywords: parcs, trace, coupling, vver 1000, safety. 1. introduction the technology progress allows the industry to push the accuracy of calculations to the new level. alongside with the progress, there is an approach to simplicity. the current trend is to couple already existing codes to enable simultaneous complex scenarios and sub-sequential analysis. the thermo-hydraulic system code trace is the next generation code being a successor to relap, both of which however have the capability of coupling with the neutronics code parcs. the parcs/trace coupling for full vver 1000 reactor has never been done before in the czech republic with the cross sections generated by scale/triton. therefore a methodology had to be developed, which proved to be a challenge on its own regarding the lack of documentation for coupling models with hexagonal geometries. most of the geometrical, material and structural data are taken from a reference vver 1000 [1]. it is important to note, that results in this paper are not final and much longer process of model development will be required in order to submit the coupled model for validation. main objective of this paper is to demonstrate in a practical example of threedimensional accident scenario of vver 1000 reactor an acquisition of parcs code for 3d reactor kinetics and its coupling with system thermo-hydraulic code trace. 2. parcs code description parcs (purdue reactor core simulator) is a threedimensional (3d) reactor core simulator which solves the steady-state and time-dependent, multi-group neutron diffusion and low order transport equations in orthogonal and non-orthogonal geometries. parcs can be coupled directly to the thermal-hydraulics system code trace which provides the temperature and flow field information to parcs during the transient calculations via the few group cross sections. parcs is available as a standalone code for performing calculations which do not require coupling to trace or relap5. parcs can also be coupled to the relap5 systems code. a separate code module, genpmaxs, is used to process the cross sections generated by lattice physics codes such as triton (scale 6.1.2), helios, or serpent into the pmaxs format that can be read by parcs. parcs is composed of several cards such as cntl (control), xsec (cross-sections) and so on. parcs code is only concerned about neutronic definitions, most importantly the macroscopic cross sections, radial and axial fuel assembly maps, mashing and other parameters [2]. the most recent version of the parcs code is 3.2 (november 2015). although parcs is a very good tool for square geometry core lattice, there are still limitations concerning hexagonal geometry. by the fall the code application maintenance program (camp) of 2015 it is not possible to calculate pin power of hexagonal assemblies. also presented at the camp by dr. guido mazzini and by the author there is a limitation using gadolinium as a burnable absorber in the core in trace. there several additional issues concerning the coupling procedure, mainly due to a lack of proper guiding material provided by the code authors. in numerous cases impact and importance of several parameters can only be found by performing a sensitivity analysis on them, which requires significant amounts of time and therefore it is difficult to build a near-perfect model [3]. 3. trace code description the trac/relap advanced computational engine (trace – formerly called trac-m) is the latest in a series of advanced, best-estimate reactor systems codes developed under u.s. nuclear regulatory commission for analysing transient and steady-state neutronic-thermal-hydraulic behaviour in light water reactors. it is the product of a long term effort to com63 http://dx.doi.org/10.14311/ap.2016.4.0080 http://ojs.cvut.cz/ojs/index.php/app marek ruščák, guido mazzini acta polytechnica ctu proceedings bine the capabilities of the nrc’s four main systems codes (trac-p, trac-b, relap5 and ramona) into one modernized computational tool. it is able to analyse large/small break loss of coolant accidents (locas) and other system transients in both pressurized water reactors (pwrs) and boiling water reactors (bwrs). the version used in this analyses is trace v5 patch 4 released in 2014. for the user point of view, there are three major steps in performing a calculation in trace and those are: input processing, initialization and results. during the input processing, trace checks the user model and checks for errors. if the model passes the processing, it is initialized and again checks for possible boundary error as well as the model will be prepared for calculation. once the calculation is finished, there are several output files, among which there is the actual output, a restart file and a binary file containing all data to be read by apt plot. this process is shown in the figure 1 [4]. figure 1. phases of a trace calculation. 4. scale code description the scale code package is a comprehensive modelling and simulation suite for nuclear safety analysis and design. scale provides a “plug-and-play” framework with several computational modules including three deterministic and three monte carlo radiation transport solvers that are selected based on the desired solution strategy. scale includes current nuclear data libraries and problem-dependent processing tools for continuous-energy and multigroup neutronics calculations, multigroup coupled neutron-gamma calculations, as well as activation and decay calculations. in particular the triton module is used for the generation of cross-sections useful for parcs code. it is a control module in computation code scale 6.1.2. it uses several computational sequences like centrm, newt, bonami, etc., that are depending on user’s requirements. triton performs 2-d calculations for square and hexagonal geometry of the fuel assembly. its main computational features are cross-section calculation and isotopic compositions of the fuel assembly. in the centrm-based discrete ordinates (sn) option, the bondarenko self-shielding method is used to determine the problem-dependent multigroup cross-sections in the unresolved resonance energy range. the unresolved resonance calculation is performed by the bonami functional module. for the resolved resonance energy range, the centrm functional module is used to determine the 1d pointwise (105 energy groups) flux solution using the sn method [5]. 5. coupling procedure coupling procedure differs greatly with the lattice of the reactor. for square pitch all the user needs are parcs and trace inputs and graphical interface called snap in order to use the auto mapping feature. this auto mapping procedure generates a “map” file to be run with the coupled calculation. this automatic process is not however available for this case. therefore in case of hexagonal pitch this procedure has to be done manually. this means manually assigning each neutronic control volume (mesh) to the one of trace’s hydraulic and heat structure meshes. to do this by hand would require a substantial amount of time and this is why to achieve mapping a secondary script done in matlab is used. the general structure of the script has four main sections. the general constants such as number of radial rows or axial planes, number of assemblies, number of planes and the vessel id defined in the trace model. in the second section there are the radial definitions of both parcs and trace. first there is parcs radial numbering (given by stand alone parcs output) and then the parcs radial weight map (0.0–1.0) where on azimuthal boundaries the assembly weight is 0.5 and doubled (two assemblies by 0.5 instead of one by 1.0) and the central node is divided into 6 by 1/6. the trace radial th map and a heat structure (hs) map are defined according to a logic set in trace. the third section of the mapping script concerns the axial meshing and mapping. it is important to note, that the number of axial levels has to be same as in the geometry definition in both parcs and trace as well as the node numbering. usually the vessel component in trace has 1 to n nodes where the core (k) itself occupies only n − k nodes and the numbering has to respect the numbering in trace for the core. the mapping file includes a table in the following columns; vessel id, trace radial id, trace axial id, parcs radial and multiplication of radial and axial weights. after the mapping file is generated it is included with the parcs and trace inputs in order for the codes to recognize the geometry. in figure 2 there is a visualization of the mapping process, where colours correspond to individual sectors. strictly speaking the nodal definitions in both parc (core) and trace (th and hs) are put together. the sequence of calculation is shown in figure 3 on page 65. in this work there are two sets of cross sections generated by the scale and serpent 2 codes. later on the results will show the differences between the two and possible ways of improvement. once the cross sections are implemented with the parcs code, 64 vol. 4/2016 parcs/trace coupling methodology for rod ejection on vver 1000 figure 2. mapping procedure. geometry definitions of parcs and trace codes have to be the same. after ensuring that the geometry is same in either code, it is possible to begin the coupling process, which begins with mapping of the geometry definitions. in order to couple parcs and trace there are four files needed. the parcs and trace inputs (in the diagram they are shown as name.inp) and trace stand alone (sa) restart file are generated by the stand alone (sa) trace and of course the mapping file. with these files it is possible to run steady state (ss) coupled (c) calculation. this calculation outputs several files including the restart files of both parcs and trace, which are necessary for transient (tr) calculation. the ss, c restart files are then used for the tr, c calculation along with the transient inputs of parcs and trace. figure 3. calculation process. 6. model description 6.1. scale the model of fuel assembly is created by triton in scale 6.1.2 and the geometrical and material parameters from a vver 1000 type npp. there are six types of fuel assemblies with different enrichment as shown in the table 1 (page 66). in the first step it is necessary to define several parameters of the fuel, such as density, temperature and enrichment as well as cladding, which consist of 99 % zr and 1 % nb. moderator is comprised from main an additional materials. the main material is water (h2o) with density of 0.73 g/cm3, temperature 575 k and the additional material is boron with 1065 ppm. inside the fuel pellets there are the central holes filled with helium. the geometrical definition and parameters of the fuel assemblies and fuel pellets are defined as following. simulation of active length of fuel assembly as infinite because calculation code triton computes only 2d (radial) geometry. the inputs are shown in figure 4 and figure 5 on page 65. fuel assembly consists of nine nodes and each node has power level that depends on the active length power distribution. the analyses in scale are performed with average power and temperature distribution, also the calculation are done at time 0, practically after refuelling without the effect of xenon and samarium. the calculations are based on endf/b-vii.0 in 238 groups. the plots below are a visualization of defined geometry, where different colours represent different materials. figure 4. inputs of f130/f200 (left) and inputs of ff36g9/ff40g9 (right). figure 5. input of f30g9 (left) and input of f40g6 (right). there are five branches considered plus a reference one. the branch information is in the table 2 on page 66. these branches are used for calculation of the differential cross sections. the branches are fuel temperature (tf), moderator temperature (tm), moderator density (dm), control rod position (cr) and boron concentration (sb). 6.2. parcs the neutronic model of vver 1000 in the parcs code is fairly simple and short, compared to system codes or mc codes, such as mcnp. most of the parameters in the model are kept default. the only important parameter specified is the nodal kernel used, in this case the triangular polynomial expansion nodal (tpen) method. this method first decouples the 3-d defined geometry into 2-d axial and radial that are coupled through transverseleakages. the radial and axial configuration as well as 65 marek ruščák, guido mazzini acta polytechnica ctu proceedings fuel 1 [%] fuel 2 [%] fuel gd [%] gd [%] gd pins [–] f200 2.0 – – – – f130 1.3 – – – – f40g6 4.0 – 3.3 5 6 f30g9 3.0 – 2.4 5 9 ff36g9 3.6 3.3 3.3 5 9 ff40g9 4.0 3.6 3.3 5 9 table 1. fuel enrichment description. tf [k] tm [k] dm [g/cm3] cr sb [ppm] reference branch 980 575 0.73 out 1065 branch 1 980 575 0.73 in 1065 branch 2 980 575 0.73 out 565 branch 3 980 575 0.83 out 1065 branch 4 980 675 0.73 out 1065 branch 5 1080 575 0.73 out 1065 table 2. scale branch structure. control rod banks positions are also defined. the control rod configuration is set so the shut down clusters are fully withdrawn from the core, and the control clusters are fully inserted. core has to have same dimensions and meshing as in the trace model, in order to couple. therefore there are 14 axial nodes and 8 radial rings as shown in figure 6 on page 66 and table 4 on page 67. the description of figure 6 is shown in table 3 on page 66 [1, 6]. figure 6. core layout. colour fa type red f130 yellow f200 green f30g9 azur f40g6 blue ff36g9 purple ff40g9 table 3. colour description for core layout. the general configuration of control and scram banks is shown in figure 7 on page 66 where different colours indicate different groups. this model however does not include reflector, due to the complexity of implementation. whole trace figure 7. bank layout. model would have to be changed in order to include one additional ring containing the reflector. 6.3. trace the vver 1000 model developed by cvr was originally prepared for relap5mod3.3 code and subsequently converted for trace v5.0 code. it represents the primary and secondary systems, as well as the safety components of the plant. the model of the primary circuit consists first of the vessel component representing the reactor pressure vessel (rpv) as illustrated in figure 8 on page 67. radially, three zones nodalization scheme is adopted with two inner zones representing the lower plenum, the core, and the upper plenum. third (outer) zone models the downcomer. axially, 37 zones are modelled with 14 of them representing the active core. further, the model of the primary circuit consists of models of each individual primary system loop subdivided into hot leg pipes, horizontal steam generators hot and cold collectors and heat exchange tube bundles, loop seal pipes, reactor coolant pumps, and cold legs. to complete the primary circuit model the pressurizer, pressurizer surge and spray lines are modelled. the pressurizer power operated relive valve and safety valves are sim66 vol. 4/2016 parcs/trace coupling methodology for rod ejection on vver 1000 ulated. trace v5.0 pipe and valve components are used to model the secondary system steam lines up to the turbine. steam dump valves and steam generator safety valves are simulated individually. feedwater system model consists of the pipes and valves located in the containment. the plant model further contains the control components and trips to simulate actions of safety grade portions of i&c. figure 8. trace rpv model nodalization. the basic idea of this model development was to develop a tool for the licensing audit calculations of dba events included in chapter 15 of the sar [7]. as such, the model was systematically validated against the number of plant start up tests including the loss of flow (rcp trips), loss of feedwater, etc. with this, and considering that best-estimate codes are more accurate than severe accident codes in phases before core degradation started, the trace v5.0 model is a reference model for initial phases of the transients simulated [8]. the transient model is a reduced version of the steady state input, in order to reduce the computational costs. also it includes the small break loss of coolant accident (sbloca), (trip number 1005) which occurs at the point of ejection when the upper part of the control mechanism breaks and is ejected from the pressure vessel. this trip is set manually and the setpoint is time. the axial core control volume distribution is shown in the table 4 on page 67. this table does not include all rpv nodes, but only the active core, since the entire nodalization is not essential for the purposes of this work. the time steps are crucial part of the model, especially when coupling. the most important parameter is that the time step in trace should be about three orders greater then the timestep set in parcs, but small enough to allow the calculation to perform [4]. from experience however, trace terminates due to high time step, yet after rigorous sensitivity analysis the problem is not solved simply by reducing the time step even further. this particular problem will be one of a subjects of further study after this work. 7. results the results suffer from several code-based issues. probably the biggest impact has so called null factor, which acts as a balancing reactivity factor. in the case during the transient calculation, if the k-effective differs axial level mesh size axial level 14 46.3 axial level 13 25.5 axial level 12 25.5 axial level 11 25.5 axial level 10 25.5 axial level 9 25.5 axial level 8 25.5 axial level 7 25.5 axial level 6 25.5 axial level 5 25.5 axial level 4 25.5 axial level 3 37.73 axial level 2 13.27 axial level 1 29.55 table 4. trace axial distribution. from 1, this null factor acts to balance reactivities to achieve k-eff = 1. the bigger the difference between k-effective actual and 1, the bigger the null factor. another possible reason for these oscillations are the cross sections themselves and more tuning may be needed. this major reason is the lack of boron reactivity feedback, which still remains to be a problem. this is suspected to be the reason why the reactivity stabilizes at 0.2 $ (see figure 17). lastly the minor source of these oscillations is the instability transition of trace from steady state to transient calculation. because the access to the source code of either parcs or trace is limited, the only way of addressing these uncertainties is through benchmarking with existing results. when the control bank ejects, there is about 1 $ of positive reactivity inserted to the system, resulting higher neutron flux, this resulting in higher fuel temperature (figure 19) and also causing a rise of the doppler temperature. therefore the doppler effect introduces negative reactivity to the system, decreasing power. this heat is being transferred to the coolant, resulting in increase of coolant temperature (figure 18). when the temperature of the coolant (in this case light water) increases, the density decreases (figure 21) resulting in lower moderation hence decrease in flux, reactivity and power. this is the general behaviour, that occurs in all the peaks. the first peak, (figure 12) is so small, because the localised rise in coolant temperature is stopped when it is cooled down by rest of the coolant in the core. however quickly after the power start rising again and the whole process described above occurs again, however much more slowly. the final oscillations show simple balancing between moderator density and moderator temperature. by comparing the results with chapter 15 of final safety analysis report of an existing vver 1000, which unfortunately cannot be shown, the results are fairly comparable also with the results of [9]. by examining the flux behaviour we can see, that 67 marek ruščák, guido mazzini acta polytechnica ctu proceedings figure 9. averaged radial normalized power profile. figure 10. averaged thermal flux. in 100 seconds the flux increases during the transient. this can be seen in figure 10 on page 68. the flux shape is actually one of the major reasons why to use 3d calculations. by examining the flux plots below, most of the core has fairly low flux, in comparison to the area around the ejection point. by using the standard approach of point kinetics, the results would be averaged over the whole core, therefore lower then the actual highest peak. in the meantime, when the control bank ejects, the driving mechanism breaks and is ejected from the pressure vessel, resulting in a small break loca of a 50 mm diameter. the loss of coolant is represented by the mass flow in the figure 22, however for the duration of the transient, sb loca has very little effect as seen from the system pressure in figure 23. there are pressure oscillations close after ejection, however it stabilizes at the end of the transient. in order to calculate the main parameter, which is amount of energy per gram (cal/g), it is necessary to integrate over one second the relative power in the figure 11. averaged thermal flux after 100 s. ejection point and then multiply by the total power at peak [w] over number of assemblies (na, 163). using the hot channel radial peak factor (pf) 1.29 (given through internal communication with vver 1000 fa specialists) and dividing the total weight of fuel (approximately 500 kg) in one assembly by the number of pins (311). the general calculation is following, having time step (t) [s], average power (p ) [w] and normalized power at ejection point (rp) [–]. first it is important to calculate power in watts (wr) for the ejection point which is wr = p · rp · pf na . after that by integrating the wr curve per time and so getting the energy per time [j], it is necessary to convert to calories (1 j = 0.239 cal) and divide by weight of fuel in one pin in grams. the expected results should be similar to [9] which is 70.5 cal/g. the results are shown in figure 13 and it is possible to see, that the maximum value is 75 cal/g and therefore below the limit of 200 cal/g and also that it is reasonably similar to the results of [9], considering the differences in tested reactors. 68 vol. 4/2016 parcs/trace coupling methodology for rod ejection on vver 1000 figure 12. core power. figure 13. maximum energy deposited in fuel pin. figure 14. flux profile cross section at ejection point. figure 15. flux profile cross section at ejection point after 100 s. figure 16. axial normalized power. figure 17. reactivity. 69 marek ruščák, guido mazzini acta polytechnica ctu proceedings figure 18. coolant temperature at ejection point. figure 19. fuel temperature at ejection point. figure 20. maximal hot rod temperature. figure 21. coolant density at ejection point. figure 22. mass flow through the break. figure 23. system pressure. 70 vol. 4/2016 parcs/trace coupling methodology for rod ejection on vver 1000 8. conclusion the rod ejection accident scenario is a reactivity initiated accident and is also considered a design basis accident (dba). the general approach is to improve the existing models and methodologies in order to increase the precision of results. this goal is being approached from two sides, first to couple different codes (neutronics with thermal-hydraulics and also with material codes) together and second to transition from 1d calculations to 3d. this is increasingly possible with the increase in computer power, which allows to run much more complex calculations. there were two main goals to this paper. first, to establish a methodology for coupling parcs with trace for reactor with hexagonal lattice and second to run a rod ejection scenario on this coupled model.the general process was to create homogenized macroscopic cross sections for six different fuel assembly types, specifications of which were provided by the research centre rez and are considered confidential. the homogenized cross sections were calculated using a scale/triton code. after the cross sections were implemented into the parcs models, the parcs geometry definition had to be mapped with the one of trace. after that, a reference steady state coupled calculation was performed for each scenario separately. when the individual steady state calculations were done, the resulting restart data were inserted into the transient models. there were several challenges when coupling these codes for hexagonal lattice. the major one, which costed the most time is a lack of a proper guide on how to perform coupling without using graphical interface snap, which does not support hexagonal lattice. this issue is currently being solved with the authors of the code and hopefully a better user guide will be available in next years. second difficulty was the absence of auto-mapping function for hexagonal lattice, which had to be substituted by a matlab script. this can also be a source of uncertainty, because it is difficult to be sure, that the mapping was done correctly. lastly the code itself has a significant setback for the rod ejection calculation, which is a lack of pin power calculation for hexagonal lattice. because of this, the power is averaged over the whole assembly and therefore is lower, than the actual value. this can be however corrected using the peak factor as done in this paper. currently there are alternative neutronic codes, that can be coupled with trace, for example dyn3d, which can be coupled not only to trace, but also with relap and athlet codes. in the end, the future work to be done is firstly to improve the user guide for coupling parcs with trace for hexagonal lattice. the work done in this paper will undergo further tuning and a number of sensitivity analyses will be performed to achieve better results. firstly a database of cross sections will be created for multiple steps in the burnup cycle and secondly the trace model will have to be further modified for coupling. acknowledgements i wish to thank ing. miroslav hrehor for allowing me to perform my internship at the research centre rez and do my master thesis. also i wish to thank ing. milos kyncl for providing and assisting with the trace model and to ing. bruno miglierini and ing. vit kopecek for technical help. i would like to also thank the u.s. nrc for helping me with several technical issues with the coupling procedure. references [1] m. ruscak. simulation of reactor kinetics in nuclear power plant using simulator parcs. bachelor’s thesis, university of hradec kralové, 2014. [2] b. miglierini, m. ruscak, g. mazzini. practical acquiring of the parcs code for 3d analysis of neutronic behavior of vver1000/v320. vver 2013 conference, 2013. [3] t. downar, y. xu, v. seker. parcs v3.2, u.s. nrc core neutronics simulator. u.s. nuclear regulatory commision, 2014. um-ners-09-0001. [4] u.s. nuclear regulatory commision. trace v5.840 manual. division of safety analysis, office of nuclear regulatory research, u.s. nuclear regulatory commission, washington, dc. [5] radiation safety information computational center at oak ridge national laboratory. a comprehensive modeling and simulation suite for nuclear safety analysis and design. oak ridge national laboratory, 2011. ornl/tm-2005/39. [6] b. miglierini, g. mazzini, m. ruscak. 3d neutronic analysis of vver1000/v-320 using parcs code. in proccedings of the 2014 15th international scientific conference on electric power engineering (epe). 2014. doi:10.1109/epe.2014.6839473. [7] u.s. nuclear regulatory commision. nureg-0800, standard review plan for the review of safety analysis reports for nuclear power plants, lwr edition. u.s. nuclear regulatory commision, 2007. [8] m. r. guido mazzini, mylos kyncl. analyses of feed water trip with sbo sequence of vver1000 reactor. journal of nuclear engineering and radiation science 2016. doi:10.1115/1.4034144. [9] j. v. javier riverola, tomas nunez. realistic and conservative rod ejection simulation in a pwr core at hzp, eoc with coupled parcs and relap. in proceedings of the 2004 international meeting on lwr fuel performance, orlando, florida. 2004. 71 http://dx.doi.org/10.1109/epe.2014.6839473 http://dx.doi.org/10.1115/1.4034144 acta polytechnica ctu proceedings 4:63–71, 2016 1 introduction 2 parcs code description 3 trace code description 4 scale code description 5 coupling procedure 6 model description 6.1 scale 6.2 parcs 6.3 trace 7 results 8 conclusion acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2017.10.0043 acta polytechnica ctu proceedings 10:43–47, 2017 © czech technical university in prague, 2017 available online at http://ojs.cvut.cz/ojs/index.php/app modelling rock mass improvement using rock bolts jan pruška czech technical university in prague, faculty of civil engineering, thákurova 7/2077, prague 6, czech republic correspondence: pruska@fsv.cvut.cz abstract. anchor support currently represents a significant reinforcing technique in underground constructions. the principles of rock bolt reinforcement action are derived from various concepts of the underground excavation stability. in recent times, rock bolt design techniques have been complemented by numerical calculations procedures. the paper describes fem modelling of a rock bolt system for mechanical and grouted bolts. keywords: rock bolts, modelling, fem. 1. introduction in recent times, rock bolting represents a significant reinforcing method, especially in tunnelling. the main effect of rock bolting is in reducing and eliminating the impact of different types of failures that occur in the rock mass (schistosity, joints, cracks, etc.) so that the reinforced rock mass can restore the equilibrium disturbed by excavating an underground work. the effect of rock bolting on the rock mass is derived using a variety of methods to determine the stability of underground excavations and the rock bolt type considered (e.g. mechanical or resin). the design and verification of the structural behaviour of rock bolts can be made by any of the following methods: • empirical methods or using rock mass classification, • analytical solution (e.g. rock reinforcement contact problem), • simple calculations based on the fenner-pacher curve theory, • mathematical modelling (numerical solution of a contact problem), • models based on discontinuity mechanics theory, • probabilistic (stochastic) approach to determine the required loading of reinforcement anchorage. of the above methods, mathematical (numerical) modelling using the finite element method (fem) is commonly used in todayâăźs practice. generally, the design of bolts is a complex task that requires the knowledge of rock bolt materials, the anchor technology and the rock mass behaviour. 2. rock bolts modelling using fem the modelling of anchors and rock bolts by fem is more complicated compared with the modelling of the rock mass itself. this is mainly caused by the fact that rock bolts are divided by the type of anchor (simply into two main groups: point-anchored and anchored along its entire length or predominantly over the entire length). thus, the fem model must necessarily take into consideration the type of rock bolt, the anchoring method and the distribution of anchoring forces in the surrounding rock. it can be used for the following task: • modelling a rock bolt as a single element, • homogenization of the rock bolt’s area. in current tunnelling practice, rock bolts attached to the rock along their entire length predominate, thus, the methods using the homogenization of the rock bolt’s area are mostly used. 2.1. modelling of rock bolts using homogenization the principle of this procedure is based on the fact that anchors create a supporting ring in the rock mass near the excavation with characteristics of a thickwalled cylinder. the area of the supporting ring is modelled so that the elements of the finite element mesh in the supporting ring change their geomechanical properties by selecting equivalent isotropic material with improved cohesion. however, this procedure is not suitable for a clearly discontinuous rock mass (connection of rock blocks using rock bolts) because it does not consider the mechanical strength of the rock bolt’s area. czech practice uses the following procedure to increase cohesion in the rock bolt’s area. the increase of cohesion due to bolting is calculated according to the equation: cs = nu + sinϕ′ ak · 2 · cosϕ′ · γkc (1) where: nu loading capacity of the bolt [kn ], ak area per rock bolt [m2], ϕ′ effective angle of internal friction [°], γkc coefficient of bolting reliability [−]. 43 http://dx.doi.org/10.14311/app.2017.10.0043 http://ojs.cvut.cz/ojs/index.php/app jan pruška acta polytechnica ctu proceedings cohesion of equivalent isotropic material: ch+s = ch + cs (2) where: ch+s total cohesion of rock increased due to rock bolting, ch original cohesion of rock, cs cohesion increased due to rock bolting. if we need to take into the account the impact of the bolt on the discontinuity surface, a so-called virtual element can be used in fem. this virtual element models discontinuity surfaces using a conventional rectangular element with a height equal to the length of the plastic area in the bolt (fig. 1). it is assumed for the shear modulus calculation that a failure will occur in the bolt and at the discontinuity interface at the same displacement. the shear modulus was expressed e.g. by h. larsson [1]: gv p = τb uf · e · ab at + τj uf · e · aj at (3) where: gv p shear modulus of the virtual element, τb shear stress at the rock bolt failure, uf block displacement at the discontinuity interface, e height of the virtual element (length of the plastic area in the bolt), ab rock bolt area, aj discontinuity interface, τj shear stress at the discontinuity interface at failure, at total area of the rock with rock bolts (ab+aj). 2.2. modelling of rock bolts as discrete elements the problem of the formulation of a finite element that would describe the stabilizing effect of the rock bolt or rock anchors on the rock mass has been solved in various approaches since early fem numerical analyses (the first models were derived around 1968). thus, a number of special elements for describing rock bolts were developed (for bolts fixed at both ends or along their own length). these special elements are connected with specific special software or they require direct implementation in a programme code by the user. in the following part, one of the most used elements (attached to the rock along their own length) will be described. the principle of this element was presented in the thesis by ömer aydan in 1988 [2]. it is essentially a beam element (special bolt element), which has four nodes, two of which are connected to the anchor rod and the other two to the surrounding rock fig. 2. the following several simplifications have been considered for deriving the stiffness matrix of this element: • the body formed by the grouting material is considered as an axisymmetric thick-walled hollow cylinder, which, due to its shear deformation, transmits tension from the anchor rod into the surrounding rock. • both bolt materials (grouting material and the anchor rod) are considered with a linear elastic behaviour, and thus the relation between the stress and strain in the anchor rod and in the grouting material are described using hooke’s law. • in terms of the coordinates of nodes, the radius of the bolt is negligible in terms of the coordinates of nodes. this presumption naturally does not apply when creating the stiffness matrix. • the bolt considers only the existence of three types of deformation: . ε0 relative longitudinal deformation of the anchor rod caused by axial displacements of the nodes of the anchor rod, . γg shear deformation of grouting caused by different axial displacements of the nodes of anchor rods and the nodes connected with the surrounding rock, . γx transverse shear deformation of grouting caused by different radial displacements of the nodes connected with the surrounding rock. the relationship of the rod bolt strain in terms of nodal displacements (the b index determines the variables associated with the bolt (rod) and the g index determines the values belonging to cement grout): εb = bb ub (4) and the relationship of the grouting ring strain using nodal displacements εg = bg ∆ug (5) where u is the matrix of displacement and b the matrix of differential operators. physical equations for the linear elastic behaviour of rods and the grouting ring can be described using the above expressions as: σb = db εb σg = dg εg (6) where d is the relevant material stiffness matrix. by discretization using finite elements the equations of virtual work (it is assumed that the stress field σ is structurally possible and the field of displacements is kinematically allowed), we get expressions for the stiffness matrix: ke = ∫ btdb d(vol) = ∫ le 0 ∫ rh 0 ∫ 2π 0 btdb dϕdrdz (7) after the integration, we obtain the stiffness matrix of the element in the form given in fig. 3, where 44 vol. 10/2017 modelling rock mass improvement using rock bolts figure 1. virtual element for the rock with bolts. figure 2. scheme of a special bolt element: 1, 2 nodes connected with surrounding rock; 3, 4 nodes connected with the bolt rod. kb = eb · π · r2 b le (8) ks = gb · π · r2 b le (9) kg = π ·gg · le 3ln( rh rb ) (10) aydan’s element has been successfully implemented in scientific and commercial programmes. fig. 4 shows an example of the application of this element to the pmd system by d. runt [3]. 2.3. modelling using geotextile elements some authors in their publications describe the ability to model grouting bolts using geotextile elements e.g. m. šejnoha [4]. these are reinforcing elements which are continuously fixed in the earth massif. this element is, therefore, a system of two node rod elements connected in the places where the line of the geotextile intersects the edge of individual network elements see fig. 5. these elements can only be loaded by tensile forces, they do not have compressive strength and cannot transmit bending moments. they exhibit linear elastic behaviour (that is dependent on a specified compressive strength) and have limited tensile strength until the moment of failure. 2.4. modelling using internal forces in the rock mass modelling using rod elements is another possibility for describing bolts that transmit only axial forces. these elements have the ability to transfer compressive load, unlike the elements for geotextile modelling. it is also possible for this element to specify the input forces in the anchor (prestress) and not consider the prestress due to compressive load. according to some authors (e.g. [5]), the bolt root can be modelled as elements without compressive strength with contact elements. the cohesion of contact elements should be determined so that the total frictional force acting on a 1 m strip corresponds to the frictional force acting on the bolt (reducing cohesion by the ratio of the bolt area to the finite element area). for modelling bolts fixed along their own length, this model is unacceptable in underground engineering [6]. 2.5. modelling using internal forces in the rock mass the use of internal forces is one of the oldest methods of modelling anchors and rock bolts in fem. the principle of modelling consists in the insertion of internal forces into the rock mass at both sides of the rock bolt (head and root). this kind of plane modelling is only used for tensioned bolts or anchors. the inserted forces simulate the bracing area in the rock mass stabilized by rock bolts. however, this kind of modelling often leads to the plastification in the internal force location and to a situation where the calculation does not converge. 3. effect of installation time it is s quite difficult to make a sufficiently accurate model of the effect of rock bolts. we can usually only estimate the maximum possible effect. this is not only due to the established simplifications in different procedures, but the installation time of rock bolts also plays an important role (time delay due to the 45 jan pruška acta polytechnica ctu proceedings figure 3. the stiffness matrix of the element figure 4. rock behaviour in the stress σxx curve without rock bolts (left) and with rock bolts (right) figure 5. dividing geotextiles on rod elements application of shotcrete lining). this effect is very difficult to describe correctly in a numerical model. rock bolts are often part of primary lining and also help to improve the final deformation. therefore, we can only determine correctly the effects of the immediate installation of bolts or bolts installed to partially deformed lining. 4. conclusion this paper gives an overview of different approaches to rock bolts modelling using fem for tasks related to underground structures. from the above mentioned methods, a survey clearly shows that the main approximation of rock bolts in fem uses modified strength and deformation parameters in the rock bolt’s area or specially derived elements (quantifying the effect of bolting for the rock mass). also, fem is not suitable for modelling a discontinuous rock mass and, therefore, a direct impact of rock bolts on discontinuities; we can say that both of the above mentioned approaches (based on the theory of continuum mechanics) are quite suitable for describing the behaviour of rock bolts. subsequently, engineers can choose such bolt properties to reach a steady state of an underground structure and also optimize its design (based on the comparison of fem modelling of different options). 5. acknowledgement this article was written as part of a study within the project supported by the competence centres program of the technology agency of the czech republic (ta cr), no. te01020168. references [1] h. larsson, t. olofsson. bolt action in jointed rock. in rock bolting : theory and application in mining and underground construction : proceedings of the international symposium on rock bolting, abisko, 28 august 2 september 1983, pp. 33–46. balkema publishers, a.a. / taylor & francis the netherlands, 1984. godkänd; 1984; 20080926 (ysko). [2] ö. aydan. the stabilisation of rock engineering structures by rockbolts. nagoya: nagoya university, 1989. 46 vol. 10/2017 modelling rock mass improvement using rock bolts [3] d. runt, j. pruška, j. novotný. modelling of rock bolts using the finite element method. in isrm regional symposium eurock 2015. international society for rock mechanics, 2015. [4] m. šejnoha. finite element analysis in geotechnical design. saxe-coburg publications, kippen, stirling, scotland, 2015. [5] z. bittnar, j. šejnoha. numerické metody mechaniky 1. české vysoké učení technické v praze, 1992. [6] m. hilar. numerická analýza tektonicky porušeného horninového masivu s primární výstrojí při aplikaci nrtm. ph.d. thesis, české vysoké učení technické v praze, 2000. 47 acta polytechnica ctu proceedings 10:44–48, 2017 1 introduction 2 rock bolts modelling using fem 2.1 modelling of rock bolts using homogenization 2.2 modelling of rock bolts as discrete elements 2.3 modelling using geotextile elements 2.4 modelling using internal forces in the rock mass 2.5 modelling using internal forces in the rock mass 3 effect of installation time 4 conclusion 5 acknowledgement references acta polytechnica ctu proceedings doi:10.14311/app.2017.8.0020 acta polytechnica ctu proceedings 8:20–23, 2017 © czech technical university in prague, 2017 available online at http://ojs.cvut.cz/ojs/index.php/app mechanical properties of cr-dlc layers prepared by hybrid laser technology petr písaříka,b,∗, miroslav jelíneka,b, jan remsaa,b, zdeněk toldec a faculty of biomedical engineering, czech technical university in prague, sitna sq. 3105, 272 01 kladno, czech republic b institute of physics of the czech of sciences, na slovance 2, 182 21 prague 8, czech republic c department of materials engineering, faculty of mechanical engineering, czech technical university in prague, karlovo namesti 13, 121 35 prague 2, czech republic ∗ corresponding author: petr.pisarik@fbmi.cvut.cz abstract. diamond like carbon (dlc) layers have excellent biological properties for use in medicine for coating implants, but poor adhesion to biomedical alloys (titanium alloys, chromium alloys and stainless steel). the adhesion can be improved by doping the dlc layer by chromium, as described in this article. chromium doped diamond like carbon layers (cr dlc) were deposited by hybrid deposition system using krf excimer laser (deposition diamond like carbon graphite target) and magnetron sputtering (deposition chromium chromium target). carbon and chromium contents were determined by wavelength dispersive x-ray spectroscopy. chromium content of cr dlc layers was 0, 1, 2, 4-5 and 15 17 at.%. the topology and roughness of layers were studied using atomic force microscopy. roughness of chromium doped dlc layers was measured on si substrates about 0.2-0.7 nm and on metallic substrates about 1-6 nm. mechanical properties were studied by nanoindentation. hardness and reduced young's modulus were reduced with rising the cr content. hardness and reduced young's modulus reached from 15.0 gpa to 31.2 gpa and from 172.7 to 271.5 gpa, respectively. films adhesion was determined by scratch test and reached 19 n for titanium substrates and for highest cr concentration. good adhesion to biomedical alloys and high dlc hardness will help to progress in the field of implantology. 1. introduction tissue grows around and into all forms of carbon [1–3]. diamond-like carbon (dlc) layers have potential applications in orthopaedics, cardiology, ophthalmology, nephrology, and other fields of medicine [1, 4–9]. dlc layers are ideal for coating stents, heart valves, because of its excellent haemocompatibility and reduced thrombus formation, and for joint implant protective films [10]. dlc have unique mechanical (high hardness = 1 − 80 gpa), tribological (low friction coefficient ≈ 0.01 − 0.7, high wear resistance), optical (transparency in ir and vis), topological (nanosmooth), and biological (chemical inertness, no cytotoxicity, good biocompatibility and haemocompatibility) properties [1, 11–13]. these properties can be modified and adapted for specific medical applications by using doping. dopants in dlc layers produce changes in hardness, coefficient of friction, resistance to wear, surface roughness, adhesion, and even biocompatibility and biological properties [13, 14]. the scheme of typical dopants and supposed change of dlc properties is in figure 1 [13]. the influence and properties of a lot of proposed elements was successfully tested and characterized till now. in our case, the doping of chromium was used to change the mechanical properties and increase the adhesion dlc layers to biomedical alloys. 2. experimental 2.1. depositions chromium doped diamond like carbon layers (crdlc) were deposited by hybrid deposition system consists of pulsed laser deposition (pld krf excimer laser) and magnetron sputtering (ms). pld technique was used for the deposition of dlc and ms technique for doping chromium. deposition parameters are summarized in table 1. figure 2 shows the schematic diagram of the deposition system used to prepare the cr-dlc film samples. silicon (100) wafers and biomedical alloy (ti-6al-4v) were used as substrates. ti-6al-4v substrate was polished before deposition (roughness was 7.7 ± 1.2 nm). 2.2. characterization chromium content of cr dlc layers was determined by a wavelength dispersive x-ray spectroscopy (wds jeol 840). more information can be found in [12]. adhesion macro scratch tester (revetest scratch tester csm co.) was used to determination of the adhesion [15]. scratch test parameters are summarized in table 2. an optical microscope was used to evaluate the critical force. hardness and reduced young's modulus nanohardness was measured on the nanotest system with 20 http://dx.doi.org/10.14311/app.2017.8.0020 http://ojs.cvut.cz/ojs/index.php/app vol. 8/2017 mechanical properties of cr-dlc layers figure 1. scheme of proposed doping elements of dlc [13]. figure 2. the scheme of hybrid deposition system: 1 substrate, 2 carbon target, 3 laser beam, 4 magnetron with cr target, 5 magnetron stopper, 6 rotation, 7 rf, 8 vacuum gauge, 9 tbm, 10 gas flow (ar). laser repetition rate laser energy density magnetron power wds cr content hardness reduced young's modulus [hz] [j·cm−2] [w] [at. %] [gpa] [gpa] cr-dlc-0 20 8 0 0 31.2 ± 1.4 271.5 ± 16.8 cr-dlc-1 40 8 40 1 24.8 ± 1.1 236.3 ± 11.4 cr-dlc-2 20 8 40 2 21.8 ± 0.8 217.7 ± 4.4 cr-dlc-3 10 8 40 4 5 17.5 ± 0.9 183.9 ± 9.5 cr-dlc-4 3 8 40 15 17 15.0 ± 0.6 172.7 ± 3.4 table 1. deposition parameters, chromium content and mechanical properties of dlc and cr doped dlc layers. indenter parameters diamond type, rockwell, radius: 200 µm normal load linearly ramped minimum 1 n and maximum 30 n scratch length 8 mm scratch speed 10 n·min−1 critical loads lc2 edge spallation critical loads lc3 spallation inside the groove table 2. scratch test parameters. 21 p. písařík, miroslav jelínek, j. remsa, z. tolde acta polytechnica ctu proceedings figure 3. afm scans of the surfaces. samples with various concentrations of chromium: afm of pure dlc (left), dlc with 15-17 at. % of cr (right). figure 4. hardness and reduced young's modulus of the dlc and cr-dlc films with various cr contents. a berkovich diamond tip. maximum indentation depth of the indenter was 10 % (about 60 nm), to minimize the effect of substrate on the resulting hardness. the presented values are averages of 20 measurements. hardness and reduced young's modulus was calculated according to [16]. 3. results and discussion cr dlc films were synthesized using hybrid system (magnetron sputtering and pulsed laser deposition) of carbon and chromium target. in dependence with deposition conditions the cr content moved from 1 at. % to ∼ 17 at. % (wds), and the thickness was 800 ± 100 nm on titanium substrate and 100 ± 30 nm on silicon substrate. afm showed that the layers were smooth, but with small amounts of random droplets see figure 3. roughness (ra) were calculated from 10×10 µm area (software nova p9) [15]. ra of our layers was measured on si substrates about 0.2-0.7 nm and on metallic substrates about 1-6 nm. adhesion diamond like carbon have poor adhesion to metal alloys [1]. baragetti [17] presented that diamond like carbon on aluminium alloy had critical load (lc3) about 3.5 n. two scratches were made in each layer and the adhesion was evaluated using a ccd micrograph. due to the substrate roughness caused by polishing, the character of load trace was not monotonous. this fact complicated the determination of critical load parameters. the first spallation presence was used as the critical load value. results of scratch tests measurement showed that the critical load (lc3) for the pure dlc films was about 14 n and for cr dlc films ranged from 15.3 n (1 at. %) to 19 n (15-17 at. %). the same trend was observed in [15]. the higher value of adhesion for pure dlc layers (compared to baragetti group) can be due to the application of the rf cleaning before the deposition and a further increase was reached by chromium doping. hardness and reduced young's modulus the applied maximum load for the film was observed at the depth of 60 nm. nanoindentation also makes it possible to specify the modulus of elasticity. the determination of the reduced young's modulus for the instrumentally performed hardness test is based on the inclusion of the elastic strain of the given ma22 vol. 8/2017 mechanical properties of cr-dlc layers terial and also of the testing indenter. the hardness and reduced young's modulus values for the diamondlike carbon film were determined to be ∼ 31.2 and ∼ 271.5 gpa, respectively see table 1 and figure 4. hardness of cr-dlc layers prepared by pld+ms decreased from 24.8 gpa (1 at. %) to 15.0 gpa (15-17 at. %), as the cr content increased, as well as reduced young's modulus of 236.3 gpa (1 at. %) to 172.7 gpa (15-17 at. %) see table 1 and figure 4. 4. conclusion this paper focuses on doped dlc films (cr-dlc films on silicon and titanium alloy). the layers were prepared using hybrid laser technology (pld+ms). the cr content moved from 1 at. % to ∼ 17 at. %. afm showed that the layers were smooth, but with small amounts of random droplets. with the cr content rise, both the hardness and young's modulus were reduced from 31.2 gpa to 15.0 gpa and 271.5 gpa to 172.7 gpa, respectively. scratch test showed that critical load lc3 rose with cr content and that the maximum critical load lc3 was 19 n. good adhesion on biomedical alloys and high hardness dlc will help to progress in the field of implantology. acknowledgements this work has been supported by the grant agency of the czech technical university in prague (grant no. sgs16/111/ohk4/1t/17 and sgs16/190/ohk4/2t/17), the grant agency of the czech republic (grant no. ga15 05864s) and the ministry of education, youth and sports of the czech republic (grant no. lo1409). references [1] r. j. narayan. functionally gradient hard carbon composites for improved adhesion and wear. ph.d. thesis, north carolina state university, 2002. [2] m. jelínek, k. smetana, t. kocourek, et al. biocompatibility and sp3/sp2 ratio of laser created dlc films. materials science and engineering: b 169(1-3):89–93, 2010. doi:10.1016/j.mseb.2010.01.010. [3] m. jelínek, t. kocourek, j. remsa, et al. diamond/graphite content and biocompatibility of dlc films fabricated by pld. applied physics a 101(4):579–583, 2010. doi:10.1007/s00339-010-5912-9. [4] t. hasebe, y. matsuoka, h. kodama, et al. lubrication performance of diamond-like carbon and fluorinated diamond-like carbon coatings for intravascular guidewires. diamond and related materials 15(1):129– 132, 2006. doi:10.1016/j.diamond.2005.08.025. [5] e. t. uzumaki, c. s. lambert, w. d. belangero, et al. evaluation of diamond-like carbon coatings produced by plasma immersion for orthopaedic applications. diamond and related materials 15:982–988, 2006. doi:10.1016/j.diamond.2005.12.006. [6] l. zhang, p. lv, z. y. huang, et al. blood compatibility of la2o3 doped diamond-like carbon films. diamond and related materials 17:1922–1926, 2008. doi:10.1016/j.diamond.2008.04.011. [7] t. hasebe, t. ishimaru, a. kamijo, et al. effects of surface roughness on anti-thrombogenicity of diamond-like carbon films. diamond and related materials 16(4-7 spec. iss.):1343–1348, 2007. doi:10.1016/j.diamond.2006.12.009. [8] g. dearnaley, j. h. arps. biomedical applications of diamond-like carbon (dlc) coatings: a review. surface and coatings technology 200(7):2518 – 2524, 2005. doi:10.1016/j.surfcoat.2005.07.077. [9] p. písařík, m. jelínek, k. smetana, et al. study of optical properties and biocompatibility of dlc films characterized by sp3 bonds. applied physics a 112(1):143–148, 2013. doi:10.1007/s00339-012-7216-8. [10] r. hauert, u. mãijller. an overview on tailored tribological and biological behavior of diamond-like carbon. diamond and related materials 12(2):171 – 177, 2003. proceedings of the 4th specialist meeting on amorphous carbon, doi:10.1016/s0925-9635(03)00019-0. [11] j. robertson. diamond-like amorphous carbon. materials science and engineering: r: reports 37(46):129 – 281, 2002. doi:10.1016/s0927-796x(02)00005-0. [12] p. gupta. synthesis, structure and properties of nanolayered dlc/dlc films. louisiana state university, 2003. [13] j. c. sánchez-lópez, a. fernández. doping and alloying effects on dlc coatings, pp. 311–338. springer us, boston, ma, 2008. doi:10.1007/978-0-387-49891-1_12. [14] p. písařík, m. jelínek, t. kocourek, et al. chromium-doped diamond-like carbon films deposited by dual-pulsed laser deposition. applied physics a 117(1):83–88, 2014. doi:10.1007/s00339-013-8206-1. [15] m. jelinek, t. kocourek, j. zemek, et al. chromiumdoped {dlc} for implants prepared by laser-magnetron deposition. materials science and engineering: c 46:381 – 386, 2015. doi:10.1016/j.msec.2014.10.035. [16] w. oliver, g. pharr. an improved technique for determining hardness and elastic modulus using load and displacement sensing indentation experiments. journal of materials research 7(6):1564–1583, 1992. doi:10.1557/jmr.1992.1564. [17] s. baragetti, l. lusvarghi, g. bolelli, f. tordini. fatigue behaviour of 2011-t6 aluminium alloy coated with {pvd} wc/c, pa-cvd {dlc} and pe-cvd siox coatings. surface and coatings technology 203(20-21):3078 – 3087, 2009. doi:10.1016/j.surfcoat.2009.03.040. 23 http://dx.doi.org/10.1016/j.mseb.2010.01.010 http://dx.doi.org/10.1007/s00339-010-5912-9 http://dx.doi.org/10.1016/j.diamond.2005.08.025 http://dx.doi.org/10.1016/j.diamond.2005.12.006 http://dx.doi.org/10.1016/j.diamond.2008.04.011 http://dx.doi.org/10.1016/j.diamond.2006.12.009 http://dx.doi.org/10.1016/j.surfcoat.2005.07.077 http://dx.doi.org/10.1007/s00339-012-7216-8 http://dx.doi.org/10.1016/s0925-9635(03)00019-0 http://dx.doi.org/10.1016/s0927-796x(02)00005-0 http://dx.doi.org/10.1007/978-0-387-49891-1_12 http://dx.doi.org/10.1007/s00339-013-8206-1 http://dx.doi.org/10.1016/j.msec.2014.10.035 http://dx.doi.org/10.1557/jmr.1992.1564 http://dx.doi.org/10.1016/j.surfcoat.2009.03.040 acta polytechnica ctu proceedings 8:20–23, 2017 1 introduction 2 experimental 2.1 depositions 2.2 characterization 3 results and discussion 4 conclusion acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2017.10.0001 acta polytechnica ctu proceedings 10:1–6, 2017 © czech technical university in prague, 2017 available online at http://ojs.cvut.cz/ojs/index.php/app nontraditional support of the western portal of the tunnel považský chlmec jan faltýnek czech technical university in prague, faculty of civil engineering, thákurova 7, prague 6, czech republic correspondence: jan.faltynek@hochtief.cz abstract. the paper is focused on general description of support elements used on the western portal of the tunnel považský chlmec in slovakia, near the city of žilina. it presents basic aspects of the method of the shell structure. in this case the shell structure in combination with pile walls was used. you can also find here detailed description of geotechnical behaviour together with the setting up of the numerical model constructed in plaxis. conclusions of important points from the entire paper including results evaluation you can find at the end of the paper. keywords: tunnel, finite element method (fem), plaxis, shell structure, pile wall, anchor. 1. introduction two-tube tunnel považský chlmec with length of about 2.2 km forms the major part of technically very challenging section žilina (strážov) žilina (brodno) of future highway d3 in the slovak republic. the route forms the western bypass of the city of žilina. the excavation in the area with flysh evolution represents for the designer and also the contractor of the tunnel solving many difficult geotechnical problems. one of them is the technical solution of the construction pit of the western tunnel portal, which was key for compliance with the construction schedule because of the planned construction process. the competition for the contract was launched and construction is still in progress in design and build mode (db) specifying the conditions according to fidic yellow book. it allows the designer and contractor to optimize technical solutions, under the conditions specified in the tender documentation. limited areas of permanent works, tight deadlines of contractual milestones of construction and complicacy of geotechnical conditions in the area led the designer and contractor of the company hochtief cz a. s. [1] to change the concept of securing the stability of the construction pit of the western portal and following excavation beside the original draft from the previous step of project documentation. after considering all options, it was decided to use a method of the shell structure. 2. method of the shell structure method of the shell structure could be classified to the hybrid cover & cut methods that combine various aspects of the excavated and cut & cover underground structures. first part of a construction process is a cutting of a ground down to the level of the lower surface of the shell structure (figure 2 stage 2). that is concreted (figure 2 stage 3) and generally backfilled after curing. the material under the shell structure could be excavated at the same time with the landscaping of the backfilled surface (figure 2 stage 4). this is very positive in terms of time of construction process. there is a big difference from the classic milan method in the shape of the supporting structure, which is in this case vaulted. the vault behaviour of the structure is very positive from the structural point of view. the option to keep the cross section the same like in the excavated part of the tunnel is also advantageous. you can use the same formwork car. method of the shell structure is particularly used for tunnels that are carried shallowly, where can be problem during excavation with breaking the cover of a tunnel, or in landslide areas where the landslide could be in a portal construction pit activated. economically, this method leads to savings earthworks and difficult enumerable reduction of the risk of construction. 3. history of this method and its modifications method of the shell structure is in czech called literally “turtle method”. the term ‘turtle’ was created during the construction of the eastern portal of branisko tunnel (1996), in the slovak republic and its author is ing. ermín stehlik. in this case the shell structure was on one tunnel tube monolithic and on the other tube prefabricated. it is another point of interest of this tunnel. method of the shell structure is quite widespread abroad, for construction in and outside of the urban areas. in the czech republic, this method was first used on the railway section zábřeh krasíkov, tunnels malá huba and hněvkovský i. 3.1. shell structure found on plain ground a shallow tunnel, where the rock strength increases very quickly with the depth, is a typical example of 1 http://dx.doi.org/10.14311/app.2017.10.0001 http://ojs.cvut.cz/ojs/index.php/app jan faltýnek acta polytechnica ctu proceedings figure 1. section situation of the d3 motorway žilina (strážov) žilina (brodno). figure 2. the construction process of the basic method of the shell structure shell structure found on plain ground figure 3. the eastern portal of the tunnel branisko, left monolithic, right prefabricated shell structure (photo by ing. libor mařík [2]) this type of construction. this leads to the use of better quality rock mass as a support for the shell structure. it means that a shell structure rests only on the plain ground. this solution was used for example in tunnels hněvkovský i and schellenberg in germany. figure 4. the shell structure on the schellenberg tunnel (photo by ing. jiří zmítko) 3.2. shell structure additionally supported in worse geological conditions the base part of shell structure is additionally supported due to uneven deformation by columns of jet grouting, micropiles or by using sn rock bolts. this solution was used for example in sudoměřice tunnel and in bypass around 2 vol. 10/2017 nontraditional support of the tunnel považský chlmec the town of jihlava (i / 38), where the shell structure was concreted on low-formwork. another example is the use of this type of structure in the central construction pit of the tunnel povážský chlmec. there was the shell structure supported by micropile walls as well as in the case of the western portal of žilina tunnel. figure 5. the shell structure supported by micropiles in the central pit of the tunnel považský chlmec (photo by ing. libor mařík [2]) 3.3. shell structure with foundations fixed to sheeting construction in the case of very unpleasant geotechnical conditions it is necessary to secure the lateral sides of the tunnel over the entire height in another way milan or pile wall. the shell structure in this case is fixed into sheeting structure and form the support (strut) level of the vertical sheeting elements to ensure the safety of the construction pit. this complex and unique solution has been used in euerwang tunnel on high-speed line nuremberg-ingolstadt in germany and považský chlmec tunnel in the slovak republic on the western portal which technical solution is the main topic of this paper. figure 6. the fixed shell structure on the euerwang tunnel (photo by ing. jiří zmítko) 4. original way of securing the western portal and start of excavation the original way of securing the western portal by tender documentation (which was not obligatory) consisted of the slope stabilization of construction pit by layer of shotcrete with two concrete reinforcing meshes. layers of surface soil were secured by nailing. sn rock bolts were used in the lower parts where occurrence of disintegrated rock was supposed. prestressed stranded anchors passing through reinforced concrete walers were among the other elements ensuring the stability of the construction pit. in this way arranged construction pit the classic cut & cover tunnel should be constructed. initiation of the excavation was assumed under the protection of micropile roof piping. 5. new way of securing the western portal the contractor hochtief cz a. s. decided to change the concept of securing the construction pit after the thorough consultation with the designer at the stage of processing of tender documentation. the method of the shell structure combined with anchored large-diameter pile walls was used for securing the portal area. pile walls work as a construction pit sheeting and also as a protection of the sides of the tunnel during the subsequent excavation under the shell structure roofing. this change reduced volume of earthworks and activities with the necessary technological pauses, e.g. concreting of the reinforced walers, implementing multilevel anchors etc. the size of the territory affected by construction and the number of anchoring elements were reduced too. the length of cut & cover tunnel was shortened to one concreting block. the risk of losing stability of a high slope decreased. the important point was also the possibility of an earlier commencement of the excavation on the western portal. the faces of the excavation pit are stabilized by anchored steal sheet pile walls. (1.) excavation of shallow sloped construction pit at a gradient of 1:1.5 at the level of drilling of the large-diameter pilot walls; (2.) drilling and subsequent concreting of the reinforced concrete piles 0,8 m in diameter length of 16 m, 18 m and 19 m with an axial distance of 1 m; (3.) excavation on the first anchor level; (4.) aking concreate walers with dimensions of 0.7 x 0.8 m, making and activation of prestressed stranded anchors length of 16 m and 18 m drilled with axial distance of 2 m; (5.) excavation of the construction pit at the level of the bottom shape of the shell structure; 3 jan faltýnek acta polytechnica ctu proceedings figure 7. original way of securing the western portal (6.) creating a niche in every individual pile, which is used for connection of the reinforcement of the shell structure with the reinforcement of piles (in each pile was added additionally glued anchor); (7.) profiling of the base of foundation pit by plane concrete into its final form, laying separating foil and geotextile; (8.) reinforcing and concreting the shell structure block by block (thickness of the structure in the crown is 0.6 m, the arch span is 10.78 m with the rise of 3.53 m, final length on southern (right on the picture) tunnel tube 37.5 m and on northern (left on the picture) tube 50 m); (9.) excavation of top heading and bench of the tunnel (excavation with horizontal sequence, unlimited length advance thanks to good workability by tunnel bagger); (10.) additional supporting of the foundation of the shell structure and shaping the sides of the tunnel by shotcrete lateral beams; (11.) making of the backfill (the shell structure was filled up after the tunnel excavation because of the need to meet a milestone for the beginning of the excavation, another reason was structural, because of the shell structure which had to be filled after making the supporting lateral shotcrete beams). 6. geological conditions at the western portal the excavation is in the upper part realized in finegrained deluvial soil with the occurrence of fragments and rubble. they passed with increasing depth into the gravel-clay soil of fluvial terraced steps. the lower part of the construction pit was excavated in the zone variously weathered flysch formation (sandstones, mudstones, siltstones, conglomerates). 7. numerical model of securing the western portal the complexity of static calculations was based mainly on uncertainty about the actual behaviour of the structural elements (prestressed stranded anchors, large diameter pile walls, shell structure) and consideration of these ideas in the mathematical model. the most important thing for the creation of a numerical model is in this case the reliable and accurate interaction between the shell structure, sheeting wall and cooperating rock mass considering all phases of construction and the relevant load cases. the connection between the shell structure and the pile wall is capable of transmitting a certain inner forces. its real stiffness but depends primarily on the quality of the execution. during the processing of the project there were a large amount of computations, whose aim was to determine the effect of input variables. secondary lining under the shell structure was examined in detail in the last phase stage simulating end of life of the construction pit supporting system where backfill weight rests directly on the secondary lining. calculations were performed by finite element method in program plaxis. the dimensions of the solved area were chosen so that the results were not affected by the boundary conditions. the finite element mesh in areas of interest was refined. all rock and soil layers were considered as plane elements with the mohr-coulomb plasticity condition. the shell structure, pile wall, shotcrete supports and secondary lining were modelled as linear plane element. to the centreline of these structural plane elements was inserted so called “reference beam”, which has a considerably smaller bending and axial stiffness. it deforms simultaneously with the structure. thanks to it, there is no need to integrate internal forces from the stress in the individual sections. the interaction between structural elements and the rock mass as well as between the individual elements together was modelled using a specially tested contact interface element. the reached ground water level and ground water flow due to the excavation of the bench was also considered in the calculation. it was very important in the numerical model to take into account all necessary load cases. the weather load (mainly temperature) and shrinkage was modelled as prescribed volume strain of finite elements. 4 vol. 10/2017 nontraditional support of the tunnel považský chlmec figure 8. new way of securing the western portal surroundings of žilina is located in a seismic zone. this effect was taken into consideration into the model using quasi-static calculation. figure 9. scheme of a basic numerical model 8. results of numerical modelling the mathematical model confirmed the smooth feasibility of designed construction system. first the slope stability of the construction pit excavated at the level of pile walls was determined by the phi-c reduction method. with “reference beam” internal forces acting on the pile wall were obtained. the bearing capacity of stranded anchors was also evaluated. as mentioned above, the greatest influence on the interaction of the whole system has a connection between the shell structure and pile walls. connection height is about 1.5 m. it is very difficult to predict how a given node will behave. therefore, it is necessary to monitor and evaluate stress at this point in principal stresses. from the structural point of view, the decisive stage is the construction phase after the completion of the excavation under the shell structure. the positive impact of the backfill on the distribution of stresses on the shell structure and pile walls was also confirmed. figure 10. envelopes of internal forces on the pile wall from left to right (nmax = -695 kn, vmax = 550 kn, mmax = 900 knm) 9. conclusion if predicted and actual geological conditions should be compared, it could be said, that sufficient consensus has been reached. geotechnical monitoring of the shell structure consisted of determining the relative deformation by strain gauge sensors located in the major blocks in the top of the vault at both surfaces. it allowed to compare the results of a numerical model with reality. an interesting finding was that the shell structure on north and south tunnel tube behaves differently in reality. a comparison of the data after the excavation under the shell structure with the results of the monitoring are quite different. both northern and southern structures in the case of a numerical model behave similarly (the upper fibres of a cross section are in tension, lower are compressed). acceptable sim5 jan faltýnek acta polytechnica ctu proceedings figure 11. distribution of principal stresses after the excavation stage (left) and after the final stage (right) (compression red and tension blue colour) figure 12. shell structure on the western portal of the tunnel povážský chlmec ilarity is evident when comparing the southern shell structures. the results from the measurement on the north shell structure are not very close. that is also the same case for other construction stages. simplifications of the numerical model (material model of construction elements and surrounding soil and rock, sub-horizontal layers etc.) and the real behaviour of the structural elements (the value of the prestressing in the anchors, the real execution of the connection between structures) may be a possible cause. as you can see even the most complex mathematical model may not always accurately capture the geotechnical behaviour of the real structure. the shell structure fixed into the sheeting structure belongs to the most unique and less conventional design solutions. the designer and contractor despite it managed to design and build a structure without significant problems during construction. generally, it could be said, that the correct optimization of the solution of the western portal had contributed to reducing the risks during construction and speeded up the construction progress. at the time of writing this paper the construction is completed successfully, backfilled and supported by final lining of monolithic concrete. it allows the above mentioned recap. references [1] hochtief cz a.s. zds a rds tunelu povážský chlmec 2015-2016. [2] l. mařík. povážský chlmec tunnel on žilina (strážov) žilina (brodno) section of the d3 motorway from design to realisation. tunel 3:88 – 103, 2015. 6 acta polytechnica ctu proceedings 10:1–6, 2017 1 introduction 2 method of the shell structure 3 history of this method and its modifications 3.1 shell structure found on plain ground 3.2 shell structure additionally supported 3.3 shell structure with foundations fixed to sheeting construction 4 original way of securing the western portal and start of excavation 5 new way of securing the western portal 6 geological conditions at the western portal 7 numerical model of securing the western portal 8 results of numerical modelling 9 conclusion references 212 acta polytechnica ctu proceedings 2(1): 212–216, 2015 212 doi: 10.14311/app.2015.02.0212 detection of diatomic molecules in the dust forming nova v2676 oph m. nagashima1, a. arai1,2, t. kajikawa1, h. kawakita1, e. kitao1, t. arasaki1, g. taguchi1, y. ikeda1 1kyoto sangyo university, koyama astronomical observatory 2university of hyogo, nishi-harima astronomical observatory corresponding author: g0837714@gmail.com abstract novae are generally considered to be hot astronomical objects and show effective temperatures up to 10,000 k or higher at their visual maximum. but, it is theoretically predicted that the outer envelope of the nova outflow can become cool enough to form molecules that would be dissociated at high temperatures. we detected strong absorption bands of c2 and cn radicals in the optical spectrum of nova v2676 oph, a very slow nova with dust formation. this is the first report of the detection of c2 and the second one of cn in novae during outburst. although such simple molecules are predicted to form in the envelope of the outflow based on previous studies, there are few reports of their detection. in the case of v2676 oph, the presence of the molecular envelope is considered to be very transient, lasting several days only. keywords: cataclysmic variables classical novae optical spectroscopy ir individual: v2676 oph. 1 introduction dust formation in the outflow of a nova had been proposed by mclaughlin (1935) to explain the rapid drop in the visual light-curve of dq her in 1934. dust formation in fh ser had also been confirmed by infrared observations by geisel et al. (1970). dq her was the first nova in which molecular absorption bands of cn in optical wavelength had been identified. the formation of molecules as the precursor to dust grains in novae is considered important for understanding how dust grains form in the outflow of novae. in the case of dq her in 1934, strong cn absorption bands of both violet and red systems had been detected merely 2 days after the visual maximum, and these absorption bands were identified for only 1 week approximately (wilson & merrill, 1935; sanford, 1935; stoy & wyse 1935; antipova 1969; sneden & lambert, 1975). since the formation of simple molecules such as cn is considered an intermediate process in the formation of dust grains from the hot atomic gas in the outflow of a nova, molecular formation in the early phase of dq her might be associated with dust formation its later phase. although cn is the first molecule observed in a nova during outburst (in the case of dq her), there are no further reports about cn in other novae. whereas, carbon monoxide (co) emission in the early phase of novae has been observed by both photometric and spectroscopic observations. in particular, the first overtone band of co (∆v = 2) has been routinely detected in near-infrared spectra of several novae, as reviewed by evans & rawlings (2008). based on previous observations, the correlation between detection of co and dust formation is noticeable. however, hydrogen (h2) or other molecules have not detected in novae during the early phase of their outburst. here we report the detection of c2 and cn in optical spectra of the classical nova v2676 oph during the early phase of its outburst. this nova could be classified as a slow nova, and it showed a rapid drop in its visual light curves about 90 days after its discovery. this is the first report of the detection of c2 in novae during outburst and the second for cn. furthermore, co emission in the near-infrared had been detected in this nova (rudy et al., 2012). 2 observations nova v2676 oph (pnv j17260708-2551454) was discovered at ut 2012 mar 25.789 (t = 0 day) by h. nishimura (reported in the central bureau electronic telegram (cbet) 3072). after the discovery of the nova, we carried out spectroscopic observations with the low-dispersion spectrograph losa/f2 (shinnaka et al., 2013) mounted on the 1.3-m araki telescope at koyama astronomical observatory on ut 2012 mar 27. on the 212 http://dx.doi.org/10.14311/app.2015.02.0212 detection of diatomic molecules in the dust forming nova v2676 oph first night, we detected narrow balmer emission lines (both hα and hβ) and narrow fe ii emission lines on a highly reddened continuum that seemed to be due to interstellar extinction. the color excess e(b − v ) was estimated by using the balmer decrement and the color of the nova (b−v ); it was determined as 0.71±0.02 and 0.72±0.06, respectively. based on the p cygni profile of the hα emission, the expansion velocity was estimated to be ∼ 800 km/s. we concluded that the object was a fe ii-type classical nova in the early phase (arai & isogai, 2012). after the first observation of v2676 oph, we continued the observation of it routinely to assess its the spectroscopic evolution (nagahsima et al., 2014). 4500 5000 5500 6000 6500 7000 7500 n or m al iz ed s pe ct ru m wavelength [å] ∆v=+1 ∆v=0 ∆v=−1 c 2 ∆v=+5 ∆v=+4 ∆v=+3 cn hβ na i hα o i v2676 oph tx psc figure 1: comparison of the spectrum of v2676 oph obtained on apr 8 with that of a typical carbon star, tx psc. tick marks indicate telluric absorption lines. after the first spectroscopic observations on ut 2012 mar 27, the emission lines in the optical spectra became fainter relative to the continuum (on ut 2012 mar 28, apr 4 and 6), while the optical brightness was increasing slowly (the optical brightness changed gradually from 12 to 11 magnitudes in the v-band, see the american association of variable star observers (aavso) database, http:// http://www.aavso.org/lcg/). no emission lines could be observed (except hα emission with a p cygni profile), but many absorption lines of fe ii and neutral atoms such as na i (5890å) and o i (7773å) were detected clearly in the spectra obtained on apr 6 (t = 12 days). those absorption lines are indicative of lower ionization and the lower temperature conditions in the outflow of the nova. prominent c2 (swan) and cn (red system) absorption bands were detected on ut 2012 apr 8 (t = 14 days), as shown in figure 1. the obtained spectrum is similar to that of a carbon star. we also plotted the spectrum of tx psc (a well-known carbon star of spectral type n0;c6,2, with teff = 3030 k; lambert et al.,1986) for comparison. we also identified weak emission lines of hα, hβ, and fe ii. based on the substructure of the c2 swan band (∆v = -1) absorption, we could derive the isotopic ratio of carbon. figure 2 shows the spectrum of the nova, the modeled spectra of 12c12c, 12c13c, and 13c13c (with an excitation temperature of 4500k). the doppler shift of the nova spectrum has been corrected by using the relative velocity of the nova to the observer, estimated as 341±87 km/s (this is derived from the velocities for hα and hβ emission peaks). the wavelengths of sub-peaks in this band cannot be explained by 12c12c only. clearly 213 m. nagashima et al. 12c13c and 13c13c contribute to form the absorption. 0.75 0.8 0.85 0.9 0.95 1 1.05 1.1 5400 5450 5500 5550 5600 5650 5700 5750 n or m al iz ed fl ux wavelength [å] 13 13 c c 13 12 c c 12 12 c c apr. 08.7 figure 2: comparison between observed and modeled spectra of the c2 (∆v = -1) absorption band. the observed spectrum is shown by dashed line (the region influenced by the hg emission from the city-light, at ∼ 5460 å, has been removed). the modeled spectra for 13c13c, 12c13c and 12c12c are shown by the thin solid lines and the sum of those lines is shown by the thick solid line. for the modeled spectra, we assumed the rotational and vibrational temperatures of 4500 k and the isotopic ratio of 12c/13c = 4. the next observations were carried out on ut 2012 apr 16 (t = 22 days). in these observations, the c2 and cn absorption bands had already disappeared and strong balmer emission lines and fe ii lines were again prominent. at that time, the spectra were typical of fe ii-type classical novae. the other difference from the previous observations was the expansion velocity derived from hα, which had increased in comparison to that before molecular formation. this higher velocity is typical of the fe ii-type novae (williams 1992). thereafter, the spectra of this nova were not unusual for an fe ii-type nova, although we continued spectroscopic monitoring observations until ut 2012 may 26. the optical light curves showed a very slow decline after the visual maximum and a rapid drop (by about six magnitudes in the v-band) at around 90 days after the discovery (this nova can be classified as a ”slow” nova). the drop in the light curves may be caused by dust formation in the outflow of the nova. 3 results & discussion the equivalent widths of hα (and also of hβ) measured in our spectra were almost constant before and after the appearance of molecular absorption bands. however, optical light curves showed a small drop of ∼ 1 magnitude before and after molecular formation. figure 3 shows the optical and near infrared light curves taken from the database of the aavso and small and moderate aperture research telescope system (smarts), the color indices of (v − i) and the equivalent width of hα and hβ. for example, the c2 absorption could markedly affect band v-band magnitudes, while cn red absorption could also affect rand i-bands. this implies that extinction of the molecules in the outflow affected both continuum and emission lines from the nova. the molecular formation zone could be in the outer region of the outflow compared with regions emitting the continuum and/or emission lines in the nova. 1.2 1.4 1.6 1.8 2 v-i (aavso) v-i (smarts) -600 -500 -400 -300 -200 -100 0 0 10 20 30 40 50 60 70 80 90 100 110 120 hα hβ 5 10 15 20 25 v-mag. (aavso) v-mag. (smarts) i-mag. (aavso) i-mag. (smarts) k-mag. (smarts) figure 3: multi band light curve of v2676 oph (data from aavso and smarts), color indices (v −i) based on the aavso and smarts database, and equivalent widths of hα and hβ measured in our spectra. upper tick marks indicate days on which spectroscopic observations were performed. furthermore, molecular formation in v2676 oph is considered very rapid (within 2 days or less) and the existence of the molecular envelope was transient (it was present at most 9 days) at around the brightness maximum in optical. why was the appearance of both c2 and cn absorption bands so transient that they could be detected on apr 8 only? we considered that the outer region of the outflow became cool enough to form molecules, since the hard ultraviolet (uv) radiation from the pseudo-photosphere of the nova in the early phase was blanketed by an giron curtainh, and the iron ions could have absorbed uv radiation strongly (shore, 2008). this picture is consistent with the spectrum taken on apr 6, which was dominated by a continuum with absorption lines indicative of lower ionization conditions. the measurements of the color indices (v − i) obtained from the aavso and smarts database also showed a redder continuum for later periods after the discovery until the molecular bands appeared. this fact also supports the later lower color temperatures until molecular formation. however, as the envelope expanded and ejected materials rarefied (i.e., became more optically thin), hard uv radiation again increased in intensity. at this time, molecules would be destroyed 214 detection of diatomic molecules in the dust forming nova v2676 oph through photo-dissociation reactions caused by uv radiation. in support of this, the later spectra showed many emission lines from ionized species, such as fe ii. however, the molecular absorption bands may have disappeared due to some opacity effects. similar behavior in terms of cn formation was observed in dq her in 1934. the appearance of cn absorption bands immediately following the optical brightness maximum was transient, persisting for approximately 1 week only (sneden & lambert, 1975). the possible dust formation about 100 days after discovery was also similar to v2676 oph. theoretical studies of chemistry in the outflow of novae suggest that formation of even more complex molecules is possible (pontefract & rawlings, 2004; evans & rawlings, 2008). it has been demonstrated that a model atmosphere could reproduce both strong cn absorption bands in optical and the co emission band in the nearinfrared, as observed in some novae (hauschildt et al., 1994). although simple molecules might be destroyed by uv radiation, more complex molecules such as polycyclic aromatic hydrocarbons (pahs) (if they formed during the transient cool phase of the outer envelope), could survive and might act as nuclei for dust formation. indeed, pah emission was detected in this nova. we performed mid-infrared spectroscopic observation using a cooled mid-infrared camera and spectrometer (comics) mounted on the subaru telescope on ut 2013 june 20 (t = 452 days). the spectrum showed pah emission at 11.4 µm (and a hint of the emission line at 7.7 µm) on the smooth continuum that could be explained by amorphous carbon grains. 1 1.5 2 2.5 3 3.5 4 4.5 5 8 9 10 11 12 13 f lu x de ns ity [1 0-1 8 w /c m 2 /µ m ] wavelength [µm] pah figure 4: the mid-infrared spectrum of v2676 oph. references [1] mclaughlin, d.b. 1935, publ. aas, 8, 145. [2] geisel, s.l., kleinmann, d.e., low, f.j. 1970, astrophysical journal, 161, l101. doi:10.1086/180579 [3] wilson, o.c. & merrill, p.w., 1935, publications of the astronomical society of the pacific, 47, 53. [4] sanford, r.f. 1935, publications of the astronomical society of the pacific, 47, 209. [5] stoy, r.h. & wyse, a.b. 1935, publications of the astronomical society of the pacific, 47, 50. [6] antipova, l.i. 1969, soviet astronomy a.j., 13, 288. [7] sneden, c. & lambert, d. 1975, monthly notices of the royal astronomical society, 170, 533. doi:10.1093/mnras/170.3.533 [8] evans, a. & rawlings, j.m.c. 2008, in classical novae, 2nd edition. edited by m.f. bode and a. evans (cambridge astrophysics series, no. 43, cambridge: cambridge university press, 2008), p.308. [9] rudy, r. j., et al. 2012, electronic telegram no. 3103, central bureau for astronomical telegrams, international astronomical union (ed., green, d.). [10] nishimura, h. 2012, electronic telegram no. 3072, central bureau for astronomical telegrams, international astronomical union (ed., green, d.). [11] shinnaka, y., kawakita, h., kobayashi, h., naka, c., arai, a., arasaki, t, kitao, e., taguchi, g., ikeda, y. 2013, icarus, 222, 734. doi:10.1016/j.icarus.2012.08.001 [12] nagashima, m., et al. 2014, astrophysical journal letter, 780, 26 doi:10.1088/0004-637x/780/1/26 [13] arai, a. & isogai, m. 2012, electronic telegram no. 3072, central bureau for astronomical telegrams, international astronomical union (ed., green, d.). [14] lambert, d.l., gustafsson, b., eriksson, k., hinkle, k.h. 1986, astrophysical journal supplement series, 62, 373. doi:10.1086/191145 [15] williams, r.e. 1992, the astronomical journal, 104, 725. doi:10.1086/116268 [16] shore, s.n. 2008, in classical novae, 2nd edition. edited by m.f. bode and a. evans (cambridge astrophysics series, no. 43, cambridge: cambridge university press, 2008), p.194. [17] pontefract, m. & rawlings, j.m.c. 2004, monthly notices of the royal astronomical society, 347, 1294. doi:10.1111/j.1365-2966.2004.07330.x 215 http://dx.doi.org/10.1086/180579 http://dx.doi.org/10.1093/mnras/170.3.533 http://dx.doi.org/10.1016/j.icarus.2012.08.001 http://dx.doi.org/10.1088/0004-637x/780/1/26 http://dx.doi.org/10.1086/191145 http://dx.doi.org/10.1086/116268 http://dx.doi.org/10.1111/j.1365-2966.2004.07330.x m. nagashima et al. [18] hauschildt, p.h., starrfield, s., allard, f. 1994, in cool stars; stellar systems; and the sun; eighth cambridge workshop, astronomical society of the pacific conference series, vol. 64; proceedings of the 8th cambridge workshop (held in athens, georgia; october 11-14; 1993; san francisco: astronomical society of the pacific (asp); 1994; edited by jean-pierre caillault), p.705. 216 introduction observations results & discussion acta polytechnica ctu proceedings doi:10.14311/app.2015.1.0051 acta polytechnica ctu proceedings 2:51–56, 2015 © czech technical university in prague, 2015 available online at http://ojs.cvut.cz/ojs/index.php/app a case for domain-independent deterministic multiagent planning michal štolba department of computer science, faculty of electrical engineering, czech technical university in prague, karlovo nám. 13, 121 35 praha 2, czech republic correspondence: stolba@agents.fel.cvut.cz abstract. the notion of planning using multiple agents has been around since the very beginning of planning itself. it has been approached from various viewpoints especially in the multiagent systems community. recently, domain-independent multiagent planning has gained more attention also in the automated planning community. in this paper, we shortly present the current state of the art, question some aspects of the research field and discuss the rising challenges. keywords: planning, multi-agent planning. 1. introduction we could trace the first mention of multiagent planning back nearly as far as strips itself – in 1980 nillson published together with konolige a paper titled “multiple-agent planning systems” [1], in which they presented a high-level extension of strips [2] towards multiple agents. since then, the topic has been active mainly in the multiagent systems community. one of the most cited works on distributed and multiagent planning is [3], which describes basics of possible coordination schemes for planning agents. there was also a large amount of work dealing with another facets of the coordination area, but usually without deeper study of the plan synthesis part (e.g., gpgp [4]) or requiring additional domain-specific knowledge (e.g., talplanner [5]). multiagent planning also often relates to planning models described by decentralized pomdps (dec-pomdps) [6], similarly as pomdps are used in single agent planning under uncertainty. the point of view on multiagent planning from the multiagent community is extensively summarized in [7, 8]. in the planning community, extensions of the pddl language intended to support multiagent planning were introduced as multiagent planning language (mapl) in [9] and as multiagent pddl (ma-pddl) in [10], but none of the extensions gained wide popularity, probably because of their complexity. in 2008, a well accepted paper on multiagent planning was published by brafman and domshlak at the icaps conference [11], which finally ignited more interest in the topic. the paper formally introduced a minimalistic extension of strips (thus forming ma-strips) and have shown that the complexity of multiagent planning is not directly exponentially dependent on the number of agents, but rather on the tree-width of their interaction graph and a minimal number of interactions needed to solve the problem. such results suggested that at least for loosely coupled problems (where the tree-width is low), the approach may be beneficial. in the following years, several multiagent planners were proposed [12–19], either using directly the ma-strips formalism, or some ad-hoc, but very similar one. rather different approach was taken by crosby et al. in [20, 21], where the agent decomposition was based not on actions, but on a variables in finite domain representation, and was used in a centralized planner, significantly improving performance over (single agent) sota mainly in well decomposable domains. a dedicated workshop distributed and multiagent planning (dmap) also took place at the icaps’13 and icaps’14 conferences. 2. ma-strips ma-strips is the most commonly used formalism for domain-independent deterministic cooperative multiagent planning. the domain independence means that the input of the planner consists of description of available operators, predicates, functions etc. describing the general mechanics of the world as well as the particular instance of the world represented by ground initial state and the particular problem represented by the ground goal facts that need to be achieved. most commonly the input is described using pddl language, where the general part is termed “domain” and is described in a domain file and the particular initial and goal configurations are termed “problem” and are described in a problem file. in the multiagent setting, additional information is typically needed to determine what are agents and which actions belong to which agent (this is not part of the ma-strips formalism). deterministic in this case relates to the action model, where each action has deterministic effects, although the multiagent planning algorithm itself may be non-deterministic in the sense that for the same input it gives different output and runs for different time this is caused by the inherent nondeterminism of communication and distributed computation. 51 http://dx.doi.org/10.14311/app.2015.1.0051 http://ojs.cvut.cz/ojs/index.php/app michal štolba acta polytechnica ctu proceedings by cooperative multiagent planning we understand a setting where a group of agents is attempting to find a joint plan, where each agent uses its own actions in order to reach some joint (and possibly some private) goals. the agents typically need to coordinate their actions and to cooperate in order to do so, although each agent is planning for itself and the agents may have some private knowledge which they do not want to reveal to other agents. the ma-strips formalism is very much the same as the original strips, except that the actions are disjunctively split (or factored) among the agents, resulting in the following definition of the multiagent planning problem: definition 1. multiagent planning problem is a quadruple π = 〈l,a, s0, sg〉, where l is a set of propositions, a is a set of agents α1, . . . , α|a|, s0 is an initial state and sg is a set of goal states. an agent α = {a1, . . . , an} is represented by a set of actions it can perform. a state s ⊆ l is a set of atoms from a finite set of propositions l = {p1, . . . , pm} which hold in s. an action is a tuple a = 〈pre(a), add(a), del(a)〉, where a is a unique action label and pre(a), add(a), del(a) respectively denote the sets of preconditions, add effects and delete effects of a from l. in the set of propositions l can further be identified subsets of public propositions known to all agents and α-internal (a.k.a. α-private) propositions known only to a specific agent α. such separation of the propositions can be derived from the problem itself (as proposed in [11]) , where a proposition is public iff it is used by any two actions of different agents and it is internal to the agent α (α-internal) iff it is used only in actions of agent α. alternatively, what propositions are public or private can be a part of the problem definition (used in [16, 17]). how the actions are partitioned among agents is not a part of the ma-strips definition and is implementation dependent, in most planners, the agents are defined as objects in pddl and an action is assigned to an agent if the agent object is part of the grounded parameters of the action. similarly to the separation of the propositions, the actions of a single agent can be seen as either public or internal. an action a ∈ α is internal iff it uses only α-internal propositions in its preconditions and effects. otherwise, the action is public, which in fact means, that the action interacts with some other agent, or agents. following the strips formalism, a solution for the multiagent planning problem is a set of sequential plans (sequence of actions, one plan for each agent), where some of the actions of different agents may be performed in parallel, although in most implementations the resulting plan is sequential (i.e. there are no parallel actions). formally, the solution is defined as follows: definition 2. a solution to multiagent planning problem ∏ is a multiagent plan p =〈 pα1 , ..., pα|a| 〉 , where each pαi is a plan of agent αi, i.e. consisting of a sequence of actions in αi ∪ {noop}. a multiagent plan p is valid, iff (1.) for each α, β ∈ a, |pα| = |pβ |. the noop actions mean that the agent is idle in the respective timestep. (a) for each time-step tk where k ∈ 〈1, ..., |pα|〉, actions ai = pαi [tk] for all i ∈ 〈1, ..., |a|〉are not mutually exclusive. (b) all actions in time-step t1 are applicable in the initial state, for all time-steps tk where k ∈ 〈2, ..., |pα|〉, actions in tk are applicable in the state resulting from time-step tk−1 and the state resulting from time-step t|pα| is in the set of goal states sg. since all the plans are required to have the same length, the ma plan can be represented by a matrix, where each row consists of a plan for single agent and each column contains all actions in the respective time-step. it is clear that the ma-strips formalism is very simplistic and minimalistic extension and can be developed further to accommodate various aspects of the multiagent systems (i.e., finer separation of public and internal actions, as discussed for example in [22]). the philosophy of the formalism is to provide the smallest possible common ground and to enable the easiest possible migration of the planning algorithm to the multiagent setting. unlike in many multiagent systems, the mastrips formalism assumes the agents to be fully cooperative. from the multiagent perspective, this may seem as an oversimplification, but similarly to the simplicity of the ma-strips, we argue to first tackle the problems of the simplest possible multiagent extension – cooperative agents – and then gradually add complexities of the multiagent nature, such as selfish agents, negotiation and such. some attempts have already been taken, for example in [23, 24]. 3. is it any good? the first question that is asked after introduction of some new or nonstandard paradigm is – what is it good for? how does it help us? in this paper, we would like to ask such questions and try to come up with some answers. from the multiagent point of view, we can see significant benefits of introducing domainindependent planning into multiagent systems, namely reuse of the wide spectrum of planning systems and techniques from the area of classical planning. on the other hand, the drawback of ma-strips for the multiagent setting is that it requires cooperation among the agents. this can be mitigated in future by means such as mechanism design, as shown in [24]. 52 vol. 2/2015 a case for domain-independent deterministic multiagent planning in the following section, we will present some of the benefits we suggest that multiagent planning may bring to the area of planning in general. 3.1. improve scalability by decomposition although being largely improved in recent years, scalability is still an issue for domain-independent planning. multiagent planning can be seen as a method of factorization of the planning problems, which has been studied in works such as [25, 26]. it has been shown that factorization of the planning problem can be beneficial, especially in the problems, where it is possible to find large independent parts – loosely coupled problems. take for example the rovers domain with several rovers. the problem may be factored in such a way that the rovers do not need to coordinate, except for the last bit, where they use shared communication channel to communicate the result. in such a case, most of the planning can be done independently (even on different machines), however some coordination is needed here (it is not possible to simply run several classical planners in parallel). this is a case for multiagent planning. given a single agent problem (e.g. from the ipc benchmark set), it is not obvious what entities should be considered as agents and how the factorization should be done. in ma-strips, the first question is not answered, but the second one has some theoretical background. once the actions are assigned to agents, the factorization is based on the interactions among the agents’ actions, effectively meaning that it is based on the causal graphs of the problem (as in [11]), the factoring is one of the best known to date, as shown in [25]. still, how to best assign actions to agents is an open question. in the fmap planner [17, 27] formalism, the actions are assigned to agents by the domain designer, as is the separation of private and shared information. this gives the domain designer more freedom, but also more responsibility in that making more information public may increase the problem complexity (as the tree width of interaction graph rises) and making less information public may render the problem unsolvable (i.e. global solution may not exist). completely different approach to factorization was taken by crosby in [20, 21], where the factorization is automatic and based on the variables of a finite domain representation (fdr) of the planning problem. the method is, again, based on causal graph structures and tries to identify agents as sets of variables which are somehow self-contained and represent the agent’s internal state. variables which do not form an internal state of any agent are understood as environment and thus public variables. classification of actions is rather more complex than in ma-strips, but can be summarized as follows. actions, which interact only with the agent variables are assigned to the particular agent, other actions are left as public, which means that they can be used by any agent. this approach was not shown to increase scalability, but was shown to increase solution efficiency, especially in some domains. note, that crosby’s approach was implemented as a centralized, single-thread planner. various kinds of factoring, and understanding gained from the research of it, may be also used for other aspects of classical planning, such as search state pruning [28]. 3.2. multi-core / cluster computing planning on multi-core machines gained wide popularity and had a separate track at the ipc2011 competition. it is definitely beneficial to be able to scale the computation using multiple cores. but, if we want to scale the computation even further and use multiple machines, we are no longer able to use shared memory (as in parallel computation). in such case, approaches of distributed computing utilizing message passing need to take place, and multiagent planning aspires to be one of them (although obviously not the only one). in this scenario, multiagent planning represented by ma-strips can be seen as an equivalent for distributed planning with factorization fixed by the partitioning of actions to particular agents. such approach has been successfully tested against sota in multi-core planning in [14] as a parallel version of mad-a*, called map-a*. again, the question of best factorization arises here, with the crosby’s variable based factorization being another candidate, however never tested in a distributed system. 3.3. privacy in multiagent systems, privacy of the data is often one of the main concerns. in ma-strips-based multiagent planning, this requirement is not one of the top priorities – the privacy (or internality) of the propositions and actions is understood rather as a way of improving effectiveness of the computation by reducing the complexity – but nonetheless if required by the application, some degree of privacy of the data can be achieved. in practice, it may be somewhat harder not to reveal the structure of the private information and it may also reduce usability of some techniques, such as distributed heuristics, but it seems to be possible. one such approach was described in [22], but without implementation or experimental evaluation. 3.4. asynchronous computation distributed computation (and multiagent as well) is inherently asynchronous and non-deterministic. this brings several challenges and complexities that need to be tackled, but once solved, some of the insights may be beneficial to classical planning as well. as a representative example, we can take the computation of a global heuristic. a heuristic in multiagent planning can be computed locally, using only a projection of the planning problem to the propositions of the respective agent. a public 53 michal štolba acta polytechnica ctu proceedings action a ∈ α can be projected to agent β by removing all α-internal propositions from pre(a), add(a) and del(a), thus retaining only public propositions (and technically also β-internal propositions, but actions of agent α contain none). such heuristics were used for example in [14]. a different approach is to attempt to compute a global heuristic estimate without exposing the internal information to other agents. this can be achieved for example by requesting other agents for their estimates of the current state and the goal, or some sub-goals, as in [29, 30]. such global heuristic computation may involve nontrivial communication, resulting in an asynchronous computation of the heuristic estimate. to our knowledge, such a phenomenon has not been studied yet. it seems to be interesting also from the point of view of classical planning, because if successfully utilized, asynchronous computation of heuristics may be used to evaluate computationally intensive heuristics, to use external solvers for heuristic estimation, sensory input (in robotics), etc. also it is interesting to observe that the global heuristic is always equally or more informative than the projected heuristic, thus we have two heuristics, the first one is less informative, gives lower estimates, but is faster, the second one is more informative, gives higher estimates, but takes significantly longer to compute. if generalized, a sequence of gradually more precise, but costlier heuristics could be used to guide search (including blind search as the fastest one), which is an interesting research topic. also, as the costly distributed heuristic is computed partly by other agents, the time while waiting for the results may be used to do some more computation such as exploration using only the local heuristic. 3.5. applications ma-strips planning may bring domain independent planning to some (more or less) new application domains. as such we see the following: • multi-robotics: slowly but steadily, domainindependent planning is penetrating the robotic research, providing the robots with high-level reasoning. on the other hand, robotic research is being extended towards multi-robotic teams. multiagent planning seems to bridge those two advancements and in the future may enable multi-robotic teams to be controlled by a high-level distributed planning system. in such scenarios, each robot would have its own planning representation of the world, thus reducing the domain and problem size and keeping the local information (i.e., sensory data) local. communication will clearly be an issue, which may encourage research of multiagent planning techniques aiming not primarily on planning speed, but on the communication requirements (this may for example discourage the use of global heuristic estimates). • orchestration of internet services: since internet services are computer programs already described in (formal) programming languages their coordination and orchestration can straightforwardly profit from multiagent planning techniques. moreover services are usually described by interfaces resembling planning actions with preconditions on the input data and effect on the data. therefore if such actions have to be ordered in a suitable fashion, beginning with initial constraints on the data and with a goal form of the data, planning is an appropriate fit. additionally, the services can be distributed over the internet and the data processing tasks can vary in the sense of particular areas of focus (e.g., financial processing, logistics services, etc.). both of these challenges are covered by the domain-independent multiagent planning. a single agent planning applications of this kind were already covered for example in [31]. • coalition planning: in recent years, many military operations involve coalitions. coalition operations are in nature cooperative, but some information cannot be disclosed among the coalition partners. this can be generally modeled by multiagent systems and in particular by multiagent planning. such a scenario is not applicable only in military, but may find uses also in business cooperation and other areas, although some more detailed privaci concerns may be necessary as illustrated in [22]. 4. challenges in the following, we will present some of the outstanding challenges of domain-independent multiagent planning we are aware of. 4.1. comparison of planners up to the present day, we know of about a half dozen domain-independent multiagent planners [12–17] and more are in the development. the biggest issue in order to compare them is that although the underlying formalisms are mostly similar, the actual language used as input differs (it is pddl with some ad-hoc definition of agents and/or public propositions). some consensus on this matter needs to be settled. similarly to the common definition language, a widely accepted set of benchmarks is needed. in the recent works, the benchmarks were typically created by ad-hoc converting some suitable ipc domains, where the agent decomposition is natural, such as logistics or rovers. only few new – multiagent specific – domains were introduced, i.e., cooperative pathfinding. in the future, it would be interesting to come up with more such domains, drawing from real world and multiagent systems applications and also designed to test some specific properties and pitfalls of multiagent planning. naturally, the best venue for such efforts would be a dedicated ipc track. the metrics used for planner comparison are also challenging. all metrics used to compare classical 54 vol. 2/2015 a case for domain-independent deterministic multiagent planning planners still apply, but there are some additional ones, such as number of exchanged messages or the amount of communicated data. further metrics can be adopted from the research of other multiagent algorithms. moreover, the comparison of multiagent planners gains a new complexity in that a single problem can be partitioned in multiple ways, and each partitioning may be beneficial for different planners. similarly, the amount of agent interaction significantly influences performance of the planners some algorithms may be better suited for loosely coupled problems while other for tightly coupled problems. this should be also reflected in the potential analysis. 4.2. which planning paradigm is the best? in classical sequential domain-independent planning, the dominating planning paradigm seems to be heuristic forward-chaining search typically utilizing one or more highly informative heuristic estimators (used by planners as fastforward, fastdownward, lama and many more), although some other approaches (such as bidirectional search) has performed well in the last ipc competition. it is not clear, whether the same holds for multiagent planning (and for all metrics). again, the only way to reliably answer this question is to perform a rigorous comparison of the planners, such as in the ipc competition. currently the most frequent approaches to multiagent planning are distributed heuristic search [14, 29, 30], partial order planning [16, 17], and various other, such as plan reuse [12] or generate-and-test [18, 19]. 4.3. heuristics one interesting area of research are the heuristic estimator for multiagent planning. as mentioned in the section about asynchronous computation, the baseline solution for heuristic computation is to use a projection of the problem to the particular agent’s propositions. this approach was used in [13, 14]. another approach is to attempt to compute (or at least approximate) the global value as if the problem was not partitioned in order to obtain better heuristic guidance. to our knowledge, the only heuristics treated this way so far were the ff heuristic in [29] and [30] and a domain transition graph based heuristic in [17]. of course, in some cases, the communication needed to compute a global heuristic may be prohibitive, in such a situation, the local heuristic would have to suffice, or a clever combination of both would need to be devised. different challenge is to come up with a heuristic specific to the multiagent planning. such heuristic may not only lead the search towards the goal, but may also lead the search in a way minimizing some multiagent metric, such as the communication load. 4.4. privacy multiagent planning can be seen also from the perspective of privacy-preserving distributed computing, although it is not the main concern of most of the ma-strips based planners. this perspective raises many questions, some of them were already addressed in [22, 32], but a comprehensive analysis of possible privacy violations in ma-strips and ma-strips based planning algorithms has not been yet presented. even the question if a ma-strips based multiagent planner can be privacy preserving (and if so how strongly) is yet to be answered. 4.5. extending complexity analysis from the theoretical perspective, the original [11]’s complexity results can give a solid groundwork for future and more detailed studies of complexity of multiagent planning. we envision two main directions: (i) problem-specific and (ii) finer complexity classes. the problem-specific study could be inspired by works as [33], where particular planning domains are analyzed from perspective of computational complexity showing which problems are easy, even if planning in general is known to be computationally hard. the idea of study of finer complexity classes, is for example parametrized analysis of classical planning by [34]. additionally, these studies can be extended by communication complexity showing how much communication would be required to solve the problems. 5. final remarks domain independent planning is a field of research bridging two distinct topics of multiagent systems and domain independent planning. we see providing a common problem definition language and set of benchmarks in order to be able to determine the state of the art and compare it with newly emerging techniques as one of the important steps to advance the field. we see such a hybrid field to be potentially beneficial to both research communities. acknowledgments this research was supported by the czech science foundation (13-22125s). references [1] k. konolige, n. j. nilsson. multiple-agent planning systems. in aaai, vol. 80, pp. 138–142. 1980. 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[34] c. bäckström, y. chen, p. jonsson, et al. the complexity of planning revisited a parameterized analysis. in aaai. 2012. 56 http://dx.doi.org/10.1609/aimag.v22i3.1581 http://dx.doi.org/10.1613/jair.1497 http://dx.doi.org/10.3233/mgs-2009-0133 http://dx.doi.org/10.1145/1838206.1838379 http://dx.doi.org/10.5220/0002702601280134 http://dx.doi.org/10.3233/978-1-61499-098-7-762 http://dx.doi.org/10.5220/0004918701780189 http://dx.doi.org/10.1007/s10489-014-0540-2 http://dx.doi.org/10.3233/978-1-61499-098-7-624 acta polytechnica ctu proceedings 2:51–56, 2015 1 introduction 2 ma-strips 3 is it any good? 3.1 improve scalability by decomposition 3.2 multi-core / cluster computing 3.3 privacy 3.4 asynchronous computation 3.5 applications 4 challenges 4.1 comparison of planners 4.2 which planning paradigm is the best? 4.3 heuristics 4.4 privacy 4.5 extending complexity analysis 5 final remarks references acta polytechnica ctu proceedings doi:10.14311/app.2017.8.0027 acta polytechnica ctu proceedings 8:27–29, 2017 © czech technical university in prague, 2017 available online at http://ojs.cvut.cz/ojs/index.php/app the current view on the use of reconstruction materials in dentistry radka vrbovaa,∗, pavel bradnaa, martin bartosa, lucie himmlovaa, tomas horazdovskyb a institute of dental medicine, first faculty of medicine, charles university and general university hospital in prague, prague, czech republic b department of physics, faculty of mechanical engineering, czech technical university in prague, prague, czech republic ∗ corresponding author: vrbova@vus.cz abstract. the hardest tissue in the human body is the enamel which covers the anatomical crowns of teeth. it must be resistant to mechanical stress and the chemical attack of many substances from food, drinks and products of the metabolism of bacteria present in the oral cavity. these low ph substances dissolve the mineral components of enamel, cause tooth demineralization, and lead to decay or erosion damage with the irreversible loss of dental hard tissues and the necessity of their reconstruction. the range of dental materials intended for dental tissue reconstruction is extensive. dental amalgam can be mechanically applied into the strongly stressed lateral segments of teeth. the use of amalgam is, however, in decline, with the possible health risks attributed to it, coupled with the need to extensively prepare tooth tissue promoting a shift towards using aesthetically and biologically favourable dental ceramic and polymeric materials instead. current developments also concentrate on these materials to reinforce this, with polymeric composite materials based on methacrylates with varying amounts of inorganic fillers at the forefront. these materials are distinguished by their good mechanical and aesthetic properties and wear resistance. however, polymerization shrinkage and a strong hydrophobic nature does not allow for their direct bonding to hard dental tissues. risks associated with the release of residual free monomers from the structure to the environment, which may cause health complications, mainly allergic reactions in sensitive individuals, have been monitored recently. further development in the field of composite materials aims to reduce or completely eliminate these negatives. keywords: hard dental tissues, tooth decay, erosion, dental materials. 1. introduction teeth are specific formations in the oral cavity which are mainly composed from hard tissues such as enamel, dentine and cementum. enamel covers the surface of anatomical crowns and is the hardest tissue in the human body due to the high content of mineral substances. the so-called prismatic structure is typical for enamel, with closely spaced prisms bound with interprismatic substance (fig. 1a). enamel prisms with a diameter of 4-8 µms are formed predominantly from hydroxyapatite nanocrystals. the interprismatic mass also contains proteins, except hydroxyapatite, which serve as a binder for the individual prisms. dentine is mineralized less than enamel, is softer, and contains about 20-30 vol. % of proteins and 20-30 vol. % of water, and is biologically active compared to enamel. typical structural elements are known as dentinal tubules, canals in the dentine where extensions of the odontoblasts exist alongside dentinal fluid (fig. 1b). figure 1. a) prismatic structure of enamel after phosphoric acid etching (cross-section); b) the surface of dentin with dentinal tubules after phosphoric acid etching (cross-section). (sem, jsm5500-lv, jeol, tokyo, japan). figure 2. a) 3d model of the damaged tooth by caries from micro-ct (skyscan 1272, bruker, belgium); b) the longitudinal cross section of a tooth with tooth decay in the enamel and dentin (optical microscope olympus, smz 2mt, japan). 27 http://dx.doi.org/10.14311/app.2017.8.0027 http://ojs.cvut.cz/ojs/index.php/app r. vrbova, p. bradna, m. bartos et al. acta polytechnica ctu proceedings figure 3. the surface of enamel exposed to drinks for 12 hours: a) intact enamel; b) coca-cola, ph 2.5; c) fanta, ph 2.8; d) sprite ph 2.7. (sem, jsm5500-lv, jeol, tokyo, japan). 2. the typical damages of hard dental tissues 2.1. tooth decay due to the effect of products of the metabolic conversions of cariogenic bacteria present in dental plaque, there is a local demineralization in the subsurface layers of enamel. if the process reaches the dentineenamel border, it will break the surface layers of the enamel and will lead to the formation of a cavity (fig. 2a, b). 2.2. erosion in the case of dental tissue erosion, the demineralization is of non-bacterial origin. it results from the attack of substances with high acidity and simple sugars content. from this point of view, so-called "soft drinks" are very aggressive with a low ph, containing e.g. phosphoric acid or citric acid (fig. 3 b, c, d). they cause the dissolution of enamel mineral components; reduce its surface hardness and resistance to abrasion or mechanical stress. the result of the above-mentioned conditions may be the loss of dental hard tissues with the necessity of their reconstruction, which is often complicated and costly. additionally, the lifetimes of reconstructions are always limited. 3. polymeric materials for the reconstruction of hard dental tissues the range of dental materials intended for the reconstruction of lost or damaged dental tissues, whether as a result of carious processes, extensive erosions or traumatic damages is very broad. the most frequently used are composites, whose matrix consists mainly of methacrylate polymers (e.g. bis-gma, tegdma, udma, bis-ema) and the fillers are ba, sr, zr ground glass with surfaces treated by silane coupling agents and amorphous sio2 added primarily for adjusting the thixotropic properties of materials and pigments. these are mainly light-cured materials containing photoinitiators of polymerization reaction, usually camphorquinone (2,3-bornandion), propanedione (1phenyl-1,2-propanedione) or lucirin tpo (biphenyl (2,4,6-trimetylbenzoyl-diphenylphosphine oxide), often in conjunction with tertiary amine co-initiators. figure 4. a) the adhesive material leaking into the dentinal tubules; b) damaged marginal seal between the filling material and enamel. (sem, jsm5500-lv, jeol, tokyo, japan). they must also contain inhibitors of the polymerization reaction. the hardening of materials occurs by radical polymerisation, when molecules of photoinitiators absorb photons of light from curing lamps forming free radicals, which initiate very fast polymerization reactions. mainly halogen or led curing lamps emitting light in the wavelength range 400 to 515 nms are currently used for curing of materials. it is very important that the spectrum of emitted light from the curing light must overlap the absorption spectrum of the photoinitiator present in material to ensure the best possible polymerization [1, 2]. composite materials are characterized by very good mechanical and aesthetic properties, and are indicated for lateral and front sections of the dental arches. however, these materials are hydrophobic and do not adhere to tooth structure comprising water. therefore, quality micromechanical binding provides adhesive systems that flow into the previously etched microrelief of dental tissue (fig. 4a). adhesive systems include hydrophilic monomers with the ability to wet moist surfaces and bond them with hydrophobic monomers which are chemically combined with a composite material after polymerization. the main disadvantage of these materials is their shrinkage due to polymerization. this may cause damage to the marginal seal, i.e. loss of contact between the dental tissue and the filling material (fig. 4b). the polymer materials also show a possible risk of residual free monomers releasing into the surrounding environment, which may give rise to health problems in susceptible individuals, especially to allergic reactions [3–6]. 28 vol. 8/2017 the current view on the use of reconstruction materials in dentistry 4. conclusion further development of polymer composites is driven by efforts to improve their mechanical and aesthetic properties, increase wear resistance, reduce polymerization shrinkage and achieve significant improvements in biocompatibility. acknowledgements this work was supported by grant prvouk p28/lf1/6 mšmt. references [1] p. bradna, l. comba, j. froněk, d. houšová. kam směřuje vývoj polymeračních lamp? česká stomatologie 110(3):61–68, 2010. [2] j. froněk. studium vlivu vlastností polymeračních lamp na vytvrzení kompozitních materiálů s různými fotoiniciačními systémy. master’s thesis. [3] m. syed, r. chopra, v. sachdev. allergic reactions to dental materials-a systematic review. journal of clinical and diagnostic research 9(10):ze04 – ze09, 2015. doi:10.7860/jcdr/2015/15640.6589. [4] i. stoeva, a. kisselova, m. zekova. allergic contact stomatitid from bisphenol-a-glycidyldimethacrylate during application of composite restorations. a case report. journal of imab annual proceeding (scientific papers) pp. 45 – 46, 2008. book 2. [5] s. gupta, p. saxena, v. pant, a. pant. release and toxicity of dental resin composite. toxicology international 19(3):225–234, 2012. doi:10.4103/0971-6580.103652. [6] m. goldberg. in vitro and in vivo studies on the toxicity of dental resin components: a review. clinical oral investigations 12(1):1–8, 2008. doi:10.1007/s00784-007-0162-8. 29 http://dx.doi.org/10.7860/jcdr/2015/15640.6589 http://dx.doi.org/10.4103/0971-6580.103652 http://dx.doi.org/10.1007/s00784-007-0162-8 acta polytechnica ctu proceedings 8:27–29, 2017 1 introduction 2 the typical damages of hard dental tissues 2.1 tooth decay 2.2 erosion 3 polymeric materials for the reconstruction of hard dental tissues 4 conclusion acknowledgements references 210 acta polytechnica ctu proceedings 1(1): 210–214, 2014 210 doi: 10.14311/app.2014.01.0210 electron acceleration in supernovae and millimeter perspectives keiichi maeda1,2 1department of astronomy, kyoto university, japan 2kavli institute for the physics and mathematics of the universe (wpi), university of tokyo, japan corresponding author: keiichi.maeda@kusastro.kyoto-u.ac.jp abstract supernovae launch a strong shock wave by the interaction of the expanding ejecta and surrounding circumstellar matter (csm). at the shock, electrons are accelerated to relativistic speed, creating observed synchrotron emissions in radio wavelengths. in this paper, i suggest that sne (i.e., ∼< 1 year since the explosion) provide a unique site to study the electron acceleration mechanism. i argue that the efficiency of the acceleration at the young sn shock is much lower than conventionally assumed, and that the electrons emitting in the cm wavelengths are not fully in the diffusive shock acceleration (dsa) regime. thus radio emissions from young sne record information on the yet-unresolved ‘injection’ mechanism. i also present perspectives of millimeter (mm) observations of sne – this will provide opportunities to uniquely determine the shock physics and the acceleration efficiency, to test the non-linear dsa mechanism and provide a characteristic electron energy scale with which the dsa start dominating the electron acceleration. keywords: acceleration of particles radiation mechanism: non-thermal supernovae: general. 1 introduction the most promising particle acceleration mechanisms require a strong shock wave, e.g., by the diffusive shock acceleration (dsa) mechanism where the particles acquire energy through repeated collisions between upand down-streams of a shock wave (fermi, 1949; blandford & ostriker, 1978; bell, 1978). supernova remnants (snrs) are believed to be the origin of cosmic rays at least up to ∼ 1015ev (e.g., bamba et al., 2003). there is one key issue in this picture for electrons – how the electrons are ‘pre-accelerated’. for the dsa mechanism to work efficiently, a particle must already have an enough kinetic energy. supernovae (sne), at the age of ∼< 1 year, also produce emissions which are believed to be originated by relativistic electrons, accelerated at a strong shock created by the expanding sn ejecta running into circumstellar matter (csm). radio emissions from sne are interpreted as the synchrotron emission, and x-rays from some sne have been suggested to be emitted through the inverse compton (ic) mechanism (e.g., chevalier & fransson, 2006 for a review). however, most of analyses on the non-thermal emissions from sne have been focusing on deriving the csm environment, rather than the acceleration mechanism (e.g., soderberg et al., 2012). in this paper, i argue that young sne provide a unique site to study the electron acceleration mechanism. i also suggest that millimeter (mm) observations, which are becoming feasible with new observatories like alma, can potentially provide essential information on this issue. 2 non-thermal emissions a situation around young sne related to the nonthermal emission is similar to that for snrs. energy transfer from the shock wave kinetic energy to relativistic particles and that to magnetic field are roughly described by equipartition (fransson et al., 1996). i adopt conventional notation – εe and εb describe constant fractions of the shock wave energy transferred to the relativistic electrons and the magnetic field, respectively. our arguments are based on modeling emissions from so-called striped envelope sne (se-sne; or sne iib/ib/ic) that are believed to be explosions of he or co stars (having lost at least the h envelope). in these sne, the radio emission is well described by synchrotron emissions with the synchrotron self-absorption (ssa) at low frequencies (see, e.g., chevalier & fransson, 2006). under some standard assumptions (björnsson & fransson, 2004; chevalier & fransson, 2006; maeda et al. 2012, 2013a), the synchrotron properties can be described by the following parameters: • p: power law index of spectral energy distribution of injected relativistic electrons. • m: power law index of shock evolution in time (r ∝ tm). 210 http://dx.doi.org/10.14311/app.2014.01.0210 electron acceleration in supernovae and millimeter perspectives table 1: characteristics of the synchrotron emission from young sne (lν ∝ ναtβ), for the adiabatic limit and for the synchrotron and ic cooling limits, respectively (maeda, 2013a). indices adiabatic syn. ic α 1−p 2 −p2 −p2 β (3m− 3) + 1−p 2 (3m− 3) + 2−p 2 (5m− 5) + 2−p 2 + δ α(p = 2) − 1 2 −1 −1 β(p = 2) (3m− 3)− 1 2 (3m− 3) (5m− 5) + δ α(p = 3) −1 − 3 2 − 3 2 β(p = 3) (3m− 3)− 1 (3m− 3)− 1 2 (5m− 5)− 1 2 + δ • δ: power law index of optical/nir sn emission in time (l ∝ tδ). • a∗: csm density scale (ρcsm ∝ a∗r −2; normalized as a∗ ∼ 1 for ṁ ∼ 10−5m�yr−1 with the mass loss wind velocity of 1, 000 km s−1). • εe: efficiency of the electron acceleration. • εb : efficiency of the magnetic field generation/amplification. note that the shock evolution (m) is mainly determined by the csm density distribution, e.g., by a self-similar solution (chevalier, 1982). table 1 shows expected synchrotron properties, lν ∝ ναtβ . from the observed properties (α, β) one can almost uniquely determine the power law indices (p, m, δ). there is a degeneracy in the other parameters (i.e., in the ‘scales’). properties of the ssa-synchrotron are described by two characteristic observables (peak date and luminosity), while these are described by the three model parameters (i.e., a∗, εe, εb). 3 efficiency of electron acceleration figure 1 shows how one can constrain the shock microphysics and csm density. an example is given for intensively observed nearby sn iib 2011dh. thanks to detailed models of the optical emission (bersten et al., 2012), the sn ejecta properties (mass and energy) have been strongly constrained – model a adopts the shock wave dynamics expected from the optical emission model. model b is shown for illustration, which assumes the dynamics so that εe ∼ 0.1, but this fails to explain the optical behavior. adopting model a, εe cannot be as large as ∼ 0.1 which has been conventionally assumed, since such a situation requires extremely large mass loss rate (a∗). then the expected thermal emission in x-rays would be much stronger than observed. indeed, from the x-ray strength, a∗ ∼< 30, thus εe ∼< 0.01 must apply. also, from the energy conservation, a∗ ∼< 2 is rejected (otherwise εb > 0.3). from these arguments, 0.005 ∼< εe ∼< 0.01 and εb ∼> 0.001 are obtained as robust constraints. this also indicates that α ≡ εe/εb < 10. 0.1 1 10 1e-3 0.01 0.1 model b model a b e e figure 1: εe and εb derived for sn 2011dh, as a function of a∗ (maeda, 2012). so, a strong constraint can be placed on εe. there is another independent argument against a large value of εe. figure 2 shows the models with small εe and large εe. large εe should produce a detectable cooling effect in radio properties, which was however not detected. this argument on the ic cooling effect should apply to sne in general. i note that sometimes a large value of εe is introduced/suggested to explain x-ray luminosities by ic up-scattered photons (e.g., chevalier & fransson, 2006), but indeed i suggest here that one has to check if such a situation is consistent with the radio (cm) properties. for example, for sn 2011dh α ∼> 30 (e.g., εe ∼< 0.3 and εb ∼ 0.01) has been suggested (e.g., soderberg et al., 2012), but as shown above this should produce a detectable change in the radio light curves that was not observed (maeda, 2012). applying the same constraint to a few other sne, it seems like that small εe is a generic feature in sne (maeda, 2013a). 211 keiichi maeda 10 100 0.1 1 10 (e) 25.0 ghz 10 100 0.1 1 10 (f) 36.0 ghz 10 100 0.1 1 10 (d) 16.0 ghz 10 100 0.1 1 10 (c) 8.4 ghz 10 100 0.1 1 10 (b) 4.9 ghz fl ux d en si ty (m jy ) day 10 100 0.1 1 10 (a) 1.4 ghz 10 100 0.1 1 10 (f) 36.0 ghz 10 100 0.1 1 10 (e) 25.0 ghz 10 100 0.1 1 10 (d) 16.0 ghz 10 100 0.1 1 10 (c) 8.4 ghz 10 100 0.1 1 10 (b) 4.9 ghz 10 100 0.1 1 10 (a) 1.4 ghz fl ux d en si ty (m jy ) figure 2: left: multi frequency radio light curves (red solid) as compared with those of sn 2011dh (maeda, 2012). the parameters are (a∗, εe, εb) = (4, 6×10−3, 5×10−2) (left; adopting model a) and (20, 0.26, 2.5×10−4) (right; model b). the synthetic light curves without the ic cooling are also shown (blue dashed). observational data are taken from soderberg et al. (2012). 4 injection and acceleration mechanisms since one can obtain both the spectral and temporal information for sne, there is essentially no degeneracy in deriving the electrons’ injected spectrum slope, p (tab. 1). one interesting issue is found from such analyses – p ∼ 3 is generally derived for young sne, unlike more evolved snrs (mostly p ∼ 2 − 2.4; e.g., bamba et al., 2003) and the standard dsa prediction in the test particle limit (p ∼ 2; e.g., ellison et al., 2000). a cause of the difference has not been clarified, and i propose that this is mainly due to totally different energies of the electrons emitting at cm wavelengths in young sne and more evolved snrs. the argument here is based on that of maeda (2013b). i note that a main difference between the synchrotron emission from sne and that from snrs is that the emitting electrons’ energy is quite different for given frequency (figure 3). typical magnetic field strength is b ∼ 1g for sne (e.g., chevalier & fransson, 2006) and 100µg for snrs (e.g., bamba, et al., 2003). this is consistent with the equipartition expectation (maeda, 2013b). at the observed frequency of ∼ 1 ghz, the emitting electrons’ energies are ∼ 50 mev and 5 gev in sne and snrs, respectively. one can estimate if these electrons satisfy an essential condition required for dsa, namely the electron’s mean free path is exceeding the shock wave width. this is satisfied by electrons with the energy ∼> 100 mev in sne and 10 mev in snrs. thus i suggest that the electrons emitting at ghz frequency are likely in the efficient dsa limit in snrs, while they cannot be efficiently accelerated by dsa in sne. 100 101 102 103 104 105 0.1 1 10 100 1000 dsa (sn) dsa (snr) alma vla /g h z figure 3: the relation between the electron’s energy and the synchrotron frequency, for b ∼ 1g typical of young sne (red-thick-solid) and ∼ 100µg typical of snrs (black-thin-solid). also shown is the minimum electron energy for the efficient dsa, adopting v ∼ 0.1c (sne; red-thick-dashed) and 0.01c (snrs; black-thindashed). the typical frequency coverage is shown by the shaded areas, for cm (‘vla’) and mm (‘alma’) observations . a unified scenario is proposed here – the steep energy spectrum of the electrons derived for young sne reflects the inefficient dsa acceleration, or in other word, the ‘injection’ spectrum. this scenario makes young sne interesting objects in studying the electron injec212 electron acceleration in supernovae and millimeter perspectives tion and acceleration mechanism, as one could directly probe the electron injection mechanism. 5 perspectives for mm observations i propose that observations of nearby young sne at mm wavelengths can potentially provide major advances in the issues discussed in this paper (see maeda, 2013b for details). on the acceleration efficiency, the ic cooling effect is more important at higher frequencies, and thus at mm wavelengths one should be able to see this effect to determine εe, or at least place much stronger upper limit than at cm wavelengths. alternatively, if εe is very small, then the synchrotron cooling becomes important, and the cooling frequency would enter into the mm wavelength. if it happens, it will provide direct estimate of b. in either case, there is a good chance to obtain additional information, and then we can solve the degeneracy between the shock physics and the csm environment (§2). another suggestion is on the electron injection. if the scenario suggested in §4 is correct, we should see the spectral flattening at high frequencies. this flattening could take place already at ∼ 100 mev (§4), then one should be able to detect this signature at mm wavelengths (fig. 3). if such a change in the electrons’ energy spectral slope is detected, this could provide direct evidence of the non-liner acceleration theory where the particles’ spectral slope is expected to become harder for higher energies (e.g., ellison et al., 2004). the energy scale for the possible transition will provide strong constraints on the acceleration theory. for nearby objects (up to ∼ 25mpc), such a signature should be detectable by alma (maeda, 2013b). 6 conclusions in this paper, i have suggested to study electron acceleration mechanisms at a strong shock wave by radio observations of nearby young sne. especially, several ideas have been presented regarding (1) the acceleration efficiency and (2) injection problem and non-linear acceleration toward the efficient dsa. the ideas include (a) to constrain the efficiency by combining radio and optical data, (b) to place an independent constraint on the efficiency by the ic cooling effect, and (c) to probe propertis of ‘injected’ electrons before entering into the efficient dsa regime. i also propose that these issues can be further advanced by mm observations. such observations are being planned – we have our too proposal of nearby sn follow-up observations by alma among the highest priority proposals in alma cycle 1, which is currently active (until early 2014). acknowledgement km thank franco giovannelli and the organizers of frascati workshop 2013 for creating the friendly and stimulating atmosphere. the work by km has been supported by wpi initiative, mext, japan, and by a grant-in-aid for scientific research for young scientists (23740141). references [1] bamba, a., et al.: 2003, apj, 589, 827 doi:10.1086/374687 [2] bell, a.r.: 1978, mnras, 182, 147 [3] bersten, m.c., et al.: 2012, apj, 757, 31 doi:10.1088/0004-637x/757/1/31 [4] blandford, r.d., ostriker, j.p.: 1978, apj, 221, l29 doi:10.1086/182658 [5] björnsson, c.-i., fransson. c.: 2004, apj, 605, 823 [6] chevalier, r.a.: 1982, apj, 258, 790 [7] chavalier, r.a., fransson, c.: 2006, apj, 651, 381 doi:10.1086/507606 [8] ellion, d.c., berezhko, e.g., baring, m.g.: 2000, apj, 540, 292 doi:10.1086/309324 [9] fermi, e.: 1949, phys. rev. 75, 1169 doi:10.1103/physrev.75.1169 [10] fransson, c., lundqvist, p., chevalier, r.a.: 1996, apj, 461, 993 doi:10.1086/177119 [11] maeda, k.: 2012, apj, 758, 81 [12] maeda, k.: 2013a, apj, 762, 14 [13] maeda, k.: 2013b, apj, 762, l24 [14] soderberg, a.m., et al.: 2012, apj, 752, 78 doi:10.1088/0004-637x/752/2/78 discussion sergio colafrancesco: ic losses are dominant w.r.t. synchrotron loses if the b-field is low. how this can match with the expectation that the ic losses are important in high b-field regions in snrs? what is a role of coulomb heating effects? keiichi maeda: on the ic cooling, i believe that we expect that in general the relative importance of the ic cooling is higher for lower b-field. note that i am 213 http://dx.doi.org/10.1086/374687 http://dx.doi.org/10.1088/0004-637x/757/1/31 http://dx.doi.org/10.1086/182658 http://dx.doi.org/10.1086/507606 http://dx.doi.org/10.1086/309324 http://dx.doi.org/10.1103/physrev.75.1169 http://dx.doi.org/10.1086/177119 http://dx.doi.org/10.1088/0004-637x/752/2/78 keiichi maeda talking about cooling, not heating/emission. here, the ic cooling rate is lic ∝ uphγ 2 and the synchrotron cooling rate is lsyn ∝ ubγ 2. then, for given observed frequency ν, if one increases b then one should decrease γ (to emit at ν), leading to lower lic. in this situation, lsyn can be large b/o the ub term. on the coulomb heating. so far i have been focusing on sne iib/ib/ic, which are believed to have relatively low density csm. i estimated the coulomb effect, and at ghz or higher frequencies, the coulomb heating is estimated to be negligible. 214 introduction non-thermal emissions efficiency of electron acceleration injection and acceleration mechanisms perspectives for mm observations conclusions acta polytechnica ctu proceedings doi:10.14311/app.2017.13.0055 acta polytechnica ctu proceedings 13:55–60, 2017 © czech technical university in prague, 2017 available online at http://ojs.cvut.cz/ojs/index.php/app micro-mechanical performance of concrete used as recycled raw material in cementitious composite vladimír hrbeka,b,∗, veronika koudelkováa,b, zdeněk prošeka,c, pavel tesáreka a czech technical university in prague, faculty of civil engineering, thákurova 7, 166 29 prague 6, czech republic b institute of theoretical and applied mechanics as cr, v.v.i., prosecká 76, 190 00 prague 9, czech republic c university centre for energy efficient buildings of czech technical university in prague, třinecká 1024, 273 43 buštěhrad, czech republic ∗ corresponding author: vladimir.hrbek@fsv.cvut.cz abstract. the reduction of industrial pollution is recently one of main goals over all fields. in civil engineering, re-cycling of structural waste provides wide opportunity contributing this effort. this paper focus on re-use of concrete waste, which after further processing can be used in new constructions as partial supplement to the mixture. to investigate the impact of re-cycled concrete addition, it is necessary to determine mechanical and structural parameters of individual phases in the “raw” material. for this purpose, grid indentation and scanning electron microscopy with energy-dispersive x-ray spectroscopy (sem, edx) are combined to determine properties of concrete sample. keywords: re-cycled concrete, grid indentation, sem, edx, image analysis. 1. introduction the waste material accumulation became worldwide environmental and economical problem. development of new effective and thrifty methods for re-use of waste products lowers the number of waste deposits while saving natural sources and environment. for example, in 2012, the countries of european union produced over 2.5 billion tones of waste and managed to recycle barely 10% of this amount. in case of czech republic, over 23million tones of waste were produced and slightly over one-fifth recycled (statistics according to environmental data center on waste). one of main problems with incorporation of waste materials in civil engineering (especially in constructions) is mistrust of designers and developers toward recycled construction waste. despite lower price of these materials (compare to traditional raw materials), final cost can be paradoxically higher due to the transportation [1–3]. the solid phase of concrete is made of coarse and fine aggregate (65 to 70%) and cement paste. typically, when recycled, coarse aggregate (30 to 50% of raw material) is to be reused, which leaves majority of the concrete as secondary waste product [4]. complete recycling of concrete (concretely use of highspeed milling) can turn this waste into fine powder (fraction < 1mm) with potential usage as micro-filler (fine ground hydration products and fine aggregate) and partial cement substitute (non-hydrated clinker in cement paste). nevertheless, the processing itself is complicated and bears addition financial costs. beyond that, due to the lack of studies and short experience of civil engineering practice with such fine powdered material, the implementation of recycled secondary waste material is not possible on large scale. m. chen et al. [5] used the recycled concrete fine powder in their study as full substitute of pulverized limestone in asphalt mixture. the results of x-ray diffraction and scanning electron microscopy (sem) showed higher content of sio2, lower caco3 content and rougher partical surface in recycled concrete fine powder, compare to of pulverized limestone. when used as filler in asphalt mixture, the water sensitivity of samples reduced while mechanical properties in high temperatures and fatigue persistence increased. however, mechanical properties in low temperature slightly decreased. in case of cement based mixtures, recycled concrete fine powder serves much as a micro-filler, which acts as coarse aggregate on the mezoand micro-scale of the material. unfortunately, studies about recycled micro-fillers are not yet carried out, although the idea seems promising as replacement of current microfilling agents [6]. moreover, the potential of nonhydrated clinker present in the powder was so far vastly ignored. y. j. kim et al. [7] presented in his study features of cement paste modified by recycled concrete fine powder as a partial replacement for a binder. according to the results, workability in mixture’s fresh state decreased with increasing level of cement compensation and the hydration process was delayed up to 2 hours. in case where 45% of cement was replaced by recycled concrete fine powder, the mechanical com55 http://dx.doi.org/10.14311/app.2017.13.0055 http://ojs.cvut.cz/ojs/index.php/app v. hrbek, v. koudelková, z. prošek, p. tesárek acta polytechnica ctu proceedings pressive strength reduced to 30% while absorptivity gain reached almost 70%. the conclusion of this study proposed limitation of the replacement to 15%. m. lidmila a k. šeps [8–10] used in their studies pulverized concrete recycled from railway sleepers. the product of recycling showed low reactive potential very likely due to very fine milling of the concrete, i.e. unfolding of non-hydrated clinkers. as the level of non-hydrated clinker is low, the material is to be possibly used for strengthening of subsoil under railway structures [11]. other potential utilization, in small-scale, can be found in cementitous composite material. the knowledge of the material on the micro-scale is thus crucial to understand the composite’s behavior on higher scale levels. with such level of investigation, it is possible to develop up-scaled prediction of macro-mechanical properties, as well as optimize the efficiency of used fine powdered concrete recycle product. this study is focused on investigation of micro-mechanical and micro-structural performance of the in-put “raw” material, i.e. concrete sleepers intended for recycle. the output would serve as comparative analysis to further planed research of the recycled pulverized concrete. 2. material on of raw materials used for the recycled pulverized concrete are railway sleepers type pb2 and sb8 (used in this study, fig. 1a). it the first step of the recycling process, crude recycled concrete gravel with dimension 0–32mm (fig. 1b) arises from the kibble of the sleepers and the metallic parts and reinforcement is separated. after selection of finer concrete gravel (fraction 0–16mm, fig. 1c), the material is milled in two steps to the final product (recycled pulverized concrete, fraction 0–1mm). the fine milling of the material was delivered by company lavaris ltd. (procedure protected by patents act) using the high speed milling technique. for a purpose of the investigation, piece of crude concrete gravel (fig. 1b) was embedded in epoxy resin and let to harden for approx. 24 hours. a part of the specimen was than severed (approx. thickness 15mm) and silica-carbon papers and diamond suspensions were used for grinding and polishing of the specimen surface. this technique ensured adequate surface roughness of the sample for both indentation and sem investigation. the coordination mark was inscribe on the polished surface, serving as origin for further orientation. this enabled positioning of both investigation techniques in the very like area of the sample. 3. investigation procedure the investigation of the crude recycled concrete consisted of three main techniques, instrumental indentation, scanning electron microscopy combined with (a) . railway sleepers, type pb8. (b) . crude recycled concrete (0 32mm) (c) . “raw” recycled concrete (0 16mm) figure 1. products of concrete recycling procedure. electron probe microanalysis (sem epma) and back scattered electron micrographs (sem bse) image processing. as a result, combination of micromechanical properties, phases chemical composition and material structure enabled proper description of the material. 3.1. scanning electron microscopy the scanning electron microscopy (sem) investigation was performed in mira ii lmu (tescan corp., brno) on polished specimens coated with thin layer of carbon necessary to ensure proper conductivity of the surface. working distance of the microscope was set closely to 15 mm and accelerating voltage was set to 15 kv – to provide good signal. chemical composition (in weight percentage) of each phase characterized by the same level of grey scale was determined using epma based on the detection of the x-rays by energy dispersive x-ray detector (edx, bruker corp., berlin) on several point of each position. due to the time-demands of technique such as phase determination by epma, image analysis of sem bse micrographs was used to obtain phase volumetric representations in viewed field, i.e. material structure. the micrographs were acquired at different magnifications. the approximate area of indentation was investigated at first at the magnification 600× to gain adequate overview. than the area was scanned with higher magnification (above 1200×) for detailed study of indented positions in several images. the sem micrographs acquired with back scattered electron detector (bse) provide information about distribution (and chemical composition) of different phases due to different spectrum of the back scattering coefficient. 56 vol. 13/2017 re-cycled concrete 3.2. quasi-static nano-indentation the technique of nano-indentation (ti 7500 series, hysitron inc.) was selected for evaluation of microscale elastic properties of the material. its principal is based on dependency of probe propagation with respect to the recorded force. the force-displacement record is further further processed and the mechanical properties are calculated from the unloading part of the record. incorporated errors of the measurement, such as creep and visco-elasticity of measured phases, is avoided by appropriate test setup [12–18]. for purpose of this study, the load function kept for further described indentation similar, i.e. the “loading” and “unloading” segments lasted 5 seconds each with an in-between 60 seconds “holding” time segment. the measurement was performed in two major steps (level i and ii) to achieve efficient investigation of phases on different material levels. as an early step, a load-controlled quasi-static grid indentation was performed with an applied force of 10mn (almost maximum capacity of the transducer) to obtain route-identification of the material. the indents were each separated by 25 µm in 10 by 10 raster, which allowed both micro-mechanical evaluation of phases in large range and estimation of further used indentation depth. however, such setting does not meet the grid-indentation specifications of heterogeneous composite [19–22], particularly does not guarantee consistent half-space influenced by each indent. the first step (level i) was thus selected as displacement-driven quasi-static grid measurement with each indent driven into 150 nm depth in two positions. the grid was constructed of 21 by 21 indents pattern with 5µm spacing. the setting ensured equal indented half-space volume of material represented on the sample surface by circle with 6×indentation depth diameter [19]. the indentation covered space over the total investigated area (given by the grid dimensions 100 by 100 µm) was computed equal to 2.81% (ratio of efficiency). the second step (also displacement-driven with identical indentation depth level ii) was therefore designed for detailed investigation of selected position. the grid consisted of 25 by 25 indents separated by 2 µm and the efficiency equaled to 17.26%. the setting (especially location and size of the position) aimed for close evaluation of phases present in cement paste. 4. results interpretation the sem epma edx analysis of 41 material positions showed five main phases present in the investigated material, i.e. low-density and high-density calcium-silica-hydrate (ld c-s-h and hd c-s-h), calcium hydroxide (ch), non-hydrated clinker and aggregate – potassium and sodium feldspar and quartz (fig. 2). the phases average weight percentage of each element are summarized in tab. 1. moreover, 1.25mm by 1.25mm area, where level i indentation positions are expected, was sequentially figure 2. micrographs with identified material phases. scanned in bse mode with high resolution (5×5 sequence of 250µm wide images). the reconstruction of this area enabled exact linkage between indentation data material structure. the histograms of nano-indentation results, the indentation modulus in particular, were normalized in respect to the 0.1 of minimum value, as captured for all studied levels in fig. 3. the preliminary study of the material (fig. 3a) proved to insufficiently detail mechanical properties of all phases. (a) . reduced modulus histogram – preliminary study. (b) . reduced modulus histogram – level i. (c) . reduced modulus histogram – level ii. figure 3. histograms of indentation modulus this can be observed in comparison with level i 57 v. hrbek, v. koudelková, z. prošek, p. tesárek acta polytechnica ctu proceedings element quartz potassium sodium ld c-s-h hd c-s-h ch clinker feldspar feldspar ca 6.65 37.37 45.37 60.05 50.18 si 46.74 29.39 27.24 6.41 12.83 0.35 12.42 al 10.09 11.06 1.42 1.33 0.50 mg 0.35 0.71 0.32 0.40 na 0.87 8.09 0.11 0.24 k 14.01 0.52 0.33 0.35 0.52 s 2.24 0.81 0.16 cl 0.36 fe 1.21 1.05 1.13 0.48 o 53.26 45.63 46.45 50.56 36.95 37.99 35.50 table 1. average weight percentage of elements present in phases. of the study (fig. 3b). the results, in this case, are more likely to depict the material structure as the occurrence of reduced modulus around 40gpa is most frequent (see [23–26]). also, several peaks corresponding to number of phases identified by sem analysis can be found in the histogram. the level ii of the material was focused mostly on the investigation of cement paste, especially on csh and ch phase. again, fig. 3c confirms predominant representation of high-density calcium-silica-hydrate in the matrix. the overall results (fig. 4), as combination of both measured levels, gives detailed view of micromechanical performance of the material. the probability density function of measured indentation moduli was subsequently processed with spectral deconvolution in order to obtain gaussian distribution of expected phases. the mechanical properties of phases 1-5 in fig. 4 were thus determined as: phase 1= 19.17 ± 3.54gpa = ld c-s-h phase 2= 39.75 ± 5.98gpa = hd c-s-h phase 3= 58.59 ± 6.34gpa = calcium hydroxide phase 4= 79.78 ± 3.11gpa = presumably aggregate phase 5= 118.35 ± 16.85gpa = clinker figure 4. histogram of overall reduced modulus results. the results are consistent with previously published researches [23–32]. based on these results, maps of indentation moduli over the investigated positions (fig. 5 –6) were constructed with bordering conditions figure 5. contour map of preliminary results. figure 6. contour map of results on level ii. identical to limits in fig. 4. in addition, it was possible to link maps of indentation data to particular segments of sem bse micrographs (fig. 7). 58 vol. 13/2017 re-cycled concrete figure 7. contour maps of level i results linked to the sem bse micrograph. 5. conclusion the raw material (railway sleeper pb8) used after milling as recycled pulverized concrete in cementitious composites was in focus of this study. the micromechanical properties and material structure of this base material were investigated with combined methodology to predict performance when used as filler and cement replacement. the scanning electron microscopy methods (sem epma edx, bse) were used to determine chemical and structural composition of the material (unknown prior to the investigation). therefore, lowand high-density calcium-silica-hydrate (ld / hd c-s-h), calcium hydroxide (ch), three aggregate types and non-hydrated clinker were identified as main material phases. the nanoindentation technique over two main material levels enabled evaluation of the indentation modulus of these phases due to the spectral deconvolution of measured data. reverse reconstruction of indented areas with respect to deconvolution results let to interlink of indentation and bse diagrams. although the combined investigation provided sufficient results, more detail should be obtained in case of aggregate phases prior to modeling of the material. list of symbols f applied force [µn] hi indentation depth [nm] er reduced modulus [gpa] h hardness [gpa] hc contact depth [nm] acknowledgements the research is financially supported by czech technical university in prague – project sgs17/168/ohk1/3t/11 and by the czech science foundation research project 17-06771s. the support is gratefully appreciated. references [1] m. stibůrek. možnosti využití stavebních odpadů. beton tks 2:45–47, 2008. 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department of vehicles, ctu in prague, horská 3, praha 2, 120 00, czech republic ∗ corresponding author: bouchkar@fd.cvut.cz abstract. the model car transport system is a laboratory intended for a practical development in the area of the motor traffic. it is also an important education tool for students’ hands-on training, enabling students to test the results of their own studies. the main part of the model car transportation network is a model in a ratio 1:87 (ho), based on component units of faller car system, e.g. cars, traffic lights, carriage way, parking spaces, stop sections, branch-off junctions, sensors and control sections. the model enables to simulate real traffic situations. it includes a motor traffic in a city, in a small village, on a carriageway between a city and a village including a railway crossing. the traffic infrastructure includes different kinds of intersections, such as t-junctions, a classic four-way crossroad and four-way traffic circle, with and without traffic lights control. another important part of the model is a segment of a highway which includes an elevated crossing with highway approaches and exits. keywords: model car transportation system (model), motor traffic, intelligent transport system (its), smart city. 1. introduction with a constant growth of people mobility, expansion of cities infrastructure, increase in traffic flow, traffic density in the cities, the efficient traffic management systems are rather crucial. telematic systems are being used for traffic monitoring and control. the road traffic is affected by a number of parameters such as weather conditions, infrastructure, human behavior, technical state of a vehicle etc. telematic systems are used in city planning, transport management, parking solutions, operation of logistics and municipal services, organization of urban space, implementation of sustainable energy sources. it can as well as be comprised under an umbrella term, the smart city initiative, and it is approached from the view of different disciplines including technical, economic, humanitarian, legal etc. [1]. traffic modeling is important in the process of traffic systems implementation. the preliminary system simulation and testing in laboratory conditions with implementation of real traffic data is safe and provides feasible results that can be used for timely corrections and improvement in traffic management schemes. there are many advantages of performing traffic studies in safe laboratory conditions. these include possibility of simulating the enforcement of services work in emergency situations, simulating dangerous human behavior in traffic with a high density, dangerous road sections or on high-speed roads, planning safer routes for dangerous goods transportation, efficient city traffic planning including city municipal transport, parking areas, better overview of city safety in general etc. rather crucial for smart city technologies is testing newly developed telematic systems on real traffic data which can be realized within the suggested model[2, 3]. the smart city solutions are to take into account the existing city architecture while dealing with efficient energy flows[4]. the mathematical (or general) models of such complex systems provided with the use of tools of virtual reality to the users are nowadays very popular and widely applied. however, the real interpretation of the models is still for many people straightforward for understanding and working with. the real models are not only convenient for demonstration and illustration purposes, but they are also especially convenient for education and training. the model car transport system described herein presents a laboratory with a physical model of infrastructure and is intended for research and analysis of road traffic monitoring and management and is realized at department of vehicle technology at faculty of transportation sciences at ctu in prague. a similar project has been realized for rail traffic model within the laboratory of transport technology[5] and is running at department of transport telematics of ctu, prague. 2. system overview the basis of the model car transportation network is a model in a ratio 1:87 (ho), based on component units of faller car system[6], e.g. vehicles, traffic lights, carriageway, parking spaces, stop sections, branch-off junctions, sensors and control sections (see figure 1[6]). the faller car system was initially designed to complement railway models. in our case it is used the other way around. the fundamental car model is 5 http://dx.doi.org/10.14311/app.2017.12.0005 http://ojs.cvut.cz/ojs/index.php/app karel bouchner, alina mashko acta polytechnica ctu proceedings figure 1. some components of the model. source: [6]. figure 2. city (left) and village (right) infrastructure. supplemented with a small railway that among other things includes two railway stations and a railway crossing. it enables simulation of real traffic situations on intraand extravilan communications with a level crossing with a rail road and a section of highway (see figures 2 through 5 for reference). see the general schematic representation of the whole model on figure 6. 2.1. technical description of the model road in the model is realized with the help of wooden plates. the technology that controls vehicles’ lateral movement is allocated within the road surface (wire rail) and under the surface. model vehicles are equipped with an electric motor, a steering apparatus, a rechargeable battery, an on/off switch on the bottom of the car to start it and a reed sensor that responds to magnet effect. permanent magnet tip is attached in a flexible way under the road so that an approaching vehicle with steering mechanism in front responds to wire magnetism which is keeping vehicles on track. 2.1.1. physical layer of the model infrastructure physical part of the model is represented by the road that is built up with the help of wooden plate elements figure 3. intravilan connection. figure 4. highway section with a trumpet-type interchange. fixed on foam plastic pieces of various constructions for modeling of different terrain levels. a wire rail is embedded into the wooden road along its length. control sections are placed under the road (see figures 7 through 9 for physical layer elements for reference). sensors are used for activation of functional elements, for example stop sections, parking spaces and branch-off junctions. they contain reed contact. sensors embedded in the road are activated by magnets on the vehicles when the vehicle goes over the sensor. the sensor gives precise feedback on the traffic control. this signal activates the control of a functional element. 2.2. traffic control • traffic node control – intersection the stop section (see figure 10) is needed to initiate vehicle full stop at road junctions (intersections and level intersections), bus stops, parking slots etc. it consists of an electromagnet embedded in the roadway. with power, its magnetic field will switch off the vehicle’s motor power via the reed sensor. for example, stop sections together with traffic lights and hw control section can control traffic on a classic four-way crossroad in a completely automated way (see figure 11). • traffic node control – navigation and routing the branch-off junction (see figure 12) is used for turning of vehicles to the right or left. when activated, the magnetic field of the branch-off junction diverts a vehicle onto a second, branching contact wire. this is done through the magnet on the vehicle’s steering slider. 6 vol. 12/2017 model car transport system figure 5. supermarket with parking. figure 6. general schematic representation of traffic real model. • parking control permanent magnet is also applied in case of longer stops such as parking (see figure 13). when the parking space is activated, an integrated electric coil briefly interferes with this magnetic field. this closes the reed sensor in the vehicle and supplies power to the motor. the vehicle starts moving. the traffic infrastructure includes different types of intersections including t-junctions, a classic fourway crossroad with traffic lights and a four-way traffic circle, bus stops etc. an important part of the model is a segment of a highway which includes an elevated crossing with highway entries and exits. the highway has two lanes in each direction. such an arrangement corresponds to common real construction of highways (according to standard [7], except for a few parts where accommodation was needed due to space constrains). the two-lane arrangement enables to simulate real traffic situations, e.g. when one vehicle enters the highway and the second one is approaching to the highway at the same time, or during overtaking of vehicles on the figure 7. road with metal wire, with control element. figure 8. physical part of road construction in different sections (left – four-way highway, right – elevated intersection) and model vehicles. highway. a disadvantage of the technology is that a movement of model vehicles does not fully correspond to a behavior of real vehicles. model vehicles move by a constant speed. the speed depends only on a status of a rechargeable battery. when the battery is fully charged the speed is the highest. during its discharging, the speed of model vehicle decreases. so far, model vehicles are neither able to start and stop smoothly, nor to vary their speed. in addition, now, their speed does not correspond to a type of real vehicles. 2.3. model operation and control there are three different ways to control the whole model: • manual control an operator activates the control of functional elements manually, using push buttons and switches. model vehicles move according to the operator’s immediate actions. • preset automatic control the functional elements are controlled by associated control sections. the operation of the control sections can be either fully automatic, depending only on their initial setting, or depending on their initial 7 karel bouchner, alina mashko acta polytechnica ctu proceedings figure 9. examples of three types of control sections (from left to right): parking space, stop section and branch-off junction – an alternative to faller elements. figure 10. stop section. source: [6]. setting in combination with signals from the associated sensors. model vehicles move according to their immediate status of functional elements controlled by the corresponding control section. operation of individual control sections is not synchronized. • program control the functional elements are controlled by an associated pc-standard module and expansion modules. the pc-standard module is connected to a pc with an appropriate sw application and interconnected with expansion modules. also sensors are connected to the modules. an operator creates on the pc programs of different traffic situations for the whole model, using the sw application. the sw application also utilizes signals from sensors for its activity. modules operate according to the running program. model vehicles move according to their immediate status of functional elements controlled via corresponding modules by the running program. figure 11. example of intersection control. source: [6]. figure 12. branch-off junction. source: [6]. 3. further perspective the current model shall be used as the basis platform for future smart systems implementation, programming based on real traffic data, or for simulation of real traffic and driver behavior. it brings several challenges for the development of software applications for intelligent transport systems, implementations of more traffic data collectors such as a camera or road section detectors and other traffic surveillance and control systems[8]. the system may serve for students’ training in a wide range of disciplines applied in transport studies and research, namely, traffic management and control, city planning and infrastructure design, software development, driver behavior and humanmachine interaction just to name a few. the current project has a goal to design a fully automatically operated system with simulation of vehicle-to-vehicle (v2v), infrastructure-to-vehicle (i2v or v2i) commu8 vol. 12/2017 model car transport system figure 13. parking space. source: [6]. nication in a way so that fully autonomous vehicles could drive in real conditions. the video-based data from the cameras that will be installed in the system will provide the traffic information for decisions to be taken by robot drivers of individual vehicles. the cameras are to be installed in the control sections and on the vehicles for strategic route planning as well as immediate reaction to the traffic in front, correspondingly. the front cameras can also be used for manual vehicle control with remote driver. thus, it is planned to simulate possible future scenarios of vehicle operation in smart cities[9]. 4. conclusion the model car transport system is one of important tools, assisting students of faculty of transportation science in their study. it facilitates understanding of matters of the intelligent transport system and the smart city. finally, it enables a comprehensive application of student theoretical knowledge in a wide range of transportation theory topics, in informatics, electronics, control theory. it is an appropriate tool for software and model hardware testing of various its applications and tools applicable in smart city projects. references [1] m. lom, o. přibyl, m. svítek. industry 4.0 as a part of smart cities. conference paper. smart cities symposium prague 2016. doi: 10.1109/scsp.2016.7501015. 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[7] čsn 73 6101 projektování silnic a dálnic. praha: český normalizační institut, 2004. [8] m. a.-p.-taylor. intelligent transport systems. university of south australia, adelaide, http://www.emeraldinsight.com/doi/abs/10.1108/ 9781615832460-031, 10.2.2017. [9] conceptualizing smart city with dimensions of technology, people, and institutions. taewoo nam & theresa a. pardo, center for technology in government, university at albany, state university of new york, u.s., https: //www.ctg.albany.edu/publications/journals/dgo_ 2011_smartcity/dgo_2011_smartcity.pdf, 10.2.2017. 9 http://dsfd.fd.cvut.cz http://www.faller.de http://www.emeraldinsight.com/doi/abs/10.1108/9781615832460-031 http://www.emeraldinsight.com/doi/abs/10.1108/9781615832460-031 https://www.ctg.albany.edu/publications/journals/dgo_2011_smartcity/dgo_2011_smartcity.pdf https://www.ctg.albany.edu/publications/journals/dgo_2011_smartcity/dgo_2011_smartcity.pdf https://www.ctg.albany.edu/publications/journals/dgo_2011_smartcity/dgo_2011_smartcity.pdf acta polytechnica ctu proceedings 12:5–9, 2017 1 introduction 2 system overview 2.1 technical description of the model 2.1.1 physical layer of the model infrastructure 2.2 traffic control 2.3 model operation and control 3 further perspective 4 conclusion references 252 acta polytechnica ctu proceedings 2(1): 252–256, 2015 252 doi: 10.14311/app.2015.02.0252 the recurrent eclipsing nova u sco: a short review r. gonzález-riestra1 1xmm science operations centre, esac, madrid, spain corresponding author: rosario.gonzalez@sciops.esa.int abstract u scorpii is the recurrent nova with the shortest inter-outburst period, only ten years. the last active phase took place at the beginning of 2010, and it provided a large amount of data from both ground-based and space observatories. this paper reviews some of the more relevant recent findings and points out some, still unanswered, questions. keywords: recurrent novae uv x-rays individual: u sco. 1 introduction u scorpii is a recurrent nova that undergoes frequent outbursts. it is also an eclipsing binary (i=83 deg, thoroughgood et al. 2001), hence the geometrical parameters of the system are known with great accuracy. the orbital period is 1.23 days (schaefer 1990). ten outbursts of this system have been observed, the first one in 1863 (schaefer 2010). outbursts take place quite regularly, every ≈ 10 years. there are two long intervals of about 20 years during which no outbursts were observed (around 1927 and 1957), but very likely they were missed because the system was too close to the sun when they took place. the system is characterised by extremely short timescales: the recurrence time is only ten years, the shortest known in this type of objects, the system goes into outburst in a few hours, and decays in a few days, with t2=1.2 days and t3=2.6 days. in accordance with all this, very high velocities of the order of 104 km s−1 have been observed. all the above implies that the primary of the system is a high-mass white dwarf. the nature of the secondary star is still unclear, but it is very likely slightly evolved, k2 iv (anupama and dewangan 2000). there is a general agreement on the sizes and masses of both stars: r2 ≈2.4 r�, m2 ≈1.3 m�, m1 ≥1.37 m� (hachisu et al. 2000, thoroughgood et al. 2001). the size of the orbit is ≈ 6.7 r�, and the distance to the system has been estimated to be 12 kpc (schaefer 2010). 1.1 before 2010: uv data iue observed u sco during the first weeks of the 1979 outburst. the uv spectrum presented at the beginning low ionisation lines, and strong p-cygni profiles in the resonance lines. later on, there was an overall increase in the ionisation level, so that in the last observations figure 1: evolution of the ultraviolet spectra of u sco during active phases. spectra of days 4-16 were taken with iue during the 1979 outburst, while the last two were taken with hst-stis in 1999. 252 http://dx.doi.org/10.14311/app.2015.02.0252 the recurrent eclipsing nova u sco: a short review the strongest lines corresponded to he ii, n v, c iv, n iv, and possibly [mg v] (williams et al. 1981, barlow et al. 1981). it is worth noting that the he ii 1640 å line was always much narrower than the others. further uv observations were obtained with hststis during the first two months of the 1999 outburst, confirming the evolution observed before (fig. 1). the last spectra showed only high ionisation broad lines and a narrow he ii line. 1.2 before 2010: x-ray data u sco was known to be a strong x-ray emitter already before the 2010 outburst. the sax observation on day 19 of the 1999 outburst, when the system was detected as a supersoft source, is described by kahabka et al. (1999). these authors fitted a model atmosphere to the sax spectrum, finding a temperature of 9×105 k and a luminosity in the range 2-20×1037 erg s−1. they also found evidence for a weak harder component, and reported variations of the order of 50% during the observation. 2 the 2010 outburst by the beginning of the 21st century the recurrence time of u sco was well established, 10±2 years (schaefer 2005). the last outburst had taken place in 1999, and the date of the next one was predicted to be 2009.3±1. in very good agreement with this prediction, the outburst took place on january 28 2010 (2010.08). the time to rise to maximum brightness was only 9 hours. the optical light curve was very complex, with many different features: there was an early decline phase, a plateau, a second decline and a second plateau. and superimposed on all this, there were short-lived phenomena: flares, dips and the resumption of eclipses (see a detailed description in schaefer et al. 2011). 2.1 x-ray observations 2.1.1 swift the 2010 outburst of u sco was followed by all the x-ray missions, swift, suzaku, chandra and xmmnewton. particularly relevant is the monitoring carried out by swift since, apart from its own scientific value, it allowed to schedule efficiently target of opportunity observations with the other missions, as described below. swift observed u sco daily during two months. supersoft x-ray emission was detected around day 12, when the flux started to increase until reaching a maximum around day 31, to decrease slowly afterwards. the appearance of the supersoft emission roughly coincided with the first extended optical plateau, while the x-ray turn-off started at the time of the second optical decline (schaefer et al. 2010). 2.1.2 suzaku suzaku took three observations of u sco, but only the last one, on day 15, provided useful data, and a surprising result: the detection of a shallow x-ray eclipse (with a depth of ≈30%) in coincidence with the predicted time of the optical eclipse. from the duration of this eclipse, takei et al. (2013) estimated the size of the x-ray emitting region in 5 r�. 2.1.3 high resolution spectroscopy: chandra and xmm-newton a further x-ray observation was taken on day 18, this time with chandra letg. unfortunately, it was short, and taken out of eclipse, and therefore the suzaku result could not been confirmed. orio et al. (2013) claimed that the deep absorption lines seen in the letg spectrum (see top panel of fig. 2) are not p-cygni profiles, but just the superposition of blue-shifted absorptions from the white dwarf atmosphere (whose shift is due to a wind) and the emission lines from the ejecta. these authors derived a temperature of 7×105 k and a luminosity of 7×1036 erg s−1. this is to be compared with the sax estimations, with a much higher luminosity. it is not clear whether the 1999 outburst was intrinsically more luminous, or if the discrepancy is just due to the different atmosphere models used for both determinations. two more x-ray high resolution spectra were obtained with xmm-newton on days 23 and 35, the second one just after the x-ray peak. the xmm-rgs spectrum of the first observation showed similar emission lines as the chandra spectrum, but no absorptions. this was interpreted by orio et al. (2013) as an indication of the end of the wind phase of the white dwarf. the model atmosphere fits presented by these authors showed an increase in the temperature of the white dwarf from day 18 to day 35, at a a nearly constant x-ray luminosity. the emission lines in the day 23 spectrum can be explained by collisional ionisation, while the situation is more complex on day 35, requiring also the existence of a strong uv field. there are some unidentified lines, and others whose presence just cannot be explained. the three grating spectra are shown in left panel of fig. 2. the x-ray light curves also provided interesting information. the first xmm observation started at orbital phase 0.8, thus a clear x-ray eclipse was expected, but an irregular light curve was seen instead, with dips and flares. these dips are an indication of clumps of 253 r. gonzález-riestra material in the re-forming accretion disk (ness et al. 2012). on the contrary, on day 35 the eclipse is very well defined. it is worth to note that on this date the depth of the eclipse is 50%, showing that the x-ray source is very extended, likely a thomsom scattering corona (ness et al. 2012, orio et al. 2013). takei et al. (2013) interpreted the difference in the x-ray eclipses of days 15 and 35 as a shrinking of the x-ray emitting region, from 5 to 4 r�. the optical eclipse mapping done by schaefer et al. (2011) also indicates a shrinking of the optical source, from 4 r� on day 20 to 2.2 r� on day 50. after day 20, the source of the optical light changes, from spherically symmetric to disk-like, first with a bright rim, later brighter at the centre. xmm-newton obtained uv photometry simultaneously with the x-ray observations. on day 23 there was a very well defined uv eclipse, and the light curve was rather smooth, with some structures, as a flare that can be coincident with a similar feature in the x-ray light curve. the depth of the minima in both uv and x-ray light curves was similar (≈50%). the eclipse on day 35 is twice as deep in the uv than in x-rays, indicating a smaller emitting area (ness et al. 2012). figure 2: x-ray high resolution spectra (left panel) and light curves (right panel) of u sco during the 2010 outburst. from top to bottom, chandra letg data taken on day 18, and xmm-newton rgs spectra and epic light curves obtained on days 23 and 35. 2.2 optical spectroscopy also interesting to mention is the evolution of the optical spectra. mason et al. (2012) reported the appearance of narrow emission lines of h, he i and he ii on day 8. the radial velocity curve of these lines (that does not mach either of the two stars) indicate that they form in an accretion flow suggesting that the disk was already re-established. mason (2011) studied the abundance in the ejecta and found a high ne/o ratio, concluding that the white dwarf belongs to the o-ne-mg class, with the implications for the further evolution of the system. finally, in this outburst, forbidden emission lines have been observed for the first time, around day 46 (mason et al. 2012, dı́az et al. 2010). this result must be taken with caution, as it can be just a selection effect, since this outburst had a much better observational coverage than the previous ones. 2.3 the evolution of the outburst as shown in the previous sections, the last outburst of u scorpii has provided a large amount of data that have given us to a more clear view of the behaviour of the system. 254 the recurrent eclipsing nova u sco: a short review models show he disk was destroyed in the outburst (drake and orlando 2010). but only a few days later there were already evidences of accretion being re-established, e.g.: • appearance of narrow optical lines on day 8, • optical and x-ray eclipses on day 15, indicating the existence of a large emitting region ≈ 4-5 r�, • optical flickering and dips in the x-ray light curve on day 23. analysis of the optical light curve shows that by day 40 the disk was already fully re-established. table 1: summary of the evolution of the 2010 outburst of u scorpii day event 0 disk destroyed 8 he, hei and heii narrow emission lines 9-15 optical flares 12 supersoft phase 15 x-ray and optical eclipses 20 end of the white dwarf mass-loss 23 x-ray dips 24 flickering 30 rim-bright disk 40 optical dips 46 nebular spectrum 3 conclusions this system represents an unique opportunity to study the outburst mechanisms of recurrent novae, as it allows us to compare data obtained in several different cycles, with relatively similar instrumentation. despite of the large amount of available data, or possibly due to that, there are still many open issues. for instance, when going through the literature, the values of the mass of the envelope cover a range of 60, even for the same outburst. williams at al. (1981) and anupama and dewangan (2000) get the same value of 10−7 m� for the 1979 and the 1989 outbursts, respectively. on the other hand, takei et al. (2013) obtain 6×10−6 m� for the last outburst. also striking is the wide range of helium abundance values (n(he)/n(h)), as high as 4.5 (evans et al. 2001), but also as low 0.07 (maxwell et al. 2012). is there is a real difference in the mass ejected in the different outbursts? how can the values of the helium abundance be reconciled? obviously, the methods used to derive this quantities and the underlying assumptions are very different. a deep, uniform analysis of all existing data is necessary to find the reason for these discrepancies and to better understand the behaviour of the system. acknowledgement i warmly thank the organisers for their kind invitation to this meeting. references [1] anupama, g. and dewangan, g.: 2000, aj 119, 1359 [2] barlow, m., brodie, j., brunt, c. et al.: 1981, mnras 195, 61 [3] dı́az, m., williams, r., luna, g. et al.: 2010, aj 140, 1860 [4] drake, j. and orlando, s.: 2010: apj, 720, l195 doi:10.1088/2041-8205/720/2/l195 [5] evans, a., krautter, j., vanzi, l. and starrfield, s.: 2001, a&a 378, 132 [6] hachisu, i.; kato, m., kato, t. and matsumoro, k.: 2000, apj 534, l189 [7] kahabka, p., hartmann, h., parmar, a. and negueruela, i.: 1999. a&a 347, 43 [8] mason, e., 2011, a&a 532, l11 [9] mason, e., ederoclite, a., williams, r. et al.: 2012, a&a 544, 149 [10] maxwell, m., rushton, m., darnley, m. et al.: 2012, mnras 419, 1465 doi:10.1111/j.1365-2966.2011.19803.x [11] ness, j.-u., schaefer, b., dobrotka, a. et al.: 2012. apj 745, 43 doi:10.1088/0004-637x/745/1/43 [12] orio, m., behar, e., gallagher, j. et al.: 2013, mnras 429, 1342 doi:10.1093/mnras/sts421 [13] takei, d., drake, j., tsujimoto, m. et al.: 2013, apj 769, l4 doi:10.1088/2041-8205/769/1/l4 [14] schaefer, b.: 1990, apj 355, l39 [15] schaefer, b.: 2005, apj, 621, l53 doi:10.1086/429145 255 http://dx.doi.org/10.1088/2041-8205/720/2/l195 http://dx.doi.org/10.1111/j.1365-2966.2011.19803.x http://dx.doi.org/10.1088/0004-637x/745/1/43 http://dx.doi.org/10.1093/mnras/sts421 http://dx.doi.org/10.1088/2041-8205/769/1/l4 http://dx.doi.org/10.1086/429145 r. gonzález-riestra [16] schaefer, b.: 2010, apjs 197, 275 doi:10.1088/0067-0049/187/2/275 [17] schaefer, b., pagnotta, a., osborne, j. et al.: 2010, atel 2477 [18] schaefer, b., pagnotta, a., lacluyze, a. et al.: 2011, apj 742, 113 doi:10.1088/0004-637x/742/2/113 [19] thoroughgood, t., dhillon, v., littlefair, s. et al.: 2001, mnras 327, 1323 [20] williams, r., sparks, w. gallagher, j. et al.: 1981, apj 251, 221 256 http://dx.doi.org/10.1088/0067-0049/187/2/275 http://dx.doi.org/10.1088/0004-637x/742/2/113 introduction before 2010: uv data before 2010: x-ray data the 2010 outburst x-ray observations swift suzaku high resolution spectroscopy: chandra and xmm-newton optical spectroscopy the evolution of the outburst conclusions acta polytechnica ctu proceedings doi:10.14311/ap.2016.4.0019 acta polytechnica ctu proceedings 4:19–21, 2016 © czech technical university in prague, 2016 available online at http://ojs.cvut.cz/ojs/index.php/app radiation degradation of pcbs in sediments: comparison between two methods ľubica darážováa,∗, andrea šagátováa,b, vladimír nečasa, marko fülöpb, bumsoo hanc a institute of nuclear and physical engineering, faculty of electrical engineering and information technology, slovak university of technology, ilkovičova 3, 812 19 bratislava, slovakia b university centre of electron accelerators, slovak medical university, ku kyselke 497, 911 06 trenčín, slovakia c eb tech co., ltd., daejeon 305-500, south korea ∗ corresponding author: lubica.darazova@stuba.sk abstract. polychlorinated biphenyls are toxic compounds which have accumulated in river sediments in eastern slovakia. bioaccumulation could cause even cancer. radiation degradation with electrons is new and perspective method to dechlorinate pcbs in sediment matrix. we tested the influence of two difference chemical pretreatments and electron irradiation on pcb contaminated sediments. keywords: pcbs, cuso4 · 5h2o, electron beam, irradiation, sediment. 1. introduction polychlorinated biphenyls (pcbs) were used as additives for lubricants, adhesives in transformers and capacitors. chemical and physical properties are following: inertness, resistance to heat, non-flammability and high dielectric constant. pcbs were produced all over the world, but the production was banned in 1998. in slovakia the producer of pcbs was the chemical company chemko strážske ltd. probably, a part of produced pcbs was released to the environment through the strážske channel. over the years the sediments in surrounding area of the chemical factory (the laborec river and the zemplínska šírava lake) were contaminated with pcbs. the contamination in strážske channel exceeds 400-times the limit, which is allowed in slovakia for the sediment matrix. behavior of pcbs in the soil is strongly dependent on the basic soil properties. great attention has been devoted to the pcbs problem in soil contamination in united kingdom, usa, canada, japan and sweden. in slovakia, the monitoring of pcbs was carried out, but specific solution has not been given [1, 2]. irradiation of pcbs with electron beam has not been used very often and it is new technology, which could provide non-combustion method for pcbs degradation in soil sediments. water is a product produced during radiolysis and it is also presented in soil samples contaminated by pcbs. the presence of oxygen during the irradiation process has negative effect to radiolytic dechlorination of pcbs contaminated sediments. soil matrice is rich of organic and anorganic compounds. the dechlorination yields of pcbs were reduced, due to the organic content of the soil, which could compete with required free radiacal reactions [3]. due to the hydrophobicity of pcb, organic-rich sediments could be easily solubilized in water with the use of organic co-solvent. during the radiolysis, hydroxyl radicals react with pcbs through the addition to the phenyl rings to produce various isomeric pcb radicals (arcl(oh)) [4]. 2. pcbs degradation with electron beam 2.1. simple pcbs irradiation with electron beam electrons interact at high energies predominantly by elastic scattering by bound electrons. the chemical changes in pcbs occur through secondary reactions of radiolytic species with pcbs (fig. 1). hydrogen atoms and hydroxyl radicals react with pcbs via addition to the phenyl rings, producing pcbs radicals. pcbs that remain in association with or within the sediment phase may undergo dechlorination by thermalized electrons [5, 6]. figure 1. mechanism of radiation dechlorination of pcbs [5]. 19 http://dx.doi.org/10.14311/ap.2016.4.0019 http://ojs.cvut.cz/ojs/index.php/app ľ. darážová, a. šagátová, v. nečas et al. acta polytechnica ctu proceedings 2.2. chemical pretreatment and electron irradiation of pcbs due to the fact that pcbs are hydrophobic compounds, it is required to use co-solvent. very often is used as co-solvent propan-2-ol, which assist with the dechlorination process of pcbs. it is essential to stabilize the formed aromatic radicals, that´s why it is need the compound which could easily donate hydrogen atoms, which could stabilize the aromatic cycle. such compounds are alcohols. trifan et al. [7] tested the influence of propan-2-ol, ethanol, thf and electron beam irradiation for pcbs destruction in transformer oils and solutions (fig. 2). the best results in terms of degree of dechlorination and convertion of pcbs into biphenyls were obtained when propan-2-ol was used. also, naoh or koh are needed for basic environment, which prevent the oxidation of cl– ions to cl2 [7]. figure 2. influence of the different solvent type on dechlorination of pcbs (dose of irradiation = 40 kgy, [pcbs] = 32.6 g dm−3, naoh = 1.5 g, temp. 65 ◦c, cb = conversion of pcb into biphenyl, dd = degree of dechlorination) [7]. 3. methodology our study was aimed to compare our method of chemical pretreatment and electron irradiation of samples to results obtained by colleagues from south korea, who chemically pretreated and irradiated the samples of soil contaminated by pcb from slovakia. they used lower energy of electrons, 2.5 mev. after irradiation at eb-tech to a dose of 400 kgy the total concentration of pcbs in soil samples decreased by more than 53 % (fig. 3). 3.1. chemical pretreatment and electron irradiation of pcbs in our experiment we used 5 mev electrons and chemical pretreatment: cuso4 · 5h2o and k2co3. 20 g of wet sediment was mixed with 2 g of k2co3, which provided stable ph ∼ 10 which is essential to keep cl– anions during reaction. our samples were rich figure 3. pcb concentration in soil sample as a function of dose irradiated by eb-tech vs. ucea trenčín. of water and water molecules are competitive compounds in the process of pcbs irradiation. due to this fact, 1 g of cuso4 · 5h2o, as a water scavenger, was added to the sample. samples were irradiated in the test tubes by electron accelerator with scanning beam uelr-5-1s at the university centre of electron accelerators (ucea) of the smu in trenčín. we applied doses: 100, 300, 500 and 700 kgy to chemical pretreated samples. determination of pcb in sediment was done at the department of toxic organic pollutants of the smu in bratislava. isotopedillution method by using of 13c-labeled standard solution was used. about 0.15 g of dried homogenized irradiated sediment was soxhlet extracted with toluene (8 hours). an 1/50 aliquot of the extract was applied on multi-layer silica column (44 % sulphuric acid/potassium hydroxide/silver nitrate on activated silica gel). the pcb extract was carefully concentrated and after dilution coupled with high-resolution mass spectrometry (hrgc). the initial total content of pcbs was intended to 1842.69 ng g−1. 4. results and discussion fig. 3 shows that with increasing dose, the total amount of pcb congeners is decreasing at our experiments. at dose 700 kgy the pcb degradation falls to 46 %. from the fig. 4, it is seen, that pcbs groups are decreasing with increasing dose, but also the increase occurs, when higher chlorinated pcbs are converting to lower ones. for environmental purposes, it is required to degrade pcb with higher efficiency. our methodology (ucea) was less efficient than the methodology of eb-tech, as we reached only 46 % decrease of total amount of pcbs at 700 kgy in comparison to their 53 % decrease at 400 kgy. it could be caused by energy of electrons and also by the chemical pretreatment method. for this purpose we are going to find better chemical pretreatment methodology in our future research. 20 vol. 4/2016 radiation degradation of pcbs in sediments figure 4. concentration of pcb groups in soil samples chemically pretreated by cuso4 · 5h2o and k2co3 as a function of increasing dose of electrons. acknowledgements the authors would like to thank for financial contribution to the research to the sut grant scheme for support of young researchers. references [1] i. danielovič, j. hecl, m. danilovič. soil contamination by pcbs on a regional scale: the case of strážske, slovakia. pol j environ stud 23(5):1547–1554, 2014. http://www.pjoes.com/pdf/23.5/pol.j. environ.stud.vol.23.no.5.1547-1554.pdf. [2] i. danielovič, j. hecl, r. mati. polychlórované bifenyly a ich obsah v životnom prostredí regiónu zemplín. informačná publikácia, košický samosprávny kraj, 2009. [3] a. singh, w. kremers. radiolytic dechlorination of polychlorinated biphenyls using alkaline 2-propanol solutions. radiation physics and chemistry 65:467–472, 2002. doi:10.1016/s0969-806x(02)00360-2. [4] d. c. schmelling, d. l. poster, m. chaychian, et al. degradation of polychlorinated biphenyls induced by ionizing radiation in aqueous micellar solutions. environmental science & technology 32(2):270–275, 1998. doi:10.1021/es9704601. [5] h. yuan, h. pan, j. shi, et al. kinetics and mechanisms of reactions for hydrated electron with chlorinated benzenes in aqueous solution. frontiers of environmental science & engineering 9(4):583–590, 2015. doi:10.1007/s11783-014-0691-8. [6] m. chaychian. radiation-induced dechlorination of pcbs and chlorinated pesticides and the destruction of the hazardous organic solvents in waste water. ph.d. thesis, university of maryland, 2007. [7] a. trifan, i. calinescu, d. martin. transformation of polychlorrinated biphenyls (pcbs) into non-hazardous products by electron beam treatment. rev roum chimie 60(10):1053–1055, 2009. 21 http://www.pjoes.com/pdf/23.5/pol.j.environ.stud.vol.23.no.5.1547-1554.pdf http://www.pjoes.com/pdf/23.5/pol.j.environ.stud.vol.23.no.5.1547-1554.pdf http://dx.doi.org/10.1016/s0969-806x(02)00360-2 http://dx.doi.org/10.1021/es9704601 http://dx.doi.org/10.1007/s11783-014-0691-8 acta polytechnica ctu proceedings 4:19–21, 2016 1 introduction 2 pcbs degradation with electron beam 2.1 simple pcbs irradiation with electron beam 2.2 chemical pretreatment and electron irradiation of pcbs 3 methodology 3.1 chemical pretreatment and electron irradiation of pcbs 4 results and discussion acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2017.9.0019 acta polytechnica ctu proceedings 9:19–25, 2017 © czech technical university in prague, 2017 available online at http://ojs.cvut.cz/ojs/index.php/app preparing of the chameleon coating by the ion jet deposition method jakub skocdopolea,∗, lucrezia aversab, martin golanc, antonin schenkd, giacomo baldib, irena kratochvilovac, ladislav kalvodaa, petr nozarb a department of solid state engineering, faculty of nuclear sciences and physical engineering, czech technical university in prague, prague, czech republic b istituto materiali per elettronica e magnetismo imem-cnr, trento, italy c institute of physics, cas in prague d department of physical electronics, faculty of nuclear sciences and physical engineering, czech technical university in prague, prague, czech republic ∗ corresponding author: jakub.skoc@gmail.com abstract. preparation of chameleon coatings using an ionized jet deposition (ijd) technique is reported in the present paper. ijd is a new flexible method for thin film deposition developed by noivion, srl. the chameleon coatings are thin films characterised by a distinct change of their tribological properties according to the external conditions. the deposited films of sic and tin materials were examined by the raman spectroscopy, sem and xps. the results of the raman spectroscopy have proved an amorphous structure of sic films. the data from xps on tin films have shown that the films are heavily oxidized, but also prove that the films are composed of tin and pure ti. the sem provided information about the size of grains and particles constituting the deposited films, which is important for tribological properties of the films. deposition of the chameleon coating is very complex problem and ijd could be ideal method for preparation of this coating. keywords: ion jet deposition, chameleon coating, raman spectroscopy, scanning electron microscope, x-ray photoelectron spectroscopy. 1. introduction chameleon coating is a multilayer film, which changes its tribological attributes according to the external conditions. the main attributes interesting from a point of view of possible industrial applications are the capability of the layer to withstand high mechanical loads without a fracture, high wear resistance and low friction coefficient of the film. ionized jet deposition is one of pvd (physical vapor deposition) methods for preparation and growth of thin films. it is based on generation and application of dense pulsed electron beam which interacts with selected target material and produces a plume of the target material. the thin layer of target material is grown by condensing the plasma plume on the appropriate substrate. the flexibility of the method stands in the set of independent parameters, variety of suitable materials for the deposition and in the possibility to use multiple ijd systems for the deposition of multilayers films. the independent parameters are distance between target and substrate, working gas, pressure during deposition, frequency of electron beam pulses and trigger voltage. all these parameters can be adjusted according to the desired deposition process outcome. the ijd technique permits to deposit both inorganic and organic materials on substrates which, again, could be of inorganic or organic nature because of low temperature of deposition plasma. it is also possible to deposit conducting and non-conducting/isolating materials and optically transparent materials. however, in order to deposit strongly isolating materials, it is necessary to add an auxiliary anode between ijd hollow cathode and target of the deposited material to electrically decouple the ijd system from the target. from these characteristics it seems to be reasonable to assume that the ijd method could be ideal technique for the chameleon coatings preparation. in this article we will focus on a part of the chameleon coatings, namely, hard metal carbides and nitrides. such deposited hard films are examined by raman spectroscopy, sem and xps and the results reported and discussed. 2. theory 2.1. ionized jet deposition the ijd working tool a pulsed energetic electron beam is generated in the ijd source. the main part of the ijd source is a hollow flight-through anode/cathode. an electron discharge trigger is attached to the anode/cathode and serves for the controlled "firing" of the main discharge and for the generation of the dense electron pulse directed towards the tar19 http://dx.doi.org/10.14311/app.2017.9.0019 http://ojs.cvut.cz/ojs/index.php/app jakub skocdopole, lucrezia aversa, martin golan et al. acta polytechnica ctu proceedings figure 1. example of an ijd source [8]. get. usually (but not necessarily), either a small channel spark or a pseudospark switch, or an electron and plasma source is connected to the main anode/cathode and used as the trigger (see figure 1). the main anode/cathode electrode has the form of a hollow cylinder with openings in both bases and oriented as follows: the base with the smaller opening towards the target and the base with the larger opening towards the substrate. the hollow cathode produces, owing to a hollow cathode effect, a dense bunch of energetic electrons [7], that leave the hollow cathode towards the target in the final stage of the process. the electrons of the bunch hitting the target surface interact with the atoms of the target transferring to them their kinetic energy and creating a plume of energetic and highly ionized target plasma. the plasma plume which is predominantly expanding in the direction perpendicular to the target surface faces the hollow cathode, enters the smaller opening in the cylinder base facing the target, expands (flies) through the hollow cylinder and leaves the hollow cathode through the larger opening base towards the substrate (therefore "flight-through cathode"). 2.2. chameleon coating it is often necessary to use a combination of different materials within one thin film layer to optimize its selected properties. for example, it is possible to combine an amorphous matrix from dlc (diamond like carbon) with small grains of hard/ultra-hard material (such as tungsten carbide) within it. the grains should have the diameter in tens of nm and distance between them in nm for the best mechanical properties. the diamond like matrix provides the high hardness and low friction of deposited thin film, while the grains of carbide prevent the film from brittle fracturing and, if required, ensure the abrasive properties of the film as e.g. in the case of cutting tools. unfortunately, the low friction of this film occurs only in humid environment, because the dlc under the friction converts to graphite. the graphite does not have good wear resistance however it could be compensated by the grains of hard material. graphite has low frictions coefficient only in humid environment. to provide low friction of the film in dry environment or in vacuum it is necessary to add another material e.g. mos2 which has a hexagonal structure similar to the graphite but is excellent lubricant for dry environment or vacuum. to provide the low friction the mos2 film has to present the hexagonal planes parallel to the friction force. fortunately, the mos2 material under friction has the tendency to orient its structure in such a way that it provides the lowest friction coefficient. such ability allows substantial reduction of the system friction coefficient in dry environment just by adding the mos2 material to the dlc matrix with carbide/nitrite particles incorporated. film composition of mos2/dlc/wc could be called chameleon because it adjusts its mechanical properties to the environment in which it is situated. in humid environment, the amorphous dlc provides low friction, the mos2 and wc are scratched from the film surface in form of a debris. in dry environment the low friction is provided by mos2, the graphitized dlc and wc are scratched from the surface [1] (figure 2). we assume that the ijd could be good choice for preparing such film. there are two options how the ijd could be used. first option uses two electron guns (ijds) with two seperated targets (each for one ijd). then the substrate must be placed in position where both plasma plumes overlap. the second option uses only one electron gun but needs rotary target holder. the rotary target holder provides opportunity 20 vol. 9/2017 preparing of the chameleon coating by the ion jet deposition method figure 2. principle of chameleon coating function [1]. name substrate film time [min] pressure [mbar] distance [mm] kuba7 silicon wafer tin 15 1.6 105 kuba9 stainless-steel tin 14 1.4 105 k2 aluminium alloy sic 30 0.6 180 table 1. deposition parameters. of deposition from multiple targets only with one electron gun (ijd). 3. experiment two types of hard material have been prepared trying to enhance the resistance of the deposited layer against fracturing, namely, the sic deposited on aluminium alloy and tin deposited on stainless steel and single crystalline silicon. all deposition events have been performed in ar atmosphere. the deposition time, process gas (ar) pressure during deposition and distance between the target and the substrate (see table 1) have been chosen as important process parameters to be adjusted for different deposited samples. the films have been analysed by raman spectroscopy, sem and xps. 4. results the thickness of films was not directly measured, but it was deduced from the xrd results and from the values obtaind for similar films, which was deposited with the same deposition parameters.the roughly estimated thickness value obtaind in such way amounted several microns. 4.1. raman spectroscopy the measurement was performed at room temperature using an excitation laser source with the wavelength of 488 nm. the exact positions of the peaks (bands) in the raman spectrum were found by using a function findpeaks in matlab r2008b. the raman spectrum of the sample k2 is shown in the figure 3: figure 3. raman spectrum of k2 sample. the measured raman band at 478 cm−1 (black arrow) is coresponding to si-si bond in amorphous phase and 1448 cm−1 (red arrow) is coresponding to c-c amourphous bond. the raman band at 829 cm−1 (green arrow) coresponds to si-c bond. the low intesity of si-c peak is not due to the number of the bonds, but it follows from the generally small raman transition probability. the bands at 1005 cm−1, 2283 cm−1 and 2889 cm−1 (blue arrows) link to the signals of si-h and c-h bonds [2]. the raman spectrum of the sample k7 and k9 is shown in figure 4. according to [3], the peaks for tin are found at 250; 330-360 and 560 cm−1 and are marked by blue arrows. observed peaks might be, perhaps, assigned to the presence of tin in the deposited layer, however, the intensity of the peaks is too weak to provide a decisive prove for tin compound existence. the peaks marked by black arrows correspond to monocrystalline silicon [6]. 21 jakub skocdopole, lucrezia aversa, martin golan et al. acta polytechnica ctu proceedings figure 4. raman spectra of samples k7 and k9. 4.1.1. discussion the raman bands, for all samples, exhibit some shifts in their position, when compared with literature [2] [3]. this fact could be caused by some small variations of the stoichiometry of deposited films. for supposed application it is necessary to obtain well-defined grains of sic or tin. unfortunately for this aim the thin films of sic are amorphous. in order to be able to grow the polycrystalline sic layers, it is necessary to heat the substrate during the deposition process, the treatment that was not performed during the present depositions. it is not possible to decide whether or not the thin films of tin are really composed of pure tin only. or consist of some mixture of metallic ti, tin and titanium oxide. all these materials exhibit rather similar raman spectra [4] and, taking in account the small intensity of the spectra obtained from present measurements it is impossible to conclude about the nature of grown material. 4.2. xrd the glancing angle xrd analysis was performed on θ / θ diffractometer x'pert operating in braggbrentano geometry with x-ray source λ (cokα )=0,179026 nm. the xrd patterns obtained with the sample k7 (figure 5) show only amorphous halo. the amorphous halo is indicative of two facts, firstly that the film is thick enough that only the diffraction signal coming from the film is registered, and secondly that the structure of the films is amourphous in its nature. the xrd patterns obtained with sample k9 (figure 6) show only presence of austenitic steel. the result indicates that the film is not thick enough to stop the diffraction signal from the substrate and that the structure of film is amorphous. 4.3. sem the sem image of sample k2 (figure 7) contains particles with size under 500 nm. the film background is homogenous despite of fact that on its surface are particles. sic is very hard material and was expected to consist of separated particles figure 5. xrd patterns obtained with sample k7 figure 6. xrd patterns obtained with sample k9 figure 7. sem micrograph of sample k2: sic layer on aluminium alloy substrate. the tin was deposited on the stainless steel and monocrystalline silicon substrates. in sem image of sample k7 (figure 8) shows particles with size under 150 nm. the film is very homogenous, in agreement with its amorphous nature confirmed by xrd. the sem image of sample k9 (figure 9) shows particles under 250 nm. the film is amourphous, as confirmed by xrd. the red lines indicate the differencies in the film structure due to existence of underlying grain boundaries present in the polycrystalline substrate. 22 vol. 9/2017 preparing of the chameleon coating by the ion jet deposition method figure 8. sem micrograph of sample k7: tin film on si single crystal substrate. figure 9. sem micrograph of sample k9: tin layer on stainless steel substrate. 4.3.1. discussion all samples exhibit some type of particles on their surface. the particles differ from each other mostly in their size. the size of particles is important for improvement of the mechanical properties. the particles should not be too large because in such case the cracks through the coating could propagate also through the particle and even not too small because the crack of the matrix may avoid the particle just by circling around it. the smallest particles are observed on the tin film deposited on single crystalline silicon. the ideal size of the particles is found to be between tens and hundreds of nanometers [1]. figure 10. xps spectra recorded with samples k7 and k9; pe = 50 ev. 4.4. x-ray photoelectron spectroscopy the xps analysis was performed using the x-ray source working at the mgkalpha energy 1253.6 ev and the wsw analyzer that can be operated at different pass energy (pe), in order to collect spectra with different resolutions. in particular, the following settings were used: 1.) pe = 50 ev to acquiring the wide range spectra used in identification of the main chemical species on the surface 2.) pe = 20 ev to acquire single peak spectra with an intermediate resolution ideal to perform a quantitative analysis and 3.) pe = 10 ev to obtain single peak spectra at high resolution. the recorded wide range spectra are shown in figure 10. in the ti-n samples, the main chemical species on the surfaces are those attended, i.e. titanium (ti2p), nitrogen (n1s), oxygen (o1s), silicon (si2p), carbon (c1s) from contaminants and boron (b1s) from the pbn sample holder. the corresponding auger electrons lines, that are always present in a typical xps spectrum, are marked, too. the o1s and c1s peaks appear quite intense, in particular compared to the n1s peak that is expected to be stronger, even taking into account its low photoemission cross section: this suggests that the films could be strongly oxidized. spectra at pe=20 ev give more indication on oxidation processes together with the quantitative evaluation of surface atomic percentages, that are collected in table 2. there is a significant amount of carbon that is present in k samples: the atomic percentages here reported take into account any contribution from the sample holder that has been properly subtracted from the c1s peak, thus the carbon percentage relies only on carbon inside or over the films. the ti2p peak is centered at a be of 459ev, the value that is typical of titanium in high oxidation state bonded to oxygen, and shows a long tail at lower bes, usually related to lower valence states. the lineshape analysis (through voigt functions, considering a shirley background) put in evidence up to three contributions to the peak: the main one (light blue peak) is at higher be and can be correlated to tio2, the second one, centered at 457.5 ev, is out of the typical region of tin (that is at even lower binding 23 jakub skocdopole, lucrezia aversa, martin golan et al. acta polytechnica ctu proceedings sample % ti2p % n1s % o1s % c1s % si2p kuba7 13.2 3.3 38.5 38.2 6.8 kuba9 7.2 1.2 24.3 67.3 table 2. chemical composition obtained from xps experiments for the k7 and k9 samples. figure 11. xps spectrum close to ti 2p line for k7 and k9 samples. figure 12. xps spectrum courses close to o1s line observed for the k7 and k9 samples. energy) and could be assigned to a mixed ti-o-n phase (oxynitrides), while the third one at 455.4 ev is the tin contribution. its intensity is low compared to the tio2 phase, suggesting that the tin layer is few nanometers deeper: the ti2p escape length in tio2 is 2.12 nm, thus the signal from the nitride layer comes from about 6-8nm below the surface. the o1s spectra for all samples show the presence of a main contribution at about 530-530.5 ev and a less intense feature at higher binding energy. the lineshape analysis, however, reveals a third contribution at around 532 ev. the most intense peak at low be can be assigned to metal oxides, i.e. titanium oxides, while the peak at higher be is typically assigned to oh groups at the surface. taking into account the indications given figure 13. xps spectra close to n1s line observed for the k7 and k9 samples; experimental values (the noisy courses) and the fit curves are shown. for more details see text. by ti2p spectra, we can assume that this component together with the other o peak can be related to oxynitrides species. this assumption can be confirmed by looking at the n1s spectra (figure 13). each nitrogen core level can be reproduced by the peak at low be ( 396-397 ev, in blue and purple) that is assigned to the nitride layer, plus one or two components at higher be that can be reasonably correlated to the oxynitrides already found in the o1s lineshape analysis. 4.4.1. discussion the measurement has been performed before the sic film has been prepared. thus only tin film is analyzed. it was expected that the measured data would originate mostly from the oxidation layer on the surface of the sample. this assumption was confirmed. the xps does not prove unambiguously that the film is composed of tin only. it seem reasonable to state that the xps proves that ti and n species are present in the film, however they could be combined in some mixture of ti, tin and ti-o-n (oxynitrides) phases. 5. conclusions both films are composed of particles created during the deposition process, the feature which is necessary for their role in the chameleon coating. the deposition of particles from these materials were successful, however further research should be performed to ensure the reliable particle size control. the thickness of films is found to be in units of micrometres. both 24 vol. 9/2017 preparing of the chameleon coating by the ion jet deposition method tin and sic films are amorphous, as proven by raman spectroscopy and xrd. further research steps towards the chameleon layer preparation should be the deposition of mos2 films and the incorporation of the mos2 into dlc matrix and their characterisation. acknowledgements the authors are indebted to ing. gianpiero tedeschi, ceo, noivion srl, for providing the deposition technology for samples preparation and to the institute of materials for electronics and magnetism, cnr, trento for its hospitality, especially to the institute director dr. salvatore iannotta, and to the trento unit head dr. roberto verucchi. this work was supported by the grant agency of the czech technical university in prague, grant no. sgs16/245/ohk4/3t/14. references [1] a. a. voevodin, j. s. zabinski. supertough wear-resistant coatings with ‘chameleon’ surface adaptation. thin solid films 370: 223-231, 2000. [2] y. inoue, s. nakashima, a. mitsuishi. raman spectra of amorphous sic. solide state communations 48(12): 1071-1075, 1983. [3] w. spengler, r. kaiser. raman scattering, superconductivity, and phonon density of states of stoichiometric and nonstoichimetric tin. physical review b 17(3): 1095-1101, 1978. [4] r. j. nemanich, c. c. tsai, g. a. n. connell. interference-enhanced raman scattering of very thin titanium and titanium oxide films. physical review letters 44(4): 273-276, 1980. [5] j.skocdopole. preparation of thin layers by ijd method and characterization of their physical-chemical properties.prague. bachelor thesis. czech technical university in prague, faculty of nuclear sciences and physical engineering, 2015. [6] j.skocdopole. construction of system for deposition of thin films by ijd method and trial deposition of chosen materials..prague. research thesis. czech technical university in prague, faculty of nuclear sciences and physical engineering, 2016. [7] s.o. leathem,et al. influence of the injection speeds on the discharge characteristics and minimization of delay time in a pseudospark discharge. the open plasma physics journal 3: 20-27, 2010. [8] p. nozar. ionized jet deposition technique, a new class of thin film deposition equipment, vacuum symposium coventry, 16 october 2014, the united kingdom [9] s.tricot, et al. dynamics of a pulsed-electron beam induced plasma: application to the growth of zinc oxide thin ïňąlms journal of physics d 41(17): 10pp., 2008 [10] m.a. raadu, j.j.rasmussen. dynamical aspects of electrostatic double layers astrophysics and space science 144: 43-71, 1988 [11] yu. d. korolev, et al. electron beams generated at different stages of the pseudospark discharge, the 27th international conference on phenomena in ionized gase, eindhoven, 18-22 july 2005, the netherlands 25 acta polytechnica ctu proceedings 9:1–7, 2017 1 introduction 2 theory 2.1 ionized jet deposition 2.2 chameleon coating 3 experiment 4 results 4.1 raman spectroscopy 4.1.1 discussion 4.2 xrd 4.3 sem 4.3.1 discussion 4.4 x-ray photoelectron spectroscopy 4.4.1 discussion 5 conclusions acknowledgements references 127 acta polytechnica ctu proceedings 1(1): 127–131, 2014 127 doi: 10.14311/app.2014.01.0127 multifrequency behaviour of the gamma-ray binary system psr b1259-63: modelling the fermi flare brian van soelen1, pieter j. meintjes1 1department of physics, university of the free state, bloemfontein 9300, south africa corresponding author: vansoelenb@ufs.ac.za abstract this paper presents a brief overview of the multifrequency properties of the gamma-ray binary system psr b1259-63 from radio to very high energy gamma-rays. a summary is also presented of the various models put forward to explain the fermi “flare” detected in 2011. initial results are presented of a new turbulence driven model to explain the gev observations. keywords: gamma-ray binaries individual: psr b1259-63. 1 introduction there are only a handful of known gamma-ray binary systems. of these, it is only in psr b1259-63 that the nature of the compact object is known due to the detection of pulsed radio emission (johnston et al., 1992a). the very high energy (vhe) nature of the source was confirmed by the h.e.s.s. telescope around the 2004 periastron passage (aharonian et al., 2005). the gammaray binary system consists of a 48 ms pulsar, in an eccentric (e = 0.87) 3.4 year orbit around a be star (e.g. johnston et al., 1992, 1994), with a spin-down luminosity of ė = 8.3 × 1035 erg s−1 (e.g. wang et al., 2004). recent spectroscopic observations have placed better constraints on the parameters of the optical companion, ls 2883, and the binary system (negueruela et al., 2011). in this paper, we present an overview of the multifrequency behaviour of psr b1259-63, and discuss some of the modelling undertaken to explain the large “flare” event detected by fermi after the previous periastron passage. 2 multifrequency observations in this section we summarize the multifrequency behaviour of the non-thermal emission originating from psr b1295-63, discussing the radio, x-ray, vhe gamma-ray and, finally, the gev gamma-ray observations. 2.1 radio behaviour psr b1259-63 was originally detected as a radio pulsar during a galactic plane survey undertaken with the 64 m parkes radio telescope (johnston et al., 1992b). observations around periastron showed that there was an increase (and variability) in the dispersion measurement of the pulsed signal and an eclipse of the pulsed signal between approximately 20 days before periastron to 20 days after (e.g. johnston et al., 1996, 1999, 2001). significantly, around periastron an unpulsed and variable radio emission was detected with a flux density of 10 − 30 mjy, higher than the flux of the pulsed signal (2 − 3 mjy). these observations are explained by the pulsar passing through and/or behind the circumstellar disc of the be star, and the formation of synchrotron emission in the pulsar wind nebula. more recently a large extended pulsar wind nebula has also been reported around periastron by moldón et al., (2011). 2.2 x-ray behaviour a non-thermal x-ray component has been detected from psr b1259-63 across the whole of the orbit, with early detections determining a luminosity of l ∼ 1034 erg s−1 around periastron (kaspi et al., 1995). combined asca, chandra, xmm -newton, bepposax, swift and suzaku observations covering the 1997 – 2010 periastron passages have shown a consistent x-ray light curve (see chernyakova et al., 2006, 2009, abdo et al., 2011, and references therein). these datasets have presented particular coverage focused on the disc crossing epochs. the observations show that the x-ray emission peaks within two narrow time bands most likely associated with the pulsar approaching the circumstellar disc. pavlov et al., (2011) also reported on an extended x-ray structure detected by chandra near apastron. the observations in the 0.5–8 kev en127 http://dx.doi.org/10.14311/app.2014.01.0127 brian van soelen, pieter j. meintjes ergy band showed an elongated structure (∼ 4′′) with a luminosity of ∼ 1032 erg s−1 (assuming a distance of 3 kpc). it is likely that this is associated with the pulsar wind nebula outflow. 2.3 vhe behaviour psr b1259-63 was detected for the first time at vhe with the h.e.s.s. telescope during the 2004 periastron passage (aharonian et al., 2005). the observations showed a variable light curve with an implied maximum flux a few days before and after periastron. the system has since been detected during 2007 (aharonian et al., 2009) and 2011 (abramowski et al., 2013), albeit with much less coverage. the h.e.s.s. observations appear to be consistent with the double peak structure observed at x-ray and radio wavelengths, but this conclusion is not definitive due to lower coverage during the pre-periastron approach of the pulsar. the spectrum of the vhe emission also appears to be consistent with a single power-law distribution, with no cut-off in the spectrum (see e.g. fig. 4 in abramowski et al., 2013). while the coverage over the last periastron passage was poor, the 2011 observations were coincident with the fermi “flare” discussed in the next section. unlike the fermi detection, the h.e.s.s. observations showed no significant change between the preand post-flare event, and morphologically the gev to tev energies are not consistent with a single power-law distribution (see fig. 5 in abramowski et al., 2013). 2.4 he behaviour due to the consistency of the radio, x-ray and vhe light curves, it was presumed (before 2010/2011) that fermi would observe similar behaviour. in fact, it was not actually clear whether the system would be detectable by fermi (e.g. chernyakova et al., 2009). the gamma-ray detection and rapid transient behaviour that were reported by fermi were completely unexpected. as reported by abdo et al. (2011), the system was relatively undetected before periastron, with a faint “brightening” phase which began around the point of the first disc crossing and lasted until ∼ 18 days after periastron. however, approximately 30 days after periastron, there was a rapid increase in the detected emission from the source showing an average flux of f (> 100mev) = (4.4±0.3stat±0.7sys)×10−10 erg cm−2 s−1, with a photon index of γ = (1.4±0.6stat±0.2sys) and a cutoff energy ec = (0.3±0.1stat±sys) gev. this “flare” was detected until > 60 days after periastron. significantly, radio, x-ray, and vhe observations showed no unusual or flaring events. most surprisingly, the daily binned light curve showed a peak luminosity of lγ ≈ 8 × 1035 erg s−1 (for isotropic emission at an assumed distance of 2.3 kpc), corresponding to ≈ 100 per cent of the spin-down power of the pulsar. the emission detected by fermi has proved difficult to reconcile with models of the system. in the next section we discuss some of the models pertaining to gev energies and the fermi flare. 3 modelling the fermi flare 3.1 effect of the infrared excess in anticipation of the fermi observations of psr b125963, van soelen & meintjes (2011), van soelen et al. (2012) and meintjes & van soelen (2012), considered the influence the infrared flux from the circumstellar disc of the be star would have on gamma-ray production. if the same population of electrons which produces the vhe emission through inverse compton scattering of the optical photons, scatters the infrared photons from the disc, it will produce emission in the gev energy range. first using an isotropic approximation (van soelen & meintjes, 2011) and then a full anisotropic approximation, which took into account the anisotropic inverse compton scattering as well as the changing flux from the circumstellar disc as measured from the pulsar’s position (van soelen et al., 2012), we showed that the scattering of the infrared photons would increased the flux at gev energies by a factor of 2. while it was thought that the large size of the circumstellar disc would result in it dominating near the disc crossing, we found that the low density of the disc mitigated this effect as the majority of the infrared emission occurred near the star and not far out in the disc. however, the flux was below what was detected by fermi. 3.2 cold pulsar wind khangulyan et al., (2012) proposed that the scattering of the cold pulsar wind (γ = 104) off photons from the circumstellar disc, after the second disc passage, could produce the observed fermi flare. the authors propose that the shock-front around the pulsar is confined within the disc, and after the pulsar leaves the circumstellar disc the opening of the shock structure will allow the inverse compton scattering of the cold pulsar wind to produce the flare. one problem presented by this model is the number of additional photons required to do this. the authors estimate that the additional photon distribution must have a luminosity of approximately 40 per cent of the stellar luminosity. this does not appear to be consistent with the observations obtained around periastron nor with the estimation of the available energy from shock-heating (van soelen et al., 2012). however, it must be cautioned that the reported observations were not undertaken at the same time as 128 multifrequency behaviour of the gamma-ray binary system psr b1259-63: modelling the fermi flare the fermi flare and new observations around the next periastron passage must be obtained, to obtain better constraints. 3.3 sph modelling a process of smooth particle hydrodynamic (sph) modelling of psr b1259-63 was undertaken by takata et al., (2012) to model the behaviour of the be star’s circumstellar disc around periastron. the simulations show that the disc should be considerably disrupted during the pulsar crossing. however, observations on 5 january 2011, approximately three weeks after periastron, showed that the mid-infrared flux was consistent with previous observations (see van soelen et al., 2012). this suggests that either the disc was not dramatically effected, or that there was a rapid recovery. in addition, the approximation of the emission light curve, while in agreement with the x-ray emission, was not in agreement with the gev light curve, as the simulations suggested that the maximum emission should occur before periastron (see figs. 3 and 5 in takata et al., 2012). 3.4 doppler boosting emission the most recent model which has been suggested is that the synchrotron emission is doppler boosting to gev energies to produce the observed flare. this was modelled in detail by kong et al. (2012). the authors consider two emission regions along the shock front: the first near the apex with a bulk lorentz factor of γ = 1 and the second along the shock tail which is mildly relativistic (γmax ≈ 2) and is beamed along the shock. these simulations are able to account for the fermi detection during the flare period, but are not so successful during the pre-periastron period – though it should be noted that the pre-periastron detections have a much lower significance. the light curve predictions also suggest only a single peaked emission around periastron for the gev energies, and a double peaked structure from the x-ray and vhe emission. however, the modelled light curves predict a higher flux for the gev and tev emission around periastron (see fig. 5 in kong et al., 2012), an effect which is not observed. 3.5 turbulence driven emission? thus far all models have presented unique problems in modelling the fermi flare. for this reason it is still necessary to consider alternative models. below we consider the possibility of a turbulence induced flare. hydrodynamical simulations of colliding wind systems have shown that kelvin-helmoltz instabilities develop along the dividing line between the pulsar and stellar winds (see e.g. bosch-ramon et al., 2012). if these instabilities form a turbulent medium, it may be possible for secondary particle acceleration to occur due to second-order fermi acceleration (see e.g. bosch-ramon & rieger 2012). it has been considered by a number of authors that under the effect of radiative cooling and second-order fermi acceleration, a maxwellian-like distribution will form (see e.g. schlickeiser 1985 for a discussion of firstand second-order acceleration with radiative cooling). such a secondary distribution of electrons may be responsible for producing the gev emission in psr b1259-63 if acceleration in the turbulent region occurs around the second disc crossing. as a first approximation we consider the modified maxwellian distribution suggested by stawarz & petrosian (2008), given by ne = n0γ 2 exp [ −1 a ( γ γeq )a] , (1) where a = 3 − q for dominant synchrotron/thomsonlimit inverse compton cooling, or a = 1.5− q for dominant klein-nishina inverse compton cooling. here q is the index of the turbulence power-law, and γeq is determined by equating the appropriate radiative cooling time to the acceleration time (see stawarz & petrosian, 2008, and reference therein, for details related to the acceleration). we calculate the best fit inverse compton emission for the electron distribution given by equation (1) for isotropic scattering (e.g. blumenthal & gould, 1970), with a blackbody distribution of photons (t? = 33 000 k). in this first approximation an isotropic radiation field has been assumed. the equilibrium energies required to fit the maxwellian distributions (equation 1) were found to be γeq = 1614.4 for the bohr approximation (q = 1), γeq = 656.30 for the hard-sphere approximation (q = 2), γeq = 1030.4 for the kolmogorov spectrum (q = 5/3) and γeq = 1196.7 for the kraichnan spectrum (q = 3/2). only a = 3 − q was considered. a constant factor was applied to match the flux level of the flare. the resulting fits are shown in fig. 1. the figure shows that the distribution produces a reasonable fit to the resulting fermi detection. initial considerations of the parameter space suggest that it may be possible to accelerated electrons to the appropriate energies around the period of periastron. a more detailed analysis is currently under-way. 129 brian van soelen, pieter j. meintjes -12 -11 -10 7 8 9 10 11 lo g ν f ν [e rg s -1 c m -2 ] log energy [ev] q=2 q=1 q=5/3 q=3/2 figure 1: inverse compton spectrum formed by a modified maxwellian electron distribution fitted to the post-periastron fermi flare detection. references [1] abdo, a.a., ackermann, m., ajello., m. et al.: 2011, apj, 736, l11 doi:10.1088/2041-8205/736/1/l11 [2] aharonian, f., et al. (h.e.s.s. collaboration): 2005, a&a, 442, 1 [3] aharonian, f., et al. (h.e.s.s. collaboration): 2009, a&a, 507, 389 [4] abramowski, a., et al. (h.e.s.s. collaboration): 2013, a&a, 551, a94 [5] blumenthal, g.r., gould, r.j.: 1970, reviews of modern physics, 42, 237 [6] bosch-ramon, v., rieger f.m.: 2012, in astroparticle, particle, space physics and detectors for physics applications proceedings of the 13th icatpp conference.. g. simone et al. (eds.), 219 [7] bosch-ramon, v., barkov, m.v., khangulyan, d., et al.: 2012, a&a, 544, 59 [8] chernyakova, m., neronov a., lutovinov a., et al.: 2006, mnras, 367, 1201 doi:10.1111/j.1365-2966.2005.10039.x [9] chernyakova, m., neronov, a., aharonian f., et al.: 2009, mnras, 397, 2123 doi:10.1111/j.1365-2966.2009.15116.x [10] johnston, s., manchester, r.n., lyne, a.g, et al.: 1992a, apj, 387, 37 [11] johnston, s., lyne, a.g, manchester, r.n., et al.: 1992b, mnras, 255, 401 [12] johnston, s., manchester, r.n., lyne, a.g, et al.: 1994, mnras, 268, 430 [13] johnston, s., manchester, r.n., lyne, a.g, et al.: 1996, mnras, 279, 1026 [14] johnston, s., manchester, r.n., mcconnel, d.: 1999, mnras, 302, 277 [15] johnston, s., wex, n., nicastro, l., et al.: 2001, mnras, 326, 643 [16] kaspi, v. m., tavani m., nagase f., et al.: 1995, apj, 453, 424 doi:10.1086/176403 [17] khangulyan, d., aharonian, f.a., bogovalov, s.v., et al.: 2012, apj, 752, l17 doi:10.1088/2041-8205/752/1/l17 [18] kong, s. w., cheng, k. s., huang, y. f.: 2012, apj, 753, 127 doi:10.1088/0004-637x/753/2/127 [19] meintjes, p.j., van soelen, b.: 2012, in multifrequency behaviour of high energy cosmic sources, f. giovannelli, l. sabau-graziati (eds.) mem. s.a.it., 83, 246 [20] moldón j., johnston s., ribó m., et al.: 2011, apj, 732, l10 [21] negueruela i., ribó m., herrero a., et al.: 2011, apj, 732, l11 [22] pavlov g. g., chang c., kargaltsev o.: 2011, apj, 730, 2 [23] schlickeiser, r.: 1985, a&a, 143, 431 [24] stawarz, l., petrosian, v.: 2008, 681, 1725 [25] takata, j., okazaki, a. t., nagataki, s. et al.: 2012, apj, 750, 70 [26] van soelen, b., meintjes, p.j.: 2011, mnras, 412, 1721 [27] van soelen, b., meintjes, p.j., odendaal, a., et al.: 2012, mnras, 426, 3135 [28] wang, n., johnston, s., manchester, r. n.: 2004, mnras, 351, 599 discussion carlotta pittori: in august 2010 agile reported the detection of gamma-ray activity above 100 mev from the psr b1259-63 region during the initial approach-to-periastron part of the orbit. fermi did not confirm this detection, but later in november reported faint gamma-ray emission before the big 2011 flare. are you aware of the agile detection and can the model explain a ∼ 100 mev emission just before the disk passage? 130 http://dx.doi.org/10.1088/2041-8205/736/1/l11 http://dx.doi.org/10.1111/j.1365-2966.2005.10039.x http://dx.doi.org/10.1111/j.1365-2966.2009.15116.x http://dx.doi.org/10.1086/176403 http://dx.doi.org/10.1088/2041-8205/752/1/l17 http://dx.doi.org/10.1088/0004-637x/753/2/127 multifrequency behaviour of the gamma-ray binary system psr b1259-63: modelling the fermi flare brian van soelen: yes i am aware of the reported agile detection, as well as the fermi follow-up analysis that did not detect emission. at this point i do not believe any model has completely accurately modelled the events that occurred around the previous periastron passage, including the reported detection by agile. 131 introduction multifrequency observations radio behaviour x-ray behaviour vhe behaviour he behaviour modelling the fermi flare effect of the infrared excess cold pulsar wind sph modelling doppler boosting emission turbulence driven emission? 26 acta polytechnica ctu proceedings 2(1): 26–32, 2015 26 doi: 10.14311/app.2015.02.0026 the space density of magnetic and non-magnetic cataclysmic variables, and implications for cv evolution m. l. pretorius 1department of physics, university of oxford, denys wilkinson building, keble road, oxford ox1 3rh, united kingdom 2previous address: school of physics and astronomy, university of southampton, highfield, southampton so17 1bj corresponding author: retha.pretorius@astro.ox.ac.uk abstract we present constraints on the space densities of non-magnetic and magnetic cataclysmic variables, and discuss some implications for models of the evolution of cvs. the high predicted non-magnetic cv space density is only consistent with observations if the majority of these systems are extremely faint in x-rays. the data cannot rule out the very simple model where long-period ips evolve into polars and account for the entire short-period polar population. the fraction of wds that are strongly magnetic is not significantly higher for cv primaries than for isolated wds. finally, the space density of ips is high enough to explain the bright, hard x-ray source population seen in the galactic centre. keywords: cataclysmic variables dwarf novae nova-likes intermediate polars polars x-rays. 1 introduction there are many remaining uncertainties in the theory of cataclysmic variable (cv) formation and evolution, as well as several serious discrepancies between predictions and the properties of the observed cv population (e.g. patterson 1998; pretorius, knigge & kolb 2007a; pretorius & knigge 2008a,b; knigge, baraffe & patterson 2011). in order to constrain evolution models, we require more precise observational constraints on the properties of the galactic cv populations. a fundamental parameter predicted by evolution theory, that should be more easily measured than most properties of the intrinsic cv population, is the space density, ρ. a few specific, important open questions concerning the formation and evolution of cvs are: (i) is the large predicted population of non-magnetic cvs at short orbital period consistent with the current observed cv sample? (ii) is there an evolutionary relationship between ips and polars? (iii) can the intrinsic fraction of mcvs be reconciled with the incidence of magnetic wds in the isolated wd population? (iv) do mcvs dominate the total galactic x-ray source populations above lx ∼ 1031 ergs−1? these questions can be addressed empirically, with reliable measurements of the space densities of the different populations of cvs. uncertainty in ρ measurements is in part caused by statistical errors, resulting from uncertain distances and small number statistics. however, the dominant source of uncertainty is most likely systematic errors caused by selection effects. fig. 1 shows some reported measurements (differing by several orders of magnitude for non-magnetic cvs). selection effects are most easily accounted for in samples with simple, well-defined selection criteria. in the absence of a useful volume-limited cv sample, a purely flux-limited sample is the most suitable for measuring ρ. whereas optical cv samples always include selection criteria based on, e.g., colour or variability, there are x-ray selected cv samples that are purely flux-limited. all active cvs show x-ray emission generated in the accretion flow. furthermore, mcvs are luminous x-ray sources, while the correlation between the ratio of optical to x-ray flux and the optical luminosity of non-magnetic cvs, implies that an x-ray flux limit does not introduce as strong a bias against shortperiod cvs as an optical flux limit (e.g. van teeseling et al. 1996). here we use 2 x-ray surveys, the rosat bright survey (rbs; e.g. schwope et al. 2002), and the rosat north ecliptic pole (nep) survey (e.g. henry et al. 2006) to construct x-ray flux-limited cv samples. we then provide robust observational constraints on the space densities of both magnetic and non-magnetic cvs, by carefully considering the uncertainties involved. 26 http://dx.doi.org/10.14311/app.2015.02.0026 the space density of magnetic and non-magnetic cataclysmic variables, and implications for cv evolution figure 1: some previously reported measurements of the space densities of different cv populations. we provide additional background on the questions listed above in section 2, present the measurements in section 3, discuss the implications of the results in section 4, and finally list the conclusions in section 5. 2 context 2.1 missing non-magnetic cvs it is not clear whether the present-day observed nonmagnetic cv population is inconsistent with theoretical expectations. population synthesis models predict that only ' 1 percent of all cvs are long-period systems (see e.g. kolb 1993). the the vast majority of cvs should therefore be intrinsically faint. pretorius et al. (2007a) and pretorius & knigge (2008b) used a specific model of kolb (1993), together with models of the outburst properties and seds of cvs, to show that, although observed cv samples are strongly biased against shortperiod systems, a currently undetected faint cv population cannot dominate the overall population to the extent predicted by this particular population synthesis model. knigge et al. (2011) used the properties of cv donor stars to conclude that the aml rate is lower above the gap and higher below the gap than predicted by the standard model. this leads to larger predicted factions of both period bouncers and long-period systems. whether this is consistent with observed cv samples is not yet known. that a large faint population of cvs exists is now clear from observations (gansicke et al. 2009; patterson 2011). however, whether observations have truly revealed a population as large as predicted remains to be seen. some predicted values of the non-magnetic cv space density are as high as 2 × 10−4 pc−3 (de kool 1992; kolb 1993); most observational estimates are much lower, but values ranging from ≤ 5×10−7pc−3 to ρ ∼ 10−4pc−3 have been reported. perhaps the most straight forward test of these predictions is to compare them to the measured space density of the galactic short-period, non-magnetic cv population. 2.2 evolutionary relationship between ips and polars in most regards, the formation and evolution of magnetic and non-magnetic cvs is believed to be similar. both types of cvs form via common envelope (ce) evolution, evolve initially from long to short porb as a result of angular momentum loss (aml), and eventually experience period bounce, when the thermal timescale of the donor becomes longer than the mass transfer time-scale. the main proposed difference between the evolution of mcvs and non-magnetic cvs actually affects only the polars, where magnetic braking (mb) is thought to be suppressed because of the very strong wd magnetic field (e.g. li & wickramasinghe 1998; townsley & gansicke 2009). the porb distributions of magnetic and non-magnetic cvs are broadly consistent with these ideas. considering polars and ips together, their porb distribution is very similar to that of non-magnetic cvs, showing a period gap in the range 2 hr<∼porb <∼ 3 hr, as well as a period minimum at around porb ' 80 min (see fig. 2). it has long been recognized that most ips are found above the period gap and most polars below (fig. 2)., which immediately suggests that ips may evolve into polars (e.g. chanmugam & ray 1984). physically this makes sense, since smaller orbital separation and lower ṁ (besides large magnetic field strength) favour synchronization. 27 m. l. pretorius figure 2: the orbital period distribution of all cvs (black), and mcvs (green). cumulative distributions are also shown for polars (red) and ips (blue). almost all ips are found at long porb, while most polars are short-porb systems. periods from ritter & kolb (2003). mb drives much higher mass-transfer rates above the period gap than gravitational radiation (gr) does below the gap; therefore, it is plausible that many accreting magnetic wds may only become synchronized after crossing the period gap. the main problem with this scenario is that the magnetic fields of the wds in ips (bip <∼ 10mg) are systematically weaker than those of the wds in polars (bpolar ∼ 10− 100mg). there are several possible solutions to this problem. perhaps the most simple (in terms of binary star evolution) is that the high accretion rates in ips partially “bury” the wd magnetic fields, so that the observationally inferred field strengths for these systems are systematically low (cumming 2002). it is also possible that the short-period polar population is dominated by systems born below the period gap. of course this does not explain why we observe so few short-period ips, although it may be that longperiod ips become unobservable once they reach short periods (see patterson 1994; wickramasinghe, wu & ferrario 1991). a way to investigate the relationship between ips and polars is to compare their respective space densities. for example, if all long-period ips evolve into short-period polars, and all short-period polars form out of of long-period ips, then their space densities should be proportional to the evolutionary time-scale associated with these two evolutionary phases. in this particular example, we would predict that ρpolar/ρip ' τgr/τmb >> 1. 2.3 intrinsic fraction of mcvs magnetic systems make up ' 20% of the known cv population (ritter & kolb 2003). taken at face value, this is a surprisingly high fraction, considering that the strong magnetic fields characteristic of ips and polars (b>∼ 106 g) are found in only ' 10% of isolated wds (e.g. kulebi et al. 2009). if these numbers really represent the intrinsic incidence of magnetism amongst cvs and single wds, the difference between them would have important implications, namely that either strong magnetic fields favour the production of cvs, or that some aspect of pre-cv evolution favours the production of strong magnetic fields (see e.g. tout et al. 2008). however, it is in reality not yet clear that magnetism is more common in cv primaries than in isolated wds. the main problem is that the observed fraction of magnetic systems amongst known cvs is almost certainly affected by serious selection biases. for example, since mcvs are known to be more luminous in x-rays than non-magnetic cvs, they are likely to be over-represented in x-ray-selected samples. conversely, polars, in particular, are relatively faint in the optical band (since they do not contain optically bright accretion disks), so they are likely to be under-represented in optically-selected samples. given that the overall cv sample is a highly heterogeneous mixture of x-ray, opticaland variability-selected sub-samples (which also typically lack clear flux limits), it is very difficult to know how the observed fraction of mcvs relates to the intrinsic fraction of magnetic wds in cvs. 28 the space density of magnetic and non-magnetic cataclysmic variables, and implications for cv evolution 2.4 galactic x-ray source populations several studies have attempted to determine the makeup and luminosity function of galactic x-ray source populations in different environments, including the milky way as a whole, the galactic centre, the galactic ridge, and globular clusters. remarkably, in all of these environments, mcvs have been proposed as the dominant population of x-ray sources above lx >∼ 1031ergs−1. in most of these studies, identifying the observed xray sources with distinct populations is very uncertain, since few of the sources have either optical counterparts or properties that allow for clear classification. the classification of observed sources rely mainly on x-ray luminosities and hardness, and statistical comparisons of observed and expected number counts. the local space densities of the relevant physical populations are likely the most important aspect of these comparisons. in effect, the relevant question is whether the extrapolation of the local space density to the environment being investigated can account for the number of sources observed there. in the case of mcvs, such extrapolations are difficult, mainly because the local space densities are quite poorly known. 3 calculating space densities 3.1 the flux-limited samples the rbs covers |b| > 30◦ to fx >∼ 10−12erg cm−2s−1, and includes 16 non-magnetic cvs, and 30 mcvs (6 ips and 24 polars). the nep covers 81 sq.deg. to fx >∼ 10−14. only 4 cvs where detected in the nep, all of them non-magnetic. the samples are presented in pretorius et al. (2007b), pretorius & knigge (2012), and pretorius et al. (2013). 3.2 the method we use the 1/vmax method (e.g. stobie et al. 1989) together with a monte carlo simulation designed to sample the full parameter space allowed by the data, as described in pretorius et al. (2007b) and pretorius & knigge (2012). we tested the method to verify that it gives reliable error estimates, and also considered various possible systematic biases (pretorius & knigge 2012; pretorius et al. 2013). 3.3 results 3.3.1 probability distribution functions the distributions of mid-plane ρ values, normalized to give probability distribution functions, from the simulations are shown in fig. 3. the best-estimate midplane space densities are 4+6 −2 × 10−6 pc−3 for nonmagnetic cvs, and 8+4 −2 × 10−7 pc−3 for mcvs. for the 2 classes of mcvs, we find 3+2 −1 × 10−7 pc−3 for ips and 5+3 −2 × 10−7 pc−3 for polars. 3.3.2 upper limits on ρ of undetected populations the ρ estimates assume that the detected populations are representative of the intrinsic populations, in that they contain at least 1 of the faintest systems present in the intrinsic populations. it is possible that even large populations of sources at the faint ends of the luminosity functions can go completely undetected in flux-limited surveys. we performed additional monte carlo simulations to place limits on the sizes of faint populations of cvs that could escape detection in the surveys we have used (see pretorius & knigge 2012; pretorius et al. 2013). fig. 4 shows the maximum allowed ρ as a function of lx , separately for possible undetected nonmagnetic, polar and ip populations. specifically, if ρn−m = 2×10−4 pc−3 (at the high end of the predicted range), we require that the majority of non-magnetic cvs have lx <∼ 4 × 1028 erg s−1. a population of undetected polars with a space density as high as 5× the measured ρpolar must have lx <∼ 1030 ergs−1. a hidden population of ips can only have ρ = 5 × ρip if it consists of systems with x-ray luminosities fainter than 5× 1030 ergs−1. 4 discussion 4.1 missing non-magnetic cvs we discuss our measured ρn−m, as well as the upper limit, in the context of the predicted (i) large total space density of non-magnetic cvs, and (ii) large predicted fraction of normal short-period cvs and period bouncers. population synthesis models predict that only a few percent of all cvs are above the period gap (kolb 1993 finds less than 1%, while knigge et al. 2011 predict 3%). although we find that long-period systems account for just over 50% of our total ρn−m, the data cannot rule out these theoretical predictions. for example, using the knigge et al. (2011) fraction of longperiod systems, and assuming that we have not significantly under-estimated the space density of longperiod cvs, the space density of short-period cvs is ' 2 × 10−6 pc−3(97/3) ' 6 × 10−5 pc−3. the upper limit on ρn−m from section 3.3.2 then implies that a short-period cv population of this size could escape detection in the two surveys, as long as these systems have lx <∼ 8 × 1028 erg s−1 (for the simple case of a hypothetical single-lx population of faint, undetected cvs). the predicted ρ = 2 × 10−4 pc−3 would require that most cvs have x-ray luminosities below lx = 4× 1028 erg s−1. 29 m. l. pretorius figure 3: the ρ distributions for non-magnetic cvs (left-hand panel) and mcvs (right-hand panel) resulting from our simulations. solid lines in both panels mark the modes, medians, and means of the distributions; dashed lines show 1-σ intervals. the probability distribution functions shown in the inset are for the whole mcv sample, polars alone (red), and ips alone (blue). reproduced from pretorius & knigge (2012) and pretorius, knigge & schwope (2013). figure 4: the upper limit on ρ as a function of x-ray luminosity for undetected populations of cvs. the left-hand panel is for non-magnetic cvs, and the right-hand panel for ips (blue) and polars (red). in the lefthand panel, the 2 upper, fine histograms show the corresponding results for the rbs (middle) and nep (top) surveys alone. note that the assumed x-ray spectra of non-magnetic cvs, polars, and ips are different, hence the different slopes. reproduced from pretorius & knigge (2012) and pretorius et al. (2013). 4.2 evolutionary relationship between ips and polars if one assumes that long-period ips are the only progenitors of short-period polars, and that all ips become polars after crossing the period gap, then the ratio of the space densities of long-porb ips and short-porb polars (ρip,lp and ρpolar,sp) will simply reflect the ratio of evolutionary time-scales. the observed logarithm of this ratio is log (ρpolar,sp/ρip,lp) = 0.32 ± 0.36. if the evolution of long-period ips is driven by mb, while that of short-period polars is driven by gr alone, the evolutionary time-scale of short-period polars should be >∼ 5× that of long-period ips (e.g. knigge et al. 2011). this is completely consistent with the ratio of the inferred space densities. in fact, at 2-σ, the uncertainties are large enough to allow both ratios exceeding 10 and ratios below one. this means that, with the best existing space density estimates for polars and ips, we cannot strongly constrain the evolutionary relationship between these two classes. nevertheless, it is interesting to note that the simplest possible model, in which short-period polars derive from long-period ips and all ips become polars, is not ruled out by their observed space densities. 30 the space density of magnetic and non-magnetic cataclysmic variables, and implications for cv evolution 4.3 intrinsic fraction of mcvs combining the space density estimates of magnetic and non-magnetic cvs to estimate the intrinsic fraction of mcvs, we find log(fmcv ) = −0.80+0.27 −0.36. this is consistent, within the considerable uncertainties, with the fraction of isolated wds that are strongly magnetic . furthermore, it is possible that the x-ray-selected cv sample is less complete for non-magnetic cvs than for mcvs. therefore, the incidence of magnetism is not obviously higher amongst cv primaries compared to isolated wds. 4.4 galactic x-ray source populations we consider if it is plausible that ips dominate x-ray source populations above lx ' 1031ergs−1, taking the galactic centre as an example. the deep chandra survey of muno et al. (2009) includes ' 9000 sources down to lx ' 1031 ergs−1, in an area of ' 10−3deg2. approximating the volume covered by the survey as a sphere of radius 150 pc, the space density of x-ray sources in the galactic centre is ρx,gc ∼ 6×10−4 pc−3, compared to the local ip space density of ρip ∼ 3 × 10−7 pc−3. the stellar space density in the galactic centre is ' 1600× higher than in the solar neighborhood. thus these densities are consistent, and we conclude that most of the x-ray sources seen in the galactic centre can indeed be explained as ips. 5 conclusions assumming that the cv samples from the rbs and nep surveys are representative of the intrinsic populations (in the sense that we detected at least one system at the faintest ends of the luminosity functions of those populations), we find ρn−m = 4+6 −2 × 10−6 pc−3 and ρmcv = 8+4 −2×10−7 pc−3 (ρpolar = 5+3 −2×10−7 pc−3 and ρip = 3+2 −1 × 10−7 pc−3). the data allow for more than half of non-magnetic cvs having 28.7 < log(lx/erg s−1) < 29.7, and being undetected. however, to reach ρn−m = 2 × 10−4 pc−3 (at the high end of the predicted range), the data requires that the majority of non-magnetic cvs have lx <∼ 4× 1028 erg s−1. the ratio of the space densities of short-period polars to long-period ips is consistent with the simple hypothesis that long-period ips evolve into short-period polars, giving rise to the entire short-period polar population. existing data cannot rule out that strongly magnetic wds have the same incidence amongst cvs as in the field. taking into account the difference in stellar density, the measured local space density of ips can account for the number of bright x-ray sources detected in the galactic centre. acknowledgement i thank the organizes for a successful meeting, and for inviting me to present this review. references [1] chanmugam g., ray a.: 1984, apj, 285, 252 [2] cumming a.: 2002, mnras, 333, 589 doi:10.1046/j.1365-8711.2002.05434.x [3] de kool m.: 1992, a&a, 261, 188 [4] gänsicke b. t., et al.: 2009, mnras, 397, 2170 doi:10.1111/j.1365-2966.2009.15126.x [5] henry j.p., et al.: 2006, apjs, 162, 304 doi:10.1086/498749 [6] knigge c.: 2006, mnras, 373, 484 doi:10.1111/j.1365-2966.2006.11096.x [7] knigge c., baraffe i., patterson j.: 2011, apjs, 194, 28 doi:10.1088/0067-0049/194/2/28 [8] kolb u.: 1993, a&a, 271, 149 [9] külebi b., et al.: 2009, a&a, 506, 1341 [10] li j., wickramasinghe d. t.: 1998, mnras, 300, 718 [11] muno m. p., et al.: 2009, apjs, 181, 110 [12] patterson j.: 1984, apjs, 54, 443 [13] patterson j.: 1994, pasp, 106, 209 [14] patterson j.: 1998, pasp, 110, 1132 [15] patterson j.: 2011, mnras, 411,2695 doi:10.1111/j.1365-2966.2010.17881.x [16] pretorius m. l., knigge c.: 2012, mnras, 419, 1442 doi:10.1111/j.1365-2966.2011.19801.x [17] pretorius m. l., knigge c.: 2008a, mnras, 385, 1471 [18] pretorius m. l., knigge c.: 2008b, mnras, 385, 1485 [19] pretorius m. l., knigge c., kolb u.: 2007a, mnras, 374, 1495 [20] pretorius, m. l., knigge, c., schwope, a. d.: 2013, mnras, 432, 570 doi:10.1093/mnras/stt499 [21] pretorius m. l., et al.: 2007b, mnras, 382, 1279 doi:10.1111/j.1365-2966.2007.12461.x 31 http://dx.doi.org/10.1046/j.1365-8711.2002.05434.x http://dx.doi.org/10.1111/j.1365-2966.2009.15126.x http://dx.doi.org/10.1086/498749 http://dx.doi.org/10.1111/j.1365-2966.2006.11096.x http://dx.doi.org/10.1088/0067-0049/194/2/28 http://dx.doi.org/10.1111/j.1365-2966.2010.17881.x http://dx.doi.org/10.1111/j.1365-2966.2011.19801.x http://dx.doi.org/10.1093/mnras/stt499 http://dx.doi.org/10.1111/j.1365-2966.2007.12461.x m. l. pretorius [22] ritter h., kolb u.: 2003, a&a, 404, 301 [23] schwope a.d., et al.: 2002, a&a, 396, 895 [24] tout c. a., et al.: 2008, mnras, 387, 897 doi:10.1111/j.1365-2966.2008.13291.x [25] townsley d. m., gänsicke b. t.: 2009, apj, 693, 1007 doi:10.1088/0004-637x/693/1/1007 [26] wickramasinghe d. t., wu k., ferrario l.: 1991, mnras, 249, 460 doi:10.1093/mnras/249.3.460 [27] warner b.: 1987, mnras, 227, 23 [28] van teeseling a., beuermann k., verbunt f.: 1996, a&a, 315, 467 32 http://dx.doi.org/10.1111/j.1365-2966.2008.13291.x http://dx.doi.org/10.1088/0004-637x/693/1/1007 http://dx.doi.org/10.1093/mnras/249.3.460 introduction context missing non-magnetic cvs evolutionary relationship between ips and polars intrinsic fraction of mcvs galactic x-ray source populations calculating space densities the flux-limited samples the method results probability distribution functions upper limits on of undetected populations discussion missing non-magnetic cvs evolutionary relationship between ips and polars intrinsic fraction of mcvs galactic x-ray source populations conclusions acta polytechnica ctu proceedings doi:10.14311/app.2017.7.0038 acta polytechnica ctu proceedings 7:38–42, 2017 © czech technical university in prague, 2017 available online at http://ojs.cvut.cz/ojs/index.php/app uhpc sandwich structures with composite coating under compressive load jan markowski∗, ludger lohaus institut für baustoffe, leibniz universität hannover, appelstraße 9a, 30167 hannover, germany ∗ corresponding author: j.markowski@baustoff.uni-hannover.de abstract. ultra-high-performance concrete (uhpc) sandwich structures with composite coating serve as multipurpose load-bearing elements. the uhpc’s extraordinary compressive strength is used in a multi-material construction element, while issues regarding the concrete’s brittle failure behaviour are properly addressed. a hollow section concrete core is covered by two steel tubes. the outer steel tube is wrapped in a composite material. by this design, uhpc is used in a materialand shape-optimised way with a low dead weight ratio[1] concerning the load-bearing capacity and stability[2]. the cross-section’s hollow shape optimises the construction’s buckling stability while saving self-weight. the composite coating on the column’s outside functions both as a layer increasing the construction’s durability and as a structural component increasing the maximum and the residual load capacity. investigations on the construction’s structural behaviour were performed. keywords: uhpc, sandwich structure, hollow tubular cross-section, composite materials, lightweight. 1. introduction the compressive strength of an ultra-highperformance concrete (uhpc) (fc) exceeds 130 n/mm2. concrete with compressive strengths of 200 n/mm2 can be unerringly produced. one of the challenges to face while working with uhpc is its brittle failure behaviour. since abrupt failure of building components is to be avoided, constructive solutions have to be found. this article addresses the design and mechanical system (see para.2.1) and the production (see para.2.3.2) of uhpc sandwich structures with composite coating, while the individual materials are described in para.2.2. the uhpc sandwich structures with composite coating exploit the concrete’s properties and provide ductile structure behaviour, as proven in experiments (see paragraph 2.3). a test set-up (see para.2.3.3) and results (see para.2.3.4) of axial compressive tests of specimens with and without composite coating are described. the results are discussed in para.2.3.4. 2. uhpc sandwich structures with composite coating as shown in figure 1, uhpc sandwich structures with composite coating consist of four layers of different materials. a hollow section uhpc core is covered by two concentrically arranged steel tubes which can be understood as lost framework and reinforcement of the uhpc core[3]. the outer steel tube is wrapped in a composite material layer. the composite layer fulfills different tasks: its fibres act as a structural component (as described in para.2.1) and its matrix protects the outer steel tube’s surface from corrosion. figure 1. uhpc sandwich structure with composite coating, cross-section. 2.1. mechanical system – conceptual approach uhpc sandwich structures with composite coating serve as multipurpose load-bearing elements, predominantly for compressive loads. axial loads are borne especially by the concrete’s cross-sectional area. the inner and outer steel tube are ideally not involved in the axial load-bearing (as far as the serviceability limit state is not exceeded), nor is the composite material layer. pure uhpc has the most favourable ratio of dead weight to compressive strength in comparison to steel. in order to increase the second moment of area and, thereby, the cross-section’s buckling resistance, a hollow cross-section design is chosen. despite the uhpc’s brittle failure behaviour, a ductile behaviour of the construction can be achieved by the interaction of the uhpc core and the construction’s other layers. after exceeding the ultimate limit 38 http://dx.doi.org/10.14311/app.2017.7.0038 http://ojs.cvut.cz/ojs/index.php/app vol. 7/2017 uhpc sandwich state, the construction is still able to bear further loads by the lateral supporting effects of the steel tubes and the composite layer. the composite material’s fibres are arranged in peripheral direction. consequently, no axial loads are borne by the composite material while buckling and detachment issues are avoided as confinement effects increase both the construction’s load-bearing capacities and its ductility. longitudinal strains of the columns under axial compression result in transverse strains. while the steel’s poisson’s ratio is larger than the concrete’s poisson’s ratio, peripheral tensile stresses occur in the concrete core. radial expansions can be converted into peripheral stresses by the composite material’s fibres, reducing the harmful tensile stresses inside the concrete core and, therefore, multiaxial compressive states of stress can be activated. due to these unidirectional properties of the composite material wrapping, the outer steel tube can be chosen with a relatively low thickness, resulting in further weight savings compared to conservative constructions. 2.2. materials 2.2.1. steel non-alloy quality steel, grade dc01, is used for the outer steel tube and conventional structural steel s235 is used for the inner steel tube. both materials have a characteristic yield stress (fy) of 235 n/mm2. 2.2.2. ultra-high performance concrete the concrete’s mix design and its basic material parameters are shown in table 1 and 2. component type kg/m3 cement cem i 52.5r hs/na 175 microsilica uncompacted 825 fine quartz powder 50/16 µm 200 sand 0.125/0.5 mm 975 water 185 superplasticizer polycarboxylate 28 w/c = 0.24 water to cement ratio w/b = 0.20 water to binder ratio table 1. the design of the concrete mix used. 2.2.3. composite material the carbon fibre reinforced plastic composite consists of • a unidirectional carbon band, which is aligned in the peripheral direction of the specimen (see fig. 2) and • a matrix of epoxy resins, which is predominantly used for rotor blades for wind turbines and shipbuilding. average density 2.323 kg/dm3 slumpflow (sf) (1) 900 mm compressive strength (2,3) 220.0 n/mm2 flexural strength(2,4) 13.3 n/mm2 (1) according to din en 12350-8[4] (2) after thermal treatment at 90 ◦c for 48 h, started 24 h after casting, cube 100 × 100 mm (3) according to din en 12390-3[5] (4) according to din en 196-1[6] table 2. the basic material parameters of the concrete used. carbon band material carbon fibre (ht) width 25.0 mm thickness 0.3 mm weight 350.0 g/m2 yarn count 400.0 tex matrix epoxy resin hexion epikote epoxy hardener hexion epikure heat-curing 12 h, 70 ◦c table 3. the composite material used. figure 2. unidirectional carbon fibre band 2.3. experimental research in order to demonstrate the composite material layer’s impact on the mechanical system, the maximum loadbearing capacities of specimens with fibre reinforced plastic composite coating (series b, two specimens) are compared to those of specimens without composite coating (series a, three specimens). 2.3.1. specimen geometry the specimen’s properties are shown in table 4. 2.3.2. production of the specimens in a first step, the steel tubes are aligned perfectly concentrically. afterwards, the concrete is placed by means of a tremie pipe (contractor method). after heat curing the concrete (see para.2.2.2), the composite material is applied onto specimen b in a hand lay-up process by means of a mounting fixture that allows rotation around the longitudinal axis. in a last step, the composite material is heat cured. a specimen of series b is shown in figure 3. 39 jan markowski, ludger lohaus acta polytechnica ctu proceedings series a: without composite layer inner steel tube: ∅outside = 133.0 mm ts, inner = 2.9 mm outer steel tube: ∅outside = 169.0 mm ts, outer = 1.0 mm concrete core: ∅mid−line = 155.0 mm tc = 19.2 mm length: l = 500 mm weight: m = 15.39 kg series b: with composite layer composite material: 8 layers of carbon band ≈ 3.0 mm (inc. matrix) weight: m = 16.64 kg in all dimensions as series a table 4. properties of the two series figure 3. specimen of series b 2.3.3. test set-up the experimental work focuses on the maximum loadbearing capacity and the ductility. the specimens are loaded in a displacement-controlled (load speed 0.25 mm/min) test by a hydraulic cylinder to failure and beyond. an integrated calotte levels possible tilting. normal force and displacement are measured. in order to introduce the load predominantly into the concrete core and to avoid cracks in the load introduction area, the specimens are grouted in a load introduction panel. the same concrete mix design as used for the specimen is used as grout material. figure 4 shows the test set-up containing the specimen, the load introduction panels, three inductive displacement transducers and protective equipment. 2.3.4. results table 5 shows the results for all specimens of series a and b. force–distance diagrams for two selected exfigure 4. experimental set-up figure 5. force–distance diagrams, two selected examples fn ,max fn , residual series a, specimen 1 1, 395 kn 554 kn series a, specimen 2 1, 389 kn 675 kn series a, specimen 3 1, 260 kn 684 kn series a average 1, 348 kn 638 kn series b, specimen 2 1, 774 kn 1, 610 kn series b, specimen 2 2, 073 kn 950 kn series b average 1, 924 kn 1, 280 kn table 5. results of the experimental research 40 vol. 7/2017 uhpc sandwich amples are presented in figure 5. positive fn -values represent compressive forces for means of presentation. figure 6 shows the two specimen after testing. the broken concrete core of the specimen shown causes a buckling formation on the outer steel tube. the inner steel tube (not shown in fig. 6) is also affected. the buckling appeared when the maximum load capacity of 1, 260 kn was reached. the specimen of series b shown appeared to be undamaged from the outside until a displacement of 10.0 mm is reached. at this point, only single sectors of the composite material failed to a limited extent. when the displacement reached 10.0 mm, a larger amount of fibres failed, causing an extensive detachment in the specimen’s middle section. under the composite layer, the outer steel tubes showed buckling formations that are comparable to those of specimen a. figure 6. series a specimen 3 (left) and series b specimen 1 (right) after testing 3. discussion as the results show, the maximum load-bearing capacity and the residual load-bearing capacity of uhpc sandwich structures can be increased by the addition of a composite layer. the ductility (in this context, the ratio of the residual force to the maximum force) is affected as well, but the results differ. while the ductility of specimen 1 of series b can be increased significantly (the residual force is at 90 % of the ultimate load), the residual force of specimen 2 is at 59 % of its ultimate load. it is still higher than the average ductility of the specimens of series a (47 %), but, in view of the large variation, the effect is negligible. it is assumed that different modes of failure are the reason for this variation. while the concrete core of specimen 1 fails first and is still supported laterally by the composite material, it is suggested that in the case of specimen 2, the fibres fail first and lead to a sudden drop in the load-bearing capacity (from 2, 073 to 950 kn) bearing capacity. table 6 shows a simple approach to estimate the construction’s load-bearing capacity. the composite layer is not taken into account because of its peripheral fibre orientation. cross-sectional area (specimen a and b) steel tubes: as = 1, 713 mm2 concrete core: ac = 7, 959 mm2 load-bearing shares (simple assumption): fn,s = as ∗ fy = 402.4 kn = 16.7 % ∗ fn,total fn,c = ac ∗ fc = 1, 750.9 kn = 81.3 % ∗ fn,total fn,total = fn,s + fn,c = 2, 153.3 kn load-bearing capacity (experimental results): specimen a: fn , average = 1, 348 kn specimen b: fn , average = 1, 924 kn table 6. maximum load-bearing capacity estimation, simple approach the construction’s maximum load-bearing capacity is overestimated by the simple approach. the reason for this could be • the load introduction by load introduction panels that exclude loads partially from the steel tubes; • effects caused by stability; or/and • imperfections (material and geometry). lindschulte proposes a more sophisticated estimation model (eq. 7-1[2]) for sandwich structures without composite coating: fn = ac ∗0.8∗fc +αs−1 ∗as,−1 ∗fy +αs+1 ∗as,+1 ∗fy (1) where is as,−1 = area of the inner steel tubes cross-section as,+1 = area of the outer steel tubes cross-section αs−1 = stability reduction factor concerning to χ(λ) according to eurocode 3, inner steel tube αs+1 = stability reduction factor concerning to χ(λ) according to eurocode 3, outer steel tube applying lindschulte’s model to the series’ geometry shown leads to fn = 1, 400 kn assuming a non-load-bearing role of the steel tubes. this estimation fits the experimental results of series a (fn,average = 1, 348 kn) well. the increase 41 jan markowski, ludger lohaus acta polytechnica ctu proceedings of the maximum load-bearing capacity of series b cannot be explained with this model. confinement effects as a result of the composite layer have to be considered as a reason for this. the composite material’s fibres bear tensile stresses in a peripheral direction, that are caused by transversal strains in a radial direction under axial compression. without these fibres, these radial strains lead to tensile stresses in the concrete-core. due to the confinement effect, all compressive stress states in the concrete core are reached, leading to a higher maximum loadbearing capacity. both specimen of series b also have a higher residual load-bearing capacity. this can be explained by the lateral supporting effect of the composite material. after the concrete is broken, it is kept in place by the inner steel tube and the outer steel tube, which is additionally supported by the composite material. 4. outlook uhpc sandwich structures with composite coating could be beneficially used, for example, in large frameworks of offshore megastructures[7]. in order to prove the concept, further investigations have to be performed. the following subjects require special attention: • the identification of different failure modes. • numerical simulations to prove the mechanical behaviour assumed. • further experimental research with different geometries in order to derive a model for the prediction of the maximal load and the residual load. • the component’s behaviour under pure bending and bending with normal force. • the component’s behaviour under cyclic load. • junctions and load introduction. list of symbols ε strain [–] ac concrete area [mm2] as steel area [mm2] fn maximum normal force [kn] fn,c normal force, concrete’s share [kn] fn,s normal force, steel tubes’ share [kn] fc concrete’s compressive strength [n mm−2] fy steel’s yield stress [n mm−2] u displacement [mm] w/c water to concrete ratio [–] w/b water to binder ratio [–] acknowledgements the research project is supported by the german research foundation (dfg) within the scope of priority programme 1542 "lightweight building with concrete". the authors would like to express their gratitude for the financial support. references [1] n. scholle, l. lohaus, n. lindschulte. weight-saving potential of hybrid tube structures. proceedings of hipermat 2016, 4th international symposium on ultra-high performance concrete and high performance construction materials 2016. [2] n. lindschulte. druckverhalten von rohren aus ultrahochfestem beton mit stahlblechummantelung. institut für baustoffe, leibniz universität hannover, appelstr. 9a, 30167 hannover, de, 1st edn., 2013. [3] n. scholle, l. lohaus. hybrid tubes: material-adapted construction with ultra-high performance concrete. iass2015 annual international symposium on future visions 2015. [4] din en 12350-8: testing fresh concrete part 8: self-compacting concrete slump-flow test; german version en 12350-8:2010. [5] din en 12390-3: testing hardened concrete part 3: compressive strength of test specimens; german version en 12390-3:2009. [6] din en 196-1: methods of testing cement part 1: determination of strength; german version en 196-1:2005. [7] n. scholle, l. lohaus. uhpc-steel hybrid tube components for application in offshore support structures. the annual international offshore and polar engineering conference (isope) 2016. 42 acta polytechnica ctu proceedings 7:38–42, 2017 1 introduction 2 uhpc sandwich structures with composite coating 2.1 mechanical system – conceptual approach 2.2 materials 2.2.1 steel 2.2.2 ultra-high performance concrete 2.2.3 composite material 2.3 experimental research 2.3.1 specimen geometry 2.3.2 production of the specimens 2.3.3 test set-up 2.3.4 results 3 discussion 4 outlook list of symbols acknowledgements references 316 acta polytechnica ctu proceedings 1(1): 316–319, 2014 316 doi: 10.14311/app.2014.01.0316 us astronomical photographic data archives: hidden treasures and importance for high-energy astrophysics rené hudec1,2, lukáš hudec2 1astronomical institute, academy of sciences of the czech republic, cz-25165 ondřejov, czech republic 2czech technical university in prague, faculty of electrical engineering, prague, czech republic corresponding author: rene.hudec@asu.cas.cz abstract we report here on an ongoing investigation of us astronomical plate archives and tests of the suitability of transportable scanning devices for in situ digitization of archival astronomical plates, with emphasis on application in high-energy astrophysics. keywords: astronomical data archives: astronomical photographic archives – spectroscopy: low-dispersion spectra – data mining. 1 introduction there are numerous important astronomical plate archives in the usa, including plate collections that are little known to the community and that have been little investigated in the past. within the framework of a czech-us collaborative project (msmt kontakt amvis me09027), we have recently analyzed some of them, obtaining test scans with the use of a portable digitizing device. digitization is a necessary step for an extended evaluation of the plate data using dedicated programs and powerful computers. several recently found large us negative archives are expected to play important role in high-energy astronomy. motivation for including this subject into multi– frequency studies of high energy sources is as follows. numerous he/vhe/uhe sources are also emitters of optical light, many of them are variable. the astronomical plate archives represent the only method how to study the behavior of the objects over very long (100 years or even more) time intervals (hudec, 1999 and 2012). in addition, huge monitoring times allows to detect and to study rare events such as flares. the databases allow to study prominent spectra and/or spectral changes as well (hudec et al., 2012). 2 the plate archives the us astronomical archival plate collections that we recently visited and investigated include those housed in the following 16 institutions: carnegie observatories pasadena, ca, lick observatory, ca yerkes observatory, wi, mt palomar observatory, ca, pari, rosman, nc (which has a collection of plates from many observatories), kpno tucson, az, cfht waimea, hi, ifa manoa, hi, usno flagstaff, az, usno washington, dc, steward observatory tucson, az, nmsu, las cruces, nm, rosemary hill observatory, university of florida, gainesville, fl and leander mccormick observatory, university of virginia, charlottesville, va, smithsonial archives washington, dc, and hazy space center, dulles, va. our estimate is that there are more than 2 million astronomical archival plates and/or negatives in these archives. there are however many us plate collections not included in our study (robbins and osborn, 2009). we performed a quality check and analyzed these plate archives with emphasis on their scientific, historical and cultural value, which we have found to be enormous. in addition to that, collaboration with several german institutes with astronomical plate archives started, with emphasis on digitization and investigation of relevant photographic collections. 3 transportable digitizing device most of the plate archives that we visited have no plate scanners and lack modern instrumentation in general. as our study includes plate digitization, it was necessary to find a solution. since we were going to travel from europe to the us by air, the obvious option was a transportable digitization device based on a digital camera with a high-quality lens and a stable tripod. this solution has the following advantages over other techniques: the device is easily transportable, and offers much faster scanning and higher repeatability than commercial flatbed scanners, because there are no mov316 http://dx.doi.org/10.14311/app.2014.01.0316 us astronomical photographic data archives: hidden treasures and importance for high-energy astrophysics ing scanner parts. the equipment that we used was as follows: camera: 21 mpx canon eos 5d mark ii, lenses: canon ef 24-70 f/2.8 l usm and canon 70200mm f4, a stable tripod, and a fomei lp-310 professional photographic light table. in last project year, a better custom-made light table based on highly homogeneous led illumination was used, and also a further improved camera (39 mpx) and lenses. the recorded images are then corrected for lens image distortions and for other effects, in order to store research-grade digital images. the achieved pixel size depends on the size of the plate, and fine pixel sizes of order of 25–30 microns or less can be typically obtained for small and medium sized plates (up to 16 cm x 16 cm or 18 cm x 24 cm) while for larger plates larger pixel sizes will be obtained (or, alternatively, the large plate may be covered by several shots to achieve smaller pixel sizes). examples of digitized 16 cm x 16 cm plates are illustrated in figs. 1 and 2 with relevant field recognition and star identification. figure 1: example of digitized bamberg southern sky patrol plate processed by astrometry.net tools: field recognition 4 general picture after visiting the us plate collections mentioned above, we offer a (subjective) list of the major problems found in these archives: (i) the list of us plate collections provided by dr. wayne osborn (robbins and osborn, 2009) was found to be incomplete. we have found valuable plate collections with plates from important telescopes that are not listed, e.g. the two hawaii plate collections in manoa (institute of astronomy) and in waimea (canada-france hawaii telescope cfht). some plate archives have been completely hidden, as their home institutes were in some extreme cases not even aware that they have plate stacks. (ii) in numerous collections, only a very rough estimate of the number of plates can be given, as no exact information about the total number of plates, etc., is available. usually, the real number of plates is higher than the previously available estimate. in general, it is very difficult to give the exact number of plates, due to lack of observation logs and inadequate organization of the plate archives. (iii) in many cases, there is no contact person responsible for the plate archive, and it is difficult to make contact. in some places, it is even difficult to get access. this situation very has a serious adverse effect on efforts to exploit these plate collections for scientific purposes. (iv) for some archives, no information is available, not even an approximate number of plates. in many archives, no plate logs are available; they have either been removed or are lost. the only available information is what is written on the plates or on the envelopes (in some cases, there is not even adequate information on the envelope). we guess that there were originally observation logs, but that these were later separated from the plates and archived in a different location. example: carnegie observatories pasadena (nearly 0.5 million plates), where the logs are probably located in the attic above the library, with difficult access. (v) damaged plates in some archives (mostly due to a partly-released or even complete released emulsion layer), probably due to improper storage (or changes in humidity/temperature over time). we point out however that even these plates can be restored using suitable chemical methods and procedures. (vi) lack of electronic records no lists of plates, the only information is on the plates and/or plate envelopes (vii) many of the archives that we visited suffer from inadequate funding, lack of devices, e.g. no scanners. (viii) we have revealed that many plates have been removed from their home: these plates are usually scattered in private homes and offices, or are being kept by observers (often abroad). numerous plates taken at us observatories have been found in european plate archives. nevertheless, we found highly valuable plates almost everywhere, and the quality of the plates (and hence their scientific potential) is mostly high or even very high, in comparison with the plates in european archives. this is true both for direct images and for spectral images (taken with an objective prism). in addition to stellar images, some of the archives that we visited also include extensive collections of solar images (e.g. carnegie observatories in pasadena) and/or planetary images (unique collection in las cruces). the storage conditions were found to vary from archive to archive, from proper temperature and humidity conditions, to less proper conditions. the main degradation sources found in the plate collections were high levels 317 rené hudec, lukáš hudec of humidity and probably also temperature variation, resulting in partial or complete release of the emulsion layer. the scientific use of the plate archives is negatively impacted by poor access to the plates at some places, and also by the fact that the plates have in most cases not yet been cataloged. figure 2: example of digitized bamberg southern sky patrol plate processed by astrometry.net tools: star identification/astrometry 5 two recently found large us archives with possible major impact on hea the two following databases found within our project are expected to play an especially important role in high-energy astronomy. baker super schmidt camera domed films with 55 degrees diameter fov, limiting magnitude up to 15, dense sampling 20 min, ∼ 110 000 negatives (fig. 5), ∼ 10 years coverage. the essential part of the archive is located at pari (pisgah astronomical research institute) in nc. baker-nunn camera networks negatives fov 30 x 5 degrees, limiting magnitude up to 16, very dense sampling ∼ few sec, more than 1 mil negatives, ∼20 years coverage (figs. 3 and 4). the part of the archive (21 boxes with ∼ 2000 film rolls) is located at smithsonian institution archive in washington, d.c. these data are suitable for wide-field studies, optical transient (ot) searches, and fast variability studies. there are up to 100 000 stars per 1 full frame. these data represent direct images suitable for photometry. however, one baker nunn kwajalein slit-less spectrograph camera for u.s. air force was designed to provide unique low–dispersive spectra. this baker nunn missal reentry tracking camera consisted of a three-element 20-inch entrance aperture corrector lens system. mounted on top of the corrector lens assembly was a multi-set of triangular prisms converting the incoming sky image into a spectrum image. figure 3: dismantling of the usaf baker–nunn camera at the evergreen aviation & space museum, mcminnville, oregon, united states figure 4: example of digitized film from sao baker– nunn camera the corrected spectral image is then projected down to a 31-inch f/1 spherical primary mirror. the focusing converging image was then reflected back up to a 4-inch x 6-inch x 0.6-inch thick mosaic of fiber-optics making film supporting focusing plate. the resulting database is expected to has applications in astronomy as well. 6 summary 16 us astronomical plate archives were visited within the amvis czech-us collaborative project. the quality of the plates and their scientific, historical and cultural value were investigated for possible inclusion in the us astronomical plate repository at pari, nc. some of these archives (e.g. baker super schmidt, baker–nunn, cfht waimea, ifa manoa) were un318 us astronomical photographic data archives: hidden treasures and importance for high-energy astrophysics known to the astronomical community before our study, and the two first mentioned are expected to play important role in high–energy astronomy because of their very wide–field and very fine time resolution. selected plates were digitized using a transportable scanning device. all the archives that we visited have plates that are scientifically valuable, and in many cases unique. the plates are however mostly hidden from the astronomical community, and the plates have not yet been catalogued. the total number of plates is higher than expected in many of the locations, the actual number of plates is unknown. as no catalogs exist, the real number of plates is very difficult to estimate, but for sure the places that we visited have more than 2 million photographic plates and/or negatives in their collections. we plan to continue these efforts, now with emphasis on german astronomical plate collections. figure 5: example of digitized film from sao baker super schmidt camera we plan to continue data mining and plate digitization in astronomical photographic plate/negative archives is as follows. (i) digitize the plate archives using a fast and transportable scanning device, as described above. this scanning method is fast and inexpensive. these are important considerations, as the archives are scattered and there are very large numbers of plates. (ii) create electronic catalogs. (iii) include these catalogs into search programs like wfpd, operated by our bulgarian colleagues (e.g. tsvetkov et al., 2005, and tsvetkov, 2009). acknowledgement the scientific part (investigation of long–term behaviour of high–energy sources) of the study is linked to the grant 13-33324s provided by the grant agency of the czech republic (gacr). the analyzes of astronomical plates are more recently supported by the gacr grant 13-39464j. references [1] hudec, r. et al., acta polytechnica, ibws2011 proceedings, 1(52), 2012. [2] hudec, r. astrophysics with astronomical plate archives, in exploring the cosmic frontier: astrophysical instruments for the 21st century. eso astrophysics symposia, european southern observatory series. edited by andrei p. lobanov, j. anton zensus, catherine cesarsky and phillip j. diamond. series editor: bruno leibundgut, eso. isbn 978-3-540-39755-7. published by springerverlag, berlin and heidelberg, germany, 2007, p.79 [3] hudec, r., an introduction to the world’s large plate archives, acta historica astronomiae, vol. 6, p. 28-40, 1999. [4] robbins, l.; osborn, w., preserving astronomy’s photographic legacy: current state and the future of north american astronomical plates. asp conference series, vol. 410, appendix b and c. edited by wayne osborn and lee robbins. san francisco: astronomical society of the pacific, 2009. 319 introduction the plate archives transportable digitizing device general picture two recently found large us archives with possible major impact on hea summary 66 acta polytechnica ctu proceedings 1(1): 66–70, 2014 66 doi: 10.14311/app.2014.01.0066 evolution of massive population iii stars sung-chul yoon1 1astronomy program, department of physics and astronomy, seoul national university, seoul, 151-742, republic of korea corresponding author: yoon@astro.snu.ac.kr abstract while the evolution of massive stars in the local universe is dominated by mass-loss, the evolution of massive population iii stars should be dominated by rotation. an important effect of rotation is rotationally-induced chemical mixing that can dramatically change the stellar structure and the nucleosynthesis. this has significant consequences in the predicted explosion types of population iii stars. keywords: stars: evolution stars: rotation stars: population iii supernovae gamma-ray bursts. 1 introduction formation of population iii (pop iii) stars marks the end of the so-called dark age in the early universe. these first stars are believed to be intrinsically massive (30 m� < m < 1000 m�), given that the primordial gas does not contain any efficient coolants in star-forming regions, and supposed to play a key role in the evolution of the early universe, in many aspects (see bromm 2013, for a recent review). neutral hydrogens and heliums are re-ionized by the first lights from these first generations of stars, and the primordial gas becomes polluted with heavy elements produced by both hydrostatic and explosive nucleosyntheses in them. although direct identification of them will be highly unlikely even with next generations of telescopes, their nature can be indirectly probed with observations of their explosions that may be luminous enough to be discovered in near future. observational studies on the surface abundances of extremely metal poor stars, which are believed to retain the nucleosynthesis signatures of pop iii stars, will also provide a good constraint on the evolution of pop iii stars. theoretical predictions that can confront future observations are indispensable for these efforts (e.g., marigo et al. 2001; heger & woosley 2002; umeda & nomoto 2003; heger & woosley 2010; kasen et al. 2011; limongi & chieffi 2012; dessart et al. 2013; whalen et al. 2013; tanaka et al. 2013). in this paper, i discuss the current understandings on the evolution of massive pop iii stars, focusing on the role of rotation in the stellar evolution (e.g., marigo et al. 2003; ekström et al. 2008; yoon et al. 2012). recent numerical studies indicate that massive pop iii stars would be born with surface rotation close to the critical value (e.g., stacy et al. 2011). 2 importance of rotation in our galaxy, stars having masses higher than about 30 m� usually undergo strong mass-loss during their evolution. this is because numerous metal lines in their atmospheres and the consequent high radiation pressure cause instabilities of various sorts at the stellar surfaces (see puls et al. 2008, for a recent review). the evolution of massive stars at high metallicity is therefore dominated by mass-loss. for instance, in the local universe, stars as massive as 120 m� at their birth would lose more than 100 m� throughout their lives, dying as 10 20 m� stars (e.g., meynet & maeder 2005). this also means that such massive stars would rapidly lose most of their initial angular momentum, rendering the effect of rotation minor or negligible in general. by contrast, it is believed that massive pop iii stars do not experience significant mass-loss because they are generally very stable (baraffe et al. 2001). in addition, lines by hydrogen and helium are too weak to drive radiation-driven winds (krticka & kubát, 2006). therefore, massive pop iii stars would retain a significant fraction of the initial angular momentum throughout their evolution. this has the following important consequence: while the evolution of very massive stars at high metallicity is dominated by mass-loss, the evolution of massive pop iii stars is dominated by rotation. an important effect of rotation is chemical mixing resulting from rotationally-induced hydrodynamic instabilities like eddington-sweet circulations that are driven by thermal imbalance inside rapidly rotating stars (e.g., maeder 2009). see brott et al. (2011b) for a 66 http://dx.doi.org/10.14311/app.2014.01.0066 evolution of massive population iii stars ! ! !" !" !"#$%&'()"&*+,"./0$,1%&&2'3"$"40-0"*5'()"&*+,"!"#$ #%&'($)*+)',) -"#$ #$% &' (%) *)+ %, -*.+/'(%)*)+%, !0/(%1",' 2,3"$%." 45"'6%("'+7' "88+6+",)$0' 9(*:"/'/%&,' /;"')%' )5"'6%("<",3"$%."' 6%;.$+,1= 65*%&'1"#071"&&%850 >%'6%("'9(*:+,1'90' )5"'6%("<",3"$%."' 6%;.$+,1= (98&"5,"-'8":0#0;' <2'#"+%+,"-'&,=0'>?@ #)(%,1'65"?+6*$'1(*/+",)'9")&"",' )5"'!<9;(,+,1'6%,3"6)+3"'6%("' *,/')5"'",3"$%." >%'65"?+6*$'1(*/+",)'9")&"",' )5"'!<9;(,+,1'6%,3"6)+3"'6%("' *,/')5"'",3"$%." figure 1: illustration for the bifurcation of massive star evolution according to their initial rotational velocity. slowly rotating massive stars develop the classical core-envelope structure such that they become red-supergiant stars during the post-main sequence phases. rapidly rotating massive stars, on the other hand, may undergo very rapid rotationally-induced chemical mixing to such an extent that quasi-chemical homogeneity can be maintained on the main sequence, if their initial metallicity is sufficiently low. in this case, stars are gradually transformed into helium stars at the end of the main sequence. see yoon & langer (2005), yoon et al. (2006), and woosley & heger (2006) for more details. recent dicussion on the observational tests and related uncertainties of the rotational mixing scenario. the hydrogen burning core is supplied with fresh hydrogenrich material from the envelope because of this mixing, resulting in a larger core size than in the corresponding case without rotation. the most extreme case is the socalled chemically homogeneous evolution (che), which is illustrated in fig. 2. with a sufficiently high rotational velocity (i.e., more than about 50% of the local keplerian value at the equatorial surface), the chemical mixing timescale by eddington-sweet circulations can be shorter than the nuclear burning time of hydrogen, so as to maintain quasi-chemical homogeneity in the star. because almost all the hydrogens in the star participate nuclear burning in this way, the star gradually becomes a massive helium star at the end of the main sequence. this is an important evolutionary channel to make massive he stars at low metallicity, while che may not play an important role at high metallicity because of rapid angular momentum loss via stellar winds (see yoon et al. 2006). this mode of evolution is also considered an important channel to produce long gamma-ray bursts from metal-poor massive stars (yoon & langer 2005; yoon et al. 2006; woosley & heger 2006). stars with the che evolve bluewards instead of evolving redwards on the hr diagram. the presence of such stars in the early universe would make significant impact on the history of reionization, because they produce more ionizing photons by several factors than stars that follow the normal evolution (yoon et al. 2012). this particular mode of stellar evolution also results in a significant diversity of pop iii star explosions, as discussed below. 3 explosions of massive population iii stars the theoretical predictions on the final fates of massive pop iii stars are given as the phase diagram in fig. 2. pop iii stars that develop the classical core-envelope structure would die as red or blue supergiant stars, and their final fates may not be much different from those 67 sung-chul yoon 10 20 35 60 100 160 250 500 0.0 0.2 0.4 0.6 0.8 final fates of rotating massive pop iii stars 10 20 35 60 100 160 250 mzams [msun] 0.0 0.2 0.4 0.6 0.8 v z a m s /v k forbidden region v zams=v crit sn iip (ns remnant) collapse to bh, or weak sn ii collapse to bh pisn (sn ii) p uls. p is n (s n ii) grb / hn (sn ibc) p uls. p is n g r b /h n (ibc) pisn (sn ibc) figure 2: final fates of massive pop iii stars for given initial masses and rotational velocities, predicted from the stellar evolution models by yoon et al. (2012). the rotational velocity on the zero-age main sequence is given in units of the keplerian value at the equatorial surface (i.e., vk = √ gm/r). we adopted the most up-to-date calibration for the rotationally-induced chemical mixing efficiency by brott et al. (2011a). the region for the chemically homogeneous evolution (che) is marked by yellow color, and the thick solid line gives the boundary between the che regime and the region for the normal evolution. different regions are marked by expected final outcomes: type ii supernovae (sn ii), collapse to black hole (bh), pulsational pair-instability supernovae (puls-pisn), pair-instability supernovae (pisn), long gamma-ray bursts (grb), hyper-novae (hn), type ib or ic supernovae (sn ibc). in the forbidden region, the equatorial surface velocity exceeds the critical velocity: vcrit = √ (gm/r)[1 − γ], where γ is the eddington factor at the stellar surface. this figure is taken from yoon et al. (2012). of non-rotating pop iii stars as predicted by heger & woosely (2002). but the limits of the initial masses for pulsational pair-instability and pair instability supernovae would move downwards for a higher initial rotational velocity, because rotational mixing tends to increase the he core masses. as explained above, in the che regime, pop iii stars would die as massive helium stars and therefore produce supernovae of type ib/c. conditions for long gamma-ray bursts can also be fulfilled in this regime for an initial mass range of 13 84 m�. while non-rotating pop iii stars would require initial masses of about 140 270 m� to produce a pair instability supernova, this mass range would decrease to about 84 190 m� in the che regime (see also chatzopoulos & wheeler 2012). another interesting prediction is that for intial masses of about 56 84 m�, a pulsational pair-instability supernova would be followed by a long gamma-ray bursts, only several years later. given that gamma-ray jets would interact with a very massive circumstellar matter in this case, extremely bright afterglow might be produced. our models also predict that the final masses of pop iii grb progenitors (∼ 10−70 m�) would be significantly more massive than those of grb progenitors in the local universe (∼ 10−20 m�; yoon et al. 2006). on the other hand, recently several authors argued for super-collapsar formation in very massive pop iii stars (m > 270 m�; mészáros & rees 2010; komissarov & barkov 2010; suwa & ioka 2011 ). however, our models show that very massive pop iii stars with initial masses larger than about 190 m� cannot retain enough angular momentum in the core to produce any rotation-powered event at their deaths. this is because such stars have a very large convective core that leads to efficient angular momentum transport from the innermost layers to the stellar surface, and because large eddington-factors of these stars facilitates mass and angular momentum loss via centrifugally driven massshedding. 68 evolution of massive population iii stars acknowledgement i am very grateful to norbert langer and alexandra dierks for their contribution to this work. references [1] baraffe, i., heger, a., & woosley, s.e., 2001, apj, 550, 890 doi:10.1086/319808 [2] bromm, v., rep. prog. phys., 2013, submitted. [3] brott, i., de mink, s.e., & cantiello, m. et al., 2011a, a&a, 530, 115 [4] brott, i., evans, c.j., & hunter, i. et al., 2011b, a&a, 530, 115 [5] chatzopoulos, e., & wheeler, j.c., 2012, apj, 760, 154 doi:10.1088/0004-637x/760/2/154 [6] dessart, l., waldman, r., livne, e., hillier, d.j., blindin, s., 2013, mnras, 428, 3227 doi:10.1093/mnras/sts269 [7] ekström, s., meynet, g., chiappini, c., hirschi, r., & maeder, a., 2008, a&a, 489, 685 [8] heger, a., & woosley, s.e., 2002, apj, 567, 532 doi:10.1086/338487 [9] heger, a., & woosley, s.e., 2010, apj, 724, 341 doi:10.1088/0004-637x/724/1/341 [10] kasen, d., woosley, s.e., & heger, a., 2011, apj, 734, 102 doi:10.1088/0004-637x/734/2/102 [11] komissarov, s.s., & barkov, m.v., 2010, mnras, 402, 25 doi:10.1111/j.1745-3933.2009.00792.x [12] krticka, j. & kubát, j., 2006, a&a, 446, 1039 [13] limongi, m., & chieffi, a., 2012, apjs, 199, 38 doi:10.1088/0067-0049/199/2/38 [14] maeder, a., 2009, physics, formation and evolution of rotating stars, astronomy and astrophysics labirary, springer [15] marigo, p., chiosi, c., & kudritzki, r.-p., 2003, a&a, 399, 617 [16] marigo, p., girardi, l., chiosi, c., & wood, p.r., 2001, a&a, 371, 152 [17] mészáros, p., & rees, m.j., 2010, apj, 715, 967 doi:10.1088/0004-637x/715/2/967 [18] meynet, g., & maeder, a., 2005, a&a, 429, 581 [19] puls, j., vink, j.s., & najarro, f., 2008, a&arv, 16, 209 [20] spruit, h., 2002, a&a, 381, 923 [21] stacy, a., bromm, v., & loeb, a., 2011, mnras, 413, 543 doi:10.1111/j.1365-2966.2010.18152.x [22] suijs, m.p.l., langer, n., poelarends, a.-j., yoon, s.-c., hger, a., & herwig, f., 2008, a&a, 481, 87 [23] suwa, y., & ioka, k., 2011, apj, 726, 107 doi:10.1088/0004-637x/726/2/107 [24] tanaka, m., moriya, t.j., & yoshida, n., 2013, arxiv:1306.3743 [25] umeda, h., & nomoto, k., 2003, nature, 422, 871 doi:10.1038/nature01571 [26] whalen, d. et al., 2013, apj, 762, 6 doi:10.1088/2041-8205/762/1/l6 [27] woosley, s.e., & heger, a., 2006, apj, 637, 914 doi:10.1086/498500 [28] yoon, s.-c., & langer, n., 2005, a&a, 443, 643 [29] yoon, s.-c., langer, n., & norman, c., 2006, a&a, 460, 199 [30] yoon, s.-c., dierks, a., & langer, n., 2012, a&a, 542, 113 discussion dorota rosinska: you have shown that evolution of low-metallicity massive stars strongly depends on rotation. such stars should rotate differentially. what laws of differential rotation you assume? how results depend on the degree of differential rotation (ωe/ωc; ωe is the angular velocity at the equatorial surface, and ωc the angular velocity at the center)? sung-chul yoon: stars always tend to rotate differentially because of the contraction of the core that results in spinning-up the central layers. the degree of differential rotation in a star is then determined by the efficiency of the angular momentum transport from the core to the envelope. i.e., we do not ”assume” the degree of differential rotation, but it is self-consistently calculated with a given prescription for the transport of angular momentum. in our models, we adopted the socalled talyer-spruit dynamo (spruit 2002) which leads to a very strong coupling between the core and the envelope, making a star rotate almost rigidly on the main sequence. stellar evolution models including the taylerspruit dynamo have proved to be consistent with observations, in terms of the predicted spin-rates of stellar 69 http://dx.doi.org/10.1086/319808 http://dx.doi.org/10.1088/0004-637x/760/2/154 http://dx.doi.org/10.1093/mnras/sts269 http://dx.doi.org/10.1086/338487 http://dx.doi.org/10.1088/0004-637x/724/1/341 http://dx.doi.org/10.1088/0004-637x/734/2/102 http://dx.doi.org/10.1111/j.1745-3933.2009.00792.x http://dx.doi.org/10.1088/0067-0049/199/2/38 http://dx.doi.org/10.1088/0004-637x/715/2/967 http://dx.doi.org/10.1111/j.1365-2966.2010.18152.x http://dx.doi.org/10.1088/0004-637x/726/2/107 http://dx.doi.org/10.1038/nature01571 http://dx.doi.org/10.1088/2041-8205/762/1/l6 http://dx.doi.org/10.1086/498500 sung-chul yoon remnants like white dwarfs and neutron stars (suijs et al. 2008). matteo guainazzi: which black hole spin distribution would you expect from your evolutionary scenario? sung-chul yoon: bimodal distribution. if a bh forms in the che regime, its kerr parameter will be close to 1. otherwise, bhs will not have any significant angular momentum, having a kerr parameter close to zero. 70 introduction importance of rotation explosions of massive population iii stars 259 acta polytechnica ctu proceedings 1(1): 259–264, 2014 259 doi: 10.14311/app.2014.01.0259 a review of astrophysical jets james h. beall1,2 1st. john’s college, annapolis, md 2space sciences division, naval research laboratory, washington, dc corresponding author: beall@sjc.edu abstract astrophysical jets are ubiquitous: this simple statement has become a commonplace over the last three decades and more as a result of observing campaigns using detectors sensitive from radio to gamma-ray energies. during this epoch, theoretical models of these sources have become more complex, moving from assumptions of isotropy that made analytic calculations possible, to fully anisotropic models of emission from the jets and their interactions with the interstellar and intra-cluster medium. such calculations are only possible because we have extensive computational resources. in addition, the degree of international cooperation required for observing campaigns of these sorts is remarkable, since the instruments include among others the very large array (vla), the very long baseline array (vlba), and entire constellations of satellite instruments, often working in concert. in this paper, i discuss some relevant observations from these efforts and the theoretical interpretations they have occasioned. keywords: astrophysical jets active galactic nuclei uhe cosmic rays quasars microquasars. 1 introduction extended linear structures, usually interpreted as astrophysical jets, are found in star-forming regions, in compact binaries, and of course in active galactic nuclei (agns), and galaxy clusters. gamma-ray bursts can be associated with astrophysical jets associated with stellar collapse where the jets is directed toward earth. we have become aware of considerable details of these results through decades of sustained observing campaigns. the results of these campaigns show similarities and significant differences in the data from some epochs of galactic microquasars, including grs 1915+105, the concurrent radio and x-ray data (beall et al., 1978) on centaurus a (ngc 5128), 3c120 (marscher (2006), and 3c454.3 as reported by bonning et al. (2009), to name a few. the bonning et al. analysis of the 3c454.3 data showed the first results from the fermi space telescope for the concurrent variability at optical, uv, ir, and γ-ray variability of that source. in combination with observations of microquasars and quasars from the mojave collaboration (see, e.g., lister et al., 2009), these observing campaigns provide an understanding of the time-dependent evolution of these sources at milliarcsecond resolutions (i.e., parsec for agns, and astronomical unit scales for microquasars). in blazar sources (see, e.g., ulrich et al. 1997, and marscher, 2006) there seems to be a confirmed connection of jets with accretion disks. in sources without large-scale linear structures (i.e., jets), as ulrich et al. (1997) note, the source variability could result from the complex interactions of the accretion disk with an x-ray emitting corona. but to the extent that ”small” jets are present in these sources, the disk-jet interaction must still be of paramount importance, since it provides a mechanism for carrying away energy from the disk. current theories (see e.g., hawley 2003 and bisnovatyi-kogan 2013 for a discussion of disk structure and jet-launch issues, respectively) suppose that the jet is formed and accelerated in the accretion disk. but even if this is true in all sources, it is still unclear whether or not astrophysical jets with shorter propagation lengths are essentially different in constitution from those that have much longer ranges, or whether the material through with the jet propagates determines the extent of the observational structures we call jets. at all events, the complexities of the jet-ambient medium interaction must have a great deal to do with the ultimate size of emitting region. this sort of argument has applications to both quasars and microquasars, especially if essentially similar physical processes occurs in all these objects (see, e.g., beall, 2003, and marscher, 2006). to some, it has become plausible that essentially the same physics is working over a broad range of temporal, spatial, and luminosity scales. hannikainen (2008) and chaty (2007) have discussed some of the emission characteristics of 259 http://dx.doi.org/10.14311/app.2014.01.0259 james h. beall microquasars, and paredes (2007) has considered the role of microquasars and agns as sources of high energy γ-ray emission. in fact, the early reports of the concurrent radio and x-ray variability of centaurus a can be plausibly interpreted as the launch of a jet from cen a’s central source into the complex structures in its core. additionally, these observations are remarkably similar to the observations of galactic microquasars and agns, including the observations from the fermi space telescope of concurrent γ-ray, ir, optical, and uv variability of 3c454.3 (bonning et al., 2009), and observations by madejski et al. (1997) for bl lac, among others. 2 concurrent variability of agn jet sources as an indication of jet launch or jet-cloud interactions i now turn to two such observations in this paper, the concurrent radio and x-ray variability of centaurus a (beall et al., 1978), and the gamma-ray, uv, and ir concurrent variability discussed by bonning et al., (2009) using the fermi lat and swift spacecraft. 2.1 radio and x-ray variability measurements of centaurus 1 (ngc 5128) the first detection of concurrent, multifrequency variability of an agn came from observations of centaurus a (beall et al., 1978, see figure 1 of that paper). beall et al. conducted the observing campaign of cen a at three different radio frequencies in conjunction with observations from two different x-ray instruments on the oso-8 spacecraft in the 2-6 kev and 100 kev xray ranges. these data were obtained over a period of a few weeks, with the stanford interferometer at 10.7 ghz obtaining the most data. beall et al. also used data from other epochs to construct a decade-long radio and x-ray light curve of the source. figure 1a of beall (2011) shows the radio data during the interval of the oso-8 x-ray observations, as well as the much longer timescale flaring behavior evident in the three different radio frequencies and at both low-energy (2-6 kev, see figure 1b of beall, 2011) and in high-energy (∼ 100 kev, see figure 1c of beall, 2011) x-rays. as noted by beall (2011), a perusal of figure 1a in that paper shows the that the radio data (represented as a “+” in that figure) generally rise during 1973 to reach a peak in mid-1974, then decline to a relative minimum in mid-1975, only to go through a second peak toward the end of 1975, and a subsequent decline toward the end of 1976. this pattern of behavior is also shown in the ∼ 30 ghz data and the ∼ 90 ghz data albeit with less coverage at the higher two radio frequencies. several points are worthy of note. first, as beall et al. (1978) show, the radio and x-ray light curves track one another. this result demonstrated the first report of concurrent radio and x-ray variability of an active galaxy. mushotzky et al. (1978), using the weekly 10.7 ghz data obtained by beall et al., (1978) demonstrate that the 10.7 ghz radio data track the 2-6 kev x-ray data on weekly time scales, also. the concurrent variability at radio and x-ray frequencies argues that the emitting region is the same for both the radio and xray light. this, as was noted by beall and rose (1980), can be used to set interesting limits on the parameters of the emitting region. in addition, the observations at the three radio frequencies (10.7 ghz, ∼ 30 ghz, and ∼ 90 ghz) clearly track one another throughout the interval whenever concurrent data are available. figure 1: fermi lat data (see, e.g., lott et al. 2013) showing the time history of the flaring from 3c454.3 (figure 1b), along with the bonning et al. (2009) data showing concurrent gamma-ray, ir, optical, and uv data (figure 1b). the time of the jet launch from the mojave ”movie” is illustrated by the arrow. the result, first reported here, shows radio flaring consistent with the bonning et al. data. a plausible hypothesis for the observations we have witnessed is that they arise from physical processes associated with the ”launch” of an astrophysical jet into the complex structures in the core of centaurus a. the timescale of the evolution from early 1973 through 1977 appears to be associated with the evolution of a larger structure over a more extended region. the observations are consistent with the interaction of the astrophysical jet with an interstellar cloud in the core of cen a. 260 a review of astrophysical jets it is clear from this discussion that a distinction needs to be made about which observational signatures are associated with the jet launch, the jet itself, and the ambient medium’s reaction to the jet. in considering such a scenario applied to microquasars, the observations of sco x-1 by fomalont, geldzahler, and bradshaw (2001), as discussed by beall et al. (2013) are extremely informative. 2.2 a jet launch coincident with the 3c454.3 multi-frequency flaring bonning et al. (2009) performed an analysis of the multi-wavelength data from the blazar, 3c454.3, using ir and optical observations from the smarts telescopes, optical, uv and x-ray data from the swift satellite, and public-release γ-ray data from the fermilat experiment. in that work, she demonstrated an excellent correlation between the ir, optical, uv and gamma-ray light curves, with a time lag of less than one day. urry (2011) noted that 3c454.3 can be a laboratory for multifrequency variability in blazars. while a more precise analysis of the data will be required to determine the characteristics of the emitting regions for the observed concurrent flaring at the different frequencies, the pattern of a correlation between low-energy and higher-energy variability is consistent with that observed for cen a, albeit with the proviso that the energetics of the radiating particles in 3c454.3 is considerably greater. a perusal of the data for 3c454.3 at milliarcsecond scales taken from the mojave vlba campaign during the bonning et al (2009) flare show that the timedependent flare occurs during launch of a new component of the jet that originates from the core. figure 2: figure 2: 3c454.3 shown at milliarcsecond scales for data taken from the mojave vlba campaign during the bonning et al (2009) flare. a perusal of the time-dependent flare clearly shows that the jet launch from the core of the agn is coincident with the double peaked flaring shown in the bonning et al. results. the pattern of variability reported by bonning et al., is consistent with the injection of relativistic particles into a region with relatively high particle and radiation densities (i.e., an interstellar cloud). the picture that emerges, therefore, is consistent with the observations of spatially and temporally resolved galactic microquasars and agn jets. it should not escape our notice that other epochs in the 3c454.3 data from mojave and fermi also show similar periods of injection of radio blobs that are associated with a double-peaked structure in the gamma-ray light. for the cen a data, and for data from 3c454.3, it is the concurrent variability that suggests that the radio to x-ray (in cen a’s case) and the ir, optical, and uv to γ-ray fluxes (in 3c454.3’s case) are created in the same region. this leads to the possibility of estimates of the source parameters that are obtained from models of these sources. vlbi observations of cores vs. jets (see, e.g., the study of bl lac by bach et al. 2006) show the structures of the core vs. jet as they change in frequency and time. it has thus become possible to separate and study the time variability of the jet and the core of agn at remarkably fine temporal and spatial scales. van der laan (1976) discussed the theoretical interpretation of cosmic radio data by assuming a source which contained uniform magnetic field, suffused with an isotropic distribution of relativistic electrons. the source, as it expanded, caused an evolution of the radio light curve at different frequencies. each of the curves in van der laan’s paper represents a factor of 2 difference in frequency, the vertical axis representing intensity of the radio flux and the horizontal axis representing an expansion timescale for the emitting region. van der laan’s calculations show a marked difference between the peaks at various frequencies. the data from cen a (as discussed more fully in beall (2008, 2010) are, therefore, not consistent with van der laan expansion (van der laan, 1976), since for van der laan expansion, we would expect the different frequencies to achieve their maxima at different times. also, the peak intensities should decline with increasing frequencies at least in the power-law portion of the spectrum. the most likely explanation for the changes in the spectrum of the cen a data at 100 kev (beall, et al., 1978) is that the emitting region suffered an injection of energetic electrons. that is, a jet-ambient-medium interaction dumped energetic particles into a putative ”blob,” or, equivalently, that there was a re-acceleration of the emitting electrons on a timescale short compared to the expansion time of the source. an analysis of the 3c120 results compared with the data from the galactic microquasar, sco x-1, under261 james h. beall taken by beall (2006) shows a similar radio evolution, with rapidly moving “bullets” interacting with slower moving, expanding blobs. it is highly likely that the elements of these sources that are consistent with van der laan expansion are the slower-moving, expanding blobs. i believe that the relativistically moving bullets, when they interact with these slower-moving blobs, are the genesis of the flaring that we see that seems like a re-acceleration of the emitting, relativistic particles. i note that a similar scenario could be operating in cen a and 3c454.3. this is not to say that the ”slower-moving blobs” are not themselves moving relativistically, since the bipolar lobes have significant enhancements in brightness due to relativistic doppler boosting for the blobs moving toward us. the true test of this hypothesis will require concurrent, multifrequency observations with resolutions sufficient to distinguish jet components from core emissions in galactic microquasars as well as for agn jets. one of the most remarkable sagas regarding the discovery of quasar-like activity in galactic sources comes from the decades long-investigation of sco x-1 by ed fomalont, barry geldzahler, and charlie bradshaw (fomalont, geldzahler, and bradshaw, et al. 2001). during their observations, an extended source changed relative position with respect to the primary object, disappeared, and then reappeared many times. we now know that they were observing a highly variable jet from a binary, neutron star system. the determinant observation was conducted using the very large array (vla) in socorro, new mexico and the vlba interferometer (see, e.g., beall, 2008) for a more complete discussion). the observations of the concurrent ir, optical, uv, and γ-ray variability of 3c454.3, and its associated jet launch in the mojave data, argue for a reinvestigation of these data sets in the near future. it is worthy of note that the milliarcsecond observations show a complex evolution of structure at parsec scales, including an apparently sharp change of directions associated with changes in the polarization of the radio light at that point in the jet’s evolution (see figure 2). furthermore, the multi-frequency flares reported by bonning et al. (2009) are consistent with the launch of another component of the astrophysical jet in the core region. regarding the acceleration region and the possible mechanisms for the collimation of the jets, a number of models have been proposed (see, e.g., kundt and gopal-krishna (2004), bisnovaty-kogan et al. (2002), romanova and lovelace (2009), and bisnovaty-kogan (2012) that might help explain the complexity present in these data. 3 concluding remarks the data discussed therein suggest a model for the jet structures in which beams or blobs of energetic plasmas propagate outward from the central engine to interact with the ambient medium in the source region. this ambient medium in many cases comes from prior ejecta from the central source, but can also come from clouds in the broad line region. the jet can apparently also excavate large regions, as is suggested by the complex structures in, for example, 3c120. the physical processes which can accelerate and entrain the ambient medium through which the jet propagates, have been discussed in detail in several venues (see, e.g., rose et al., 1984, 1987, beall, 1990, beall et al., 2003, and beall, 2010). in this paper, we have focused on the patterns of concurrent radio and x-ray variability for cen a, and on concurrent radio, gamma-ray, optical, ir, and uv variability for 3c454.3. the data can be interpreted as being associated with a jet-launch scenario for these sources, and this paper represents the first report of the association between a jet launch in the mojave data for 3c454.3 and its gamma-ray flare from fermi. in addition, jets from microquasars show similar patterns of variability to those of agns. these data require us to abandon our assumptions both of spherical symmetry and of single-zone productions in our models of these sources. acknowledgement the author gratefully acknowledges the support of the office of naval research for this research. this research has made use of data from the mojave database that is maintained by the mojave team (lister et al., 2009, aj, 137, 3718). the author gratefully acknowledges the data provided by the mojave team for this paper. references [1] the pierre auger collaboration, et al., 2007, science 318, 938. doi:10.1126/science.1151124 [2] bach, u., villata, m., raiteri, c. m., et al. 2006, ”structure a variability in the vlbi jet of bl lacertae during the webt campaigns (1995-2004),” a&a, 456, 105. 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[46] zanni, c., murante, g., bodo, g., massaglia, s., rossi, p., ferrari, a., 2005 astron. astrophys., 429, 399. discussion todor stanev do you expect an important role of magnetic reconnection in the particle acceleration in the jets? jim beall i do believe that magnetic reconnection plays an important part in particle acceleration, not only in the launch region, but also in the jet itself. if you recall the data i showed for 3c454.3, the region where the get seems to turn shows marked variations in brightness that seem to be associated both with the blobs from the jet traversing that region, and with the jet luminosity at the ”elbow.” in fact, it seems likely that the acceleration and collimation region of the jet in 3c454.3 extends many parsecs from the launch region. carlotta pittori can you say something about the southern jet of the crab nebula? as i actually showed at a previous vulcano workshop and promptly noticed by w. kundt, the jet orientation for x-ray images from chandra changed in the ten years from 20002010. how can this be explained? by kink instabilities? where does the field close in this system? jim beall this is only a conjecture, but if we look at the large distance where the jet in 3c454.3 changes direction, it appears that magnetic field structures can extend out to several parsecs and maintain their coherence, at least in agn sources. if the same is true in the crab, then the magnetic fields might be responsible for the changes in the jet orientation. wolfgang kundt can you tell me when and where geoff burbidge argued in favor of hadronic jets on the grounds of their stability? in my model with gopal, a pair-plasma jet is thought to be stable on the grounds of their exb drift, for example in new astronomy reviews, 2008, 52, pp 364-369. jim beall geoff burbidge’s article was published in astrophysical journal, 1956, 124, 416. that paper was in the main an argument that the energy supplied to the giant radio lobes must be of order 1060 ergs, and could only be delivered by hadronic jets. 264 introduction concurrent variability of agn jet sources as an indication of jet launch or jet-cloud interactions radio and x-ray variability measurements of centaurus 1 (ngc 5128) a jet launch coincident with the 3c454.3 multi-frequency flaring concluding remarks 200 acta polytechnica ctu proceedings 1(1): 200–204, 2014 200 doi: 10.14311/app.2014.01.0200 high-resolution x-ray spectroscopy of galactic supernova remnants satoru katsuda1, hiroshi tsunemi2 1riken (the institute of physical and chemical research), 2-1 hirosawa, wako, saitama 351-0198, japan 2department of earth and space science, graduate school of science, osaka university, 1-1 machikaneyama, toyonaka, osaka, 560-0043, japan corresponding author: katsuda@crab.riken.jp abstract high-resolution x-ray spectroscopy of galactic supernova remnants (snrs), based on grating spectrometers onboard xmm-newton and chandra, has been revealing a variety of new astrophysical phenomena. broadened oxygen lines for a northwestern compact knot in sn 1006 clearly show a high oxygen temperature of ∼300 kev. the high temperature together with a lower electron temperature (kte ∼ 1 kev) can be reasonably interpreted as temperature non-equilibration between electrons and oxygen behind a collisionless shock. an ejecta knot in the puppis a snr shows blueshifted line emission by ∼1500 km s−1. the line widths are fairly narrow in contrast to the sn 1006’s knot; an upper limit of 0.9 ev is obtained for o viii lyα, which translates to an oxygen temperature of kto < 30 kev. the low temperature suggests that the knot was heated by a reverse shock whose velocity is ∼4 times slower than that of a forward shock. anomalous intensity ratios in o vii heα lines, i.e., a stronger forbidden line than a resonance line, is found in a cloud-shock interaction region in puppis a. the line ratio can be best explained by the charge-exchange emission that should arise at interfaces between the cold/warm clouds and the hot plasma. there are several other targets for which we plan to analyze high-quality grating data prior to the operation of the soft x-ray spectrometer onboard astro-h. keywords: supernova remnants x-rays individuals: sn 1006, puppis a. 1 introduction there are many scientific motivations to perform highresolution x-ray spectroscopy of supernova remnants (snrs), since the x-ray emission is often dominated by thin thermal emission, namely line emission. for instance, accurate line widths allow us to measure the degree of temperature equilibration between electrons and ions as well as efficiency of particle acceleration. resolved fine structures of lines give us insight into emission processes. we can also reveal ejecta dynamics and measure abundances in unprecedented detail. current challenges are based on grating spectrometers onboard xmm-newton and chandra. both of the grating systems are slitless. therefore, while they work well for point-like sources or compact extended sources, they are generally not suitable for largely extended sources because off-axis emission along the dispersion direction is detected at wavelength positions shifted with respect to the on-axis source. most snrs in the large/small magellanic clouds are small enough for the gratings, and indeed over 10 papers have been published in literature (e.g., burrows et al. 2000; rasmussen et al. 2001; behar et al. 2001; van der heyden et al. 2001; 2002; 2003; flanagan et al. 2004; kosenko et al. 2008; 2011; broersen et al. 2011). a problem here is that we can not deduce spatial information, since the grating spectra are integrated for the entire snr. this causes serious ambiguities of interpretations. therefore, it is important to observe large galactic snrs for spatiallyresolved high-resolution spectroscopy. however, in this case, grating spectra suffer from spectral degradation due to the spatial extent of the source. the only solution to this dilemma is to focus on locally bright and compact features in large snrs. we here review a few successful examples of high-resolution spectroscopy of galactic snrs with the xmm-newton’s reflection grating spectrometer (rgs: den herder et al. 2001). the rgs has a large dispersion angle and is more suitable for extended sources than chandra’s grating spectrometer, although there are some nice results (lazendic et al. 2006; rutherford et al. 2013). 2 fast-moving knots in sn 1006 and puppis a a pioneering work employing the rgs to obtain highresolution spectra from galactic snrs was given by vink et al. (2003) who observed a northwestern (nw) knot in sn 1006 (see fig. 1 (a)). as shown in fig. 1 200 http://dx.doi.org/10.14311/app.2014.01.0200 high-resolution x-ray spectroscopy of galactic supernova remnants (b), the rgs spectrum showed several emission lines, including o vii heα forbidden and resonance, from the knot. by taking account of the degradation effect due to spatial extent, vink et al. (2003) found intrinsic broadening of o k-shell lines to be σ = 3.4 ± 0.5 ev. this value was later revised by the same group based on additional deep observations to σ = 2.4±0.3 ev (broersen et al. 2013). the amount of broadening corresponds to an oxygen temperature of 275+72 −63 kev, if attributed to thermal doppler broadening. on the other hand, the electron temperature is derived to be ∼1.35 kev, which is ∼200 times lower than the oxygen temperature. this temperature discrepancy is the evidence for temperature non-equilibration as is expected behind collisionless shocks; the ion temperatures (kti) immediately behind collisionless shocks are given by kti =3/16miv 2 sh, where mi is an elemental mass and vsh is a shock speed, so that the initial temperatures are proportional to particle masses. 18 19 20 21 22 23 0 0.5 1 c ou nt s s− 1 å − 1 wavelength (å) knot knot bek (a) sn 1006 (c) puppis a (b) (d) o viii ly α r f figure 1: (a) x-ray view of sn 1006 in 500–599 ev, taken from broersen et al. (2013). the rgs field of view is within the two lines. (b) rgs spectra zooming into o vii heα from the nw knot in sn 1006. the positions of the resonance (21.6å), the intercombination (21.8å), and the forbidden line (22.1å) are indicated. this figure is taken from broersen et al. (2013). (c) x-ray view of the puppis a snr in 0.5–5 kev. the rgs spectra taken from the two solid lines (hosting the knot) or dashed lines (hosting the bek) are shown in fig. 1 (d) and fig. 3, respectively. (d) rgs spectrum zooming into o k-shell lines from the ejecta knot in puppis a. in the model, the contribution from the knot is indicated as a blue line. rest-frame positions of the o viii lyα (18.9å) and the o vii heα lines are indicated as dashed lines. the blueshifts are clearly visible. along the same line, another interesting target is an ejecta knot in the northeast of the puppis a snr (see, fig. 1 (c)) whose southern portion is positionally coincident with an optical o-rich filament (the so-called ω filament which is not shown here: winkler & kirshner 1985). based on nondispersive x-ray ccds onboard xmm-newton (epic: turner et al. 2001, strüder et al. 2001), katsuda et al. (2008) noticed blueshifted k-shell lines from the knot. however, the poor energy resolution of the epic data (e/∆e ∼ 20) prevented them from conclusive arguments; the doppler velocity ranged from 1700+700 −800 km s−1 at the south of the knot, which agrees with the optical measurement of ∼1500 km s−1 (winkler & kirshner 1985), to 3400+1000 −800 km s−1 at the north of the knot. since the knot is bright and compact (∼5 times brighter than its surroundings and the size is smaller than ∼3′), the rgs is capable of yielding a highresolution spectrum at a level of e/∆e ∼ 150. thus, we performed a new xmm-newton observation, and ob201 satoru katsuda, hiroshi tsunemi tained a nice rgs spectrum as shown in fig. 1 (d). it exhibits prominent k-shell lines, including o vii heα and o viii lyα. the lines are clearly blueshifted by 1480±140±60 km s−1 (the first and second term errors are measurement and calibration uncertainties, respectively), which is fully consistent with optical measurements for the ω filament. it is also found that doppler velocities are uniform within the knot (katsuda et al. 2013b). in addition, the rgs spectra enabled us to measure line broadening to be σ < 0.9 ev at o viii lyα, indicating an upper limit of an oxygen temperature of 30 kev. interestingly, the temperature for puppis a’s knot is at least an order-of-magnitude lower than that for sn 1006’s knot, even though they are comparably fast moving. to interpret the dramatic difference in oxygen temperatures between sn 1006’s knot and puppis a’s knot, we investigate temperature equilibration through coulomb interactions behind collisionless shocks. in the calculation, we assume elemental abundances and the ionization timescales given in the literature (table 1 in broersen et al. 2013 for sn 1006 and table 2’s case-b in katsuda et al. 2013b for puppis a). also, since the degree of electron heating at collisionless shocks is not yet understood well, we examined various initial electronto-proton temperature ratios, te/tp = β, for each shock speed. in this way, we ran calculations for several shock speeds. figure 2 shows the best-representative models, where temperature curves are illustrated only for electrons (black) and oxygen (red). for comparison, we also show model curves with β = me/mp. we find that the data require fairly different shock speeds; a fast (∼3000 km s−1) shock for sn 1006 and a relatively slow (∼600 km s−1) shock for puppis a. also, the values of β are obtained to be ∼0.06 for sn 1006 and less than ∼0.5 for puppis a. these values roughly agree with an observational trend shown in van adelsberg et al. (2008). furthermore, the value for sn 1006 is consistent with a direct measurement from hα spectroscopy for the optical filament (β < 0.07: ghavamian et al. 2002). on the other hand, it is interesting to note that our best-estimated value of 0.06 is higher than a recent theoretical prediction, β = 0.023 for vsh > 1500 km s−1 (ohira & takahara 2008). we will revisit this issue in our future work. the inferred shock velocity for sn 1006 is roughy consistent with the proper motion of the shock ahead of the knot (winkler et al. 2003; katsuda et al. 2013a). therefore, a simple interpretation is that the knot was heated by a forward shock. however, we cannot rule out a possibility of reverse-shock heating. in fact, the knot shows an elevated si abundance, suggesting that it originates from the sn ejecta that should have been heated by a reverse shock. in this case, the (reverse) shock velocity should be higher than ∼5000 km s−1 (see, the discussion in katsuda et al. 2013b). such a fast shock requires an oxygen temperature of ∼800 kev. this is significantly higher than the x-ray measurement, thereby a significant fraction of the shock energy may go into cosmic-rays. if true, this suggests efficient cosmic-ray acceleration at the reverse shock. as for puppis a, the inferred shock velocity, ∼600 km s−1, is less than the shocked-gas velocity, ∼2000 km s−1, derived by the doppler velocity of ∼1500 km s−1 and the optical proper motion of ∼1250 km s−1 at a distance of 2.2 kpc (winkler et al. 1988). this is consistent with a picture that the ejecta knot was heated by a reverse shock rather than a forward shock, since the forward shock velocity should be ∼2700 km s−1 (= 4/3 × 2000 km s−1 according to the rankine-hugoniot relation). 109 1010 1011 1012 1 10 100 kt ( ke v ) net (cm−3s) oxygen electron vshock = 3000 km s−1 solid line: β = 0.06 dotted line: β = me/mp 109 1010 1011 1012 0.1 1 10 kt ( ke v ) net (cm−3s) oxygen electron vshock = 600 km s−1 solid line: β = 0.5 dotted line: β = me/mp sn 1006 puppis a figure 2: temperature histories for electrons (black) and oxygen (red) as a function of net. left and right panels are for the nw knot in sn 1006 and the ejecta knot in puppis a, respectively. 202 high-resolution x-ray spectroscopy of galactic supernova remnants 3 cloud-shock interaction regions in puppis a possible sites for high-resolution spectroscopy of galactic snrs are not only ejecta knots but also swept-up ism regions. in particular, cloud-shock interaction regions in the puppis a snr are promising, because they are bright and compact (e.g., hwang et al. 2005). so far, rgs spectra taken from the bright eastern knot (bek) and the northern knot were presented by katsuda et al. (2012). in fig. 3, we show one of the bek spectra focusing on the o vii heα lines, which was taken from the region within the dashed lines in fig. 1 (c). interestingly, we see a strong forbidden line with respect to the resonance line. this is inconsistent with any thermal emission models with reasonable plasma parameters (i.e., kte = 0.3–0.7 kev and net = 1010–5×1011 cm−3 s). after considering a few possibilities to reproduce the anomalous line ratio, katsuda et al. (2012) proposed that the charge-exchange emission, that arises at interfaces between the cold/warm clouds and the hot plasma, can best explain the data. this would support a long-standing expectation of chargeexchange x-ray emission in snrs. 21 21.5 22 22.5 0 0.5 1 c ou nt s s− 1 å − 1 wavelength (å) puppis a bek f i r figure 3: rgs spectrum zooming into o vii heα from the bek in puppis a. the positions of the resonance (21.6å), the intercombination (21.8å), and the forbidden line (22.1å) are indicated. 4 future prospects there are several nice rgs data sets of galactic snrs that are waiting for being analyzed. these include bright shells of rcw 86, an equatorial belt in g292.0+1.8, and a southwestern knot in the cygnus loop. in addition, it is worth while to search for other promising targets and observe them. we are planning to analyze the rgs data in the near future. the rgsbased study is a great pathfinder for the coming highresolution x-ray spectroscopy of snrs with the nondispersive soft x-ray spectrometer (mitsuda et al. 2010) onboard astro-h (takahashi et al. 2012). acknowledgement s.k. is supported by the special postdoctoral researchers program in riken. this work is partly supported by a grant-in-aid for scientific research by the ministry of education, culture, sports, science and technology (23-000004 and 24-8344). references [1] behar, e., et al. 2001, a&a, 365, l242 [2] broersen, s., et al. 2011, a&a, 535, 11 [3] broersen, s., et al. 2013, a&a, 552, 9 [4] burrows, d. n., et al. 2000, apj, 543, l149 doi:10.1086/317271 [5] den herder, j. w., et al. 2001, a&a, 365, l7 [6] flanagan, k. a., et al. 2004, apj, 605, 230 doi:10.1086/382145 [7] ghavamian, p., et al. 2002, apj, 572, 888 doi:10.1086/340437 [8] hwang, u., et al. 2005, apj, 635, 355 doi:10.1086/497298 [9] katsuda, s., et al. 2008, apj, 678, 297 doi:10.1086/586891 [10] katsuda, s., et al. 2012, apj, 756, 49 doi:10.1088/0004-637x/756/1/49 [11] katsuda, s., et al. 2013a, apj, 763, 85 doi:10.1088/0004-637x/763/2/85 [12] katsuda, s., et al. 2013b, apj, 768, 182 doi:10.1088/0004-637x/768/2/182 [13] kosenko, d., et al. 2008, a&a, 490, 223 [14] kosenko, d., et al. 2011, a&a, 532, 114 [15] lazendic, j. s., et al. 2006, apj, 651, 250 doi:10.1086/507481 [16] mitsuda, k., et al. 2010, spie, 7732, 773211 [17] ohira, y., & takahara, f. 2008, apj, 688, 320 doi:10.1086/592182 [18] rasmussen, a. p., et al. 2001, a&a, 365, l231 [19] rutherford, j., et al. 2013, apj, 769, 64 doi:10.1088/0004-637x/769/1/64 203 http://dx.doi.org/10.1086/317271 http://dx.doi.org/10.1086/382145 http://dx.doi.org/10.1086/340437 http://dx.doi.org/10.1086/497298 http://dx.doi.org/10.1086/586891 http://dx.doi.org/10.1088/0004-637x/756/1/49 http://dx.doi.org/10.1088/0004-637x/763/2/85 http://dx.doi.org/10.1088/0004-637x/768/2/182 http://dx.doi.org/10.1086/507481 http://dx.doi.org/10.1086/592182 http://dx.doi.org/10.1088/0004-637x/769/1/64 satoru katsuda, hiroshi tsunemi [20] strüder, l., et al. 2001, a&a, 365, l18 [21] takahashi, t., et al. 2012, spie, 8443, 84431z [22] turner, m. j. l., et al. 2001, a&a, 365, l27 [23] van adelsberg, m., et al. 2008, apj, 689, 1089 doi:10.1086/592680 [24] van der heyden, k. j., et al. 2001,a&a,365,l254 [25] van der heyden, k. j., et al. 2002,a&a,392,955 [26] van der heyden, k. j., et al. 2003,a&a,406,141 [27] vink, j., et al. 2003, apj, 587, l31 doi:10.1086/375125 [28] winkler,p.f. & kirshner,r.p. 1985,apj,299,981 [29] winkler, p. f., et al. 1988, iau colloq. 101: 65 [30] winkler, p. f., et al. 2003, apj, 585, 324 doi:10.1086/345985 204 http://dx.doi.org/10.1086/592680 http://dx.doi.org/10.1086/375125 http://dx.doi.org/10.1086/345985 introduction fast-moving knots in sn 1006 and puppis a cloud-shock interaction regions in puppis a future prospects acta polytechnica ctu proceedings doi:10.14311/app.2017.8.0005 acta polytechnica ctu proceedings 8:5–7, 2017 © czech technical university in prague, 2017 available online at http://ojs.cvut.cz/ojs/index.php/app adhesion of biocompatible tinb coating tomas kolegara,∗, martin matouseka, monika vilemovab, vladimir starya a faculty of mechanical engineering, department of material engineering, czech technical university in prague, charles square 13, 121 35 prague, czech republic b institute of plasma physics as cr, v.v.i. ∗ corresponding author: tomas.kolegar@fs.cvut.cz abstract. preparation of a coating with a high quality requires good adhesion of the film to the substrate. the paper deals with the adhesion of biocompatible tinb coating with different base materials. several materials such as titanium cp grade 2, titanium alloys ti6al4v and stainless steel aisi 316l were measured. testing samples were made in the shape of small discs. those samples were coated with a tinb layer by using the pvd method (magnetron sputtering). onto the measured layer of tinb an assistant cylinder was stuck using a high strength epoxy adhesive e1100s. the sample with the assistant cylinder was fixed into a special fixture and the whole assembly underwent pull-off testing for adhesion. the main result of this experiment was determining the strength needed to peel the layer and morphology and size of the breakaway. as a result, we will be able to determine the best base material and conditions where the coating will be remain intact with the base material. keywords: pvd coating, adhesion, biocompatible materials, pull-off test, titanium, niobium. 1. introduction today's civilized society brings human kind many simplifications for numerous activities in every day life. due to this comfort more civilization illnesses appear, such as obesity, cancer, atherosclerosis, heart attack and of course musculoskeletal diseases including joints and tendons, but also chronic stress and sports injuries. high life expectancy on other hand causes faster aging of tissue and makes surgery of hip, shoulder and other joints necessary. one solution to help with this this health problem is the utilization of implants. nowadays, commonly used implants are made of lower quality material like stainless steel and these implants are coated with a high quality biocompatible material such as tinb and also other titanium alloys. the most important property of the coating is its adhesion to the base material. the main quality of the parameters for adhesion are the substrate structural properties, purity and surface condition at the time of coating formation, the value of the kinetic energy of falling atoms and ions to the surface, and also the forming a coating on the support surface and the other deposition parameters. according to g. rosenman, the minimum adhesion strength of biocompatible coating to the substrate is 15 mpa [1] and cells have better conditions for adhesion, growth and their establishment in the living body. the goal of this experiment is to verify if the coating is capable of withstanding the strain which can be found in the human body. each human body and conditions inside are very individual and results depend on many factors [2]. 2. materials and procedure for the experiment three base materials were used. titanium cp grade 2, ti6al4v and stainless steel aisi 316l. chemical composition of these materials is shown in table 1, 2 and 3 [3]. experimental samples (discs of diameter 14 mm and 4 mm thickness) were coated with tinb (61 wt % of ti and 39 wt % of nb). the pvd magnetron sputtering method was used [4]. final thickness of the coating was measured by calotest (csm, switzerland) and it was 2.4 ± 0.1 µm. the screw according to din 931 with a 10 mm metric thread was used as the assistant cylinder. these two parts were glued together with high strength epoxy adhesive e1100s with a minimum ultimate tensile strength 70 mpa. the glue was cured in a furnace for 1 hour at 140 ◦c. the pull-off test performed on the available samples was according to en iso 4624:2003 [5, 6]. table 4 presents the measured parameters of the experiment, especially the force when the tested sample was broken. ultimate tensile strength and the average value were measured. 3. results of the experiment (morphology and tensile strength) results of the experiments can be seen in figures 1, 2 and 3. note: the most suitable sample was chosen. black spot indicates detachment of coating. 5 http://dx.doi.org/10.14311/app.2017.8.0005 http://ojs.cvut.cz/ojs/index.php/app t. kolegar, m. matousek, m. vilemova, v. stary acta polytechnica ctu proceedings c n mn p s cr mo ni cu si fe 0.022 0.094 1.75 0.021 0.001 17.18 2.71 14.1 0.06 0.35 rest table 1. chemical composition of aisi 316l [wt %] c n o h fe al v ni mo other ti 0.08 0.03 0.25 0.0125 0.3 rest table 2. chemical composition of titanium cp grade 2 [wt %] figure 1. stainless steel aisi 316l. figure 2. titanium cp grade 2. 4. conclusions • average uts on fe-material was measured (22.48 mpa). . coating stayed on sample. • average uts on ti-material was measured (19.30 mpa). . coating was partially removed. • average uts on ti6al4v-material was measured (34.27 mpa). . large part of coating was removed. figure 3. titanium alloys ti6al4v. ultimate tensile strength of epoxy adhesive e1100s specified by manufacturer is 70 mpa. in the experiment, we achieve 34 mpa in the best result (ti6al4v) while all three testing materials should withstand more than 15 mpa which is the minimum adhesion strength according to g. rosenman [1]. each human body is very individual and we must be sure that implants withstand all the processes in the human body. high values of uts approaching the values set up by manufacturer are more than is required. generally, on a surface with high roughness the adhesion is also higher. further experiments will involve changing parameters of the surface roughness, combination of epoxy adhesion and film adhesion. an ultimate tensile strength of coating at least higher than 50 mpa should be achieved. list of symbols f force [n] ra roughness average [µm] uts ultimate tensile strength [mpa] acknowledgements this work was supported by project lo1207, program npu1, from the ministry of education, youth and sports of the czech republic and by czech science foundation project no. 15-01558s. 6 vol. 8/2017 adhesion of biocompatible tinb coating c n o h fe al v ni mo other ti 0.08 0.05 0.13 0.012 0.25 6.5 4.5 rest table 3. chemical composition of ti6al4v [wt %] sample f ra uts average uts stand. deviation [n] [µm] [mpa] [mpa] [mpa] aisi 316l 1 1540 0.06 ± 0.01 19.62 22.48 2.592 1950 24.84 3 1990 25.35 ti (cp2) 1 1040 13.25 19.30 4.942 1490 18.98 3 1990 25.35 ti6al4v 1 3160 40.25 34.27 4.902 2630 33.50 3 2220 28.28 table 4. table of measured parameters references [1] n. eliaz, o. ritman-hertz, d. aronov, et al. the effect of surface treatments on the adhesion of electrochemically deposited hydroxyapatite coating to titanium and on its interaction with cells and bacteria. journal of materials science: materials in medicine 22(7):1741–1752, 2011. doi:10.1007/s10856-011-4355-y. [2] e. eisenbarth, d. velten, m. müller, et al. biocompatibility of β-stabilizing elements of titanium alloys. biomaterials 25(26):5705 – 5713, 2004. doi:http://doi.org/10.1016/j.biomaterials.2004.01.021. [3] s. guo, j. zhang, x. cheng, x. zhao. a metastable β-type ti-nb binary alloy with low modulus and high strength. journal of alloys and compounds 644:411 – 415, 2015. doi:http://doi.org/10.1016/j.jallcom.2015.05.071. [4] k.-t. k. chiang, r. wei. growth morphology and corrosion resistance of magnetron sputtered cr films. surface and coatings technology 206(7):1660 – 1665, 2011. doi:http://doi.org/10.1016/j.surfcoat.2011.09.020. [5] b. jönsson, s. hogmark. hardness measurements of thin films. thin solid films 114(3):257 – 269, 1984. doi:http://dx.doi.org/10.1016/0040-6090(84)90123-8. [6] x. dai, a. zhou, l. feng, et al. molybdenum thin films with low resistivity and superior adhesion deposited by radio-frequency magnetron sputtering at elevated temperature. thin solid films 567:64 – 71, 2014. doi:http://doi.org/10.1016/j.tsf.2014.07.043. 7 http://dx.doi.org/10.1007/s10856-011-4355-y http://dx.doi.org/http://doi.org/10.1016/j.biomaterials.2004.01.021 http://dx.doi.org/http://doi.org/10.1016/j.jallcom.2015.05.071 http://dx.doi.org/http://doi.org/10.1016/j.surfcoat.2011.09.020 http://dx.doi.org/http://dx.doi.org/10.1016/0040-6090(84)90123-8 http://dx.doi.org/http://doi.org/10.1016/j.tsf.2014.07.043 acta polytechnica ctu proceedings 8:5–7, 2017 1 introduction 2 materials and procedure 3 results of the experiment (morphology and tensile strength) 4 conclusions list of symbols acknowledgements references acta polytechnica ctu proceedings doi:10.14311/ap.2016.4.0073 acta polytechnica ctu proceedings 4:73–79, 2016 © czech technical university in prague, 2016 available online at http://ojs.cvut.cz/ojs/index.php/app cfd simulation of upward subcooled boiling flow of freon r12 tomas romsy∗, pavel zacha faculty of mechanical engineering, czech technical university in prague, technická 4, prague 6, 166 07, czech republic ∗ corresponding author: tomas.romsy@fs.cvut.com abstract. subcooled flow boiling under forced convection occurs in many industrial applications to maximize heat removal from the heat source by the very high heat transfer coefficient. this work deals with cfd simulations of the subcooled flow boiling of refrigerant r12 solved by code ansys fluent r16. the main objective of this paper is verification of used numerical settings on relevant experiments performed at debora test facility. also comparisons with previously provided simulation on nri rez are presented. data outputs from this work are basis to subsequent calculations of steam-water mixture cooling of pb-li eutectic. keywords: cfd simulation, subcooled boiling, freon r12. 1. introduction subcooled flow boiling can solve a wide range of high power thermal challenges in the following areas: central processing unit (cpu) and computational application specific integrated circuits (asics) [1, 2], ultra-high brightness light emitting diodes (leds) and lasers sources [3], automotive power electronics [4], or avionics [5], where research findings have made possible the development of a new generation of cooling hardware, which promises order of magnitude increase in heat dissipation compared to today’s cutting edge cooling schemes. subcooled flow boiling is expected to realize the high heat flux cooling for electronic devices [6, 7], or for high heat flux cooling in microgravity [8]. subcooled flow boiling also plays a significant role in case of cooling of plasma facing component of thermonuclear reactors. the subcooled flow boiling is a high complex form of two-phase liquid flow in which the single-phase flow merges into the two-phase flow and back to the singlephase flow. a local appearance and disappearance of the two-phase flow allow the use of the latent heat of vaporization for a heat removal from the surface into the liquid without causing the boiling crisis. in comparison with a single-phase fluid it represents the efficient way to improve heat transfer from the wall to the flowing liquid. therefore, it provides an ability of the sufficient heat transfer at high heat fluxes. detailed investigation of this phenomenon can help for its better understanding to make wider improvement. one of the perspective application of subcooled flow boiling is so-called cold trap facility. this device serves for corrosion impurity separation from the metal eutectic (pb, pb-li), whereas the separation is carried out by the cooling of the eutectic down to the required temperature. this facility considered in centre of advanced nuclear technology project (canut) [10] provides sufficient heat removal from eutectic across the wall to the subcooled steam-water mixture (cooling loop) and subsequently to the tertiary water loop providing ultimate heat removal. part of the cold trap design is cooling concept which strongly depends on steam-water mixture behaviour. description of cooling loop behaviour is provided with the aid of computational fluid dynamics (cfd) code ansys fluent. verification of suitable numerical settings and computational methodology should be determined in the first step. selected experiments on the debora facility were used for this purpose. a detailed description of this device can be found in [11] and [9]. debora project at cea grenoble (french alternative energies and atomic energy commission) was focused on determination of subcooled boiling character by using refrigerant dichlorodifluoromethane (r12). provided experiment outputs contain especially void fraction, gas velocity, temperature of the fluid and the bubble size. because both freon r12 and water have relative physical similarity, we can use this freon for his lower operating parameters in comparison with water. this leads to a much simpler and safer measurements. the geometry of the debora facility is shown in figure 1. it is the vertical heated tube with inner diameter of 19.2 mm. freon is heated along the 3.5 m long part of pipe. the rest of 5 m long tube is unheated. it corresponds to 1 m of inlet section and 0.5 m of outlet section. a radial profile of the vapour volume fraction and its speed at the end of the heated part were measured by optical probe. additionally, the bubble size profiles were available for this area. axial wall temperatures were measured using thermocouples, but radial temperatures were not available [9]. cfd model was performed based on the debora facility parameters and the boundary conditions and standard numerical approaches were set according to the experiment conditions. the results from simula73 http://dx.doi.org/10.14311/ap.2016.4.0073 http://ojs.cvut.cz/ojs/index.php/app tomas romsy, pavel zacha acta polytechnica ctu proceedings figure 1. a) sketch of the debora test geometry [9] b) computational model sketch with detail of used grid. measured quantity p [mpa] v [m/s] qw [w/m2] tin [◦c] tsat [◦c] case 1 3.008 0.86 58 260 67.89 94.24 case 2 3.006 0.891 58 260 73.7 94.21 table 1. specified conditions of the chosen experimental cases. tions provided at nuclear research institute rez (nri rez) [12] are included in section 4 for better comparison. main purpose of this comparison are different approaches in calculation which lead to opportunity to compare them. while the previous simulation from nri rez was performed at ansys fluent r13, presented work was processed at ansys fluent r16. earlier release 13 missed necessary subcooling numerical models and these calculations were implemented by individual user defined functions (udf). however, starting from release 15, numerical models describing this phenomenon are implemented internally. the main output of this work is to determine selected parameters (pressure, velocity, temperatures) in the specified geometry position and compare them against the measured experimental data provided from debora test facility and against the previously calculated values from nri rez simulations. 2. model description simulation model is based on geometry of the debora experimental device. the 10° v-cutout, shown in figure 1, was considered due to the symmetry and to reduce the computational time. slightly rough hexahedral grid was tested in the first steps of the calculation. further enhancements were applied after obtaining better knowledge of the calculation behaviour on this grid. also much finer grid was tested. this grid was only the 10° v-cutout of tested geometry for computational time reduction due to a greater number of cells closer to the wall. however, there was a too small angle of the tip in the axis (centre of the tube) and therefore a smooth auxiliary wall had to be applied with tip neglecting. as the result, flow cross-section has been little changed, but hexahedral cells could be used for the whole geometry. the simulation results for this grid were similar to case with 45° v-cutout geometry. the 45° v-cutout grid has been selected for the final evaluation of the results, because no geometry adjustments are used there. also, it is not appropriate to use fine meshes with respect to numerical approach for the used boiling computational model, like accretion of bubbles near the wall etc. this final grid contains 90 000 hexahedral cells with their higher concentration in the wall vicinity, shown in figure 1. cells quality was evaluated for equi-size skew, which does not exceed value of 0.5 and by the aspect-ratio with the maximum of 10.3. two cases with different boundary conditions based on the debora experiments are considered. the first one – case 1 – has a much larger subcooled inlet state then the case 2, but the saturation conditions and heat flux are almost the same for both of them. the main specified conditions of these experiments are given by table 1 and setting of the boundary conditions in the simulation model corresponds with them. usage of these two experimental cases brings the opportunity to try if the simulation model can solve various ratio of boiling states; from subcooled up to almost boiling crisis – departure from nucleate boiling (dnb). the physical values setting for the freon r12 liquid and vapour is based on these conditions. therefore linear functions depending on the temperature were used for density, specific heat capacity, viscosity and thermal conductivity. these quantities were obtained from nist database [13]. 74 vol. 4/2016 cfd simulation of upward subcooled boiling flow of freon r12 setting of the model boundary conditions: • inlet: velocity inlet – the same speed for both phases turbulence intensity – estimated at 3 % • outlet: pressure outlet – 0 backflow volume fraction • heated wall: heat flux the measuring line was created on one symmetry wall directly at the interface between the heated and outlet non-heated section. this line contains 11 radially distributed evaluation points which were created to monitor calculated data. they are situated in the centre of each appropriate cell and their distribution is shown in the figure 2. radial distance in [m] . ........ . . figure 2. distribution of the evaluation points. 3. solver setting the solver setting is mainly supported by the documentation [12, 14–17]. time dependent solution has to be considered with time step set to 0.01 s for each calculated case. the realizable k−ε turbulence model with the non-equilibrium wall function has been chosen. this turbulence model is solved for each phase, but the wall function is calculated only for singlephase. the multiphase coupled model was used for the sequential solution of velocity and pressure fields and the least square cell-based method was set for gradients. all discretization schemes were set to 1st order, otherwise the simulation is very susceptible to the formation and disintegration of bubbles along the simulation domain. calculations were carried out on the sixteen intel xeon e5-2660 processors. the complete final calculation took about 8 hours of computer time for each case. setting of boiling in ansys fluent 16: multiphase eulerian – rpi boiling model this boiling model is appropriate to use for subcooled flow, where condition tsat − tbulk > 3 k is fulfilled. model does not calculate the vapour temperature, but it is fixed at the saturation temperature instead. as an alternative, non-equilibrium boiling model can be used with criterion tsat − tbulk ≤ 3 k, where the vapour temperature is included in the solution process. this condition could occur at the higher model levels, where the liquid can be already sufficiently heated close to saturation conditions. however, the rpi boiling model is considered for the next steps of this work. figure 3 shows scheme of the selected submodels for rpi boiling model. figure 3. scheme of the selected sub-models for rpi boiling model. the wall lubrication model (effect of virtual mass) was neglected, because of big impact to the solution grid fineness, which is not very desirable for the model. 4. calculation results the results of performed calculations are compared with data calculated by nri rez (ansys fluent r13) and measured values from the debora experiment given by [12]. the radial profiles of important monitored parameters for both calculated cases are described below. these profiles are obtained from measuring line, for contained evaluation points respectively. dispositional (orthographic) views are also given for contours of selected parameters in the computational area, see figure 12. if they had not been applied, the side view would have showed just the long and thin tube with nothing obvious to see in the 75 tomas romsy, pavel zacha acta polytechnica ctu proceedings computational domain. given the similarity of the boiling behaviour of both cases, only results for case 1 are listed. 4.1. radial temperature profiles unfortunately due to unavailable experimental data for radial temperature profiles, only the comparison with the results provided by nri rez is mentioned. for both calculated cases the evaluated radial profile has a similar course, as shown in figure 4 for case 1 and in figure 5 for case 2. . . . . . . . . . . . . . . . . . . . . . . figure 4. radial temperature profile, case 1. . . . . . . . . . . . . . . figure 5. radial temperature profile, case 2. only slight differences between the temperature values obtained by nri rez and calculation up to 0.78 ◦c in the fluid midstream and up to 1.36 ◦c near the wall for case 1 are seen. as regards the case 2, here is obvious difference between temperature profiles in the midstream. the nri rez results for case 2 are very close to saturation temperature almost across the whole radial profile, except the wall. the character of ctu calculation temperature profile has a decreasing character up to the axis – tube centre (little bit subcooled state). but near the axis the temperature difference does not exceed 0.58 ◦c. 4.2. radial void fraction profile radial profiles for void fraction with measured data and also nri rez results have been compared already. this is shown in figure 6 (case 1 ) and figure 7 (case 2 ). in case 1 is seen that all the data are in good agreement near the wall, but the curves have slightly different courses closer to axis – tube centre. in case of this work, such disagreement is probably caused by collecting of non-condensed secondary phase in the midstream and efforts of liquid go closer toward the wall. this can be seen from void fraction rendered along the calculated area in section 4.5, where the results evaluation along the model is described. figure 6. radial void fraction profile, case 1. figure 7. radial void fraction profile, case 2. 76 vol. 4/2016 cfd simulation of upward subcooled boiling flow of freon r12 the greater disagreement with other data is obvious for case 2. the ctu simulation model estimates almost abrupt increase of void fraction near the heated wall pretty well. but in the midstream the void fraction profile is lower than the others. this probably correlates with the results for temperature quite well. for the treatment of such disagreements it would be possible to try a different approach in model setting, or using of non-equilibrium boiling model for a future simulations respectively. 4.3. radial vapour velocity profile vapour velocity radial profile comparison for case 1 is illustrated in figure 8. it can be seen that the ctu and nri rez results have similar character, but compared with the measurement they have a higher drop of vapour velocity in the fluid midstream. this is probably caused by using of non-ideal single phase wall function. this will be the subject of further investigation in a future simulations. figure 8. radial vapour velocity profile, case 1. in case 2, see figure 9, it is well visible that curve of ctu calculation follows the measured data near the wall pretty well. on the other hand, the same problem as in the previous case occurs in the fluid midstream. figure 9. radial vapour velocity profile, case 2. 4.4. radial bubble diameter profile very important parameter in terms of subcooled boiling behaviour is diameter of vapour bubbles. for experiment measured data; the sauter mean diameter is defined as the diameter of a sphere, which has the same ratio of volume and surface area as the base particle (bubble) and the average diameter dg is the diameter of the bubble, which would have the equivalent two-phase flow with the same density of bubbles amount, the same density of interfacial area and the same diameter for all bubbles, see [12]. for case 1, the results are depicted in figure 10. here is evident that the bubble diameters along the radius have similar size and character as the measured data. it means that the bubbles are breaking up near the wall and their diameter is increasing in the centre of the stream. slighter difference in the fluid midstream probably corresponds with the above listed diagram for void fraction. also bubble diameter along the tube for this case is shown in figure 12. figure 10. radial bubble diameter profile, case 1. figure 11. radial bubble diameter profile, case 2. the results for case 2, shown in figure 11, are in very good agreement with experiment – dg and nri rez near the wall. whereas the bubble diameters in the midstream have little bit lower values compared with other data, they are still in very good agreement 77 tomas romsy, pavel zacha acta polytechnica ctu proceedings figure 12. axial liquid temperature, void fraction and bubble diameter, case 1. with experiment – dg, because it is seen, that the character of radial profile have nearly the same course. smaller values for bubbles are probably caused by lower temperature of fluid in the midstream, shown in figure 5. therefore, the question of using the non-equilibrium boiling model arises here. 4.5. axial results profiles figure 12 (case 1 ) due to similarity of the both cases of boiling behaviour mentioned above is presented for a better understanding, how the fluid behaves along the tube and where boiling occurs. there are orthographic views for the fluid temperature, void fraction and bubble diameters. there is visible, that all of these quantities correlate with each other. for example; the fluid temperature approximately in the middle of the model reaches the saturation condition near the wall and the void fraction begins to occur there. the higher level the fluid stream in tube reaches, the higher temperature in the midstream occurs. it also means, that the higher concentration of void fraction is there. on the profile of bubble diameters, it is shown how to bubbles reach the greater diameters in the higher levels of the tube and how they are trying to get into the centre of fluid stream. considering this view, it should be observed, that for setting of bubbles modelled by yao-morel model, the smallest possible value of the bubbles have to be set. for both cases this value was set to −105 m. 5. conclusion the main goal of this work was to try the possibility to utilize the newly implemented boiling models in cfd solver ansys fluent r16. the debora experiments were chosen for comparison with data received from these simulations. some of the earlier works, such as nri rez (fluent r13), were also tried to simulate boiling experiments by using older version of fluent cfd solver. however, suitable boiling models had to be supplied by user defined functions (udf) in this earlier version. for comparison, how the solver with newly implemented boiling models (fluent r16) stand up against the older version with udf’s, the evaluation of these data compared with ctu is described. two cases (case 1 and case 2 ) of debora experiments were chosen. only inlet conditions for fluid temperature are different in these two cases. it means that the first one is much more subcooled case then the second one. the comparison between measured data, calculations from nri rez and results of this work are in the high level of agreement. only few small differences occur. the vapour velocity disagreement of both calculations can be primarily caused 78 vol. 4/2016 cfd simulation of upward subcooled boiling flow of freon r12 by numerical approach, or by using of non-ideal single phase wall function respectively. also, for radial void fraction closer to axis the curve have slightly different course compared to experiment and nri rez. this disagreement is probably caused by collecting of non-condensed secondary phase in the midstream and effort of liquid go closer toward the wall. and also in case 2 the void fraction has the lower values in the fluid midstream. therefore the question of using the non-equilibrium boiling model arises here. it will be subject of further research, because it can treat the fact, that for case 2 the midstream fluid temperature could reache the values close to saturation conditions in the measured section. after evaluation of all the results we can say, that the cfd solver with newly implemented boiling models (fluent r16) can be successfully used for problems where the subcooled boiling occurs. but some minor deficiencies are still present. unfortunately due to very complicated and sensitive numerical model used for description of this boiling phenomenon, there are many degrees of freedom at the model settings. also variables that are tracked interact with each other and individual numerical approaches have limited validity. this leads to time-consuming study to solve these minor deficiencies and verifying all of the used hypotheses (and their interaction) by experiments. all the knowledge and experiences described above are currently being used for simulations of more complicated situation of subcooled boiling of water, which occurs for example in pb-li cold trap cooling system, and they are continuously being improved. list of symbols p pressure [mpa] v inlet liquid velocity [m/s] qw heat flux [w/m2] t temperature [°c] tin inlet liquid temperature [°c] tbulk bulk liquid temperature [°c] tsat saturation temperature [°c] % liquid density [kg/m3] cp specific heat capacity [kj/kg k] ν dynamic viscosity [pa s] λ thermal conductivity [w/m k] references [1] s. g. kandlikar. high flux heat removal with microchannels – a roadmap of challenges and opportunities. heat transfer engineering 26(8):5–14, 2005. doi:10.1080/01457630591003655. [2] n. khan, k. c. toh, d. pinjala. boiling heat transfer enhancement using micro-machined porous channels for electronics cooling. heat transfer engineering 29(4):366–374, 2008. doi:10.1080/01457630701825481. [3] a. bar-cohen, p. wang, e. rahim. thermal management of high heat flux nanoelectronic chips. microgravity science and technology 19(3):48–52, 2007. doi:10.1007/bf02915748. [4] f. ramstorfer, h. steiner, g. brenn, et al. subcooled boiling flow heat transfer from plain and enhanced surfaces in automotive applications. asme j heat transfer 130(1), 2008. doi:10.1115/1.2780178. [5] i. mudawar. assessment of high-heat-flux thermal management schemes. ieee transactions on components and packaging technologies 24(2):122–141, 2001. doi:10.1109/6144.926375. [6] j. l. parker, m. s. el-genk. enhanced saturation and subcooled boiling of fc-72 dielectric liquid. international journal of heat and mass transfer 48(18):3736–3752, 2005. doi:10.1016/j.ijheatmasstransfer.2005.03.011. [7] y. madhour, j. olivier, e. costa-patry, et al. flow boiling of r134a in a multi-microchannel heat sink with hotspot heaters for energy-efficient microelectronic cpu cooling applications. ieee transactions on components, packaging and manufacturing technology 1(6):873–883, 2011. doi:10.1109/tcpmt.2011.2123895. [8] k. suzuki, h. kawamura. microgravity experiments on boiling and applications: research activity of advanced high heat flux cooling technology for electronic devices in japan. annals of the new york academy of sciences 1027(1):182–195, 2004. doi:10.1196/annals.1324.017. [9] e. krepper, r. rzehak. cfd for subcooled flow boiling: simulation of debora experiments. nuclear engineering and design 241(9):3851–3866, 2011. doi:10.1016/j.nucengdes.2011.07.003. [10] o. frybort. background report – pb-li17 cold trap. nuclear research centre rez, 2014. [11] j. garnier, e. manon, g. cubizolles. local measurements on flow boiling of refrigerant 12 in a vertical tube. multiphase science and technology 13(1&2), 2001. doi:10.1615/multscientechn.v13.i1-2.10. [12] l. vyskočil. simulation of the subcooled boiling in fluent 13. nuclear research institute rez, 2011. [13] e.w. lemmon, m.o. mclinden, d.g. friend. thermophysical properties of fluid systems. in nist chemistry webbook, nist standard reference database number 69, p. linstrom, w. mallard (eds.). national institute of standards and technology. http://webbook.nist.gov. [14] t. romsy, p. zacha. cfd simulation of subcooled boiling, 2016. erin, the 8th international conference for young researchers and phd students, april 23–25, 2014, blansko, czech republic. [15] ansys, inc. fluent user’s guide, 2013. release 15.0. [16] ansys, inc. fluent theory guide, 2013. release 15.0. [17] ansys, inc. fluent multiphase 15.0 optional lecture 03 – wall boiling models: training materials, 2014. 79 http://dx.doi.org/10.1080/01457630591003655 http://dx.doi.org/10.1080/01457630701825481 http://dx.doi.org/10.1007/bf02915748 http://dx.doi.org/10.1115/1.2780178 http://dx.doi.org/10.1109/6144.926375 http://dx.doi.org/10.1016/j.ijheatmasstransfer.2005.03.011 http://dx.doi.org/10.1109/tcpmt.2011.2123895 http://dx.doi.org/10.1196/annals.1324.017 http://dx.doi.org/10.1016/j.nucengdes.2011.07.003 http://dx.doi.org/10.1615/multscientechn.v13.i1-2.10 http://webbook.nist.gov acta polytechnica ctu proceedings 4:73–79, 2016 1 introduction 2 model description 3 solver setting 4 calculation results 4.1 radial temperature profiles 4.2 radial void fraction profile 4.3 radial vapour velocity profile 4.4 radial bubble diameter profile 4.5 axial results profiles 5 conclusion list of symbols references acta polytechnica ctu proceedings doi:10.14311/app.2017.7.0053 acta polytechnica ctu proceedings 7:53–57, 2017 © czech technical university in prague, 2017 available online at http://ojs.cvut.cz/ojs/index.php/app righting of chinese mitten crabs (eriocheir sinensis) and their models tobias riphausa,∗, florian hoffmannb, susanna labischb a international degree course in biomimetics, city university of applied science bremen, germany b biomimetics-innovation-centre (b-i-c), city university of applied science bremen, germany ∗ corresponding author: t. riphaus via slabisch@bionik.hs-bremen.de abstract. the usage of unmanned underwater vehicles for marine tasks is continuously growing and bioinspired stabilizing systems shall help them to gain and keep a stable position during work. therefore the righting maneuver of e. sinensis has been studied. these crabs are able to perform a 180◦-rotation with an angular velocity of 4.30 s−1 when falling underwater from a supine starting position. high-speed particle image velocimetry has shown, that propulsive forces with a peak of 0.021 ± 0.001 n were produced by the hind legs to initiate and stop the rotation. in a numerical multibody simulation a constant force of 0.009 n acting for 0.2 s leads to the same rotation. in order to prove this mechanism, it was implemented into a robotic system. its mean density of 1.15 g/cm3 deviates not more than 4% from the biological and numerical models. it can complete a 180◦-turn within 1.03 ± 0.12 s with a rotational velocity of up to 4.25 s−1. keywords: biomimetics, righting maneuver, robot stabilization. 1. introduction since the first success of unmanned underwater vehicles (uuvs) their field of operation is constantly spreading. in spite of all the technology, the optimization of their stabilizing behavior is still a crucial topic in contemporary research. the reason why even modern uuvs are facing problems in holding stable positions, to carry out their tasks or turn around quickly once they have lost their preferred position, is the kind of drive that is mainly used. propeller and pump-jets are advantageous for high-speed locomotion, but too expensive for the use in a seldom occurring event. to gain new ideas for solving such problems more and more research groups are investigating biological models. first successful terrestrial applications are the amphibious robot robocrab by krummel et al., which adapts the skills of horseshoe crabs within the surf zone, or the four-wheeled autonomous robot by changsiu et al., which is capable of mid-air reorienting during free fall modeled on geckoes (hemidactylus platyurus) [1][2]. but the usage of biological models for improving aquatic robots is poorly documented. thus, the research group dfu of the department of biomimetics in bremen started investigations on different benthic marine animals. the aim was to search for a strategy that allows a quick turning or efficient reorientation manoeuvre which uses neither a propeller nor is driven by a permanent rotation. especially promising seemed the crustacean order decapoda. therefore the species eriocheir sinensis has been studied in terms of their mid-water righting behavior [3][4]. besides the morphological biomechanical analysis, also the necessary figure 1. individual of e. sinensis (a, [3] modified), its numerical model (b) [4] and the adapted robot o-typus (c) with an insight into its electronic devices (d) [5]. hydrodynamic forces have to be determined to show, that such a strategy is transferable to a technical application. 2. material and methods 2.1. eriocheir sinensis biological model in total 52 rotations of 10 individuals of e. sinensis were analysed. the rotations are differentiated in the kinematic approach with 39 analysed rotations (28/11, initialising/stopping rotation) and for the piv (particle image velocimetry) analysis with 13 analysed rotations (9/4, initialising/stopping rotation). 53 http://dx.doi.org/10.14311/app.2017.7.0053 http://ojs.cvut.cz/ojs/index.php/app t. riphaus, f. hoffmann, s. labisch acta polytechnica ctu proceedings 2.1.1. kinematic analysis to observe the natural righting behavior of e. sinensis (figure 1a), the crabs were held in a supine starting position at the height of 0.60 m within a freshwater tank with a ground area of 0.6 × 0.6 m and a high of 0.8 m and released by starting device. the body movement was high speed recorded (250 fps) to be able to manually track two points on the body and the tip of the 5th leg. furthermore falling speed, body angle and leg trajectories were plotted and the speed of the leg tip calculated. the leg induced force (fd) during the strike was calculated by fd = 1 2 ρ v2 c cd a (1) where ρ = water density, vc = leg centroid velocity, a = area of 5th legs and cd = drag coefficient from static force balance measurements. centroid velocity was calculated by: vc = vt lbc lbt (2) where vc = velocity of centroid, vt = velocity of leg tip, lbc = length leg base to leg centroid and lbt = length leg base to leg tip. lbt and lbc were experimental determined on 15 legs of 10 dead e. sinensis. centroid x/y coordinates were calculated with an integrated function of imagej from the leg area. the resulting factor for lbc / lbt was calculated as 0.44 ± 0.016. 2.1.2. piv measurements and force calculation the tank of the kinematic study was also used for the piv measurements. crabs were released by hand or the automated release fixture. the laser sheet was placed vertically in the middle of the tank illuminating a surface of approx. 0.3 x 0.3 m. flow was visualized by polyamid seeding particles (ø 57 µm, density: 1.03 g/cm3). images were taken by a high speed camera (photron apx-rs) at an interframing time of 3.75 ms (266 fps). crabs were released at different distances to the lasersheet depending on the leg length. we used falling sequences where the last leg was cutting the lasersheet at positions from 50 to 80% of the leg. by measuring the bound circulation forces around the leg the generated propulsion force was calculated by using the kutta-joukowski theorem. circulation γ was measured from the beginning to the end of the leg stroke in order to get a temporal resolution of the circulation. in pivlab [6] under matlab (the mathworks inc.) the piv images were analysed and the spanwise circulation (circulation around the spanwise axis) was derived by a loop integral of the tangent velocity: γ = ∮ svort vt dsvort = ∫ avort ωz davort (3) where svort = circular path around the vortex core, vt = tangential velocity, avort = area of the vortex core and ωz = spanwise vorticity. the vortex ring momentum calculation model was used to estimate an average propulsive force [7][8]. the average propulsive force of the leg stroke was herewith calculated by f = 2 γ ρ ν b (4) where γ = circulation, ρ = fluid density, ν = fluid velocity and b = leg length. the force results were multiplied by two, to get the force of both legs. 2.2. simulated model for the numerical simulation of the physical behavior the software adams (rel. 2013.1) was used. it allows to describe the limb movement as a motion of mechanically joint rigid bodies, builds the relevant differential equations of motion and solves them numerically. basis for the multi-body-analysis is experimental gained data of an individual crab showing the dimensions, the mass, the density and the center of mass (com) of each limb. all body-parts are connected with rigid joints. in order to simulate the correct falling speed in water the gravity was reduced to 1.09 m/s2. during movement the crabs hold their front legs close to the carapax and stretching out only the hind legs. therefore the main body can easily be simplified to a cylinder. with a diameter of 6.00 cm and a height of 2.75 cm for carapax and front legs such a cylinder will have an average density of 1.12 g/cm3 which leads to a mass of 87 g. this data fits the measured crab very well. the hind leg pair is modelled with the merus/ischium, the propodus/carpus and the dactylus in their measured length and density. the parts of the leg are conical so that the com of these parts matches also (figure 1b). the acting forces are applied on the joint between dactylus and propodus with a constant value for a given duration (thick arrows in fig 1b). 2.3. o-typus technical model 2.3.1. setup to confirm whether the observed mechanism is correctly understood, the basic model is implemented in the purpose-built robot o-typus (figure 1c+d) [5]. its chassis is congruent with the carapace of an individual of e. sinensis and scaled 2:1, to house drive units and the sensor system. the actuating elements consist of two waterproof hk15328d servo motors (max torque: 12,8 kg.cm) with a 6v external power supply (2250 ma). their control is based on the pwm-signals from an arduino uno. the position tracking is enabled by the mpu6050 accelerometer. the real time operation sends the current yaw-/ pitch-/ roll-angles (ypr) to the arduino uno to log the data (~27 hz). 54 vol. 7/2017 righting of chinese mitten crabs (eriocheir sinensis) and their models figure 2. a. stereotypical turning maneuvers observed for e. sinensis after a virtually moment-free start ([9], modified). ∆t = 0.2 s. b. positions of body axis, right 5th leg and com for one individual (e. s.). origin of ordinates on caudal end of carapace at starting point. ∆t = 0.02 s (0.1 s for body axis). 2.3.2. experiments the performed tests are adapted by the setup of the experiments carried out in section 2.1.1. therefore, the robot starts in a supine position at the top of the tank. to initialize the turning, the two legs will strike simultaneously. the appropriate kinematic parameters were determined by an iteratively change of the striking angle θ at the maximum angular striking velocity ω of 4.76 s−1. the acting forces were kinematically analyzed by equation 1. 3. results falling e. sinensis show a body rotation along the transversal axis during free fall (see figure 2a). the rotation is initiated directly after release by one stroke cycle of the 5th leg pair followed by a passive phase and stopped again with the 5th leg pair by a counterstrike. the turn is completed in 0.76 ± 0.15 s which leads to a mean angular velocity of ω = 4.3 s−1. in figure 2b the trace of one individual turning maneuver is given in sagittal view. besides the reorientation of the body axis the trajectory of the 5th leg tip is given to show the efficient strike and counterstrike. the moving location of the com testifies the complex interaction of the acting forces. 3.1. force calculation of 5th leg pair by kinematic approach and piv forces which are induced by moving the hind leg pair in phase 2a (see figure 3b) have a maximum at 33% of the stroke with a mean force of 0.018 ± 0.004 n and a maximum peak of 0.023 ± 0.002 n. the total momentum during phase 2a is 0.0036 ns. for phase 3a the maximum is at 35% of leg stroke with a mean force 0.015 ± 0.003 n and a maximum peak of 0.018 ± 0.002 n. figure 3. results from experiment and multibodysimulation for a: body angle and b: induced forces. experimental forces are mean values of the force calculations. body angle of experimental data (blue rhomb), n=8 and multibody-simulation (red line) are showing the same trend in time and angle. the calculated forces are 1.5 to 2 times higher (piv: 2a n=9, 3a n=3, kinematic: 2a n=28, 3a n=11) than the forces used in the simulation. the piv data shows that propulsive forces were produced by the hind legs to initiate and stop the rotation. peak forces to initiate the rotation (45% of stroke cycle) of 0.021 n (± 0.001 n) (n=9) lead to a total momentum of 0.0021 ns. 3.2. simulation the simulation shows that a constant force of 0.0045 n acting on each hind leg respectively for 0.2 s leads to a successful rotation within time. this means that the total momentum of 0.0018 ns causes a constant angular velocity of 3.99 s−1 in the middle part of the turning maneuver, part 2b in figure 3. 3.3. technical model the experiments confirmed the assumption, that a stroke with two legs is sufficient for a 180◦-turn (see figure 4b). the correspondent kinematic parameters are θ = 2.44 and ω = 4.76 s−1. the complete turn was herewith possible within 1.00 ± 0.12 s (n = 6). an exemplary run is shown in figure 4b. the turn is divisible into the same five phases like e. sinensis. during the initializing (1) the chassis is descending in a supine position( till i/ 2). the striking phase (2a) includes the active leg movement and thereby the acceleration (i-ii/ 2-3). the following gliding phase (2b) describes the passive turning based on inertia (ii-iii/ 55 t. riphaus, f. hoffmann, s. labisch acta polytechnica ctu proceedings figure 4. turning movement of o-typus. b. observed turning behavior with five characteristic phases (initializing pi=̂1, striking ps=̂2a, gliding pg=̂2b, counterstriking pc=̂3a, descending pd=̂3b); ø∆t = 0,36 s. a. course of the body axis and trajectory of the left leg’s tip. leg positions at the end of pi (i), end of ps (ii) and the target position (iii) in grey. iv shows the maximal overwinding; ∆t = 0,2 s (0,1 s during passive turning (ii-iii)). c. angle of attack (green) and angular velocity (red) of three runs. 3-5). when the horizontal target position is reached, the counterstriking phase (3a) starts with a counter movement of the legs, to stop the rotation (iii-iv/ 5-6). thereafter the chassis descends (3b) in a stable up-right position. additionally figure 4a shows the trajectory of one striking leg. point i poses the initiation of the stroke, point ii the end of the stroke, point iii the end of the 180◦-turn and thereby the initiation of the counterstrike and point iv the eventual slight overwinding. the graphs of figure 4c are describing the shifting of the angle of attack and its angular velocity. the same phases can be located here. the stroke starts at 2000 ms and leads over from phase 1 to 2a. the maximal angular velocity of 4.06 s−1 is reached at the end of 2a. subsequently ω starts decreasing within phase 2b. after approximately 1150 ms the turn is completed and results in phase 3a, which prevents overwinding. afterwards the body descends stable with minor wobbling. the force which is induced by the moving leg pair in phase 2a has a mean value of 0.0175 n. this propulsive force is acting over 0.51 s, which results in a total generated momentum of 0.00898 ns. figure 5. comparison of characteristic turning phases of (a) e. sinensis, (b) o-typus and (c) the simulated model. phases: 1 (yellow), 2a (blue), 2b (green), 3a (orange) and 3b (grey). 4. conclusions 4.1. comparison of e. sinensis with its models a qualitative comparison of the righting behavior of e. sinensis with the turning characteristics of the adapted robot and the multibody simulation is shown in figure 5. o-typus shows like the biological model a complete rotation after a single stroke. its rotation could be divided into the exact same phases like e. sinensis. the correspondence can be seen by comparing the phases. the duration of some phases are partially varying. a considerable difference between a and b can be seen within phase 2b. according to this, the larger o-typus needs significantly more time to complete the passive turn. a full comparison of the designed and measured parameters is shown in table 1. while the numerical model could be simulated in the original scale, the robot had to be enlarged to fit all devices. however the mean density was adapted. the measured propulsive forces of e. sinensis deviate among the two methods. while the piv data corresponds closely to the values gained by the simulation, the kinematic approach matches nicely with the acting forces on o-typus. due to the considerable bigger dimensions of o-typus, the total momentum to generate a similar turn is about 3 times of e. sinensis value. herewith o-typus reaches a mean angular velocity of 3.93 ± 0.30 s−1, which deviates no more than 8.6% of its model. 4.2. transferability to aquatic robots as the motivation for this study showed, there is a desperate need in new stabilizing systems for technical aquatic applications. the observed and tested righting behavior of e. sinensis revealed a high efficiency and therefor a potential approach to solve the problem. in order to verify, whether this new stabilizing system is an optimization compared to current methods, further 56 vol. 7/2017 righting of chinese mitten crabs (eriocheir sinensis) and their models parameter e. sinensis simulation robot kinematic piv scaling 100% 100% 200% mass [g] 88.65 87 220 mean density [g/cm3] 1.12 1.12 1.15 mean force [n] 0.018 0.011 0.009 0.0175 ± 0.004 ± 0.003 momentum [ns] 0.0036 0.0021 0.0018 0.00898 mean angular vel. [s−1] 4.30 3.99 3.93 ± 0.30 table 1. summary of relevant parameters regarding the turning of e. sinensis, its numerical model and its technical application. values of the measured force are mean values during the striking phase 2a. the angular velocity is the averaged value of the whole turn. figure 6. smart-e. modern thruster-driven auv, which is declared to be highly efficient [10]. studies have to be carried out. thus, two different efficiencies have to be compared. on one hand, the power consumption has to be evaluated. at the current state of research no statement is possible, as there is no data about robotic devices, driven by thruster, available to compare it with. it would be worth to use an existing modern robot like the auv smart-e [10], to compare the two mechanisms at the same chassis. on the other hand, time efficiency has to be considered. most remotely operated vehicles are featuring slow turning capabilities. unfortunately non published the specific turning kinematics. however, the maximum angular velocity of 4.25 s−1 reached by otypus is considerably (visually) higher than of most found thruster-driven devices. acknowledgements special thanks are referring to the department of biomimetics in bremen, which made these studies possible. references [1] g. krummel, k. n. kaipa, s. k. gupta. a horseshoe crab inspired surf zone robot with righting capabilities. in proc. of the asme 2014 int. design eng. tech. conf. & comput. and inform. in eng. conf. 2014. doi:10.1115/detc2014-34679. [2] e. chang-siu, t. libby, m. tomizuka, r. j. full. a lizard-inspired active tail enables rapid maneuvers and dynamic stabilization in a terrestrial robot. ieee/rsj int conf of intelligent robots and syst 2011. doi:10.1109/iros.2011.6094658. [3] p. bausch, f. hoffmann, a. b. kesel. im freien fall unter wasser lagestabilisierung bei wollhandkrabben. eine experimentelle analyse. in bionik: patente aus der natur (ed. a.b. kesel und d. zehren). 155-161, bionik-innovations-centrum, bremen, 2015. [4] f. hoffmann, s. labisch, r. sonntag, et al. analysis of underwater righting reflex of the chinese mitten crab (eriocheir sinensis) using piv and numerical multibody simulation. in proceedings of the annual main meeting of the society for experimental biology, prag, [a 11.68]. 2015. [5] t. riphaus, s. labisch. konstruktion des biologisch inspirierten roboters o-typus zur implementierung und untersuchung des stabilisationsverhaltens von decapoden. in proceedings of the 7. ieee germany student conference, magdeburg. 2016. [6] w. thielicke, e. j. stamhuis. pivlab towards userfriendly, affordable and accurate digital particle image velocimetry in matlab., 2014. j.openres.softw.,2,. doi:http://doi.org/10.5334/jors.bl. [7] j. m. v. rayner. a new approach to animal flight mechanics. j exp biol 80:17–54, 1979. [8] g. r. spedding, j. m. v. rayner, c. j. pennycuick. momentum and energy in the wake of a pigeon (columba livia) in slow flight. j exp biol 111:81–102, 1984. [9] p. bausch, p. berger, f. hoffmann, a. b. kesel. how to land on your legs underwater righting reflexes in crabs and crayfish. in proceedings of the annual main meeting of the society for experimental biology, prag, [a 11.67]. 2015. [10] b. meyer, k. ehler, c. osterloh, e. maehle. an autonomous omnidirectional underwater robot. journal of behavioral robotics pp. 204–210, 2013. 57 http://dx.doi.org/10.1115/detc2014-34679 http://dx.doi.org/10.1109/iros.2011.6094658 j. open res. softw., 2,. http://dx.doi.org/http://doi.org/10.5334/jors.bl acta polytechnica ctu proceedings 7:53–57, 2017 1 introduction 2 material and methods 2.1 eriocheir sinensis biological model 2.1.1 kinematic analysis 2.1.2 piv measurements and force calculation 2.2 simulated model 2.3 o-typus technical model 2.3.1 setup 2.3.2 experiments 3 results 3.1 force calculation of 5th leg pair by kinematic approach and piv 3.2 simulation 3.3 technical model 4 conclusions 4.1 comparison of e. sinensis with its models 4.2 transferability to aquatic robots acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2016.3.0047 acta polytechnica ctu proceedings 3:47–50, 2016 © czech technical university in prague, 2016 available online at http://ojs.cvut.cz/ojs/index.php/app structural and mechanical characterization of deformed polymer using confocal raman microscopy and dsc birgit neitzel∗, florian aschermayer, milan kracalik, sabine hild johannes kepler university linz, institute of polymer science, altenberger straße 69 4040 linz, austria ∗ corresponding author: birgit.neitzel@jku.at abstract. polymers have various interesting properties, which depend largely on their inner structure. one way to influence the macroscopic behaviour is the deformation of the polymer chains, which effects the change in microstructure. for analyzing the microstructure of non-deformed and deformed polymer materials, raman spectroscopy as well as differential scanning calorimetry (dsc) were used. in the present study we compare the results for crystallinity measurements of deformed polymers using both methods in order to characterize the differences in micro-structure due to deformation. the study is ongoing, and we present the results of the first tests. keywords: raman spectroscopy, dsc, deformed polymers, crystallinity. 1. introduction deformed polymers are used in a wide range of applications because of their advanced mechanical properties. especially, materials with high strength gained increasing importance. in order to obtain high strength materials with tailored properties, the morphology can be controlled by varying process conditions. nevertheless, there is still a lack of knowledge in the correlation between mechanical properties, processing conditions and polymer morphology. the microscopic structure is claimed as a crucial factor, which determines the mechanical properties. furthermore, mechanical performance is influenced by the crystallinity and chain orientation of a polymer. therefore, characterization of these parameters is important for developing new tailored polymeric materials. wide angle x-ray diffractometry or differential scanning calorimetry (dsc) are common methods to determine the overall crystallinity of polymers. besides, also spectroscopic techniques such as raman spectroscopy [1–4] have shown to be suitable techniques to observe process-induced changes in crystallinity [5] and chain orientation [3]. raman spectroscopy uses polarized laser light to scan the surface of a sample. when changing the polarization angle of the filter, the intensities of certain structure-related peaks in the raman spectrum change as well, which allows conclusions on the polymer morphology. the degree of orientation can be determined from these peaks via a material specific orientation function fcz, such as equation (1) for pp [3]: fcz = 0.134 · ∑ i973∑ i998 − 0.182 (1) see [3] it reveals if the polymer chains are randomly distributed (fcz = 0), in deformation direction (fcz > 0) or perpendicular to it (fcz < 0). as fcz depends on the polarization filter’s orientation, the differences in multiple spectra must be considered when determining its value. crystallinity α is supposed to be independent of the filter’s orientation. its equation sums up peaks of multiple spectra to determine its value. αraman = ∑ i809∑ i809 + ∑ i841 (2) see [5] directed force is applied to the samples, this direction is designated mod (machine orientation direction). for each sample two raman spectra were recorded. the first with polarisation set to 0 degrees, emitting laser light perpendicular to the mod. the second spectrum is taken with the laser aligned parallel (i.e. 90 degrees) to the direction of the applied force. we took 40000 measurements of one sample per mod to see if mod-dependent peaks in the spectra can be identified. the relative intensities were determined by integration of raman bands. the crystallinity was then calculated via equation (2) from the sum-spectra of both measurement series, parallel and perpendicular to the deformation direction. the second method we used, was dsc. from dsc curves, first the enthalpy of the sample can be calculated and from these results the crystallinity as well. one goal of this study is to compare the crystallinity values, obtained from both methods, because each technique is best suited for different surface textures. if a sample has a very smooth surface, then confocal raman microscopy can be used; for more irregular surfaces, measuring with dsc is more suitable. to characterize the polymer sample also on a macro-structure level, mechanical tests are ongoing. 47 http://dx.doi.org/10.14311/app.2016.3.0047 http://ojs.cvut.cz/ojs/index.php/app b. neitzel, f. aschermayer, m. kracalik, s. hild acta polytechnica ctu proceedings the mechanical investigation focuses on tensile testing. with this method, tensile strength and fracture behaviour can be analysed. furthermore, hardness tests could give us important data to build up the comprehensive material overview of the polymer samples in microand macro structure scales. 2. experimental in this current study, raman measurements were done with a confocal raman microscope (alpha300r, witec gmbh, ulm, germany). for the measurements, polarized laser light of 532 nm and 16 mw power was used. a polarization direction of 0° or 90° was applied. the graphical analysis has been performed using an objective with 20x magnification. with this equipment and these settings, polypropylene (pp) samples with different degrees of deformation in one direction were analyzed. furthermore, polyethylene terephthalate (pet) samples were investigated. the second method in this study was dsc using the instrument perkin elmer, dsc 8000, usa . both, heating rate and cooling rate were set at 10 k/min. the pp samples were heated from 25°c to 250°c, and the pet samples from 25°c to 300°c. the crystallinity was calculated from the first heating curve, to detect the influence, caused by the deformation. 3. results and discussion 3.1. crystallinity determined by confocal raman microscopy in the case of pp, the peaks at 809 cm−1 and 841 cm−1 provide information about the crystallinity, whereas the peaks at 973 cm−1 and 998 cm−1 are important for determining the chain orientation. these peaks change their orientation depending on the polarization of the laser light. all results were calculated via integration of the peak areas (see figure 1). figure 1. material specific (pp) peaks for crystallinity and chain-orientation calculation. in figure 2, the crystallinity distribution of a nondeformed (figure 2a), deformed pp (figure 2b), and highly deformed pp (figure 2c) can be seen. the deformed sample has a higher degree of crystallinity compared to the non-deformed sample. thus, the random distribution of the polymer chains, in the nondeformed sample changed to a well-ordered structure in the highly deformed pp-sample. (a) . non-deformed pp (b) . deformed pp (c) . highly deformed pp figure 2. crystallinity α calculated from raman spectra. confocal raman microscopy can be a powerful tool for structural analysis of further polymeric materials, but in this case it is necessary to find material specific structure relevant peaks. two peaks could be interesting for the second investigated material, pet: 998 cm−1 and 1096 cm−1. [6] furthermore, material specific formulas for crystallinity and orientation are required. 3.2. crystallinity obtained by dsc a big advantage of differential dsc is that samples can be produced very easily and in the same way for every polymer. also the crystallinity can be calculated with the same equation. so far, pet has been analysed as a second polymeric material. in figure 3, the crystallinity, calculated from dsc measurements, can be seen. these first measurements show, that crystallinity increases with high deformation. in a next step, these results will be compared to the crystallinities, determined via confocal raman microscopy. macrostructure will be characterized via tensile test. the aim is to find a relationship between the microand 48 vol. 3/2016 structural and mechanical characterization of deformed polymer figure 3. crystallinity of pet samples, measured via dsc; (a) amorphous, (b) deformed and (c) highly deformed pet. macro-structure behaviour of different polymeric materials. 3.3. comparison of crystallinity calculated by confocal raman microscopy and dsc using both methods, dsc and raman spectroscopy, it was found that the crystallinity α (figure 4) increases with higher deformation, but they have different increase rates. figure 4. crystallinity α calculated from raman spectra and dsc measurements for a non-deformed, b deformed and c highly deformed polypropylene (pp). the highest crystallinity and best correlation between both measuring methods was observed in the most deformed polypropylene c. it was furthermore observed that deformation of pp causes changes in its microstructure. 3.4. orientation distribution obtained by confocal raman microscopy figure 5 shows that the orientation function for the polymer chains, determined via confocal raman microscopy, increases with higher deformation. the polymer chains realign under stress by positioning themselves parallel to each other, resulting in a restructured polymer. this effect was also observed for the crystallinity, which also increases. a denotes non-deformed pp without orientation, b deformed polymer with weak orientation and c highly deformed pp with a strong chain-orientation along the deformation direction, 0° perpendicular, 90° parallel to mod. figure 5. orientation function calculated from raman spectra and dsc measurements for a nondeformed, b deformed and c highly deformed polypropylene (pp). (a) . parallel to mod, nondeformed (b) . perpendicular to mod, nondeformed (c) . parallel to mod, deformed (d) . perpendicular to mod, deformed (e) . parallel to mod, highly deformed (f) . perpendicular to mod, highly deformed figure 6. orientation distribution of pp samples calculated from confocal raman spectra figure 6 shows the orientation distribution of the pp-samples at different mods and degrees of deformation. the scanned area was 200 µm× 200 µm. it can be seen that the deformation changes the chain orientation from a random distribution to a well-defined periodic arrangement for both laser polarizations. the random distribution of the chains in non-deformed pp changes to an uniform distribution in highly deformed pp. 49 b. neitzel, f. aschermayer, m. kracalik, s. hild acta polytechnica ctu proceedings 4. summary with the two selected methods, dsc and raman spectroscopy, crystallinities were calculated. it was shown that the crystallinity and orientation of the chains increase with the deformation of the polymer. the effect of increasing crystallinity caused by deformation was shown for both polymeric materials, pp and pet. 5. outlook in the current study, pp as one important polymeric material, was analyzed and the results show that confocal raman microscopy and differential scanning calorimetry are powerful tools for the characterization of the inner structure. because of the different chemical behavior of different polymers, it is necessary to find useful structure peaks and mathematical formulations for crystallinity and orientation. finding solutions for other polymer, like pet, will be a subject of future work. in addition, relationships between mechanical properties and the polymer morphology/structure on a microand macro-scale are currently under investigation. acknowledgements robert führicht for latex typesetting. references [1] j. martin, m. ponçot, p. bourson, et al. study of the crystalline phase orientation in uniaxially stretched polypropylene by raman spectroscopy: validation and use of a time-resolved measurement method. polymer engineering & science 51(8):1607–1616, 2011. doi:10.1002/pen.21944. [2] m. fernandez, j. merino, m. gobernado-mitre, j. pastor. molecular and lamellar orientation of α-and β-transcrystalline layers in polypropylene composites by polarized confocal micro-raman spectroscopy: raman imaging by static point illumination. applied spectroscopy 54(8):1105–1113, 2000. [3] j. rodríguez-cabello, j. merino, l. quintanilla, j. pastor. deformation-induced conformational changes in stretched samples of amorphous poly (ethylene terephthalate). journal of applied polymer science 62(11):1953–1964, 1996. [4] j. rodríguez-cabello, j. merino, t. jawhari, j. pastor. rheo-optical raman study of chain deformation in uniaxially stretched bulk isotactic polypropylene. journal of raman spectroscopy 27(6):463–467, 1996. [5] a. s. nielsen, d. batchelder, r. pyrz. estimation of crystallinity of isotactic polypropylene using raman spectroscopy. polymer 43(9):2671–2676, 2002. [6] s. yang, s. michielsen. orientation distribution functions obtained via polarized raman spectroscopy of poly (ethylene terephthalate) fibers. macromolecules 36(17):6484–6492, 2003. doi:10.1021/ma034486q. 50 http://dx.doi.org/10.1002/pen.21944 http://dx.doi.org/10.1021/ma034486q acta polytechnica ctu proceedings 3:47–50, 2016 1 introduction 2 experimental 3 results and discussion 3.1 crystallinity determined by confocal raman microscopy 3.2 crystallinity obtained by dsc 3.3 comparison of crystallinity calculated by confocal raman microscopy and dsc 3.4 orientation distribution obtained by confocal raman microscopy 4 summary 5 outlook acknowledgements references 181 acta polytechnica ctu proceedings 1(1): 181–188, 2014 181 doi: 10.14311/app.2014.01.0181 magnetorotational explosions of core-collapse supernovae gennady s. bisnovatyi-kogan1, sergey g. moiseenko2, nikolay v. ardeljan3 1space research institute, profsoyuznaya str. 84/32, moscow 117997, russia, and national research nuclear university ”mephi”, kashirskoye shosse, 31, moscow 115409, russia 2space research institute, profsoyuznaya str. 84/32, moscow 117997, russia 3department of computational mathematics and cybernetics, moscow state university, vorobjevy gory, moscow b-234, russia corresponding author: gkogan@iki.rssi.ru abstract core-collapse supernovae are accompanied by formation of neutron stars. the gravitation energy is transformed into the energy of the explosion, observed as sn ii, sn ib,c type supernovae. we present results of 2-d mhd simulations, where the source of energy is rotation, and magnetic field serves as a ”transition belt” for the transformation of the rotation energy into the energy of the explosion. the toroidal part of the magnetic energy initially grows linearly with time due to differential rotation. when the twisted toroidal component strongly exceeds the poloidal field, magneto-rotational instability develops, leading to a drastic acceleration in the growth of magnetic energy. finally, a fast mhd shock is formed, producing a supernova explosion. mildly collimated jet is produced for dipole-like type of the initial field. at very high initial magnetic field no mri development was found. keywords: core-collapse supernova magnetorotational mechanism numerical modeling. 1 introduction supernova is one of the most powerful explosion in the universe which releases about 1051 erg both in radiation and kinetic energies. sne explode at the end of evolution of massive stars, with initial mass larger than ∼ 8m�. a thermonuclear explosion of c-o degenerate core with total disruption of the star takes place in sn ia, what happens when initial mass of the star does not exceed ∼ 12m�, when electrons are degenerate in the carbon-oxygen core . for larger initial masses the evolution proceeds until the formation of the iron core, and the star collapses due to a loss of a hydrodynamic stability. during the core collapse and formation of a neutron star, gravitational energy release ∼ 6 · 1053 erg, is carried away by neutrino. the first mechanism suggested in [15] for the explanation of the explosion in a corecollapse sn was connected with a neutrino deposition. the huge energy flux carried by neutrino heats the infalling outer layers, reverse the direction of motion, and leads to formation of the shock wave, producing explosions of sn ii, sn ib,c types. later more accurate calculations revealed that the energy of such explosion is not enough for the explanation of observations. many modifications of the neutrino model have been calculated during years, but the problem is not yet clear. the review of the problem may be found in the book [8]. in recent simulations (see details in [23]) we have found that for extremely strong initial magnetic field h0 = 1012g a prompt supernova explosion occurs, and collimated jet is formed in agreement with [28]. for case when the initial magnetic field is weaker h0 = 109g we have identified, after the linear growth of the poloidal magnetic field due to differential rotation, the exponential field growth due to the magnetorotational instability of tayler type [30]. we call the combination of differential rotation with the tayler type mri instability, as magneto-differential-rotation instability (mdri). the ejection due to the explosion is only weakly collimated, while [28] had obtained a strong collimation in this variant also. in these simulations we considered a uniform magnetic field along the rotational axis, as the initial field configuration, similar to [28]. 2 magnetorotational mechanism of explosion in magnetorotational explosion (mre) the transformation of the rotational energy of the neutron star into explosion energy takes place by means of the magnetic field [7]). neutron stars are rotating, and have magnetic fields up to 1013 gs, and even more. often one or two-side ejections are visible. that indicate to nonspherical form of the sne explosions. in differentially 181 http://dx.doi.org/10.14311/app.2014.01.0181 gennady s. bisnovatyi-kogan, sergey g. moiseenko, nikolay v. ardeljan rotating new born neutron stars radial magnetic field is twisted, and magnetic pressure becomes very high, producing mhd shock by which the rotational energy is transformed to the explosion energy. calculations of mre have been done in [12], using one-dimensional nonstationary equations of magnetic hydrodynamics, for the case of cylindrical symmetry. the energy source is supposed to be the rotational energy of the system (the neutron star, and surrounding envelope). the calculations show that the envelope splits up during the dynamical evolution of the system, the main part of the envelope joins the neutron star and becomes uniformly rotating with it, and the outer part of the envelope expands with large velocity, carrying out a considerable part of rotational energy and rotational momentum. mre has an efficiency about 10% of the rotational energy, the ejected mass is ≈ 0.1 of the star mass, explosion energy ≈ 1051 erg. ejected mass and explosion energy depend weekly on the parameter α = emag/egrav at initial moment. explosion time depends on α as texpl ∼ 1√ α . small α is difficult for numerical calculations with explicit numerical schemes because of the courant restriction on the time step, hard system of equations, where α determines a hardness. 3 2-d calculations the numerical method used in simulations is based on the implicit operator-difference, completely conservative scheme on a lagrangian triangular grid of variable structure, with grid reconstruction (fig.1). the implicitness of the applied numerical scheme allows for large time-steps. it is important to use the implicit scheme in the presence of two strongly different time-scales: the small one due to huge sound velocity in the central parts of the star, and the big one determining the evolution of the magnetic field. the method applied here was developed and its stability was investigated in the papers of [5], [6], [1]. the scheme is fully conservative, what includes conservation of mass, momentum and total energy, and correct transitions between different types of energies. it was tested thoroughly with different tests by [2]. in the calculations of magnetorotational corecollapse supernova performed by [3], magnetohydrodynamic (mhd) equations with self-gravitation, and infinite conductivity have been solved using the numerical scheme as described above. the problem has an axial symmetry ( ∂ ∂φ = 0), and the symmetry to the equatorial plane (z=0). initial toroidal current jφ was taken at the initial moment (time started now from the stationary rotating neutron star) producing hr, hz according to biot-savart law b = 1 c ∫ v j×r r3 dv . initial magnetic field of quadrupole-like symmetry is obtained at opposite directions of the current in both hemispheres. neutrino cooling was calculated using a variant of a fluxlimited method, [3]. figure 1: example of the triangular grid magnetic field is amplified due to twisting by the differential rotation, and subsequent development of the magnetorotational instability. the field distribution for initial quadrupole-like magnetic field with α = 10−6, at the moment of the maximal energy of the toroidal magnetic field is represented in fig.1. the maximal value of bφ = 2.5·1016 gs was obtained in the calculations. the magnetic field at the surface of the neutron star after the explosion is b = 4 ·1012 gs. time dependence during the explosion of rotational, gravitational, internal, and kinetic poloidal energies is given in figs.3. almost all gravitational energy, transforming into heat during the collapse, is carried away by weakly interacting neutrino. the total energy ejected in the kinetic form is ∼ 0.6 · 1051 erg, and the total ejected mass is equal to ∼ 0.14m�. figure 2: toroidal magnetic field distribution at the moment of its maximal energy for the initial quadrupole field . 182 magnetorotational explosions of core-collapse supernovae time, s 0 0.1 0.2 1 2 3 4 erot[1051 ergs] ekinpol[1051 ergs] figure 3: time dependence of rotational, kinetic poloidal, and magnetic energies during explosion for a dipole -like field, from [22]. the simulations were done for the initial poloidal magnetic field of quadrupole [3] and of dipole [22] types of symmetry. before the collapse the ratios between the rotational and gravitational, and between the internal and gravitational energies of the star had been chosen as: erot egrav = 0.0057, eint egrav = 0.727. the initial magnetic field was ”turned on” after the collapse stage. the ratio between the initial magnetic and gravitational energies was chosen as 10−6. the initial poloidal magnetic field in the center, at start of the evolution of the toroidal field was ∼ 3.2× 1013g. time, s e je ct ed m as s/ m su n 0 0.1 0.2 0.30.00 0.04 0.08 0.12 figure 4: time dependence of the ejected mass during the magnetorotational explosion with initial dipole magnetic field, from [22]. the magnetic field works as a piston for the originated mhd shock. the time dependence of the ejected mass and energy is given in figs. 4, 5. during the magnetorotational explosion ∼ 0.14 m� of the mass and ∼ 0.6 · 1051ergs (∼ 10% of the rotational energy) are ejected. the simulation of the mr supernova explosion for various initial core masses and rotational energies was done by [11]. the initial core mass was varied from 1.2m� to 1.7m�, the initial specific rotational energy erot/mcore, was varied from 0.19 × 1019 to 0.4 × 1019 erg/g. the explosive energy increases with the mass of the core, and the initial rotational energy. the energy released in mr explosion, (0.5−2.6)×1051 erg, is sufficient to explain supernova with collapsing cores, types ii and ib. the energies of type ic supernovae could be higher. time, s e je ct ed e n er g y [1 051 er gs ] 0.2 0.3 0.1 0.2 0.3 0.4 0.5 0.6 figure 5: time dependence of the ejected energy during the magnetorotational explosion with initial dipole magnetic field, from [22]. 4 magnetorotational instability magnetorotational instability (mri) leads to exponential growth of magnetic fields. different types of mri have been studied by [31], [27]. mri starts to develop when the ratio of the toroidal to poloidal magnetic energies is becoming large. in 1-d calculations mri is absent because of a restricted degree of freedom, and time of mr explosion is increasing with α as texpl ∼ 1√ α , α = emag0 egrav0 . due to development of mri the time of mr explosion depends on α much weaker. the mr explosion happens when the magnetic energy is becoming comparable to the internal energy, at least in some parts of the star. while the starting magnetic energy linearly depends on α, and mri leads to exponential growth of the magnetic energy, the total time of mre in 2-d is growing logarithmically with decreasing of α, texpl ∼ − logα. these dependencies are seen clearly from 1-d ([12]) and 2-d calculations ([3], [22]) giving the following explosion times texpl (in arbitrary units): α = 0.01, texpl = 10, α = 10−12, texpl = 106 in 1d, and α = 10−6, texpl ∼ 6, α = 10−12, texpl ∼ 12 in 2-d. the dependence of the explosion time is shown 183 gennady s. bisnovatyi-kogan, sergey g. moiseenko, nikolay v. ardeljan in graphs for the quadrupole ([21]), and dipole ([22]) configurations of the magnetic field. the qualitative picture of the mri in 2d, and the example of the analytical toy model with an exponential growth of the magnetic field, have been presented by [3], [22]. 5 jet formation in mre jet formation in mre happens when the initial magnetic field is of a dipole-like structure. 2-d calculations with the initial dipole-like magnetic field gave almost the same values of the energy of explosion ∼ 0.5 · 1051 , and ejected mass ≈ 0.14m�, but the outburst was slightly collimated along the rotational axis [22], see fig.6 and fig.7. r, km z, km 0 2000 4000 6000 0 1000 2000 3000 4000 5000 6000 7000 0 500 10000 200 400 600 800 1000 figure 6: time evolution of the velocity field (outflow) for the time moment t = 0.075s, from [22] 0 500 10000 200 400 600 800 1000 r, km z, km 0 2000 4000 6000 0 1000 2000 3000 4000 5000 6000 7000 figure 7: time evolution of the velocity field (outflow) for the time moment, t = 0.25s, from [22]. simulations of the mr supernova have been made with equation of state suggested in [25]. a comparison of our results for the initially uniform magnetic field, using a lagrangian scheme, with the results in [28] and [29], using an eulerian scheme for the same initial and boundary conditions, shows good agreement for a strong initial field (h0 = 1012g), while for a weaker field (h0 = 109g) we get mildly collimated jet-like explosion; see also [13]. details of the results of these simulations will be published elsewhere ([23]). mri is developed in the case of a weaker initial magnetic field, and it is not present in the calculations with stronger field, see fig.8 and fig.12. we have made simulations for the initial magnetic fields h0 = 109, 1012g. the ratio of the initial rotational energy to the absolute value of the gravitational energy was taken erot0/egrav0 = 1%, 2%. when the initial magnetic field is moderate (h0 = 109 g) and erot0/egrav0 = 1% mdri develops what means exponential growth of all components of the magnetic field (fig.8). figure 8: developed mdri due to convection and mri/tayler instability at t = 267ms for the case h0 = 109g, erot0/egrav0 = 1% (contour plot the toroidal magnetic field, arrow lines force lines of the poloidal magnetic field). at fig.9 the lagrangian triangular grid and the ratio of the toroidal magnetic energy to the poloidal one (etor/epol) is represented for the case h0 = 109 g, and erot0/egrav0 = 1%. the toroidal magnetic energy dominates over the poloidal one in the significant part of the region where new neutron star is forming. the mdri is well-resolved on our triangular grid. in the case (h0 = 1012 g) and erot0/egrav0 = 1% there is no regions of domination of etor over epol. the fig.10 represents a time evolution of rotational, magnetic poloidal and toroidal energies for mr explo184 magnetorotational explosions of core-collapse supernovae sion when h0 = 109g. the fig.11 is the same data plot as the fig.10 but zoomed and the vertical axis is in logarithmic scale. the straight dash-dotted line at the fig.11 shows the exponential growth of the toroidal and poloidal magnetic energies with the time due to the mdri. the rotational energy has two maxima. the first contraction is accompanied by the strong growth of the rotational energy due to angular momentum conservation, maximum of which coincides with the first maximum of the density. the first contraction, and the subsequent bounce, happens when the magnetic field is growing slowly, and the angular momentum losses from the stellar core are small. development of the magnetorotational instability leads to a rapid growth of the magnetic field, large angular momentum flux from the core, what stops the expansion, and leads to the second contraction phase. in this case the contraction is not transforms into expansion, because of the rapid decrease of the rotational energy due to strong angular momentum flux outside from the core. r,cm z ,c m 0 5e+060 2e+06 4e+06 6e+06 eetor/eepol 1000 800 600 500 200 100 10 1 time= 0.265sec figure 9: the lagrangian triangular grid and the ratio of the toroidal magnetic energy etor to the poloidal magnetic energy epol, etor epol at t = 265ms for the case h0 = 109g, erot0/egrav0 = 1%. the energy of the poloidal magnetic field grows due to the contraction until the time t ≈ 0.225 sec. then it slightly decreases because of the formation of the bounce shock, and its motion outwards. the toroidal magnetic energy grows as quadratic function because of wrapping of the magnetic force lines (toroidal component of the magnetic field grows linearly). starting from t ≈ 0.3 sec both the poloidal and toroidal magnetic energies begin to grow exponentially due to mdri. at t ≈ 0.36 sec both magnetic energies comes to saturation. the mhd shock wave develops what leads to the mr explosion. the mr explosion develops in all directions without formation of a collimated flow. the mr explosion for an extremely high initial magnetic field (h0 = 1012 g) is developing in a qualitatively different way. the initial magnetic field is so strong that it grows strongly during the first contraction, and the explosion happens before the development of mdri happens (fig.12). at the fig.13 the time evolution of the rotational, poloidal magnetic and toroidal magnetic energies are represented. the rotational energy has one extremum at t ≈ 0.32 sec corresponds to the maximal contraction, accompanied by a corresponding growth of of the toroidal and poloidal magnetic energies. the poloidal magnetic field grows due to the contraction, the toroidal magnetic field appears due to the differential rotation and is amplified both due to the differential rotation and the contraction of the core. the strong initial magnetic field leads to a rapid loss of the angular momentum from the core already during the contraction phase. the centrifugal force becomes unimportant, and the first contraction is not followed by any bounce, leaving behind a dense slowly rotating neutron star core. we have got here a prompt explosion. the force lines of the magnetic field play the role of ’rails’. the matter moves along the force lines. the magnetic pressure dominates a the periphery of the core. the mr explosion develops mainly along the axis of rotation, and the collimated flow (protojet) is formed. the mr explosion results in the collimated jet. the degree of jet collimation is approximately the same as in [28]. for the case when erot0/egrav0 = 1% and h0 = 109 g the mr supernova explosion energy is ∼ 4× 1050 erg, for the h0 = 1012 g the mr supernova explosion energy reaches the value of ∼ 8 × 1050erg. the explosion energy resulted in the simulations by lagrangian method are close to those ones found in the simulations made by eulerian scheme [28] (excluding the case of b0 = 1012g and erot0/|egrav0| = 1/%). 6 asymmetry of the explosion it is known from the observations that the shapes of core collapse supernovae are different. from our simulations it follows that mr supernova explosion arises after development of the mri. the development of the mri is a stochastic process and hence the resulting shape of the supernova can vary. we may conclude that mr supernova explosion mechanism can lead to different shape of the supernova. it is important to point out that mr mechanism of supernova explosion leads always to asymmetrical outbursts. the simulations of the mr supernova explosions described here are restricted by the symmetry to the equa185 gennady s. bisnovatyi-kogan, sergey g. moiseenko, nikolay v. ardeljan torial plane. while in reality this symmetry can be violated due to the mri, simultaneous presence of the initial dipole and quadrupole -like magnetic field ([33]) and initial toroidal magnetic field ([10]). the violation of the symmetry could lead to the kick effect and formation of rapidly moving radio pulsars. a kick velocity, along the rotational axis, formed due to magnetohydrodynamic processes in presence of the asymmetry of the magnetic field, by estimations [9] does not exceed 300km/sec. figure 10: time evolution of rotational energy erot (solid line), magnetic poloidal energy emagpol (dashed line) and magnetic toroidal energy emagtor (dashdotted line) for the case h0 = 109g, erot0/egrav0 = 1%. when rotational and magnetic axes do not coincide the whole picture of the explosion process is three dimensional. nevertheless, the magnetic field twisting happens always around the rotational axis, so we may expect the kick velocity of the neutron star be strongly correlated with its spin direction. during the phase of mre explosion the regular component of magnetic field may exceed temporally 1016 g [3], [22], when the neutrino cross-section depends on the magnetic field strength. the level of the anisotropy of the magnetic field relative to the plain perpendicular to the rotational axis [20] may be of the order of 50%, leading to strong anisotropy of the neutrino flux. the kick velocity due to the anisotropy of the neutrino flux may reach several thousands km/c [9], explaining appearance of the most rapidly moving radio pulsars [32]. simultaneously, because of the stochastic nature of mri, the level of the anisotropy should be strongly variable, leading to a large spreading in the the neutron star velocities. this prediction of mr explosion differs from the models with a powerful neutrino convection, where arbitrary direction of the kick velocity is expected ([14],[17]). it was claimed in [16], that proto-neutron star (pns) convection was found to be a secondary feature of the corecollapse phenomenon, rather than a decisive ingredient for a successful explosion. figure 11: zoomed time evolution of rotational energy erot (solid line), magnetic poloidal energy emagpol (dashed line) and magnetic toroidal energy emagtor (dash-dotted line) for the case h0 = 109g, erot0/egrav0 = 1%. straight dash-dotted line shows exponential growth of the toroidal and poloidal magnetic energies. analysis of observations of pulsars shows that rotation and velocity vectors of pulsars are aligned, as is predicted by the mr supernova mechanism. this alignment was first found in [26], and was confirmed with reliability, increasing with time, in the papers [18],[19],[24]. the alignment of the vectors can be violated in the case when the supernova explodes in a binary system. 7 conclusions in mre the efficiency of transformation of rotational energy into the energy of explosion is ∼ 10%. mri strongly accelerates mre, at lower values of the initial magnetic fields. jet formation is possible for dipolelike topology of the field. mre energy is not sensitive to the details of the equation of state, model of the neutrino transfer, and to the choice of the numerical scheme. the observed alignment of the rotation and velocity vectors of pulsars follows directly from the mre supernova model. 186 magnetorotational explosions of core-collapse supernovae r,cm z ,c m 0 1e+06 2e+060.0e+00 5.0e+05 1.0e+06 1.5e+06 2.0e+06 time= 0.328sec figure 12: absence of mdri at t = 328ms for the case h0 = 1012g, erot0/egrav0 = 1% (contour plot the toroidal magnetic field, arrow lines force lines of the poloidal magnetic field). 0.1 0.15 0.2 0.25 0.3 0.35 time, sec e rot e magpol e magtor 10 8 6 4 2 e n e rg y x 1 0 5 0 , e rg figure 13: time evolution of rotational energy erot (solid line), magnetic poloidal energy emagpol (dashed line) and magnetic toroidal energy emagtor (dashdotted line) for the case h0 = 1012g, erot0/egrav0 = 1%. acknowledgement the work of sgm and gsbk was supported partially by rfbr grant 11-02-00602, grant for leading scientific schools nsh-5440.2012.2 and 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[28] takiwaki, t., kotake, k., nagataki, s., & sato, k. 2004, apj, 616, 1086 doi:10.1086/424993 [29] takiwaki, t., kotake, k., & sato, k. 2009, apj, 691, 1360 doi:10.1088/0004-637x/691/2/1360 [30] tayler, r. 1973, mnras, 161, 365 [31] velikhov, e.p. 1959, j. exper. theor. phys., 36, 1398 [32] vlemmings, w.h.t., et al. 2005, mem. sai, 2005, 76, 531 (astro-ph/0509025) [33] wang, j. c. l., sulkanen, m. e., lovelace, r. v. e., 1992, apj, 390, 46 188 http://dx.doi.org/10.1086/589759 http://dx.doi.org/10.1111/j.1365-2966.2006.10517.x http://dx.doi.org/10.1111/j.1365-2966.2012.21083.x http://dx.doi.org/10.1086/424993 http://dx.doi.org/10.1088/0004-637x/691/2/1360 introduction magnetorotational mechanism of explosion 2-d calculations magnetorotational instability jet formation in mre asymmetry of the explosion conclusions 282 acta polytechnica ctu proceedings 2(1): 282–285, 2015 282 doi: 10.14311/app.2015.02.0282 diagnostic of the symbiotic stars environment by thomson, raman and rayleigh scattering processes m. sekeráš1, a. skopal1 1astronomical institute of the slovak academy of sciences, 059 60 tatranská lomnica, slovakia corresponding author: msekeras@ta3.sk abstract symbiotic stars are long-period interacting binaries consisting of a cool giant as the donor star and a white dwarf as the acretor. due to acretion of the material from the giant’s stellar wind, the white dwarf becomes very hot and luminous. the circumstellar material partially ionized by the hot star, represents an ideal medium for processes of scattering. to investigate the symbiotic nebula we modeled the wide wings of the resonance lines ovi λ1032å, λ1038å and heii λ1640å emission line in the spectrum of ag dra, broadened by thomson scattering. on the other hand, raman and rayleigh scattering arise in the neutral part of the circumstellar matter around the giant and provide a powerful tool to probe e.g. the ionization structure of the symbiotic systems and distribution of the neutral hydrogen atoms in the giant’s wind. keywords: symbiotic stars spectroscopy. 1 introduction symbiotic stars are interacting binary systems consisting of a cool giant and a hot compact star most probably a white dwarf. typical orbital periods are between 1 and 3 years, but can be significantly larger. a small part of the material lost by the giant in the form of a stellar wind is accreted by the compact star. this process generates a very hot (th ∼ 105 k) and luminous (lh ∼ 102 − 104 l�) source of radiation, which then ionizes a fraction of the neutral wind from the giant, giving rise to nebular emission in the so-called symbiotic nebula. we distinguish two stages of symbiotic systems. a quiescent phase, when the hot star releases its energy with constant rate and spectral energy distribution, and active phase, when the symbiotic system significantly changes its radiation and brightens up by a few magnitudes in the optical. the partially ionized circumbinary environment represents an ideal medium for scattering processes. in particular we measure effects of the raman and rayleigh scattering of the hot star radiation on neutral atoms of hydrogen and the thomson scattering on free non-relativistic electrons. fig. 1 shows a sketch of simplified ionization structure of symbiotic stars with location of scattering processes in their environment. in this contribution we present the effects of these three types of scattering processes observed in the spectra of symbiotic stars and their possible applications. thomson scattering raman scattering & rayleigh white dwarf red giant ) h ii region (symbiotic nebula h i region figure 1: simplified ionization structure of a symbiotic star. while the radiation from emission region in a vicinity of the white dwarf is thomson scattered in ionized part of the circumstellar environment, in the neutral part it can be raman or rayleigh scattered. 2 thomson scattering the scattering of photons on free low-energy electrons (hν << mec 2) has a very small cross section σt = 6.652 × 10−25cm2 and is wavelength independent. within the symbiotic nebula this scattering will attenuate the cores of emission lines redistributing the scattered part of the line into its wings. the wings be282 http://dx.doi.org/10.14311/app.2015.02.0282 diagnostic of the symbiotic stars environment by thomson, raman and rayleigh scattering processes come very broad, because of a large thermal motions of free electrons, and due to the small value of σt , they will be faint. nevertheless the densest portions of symbiotic nebula with high electron concentration (ne ∼ 108 − 1012cm−3) together with the extremely intense emission lines of highly ionized elements formed in the vicinity of the hot star (e.g. ovi λ1032å, λ1038å) allow us to observe these broad wings. figure 2: a model (solid line) of the observed (dotted line) profile of the ovi λ1032, λ1038å emission lines as a result of thomson scattering in the spectra of ag dra during active and quiescent phase. to model the broad faint wings of the line profile, we adopted the theoretical line profile used already by castor et al. (1970). we applied modeling to the resonance dublet ovi λ1032å, λ1038å and heii λ1640å emission line in the (far-)uv spectra of symbiotic star ag dra. we determined the mean electron optical depth, τe, and electron temperature, te, of the symbiotic nebula during different levels of activity (fig. 2). during quiescent phases the mean value of τe = 0.056 ± 0.006 and te = 19 200 ± 2300 k. on the other hand during active phases τe = 0.64±0.11 and te = 32 300±2000 k (see sekeráš & skopal, 2012 in detail). during the active phases the hot star significantly enhances its stellar wind. this is supported by the broadening of the hα wings (skopal, 2006) as well as by the x-ray/uv flux anticorrelation observed in ag dra during active and quiescent phases (skopal et al., 2009). the material of the wind is ionized by the luminous hot star, which enhances radiation from the symbiotic nebula, which dominates the spectral region u. this produces a significant supplement of free electrons into the nebula. because τe depends on the electron concentration along the line of sight throughout the symbiotic nebula in direction to the hot star, its increase during the active phase indeed indicates the presence of an enhanced mass loss from the white dwarf (fig. 3). 8 9 10 11 0.1 0.3 0.5 0.7 0.9 1.1 1.3 u m ag ni tu de τe figure 3: during the transition from the quiescent phase (u∼11; bottom left) to the active phase (u∼8; top right), the ionized enhanced stellar wind from white dwarf represent a new source of free electrons into the nebula, which increases the electron optical depth τe. 3 raman scattering unlike the thomson scattering on free electrons, which acts in the ionized part of the red giant’s wind (symbiotic nebula), raman scattering arises within its neutral part around the giant. the raman scattering on neutral atoms of hydrogen represents the most pronounced effect. it is a process when a photon excites the atom from its ground state to an intermediate state, which is immediately stabilized by a transition to a different lower main energy level, resulting in emitting a photon of a different frequency. we can observe several raman emission features originated from scattered photons of intense emission lines, such as heii λ6545å (e.g. lee et al., 2003), cii λ7023å, λ7054å (schild & schmid, 1996), heii λ4851å (van groningen, 1993), etc. however the most conspicuous example of raman scattering are broad emission lines at λ6825å and λ7082å (e.g. schmid et al., 1989) (see fig. 4). these are the characteristic features in the spectrum of about half of symbiotic stars. they are result of scattering of resonance dublet of ovi λ1032å, λ1038å in the far-uv on neutral hydrogen atoms. their nature was confirmed by near-simultaneous uv and optical observations and spectropolarimetry (e.g. schild & schmid, 1996, birriel et al., 2000). the raman scattered lines often show a multi-peaked structure, even when the parental ovi emission line is single-peaked. to explain the line profile differences between the ovi lines and their raman scattered counterparts, schmid et al. (1999) suggest doppler shifts introduced in the raman scattering process and non-isotropic ovi line emission. moreover, the evolution of raman lines during quiescent and active phases can reveal the ionization structure around the hot star (e.g. skopal et. al., 2006, fig. 4 here). one of the main characteristic feature of the ramanscattered lines is the broadening according to the 283 m. sekeráš, a. skopal parental uv line. the wavelength λ0 of raman scattered radiation is associated with the wavelength λi of the incident uv radiation, which leads to the broadening of the raman emission lines by factor of λ0/λi. for example the raman emission at 6545å is broadened according to the parental heii λ1025å line by a factor of ≈ 6.4. as raman scattering is a dipol process we can observe strong polarization of the broad emission lines λ6825å and λ7082å. the amount and orientation of the polarization are expected to change periodically due to the orbital motion. this fact can be helpful to provide information concerning the orbital parameters, such as binary period, orientation and inclination of the orbital plane (e.g. schild & schmid, 1996, harries & howarth, 1996). 0 1 2 fl ux [ 10 -1 2 e rg c m -2 s -1 a -1 ] fuse: 05/07/2002 ϕ = 0.95 optical: 07/08/2002 ϕ = 0.99 o vi λ1032/500 raman λ6825 0 1 2 fuse: 22/10/2002 ϕ = 0.09 optical: 02/02/2003 ϕ = 0.23 o vi λ1032/150 raman λ6825 0 1 2 -200 -100 0 100 200 radial velocity [km/s] fuse: 04/08/2003 ϕ = 0.47 optical: 31/07/2003 ϕ = 0.47 o vi λ1032/400 raman λ6825 figure 4: comparison of the ovi λ1032å line profiles with their raman scattered counterparts along the outburst in symbiotic star z and (adopted from skopal et al., 2006). in addition to polarimetric measurements, the scattering geometry can be estimated by the raman scattering efficiency. it is defined as the ratio of the number of raman scattered photons to the number of emitted far-uv photons. this parameter strongly depends on the size of the neutral scattering region as ’seen’ by the original photons and thus also on the mass loss rate from the cool giant in symbiotic stars (e.g. lee et al., 2003). 4 rayleigh scattering in the case of rayleigh scattering, a photon excites an atom from its ground state to an intermediate state, which is immediately stabilized by a transition to a true bound state and this is followed by immediate reemission of a photon of the same wavelength. thus when the far-uv radiation from/around the hot star goes through the neutral part of the giant’s wind, it can be rayleigh scattered on the neutral hydrogen atoms. effect of this type of scattering is well observed in the form of strong attenuation of the continuum around hydrogen lines of the lyman series. the presence of rayleigh scattering was observed in the spectra of symbiotic stars with a high orbital inclination at/around the position of the inferior conjunction of the giant (e.g. eg and, vogel, 1991). figure 5 demonstrates this case for eg and. 1 3 5 7 9 3.1 3.2 3.3 3.4 3.5 3.6 fl ux [ 10 -1 3 e rg c m -2 s -1 a -1 ] log wavelength [a] eg and 13/01/1983 ϕ=0.95 figure 5: strong attenuation of the far-uv continuum by rayleigh scattering process (gray area) in the spectrum of eg and, close to the inferior conjunction of the giant. the cross section for raman scattering depends strongly on wavelength with λ−4, which is well demonstrated by monte carlo simulations by schmid (1995), and increases sharply toward lyman resonance transitions. therefore it is easily observable in far-uv spectrum (1200 å 1400 å) and not distinguishable in the optical. in optically thick environment the photon may undergo many rayleigh scatterings until it is raman scattered and escape the neutral region. rayleigh scattering arises in the neutral wind around the giant. if we consider the simplified steady284 diagnostic of the symbiotic stars environment by thomson, raman and rayleigh scattering processes state ionization model of the symbiotic star (see fig. 1), where the hi zone is approximately cone-shaped with the giant at its top (seaquist et al., 1984), it is obvious that the effect of rayleigh scattering will depend on the ionization structure, the inclination of the orbital plane and the orbital phase. it is therefore most easily measurable in eclipsing systems as periodic changes in the far-uv radiation. through the rayleigh scattering we can measure the neutral hydrogen column density in the line of sight as a function of the orbital phase, with typical values of 1020 − 1024cm−2. in this way, it is possible to explore the density and distribution of the neutral hydrogen and thus also the velocity structure in outer atmosphere of a red giant (e.g. vogel, 1991, pereira et al., 1999). 5 conclusions the nature of the environment around the symbiotic stars, with a fully ionized and also the neutral part of the circumstellar matter, allow us to investigate symbiotics through the effects of scattering processes. thomson scattering on free electrons can diagnose the symbiotic nebula by determining the mean electron optical depth, τe, and electron temperature, te. while the nebula is directly connected with the changes of the hot star properties, the effects of thomson scattering can provide some information also about the symbiotic nebula during the quiescent and active phase. raman and rayleigh scattering on neutral hydrogen atoms arises in the neutral part of the giant’s wind. they are observed in a form of broad emission features and an attenuation of the far uv continuum, respectively. these processes are useful to examine the ionization structure of the symbiotic systems, kinematics of the scattering and emitting region, the structure and density distribution of the neutral hydrogen in the giant’s wind and thus its mass loss rate. because the environment, where these scattering processes arises, is a result of the interaction between the giant and the white dwarf, they can provide also information about orbital parameters and physical properties of the two stars independently of other approaches. acknowledgement the research was supported by a vega grant of the slovak academy of sciences no. 2/0002/13. references [1] birriel, j. j., et al.: 2000, apj 545, 1020. doi:10.1086/317851 [2] castor, j.i., smith, l.f., van blerkom, d.: 1970, apj 159, 1119. doi:10.1086/150393 [3] harries, t. j., howarth, i. d.: 1996, a&a 310, 235. [4] lee, hee-won., et al.: 2003, apj 598, 553. doi:10.1086/378886 [5] pereira, c. b., et al.: 1999, a&a 344, 607. [6] schild, h., schmid, h.m.: 1996, a&a 310, 221. [7] schmid, h. m., et al.: 1989, a&a 211, l31. [8] schmid, h. m., et al.: 1999, a&a 348, 950. [9] seaquist, e. r., taylor, a. r., button, s.: 1984, apj 284, 202. doi:10.1086/162399 [10] sekeráš, m., skopal, a..: 2012, mnras 427, 979. doi:10.1111/j.1365-2966.2012.21991.x [11] skopal, a.: 2006, a&a 457, 1003. [12] skopal, a., et al.: 2006, a&a 453, 279. [13] skopal, a., et al.: 2009, a&a 507, 1531. [14] van groningen, e.: 1993, mnras 264, 975. [15] vogel, m.: 1991, a&a 249, 173. 285 http://dx.doi.org/10.1086/317851 http://dx.doi.org/10.1086/150393 http://dx.doi.org/10.1086/378886 http://dx.doi.org/10.1086/162399 http://dx.doi.org/10.1111/j.1365-2966.2012.21991.x introduction thomson scattering raman scattering rayleigh scattering conclusions 188 acta polytechnica ctu proceedings 2(1): 188–191, 2015 188 doi: 10.14311/app.2015.02.0188 swsex stars, old novae, and the evolution of cataclysmic variables l. schmidtobreick1, c. tappert2 1european southern observatory, casilla 19001, santiago 19, chile 2departamento de f́ısica y astronomı́a, universidad de valparáıso, avda. gran bretaña 1112, valparáıso, chile corresponding author: lschmidt@eso.org abstract the population of cataclysmic variables with orbital periods right above the period gap are dominated by systems with extremely high mass transfer rates, the so-called sw sextantis stars. on the other hand, some old novae in this period range which are expected to show high mass transfer rate instead show photometric and/or spectroscopic resemblance to low mass transfer systems like dwarf novae. we discuss them as candidates for so-called hibernating systems, cvs that changed their mass transfer behaviour due to a previously experienced nova outburst. this paper is designed to provide input for further research and discussion as the results as such are still very preliminary. keywords: cataclysmic variables dwarf novae novae sw sextantis stars. 1 introduction in the following we briefly review our current knowledge on the populations of old novae and sw sextantis stars especially in the context of the evolution of cataclysmic variable stars (cvs). 1.1 evolution of cvs in general, the evolution of stable interactive binaries is driven by angular momentum loss which as such determines the mass transfer rate. the loss of angular momentum as a source for stable mass transfer implicates that the general evolutionary direction for cvs is from longer to shorter orbital periods. according to the standard model of cv evolution, the main source of angular momentum loss for cvs with orbital periods porb > 3 h is magnetic braking. due to the continuous mass transfer, the secondary is pushed out of thermal equilibrium and becomes bloated. at a period of about 3 h, i.e. at the upper edge of the period gap, the secondary becomes fully convective, magnetic braking ceases and only the much weaker braking through the emission of gravitational radiation continues. therefore, the mass transfer is greatly diminished which allows the secondary star to relax into a state of thermal equilibrium and to contract to a volume corresponding to its mass. it thus loses contact with its roche lobe and mass transfer stops completely. the angular momentum loss via gravitational radiation shrinks the orbit of the now detached and therefore hardly detectable binary until the secondary star fills its roche lobe again at an orbital period of about 2 h, i.e. at the lower edge of the gap and the binary continues as a low mass transfer cv below the gap. for an extensive review of the current understanding of cv evolution, see knigge et al.̃(2011). 1.2 swsex stars this sub-class was originally defined by thorstensen et al. (1991). incorporating eclipsing nova-like stars with single-peaked emission lines, high velocity line wings, strong he ii emission but no polarisation, and transient absorption features at orbital phases around φ = 0.5. the radial velocity curves show an offset of 0.2 cycle with respect to the phase defined by the eclipse. today, sw sex stars are considered novalike stars with an extremely high mass transfer rate (see e.g. rodŕıguez-gil et al. 2007a,b). this is supported by the high temperatures found for the white dwarfs in these systems (townsley & gänsicke, 2009). while sw sex stars used to be considered as rare and strange objects with unusual behaviour, it has been shown in the last years that they in fact represent the dominant cv population in the orbital period range between about three and four hours (schmidtobreick et al., 2012). from a sample of cvs that was purely seleceted to have an orbital period between three and four hours, they find that the percentage of sw sex stars in this range must exceed 85%. according to the standard model, all long period cvs have to evolve through this period range before entering the gap and will thus share the sw sex characteristics 188 http://dx.doi.org/10.14311/app.2015.02.0188 sw sex stars, old novae, and the evolution of cataclysmic variables during that time. this makes the sw sex phenomenon a phase in the secular evolution of cvs (schmidtobreick et al. in preparation). 1.3 old novae classical novae are cvs that experience a thermonuclear runaway on the surface of the white dwarf where the accreted material has reached a critical mass. in this process, the binary is not destroyed and mass transfer is usually re-established within a few months. it is therefore safe to assume that nova explosions are recurrent events and part of the evolution of every cv with a sufficiently high mass transfer rate to accumulate the necessary material on the white dwarf surface within its lifetime. inbetween the nova events, the behaviour of the cv depends on properties like orbital period, mass-transfer rate, and magnetic field of the white dwarf that also determine its subtype. in addition, the hibernation model predicts changes of the mass transfer rate in the evolution of the preand postnova: after an initial phase of high mass transfer rate – due to the secondary star being driven strongly out of thermal equilibrium via irradiation from the eruptionheated white dwarf – which increases the distance between the two stars, the binary should descend into a long state of low mass transfer once the white dwarf cooled down and the secondary is allowed to relax more towards thermal equilibrium. (shara et al., 1986; prialnik & shara, 1986). some potential evidence for hibernation has been presented in the form of old nova shells around cvs that have previously been known as low mass transfer systems, i.e. z cam (shara et al., 2007) and at cnc (shara et al., 2012). however this interpretation is not exclusive, as the presence of nova shells around dwarf novae could also indicate that all types of cv (including dwarf novae) can experience a nova explosion during their lifetime without necessarily undergoing cyclic changes of the mass transfer rate. 2 what can we learn combining our knowledge from old novae and swsex stars? several years ago, we conducted a project investigating old novae which had experienced large outburst amplitudes (schmidtobreick et al., 2003; 2005). the idea behind this was that since the absolute magnitude of a nova explosion depends mainly on the mass of the white dwarf (livio, 1992) it thus differs only slightly for different systems. thus, novae with large outburst amplitudes are likely to be intrinsically faint cvs and therefore candidates for low mass transfer rate systems. two systems of our sample, v842 cen and xx tau, show spectroscopic properties that, compared to other old novae, indicate a rather low mass transfer rate. the spectra present comparatively strong balmer emission lines from hα bluewards down to h11. a weak heii emission line is present at 469 nm and indicates a hot component in the system, but at the same time the presence and strength of the hei series is evidence for a significant amount of cooler material. we note that most old novae do not share these characteristics (e.g., ringwald et al. 1996, tappert et al. 2012, 2014). for xx tau, preliminary analysis of long-term photometric monitoring covering a range of 150 d furthermore shows a probable dwarf-nova like outburst with an amplitude of ∼0.8 mag and a duration of ∼10 d (tappert, private communication), which represents additional evidence for a comparatively low mass transfer rate. the approach by tappert et al. (2012) to re-discover lost old novae revealed two more such candidates from spectroscopy: v2109 oph and v728 sco. in particular v728 sco which has also been observed photometrically can be considered a low mass transfer system as it also shows frequent stunted dwarf novae outbursts (tappert et al., 2013a). for three of these low mass transfer old novae, i.e. v842 cen, v728 sco, and xx tau, at least rough values for the orbital period could be established (woudt et al., 2009; luna et al., 2012; tappert et al., 2013a; rodŕıguez-gil & torres, 2005) and they all fall into the range of sw sex stars, i.e. between 3 and 4 hours. on the one hand, this period range is thus dominated by high mass transfer cvs. on the other hand, also the majority of old novae have high mass transfer rates. in fact, the period distribution of old novae shows a distinctive peak in the 2.8-5 h period range, while few novae are found in the period bins that are dominated by dwarf novae (e.g., tappert et al. 2013b, schmidtobreick & tappert 2014). thus, the indicated low mass transfer nature of v842 cen, v728 sco and xx tau is somewhat surprising. for the sake of completeness, we point out that other low mass transfer novae are known. for example, the system v446 her (porb = 4.97 h, thorstensen & talor, 2000) shows frequent ”stunted” dwarf-nova like outbursts (honeycutt et al. 1998). however, we here restrict our discussion to systems with orbital periods in the 3-4 h range. when thinking about a reason why several old novae would appear as low mass transfer systems even though some of them are even situated in the sw sex regime, hibernation comes naturally to mind. it seems an attractive explanation that these cvs have indeed been sw sex stars which due to the nova explosion in the recent past were pushed into a low state and thus experience low mass transfer rates at the moment. in 189 l. schmidtobreick, c. tappert this context we point out that v728 sco in outburst presents a triangular eclipse shape that is typical for sw sex stars (tappert et al., this volume). 3 conclusions and future work cvs with orbital periods between 3 and 4 hours are in general sw sex stars and as such experience high masstransfer rates. however, we find evidence for three old novae with orbital periods in this range that should be of sw sex type but are instead low mass transfer systems. we tentatively conclude that the mass transfer of these systems was changed by the nova eruption, similar to what is proposed in the hibernation scenario. whether the mass transfer in these systems will completely cease or just remain on a low level before rising up again remains to be investigated. to better understand the recent mass transfer history and to thus shed light on the evolutionary state of these systems, it would be important to measure the dynamical masses and the temperatures of the two components in order to determine the binary solution. for this, it is valuable to have a system like v728 sco in the sample which as eclipsing binary offers more opportunities to determine the binary parameters. to test the idea that the low mass transfer novae in the sw sex regime are affected by hibernation, it will be essential to check for the few existing dwarf novae in the sw sex range whether they have experienced a nova eruption in the past. we thus started an observational project to look for nova shells around these objects. other possible candidates for hibernating cv could be found among the so-called pre-cvs, white-dwarf/mainsequence binary which are detached systems. examples for such candidates are ltt 560 (tappert et al., 2011) and qs vir (drake et al., 2014, and references therein) which both have orbital periods in the sw sex range and show evidence of accretion. if indeed a significant number of these low mass transfer systems in the sw sex regime can be shown to have experienced a nova outburst in the past, this would not only be strong evidence for the hibernation scenario but at the same time also prove the evolutionary significance of the sw sex stars as it would yield a natural explanation for the few remaining cvs in the 3-4 h period range that do not follow the sw sex behaviour. acknowledgement this research was supported by fondecyt regular grant 1120338 (ct). we gratefully acknowledge the use of the simbad database, operated at cds, strasbourg, france, and of nasa’s astrophysics data system bibliographic services. we thank an anonymous referee for valuable comments. references [1] knigge c., baraffe i., patterson j., 2011, apjs 194, 28 doi:10.1088/0067-0049/194/2/28 [2] drake j. j., garraffo c., takei d, gänsicke b., 2014, mnras 437, 3842 doi:10.1093/mnras/stt2186 [3] honeycutt r. k., robertson j. w., turner g. w., henden a. a., 1998, apj 495, 933 doi:10.1086/305299 [4] livio, m. 1992, in vina del mar workshop on cataclysmic variable stars, edited by n. vogt, asp conf. ser. 29, 4. [5] luna g. j. m., diaz m. p., brickhouse n. s., moraes m., 2012, mnras, 423, l75. doi:10.1111/j.1745-3933.2012.01260.x [6] prialnik, d., & shara, m. m. 1986, apj, 311, 172. doi:10.1086/164763 [7] ringwald f. a., naylor t., mukai k., 1996, mnras 281, 192 doi:10.1093/mnras/281.1.192 [8] rodŕıguez-gil p., schmidtobreick l., gänsicke b. t., 2007a, mnras 374, 1359 doi:10.1111/j.1365-2966.2006.11245.x [9] rodŕıguez-gil p., gänsicke b. t., hagen h.-j. et al., 2007b, mnras 377, 1747 doi:10.1111/j.1365-2966.2007.11743.x [10] rodŕıguez-gil p., torres m. a. p., 2005, a&a, 431, 289 [11] schmidtobreick l., tappert c., saviane i., 2003a, mnras 342, 145 doi:10.1046/j.1365-8711.2003.06523.x [12] schmidtobreick l., tappert c., bianchini a., mennickent r. e. 2003b, a&a, 410, 943. [13] schmidtobreick l., tappert c., bianchini a., mennickent r. e., 2005, a&a, 432, 199. [14] schmidtobreick l., rodŕıguez-gil p., gänsicke b. t., 2012, mem. s.a.it. vol. 83, 610 [15] schmidtobreick l., tappert c., 2014, stella novae: future and past decades. woudt & ribeiro (eds), asp in press [16] shara, m. m., livio, m., moffat, a. f. j., & orio, m. 1986, apj 311, 163 190 http://dx.doi.org/10.1088/0067-0049/194/2/28 http://dx.doi.org/10.1093/mnras/stt2186 http://dx.doi.org/10.1086/305299 http://dx.doi.org/10.1111/j.1745-3933.2012.01260.x http://dx.doi.org/10.1086/164763 http://dx.doi.org/10.1093/mnras/281.1.192 http://dx.doi.org/10.1111/j.1365-2966.2006.11245.x http://dx.doi.org/10.1111/j.1365-2966.2007.11743.x http://dx.doi.org/10.1046/j.1365-8711.2003.06523.x sw sex stars, old novae, and the evolution of cataclysmic variables [17] shara m. m., martin c. d., seibert m., rich r. m., salim s., reitzel d., schiminovich d., deliyannis c. p., sarrazine a. r., kulkarni s. r., ofek e. o., brosch n., lépine s., zurek d., de marco o., jacoby g., 2007, nature 446, 159 [18] shara m. m., mizusawa t., wehinger p., zurek d., martin c. d., neill j. d., forster k., seibert m., 2012, apj 758, 121. [19] tappert c., gänsicke b. t., schmidtobreick l., ribeiro t., 2011, a&a 532, 129 [20] tappert c., ederoclite a., mennickent r. e., schmidtobreick l., vogt n. 2012, mnras, 423, 2476. [21] tappert c., vogt n., schmidtobreick l., ederoclite a., vanderbeke j., 2013a, mnras 431, 92 [22] tappert c., schmidtobreick l., vogt n., ederoclite, a., 2013b, mnras 436, 2412 [23] tappert c., vogt n., della valle m., schmidtobreick l., ederoclite a., 2014, mnras in press [24] thorstensen j. r., taylor c. j., 2000, mnras 312, 629 [25] thorstensen j. r., ringwald f. a., wade r. a., schmidt g. d., norsworthy j. e., 1991, aj 102, 272 [26] townsley d. m., gänsicke b. t., 2009, apj 693, 1007 [27] woudt, p. a., warner, b., osborne, j., page, k. 2009, mnras, 395, 2177. discussion vitaly neustroev: you claim that some old novae have a very small mass transfer rate. does it mean that these stars should show dn-type outbursts? linda schmidtobreick: yes, indeed. and some stars do show such outbursts. we don’t yet have a lot of data but we have observed several outbursts for v728 sco. 191 introduction evolution of cvs swsex stars old novae what can we learn combining our knowledge from old novae and swsex stars? conclusions and future work acta polytechnica ctu proceedings doi:10.14311/app.2017.7.0012 acta polytechnica ctu proceedings 7:12–17, 2017 © czech technical university in prague, 2017 available online at http://ojs.cvut.cz/ojs/index.php/app microscopic features of cement paste modified by fine perlite vladimír hrbeka,∗, veronika koudelkováb, pavel padevěta, petr šašekb a czech technical university, thakurova 6, prague 6, czech republic b institute of theoretical and applied mechanics as cr, v.v.i., prosecka 76, prague 9, czech republic ∗ corresponding author: vladimir.hrbek@fsv.cvut.cz abstract. the use of waste material and replacement of binder element in cementitious composites is in focus of material development. perlite in the construction industry is usually used in form of lightweight aggregate enhancing the insulating performance of concrete. this paper focuses on integration of fine perlite into the cement matrix and possible replacement of the cement binder in the composition of the material. the macromechanical performance of the modified paste is tested on specimens with 5, 10, 15 and 20 % fine perlite substitution and pure cement sample. to distinguish the effect of the perlite on the microstructural level, pure cement material and specimen containing 10 % of fine perlite are investigated by the electron microscopy. furthermore, the mechanical properties of individual phases are examined and compared on same samples by instrumented indentation. the presented results enabled estimation of fine perlite impact on the macro and microscopic performance of the material. keywords: perlite, sem, nanoindentation, mechanical properties. 1. introduction reduction of energy consumption and co2 emissions production is the important challenge in the concrete industry at the present time, thus the focus on the utilization of different waste material partly replacing cement binder plays the key role in the field of research [1]. among supplementary materials perlite represents an expanded natural material produced through loosing water during heating of source material hydrous volcanic glass. most frequently is perlite with its porous structure used as a filler in the lightweight concretes. moreover, it also improves concrete insulation properties and fire resistance. processing of the crude natural volcanic material and the production of perlite lead into creation of perlite fines currently considered as the waste material [2]. the quality of concrete containing fine perlite is possible to asses from micro as well as from macro aspects. the most important phenomenon observable on the microscale is pozzolanic activity of perlite fines since creation of chemical bond between perlite particles and surrounding binder is crucial in developing proper mechanical properties of the cementitious mixture. the second important parameter is the total volume of fine perlite in cement which influences the mechanical behavior of cement composite. yu et. al [3] determined the most effective perlite content equal to 15 % since it significantly improved the compressive strength of concrete (about 16 mpa higher than in the case of concrete without perlite fines addition). similar results obtained rózycka and pichór [4] during testing of perlite waste addition effect on the properties of autoclaved aerated concrete. the compressive strength didn´t significantly changed up to 30 % of volume addition of perlite waste particles. on the contrary oktay et. al [5] determined that the highest compressive strength showed the concrete specimens with no addition of expanded perlite. the results mentioned above [3–5] confirms the importance of chemical availability of perlite particles resulting in creation of interface between perlite and surrounding binder. hence, this paper is aimed at the investigation of macroscopic and microscopic mechanical properties of cementitious paste containing different volume (5, 10, 15 and 20 %) of crushed fine perlite particles. the results of macroscopic testing determined samples for microscopical evaluation, where specimen with 10 % substitution of perlite is compared to pure cement paste. the microstructure is observed implementing the scanning electron microscope (sem), which enables defining different chemical phases and their interfaces in the material. the micromechanical properties of individual phases are determined from histograms of the results provided by instrumented indentation. 2. materials and methods the macroscopic specimens were prepared from ordinary portland cement cem i 42.5r mixture with water to cement ratio 0.35. the composition according to declaration of fine perlite producer is summarized in table 1. the specimens were placed in 40× 40× 160mm forms, cast out 24 hours after mixing and stored until 28 days of total age in water 12 http://dx.doi.org/10.14311/app.2017.7.0012 http://ojs.cvut.cz/ojs/index.php/app vol. 7/2017 to prevent the carbonation. the sets of specimens are denoted as modcem x.xx, where x.xx stands for the fine perlite percentage substitution and specimen modcem 0.00 served as reference sample (pure cement paste). the residues from macroscopic testing were embedded in epoxy resin and used for microscopic investigation. the samples were sectioned by diamond cut-off wheel and the surface grinded and polished using silica-carbon papers (grid roughness p1200, p2400 and p4000 according to european p-grade system) and the 3 µm diamond suspension. chemical content granulometry sio2 min 66 % > 0.2 mm max 10 % al2o3 max 18 % > 0.1 mm min 50 % fe2o3 max 3 % < 0.1 mm max 50 % cao + mgo max 6 % na2o + k2o max 8 % table 1. composition of used fine perlite. 2.1. macromechanical testing to determine macromechanical properties i.e. bending strength and compressive strength, 6 macroscopic specimens (40× 40× 160mm dimensions) were tested from each type of cement mixture. tensile stresscapacity was determined from load-controlled threepoint bending test with support spacing of 100 mm (l) from the equation 1, where ft is maximum applied force causing bending moment, b and h are dimensions of sample cross-section. σt = 3 2 ftl bh2 (1) compressive strength of samples was determined by load-controlled compression test with pressure pad area of 40×40mm and calculated according the equation 2, where fc is maximum applied compressive force and a is the pressure pad area (40× 40 mm). σc = fc a (2) figure 1. macroscopic tests setting – three-point bending (left), compression (right). 2.2. sem sem investigation was performed in mira ii lmu (tescan corp., brno) on polished specimens coated with thin layer of carbon necessary to ensure proper conductivity of the surface. working distance of the microscope was set closely to 15 mm and accelerating voltage was set to 15 kv to provide good signal. the micrographs were acquired at different magnifications. each specimen was investigated at first at the magnification 600× to gain good overview and than at higher magnifications (above 1200×) for better study of different phases in detail. sem micrographs acquired with back scattered electron detector (bse) provide information about distribution and chemical composition of different phases since back scattering coefficient strongly depends on the atomic number z. 2.3. nanoindentation to determine micromechanical properties, instrumented grid indentation using nanoindenter ti 750 serie (hysitron inc.) was performed on the polished specimens with high surface quality. in this technique mechanical properties are calculated from the unloading part of the force-displacement dependency diagram. one of the most important parameter which can be directly calculated is hardness h defined as pressure the maximum force (pmax) under the contact area of the tip (ac). h = pmax ac (3) the reduced (effective) elastic modulus (er) of measured volume of the material follows from the relationship between the unloading stiffness (s) and contact area with respect to the probe geometry (β) [6]. er = √ π 2β s√ ac (4) a grid indentation of 10× 10 pattern with equally spaced indents (30 µm separation) was used in order to cover sufficient area containing all material phases. the load-controlled protocol of indentation consisted of loading holding unloading parts lasting 5 25 5 seconds respectively and reaching maximum applied force of 2.5 mn. to obtain proper statistical set of measured data, four grid indentations were performed on different places of each sample. mechanical properties of individual phases in the heterogeneous cement paste can be obtained from the histograms with equally spaced bins of the examined property by statistical deconvolution [7, 8]. in this paper, we assess the properties of individual phases based on difference between histograms of indentation results of modified cement paste (modcem 0.10) and referential specimen (modcem 0.00). 13 v. hrbek, v. koudelková, p. padevět, p. šašek acta polytechnica ctu proceedings 3. results 3.1. three-point bending, compression the results of macromechanical testing (table 2) correspond with results presented by oktay et. al [5], i.e. referential pure cement samples reach the highest values of compressive (81.89± 9.68 mpa) and tensilestress capacity (1.50 ± 0.83 mpa). mod/ref value present deviation of specimen mechanical property from referential sample. at this preliminary stage of research, perlite modification of cement paste with similar macromechanical performance was in focus. according to table 2, specimens containing 10% of fine perlite substitution (modcem 0.10) exhibit lowest decrease of both compressive and tensile-stress capacity (about 8% and 4% respectively). this led to selection of modcem 10 sample for microscopic investigation of the material. compressive strength specimen σc [mpa] denotion mean stat.dev. mod/ref modcem 0.00 81.89 9.68 1.00 modcem 0.05 64.01 13.28 0.78 modcem 0.10 76.04 10.84 0.93 modcem 0.15 64.69 5.42 0.79 modcem 0.20 70.80 11.32 0.87 tensile strength specimen σt [mpa] denotion mean stat.dev. mod/ref modcem 0.00 1.50 0.83 1.00 modcem 0.05 1.36 0.63 0.91 modcem 0.10 1.45 0.52 0.97 modcem 0.15 1.57 0.34 1.05 modcem 0.20 1.56 0.30 1.04 table 2. macromechanical test results. 3.2. sem investigation the sem equipped with energy dispersive x-ray detector (edx) enables to distinguish different phases in the studied cementitious materials (modcem 0.00, modcem 0.10). based on the bse micrographs and epma (electron probe microanalysis using edx) was possible to determine five phases clinker, csh gels, portlandite (calcium hydroxide ch), pores and perlite particles (see in the figure 2). clinkers in the gray-scaled bse micrographs represent the most intensive particles because of their dense structure and high iron and calcium content. around clinker particles is possible to determine darker well defined area called high density calcium-silica-hydrate gel (hd-csh) as a result of progressive slow hydration of non-hydrated clinker cores. the hd-csh gel represent the first one modification of csh, the second one is low density csh gel (ld-csh) with more opened porous structure. figure 2. bse micrograph showing microstructure of cement paste with perlite addition. the size of perlite was variable from approximately 200 µm of undamaged whole particles to perlite fines i.e. internal lamella of perlite with various size. the interface between undamaged perlite particles and surrounding cement matrix was sharp and didn’t show in most cases any chemical reaction detectable at microscale in the sem (figure 4). nevertheless, according to literature perlite fines are considered as good pozzolanic material [3]. in our investigation chemical reaction between crushed internal lamella and cement was observed in the specimen modcem 0.10. the evident structural disintegration coupled with decreasing content of si in affected perlite lamella is demonstrated in the figure 3. figure 4. bse micrograph showing microstructure of cement paste with perlite addition. 14 vol. 7/2017 figure 3. sem – eds of integrated perlite fines – si content decrease and detail of lamella integration. the decreasing tendency of si content can be also presented on epma with edx analysis results of perlite lamella and cement matrix boundary. figure show positions of testing and table describes weight percentage containment (wpc) of individual elements in each position of interest. position 4 with si containment 10.55% of weight correspond to csh phases and position 8 (si wpc 16.31%) to pure perlite [9]. figure 5. epma – edx of perlite lamella and cement. pos 8 7 6 5 4 ca 9.80 27.61 23.49 23.25 23.91 si 16.31 15.38 13.96 11.39 10.55 al 3.11 2.88 3.34 2.27 4.22 mg 0.05 0.06 0.23 0.19 0.21 k 4.09 1.75 4.29 4.65 5.28 na 0.76 1.26 1.12 0.80 0.73 s 0.30 0.40 0.36 0.71 0.42 fe 1.17 0.51 0.86 0.82 1.22 o 64.42 50.15 55.85 55.91 53.47 table 3. epma – edx chemical elements analysis (weight percentage containment in %). 3.3. instrumented indentation the previously described phases of the material (ld csh, hd csh, ch and clinker) and their mechanical properties (reduced modulus) can be observed in the indentation histogram. the ld csh forms a significant peak close to 30 gpa, which is consistent with previous research (21.7 ± 2.2 gpa [7, 8, 10]). measured indentation modulus of the hd csh phase is close to 40 gpa (occurrence of peaks in range 35 to 50 gpa) and the ch reaches up to 60 gpa, according to the figure 6. more accurately, using statistical deconvolution, reduced modulus estimation reach 27.34± 2.56 for ld csh phase, 39.29± 4.11 gpa for hd csh phase and 61.54± 4.30 gpa for ch phase. compare to phase reduced modulus measured on samples with w/c ratio 0.5 (29.4± 2.4 gpa), hd csh phase reaches peak of indentation modulus naturally higher due to lower w/c ratio. the clinker phase is not in focus of this study due to its high stiffness and interference with other “low stiffness” phases, which misrepresent the result of indentation (collected data above 90 gpa are therefore excluded). figure 6. modcem 0.00 – reduced modulus histogram. figure7 represents the indentation data measured on specimen modcem 0.10. significant peaks of the cementitious matrix are slightly shifted to lower values due to the interaction of fine perlite residues to the composite structure and formation of “low stiffness" phase. the formation is not yet known to authors and will be subjected to further investigation. the 15 v. hrbek, v. koudelková, p. padevět, p. šašek acta polytechnica ctu proceedings individual phases of the cement form the peaks around 25 gpa for ld csh phase, 33 gpa for hd csh and 49 gpa in case of ch phase. the young’s modulus of expanded perlite is assumed to be equal 20 gpa [11, 12], which correspond to the peak found in the histogram. there is very significant peak formed around 10 gpa, which the authors have not been able to identify yet. the lower indentation modulus of all phases in modified cement paste also explains the decrease of macroscopic performance of the composite. figure 7. modcem 0.10 – reduced modulus histogram. 4. conclusions cementitious composite with substitution of fine perlite was investigated on macro and microscopical level. the macro mechanical testing of specimens showed slight decrease of material strength for all cases of cement replacement. the lowest decrease was observed on modcem 0.10, which was thus tested on microscopical level. the investigation of microstructure with sem proved the interaction of fine perlite lamella with cement matrix and integration of whole perlite particles. the instrumented indentation enabled to identify impact of cement paste modification due to comparison of referential data and reduced modulus of phases in specimen modcem 0.10. more detailed interaction of fine perlite and cement composite will be further researched. list of symbols σt bending strength [mpa] ft maximum applied force causing bending moment [n] b specimen cross section width [m] h specimen cross section height [m] σc compressive strength [mpa] fc maximum applied compressive force [n] a pressure pad area [m−2] l spacing of supports [m] h hardness [gpa] pmax maximum force [pa] ac contact area under the tip [nm−2] er reduced elastic modulus [gpa] β probe geometry constant [–] s stiffness [nm−1] acknowledgements the research has been supported by czech science foundation (project no. p105/12/g059). references [1] paris j.m., roessler j.g., ferraro c.c., deford h.d., townsend t.g., a review of waste products utilized as supplement to portland cement in concrete. j clean product 121:1 – 18, 2016. doi: 10.1016/j.jclepro.2016.02.013 [2] kotwica l., pichór w., nocún-wczelik w., study of pozzolanic action of ground waste expanded perlite by means of thermal methods. j therm anal calor 123:607 – 613, 2016. doi: 10.1007/s10973-015-4910-8 [3] yu l.h., ou h., lee l.l., investigation on 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10.1016/j.compositesb.2016.02.034 16 acta polytechnica ctu proceedings 7:12–16, 2017 1 introduction 2 materials and methods 2.1 macromechanical testing 2.2 sem 2.3 nanoindentation 3 results 3.1 three-point bending, compression 3.2 sem investigation 3.3 instrumented indentation 4 conclusions list of symbols acknowledgements references 331 acta polytechnica ctu proceedings 2(1): 331–333, 2015 331 doi: 10.14311/app.2015.02.0331 the golden age of cataclysmic variables and related objects ii concluding address f. giovannelli1 1inaf istituto di astrofisica e planetologia spaziali, area di ricerca di tor vergata, via fosso del cavaliere, 100 i00177 roma, italy corresponding author: franco.giovannelli@iaps.inaf.it abstract before to conclude officially this workshop — far from me the idea to attempt some concluding remarks already dealt at the meeting with various burning by joseph patterson, mariko kato, dmitry bisikalo, and rené hudec —, i would like to comment few highlights coming out from our fruitful week of discussions about the golden age of cataclysmic variables and related objects ii, without any pretension of completeness. keywords: cataclysmic variables and related objects photonic astrophysics. undoubtedly the advent of spacecrafts gave a strong impulse to astronomy; starting roughly from middle 1970ies almost all the electromagnetic spectrum was continuously surveyed by the many space experiments. the cataclysmic variables (cvs) historically were the first systems for triggering the studies of the accretion disk around white dwarfs (wds) starting from the 1960’s with the schools of warsaw (poland) and cambridge (uk). however, they lost rapidly their primeval importance because of the advent of the first x-ray space experiments that, with their limited sensitivity, were mostly detecting x-ray binary systems (xrbs) that showed x-ray emissions abundantly over the thresholds of their detectors. this thanks to the presence of neutron stars or black holes as companions of the optical low–mass or high–mass stars. the x-ray emission of cvs is about 2–3 orders of magnitude lower than that of xrbs. thus the bulk of cvs observations was coming for long time from optical and uv regions, and sometimes from ir and seldom from radio bands. in the last decade results coming from the new generation satellites, especially in the hard x–ray and γ– ray regions, renewed the interest of scientific community about cvs. this, together with the new developments in searching for the progenitors of type ia supernovae was the main reason for an new explosion of interest about cvs. during this week of a deep discussion about cvs and related objects, mostly through the physical processes occurring inside those systems, has shown the most powerful way for a better and faster development of our knowledge of the cataclysmic processes, rather usual in the universe. among the many experimental and theoretical results discussed during this workshop, i would like to remark one important old idea developed by vladimir lipunov in 1980’s, that, in my opinion, will stress the future of cvs physics. figure 1: lipunov’s diagram for gravimagneti rotators calculated for 1 m� white dwarf. the positions of the polars am her and ae aqr, and the intermediate polars dq her, ei uma, and ss cyg are marked (after lipunov, 1987). this is the physical description of cvs as gravimagnetic rotators (lipunov, 1982, 1987). in this way the behaviour of cvs is completely determined by the spin period of the wd and the gravimagnetic parameter y = ṁ/µ2, where ṁ is the mass accretion rate onto 331 http://dx.doi.org/10.14311/app.2015.02.0331 f. giovannelli the gravimagnetic rotator having mass m, and µ is its magnetic moment. this is valid also for all the compact objects, like neutron stars and black holes. in this way, the behavior of cvs is completely independent of the optical phenomena which until now have been those which prevailed in cataloging such systems in different classes. figure 1 shows a part of lipunov’s diagram where polars (pcvs), intermediate polars (ipcvs) and non–magnetic cvs (nmcvs) are situated. as discussed by giovannelli & sabau-graziati (this workshop), it appears evident that the most suitable approach for studying cvs from a physical point of view is to consider them as gravimagnetic rotators. the detection of several sw sex systems having orbital periods inside the so-called ’period gap’ opens a new interesting problem about the continuity in the evolution of cvs. are the ipcvs and pcvs smoothly connected via the sw sex-like systems placed just in between? in order to fully understand the emission properties and evolution of cvs, the mass–transfer process needs to be clearly understood, especially magnetic mass transfer, as well as the properties of magnetic viscosity in the accretion discs around compact objects. consequently, the investigation on the magnetic field intensities in wds appears crucial in understanding the evolution of cvs systems, by which it is possible to generate classical novae (e.g., isern et al., 1997) and type-ia supernovae (e.g., isern et al., 1993). in those catastrophic processes the production of light and heavy elements, and then the knowledge of their abundances provides strong direct inputs for cosmological models and cosmic ray generation problems. after this workshop it appears evident, once more, the importance of multifrequency astrophysics. however, there are many problems in performing simultaneous multifrequency, multisite, multiinstrument, multiplatform and multienergy measurements due to: i) objective technological difficulties; ii) sharing common scientific objectives; iii) problems of scheduling and budgets; iv) politic management of science. during this fruitful workshop, we hope to have demonstrated once more the “vulcano theorem” enunciated in 1984 in my concluding address: it is possible to develop science seriously even if smiling. but, as you probably suspected, this workshop has been organized under ”peaceful surroundings”. therefore, i would like to mention and to support tolstoy’s philosophy. ”think again”, was written by tolstoy in 1904, at the beginning of the russian-japanese war. the human conditions described by tolstoy in the war are ”like spiders in a glass”. this is a pamphlet that is not only against carnage...: ”again the suffering that benefits nobody, again the lies, and again the universal process of stupidity, the turning of men into beasts...” ...but that is also against the racism and the hypocrisy of the educated: ”scholars ... deal extensively with the laws of the migration of peoples, the relationship between the white and the yellow race, between buddhism and christianity, and according to their deductions and considerations justify the killing of men ...”. ... it shows unequivocally the tolstoyan base of gandhi’s non-violence doctrine: ”i cannot act in any other way than god requires of me, and therefore, as a man, i cannot take part in any war, neither directly nor through a third party, nor by giving orders, nor by cooperating in any form, nor by encouraging doing it: i cannot, i will not and i will not do it.” and finally, i would like to conclude with few wonderful words of dr daisaku ikeda (2001) – president of the soka gakkai international (sgi) – reported in the booklet for today and tomorrow the thought of 30th may: “the one who has many friends has greater opportunities for growth. in this way, one both makes society a better place, and lives happier and more satisfied. in all cases, human relations, the inter-personal interaction and communication are of vital importance. we must establish and nurture friendship and contacts with many people, both in our environment, and in society in general. in this manner our life will open up and will flourish”. we could go back to early childhood when we were as the ”little prince”. one sees clearly only with the heart. what is essential is invisible to the eye. (from ”the little prince” by antoine de saint exupéry). acknowledgement it is my pleasure to thank: the scientific organizing committee (dmitry bisikalo, massimo della valle, rené hudec, lola sabaugraziati, giora shaviv, paula szkody); the informatic operator (francesco reale alias figaro, the factotum) for helping in informatics and solving all related problems during the preparation and development of the workshop and in palermo, at the registration desk; 332 the golden age of cataclysmic variables and related objects ii concluding address the executive secretary daniela giovannini, who smoothed all the inconveniences occurred at the registration desk with professionalism and moreover with her nice smile. daniela giovannini (biologist) said goodbye to everybody and left in hurry tuesday afternoon because of the start of an important experiment in her institute (cnr/ibcn) about “treatment with a new generation of adjuvant to enhance the human immune response against the major infections”; the directors of the inaf-istituto di astrofisica e planetologia spaziali, roma, italy; intadepartamento de cargas utiles y ciencias del espacio, madrid, spain; ras-institute of astronomy, moscow, russia; inaf-osservatorio astronomico di capodimente, napoli, italy; ascr/astronomical institute, ondřejov & czech technical university, prague, czech republic; technion-department of physics, haifa, israel; department of astronomy, university of washington, seattle, wa, usa; the actresses lisa colosimo and flavia giovannelli; the violinist alessandro perpich and pianist valentina usai; the staff of the splendid hotel la torre. on behalf of the soc and loc, i am pleased to thank all participants and especially the speakers for their active contributions in rendering this workshop updated with their talks, alive with their discussions, and friendly with their attitudes. a special thank to the three colleagues and friends (joseph patterson, dmitry bisikalo, mariko kato, and rené hudec) who kindly accepted the not easy task of making the concluding remarks of the workshop. finally, on behalf of all participants, i would like to express my warm tanks to the chef, mr daniele inzerillo, who prepared for us a large number of delicacies. i hope to meet all of you once again during our next palermo workshop. references [1] giovannelli, f., sabau-graziati, l.: 2015, this volume. [2] ikeda, d.: 2001, for today and tomorrow the thought of 30th may, edizioni esperia. [3] isern, j., hernanz, m., garćıa-berro, e.: 1993, in white dwarfs: advances in observation and theory, m.a. barstow (ed.), kluwer academic publ., dordrecht, holland, nato asi ser., c403, 139. [4] isern, j., hernanz, m., abia, c., josé, j.: 1997, in frontier objects in astrophysics and particle physics, f. giovannelli & g. mannocchi (eds.), italian physical society, editrice compositori, bologna, italy, 57, 113. [5] lipunov, v.m.: 1982, ap&ss, 85, 451-457. doi:10.1007/bf00653467 [6] lipunov, v.m.: 1987, ap&ss, 132, 1-51. doi:10.1007/bf00637779 333 http://dx.doi.org/10.1007/bf00653467 http://dx.doi.org/10.1007/bf00637779 acta polytechnica ctu proceedings doi:10.14311/app.2015.1.0001 acta polytechnica ctu proceedings 2:1–7, 2015 © czech technical university in prague, 2015 available online at http://ojs.cvut.cz/ojs/index.php/app reinforced encoding for planning as sat tomáš balyo∗, roman barták, otakar trunda department of theoretical computer science and mathematical logic, faculty of mathematics and physics, charles university, malostranske namesti 2/25, praha 1, czech republic ∗ corresponding author: biotomas@gmail.com abstract. solving planning problems via translation to satisfiability (sat) is one of the most successful approaches to automated planning. we propose a new encoding scheme, called reinforced encoding, which encodes a planning problem represented in the sas+ formalism into sat. the reinforced encoding is a combination of the transition-based sase encoding with the classical propositional encoding. in our experiments we compare our new encoding to other known sas+ based encodings. the results indicate, that he reinforced encoding performs well on the benchmark problems of the 2011 international planning competition and can outperform all the other known encodings for several domains. keywords: planning, satisfiability, encoding. 1. introduction planning is the problem of finding a sequence of actions – a plan, that transforms the world from an initial state to a state that satisfies some goal conditions. the world is fully-observable, deterministic and static (only the agent we make the plan for changes the world). the number of possible states of the world as well as the number of possible actions is finite, though possibly very large. we will assume that the actions are instantaneous (take a constant time) and therefore we only need to deal with their sequencing. actions have preconditions, which specify in which states of the world they can be applied, as well as effects, which dictate how the world will be changed after the action is executed. one of the most successful approaches to planning is encoding the planning problem into a series of satisfiability (sat) formulas and then using a sat solver to solve them. the method was first introduced by kautz and selman [1] and is still very popular and competitive. this is partly due to the power of sat solvers, which are getting more efficient year by year. since then many new improvements have been made to the method, such as new compact and efficient encodings [2–5], better ways of scheduling the sat solvers [3] or modifying the sat solver’s heuristics to be more suitable for solving planning problems [6]. in this paper we present a new encoding scheme. it is inspired by the sase transition-based encoding [2], which was the first sat encoding based on the sas+ planning formalism. the motivation for our work is to make the sase encoding more robust by incorporating the strengths of older encoding schemes. we will prove the correctness of our encoding and compute an upper bound on the size of the encoded formula. in the experimental section of the paper we compare our new encoding to other sas+ encodings on benchmark problems from the 2011 international planning competition (ipc) [7]. 2. preliminaries in this section we give the basic definitions of satisfiability, and planning with parallel plans. 2.1. satisfiability a boolean variable is a variable with two possible values true and false. a literal of a boolean variable x is either x or ¬x (positive or negative literal). a clause is a disjunction (or) of literals. a clause with only one literal is called a unit clause and with two literals a binary clause. an implication of the form x ⇒ (y1 ∨ · · · ∨ yk) is equivalent to the clause (¬x∨y1∨· · ·∨yk). a conjunctive normal form (cnf) formula is a conjunction (and) of clauses. a truth assignment φ of a formula f assigns a truth value to its variables. the assignment φ satisfies a positive (negative) literal if it assigns the value true (false) to its variable and φ satisfies a clause if it satisfies any of its literals. finally, φ satisfies a cnf formula if it satisfies all of its clauses. a formula f is said to be satisfiable if there is a truth assignment φ that satisfies f . such an assignment is called a satisfying assignment. the satisfiability problem (sat) is to find a satisfying assignment of a given cnf formula or determine that it is unsatisfiable. 2.2. planning in the introduction we briefly described what planning is, in this section we give the formal definitions. we will use the multivalued sas+ formalism [8] instead of the classical strips formalism [9] based on propositional logic. a planning task π in the sas+ formalism is defined as a tuple π = {x,o, si , sg} where • x = {x1, . . . , xn} is a set of multivalued variables with finite domains dom(xi). 1 http://dx.doi.org/10.14311/app.2015.1.0001 http://ojs.cvut.cz/ojs/index.php/app t. balyo, r. barták, o. trunda acta polytechnica ctu proceedings • o is a set of actions (or operators). each action a ∈ o is a tuple (pre(a), eff(a)) where pre(a) is the set of preconditions of a and eff(a) is the set of effects of a. both preconditions and effects are of the form xi = v where v ∈ dom(xi). • a state is a set of assignments to the state variables. each state variable has exactly one value assigned from its respective domain. we denote by s the set of all states. si ∈ s is the initial state. sg is a partial assignment of the state variables (not all variables have assigned values) and a state s ∈ s is a goal state if sg ⊆ s. an action a is applicable in the given state s if pre(a) ⊆ s. by s′ = apply(a, s) we denote the state after executing the action a in the state s, where a is applicable in s. all the assignments in s′ are the same as in s except for the assignments in eff(a) which replace the corresponding (same variable) assignments in s. if p = [a1 . . . ak] is a sequence of actions, then apply(p, s) = apply(ak, apply(ak−1 . . . apply(a2, apply(a1, s)) . . . )). a sequential plan p of length k for a given planning task π is a sequence of k actions p such that sg ⊆ apply(p, si). 2.3. parallel plans a parallel plan p with makespan k for a given planning task π is a sequence of sets of actions (called parallel steps) p = [a1, . . . , ak] such that e(a1)⊕· · ·⊕e(ak) is a sequential plan for π, where e is an ordering function, which transforms a set of actions ai into a sequence of actions e(ai) and ⊕ denotes the concatenation of sequences. let us denote by sj the world state in between the parallel steps aj and aj+1, which is obtained by applying the sequence e(aj) on sj−1, i.e., sj = apply(e(aj), sj−1) (except for s0 = si). in this paper we will use the ∀-step parallel planning semantics [10], which requires that each action a ∈ aj is applicable in the state sj , the effects of all actions are applied in sj+1 and all possible orderings e of the sets aj make valid sequential plans (hence the name ∀-step semantics). to ensure, that each ordering of the sets of actions in a parallel plan leads to a valid sequential plan, it is sufficient to check that the actions in each set are pairwise independent [3]. we say that two actions a1 and a2 are independent if they do not share common variables, i.e., scope(a1) ∩ scope(a2) = ∅, where scope(a) ⊆ x is a set of all state variables that appear in pre(a) and eff(a). note, that the pairwise independence of actions is a sufficient but not a necessary condition for the parallel steps in a ∀-step semantics plan, as the following example demonstrates. example 1. let a1 and a2 be two actions such that pre(a1) = pre(a2) = {x = 1}, eff(a1) = {y = 2}, and eff(a2) = {z = 2}. clearly, a1 and a2 are not sp1 planningassat (π) sp2 k := 0 sp3 repeat sp4 k := k + 1 sp5 f := encodetaskwithmakespan(π, k) sp6 until issatisfiable(f ) sp7 p := extractplan(getsatassignment(f )) sp8 return p figure 1. pseudo-code of the basic planning as satisfiability algorithm. independent (they share the variable x), however, they can be ordered arbitrarily to achieve the same changes between two given states. the pairwise independence of actions in each step of a parallel plan also implies that they can be executed in parallel (at the same time). 3. finding plans using sat the basic idea of solving planning as sat is the following [1]. we construct (by encoding the planning task) a series of sat formulas f1, f2, . . . such that fi is satisfiable if there is a parallel plan of makespan ≤ i. then we solve them one by one starting from f1 until we reach the first satisfiable formula fk. from the satisfying assignment of fk we can extract a plan of makespan k. the pseudo-code of this algorithm is presented in figure 1 the method was first introduced by kautz and selman [1] and is still very popular and competitive. this is partly due to the power of sat solvers, which are getting more efficient year by year. since then many new improvements have been made to the method, such as new compact and efficient encodings [2–5], better ways of scheduling the sat solvers [3] or modifying the sat solver’s heuristics to be more suitable for solving planning problems [6]. clever ways of solver scheduling [3] can significantly improve the performance of the planning algorithm at the cost of possibly longer-makespan plans. nevertheless, we will use the basic one-by-one scheduling since we are interested only in comparing the properties of encodings, i.e., the construction of the formulas. in the following section we will describe how a formula encoding a planning task can be constructed using our new reinforced encoding. 4. reinforced encoding our goal is (given a planning task π = {x,o, si , sg} and an integer k) to construct a cnf formula fk such that fk is satisfiable only if there is a parallel plan of at most k steps for π. we also want to construct fk in a way, that in the case it is satisfiable, we can easily extract a plan from its satisfying assignment. before we describe the formula, we need to introduce the notion of transitions [2]. 2 vol. 2/2015 reinforced encoding for planning as sat a transition represents a change of a state variable x ∈ x from one value to another from its domain dom(x) or from an arbitrary value to a specific value. there are the following three kinds of transitions. • an active transition changes the value of the variable x from d to e such that d 6= e, {d, e} ⊆ dom(x), it is denoted by δx: d→e. an action a has an active transition δx: d→e if (x = d) ∈ pre(a) and (x = e) ∈ eff(a). • a prevailing transition conserves the value of the variable x (if it was d, then it remains d, d ∈ dom(x)), it is denoted by δx: d→d. an action a has a prevailing transition δx: d→d if (x = d) ∈ pre(a) and there is no assignment related to x in eff(a). • a mechanical transition changes the value of the variable x from any value to the value d (d ∈ dom(x)), it is denoted by δx: ∗→d. an action a has a mechanical transition δx: ∗→d if (x = d) ∈ eff(a) and there is no assignment related to x in pre(a). example 2. the action a with preconditions pre(a) = {x = 1, y = 3} and effects eff(a) = {y = 1, z = 2} has one active transition (δy: 3→1), one prevailing transition (δx: 1→1), and one mechanical transition (δz: ∗→2). the transition set of an action a is the set of all transitions that a has, it is denoted by ∆a. by ∆p we will mean the set of all possible prevailing transitions of a planning task, i.e., ∆p = {δx: d→d | x ∈ x, d ∈ dom(x)}. the set of all transitions ∆ is the union of all the prevailing transitions and the transition sets of all the actions ∆ = ∆p ∪ {∆a | a ∈ o}. by ∆x ⊆ ∆ where x ∈ x we will denote the set of all transitions related to the variable x. the constructed formula fk will have the following three kinds of boolean variables. • action variables ati indicating whether the i-th action is used in the t-th step. we will have one such variable for each action from the description of the planning task and for each of the k parallel steps. • assignment variables btx=v indicating whether the value of the variable x is equal to v in the end of the t-th step (after applying the actions of the t-th step). we will have one such boolean variable for each state variable x ∈ x and each value v ∈ dom(x) for each of the k parallel steps. • transition variables ctδ (or ctx: d→e where δ = δx: d→e) indicating whether the transition δ occurred during the t-th step. we will have one such variable for each δ ∈ ∆ for each of the k parallel steps. now we are ready to define the clauses contained in fk. the following set of binary clauses will enforce, that at most one value is assigned to each state variable x ∈ x. (¬btx=vi ∨ ¬btx=vj ) ∀x ∈ x, vi 6= vj , {vi, vj} ⊆ dom(x), ∀t ∈ {1, . . . , k} (1) the following three kinds of clauses connect the assignment variables with the transition variables. the first set of clauses ensures that each transition δx: d→e (including prevailing transitions δx: e→e and mechanical transitions δx: ∗→e) implies that x = e at the end of each step. (¬ctδx: d→e ∨ btx=e) ∀δx: d→e ∈ ∆,∀t ∈ {1, . . . , k} (2) similarly, we need to add clauses for each transition δx: d→e (except for mechanical transitions) to enforce that x = d holds at the end of the previous step, except for the first step, where we explicitly disable all the transitions that are not compatible with the initial state (using the clauses from equation 8). (¬ctδx: d→e ∨ bt−1 x=d) ∀δx: d→e ∈ ∆, d 6= ∗,∀t ∈ {2, . . . , k} (3) the third kind of clauses is needed to guarantee, that if a variable x has the value v then there is a transition which changes the value of x to v. (¬btx=v ∨ ctδ1 ∨ · · · ∨ ctδm ) ∀x ∈ x, v ∈ dom(x),δ1, . . . , δm transform x to v, ∀t ∈ {1, . . . , k} (4) next we describe the clauses that connect the action variables with the transition variables. if an action a is selected, then all the transitions in its transition set ∆a must be selected as well. this implication is expressed via the following clauses. (¬at ∨ ctδ) ∀a ∈ o,∀δ ∈ ∆a,∀t ∈ {1, . . . , k} (5) also we need to make sure, that transitions (except for prevailing transitions) cannot happen without actions that have them in their transition sets. the following set of clauses will ensure this. (¬ctδ ∨ ats1 ∨ · · · ∨ atsm ) ∀δ ∈ (∆ \∆p), support(δ) = {s1, . . . , sm}, ∀t ∈ {1, . . . , k} (6) by support(δ) we mean the set of indices of actions that have δ in their transition set, i.e., support(δ) = {i | ai ∈ o; δ ∈ ∆ai }. next we need to deal with the interfering actions inside a parallel step. as discussed earlier, it is sufficient to ensure, that only pair-wise independent actions are together in each parallel step. we will achieve this by disabling all pairs of non-independent (interfering) actions. to extrude interfering actions from the parallel 3 t. balyo, r. barták, o. trunda acta polytechnica ctu proceedings steps we will add binary clauses for all the interfering action pairs. (¬ati ∨ ¬atj) ∀ai, aj ∈ o, ai, aj not independent, ∀t ∈ {1, . . . , k} there might be a plenty of interfering action pairs producing a lot of clauses. but if we look carefully at the clauses we have already described, we can see, that most of the interfering actions cannot occur together anyway as we will show via the following notion of compatible actions. two sets of conditions (assignments) are compatible if they assign the same values to the variables they share. two actions a1 and a2 are compatible if the preconditions of a1 are compatible with the preconditions of a2 and also the effects of a1 are compatible with the effects of a2. due to the clauses that enforce, that actions imply their transitions (5) and their connection to assignment variables (2 and 3) together with the clauses that forbid a state variable to have more than one value 1, actions that are not compatible cannot be in a parallel step together. therefore it is enough to suppress compatible interfering action pairs. (¬ati ∨ ¬atj) ∀ai, aj ∈ o, ai, aj compatible and not independent, ∀t ∈ {1, . . . , k} (7) lastly, we add the clauses that enforce the initial state to hold in the beginning and the goal conditions to be satisfied in the end. as for the initial state, we will disable all the transitions that are not compatible with the initial state, i.e., if a variable x has the value d in the initial state, then all the transitions that change x from a value other than d are disabled by using a unit clause. note, that mechanical transitions are always compatible with the initial state (or any other state) and therefore no mechanical transition is disabled. (¬c1 δx: d→e ) ∀δx: d→e ∈ ∆, (x = d) /∈ si (8) to encode the goal conditions we will use unit clauses with assignment variables. fore each goal condition (x = v) ∈ sg we will have a unit clause (bkx=v) which forces the value of x to be v after the last parallel step. (bkx=v) ∀(x = v) ∈ sg (9) the formula fk for the reinforced encoding is a conjunction of the clauses defined in equations 1, 2, 3, 4, 5, 6, 7, 8, and 9. a ∀-step parallel plan can be extracted from any satisfying assignment of fk in the following way. let φ be a satisfying assignment of fk. pφ is a sequence of action sets such that its t-th set contains those actions ai ∈ o for which φ(ati) = true. 4.1. correctness in this subsection the prove the correctness of our encoding, i.e., the following proposition. proposition 1. if the formula fk obtained using the reinforced encoding of the planning task π is satisfied by a truth assignment φ then pφ is a valid ∀-step parallel plan of makespan k for the planning task π. proof. the requirements for the action sets given by the ∀-step semantics are clearly satisfied: • the preconditions of actions in each parallel step are satisfied due to 2, 3, and 5 • the effects are propagated also due to 2, 3, and 5 • the actions can be ordered arbitrarily thanks to 7 it remains to prove that psφ = [e(a1)⊕· · ·⊕e(ak)] is a valid (sequential) plan for π, where ⊕ denotes the concatenation of sequences and e is an arbitrary ordering of an action set. let us observe, that the transitions of the state variables are consistent at each step, i.e., exactly one transition is allowed for each state variable (due to 2, 3, and 1) and a non-prevailing transition cannot happen without an action that has it (thanks to 6). prevailing transitions do not have to be supported by any actions since they are used to preserve the values of the variables that are not changed in the given step by any actions. furthermore, all the transitions between two neighboring parallel steps must be compatible due to 2, 3, 4 and 1. note, that it may happen, that a variable x has no value assigned in the end of a step t (all the btx=v, v ∈ dom(x) are false) and no transition related to the variable is selected (all the ctδ, δ ∈ ∆x are false). however, this can only occur for variables that are not used in the goal conditions or by actions that appear in the t-th step or later. since the action variables imply the proper transition variables thanks to 5, the actions must be applicable if their action variable is true and also the transition connected to the action must happen. thanks to 8 only transitions compatible with the initial state can happen in the first step and because of 2 and 9 only transitions that change the variables to their goal values are allowed in the last step. this fact together with the consistency of the transitions during all the k steps implies the validity of psφ for the planning task π. the reversed implication, which is that if pφ is a valid ∀-step parallel plan of makespan k, then φ satisfies fk, does not hold. this is because pφ may contain a non-independent pair of actions in one of its steps and still be a valid ∀-step plan (see example 1). such a pair of actions would make one of the clauses of type 7 unsatisfied. 4 vol. 2/2015 reinforced encoding for planning as sat 4.2. size of the encoded formula the size of the formulas will of course depend on the parameters of the planning task being encoded. we will use the following quantitative properties of a planning task π = (x,o, si , sg) to compute the upper bounds. • n the number of actions (n = |o|). • v the number of state variables (v = |x|). • d the maximum domain size of any state variable (d = maxx∈x{|dom(x)|}). • p the maximum number of preconditions or effects an action has (p = maxa∈o{|pre(a)|, | eff(a)|}) typically, the number of actions is much higher than the other parameters. from these values we can compute the following upper bounds related to the planning task. • the number of assignments is at most vd. • the number of transitions is at most v(d2 +d) since there are at most d(d− 1) active, d prevailing, and d mechanical transitions for each variable. from these bounds it is apparent, that fk (a formula for makespan k) has at most kn action variables, kvd assignment variables, and kv(d2 + d) transition variables. therefore the total number of boolean variables in fk is k(n+ vd(d+ 2)). now let us compute an upper bound on the number of clauses in fk. we will count separately the number of unit clauses (clauses with one literal), binary clauses (clauses with two literals), and horn clauses (clauses with at most one positive literal). the formula fk obtained by the reinforced encoding is the conjunction of the clauses defined in equations 1, 2, 3, 4, 5, 6, 7, 8, and 9 • there are at most kvd2 clauses of the type 1 – one for each step and variable and two different values from its domain. these clauses are binary and horn. • there are at most kv(d2 + d) clauses of the both type 2 and type 3 – one for each step and transition. these clauses are binary and horn. • there are at most kvd clauses of the type 4 – one for each step and assignment. • there are at most 2knp clauses of the type 5 – one for each step, action and each of its transitions (there are at most 2p transitions connected to each action). these clauses are binary and horn. • there are at most kv(d2 + d) clauses of the type 6 – one for each step and transition. • there are at most kn2 clauses of the type 7 – one for each step, and each pair of compatible interfering actions (at most each pair of actions). these clauses are binary and horn. • there are at most v(d2 + d) clauses of type 8 – one for each transition that is not compatible with the initial state (at most all the transitions). these are unit clauses. • there are at most v clauses of the type 9 – one for each goal condition. these clauses are unit. in total we have k(n2 +2np+4vd2 +4vd)+vd2 +vd+v clauses, from which vd2 + vd+ v are unit clauses and k(n2 + 2np+ 3vd2 + 2vd) are both binary and horn clauses. 5. experimental evaluation to evaluate the performance of our new reinforced encoding, we compared it with three other sas+ based encodings of planning as sat. we ran experiments with a 30 minutes time limit using the following four encodings. • reinforced encoding (reinf). a java implementation of our new reinforced encoding as described in the previous section. • direct encoding (dir). we implemented a simple encoding based on the historically first encoding of planning as sat [1]. we adapted it for the sas+ formalism. this encoding is similar to our reinforced encoding but uses only action and assignment variables. • sase encoding (sase). our java implementation1 of the transition-based sase encoding [2]. this encoding uses only action and transition variables. • r2∃-step encoding (r2∃). the original java implementation of the r2∃-step encoding [5]. this encoding differs from the previous three encoding significantly since it uses a different parallel planning semantics. the r2∃-step encoding allows more actions inside the parallel steps, therefore it often finds plans with much lower makespans. lower makespan indicates that fewer sat solver calls are required to find a plan, however, it does not say anything about its length, i.e., the total number of actions it contains. 5.1. experimental setting to compare the performance of the encodings we created a simple script, which iteratively constructed and solved the formulas for time steps 1, 2, . . . until a satisfiable formula was reached (see figure 1). for each encoding we used the same sat solver – lingeling[11] (version ats). the time limit was 30 minutes for the sat solving part, i.e., the total time the sat solver could spend solving the formulas f1, f2, . . . for each problem instance was 30 minutes. the time required for the generation of f1, f2, . . . is usually negligible compared to the time required to solve them and therefore we will ignore it. hence the overall planning time could exceed the given time limit for a problem instance. 1the original sase implementation cannot be used since it does not support the format of the latest benchmark problems 5 t. balyo, r. barták, o. trunda acta polytechnica ctu proceedings domain dir sase reinf r2∃ barman 4 4 4 8 elevators 20 20 20 20 floortile 16 11 18 18 nomystery 20 10 20 6 openstacks 0 0 0 15 parcprinter 20 20 20 20 parking 0 0 0 0 pegsol 10 6 10 19 scanalyzer 14 12 15 9 sokoban 2 2 2 2 tidybot 2 2 2 2 transport 16 17 18 13 visitall 12 9 10 20 woodworking 20 20 20 20 total 156 133 159 172 table 1. the number of problems (out of 20) in each domain that the encodings solved within the time limit (30 minutes for sat solving). the experiments were run on a computer with intel i7 920 cpu @ 2.67 ghz processor and 6 gb of memory. the benchmark problems of the ipc are organized into domains. each domain contains 20 problems and there are 14 domains which results in a total of 280 problems. the benchmark problems are provided in the pddl format, however, the encodings require input in the sas+ format. we used helmert’s translation tool, which is a part of the fast downward planning system [12], to obtain the sas+ files from the pddl files. the translation is very fast requiring only a few seconds for all domains. 5.2. experimental results the number of solved instances in presented in table 1. looking at the results from the perspective of the domains, we can observe, that the elevators, parcprinter, and woodworking domains are entirely solved by every encoding. on the other hand, the parking domain is so difficult that not even a single problem is solved by any of the encodings. the openstacks domain is very difficult for all but the r2∃-step encoding. the sokoban and tidybot domains are also very hard for all of the encodings, only two of the twenty problems are solved by each encoding. if we compare the encodings, we can observe that the r2∃-step encoding has the highest total number of solved instances followed by our new reinforced encoding. as for the individual domains, the r2∃-step encoding solves strictly more problems than the other encodings in four cases. the reinforced encoding achieves this for three domains, while the direct and sase encoding cannot outperform the other encoddomain dir sase reinf r2∃ barman 121 121 121 84 elevators 190 190 190 85 floortile 302 181 344 169 nomystery 347 119 347 30 openstacks 93 parcprinter 261 261 261 30 parking pegsol 222 131 222 158 scanalyzer 83 61 95 17 sokoban 60 60 60 27 tidybot 14 15 15 6 transport 221 242 262 55 visitall 223 110 146 34 woodworking 68 68 68 33 table 2. the sum of makespans of the plans found within the time limit for each domain. lower makespan means fewer sat solver calls, it does not indicate better plan quality. domains with the same number of solved problems for each encoding are highlighted. ings in any of the domains. the reinforced encoding solves the same number of problems as any other encoding in seven cases. except for the visitall domain, the reinforced encoding is never worse than the direct or sase encoding. looking at the makespans of found plans displayed in table 2 we can observe that the makespans for the r2∃-step plans are indeed significantly lower than the makespans of plans found by the other three encodings. as expected, in the cases when the direct, sase, and reinforced encodings solve all the problems (or solve the same problems) their total makespans are identical. this is due to the fact that these three encodings use the same ∀-step parallel planning semantics. the times required to solve the problems are presented in table 3. if we look at the results for the domains, where each encoding solved the same number of problems, i.e., the highlighted domains, we can notice, that except for the parcprinter and sokoban problems, the runtime of the r2∃-step encoding is much higher than the runtime of the other methods. if we also look at table 2, which contains the total makespan of the found plans, we can deduce, that lower makespan, i.e., fewer sat calls does not necessarily mean faster planning, especially not in the case of the easy domains. nevertheless, for the domains, where r2∃-step significantly outperformed the other methods – openstacks, pegsol, and visitall, the makespans are much lower than the makespans of the other methods, despite the fact, that they solved fewer problems. 6 vol. 2/2015 reinforced encoding for planning as sat domain dir sase reinf r2∃ barman 2041.44 1549.31 1680.53 2999.30 elevators 33.96 55.72 38.10 288.29 floortile 7327.15 2083.57 1206.18 1380.63 nomystery 3798.45 1377.23 1894.65 1927.40 openstacks 2679.68 parcprinter 10.07 28.25 12.15 4.24 parking pegsol 3796.11 2096.72 4971.97 830.72 scanalyzer 1401.21 831.80 1435.19 1118.89 sokoban 592.03 1337.87 857.04 550.18 tidybot 75.68 74.02 118.09 480.85 transport 1404.23 3554.90 3203.62 7418.01 visitall 2380.76 683.25 726.53 14.24 woodworking 2.57 2.04 3.84 138.61 table 3. the time in seconds required to solve all the problems that were solved within the time limit. the presented time is the sum of times the sat solver alone required, formula generation time is not included. domains with the same number of solved problems for each encoding are highlighted. 6. conclusion in this paper we have introduced a new encoding of a planning problem represented in the sas+ formalism into sat. our new encoding performs well on the benchmark problems of the 2011 international planning competition. it can strictly outperform all the other evaluated sas+ encodings in three domains and solve the same number of problems as any other encoding for seven domains out of fourteen. on the remaining four domains our encoding is outperformed by the r2∃-step encoding which uses a different parallel planning semantics. as for future work, we believe that the reinforced encoding can be improved by decreasing the number of its clauses by using a more compact way of encoding of the action interference constraints. acknowledgements the research is supported by the czech science foundation under the contract p103/10/1287 and by the grant agency of charles university under contracts no. 600112 and no. 390214. this research was also supported by the svv project number 260 104. references [1] h. a. kautz, b. selman. planning as satisfiability. in ecai, pp. 359–363. 1992. http://citeseerx.ist.psu. edu/viewdoc/summary?doi=10.1.1.35.9443. 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[7] c. l. lópez, s. j. celorrio, á. g. olaya. the deterministic part of the seventh international planning competition. artificial intelligence 2015. doi:10.1016/j.artint.2015.01.004. [8] c. bäckström, b. nebel. complexity results for sas+ planning. computational intelligence 11:625–656, 1995. doi:10.1111/j.1467-8640.1995.tb00052.x. [9] r. fikes, n. j. nilsson. strips: a new approach to the application of theorem proving to problem solving. artif intell 2(3/4):189–208, 1971. doi:10.1016/0004-3702(71)90010-5. [10] a. blum, m. l. furst. fast planning through planning graph analysis. artificial intelligence 90(12):281–300, 1997. doi:10.1016/s0004-3702(96)00047-1. [11] a. biere. lingeling and plingeling home page, 2015. http://fmv.jku.at/lingeling/. [12] m. helmert. the fast downward planning system. journal of artificial intelligence research (jair) 26:191–246, 2006. doi:10.1613/jair.1705. 7 http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.35.9443 http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.35.9443 http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.182.9561 http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.182.9561 http://dx.doi.org/10.1016/j.artint.2006.08.002 http://aaai.org/ocs/index.php/icaps/icaps09/paper/view/732 http://aaai.org/ocs/index.php/icaps/icaps09/paper/view/732 http://dx.doi.org/10.1109/ictai.2013.131 http://dx.doi.org/10.1016/j.artint.2012.08.001 http://dx.doi.org/10.1016/j.artint.2015.01.004 http://dx.doi.org/10.1111/j.1467-8640.1995.tb00052.x http://dx.doi.org/10.1016/0004-3702(71)90010-5 http://dx.doi.org/10.1016/s0004-3702(96)00047-1 http://fmv.jku.at/lingeling/ http://dx.doi.org/10.1613/jair.1705 acta polytechnica ctu proceedings 2:1–7, 2015 1 introduction 2 preliminaries 2.1 satisfiability 2.2 planning 2.3 parallel plans 3 finding plans using sat 4 reinforced encoding 4.1 correctness 4.2 size of the encoded formula 5 experimental evaluation 5.1 experimental setting 5.2 experimental results 6 conclusion acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2016.5.0051 acta polytechnica ctu proceedings 5:51–58, 2016 © czech technical university in prague, 2016 available online at http://ojs.cvut.cz/ojs/index.php/app criticallity of transportation infrastructure in czech republic jan procházka∗, dana procházková czech technical university in prague, faculty of transportation sciences, praha, czech republic ∗ corresponding author: prochazka@fd.cvut.cz abstract. the paper deals with transportation infrastructure criticality because this quantity determines the state capability to overcome the critical conditions and to ensure the inhabitants survival. the criticality rates for individual types of transportation infrastructure and for the entire transportation infrastructure are determined by data from experts from the areas: transportation; transportation management in the territory; supply chains; public administration; and the integrated rescue system. the experts assessed 14 factors, which have been often used in the developed world countries, from the view of human security and development. the result values and their interpretations were determined by using the multiatribute utility theory. keywords: transportation infrastructure, system of systems, interdependences, criticality, interoperability, serviceability, human survival, state stability. 1. introduction transportation infrastructure belongs to the basic systems that make up the critical infrastructure in the european union, in developed countries and also in the czech republic. we understand the system of transportation in accordance with the current knowledge as a system of systems, i.e. the set of several overlapping systems. the most important property in such type of system is the interoperability of partial systems that depends on interdependences [1]. the interdependences are the cause of different emergent phenomena and cascade failures that cause that the transportation infrastructure does not fulfil its function, i.e. the serviceability of territory at normal, abnormal and critical conditions if it is threatened the human survival and state stability. therefore, it is necessary to manage the transportation infrastructure by the way so infrastructure criticality is acceptable [1]. the subject of this work is to assess the criticality of the transportation infrastructure in the czech republic on the basis of the integral safety concept, which aims are security and development of the territory concerned and its inhabitants. the concept used is based on the knowledge of the reality of the dynamic development of the human system and it considers that human system conditions are normal, abnormal and critical. the capability to overcome the critical conditions, ensuring the survival of humans and to have capability to stabilize the situation we consider crucial, and therefore, we use the degrees of criticality, which are aimed at assessing the aspects associated with the given properties. for the assessment of the criticality we use criteria used in developed countries, multi-objective approach and data from experts whose qualification is certified according to the criteria used in the european union [1]. 2. current knowledge on transportation infrastructure the transportation system, which provides transportation of humans and goods, includes all modes of transportation, which, under the coordination of individual transportation systems work together and create a logistical network [2]. transportation logistics network is currently becoming a very important area of business activities, and not just in a regional and national scale, but also in international relations. the transportation network can be combined with energy as one of the foundations of the economic prosperity of states and organizations. transportation systems for their activities require a considerable amount of funds, namely in investment, operational and administrative domains, which typically create autonomous sub-sectors, the activity of which is mutually coordinated. the characteristic feature of those technological units is their capability to own separate activity, and therefore, they are often considered as closed units, which is projected to the safety concept, and at risk management it is used the simplest approach, i.e. the procedures for a closed system, in which the sources of the risk are only the technical problems and the human factor [2]. infrastructure of transportation network is currently mostly owned by the public sector; however, its development is largely determined by the requirements of private sector. simultaneously it constitutes an important element of the economy and of the citizens living way. transportation network has considerable international political influence, as it allows the international cooperation of individual states and tourism, which contributes to the integration of citizens. the transportation system is built from the separate parts that make up certain units that provide certain 51 http://dx.doi.org/10.14311/app.2016.5.0051 http://ojs.cvut.cz/ojs/index.php/app jan procházka, dana procházková acta polytechnica ctu proceedings circuit activities separately. in the serviceability of territory (creating the space for human lives, i.e. the human system), however, the units are together overlapped, i.e. they create a system of systems [2] [3] [4]. the ground of each unit is a specific type of transportation vehicles, accompanied by complex of operational and administrative procedures. the characteristic feature of these technological units is their specialized capability to separate operation, i.e. the autonomy. the capability to separate operation, however, it is only apparent, the sub units are bound by the rules of their transport logistics, which not only coordinates their activities, but also makes it economically more profitable because it reduces operating costs. on the other hand, however, it is the source of internal dependencies "interdependences" [2] [5]. safety of transportation networks in the past was provided only by a simple guarding. the guarding was mostly limited to surveillance, which should prevent the mostly property-related offences, such e.g. theft of the goods transported, pertinently endangering the personal safety of transported passengers. on the basis of current knowledge, it is the aim of the safety management of the transportation system, which consists of objects and infrastructures, ensuring the security and development of both, the transportation system and transportation system vicinity, because it is the only way how to ensure the safe human system. on the basis of current knowledge, this means that we have to apply the tools of the disciplines that provide safety management of complex systems [1] [2] [5]; i.e. the multi criterial approach based on expert assessment of the possible variants that describe the behaviour of systems of systems designated by different behaviour of individual subsystems [4]. 3. transportation system of the czech republic in the czech republic a transportation network is in essence created from the following areas: railway transport system; road transport system; air transport; and water transport. the main activities of the business in the transport are passenger and freight transport. ensuring the rational operation requires considerable resources – labour, capital, information, as in a modern industrialized society, it is common that the interconnection and operation of transportation networks and the provision of transport services is an important part of near all other activities. for example, the provision of transport services in the major cities is done by public transportation, which is based on mostly coordinated cooperation of road and rail transport, sometimes supplemented by underground transportation; it is basically so called integrated transportation system. in the last few decades, the transportation infrastructure has become a major target of terrorist attacks. in particular, it clearly reflected in the air transport, in which they terrorists originally focused only on the kidnappings, and the destruction of the aircraft in flight [5]. after the attacks of 11 september, 2001 on the trade centre in new york, it turned out that the aircraft may change in the gun with the devastating consequences of the disastrous. from historical practice it is known that natural disasters (in our condition floods, storms, earthquakes and landslides) often caused considerable and practically difficult to quantify damages to parts of the rail and road transport of technological system and the environment. therefore, it can be concluded that transportation systems are particularly vulnerable to attacks and natural disasters. they are very favourite target of the activities of terrorists who by attack on any of transportation systems can cause considerable damage to property and infrastructure devices. it is necessary to recognize that the indirect impacts of attacks and their consequences can even endanger the stability of the economics of the industry, or even the state. from the above analysis it follows that it is necessary to devote to the issue of the destruction of the infrastructure of transportation systems special attention, to define particularly sensitive parts and to search for ways how to reduce the danger of the destruction of important parts, which can cause the interruption of the activity of transportation systems, and by such way to prevent the economic destabilization not only of the relevant business units, but also to other bodies in the industrial areas. if we want to manage the transportation system safety with the aim of ensuring its security and development, so we need to know the priority aspects, on which the achievement of objectives depend and on that we need to focus the attention, i.e. the measures and activities [1] [5]. from the same source it follows that the critical spots in the technological system (object, the infrastructure, plant, facility, territory) are the places where the basic technological processes are under way and for which there are in force specific provisions for the safety under normal, abnormal and critical conditions (e.g. single-level crossing of the railways and roads in the village). table 1 gives the important parts of the transportation system, separated for four of its parts according to criteria of governmental decree no. 2000 col. [6]. 4. method for determining the transportation infrastructure criticality according to the detailed discussion given in [1] [5] the criticality of item means that item has simultaneouslygreat importance and great vulnerability. it means that we work with conflict criteria at the solution of problem. therefore, we use the strategy defined by the keeny and raiffa [7] defined by the mathematical statement "the higher, the worse, i.e. the higher criticality in our case". as it has been already said above, the transportation system as a whole and its basic subsystems are 52 vol. 5/2016 criticallity of transportation infrastructure in czech republic rail transport road transport air transport water transport linear constructions tracked rail network, the electrification network of railways, railway security network the full road network the system of interconnected airports the waterways objects the railway station buildings for traffic management, expedition of passengers and redeployment of goods, the depots of vehicles, railway workrooms, railway bridges, railway tunnels, track divisionmaintenance rail network objects and networks for security techniques, railway workrooms for servicing the locomotives of all kinds and fleet, the power stations of power supply to rail network, fuel stations road bridges, road tunnels, stocks of fuels for road transport purposes, objects for the management of road traffic, gas fuel stations for road vehicles, objects for the management of emergency situations in road network airport runways for takeoff and landing, a network of runways, lighting, passing ways, aircraft stands, buildings for service stations for air transport, buildings for normal preflight preparationinput and general examination, air traffic control objectsairport, regional, the radar station for air traffic control, radar stations for meteorology, computing centre of the airport, the equipment for operational information sharing for regional service, facilities for the sharing of operational information for the management of the activities of the airport, centre for intervention in emergency traffic incidents (accidents...), gas stations and fuel warehouse ports with the devices, the locks, docks, hydrometeorology stations, objects for maintenance of vessels, fuel stations fittings rail vehicleslocomotives and cars, equipment of the rail stations and the tracked rail network, device for controlling the places on the crossing the road and rail transport, the equipment for operational information sharing within the operation of the rail system, facilities for the supply of fuel for vehicles the equipment for operational information sharing with the road network, equipment of centres with technique for solving incidents, buses, freight cars, personal cars aircrafts ships table 1. important parts of transport system 53 jan procházka, dana procházková acta polytechnica ctu proceedings systems of systems, i.e. it goes on several mutually penetrating systems of different nature (technical, organizational, financial, human resources, and legal), and therefore, the criticality assessment cannot be performed from a single indicator, but it is necessary to use a multicriterial approach and information from experts [4]. in accordance with the procedures that are used in the usa, australia, canada etc. [8] [9] [10] [11] [12] [13] there were used the following factors important to the criticality of transportation infrastructure: • a rate of capability of defence, • a rate of vulnerability to attack, • a rate of threat to the health and lives of humans, • a rate of accidents impact on the environment • a rate of cost of exchange or repair, • a rate of exchange time or repair time, • a rate of importance for ensuring the life-saving or emergency functions in the territory, • a rate of importance for ensuring the functions of administration and local government, • a rate of importance for ensuring the functions of army and police, • a rate of redundancy or replacement services, • a rate of importance for ensuring the communication functions, • a rate of impact of frontage failure on the economy of region (state), • a rate of importance of operability, • a rate of importance in the field of symbolic, cultural, etc. with the facts in the documentation of the czech republic transportation system [14] and with the data from 5 experts, selected according to the criteria of the eu given in [1], from the areas: transportation; transportation management in the territory; supply chains; public administration; and the integrated rescue system, there wereobtained verbal assessments of the factors mentioned above. since the main human objective is the human safety and the human development [3], for which the safe transportation infrastructure is vital, the criticality of the transportation infrastructure is understood from the perspective targeted on the human survival. therefore, the verbal assessment was transferred with the help of five experts participating in the panel discussion into the numerical scale that is analogue to the scale using in the risk assessment in the čsn standards [1] – allocate number of points is the following: (1.) if factor does not contribute to the criticality of the transport system (the expected damages are less than 5%), (2.) if factor contributes little to the criticality of the transport system (damages are expected at an interval of 5-25%), (3.) if factor contributes intermediately to the criticality of the transport system (the expected damages are in the interval 25-45%), (4.) if factor contributes significantly to the criticality of the transport system (the expected damages are in the interval of 45-70%), (5.) if factor contributes very significantly to the criticality of the transport system (the expected damages are in the interval of 70-95%), (6.) if factor significantly contributes most significantly to the criticality of the transport system (the expected damage is greater than 95%). the resulting evaluation of criticality for each partial system provided that we give equal weight to all factors, can have the values 0 to 70 and for the entire system having 4 subsystems, provided that their equivalence the criticality values are between 0 and 280. if we again apply approach used in the čsn standards [1], we get the data given in table 2. 5. results – the criticality rates for transport infrastructure the verbal assessments of factors for rail transport, obtained from the experts are the following: (1.) rate of capability of defence – defence of the railway is difficult due to the considerable complexity of rail system. at the attack of terrorists on main parts or at natural disaster there may originate extensive damages that stop the operation of a large part of the system. the human factor and organizational accident can also cause considerable damages. (2.) rate of vulnerability to attack – vulnerability of the railway is significant because some parts of it can be quite simply destroyed extensively (railway stations complexes, bridges, tunnels, etc.). (3.) rate of threat to the health and lives of humans – the health and lives of passengers are threatened by practically in all the accidents of the rail system. (4.) rate of impact of railway accidents on the environment – railway accidents can have significant impacts on the environment, in particular in the transport of fuels, chemicals and other raw materials. removal of contamination of environment is the most difficult and expensive. (5.) rate of cost of exchange or repair – elimination of damages caused by the destruction of the track system can be mostly done without the big cost. expensive there are large constructions repairs of faulty bridges and tunnels. repair or restoration of the destroyed railway vehicles is often expensive. 54 vol. 5/2016 criticallity of transportation infrastructure in czech republic criticality rate score the percentage of the total value individual transport systems entire transport system the extremely high more than 95% more than 66.5 more than 266 very high 70 95% 49 66.5 196 266 high 45 70% 31.5 49 126 196 medium 25 45% 17.5 31.5 70 126 very small 5 25% 3.5 17.5 14 70 insignificant / negligible less than 5% less than 3.5 less than 14 table 2. value scale used for determining the criticality rate (6.) rate of exchange time or repair time – repair of tracks is quite fast. large construction repairs of bridges and tunnels require a longer period of time. (7.) rate of importance for ensuring the life-saving or emergency functions in the territory – in emergency situations the railway is very important and often essential for the provision of emergency activities. (8.) rate of importance for ensuring the functions of administration and local government – for the provision of administrative and self-government the railway is important, but its services for the activities in most cases can be replaced by other means of transport. (9.) rate of importance for ensuring the functions of army and police – to the needs of the army and the police the railroad is necessary. (10.) rate of redundancy or replacement services – backup at the railway is quite high (multiple lines), particularly in the transport of basic industrial raw materials, food, and fuel in larger quantities. very important it is container shipping service linking the various transport systems and container transport requires its own technology. in particular, the important role has container transport in international traffic, which of course, requires the construction of investment-intensive terminals, which can be a sensitive target for terrorists. (11.) rate of importance for ensuring the communication functions – in emergency case, the communication network of railway can be used because it is independent. (12.) rate of impact of frontage failure on the economy of region (state) – disturbed rail traffic system adversely affects the smooth running of the regional or national economy and causes considerable economic losses. (13.) rate of importance of operability – railway operability is very important on both, the national and the international scales. (14.) rate of importance in the field of symbolic, cultural, etc. – .-the level and status of the railway transport system is testimony to the cultural and technical level of the state. the verbal assessments of factors for road transport, obtained from the experts are the following: (1.) rate of capability of defence – protection of the road network is complex because even if the disruption of the custom paths can be quite easily and quickly repair, sensitive parts such as bridges and tunnels it is necessary against terrorist attacks and natural disasters especially consistently to protect, because their damage or destruction often requires a long-term repair, mostly investment-intensive. the human factor and organizational accident can also cause considerable damages. (2.) rate of vulnerability to attack – vulnerability of road network is particularly determined by sensitive parts of system, such as bridges, tunnels, etc. (3.) rate of threat to the health and lives of humans health and lives are threatened by accidents during the operation, the number of which can be in the effective management of road traffic minimised. (4.) rate of impact of road accidents on the environment road transport heavily polluting environment by gases that vehicles emit into the atmosphere. when accidents of vehicles, environmental pollution is caused in particular by spilling fuel and possibly also by transported goods that is spread in nature. (5.) rate of cost of exchange or repair – except the costly repairs of sensitive network places – bridges tunnels etc. it is possible to perform repairs of damages of road network fairly cheaply and in a short time. (6.) rate of exchange time or repair time – outside the repairs of bridges, tunnels and other expensive parts it is possible to perform the repairs of network in a relatively short time. (7.) rate of importance for ensuring the life-saving or emergency functions in the territory – the serviceability of roads is very important, in the case of an emergency it can be solved by the finding a replacement path that avoids the damaged parts. (8.) rate of importance for ensuring the functions of administration and local government – road transport is very necessary. 55 jan procházka, dana procházková acta polytechnica ctu proceedings (9.) rate of importance for ensuring the functions of army and police – road transport is fundamentally necessary. (10.) rate of redundancy or replacement services – there are, in principle, to allow mutual logistic redundancy compensation of individual transport systems. (11.) rate of importance for ensuring the communication functions – communication function can be partially replaced by road transport. (12.) rate of impact of frontage failure on the economy of region (state) – the status and level of road system quality has a significant influence on the economy. (13.) rate of importance of operability – operability of the road system is very important and fundamentally affects all activities in the region and state. (14.) rate of importance in the field of symbolic, cultural, etc. – the level of road transport and the road system is indicative of the level of the state and its tradition. the verbal assessments of factors for air transport, obtained from the experts are the following: (1.) rate of capability of defence – defence of air transport is very exacting and difficult, because the air transport system is currently the most complex of all transport systems. the accidents in aviation occur from reasons, the technical reasons and the meteorological reasons. however, the air transport is also a hot target for terrorists. the human factor and organizational accident can also cause considerable damages. (2.) rate of vulnerability to attack – the vulnerability is high. preparation of the aircraft for operation is very challenging. therefore, the flight and technical staff is carefully selected and trained. defence against terrorist attacks is performed while the intensely, but not always successfully. (3.) rate of threat to the health and lives of humans – in air accidents it is occurred injury and death of crew and passengers. therefore, the prevention of accidents in the aviation service is very important. (4.) rate of impact of road accidents on the environment – air accidents are almost always accompanied by considerable local damage to the environment, but mostly they are hitting a small part of the territory. (5.) rate of cost of exchange or repair – replacement of the damaged aircraft technique is almost always very costly financially and time often challenging. repairs require a considerable supply of spare parts and the repair work of specialists. the repair needs to be carried out at the manufacturers or authorised service companies. (6.) rate of exchange time or repair time – the time length of the repair is different according to the nature of the fault. (7.) rate of importance for ensuring the life – saving or emergency functions in the territory – aviation equipment has a significant position and it is almost necessary in response to real crisis situations of all kinds. (8.) rate of importance for ensuring the functions of administration and local government – it is important only conditionally. (9.) rate of importance for ensuring the functions of army and police – it is very necessary. (10.) rate of redundancy or replacement services – air transport has a conditional redundancy that from its independence on surface networks enables to solve specific tasks. (11.) rate of importance for ensuring the communication functions – in some exceptional cases, air transport is the only viable solution. (12.) rate of impact of frontage failure on the economy of region (state) – air transport is of great importance for the economy, because outside of its own air traffic it is driving article of industry. (13.) rate of importance of operability – it is very important. (14.) rate of importance in the field of symbolic, cultural, etc. – air transport in the czech republic has a significant position in the world. the aviation industry of the czech republic has a long tradition, the products have been exported into all world. the verbal assessments of factors for water transport, obtained from the experts are the following: (1.) rate of capability of defence – with regard to the limited scope of the network of waterways in the czech republic this transport system can be probably successfully protected. the human factor and organizational accident can also cause damages. (2.) rate of vulnerability to attack – the vulnerability of the network of waterways, in particular, determines the sensitive parts – locks, weirs, etc., that ensure the navigability of the waterways. (3.) rate of threat to the health and lives of humans – the health and lives of the residents and workers in the transport sector are threatened to the extent that is usual in other sectors of industry. (4.) rate of impact of road accidents on the environment – in particular the pollution of waterways by fuels, which adversely affects the environment and the biosphere in the water and on the banks of streams. (5.) rate of cost of exchange or repair – ship accident are not common, there is not usually a large extent of the damages. 56 vol. 5/2016 criticallity of transportation infrastructure in czech republic factor rail transport road transport air transport water transport entire transport 1 5 4 5 2 16 2 5 4 5 3 17 3 4 3 4 3 14 4 4 3 3 2 12 5 4 3 5 2 14 6 3 2 4 3 12 7 5 5 4 1 15 8 3 3 2 2 10 9 5 5 5 2 17 10 3 2 3 1 9 11 3 2 3 1 9 12 5 4 4 4 17 13 4 4 4 2 14 14 3 2 3 2 10 all factors 56 46 54 30 186 table 3. rates of criticality of the transportation infrastructure in the czech republic (6.) rate of exchange time or repair time – the ship drives are mostly repairs in the port, where the repair can be done in a short time. repair of damage to the ship’s body can usually only be performed in the dock, which prolongs the time of repair. (7.) rate of importance for ensuring the life-saving or emergency functions in the territory – it is needed only for the rescue work of a certain character. (8.) rate of importance for ensuring the functions of administration and local government – small. (9.) rate of importance for ensuring the functions of army and police – it is significant only in special cases. (10.) rate of redundancy or replacement services – water transport can be replaced by the rail or road transport. (11.) rate of importance for ensuring the communication functions – small. (12.) rate of impact of frontage failure on the economy of region (state) – water transport by its low expenses positively affects the transport of raw materials and objects of large dimensions. (13.) rate of importance of operability – smaller, only to a limited extent. (14.) rate of importance in the field of symbolic, cultural, etc. – water transport has probably the longest tradition and by this reason it is one of the symbols of the country. the criticality rates determined by the methodological procedure described above are in table 3. table 3 shows that the greatest criticality has rail transport, which is followed by air transport, road transport and water transport. according to the chosen concept, the chosen scale and evaluation provided by the experts the criticality rates of both, the rail transport and the air transport are very high, the criticality rate of road traffic is high and the criticality rate of water transport is medium, and the criticality rate of whole transport system is high. from table 3 it is clear that to the size of criticality rate mostly contributes the vulnerability to attacks, little capability of protection (the protection of long linear networks in the territory is always a big problem), the importance for ensuring the functions of the army and the police, a big impact of transport failure on the economy of region (state), and the big importance for ensuring the emergency and rescue functions in the territory. 6. proposals for transport infrastructure safety improve from the above analysis of transportation systems criticality it follows that the transport domain is one of the most important areas of economic and social item in the state. to its high criticality it contributes that in recent decades this fact was discovered by population groups in different states, which are not satisfied with the current state establishment in the home countries, they create their own political ideology, which most strongly criticizes the current political establishment of the state in which they live and they try to their views and goals in life violently to enforce by illegal way and as onerous procedure for the enforcement of their ideology, they consider terrorism. terrorism currently has a global form, so its international character leads to the need for protection against attacks. in the reference case, the facts: the great importance of transport systems, the high vulnerability of transport systems, and also the fact that part of the transportation systems can be used as an offensive weapon with great destructive potential, as shown by the attacks of 11 september 2001 in new york. it is, therefore, necessary to develop countermeasures for reducing the eventuality of attack on transportation systems and to avert the devastating impacts on the functioning the state and on the lives of citizens. in conjunction with the results of the project the focus of the european union [15], it is necessary to consider that not only terrorism, but also a natural disasters, organizational accidents and corruption associated with the areas of control often to cause the extensive damages on the transportation systems. preparation and production of protective measures requires substantial investment, and therefore, it is necessary to coordinate investment and to combine possible measures for protection of transportation systems against possible damages caused by disasters [16] and terrorist activities, as required by the principles of strategic management [3]. 57 jan procházka, dana procházková acta polytechnica ctu proceedings due to the fact that neither terrorism nor natural disasters cannot be averted, the main goal of our activities are mitigating measures at occurrence of disasters in order to reduce the losses of human lives and minimize damages [1] [3]. because of the importance of transportation systems is considerable, it is very urgently to implement the measures and activities that will allow the critical infrastructure elements of transportation systems effectively protect against the activities of international terrorism and other disasters. it is necessary to carry out measures that reduce the vulnerability, ensure a rapid response to the failure of critical elements, and ensure the continuity of critical elements, and on the quick recovery of the other important elements. it is necessary to: • to estimate the vulnerability of functional elements of transportation system in the context of the assessment of transportation system objects such as bridges, tunnels, roads, highways, airports and waterways, • to provide for the most vulnerable part of the infrastructures, the failure of which can cause the most economic damages in infrastructure up to make impossible the transport activity, • to create scenarios of the impacts of major disasters and possible attacks, and on the basis of scenarios of impacts to determine the rapid-response procedures and the procedures for ensuring the transportation services as much as possible, • to improve measures and activities in the areas of planning, with the aim of improving the protection against possible disasters and terrorist attacks on the transportation system, and not only from the point of view of individual systems, but in the logistics context to link measures and to increase the safety of the entire system taken in as a system of systems, • to provide for measures that reduce the option of attack on transportation infrastructures and improve the detection of attacks on transportation infrastructure, • to estimate the amount of investment and labour costs, which will require the implementation of mitigation measures in case of failure of transportation infrastructures and to propose a procedure for their application, • to perform the analysis and to design the transportation infrastructure network protection against damages, which may arise as a result of possible disasters and to perform the estimation of operational inability origin as a result of their putting out of operation of functional elements or their parts, • to reduce the frequency of interruptions of transportation systems operation as a result of a failure that cannot be reliably in advance revealed during the regular inspections. 7. conclusions the above facts show that it is urgent to pay increased attention to safety issues of transportation systems. it needs to be processed as a separate concept of transportation system safety, so the concept of critical infrastructure safety, as it shows the work [17]. from the professional point of view, it is necessary to include into safety issues solution a systemic approach, which enables us to support the technological development of transportation systems, to consider the needs of people and transportation serviceability, the environmental protection and to reduce economic damages. it is necessary to define by the expert way the issues that are necessary to solve, and to process the relevant studies in the meaning of the above recommended procedure. references [1] d. procházková. základy řízení bezpečnosti kritické infrastruktury, 2013. isbn 978-80-01-05245-7. [2] d. procházková. bezpečnost kritické infrastruktury, 2012. isbn 978-80-01-05103-0. [3] d. procházková. strategické řízení bezpečnosti území a organizace, 2011. isbn 978-80-01-04844-3. [4] d. procházková. analýza a řízení rizik, 2011. isbn 978-80-01-04841-2. [5] d. procházková. safety of complex technological facilities, 2015. isbn 978-3-659-74632-1, id: 200303845. [6] czech republic, czech code. [7] r. l. keeny, h. raiffa. decision analysis with multiple conflicting objectives, 1976. [8] ema. critical infrastructure emergency risk management and assurance [online], 2003. www.ema.gov.au. [9] fema. promoting critical infrastructure protection by emergency managers and first responders [online], 2005. www.usfa.fema.gov. [10] saic. a guide to highway vulnerability assessment for critical asset identification and protection, 2002. national cooperative highway research program project 20-07/task 151b. [11] s. (sandia national laboratories). vital area identification, 2005. albuquerque. [12] us. the national strategy for the physical protection of critical infrastructures and key assets, 2000. [13] us. guide for critical infrastructure protection, 2005. [14] ministerstvo dopravy čr [online]. http://www.mdcr.cz. [15] eu. focus project [online], 2012. http://www.focusproject.eu/documents/14976/ -5d763378-1198-4dc9-86ffc46959712f8a. [16] fema. guide for all-hazard emergency operations planning, 1996. [17] us. us critical infrastructure conception, 2001. 58 www.ema.gov.au www.usfa.fema.gov http://www.mdcr.cz http://www.focusproject.eu /documents /14976/-5d763378-1198-4dc9-86ffc46959712f8a http://www.focusproject.eu /documents /14976/-5d763378-1198-4dc9-86ffc46959712f8a acta polytechnica ctu proceedings 5:51–58, 2016 1 introduction 2 current knowledge on transportation infrastructure 3 transportation system of the czech republic 4 method for determining the transportation infrastructure criticality 5 results – the criticality rates for transport infrastructure 6 proposals for transport infrastructure safety improve 7 conclusions references 199 acta polytechnica ctu proceedings 2(1): 199–204, 2015 199 doi: 10.14311/app.2015.02.0199 a new review of old novae a. pagnotta1 1american museum of natural history corresponding author: pagnotta@amnh.org abstract novae are certainly very exciting at the time of their eruptions, and there is much work left to be done in understanding all of the details of that part of their existence, but there is also much to be learned by looking at the systems decades, centuries, and even millennia after their peak. i give a brief overview of some of the history and motivation behind studying old novae and long-term nova evolution, and then focus on the state of the field today. exciting new results are finally starting to shed some light on the secular behavior of post-nova systems, although as is often the case in astronomy, the observations are at times conflicting. as is always the case, we need more observations and better theoretical frameworks to truly understand the situation. keywords: novae dwarf novae cataclysmic variables photometry. 1 introduction novae have been observed at least as far back as 134 b.c. (fontanille 2007), and we continue to observe them and their kin—recurrent novae (rne), dwarf novae (dne), and the rest of the cataclysmic variables (cvs)—to this day. just after discovery, near peak, and for the few weeks following peak, they are quite exciting and tend to receive a good amount of observational coverage from both amateur and professional astronomers. a month or two after peak, however, the number of observations decreases sharply, due to various reasons including limited telescope time and short attention spans among astronomers who understandably want to move on to the next exciting object. because of this, the long-term, secular behavior of novae is poorly understood. in this conference review, i examine what little we know about novae in the decades, centuries, and millennia following their eruptions and describe efforts to remedy this lack of knowledge and understand how novae fit into the broader context of cv evolution in particular and binary evolution in general. 2 the first decades a classical nova (cn) occurs in a cv when enough accreted gas accumulates on the surface of the white dwarf (wd) that a critical temperature/pressure is reached and a thermonuclear runaway is ignited, causing a dramatic brightening of the system, which we see as a nova. immediately after a nova eruption, two effects compete to affect the brightness of the system. these effects were quantified into the hibernation model originally proposed by shara et al. (1986) to explain the low observed space-density of novae. although more recent surveys seem to show that the low numbers of observed novae at the time may have been due to observational limitations and biases, (e.g. gänsicke et al. 2009), the physical effects described by the hibernation model remain valid and should be investigated. there are two potent effects we must consider. first, the mass ejection during the nova eruption leads to an increase in the angular separation between the accreting wd and its companion star. this leads to a decrease in the accretion rate and therefore the brightness of the star, since the observed brightness is due predominantly to the accretion luminosity. the second effect is that the thermonuclear runaway on the surface of the wd greatly increases its temperature; the hot wd then irradiates the companion star. the companion in turn puffs up, overflows its roche lobe more than it was previously doing, and the accretion rate increases, causing the system to get brighter. so the two main observable consequences following a nova eruption act in opposite directions and potentially cancel each other out. the question is which effect dominates, and why. collazzi et al. (2009) and schaefer & collazzi (2010) obtained the preand post-eruption magnitudes of a sample of 25 classical and 6 recurrent novae using a combination of published literature magnitudes, archival plates, and amateur observations. defining ∆m as the average difference between the preand post-eruption magnitude values for a given system, mpre − mpost, they found ∆mavg = +0.16 mag and 199 http://dx.doi.org/10.14311/app.2015.02.0199 a. pagnotta ∆mmed = +0.13 mag, with σ = 0.42 mag for the 25 cne with their 25 observed eruptions. adding in the 6 rne, for a total of 44 eruptions, they found ∆mavg = +0.03 mag and ∆mmed = +0.03 mag, with σ = 0.43 mag. both results are consistent with ∆m = 0, implying that the increase and decrease in the accretion rate after an eruption do in fact cancel each other out in most systems. a textbook example of this is qz aurigae, nova aurigae 1964, the preand post-eruption light curve of which can be seen in figure 1. qz aur has a ∆mavg ≈ 0.03 mag, essentially equal to zero. figure 1: qz aurigae, nova aurigae 1964, is a textbook example of the findings of collazzi et al. (2009), who showed that for nearly all novae, the preand posteruption magnitudes are identical, and that on average ∆m = mpre −mpost across all nova eruptions is consistent with 0. figure reproduced with permission from collazzi et al. (2009). there are of course a few systems that do not follow the trend. christened the v1500 cyg stars by schaefer & collazzi (2010) after their prototype system, these stars show large positive ∆m values years-to-decades after their eruptions have ended, i.e. they return to a quiescence that is brighter than their pre-eruption quiescence value. there are eight systems that fall in this category, with ∆m values ranging from 3.01 to > 4.96. the underlying physical mechanism causing this phenomenon is not well understood but it has been postulated that all of the systems are magnetic, either polars or intermediate polars. the magnetic field funnels the accreting material onto a small portion of the wd surface, leading to sustained nuclear burning, which irradiates the companion for a much longer time period than after a normal nova eruption and again causes an increase in the accretion rate and correspondingly the brightness of the system. this theory addresses the basics of the observations, but needs more thorough computational work to confirm its validity. neeley & schaefer (private communication) examined seven old novae that had a large number of magnitudes in the literature covering the decades since their eruptions. taking all seven together, they found an average decline of 1.4 mags per century, in line with the hibernation model, which predicts an average decline of 1 mag per century. johnson et al. (2013) examined one of the systems, v603 aquilae (nova aquilae 1918) in much greater detail and found an overall decline in brightness of 0.44 ± 0.04 mags per century since 1938, again as expected from the hibernation model. the light curve for v603 aql showing this decline can be seen in figure 2. this average behavior, however, hides the unusual behavior of a few systems among the more vanilla ones, including novae that appear to have undergone state changes, as well as at least one system (q cygni) that is getting brighter on a secular timescale (schaefer, private communication). figure 2: this yearly binned light curve for v603 aquilae (nova aquilae 1918) clearly shows a secular fading in both b− and v -band, as predicted by the hibernation model. the calculated decline rate for v603 aql is 0.44± 0.04 mags per century. figure reproduced with permission from johnson et al. 2013. the variety of behaviors seen when looking at even just a small number of systems indicates that a full survey is necessary to understand what is actually happening to post-nova cvs. i am embarking on a project to examine a much larger sample of post-nova systems in an attempt to obtain complete statistics on what novae do in the decades and centuries following their eruptions. i am working to combine historical magnitudes from archival plates and the published literature with a modern magnitude survey of all galactic novae. the bulk of the plate magnitudes will come from the collection of the harvard college observatory, which has ∼500,000 plates dating back to the 1890s and ranging from b=14 to b=18 in limiting magnitude. although at this point, much of the work done using the harvard plates must be completed by hand, the observatory has 200 a new review of old novae embarked on a scanning project known as dasch (digital access to a sky century at harvard) led by josh grindlay. eventually, the entire plate collection will be scanned and available digitally, which should provide an efficient way to study the long-term behavior of many systems, including novae. figure 3: this finder chart for v906 ophiuchi (nova ophiuchi 1952) from the online version of the catalog and atlas of cataclysmic variables (downes et al. 2001), showing the location of the nova eruption. there are at least three stars equally near the middle of the identifying circle, any of which could be the quiescent counterpart of the nova, or the actual counterpart could be a fainter star that is not easily detected in this relatively shallow image. there are many nova systems with finder charts as (un)clear as this one, which necessitates photometric and spectroscopic follow-up to identify the proper counterpart before any information can be obtained about its secular brightness history. one big challenge in this endeavor is identifying the quiescent counterpart of the nova eruption. although the eruptions are bright and, after detection, easy to identify, once the systems have faded, it becomes much more difficult. the positions and finder charts given for novae are often unclear, with multiple stars, or none at all, visible at the marked location, an example of which can be seen in figure 3, which shows the v906 ophiuchi finder chart given in the online version of the downes et al. (2001) catalog and atlas of cataclysmic variables. deep, multi-color images of the fields are required first, to identify blue stars which are more likely to be cvs. if in fact hibernation is correct and most novae spend their decades post-eruption fading in brightness, the practice of identifying the counterparts is more challenging, because they are fainter than they were before the eruption, and will continue to fade. when imaging is not enough to identify post-eruption nova counterparts, spectroscopy is required as well. the team of tappert, schmidtobreick, ederoclite, and vogt is currently in the middle of a dedicated effort to identify the counterparts of old novae using a combination of photometry and spectroscopy. tappert, schmidtobreick, and ederoclite also have papers in this proceedings, so i will leave the details of their work to them. figure 4: the expected cycle of a cv both pre-and post-nova eruption according to the hibernation model. before and after the nova, the system would be classified as a nova-like cv, with a high accretion rate but no known eruptions. as the accretion rate falls off post-eruption, the system transitions through a series of other types of cvs, from nova to nova-like to dwarfnova (perhaps multiple types of dwarf novae), before eventually settling into a state of hibernation in which the accretion essentially goes to zero. after a long period of time during which only gravitational radiation is actively moving the stars close together again, the companion once again overflows its roche lobe, accretion restarts at a low level, and the system moves up the left side of the figure, returning to dwarf nova and then nova-like states before likely undergoing another classical nova eruption tens of thousands of years after the first. 3 the centuries to millennia following eruption in addition to the long-term fading of a post-nova system, another prediction of the hibernation model is that a system should undergo state changes, passing through various other types of cvs on its way into and out of hibernation (shara et al. 1986). it is expected to be a cycle such as that depicted in figure 4, in which a 201 a. pagnotta system changes state as the accretion rate changes. immediately post-eruption, the system should appear as a nova-like variable, eventually transitioning to a dwarf nova, and then finally going into hibernation proper when the accretion essentially stops. after a time period that could be on the order of thousands of years, gravitational radiation will grind down the system, return it to contact, and restart the accretion, propelling the system back through the cycle from dwarf nova to nova-like and eventually back to another nova eruption. this is another testable prediction of the hibernation theory. figure 5: three images of the classical nova shell surrounding the dwarf nova z camelopardalis; from left to right they are in far-uv (1516 å) and near-uv (2267 å) from galex and in [nii] from the kpno 4m telescope. shell expansion has not yet been detected, but an upper limit of 0.17” per year, corresponding to a velocity no greater than 138 km s−1 has been placed on the expansion of the shell by shara et al. 2012b. the nova eruption of z cam is quite possibly associated with a guest star recorded by chinese astronomers in 77 b.c. (johansson 2007). figure reproduced with permission from shara et al. 2012b. at this time, we have regular post-eruption observations of novae going only approximately one century past their eruptions, for old novae with good coverage on the harvard plates. there are two systems in which both classical and dwarf nova eruptions have been observed. gk persei (nova persei 1901) was the first system in which this behavior was observed, when dwarf nova eruptions began approximately 50 years after the cn eruption. the second system is v1017 sagittarii, which was considered to be recurrent for many years, but upon closer examination it was realized that v1017 sgr had a dn eruption first, in 1901, followed by a cn eruption in 1919, followed by future dn eruptions, starting back up again in 1973 (webbink et al. 1987). this state change occurs on a much faster timescale than predicted by the hibernation model. the century of data from harvard is an excellent resource, but is not, however, the limit of how far forward—or backward, depending on your perspective—we can look. thus far there are two systems currently known to be dwarf novae around which very old nova shells have been discovered. the first was z cam, originally discovered by shara et al. (2007), and followed up with better observations of the shell published in shara et al. 2012b; figure 5 shows three of those images, the nearand far-uv images from galex (1516 å and 2267 å, respectively), and the [nii] optical emission image taken at the kitt peak 4-m telescope. shell expansion has been undetectable thus far, with an upper limit of 0.17” per year, corresponding to an expansion velocity no greater than 138 km s−1. it is likely that the nova eruption that caused the shell was recorded as a guest star by chinese astronomers in 77 b.c. (johansson 2007). 1’ fuv nuv[nii] figure 6: three images of the classical nova shell surrounding the current dwarf nova at cancri; from left to right they are in [nii] from the kpno 4m telescope, far-uv (1516 å) and near-uv (2267 å) from galex. at cancri is the second dwarf nova, after z cam, which has had a classical nova shell detected around it. figure reproduced with permission from shara et al. 2012c. the second dwarf nova to have a shell is at cancri, shown in figure 6 in the same three bands as the z cam image. additionally, at cancri’s shell has knots, as seen more clearly in the hα + [nii] image in figure 7. the knots are likely caused by the same rayleigh-taylor instabilities that cause the knots in gk per (shara et al. 2012a) and t pyx (shara et al. 1997, schaefer et al. 2010, toraskar et al. 2013). most recently, patterson et al. (2013) have pointed to the star bk lyncis as a third system that has changed state. pre-2005, bk lyn appeared to be a normal nova-like star, with a wd accreting at a relatively high rate, but nothing else special about the system. starting in 2005, however, and very well defined by 2011 and 2012, bk lyn transitioned into being an er umatype dwarf nova. (for more details about bk lyn and the contributions of the excellent observers of the center for backyard astrophysics to this discovery, see joe patterson’s proceedings paper from this meeting.) bk lyn is possibly associated with a nova recorded in 101 a.d., and therefore it took approximately 2000 years for it to make its transition, as opposed to just a few decades for gk per and v1017 sgr. this is loosely theorized to be due to the large difference in the orbital periods of the systems, with bk lyn being quite short (107.97 minutes; patterson et al. 2013) and gk 202 a new review of old novae per and v1017 sgr having long orbital periods (1.997 days and 5.780 days, respectively; pagnotta & schaefer 2013), but the effect is still not well understood and needs a more solid theoretical grounding. figure 7: this hα + [nii] image of the classical nova shell around at cancri clearly shows the knots present. the knots are likely caused by rayleigh-taylor instabilities and are similar to those seen in gk per (shara et al. 2012a) and t pyx (shara et al. 1997, schaefer et al. 2010, toraskar et al. 2013) figure reproduced with permission from shara et al. 2012c 4 conclusion with even just a small sample of post-eruption novae for which the long-term behavior has been studied, we see a wide variety of behaviors. some novae fade, as predicted by the hibernation model; others brighten, in direct opposition to the theory. some appear to stay at the same magnitude for many decades following their eruption; perhaps they will continue in this fashion, or perhaps a change will set in many years later. a number of systems have been seen to change states, from novae to dwarf novae, on time scales varying from approximately 50 years to approximately 2000 years. taken all together, the lesson from this review is that the longterm behavior of post-eruption novae is poorly understood, and most certainly in need of more study, both observationally and theoretically. references [1] collazzi, a. c., schaefer, b. e., xiao, l., pagnotta, a., kroll, p., löchel, k., & henden, a. a.: 2009, aj, 138, 1846 [2] downes, r. a., webbink, r. f. shara, m. m., ritter, h, kolb, u., & duerbeck, h. w.: 2001, pasp, 113, 764 doi:10.1086/320802 [3] fontanille, j.-p.: 2007, jrasc, 101, 11 [4] gänsicke, b. t., et al.: 2009, mnras, 397, 2170 doi:10.1111/j.1365-2966.2009.15126.x [5] johansson, g. h. i.: 2007, nature, 448, 251 doi:10.1038/448251d [6] johnson, c. b., schaefer, b. e., kroll, p., & henden, a.: 2014, apjlett, 780, 25 doi:10.1088/0004-637x/780/1/25 [7] pagnotta, a., & schaefer, b. e.: 2014, apj, 788, 164 doi:10.1088/0004-637x/788/2/164 [8] patterson, j., et al.: 2013, mnras, 434, 1902 doi:10.1093/mnras/stt1085 [9] schaefer, b. e. & collazzi, a. c.: 2010, aj, 139, 1831 [10] schaefer, b. e., pagnotta, a., & shara, m. m.: 2010, apj, 708, 381 doi:10.1088/0004-637x/708/1/381 [11] shara, m. m., livio, m., moffat, a. f. j., & orio, m.: 1986, apj, 311, 163 doi:10.1086/164762 [12] shara, m. m., zurek, d. r., williams, r. e., prialnik, d., gilmozzi, r., & moffat, a. f. j.: 1997, aj, 114, 258 [13] shara, m. m., et al.: 2007, nature, 7132, 159 [14] shara, m. m., et al.: 2012a, aj, 143, 143 [15] shara, m. m., et al.: 2012b, apj, 756, 107 [16] shara, m. m., et al.: 2012c, apj, 758, 121 [17] toraskar, j., mac low, m.-m., shara, m. m., zurek, d. r.: 2013, apj, 768, 48 [18] webbink, r. f., livio, m., truran, j. w., & orio, m.: 1987, apj, 314, 653 discussion ed sion: q cyg may not be so odd. we found that nova oph 1848 (v841 oph) and di lac (nova lac 1910) are also accreting at a high rate and probably getting brighter. ashley pagnotta: very interesting, and a great example of why we need a large-scale survey of novae to see what’s normal and what’s not. thank you for pointing out those two systems. jan-uwe ness: very encouraging results from historical plates. in addition to the harvard plates, smaller historical observatories such as hamburg may 203 http://dx.doi.org/10.1086/320802 http://dx.doi.org/10.1111/j.1365-2966.2009.15126.x http://dx.doi.org/10.1038/448251d http://dx.doi.org/10.1088/0004-637x/780/1/25 http://dx.doi.org/10.1088/0004-637x/788/2/164 http://dx.doi.org/10.1093/mnras/stt1085 http://dx.doi.org/10.1088/0004-637x/708/1/381 http://dx.doi.org/10.1086/164762 a. pagnotta have scanned plates that could fill more gaps. marina orio and rene hudec made similar comments about smaller observatories including asiago and bamburg. rene hudec also commented on his presentlyactive project to document what exists in current plate archives and help facilities get their collections scanned into a digitally-accessible form. 204 introduction the first decades the centuries to millennia following eruption conclusion 108 acta polytechnica ctu proceedings 1(1): 108–112, 2014 108 doi: 10.14311/app.2014.01.0108 mass accretion processes in young stellar objects: role of intense flaring activity salvatore orlando1, fabio reale2,1, giovanni peres2,1, andrea mignone3 1inaf osservatorio astronomico di palermo, piazza del parlamento 1, 90134, palermo, italy 2dip. di fisica e chimica, università degli studi di palermo, piazza del parlamento, 1, 90134, palermo, italy 3dip. di fisica generale, università degli studi di torino, via pietro giuria 1, 10125, torino, italy corresponding author: orlando@astropa.inaf.it abstract according to the magnetospheric accretion scenario, young low-mass stars are surrounded by circumstellar disks which they interact with through accretion of mass. the accretion builds up the star to its final mass and is also believed to power the mass outflows, which may in turn have a significant role in removing the excess angular momentum from the star-disk system. although the process of mass accretion is a critical aspect of star formation, some of its mechanisms are still to be fully understood. on the other hand, strong flaring activity is a common feature of young stellar objects (ysos). in the sun, such events give rise to perturbations of the interplanetary medium. similar but more energetic phenomena occur in ysos and may influence the circumstellar environment. in fact, a recent study has shown that an intense flaring activity close to the disk may strongly perturb the stability of circumstellar disks, thus inducing mass accretion episodes (orlando et al. 2011). here we review the main results obtained in the field and the future perspectives. keywords: accretion, accretion disks mhd stars: circumstellar matter stars: flare stars: pre-main-sequence x-rays: stars. 1 introduction observations in the x-ray band reveal that low-mass pre-main-sequence stars are strong sources with x-ray luminosities 3−4 orders of magnitude greater than that of the present-day sun. the source of this high-energy radiation is plasma with temperatures of 1 − 100 mk in the stellar outer atmospheres (coronae), heated by magnetic activity analogous to the solar one but higher by factors up to 106. such a magnetic activity manifests through very different phenomena that may occur in several places of the stellar atmosphere and circumstellar environment. the young star interacts with its disk in a complex fashion, with accretion and ejection of collimated outflows. strong magnetic fields are believed to connect the star with a keplerian circumstellar disk, funneling accretion onto limited portions of the stellar surface (e.g. hartmann 1998) where shocks are produced by the impact of the accretion streams (e.g. orlando et al. 2010). x-ray flares are violent manifestations of the stellar magnetic activity and are triggered by an impulsive energy input from coronal magnetic fields. x-ray observations in the last decades have shown that flares in young stellar objects (ysos) have amplitudes much larger than solar analogues and occur much more frequently. examples of these flares are those collected by the chandra satellite in the orion star-formation region (coup enterprise; favata et al. 2005). in the sun, such energetic events are often associated to coronal mass ejections and give rise to perturbation effects of the interplanetary medium, broadly known as space weather effects. similar phenomena are expected to occur in young stars, and may affect the circumstellar environment. for instance, favata et al. (2005) analyzed the most energetic flares observed by coup and found that these flares might be hosted in magnetic loops extending several stellar radii, much larger than ever observed in older stars. since the central star is surrounded by a circumstellar disk accreting material onto the star, it is natural to ask whether strong flaring activity involves the disk and even perturbs its stability, possibly affecting the mass accretion to the star. at the present time, in fact, it is unclear where these flares occur. the differential rotation of the disk together with the interaction of the disk with the magnetosphere may cause magnetic reconnection close to the disk’s surface, triggering large-scale flares there. in this case, the flares may perturb the stability of the circumstellar disk causing, in particular, a strong local overpressure. the pressure gradient force might be able to 108 http://dx.doi.org/10.14311/app.2014.01.0108 mass accretion processes in young stellar objects: role of intense flaring activity push disk’s matter out of the equatorial plane into funnel streams, thus providing a mechanism to drive mass accretion that differs from that, commonly invoked in the literature, based on the disk viscosity (romanova et al. 2002). bright flares close to circumstellar disks may therefore have important implications for a number of issues such as the transfer of angular momentum and mass between the star and the disk, the powering of outflows, and the ionization of circumstellar disks. recently, we have investigated the idea that an intense flare close to an accretion disk may perturb the stability of the disk and trigger mass accretion onto the star (orlando et al. 2011). in this paper we review our findings and present preliminary results of a study investigating the effects of a storm of small-to-medium flares on the stability of accretion disks (orlando et al. 2014). in sect. 2 we describe the mhd model; in sect. 3 we present the results; in sect. 4 we draw our conclusions. 2 the mhd model the model describes a classical t tauri star (ctts) of mass m∗ = 0.8m� and radius r∗ = 2r� located at the origin of a 3d spherical coordinate system (see orlando et al. 2011 for a detailed description). we adopted the initial conditions introduced by romanova et al. (2002). in particular, we assumed the rotation period of the star to be 9.2 days as representative of cttss. the initial unperturbed stellar atmosphere is approximately in equilibrium and consists of three components: the stellar magnetosphere, the extended stellar corona, and the keplerian disk. initially, the magnetosphere is assumed to be force-free, with dipole topology and magnetic moment aligned with the rotation axis of the star. the magnetic moment is chosen in order to have a magnetic field strength of the order of 1 kg at the stellar surface. the isothermal disk is cold, dense and rotates with angular velocity close to the keplerian value; its rotation axis (coincident with the rotation axis of the star) is aligned with the magnetic moment. the disk is initially truncated by the stellar magnetosphere at the radius rd where the ram pressure of the disk is equal to the magnetic pressure; for the adopted parameters, rd = 2.86r∗ and the co-rotation radius is located at rco = 9.2r∗. the corona is initially isothermal with temperature t = 4 mk and at low density. the system is described by the time-dependent mhd equations of mass, momentum, and energy conservation in a 3d spherical coordinate system, extended with gravitational force, viscosity of the keplerian disk, thermal conduction (including the effects of heat flux saturation), coronal heating (via a phenomenological term), and radiative losses from optically thin plasma (see orlando et al. 2011 for more details). the phenomenological heating is prescribed as a component, describing the stationary coronal heating, plus a transient component, triggering the flares. the calculations were performed using pluto (mignone et al. 2007), a modular, godunov-type code for astrophysical plasmas. 3 results as a first application of the model, we have investigated the effects of a single bright flare on the stability of the disk (orlando et al. 2011). the initial heat pulse triggering the flare is released at a distance of 5r∗, namely in a region comprised between the truncation and corotation radii, and it is supposed to be indicative of a likely area of magnetic reconnection. heat pulses occurring closer to the inner disk edge are expected to produce analogous perturbations on the disk dynamics. we followed the evolution of the star-disk system for ≈ 2 days. after the heat pulse has been released, an mhd shock wave develops above the disk and propagates away from the protostar. the heat deposition determines a local increase of temperature and pressure. disk material is heated and expands with a strong evaporation front which is channeled along the magnetic field lines towards the central star. after ∼ 1 hour a hot loop forms linking the disk with the star (see upper panel in fig. 1). the loop has an effective maximum temperature of ≈ 100 mk (at the peak of emission measure) and a length of ≈ 1012 cm; these values are in good agreement with those inferred from the analysis of the brightest flares observed by coup (favata et al. 2005). the overheating of the disk surface makes a significant amount of material expand and be ejected in the magnetosphere. a small fraction of this material fills the loop whose density increases from 108 cm−3 to 1010 cm−3. on the other hand, most of the expanding disk material is not confined by the magnetic field and is ejected away from the star, carrying away mass and angular momentum. during the evolution of the hot loop, an overpressure wave develops where the heat pulse has been injected. this overpressure travels through the disk and reaches the opposite boundary after ≈ 5 hours. there the pressure gradient force drives the material out of the disk and channels it into a funnel flow. then the gravitational force accelerates the escaped material toward the central star where the stream impacts ≈ 25 hours after the injection of the heat pulse. the accretion flow persists until the end of the simulation (t = 48 h). the lower panel in fig. 1 shows a cutaway view of the stardisk system after the impact of the stream onto the stellar surface. we found that the dynamics and physical characteristics of the accretion stream triggered by the flare closely recall those of streams driven by the accumulation of mass at the disk truncation radius under 109 salvatore orlando et al. the effect of the viscosity and pushed out of the equatorial plane because of the growing pressure gradient there (e.g. romanova et al. 2002). figure 1: effects of a single bright flare on the stability of the circumstellar disk. cutaway views of the star-disk system showing the mass density of the disk (yellow-green) at t = 2.0 hours (upper panel) and at t = 45 hours (lower panel) since the injection of the heat pulse. the upper panel also over-plots the threedimensional volume rendering of the plasma temperature (mk), showing the flaring loop (in red) linking the inner part of the disk with the star. the lower panel shows the accretion stream triggered by the flare in the side of the disk opposite to the flaring loop. selected magnetic field lines are overplotted in red. from the simulation, we derived also the mass accretion rate and found ṁ > 2.5 × 10−10 m� yr−1. we compared this rate with those inferred from optical observations of cttss (herczeg & hillenbrand 2008; curran et al. 2011) and found a good agreement. we concluded that a bright flare as those frequently detected in ysos (e.g. coup observations) can be an efficient mechanism to trigger accretion onto the protostar itself with accretion rates on the same order of those commonly measured in cttss. as a follow-up of the previous study, we explored in more details the possibility that significant mass accretion in young stars can be triggered by a storm of small-to-medium flares (as those frequently observed) occurring on the accretion disk (orlando et al. 2014). to this end, we performed a 3d mhd simulation analogous to that described in orlando et al. (2011) but considering a storm of flares distributed randomly in proximity of the disk surface instead of a single bright flare. we found that each simulated flare follows an evolution similar to that of the single bright flare described in orlando et al. (2011). the main difference is that, now, interactions between next flares may occur. figure 2 shows cutaway views of the star-disk system after ≈ 26 hours. during the system evolution, the flares build up an extended corona linking the star with the disk. at the same time, the disk is strongly perturbed by the flares and, after ≈ 20 hours, several funnel streams develop, accreting substantial mass onto the star (see lower panel in fig. 2). the streams persist until the end of the simulation and last for a time longer than the typical interval between flares. the simulated mass accretion rate is 10−10 < ṁ < 10−9 m�yr−1, again in good agreement with the values inferred from optical observations of cttss (herczeg & hillenbrand 2008; curran et al. 2011). 4 conclusions we investigated the effects of an intense flaring activity on the stability of a circumstellar disk surrounding a magnetized ctts. to this end, we developed a 3d mhd model including, for the first time, all key physical processes, most notably the thermal conduction and the radiative losses from optically thin plasma. as a first step, we analyzed the perturbation induced by a single bright flare occurring in proximity of the disk (orlando et al. 2011). then, we investigated the effects of a storm of small-to-medium flares distributed randomly close to the disk (orlando et al. 2014). our findings lead to the following conclusions: (a) flares occurring close to the circumstellar disk can trigger substantial and persistent accretion flows, similar to those caused by the disk viscosity; (b) an intense flaring activity close to the disk builds up an extended corona linking the star to the disk. in the case of a single bright flare occurring close to the disk, the simulations suggest that mass accretion events associated with x-ray flares should be observed. however, correlation between uv/optical accretion tracers and x-ray flux is rarely seen (stassun et 110 mass accretion processes in young stellar objects: role of intense flaring activity al. 2006). on the other hand, such a correlation is not prediced by simulations describing a continuous flaring activity close to the disk (orlando et al. 2014). in this case, the streams are triggered by the first flares at the beginning of the simulation and then are continuously powered by the following ones. at regime, therefore, no clear correlation between uv/optical accretion tracers and x-ray flux is foreseen. in the light of the above findings, we suggest that the flaring activity common to ysos may turn out to be important in the exchange of angular momentum and mass between the circumstellar disk and the central protostar. figure 2: effects of a storm of flares on the disk stability. cutaway views of the star-disk system showing the mass density of the disk (blue) at t = 26 hours. the panels over-plot the three-dimensional volume rendering of the plasma temperature (in mk), showing the flaring loops (in red) linking the inner part of the disk with the star. the upper panel shows the star-disk system observed edge-on, whereas the lower panel shows the system observed pole-on. selected magnetic field lines are overplotted in the upper panel. acknowledgement we thank the referee for constructive and helpful criticism. pluto is developed at the turin astronomical observatory in collaboration with the department of general physics of the turin university. we acknowledge the cineca award n. hp10a4zcv5,2012 for the availability of high performance computing resources and support. references [1] curran, r.l., argiroffi, c., sacco, g.g., orlando, s., peres, g., reale, f., maggio, a.: 2011, a&a 526, a104 [2] favata, f., flaccomio, e., reale, f., micela, g., sciortino, s., shang, h., stassun, k.g., feigelson, e.d.: 2005, apjs 160, 469 doi:10.1086/432542 [3] hartmann l.: 1998, accretion processes in star formation. new york: cambridge university press; cambridge astrophysics series; 32 [4] herczeg, g.j., hillenbrand, l.a.: 2008, apj 681, 594 doi:10.1086/586728 [5] mignone, a., bodo, g., massaglia, s., matsakos, t., tesileanu, o., zanni, c., ferrari, a.: 2007, apjs 170, 228 doi:10.1086/513316 [6] orlando, s., sacco, g.g., argiroffi, c., reale, f., peres, g., maggio, a.: 2010, a&a 510, a71 [7] orlando, s., reale, f., peres, g., mignone, a.: 2011, mnras 415, 3380. [8] orlando, s., reale, f., peres, g., mignone, a.: 2014, in preparation [9] romanova, m.m., ustyugova, g.v., koldoba, a.v., lovelace, r.v.e.: 2002, apj 578, 420 doi:10.1086/342464 [10] stassun, k.g., van den berg, m., feigelson, e., flaccomio, e.: 2006, apj 649, 914 doi:10.1086/506422 discussion matteo guainazzi: what kind of x-ray variability does your model predict? is it observed in the coup sources? salvatore orlando: the x-ray variability depends on the rate of flares triggered. in the simulation with a single flare, we observe the fast rise and subsequent slow decay of x-ray emission characteristic of flares. in the simulation with a storm of flares, we observe a background emission due to the many small flares evolving simultaneousluy with superimposed local peaks due to the brightest flares. the x-ray variability found is consistent with that of ysos observed by coup. 111 http://dx.doi.org/10.1086/432542 http://dx.doi.org/10.1086/586728 http://dx.doi.org/10.1086/513316 http://dx.doi.org/10.1086/342464 http://dx.doi.org/10.1086/506422 salvatore orlando et al. david buckley: do you believe that your magnetospheric disk interaction models could be extrapolated to intermediate polar class of cvs, where accretion occurs from disrupted disc onto a magnetic white dwarf? salvatore orlando: our model can be extrapolated (with appropriate scaling) to systems in which mass accretion occurs from a circumstellar disk, if relativistic effects can be neglected. 112 introduction the mhd model results conclusions acta polytechnica ctu proceedings doi:10.14311/app.2017.12.0099 acta polytechnica ctu proceedings 12:99–103, 2017 © czech technical university in prague, 2017 available online at http://ojs.cvut.cz/ojs/index.php/app artificial neural network for models of human operator martin růžek czech institute of informatics, robotics and cybernetics (ciirc), czech technical university in prague, zikova 4, 166 36 prague 6, czech republic correspondence: martin.ruzek@cvut.cz abstract. this paper presents a new approach to mental functions modeling with the use of artificial neural networks. the artificial neural networks seems to be a promising method for the modeling of a human operator because the architecture of the ann is directly inspired by the biological neuron. on the other hand, the classical paradigms of artificial neural networks are not suitable because they simplify too much the real processes in biological neural network. the search for a compromise between the complexity of biological neural network and the practical feasibility of the artificial network led to a new learning algorithm. this algorithm is based on the classical multilayered neural network; however, the learning rule is different. the neurons are updating their parameters in a way that is similar to real biological processes. the basic idea is that the neurons are competing for resources and the criterion to decide which neuron will survive is the usefulness of the neuron to the whole neural network. the neuron is not using "teacher" or any kind of superior system, the neuron receives only the information that is present in the biological system. the learning process can be seen as searching of some equilibrium point that is equal to a state with maximal importance of the neuron for the neural network. this position can change if the environment changes. the name of this type of learning, the homeostatic artificial neural network, originates from this idea, as it is similar to the process of homeostasis known in any living cell. the simulation results suggest that this type of learning can be useful also in other tasks of artificial learning and recognition. keywords: neural network, artificial neuron, learning algorithm, mental model. 1. introduction for many practical applications it would be useful to have a model of a human operator, as for example in transport engineering where the human operator plays a principal role. unfortunately, the human factor causes also most of the accidents; therefore it would be helpful to understand the processes in the human brain. due to the complex nature of the human brain it is however very difficult to model and predict its behavior. he need to dispose of the model of a human operator, as the most complex part of many transport systems, stays behind this research. the transport and traffic simulations are on high level, but the principal component of any transport system – the human factor – is still quite unexplored. the main obstacles are the lack of knowledge about the mental processes and the differences between the typical artificial intelligence approaches and the biological neural networks. the artificial neural networks (ann) seems to be a promising method for these models because the architecture of the ann is directly inspired by the biological neuron and also because it is data driven method. the classical paradigms of artificial neural networks are however not suitable for direct use because they simplify too much the real processes in biological neural network, but it is possible to update the learning algorithm so that it corresponds more frankly to the biological reality. the neural networks have already been used in several projects aimed at mental models. one of them is the blue brain project[1, 2] with the goal to simulate the whole brain. this model should be so detailed that every cell is depicted on a molecular level. according to the authors, processes like consciousness, creativity, emotions or aggression will emerge. another research aimed at the models of human mind is the darpa synapse project with opposite attitude. the idea behind this research is to create a neural network with architecture similar to biological neural network, however with strongly simplified neurons[3]. the expectation is not to model mental processes, but to use the principles of biological neural network to improve the computer architecture. the way of information processing in the computer architecture is different from biological networks. in brain, the memory and computation are distributed; there is no central processor or memory. the expectation of synapse project is to understand the advantages of this architecture and to use them in formation of novel computer architecture. with respect to both above mentioned researches, the target of this paper is to find such an equilibrium point where the model of the neuron is still faithful enough so that it can model the strong processes and 99 http://dx.doi.org/10.14311/app.2017.12.0099 http://ojs.cvut.cz/ojs/index.php/app martin růžek acta polytechnica ctu proceedings yet be simple enough to be practically realizable. 2. new solution – homeostatical neural network the need for a compromise between the complexity of biological neural network and the practical feasibility of the artificial network led to a proposal of new learning algorithm. the idea is based on the classical multilayered neural network (mlp), the difference is in the learning process. the neurons are updating their parameters in a way that is similar to the real biological processes. the basic idea is that the neurons are in competition for resources and the survival criterion is the usefulness of the neuron to the whole neural network. the neurons are not using any "teacher" or other kind of superior system, they have the same information as the biological neuron. the learning process can be seen as searching of some equilibrium position which represents a state where the importance of the neuron for the neural network is maximal. this position can change in time if the environment changes. the name of this type of learning, the homeostatic artificial neural network, is derived from the similarity of this idea to the process of homeostasis that is known from biology. the proposed neural network is based on the idea of mcculloch-pitts neuron: y = φf n∑ i=0 xiwi (1) where f is the transfer function. the sigmoid transfer function was used: f(x) = 2 (1 + e−α∗x)− 1 (2) as the similarity to biological neuron was the basic requirement, the back propagation algorithm is not a solution because a higher structure (or teacher) is used to train the neuron. in the so called homeostatic neuron, the unit is using its proper forward connection to improve its function. the ’axon’ in this model has two functions – the first one is the transmission of the output to higher layer (as in back propagation), the second is the transmission of the utility information from higher layer to the lower one. the idea of this type of training is that the neuron improves its relative importance in the network, in other words, it is trying to maximize the part of its output signal that is accepted by other neurons. this idea corresponds to the biological reality because the information transmitted by the axon has the form of energy (and is inseparable from energy). therefore, the neuron knows which part of its output energy was accepted by other neurons. the process of learning can be described by the following algorithm: first, the neuron computes its output with its initial random weight. then the neurons in the higher layer set their weights according to their level of contentment with the reference neuron. in the next step, the neuron changes its weights accordingly. several possibilities of the weight change are described later in this paper. then the new output is computed. the neurons in the higher layer read the output and re-calculate their inputs weights. the reference neuron then decides which setting was better. this algorithm has several variants, all of them are using at least 2 successive values of the level of acceptance. this implies that the neuron must be equipped with a memory. the process of learning is on figure 1. from the point of view of the reference neuron the learning is described by the following algorithm: (1.) random initial weights (2.) output with initial weights for the first input (3.) neurons in higher layer (output neurons) compute the utility of the reference neuron and set their input weights accordingly. if they are satisfied with the output of the reference neuron, they increase their weights, otherwise they decrease them. there are several ways how to compute the utility, some of them are described in eq. 3 6. (4.) the reference neuron changes one (or more) of its input weights (5.) the reference neuron repeats the forward phase with the same data but with changed weight (6.) the output neurons compute the utility (as in step 3) (7.) the reference neuron evaluates the change in the step 4 – if it improves the utility, it will keep it, otherwise it will change the weights in the opposite direction (8.) neuron repeats steps 2 to 7 with all the connections and all the inputs. 3. criterions for optimization several methods can be used for the calculation the importance of the neuron. the basic difference among them is the number of output neurons for which the reference neuron is ’working’. the first extreme is a neuron that is optimizing its function for all output neurons. this neuron is ’reading’ all output weights without taking any particular weight into consideration. the opposite extreme is neuron that works only for one neuron in the higher layer; in other words is optimizing its function to improve its utility for this particular neuron. apart from these options, many other compromise criterions can be defined. 100 vol. 12/2017 artificial neural network for models of human operator figure 1. wide figure[4, 5]. 101 martin růžek acta polytechnica ctu proceedings this first idea corresponds to a neuron that is finding such a weight vector ∧ = w1, w2, ..., wn for which the sum of the absolute values of the output weights is maximal. this idea corresponds firmly to the biological reality because the neuron has only one axon and therefore it can only be aware of the total amount of the signal that is accepted by other neurons, not of the particular weights. in the case of the artificial neuron we also expect negative weights; therefore the neuron sums the absolute or square values. the utility q is: q = n∑ j=1 |woj | (3) respectively: q = n∑ j=1 (woj )2 (4) eq. 4 puts stress on great values and reduces the importance of the small ones. this may be in certain cases advantage for the learning but does not correspond to the biological reality. 4. searching the one neuron maximum – the second extreme the other type of training is based on the presumption that the neuron is increasing its importance to only one neuron in the higher layer, therefore it maximizes the function: q = max|woj |; j ∈ [0, 1, ...n] (5) the problem is that if max|wo| = 1, no further improvement is possible and the training stops. in a real situation, we expect networks with many neurons where this value will be reached very soon and then the learning stops. this is not the desired behavior; therefore in that case there should be used an additional condition that ensures the continuation of the training. the solution of this problem is to use a compromise that takes into consideration more than one output neuron but not all of them. this can be done by optimization of some given number of maximal output weights: u = max(|wo|) +max(wo −max(wo)) + ...; (6) where w = {w0, ..., wn−1}. 5. variants of homeostatical neural network apart from the above mentioned variant, several other options must be defined. first of all, the question of the weight range arises at least three options. the most biologically plausible is to limit the weights to interval between 0 and 1. that means that the weights cannot change the polarity of the signal and no gain of the output energy is allowed. this option is the most biologically plausible as the axon, synapse and dendrites are mostly working only as transmitters of the signal. on the other hand, some operation with the signal may be done also on the level of the axon dendrit transmission, for example the inhibitory weights can reduce the neuron potential. this leads to the second idea, to limit the weights to interval 〈−1; 1〉. the last option is not to limit the weights at all, that means the signal can be multiplied by any real number. this is the least biologically plausible method; on the other hand it will bring the highest computational power. the other question is how to choose the weights that should be updated. the first option that comes to mind is to update the weight that has the greatest influence on the result; that means the most sensitive weight. this will lead to highest increase of the utility of the neuron in the next step, however it may not be the best option from the global point of view. making the step always in the direction of the highest gradient may lead to falling into local extreme. also, to find the most sensitive weight consumes a lot of computational power. alternative option is to choose the updated weight randomly or in given order. these two options lead to similar results. it is also possible to imagine the combination of these algorithms, for example by using several steps to update of the most sensitive weights which may be followed by randomly chosen weights. because of the complex nature of the neural networks, it is not possible to say which method is the best for arbitrary data. 6. problems of the idea of homeostatical learning the described idea has two principal limitations. the first one is the learning of the highest layer. the idea of a learning algorithm is that neurons in certain layers are updating their weights according to the higher layer. this can be used for all layers except for the highest as it does not have any output neuron. the solution for practical simulation and testing is that the highest layer was not trained by the homeostatic learning algorithm, but by the back propagation. this is of course is an alteration of the original idea, but for the sake of the practical realization it is the easiest way how to program the homeostatic learning for the rest of the network. in the case of practical application, for example in robot, this problem will be solvable in a natural way because the network will be part of a closed loop. the second problem is the delay question. the reference neuron is updating its input weights accord102 vol. 12/2017 artificial neural network for models of human operator ing to its output weights, but in each neural cell the output calculation takes some time., meanwhile the inputs are changing this means that the forward information will not ’meet’ the information about the utility in the same time. this problem has two solutions. the first is to set the dynamics of inputs to a lower level, so that the speed of information processing in the whole system is significantly higher than the changes of the input signals. the other possibility is to equip the neurons with a memory that stores the previous inputs so that it is possible to recall them when the information about the utility reaches the neuron. 7. conclusions in this paper, the idea of a new learning algorithm for neural networks is presented. this algorithm can cope with some disadvantages that the classical neural networks paradigms have. the proposed algorithm has several variants; from the point of view of the optimization criterion it is optimization for all input weights vs. optimization for only one input weights, and many compromising solutions. from the point of view of the weight update, the possible variants are the update of the weight with the highest sensitivity to the change, of randomly chosen weight of consequently chosen weight. from the point of view of the weight range, it is possible to define weights as positive numbers smaller than 1, as either positive or a negative number in absolute value smaller than 1, or as real numbers. every variant has its pros and cons. due to the high number of variants which is even multiplied by the amount of data (for neural networks as for data driven method it is important which data is fed into the network. the same network can work well with one data and wrongly with another) it is difficult to decide which variants is the most promising. larger testing is needed to understand the quality of the methods. despite the fact that the initial tests showed that the signal prediction task is better fulfilled by back propagation algorithm, the homeostatic neural network seems as a promising method for modeling on mental processes. of course it is not possible to create a model of the complete consciousness, but it is possible to concentrate on some specific region. transport engineering brings many possible applications as the human factor is the most important part in many transport systems. several processes may be modeled and predicted by the use of this network. typical example is a car driver, who is often making decisions with a lack of information and with the use of prior data. it is difficult to understand the factors that make for example the decision whether to cross the crossroads if the traffic light is orange and will become red in a short period of time. in such situations some drivers react differently even if the conditions are the same. the analytical way to explain such states is quite complex, but it is possible to collect enough data (either from real traffic or from a simulator) to predict this type of decisions. the neural network is a good tool to process this data. once the network is trained for certain tasks in a driver’s decision making processes, it can be used for the investigation of other decisions and behavioral predictions. this can help to explain and avoid danger situations caused by aggressiveness, fatigue, emotions and others. references [1] m. růžek, t. brandejský. model of biological ann based on homeostatic neurons. in 12th wseas international conference on neural networks (nn’11), isbn 978-960-474-292-9, athens 2011, pp. 66 69. [2] m. růžek. artificial neural network inspired by homeostasis in biological networks. in proceedings of 17th international conference on soft computing (mendel 2011), isbn 978-80-214-4302-0, pp. 232 235. [3] m. růžek. artificial neural networks for models of driver’s brain functions. in 20th anniversary of the faculty of transportation sciences, czech technical university in prague – selected papers, čvut v praze, 2013, isbn 978-80-01-05320-1, pp. 207 211. [4] m. růžek. modeling of eeg signal with homeostatic neural network. in nostradamus 2013: prediction, modeling and analysis of complex systems, ostrava 2013, isbn 978-3-319-00541-6 pp. 175 180. [5] m. růžek, t. brandejský. model of homeostatic artificial neuron. in neural networks, fuzzy systems & evolutionary computing, isbn 978-960-474-195-3, athens 2010, pp. 145 148. 103 acta polytechnica ctu proceedings 12:99–103, 2017 1 introduction 2 new solution – homeostatical neural network 3 criterions for optimization 4 searching the one neuron maximum – the second extreme 5 variants of homeostatical neural network 6 problems of the idea of homeostatical learning 7 conclusions references 323 acta polytechnica ctu proceedings 2(1): 323–324, 2015 323 doi: 10.14311/app.2015.02.0323 concluding remarks d. v. bisikalo1 1institute of astronomy ras, moscow, russia corresponding author: bisikalo@inasan.ru keywords: cataclysmic variables. there are four persons making the concluding remarks of the conference, so i restricted myself by the 1-st part when we discussed the cvs. i am pleased to note that the scientific level of our meeting has been quite high. i have heard all 46 talks on cvs and only one has not been new for me, my own ”magnetic cvs behaviour: a review”. i would like to stress the very good organization of the conference program, which, in particular, appeared as a balanced number of reviews and contributed talks. i was really impressed by the review talks made by franco giovannelli & lola sabau-graziati, hans ritter & ulrich kolb, joseph patterson, margaretha pretorius, pietro parisi, christian knigge, edward sion, paula szkody, klaus reinsch, and solen balman. all these talks contain the absolutely comprehensive material available today on the discussed problems. i do not reproduce their content, since all of them can be found in this book of proceedings, but i want to note that any researcher, studying cvs, independently on the age and scientific status, will find them useful. nonetheless i would like to focus on one of the important problems addressed in the talks. this is the problem raised by hans ritter. we all know that during the last 30 years the rkcat (undoubtedly, the most known catalogue of cvs and related objects) has been annually supplemented at a rate of approximately 9% a year. however, in his talk, hans ritter has drawn our attention to two important points: 1) servicing the data base is now practically a full-time job, 2) he will not be able to provide this service any longer. at this point a natural question for the entire binary community arises, who will continue with rkcat and in what form?”. happily the authors of this catalogue still intensively work and we have time to solve this problem. but, of course, we all should think about it. the contributed talks have been well prepared and delivered, and the reader may see it when reading the papers in the book of proceedings. like for the reviews i will not retell the content of even the most interesting talks, but i will try to pick out general tendencies that came out after analysis of the contributions. besides, i will highlight the results that might be useful for a broader community of researchers, studying objects and problems other than cvs. the first point, i would like to mention, is grand astronomical progress in south africa. its scientists have presented 7 talks, some of which delivered by very young persons, and all of them have excited an intense interest of the audience. undoubtedly, this burst of activity is concerned with the development and launch of the salt. and, in spite of the fact that david buckley still regards the obtained results as an appetizer, they obviously have become the main course in various fields of astrophysics. the presented talks revealed that theoretical studies and interpretations (both in the number of delivered talks and excited interest) noticeably dominate over observations. indeed we have heard wonderful analytical and numerical contributions that not only explain available observational data both from the space and ground-based instruments but also possess the great potential for the predictions, analysis and generalization of upcoming data. i do not want to focus on a particular work since i hope that the reader will first get familiar with the original papers but not these concluding remarks however, i must emphasize noticeable progress in observational methods. this has become my discovery that by using quite simple modifications one can significantly improve the quality of doppler tomography (see the papers of e. kotze, m. uemura, s. zharikov, s. potter, d. kononov) observational works have mostly followed a conservative way of data accumulation, which, undoubtedly, is the most important part of any research and the basics of scientific progress. at the same time we have got some surprises: .odendaal has told us that cal83 is a new candidate to ae aqr-like system; j. v. hernandez santisteban et al. have confirmed the detection of a sub-stellar donor star in a cv; d. chochol et al. have reported the superoutburst observations of a possible new helium-rich dwarf nova 2013 in hercules. the next important aspect in the development of observations is concerned with the effective usage of surveys (see, e.g., the wonderful paper of p. szkody about cvs from sdss and the paper of p. cartera about cvns in 323 http://dx.doi.org/10.14311/app.2015.02.0323 d. v. bisikalo sdss) and data from space telescopes (see e.g. the papers of p. parisi, s. balman, m. kotze, v. suleimanov, a. semena). resuming my concluding remarks i would like to stress that the conference was not only interesting, but also useful. i thank the organizers of the conference not only for the nice time, spent in the marvelous place, but also for the opportunity to discuss all the results in the friendly atmosphere. as i found when talking with many participants, we all are interested in the continuation of this series of meetings and in making this conference recurrent. franco giovannelli & lola sabau-graziati and their team are the main engine of the conference and i hope that they will continue this hard job, the organization of cvs conferences. 324 acta polytechnica ctu proceedings doi:10.14311/app.2017.11.0091 acta polytechnica ctu proceedings 11:91–96, 2017 © czech technical university in prague, 2017 available online at http://ojs.cvut.cz/ojs/index.php/app measurement methods of internal stress in continuous welded rail petr vnenk∗, bohumil culek department of transport structures, faculty of transport engineering, university of pardubice, studentska 95, pardubice, czech republic ∗ corresponding author: petr.vnenk@student.upce.cz abstract. this paper deals with the problem of internal rail stress estimation. it is based on a detailed research of contemporary situation in the field, presents basic outlines of the problem and sums up the major research areas and their possible applications in the current state of railway infrastructure management. the directions are divided into four categories in the paper: displacement methods, rail shifting methods, methods based on acoustoelastic effect and methods based on magnetoelastic effect. particular methods, both scientific and industrial, are presented in their section respectively. every method that is presented within the scope of this paper is briefly described and its advantages and disadvantages are mentioned. in the end, potential of application of some of the presented methods in the practical use is discussed. keywords: rail, stress, measurement, cwr, temperature. 1. introduction continuous welded rail, abbreviated as cwr, is a part of railway superstructure that has been under continuous development since its beginnings in the first half of the twentieth century. the implementation of continuous welded rail caused a major qualitative improvement in the railway transport. one of the biggest challenges of this improvement was the mitigation of the thermal stress effect. at the moment of welding, there is a certain stress state which equals a temperature called "neutral temperature". the neutral temperature is characterized by the attribute that when a welded rail reaches the neutral temperature, the rail is free from thermal stress. when the rail temperature is higher than the neutral temperature, compressive stress in rail emerges. at extreme temperatures, the compressive stress induced in the rail can cause railway track buckling, also known as "sun kink". on the contrary, when the rail temperature is lower than the neutral temperature, tensile stress in rail emerges. at extreme temperatures, the tensile stress induced in the rail can cause the rail to be pulled apart. the thermal stress in the continuous welded rail is expressed as σ = −α · ∆t · e (1) where α = 12 · 10−6k−1 (2) and e = 210 · 109pa (3) in the czech republic, the permitted welding temperature for rails ranges from 17 °c to 23 °c [1]. considering the minimum rail temperature at -30 °c and the maximum rail temperature at 60 °c (for rails situated in the sunny locations and thus heated), the difference between the extreme temperature and the neutral temperature can reach as many as 53 °c [2]. assuming such conditions, the thermal stress in continuous welded rail is σ = 133.56 · 106pa (4) this stress as such is not high enough to cause failure of continuous welded rail. however, there are other contributing factors, too. the thermal stress is cyclic, repeating in both daily and yearly cycles. trains cause further heavy dynamic loading which is cyclic, too. all these loadings are countable. it appears, however, that the neutral temperature is not constant over a period of time. based on the location of the rail, it seems the neutral temperature is shifting. this shift of the neutral temperature can reach even several degrees of celsius. this difference has to be added to the ∆t parameter, eventually increasing the thermal rail stress. combinations of the above mentioned stresses and their cyclic character have adverse influence on continuous welded rails. unlike the other items, it is very difficult to predict the shift of the neutral temperature. this article presents contemporary findings in 91 http://dx.doi.org/10.14311/app.2017.11.0091 http://ojs.cvut.cz/ojs/index.php/app petr vnenk, bohumil culek acta polytechnica ctu proceedings this area and suggests possible solutions to the evaluation of the thermal stress in rail with the influence of the neutral temperature shift. 2. displacement methods displacement methods of cwr stress measurement work on the simple mechanical assumption of change of a relative distance between two points of material, rail in this case. in newton’s mechanics, relatively stiff rail behaves according to hook’s law if unfastened. however, even the fastening systems ensure only a certain level of rigidity and, provided the axial force in rail is strong enough, a relative displacement can be observed even at fastened rail sections. as of now, some solutions using displacement are available as presented below. 2.1. research of the gdansk university of technology this research was carried out in the mid-seventies at the gdansk university of technology, poland. a displacement sensor with accuracy of 0.001mm has been applied for a measurement on a 200mm long rail section. next to the cwr measurement spot, a reference rail section has been placed. this reference rail section has been set free to lengthen and shorten. it was able to observe soon that the stress varies over different sections of the rail even if the temperature of the rail was the same at measured locations. it is believed that this effect is caused by slip of rail in the fastening points. such a slip can be derived by several sources like tangential forces of traffic, but also by exposure of some parts of rail to sun and location of some parts of rail to completely shady place. such a different exposure to sun results in different daily mean temperature and, subsequently, to a stress gradient in the rail. this revealing is a good asset of the measurement. the method itself, however, is not suitable for commercial use as it is very costly, time demanding and requires a reference rail to be placed at the measurement spot. nevertheless, it can well serve as a reference method for calibration of other methods [3]. 2.2. the calibrated length of rail method this method, which has supposedly been applied in china since 1982, consists of length comparison of a freely placed 50m long steel strip with a selected rail section originally of the same length. the principle of this method, however, appears to be the same like in the research of the gdansk university of technology, only modified for a bigger span. author’s daring claim that this method is the only one in the world without "fatal" shortcomings and can be operated widely does not match the state of the research in this area after the publication date, because much of research has been emerging in this field [4]. 2.3. measurement with bi-directional fbg strain sensors one of the most recent papers covering this topic, from 2015, presents a method of displacement measurement in a rail web in longitudinal and vertical direction using optic fbg sensors. in general, this measurement resembles strain gauge measurement. however, the advantage of fbg sensors is the possibility to measure even temperature, apart from mere strain. moreover, it is characterized by its longer durability in relation to strain gauge sensors. provided the sensor is attached to the rail web prior to cwr installation (i.e. at rail neutral temperature or known rail tension), it keeps measuring the deformation as long as the sensor works. this can be theoretically reached using strain gauges only, although the durability appears as an important factor here. [5]. 2.4. vortok measure and detect another option to measure displacement can be reached by using the measure and detect sensor produced by vortok ltd. company. simple sensor that is able to measure variety of parameters is made ready to be screwed into a drilled hole in a rail web. the manufacturer does not share any detailed information of what principles this device uses. it looks, however, like a piezoelectric sensor with accessories that is incorporated into a screw-in dowel. this robust shape most probably predicts a displacement method with a long durability. a disadvantage of this method is a necessity of calibration. after calibration is made, either by fastening of the device on a stress-free rail or by using another calibration method, the value of internal stress in rail can be obtained in a stable manner. the options to set up automatic data transmission via email or wi-fi even from a moving train make this sensor a user-friendly device [6]. 3. rail shifting methods rail shifting methods of cwr stress measurement use the principle of bending stiffness dependence on axial tension in rail. provided the rail is in tension, the higher force is necessary to be applied to laterally shift the rail the higher the axial force in the rail is. apparently, these methods are viable only when the rail is in tension, otherwise failure by buckling may appear. 3.1. verse method this method has been presented by vortok ltd. company from the united kingdom. in the first place, the rail shall be released from all fastenings in the length of 30m. afterwards, the central part of the unfastened section of the rail shall be lifted into a certain height. the force that is applied to move the rail is related to the axial force in the rail. wide use of this method is restricted by some disadvantages. the most obvious one is the limitation to tensile stress in the measured rail. the operator needs 92 vol. 11/2017 measurement methods of internal stress in cwr to be sure that there is a tensile stress as problems with re-installation of the rail into the original position could occur, leave apart the danger of operator’s injury or material damage. another disadvantage is the need to unfasten the rail in the length of 30m. this operation requires additional time and subsequently prolongs track closure. moreover, in case the measured section is situated in radius, re-installation of the rail can be difficult, too, as the condition of tensile stress in rail is inevitable. even though the disadvantages are important, this method appears to be one of the most widely spread to measure the cwr stress nowadays. the manufacturer produces their own device for rail lifting and stress measurement. light aluminium design of this device enables easy transport, manipulation and operation [7]. 3.2. use of tamping machine in a paper published in 2009, scientists at gdansk university of technology look into a possibility to use tamping machine for stress measurement in cwr. tamping machine is capable to shift rails in both vertical and horizontal direction when adjusting railway track geometry. the goal of this research was to use this movement to estimate the stress in cwr. the principle of this measurement is similar like at the verse method, including the disadvantages. however, the disturbing signal produced by the operation of tamping machine turned out to be too high and the attempts to get some cwr stress data were unsuccessful [8]. 4. methods based on acoustoelastic effect the acoustoelastic effect is a physical effect of change of sound velocity based on change of mechanical stress in elastic continuum. for measurements, ultrasound is used. the definition, however, can cover vibration, as mechanical waves, too. in such a case, vibrometer is used as a measurement tool. it is always necessary to execute a calibration measurement on the site with known temperature and mechanical stress as the sound velocity in rail is not dependent only on mechanical stress, but on steel microstructure, too. this calibration measurement can be performed either prior to cwr installation, or once the cwr is installed. however, in the latter only when the rail is cut, unfastened and welded again. as a consequence, similar disadvantage like at displacement methods emerges, which means the need to start the measurement at the neutral temperature. the advantage, on the contrary, is in possible high durability of ultrasound sensors and their attachment to measurement place. 4.1. j. szelazek measurement j. szelazek has worked out two ultrasound measurement procedures in the nineties of the twentieth century. in the first procedure, ultrasound signal is emitted vertically from the top of rail head to the bottom of rail foot where it reflects off and moves back to the receiver, which can be the same device as the emitter. the second procedure uses separate emitter and receiver and those are placed to the side of rail head horizontally next to each other. the signal moves through the rail head lengthwise, in this case. the first procedure showed as inapplicable, as the signal dispersed due to uneven rail head profile. the major cause of this unevenness is the rail head wear. placing the signal receiver to the bottom of rail foot did not improved the results and, moreover, the signal path was reduced to one half. the second procedure appeared more promising, but even at this measurement, disturbances appeared and the results were influenced by strong dispersion [9]. 4.2. research of the university of florence in 2007, italian scientists worked out the second procedure of j. szelazek. this procedure has been tested for 2 years and appears viable. however, it shares all the common disadvantages of ultrasound methods and the necessity to perform calibration measurement in situ [10]. 4.3. measurement of stresses using the polarization of rayleigh surface waves two us researches work with measurement of polarization of rayleigh surface waves. measurement of dependence of the rayleigh surface waves polarization on change of mechanical rail stress turned out not just to be more robust than measurement of dependence of the rayleigh surface waves velocity on change of mechanical rail stress, but also to be easier to detect. michael d. a. junge’s work deals with general measurement of mechanical stress in material based on rayleigh surface waves [11]. stefan hurlebaus’ report deals with application of this procedure on rail stress research. on page 42, the author presents that the polarization of rayleigh surface waves is dependent on the value of axial force in rail and that a further research is recommended [12]. 4.4. vibration measurement two publications, a diploma thesis and a research report, deal with vibration measurement of the internal rail stress. both of them are outputs of one research. the results of the rail stress determination from wave lengths of vibrating sensor seem well in laboratory conditions when a steel bar or a new rail is used. in case of worn rail, and this is very important for practical application, it is possible to get certain results, but these results are not that clear like in the previous cases. when performing a field measurement, the results of measurement on worn rail are much worse. 93 petr vnenk, bohumil culek acta polytechnica ctu proceedings as of yet, this method is inapplicable. in case it is possible to diminish the problems, this method can have a good perspective [13, 14]. 5. methods based on magnetoelastic effect the magnetoelastic effect is inverse magnetostrictive effect. it is presented as a change of magnetic susceptibility in relation to change of mechanical stress in material. the magnetoelastic effect is also known as the villari effect. 5.1. measurement of magnetic hysteresis in the nineties, d. utrata tested the possibility of application of the magnetoelastic effect on the rail stress measurement. he measured the dependence of coercivity and remanence of the magnetic field in rail on different surface conditions, like scaled rail surface, or milled rail surface. however, the comparison of the obtained data has shown various results based on the input type of rail. surprisingly, the only data that match were on the milled running surface and scaled base underside. d. utrata assumes, in the end, that either a calibration measurement or a new approach to data interpretation has to be delivered in order to make this method applicable. a. wegner needed 400 calibration sensors to run a measurement of 3 meters long rail using the hysteresis method, which does not contribute to viability of this method [14, 15]. 5.2. measurement of magnetic barkhausen effect japanese scientists tsuchima and enokinozo have studied the reaction of the barkhausen noise on mechanical stress in steel plate. it is possible to get some relation from the graphs of the mutual dependence. however, the authors found it difficult to estimate the influence of time and get a value of the mechanical stress. additionally, they suggest application of the chaos theory to get a better interpretation of the results [16]. measurement using the barkhausen noise together with measurement of magnetic permeability is used by elektro-thermit gmbh & co. kg. company to determine the internal rail stress. their method requires calibration on a rail test sample. their product has been awarded a certificate of the railway infrastructure manager of denmark, banedanmark. on the contrary, the manufacturer appears not interested in sale of their product and only offers to perform a measurement in the required section by themselves [3, 17]. 5.3. metal magnetic memory measurement collective of authors from the university of nanjing have carried out an experiment of measuring the effect called metal magnetic memory on a steel test sample. figure 1. dhp(y)/dx for applied tensile stress on a steel sample [18]; see the linear dependence in the elastic region. contrary to the barkhausen noise, this effect does not require external magnetization of the sample. the effect is activated anywhere in the natural magnetic field of the earth. when the loading increases, the rotation of the weiss magnetic domains in metals raises the value of the magnetic flux around the central point and this increases the gradient of the magnetic flux which is in elastic region linearly dependent on the stress in the sample. graph of the linear dependence of the magnetic flux gradient on the mechanical stress in a steel sample as measured by wang, et. al. [18] is presented in figure 1. the linear dependence in the elastic region is completely sufficient, because rail geometry deformation emerges earlier than plastic deformation in the axial direction of the rail [18]. 6. discussion every presented method has its limitation. displacement methods can be related to widely spread strain gauge measurement. this has been used in many industrial application; in this case, however, the biggest problem seems to be the necessity to measure an absolute value of stress, not a relative one, which is the common purpose of strain gauge measurement in general. rail shifting methods, especially the verse method, seem to be one of the most reliable at this time. the reliability is underpinned by its simplicity. however, more extensive use of this method is very limited through many disadvantages it inevitably drags along. movement of a rail is a complicated operation, which needs expensive tools and is very time demanding. re-installation of the rail requires verification that the rail is in good condition after the measurement and safety of operation on the track is kept. additionally, performing such a measurement in radius is much more complicated and performing such a measurement when the rail is under a compressive stress is excluded. considering the compressive stress 94 vol. 11/2017 measurement methods of internal stress in cwr in rail is usually during the day, the limitations of use of this method are vast. methods based on the acoustoelastic and magnetoelastic effect have the potential to become viable in the non-destructive rail stress measurements. as of yet, however, the problems of getting clear values of stress were not mitigated and residual stresses, difficult conditions of field measurement, or even various conditions of the rail itself (wear, rust, etc.) prevented the research to reach a satisfactory solution. 6.1. further research orientation out of the contemporary possibilities, there are some ways to be worked out in order to either get satisfactory results or to discover serious obstacles that hinder their application. (1.) application of the metal magnetic memory method on internal rail stress measurement seem like a research that has not been done yet. as mentioned in section 5, this method does not require external magnetization [18] and this is a great asset for possible further application. if the measurement of the gradient of magnetic flux in rail results in a similar dependence as shown in figure 1, the potential of applicability of this method would be great [18]. (2.) vibration measurement, or modal analysis could also offer an approach to obtain data of internal stress in cwr. however, damping caused by rail fastenings could have too high influence on the results. as presented in section 4, the vibration analysis performed by the team of the university of illinois at urbana-champaign did not produce any applicable results even after they released some fastenings, which is a procedure that is disadvantageous as such [13, 14]. (3.) provided the stress could be evaluated from a surface layer of rail only, i. e. there was a mutual relation between surface stress and stress in the core of the rail, another possible method emerges. method of dependence between passive layers formation in electrolyte and mechanical stress in the surface layers has been shown in an experiment of stainless steel in a normal sulphuric acid bath [19]. (4.) another option is to work out the measurement of stresses using the polarization of rayleigh surface waves. the papers presented in section 4 suggest a further research can have a potential to find out a viable solution [11, 12]. (5.) finally, if strain gauge is able to measure a quasi static load change in a long term, a statistical evaluation of measurements could provide a range of data that could be sufficient for prediction of neutral temperature level and subsequently stress level in rail. if such a research is biased towards a certain limited location, an applicable methodology can be a reasonable output. 7. conclusions many attempts to measure the neutral temperature or stress in continuous welded rail have been undertaken. displacement and rail shifting methods like vortok measure and detect or verse have been finalized in an applicable product and offered on market. these methods come out of mechanical solutions. however, these methods suffer from many limitations and requirements of special conditions that are necessary to be created in order to use the methods. later in time, non-destructive methods based on the acoustoelastic or magnetoelastic effects have been tested. as of yet, none of them reached the level of being reliable enough to compete with the above mentioned ones. nevertheless, there are some promising results and further research in this area, as described in section 6, could prove viability of some of them. provided strain gauges are capable of measuring quasi static load in a long term and a thorough strain gauge measurement is done, the value of stress in a continuous welded rail could be estimated on a certain geographical area even from statistical data measured by strain gauges. finally, the possible solution can lurk in another method of stress evaluation in a continuum, a method which is not yet known to the authors of this paper. acknowledgements the work was created in connection with the scientific research project of the university of pardubice no. sgs_2017_009. the authors are grateful for the support. references [1] sprava zeleznicni dopravni cesty, s.o. szdc s 3/2 bezstykova kolej, 2013. kapitola iii, cast a, bod 110. [2] sprava zeleznicni dopravni cesty, s.o. szdc s 3 dil iv zeleznicni svrsek. kolejnice ve zneni zmen c. 1 a 2, 2014. kapitola iv, bod 43. [3] w. koc, et. al. okreslanie wartosci sil podluznych w szynach toru bezstykowego. problemy kolejnictwa 163, 2014. [4] l. xing-han. the "calibrated length of rail" method for measuring rail temperature stress. arema proceedings of the 2000 annual conference 2000. [5] p. wang, et. al. longitudinal force measurement in continuous welded rail with bi-directional fbg strain sensors. smart materials and structures 25, 2015. [6] vortok, ltd. vortok measure and detect, 2012. vortok, [2016-06-07], http://www.vortok.com/rail-stress-management/ vortok-measure-and-detect-2. [7] vortok, ltd. verse, 2012. vortok, [2016-05-31], http: //www.vortok.com/rail-stress-management/verse. [8] w. koc, a. wilk. investigations of methods to measure longitudinal forces in continuous welded rail tracks using the tamping machine. journal of rail and rapid transit 223(1), 2009. 95 http://www.vortok.com/rail-stress-management/vortok-measure-and-detect-2 http://www.vortok.com/rail-stress-management/vortok-measure-and-detect-2 http://www.vortok.com/rail-stress-management/verse http://www.vortok.com/rail-stress-management/verse petr vnenk, bohumil culek acta polytechnica ctu proceedings [9] j. szelazek. monitoring of thermal stresses in continuously welded rails with ultrasonic technique, 1998. ndtnet, 1. june 1998, [2016-05-30], http://www. ndt.net/article/dresd97/szelazek/szelazek.htm. [10] d. vangi, a. virga. a practical application of ultrasonic thermal stress monitoring in continuous welded rail. experimental mechanics 47, 2007. [11] m. d. a. junge. measurement of applied stresses using the polarization of rayleigh surface waves, 2003. atlanta, georgia, school of civil and environmental engineering, institute of technology. [12] s. hurlebaus. determination of longitudinal stress in rails, 2011. college station, texas, transportation research board of the national academies. [13] r. weaver. vibration measurement of rail stress, 2006. champaign, illinois, transportation research board of the national academies. [14] m. j. koob. the development of a vibration technique for estimation of neutral temperature in continuously welded railroad rail, 2005. champaign, illinois, university of illinois at urbana-champaign. [15] d. utrata, a. strom, m. negley. stress measurement in railroad rail using ultrasonic and magnetic technique. review of progress in quantitative nondestructive evaluation 14, 1995. [16] y. tsuchida, m. enokinozo. stress evaluation by chaotic characteristics of barkhausen noise. review of quantitative nondestructive evaluation 22, 2003. [17] a. wegner. stress-free temperature monitoring using different measuring technologies – experiences and assessment. proceedings of 10th international heavy haul association conference 2013. [18] p. wang, et. al. stress measurement using magnetic barkhausen noise and metal magnetic memory testing. 17th world conference on nondestructive testing 2008. [19] f. navai. effects of tensile and compressive stresses on the passive layers formed on a type 302 stainless steel in a normal sulphuric acid bath. journal of materials science 30, 1995. 96 http://www.ndt.net/article/dresd97/szelazek/szelazek.htm http://www.ndt.net/article/dresd97/szelazek/szelazek.htm acta polytechnica ctu proceedings 11:91–96, 2017 1 introduction 2 displacement methods 2.1 research of the gdansk university of technology 2.2 the calibrated length of rail method 2.3 measurement with bi-directional fbg strain sensors 2.4 vortok measure and detect 3 rail shifting methods 3.1 verse method 3.2 use of tamping machine 4 methods based on acoustoelastic effect 4.1 j. szelazek measurement 4.2 research of the university of florence 4.3 measurement of stresses using the polarization of rayleigh surface waves 4.4 vibration measurement 5 methods based on magnetoelastic effect 5.1 measurement of magnetic hysteresis 5.2 measurement of magnetic barkhausen effect 5.3 metal magnetic memory measurement 6 discussion 6.1 further research orientation 7 conclusions acknowledgements references 66 acta polytechnica ctu proceedings 2(1): 66–70, 2015 66 doi: 10.14311/app.2015.02.0066 multifrequency behaviour of polars k. reinsch1 1georg-august-universität göttingen, institut für astrophysik, friedrich-hund-platz 1, 37077 göttingen, germany corresponding author: reinsch@astro.physik.uni-goettingen.de abstract cataclysmic variables emit over a wide range of the electromagnetic spectrum. in this paper i will review observations of polars in relevant passbands obtained during the last decade and will discuss their diagnostical potential to access the physics of the main components within the binary systems. this will include a discussion of intrinsic source variability and the quest for simultaneous multi-frequency observations. keywords: cataclysmic variables polars optical spectroscopy photometry ir uv x-rays. 1 introduction in the standard model of magnetic cataclysmic variables a polar is described as a semi-detached binary in which matter from a roche-lobe filling late-type mainsequence star is coupled outside the circularization radius to the field lines of a highly-magnetic white dwarf and channeled onto small accretion region(s) in the vicinity of the magnetic pole(s). the system components can be observationally disentangled – at least to some degree – by their characteristic emission in different wavelength regimes (see fig. 1). the contribution of the binary components to the spectral energy distribution of the system will be discussed individually and in the context of multi-frequency observations. the basic understanding of polars has not much changed since the comprehensive review by warner (1995). i will, therefore, concentrate on recent observational findings which led to a more detailed understanding of the physical processes involved. figure 1: schematic view of a polar and main emission regions (iris traulsen, adapted from marc garlick). 66 http://dx.doi.org/10.14311/app.2015.02.0066 multifrequency behaviour of polars 2 binary components and spectral energy distribution 2.1 secondaries in polars secondaries in polars emit most of their light in the red and near ir. but even in these bands their luminosity is generally outshone by other components within the binary. the secondaries are, therefore, best studied when the system happens to be in a low state of accretion or during eclipses of the accretion region and the white dwarf. optical and near-ir studies have revealed that their spectra are consistent with normal late-type dwarfs (or brown dwarfs). contrary to similar studies of non-magnetic systems, no significant abundance anomalies have been found from near-ir spectroscopy of the secondaries in polars. this provides evidence that magnetic and non-magnetic systems might take different evolutionary paths (harrison et al., 2005). spectroscopy during the eclipse of the primary combined with phase-resolved near-ir photometry of polars can be used to determine the spectral type and the brightness of the secondary, as has been shown e.g. for the long-period polar v1309 ori (reinsch at al., 2006). using calibrations of the surface brightness of m-dwarfs (e.g. beuermann 2000), from the brightness of the secondary an accurate distance of the system can be derived. 2.2 circumbinary dust disk mid-infrared observations of the short-period polar ef eri with the infrared spectrograph (irs) on the spitzer space telescope have revealed the presence of emission in the 3–14 µm wavelength range which exceeds the combined spectral energy distribution of the white dwarf, the l5-type brown dwarf secondary, and cyclotron radiation from the b ≈ 13 mg white dwarf magnetic field. hoard et al. (2007) concluded that the spectrum longward of ∼ 5 µm is dominated by emission from a circumbinary dust disk. in a later paper, however, harrison et al. (2013) argue based on combined ground-based, spitzer, wise, and herschel observation of the two polars am her and ef eri that their midinfrared excess can be fully explained by optically thin emission from the lowest cyclotron harmonics (n ≤ 3). 2.3 stellar activity of the secondary from long-term observations of polars it is well known that most systems alternate between low and high states and sometimes also intermediate states of accretion. the best studied system is am her with a record of 33 years of optical observations by the american association of variable star observers (aavso, http://www.aavso.org/). due to the absence of an accretion disk in polars such changes must reflect the immediate response of the system on variations of the mass-transfer rate. stellar activity of the donor star has been proposed as a possible mechanism to induce such variations (e.g. hessman et al., 2000). in a recent analysis of shortand long-term brightness variations of five polars, periodic variations were indicated which could result from magnetic cycles in the secondary stars (kalomeni, 2012). kafka et al. (2010) present another piece of evidence for stellar activity from phase-resolved high-resolution spectroscopy of polars in low states. they interprete two bright satellites of the central low-velocity component in the hα doppler map of bl hyi as prominencelike magnetic loops kept in place by magnetic field interactions between the white dwarf and the donor star. howell et al. (2006) have monitored the long-term variation of the hα equivalent width of ef eri during its extended low state and attribute the observed behaviour to chromospheric activity of the secondary. in addition they identify magnetic field interaction between the two stars as a probable mechanism that would concentrate stellar activity on the white dwarf facing hemisphere of the secondary. 2.4 coupling region partially ionized matter released from the secondary over the l1 point is assumed to follow initially a freefalling trajectory. at the magnetospheric radius of the white dwarf the ram pressure of the infalling gas is balanced by the magnetic pressure and the accretion stream couples to the magnetic field of the white dwarf. observationally little information is available on the processes occurring in this coupling region. xmm-newton observations of v1309 ori show a prolonged ingress of the soft x-ray light curve with xray emission being visible beyond the geometrical occultation of the white dwarf. schwarz et al. (2005) have suggested plasma heated by shocks in the coupling region as a possible origin of the x-ray emission seen during eclipse. 2.5 accretion curtain beyond the coupling region accretion becomes magnetically controlled and the initially narrow ballistic stream is diverted off the orbital plane and forms a broad curtain-like structure through which matter is channeled into the vicinity of the magnetic poles of the white dwarf. the accretion stream and curtain are characterized by an optical and uv emission-line spectrum comprising strong lines of neutral hydrogen and of ionized helium. using relatively simple geometrical assumptions indirect imaging techniques, like doppler 67 k. reinsch tomography, roche tomography, accretion stream mapping, and eclipse mapping, have been well established to confine the geometrical parameters of different parts of the accretion stream (e.g. schwope et al., 2004). 2.6 white dwarf white dwarfs in polars have typical temperatures in the range 10,000–15,000 k. their photospheric emission has been well studied in the uv and far-uv spectra of polars in low states of accretion. based on a far-uv spectroscopic survey with hst/ stis, araujo-betancor et al. (2005) found that at any given orbital period white dwarfs in magnetic cvs are colder than those in non magnetic cvs with the differences in effective temperatures being largest for systems above the period gap. as a possible explanation they suggest that magnetic braking in polars might be reduced. the magnetic field structure of several white dwarfs in polars has been studied in detail using phase-resolved circular spectropolarimetry and zeeman tomography (e.g. beuermann et al., 2007 and references therein). in most cases the field structure departs significantly from that of a dipole field and higher order multipoles are required to adequately describe the observed zeeman spectra. accreting material is flowing preferentially along field lines reaching far out towards the ballistic part of the stream. depending on field geometry and accretion rate, this leads to one or more accretion regions on the white dwarf. 2.7 accretion region in the standard scenario actual accretion onto magnetic white dwarfs occurs in a comparably small region of some 0.1% of the white dwarf surface. infalling matter forms an accretion column with a shock front at some height above the white dwarf where the supersonic gas of the accretion stream is decelerated to a subsonic settling flow. the shock height and the depression of the post-shock flow below the photosphere depends on the magnetic field strength, the mass of the white dwarf, and the specific accretion rate. the structure of the accretion column has been studied theoretically. the emergent spectra have been calculated by radiationhydrodynamical models as a function of mass-flow rate and field strength (e.g. fischer & beuermann, 2001; beuermann, 2004). at low mass-flow rates, the shock stands above the atmosphere and bremsstrahlung radiation is the main cooling process. for larger accretion rates the emission is hidden behind the column density and reprocessed as soft x-rays. irradiation is the principle cause for a large heated accretion cap extending far beyond the impact region of the accreted material. calculations with a standard lte stellar atmosphere code including an angledependent external radiation source have demonstrated that much of the reprocessed light appears in far uv and not at soft x-rays (könig et al., 2006). observationally the temperature distribution in the accretion spot could be demonstrated by the analysis of the chandra letg spectrum of am her. this data provide enough photons to show that the soft x-ray component cannot be fitted by a single temperature model (beuermann et al., 2012). at high mass-transfer rates sufficiently dense blobs can penetrate to large optical depths in the white-dwarf atmosphere submerging the shock below the photosphere. the most extreme example of this “blobby accretion” case has been observed in v1309 ori. its soft x-ray light curve obtained with xmm-newton shows x-ray emission occuring in flares of typically 10 s time scale. this corresponds to a length scale of 4 × 109 cm for the infalling filaments (schwarz et al., 2005). short-term spectral variability at x-ray wavelengths could be studied in the rosat pspc high state observation of am her which contains 1.3 million counts. significant variability on time scales down to 200 ms could be detected. correlated spectral and count-rate variations are seen in flares on time scales down to 1 s, demonstrating the heating and cooling associated with individual accretion events (beuermann et al., 2008). some polars are even bright enough that x-ray emission line spectroscopy is feasible with current instruments. the high-diagnostics potential of such observations has been demonstrated e.g. by girich et al. (2007), mauche et al. (2003), mauche et al. (2009). 3 variability of spectral energy distribution several polars have been studied at different epochs yielding that their optical, uv, and x-ray fluxes can vary by more than an order of magnitude. as an example, fig. 2 shows the integrated energy flux νfν from the infrared to the hard x-ray regime of the polar rx j1007.5-2017 collected with various instruments between 1992 and 2001. the left panel shows spectrophotometry of 1992, 1997, and 2000 (black curves), supplemented by the 2mass j, h, and k-band fluxes of 1999 (red filled squares) and the visual and ultraviolet fluxes measured with the optical monitor on board of xmm-newton simultaneous to the 2001 x-ray observation (red filled circles). the green curve represents the adjusted flux distribution of the dm3 star lhs58 which is expected to match the spectral type of the donor star and to be the main contribution to the 1997 low-state spectrum. 68 multifrequency behaviour of polars figure 2: spectral energy distribution of the polar rx j1007.5-2017 at different epochs (from: thomas et al. 2012) the right panel shows the incident spectra for the 2001 xmm pn observation (red filled circles) and the 1992 rosat pspc observation (open circles). the dashed curve illustrates the “source spectrum” corrected for interstellar absorption (thomas et al., 2012). 4 conclusions x-ray and optical surveys, like the rosat all-sky survey and the sloan digital sky survey, let to the discovery of a large number of new polars during the recent two decades increasing the sample size to 135 polars with known orbital periods (ritter & kolb 2003; 2013). in addition, sensitivity and instrumental capabilities at all wavelengths have been significantly enhanced. taking both together, polars provide an important lab to quantitatively understand individual systems and physical processes of matter under extreme conditions. in this sense, we are indeed currently in a “golden age” of research on polars. there are still several fundamental questions remaining open. among these are: do we fully understand the evolution of polars? what drives accretionrate variations? do polars with fields as high as in single white dwarfs exist? what is the origin of the magnetic field in polars? what exactly happens in the coupling region? multi-frequency observations are a powerful tool to improve our understanding of polars in future. due to large variations of the accretion rate on different time scales simultaneous coverage at different wavelengths is mandatory. acknowledgement i thank the many people who contributed to increase our knowledge on polars over the last decade, and kindly acknowledge fruitful and long-lasting collaborations with klaus beuermann, vadim burwitz, essam el-kholy, fabian euchner, boris gänsicke, yonggi kim, axel schwope, robert schwarz, hans-christoph thomas, iris traulsen, and fred walter which led to several papers i have referred to in this contribution. references [1] araujo-betancor, s., gänsicke, b.t., long, k.s., beuermann, k., de martino, d., sion, e.m., szkody, p., 2005, apj 622, 589 doi:10.1086/427914 [2] beuermann, k., 2000, new astr. rev. 44, 93 doi:10.1016/s1387-6473(00)00020-8 [3] beuermann, k., 2004, aspc 315, 187 [4] beuermann, k., el kholy, e., reinsch, k., 2008, a&a 481, 771 [5] beuermann, k., euchner, f., reinsch, k., gänsicke, b.t., 2007, a&a 463, 647 [6] beuermann, k., burwitz, v., reinsch, k., 2012, a&a 543a, 41 [7] fischer, a., beuermann, k., 2001, a&a 373, 211; 69 http://dx.doi.org/10.1086/427914 http://dx.doi.org/10.1016/s1387-6473(00)00020-8 k. reinsch [8] girich, v., rana, v.r., singh, k.p., 2007, apj 658, 525 [9] harrison, t.e., howell, s.b., szkody, p., cordova, f.a., 2005, apj 632, l123 doi:10.1086/498067 [10] harrison, t.e., hamilton, r.t., tappert, c., hoffman, d.i., campbell, r.k., 2013, aj 145, 19. [11] hessman, f., gänsicke, b., mattei, j., 2000, a&a 361, 952 [12] hoard, d.w., howell, s.b., brinkworth, c.s., ciardi, d.r., wachter, s., 2007, apj 671, 734. doi:10.1086/522694 [13] howell, s.b., walter, f.m., harrison, t.e., huber, m.e., becker, r.h., white, r.l., 2006, apj 652, 709 doi:10.1086/507603 [14] kafka, s., tappert, c., ribeiro, t., honeycutt, r.k., hoard, d.w., saar, s., 2010, apj 721, 1714 doi:10.1088/0004-637x/721/2/1714 [15] könig, m., beuermann, k., gänsicke, b., 2006, a&a 449, 1129 [16] mauche, c.w., liedahl, d.a., fournier, k.b., 2003, apj 588, l101 doi:10.1086/375684 [17] mauche, c.w., 2009, apj 706, 130 doi:10.1088/0004-637x/706/1/130 [18] reinsch, k., kim, y., beuermann, k., 2006, a&a 457, 1043 [19] ritter, h., kolb, u., 2003, a&a 404, 301 (update rkcat7.20, 2013) [20] schwarz, r., reinsch, k., beuermann, k., burwitz, v., 2005, a&a 442,271 [21] schwope, a., staude, a., vogel, j., schwarz, r., 2004, an, 325, 197 [22] thomas, h.-c., beuermann, k., reinsch, k., schwope, a.d., burwitz, v., 2012, a&a 546, a104 [23] warner, b., 1995, cataclysmic variable stars, cambridge university press discussion david buckley: would you care to comment on the fact that a certain fraction of polars have no directly (observed) determined magnetic fields (e.g. from either detection of cyclotron features or spectropolarimetry) and have been suggested to be low-field systems. klaus reinsch: the magnetic field of polars must be strong enough that spin and orbital motion of the white dwarf will be synchronized. direct measurements of magnetic fields in polars require the systems to be in a suitable state that either cylcotron or zeeman features can be detected. therefore, there are still many polars lacking field determinations. augustin skopal: how does the sed look like with the unabsorbed supersoft x-rays and dereddened far-uv fluxes? does your unabsorbed x-ray model fit the far-uv fluxes? klaus reinsch: the “source spectrum” of the x-ray components corrected for interstellar absorption is included as the dashed curve in fig. 2. it is two orders of magnitude below the observed flux in the uv. raymundo baptista: you showed doppler tomograms of hα emission from am her in low state by kafka et al. (2010), with side emission apparently from l4 and l5 regions. an alternative explanation for that is zeeman-doppler spliting produced by the magnetic field of the donor star, of the order of a few kg. klaus reinsch: yes, i agree. linda schmidtobreick’s comment: you showed the prominences detected by stella kafka in am her. we detected the same structure in the low state of bb dor which is a non-magnetic vy scl system. so we think it unlikely that the magnetic field of the wd has an influence on this structure and it should rather be explained by a combination of the magnetic field of the secondary plus the roche potential. klaus reinsch: this is indeed interesting to note. in the final remarks of their paper kafka et al. (2010) state that prominence-like structures are likely present in disk cvs above the period gap as well. but their stability and evolution with time is expected to be different. 70 http://dx.doi.org/10.1086/498067 http://dx.doi.org/10.1086/522694 http://dx.doi.org/10.1086/507603 http://dx.doi.org/10.1088/0004-637x/721/2/1714 http://dx.doi.org/10.1086/375684 http://dx.doi.org/10.1088/0004-637x/706/1/130 introduction binary components and spectral energy distribution secondaries in polars circumbinary dust disk stellar activity of the secondary coupling region accretion curtain white dwarf accretion region variability of spectral energy distribution conclusions 60 acta polytechnica ctu proceedings 2(1): 60–65, 2015 60 doi: 10.14311/app.2015.02.0060 on the influence of magnetic field on accretion processes in cvs d. v. bisikalo1, a. g. zhilkin1 1institute of astronomy ras, moscow, russia corresponding author: bisikalo@inasan.ru abstract we consider the influence of such parameters as the value of the proper magnetic field ba and the spin-rotation velocity of the white dwarf on accretion processes in cvs. the results of 3d mhd simulations have shown that the accretion rate is a non-monotonic function of ba: with growing ba it raises in the intermediate polars and decreases in the polars. the maximal accretion rate occurs in the systems, transiting from the stage of intermediate polars to polars; it’s value reaches ∼60% of the initially set mass transfer rate. we have also shown that the acretion rate decreases with the growing spin-rotation velocity of the white dwarf. keywords: cataclysmic variables intermediate polars polars accretion mhd simulations. 1 introduction two main classes of cvs where magnetic field significantly influences accretion processes are the intermediate polars and polars [1]. these are semi-detached binary systems, consisting of a low-mass late type star (donor-star) and a white dwarf (accretor). in the polars, white dwarfs possess strong magnetic fields (ba ≈ 107–108 g on the surface). observations show that no accretion disks form in polars. instead, the material, issuing from the l1 point, forms a collimated stream, that flows onto one of the magnetic poles of the accretor along magnetic field lines[1, 2]. in the intermediate polars, white dwarfs possess relatively weak magnetic fields (ba ≈ 104–106 g on the surface). these systems occupy an intermediate stage between the polars and non-magnetic cvs. we should note that in these systems the spin-rotation periods of the primaries may be significantly shorter than the orbital periods [3]. in our previous works [4, 5] we, by means of 3d mhd simulations, studied the flow structure of the intermediate polars and polars, having various surface magnetic fields of the accretors ba, ranging from 105 to 108 g. this allowed us to investigate conditions of accretion disk formation and to find a criterion, separating two types of the flow, corresponding to the intermediate polars and polars. besides, in [5] we also investigated how the spin-rotation velocity of the accretor influences the mhd flow structure in the system.in this paper we summarize the results of our previous research and show how the accretion rate varies, depending on two parameters as the magnetic induction on the surface of the white dwarf and its spin-rotation velocity. 2 what happens with the flow structure if ba grows to investigate how the value of the accretor’s surface magnetic field influences mhd flow structure in a close binary system we have conducted 7 computational runs with various ba: 105 g (model 1); 5 × 105 g (model 2); 106 g (model 3); 5 × 106 g (model 4); 107 g (model 5); 5 × 107 g (model 6); and 108 g (model 7). we suppose that the accretor’s magnetic field is of dipole type and the axis of the field is inclined with respect to the accretor’s rotation axis by θ = 30◦. for the computations we used our method of investigations of mass transfer processes in semi-detached binary systems where the accretors possess strong magnetic fields [4, 5]. the method is based on the idea that plasma dynamics is determined by the relatively slow average motion of plasma against a background of which very rapid mhd waves propagate. the equations, describing the slow motion of plasma, are derived via a procedure of averaging over the rapidly propagating pulsations. strong external magnetic field in this approach plays a role of an effective liquid interacting with plasma. in the equation of motion the mean electromagnetic force is analogous to the friction force working in multi-component plasma. in our numerical model we take into account processes of the magnetic field diffusion caused by the currents’ dissipation in turbulent vortices and magnetic buoyancy. besides, when averaging the induction equation over the rapidly propagating mhd waves we introduce an additional source of magnetic field dissipation (wave dissipation). in order to see how namely the magnetic induction influences the solution we fix all the other param60 http://dx.doi.org/10.14311/app.2015.02.0060 on the influence of magnetic field on accretion processes in cvs eters. the computations have been conducted for a close binary system, having parameters of the ss cygni (see [6]). the donor-star (red dwarf) in this system has the mass md = 0.56m� and the effective temperature of 4000 k. the accretor (white dwarf) has the mass ma = 0.97m� and the temperature of 37000 k. the orbital period of the system is porb = 6.6 hours and its binary separation is a = 2.05r�. we have supposed that the accretion disk forms in the system for the first time so the accretor’s spin-rotation is synchronous, i.e. pspin = porb. the influence of the accretor’s spinrotation on a formed accretion disk is considered in the next section. the obtained solutions can be divided into two groups. the first group contains models 1, 2, and 3 with relatively weak magnetic fields where accretion disks form. these models correspond to the intermediate polars. in the models of the second group (models 4–7) magnetic fields are strong and there are no accretion disks formed. these models represent the polars. the typical flow structure in the intermediate polars is shown in fig. 1, where we plotted an iso-surface of density logarithm lgρ = −4.5 (in units of ρ(l1)) obtained in the model 1 (ba = 105 g). in figure we also show the magnetic flow lines originating on the accretor’s surface, accretor’s rotation axis (thin straight line) and magnetic axis (bold inclined line). we note that all the gas dynamic structures found in our previous studies [7, 8, 5], including the accretion disk and shock waves exist in the obtained solution. near the accretor the magnetosphere forms and material is accreted via funnel flows. with growing magnetic field the outer radius of the disk significantly decreases. the efficiency of the magnetic breaking and angular momentum transfer increases. the region of the magnetosphere becomes significantly larger. finally, when ba = 106 g (model 3, fig. 2) the accretion disk almost degenerates. the material makes only 1–2 cycles before falling onto the accretor. to describe this flow structure a term ”spiralodisk” is more appropriate, since the velocity distribution in this structure strongly differs from the keplerian. the outer radius of this ”spiralo-disk” is approximately 0.1a and the inner radius is determined by the size of the magnetosphere. a significant part of the ”spiralo-disk” is accretion funnel flows. we can state that this solution is an ultimate case of the intermediate polars. to determine a criterion, separating the intermediate polars and polars let us consider behavior of the gas stream, issuing from the l1 point into the roche lobe of the accretor. taking into account that the gas in the stream moves with supersonic velocity we can consider its behavior using the ballistic approach and omitting the effects of pressure and magnetic field [9, 10]. figure 1: 3d flow structure for the model 1 with ba = 105 g. iso-surface of density (lgρ = −4.5 in units of ρ(l1)) and magnetic field lines are shown. the accretor’s rotation axis is shown by the thin straight line, magnetic axis — by the bold inclined line. figure 2: the same as in fig. 1 for the model 3 with ba = 106 g. figure 3: the same as in fig. 1 for the model 5 with ba = 107 g. 61 d. v. bisikalo, a. g. zhilkin 9 ,1 0 y ea r a m m e & , g a b 9 , 1 0 y ea r m m e & , g a b ,% a m m & & , g a b 9 ,1 0 y ea r a m m e & orb spinp p figure 4: the dependence of the accretion rate ṁa on value of the magnetic field ba (left row) in absolute values (top panel) and in % from the mass transfer rate ṁ (bottom panel). the vertical lines limit the specific amplitudes of the accretion rate variations. in the upper-right panel we show the mass transfer rate as a function of the value of magnetic field. in the lower-right panel we show the mass transfer rate as a function of the accretor’s spin velocity in the model with ba = 105 g. analysis of the trajectories [11] shows that the stream comes very close to the accretor’s surface at a distance rmin. if rmin is larger than the radius of the magnetosphere rm the magnetic field does not strongly influence the motion of material. the stream rounds the star and intersects itself at a certain point. the further evolution of this flow results in the formation of an accretion disk in the system. if rmin is smaller than rm then the magnetic field starts to strongly influence the stream at a certain part of its trajectory. the action of electromagnetic forces in this region slows the stream down and causes loss of the angular momentum. as a result the stream can not round the star and form an accretion disk. thus, the boundary between the intermediate polars (with accretion disks) and polars (no accretion disks) is determined by the relation rm = rmin. if we calculate both the radii using the parameters of the ss cygni we see that this relation is satisfied at ba ≈ 106 g separating the two regimes of the flow. we should note that this estimate of magnetic field induction, separating the intermediate polars and polars, is more or less general for cvs, since the value of q there varies insignificantly. the typical flow structure in the polars is shown in fig. 3, where, like in figures 1 and 2, we plotted an iso-surface of density logarithm lgρ = −4.5 (in units of ρ(l1)) for the model 4 (107 g). in the models 4–7 no accretion disks form and material is accreted via funnel flows. in the models 4 and 5 the stream, starting at l1 point, splits into two flows when approaching the accretor’s surface. then the first flows falls onto the northern magnetic pole of the accretor, the second — onto the southern. with growing magnetic field the difference between the two poles becomes more obvious. in the model 6, for example, only one accretion flow occurs that then ends up at the southern pole. in the model 7 the magnetic field is so strong that it almost entirely controls the flow structure in the roche lobe of the accretor. the field captures material in the immediate vicinity of the inner lagrangian point and, splitting 62 on the influence of magnetic field on accretion processes in cvs it into two flows — more powerful southern and weaker northern, — transports it onto the accretor’s surface along the field lines. let us now consider how the described variations of the flow structure influence the accretion rate. in the upper-left panel of fig. 4 we plot the computed accretion rate ṁa as a function of ba. the vertical lines show specific values of the accretion rate variations. the main feature of this function is that it is non-monotonic. for the values ba < 106 g the accretion rate increases with increasing magnetic field and the amplitude of its variations decreases. at the point ba = 106 g the accretion rate approaches its maximal value. this can be explained by the fact that in the solutions with accretion disks the accretion rate is controlled by processes of angular momentum transfer. the growing magnetic field in intermediate polars makes magnetic breaking more effective and, hence, increases ṁa. after the point of maximum the accretion rate starts to decrease while ba continues to grow. as we have shown above starting at the point ba = 106 g no accretion disk forms in the system and the material is accreted via funnel flows. in this case the accretion rate is determined by the throughput of the stream. when the magnetic field grows the width of the stream decreases and the accretion rate falls. dependence of the accretion efficiency ṁa/ṁ on the induction of magnetic field is shown in the lower-left panel of fig. 4. with growing ba this value increases monotonically. in a transit range of magnetic field values (near ba ≈ 106 g) this dependence has an inflection point. the lowest effectiveness of accretion (30–40%) is approached for intermediate polars with weak magnetic fields. this may be explained by the fact that their disks are large and a portion of material may leave the accretor’s roche lobe and form a common envelope of the binary system [12]. in the polars (with strong magnetic fields) almost all the material, flowing from the l1 point onto the accretor, is accumulated and ”enclosed” in the stream. thus, in such systems the effectiveness of accretion reaches almost 100%. the non-monotonic shape of the ṁa(ba) curve, obtained in our calculations, does not contradict the monotonic character of the dependence of the accretion effectiveness on ba. the point is that the mass transfer rate ṁ also depends on the value of magnetic field. this dependence is shown in the upper-right diagram of fig. 4. in figure one can see that with growing ba the value of ṁ monotonically decreases. in a range of weak fields (ba < 106 g, intermediate polars) this decrease is almost invisible. however in strong fields (ba > 106 g, polars) the value of the mass transfer rate sharply drops (by almost an order of magnitude). this behavior may be explained by the fact that the strong magnetic field of the accretor generates extra-pressure in the vicinity of the l1 point, which prevents the outflow of the material from the donor’s envelope. thus, with growing ba the throughput of the accretion stream decreases. 3 influence of the asynchronous rotation of the white dwarf on the accretion rate the influence of the accretor’s spin-rotation on mhd flow structure in cvs can be characterized by a relation between the radius of the magnetosphere rm and the co-rotation radius rc that is a distance at which the velocity of rotation of the field lines is equal to the rotation velocity of material in the accretion disk. if the accretor rotates relatively slow (rc > rm) then the rotation velocity of the field lines at the boundary of the magnetosphere is lower than the keplerian. so the material, captured by the magnetic field can freely fall onto the accretor’s surface. this regime may be called the ”accretor” regime. when the accretor’s rotation is fast enough (rc < rm) a centrifugal barrier occurs at the boundary of the magnetosphere. this barrier prevents material from free falling onto the surface. this regime we may call the ”propeller”. in systems of ae aqr type the rotation of the white dwarf is so fast that no accretion disk can form. thus, for this regime we can use a term ”super-propeller” [5]. to investigate how the asynchronous rotation of the accretor influences the flow structure in a close binary system we conducted four runs of 3d numerical simulations for the period relation of pspin/porb: 0.1 (”accretor”); 0.033 (”equilibrium rotation”); 0.01 (”propeller”); and 0.001 (”super-propeller”) [5]. to preserve the generality we again use the parameters of the ss cygni. in all the runs the magnetic induction was set equal to 105 g and the magnetic axis is inclined by θ = 30◦. in the models ”accretor” and ”equilibrium rotation” the flow structure is analogous to the model 1, considered in previous section. in the ”propeller” regime, in the inner regions of the disk a magnetospheric cavity forms. its radius is 0.05–0.1a. at the boundary of this cavity the kelvin–helmholtz instability develops. since there is almost no material in the cavity, the accretion rate in the ”propeller” model is close to zero. on the other hand the mass transfer continues and the mass of the disk grows. in the systems with an appropriate combination of parameters (disk mass, value of magnetic field and rotation velocity) the material at the boundary of the cavity at a certain moment can push the magnetosphere down and initiate a short period of accretion. after the exceptional mass has been removed the system returns into the ”propeller” regime. in the ”super-propeller” regime the material, issuing from the inner lagrangian point, is captured by the 63 d. v. bisikalo, a. g. zhilkin rapidly rotating accretor’s magnetosphere, acquires additional angular momentum and gets ejected out of the accretor’s roche lobe. the accretion rate in this case is almost zero. in the lower-right panel of fig. 4 we show dependence of the accretion rate on the parameter porb/pspin that characterizes the spin velocity of the accretor. the data shown are for the case of ba = 105 g. in figure one can see that with the growing accretor’s spin velocity the accretion rate decreases. if the white dwarf rotates relatively slow, which corresponds to the ”accretor” regime, the accretion rate decrease is not significant in comparison with the case of synchronous rotation. however, when the spin velocity overcomes an equilibrium value the system enters the ”¡propeller” regime and the accretion rate sharply drops. in fact, in the ”propeller” and ”super-propeller” regimes the accretion rate is determined by the accretion of material from the common envelope but not of the material, issuing from the inner lagrangian point. 4 conclusions using the results of 3d numerical mhd simulations, we have considered the influence of the magnetic field value ba and the spin-rotation velocity of the white dwarf on accretion processes in cvs. analyzing the results of seven computational runs for models with ba equal to 105 g, 5 × 105 g, 106 g, 5 × 106 g, 107 g, 5 × 107 g and 108 g, we have found that the accretion rate ṁa is a non-monotonic function of ba. in the intermediate polars the growing ba makes the magnetic breaking more effective and, hence, increases ṁa. in the polars, where the accretion rate is determined by the throughput of the funnel flow the accretion rate decreases with growing ba. the maximal value of the accretion rate occurs in the systems at a stage of transit between the intermediate polars and polars. this value is ∼60% of the initially set mass transfer rate. we should note that unlike the absolute value of ṁa the accretion efficiency ṁa/ṁ monotonically increases with the increasing induction of magnetic field. in the polars it approaches the value of almost 100%. this is because the mass transfer rate decreases with growing ba, since strong magnetic field prevents the inflow of material into the accretor’s roche lobe through the inner lagrangian point. analyzing the results of four computational runs of 3d mhd simulations with various values of the period ratio (pspin/porb = 0.1, 0.033, 0.01 0.001) and fixed ba = 105 g we found that the accretion rate decreases with the increasing spin-rotation velocity. it is interesting to note that even in a system, living in the ”propeller” regime, the average accretion rate may be not equal to zero if the combination of such parameters as the disk mass, value of magnetic field and rotation velocity can cause quasi-periodic accretion bursts (falling of the portion of the disk mass onto the accreting star). acknowledgement this work was supported by the basic research program of the presidium of the russian academy of sciences, the russian foundation for basic research research (projects 11-02-00076, 12-02-00047, 13-02-00077, 13-02-00939), the federal targeted program ”science and science education for innovation in russia 2009– 2013”. references [1] warner, b. : 1995, cataclysmic variable stars, cambridge university press, cambridge. doi:10.1017/cbo9780511586491 [2] campbell, c.g. : 1997, magnetohydrodynamics in binary stars, kluwer acad., dordrecht. [3] norton, a.j., , wynn, j.a., somerscales, r.v. : 2004, apj 614, 349. doi:10.1086/423333 [4] zhilkin, a.g., bisikalo, d.v. : 2010, astron. rep. 54, 1063. [5] zhilkin, a.g., bisikalo, d.v., boyarchuk, a.a. : 2012, phys. usp. 55, 115. [6] giovannelli, f., gaudenzi, s., rossi, c., piccioni, a. p.j. : 1983, acta astronomica 33, 319. [7] zhilkin, a.g., bisikalo, d.v. : 2009, astron. rep. 53, 436. [8] zhilkin, a.g., bisikalo, d.v. : 2010, advances in space research 45, 437. doi:10.1016/j.asr.2009.09.006 [9] boyarchuk, a.a., et al. : 2002, mass transfer in close binary stars, taylor & francis, london. [10] fridman, a.m., bisikalo, d.v. : 2008, phys. usp. 51, 551. [11] lubow, s.h., shu, f.h. : 1975, apj 198, 383. doi:10.1086/153614 [12] sytov, a.yu., et al. : 2007, astron. rep. 51, 836. doi:10.1134/s1063772907100083 64 http://dx.doi.org/10.1017/cbo9780511586491 http://dx.doi.org/10.1086/423333 http://dx.doi.org/10.1016/j.asr.2009.09.006 http://dx.doi.org/10.1086/153614 http://dx.doi.org/10.1134/s1063772907100083 on the influence of magnetic field on accretion processes in cvs discussion christian knigge: you seemed to use ss cyg as a prototype of dn intermediate polar, out what is the empirical evidence for this? dmitry bisikalo: we just used ss cyg as a dn illustrative example. the parameters of the ss cyg (components masses, radii, separation, etc.) are quite typical for intermediate polars, so we can consider these numerical results as a good illustration of the flow structure in the intermediate polars. giora shaviv: if i saw right, you do not have radiative transfer, only local energy sinks. how it works when the disk is optically thick? dmitry bisikalo:yes, this approach is strictly applicable for optically thin disks. if you consider optically thick disks it is necessary to add the radiative transfer in some form to the model, and we are working on this. fortunately, we can use the assumption of optically thin disks for a significant sample of cvs. 65 introduction what happens with the flow structure if ba grows influence of the asynchronous rotation of the white dwarf on the accretion rate conclusions acta polytechnica ctu proceedings doi:10.14311/app.2016.5.0026 acta polytechnica ctu proceedings 5:26–28, 2016 © czech technical university in prague, 2016 available online at http://ojs.cvut.cz/ojs/index.php/app traffic management system in terms of data exchange dušan kamenický czech technical university in prague, faculty of transportation sciences, department of transport telematics, konviktská 20, prague, czech republic correspondence: kamendus@fd.cvut.cz abstract. prediction of train running and it support of conflict resolution decision for an efficient use of the existing railway infrastructure is needed. to meet these requirements standardized interfaces between infrastructure managers and railway undertakings and infrastructure description are indispensable. keywords: technical specification for interoperability, infrastructure description, traffic planning, dispatching, train controlling, telematics applications, railml, railtopomodel. 1. introduction with the increasing demand for freight and passenger transport railway subjects aims on increasing the capacity by a reduction of delays and improved traffic fluidity. to meet these requirements modern traffic management systems are implemented. they are based on a prediction of train running and automatic conflict resolution. standardized interfaces shall ensure the needs of the railway undertakings, infrastructure managements and customers of different european countries in traffic planning, dispatching and train control. directive 2008/57/ec of the european parliament and of the council of 17 june 2008 on the interoperability of the rail system within the community defines in annex ii infrastructure, energy, control-command and signaling and rolling stock as structural subsystems of the rail system, and traffic operation and management, telematics applications for passenger and freight services as a functional subsystem of the rail system. specifications are drafted by the european railway agency and adopted in a decision by the european commission, to ensure the interoperability of trans-european rail system. the paper describes data exchange with focus on the operational management. 2. architecture of management process railway management process can be divided into three basic layers: traffic planning, dispatching and train controlling. traffic planning can be characterized as conceptual planning, strategic network development, service planning and infrastructure planning in long term, timetabling in short term. timetabling is a process concerning transport demand by customer and path requests and allocation by railway undertakings and infrastructure managers [1]. traffic planning is influenced by transport demands, infrastructure parameters, rolling stocks parameters and path fees. traffic planning shall define rules, like priorities of the train and competence between railway undertakings and infrastructure managers. the aim of dispatching is at close approximation to scheduled state, from which railway system has been deflected by external influences. it support is necessary in dispatcher decision processes [2]. quality and timeliness of information is necessary condition the dispatcher could correctly decide the intervention, which leads to fulfillment of the planned timetable. infrastructure manager dispatcher is responsible for changing train sequences to minimalize deviations from the timetable. railway undertaking dispatcher is responsible for skipping a commercial stop, breaking a connection, providing rolling stock and staff. major incidents forcing those decisions like rolling stock or infrastructural failures with a following adaptation of the timetable require a close communication between infrastructure manager and railway undertaking. train controlling is process train route setting, shunting route managing and setting, infrastructure elements controlling to ensure railway safety. routine processes can be performed by automatic train route setting system. automatic train control system, which is designed to eliminate human error, can be supplemented by automatic train operation system or driver advisory system. a train movement is adjusted to achieve defined points of infrastructure in defined time slot and therefore to reduce energy consumption and increase the capacity at a time. 3. standardized interfaces technical parameters of infrastructure elements, like line layout, track parameters, switches and crossings, 26 http://dx.doi.org/10.14311/app.2016.5.0026 http://ojs.cvut.cz/ojs/index.php/app vol. 5/2016 traffic management system in terms of data exchange platforms etc. are defined by 1299/2014/eu "technical specifications for relating to the infrastructure subsystem of the rail system in the european union" [3]. with exception of the requirements for infrastructure register, infrastructure description, methodology of representation or the storage of infrastructure data are not defined. infrastructure register is defined by 2014/880/eu "specifications of the register of railway infrastructure" [4]. this specification concerns data about the infrastructure structural subsystem the energy structural subsystem, and the trackside control-command and signaling subsystem. railway network shall be subdivided into sections of line and operation points. section of line means the part of line between adjacent operational points and may consist of several tracks. operational point means any location for train service operations, where train services may begin and end or change route and where passenger or freight services may be provided [5]. the structural subsystems specific requirements that interoperable railway lines and rolling stock must meet and many of these requirements must be stored in the infrastructure registers and the register of rolling stock. comparing those registers should make it clear which lines accept which rail vehicles. telematics applications for passenger and freight services subsystem equipment are defined by 2006/62/es "technical specification for interoperability relating to the telematics applications for freight subsystem of the trans-european conventional rail system" (tsi taf) [6] and 2011/454/es "technical specification for interoperability relating to the subsystem telematics applications for passenger services of the transeuropean rail system" (tsi tap) [7]. the specifications relating to telematics applications define architecture of information system and interfaces among subjects: infrastructure manager, railway undertaking and customer. processes and data exchanges to allocating of train path and monitoring train movement are defined. fundamental processes are: • train path request – path departure point, path departure time, path destination point, command and control system including on-board radio equipment, train weight and length, braking system and braking performance, maximum speed, rid numbers relating to any dangerous goods, information concerning exceptional gauging etc., • train composition message – for the preparation of the train, railway undertaking must have access to the infrastructure restriction notice, to technical wagon data (rolling stock reference database), to the dangerous goods reference file and to current updated information status on the wagons (the wagon and intermodal unit operation database), • train ready – message must be sent to indicating, that train is ready for departure, • train running information and train running forecast. interface between telematics applications and subsystem "control-command and signaling" is not specified, although interlocking equipment need data on the parameters of the trains. interface between telematics applications and subsystem "infrastructure" is given with the train path data definition and via the infrastructure restriction notice database. interface between telematics applications and subsystem "rolling stocks" is only given via the rolling stock reference database. the procedures enabling a coherent operation of various structural subsystems during both normal and degraded operation are defined by 2015/995/eu "technical specification for interoperability relating to the operational and traffic management subsystem of the rail system in the european union" [8]. from the viewpoint of the data exchange, relevant is requirement of route book and timetables. the development of the route book is the responsibility of ru and should be prepared in the language of the railway undertaking. however, the format of the route book is not specified. each train must be identified by a train running number. the train running number is given by the infrastructure manager when allocating a train path. the train running number format is defined in commission decision 2012/88/eu. interfaces among different railway it applications weren’t specified. the railml.org initiative was founded in 2002 in order to create an interface to enable heterogeneous railway applications to communicate with each other [9]. the result has been the development of the railway markup language railmlwhich delivers a universally applicable data exchange format. the railml standard has been developed by infrastructure managers, railway undertakings, software and consulting firms and academic institutions from number of countries. the railml specification contains subschemas for four main areas: infrastructure, timetable, rolling stock and interlocking. railml can be seen as a direct use case of the railtopomodel. the railtopomodel is a logical object model to standardize the representation of railway infrastructurerelated data [10]. on the other side, railtopomodel will become international railway standard (irs) in spring 2016, compiled by uic, the largest railway organization worldwide. sncf réseau has worked since 2011 to develop its global model for railways business objects, ariane model, based on the same principles as railtopomodel. ariane model now covers the description of all functional objects and properties of the network, at track and lines levels, including topology, referencing and topography, routes description and signaling. on 2016-2017 catenary and power supply network, and finalization of rolling stock will be added. 27 dušan kamenický acta polytechnica ctu proceedings 4. coclusion technical specification for interoperability defines technical requirements of infrastructure, energy and rolling stocks parameters and control-command and signaling system to enable interoperable vehicle driving on interoperable infrastructure. necessary data exchange relating to path allocation and operation and management processes relating to ensure safety are defined in function subsystems. however, data exchange relating to dispatching is not standardized. infrastructure description, specified in infrastructure register, is not sufficient for prediction of train running. data exchange relating to train parameters, like traction characteristics, and timetable requirements is not specified. infrastructure managers and railway undertakings interface ensuring dynamic data exchange relating to breaking a connection is not defined in technical specification for interoperability. railml seems to become effective tool for data exchange supporting prediction of train running and conflict resolution decision among others. references [1] birgit jaekel, thomas albrecht. operational railway management as part of an integrated railway management process, 2014. euro žel, žilina. [2] d. kamenický. optimalizační algoritmy pro systémy řízení a zabezpečení železniční dopravy, studie k disertační práci, 2015. čvut v praze, fakulta dopravní, praha. [3] 1299/2014/eu ”technical specifications for relating to the infrastructure subsystem of the rail system in the european union”. [4] 2014/880/eu ”specifications of the register of railway infrastructure”. [5] j. barnet. rail infrastructure informational description, master thesis, 2011. ctu in prague, faculty of transportation sciences, prague. [6] 2006/62/es ”technical specification for interoperability relating to the telematics applications for freight subsystem of the trans-european conventional rail system”. [7] 2011/454/es ”technical specification for interoperability relating to the subsystem telematics applications for passenger services of the trans-european rail system”. [8] 2015/995/eu ”technical specification for interoperability relating to the operational and traffic management subsystem of the rail system in the european union”. [9] railml website [online]. [2016-01-25], http://www.railml.org/en/. [10] railtopomodel website [online]. [2016-01-25], http://www.railtopomodel.org/en/. 28 http://www.railml.org/en/ http://www.railtopomodel.org/en/ acta polytechnica ctu proceedings 5:26–28, 2016 1 introduction 2 architecture of management process 3 standardized interfaces 4 coclusion references untitled vol. 5/2016 scientific student conference interoperability of railway transport iricon 2016 may 4, 2016 prague, czech republic organised by: faculty of transportation sciences czech technical university in prague sponsored by: internal grant agency ctu svk 46/16/f6 guest editors: jitka jírová (faculty of transportation sciences, czech technical university in prague) jitka řezníčková (faculty of transportation sciences, czech technical university in prague) local organizing comittee: vít fábera tomáš brandejský jitka jírová dušan kamenický petr koutecký jitka řezníčková jan zelenka scientific comittee: tomáš brandejský (faculty of transportation sciences, czech technical university in prague) pavel hrubeš (faculty of transportation sciences, czech technical university in prague) josef jíra (faculty of transportation sciences, czech technical university in prague) zdeněk kaufmann (tp iži — technology platform, interoperability of railway infrastructure) martin leso (faculty of transportation sciences, czech technical university in prague) jiří masopust (faculty of electrical engineering, university of west bohemia) petr nachtigall (jan perner transport faculty, university of pardubice) otto plášek (faculty of civil engineering, brno university of technology) jindřich sadil (faculty of transportation sciences, czech technical university in prague) richard svoboda (brno university of technology, faculty of civil engineering) jaroslav vašátko (railway research institute, j.s.c., prague) martina vitteková (faculty of transportation sciences, czech technical university in prague) iii acta polytechnica ctu proceedings book title: scientific student conference interoperability of railway transport — iricon 2016. series title: acta polytechnica ctu proceedings. volume: 5. date of issue: october 15, 2016. published by: czech technical university in prague, zikova 4, 166 36 praha 6 – dejvice, czech republic. ič: 68407700. editorial office: ctu — central library, technická 6, 160 80 praha 6, czech republic. acta@cvut.cz . head of the editorial board: zbyněk škvor (ctu in prague, czech republic). editor-in-chief: iva adlerová (ctu in prague, czech republic). editorial board: robin arthur healey (rectorate of ctu in prague, dept. of external affairs, ctu in prague, czech republic), taťána jarošíková (faculty of biomedical engineering, dept. of natural sciences, ctu in prague, czech republic), milan jirásek (faculty of civil engineering, dept. of mechanics, ctu in prague, czech republic), jitka jírová (faculty of transportation sciences, dept. of mechanics and materials, ctu in prague, czech republic), petr jizba (faculty of nuclear sciences and physical engineering, dept. of physics, ctu in prague, czech republic), pavel kalina (faculty of architecture, dept. of theory and history of architecture, ctu in prague, czech republic), zuzana masáková (faculty of nuclear sciences and physical engineering, dept. of mathematics, ctu in prague, czech republic), jan píchal (faculty of electrical engineering, dept. of physics, ctu in prague, czech republic), miroslav sýkora (klokner institute, dept. of structural reliability, ctu in prague, czech republic), radek šulc (faculty of mechanical engineering, dept. of process engineering, ctu in prague, czech republic). graphic design and typesetting: tomáš hejda (faculty of chemical engineering, dept. of mathematics, university of chemistry and technology prague), jan řezníček (faculty of information technology, ctu in prague, czech republic), j. jílek (ctu in prague, czech republic). available on-line at http://ojs.cvut.cz/ojs/index.php/app each article is assigned a digital object identifier doi:10.14311/app.2016.5.〈4-digit article page number〉 issn 2336-5382 isbn 978-80-01-06022-3 (1st edition) © czech technical university in prague, 2016. reprinting is allowed only with the agreement of the editorial office and with citation of source. iv vol. 5/2016 preface the papers of the scientific student conference interoperability of railway transport – iricon 2016 are concerning new results coming from the project interoperability of railway infrastructure competence network (iricon) which was coordinated by technical university in brno in collaboration with the technological platform "interoperability of railway infrastructure" and was closed in 2015. the project was supported by the operational programme of the ministry of education, youth and sports – education for competitiveness under the priority axis – tertiary education, research and development. the aim of the project was to improve the quality of human resources for research and development and improve training of target groups, consist of students and young professionals in the field of interoperability of railway infrastructure. necessity of the project was supported by the research interests of students and academic staff of technical universities and research institutes. the project enables to involve young experts in advisory groups for members of the governing organs of the european network interoperability, which was established for the preparation and implementation of the interoperability of the european rail system. the project has set up expert groups called the mirror groups, which reflect the activities of the trans-european rail networks in the areas of: (i) infrastructure, (ii) management and safety, (iii) international railway research board, (iv) energy, (v) system solutions and (vi) research. preparing members of the target groups took the form of: (i) traineeships for young researchers at foreign universities or at european institutes aimed in railway interoperability (ii) organizing workshops and training sessions with the participation of domestic and foreign experts, (iii) participation in international conferences on railway transport, (iv) fellowships at leading foreign railway sites, (v) workshops aimed at exchanging the latest information about news in railway transport and (vi) additional training focused on project management in the field of railway research. besides the aforementioned activities there was created portfolio of complementary activities for the target groups aimed at transfer of technology and improving knowledge in the field of control and legislation in the field of research and development and for evaluation of scientific results. the aim of our conference is to exchange knowledge and connections of scientific and technical potential of universities, research and design institutes together with professional potential of construction and manufacturing companies to promote innovation in rail services in accordance with the requirements of technical specifications for interoperability trans-european rail system in subsystems of infrastructure, energy and management and safety. an important aim of the conference is also to contribute to higher education and professional qualifications of future graduates master and doctoral studies, which will enable them to participate on the management, coordination and other activities in the area of interoperability of the european rail system. otto plášek for the scientific comittee v 303 acta polytechnica ctu proceedings 1(1): 303–306, 2014 303 doi: 10.14311/app.2014.01.0303 atmosphere-space interactions monitor (asim): state of the art pere blay1, lola sabau-graziati2, vı́ctor reglero1, paul h. connell1, juana m. rodrigo1, juan m. macián1, josé t. biosca1, chris eyles1 1ipl, university of valencia, valencia, spain 2inta, madrid, spain corresponding author: pere.blay@uv.es abstract atmosphere-space interactions monitor (asim) mission is an esa payload which will be installed in the columbus module of the international space station (iss). asim is optimized to the observation and monitoring of luminescent phenomena in the upper atmosphere, the so called transient luminous event (tles) and terrestrial gamma ray flashes (tgfs). both tles and tgfs have been discovered recently (past two decades) and opened a new field of research in high energetic phenomena in the atmosphere. we will review the capabilities of asim and how it will help researchers to gain deeper knowledge of tgfs, tles, their inter-relationship and how they are linked to severe thunderstorms and the phenomena of lightning. keywords: space instrumentation atmosphere physics thunderstorms lightning high energy radiation. 1 introduction an increasing number of luminous phenomena related to lightning and severe thunderstorms have been discovered in the last decades. on the one hand, the transient luminous events (tles) are luminous phenomena produced at the stratosphere and mesosphere. tles studies were pushed forward with the advent of high speed cameras (capable of several tens of frames per second, see, e.g. franz et al. 1990). more recently, with very high speed cameras (thousands of frames per second, see for example cummer et al. 2006) these events have been very well characterized in terms of evolution and fine structure. on the other hand terrestrial gammaray flashes (tgfs) are very bright high energy atmospheric emissions (x-ray and γ-ray) and were discovered by astrophysical missions in orbit. according to their appearance, tles are classified mainly as blue jets, red sprites or elves. while blue jets and red sprites are directly linked to lightning and are due to air breakdown, elves are generated by electromagnetic pulses which produce ionization at around 90 km of altitude. elves have a typical lifetime of ∼1 millisecond. red sprites are generated at the mesosphere (55-80 km) and last from a few milliseconds to tens of milliseconds. blue jets are upward propagating jets of light from the cloud tops up to ∼37 km, with a velocity of ∼100 km/s and last for a few tens of a second. an schematic cartoon of these phenomena can be seen in fig. 1. with regards to tgfs, first reported detections by the batse team were published by fishman et al. (1994). tgfs are bright outburst of gamma-ray radiation produced in the atmosphere and which can be seen from space-based instruments. tgfs are thought to be produced by bremsstrahlung emission from energetic runaway electrons accelerated by electric fields on top of thunderclouds. the possible connection of tgfs to electron avalanches produced by cosmic-rays in the atmosphere, or a combination of both the cosmic-ray induced electron avalanches and the electrical discharges during lightning, is also a possibility under discussion. tgfs last typically a few hundreds of microseconds and have been observed from 40 kev up to 80 mev. for an up to date review see dwyer et al. 2012. several astrophysical high energy missions, other than batse, have detected tgfs: rhessi (designed to observe high energy emissions from the sun), fermi and agile (both dedicated to the observations of gamma-ray bursts in the universe). fermi team has even performed dedicated observations of the earth atmosphere and designed specific software and data analysis techniques in order to characterize the tgf emission. agile has been specially productive in the field of tgf detection and characterization in the 400 kev30 mev energy range. the agile team has released its own tgf catalogue available online1 (marisaldi et al. 2013). this new filed of atmospheric physics has gained 1http://www.asdc.asi.it/mcaltgfcat 303 http://dx.doi.org/10.14311/app.2014.01.0303 pere blay et al. an increasing attention from atmosphere physicist and high energy astrophysicists, and is being considered seriously by space agencies as a tool to understand atmospheric phenomena related to cloud formation, severe thunderstorms and lightning, and climate changes. in the wake of this new scientific field of research, the atmosphere-space interactions monitor (asim) was conceived as one of the first space missions dedicated and designed specifically for the observation and analysis of tgfs and tles. asim is an esa mission with contributions from spain, denmark, norway, france, italy and poland. figure 1: schematic view of tles associated to the lightning phenomena. 2 the asim mission asim was conceived as a monitoring mission with imaging and spectroscopic capabilities, with the goal to locate tgfs and tles and provide empirical data which will help to understand the nature of these emissions and the possible links between them. although tles are known to be related to lightning in severe thunderstorms, the relationship of tgfs with both of them is up to now only an hypothesis. there is some evidence that points to the relationship between tgfs and lightning, but a clear understanding is still lacking (oostgaard 2013). asim will be located at the nadir looking columbus external payload facility (cepf) on the columbus module of the international space station. asim will continuously monitor the earth atmosphere from an iss altitude of around 400 km. at a global scale, there are approximately 40 lightning flashes per second, as estimated from data taken by the optical transient detector (otd, christian et al. 1996) and lightning imaging sensor (lis, christian et al. 1999), and they are related to changes in the chemistry of the nox molecules in the atmosphere (very determinant for the ozone generation) and are thought of being of great interest for climate change studies (see for example reeve and toumi 2006, and price and rind 2012). for this reason, the observation and monitoring of lightning and related phenomenology has become of great interest for the scientific community. asim will also address this questions and provide for the first time strictly simultaneous optical and high energy data of the phenomenology related to severe thunderstorms. asim science goals will focus on understanding the physics behind tles and tgfs, get a deeper knowledge on how they are related to lightening, and understand their impact on atmospheric processes and possible links to climate determining factors. other science related to high-altitude cloud formation, cloud electrification, nox production, meteors, auroras, etc. will also be in the scope of the mission. 3 the asim instruments the asim payload will consist of two main instruments, the modular x and gamma-ray sensor (mxgs) and the modular multi-spectral imaging array (mmia). fig. 2 shows the location of the instruments on the cepa (columbus external payload adapter) which will be attached to the columbus nadir cepf. figure 2: schematic view of the mxgs and mmia instruments in the cepa. mxgs is a coded mask imaging instrument with two detector layers: one with imaging capabilities (led, low energy detector), operating in the 15−400 kev energy range, and a second layer (hed, high energy detector) sensitive to higher energy photons (200 kev−20 mev), but without imaging capabilities. the optimal 304 atmosphere-space interactions monitor (asim): state of the art energy resolution (10% at 60 kev for the led and 15% at 662 kev for the hed), the good angular location accuracy (0.7◦ for point sources and 2◦ for diffuse sources), and a time resolution bellow 5µs, make from mxgs an ideal instrument to locate tgfs and characterize their emission in terms of duration, shape of the light-curve and their spectral behavior. mxgs will have a field of view of 80◦×80◦, which at zero response becomes 147◦×147◦, i.e., the size of the earth disc as seen from the iss. table 1 summarizes the mxgs capabilities and in fig. 3 a mxgs model is depicted. table 1: summary of mxgs capabilities. led hed collecting area 1024 cm2 900 cm2 energy range 15−400 kev 200 kev−20 mev energy resolution 10% at 60 kev 15% at 662 kev time resolution ≤5 µs ≤5 µs location accuracy 0.7 (point src.) (degrees) 2 (diffuse) fov 80◦×80◦ † †147◦×147◦ at zero response mxgs may be the very first instrument locating tgfs with an error circle of ∼15 km radius, solving for the first time the ambiguities in the tgf association with lightning activity. mxgs will also be capable to observe for the very first time the tgf spectra in a very wide energy range, from 15 kev up to 20 mev. mxgs is also the very first coded mask imaging instrument specially designed to observe high energy atmospheric phenomena. figure 3: schematic view of the mxgs instrument. mmia will consist of 3 photometers and two imaging cameras. the 3 photometers are working in the wavelengths 337 nm, 180−230 nm, 777.4 nm, and the cameras are optimized to work at 337 nm and 777.4 nm, covering altogether from the near uv up to the near ir. the time resolution is 83 ms for the cameras and 0.01−0.1 s for the photometers. the cameras will have an spatial resolution between 0.4 and 0.5 km and both the photometers and the cameras have the same field of view, namely, 61.4◦. mmia will provide invaluable data on the optical emission (as opposed to the high energy emission) from severe thunderstorms. table 2 summarizes the capabilities of mmia and in fig. 4 a model of the instrument is depicted. figure 4: schematic view of the mmia instrument. table 2: summary of mmia capabilities. cameras photometers fov (nadir) 61.4◦ 61.4◦ num. of pixels 1024×1024 −− pixel resolution 0.4−0.5 −− (nadir) (km) time resolution (ms) 83 0.01−0.1 spectral bands 337/5 337/5 (center/width) 777.4/5 777.5/≤5 (nm) 205/50 mxgs and mmia will both work together in their triggering mode, and will be able to send their triggering information to each other. fig. 5 depicts a schematic view of the instruments capabilities and their interconnected cross-triggering system. they will offer the first strictly simultaneous observation of luminous emissions associated to thunderstorm activity. 305 pere blay et al. figure 5: schematic view of the mmia and mxgs configurations, depicting the instruments capabilities and the interconnected cross-triggering. 4 mission status asim mission was approved by esa during 2005. now (2013) the critical design review (cdr) is on going and the manufacturing is expected to start along 2013. the flight model should be ready by the end of 2014 and the launch of the mission is expected by mid-2015. the nominal operational life of the mission is 2 years, with possible extensions. 5 conclusions asim will help the scientific community to solve some of the problems related to tle and tgf formation, a quite recent new filed of atmospheric physics which has been also called high energy atmosphere physics (see dwyer et al. 2012). the capabilities of its instruments and the wavelength coverage will allow the location tgfs with high accuracy and disentangle their links to tles, thunderstorms and lightning. acknowledgement pere blay acknowledges funding from the spanish ministerio de economia y competitividad through project aya-2011-29936-c05. references [1] christian h.j., driscoll k., goodman s.j., blakeslee r.j., mach d., and buechler d. 1996, proc. 10th int. conf. on atmospheric electricity, osaka, japan, icae, 368371 [2] christian h.j., blakeslee r.j., goodman s.j., et al. 1999, proc. 11th int. conf. on atmospheric electricity, guntersville, al, international commission on atmospheric electricity, 746749 [3] cummer, s. a., n. jaugey, j. li, w. a. lyons, t. e. nelson, and e. a. gerken 2006, geophys. res. lett., 33, l04104 doi:10.1029/2005gl024969 [4] dwyer j.r., smith d.m., and cummer s.a. 2012, space sci rev, 173, 133 doi:10.1007/s11214-012-9894-0 [5] fishman g.j., bhat p.n., mallozzi r. et al. 1994, science, 264, 1313 doi:10.1126/science.264.5163.1313 [6] franz r.c., nemzak r.j., and winkler j.r. 1990, science, 249, 4851, 1990 [7] m. marisaldi, f. fuschino1, m. tavani et al. 2013, j. geophys. res. space physics, http://dx.doi.org/10.1002/2013ja019301 [8] n. ostgaard, t. gjesteland, b. e. carlson, a. b. collier, s. a. cummer, g. lu and h. j. christian 2013, geophys. res. lett.,vol. 40, 1-4 doi:10.1029/2012gl054022 306 http://dx.doi.org/10.1029/2005gl024969 http://dx.doi.org/10.1007/s11214-012-9894-0 http://dx.doi.org/10.1126/science.264.5163.1313 http://dx.doi.org/10.1029/2012gl054022 introduction the asim mission the asim instruments mission status conclusions acta polytechnica ctu proceedings doi:10.14311/app.2017.11.0043 acta polytechnica ctu proceedings 11:43–48, 2017 © czech technical university in prague, 2017 available online at http://ojs.cvut.cz/ojs/index.php/app netgraph – an efficient tool for periodic timetabling and capacity planning zdeněk michl∗, michal drábek, rudolf vávra department of logistics and management of transport, faculty of transportation sciences, ctu in prague, czech republic ∗ corresponding author: michlzde@fd.cvut.cz abstract. a netgraph is a graphical representation of a periodic timetable concept for given spatial or modal scope. it adds a third dimension to the network plan – time. despite no strictly prescribed rules, a common practice notation has been settled over time. the authors propose an extension of a netgraph for planning of periodic railway capacity and operation, including related notation rules. keywords: edge, netgraph, node, notation, periodic timetable, periodic capacity. 1. introduction a netgraph, also called interval graphic, is a graphical representation of a periodic (interval) timetable concept of public transport lines in a designated geographical or political territory, for a transport segment or a production unit. it displays at first traffic and transport coherence of the individual lines in transfer nodes and their service function on the territory (number of stops). netgraph was made up of a combination of a network plan, as usually used in urban and local public transport, and a timetable, as used in railway timetabling. while the former reproduces the course of the lines and partially the intervals in the entire network, the latter shows in detail the scheduled traffic on a part of the network (mostly a single railway line). the netgraph adds a third dimension to the two-dimensional network plan – time schedule. thus, in a netgraph, periodic timetable with small to medium intervals in an area can be clearly illustrated. there are no strictly prescribed rules for creation of netgraphs, but over time a common practice notation has been settled. since the level of detail, the purpose of creation and highlighted properties in netgraphs varies, it is always necessary to provide a legend of line (edge) types and colours and time notation used in it. a netgraph may be created as an output of a specialised transport planning software package as german fbs – fahrplanbearbeitungssystem (fbs/iplan) [1], swiss viriato [2] and others, enabling an overview of the constructed lines and connections at a glance for comparison. a basic netgraph can even be constructed using a spreadsheet office suite or another graphic tool. a netgraph consists of following elements: • nodes, including their names, • edges, including the timetable notation for each public transport line and number of stops, • technological processes, • a legend. further, the authors propose an extension of a netgraph for planning of periodic railway capacity and operation. for such purpose, it is necessary to establish rules for notation of passing through defined station at a certain time, for alternative routing of a train path, for a train path for one direction only, and for crossing of the opposite trains on a single-track line. a proposal of related rules is presented and discussed consequently. 2. common practice in netgraph notation there emerged two main producers of netgraphs – irfp. e. k. (institut für regional-und fernverkehrsplanung), and sma und partner ag. both companies provide timetabling software tools. their rules for netgraph notation are mostly equal, except some differences (e.g. as in figure 9 by sma and as in figure 10 by irfp). both companies publish on their websites a short introduction into purpose and notation of a netgraph. irfp uses a term "interval graphic", sma uses "netgraph". the authors of this paper have decided for the term "netgraph" for the sake of brevity. irfp notes that a netgraph displays at least • geographical offer : (public transport) "line network and line management, main railway stations, operated access points per line • temporal offer : interval frequence per line as well as offer density on certain lines through overlap, arrival and departure times of the clocked lines in main railway and intermediate stations, determining of lines and transition periods in the stations • qualitative offer : long-distance traffic and local transport services with regard to the lines, single products or train categories if applicable train operation company, number of train changing per relation" [3] 43 http://dx.doi.org/10.14311/app.2017.11.0043 http://ojs.cvut.cz/ojs/index.php/app z. michl, m. drábek, r. vávra acta polytechnica ctu proceedings sma stresses that "a single line represents each service (and the corresponding journey in the reverse direction) connecting the stations it passes through on its trip, with the arrival and departure times for each station indicated. the repeating aspect of the timetable means that a whole repeating family of trains can be covered by one line on the diagram. the connection times between services at a station can be seen at a glance, giving a powerful method for improving the coordination between services." [4] the notation rules represented below generally come out from both above mentioned netgraph providers. besides fbs/iplan and viriato, some other timetabling software tools, e.g. railsys [5], optitakt [6] and takt [7] can produce netgraphs as well. 2.1. nodes nodes of a netgraph represent important places of a transport network from the passenger’s (user’s) point of view. each node must be named (in full or using an abbreviation – for the recommended place for descriptor see figure 1). optionally, a minimum transfer (connection) time in a node in minutes may be added to the description. node importance category, frequency of service and other attributes may be displayed using different borderline and filling types and colours. a netgraph node depicts a timetable and network node, which is: • station of origin or destination of at least one public transport line (or a part of its services), • transfer (interchange) station to other public transport lines and transport systems, even if they are not included in the netgraph • stations, where splitting or joining of trains or lines or other technology operation regularly takes place, • other important places in the network, such as regional or tariff borders, crossing of trains etc., according to the purpose of creation of the netgraph. 2.2. edges a netgraph edge represents a public transport line (or its branches and joint sections) from the passenger’s point of view. edges of lines passing more nodes are connected through the node boxes. important edge attributes are line style and colour, which depicts the service function and interval of the public transport line. 2.3. edge colour colours are usually used (according to the purpose of creation of the netgraph) for: • transport segment of the line (e.g. train category), • operation periods (e.g. lines operating only on working days, during peak hours etc.). figure 1. a node of a netgraph including name descriptor and the minimum transfer time (5 min). • different transport modes (each mode in another colour), • different train operating companies, public transport authorities etc., where applicable, • in exceptional cases individual lines can be distinguished by the line colour. there is no single correct way to colorize the lines. the selected scheme therefore has always to be explained in the legend next to the netgraph. yet there are common conventions. the authors recommend getting inspired from corresponding use cases. 2.4. edge line style a line style of the edge distinguishes the interval between services of the line. line style can even show public transport lines operated without a fixed interval. generally, one solid line represents one service per period (usually one hour). by convention, the line styles as shown in figure 2 are used. different line styles are used e.g. in urban public transport systems. in each case, the line styles used in a netgraph should always be explained in the legend. 2.5. timetable notation edges representing public transport lines with periodic timetable are complemented with time labels. they indicate periodic arrival and departure times to and from each node, noted only in minutes. the descriptors are usually on the side of travel direction from the edge – on the right in most cases, on the left e.g. for swiss railways. the descriptor closer to a node shows the arrival time, the one further from the node then the departure time respectively, as shown in figure 3. both lines (black and red) operate every hour between a and c. trains of the black line leave a every hour at minute 31 for c. they arrive to b in minute 44 and leave in minute 45, while arriving to c in minute 59. in the opposite direction the trains leave c every hour at minute 1, call at b between minutes 15 and 16 and reach a in minute 29. trains of the red line leaves a every hour at minute 4, does not call at b and arrive to c at minute 27. in the opposite direction the trains of the red line leave c every hour in minute 33, do not call at b and reach a in minute 56. if trains of a public transport line 44 vol. 11/2017 netgraph figure 2. line style usage convention according to the service interval. do not call at the node, the time descriptor is usually omitted also (see the red line at node b in figure 3). in some cases of netgraphs for timetabling or railway capacity planning purpose, it may also be useful to indicate the passage time as departure times. it is then useful to distinguish the nodes where the train calls with dots in the colour of the respective line at intersections of the edge and node borders (see figure 4). this is also useful for lines with fixed stops but non-periodic timetable, where minute descriptors are omitted. in the case of two-hours-interval the descriptor must also indicate the parity of the hour of operation (even or odd). by convention, the minute-descriptor of an even hour is descripted by normal font and that of an odd hour in italics, see figure 5. the public transport line is operated every two hours between a and c. for c it leaves a every even hour at minute 40, arrives at b every odd hour in minute 14, calls until minute 16 and arrives at c every odd hour in minute 59. in the opposite direction, it leaves c every even hour at minute 01, calls at b every even hour in minutes 44 to 46 and reaches a every odd hour in minute 20. alternative resolution of even and odd hours is also possible, e.g. by underscoring the minute descriptor for an odd hour. in the case of public transport lines operated every timetable period (60 minutes), there is no need to distinguish between odd and even hours, see both lines in figure 4. for public transport lines operated more than once a timetable period (with notation as in figure 2), only one minute-descriptor for all trains of the line within a period is provided, other service times of the period are omitted – see figure 6. the public transport line is operated twice an hour (every 30 minutes) between nodes a and c. for c it leaves a every hour in minute 03 and 33, arrives at b every hour in minute 29 and 59, calls there until minute 30 and 00 and arrives at c every hour in minute 58 and 28. in the opposite direction, it leaves c every hour in minute 02 and 32, calls at b every hour in minutes 30 and 00 to 31 and 01 and reaches a every hour in minute 57 and 27. descriptors for the opposite direction are calculated symmetrically along the main timetable axis (as complements to 60 or another timetable period). 2.6. number of serviced stops intermediate stops of a public transport line are shown on the edge between nodes. either each serviced stop is indicated with a solid dot (see figure 7) or grouped in an empty circle with number of grouped stops next to it or in it (see figures 8 and 9). 2.7. branching of a public transport line branching (some trains continue from a node in other direction than the others) is indicated in the node with two connections between the edges, see figure 10. trains arrive from b to a every hour in minute 29. they continue every odd hour in minute 32 for d, and every even hour in minute 32 for c. the situation is similar in the opposite direction. the line style shows the doubled service interval of the branches. 2.8. splitting and joining of trains (units) in this case, each train splits into two parts, and each part continues to a different destination, see figure 11. trains from b arrive in a every hour in minute 28 and split in two parts, one leaving for d every hour in minute 32, the other one continuing to c every hour in minute 35. the situation is similar in the opposite direction. the form of linking of edges in a node can also indicate a u-turn of trains of the public transport line. 3. proposed additional rules for netgraph creation and discussion thereof 3.1. different service routing in each direction in some cases, a route of a public transport line contains sections, where services operate in one direction only. such sections are distinguished with arrows. time descriptors are displayed in particular direction only. in figure 12, the line is from a to f routed through nodes b, c and e, and in the opposite direction through nodes e, d and b. 3.2. alternative routing of freight train paths lindner and von redern [8] found that it was necessary to let periodic time windows ("canals") free for freight 45 z. michl, m. drábek, r. vávra acta polytechnica ctu proceedings figure 3. example of timetable descriptors. figure 4. extended timetable descriptors example. serviced nodes are distinguished with dots of respective colour of the line. figure 5. extended timetable descriptors example – 2h interval with odd hour distinguished. figure 6. extended timetable descriptors example – 30 min interval. figure 7. three serviced intermediate stops between nodes a and b, each one indicated separately with a solid dot. 46 vol. 11/2017 netgraph figure 8. five intermediate stops between nodes a and b, the number of grouped stops is indicated with a numeric descriptor next to an empty circle. figure 9. five intermediate stops between nodes a and b, the number of grouped stops is indicated with a numeric descriptor in an empty circle. trains. these should be wide enough for required number of freight train paths per hour. these time windows should be preferably connected in nodes (if needed, among more than two railway lines to enable use by freight trains with various origin/destination stations). in the case of insufficient capacity, the authors proposed reviewing the structure of passenger transport offer. this idea was further worked up by drábek [9], who proposed a concept of network-bound periodic freight train paths with possible alternative routing in node station. this concept is, to a certain extent, an analogy to integrated periodic timetable (ipt) in passenger transport. ipt is a network-wide, periodic offer of services and transfer connections between them. the concept of network-bound periodic freight train paths is a network-wide periodic offer of capacity that meets various requirements of freight train operating companies. unified period and unified (zero) symmetry are maintained in this concept as well. alternative routing through nodes represents an analogy to transfer connection in ipt. this alternative routing is illustrated in figure 13. the train paths from b through a are routed alternatively in the direction of c and d. a freight train can pass through node a (without stop) in minute 32, either in direction c or d. in the opposite direction, a freight train can pass through node a in minute 31, either from direction c or d. 3.3. train crossing on a single-track line for planning of periodic capacity of single-track lines, a key information is in which stations the trains in opposite directions periodically cross (meet) each other – at present day, or in a certain planning time horizon. in the case of ipt, a unified axis (minute) of symmetry is kept, and the crossing happens (at least approximately) in symmetry time. in europe, a convention on zero symmetry is kept. due to mathematical prefigure 10. branching of a public transport line into two directions. figure 11. splitting and joining of trains (units). conditions of ipt, two groups of nodes with symmetry time always occur – for instance, for 60 min interval, zero symmetry means one group of nodes (stations) with symmetry time in minute 00, and another group of nodes with symmetry time in minute 30. necessity of displaying time descriptors in such symmetrical crossing stations depends on level of detail for the capacity planning. for timetabling purposes, the descriptors are necessary for information about (real or required) scheduled running time between neighbouring symmetry nodes. for conceptual planning, name of the node station may be sufficient, as such symmetrical crossing stations change only after implementation of a new ipt-stage (shortening of travel times). figure 14 shows, beyond a "common" netgraph, depiction of periodic crossing of fast train in station katovice. as this crossing is not connected with a commercial stop and the netgraph serves for conceptual level of detail only, time descriptors for katovice are omitted. 3.4. discussion besides timetable concept elaboration and presentation, netgraphs can be used for railway capacity planning as well. the authors further propose additional rules to address various aspects of railway capacity planning. in many cases, the meaning of an edge, displayed in a netgraph, shifts from periodic service to periodic capacity, which may (fully or partially), or may not, be really used by a train (service). 47 z. michl, m. drábek, r. vávra acta polytechnica ctu proceedings figure 12. operation of a public transport line in a certain direction only. figure 13. alternative routing of freight trains (alternative train paths – from a either to c or d). figure 14. example of descriptor of a crossing station (zero symmetry) [10]. 4. conclusions a netgraph is a very useful tool not only for public transport (mostly railway) timetable conceptual planning. for periodic railway capacity planning (either periodic passenger service with half interval during the peak hours, or periodic capacity for freight trains, even with alternative routing). netgraph cannot replace classical timetabling tools, but can enable an easy and rapid overview for network capacity planning. for codification of necessary descriptors of symmetrical periodic crossing in terms of group of symmetry nodes, all-day or peak only incidence and planning horizon, further research is needed. references [1] irfp e.k. institut für regional-und fernverkehrsplanung. our product: the timetable construction system fbs [online]. [cit. 2017-03-24] "http://irfp.de/our-product-fbs.html". [2] s. und partner ag: viriato, zlr. the versatile timetable system [online]. [cit. 2017-03-23] "http://www.sma-partner.com/en/solutions/viriatoand-zlr/overview". [3] irfp e.k. institut für regional-und fernverkehrsplanung. timetable creation [online]. [cit. 2017-03-23] "http://irfp.de/timetable-creation.html". [4] s. und partner ag: viriato, zlr. netgraph. focussing on service [online]. [cit. 2017-03-23] "http://www.sma-partner.com/en/solutions/viriatoand-zlr/functionality/netgraph". [5] r. m. c. gmbh. railsys [online]. [cit. 2017-03-17] "http://www.rmcon.de/railsys-en/". [6] w. h. et al. optitakt [online]. [cit. 2017-03-24] "http://www.optitakt.de/". [7] c. o. t. f. s. u dresden. timetabling [online]. [cit. 2017-03-23] "https://tudresden.de/bu/verkehr/ila/vkstrl/forschung/ forschungsfelder/fahrplanung?set_language=en". [8] h.-r. lindner, h. von redern. güterzüge im taktfahrplan – möglichkeiten und grenzen. die bundesbahn 10/1989. [9] m. drábek. periodic freight train paths in network. doctoral thesis, 2014. "http://takt.fd.cvut.cz/cargo/drabek_thesis.pdf". [10] m. drábek. netgraphs of regional railway public service in chosen czech regions. ctu in prague, undated. unpublished. 48 acta polytechnica ctu proceedings 11:43–48, 2017 1 introduction 2 common practice in netgraph notation 2.1 nodes 2.2 edges 2.3 edge colour 2.4 edge line style 2.5 timetable notation 2.6 number of serviced stops 2.7 branching of a public transport line 2.8 splitting and joining of trains (units) 3 proposed additional rules for netgraph creation and discussion thereof 3.1 different service routing in each direction 3.2 alternative routing of freight train paths 3.3 train crossing on a single-track line 3.4 discussion 4 conclusions references 251 acta polytechnica ctu proceedings 1(1): 251–254, 2014 251 doi: 10.14311/app.2014.01.0251 testing f (r)-theories by binary pulsars mariafelicia de laurentis1,2, ivan de martino2,3 1dipartimento di scienze fisiche, università di napoli ”federico ii” 2infn sez. di napoli compl. univ. di monte s. angelo, edificio g, via cinthia, i-80126 napoli, italy 3departamento de fisica teorica, universidad de salamanca, 37008 salamanca, spain corresponding author: felicia@na.infn.it abstract using the post-keplerian parameters to obtain, in the minkowskian limit we obtain constraints on f(r)-theories of gravity from the first time derivative of the orbital period of a sample of binary stars. in the approximation in which the theory is taylor expandable, we can estimate the parameters of an an analytic f(r)-theory, and fulfilling the gap between the general relativity prediction and the one cames from observation, we show that the theory is not ruled out. keywords: gravitation binary pulsar systems f(r)-theories gravitational waves. 1 introduction astrophysical systems like neutron stars (ns), coalescing binary systems, black holes (bhs), and white dwarfs (wds), are the most promising to study the gravitational waves (gws) emision. indeed, studying the binary system b1913+16, known as the hulsetaylor binary pulsar, the first time derivative of the orbital period was measured to be different from zero [13, 18], as predicted by general relativity (gr) when gravitational radiation is emitted. this measurements was confirmed by study in other relativistic binary systems. the agreement between gr and the observation is at the order of ∼ 1%. however, using the extended theories of gravity (etg) it should be possibile to explain the observational results as shown in [10, 12] where, starting from a class of analytic f(r)-theories it is possible evaluate the first time derivative of the orbital period and compare it with the data. this approach permit both to test the etgs both to explain the gap between observation and the theoretical prediction. this paper was organized as follow: in sec. 1 we calculate the quadrupole emission for an analytic f(r)-lagrangian using the weak-field limit; in sec 2 we compare the theoretical prediction with the observed data. finally in sec 3 we give our conclusions and remarks. 2 the first time derivative of the orbital period in the f(r)-theories the simplest extension to gr is the f(r)-gravity, in which, the lagrangian is an arbitrary function of ricci scalar [2]. starting from the field equations in f(r)gravity (for details see [2, 16, 4, 5]) f ′(r)rµν − f(r) 2 gµν − f ′(r);µν + +gµν�gf ′(r) = x 2 tµν , (1) 3�f ′(r) + f ′(r)r− 2f(r) = x 2 t , (2) where tµν = −2√ −g δ( √ −glm) δgµν is the energy momentum tensor of matter (t is the trace), x = 16πg c4 is the coupling, f ′(r) = df(r) dr , �g = ;σ ;σ, and � = ,σ ,σ. it is possibile, in the minkowskian approximation of an analytic f(r)-lagrangian1, f(r) = ∑ n fn(r0) n! (r−r0)n ' ' f0 + f ′0r+ f ′′0 2 r2 + ... (3) to compute the quadrupole emission due to gws [10, 11]. furthermore, it is possible calculate the energy 1for convenience we will use f instead of f(r). all considerations are developed here in metric formalism. from now on we assume physical units g = c = 1. 251 http://dx.doi.org/10.14311/app.2014.01.0251 mariafelicia de laurentis, ivan de martino momentum tensor of gravitational field in f(r)-gravity that assumes the following form tλα = f ′0k λkα ( ḣρσḣρσ ) ︸ ︷︷ ︸ gr − 1 2 f ′′0 δ λ α ( kρkσḧ ρσ )2 ︸ ︷︷ ︸ f(r) . (4) to be more precise, the first term, depending on the choice of the constant f ′0, is the standard gr term, the second is the f(r) contribution. it is worth noticing that the order of derivative is increased of two degrees consistently to the fact that f(r)-gravity is of fourthorder in the metric approach [10]. in this contest, we can write the total average flux of energy due to the gws integrating over all possible directions as 〈 de dt 〉 ︸ ︷︷ ︸ (total) = g 60 〈 f ′0 (... q ij ... qij ) ︸ ︷︷ ︸ gr − f ′′0 (.... q ij .... q ij ) ︸ ︷︷ ︸ f(r) 〉 , (5) where we point out that for f ′′0 → 0 and f ′0 → 4 3 , the previous equation becomes 〈 de dt 〉 ︸ ︷︷ ︸ (gr) = g 45 〈... q ij ... qij 〉 , (6) that is the prediction of gr [14, 17]. in order to evaluate the above expressions for the flux it is necessary to form explicit expressions for 〈... q ij ... qij 〉 and〈.... q ij .... q ij 〉 for the system under consideration. for our purposes we consider a binary pulsar system. if we assume a keplerian motion of the stars in the binary system, wherewe mp is the pulsar mass, mc the companion mass, and µ = mcmp mc +mp is the reduced mass, it is possible to compute the time average of the radiated power computing the first time derivative of the orbital period [11] ṗb = − 3 20 ( t 2π )− 5 3 µg 5 3 (mc +mp) 2 3 c5(1− ε2) 7 2 × × [ f ′0 ( 37ε4 + 292ε2 + 96 ) − f ′′0 π 2t−1 2(1 + ε2)3 × × ( 891ε8 + 28016ε6 + 43520ε2 + 3072 )] , (7) where ε is the orbital eccentricity and t is the orbital period of the binary. 3 methodology and data analysis knowing exactly the lagrangian that describes the system, we can predict the orbital period decay, however,we want understand how well the relativistic binary systems can fix bounds on f(r) parameters using eq. (7), and getting an estimation of the second derivative of the lagrangian with respect to ricci scalar, f ′′0 . we use the following prescription, the difference between the first derivative of the binary observed period variation (ṫbobs ± δ) and the theorethical one obtained by gr, ∆ṫb = ṫbobs − ṫgr, is fulfilled imposing that: ṫbobs − ṫgr − f ′′0 ṫbf(r) = 0, (8) ṫbobs ± δ − ṫgr − f ′′0±δ ṫbf(r) = 0, (9) where δ is the experimental error, that we propagate on the ṫbobs , into an uncertainty on f ′′0±δ . in this way, the extra contribution to the loss of energy due to the emission of gws radiation in the etgs regime can provide to fill the difference between theory and observations. we select a sample of observed relativistic binary pulsars (see their references reported in tab. 1 of [11]) for which we compute the correction ṫbf(r) , the difference ∆ṫgr between ṫbobs and ṫgr (equal to the correction −f ′′0 ṫbf(r) ), the corresponding f ′′0 solution of (8), the interval centered on f ′′0 and finally, the interval centered on f ′′0 and computed from the difference: f ′′0+δ −f ′′0−δ 2 , all results are reported in tab. 1. in fig. 1 we show, for sake of convenience, in logarithmic scale, the absolute values of f ′′0 reported in tab.1 versus the ratio ṫbobs ṫgr . there are six binaries in tables, for which the etgs are not ruled out 0.04 ≤ f ′′0 ≤ 38, getting 0.5 ≤ ṫbobs ṫgr ≤ 1.5. for those systems the difference between ṫgr and ṫbobs can be explained adding an extra contribution that comes out from the f(r)-thoery. instead for most of binaries we have f ′′0 values that can surely rule out the theory, since taking account of the weak field assumption we obtain 38 ≤ f ′′0 ≤ 4 × 107. from this last values to the first ones, there is a jump of about four up to five order of magnitude on f ′′0 . the origin of these strong discrepancies, perhaps, is due to the extreme assumption we made, to justify the difference between the observed ṫbobs and the predicted ṫgr using the etgs. 252 testing f(r)-theories by binary pulsars table 1: upper limits of f ′′0 correction to ṫgr of binary relativistic pulsars assuming that all the loss of energy is caused by gravitational wave emission. we reported the j-name of the system,the difference ∆ṫgr between ṫbobs and ṫgr equal to the correction −f ′′0 ṫbf(r) , the correction ṫbf(r) , the corresponding f ′′0 solution of (8), the interval centered on f ′′0 and computed from the difference f ′′0+δ −f ′′0−δ 2 ,where f ′′0±δ ,are the corresponding solutions of ( 8) taking account of the experimental errors ±δ on the observed orbital period variation ṫbobs . name ∆ṫgr ṫbf(r) f ′′0 ±∆f ′′0 j2129+1210c -2.17e-14 6.01e-13 3.61e-02 8.32e-02 j1915+1606 -2.04e-14 2.10e-13 9.74e-02 4.77e-03 j0737-3039a -4.23e-15 1.86e-14 2.28e-01 9.15e-02 j1141-6545 -1.65e-14 3.88e-15 4.25e+00 6.44e+00 j1537+1155 5.39e-14 1.42e-15 -3.79e+01 7.03e-02 j1738+0333 -1.56e-15 1.06e-16 -1.47e+01 2.92e+01 j0751+1807 1.41e-13 8.98e-16 -15.7e+01 1.002e+01 j0024-7204j -5.22e-13 3.13e-16 1.67e+03 4.15e+02 j1701-3006b -5.03e-12 8.81e-16 5.71e+03 7.04e+01 j2051-0827 -1.55e-11 4.77e-16 3.24e+04 1.68e+03 j1909-3744 -5.47e-13 2.62e-18 2.09e+05 1.14e+04 j1518+4904 2.41e-13 3.42e-19 -7.05e+05 6.43e+03 j1959+2048 1.47e-11 1.07e-17 -1.38e+06 7.51e+04 j2145-0750 4.01e-13 1.00e-19 -4.00e+06 2.99e+06 j0437-4715 1.59e-13 1.04e-19 -1.57e+06 2.73e+06 j0045-7319 3.02e-07 1.11e-16 2.74e+9 8.13e+07 j2019+2425 -3.00e-11 1.11e-22 2.71e+11 5.41e+11 j1623-2631 4.00e-10 2.02e-23 -1.98e+13 2.97e+13 4 discussion and remarks in this paper, we develop expressions for quadrupole gravitational radiation in f(r)-gravity theory using the weak field technique and apply these results, which are applicable in general, to a sample of a binary pulsars, though their orbits are eccentric. here, we seen that, where the gr theory is not enough to explain the gap between the data and the theoretical estimation of the orbital decay, there is the possibility to extend the gr theory with a generic f(r)theory to cover the gap. according to eq. (7),we have selected a sample of relativistic binary systems for which the first derivative of the orbital period is observed, we have computed the theoretical quadrupole radiation rate, and finally we have compared it to binary system observations. from tab. 1, it is seen that the first five systems have masses determined in a manner quite reliable, while for the remaining sample, masses are estimated by requiring that the mass of the pulsar is 1.4m� and, assuming for the orbital inclination one of the usual statistical values (i = 60◦ or i = 90◦ ), and from here comes then the estimate of the mass of the companion star. so a primary cause of major discrepancies, not only for the etgs, but also for the gr theory, between the variation of the observed orbital period and the predicted effect of emission of gravitational waves, could be a mistake in the estimation of the masses of the system. in addition, other causes may be attributable to the evolutionary state of the system, which, for instance, if it does not consist of two neutron stars may transfer mass from companion to the neutron star. in our sample, there are only five double ns that can be used to test gr and etgs. taking into account of the strong hypothesis we made, the etg correction to ṫgr can also include the galactic acceleration term correction ([7], [8]). here, we give a preliminary result about the energy loss from binary systems and we show that, when the nature of the binary systems can exclude energy losses due to trade or loss of matter, then, we can explain the gap between the first time derivative of the observed orbital period and the theoretical one predicted by gr, using an analytical f(r)-theory of gravity. 253 mariafelicia de laurentis, ivan de martino 10 0 10 5 10 10 10 15 10 −4 10 −2 10 0 10 2 10 4 10 6 10 8 10 10 f ′′0 ṫ b o bs ṫ g r ṫbobs ṫgr = 1.5 ṫbobs ṫgr = 0.5 f ′′ 0 = +0.04 f ′′ 0 = 38 psrj1537+1155 psrj1738+0333 f ′′ 0 = 14.71 psrj2129+1210c figure 1: in figure there are shown, for sake of convenience, in logaritmic scale, the absolute values of f ′′0 reported in tab. 1 versus the ratio ṫbobs ṫgr . we must note that for five binaries the etgs we are probing is not ruled out 0.04 ≤ f ′′0 ≤≈ 38, for those systems the difference between ṫgr and ṫbobs is tiny, indeed we get 0.5 ≤ ṫbobs ṫgr ≤ 1.5. instead for most of binaries we have f ′′0 values that can surely rule out the theory, since taking account of the weak field assumption we obtain 38 ≤ f ′′0 ≤ 4× 107. from this last values to the first ones, there is a jump of about four up to five order of magnitude on f ′′0 . references [1] bogdanos c., capozziello s., de laurentis m., nesseris s., 2010, astrop. phys. , 34, 236. doi:10.1016/j.astropartphys.2010.08.001 [2] capozziello s., de laurentis m., 2011, physics reports 509, 167. doi:10.1016/j.physrep.2011.09.003 [3] capozziello s., corda c., de laurentis m., 2008 phys. lett. b 669, 255-259 . doi:10.1016/j.physletb.2008.10.001 [4] capozziello s., francaviglia m., 2008, gen. rel. grav. 40,357. doi:10.1007/s10714-007-0551-y [5] capozziello s., de laurentis m., faraoni v., 2009 ,the open astr. jour , 2, 1874. 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[15] maggiore m., 2007, gravitational wawes: theory and experiments, oxford univ. press, oxford. doi:10.1093/acprof:oso/9780198570745.001.0001 [16] nojiri s., odintsov s.d., 2011, physics reports 505, 59. doi:10.1016/j.physrep.2011.04.001 [17] weinberg s., 1972 ,”gravitation and cosmology”, john wiley & sons, inc., new york. [18] weisberg j.m., nice d.j. and taylor j.h., 2010, apj, 722,1030-1034. doi:10.1088/0004-637x/722/2/1030 [19] will c.m, 1993, ”theory and experiment in gravitational physics” cambridge university press, uk. doi:10.1017/cbo9780511564246 254 http://dx.doi.org/10.1016/j.astropartphys.2010.08.001 http://dx.doi.org/10.1016/j.physrep.2011.09.003 http://dx.doi.org/10.1016/j.physletb.2008.10.001 http://dx.doi.org/10.1007/s10714-007-0551-y http://dx.doi.org/10.1103/physrevd.58.042001 http://dx.doi.org/10.1016/j.astropartphys.2011.08.006 http://dx.doi.org/10.1093/mnras/stt216 http://dx.doi.org/10.1111/j.1365-2966.2012.21253.x http://dx.doi.org/10.1093/acprof:oso/9780198570745.001.0001 http://dx.doi.org/10.1016/j.physrep.2011.04.001 http://dx.doi.org/10.1088/0004-637x/722/2/1030 http://dx.doi.org/10.1017/cbo9780511564246 introduction the first time derivative of the orbital period in the f(r)-theories methodology and data analysis discussion and remarks acta polytechnica ctu proceedings doi:10.14311/app.2016.3.0025 acta polytechnica ctu proceedings 3:25–29, 2016 © czech technical university in prague, 2016 available online at http://ojs.cvut.cz/ojs/index.php/app experimental validation of cumulative surface location error for turning processes adam k. kiss∗, daniel bachrathy budapest university of technology and economics, department of applied mechanics, 1111 budapest, muegyetem rkp. 5., hungary ∗ corresponding author: kiss_a@mm.bme.hu abstract. the aim of this study is to create a mechanical model which is suitable to investigate the surface quality in turning processes, based on the cumulative surface location error (csle), which describes the series of the consecutive surface location errors (sle) in roughing operations. in the established model, the investigated csle depends on the currently and the previously resulted sle by means of the variation of the width of cut. the phenomenon of the system can be described as an implicit discrete map. the stationary surface location error and its bifurcations were analysed and flip-type bifurcation was observed for csle. experimental verification of the theoretical results was carried out. keywords: surface quality, surface location error, implicit map, flip bifurcation. 1. introduction in the industry for production and manufacturing, turning is a widely used method. however, high material removal rate often cannot be accomplished in practice due to the instability of the cutting process. this machining process induces harmful vibrations which are responsible for unacceptable surface quality [1]. the phenomenon is the self-excited vibration, the so-called chatter vibration which comes from loss of stability of the periodic vibration due to the surface regeneration effect [2]. this effect can be modelled with delayed differential equations (dde) [3]. the so-called stability chart [4] presents the chatterfree (stable) parameter domain which is usually illustrated along the parameters of the spindle speed and the depth of cut, calculated by numerical methods [5, 6]. the most productive parameter regions are located between the pockets of the the so-called lobes [7], where large cutting force can occur due to the high depth of cut ratio which leads substantial surface location error (sle). the stability chart is usually used at the roughing processes. due to the fact, that sle is relevant at finishing operations [8], its effect is usually neglected in the model of roughing operations, but despite this, it can has significant impact on the surface quality in case of consecutive immersions which is typical for roughing operation. we introduce a new type of surface error calculation which considers the effect of the series of sles during several consecutive immersion. the oversize of a workpiece is removed with several consecutive immersions in roughing machining process. at every immersion, the machined surface differs from the desired one due to the sle. the actual offset error slei modifies the subsequent immersion. this modified immersion generates different cutting force figure 1. sdof turning dynamical model. which leads another, modified offset error slei+1. during this process, the sle can be accumulated and leads a new surface quality parameter denoted by the cumulative surface location error (csle) [9, 10]. the evaluation of the series of sles values and its stability problems are investigated in turning operations on a single-degree-of-freedom (sdof) mechanical model. 2. evolution of cumulative surface location error the constructed mechanical model and the main steps of the sle computation are summarised as follows. 2.1. mechanical model in the mechanical model, the workpiece is considered as a rigid body and the cutting tool is considered as single-degree-of-freedom (sdof) model, as shown in figure 1. in the figure, some technological parameters which correspond to the material removal rate and 25 http://dx.doi.org/10.14311/app.2016.3.0025 http://ojs.cvut.cz/ojs/index.php/app adam k. kiss, daniel bachrathy acta polytechnica ctu proceedings figure 2. evolution of the surface location error (sle) in consecutive immersions. figure 3. schematic representation of map. the main mechanical parameters are represented. w is the width of the chip, h is the chip thickness or the feed per revolution, vf is the feed movement, ω is the spindle speed, f is the cutting force generated from the material removal process, s is the stiffness of the tool-holder and sle is the surface location error which is the distance between the machined and desired surface. the contour of the surface is defined by the position of the turning insert. in other words, the sle is the static displacement error of the turning insert which is generated by the f constant cutting force. in this model, the so-called surface regeneration effect of the machining process is not considered. this effect would lead to time dependent cutting force function, which may result stability problems, described in details in [11]. in this study, only the static displacement is analysed, hence exclusively the stationer component of the chip thickness is used. note, that in case of the surface regenerative models, the tool is considered to be flexible in the direction of the feed velocity [12], but, in our case it is flexible perpendiculat to the feed velocity. 2.2. calculation of csle the model considers the evolution of the width of the chip as follows: in roughing operation the desired width of cut w0 is modified by the actual slei and the subsequent slei+1 (see figure 2): wi+1 ( slei, slei+1 ) = w0 + slei − slei+1. (1) in the stationary position the spring force is equal to the cutting force: s · slei+1 = fi+1, (2) so the subsequent surface error can be calculated as: slei+1 = 1 s f ( wi+1 ( slei, slei+1 )) . (3) the above implicit discrete map describes how one sle develops into another sle over of a subsequent immersion. the sle values follow each other one by one. these series of the sles can converge to a fix point (see figure 3 ; where 45 degree line represents points where slei=slei+1 ) which is denoted by csle. this solution is obtained by two different methods. one of them is a numeric iteration of the map and the other one is a numerically calculated roots of the analytical function defined by eq.(3), made by multi-dimensional bisection method [13]. the characteristic of the map is highly dependent on the applied cutting force model. some typical approximations of the cutting force as the linear, the threequarter rule [14] and the cubic polinomial rule [15] were investigated. 2.3. stability analysis of csle the stability of the investigated system is determined based on the derivative of eq. 3 with respect to w width of cut at the fix points [16]. fold type bifurcation occurs, when the derivative is greater than 1, which leads to stabiliy loss of the csle and also could lead to bistable region. another case, when the derivative is smaller than -1, this situation corresponds to the period doubling or flip-type bifurcation. after some mathematical transformation, it can be proved that only the flip-type bifurcation can occure for this model. it happens when the derivative is equals to −s/2. for a realistic environment, the s stiffness is positive and the derivative of the turning force function is also positive for the usual turning force models, therefore the mechanical model is stable. but, there can exist some special cases, when the cutting force in the function of the width of cut turn out to negative sign (and its derivative is also negative) in real machining environment, not only in mathematical sense. for instance, in our experimental measurement (see in details in the following subsection), the resultant cutting force was negative due to the edge geometry of the applied insert. fold or saddle node bifurcation point cannot be obtained for physical meaningful parameters [10]. 26 vol. 3/2016 cumulative surface location error for turning processes figure 4. the softened tool mount and the fabricated strain gauges. 1st strain gauge 2nd strain gauge 3rd strain gauge 4th strain gauge def. calib. coeff.[ deformation strain value ] = [ mm µm/m ] -1.56·104 1.06·104 0.75·104 -0.57·104 force calib. coeff.[ force strain value ] = [ n µm/m ] -8.64·106 6.76·106 4.56·106 -3.38·106 stiffness[ force calib. coeff. deformation calib. coeff. ] = [ n mm ] 533.22 636.71 614.07 593.43 averaged stiffness[ n mm ] 599.36 table 1. calibration coefficients and the calculated stiffness. figure 5. experimental setup. 3. experimental validation experimental verification of the theoretical results was carried out in a 2 axes universal turning machine center. the development of the resulted offset error along the direction of the width of the chip (sle) was investigated on a given workpiece during five consecutive immersions, as shown in figure 5. for better measurement conditions, a softened tool holder figure 6. measured sign (blue) and averaged values (red lines) of force calibration. was used which results higher offset error. the tool holder is described in details in [17]. the generated cutting force and the deformation were measured by means of strain gauges installed on the softened tool holder (see figure 4). according to the strain gauges, the measurement instrument had to be calibrated for force and for deformation measurement. the force calibration was carried out as the artificial deformations and forces were generated meanwhile the signals from the strain gauges were recorded (see figure 6). the calibration coefficients (presented in table 1) were determined from the averaged measured data by applying linear regression by means of ordinary least squares approach. the calculated stiffness: s ∼= 600 n/mm. we carried out a series of measurements using different turning process parameters. in a single test, 5 consecutive cuts were performed with fixed cutting 27 adam k. kiss, daniel bachrathy acta polytechnica ctu proceedings figure 7. calculated csle function and the measured displacements (blue stars denote the converged solutions; colored stars denote the period doubling characteristics). figure 8. measured period doubling characteristics; parameters: w = 4 [mm], h0 = 0.2 [mm]. parameters to investigate the development of sle series. the measured cutting force was negative due to the applied insert. the reason of this phenomenon may come from geometrical setup, the edge geometry and the shape of the inserts. a fitting method was applied based on the measured averaged cutting forces to determine the specific cutting coefficients of the widely used polynomial cutting force function [15]. the material of the workpiece was almgsi0.5 aluminium alloy. the csle is calculated numerically from eq.( 3) for the fitted specific cutting coefficients and it is compared to the measured values. for the computed csle, flip-type bifurcation is occurred at certain parameter points at h0 = 0.17 [mm] and 0.23 [mm] (see figure 7) where the stable solution becomes unstable and creates a periodic-2 solution. between these parameter points, the series of the meeasured sle values do not converge to a certain value, but they alternate around 2 values (see figure 8). however, the amplitude of the measured alternating (bifurcating) displacement error has much smaller amplitude than the calculated one. the source of this large error could be the small number of consecutive cuts in the measurements, while the theoretical amplitude refers to the steady state amplitude after an infinite number of cuts. 4. conclusions in the present study, we show a new type of stability problem, which can occur during roughing process. this csle-stability problem leads to an unpredictable final surface location error, which can affect the finishing operation substantially. an experimental verification of the theoretical results was carried out in a softened machine tool environment, where a correlation was found between the measured and the computed values. in most of the cases, the series of csle values of one roughing test converged to certain value, showing also good correlation with the computed csles. in the predicted parameter range of period-2 solutions, the series of the measured deformations shows alternating characteristic. we can state, that the predicted phenomenon is validated by measurements. list of symbols sle surface location error [mm] csle cumulative surface location error [mm] w width of cut [mm] w0 pre-set width of cut [mm] h feed per revolution [mm/rev] h0 pre-set feed per revolution [mm/rev] v f feed movement [mm/s] ω spindle speed [ rpm] f cutting force [n] s stiffness of the tool-holder [n/mm] acknowledgements this paper was supported by the hungarian scientific research fund otka pd-112983 and the janos bolyai research scholarship of the hungarian academy of sciences. references [1] s. a. tobias. machine tool vibration. blackie and son, ltd, london 1965. [2] j. tobias, l. spacek. self-excited vibrations on machine tools, 1954. [3] g. stepan. retarded dynamical systems: stability and characteristic functions. longman scientific and technical 1989. [4] y. altintas, e. budak. analytical prediction of stability lobes in milling. cirp annals manufacturing technology 44(1):357–362, 1995. [5] t. insperger, g. stepan. semi-discretization for time-delay systems. springer, applied mathematical sciences 2011. [6] d. bachrathy, g. stepan. improved prediction of stability lobes with extended multi frequency solution. cirp annals manufacturing technology 62(1):411–414, 1995. [7] m. j. reith, g. stepan. optimization of material removal rate for orthogonal cutting with vibration limits. periodica polytechnica-mechanical engineering 56(2):91–97, 2012. doi:10.3311/pp.me.2012-2.03. 28 http://dx.doi.org/10.3311/pp.me.2012-2.03 vol. 3/2016 cumulative surface location error for turning processes [8] w. a. kline, r. e. devor, i. a. shareef. the prediction of surface accuracy in end milling. asme journal of engineering for industry 104(3):272–278, 2009. [9] a. kiss, d. bachrathy. explicit model of cumulative surface location error for milling processes. 12th hungarian conference on theoretical and applied mechanics 2015. [10] a. kiss. cumulative surface location error for cutting processes. msc thesis, bme, department of applied mechanics 2014. [11] r. szalai, g. stepan. lobes and lenses in the stability chart of interrupted turning. journal of computational and nonlinear dynamics 1(3):205–212, 2006. [12] t. insperger, d. a. w. barton, g. stepan. criticality of hopf bifurcation in state-dependent delay model of turning processes. international journal of non-linear mechanics 43(2):140–149, 2013. doi:10.1016/j.ijnonlinmec.2007.11.002. [13] d. bachrathy, g. stepan. bisection method in higher dimensions and the efficiency number. periodica polytechnica-mechanical engineering 56(2):81–86, 2012. [14] o. kienzle. spezifische schnittkrafte bei der metallbearbeitung. werkstattstechnik und maschinenbau 47(1):224–225, 1957. [15] h. shi, s. tobias. theory of finite-amplitude machine-tool instability. international journal of machine tools and manufacture 24(21):45–69, 1984. [16] j. t. sandefur. discrete dynamical systems : theory and applications 1990. [17] m. j. reith, d. bachrathy, g. stepan. stability properties and optimization of multi-cutter turning operations. asme, international design engineering technical conferences and computers and information in engineering conference: detc2013: portland, usa, oregon 2013. doi:10.1115/detc2013-12347. 29 http://dx.doi.org/10.1016/j.ijnonlinmec.2007.11.002 http://dx.doi.org/10.1115/detc2013-12347 acta polytechnica ctu proceedings 3:25–29, 2016 1 introduction 2 evolution of cumulative surface location error 2.1 mechanical model 2.2 calculation of csle 2.3 stability analysis of csle 3 experimental validation 4 conclusions list of symbols acknowledgements references 189 acta polytechnica ctu proceedings 1(1): 189–193, 2014 189 doi: 10.14311/app.2014.01.0189 are supernovae responsible for the gamma ray spectrum from the galactic center? todor stanev1 1bartol research foundation and department of physics and astronomy, university of delaware, u.s.a. corresponding author: stanev@bartol.udel.edu abstract we discuss the supernova remnants distribution as a function of the galactic longitude and compare their positions to that of the detected tev gamma ray sources. a large fraction of these sources either coincide or a close by known supernova remnants. if we look within 10o of the galactic center most identified sources are combinations of supernova remnants with giant molecular clouds. the diffuse gamma ray flux from the direction of the galactic center is much smaller. keywords: supernova remnants cosmic rays acceleration tev gamma rays. 1 introduction i will start this write-up the same way as i started the talk: with the answer to the question, which is i do not know. what i will write about is what we know about different types of supernova remnants (snr) from which tev γ rays are detected and what theoretical research says about the way these γ rays are produced in them. the main reason we are interested in supernova remnants is they are supposed to be the sources of galactic cosmic rays, which are accelerated at the supernova shocks. the outer regions of the expanding supernovae initially move with high velocity, approximately 0.1c. the expansion velocity is supersonic and a shock is formed in front of the remnant. the shock collects the interstellar matter that it meets during the expansion. once the shock radius is close to 1 pc, when the remnant is about 1,000 years old, the amount of the swept-up material becomes too much and the remnants velocity decreases. this is the time when cosmic rays are accelerated at the shock front. the magnetic field at the shock front is significantly higher than that of the interstellar medium and only a small fraction o(few %) of the kinetic energy of the remnant can supply all galactic cosmic rays (ginzburg & syrovatskii, 1964). 2 supernova remnants the latest supernova remnant catalog that i am familiar with is that of d.a. green (green 2009). it contains 274 supernova remnants studied in radio and gives their location, power at 1 ghz, and spectral indexes. ten of these snr have longitude less than 10o from the galactic center. all of them are closer than 1.5o from the galactic plane. the directions of all 274 these supernova remnants are shown in fig. 1 where the ones from supernovae after 1,000 ad (and the galactic center) are indicated. one can easily see that most of the remnants are very close to the galactic plane and a few are more than 5o away from it. one can also see that the supernova remnants density is much higher in the inner 60o of the galaxy. figure 1: locations of the supernova remnants in d.a. green catalog. the question if these snr are the sources of cosmic rays and of high energy γ-rays becomes very reasonable. to approach this question we will have a look at the galactic sources of high energy γ-rays, the tevcat of the university of chicago (http://tevcat.uchicago.edu). this catalog contains 145 gamma ray sources that have been discovered by the atmospheric cherenkov gamma ray telescopes. these devices consist of several mirror telescopes that observe the cherenkov light emitted by the cascade developing after the high energy gamma rays interact with atmosphere. the shape of the image of the cascade is enough to differentiate between atmospheric γ-ray cascades and cascades from the interactions of cosmic ray 189 http://dx.doi.org/10.14311/app.2014.01.0189 todor stanev protons and nuclei in the atmosphere. when the cascade is observed by more than one telescope the angular resolution is a small fraction of a degree. the threshold energy for these devices depends on the size of the mirrors and is often of o(100 gev). the three major tev gamma ray atmospheric telescopes are, in order of their completion, hess (hinton et al, 2004), veritas (hanna et al., 2008), and magic (boria tridon et al., 2010). hess and veritas have each four 12 m. telescopes and magic has two 17 m. telescopes. the average detection threshold for 12 m. telescopes is between 100 and 200 gev while the 17 m. telescopes can come down to 60 gev. tevcat has identified 64 galactic sources listed by different types in table 1. table 1: galactic tev gamma ray sources pulsar wind nebulae 31 shell supernova remnants 12 x-ray binaries 3 gamma ray binaries 1 massive stars 3 snr/molecular clouds 8 globular clusters 1 unidentified 5 it is not obvious that the different distribution of the tev gamma ray sources within 10o of the galactic center is important. out of the ten sources four are snr with close by molecular cloud, three are unidentified (which includes the galactic center and the galactic center ridge), one is pulsar wind nebula. there is also one shell snr and one globular cluster. since all of these objects are at the approximately the same distance from us we expect the gamma ray fluxes from them to correspond to the overall luminosity of the sources. the number of the detected gamma rays will also correspond to the threshold energy for detection. the exact threshold energy for detection by the tev air cherenkov telescopes depends on the size of the telescope mirrors and on the positions of the source and the telescope. to demonstrate this we show in fig. 2 the galactic plane and the sources in equatorial coordinates. the telescopes are sensitive to source elevation down to 30o but the lower the source is the higher the energy threshold. the hess telescope is in namibia at 23 degrees south, veritas is in arizona, u.s.a. at 31 degrees north, and magic is at 29 degrees north. all of them can observe most of the galactic plane, but the location of hess is the best one for observations of the galactic center. this is the reason it was constructed in namibia. figure 2: the vicinity of the galactic plane and the galactic sources of tev gamma rays in equatorial coordinates. figure 3 shows the galactic sources of tev gamma rays (circles) overlayed on the supernova remnants. the sources from the first fermi/lat catalog (abdo et al (2010)) are shown with triangles. if we had a look at the later fermi/lat catalogs we would find many more galactic sources the coincide with supernova remnants. one should not forget that fermi/lat is sensitive to γrays of energy 0.1 to 100 gev and the average spectral slope of the gamma ray sources is about 2.5. this means that are 30,000 more gamma rays above 100 mev than are gamma rays above 100 gev. this number varies, of course, from source to source depending on the spectral index of the source. figure 3: the position of the galactic tev gamma ray sources (circles) are overlayed on top of the supernova remnants from the green’s catalog. the triangles show the the fermi/lat gamma ray sources from its first catalog (abdo et al (2010)). 2.1 pulsar wind nebulae pulsar wind nebulae are formed by the highly relativistic mhd winds expelled by the rotating neutron stars. such objects can accelerate all kinds of charged particles, from electrons to heavy nuclei if there is a proper injection mechanism close to the neutron star. the best studied pwn is that of the crab. the gamma ray fluxes from different sources are often given in crab units that describe the ratio of their emission to that of the crab. its gamma ray energy spectrum is best described by the synchrotron self compton model that is based only on electron acceleration. electrons emitted by the source suffer from synchrotron energy loss to photons. these photons than go through inverse compton interactions 190 are supernovae responsible for the gamma ray spectrum from the galactic center? with the accelerated electrons that pushes them to tev energies. it is not expected that such sources can produce very high energy gamma rays when the inverse compton cross section decreases in the klein-nishina regime. the studies of most tev pulsar wind nebulae, however, agree better with leptonic models than with cosmic ray interaction models. the same is true for many other tev gamma ray sources such as geminga and vela x. 2.2 shell supernova remnants shell supernova remnants sources are the typical supernova remnants as we imagine them. the shock wave from supernova explosion heats up the interstellar material as it propagates through it. when we observe such a remnants we see mostly its outer edge that is brighter than the inside of the remnant. in optical astronomy this effect is called limb brightening. the observations also indicate higher magnetic fields at the limbs of the remnants. a very good introduction to the processes in the remnant is given in (reynolds, 1998) where the author deals only with electron acceleration. the acceleration of protons and heavier nuclei is discussed by (caprioli, amato & blasi, 2010). in order to understand the limb brightening one has to assume that there is electron acceleration at the edge of the remnants where the electrons have synchrotron energy loss in the higher magnetic field. the shell supernova tev gamma ray sources in tevcat include ic443, sn1006, cassiopeiaã, and tycho. 2.3 snr/molecular clouds this type of sources includes supernova remnants that have massive molecular clouds nearby. very often the tev gamma ray sources do not point at the center of the remnant, rather at the molecular cloud itself or at the side of the remnant that is close to the cloud. figure 4: positions of the molecular clouds and the number of tev gamma rays observed by the hess experiment (aharonian et al, 2006). most of these sources were first observed by the hess experiment and carry its name. an extremely interesting analysis was performed by hess after its observation of the galactic center ridge (aharonian et al, 2006). this analysis is illustrated in fig. 4. hess saw that the number of tev gamma rays coming from the vicinity of the galactic center has gaps, i.e. there would be hundreds of gamma rays coming from certain direction, then almost no gamma rays, then again hundreds of gamma rays. the exact directions of the locations that produced hundreds of gamma rays coincided with the positions of several giant molecular clouds containing 2-4×107 m�. at the 8.5 kpc distance of the galactic center 0.2o longitudinal difference corresponds to a distance of 30 pc. the analysis concluded with the idea that cosmic rays were accelerated in the galactic center (maybe at sagittarius a east) many years ago and started diffusing away from it. when they diffuse into one of the huge molecular clouds they interact with the matter there, generate neutral mesons that decay into gamma rays. these cosmic rays were able to diffuse at distances up to 100 pc but have not yet reached the molecular cloud at a distance of 200 pc. this leads to an estimate of the time of the supernova remnant acceleration episode of 104 years ago. an acceleration episode could be similar to the movement of a small molecular cloud to the black hole sgr a∗ in the galactic center that is being observed now (gillesen et al, 2013). the absorption of large amount of matter by the black hole can easily create a shock and thus accelerate cosmic rays for a relatively short time. there are now analyses of different gamma ray sources that require the existence of molecular clouds nearby. a very interesting one is that of (torres, rodriguez marrero & decea del pozo, 2010) of the supernova remnant ic443. the gamma ray emission of this source is known from the 1990’s when it was detected by egret. it was observed by the tev cherenkov telescopes magic and veritas at a slightly different (0.4o) direction. the direction of the same source from agile and fermi/lat are consistent with this of egret. the authors of this analysis identify two (or maybe three) different sources: ic443 as seen by the tev telescopes and a molecular cloud in front of it that is observed in the gev energy range. it is also possible that another, relatively small molecular cloud also emits gamma rays. the existence of different sources solves the problem with the different spectral indices in the gev and the tev gamma ray emission. 3 other possible ideas an old paper (berezinsky et al, 1993) looks at the possibility that there would be a strong diffuse radiation from the central region of the galaxy. the paper uses a matter density study (bloemen, 1989) that determined that 191 todor stanev the matter density in the inner 300 pc of the galactic plane within b < |2|o is 38 nucleons cm−3. the matter density strongly decreases to reach less than 1 per cm3 at distances greater than 6 kpc. assuming that the matter distribution in the galaxy is symmetric (which it is not) the paper used the data of bloemen to calculate the column density in different directions. the highest column density for latitude less than 2o around the galactic center reached 8×1022 cm−2. using this mapping the paper provides the ratio between the γray flux and cosmic ray flux as a function of the galactic longitude. in the vicinity of the galactic center and for energies less than 10 tev this ratio is still less than 10−4. it is easy the explain that: a column density of 8×1022 cm−2 is slightly more than 0.1 g/cm2 when the proton interaction length is about 50 g/cm2. this means that only 0.6% of the galactic cosmic rays interact and generate gamma rays that we will see coming from the direction of the galactic center. in all other directions the diffuse gamma ray flux is smaller. i refer to this paper because it attempted to calculate in an easy way the diffuse gamma ray flux. the contemporary attempts to do this are much more sophisticated and involve measurements and subtraction of all known gamma ray sources. the fermi bubbles were discovered in this way (su, slatyer & finkbeiner, 2010). the bubbles cover an area much larger than the galactic center region and are still discussions of the origin of the gamma ray emission from them. one example of the fermi/lat studies of the diffuse gamma ray radiation in the galaxy is in (ackermann et al, 2012) where the small and large scale anisotropy is studied and the existence of unknown gamma rays sources is discussed. in other papers the diffuse radiation is searched for for possible dark matter signatures. 4 discussion and conclusions the gamma ray energy spectra from the detected gamma ray sources are very often compared to leptonic and hadronic models models of gamma ray production. some of the sources, typically the pulsar wind nebulae, fit better the leptonic models, where electrons accelerated at the source have inverse compton collisions with synchrotron photons emitted by the same electrons in propagation of the magnetic fields around the source. even if these so called ssc models fit much better the gamma ray emission in a wide energy range, there are still problems that are hard to solve. the main problem (for me) is that it is difficult to imagine a mechanism that only injects electrons and not charged nuclei in the acceleration site. one possible answer is that both electrons and cosmic rays are accelerated but the matter density around the source is very low and the cosmic rays do not interact to produce neutral mesons and gamma rays. electrons, on the other hand, emit synchrotron radiation in their propagation in magnetic fields and these low energy photons are the target for inverse compton interactions. there are, of course, several sources that fit better the hadronic interaction models and this is true for more of the combinations of supernova remnants and molecular clouds. the proof of π0 origin of the gamma rays in a source is the decrease of the flux at energies lower than 70 mev, one half of the π0 mass. this however does not happen if there is a significant contribution to the gamma ray flux from bremsstrahlung. this is one of the reasons that for most of the gamma ray sources the hadronic origin is suspected, but not proven. the suspicion is usually because of the existence of large matter density around or in front of the source. figure 5: the neutrino flux that corresponds to the gamma ray flux of ic 443. no neutrinos attributed to this sources have been observed yet. the only way we can be certain that the gamma ray generating process is hadronic is if we observe also neutrinos from the same object. gamma ray and neutrino productions are closely related as shown in eq. 1 that gives the shape of the gamma ray and neutrino fluxes as a function of the same astrophysical parameters. dnγ(ν) deγ(ν) = dncr decr fa σinel mn [ρr]cν 2znπ0(π±) γ + 1 (1) the term fa accounts for the differences between proton-proton and nuclei interactions and the term cν , that is less than 1, accounts for the different kinematics 192 are supernovae responsible for the gamma ray spectrum from the galactic center? of gamma ray and neutrino production. the gamma ray flux is higher than that of neutrinos. the big problem is the tiny neutrino interaction cross section that requires huge detectors similar to the 1 km3 icecube detector at the south pole. in some cases the low neutrino cross section is an advantage as all neutrinos generated by a source will not be absorbed and will be visible by the neutrino telescopes. acknowledgement this work is supported in part by the us department of energy contract ud-fg02-91er40626. references [1] abdo, a.a. et al. (fermi/lat collaboration): 2010, ap.j. suppl. 188, 429. doi:10.1088/0004-637x/715/1/429 [2] aharonian, f. et al. (hess collaboration): 2006, nature, 439, 695. doi:10.1038/nature04467 [3] ackermann, m (fermi/lat collaboration(: 2012, phys. rev. d85:109901; arxiv:1202.2856 [4] berezinsky, v.s., gaisser, t.k., halzen, f., & stanev, t.:1993, astropart. phys. 1, 281. doi:10.1016/0927-6505(93)90014-5 [5] bloemen, h.:1989, ann. rev. astron. astrophys. 27, 469. doi:10.1146/annurev.aa.27.090189.002345 [6] boria tridon, d. et al. (magic collaboration):2010, nim a623, 437. [7] caprioli, d., amato, e. & blasi:2010, p.:astropart. phys., 33, 160. [8] gillesen, s. et al.:2013, arxiv:1306.1374 [9] ginzburg, v.l. & syrovatskii, s.i.:1964, the origin of cosmic rays (pergamon press, oxford). [10] green, d.a.:2009, bull. astr. soc. india, 37, 45. astro-ph:0905.3699v2 [11] hanna, d. et al. (veritas collaboration):2008, nim a588, 26. [12] hinton, j.a. et al. (hess collaboration):2004, new astr. rev. 48, 331. doi:10.1016/j.newar.2003.12.004 [13] reynolds, s.p.:1998, apj 493, 375. [14] su m., slatyer t. r.& finkbeiner d. p.:2010, apj, 724, 1044. doi:10.1088/0004-637x/724/2/1044 [15] torres, d., rodriguez marrero, a.y. & de cea del pozo, e.:2010, mnras, 408, 1257. doi:10.1111/j.1365-2966.2010.17205.x discussion carlotta pittori: is there any possible correlation between your viewgraph of the galactic center with the idea of an episode of cosmic ray acceleration 106 years ago and the 511 kev integral map shown by ubertini? todor stanev: as piero ubertini said himself it is a matter of electron and positron density in the annihilation site. there is, of course, also the problem of their diffusion. how far these particles would go away from the acceleration site in a million years and how big the annihilation area would be? the hess analysis of the galactic ridge discusses the diffusion in a location where we expect very irregular and strong magnetic fields and thus fast diffusion. 193 http://dx.doi.org/10.1088/0004-637x/715/1/429 http://dx.doi.org/10.1038/nature04467 http://dx.doi.org/10.1016/0927-6505(93)90014-5 http://dx.doi.org/10.1146/annurev.aa.27.090189.002345 http://dx.doi.org/10.1016/j.newar.2003.12.004 http://dx.doi.org/10.1088/0004-637x/724/2/1044 http://dx.doi.org/10.1111/j.1365-2966.2010.17205.x introduction supernova remnants pulsar wind nebulae shell supernova remnants snr/molecular clouds other possible ideas discussion and conclusions 49 acta polytechnica ctu proceedings 1(1): 49–55, 2014 49 doi: 10.14311/app.2014.01.0049 planck 2013 cosmology results: a review josé alberto rubiño-mart́ın1,2 on behalf of the planck collaboration 1instituto de astrofisica de canarias, e-38200 la laguna, tenerife, spain 2departamento de astrofisica, universidad de la laguna, e-38206 la laguna, tenerife, spain corresponding author: jalberto@iac.es abstract this talk presents an overview of the cosmological results derived from the first 15.5 months of observations of the esa’s planck mission. these cosmological results are mainly based on the planck measurements of the cosmic microwave background (cmb) temperature and lensing-potential power spectra, although we also briefly discuss other aspects of the planck data, as the statistical characterization of the reconstructed cmb maps, or the constraints on cosmological parameters using the number counts of galaxy clusters detected by means of the sunyaev-zeldovich effect in the planck maps. all these results are described in detail in a series of papers released by esa and the planck collaboration in march 2013. keywords: cosmology: observations cosmic microwave background cosmological parameters galaxy clusters. 1 introduction planck1 (tauber et al., 2010; planck collaboration i, 2011) is the third generation space mission to measure the anisotropy of the cosmic microwave background (cmb). it observes the sky in nine frequency bands covering 30–857 ghz with high sensitivity and angular resolution from 31’ to 5’. the low frequency instrument (lfi; mandolesi et al., 2010; bersanelli et al., 2010; mennella et al., 2011) covers the 30, 44, and 70 ghz bands with amplifiers cooled to 20 k. the high frequency instrument (hfi; lamarre et al., 2010; planck hfi core team, 2011a) covers the 100, 143, 217, 353, 545, and 857 ghz bands with bolometers cooled to 0.1 k. polarisation is measured in all but the highest two bands (leahy et al., 2010; rosset et al., 2010). a combination of radiative cooling and three mechanical coolers produces the temperatures needed for the detectors and optics (planck collaboration ii, 2011). two data processing centres (dpcs) check and calibrate the data and make maps of the sky (planck hfi core team 2011b; zacchei et al., 2011). planck ’s sensitivity, angular resolution, and frequency coverage make it a powerful instrument not only for cosmology, but also for galactic and extra-galactic astrophysics. this talk focuses on the main cosmology results based on planck ’s second data release, which covers data acquired in the period 12 august 2009 to 27 november 2010, and will be quoted here as nominal mission maps. all these results and the associated data products are described in a series of papers released in march 2013 (planck collaboration i–xxix, 2013). because the analysis of the polarization data is not yet as mature as the analysis of the temperature data, the planck polarization results are not included in this release. 2 overview of planck 2013 cosmology results the main goal of the planck mission is to determine with great precision the key cosmological parameters describing our universe, using the information encoded in the cmb anisotropies on intermediate and small angular scales over the whole sky (tauber et al., 2010; planck collaboration i, 2013). the data processing pipelines and beam calibrations employed by the lfi and hfi dpcs to create and characterize the nine full-sky maps based on the first 15.5 months of operations are described in planck collaboration ii–ix (2013). these nine maps (see table 1) allow robust reconstruction of the primordial cmb temperature fluctuations over nearly the full sky, as well as new constraints on galactic foregrounds, including thermal dust and line emission from molecular carbon monoxide (planck collaboration xii 2013). four different component separation methods were optimized to produce 1 planck (http://www.esa.int/planck) is a project of the european space agency (esa) with instruments provided by two scientific consortia funded by esa member states (in particular the lead countries france and italy), with contributions from nasa (usa) and telescope reflectors provided by a collaboration between esa and a scientific consortium led and funded by denmark. 49 http://dx.doi.org/10.14311/app.2014.01.0049 josé alberto rubiño-mart́ın a cmb map based on the planck maps alone, i.e., without the addition of any other external data. figure 1 shows the map produced by the smica method. 2.1 the planck cmb likelihood and the angular power spectrum a complete statistical description of the two-point correlation function of the cmb temperature fluctuations measured by planck which accounts for all known relevant uncertainties, is encoded in the planck likelihood (planck collaboration xv 2013). we follow a hybrid approach to construct this likelihood, using a exact approach at large scales (` < 50), and a pseudo-c` approach at small scales (` > 50). this function can be used to derive our estimate of the cmb angular power spectrum (see figure 2), which constitutes an extremely precise measurement over three decades in multipole range, allowing to see seven acoustic peaks. in this plot, our main source of error at ` ≤ 1500 is due to cosmic variance. the measurements are in excellent agreement with previous experiments (wmap, act, spt). we have validated our likelihood through an extensive list of consistency tests, assessing the impact of residual foreground and instrumental uncertainties on the final results. although the polarization data are not released, we have checked that the best-fit λcdm cosmology is in excellent agreement with preliminary planck ee and te polarisation spectra. table 1: planck performance parameters determined from flight data (planck collaboration i 2013). channel νcenter fwhm noise sensitivity [ghz] [arcmin] [µkrj s 1/2] 30 ghz 28.4 33.16 145.4 40 ghz 44.1 28.09 164.8 70 ghz 70.4 13.08 133.9 100 ghz 100 9.59 31.52 143 ghz 143 7.18 10.38 217 ghz 217 4.87 7.45 353 ghz 353 4.70 5.52 545 ghz 545 4.73 2.66 857 ghz 857 4.51 1.33 2.2 lensing by large scale structure on small (arcminute) angular scales, the cmb anisotropies are perturbed by gravitational lensing, primarily sourced by the large scale structure of the universe at relatively high redshifts (peaking at z ∼ 2). planck provides a 25σ detection of this effect (planck collaboration xvii, 2013), allowing also a lensing potential reconstruction at the map level on almost the full sky. the power spectrum of this lensing potential map is used to construct a “lensing” likelihood, which is included in our cosmological analyses. the inclusion of the planck lensing reconstruction can break degeneracies inherent in the temperature data alone, especially the geometric degeneracy in nonflat models, thus providing a strong constraint on spatial curvature without the need of including polarization data. 2.3 cosmological parameters: base model the planck measurements of the cmb temperature and lensing-potential power spectra are well described by the standard spatially-flat six parameter λcdm model with adiabatic scalar perturbations (planck collaboration xvi, 2013). the computed values of the six key parameters within this model are summarized in table 2, together with the values obtained for some derived parameters, as ωλ, ωm, σ8 or the hubble constant h0. the most remarkable result is that, for this cosmology, we find a low value of the hubble constant, which is in tension with recent direct measurements of h0, but is in excellent agreement with constraints from baryon acoustic oscillation surveys (see planck collaboration xvi (2013) for a detailed discussion). we also find a higher value of the matter density, as compared to previous measurements. 2.4 cosmological parameters: beyond the base model multiple one-parameter extensions to the base (sixparameter) model have been discussed in planck collaboration xvi (2013). however, the main result is that there is no compelling evidence for any of these extensions. for example, including curvature, we find that the universe is consistent with spatial flatness to subpercent level precision (ωtot = 1.0010± 0.0065, at 95%, using planck +lensing+wp+highl+bao). there is no evidence for additional neutrino-like relativistic particles beyond the three families of neutrinos in the standard model. using bao and cmb data, we find neff = 3.30 ± 0.27 for the effective number of relativistic degrees of freedom, in excellent agreement with the standard value of 3.046, and an upper limit of 0.23 ev for the sum of neutrino masses. in addition, we find no evidence for dynamical dark energy; using bao and cmb data, the dark energy equation of state parameter 50 planck 2013 cosmology results: a review is constrained to be w = −1.13+0.13 −0.10. finally, deviations from the simplest inflationary scenario are discussed in planck collaboration xvi and xxii (2013), including tests of specific inflationary models, isocurvature modes, and broken scaleinvariance. as a main result, planck rules out exact scale invariance (ns = 1) at high significance. the deviation of the scalar spectral index from unity is insensitive to the addition of tensor modes and to changes in the matter content of the universe. we find a 95% upper limit of r < 0.11 on the tensor-to-scalar ratio (see figure 3), which allows us to constrain a number of inflationary models. figure 1: the smica method estimates the cmb over about 97% of the sky, with the remaining area replaced with a constrained gaussian realization. figure taken from planck collaboration i (2013). figure 2: cmb temperature angular power spectrum from planck. the shaded area represents cosmic variance. the solid line corresponds to the best-fit six-parameter λcdm model quoted in the text. figure taken from planck collaboration i (2013). 51 josé alberto rubiño-mart́ın table 2: cosmological parameter values for the six-parameter base λcdm model. column 2 give the results for the planck temperature and lensing potential power spectra, while column 3 adds the information from wmap polarization (wp) at low multipoles (` ≤ 23), and other small scale cmb experiments (highl). see planck collaboration xvi 2013 for details. the last four rows are derived parameters. planck + lensing + wp + highl ωbh 2 0.02217 ± 0.00033 0.02218 ± 0.00026 ωch 2 0.1186 ± 0.0031 0.1186 ± 0.0022 100θmc 1.04141 ± 0.00067 1.04144 ± 0.00061 τ 0.089 ± 0.032 0.090+0.013 −0.014 ns 0.9635 ± 0.0094 0.9614 ± 0.0063 ln(1010as) 3.085 ± 0.057 3.087 ± 0.024 ωλ 0.693 ± 0.019 0.693 ± 0.013 ωm 0.307 ± 0.019 0.307 ± 0.013 h0 67.9 ± 1.5 67.9 ± 1.0 σ8 0.823 ± 0.018 0.8233 ± 0.0097 figure 3: marginalized joint 68% and 95% confidence level regions for ns and r from planck in combination with other data sets, compared to the theoretical predictions of some inflationary models. taken from planck collaboration xxii (2013). 2.5 gaussianity and statistical isotropy the statistical isotropy and the gaussian nature of the planck cmb maps has been rigorously examined in several papers (planck collaboration xxiii–xxvi, 2013). as a main result, there is no detection of primordial non gaussianity, obtaining the following values for the primordial local, equilateral and orthogonal bispectrum amplitudes: f local nl = 2.7 ± 5.8, f equil nl = −42 ± 75, and fortho nl = −25 ± 39 (all at 68% confidence level). the integrated sachs-wolfe-lensing bispectrum signal is detected at 2–3 sigma level. the planck data also provides stringent new constraints on cosmic strings and other defects (planck collaboration xxv 2013). alternative geometries and non-trivial topologies have also been analyzed (planck collaboration xxvi 2013). deviations from isotropy have been found in planck 52 planck 2013 cosmology results: a review cmb maps, essentially confirming all the low-angle cmb anomalies previously seen in wmap data at similar levels of significance (about 3σ): the low quadrupole amplitude; the quadrupole-octupole alignment; the hemispherical asymmetry; and the “cold spot”. finally, planck notes a further low-angle discrepancy, related to the deficit of power in the angular power spectrum in the multipole range 20 ≤ ` ≤ 40, which could be connected with some of the previous features (e.g. the hemispherical asymmetry or the low observed variance). 2.6 galaxy clusters planck maps can also be used to detect and study galaxy clusters by means of their sunyaev-zeldovich (sz) effect (sunyaev & zeldovich, 1972). early planck results on clusters were published in planck collaboration viii-xii (2011). in march 2013, we have released the planck catalogue of sz sources (psz1) based on the nominal mission maps (planck collaboration xxix 2013). psz1 contains 1227 entries, out of which 861 are confirmed galaxy clusters, and it constitutes the largest all-sky cluster catalogue to date (see figure 4). in planck collaboration xx (2013), we present constraints on cosmological parameters using the number counts (n(z)) for a well-defined and 100% reliable subsample of 189 galaxy clusters from psz1. using a xray calibrated mass-sz flux relation, and assuming a 20% bias between the x-ray determined mass and the true mass of a cluster, we derive constraints on the matter power spectrum amplitude and the matter density parameter, obtaining σ8(ωm/0.27)0.3 = 0.78±0.01, with one-dimensional ranges σ8 = 0.77 ± 0.02 and ωm = 0.29 ± 0.02. these results are in excellent agreement with the cosmological constraints from the angular power spectrum of the thermal sz emission. in planck collaboration xxi (2013), we construct the first all-sky maps of the thermal sz emission, and compare their power spectrum to theoretical models, finding σ8(ωm/0.27)0.3 = 0.784 ± 0.016. however, these results favour somewhat low values of σ8 and ωm as compared to the cmb based analysis (see table 2). this tension can be eliminated by relaxing the assumption on the mass bias between x-ray and true mass up to 45%, although such a high value is difficult to reconcile with the known observed properties of galaxy clusters. there is another interesting possibility to reconcile these values, which consists in allowing for a component of massive neutrinos with a total mass (for the sum of the three species) of the order to ∑ mν ≈ 0.22 ev (see marulli et al. 2011 for a review of how cosmological observations can be affected by the inclusion of neutrino masses). these possibilities will be further investigated in future releases. figure 4: distribution of the 1227 planck sz clusters and candidates across the sky, shown in mollweide projection in galactic coordinates. the masked regions in black color (point-sources, magellanic clouds and galactic emission) cover a total area of 16.3%, and are not used by the sz cluster-finder algorithms. taken from planck collaboration xxix (2013). 3 conclusions the main conclusions of the planck 2013 release are: • the planck data are in remarkable agreement with a flat six-parameter λcdm model; • we detect with high significance lensing of the cmb by intervening matter, providing evidence for dark energy from the cmb alone; • the best-fit model contains a weaker cosmological constant (∼ 69%), and more cold dark matter (∼ 26%) than previously estimated; and we firmly establish a deviation from scale invariance for primordial matter perturbations; • we find a low value of the hubble constant, in tension with other astrophysical measurements; • there is no compelling evidence for extensions of the base model; • we find no evidence for significant deviations from gaussianity in the statistics of cmb anisotropies; however, we confirm the anomalies at large angular scales first detected by wmap and find a deficit of power at low-multipoles with respect to our best-fit model. • the planck sz cluster measurements are in tension with the cmb results; although this conclusion is limited by our knowledge of the observablemass relations, it might provide hints for physics in the neutrino sector. 53 josé alberto rubiño-mart́ın the next (2014) release of planck data, which will include the full-mission maps and the polarization information, will provide a significant improvement in data quality and the level of systematic error control, helping in answering these open questions. acknowledgement the development of planck has been supported by: esa; cnes and cnrs/insu-in2p3-inp (france); asi, cnr, and inaf (italy); nasa and doe (usa); stfc and uksa (uk); csic, micinn, ja and res (spain); tekes, aof and csc (finland); dlr and mpg (germany); csa (canada); dtu space (denmark); ser/sso (switzerland); rcn (norway); sfi (ireland); fct/mctes (portugal); and prace (eu). a description of the planck collaboration and a list of its members, including the technical or scientific activities in which they have been involved, can be found at http://www.sciops.esa.int/index.php?project=planck. jar-m is grateful to the organizers of the frascati workshop 2013 for the invitation to present these results. jar-m acknowledges support from the project aya2010-21766-c03-02 of the spanish ministry of science and innovation (micinn). references [1] bersanelli, m., mandolesi, n., butler, r. c., et al. 2010, a&a, 520, a4 [2] mandolesi, n., bersanelli, m., butler, r. c., et al. 2010, a&a, 520, a3 [3] mennella et al. 2011, a&a, 536, a3 [4] lamarre, j., puget, j., ade, p. a. r., et al. 2010, a&a, 520, a9 [5] leahy, j. p., bersanelli, m., d’arcangelo, o., et al. 2010, a&a, 520, a8 [6] marulli, f. et al. 2011, mnras, 418, 346 [7] planck collaboration i. 2011, a&a, 536, a1 [8] planck collaboration ii. 2011, a&a, 536, a2 [9] planck collaboration viii. 2011, a&a, 536, a8 [10] planck collaboration ix. 2011, a&a, 536, a9 [11] planck collaboration x. 2011, a&a, 536, a10 [12] planck collaboration xi. 2011, a&a, 536, a11 [13] planck collaboration xii. 2011, a&a, 536, a12 [14] planck hfi core team. 2011a, a&a, 536, a4 [15] planck hfi core team. 2011b, a&a, 536, a6 [16] planck collaboration i. 2013, a&a submitted [17] planck collaboration ii. 2013, a&a submitted [18] planck collaboration iii. 2013, a&a submitted [19] planck collaboration iv. 2013, a&a submitted [20] planck collaboration v. 2013, a&a submitted [21] planck collaboration vi. 2013, a&a submitted [22] planck collaboration vii. 2013, a&a submitted [23] planck collaboration viii. 2013, a&a subm. [24] planck collaboration ix. 2013, a&a submitted [25] planck collaboration x. 2013, a&a submitted [26] planck collaboration xii. 2013, a&a submitted [27] planck collaboration xiii. 2013, a&a subm. [28] planck collaboration xiv. 2013, a&a subm. [29] planck collaboration xv. 2013, a&a submitted [30] planck collaboration xvi. 2013, a&a subm. [31] planck collaboration xvii. 2013, a&a subm. [32] planck collaboration xviii. 2013, a&a subm. [33] planck collaboration xix. 2013, a&a subm. [34] planck collaboration xx. 2013, a&a submitted [35] planck collaboration xxi. 2013, a&a subm. [36] planck collaboration xxii. 2013, a&a subm. [37] planck collaboration xxiii. 2013, a&a subm. [38] planck collaboration xxiv. 2013, a&a subm. [39] planck collaboration xxv. 2013, a&a subm. [40] planck collaboration xxvi. 2013, a&a subm. [41] planck collaboration xxix. 2013, a&a subm. [42] rosset, c. et al. 2010, a&a, 520, a13 [43] sunyaev, r. a. & zeldovich, y. b. 1972, comments on astrophysics and space physics, 4, 173 [44] tauber, j. a. et al. 2010, a&a, 520, a1 [45] zacchei et al. 2011, a&a, 536, a5 54 planck 2013 cosmology results: a review discussion marco regis: can the large scale anomalies be due to galactic foregrounds ? jose alberto rubiño-martin: we have performed a large number of tests using the four cmb maps produced by the different component separation algorithms, and the consistency of the results strongly favors a cosmological origin for the anomalies. in addition, the agreement between planck and wmap rules out a possible explanation based on systematic effects. 55 introduction overview of planck 2013 cosmology results the planck cmb likelihood and the angular power spectrum lensing by large scale structure cosmological parameters: base model cosmological parameters: beyond the base model gaussianity and statistical isotropy galaxy clusters conclusions acta polytechnica ctu proceedings doi:10.14311/app.2016.3.0039 acta polytechnica ctu proceedings 3:39–42, 2016 © czech technical university in prague, 2016 available online at http://ojs.cvut.cz/ojs/index.php/app database finliv – focus on staircase method maxim lutovinov∗, jan papuga, milan růžička faculty of mechanical engineering of czech technical university in prague, technická 4, prague, czech republic ∗ corresponding author: maxim.lutovinov@fs.cvut.cz abstract. this paper introduces finliv database focused on gathering, manipulating and providing experimental static and fatigue data. one of its new features is the ability to include also fatigue tests realized by the staircase method. keywords: fatigue, database, staircase method, fatigue limit. 1. introduction finliv is a database intended to gather the information about static and stressand strain-controlled fatigue experiments and also about parameters of various static and fatigue material models derived from such data. it was developed to resolve the problems of researchers in a lack of quality data, their frequent second-hand use, and the way the data items are selected for various benchmark tests. by now, finliv has a large set of tools for entering and processing data. one of the newest tools is the staircase method [1] that automatically evaluates data from stress-controlled experiments and provides the fatigue limit as a result. 2. finliv finliv is represented by two parts, which closely cooperate, but which also can be used as standalone applications. the first part serves for entering data, while the second part serves for listing them. the second part is implemented as a web database and can be found at www.fadoff.cz/page/finliv. it allows the user to have permanent access to the raw experimental data or to the final regression curves retrieved from them. in the web part there are no tools to process the data, it simply retrieves and lists them on a request in a raw or processed form. the first part, finliv.vba [2], was built as an ms excel application written in visual basic for applications, and its main purpose is to enter experimental data and to process them. data processing is necessary in order to e.g. perform regression analysis or to prepare automated input for fatigue solvers, if a particular experimental data set has to be used within a benchmark of a particular calculation method. the ms excel platform was chosen because of its prevalence among engineers, and because it is substantially cheaper than matlab. the application can be accessed from any computer which has ms excel installed. the main interface of finliv.vba is represented by "form" sheet (see the part of the input interface in figure 1). it consists of a group of tables, combo boxes and buttons. data input is realized by filling in the salmon colored cells and by choosing options from the combo boxes. in the first non-empty column there are designations that correspond to the input parameters. the same designations are used in the datasheets (figure 2), where the material data are stored. the datasheets are placed in the same workbook as the “form” sheet, and their structure is quite simple: in the second column there are designations that represent the entered parameters, while the other columns contain information on individual curves of the particular data set. finliv works with data from static experiments and stressand strain-controlled fatigue experiments. since each type of experiments uses a different set of parameters, the “form” sheet changes its structure to accommodate the selected type. in addition to entering data, finliv.vba can also process data. with the help of the application the user can easily perform regression analyses of experimental data and obtain corresponding regression parameters. for strain-life data there are two types of a regression analysis. the first one is carried out in a conventional way, which is represented by linear regression using the least square method. the second method performs non-linear regression according to the 3-d method [3]. the 3-d method is based on the assumption that one value of the total strain amplitude corresponds to only one stress amplitude and to a single number of cycles. if this assumption is valid a curve can be drawn in a 3-dimensional space which besides formulation of the total characteristics depicts also plastic and elastic strain. unfortunately, implementing such a non-linear 3-d regression in excel is problematic, therefore it was decided to carry out the regression analyses in an external matlab application. to prepare an input file for that application the user should click "input for regression" button. to load the resulting parameters of the regression analysis to finliv, the user should click "get regression data" button and choose the output file created by the matlab application. for stress-life data, there are also two types of a regression analysis, but unlike the case of strain-life 39 http://dx.doi.org/10.14311/app.2016.3.0039 http://ojs.cvut.cz/ojs/index.php/app m. lutovinov, j. papuga, m. růžička acta polytechnica ctu proceedings figure 1. "form" sheet of finliv with sample data. figure 2. a datasheet of finliv with sample data. data both types of the regression are carried out in the excel environment. regression analyses of s-n data as well as a visual display of obtained e-n curves are available in another workbook that is generated automatically from finliv. individual sheets in that workbook correspond to the data sets that were selected for processing. at each page of such data there are two plots of regression curves. in the case of strain-life data, there are two manson-coffin curves, constructed on the basis of parameters obtained from conventional regression analyses and 3-d analyses. in the case of stress-life data, the first regression curve is represented by the basquin relation and another one by kohout-vechet relation. for s-n data, a new tool is also available in the same workbook the staircase method, which is described below. 2.1. staircase method the staircase method is a standardized method [1] for carrying out and evaluating experiments in order to determine the fatigue limit related to a given number of cycles. the essence of the method is to increase or decrease the load amplitude of a new test specimen by a constant pre-selected value based on the response of the previous specimen, i.e. whether a failure occurred during the previous experiment. for evaluation, the staircase method uses a special table (figure 3) constructed according to the standard iso 3800 [1]. the rows of the table represent stress levels and the columns represent the order of the experiments. a symbol “o” is entered into the appropriate cell when a failure doesn’t occur, and a symbol “x” in the opposite case. if the essence of the method has been respected, then the symbols “x” and “o” are arranged in the table into the sequence that 40 vol. 3/2016 database finliv – focus on staircase method figure 3. process of generating the output table according to the staircase method in finliv. looks like stairs. in case of a suitable choice of the first stress level and a step size between individual levels, the stress levels after several experiments (about 10 experiments) will oscillate around a certain value – the fatigue limit. the fatigue limit and the corresponding standard deviation can be then calculated with the help of the equations given by the standard [1], which uses values from the left part of the table (figure 3, columns 3 to 8). the fatigue limit labeled as sig_50 in figure 3 is calculated according to: sig_50 = σao + ∆σall( a c + x) (1) here, σao is the lowest load amplitude level at which the first non-failed specimen appears, given that the total sum of the non-failed specimens is less than the total sum of the failed specimens. in the opposite case, the load level corresponding to the first failed specimen should be input. the standard doesn’t specify how to proceed in the case when the total sums of the failed and non-failed specimens are equal. the observations show that in such case the result will be the same regardless of whether failed or non-failed specimens are selected as the reference type. the algorithm implemented in finliv in cases like that calculates the fatigue limit in two steps, with the failed and non-failed specimens as the reference type and then chooses the lowest fatigue limit. the parameter ∆σall is the step size between individual load levels. a and c are sums of the values in the columns z*f and f. the parameter x is equal to +0.5 when the number of the non-failed specimens is lower than the number of the failed specimens, or -0,5 in the opposite case. it is worth noting that the method can be applied to the load levels represented by stress as well as to the levels represented by forces. the result is then the fatigue limit or the force corresponding to the fatigue limit. the parameter s(sig) is a standard deviation of the distribution and is calculated according to: s(sig) = 1.62∆σall( ce −a2 c2 + 0.029) (2) where e is the sum of values in the column zˆ2*f. the staircase method was extended in finliv, and the application analyzes whether the data are suitable for being processed by the staircase method at all. thanks to that, the method can also be used on data that were obtained from experiments initially unintended to be processed by the staircase method. if the data are suitable for the method, finliv automatically draws an output table (figure 3). the algorithm that analyzes data from the selected load levels assembles various combinations that differ 41 m. lutovinov, j. papuga, m. růžička acta polytechnica ctu proceedings in the step size between levels and/or by the number of levels. then individual combinations are checked according to the criteria that say whether individual sets of load levels and corresponding sets of specimen states (failure, non-failure) are suitable for evaluation by the staircase method. these criteria are derived from the essence of the method. for example, one of the criteria says that if the number of the non-failed specimens at the highest load level is more than one, then such set of load levels and specimen states is not suitable for the staircase method. according to the method, the appearance of a non-failed specimen leads to increasing the value of load amplitude. if the experiment with such specimen is not the last one, then the next experiment should be carried out at a higher level of load amplitude, which means that the level of load amplitude corresponding to the non-failed specimen in question is not the highest level in the particular set of experiments. in the other case, if the experiment at the highest load level with non-failed is the last one, then at the same level there cannot be other non-failed specimens. after selected data are checked by the criteria, finliv provides the user the list of possible combinations of load levels and corresponding steps to choose from (figure 3). the combinations are ordered by the number of included specimens. for each combination there are also listed corresponding step sizes and calculated fatigue limits. while selecting a combination it is important to check the ratio of the fatigue limit and the step size. the smaller is the ratio, the more accurate the estimate should be. the drawback of the implemented check is that it works only with load levels and can’t take into account the possibility of individual specimens being removed from evaluation in order to assemble a suitable combination of the load levels and specimen states. 3. summary the paper refers to finliv database for storing, listing and manipulating experimental static and fatigue data. it describes its newest feature, which is the ability to process data by the staircase method. acknowledgements the authors thank for the support of their research by sgs14/181/ohk2/3t/12 ctu grant. references [1] iso 3800:1993(e). threaded fasteners axial load fatigue testing test methods and evaluation of results. switzerland: international organization for standardization, 1993. [2] j. papuga, m. lutovinov. help for finliv.vba excel database [ver. c], fme ctu in prague & evektor, spol. s.r.o., prague 2014. [3] a. nieslony, c. dsoki, h. kaufmann, p. krug. new method for evaluation of the manson–coffin–basquin and ramberg–osgood equations with respect to compatibility. international journal of fatigue 30(1011):1967–1977, 2008. doi:10.1016/j.ijfatigue.2008.01.012. 42 http://dx.doi.org/10.1016/j.ijfatigue.2008.01.012 acta polytechnica ctu proceedings 3:39–42, 2016 1 introduction 2 finliv 2.1 staircase method 3 summary acknowledgements references 255 acta polytechnica ctu proceedings 1(1): 255–258, 2014 255 doi: 10.14311/app.2014.01.0255 probing gravitational theories with eccentric eclipsing detached binary stars leopoldo milano1,2, rosario de rosa1,2, mariafelicia de laurentis1,2, fabio garufi1,2 1dipartimento di fisica universita’ federico ii di napoli complesso universitario di monte s. angelo, via cinthia, i 80125, napoli, italy 2sezione infn di napoli,complesso universitario di monte s. angelo, via i 80125, napoli, italy corresponding author: milano@na.infn.it abstract in this paper, we compare the effects of different theories of gravitation on the apsidal motion of eccentric eclipsing detached binary stars. the comparison is performed by using the formalism of the post-newtonian parametrization to calculate the theoretical advance at periastron and compare it to the observed one, after having considered the effects of the structure and rotation of the involved stars. a variance analysis on the results of this comparison shows that no significant difference can be found due to the effect of the different theories under test with respect to the standard general relativity (gr). it will be possible to observe differences, as we would expect, by checking the observed period variation on a much larger lapse of time. keywords: relativity gravitation eclipsing binaries apsidal motion. 1 introduction the problem of the motion of two bodies under their mutual gravitational attraction and the study of binary stellar systems has always been the ideal test bed for the theories of gravitation. several authors in the last decades dedicated a lot of work in analyzing, both on the theoretical and the experimental point of wiew, the phenomenon of the periastron precession in binary systems to test various gravitational theories [1, 2] as well as to find correction to the newtonian and general relativistic behaviour of the systems due to stellar form factors, spin, tides and other phenomena [3]. the classical effect of general relativity (gr) on the apsidal motion rate at periastron is well known since long time and described by levi-civita in a famous paper in 1937 [4, 5]. another possible formulation of the problem, that allows also to test other gravitational theories besides gr, is the use of parametrized post newtonian (ppn) formalism [6, 7]. using this formalism, the different gravitational theories can be compared side by side on the basis of a set of post-newtonian (pn) parameters: the masses, the system major semiaxis and the eccentricity. thus, using a sample of eccentric eclipsing detached binary (eedb) systems, for which masses and orbital parameters are known with sufficient precision, it is possible to compare the apsidal motion rate at periastron ω̇th, as expected in the different theories with the observations in order to verify whether the observations can select one theory or another. the choice of this class of stellar objects to test the theories is dictated by the circumstance that the both orbital and the structural parameters can be precisely determined by observation of the eclypses (the only alea being the orbit plane observation angle), and that in close stellar orbits, the gravitational field is supposed to be strong, thus enhancing eventual effects of deviations from the classical theory. there are so many aspects of binary star evolution and angular momentum exchange (see, e.g., [8]) that any attempt to dig out from these other effects the subtleties of gr could seem futile, nonetheless, we consider useful to ascertain whether from the observation of many binary systems, some statistical information about the prevalence of one or another theory of gravitation can be extracted and to ground the bases of a method that can be used when other more significant data will be available. 2 advance at periastron the idea of considering relativistic gravitational tests in terms of a metric expansion is originally based on a work by shiff [9] who expanded the single body metric in terms of the ratio between the geometrized mass mg = gm/c2 and the distance r: g00 = 1− 2α mg r + 2β (mg r )2 255 http://dx.doi.org/10.14311/app.2014.01.0255 leopoldo milano et al. g0k = 0 gik = − ( 1 + 2γ mg r ) δik i, k = 1, 2, 3 (1) four new parameters α′, α′′, α′′′, ∆ were then introduced to account for relative velocities and accelerations. for general relativity all the parameters are equal to 1 and the advance at periastron can be verified to reproduce the ”classical” formula by levi civita [4]. we conveniently modified the ’classical’ formula by introducing a factor (kth) to take into account the dependance on the theory dependent pn terms in order to test the different relativistic theories. [10]: ω̇rel = 1.8167× 10−4kth (m p ) 2 3 c2(1− e2) (2) where: kth = αth(8∆th − αth + 2α ′′ th − α ′′′ th − γth − α ′ th) 2 (3) the pn parameters, are calculated for the different gravitational theories , i.e. the general relativity (’classical’ term), the nordvedt, the brans-dicke theories and the so called f(r) theories that take into account higher order terms of the ricci scalar r, giving a general expression for the relativistic term ω̇rel, that contributes to the advance at periastron. using for the different theories the appropriate values of αth, α ′ th, α ′′ th, α ′′′ th,∆th and γth the numerical values of kth can be obtained [10]: kth =  kgr = 3→ (general −relativity) kbd = 19 7 → (brans−dicke) knd = 11 4 → (nordvedt) kf(r) = 13 4 → (f(r)) (4) 3 comparison with experimental data to test the effects of deviations from the gr, we choose to study binary stellar systems with small radius orbits, so that the gravitational field is strong enough to evidence these deviations, if any. among the various binary stars catalogues available in literature, we choose a sample of eccentric eclipsing detached binary stars such that the period, the eccentricity, the masses of the components, and, possibly, the observed internal structure function are known with a good precision as e.g. [11]. for these systems the passage at periastron precedes in a way that is precisely predictable from the gravitational theory, once given the stellar parameters such as masses, radius of the components, and orbital parameters. to compare the global rate of theoretical apsidal motion in a binary system with the measured one we must take into account the individual contributions of each component due to tidal and rotational distortions, and the general relativistic term ω̇th, where the index th indicates the theory under test (e.g. ωgr for general relativity). assuming that rotation of both components of an eclipsing binary system is perpendicular to the orbital plane, the apsidal motion rate, ω̇ is given by the following simple relation [12]: ω̇obs = ω̇cl + ω̇rel (5) where ω̇cl is the classical newtonian term and ω̇rel is the relativistic contribution of eq. 2 and eq. 4. the dependance of ω̇cl on the binary system parameters is expressed through the internal second-order constants (isc): k̄2th = c21k21th + c22k22th c21 + c22 (6) k̄2obs = ω̇cl c21 + c22 (7) where the parameters c2i are related to the masses and the orbital eccentricity of the binary system. it must be noticed that the individual isc’s k2,i cannot be obtained from the observations although they can be interpolated from evolutionary codes like those used in [13, 14]. so we can evaluate a mean model dependent k̄2th and a mean observation dependent k̄2obs, and compare them to test the evolution stellar models from the observations of apsidal motion. taking into account that k̄2obs is generally smaller than k̄2th (this means that the evolution models predict stellar cores less dense than those found by observed data), remembering eq. 2 and eq. 5 we see that ω̇cl will vary according to ω̇th. in this way we can test the different relativistic theories of gravitation by verifying whether the agreement of the model dependent mean isc, with those coming from observations is significantly improved by using the different ω̇th relativistic terms. in fig. 1 we show the trend of the apsidal motion rate ω̇gr vs ω̇gr,bd,nd,f(r). it results: ω̇bd ∼= 0.92ω̇gr, ω̇nd ∼= 0.90ω̇gr, ω̇f(r) ∼= 1.10ω̇gr. obviously the numerical coefficients are the ratios kth kgr (see eq. 4). it is interesting to note that the f(r) theory gives a relativistic contribution that is slightly higher than gr,bd and nd. moreover, gr is ≈ the mean between f(r) and bd and nd theories. it is also evident that there is no significant difference among the theories under test within the errors. in fig. 2 we show the trend of observed ω̇obs vs ω̇rel for different relativistic terms (th ≡ gr,bd,nd, f(r)). the red line is the trend of ω̇rel = ω̇obs. it is evident that, apart 256 probing gravitational theories with eccentric eclipsing detached binary stars from a few systems, the relativistic term is always less than the observed one. 0 0.5 1 1.5 2 2.5 3 3.5 x 10 −3 0 0.5 1 1.5 2 2.5 3 3.5 x 10 −3 dω/dtgr d ω /d t g r ,b d ,n d ,f( r ) dω/dt gr dω/dt bd dω/dt nd dω/dt f(r) figure 1: ω̇gr vs ω̇gr,bd,nd,f(r) for the different relativistic theories (gr,bd,nd,f(r)): ω̇gr = ω̇gr, ω̇bd ∼= 0.92ω̇gr, ω̇nd ∼= 0.90ω̇gr, ω̇f(r) ∼= 1.10ω̇gr fig. 3 shows the isc’s comparison: log(k̄2obsth ) vs log(k̄2stellarmodel) are shown for different relativistic terms. the blue line is the trend of log(k̄2stellarmodel) = log(k̄2obsth ). it is evident that, apart from a few systems, log(k̄2stellarmodel) is always greater than the observed one. so the stellar core densities derived from the observations is higher than those coming from stellar model prevision. −0.02 0 0.02 0.04 0.06 0.08 0.1 0.12 0 0.5 1 1.5 2 2.5 3 3.5 x 10 −3 dω obs /dt dω re l/d t dω rel /dt=dω obs /dt figure 2: observed ω̇obs vs ω̇rel for different relativistic terms (th ≡ gr,bd,nd, f(r)). the red line is the trend of ω̇rel = ω̇obs. it is evident that apart from some systems the relativistic term is always less than the observed one. 4 discussion and conclusion using data coming from apsidal motion rate of eedb we compared the variation of the relativistic term of the apsidal motion rate due to different theories of gravitation, that accordingly produces variation of classical newtonian term (see eq. 5). the results of this comparison was that we could not find any significant difference due to the effect of the different theories under test with respect to the standard general relativity. since the advance at periastron accumulates, the trend and the amount of this motion can be better determined by the observation of more orbits (or a larger fraction of orbit). longer observations also improve the determination of isc’s; thus, it would be possible, perhaps, to observe more significant differences, by checking the period variation on a much larger lapse of time and verifying the assumptions of syncronous orbital and rotation motion of the binary star components. a lot of observing work are producing new and more accurate data and step by step it is getting a better agreement between theory and observations. −4.5 −4 −3.5 −3 −2.5 −2 −1.5 −1 −2.6 −2.4 −2.2 −2 −1.8 −1.6 log k2obs th lo g k2 s te lla r m od el hp aur bm mon hh car v1765 cyg δ ori ι ori log k2 obs th =log k2 stellar model figure 3: internal second order structure constants (isc) log(k̄2obsth ) vs log(k̄2stellarmodel) for different relativistic terms (th ≡ gr,bd,nd, f(r)). the blue line is the trend of log(k̄2stellarmodel) = log(k̄2obsth ). it is evident that apart from some systems log(k̄2stellarmodel) is always greater than the observed one. so the stellar core according to the observations is more dense than the stellar model prevision. acknowledgement this work was supported by infn grant 2011.the authors gratefully acknowledge the referee prof. todor stanev for his useful suggestions. references [1] breen b., 1973, j. phys. a: math., nucl. gen., 6, 150. 257 leopoldo milano et al. [2] l. lin-sen, 2010, astrophys. space sci., 327, 59. doi:10.1007/s10509-010-0267-4 [3] giménez a., claret a., 2010, astron. and astroph. 519, a57. [4] levi civita t., 1937, amer. j. of math., 59, 225. doi:10.2307/2371404 [5] giménez a., 1985, astroph. j., 297, 405. doi:10.1086/163539 [6] thorne k.s., will c.m., 1971, astrophys. j., 163, 595. doi:10.1086/150803 [7] nordtvedt k., 1969, phys. rev., 180, 1293. doi:10.1103/physrev.180.1293 [8] biermann, p., et al. 1985, astrophys. j., 293, 303 [9] schiff l.i. , 1967, relativity theory and astrophysics vol 1, ed j ehlers 1967 (philadelphia: american mathematical society) [10] m. de laurentis,r. de rosa,f. garufi & l. milano,2012,mnras, 424, 2371-2379 [11] bulut i., demircan o., 2007, mon. not. r. astron. soc. 378, 179. [12] kopal z., dynamics of close binary systems. reidel,dordrecht (1978) [13] claret a., giménez a., 1992, astron. and astroph. suppl. 96, 255. [14] claret, a. 2004, a&a, 424, 919 doi:10.1111/j.1365-2966.2007.11756.x discussion jim beall: what systems would give a proper test? leopoldo milano: the systems that give a proper test are about eleven. carlotta pittori: do you think that it could be useful to study in more datail the outliers? which kind of obsevations could help to constrain the theory? leopoldo milano: many group of astronomers are doing new observations on the outliers with the aim of improving the knowledge on the rotations velocities and other parameters that can be the cause of the failure of the simple model that is generally adopted. 258 http://dx.doi.org/10.1007/s10509-010-0267-4 http://dx.doi.org/10.2307/2371404 http://dx.doi.org/10.1086/163539 http://dx.doi.org/10.1086/150803 http://dx.doi.org/10.1103/physrev.180.1293 http://dx.doi.org/10.1111/j.1365-2966.2007.11756.x introduction advance at periastron comparison with experimental data discussion and conclusion acta polytechnica ctu proceedings doi:10.14311/app.2016.5.0044 acta polytechnica ctu proceedings 5:44–46, 2016 © czech technical university in prague, 2016 available online at http://ojs.cvut.cz/ojs/index.php/app railway infrastructure capacity management for ad hoc trains on the sždc network petr nachtigall∗, martin škárek dopravní fakulta jana pernera, univerzita pardubice, studentská 95, 532 10 pardubice ∗ corresponding author: petr.nachtigall@upce.cz abstract. the paper describes basic principles of ad hoc routes assignment. each carrier has its own interface called information system kadr with sždc for capacity requests management. the paper describes all steps which must be done before train departure keywords: informition system, carrier, ad hoc train, railway infrastructure capacaty. 1. introduction for last couple of years is growing volume of request from railway operators to infrastructure manager (sždc) for so called ad hoc trains [1]. these trains are operated out of the regular construction of the timetable. to make this agenda easier, there was developed an information system called isoř kadr. the effects of the implementation of this system were in much faster and streamline assignment of the ad hoc routes. faster is as well the distribution of the time table to all supporting information systems. among the most important systems which are linked with kadr belong isoř (information system for operative control), compost (train composition) and aport (preparedness and composition of the train). during autumn 2015 kadr stopped being just a planning program, but it was implemented into the basic operation of the traffic. via kadr are constructed deflecting routes by traffic incidents and by traffic situation which requires that the train is going on different than basic route. the sždc has also competency to set the request instead of the carrier and assign the route and the capacity in one step. 2. basic information about kadr information system kadr has two main parts. there is a web part and a desktop part. the access rights for both parts of the system are restricted by roles in the system. there are five basic roles: • carrier – in this role are open up all functions for carriers. this role is available only as a web part, • billing – in this role you can use all functions of the program, • oss – one stop shop – construction of the route, • da – dispatcher – construction of the route, • tnack restrictions – construction of the time table by track restriction (available only in desktop part). each user must be authorized before entering the kadr software. for authorization there is used the logserver. on the logserver are saved all types of roles and for each role are assigned users. after successful authorization the logserver returns back the role (or more roles) for the user. the system automatically starts the interface with functions and data for user’s role [2]. 3. web part of the application the web part of kadr is available anywhere on public internet at the website of the infrastructure manager (http://provoz.szdc.cz/kadr). this part is used mainly by carriers pro submitting of their new requests for assignment of the capacity, for activation and deactivation of trains, for statistics and so on. on the figure 1 is a basic screen of the web part for the carrier after log in. the basic screen has some bookmarks with groups of functions: • requests/djř (day time table) – on this bookmark are all routes of the carrier (including routes from the year time table). each row is one train and carrier can see their basic information and the application status. at the end of each row are icons for work with each request. • activation – this bookmark is for changing of the train status. the carrier can activate the train (train will be really operated), deactivate the train (carrier doesn’t use the capacity that day or period), put together some routes and submit a request for re-activation of the detached train. • train preparation – carrier can set the analysis of the train and preparation of the train for the departure. both information are absolutely necessary for train operation. • restrictions – link for domin (database of infrastructure restrictions) application, where can be find actual track restrictions and other incidents. • carriers – the list of carriers is in register of the infrastructure manager (sždc). this list is in the system called kafr (catalogue of carriers). each carrier can find here basic information about other carriers as well as contacts for responsible people. 44 http://dx.doi.org/10.14311/app.2016.5.0044 http://ojs.cvut.cz/ojs/index.php/app vol. 5/2016 railway infrastructure capacity managment for ad hoc trains figure 1. basic screen of the web part [2]. • numbered lists – there is an overview of all available numberings, such as: . numbered list of engines – information from the information system revoz (register of engines) – basic information about engines and special vehicles. . services – there is a numbered list of services, which are offered by infrastructure manager for ad hoc routes. . operations – numbered list of acts, this can be requested by a carrier. . products – numbered list of products, this can be assigned to a request. • archive – archive of data from previous time tables. • statistics – carrier has to available 4 types of statistics: . charges for capacity assignment – carrier can see financial costs for capacity assignment for any period. . set of capacity requests – list of all capacity requests. requests can be filtered or exported according to various parameters. . set of time tables – list of all assigned requests. requests can be filtered or exported according to various parameters. . day set of time tables – day list of all route of trains. trains can be filtered or exported according to various parameters. probably the most important feature of the web part of kadr is for the carrier submitting of the application. the carrier must fill in step-by-step the following information: • schedule – carrier must choose days for which he submits the request. there are three types of requests. type c – less than 3 working days (the train departs less than 3 days from the request). type b – more than 3 working days (the train departs 4 of more days after the request). this distribution is for easier preferences by capacity assignment. • route of the request – carrier finds his requested route on the map. • basic information about a train – carrier must fill in all mandatory data such as: train engines, weight, length, used type of the brake, category and type of the train. • extending data – here are stated other train parameters, transport of dangerous goods, special load, military transport, services on the train. after completing the entire information carrier is allowed to forward the application to the interfaces of the infrastructure manager. next possibility for submit of the request is via data communication, when carrier has its own software which is compatible with kadr. the data interface is in the format of tsi taf. this requires bidirectional communication between kadr and carrier’s information system. this interface shall be in defined format tsi taf with annexes. nowadays is valid version 5.17 and during this year will be implemented new version 2.1. the web part of the application is for the infrastructure manager very similar to the carrier’s web part. the infrastructure manager doesn’t use this part very often, because most of his usually used functions are in the desktop part. 4. desktop part of the application the desktop part of kadr is available only for infrastructure manager. the installation is executed 45 petr nachtigall, martin škárek acta polytechnica ctu proceedings figure 2. desktop part of the application [2]. through the web part. the work with the desktop part is possible only from the sždc intranet. the infrastructure manager is setting up the time table of ad hoc trains in this part of the application. here can be also authorized the carrier’s request for the ad hoc trains or prepare the aids of the time table like dispatch or tabular time table. the basic screen of the desktop part after authorization is at the figure 2. 5. conclusions the information system kadr becomes during its existence one of the most important planning tool for basic operation of the railway transport. without its existence isn’t nowadays possible ride of any train, because kadr secures activation and deactivation of trains and information system isoř on request of kadr establish the route into all other information systems. the sždc offers to all carriers the web part of kadr for free. each carrier pays only charges and fees for capacity assignment and using of railway infrastructure according to the czech law [3]. references [1] document of infrastructare manager "prohlášení o dráze celostátní u regionální". [2] internal materials of the company oltis group a.s. [3] law nr. 266/1994 sb., o dráhách. 46 acta polytechnica ctu proceedings 5:44–46, 2016 1 introduction 2 basic information about kadr 3 web part of the application 4 desktop part of the application 5 conclusions references 217 acta polytechnica ctu proceedings 2(1): 217–221, 2015 217 doi: 10.14311/app.2015.02.0217 the slow nova v1280 sco: a short review h. naito1 1graduate school of science, nagoya university, furo-cho, chikusa-ku, nagoya 464-8602 corresponding author: naito@phi.phys.nagoya-u.ac.jp abstract we summarize the results of the extremely slow nova v1280 sco and discuss the approach using discrete multiple blueshifted absorption lines, such as metastable he i* and na i d, detected in our high-resolution spectra. keywords: cataclysmic variables classical novae optical spectroscopy photometry individual: v1280 sco. 1 introduction a nova occurs in a binary system consisting of a white dwarf (wd) and a normal star. the correlation between the wd mass and the decline rate of the light curve is widely accepted (e.g. hachisu and kato 2006), and the nova which occurs on low mass wd tends to decline in brightness at a slow rate and shows slow ionization evolution of its ejacta. a slow nova provides us a good opportunity to perform detailed observation over a long duration. in this review, we summarize the results of the extremely slow nova v1280 sco and discuss the approach using discrete multiple blue-shifted absorption lines, such as metastable he i* and na i d, detected in our high-resolution spectra. 2 summary of v1280 sco 2.1 initial observations v1280 sco was independently discovered by two japanese amateur astronomers (y. nakamura and y. sakurai) at the position of r.a. = 16h57m41s.0, decl. = −32◦20’ 36”.4 (equinox 2000.0) on 2007 february 4 at ninth visual magnitude (yamaoka et al. 2007). it was identified as a classical nova by naito and narusawa (2007) from a low resolution spectrum obtained on february 5.87 (one day after the discovery). it reached the maximum brightness of v = 3.78 mag on february 16, which was 11.3 days after its discovery (munari et al. 2007). the amplitude (a), the magnitude difference between pre-outburst and maximum brightness, is 15 mag or larger because das et al. (2008) noted that no star was visible down to b and r magnitudes of 20.3 and 19.3, respectively, at the position on pre-discovery plates. the notable point is that v1280 sco showed a remarkable formation of dust in its very early phase (das et al. 2007). this dust formation was directly detected using eso’s very large telescope interferometer (vlti) by chesneau et al. (2008) and this result was published as the eso press release (eso0822: http://www.eso.org/public/news/eso0822/). 2.2 infrared observation das et al. (2007) observed the nova in the nir region on 2007 march 4.95, and found that the continuum in the 1.08-2.35 µm region had risen sharply, indicating dust formation in the nova ejecta. das et al. (2008) suggested that the dust was in clumps from nir studies of v1280 sco. puetter et al. (2007) reported spectroscopic observations in the visual-nir regions carried out in 2007 may, and found that the nova was in a very low-excitation state showing strong c i lines and no discernible he i emission. 2.3 high spatial resolution observation high spatial resolution monitoring of the dust formation event was performed using the vlti during the first four months following the discovery, indicating that the dusty shell expanded regularly (chesneau et al. 2008). chesneau et al. (2012) revealed the presence of a dusty hourglass-shaped bipolar nebula around v1280 sco based on mid-infrared imaging observations taken in 2010 and 2011 using the vlt spectrometer and imager for the mid-infrared (visir). 2.4 high quality photometric observation hounsell et al. (2010) published the data set of v1280 sco observed by the solar mass ejection imager (smei) on board the coriolis satellite. thanks to high quality and high time resolution, they revealed that there had been three major but short episodes of brightening near 217 http://dx.doi.org/10.14311/app.2015.02.0217 h. naito maximum light (before 2007 february 20), which had not been detected by any ground-based telescopes. 2.5 distance and wd mass parameters (distance and wd mass) of v1280 published in the literature are controversial (see table 1). hounsell et al. (2010) estimated the distance to be 630 ± 100 pc by measuring the condensation time of dust grains, assuming that the condensation temperature of the dust was 1200 k and the ejection velocity was about 600 km s−1. they also noted that wd mass of 0.6 m� is likely to be an upper limit. on the other hand, chesneau et al. (2008) derived the distance of 1.6 ± 0.4 kpc from direct observations of the size of the expanding shell with velocity of ∼500 km s−1. the discrepancy between these two estimations of distance is over a factor of two, which could result from the complexity in the physical conditions (temperature and velocity) of the dust shell. das et al. (2008) estimated the distance to be 1.25 kpc using the maximum magnitude versus rate of decline (mmrd) relation, and inferred that the higher mass end (1-1.25 m�) may be supported when the amplitude a and the expansion velocity values observed inv1280 sco were taken into account. 3 our results in this section, our results of v1280 sco are summarized. especially we focus on multiple absorption lines detected in high-resolution spectra (see sadakane et al. 2010, naito et al. 2012, naito et al. 2013 for details). photometric and spectroscopic observations have been conducted from february 2007 to july 2013 over table 1: parameters of v1280 sco in the literatures distance [kpc] wd mass [m�] reference 0.63 ± 0.10 < 0.6 hounsell et al. 2010 1.6 ± 0.4 — chesneau et al. 2008 1.25 1.0-1.25 das et al. 2008 1.1 ± 0.5 < ∼0.6 naito et al. 2012 0 2 4 6 8 10 12 14 16 18 0 500 1000 1500 2000 2500 m a g n it u d e s days since maximum v723 cas (vis) －2.8 mag. v1280 sco (v) nebular phase v1280 sco v723 cas figure 1: comparison of light curve and nebular phase between slow novae v1280 sco and v723 cas. light curve of v723 cas is collected from aavso database (http://www.aavso.org) and is shifted by −2.8 mag. v723 cas entered the nebular phase about 18 months after maximum (iijima 2006), while v1280 sco took about 50 months (about three times longer than v723 cas) to enter the nebular phase (naito et al. 2012). 218 the slow nova v1280 sco: a short review six years. photometry in b, v , rc, ic, and y band was carried out with a 0.51-m reflector at osaka kyoiku university. low-resolution spectroscopy (r∼1000) was carried out mainly with 2.0-m nayuta telescope at nishi-harima astronomical observatory and highresolution spectroscopy (r∼60000) was carried out with 8.2-m subaru telescope. according to iijima 2006, v723 cas had been the slowest nova, spending a long time to enter the nebular phase. to compare the evolution between v1280 sco and v723 cas, the light curves and the nebular phases of them are shown in figure 1. v723 cas declines in brightness at very slow rate gradually, while v1280 sco keeps its brightness (v∼10) for 2000 days. v723 cas entered the nebular phase, defined by the appearance of both [o iii] 4959 and 5007, about 18 months after maximum (iijima 2006), while v1280 sco took about 50 months (about three times longer than v723 cas) to enter the nebular phase (naito et al. 2012). considering that v723 cas had the longest transitional time to enter the nebular phase, we conclude that v1280 sco is going through the slowest spectral evolution among known classical novae. hachisu and kato (2004) estimated the mass of v723 cas to be 0.59 m� by fitting their theoretical light curve. our results suggest that the mass of a wd in v1280 sco system might be 0.6 m� or lower by comparing evolution rates between v1280 sco and v723 cas. 0 1 2 3 4 5 5870 5875 5880 5885 5890 5895 5900 in te n s it y + c o n s t. wavelength [ ]å 2009 jun. 16 2010 jul. 1 2011 aug. 6 na d1(is)na d2(is)na d1na d2 2013 jul. 1he i figure 2: multiple high velocity absorption lines associated with na i d1 and d2. we found discrete multiple blue-shifted na i d lines, ranging from −650 to −900 km s−1, on high-resolution spectra (figure 2). some components weakened significantly from 2009 to 2011 and almost all lines disappeared in 2013. similar absorption lines associated with ca ii h and k and metastable he i* 3188 and 3889 are shown on our spectra. metastable he i* 3889 absorption lines were detected for the first time in 2011 (four years after the maximum light) and have been shown until 2013 (figure 3). this is the first detection of he i* absorption lines in the ejected (circumstellar) gas around novae (naito et al. 2013). we suggest that the complex evolutions of multiple absorption lines are due to combined changes in physical conditions, such as the density, recombination and ionization rate. survival time of these absorption lines in v1280 sco is an order of years, which is much longer than those observed among fast novae (an order of weeks or months; williams and mason 2010). this can be related to the fact that the ionization evolution of v1280 sco is very slow. the behavior associated with he i* in 2011 can be understood that the number of ultraviolet photons had increased significantly to produce singly-ionized helium as the central photosphere shrinks to become hotter, and the disappearance of na i d lines low excitation lines in 2013 can be caused by an additional increase in the number of ultraviolet photons. 0 1 2 3 4 5 3820 3830 3840 3850 3860 3870 3880 3890 3900 in te n s it y + c o n s t. wavelength [ ]å 2009 jun. 16 2011 aug. 6 2012 mar. 20 2013 jul. 1 h e i * 3 8 8 9 h i 3 8 3 5 h i 3 8 3 5 ( h 9 ) h i 3 8 8 9 ( h 8 ) s i ii 3 8 5 6 s i ii 3 8 6 3 f e i i 3 8 2 5 s i ii 3 8 5 4 figure 3: multiple high velocity absorption lines associated with he i* 3889. 4 discussion and conclusions as described in the previous section, metastable he i* absorption lines could survive longer than low excita219 h. naito tion lines, such as na i d and ca ii h and k. this means that metastable he i* absorption lines are more likely to be detected than low excitation lines in the late phase. moreover metastable he i* lines are often used to derive physical parameters of nebulosity because the processes of transition are well understood. these characteristics of metastable he i* lines have advantage in measuring helium composition and the mass of the ejected material, and in studying nova shell structures. using metastable he i* lines in v1280 sco, we show that the ejected shell consists of numerous clumpy gas which cover a significant part of the continuum emitting radiation region as figure 4 (naito et al. 2013). we postulate that this approach is very useful to research nova shell. we reveal that v1280 sco is the extremely slow nova and is available for high-resolution spectroscopic observation for long periods. we attempt to obtain new results of v1280 sco by follow-up observations. free-free emission region clumpy gas photosphere { observer figure 4: schematic of ejected shell producing absorption lines of metastable he i* (see naito et al. 2013 for details). acknowledgement i thank dr. akito tajitsu, prof. kozo sadakane and dr. akira arai for longstanding observations and valuable discussion. i also thank prof. iijima for his useful comments. references [1] chesneau, o., et al.: 2008, a&a 487, 223. [2] chesneau, o., et al.: 2012, a&a 545, a63. [3] das, r. k., et al.: 2007, cbet 866, 1. [4] das, r. k., et al.: 2008, mnras 391, 1874. doi:10.1111/j.1365-2966.2008.13998.x [5] hachisu, i., kato, m.: 2004, apj 612, l57. doi:10.1086/424595 [6] hachisu, i., kato, m.: 2006, apjs 167, 59. doi:10.1086/508063 [7] hounsell, r., et al.: 2010, apj 724, 480. doi:10.1088/0004-637x/724/1/480 [8] iijima, t.: 2006, a&a 451, 563. [9] munari, u., et al.: 2007, cbet 852, 1. [10] naito, h., narusawa, s.: 2007, iau circ. 8803, 2. [11] naito, h., et al.: 2012, a&a 543, a86. [12] naito, h., et al.: 2013, pasj 65, 37. [13] puetter, r. c., et al.: 2007, iau circ. 8845, 1. [14] sadakane, k., et al.: 2010, pasj 62, l5. [15] williams, r., mason, e.: 2010, ap&ss 327, 207. doi:10.1007/s10509-010-0318-x [16] yamaoka, h., et al.: 2007, iau circ. 8803, 1. 220 http://dx.doi.org/10.1111/j.1365-2966.2008.13998.x http://dx.doi.org/10.1086/424595 http://dx.doi.org/10.1086/508063 http://dx.doi.org/10.1088/0004-637x/724/1/480 http://dx.doi.org/10.1007/s10509-010-0318-x the slow nova v1280 sco: a short review discussion alessandro ederoclite: how long after the explosion do you see the emergence of the he i* lines? hiroyuki naito: our latest spectrum taken in july 2013, shows the emergence of the he i* lines which indicates that these lines have appeared until at least 6.5 years after maximum. 221 introduction summary of v1280 sco initial observations infrared observation high spatial resolution observation high quality photometric observation distance and wd mass our results discussion and conclusions 231 acta polytechnica ctu proceedings 1(1): 231–234, 2014 231 doi: 10.14311/app.2014.01.0231 data analysis of globular cluster harris catalogue in view of the king models and their dynamical evolution. i. theoretical model marco merafina1, daniele vitantoni1 1department of physics, university of rome la sapienza, piazzale aldo moro 2, i-00185 rome, italy corresponding author: marco.merafina@roma1.infn.it abstract we discuss the possibility to analyze the problem of gravothermal catastrophe in a new way, by obtaining thermodynamical equations to apply to a selfgravitating system. by using the king distribution function in the framework of statistical mechanics we treat the globular clusters evolution as a sequence of quasi-equilibrium thermodynamical states. keywords: globular clusters gravothermal catastrophe king models thermodynamical stability. 1 introduction globular clusters (gcs) are stellar systems with masses within the interval 104 − 106 m�, containing a number of stars of the order of 105. they are considered as nearly spherical systems due to their low values of eccentricity e; at least 50% of gcs have e < 0.1 and there are no clusters with e > 0.2. the core radius rc, namely the radial coordinate at which the brightness becomes one half of the corresponding value at the center of the system, is almost 10 pc, whereas the tidal radius rt, which is the biggest spatial extension of the cluster allowed by the external tidal field, is typically around 50 pc. for their symmetry and age, there is the possibility to test the evolution of a gcs studying a classical single mass king model (king, 1966) in relation to thermodynamical instability phenomena. in fact, in the analysis of the evolution of gcs, stellar encounters strongly contribute in phase space mixing of stellar orbits. in this scenario, thermodynamics plays a centrale role in the gravitational equilibrium and stability of these clusters, being the average binary relaxation time shorter than their old absolute age which ranges between 10 to 13 gyr. this means that fokker-planck approximation, which takes into account the nature of collisions in globular clusters, can determine the distribution function relevant for obtaining the equilibrium configurations of these systems, whereas the tidal effects due to the presence of galactic gravitational potential are responsible of the confination of the cluster. on the other hand, the observations of the luminosity profiles of different gcs (king, 1962) show similar curves depending only on different values of the star concentration, giving the possibility to fit them by an empirical law and suggesting a unique distribution function for the whole sample of clusters (king, 1966). this effect can be described as a change of the main parameters of the cluster during the dynamical evolution (horwitz & katz, 1977), which maintains the form of the distribution like in a sort of reversible trasformation of a gas in thermodynamic equilibrium, also in accordance to numerical simulations existing in literature which result in keeping unchanged the distribution of velocities of stars for a wide range of values of the concentration during the time evolution driven by the fokker-planck equation. therefore, the evolution of globular clusters can be studied by considering small thermodynamic transformations which keep constant the functional form of the velocity distribution of stars like in the framework of boltzmann statistical mechanics. it is important to note that while the equilibrium is given by the form of the distribution which depends on the fokker-planck equation and consider the real nature of collisions, thermodynamics plays a role in the tidal effects acting on the confination of the system, due to a two competitive phenomena: one given by stellar encounters which tend to refresh the tail of high velocities in the distribution and one due to evaporation of stars which prevents the formation of it, maintaining the system in a sort of thermodynamical equilibrium with the same distribution function even if in presence of a cutoff in the velocity of the stars. 2 the effective potential the king df characterizing the energy distribution of stars with the same mass m in a model that describes 231 http://dx.doi.org/10.14311/app.2014.01.0231 marco merafina, daniele vitantoni a spherically symmetric system with isotropic velocity distribution may be written as f(ε) = b [ e−(ε+mϕ)/kθ − e−(ψ+mϕ)/kθ ] for ε ≤ ψ , f(ε) = 0 for ε > ψ . (1) here ψ = m (ϕr − ϕ) is the energy cutoff, corresponding to the maximum kinetic energy that a star can have at a given radial coordinate r, while ϕ is the gravitational potential. this energy is sufficient to reach the border of the equilibrium configuration r = r, being also the difference between the value of the gravitational potential at the edge of configuration and the same quantity evaluated at a generic distance r from the center. the quantity θ is the thermodynamic temperature of the system while b is a constant of normalization. the behavior of equilibrium solutions for king models has been also analyzed by merafina & ruffini (1989) by solving the poisson equilibrium equation in newtonian regime. we can see the presence of a maximum value of the total mass m at increasing values of the central gravitational potential w0, which denotes the arising of a sort of thermodynamic instability at w0 = 1.35 (fig.1). figure 1: mass in function of w0 for families of solutions at different values of the velocity dispersion (merafina & ruffini, 1989). over this value, we can think the system can evolve towards the loss of thermodynamical equilibrium (gravothermal catastrophe), in accordance to the expected evolution of lynden-bell & wood (1968). in order to consider thermodynamic transformations in the framework of statistical mechanics, it is possible to describe the king df like a maxwell-boltzmann one by introducing an effective potential. in this way the evolution of the king models can be treated as a succession of quasi-equilibrium stages by a thermodynamic theory formally equivalent to the classical one. then, the expression of the effective potential is given by φ = −kθ ln [ 1− e(ε−ψ)/kθ ] (2) and the distribution function can be expressed as f = be−h/kθ , (3) where h = ε + mϕ + φ is the single particle hamiltonian of the system which includes also the gravitational energy of the single star. the effective potential is a screen potential which restricts the phase space of the available velocities for the stars and takes into account the effect of the tidal forces on the system. in this way, the kinetic temperature t connected with the average velocity of the stars, depending on the radial coordinate r, becomes distinguished from the thermodynamic temperature θ, constant all over the equilibrium configuration. from the modified boltzmann df of eq.3, we can deduce the generalized thermodynamical quantities, as the energy u , the thermodynamical pressure π and the entropy s, related to a shell with radial coordinate r. we get n = av ∫ ψ 0 f √ ε dε , (4) u = av ∫ ψ 0 fh √ ε dε , (5) π = 1 3 a ∫ ψ 0 fε3/2 dh dε dε , (6) s = kav ∫ ψ 0 f(1− ln f) √ ε dε , (7) where we have replaced the costant b with a, being b = aeα/kθ and now f = ae(α−h)/kθ, while α = µ + mϕ is the chemical potential in presence of the gravitational potential ϕ. in this way we can rewrite the first law of thermodynamics and obtain a new form for the eulero expression, that include the extensive and intensive quantities. we can get also an equation of state formally equivalent to classical one which involves the thermodynamical quantities, valid for a shell with radial coordinate r. we have du = θds −πdv + αdn +n〈dh〉 , (8) u = θs −πv + αn , (9) πv = nkθ (10) 232 data analysis of globular cluster harris catalogue... part i and, for kinetic quantities like temperature t and pressure p , pv = nkt . (11) finally, by integrating the expression of u containing the single particle hamiltonian h (eq.5) all over the configuration, we can find an additional term eeff in the expression of the total energy etot of the system, called effective energy etot = ekin + egr + eeff , (12) where ekin and egr are the total kinetic energy and the total gravitational energy, respectively. the partecipation of the effective potential in the total energy corresponds to the account of the tidal potential which determines a finite radius of the cluster. moreover, eq.5 defines the energy of the test shell but, for calculating egr, we need to use the expression egr = 1 2 ∫ r 0 ρϕ dv . (13) 2.1 the gravothermal catastrophe thermodynamical instability of a selfgravitating spherical system was first studied by lynden-bell & wood (1968), by considering an isothermal sphere (core) confined in a spherical box. using the classical form of the virial theorem, including a boundary term due to spatial truncation of the density prophile, it is possible, for that system, to calculate the critical value of the central gravitational potential w0 = 6.55 after that thermodynamical instability, known as gravothermal catastrophe, onsets. it is important to note that such instability takes place only in presence of an external thermal bath exchanging heat with the core and driving the system towards the dynamical collapse. with the introduction of the effective potential, we can repeat this analysis for king models and get another critical value for the central gravitational potential w0 = 6.9, which differs from the one obtained by katz (1980) w0 = 7.4, due to the additional term in the total energy etot (see eq.12). the most interesting results concern the profile of specific heat for different values of w0 (see merafina et al., in preparation). by analyzing the behavior of the specific heat all over the configuration, we found different results. the expression of the specific heat cv = (dq/dθ)v arises from eq.8, being constant n and v , by using the expression dq = du −n〈dh〉 . (14) • for w0 < 1.35, we have equilibrium configurations with positive heat capacity all over the system. there are not existing conditions for an evolution of the system towards the critical value corresponding to the onset of the gravothermal catastrophe (w0 = 6.9). further, this particular value (w0 = 1.35) corresponds to one concerning the first maximum mass we found among the equilibrium solutions (see fig.1). • for w0 > 1.35, the system shows an external halo with negative heat capacity and an internal core with a positive value. the system can evolve by increasing the value of w0 until reaching the critical value in which the gravothermal instability onsets. these evolution can take place without the necessity of the presence of an external thermal bath, differently from the previously requested condition in the lynden bell & wood model. results showing the specific heat profiles in function of the radial coordinate for different values of w0 are summarized in fig.2. 0.00 0.25 0.50 0.75 1.00 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 c v/ n k r/r figure 2: behaviour of the specific heat in function of the radial coordinate for different values of w0. 2.2 preliminar observational evidences the stability of the king models was analyzed in detail by katz in 1980, with the same investigation carried out by lynden-bell & wood for the isothermal sphere. katz introduced a new parameterk, which corresponds essentially to the ratio between the escape velocity and the dispersion velocity, both calculated at the center of the cluster. this parameter is directly connected with w0. calculations performed by katz showed that models become thermodynamically unstable over the value k = 8.1 (w0 = 7.4). but, analyzing the sample of data coming from peterson & king (1975) and peterson (1976), katz highlighted an unexplainable gap between the expected value of the sample, k = 7.8 equivalent 233 marco merafina, daniele vitantoni to w0 = 6.9, and the one corresponding to the onset of gravothermal instability, resulting at w0 = 7.4 (see fig.3). figure 3: distribution of galactic gcs at different values of k (katz, 1980). we indeed expect that gcs had enough time to undergo the gravothermal catastrophe and, therefore, the distribution of gcs in terms of k or w0 should peak exactly in correspondence to the critical value. in fact, the primeval gaussian distribution, approaching the critical value during the evolution, deforms in a non-symmetric gaussian curve due to the effect of gravothermal catastrophe which progressively subtracts the collapsed gcs with values of w0 larger than the critical value. for these reasons, the resulting distribution must present a maximum which corresponds to the critical value. it is remarkable to note that, with the introduction of the effective potential in the study of the thermodynamical instability, we obtain a critical value, w0 = 6.9, that bridges this gap and corresponds exactly to the expected value of the sample. this correspondence becomes much more evident by considering the sample of harris (1996) with 127 clusters (if we exclude the pcc ones), as well becomes more evident the non-symmetric form of the distribution. on the other hand, by making a z-test in order to verify the statistical significance of the gap between the stability limit w0 = 7.4 expected by katz and the peak value of the distribution at w0 = 6.9, it can be shown that these two values are not compatible within a confidence level of 95%. 3 conclusions • the additional (positive) contribution of the effective potential on the total energy, considering also the virial condition 2ekin +egr = 0, implies that etot = −ekin + eeff . this enables us to construct models in which the core has a positive heat capacity, allowing to assume the possibility of a survival of the system from the gravothermal cathastrophe which could explain the existence of post core-collapsed objects (pcc). • the model is selfconsistent and admits regions with positive and negative heat capacity which can exchange energy and produce gravothermal instability, without the necessity to assume an external bath as in the lynden-bell & wood model. • we obtain a new critical value for the onset of gravothermal instability by the presence of the effective potential. this value coincides with the value of k (or, equivalently, to w0) corresponding to the peak of the gcs distribution, removing the unexplainable difference outlined by katz. this is an observational evidence of the effects due to the presence of the effective potential, confirmed in the analysis of data of more than 150 gcs contained in the last version of catalogue recently published by harris in 2010 (see also harris, 1996). finally, it may be useful to consider some unsolved problems and perspectives in order to develop the analysis of thermodynamical instabilities of gcs. • the model is not a multimass one and does not take into account the effects in the formation of binary stars. at moment this is a preliminary model which has to be improved. • the new possibility of measuring transverse velocities of the stars in gcs opens important perspectives on the knowledge of the distribution of the star orbits and their eccentricity, in order to better develop n-body simulations in supporting the validity of the model. references [1] cohn, h.: 1980, apj, 242, 765. doi:10.1086/158511 [2] harris, w.e.: 1996, aj, 112, 1487. [3] horwitz, g., katz, j.: 1977, apj, 211, 226. doi:10.1086/154924 [4] katz, j.: 1980, mnras, 190, 497. doi:10.1093/mnras/190.3.497 [5] king, i.: 1962, aj, 67, 471. [6] king, i.: 1966, aj, 71, 64. [7] lynden bell, d., wood, r.: 1968, mnras, 138, 495. doi:10.1093/mnras/138.4.495 [8] merafina, m., ruffini, r.: 1989, a& a 221, 4. [9] merafina, m., fragione, g., piscicchia k.: in preparation. [10] peterson, c.j.: 1976, aj, 81, 617. [11] peterson, c.j., king, i.: 1975, aj, 80, 427. 234 http://dx.doi.org/10.1086/158511 http://dx.doi.org/10.1086/154924 http://dx.doi.org/10.1093/mnras/190.3.497 http://dx.doi.org/10.1093/mnras/138.4.495 introduction the effective potential the gravothermal catastrophe preliminar observational evidences conclusions acta polytechnica ctu proceedings doi:10.14311/app.2017.12.0038 acta polytechnica ctu proceedings 12:38–41, 2017 © czech technical university in prague, 2017 available online at http://ojs.cvut.cz/ojs/index.php/app analysis of stored data help to propose and generate new tracks václav jirkovský czech technical university in prague, faculty of transportation sciences, department of vehicle technology. konviktská 20, 110 00 prague correspondence: vaclav.jirkovsky@volny.cz abstract. during experiments on vehicle simulators a large amount of data is stored. on these data, it is possible to trace some similarities in the behavior of drivers in certain areas or when performing the same task. we can assume that if the driver performs a certain type of experiment, his behavior exhibits certain traits. these elements of common behavior can be used to create virtual track for experiments. which elements and how they can be used is described in this article. the algorithm for automatic creation of virtual track based on type of experiment is provided. it will help us to define the purpose of measurement and the track could be generated automatically. keywords: vehicle simulator, measurement, virtual track. 1. introduction conducting of experiments is one of the most important things related to the operation of driving simulators at the faculty of transportation sciences. there are many types of them. they are focused on investigating the driver behavior in fatique[1–3], testing systems for predicting microsleeps[4], humanmachine interaction[5], interaction with surrounding traffic[6, 7] and many more. during their performing the data are collected, which are mostly of a technical nature, and on the other hand, they give us information about the state of the monitored driver. all information about a vehicle, terrain and environment belong the first category. the data collected on a driver's body such as heart beat, eeg, eye view direction, blink rate etc. belong to the second category[8]. after the data evaluation we are able to describe a driver's behavior in particular states of experiment and determine a crisis situations or areas. hierarchical structure of measured data is shown on fig.1 figure 1. hierarchical structure of measured data 2. measured data analysis if the amount of the measured data is sufficiently large, we are able to detect with help of mathematical tools how the drivers react to a given event, incentive or transport situation. we can find out if their reactions are different or identical. it is possible, based on the analysis and using of appropriate tools, to predict the behavior of drivers on virtual tracks if these tracks are only modeled or created without performing of experiments with data collection[9]. on the basis of structure and character of the track, it is possible to determine the place and the moment when the driver will have to solve the crisis situation. but the opposite procedure will be applied. it means to design and construct a virtual track according to the requirements of the type of crisis. on thus obtained results we can create new scenarios with a specific focus. for example the results of the analysis shows that while operating the radio in rugged terrain, a large percentage of drivers do not hold the vehicle on an ideal path and run off the road. it's evident that the equally segmented track is suitable for testing similar devices. based on these derived addiction it is possible to design and construct virtual tracks with greater efficiency. it will also allow it to specialize in one specific purpose. the figure no.2 shows the experimental track. the yellow marks show the places where the value of deviation from the ideal track overlaps 1.5 m. this value indicates that the car doesn't stay in the correct road lane and crosses the middle line to the opposite road lane. this can cause a serious traffic situation with possibility of frontal collision. the chart shows that the deviation from the ideal track is caused by splitting the focus of the driver between the driving and the device manipulation. for more detailed evaluation we can divide the tasks into easy and difficult. an easy task can be done by one 38 http://dx.doi.org/10.14311/app.2017.12.0038 http://ojs.cvut.cz/ojs/index.php/app vol. 12/2017 analysis of stored data help to propose and generate new tracks figure 2. experimental track or two clicks (touch). difficult task needs more time to finish. 3. identifying common elements of behavior during data analysis i've found out that the common behavior patterns could be observed for most types of experiments. with a very high probability we can assume that if the driver performs the measurements under the same conditions as the previous driver, his behavior will show the same elements. by examining the experiments i received the following connection: • assistive devices ragged track • research on driver fatigue monotonous, minimally curved track with minimal distractions • research driver's responsiveness to sound stimuli monotonous track • research driver's responsiveness to obstacles in riding complex traffic situations, poor visibility • driving precision special testing polygon (slalom, braking precision, ride along a line etc.) • overtaking research track with poor clarity (bend, curl, heavy traffic) individual connections determine the characteristics. with their help, i set up a knowledge base. that gives us an idea of which features driver's behavior occur in different types of experiments. that will tell us what behavior we expect on a particular experiment, but in the opposite case it will help us, when examining on the basis of activity to determine the shape of table 1. experimental characteristics the track. characteristics of each experiment can be found in the following table. experiment properties c om pl ex tr ac k r ec om m en de d ve lo ci ty su rr ou nd in g tr affi c li gh t tr affi c h ea vy tr affi c g oo d vi sib ili ty c or ru ga te d tr ac k o bj ec ts reaction speed on sound effects no no yes no no reaction speed on road obstacles yes yes no yes no yes the accuracy of the ride yes no no no driver's fatigue no 90130 km/h no yes no no assistance devices handling yes 50 km/h no overtaking yes yes no yes no yes 4. proposal track by type of experiment a knowledge base is essential for the design of the track, which i described in the last chapter. to create the track i proposed algorithm, which is described below. the entire process can be divided into several steps: • the user determines variables, which aims to examine • distinguishing features are loaded from knowledge base • the track is automatically proposed based on those distinguishing features • the result is a polyline characterizing the road axis in space first the user enters input data. it is primarily a type of experiment. the next steps of propose are fully automatic and the user cannot affect them. based on input values the distinguishing features are obtained from the knowledge base. it means maximal velocity, curvature, longitudinal profile etc. these go to input of an algorithm as a specific values (arcs with radius from 100m to 350m, narrow lines of length from 200m to 1000m etc.). based on the input data the algorithm proposes the whole track. geometric properties of individual building elements comply with standard csn 73 6101. the result is a polyline that determines the 39 václav jirkovský acta polytechnica ctu proceedings points of the track. it can be subsequently imported into other software where it's finalized[10]. 5. the algorithm for automatic generation of experimental tracks the automatic generation of a track is as old as the game industry itself. the limitation of existing platforms did not allow the distribution of a large predefined content. ad-hoc algorithmic procedures were widely used to generate game content on-the-fly. it was called procedural content generation. now, when the distribution of games is not limited by memory space, the automatic generation of the content is still used mainly to reduce design costs. the principle of automatic generation of the track is based on evolutionary computation to evolve track for a simple two-dimensional car racing game[11]. the basic idea is to connect individual segments (lines, arcs) so that they don't cross the other parts. and they gradually point to beginning. the connection of the parts is strictly defined. for example between each line and arc must be placed a spiral. the segment properties are based on the rules mentioned above. additional parameters slope can be added to extend into 3d space to make virtual track more realistic[12]. 6. pseudo-code as described in section four, the type of track is dependent on the type of experiment. it means each track consists of segments with a specific property. the track properties are set at the beginning of the algorithm. in the next step the random segment is randomly selected from the segment list. the list consists of narrow lines of different length and arcs of different length and radius. the end point of the previous segment is the start point of a next segment. the algorithm takes into account the crossing of the segments. if the actual segment crosses the track it is not placed and other segment is chosen. in some cases the algorithm doesn't offer the closed track, therefore, it must be done manually. to reduce the occurrence of these cases, the number of right-handed arcs is bigger than the number of left=handed arcs. once the track has been built, it needs to be smoothen. in this step the spirals are placed between a line and an arc to provide a fluent transition. (1.) var property = settrackproperty() (2.) track=null (3.) object=choose random object from list (4.) object.properties=random(property) (5.) end_point=object.endpoint() (6.) start_point=object.startpoint() (7.) track.insert(object) (8.) do while (start_point != point) (9.) object2=choose random object from list (10.) object2.properties=random(property) (11.) object2.start_point=end_point (12.) point=object2.endpoint() (13.) if(collision(track, object2) == false and suitableposition(object2, start_point) == true) (14.) track.insert(object2) (15.) calculate parts connection (16.) if(distance(start_point, point) < minimal_distance) (17.) break (18.) else (19.) point=end_point (20.) loop (21.) connect point and start_point (22.) track.smooth() 7. conclusions the measurements on a vehicle simulator and data analysis confirmed that the use of assistance and media devices can directly cause a dangerous behavior of the driver. those devices should be designed to reduce driver's attention and thereby minimize dangerous situations on the road. thorough tests could help us to determine which devices are suitable for using in a car and which are not. the analysis also showed that there are connections between driver's behavior and track's properties and conditions of the measurement. the connections define rules that can be used during creating of the new tracks. in the future we will be able to define purpose of measurement and the track will be generated automatically. the described algorithm for creating a track is one of many possible. it's easy to understand and fulfills the purpose. a number of track were generated during testing. many of the tracks were useful and usable as a basis of a new created track. their use is one of the steps to improve the quality of experiments on vehicle simulators. references [1] p. bouchner, r. pieknik, s. novotny, et al. fatigue of car drivers – detection and classification based on the experiments on car simulators. 2006, wseas transactions on systems, 5 (12), pp. 2789-2794. [2] p. bouchner. a complex analysis of the driver behavior from simulated driving focused on fatigue detection classification. 2006, wseas transactions on systems, 5 (1), pp. 84-91. [3] p.bouchner, m.hajny, s.novotny, et al. car simulation and virtual environments for investigation of driver behavior. 2005, neural network world, 15 (2), pp. 149-163. 40 vol. 12/2017 analysis of stored data help to propose and generate new tracks [4] p. spurny, j. andrs, p. bouchner, et al. testing a system for predicting microsleep. 2016 lekar a technika, 46 (2), pp. 51-54. [5] p. bouchner, s. novotny. system with driving simulation device for hmi measurements. 2005, wseas transactions on systems, 4 (7), pp. 1058-1063. [6] m. matowicki, o. pribyl, p. bouchner. pragmatic overview of surrounding traffic implementation into driving simulator. elektro 2016 – 11th international conference, proceedings, art. no. 7512111, pp. 423-428. [7] h. klee. microscopic car modeling for intelligent traffic and scenario generation in the ucf driving simulator. [8] p. bouchner. driving simulators for hmi research. phd. thesis, ctu. [9] r. pieknik. the methodology of creating scenario roads of driving simulator for different types of experiments. conference of driver-car interaction and interface 2009. [10] a. orlicky. automatic generation of road infrastructure in 3d for vehicle simulators. diploma thesis, prague 2016. [11] j. togelius, r. de nardi, s. lucas. making racing fun through player modeling and track evolution. [12] d. loicono, l. caramone, p. lanzi. automatic track generation for high-end racing games using evolutionary computation. 41 acta polytechnica ctu proceedings 12:38–41, 2017 1 introduction 2 measured data analysis 3 identifying common elements of behavior 4 proposal track by type of experiment 5 the algorithm for automatic generation of experimental tracks 6 pseudo-code 7 conclusions references 56 acta polytechnica ctu proceedings 1(1): 56–65, 2014 56 doi: 10.14311/app.2014.01.0056 multi-frequency study of the sz effect in cosmic structures sergio colafrancesco1 1university of the witwatersrand, johannesburg (south africa) corresponding author: sergio colafrancesco. email: sergio.colafrancesco@wits.ac.za abstract the sunyaev-zel’dovich effect (sze) is a relevant probe for cosmology and astrophysics. a multi-frequency approach to study the sze in cosmic structures turns out to be crucial in the use of this probe for astrophysics and cosmology. astrophysical and cosmological applications to galaxy clusters, galaxies, radiogalaxies and large-scale structures are discussed. future directions for the study of the sze and its polarization are finally outlined. keywords: cosmology cmb dark matter dark energy cosmic magnetism cosmic structures: galaxy clusters, galaxies, radio galaxies. 1 introduction galaxy clusters, the largest gravitationally bound structures in the universe, are the representative systems of the distribution of lss in the universe. the description of these cosmic structures is continuously enriching of physical details regarding their matter and field content. dark matter (dm) is the dominant form of matter that creates the potential wells of cosmic structures, from those on the largest scales down to galactic and sub-galactic scales. if we consistently take into account the fundamental nature of dm particles, we are inevitably bound to consider the effects of their annihilation or decay on the structure and evolution of the dm clumps [13]. baryonic material collected in the dm potential wells of lss is likely heated and shocked by large–scale shock waves found in cosmological simulations (see, e.g, [53]) which produce a complex distribution of mach numbers. the presence of shock waves with relatively high mach numbers naively suggest that fermi-like acceleration might take place in lss thereby accelerating cosmic rays (crs) that can be efficiently confined mainly in galaxy clusters [8], while they can diffuse out of galactic and sub-galactic structures [8, 9]. radio observations and magneto-hydrodinamic (mhd) simulations also suggest that magnetic fields are associated to the distribution of baryonic material collected in lss potential wells. the seed magnetic field is likely amplified and made turbulent by the coupling of gravitational collapse and mhd processes during the formation of galaxy clusters. evidence for wide-scale and turbulent intra-cluster magnetic field is indicated by the diffuse radio synchrotron emission found in many clusters (with typical values b ∼ 0.1 − 2 µg), by radio relics in dynamically active clusters (with typical values b ∼ 0.2 − 5 µg) and by faraday rotation measurements of background and embedded polarized radio sources (with typical values b ∼ 1− 50 µg [27]. very massive dm clumps that collapse at high redshifts (z ∼ 6−7) often contain the most massive black holes (bhs) at their centers [35]. the agn descendants of these ancient supermassive bhs (smbhs) are found to be part of the massive galaxies located at the centers of the most massive galaxy clusters we observe at the present epoch, like, e.g., cena, m87/virgo, perseus, a262, a4059, with their radio lobes penetrating the icm for tens or hundreds of kpcs. it is often observed that the radio jets/lobes end up in approximately spherical bubbles of relativistic plasma (likely containing relativistic or mildly-relativistic plasmas) that appear as cavities in the x-ray images of galaxy clusters [3] with dimensions ranging from a few kpcs (as in perseus) to ∼ 100 kpc. the combination of high-resolution radio and x-ray images indicates that the relativistic plasma found inside the x-ray cavities is connected with the jet/lobe structure of the central agn and with the history of the ejection events and mechanisms from the central agn. the cluster cores which host such non-thermal phenomena (bhs, cavities, magnetic fields) are found to be systematically cooler than the outer regions of the clusters with the inner temperature setting at a value ∼ 1 3 − 1 2 of the outer temperature, usually consistent with the virial expectation. it seems that a heating agent of nongravitational and non-thermal nature – with a heating rate that is able to accomodate itself to the cooling rate 56 http://dx.doi.org/10.14311/app.2014.01.0056 multi-frequency study of the sz effect in cosmic structures of the intra-cluster plasma – is present in the cluster’s cool core so as to maintain it in a quasi-stationary, warm configuration [16]. the previous evidence indicates that galaxy clusters are the largest storage rooms for cosmic material (galaxies, dm, hot thermal baryonic plasma, non-thermal and relativistic plasma, bhs, magnetic fields, crs). in this sense they can be considered as the largest multidisciplinary laboratories in the universe where one can efficiently study some of the most interesting aspects of the astrophysics of lss: the nature of dm, the origin and distribution of crs, the impact of magnetic fields on lss, the impact of bhs on lss, the interplay between thermal and non-thermal phenomena in lss. two ways can be identified to proceed in this study: i) a multi-technique, single-purpose approach that require to integrate the analysis of various observations performed in various frequency bands; ii) a singletechnique, multipurpose approach that is able to provide detailed physical information based on a single observational technique. we will discuss, in the following, some of the results obtainable through the last approach by using the physical information contained into the sze. 2 the physics of the sz effect the sze is produced by the inverse compton scattering (ics) of cmb photons off the electrons confined in the atmospheres of cosmic structures. manifestations of the sze include: i) spectral distortions of the cmb due to up-scattering of cmb photons induced by high-e electrons (thermal, non-thermal and relativistic sze); ii) spectral distortion of the cmb due to a bulk motion of the electronic plasma w.r.t. the hubble flow (kinematic sze); iii) polarization of the cmb due to dynamical and plasma effects (sze polarization). the sz effect. the spectral distortion of the cmb spectrum observable in the direction of a galaxy cluster writes [56, 2, 6] as ∆i(x) = 2 (ktcmb)3 (hc)2 y g(x) , (1) where ∆i(x) = i(x) − i0(x), i(x) is the up-scattered cmb spectrum in the direction of the cluster and i0(x) is the unscattered cmb spectrum in the direction of a sky area contiguous to the cluster. here x ≡ hν/ktcmb , h is the planck constant, k is the boltzmann constant, tcmb = 2.726 k is the cmb temperature today and ν is the observing frequency. the comptonization parameter y is y = σt mec2 ∫ ped` (2) in terms of the pressure pe contributed by the electronic population. here σt is the thomson cross section, me the electron mass, and c the speed of light. the spectral function g(x) of the sze is g(x) = mec 2 〈εe〉 { 1 τe [∫ +∞ −∞ i0(xe−s)p (s)ds− i0(x) ]} (3) [6] in terms of the photon redistribution function p (s) and of i0(x) = i0(x)/[2(ktcmb)3/(hc)2] = x3/(ex − 1) . (4) the quantity 〈εe〉 ≡ σt τe ∫ ped` = ∫ ∞ 0 dpfe(p) 1 3 pv(p)mec , (5) where fe(p) is the normalized electron momentum distribution function, is the average energy of the electron plasma [6]. the optical depth along the line of sight ` of the electron population with number density ne is τe = σt ∫ d`ne . (6) the photon redistribution function p (s), with s = ln(ν′/ν) in terms of the cmb photon frequency increase factor ν′/ν, can be calculated by repeated convolution of the single-scattering redistribution function, p1(s) = ∫ dpfe(p)ps(s; p), where ps(s; p) depends on the physics of inverse compton scattering. the previous description is relativistically covariant and general enough to be applied to both thermal and nonthermal plasma, as well as to a combination of the two (see [6, 17, 38] for details). kinematic sz effect. the velocity (or kinematic) sze (hereafter ksze) arises if the plasma causing the thermal, or non-thermal, sze is moving relative to the hubble flow. in the reference frame of the scattering particle the cmb radiation appears anisotropic, and the effect of the ics is to re-isotropize the radiation slightly. back in the rest frame of the observer the radiation field is no longer isotropic, but shows a structure towards the scattering atmosphere with amplitude ∝ τevt/c, where vt is the component of the peculiar velocity of the scattering atmosphere along the line of sight [55, 51]. the brightness change of the cmb due to the ksze is given by ∆i i = −τeβt xex ex − 1 (7) with βt ≡ vt c [55, 40]. a general relativistic description of the ksze has been given in the framework of the general boltzmann equation [29] and in the relativistic covariant formalism [38]. sz effect polarization. the ics process yields naturally a polarized upscattered radiation field. the polarization π of the sze arises from various dynamical 57 sergio colafrancesco and plasma effects [54, 22, 32, 1]: galaxy clusters transverse motion (πk ∝ β2 t τ in the rayleigh-jeans, rj, regime), transverse motions of plasma within the cluster (πv ∝ βtτ 2 in the rj regime) and multiple scattering between electrons and cmb photons within the cluster (πth ∝ θτ2 in the rj regime for the thermal sze with θ = kte/mec 2). a general, covariant, relativistic derivation of the sze polarization for thermal, non-thermal and relativistic plasma can be derived [22] and generalizes the non-relativistic derivation [54] in a way similar to the general derivation of the sze [6] previously discussed. 3 astrophysical and cosmological impact studying the sze in the atmospheres of various cosmic provides many insights on their energetics, pressure and dynamical structure. the combination of sze with other emission mechanisms related to the same particle distribution (i.e., synchrotron, high-e ics and bremsstrahlung emission) provides further information on the radiation, matter and magnetic fields that are co-spatial with the electrons producing the sze. these properties of the sze concern various cosmic structures, from galaxy clusters to radiogalaxy lobes, from galaxy halos to supercluster. the redshift-independent nature of the sze allows to use it as a powerful cosmological probe yielding constraints on the evolution of cluster abundance, cosmological parameters, the dark energy equation of state, the homogeneity of the universe, the properties of cosmological magnetic fields [14]. observations aimed at these challenges must however exploit the whole physical information contained in the spectral and spatial features of the sze. 3.1 simple sze astrophysics the sze has been searched in galaxy clusters since it was originally proposed [55, 56] using various techniques (see [14] for a recent review). ground-based sze experiments (e.g., spt, act, apex, ami, gbt, among others) provided excellent results in terms of imaging and blind search surveys with their low frequency, multiple-band observations, but they do not have neither true spectroscopic capabilities nor a wide frequency band, and they are not sensitive to the high-ν range ( ∼> 400 ghz) of the sze signals, which is crucial to exploit the astrophysical information contained in the sze (see fig. 1, [14]). ground-based instruments widely improved the source statistics (crucial to obtain cosmological information using the sze) and the angular resolution of sze images (crucial to disentangle the extended sze signal from point-source contamination), but add little to the physical specification of the detected sze sources, and therefore they need x-ray and optical follow-up to fully characterize the physical parameters derived from sze observations. figure 1: thermal sze spectrum of galaxy clusters with different plasma temperatures (as indicated) and the same value of the optical depth. the low-ν part (ν ∼< 220 ghz) of the spectrum depends mostly on the total compton parameter y ∝ ∫ d`ptot with no strong spectral dependence on the temperature. the high-ν part of the spectrum (at ∼> 300 ghz) shows a strong spectral dependence from the plasma temperature [17]. typical frequency bands where the sze is observed from the ground are shown as blue-cyan bands, while the region accessible from space observations is shown by the gray shaded area. 3.2 the road to astrophysics the planck satellite allowed for the first time to access a wide frequency range in the study of the sze. planck early sze science observations yielded 189 sze sources with s/n > 6 which provides the first sze measure for ∼ 80 % of the known galaxy clusters, and 20 additional new clusters (see arnaud at this meeting). this is the largest sample so far of sze detected clusters. planck detected sze clusters are followed-up with a multi-frequency observation program in the x-rays, sze, optical bands to obtain confirmation, redshift estimation and estimates of the global physical parameters. these results show that the sze selection is a very powerful method for the detection of new distant and very massive clusters. planck also unveiled a population of dynamically perturbed clusters at z ∼> 0.3, 58 multi-frequency study of the sz effect in cosmic structures possibly underrepresented in x-ray surveys. the information collected so far strengthen our overall view of the icm properties and mass content in galaxy clusters. most of these results are discussed in the early and intermediate papers [41, 42, 43, 45, 46, 47] and more analyses are still coming. the use of planck sze results also allowed to extend the sample of radio halo clusters with combined radio, sze and x-ray data [23]. the correlation ysz−lx probes the existence of a substantial amount of non-thermal pressure in clusters that also requires a correlation x ∝ l−0.96x between the nonthermal to thermal pressure ratiox = pnon−th/pth and the cluster x-ray luminosity lx [23]. the nature of the non-thermal electron population could be only probed with additional high-ν spectroscopic studies. the herschel satellite (co-eval with planck) has been able to observe the sze in a few pointed clusters with the spire instrument equipped with an fts spectrometer working in the frequency range ∼ 600 − 1200 ghz. the possibility to have sensitive spectroscopic measurements in these high-frequency bands opens the way to the deep astrophysical exploitation of the sze. as an example, the additional data points on the sze spectrum of the bullet cluster observed with herschelspire [60] allowed to establish a number of properties for the thermal and non-thermal plasma superposition in the atmosphere of this strong merging cluster (see, e.g. [18, 48, 49]. a consequence of such superposition is that cluster temperature distribution has a high value of of the temperature standard deviation σt , as in the case of the bullet cluster where it is found σt = 10.6 ± 3.8 kev [50]. this result shows that the temperature distribution in the bullet cluster is strongly inhomogeneous along the line of sight and provides a new method for studying galaxy clusters in depth. these studies have been possible because the access to the very high-ν part of the sze spectrum contains detailed information on the relativistic effect on the single thermal plasma and on the presence of additional plasmas of either thermal or non-thermal nature (see discussion in [18, 21]. planck and herschel observations of the sze opened a rich field of investigation that will fully blossom in the next years with the full exploitation of spatiallyresolved spectroscopic sze observations. in the following we discuss some of the astrophysical and cosmological studies that will be possible with a spectropolarimetric study of the sze. 4 astrophysical impact studying the sze in various cosmic atmospheres provides many insights on their energetics, pressure and dynamical structure. the combination of sze with other emission mechanisms related to the same particle distribution (i.e., synchrotron, high-e ics emission, bremsstrahlung emission) provides further information on the radiation, matter and magnetic fields that are co-spatial with the electrons producing the sze in various cosmic structures. 4.0.1 galaxy clusters precise observations of the sze at mm and sub-mm wavelengths are crucial for unveiling the detailed structure of cluster atmospheres, their temperature distribution, and the possible presence of suprathermal and/or nonthermal plasma because the high-frequency part (i.e. at ν ∼> 350 ghz or x ∼> 6) of the sze spectrum is more sensitive to the relativistic effects of the electron momentum distribution [6, 10, 17]. this is even more so for galaxy clusters with a complex plasma distribution as found for powerful merging clusters, like the exemplary case offered by the bullet cluster (1es0657-56) [18]. powerful merging events in galaxy clusters can, in fact, produce an additional high-t plasma distribution (if the electron acceleration time scale at the merging shocks is longer than their equilibration time scale [57]), or an additional nonthermal population (produced either in a merging process with a very short acceleration time scale or by secondary electrons produced by p-p collisions, after the high-e protons have been accelerated by the merging and accumulate in the cluster region on long time scales [58]). the quasi-stationary case provided by the competition between particle thermalization and stochastic acceleration and momentum diffusion [26] can develop a subrelativistic electron distribution tail and can produce suprathermal regions in the cluster atmosphere. a quantitative estimate of the temperature inhomogeneity (stratification) along the line of sight is possible using sze data only providing a measure the temperature standard deviation of the cluster plasma along the line of sight. we found that the bullet cluster has a temperature standard deviation of 10.6±3.8 kev [50]. this result (obtained for the first time with sze measurements) shows that the temperature distribution in the bullet cluster is strongly inhomogeneous and provides a new method for studying galaxy clusters in depth. study of the multifrequency (from ∼ 30 to ∼ 850 ghz) sze signal observed in the bullet cluster shows, in fact, the presence of a thermal plasma at ∼ 13.9 kev coexisting with a second plasma component, either at higher temperature (≈ 25 kev) or, more preferably, of a nonthermal origin [18] (see fig.2). additional observations of the bullet cluster at ν ∼ 400 ghz with a precision ∼< 1% of the expected signal will be able to further distinguish between the two cases of non-thermal powerlaw or suprathermal tail [18]. sze observations over a wide frequency range, and especially with high sensitivity in the high-ν range, can 59 sergio colafrancesco also add relevant information on the electron distribution function (df) in the icm, a subject that even though relevant for a proper analysis of the sze has not been addressed in details so far. the relativistic kinetic theory, on which the df derivation is based, is still a subject of numerous debates (see discussion in [48]). sze observations can separate the sze spectrum caused by a departure from the diffusive approximation based on the kompaneets approach [31] from those which are due to using a relativistic correct df instead of a maxwell-boltzman df (see fig.3) and therefore set constraints to the actual electron df [48]. this analysis is best fulfilled in hot massive clusters because the sze intensity change due to using a relativistic correct df instead of a maxwell-boltzman df are much larger in hot clusters due to the fact that relativistic sze corrections scale as ∝ t 5/2. a method used to derive the df of electrons using sze multi-frequency observations of massive hot clusters [48] makes use of fourier analysis representation of the approximate electron df whose parameters are best fitted using observations in the (optimal) frequency channels at 375, 600, 700, 857 ghz. figure 2: the sze spectrum at the bullet cluster center modeled with a thermal plus nonthermal plasma: thermal plasma with kt = 13.9 kev and τ = 1.1×10−2 (dot-dashed); nonthermal plasma with p1 = 1, s = 2.7 and τ = 2.3 × 10−4 (dotted); total sze produced by the sum of the two plasmas (solid). a morphological analysis of the sze observed at various frequencies adds relevant information to assess the pressure and energy density structure of cluster atmospheres. morphological sze differences are particularly evident for clusters undergoing violent mergers that create large inhomogeneities of the electron df. sze intensity maps of merging clusters obtained from hydrodynamical simulations show that the morphology of the sze intensity maps observable with laboca (at 345 ghz) and herschel-spire (at 857 ghz) are rather different [49] (see fig.4). for a bullet-like cluster, the sze intensity map at 857 ghz has a spatial feature caused by the presence of the cold bullet-like substructure seen also in the x-ray surface brightness map. however, this cold substructure is not present in the sze intensity map at 345 ghz. this is a consequence of the relativistic effects of the sze and shows that observations of the sze intensity maps at very high frequencies can reveal complex pressure substructures within the atmospheres of massive galaxy clusters. this result shows that the analysis of the sze signal at 857 ghz, correlated with lower-ν observations offers a promising method for unveiling high-t regions in massive merging clusters using available experiments like, e.g laboca and herschel-spire. figure 3: the sze intensity spectra for a massive cluster with a temperature of kte = 15.3 kev for juttner (solid) and maxwell-boltzman (dashed) dfs. the non-relativistic sze spectrum (solid) is also shown for comparison. figure from [48] in more relaxed clusters spectroscopic measurements of the sze over a wide frequency band allow to derive precise information on the temperature distribution and on the cool-core nature independently of x-ray priors [17] and hence reconstruct the full set of cluster physical parameters [25]. spectro-polarimetry measurement of the sze are able to add further information on the transverse plasma motions within the cluster and on the pressure substructure of the plasma. sze polarization signals in galaxy clusters are quite low and typically below mjy (or ≈ µk) level even for high-t clusters [14] but it is interesting to note that the sze polarization in cluster has quite different spectra w.r.t. the intensity sze spectrum, a property that requires to use spectropolarimetry to fully disentangle the physical information contained in the sze. combining intensity and polarization observations of the sze can uncover unique 60 multi-frequency study of the sz effect in cosmic structures details of the 3d (projected and along the line of sight) velocity structure of the icm, of its 3d pressure structure and of the influence of a structured magnetic field in the stratification of the icm, and therefore provides a full tomography of cluster atmospheres. analogously, the combination of the intensity and polarization observations of the kinematic sze (and its frequency dependence) can yield crucial information on the 3d distribution of the cosmological velocity field traced by galaxy clusters. specifically, the ratio ∆ith/πth yields direct information on the plasma optical depth τ , and the ratio ∆ith/πv on the combination τ ·βt, thus allowing to use intensity and polarization sze measurements to fully disentangle the pressure and velocity structure of the cluster atmospheres. sze polarization measurements are quite difficult to obtain with present-day experiments and they are also at the limit of next generation experiments. however, stacking analysis of even small samples (order of ∼ 20) of hot and dense galaxy clusters would allow to determine statistically the polarization signals of the thermal sze for clusters with kt > 10 kev and τ > 0.03 in the optimal frequency range ≈ 90− 250 ghz. figure 4: from top to bottom. the sze signal to noise ratio map for the cluster 1e0657-558 at 345 ghz smoothed to the resolution of laboca and at 857 ghz smoothed to the resolution of herschel-spire. figures from [49]. 4.0.2 cluster cavities the atmospheres of galaxy clusters often show the presence of bubbles filled with high-e particles and magnetic field that are sites of bright radio emission and produce cavities in their x-ray emission distribution. cavities with diameters ranging from a few to a few hundreds of kpc have been observed by chandra in the x-ray emission maps of several galaxy clusters and groups [3, 34]. while the properties of these cavities and of the relativistic plasma they contain is usually studied by combining x-ray and radio observations, an alternative and efficient strategy is to study the consequences of the sze produced by the high-energy electrons filling the cavities [7, 39] whose amplitude, spectral and spatial features depend on the overall pressure and energetics of the relativistic plasma in the cavities. as an example, the overall sze observable along the line of sight (los) through a cluster containing cavities (see fig.5 for the case of the cluster ms0735.6+7421) is the combination of the non-thermal sze produced by the cavity and of the thermal sze produced by the surrounding icm. due to the different ν-dependence of the thermal and non-thermal sze, the non-thermal sze from a cluster cavity shows up uncontaminated at frequencies ν ≈ 220 ghz: at this frequency, in fact, the overall sze from the cluster reveals only the ics of the electrons residing in the cavities without the presence of the intense thermal sze dominating at lower and higher frequencies. the cavity’s sze becomes dominant again at very high-ν (x ∼> 14 or ν ∼> 800 ghz) where the nonthermal electrons dominate the overall ics emission (see fig.5). the cavity’s sze is more spatially concentrated than the overall cluster sze because it is only emerging from the cavity regions: this fact allows to study the overall energetics and pressure structure of the cavity’s high-e particle population and the b-field structure in combination with x-rays and radio images. the observation of the crossover of the non-thermal sze from the cavities (which depends on the value of emin(p1) or, equivalently, on the value pcavity) provides a way to determine the total pressure and hence the nature of the electron population within the cavity [7], an evidence which adds crucial, complementary information to the x-ray and radio analysis. if cluster cavities contain a high-t (∼ 109 − 1010 k) plasma, the sze flux from cocoons in the central part of a distant elliptical and a nearby galaxy cluster are of the same order. for a high-t plasma, the cocoon’s sze spectrum is rather flat at high-ν resembling the shape of the non-thermal sze from cavities. in this high-t plasma model, however, no strong radio emission at ν ∼> 1 ghz (as instead observed) is expected from the cocoon, unless the cocoon’s b-field is very high b ∼> 103µg. 4.0.3 radiogalaxy lobes studies of (giant) radio-galaxy (rg) lobes (see, e.g., [28, 30, 24, 4]) have shown that these extended structures contain relativistic electrons that are currently available to produce both low-ν synchrotron radio emission and ics of the cmb (as well as other radiation background) photons. as a consequence, an sze from the lobes of rgs is inevitably expected [11]. such nonthermal, relativistic sze has a specific spectral shape that depends on the shape and energy extent of the spectrum of the electrons residing in rg lobes. the 61 sergio colafrancesco sze emission from rg lobes is expected to be co-spatial with the relative ics x-ray emission [11] and its spectral properties are related to those of the relative ics x-ray emission. in fact, the spectral slope of the ics x-ray emission αx = (α− 1)/2 (where fics ∼ e−αx ) can be used to set the electron energy spectral slope α (where ne ∼ e−α) necessary to compute the sze spectrum, and to check its consistency with the synchrotron radio spectral index αr = (α − 1)/2 (where fsynch. ∼ e−αr ), that is expected to have the same value [11, 19]. figure 5: top. the geometry of the cavities in the cluster ms0735.6+7421. bottom. the sze spectrum has been computed at a projected radius of ≈ 125 kpc from the cluster center where the los passes through the center of northern cavity. the thermal sze (blue), the non-thermal sze from the cavity (black) and the total sze (red) are shown. the non-thermal sze is normalized to the cavity pressure p = 6 · 10−11 erg cm−3, and is shown for various values of p1. figure from [7] the sze in rg lobes has not been detected yet: only loose upper limits have been so far derived on the sze from these sources (see [2, 59]). a detection of the sze from rg lobes can provide a determination of the total energy density and pressure of the electron population in the lobes [11] allowing to determine the value of emin once the slope of the electron spectrum is determined from radio and/or x-ray observations. sze measurements provide a much more accurate estimates of the electron pressure/energy density than with other technique like ics x-ray emission or synchrotron radio emission, since the former can only provide an estimate of the electron energetics in the high-energy part of the electron spectrum, and the latter is sensitive to the degenerate combination of the electron spectrum and of the magnetic field in the radio lobes. the combination of sze observations (that depend on the electron distribution and on the known cmb radiation field) and the radio observations (which depend on the combination of the electron distribution and of the magnetic field distribution) provides an unbiased estimate of the overall b-field in the lobe by using the ratio fradio/fsze ≈ eb/ecmb , that is more reliable than that obtained from the combination of ics x-ray (or gamma-ray) and radio emission [19]. the spatially resolved study of the sze and synchrotron emission in rg lobes also provide indication on the radial behaviour of both the leptonic pressure and of the magnetic field from the inner parts to the boundaries of the lobes. study of the pressure evolution in rg lobes can provide crucial indications on the transition from radio lobe environments to the atmospheres of giant cavities observed in cluster atmospheres, which seem naturally related to the penetration of rg jets/lobes into the icm (see fig.5). a substantial sze polarization is also expected in rg lobes due to both coherent transverse motions of the plasma along the jet/lobe direction and to the electron pressure substructures induced by e.g. plasma inhomogeneities and magnetic field turbulence. the transverse velocity-induced polarization is πv ∝ τrel(βtτrel), and the multiple scattering induced polarization is πτ ∝ τrelprel where prel is the pressure of the relativistic electron distribution. observations of the sze and its polarization in rg lobes can yield, therefore, direct information on electrons τrel and βt in the rg lobe. 4.0.4 galaxies hot gas trapped in a dm halo can produce a sze. a typical galaxy halo might hence show an integrated thermal sze at the level of ∼< 0.5 mjy arcmin−2 from a plasma with t ∼ 106 k and density ne ∼ 10−3cm−3 extended for ∼ 50 kpc in the inter-stellar medium. a measurement of galaxy halo sze would provide direct information on the mass, spatial distribution and thermodynamic state of the plasma in a low-mass galactic halo, and could place important constraints on current models of galaxy formation. detecting such an extended, low-amplitude signal will be challenging, but possible with sensitive all-sky sze maps. an sze is also expected from galaxy outflows swept by galaxy (hyper-)winds. a thermal sze is expected to arise from the shocked bubble plasma in a galaxy wind described by a simple, spherical blast wave model [52]. 62 multi-frequency study of the sz effect in cosmic structures however, such simple recipe for the sze from galaxy winds is to be modified by the presence of cosmic rays and magnetic field in the expanding wind leading on one side to a more complex sze spectrum, and on the other side to an amplification of the overall sze at high frequencies, thus increasing the detection probability. sze observations from galaxy winds will be possible with high-sensitivity and high-resolution telescopes like alma and ska.     figure 6: top. thermal sze spectrum (continuous line in bottom panel), compared to the atmospheric transmission of a dry, cold, atmosphere (top panel, pwv= 0.5 mm), and to spectra of the nonthermal sze (dotted line), of cmb anisotropy and kinematic sze (dashed line), and of dust anisotropy (dotdashed line). the parameters of the different spectra are for τth = 5 × 10−3, te = 8.5 kev, ∆tcmb = 22µk, τnon−th = 10−4, α = −2.7, p1 = 1.4 mev/c, ∆id(150ghz) = 600 jy. the frequency coverage of different experiments [25] is shown as dotted horizontal lines, labelled with the experimental configuration number. bottom. simulated data sets for the spectroscopic configurations ec5 (differential cold fts on a l2-orbit satellite, with cold telescope). 5 cosmology and fundamental physics the redshift-independent nature of the sze allow to use this effect as a powerful cosmological probe [14] by using both the redshift evolution of cluster abundance and direct probes of cosmological parameters. the recent planck results however indicate a tension at ∼ 3σ level on the value of σ8 vs. ωm comparing sze counts and cmb results that might be mitigated by other cosmological effects like, e.g., a more complex cluster bias dependence, the existence of massive neutrinos with ∑ mν ∼ 0.2 ev (see rubino-martin in these proceedings) or by addressing more in details the complex astrophysics issues still pertaining cluster formation and evolution. this last possibility requires to have new high-quality sze cluster samples studied with full spectra-polarimetric capabilities and in the high-ν range of the sze spectra where the astrophysical effects are more evident. fig.6 shows the frequency coverage requirements for a full spectroscopic sze mission and the outcomes of a simulated thermal sze spectrum from a kte = 8.5 kev cluster [25]. the sensitivity and spectro-polarimetry power of such an experimental configuration (ec5) will allow to address fundamental questions in cosmology and fundamental astrophysics with the sze, in addition to standard cosmological probes [14]. a wide discussion on the cosmological relevance of the sze for probing the concordance cosmological scenario, the search for the nature of dm, cosmological magnetic fields and cosmological velocity fields can be found in [14]. here we discuss the additional application of the spectro-polarimetric observations of the sze to measure the cmb multiplies and test the assumption of the homogeneity of the universe. high-sensitivity observations of the (thermal and kinematic) sze and its polarization can be used to test the homogeneity of our universe through probes of the copernican principle (cp) [33, 23]. in fact, large variations of the thermal sze induced by cmb photons that have temperature significantly different from the blackbody temperature we observe directly, and that arrive at galaxy cluster from points inside our light cone, could indicate a violation of the cp and homogeneity. analogously, large variations of the cmb dipole measured by the cluster ksze could also indicate a violation of the cp and homogeneity. the sze polarization contains more refined information on the cmb temperature and thus it can also provide a powerful probe of cp and homogeneity. thus, observations of large sze and of its polarization w.r.t. to the expectations of the sze produced from a pure blackbody cmb spectrum might provide indications of a non-flrw universe. the combination of an ec5 experiment with radio observations of the low-ν part of the sze spectrum can also be used to probe even more fundamental aspects of astrophysics, like the fundamental properties of the photon. measurements of the sze with sensitivity of order of < 0.1 jy, in the range ∼ 10 − 50 ghz, can set very stringent constraints on the photon decay time [20]. this frequency band is also the one less affected by other sources of astrophysical contamination, and it will be best explored with the advent of the high63 sergio colafrancesco sensitivity ska telescope. at higher frequencies (i.e., ∼ 120 − 180 and ∼ 200 − 300 ghz), there are other spectral windows where the sze method is again competitive if not advantageous in this respect. the necessary sensitivity in the high-ν range can be achieved with the next coming millimetron space mission [14], whose ec5-like configuration is also advantageous in disentangling other astrophysical sources of contamination (e.g., sources of non-thermal sze and/or special distortions of the sze due to multiple-temperature regions) that could contaminate these measurements. figure 7: difference between the standard thermal sze calculated for a2163 in 2–45 ghz range and integrated in a 1 arcmin2 area and the one modified for values e∗ = 5×10−9 ev (solid line), 3×10−9 ev (dash 3 dots), 2 × 10−9 ev (dot-dashed), and 1 × 10−9 ev (dashed). the sensitivity achievable with ska in ∼ 30 hours (red thick solid line) and in ∼ 260 hours (blue thick long-dashed line) are shown [20] 6 future directions the impact of sze observation for astrophysical and cosmological application is steadily increasing since the advent of dedicated experiments and large-scale surveys in the mm and sub-mm frequency range. the quality and the spatial resolution of sze images is reaching the level of arcsecs and the spectral coverage in systematically extending in the sub-mm region where highest spatial resolutions can be achieved with a wide spectral coverage able to decipher the physical details of the electron distribution in the atmosphere of cosmic structures. new paths of theoretical investigations are underway and concern both the detailed study of the ics mechanism and the impact of the various plasma structure and fields in cosmic structures. the possibility to perform precise measurements of the various sze signals and to extract the relevant astrophysical information depends crucially on the capability to have spatially-resolved spectral (and polarization) observations of sze sources over a wide ν band, from radio to sub-mm. in particular, the important condition for such study is to have a wide-band continuum spectroscopy (polarimetry) and especially a good spectral coverage and sensitivity in the high-ν band, where most of the astrophysical effects reveals more clearly. spatially resolved spectroscopic and polarimetric observations of the sze in the frequency range from ∼ 100 ghz to ∼ 1 thz, complemented by analogous observations in the radio band ∼ 1 − 30 ghz, are the key to improve our understanding of the structure of cosmic atmospheres through analysis of the intensity and polarized sze signal, and will allow to use this technique to probe the fundamental properties of the universe. acknowledgement s.c. acknowledges support by the south african 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http://dx.doi.org/10.1146/annurev.aa.18.090180.002541 http://dx.doi.org/10.1086/498227 http://dx.doi.org/10.1086/591723 introduction the physics of the sz effect astrophysical and cosmological impact simple sze astrophysics the road to astrophysics astrophysical impact galaxy clusters cluster cavities radiogalaxy lobes galaxies cosmology and fundamental physics future directions acta polytechnica ctu proceedings doi:10.14311/app.2017.7.0022 acta polytechnica ctu proceedings 7:22–28, 2017 © czech technical university in prague, 2017 available online at http://ojs.cvut.cz/ojs/index.php/app validation and implementation of failure parameters in integrated simulations for short fibre reinforced polypropylene anna kalteisa,∗, martin reitera, michael jerabekb, zoltán majora a institute of polymer product engineering, johannes kepler universität linz, altenberger straße 69, linz, austria, 4040 b borealis polyolefine gmbh, st.-peter-straße 25, 4021 linz ∗ corresponding author: anna.kalteis@jku.at abstract. nowadays short fibre reinforced polymers are often used in load carrying structural parts. compared to continuous fibre reinforced polymers they exhibit a more complex morphology. hence the determination of the strength is a difficult but important task. therefore this was the objective of this research. the strength of short fibre reinforced polymers was numerically determined for low-speed to high-speed strain rates for specimens with different fibre orientations. for the failure modelling the micromechanical approach “first pseudo grain failure” in digimat was used. the parameters for the material and failure description were determined with the reverse engineering method. integrated finite element simulations were performed to validate the material and failure models by tensile and bending tests with different specimens. the comparison of the results of the experiments and simulations showed low deviation. keywords: integrated simulation, short fibre reinforced polymer, failure prediction, first pseudo grain failure, reverse engineering. 1. introduction the prediction of the stiffness of a fibre reinforced material is not a difficult task anymore because there is appropriate software available. one is the software digimat mf. there only few input parameters are needed to calculate the stiffness [1]. however the prediction of the strength of a material especially for short fibre reinforced polymers is not trivial. to perform simulations is very common nowadays, therefore in this research the strength of short glass fibre reinforced polypropylene is determined by integrated finite element simulations. there are many different approaches how to predict the strength of a discontinuous fibre reinforced polymer. for example there is an analytical model where the strength of fibre reinforced composites can be calculated without any numerical software [2]. furthermore there are several continuum mechanics based models [3], [4], [5], [6], [7], [8], [9] and micromechanics based models [10], [11], [12], [13]. in the paper also a micromechanics based model is used namely the “first pseudo grain failure”(fpgf) model [1]. a short description is given in section 4 of this paper. the investigated material is a short glass fibre reinforced polypropylene. the glass fibre content is 32 w%. because of the short fibre reinforcement the fibre distribution and orientation in the different specimens have to be investigated. the fibre orientations were determined by the computer tomographic method. the raw data from the computer tomographic measurements of the fibre orientation were provided by fh-oö (wels, austria). therefore a description of the ct-measurements is not given here but in [14]. the experimental data were provided by borealis polyolefine gmbh (linz, austria). standardized tensile (iso 527) and bending tests (iso 178) were performed with the corresponding strain rate and specimen. specimens with different complexity of fibre distribution and orientation were used. specimens with a uniform fibre distribution and orientation (ud multi tool) are used for the reverse engineering. for the validation specimens with a more complex fibre distribution and orientation are used. a description of the used specimens is given in section 2. the modelling of the material behaviour is an other part of this paper. elastic-plastic (low-speed strain rates) and elastic-viscoplastic (low-speed and highspeed strain rates) material models are generated. the needed material parameters are reverse engineered based on experimental data of tensile tests. in section 4 the determination of an appropriate failure criterion is described. because the polymer is reinforced with short fibres classical failure criteria (e.g. tsai-hill) cannot be simply used because they are just valid for continuous fibre reinforced composites [1]. hence as already mentioned above the “first pseudo grain failure” (fpgf) model has to be used. the failure parameters are determined with the method of reverse engineering with data of tensile tests. the results of the material and failure modelling are validated with tensile tests with specimens with different fibre distribution and orientation as the reverse engineered ones. further validation follows by per22 http://dx.doi.org/10.14311/app.2017.7.0022 http://ojs.cvut.cz/ojs/index.php/app vol. 7/2017 forming bending tests. the appearing difficulties in the simulation of the bending test have to be solved and are described in section 5.3. the validation of the material and failure model is done for low-speed and high-speed strain rates. in this paper only the results for the low-speed strain rates are discussed. 2. specimens as already mentioned different types of specimens are used for the experiments and simulations. an overview of the used specimens is given in fig. 1. figure 1. overview of different specimens. different specimens are used because of their different complexity of fibre orientation. the ud-multitool was designed by borealis polyolefine gmbh (linz, austria). the specimens, which can be milled out of the tool have a highly uniform fibre distribution. the specimens 0◦, 45◦ and 90◦ are relevant for this paper. because of the uniform fibre distribution these specimens are used for the reverse engineering of the material and failure parameters. for the validation of the material and failure models the iso-mps specimen and the plate specimens are used. the plate specimens were milled out of a plate at different positions and under the angles 0◦, 45◦ and 90◦. these specimens do not show a uniform fibre orientation through the thickness as the ud-multitool specimens. 3. material modelling for the material modelling the software digimat mf 6.0.1 is used. for the glass fibre a linear elastic, isotropic material model is chosen. therefore only the elastic modulus and the poisson’s ratio have to be defined. for the elastic modulus and poisson’s ratio the following values are used: e = 72000 mpa, ν = 0.2 the aspect ratio (ar) of the fibres is set to 35 [15]. an elastic-plastic (ep) and also an elastic-viscoplastic (evp) material model are generated for the matrix. the ep and evp material models for the matrix need more input parameters than an elastic one. the j2plasticity model is chosen (eq. 1). for the hardening the exponential and linear law is used (eq. 2). where r(p) is the hardening stress, p is the accumulated plastic strain, k is the linear hardening modulus, r∞ is the hardening modulus,m is the hardening exponent [1]. σeq = √ j2(σ) (1) r(p) = kp+r∞[1 − exp(−mp)] (2) for the elastic-viscoplastic model some additional parameters for considering the viscoplasticity have to be determined. a law for the accumulated plastic strain rate ṗ has to be chosen. in this paper the current yield norton law is chosen (eq. 3). there the ṗ is dependent on the initial yield stress σy, the viscoplastic stress f , the hardening stress r(p), the viscoplastic coefficient η and the viscoplastic exponent m. ṗ = σy η ( f σy +r(p) )m (3) a detailed description is given in [1]. the above described parameters are reverse engineered with the software digimat mx 6.0.1. with this procedure the parameters for an artificial matrix are generated. they are artificial because the real behaviour of the matrix (e.g. micro-cracking) and the influence of the fibre on the matrix (e.g. fibre-matrix debonding) are inherently considered. these influences cannot be modelled by using the parameters for the neat matrix material [16]. in the reverse engineering procedure experimental stress-strain curves are needed where the microstructure (fibre orientation) is known. the parameters of the matrix are fitted to the experimental curves in an iterative optimization process [1]. the results of the material modelling for the ep and evp material models for different strain rates are summarized in table 1 and table 2. 4. failure modelling the intralaminar failure criteria, which are available in digimat are only valid for a lamina layer with unidirectional aligned fibres [1]. they can be non-interactive, interactive or failure mode based [17]. the failure criterion, which is used is tsai-hill 3d transversely isotropic (stress based, interactive). this failure criterion assumes that the strength in tension and compression is identical and the material is isotropic in the 2-3 plane [1]. direction 2 and 3 are the directions transverse to the fibre orientation (direction 1). for the description of the failure criterion three input parameters have to be defined: 23 a. kalteis, m. reiter, m. jerabek, z. major acta polytechnica ctu proceedings strain rate [s−1] 10−4 10−3 10−2 young’s modulus [mpa] 1800 1800 1800 poisson’s ratio [-] 0.42 0.42 0.42 yield stress [mpa] 4 4 4 hardening modulus [mpa] 10.3 13.3 14.2 hardening exponent [-] 154.5 208.2 188.3 linear hardening modulus [mpa] 22.6 19.3 32.2 table 1. reverse engineered elastic-plastic material parameters for low-speed strain rates. strain rate [s−1] low-speed (10−4, 10−3, 10−2) high-speed (0.1, 1, 14) young’s modulus [mpa] 1800 1800 poisson’s ratio [-] 0.42 0.42 yield stress [mpa] 3 4 hardening modulus [mpa] 12 9 hardening exponent [-] 145 85 linear hardening modulus [mpa] 22 60 viscoplastic coefficient [mpa*s] 51 71 viscoplastic exponent [-] 4 6 plastic strain multiplier [-] 1.5 1.9 table 2. reverse engineered elastic-viscoplastic material parameters for low-speed and high-speed strain rates. • axial tensile strength x (tensile strength in fibre direction) • in-plane tensile strength y (transverse tensile strength) • transverse shear strength s the failure criterion is defined as follows [1]: fa(σ) = σ2 11 x2 − σ11(σ22 + σ33) x2 + σ2 22 + σ2 33 y 2 +( 1 x2 − 2 y 2 ) σ22σ33 + σ2 12 + σ2 13 s2 +( 4 y 2 − 1 x2 ) σ2 23 (4) because a short fibre reinforced polymer with nonperfectly oriented fibres is used and not a lamina layer with perfectly aligned fibres the “first pseudo-grain failure” (fpgf) method has to be used. with this method the failure criteria for laminates can also be used for composites with complex fibre orientation distributions because the short fibres are divided into regions with uniform fibre orientations [1]. this is illustrated in fig. 2. the strength values are reverse engineered based on the ep material model. also here the data of the tensile tests of specimens of the ud multi tool are used. in fig. 3 the strength parameters of the failure figure 2. illustration of the first pseudo-grain failure method [16]. modelling for different strain rates are illustrated. it can be seen that the higher the strain rate the higher the strength is. 5. integrative simulation 5.1. interface digimat cae for the validation of the material model integrative digimat-abaqus fem simulations are performed. for these simulations an interface is needed. thus the software digimat cae 6.0.1 is used. with this software the material properties are calculated depending on the fibre orientation either before or during the simulation. this depends on which so24 vol. 7/2017 figure 3. illustration of strength parameters for different strain rates. lution is chosen in digimat cae. in the following the solutions are described [1]: • micro solution: the material properties are calculated at every iteration and not kept constant. as a result the simulation takes very long. • hybrid solution: the material properties are precomputed before the simulation. this enables that the simulation is sped up. with this method so called hybrid parameters are computed during the generation of the material input file. integrative simulations (tensile test) with the two different solution methods are performed. the results show that there is nearly no difference between micro and hybrid solution (fig. 4). the hybrid solution is chosen for the further simulations because of the lower computation time. figure 4. comparison of hybrid and micro solution method. 5.2. simulation model of tensile test as the specimens show a fibre orientation distribution through the thickness this distribution has to be discretised for the simulation model. the discretised distribution can be modelled with a layer build-up of the specimen (fig. 5). the layers can be modelled by partitioning the specimen through the thickness and assigning a material model with a specific fibre orientation to every partition. the number of layers can vary. the higher the number of layers the more accurate the fibre distribution is described. furthermore the higher the number of layers is the higher the computation time because more different material models have to be assigned. for the tensile tests a 4 layer model is used. therefore for the simulation of a tensile test a 4 layer model with the hybrid solution method is used. in this simulation model no problems occur. figure 5. simulation model with 4 layers. 5.3. simulation model of bending test because there are no problems in the simulation model of the tensile test these settings are also applied for the bending tests. therefore a bending test with a 4 layer model and with the hybrid solution method is modelled. in contrast to the tensile test simulation model, the bending simulation model shows various complications by using exactly the same simulation approach. therefore the settings of the simulation model of a tensile test cannot be directly transferred to the simulation model of the bending test. the simulation model of the bending test has to be modified. the problems in the simulation model of the bending test (hybrid solution, 4 layers) and the belonging solutions are explained in table 3. figure 6. specimen with global fibre orientation. the soft contact, which is used is a normal behaviour with exponential law between the master and slave surface. this contact is implemented in abaqus. two parameters (clearance and pressure) have to be defined. with this contact the pressure of the stamp 25 a. kalteis, m. reiter, m. jerabek, z. major acta polytechnica ctu proceedings problem solution 1 fibre orientation does not rotate with specimen applying global fibre orientation (fig. 6) 2 elements break under pressure soft contact between specimen and stamp (fig. 7) 3 “kink” in curve (fig. 8) 10 layer model instead of 4 layer model 4 considering of different behaviour under compression [18] adjustment of hardening modulus for the upper 5 layers (fig. 9) 5 no valid hybrid parameters could be determined for some layers using micro solution 6 specimen breaks at a too low stress adjustment of failure parameters table 3. description of the problem and the corresponding solution for the bending simulation model. figure 7. illustration of position of applied soft contact. figure 8. illustration of kink in curve. figure 9. adjustment of material behaviour under compression; a) illustration of layers for the adjustment; b) red curve: stress-strain curve without adjustment, green curve: stress-strain curve with adjustment. is distributed over a larger area of the specimen [19]. therefore the stress concentrations on the specimen are lowered and as a result the specimen does not break under pressure anymore. the parameters are investigated by performing simulations of a bending test to find the most appropriate ones to solve the problem that the elements break under pressure. the reason for using a 10 layer model for the bending test is that the stress differences in the 10 layer model are lower than in a 4 layer model. this is because in the 10 layer model there is a smoother transition between the different fibre orientations. therefore the “kink” in the curve vanishes. unlike in the simulation model of the tensile test in the simulation model of the bending test the hybrid solution cannot be used. the reason for this is that in some layers no valid hybrid parameters could be determined by the software. therefore the micro solution has to be chosen. the failure parameters are reverse engineered for the ep material model. but for the bending tests the evp material model is used in order to consider the strain rate dependency. therefore the reverse engineered failure parameters may not fit as well for the evp material model as for the ep material model. as a result the parameters are fitted as it is described in section 6.2. 6. results 6.1. results of tensile test fig. 10 shows the result of the tensile test for a lowspeed strain rate. the material model is the elasticplastic one with strain rate of 10−3s−1. as the strain rate for the simulation is implemented in the elasticplastic material model also for the low-speed strain rates the dynamic explicit solver can be used to speed up the simulation. also an uncoupled simulation was performed in digimat mf. a difference in the failure stress between the simulations performed in abaqus 6.14 (integrative simulation) and digimat mf can be seen. 26 vol. 7/2017 figure 10. comparison of stress-strain curves of experiment, simulation performed in digimat mf 6.0.1 and simulation performed in abaqus 6.14. the reason for this is that the specimen shows a stress concentration in the region of the shoulder (fig. 11). figure 11. illustration of location of stress concentrations; a) specimen before failure, b) failed specimen. the notch effect at the specimen typically does not appear in reality. in reality the iso-mps specimen should break in the middle. in the simulation this cannot be realized in any specimen. because the simulation model itself cannot be modified to consider the notch effect, the failure parameters have to be fitted. therefore the parameters are fitted in this way that the notch effect can be considered. this fitting is performed by lowering the strain rate dependency by a power of 10. this means that the failure parameters for the strain rate of 10−2 s−1 are used for the elasticplastic material model for the strain rate of 10−3 s−1. in the following figure the results of the experiment and the simulation with the fitted failure parameters are shown. figure 12. comparison of stress-strain curves of experiment and simulation performed in abaqus 6.14 with fitted failure parameters. 6.2. results of bending test the bending test simulations are performed with the elastic-viscoplastic low-speed material model. because of the viscoplastic behaviour the strain rate has to be defined in abaqus. the static solver is used. in the following the result of the bending test for a specimen with misaligned fibres with the not fitted failure parameters is shown (fig. 13). as it can be seen the specimen breaks at a lower force than the experiment. one reason for this is described in section 5.3. figure 13. comparison of force/thicknessdisplacement curves of experiment and simulation. therefore the in-plane tensile and the transverse shear failure parameters are fitted. the axial tensile strength is kept constant because it has the most influence on layers with fibres, which are nearly perfectly aligned. in table 4 the previous and the fitted parameters are listed for the two different parameters. the difference between the previous and the fitted value is not very big. nevertheless these little increased values have a huge influence on the global result, which is illustrated in fig. 14. strength parameter previous value fitted value in-plane tensile strength 22.83mpa 25.32mpa transverse shear strength 13.38mpa 13.60mpa table 4. previous and fitted strength parameters. figure 14. comparison of force/thicknessdisplacement curves of experiment and simulation with fitted strength parameters. 27 a. kalteis, m. reiter, m. jerabek, z. major acta polytechnica ctu proceedings 7. conclusion with the knowledge of the specimen’s fibre orientation simulations can be set up, where this fibre orientation is considered. material and failure modelling was done for specimens with different fibre orientations and for low-speed to high-speed strain rates. it was shown that it is possible to determine failure in tensile and bending tests with low deviation (fig 15). figure 15. illustration of the deviation between failure force in the experiment and in the simulation. many influences have to be considered in a bending test (e.g. applying a global fibre orientation). little different values of failure strength parameters have a huge influence on the failure strength of the specimen. acknowledgements i would like to thank dr. michael jerabek for the possibility to do this research in cooperation with borealis polyolefine gmbh. references [1] digimat documentation 6.0.1. 2015. 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[19] abaqus analysis user’s guide, 6.14. 2014. 28 acta polytechnica ctu proceedings 7:22–28, 2017 1 introduction 2 specimens 3 material modelling 4 failure modelling 5 integrative simulation 5.1 interface digimat cae 5.2 simulation model of tensile test 5.3 simulation model of bending test 6 results 6.1 results of tensile test 6.2 results of bending test 7 conclusion acknowledgements references 55 acta polytechnica ctu proceedings 2(1): 55–59, 2015 55 doi: 10.14311/app.2015.02.0055 cataclysmic variables from sdss: a review and a look forward to lsst p. szkody1 1department of astronomy, university of washington, usa corresponding author: szkody@astro.washington.edu abstract the past and current projects of the sloan digital sky survey are reviewed in the context of applicability and results for cataclysmic variables. ongoing and future time domain surveys that will have impact on the field are also briefly discussed. keywords: cataclysmic variables dwarf novae intermediate polars polars optical photometry spectroscopy. 1 introduction cataclysmic variables (cvs) have been discovered in a variety of ways, with the largest numbers of new systems coming from survey work. the mode of operation and the limiting magnitude of each survey determines the different types of cvs that are discovered. the palomar-green survey (green et al. 1986) used blue color to find objects that were brighter than 16th magnitude, hence it found many bright novalike systems. x-ray surveys such as rosat (voges et al. 1999) identified those systems with high x-ray flux, hence many polars and intermediate polars were found. the hamburg survey (hagen et al. 1995) searched for emission line objects down to about 18th magntitude, finding sw sex stars, intermediate polars and long period dwarf novae. the sloan digital sky survey (sdss) used both photometry (to 22nd magnitude) and spectroscopy (to 19th magnitude) to identify all types of cvs, especially those with low accretion rates and orbital periods below the gap (szkody et al. 2011). current and future surveys are concentrating on variability as a means of identification and as a result are turning up many dwarf novae. the combination of all of these results will ultimately end in a better understanding of the total space density of all types of cvs. 2 sdss the sdss project (york et al. 2000) began taking data in 2000 and has continued to the present time, undergoing changing modes of operation and target selection. the past results on cvs from this survey, as well as current and future plans are summarized below. 2.1 sdss i,ii legacy survey sdss i started with a goal to photometrically survey the entire north galactic cap, as well as obtain spectra of a selected subset of objects within what is termed the legacy survey. in addition to the north cap, a 2.5 degree wide stripe centered on the celestial equator (stripe 82) was also included (see figure 1). the initial plan for completion in 5 years was extended (sdss ii) in order to finish the original footprint so the legacy encompassed the years 2000-2008 and the final entire database was released as data release 7 (dr7; www.sdss.org/dr7/). this database includes imaging data of 230 million objects, using 54 second integrations in 5 filters (ugriz). based on the colors obtained, targets (primarily quasars and galaxies) were selected by a variety of groups, within restrictions of fiber spacing, brightness limits, etc. for 1 hour integration spectra with wavelength coverage from 3800-9200å at a resolution of about 2000. plug plates were than drilled and threaded to accomodate 640 fibers resulting in 1.37 million spectra in dr7, including 225 thousand stars. since the sources receiving spectra were determined by color, and cvs have a broad range of colors (szkody et al. 2003), the main source of cv spectra turned out to be objects taken from quasar loci, which span a broad range of colors outside the main sequence footprint. the resulting computer and eye searches of all the legacy spectra for balmer lines turned up 285 cvs which included 30 polars, 6 ips and 9 systems containing pulsating white dwars. a list of these sources can be found in szkody et al. (2011) as well as on the web with links to the spectra (http://www.astro.washington.edu/users/szkody/cvs/). from 2000-2011 extensive followup on more than 300 55 http://dx.doi.org/10.14311/app.2015.02.0055 p. szkody nights was conducted on this list by many observers using apo, la palma, usno, steward, mdm, mmt observatories. as a result of this work, 151 orbital periods were determined that allowed specific classification of these objects and how they fit into population models. gänsicke et al. (2009) used 126 of the periods known at that time to reach major conclusions that enforced the magnetic braking model and confirmed the population synthesis predictions of howell et al. (2001): i.e. the majority of the disk accreting systems exist below the period gap and a period spike appears at the minimum period. the sdss results showed that the discrepancies in the past were primarily due to selection effects that favored the discovery of bright, long period systems, while sdss was able to uncover the larger population of faint sources. however, the percentages under the gap are slightly less and the period spike occurs at a slightly longer period than predicted so adjustments to the angular momentum losses are needed (knigge 2011). the fainter magntitude limit reached by sdss also revealed large numbers of accreting pulsators and low accretion rate polars (larps). of the 16 known accreting pulsators, 9 were found in the legacy survey. all of these showed broad balmer absorption lines surrounding the emission, providing a clear optical signature of the white dwarf. followup optical and uv results on the set of accreting pulsators has led to three major results. the first is that the instability strip is much wider than h atmosphere pulsating white dwarfs (zz ceti) (szkody et al. 2010). arras et al. (2006) attribute this to the existence of a he instability strip as well as hydrogen. because many of the accreting white dwarfs in this wide instability strip are observed not to pulsate, followup long term observations were conducted. this led to the second result that objects can stop pulsating, usually after an outburst, but sometimes when no outburst has occurred! e.g. sdss0745+45 (eq lyn, mukadam et al. 2013). third, in followup observations with hst, it was discovered that the pulsation that was present after outburst in v455 and appeared in the emission lines, not the continuum, presenting problems for a physical mechanism (szkody et al. 2013). of the 9 known larps, 7 were found in sdss. these objects have prominent humps due to cyclotron harmonics at high fields and low optical depth (wickramasinghe & ferrario 2000). finding larps in sdss data is not easy as the dependence of the harmonics on field strength moves them into different regions of color space. schmidt et al. (2005) calculated the sdss color ranges for various fields but there has as yet been no systematic search to find all the candidates. they estimate these could be a major contribution to the magnetic white dwarf population. several groups are using the sdss database to further understand the properties of the known cvs and to find further ones. a large hst program led by boris gänsicke is obtaining uv spectra of 40 cvs (including 12 discovered by sdss) to characterize the temperature of the white dwarf and the mass accretion rates over a variety of orbital periods and compositions. when combined with gaia distances, the masses of these white dwarfs will also be determined, leading to improved understanding of the evolution of close binaries. carter et al. (2013) discovered 29 new cvs in doing spectroscopic followup of am cvn candidates selected by color in the photometric database. breedt and gänsicke (2011) are obtaining spectra of the faintest cvs in sdss that are being discovered by crts (see section below). 2.2 sdss ii: segue and sn besides the legacy extension, sdss ii (2005-2008) contained the sloan extension for galactic understanding and exploration (segue) and a supernova survey. these results are contained in dr8. the main goal of segue was to explore the milky way, including its structure, history, kinematics, evolution and dark matter by mapping the positions, velocities, composition and temperatures of 240,000 stars (see figure 1 for the coverage compared to legacy). while this survey mainly targetted white dwarfs and giants, several cvs were included as their colors matched those of white dwarfs. the sn survey involved repeated imaging of the same region of the sky (stripe 82) during 3 months of each year. the repeat imaging over 275 deg2 with about 20 measurements on each object identified hundreds of new transient sources (sako et al. 2008). while the main goal was identifying sn, other types of variable stars were also found. the analysis of all the light curves of variable objects is ongoing and may include some cvs. a catalog of 13,051 variable sources brighter than g=20.5 from 1998-2007 stripe 82 data is in sesar et al. (2007). bhatti et al. (2010) provide a catalog of light curves for 221,842 point sources for half of the entire stripe 82 data. 2.3 sdss iii: segue-2, boss, apogee and marvels sdss iii (2008-2014) continues the segue project and adds 3 new enterprises. segue-2 obtained spectra of 119,000 stars with a concentration on the stellar halo with distances of 10-60 kpc. these data appeared in dr8 while dr9 updated the stellar parameters and added catalogs. the baryon oscillation spectroscopic survey (boss) updated the sdss fibers to 1000 and is producing spectra of many galaxies and quasars. while the target selection is not as optimal as it was in the 56 cataclysmic variables from sdss: a review and a look forward to lsst legacy survey, there are a few cvs which emerge in the spectra. dr9 was the first public release of boss spectra, while dr10 contains the latest data. the apo galactic evlution experiment (apogee) uses ir spectra to observe red giants throughout the galaxy. the first data appear in dr10. finally, the multiobject apo radial velocity exoplanet large-area survey (marvels) is monitoring the radial velocities of 11,000 bright stars to look for planets. 2.4 sdss iv: the future plans are underway to extend sdss from 2014-2020 with a continuation that involves apogee-2, eboss and manga. apogee-2 will continue the milky way exploration using apo and extend to the south with a 2.5m telescope at las campanas. eboss will continue boss but add 2 segments of interest to cvs: a time-domain spectroscopic survey (tdss ) that will obtain spectra of 100,000 variable sources and the spectroscopic identification of erosita sources (spiders). mapping nearby galaxies at apo (manga) will obtain spatially resolved spectra of 10,000 nearby galaxies. 3 current surveys several surveys are now ongoing and searching for objects that vary. the catalina real-time transient survey (crts; drake et al. 2009) consists of 3 telescopes: a 1.5m on mt. lemmon, a 0.7m on catalina and a 0.5m at siding springs. at the time of this meeting, 1022 potential cvs with outburst magnitudes brighter than 17 were posted on the web page nesssi.cacr.caltech.edu/catalina/brightcv.html. this page has a column that denotes whether the object is in the sdss photometric database and provides a direct link to sdss. since most of the objects were found at outburst, the quiescent magnitudes tend to be very faint (20-22nd mag) and therefore difficult to followup spectroscopically. several groups are now following up on the crts sources (woudt et al. 2012; thorstensen & skinner 2012). as noted above, elme breedt is also leading a project using gemini and other large telescopes to categorize the faintest crts sources that have 5 color photometry in sdss. the panoramic survey telescope & rapid response system (panstarrs1) is using an 1.8m telescope on haleakala, hawaii to complete a 2010-2013 northern sky transient survey which observes the available sky several times a month (tonry et al. 2012). one of the 12 key projects involves variables and explosive transients. a data release is planned for 2014 and a second telescope is under development. the palomar transient factory (ptf) involves the palomar 48 in schmidt telescope to image the sky from 2009-2014 to a magnitude of 21 on timescales from minutes to years to find new transients and variables, including cvs (rau et al. 2009). recent improvements (called iptf) have led to pipeline products that provides candidates within 30 minutes for spectroscopic followup. the followup spectroscopy is accomplished with the palomar 1.5m and other telescopes and some of that is available in wiserep (yaron & gal-yam 2012). skymapper is a robotic 1.35m survey telescope in siding springs, australia that is imaging the entire southern sky 36 times over 5 years in a series of 6 filters to 22nd mag that will provide spectral types of stars as well as variability information (keller et al. 2007). the data will be made public through the virtual observatory. in addition to these single, wide-field telescope surveys, there are 2 all-sky surveys which are using 2-4 cameras to image the sky. one is the all sky automated survey (asas) which images the entire sky to 14th mag in v and i bands from las campanas, chile and haleakala, maui (pojmanski 1997). the other is the mobile astronomical system of telescope-robots (master) which images to 19th mag at sites from russia and argentina (lipunov et al. 2010). 4 the future: lsst the future for investigation of variability lies with the large synoptic survey telescope (lsst). this 8.4m telescope situated in chile will image 18,000 deg2 of sky about 1000 times over 10 years (2020-2030). it uses 6 filters and will reach r = 24.5 mag on single nights and 27.5 on co-added images. the survey will produce alerts within minutes of observation as well as long term catalogs that will be made public. details may be found in the online science book (http://www.lsst.org/lsst/scibook). this survey will go several magnitudes fainter than sdss and should be able to find the population of period bounce systems that are predicted by models, as well as find unusual long term variability such as found by honeycutt et al. (2003) during their long term monitoring with roboscope. however, the difficulty lies in planning spectroscopic followup for 24-25 mag objects, which will require a lot of observing time on the largest telescopes available. time series photometry for short period, low amplitude variables will still be possible, but the problems of smart classification to pick out interesting variables from the multitude each night and the ensuing spectral confirmation remain to be solved. 5 conclusions the sdss has provided a significant database of cvs including a consistent set of medium resolution spectra for 285 systems and a photometric database that 57 p. szkody likely contains many more at faint magnitudes down to 22. due to the fainter magnitude limit compared to previous surveys, followup observations resulted in a large change in the observed orbital period distribution of cvs that has resolved some discrepancies in close binary evolution. current and future surveys rely on discoveries based primarily on variability, and so uncover large numbers of dwarf novae. as these surveys push further into the galactic plane and to fainter magnitudes with larger telescopes, the true space density of cvs and the distribution among types will become better known. however, the detailed information that comes from spectroscopy will be difficult to obtain for the faintest systems. acknowledgement the work with sdss data took place by many individuals over many years, starting with the sdss collaboration and extending to the public. support for the cv part of the research was provided by nsf grants ast 97-30792, ast 0607840 and ast 1008734 and nasa grant hst-go-12870.07a. funding for sdssiii has been provided by the alfred p. sloan foundation, the participating institutions, the national science foundation, and the u.s. department of energy office of science. the sdss-iii web site is http://www.sdss3.org/. sdss-iii is managed by the astrophysical research consortium for the participating institutions of the sdss-iii collaboration including the university of arizona, the brazilian participation group, brookhaven national laboratory, carnegie mellon university, university of florida, the french participation group, the german participation group, harvard university, the instituto de astrofisica de canarias, the michigan state/notre dame/jina participation group, johns hopkins university, lawrence berkeley national laboratory, max planck institute for astrophysics, max planck institute for extraterrestrial physics, new mexico state university, new york university, ohio state university, pennsylvania state university, university of portsmouth, princeton university, the spanish participation group, university of tokyo, university of utah, vanderbilt university, university of virginia, university of washington, and yale university. references [1] arras, p., townsley, d. m. & bildsten, l.: 2006, apj 643, l119. doi:10.1086/505178 [2] bhatti, w. a. et al. : 2010, apjs 186, 233. doi:10.1088/0067-0049/186/2/233 [3] breedt, e. & gänsicke, b. t.: 2011, aspcs 447, 203. [4] carter, p. j. et al.: 2013, mnras 429, 2143. doi:10.1093/mnras/sts485 [5] drake, a. j. et al.: 2009, apj 696, 870. doi:10.1088/0004-637x/696/1/870 [6] gänsicke, b. t. et al.: 2009, mnras 397, 2170. doi:10.1111/j.1365-2966.2009.15126.x [7] green, r. f., schmidt, m. & liebert, j.: 1986, apjs 61, 305. [8] hagen, h. j. et al.: 1995, aaps 111, 195. [9] honeycutt, r. k. et al.: 2003, aspcs 292, 329. [10] howell, s. b., nelson, l. a. & rappaport, s.: 2001, apj 550, 897. doi:10.1086/319776 [11] keller, s., bessell, m., schmidt, b. & francis, p.: 2007, aspcs 364, 177. [12] knigge, c.: 2011, aspcs 447, 3. [13] lipunov, v. et al.: 2010, adast 30l. [14] mukadam, a. s. et al.: 2013, aj 146, 54. [15] pojmanski, g.: 1997, aa 47, 467. [16] rau, a. et al.: 2009, pasp 121, 1334. [17] sako, m. et al.: 2008, aj 135, 348. [18] schmidt, g. d. et al.: 2005, apj 630, 1037. [19] sesar, b. et al.: 2007, apj 134, 2236. [20] szkody, p. et al.: 2003, aj 126, 1499. [21] szkody, p. et al.: 2010, apj 710, 64. doi:10.1088/0004-637x/710/1/64 [22] szkody, p. et al.: 2011, aj 142, 181. [23] szkody, p. et al.: 2013, apj 775, 66. doi:10.1088/0004-637x/775/1/66 [24] thorstensen, j. r. & skinner, j. n.: 2012, aj 144, 81. [25] tonry, j. l. et al.: 2012, apj 750, 99. doi:10.1088/0004-637x/750/2/99 [26] voges, w. et al.: 1999, aap 349, 389. [27] wickramasingeh, d. t. & ferrario, l.: 2000, pasp 112, 873. [28] woudt, p. et al.: 2012, mnras 421, 2414. [29] yaron, o. & gal-yam, a.: 2012, pasp 124, 668. doi:10.1086/666656 [30] york, d. g. et al.: 2000, aj 120, 1579. 58 http://dx.doi.org/10.1086/505178 http://dx.doi.org/10.1088/0067-0049/186/2/233 http://dx.doi.org/10.1093/mnras/sts485 http://dx.doi.org/10.1088/0004-637x/696/1/870 http://dx.doi.org/10.1111/j.1365-2966.2009.15126.x http://dx.doi.org/10.1086/319776 http://dx.doi.org/10.1088/0004-637x/710/1/64 http://dx.doi.org/10.1088/0004-637x/775/1/66 http://dx.doi.org/10.1088/0004-637x/750/2/99 http://dx.doi.org/10.1086/666656 cataclysmic variables from sdss: a review and a look forward to lsst figure 1: the sky coverage of the legacy and segue surveys in galactic (left) and celestial (right) coordinates, from www.sdss.org/dr7. discussion linda schmidtobreick: wouldn’t you expect the pulsating white dwarfs in accreting systems to be hotter than single white dwarfs? due to accretion, you would get a thin hot layer that influences the measured temperature but not necessarily the pulsation. paula szkody: yes, most accreting white dwarfs are indeed observed to be hotter than single white dwarfs. however, there are several parameters that could affect the pulsations (instability strip) in these accreting white dwarfs besides the temperature. the accreting ones are spun up by the accretion and the composition of the atmosphere is different due to the mass transfer from the secondary. right now, we don’t have enough data to distinguish which of these parameters are determining whether an accreting white dwarf will pulsate or not. 59 introduction sdss sdss i,ii legacy survey sdss ii: segue and sn sdss iii: segue-2, boss, apogee and marvels sdss iv: the future current surveys the future: lsst conclusions 246 acta polytechnica ctu proceedings 2(1): 246–251, 2015 246 doi: 10.14311/app.2015.02.0246 recurrent novae — a review k. mukai1,2 1cresst and x-ray astrophysics laboratory, nasa/goddard space flight center, greenbelt, md 20771, usa 2department of physics, university of maryland, baltimore county, 1000 hilltop circle, baltimore, md 21250, usa corresponding author: koji.mukai@nasa.gov abstract in recent years, recurrent nova eruptions are often observed very intensely in wide range of wavelengths from radio to optical to x-rays. here i present selected highlights from recent multi-wavelength observations. the enigma of t pyx is at the heart of this paper. while our current understanding of cv and symbiotic star evolution can explain why certain subset of recurrent novae have high accretion rate, that of t pyx must be greatly elevated compared to the evolutionary mean. at the same time, we have extensive data to be able to estimate how the nova envelope was ejected in t pyx, and it turns to be a rather complex tale. one suspects that envelope ejection in recurrent and classical novae in general is more complicated than the textbook descriptions. at the end of the review, i will speculate that these two may be connected. keywords: cataclysmic variables symbiotic stars recurrent novae individual: t pyx. 1 introduction nova eruptions are understood to be powered by thermonuclear runaway (tnr) on the surface of accreting white dwarfs. hundreds of objects in the galaxy have been seen to experience one nova eruption: these are called classical novae (cne). recurrent novae (rne) are objects that have been seen to experience multiple nova eruptions. there are currently 10 confirmed rne in the galaxy. between 10−6 and 10−4 m� of hydrogen rich material needs to be accreted to reach the critical temperature and density required for tnr. the critical mass is lower for more massive white dwarfs with higher gravity. therefore, we expect rne to contain near chandrasekhar mass white dwarf accreting at a high rate. this makes rne candidate progenitors of type ia supernova. for this reason, and because the recurrent nature of these objects allows studies that one cannot undertake for cne, rne have become the subject of intensive study. it is impossible to present a comprehensive review of rne in the space allotted; for that, the readers are referred to schaefer (2010) and anupama (2013). in this review, i will present selected highlights from multiwavelength campaigns on recent rn outbursts, highlighting the work of the e-nova collaboration1. i will also include results on several cne: some of these system may be unrecognized or unconfirmed rne, and others provide a useful comparison. i will also present some quiescent observations. i will discuss implications on the white dwarf mass, the ejecta mass, the quiescent accretion rate,and the evolutionary scenarios for rne and cne. 1.1 x-ray bursts: a cautionary tale although rne provide a unique opportunity to compare multiple nova episodes and possibly to compare accreted vs. ejected mass, only a handful of eruptions are observed for each system. this is in stark contrast to the studies of x-ray bursts, which are tnrs on accreting neutron stars. for example, linares et al. (2012) studied 398 x-ray bursts detected from the transient x-ray binary in the globular cluster, terzan 5, as the accretion changed by a factor of ∼5. this allowed these authors to study the relationship between the persistent luminosity, the burst recurrence time and the burst fluence, and thereby test the theory of tnr on neutron stars. unfortunately, analogous tests have not been possible yet in the case of rne. yet, even in the case of x-ray bursts, puzzles remain (galloway et al. 2008). one is the burst oscillations observed during the decay. the drifting period of burst oscillations reflect the spin period of the neutron star atmosphere, which changes as the atmosphere expands and then contracts during the course of a burst. the presence of the oscillations during the decay, however, requires inhomogeneous burning over the neutron star 1https://sites.google.com/site/enovacollab/ 246 http://dx.doi.org/10.14311/app.2015.02.0246 recurrent novae — a review surface, even though one might expect uniform burning at this stage. the other is that pairs of bursts can occur with very short (<10 min) recurrence times, much too short to have accreted sufficient fuel for a new burst, judging by the persistent x-ray luminosity. this requires a reservoir of unburnt fuel on or very near the neutron star surface. thus, our theoretical understanding of x-ray bursts appears incomplete. it may well be that the current theories of nova outbursts are equally incomplete regarding, e.g., the recurrence times of rne. 2 selected recent results 2.1 ejecta geometry montez et al. (in preparation) have detected extended x-ray emission in the chandra observations of rs oph obtained in 2009 and 2011. these structures are wellseparated from the central x-ray source in the e-w direction, and were seen to expand from 2009 to 2011. this x-ray emitting bipolar outflow appears to follow the same angular expansion curve inferred for radio and hubble space telescope (hst) bipolar structures observed earlier. the implied current expansion velocity is very high (of order 4,000 km s−1. one possible origin of the bipolar flow is that rs oph produced a true, well-collimated, jet near the time of nova eruption. another is that an initially spherical ejecta encountered an equatorial torus and slowed down except in the polar directions. since rs oph is an rn in a symbiotic binary, the wind of the giant mass donor is a potential source of such a torus (mohamed et al. 2013). however, similar shaping of the ejecta might also occur in cataclysmic variables (cvs), with a roche-lobe filling mass donor on or near the main sequence. in a series of simulations of the 2010 eruption of u sco by drake & orlando (2010), the accretion disk is destroyed by the blast wave. this interaction causes the ejecta to expand away from the orbital plane. one particular simplifying assumption used by these authors, that of a uniform density disk, is a cause for concern, and independent simulations are needed to confirm their results in general. nevertheless, the possibility that disk-blast wave interactions create bipolar outflow should be kept in mind for all novae, whether the underlying binary is a symbiotic system or a cv. the above-mentioned results on rs oph and u sco are both about the outflow during the most recent outbursts of rne, and may apply to cne as well. in contrast, one type of study unique to rne is the analysis of light echoes produced by ejecta from previous outbursts, as sokoloski et al. (2013) did for t pyx. the arrangement of the echo location on the sky and the progression of echos from east to west suggest a ringlike structure from a previous outburst. the delay times for echoes along the north-south axis suggest a distance of 4.8±0.5 kpc for t pyx. moreover, the time lags between different echoes suggest that the ring is inclined ∼30–40◦ relative to the plane of the sky. this is most likely to reflect the binary inclination, somewhat higher than values previously inferred for t pyx. regardless of the precise inclination angle, the very fact that an equatorial ring was formed by the ejecta is worth noting. 2.2 novae in symbiotic systems four of the known galactic recurrent novae are in symbiotic binaries: rs oph, t crb, v745 sco, and v3890 sgr. they are all s type systems: they have a normal red giant mass donor, an orbital separation of order 1 au, and an orbital period of order 1–2 years. in the other subtype, the d (dusty) type, the mass donor is an agb star; the d type systems have a much wider orbit than the s type systems. before 2010, all known tnr events in symbiotic systems were either a very slow “symbiotic novae,” or very fast rne (mikolajewska 2008). it is important to note that tnr can lead to a quasi-static configuration without explosive mass loss in symbiotic novae. also noteworthy is the fact that the accretion rate is high enough in the 4 s type symbiotic systems to produce rne. if we take 10−7 m� yr−1 as the typical wind mass loss rate of a normal red giant, then this implies either roche-lobe overflow or a very efficient mechanism to capture the wind, such as wind roch-lobe overflow (mohamed & podiadlowski 2010), although m giants in symbiotic binaries may have higher mass-loss rates (seaquist & taylor 1990). in march 2010, a d-type symbiotic system, v407 cyg, became a nova. it was noteworthy for being the first nova to be detected as gev γ-ray source with fermi lat (abdo et al. 2010); it was the subject of an intensive multiwavelength from radio to x-rays (nelson et al. 2012; chomiuk et al. 2012). the x-rays were predominantly from the shock between the nova blast wave and the wind of the mira type mass donor; interestingly, v407 cyg became x-ray bright after the gev signal faded. the thermal emission from the flashionized agb wind was the dominant source of radio signal. while we learned a lot about the nova event, we are left with one important question: how often does v407 cyg experience nova eruptions? is it an unrecognized rn, or are the eruptions much less frequent? 2.3 long period cvs darnley et al. (2012) proposed to classify novae into red giant, sub-giant, and main sequence systems. the orbital periods of the “sub-giant” systems are in the range 10 hrs to 6 days. according to the ritter & kolb catalog version 7.20 (ritter & kolb 2003), there are 46 247 k. mukai systems (excluding one uncertain entry in the catalog) in this orbital period range. of the confirmed rne, v894 cra, u sco and ci aql belong to this group, and a fourth, v2487 oph, probably has an orbital period in this range. the evolution of such long-period cvs has not been studied extensively to date, compared to those with periods under 10 hrs, for which the basic framework and much more have been established (knigge et al. 2011). cvs with similarly long orbital periods include several novae not known to be recurrent (gk per, v1017 sgr), supersoft sources (e.g., mr vel, cal 83), and v sge and other systems that may be related to supersoft sources. one possibility is that these systems are currently undergoing thermal timescale mass transfer (ttmt; schenker et al. 2002) or did so in the past. the evolution of cvs with p>10 hrs should be studied further, and a search for additional nova outbursts of systems like gk per may be worthwhile. 2.4 quiescent x-ray observations figure 1: possible white dwarf mass of v2487 oph for the magnetic (dashed line) and the non-magnetic (solid line) cases of the known recurrent novae, t crb and v2487 oph are bright hard x-ray sources detected with integral and with swift bat (see, e.g., baumgartner et al. 2013). when such hard x-ray emission is detected, it can constrain both the white dwarf mass and the accretion rate in quiescence. in both magnetic and nonmagnetic cases, hard x-rays are generated when the supersonic accretion flow encounters the white dwarf surface and is shock-heated. the hot plasma must radiatively cool before settling onto the white dwarf surface. in the case of magnetic cvs, the accretion flow is radial and its velocity is approximately at the freefall value (aizu 1973). the observed x-ray spectrum is multi-temperature in nature, with shock temperatures typically in the 10–50 kev range. the spectral curvature in the hard x-ray range is a reliable measure of the shock temperature, and can be used to infer the white dwarf mass (see, e.g., yuasa et al. 2010). the situation may well be more complex for the boundary layer of non-magnetic cvs, but the maximum temperature is unlikely to exceed that expected for strong shocks from keplerian velocity, and quiescent dwarf novae appear to follow the “keplerian strong shock” relationship (byckling et al. 2010). some authors have speculated that v2487 oph may contain a magnetic white dwarf, based solely on its bright, hard x-ray emission. however, i measure a shock temperature of ∼50 kev using the bat survey 8-channel spectrum, which corresponds to either a ∼0.9 m� magnetic white dwarf or a >1.24 m� non-magnetic white dwarf. given the rn nature of this object, and given that repeated xmm-newton observations have not revealed a spin modulation, the latter interpretation seems more likely. 2.5 high mass transfer rate: evolutionary or temporary? patterson et al. (2013) made the following simple prediction, based on average secular mass accretion rate: cvs above the period gap should experience nova eruptions once every 10,000 years or so, while those below the gap should recur once every 1 million year. for these normal cvs to be a rn, the accretion rate needs to be elevated by many orders of magnitudes above the secular mean. this is in contrast to the long-period systems with a sub-giant mass donor, where ttmt may drive a very high accretion rate. similarly, in symbiotic systems, the mass loss rate from the donor is high enough, although the fraction that can be captured by the white dwarf is highly uncertain. let us now consider the census of known classical and recurrent novae of various types with the above expectations in mind. among symbiotic stars, most novae are extremely slow and often referred to as symbiotic novae, the slowness suggestive of low-mass white dwarfs. the four wellknown symbiotic recurrent novae are all in s-type systems: the short recurrence time and the high velocity of the ejecta both suggest these to have massive white dwarfs. the bifurcation of novae in symbiotic stars into two such extreme groups is very different from the situation in cvs, and should be investigated further. although only a single outburst is known, the outburst properties of v407 cyg makes it a candidate rn in this context. since it is in a d type symbiotic system, with a much greater binary separation, an estimate of its quiescent accretion rate, when one is obtained, may tell us about how white dwarfs accrete in symbiotic stars, not to mention providing a clue as to its likely recurrence time. u sco, ci aql, and v894 cra are long-period systems; v2487 oph may belong in this group, although its orbital period has not been determined yet. the ttmt 248 recurrent novae — a review scenario points to the general framework for why these systems can be rne. more research is needed to understand why some long-period systems are sss, others rne, and yet others cne (as far as we know). this leaves im nor (in the period gap) and t pyx (below the gap) in the orbital period range with very low secular accretion rate and with only a small number of known classical novae (v1794 cyg, v per, ...). it is interesting to note that, while many classical novae are known above the gap (orbital period in the 3–10 hr range), no rne are known in this regime. this could purely a matter of small number statistics. at the same time, it could be that the mechanism elevating the accretion rate of t pyx and im nor far above the secular mean is much less effective for systems above the gap. 2.6 multi-wavelength observations of t pyx multi-frequency radio monitoring of novae is a powerful technique that allows us to estimate the total ejected mass in a relatively simple manner, although complications often arise (see, e.g., roy et al. 2012 and references therein.) the nova ejecta is initially optically thick in the radio, thus the brightening traces the angular expansion of the ejecta. as the ejecta becomes optically thin, first from the highest frequencies then progressively to lower frequencies, this allows the amount of mass to be estimated, as long as we have a handle on the temperature and clumping of the ejecta. at the same time, we know that early x-ray emission from novae is likely due to shocks in the ejecta (e.g., o’brien et al. 1994). with this mind, the e-nova collaboration has begun multi-wavelength observations of recent novae using the much improved karl g. jansky vla in the radio, and swift and other observatories in the x-rays. t pyx is one of the major targets of the e-nova collaboration. the radio and x-ray results are presented by nelson et al. (2014) and chomiuk et al. (2014), respectively. t pyx was largely undetected in the radio for the first ∼60 days since the discovery of the 2011 outburst, then started to rise around day ∼100. it was also x-ray faint during the first several months, and then started to rise slightly after the onset of the radio rise. the x-ray photons detected with swift xrt are a mixture of optically thin emission from the shocked shell and the supersoft emission from the still nuclear burning surface. it is difficult to disentangle the two using short snapshot observations, hence it is difficult to determine, e.g., the turn-on time of the supersoft emission. in the optical, t pyx remained near peak optical magnitude for 2 or 3 months, depending on where one defines the peak to have ended and decline to have begun. this implies a large photospheric radius, perhaps of order 5×107 km (∼ a third of an au; assuming a blackbody with a temperature of 10,000k, a distance of 4.8 kpc, and av ∼ 1.0, this radius corresponds to a visual magnitude of v∼7.9). this is well outside the central binary, yet it only takes of order 1 day for matter traveling at 600 km s−1 to reach this distance. for a shell at a distance of 1×108 km to have an optical depth of 1, which implies a column density of order 3 g cm−2 (assuming electron scattering dominates the opacity), the total mass of the shell must be greater than ∼ 5.0 × 10−7 m�. so the duration of optical peak implies either a continuous ejection of ∼ 5× 10−7 m� per day for several months, or that t pyx went into a quasi-static, red giant-like configuration during the peak, and ejected the extended atmosphere with a significant delay. in the latter case, the total mass of the extended atmosphere must be much greater than 5×10−7 m�, because it is a filled sphere and not a thin shell. schaefer et al. (2013) presented the detailed visual light curve of t pyx during the initial rise. until it reached v∼7.7, it can be modeled well assuming a uniform expansion of the ejecta, then the observed brightness drops below this model. if we equate this instance with the time when the optical depth of the ejecta dropped below 1.0, we infer that an initial shell of ∼ 6× 10−7 m� was ejected. such a small ejection is easy to hide in the radio data, although there is one detection on day 17 that could be interpreted as due to this. on the other hand, continuous mass ejection of ∼ 5 × 10−7 m� per day is difficult to reconcile with the deep radio non-detections followed by rapid brightening around day 60. rather, the radio data are consistent with a prolonged period of quasi-stationary configuration, and a delayed ejection of a more massive (of order 10−5 m�) shell. in addition, if the latter system was ejected with a larger velocity, then we expect shocked x-ray emission when it catches up with the initial ejecta. the existing x-ray data are broadly consistent with such a picture. 2.7 the cause of elevated mass transfer rate for t pyx, we have a clear-cut case that the mass transfer rate is elevated by several order of magnitudes above the evolutionary mean (gilmozzi & selvelli 2007). the same presumably applies to im nor as well. irradiation of the donor is often invoked as the explanation. however, theorists have long concluded that this requires hard photons, and therefore theoretical studies largely focus on x-ray binaries (see, e.g., hameury et al. 1986; king 1989). to quote from ritter (2000), “energy emitted in certain spectral ranges, 249 k. mukai as e.g., euv radiation and soft x-rays, is unlikely to reach the photosphere of the donor.” to reach the photosphere of k or m type dwarfs, irradiating flux needs to be able to penetrate above 1024 cm−2 of column, thus requiring strong flux above 10 kev. therefore, supersoft sources or photospheric emission of otherwise very hot white dwarf only irradiates the chromosphere and above, and not the photosphere. the irradiation mechanism studied in above-mentioned papers cannot work when the irradiating flux is in the form of soft x-ray and euv photons. while v1500 cyg is sometimes taken as an example of a system that is experiencing enhanced accretion rate due to irradiation, this is not necessarily the case. this is because one well-established effect of irradiation is to increase the luminosity of the existing structures, be it the secondary or the accretion disk. in fact, according to somers & naylor (1999), the elevated brightness of v1500 cyg, which is currently an asynchronous polar due to its 1975 nova eruption, is due to an orbitally modulated component, not due to the spin modulated component. since accretion luminosity should be modulated on the spin period, we know that the extra light is due to the irradiated face of the secondary. in this picture, the reflection of the gradually decreasing postnova white dwarf flux explains the secular changes in the brightness of v1500 cyg, without invoking variable accretion rate. thomas et al. (2008) obtained phase-resolved kband photometry of old novae of various ages since outburst. they were also able to interpret their results without invoking changing accretion rate. rather, in their interpretation, variable irradiation, and hence variable reflection, changes the brightness of the existing structures, the accretion disk and the secondary. at a minimum, this points out that an enhanced brightness is insufficient to prove an enhanced accretion rate. these studies also suggest that irradiation by a postnova does not lead to enhanced mass transfer, although they do not yet constitute a solid proof. if not irradiation, what other mechanisms can potentially enhance the mass transfer? here i speculate that the post-nova common envelope phase might be ultimately responsible, as follows. the ring geometry of the ejecta of t pyx (§2.1) suggests that the secondary plays a role in shaping the geometry of the ejecta. in symbiotic systems, slow novae can stay in the “plateau” phase for decades, whereas no such example is known among cvs, again implying that the secondary plays a role in ejecting the nova envelope. the potential role played by the common envelope system was first pointed out by macdonald (1980) and later studied quantitatively by, e.g., livio et al. (1990). the general consensus is that the common envelope phase can contribute to the ejection of the nova envelope, but only if the ejecta is moving more slowly than the orbital motion of the mass donor. the multi-wavelength data on t pyx indeed suggests that the envelope may have been in a quasi-stationary configuration for 2–3 months, as it is for decades in slow symbiotic nova. the possibility that many novae in cvs may not be able to eject the bulk of the envelope, if it were not for the common-envelope phase, should therefore be investigated. if common envelope phase is an important factor in the envelope ejection process, then this implies that the binary must have lost angular momentum. while the orbital period of t pyx was seen to increase after the 2010 eruption (patterson et al., this volume), the period increase only constrains the combination of ejected mass and angular momentum loss. if more than the minimum allowed mass was ejected, so was angular momentum. so the proposed scenario is that of a common-envelope interaction during the nova eruption, resulting in an impulsive angular momentum loss, which drives a higher accretion rate for the ensuing decades. i suggest that such a scenario deserves serious, quantitative analysis. 3 summary and conclusions recent rn eruptions have be subjected to intense multi-wavelength observing campaigns using advanced facilities, including the karl g. jansky vla, many ground-based optical/ir telescopes, hst, swift and other x-ray observatories. although not a new discovery as such, recent images of nova ejecta demonstrates once again that they are not spherically symmetric. in the case of t pyx, the multi-wavelength data strongly suggest that there was a initial, small ejection and a much larger, delayed ejection. these facts together suggest that the binary companion, via common-envelope interaction, may be involved in the ejection process. it is expected that rne harbor massive white dwarfs accreting at a very high rate. while the accretion rate may be high in some subset of rne for evolutionary reasons (symbiotic rne, and long-period systems), this is definitely not the case for t pyx and im nor. unfortunately, there is a serious difficulty with the commonly invoked mechanism of irradiation, when the irradiation is by soft x-ray and euv photons. i presented a possible scenario involving impulsive angular momentum loss during the common envelope phase. acknowledgement i thank my colleagues in the e-nova collaboration, particularly drs. nelson, sokoloski, and chomiuk, for stimulating discussion. 250 recurrent novae — a review references [1] abdo, a.a. et al.: 2010, sci. 329, 817. doi:10.1126/science.1192537 [2] aizu, k.: 1973, prog theor. phys. 49, 1184. doi:10.1143/ptp.49.1184 [3] anupama, g.g.: 2013, iausymp 281, 154. 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[35] yuasa, t. et al.: 2010, a&a 520, a25 discussion christian knigge: regarding the possibility that irradiation might enhance the mass accretion rate: i think that an enhancement is possible, at least via irradiation-driven winds from the donor. in fact, this model was first proposed for sss, where the irradiation is all in the soft x-ray/euv regime. koji mukai: it is true that the viability of irradiation-driven wind to enhance the accretion rate is a separate issue. i feel that sometimes the direct effect of irradiation is often too casually invoked as the explanation, and my intent was to advise caution. 251 http://dx.doi.org/10.1126/science.1192537 http://dx.doi.org/10.1143/ptp.49.1184 http://dx.doi.org/10.1088/0004-637x/761/2/173 http://dx.doi.org/10.1088/0004-637x/788/2/130 http://dx.doi.org/10.1088/0004-637x/746/1/61 http://dx.doi.org/10.1088/2041-8205/720/2/l195 http://dx.doi.org/10.1086/592044 http://dx.doi.org/10.1093/mnras/241.3.365 http://dx.doi.org/10.1088/0067-0049/194/2/28 http://dx.doi.org/10.1088/0004-637x/748/2/82 http://dx.doi.org/10.1086/168836 http://dx.doi.org/10.1088/0067-0049/187/2/275 http://dx.doi.org/10.1088/0004-637x/773/1/55 http://dx.doi.org/10.1046/j.1365-8711.2002.05999.x http://dx.doi.org/10.1088/2041-8205/770/2/l33 introduction x-ray bursts: a cautionary tale selected recent results ejecta geometry novae in symbiotic systems long period cvs quiescent x-ray observations high mass transfer rate: evolutionary or temporary? multi-wavelength observations of t pyx the cause of elevated mass transfer rate summary and conclusions 293 acta polytechnica ctu proceedings 2(1): 293–296, 2015 293 doi: 10.14311/app.2015.02.0293 non relational models for the management of large amount of astronomical data b. l. martino1, m. federici 2 1associated inaf iaps 2istituto di astrofisica e planetologia spaziali, inaf iaps via fosso del cavaliere 100, 00133 roma, italy corresponding author: brunolmartino@iasi.cnr.it abstract the objective of this paper is the comparison between two different database typologies: the relational and the nonrelational architecture, in the context of the applications related to the use and distribution of astronomical data. the specific context is focused to problems quite different from those related to administrative and managerial environments within which were developed the leading technologies on which are based the modern systems of massive storage of data. the data provided by astronomical instrumentation are usually filtered out by the front-end system (trigger, anticoincidence, dsp etc.), so they do not require special controls of congruence. moreover, the related storage systems must be able to ensure an easy growth, minimizing human systemistic interventions and automating the related actions. the use of a non-relational architecture (nosql), offers great advantages during the insertion of informations within a data base, while the response speed of the queries is mainly tied to their type and complexity. keywords: dbms mongodb nosql mysql gsc catalog. 1 using dbms a careful planning of the use of a file system allows to store informations in a rational way but, whatever the criterion used to organize an archive based only on files, it is not possible to build a search system that can guarantee sufficient flexibility. the user of a database, conceived in this manner, is required to know in detail its structure and it’s organization. almost any application focused on the analysis of astronomical data may read data in fits format (wells et al., 1981) fits format allows to add to a collection of data a set of additional information used to allow their better characterization. a dbms (data base management system) is a software infrastructure designed to operate on large data sets with the goal of optimizing the data: • storage • access • sharing • protection a dbms allows, through the use of its command language, the imposition of constraints of consistency, the creation of indexes to improve performance and retrieval of data independently from their physical representation. well-known examples of the use of advanced databases in high-energy physics are represented by spires (bourne et al., 2003), opera (agafonova et al., 2009) and !chaos (bisegni, 2012) and is consolidated practice their adoption as a storage medium in the ground segment. 1.1 relational models the relational databases today are the most popular; the model they are based it was proposed by codd (1970). the relational model allows access to data at the logical level by providing a complete independence from their physical organization. the logical representation of the data in the relational model is based on the concept of relationship, in algebraic terms; it is common to use the term ”table” in place of ”algebraic relationship” and the term ”relationship” to indicate an association between the data. the language sql (date et al., 1997) is the standard language ”de facto” for defining, manipulating and interrogation of relational databases; this is a declarative language and not procedural. in the relational model a logical unit of work is defined transaction and is constituted by a sequence of operations of reading and writing, which must meet some properties, known as acid property (from atomicity, consistency, isolation, durability). atomicity: a transaction is an atomic unit of operations, validated or canceled depending on whether or not they reach a successful conclusion (rollback / commit); consistency: at 293 http://dx.doi.org/10.14311/app.2015.02.0293 b. l. martino, m. federici the end of the transaction if the initial state is correct, even the final state must be; isolation: the action of a transaction should not interfere with each other; duration: the effects of a transaction must be persistent; the most popular relational databases are oracle (kunh, 2010) and mysql (schwartz, 2012)). 1.2 non relational models recently, have been developed a series of new dbms systems, to provide an high horizontal scalability, in order to achieve high performance in the read / write operations of database distributed across multiple servers geographically delocalized (cloud). many of these new systems are called nosql data stores. the definition of nosql, which stands for ”not only sql” or ”not relational”, was used for the first time in 1998 for an open source relational database. the non-relational systems, do not attempt to provide the classic acid guarantees, typical of relational databases, but embrace the model b.a.s.e. (basic available, soft-state, eventual consistency) where some constraints are relaxed: basic available: the database can operate even if a part is no longer available; soft-state: the system status may change over time even in absence of input data; eventual consistency: the data may not be updated immediately, but will be consistent throughout the system within a finite time as stated by the cap theorem (lynch et al., 2002) can not be achieved consistency, availability and partition tolerance at the same time but only two of these features at a time. the systems based on nonrelational architecture follow the base model and are those that allow to overcome the major limitation of the rdbms (relational data base management system): the scalability. in many areas, consistency and/or availability offered by relational databases are not essential, e.g. in astronomy and astrophysics applications. in a highly available and tolerant partitioning system, alteration of data base will reach all nodes not instantly but within a finite time, if a reading is done on a node is not synchronized with the last write, it returns the last valid data (stale data). 2 dbms nosql a property of nosql database is to be free of pattern (schemaless), with consequent advantages and disadvantages like ease of deployment, but sometimes, more difficulties to construct complex queries. the nonrelational logical models not have the same expressive power of the relational model and can be classified into four main families: key-value, column-family, document store and graph. the most common are the mongodb nosql architecture (chodorow, 2011), hbase (george, 2011) and cassandra (lakshman et al., 2010); their characteristics are summarized in figure 1. mongodb is a document-oriented database, which is based on aggregates, which may have a structure with multiple hierarchical levels and groups that can be variously indexed. mongodb uses the javascript language, which is inherently single-threaded while cassandra uses the language cql (cassandra query language, simplified variant of sql), which allows the management of distributed databases. both rely on the file system of the machine on which they were installed without introducing data abstractions. hbase is a distributed database (based on the project hadoop (sammer, 2012) written in java, that uses storage devices located on different hosts also geographically distributed, interconnected via networks. hbase uses hdfs, a portable and scalable distributed file system initially developed for the framework hadoop. figure 1: key features of nosql architectures. to carry out complex operations is used the mapreduce paradigm (dean et al., 2004), which allows to divide the computation in many elaborations of lower complexity to achieve the processing of large datasets in parallel on multiple cores/cpu/computer. 3 mysql vs mongodb in order to evaluate the possible use with advantage of non-relational architectures in astronomy and astrophysics area we realized the mdbirs system (showed in figure 2), composed by 10 pc, equipped with intel i7, 16gbytes of ram, 2 hd 1tbytes connected via ethernet lan at 1gbit of speed figure 2: the mdbirs system. 294 non relational models for the management of large amount of astronomical data in order to assess objectively the behavior of the system tests were carried out so as not to make use of the advanced characteristics of the query languages used. for this reason it was chosen as the working set of data the gsc catalog (stars general catalog). gsc is composed of a single table and is used primarily to provide support for the planning and guiding stars of the hst observations (dalcanton, 2009), the jwst (gardner at al., 2006), gaia (busso et al., 2012) and some groundbased telescopes of new generation. furthermore, the services provided by the machines on which they are installed databases are not affected by processes (user or system) not strictly essential to their functioning and the software installed on the machines which host the test has been aligned to the same versions. the databases under test are the two most widely used open source architectures in the world, mysql for its versatility, speed and diffusion and mongodb for its attitude to the horizontal growth (scalability) and its robustness. starting from the data available, have been generated some of the series of samples of increasing amplitude, so as to highlight the behavior of the two systems both with regard to populate the database, than to retrieve the information of interest. the test was performed using queries significant from the point of view of the community of users, which allow to highlight the behavior of databases in real operating conditions. in particular: • query1: selection on healpix (mapping system applied to the celestial sphere) • query 2: selection of objects in a spatial region which satisfy some conditions in magnitude and color • query 3: selection of non-stellar objects which meet a condition in color • query 4: computing of the average magnitudo, resolution of one degree square 4 test results the database engine installed by default in mysql since version 5.0 (innodb) is transactional, acid compliant and uses the row-level locking strategy (constraint of exclusive use for the time necessary to perform the required actions). the data entry test was made from data provided in csv (text data whose fields are marked by the separator character comma.) as shown by the graph in figure 3, the behavior of mongodb is faster than mysql, thanks to the absence of checks carried out on the integrity of the data; information are stored in files as key-value pairs. figure 3: insert performances vs data set size. regarding the execution time of the test queries, the results are shown in the graph of figure 4 and are substantially comparable. in the case of the gsc catalog , the occupation of the relevant mysql table is about 116 gbytes of data collection while the corresponding mongodb occupies about 657 gbytes (the size is 5 times bigger). the tables were not associated with indexes, so searches are performed on the whole set of data with a time dependent to amplitude of the set itself. figure 4: queries performances vs data set size. 5 future activities mongodb is a good choice to obtain good safety and performances using local data sharding. our goal is to achieve a full geographic data delocalization (cloud) in order to obtain: continuity of access to data (total disaster recovery) to the user community optimized average access time we evaluated two possible scenarios: hbase: hadoop based nosql data base cassandra: the apache cassandra database hbase is a masterslave system including two types of machines: hmaster: access control hregionserver: local data replication by using the stargate plugin can be achieved local caches able to speed up response times by minimizing the network traffic to the master. every region server keeps a copy of the data so the system ensures very 295 b. l. martino, m. federici high reliability. it is a nosql solution initially developed by facebook which has in the p2p architecture its focal point. 6 conclusions mongodb has proven much more efficient and fast in data entry and is particularly suited to the management of flows of data to be stored without downtime. in the configuration we used the rate of loading is about 15000 rows/sec (the average rows length is 256 bytes). by using more powerful hardware such as high speed network interfaces (10 gbit/sec or higher) it’s possible to greatly increase the speed of data acquisition. regarding the search of data in the database the results show a behavior strongly dependent on the number of records and the presence or absence of critical elements such as complex calculations. the command language of mongodb does not contain any advanced mathematical primitives as in the case of mysql, which can rely on a large library of mathematical functions. the use of mongodb on structures designed to be handled by a rdbms is inappropriate because it does not exploits the potential of its schemaless organization (nosql). acknowledgement we thanks paola marrese by asdc-asi for providing us the test data, , the director of iaps-inaf prof pietro ubertini, and a special thanks to franco giovannelli and his staff for allowing us to participate to this workshop references [1] wells, d. c., greisen, e. w., and harten, r. h.: 1981, a flexible image transport system. astronomy & astrophysics supplement series, 44, 363-370. [2] bourne, c. p., hahn, t. b.: 2003, a history of online information services, 1963-1976, mit press. [3] agafonova, n., et al.: 2009, the detection of neutrino interactions in the emulsion/lead target of the opera experiment, journal of instrumentation. doi:10.1088/1748-0221/4/06/p06020 [4] bisegni, c.: 2012, thesis in computer science, university tor vergata, rome. [5] codd, e.f.: 1970, a relational model of data for large shared data banks, communications of the acm 13 (377-387). doi:10.1145/362384.362685 [6] c. j. date, c. j., darwen, h.: 1997, a guide to the sql standard: a users guide to the standard database language sql, addison wesley. [7] kuhn, d.: 2010, pro oracle database 11g administration, apress. doi:10.1007/978-1-4302-2971-1 [8] schwartz, b.: 2012, high performance mysql: optimization, backups, and replication, o’reilly. [9] lynch, n., gilbert, s.: 2002, brewer’s conjecture and the feasibility of consistent, available, partition-tolerant web services, acm sigact news, volume 33 issue 2, pg. 51-59. [10] chodorow, k.: 2011, scaling mongodb, o’reilly. [11] george, l: 2011, hbase: the definitive guide, o’reilly. [12] lakshman, a., malik, p.: 2010, cassandra: a decentralized structured storage system, acm sigops operating systems review, vol. 44, no.2. doi:10.1145/1773912.1773922 [13] sammer, e.: 2012, hadoop operations, o’reilly. [14] dean, j., ghemawat, s.: 2004, mapreduce: simplified data processing on large clusters, in osdi’04: sixth symposium on operating system design and implementation, san francisco, ca, december. [15] dalcanton, j.j.: 2009, 18 years of science with the hubble space telescope, nature jan 1;457(7225):41-50. [16] gardner, j. p., at al.: 2006, the james webb space telescope, space science reviews. doi:10.1007/s11214-006-8315-7 [17] busso, g., de angeli, f., montegriffo, p.: 2012, the gaia photometric data processing, in proc. spie 8442, space telescopes and instrumentation 296 http://dx.doi.org/10.1088/1748-0221/4/06/p06020 http://dx.doi.org/10.1145/362384.362685 http://dx.doi.org/10.1007/978-1-4302-2971-1 http://dx.doi.org/10.1145/1773912.1773922 http://dx.doi.org/10.1007/s11214-006-8315-7 using dbms relational models non relational models dbms nosql mysql vs mongodb test results future activities conclusions 156 acta polytechnica ctu proceedings 2(1): 156–160, 2015 156 doi: 10.14311/app.2015.02.0156 outburst properties of v1504 cyg and v344 lyr j. k. cannizzo1,2 1cresst, dept. of phys., umbc, baltimore, md 21250, usa 2nasa/gsfc/astroparticle physics lab., greenbelt, md 20771, usa corresponding author: john.k.cannizzo@nasa.gov abstract i begin by reviewing dwarf novae and the disk instability theory, and then present an overview of three ideas for producing superoutbursts in the su uma stars − the thermal tidal instability, irradiation-induced secondary mass overflow, and the plain vanilla disk limit cycle instability. i discuss the properties of the outbursts in two su uma systems observed by kepler in the context of the three theories. i conclude with a look beyond the su uma systems. keywords: cataclysmic variables dwarf novae optical photometry individual: v1504 cyg, v344 lyr, u gem, ss cyg. 1 introduction 1.1 disk instability model the outbursts in dwarf novae (dne), a subclass of cataclysmic variables (cvs warner 1995), are thought to be due to a limit cycle instability operating within the accretion disk (smak 1984) that episodically stores up matter (during quiescence) and dumps a fraction of it onto the accreting wd (during outburst). the basic disk instability model is now generally accepted as the correct explanation for dwarf nova outbursts (smak 1984, cannizzo 1993a, lasota 2001). the strongest point of the model is its prediction of a dividing line between novalike and dwarf nova systems (smak 1984), a line which has no free parameters1. for the systems above this line, the rate of accretion feeding into the outer disk is such that the entire disk is always in a hot, ionized state. other strengths of the theory include its natural ability to account for slow and fast decays of dn outbursts as being due either to viscous or thermal decay, the natural tendency of the model to produce alternating long and short outbursts (cannizzo 1993b), and the fact that the outer disk radius router continues to contract in quiescence following a dn outburst, whereas in the secondary mass-overflow model there is a one-to-one relation between disk luminosity and router (ichikawa & osaki 1992). smak (1984) was the first to use the relation between the fast rate of decay in dwarf nova outbursts and orbital period (i.e., the bailey relation) to constrain the value of the shakura-sunyaev (1973) alpha parameter in the ionized disk, αhot ' 0.1. the high fidelity kepler data now enable us to trace deviations from strictly exponential decay for the fast decays (cannizzo et al. 2012=c12), and also to utilize the slow rate of decay, wherein the entire disk resides in a hot, ionized state. the fact that the viscous and thermal time scales differ by a factor h/r means that this ratio can be directly calculated to be ∼0.01 − 0.02 (cannizzo 1998; menou, hameury, & stehle 1999). although successful as a model for outbursts, it has been known for some time that there must be various add-ons to the theory in order to have a complete theory for the accretion disk, including something like a coronal siphon flow (meyer & meyer-hofmeister 1994) or some other type of evaporative instability (e.g., shaviv & wehrse 1986) to transfer matter from the disk onto the wd during quiescence to get accretion. 1.2 su uma systems the su uma stars represent short orbital period dwarf novae (dne), porb < 2 hr, displaying normal outbursts (nos) and superoutbursts (sos) long outbursts with superhumps (shs). although the contrast between nos and sos seems quite dramatic, van paradijs (1983) has shown that the properties of sos, seen in systems below the period gap, can be understood as a natural continuation of the long outbursts in dne above the period gap porb > 3 hr. in addition, sos are defined by the existence of shs, modulations in the outburst 1 for ∼10 yr it appeared that the best studied dwarf nova, ss cyg, was on the wrong side of the line (harrison et al. 1999, schreiber, hameury, & lasota 2003). however, this prognosis was based on an incorrect distance (159 ± 12 pc). the corrected distance (114± 2 pc) reconciles data and theory (miller-jones et al. 2013). 156 http://dx.doi.org/10.14311/app.2015.02.0156 outburst properties of v1504 cyg and v344 lyr light curve at periods a few percent longer than the orbital period. the prevailing theory is that when the outer disk edge surpasses the point of 3:1 resonance with the binary orbit, an eccentric disk develops that precesses progradely in the binary frame (whitehurst 1988). more recent work indicates that the disk is not statically eccentric but has a fundamental “breathing” mode − oscillating between circular and eccentric (montgomery 2012ab). these shs are referred to as positive superhumps (pshs) since their periods exceed porb; there also exist negative superhumps (nshs) with periods a little less than porb thought to be indicative of a tilted, retrogradely precessing disk (patterson et al. 1993). the two disk states can and sometimes do coexist (montgomery 2012ab). in recent years three scenarios have emerged to account for sos: (i) the thermal-tidal instability (tti osaki 1989) which combines the normal disk instability model and the precessing disk model, (ii) irradiationinduced enhanced mass overflow from the mass-losing star, and (iii) the plain disk instability (pdi) model (cannizzo et al. 2010=c10). in a recent series of papers osaki & kato (2013abc) have discussed the relative merits of these three models within the context of recent kepler observations of v1504 cyg and v344 lyr. 2 kepler observations of v1504 cyg and v344 lyr c12 find that the durations of the nos within a supercycle increase monotonically, whereas the durations of the quiescence intervals between nos can either increase monotonically, or else increase to a local maximum about half way to the next so, and then decrease. these supercycles for which the recurrence times for nos reach a maximum roughly half way between sos present a challenge for the tti model, in which one expects a monotonic increase in disk mass, angular momentum, and triggering radius as the next so approaches. osaki & kato (2013c) acknowledge this difficulty, and also point out that this criticism, given by c12, applies equally well to the pdi calculations presented by c10. this is a good point − in fact the calculations presented in c10 did not show any variation in recurrence time for nos. figure 1 shows the latest pdi calculations of sos in a su uma. ok13a discuss the amplitude of the precursor dip and note that, at shorter wavelengths, the dip tends to stand out more strongly. one of the best examples is examples is in oy car (porb = 1.51 hr; mauche & raymond 2000) where simultaneous visual and euv (∼100 å) observations are presented of a so. embedded precursors which are simultaneously small in the optical and large in the euv provide a challenge to both tti and pdi models. in the tti model, the amplitude of the precursor can be precisely controlled by tweaking the strength of the increase in tidal torque accompanying the crossing of the 3:1 resonance radius by router, and the delay time ttti, delay accompanying its onset. for the sos there can be no precursors, as in the early tti models (e.g., ichikawa, hirose, & osaki 1993), or there can be precursors which are well-defined both in v and euv wavelengths, e.g., in models with ttti, delay = 1.75 d (schreiber, et al. 2004, see their fig. 3). v344 lyr figure 1: a comparison of kepler data on v344 cyg with two sos in a long light curve using the pdi model. a new idea introduced in ok13a is that the presence of time-varying nshs, when they are present, gives us a handle on variations in the outer disk radius. larwood (1998) gives a general expression for the (nodal) precessional frequency associated with a tilted disk ωnpr = ωorb − ωnsh which is based on the surface density distribution σ(r) and accretion disk tilt angle θ, ωnpr = 2πνnpr = −3 4 gm2 a3 ∫ σr3dr∫ σωr3dr cos θ, (1) where m2 is the secondary mass and a the binary separation. for a nominal power law distribution in surface density, νnpr νorb = −η 3 7 q√ 1 + q (router a )3/2 cos θ, (2) where η is a constant of order unity. the observed νnsh(t) variations in v1504 cyg reveal a disk contraction between nos, superposed on a longer term expansion between sos. this approach was criticized by smak (2013) who argued that, if one takes the time derivative of larwood’s pnsh equation, one obtains an expression with five terms (see his eqn. [7]), only one of which is related to the outer disk radius. 157 j. k. cannizzo the disk instability model provides a definitive prediction of σ(r) at all times, and therefore one may carry out a calculation of the long-term accretion disk evolution relevant for an su uma system, and do a direction comparison of νnsh calculated using the exact and approximate expressions. the results are shown in figure 2. the panels show the light curve, disk mass, disk radius, and the exact and approximate calculations for νnsh. the approximate expression gives a very good representation of the exact formula, which vindicates the underlying idea of ok13a and supports their use of νnsh(t) to track router(t). figure 2: a calculation of the pdi model for a su uma system. shown are (i) the kepler light curve, (ii) the disk mass, in units of 1023 g, (iii) the outer disk radius, in units of 1010 cm, (iv) the nsh frequency calculated using the larwood formula, and (v) the nsh frequency calculated using the approximate formula given ok13abc, taking η = 1.5. there is actually only one small difference between osaki’s tti and the pdi model for superoutbursts, namely, in the tti model there is a strong and brief contraction of the outer disk radius due to the (hypothesized) strong increase in tidal torque from the secondary star. an obvious way to distinguish between the two models is to closely monitor the outer disk radius in an eclipsing su uma star during the early onset of a so to see if there is a sudden, brief decrease in router. smak (2013) argues that this observation has already been carried out and shows no such contraction; ok13c counter that the time period in question was well into the so and therefore missed the start of the so. 3 looking beyond the su uma systems ok13abc argue that the precursor in sos is due to the onset of the tti, whereas i have argued it is a natural consequence of the pdi. an obvious next step would be to look in detail at long dn outbursts in systems where we do not expect the tti to operate − systems above the period gap where the outer disk radius never reaches the 3:1 radius. even if one accepts the fundamental tenet of the tti, namely a causal relation between whitehurst’s precessing eccentric disk and the di, it could not possibly be a factor because the disks in long period systems are never subject to the 3:1 instability. if one sees precursors in long dn outbursts in these systems, then they must be due to a more general aspect of the di. the two best studied dne are both longward of the period gap and should not experience the 3:1 instability − u gem and ss cyg. they have > 100 yr light curves, but unfortunately the bulk of the data is not precise enough to enable any statement to be made about the detailed shape of the outburst onset. cannizzo (2012) examined the aavso data for these systems, and found brief stretches of the light curves for which the number of observations entering into daily means exceeds ∼103, concomitant with the use of digital photometry. although still not on a par with kepler data, these data do afford a more detailed look at the outburst light curve shapes than the usual data. there are two long outbursts in u gem and one in ss cyg for which a statement can be made concerning the shape of the outburst light curve during outburst onset; one of the u gem outbursts shows no obvious precursor, one does, and the one ss cyg long outburst also does. in fact, the flattened shape of the precursor is similar to that seen in fig. 13 (panel 2) of cannizzo (1993b) in modeling ss cyg. these outbursts provide a hint that embedded precursors during long outbursts may not be confined to the su uma systems. 4 summary kepler light curves of short period dne have resparked interest in the nature of sos and led to the question: is the thermal-tidal instability needed, or can the plain vanilla version of the accretion disk limit cycle do the job all by itself? a detailed time-resolved study of an eclipsing su uma system during so onset should settle the question − if there is a dramatic contraction of the disk at so onset, the tti would be preferred; if not, the plain di model would be sufficient. also, recent work 158 outburst properties of v1504 cyg and v344 lyr by osaki & kato has shown that the time varying nsh frequencies νnsh(t) can be taken as a surrogate for the outer disk radius variations router(t). finally, it may be necessary to look beyond the short period dwarf novae to gain perspective on the nature of embedded precursors in long outbursts. references [1] cannizzo, j.k.: 1993a, in accretion disks in compact stellar systems ed., j.c. wheeler (singapore: world scientific), 6. [2] cannizzo, j.k.: 1993b, apj, 419, 318. doi:10.1086/173486 [3] cannizzo, j.k.: 1998, apj, 494, 366. doi:10.1086/305210 [4] cannizzo, j.k.: 2012, apj, 757, 174. doi:10.1088/0004-637x/757/2/174 [5] cannizzo, j.k., smale, a.p., wood, m.a., et al.: 2012, apj, 747, 117. doi:10.1088/0004-637x/747/2/117 [6] cannizzo, j.k., still, m.d., howell, et al.: 2010, apj, 725, 1393. doi:10.1088/0004-637x/725/2/1393 [7] harrison, t.e., mcnamara, b.j., szkody, p., et al.: 1999, apj, 515, l93. doi:10.1086/311982 [8] ichikawa, s., hirose, h., osaki, y.: 1993, pasj, 45, 243. [9] ichikawa, s., osaki, y.: 1992, pasj, 44, 15. [10] larwood, j.: 1998, mnras, 299, l32. doi:10.1046/j.1365-8711.1998.01978.x [11] lasota, j.-p.: 2001, new astron. rev., 45, 449. doi:10.1016/s1387-6473(01)00112-9 [12] mauche, c.w., raymond, j.c.: 2000, apj, 541, 924. doi:10.1086/309489 [13] menou, k., hameury, j.-m., stehle, r.: 1999, mnras, 305, 79. [14] meyer, f., meyer-hofmeister, e.: 1994, a&a, 288, 175. [15] miller-jones, j.c.a., sivakoff, g.r., knigge, c., et al.: 2013, science, 340, 950. doi:10.1126/science.1237145 [16] montgomery, m.m.: 2012a, apj, 745, l25. doi:10.1088/2041-8205/745/2/l25 [17] montgomery, m.m.: 2012b, apj, 753, l27. doi:10.1088/2041-8205/753/2/l27 [18] osaki, y.: 1989, pasj, 41, 1005. [19] osaki, y., kato, t.: 2013a, pasj, 65, 50. [20] osaki, y., kato, t.: 2013b, astro-ph/1305.5877. [21] osaki, y., kato, t.: 2013c, astro-ph/1309.3722. [22] patterson, j., thomas, g., skillman, d.r., diaz, m.: 1993, apjs, 86, 235. [23] schreiber, m.r., hameury, j.-m., lasota, j.-p.: 2003, a&a, 410, 239. [24] schreiber, m.r., hameury, j.-m., lasota, j.-p.: 2004, a&a, 427, 621. [25] shakura, n.i., sunyaev, r.a.: 1973, a&a, 24, 337. [26] shaviv, g., wehrse, r.: 1986, a&a, 159, l5. [27] smak, j.: 1984, acta astron., 34, 161. [28] smak, j.: 2013, acta astron., 63, 109. [29] van paradijs, j.: 1983, a&a, 125, l16. [30] warner, b.: 1995, cataclysmic variables (cambridge: cambridge univ. press). [31] whitehurst, r.: 1988, mnras, 232, 35. discussion margaretha pretorius: if there is no tidal instability how do you make superhumps? john cannizzo: the tidal instability is not being disputed. in both osaki’s thermal-tidal instability model and the plain disk instability model superhumps are due to a precessing eccentric disk which becomes manifest when the outer disk radius surpasses the the 3:1 radius. the question is then whether this has anything to do with the superoutbursts: osaki would say yes, i would say no. raymundo baptista: yesterday i showed inferred αcold values for three quiescent su uma stars, which show αcold ∼ 10−1 ∼ αhot. my interpretation is that this eliminates the possibility of the disc instability model to explain their outbursts. could you please comment on this? john cannizzo: what’s the temperature of that gas? raymundo baptista: about 6000 k. 159 http://dx.doi.org/10.1086/173486 http://dx.doi.org/10.1086/305210 http://dx.doi.org/10.1088/0004-637x/757/2/174 http://dx.doi.org/10.1088/0004-637x/747/2/117 http://dx.doi.org/10.1088/0004-637x/725/2/1393 http://dx.doi.org/10.1086/311982 http://dx.doi.org/10.1046/j.1365-8711.1998.01978.x http://dx.doi.org/10.1016/s1387-6473(01)00112-9 http://dx.doi.org/10.1086/309489 http://dx.doi.org/10.1126/science.1237145 http://dx.doi.org/10.1088/2041-8205/745/2/l25 http://dx.doi.org/10.1088/2041-8205/753/2/l27 j. k. cannizzo john cannizzo: one of the limitations of the standard disk instability theory has always been that it’s not a complete theory certainly it doesn’t work at all to describe the quiescent state. if you take the model seriously, the plots in the movie i showed indicate temperatures in quiescence of ∼2000-3000 k. also, we see significant evidence for accretion during quiescence mainly through soft x-ray emission, which is not predicted by the di theory since in quiescence the accretion rate would be zero. so the bottom line is that the quiescence gas you observe must be due to something extrinsic to the di model. 160 introduction disk instability model su uma systems kepler observations of v1504 cyg and v344 lyr looking beyond the su uma systems summary acta polytechnica ctu proceedings doi:10.14311/app.2016.6.0034 acta polytechnica ctu proceedings 6:34–39, 2016 © czech technical university in prague, 2016 available online at http://ojs.cvut.cz/ojs/index.php/app the dubins traveling salesman problem with constrained collecting maneuvers petr váňa∗, jan faigl dept. of computer science, czech technical university in prague, technická 2, 166 27 prague, czech republic ∗ corresponding author: vanapet1@fel.cvut.cz abstract. in this paper, we introduce a variant of the dubins traveling salesman problem (dtsp) that is called the dubins traveling salesman problem with constrained collecting maneuvers (dtsp-cm). in contrast to the ordinary formulation of the dtsp, in the proposed dtsp-cm, the vehicle is requested to visit each target by specified collecting maneuver to accomplish the mission. the proposed problem formulation is motivated by scenarios with unmanned aerial vehicles where particular maneuvers are necessary for accomplishing the mission, such as object dropping or data collection with sensor sensitive to changes in vehicle heading. we consider existing methods for the dtsp and propose its modifications to use these methods to address a variant of the introduced dtsp-cm, where the collecting maneuvers are constrained to straight line segments. keywords: dubins vehicle, dtsp, non-holonomic vehicle, path planning, collecting maneuver. 1. introduction the dubins traveling salesman problem (dtsp) [1] has been subject of studies for many years. the problem is to find a shortest tour visiting a given set of target locations by the dubins vehicle [2] that models a fix-wing aircraft (or car-like vehicle) with a limited minimal turning radius and a constant forward velocity. the herein studied path planning problem is motivated by surveillance and rescue missions to visit a set of target locations by a fix-wing aerial vehicle. such a problem can be formulated as the variant of the combinatorial optimization the traveling salesman problem (tsp) where the target locations have to be connected by dubins maneuvers. the problem is to determine a sequence of visits to the targets together with the most suitable heading of the vehicle at each target location such that the total tour length is minimized. dubins proved that the optimal path between two configurations of the dubins vehicle can be found analytically by enumerating six possible maneuvers [2]. however, it is necessary to determine the optimal headings to find the optimal maneuver analytically, which in fact depend on the optimal sequence of visits to the targets. therefore, this challenging problem is called the dubins traveling salesman problem (dtsp) rather than just the tsp to distinguish computational challenges arising from the constrained turning radius. the dtsp is np-hard [3] as for zero minimal turning radius, the problem becomes the tsp. even though an ordinary formulation of the dtsp provides a solution of the problem to visit a set of target locations by an aircraft, the problem considered in this paper requires to perform specific collecting maneuvers to reliably accomplish the mission. such a requirement is arising from the need to get a visual contact with the drop-off location. then, the vehicle figure 1. an example of the dtsp-cm instance with straight collecting maneuvers in a cargo airdrop mission. target locations are depicted as small green disks, maneuvers are the red straight line segments, and particular trajectories for a reliable cargo drop-off are in green. the blue curves stand for the dubins maneuvers which connect the determined collecting maneuvers into the closed tour. is required to move straight ahead during the dropping phase itself. an example solution of such an object dropping mission is depicted in fig. 1. in this paper, we propose a novel extension of the ordinary dtsp to address the requirement of specific maneuvers for target accomplishment. the introduced problem is called the dubins traveling salesman problem with collecting maneuvers (dtsp-cm). further, we study a special case of the dtsp-cm where collecting maneuvers are straight line segments (denoted as the dtsp-scm) and we propose modifications of the existing approaches to the dtsp to solve this constrained variant of the dtsp-cm. the paper is organized as follows. an overview of the related work with various dtsp approaches 34 http://dx.doi.org/10.14311/app.2016.6.0034 http://ojs.cvut.cz/ojs/index.php/app vol. 6/2016 dubins traveling salesman problem is provided in section 2. the proposed problem is formally introduced in section 3 and modifications of the existing dtsp approaches are proposed in section 4. evaluation of the proposed modifications of the existing approaches is presented in section 5. finally, conclusion remarks are in section 6. 2. related work various approaches have been proposed to address the dtsp that can be broadly divided into three main classes: 1) decoupled approaches, 2) sampling-based approaches, 3) and evolutionary-based approaches. in this section, a brief overview of representative approaches of each particular class is provided to clarify the context for the introduced dtsp-cm and proposed modifications of the particular approaches to solve the constrained variant of the dtsp-cm with straight line collecting maneuvers. probably the first approach to the dtsp is the alternating algorithm (aa) introduced in [1] which can be categorized as a decoupled approach, since it addresses the dtsp in two consecutive steps. first, a sequence of visits to the targets is determined, e.g., by a solution of the underlying euclidean tsp without considering the vehicle motion constraint. in the second step, headings at the target locations are established by applying straight line segments for even segments followed by closing the tour with dubins maneuvers for odd segments. a similar decoupled strategy has been utilized also in the receding horizon based approach [4] with the horizont of two or three target locations ahead which significantly outperforms the original aa in terms of the solution quality. sampling-based approaches [5, 6] represent the second class of existing solutions for the dtsp. these approaches are based on sampling of possible headings into a finite discrete set and the dtsp is transformed into the generalized asymmetric tsp (gatsp) and further into the atsp by noon-bean transformation [7]. the resulting atsp can be then solved by existing solvers such as concorde [8] or lkh [9]. the third class of the dtsp approaches are evolutionary based algorithms [10]. they encoded solutions into individuals in the population and used similar crossover and mutation operators like for the standard tsp. although evolutionary based approaches can be computationally demanding, their main advantage is the ability to search for a global optima. in [11], authors proposed a novel memetic algorithm to improve performance of the evolutionary based approach by utilizing a local optimization method. recently, the evolutionary multi-objective algorithm nsga-ii [12] has been deployed in [13] to address path planning for data collection in a wireless sensor network that is formulated as the dtsp. the authors consider a bi-objective optimization criterion where the first objective is to minimize the tour length and the second objective is to maximize the time when the vehicle is in the vicinity of the sensor. the problem formulation [13] is similar to the herein proposed dtsp-cm in the considering an influence of the executed maneuver type at the target locations; however, the authors of [13] do not explicitly constraint the type of the maneuver and rather optimize a bi-objective function. on the other hand, the proposed dtsp-cm problem formulation explicitly restricts the collecting maneuver to be one of the specified type with the aim to improve robustness of the solution execution and the mission accomplishment. 3. problem statement the proposed formulation of the dubins traveling salesman problem with collecting maneuvers (dtspcm) is motivated by surveillance missions with fixwing aerial vehicles where the goal is not only to visit the target locations but also to execute a prescribed maneuver for each target. similarly to the dtsp, also in the dtsp-cm, the dubins vehicle [2] is constrained to move only forward at the constant speed v and has the minimum turning radius restricted to ρ. the motion of the vehicle can be described as: ẋ ẏ θ̇  = v  cos θ sin θ u ρ  , |u| ≤ 1, (1) where u is the control input. the state of the dubins vehicle q is represented as a triplet q = (x, y, θ) and q ∈ se(2), where (x, y) ∈ r2 is the vehicle position in a plane and θ ∈ s1 is the vehicle heading at (x, y). the dtsp-cm stands to determine a closed shortest curvature-constrained tour for which all targets are accomplished by executing a maneuver from a specified set of acceptable collecting maneuvers. each target i is defined by its position pi in the plane and a set of collecting maneuvers ti. the dtsp-cm is formally defined for n target locations p = (p1, . . . , pn) where pi ∈ r2, corresponding collecting maneuvers t = {t1, . . . , tn}, and the minimal turning radius of the vehicle ρ. in the dtsp-cm, the dubins vehicle is requested to accomplish all given targets by the particular collecting maneuvers joined in a single closed tour satisfying the vehicle motion constraints (1). therefore, it is necessary to determine both the collecting maneuvers m = {τi, . . . , τn}, τi ∈ ti, and the sequence of the visits to the targets σ = {σ1, . . . , σn}. notice, a set of possible collecting maneuvers ti can be infinite, e.g., as an arbitrary selection of the heading angle θ from the set θ ∈ (0, 2π〉 in the ordinary dtsp. the final closed tour consists of the particular collecting maneuvers that are connected to a closed tour by the shortest possible paths respecting the vehicle motion (1), i.e., dubins maneuvers. therefore the total tour 35 petr váňa, jan faigl acta polytechnica ctu proceedings (a) . crossing maneuver (b) . dropping maneuver figure 2. examples of straight collecting maneuvers of the dtsp-scm length l givenm and σ can be computed as: l(m,σ) = n−1∑ i=1 d(g(τσi ), f(τσi+1)) +d(g(τσn ), f(τσ1)) + n∑ i=1 l(τi), (2) where the function f(τi) represents the starting point of the maneuver τi, and g(τi) is the end point of the maneuver, d(g(τi), f(τj)) stands for the length of the dubins maneuver connecting collecting maneuvers τi and τj , and the symbol l(τi) stands for the length of the τi maneuver. the aim is to minimize the tour length in the dtsp-cm, and therefore, the problem is formulated as the optimization problem formalized in problem 3.1. based on the set of collecting maneuvers, the optimization problem can be both discrete or piecewise continuous. the problem is not always fully continuous, since the length of the dubins maneuver is also only piecewise continuous [14]. problem 3.1 (dtsp-cm). minimizem,σ l(m,σ) subject to ∀i : τi ∈ ti 3.1. dtsp-cm with straight collecting maneuvers (dtsp-scm) based on the proposed formulation of the dtsp-cm we further investigate its special variant with collecting maneuvers limited to straight line segments with a prescribed length and its relative position to the particular target location. such limited maneuvers are motivated by real scenarios where the vehicle needs to stay at its course while data from sensors are measured or a cargo is dropped to the particular target location. we call this problem the dtsp with straight collecting maneuvers and denote it as the dtsp-scm in the rest of this paper. having the possible fulfilling maneuver as a straight line segment for which the supporting line is passing the target location, the problem is to find the segment orientation for each target, i.e., we need to determine the vehicle heading at each target location as in the ordinary dtsp. the considered straight collecting maneuver can be specified by two parameters that determine its relative position to the target location: 1) the parameter α represents a distance of the maneuver starting point to the target location; 2) and the parameter β that specifies the end of the collecting maneuver from the target location. for both parameters we can consider positive or negative values according to their relative location to target location defined by the vehicle motion. thus, positive values denote the positions of the maneuver endpoints that are on the approaching side to the target locations (the vehicle is approaching the target), while the negative values denote the position of the endpoints for which the vehicle recedes the target location. the length of the maneuver l(τ) is always positive, and therefore, we define it as l(τ) = |α− β|, l(τ) > 0. (3) we consider two types of straight collecting maneuvers. the first type represents a situation where the target location lies directly on the collecting maneuver. such a maneuver is called the crossing maneuver (α > 0 and β < 0) and it is motivated by robotic scenarios to precisely measure some location and where the vehicle cannot change its heading during the sensing phase. an example of the crossing maneuver is depicted in fig. 2a. the second type of straight collecting maneuvers is a so-called dropping maneuver. in this case, the target location is at the maneuver direction, but not necessary on the maneuver itself, α > 0 and β > 0. this type of maneuver can be used for planning object dropping, since the vehicle is required to focus the target while moving on the straight line segment to ensure a precise airdrop location [15]. after the object dropping, the vehicle can continue in an arbitrary direction and does not need to achieve the target location itself. an example of the dropping maneuver is depicted in fig. 2b. formally, the dtsp-scm is a variant of the optimization problem 3.1. in contrast to the general dtsp-cm, the straight collecting maneuvers have a constant length, since the parameters α and β are fixed for the particular problem instance. therefore, the total length l(m) of all collecting maneuvers can be determined l(m) = n∑ i=1 l(τi) = n∑ i=1 |α− β| (4) and its value is constant that can be computed before solving the particular instance of the dtsp-scm. this property enables to consider only dubins maneuvers length while minimizing the tour length, which significantly simplifies possible methods addressing the dtsp-scm. furthermore, it is possible to modify approaches for the dtsp to address the proposed dtsp-smc due to the constant length of the collecting maneuver. proposed modifications for existing dtsp approaches are presented in the next section. 36 vol. 6/2016 dubins traveling salesman problem 4. solution of the dtsp-scm the main advantage of the proposed dtsp-scm over more general the dtsp-cm is that it is possible to solve the dtsp-scm in a relatively straightforward way by approaches developed for the dtsp with only slight modifications. this is possible because the length of the individual collecting maneuver is constant, and thus the sum of all collecting maneuvers (4) is a constant value and it is not needed to consider individual lengths during the optimization process. hence, we can build solvers for the dtsp-scm on the existing methods for the dtsp. the proposed modifications of the selected representative approaches for each of three classes of the dtsp approaches introduced in section 2 are presented in this section. for decoupled approaches, we can still utilize a solution of the euclidean tsp to determine the sequence of visits to the targets. however, we can expect that such a sequence can be farther from the optimum when the collecting maneuvers are getting longer. a modification of the original aa [1] to utilize collecting maneuvers is straightforward. in solving the ordinary dtsp, vehicle headings are determined for each target location. instead of that, we can determine directions of collecting maneuvers using the direction in odd segments. then, we can connect even segments by the dubins maneuvers to create a closed tour, and thus solve the dtsp-scm. the sampling-based approach [5] is another option how to address the proposed dtsp-scm. instead of sampling the vehicle heading at each target location, we need to sample a set of collecting maneuvers corresponding to each particular target location in the dtsp-scm. in [5], the authors propose to create a distance graph which is an instance of the gatsp. for the proposed dtsp variant with collecting maneuvers, the resulting instance of the gatsp has twice the number of vertices. however, the size of the gatsp instance can be reduced back to the original size by a simple transformation as follows. the length of the utilized collecting maneuver is added to the length of the dubins maneuver which together form a continuous path. similarly the evolutionary-based algorithms [10, 11] are also suitable for a straightforward extension to address the proposed dtsp-scm. here, the chromosome representing the individual needs to be modified to contain the collecting maneuver instead of the vehicle heading only. in addition, the evaluation function needs to be modified to respect the total tour length according to (2). 5. evaluation results several problem instances of the dtsp-scm have been solved to demonstrate usability of the newly proposed variant of the dtsp with straight collecting maneuvers. a small quadrotor helicopter has been simulated in the realistic gazebo simulator [16] with a physical model based on the pixhawk flight controller. figure 3. simulated quadrotor helicopter dropping red ball object. an example of graphical output from the simulator is depicted in fig. 3. the helicopter has been requested to fly at the altitude 20 m with the speed 5 m.s-1 and the minimal turning radius has been set to ρ = 7 m to imitate realistic conditions of a real aircraft. the dropping distance has been set to approximately 9.5 m and the used straight collecting maneuvers have the parameters: α = 12 m and β = 8 m. we have conducted two types of evaluation scenarios. the first scenario is designed to validate a precision of the airdrop maneuvers without and with the explicit consideration of the straight collecting maneuvers in the proposed dtsp-scm. the second scenario is designed to evaluate performance of the proposed modifications of the existing solutions for the dtsp in solving instances of the introduced dtspscm. 5.1. precision of the airdrop first, we found solutions of the ordinary dtsp and the proposed dtsp-scm using the sampling based approach with the lkh solver [17]. the evaluation instance is generated for 10 target locations (n = 10) with the relative density d = 0.3 defined as: d = ρ √ n s , (5) where s is the size of the bounding box where all target locations are positioned. we consider solution of 10 random instances with n = 10 solved by each particular approach (dtsp and dtsp-scm) to obtain statistically significant results. hence, 100 simulated intersection of the dropped object with ground have been recorded. in the used simulated environment, a limited precision of the gps and all the motors were considered except of air resistance. the obtained locations of the dropped object are visualized in fig. 4 where a direction of all dropping maneuvers is rotated to the same orientation to draw all locations within the same figure. it can be easily seen that objects dropped without explicit consideration of the straight collecting maneuvers (i.e., a solution of the ordinary dtsp) are often dropped with a longer distance error from the desired target 37 petr váňa, jan faigl acta polytechnica ctu proceedings ● ● ●● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ●● ● ● ● ● ●● ● ● ●● ● ● ● ● ●● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ●● ● −10 −5 0 5 −5 0 5 10 xerror [m] y e rr or [m ] method ● dtsp dtsp−scm airdrop target figure 4. locations of the dropped object to the target location as a result of the performing dtsp solution and a solution of the newly introduced dtspscm. all measurements are normalized by the position of the airdrop and the target location. 0.0 2.5 5.0 7.5 10.0 12.5 0255075100 percent of dataset [%] e rr or d is ta nc e [m ] solver dtsp dtsp−scm figure 5. precision of the airdrop maneuver obtained by solution of the dtsp and the newly proposed formulation of the dtsp-scm location than for the case when the trajectory is found as a solution of the dtsp-scm. the distance of the real position of the dropped object to its expect position at the target location is depicted in fig. 5. the error for the solutions of the dtsp-scm is almost half of the error for the dtsp. 5.2. performance of dtsp approaches in the dtsp-scm the proposed modifications of the existing approaches for the dtsp to address the introduced dtsp-scm have been evaluated in a series of dtsp-scm instances according to the number of target locations n and their density d determined by (5). six algorithms for the dtsp-scm has been compared. in particular, the proposed modifications of approaches: alternating algorithm (aa) [1], local iterative optimization [18], genetic algorithm [10], memetic algorithm [11], and sampling-based approach [6] with the lkh solver [17] (denoted as the sampling+lkh) and with the optimal concorde solver [8], denoted as the sampling+concorde. due to high computational requirements of the genetic, memetic, and sampling● ● ● ● ● ● ● ● ● ● ●● ● ● ● ● ● ● ● ● ● ● ● ● 0 1 2 3 10 20 50 100 number of target positions r at io o f s ol ut io n le ng th to th e re fe re nc e le ng th method ● ● ● ● ● ● aa lio genetic memetic sampling+lkh sampling+con. figure 6. average ratios of the solution length to the reference length of the best found solution depending on the number of target locations with the relative density d = 0.3 ● ● ● ● ●● ● ● ● ● ●● ● ● ● ● ● ● 0 1 2 0.2 0.3 0.4 density of target positions r at io o f s ol ut io n le ng th to th e re fe re nc e le ng th method ● ● ● ● ● ● aa lio genetic memetic sampling+lkh sampling+con. figure 7. average ratios of the solution length to the reference length of the best found solution depending on the density of n = 20 target locations based algorithms, their computational time has been limited to 1 hour. for sampling based approach, a series of solutions is found with iteratively increasing number of samples until the time limit is reached [19]. therefore, all the algorithms provide a solution, albeit more demanding approach (i.e., based on the concorde solver) for less samples than faster heuristic lkh solver, which solves more iterations within the same time limit. the considered number of target locations n has been selected from the set n ∈ {10, 20, 50, 100} and the density d has been selected from the set d ∈ {0.2, 0.3, 0.4}. for each particular scenario defined by the n and d, 20 random instances were generated. for each problem instance, the best found solution (among all solutions found by all approaches) has been selected as a reference solution that allows to aggregate the results into a single solution quality indicator as the ratio of the length of the found tour to the length of the reference solution. average ratios and standard deviations (shown as error bars) are presented in fig. 6 and fig. 7 depending on the number of target locations n and for the particular density of the target locations d, respectively. the results indicate that superior solutions 38 vol. 6/2016 dubins traveling salesman problem are found by the sampling-based approaches. the lkh heuristic approach provides better results than the concorde solver because of limited computational time, and thus solutions with finer discretizations are not computed within the dedicated time limit by the optimal concorde solver. 6. conclusion in this paper, we propose a new variant of the dtsp formulation to address specific requirements of the collecting maneuvers to accomplish visitation of each target location. the proposed variant of the dtsp is called the dubins traveling salesman problem with collecting maneuvers (dtsp-cm). further, we considered collecting maneuvers restricted to straight line segments with fixed length and its specified relative position to the target that is denoted as the dtsp with straight collecting maneuvers (dtsp-scm). for the dtsp-scm, we propose modifications of the existing approaches to the dtsp to address the prescribed constrained collecting maneuvers. the proposed modifications of the algorithms have been evaluated in several scenarios including evaluation of the proposed problem formulation to the precision of the cargo delivery in airdrop maneuvers. the presented results indicate that the proposed formulation improves the reliability of the mission accomplishment. moreover, the presented evaluation results also provide an overview of the performance of particular approaches to the dtsp in the dtsp-scm formulation, where the sampling based methods provide the best found solution among the evaluated approaches. acknowledgements the presented work has been supported by the czech science foundation (gačr) under research project no. 16-24206s. the support of grant no. sgs16/235/ohk3/3t/13 to petr váňa is also gratefully acknowledged. references [1] k. savla, e. frazzoli, f. bullo. on the point-to-point and traveling salesperson problems for dubins’ vehicle. in proceedings of the american control conference, pp. 786–791. ieee, 2005. doi:10.1109/acc.2005.1470055. 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[15] c. w. hewgley, o. yakimenko. precision guided airdrop for vertical replenishment of naval vessels. in proceedings of the 20th aiaa aerodynamic decelerator systems technology conference, pp. 4–7. 2009. doi:10.2514/6.2009-2995. [16] gazebo robot simulator. [cited 31 oct 2016]. [17] k. helsgaun. lkh solver 2.0.7. www.akira.ruc.dk/~keld/research/lkh. [cited 31 oct 2016]. [18] p. váňa, j. faigl. on the dubins traveling salesman problem with neighborhoods. international conference on intelligent robots and systems (iros) pp. 4029–4034, 2015. doi:10.1109/iros.2015.7353945. [19] p. váňa, j. faigl. on sampling based methods for the dubins traveling salesman problem with neighborhoods. acta polytechnica ctu proceedings 2(2):57–61, 2015. doi:10.14311/app.2015.1.0057. 39 http://dx.doi.org/10.1109/acc.2005.1470055 http://dx.doi.org/10.2307/2372560 http://dx.doi.org/10.1109/cdc.2007.4434503 http://dx.doi.org/10.1109/cdc.2006.376928 http://dx.doi.org/10.1109/acc.2011.5991501 http://dx.doi.org/10.2514/1.48949 http://dx.doi.org/10.1287/opre.39.4.623 http://dx.doi.org/10.1016/s0377-2217(99)00284-2 http://dx.doi.org/10.1109/isie.2012.6237270 http://dx.doi.org/10.1016/j.cja.2014.04.024 http://dx.doi.org/10.1109/4235.996017 http://dx.doi.org/10.1016/j.cor.2016.07.004 http://dx.doi.org/10.1137/100816079 http://dx.doi.org/10.2514/6.2009-2995 www.akira.ruc.dk/~keld/research/lkh http://dx.doi.org/10.1109/iros.2015.7353945 http://dx.doi.org/10.14311/app.2015.1.0057 acta polytechnica ctu proceedings 6:34–39, 2016 1 introduction 2 related work 3 problem statement 3.1 dtsp-cm with straight collecting maneuvers (dtsp-scm) 4 solution of the dtsp-scm 5 evaluation results 5.1 precision of the airdrop 5.2 performance of dtsp approaches in the dtsp-scm 6 conclusion acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2017.12.0074 acta polytechnica ctu proceedings 12:74–78, 2017 © czech technical university in prague, 2017 available online at http://ojs.cvut.cz/ojs/index.php/app correlation analysis of vehicle frontal impact parameters josef mík∗, jana kadlecová department of vehicle technology, čvut v praze, konviktská 20, 12000 prague ∗ corresponding author: mikjosef@fd.cvut.cz abstract. the article considers a possible improvement of road vehicle safety by using ecall – a system which initiates an emergency call in case of traffic accident. a possible way of better description of a frontal impact accident of a vehicle is examined and enriched by the information from the onboard e-call unit. in this article, we analyze results of frontal crash tests with different types of barriers and overlapping area and look for the correlation between the individual vehicle and collision parameters in order to provide a better description of the severity of the accident by the ecall system. the relation among the selected parameters is described using the correlation analysis. keywords: crash test, ecall, cotrrelation. 1. introduction in a long-term timeframe, there is a noticeable decrease in the amount of road accidents, casualties and fatal victims of road accidents in europe. it is mostly achieved thanks to newly accepted legislation measures increasing road safety and minimizing consequences of road accidents. one of these measures is a mandatory installation of the ecall system – an emergency call system in case of a road accident. the system is to be launched and installed on all newly produced vehicles in april 2018 [1]. based on the information obtained from passive safety systems installed in the vehicle and additional ecall system sensors, the ecall system automatically detects a road accident and consequently sends a minimum set of data (msd) message to the emergency service and at the same time establishes a voice communication with 112 service. the operator of the emergency service decides on the measures and safety services dispatched to the accident site based on the information obtained in the msd message and from oral communication with passengers of the crashed vehicle. the basic structure of the msd message contains the following information about the vehicle and the accident: • version of msd format; • identification of msd; • ecall activation type; • vin code for vehicle (iso 3779)[2]; • fuel type; • vehicle direction; • recent vehicle location n1, n2; • number of passengers (locked safety belts). a set of additional information, which can be used for more accurate description of consequences of accident, is described in this article. this was concluded based on results of series of crash tests. 2. experiment description a background for the analysis is data obtained from a series of front impact collision of vehicles category m1. results of these crash tests are shown in table 1. calculated resulting injury criteria coefficient hic and 3 ms head criteria are represented in table 2. as it can be seen from table 1 and 2, the constellations of test vehicles contain vehicles equipped with different types of passive safety systems. activation of the frontal airbag was a monitored parameter. as for the collision partner, a solid barrier with a 100 the following parameters were selected for correlation analysis: • overlapping of the frontal area – indicates overlapping of the frontal area of the test vehicle with the collision partner in percent; • collision velocity (v0) – calculated based on the video of the collision from high-speed cameras; • the weight of the vehicle – calculated based on measurements performed on separate wheels with plate weights; • kinetic energy of the vehicle(ek) – is calculated based on the vehicle weight, and collision velocity (equation 1) ek = 1 2 ∗m ∗ v 2, (1) where: ek – kinetic energy of the vehicle right before the collision [j], m – weight of the vehicle [kg], v – collision velocity [m*s−2]; • maximal acceleration of the vehicle body – estimated from the acceleration of the vehicle body during the collision in x direction; 74 http://dx.doi.org/10.14311/app.2017.12.0074 http://ojs.cvut.cz/ojs/index.php/app vol. 12/2017 correlation analysis of vehicle frontal impact parameters test vehicle collision vehicle characteristics partner overlap [%] front airbag v0 [km*h−1] m [kg] ek [kj] amax [m*s−2] deformation [mm] škoda fabia rigid barrier 100 yes 50,77 1208 120,13 620 447 škoda favorit rigid barrier 100 no 47,3 875 75,53 459,8 508 škoda rapid rigid barrier 100 yes 45 1235 96,48 378,1 416 škoda rapid rigid barrier 100 no 14,92 1122 9,64 115,6 127 renault 5 gtd rigid barrier 40 no 52,2 938,6 98,67 345 700 škoda roomster deform. barrier 40 yes 56,18 1542 187,76 no no škoda fabia škoda octavia 33 yes 50 1157 111,59 176 815 table 1. results of crash tests – characteristic of test vehicles. test vehicle collision partner driver manikin other manikin amax [m*s−2] hic [-] location in the vehicle amax [m*s−2] hic [-] škoda fabia solid barrier 510 302,94 passenger 2277 4139 škoda favorit solid barrier 2490 2205,6 child p3 708,8 706 škoda rapid solid barrier 460 180,42 ne ne no škoda rapid solid barrier no ne ne ne no renault 5 gtd solid barrier no no child p3 856,6 612 škoda roomster deform. barrier no 359,25 passenger ne 358,64 škoda fabia škoda octavia no no no no no table 2. results of crash tests – hic criteria of test manikins. • resulting deformation of the vehicle (ξ) – estimated from visual documentation of the test vehicle after the accident (photos and video from the high-speed cameras), based on comparison with photo documentation of the test vehicle before the collision; • maximal acceleration of the manikin driver’s head (amax) – estimated from the acceleration acting on the driver manikin’s head during the impact. the resulting acceleration curve was calculated from the accelerations of the manikin head in x, y and z axis based on the equation 2. the overall progress of result acceleration was then filtered with standard frequency filter cfc1000. acelk = √ a2 x + a2 y + a2 z, (2) where: acelk – resulting acceleration [m*s−2], ax, ay, az – acceleration of the manikin head measured in x, y, z [m*s−2]; • hic criteria value for the driver manikin – head injury criteria was calculated from the result acceleration curve based on the equation 3. in case there was no contact of the manikin head with the vehicle interior the time interval of 36 ms was selected for time t1 to t2 (estimated from the speed camera recordings), in casethere was a collision, a 15 ms time interval t1 to t2 was selected. hic criteria value is then the maximum value in the selected interval for the result acceleration hic = [ 1 t1 − t2 ∗ ∫ t2 t1 adt ]2,5 ∗ (t1 − t2), (3) where: hic – value of head injury criteria [-], a – result acceleration [g], t1 – start of the time interval for hic calculation, t2 – end of the time interval for hic calculation. results of crash tests are summarized in tables 1 and 2. 3. correlation analysis of crash test results analysis of the crash tests results was made based on correlation analysis between selected parameters presented in table 1 and 2, considering if it is suitable to use them for estimation of the severity of the accident by ecall system. the relationship between parameters is expressed by a regression function using the least squares method (equation 4) 75 josef mík, jana kadlecová acta polytechnica ctu proceedings yi − ŷi yi − ayx + byx ∗ xi ... yi − ŷi · yi − ayx − byx ∗ xi ... f ( ayx, byx ) = 1 n ∑n i=1 (4) where: ŷi – aligned value (theoretical), yi – emperical values, ayx, byx – equation coefficients. the reliability of regression estimation was determined by the correlation coefficient ryx. correlation dependency is considered to be poor tight if the value of correlation coefficient is lower than 0.3. in case the value is higher than 0.8, then correlation dependency is considered to be very tight. if the value lies in the interval from 0.3 to 0.8 the correlation dependency is considered to be mild tight. 4. dependency of the vehicle body deformation from collision velocity the dependency of the deformation of the vehicle from collision velocity is represented in figure 1. from all the crash tests, only collisions with a rigid barrier with 100% overlapped were used for correlation analysis. these values are shown in the figure as blue dots with a line regression function. it is possible to calculate correlation coefficient from the coefficient of determination r2 as it is shown in the equation 5. r = √ r2 = √ 0, 9433 = 0, 9712 (5) the resulting value of the regression coefficient is r = 0.9712 so we can consider correlation dependency as very tight. but a small amount of input data has to be considered. in figure 1 there are also shown other values which were not used for the regression analysis because of a different type of collision parameters. these are the data obtained from crash tests with the vehicle renault 5 gdi, which was collided with a rigid barrier with 40% overlap, and a skoda fabia sedan, which was collided with another vehicle skoda octavia with 33% overlap. it is visible from figure 1 that the value of the resulting deformation is increasing with decreasing overlap of the collision area. 5. the dependency of maximum acceleration on the kinetic energy of the vehicle the next parameter that we consider in this research is the dependency between the value of maximum acceleration, which impacts the vehicle body during a crash, and the value of the vehicle’s kinetic energy, which is transformed into the vehicle’s body deformation during the collision. the values for individual vehicles are incorporated in the graph in figure 2. as in the case of the dependency between vehicle body deformation and collision velocity, the regressive function describes only vehicles that suffered a frontal collision with a rigid barrier with 100% overlap. the values that are applicable to personal vehicles can also be found in the graph. to describe the dependency, an exponential regression function was chosen as it has a higher value for correlation coefficient than a linear function. the graph describing the equation of the regression function can be found in figure 2, as well as the determination coefficient r2. equation 6 establishes the value of the correlation coefficient through a determination coefficient. r = √ r2 = √ 0, 9103 = 0, 954 (6) according to the resulting value of the correlation coefficient r = 0,954 it is possible to see the correlation dependency as very high. however, it is necessary to take into consideration the small number of input values on the basis of which the function was solved. the graph in figure 2 also contains the values of acceleration for the renault 5 gtd (40% overlap) and the škoda fabia sedan (33% overlap) with the lesser percentage of overlap. the graph demonstrates that the maximum acceleration influencing these vehicles with an equal value of the consumed kinetic energy in the case of vehicles with a 100% overlap is smaller. in the case of the škoda fabia sedan, however, part of the kinetic energy was given to another vehicle, škoda octavia, that was moved by 9,28 meters from its original position before the crash, as well as accelerated. for this reason, the tested vehicle was slowed down, which does not happen in case of frontal collision with a rigid barrier. this parameter, therefore, should not be considered. 6. the dependencies of the hic criteria on the speed of the vehicle in this chapter, the correlation of the possibility of the driver’s head injury and the speed of the vehicle is considered in order to study the severity of injuries passengers of the car can suffer during a traffic accident. as it is clear from the results of the car collision that an airbag can have a serious impact on the resulting value of the hic criteria, the correlation assumes that the airbag was used. the graph in figure 3 presents values which also include the values established for the passenger mannequin. the linear regression function presented in the graph is applicable to a restraint system of a vehicle that has an airbag. the quantity of the correlation coefficient is calculated by equation 7. r = √ r2 = √ 0, 9024 = 0, 9499 (7) 76 vol. 12/2017 correlation analysis of vehicle frontal impact parameters figure 1. dependency of the deformation of the vehicle from collision velocity. figure 2. dependency of the deformation of the vehicle from collision velocity. 77 josef mík, jana kadlecová acta polytechnica ctu proceedings figure 3. dependency of the deformation of the vehicle from collision velocity. according to the resulting value of the correlation coefficient r = 0,9499 it is possible to see the correlation dependency as very high. however, it is necessary to take into consideration the small number of input values on the basis of which the function was solved. to establish the regression function we have used only 3 input hic values. in the case where we need to describe the seriousness of the injuries suffered by the passengers in a traffic accident, it is better to use the ais method instead of the resulting value of the criterion of the injuries. the ais method describes the seriousness of the injuries suffered by individual parts of the human body using a number in the 0 – 6 interval. this possibility is considered in the research described by [3] in the bibliography. 7. conclusion the problem of the estimation of the consequences arising from a vehicle collision was solved on the basis of establishing a dependency between the chosen vehicle parameters. in the chosen pairs of the parameters one quantity was possible to measure with ecall while another quantity was to be estimated. the dependency between the chosen parameters was described with a regression function and its accuracy was verified on the basis of the value of the correlation coefficient. in all three cases the accuracy of the quantity’s dependence of the correlation coefficient rxy was verified. in all three cases, this coefficient came out to be higher than 0,8 and therefore we can establish the dependency between the chosen quantities to be very high. however, in considering the outcome we also have to bear in mind the low number of quantities on the basis of which the correlation analysis was carried out. this fact considerably undermines the reliability of the results. to strengthen the reliability of the results it is necessary to carry out a correlation on the basis of a higher number of crash tests with different impact velocities. another way to acquire data for statistical analysis is to create a fem or multi-body model and to carry out a simulation of collision impact with simulation software. this would also allow for the repetition of the scenario with the same model and an easy change in input parameters. references [1] e. 2015/758. požadavky na schválení typu pro zavedení palubního systému ecall využívajícího linku tísňového volání 112. [2] i. 3779:2009. road vehicles – vehicle identification number (vin) – content and structure. [3] h. kovandová, h. krejčí, j. kovanda. crash tests of vehicles – selected experiments and analysis. in: transactions on transport sciences, vol. 6, no. 4/2013, issn: 1802-9876, pg. 193-202. 78 acta polytechnica ctu proceedings 12:74–78, 2017 1 introduction 2 experiment description 3 correlation analysis of crash test results 4 dependency of the vehicle body deformation from collision velocity 5 the dependency of maximum acceleration on the kinetic energy of the vehicle 6 the dependencies of the hic criteria on the speed of the vehicle 7 conclusion references 192 acta polytechnica ctu proceedings 2(1): 192–196, 2015 192 doi: 10.14311/app.2015.02.0192 the role of magnetic field for quiescence-outburst models in cvs s. de bianchi1,2, v. f. braga3, s. gaudenzi1 1dept. of physics, università degli studi di roma ”la sapienza”, piazzale a. moro 5, 00185 rome, italy 2école normale supérieure, cnrs-umr 8547, rue d’ulm 45, 75005 paris, france 3dept. of physics, università tor vergata, via della ricerca scientifica 1, 00133 rome, italy corresponding author: silvia.debianchi@uniroma1.it abstract in this paper we present the elementary assumptions of our research on the role of the magnetic field in modelling the quiescence-outbursts cycle in cataclysmic variables (cvs). the behaviour of the magnetic field is crucial not only to integrate the disk instability model (osaki 1974), but also to determine the cause and effect nexus among parameters affecting the behavior of complex systems. on the ground of our interpretation of the results emerging from the literature, we suggest that in models describing dne outbursts, such as the disk instability model, the secondary instability model (bath 1973) and the thermonuclear runaway model (mitrofanov 1978), the role of the magnetic field is at least twofold. on the one hand, it activates a specific dynamic pathway for the accreting matter by channelling it. on the other hand, it could be indirectly responsible for switching a particular outburst modality. in order to represent these two roles of the magnetic field, we need to integrate the disk instability model by looking at the global behaviour of the system under analysis. stochastic resonance in dynamo models, we believe, is a suitable candidate for accomplishing this task. we shall present the mhd model including this mechanism elsewhere. keywords: cataclysmic variables dwarf novae intermediate polars magnetic field disk instability model secondary instability model thermonuclear runaway model. 1 introduction in the last decades, methods and models used to describe dne outburst cycles achieved several results (bath & van paradijs 1983, cannizzo & mattei 1998, cannizzo 2012). however, there is no unique model that predicts these cycles (smith 2007), as well as their underlying mechanism. the limit-cycle model for dwarf nova outbursts is generally accepted, but there are still uncertainties about how to explain all the details, especially for the su uma stars (smith 2007). even if we know that the secondary stars in dne are magnetically active, yet it is unknown how they maintain a dynamo in the presence of tidal forces or whether there is differential rotation (smith 2007). crawford et al. (2008), for instance, when presenting their results of the detection of the first observed outburst of dw cnc, asked the question of what caused the outburst observed. dw cnc experienced a magnitude change of ∼4 mag showing a behavior similar to those systems whose outbursts are due to disk instability. however, with respect to pspin and porb, dw cnc shares characteristics of those systems whose outbursts are due to a mass transfer event. therefore, a decision for which mechanism was responsible for the first detected outburst could not be made. precisely in order to overcome this kind of problems of undecidability a deeper analysis of our models’ ability to explain dne outbursts is needed. 2 disk instability model and secondary instability model: some open questions each of our best available models, such as the disk instability model (dim), the secondary instability model (sim) and thermonuclear runaway (tnr) model, taken separately, shows weaknesses in predicting the outburst-quiescence cycles of many systems, such as v513 cas for outbursts during standstills (hameury & lasota 2014). dim is generally accepted as reproducing an explanation of dne outbursts and invokes an intrinsic modulation of the accretion rate in the disk. however, as seen in the case of dw cnc, the model does not satisfactorily account for all systems and suffers of some structural problems. the first one has been inherited by the disk model of shakura & sunyaev (1973) which was based on a constant value of α, a parameter that stores all unknown information of the complex friction processes. a constant α does not account for a reliable outburst amplitude (smak, 1984) and the pa192 http://dx.doi.org/10.14311/app.2015.02.0192 the role of magnetic field for quiescence-outburst models in cvs rameter appeared to be an ad hoc solution for strictly regular outbursts (bath 2004). studies concerning its variation or a turbulent α parameter aimed to the appropriate simulation of the disk instability by applying suitable corrections (latter & papaloizou 2012; penna et al. 2013; potter & balbus 2014). a second problem consists in the fact that dim does not appropriately model the observed fluctuations, due to the assumption of a constant value for mass transfer rate ṁ. third, dim shows weaknesses in predicting global changes in the disk structure, such as whether thin disk accretion can make a transition to advection-dominated flow (narayan & yi 1994, 1995a, 1995b) or how an accretion disk creates and powers jets at its center (king 2012). as an alternative to dim, sim assumes a mechanism triggering the outbursts based on mass transfer modulation from the secondary star (bath et al. 1974). by combining dynamical instabilities of the secondary, time evolution of the accretion disk together with thermonuclear burning due to accreted material, sim produces reliable results: outbursts during standstills in z cam systems can be explained only by appealing to instabilities in the flow from the secondary (hameury & lasota 2014). it seems then that the sim is decisive in predicting cvs like z cam variables and, possibly, su uma variables. nevertheless, its application has not been always straightforward. osaki (1985), for instance, explained the superoutbursts of su uma variables by using sim, but later denied this possibility in favor of a thermal-tidal-instability (tti) model (osaki 1989). finally, smak (1996) suggested a hybrid ttisim model, which is still debated today (osaki & kato 2013). 3 magnetic field and outburst modality in order to enrich the theoretical scenarios in dealing with dne outburst-quiescence cycle, the (tnr) model shows interesting implications, even if the improvement of the tnr model launched by shara (1982) never brought to a decisive point. tnr introduces a time parameter describing the recurrence of outbursts accounting for both classical novae (cne) and dne outbursts and their differences (shara 1982) and also accounts for dnos and x-ray flux variations concerning some representative systems like u gem, ss cyg, ex hya, z cam, cy cnc, ah her, cn ori, kt per. more importantly, tnr can be associated to one of the processes that causes nitrogen to carbon enhancement observed on the surface of the wd in vw hyi and u gem (sion 2014). in tnr models, outbursts are the product of thermonuclear burning onto the wd surface (mitrofanov 1978; 1980). in the presence of a magnetic field of the order of 3 · 106 − 3 · 107 g on the surface of the degenerate component, a local accumulation of hydrogen could generate, through thermal instability, a thermonuclear burning responsible of the outburst. for this process to occur, an intense magnetic field 106−108 g is required; primaries with such fields do exist in intermediate polars (ips) and polars that owe their properties to the very primary magnetic field (patterson 1994). therefore, the magnetic field strength on the wd surface allows us to distinguish novae, dwarf novae and novalike stars. in exploring models that go beyond the use of tnr alone, livio (1983) emphasized the importance of the magnetic fields in cvs (livio & verbunt 1988; meyer-hofmeister et al. 1996). as livio (1983) shows, nova explosions can be inhibited if the magnetic field strength is over a certain limit, explaining in this way why active novae are absent among polars while novae can be found among the ips. figure 1: upper limits on the magnetic field strength for which nova outbursts can occur (provided that matter is confined to polar caps) as a function of the accretion rate, for various white dwarf masses. reproduced from livio, m., 1983, astronomy and astrophysics, 121, l7, with permission from astronomy and astrophysics, c©eso. livio’s approach allows one to determine upper limits on the magnetic field strength for which nova outbursts can occur when polar caps are present. these upper limits can be represented as a function of the accretion rate and for different wd masses (see figure 1). even if a magnetic field more intense than a certain critical value could inhibit nova explosions (livio 1983), a magnetic field is strictly correlated, on an intensity-dependent scale, to different modes of accretion and thermonuclear runaways. these modes in193 s. de bianchi, v. f. braga, s. gaudenzi clude the channeling of the accreted matter into magnetic polar caps, the alteration of radiative and conductive opacities, interference with the development of convection that likely is also present in non magnetic cvs. furthermore, the magnetic field of the wd itself may affect the nova outbursts, e.g. by enhancing mass loss in the equatorial plane (livio et al. 1988; prialnik & livio 1995). this reveals how the magnetic field activates different and specific dynamic pathways. in other words, to focus on the role of a variable magnetic field and its effects can be helpful for identifying common features of both magnetic and non-magnetic cvs, as well as for obtaining crucial information on the magnetic field responsible for switching a particular outburst modality. indeed, according to livio (1983), the pressure at the base of the accreted matter is the physical parameter that determines the outcomes of a tnr: pb = gmwd r2 wd ∆macc acap (1) when pb exceeds a critical value of some 1019dyne/cm2, ignition occurs. it should be noted that, for higher cno abundances, the critical pressure for the occurrence of an outburst is lower. this fact leads us to the investigation of cno abundances and of the presence of material accumulating during quiescence, in order to speculate on the triggering of an outburst. it means that a function of “prediction” and not of mere “description” can be added to the model. figure 2: e(b-v) varies with both the orbital phase and the percentile quiescence time (t%). 2175 å absorption bump is believed to be caused by c60. e(b-v) takes the highest value at the “heart” of the quiescence. c60 molecules could be reasonably supposed to be affecting the modulation of time elapsing between the outbursts. reproduced from gaudenzi et al. (2011), astronomy and astrophysics 525, a147 with permission from astronomy and astrophysics, c©eso. more importantly, the analysis of uv spectra revealed the presence of fullerenes as an intrinsic source of reddening in ss cyg (gaudenzi et al. 2011, see figure 2). density gradients in the disk may influence the accumulation of molecules in specific sites of the disk itself where they are accreted and/or ejected during the quiescence-outburst cycle. along with accretion/ejection mechanisms, a major role in determining the structure and the stability of the disk might be played by the solid-gaseous phase transitions of fullerenes (gaudenzi et al. 2012) at 5855 k (hussien et al. 2008). on the ground of the hypothesis that the quiescence-outburst phase transition is the effect of a stochastic resonance mechanism, the presence of material accumulated in the disk could trigger wave amplification within the disk even during quiescence. this would introduce in our modelling of the disk instability an amplification of stochastic resonance induced by turbulent fluctuations (benzi & pinton 2011). 4 magnetic field and accretion behaviour seminal numerical studies of the influences of a magnetic field on the flow structure were performed in the early 1990s either in the frame of simplified models (king 1993; wynn & king 1995; wynn et al. 1997; king & wynn 1999; norton et al. 2004; ikhsanov et al. 2004; norton et al. 2008) or in a limited region of the stellar magnetosphere (koldoba et al. 2002; romanova et al. 2003, 2004). in the last few years, there have been attempts to develop a comprehensive 3d numerical model to calculate the flow structure in close binaries (zhilkin & bisikalo 2009, 2010; bisikalo & zhilkin 2012). according to our interpretation of the results of 3d mhd simulation of bisikalo & zhilkin (2012), it emerges that the value of the magnetic induction on the surface of the accreting star activates the dynamic pathway and, in doing so, distinguishes two different modalities of accretion. in the numerical model, bisikalo & zhilkin (2012) take into account radiative heating and cooling,as wells as diffusion of the magnetic field due to dissipation of currents in turbulent vortexes, magnetic buoyancy, and wave mhd turbulence. the interesting result consists in that if the magnetic field induction grows, the cross-section of the stream decreases and the accretion rate decreases as well, thereby influencing density and pressure, and thus affecting the possibility of triggering an outburst. we interpret the nonmonotonic variation of the magnetic field behaviour and its consequences reported in (bisikalo & zhilkin 2012) as stochastic oscillations of b. in particular, magnetic field values around 106g could originate phase transitions between polar and ip. based on our previous work (gaudenzi et al. 2012), we suggest that the dynamic pathway of accretion can account for instabilities of many cvs and discloses the possibility of acquiring an ability to predict their behaviour as transition ob194 the role of magnetic field for quiescence-outburst models in cvs jects. the physics behind this process concerns the amplification of stochastic resonance induced by turbulent fluctuations, i.e. the amplitude of the external periodic perturbation needed for stochastic resonance to occur is much smaller than the one estimated by the equilibrium probability distribution of the unperturbed system (benzi & pinton 2011). 5 discussion and conclusion in order to enrich the theoretical scenario we stressed the relevance of the accretion process that may change the surface magnetic field of an accreting wd significantly (cumming 2002), and, in agreement with (livio 1983), that the magnetic field of the wd itself may affect the nova outburst. we also remarked that the accumulation of some type of molecules, such as c60 molecules within the disk is important to understand the behavior of dne in quiescence and that the cno abundances also depend on the presence of a magnetic field (even of a weak one). if we want to predict the behavior of a complex system, such as dne outbursts cycles, neither dim nor any other model taken alone is sufficient. in fact, dim explains why thermal instability leads to outbursts, but it does not provide exact information at the level of the global system in the quiescent phase, e.g. whether there are transition from a state to another, and how to determine time-dependent α variations. in order to obtain more information about the dne outburst-quiescence cycle, we shall work on a model that is able to predict future states of these systems, as well as to describe the present state, without appealing to ad hoc assumptions. as previously stated, local, small-scale turbulences, may likely represent the physical outcome of stochastic oscillations of the magnetic field. among the other mechanisms, such as a strong differential rotation and vertical density gradients (pudritz 1981b), turbulences can be responsible for the generation of a large-scale magnetic field (pudritz 1981a). therefore, the development of mean field dynamo theory is crucial in order to overcome some difficulties related to current models in accounting for the role of the magnetic field generated in a turbulent medium. this theory, if appropriately modified, could allow us to calculate the overall structure of the global field without any detailed knowledge of the small-scale turbulence. even if this echoes earlier works by shakura & sunyaev (1973) there is a characteristic of the mean field dynamo theory that makes it different from earlier theories. it builds up a large-scale magnetic field trying to achieve a global redistribution of the angular momentum, whereas other models assume a smallscale field and a local viscosity. therefore, in order to gain knowledge on the underlying mechanism producing outbursts, such as the one presented in dw cnc, we will 1) simulate the interaction among thermonuclear runaway and magnetic field leading to a specific modification of the α viscosity parameter to be integrated in the model (switching the outburst modality); 2) model dynamic pathways of accretion due to the magnetic field behaviour by means of a mhd model that takes into account stochastic resonance in a mean field dynamo model. we shall present and discuss this simulation elsewhere, but we presented here the theoretical assumptions that inspired our study. references [1] bath, g.t.: 1973, nphs, 246, 84. 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[54] zhilkin, a.g., bisikalo, d.v.: 2010, adspr, 45, 2420. 196 http://dx.doi.org/10.1093/mnras/sts185 http://dx.doi.org/10.1093/mnras/stu519 http://dx.doi.org/10.1086/377514 http://dx.doi.org/10.1086/421867 http://dx.doi.org/10.1086/160376 http://dx.doi.org/10.1086/131295 http://dx.doi.org/10.1080/00107510601181175 http://dx.doi.org/10.1093/mnras/286.2.436 introduction disk instability model and secondary instability model: some open questions magnetic field and outburst modality magnetic field and accretion behaviour discussion and conclusion 303 acta polytechnica ctu proceedings 2(1): 303–307, 2015 303 doi: 10.14311/app.2015.02.0303 gloria and study of cataclysmic variables r. hudec1,2, v. šimon1,2 1astronomical institute, academy of sciences of the czech republic, cz-25165 ondřejov, czech republic 2czech technical university in prague, faculty of electrical engineering, prague, czech republic corresponding author: rene.hudec@asu.cas.cz abstract we report here on the ongoing eu fp7 project gloria (global robotic-telescopes intelligent array) with emphasis on possibility of investigation of cataclysmic variables by users. gloria will enable the first free and open-access network of robotic telescopes in the world. we show several examples of the not often used topics (but suitable for gloria) for the studies of activity of cataclysmic variables, e.g. search for outbursts in intermediate polars and the fluctuations of brightness in their quiescence, and investigation of the optical counterparts of supersoft x-ray sources. keywords: catalysmic variables: optical photometry spectroscopy: low-dispersion spectra data mining robotic telescopes project eu fp7 gloria. 1 introduction the research in astronomy poses two main challenges, namely 1) the immensity of the sky and 2) the huge amount of astronomical data being gathered. in fact, astronomers are nowadays facing great difficulty in finding the resources to analyze the increasing flood that modern astronomy instruments generate,insufficient computing power, insufficiently powerful software tools, and not enough hours in the day. the sky comprises 40 000 square degrees (144 million square minutes). the future professional projects (like lsst) intend to observe a very significant fraction of it on a regular basis. in order to meet the abovementioned challenges, an increasing number of astronomical projects have begun to try to foster citizen participation to help analyze data by using collaborative internet application (the so-called web 2.0). gloria stands for “global robotic-telescopes intelligent array”. gloria will be the first free and open-access network of robotic telescopes in the world. it represents a web 2.0 environment where users can do research in astronomy by observing with robotic telescopes, and/or by analyzing data that other users have acquired with gloria, or from other free access databases like the european virtual observatory (http://www.euro-vo.org). the gloria project (http://gloria-project.eu) represents the first attempt to create a network of about 20 robotic telescopes for public access, education, science and much much more. the gloria network has 17 network telescopes now. few others are expected to join us soon. many internet communities have already formed to speed-up scientific research, to collaborate in documenting something, or for social projects. research in astronomy can benefit from attracting many eyes to the sky – to detect something in the sky requires looking at the right place at the right time. our robotic telescopes can search the sky, but the vast quantities of data they produce are far greater than astronomers have time to analyze. gloria will provide a way of putting thousands eyes and minds on an astronomy problem. gloria is intended to be a web 2.0 structure, with the possibility of doing real experiments. the community will not only generate content, as in most web 2.0, but it will control telescopes around the world, both directly and via scheduled observations. the community will take decisions for the network and it will give “intelligence” to gloria, while the drudge work (such as drawing up telescope schedules that satisfy various constraints) will be done by algorithms that will be developed for the purpose. gloria project will define free standards, protocols and methodology for the following purposes: controlling robotic telescopes and all related instrumentation (i.e. cameras, filter-wheels, domes...); giving web access to the network: access to an arbitrary number of robotic telescopes via a web portal; conducting online experiments (it will be possible to design specific web environments for controlling telescopes for research on a specific scientific issue; conducting off-line experiments (it will be possible to design specific web environments for analyzing astronomical meta-data produced by gloria or other databases). 303 http://dx.doi.org/10.14311/app.2015.02.0303 r. hudec, v. šimon the main part of gloria is represented by providing access to robotic telescopes, online experiments (observations), offline experiments (research on acquired images), live broadcasting, social network + community, and support. 2 objectives and benefits of gloria project the world-wide network of robotic telescopes (gloria) does not intend to compete with lsst, but our underlying idea is that “the more eyes we put on the sky, the more and greater the scientific discoveries will be achieved”. thus, in order to try to improve the way we are doing astronomy research, this project aims to build the first open access world wide network of robotic telescopes to serve citizens from around the world for free, but competing for observing time. hence gloria is indeed an “intelligent array” and it bases its intelligence in its community of users (sanchez moreno et al. 2013). like most web 2.0 projects, gloria implements a reputation-based scoring system to reward user contributions, driven by parameters such as the quality of the gathered and processed images, number of hours invested in the observations etc., as well as the votes granted by the rest of the community that finally evaluate the quality of the work done. at the end of month 24 (october 2013) the consortium produced a standard for adding new telescopes and experiments to this network. unlike other private, profitable ongoing initiatives, the gloria network will provide a free, twofold service to the community: 1. giving citizens (including professional and amateur astronomers) free access to a network of 17 robotic telescopes spread into 4 continents and both hemispheres. in the case of professional astronomers, preferences will be given to those of developing countries who will lack astronomical facilities in their own nations. 2. giving citizens an easy web access to all the data collected by the robotic telescopes. beside giving service to research, in order to manage gloria main objective of fostering astronomical research, by allowing near continuous observations of a given target thanks to its world-wide network facilities, the following additional tangible outcomes will be pursued: 1. gloria will enable more telescopes to join, producing methodology, standards, software, and documentation oriented to teach professional and amateurs astronomers to robotize their telescopes and to integrate them into the gloria network. 2. gloria will enable more research goals to be pursued, producing methodology, standards, software, and documentation oriented to teach amateurs and professional astronomers to design new web experiments and to integrate them into the gloria network. 3. gloria will encourage participation in order to increase in number its community of citizen scientists. newcomers are very welcomed. 4. gloria will give free access to knowledge to everybody. all knowledge (software, manual, standards, documents, astronomical images, data, etc) produced by the gloria consortium and by the community will have copyleft licenses. 5. gloria intends to continue in the future. since the maintenance of the distributed telescopes is paid by their owners, the cost of the core part is very cheap. the consortium believes that an economic model based in public and private subventions and donations is very possible for a project like this to go on beyond 2014. 2.1 impact the gloria partners really believe in the enormous power of astronomy as a center of interest in scientific and human training of our young people. the children of today will be the astronomers of tomorrow. in order to enroll newcomers and awaken interest in astronomy among children, during 2012-14 we will organize the live internet broadcast of 5 astronomical events: 4 eclipses and a transit of venus, which will be made from the gloria network, with associated activities in all schools of the partners countries, with the aim of getting students and teachers participating in researchbased science education and improving their motivation to push the barriers of science education further. 2.2 the gloria consortium gloria brings together a critical mass of scientific and technological leaders with extensible expertise in tic and astronomical research, dissemination and developed of technological projects. the gloria consortium includes these institutions: upm–technical university of madrid computer science faculty–spain (project coordination); auav–astronomical institute, academy of sciences of the czech republic; csic–spanish research council–spain (scientific coordination); cvtu– czech technical university–czech republic, ip-ascr– institute of physics of the academy of sciences of the czech republic; iac–astrophysic institute of canarias–el teide observatory–spain; inaf–istituto nazionale di astrofisica–italy; sao–special astrophysical observatory–russia; ucdnuid–university college dublin–ireland; uc–university of chile– chile; uma–university of málaga–spain; uoxf– oxford university–u.k.; uniwarsaw–university of warsaw–poland. 304 gloria and study of cataclysmic variables 3 gloria and cataclysmic variables cataclysmic variables (cvs) represent important targets for gloria observations mainly because of the unique properties of their activity. the category of these systems as a whole displays a very broad range of the optical activity observable by the common ccd photometers (e.g. flickering on the timescale of minutes and the amplitude of several tenths of magnitude, the orbital modulation sometimes with the amplitude of more than a magnitude, outbursts or transitions between the high and low states on the timescales of days or weeks and the amplitudes of several magnitudes). the basic properties of the optical emission of cvs need to be taken into account for the optimal strategy of monitoring of these targets in citizen science mode. we will show some examples of activity of various types of cvs which can be the targets for the gloria observers. 3.1 distribution of cataclysmic variables the sky generally, celestial objects are not distributed uniformly in the sky mainly because of the structure of our galaxy. as shown by warner (1995), most known cvs are observed over the whole sky. the reason is that cvs represent rather low-luminosity population, hence only the nearby such objects are bright enough to be observed. this relatively isotropic distribution implies that all the observatories at any latitude, included in gloria, will have the opportunity to choose a sufficiently large amount of suitable cvs. only known classical novae concentrate toward the galactic plane and the galactic bulge. the reason for this is the high optical luminosity in their explosion, hence even the objects at the large distances from the earth are observable. the direction toward the center of the galaxy is therefore also suitable for searching for explosions of classical novae with a monitor. this favors the gloria observatories located in the southern hemisphere. 3.2 peak magnitudes of cvs the catalog of downes et al. (2001) shows that the apparent peak magnitudes of various cv types differ from each other. the reason is that there is a significantly different typical optical luminosity of each type. the explosions of classical novae dominate in the brightest part of the distribution. their peak magnitude is often near 9–10 (but some of them may be even brighter), so they are easily detectable even by small-aperture, wide-field monitors. the brightest known dwarf novae in outburst can reach about 10–12 mag but about 13 mag is more typical. cvs with strongly magnetized white dwarf (polars) (e.g. warner 1995) are rather faint systems (13–20 mag), so their observing requires the largest available telescopes. 3.3 prospects of cv investigation by gloria below we give some examples of cv types suitable for investigation within gloria project. dwarf novae with their large-amplitude (3–5 mag) outbursts with the typical recurrence time from weeks to months (e.g. warner 1995; hameury et al. 1998) are definitely very promising targets even for the modest telescopes included in gloria. we also emphasize the need of observing the outbursts of some intermediate polars (i.e. cvs with a mildly magnetized white dwarf) (e.g. warner 1995). these systems may be promising gloria targets, as illustrated by the optical activity of do dra/2a 1150+720 (fig. 1). what typical features of activity of this object can be explored by gloria telescopes? (1) rare strong outbursts with the amplitude of 4–5 mag, lasting for several days and separated by ∼800 days. these short outbursts with the decay faster than exponential and faster than predicted for non-magnetic dwarf novae are in accordance with angelini & verbunt’s (1989) model for outbursts in a disk with a large missing inner region (caused by the magnetosphere of the white dwarf). (2) strong and unpredictable fluctuations on a timescale of several days or weeks (much longer than the orbital period). figure 1: the light curve of the intermediate polar do dra with rare outbursts. the points denote the one-day means, the v symbols represent the upper limits of brightness. adapted from šimon (2000). the short outbursts or rather flares of uncertain origin may occur in various intermediate polars but since they are short, extensive monitoring is needed to detect them (van amerongen & van paradijs 1989; warner 1995). supersoft x-ray sources (van den heuvel et al. 1992) represent another example of interesting objects to be investigated by gloria experiments. since the very soft x-ray emission is easily absorbed both inside 305 r. hudec, v. šimon the source and the interstellar medium, only a small fraction of supersoft x-ray sources is observable in the x-ray spectral region. the specific spectroscopic properties of their optical emission give a hope to discover more such objects even without detecting their x-ray emission (steiner & diaz 1998). the system v sge (herbig et al. 1965) can be considered to be a member of this category (steiner & diaz 1998; greiner & van teeseling 1998). since it is relative bright (10– 13 mag(v )), it is easily observable by various robotic telescopes. v sge shows a very complicated long-term optical activity. its outbursts were observed mainly in the first half of the twentieth century (šimon & mattei (1999). however, this activity changed considerably with time. a segment of the long-term activity starting in the 1960s is displayed in fig. 2). it consists of the one-day means of brightness. the high/low state transitions occurred in some time segments. they were separated by the so-called flat segments (e.g. in the surroundings of jd 2 445 000), in which only small fluctuations of brightness remained. we can thus conclude that observing at various times captures this object in different phases of its strong activity. figure 2: long-term activity of the supersoft xray source v sge in the optical band (aavso and afoev data). notice the large variations of activity on the timescale of several years (from the segments with only small fluctuations to the segments with the large-amplitude transitions between the high and low states). adapted from šimon & mattei (1999). dividing the series of the optical (ccd and photoelectric) observations of v sge, obtained at different nights (fig. 3), into the groups according to the states of the long-term activity enables to obtain the profile of the typical orbital modulation (the orbital ephemeris of smak 1995 was used). this figure shows the intensity transfered from magnitudes. although the profile of the orbital modulation can be resolved in the individual night series, the orbital period of 12.34 hours (smak 1995) does not enable to observe the whole period from a given observing site, so a multisite campaign is desirable, especially because the orbital modulation and the long-term variations of brightness are superimposed. this means that observations obtained by gloria may provide valuable information. when the intensity of a given nigh series belonging to a given such group is shifted to match a template, it is possible to obtain the mean profile of the orbital modulation characteristic for the group (fig. 3). figure 3: photoelectric and ccd observations of v sge in the intensity scale, folded with the orbital phase (the orbital ephemeris of smak 1995). the sorting into three groups is marked. adapted from šimon et al. (2002). the cv in pegasus (found in eruption in may 8, 2010) can serve as an excellent example of a bright object which can be easily detected by amateurs and small cameras and/or telescopes. this object represented a rare type of dwarf nova with very long recurrence time (67 years) but with very large amplitudes (hudec 2010). we note that the second flare of this cv in 1942 was found using astronomical photographic archives which are perfectly suitable for this kind of work, i.e. searches for past historical flare events (hudec 1999 and 2007, hudec et al. 2012). only very few such objects are known because they can easily escape detection. in this regard, we also note that the sky contains many yet unknown transients of various kinds. they are identified and attract attention only when they show themselves in their outbursts. the follow-up observations then can lead to the study of their type. one such object detected by a very small telescope is the lǐska flare star. it is an example of the astronomical discovery of a very rare event with a small amateur telescope (lǐska et al. 2014). we suggest that this object is a new flare dme star of uv ceti type based on its color, x-ray emission, and the properties of the optical light curve of the flare, identical with the x-ray sources rx j1118.3+1347 and 2e 1115.8+1403.in simbad, these 2 sources are given as different sources but this is probably wrong and it is only one source. the object may be similar to the exosat x-ray source exo 020528+1454.8 = g 035–027 with a dme flare star counterpart detected by hudec et al. (1988). 306 gloria and study of cataclysmic variables figure 4: photoelectric and ccd observations of v sge from fig. 3, now sorted into three groups according to their mean intensity (the orbital ephemeris of smak 1995). the individual curves of each group were shifted in intensity to match the template. the scale of the ordinate is identical for all three plots. adapted from šimon et al. (2002). the observations and research of cvs are expected to be supervised by gloria scientists. everybody can join us at http://gloria-project.eu, and this involves both citizen science and students, as well as professional astronomers. typically 20 percent of robotic telescopes (rts) observing time is expected to be used for gloria users, but some telescopes such as bart will provide up to 40 percent. acknowledgement gloria is project supported by the eu fp7 program (no. 283783). the scientific part of the study is linked to the the ga cr grant 13-39464j (optical analyses) while the long–term x–ray analyses are related to ga cr grant 113-33324s. we used the observations from the aavso international database (massachusetts, usa) and the afoev database operated in strasbourg, france. we thank the variable star observers worldwide whose observations contributed to this analysis as well. references [1] angelini, l., verbunt, f., 1989, mnras, 238, 697 doi:10.1093/mnras/238.3.697 [2] downes, r. a., et al., 2001, pasp, 113, 764 doi:10.1086/320802 [3] greiner, j., van teeseling, a., 1998, a&a, 339, l21 [4] hameury, j.-m., et al., 1998, mnras, 298, 1048 doi:10.1111/j.1365-8711.1998.01773.x [5] herbig, g. h., et al., 1965, apj, 141, 617 doi:10.1086/148149 [6] hudec, r., 2010, the astronomer’s telegram, no. 2619 [7] hudec, r., et al., 1988, astronomical institutes of czechoslovakia, bulletin (issn 0004-6248), vol. 39, no. 5, p. 296-302 [8] hudec, r., et al., 2012, acta polytechnica, ibws2011 proceedings, 1(52) [9] hudec, r., 2007, in exploring the cosmic frontier: astrophysical instruments for the 21st century. eso astrophysics symposia, isbn 978-3-540-39755-7. germany, p.79 doi:10.1007/978-3-540-39756-4 25 [10] hudec, r., 1999, an introduction to the world’s large plate archives, acta historica astronomiae, vol.6, 28 [11] lǐska, j., et al., 2014, acta polytechnica, accepted [12] sanchez moreno, f. m., et al., 2013, glorora documentation package [13] šimon, v., mattei, j. a., 1999, a&as, 139, 75 [14] šimon, v., 2000, a&a, 360, 627 [15] šimon, v., hric, l., petŕık, k., et al., 2002, a&a, 393, 921 [16] smak, j., 1995, acta astron., vol. 45, 361 [17] steiner, j. e., diaz, m. p., 1998, pasp, 110, 276 doi:10.1086/316139 [18] van amerongen, s., van paradijs, j., 1989, a&a, 219, 195 [19] van den heuvel, e. p. j., et al., 1992, a&a, 262, 97 [20] warner, b., 1995, cataclysmic variable stars, cambridge univ. press, cambridge doi:10.1017/cbo9780511586491 307 http://dx.doi.org/10.1093/mnras/238.3.697 http://dx.doi.org/10.1086/320802 http://dx.doi.org/10.1111/j.1365-8711.1998.01773.x http://dx.doi.org/10.1086/148149 http://dx.doi.org/10.1007/978-3-540-39756-4_25 http://dx.doi.org/10.1086/316139 http://dx.doi.org/10.1017/cbo9780511586491 introduction objectives and benefits of gloria project impact the gloria consortium gloria and cataclysmic variables distribution of cataclysmic variables the sky peak magnitudes of cvs prospects of cv investigation by gloria 132 acta polytechnica ctu proceedings 1(1): 132–138, 2014 132 doi: 10.14311/app.2014.01.0132 auger highlights antonella castellina 1 for the pierre auger collaboration 2 1istituto nazionale di astrofisica oato and infn torino 2observatorio pierre auger, av. san mart́ın norte 304, 5613 malargüe, argentina (full author list : http://www.auger.org/archive/authors 2012 12.html) corresponding author: castellina@to.infn.it abstract the pierre auger observatory has been designed to investigate the origin and nature of the ultra high energy cosmic rays using a hybrid detection technique. a review of selected results is presented, with the emphasis given to the measurement of energy spectrum, mass composition and arrival directions. keywords: ultra high energy cosmic rays sources energy spectrum primary composition. 1 introduction understanding the sources, nature and propagation properties of the cosmic rays at ultra high energies (e > 1018 ev) is one of the key questions in astroparticle physics. from the experimental point of view, their study can be performed indirectly, by exploiting the extensive air showers they produce by interacting with the nuclei in the earth atmosphere. among the different features characterizing the spectral shape, the region between' 1018−1019 ev is thought to host the transition from galactic to extragalactic cosmic rays. different models explain it as due to e+/e− pair production of protons with the photons of the cosmic microwave background (cmb) [1], or more traditionally to the intersection of a steep galactic component and the onset of a flatter extragalactic one [2]. at even higher energies, above ' 4 1019 ev, a cut-off in the cosmic ray flux is expected, due to photo-pion production of extragalactic protons in the cmb (the ”gzk cut-off” [3]) , although the same feature could arise when reaching the limits in the maximum energy of the sources. however, the all particle spectrum cannot provide a discrimination among the different hypotheses, and the determination of the primary composition is mandatory to reach any reliable conclusion. the analysis of the arrival directions and their anisotropy can give further insight into the sources and provide information about the magnetic fields which the ultra high energy (uhe) cosmic rays experience during their travel to earth. the pierre auger observatory has been specifically designed to investigate the origin and the nature of ultra high energy cosmic rays. it is located in malargüe, argentina, and consists of a surface array (sd) of 1660 water cherenkov stations on an area of ' 3000 km2, overlooked by 27 air fluorescence telescopes (fd) grouped in four sites [4]. thanks to the possibility of combining the information from the surface array, measuring the lateral distributions of secondary particles at the ground, and the fluorescence telescopes, observing the longitudinal profile, the reconstruction capabilities are enhanced with respect to the individual detector components. 2 the energy spectrum the energy spectrum above 2.5 1018 ev has been determined using the data from the sd [5], considering only events up to 60◦. the exposure is obtained by integrating the number of active stations over time; the overall acceptance uncertainty is ' 3% [6]. the energy calibration is derived directly from data, using a subset of high quality hybrid events, i.e. events reconstructed by both the fd and the sd [7]. despite the low duty cycle of the fd, the energy spectrum could be extended to 1018 ev using hybrid events, thanks to the good energy resolution and low threshold, thus investigating the transition region in detail [8]. the total systematic uncertainty in the energy scale is about 22%, the main contribution coming from the uncertainty in the fluorescence yield (14%) and in the reconstruction of the longitudinal profile (10%). the sd and hybrid spectra can be combined using a maximum likelihood method, since both have the same systematic uncertainties in the energy scale. the normalization uncertainties are on the contrary independent and have been used as additional constraints in the 132 http://dx.doi.org/10.14311/app.2014.01.0132 auger highlights procedure. the resulting spectrum is shown in fig.1; a fit with three power laws is shown by the dashed lines, while the solid line indicates the result of a fit with two power laws and a smooth function. the ankle feature is present at an energy of 1018.62 ev; the cutoff is clearly seen with a significance of 20 σ. different astrophysical models can be compared to our data; however, the energy spectrum can be described by both a heavy or proton composition at the highest energies and the information must be complemented by independent measurements of the primary composition. figure 1: the combined auger energy spectrum. only statistical uncertainties are shown. the systematic uncertainty on the energy scale is 22%. a comparison of the auger results with data from hires, telescope array and yakutsk has been recently performed [9]. the various fluxes can be rescaled assuming that any difference among them be due solely to energy scale and not to aperture calculations or energy resolution. the differences found are entirely consistent with the systematic energy uncertainties quoted by the experiments. 3 the nature of the primaries the most direct information about composition can be obtained by measuring the longitudinal development of showers in hybrid events, thus determining the mean depth of maximum development, xmax, and its fluctuation, rms(xmax). for each event, xmax depends on the depth of first interaction of the primary in the atmosphere and on the subsequent development of the shower; for this reason, the interpretation of the results in terms of composition is complicated by the uncertainties in the hadronic interaction models used in simulations. about six years of hybrid data have been analyzed applying fd quality cuts and ensuring that no bias with respect to the cosmic ray composition is introduced in the data sample [10]. having been corrected for the detector resolution, the xmax and its fluctuations are detector independent and can be directly compared to the predictions of different models, as shown in fig.2. both observables show a change for e > 5 1018 ev towards an increasingly heavy composition in comparison to the model predictions. the average resolution of xmax ' 20 g/cm2 in the considered energy range. figure 2: < xmax > and rms(xmax) as a function of energy, compared with the predictions of air shower simulations using different hadronic interaction models. in general, different values for rms(xmax) are allowed for different combinations of elements [11]. the fluctuations predicted by the considered hadronic interaction models and shown in fig.2 are evaluated only for pure compositions . a different conclusion, leading to a light composition up to the highest energies, has been drawn from the data of the hires and telescope array. however, a direct comparison of their results with the auger ones is not possible, as the detector biases are included in their simulation. furthermore, their dataset is smaller that that of auger. a lenghty discussion about this comparison can be found in [12] (and refs. therein). starting from an extension of the heitler model of ex133 antonella castellina tensive air showers [13], a method for interpreting the results of xmax and rms(xmax) in terms of mass composition has been developed. as discussed in [14], xmax is only function of the mean logarithmic mass < ln a >, and as such it carries information on the average composition. on the contrary, both the shower-to-shower fluctuations and the dispersion in the mass distribution contribute to rms(xmax). this information can be used to extract the < lna > and its variance from the observables and to build the plots shown in fig.3, where the size of the data points increases with increasing energy. the energy evolution of the composition is common to all models; reduced systematic uncertainties will allow in the future to test or even exclude some of them. different consequences from the astrophysical point of view can be derived from this comparison. extragalactic sources of protons seem to be disfavored by our composition result, within the uncertainties on the hadronic interaction models used to interpret the data. in a propagation scenario, nuclei from nearby sources could produce small mass dispersion at earth, as propagation would not be able to degrade mass and energy. if on the other hand the proton component is depleted by the reach of a rigidity dependent end of the injection spectrum, and if sources are uniformly distributed, hard injection spectra with low energy cutoff, together with local sources, could explain the data [15, 16]. figure 3: the pierre auger data in the (< lna > , σ2 lna) plane for different hadronic interaction models. grey contours limit the allowed region, the systematic uncertainties are shown by the black lines. 4 primary photons and neutrinos uhe primary photons and neutrinos can provide invaluable information about the astrophysics of cosmic rays. their detection would be a direct proof of the gzk cutoff; limits on exotic models [17] and tests for new physics [18] could be obtained from a positive or negative result on their detection. in both cases, their search is based on the characteristic features of the showers they produce in comparison to the hadronic ones. primary photons produce late developing showers, a characteristic further enhanced by the lpm effect [19]. the deeper xmax, observable by the fd, is associated to a more dispersed distribution of the arrival time of the particles at ground level. at a given distance from the shower axis, the arrival time of the first particles is delayed with respect to a planar shower front and the radius of curvature is thus expected to decrease for photon induced showers. these observables can be recorded by means of the sd. the upper limits derived from both the sd and the hybrid data collected by auger are shown in fig.4 and discussed in [20, 21]. astrophysical scenarios are favoured with respect to top-down models. figure 4: upper limits on the integral photon flux. different expectations are shown for comparison. primary neutrinos can produce showers characterized by a significant electromagnetic component; the huge hadronic background can be eliminated by looking at large zenith angles. ντ can interact by charged current in the earth crust, producing earth-skimming upward-going showers; neutrinos of any flavour can interact in the atmosphere by neutral or charged current giving rise to downward-going events. assuming a diffuse neutrino flux ' k e−2, 90% cl limits on their flux are obtained, as shown in fig.5. neutrinos from point sources were also searched for, over a broad declination range (north of ' −65◦ and 134 auger highlights south of ' 55◦), for eν > 1017 ev. for a differential neutrino flux ' kps e −2, 90% cl limits of ' 5 · 10−7 and 2.5 · 10−6 gev cm−2 s−1 have been obtained for up-going and down-going events respectively [22]. figure 5: differential and integrated upper limits on the single flavour e−2 ν flux (thin black lines: downward-going ν, thick red line: earth-skimming ν). different models are shown for comparison and discussed in [23] 5 anisotropies the spatial distribution of the arrival directions of uhe cosmic rays as a function of energy is a key observable to provide information about their sources and nature, complementary to those of energy spectrum and composition. particles of uhe are most probably extragalactic, and if the observed cutoff in the spectrum can be attributed to the gzk propagation effect we could expect their sources to be confined in our courtyard, within about 100 mpc. in 2007 [24] the auger collaboration reported the observation of a correlation between the arrival directions of the highest energy cosmic rays and the positions of nearby agn from the véron-cetty-véron catalogue [25]. the result came from an analysis of indipendent data with a priori parameters determined from an exploratory scan; this allowed to avoid the use of penalty factors which would be needed in a posteriori analyses. the most recent update of this search is shown in fig.6 [26]: the fraction of correlating cosmic rays is (33±5)% (28 events correlating out of a total of 84). the probability of this correlation to occur by chance if the true distribution of arrival directions is isotropic stays below 1%. the independent averages of 10 consecutive events are also shown (black dots). a recent comparison of our result with the telescope array and the yakutsk ones showed that the correlating fractions are compatible [27]. more data are necessary to show whether this correlation is statistically significant or not. figure 6: the correlating fraction as a function of the total number of time-ordered events. different confidence levels are shown, together with the isotropy value p=0.21 and the current estimate of all data, pdata = 0.33± 0.05. another possible scenario is that the anisotropy is dominated by cosmic rays originating from the vicinity of centaurus a, the nearest active galaxy with an estimated distance of about 3.8 mpc, since 19 events out of 7.6 expected have arrival directions within 24◦ of cena. a kolmogorov-smirnov test shows that the chance probability for this to happen is 4%. directionally aligned events, or ”multiplets”, can be expected from the same source after deflection in the magnetic fields, showing a correlation between the arrival direction and the inverse of energy. the largest multiplet found was one 12-plet, but also in this case the probability for it to come from an isotropic distribution is ' 6% [28]. potential sources of galactic cosmic rays have been looked for by performing a blind search for neutron primaries [29]. in fact, due to the relativistic time dilatation the uhe neutron mean decay length is (9.2× e/eev ) kpc; above 2 eev, neutron emitters can be searched for in the whole galaxy. auger can detect neutron showers by a simple search for an excess of proton-like showers from a specific direction in the sky. no candidates have been found, bringing to a median flux upper limit of 0.0114 n km−2 yr−1 above 1 eev. the absence of a neutron flux from the galaxy, which could be expected in the hypothesis of sources steadily emitting protons and neutrons with similar luminosity, could be a hint that the sources at eev energy could be e.g. extragalactic, transient or weak but densely distributed. the large scale distribution of the arrival directions of cosmic rays is another fundamental tool in the search for their origin. the results from a study performed using data from the sd array are shown in fig.7 [30]. no significant anisotropies are observed, resulting in the most stringent bounds on the first harmonic amplitude above 2.5 1017 ev. 135 antonella castellina figure 7: equatorial dipole component (top) and phase of the first harmonic (bottom) as a function of energy. the obtained limits already exclude some of the galactic models (labeled in fig.7 with a and s, indicating antisymmetric and symmetric galactic magnetic fields), according to which the cosmic rays at these energies are galactic and can escape by diffusion and drift motion. in the model labeled gal, cosmic rays are assumed to be galactic at all energies, and the anisotropy is due to purely diffusive motion caused by the turbulent component of the galacic magnetic field. in extragalactic models, the transition is put at the second knee and the cosmic rays large scale distribution is influenced by the relative motion of the observer with respect to the frame of the source. assuming that the frame in which the cosmic ray distribution at these energies is isotropic is coincident with the cosmic microwave background rest frame, a small anisotropy (the extragalactic compton-getting effect, labeled c-g xgal) is expected. interestingly, the phase of the first harmonic shows a smooth transition between a common phase of ' 270◦ below 1 eev and ' 100◦ above 5 eev. a consistency of the phase in ordered energy intervals can indeed be expected in presence of a real underlying anisotropy, standing out of the background more prominently than the amplitude. however, no confidence level can for the moment be assigned to this result, being an ”a posteriori” observation. the study of the large scale anisotropy has been performed for the first time with auger data as a function of both the right ascension and the declination and expressed in terms of dipole and quadrupole amplitudes [31]. no significant deviations from isotropy are detected. under the hypothesis that any anisotropy is dominated by these moments, the 99% cl upper limits can be derived, as shown in fig.8. figure 8: 99% cl upper limits on the dipole and quadrupole momenta as a function of energy. as an example of the power of the measurement to discriminate among different astrophysical models, the experimental limits are compared in the figure with the expectations from a toy model, in which the sources of protons and iron are stationary and uniformly distributed in the galactic disk. being the expected amplitudes for protons largely above the allowed upper limits, we can exclude this scenario for the light component of eev primary cosmic rays. 6 future developments the pierre auger observatory has reached a cumulative exposure of more than 26000 km2sr yr. new information about the characteristics of the primary cosmic rays have been derived, opening at the same time more questions and pointing to the need of an 136 auger highlights extension of the life time of auger. the measurement of composition of the primary particles from the eev region up to the highest energies has emerged as the key for hitting the hottest scientific questions: a) understand the origin of flux suppression discussed in sect.2, if due to the reach of the maximum energy at injection or to the gzk effect (a clear signature of which would be the observation of a flux of primary photons and neutrinos); b) perform composition driven anisotropy searches; c) determine the energy at which the transition from galactic to extragalactic sources of cosmic rays takes place. acknowledgement the author wishes to thank the organizers for the warm hospitality in mondello and the stimulating and interesting discussions during the workshop. references [1] v. berezinski, a. gazizov, s. grigorieva: 2006, phys.rev.d 74, 043005. doi:10.1103/physrevd.74.043005 [2] a.m. hillas: 2006, in cosmology, galaxy formation and astroparticle physics on the pathway to the ska klöckner, h.-r., jarvis, m., rawlings, s. (eds.) april 10th-12th 2006, oxford, united kingdom. [3] k. greisen: 1966, phys. rev. lett. 16, 748; g.t. zatsepin and v.a. kuzmin: 1966, sov. phys. jetp lett. 4, 78. [4] the pierre auger collaboration: 2010, nucl.instr.meth. a 620, 227. doi:10.1016/j.nima.2010.04.023 [5] the pierre auger collaboration: 2008, phys.rev.lett. 101, 061101. doi:10.1103/physrevlett.101.061101 [6] the pierre auger collaboration: 2010, nucl.instr.methods a613, 29. [7] r. pesce for the pierre auger collaboration: 2011, in 32nd int.cosmic ray conf., beijing, china; arxiv:1107.4809. [8] m. settimo for the pierre auger collaboration: 2012, eur.phys.j. plus 127, 87. [9] b. dawson et al. for the pierre auger, telescope array and yakutsk collaborations: 2012, in int.symposium on future directions in uhecr physics, cern. [10] p. facal for the pierre auger collaboration: 2011, in 32nd int.cosmic ray conf., beijing, china; arxiv:1107.4804. [11] k.-h. kampert, m. unger: 2012, astrop.phys. 35, 660. [12] e. barcikowski et al. for the pierre auger, telescope array and yakutsk collaborations: 2012, in int.symposium on future directions in uhecr physics, cern. [13] j. matthews: 2005, astrop.phys. 22, 387. [14] the pierre auger collaboration: 2013, jcap 02, 026. [15] a.m. taylor, m. ahlers, f.a. aharonian: 2011, phys.revd84, 105007. doi:10.1103/physrevd.84.105007 [16] d. allard: 2012, astrop.phys. 39-40, 33. doi:10.1016/j.astropartphys.2011.10.011 [17] p. battacharije and g. sigl: 2000, phys.rep.327, 109. [18] m. galaverni and g. sigl: 2008, phys.rev.lett.100, 021102. [19] l.d. landau, i.ya. pomeranchuk: 1953, dokl.acad.nauk.92, 553; a.b. migdal: 1956, phys.rev.103, 1811. [20] the pierre auger collaboration: 2009, astrop. phys.31, 399; 2008, astrop. phys.29, 243. [21] v. scherini for the pierre auger collaboration: 2012, in int.symposium on future directions in uhecr physics, cern. [22] the pierre auger collaboration: 2012, apj.lett. l4, 755. [23] the pierre auger collaboration: 2013, adv.in high energy phys.res., 2013, 708680. [24] the pierre auger collaboration: 2007, science 318, 938. [25] m.p. véron-cetty, p. véron: 2006, a&a 445, 773. [26] k.-h. kampert for the pierre auger collaboration: 2011, highlight talk in 32nd int. cosmic ray conf., beijing, china. [27] o. deligny et al. for the pierre auger, telescope array and yakutsk collaborations: 2012, in int.symposium on future directions in uhecr physics, cern. 137 http://dx.doi.org/10.1103/physrevd.74.043005 http://dx.doi.org/10.1016/j.nima.2010.04.023 http://dx.doi.org/10.1103/physrevlett.101.061101 http://dx.doi.org/10.1103/physrevd.84.105007 http://dx.doi.org/10.1016/j.astropartphys.2011.10.011 antonella castellina [28] the pierre auger collaboration: 2012, astrop.phys. 35, 354. doi:10.1016/j.astropartphys.2011.10.004 [29] the pierre auger collaboration: 2012, apj 760, 148. doi:10.1088/0004-637x/760/2/148 [30] the pierre auger collaboration: 2011, astrop.phys. 34, 627. doi:10.1016/j.astropartphys.2010.12.007 [31] the pierre auger collaboration: 2012, apj.suppl. 203, 34; 2013, apj.lett. 762, l13. 138 http://dx.doi.org/10.1016/j.astropartphys.2011.10.004 http://dx.doi.org/10.1088/0004-637x/760/2/148 http://dx.doi.org/10.1016/j.astropartphys.2010.12.007 introduction the energy spectrum the nature of the primaries primary photons and neutrinos anisotropies future developments 46 acta polytechnica ctu proceedings 2(1): 46–49, 2015 46 doi: 10.14311/app.2015.02.0046 transient processes in a binary system with a white dwarf. d. a. kononov1, d. v. bisikalo1, v. b. puzin1, a. g. zhilkin1 1institute of astronomy of the ras, moscow, russia corresponding author: dkononov@inasan.ru abstract using the results of 3d gas dynamic numerical simulations we propose a mechanism that can explain the quiescent multihumped shape of light curves of wz sge short-period cataclysmic variable stars. analysis of the obtained solutions shows that in the modeled system an accretion disk forms. in the outer regions of the disk four shock waves occur: two arms of the spiral tidal shock; “hot line”, a shock wave caused by the interaction of the circum-disk halo and the stream from the inner lagrangian point; and the bow-shock forming due to the supersonic motion of the accretor and disk in the gas of the circum-binary envelope. in addition, in our solutions we observe a spiral precessional density wave in the disk. this wave propagates from inside the disk down to its outer regions and almost rests in the laboratory frame in one orbital period. as a results every next orbital period each shock wave passes through the outer part of the density wave. supplying these shocks with extra-density the precessional density wave amplifies them, which leads to enhanced energy release at each shock and may be observed as a brightening (or hump) in the light curve. since the velocity of the retrograde precession is a little lower that the orbital velocity of the system, the same shock wave at every next orbital cycle interacts with the density wave later than at the previous cycle. this causes the observed shift of the humps over binary phases. the number of the shock waves, interacting with the density wave determines the largest number of humps that may be observed in one orbital period of a wz sge type star. keywords: cataclysmic variables dwarf novae optical spectroscopy photometry numerical simulations individual: ss cyg ≡ bd+42◦ 4189a. 1 introduction wz sge stars (subclass of su uma stars) are deeply evolved close binary systems with very short orbital periods (∼ 80) min and low mass ratios of the components (q < 0.1). as a rule these objects are very faint (fainter than 15m). the most remarkable observational feature of these stars in quiescence are their multi-humped orbital light curves. usually astronomers call them double-humped.there are a number of models proposed to explain a double-humped shape of the light curves of wz sge stars. one of them, advocated by aviles et al. (2010), explains the formation of the humps by visibility conditions of two arms of the tidal spiral shock caused by the action of the 2:1 tidal resonance. another model, mentioned, for instance, by wolf (1998) and silvestri et al., (2012) supposes that the stream from the inner lagrangian point may overflow the disk after it ricochets on the disk rim and forms an additional hot spot at the opposite side of the disk, i.e. two hot spots give two humps in the light curve. these two models would have worked perfectly had not there been a number of observational facts that cannot be explained by them. we, above, deliberately called the light curves of wz sge stars “multihumped” instead of commonly used “double-humped”, since there are a number of works where observers report from one to four humps in one orbital period (see, e.g., araujo-betancor et al. (2005), katysheva & shugarov (2009), kitsionas et al. (2005), and pavlenko (2009)). besides, as shown in the mentioned papers, even in the same system the number of the humps varies depending on the epoch of observations. thus, only two bright sources, implied by the models, described above, are not enough to explain more than two humps. one more feature of the humps that cannot be explained by, say, the visibility conditions of the arms of the tidal shock is that they tend to shift over binary phase when observed in time-distant nights. the point is that the positions of the tidal shock arms in an accretion disk are determined by the roche potential and fixed in the coordinate frame, rotating along with the binary. thus, their contributions must always be observed at the same binary phases in every night. it is commonly accepted that the most important observational manifestations of cvs and, in particular, wz sge stars, come from gas dynamical processes initiated by the mass transfer between the components. 46 http://dx.doi.org/10.14311/app.2015.02.0046 transient processes in a binary system with a white dwarf. therefore, in order to understand what happens in wz sge stars and what structures and processes contribute to their observational manifestations one needs to study gas dynamics of these objects. in this paper we, by means of 3d gas dynamic simulations, aim to investigate the flow structure in a representative of wz sge stars and propose a physical model, capable to explain the multi-humped nature of their light curves. in section 2 we describe the simulations and show their results. in section 3, based on the results of gas dynamic simulations we consider a physical model of the formation of the humps in orbital light curves of wz sge stars. in section “conclusions” we summarize our results and discuss further studies. 2 numerical simulations to investigate flow structure in wz sge stars we have performed gas dynamic simulations of a close binary system having the parameters of v455 and. this system is one of the brightest representatives of the class and has well defined parameters (araujo-betancor et al. (2005)) that are as follows. the secondary mass is msec = 0.07m�, and the primary mass (white dwarf) is mwd = 0.6m�. the binary separation of the system is a = 0.56r�. the mass transfer rate in the system is estimated as ṁ = 10−11m�/year. the orbital period of the system is porb = 81.08 min. to model the flow structure in the v455 and system we use a numerical hd/mhd code “nurgush” (zhilkin & bisikalo (2009)). when modeling, we set the magnetic field of the accretor equal to zero, so this is a pure gas dynamic case. the simulations are conducted until the system enters a quasi-stationary regime, i.e. the total mass of the disk varies by less than 2% in one orbital period. approaching this regime takes approximately 30 orbital periods. the results of the calculations are shown in fig. 1. in its panels we show density distributions in the equatorial plane of the system for two time moments τ = 33.53porb and τ = 34.13porb. analysis of the obtained results shows that an elliptic accretion disk forms in the system. one also can see that in the disk a spiral density wave occurs. its approximate location is denoted by the white dashed lines. this precessional density wave, first described by bisikalo et al. (2004), occurs due to the retrograde precession (apsidal motion) of elliptic flow lines, caused by the tidal action of the secondary. the angular velocity of the retrograde precession of this wave (its absolute value) is only a few precent lower than the orbital angular velocity of the system. this means that in the observer’s frame in one orbital period this wave almost rests and, in the frame, rotating with the binary, it makes almost a round during the same period. this effect is wellseen in fig. 1 where at two time moments, separated by approximately 0.6porb, the outer part of the wave occupies almost opposite locations. figure 1: density distributions in the equatorial plane of the system for two time moments τ = 33.53porb (top panel) and τ = 34.13porb (bottom panel). the white dashed line denotes the position of the precessional density wave. 3 model of the multi-humped light curve in fig. 1 one can clearly see that the precessional spiral density wave in our solution propagates down to the outer regions of the accretion disk. from our previous studies (see, e.g., bisikalo & kononov (2010), kononov et al. (2012)) we know that in the outer regions of the disk four most powerfull shock waves should form. these are: two arms of the spiral tidal shock; “hot line”, a shock wave caused by the interaction of the circumdisk halo and the stream from the inner lagrangian point; and the bow-shock forming due to the supersonic motion of the accretor and disk in the gas of the 47 d. a. kononov et al. circum-binary envelope. since, as we noted above, the precessional density wave rests in the observer’s frame and propagates down to the outer regions of the disk, the latter four shocks must pass through its outer part in every next orbital period. when a shock wave passes through the outer part of the density wave it is supplied with extra-density. this must cause amplification of the shock wave. indeed in fig. 2, where we plot pressure distributions for the time moments, corresponding to those of fig. 1, we can see that in the location of the outer part of the density wave pressure increases. the cases shown in fig.2 correspond to time moments when the density wave interacts with the arms of the tidal shock. in the upper panel of fig. 2 the density wave interacts with the closer-to-donor arm of the tidal shock. note that at this moment the pressure in the location of the opposite arm is remarkably lower. in the lower panel the farther-from-donor arm of the tidal shock is more pronounced, while the opposite arm is “inactive”. summarizing the above behavior, we can propose the following model that can explain the formation of the humps in quiescent orbital light curves of wz sge stars. in a system an elliptic accretion disk forms. under the tidal action of the secondary, in this disk, a precessional density wave develops. this wave propagates down to the outer regions of the disk. since the angular velocity of the retrograde precession of the wave is only a few percent lower than the orbital angular velocity of the system, the density wave almost rests in the observers frame. thus the four main shock waves also located in the outer regions of the disk consecutively pass through the outer part of the density wave every next orbital period. when the shock wave interacts with the density wave it is supplied with extra-density and, hence, amplified. the amplification of the shock wave must increase the energy release at the shock that should be seen as a hump in the light curve of the system. we should note that our model can explain not only the origin of the humps but, as well, their behavior. in particular, in the frame of our model, we can easily explain why the humps shift over binary phases from night to night. the point is that the angular velocity of the retrograde precession is not exactly the same as the orbital velocity of the system, it is a little lower. thus, at the next round a shock wave approaches the outer part of the density wave a little later than at the previous one and the interaction between the two waves starts later. the corresponding humps, hence, must shift forward over the binary phase. one more feature of the wz sge light curves that should be explained is the varying number of the humps. we show above that in principle one can observe not two but up to four humps in the light curve, since the density wave may interact with four most powerful shock waves located in the outer regions of the disk. however, the number of seen humps may depend on the visibility conditions of the amplified shock. for example, if a shock is amplified when it is situated at the moment of observations at the opposite from the observer side of the disk, radiation from the shock must pass through the disk and, in principle, may be reduced. besides, the shocks may be of different power. in order to have a clear answer to this question we need to have more observations. figure 2: pressure distributions in the equatorial plane of the system for two time moments τ = 33.53porb (top panel) and τ = 34.13porb (bottom panel). the black dashed line denotes the position of the precessional density wave. 4 conclusions we have performed 3d gas dynamic simulations of the flow structure in a short-period cataclysmic variable star of wz sge type. analysis of the results has shown 48 transient processes in a binary system with a white dwarf. that in the system an elliptic accretion disk forms. in the disk a precessional density wave develops. the wave propagates down to the outer regions of the disk. bisikalo et al. (2004) showed that the angular velocity of the retrograde precession of this wave is only a few percent lower than the orbital velocity of the system. thus, the precessional density wave almost rests in the observer’s frame. from our previous studies we know that in the outer regions of the disk four shock waves form. every next orbital cycle these shocks consecutively pass through the outer part of the density wave. as a result every shock is supplied with extra-density and amplified. the amplification of a shock causes enhanced energy release and may be seen in the light curve as a hump. since the density wave retrogradely precess with the velocity that is a little lower than the orbital velocity, at every next round the same shock passes through the outer part of the density wave a little later. thus the corresponding hump in the light curve at every next round occurs at a later binary phase. this, in the frame of our model, allows us to explain the observational fact that the humps shift over binary phases when observed at different nights. the variable number of the humps may be explained by the visibility conditions of the amplified shock wave. our studies of wz sge stars continue and with further accumulation of observational results we expect to have a more detailed model of the formation of light curves in these cataclysmic variable stars. acknowledgement this work is supported by the russian foundation for basic research (projects 11-02-00076, 12-02-00047, 1302-00077, 13-02-00939, 12-02-31031), the federal targeted program ”science and science education for innovation in russia 2009–2013”, grant of the president of russia for young scientists (mk-2432.2013.2) references [1] aviles, a.; zharikov, s.; tovmassian, g. at al.: 2010, the astrophysical journal, 711(1), 389-398. doi:10.1088/0004-637x/711/1/389 [2] wolf s., et al.: 1998, a&a, 332, 984. [3] n. m. silvestri, p. szkody, a. s. mukadam et al.: 2012, the astronomical journal, doi:10.1088/0004-6256/144/3/84 [4] araujo-betancor, s.; gänsicke, b. t.; hagen, h.-j. at al.: 2005, a&a, 430, 629-642. [5] katysheva, n., shugarov, s.: 2009, journal of physics: conference series, 172(1), 12-44. [6] s. kitsionas, o. giannakis, e. harlaftis, h., et al.: 2005, the astrophysics of cataclysmic variables and related objects, asp conference series, 330, [7] pavlenko e.,: 2009, journal of physics: conference series, 172(1), id. 012071. [8] a.g. zhilkin, d.v. bisikalo: 2009, asp conference series, 406, 118 123. [9] d. v. bisikalo, a. a. boyarchuk, p. v. kaigorodov, o . a. kuznetsov, and t. matsuda: 2004, astronomy reports, 48(6), 449456. doi:10.1134/1.1767212 [10] bisikalo, d. v., kononov, d. a.: 2010 memorie della societ? astronomica italiana, 81, 187 [11] kononov, d. a., giovannelli, f., bruni, i. and bisikalo, d. v.: 2012, astronomy & astrophysics, 538, id.a94, 7 49 http://dx.doi.org/10.1088/0004-637x/711/1/389 http://dx.doi.org/10.1088/0004-6256/144/3/84 http://dx.doi.org/10.1134/1.1767212 introduction numerical simulations model of the multi-humped light curve conclusions 264 acta polytechnica ctu proceedings 2(1): 264–268, 2015 264 doi: 10.14311/app.2015.02.0264 on the 2011 outburst of the recurrent nova t pyxidis l. izzo1, m. della valle2, a. ederoclite3, m. henze4 1dip. di fisica, sapienza university of rome, p.le a. moro 2, rome; icranet, p.le della repubblica 10, pescara, italy 2inaf osservatorio di capodimonte, salita moiariello 16, 80131 napoli, italy 3centro de estudios de fisica del cosmos de aragón, plaza san juan 1, planta 2, teruel, e44001, spain 4european space astronomy centre, p.o. box 78, 28692 villanueva de la cañada, madrid, spain corresponding author: luca.izzo@gmail.com abstract we discuss the nebular phase emission during the 2011 outburst of the recurrent nova t pyxidis and present preliminary results on the analysis of the line profiles. we also present some discussions about the binary system configurations and the x-ray emission, showing that the white dwarf mass should be larger than 0.8 m�. keywords: spectroscopy x-rays individual: t pyx. 1 introduction the 2011 outburst of the recurrent nova t pyx shed more light on some interesting characteristics of this peculiar nova. the optical evolution was not very different from the last (1966) outburst, showing the 8 days ’shoulder’ before the final rise to the maximum luminosity, which was reached ∼ 30 days from the initial outburst. the distance of t pyx was determined from light echoes to be 4.8 ± 0.5 kpc [1], which is marginally consistent within the errors with the value of 3.5 ± 0.4, estimated from the application of the maximum magnitude versus rate of decline (mmrd) relation [2] to the light curve of the 1966 outburst [3]. the presence of a clumpy ring with an inclination of 30-40 degrees, embedded within a spherical shell, was also recently reported using hubble space telescope observations during the nebular phase [1]. a variation of the orbital period after the 2011 outburst was detected, indicating an ejected mass in the outburst of 3 × 10−5 m� [4]. also, the orbital period was found to have increased during the long quiescence period between the last two outbursts of t pyx. moreover, with respect to the 1966 epoch, when no advanced x-ray and radio detectors were available, we have now a good knowledge of the emission of t pyx at these frequencies during its outburst [5]. in the previous work [6], we have presented the early spectral evolution, up to two months after the initial outburst. we have shown the hybrid spectral transition he/n fe ii he/n and initial ejecta velocities. in this work, we present the optical observations of the nebular phase, which was marked by the onset of the supersoft x-ray source (sss) emission which happened during the seasonal gap [7]. the optical observations presented here were obtained with the high-resolution spectrograph sarg, mounted on the 3.5m telescope nazionale galileo (tng) located at la palma. spectra with power resolution of r = 29000 were obtained but the complete sample, which includes also observations obtained at the very large telescope, will be presented in ederoclite et al. in preparation. then we briefly discuss the x-ray emission and finally we provide an updated scheme of the binary system configuration in t pyx, taking into account the recent results. 2 the nebular phase of t pyx the nebular phase in novae is easily recognized by the absence of absorption lines and the presence of forbidden emission lines, e.g. [oiii] and [nii], in the optical spectra, similarly to planetary nebula cases. indeed, these emission lines are due to photo-ionization of the ejecta by the central hot white dwarf (wd). at the same time, forbidden transitions originate mainly in rarified environments, so that any structure observed in corresponding line profiles is mainly due to the distribution of the material and not to self-absorption mechanisms. the analysis of the [oiii] 4959-5007 and [nii] 5755 λλ profiles can consequently provide hints on the geometry of the ejecta. these line profiles are well described by a large flat top base (vexp,1 ≈ 1700 km/s) overlapped by a saddleshaped and brighter emission, with vexp,2 ≈ 600 km/s, see fig. 1 for the case of the [oiii] 5007 λ. the high resolution allows us to detect a castellated structure on 264 http://dx.doi.org/10.14311/app.2015.02.0264 on the 2011 outburst of the recurrent nova t pyxidis the top of the profiles which is due to the presence of a clumpy ejecta structure. the saddle-shaped configuration suggests a non-spherical geometry of the ejecta, as generally discussed in [8] and further developed in several publications. in order to estimate the asphericity from forbidden lines we have used a method first presented in [9]. an analytic expression for the line profiles i(l) from an anisotropic distribution of the nova ejecta was provided: i(l) = k ( c v0 )α+1 ×[ lα−2 2 − lα−2 1 α− 2 l2(1 − 3cos2i) + lα2 − lα1 α (1 − sin2i) ] , l ≤ l1;[ lα−2 2 − lα−2 α− 2 l2(1 − 3cos2i) + lα2 − lα α (1 − sin2i) ] , l ≥ l1; (1) where c is the speed of light, v0 is the velocity at the inner radius of the expanding shell (we have assumed the maximum velocity observed for the bright profile, v0 ≈ 600 km/s), α describes the variation of the shell’s velocity with the distance form the wd, and l = (λ− λ0)/λ0 = v/c is our variable. the quantity -0.006-0.004-0.002 0.000 0.002 0.004 0.006 0 5.´ 10-13 1.´ 10-12 1.5´ 10-12 displacement l from @oiiid λλ 5007 fl ux he rg �c m 2 � s� a ng l figure 1: the saddle-shaped profile in the [oiii] 5007 line and the best fit (red line) obtained with the method described in the text. l2 = v2/c corresponds to the zero intensity along the wing of the line profile. a grid-search algorithm was developed to find the best fit for the inclination angle i that the binary orbital plane forms with the line of sight. for all the three lines, we find a large angle i ≈ 60 ± 10 degree, see also tab. 1, which contrasts some estimation of the i given before the 2011 outburst [14], and suggests a larger value for the binary inclination angle, see also [1]. we note that this approach does not take into account the presence of a clumpy structure. a more detailed approach is needed, and will be presented elsewhere. 3 the configuration of the binary system it is clear from the nebular spectra that the emission in t pyx is well described by quasi-spherical ejecta, which give rise to the larger flat top profile, and an equatorial ring of material, which is slower and gives rise to the saddle-shaped profiles. the origin of this equatorial ring is unknown: it is possible that it formed during the interaction of the main ejecta with pre-existing gas, which is equatorially distributed around the binary system (an hypothesis presented in [13]); another possibility can be that the early interaction of the ejecta with the secondary star could influence the subsequent expansion [10], but other mechanisms can also work. in all cases, it is necessary to shed light on the observed asphericity in t pyx, and the first ingredient we need is the mass of the wd. table 1: fit results of the line profiles of [oiii] 4959, 5007 and [nii] 5755, using the approach developed in [9]. ident. wavelength ( å) i (degree) v1 (km/s) α k (erg/cm2/s/ang) 4959 59 ± 10 465 ± 6 -2.2 ± 0.3 (3.7 ± 0.1) × 10−15 5007 59 ± 10 477 ± 6 -2.3 ± 0.3 (1.09 ± 0.03) × 10−14 5755 60 ± 10 444 ± 6 -2.8 ± 0.3 (6.8 ± 0.5) × 10−16 265 l. izzo et al. � � � � � � � � figure 2: the location of the t pyx parameters (green stars) with respect to the m 31 nova correlations presented in [20]: (1) the ton-toff correlation, for t pyx measured from the time of maximum brightness; (2) the kt-toff relation, with kt given by the average value kt=55.7 ev; (3) the relation between ton and t2 and (4) the correlation between ton and the expansion velocity as measured from the hα emission line. x-ray observations of the sss phase can in principle provide this physical quantity. the sss emission is caused by stable hydrogen burning in the matter that was not ejected during the nova outburst. this burning matter forms an envelope around the wd and can be observed once the ejected material becomes optically thin to soft x-rays. the duration of the sss phase is inversely related to the wd mass [11], since more massive wds need to accrete less matter to trigger an outburst. given its recurrent nature, t pyx is expected to contain a massive wd. the sss phase of t pyx was first detected about 90 days after the outburst [7], when an increase in soft x-rays was observed by the swift-xrt [12]. t pyx remained in this state for ≈ 200 days, although around day 190 the luminosity started to decline. we do not enter here into a discussion on the details of this emission, which will be described in a forthcoming work. what is interesting in the context of this work is the relatively short duration of the sss phase, which suggest a large wd mass. observations obtained before 2011 reported a wd mass of 0.7 m� and a mass ratio of q = 0.2 [14], which implies a secondary star of 0.14 m�. the orbital period was observed with high accuracy to be porb = 1.8295 hrs. with some assumption on the type of the companion star, we can infer the mass of the secondary which overfills its roche lobe: ∆m2 = ρ2 4 3π(∆r2)3, where ρ2 is the density and ∆r2 the dimensions of the overfilling envelope of the secondary. from the q value and the absence of signatures of the secondary in optical spectra during the quiescent phase [3], we can assume that it is a low-mass m-type star. its density can be well approximated using the eggleton formula ρ2 = 107porb(hrs) −2 g cm−3 [15], while its radius can be derived from the patterson et al. formula r2 = 0.62m0.61 2 r� [16]. the roche lobe size of the secondary is related to the q parameter through l2 = 1.631q1/3m 1/3 1 porb(hrs) 2/3 1010 cm [17]. the result is that the envelope overfilling the secondary roche lobe is related to the wd mass. from the recent estimate of the mass ejecta after the 2011 266 on the 2011 outburst of the recurrent nova t pyxidis outburst (3 × 10−5m�, [4]), and assuming that every observed t pyx outbursts ejected similar masses, we obtain a total mass ejected from t pyx of about 2× 10−4 m�. if this mass was accreted onto the wd from the secondary we can obtain a lower limit on the wd mass from the above formula. this results in mwd ≥ 0.8 m�. 4 final considerations our last result requires some important remarks. first, it was proposed that the mass ejected in t pyx is larger than the one accreted [3]. we do not consider this possibility in this work, i.e., that all the accreted mass results to be the ejected one. we know that this is not exactly true, since the sss phase is due to the burning of accreted hydrogen which is not ejected in the nova outburst. the mass of this burning envelope can be estimated assuming that the energy source for the sss phase is the gravitational contraction [18], but this contribution can be neglected. second, there is the possibility that a fraction of the matter overfilling the secondary’s roche lobe would flow out from the external lagrangian point l2, leading to the formation of an asymmetric structure around the binary system. such a mechanism would operate only if the primary roche lobe has been filled by accreted gas, leading to the detection of transient heavy elements in absorption (thea) during the early phases of the outburst [19], whose presence, in this case, is not certain. moreover, in tight systems where the secondary is less massive than the wd (q < 1) also centrifugal forces can contribute to such an outflow. there is again observational evidence that t pyx shares many properties common to other novae. as an example of this, we compare t pyx to the unbiased sample of m31 novae [20], in particular with respect to the correlations between some optical and x-ray observables: the blackbody temperature of the sss emitter, the turn-on and turn-off time of the sss emission, the expansion velocity of the ejecta and the t2 determined from the r-band light curve [21], see fig. 2. we see that t pyx lies in the same region occupied by classical novae observed in m31. all these results need to be figured in a unique picture for t pyx and can shed more light on cv systems with a mass ratio of less than unity. further results and conclusions will given in izzo et al. (2015) in preparation. acknowledgement we are very grateful to r. williams, s. n. shore, e. mason, d. de martino and r. gilmozzi for precious discussions. m.h. acknowledges support from an esa fellowship. references [1] sokoloski, j. l., crotts, a. p. s., uthas, h., et al.: 2013, apj 770, 33. [2] della valle, m. & livio, m.: 1998, apj 506, 818. [3] selvelli, p., cassatella, a., gilmozzi, r., et al.: 2008, a&a 492, 787. [4] patterson, j., oksanen, a., monard, b., et al.: 2013, arxiv/1303.0736. [5] nelson, t., chomiuk, l., roy, n., et al.: 2012, apj, 785, 78. [6] izzo, l., et al.: 2012, mem. s.a.it. s. 83, 830. [7] schwarz, g. j., osborne, j. p., page, k., et al.: 2011, atel 3754. [8] payne-gaposchkin, c. h.: 1957, the galactic novae. [9] vainu bappu, m. k. & menzel, d. h.: 1954, apj 119, 508. doi:10.1086/145857 [10] hutchings, j. b.: 1972, mnras 155, 357. doi:10.1093/mnras/155.3.357 [11] starrfield, s., truran, j. w., sparks, w. m., et al.: 1991, in extreme ultraviolet astronomy, ed. r. malina & s. bowyer, newyork, pergamon, p. 168. [12] gehrels, n., et al.: 2004, apj, 611, 1005. doi:10.1086/422091 [13] williams, r.: 2012, aj, 144, 98. [14] uthas, h., knigge, c. & steeghs, d.: 2010, mnras, 409,237. doi:10.1111/j.1365-2966.2010.17046.x [15] eggleton, p. p.: 1983, apj, 268, 368. doi:10.1086/160960 [16] patterson, j., kemp, j., harvey, d. a., et al.: 2005, pasp, 117, 1204. doi:10.1086/447771 [17] paczynski, b.: 1971, ara&a, 9, 183. doi:10.1146/annurev.aa.09.090171.001151 [18] krautter, j., oegelman, h., starrfield, s., et al.: 1996, apj, 456, 788. [19] williams, r., mason, e., della valle, m., et al.: 2008, apj, 685, 451. doi:10.1086/590056 [20] henze, m., pietsch, w., haberl, f., et al.: 2014, a&a, 563, a2. [21] della valle, m. & livio, m.: 1995, apj, 452, 704. 267 http://dx.doi.org/10.1086/145857 http://dx.doi.org/10.1093/mnras/155.3.357 http://dx.doi.org/10.1086/422091 http://dx.doi.org/10.1111/j.1365-2966.2010.17046.x http://dx.doi.org/10.1086/160960 http://dx.doi.org/10.1086/447771 http://dx.doi.org/10.1146/annurev.aa.09.090171.001151 http://dx.doi.org/10.1086/590056 l. izzo et al. discussion christian knigge: the system parameter we estimated in uthas et al. 2011 should be used with caution. they are based on a radial velocity study, and parameters resulting from such studies are known to be biased quite often. christian knigge: you consider r2 and l2 differing by ≈ 20%. are you really saying this could be the physical configuration, i.e. that the donor is overflowing the roche lobe by this amount ? luca izzo: the results that i’ve presented comes out naturally from formulae and physical considerations based on fixed binary parameters, as mwd = 0.7 m�, q = 0.2, porb = 1.8295 hrs, and considering the donor as a low mass m-type star. following a work by patterson et al., we obtain the r2 value, while from paczynski the value of l2. their interpretation depends obviously on a binary parameter as q presented in the uthas et al. work. anyway, other works consider that value of q as the actual one working in t pyx case. 268 introduction the nebular phase of t pyx the configuration of the binary system final considerations acta polytechnica ctu proceedings doi:10.14311/app.2017.12.0050 acta polytechnica ctu proceedings 12:50–55, 2017 © czech technical university in prague, 2017 available online at http://ojs.cvut.cz/ojs/index.php/app the relation between driver behavior and intelligent transport system alica kalašováa,∗, simona kubíkováb, ján kapustab a univerzitná 1,âă010 26, žilina, slovak republic, supervisor of department of road and urban transport b univerzitná 1,âă010 26, žilina, slovak republic, phd. student ∗ corresponding author: alica.kalasova@fpedas.uniza.sk abstract. the main objective of slovakia’s transport policy is to reduce the number of traffic accidents and increase safety on our roads. implementation of intelligent transport systems presents one of the possibilities how to meet this goal. acceptance of these systems by motor vehicle drivers and other road traffic participants is necessary in order for them to fulfill their purpose. only if the drivers will accept intelligent transport systems, it is possible to flexibly and effectively manage road traffic flexibly and effectively. from the perspective of a driver it concerns, in particular, the possibility of using alternative routes when traffic accidents or other obstacles occurs on the route that would significantly affect the continuity and safety of road traffic. thanks to these technologies, it is possible to choose the appropriate route while driving, of course based on the criterion, which the driver considers the most important during the transport from origin to destination (driving time, distance from origin to destination, fuel consumption, quality of infrastructure). information isare provided to the driver through variable message signs or directly in the vehicle (rds-tmc). another advantage of intelligent transport systems is a positive impact on psychological well-being of the driver while driving. additional information about the possible obstacles, weather conditions and dangerous situations that occur on the roads as well as alternative routes are provided to the driver well in advance. this paper is mainly focused on how the drivers perceive the influence of intelligent transport systems in žilina region. keywords: safety, accidents, its. 1. introduction the slovak republic has implemented into its legislation aâălegally binding act of the european union, directive 2010/40/eu of the european parliament and of the council of 7 july 2010 on the framework for the deployment of intelligent transport systems in the field of road transport and for interfaces with other modes of transport. thereby, the slovak republic committed to meet the conditions of implementation and use of intelligent transport systems. however, effort exists to improve this situation through the adoption of act no. 317/2012 coll. on intelligent transport systems in road transport and amending and supplementing certain acts, as well as by adopting national road safety plan of the slovak republic in the years 2011-2020 and program to support the development of intelligent transport systems and national system of traffic information. the basis of these documents is a promising creation of a so-called vision of zero accident rate in the field of transport safety. in this context that regarding the european area of road traffic safety, the european commission enforces reduction of the number of road traffic fatalities by half in comparison with 2010 as a priority for years 2011 to 2020[1]. in recent years, the slovak republic managed to lower the number of traffic accidents. the ambitious objective of slovakia’s commitment under the european road safety charter to reduce the number of fatalities in road accidents to half by 2010, in comparison with 2002, when 610 people were killed on our roads, was not fulfilled. we managed to lower the number to 345 fatalities, which represents a 43% decrease. even though we failed to meet the commitment mentioned above, subsequent statistical indicators of accident rates in the slovak republic, which show a downward trend, are positively perceived in slovakia as well as in the european union[2]. figure 1 shows the decrease of accident rates on our roads from 2000 to 2015. this decrease is caused by the development and evolution of automotive technology (especially the development of active and passive safety of vehicles) as well as by better and more suitable roads, better traffic signs, proper traffic management and organizational measures and also by the use of intelligent transport systems. the rapid decline of accident rate between 2008 and 2009 was caused by the adoption of new laws and decree in our legislative related to traffic safety[3]. this paper is focused mainly on the analysis of traffic situation in žilina region, specifically on the section between the city of bytča and ivachnová village. the intensity of vehicles on this section is one of the highest in the region. the application of intelligent transport systems is one of the most effective 50 http://dx.doi.org/10.14311/app.2017.12.0050 http://ojs.cvut.cz/ojs/index.php/app vol. 12/2017 the relation between driver behavior and intelligent transport system figure 1. evolution of accident rates in the slovak republic from 2000 to 2015. city road name intensity traffic accidents ružomberok d1 19636 3 hlboké nad váhom-strečno i/18 26087 86 šútovo-kraľovany i/18 21225 5 kraľovany-dolný kubín i/70 13766 8 vrútky-kraľovany i/18 21758 33 kraľovany-sliače i/18 21861 31 žilina-terchová ii/583 5366 39 terchová-párnica ii/583 3843 5 table 1. traffic accident rates on the selected sections of road network in 2014. ways how to improve traffic situation on this road section. 2. characteristic of the selected section of road network selected section of road network consists of the first and second class roads and d1 motorway. the statistics of traffic accidents based on the section and type of road are listed in the following table 1. presented data are from 2014. the table shows that the most traffic accidents in 2014 on a selected road network were documented on the section from hlboké nad váhom to strečno (road i/18), where the highest intensity of vehicles per 24 hours was also recorded. 2.1. measurement of road network safety indicators of accident rates usually determine the safety on roads. safety is the result of risk and level of risk influence. the risk is expressed by the number of traffic accidents and the level of risk influence is specified through intensity. the indicators always refer to the same period of time, which is in this case one year. this enables us to determine the frequency of traffic accidents. frequency = n/t (1) where: n – number of traffic accidents, t – time period [year]. relative accident rate represents the probability of occurrence of traffic accident on individual sections. it is calculated as the ratio between the number of traffic accidents and traffic performance. when calculating this indicator, it is necessary to distinguish whether it is a calculation of relative accident rate at intersections or on sections between intersections[4]. r = n 365 ∗ rpdi ∗ t ∗ 106 (2) for intersections r = n 365 ∗ rpdi ∗ l ∗ t ∗ 106 (3) for sections where: n – number of traffic accidents, rpdi – annual average of daily intensities, t – time period, l – length of examined section. another indicator that is used to assess safety is the density of traffic accidents. it refers to the number of road accidents and to the section length, thus intensity is not taken into account. it is calculated according to the formula: h = n l ∗ t (4) where: h – density of traffic accidents, n – number of traffic accidents, t – time period, l – length of examined section. frequency of accidents, relative accident rate and density of accidents were the indicators used in this paper. these indicators were calculated based on the statistical data from 2014 and they are listed in table 2[5, 6]. as shown in table 2, relative accident rate which depends on the transport performance is not critical in any case. however, it should be noted that property damage only (pdo) accidents were not taken into account since these statistics were not available. otherwise, the values of relative accident rate would be much higher. the highest value of density of traffic accidents (3,1 accidents per one kilometer) was recorded on the stretch from hlboké nad váhom to strečno, which is the section of road i/18. also in this case, property damage only accidents were not included in the calculation. this analysis shows that it is necessary to solve the traffic situation on the given section mainly through long-term organizational measures as well as by the application of intelligent transport systems. voluntary acceptance of intelligent transport systems by drivers and other road traffic participants is necessary in order for it to be effective. a questionnaire survey, whose main goal was to identify the requirements and needs of road users as well as the level of perception of intelligent transport systems was conducted within the žilina region. 51 a. kalašová, s. kubíková, j. kapusta acta polytechnica ctu proceedings section rpdi [veh/24] l [km] n [number of accidents] r [%] h [number of accidents/km/year] ružomberok 19636 3,06 3 13,67899 0,980392157 hlboké nad váhomstrečno 26087 27,7 86 32,60631 3,104693141 šútovo-kraľovany 21225 5,2 5 12,41155 0,961538462 kraľovany-dolný kubín 13766 18,1 8 8,79652 0,44198895 vrútky-kraľovany 21758 15,7 33 26,46686 2,101910828 kraľovany-sliače 21861 27,2 31 14,28334 1,139705882 žilina-terchová 5366 24,5 39 81,27463 1,591836735 terchová-párnica 3843 20,4 5 17,47337 0,245098039 table 2. accident indicators. 3. analysis of perception of intelligent transport systems from the perspective of drivers the survey was conducted electronically through a website, as well as by personal conversations with residents who possess a valid driving license. data was collected during one month, from april 13, 2015 to may 19, 2015. subsequently, the data were was analyzed and evaluated graphically and in tables. the tool called ,"sample size calculator" was used to determine the minimum sample size that is required for sufficient explanatory power of the survey, in which we set up the population of registered drivers in žilina region to 414âă678, we chose 95% confidence interval and permissible margin of error was 5% (fig. 2). the minimum sample was set to 384 respondents. a total of 519 respondents participated in the survey and therefore we can conclude, that the survey has sufficient explanatory power. figure 2. sample size calculator. the first part of the questionnaire was focused on the categorization of respondents according to age, sex and also according to the fact whether the respondent is an active or passive driver (fig. 3). the purpose and regularity of trips made during the week were also examined (fig. 4). when choosing the route, every driver prefers the figure 3. percentage of active and passive drivers. route that best suits his needs and criteria. essential criteria for choosing the most appropriate route are fuel consumption, time accessibility of the destination point, distance between origin and destination, the danger of traffic accidents and so on. the evaluation scale was as follows: 0 – irrelevant, 1 – the least significant, 5 – the most important. weighted arithmetic means of importance of the individual criteria are calculated from these values and they are graphically processed in table 3, which shows that the most important criterion for men in both age categories is the quality of transport infrastructure. time accessibility of destination point is the most important for women. traffic congestions and delays on roads can be eliminated through the opportunity to use an alternative route, to which the vehicle will be diverted via information systems installed directly in the vehicle (rds-tmc) or through information systems situated on roads that contain information for the entire traffic flow (variable message signs). in the questionnaire survey, respondents were also asked whether they would be willing to take an alternative route offered by the information system in case of traffic accident or the occurrence of other obstacles on their original route. the results are shown in fig. 5. the most frequent answer of the respondents was that they are willing to accept an alternative route even if the distance to the destination would rise by 52 vol. 12/2017 the relation between driver behavior and intelligent transport system figure 4. purpose of transport and routing of traffic flows. criteria men 18-26 men 27-65 women 18-26 women 27-65 fuel consumption 3,59 3,52 3,10 3,75 travel time 3,79 3,95 3,96 4,13 distance between origin and destination 3,27 3,39 3,40 3,75 quality of road infrastructure 3,80 3,97 3,90 4,13 possibility of congestion 3,48 3,66 3,42 3,50 possibility of road accident 3,23 2,71 3,52 3,69 table 3. criteria for route selection – weighted arithmetic mean. figure 5. acceptability of alternative route due to the length of detour (left) and with respect to the prolongation of travel time (right). 53 a. kalašová, s. kubíková, j. kapusta acta polytechnica ctu proceedings more than 2 km and time accessibility would increase in the interval from 6 to 10 minutes. sufficient awareness of drivers concerning road traffic situation is another way how to avoid roads with high intensities, congestions or road sections where a traffic accident occurred. therefore, when planning the transport, it is appropriate to have the actual current traffic information, which can be detected by the driver before or during the trip through information technologies. according to the survey, the most frequently used means of obtaining traffic information is the radio, as can be seen in figure 6. figure 6. information systems used and their share in obtaining traffic information. in the next question, we asked whether the drivers would appreciate intelligent transport systems, which would alert them about the possible dangers and which of the dangerous situations are the most important to them. the survey shows that women consider the information about black ice and fog to be the most important and the least important are for them warnings concerning dead-end streets. men consider the information about traffic accident and black ice to be the most important and the least important are for them warnings about dead-end streets and one-way roads. the evaluation scale was as follows: 0 – irrelevant, 1 – the least significant, 5 – the most important. weighted arithmetic means, which is processed graphically in figure 7, were calculated based on the values obtained. figure 7. evaluation of the importance of warning for driver about dangerous traffic situation. the last part of the questionnaire was devoted to active safety components. it was examined, whether the drivers consider the presence of selected components of active safety to be necessary. active safety components are defined as systems and devices that help to prevent the occurrence of traffic accidents and thus increase the safety of road transport. table 4 shows which systems the respondents consider to be the most important. the evaluation scale was as follows: 0 – irrelevant, 1 – the least significant, 5 – the most important. the weights of importance were evaluated based on the weighted arithmetic mean. an overall analysis of the questionnaire shows that the drivers are open to new possibilities of using intelligent transport systems, especially with regards to the increase of their own safety while travelling by a motor vehicle. 4. conclusion building telematics technologies is expensive. it is therefore not possible to apply them to all roads. however, they should form an essential part of motorways, expressways and first class roads, mainly because of high traffic intensity on these roads. several cities, where it will be necessary to solve growing traffic problems by building intelligent transport systems, were chosen, based on the analysis of traffic – transport processes, which was performed within the implementation of the national system of traffic information in individual regional capitals and in some larger cities. sections of road network that pass through the cities of žilina, martin and ružomberok were examined in this paper. these sections have the highest vehicle intensities and it is therefore necessary to solve the traffic issues through the use of intelligent transport systems. telematics technologies have a positive impact on traffic and its characteristics (intensity, density, speed, etc.) reduction in the number of traffic accidents can be achieved by reducing the intensity on congested road sections. the use of variable message signs to inform drivers about the current traffic situation and possible obstacles that affect the road capacity appears to be a suitable tool for reducing the risk of traffic accidents. intelligent transport systems have a positive effect on psychological well-being of the driver while driving. awareness plays an important role not only in warning about current traffic situation but also in planning the transport before it actually starts. drivers can check road traffic situation thanks to the topical and real-time information. road safety may be improved by reducing the number of traffic accidents and minimize their consequences and thus society can save hundreds of thousands of euros. the main benefit of reducing the number of traffic accidents remains minimization or complete elimination of the negative impacts of traffic accidents on mental and physical aspects of people involved in traffic accidents as well as their relatives. 54 vol. 12/2017 the relation between driver behavior and intelligent transport system ecall 3,54 3,55 4,13 3,94 parking assistant 1,89 2,26 2,63 3,00 cruise control 2,66 2,76 2,85 3,06 stop & go 1,66 1,61 2,67 2,94 automatic system for the detection of congestions 2,86 3,11 3,35 3,88 lane departure 2,56 3,16 3,63 3,81 maintaining safety distance 3,86 2,97 3,23 3,56 traffic signs 2,42 2,37 2,60 3,19 table 4. evaluation of the importance of active safety systems’ application in vehicles. references [1] a. kalašová, j. paľo. dopravné inžinierstvo – organizácia a riadenie. žilina: edis vydavateľstvo žu v žiline, 2003. 162 s., isbn 80-8070-076-1. [2] národný plán sr pre becep 2011-2020, july 2014. [online] available from: http://www.becep.sk/file_download/6/narodny_ plan_sr_pre_becep_2011_2020.pdf. [3] j. ondruš, s. kubíková. interakcia bezpečnosti a inteligentných dopravných systémov. vedecký – recenzovaný online časopis – svet dopravy, july 2014. issn 1338-9629, available from: http: //www.svetdopravy.sk/interakcia-bezpecnosti-a\ -inteligentnych-dopravnych-systemov. [4] centrum dopravního výzkumu. jak lze měřit bezpečnost? [online], available from: http://www.czrso.cz/clanky/ jak-lze-merit-bezpecnost-1-cast. [5] road accident investigation guidelines for road engineers, 2007. available from: http: //www.who.int/roadsafety/news/piarc_manual.pdf. [6] m. sørensen, r. elvik. black spot management and safety analysis of road networks – best practice guidelines and implementation steps. tøi report 919/2007, oslo, july 2014. isbn 978-82-480-0809-5, available from: http: //www.toi.no/getfile.php/publikasjoner/t%d8i% 20rapporter/2007/919-2007/919-2007-nett.pdf. 55 http://www.becep.sk/file_download/6/narodny_plan_sr_pre_becep_2011_2020.pdf http://www.becep.sk/file_download/6/narodny_plan_sr_pre_becep_2011_2020.pdf http://www.svetdopravy.sk/interakcia-bezpecnosti-a\-inteligentnych-dopravnych-systemov http://www.svetdopravy.sk/interakcia-bezpecnosti-a\-inteligentnych-dopravnych-systemov http://www.svetdopravy.sk/interakcia-bezpecnosti-a\-inteligentnych-dopravnych-systemov http://www.czrso.cz/clanky/jak-lze-merit-bezpecnost-1-cast http://www.czrso.cz/clanky/jak-lze-merit-bezpecnost-1-cast http://www.who.int/roadsafety/news/piarc_manual.pdf http://www.who.int/roadsafety/news/piarc_manual.pdf http://www.toi.no/getfile.php/publikasjoner/t%d8i%20rapporter/2007/919-2007/919-2007-nett.pdf http://www.toi.no/getfile.php/publikasjoner/t%d8i%20rapporter/2007/919-2007/919-2007-nett.pdf http://www.toi.no/getfile.php/publikasjoner/t%d8i%20rapporter/2007/919-2007/919-2007-nett.pdf acta polytechnica ctu proceedings 12:50–55, 2017 1 introduction 2 characteristic of the selected section of road network 2.1 measurement of road network safety 3 analysis of perception of intelligent transport systems from the perspective of drivers 4 conclusion references 205 acta polytechnica ctu proceedings 1(1): 205–209, 2014 205 doi: 10.14311/app.2014.01.0205 expected hard x-ray and soft gamma-ray from supernovae keiichi maeda1,2, yukikatsu terada3, aya bamba4 1department of astronomy, kyoto university, japan 2kavli institute for the physics and mathematics of the universe (wpi), university of tokyo, japan 3department of physics, saitama university, japan 4department of physics and mathematics, college of science and engineering, aoyama gakuin university, japan corresponding author: keiichi.maeda@kusastro.kyoto-u.ac.jp abstract high energy emissions from supernovae (sne), originated from newly formed radioactive species, provide direct evidence of nucleosynthesis at sn explosions. however, observational difficulties in the mev range have so far allowed the signal detected only from the extremely nearby core-collapse sn 1987a. no solid detection has been reported for thermonuclear sne ia, despite the importance of the direct confirmation of the formation of 56ni, which is believed to be a key ingredient in their nature as distance indicators. in this paper, we show that the new generation hard x-ray and soft γ-ray instruments, on board astro-h and nustar, are capable of detecting the signal, at least at a pace of once in a few years, opening up this new window for studying sn explosion and nucleosynthesis. keywords: nuclear reaction nucleosynthesis abundances supernovae: general. 1 introduction supernova (sn) explosions trigger (or are triggered by) explosive nucleosynthesis, and they are believed to be main production sites of heavy elements in the universe. the resulting yields are sensitive to explosion mechanism(s), and thus studying nucleosynthesis products is important to uncover the still-debated explosion mechanism. especially important is the production of 56ni – this is the origin of fe (as a result of the radioactive decay chain 56ni → 56co → 56fe), and the decay is believed to provide a source of emissions from (many classes of) sne through thermalization of emitted γ-rays and positrons. in type ia supernovae (sne ia), about half of an exploding white dwarf in mass is processed into 56ni, supporting their huge luminosities as distance indicators. however, the most direct evidence in this scenario is still missing – there has been no solid detection of the decay γ-rays from sne except for sn 1987a (e.g., dotani et al., 1987; sunyaev et al., 1987). especially, no solid detection has been reported for sne ia (see milne et al., 2004 for a review). from a theoretical point of view, studying this high energy emission has been restricted to one-dimensional models (see milne et al., 2004, for a review) despite the importance of multi-dimensional structures of the explosion both in theory and observation (e.g., kasen et al., 2009; maeda et al., 2010a). most previous studies also focused on the emission in the mev range. in this paper, we present our radiation transfer simulations of the high energy emission based on the state-of-the-art sn ia explosion models. we extend our analysis to hard x-ray and soft γ-ray regimes, for which dramatic improvement is expected in the observational sensitivities thanks to new generation observatories like nustar (koglin et al., 2005) or astro-h (takahashi et al., 2010). we predict that these telescopes are capable of detecting the radioactive decay signals from sne ia, at a rate of once in a year or at least once in a few years. we also briefly comment on perspectives for core-collapse sne. 2 expected characteristics we performed radiation transfer simulations (maeda et al. 2012) based on a series of two-dimensional delayed detonation models by kasen et al. (2009). the delayed detonation model is among the most popular scenarios for sne ia, resulting from a near-center ignition of thermonuclear sparks within a chandrasekhar-mass white dwarf (khokhlov, 1991). conditions for the initial triggers have not been clarified from the first principle (e.g., seitenzahl et al., 2013), thus kasen et al. (2009) adopted various conditions (i.e., distribution of the sparks) and produced a range of the ejecta models. in this scenario, different initial conditions can be associated with observed diversities. figure 1 shows examples of the ejecta structure. the model dd2d asym 04 is for bright sne ia (resulting in ∼ 1m� of 56ni), while 205 http://dx.doi.org/10.14311/app.2014.01.0205 keiichi maeda, yukikatsu terada, aya bamba figure 1: examples of the delayed-detonation models (kasen et al., 2009; see also maeda et al., 2010b). the mass fractions of si (left) and 56ni (right) are shown, on a logarithmic scale. the axes are in 10, 000 km s−1. dd2d iso 04 is for fainter ones (∼ 0.4m� of 56ni). generally, this model sequence predicts more asymmetric structure for fainter sne ia (note that the initial condition of model dd2d asym 04 is indeed more asymmetric than dd2d iso 04, but the post-explosion ejecta are less asymmetric). examples of the synthetic spectra are shown in figure 2. the spectra are characterized by the decay lines, compton scattering continuum, and the low energy cut off by the photoelectric absorption. at 20 days, the decay lines from 56ni → 56co (e-folding time of ∼ 8.8 days) are more important than those from 56co→ 56fe (∼ 113 days) as characterized by strong lines in the soft γ-ray range (e.g., the 158 kev line followed in strength by the 270 and 480 kev lines). later on, the strong lines are mostly in the mev range (i.e., the 847 kev line as the strongest) except for the annihilation line. the cut off energy becomes higher as time goes by due to the increasing contribution to the emission from the deeper part where the mean atomic number and photoelectric cross sections are larger. thus, overall the spectra evolve from soft to hard as time goes by. this indicates that follow-up of sne at relatively early phases is important in hard x-ray and soft γ-ray range. the classical 1d model w7 has lower energy cut off than the delayed detonation model sequence, since the w7 model has less extended explosive nucleosynthesis in the surface layer than the delayed detonation model. due to increasing transparency, the emission is sensitive to model variants (including the viewing direction) in the earlier phase, while the mass of 56ni plays a dominant role in the later phase. thus, observation at relatively early phases in (relatively) soft bands can provide unique diagnostics in clarifying the explosion mechanism(s). in the multi-d delayed detonation model sequence, a unique prediction is that faint sne ia should show larger variations in the high energy emission arising from various viewing directions than brighter ones. such prediction can be tested once there are at least a few samples of high energy emission detected for sne ia. another diagnostics using the ‘soft’ bands includes the surface composition analysis from the photoelectric absorption, e.g., the different behavior shown for the w7 model and delayed detonation models. 3 observational perspectives table 1 summarizes expected detectability of extragalactic sne ia at various band passes by a few current and future instruments. while detecting the mev lines from the 56co decay has been challenged in the past, even with spi on board integral and 106 s exposure (roques et al., 2003; see also isern et al., 2013), this is limited to extremely nearby sne ia up to 5 6 mpc (or 8 mpc for extremely bright sne ia). such nearby events are expected only once in a decade. this frustrating situation in the mev range will be improved only when the sensitivity is improved by an order of magnitude, hopefully by proposed new generation observatories like grips (greiner et al., 2012; see also summa et al., 2013). we propose that new generation hard-x and/or soft γ-ray instruments can change the situation. in hard xrays, nustar has been already launched. astro-h is scheduled for launch in 2015, which will be attached with hxi (hard x-ray) and sgd (soft γ-ray). these instruments are expected to have 106 s exposure sensitivities sufficient to reach sne ia at 15 (conservative estimate) or 25 mpc (optimistic estimate) (tab. 1). the ‘line detection’ is more challenging, and we estimate the distance for the 5σ detection of the 158 kev line is ∼ 10 − 15 mpc for model dd2d asym 04 and ∼ 3 − 5 mpc for the other two models shown in table 1. figure 3 shows simulations for expected signals from sne ia at 15 mpc, by convolving the synthetic spectra 206 expected hard x-ray and soft gamma-ray from supernovae 10 10020 50 20010 − 8 10 − 7 10 − 6 10 − 5 10 − 4 no rm al iz ed c ou nt s s k ev energy (kev) dd2d_asym_04_dc2 15mpc 10 10020 50 20010 − 8 10 − 7 10 − 6 10 − 5 10 − 4 no rm al iz ed c ou nt s s k ev energy (kev) w7 15mpc 10 10020 50 20010 − 8 10 − 7 10 − 6 10 − 5 10 − 4 no rm al iz ed c ou nt s s k ev energy (kev) dd2d_iso_04_dc3 15mpc figure 2: detector response simulations for an exposure of 106 seconds for selected models (tab. 1), for hxi (black) and sgd (red) on board astro-h. the model spectra at 20 days after the explosion are used as input, placed at distances of 15 mpc. the sensitivity curves are adopted from kokubun et al. (2010), tajima et al. (2010), and takahashi et al. (2010). note that the photon count is very low in the hxi band for all the models at this distance, thus an apparent detection by hxi (left panel) just comes from the statistical fluctuation. table 1: expected detectability (for an exposure of 106 s centered at the peak date in each band pass). shown here are limiting distance and the expected number of sne ia within the distance (shown in parenthesis). ‘cons’ and ‘opt’ are conservative and optimistic estimates, respectively. see maeda et al. (2012) for details. dd2d asym 04 w7 dd2d iso 04 m(56ni)/m� 1.02 0.64 0.42 band (kev) instrument mpc (sne year−1) 60–80 hxi 13.9 (0.43) 17.7 (0.96) 10.5 (0.09) nustar (cons.) 13.0 (0.43) 16.5 (0.70) 9.7 (0.09) nustar (opt.) 18.4 (1.13) 23.3 (2.52) 13.8 (0.43) 158 spi 4.6 (<0.09) 2.9 (<0.09) 2.3 (<0.09) sgd (cons.) 22.2 (2.09) 14.2 (0.43) 11.4 (0.09) sgd (opt.) 38.5 (6.70) 24.6 (2.96) 19.7 (1.57) 200–460 spi 3.7 (<0.09) 2.7 (<0.09) 2.3 (<0.09) sgd (cons.) 11.6 (0.09) 8.6 (0.09) 7.1 (0.09) sgd (opt.) 20.2 (1.74) 14.8 (0.43) 12.3 (0.26) 812 spi 4.3 (<0.09) 2.6 (<0.09) 2.0 (<0.09) grips 16.8 (0.87) 10.0 (0.09) 7.6 (0.09) 847 spi 7.7 (0.09) 5.4 (<0.09) 4.6 (<0.09) grips 29.8 (4.52) 21.0 (2.00) 18.0 (1.04) and designed sensitivity curves of hxi and sgd (here the adopted sensitivity curve corresponds to our ‘optimistic’ case). in 2011-2012, 6 sne ia were discovered within ∼ 20 mpc, 3 of which were within ∼ 15 mpc (from the asiago sn catalog; barbon et al., 1999). most of these were discovered soon after the explosion, and especially the nearest ones were all discovered within a week after the explosion – thus, too follow-up at the hard x and soft γ-ray peak (2 3 weeks after the explosion) is feasible. with 106 s exposure, we predict that a few (optimistic) or one (conservative) sne ia per year are reachable by astro-h. 207 keiichi maeda, yukikatsu terada, aya bamba 100 1000 1e-8 1e-7 1e-6 sgd hxi (b) 60 day fl ux [c m 2 s 1 k ev 1 ] energy (kev) 100 1000 1e-8 1e-7 1e-6 sgd hxi (a) 20 day fl ux [c m 2 s 1 k ev 1 figure 3: examples of synthetic spectra at (a) 20 days and (b) 60 days after the explosion. shown here are angleaveraged spectra for models dd2d asym 04 (red line), w7 (gray; nomoto et al., 1984), and dd2d iso 04 (dark blue) (at 10 mpc). the angle-dependent spectra seen from two opposite directions are shown for dd2d iso 04 (green and cyan) at 20 days. at 60 days the angle dependence is small. the angle dependence is small for dd2d asym 04 at both epochs. sensitivity curves for an exposure with 106 seconds of hxi and sgd on board astro-h (tajima et al., 2010; takahashi et al., 2010) are shown by black lines. 4 discussion and conclusions according to our simulations of radioactive decay signals from sne ia, the new generation hard x-ray and soft γ-ray observatories (either nustar or astro-h) are expected to be capable of detecting these signals from sne ia up to ∼ 20 mpc with 106 s exposure. this will hopefully lead to nearly annual detections, dramatically changing the field. we thus propose follow-up of nearby sne ia by these telescopes in a too mode. with a standard set up with a few 105 s exposure, the detection will be limited to extremely nearby objects (i.e., up to ∼8 12 mpc for sne ia with average brightness), but still there is a good chance of first solid detection of the signal from sne ia. once detected, it will provide various diagnostics on explosive nucleosynthesis and explosion mechanisms, and here a combination of hard x-ray and soft γ-ray will be essential. a similar argument applies for core-collapse sne. by combining results from similar simulations for corecollapse sne (maeda, 2006) and those obtained for sne ia (maeda et al., 2012), we find the following. sne iip (an explosion of a red supergiant) is not a promising target in the soft bands, since the thick h envelope is still opaque in the early phase where the 56ni decay can provide the strong emission in these band passes. among different types of core-collapse sne, sne iib/ib/ic (an explosion of a he or c+o star) are most promising. we estimate that the peak date in the high energy emission will be similar to (or a bit delayed than) that of sne ia (i.e., 2 3 weeks since the explosion). the smaller amount of 56ni here (typically ∼ 0.1m�), taking into account the delayed peak date, results in the expected horizon as 5 − 8 mpc (conservative and optimistic). such nearby objects are rare, but expected to be discovered at a rate of a few in a decade according to the past statistics. acknowledgement km thanks franco giovannelli and the organizers of frascati workshop 2013 for creating the friendly and stimulating atmosphere. the work by km has been supported by wpi initiative, mext, japan. the authors acknowledge financial support by grant-in-aid for scientific research from mext (22684012, 23340055, 23740141). references [1] barbon, r., et al.: 1999, a&as, 139, 531 [2] dotani, t., et al.: 1987, nature, 330, 230 doi:10.1038/330230a0 208 http://dx.doi.org/10.1038/330230a0 expected hard x-ray and soft gamma-ray from supernovae [3] greiner, j., et al.: 2012, exp. astron., 34, 551 doi:10.1007/s10686-011-9255-0 [4] isern, j., et al.: 2013, a&a, 552, 97 [5] kasen, d., röpke, f.k., woosley, s.e.: 2009, nature, 460, 869 doi:10.1038/nature08256 [6] koglin, j.e., et al.: 2005, proc. spie, 5900, 266 [7] kokubun, m., et al.: 2010, proc. spie, 7732, 33 doi:10.1117/12.857933 [8] khokhlov, a.: 1991, a&a, 245, 114 [9] maeda, k.: 2006, apj, 644, 385 [10] maeda, k., et al.: 2010a, nature, 466, 82 doi:10.1038/nature09122 [11] maeda, k., et al.: 2010b, apj, 712, 624 doi:10.1088/0004-637x/712/1/624 [12] maeda, k., et al.: 2012, apj, 760, 54 doi:10.1088/0004-637x/760/1/54 [13] milne, p.a., et al.: 2004, apj, 613, 1101 doi:10.1086/423235 [14] nomoto, k., thielemann, f.-k., yokoi, k.: 1984, apj, 286, 644 [15] roques, j.p., et al.: 2003, a&a, 411, l91 [16] seitenzahl, i.r., et al.: 2013, mnras, 429, 1156 [17] summa, a., et al.: 2013, a&a, 554, 67 [18] sunyaev, r.a., et al.: 1987, nature, 330, 227 doi:10.1038/330227a0 [19] tajima, t., et al.: 2010, proc. spie, 7732, 34 [20] takahashi, t., et al.: 2010, proc. spie., 7732, 27 209 http://dx.doi.org/10.1007/s10686-011-9255-0 http://dx.doi.org/10.1038/nature08256 http://dx.doi.org/10.1117/12.857933 http://dx.doi.org/10.1038/nature09122 http://dx.doi.org/10.1088/0004-637x/712/1/624 http://dx.doi.org/10.1088/0004-637x/760/1/54 http://dx.doi.org/10.1086/423235 http://dx.doi.org/10.1038/330227a0 introduction expected characteristics observational perspectives discussion and conclusions 325 acta polytechnica ctu proceedings 2(1): 325–327, 2015 325 doi: 10.14311/app.2015.02.0325 golden era of cataclysmic variables and related objects ii: concluding remarks r. hudec1,2 1astronomical institute, academy of sciences of the czech republic, cz-25165 ondřejov, czech republic 2czech technical university in prague, faculty of electrical engineering, prague, czech republic corresponding author: rene.hudec@asu.cas.cz abstract the cataclysmic variables continue to represent important part of modern astrophysics, as demonstrated, already second time, by the productive and successful conference golden era of cataclysmic variables and related objects ii held on mondello (palermo) in september 2013. keywords: stars variable stars cataclysmic variables. 1 introduction cataclysmic variables (cvs) belong to important targets of modern (and multispectral) astrophysics, as they represent in many aspects the true astrophysical laboratory, in which various physical processes and models can be observed, investigated, simulated, and tested. they even may play a significant role in the generation of galactic x-ray background, a consequence of surprisingly large fraction of cvs in hard x-ray sources detected by esa satellite integral. despite of large theoretical and experimental progress in the last years there are still numerous important questions to be answered. even for well known and studied objects such as ss cyg or ae aqr, there are still different opinions in the astronomical community about their nature and involved physical processes. the cv field is quite complex and interdisciplinary, and it is obvious that only such complex approach has a chance to deliver reliable answers to our questions. many important questions regarding investigation of cvs and related objects were addressed in mondello, not only during scientific sessions, but also during numerous discussions among participants (see figs. 2 to 11). there were plenty opportunities for such discussions during coffee breaks, breakfasts, lunches and dinners, and also during various social activities, all this enjoying the pleasant atmosphere of sicily (including delicious food, coffee, and wine). 2 few subjective comments the cataclysmic variables conference in mondello (palermo) represents, in my opinion, another successful workshop confirming prof. franco giovanelli/vulcano theorem (adopted from his famous vulcano frascati workshops), namely that one can make serious science even keeping smiling. as already confirmed before by long series of vulcano frascati (and analogous) workshops, the key for successful and highly productive workshop is as follows: nice environment, dining and staying together, enough (and good) wine (or beer in countries like czech republic). and great participants and speakers, of course, and last but not least experienced conference organizers. there was really wide international participation of both field leaders as well as young scientists in our conference many of them have got the possibility to meet the famous senior scientists they know from the literature for the first time. personally, i have much appreciated the high quality of most of invited and review talks. in my opinion, this is nowadays not very common as we face the problem of declining quality of such talks at numerous other conferences. there were nice carefully prepared reviews by various key speakers covering both theory as well as experiment. these lectures have illustrated and demonstrated the great progress both in theory as well as observation/experiment of cvs (although there was lively discussion this time whether we indeed have enough new observations or not) and related objects over last years. in addition to that, some new and mostly detailed observations on particular cvs were presented and discussed, mostly by young scientific fellows. as for the previous cv conference, the frequent conclusion of numerous speakers was as follows: we need more and better data, as well as better statistics. in addition to that, prospects of some of the new future 325 http://dx.doi.org/10.14311/app.2015.02.0325 r. hudec experiments/projects still need to be better formulated. in general, there is a need for better monitoring, multilambda observations, and in this direction robotic telescopes (rts, e.g. gloria) and novel space experiments may help, as also proposed and discussed during the workshop. 3 selected pictures in my opinion, the following selected pictures taken during the workshop may much better illustrate the scope, atmosphere, and success of the conference better than any further words. the authors of these pictures are rene hudec (21 pictures) and nataly katysheva (6 pictures). figure 1: the japanese delegation wearing traditional kimono during the welcome reception (left). the delicious italian coffee served during the coffee breaks at the hotel terrace (right). figure 2: meeting colleagues and having discussions represented, apart from scientific sessions, a key point of the workshop. figure 3: scientific talks: the discipline of conference participants in attending was great. figure 4: sometimes, the discussion among participants was quite lively figure 5: the main conference organizers and key persons at action (left) and russian delegation enjoying the delicious wine of sicily at the dinner (right). figure 6: the workshop artistic performance (left) and the beautiful sunset observed from the conference hotel terace (right). figure 7: are these conference participants both showing their estimate of the accretion disk radius? 326 golden era of cataclysmic variables and related objects ii: concluding remarks figure 8: the delicious desserts served after the meals surely contributed to the workshop success as well. figure 9: some (not all) conference participants enjoyed mare and swimming pool bar. figure 10: sometimes the discussions continued either at hotel bar, or at dining. figure 11: these two pictures surely belongs to the workshop highlights. figure 12: the etruscan wine tasting and departure for the conference trip. figure 13: the conference trip to segesta. 4 conclusions the international workshop golden era of cataclysmic variables and related objects ii, apart of presenting and discussing numerous contributions, promoted in an effective way further contacts among international cvs community. in my opinion, we definitely need this type of workshop, probably every two years. if compared to the situation two years ago when various future meeting sites were proposed and discussed, i have got feeling that this time, the opinion of majority of participants was quite unique: franco giovanelli and his team and palermo/sicily remains to be the best option. to conclude, lets hope to meet again in palermo in 2015 for at least so productive cv workshop as was the one in 2013. acknowledgement our thanks are due to the conference organizers, speakers and participants, restaurant and hotel staff, artists, and to all those not mentioned above but who contributed to the success of our conference as well. the authors of pictures in this contribution are rene hudec and nataly katysheva. my personal involvement in cv astrophysics (and consequently in the involvement in the cv mondello (palermo) conference) was supported by grants 13-39464j and 13-33324s by gacr. 327 introduction few subjective comments selected pictures conclusions acta polytechnica ctu proceedings doi:10.14311/app.2017.8.0017 acta polytechnica ctu proceedings 8:17–19, 2017 © czech technical university in prague, 2017 available online at http://ojs.cvut.cz/ojs/index.php/app microscopic evaluation of the quality of dental replacement františka pešlováa,b,∗, daniela koštialikováa, richard veselýc, andrej dubeca, maxim puchninb a faculty of industrial technology in púchov, alexander dubček university of trenčín, i. krasku 491/30, púchov, slovakia b czech technical university in prague, faculty of mechanical engineering, karlovo nám. 13, 121 32 prague 2, czech republic c czech technical university in prague, faculty of mechanical engineering, department of enterprise management and economics, karlovo nám. 13, 121 32 prague 2, czech republic ∗ corresponding author: abstract. the permanent tooth replacements including metal-ceramic crowns are a convenient solution for the renewing of the original function of the whole set of teeth as well as of the natural appearance. development and preparation of suitable tooth replacement presents a real challenge for dental engineers as the replacement has to meet all the conditions and requirements of the dental medicine as well as patient's needs and wishes. the preparation of the metal-ceramic crown is a sophisticated process and the dental engineer has to prepare always a unique replacement on demand. in dental medicine there is a wide spectrum of inorganic and organic materials used for manufacturing of dental replacements. each of the material has specific properties leading to distinct applications. besides the material properties, the attention has to be paid to the aesthetic function and biocompatibility of the material to ensure the complete restoration of the whole set of teeth. keywords: metal-ceramic crown, dental replacement, microstructure. 1. introduction the presented paper focuses on investigation of the randomly selected tooth replacements which were no longer suitable to be used in mouth cavity in a satisfying and predetermined way. from the aspect of materials engineering, the mentioned dental condition or phenomenon is based on the critical state of the dental replacement and it can be the cause of the occurrence of many other critical states. the given critical states lead to degradation of tooth replacements and it can even end in tooth replacement's fracture causing the functional and aesthetic defect as well. [1–4]. the main scope of the performed microscopic observations was to identify the initiation stimulus for rupture of a tooth replacement – the child patient's crown, which was used as tooth replacement after the injury. in relation to the given metal-ceramic crown (of the type seen in the 1), there was the rupture of the ceramic layer and its subsequent delaminating. the crown was supplied without any specific information (producer, chemical composition, life duration, etc.) 2. materials the metal-ceramic crown is a complex composite material with chemical composition given in the table 1 and 2. the identification of the phase composition was evaluated using scanning electron microscope (sem) jsm-7600f (jeol, jp) with energy dispersive spectroscopy (eds) x-max 50 mm2 (oxford instruments, gb) was used.the metallographic preparation of samples was performed in a standard and normalized way in order to carry out the microscopic investigation of the microstructure of used materials in their crosssections (fig. 2 ). metallographic preparation was made in standard way – selection of the material from the defect and from the intact location followed by grinding, polishing and etching. according to the fact mentioned herein before, the precise investigation of ceramics and metal microstructure were carried out. the detailed images (fig. 3) revealed the sites with the altered roughness. the occurrence of cavities was identified in sites where the metal was in contact with the ceramic layer and it led to the rupture of the ceramic layer. based on the microscopic observations, the penetration of ceramic material into the irregular areas of metal originating from the manufacturing process has been identified. that led to the deteriorative effect in relation to the metal – ceramics contact (fig. 4). the microscopic observation of the metal alloy quality revealed that the low-quality heterogeneous metal alloy was used for manufacturing of selected metalceramic crown which caused defects in contact layer between metal and ceramic material during manufacturing. besides the distribution of individual phases, 17 http://dx.doi.org/10.14311/app.2017.8.0017 http://ojs.cvut.cz/ojs/index.php/app f. pešlová, d. koštialiková, r. veselý et al. acta polytechnica ctu proceedings weight % o si sn k co al cr zr ti na sample nr.1 – ceramic 34,2 14,8 8,5 6,4 6,3 5,2 2,9 2,7 2,6 2,2 table 1. chemical composition of the ceramic part of the crown obtained by edx analysis weight % co cr w o si v s al k sample nr.1 – metal 57,2 22,7 6,9 2,1 1,4 0,6 0,5 0,5 0,3 table 2. chemical composition of the metal part of the crown obtained by edx analysis figure 1. metal-ceramic crown, detail. figure 2. metal-ceramic crown detailed image. figure 3. rupture in the site of the metal-ceramic material contact. figure 4. penetration of ceramic material. the low quality of materials was confirmed by chemical composition. the character of the microstructure (fig. 5) exhibits the imperfect primary crystallisation of co-cr alloy with the occurrence of undesired phases. [5–8]. 3. conclusion based on the microscopic evaluation, it can be concluded that individual microstructure of the metal alloy and contact layer between metal alloy and ceramic material indicates that evaluated metal alloy was not suitable for manufacturing of tooth replacement. the most significant evidence was obtained from the contact layer between metal and ceramic material where cavities were observed. the cavities are a source of oxidation which can lead to further degradation of the metal alloy. with the reference to the obtained results, we can come to the conclusion that the low quality materials used for manufacturing process of the metal-ceramic crown had caused defects that led to the degradation of the replacement. moreover, there was a danger of damaging other teeth as well as danger of ingestion or inhaling parts of metal or ceramic material. the work is showing an example that may occur in practice thus a regular inspections of patients tooth replacements are recommended. 4. results regulatory requirements for control samples preparation during the manufacturing of the replacements should be initiated despite the fact that preparation of samples for potential defect detection control may not be economical manufacturing process even though it is in the best interest of the patients. 18 vol. 8/2017 microscopic evaluation of the quality of dental replacement figure 5. microstructure of co-cr alloy . acknowledgements this work was supported by the slovak grant agency kega 006tnuad-4/2014, ministry of education youth and sport of the czech republic program npu1, project no lo127 references [1] y. wang. bioadaptability: an innovative concept for biomaterials. journal of materials science & technology 32(9):801 – 809, 2016. bioadaptation of biomaterials, doi:http://dx.doi.org/10.1016/j.jmst.2016.08.002. [2] h. hubálková, j. krňoulová. materiály a technologie v protetickém zubním lékařství. galén, praha, 2009. [3] r. g. craig, j. m. powers. restorative dental materials. mosby, st. louis, missouri, 11th edn., 2002. [4] h. hubálková, j. charvát, t. dostálová. faktory ovplyvňujúce životnosť fixnej zubnej náhrady. progresdent, 5th edn., 2004. [5] j. giacchi, c. morando, o. fornaro, h. palacio. microstructural characterization of as-cast biocompatible co-cr-mo alloys. materials characterization 62(1):53 – 61, 2011. doi:http://dx.doi.org/10.1016/j.matchar.2010.10.011. [6] j. giacchi, o. fornaro, h. palacio. microstructural evolution during solution treatment of co-cr-mo-c biocompatible alloys. materials characterization 68:49 – 57, 2012. doi:http://dx.doi.org/10.1016/j.matchar.2012.03.006. [7] j. e. lemons, f. misch-dietsch, m. s. mccracken. biomaterial for dental implants. in dental implant prosthetics, chap. 4, pp. 66–94. mosby, st. louis, missouri, 2015. [8] j. b. suzuki, d. lynn, l. d. terracciano-mortilla, c. e. misch. maintenance of dental implants. in dental implant prosthetics, chap. 34, pp. 964–981. mosby, st. louis, missouri, 2015. 19 http://dx.doi.org/http://dx.doi.org/10.1016/j.jmst.2016.08.002 http://dx.doi.org/http://dx.doi.org/10.1016/j.matchar.2010.10.011 http://dx.doi.org/http://dx.doi.org/10.1016/j.matchar.2012.03.006 acta polytechnica ctu proceedings 8:17–19, 2017 1 introduction 2 materials 3 conclusion 4 results acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2017.12.0104 acta polytechnica ctu proceedings 12:104–107, 2017 © czech technical university in prague, 2017 available online at http://ojs.cvut.cz/ojs/index.php/app the future of passenger air transport – very large aircraft and out key human factors affecting the operation and safety of passenger air transport petra skolilová ctu in prague, faculty of transportation sciences, czech republic correspondence: skolilova@fd.cvut.cz abstract. the article outlines some human factors affecting the operation and safety of passenger air transport given the massive increase in the use of the vla. decrease of the impact of the co2 world emissions is one of the key goals for the new aircraft design. the main wave is going to reduce the burned fuel. therefore, the eco-efficiency engines combined with reasonable economic operation of the aircraft are very important from an aviation perspective. the prediction for the year 2030 says that about 90% of people, which will use long-haul flights to fly between big cities. so, the a380 was designed exactly for this time period, with a focus on the right capacity, right operating cost and right fuel burn per seat. there is no aircraft today with better fuel burn combined with eco-efficiency per seat, than the a380. the very large aircrafts (vlas) are the future of the commercial passenger aviation. operating cost versus safety or co2 emissions versus increasing automation inside the new generation aircraft. almost 80% of the world aircraft accidents are caused by human error based on wrong action, reaction or final decision of pilots, the catastrophic failures of aircraft systems, or air traffic control errors are not so frequent. so, we are at the beginning of a new age in passenger aviation and the role of the human factor is more important than ever. keywords: vla, human factors, air transport, air traffic, security. 1. introduction the study of human factors deals with people’s interaction with their environments. in the case of general aviation, this is the study of pilot performance with influence on the emotions, stress, fatigue or design of cockpits or interaction with crewmembers, air traffic control personnel and others. fatigue is an expected aspect of life and there is a very thin border between normal daily fatigue (for example fatigue from lack of sleep) and the tough fatigue from continuous stress. typically, there are no significant marks or consequences. but the aftermath in general aviation environment can be disastrous[1]. 1.1. introduction of the air traffic markets situation aviation mega-cities (amcs) are the new phenomenon for the long-haul market. for long-haul flights we are rating the traffic over 2,000 nautical miles. more than 40% of all long-haul passengers are beginning or ending, at amcs and another 20% are between these 40 amcs. more than another 40 cities have a potential to grow to amcs. by the gmf of airbus, the a380 is the choice number one for these markets. the current markets situation is represented figure 1. the wide-body aircraft is a very important aspect of the aircraft market as well. these aircraft types are more focused on the routes for amcs than to the secondary cities. this is the right market opportunity for the highly efficient new a350xwb as well as the a330neo by the airbus. 1.2. very large aircraft according to airbus statistics before the vlas age, one million passengers per month were served by 200 aircrafts per day. but today, it is only 100 vla aircrafts connecting the world’s major hub. the passengers prefer to fly on a vla due to the high travel comfort which only the vla aircraft can provide. one of the most comfortable seating configurations is the 407 pax (passengers) on board configuration. but also, the maximum capacity of 853 seats are a highly desired option for airlines, which means they have the flexibility to mix comfort with revenue and cost per seat. but today, a typical configuration of an a380 is between 500-520 seats in three or four traveling class configurations. the capacity flexibility is one of the best airline tools how to optimize the right capacity at the right time between mature and expanding markets and their reaction you can read from figure 2[2]. we can find three key reasons why the vlas aircraft are the future of aviation: a) solution to sky congestion: reducing the number of aircrafts needed for transport of the ever-growing air passenger volume; b) solution to the changing demand at different times of the day; c) solution to overcrowded slots at the megacities airports. the standard solution, which is often implemented 104 http://dx.doi.org/10.14311/app.2017.12.0104 http://ojs.cvut.cz/ojs/index.php/app vol. 12/2017 the future of passenger air transport – very large aircraft figure 1. concentration of flights in long-haul markets [source: oag, airbus]. figure 2. new deliveries of vla by region [source: oag, airbus]. 105 petra skolilová acta polytechnica ctu proceedings on routes via the a380, is operating the a380 during peak hours and other aircraft types at non-peak hours during the day on the same routes. in fact, 50% of airports-pairs served by the a380 have another aircraft type in operation also. for the airlines it means to target the right capacity at the right time during the day, for example a combination of the a380 and a350xwb aircraft. the main markets for the vla are the long-haul markets between amcs. in these days they are mainly on the asia/pacific area, where more than 25% flights are served by the a380[2–4]. "from today until 2033, we forecast the demand for nearly 1,230 vla globally. the two largest markets are asia-pacific and the middle east. asia-pacific will represent 47% of the demand for vla over the next 20 years which can be clearly seen by the large number of vla today flying to from and within asiapacific followed by the middle east which today has the largest fleet and largest backlog"[2]. by the gmf prediction we can await an annual growth of the passenger air traffic around 4.7% and it will assist approximately 30,555 passengers’ aircraft during next 20 years. "this demand represents $4.4 trillion at book value over the next 20 years. of this demand for passenger aircraft, 40% will be for the replacement of ageing, less fuel efficient aircraft and 60% of the demand will be for growth"[3]. 2. human factors training in aviation safety means different things to different people. to the traveling public, the term ’safety’ means, at its most fundamental level, that we want to reach our destinations without getting hurt. since people do get hurt on occasion, it is logical to conclude that we are willing to accept some risk in traveling. whether consciously or subconsciously, we know that there is some chance, albeit a minute one, that we could be hurt or killed while traveling. today we average about 50 accidents in commercial airlines each year, but it means that the odds of any one flight being your last one are remote ( 1 to 4,9 million ) and given that more people are killed or injured in automobile accidents (approximately 44,000 deaths per year), ’safe’ is a reasonable word to apply to commercial aviation[5]. one of the first airlines to accept the human factors training was continental airlines. they accepted the existing format and settled the special training called cockpit resource management, evolved into their original training initially titled "crew coordination concepts", and later changed to "maintenance resource management"[6]. it covers communications with pilots, pilots with cabin crewmembers, labor with management, and companies with the government. the icao (international civil aviation organization) is a specialized agency of the united nations, which codifies the principles and techniques that aviation has developed and implemented a special human factors training called sms program: safety management system. the definition of sms: "a dynamic risk management system based on quality management system (qms) principles in a structure scaled appropriately to the operational risk, applied in a safety culture environment." 2.1. human reliability analysis (hra) it is well-established that the largest contribution to the probability of system failure in complex systems is human error. a risk assessment method of any value in aviation must consider human interactions and errors on system reliability. hra models human behavior to better understand human interactions with the rest of the system. a popular method used for human performance reliability prediction is the technique for human error rate prediction (therp). therp are model with five steps: (1.) define the system or process. (2.) identify and enumerate all human operations performed and their relationships to the system. (3.) predict error rates for each human operation. (4.) determine the effect of human error on the system. (5.) make changes that will reduce the system failure rate. pra has been used in chemical, transport, energy, aerospace and military. the first step in an hra is a study to identify the hazards and their impact on the scale. the scale is uses fault trees (ft). the hazards are assigned probabilities, which are propagated up the tree to establish the probability of the undesired top hazards. in practice, it means that the phase of mapping the hazards and human failures is followed by a practical modeling scenario with deductive logic and probabilistic tools called fault trees (ft). the ft are composed of three levels: the top level called type of accident which defines the accident scenario, the middle level are the intermediate failures causing the top level and the basic level are the failures that cause the top level to occur. developing the scenario is a highly sophisticated method with extreme dependency on source data of the model and good subject knowledge. it could be supported by probabilistic software tools. 3. conclusion in these days, more than 300 airbus a-380 are in service operating under the flags of 13 carriers: singapore airlines, emirates, qantas, air france, lufthansa, korean air, china southern airlines, malaysia airlines, thai airways international, british airways, asiana airlines, qatar airways and etihad airways using airports all over the world. one part of responsibility for passengers’ security is on airport management, how to manage the airports runways, taxiways, gate areas 106 vol. 12/2017 the future of passenger air transport – very large aircraft and terminals to accommodate this large and heavy aircraft and more than 700 pax per flight. what has resulted has been a relatively seamless, trouble-free adaptation on the part of airports, which reflects the results of thoughtful planning and analysis. the other part is the human factor. commercial air carriers continue to restructure. the world’s economic downturn beginning in 2008 forced many air carriers out of business and other to consolidate. as a result, air traffic management has had to, and will continue to, become more flexible in its contractual obligations with air carriers, their aeronautical revenue structures, and increasingly the use of their facilities. in the past, aviation safety improvements were characterized by a fly-crash-fix-fly approach. sometimes the causes would be weather-related or a mechanical failure, but more often the causes would be determined to be human factor error – usually the pilot. today we realize that it is much more productive to engineer a system in which causes of failure have been prevented. as one might imagine, there are many elements to this engineering effort. the understanding of hazard identification, risk management, system theory, human factors engineering, organizational culture, quality engineering and management, quantitative methods and decision theory must have a pillar in aviation development. the requirement for vlas will grow with the world’s network and as more people have the ability to fly. by 2032, this demand will result in a need for more than 1,300 vlas. given the projected growth in asia-pacific, both economic and air passenger traffic growth, the region’s demographics and urbanization trends, it is unsurprising that the region’s airlines will take 47% of these aircraft over the next 20 years. the middle east will be the second largest region in terms of demand for vlas, at 26%. this can be seen today in the size of the backlog of a380s within middle eastern carriers[3]. references [1] s. young. airport: planning & management 6th edition. mcgraw-hill companies, inc., 2011. [2] a. 2015. airbus. [online], přístup získán may 2015. [3] iata. the international air transport association. [online]. available: http://www.iata.org/about/pages/index.aspx. accessed mar 2015. [4] wttc. world travel & tourism council. [online]. available: www.wttc.org. přístup získán mar 2015. [5] the economist group. [online]. available: http://www.eiu.com/home.aspx. přístup získán apr 2015. [6] r. stanley, r. trollip. human factors for general aviation. jeppesen sanderson, 2011. 107 http://www.iata.org/about/pages/index.aspx www.wttc.org http://www.eiu.com/home.aspx acta polytechnica ctu proceedings 12:104–107, 2017 1 introduction 1.1 introduction of the air traffic markets situation 1.2 very large aircraft 2 human factors training in aviation 2.1 human reliability analysis (hra) 3 conclusion references acta polytechnica ctu proceedings doi:10.14311/app.2017.10.0048 acta polytechnica ctu proceedings 10:48–51, 2017 © czech technical university in prague, 2017 available online at http://ojs.cvut.cz/ojs/index.php/app shear strength of soils from the dobkovičky landslide in the central bohemian uplands determinated by laboratory tests jakub roháča,b,∗, petr kyclb, jan boháča, david mašína a charles university, faculty of science, albertov 6, prague b czech geological survey, klárov 3, prague ∗ corresponding author: jakub.rohac@geology.cz abstract. the aim of this paper is to evaluate soil shear strength from the dobkovičky landslide. the landslide was activated on june 6, 2013 after heavy rains and the d8 motorway, which was under construction at the time, was damaged. the laboratory tests were carried out on two types of soils, clay and tuff, both from the surface of the rupture. critical and residual friction angles were evaluated on both types of soils. keywords: laboratory tests, the dobkovičky landslide, shear strength. 1. introduction the plans for a highway connecting prague with dresden have been debated since the 1930s. construction of the first 4.2 km of the 92 km long d8 motorway began with section 0806 řehlovice ústí nad labem nearly 50 years later in 1984. the construction of the last remaining 16.2 km in section 0805 lovosice řehlovice was launched in 2007. this section crossed the central bohemian uplands, which represent the most geologically complex and problematic part of the motorway, in particular due to the presence of a large area of dormant and abandoned landslides known as “prackovice landslides” intersected by the motorway. on friday, june 7, 2013 the works were interrupted and the motorway under construction was damaged when heavy rains triggered a landslide near the village of dobkovičky (km 56.3-56.5). the damage was not restricted only to the motorway but also affected the single-track railway connecting lovosice with teplice between the stations dobkovičky and radejčín. the railway track was completely destroyed in the length of 200 m, including a 6 m high embankment. 2. dobkovičky landslide dobkovičky landslide (fig. 1) was activated in the well-mapped landslide area of prackovice, which is characterized by landslides in its geological history. the whole area from the river labe up to the railway embankment is covered by morphologically visible landslides. on june 3, 2013, the first cracks were observed in the area of dobkovičky quarry after 4 days of heavy rains. the dobkovičky landslide was activated in the evening of june 6, 2013. 2.1. landslide description the planar surface of the rupture was detected at a depth of 4-6.5 m below ground level by an engineeringgeological survey provided by the company az consult figure 1. picture of the landslide after collapse (summer 2013). [1]. the length was 470 meters and width was 200 meters. the maximum length of the movement in the center of the landslide was 49 m and the maximum speed of the movement was 1 meter per hour on june 8 [2]. 2.2. geology and hydrogeology the surface of the rupture of dobkovičky landslide consists of siltstone, altered basalt, tuff and anthropogenic material. the anthropogenic material is located in the center of the landslide in the form of a railway embankment and on the top of the landslide in the form of a dump of material from dobkovičky quarry. weathered chalky siltstone (clayey character, r6) represented the slip surface from the toe to the railway line as documented in the minor scarp and in the place of the railway embankment. basalt (r4) was 48 http://dx.doi.org/10.14311/app.2017.10.0048 http://ojs.cvut.cz/ojs/index.php/app vol. 10/2017 shear strenght of soils from the dobkovičky landslide figure 2. map of the landslide with sampling points. type sample depth [m] wl wp ip kts hv-p220 8.3m 59.3 24.8 34.5 kts1 hv-p212 2.3m 62.5 25.5 37 tf ig 208 6.5m 87.1 52.2 34.9 table 1. index properties documented above the railway embankment up to the main scarp. neogene tuff had a sandy clay character with pieces of coal. it had an irregular representation in the surface of the rupture. the main source of groundwater is quaternary debris. the stratum of fresh and weathered siltstone is considered as a hydrogeological insulator [3]. the general groundwater flow direction is from north-west to south-east. hydrogeology of dobkovičky quarry is characterized by fissured permeability, it can be assumed that most of the rainwater is drained through the fissures to the top of the impermeable siltstones and drained to the slope under the quarry. 3. sample description the laboratory tests were carried out on two types of soils, clay and tuff, both from the slip surface. samples were taken during the engineering-geological research in 2014 and 2015. there are 3 main ig types of material: kts1-cretaceous silt stone, olive-colored (borehole hv-p 212), kts-cretaceous siltstone, bluegray colored (borehole hv-p 220), and tf -tertiary tuff, yellow colored (borehole ig 208) [1]. sampling points are labeled on fig. 2. the grading curves of the three main ig types are shown in fig. 3. index properties are described in tab. 1, where wl is the liquid limit, wp is the plastic limit and ip is the plasticity index. 4. sample preparation and test procedures all of the laboratory tests were carried out in the laboratory of soil mechanic at the faculty of science. critical parameters were evaluated in a conventional triaxial apparatus and in a shearbox. residual parameters were evaluated in a ring shear apparatus. all of the laboratory tests were carried out on reconstituted soils the material was thoroughly mixed with distilled water until a homogeneous slurry was obtained. the final water content of the slurry was approximately 1.5 times higher than the liquid limit of the material. the material was then sieved through a 2 mm mesh sieve. 4.1. triaxial test ciup triaxial test (consolidated isotropic undrained with pore pressure measurement). the slurry was poured carefully into a consolidometer and consolidated to a vertical stress of approximately 80 kpa. the load was applied with increments of 10 kpa every day to prevent squeezing up the sides of the piston. after two weeks, the sample was extruded and its ends were trimmed to the correct length of 78 mm, the diameter was 38 mm. the sample was measured and the mass of the sample was determined. then the sample was placed into the triaxial cell on a base pedestal. radial drains were used for all of the samples to reduce the duration of the tests. top drainage was not used. the samples were isotropically consolidated and sheared in an undrained condition with pore pressure measurements. 4.2. shear box test tests were carried out on reconstituted slurry, which was consolidated to a vertical stress of 60 kpa, 100 kpa and 150 kpa. the speed of shear was lower than 0.0013 mm/min to ensure drained conditions. 4.3. ring shear test a bromhead ring shear apparatus was used for evaluation of the residual shear strength. the test was carried out, similarly to the previous apparatus, on reconstituted soils. slurry was consolidated to a vertical stress of 30 kpa, 50 kpa and 70 kpa. the speed of shear was lower than 0.023 mm/min. 5. results table 2 shows the results, the types of tests and the used stresses. eighteen laboratory test were carried out for evaluation of the shear strength of the soils from the dobkovičky landslide. nine of them were ciup (consolidated isotropic undrained with pore pressure measurement) triaxial tests, three were shear box tests and six were ring shear tests. the results of the three selected tests are shown in fig. 4. the shear parameters of the tuff are slightly higher than the clays. the residual parameters are approximately 49 j. roháč, p. kycl, j. boháč, d. mašín acta polytechnica ctu proceedings figure 3. grading curves of the three main ig types of soils, after [4] table 2. results of shear strength tests 1 2 of the critical parameters of the same sample, which confirms the known theory of soil mechanics [5]. 6. conclusions the presented laboratory tests confirm that shear strength depends on the movement of the slip surface. residual shear strength reached a value of between 13 and 15◦, which is approximately one half of the critical values [5]). it is planned to extend the number of laboratory tests within the study to create a detailed numerical model of the dobkovičky landslide. references [1] j. suchý. d8 odstranění sesuvu v km 56,300 56,500 + stabilizace oblasti kolem stavby d8 0805. tech. rep., az consult, 2014. [2] p. kycl. situační zpráva o sesuvu dobkovičky d8 km 56,300 ke dni 11. června 2013. tech. rep., česká geologická služba, 2013. [3] j. záruba. inženýrskogeologický doprůzkum, stabilitní výpočty v km 56,160 56,660. tech. rep., arcadis geotechnika, 2011. [4] z. j. s. j. plšková, m. předběžná zpráva d8-0805 lovosice-řehlovice, ii. etapa inženýrsko-geologického dopruzkumu v oblasti sesuvu dobkovičky a v jeho předpolí v km 56.300-56.500. tech. rep., 2016. [5] j. atkinson. the mechanics of soils and foundations. crc press, 2007. 50 vol. 10/2017 shear strenght of soils from the dobkovičky landslide figure 4. the results of performed tests 51 acta polytechnica ctu proceedings 10:48–51, 2017 1 introduction 2 dobkovicky landslide 2.1 landslide description 2.2 geology and hydrogeology 3 sample description 4 sample preparation and test procedures 4.1 triaxial test 4.2 shear box test 4.3 ring shear test 5 results 6 conclusions references acta polytechnica ctu proceedings doi:10.14311/app.2015.1.0029 acta polytechnica ctu proceedings 2:29–33, 2015 © czech technical university in prague, 2015 available online at http://ojs.cvut.cz/ojs/index.php/app x-copter studio michal koutný∗, ondřej pilát, patrik černý, maroš kasinec charles university in prague, faculty of mathematics and physics, ke karlovu 3, prague, czech republic ∗ corresponding author: xm.koutny+pair@gmail.com abstract. we present a project that aggregates various existing robotic software and serves as a platform to conveniently control a quadrocopter, mainly for research or educational purposes. user interface runs in a browser and other components are also made with portability in mind. we provide a common interface that unifies different quadrocopter models and we implemented it for the parrot ar.drone 2.0. the platform is data oriented, i.e., it is based on dataflow between user objects. we implemented several such objects for: data recording and replaying, inertial and visual localization and following a given path. keywords: robotics, quadcopter, ide. 1. motivation despite the fact that cheap hardware (such as parrot ar.drone 2.0 [1] or ready kits [2]) is available, there are not many possibilities for application programmers to develop software for these robots without need to distinguish between individual models. our aim is to provide a platform for development of software for quadrocopters. the target users are ai programmers or students and expected tasks are general algorithms (basic example in figure 1) for quadrocopters. the result should work with any robot compatible with our software (section 4.3). the testing should be further simplified by running the application without physical access to a quadrocopter either by using a simulator or data previously captured during live flights. at(xcheckpoint.reachedcheckpoint) { xcheckpoint.checkpoint = nextcheckpoint(); },; // x = 3 m, y = 1 m, z = 1.5 m var cp0 = checkpoint.new(3, 1, 1.5); xcheckpoint.checkpoint = cp0; figure 1. sample script that flies through computed checkpoints. it assumes there is created a dataflow graph with the node xcheckpoint in it. 2. related work 2.1. middleware for robotics probably most popular middleware for robotics is the robot operating system (ros) [3]. it supports communication between objects using publish–subscribe mechanism. it is open source, mainly targeted on linux platforms. similar project is urbi sdk, developed by former private company gostai [4]. the current maintainer is aldebaran robotics [5], unfortunately the community around urbi sdk is much smaller and much less active in comparison with ros. reasons why we chose urbi sdk despite this fact are in the section 3.2. 2.2. parrot ar.drone 2 api part of our project is an api for parrot ar.drone 2. various other projects are dealing with this. there is the official sdk [6] with c++ api, controltower [7] that provide java interface, a ros package ardrone_autonomy [8], uobject for urbi sdk [9] or implementation of czech technical university [10]. because none of the aforementioned fit to our requirements for os portability, stability, functionality or documentation, we implemented our own (see the section 4.3). 2.3. ground control system there is official application for parrot ar.drone [11] intended for mobile phone users allowing manual control and displaying only limited data from sensors. the pc application controltower [7] allows controlling quadrocopter with specialized computer peripheries and has airplane-like gui. more complex application is qgroundcontrol [12] that cooperates with pixhawk project [13] that encompasses own hardware and uses visual localization. 3. used technology 3.1. architecture our system is divided into three components. first interacts directly with a user, second controls the robot and the last one connects the former two. the components are separate processes that communicate with each other over network, with intention to run components on different machines. 29 http://dx.doi.org/10.14311/app.2015.1.0029 http://ojs.cvut.cz/ojs/index.php/app m. koutný, o. pilát, p. černý, m. kasinec acta polytechnica ctu proceedings 3.1.1. client client software is used to create and launch user scripts, edit them, manually control the robot and visualize data (e.g., directly from quadrocopter’s sensors). despite the technology challenges the client is thin – running in a web browser.1 3.1.2. server the server component conveys communication between the client and the actual control machine (further onboard). its task is to control the access to the onboard and monitor quality of the connection between the client and the onboard. in the case the overall connection latency (client–server and server– onboard) exceeds preset limits, a warning message is shown to the user. if the connection is lost, onboard execution is correctly terminated and user is notified too. 3.1.3. onboard the onboard is the main executive component. the robot control and data processing run here because it is closest to the robot. the onboard is executing commands obtained from the client and sends back various data selected by user. the onboard component is supposed to run under normal operating system.2 our implementation exploits a pc that communicates with quadrocopter via wi-fi. we did not test the onboard component directly on a robot.3 3.2. urbi sdk urbi sdk is a c++ middleware for robotics, which we based the onboard component on. basically it provides support for communication between user objects (uobjects) and schedules user jobs. communication is possible via so called uvars, which are slots of uobjects. sender just writes to these slots and a receiver’s callback handles the change of uvar’s value. this allows both apparently asynchronous communication and really asynchronous when a thread pool is used to run the callbacks. further, uobjects can run in different processes and urbi sdk ensures transparent messaging via tcp or udp sockets. orchestration user scripts (written in urbiscript) can be executed by the urbi runtime. urbiscript is a prototype-based object-oriented language conceptually similar to javascript. it is possible to implement uobject functionality exclusively in the urbiscript. we chose urbi sdk because of its portability (linux and windows systems are supported) and the own 1google chrome is strongly recommended, though mozilla firefox will also get by (without visualization of video data). 2we support windows 7/8 and ubuntu 14.04 systems. 3the hardware of parrot ar drone 2.0 theoretically should be able to run our onboard with limited performance. scripting language and runtime. considered alternative was robot operating system. 3.3. nodejs nodejs is a server-side javascript engine. recently, it became quite popular among developers of interactive web applications and various modules [14] exist that extend core functionality. it suited our needs for the server component. 3.4. html5 thanks to the standardization efforts many features that were earlier common only for desktop applications or via third party plug-ins (flash, java applets, native plug-ins) are now implemented directly in the browser, generally referred to as html5. to make client as multi-platform as possible, we decided to implement the client for the browser using aforementioned html5 technologies. most importantly, we use the web socket api [15] for sending data back to the client and <video> tag to display streamed video. 3.5. c++ the executive parts of the onboard component are written in c++ (which is consequence of using urbi sdk). we utilize features of the c++11 standard, mainly for threading and memory management. 4. features 4.1. dataflow graph we chose dataflow driven approach to describe the user’s program. it consists of units of operation which we call nodes. each node has at least one input or at least one output. any operations are either results of changes on the node inputs or are launched by an external event hidden in a node (e.g., timer expiration, a socket received data). urbi sdk itself encourages dataflow control by its communication paradigm. we extended the original mechanism to ensure syntactic and semantic compatibility between nodes. dataflow bears following advantages for the end user. 4.1.1. implicit parallelism nodes are implicit synchronized on outputs-inputs connections, thus avoiding excessive user effort. this also ensures certain level of scalability on multiple cpus machines. 4.1.2. separating abstraction levels user can concentrate on high level objectives only, implementation details are hidden in particular nodes. on the other hand, a creator of a node is limited by a particular node interface. 30 vol. 2/2015 x-copter studio 4.1.3. visual development each node defines an interface, which makes it possible to check what connections are possible (not only syntactically but also semantically with flat system of semantic types). in the end, user can assemble various dataflow graphs interactively (figure 2). 4.1.4. in-time data inspection selected outputs of the nodes can be connected with a special node that resends the data to the gui where the data are visualized on the fly (figure 2). 4.2. scripting only the dataflow graph of nodes is not enough for controlling the quadrocopter. thus, a user can write scripts that are sent and executed on onboard. in such scripts it’s possible to handle events occurring within the dataflow graph or manually affect the dataflow. the script execution can be paused and resumed (that is a feature of urbiscript, for instance useful for intentionally infinite loops, see figure 3 for urbiscript console). 4.3. quadrocopters unified api the x-copter studio supplies a unified application programming interface (denoted as xci) for data acquisition and control of quadrocopters. it is ready for common sensors on quadrocopters and it can be easily extended. we implemented the xci for one of the most popular quadrocopters – parrot ar.drone 2.0 and also a simulated quadrocopter in the v-rep simulator [16]. our parrot implementation exploits asynchronous socket events handling, which allows us to detect connection failures and attempt to restore the connection with the quadrocopter. the implementation further supports full configuration of the parrot ar.drone 2.0 and reading data from all available sensors. we use parrot for its price, robustness and popularity among research groups. the cost for this is extensibility – neither the hardware nor the software on parrot can be expanded. we have to deal with unreliable communication over wi-fi and connection failures. physical dimensions of the ar.drone 2 with the hull are 53 cm × 52 cm and it weights 420 g including battery. it can fly for about 15 minutes with the more powerful battery version (1500mah). the ar.drone 2 provides a video stream in high definition (720p) with 30fps from front camera aimed forward or qvga stream with 60fps from bottom camera aimed to the ground, three-axes gyroscope, three-axes accelerometer, three-axes magnetometer, pressure sensor and ultrasound altimeter. the quadrocopter sends raw and adjusted measurement from gyroscope, accelerometer, ultrasound sensor and pressure sensor with frequency of 200hz. all data from these sensors are accessible for user in the dataflow. quadrocopter firmware uses aforementioned sensors to maintain tilt, rotation and vertical velocity of the quadrocopter according to an external fly control command. this control command u = (φ, θ, ψ, ż) ∈ [−1, 1]4 consists of quadrocopter’s roll φ, pitch θ, yaw rotation speed ψ and vertical velocity ż. it should be sent every 30ms for smooth movements. 4.4. data recording and replaying thanks to the structured dataflow architecture, any potentially interesting data can be captured and stored in a file. we use text format which is portable and allows convenient processing with data processing tools. data that are inherently binary aren’t supported, however, special case – image data are stored with compression to a separate video file. recorded data are stored together with timestamps, so it’s possible to “replay” them and test or debug system’s response with real timing. 4.5. localization in unknown environment we use quadrocopter’s onboard sensors to give the end user information about the absolute position of the aircraft. the localization is a node within the dataflow graph, thus it runs exclusively on the onboard component. 4.5.1. inertial localization our implementation expects accelerometers with aggregated outputs in a form of horizontal velocity, altimeter and drone pose sensors (i.e., accelerometers for roll and pitch angles and gyroscope and/or magnetometer for yaw angle). sensor data are filtered with extended kalman filter (ekf) where we engage physical model based on a similar project [17]. 4.5.2. visual localization to enhance absolute position estimate, we are processing the video stream from the onboard camera. we use monocular slam framework ptam [18] that estimates camera pose and position for each frame and also builds a map of observed feature points. first, the map is initialized from a pair of images where point correspondence is based on optical flow and the initial scale is estimated thanks to inertial localization. after the initialization, the scale estimate is continuously refined using method described in [17]. data both from inertial and visual localization are merged together in the extended kalman filter (see figure 4) in separate update steps. the result from ptam (absolute position and quadrocopter’s pose) is used to update the ekf, taking into account a delay of visual localization and then recalculating prediction of the current state. 31 m. koutný, o. pilát, p. černý, m. kasinec acta polytechnica ctu proceedings figure 2. x-copter studio gui. top navbar indicated connection quality, upper half is filled with widgets that display data from the onboard, lower half accommodates the editor with repository of nodes on the left, and connected nodes on the right. at the bottom there are (left to right): dataflow control buttons, scripting console control buttons and manual flight control buttons. the scripting console is hidden. figure 3. urbiscript console. on the top – script editor, middle – output of executed script, bottom left – script execution control (surrounded with other controls of x-copter studio). 4.6. following given path path is represented as a list of checkpoints that drone strictly follows. a checkpoint is a composite of world coordinates and a tangential vector.4 4this vector specifies direction of quadrocopter’s heading at the given checkpoint. this is intended to be used when checkpoints should be followed as a spline curve. checkpoint attainment quadrocopter achieves a checkpoint when it is in a sphere around the checkpoint with 10 cm diameter (according to the localization) and immediately continues to the next checkpoint in the queue. flight to the checkpoint is controlled by 200hz pid regulator.5 regulator uses data from the extended kalman filter in order to eliminate data delay and predict quadrocopter’s current position in the world. output from the regulator is sent directly to the quadrocopter. 4.7. runtime configuration any configuration (parameters, calibration constants, settings, etc.) is stored in human readable and editable text files and loaded during runtime, thus avoiding necessity of recompilation to apply changes. the same configuration can be accessed both from c++ and urbiscript api. 5. conclusion we hope the x-copter studio will become used (at least) for educational purposes since it provides sufficiently high-level interface, for instance to prototype planning algorithms. 5the frequency is induced by the dataflow, in this case it is refreshed frequency of data from quadrocopter. 32 vol. 2/2015 x-copter studio -0.5 0 0.5 1 1.5 2 2.5 3 -1 -0.5 0 0.5 1 1.5 2 2.5 3 x [m] -0.5 0 0.5 1 1.5 2 2.5 3 -1.5 -1 -0.5 0 0.5 1 1.5 2 2.5 x [m] y [m ] heading path heading path figure 4. example of a path tracked by the ekf. a square 2m × 2m was set up and quadrocopter was navigated (with pid controller) to its vertices using inertial localization only. supposed position is shown on the left plot. as you can see, plot on the right shows more realistic pose and position thanks to visual localization (applied to the original data). alas, we don’t have any reference data for absolute comparison. if it is proved as useful and practical, it is possible to extend the software for other types of robots, not only quadrocopters. references [1] parrot. ar.drone 2.0. parrot new wi-fi quadricopter. [2015-02-08], http://ardrone2.parrot.com/. [2] bitcraze. the crazyflie nano quadcopter. [2015-02-08], http://www.bitcraze.se/crazyflie/. [3] open source robotics foundation. ros.org. [2015-02-08], http://www.ros.org/. [4] gostai. urbi. [2015-02-08], http: //www.gostai.com/products/jazz/urbi/index.html. [5] aldebaran. aldebaran/urbi. [2015-02-08], https://github.com/aldebaran/urbi. [6] ardrone open api platform. [2015-02-08], https://projects.ardrone.org/. [7] javadrone – ar.drone java api – google project hosting. [2015-02-08], https://code.google.com/p/javadrone/. [8] m. monajjemi. ardrone_autonomy – ros wiki. [201502-08], http://wiki.ros.org/ardrone_autonomy. [9] urbiforge projects/urbi 4 ar drone. [2014-08-28], http://www.urbiforge.org/index.php/projects/ urbi4ardrone. [10] t. krajník, v. vonásek, d. fišer, j. faigl. ar-drone as a platform for robotic research and education. in research and education in robotics eurobot 2011, pp. 172–186. springer science + business media, 2011. doi:10.1007/978-3-642-21975-7_16. [11] ar.freeflight. [2015-02-08], http://ardrone2.parrot.com/#freeflight. [12] l. meier. qgroundcontrol gcs. [2015-02-08], http://www.qgroundcontrol.org/. [13] l. meier, p. tanskanen, l. heng, et al. pixhawk: a micro aerial vehicle design for autonomous flight using onboard computer vision. auton robot 33(1-2):21–39, 2012. doi:10.1007/s10514-012-9281-4. [14] node packaged modules. [2015-02-08], https://www.npmjs.org/. [15] a. m. i. fette. rfc 6455 – the websocket protocol. [2015-02-08], http://tools.ietf.org/html/rfc6455/. [16] c. r. gmbh. coppelia robotics v-rep: create. compose. simulate. any robot. [2015-02-08], http://www.coppeliarobotics.com/. [17] j. engel, j. sturm, d. cremers. camera-based navigation of a low-cost quadrocopter. in 2012 ieee/rsj international conference on intelligent robots and systems. ieee, 2012. doi:10.1109/iros.2012.6385458. [18] g. klein, d. murray. parallel tracking and mapping for small ar workspaces. in 2007 6th ieee and acm international symposium on mixed and augmented reality. ieee, 2007. doi:10.1109/ismar.2007.4538852. 33 http://ardrone2.parrot.com/ http://www.bitcraze.se/crazyflie/ http://www.ros.org/ http://www.gostai.com/products/jazz/urbi/index.html http://www.gostai.com/products/jazz/urbi/index.html https://github.com/aldebaran/urbi https://projects.ardrone.org/ https://code.google.com/p/javadrone/ http://wiki.ros.org/ardrone_autonomy http://www.urbiforge.org/index.php/projects/urbi4ardrone http://www.urbiforge.org/index.php/projects/urbi4ardrone http://dx.doi.org/10.1007/978-3-642-21975-7_16 http://ardrone2.parrot.com/#freeflight http://www.qgroundcontrol.org/ http://dx.doi.org/10.1007/s10514-012-9281-4 https://www.npmjs.org/ http://tools.ietf.org/html/rfc6455/ http://www.coppeliarobotics.com/ http://dx.doi.org/10.1109/iros.2012.6385458 http://dx.doi.org/10.1109/ismar.2007.4538852 acta polytechnica ctu proceedings 2:29–33, 2015 1 motivation 2 related work 2.1 middleware for robotics 2.2 parrot ar.drone 2 api 2.3 ground control system 3 used technology 3.1 architecture 3.1.1 client 3.1.2 server 3.1.3 onboard 3.2 urbi sdk 3.3 nodejs 3.4 html5 3.5 c++ 4 features 4.1 dataflow graph 4.1.1 implicit parallelism 4.1.2 separating abstraction levels 4.1.3 visual development 4.1.4 in-time data inspection 4.2 scripting 4.3 quadrocopters unified api 4.4 data recording and replaying 4.5 localization in unknown environment 4.5.1 inertial localization 4.5.2 visual localization 4.6 following given path 4.7 runtime configuration 5 conclusion references acta polytechnica ctu proceedings doi:10.14311/app.2017.10.0061 acta polytechnica ctu proceedings 10:61–64, 2017 © czech technical university in prague, 2017 available online at http://ojs.cvut.cz/ojs/index.php/app stabilization and specification of characteristic value of geotechnical parameters ivan vaníček czech technical university in prague, faculty of civil engineering, thákurova 7, prague 6, czech republic correspondence: ivan.vanicek@cvut.cz abstract. soil stabilization, process, by which the soil properties are improved, is getting growing attention during last periods. paper is focused on application for earth structures. methods of soil stabilization together with their applicability for different soils are discussed firstly. the sensitivity of the process of the cautious estimation of the characteristic values of geotechnical parameters which are subsequently used for the design, is shown on the base of the logical scheme of the geotechnical design report for earth structures. finally the need of the laboratory tests for the closer specification of the soil stabilization technology and also for the determination of the geotechnical data are discussed. keywords: stabilization, earth structure, eurocode 7, characteristic value. 1. introduction soil stabilization is one of the techniques of soil improvement. together with soil reinforcement it is the most applied techniques. soil improvement methods can be divided with respect to the characteristic values specification into two basic subgroups [1]: • diffusive soil improvement; • discrete soil improvement. in the first case, typical for soil stabilization, the design counts with new properties for geological environment in general, better speaking for individual quasi-homogeneous layers of the environment. in the second case soil properties remain the same (on the previous values), as well the parameters of the reinforcing element (most often geosynthetics). the behaviour of the composite therefore depends on the interaction of these two basic materials and limit states of failures should be determined also for this reinforcing element. for earth structures the improvement of individual soil layers of fill or surface of ground (subsoil) has dominant application role. it is typical for earth structures of transport engineering including airports. however it is connected also with subsoil improvement of large halls, especially when they are founded partly in cut and partly on fill. soil stabilization, mixing soil particles with different additives, can be realized either on site or in mixing centre situated of-site with subsequent transport on the place of application. additives are different, either according to the soil type or with respect of certain demands. the following additives can be applied [2]: • lime, cement, plaster; • industrial by products as fly ash, energy-gypsum, slag (cinder) [3]; • additional soil mechanically stabilized soil; • several chemical additives (resins, bitumen). the paper is for the simplicity focused on the first possibility, on the application of lime or cement, whereas cement is typical for coarse soil and lime for fine grained soils. the recommendation in this direction are often connected with index plasticity. for cement application, the higher strength is reached, however the strength is rising up relatively slowly. the final result is more typical for road base or subbase or for layers just below floor (e.g. for above mentioned halls), where higher strength is typically demanded. subsequently more attention is focused on lime application, which is more suitable for fine soils and where quicker effect of application is needed. 2. lime stabilization quick lime is preferred for soil stabilization as it hydrates with water and calcium hydroxide develops. this reaction is exothermic. heat is created as the result of quick lime reaction with wet soil having quick impact on soil compaction which is after that for dryer soil better. therefore generally two impacts are distinguished, short term (having positive impact on soil compaction) and long term (having positive impact on strength increase). while short term effect occurs for any quick lime application, long term effect need a certain percentage of quick lime. broadly speaking this boundary is close to 2 % of added lime to dry soil. particle flocculation starts for higher content of lime, changing grain size distribution curve with lower content of fine particles. soil creates clods and change the structure. therefore for fine grain soils the stabilization process with lime leads to: • better soil workability; • strength increase. from this point of view the long term effect is more important for increase of characteristic values of geotechnical parameters, however requires higher lime 61 http://dx.doi.org/10.14311/app.2017.10.0061 http://ojs.cvut.cz/ojs/index.php/app ivan vaníček acta polytechnica ctu proceedings content, usually in the range of 2 to 6 %. stabilization effect connected with long term effect is therefore connected with reaction of lime with clay particles, subsequently leading to: • growth in strength and the subsoil bearing capacity in general due to the creation of cementing bonds resulting from soil-lime pozzolanic reaction, which, however, develop in time and therefore strength tests are performed with a time lag of mostly 28 days; • reduction in the susceptibility to swelling and shrinkage; • reduction in moisture content is caused both by absorption, extraction of moisture from the surrounding soil after the lime type is changed into slake lime, and by hydration heat as was mentioned above. at the same time, however, the new reaction of lime with clay minerals leads to a change in plasticity thus affecting another factor relevant for workability. • two additional notes are needed [4]; • for higher content of quick lime (above 4-6 %) there is no another reduction of plasticity; • for the development of cementing bonds clay particles are needed, at least 10 %. 3. technology effect the application of lime stabilization on site prevails at the present days. after areal spreading of the demanded content of lime the soil and lime are mixed with the help of cutter or another machineries to the required depth. such modified layer is levelled by grader or bulldozer and subsequently compacted by roller. a great attention is devoted to the moisture content at the time of compaction, to guarantee that soil after compaction will not have more than 5 percent of air in pores. therefore some specialists devote a great attention to the mellowing maturing, to the time between soil and lime mixing and final compaction. therefore after mixing only light compaction is recommended and after about 24 to 72 hours, followed by second mixing supplemented by additional increase of water the final compaction is applied. it is consistent that this recommendation has some limits, as for example for improvement of railway tract subsoil during rail heave [5]. from this technology point of view the application of lime stabilization is not recommended during late autumn, as stabilized soil can freeze. for such case addition of cement is recommended after short delay after lime. nevertheless this recommendation has general character, as lime can prepare soil for cement application. this effect can be reached by some special mixed additives, e.g. for the czech republic it is dorosol, which contains a certain percentage of lime and the rest are hydraulic ingredients. in this case a special attention is devoted to dusting lowering. 4. logical scheme of the earth structure design contemporary version of the eurocode 7 geotechnical design contains four important steps of the geotechnical structure design and performance as specification of geological model, geotechnical model, calculation model and ending with structure realization, performance connected with structure control. these four phases can be used for the specification of the risk with which the design and performance is connected, first of all from the point of view of uncertainties and indeterminateness. for earth structures this scheme is presented in fig. 1 in the form of the geotechnical design report gdr. geological model should be specified not only for the ground but also for the borrow pit, from which fill will be constructed. geotechnical investigation report or ground investigation report gir includes not only geological model but also results of the laboratory and field tests of the ground and theirs presentation in the form of measured or derived geotechnical parameters. from the borrow pit technological samples are taken and subsequently tested in the laboratory. firstly it is compaction test most often proctor standard test. range of the geotechnical investigation as well the range of performed tests is connected with risk, with which the design and performance is connected. eurocode 7 recommends three degrees for this specification with the help of so called geotechnical categories (gc), when into: • 1 gc belongs simple structures in modest geological and geotechnical conditions. these structures are designed with the help of up to day experiences calculation of limit states is not needed. generally into first gc belongs fills with high up to 3 m with good ground conditions. however this is not the case for the ground of fill which need improvement soil stabilization. • 3 gc belongs very demanding structures, or structures with complicated ground from the geological and geotechnical point of view or structures connected with extreme risk for surrounding environment in the case of failure. • 2 gc belongs all other structures. for the structures falling into 3 gc it is necessary always determine mechanical-physical properties either for ground or for fill. for material of fill the test are performed on samples compacted in laboratory and for which moisture content and dry density are falling into the accepted (recommended) range. the same procedure is recommended for structure which belongs to the second gc, however with one exception. in the case of 2 gc connected with lower risk the mechanical-physical properties can be selected from 62 vol. 10/2017 specification of characteristic value of geotechnical parameters figure 1. gdr scheme for earth structures. standard tables, based on the correlation of index properties and mechanical-physical properties. such correlation for compacted fill is e.g. used in codes for small dams, (in the cr it is e.g. code čsn 75 2410 small water reservoirs). however for stabilized soils, firstly where the strength increase is needed, such tables can be hardly used, as there is a shortage or correlations, and therefore lab tests are needed. in this case very good cooperation between project designer and specialist on the lab tests is preferred, namely with respect to the tests performed on samples which were prepared by the nearly same way as is proposed stabilization on the site. 5. sensitivity of the characteristic values determination as stated already lab tests of the mechanical-physical properties are needed for structures belonging into 3 gc and 2 gc with moderate and high risk. after that the results are statistically evaluated and after subsequently they are used for the determination of the characteristic values of the geotechnical parameters. here ec 7 defines the characteristic values as “selected as a cautious estimate of the value affecting the occurrence of the limit state”. this expression should be taken very carefully, as: • “selected” emphasizes the engineering judgement; • “cautious estimate” emphasizes that a certain conservatism should be applied; • “limit state” emphasizes that selected values should have close relation to the given limit state. nevertheless also other aspects are playing significant role during the process of selection: • amount of information and a degree of confidence to these information; • volume of ground affected by expected limit state of failure (e.g. slip surface) or an ability of the structure to transfer loads when both aspects are connected with an ability to compensate weaker zones by stronger ones [6]. therefore for slip surface of failure with significant length it is possible to select characteristic values closer to the average ones, while for shorter slip surface the cautious estimate will be more conservative. for all that it can be specified that the characteristic values selection is one of the most sensitive task of the project designer. probability of verification of the selected values is very low. quality of ground is usually not controlled, only of the fill. but even for the fill the control is going mostly via quality of compaction, whether the moisture content and dry density are falling into prescribed range. direct tests of mechanical-physical properties are rather exceptional, e.g. permeability control for sealing system of landfills. 63 ivan vaníček acta polytechnica ctu proceedings 6. laboratory measurement laboratory tests are needed for soil stabilisation where the main aim is strength increase. optimal content of the additives should be determined as well with mechanical-physical properties, from which the designer selects characteristic values presented some interesting results performed within her diploma theses, devoted to the additives research [7]. for given soil, characterized as clay with low plasticity (cl) she concluded, that it is material unsuitable or conditionally suitable which is also sensitive to freezing. she tested stabilization by quick lime, cement and dorosol, and she focused on: • impact of lime on change of consistency limits; • impact of additives amount on increase of bearing capacity (controlled via cbr test); • impact of additives amount and time on the resistivity of stabilized soils to freezing cycles. follow-up diploma and phd thesis are focused not only on classical soils, but also on alternative aggregates as flying ash, when different additives are tested [3]. last attention is devoted to the combination of additives with randomly distributed short synthetic fibres. the evaluation of these tests is mostly via possibility of application of stabilized soils into road foundation road base or sub-base layers, e.g. according to the czech code čsn 73 6125. up to now lower attention is devoted to the additives application for the own fill of the earth structure. there the properties as shear parameters and deformation modulus have more informative impact, as they are needed for the solution of the limit states uls ultimate limit state or sls serviceability limit state. the problem is very sensitive, as a stiffness increase is function of time and the designer should solve the limit states not only for short term conditions, but also for long term conditions. 7. conclusion the application of different additives for improvement of soil properties has growing tendency. from the view of lime application on fine grained soils it is necessary to distinguished between improvement from the workability view with short term effect and improvement with long term effect, when strength increase has permanent character. in the course of additives application for earth structures of transport engineering the designer should firstly specify his/her intention, whether the process of improvement is focussed on the ground and own fill or on the construction layers, as the applications (and conditions) are different. time effect is playing important role as properties are time dependant, mostly from the view of long term bonds having decisive impact on the mechanical-physical properties. therefore the lab tests are needed pro for demanding structures with moderate and high risk and should be focused not only on the specification of additive amount but also on the determination of soil properties reported as characteristic values. from the view of safety the designer of earth structures has to face many uncertainties with respect to the soil properties, (compared to the designer dealing with man-made construction materials) as subsequent control of the compacted soil is limited and therefore the transfer of experiences from one site to the another one is limited. acknowledgements the paper was prepared with financial support from the research project te0 120168 of the technology agency of the cr: cesti centre of effective and sustainable transport infrastructure. references [1] i. vaníček. significance of geotechnical investigation for geotechnical structure design. in proceedings of 43rd conference with international participation: foundation engineering. brno, cgts cice, pp. 27–40. brno, 2015. [2] m. vaníček, i. vaníček. earth structures in transport, water and environmental engineering. springer, dordrecht, 2008. [3] v. mráz. research of stabilized fly ash properties and its application for earth structures of transport engineering (in czech). ph.d. thesis, czech technical university in prague, faculty of civil engineering, 2016. [4] h. m. greaves. an introduction to lime stabilization. in lime stabilisation. proceedings of the seminar held at loughborough university, civil ans building engineering dept on 25 september 1996. [5] j. h. smith. construction of lime or lime plus cement stabilized cohesive soils. in lime stabilisation. proceedings of the seminar held at loughborough university, civil ans building engineering dept on 25 september 1996. [6] i. vaníček. jubilee volume: andreas anagnostopoulos 50 years of service at the national technical university of athens, chap. earth structures sensitivity of the design to the characteristic geotechnical parameters values determination., pp. 543–552. tsotras, 2015. [7] vrbová. soil stabilization. master’s thesis, czech technical universiti in prague, faculty of civil engineering, 2008. 64 acta polytechnica ctu proceedings 10:61–64, 2017 1 introduction 2 lime stabilization 3 technology effect 4 logical scheme of the earth structure design 5 sensitivity of the characteristic values determination 6 laboratory measurement 7 conclusion acknowledgements references 307 acta polytechnica ctu proceedings 1(1): 307–310, 2014 307 doi: 10.14311/app.2014.01.0307 recent results from the safir project1 m. sánchez-portal1,2, m. castillo-fraile1,2, c. ramos almeida3, p. esquej4,5, a. alonso-herrero5, a. m. pérez garćıa3, j. acosta-pulido3, b. altieri1, a. bongiovanni3, j. m. castro cerón1, j. cepa3, d. coia1, l. conversi1, j. fritz6, j. i. gonzález-serrano5, e. hatziminaoglou7, m. pović8, j. m. rodŕıguez espinosa3, i. valtchanov1 1european space astronomy centre (esac)/esa, madrid, spain 2isdefe, madrid, spain 3instituto de astrof́ısica de canarias, la laguna, tenerife, spain 4centro de astrobioloǵıa, inta-csic, madrid, spain 5instituto de f́ısica de cantabria, csic-uc, santander, spain 6sterrenkundig observatorium, universeit gent, belgium 7european southern observatory, garching bei münchen, germany 8instituto de astrof́ısica de andalućıa, granada, spain corresponding author: miguel.sanchez@sciops.esa.int abstract the “seyfert and star formation activitiy in the far-infrared” (safir) project is aimed at studying the physical nature of the nuclear ir emission and star formation properties of a small sample of nearby seyfert galaxies observed with the pacs and spire instruments on board the herschel space observatory. in this paper, we review the achieved results, that reveal the importance of the far-ir range to improve the quality and reliability of the estimates of basic agn torus parameters, and describe some preliminary outcome from the on-going work on the dust properties of resolved agn host galaxies. keywords: agn seyfert sed ir. 1 introduction coeval agn and starburst phenomena can be assessed by means of the analysis of dust in the infrared (ir) domain. in this range, in particular in the mid-ir (mir) and far-ir (fir), dust contributes to most of the thermal emission. the unified model considers a central agn engine and a broad-line region (blr) obscured by a thick dust torus. the dust grains re-radiate in the ir the absorbed uv/optical photons. as it has been well characterised by existing facilities (eg. spitzer, trecs), the dusty torus emission peaks in the mir (730µm) and it extents to the fir, where the contribution related to the star formation (sf) becomes dominant. thus, agreeing to [8], the sed of seyfert galaxies in the mir and fir range can be solely explained by the dust thermal re-radiation of higher energy photons. therefore, dust thermal emission should be made-up of three different contributions: (a) warm dust heated by the agn (120-170 k); (b) cold dust heated by the star formation (40-70 k, and (c) very cold dust heated by the general interstellar radiation field (15-25 k). until now it has been poorly constrained due to the limited spatial resolution and spectral coverage of the existing facilities. the herschel observatory [5] provides new performances and capabilities to study the emission of nearby galaxies in fir ans sub-mm regions: the pacs [9] photometer allows to image in the 70, 100 and 160µm with unprecedented spatial resolution (5.5 arcsec at 70µm) and the spire [3] photometer permits to image in the 250, 350, 500µm bands, a formerly unexplored region, at a relatively high spatial resolution. these instruments provide, on the one hand the characterisation of the agn sed minimizing the contamination by the host galaxy (pacs) and on the other, the assessment of the cold and very cold dust components (spire) both across the host galaxy and the nuclear and circum-nuclear regions. pacs and spire data can 1herschel is an esa space observatory with science instruments provided by european-led principal investigator consortia and with important participation from nasa. 307 http://dx.doi.org/10.14311/app.2014.01.0307 miguel sánchez-portal et al. be used to fit seds sampling both the emission peak and the rayleigh-jeans tail of the thermal emission of the cold and very cold dust components. the fitted seds can be used to derive dust masses and temperatures and the star formation rate (sfr). the relatively high spatial resolution of these instruments makes possible to map the different emission regions (e.g. nucleus, arms, inter-arm region). in this context, two foci of interest drive the safir study: (a) the dusty torus: current models consider either smooth or clumpy dust distributions. any agn ir model should consider three constituents when applied to describe the actual sed of seyfert galaxies: agn, starburst and host galaxy. the starburst contribution can be constrained by the use of fir data. in addition, both the torus and starburst emission overlap smoothly in the fir. therefore, the use of fir data with high spatial resolution and wide spectral range coverage is fundamental to discriminate the modelled torus characteristics. (b) the nuclear activity and star formation coexistence: the interrelationship between accretion onto massive black holes and the star formation is a topic fundamental to understand the formation and evolution of galaxies. fir data from herschel pacs and spire allow to characterise and map the dust distribution and temperature and the star formation activity across galaxies hosting agns. it can contribute to constraint the current agn formation and evolution models. 2 sample of galaxies and technical implementation the safir collection of nearby galaxies is constituted by 18 seyfert galaxies sampled to represent different nuclear clases (seyfert 1.x & seyfert 2). only objects with available high-resolution mir data (groundbased or spitzer) were selected. in addition, all the objects have available optical, nir, x-radio and radio data. this allows to construct a complete multiwavelength sed for all the objects of the sample. ten galaxies of the sample are barred spiral/lenticulars and five are peculiar/interacting systems. four objects are confirmed luminous or ultra-luminous ir galaxies (lirg/ulirg). the observations were performed in the pacs and spire scan map modes adjusting the mapped areas to fit the host galaxy and a background region within the surveyed area. the achieved 1σ sensitivities were approximately, 3.6, 3.9 and 3.9 mjy/beam for pacs at 70,100 and 160µm and 5.5, 7.6 and 6.4 mjy/beam for spire at 250, 350 and 500µm. with these sensitivities it was possible to map both the nuclear and circum-nuclear regions and also large areas within the galaxy disks. 3 results this section presents some already published results from the safir project [10, 2, 1] for three objects of the sample. 3.1 ngc 3081 this galaxy was studied in the context of the safir project combining pacs/spire data with groundbased high-resolution nir/mir data [10]. this object is an early-type barred spiral ((r)sab0/a(r)). it comprises a series of well defined nested star-forming annular-like features: nuclear (r1, 2.3 kpc), inner (r2, 11 kpc) and outer (26.9 kpc) rings. the inner ring (r2) is evidently resolved in the images up to 250µm. the nuclear sed was fitted combining unresolved fir fluxes (r≤ 1.7 kpc) together with integrated nir and mir data. a clumpy model was applied to simulate the nuclear torus emission [4] to assess how the the torus parameters have to be modified, in particular the torus size to account for the fir emission. as a result, it was obtained that the torus outer radius must be notably increased: ro = 4+2 −1 pc vs. ro = 0.7± 0.3 pc obtained using only nir and mir data [11]. also the radial distribution of clouds (defined by the power-law index of the radial density profile q) flattens when the fir data are included in the simulation: (q = 0.2 vs q = 2.3). other model parameters (width of the angular distribution, inclination angle, optical depth, number of clouds) are in agreement with those obtained without fir data. at larger scales (1.7 kpc≤ r≤ 5.4 kpc), the fir emission is well characterised by cold dust thermal emission at t = 28± 1 k (assuming a grey blackbody with emissivity β= 2) likely heated by young stars in r1. the fir emission of the outer part of the galaxy can be reproduced with very cold dust (t = 19± 3 k) heated by the interstellar radiation field. 3.2 mrk 938 this galaxy contains a seyfert 2 agn and presents a significant starbust activity. this object is a morphologically peculiar galaxy that has been proposed to be the remnant of a gas-rich merging of two unequal mass galaxies [12]. it is classified as lirg due to its large ir luminosity. a multi-wavelength study was performed for this object combining x-ray, nir, mir and pacs/spire fir data in the context of the safir project [2] in order to characterise the origin and nature of its strong emission in the ir range. the agn bolometric contribution to the mir and the total ir luminosity is small [lbol(agn)/lir∼ 0.02] as observed in the component decomposition of the mir spitzer/irs spectrum, which is in agreement with previous estimations. the mips 24µm and pacs 70µm images indi308 recent results from the safir project cate that the major part of the star formation activity is concentrated in a compact obscured region of ≤ 2 kpc. fir data have been used to constraint the cold dust emission with unprecedented accuracy. in order to derive the dust properties, the integrated ir sed has been fitted. it has been found that the mir to fir spectrum can be properly modelled by a two-component sed: two modified blackbodies with fixed emissivity (β= 2) and temperatures tw = 67 k and tc = 35 k for the warm and cold dust components, respectively. in addition, a single blackbody component sed was fitted to the fir-only spectrum with a modified blackbody of β= 2 and t = 36.5 k. this value has been used along with the spire flux at 250µm to derive the dust mass using eq. 2 considering an absorption coefficient κ250µm = 4.99 cm2g−1. the value obtained for the dust mass, mdust = 3× 107 m�, is consistent with those derived for local ulirgs and other ir-bright galaxies. 3.3 ngc 1365 ngc 1365 (fig. 1, left) is a supergiant barred spiral galaxy (sb(s)b). it is a nearby (18.6 mpc) lirg harboring a seyfert 1.5 type nucleus. the inner linblad resonance (ilr) region of the galaxy contains a powerful nuclear starburst ring with an approximate diameter of 2 kpc. we have probed the nuclear and circum-nuclear activity of this galaxy in the ir [1]. the strong star formation activity in the ring is resolved by the herschel/pacs imaging data that shows some substructures (super star clusters), as well as by the spitzer 24µm continuum emission, [ne ii]12.81µm line emission, and 6.2 and 11.3µm pah emission. the active galactic nucleus (agn) is the brightest source in the central region up to λ∼ 24µm, but it becomes increasingly fainter in the fir when compared to the emission originating in the ir clusters located in the ring. we modelled the agn unresolved ir emission with a clumpy torus model and estimated that the agn contributes only in a small fraction (∼ 5%) of the ir emission produced in the inner ∼ 5 kpc. the estimated torus size is ∼ 5 pc. we fitted the non-agn 24–500µm sed of the region within the ilr and found that the dust temperature and mass are similar to those of other nuclear and circum-nuclear starburst regions. finally, the comparison of the ir-derived sfr with that obtained from hα observations indicates that ∼ 85% of the ongoing star formation within the ilr is taking place in dust–obscured regions. 4 dust properties of resolved agn host galaxies a study of the dust properties of spatially well-resolved agn hosts has been started (sánchez-portal, castillofraile et al. in preparation). within the safir sample, four galaxies (ngc 1365, ngc 4258, ngc 1566 and ngc 5728) have an apparent size large enough to allow a detailed analysis of the spatial dust properties, notably its temperature and mass that can be directly compared with the star formation characteristics. for these objects, the spatial resolution of the observations is being exploited to produce maps of the dust mass, temperature, and sfr. in this section some examples of the activities currently on-going are shown. assuming an optically thin emission, the flux density can be expressed as fν ∝ νβb(ν, tdust) where β is the dust emissivity. as already stated, several dust components with different temperature should be generally considered, so the flux density can be expressed as: fν(λ) = n∑ i=1 ni λβ+3(ehc/λkti − 1) (1) figure 1: temperature maps of ngc 1365 (left) and ngc 1566 (right). average temperatures range from ∼ 17–18k in the inter-arm regions to t∼23–24k in the bright spots within the spiral arms. the highest average dust temperatures are observed in the central region of ngc 1365 with t∼26k. where ni are the normalization constants and ti are the temperatures of the different components. the procedure devised to generate temperature maps includes the following steps: after a standard reduction procedure (see [1] for a description), the pacs 70, 100 and 160µm and spire 250 and 350µm maps have been convolved to the resolution of the spire 500µm images and resampled to the largest pixel size (that of the spire 500µm maps, set to 14 arcsec). the images have been spatially registered and used as input to an idl procedure that performs a least-squares fit to either one or two dust components (n = 1 or 2) at each pixel in order to cope with the cold or/and very cold dust components. in the maps shown in fig. 1 we have used a single temperature component with a fixed emissivity β= 2 to create the temperature maps of ngc 1365 (top) and 309 miguel sánchez-portal et al. ngc 1566 (bottom). the latter is a bright (lc ii-iii), nearby (11.83 mpc) sab(s)bc spiral galaxy harboring a seyfert type 1.5 nucleus. figure 2: sfr map of ngc 1365, obtained from the grey body ir luminosity integrated between 8 and 1000µm. the temperature maps generated closely follow the topology of the star formation regions, with the highest temperatures corresponding to areas of high sf activity, as observed by comparison with the morphology of 70µm and optical hα images. in fact, we have created sfr maps by integrating the grey body sed at the best-fit temperature and applying standard scaling relations [7]. in fig. 2 we show the sfr map of ngc 1365. there is an excellent agreement with the structures revealed by the dust temperature map and the sfr density. in agreement with [1], it is observed that the most intense star formation is taking place in the circum-nuclear region (within the ilr). outstanding formation rate is also taking place in the spiral arms. figure 3: ngc 4258 temperature (left) and dust mass (right). the spatial distribution of dust mass (projected dust density) can be obtained from the temperature maps, using the expression: mdust = d2 lfν κνbν (tdust) (2) adapted from [6], where dl is the luminosity distance and fν is extracted from the spire flux map at 250µm assuming an absorption coefficient κ250µm = 4.99 cm2g−1. in fig. 3 we present the dust temperature and mass maps of ngc 4258, a bright (lc ii-iii), nearby (7.44 mpc) sab(s)bc spiral galaxy hosting a liner/seyfert 1.9 nucleus. the pixel scale is 0.36 kpc−2. 5 conclusions the high spatial resolution herschel pacs & spire observations are demonstrating the importance of the fir to improve the quality and reliability of the agn torus fits. parameters as important as the torus radius and cloud radial distribution have a strong dependency of this spectral range. moreover, the fir data are crucial to characterise the starburst contribution and to constrain the dust properties. the quality of the herschel data is allowing us to study the spatial distribution of dust within the galaxies, thus permitting to characterize the variation of dust properties (temperature, dust mass) and sfr with the nuclear distance. acknowledgments we would like to acknowledge the herschel project scientist, göran pilbratt, for making possible the implementation of this project kindly providing the required guaranteed time from the ps budget. references [1] alonso-herrero, a., sánchez-portal, m., ramos almeida, c., et al. 2012, mnras, 425, 311 doi:10.1111/j.1365-2966.2012.21464.x [2] esquej, p., alonso-herrero, a., pérez-garćıa, a. m., et al. 2012, mnras, 423, 185 doi:10.1111/j.1365-2966.2012.20779.x [3] griffin, m.j., abergel, a., abreu, a. et al. 2010, a&a, 518, l3 [4] nenkova m. et al., 2008, apj, 685, 147 doi:10.1086/590482 [5] pilbratt, g.l., riedinger, j.r., passvogel, t. et al. 2010, a&a, 518, l1 [6] hildebrand r. h., 1983, q. j. r. astron. soc., 24, 267 [7] kennicutt, jr., r. c., araa, 1998, 36, 189 doi:10.1146/annurev.astro.36.1.189 [8] pérez garćıa a. m., rodŕıguez espinosa j. m., 2001, apj, 557, 39 doi:10.1086/321675 [9] poglitsch, a., waelkens, c., geis, n. et al. 2010, a&a, 518, l2 [10] ramos almeida, c., sánchez-portal, m., pérez garćıa, a. m., et al. 2011, mnras, 417, l46 doi:10.1111/j.1745-3933.2011.01117.x [11] ramos almeida c. et al., 2011, apj, 731, 92 doi:10.1088/0004-637x/731/2/92 [12] schweizer f., seitzer p., 2007, aj, 133, 2132 310 http://dx.doi.org/10.1111/j.1365-2966.2012.21464.x http://dx.doi.org/10.1111/j.1365-2966.2012.20779.x http://dx.doi.org/10.1086/590482 http://dx.doi.org/10.1146/annurev.astro.36.1.189 http://dx.doi.org/10.1086/321675 http://dx.doi.org/10.1111/j.1745-3933.2011.01117.x http://dx.doi.org/10.1088/0004-637x/731/2/92 introduction sample of galaxies and technical implementation results ngc 3081 mrk 938 ngc 1365 dust properties of resolved agn host galaxies conclusions 35 acta polytechnica ctu proceedings 2(1): 35–40, 2015 35 doi: 10.14311/app.2015.02.0035 white dwarfs in cataclysmic variables: an update e. m. sion1, p. godon1 1department of astrophysics & space science, villanova university, 800 lancaster ave., villanova, pa 19085, usa corresponding author: edward.sion@villanova.edu abstract in this review, we summarize what is currently known about the surface temperatures of accreting white dwarfs in nonmagnetic and magnetic cataclysmic variables (cvs) based upon synthetic spectral analyses of far ultraviolet data. we focus only on white dwarf surface temperatures, since in the area of chemical abundances, rotation rates, wd masses and accretion rates, relatively little has changed since our last review, pending the results of a large hst go program involving 48 cvs of different cv types. the surface temperature of the white dwarf in ss cygni is re-examined in the light of its revised distance. we also discuss new hst spectra of the recurrent nova t pyxidis as it transitioned into quiescence following its april 2011 nova outburst. keywords: cataclysmic variables dwarf novae intermediate polars spectroscopy uv individuals: t pyx, cc syg. 1 introductory overview the white dwarfs in cataclysmic variables (hereafter cvwd) are the central engines of the observed outbursts, either as potential wells for the release of gravitational energy during accretion (dwarf nova dn), or as the sites of explosive thermonuclear runaway (tnr) shell burning (classical novae), steady shell burning (supersoft x-ray binaries) or instantaneous collapse and total thermonuclear detonation if the wd reaches the chandrasekhar limit (type ia supernova? sn ia). therefore, the accreting wds serve as probes of explosive evolution and accretion physics and diffusion, as they bear the thermal, chemical and rotational imprint of their long term accretion and thermonuclear history (sion 1991 & 1995; townsley & bildsten 2003; townsley & gänsicke 2009). deeper physical insights however require a larger number of chemical abundances, rotation rates, surface temperatures, mass accretion rates, and masses for each spectroscopic subclass of cvs. only then can any definitive conclusions be drawn. in order to adequately sample the parameter space (mwd, i, ṁ , teff , porb) of the dns, nova-like variables, and magnetic cvs, a large go program was approved in cycle 20 (b.gänsicke, principal investigator) to secure high quality cos spectra for cv classes underrepresented in the current overall cv sample. the data is in hand and undergoing analysis at the time of this writing. ultimately, we hope to obtain data for > 30 cvs per class. therefore, in this review our focus is restricted to white dwarf surface temperatures, since in the area of chemical abundances, rotation rates, wd masses and accretion rates, relatively little has changed since our last review in the 2011 palermo meeting proceedings. in section 2, our fuv analysis techniques are briefly summarized, section 3 we address how the revised (shorter) distance to ss cygni affects the results of our analysis of the fuse + hst stis spectra of ss cygni by sion et al. (2010), in section 4 we tabulate and display the current distribution of cvwd surface temperatures versus orbital period and in the final section, we include some remarks on new hst spectra of the recurrent nova t pyxidis. 2 synthetic spectral analysis of fuv spectra of cvwds we have modeled the fuv spectra of wds in cvs during dn quiescence and nova-like low states from iue, fuse and hst (fos, ghrs, stis, cos) archival data, and through our ongoing collaboration with past hst surveys led by b.gänsicke, and p. szkody. the iue archival spectra and high quality fuse and hst fuv data are fit with the latest versions of the tlusty/synspec model photosphere code and model accretion disk codes (hubeny 1988; hubeny & lanz 1995). we are taking into account the bl explicitly in our modeling of the fuv spectra of disk accreting systems, by replacing the very inner rings of the standard accretion disk model with high temperature rings in agreement with the temperature and density in the boundary layer. this improves the model fitting at the shorter wavelengths. as an example of our cvwd 35 http://dx.doi.org/10.14311/app.2015.02.0035 e. m. sion, p. godon photosphere fitting, in fig.1, we display a wd solar composition fit to the hst stis spectrum of v442 cen with e(b-v) = 0.10. the wd model has t=47,000k ±2000k and log(g) = 8.3±0.2, vsin(i) = 300km/s ±50 km/s (sion et al. 2008). figure 1: the best-fitting single temperature wd fit to the hst/stis spectrum of v442 cen. the model consists of a 47,000k wd with log(g)=8.3 and scaled to a distance of 328pc. 3 the white dwarf in ss cygni:the vlbi and corrected hubble fgs distance schreiber & lasota (2007), and references therein, pointed out that the previously published hubble fgs parallax of ss cygni, with a distance of 166 +/12 pc, posed serious problems for the disc instability model (dim) of dwarf nova outbursts because it would fail to explain the absolute magnitude during outburst. with the new vlbi-derived distance (miller-jones et al. 2013) to ss cygni (114 pc), and the corrected hst distance (nelan & bond 2013) of 120 pc instead of 166 pc, the concern for the validity of dim is alleviated. an obvious question is: how does the new, shorter distance affect the analysis of sion et al. (2010) regarding their detection of the wd during quiescence and their derivation of the wd’s surface temperature from the modeling of fuse + hst stis spectra? to answer this question, we have carried out model fits using the new distance in the range of 114 pc to 120 pc. when we used single, steady state disk models, we obtained the same results as in sion et al. (2010). the disk does not fit the flux in the shorter wavelengths unless the accretion rate is very large, which is inconsistent with dwarf nova quiescence, and the model-derived distance is far too large. when we combined a model disk with a model white dwarf photosphere (using the bitner et al. wd mass as in sion et al. (2010), the correct distance to ss cygni was obtained for a low accretion rate and a lower temperature white dwarf. however, the fits with the wd + disk are inferior to the fits where the wd dominates the fuv flux. in order to match the best fit solution to the fuse + hst stis wavelength range in sion et al. (2010), the white dwarf must have teff = 45000k 50000k for a distance of 112 pc to 120 pc and a white dwarf mass mwd = 0.95m�. this value of the wd mass is within the error range of the bitner et al. mass. our conclusion about the teff of the wd in ss cygni during quiescence is essentially unchanged from sion et al. (2010). 4 the current distribution of cv white dwarf surface temperatures versus orbital period the effective temperatures teff of the wds (obtained during dwarf nova quiescence and nova-like low states when the wd is exposed), are the critical key to revealing the thermal response of the wd to mass accretion and the long term accretion rate < ṁ > which, through compressional heating by the accreted material, is linked to the wd surface temperature (sion 1995; townsley & bildsten 2003; townsley & gänsicke 2009). in table 1, we have tabulated what we regard as the most reliably secured wd surface temperatures. the first column gives the name of the cv, the second column the cv subtype, the third column, the orbital period in minutes, the fifth column the wd teff and the sixth column, the reference for the temperature. these temperatures are derived from a variety fuv spectra and all are known with a precision of at least 3000k and in the majority of cases, better that +/2000k. with the paucity of fuv-derived temperatures per cv subclass, one should not ignore usable fuv data from iue, fuse and hut because the ”quality” of the data is deemed inferior to hst stis and cos. the current distribution of wd teff against the orbital period porb is displayed in fig.2. (see also fig.4 in townsley & gänsicke 2009). note the continued relatively sparse coverage in temperature of the wds in cvs above the period gap (porb > 3hr), compared with the coverage below the period gap. it is possible that the apparent trend toward higher temperatures with increasing porb (i.e. higher long term average < ṁ >) could be due to observational selection since the wd teff ’s were derived primarily in the fuv where the planckian peak occurs for hotter accreting wds. for example, copperwheat et al. (2010) derive a teff = 10 − 15, 000k in the optical for the wd in the eclipsing dn ip peg which has porb = 3.8 h while the eclipsing system sdss1006 with porb = 4.46 h, has teff = 16, 500 ± 2000k (southworth et al. 2009) in the optical. both of these objects should contain much hotter wds (higher < ṁ >) for their orbital periods. 36 white dwarfs in cataclysmic variables: an update table 1: the temperature of cv white dwarfs system cv type porb (h) teff (k) ref sdss1507 dn/su 1.11 11000 szkody et al. (2010a) gw lib dn/wz 1.280 14700 szkody et al. (2002a) bw scl dn? 1.304 14800 gänsicke et al. (2005) ll and dn/wz 1.321 14300 howell et al. (2002) pq and dn/su 1.34 12000 szkody et al. (2010a) ef eri am 1.350 9500 szkody et al. (2010b) sdss j1610-0102 dn? 1.34 14500 szkody et al. (2007) v455 and dn/su 1.35 10500 szkody et al. (2013) hs2331+3905 dn 1.351 10500 araujob-betancor et al. (2005a) al com dn/wz 1.361 16300 szkody et al. (2003) wz sge dn/wz 1.361 14900 sion et al. (1995) sw uma dn/su 1.364 13900 gänsicke et al. (2005) sdss0919 dn 1.36 13500 szkody (2014) sdss1035 dn? 1.37 10500 southworth et al. (2006); littlefair et al. (2006b) hv vir dn/wz 1.370 13300 szkody et al. (2002b) sdss1339 dn 1.38 12500 szkody et al. (2010a) sdss2205 dn 1.38 15000 szkody et al. (2010a) wx cet dn/wz 1.399 13500 sion et al. (2003) t leo dn/su 1.41 16000 hamilton & sion (2004) eg cnc dn/wz 1.44 12300 southworth et al. (2006) xz eri dn/su 1.468 15000 szkody et al. (2010a) sdss1514 dn 1.48 10000 szkody (2014) dp leo am 1.497 13500 schwope et al. (2002) v347 pav am 1.501 11800 araujob-betancor et al. (2005b) bc uma dn/su 1.503 15200 gänsicke et al. (2005) ek tra dn/su 1.509 18000 godon et al. (2008) vy aqr dn/wz 1.514 14500 sion et al. (2003) oy car dn/su 1.515 15000 cheng et al. (2000) sdss0131 dn/su 1.63 14500 szkody et al. (2010a) vv pup am 1.674 11900 araujob-betancor et al. (2005b) v834 cen am 1.692 14300 araujob-betancor et al. (2005b) ht cas dn/su 1.768 14000 wood et al. (1992) vw hyi dn/su 1.783 22000 godon et al. (2008) cu vel dn/su 1.88 18500 gänsicke & koester (1999) mr ser am 1.891 14200 araujob-betancor et al. (2005b) bl hyi am 1.894 13300 araujob-betancor et al. (2005b) st lmi am 1.898 10800 araujob-betancor et al. (2005b) ar uma am 1.93 20000 schmidt et al. (2005) rej1225 dn/su 1.99 12000 szkody et al. (2010a) ef peg dn/wz 2.01 16600 howell et al. (2002) dv uma dn/su 2.138 20000 feline et al. (2004) hu aqr am 2.084 14000 gänsicke (1999) qs tel am 2.332 17500 rosen et al. (2001) sdss j1702+3229 dn/su 2.402 17000 littlefair et al. (2006a) tu men dn 2.813 28000 sion et al. (2008) am her am 3.094 19800 gänsicke et al. (1995) mv lyr nl/vy 3.176 45000 godon et al. (2012) dw uma nl/vy 3.279 50000 araujob-betancor et al. (2003) tt ari nl/vy 3.301 39000 gänsicke et al. (1999) ip peg dn 3.80 15000 southworth et al. (2009) vy scl nl 3.99 45000 hamilton & sion (2008) v1043 cen am 4.190 15000 araujob-betancor et al. (2005a) ww cet dn 4.220 26000 godon et al. (2008) ugem dn/ug 4.246 30000 sion et al. (2001) ssaur dn/ug 4.391 34000 godon et al. (2012) sdss1006 dn 4.46 16500 southworth et al. (2009) v895 cen am 4.765 14000 araujob-betancor et al. (2005b) rx and dn/zc 5.037 34000 sion et al. (2001) ss cyg dn/ug 6.60 47000 sion et al. (2010) vy scl nl 3.99 45000 hamilton & sion (2008) em cyg dn/zc 6.98 40000 godon et al. (2012) tt crt dn 7.30 29000 sion et al. (2008) ru peg dn 8.99 70000 godon et al. (2012) v442 cen dn 11.04 47000 sion et al. (2008) 37 e. m. sion, p. godon figure 2: effective white dwarf temperature as a function of the orbital period. the references for the indivdual temperatures can be found in sion et al. 2008, townsley & gänsicke 2009, araujo-betancor 2005a & b and references therein). the traditional magnetic braking above the period gap is shown between the parallel diagonal solid lines. on the right hand side are the time-averaged accretion rates correpsonding to the temperature scale on the left hand side of the diagram. shown for comparison between the dotted lines is the long term evolutionary path of a 0.8 solar mass white dwarf (with an initial core temperature of 30 million degrees k) which has undergone 1000 nova outburst cycles accreting at the long term rate of 10−8m�/yr 5 the hst cos + stis spectra of the recurrent nova t pyxidis:a progress report the interstellar reddening e(b-v) of the recurrent nova t pyxidis is a critical parameter in the determination of the best-fitting model parameters. the uv spectra exhibit a minimum near 2175 a which is due to the interstellar extinction. since the galex spectrum is the most reliable (i.e. highest s/n ratio) in that wavelength region, we used the galex spectrum to determine e(b-v). the value of e(b-v) for which the 2175 a feature disappears from the dereddened spectrum, e(b-v)=0.35, is taken as the e(bv) value towards t pyx (see fig.3). we use this value to deredden the iue, galex and hst spectra, and we also consider the effects of different reddening values on our results. the hst stis and cos spectra obtained in december 2012 and july 2013 are identical. thus, we co-added them to improve the signal-to-noise (s/n). we have found that the pre-outburst iue and galex spectra together with the hst post-outburst spectrum. we note that the uv flux has remained constant not only before the outburst, but it has now come back precisely to the same value. this is an indication that 1600 1800 2000 2200 2400 2600 2800 1e-14 1e-13 1e-12 e(b-v) 0.00 0.10 0.20 0.25 0.30 0.35 0.40 0.45 0.50 a log (flux) er g/ s/ cm ^2 /ao o figure 3: the merged galex-iue spectrum of t pyx has been dereddened for different values of e(b-v) as indicated on the right. the 2175å feature associated with the reddening is clearly seen in absoprtion for low balues of e(b-v), and it appears as extra flux for large values of e(b-v). we deduce that the reddening towards t pyx must be e(b-v)=0.35, the value for which the 2175å feature vanishes. 38 white dwarfs in cataclysmic variables: an update the mass accretion rate remained constant before and after the outburst (see godon et al. 2014). 0.4 0.6 0.8 1 1.2 1.4 1.6 1e-06 1e-05 0.0001 0.001 mwd/msun envelope mass (msun) e(b-v)=0.45 luv e(b-v)=0.35 luv e(b-v)=0.25 luv e(b-v)=0.50 disk e(b-v)=0.35 disk [1] e(b-v)=0.50 e(b-v)=0.25 disk [2] accreted accreted accreted accreted accreted accreted ejected ejected extrapolation figure 4: the mass of the accreted envelope and ejected envelope are shown as a function of the wd mass for different values of the reddening for t pyx. our disk model results are drawn with a solid line (“disk”). the lower limit of the accreted envelope inferred from the uv flux is shown (dotted line, “luv”). in comparison we show the ejected envelope [1] as estimated by nelson et al. (2012) (square symbol), as well as [2] computed in patterson et al. (2013) (dashed line). the accreted envelope is larger than the ejected envelope, except for a value of e(b-v)=0.25 combined with a wd mass ∼ 0.9m� and larger. however, the best results were obtained for a larger value of the reddening e(b − v ) > 0.30. using the value of the reddening we derived, together with the new distance estimate of 4.8 kpc (sokoloski et al. 2013), we fit the observed (back-toquiescence) hst spectra with disk models for different wd masses. we then computed the accreted mass over a period of 44yrs, which we then compared to the estimates of the ejected envelope mass (during the 2011 outburst). we recapitulate our results in fig.4, where we also consider different reddening values for the sake of completness. our main finding is that the mass of the accreted material between the last two outbursts is larger than the mass of the ejected envelope in the last outbursts, unless the reddening is e(b-v)=0.25 (or smaller) and the mass of the wd is ∼ 0.9m� or larger. however, such a small value of the reddening leads to a very bad fit and has therefore to be rejected. acknowledgement this research is supported by hst grants go12799.01a and go12890.01a, both to villanova university. the archival data (iue, fuse & galex) analysis was supported by the national aeronautics and space administration (nasa) under grant number nnx13af11g issued through the office of astrophysics data analysis program (adap) to villanova univesity. references [1] araujo-betancor, s., et al.: 2005a, a&a, 430, 629 [2] araujo-betancor, s., et al.: 2005b, apj, 622, 589 [3] araujo-betancor, s., et al.: 2003, apj, 583, 437 doi:10.1086/345098 [4] cheng, f. h., et al.: 2000, apj, 542, 1064 [5] copperwheat, c., marsh, t. r., dhillon, v., littlefair, s.. hickman, r., gnsicke, b., southworth, j.: 2010, mnras, 402, 1824 [6] feline, w. j., et al.: 2004, mnras, 347, 1173 [7] gänsicke, b. t., et al.: 2005, apj, 629, 451 doi:10.1086/431271 [8] gänsicke, b. t.: 1999, in annapolis workshop 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doi:10.14311/app.2017.12.0068 acta polytechnica ctu proceedings 12:68–73, 2017 © czech technical university in prague, 2017 available online at http://ojs.cvut.cz/ojs/index.php/app cross-study research on utility and validity of driving simulator for driver behavior analysis michal matowicki∗, ondřej přibyl czech technical university, faculty of transportation sciences, na florenci 25, prague 1, czech republic ∗ corresponding author: matowicki.m@gmail.com abstract. driving is one of the most ordinary and universal everyday tasks and, at the same time, one of the most complex and dangerous. it requires a full range of sensory, perceptual, cognitive, and motor functions, all of which can be affected by a wide range of stressors and experience levels. therefore, exploring of human behaviour while controlling a vehicle is a crucial task in improving traffic safety. experimental studies can always be conducted with on-road tests, however, using a simulator is safer and more cost-effective. the main goal of this paper is to demonstrate if and under what conditions could a driving simulator provide sufficient results required for a proper study of driver behavior. it discusses its limits and advantages. overall, the research reviewed in this paper indicates that simulator driving behaviour approximates (relative validity of speed and lateral position of vehicle on road), but does not exactly replicate (absolute validity), on-road driving behaviour. keywords: driving simulator, relative validity, absolute validity, cross-study analysis. 1. introduction with rapid growth of technological advancement, not only executive elements or traffic engineering like intelligent transport systems, is shifting toward more sophisticated and ecologically and economically valid solutions. another example is research on the transport subjects which is reaching for advanced tools like driving simulators. however, although supported in many government and scientific organizations [1] the usefulness and validity of results achieved in a driving simulator remain a very hot topic in transport engineering especially when driving behaviour and the psychology of the drivers are concerned. although this topic seems to be of significant importance (either supports of diminishing effects and results of driving simulator experiment), it seems to be often forgotten during the execution of the experiment, which may, at times, lead to meaningless conclusions. hence, it is very important to produce a literature review on the topic in search of significant data to utilize simulator results to make conclusions on real-world driver behaviour. this document includes a table (table 1) which summarizes the most important papers on the topic, as well as further analysis and description of chosen papers, all of which are divided into several categories depending on the kind of validity confirmed and parameters measured in the experiment. the table allows for quick familiarization with the most important research reviewed and introduces an in depth analysis of specific studies. chapter two consists of subsections that are dedicated to particular kinds of validity which can and should be of concern when setting up a driving simulator experiment. this paper produced a cross-study review and analysis of confirmed and proven validity measures for driving simulator experiment measures, and is concluded by summarizing recommendations and predictions regarding the future of driving simulators as tools for various studies on driving behaviour. 2. reviewed studies despite the importance that is placed on the simulator as a tool for investigating driver performance and behaviour, an extensive literature review reveals that only a very limited number of studies specifically evaluating the behavioural validity of simulators was conducted [3]. majority of the available literature on simulator validation has focused on measures of speed, lateral position, and braking responses (table 1). the following summary of literature research reviews literature on the given topic with a specific emphasis on the description of performance measures, the tasks and conditions, and first of all the evidence provided for behavioural validity of each simulator and the behavior of the subject "driving" it. 2.1. speed adjustment speed is one of the most studied measures of driver behaviour and, thus, one of most commonly analised with respect to simulator validity [2, 5–7, 9–11, 13, 15, 17]. overall, these studies rather consistently provide results confirming relative but not absolute validity of speed data in driving simulators. most explanatory ones were for example klee et al. [10] who examined the validity of a fixed-base simulator with respect to forward speed. 30 drivers drove the same road section on both driving simulator and in an instrumented vehicle. the speed values were measured at 16 points, while measures in 10 of them were similar in both observed environments. while authors do not conclude relative validity of the simulator and focus on lack of 68 http://dx.doi.org/10.14311/app.2017.12.0068 http://ojs.cvut.cz/ojs/index.php/app vol. 12/2017 cross-study research of driving simulator authors achieved validity bella, 2008 [2] speed and complexity of menoeuvre mullen, charlton, devlin & bedard,2011 [3] relative validity of lateral position across different road types watts & quimby, 1979 [4] relativity of perceived hazard risk blaauw, 1982 [5] absolute speed validity & relative lateral displacement riemersma et al., 1990 [6] relative validity of speed reduction measures tornros, 1998 [7] relative validity of speed and lateral positioning wade & hammond, 1998 [8] relative validity of lane position reed & green, 1999 [9] absolute validity on speed control relative validity on effects of phone dialling klee et al., 1999 [10] relative validity of vehicle velocity bittner et al., 2002 [11] relative validity of speed in road curves blana & golias, 2002 [12] no relative validity confirmed godley et al., 2002 [13] relative validity of speed countermeasures lee, lee & cameron, 2003 [14] relative validity of visual attention-age relation bella, 2005 [15] relative validity of speed mcavoy 2007 [16] no validity for speed adjustment in work zones shinar & ronnen 2007 [17] relative validity of speed adjusting table 1. studies elaborating driving simulator validity. absolute validity, it can be observed that speeds in all 10 locations with similar results were 5-10 km/h slower for simulated driving. moreover, described speed difference provided the same results in each case which implies relative validity of simulator. another author providing evidence for the relative validity of the simulator maintained speed and its correlation with the on-road speed is bella [2, 15]. bella provides two studies in which the simulators were designed with identical road sections to those where real driving was performed by experiment participants. in real world experiments, speed measurement points were specified along the road instead of the instrumented vehicle driving record. the first experiment was conducted on road section with construction zone with speed limitation applied, whereas the second study described a highway in normal condition. a bilateral z-test for non-matched samples was performed in both data sets to estimate whether the recorded driving speeds were significantly different from those in the simulator. consistently with other researches bella conducted that simulator speeds were different than in the real world, although in [2] mean speed in simulator was higher than in real world while in [15] it was on the contrary. nevertheless, the differences in simulator speeds and in the field speeds for each point were not significantly different, demonstrating at minimum, interactive relative validity of static simulator tool for assessing speed. unconventional way of assessing speed as simulator validity indicator was presented by shinar & ronen [17] where two speed related measures were observed: speed estimation (i.e. driver perception of the current speed of vehicle) and production of speed (i.e. adjustment of the vehicle’s speed to reach predetermined speed). sixteen participants took part in an experiment where both on-road driving and simulator (with shielded speedometer) drives were performed. during each drive, participants were repeatedly asked to estimate the current speed of vehicle and produce seven different speeds 69 michal matowicki, ondřej přibyl acta polytechnica ctu proceedings ranging from 40 to 100 km/h. authors found that estimated speeds differed significantly in the simulator only for speeds of 40 and 100 km/h where the first speed was always estimated higher in simulator and latter one was always estimated lower. on the other hand, second investigated variable showed consistently higher produced speeds in simulator with a difference of approximately 25 km/h. conclusions of multiple analysed studies pointed out towards absolute validity of most of the estimated speeds and vast majority of the studies suggests relative validity for speed produced in the simulator. this seems to form firm foundations for performing scientific research with the use of driving simulators. 2.2. lateral position of vehicle second often validated measure in driving simulators is the lateral position of the vehicle on the simulated road. one of the most interesting studies in the matter of lateral positioning validity was performed by wade and hammond [8]. the authors measured positioning deviation as distance from the centreline of the road. in a tested sample of 26 participants, the mean deviation of the distance from centreline was statistically significantly larger in virtual driving when compared to the on-road conditions. authors determined that in the real-world it is clear that drivers travel more accurately near the centreline when compared to virtual driving experience. this led to the conclusion that absolute validity was not demonstrated in performed research. nevertheless, mullen, charlton, devlin and bedard [3] noticed that similar to results in speed researches mean deviations on different types of roads showed a familiar pattern when compared to the real environment (i.e. the road with the highest mean deviation in the real-world was also the road with the highest mean deviation in the virtual environment) which indicates relative validity. however, it is a noticeable fact that data were collected on straight road section with a relatively low speed limit of 35 mph (approx. 56 km/h). although blana & golias [12] tried to answer the question of speed influencing validity of lateral displacement measures, it remains unknown. in their research scientists conducted experiment in a simulator where distance between front wheel of the vehicle on the passenger side and white line determining the edge of the road were measured. a sample of 100 on-road drivers in vehicle instrumented with video cameras pointing toward road surface drove on elaborated highway section, while corresponding road was also driven in simulator by 100 participants. the results clearly pointed towards higher average distance measured in the real-world conditions when compared to simulator on both straight sections and curves. this indicates that drivers in the on-road conditions position their vehicles closer to the centreline of the road lane. this conclusion was further confirmed by smaller standard deviations (sd) for the measures recorded in real-world conditions. although the direction of displacement from the road edge was consistent for curved and straight road sections for all speeds measured, the magnitude of the difference in displacement and sd varied depending on speed, leading to inconclusive results in terms of absolute or relative validity of the simulator. again, these results seem to provide strong argument supporting driving simulators as recommended research tools. 2.3. road safety countermeasures the simulator validity for evaluating the relation of a driver with road design and traffic control devices was another issue often examined. riemersma et al. [6] performed two experiments aiming to understand, explain and describe these effects. first, the daimlerbenz driving simulator in which the authors created a scenario where the driver entered a dutch village area. the goal of the simulator was to register changes in desired speed adjustments. the drivers were divided into 2 groups and drove the car either on a normal road or on the same road with special speed reduction objects. three infrastructure changes were implemented in the second scenario, those being a median strip, coloured asphalt and a portal gate. analysis of the vehicle speeds in both scenarios revealed decreased speed values in the scenario with special objects installed. moreover, faster drivers reduced their speeds more than average speed drivers. the second experiment involved both real-world driving and in-simulator ride, utilizing their university driving simulator to perform speed reducing infrastructure testing. the experiment was conducted on 24 participants, of which each drove through the village entrance 12 times. infrastructure changes varied during the rides. driving speed comparison between real-world and simulator revealed higher average speed of in-simulator driving than on road conditions at distance of approx. 400 meters from village entrance. nevertheless, an interesting finding was that, after the 400m distance point, simulator drivers reduced their speed to a greater extend, thus entering the village at a lower speed. the magnitude of speed reduction was evaluated as 3 times bigger for simulator than real-world (25,7% and 8,6% respectively) this can indicate similarly to chapter 2.5.2, relative but not absolute validity of the simulator results. due to these results, the authors concluded that a simulator was an effective tool for evaluating speed reduction methods. following research of mcavoy and datta [16], found in their study that amos ii simulator has contrary results for evaluating night traffic speed control devices in construction zones. a model construction zone on a highway with warning lights was designed and implemented into simulator conditions. 35 drivers took a ride in simulator at the road described above, while in naturalistic study vehicles average speed was measured on road with the use of radar guns. the summary of the results led the researchers to conclude that the simulator failed to provide proof for relative 70 vol. 12/2017 cross-study research of driving simulator or absolute validity standards. a suggesteed explanation for that fact was the subject’s perception of risk associated with driving through real-world work zones at night in a simulator-provided experience. the final paper reviewed in relation to drivers’ behaviour studied using a simulator was the work of godley et al. [13]. in this study again, similarly to riemersma work [6], relative validity of simulator results was found for effect of rumble strips as a speed-reduction method. 2.4. complex driving behaviours another examined validity in short history of driving simulator experiments is in relation to the complex behaviours of drivers such as tasks of divided attention. reed & green [9] investigated validity of the driving simulator constructed at the university of michigan transportation research institute. the goal of the study was to assess decrements in driving performance during a manual car phone task. participants of the experiment were divided into 2 groups: young (2030 years) and old (above 60 years old), and received instructions to dial a manual phone call during vehicle driving tasks both in the on-road setting and in a simulator. conclusions of the work exhibited that speed parameters (sd of speed and throttle position) were similar in both studied conditions, whereas lane positioning (i.e. mean lateral speed and sd of steering wheel position) varied significantly more in the simulator. overall, for seven of the ten variables measured in the experiments, a significant correlation was found between two environments (authors provide correlation factor r in range of 0.43 to 0.76). for all experiment participants regardless of the age group, the phone task resulted in a decrease in driving abilities in terms of speed control and lateral position of vehicle. worth mentioning is the fact that, just like in the previous studies, the magnitude of impact of particular driving conditions was higher in a simulator than on the road. additionally, older drivers seemed to suffer more from the phone dialing task than young drivers in terms of driving abilities. few other studies in this area were performed, but ones worth mentioning are lee, lee and cameron [14] (distinguishing between drivers with different abilities), lee & lee [18] (identifying drivers at risk of future violations), reimer et al. [19] and salvucci et al. [20] who investigated an iphone, button style flip-phone and ipod distraction on driver performance. results of these works however, are based on very few participants therefore, although similar to the ones described above, they will not be summoned in detail here. 2.5. physiological measures the final validity of importance investigated in this research are psychological responses. while the previous sections described a direct way of measuring driving behaviour, physiological responses are commonly used to determine driver awareness and thus produce some useful insight into driver behaviour and reactions across both simulator and real-world contexts. an important condition that is required from driving simulators all over the world is that simulations should induce and provoke similar physiological responses as real-world experience. first research conducted on that topic was watts and quimby [4], who used skin conductance measures for subjects in a transport and road research laboratory simulator. due to the technological limitations of the time, driving simulation was performed in form of film presented to the drivers that was placed to create the impression of being the driver of a vehicle while film from a drive on a road was displayed. the presented film was 30 minutes long, and consisted of 45 hazardous situations for which drivers had to evaluate the perceived danger on a 10-point scale. conductance of skin was measured for all participant during the film. the measurements were then repeated for a real-world driving condition and the results were compared. in the presented results, the risk assessments were comparable for both environments, and moreover, the perceived risk ratings were positively associated with skin conductance levels (r = 0.78). a more recent study performed by slick et al. [21] utilized measures of both skin conductance and heart rate in both simulator and real-world conditions. the experiment performed in drivesafety ds600c motionbased simulator was made of four specific driving tasks performed on both stop sign and intersection traffic lights. the study suggests that there are no significant differences between skin conductance and heart rate measures between the simulator and realworld driving conditions. the only exception for female drivers was that their mean change in heart rate was significantly smaller in the simulated condition than while driving on-road. the most recent study is one analyzed by mullen, charlton, devlin and bedard [3]. bedard in his research measured heart rate and respiration for participants exposed to threatening situations while driving stisim 400 simulator. two intersection scenarios were investigated in depth with the green light turning into orange immediately after driver approached the intersection, and vehicle appearing from the right shoulder of the road in front of the driver. results indicated high measure of presence of the simulator. in each observed situation, the driver’s heart rate increased significantly, and 65% of the participants showed increased respiration rates in risky situations. overall, these studies suggest that physiological measures in general demonstrate the validity of simulators. overall, these studies suggest that physiological measures in general demonstrate the validity of simulators. 3. summary and conclusions many studies have investigated the validity of simulators for examining driving behaviour, leading to a 71 michal matowicki, ondřej přibyl acta polytechnica ctu proceedings conclusion that there is sufficient evidence to suggest that simulators provide a valid tool for assessing various driving performance measures. some important findings on the topic of simulator implementation can be found in studies like [22–24], where overwhelming majority of the studies suggest that there are no obstacles in using driving simulators. these studies suggest that simulators provide approximated behaviour and reactionsof the drivers (relative validity) , despite the fact that it does not replicate on-road behaviour (absolute validity) exactly. the studies under review confirm a moderate to high correlation of the simulator-achieved results for a whole range of cognitive measures of drivers as well as physiological reactions. this implies that a driving simulator is a sufficient tool for majority of research purposes, and thus may be accepted as suitable for the collection of desired data. nevertheless, it is of high importance to remember that the fact that a driving simulator was shown to be valid in a previous setting does not guarantee it will be suitable for the next one. therefore, it is highly recommended to perform additional simulator validation of the results for each specific setting and the scenario designed for a given simulation experiment. this ensures the researchers are capable of utilising this data to create solutions applicable to real-world situations. on road measures will surely be necessary and prevail over the driving simulator method in research situations in which absolute validity of measured variables is required. although there are certain drawbacks and limitations to current simulator deployment in transport research, there is a confident promise of overcoming this issue through technological advancement and, thus, broadening of simulator validity by creating better imitation and perceptual projection of on-road condition for simulator drivers. this will undoubtedly increase the role of simulators as a tool for driving performance measurements as well as for studying driving behaviour. such studies are certainly important not only with respect to the traditional transportation research tasks (transportation planning, study of traffic flow characteristics, optimization of traffic control and others), but also for new and emerging tasks such as within the field of smart cities (resp. smart mobility), study of drivers using cooperative vehicles by přibyl and svítek [25]. the authors of this paper place high importance on understanding which real-life conditions a driving simulator can replicate and aimed to provide guidance in this respect through this document. acknowledgements this article was supported by funds from grant no. sgs16/186/ohk2/2t/16. references [1] e. commission. white paper roadmap to a single european transport area – towards a competitive and resource efficient transport system. tech. rep., european union commission, 2011. date-added: 2011-03-08. 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[7] j. tornros. driving behaviour in real world in a real and simulated road tunnel – a validation study. accidents analysis & prevention 30:497–503, 1998. [8] m. g. wade, c. hammonds. simulator validation: evaluating driver performance in simulation and the real world. tech. rep., minnesota department of transportation, 1998. [9] m. p. reed, p. green. comparison of driving performance on-road and in low-cost simulator using a concurrent telephone dialling task. ergonomics 42:1015–1037, 1999. [10] h. klee, c. bauer, e. radwan, h. al-deek. preliminary validation of driving simulator based on forward speed. transportation research record 1689:33–39, 1999. [11] a. c. bittner, o. simsek, w. h. levison, j. l. campbell. on-road versus simulator data in driver model development: driver performance model experience. transportation research record 44:303–313, 2002. [12] e. blana, j. golias. differences between vehicle lateral displacement on the road and in a fixed-base simulator. human factors 44:303–313, 2002. [13] s. t. godley, t. j. triggs, b. n. fildes. driving simulator validation for speed research. accident analysis & prevention 34:589–600, 2002. [14] h. c. lee, a. h. lee, d. cameron. validation of driving simulator by measuring the visual attention skill of older adult drivers. american journal of occupational therapy 57:324–328, 2003. [15] f. bella. validation of driving simulator for work zone design. transportation research record 1937:136–144, 2005. [16] d. s. mcavoy, k. l. schattler, t. k. datta. driving simulator validation for nighttime construction work zone devices. transposrtation research record 2015:55–63, 2007. 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[21] r. slick, d. f. evans, e. kim, j. steele. using simulators to train novice teen drivers: assessing psychological fidelity as a precursor to transfer of training. paper presented at driving simulation conference – tsukuba 2006. [22] p. bouchner, s. novotny, r. pieknik, o. sykora. assessments of grade crossing warning and signalization devices driving simulator study. pp. 179–183. 2011. cited by 0. [23] p. bouchner, s. novotny. development of advanced driving simulator: steering wheel and brake pedal feedback. pp. 170–174. 2011. cited by 0. [24] a. mashko, p. bouchner, d. rozhdestvenskiy, s. novotny. virtual traffic signs – assessment of an alternative adas user interface with use of driving simulator. advances in transportation studies 1:37–50, 2016. cited by 0. [25] o. pribyl, m. svitek. system-oriented approach to smart cities. proceedings of the first ieee international smart cities conference isbn 978-1-4673-6551-2 2015. 73 acta polytechnica ctu proceedings 12:68–73, 2017 1 introduction 2 reviewed studies 2.1 speed adjustment 2.2 lateral position of vehicle 2.3 road safety countermeasures 2.4 complex driving behaviours 2.5 physiological measures 3 summary and conclusions acknowledgements references 79 acta polytechnica ctu proceedings 1(1): 79–83, 2014 79 doi: 10.14311/app.2014.01.0079 forty years of x-raying narrow-line seyfert 1 galaxies thomas boller1 1max-planck-institut für extraterrestrische physik, garching, germany corresponding author: bol@mpe.mpg.de abstract forty years after the discovery of narrow-line seyfert 1 galaxies (nls1s), and 20 years since the discovery of the remarkable ultrasoft soft x-ray emissions of nls1s, strategic publications improved the understanding of the seyfert phenomenon more generally. new theoretical models emerged from the observations and stimulated the discussions on the innermost regions of agn. nls1s are an amazing class of agn for x-ray, optical and multiwavelength science. keywords: x-rays: narrow-line seyfert 1 galaxies. 1 introduction type 1 agn are divided into broad line seyfert 1 galaxies (bls1s) and narrow-line seyfert 1 galaxies (nls1s). bls1s are associated with the name of carl seyfert (∗11.2.1911, cleveland, ohio, usa+13.6.1960, nesville,tenneesee,usa). seyfert (1943) discovered that the fwhm of the optical emission lines reaches values up to about 10000 km s−1. such high values were thought to originate from the broad-line region in the gravitational potential of supermassive black hole. the fwhm values of bls1s are much higher than the velocity dispersions found from the rotation curves of normal galaxies, which reach only a few 100 km s−1. observations of such high velocities in bls1s suggested in addition that the observer has a direct view to the central regions of active galaxies. in 1970, fritz zwicky (∗14.2.1898, bulgaria,+ 8.2.1974, pasadena,califormia,usa) made the fundamental discovery that type 1 agn exhibit fwhm of the optical permitted lines much less than that what was observed in bls1s, with fwhm values down to 500 km s−1 (zwicky 1970). these objects were sometimes misclassified with normal galaxies, however the presence of very strong fe ii multiplet emission definitely revealed the type 1 agn nature of these sources, as fe ii multiplet emission is only occurring in high density regions with densities larger than 109 cm−3. such densities are only present in the blr clouds or the accretions disc in agns. 2 historical review 1970, zwicky the first note on a new class of type 1 agn with unusually narrow permitted lines was made by zwicky (1970). 1978, davidson and kinman davidson and kinman (1978) investigated the optical spectrum of mrk 359. they pointed out that the object has unusually narrow permitted optical lines and that ’this object merits further observations‘. figure 1: fwhm value of the hβ line versus the rosat photon index for broad line seyfert 1 galaxies and narrow-line seyfert 1 galaxies. below 2000 km s−1 the photon index reach values up to about 5. 79 http://dx.doi.org/10.14311/app.2014.01.0079 thomas boller 1985, osterbrock and pogge osterbrock and pogge (1985) came up with the first definition for nls1s. they defined nls1s as follows: (i) the permitted optical lines are only slightly broader than the forbidden lines; (ii) strong fe ii multiplet emission, centered between 4400 and 4500 å; (iii) a ratio of the o iii line to hβ line with less than 3; (iv) having fwhm of the hβ line between 500 and 2000 km s−1 (added by goodrich 1989). 1992, puchnarewicz based on einstein ipc data (puchnarewicz 1992) reported for the first time on remarkable ultra-soft x-ray spectra of nls1s. the sample size of nls1 with steep soft x-ray spectra reached a number of 53. 1996, boller, brandt, fink in 1996, we (boller, brandt, fink, 1996) published a relation between the fwhm value of the hβ line and the rosat 0.1-2.4 kev photon index (c.f. fig. 1). while bls1 have their photon indices confined to a fairly narrow range around about 2.3, the rosat photon index rises steeply below fwhm values of 2000 km s−1. this plot demonstrates that the emission within a few rg around the black hole determines the velocity distribution at much larger scales in the blr. the main reasons for the steep x-ray spectra of nls1s are the low masses of the black hole and the very high accretion rates (boller et al. 1996). there appears also a zone of avoidance (fwhm values larger than 2000 km s−1 and photon indices larger than 3), where so seyfert galaxies have been observed so far. 1997, boller, brandt, fabian, fink figure 2: 30 day rosat monitoring of the nls1 galaxy iras 13224-3809 taken in 1997. persistent, giant and rapid amplitude variations have been detected with the maximum amplitude variation with a factor of about 57. in 1997 we have performed a 30 day rosat monitoring observation of the nls1 galaxy iras 13224-3809 (boller et al. 1997). giant, persistent and rapid amplitude variations have been discovered with a maximum amplitude variation with a factor of 57 within about 1200 seconds. the peak in luminosity corresponds to about 1044 erg s−1 within that time scale. the most plausible explanation for these extreme x-ray variability is flux boosting of hot spots orbiting the central black hole. as the flux boosting factor scales with the doppler-factor to the power of 3+γ, and as the photon index of the object is very steep with 4.4, such extreme variability is a natural consequence of flux boosting in steep spectrum nls1s (fig. 2). figure 3: sharp spectral cut-off detected in 1h0707495. a power-law plus absorption edge model, a partial covering model and an ionized reflection disk model are shown with their corresponding residua. 2002, boller, fabian, sunyaev, trümper in 2002 we (boller et al., 2002) have discovered the first sharp spectral drop at around 7 kev without any noticeable fe k emission. the spectral drop is of the order of about 2. two explanations have been discussed in this paper, (i) a partial covering scenario, and (ii) a reflection dominated spectrum. while the partial covering model appears to be ruled out in subsequent analysis, the reflection model of ross and fabian (2005) might explain this type of new spectral cut-offs, now also discovered in other nls1s. 2003, pounds high velocity outflows with substantial fractions of the speed of light have been detected by (pounds et al. 2003). in the nls1 galaxy pg 1211+143 outflow velocities up to 0.2 c have been seen. this discovery fits into the general scheme of understanding nls1 as super-eddington accreeting sources, which naturally leads to substantial mass outflows. 80 forty years of x-raying narrow-line seyfert 1 galaxies 2004, miniutti, fabian triggered by the detection of sharp spectral drops in the high energy spectra of nls1s, a new spectral model has been developed, named as the ’light bending model’ by miniutti and fabian (2004). in this model a hot and compact region, emitting a power law spectrum, is located very close to the central black hole and changes its scale height above the black hole. the compact source illuminates the accretion disc and gives rise to reflection spectrum. when the scale height of the compact object is small (2 rg) then most of the power law photons from the compact object are bent towards the black hole and the power law contribution for the external observer is small. the spectrum is in this case reflection dominated and especially the strongest x-ray emission lines, the fe k and fe l lines become visible (fabian et al. 2009). when the source height is increasing, a smaller amount of photons is lost into the black hole and the powerlaw component increases in strength for the external observer. while the power law component varies by a factor of about 70, the reflection component is less variable with a factor of about 4. this model can explain the missing relation between the illumination strength of the disc and the strength of the fe k line as well as most of the spectral and timing properties we have observed from the innermost regions of agns. 2007, komossa in 2006, komossa (2006) carried out a systematic search for radio-loud nls1s. the analysis confirmed that most nls1s are radio-quiet objects and that only 2 per cent can be considered as radio-loud with an r value greater than 100. the physical reasons for the radio-loudness of some nls1s are still under discussions. 2008, foschini; malizia foschini (2008) and malizia (2008) first reported on the detection of nls1s at hard (2-20 kev) x-rays. this was something unexpected given the steep x-ray spectra of these objects. 2009, fabian a very important discovery, strongly supporting the light bending model from miniutti and fabian (2004), was the detection of a 30 second reverberation lag between the direct hard power-law emission and the soft fe l line reflection component, with the power-law emission leading the soft x-ray emission (fabian et al. 2009). the light bending model predicted that the external observer first sees the direct power-law emission from a compact and hot region sitting very close to the central black hole and after that the reflection component, which has a longer light travel time. in addition, both the fe k and the fe l line were seen for the first time in a agn. the lines are strongly relativistic and the intensity of the lines are in the ratio of 20 to 1, in excellent agreement with atomic physics. 2012, foschini the first detection of gamma-ray emission from a nls1 galaxy was reported by foschini et al. (2012, and references therein). a more than three years monitoring programme ranging from the radio to gamma-rays on the nls1s pmn j0948+0022 revealed the detection of emission above 100 mev. the observations revealed the presence of a powerful relativistic jet with isotropic emission of about 1048erg s−1. a complete understanding of the radio and gamma-ray emission has still not emerged from the few available data. 3 upcoming nls1 erosita science 3.1 survey science erosita will carry out a 4 years all-sky survey in the 0.3 to 10 kev band. the expected number of new nls1s which will be detected is 100000, exceeding the presently known number of nls1s by orders of magnitude. these objects can easily disentangled from other objects, via their distribution in the fwhm photon index plane (c.f. fig. 1). as most of the source will have only moderate count rate statistics during the survey observations, the survey science will be restricted to statistical analysis on different science cases discussed below. • eigenvector 1 analysis brandt and boller (1999) have carried out an eigenvector 1 analysis of broad and narrow-line seyfert 1 galaxies. we found that nls1s lie towards one extreme of the primary eigenvector, and that steep x-ray spectra, narrow optical permitted lines, extreme x-ray variability, week [o iii] emission, strong fe ii multiplet emission are correlated. these type of analysis will be applied to the erosita nls1s with much higher data statistics, probing the physical mechanism lying behind these correlations with much better statistics. • super-eddington accretion and comptonization most nls1 galaxies are accreeting above the eddington limit. the relation between supereddington accretion, outflows, steepness of the x-ray spectra and comptonization has been summarized by boller (2011). the basic idea is that super-eddington accretion results into a strong uv luminosity and a stronger and to higher energies shifted planck-emission from the accretion 81 thomas boller disc. the stronger the uv disc luminosity is and the larger the solid angle subtended by the reflector is, the greater is the cooling by the seed photons incident on the plasma, the lower is the plasma electron temperature, and the steeper are the x-ray photon indices, consistent with the observations. again, with the much larger number of new nls1s detected in the erosita survey observations, a better physical understanding of the relation between all these properties will emerge. other science cases which will be addressed during the erosita survey observations will be, (i) search for extreme x-ray variability and nonlinear x-ray variability, (ii) αox science and the global x-ray baldwin effect, and (iii) multiwavelength source population properties. 3.2 erosita science with pointed observations after the 4-years period of survey observations, pointed observations will be carried out with erosita, allowing for much higher count rate statistics. some of the basic science drivers for the nls1 research will be: • relativistic iron k and l lines and reverberation lag physics erosita will offer a unique possibility to further study the behavior of matter under strong gravity on a much larger sample as presently known. • erosita black hole growth studies nls1 galaxies with their proven high accretion rates are the objects with the highest black hole growth rates and are therefore ideally suited for such studies. • accretion disc physics and nls1 science erosita will open a new window on the study of relativistic disc reflection. strong x-ray reflection is a natural consequence for the existence of a compact corona close to the central black hole system. erosita will offer much more nls1 and agn science. among them are: (i) high velocity outflows and feedback processes, (ii) strong gravity in the highcurvature regime and gr tests (boller and müller, 2013), (iii) intensive starbursts, super-solar metallicities, and relation to agn accretion, (iv) extending the luminosity function of nls1s to higher redshifts, (v) models for the black hole regions of nls1s, (vi) the origin of the extreme soft x-ray excesses, (vii) thin and thick accretion discs in nls1s, (viii) the accretion disc coronae of nls1s, (ix) the origin of the extreme x-ray variability in nls1, (x) interpretation of line and continuum correlations, and (xi) models for the broad line region and the x-ray and uv absorption. acknowledgement tb is grateful to luigi foschini for critical reading of the manusscript and franco giovannelli for organizing and leading the workshop. references [1] seyfert, c: 1943, apj 97, 28. doi:10.1086/144488 [2] zwicky, f.: 1970, pasp 82, 93. doi:10.1086/128889 [3] davidson, k., kinman, t.d.: 1989, apj 225, 776. 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[18] brandt, w.n., boller, th.,: 1999, aspc 175, 265. [19] boller, th.: 2011, the x-ray universe 2011, presentations of the conference held in berlin, germany, 27-30 june 2011 82 http://dx.doi.org/10.1086/144488 http://dx.doi.org/10.1086/128889 http://dx.doi.org/10.1046/j.1365-8711.2002.05040.x http://dx.doi.org/10.1111/j.1365-2966.2005.08797.x http://dx.doi.org/10.1111/j.1365-2966.2003.07164.x http://dx.doi.org/10.1111/j.1365-2966.2004.07611.x http://dx.doi.org/10.1038/nature08007 forty years of x-raying narrow-line seyfert 1 galaxies [20] boller, th., müller, a., fias interdisciplinary science series, isbn 978-3-319-00046-6, springer international publishing switzerland 2013, p. 293. discussion matteo guainazzi’s comment: do you believe that measurements of black hole masses in nls1s are nowadays robust? thomas boller: after the discovery of the extreme x-ray properties of nls1s all reverberation measurements have shown that nls1s lie at the lower mass end of the mass distribution in agn. especially the papers by b. peterson are extremely helpful in that matter. 83 introduction historical review upcoming nls1 erosita science survey science erosita science with pointed observations 71 acta polytechnica ctu proceedings 2(1): 71–75, 2015 71 doi: 10.14311/app.2015.02.0071 photometry and multipolar magnetic field modeling of polars: by camelopardalis and fl ceti p. a. mason1,2, a. g. zhilkin3, d. v. bisikalo3, s. gomez1, j. morales1, e. l. robinson4 1department of physics, university of texas at el paso, el paso tx, usa 2department of mathematics and physical sciences, new mexico state university dacc, las cruces nm, usa 3institute of astronomy, russian academy of sciences, moscow, russia 4department of astronomy, university of texas at austin, austin tx, usa corresponding author: pmason@nmsu.edu abstract we present new broad band optical photometry of two magnetic cataclysmic variable stars, the asynchronous polar by camelopardalis and the short period polar fl ceti. observations were obtained at the 2.1-m otto struve telescope of mcdonald observatory with 3s and 1s integration times respectively. in an attempt to understand the observed complex changes in accretion flow geometry observed in by cam, we performed full 3d mhd simulations assuming a variety of white dwarf magnetic field structures. we investigate fields with increasing complexity including both aligned and non-aligned dipole plus quadrupole field components. we compare model predictions with photometry at various phases of the beat cycle and find that synthetic light curves derived from a multipolar field structure are broadly consistent with optical photometry. fl ceti is observed to have two very small accretion regions at the foot-points of the white dwarf’s magnetic field. both accretion regions are visible at the same time in the high state and are about 100 degrees apart. mhd modeling using a dipole plus quadrupole field structure yields quite similar accretion regions as those observed in fl ceti. we conclude that accretion flows calculated from mhd modeling of multi-polar magnetic fields produce synthetic light curves consistent with photometry of these magnetic cataclysmic variables. keywords: cataclysmic variables magnetic fields polars optical photometry individual: by cam individual: fl cet. 1 introduction we report on new observations of magnetic cataclysmic variable stars (mcvs). photometric observations of two mcvs: by cam and fl ceti, were obtained at mcdonald observatory using the otto struve 2.1-m telescope. the data consists of 1 or 3 second exposures and no dead time. a broad band filter covering the visual spectrum, bvr, was used in order to obtain the highest possible time resolution. we describe progress towards understanding the structure of accretion flows in mcvs, obtained through the use of full 3d mhd simulations assuming a variety of magnetic field structures. 2 by camelopardalis by cam contains a highly magnetic white dwarf which rotates slightly (about 1 %) faster than its companion (donor star) orbits (mason et al. 1989, 1998; silber et al. 1992, 1997). hence, it is a member of the class of mcvs known as asynchronous polars or by cam stars. the binary period is 201.3 minutes while the white dwarf spin period is 199.3 minutes (mason et al. 1995). a precise orbital ephemeris was derived by schwarz et al. (2005). the vast majority of polars are synchronously locked due to the strong magnetic field of the white dwarf primary. despite the fact that by cam has a magnetic field typical of polars, it currently is in an unlocked state. mason and chanmugam (1992) and pavlenko et al. (2012) provide evidence for the synchronization of the orbit and spin periods in 1600 and 250 years respectively. by cam is well suited for the study of magnetic field structure in mcvs because we are able to take advantage of asynchronous rotation of the white dwarf with respect to the binary. synchronized polars, by definition, only accrete from one azimuthal angle. in by cam, accretion flow occurs at all azimuthal angles as the field structure moves with respect to the orbital plane over the course of a 14 day beat cycle (mason et al. 1995, 1998), see figure 1. other extensive photometric studies of by cam were conducted by andronov et al. (2008) and babina et al. (2010). 71 http://dx.doi.org/10.14311/app.2015.02.0071 p. a. mason et al. figure 1: by cam photometry. top: b-band from 7 february 2013. bottom: r-band from 12 february 2013. cycle to cycle variations are clearly evident. 2.1 by cam: mhd modeling a complex magnetic field model was proposed to explain optical photometry and polarimetry of by cam by mason et al. (1998). full 3d mhd simulations of flow structure in asynchronous polars were carried out using the russian supercomputer facility (zhilkin et al. 2012). 0 0.2 0.4 0.6 0.8 1 0 0.2 0.4 0.6 0.8 1 in te ns ity phase beat phase = 0.3i=30o i=60o i=90o i=120o i=150o figure 2: synthetic light curves derived from a nonaligned dipole plus quadrupole field model at beat phase 0.3 are shown as an example. these light curves resemble some of the observed light curves of by cam, see figure 1. calculations of accretion flow patterns at 10 beat phases were performed. for pure dipole and dipole plus quadrupole field components of various dipole/quadrupole field strength ratios. figures 2 and 3 show synthetic light curves of a by cam model at two beat phases. 0 0.2 0.4 0.6 0.8 1 0 0.2 0.4 0.6 0.8 1 in te ns ity phase beat phase = 0.8i=30o i=60o i=90o i=120o i=150o figure 3: same as figure 2, except for beat phase 0.8. these light curves resemble some of the observed light curves of by cam, see figure 1. figure 4 shows the magnetic induction at the surface of the white dwarf for a misaligned dipole plus quadrupole magnetic field model, currently the best fitting model for by cam. figure 5 shows the location of accretion regions for an aligned dipole plus quadrupole field structure at a particular beat phase. b/ba 0 0.5 1 1.5 2 φ/π 0 0.2 0.4 0.6 0.8 1 θ/ π 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 figure 4: the magnetic induction over the surface of the model white dwarf in by cam. dark areas represent locations of potential accretion. there are three regions, two near the magnetic equator and one at the north magnetic pole. this is a non-aligned dipole plus quadrupole field model. figure 5: an example model of by cam with dipole plus quadrupole components with accretion regions, just below the magnetic equator (see right panel) and the magnetic north pole. 72 photometry and multipolar magnetic field modeling of polars: by camelopardalis and fl ceti 3 fl ceti the original optical observations of this synchronized polar revealed deep eclipses on a period of 87 min (szkody et al. 2002). it has been observed undergoing ∼ 6 mag. eclipses in the high state, with ∼ 1.5 mag. eclipses in the low state. it has been the subject of numerous additional investigations (dubkova et al., 2003; wiehahn et al. 2004, woudt et al., 2004, schmidt et al., 2005, o’donoghue et al. 2006). katysheva and shugarov (2012) provide a concise review of past work and present additional data. we obtained optical photometry of fl ceti, during both high and low states, using the mcdonald observatory otto struve 2.1-m telescope. fast photometry allows us to resolve detailed eclipse structure resulting from the very small accretion regions on the surface of the white dwarf. eclipse’s of the accretion spots have ingress and egress times of 2-3 seconds, indicating a very small structure. polars are known for exhibiting high and low brightness states, corresponding to high and low accretion rates respectively. in some cases, low states may be periods of near zero accretion. broad-band (bvr) photometry was collected in august and september 2011 during which time fl ceti was in the high state with two active accretion regions (see figure 6). low state light curves were obtained on six nights in october 2013. v-band photometry obtained 7 october 2013 is shown in figure 7. the light is dominated by the white dwarf at a relatively constant brightness. the low amplitude variability in the out of eclipse light curve may be due to ellipsoidal variations of the secondary. that is, the visible surface area of the non-spherical donor star changes over the course of the orbital period. during the eclipse the back side of the secondary provides the only contribution to the light during both high and low states. figure 6: broad-band (bvr) photometry obtained during a high accretion state of fl ceti. outside of eclipse the light curve is dominated by cyclotron emission. during the eclipse the back side of the secondary and the disappearance of the accretion stream provide the only contribution to the optical light. in order to compare the high and low states a close up view of eclipses of the white dwarf by the secondary star are shown in figure 8, for the high state (top) and the low state (bottom). the high state eclipse involves two sharp drops during ingress and a stand still in between. the egress is symmetric about mid-eclipse of the two spots. this is interpreted as the eclipse of two small accretion spots on the surface of the white dwarf (o’donoghue, et al. 2006). one spot is eclipsed for a somewhat longer period of time than the other. low state flux is flat during the deep eclipse while the high state light curve shows a gradual decline. an explanation for the gradual fading during eclipse has been given by o’donoghue et al., (2006), that light from the accretion stream is being gradually blocked during the eclipse. figure 7: v-band photometry obtained during a low accretion state of fl ceti. figure 8: eclipses of the white dwarf by the secondary star are shown for the high state (top red points) and low state (bottom blue points). the orbital phase is shown, with phase 0 being the time of mid-eclipse. 3.1 fl ceti: mhd modeling as described in the previous section, zhilkin et al. (2012) performed calculations of mhd accretion flow in an asynchronous polar at 10 beat phases. the same calculations are used to model the field structure synchronous polars. conveniently, fl ceti has a measured magnetic field strength very similar to by cam. a first 73 p. a. mason et al. attempt at an accretion flow model resembling fl ceti is shown in figure 9. in this case, for mhd calculations, a strong quadrupole field component is assumed to be aligned with a dipole component. the donor star is at the right edge of the image and the stream phasing is similar to the pre eclipse dip observed in the light curve of fl ceti. plasma flows onto two small regions located about 100 degrees apart on the surface of the white dwarf. similar features occur for calculations involving non-aligned dipole plus quadrupole magnetic field components. figure 9: model accretion flow in fl ceti. the magnetic field structure is calculated for a centered dipole plus strong quadrupole, where the component axes are aligned. the donor star is off the far right edge on the horizontal axis. the densest part of the flow is shown in brown. accretion stream phasing is consistent with the observed pattern in fl ceti, specifically the pre-eclipse dip and the existence of magnetic poles separated by ∼ 100 degrees. in the model, one spot is at the magnetic north pole while the other is near the intersection of the magnetic equator and the orbital plane. similar flow patterns result for misaligned dipole and quadrupole components as well. 4 conclusions it is clear that pure dipolar fields are inconsistent with observations of both by cam and fl ceti. by modeling accretion flows in these binaries with higher order complex fields we are able to explain the evolution of the light curves of by cam and the location of accretion spots in fl ceti. actual fields may consist of many multipolar components. however, simple assumptions about complex field components yield results that are quite consistent with observations. acknowledgement special thanks go to franco giovannelli for the invitation to present this work in palermo and to the referee, klaus reinsch, for suggestions that improved the manuscript. we thank david buckley for discussions concerning salt data on fl ceti. this work is supported, in part, by nsf/paare grant no. 0958783. ag and db were supported by rfbr grants. references [1] andronov, i. l., et al., 2008, cejph, 6, 385 [2] babina, ju., andreev, m., pavlenko, e., 2010, aipc, 1273, 313 [3] dubkova, d. n.; kudryavtseva, n. a.; hirv, a., 2003, ibvs, 5389, 1 [4] katysheva, n., shugarov, s. 2012, memorie della societa astronomica italiana, 83, 670 [5] mason, p. a., liebert, j., and schmidt, g. d., 1989, apj, 346, 941 [6] mason p. a. and chanmugam g, 1992, aspc, 29, 216 [7] mason, p. a., et al., 1995, aspc, 85, 496 [8] mason, p. a., et al. 1998, mnras, 295, 511 [9] o’donoghue, d., et al., 2006, mnras, 372, 151 doi:10.1111/j.1365-2966.2006.10834.x [10] pavlenko, e., et al, 2013, aspc, 469, 343 [11] schmidt, g. d., et al., 2005, apj, 620, 422 doi:10.1086/426807 [12] schwarz, r., schwope, a. d., staude, a., remillard, r. a., 2005, a & a, 444, 213 [13] silber, a. d., et al., 1992, apj 389, 704 doi:10.1086/171243 [14] silber, a. d., et al., 1997, mnras, 290, 25 doi:10.1093/mnras/290.1.25 [15] szkody, p. et al. 2002, aj, 123, 430 [16] wiehahn, m., potter s. b., warner, b., woudt, p. a., 2004, mnras, 355, 689 doi:10.1111/j.1365-2966.2004.08346.x [17] woudt, p. a., warner, b., pretorius, m. l., 2004, mnras, 351, 1015 doi:10.1111/j.1365-2966.2004.07843.x [18] zhilkin a, et al.: 2012, astron. rep. 52, 4, 318 74 http://dx.doi.org/10.1111/j.1365-2966.2006.10834.x http://dx.doi.org/10.1086/426807 http://dx.doi.org/10.1086/171243 http://dx.doi.org/10.1093/mnras/290.1.25 http://dx.doi.org/10.1111/j.1365-2966.2004.08346.x http://dx.doi.org/10.1111/j.1365-2966.2004.07843.x photometry and multipolar magnetic field modeling of polars: by camelopardalis and fl ceti discussion paula szkody: how do the new light curves of by cam compare to those obtained in the past, by you and silber et al.? paul mason: the new light curves resemble the light curves from 25 years ago. however, it should be noted that recently e. pavlenko and co-workers suggest that by cam is in the process of synchronizing in about 250 years, a lot faster than the 1600 years estimated by mason and chanmugam (1992). 75 introduction by camelopardalis by cam: mhd modeling fl ceti fl ceti: mhd modeling conclusions 205 acta polytechnica ctu proceedings 2(1): 205–211, 2015 205 doi: 10.14311/app.2015.02.0205 theory of nova outbursts and type ia supernovae m. kato1, i. hachisu2 1department of astronomy, keio university, hiyoshi, yokohama 223-8521, japan 2department of earth science and astronomy, college of arts and sciences, the university of tokyo, komaba, meguroku, tokyo 153-8902, japan corresponding author: mariko@educ.cc.keio.ac.jp abstract we briefly review the current theoretical understanding of the light curves of novae. these curves exhibit a homologous nature, dubbed the universal decline law, and when time-normalized, they almost follow a single curve independently of the white dwarf (wd) mass or chemical composition of the envelope. the optical and near-infrared light curves of novae are reproduced mainly by free-free emission from their optically thick winds. we can estimate the wd mass from multiwavelength observations because the optical, uv, and soft x-ray light curves evolve differently and we can easily resolve the degeneracy of the optical light curves. recurrent novae and classical novae are a testbed of type ia supernova scenarios. in the orbital period versus secondary mass diagram, recurrent novae are located in different regions from classical novae and the positions of recurrent novae are consistent with the single degenerate scenario. keywords: novae light curve analysis wd mass uv x-rays type ia supernova. 1 introduction a nova outburst is a thermonuclear runaway event on a white dwarf (wd). after unstable hydrogen shellburning sets in, the envelope greatly expands and the wd becomes very bright and reaches its optical maximum. after the maximum expansion of the photosphere, the wd’s radius shrinks with time. free-free emission of the expanding ejecta dominates the spectrum in the optical and infrared regions. a large part of the hydrogen-rich envelope is blown away in the wind and the photosphere gradually moves inside. figure 1: light curves of various speed classes of novae. from left to right, v1500 cyg, v838 her, v1668 cyg, pw vul, v723 cas, and pu vul (horizontal line and inset), which are taken from kato et al. (2013). figure 1 shows v and visual light curves of six wellobserved galactic classical novae with different speed classes. v1500 cyg is an exceptionally bright (superbright), very fast nova (t2 = 2.9 days and t3 = 3.6 days). here, t2 (t3) is the time in days in which a nova decays by two (three) magnitudes from its maximum. v838 her is a very fast nova, one of the fastest novae except for the superbright novae. it exhibits a normal super-eddington luminosity in the early phase. v1668 cyg is a fast nova with t2 = 12 days and t3 = 25 days. pw vul is a slow nova that exhibits oscillatory behavior in the early phase. v723 cas is a very slow nova that features multiple peaks in the early stage. pu vul is a symbiotic nova with a flat optical peak lasting eight years. its light curve for the first 20 years is plotted in the inset of figure 1. many classical novae exhibit super-eddington luminosity, i.e., the peak luminosity exceeds the eddington luminosity at the photosphere. ledd ≡ 4πcgmwd κ , (1) where κ is the opal opacity. if we assume mwd = 1.0 m� and κ = 0.4, the eddington luminosity is ledd = 1.258× 1038 erg sec−1. because of space limitations, in this paper we concentrate on the light curve analysis of the decay phase of novae and their relation to type ia supernova (sn ia) progenitors. for other important issues readers may re205 http://dx.doi.org/10.14311/app.2015.02.0205 m. kato, i. hachisu fer to a brief summary of super-eddington luminosity and a theoretical explanation of maximum magnitude versus rate of decline of novae (the mmrd relation) by kato (2012), the common path in the color-color diagram by hachisu and kato (2014), and information on instability of the shell flash by kato & hachisu (2012). 2 nova light curves 2.1 the universal decline law decay phases of novae can be followed based on the theory of optically thick winds. in this method, we construct a sequence of steady-state solutions with assumed chemical composition of the hydrogen-rich envelope. we solve the equations of motion, mass continuity, radiative diffusion, and conservation of energy, from the bottom of the hydrogen-rich envelope through the photosphere assuming a steady-state (kato & hachisu 1994). optical and infrared light curves are calculated by assuming free-free emission originating from the optically thin ejecta outside the photosphere. the flux of free-free emission is estimated by fν ∝ ∫ nenidv ∝ ∫ ∞ rph ṁ2 wind v2windr 4 r2dr ∝ ṁ2 wind v2phrph , (2) during the optically thick wind phase, where fν is the flux at frequency ν, ne and ni are the number densities of electrons and ions, respectively, rph is the photospheric radius, ṁwind is the wind mass-loss rate, vph is the photospheric velocity, and ne ∝ ρwind and ni ∝ ρwind. these ṁwind, rph, and vph values are calculated from our optically thick wind solutions. figure 2 shows light curves calculated for various wd masses. as the wind mass-loss rate quickly decreases with time, the flux at a given wavelength also decreases with time. note that the shape of the light curve is independent of the frequency (or wavelength), whereas the absolute magnitude, i.e., proportionality constant of equation (2), depends on it. this wavelength-free light curve shape is one of the characteristic properties of free-free emission (see hachisu & kato 2006). for a more massive wd, its light curve decays more quickly mainly because of its smaller envelope mass, so the evolution time scale is shorter. these light curves follow a common shape, known as the universal decline law. if we plot these light curves on logarithmic time since the outburst, and shift them in the vertical and horizontal directions (i.e., normalize them in the time direction by a factor of fs and also shift the magnitude in the vertical direction by −2.5 log fs), all the light curves essentially converge into a single curve (hachisu & kato 2010) independently of the wd mass and chemical composition, as demonstrated in figure 3. this figure also shows the uv 1455 å narrowband light curve. this narrow band is defined observationally to avoid prominent emission and absorption lines in the nova uv spectra and to represent the continuum flux (cassatella et al. 2002). we see that our model light curves of the uv 1455 å band also converge into a single curve by using the same timescaling factor of fs. in our model light curves, x-ray turnoff times do not converge, because the supersoft xray phase (duration of steady nuclear burning) depends on the hydrogen burning rate, which does not follow the times-scaling law. figure 2: free-free emission model light curves for various wd masses from 0.55 m� to 1.2 m� in steps of 0.05 m� (taken from hachisu & kato 2010). open squares denote observational y magnitudes of v1668 cyg that show good agreement with the 0.95 m� model. this figure also shows the fundamental properties of nova evolution. after the optical maximum, the photospheric temperature rises with time, and the main emitting wavelength region shifts from optical to uv and then to supersoft x-ray. the optically thick winds continue until the photospheric temperature reaches log t (k) > 5.2 (which corresponds to the opacity peak from iron ionization). the wd photosphere emits supersoft x-rays until hydrogen nuclear burning stops. after the hydrogen burning is extinguished, the nova enters a cooling phase and finally the wd becomes dark. figure 4 shows the same light curves as in figure 3 but on a real time scale. in a light curve analysis, we can select the best-fit theoretical model by comparing observed data with our model, but only the optical light curve is not enough. this is because the main body of the optical light curve follows the same decline rate of t−1.75 and we can fit several light curves of different wd masses with the optical data as seen in figure 4. if we have information other than optical, such as the uv 1455 å light curve or x-ray turn-on and turnoff times, 206 theory of nova outbursts and type ia supernovae we can accurately select one model light curve. this is why multiwavelength observations are important for determining the nova parameters. figure 3: free-free emission model light curves for various wd masses from 0.7 m� to 1.3 m� by steps of 0.05 m� (taken from hachisu & kato 2010 ). the time scales are squeezed or stretched by a factor of fs along time to fit each other and the magnitude is shifted by −2.5 log fs. the two straight lines indicate the decline rates of free-free flux, i.e., fν ∝ t−1.75 (dash-dotted line) and fν ∝ t−3 (solid line). observational data of v1974 cyg are superposed. 2.2 chemical composition of ejecta classical novae exhibit heavy element enrichment in their ejecta by a few to several tens of percent by mass (see table 1 in hachisu & kato 2006 for a summary). this enhancement is considered to originate the wd by diffusion of accreted hydrogen into a core during a quiescent phase (e.g., prialnik 1986). as the surface of a wd core is eroded during nova outbursts and blown off in the nova wind, the wd mass decreases after each nova outburst. however, in recurrent novae, the composition of ejecta is very different from those of classical novae and is almost solar. theoretical calculations showed that a certain amount of processed helium is added to the wd after every outburst (prialnik & kovetz 1995). the wd could then experience repeated weak helium shell flashes. the resultant wind is weak, and a large part of the envelope mass remains on the wd (kato & hachisu 2004). therefore, the wd increases its mass in recurrent novae such as u sco and rs oph. note that helium burning produces a substantial amount of ne and mg (shara & prialnik 1994). therefore, even if the wd is made of carbon and oxygen, the hydrogen-rich envelope could be contaminated with neon. mason (2011) claimed that the high ne/o line ratios observed in u sco indicate that u sco harbors an onemg wd. mikolajewska criticized mason’s results because mason neglected the effects of collisions on the line formation. with these effects included, the u sco abundance is consistent with the solar abundance ratio. mason (2013) accepted mikolajewska’s criticism and revised her results on the u sco wd, which previously had been totally erroneous because of her misinterpretation and miscalculation. here, we note that (1) high ne/o line ratios could be an indication of a massincreasing wd and (2) strong ne lines are detected even in the slow nova v723 cas (iijima 2006), which harbors a low-mass wd (0.5–0.6 m�). figure 4: same as figure 3 but on a real time scale and using absolute v magnitude (taken from hachisu & kato 2010). model uv and supersoft x-ray light curves are plotted only for three wd masses (1.1 m�, 1.05 m�, and 1.0 m�) to improve readability. 2.3 wd masses estimated by light curve fittings figures 3 and 4 also show light curves of the classical nova v1974 cyg. the optical light curve decays as t−1.75 from the optical peak at t = 2.67 days (point b). there are two different sets of v -magnitudes obtained by using different v filters that have different degrees of emission line contamination. as the nova enters the nebular phase, strong emission lines such as [o iii] contribute to the v band flux and the v magnitude becomes much brighter than the continuum flux. (the theoretical free-free flux mimics the continuum flux.) after the optically-thick winds stops, the magnitude decays as t−3, which represents free-expansion of the ejecta. because the optical, uv, and supersoft x-ray light curves have different dependencies on the wd mass, we can choose a best-fit model that reproduces these three light curves simultaneously as shown in figure 4. in this way, we have estimated the wd mass in many classical novae, recurrent novae, and a symbiotic nova. such 207 m. kato, i. hachisu examples are shown in figure 1 by the solid lines that represent theoretical light curve models. these wd masses are mwd = 1.20 m� for v1500 cyg, 1.35 m� for v838 her, 0.95 m� for v1668 cyg, and 0.6 m� for v723 cas. note that some classical novae have optical light curves that are similar to those of recurrent novae. for example, v838 her and u sco have very similar optical and uv 1455 å light curves. also the classical nova v2491 cyg exhibits a similar optical decline to that of rs oph. v838 her and v2491 cyg are classical novae and their wd masses are decreasing because of heavy element enhancement in their ejecta, whereas u sco and rs oph have mass-increasing wds. this indicates that classical and recurrent novae share different histories of binary evolution even if they exhibit very similar light curves. 2.4 very slow novae with a flat optical peak pu vul is a symbiotic nova that exhibits a long-lasting flat optical peak followed by a slow decline, as shown in figure 1. the evolution of this nova can be followed by using a quasi-evolution model of an outburst on a ∼ 0.6 m� wd (kato et al. 2011). in less massive wds (≤ 0.6 m�, which depends slightly on the chemical composition) the optically thick winds are not accelerated and thus the nova evolution is extremely slow. in contrast, in the majority of classical novae (that occur on a wd of mwd > 0.7 m�) the strong optically thick winds are accelerated and carry most of the envelope mass in a short time. as a result, their evolutions are fast and exhibit a sharp optical maximum. the very slow novae v723 cas, hr del, and v5558 sgr exhibit a flat pre-maximum phase with no indication of a strong wind-mass loss, followed by a smooth decline with massive winds after some oscillatory phase of the optical light curves. this sequence can be understood as a transition from initial quasi-static evolution to optically thick wind evolution. the oscillatory behaviors in the light curves may correspond to a relaxation process associated with the transition from static to wind solutions (kato and hachisu 2011). 3 type ia supernova scenarios type ia supernovae are characterized in principle by spectra without hydrogen lines but with strong si ii at maximum light. it is commonly agreed that the exploding star is a mass-accreting carbon-oxygen (c+o) wd. however, whether the wd accretes hand he-rich matter from its binary companion (the so-called single degenerate (sd) scenario) or whether two c+o wds merge (the so-called double degenerate (dd) scenario) has not yet been clarified. in the dd scenario, proposed by webbink (1984) figure 5: initial and final binary parameter ranges for the sd scenario (taken from hachisu et al. 2012). m2 is the companion mass and porb is the orbital period. an initial binary system inside the region encircled by a solid line (labeled initial) evolves to explode as a type ia supernova when it reaches within the region labeled final. the recurrent novae u sco and t crb are located inside the final region, whereas the classical novae v2491 cyg, v838 her, and gk per lie outside. most classical novae and cataclysmic variables are located much below the left-side final region. 208 theory of nova outbursts and type ia supernovae and iben & tutukov (1984), intermediate-mass (3– 8 m�) binaries undergo the first and second common envelope phases and finally become a double wd system. if the orbital period is very short (i.e., if they merge within a hubble time), and the total mass exceeds the chandrasekhar mass, it could explode as a sn ia. after introduction of this traditional dd scenario, many modified binary evolution scenarios have been proposed, but the evolutionary path is essentially governed by the efficiency parameter of the common envelope evolution (e.g. webbink 2008), which is not well constrained yet. the original idea of the sd scenario was proposed by whelan & iben (1973), but this idea was soon abandoned by one of its originators (iben) in 1984 because of its too small contribution to sne ia. these studies are based on the old opacity, which was revised at the beginning of the 1990s. the new opacities have a large peak at log t (k) ∼ 5.2, which brought drastic change in stellar structures. the presence of strong optically thick winds has changed nova theory, and we now can well reproduce observational light curves. hachisu et al. (1996) adopted these optically-thick winds as a new elementary process in binary evolution, which was dubbed the accretion wind evolution. the wd accretes matter from the accretion disk and, at the same time, blows optically thick winds. these winds carry mass and angular momentum, so mass transfer in the binary can be stabilized, and, in most cases, the so-called second common envelope evolution does not occur. wds can grow in mass to the chandrasekhar mass and explode as a sn ia. therefore, the essential difference between the dd and sd scenarios resides in the assumption of the second common envelope evolution or accretion wind evolution. the physical process of the latter is well studied in nova outbursts, whereas the former process is not established yet (see e.g., webbink 2008, kato and hachisu 2012 for more details). 3.1 wd+rg channel and wd+ms/sg channel in the sd scenario, there are two known channels to sne ia, i.e., the wd+ms (including sub-giant (sg) companion) channel and the wd+rg channel. the wd+rg channel was proposed by hachisu et al. (1996), who called it the symbiotic channel. in this symbiotic channel we start from a very wide binary consisting of a pair of ms stars. the primary evolves to an asymptotic giant branch (agb) star. the binaries undergo a common envelope-like evolution during the superwind phase of the primary agb star. the orbital period of the binary shrinks to 30 – 800 days, with the binary now consisting of a co wd and a ms star. then the secondary ms star evolves to a rg with a helium core and fills its roche lobe. as the mass-transfer rate exceeds the critical rate, the binary undergoes the accretion wind evolution phase. the mass-transfer rate gradually decreases and the optically thick winds stop. then all the accreted matter is burned on the primary co wd. the binary enters a persistent supersoft x-ray source (sss) phase. the mass-transfer rate further decreases to below the minimum rate of steady hydrogen burning (ṁst) and the binary enters a recurrent nova phase. depending on the binary parameters, the wd mass reaches mwd = 1.38 m� during one of the three phases (wind, sss, and recurrent nova) and explodes as a type ia supernova. the ms/sg channel was proposed by li & van den heuvel (1997) and further investigated by hachisu et al. (1999b) and others. in this channel, a pair of ms stars evolves to a binary consisting of a helium star plus a ms star after the first common envelope evolution. the primary helium star further evolves to a helium rg with a co core and fills its roche lobe followed by a stable mass-transfer from the primary helium rg to the secondary ms star. the secondary ms star increases its mass and is contaminated by the primary’s nuclear burning products. the primary becomes a co wd after all the helium envelope is transferred to the secondary ms star. then, the secondary evolves to fill its roche lobe and mass transfer begins from the secondary ms (or subgiant) star to the primary co wd. then, the system enters an accretion wind evolution phase. after that, the binary becomes a persistent sss and evolves finally to a recurrent nova. during these three periods, the primary wd increases its mass to the chandrasekhar mass. depending on the binary parameters, the wd will explode as a sn ia during one of the three phases (wind, sss, and recurrent nova). some binaries are identified by the last phase of each channel. in the symbiotic channel, the smc symbiotic x-ray source smc3 corresponds to the sss phase and symbiotic recurrent novae such as rs oph and t crb correspond to the last phase. in the ms/sg channel, v sge and rx j0513.9−6951 correspond to the accretion wind evolution phase. steady-burning supersoft sources such as cal 87 and cal 83 correspond to the sss phase. u sco and v394 cra are recurrent novae in the ms/sg channel. 3.2 evolutionary status of novae and recurrent novae figure 5 shows the orbital period versus secondary mass diagram for sn ia progenitor binaries for both the wd+ms/sg channel and the wd+rg channel. the upper region labeled “initial” for each channel indicates that binaries will evolve downward with the secondary mass decreasing and the wd mass increasing and even209 m. kato, i. hachisu tually explode as a sn ia in the “final” region. the present positions of u sco and t crb in this diagram are very consistent with the final stage. from light curve analysis, the wd mass is determined as massive as 1.37 m� for u sco and 1.35 m� for rs oph and t crb. these values are also consistent with this final stage toward a type ia sn. in contrast, classical novae v838 her and v2491 cyg are located much below the final region, indicating that neither is on the way toward a sn ia explosion in the wd+ms/sg channel nor in the wd+rg channel, even though they harbor a massive wd near the chandrasekhar mass. in this way, the present status of these recurrent novae and classical novae can be understood from the difference in binary evolution that is consistent with the sd scenario. however, in the original dd scenario, there are no known paths to recurrent novae, especially for the rs oph type systems. thus, studies of cvs are important in understanding the nature of progenitors of sne ia. references [1] cassatella, a., altamore, a., gonzález-riestra, r.: 2002, a&a 384, 1023 [2] della valle, m.: 1991, a&a 252, 9 [3] hachisu, i., kato, m.: 2001, apj 558, 323 doi:10.1086/321601 [4] hachisu, i., kato, m.: 2006, apjs 167, 59 doi:10.1086/508063 [5] hachisu, i., kato, m.: 2010, apj 709, 680 doi:10.1088/0004-637x/709/2/680 [6] hachisu, i., kato, m.: 2014, apj 785, 97 doi:10.1088/0004-637x/785/2/97 [7] hachisu, i., kato, m., nomoto, k.: 1996, apj 470, l97 [8] hachisu, i., kato, m., nomoto, k.: 1999a, apj 522, 487 [9] hachisu, i., kato, m., nomoto, k.: 2008, apjl 683, 127 doi:10.1086/591646 [10] hachisu, i., kato, m., nomoto, k.: 2012, apjl 756, l4 doi:10.1088/2041-8205/756/1/l4 [11] hachisu, i., kato, m., nomoto, k., umeda, h.: 1999b, apj 519, 314 doi:10.1086/307370 [12] iben, i., jr., tutukov, a. v.: 1996, apjs 105, 145 [13] iijima, t.: 2006, a&a 451, 563 [14] kato, m., hachisu, i.: 1994, apj 437, 802 [15] kato, m., hachisu, i.: 2004, apj 613, l129 doi:10.1086/425249 [16] kato, m., hachisu, i.: 2009, apj 699, 1293 doi:10.1088/0004-637x/699/2/1293 [17] kato, m., hachisu, i.: 2011, apj 743, 157 [18] kato, m, hachisu, i.: 2012, basi 40, 393. [19] kato, m., hachisu, i., cassatella, a., gonzález-riestra r.: 2011, apj 727, 72 [20] kato, m, hachisu, i., henze, m.: 2013, apj, 779, 19 [21] mason, e.: 2011, a&a 352, l11 (2013, a&a 556, c2: corrigendum) [22] prialnik, d.: 1986, apj 310, 222 [23] prialnik, d., kovetz, a.: 1995, apj 445, 789 [24] shara, m., m., prialnik, d.: 1994, aj 107, 1542 [25] webbink, r. f.: 1984, apj 277, 355 [26] whelan, j., iben, i., jr.: 1973, apj 186, 1007 [27] webbink, r. f.: 2008, assl 352, 233 discussion jan-uwe ness: for a sn ia, the wd has to be a co wd, thus, the progenitor main-sequence was low mass star. are there really enough co wds with highenough initial mass? furthermore, the companion will initially be of rather low mass in order not to evolve faster than the primary star. can such a companion feed a lot of mass onto the co wd until the sn ia explosion? thus a sd progenitor will have a very low mass companion just before sn ia explosion. mariko kato: a co wd originates from less massive stars (m at zams < 8 m�). the companion mass is smaller than this value. we have calculated the binary evolution including mass exchange, mass-loss from the binary, wind mass-loss from the wd, and so on. as you see in figure 5, the mass permitted for the secondary is rather limited. especially in the wd+rg channel, the initial companion mass is small, i.e., 1 – 3 m� when the secondary had just filled its roche lobe, and 0.5 – 1.0 m� just before the sn ia explosion. for the primary wd, the initial 0.9 – 1.07 m� wd becomes a type ia sn. we calculated the expected number of sne ia and showed that the two channels have enough sne ia. their delay time distribution is also consistent with observation (hachisu et al. 2008). 210 http://dx.doi.org/10.1086/321601 http://dx.doi.org/10.1086/508063 http://dx.doi.org/10.1088/0004-637x/709/2/680 http://dx.doi.org/10.1088/0004-637x/785/2/97 http://dx.doi.org/10.1086/591646 http://dx.doi.org/10.1088/2041-8205/756/1/l4 http://dx.doi.org/10.1086/307370 http://dx.doi.org/10.1086/425249 http://dx.doi.org/10.1088/0004-637x/699/2/1293 theory of nova outbursts and type ia supernovae giora shaviv: we do a calculation of one flash and find that 10−7m� is accreted. here we infer that this system will be a sn ia. but we saw only how a 10−7m� are accreted and infer that the same is true with 0.1 m�. this is an extrapolation over 6 orders of magnitude!! hence can easily be wrong. mariko kato: in our estimate based on the light curve analysis, the accumulation efficiency in typical recurrent novae (u sco and rs oph) is about 50%. the accreted matter in one nova outburst is 1.2× 10−6m� for u sco, and 1−2×10−6m� for rs oph. as the wd is estimated to be 1.35 m� for rs oph and 1.37 m� for u sco, we can expect a continuous mass increase of the wds, not just from an extrapolation from 1.3 m�. 211 introduction nova light curves the universal decline law chemical composition of ejecta wd masses estimated by light curve fittings very slow novae with a flat optical peak type ia supernova scenarios wd+rg channel and wd+ms/sg channel evolutionary status of novae and recurrent novae acta polytechnica ctu proceedings doi:10.14311/app.2017.7.0048 acta polytechnica ctu proceedings 7:48–52, 2017 © czech technical university in prague, 2017 available online at http://ojs.cvut.cz/ojs/index.php/app parameters affecting the structural analysis of a tunnel structure exposed to fire omid pourana,∗, reinhard harteb, carsten peterc a structural engineering, institute for statics and dynamics of structures, bergische universität wuppertal, wuppertal, germany b structural engineering, institute for statics and dynamics of structures, bergische universität wuppertal, wuppertal, germany. kraetzig & partner engineering consultants, bochum, germany c imm maidl & maidl engineering consultants, bochum, germany ∗ corresponding author: pouran@uni-wuppertal.de abstract. behaviour of cut-and-cover tunnels exposed to fire should be analysed by using a realistic structural model that takes account of mechanical and thermal effects on the structure. this has been performed with the aid of finite element (fe) software package called sofistik in parallel, for two types of elements as a scope of research project financed by the german bundesanstalt für straßenwesen bast. since the stiffness of the structure at elevated temperatures is highly affected, a realistic model of structural behaviour of the tunnel could be only achieved by considering the nonlinear analysis of the structure. this has been performed for a 2–cell cut and cover tunnel by taking account of simultaneous reduction of stiffness and strength and the time-dependent increasing indirect effects due to axial constraints and temperature gradients induced by elevated temperatures. the thermal analyses have been performed and the effects were implemented into the structural model by the multi-layered strain model. the stress–strain model proposed by en 1992-1-2 is implemented for the elevated temperature. since there was sufficient amount of polypropylene fibres in the concrete mixtures, modelling of spalling was excluded from the analysis. the critical corresponding stresses and material behaviour are compared and interpreted at different time stages. the main parameters affecting the accuracy and convergence of the results of structural analysis for the used model are identified: defining a realistic fire action, using concrete material model fulfilling the requirements of fire situation in tunnels, defining appropriate time intervals for load implementations. these parameters along with other parameters, which influence the results to a lesser degree, are identified and investigated in this paper. keywords: cut-and-cover, tunnel, fire, thermal effects, structural analysis, accidental situation. 1. introduction in the event of fire, structural behaviour of road tunnels are affected by imposed thermal effects to high extent and this might, consequently, result in failure of the structure. this risk can be minimized by means of appropriate fire design, which provides sufficient passive fire protection for the structure. using an advanced method proposed by en 19921-2 [1] to analyse and design such structures, the following three main areas should be taken into account: • determination of fire action for the specified project, • using realistic material model for the concrete exposed to high temperatures, • defining a thermo-mechanical model which takes account of indirect-thermally induced effects corresponding to the specified fire action and the strength reductions due to temperature rise simultaneously. in germany, the nominal fire exposure curve adopted in for tunnel structures is called ztv–ing fire exposure curve [2]. this can be assumed as fire action applied to the compartmented tunnel structure for accidental fire design situation. further, regarding the material behaviour of reinforced concrete structures, spalling of concrete is an important phenomenon. this implies reduction of cross section thickness and, as a consequence, decrease of concrete cover, which may expose the reinforcement directly to the fire. to avoid spalling, in germany, polypropylene fibres (pp fibres) are usually used to improve the concrete for both new road and railway tunnels. the addition of pp fibres, verifiably, reduces the inner pore pressure in the concrete and by this decreases the risk and impact of concrete spalling [3], as various small and large–scale fire tests have shown. the stress– strain curve proposed by en 1992-1-2 [1] for concrete exposed to elevated temperatures is for the concrete without pp fibres and the heating rate between 2 to 50 k · min−1 which occurs at the fire growth stage for building structures. further, the residual strength at cooling phase should be accordingly determined for each specified project condition. therefore, number of tests have been carried out at bergische universität wuppertal to investigate the effects of polypropylene 48 http://dx.doi.org/10.14311/app.2017.7.0048 http://ojs.cvut.cz/ojs/index.php/app vol. 7/2017 parameters affecting structural analysis of tunnel exposed to fire figure 1. nominal fire exposure curve vs ztv-ing fire exposure curve [4]. fibres on material behaviour of concrete at high temperatures. for verification of ultimate limit states (uls) criteria, structural calculations are performed for fire as an accidental design situation. in this case, the indirect thermal effects result from forces due to restrained thermal elongation or rotation dependent on the temperature distribution in the concrete cross section. the superposition of thermal effects in different time steps with the most unfavourable effects from standard load combinations is performed as so called thermo–mechanical analysis. this analysis is based on an iterative nonlinear calculation to determine the internal forces by considering the simultaneous decrease of stiffness of each cross section exposed to elevated temperatures, and considering the indirect thermally– induced effects. the conditions of equilibrium and compatibility are met by taking into account the maximum permissible temperature–dependent strains for reinforcement and concrete given in en 1992-1-2 [1]. to be able to analyse a 2–cell cut-and-cover tunnel exposed to fire, an advanced analysis according to [1] has been performed by taking into account a model with a simplified boundary condition and nominal ztv-ing fire exposure curve [2]. 2. ztv-ing fire exposure curve for tunnel structures in germany the most common fire exposure curve used for building fire structural design is the iso fire exposure curve according to en 1991-1-2 [1] whereas for road tunnels in germany the ztv-ing curve according to [2] has to be applied. the tunnel shall be designed for the fire situation by considering the high possibility of several vehicles being involved in fire ignition and the fire rate of 100 mw [5]. thus, the increase rate of temperature within the first five minutes from the start of ignition (fig. 1) is very high. further, for the long period of 25 minutes, temperature values remain constant, 1200 ◦c, which is completely different from the nominal fire exposure curve having a continuous trend where the ambient temperature reaches 1006 ◦c, gradually, after 90 minutes. 3. effects of polypropylen fibres on concrete behaviour at elevated temperatures the material properties according to en 1992-1-2 [1] are based on the thermo–mechanical tests for the concrete without pp fibres and they correspond to the defined heating rate between 2 to 50 k · min−1. the latter is to be used for the structural calculations for tunnel structures having pp fibres in concrete’s admixtures only conditionally due to the following reasons: • the german fire exposure curve for tunnel structures includes a cooling phase, which means it comprises a growth, a full and a cooling phase. considering the latter, the changes in the strength of concrete in the cooling phase and the residual deformations are not considered • the addition of pp fibres has influence on the thermal and thermomechanical behaviour of concrete through the experimental investigation at bergische universität wuppertal, influences of specific boundary conditions on the thermomechanical properties were investigated. these were conducted with steady and transient tests according to the recommendation of rilem technical committee [6] on the test specimen with the diameter of 60 mm and the height of 180 mm. for the samples, the concrete strength class of c30/37 with pp fibres and without pp fibres were used. a volume of 2 kg · m−3 of pp fibres has been added to the concrete mixtures to prevent spalling. further, tests were carried out with drying boundary condition and moisture content of 3 % at the beginning of test. to perform the compressive test a static compression and tension machine with a maximum compressive force of 250 kn has been used. heating has been performed via a tube furnace with a maximum temperature of 1200 ◦c. two different deformation measuring devices were installed, one on the specimen and the other one on the test equipment. regarding the material behaviour, the total strain suggested in en 1992-1-2 is according to [7] and has the following components: εtot = εth(t ) + εσ(σ, σ, t ) + εcr(σ, t, t) + εtr(σ, t, t) (1) where εth, thermal strain including shrinkage, εσ, instantaneous stress related strain, εcr, creep strain or time dependent strain and εtr, transient strain, all resulted from transient and steady tests. it should be noted that en 1992-1-2, however, proposes an implicit model. to be able to model the behaviour of concrete by an explicit strain model, the transient creep component should be defined. transient creep of concrete is the creep that occurs during first time heating at a constant rate under load. 49 o. pouran, r. harte, c. peter acta polytechnica ctu proceedings figure 2. comparison of reduction of compressive strength from the tests results with pp fibres with en 1992-1-2 results for the specimen without pp fibres [9]. further for the drying boundary condition [8]: εt,0,dtr,cr = εt,σ,dtr,tot − εt,0,dtr,th − εt,0,dtr,sh − εt,σ,dco,el (2) where εt,0,dtr,cr , transient creep for drying concrete during heating, εt,σ,dtr,tot, total measured transient strain, εt,0,dtr,th , thermal strain (free thermal strain), εt,0,dtr,sh , drying shrinkage strain and εt,σ,dco,el , elastic strain. for drying concrete heated without load (i.e. σ = 0) εt,0,dtr,th + εt,0,dtr,sh = εt,0,dtr,tot (3) therefore: εt,0,dtr,cr = εt,σ,dtr,tot − εt,0,dtr,tot − εt,σ,dco,el (4) two main influences, rising from addition of pp fibres to the concrete mixture, are on the compressive strength of concrete evaluated from the steady state tests unstressed during the heating and on total transient strain, εt,σ,dtr,tot, for specimen under different stress levels, α. the latter corresponds to the ratio of constant value of stress imposed during the heating phase to the compressive strength of concrete specimen at the ambient temperature. results of steady tests as shown in figure 2 from tests at 20, 200, 400, 600 and 750 ◦c compared to the test results from en 19921-2 curves represents a good agreement between the compressive strength reduction of concrete specimen with pp fibres with a compressive strength of 63 mpa. further, there is a considerable influence of loading on the reduction of compressive strength. figure 3 shows the values of thermal strains for concrete specimen with pp fibres. the results show that the addition of pp fibres has an influence on the thermal strain course. thermal strains for pp fibres concrete in the range of 200 ◦c to 300 ◦c shows a small amount of deviation compared to the values of en 1992-1-2. further, the maximum value of thermal strains for pp fibres concretes is lower compared to the en 1992-1-2 curve. in range of 200 to 300 ◦c the drying shrinkage and thermal elongation components figure 3. total strain vs. temperature for specimen with pp fibres [9]. are of main role and due to this there is not large deviation in this range. the lower value of maximum thermal strain for pp fibres concrete is due to a smaller crack divisions at pp fibres concrete and composition of less internal stress. 4. thermo-mechanical analysis in order to find the best approximation of heat transfer through the concrete elements, thermal analyses of five concrete samples with different thicknesses of 80, 90, 100, 110, 120 and 130 cm have been performed for 90 minutes. time intervals of 5 minutes, were chosen to read the absolute temperatures after each step. the absolute temperature values corresponding to each time interval are evaluated at different layers across the thickness of concrete. 1d heat transfer is considered by assuming concrete as a homogeneous and isotropic material. fourier’s law of heat conduction is used for the energy transfer in temperature gradients directions with the upper limit of thermal conductivity of concrete, λc = 1.951 w · k−1 · m−1, for the normal concrete corresponding to 10 ◦c according to en 1992-1-2. further, absolute temperature values as a function of time according to ztv-ing fire exposure curve were applied on the exposed edges. the newton’s law of convection with the heat convection coefficients of correspondingly α = 25 and 50 wm2k are used. the convective coefficient of heat transfer is, however, an experimental value and it is not a material constant. however, the comparison of results by the authors shows that using each coefficient influence the temperature fields in the fire growth phase for the fire exposed boundary up to 10 minutes by 5 %. therefore, the effects are negligible in this range. to find the effect of emissivity ε [10], studied two values of emissivity of 0.7 and 0.8 on the absolute temperatures within a concrete element and concluded that for the depth of 10 cm from the exposed surface there was a temperature difference of 10 ◦c was observed and for the deeper layers there is no influence on the absolute values of temperature. thus, stefan–boltzmann law is assumed with the emissivity grade of 0.8. figure 4 shows how temperature values vary as a function of time on 6 layers across the half–thickness of an 80 cm 50 vol. 7/2017 parameters affecting structural analysis of tunnel exposed to fire figure 4. temperature curves for an 80-cm-thick element at different nodes [11]. thick concrete element till 90 minutes. it is to be noted that temperatures on the boundary, corresponding to node 51, tend to increase with a slower rate compared to the temperatures according to ztv-ing curve up to 30 minutes. having thermal analysis performed for different concrete elements, the thermal effects are implemented to the mechanical model. this is performed by combining the multi-layered strain model [12], taking account of strain induced by thermal effects. the latter takes account of the reduced material strength corresponding to thermal and loading effects at each previous time step as initial governing state for the subsequent step of analysis. based on 10-minute time intervals and nonlinear analysis, the stresses according to each time step plotted and the ultimate limit strength verification of the system for the whole duration of 90 minutes is carried out. the critical points at which the plastic hinges might be developed are identified and possibility of failure by developing further plastic hinges. by reaching 30 minutes, as shown in figure 6, the maximum relative bending moment has reached a value of −2514 knm, approximately 73 % greater than a moment imposed just after the ignition of fire (fig. 5). this is due to the simultaneous increase of tensile stresses on the upper part of beam due to thermal induced bending moments and reduced cross sectional properties. the following parameters can affect the result of nonlinear analysis: • time intervals chosen for thermal analysis (minor influence for intervals of 1 to 10 minutes) • time intervals for load implementations (causes unavoidable inaccuracies for time intervals being larger than 2 minutes) • residual force tolerance at nodes (influences the convergence of fe solution and accuracy of effects of actions) mesh generation (influences both convergence and accuracy of results). 5. conclusions to be able to analyse a 2-cell cut-and-cover tunnel structure with a sufficient amount of pp fibres in the concrete mixtures, three main areas with the corresponding parameters, which can influence the results, figure 5. bending moment diagram for t = 0 [4]. figure 6. bending moment diagram for t = 30 [4]. have been investigated. these main areas were defining a realistic fire action, using an appropriate material model and utilizing a thermo-mechanical model that can represent the nonlinear structural behaviour of the structure. the most important parameters according to each area were highlighted and discussed in this paper. acknowledgements the paper has presented some global results from the research project "verification the fire design of cut-andcover tunnel via nonlinear analysis", financed by bast, and the results of test series carried out at the bergische universität wuppertal. the support by bast is gratefully acknowledged. references [1] en 1992-1-2: design of concrete structures part 1-2: general rules structural fire design 2004. [2] b. f. s. bast. zusätzliche technische vertragsbedingungen und richtlinien für ingenieurbauten (ztv-ing). chap. 3, 5. 2013. [3] k. pistol, f. weise, b. meng. polypropylen-fasern in hochleisungbetonen. betonund stahlbetonbau 107(7):476–483, 2012. doi:10.1002/best.201200024. [4] o. pouran, r. harte, c. peter. nonlinear structural analysis of cut-and-cover tunnels exposed to fire. applications of structural fire engineering asfe’15 2015. doi:10.14311/asfe.2015.005). [5] a. schlüter. passive fire protection for tunnels: guidelines, parameters, reality and suitable measures. tunnel pp. 22–23, 2014. [6] recommendation of rilem tc-htc: mechanical concrete properties at high temperatures-modelling and applications. materials and structures pp. 841–583, 2007. doi:10.1617/s11527-007-9285-2. 51 http://dx.doi.org/10.1002/best.201200024 http://dx.doi.org/10.14311/asfe.2015.005) http://dx.doi.org/10.1617/s11527-007-9285-2 o. pouran, r. harte, c. peter acta polytechnica ctu proceedings [7] y. anderberg. mechanical behaviour at fire of concrete and hyperstatic concrete structures. lund institute of technology, lund, 1976. [8] rilem tc 129-mht: test methods for mechanical properties of concrete at high temperatures recommendation: part 7: transient creep for service and accident conditions. materials and structures 31:290–295, 1998. [9] c. peter. tragverhalten von verkehrstunneln im brandfall mit einer innenschale aus pp-faserbeton. bergische universitaet wuppertal, wuppertal, 2016. [10] c. peter, j. knief, j. schreyer, a. piazolla. rechnericher nachweis des baulichen brandschutzes für tunnel in offener bauweise, schlussbericht zum fe projekt 15.0502/2010erb im auftrag des bmvbs/ der bast 2013. [11] o. pouran, r. harte. prüfgutachten hh 02/14-1 zum forschungsprogramm straßenwesen fe 15.0582/2013/frb erprobung des rechnerischen brandschutznachweises für tunnel in offener bauweise 2014. [12] r. harte, w. b. krätzig, s. y. noh, y. s. petryana. on progressive damage phenomena of structures. computational mechanics 25:404–412, 2000. doi:10.1007/s004660050487. 52 http://dx.doi.org/10.1007/s004660050487 acta polytechnica ctu proceedings 7:48–52, 2017 1 introduction 2 ztv-ing fire exposure curve for tunnel structures in germany 3 effects of polypropylen fibres on concrete behaviour at elevated temperatures 4 thermo-mechanical analysis 5 conclusions acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2017.11.0080 acta polytechnica ctu proceedings 11:80–85, 2017 © czech technical university in prague, 2017 available online at http://ojs.cvut.cz/ojs/index.php/app cant excess and cant deficiency evaluation in relation to train speed jan valehrach∗, tomáš říha, erik dušek brno university of technology, faculty of civil engineering, institute of railway structures and constructions, veveří 331/95, 60200 brno, czech republic ∗ corresponding author: valehrach.j@fce.vutbr.cz abstract. the paper deals with the evaluation of measured speeds of the vehicles on the chosen sections of the railway network in the czech republic. in connection with the phased implementation of the avv system (automatic train operation) and the european etcs system, the dispersion of the train speeds will be reduced. measured data were used to assess the weighted average of the cant excess or cant deficiency in relation to the rail defects. within the framework of the research, the speeds were statistically evaluated covering more aspects like types and sorts of the trains, their operators, daytime and more. keywords: speed/velocity, track, train, cant excess, cant deficiency. 1. introduction in the present time speeds of trains passing through the certain place vary. the variability of the speeds depends on more factors, such as train composition and category, actual traffic situation, daytime and the location. in connection with the implementation of new systems for train controlling and commanding the speed profile of the trains will likely become more efficient. by author’s opinion, this will eventually lead to the consolidation of speeds into a narrow range, or eventually into more ranges based on above mentioned factors. the knowledge of the speeds is necessary for a number of fields in railway industry. some of the applications, where the knowledge of speed is critical, are the studies [1–3]. this article focuses on the influence of speed on development of defects on superstructure by studying speed ranges in certain places of the railway infrastructure. as a starting point for the study an expected relation between the appearance of the long-pitch corrugation in curves of small radii and the speed range of the passing vehicles was considered. authors believe that the speed range in certain places of the infrastructure can affect the functionality of the superstructure and under specific circumstances the inappropriate speed range could result in the development of the defects of the rails. by minimizing inconvenient combinations of track parameters and the speed ranges the positive effect will likely appear, which in some extents gives us the potentiality for the optimization. one of the possible parameters, influencing the appearance of the long pitch corrugation in curves of small radii is considered to be cant excess or cant deficiency. in standard conditions, where negative effects on infrastructure do not take place, both values are designed with the design of the track itself. in such case, the designer uses some initial expectations of future traffic composition to calculate cant values. then uses (still only as an expectation) the track speed and calculates the values of cant excess or cant deficiency. nevertheless, this method could not take in account the real speeds of vehicles, which may be far different from the expectations. this lack of feedback could result in some negative effects on infrastructure, and one of them is believed to be the long pitch corrugation defect. this work focuses on evaluation of the cant excess or cant deficiency values obtained through the speed measurements while the relation to long pitch corrugation is studied. 2. methods of measurement and evaluation 2.1. location selection the appearance of the long pitch corrugation was the basis for the selection of the locations for measurements. extensive measurements took place in first track of the line from brno to česká třebová between stations adamov and brno-maloměřice. four locations were selected as follows: • line no. 326 (brno – česká třebová) near babice nad svitavou, km 169.0, in approximately 200 metres the first turnout of the marshalling yard brnomaloměřice follows; • line no. 326 (brno – česká třebová) near babice nad svitavou, km 167.0, the position of the automatic block signal; • line no. 326 (brno – česká třebová) near former branching station hady, km 161.7; • line no. 326 (brno – česká třebová) near former branching station hady, km 162.8, currently the babice stop. 80 http://dx.doi.org/10.14311/app.2017.11.0080 http://ojs.cvut.cz/ojs/index.php/app vol. 11/2017 cant excess and cant deficiency evaluation figure 1. the locations positioning of speed recording device (red points) and automatic image recorder (blue points). in all four locations the significant long pitch corrugation defect occurs. 2.2. measurement method all the measurements were continual and lasted several days. measurements were conducted in pairs, since there are two recording devices at disposal. the pair includes the measurements in km 169.0 and in km 167.0, the second pair consists of measurements in km 161.7 and in km 162.8. by pairing it is possible to get the speed of one particular train in two locations. following data were recorded for each passing train: • speed; • image record; • date and time of the recorded event. as a measuring instrument for recording speeds and directions of the trains were used drs devices (frequency modulation radar units). the main advantage of the device is the possibility of the quick on site installation and measuring without any external energy supply due to device batteries. on the other hand, the device tends to record more than one speed of the train, partially due to the response time and partially due to the length of the trains, which exceeds the length of the vehicles for which was the device initially built1. thus, longer trains inflict more measurements and more records. such sets of records then have to be sorted out by hand, which is very challenging work. the aim is to choose only the speed record which corresponds with the speed of a head of a train. mostly, the first record is chosen, but in some cases careful consideration has to take place. 2.3. evaluation following information about the particular train is then searched out based on the image record, train traffic diagram, timetable, train compositions list and others: • category (freight train, express, passenger train and more – see below); • train number; • composition of the train or train set, types of couches; • train mass; • delay. despite of a great amount of endeavour some trains could not be identified satisfactory. also some measurements had to be edited out, mainly the ones anticipated faulty. at the end, in every of all four locations at least 200 trains were evaluated. 2.4. calculation method a weighted average of all speed measurements in a location was calculated, when the mass of each train was taken into consideration [4]. the masses of the trains for the purposes of the weighted averages have been obtained by two ways. the weight of passenger trains was taken from the passenger carriages charts and from the train compositions list. the mass of the freight trains could be found out from the mass normative (which is the maximum allowable mass of the whole train). the problem is that the actual load on the train is not known and thus it is not possible to know the right mass of the train. therefore, the masses of the freight trains were estimated with the value 1000 tonnes2. this approach brings the significant amount of uncertainty to the result, but on the other hand, according to higher order masses of the freight trains in comparison to the passenger trains, the sensitivity of the result to the changes of the masses is quite small. then for the calculation of the cant excess and the cant deficiency the following scheme was used [5]. values of radii and cant for every location were taken from superstructure plotted outline [6]. 1used devices were initially designed for recording of road vehicles not exceeding the length of 18 metres. 2despite of a fact that this simplification seems to be very disproportional, the results later imply, that the influence of a train masses on the cant excess and cant deficiency is not as strong as was expected. in the present time authors believe, that the influence of the train mass could be negligible. by all means, this still certainly has to be researched more thoroughly. 81 j. valehrach, t. říha, e. dušek acta polytechnica ctu proceedings assuming a formula for ideal speed of trains v = √ d × r 11, 8 (1) the cant excess of a single train then applies ei = d − 11, 8 × v 2 i r (2) and analogically for the cant deficiency ii = 11, 8 × v 2 i r − d (3) ideally, the effects of cant excess and deficiency should be balanced, especially with respect to train masses. however, in general in some locations cant excess could markedly dominate. that happens for example in the vicinity of stations, where the trains are accelerating and have not yet reached the track speed or where the greatest amount of the track load consists of slow moving trains (usually freight trains). in such locations this leads to the overload of the railway superstructure and consequently results in increased occurrence probability of the defects. 3. outputs 3.1. speed ranges the first look at the cleaned data inserted into the charts is promising. the charts below show all four evaluated locations. the speed ranges are apparent, and also the variability could be easily distinguished. the width of the ranges could be identified as well. the width of the speed range has to be considered separately from the variability. the variability describes more the whole dispersion of the measured values, whileas the speed range could be in most cases identified not only once (namely in first chart there could be identified the upper speed range between 65 and 80 and the down speed range between 45 and 60). the speed range however should include the category of trains, as described below in figure 3 and thus it is more complicated to identify it soundly. some of the records are separated from the speed ranges. these records are considered as anomalies; since they are isolated, the passes of the trains corresponding to these records cannot have any significant influence on the infrastructure3. for these purposes, these records are in further analysis neglected. the following table differentiates the speeds by category of the train, showing the average values. 3.2. cant excess and deficiency values for every single train the cant excess or cant deficiency was calculated. in the following charts also the categories of the trains were distinguished, showing clearly the anomalies in the traffic. 3in general, exemptions to this statement could be found, such as special transports of overweighed cargo or other similar uncommon traffic. as it could be seen in the figure 3, the highest cant deficiency values (as high as 130 mm) are used by passing trains in location 162.8. however, this happens to be the location with the least cant of all monitored locations. on the contrary, the least cant deficiency values are used by passing trains in location 169.0. in this case the track alignment consists of compound curve of a radius 375 m with cant 140 mm in first part and radius 382 and cant 100 mm in second part of the curve. by the end of the curve could be found the train stop. measuring section was located in first part of this curve. as a result of evaluation according to chapter 2.4 just one value of the weighted cant deficiency was calculated. in case of our locations, cant excess did not take place. this value could be used for specifying the influence of all the trains passing through certain location on the infrastructure [4]. in case of a need of a closer in-depth analysis, it is necessary to divided the trains into more groups or categories and to calculate the weighted averages afterwards. 3.3. selected categories the categories were selected based on the categories of the passenger trains (also private companies were taken into account). in the future it could be expected, that the speeds will consolidate within these categories rather than by the construction and design properties of the trains. for every category the weighted cant excess (formula 4) or cant deficiency (formula 5) were calculated by e = ∑n i mi × ei∑n i mi (4) i = ∑n i mi × ii∑n i mi (5) results calculated using formulas 4 and 5 are displayed in table 2. when comparing the weighted cant excess or cant deficiency for train categories with the weighted value for all the trains according to table 3 it could be stated, that at least for the rough estimate for the purposes of setting curves against each other is this value conclusive enough. 3.4. train speed/velocity and long-pitch corrugation in a framework of the research also the rate of long pitch corrugation development was monitored in locations 161.7 and 162.8. the corrugation tends to develop quicker in the curve with 162.8, which is the curve where higher values of cant deficiency were found [7]. 82 vol. 11/2017 cant excess and cant deficiency evaluation figure 2. first look at cleaned data from all locations. based on this finding and also using experience from the preceding investigation it is to expect that in case of curves with locations 167.0 and 169.0 the long pitch corrugation will be developing faster in curve with 167.0. 4. conclusions the research was aimed to confirm or disprove the possibility, that cant excess or cant deficiency could influence the development of certain defects of the railway superstructure. in order to be able to calculate the actual values of cant excess or cant deficiency it was necessary to measure speed of passing vehicles. four locations were chosen, both for comparison and for variability monitoring. the speeds were measured with the recording devices and the trains were divided into categories of the similar speed range. after that, the calculation of the cant deficiency or cant excess was executed with double results. first set of results uses as a basis all the speed measurements across all the train categories. second set of results respects the averages in categories detached. the results show, that even the simpler first method could bring satisfactory results. further research should focus on the long-term monitoring of train speeds in order to be able to calculate cant deficiency or cant excess more precise. also more emphasis should be put into the differentiation of masses of the trains. acknowledgements research reported in this paper was supported by competence centres program of technology agency of the czech republic (ta cr), project centre for effective and sustainable transport infrastructure (no. te01020168), student’s grant tender of the faculty of the civil engineering of the brno university of technology (fast-j-15-2860) and railway infrastructure administration, state organization (sždc s. o.). references [1] c. esveld. c. modern railway track. 2nd ed. zaltbommel: mrt-productions, 2001. isbn 90-800-3243-3. [2] j. smutny, i. vukusic, v. tomandl, et al. analýza dynamických účinků v srdcovkové části výhybek, stavební obzor. stavební obzor, thákurova 7, 166 29 praha 6, 2013. issn 1210-4027. [3] p. guziur. design parameters of buffer stops. acta polytechnica ctu proceedings. 2016. issn: 2336-5382. 83 j. valehrach, t. říha, e. dušek acta polytechnica ctu proceedings figure 3. the comparison of the cant excess and cant deficiency in locations (cant deficiency [+] / cant excess [-]). 84 vol. 11/2017 cant excess and cant deficiency evaluation km average speed v [kph] for the train category eurocity train1 express train2 euronight train3 fast train4 fast train suspended5 passenger train6 railjet train7 regiojet train8 freight train other trains9 161.7 72.93 67.80 73.00 68.87 72.13 68.47 71.13 72.80 58.27 53.00 162.8 73.50 73.40 73.00 69.57 72.75 70.09 71.60 73.80 61.64 60.67 167.0 75.18 75.18 75.18 75.18 75.18 75.18 75.18 75.18 75.18 75.18 169.0 78.68 78.68 78.68 78.68 78.68 78.68 78.68 78.68 78.68 78.68 1 the highest category of passenger trains, calling only at major stations not less than 60 km away from each other, usually consisting of the locomotive and six to eleven coaches (hereinafter ec). 2 the category of passenger trains, calling only at major stations not less than 60 km away from each other, usually consisting of the locomotive and three to six coaches (hereinafter ex). 3 the highest category of passenger trains running during nigh time, calling only at major stations not less than 60 km away from each other, usually consisting of the locomotive and more than ten coaches (hereinafter en). 4 the category of passenger trains below express trains, calling at major stations and junctions, usually represented by the train set interpanter (hereinafter r). 5 the category of passenger trains not calling at all stops, usually consisting of the locomotive and three to six coaches (hereinafter sp). 6 the usual passenger train calling at all stops. the train composition differs, both locomotive with coaches and train sets are used to cover this category (hereinafter os). 7 the same category of passenger trains as the eurocity trains, consisting exclusively of the locomotive and train set siemens viaggio comfort (hereinafter railjet). 8 the same category of passenger trains as the eurocity trains, consisting exclusively of the locomotive and six to eleven coaches (hereinafter regiojet). 9 covers all other categories of trains, or trains which do not have specific category, for example maintenance machines etc. table 1. average speeds of the displayed train categories. km weighted cant deficiency [+] / cant excess [-] in millimetres for the categories of trains eurocity train express train euronight train fast train fast train suspended passenger train railjet train regiojet train freight train other trains 161.7 100 82 100 72 92 74 89 99 21 -4 162.8 122 124 118 93 114 98 110 124 52 42 167.0 90 98 96 90 83 78 94 95 58 86 169.0 56 57 49 56 57 34 54 55 8 51 table 2. cant deficiency and cant excess values. km average speed v [kph] for all trains 161.7 67.84 162.8 70.00 167.0 75.18 169.0 78.68 table 3. average speeds of all trains. [4] j. valehrach, j. smid. monitoring of track sections with long-pitch corrugation, proc. int. conf. on interoperability of railway transport – iricon 2016. czech technical university in prague, 2016. isbn 978-80-01-05939-5. [5] c. . 6360-1. geometrical characteristics of railway tracks – part 1. layout. 2008. [6] s. o. szdc. superstructure plotted outline, internal km all trains 161.7 57.1 162.8 83.1 167.0 73.7 169.0 27.7 table 4. cant deficiency values (not rounded). information system output. valid for 2017-04. brno, 2017. [7] j. valehrach. long-pitch corrugation – monitored track sections, proc. int. conf. on young scientist. technical university of kosice, faculty of civil engineering, kosice, 2016. isbn 978-80-553-2537-8. 85 acta polytechnica ctu proceedings 11:80–85, 2017 1 introduction 2 methods of measurement and evaluation 2.1 location selection 2.2 measurement method 2.3 evaluation 2.4 calculation method 3 outputs 3.1 speed ranges 3.2 cant excess and deficiency values 3.3 selected categories 3.4 train speed/velocity and long-pitch corrugation 4 conclusions acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2016.3.0015 acta polytechnica ctu proceedings 3:15–18, 2016 © czech technical university in prague, 2016 available online at http://ojs.cvut.cz/ojs/index.php/app flexibility test of the tools of angioplasty istván hajdu∗, dóra károly, liza pelyhe budapest university of technology and economics, faculty of mechanical engineering, department of materials science and engineering, bertalan lajos u. 7, 1111, budapest, hungary ∗ corresponding author: hajdu999@gmail.com abstract. coronary angioplasty is a procedure used to treat the narrowed coronary arteries. physicians operate with many different tools during the intervention, the main devices are the following: guidewires, guiding catheters, balloon catheters and stents. one of the most important properties of the tools of angioplasty is flexibility. this article introduces a flexibility measuring device and a testing method. with the help of this the flexibility of the tools of angioplasty can be compared easily. keywords: angioplasty, coronary stent, balloon catheter, flexibility. 1. introduction coronary artery disease (cad) is a leading cause of morbidity and mortality in both developing and developed countries [1]. during life fat and cholesterol deposits (called plaques) emerge inevitably in the coronary vessel walls. they gradually increase over time, first they cause coronary stenosis, then complete blockage of the coronary vessels. percutaneous transluminal coronary angioplasty (ptca) is an invasive procedure performed to reduce blockages in coronary arteries [2]. during the process a balloon catheter is guided into the narrowed section of the coronary artery. physicians expand this at the place of the stenosis, thereby the balloon compresses the plaque and ensures continuous flow of blood. usually, a metal mesh is also expanded with the balloon, called stent (fig. 1). stents are commonly made of cocr, ptcr or stainless steel. the most important requirements for stent materials are biocompatibility and hemocompatibility. the stent sholuld not produce a toxic, injurious or immunologic response in living tissue. it is also important for physicians to see the stent during the intervention, therefore it should be visible under x-rays. mri compatibility is another main property of stents, that is why these devices are made from non-magnetic metals (for example austenitic stainless steel). the stent should be flexible to follow the curves of the vessel during the pulsation of the heart, but it should be rigid to compress the plaque [2, 3]. sometimes arteries will still become clogged and narrowed again even with a stent. this phenomenon is called restenosis [4]. low endothelial shear stress can affect the emergence of atherosclerosis and restenosis. stent design, strut thickness and so flexibility have a main role in this field [5]. stent and stent-system properties are mechanical parameters which are important during the deployment, dilation and long term use of the devices. we figure 1. balloon catheter, stent-system and stent. can say that flexibility is one of the most important properties of the tools of angioplasty. the catheter and with it the stent have to follow the curves of the vascular network without damaging or losing their function during deployment, and the stent has to follow the curves of the vessel after dilation at the place of the stenosis. there are many methods to measure the flexibility of the tools of angioplasty, for example the company certiga engineering solutions developed a special equipment. they push the tested tool into a spiral tube as long as it breaks. then they measure the radius at the place of the breaking; they characterize the flexibility with this value [6]. in the study of mori and saito the four-points bending test was utilized. acryl resin bars were attached to both ends of the stent prior to bending measurements, thereby allowing for application of a constant moment to the specimen. through contact with the punch on the acryl resin bars, stent bending with a constant moment and without radial deformation can be achieved [7]. another method is based on the mechanical model of a loaded beam. dr. péter szabadíts characterize the flexibility with the flexural strength of the beam. 15 http://dx.doi.org/10.14311/app.2016.3.0015 http://ojs.cvut.cz/ojs/index.php/app i. hajdu, d. károly, l. pelyhe acta polytechnica ctu proceedings figure 2. the used mechanical model. we also used this model, but by us to the characterization of flexibility the reciprocal of the flexural strength was used [8]. the european standard (en) does not contain any method for the test of the flexibility. the u.s. food and drug administration recommends to wrap the catheter around a series of mandrels with successively smaller radius until the catheter kinks or the lumen collapses. but there is no standard for the stents flexibility measurement [9]. our aim was to design and manufacture a device which is appropriate for the measurement of the flexibility. the method should be fast and easy and adaptable to all kind of tools of angioplasty. 2. materials and methods for the flexibility tests a mechanical model of a loaded beam was used (one end of the cantilever beam was clamped and the other end was loaded; fig. 2). the following equation was used to describe the model: ei = fl3 3f (1) with the help of this equation, the flexibility can be easily calculated: flexibility = 1 ei (2) for the tests a measuring device was needed (fig. 3), which can be impacted into the testing machine (instron 5965). after designing the device it was fabricated with polyjet and fff rapid prototyping technologies. this device consist of a bottom and a top holder, 33 guide bushes, 2 bolts and 5 spacers. all of the guide bushes had a mortise in their centers with different diameters from 0,4 to 4,7 millimeters. the spacers have the following length: 1, 2, 4, 8 and 16 millimeters; with these between 1 and 31 millimeters all discrete quantities can be assembled. several stents, balloon catheters and stent-systems were investigated as a loaded beam in the model. the investigated devices were the following: 3 balloon catheters with different diameters; 10 stentsystems, from these 6 were the same type, they had difference just in the size of the diameter; and 5 stents figure 3. flexibility measuring device. figure 4. the investigated stents. with different diameters and strut widths (fig. 4). the investigated stents were made from platinum chromium (ptcr) or cobalt chromium (cocr) alloys. the investigations were performed by stereo microscope and testing machine. first, the diameter of the tools was measured with the help of the stereo microscope. then it was impacted into the right size guide bush, which was embed to the top holder. the guide bushes had a bigger outside diameter as the diameter of the mortise on the top holder, so they were fitting tight. after that, the required length was adjusted with the spacers and the whole measuring device with the tested tool was impacted into the testing machine. the tested tools were loaded with a perpendicular force at the end. the load force, the length and deflection of the beam was measured. after the tests the flexibility was calculated with the help of equation (2). every device was loaded five times, between the measurements each was rotated about its axis with the same angles. 16 vol. 3/2016 flexibility test of the tools of angioplasty table 1. the results. figure 5. correlation between flexibility and diameter in the case of stent-systems. 3. results table 1. contains basic information and flexibility of the investigated devices. diameter and length are the nominal sizes. the maximum deflection was 20% of the clamping length, but it was maximized in 3 millimeters. strut width was measured with the help of stereo microscope. three main factors influences the flexibility of the tools of angioplasty, these are the followings: diameter, material and stent pattern. primarily the relationship between the flexibility and diameter was investigated. the results show that between the flexibility and the diameter there is a power function dependence (fig. 5). in the case of stents the relationship between flexibility and strut width was also investigated; between figure 6. correlation between flexibility and strut width in the case of stents. the flexibility and the strut width there is a power function dependence (fig. 6). based on the measurement the flexibility of stents and balloon catheters are about with one order of magnitude greater than the flexibility of stent-systems (fig. 7). 4. conclusions based on the experiments the measuring device is adequate for the investigation of flexibility. in every case the relative standard deviation of the data is lower than 10% (in most cases it is lower than 1%), so the data is homogeneous, this means that it isn’t needed to measure more times in different positions. the flexibility depends on the diameter, raw material, 17 i. hajdu, d. károly, l. pelyhe acta polytechnica ctu proceedings figure 7. correlation between flexibility and diameter of the investigated devices. strut width and stent pattern. the manufacturers don’t give any concrete, numerical data about flexibility of the tools of angioplasty, that’s why these investigations are important. with the help of these results, physicians can easily compare stents or stent-systems according to flexibility, and they can purposefully choose the most suitable tools for the intervention. list of symbols e young’s modulus [mpa] i moment of inertia of beam [mm4] f force [n] l length of beam [mm] f deflection of beam [mm] references [1] k. iwasaki, et al. prevalence of subclinical coronary artery disease in ischemic stroke patients. journal of cardiology 65:71–75, 2015. doi:10.1016/j.jjcc.2014.04.004. [2] g. mani, m. d. feldman, d. patel, c. m. agrawal. coronary stents: a materials perspective. biomaterials 28:1689–1710, 2007. [3] d. jános. az értágítóbetétek anyagainak fejlödése. kohászat 5-6:44–48, 2013. [4] l. petrini, f. migliavacca, f. auricchio, g. dubini. numerical investigation of the intravascular coronary stent flexibility. journal of biomechanics 37:495–501, 2004. [5] k. c. koskinas, et al. role of endothelial shear stress in stent restenosis and thrombosis : pathophysiologic mechanisms and implications for clinical translation. journal of the american college of cardiology 59:1337–1349, 2012. doi:10.1016/j.jacc.2011.10.903. [6] stm-3015 stent system testing ”flexibility” according to din en 14299:2004 7.3.3. http://www.certiga.com/en/stm_3015.en.htm. [7] k. mori, t. saito. effects of stent structure on stent flexibility measurement. annals of biomedical engineering 33:733–742, 2005. [8] p. szabadíts, z. puskás, j. dobránszky. flexibility and trackability of laser cut coronary stent systems. acta bioeng biomech 11:8–11, 2009. [9] class ii special controls guidance document for certain percutaneous transluminal coronary angioplasty (ptca) catheters, 2010 september. http://www.fda.gov/ medicaldevices/deviceregulationandguidance/ guidancedocuments/ucm225145.htm. 18 http://dx.doi.org/10.1016/j.jjcc.2014.04.004 http://dx.doi.org/10.1016/j.jacc.2011.10.903 http://www.certiga.com/en/stm_3015.en.htm http://www.fda.gov/medicaldevices/deviceregulationandguidance/guidancedocuments/ucm225145.htm http://www.fda.gov/medicaldevices/deviceregulationandguidance/guidancedocuments/ucm225145.htm http://www.fda.gov/medicaldevices/deviceregulationandguidance/guidancedocuments/ucm225145.htm acta polytechnica ctu proceedings 3:15–18, 2016 1 introduction 2 materials and methods 3 results 4 conclusions list of symbols references 151 acta polytechnica ctu proceedings 1(1): 151–156, 2014 151 doi: 10.14311/app.2014.01.0151 argo-ybj: highlights and prospects giuseppe di sciascio1 on behalf of the argo-ybj collaboration 1infn sezione di roma tor vergata, viale della ricerca scientifica 1, i-00133 roma corresponding author: disciascio@roma2.infn.it abstract the argo-ybj experiment has been in stable data taking for 5 years at the yangbajing cosmic ray laboratory (tibet, p.r. china, 4300 m a.s.l., 606 g/cm2). with a duty-cycle greater than 86% the detector collected about 5·1011 events in a wide energy range, from few hundreds gev up to the pev. a number of open problems in cosmic ray physics has been faced exploiting different analyses. in this paper we summarize the last results in gamma-ray astronomy and in the cosmic ray physics and introduce the lhaaso project, mainly driven by the chinese community, to study the cosmic ray physics up to 1017 ev. keywords: cosmic rays extensive air showers gamma-ray astronomy argo-ybj. 1 the argo-ybj experiment argo-ybj is a full coverage air shower detector located at the yangbajing cosmic ray laboratory (4300 m a.s.l., 606 g/cm2, tibet, pr china) devoted to the study of gamma rays and cosmic rays. exploiting the high altitude and the full coverage technique, argoybj can detect gamma rays with an energy threshold as low as a few hundred gev. the detector consists of a ∼74×78 m2 carpet made of a single layer of resistive plate chambers (rpcs) with ∼93% of active area, surrounded by a partially instrumented (∼20%) area up to ∼100×110 m2. the detector has a modular structure with a high granularity, that provides a detailed view of the shower front. the smallest space-time unit, called “pad”, has a size of 55.6×61.8 cm2. the time and the location of each fired pad are recorded and used to reconstruct the position of the shower core and the arrival direction of the primary particle. the point spread function (psf), the pointing accuracy and the energy calibration of the detector have been evaluated using the moon shadow technique, i.e. the deficit of cosmic rays in the moon direction. for a detailed description of the detector performance see (aielli et al., 2006, aielli et al. 2009, bartoli et al., 2011). since november 2007 to january 2013 the argoybj experiment monitored with high duty cycle (∼86%) the northern sky at tev photon energies. with a cumulative sensitivity ranging from 0.24 to ∼1 crab units, depending on the declination, six sources have been observed with a statistical significance greater than 5 standard deviations (s.d.) in the declination band from -10◦ to 70◦. in this paper the last results obtained in gamma-ray astronomy and in the study of the anisotropy in the cr arrival direction distribution are summarized. 2 northern sky survey the argo-ybj data used in this analysis were collected from november 2007 to january 2013, with a total observation time of 1670.45 days. the total number of events selected with a zenith angle less than 50◦ is about 3×1011. they are used to fill a map in celestial coordinates (right ascension and declination) with 0.1◦×0.1◦ bins, covering the declination band from -10◦ to 70◦. )σ significance ( -5 0 5 10 15 20 n um be r of e nt rie s pe r bi n 1 10 210 310 410 510 610 all region °<l<90°,20°|b|<2 figure 1: significance distribution of the whole sky map (thick solid line). the thin solid line represents the best gaussian fit. the significance distribution of the galactic plane region with | b |< 2◦ and 20◦ < l < 90◦ is shown by the thick dotted line. the thin dotted line represents the best gaussian fit for the same region. 151 http://dx.doi.org/10.14311/app.2014.01.0151 giuseppe di sciascio figure 2: significance map of the northern sky as seen by the argo-ybj experiment in vhe γ-ray band. the two dotted lines indicate the galactic latitudes b = ±5◦. galactic longitude (deg) g al ac tic la tit ud e (d eg ) -10 -5 0 5 10 -4 -2 0 2 4 6 °dec=50 °dec=40 °dec=30 °dec=20 °dec=10 °dec=0 2030405060708090 figure 3: significance map of the galactic plane region with | b |< 10◦ and 20◦ < l < 90◦ obtained by the argo-ybj experiment. the circles indicate the position of all the known vhe sources. the open stars mark the location of the gev sources of the second fermi-lat catalog. the significance distribution of the whole map bins is shown in fig.1. the distribution, with a mean value of 0.002 and σ=1.02, closely follows a standard gaussian distribution except for a tail with large positive values, due to the excesses from several gamma ray emission regions, shown in fig. 2. table 1 lists the locations of all the regions with pre-trial significance greater than 4.5 s.d. . for each independent region, only the coordinates of the pixel with the highest significance are given. based on the distribution of negative points (fig. 1), a significance threshold of 4.5 s.d. corresponds to ∼ 2 false sources in our catalog. the significance distribution of the inner galactic plane region (longitude 20◦ < l < 90◦ and latitude | b |< 2◦) is also shown in fig. 1. the gaussian fit of the distribution has a mean value of 0.40 and σ=1.04. also in this case a tail of significant excesses is present. the locations of all the excesses with a significance greater than 4.0 s.d. are listed in table 1. fig. 3 shows the inner galactic plane region in galactic coordinates. since the smoothing radius is larger than the bin width, the significances in adjacent bins are correlated, and a monte carlo simulation is necessary to correctly evaluate the post-trial probabilities. according to our simulations, the probability to have at least one source with a pre-trial significance > 5.1 σ (> 4.0 σ) anywhere in the map (in the inner galactic plane) due to background fluctuations is 5%. however, since in the sky region monitored by argo-ybj only ∼70 known vhe emitters exist, the post-trial significance increases for the candidate sources associated to a counterpart. detail about different sources are discussed in chen s. et al. (2013). 152 argo-ybj: highlights and prospects table 1: location of the excess regions argo-ybj ra dec s associated name (deg) (deg) (s.d.) tev source j0409−0627 62.35 -6.45 4.8 j0535+2203 83.75 22.05 20.8 crab nebula j1105+3821 166.25 38.35 14.1 mrk 421 j1654+3945 253.55 39.75 9.4 mrk 501 j1839−0627 279.95 -6.45 6.0 hess j1841-055 j1907+0627 286.95 6.45 5.3 hess j1908+063 j1910+0720 287.65 7.35 4.3 j1912+1026 288.05 10.45 4.2 hess j1912+101 j2021+4038 305.25 40.65 4.3 ver j2019+407 j2031+4157 307.95 41.95 6.1 mgro j2031+41 tev j2032+4130 j1841-0332 280.25 -3.55 4.2 2.1 sky upper limits excluding the sources listed in table 1, we can set upper limits to the γ-ray flux from all other directions in the sky. to estimate the response of the argo-ybj detector we simulated a source located at different declinations, with a power law spectrum in the energy range 10 gev 100 tev and different spectral indices. the number of events is transformed into a flux using the results of the simulation. the 95% c.l. upper limits to the flux of γ-rays with energies above 500 gev for each bin are obtained. the upper limits as a function of the declination are shown in fig. 4 for different photon spectral indices. the limits range between 9% and 44% icrab and are the lowest obtained so far. the lowest limit for a spectral index −2.0 (−3.0) is 5% (9%) icrab, where the crab unit is defined as 5.77×10−11 cm−2 s−1. 3 cosmic ray anisotropy the cr arrival direction distribution and its anisotropy has been a long-standing problem ever since the 1930s. in fact, the study of the anisotropy is a powerful tool to investigate the acceleration and propagation mechanism determining the cr world as we know it. the anisotropy in the cr arrival direction distribution have been observed by different experiments with increasing sensitivity and details at different angular scales. current experimental results show that the main features of the anisotropy are uniform in the energy range (1011 1014 ev), both with respect to amplitude (10−4 10−3) and phase ((0 4) hr). the existence of two distinct broad regions, one showing an excess of crs (called “tail-in”), distributed around 40◦ to 90◦ in r.a., the other a deficit (the “loss cone”), distributed around 150◦ to 240◦ in r.a., has been clearly observed. dec (deg) -10 0 10 20 30 40 50 60 70 9 5% c .l . u pp er l im it (c ra b un it) -210 -110 1 =-3.0α =-2.6α =-2.0α figure 4: 95% c.l. flux upper limits for energy above 500 gev, averaged over the right ascension, as a function of the declination. the different curves indicate a different power-law spectral index. the origin of the cr anisotropy is still unknown. unlike predictions from diffusion models, the cr arrival distribution in sidereal time was never found to be purely dipolar. even 2 harmonics were necessary to properly describe the r.a. profiles, showing that the cr intensity has quite a complicated structure unaccountable simply by kinetic models. in the last years the milagro (abdo et al. 2008) and argo-ybj (di sciascio, 2013) collaborations reported evidence of the existence of a medium angular scale anisotropy contained in the tail-in region. the 153 giuseppe di sciascio observation of similar small scale anisotropies has been recently claimed also by the icecube experiment (abbasi et al., 2011) in the southern hemisphere. in fig. 5 the argo-ybj sky map in galactic coordinates as obtained with 4.5 years data is shown. the color scale gives the statistical significance of the observation in s.d. . the map center points towards the galactic anti-center. the maps have been smoothed with an angle given by the psf of the detector for crinduced showers. 0 ◦360 ◦ -15 -12 -9 -6 -3 0 3 6 9 12 15 figure 5: argo-ybj sky-map in galactic coordinates. the statistical significance of the observation in s.d. is shown. the map center points towards the galactic anti-center. data have been recorded in 1587 days out of 1656, for a total observation time of 33012 hrs (86.7% dutycycle). a selection of high-quality data reduced the data-set to 1571 days. the zenith angle cut (θ ≤ 50◦) selects the dec. region δ ∼ -20◦÷ 80◦. according to the simulation, the median energy of the isotropic cr proton flux is e50 p ≈1.8 tev (mode energy ≈0.7 tev). no gamma/hadron discrimination algorithms have been applied to the data. therefore, the sky map is filled with all crs possibly including photons, without any discrimination. figure 6: one-dimensional projection in right ascension of the two-dimensional cr sky map in local solar time. the red line shows the best-fit to argo-ybj data (crosses). in spite of the fact that the bulk of snr, pulsars and other potential cr sources are in the inner galaxy surrounding the galactic centre, the excess of cr is observed in the opposite, anti-centre direction. as stressed in erlykin & wolfendale (2013), the fact that the observed excesses are in the northern and in the southern galactic hemisphere, favors the conclusion that the cr at tev energies originate in sources whose directions span a large range of galactic latitudes. the right side of the map is full of few-degree excesses not compatible with random fluctuations (the statistical significance is up to 7 s.d.). the observation of these structures is reported by argo-ybj for the first time. so far, no theory of crs in the galaxy exists which is able to explain both large scale and few degrees anisotropies leaving the standard model of crs and that of the local galactic magnetic field unchanged at the same time. 3.1 the compton-getting effect the origin of cr anisotropies is still unknown therefore, the observation of an expected anisotropy is important to check the reconstruction algorithms, the exposure and background calculations and the stability of the detector performance. a well-known expected anisotropy is the so-called compton-getting (cg) effect, a dipole anisotropy in the local solar frame, due to the earth’s motion around the sun (compton & getting, 1935). a significant signal compatible with cg is seen by argo-ybj in solar time above ∼ 8 tev to avoid additional effects due to heliospheric magnetic field and solar activity. in fact, we found that including lower energy events results in much larger modulation amplitudes than those obtained when these events were excluded. fig. 7 shows the solar variations observed by argo-ybj together with the sinusoidal curve best fitted to the data. the fair agreement between data and calculations (φ = 6:00 hr, a = 9.7 · 10−5) make us confident about the capability of argo-ybj in detecting anisotropies at a level of 10−4. 4 prospects: the lhaaso experiment a new experiment has been proposed by the chinese community to face the open problems in galactic cosmic ray physics. the lhaaso experiment is a multicomponent extensive air shower array constituted by: (1) an array consisting of 5137 scintillators (1 m2 each) 15 m away from each other (km2a) and 1200 muon detectors (40 m2 each). the total effective area of the muon detector is about 48,000 m2. (2) a water cherenkov detector array (wcda) consisting of 4 water ponds, 150 × 150 m2 each. the pond depth is 154 argo-ybj: highlights and prospects about 4.5 m. each pond is subdivided into 30 × 30 = 900 cells sized 5 × 5 m2 each, separated by black plastic curtains. an 8 inches pmt looks upward at the bottom of each cell to collect cherenkov photons produced by secondary charged particles in the water pond. (3) 24 wide field of view cherenkov telescope array (wfcta). (4) shower core detector array (scda) with an effective area of 5000 m2. figure 7: layout of the lhaaso experiment. the water ponds allow to improve the sensitivity to γ-ray sources down to a percent of the crab nebula flux in the tev energy region. the km2a array will extend the search for γ-ray sources in the 100 tev region with an unprecedented sensitivity. in fact, exploiting the shower muon content measurement, the detection of photon-induced showers is basically background free above few tens tev. the sensitivity of lhaasowcda + lhaaso-km2a (green line) for detection of point gamma ray sources is compared to other experiments or projects in fig. 8. the observation times is 1 year and 50 hour for wide field-of-view detectors and iact, respectively. the wfcta and the scda will allow, in addition, to study the cosmic ray physics up to the 1018 ev region, thus investigating the transition between galactic and extra-galactic cr components and the elemental composition above pev energies. the different elements of the experiment (scintillators, water pool, wide field of view cerenkov telescopes, neutron detectors) have been successfully tested at the yangbajing laboratory exploiting the cr beam provided by the argo-ybj detector. the installation of the detectors is expected to start between 2-3 years and finish in about 5 years. the proposed site is located in china, in the yunnan province, at an altitude of about 4300 m a.s.l. . figure 8: the sensitivity of lhaaso-wcda + lhaaso-km2a (green line) compared to other experiments or projects. the observation times is 1 year and 50 hour for wide field-of-view detectors and iact, respectively. 5 conclusions the argo-ybj detector exploiting the full coverage approach and the high segmentation of the readout is imaging the front of atmospheric showers with unprecedented resolution and detail. the digital and analog readout will allow a deep study of the cr phenomenology in the wide tev pev energy range. the results obtained in the low energy range (below 100 tev) predict an excellent capability to address a wide range of important issues in astroparticle physics. in this paper we summarized the last results in gamma-ray astronomy and in the study of the cr anisotropy. the new experiment lhaaso, mainly driven by the chinese community, to study the cosmic ray physics up to 1018 ev has been introduced. references [1] abbasi r. et al.: 2011, apj 740, 16. [2] abdo a.a. et al.: 2008, phys. rev. lett. 101, 221101. [3] aielli, g. et al.: 2006, nim a562, 92. [4] aielli g. et al.: 2009, nim a608, 246. doi:10.1016/j.nima.2009.07.020 [5] aielli, g. et al.: 2010a, apj 714, l208 doi:10.1088/2041-8205/714/2/l208 [6] amenomori m. et al.: 2010, astrophys. space sci. trans. 6, 49. doi:10.5194/astra-6-49-2010 [7] bartoli, b. et al.: 2011a, phys. rev. d84, 022003. doi:10.1103/physrevd.84.022003 155 http://dx.doi.org/10.1016/j.nima.2009.07.020 http://dx.doi.org/10.1088/2041-8205/714/2/l208 http://dx.doi.org/10.5194/astra-6-49-2010 http://dx.doi.org/10.1103/physrevd.84.022003 giuseppe di sciascio [8] compton a.h. and getting i.a., 1935, phys. rev. 47, 817. [9] di sciascio g., 2013, epj 52, 04004. [10] erlykin a.d. and wolfendale a.w.: 2013, arxiv:1303.2889. [11] chen, s. et al.: 2013, icrc 2013, id 586. discussion c. munoz-tunon: could you extend a little on the details of the site for the new experiment in china ? g. di sciascio: in principle the new project lhaaso will be located in two different high altitude sites. the lawca (large water cherenkov array) experiment is a possible upgrade of the argo-ybj experiment at the yangbajing laboratory in tibet. the detector will consist in a large water pond l-shaped around the argo-ybj building with an area of about 23,000 m2 and is focused to study gamma-ray astronomy between 100 gev and 30 tev. the detector structure is identical to the wcda of the lhaaso project, therefore this experiment is the phase-0 of the lhaaso project. the data taking is expected to start a couple of years after the start of the construction. j. beall: will the new facility be at the same altitude ? g. di sciascio: yes, the lhaaso experiment will be located at an altitude of about 4300 m a.s.l. in the yunnan province, similar to the altitude of the yangbajing laboratory where is located the argo-ybj detector. c. pittori: can you say something more about the possible connection between observed anisotropies and the heliosphere ? g. di sciascio: as discussed in (amenomori et al., 2010), the main regions of the medium scale anisotropy can be described as two intensity enhancements placed along the hydrogen deflection plane, which contains the directions of the interstellar wind velocity and the interstellar magnetic field surrounding the heliosphere, each symmetrically centered away from the heliotail direction. the separation angle between the heliotail direction and each enhancement monotonously decreases with increasing energy in an energy range 4 30 tev. the msa being placed along the hdp suggests that it is possibly caused by the modulation of galactic cosmic rays in the magnetic field of the heliotail within ∼ 70 au to ∼ 340 au from the sun. 156 the argo-ybj experiment northern sky survey sky upper limits cosmic ray anisotropy the compton-getting effect prospects: the lhaaso experiment conclusions 76 acta polytechnica ctu proceedings 2(1): 76–80, 2015 76 doi: 10.14311/app.2015.02.0076 probing the accretion processes in soft x-ray selected polars i. traulsen1, k. reinsch2, a. d. schwope1 1leibniz-institut für astrophysik potsdam (aip), an der sternwarte 16, 14482 potsdam, germany 2institut für astrophysik, georg-august-universität göttingen, friedrich-hund-platz 1, 37077 göttingen, germany corresponding author: itraulsen@aip.de abstract high-energy data of accreting white dwarfs give access to the regime of the primary accretion-induced energy release and the different proposed accretion scenarios. we perform xmm-newton observations of polars selected due to their rosat hardness ratios close to -1.0 and model the emission processes in accretion column and accretion region. our models consider the multi-temperature structure of the emission regions and are mainly determined by mass-flow density, magnetic field strength, and white-dwarf mass. to describe the full spectral energy distribution from infrared to x-rays in a physically consistent way, we include the stellar contributions and establish composite models, which will also be of relevance for future x-ray missions. we confirm the x-ray soft nature of three polars. keywords: cataclysmic variables polars spectroscopy photometry x-rays individual: ai tri, qs tel, rs cae. 1 introduction accretion onto magnetic white dwarfs involves plasma under extreme physical conditions, in particular high temperatures up to millions of kelvin. x-ray observations of the discless am her-type systems (“polars”) provide direct insight into the accretion processes and the opportunity to study related system properties. hard x-ray emission (e > 0.5 kev) arises from the cooling accretion column above the white dwarf and soft (e < 0.5 kev) from the heated accretion region on the white-dwarf surface. model calculations and recent spectral analyses reveal complex structures of the emission regions and a wide range of temperatures and densities. several systems are found at excesses of soft over hard x-ray flux by factors up to 1 000, which can be interpreted as a sign of inhomogeneous accretion. full understanding of the accretion processes and the binary system requires multi-wavelength data, since the different system components dominate the spectral energy distribution (sed) at different wavelengths from infrared up to x-rays. in a campaign of dedicated xmm-newton and optical observations of selected am her-type systems, we spectrophotometrically study the parameters and flux contributions of their components. we concentrate on the conditions in the emission regions in the post-shock accretion column and on the heated white dwarf, flux and luminosity ratios and their strong dependence on the choice of the underlying spectral models. here, we summarize our work on three soft x-ray selected polars and our efforts to establish consistent multi-wavelength models. 2 observed seds starting in 2005, we obtained xmm-newton x-ray and ultraviolet data of ai tri, qs tel, and rs cae (obs. ids 0306840901, 0306841001, 0404710401, 0554740801; traulsen et al., 2010, 2011, 2014), covering one to five orbital cycles per object. on the basis of optical monitoring, the too observations were triggered during high and intermediate high states of accretion. optical photometry and – for qs tel – spectroscopy were performed (quasi)simultaneously. to construct the full long-term seds of the objects from infrared to x-ray wavelengths, we use publicly available archival data, in particular of the wise, 2mass, hst, fuse, and rosat archives. figure 1 shows all data, the main system components being marked according to their approximate flux maxima in the middle panel as follows: a. the secondary star in the ir, b. the cyclotron emission in the ir to optical, c. the accretion stream and d. the white-dwarf primary in the optical to uv, e. the accretion-heated region on the white-dwarf surface in the extreme uv to the soft x-ray regime, and f. the post-shock accretion column in hard x-rays. all panels include both high-state and low-state data, identifiable by their different flux levels. the distinct soft x-ray / euv components at high states are clearly visible, compared for example to the low-state sed of ef eri 76 http://dx.doi.org/10.14311/app.2015.02.0076 probing the accretion processes in soft x-ray selected polars (schwope et al. 2007) or to the serendipitously discovered x-ray hard polar 2xmmp j131223.4+173659 (vogel et al. 2008). at times of high soft x-ray flux, the optical and soft x-ray data show pronounced short-term variability (“flickering”). 10 5 10 4 10 3 10 2 10 1 wavelength [å] 10−6 10−5 10−4 10−3 10−2 10−1 ai tri xmm rosat euve galex fuse iue hst ground 2mass wise 10−6 10−5 10−4 10−3 10−2 10−1 e ne rg y flu x [ k ev c m− 2 s − 1 ] qs tel a b c d e f 10−6 10−5 10−4 10−3 10−2 10−1 energy [kev] 0.001 0.01 0.1 1.0 10.0 rs cae figure 1: observed seds of three x-ray soft polars from ir to x-rays: our xmm-newton and optical plus archival data at different epochs and accretion states (cf. traulsen et al. 2010, 2011, 2014). the x-ray spectra are unfolded using the best-fit models. 3 multi-wavelength modeling to consistently describe the spectral energy distribution and multi-band light curves, we synthesize spectral models for one uniform set of system parameters and calculate the corresponding light curves. for the different system components, we adopt (referring to the labels used in sect. 2 and fig. 1): a. a phoenix stellar atmosphere model of an m star (hauschildt & baron 1999), b. a cyclotron component of a stratified postshock accretion column (fischer & beuermann 2001), c. a simplified accretion-stream component of a 3d binary model (staude et al. 2001), d. a non-lte whitedwarf atmosphere model (werner & dreizler 1999), e. a singleor multi-temperature black body or hot whitedwarf atmosphere, f. a singleor multi-temperature plasma model of the accretion column. the input parameters of the models are determined from observational data, where possible, and estimated as typical values for primary white dwarf and secondary m star otherwise (e.g. townsley & gaensicke 2009, knigge 2006). in particular, orbital period, inclination, and magnetic field strength are available from optical spectroscopy and polarimetry; parameters of the accretion-induced emission are fitted to the x-ray spectra (cf. sect. 4 and table 1). table 1: parameters of the best fits to the xmmnewton spectra. bolometric rosat flux ratios at fixed temperatures are calculated from (a)schwarz et al. 1998, (b)schwope et al. 1995, (c)burwitz et al. 1996, applying corrections of κbb = 2.5, κbr = 4.8. ai tri qs tel rs cae black body multi-t single-t single-t plasma model multi-t two-t single-t nh,ism [cm−2] ∼ 1020 ∼ 1020 2±1×1019 ktbbody [ev] 44 ± 5 20 ± 4 36 ± 1 ktplasma [kev] 1 .. 20 0.2 .. 4.0 7 ± 3 nh,intr [cm−2] 3±2×1023 1022..1023 ∼ 4×1023 log ṁ [m�/yr] −11..− 9 ∼ −10 ∼ −10 fbb/fplasma ∼ 3..200 ∼ 10..100 ∼ 11 fbb/fbr, rosat ∼ 170(a) ∼ 80(b) ∼ 90(c) synthetic cyclotron light curves are derived from the phase-dependent model spectra of the accretioncolumn (b) by folding them with the johnson and xmm-newton om filter bandpasses, and white-dwarf and accretion-stream light curves from model components c and d. by comparing them with observational data, we determine their respective phase shifts and intensities. as an example, fig. 2 shows the cyclotron spectra calculated for rs cae, fig. 3 the corresponding synthetic ubv ri light curves. using a consistent set of parameters for all components, we thus establish a physically realistic and consistent multi-wavelength model of the whole binary system, missing only the unknown spectral contribution of the accretion stream. we describe its successful application to the multi-wavelength data of rs cae in traulsen et al. (2014). the most relevant limitation relates to the different observational epochs of the high-state data. while we need low-state spectra in the ir and uv to identify the secondary and (unheated) 77 i. traulsen, k. reinsch, a. d. schwope primary star, all high-state data should be, ideally, observed simultaneously, due to the high variability of the accretion processes. 4 probing the accretion processes in x-rays as described above, x-ray data are not the sole, but the main source of information on the accretion mechanisms in magnetic cvs. they give us access to characteristic determinants of the systems and their evolution like mass accretion rate, component masses, and bolometric fluxes / luminosities, which let us distinguish between accretion scenarios like standing or buried shocks or inhomogeneous accretion. these objectives, however, are limited by the complexity of the puzzle and by the energy resolution and signal-to-noise ratio of currently available x-ray data. several model approaches have been developed, each of them focusing on different aspects, as cropper et al. (1999, mass determination and effective spectral fitting), fischer & beuermann (2001, column structure, sed coverage). mass and flux determination particularly depend on the underlying spectral models (see also cropper et al. 1999). 103 104 105 wavelength [å] 10−21 10−20 10−19 10−18 10−17 10−16 10−15 s im ul at ed fl ux [e rg c m− 2 s − 1 å − 1 ] u b v r i figure 2: spectral models of cyclotron emission for b = 36 mg, ṁ = 0.01, 1.0 g cm−2 s−1, and the estimated accretion geometry of rs cae: orbital mean (red) and phase-resolved from ϕmag = 0.0 (light) to 0.5 (dark). dotted: johnson filter bandpasses, used to derive the light curves in fig. 3. the x-ray soft component is usually modeled by an absorbed black body, which is on the one hand an appropriate approach for ccd data, not resolving the line features. on the other hand, non-lte processes and the metal-richness of the hot photosphere have a non-negligible effect on the spectral continuum in the uv to x-rays (cf. rauch 2003 and references therein). the relevance of consistent non-lte modeling, considering line-blanketing by heavier elements has been demonstrated for non-accreting hot white dwarfs (e.g. traulsen et al. 2005). bolometric fluxes derived from black-body fits to xmm-newton data are lower by factors up to five than from non-lte models, which typically yield lower effective temperatures and hydrogen absorptions. multi-temperature models, designed to reproduce the temperature gradient in the heated accretion region, result in increased bolometric fluxes by at least 50 % with respect to single-temperature models. to illustrate the differences between the models, we simulate pointed observations with the upcoming erosita mission (merloni et al. 2012), which will have a higher effective area at energies between 0.2 and 2 kev than 0.0 0.5 1.0 1.5 magnetic phase 21 20 19 18 17 m ag ni tu de ( v eg a sy st em ) 0.0 0.5 1.0 1.5 x−ray dip phase figure 3: simulated and observed light curves of rs cae. left: ubv ri models (bottom to top), derived from the spectra in fig. 2. right: smarts/b, xmm-newton/u and uvw1. dashed: cyclotron emission, dotted: white dwarf and accretion stream. xmm-newton and rosat. figure 4 shows 50 ks erosita spectra based on fits to the xmm-newton data of ai tri (reduced χ2 between 0.96 and 1.01). for the x-ray hard post-shock spectra, we adopt the radiation-hydrodynamic models by fischer & beuermann (2001). they are valid for shallow accretion columns, both for the shock scenario dominated by bremsstrahlung cooling and the bombardment scenario dominated by cyclotron cooling. to make them available for automated spectral fitting of x-ray ccd and grating data, we incorporate their temperature and density distributions in xspec, using the local mass flow densities ṁ and magnetic field strengths b listed in their paper and white-dwarf masses mwd = 0.6, 0.8, and 1.0m�. we parameterize the distributions for 30 layers of a stratified column, and add up 30 apec plasma components to the final combined column spectrum. our models include velocity shifts and broadening of the emission lines by stream motion, gravity, orbital motion, and the changing viewing angle. adding a pexmon reflection component (nandra 2007) and multiplying it with the same orbital velocity term, we get a comprehensive description of the phase-dependent emission induced by the accretion column, in particular of the iron lines between 6.4 and 6.9 kev. figure 5 shows composite accretion-column models compared to 78 probing the accretion processes in soft x-ray selected polars a 50 ks synthetic spectrum of an (illustrative) polar with similar fluxes to am her. the models include the same reflection term and different column parameters. the unabsorbed bolometric fluxes of the column models are typically by about 50 % higher than of the corresponding single-temperature fits. the intrinsic absorption and reflection components have a considerable impact on the fluxes (cf. cropper et al. 1999), which increase by factors up to 15 compared to pure plasma models. the soft-to-hard flux ratios, thus, may significantly vary for the same object and observation, depending on the model choice. 0.0 0.5 1.0 1.5 2.0 2.5 n or m al . c ou nt s s− 1 k ev − 1 −5 0 energy [kev] ∆χ 0.2 0.3 0.4 0.5 + single−t black body + multi−t black body + nlte wd atmosphere figure 4: synthetic spectra of a 50 ks erosita pointed observation, based on different models for the heated white dwarf in the x-ray soft polar ai tri. 3 4 5 6 7 8 9 10 energy [kev] 10 −2 10 −1 1 n or m al iz ed c ou nt s s− 1 k ev − 1 – mwd = 0.6 – 1.0 mo • – b = 100 – 10 mg – m = 0.01 – 100 g cm–2 s–1 ˙ figure 5: simulated 50 ks erosita observation of a polar at ṁ = 1.0 g cm−2 s−1, b = 30 mg, mwd = 0.8m�, scaled to the xmm-newton spectrum of am her (black: data, yellow: model). each colored line represents one parameter that is varied while the other parameters and the reflection component are fixed (dashed: lowest, solid: highest value). 5 conclusions our xmm-newton observations confirm the soft x-ray excess of the three selected polars and indicate inhomogeneous accretion processes. we develop composite models including the contributions of the stellar atmospheres and the x-ray emitting accretion regions for a physically realistic description of the binary system and the accretion processes. simultaneous multi-λ observations are relevant for a fully consistent sed fitting. the upcoming erosita survey will significantly increase the total number of known systems and of systems for that reliable soft-to-hard ratios can be derived. survey and pointed observations will enable us to better distinguish between different models of the accretion processes, refine them, and push our knowledge about the physical properties of the x-ray emission regions of polars. acknowledgement our research was supported by dlr under grant numbers 50 or 0501, 50 or 0807, and 50 or 1011. references [1] burwitz, v. et al.: 1996, a&a 305, 507 [2] cropper, m. et al.: 1999, mnras 306, 684 doi:10.1046/j.1365-8711.1999.02570.x [3] fischer, a., beuermann, k.: 2001, a&a 373, 211 [4] hauschildt, p. h., baron, e.: 1999, j. comp. appl. math. 109, 41 doi:10.1016/s0377-0427(99)00153-3 [5] knigge, c.: 2006, mnras 373, 484 doi:10.1111/j.1365-2966.2006.11096.x [6] merloni, a. et al.: 2012, arxiv:1209.3114 [7] nandra, k. et al.: 2007, mnras 382, 194 doi:10.1111/j.1365-2966.2007.12331.x [8] rauch, t.: 2003, a&a 403, 709 [9] schwarz, r. et al.: 1998, a&a 338, 465 [10] schwope, a. d. et al.: 1995, a&a 293, 764 [11] schwope, a. d. et al.: 2007, a&a 469, 1027 [12] staude, a., schwope, a. d., schwarz, r.: 2001, a&a 374, 588 [13] townsley, d. m., gänsicke, b. t.: 2009, a&a 693, 1007 [14] traulsen, i. et al.: 2005 in 14th european workshop on white dwarfs, d. koester & s. moehler (eds.), asp conf. ser. 334, 325 [15] traulsen, i. et al.: 2010, a&a 516, a76 [16] traulsen, i. et al.: 2011, a&a 529, a116 79 http://dx.doi.org/10.1046/j.1365-8711.1999.02570.x http://dx.doi.org/10.1016/s0377-0427(99)00153-3 http://dx.doi.org/10.1111/j.1365-2966.2006.11096.x http://dx.doi.org/10.1111/j.1365-2966.2007.12331.x i. traulsen, k. reinsch, a. d. schwope [17] traulsen, i. et al.: 2014, a&a 562, a42 [18] vogel, j. et al.: 2008, a&a 485, 787 [19] werner, k., dreizler, s.: 1999, j. comp. appl. math. 109, 65 discussion christian knigge: your data on ai tri seems to show that the hard x-ray flux actually decreases as the accretion rate (and soft x-rays) increase. what is the interpretation of this? klaus reinsch: it is related to the response of the accretion shock height to the changing specific accretion rate. in addition, a higher fraction of inhomogeneous accretion events can suppress x-ray hard emission. the first observation of ai tri during an extremely soft state does not cover a full binary orbit. 80 introduction observed seds multi-wavelength modeling probing the accretion processes in x-rays conclusions acta polytechnica ctu proceedings doi:10.14311/app.2016.5.0047 acta polytechnica ctu proceedings 5:47–50, 2016 © czech technical university in prague, 2016 available online at http://ojs.cvut.cz/ojs/index.php/app current challenges for research activities in the field of railway infrastructure otto plasek brno university of technology, faculty of civil engineering, veveri 95, brno, czech republic correspondence: plasek.o@fce.vutbr.cz abstract. the increasing importance of railway systems, arising from both national and european strategic documents, leads to increasing demands on its infrastructure. transport development in the european territory is defined in the strategy summarized in the white paper, which understands the rail sector as a key sector in terms of sustainability of the transport development due to the suppression of congestions, carbon oxide emissions, reliability and safety of transport. the paper is aimed in the specific technical aspects of rail infrasructure, which emphasize their ability to contribute to an efficient and sustainable transport in europe and justify motivations for increasing the attractiveness of rail transport. current challenges for research activities in the field of railway infrastructure that will led to achievement of the objectives which were defined in the white paper and further specified in the particular requirements of rstakeholders in the railway industry are discussed. keywords: railway infrastructure, railway research, interoperability of railway infrastructure, noise and vibrations. 1. introduction the increasing importance of railway systems, arising from both national and european strategic documents, leads to increasing demands on its infrastructure. transport development in the european territory is defined in the strategy summarized in the "white paper roadmap to a single european transport area towards a competitive and resource efficient transport system", which was released by the european commission in 2011 [1]. this strategy defines the "roadmap to a single european transport area" with the aim of creating a competitive and efficient transport system. it is evident that an efficient transport systems are crucial for the competiveness of european enterprises in the global economy. this fact is supported by an argument that transport and freight storage costs are 10-15 % of the final products costs. every european household gives approximately 13.2 % of its budget on transport-related products and services. annual congestion costs in europe are approximately 1 % of gross domestic product. transport development strategy is based on several obvious assumptions. it is expected a continuous increasing of the oil price which should be at least twice in 2050 compared to 2005, while transport is dependent on oil for at least 96 %. greenhouse gas emission should be reduced 80 % bellow the level of 1990 to limit the global temperature change to 2c. alarming is also fact that exceeding of capacity limits for road and air transport requires annually 1 % additional costs in europe. the increase of freight transport up to 40 % in 2030 and up to 80% in 2050 is assumed. moreover, the infrastructure is developed unevenly in the eastern and western parts of european union. nowadays only about 4,800 km of motorways, no high speed railway lines and conventional railway lines often in poor conditions are in the new member states [2]. the white paper for transport understands the rail sector as a key sector in terms of sustainability of the transport development due to the suppression of traffic congestions, carbon oxide emissions, reliability and safety of transport. the necessity of transformation of rail transport to become more attractive and to increase its market share of passenger and freight transport for medium distance (up to 300 km) in 2050 is defined in this strategic plan. in this respect, the white paper puts a very ambitious goal of moving the road transport to rail or water transport in volume of 50 % [1]. the specific technical aspects of rail transport, which emphasize their ability to contribute to an efficient and sustainable transport in europe and justify motivations for increasing the attractiveness of rail transport are stated bellow in the paper. 2. current role of rail transport rail passenger transport in europe now plays an important role in long-distance transport (from 300 km) through especially high-speed trains [3]. the definition of high-speed services is not easy and involves a wide range factors which determine operator access to the passenger rail transport. nevertheless, it is usually considered the rail traffic at speeds higher than 230 km/h. the rail passenger services not quite successfully compete in individual transport for medium 47 http://dx.doi.org/10.14311/app.2016.5.0047 http://ojs.cvut.cz/ojs/index.php/app otto plasek acta polytechnica ctu proceedings distances up to 300 km so far, because the quality of service reflects considerable time losses when access to rail transport and while waiting for the connection or transfer. the exception is train connection in directions where there is no motorway available. that is why rail operators is really interested in the connection praha ostrava. it should be noted that in the mid-range distances will play a crucial role, yet unmentioned aspects of the definition of high-speed traffic, which express the quality of transport: • infrastructure (including civil works, tracks, overhead catenary systems, etc.) • comfortable railway stations (their location, functional design, equipment, etc.) • rolling stock (technically advanced, comfortable, modern design etc.) • operation (planning, management, regulations) • advanced command and control systems (etcs) • sophisticated strategy of maintenance • coherent financing, marketing and management the suburban transport in big cities or regional centers is another important area of the positive benefits of passenger rail transport. the inclusion of railway transport into integrated transport systems reflects this trend. rail freight transport currently plays an important role in the field of long distance, especially the transcontinental transport. the effectiveness of the railways in this regard improves the use of combined systems. however, it must be noted that the increase in the medium-range transport (mainly domestic) will always run into tough competition with road transport. 3. research activities in the field of railway infrastructure research, development and innovation will be focused on construction of high-speed lines, increasing speeds on existing rail lines, increasing reliability and durability of tracks with mixed railway traffic in the context of increasing service load and capacity. the current issue of interoperability of the railway infrastructure is taken into particular consideration. rail infrastructure is in the state that corresponds a history of its construction, maintenance, renovation and modernization. rail infrastructure must be considered as a very complex system, which consists of components that are of different technical, technological advancement and age. railway track consists both from earthworks such as embankments or cuts and from advanced railway superstructure both ballasted and ballastless, characterized by the controlled vertical stiffness of substructure and with resilient rail fastening. most of tracks are of different age and technological advancement due to the gradual replacement of the structural elements except completely renewed or modernized tracks. the main activities of research activities aimed in the development of rail infrastructure, both railway and urban lines are focused on requirements of speed increase, higher service and axle load, safety and security, ride comfort while meeting economic needs, environmental requirements, i.e. reduction of noise, suppression of vibrations spreading, especially in urban areas, also reducing costs of maintenance and repair works, reducing energy consumption and consumption of raw materials. calls for research projects both national and european correspond to the above specified requirements. the focus of the research activity in the czech republic is the preparation of technological solutions for the design and construction of high-speed railways. an important part of the process is development of decision-making strategies for choosing the most appropriate design solutions for high speed lines (eg. ballasted vs. ballastless track). research activities are focused on the preparation of new structural and technological solutions to meet the requirements of the railway superstructure and substructure of high-speed lines, especially on the structural design of switches and crossings. the attention must be paid to the particular design of the high speed structures as well as to encourage the readiness of domestic suppliers in railway industry and track work contractors. it is also important to pay attention to the management and maintenance systems of high speed lines, focusing on the question both of logistics and policy-warranty service and preventive maintenance in defined cycles. regarding railway substructure, earthworks were constructed usually in the 19th century. structural layers ensuring the strength of substructure corresponding to the increased axle load were constructed only for certain lines in the second half of the 20th century. at the turn of the millennium corridor lines have been modernized, where structural layers in the active zone of the embankment have been constructed or completely renewed. most sections outside the corridor lines does not comply with current requirements due to the age of the earthworks and their original design load. remedying this situation will require high costs and be associated with a number of construction works, which require service disturbance or interruption. most of the research is therefore concentrated on finding technologies, increasing strength of substructure especially for weak, compressible or unstable soils. the solutions are searched with help static and dynamic analyses and an optimization of the stiffness of the railway substructure, development of advanced mobile diagnostic methods and tools. solutions which help to minimize service disturbances and interruptions at the least possible cost of works and consumption of materials are looked for. railway superstructure was many times renewed in comparison with rail substructure in the same section. whenever rail superstructure was usually renewed with up to date technology. this means that except 48 vol. 5/2016 current challenges for research activities in the field of railway infrastructure figure 1. test section in plana nad luznici railway station under sleeper pads corridor lines railway superstructure is a mixture of components of different age and technology along the track. it can be state that currently used technology of ballasted tracks are comparable with ones used in high speed lines. design of switches and crossings (s&c) is the only exception. however, the domestic s&c manufacturer in its applied research activities intensively develops new technologies. an optimized geometry, the application of the elastic elements in order to suppress the influence of variable vertical stiffness along the length of the switch and crossing, development of resistant materials for crossings, operational systems of the new generation, trough sleepers, monitoring systems for switches and movable frogs are the subject of research activities. a certain barrier to the development is the fact that high speed switches crossings is not easy tested by high-speed traffic in the czech republic. rail infrastructure administration faces with the development of rail defects which are usually connected to the increase of axle load and service speed. this can be remedied by increasing the rail steel quality, but in the surface layer resisting to fatigue loading. thus new types of rail defects occur and appropriate preventive measures are looked for. the short pitch corrugation occurs in lower rail in tight curves which during wheelset passages besides vibrations causes also noise emission to the track vicinity. the only effective measure is the grinding or milling of the rails, which is very costly, requires service interruption and significantly reduces the life span of the rails. effective measures may consist in an additional elasticity of the track or modification of the friction coefficient at the wheel-rail contact. a significant part of the research work is aimed at the technology procedures and management of construction activities and maintenance work. next subject of research works in this field is the development of advanced technological processes, rules of guarantee service and preventive maintenance strategy. innovative and advanced technological procedures for figure 2. investigation of rail corrugation by the salamander device havlickuv brod maintenance and reconstruction of tracks, logistics issues, management and strategy of maintenance are developed. 4. environmental aspects of rail infrastructure outcomes of research activities lead to the development of such infrastructure design, which also contributes to the reduction of negative impacts on the track vicinity, while solutions are being sought especially economically efficient from a lifecycle. cost perspective. railway tracks mainly in urban areas are perceived as a significant source of noise and vibrations. with a growing number of rail vehicles is adversely affected track vicinity. this negative influences were worse if any measures would not be used. the negative effects in some cases led to an idea to shift of tracks away of city centre, which, however, caused more difficult access of passengers to rail transport. this approach has been modified but this requires significant suppression of noise emission and vibrations propagation. the noise caused by traffic not only adversely contributes to the emergence of various diseases. in principle, the noise generated by the rail transport can originate from wheel-rail contact, from vehicle engines and aggregates, which is significant for low service speed, and aerodynamic noise, which is significant for high service speed. the noise originated from wheelrail contact rolling, squeezing, impact is a common trouble for every rail vehicle which increases necessity to deal with this phenomenon. for this reason, not only an influence of parameters of rolling stock but also railway superstructure and substructure parameters and service conditions are investigated. vibrations that spread to building structures and building foundations are other loads. vibrations are induced by dynamic effects from the train movement across the track irregularities. characteristics of dynamic effects are related to number, weight and tech49 otto plasek acta polytechnica ctu proceedings figure 3. installation of noise barriers in usti nad orlici nical state of vehicles, service speed, running dynamic, track structure, track quality, track alignment, geological parameters, etc. preventive measures at the vibration source are most economical and most effective. if it is not possible to prevent structural vibrations at a source, then this source should be completely separated from adjacent structures. special elastic rail fastening, under sleeper pads, under ballast mats, mass-spring systems or ballastless track are used as measures in track structure for vibration mitigation. most types of these measures are available today as deliveries of recent applied research projects. higher initial costs, which are usually outweighed by non-financial benefits, are currently an obstacle for immediate application of measures preventing vibration propagation. 5. conclusions railway infrastructure administration put emphasize on research and development and implementation of research results into practice. it is considered as the fundamental way to increase the competitiveness of rail transport in the process of developing a sustainable and efficient transport. stakeholders (manufacturing companies, contractors, design companies, universities and research centres), which participate in the investment production and maintenance of the railway infrastructure in the czech republic, in the areas of infrastructure, energy, control, command and signalling, have jointed their efforts in the czech technology platform. its objective is the linkage of the scientific and technical potential of universities, research and project institutes together with the production potential of construction and manufacturing companies for the implementation of the following areas of activities [4]: • support of innovation and increase of competitiveness of the members of the association • structuring and support of implementation of development, research and testing projects ensuring the existing production of the members of the association with the requirements of the technical specifications of interoperability of the trans-european railway system in the sub-systems of infrastructure, energy, control, command and signalling • acquiring of financial funds for implementation of these projects • international (european) activities related to the creation of new regulations for construction, production and maintenance and related tests and evaluation of the railway industry a variety of projects supported by national funding agencies, particularly the technology agency of the czech republic, grant agency of the czech republic or by the ministry of industry and trade are solved nowadays. the project "centre for efficient and sustainable transport infrastructure -cesti" belongs among the most important projects, aimed in railway infrastructure. the ministry of education, youth and sports promotes the development of science and research projects through operational programs, an example is the project "interoperability of railway infrastructure competence network (iricon)". projects can be also supported from different european funding programmes. the leading european companies in railway industry joined efforts in a joint undertaking shift2rail within the eu framework programme for research and innovation ? horizon2020. the shift2rail has just opened the first calls for project of applied research for its members and in the form of opencalls to non-members, too. with respect to the current support of science and research projects at national and european level, with regard to available facilities of newly built research centres, excellent research teams, it can be assumed that research activities will achieve the objectives which were defined in the white paper and further specified in the particular requirements of railway companies, contractors and manufacturers in the railway industry. acknowledgements the article was processed under financial support of the project lo1408 "admas up advanced building materials, structures and technologies" supported by ministry of education, youth and sports within the "national programme for sustainability i". references [1] european commission. white paper on transport. towards a competitive and resource efficient transport system., 2011. http://www.transforum-project.eu/ transforum/white-paper-on-transport.html. [2] union internationale chemins de fer (uic). rail and sustainable development, 2001. http://www.uic.org. [3] union internationale chemins de fer (uic). high speed rail, fast track to sustainable mobility, 2009. http://www.uic.org. [4] www.sizi.cz, 2016. http://www.sizi.cz/introduction. 50 http://www.transforum-project.eu/transforum/white-paper-on-transport.html http://www.transforum-project.eu/transforum/white-paper-on-transport.html http://www.uic.org http://www.uic.org http://www.sizi.cz/introduction acta polytechnica ctu proceedings 5:47–50, 2016 1 introduction 2 current role of rail transport 3 research activities in the field of railway infrastructure 4 environmental aspects of rail infrastructure 5 conclusions acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2016.5.0059 acta polytechnica ctu proceedings 5:59–62, 2016 © czech technical university in prague, 2016 available online at http://ojs.cvut.cz/ojs/index.php/app the priority of international freight expresses in the overlapping section of rfc 7 and rfc 9 kolín–česká třebová přemysl šrámek∗, tatiana molková university of pardubice, jan perner transport faculty, department of transport technology and control, studentská 95, 532 10 pardubice, czech republic ∗ corresponding author: st24539@student.upce.cz abstract. this article deals with the railway traffic control and railway traffic equipment in terms of implementation of tsi. the main goal is to research the optimal priority of international freight expresses, especially in the overlapping sections, which represent critical line sections of european freight corridors. this article shows this problem in the context in the overlapping section of rfc 7 and rfc 9 kolín–česká třebová. there are mentioned possibilities of using of etcs and implementation of automatic train paths. keywords: capacity, freight expresses, freight corridors, overlapping section, periodic timetable, traffic control . 1. introduction one of the aims of european transport policy is to redirect capacity of road freight traffic to other modes of transport, where rail transport is an interesting and environmentally friendly alternative, especially in terms of speed, availability and amount of transported cargo. on the other hand, by customers is required delivery time is guaranteed by carriers, what is unfortunately not always possible, especially due to high utilization of european rail infrastructure. the solution is the construction of new lines, increasing the capacity of existing lines through construction and reconstruction measures or through operational measures. among the operational measures may be included alternative routing of trains as well as changing priorities of a particular type of train, e.g. train category fex (freight express). 2. aims the aim of this article is to find the optimal priority of international freight expresses to satisfy the customers required delivery times due to optimization of railway traffic control. unfortunately, in the czech republic is very problematic to buy up structural land for extension of railway infrastructure, e. g. for construction of another track line. the railway infrastructure of the overlapping section of rfc 7 and rfc 9 kolín – česká třebová is equipped with the new modern signalling system with etcs level 2. the possibility to improve the signalling system is only implementation etcs level 3 or implementation of automatic train paths. however, the easiest variant is to change the priority of freight expresses and observe the passenger transport impact. 3. materials and methods there are mentioned some information about european freight corridors rfc, leaflet uic 406, deadline cargo mode and the simulation method with using of simulation tool simut (average delay increment (adi) calculation). 3.1. rail freight corridors (rfc) on the basis of the european parliament and the eu council no. 913/2010 for competitive freight and no. 1316/2013, which is created the connecting europe facility (cef), is the gradual establishment of rail freight corridors rfc (rail freight corridors). the main aim of the operation of these corridors is to strengthen the competitiveness of railways synergies between rail systems and harmonization of allocation interstate freight routes by national infrastructure managers [1]. czech republic, respectively czech railway infrastructure administration (cria), as the infrastructure manager and allocator of capacity, is a member of rfc 5 (baltic – adriatic), rfc 7 (orient / east-mediterranean), rfc 8 (north sea – baltic) and rfc 9 (czechoslovak corridor). each corridor is operated on the basis of the corridor information document (cid); each corridor has a single point of contact (c oss) and allows carriers to apply international prearranged train paths facilitated freight via c-oss. the request must be filed by information system rne pcs and must include the cross-border section. this paper deals with the model study of common section of rfc 7 and rfc 9 kolín – česká třebová. 3.1.1. rfc 7 rfc 7 – the orient corridor runs from central europe to easternand southern-europe connecting 7 member states – czech republic, austria, slovak republic, 59 http://dx.doi.org/10.14311/app.2016.5.0059 http://ojs.cvut.cz/ojs/index.php/app přemysl šrámek, tatiana molková acta polytechnica ctu proceedings hungary, romania, bulgaria and greece. the total length of main lines is approx. 3 900 km and the length of alternative and connecting lines is almost 2 500 km altogether. most limiting factors of rfc 7 are low capacity, speed limit, limited length of trains, limited axle load, not electrified sections and lack of adequate safety equipment (signalling track circuits with 25 hz frequency, etcs, gsm-r, etc.). the railway infrastructure managers and capacity allocation companies responsible for establishing and running rfc 7 are committed to offer reliable, high-quality, competitive transport services in order to increase the market demand, to operate the infrastructure costeffectively on the long run through harmonization of technical and procedural conditions and to facilitate the environmentally sustainable development of the european economy and the achievement of a better quality of life for its people [2]. in capacity analysis, published in implementation plan of rfc 7 [2], there are found lines with capacity utilization higher than 90 %. the longest sections with this high capacity utilization are situated in the czech republic, concretely sections poříčany – pardubice (65 km) and choceň – česká třebová (25 km). the whole second section is in the model study, which results are published in this paper, included. the first section is included, too, without the section kolín – poříčany (20 km). 3.1.2. rfc 9 rfc 9 the czech-slovak rail freight corridor (cs corridor) runs from prague to čierna nad tisou (slovak-ukrainian border) connecting 2 member states czech republic and slovak republic. the total length of main lines is 972 km and the length of alternative and connecting lines is 276 km altogether. most limiting factors of rfc 9 are the same as in rfc 7. in capacity analysis, published in implementation plan of rfc 9 [3], there are found lines with capacity utilization higher than 90 %. 3.2. the uic 406 leaflet the uic 406 leaflet, completely overhauled in june 2013, is one of the most important documents about the capacity calculation. there is published the approach to calculate capacity consumption by compressing a timetable and to evaluate the number of possible train path to a node, line or corridor. the compression method consists of five steps – defining infrastructure and timetable boundaries, defining section for evaluation, calculation capacity consumption, evaluating capacity consumption and evaluation available capacity. corridor is in the uic 406 defined as the main international and national connection and thus usually stretches over several hundred kilometres. corridors may overlap with one another overlapping section is illustrated on the figure 1 [4]. 3.3. the deadline cargo mode periodic schedules are in rail passenger transport commonplace. in order to satisfy also the end users of figure 1. overlapping train path line sections [4] rail freight and ensure its full competitiveness, it is necessary to trace freight trains in the fixed periodic routes, which would have removed one of the main rail transport disadvantages against road transport. to increase the transport speed and reduce the overall delivery time is also desirable international freight train paths were carried out accurately under deadline cargo for this purpose it may be necessary to change the current train priority so that international freight expresses (fex) weren’t in their paths delayed by stop passenger trains. this measure in addition to increasing the speed of transport and reducing the overall delivery time also contributes to reducing the overall energy efficiency of rail freight, which will no longer need to waste power electricity due to start heavy freight trains. 3.4. the simulation method in this part is said something about the simulation method in context of the transit of international freight express trains (fex) in the deadline cargo mode in the overlapping section of rfc 7 and rfc 9 kolín česká třebová. in this section there is a total amount of 14 railway stations, in which it is possible overtaking trains (double track line). interstation sections are divided by an automatic block into track sections; all crossing safety devices are equipped with gates. for simulation was used simulation tool simut, developed by pavel krýže, phd. from cria. the simulation tool simut is developed in visual basic, therefore it can run on every pc with microsoft office. it has to be input the option of a simulation, then railway stations and their shortcuts, station tracks for each railway station, line tracks, connection of station and line tracks, length of interstation departments and amount of line departments, type of train for priority, type of train for each number of a train and the path of each train. the simulation program simut can put new paths in a daily timetable with the solution for arisen path conflicts. it was created the daily timetable, which 60 vol. 5/2016 the priority of international freight expresses variant adi long distance passenger transport regional passenger transport fex total low priority 5,40 10,17 5,12 5,87 d1 priority 7,22 19,35 -5,63 4,03 high priority 7,67 18,57 -8,49 3,14 table 1. adi for different variants of priority of fex trains. included the amount of 384 trains, concretely 160 express trains, 34 speed-up passenger trains, 46 stop passenger trains and 144 fex trains. the passenger transport trains were concentrated especially from 6 am to 23 pm. all trains were conducted in periodic timetable. within the simulation program was established the average delay increment (adi). the average delay increment was calculated by dividing the difference between total output and total input delay and the total number of trains. this indicator was calculated as an ongoing basis for each simulation run, so the total for the entire graph (all simulation runs). the indicator was also calculated for different types of transport, i.e. for long-distance passenger transport, regional passenger transport and freight transport. in order to achieve measurable results of transiting fex within rfc in the deadline cargo mode, the freight transport segment was narrowed to only those trains, for which it was created recurring schedule, counting 3 pairs of trains every hour (the period 20 min) on the prescribed speed of 100 km/h (total amount of 144 trains). in the real operation the other freight trains shouldn’t delay the trains of fex category. three variants were created, reflecting the priority of fex. the first variant corresponded more or less simultaneous the operation in the czech republic, when international freight trains of the fex category were delayed by all passenger trains including stop passenger trains. in the second variant there was set priority of trains, corresponding to the transport prescription d1 of cria [5], when the fex trains overtook stop passenger trains. and in last, the third variant, received international freight trains of the fex category the second highest priority, not just for passenger trains category ex (expresses). as part of the simulation was set for all simulation runs random entry delay based on the exponential probability distribution. there were solved conflicts of station tracks, freight trains were allowed to ride before their schedule time (in the case of free capacity). for each variant was made a total of 365 runs of simulation (for a daily timetable) [6]. 4. results and discussion depending on the other train categories (multicriteria decision model – stop passenger train has got coefficient 1) it was set low priority of fex trains (0,5), priority of trains by transport prescription d1 made figure 2. average delay increment for different variants of fex train priority. by cria (1,7) and high priority of fex trains (1,9) – results are displayed in table 1. for each run of simulation had to be done 34 500 solutions by simulation program simut, it meant in total to count for 365 runs of simulation 12 592 500 solutions for each variant for three different variants had to be counted 37 777 500 solutions of daily timetable with random entry delay based on the exponential probability distribution. the most important is the total average delay increment this indicator is getting lower with the increasing priority of fex trains. in this research we are getting back to the beginning of railways in the territory of czech republic to years, when cargo transport was preferred. in the growing amount of fex trains, which are operated on rfc, it seems quite logical to overtake some passenger transport trains by fex trains to balance the railway transport. for cria is this model convenient because of higher fee payed for using of the railway infrastructure (which is generated by fex trains). and especially the high priority of fex trains fulfil at most the terms of deadline cargo mode, which is preferred by customers. the results from table are illustrated on figure 2. to explain more the results presented on the figure 2, it must be said the adi is for fex trains negative (d1 and high priority) due to allowing of riding before their schedule time. the schedule was created and new paths were included on the basis of simultaneous operation (low priority). therefore, if the fex train could overtake in other variants the train of another category, it did it and then the average delay increment of fex was negative, on the other hand the adi 61 přemysl šrámek, tatiana molková acta polytechnica ctu proceedings of the other train category increased. from the table 1 is apparent, too, all train categories couldn’t be conducted in the overlapping section of rfc 7 and rfc 9 kolín česká třebová in periodic timetable. if will be possible to conduct in periodic timetable the fex trains, it is inappropriate to conduct the same way the other train categories due to increasing adi. the solution could be to adjust paths of stop passenger trains – e.g. incorporate some deviation of periodic timetable. in long distance passenger transport the operation could be solved by consolidation of expresses, because nowadays short passenger expresses (locomotive + 3 wagons) often cause the stopping of heavy fex trains due to overtaking. if the passenger express will have more wagons and to final directions will be divided in intermediate station, it resists more capacity for fex and stop passenger trains. in the section kolín česká třebová is this rule very good applicable – present expresses could go together in one longer express train from prague to česká třebová, where will be divided to directions brno and ostrava. in the context of implementation of tsi-taf in the overlapping section, it means operation of etcs level 2 with full-functional gsm-r, it is offered the extension to etcs level 3 or the implementation of automatic train paths on etcs level 2. both variants enable to improve the railway traffic control technology and to reduce the amount of human interventions to do the railway traffic safer and well-timed. 5. conclusions to boost the attractiveness of railway freight transport it has to be kept customers required delivery times. to keep the customers required delivery times railway freight transport has to be operated in deadline cargo mode. to operate the railway freight transport in deadline cargo mode it has to be guaranteed train path stability. to guarantee the train path stability for freight trains it has to be increased priority of freight trains (expresses). in the overlapping section of rfc 7 and rfc 9 kolín česká třebová there could be generated free capacity due to expresses consolidation (passenger transport), too. acknowledgements this paper is supported by sgs_2016_008. references [1] čech r., šlachtová m. železniční nákladní koridory rfc. železniční magazín czech raildays, m-presse plus, s.r.o. issn 1212-1851, praha, 2015. [2] rfc 7 orient corridor : implementation plan of rail freight corridor 7 orient corridor [online]. [2015-11-20], http://www.rfc7.eu/ckfinder/ userfiles/files/rfc%207%20cid%20all%20books/ rfc%207%20cid%20book%205%20implementation% 20plan%202015-02-16%20approved.pdf. [3] rfc 9 corridor : implementation plan of rail freight corridor 9 [online]. [2015-11-23], http://www. szdc.cz/rfc9/soubory/knihy/05-cid-kniha5.pdf. [4] international union of railways (uic). uic code 406, 2nd edition. paris: uic, 2013. isbn 978-2-7461-2159-1. [5] sždc d1 dopravní a návěstní předpis. praha: sždc, s. o., 321 s., účinnost od 1. 7. 2013. [6] p. šrámek. kapacitní možnosti trati 031 v úseku pardubice hl. n. – hradec králové hl. n. astrophys j pp. 168–178, 1/2015. http: //pernerscontacts.upce.cz/38_2015/sramek.pdf. 62 http://www.rfc7.eu/ckfinder/userfiles/files/rfc%207%20cid%20all%20books/rfc%207%20cid%20book%205%20implementation%20plan%202015-02-16%20approved.pdf http://www.rfc7.eu/ckfinder/userfiles/files/rfc%207%20cid%20all%20books/rfc%207%20cid%20book%205%20implementation%20plan%202015-02-16%20approved.pdf http://www.rfc7.eu/ckfinder/userfiles/files/rfc%207%20cid%20all%20books/rfc%207%20cid%20book%205%20implementation%20plan%202015-02-16%20approved.pdf http://www.rfc7.eu/ckfinder/userfiles/files/rfc%207%20cid%20all%20books/rfc%207%20cid%20book%205%20implementation%20plan%202015-02-16%20approved.pdf http://www.szdc.cz/rfc9/soubory/knihy/05-cid-kniha5.pdf http://www.szdc.cz/rfc9/soubory/knihy/05-cid-kniha5.pdf http://pernerscontacts.upce.cz/38_2015/sramek.pdf http://pernerscontacts.upce.cz/38_2015/sramek.pdf acta polytechnica ctu proceedings 5:59–62, 2016 1 introduction 2 aims 3 materials and methods 3.1 rail freight corridors (rfc) 3.1.1 rfc 7 3.1.2 rfc 9 3.2 the uic 406 leaflet 3.3 the deadline cargo mode 3.4 the simulation method 4 results and discussion 5 conclusions acknowledgements references 277 acta polytechnica ctu proceedings 2(1): 277–281, 2015 277 doi: 10.14311/app.2015.02.0277 what powers the 2006 outburst of the symbiotic star bf cygni? a. skopal1, m. sekeráš1, n. a. tomov2, m. t. tomova2, t. n. tarasova3, m. wolf4 1astronomical institute of the slovak academy of sciences, 059 60 tatranská lomnica, slovakia 2institute of astronomy and nao, bulgarian academy of sciences, 4700 smolyan, bulgaria 3crimean astrophysical observatory, 298409 nauchny, crimea, russia 4astronomical institute, charles university prague, 180 00 praha 8, v holešovičkách 2, czech republic corresponding author: skopal@ta3.sk abstract bf cygni is a classical symbiotic binary. its optical light curve occasionally shows outbursts of the z and-type, whose nature is not well understood. during the 2006 august, bf cyg underwent the recent outburst, and continues its active phase to the present. the aim of this contribution is to determine the fundamental parameters of the hot component in the binary during the active phase. for this purpose we used a highand low-resolution optical spectroscopy and the multicolour ubv rcic photometry. our photometric monitoring revealed that a high level of the star’s brightness lasts for unusually long time of > 7 years. a sharp violet-shifted absorption component and broad emission wings in the hα profile developed during the whole active phase. from 2009, our spectra revealed a bipolar ejection from the white dwarf (wd). modelling the spectral energy distribution (sed) of the low-resolution spectra showed simultaneous presence of a warm (< 10 000 k) disk-like pseudophotosphere and a strong nebular component of radiation (emission measure of ∼ 1061 cm−3). the luminosity of the hot active object was estimated to > 5−8×103 l�. such high luminosity, sustained for the time of years, can be understood as a result of an enhanced transient accretion rate throughout a large disk, leading also to formation of collimated ejection from the wd. keywords: binaries: symbiotic optical spectroscopy photometry individual: bf cyg. 1 introduction symbiotic stars (sss) are the largest interacting binary systems with known orbital periods in order of years. they consist of a cool giant and a wd accreting from the giant’s wind. accretion process heats up the wd to 1 − 2 × 105 k and makes it as luminous as a few times 103 l�, whose photons ionize a large fraction of the neutral giant’s wind, giving rise to nebular emission. as a result the spectrum of sss consists of three basic components of radiation – two stellar and one nebular. if a symbiotic system releases its energy approximately at a constant rate and the temperature, it conforms the so-called quiescent phase. the stage, when the system brightens up in the optical by a few magnitudes and/or shows signatures of a mass-outflow, is named an active phase. bf cyg is an eclipsing symbiotic binary with an orbital period of 757.2 d (e.g. pucinskas 1970; fekel et al. 2001). the binary consists of a late-type m5 iii giant (mürset & schmid, 1999) and a hot luminous compact object (mikolajewska et al., 1989). its eclipsing nature was revealed by optical photometry of skopal (1992). historical light curve of bf cyg is characterized by a aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa slow, symbiotic-nova-type outburst (1895–1960), with superposed eruptions of the z and-type (1920, 1989, 2006) and short-term flares (e.g. skopal et al., 1997; leibowitz & formiggini, 2006). the last, 1989 z andtype eruption was described by cassatella et al. (1992), skopal et al. (1997). the uv/optical continuum cooled to ∼ 20 000 k, and its source expanded to ∼ 7r� having a luminosity of ∼ 10 000l�. (see also skopal, 2005). the line spectrum showed violet-shifted absorption lines indicating a mass outflow from the hot component at 100–500 km s−1. the recent, 2006 outburst of bf cyg was first reported by munari et al. (2006). spectroscopic observations by sitko et al. (2006), iijima (2006) and mckeever et al. (2011) indicated appearance of strong p-cyg type of hi, hei line profiles from the beginning of the outburst. photometric observations indicated a high level of the star’s brightness for much longer time than was observed for the previous, 1989-92, active phase (siviero et al. 2012; skopal et al., 2012). recently, skopal et al. (2013) reported an evidence of highly-collimated bipolar ejection from bf cyg. 277 http://dx.doi.org/10.14311/app.2015.02.0277 a. skopal et al. 9 10 11 12 13 47000 48000 49000 50000 51000 52000 53000 54000 55000 56000 1990 1995 2000 2005 2010 m ag ni tu de julian date 2 400 000 bf cyg q u i e s c e n t p h a s e a c t i v e p h a s eact. phase v b u figure 1: the ubv light curves of bf cyg from 1986 to the present. they cover the last, 1989-93, and the present, 2006-13, active phases. during the quiescent phase (∼ 1994 − 2006), the star was by 2–3 mag fainter, being characterized with the wave-like orbitally-related variation. in this contribution we analyze our optical spectroscopy and ubv rcic photometry from the current active phase of bf cyg, with the aim to determine fundamental parameters of the hot component. we point the problem of its high luminosity, which sustains for a long time of years. 2 observations broad-band photoelectric ubv and ccd ubv rcic photometry of bf cyg was carried out by 0.6-m telescopes at the skalnaté pleso and stará lesná observatories of astronomical institute of the slovak academy of sciences (see skopal et al. 2012 for details). the data are plotted in fig. 1. the high-resolution spectroscopy was carried out by the single dispersion slit spectrograph mounted at the coudé focus of the 2-m rcc telescope of the rozhen national astronomical observatory and at the ondřejov observatory. the low-resolution (r ∼ 1000) spectroscopic observations were secured by the 2.6-m shajn telescope, operated by the crimean astrophysical observatory. spectroscopic observations were dereddened with eb−v = 0.35 and the resulting parameters were scaled to a distance of 3.8 kpc (e.g. skopal, 2005). 3 analysis and results 3.1 modelling the sed in the optical assuming that the optical continuum consists of the three basic radiative components of radiation (see sect. 1), the resulting flux in the continuum, f (λ), can be expressed as their superposition, f (λ) = fwd(λ) + fn(λ) + fg(λ), (1) where fwd(λ) is the flux from the wd’s pseudophotosphere, fn(λ) is the flux from thermal plasma and fg(λ) represents the flux from the giant. for effective temperatures, t eff wd ∼ 5 000− 10 000 k, an atmospheric model, fλ(t eff wd), is needed to fit the radiation of the warm pseudophotosphere. otherwise, a simple blackbody radiation is satisfactory. the nebular radiation in the continuum can be approximated by processes of recombination and thermal bremsstrahlung in the hydrogen/helium plasma for case b. finally, radiation from the giant is represented by an appropriate synthetic spectrum, fλ(t eff g ). then eq. (1) can be expressed as, f (λ) = θ2 wdfλ(t eff wd) + knελ(te) + θ2 gfλ(t eff g ), (2) where θwd = rwd/d and θg = rg/d are angular radii of the wd pseudophotosphere and the giant, respectively. the factor kn (= the observed emission measure in cm−5) scales the volume emission coefficient ελ(te) of the nebular continuum to observations. constant electron temperature, te, throughout the nebula is assumed. physical parameters of the model spectrum (2), θwd, θg, t eff wd, t eff g , kn and te, are given by the solution of eq. (2), which corresponds to a minimum of the reduced χ2 function. the sed-fitting analysis was described by skopal (2005) and skopal et al., (2011). 3.2 physical parameters example of a low-resolution (3400 – 7000 å) spectrum, taken around a brightness maximum (23/10/2008), is depicted in fig. 2. the model sed shows that the spectrum is dominated by the radiation of the warm wd pseudophotosphere (denoted as the warm stellar component (wsc) by skopal et al., 2011) and the nebular continuum. the light from the giant becomes more significant for λ > 6 600 å. 278 what powers the 2006 outburst of the symbiotic star bf cygni? the wsc is produced by a source with t eff wd ∼ 8 500 k, the effective radius of ∼ 25r� and the luminosity of ∼ 3000l�. the nebular component was characterized with a high emission measure of em = 4πd2 × kn ∼ 2.6 × 1061(d/3.8 kpc)2 cm−3, radiated at te ∼ 30 000 k, which correspond to the luminosity ln ∼ 5100l�. thus the lower limit of the total hot component luminosity was ∼ 8 100l�, because only a fraction of the burning wd radiation can be converted to the wsc and the nebular emission. 0 1 2 3.6 3.7 3.8 3.9 lo g( fl ux ) [e rg c m -2 s -1 a -1 ] + 1 3 log(wavelength) [a] bf cyg23/10/2008 h ei h α h β h γ h δ h ei obs. fg(λ) fwd(λ) fn(λ) sed figure 2: an example of the low-resolution spectrum (gray line) and its model (heavy solid line) taken during the 2006-13 active phase of bf cyg, on 23/10/2008. the model sed and its components of radiation here represent a graphic form of eq. (1) with the same denotation in keys. 3.3 a disk-like shape of the wd pseudophotosphere the shape the wd pseudophotosphere cannot be spherical, because of the simultaneous presence of the strong nebular emission in the spectrum. if it were a sphere, its radiation would not be capable of giving rise to the observed nebular emission. on the other hand, the presence of the strong nebular emission in the spectrum constrains the presence of a hot ionizing source in the system. this type of the spectrum (called as twotemperature type) suggests that the wd pseudophotosphere has a form of a disk. when viewing the disk under a high inclination, its outer rim simulates the warm photosphere (producing the wsc), while the material above/below the disk is ionized by the hot central source and thus converts a fraction of its radiation to the nebular emission (see skopal 2005 and skopal et al. 2011 in detail). 3.4 collimated mass ejection figure 3 shows evolution of the hα profiles from the 2006 august eruption to 2013 april. the broad wings expanding to ∼ ±2000 km s−1 in hα were present in all spectra. significant variations were observed mainly at/around the line cores. (i) during the whole active phase, a sharp absorption developed on the blue side of the profile. (ii) during 2009, additional satellite emission components appeared at the position of a few times ±100 km s−1 to the hα emission core. (iii) during 2012 september, the satellite components were placed nearly symmetrically with respect to the hα central emission. 1 2 3 4 5 6 7 -2000 -1000 0 1000 2000 lo g( fl ux ) + c on st . heliocentric radial velocity [km s-1] 30/08/2006 05/06/2009 02/09/2012 bf cyg hα dd/mm/yyyy +5.0 +4.1 +2.5 +1.0 const. 03/11/2012 [n ii] f ei i 24/04/2013 figure 3: evolution in the hα line profile along the outburst. the filled curves represent the jet emission components (sect. 3.4). fluxes are in 10−13 erg cm−2 s−1 å−1. their radial velocities of ± ∼ 370 km s−1 and fluxes of ∼ 1.4× 10−11 erg cm−2 s−1 were around a maximum (see skopal et al., 2013 in detail). (iv) the presence of satellite components and their properties were unstable in the spectrum. during two months after their 279 a. skopal et al. best pronounced stage (on 02/09/2012), they practically disappeared on 2012 november 3rd. however, in 2013 april, they re-appeared again (fig. 3). the relatively small width of the (well measured) satellite components (fwhm ∼ 245 km s−1) and their radial velocities suggest that these emissions were produced by radiation of a highly collimated ejection by the central star. 4 concluding remarks according to the elements of the spectoscopic orbit (fekel et al., 2001), the mass of the wd in bf cyg is as low as ∼ 0.55 − 0.6m�. during the quiescent phase, the luminosity of the hot component was estimated to ∼ 10 000l� for d = 3.8 kpc (e.g. mikolajewska et al., 1989). this quantity suggests that the source of such the energy output is caused by a stable hydrogen burning on the wd surface at the accretion rate of ∼ 1.4×10−7m� yr−1 for the 0.55−0.6m� wd (e.g. shen & bildsten, 2007). during active phase, the luminosity of the hot component can be > 10 000l� (e.g. cassatella et al., 1992), however, with difficulties of its precise determination as mentioned in sect. 3.2. in addition, (i) a significant extension and thus cooling of the wd pseudophotosphere is indicated by modelling the sed (sect. 3.2), (ii) an enhanced mass-loss rate from the wd is evidenced by the broad hα wings with a violet-shifted absorption component, and (iii) an enhancement of the accretion rate onto the wd is required by the satellite emission components. the presence of bipolar jets confirms the presence of a disk around the accretor during the outburst. these observational properties are consistent with evolution of burning wds in the h-r diagram, when the accretion rate increases above the stable burning regime. the accretion at ≈ 2 × 10−7m� yr−1 throughout the disk during the outburst can sustain the high luminosity of the burning wd at ≈ 10 000l� (see fig. 2 of shen & bildsten, 2007). the case of the current bf cyg active phase lead us to a speculation that the simultaneous presence of the enhanced mass outflow and mass infall during some active phases of sss can reflect a new type of the accretion process, which can sustain a high luminosity of their hot components for a long time of years. similar properties with mass outflow/infall and jets were also observed during the 1977–1984 active phase of ch cyg (e.g. skopal et al. 2002). it is of interest to note that the enhanced mass outflow, sometimes followed with jet-like components, and emergence of a warm pseudophotosphere simulated by the irradiated disk are also observed during optical high states of supersoft x-ray sources (e.g. southwell et al. 1996; hutchings et al. 2002; hachisu & kato, 2003). acknowledgement this research was supported by the slovak-bulgarian research and development cooperation project skbg-0015-10, by the grant bstc no. 01/14 bulgariaslovakia, by the grant do 02-85 of bulgarian scientific research fund, by the research program msm0021620860 of the ministry of education of the czech republic and by a grant of the slovak academy of sciences vega no. 2/0002/13. references [1] cassatella a., et al.: 1992, a&a 258, 368 [2] fekel, f. c., et al.: 2001, aj 121, 2219 [3] hachisu, i., kato, m.: 2003, apj, 588, 1003 doi:10.1086/374303 [4] hutchings et al.: 2002, aj, 124, 2833 [5] iijima, t.: 2006, cbet no. 633 [6] leibowitz, e. m., formiggini, l.: 2006, mnras 366, 675 doi:10.1111/j.1365-2966.2005.09895.x doi:10.1086/662076 [7] mckeever, j., et al.: 2011, pasp 123, 1062 [8] mikolajewska, j., et al.: 1989, aj 98, 1427 [9] munari, u., et al.: 2006, cbet no. 596 [10] mürset, u., schmid, h. m.: 1999, a&as 137, 473 doi:10.1086/513457 [11] pucinskas, a.: 1970, bull. vilnius univ. astron. obs. no. 27, 24 [12] shen, k. j., bildsten, l.: 2007, apj 660, 1444 [13] sitko, m. l., et al.: 2006, iauc no. 8746 [14] siviero, a. et al.: 2012, baltic astron. 21, 188 [15] skopal a.: 1992, ibvs no. 3780 doi:10.1093/mnras/292.3.703 [16] skopal, a.: 2005, a&a 440, 995 doi:10.1046/j.1365-8711.2002.05715.x [17] skopal, a., et al.: 1997, mnras 292, 703 [18] skopal, a., et al.: 2002, mnras 335, 1109 doi:10.1002/asna.201111655 [19] skopal, a., et al.: 2011, a&a 536, id. a27 [20] skopal, a., et al.: 2012, astron. nachr. 333, 242 doi:10.1086/177931 280 http://dx.doi.org/10.1086/374303 http://dx.doi.org/10.1111/j.1365-2966.2005.09895.x http://dx.doi.org/10.1086/662076 http://dx.doi.org/10.1086/513457 http://dx.doi.org/10.1093/mnras/292.3.703 http://dx.doi.org/10.1046/j.1365-8711.2002.05715.x http://dx.doi.org/10.1002/asna.201111655 http://dx.doi.org/10.1086/177931 what powers the 2006 outburst of the symbiotic star bf cygni? [21] skopal, a., et al.: 2013, a&a 551, id. l10 [22] southwell et al.: 1996, apj, 470, 1065 discussion dmitry bisikalo: if you have a wind from the wd during the quiescence, how can you accumulate matter to form a huge accretion disk? augustin skopal: the presence of a wind from the wd is indicated even during quiescent phase by, for example, the broad hα wings. the presence of a large disk-like formation during active phases is observationally confirmed. however, its creation is not well understood yet. 281 introduction observations analysis and results modelling the sed in the optical physical parameters a disk-like shape of the wd pseudophotosphere collimated mass ejection concluding remarks acta polytechnica ctu proceedings doi:10.14311/app.2017.13.0020 acta polytechnica ctu proceedings 13:20–28, 2017 © czech technical university in prague, 2017 available online at http://ojs.cvut.cz/ojs/index.php/app comparison of methods used for seismic analysis of structures petr čada∗, jiří máca czech technical university in prague, faculty of civil engineering, thákurova 7, 166 29 prague 6, czech republic ∗ corresponding author: petr.cada@fsv.cvut.cz abstract. this paper investigates effects of the seismic load to a structure. the article describes main methods of the definition and practical application of the seismic load based on the standard eurocode 8. there was made a comparison of all methods using the same structure. a simple two-storeyed concrete 2d-frame with fixed joints was chosen. a one another model with rigid beams for some calculations was defined. the second model can be used for hand-calculations as a cantilever with two masses. the paper describes main dynamic properties of the chosen structure. seismic load was defined by lateral force method, modal response spectrum, non-linear time-history analysis and pushover analysis. the time-history analysis is represented by accelerograms. there were made linear and non-linear calculations. keywords: seismic response, lateral force method, response spectrum, time-history analysis, pushover analysis. 1. introduction for a calculation of the seismic response, the linear state of stress is commonly used. by complicated cases or by higher importance of the structure is recommended to use some non-linear method. in eurocode 8 (ec8) [1] these four methods of analysis are defined: • lateral force method, • modal response spectrum analysis, • non-linear time-history (dynamic) analysis, • non-linear static (pushover) analysis. for the better results of the behaviour of the structure, there is necessary to use one of the non-linear methods. the ductility is the most important property for a non-linear calculation. the ductility describes the behaviour of the plastic deformations, which cause irreversible deformations and this fact can be used for the reduction of the seismic load. calculations in this article describe the possibilities of the ec8 [1]. for this purpose, a two-storeyed concrete frame with a uniformly distributed load was used. this simple structure was chosen because of the possibility to applicate all methods. only a simple model can fulfil all conditions of all methods. lateral force method is a static and linear (linear behaviour of the material) calculation using horizontal forces as a seismic load. by this method a high frequency cannot have the decisive influence. the base shear force can be defined for each direction. distribution of horizontal forces is linear increasing or responds to the eigenvalues. the each storey must be rigid in its plane [1]. modal response spectrum analysis is a dynamic method with the same geometrical rules as by the lateral force method. for the horizontal displacement, all decisive eigenmodes should be used, but at least 90 % of the total mass. responses from different directions can be combined by the srss (square root of the sum of the squares) or cqc (complete quadric combination) method. this is possible only when eigenmodes are mutually independent [1]. worked examples for lateral force method and modal response spectrum are shown in [2]. non-linear time-history (dynamic) analysis is based on direct numerical integration of the differential equation of motion. motion of the base is represented by the accelerogram [1]. this method is described in [3, 4] as well. non-linear static (pushover) analysis is the last type of the seismic calculations from the ec8 [1]. in this standard, the most used version – n2 method – is used. the name was derived from “nonlinear” for “n” and “2” for two mathematical models. method was developed by prof. peter fajfar. three steps of analysis are defined. the first step is used for determination of the stiffness, strength and ductility of the structure. the first mathematical model – mdof (multiple degree of freedom) system – with linear increasing horizontal load is used. for the structure, an equivalent sdof (single degree of freedom) system in the second step must be defined. the behaviour must be same as by the mdof system. the ratio between base shear force and the top-displacement is the base for the non-linear characteristics. the maximal displacement is calculated in the third step by the sdof system. the pushover analysis is regularly used for the first eigenmode (the first eigenmode must 20 http://dx.doi.org/10.14311/app.2017.13.0020 http://ojs.cvut.cz/ojs/index.php/app vol. 13/2017 comparison of methods used for seismic analysis of structures be decisive), but theoretically is it possible to use this method for any eigenmod [5]. for example [6–9] are dealing with this issue as well. 2. structure and load 2.1. model of the structure and material parameters for sample-calculations of the response to the seismic load, a simple two-storeyed concrete frame with fixed joints was chosen. the width is 6 m and the height is 4 m for one storey. the total height is 8 m. the frame consists of two cross-sections. columns are square cross-section of the dimension 40×40 cm. the reinforcement is distributed symmetrical to all sides and the bar diameter is ø20. cross-beams are rectangular cross-section of the dimension 40×60 cm and the reinforcement is placed only at the bottom and on the top of cross-section. it is used 3ø20 for one side. the shear-reinforcement is not considered. figure 1. 2d-model of the construction. figure 2. cross-sections. concrete strength class is c30/37 and the reinforcement strength class is b500b. main material parameters for linear and non-linear behaviour were defined according to [10], chapter 3.1 and 3.2. the most important parameters for the non-linear analysis of the concrete are compressive strength fck =30 mpa, tension strength (5% fractile) fctk0,05 =2.0 mpa and design strain εcd =0.077 ‰. for the reinforcement are the most important yield strength fyk = ftk =500 mpa and strain εyk =2.5 ‰. 2.2. load 2.2.1. vertical load the frame is loaded by the self-weight (6 kn/m for the cross-beam and 4 kn/m for the column) and a uniformly distributed load 10 kn/m, which represents fittings and imposed loads together. for the simplification, only one load for one combination was defined. safety factors are not considered as well. figure 3. vertical load. 2.2.2. seismic load seismic load was represented by horizontal elastic response spectrum, which is defined in [1], chapter 3.2.2.2. initial conditions were chosen randomly, because the structure is fictional. the design peak ground acceleration (pga) is 0.8 m/s, that is 0.0815g. the response spectrum is type 1, which corresponds in the czech republic to the moravia and silesia. the frame is based in the ground type b, which corresponds to the very dense sand, gravel, or very stiff clay, with the average shear wave velocity vs,30 =360 m/s. the importance class of the structure is class ii, which describes ordinary buildings with the importance factor γ,i = 1.0. because the comparison has to follow the original response of the structure, it was necessary to use the 21 petr čada, jiří máca acta polytechnica ctu proceedings figure 4. horizontal elastic response spectrum. behaviour factor q=1. with the behaviour factor q=1, it cannot to occur to an undesirable dissipation of the energy and to the change of the result. the effective damping (ξ) follows the recommendation of the standard [1] and it is ξ=5 %, which implies that the damping correction factor is equal to 1 (η=1). 2.3. software for following calculations was necessary to use some software: • for the definition of the accelerogram, a programm seismoartif [11] was used. seismoartif [11] is made by the company seismosoft (pavia, italy). this programm allows a conversion of the response spectrum to the one or more accelerograms. accelerograms are used for the time-history analysis. • for calculations with accelerograms, a programm seismostruct [12] was used. this programm is made by seismosoft as well. seismostruct [12] uses the finite element method (fem) and can use material and geometrical nonlinearities of the structure. seismostruct [12] was used for calculations of the eigenvalues, eigenmodes and seismic response. • for the pushover analysis, a programm stab2dnl [13] was used. this programm is made by dr. uwe pfeiffer from university in hamburg. stab2d-nl [13] can compute 2d frames with the finite element method including material and geometrical nonlinearities. this software can consider the third-order-analysis as well. 3. dynamic properties of the structure 3.1. eigenmodes for the first rough estimation of eigenmodes, a simplified method which neglects all deformations of the cross-beam can be used. that means the crossbeam has an infinite stiffness. this simplification allows to analyse the structure as a system with two degrees of freedom. computational model is now a cantilever with two masses. the first eigenfrequency is f0,1 =6.62 hz and the second eigenfrequency is f0,2 =16.87 hz. eigenmodes were normalized to the roof level. the first eigenmode is defined as φ1 =(0.632;1)t and the second mode is φ2 =(-1.383;1)t. figure 5 shows the first and the second eigenmodes. figure 5. the first and the second eigenmode. 3.2. eigenmodes – fem (1.) frame with flexible beams (2.) frame with rigid beams for following calculations, an infinitely stiff model was chosen. this effect was defined with a very high 22 vol. 13/2017 comparison of methods used for seismic analysis of structures eigenmode 1 2 eigenfrequency [hz] 5.830 15.853 eigenperiod [s] 0.1715 0.0631 effective modal mass wxi/wx,tot [%] 92.50 7.49 σ 99.99 table 1. eigenvalue properties of the pliable stiff cross-beam. stiffness of the cross-beam, specifically with the enlargement of the e-modulus in factor 40. from this moment are eigenvalues almost constant. eigenmode 1 2 eigenfrequency [hz] 6.749 16.870 eigenperiod [s] 0.1482 0.0593 effective modal mass wxi/wx,tot [%] 95.16 4.83 σ 99.99 table 2. eigenvalue properties of the infinitely stiff cross-beam. 4. lateral force method of analysis this method is a static analysis using linear behaviour of the material, where horizontal forces represent the seismic load act to the joints of the structure. it is possible to use this method only for structures with small relevant eigenmodes. this method is used for a direct design of the cross-section, mostly from concrete and steel [14]. for this calculation, a proceeding according to [1], chapter 4.3.3.2 was used. the most important results are the design spectrum sd(t 1) =2.4 m/s2 and the base shear force fb =57.6 kn. these values approach to horizontal forces acting on all storeys. the force for the first storey is f1 =24.16 kn and for the roof level is f2 =33.44 kn. values φ1 and φ2 were assumed from the hand calculation in the chapter 3.1. final horizontal displacements are u1 =0.00109 m for the first storey and u2 =0.001724 m for the second one. 5. modal response spectrum analysis 5.1. hand calculation input values for the structure and the response spectrum are equivalent to calculations in chapters 2.2.2 and 3.1. in the first step, horizontal forces for the first eigenmode f1 =22.98 kn, f2 =31.78 kn and for the second eigenmode f1 = 4.32 kn, f2 =-2.73 kn were defined. in the second step, horizontal displacements for the first eigenmode u1 =0.001037 m, u2 =0.00003 m, and for the second eigenmode u1 =0.00003 m, u2 =0.000022 m were derived. from these results of the both eigenmodes, the total displacement was calculated. srss method was used for the combination of displacements and target results are u1 =0.001038 m a u2 =0.001639 m. results show that the effect of the second eigenmode is marginal. as a proof, a calculation of effective modal masses can be used – meff 1 =22.815 and meff 2 =1.184. 5.2. fem – linear horizontal displacements were analysed by the fem program with linear material behaviour of two versions: (1.) frame with flexible beams uk,max =0.00209002 m, uk,min = -0.00230263 m. (2.) frame with rigid beams uk,max =0.00157214 m, uk,min = -0.00158202 m. 6. time-history analysis the time-history analysis is another option, how the seismic response can be calculated. a model for this calculation is imposed by up to three different accelerograms which describe the behaviour for each direction. usually is this method used as non-linear analysis. to compare this method with two linear methods is necessary to use synthetic accelerograms. these accelerograms describe the response spectrum. ec8 [1] defines, that for the determination of the design seismic action, an average value from at least seven nonlinear calculations has to be used. by fever calculations, the worst result has to be chosen. calculation of motion equations is the main principle of this method and it is calculated by the modal analysis or by the direct integration. modal analysis allows the calculation with only linear material behaviour. with the direct integration is possible to use material non-linearities. 6.1. horizontal load this method is usually used for a calculation of any past known earthquake with defined accelerogram. for this comparison, accelerograms from the spectrum have to be defined. in fact, it is a reverse procedure. for the simplification, only one accelerogram was chosen. deformation calculated by this method has almost the same value as the result by the response spectrum method. the software seismoartif [11] was chosen to generate the optimal accelerogram. for the generation of synthetic accelerogram, a direct integration according to [15] (this method will be not described in this article) was used. 23 petr čada, jiří máca acta polytechnica ctu proceedings figure 6. acceleration. figure 7. velocity. figure 8. displacement. input values for the generation of synthetic accelerogram by programm seismoartif [11]: (1.) response spectrum according to chapter 2.2.2 (2.) earthquake parameters • inter-plate regime of far-field • moment magnitude – 5 • distance between the main event and the station – 5 km • soil type – generic soil (vs,30 =310 km/s) (3.) generation of the synthetic accelerogram • smallest period of desired response spectrum – 0.02 s • largest period of desired response spectrum – 3 s • damping – 5 % • time-step – 0.01 s • pga – 0.0815g =0.8 m/s 6.2. comparison to the response spectrum programm seismoartif [11] allows a comparison of time-history charts with the assigned response spectrum – figures 9, 10 and 11. 6.3. linear time-history analysis horizontal displacements were analysed by the fem program seismostruct[12] for two versions of the structure. (1.) frame with flexible beams uk,max =0.00236767 m, uk,min = -0.00283672 m. (2.) frame with rigid beams uk,max =0.001553 m, uk,min = -0.001545 m. 6.4. non-linear time-history analysis total horizontal displacement was calculated by the same software, but in this version, the non-linear material behaviour was used. (1.) frame with flexible beams uk,max =0.00238527 m, uk,min = -0.00253367 m. 24 vol. 13/2017 comparison of methods used for seismic analysis of structures figure 9. comparison of the acceleration. figure 10. comparison of the velocity. figure 11. comparison of the displacement. (2.) frame with rigid beams uk,max =0.00155074 m, uk,min = -0.00154217 m. 7. non-linear static (pushover) analysis pushover analysis is the last method of the ec8 [1] for the calculation of the seismic response. this nonlinear method is based on constantly acting self-weight and monotonically increasing horizontal load. in this method, plastic mechanism has to be defined by plastic regions. sufficient ductility has to be ensured by good construction design of plastic regions. remaining regions must have a big capacity and they have to stay elastic while plastic regions are overloaded. this procedure determines members for the energy dissipating. “soft” regions are for dissipating and “hard” are elastic [14]. 7.1. hand calculations of the frame with flexible beams by pushover analysis for determination of the pushover curve is necessary to monotonically increase the horizontal load up to the collapse of the structure or up to the maximal allowed horizontal displacement. according to [1] is necessary to applicate at least two different distributions of the lateral load (constant, linear, modal, . . . ). for a simple comparison, only one distribution of the lateral load was used. in this case a modal distribution of the first eigenmode was chosen. the pga is 0.8 m/s2. the product of the pga and the total weight defines the base shear force fb =20.48 kn. the base shear force is distributed to the structure according to normalized eigenmodes. pushover curve describes the relation between the base shear force fb and the horizontal displacement d of the roof level. this capacity curve shows the yielding progress of the structure and regions of plastic hinges. for calculation of the monotonically increasing load, the software stab2d-nl [13] was chosen. iterative non-linear calculation determined the maximal load by collapse of the structure by 7.4-times of the assigned lateral load – f1 =60.83 kn a f2 =82.88 kn. lateral load was increased in 50 steps, respectively by 2.0 %. vertical load acts in all steps by 100 %. the collapse of the structure occurs by the depletion of the crosssection strength by the member nr. 3. this method determines the capacity curve for the member nr. 6 in the roof level. horizontal displacement of the multiple degree of freedom (mdof) system in the roof level before collapse is 37.7 mm. for the transformation to the single degree of freedom (sdof) system is necessary to 25 petr čada, jiří máca acta polytechnica ctu proceedings figure 12. example of the time-chart of the displacement. figure 13. capacity curve of the model with flexible beams. determine transformation factor γ. γ was counted according to procedure in [1], annex b, and is equal to 1.127. next step in this method is the determination of the deformation energy. deformation energy is the surface under the capacity curve of sdof system (figure 13). this surface was calculated by integration of sextic equation which was defined by programm excel (equation of the capacity curve). deformation energy em was transformed to the deformation energy of the sdof system by transformation factor γ. em* is equal to 2875.07 knmm. figure 14. determination of the idealized elastoperfectly plastic force-displacement relationship for the frame with flexible beams. determination of the idealized elasto-perfectly plastic force-displacement relationship is shown in figure 14. the period of the idealized sdof system was determined (t* =0.362 s). with this period, the horizontal displacement can be calculated. because the period of the sdof is smaller than the tc -value of the response spectrum, a short period range can be considered. because the calculated displacement of the sdof was too far from the dm*, iterative solution was used. after the fifth iteration, the target displacement is equal to d∗et =4.37 mm. the displacement was transformed by the transformation factor to the target displacement of the mdof system – dt = 4.92 mm. 26 vol. 13/2017 comparison of methods used for seismic analysis of structures figure 15. capacity curve of the model with rigid beams. 7.2. hand calculations of the frame with rigid beams by pushover analysis the weight of the system stays identical. distribution of lateral forces is changed with dependence on the stiffness. collapse of the stiff structure occurs by the same member by 7.3-times of the assigned lateral load – f1 =59.641 kn a f2 = 81.76 kn. the capacity curve for the member nr. 6 in the roof level was determined (figure 15) horizontal displacement of the mdof system in the roof level before collapse is 31.4 mm. for the transformation to the sdof system is necessary to determine transformation factor γ. γ was counted according to procedure in [1], annex b, and is equal to 1.129. deformation energy em* was determined by the same procedure as in previous chapter. em* is equal to 2428.84 knmm. determination of the idealized elasto-perfectly plastic force-displacement relationship is shown in figure 16. the target displacement of the sdof system was determined as in previous chapter. six iterations were needed for the target displacement of the sdof – d*et =3.02 mm. the transformed displacement of the mdof system is equal to dt = 3.41 mm. 8. comparison and conclusion all calculation methods for the seismic response according to ec8 [1] were compared. a simple model of the two-storeyed frame was used. the main line of the comparison was in the following way: • hand calculations of dynamical characteristics were made. figure 16. determination of the idealized elastoperfectly plastic force – displacement relationship for the frame with rigid beams. • these values were used for the hand calculation of the response from the spectrum. • results of the response spectrum were compared with software results. • an accelerogram for time-history analysis with identical displacements to the response spectrum method was determined • the chosen accelerogram was used for further calculations. • results were compared with lateral force method and pushover analysis. all results correspond with the theory that the stiffer structure approaches to smaller deformations. the lateral force method is the most conservative method for the seismic response. this method is used for simple symmetrical structures with predictable behaviour. the lateral force method is the simplest method and it is not necessary to use special software or complicated calculations. a disadvantage of this method is only linear material behaviour. the next linear method is the response spectrum method. this method gave us better results. this is a good choice for more complicated structures. members of the structure are with this analysis not so much oversized. it is widely used in the practice. the non-linear behaviour changes results significantly. non-linear methods according to ec8 [1] are the time-history analysis and the pushover analysis. both of them are particular and it is necessary to use of some specialized software. the time-history analysis is suitable for the analysis of past known earthquakes. the pushover analysis is better for design of new structures or reconstructions. all results are compared in table 3. identical results of the horizontal displacement between linear calculations and dynamic calculation were achieved. on the other hand, a bigger difference between the pushover analysis and other methods was found out. this difference implies to a requirement to be cautious by this method and to focus on initial 27 petr čada, jiří máca acta polytechnica ctu proceedings calculation method horizontal displacement of the structure [mm] cross-beam stiffness linear non-linear lateral force method flexible rigid 1.724 d response spectrum method flexible 2.303 a rigid 1.582 a, 1.639 d time-history analysis flexible 2.837 a 2.534 b rigid 1.545 a 1.543 b pushover analysis flexible 4.92 b,c rigid 3.41 b,c (a) calculation by software seismostruct [12] (b) calculation by software stab2d-nl [13] (c) hand calculation table 3. comparison of calculations of the seismic response. values. for these reasons, it is necessary to focus more on this method and to study another new conditions and inputs of its application. acknowledgements the authors gratefully acknowledge support from the czech technical university in prague, project sgs17/043/ohk1/1t/11 development and application of advanced algorithms for numerical analysis and modelling of structures and materials. references [1] en 1998-1: eurocode 8: design of structures for earthquake resistance: part 1: general rules, seismic actions and rules for buildings, 2004. [2] p. bisch, e. carvalho, h. degee, et al. eurocode 8: seismic design of buildings: worked examples. 1 lisbon: publications office of the european union 2012. [3] a. faroughi, m. hosseini. simplification of earthquake accelerograms for quick time history analyses by using their modified inverse fourier transforms. procedia engineering 14:2872–2877, 2011. doi:10.1016/j.proeng.2011.07.361. [4] e. kalkan, a. k. chopra. practical guidelines to select and scale earthquake records for nonlinear response history analysis of structures. us geological survey open-file report 2010. [5] p. fajfar, m. fischinger. n2 a method for non-linear seisimic analysis of regular buildings. proceedings of ninth world conference on earthquake engineering pp. v111–v116, 1988. doi:10.1002/eqe.144. [6] a. k. chopra, r. k. goel. a modal pushover analysis procedure for estimating seismic demands for buildings. earthquake engineering 31(3):561–582, 2002. doi:10.1002/eqe.144. [7] p. fajfar. a nonlinear analysis method for performance based seismic design. earthquake spectra 16(3):573–592, 2000. [8] v. kilar, p. fajfar. simplified push-over analysis of building structures. eleventh world conference on earthquake engineering acapulco (1011):2872–2877, 1996. [9] m. kreslin, p. fajfar. the extended n2 method taking into account higher mode effects in elevation. earthquake engineering 40(14):1571–1589, 2011. doi:10.1002/eqe.1104. [10] en 1992-1-1: eurocode 2: design of concrete structures: part 1-1: general rules and rules for buildings, 2004. [11] programm seismoartif. http://www.seismosoft.com/seismoartif. [12] programm seismostruct. http://www.seismosoft.com/seismostruct. [13] programm stab2d-nl. http://www.u-pfeiffer.de/. [14] h. bachmann. erdbebensicherung von bauwerken. basel boston berlin: birkhäuser verlag, 2002. [15] b. halldorsson. calibration of the specific barrier model to earthquakes of different tectonic regions. bulletin of the seismological society of america 95(4):1276–1300, 2005. doi:10.1785/0120040157. 28 http://dx.doi.org/10.1016/j.proeng.2011.07.361 http://dx.doi.org/10.1002/eqe.144 http://dx.doi.org/10.1002/eqe.144 http://dx.doi.org/10.1002/eqe.1104 http://www.seismosoft.com/seismoartif http://www.seismosoft.com/seismostruct http://www.u-pfeiffer.de/ http://dx.doi.org/10.1785/0120040157 acta polytechnica ctu proceedings 13:21–29, 2017 1 introduction 2 structure and load 2.1 model of the structure and material parameters 2.2 load 2.2.1 vertical load 2.2.2 seismic load 2.3 software 3 dynamic properties of the structure 3.1 eigenmodes 3.2 eigenmodes – fem 4 lateral force method of analysis 5 modal response spectrum analysis 5.1 hand calculation 5.2 fem – linear 6 time-history analysis 6.1 horizontal load 6.2 comparison to the response spectrum 6.3 linear time-history analysis 6.4 non-linear time-history analysis 7 non-linear static (pushover) analysis 7.1 hand calculations of the frame with flexible beams by pushover analysis 7.2 hand calculations of the frame with rigid beams by pushover analysis 8 comparison and conclusion acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2017.12.0032 acta polytechnica ctu proceedings 12:32–37, 2017 © czech technical university in prague, 2017 available online at http://ojs.cvut.cz/ojs/index.php/app microsleeps and their detection from biological signals martin holub∗, martina šrutová, lenka lhotská czech technical university in prague, faculty of electrical engineering, department of cybernetics, karlovo nám. 13, praha 2, 121 35, czech republic ∗ corresponding author: martinholub@mail.com abstract. microsleeps (ms) are a frequently discussed topic due to their fatal consequences. their detection is necessary for the purpose of sleep laboratories, where they provide an option for the quantifying rate of sleep deprivation level and objective evaluation of subjective sleepiness. many studies are dealing with this topic for automotive usage to design a fatigue countermeasure device. we made a research of recent attitude to the development of the automated ms detection methods. we created an overview of several ms detection approaches based on the measurement of biological signals. we also summarized the changes in eeg, eog and ecg signals, which have been published over the last few years. the reproducible changes in the entire eeg spectrum, primarily with the increased activity of delta and theta, were noticed during a transition to fatigue. there were observed changes of blinking rate and reduction of eye movements during the fatigue tasks. ms correspond with variations in the autonomic regulation of the cardiovascular function, which can be quantified by hrv parameters. the decrease in hr, vlf, and lf/hf before falling asleep was revealed. eeg signal, especially its slow wave activity, considered to be the most predictive and reliable for the level of alertness. in spite of the detection from eeg signal is the most common method, eog based approaches can also be very efficient and more driver-friendly. besides, the signal processing in the time domain can improve the detection accuracy of the short events like ms. keywords: microsleep, automatic detection, detection methods, eog, eeg, ekg. 1. introduction microsleeps (ms) are mostly detected for the purpose of automotive usage and sleep laboratories. the evaluation of ms stages in the sleep laboratories provides an option for the quantifying rate of sleep deprivation level and an objective assessment of subjective sleepiness. they use the definition of microsleep (ms), which arose as a consequence of the evaluation method of the psg (polysomnography) or mslt (multiple sleep latency test) data, where each 30-second epoch is evaluated separately. to score this epoch as a sleep, more than 15 seconds should show some sleep activity [1]. episodes with sleep symptoms, but with a short duration between 3 and 15 seconds are defined as the microsleeps. the epoch would be classified as a sleep in case, they were longer than 15 seconds [2]. the theta rhythm with slow eye movements, k complexes, or sleep spindles is included. microsleeps can be accompanied by the same behavioural changes as during the states of drowsiness. in spite of the short duration of microsleep, it can be inconvenient and dangerous for many people and for their surroundings to perform monotonous activity while being tired and drowsy. many studies are dealing with this topic for automotive usage to design a fatigue countermeasure device. for those kinds of studies, it isn’t important to distinguish the events of drowsiness, microsleeps or fatigue correctly by the terminology, because all the states associated with the loss of attention are dangerous in the traffic. in the studies observed, the mentioned lapses of responsiveness are often manifested by absent or prolonged responses and short suspension of performance. these symptoms are accompanied by droopy eyes, slow eyelid closure, head nodding and many changes in biological signals. we aimed to make a research of recent attitudes for the development of the automated ms detection methods. we created an overview of several ms detection approaches based on biological signals measurement. they are mostly based on the processing eeg (electroencephalography) signals or eeg accompanied by eog (electrooculography) signals. we also summarized the changes in the basic biological signals like eeg, eog and ecg (electrocardiography), which have been published over the last few years. 2. signal changes 2.1. changes in eeg signals the changes in the entire eeg frequency spectrum were noticed during transition to fatigue [3–8]. the increased activity of delta and theta rhythms was primarily observed. it could reflect decreased cortical arousal during a monotonous task. the removing noise from the eeg is crucial for observation of right changes. the filtration method is especially necessary 32 http://dx.doi.org/10.14311/app.2017.12.0032 http://ojs.cvut.cz/ojs/index.php/app vol. 12/2017 microsleeps and their detection from biological signals for the low frequency signal, which is influenced by artefacts from activities such as breathing and movement. slow activity is prone to be also contaminated by eye movement artefact. various areas of the brain behaved differently during fatigue, during its onset delta and theta activity were present mostly in the frontal, central, and parietal areas of the brain with some anterior alpha and posterior beta [3, 5]. the occipitoparietal alpha spread to anterior areas and become more centrofrontal and temporal alpha during drowsiness [3]. the reproducibility of the eeg magnitude changes in the delta, theta, alpha and beta bands during drive fatigue was tested [9]. there was revealed high reproducibility for the delta and theta bands [9]. during the transitional phase to fatigue, there were no significant differences in the delta and alpha magnitudes across the entire brain [9]. the inter-hemispheric eeg coherence analysis was shown in the study of jap et al. [10], which revealed that the interhemispheric coherence level was significantly higher at the frontal and occipital sites compared to the central, parietal, and temporal sites throughout the driving sessions. 2.2. changes in eog signals due to the rich connectivity between eyes and brain, different eog analyses can be used to identify the states similar to fatigue. the changes of the blinking rate were observed during the fatigue tasks in the published papers [5]. the work of lal [5] showed, that the fast eye movements and conventional blinks during alert were replaced by no eye movements and small fast rhythmic blinks during fatigue, and that the slow blinks were present during deeper drowsiness for all the subjects. wierwille et al. [11] declared that slow eyelid closure is a reliable estimate of the level of drowsiness. in the study of papadelis et al. [12], a significant increase of eye blinks’ number and duration before driving errors was observed. 2.3. changes in ecg signals the stages of microsleeps, drowsiness or fatigue correspond with the changes in the autonomic regulation of the cardiovascular function. the hrv (heart rate variability) parameters are used to quantify these changes. the gradual and sustained decrease in the vlf range minutes before falling asleep was observed [13]. the ratio lf/hf, which corresponds with the sympathovagal balance, was very low compared to baseline wake values for about 5 minutes before the event [13]. it is a sign of the relaxation taking over. there is a significant surge in this balance after the falling asleep due to the increased stress [13]. heart rate decreased significantly during falling asleep event [13] and fatigue (by 7 bpm) [5]. same changes in the heart rate were observed during prolonged and monotonous driving [14]. also, the rr variability declined over the falling asleep events in the study of furman [13]. 3. detection approaches in the following section, there are presented and compared (table 1) detection methods, which have been issued during the recent years and which were dealing with the states similar to microsleeps. 3.1. detection by vuckovic et al. (published 2002) the classification of the drowsy states from full spectrum eeg recordings was published in the paper [15]. parameterization was based on a moving cross-spectral density csd. the interhemispheric csd was calculated from homologous contralateral electrodes [15]. the intrahemispheric csd was calculated from electrodes on the left hemisphere [15]. the csd was computed for the signal lasting one epoch and the signal from the same electrode delayed by 50% of the epoch [15]. the main reason to choose interhemispheric csd was that the correlation of homologue electrodes decreases in drowsiness [16]. as well as the intrahemispheric csd detects the transition from alertness to drowsiness and vice versa because of the changes in the synchronization of the intrahemispheric eeg signal [16]. a convolutional neural network, auto encoder, and linear dynamic system described accurately in [15] were used for classification. 3.2. detection by golz et al. (published 2007) different ms detection approaches and their combinations were presented in the paper [17]. the adaptive signal processing was based on the signals of the brain electric activity, variation in the pupil size, and eyelid movements. totally fifteen signals had been acquired, seven signals of eeg, vertical and horizontal eog channel, and six signals from eye tracking system (the pupil size and two coordinates of eye gaze on the plane of projection). the features were firstly extracted in the frequency domain by one linear method of modified periodogram and then in the state space by using the nonlinear method of the delay vector variance (dvv) [18]. the results show, that periodogram was more powerful for the feature extraction than the dvv method and that the complementing power spectral density by dvv features showed only small improvements. the achieved features were used as input vectors in automatically learning classification algorithm, where the decision process was based on the support vector machines and three learning vector quantization neural networks. all algorithms were optimized to gain maximal classification accuracy. the best results were achieved by the fusion of features for all available signals. proposed methods were created with respect to the future online driver monitoring. [17] 33 m. holub, m. šrutová, l. lhotská acta polytechnica ctu proceedings figure 1. the table shows the comparison of the published methods. 3.3. detection by furman et al. (published 2008) this study [13] checked a feasibility of the new detection approach based on the processing of the ecg signal, where the trends of heart rate variability (hrv) were observed. the hrv parameters were used to look into the changes in the autonomic regulation of the cardiovascular function. physiological changes were quantified before, during and after an event of microsleep. it was found that there was a gradual and sustained decrease in the vlf range minutes before falling asleep events. the sympathovagal balance, which was represented by lf/hf, was very low compared to baseline wake values for about 5 minutes before the events. it was a sign of relaxation taking over. after falling asleep there was a surge in this balance signifying an increased stress aimed to overcome the drowsiness. the mean hr and rr variability decreased during falling asleep events, thus confirm the autonomic changes. during a microsleep, there was a decrease in sympathetic drive with increased vagal activity. [13] 3.4. detection by tsai et al. (published 2009) real-time system [19] for detecting a drowsy driver was based on eeg signal processing. six channels of eeg were acquired by a novel active dry electrode non-invasive system. the wavelet transform using daubechies 2 wavelet was applied on the each channel. integrated eeg and zero crossing features were computed from the scales corresponding to the frequency of beta, alpha and theta rhythms. all 36 parameters were used as an input vector to the artificial neural network with 18 hidden nodes and two output nodes. system rejected the detection accuracy of drowsy subject 90.9% with too many warnings signals while the subject was totally alert. [19] 3.5. detection by garcés et al. (published 2010) the automatic detector of drowsiness episodes [20] was based on the processing of one eeg channel. the butterworth band-pass filter with cut-off frequencies of 0.5 and 60 hz was used for signal pre-processing 34 vol. 12/2017 microsleeps and their detection from biological signals and then segmented into the blocks of duration of 1, 2, 5 and 10 seconds. the length of 5 seconds was considered the most appropriate to take the decision in the real time. twelve features were computed from spectral analysis and wavelet decomposition of the eeg signal: the central frequency, the first quartile frequency, the maximum frequency, the total energy of spectrum, the power of theta and alpha bands, the number of zero crossing and the integrated eeg for scales 3, 4 and 5 of daubechies 2 wavelet transform. in this case, the equivalent filters of mentioned scales had approximately same frequency as beta, alpha and theta rhythms. twenty-five neural networks with three layers (first 12 neurons, last one neuron with tan-sigmoid function, hidden-between 5 and 30 neurons) were tested for classification. the best architecture was neural network (12-20-1), for which 81.7% of drowsiness was detected. [21] 3.6. detection by peiris et al. (published 2011) the detection of lapses in responsiveness [22] was based on the signal processing of the electroencephalogram. the features using spectral analysis (mean spectral power, normalized spectral power and spectral power ratios) and three nonlinear methods (fractal dimension, approximate entropy and lempel-ziv complexity) were calculated and compare to determine their efficacy in eeg-based detection of lapses. principal component analysis was used to reduce the dimensionality of the feature matrix. the detector model created using mean spectral power features achieved the best failure state estimation performance. there were reached sensitivity 73.5% and selectivity 25.5%. among the nonlinear approaches the detector based on the lempel-ziv complexity showed the highest performance. the performance of the detector was no greater by complementing spectral power features by lempel-ziv complexity. it was similar to the finding of [17]. the use of meta-learner weights showed a marginally higher performance than the use of constrained least-squares metalearner weights. [22] 3.7. detection by zhu et al. (published 2014) the drowsiness detection [23] was based on two electrooculography signals, vertical and horizontal. these signals were filtered, saturated and normalized during pre-processing. the automatic detection technic of slow eye movements [21, 24, 25] based on wavelet transform was used for the feature detection. the features were slow eye movements number, duration of closing phase of blink, peak and mean velocities in closing and opening phases of blink. the wavelet transformation was also used for the extraction of two other features ratio of low and high frequency on both horizontal and vertical eogs. the linear dynamic system was used for post processing. the convolution neural network, which was described in detail in the paper [23], was used for classification. 3.8. detection by holub et al. (published 2015) ms classifier [26] was based on the independent analysis of each couple of eog signals in the time domain. the basic idea of the algorithm was to observe differences between the minimum and maximum values in the appropriately chosen sliding window, which was gradually applied to the entire length of both eog signals. by merging information from the signals of the left and the right eye was calculated the final feature, which was able to appropriately distinguish ms events from the rest of the awake signal [26]. this parameter highlighted ms states, which were manifested in the signal as the episodes without significant eye movements and fast changes. these episodes were terminated by the specific high-frequency wave, which corresponded to blink and arousal from the ms. the final parameter was thresholded to distinguish ms from the rest of the signal. the differences between fix and adaptable thresholding were presented. the fix threshold was appropriately set as the same value for all signals, while the calculation of adaptable threshold was based on the signal similarity of ms episodes with sleep stages nrem1 and nrem2, which were used to set this limit value. there were reached precision 67% and recall 82% in the method of adaptable thresholding. 4. conclusions eeg signal has been considered by the researchers as the most predictive and reliable for the level of alertness. previous studies confirmed that detection of changes in slow wave activity might form the basis of a fatigue countermeasure device. these approaches use mostly wavelet transform or methods of periodogram, which can be suitable for the detection of drowsy states, but they can provide the insufficient time resolution for detection of very short events like ms. to achieve a good frequency resolution, it is necessary to analyse the longest possible part of the signal. but the time resolution deteriorates with increasing length of the sliding window, which is applied to the analysed signal. this fact is undesirable due to the ms detection of an event that may take only 3 seconds. the window has to be always chosen long enough with respect to the stationarity of the signal and frequency resolution. the same problem of insufficient time resolution can be in the detection from ecg, where the heart rate variability is counted. in comparison to detection in the frequency domain, the signal processing in the time domain can provide higher accuracy and faster response of the detector. in spite of the detection from eeg signal, it is the most common method, eog based approaches can also be very efficient and more driver-friendly. the electrodes for the measuring of eog signals can be 35 m. holub, m. šrutová, l. lhotská acta polytechnica ctu proceedings easily implemented for example to headbands, hats or glasses, which provide unlike measuring multi-channel eeg greater comfort and easy application for the drivers. acknowledgements this work has been supported by the project grant application no. sgs16/231/ohk3/3t/13 of the czech technical university in prague. references [1] c. iber. the aasm manual for the scoring of sleep and associated events: rules, terminology and technical specifications. american academy of sleep medicine, 2007. 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[20] a. g. correa, e. l. leber. an automatic detector of drowsiness based on spectral analysis and wavelet decomposition of eeg records. in 2010 annual international conference of the ieee engineering in medicine and biology, pp. 1405–1408. ieee, 2010. [21] e. magosso, m. ursino, a. zaniboni, et al. visual and computer-based detection of slow eye movements in overnight and 24-h eog recordings. clinical neurophysiology 118(5):1122–1133, 2007. doi:10.1016/j.clinph.2007.01.014. [22] m. t. peiris, p. r. davidson, p. j. bones, r. d. jones. detection of lapses in responsiveness from the eeg. journal of neural engineering 8(1):016003, 2011. [23] x. zhu, w.-l. zheng, b.-l. lu, et al. eog-based drowsiness detection using convolutional neural networks. in ijcnn, pp. 128–134. 2014. 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[25] e. magosso, m. ursino, a. zaniboni, e. gardella. a wavelet-based energetic approach for the analysis of biomedical signals: application to the electroencephalogram and electro-oculogram. applied mathematics and computation 207(1):42–62, 2009. doi:10.1016/j.amc.2007.10.069. 36 http://dx.doi.org/10.1016/j.sleep.2006.06.011 http://dx.doi.org/10.1097/00004691-198710000-00002 http://dx.doi.org/10.1017/s0048577201393095 http://dx.doi.org/10.1016/0013-4694(87)90096-4 http://dx.doi.org/10.1016/j.ijpsycho.2004.07.001 http://dx.doi.org/10.1016/0001-4575(94)90019-1 http://dx.doi.org/10.1016/j.clinph.2007.04.031 http://dx.doi.org/10.1016/s1350-4533(02)00030-9 http://dx.doi.org/10.1177/155005949602700207 http://dx.doi.org/10.1007/s11265-007-0083-4 http://dx.doi.org/10.1016/j.clinph.2007.01.014 http://dx.doi.org/10.1016/j.medengphy.2006.01.002 http://dx.doi.org/10.1016/j.amc.2007.10.069 vol. 12/2017 microsleeps and their detection from biological signals [26] m. holub, m. šrutová, l. lhotská. adaptable microsleep detection based on eog signals: a feasibility study. in computational intelligence for multimedia understanding (iwcim), 2015 international workshop on, pp. 1–4. ieee, 2015. 37 acta polytechnica ctu proceedings 12:32–37, 2017 1 introduction 2 signal changes 2.1 changes in eeg signals 2.2 changes in eog signals 2.3 changes in ecg signals 3 detection approaches 3.1 detection by vuckovic et al. (published 2002) 3.2 detection by golz et al. (published 2007) 3.3 detection by furman et al. (published 2008) 3.4 detection by tsai et al. (published 2009) 3.5 detection by garcés et al. (published 2010) 3.6 detection by peiris et al. (published 2011) 3.7 detection by zhu et al. (published 2014) 3.8 detection by holub et al. (published 2015) 4 conclusions acknowledgements references 103 acta polytechnica ctu proceedings 1(1): 103–107, 2014 103 doi: 10.14311/app.2014.01.0103 the formation of massive stars: from herschel to near-infrared paolo persi1, mauricio tapia2 1institute for space astrophysics and planetology (iaps/inaf) via fosso del cavaliere 100, 00133 roma, italy 2instituto de astronomia,unam, apartado postal 877,ensenada, baja california, cp22830, mexico corresponding author: paolo.persi@iaps.inaf.it abstract we have studied a number of selected high mass star forming regions, including high resolution near-infrared broadand narrow-band imaging, herschel (70, 160, 250, 350 and 500 µm) and spitzer (3.6, 4.5, 5.8 and 8.0 µm) images. the preliminary results of one of this region, iras 19388+2357(mol110) are discussed. in this region a dense core has been detected in the far-infrared, and a young stellar cluster has been found around this core. combining near-ir data with spitzer and herschel photometry we have derived the spectral energy distribution of mol110. finally comparing our h2 and kc narrow-band images, we have found an h2 jet in this region. keywords: star formation circumstellar matter giant molecular clouds infrared. 1 introduction the formation of high-mass stars defined as those with masses greater than 8 msun is still controversial. one of the crucial problem is to understand if high-mass stars can form through(disk) accretion like low-mass stars. such stars reach the zero-age main sequence (zams) still undergoing heavy accretion, and their powerful radiation pressure should halt the infalling material, thus inhibiting growth of the stellar mass beyond about 8 msun (e. g., palla & stahler 1993). recently, various studies have proposed a solution to this problem based on non-spherical accretion and high accretion rates (e. g., mckee & tan 2003; bonnell et al. 2004; kuiper et al. 2010). in addition at difference of low-mass stars, high-mass stars forms in cluster. therefore the comprehension of the massive star formation process requires good observational knowledge of the star-forming environment and of the evolutionary steps through which ob star formation occurs. this can be made combining observations at different wavelengths from near to far-infrared and millimeter wavelengths. thanks to the spitzer and herschel satellites that operate from the mid-ir to the sub-millimeter it is now possible to have a broad observational coverage of these high mass star formation regions. we have selected a number of these regions reported in table 1 with typical characteristics of high-mass star formation(i.e. presence of water and methanol maser sources, radio and millimeter emission , ammonia cores, (molinari et al.1998, molinari et al. 2000). the type high(h) and low(l) reported in table 1 are taken from molinari et al.1996.. we obtained subarcsec resolution nearinfrared broadand narrow-band images of the source of table 1. these observations are compared with far-ir images from the herschel infrared galactic plane survey (hi-gal, molinari et al. 2010) supplemented with spitzer/irac archive images. the observations are described in section 2, while in section 3 we report the preliminary results of iras 19388+2357(mol110). all the results will be discussed in forthcoming papers. table 1: sample of the observed high-mass protostars source type iras α(2000) δ(2000) d l h m s ◦ ′ ” kpc l� mol83 h 18566+0408 18 59 10.0 04 12 15 6.8 1.02 × 105 mol98 l 19092+0841 19 11 37.4 08 46 30 4.5 9.20 × 105 g45.07+0.13 19110+1045 19 13 22.6 10 50 53 9.7 1.42 × 105 g45.47+0.13 19 14 08.3 11 12 32 6.0 3.80 × 105 mol110 h 19388+2357 19 40 59.4 24 04 39 4.3 1.48 × 104 103 http://dx.doi.org/10.14311/app.2014.01.0103 paolo persi, mauricio tapia 1.1 near-infrared images near-infrared images through narrow-band h2 (λo = 2.122µm, ∆λ = 0.032µm ) and kcont ( λo = 2.270µm, ∆λ = 0.034µm) filters, as well as through standard broad-band jhks filters, were collected on the nights of 2008 july 12 and 14 using the near infrared camera spectrometer (nics) attached to the 3.58m telescopio nazionale galileo (tng) at the observatorio del roque de los muchachos on la palma island. nics has a hgcdte hawaii 1024 ×1024 array and was used in the sf (small field) configuration with a plate scale of 0.13 arcsec/pixel. in each band, 9 dithered frames spaced by 10 arcsec were taken and coadded, for total on-source integration times of 630 s, 540 s and 360 s for j , h, and ks, respectively. the total integration time for each of the narrow-band (h2 and kcont) filters was 1170 s. all images were calibrated using photometric standard stars from hunt et al. (1998) and persson et al. (1998). the measured fwhm of the point-spread function (psf) is between 0.6 arcsec and 0.8 arcsec. jhk photometry was obtained using daophot (stetson 1987) within iraf in the standard way, with an aperure of 1 arcsec. for the crowded regions, we used the psf procedure, also within iraf. 1.2 hi-gal images hi-gal is a herschel open time key-project (molinari et al.2010) aiming at mapping the galactic plane with the pacs (70 and 160 µm, poglitsch et al. 2010) and spire (250, 350, and 500 µm, griffin et al. 2010) photometers on board the herschel satellite (pilbratt et al. 2010). our target were observed by spire+pacs in parallel mode at a scan speed of 60 arcsec/s. the data were reduced using the hi-gal standard pipeline (traficante et al. 2011). the images have pixel sizes 3.2 arcsec, 4.5 arcsec, 6 arcsec, 8 arcsec, and 11.5 arcsec , at 70, 160, 250, 350, 500 µm respectively. from the images, we performed the source extraction and photometry using the curvature threshold extractor package (cutex, molinari et al. 2011). 1.3 spitzer/irac archive images flux-calibrated images of our regions were retrieved from the glimpse (benjamin et al. 2003, churchwell et al. 2009) survey taken at 3.6, 4.5, 5.8 and 8 µm with irac on board the spitzer space observatory . the flux densities of the mid-infrared counterparts of our sources were extracted from the glimpse catalogue. 2 iras19388+2357(mol110) iras 19388+2357 is associated with h2o maser emission (palla et al. 1991; brand et al. 1994), a radio source (hughes & macleod 1994; molinari et al. 1998) and dense molecular gas traced in nh3 by molinari et al. (1996) who renamed as mol110. methanol maser was also detected from this source (schutte et al. 1993; slysh et al. 1994). zhang et al. (2005) detected a co outflow. the centroid of their co emission is approximately 29 arcsec south of the iras position. finally beltran et al.(2006) detected a dense core at 1.2mm with a mass of 167 msun. the presence of a uchii region, of water and methanol maser, confirm that mol110 is an high-mass star forming region. figure 1 shows our ks-band image including the positions of the mentioned sources. figure 1: ks-band image of iras19388+23657. the plus indicates the position of the iras source, while the open circle and the cross give the positions of the 6cm radio continuum and the msx source respectively. the contours represent the 1.2mm emission. a point-like far-infrared sources has been detected in the hi-gal herschel images from 70 to 500 µm at the position α2000 = 19h40m59.2s, δ2000 = +24◦ 04’ 44.9”. we have analyzed our jhks and spitzer images within an area of approximately 20” × 20” around this position. figure 2 reports the color-coded jhks and spitzer images of this area. within the herschel beam we found several very red sources. we have obtained the jhks photometry of these sources around the herschel peak position. from this photometry, we have obtained the j −h versus h − ks diagram illustrated in figure 3. more than 14 objects show significant near-infrared excess, suggesting the presence of a young stellar cluster in this region. the positions of these sources are marked in fig.2 (left panel). at least six of these sources are identified with the mid-ir spitzer sources (see fig.2 right panel). 104 the formation of massive stars: from herschel to near-infrared figure 2: (left panel) jhks color-coded image of iras19388+2375 obtained combining the j (blue), h (green), and ks (red) individual images the contours show the 70 µm herschel observation. the symbol (+) marks the positions of the sources with near-ir excess, while the crosses indicate sources not detectect in j but with h-ks greater than 3(right panel) color-coded spitzer image obtained combining the [3.6] (blue), [4.5] (green), and [8.0] µm (red) individual irac images. the central position of the two images is α2000 = 19h40m59.1s, δ2000 = +24◦ 04’ 45.4”. north is at the top and east to the left. figure 3: j-h versus h-ks diagram relative to a region of 20arcsec around the iras position. on the base of the position coincidence, we have identified the far-ir source with a near-ir and spitzer source. at the same position a source has been detected also with the wise satellite. combining data from near-ir to millimeter spectral region we have constructed the spectral energy distribution (sed) of mol110 reported in figure 4. the sed has been fitted with the infalling envelope+disc+central source radiation transfer model described by robitaille et al. (2006) by using the fitting tool of robitaille et al. (2007). the parameters of the model that best fit the sed are reported in table 2. figure 4: spectral energy distribution (sed) of mol 110 the observed luminosity of 1.13 104 l� correspond to that of a b1-2 zams star reddened by 50.2 magnitudes of extinction in v. 105 paolo persi, mauricio tapia figure 5: (left panel) h2 narrow-band image of iras 19388+2357. (right panel) narrow-band kc image of the same region. north is at the top and east to the left. table 2: physical parameters of mol 110 derived from the robitaille et al. (2007) model. stella mass (msun) 12.94 stellar temperature (k) 12262 envelope accretion rate (msun/yr) 3.61 10−3 envelope outer radius (au) 1.0 105 envelope cavity angle (deg) 25.1 disk mass (msun) 1.4910−1 disk outer radius (au) 23.1 disk accretion rate (msun/yr) 5.18 10−6 av 50.2 d(kpc) 4.7 lbol 1.13 104 l� from the comparison of our narrow-band images centered on the h2 (λo = 2.122µm), and nearby continuum kcont ( λo = 2.270µm), we have found an h2 jet at the position α2000 = 19h40m59.7s, δ2000 = +24◦ 04’ 49.0” . a nearby source at the position α2000 = 19h40m59.5s, δ2000 = +24◦ 04’ 47.6” with near-ir excess could be the young stellar object (yso) driving the observed outflow. this is illustrated in figure 5. similar observations obtained by varricatt et al. (2010) report three different h2 jets in the region not detected in our images. the positions of these h2 jets are very far from the co outflow observed by zhang et al. (2005), indicating that another yso is responsible of driving this outflow. 3 conclusions in order to understand the physical processes that involve high massive star forming regions, the comparison of observations at different wavelengths from near-ir to millimeter are fundamental. we have here reported an example of this combined analysis including near-ir images, spitzer data from 3.6 to 8 µm and herschel images in five bands from 70 to 500 µm, relative to the star forming region iras 19388+2357. from this analysis the following conclusions can be made: 1) a very dense and cold core has been detected from the farir herschel images, in proximity of the iras source. 2) within the dense core the near-ir images show the presence of a young stellar cluster of at least 15 members in a radius of 20 arcsec. 3) the far-ir peak has been identified with a bright spitzer and near-ir source. combining the photometry from 1.25µm to 1.2mm, we have derived its spectral energy distribution(sed). the measured total luminosity indicates that the source is a 106 the formation of massive stars: from herschel to near-infrared b1-2 zams with av =50.2 4) finally, the narrow-band image centered on the h2 line at 2.122 µm shows the presence of an h2 jet in proximity of the iras source. references [1] beltran, m. t., brand, j., cesaroni, r., et al.: 2006, a&a, 447, 221 [2] benjamin, r. a., churchwell, e., babler, b. l., et al.: 2003, pasp, 115, 953 doi:10.1086/376696 [3] bonnell, i. a., vine, s. g., bate, m. r.: 2004, mnras, 349, 735 doi:10.1111/j.1365-2966.2004.07543.x [4] brand j., et al.: 1994, a&as, 103, 541 [5] churchwell, e., babler, b. l., meade, m. r., et al.: 2009, pasp, 121, 213 doi:10.1086/597811 [6] griffin, m. j., abergel, a., abreu, a., et al.: 2010, a&a, 518, l3 [7] hughes v. a., macleod g. c.: 1994, apj, 427, 857 [8] hunt, l. k., mannucci, f., testi, l., et al.: 1998, aj, 115, 2594 [9] kuiper, r., klahr, h., beuther, h., henning, t.: 2010, apj, 722, 1556 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wood, k.: 2007, apjs, 169, 328 [23] schutte a. j., van der walt d. j., gaylard m. j., macleord g. c.: 1993, mnras, 261, 783 [24] slysh v. i., dzura a. m., valtts i. e., gerard e.: 1994, a&as,106, 87 [25] stetson, p. b.: 1987, pasp, 99, 191 [26] traficante, a., calzoletti, l., veneziani, m., et al.: 2011, mnras, 416, 2932 [27] varricatt, w. p., davis, c. j., ramsay, s., todd, s. p.: 2010, mnras, 404, 661 bibitem zhang q., hunter t. r., brand j., et al.: 2005, apj, 625, 864 107 http://dx.doi.org/10.1086/376696 http://dx.doi.org/10.1111/j.1365-2966.2004.07543.x http://dx.doi.org/10.1086/597811 http://dx.doi.org/10.1088/0004-637x/722/2/1556 http://dx.doi.org/10.1086/346149 introduction near-infrared images hi-gal images spitzer/irac archive images iras19388+2357(mol110) conclusions 50 acta polytechnica ctu proceedings 2(1): 50–54, 2015 50 doi: 10.14311/app.2015.02.0050 optical photometry of lmxbs: uw crb (=ms 1603+260) and v1408 aql (=4u 1957+115) p. a. mason1,2, e. l. robinson3, s. gomez1, j. v. segura1 1department of physics, university of texas at el paso, 500 w. university, el paso, tx, usa, 79902 2department of mathematics and physical sciences, new mexico state university dacc, las cruces, nm, usa, 88003 3department of astronomy, university of texas at austin, 1 university station c1400, austin, tx, usa, 78712 corresponding author: pmason@nmsu.edu abstract we present new optical observations of v1408 aql (= 4u 1957+115), the only low mass x-ray binary, black hole candidate known to be in a persistently soft state. we combine new broadband optical photometry with previously published data and derive a precise orbital ephemeris. the optical light curves display sinusoidal variations modulated on the orbital period as well as large night to night changes in mean intensity. the amplitude of the variations increases with mean intensity while maintaining sinusoidal shape. considering the set of constraints placed by the x-ray and optical data we argue that v1408 aql may harbor a very low mass black hole. optical light curves of uw crb display partial eclipses of the accretion disk by the donor star that vary both in depth and orbital phase. the new eclipses of uw crb in conjunction with published eclipse timings are well fitted with a linear ephemeris. we derive an upper limit to the rate of change of the orbital period. by including the newly observed type i bursts with published bursts in our analysis, we find that optical bursts are not observed between orbital phases 0.93 and 0.07, i.e. they are not observable during partial eclipses of the disk. keywords: low mass x-ray binaries photometry individual: v1408 aql individual: uw crb. 1 introduction low mass x-ray binaries (lmxbs) are composed of a low mass donor and a compact object, either a black hole or a neutron star primary. the compact star accretes matter from the donor via an accretion disk. we present updates on our previous work on the black hole candidate v1408 aql and the lmxb containing a neutron star, uw crb (see mason et al. 2012). 2 v1408 aql v1408 aql is a lmxb whose secondary star fills its roche lobe and the compact object is a black hole candidate. this is the only lmxb black-hole candidate known to be in a persistently active soft state. there are only three other x-ray binaries that have been found to be persistently active, lmc x-1, lmc x-3 and cyg x1. but unlike v1408 aql, these are all high mass x-ray binaries. v1408 aql has always remained in the spectrally soft, disk dominated, x-ray state, never reaching the low/hard state. the persistent activity of v1408 aql and the disk-dominated soft x-ray spectrum are likely to be related. 2150 2151 2152 2153 2154 2155 0.02 0.025 0.03 0.035 0.04 0.045 0.05 hjd − 2454000 r e la ti v e i n te n s it y figure 1: optical photometry of v1408 aql on four nights during 10-15 august 2012. intensity is shown relative to the combined light of stars labeled 6 and 8 of figure 1 in bayless et al. (2011). an orbital sine wave like variation is seen each night as well as a large drop in brightness between the second and third night. once night to night variations are removed, the light curve is nearly sinusoidal. the optical light curve of v1408 aql shows a sinusoidal orbital modulation at the system’s 9.33 hr orbital 50 http://dx.doi.org/10.14311/app.2015.02.0050 optical photometry of lmxbs: uw crb (=ms 1603+260) and v1408 aql (=4u 1957+115) period. this period was first measured by thorstensen (1987) and it was later confirmed by bayless et al. (2011) and mason et al. (2012). we obtained new high-speed optical photometry of v1408 aql using the 2.1m otto struve telescope of mcdonald observatory on four nights in august 2012 and five nights in july 2012. photometry was obtained using the argos ccd photometer (mukadam and nather, 2005) and a bvr (broad-band) filter with a time resolution of 10 seconds. the light curves from august 2012 are shown in figure 1. strong night to night transitions such as these are often observed. as part of our analysis, nightly mean variations were removed and normalized to form a data set for period analysis. once normalized and phased the orbital modulation becomes apparent, and a clear sinusoidal modulation is observed. 2.1 updated ephemeris for v1408 aql using all the available optical photometry for a total of 126 hours of observation spread over 29 nights and 5 years, we construct a phase dispersion minimization periodogram (stellingwerf, 1978), see figure 2. preliminary results were presented by gomez et al. (2013). we derive a precise orbital period of 0.388893(3) days. this improved period is consistent with previously published results. figure 2 also shows previous period determinations with error bars (thorstensen 1987; bayless et al. 2011; mason et al. 2012). the improved ephemeris for the time of maximum flux is t (hjd) = 2454621.829(4) + 0.388893(3)e 0.15 0.2 0.25 0.3 0.35 0.4 0.38820.38840.38860.38880.3890.38920.3894 p h a s e d is p e rs io n period (days) thorstensen, 1987 current period mason, 2012 bayless, 2011 figure 2: period search results using phase dispersion minimization. previously published results are also shown. 2.2 the nature of the compact star multi-wavelength datasets yield a variety of constraints on the compact component of this binary. the x-ray data show a soft spectrum well described by a multitemperature blackbody and in 15% of the observations an additional non-thermal steep power law component is seen (nowak et al. 2012). the optical intensity also occasionally increases sharply; see the histogram of nightly mean brightness in figure 3. the distance of v1408 aql is likely d > 7 kpc, but the compact star mass remains largely unconstrained by x-ray data (nowak et al. 2012). radio emission is 330-810 times fainter than expected in relation to other x-ray binaries (russell et al. 2011). 0 0.005 0.01 0.015 0.02 0.025 0 2 4 6 8 10 12 14 16 increase from minimum intensity c o u n ts ( n ig h ts ) figure 3: histogram of nightly optical intensity variations. in order to examine the sinusoidal modulation, light curves were shifted by the amounts shown in the histogram. zero on this linear scale is the relative intensity of the minimum observed nightly mean. for example, the first night shown in figure 1 is shifted down by approximately 0.013 to remove the night to night variation. on most nights v1408 aql remained near minimum intensity, while on one night (14 july 2012) it showed a particularly high intensity. the optical data suggest that the orbital modulation is dominated by the heated face of the donor star. even as strong night to night intensity fluctuations occur, a strong sinusoidal modulation remains at all brightness levels. bayless et al. (2011) estimated a mass ratio of q = m2/m1 > 0.3. the primary is most likely a black hole, since the system has never shown a type-i x-ray burst since its discovery in 1973, and the black hole candidate has a fast spin and a spectrum typical for black holes (nowak et al. 2008). fits of models to the optical light curve give strong evidence for a low mass primary with a mass of m < 7m�, as well as a low inclination of i = 11o − 25o. in summary, a low mass black hole model is consistent with both optical and x-ray emission studies of v1408 aql. 3 uw crb the lmxb uw crb contains a neutron star primary. the optical light curve displays partial eclipses of the disk by the donor and frequent bursts (mason et al. 51 p. a. mason et al. 2008, 2012, hakala et al. 2009). the appearance of type i x-ray bursts, either as x-rays or reprocessed as optical emission clearly indicates that the compact object is a neutron star. the secondary star is likely significantly evolved. an hst/cos uv spectrum uw crb was obtained (froning et al. 2012) in which the c iv lines are totally absent. we obtained optical photometry at the mcdonald observatory otto struve 2.1-m telescope in june 2013, using the argos ccd photometer (mukadam and nather, 2005), a bvr filter, and 10-second time resolution (see figure 4). several bursts are easily noticed. the burst frequency varies from night to night. 0.00 0.05 0.10 0.15 0.20 0.0 1.0 2.0 3.0 r e la � v e i n te n si ty orbital phase 2013, june 10 2013, june 11 2013, june 12 2013, june 13 figure 4: optical photometry of uw crb on four successive nights in june 2013 is shown, compared to star c in the finder chart of hakala et al. (1998). the light curves taken on the first three nights have been shifted in intensity relative to the fourth by increments of 0.05. the phase and shape of the partial eclipses fluctuate over a 5.5 day beat cycle. see mason et al. (2008, 2012) and hakala et al. (2009). 3.1 an updated ephemeris for uw crb we measured 7 new eclipse times which we combined with 56 eclipses listed by mason et al. (2008, 2012) for a total of 63 eclipses. the new best-fit linear ephemeris is: t (hjd) = 24553118.83822(47) + 0.077067211(17)e where e is the eclipse number. we also fit a quadratic function to the eclipses in the o-c diagram (see figure 5). the quadratic term is not significant, but allows the derivation of an upper limit to the period derivative of (at 90 % confidence): |dp/dt| < 3.0× 10−11s/s. 3.2 optical bursts as a mixture of hydrogen and helium accumulates on the neutron star, some hydrogen fuses steadily while a he-rich layer builds up. when this atmosphere becomes dense and hot enough, it suddenly fuses into carbon and emits a type i x-ray burst (e.g. strohmayer and bildsten, 2006). 0 -0.02 -0.01 0 0.01 0.02 figure 5: o-c diagram for 63 eclipses of uw crb with respect to the best-fit linear ephemeris. the dashed line is the best-fit quadratic function. however, the quadratic term is not statistically significant. the vertical scatter of the points is due to periodic phase shifting of the eclipses. figure 6: photometry of a burst that occurred on june 12, 2013. the intensity, compared to star c in figure 1 of hakala et al. (1998), of this typical burst rises to peak in 10-20 s and declines in about 50 s. uw crb is a frequent x-ray burster (hakala et al., 2005). optical bursts are observed (e.g. hynes, robinson, and jeffery, 2004) because regions in the accretion disk absorb x-rays and are heated. the heated areas emit optical radiation and we see a burst while the optical emission regions cool. an example optical burst is shown in figure 6. when combined with published timings (see mason et al. 2012 and hakala et al. 2009 and references therein) 40 bursts events have been ob52 optical photometry of lmxbs: uw crb (=ms 1603+260) and v1408 aql (=4u 1957+115) served. these are shown as a function of orbital phase in figure 7. notice that there is a partial eclipse zone where no bursts have been detected. if the bursts were being reprocessed off the secondary star, the number and strength of the bursts would have decreased slowly before and increased slowly after eclipses. instead, the number drops abruptly at the edge of the eclipse. this is strong evidence that the optical burst emission region is the disk and not the heated face of the secondary. figure 7: distribution of 40 type i bursts as a function of phase. no bursts occurred between phase 0.93 and 1.07 due to partial eclipses of the accretion disk by the companion. 4 conclusions a new ephemeris is derived for v1408 aql. the light curves show sinusoidal modulation with night-to-night changes in mean brightness. our models favor a black hole primary with a mass less than 7 solar masses. we suggest that the orbital modulation is caused primarily by the changing aspect of the heated face of the donor star. v1408 aql (= 4u 1957+115) is likely a black hole, and there is increasing evidence that it is near the lower end of the observed black hole mass distribution. analysis of new and previously published partial eclipses of the accretion disk in uw crb are well fitted with a linear ephemeris. in addition, there is ∼ 5 day periodic shifting around phase 0 due to precession of an elliptical accretion disk (mason et al. 2008, 2012; hakala et al. 2012). an upper limit on the rate of change of the orbital period based on the best fit quadratic ephemeris is derived, see equation 1. strong bursts are seen at all phases outside of eclipse indicating that the disk is the source of the re-processing responsible for the optical bursts, rather than the heated face of the donor star. acknowledgement we thank tom maccarone for discussions concerning the nature of the compact object in v1408 aql (=4u 1957+115). we thank the referee, hans ritter, for a careful review and many helpful comments and corrections. this work is supported by nsf/paare grant no. 0958783. references [1] bayless, a. j., robinson, e. l., mason, p. a., robertson, p., 2011, apj, 730, 43. doi:10.1088/0004-637x/730/1/43 [2] froning, c. s., 2012, aas, 219, 153.12. [3] gomez, s., mason, p. a., robinson, e. l., 2013, aas, 221, 142.25. [4] hakala, p., et al., 2005, mnras, 356, 1133. [5] hakala, p., hjalmarsdotter, l., hannikainen, d. c., muhli, p., 2009, mnras, 394, 892. [6] hynes, r. i., robinson, e. l., jeffery, e., 2004, apj, 608, 101. doi:10.1086/422471 [7] mason, p. a., robinson, e. l., gray, c. l., hynes, r. i., 2008, apj, 685, 428. doi:10.1086/590381 [8] mason, p. a., robinson, e. l., bayless, a. j., hakala, p. j., 2012, aj, 144, 108. [9] mukadam, a. s., nather, r. e., 2005, japa, 26, 321. [10] nowak, m. a., et al., 2008, apj, 689, 1199. doi:10.1086/592227 [11] nowak, m. a., wilms, j., pottschmidt, k., schulz, n., maitra, d., miller, j., 2012, apj, 744, 107. doi:10.1088/0004-637x/744/2/107 [12] russell, d. m., et al., 2011, apj, 739, l19. doi:10.1088/2041-8205/739/1/l19 [13] stellingwerf, r. f., 1978, apj, 224, 953. [14] strohmayer, t., bildsten, l., 2006, in ”compact stellar x-ray sources”, ed. by w. lewin and m. van der klis, (cambridge: cambridge univ. press) p. 113. doi:10.1017/cbo9780511536281.004 [15] thorstensen, j. r., 1987, apj, 312, 739. discussion simone scaringi: could the absence of type i x-ray bursts at specific phases be used to constrain the size of the secondary star and/or the emitting region giving rise to the burst. 53 http://dx.doi.org/10.1088/0004-637x/730/1/43 http://dx.doi.org/10.1086/422471 http://dx.doi.org/10.1086/590381 http://dx.doi.org/10.1086/592227 http://dx.doi.org/10.1088/0004-637x/744/2/107 http://dx.doi.org/10.1088/2041-8205/739/1/l19 http://dx.doi.org/10.1017/cbo9780511536281.004 p. a. mason et al. paul mason: yes, especially when combined with information about the mass ratio. currently, we do constrain the optical burst emission region to the disk. we exclude significant re-processing off of the companion star. 54 introduction v1408 aql updated ephemeris for v1408 aql the nature of the compact star uw crb an updated ephemeris for uw crb optical bursts conclusions acta polytechnica ctu proceedings doi:10.14311/app.2016.3.0078 acta polytechnica ctu proceedings 3:78–81, 2016 © czech technical university in prague, 2016 available online at http://ojs.cvut.cz/ojs/index.php/app validation of cutting force characteristic for complex toolpath henrik tamás sykora∗, attila kovács, dániel bachrathy budapest university of technology and economics, department of applied mechanics, muegyetem rkp. 5., 1111 budapest, hungary ∗ corresponding author: henrik.sykora@gmail.com abstract. in the design phase of the milling process, there is a great need for the prediction of the cutting force, the required torque and power of the spindle. these informations could be used to optimize the tool path and improve the material removal rate. in this work, we present our dexel based simulation software, its modules, calculations steps and the simulation method. different force models were analysed to describe the specific force as a function of the local chip thickness. the models were fitted to the measured force data. then the selected force model was validated in case of a complex tool path. keywords: milling experiment, cutting force prediction, cwe simulation, force model fitting. 1. introduction in the design of the milling process of a product, cam (computer-aided manufacturing) softwares can be used to generate a tool path, which is appropriate for the geometric requirements. however, these algorithms do not take the dynamic effects into account. in order to make optimal usage of the machines, it is indispensable to know the connection between the milling tool and workpiece (cwe – cutter workpiece engagement) accurately. however, the commercially available cam programs usually do not provide a possibility to obtain this kind of data. even if the cwe is known, it is also essential to use an appropriate cutting force function to predict the resultant cutting forces and moments, the resultant vibrations, the surface quality and the stability properties [1, 2]. the main goal of our work is to develop a program capable of a generic 3 axis milling simulation with a cylindrical milling tool along with the computation of forces and the moments. the experimental validation of the theoretical model is also presented. 2. software the software (see in figure 5) is made up of loosely interconnected modules, such as the solid-modeling, cwe calculation, force calculation and the graphics module. this makes the development and the extension of the program manageable. all of the settings are stored in an user friendly input file, which enables to easily set up simulations. the software was implemented in c++ programming language and it is crossplatform (windows, linux). in the following sections these modules will be presented briefly. 2.1. dexel method the solid modelling module utilises the multi-dexel solid modelling technique [3], which is a promising discretization technique for this field, because of its many advantages, such as it is relatively easy to implement compared to other techniques [4] (e.g. the constructive solid geometry technique). it is sufficiently accurate, fast and there is potential for even more speedup by utilizing the parallell processing powers of the gpus [5], since it comprises many separately calculable parts. the dexel model represents the solid body by choosing a plane with a grid and then storing the intersection points of the body and the infinite straight lines projected from this grid [6] as presented in figure 1 for planar bodies. this can be extended by using three perpendicular plaines in 3d for 3 dimensional bodies (presented in figure 2.), which leads to the so-called tridexel representation. depthy y x depthx dexelpoint dexelplane x1 x2 ... dx2,1 dx2,2 px2,1 px2,2 figure 1. multi-dexel method in two dimensions by applying two perpendicular dexeplanes (blue and green). the definition of boolean operations between the tridexels (subtraction, intersection, union) is also nec78 http://dx.doi.org/10.14311/app.2016.3.0078 http://ojs.cvut.cz/ojs/index.php/app vol. 3/2016 validation of cutting force characteristic for complex toolpath figure 2. three dexel planes in the tri-dexel method to represent spatial bodies. essary. this is done by defining the operations between individual dexels. the concept of the operations between dexels is shown in figure 3. d2 d1 ⋂ d2d1 ⋃ d2 d1/d2 p1,1 p1,2 p1,3 p1,4 p1,5 p1,6 p2,1 p2,2 p2,3 p2,4 p2,3 p1,5 p2,3 p2,4 p1,1 p1,2 p1,3 p1,4 d1 figure 3. boolean operations defined between dexels. 2.2. simulation process the use of primitives, like cylinders and cuboids combined with the boolean operations allows to create simpler workpieces. after defining a milling tool and a workpiece their intersection tridexel can be calculated, which can be used to calculate the cwe. it is represented by a matrix in figure 4. this cwe matrix can be used for calculating the forces, using various force models and edge geometries. these steps are combined into a simulation by utilizing a standard g-code input. the toolpath is subdivided into smaller segments. for every segment the cylindrical tool and swept volume are created to intersect the workpiece with. these intersections can be used to calculate the cwe and the forces. 3. force models in the calculation of the cutting force components, it is essential to determine the relationship between the local chip thickness and the specific force. in principle, this relationship is based mainly on the material properties. the cutting force is often modelled via fem simulations, where the material model is usually based on an elastic-viscoplastic constitutive equation extended with thermal softening effect, where the yield stress of the material is governed by the expression proposed by johnson and cook [7]. in spite of the advanced material description, the resultant cutting figure 4. main steps in generating the cwe from left to right: intersection tridexel, cwe calculation, cwe matrix. figure 5. screenshot of the software on windows showing the simulation of the later measured case. force has a large sensitivity on the parameters of the fem models. in practice, the resultant cutting force is measured and then different force models are fitted, from the simplest linear (1), the shifted-linear (2), the three quarter (3) and to the third order polynomial (4) force model. the resultant coefficients are in close correlation with the material properties. flinear = k w h, (1) fshift = kshift w (h + hshift), (2) fthree−quarter = k3/4 w h3/4, (3) fpolynomial = w (q0 + q1 h + q2 h2), (4) where k is the cutting force coefficient, w is the chip width, h is the chip thickness and qi are the polynomial coefficients. 4. experimental validation several cutting force measurements were carried out with constant radial and axial depth of cut with the measurement setup presented in [8] and in figure 7 8. different force models were fitted for each measurement, based on the time function of the cutting force, shown in figure 6. the coefficients shown in table 1 have good agreement with ones presented in the literatures [2]. it is found, that cutting force models with multiple parameters can lead to smaller error, however, the linear and three-quarter rule have smaller 79 h. t. sykora, a. kovács, d. bachrathy acta polytechnica ctu proceedings cases linear shifted 3/4 cubic k kshift hshift k34 q0 q1 q2 1 696.6125 307.4470 0.5448 976.3130 31.6258 2011.2 -1854.6 2 840.1928 503.9487 0.3957 1008.88 56.4329 902.4742 -58.1335 3 784.6768 479.4080 0.4464 871.3346 65.2098 479.9361 271.2453 4 788.5676 785.1263 0.0206 1146 21.6363 765.2083 1080.9 5 939.6342 664.7103 0.2348 1159 34.7089 915.6962 365.1631 6 828.4690 624.2070 0.2230 940 28.8512 2842.1 -2359.6 7 869.1109 750.2941 0.0597 1289.6 14.7875 872.2752 1175,3 8 978.0969 877.4342 0.0625 1231.6 17.8862 2438.7 -1546.7 9 841.3138 728.3958 0.1023 972.35 19.9137 1591.9 -727.6420 average 840.7416 635.6635 0.2322 1066.12 32.3391 1424.388 -603.671 deviation 83.8457 177.8332 0.1906 144.1859 17.55409 834.6887 1218.447 rel. dev. 0.09973 0.2798 0.8208 0.1352 0.5428 0.5856 -2.0184 table 1. fitted parameters where the units are given as follows: k [ n/mm2] , kshift [ n/mm2] , hshift [mm], k3/4 [ n/mm7/4] , q0 [n/mm], q1 [ n/mm2] , q2 [ n/mm3] . time (s) 0 0.005 0.01 0.015 |f (t )| 0 100 200 300 400 fitted force signal measured linear shifted linear k34 cubic figure 6. comparison of various fitted force models and the actual measured force signal. deviations between different measurement conditions. hence, these two are good candidates for force computations at complex cwe-s. figure 7. workpiece after the measurements; on the lower side the variable axial-radial depth test can be seen. furthermore, a continously changing axial and radial immersion toolpath was used as shown in figure 7. the cwe was computed based on the multi-dexel solid modelling and the resultant cutting force is computed for the linear force model. figure 9 presents the measured and the calculated forces, respectively. finally, an experiment was conducted with a more figure 8. the tool used for the force measurements. time [s] |f o rc e | [n ] 0 100 200 300 measured 5 simulated 050 figure 9. comparison of the measured and the simulated force signal in the case of continously varying axial and radial immersions. � maximum force, � average force, � minimum force. complex toolpath, where a circular arc was investigated (see figure 5), on which we tested the force signal by upand downmilling procedures as well. based on the very good agreement shown in figure 9-11, it can be stated, that the simplest force functions (linear, three-quarter) can be used to compute the resultant cutting force precisely in milling, if the cwe is determined with high accuracy. during testing of the more complex toolpaths (figure 5, figure 10-11) we were faced with the fact that our calculated force signal was qualitatively the same as the measured, but the exact timings were not 80 vol. 3/2016 validation of cutting force characteristic for complex toolpath time [s] 0 5 10 15 20 |f o rc e | [n ] 0 50 100 figure 10. comparison of the measured and the simulated force signal in the case of downand upmilling. measured forces: � maximum, � average, � minimum; calculated forces: � maximum, � average, � minimum. correct. after investigating the effect it was concluded that this comes from the fact that in our calculations it was assumed that the milling machine can follow the prescribed speeds perfectly and did not take into account the control. in reality, this assumption is false, since these machines have their own control module, which could smooth out the sharp direction changes due to the limited jerk and accelerations. the milling machine, on which the measurements where carried out, is not an industrial machine, and in case of sharp direction changes the machine slows down and when it runs off of the material it speeds up more considerably. because the goal of the measurement was to validate only the forces the time signal was corrected by inserting small delays where needed, with the sum of 0.675 seconds. 5. conclusions and future research from the experiments it can be concluded that the calculation of the forces is sufficiently good and the results are promising. there is a good agreement between the calculated and the measured data despite not taking the control dynamics of the milling machine into account. the computed results can be used for feed optimization, also the cutting process of a workpiece can be investigated deeper to avoid the chatter and stability losses, so it can also help manufacture items of higher quality faster. in the future the expansion of the software with the above mentioned functions will be pursued. acknowledgements this paper was supported by the hungarian scientific research fund otka pd-112983 and the janos bolyai research scholarship of the hungarian academy of sciences. time [s] 0 5 10 15 20 |f o rc e | [n ] 0 50 100 figure 11. comparison of the corrected measured and the simulated force signal in the case of downand upmilling. measured forces: � maximum, � average, � minimum; calculated forces: � maximum, � average, � minimum. references [1] y. altintas, p. kersting, d. biermann, et al. virtual process systems for part machining operations. cirp annals manufacturing technology 63(2):585–605, 2014. doi:10.1016/j.cirp.2014.05.007. [2] d. bachrathy. cutting dynamics and surface quality. ph.d thesis, budapest university of technology and economics, 2013. [3] y. ren, w. zhu, y.-s. lee. feature conservation and conversion of tri-dexel volumetric models to polyhedral surface models for product prototyping. computer-aided design and applications 5(6):932–941, 2008. doi:10.3722/cadaps.2008.932-941. [4] x. qian, j. villarrubia. general three-dimensional image simulation and surface reconstruction in scanning probe microscopy using a dexel representation. ultramicroscopy 108(1):29–42, 2007. doi:10.1016/j.ultramic.2007.02.031. [5] b. tukora. material removal simulation and cutting force prediction of multi-axis machining processes on general-purpose graphics processing units. ph.d thesis, budapest university of technology and economics, 2012. [6] r. anderson. detecting and eliminating collisions in nc machining. computer-aided design 10(4):231–237, 1978. doi:10.1016/0010-4485(78)90058-1. [7] g. r. johnson, w. h. cook. fracture characteristics of three metals subjected to various strains, strain rates, temperatures and pressures. engineering fracture mechanics 21(1):31–48, 1985. doi:10.1016/0013-7944(85)90052-9. [8] i. biró, m. czampa, t. szalay. experimental model for the main cutting force in face milling of a high strength structural steel. periodica polytechnica mechanical engineering 59(1):16–22, 2014. doi:10.3311/ppme.7516. 81 http://dx.doi.org/10.1016/j.cirp.2014.05.007 http://dx.doi.org/10.3722/cadaps.2008.932-941 http://dx.doi.org/10.1016/j.ultramic.2007.02.031 http://dx.doi.org/10.1016/0010-4485(78)90058-1 http://dx.doi.org/10.1016/0013-7944(85)90052-9 http://dx.doi.org/10.3311/ppme.7516 acta polytechnica ctu proceedings 3:78–81, 2016 1 introduction 2 software 2.1 dexel method 2.2 simulation process 3 force models 4 experimental validation 5 conclusions and future research acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2017.11.0074 acta polytechnica ctu proceedings 11:74–79, 2017 © czech technical university in prague, 2017 available online at http://ojs.cvut.cz/ojs/index.php/app high speed railway lines – future part of czech railway network? lukáš týfaa,b,∗, david vodákb a department of logistics and management of transport, ctu in prague, faculty of transportation sciences, prague, czech republic b department of transportation systems, ctu in prague, faculty of transportation sciences, prague, czech republic ∗ corresponding author: tyfa@fd.cvut.cz abstract. the paper first describes high speed rail generally and explains the relationship between high speed and conventional railway networks (according to the vehicle types in operation on the network). the core of the paper is comprised of the methodology for choosing the best route for a railway line and its application to the high speed railway connection praha – brno. the algorithm used assumes the existence of more route proposals, which could be different in terms of the operational conception, line routing or types of vehicles used. the optimal variant is the one with the lowest daily cost, which includes infrastructure and vehicle costs; investment and operational costs. the results from applying this model confirmed the assumption, that a dedicated high speed railway line, only for high speed trains, has the same or lower investment costs than a line for both high speed and conventional trains. furthermore, a dedicated high line also has a lower cost for infrastructure maintenance but a higher cost for buying high speed multiple units. keywords: high speed rail, railway network, investment costs, operational costs. 1. railway network every transportation system has its advantages and disadvantages. this means, that there is a perfect match for every single transport demand (from the sum of all costs – externalities included). in general, railway transportation appears to be the perfect transport system for the backbone of urban and intra-urban transport, regional transport and long-distance continental transport. it also works perfectly in freight transport, as a part of combined transport, or regular transportation between branches. currently railway transportation in the czech republic cannot work optimally because [1]: • conventional mainlines, with standard parameters (from the czech point of view – railway lines with more tracks, maximal speed 120-160 km/h and electric traction), are almost out of capacity and routes in graphical time table cannot be assigned appropriately • some segments have high transport potential due to their route, but their technical and operational parameters (low capacity, low speed, low load class, often speed restrictions) make them unattractive for carriers (transport operators) • on some routes the traffic demand is not satisfied at all or it is satisfied by another mode of transportation, despite the fact, that these routes have perfect conditions for railway transportation • on the other hand, there are some effectively obsolete sections of railway line without any current use and without hope for future transport potential one possible solution, which could improve this situation, is the building of a high speed railway system [1]. the high speed railway system and the conventional railway system are both parts of the general railway system – see directive eu about interoperability of railway system [2]. high speed railway lines include [2]: newly built lines for speeds up to 250 km/h, upgraded conventional lines for speeds of approximately 200 km/h and upgraded conventional lines with special attributes. new high speed railway line is an adhesion railway line (usually track gauge 1,435 mm and electric traction), which has sections with a maximal speed of 250 km/h, and which are long enough so vehicles can reach this speed. combinations of these sections and upgraded sections (usually for the speeds up to 200 km/h) create the high speed railway network. the complexity of railway transportation is mainly caused by strong connectivity between the subsystems, the different life-cycle of the subsystems and the long time period between launch of a project and launch of its regular operation. changes in the operational conception (train traffic schedule) could be made in days and usually lasts one year. it usually takes several years to prepare a railway vehicle for operation and its servisable life is usually between twenty and thirty years. the building of new railway infrastructure usually takes a period of between ten and twenty years (including design and all administrative procedures) and its period of operation could be (with the proper maintenance) a hundred years 74 http://dx.doi.org/10.14311/app.2017.11.0074 http://ojs.cvut.cz/ojs/index.php/app vol. 11/2017 high speed railway lines or so. the situation is similar for high speed railway systems, but due to the higher speed, the connection between subsystems is even stronger. 2. typology of the connection between high speed and conventional network according to the strength of connection between the high speed and conventional railway system, we may establish four basic approaches [1]: (1.) completely segregated railway systems high speed and conventional lines networks are completely separated and independent in this case. conventional trains are using only the conventional lines and high speed trains are using only high speed lines. the advantage of this approach is that we can adjust the parameters of the line to meet the requirements of only high speed trains which usually leads to a smaller radius of curves, higher longitudinal gradients and lower investment costs. the main disadvantage is the impossibility of profiting from connection between the networks and the necessity of a really strong transport flow (passengers and so trains) to make this kind off network economically effective. we can find this kind of network in japan where the main reason for existence of this system is different track gauge on both railway networks. (2.) conventional trains are operating on the high speed railway network in this case, the conventional and high speed networks are connected, but only conventional trains (freight or passenger) are using both networks. conventional trains have to be modified for the operations on the high speed railway lines, especially for the safe passing of high speed trains. the main advantages of this approach are: the possibility of bypassing sections where demand exceeds capacity on the conventional network, the speeding up of conventional trains and/or increasing the usage of high speed railway lines. the disadvantage of this approach is the necessity of adjusting the parameters of the high speed lines to conventional vehicles, which have lower transport performance and lower maximum vehicle speed. in order to adjust the specifications we have to increase the radius of curves and reduce longitudinal gradients which results in higher investment costs. this approach also increases the wear on railway infrastructure (especially rails), because the speeds of the operated trains are different. the higher the difference between speeds is, the more strained the superstructure is. at the same time, the capacity of the high speed line is decreased, because parallel graphical timetables cannot be designed due to the difference in the speeds of trains which results in the need to build passing points, which allow trains with different speeds to pass each other. an example of this solution can be seen in spain, where there are different track gauges on conventional and high speed networks, but where some of the conventional trains are also operating on the high speed lines. these vehicles can change their wheel gauge thanks to the special construction on undercarriage and superstructure. (3.) high speed trains are operating on conventional network in this case the conventional and high speed networks are connected, but only the high speed trains are using both networks; they use the high speed line for the most of train line length. this solution allows high speed trains to get closer to the place of traffic demand without the need for a high speed line. on the other hand, the operation of high speed multiple units on the conventional line is more expensive than operating just conventional trains. the difference in costs is not big, but it still exists – mainly because of higher performance, higher safety and reliability requirements, and better parameters of high speed multiple units (higher mechanical body resistance to air pressure – train passing, running in tunnels). another disadvantage is the higher probability of accidents on the conventional network, which can lead to transition of delay from the conventional network to the high speed network. an example of this solution could be found in france, where high speed trans operate on conventional lines in order to get closer to city centres or to reach destinations, which are not yet connected to high speed network. (4.) high speed and conventional trains are both operating on both networks in this case the conventional and high speed networks are connected and both types of trains are using both networks. this solution uses the maximal potential of connection of both networks. it also brings the possibility of building the high speed railway line in stages, so that non-related segments of the high speed railway line can be built in order to fix the worst issues on the conventional network. the disadvantages are: the higher probability of the accidents on the conventional network, which can lead to a delay in the transition from the conventional network to the high speed network, complicated train traffic schedule designs and stricter the need for higher specifications of the high speed line. this solution also results in the less efficient use of high speed multiple units, because they cannot reach their full potential on the conventional network. an example of this approach can be found on many newly built and upgraded high speed railway lines in germany. the traffic consists not only of the high speed multiple units and regional passenger trains but also of the freight trains (mostly in the night) – see below. 75 lukáš týfa, david vodák acta polytechnica ctu proceedings 3. freight trains on high speed lines there are two basic approaches to freight operation on high speed lines. those approaches are distinguished according to the parameters of freight trains (length, weight, weight on axle, performance). in the first case, the freight train has the same parameters as the passenger train; the difference is only in the transported goods (mail, goods on pallets, special small containers). in the second case the freight train consists of classic freight carriages. in the first case, the freight trains can be treated as passenger trains. in the second case, we have to make some special safety measures to allow for the coexistence of the freight and passenger trains, which usually means, to the determination of different time periods for passenger trains and for freight trains. 4. general benefits of high speed railway network • quantitative – high speed railway lines increase the overall capacity of the railway network but it is important connecting new lines to the nodes must not cause traffic restrictions or instability on the existing conventional network • qualitative – better transportation by cutting the travel times between main cities • economical for railway carriers (transportation operators) – high speed multiple units are going to run more distance per day and more distance on high speed lines • decreasing intensity of road transportation by moving the transport flows to the railway with significant benefits in terms of safety and the environment • establishing connections between trains in periodical timetable • secondary effects – increasing share of electrical traction (and so production decreasing of carbon dioxide and harmful exhalation, possibility of independence from hydrocarbon fuels) 5. methodology of project comparison and its application the main goal of this paper was to introduce an algorithm for choosing the best route for a railway line and its application to the high speed railway connection praha – brno. the algorithm assumes the existence of more route proposals, which could be different in operational conception, line routing or types of vehicles. the optimal variant (according to the algorithm) is the one with the lowest daily cost, which includes infrastructure and vehicle costs and investment and operational costs. the algorithm does not take into account the dividing of the railway transportation among subjects – railway transportation is assumed as one unit. this unit consists of owning the railway, operating the railway and operating the railway transportation. the methodology is showed in the six connected tables. the first table, "operation", shows operated lines on the line; then number of trainsets (multiple units) and the daily time costs of trains in operation are calculated. the second table, "trainsets", shows the trainsets (multiple units) operated on the line and its results are the daily costs for buying trainsets. table iii. "unitary infrastructure investment costs", shows aggregated investment costs for the infrastructure – it is used for drawing up a budget. table iv. "infrastructure and operation on it" is designed to calculate the infrastructure investment and train operational costs according to energy consumption. table v. "track sections with constant operation" calculates the infrastructure and trains operating costs and the infrastructure investment costs. table vi. "recapitulation" shows the unitary daily costs from the previous tables and also divides daily the cost between the subjects on the railway (infrastructure manager, carriers – railway operators, infrastructure owner). to demonstrate the algorithm, two variants of high speed railway line praha – brno were evaluated. the first variant is from the study designed by sudop praha ag in 2010 [3], which is called study 2010. for the purposes of demonstration, variant h4 from the study 2010 was chosen. the second variant is from the study lt 2012, which is study 2010 with alterations from lukáš týfa, one of the authors of this paper [1]. study 2010 assumes following operational conception – the high speed railway network will be used both by high speed and conventional trains, high speed trains will be mainly operated on the high speed railway lines and only temporarily on conventional lines, conventional trains will use connections between both networks to enter the high speed lines and use them for the part of their journey. variant lt 2012 assumes the same horizontal alignment as study 2010, but makes changes in the vertical alignment – maximal proposed longitudinal gradient value is 35‰. this is a consequence of the change in the operational conception – line segment benešov – brno will be used only by high speed multiple units, those which can be easily operated in such a longitudinal gradient. a change in the operational conception could also cause a lowering of the minimal curve radius, but the author of variant lt 2012 thinks, that this value was defined too benevolent in study 2010, so no changes were made in this value [4, 5]. both variants have same maximal proposed speed – 350 km/h, although regular operations with a speed 350 km/h do not seem very effective (only a minor cutting in travel times , but a major increase of the operational costs). the main design parameters (track geometry characteristic) of study 2010 and lt 2012 are displayed in table 1. 76 vol. 11/2017 high speed railway lines study/variant study 2010 lt 2012 parameter var. v7 var. h4 line speed limit (max. train speed) v [km/h] 300 330 350 350 350 350 300 350 standard speed of the slowest trains vmin [km/h] 140 160 160 200 200 200 300 350 max. cant dmax [mm] 180 173 163 157 65 150 180 170 max. cant deficiency imax [mm] 150 100 100 80 80 130 150 80 max. cant excess emax [mm] 109 109 109 80 18 80 -* -* min. radius of curve rmin [m] 3 220 4 720 5 500 6 100 10 000 5170 3 220 5 790 type of transition curve and cant transition nonlinear nonlinear nonlinear non-linear non-linear nonlinear nonlinear nonlinear max. longitudinal gradient smax [%] 20 20 20 20 20 20 35 35 track construction slab slab slab ballasted ballasted slab slab slab max. length of tunnel by tsi lt,max [km] 5 5 5 5 5 5 20 20 * cant excess evaluation is considered only in large longitudinal gradients when high speed train cannot reach line speed table 1. main design parameters of study 2010 and lt 2012. evaluation of variants study 2010 and lt 2012 shows that the daily costs of study 2010 are about a million czk higher (see table 2 and figure 1). the daily costs for buying trainsets are 3% higher for variant lt 2012 – it considers only the high speed multiple units operation with the speed 300 km/h or 350 km/h. both variants have the same investment infrastructure costs. variant lt 2012 assumes slab track, which is more expensive than ballasted track, but this variant saves money on building tunnels and bridges. thanks to the maximal longitudinal gradient 35‰ (in comparison with 20‰ in study 2010) these structures are less frequent in this variant lt 2012. slab track also resulted in a 7% cost decrease, because maintenance costs are reduced. a negligible part of the costs includes: trainsets cleaning, overheads and investment to estates (estates costs are divided into long time). the different composition of costs for both of the variants causes different cost ratios between the infrastructure manager and railway transportation operator. in the study 2010, the railway transportation operator spends 60% of overall costs. in the variant lt 2012, the railway transportation operator spends 67% of overall costs. this distribution also has an impact on infrastructure manager’s daily costs for the unit of line length and the railway transportation operator daily costs, because the costs are 22% lower for variant lt 2012 and, because the costs are 14% higher for variant lt 2012. the difference in infrastructure investment costs are approximately three billions czk. 6. conclusion there is no doubt that the building and operation of a high speed railway system is one of the key steps to improve railway transportation functionality [6]. we can testify this not only in the states of western europe or japan, but also in morocco and saudi arabia. there are many different approaches to the relationship between current conventional networks and newly built high speed networks. these approaches are based on demographical structure, terrain configuration, conventional network parameters and the overall goals for railway transportation [7]. the choice of approach affects the relationship between the infrastructure and the train parameters. the different approaches to conventional/high speed train integration were described in the first part of this paper and applied on the planned high speed railway line praha – brno, which should create the best transport route in the czech republic. the best variant was chosen according to the total intern costs – investment and operational. for the purpose of calculation unit costs were used t. the values of these costs are different, according to what sources and assumptions are made and are subject to debate [8–10], but every cost which was used, is considered as representative and approximately the correct value. not only was a theoretical model made, but a fully functional and editable model was created for the final calculation of costs. the application of the model has confirmed the assumption that a high speed railway line, for use exclusively for high speed trains has the same or lower 77 lukáš týfa, david vodák acta polytechnica ctu proceedings study – variant study 2010 – var.h4 lt 2012 item costs per day costs per day costs per day [czk/day] [%] [czk/day] [%] hs train operation (time costs) 263 571 2 258 844 2 train-sets acquisition 1 365 766 10 1 713 718 13 investment to line-section infrastructure 3 086 366 22 2 933 059 22 investment to estates 11 707 0 11 798 0 electric power for trains 3 238 855 23 3 176 694 24 hs line infrastructure maintenance 2 521 874 18 1 464 062 11 hs train-sets maintenance 3 542 821 25 3 473 389 26 hs train-sets cleaning and overhead costs 162 182 1 145 023 1 sum 14 193 142 13 176 587 costs per day of railway transportation subjects: infrastructure manager 40% 33% carriers 60% 67% infrastructure owner 0% 0% infrastructure investment costs: total investment costs of infrastructure construction [mil. kč] 96 348 93 162 total costs of purchase estates [mil. kč] 427 431 table 2. costs evaluation of variants. figure 1. cost evaluation of variants. 78 vol. 11/2017 high speed railway lines investment costs than line for both high speed and conventional trains (the higher cost for slab track are compensated for by reduced number/length of the tunnels and bridges). a high speed line also has lower cost for infrastructure maintenance but a higher cost associated with buying only high speed multiple units. the costs for a line exclusively for high speed trains (high speed multiple units) is lower in the absolute terms in comparison to line for both high speed and conventional trains. the selection of the optimal variant seems to be clear, but minimization of total internal costs does not have to be the only important parameter. for example, we can decide only according to the total infrastructure manager investment costs. this shows strong connectivity between the railway transportation subsystems and the importance of continuous coordinated planning and design of the whole railway system. after we overcome all of the obstacles, society, as a whole, will be rewarded with a safe, environmentally friendly and highly effective railway system. this is why it is necessary to examine possible parameters (operational conception, line parameters, safety devices, train sets) of the new lines, before we even assign routing studies to the designers. furthermore, we cannot omit an evaluation of the time period between launching the project and launching its regular operation for all subsystems and their life cycles. it is possible to conclude that a high speed railway system in the czech republic should consist of several line types (or line segments). the parameters of these types should be based on the anticipated types of trainsets (multiple units), which are going to use the system. some segments should have parameters that fit both conventional trains and high speed trains, while others should be designed only for high speed trains. the individual types of lines should therefore be designed with different parameters. decisions on the types of high speed lines segments will significantly affect current and future conditions of the conventional railway network, which, once again, shows us strong connectivity between different parts of railway system. references [1] l. týfa. service of high speed railway systems in region [associate professor lecture]. praha: czech technical university in prague, 2013. isbn 978-80-01-05199-3. [2] l. týfa. nejnovější trendy v oblasti infrastruktury vysokorychlostních tratí. in odborná konference vysokorychlostní železniční doprava ve světě a v české republice [cd-rom]. praha: sudop praha a.s., 2007. [3] directive eu no. 2008/57/ec on the interoperability of the rail system within the community. [4] čsn en 13803-1:2010. railway applications – track – track alignment design parameters – track gauges 1435 mm and wider – part 1: plain line, 2010. [5] j. tvrdík. podmienky pre výstavbu vrt praha – brno. in 7. fórum koľajovej dopravy. bratislava: spoločnosť pskd – prevádzka a stavby koľajovej dopravy, 2011. isbn 978-80-88973-68-3. [6] j. ilík, j. kušnír, r. čech. budoucnost české železnice v dokumentech evropské dopravní politiky. železniční magazín. praha: m-presse plus, 2012, roč 18, č 1. issn 1212-1851. [7] uic. high speed rail: fast track to sustainable mobility. paris: uic, high speed department. 2nd ed. updated. isbn 2-7461-1452-6. [8] j. pohl. rychlá železniční osobní doprava: díl dvacátý: náklady a výnosy poprvé. železniční magazín. praha: m-presse plus, 2010, roč 16, č 1. issn 1212-1851. [9] j. pohl. rychlá železniční osobní doprava: díl dvacátý první: náklady a výnosy podruhé. železniční magazín. praha: m-presse plus, 2010, roč 16, č 2. issn 1212-1851. [10] i. c. gmbh, inrets. traffic and profitability for a western european high speed train network, 1994. 79 acta polytechnica ctu proceedings 11:74–79, 2017 1 railway network 2 typology of the connection between high speed and conventional network 3 freight trains on high speed lines 4 general benefits of high speed railway network 5 methodology of project comparison and its application 6 conclusion references 297 acta polytechnica ctu proceedings 2(1): 297–302, 2015 297 doi: 10.14311/app.2015.02.0297 study of cataclysmic variables with the satellites loft and gaia r. hudec1,2, v. šimon 1,2 1astronomical institute, academy of sciences of the czech republic, cz-25165 ondřejov, czech republic 2czech technical university in prague, faculty of electrical engineering, prague, czech republic corresponding author: rene.hudec@asu.cas.cz abstract the goal of this paper is to discuss the capabilities of esa satellite missions gaia (already in space) and loft (considered for the esa m4 slot) for investigation of cataclysmic variables (cvs). both gaia and loft can contribute to study of cvs and related objects. spectrophotometry and low dispersion spectroscopy are the most important for cv analyses with the observations of gaia. we present the possible strategies for investigation of cvs in the sampled photometric and spectroscopic data provided by gaia. e.g. statistical properties of the long-term activity of various types of cvs can be determined from them. loft can be a promising satellite to provide a sensitive x-ray monitor which will enable to investigate the little studied long-term activity of various types of cvs (especially the magnetic ones) in the x-ray band. keywords: cataclysmic variables dwarf novae intermediate polars x-rays loft gaia. 1 introduction although two esa satellite missions, namely gaia (already in space) and loft (considered for the esa m4 slot), focus on different science areas, both of them can effectively contribute to the investigation of cataclysmic variables (cvs). 2 the esa gaia mission the esa satellite gaia was successfully launched on december 19, 2013. gaia is an ambitious mission to chart a three-dimensional map of our galaxy, in the process revealing the composition, formation and evolution of the galaxy (e.g. de boer et al. 2000 and eyers et al. 2013). gaia will provide unprecedented positional and radial velocity measurements with the accuracies needed to produce a stereoscopic and kinematic census of about one billion stars in our galaxy and throughout the local group. this amounts to about 1 percent of the galactic stellar population. 2.1 astrophysics with gaia motivation of this paper is to outline the possibilities of performing astrophysics of cvs with gaia data. this is not trivial as the main goal of the gaia mission is to create a catalog. the photometric sampling provided by gaia will not be optimal for many variable astrophysical sources. however, the fine spectrophotometry (in reality ultra low resolution spectroscopy) provided by bp/rp photometers will be unique and important for many astrophysical investigations with gaia, including cataclysmic variables science (hudec & šimon 2007a and 2007b; hudec et al. 2012 and 2013). even the sampled gaia data can provide the following properties of the object: 1. determination of amplitude of the brightness variations 2. statistical distribution of brightness 3. absolute magnitude (from distance determined from parallax (and interstellar extinction)). note that these items 1, 2, and 3 even enable to establish the physically justified sequence of types of cvs. 4. color indices and their time variations in principle, it is also possible to search for the periods and cycles of the brightness variations, but one has to very cautious in doing this procedure using the sampled data from gaia. the sampling of the data provided by gaia is not optimal for the astrophysical work; these observations are not dense enough and not equidistantly distributed. typically, one can expect 50–100 photometric points over 5 years. however, additional data can be provided by the ground-based experiments. we propose the following two approaches for classification and verification of cvs in the sampled gaia data. the first one is to search for outbursts or high/low 297 http://dx.doi.org/10.14311/app.2015.02.0297 r. hudec, v. šimon state transitions, i.e. analysis of parameters of the light curve not strongly dependent on sampling. 2.2 statistical properties of the long-term activity of cvs, perspectives of histograms for gaia we investigated the profiles of the light curves of cvs of various types with real observations (daily means) from the afoev database (http://cdsarc.ustrasbg.fr/afoev/). we approximated the gaia sampling of these light curves by selecting the data separated by ∼ 20 days. this procedure yielded the amount of the sampled data and the length of the mapped time segment which roughly corresponded to the expected time of observing with gaia. we investigated which methods can yield correct identification of a cv of a given type almost independent of the sampling. we identified the impact of the gaia data on the investigation of cvs which are very active objects. figure 1: comparison of the observed (one-day means of afoev data (crosses)) and the approximated sampled data of the dwarf nova ah her (closed circles). heavily sampled data are expected from the gaia satellite. notice that it is difficult to resolve the profile of the light curve in the sampled data. the basic profile of the high state and its fluctuations in e.g. novalike systems and supersoft x-ray sources can be plausibly mapped by the sampled gaia data. only the high state/low state transitions may be missed or covered by only one or two data points by gaia. on the other hand, we expect the large outbursts of a dwarf nova to be covered by only a very few data points while short outbursts may be missed in the gaia data set. moreover, the individual outbursts will be probably captured in different phases due to their short duration. gaia will therefore usually not provide us with information about the profile of the outburst. however, we found that the statistical distribution of magnitudes of a given cv is only slightly distorted if a long time (several years) interval is covered. the statistical distribution of brightness and its parameters like the standard deviation, skewness, excess are a representative description of the properties of the long-term activity of cvs. these histograms are only slightly distorted by the sampling of the gaia observations if a long (several years) time segment is covered by the observations. nevertheless, a distortion may appear if the recurrence time of outbursts is close to the sampling time of gaia. figure 2: statistical distribution of the observed data in the dwarf nova ah her (from fig. 1). figure 3: statistical distribution of the approximated sampled gaia data of the dwarf nova ah her (from fig. 1). notice that the histogram is not dramatically influenced by the sampling and is similar to that of the observed data. we conclude that the sequence of the types of cvs, justified by the physics of accretion onto the compact objects, is reflected in the statistical distribution of the long-term brightness variations, hence it is suitable for analysis of the sampled gaia data. the importance of this sequence can be even enhanced if the apparent magnitudes are transformed to the absolute magnitudes by using the distances of cvs determined from the parallaxes. 2.3 rare flares (different from outbursts in dwarf novae) in cvs we investigated the possibility to detect and identify rare phenomena in cvs with gaia. deep monitoring of a large number of objects can lead to discoveries of more cvs with flares (phenomena different from outbursts) like the event discovered by van amerongen 298 study of cataclysmic variables with the satellites loft and gaia & van paradijs (1989) and to detection of additional such events even in the already known systems. we investigated some cvs in bamberg photographic plates which have a similar sampling and coverage as the expected gaia data. we conclude that the discovery of such events is possible with the monitoring planned for gaia (šimon 2010). 2.4 ultra-low dispersion spectra the ultra-low dispersion spectra will be provided by gaia rp/bp photometers. they can play a significant role in (1) searches for prominent spectroscopic variability, and (2) searches for objects with prominent (maybe variable) spectral features. as an example, the spectra of the outburst of v407 cyg (munari et al. 2011) (or of an analogous object) could be investigated by gaia. the spectral appearance of v407 cyg in outburst was a highly peculiar one. the spectrum of this event is completely different from those ever recorded for this object and other symbiotic mira variables. the white dwarf companion to the mira variable experienced an outburst similar to that of classical novae, and its ejecta were moving in the circumstellar environment already filled by the ionized wind of the mira. we summarize several types of cvs and the related systems with very bright emission lines: • supersoft x-ray sources in the optical high states (steiner & diaz 1998). their spectra display intense and broad emission lines (especially hα) (e.g. v sge (herbig et al. 1965)). such systems also often have balmer jump in emission. • classical novae in the nebular phase of the outburst (late phase, weeks to months after the maximum of the optical luminosity) – faint continuum, very strong emission lines of various elements (e.g. munari et al. 2013). • symbiotic systems: very strong emission lines (mainly hα) (e.g. davidson et al. 1978). • dwarf novae: highly variable strong balmer jump (e.g. walker & chincarini 1968). 2.5 the color indices from rp/bp spectra the color indices can be determined from the spectra obtained by the gaia rp/bp photometers. significant results are expected to be immediately available, there is no need to wait for years as in the photometry case. the color indices give important information on the spectral energy distribution. these indices can play an important role in classification and/or verification of cvs. they are also important for a search for the common properties of the sources of a given kind (e.g. to help identify a source as a cv and/or to study the evolution of the spectral profile with time as the cv undergoes various states of its activity). not only the color indices of the object at a given time, but also the time evolution of these indices are therefore important. they can also play a role in resolving among the individual radiation mechanisms (e.g. cyclotron radiation versus thermal emission). even variations of strong emission lines with respect to the continuum can be resolved by the color indices (e.g. hα changes, changes of balmer jump between emission and absorption). the color indices are also important for forming a representative ensemble of events (e.g. outbursts) in a given cv or in a given type of cvs. this is possible also for faint objects detected by gaia (down to mag ∼ 20). the parallaxes from gaia will enable to determine the distances, and hence the absolute magnitudes of many cvs. it will be therefore possible to determine and study the relation between the color indices and the absolute magnitudes of cvs of various types. 2.6 searches for cycles or periods variograms allow us to search for characteristic timescales or quasiperiods which extend just for several epochs of the cycle. variogram characterizes the spatial continuity or roughness of a data set. variograms are important for a search for the superorbital cycles in the long-term activity of cvs and low-mass x-ray binaries because their long-term activity is not periodic. standard period searches therefore often reveal nothing. we used the light curves of cvs of various types for testing. we applied the real observations from the afoev database for feasibility study of gaia data. we approximated the gaia sampling of these light curves by the data separated by ∼20 days. we investigated how the variograms were modified by the sampling. an example, the supersoft x-ray source v sge, is displayed in fig. 4. the amount of the sampled data and the length of the mapped time segment roughly correspond to the expected time of observing of gaia. since the duration of the high or the low state is longer than the time interval between the sampled data, the profile of the light curve is still recognizable. our tests showed that, in the case of the gaia data, variograms are suitable especially for investigation of the types of cvs which display the light curves, whose profiles are gradual. v sge with its alternating high and low states and the amplitude of the brightness variations similar for the individual epochs of the cycle is a good example (figs. 5 and 6). similar results can be obtained for novalike cvs with the episodes of the high and low states. however, it is necessary to be cau299 r. hudec, v. šimon tious in the case of dwarf novae because the outbursts, although with a large amplitude, are usually considerably shorter than the intervals of quiescence. this implies that some outbursts may be missed in the sampled data. it also emerged that binning of the data for variograms (fig. 6) plays a big role. fine binning yields a more precise determination of the cycle-length, but artifacts and false detections can appear, especially if the data are sampled. figure 4: comparison of the observed (one-day means of afoev data (crosses)) and the approximated sampled data (closed circles) of the supersoft x-ray source v sge. heavily sampled data are expected from the gaia satellite. figure 5: variogram for the observed data of the supersoft x-ray source v sge (from fig. 4). n bins refers to the number of bins used for constructing the variogram. the orbital modulation of cvs can be searched for and investigated in the sampled gaia data only under some favorable circumstances. although the length of the orbital period of cv can be considered to be stable (or at most slightly variable) during the proposed lifetime of gaia, it is reasonable to expect that the orbital modulation will be influenced by the long-term variations (e.g. outbursts, episodes of the high and low states). the amplitude of the long-term variations can often be considerably larger than that of the orbital modulation even in the case of a significant amplitude of this modulation (e.g. > 0.8 mag). only if the long-term level of brightness remains almost stable (or if the limited segments of the long-term light curve are selected), the orbital modulation can emerge. a preferable situation will be to fold the gaia data with the already known orbital period, and to investigate if and how the profile of the modulation varies with the changes between the states of activity. the orbital period can be determined for example by the follow-up observations of the ground-based telescopes. this is one of the cases of co-operation of the orbital and the ground-based observing components of the project. figure 6: variogram for the approximated gaia data of the supersoft x-ray source v sge (from fig. 4). a higher value of n bins can yield a better determination of the possible cycle-length, but also a bigger noise. 3 esa loft loft (the large observatory for x-ray timing) is specifically designed to exploit the diagnostics of very rapid x-ray flux and spectral variability in compact objects, yielding unprecedented information on strongly curved spacetimes and matter under extreme conditions of density and magnetic field strength (feroci et al. 2012; brandt et al. 2013). loft is designed to investigate variability from submillisecond quasi-periodic oscillations (qpo) to years long transient outbursts. loft belongs to esa medium class mission (m3 candidate) and was selected in feb 2011 to complete a 3year phase 0/a study. final down selection on jan 2014 (5 competed for single launch slot) was however negative for loft. there is however a chance to be selected for m4. loft is observatory-type mission: there are 2 onboard instruments and science data center provided by the community. the originally expected launch date was 2022–2024 timeframe (soyuz launcher) with 4+1 years mission lifetime and low earth orbit (550 km, 2 ground stations, 7 gb science data/orbit). the loft large area detector (lad) has an effec300 study of cataclysmic variables with the satellites loft and gaia tive area ∼20 times larger than any largest predecessor, uniquely combined with a ccd-class energy resolution. loft lat is suitable for observing cvs in a wide xray band of 1–40 kev, with very fine time resolution. the loft wide field monitor (wfm) has a 4 steradian field of view at soft x-rays to discover and localize x-ray transients and impulsive events and to monitor spectral state changes, triggering follow-up observations and providing a wealth of science in its own. loft wfm is suitable for monitoring of the outbursts and high states of cvs in the energy range of 1–50 kev. 3.1 the loft science the main loft science is the study of neutron star structure and equation of state of ultradense matter (3+ independent methods): (1) neutron star (ns) mass and radius measurements, and (2) neutron star crust properties. in addition to that, strong gravity and the mass and spin of black holes will be investigated (5 independent methods), e.g. quasi-periodic oscillation evolution, and, in the time domain, nfe line tomography and reverberation studies in bright active galactic nuclei (agns) and black hole candidates (bhcs) ((also vs nss), and relativistic precession. loft observatory science is expected to provide important observations for virtually all classes of relatively bright sources, including: x-ray bursters, high-mass x-ray binaries, xray transients (all classes), cvs, magnetars, gamma-ray bursts (grbs), nearby galaxies (smc, lmc, m31...), agn etc. in general, cvs are x-ray emitters (e.g. warner 1995). however, the properties and time evolution of their x-ray spectra strongly depend on the type and the state of activity of a given cv. although some general relations exist for a given cv type, each cv is really specific in this regard. since the proposed band of loft is 1–50 kev, those cvs with the hardest xray spectra will be the suitable targets for observing with this satellite. for example, the intermediate polars usually display the significantly larger values of kt in comparison with non-magnetic cvs (šimon et al. 2006). magnetic cv often have the hard x-ray intensity correlated with the mass accretion rate onto the white dwarf because of the accretion rate onto the magnetic poles of the wd. the dwarf nova outbursts of intermediate polars can be therefore accompanied by a strong increase of the hard x-ray intensity (e.g. do dra (szkody et al. 2002), gk per (fig. 7)). in polars (e.g. warner 1995), the hard x-ray intensity strongly increases in the optical high states. however, the case of the polar am her shows that the relation of the optical and hard x-ray emission varies for the individual episodes of the high state (šimon 2011). this emphasizes the necessity to obtain the observations of a larger ensemble of cvs to study the processes and their parameters which influence the behavior of cvs on the long timescales. we therefore expect the satellites like loft to be important instruments for investigation of magnetic cvs. figure 7: outbursts of the intermediate polar gk per in the optical band (afoev data) (a) and the hard xray band (bat/swift data (data of krimm et al. 2013). 4 conclusions both gaia and loft can contribute to study of cvs and related objects. spectrophotometry and low dispersion spectroscopy are the most important for cv analyses with the observations of gaia. both the loft wfm x-ray monitoring in the energy range of 1–50 kev and detailed lad observations with fine time resolution are important in this research field. gaia was launched on december 19, 2013. loft was not selected in competition with other esa m3 candidates, but it remains a promising candidate for m4. it is true that dense series of observations covering the intervals of several years are necessary to investigate the rising and decaying branches of outbursts and high/low states. nevertheless, a properly chosen strategy can yield suitable results of analyses even in the sampled photometric and spectroscopic data provided by gaia. we propose to obtain the statistical properties of the long-term activity of a representative ensemble of events (e.g. outbursts, high and low states) in a very large ensemble of a given type of systems. this is important for our understanding of the physical processes involved in the systems distributed in various regions of the galaxy which may have a different history of the star formation and chemical evolution. we also argue in favor of the development of more sensitive x-ray monitors that will be able to detect the long-term activity of cvs. even the available short data series obtained by the pointed x-ray observations 301 r. hudec, v. šimon of some known cvs revealed the very large variety of properties of their x-ray emission (e.g. warner 1995). the current monitors are able to observe mainly the luminous binary x-ray sources which contain the ns or the bh. since only a very few cvs are detectable by these instruments, loft can be a promising satellite to provide a sensitive x-ray monitor. acknowledgement we acknowledge partial support by ga cr grants 1339464j and 13-33324s. we also acknowledge the use of public data from swift/bat transient monitor provided by the swift/bat team. we also used the observations from the aavso international database (massachusetts, usa) and the afoev database operated in strasbourg, france. references [1] brandt, s., et al., 2013, eas publications series, volume 61, 2013, pp.617-623 doi:10.1051/eas/1361098 [2] davidson, k., et al., 1978, apj, 220, 239 [3] de boer,k., gilmore, g., hog, e., lattanzi, m.g., lindegren, l., luri, x., mignard, f., face, o., ferryman, m., de zeeuw, p.t. (eds) gaia concept and technology study report (the ’red book’) esa-sci(2000)4 pp381 (esa paris) 2000 [4] hudec, r. et al., 2013, acta polytechnica, vol. 53, no. 3, p.30 [5] eyer, l. et al., 2013, central european astrophysical bulletin, p. 115-126 [6] feroci, m. et al., 2012, experimental astronomy, volume 34, issue 2, pp.415-444 doi:10.1007/s10686-011-9237-2 [7] herbig, g. h., et al., 1965, apj, 141, 617 doi:10.1086/148149 [8] hudec, l., 2007, algorithms for spectral classification of stars, bsc. thesis, charles university, prague [9] hudec, r., šimon, v., 2007a, specific object studies for cataclysmic variables and related objects esa gaia reference code gaia-c7-tn-aio-rh-001-1. [10] hudec, r., šimon, v., 2007b, specific object studies for optical counterparts of high energy sources. esa gaia reference code gaia-c7-tn-aio-rh-002-1. doi:10.14311/ap.2013.53.0799 [11] hudec, r., šimon, v., hudec, l., 2013, acta polytechnica, vol 53, supplement, p.798 doi:10.1088/0067-0049/209/1/14 [12] krimm, h. a., et al., 2013, apjs, 209, 14 doi:10.1111/j.1745-3933.2010.00979.x [13] munari, u., et al., 2011, mnras, 410, l52 doi:10.1093/mnras/stt1340 [14] munari, u., et al., 2013, mnras, 435, 771 [15] šimon, v., mattei, j. a., 1999, a&as, 139, 75 [16] šimon, v., 2000, a&a, 360, 627 [17] šimon, v., et al., 2006, iaus, 230, 66 [18] šimon, v., hric, l., petŕık, k., et al., 2002, a&a, 393, 921 [19] šimon, v., 2010, advances in astronomy, article id. 382936 doi:10.1016/j.newast.2011.03.001 [20] šimon, v., 2011, newa, 16, 405 [21] steiner, j. e., diaz, m. p., 1998, pasp, 110, 276 [22] szkody. p., et al., 2002, aj, 123, 413 [23] van amerongen, s., van paradijs, j., 1989, a&a, 219, 195 [24] walker, m., & chincarini, g., 1968, apj, 154, 157 302 http://dx.doi.org/10.1051/eas/1361098 http://dx.doi.org/10.1007/s10686-011-9237-2 http://dx.doi.org/10.1086/148149 http://dx.doi.org/10.14311/ap.2013.53.0799 http://dx.doi.org/10.1088/0067-0049/209/1/14 http://dx.doi.org/10.1111/j.1745-3933.2010.00979.x http://dx.doi.org/10.1093/mnras/stt1340 http://dx.doi.org/10.1016/j.newast.2011.03.001 introduction the esa gaia mission astrophysics with gaia statistical properties of the long-term activity of cvs, perspectives of histograms for gaia rare flares (different from outbursts in dwarf novae) in cvs ultra-low dispersion spectra the color indices from rp/bp spectra searches for cycles or periods esa loft the loft science conclusions acta polytechnica ctu proceedings doi:10.14311/app.2016.3.0030 acta polytechnica ctu proceedings 3:30–34, 2016 © czech technical university in prague, 2016 available online at http://ojs.cvut.cz/ojs/index.php/app comparison of optical methods for characterization of glass mosaic weathering vladimir kocour∗, veronika petranova, jaroslav valach institute of theoretical and applied mechanics, prosecka 76, 190 00 prague, czech republic ∗ corresponding author: kocour@itam.cas.cz abstract. the work presented in this paper is a part of research project supported by ministry of culture and devoted to conservation of precious mosaic of "last judgement" on st. vitus cathedral in prague. the aim of the project is to enhance external protective polymeric coating on glass tesserae of mosaic and also to develop optical method for assessment of coating’s surface conditions. the paper concentrates on comparison of various methods for surface evaluation assesses their advantages and disadvantages and also discusses their suitability for long term monitoring of coating state, namely reflectometry, scanning probe microscopy, nanoindentation, white-light interferometry, and scanning electron microscopy. it is found that white-light interferometry and reflectometry methods are the most suitable ones for monitoring purposes. keywords: surface, reflectance, photometry, roughness, glass mosaic. 1. introduction this study is a part of extensive research project devoted to protection of medieval glass mosaic found on southern entrance to st. vitus cathedral on prague castle. finished at the end of 14th century, the last judgement mosaic at south wall of st. vitus cathedral in prague castle complex belongs to the oldest outdoor glass mosaics in the czech republic (see [1] and [2] for more details on mosaic history and conservation). although glass is being considered as a stable material, the reality is more complicated, especially if we are interested in long-time properties of the glass. historical glass is different from contemporary in chemical composition and production technology. therefore, its resistance to elements is generally lower and gradual degradation takes part. as tesserae of glass mosaics are exposed to various harmful effects like temperature fluctuation, rain, frost, chemicals, their durability can be improved with some protection coating [3]. based on previous restoration works, the glass is protected by a two-layer polymeric coating system and an additional new layer is now under development. but also the resistance of the coating itself is limited and has to be carefully selected. especially for monitoring of the state of this new layer, a novel monitoring method is being developed. 2. specimen treatment and characterization 2.1. artificial weathering of specimens as a first step, glass specimens microscopy slides dip coated with the polymeric coating were prepared and artificially weathered to observe and measure effects of degradation process. one half of every slide was covered with the investigated protective layer, while second half was left uncoated and unprotected. changes in appearance were measured after one week, two weeks, three weeks, four weeks, five weeks, and six weeks of artificial weathering of high humidity and temperature [4]. 2.2. sufrace characterization of specimens it is known that surface roughness and angular reflectivity are closely related to surface topography and can be used to characterize the degree of degradation. changes of the surface were visible and the deployed optical and mechanical methods showed, that these changes are also measurable [5] [6]. one variable which is used for characterization of surface topography is arithmetic roughness. arithmetical roughness is defined as ra = ∑n i=1 |zi − z̄| n , (1) where n is the number of points in the area, where roughness is calculated, zi is the measured z-coordinate for each individual point, z is the arithmetic mean of zi of all points in the area. our measurement shows that this variable increases with time of artificial weathering. another useful measure explained later is the change of reflectivity of surface from "specular" to "lambertian", i. e. evenly matt.the reflectance property enables to determine quantitatively "position" of surface on gloss to matt scale for given wavelength (surface may appear rough at shortwaves while being glossy at longwaves of electro-magnetic radiation). this position can be used to indicate surface roughness. various methods 30 http://dx.doi.org/10.14311/app.2016.3.0030 http://ojs.cvut.cz/ojs/index.php/app vol. 3/2016 comparison of optical methods were used to characterize changes occurring on the surface and inside the weathered coating. the changes result in visual degradation, loss of transparency and gloss. to study these surface alterations, we used digital microscopy, scanning probe microscopy (spm), scanning electron microscopy (sem), interferometry in white light and angle depended reflectometry. the following chapter concentrates on selection of suitable non-invasive methods for assessment and monitoring of coating properties. 3. used optical and mechanical methods 3.1. digital microscope a 3d microscope (hirox kh-7700) with focus stacking technique was used. this technique overcomes the limitation of optical microscopy of shallow depth of focus by combining multiple images with different focus. the visual changes documented by this digital microscope with enhanced depth of focus can be associated with roughening of the coating surface due to chemical and physical effects. one can observe snowflake like surface features, formation of rounded protrusions, ridges and valleys as surface "wrinkles". the changes (see fig. 1) are visually assessed by naked eye as gradual decrease of gloss and also as diffuse appearance of the surface. 3.2. nanoindenter with spm degradation of coating surface was also studied by nanoindenter (hysitron ti-750). it was found that hardness increases significantly with duration of artificial weathering. spm technique is integrated into the device, therefore the microtopography of the coating can be analyzed as well. increase of surface irregularities and overall roughness was observed as demonstrated in fig. 2. spm is not limited in its resolution to features comparable to wavelength of visible light as optional microscopy and can be used to measure fine surface microtopography. on the other hand, a significant drawback of this technique is the horizontal size of analyzed area, which is limited to 80 µm. therefore the investigated area cannot be considered representative. 3.3. scanning electron microscopy the scanning electron microscope mira ii. (tescan, brno, czech republic) was used for observation. however sem overcomes the limitation of optical microscope in terms of resolution and depth of filed. in order to observe the coating in sem, it is necessary to add a thin conductive layer (several nanometers of carbon or gold). therefore the method cannot be considered as non-invasive technique. a backscattered electron detector and a secondary electron detector are predominantly used to record a signal and form an image of observed surface. contrast in former is based on difference of atomic number in viewfield, while the topography is mostly recorded by the later. as seen on fig. 3, the only atomic contrast is presence of dust particles (upper part) and as concerns the topography of initial coating’s surface, this technique renders it flat (lower part of figure). 3.4. white-light interferometry white-light interferometry is an interferometric methods. interferometric methods use interference of light. interferometric devices are often constructed as michelson interferometer. the first experiments with white-light interferometry have been done by a. a. michelson [7]. however, the method became practically usable about 20 years ago in connection with the development of computational data processing. the name of this method is a bit improper. a better name would be "interferometry in polychromatic light" because the used light does not need to seem as "white" for human eye, and it is not necessary so that the light must be visible. a light source of time-incoherent radiation with continuous spectrum is used for this method. light bulb, led diode or super-luminescence diode can be such a light source. it is being utilized that coherence length of the radiation is short, and interference effect occurs only if difference of optical paths in the interferometer is comparable with the coherence length [8]. the measured specimen is situated in one arm. reference mirror is situated in second arm. the specimen is equipped with a fine linear stage. camera recording interfered light detects a series of snaps while the specimen moves on the stage. graphical dependence of intensity of detected radiation (light) is called correlogramme. the serie of snaps is then processed with help of various methods, usually hilbert transformation [9]. the result of this processing is the maximum of envelope of the correlogramme. position of maximum of the correlogramme is equal to z-coordinate (height coordinate) of measured point. we measured artificially weathered microscope slides coated with protective layer with zygo white-light interferometer (fig. 4). correspondence between roughness and weathering is then summarized in fig. 5. it shows that as weathering continues, the surface roughness increases. 3.5. reflectometry surfaces of real objects can be divided to three groups: 1. optically smooth, 2. transition, 3. optically raw. for optically smooth surfaces h < λ/4 and ∆ϕ < π in a cell of resolution. for optically raw surfaces h > λ/2 and ∆ϕ > 2π, where h is relief height, λ is wavelength of the radiation and ∆ϕ is phase shift detected in a cell of resolution. the transition surfaces have properties as smooth or raw surfaces in dependence of conditions and behave variously. a 31 v. kocour, v. petranova, j. valach acta polytechnica ctu proceedings figure 1. digital microscope depiction of progressing surface degradation. figure 2. gradual change of coating’s surface measured by scanning probe microscopy mode in nanoindenter. color coded vertical scale changes from image to image, thus height difference between the lowest and highest point is given. figure 3. sem image of the unweathered coating using (a) the bse detector and (b) the se detector. quite general access includes a possibility of reflection of incidence ray out of incidence plane. reflected radiation is completely described with bidirectional reflectance distribution function (brdf)[10] [11]. most of surfaces (except special cases like crocidolite, chatoyant glitter etc.) behaves "more orderly"; it is sufficient to study its reflectivity properties in plane of incidence ray. for angle dependent reflectometry, we have designed and built a programmable automatic device. reflectometry, in our interpretation, is measuring of relative intensity of reflected light in dependence on incidence angle φ and angle of observation ε, therefore the device is capable to create a two-dimensional map of relative intensity of reflected radiation in dependence on incidence angle and angle of observation ". as follows from 4, where the relative intensity is expressed as false colour [12], the more weathered surface the more dispersed peak in reflectivity map can be found. although this expression is more qualitative than quantitative, it is visible from such a map, how studied surface is specular or matt. the map shows relative intensity of light measured by reflectometer for given pair of incidence and observation angles (φ, ε), fig. 6. the maxima are naturally found for conditions, where the two angles are equal. increase in coating’s surface roughness is associated with widening of maximum’s profile. 32 vol. 3/2016 comparison of optical methods figure 4. three artificially weathered specimens of glass covered with protective layer: 0 weeks (non-weathered), 3 weeks, and 5 weeks old. above: horizontal profiles, below: false color height maps. field of view 3.53×2.56 mm. figure 5. relation between area roughness and duration of accelerated weathering of coating. 4. discussion and conclusions several methods capable of analyzing surface properties were applied in this presented comparative study. the major goal was to evaluate advantages and disadvantages of these methods and select the most suitable one for documentation and quantitative characterization of surface degradation. these methods will be exploited for long-term monitoring of protective coating of glass mosaic. it was confirmed that optical methods can be used to quantify changes the changes induced by the degradation of polymeric coating induced by accelerated weathering. it was found that the necessity to cover the surface of the specimen with a conductive layer excludes sem from further consideration, because this additional layer alters the specimen properties. the applied technique has to be non-invasive, if long-term monitoring has to be achieved. spm correctly indicates the surface topography changes, but the area studied was too small to be representative for the whole coating. digital microscope can document coating modifications as weathering progresses. but in long-term monitoring based on observation of microscopy slides, there is a possibility the results would be hindered by degradation of glass below coating as the digital microscope is the only method in compared set that relies on transparency of the set microscopy slide coating. therefore methods quantifying the coating roughness as a measure of its degradation are the best candidates for reliable monitoring. a suitable method is direct roughness measurement as in white light interferometry or indirect expression of roughness in surface reflectivity as measured by reflectometer. acknowledgements this research is supported by the czech ministry of culture project #df12p01ovv017 "technology of maintenance and conservation of the last judgment mosaic and methods of medieval and archaeological glass conservation". authors also thank to dr. vítů from faculty of transportation sciences of the czech technical university for performing measurement on white light interferometer and to dr. šašek from cet telč for nanoindentation. references [1] j. valach, v. kocour, m. černý. investigation of optical properties of protective layer for glass medieval mosaic of last judgement from st. vitus cathedral in prague. in the 23rd international congress on glass, book of abstracts. praha, česká sklářská společnost, 2013. pp. 154-155. isbn 978-80-904044-3-4. 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[8] peter de groot, leslie deck, surface profiling by analysis of white-light interferograms in the spatial frequency domain, j. of modern optics, 42, issue 2, pp 389-401, 1995 [9] r. onodera, h. watanabe, y. ishii. interferometric phase-measurement using a one-dimensional discrete hilbert transform, optical review, 12, (2005). doi:10.1007/s10043-005-0029-7 [10] v. kocour, j. bryscejn, d. vrba, j. valach. determination of optical properties of surfaces for use in cultural heritage protection. in o. jiroušek, d. kytýř. (ed.). experimental methods and numerical simulation in engineering sciences. praha: czech technical university, faculty of transportation sciences, 2012, pp. 15-18. isbn 978-80-01-05062-0. [11] j. valach, v. petráňová, d. majtás, v. kocour. surface degradation of coating measured by means of emissivity change. in e-proceedings of quantitative infrared thermography, bordeaux, university of bordeaux, 2014. http://www.qirt.org/archives/qirt2014/qirt [12] v. kocour, j. valach. semiautomatic surface reflectance measurement for monitoring of material weathering. in d. kytýř, p. zlámal, m. růžička. (ed.). proceedings of xiiith youth symposium on experimental solid mechanics. praha: czech technical university in prague, faculty of transportation sciences, 2014, pp. 56-60. isbn 978-80-01-05556-4. 34 http://dx.doi.org/10.1016/s0079-6700(00)00030-7 http://dx.doi.org/10.1007/s10043-005-0029-7 acta polytechnica ctu proceedings 3:30–34, 2016 1 introduction 2 specimen treatment and characterization 2.1 artificial weathering of specimens 2.2 sufrace characterization of specimens 3 used optical and mechanical methods 3.1 digital microscope 3.2 nanoindenter with spm 3.3 scanning electron microscopy 3.4 white-light interferometry 3.5 reflectometry 4 discussion and conclusions acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2019.22.0145 acta polytechnica ctu proceedings 22:145–149, 2019 © czech technical university in prague, 2019 available online at http://ojs.cvut.cz/ojs/index.php/app lightweight uhpc facade panel with led display michal ženíšeka,b,∗, aleš vodičkab, tomáš vlacha,b, lenka laiblováa,b, petr hájeka,b a czech technical university in prague, faculty of civil engineering, thákurova 7, 166 29 prague 6, czech republic b czech technical university in prague, university centre for energy efficient buildings, třinecká 1024, 273 43 buštěhrad, czech republic ∗ corresponding author: michal.zenisek@fsv.cvut.cz abstract. this article deals with the development of lightweight uhpc facade panel which also can serve as an led display. combining both functions into one gives the potential ability to create a led display on the facade of the building. the article describes the experiences of the development and production of a small-format prototype facade panel, including the installation of leds and their electrical connection. the result of the research is the prototype of a 16mm thick small-format lightweight facade panel with a led display for displaying text on two parallel lines. thanks to the waffle lightening, the total material and weight savings are approximately 58% compared with the full cross section. keywords: facade panel, uhpc, led diodes, led display. 1. introduction ultra-high performance concrete (uhpc) is cement composite that is characterized by very high strength and long durability. these two benefits make uhpc an excellent material that can be used across a wide range of applications from the field of construction and architecture. on the other hand, the uhpc price is several times higher than conventional concrete and therefore its use is reserved for the most demanding buildings or for buildings where long durability is expected. very often, the uhpc does not make the entire construction, but only the part which is most suitable for uhpc use. as an example, it can be mentioned joints of prefabricated elements, thin facade elements, staircases, balconies, etc. structures and products from uhpc are characterized by their subtle design [1]. this is possible due to high strength, but also due to other specific properties. the maximum aggregate size is very small in the most uhpc mixtures and does not limit the reinforcement or shape of the element. conventional steel reinforcement can be replaced by alternative reinforcement methods, for example by using technical textiles or dispersed reinforcement (fibres, microfibres) [2]. the high cement content and low porosity can better protect reinforcement against corrosion and it is therefore possible to reduce the cover depths of reinforcement. all these features make it possible to reduce the dimensions of uhpc elements. to sum up, the element’s self-weight will decrease as well as price due to lower material consumption [3]. the production of thin uhpc panels has been very popular lately. the main effort of most manufacturers remains to achieve the required properties while maintaining the minimum weight of the panel. nowadays, the facade panels with a thickness of 9mm can be seen [4]. however, reducing the thickness cannot be done indefinitely, and therefore other methods are also successfully used to reduce the weight of the panel. an example may be lightening the panel with polystyrene [5] or reinforcement with ribs [6]. 2. panel description and its production this paper deals with lightweight facade panel with dimensions of 500 × 600mm from uhpc with led display. lightening is realized by weakening the panel to 5mm on the front and reinforcing with 11mm high ribs on the back. the reinforcing ribs are conical, 6-9mm thick and are oriented in two perpendicular directions with an axial distance of 50mm. the space between ribs is used to fit leds (light emitting diodes) in a regular raster of 16.7mm, interconnecting them and creating an led display. compared to the full cross section, the material and weight savings are more than 58%. the overall geometry of the panel can be seen from the section in figure 1. the shape of the panel was optimized during development. the resulting solution is a compromise between the various requirements during production and subsequent placement on the facade. in the first place, it is necessary to ensure sufficient stiffness of the panel against wind load. therefore, the distance of the ribs cannot be too large to provide sufficient stiffness, but not too small to achieve the highest degree of lightening. the location of the ribs also strongly affects the way of panel reinforcement. it was considered using both dispersed reinforcement (fi145 http://dx.doi.org/10.14311/app.2019.22.0145 http://ojs.cvut.cz/ojs/index.php/app m. ženíšek, a. vodička, t. vlach et al. acta polytechnica ctu proceedings figure 1. panel section. material [kg/m3] proportions by weight cement 42.5 r 650 1 microsilica 40 0.06 quartz sand 0/0.6mm 1200 1.85 quartz powder 250 0.38 superplasticizers 25 0.04 water 182 0.28 table 1. used mixture. bres, microfibres) and textile reinforcement which also proved to be more appropriate. its main advantage is to ensure a higher stiffness of the panel and a uniform quality of reinforcement. in case of using dispersed reinforcement, it is possible to create ribs of any shape, but with a high fibres content (120 kg/m3) the overall bending stiffness is significantly smaller. other disadvantages include deterioration of workability and the risk of uneven reinforcement. for these reasons, the use of textile reinforcement has been preferred. at the beginning of the research, textile reinforcements from glass, carbon and basalt were tested [6]. these reinforcements had to be manually manufactured for research purposes (the textile was weaved) because commercially available products did not match the shape of the panel. the axial distance ribs as well as leds were in the first variant 24mm (i.e. only one led was placed between the ribs). this spacing of the leds would require a large distance from the viewer. for these reasons, a new solution has been sought which resulting in an axial distance of 50mm for ribs and 16.7mm for leds (fig. 1). the axial distance of the 50mm ribs was chosen intentional because excluding textile reinforcements that would have to be hand-made again, it was possible to use commercially available steel welded mesh. this has proved to be the best, both in terms of price and production. the uhpc mixture was gradually tuned to the final form during development (table 1). the most important task was to achieve a high degree of consistency so that the panel surface was sufficiently smooth and the ribs were correctly concreted. on the contrary, the high strength of the uhpc is not so necessary for the functionality of the panel because the panel will be stressed on the facade mainly by bending which is ensured by reinforcement. the w/c ratio of the mixture was therefore adjusted from the usual values of 0.25 to 0.28 and the content of cement and microsilica was reduced. it was also essential to use only small aggregate size (again due to the correct concreting of the ribs) and therefore quartz sand with a maximum grain size of 0.6mm was used. the panel was produced by casting uhpc into the mould and short vibration on the vibrating table. the panel was oriented upward during the casting and due to the high degree of consistency, it was not necessary to modify the view side (fig. 2). the reinforcement was inserted into the mould before casting and a short vibration ensured reliable filling of the ribs with concrete. the opposite approach, pushing the reinforcement into the concrete after casting has also been tested. however, this approach was not very successful due to the high viscosity of the uhpc mixture, which prevented the pushing of the reinforcement. fixing the leds to the panel was preceded the creation of holes for the leds. these could be set up in the panel in two ways: using a special punch during casting or cutting through a water jet on a hardened specimen. the first way, the special punch is driven into the panel during casting and will pull out after several hours when the mixture begins to harden. this solution is very cheap, but it puts increased demands on the precise location of the punch during casting. in contrast, the second way, the cutting of water jet is much more expensive, but the resulting quality of the holes is at a higher level. the leds with a diameter of 5mm were then placed in the holes and fixed by the hot-melt adhesive. the advantage of this solution is the possibility of easy repairs when necessary because hot-melt adhesive can be relatively easily removed. for these reasons, the epoxy resin proved to be inappropriate because it cannot be removed and can pass through holes around leds. 3. led display the led panel has 2 sections located one under the other, each consisting of 10 rows and 32 columns. therefore, there are 20 rows and 32 columns in total (fig. 3 and 4). every single row and column are represented by a wire, which is isolated from the others. 146 vol. 22/2019 lightweight uhpc facade panel with led display figure 2. ready mould (left), mould after casting (right). figure 3. front side of panel. figure 4. back side of panel. 147 m. ženíšek, a. vodička, t. vlach et al. acta polytechnica ctu proceedings figure 5. block diagram of led panel. figure 6. lit led panel. 148 vol. 22/2019 lightweight uhpc facade panel with led display at every crossing of row and column conductor there is led which has one lead (anode – positive) connected to the row conductor and the other lead (cathode – negative) to the column conductor. in total, there are 640 leds connected into a matrix form. the matrix display allows controlling the individual leds (pixels) by activating particular row and column. it is also possible to turn on multiple pixels simultaneously as long as the pixels are in the same row or column. displayed content is being drawn sequentially row by row and every row is active only for 1ms (time division multiplex). the full display is then formed in 20ms. this row switching repeats infinitely to keep the content of the display visible. the switching process is not noticeable to an observer, since human eye is relatively slow and is not capable to see such fast transitions. continuous current consumption of single led is maximally 20ma. time division multiplex keeps the led on only for 1/20 of the whole image time. this will obviously affect the brightness of led. the led can be overloaded up to five times (100ma) for a very short time, thus brightness drop can be compensated. even though the achieved brightness is still lower than the maximal possible, it is still good enough to ensure good readability of the display. the whole display assembly is powered from 5v power supply. the maximal current consumption of all leds on at maximal brightness is 3.2a; that gives the total power consumption 16w (consumption of electronics is negligible in comparison with the display). arduino uno microcontroller controls the matrix display (fig. 5). the additional electronics had to be developed for arduino uno in order to connect all the leds since this platform does not have enough output pins to connect all the rows (20) and all the columns (32). the electronics uses shift registers to convert serial information to parallel allowing the direct connection to rows/columns. every shift register has 8 outputs so connection of several shift registers in series is needed to serve all the lines while requiring only 3 output pins of arduino uno. there are 3 shift registers controlling the rows and 4 shift registers controlling the columns. software controlling the arduino uno was written in c programming language. code contains repertoire of characters for easy letters and digits displaying. there is text string being stored in the memory of microcontroller, which is being displayed on 2 lines of led panel. the text scrolls from right to left in case the string is longer than the width of the panel. the versatility of this solution allows easy extension in the future, such as displaying the current date and time (fig. 6) or information from a superior system. 4. summary the subject of this research was the development and experimental fabrication of a small format prototype lightweight uhpc facade panel with led display. the main effort was to find an optimal shape between sufficient stiffness, easy rib reinforcement and the densest distribution of leds forming the led display. the resulting panel has dimensions of 500 × 600mm with a total thickness of 16mm (5mm plate, 11mm ribs) and 50mm axial spacing. thanks to this solution, compared to the full cross section, the material and weight savings are more than 58%. the led display consists of two parallel fields for displaying text. the led display function is controlled by the arduino uno microcontroller, which has been specially programmed for this purpose. acknowledgements the authors thank university centre for energy efficient buildings ctu in prague for provision of laboratories for experimental work. this work has been supported by the ministry of education, youth and sports within national sustainability programme i, project no. lo1605 and by the tacr 11124 311 3111702a124 subtle concrete furniture and small structures for the railways stations. references [1] e. vicenzino, v. perry, t. chow, et al. first use of uhpfrc in thin precast concrete roof shell for canadian lrt station. pci journal 50:50–67, 2005. doi:10.15554/pcij.09012005.50.67. [2] k. hannawi, h. bian, w. prince-agbodjan, b. raghavan. effect of different types of fibers on the microstructure and the mechanical behavior of ultra-high performance fiber-reinforced concretes. composites part b: engineering 86:214–220, 2016. doi:10.1016/j.compositesb.2015.09.059. [3] m. robentrost, g. wight. experience and applications of ultra-high performance concrete in asia. in proceedings of second international symposium on ultra high performance concrete, pp. 19–30. 2008. [4] g.tecz engineering gmbh. https://www.gtecz-engineering.com/. [5] a. chira, a. kumar, t. vlach, et al. textile-reinforced concrete facade panels with rigid foam core prisms. journal of sandwich structures and materials 18:200–214, 2015. doi:10.1177/1099636215613488. [6] l. laiblová, t. vlach, m. ženíšek, et al. lightweight trc facade panels with the leds. in special concrete and composites 2017, vol. 760 of key engineering materials, pp. 141–146. trans tech publications, 2018. doi:10.4028/www.scientific.net/kem.760.141. 149 http://dx.doi.org/10.15554/pcij.09012005.50.67 http://dx.doi.org/10.1016/j.compositesb.2015.09.059 https://www.gtecz-engineering.com/ http://dx.doi.org/10.1177/1099636215613488 http://dx.doi.org/10.4028/www.scientific.net/kem.760.141 acta polytechnica ctu proceedings 22:145–149, 2019 1 introduction 2 panel description and its production 3 led display 4 summary acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2017.10.0052 acta polytechnica ctu proceedings 10:52–55, 2017 © czech technical university in prague, 2017 available online at http://ojs.cvut.cz/ojs/index.php/app testing of soil improvement below a hall floor monika súľovská∗, peter turček, zuzana štefunková stu bratislava, faculty of civil engineering, radlinského 11, bratislava, slovakia ∗ corresponding author: monika.sulovska@stuba.sk abstract. construction of a hall situated in north-west slovakia was prepared in winter. after finishing an elementary ground adaptation in february there were extreme weather conditions: the precipitation exceeded by over 300 % the normal long term measured amount. this indisposition of climate led to a stopping of the construction work. checks on the required quality of the base course using static load tests showed a failure in performance of compaction. the sampling and laboratory testing of the original and the improved soils are discussed in the paper. keywords: quality control, base course, laboratory analysis, consolidation test, cement content. 1. introduction the required intensity of deadlines for finishing the construction regardless of limiting conditions (e.g. climatic, occurrence of soils with low bearing capacity) conduced on a good number of occasions to improvement efforts on the base course. these are usually similar technologies to those used in road construction. among the most common technologies there is included adding a certain percentage of lime or cement to fine-grained soils6 (so-called soil stabilization). mixing cement with soil and compaction reduces the compressibility of the treated soil. subsequent quality control of any improvement is mostly done by static load test. the underlying layers of the future floors were simply improved within the rainy weather and were not protected. the adverse weather situation caused the suspension of work. checking the quality of the underlying layers using static load tests after a week of completion of the stabilization showed a failure to satisfy the required improvement. it was necessary to immediately resolve the situation and help meet deadlines. consequently soil was taken from the unsatisfactory area of the hall and then a check was made on the properties of both treated and untreated layers by laboratory tests. 2. laboratory tests on the construction site of the hall there were removed undisturbed soil samples improved by cement, but also the original soil without improvement by cement. on the original soil samples there were firstly detected the basic descriptive soil characteristics. based on stn 72 1001 [1] there was soil classified as firm clay with low plasticity (f6 cl). the original moisture of the soil was more than 20 %. the moisture of the improved soil with cement did not differ from the moisture of the original soil. to determine the optimum moisture of soil for compaction there was used the proctor test (see fig. 1). the optimum moisture of the tested soil was determined wopt = 15.2 %. this corresponds to the maximum dry density of soil ρmax = 1.768 g/cm3. the collected samples had significantly higher moisture content than its optimal moisture. 2.1. oedometric tests on undisturbed soil samples compressibility testing on the oedometer was made to the original and also the improved soil. at improvement there was added into the soil 5 % of cement. the results of average deformation characteristics of original and improved soil are collected in table 1. on the basis of laboratory results there can be determined for the improved soil the following increases in deformation characteristics: • at stresses in soil σ ≤ 50 kpa eoed stab eoed not = 32.25 5.10 = 6.32 (1) • at stresses in soil 50 ≤ σ ≤ 100 kpa eoed stab eoed not = 14.13 5.70 = 2.47 (2) • at stresses in soil 100 ≤ σ ≤ 200 kpa eoed stab eoed not = 15.19 9.62 = 1.58 (3) based on the stress range of its own weight and the expected load from buildings, the improvement of soil after adding cement was 1.5 to 2 times. the recommended values in the deformation modulus of improved soil should not exceed the value eoed = 14 mpa. 2.2. compression tests of soil without improvement at compaction after the proctor standard artificially prepared samples of soil compacted after proctor standard were tested in the oedometer. the 52 http://dx.doi.org/10.14311/app.2017.10.0052 http://ojs.cvut.cz/ojs/index.php/app vol. 10/2017 testing of soil improvement below a hall floor figure 1. determination of the maximum bulk density versus optimum moisture. table 1. deformation modulus of soils determined from compression tests. resulting average oedometer modulus corresponding to different moistures of tested soils are evaluated in table 2. effects of changes in soil moisture on the oedometric modulus are shown in fig. 2. the plasticity index of soil was ip = 10 % (liquid limit wl = 30.5 % and plasticity limit wp = 20.5 %). until the soil was in natural conditions and firm consistency, there were found very small values of deformation characteristics of tested soil to all extents of normal stresses. increasing the soil stiffness (higher values of eoed) was possible by observing a decreasing of soil moisture below 17 %. the consistency of soil at such moisture was ic = 1.4. decreasing the moisture below 17 % significantly increases the oedometric modulus of the soil. analysis of fig. 2 taking into account fig. 1 can be stated as: • exceeding the optimum moisture wopt occurred in samples loaded by normal stress of 50 kpa with rapid decreasing of eoed. • with an increasing of moisture there was observed a significant decreasing of eoed at lower normal stress (< 50 kpa) more than 5-times. exceeding the value of normal stress 50 kpa and with increasing the moisture there was found a decreasing of eoed approximately 3.5 times. in the test range of normal stress eoed was significantly affected by moisture. in fig. 3 there is shown the relationship between deformation characteristics and normal stresses of soil at various initial moistures of the soil. also in this evaluation it was proved the moisture was around 17 % than the limit. at lower moisture there were detected significantly different deformation parameters, especially at normal stress to 50 kpa. 2.3. improving the soil characteristics by adding cement at natural moisture of soil 20 % of collected samples had values of deformation modulus of the original soil which were very low. the addition of cement decreased soil moisture, whilst increasing the deformation parameters was not sufficient. very interesting results were obtained from evaluation of the effect of the 5 % addition of cement (see fig. 4). at lower humidity than 20.5 % (on a stiff consistency) there was observed a significant effect of the cement. at the same time, the effect of improvement increases with lower levels of normal stress. adding 5 % of cement at optimal soil moisture brought a 2-3 times increase of eoed compared to unimproved soil. exceeding soil moisture w = 20.5 % the soil fell into firm consistency and the impact effect of the cement was shown to be very low to negligible. cement added to the soil with natural moisture, got between 19 to 21 %, extracting a portion of water by hydration. this will inevitably reduce soil moisture itself. it can be assumed that the excess of the initial 53 m. súľovská, p. turček, z. štefunková acta polytechnica ctu proceedings table 2. average oedometric modulus of tested soils at various moistures. figure 2. relationship between moisture of tested soil and oedometric modulus. moisture content in excess of the wopt + 5 % there is to be expected a significant decrease in the effect of stabilization. 3. the quality assessment and the cement content in the soil cement and its contents were independently tested in the stabilized soil. laboratory tests of the physicalmechanical properties of cement were carried out and supplemented by x-ray and dta analysis. stabilized soil was investigated by dta and also x-ray analysis. tested cement was used to stabilize the soil at the construction site. the studied properties and the results derived are as follows: • after 28 days, the compressive strength reached 48.3 mpa. this value is in accordance with the requirements of the class of cement 42.5 mpa according to en 197-1. • bulk density with a timed deposit of test samples slightly increased. after 28 days, the bulk density reached a value around 2190 kg/m3, which corresponded with standard values. • by testing the cement slurry, it was found that the cement needs to reach a slurry of normal density of 32 % water. • the initial set of cement slurry observed after 195 minutes, in which it fulfilled the limit set by the standard. final set occurred after 250 minutes. • soundness of the testing cement went to 6.0 mm, which met the standard set limit of 10 mm. • bulk density of fresh cement mortar was 2160 kg/m3. all testing samples had low scatter from the average. • x-ray analysis showed the incidence of cement clinker minerals as alite c3s (tricalcium silicate), belite c2s (dicalcium silicate), brownmillerite c4af (tetracalciumaluminoferrite), c3a (tricalcium aluminate) and gypsum setting regulator (caso4 ·2h2o). furthermore, there occurred silica (sio2), calcite (caco3) and portlandite ca(oh)2. according to this composition it can be assumed that this was portlandcomposite cement cem ii. tests of the stabilized soil by method x-ray and dta showed that samples taken from the hall after the defective area of the static load tests contained less than 2 % of cement. by applying the same method to artificially prepared samples there was found good correspondence with the percentage of added cement. 4. conclusion based on the compressibility tests on the stabilized soil it is clear that the high moisture of the soil after the rainy season had not reached sufficiently an increasing of the soil parameters. they showed two reasons for failure to satisfy the criteria required for static load testing. the first reason was lower cement content as determined by the project. instead of 5 % of the cement which was in samples taken from the 54 vol. 10/2017 testing of soil improvement below a hall floor figure 3. dependence of normal stress and moisture on deformation characteristics of soil. figure 4. influence of addition 5 % cement into the soil on eoed at optimum moisture. subgrade in the construction site there was found only a cement content in the range of 1 to 2 %. significant precipitation caused the natural soil moisture to exceed 20 %. laboratory experiments have shown that at such a level of moisture in the not improved original soil the deformation properties of soil are very low. mixed cement decreased soil moisture, but it reflected only a small increase of eoed. deformation parameters of soil by adding cement may bring an increase 2-3 times. this effect is linked to the initial soil moisture before stabilization; value wopt should not be exceeded by 5 %. in exceeding this moisture there is not expected a major stabilization effect. 5. acknowledgement the paper is a part of results from a research project supported by the slovak agency vega no. 1/0882/16 “influence of boundary conditions to limit states of geotechnical structures”. [2] [3] [4] [5] [1] references [1] stn 72 1001: classification of soil and rock in engineering geology and geotechnics (in slovak), 2010. [2] p. turček, m. súľovská, z. štefunková. evaluation of soil layers improving below the production hall (in slovak). svf stu bratislava, 2016. [3] p. turček, m. súľovská, z. štefunková. experimental research of deformation properties of fine-grained soil situated in hall subgrade (in slovak). svf stu bratislava, 2016. [4] p. turček, j. frankovská, súľovská. geotechnical design according to eurocodes (in slovak). engineering advisory centre of slovak chamber of civil engineers, 2010. [5] en 197-1: cement. part 1: composition, specifications, and conformity criteria for common cements, 2012. 55 acta polytechnica ctu proceedings 10:53–56, 2017 1 introduction 2 laboratory tests 2.1 oedometric tests on undisturbed soil samples 2.2 compression tests of soil without improvement at compaction after the proctor standard 2.3 improving the soil characteristics by adding cement 3 the quality assessment and the cement content in the soil 4 conclusion 5 acknowledgement references acta polytechnica ctu proceedings doi:10.14311/app.2016.3.0001 acta polytechnica ctu proceedings 3:1–6, 2016 © czech technical university in prague, 2016 available online at http://ojs.cvut.cz/ojs/index.php/app statistical evaluation of fatigue data of components chi nghia chung∗, zoltan major johannes kepler university linz, institute of polymer product engineering, altenbergerstraße 69, 4040 linz, austria ∗ corresponding author: chi_nghia.chung@jku.at abstract. a variety of steels, cast iron grades and other metals have long been used for the production of machine components. in recent years, however, new materials such as sintered materials and plastics become increasingly important. because of the large number of different fibers, matrices, stacking sequences, processing conditions and processes and the variety of resulting material configurations it is not possible to rely on proven fatigue models for conventional materials. moreover, the development of models, which are valid for all composites are generally extremely difficult. in this work, a possible application of high-performance composites as materials for machine elements are investigated. this study attempts to predict the fatigue behavior and the consequent durability, based on laboratory measurements. using the statistics program jmp, the aquired data was subjected to a reliability analysis in order to ensure the plausibility, validity and accuracy of the measured values. keywords: fatigue models, statistical evaluation, reliability, techniques of parameter fitting, jmp. 1. introduction there was a tremendous advance in the field of plastics took in the past few decades. plastics have become an integral part of our daily life. polymers are flexible materials which can cover a great range of applications. they replace more and more metal, glass, wood and other materials. the development of novel artificial materials often opens a door to new technologies. electrical and automotive industry are hard to imagine without plastics, the use of polymer materials has revolutionized medicine. cyclically stressed components have a limited durability, therefore it is important to perform fatigue tests or simulations on critical components to predict their lifetime. figure 1 shows representative loading patterns with constant amplitudes for the whole loading level [1]. figure 1. constant amplitude loading patterns. fatigue tests are performed to study the relationship between the fatigue resistance of a material, component or structural element and cyclic loading [2]. it is a slowly progressing damage process. the strengths lie far below the static strength and yield strength. the objective of this work was to create high cycle fatigue curves from experimental datasets, using suitable material models, similar to the wöhler curve. the curve should as closely as possible reflect the experimental values. since specimens and test conditions are never 100 % identical, in two discrete measurements there is always scattering in the results, which can span over a decade in fiber reinforced polymers [1]. therefore to handle and correctly interpret experimental results, statistical methods must be used. 2. fatigue models in general, fatigue models are quantifications of physical material properties. they are independent of the shape of the component and are usually based on experimentally acquired data. the aim of all models is to predict how a component behaves in certain conditions. the relationships are reproduced mathematically afterwards, therefore they are mathematical models. in this work, the focus will be on presenting fatigue models [3]. in order to design a component correctly in terms of fatigue, a complete set of experimentally acquired data is usually required. since this is not possible for reasons of time and cost, engineers have to rely on predictive models. this models predict the durability n (in cycles) under a given cyclic loading. 1 http://dx.doi.org/10.14311/app.2016.3.0001 http://ojs.cvut.cz/ojs/index.php/app chi nghia chung, zoltan major acta polytechnica ctu proceedings 2.1. the basquin model the basquin fatigue model is a linear regression model. in a logarithmic scale, the durability (logn) is plotted versus the stress amplitude (log∆σ). logn = a−b · log∆σ, ∆σ ≥ ∆σ0, b ≥ 0 (1) a and b are material parameters that need to be approximated with appropriate fitting methods. in the model, ∆σ is limited by ∆σ0, the endurance limit. in this work, the endurance limit is at 406 cycles. ∆σ0 itself has no direct influence on the model, since it is not considered in the formula. 2.2. the strohmeyer model in contrast to the basquin model the models of strohmeyer and weibull are nonlinear. they are shaped by smoothing a piecewise linear function. again a and b are the material parameters. logn = a−b · log(∆σ −∆σ0), b ≥ 0 (2) 2.3. the weibull model the weibull model is more complex, since there are more parameters to fit. log(n+d) = a−b·log ( ∆σ −∆σ0 ∆σst −∆σ0 ) , b ≥ 0 (3) a, b and d describe the material parameters and ∆σst denotes the ultimate strength. 2.4. the bastenaire model logn −b ∆σ −∆σ0 = a · exp[−c · (∆σ −∆σ0)] (4) a, b and c express the material parameters. this model will be henceforth denoted as bastenaire1, because one can find another model of bastenaire in the literature, which is called bastenaire2 here. it is quite similar to equation 4 and it is also discussed for the purpose of comparison. n = a · exp[−c · ∆σ −∆σ0 b ]/(∆σ −∆σ0) (5) 3. techniques of parameter fitting in this section, two exemplar fitting methods are discussed in greater detail [4–6]. both methods are current and proven estimation methods in statistics. 3.1. the method of least squares (l2-norm) the method of least squares is a mathematical standard procedure for compensation calculation. here a function is determined, which fits a point cloud as close as possible. a point cloud is a scatter plot, a graphical representation of statistical measurements in a coordinate system. to illustrate the method, the basquin model is used as an example. consider a dependent variable y (in this case, logn), which depends on one or several variables (in this case a, b and log∆σ). the relationship between the dependent variable and the arguments are described in a model function f [7]. the model function f can be linear, as in this case, but it can also have any other shape (parabolic, exponential, ...). the general form is: y(x) = f(x; a1.........am) (6) in this case: logn = a−b · log∆σ, ∆σ ≥ ∆σ0, b ≥ 0 (7) the parameters a and b should be adapted such that bad data points (outliers) have only little effects on the fitting. if no unique solution that perfectly fits the point cloud can be found, then a compromise solution with the smallest overall deviation from the point cloud is the valid criterion. for this purpose, the sum of the squares of the differences between the model function f and the measured values yi is formed. n∑ i=1 (f(xi; a1.........am)− yi)2 (8) the parameters a and b are adapted until the sum of squares becomes minimal. in this work the fitting procedure carried out with the help of a solver implemented in excel. 3.2. the method of least absolute deviations (l1-norm) the l1 norm is a more robust fitting method. outliers are not so strongly weighted. in principle, this method works similarly to the previously explained method of least squares. instead of the sum of the squares, the absolute sum of the differences between the model function f and the measured values yi is calculated. abs ‖ n∑ i=1 (f(xi; a1.........am)− yi) ‖ (9) subsequently, the parameters a and b are also be adjusted until the absolute sum is minimized. 4. reliability the reliability is a measure of the accuracy of the measurement, as well as for the reliability of data. measurement series with very high reliability are therefore almost free of random errors, which means that they are repeatable at any time under the same measurement conditions and thereby provide approximately the same results [8, 9]. therefore, one gets a high reliability, when performing controlled and standardized measurements. to examine the reliability, different techniques can be used [10]. some known techniques are: 2 vol. 3/2016 statistical evaluation of fatigue data of components • test retest reliability • parallel forms reliability • split half reliability • internal consistency reliability. reliability analysis can be easily carried out with various computer programs. known programs are spss or, as used in this work, jmp. 5. evaluation of fatigue data two different materials were tested. material#1 is a glass fiber reinforced, semi-crystalline thermoplastic. material#2 is a carbon fiber reinforced, semicrystalline thermoplastic, where the bearing was simulated as a compliant bearing. material#3 is the same material as material#2, however it has been simulated as a rigid bearing. 5.1. fatigue models figure 2 exhibits the applied fatigue models for material#1. since for material#2 the ultimate strength and the load at the endurance limit could not be investigated, only the basquin model could be applied 3. in the case of material#3 the fitting with the strohmeyer and the weibull model resulted in the same curve 4. figure 2. applied fatigue models for material#1. figure 3. applied fatigue models for material#2. figure 4. applied fatigue models for material#3. the software minimized the deviation between the models and the measured values. by comparing the obtained model parameters the optimal model can be easily determined, not only qualitatively but also quantitatively. figure 5 and 6 show the optimal fatigue models for the materials. since for material#2 only the basquin model was applied, it was omitted below. figure 5. optimal fatigue model for material#1. figure 6. optimal fatigue model for material#3. 5.2. reliability analysis in jmp in jmp custom tables can be created, or files of different formats (excel, sas, text files, ...) can be processed. for a correct data input, the appropriate settings in the import wizard must be applied. in order to determine the best distribution for the measured values, a life distribution is performed [11]. the program returns a table with the appropriate distributions for the respective materials. jmp sorts them in descending order, the best model being on the top. the distribution of the measured data is weighted by 3 criteria.the akaike information criterion (aic), the bayesian information criterion (bic) and loglikelihood (maximum probability) used as estimation methods for the selection of models in statistics. 3 chi nghia chung, zoltan major acta polytechnica ctu proceedings tables 1, 2 and 3 show the "model comparisons" for each materials. according to the distribution analysis in jmp, the measured values from material#1 follows a natural logarithmic distribution, from material#2 a frechet distribution and from material#3 a weibull distribution. distribution aicc loglike bic lognormal 503.65435 498.90435 504.79323 weibull 503.66928 498.91928 504.80816 frechet 505.47261 500.72261 506.61149 loglogistic 505.74393 500.99393 506.88280 exponential 518.31862 516.08332 519.02776 lev 538.93104 534.18104 540.06992 logistic 546.20378 541.45378 547.34266 normal 550.30607 545.55607 551.44495 sev 563.21318 558.46318 564.35205 table 1. comparisons of the distribution of the measured data for material#1. distribution aicc loglike bic frechet 297.91571 292.41571 297.21150 lognormal 299.16181 293.66181 298.45760 loglogistic 300.06766 294.56766 299.36345 weibull 301.43592 295.93592 300.73171 exponential 305.28286 302.83841 305.23631 lev 320.31613 314.81613 319.61192 logistic 326.13477 320.63477 325.43056 normal 329.93763 324.43763 329.23342 sev 337.33938 331.83938 336.63517 table 2. comparisons of the distribution of the measured data for material#2. distribution aicc loglike bic weibull 407.25467 402.16376 407.44188 loglogistic 409.33460 404.24369 409.52181 lognormal 409.74094 404.65003 409.92814 frechet 415.33191 410.24100 415.51912 exponential 416.03305 413.69971 416.33877 lev 432.98688 427.89597 433.17409 logistic 439.48273 434.39182 439.66994 normal 439.98404 434.89313 440.17125 sev 445.68650 440.59560 445.87371 table 3. comparisons of the distribution of the measured data for material#3. based on this findings, the durability evaluation was performed. tables 4, 5 and 6 display the mean estimates for each material. σ is the standard deviation from the measured data, β0 and β1 denote positional and shape parameters. stderror stands for standard error and describes the standard deviation, but from the estimate function. additionally the tables show for each estimation the 95 % confidence interval. µ is the estimated average value of cycles, which is dependent on the loading. strictly speaking, this designation should be logµ, because the relations are from logarithmic nature. par. estimate stderror low95 % up95 % β0 79.25810 6.98804 64.83945 93.67675 β1 -14.17640 1.44835 -17.16484 -11.18796 σ 0.92287 0.14971 0.69226 1.31688 table 4. mean estimations for material#1. µ = 79.2581− 14.1764 · log(loading) (10) par. estimate stderror low95 % up95 % β0 126.9802 20.30212 82.06733 168.4553 β1 -22.7520 3.98476 -30.90587 -13.9531 σ 0.6862 0.15914 0.45366 1.1457 table 5. mean estimations for material#2. µ = 126.9802− 22.75204 · log(loading) (11) par. estimate stderror low95 % up95 % β0 86.41222 14.55656 53.28540 112.4174 β1 -14.89503 2.94585 -20.13681 -8.1682 σ 1.18943 0.25506 0.80843 1.8933 table 6. mean estimations for material#3. µ = 86.41222− 14.89503 · log(loading) (12) figures 7, 8 and 9 illustrate the quantile analysis. the quantile is a measure of location in statistics. it represents a threshold. a certain amount of the value is below, the residual amount above this threshold. using the example of material#1 the threshold is 21675.91 cycles. that means here that for a loading of 132.5 there is a 50 % probability of failure. the blue dotted lines represent the 95 % confidence interval. jmp can also perform a custom estimate. for example, the failure probability at the endurance limit is calculated for each material. the results are given in figures 10, 11 and 12. in the case of material#3 the material can sustain 4.7*106 cycles at a loading of 93 and a failure probability of 5 %. at a loading of 93 and cycles of 406 the failure probability becomes 4.4 %. 6. conclusion due to the very large scatter of the measurement results, the determination of the most suitable model 4 vol. 3/2016 statistical evaluation of fatigue data of components figure 7. quantile analysis for material#1. figure 8. quantile analysis for material#2. figure 9. quantile analysis for material#3. 5 chi nghia chung, zoltan major acta polytechnica ctu proceedings figure 10. custom estimate for material#1. figure 11. custom estimate for material#2. figure 12. custom estimate for material#3. to fit the experimental data was a major challenge. in some cases, one gets very unsatisfactory graphical representations of the models. by the application of several fatigue models, at least one suitable model for each material could be found. by the implementation of statistical analysis also the large outliers could be included in the parameter estimation. jmp is a powerful tool for statistical analysis. especially, the analysis of the failure probability is a very important feature. the estimate in jmp within the experimental data range leads to slightly different prediction than that of the fatigue models. extrapolation with jmp out of range is highly dependent on the quality of the measurement data and therefore, does not always leads to plausible results. list of symbols a,b,c,d material parameter n number of cycles ∆σ loading amplitude ∆σ0 loading amplitude at endurance limit ∆σst loading amplitude at ultimate strength σ standard deviation β0 positional parameter in jmp β1 shape parameter in jmp µ estimation for the mean value references [1] t. k. anastasios p. vassilopoulos. fatigue of fiber-reinforced composites. springer verlag london limited, london, 2011. [2] m. v. dieter radaj. ermüdung grundlagen für ingenieure. springer verlag berlin, heidelberg, 1995, 2003, 2007. [3] a. f.-c. e. castillo. a unified statistical methodology for modeling fatigue damage. springer science + business media b. v., 2009. [4] u. heidelberg. http://www.physi.uni-heidelberg.de/einrichtungen/ ap/elearning/index.php/animationen/37-anpassungvon-funktionen-an-messdaten/52-die-methode-derkleinsten-fehlerquadrate 2013. universität heidelberg, 2013. [5] s. j. m. c. r. a. schneider. best practice guide on statistical analysis of fatigue data. twi cambridge, uk, 2003. [6] a. s. for testing, materials. statistical analysis of fatigue data. astm, 1981. [7] s. j. miller. the method of least squares. brown university, providence, ri 02912, us. [8] t. e. j. s. udo kuckartz, stefan rädiker. statistik eine verständliche einfürung. springer fachmedien wiesbaden gmbh, 2010. [9] a. f. hans diefenbacher. statistik eine verständliche einfürung. andreas frank & ventus publishing aps, 2006. [10] p. schmolck. methoden der reliabilitätschätzung. universität der bundeswehr münchen, 2007. [11] s. i. inc. jmp version 11 documentation. sas institute inc., 2013. 6 acta polytechnica ctu proceedings 3:1–6, 2016 1 introduction 2 fatigue models 2.1 the basquin model 2.2 the strohmeyer model 2.3 the weibull model 2.4 the bastenaire model 3 techniques of parameter fitting 3.1 the method of least squares (l2-norm) 3.2 the method of least absolute deviations (l1-norm) 4 reliability 5 evaluation of fatigue data 5.1 fatigue models 5.2 reliability analysis in jmp 6 conclusion list of symbols references acta polytechnica ctu proceedings doi:10.14311/app.2017.12.0083 acta polytechnica ctu proceedings 12:83–93, 2017 © czech technical university in prague, 2017 available online at http://ojs.cvut.cz/ojs/index.php/app an intelligent co-driver surveillance system mădălina toma, mirela popa, leon rothkrantza,b,∗ a intelligent interaction, delft university of technology, mekelweg 4, delft, the netherlands b faculty of transportation sciences, czech technical university in prague, konviktská 20, prague 1, czech republic ∗ corresponding author: l.j.m.rothkrantz@tudelft.nl abstract. in recent years many car manufacturers developed digital co-drivers , which are able to monitor the driving behaviour of a car. sensors in the car measure if a car passes speed limits, leaves its lane, or violates other traffic rules. a new generation of co-drivers is based on sensors in the car which are able to monitor the driver behaviour. driving a car is a sequence of actions. in case a driver doesn’t show one of the actions the co-driver generates a warning signal. experiments in the car simulator torc were performed to extract the actions of a car driver. these actions were used to develop probabilistic models of the driving behaviour. a prototype of a warning system has been developed and tested in the car simulator. the experiments and test results will be reported in this paper. keywords: behaviour analysis, car simulator, surveillance, bayesian reasoning. 1. introduction in recent years we can observe an increase of investments of car producers and researchers in the development of self-driving cars. the first prototypes of such cars have been tested. however, there is a long way to go before such cars appear on the highways and in the cities. it can be expected that at least for the coming 10 years cars with a human driver will prevail. also in the regular driver-car interaction a lot of automatization will take place. navigation systems support the driver to find the shortest route from his starting point to its destination. smart interfaces support the drivers to use their phones, radio and other digital devices in their cars. this paper focuses on the development of systems increasing the safety of the car driver and other road users. at delft university of technology there is a project running on the development of surveillance angels and guardian angels [1], [2]. one of the applications is a digital automated car driver assistant. such an assistant is able to supervise the driving behaviour of a car driver. the system permanently receives the position of the steering wheel, position of the pedals, gear lever, but also information of body movements of the driver, its facial expression and speech and information from the environment. this information is provided by sensors in the car and sensors attached to car parts and (manual-) instruments. the goal of the system is to make assessments of the possible dispositions/situation of the car driver, that is to say a semantic interpretation of the driving behaviour. in case the driver overlooks important information, violates traffic rules or driving rules, the system will generate an alert or support the driver. the next step is that the car takes over the driving of the car, however this is postponed to future work. the full model is based on information of the car driver, but also on the environment and state of the car. from the movements of the car driver we make assessments of the state of the car driver, his goals and intentions. information on the state of the driver can also be assessed by physiological measures and eeg measurements [3],[4]. this study focuses on the assessment of the visual behaviour of the car driver. a car driver can show many body movements. we can order them in specific categories: • core driving movements. these movements are related to driving a car. • peripheral movements. these movements are related to interaction with peripheral devices in the car such as radio, phone, navigation devices. • signal movements. these movements express some message from the driver about his physical or emotional state (stretching the arms, showing a fist) or messages to other drivers on the road. • random movements. these movements have no specific semantic meaning related to car driving such as scratching the nose. core driving movements can be composed of one single action or a sequence of actions. take, for example, the actions of the car driver such as starting a car, parking, speeding up, slowing down, taking a turn. all these actions are characterized by a sequence of information of the car sensors. after an action the next step is to classify the action. it is necessary to realise that the classification is context sensitive. the classification is usual an ambiguous probabilistic process. in case of an ongoing sequence of actions we have to predict the future and compute the probability of possible next steps. it can also happen that a driver 83 http://dx.doi.org/10.14311/app.2017.12.0083 http://ojs.cvut.cz/ojs/index.php/app m. toma, m. popa, l.j.m. rothkrantz acta polytechnica ctu proceedings skips one or more action on purpose or by mistake. take, for example, the case when a driver wants to take over but does not view the mirror to check if the next lane is free. this can result in a dangerous situation and an alert has to be generated. in this paper a rule based system and a bayesian reasoning system will be used to compute the probability of missing or future steps in an action string. by experiments using a driving simulator all possible sequences of actions are computed as well as corresponding probabilities of those actions. in the next section we discuss related work. in section 3 a model of a digital co-driver is presented. in section 4, experiments with the car driver simulator torc are discussed. then we present an analysis of the recorded data in section 5. this paper is concluded with a final discussion. 2. related work according to the statistics published by the national highway traffic safety administration in 2005 a big number of collisions (78%) and near-collisions (65%) are associated with driver inattention. distractions caused by secondary tasks, such as manipulation of the navigation system, radio, or phone, seems to be the main source of the inattention [5]. in [6] merat and jamson present their findings regarding driver’s ability to respond to sudden and unexpected events while driving under normal conditions and also while performing other tasks. their conclusions highlight that the reaction time increases with 200ms when the driver is using in-car systems. head pose estimation reveals important information about the driver’s focus of attention. in [7] doshi and trivedi present their findings regarding head dynamics and eye gaze as important cues in predicting driver’s intent of changing lanes. regarding human pose estimation, currently there are various representation methods of a human in an image, such as silhouettes [8], bounding boxes, or sticks representing the limbs of a person [9][10]. one of the most popular approaches is using the pictorial structures model which stands as a collection of human body parts [11]. building intelligent systems have become a trend in current researches. in driving domain, they have been called advanced driver assistance systems [12]. according to adarsha [13], these systems support the driver’s sensing ability and they are able to track and detect the error/lapses of drivers. assistive intelligent systems are usual a combination of several tools. for example, benoit, [14] developed in his work an assistive driving system for assessing the driver’s fatigue. his system uses two platforms, namely openinterface developed in c++ for signal processing and icare. icare is a conceptual component model for multimodal input/output interaction. thus, the benoit’s system combines multimodal signal processing analysis with a multimodal interaction. our system can be built on a similar system architecture, and it tries to enhance the novice driver’s skills. many other intelligent systems have been developed [15]. the cognitive model of the driver had been investigated in order to implement driver’s mental activity in this system. the driver’s mental activity is analyzed when a driver interacts dynamically with the driving environment. therefore, the driver mental representation of a situation can be illustrated as an instance in the working memory. we also investigate in building a cognitive model of drivers. we analyzed driver’s mental activity as a result of the following activities: gaze and head activities related to the traffic situation, the car controller (steering wheel, pedals, and gear shifter) related to body parts movements, and states of the car on the road. other intelligent systems have been implemented based on a stochastic model. in these cases, driver behavior has been modeled using collected data from the driving simulator. liang used this model in his work [16], to detect driver distraction in real-time. he modeled driver behavior based on parameters extracted from driver’s eye movements, states of the steering wheel and car position on the lane. in a similar way giusti [17] detected driver sleep-attacks by using acquired data from the steering wheel. wang and gao estimated in [18] the state of a car on the road with a rule-based expert system. the expert system typically provides the intelligence of the system, and it fits very well with the requirements of our system. according to the last decade researches, an expert system can be classified in the following six methodologies: rule-based systems, knowledge-based systems, intelligent agent (ia), database methodology, inference engine, system-user interaction. a basic rule-based expert system has the following entities: a knowledge base which contains data from experts, an inference engine which contains the evaluation of decision making components of the system. we estimated the driver’s intent with a rule-based expert system. moreover, our framework developed with a rule-based expert system reasons like a driver providing full assistance for novice drivers in terms of the driving skills enhancement. fletcher proposed an assistive driver system [19], which monitors driver activity using vision sensors. the set of sensors used in fletcher’s system tracks the eye movements and the body parts of the driver. however, it is difficult to estimate driver intentions from the data of the visual sensors. therefore, our system estimates driver’s intention by correlating data from visual sensors with states of cars and information about the driving environment. the gaming industry provides driving simulations which simulate 3d traffic environments. if a simulation of a 3d traffic environment is used, the data about a driving situation can be read easily. in our work, we used torcs simulator because it can be extended and adapted to the proposed system. this simulator is an excellent 84 vol. 12/2017 an intelligent co-driver surveillance system open source tool, and it gave us an opportunity to configure the tracking sensors and the car controller. in prior research, we tried to determine the driver’s interaction intent by analysing only single body parts, e.g. pose estimation [20], gaze detection, or facial expression. compared with these, we take into account more aspects and actions, such as body postures, gaze direction, and head orientation during interaction with the car and the surrounding environment. a lot of intelligent systems that provide assistance to the drivers were presented above. some of them present good solutions. still an intelligent system with a complex reason which trains the novice driver’s skills need to be developed. thus, we present the implementation of an intelligent system that has the capability to assist a novice driver and it tries to enhance his skills at the same time. 3. model in figure 1 we display a model/architecture of our digital co-driver system. the behaviour of the driver is assessed by parallel sensor systems: kinect, gaze tracker, car-driver interaction tools. the sensory input system has been configured and synchronized with clock cycle of torcs tool in order to track the driver activities. the torcs simulator is extended on three modular levels of abstraction. in the first level, the system detects sequences of postures. the driver action can be recognized from sequence of postures in the second level. in the last level, a driver’s intention is estimated based on the driver’s actions related to the traffic situation. the set of possible intentions had been stored in the database. when a driver’s intention is wrong the proposed framework sends feedback alarms. a gui interface has been built in order to allow monitoring the functionality of the whole system and also driver’s activity. the main reasoning module of the system implements the intelligence of the system, and it is responsible to predict driver’s intent. in one of the developed prototypes a rule based system was used to assess the most probable scenario. every rule has a tally attached to it to indicate the probability or importance of the rule. some scenarios are composed of one single action. an observed action triggers the most probable rule. this activated rule is allowed to fire and the right hand site of the if-then rule is generated. some scenarios consist of only one action. if this is a dangerous driving action an alert has been generated by a rule. an example is a driver who doesn’t look at the road in front of him. in other cases observed action(s) trigger the most probable scenario and then specific rules check if important actions are missing or dangerous actions can be expected. an example is a car driver switching on his signal lights to change his lane who did not check of the next lane is free to insert. the problem with rule based systems is that all the rules have to be defined in advance. so the system is not prepared for new, unexpected situations. another problem is that sensors are not able to assess all the ongoing actions. these errors in the sensor observation can be caused by failing technology or because vision field of sensors is occluded. the actions are ambiguous or the lighting conditions are too bad. a gaze tracker, for example, usually misses a significant amount of actions. by sudden, large movements of the head the gaze tracker has to be calibrated again. nevertheless, the biggest problem to solve is that actions are usually distributed in time and that some car drivers start actions simultaneously. it is necessary to realize that the proposed system will be used as a decision support system. ultimately car drivers themselves remain responsible for their actions. to improve the performance of the assessment of the actions a bayesian probabilistic system will be used. to compute the values of the entries in conditional probabilities tables experts are needed to set these values or huge amount of data generated in experiments testing the system is needed to compute the values. the robustness of the system is increased with a fusion method based on multiple sensors compared with a fusion method based on a single sensor. multi-sensor data fusion allows the combination of information with different physical characteristics to enhance the understanding of the driver’s action. the information read from the set of sensors are fundamental in the decision-making module of the system. our fusion method seeks to combine information from multiple sensors. table 1 presents a hierarchy of driver activity in a bottom-up way, and it represents the foundation of our system architecture . the first level needs to detect the driver postures, and then sequences of postures define the driver action. in the third level, one action or a set of actions predict the driver’s intention. a human intention can be defined as an anticipated outcome that guides planned actions. it is the goal or purpose behind an action or a set of actions that a person is following. driver’s intention can be predicted from the driving actions. driving actions in a scenario can be recognized based on a set of postures performed over time. thus, the system architecture depicted in figure 1 defines an assistive car driving system based on multi-steps of processing: tracking driver behavior, predicts driver’s future behavior, decide if the driver’s intention is correct or not, and issue corrective feedback. 4. simulation environment the main tool in our system is torcs simulator. it is one of the most popular 3d open source car simulators written in c++ and available under gpl license. it can be used as an ordinary car racing game, as an ai racing game. the main advantage of the software is that we could extend the platform by configuring inside all sensors used for tracking the driver’s behavior. we selected torcs in our research for the following reasons. torcs is an advanced fully 85 m. toma, m. popa, l.j.m. rothkrantz acta polytechnica ctu proceedings figure 1. architecture of the digital co-driver system. levels of driver activity driver interaction with car controller and traffic situations driving intent interaction: is a mental state of any driver who executes an action or set of actions into a driving situation based on driving environment and the car states. driver’s actions: are made up of multiple body-part gestures such as eyes, head, arms, legs motion, and each body-part gesture is an elementary event of motion and can be composed of a sequence of instantaneous poses at each moment of time. driver’s postures: are instantaneous configuration of the body part in the space of interaction. table 1. hierarchy of driver activity. customizable simulator, and it can be adapted for our application; it features a sophisticated physics engine as well as a 3d graphics engine for the visualization of the virtual environment; and it has a modular software architecture, hence the new controlling and sensing devices allow a straightforward integrating. according to the features of the proposed system, we design tracks which simulate some driving scenarios. therefore, the creation of a customized track involves the use of other tools. in the first step, we used the trackeditor tool for creation a track. it can design the tracks adding straight or curves and some parameters can be configured such as length, radius or banking. all of this information can be stored in an xml file. in addition, the trackeditor tool generates a file with extension. ac, and this file stores the 3d description of the track. the 3d description of the track was edited with the blender tool. blender tool allows us to add elements such as traffic signs and to insert textures. we added billboards to one of the tracks. looking at the driver and understanding his actions is the first task of our system. we used a marker-less system to track upper body activity, and the driver’s upper limb motion can be tracked by the kinect device. this device has been developed by microsoft as a game console input device. comparing with other video cameras [21] used for recognizing human’s movement activities, it tracks the motion of the subject through a combination of hardware and software technologies and achieves a high accuracy tracking of the body with a rate of 30 fps (frames per seconds). in our system, we used for eye movements and head orientation the eyelink ii device produced by the canadian sr research, which uses pupil and cornea reflection tracking mode. this system provides a comprehensive tracking based on smart vision sensors which consist of a head-mounted camera system and two pcs for processing data and running experiments. 86 vol. 12/2017 an intelligent co-driver surveillance system on the head-mounted device, both left and right eye pupil position and the head orientation relative to computer monitor can be tracked. combining the position of the head with the pupil movement relative to the screen, enabled recording of the gaze direction. sensors for physical driver’s interaction. our proposed system (see figure 2, figure 3 and figure 5) presents the architecture of a driver-computer interaction system. the driver can interact naturally with the simulated traffic environments through physical input/output interaction interfaces by bridging the gap between the digital and physical world. figure 2. car driving simulator. figure 3. typical scene from the curved race track with billboards. figure 4. the curved race track driven by the participant. markers indicate locations of the billboards, if enabled. we used as an input interface a logitech g27 joystick controller based on a steering wheel, a gear shifter and three pedals (clutch, brake and throttle). it is similar to the usual basic control system from a cockpit car and the driver’s actions can be performed in the same way as in the real car. the driving scenes are displaying on an output interface which is a large screen like a real car windshield. we used a tv screen with 56 inches size diameter as a large screen. four optical sensors of head camera from the eyelink ii device had placed in the four corners of the screen. 5. experiments in this section we discuss two experiments testing respondents in our driving simulator torc as displayed in figure 5. several devices were used to assess the actions of the driver. in section 5.2 we discuss the assessment procedures using the kinect device. but first we discuss the experimental results of assessment of driving actions using the car sensors as discussed in section 3. data captured with video cameras and gaze tracker has been captured with different sampling rates. the same holds with data from the kinect sensor. recorded data is redundant, full of errors and missing data. the different data streams have to be fused. different types of data fusion using multiple, multimodal streams have been discussed in [22]. figure 5. test person in action in the driving simulator. 5.1. experiment 1 detection behaviour actions in total, there were 23 students invited to take part in the driving experiment. they were supposed to drive for two hours, one hour on a simple trajectory and one hour on a curved trajectory. they had to drive with different speed, normal speed and as fast as possible. along the simple trajectory we placed some billboards. there were other car drivers on the road driving both in a low and high speed speed. this forced our test persons to take over at regular times. we were interested in isolated driving actions or scenarios composed of single driving actions. in figure 6 we show a state diagram of all possible states of actions and transition between states. during driving, the students got phone calls or visual messages about routing. the billboards along the road caught the car driver’s attention. 87 m. toma, m. popa, l.j.m. rothkrantz acta polytechnica ctu proceedings figure 6. the state diagram describing the driver intention and rules used on each transition. 5.1.1. analysis experiment 1 the goal of our system is to generate alerts in case of dangerous driver’s behaviour. we analyse single actions and scenarios composed of actions. a car driver is supposed to watch the road all the time. in case he/she is looking left or right for some time, looking to the billboards of traffic signs for a long time or looking at their telephone or routing devices for a long time, a dangerous situation can occur. assessing the gaze direction via the gaze tracker is rather complicated. fast movements over large view angles were difficult to track. sensors in the car detected if the car left its lane. in the next section we discuss our experiments using kinect to assess the position of the head and gaze direction. parallel to the single action analysis we researched possible scenarios. as soon as the action has been detected we find the position in the state diagram and research if this state is the starting point of a scenario or a point within a scenario. if an important preceding action is missing an alert can be generated. in case of the take-over scenario it can happen that the driver switch on the signal lights but monitoring the side mirror is not detected. to reduce the amount of false alarms we used a probabilistic approach. states in the state diagram and transition between states have probability numbers depending on the frequencies in all recordings during the experiment. we choose the most probable states and transition path to compute the possibility of an alert. we tested 23 drivers during 2 hours (high-low speed). on average, every minute a single action or a scenario of average 3 actions of the driver was required or generated. in total we observed 16.500 single actions and 48.230 single action in scenarios (see table 2). we focused on the actions with most frequent errors. 5.2. experiment using kinect this section presents one of the first digital co-driver experiments [23]. we present adapted behavioral models, which are based on our driving experience and also on information gathered from experts in the field. we assess both head pose and body pose in terms of orientation and hands position in relation to the car environment. we defined normal vs. dangerous driver’s behavior, on several levels of importance. normal driver’s behavior implies both hands on the steering wheel and the head orientation frontal. there are variations from these rules, such as: head orientation can be left or right for a small amount of time in case of lane changing or observation of the surroundings, etc. the dangerous behavior appears in case the driver keeps looking in any other direction than frontal for a long period of time. regarding hands position, the driver is supposed to have at least one hand on the steering wheel, while the other one can rest, be used for changing the gears or 88 vol. 12/2017 an intelligent co-driver surveillance system high speed low speed actions/scenarios frequency errors correct alert frequency correct alert looking around 121 72% 276 81% switching lanes 322 68% 158 77% giving priority 220 75% 118 87% speed limit 296 92% 88 94% table 2. frequency of detected actions generated alerts. for manipulating the radio or the navigation system. if the driver doesn’t keep his hands on the steering wheel for a long period of time, it is considered to be a dangerous action. furthermore, if besides having no hands on the steering wheel, the driver is also looking in another direction we consider this behavior very dangerous and the system will generate an alert to notice the driver. a very important aspect taken into consideration while defining the behavioral models is the temporal evolution of an action. under normal circumstances, stretching arms (no hands on the steering wheel) or looking in another direction, these activities are performed by a driver without any serious consequences. still, in case any of these actions are performed for a longer period of time they might affect the level of concentration and of the driver’s attention. what is more, in case of an unexpected or sudden event, the driver’s speed of reaction might not be high enough, which could lead to an accident. figure 7. architecture of a digital car driver system. kinect software module was used for pose estimation. the output of the pose estimation module, consisting of the location and orientation of each body part (see figure 8) was used to assess the relation between the different body parts (e.g. arms relative to the torso, lower arms relative to upper arms). we computed the angles between the different body parts on a frame basis and also for an image sequence. the angles between head, torso and the vertical axis together with the one between head and torso provide relevant information regarding head and body position.we considered three basic orientations of the head relative to the body: straight, left, and right. the goal of our research is to test the possibility to train the basic car driver scenarios using a driving simulator and serious gaming. in table 4 we displayed figure 8. model of the upper body used for pose estimation. figure 9. hierarchy of driver activity. figure 10. incorrect detection of left arm. 89 m. toma, m. popa, l.j.m. rothkrantz acta polytechnica ctu proceedings figure 11. motion angles pattern for two actions: (a) hands on the steering wheel (b) hands raised . some of the basic scenarios. every scenario is composed of a sequence of basic actions. we selected the following basic actions: • browsing (moving position of head, looking into the mirror, inspecting the routing device or radio) • tuning (radio, mobile phone etc.) • activating (touching eyes, mouth, pulling ears etc) • selecting (putting the gear by hand in the requested position) • waving (putting left/right hand up) • driving (turning the steering wheel). we evaluated the performance of the body pose estimation module on the driving recorded data, by inspecting visually the accuracy of each detection (see figure 9 and figure 10), and we achieved 89% correct upper body detections and 58% correct upper body parts estimation. kinect proved to be very succesfull with limb detection and person tracking. another important thing about kinect’s limb tracking is that kinect is aware of occlusion and for example if one of the arms is missing, the stick configuration shows that a limb is missing. in short two main problems that are faced with kinect and the video tool are undetected limbs (see table 5) and frames with no detection at all respectively (see table 6). the detection heavily depends on the exposed action. in table 3, we show the average missing rate. the car environment can be divided into a number of regions of interest for the driver. the most important and used ones are the ’steering wheel’ (1) and the ’gears’ (2) regions, followed closely by the ’contact’ (3) and ’brake’ (4) regions. other secondary regions are the ’navigation system’ (5), the ’radio’ (6) and the ’drawer’ (7) ones. a visual representation of the defined regions can be found in figure 12. figure 12. regions of interest inside a car. we included a graphical division of the inside the car environment into regions of interest. the driver’s interaction with each object inside the car can be assessed by determining hands’ position inside a region. this information is used in combination with the motion pattern analysis in order to extract a first semantical interpretation of the possible action of the driver. movements characterized by a certain speed and amplitude, inside a specific region of interest are very likely to depict a certain type of action. for example movements having a low speed and a small amplitude associated with the ’steering wheel’, ’radio’ or ’navigation system’ regions of interest are very probable to correspond to driving, manipulating the radio or the navigation system actions. on the other hand, movements with a high speed and a large amplitude inside the ’drawer’ or the ’gears’ regions are associated with picking an item from the drawer or changing the gears actions. the rois, the transitions between the different rois, the associated type of movements, and the probable behavioural interpretation are presented in figure 13. the reasoning step was implemented using a rulebased system which received as input, features from the previously described modalities (body pose estimation, face detection, and regions of interest assessment). on a time basis the extracted features were observed and using the rules containted in the statebased model we were able to distinguish normal from potential dangerous and dangerous driving behavior. conclusion regarding the driving behavior was generated. we employed the state-based model depicted in figure 13. 90 vol. 12/2017 an intelligent co-driver surveillance system browsing tuning activating selecting waving driving browsing 23 22 55 0 0 0 tuning 0 95 5 0 0 0 activating 5 10 77 3 0 5 selecting 0 17 0 83 0 0 waving 0 0 0 14 85 0 driving 25 0 25 0 0 50 table 3. confusion matrix of selected driving actions. driver action behavior detected by kinect,video and car sensors s1 starting the car 1.check if the gear is in neutral position hand movements, movements of head (turn down) 2.start the car hand motion 3.press the throttle a little bit movements of right leg s2 driving away 1.start the car see s1 2.press the clutch to the floor movement of left leg 3.enter the gear shift in position 1 hand movement (movement of the head) 4. press the throttle pedal movement of right leg 5.release the clutch movement of left leg s3 driving away from parked position 1.start the car see s1 2.look in the inner and outer mirror if next lane is free turning the head right/left 3.switch on the left signalling light movement of hand 4.press the clutch to the floor movement of left leg 5.enter gear shift in position 1 movement of hand, movement of leg 6.press slowly the throttle pedal movement of right leg 7.turn the steering wheel to the left or right hand movement 8.turn the steering wheel to the neutral position after reaching the next lane hand movement table 4. scenarios and behavioural cues. total number of frames kinect 16% 574 video-tool 32% 574 table 5. rate of frames without detected limbs. 91 m. toma, m. popa, l.j.m. rothkrantz acta polytechnica ctu proceedings upper right arm upper left arm lower right arm lower left arm browsing 12 74 20 74 tuning 10 2 12 2 activating 2 0 2 0 selecting 12 7 12 7 waving 16 75 21 75 driving 17 20 17 20 table 6. kinect rate of frames without detected limb relative to all frames in the dataset. figure 13. state based model. we used 3 seconds as a threshold for the critical duration of a secondary action or body part orientation, such as head orientation, lower arms position, or body pose orientation. 6. conclusions in the framework of designing a digital co-driver system, we performed a study to assess the actions of a driver. based on the experiments in a car simulator it was shown that the action of the car driver can be assessed in more than 80% of the cases. but we stress the fact that the results are based on the experiments in a car simulator using students as testpersons doing their best to drive according to the rules. in real life situations the results could be quite different. we expect that in real life situations the recognition rate could be quite lower because of bad lighting conditions in the car, occlusions, position of the car driver. that is the reason that we used multiple multimodal sensors. this implies sensor fusion. to assess the driver actions we used a probabilistic rule based system. the limitation of such a system is that all possible (string of) actions have to be defined in advance. unexpected rare actions will not be detected. this is also caused by the fact that our system selects the most probable actions. in case that our system has to detect some actions with a high priority to send an alert such actions have to be labeled with a higher priority in our system. the designed system can be used as a decision support system in current cars. the car has to be equipped with some sensors and a microprocessor to process the data and generating alerts. the results can also be used in the design of self driving cars in the near future by modelling a digital car driver according to a human model. the basic positions of individual body parts, along with the motion temporal patterns and the corresponding regions of interest were fused in order to draw a conclusion regarding the driver’s possible type of activity. references [1] l. rothkrantz. surveillance angels. neural network world 24:1–25, 2014. 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[23] m. popa, l. rothkrantz. assessment of behaviour in serious games of driving simulators. international journal of intelligent games & simulation 6(2), 2011. 93 http://dx.doi.org/10.1007/978-3-319-12352-3 http://dx.doi.org/10.4236/jtts.2012.21007 http://dx.doi.org/10.1007/bf02884432 http://dx.doi.org/http://dx.doi.org/10.1016/j.ssci.2009.03.014 http://dx.doi.org/10.1109/tits.2007.895298 http://dx.doi.org/10.1109/tits.2008.2011707 http://dx.doi.org/10.1016/j.patrec.2013.01.002 acta polytechnica ctu proceedings 12:83–93, 2017 1 introduction 2 related work 3 model 4 simulation environment 5 experiments 5.1 experiment 1 detection behaviour actions 5.1.1 analysis experiment 1 5.2 experiment using kinect 6 conclusions references acta polytechnica ctu proceedings doi:10.14311/app.2018.14.0008 acta polytechnica ctu proceedings 14:8–13, 2018 © czech technical university in prague, 2018 available online at http://ojs.cvut.cz/ojs/index.php/app uncertainty propagation for lwr burnup benchmark using sampling based code scale/sampler daniel campolinaa,b,∗, jan fryborta a department of nuclear reactors, czech technical university in prague, prague, czech republic b nuclear technology development center, belo horizonte, brazil ∗ corresponding author: campolina@cdtn.br abstract. sampling based method is adopted in many fields of engineering and it is currently used to propagate uncertainties from physical parameters and from nuclear data, to integral indicators of nuclear systems. the total uncertainty associated with a model simulation is of major importance for safety analysis and to guide vendors about acceptable tolerance limits for nuclear installations parts. this work presents some calculations to propagate uncertainties for a nuclear reactor fuel element modeled in scale/triton, using the sampling tool scale/sampler. results showed that the influence of input uncertainties on kinf is more pronounced in the fresh core other than the depleted core and the contribution from studied manufacturing uncertainties is smaller than the contribution of nuclear data uncertainties. keywords: random sampling, uncertainty, nuclear fuel burnup, oecd uam lwr. 1. introduction in a global picture, uncertainty quantification (uq) is the process of characterizing input uncertainties, forward propagating these uncertainties through a computational model, and performing statistical assessments on the resulting responses. in this forward propagation, illustrated in fig.1, probabilistic or interval information on parametric inputs are mapped through the computational model to assess statistics or intervals on outputs. the computational model for the present application is the triton control module of scale, composed of cross-section processing codes, a neutron transport solver, and point depletion code [1]. for uq, components of the input parameter are considered to be uncertain as specified by probability distributions (e.g., normal, beta). by stipulating specific distributional structure to the inputs, distributional structure for the outputs (i.e, response statistics) can be inferred [2]. uncertainties and correlations among experimentally measured cross-sections constitute the so called nuclear data covariance libraries. the sampling based approach can be used to sample physical parameters like dimensions and densities and also joint probability density functions given in the nuclear data covariance libraries. the last produces a random sample for the nuclear cross-sections that are used in a transport calculation. one of the most important results of a nuclear reactor simulation is neutron multiplication factor (k) that measures the balance between production of neutrons and their absorption in the core and leakage out of the core. the multiplication factor is a key figure 1. realizations from specified input distributions being run in a model simulation. indicator for the changes in the neutron flux that also causes changes in the power level of the reactor. if the multiplication factor is lower than 1, the system is subcritical and its power is decreasing. loss of neutrons is greater than their production and it is impossible to maintain the fission chain reaction. a uq analysis can provide the level of confidence in the multiplication factor predicted by simulations and identify through a sensitivity analysis the key parameters whose uncertainties contribute most to the multiplication factor uncertainty. there are always safety margins to multiplication factor in criticality calculations. it reflects the uncertainty in all calculations. when subcriticality of some system must be certified the calculations are usually aimed at maximum multiplication factor by 5 % below the criticality. the uq process can verify that such a margin is sufficient for any design systems. since sampling based method only requires forward 8 http://dx.doi.org/10.14311/app.2018.14.0008 http://ojs.cvut.cz/ojs/index.php/app vol. 14/2018 uncertainty propagation for lwr burnup benchmark calculations, it presents some advantages if compared to adjoint-based perturbation theory approach used by e.g. tsunami modules of scale. in problems with significant second order effects, the first order perturbation theory is not valid while the sampling method is fully applicable. the typical example of the second order effect is when the perturbation in the transport operator causes significant perturbation in the flux solution of the boltzmann transport equation (see chapter 6 of [1]). scale/sampler is one of the sampling tools available at department of nuclear reactors of cvut for uncertainty qualification. it is at first used in this study to assess the impact of covariance data on nuclear transport and depletion of a lwr benchmark 2d model. then effect of geometric uncertainties, also known as manufacturing uncertainties, are quantified for the same model. 2. uncertainty analysis from nuclear data in this section, cross-section perturbation using scale/sampler is performed for a benchmark proposed by the oecd expert group uncertainty analysis in modeling (uam) [3]. resulted values of propagated uncertainties to kinf are compared against participant-averaged results for the uam benchmark. 2.1. methodology the uam benchmark has several exercises. this analysis is using fuel pin model of a typical pwr fuel rod in 15x15 assembly design from exercise i-b. this was proposed in scope of the uam benchmark in order to address uncertainties in nuclear fuel depletion calculation due to the basic nuclear data as well as the impact of processing the nuclear and covariance data. summary and discussion of neutronic cases performed by the participants is available in [4]. participantaveraged results for exercise i-b are reproduced here for comparison purposes despite different input covariance cross-sections were used by the participants. the uam expert group selected 44-group scale 6.0 covariance library as a reference library for the benchmark exercise since it was the most complete and up-to-date compilation at the time of that review. calculations in scale are conducted according to control modules and sequences. this study adopted t-depl sequence of triton that couples 2d multigroup deterministic transport in newt with origen module depletion. the lattice physics calculations are based on the 56-group energy structure of the v7-56 scale library. this library is recommended for light-water reactor calculations. trace amount of 231 nuclides was added in the inventory of burnable materials (addnux=3 card of triton) in order to reach more realistic fuel depletion from the very beginning. infinite lattice of the elementary cell was simulated by reflective boundary condition adopted for all sides of the model. parameter value hfp conditions fuel temperature (k) 900 cladding temperature (k) 600 moderator (coolant) temperature (k) 562 moderator (coolant) density (g/cm3) 0.7484 configuration unit cell pitch (mm) 14.427 fuel pellet diameter (mm) 9.391 fuel pellet material uo2 fuel density (g/cm3) 10.283 fuel enrichment (w/o) 4.85 cladding outside diameter (mm) 10.928 cladding thickness (mm) 0.673 cladding material zircaloy-4 cladding density (g/cm3) 6.55 gap material he moderator material h2o operating history operating cycle 1 burn time (days) 1825 final burn-up (gwd/mtu) 61.28 specific power (kw/kgu) 33.58 table 1. configuration and conditions of burnup pin-cell test problem. materials: pitch of unit cell reflective boundary r ef le ct iv e b o u n d ar y r ef le ct iv e b o u n d ar y figure 2. triton model model for exercise i-1b pin cell. the scale/sampler module was used to perform uncertainty propagation/analysis of neutron multiplication factor for the pin-cell shown in fig. 2. data library for cross-section perturbations is described in the following section. cross section covariance library the new 56-group (56groupcov) scale 6.2 covariance library [1], recommended by scale 6.2.1 for 9 daniel campolina, jan frybort acta polytechnica ctu proceedings kinf 0 gwd/mtu 60 gwd/mtu sampler sampler ref.* ∆ mean val 1.4023 0.9063 0.9030 0.4 % rsd (%) 0.55 0.58 0.89 sd (pcm) 770 529 805 52 % table 2. initial condition and final burnup uncertainties in kinf for cases uam* (using 44groupcov) and sampler (using 56groupcov). all applications, was used by sampler uncertainty module. the data correspond to relative uncertainties assembled from a variety of sources, including highfidelity covariance evaluations from endf/b-vii.1 [5] as well as approximate uncertainties obtained from the collaborative projects involving bnl, lanl, and ornl. the assumptions in generating the data, the library contents, and processing procedure for the scale 56-group covariance libraries can be found in [1]. comparison between the two libraries adopted by sampler and adopted by uam participants results are provided in next section in terms of relative standard deviation for some reactions, in order to clarify the difference and the probable impact on results. the number of samples/realizations in sampler code was set to 1000, that is the maximum allowed for cross section sampling. recent works show that 1000 calculations are enough to permit optimal convergence of the statistical moments of significance for any given response function [6–8]. it is important to emphasize that only cross-section (xs) perturbation is performed and it is the most important contributor to the overall uncertainty. neither fission yield perturbation nor decay data perturbations are performed. some studies considering the same benchmark concluded that contribution of decay data uncertainty for burnup calculations is negligible [9]. on the other hand, other studies stated that fission yield and decay data uncertainties may be relevant for uncertainties of nuclide densities in depletion calculations [10]. 2.2. results propagation of uncertainties to initial and final burnup results performed using sampler are presented in tab. 2 together with the mean value of kinf calculated. participant-averaged result from exercise i-b benchmark (only available for the final burnup condition in [4]) is presented in the fifth column for comparison purposes. the symbol ∆ is the difference between the value calculated in this study by sampler and the reference value. note that sd stands for 1 standard deviation and rsd is the relative standard deviation. the difference between kinf mean value calculated by sampler and the reference value was 0.4 %. it is typical difference that can be observed for criticality calculations of identical systems by different codes and xs libraries [11]. this multiplication factor value is the average for the all the 1000 perturbed cases. as the xs perturbations are based on normal distribution, figure 3. running kinf average and standard deviation as function of sample size for simulation on sampler. the mean value should stay almost the same as for a single case with average xs values. in order to provide information on sampling convergence, fig. 3 presents the average kinf and the running standard deviation as function of the sample size. it is possible to see that after 800 samples, the average of calculated kinf mean values is stabilized. in contrast to the above results, the increase of propagated uncertainty in kinf for burnup 60 gwd/mtu was 52 %. sampler estimated 529 pcm for the standard deviation on kinf and the reference value is 805 pcm. the large difference (scale x uam reference) can be explained by use of different versions of the covariance libraries. there are also sampler results for the fresh fuel and the final burnup. the estimated standard deviation on kinf was 770 pcm for the fresh fuel and it decreased to 529 pcm for the final burnup. the 31 % difference comes from change of fuel composition during depletion. each nuclide has its own xs covariance data thus the total uncertainty changes during depletion. the reason for the uncertainty decrease is discussed below and the main contributors to this phenomenon are illustrated in fig. 4 and 5. it is possible to select the main contributors to the overall kinf uncertainty. these examples can also serve as the explanation for differences between the current and the older version of the xs covariance library in scale; and they are also the reason for computed uncertainty decrease during fuel depletion. the usual main contributor to the calculation uncer10 vol. 14/2018 uncertainty propagation for lwr burnup benchmark variable distribution value std.dev lower cutoff upper cutoff fuel_radius normal 0.46955 2.1667e-4 0.4689 0.4702 gap_thickness normal 0.00955 1.3333e-3 0.00555 0.01355 clad_thickness normal 0.0673 1.3333e-3 0.0633 0.0713 all dimensions in cm. table 3. parameters for variable sampling in sampler input file. figure 4. 235u nubar relative standard deviation by energy for covariance libraries 44groupcov and 56groupcov. tainty is average fission neutron yield (nubar) of the major fissile nuclides, i.e. 235u and 239pu. fig. 4 and fig. 5 shows the relative standard deviation of this fission neutron yield in 44-groupcov and 56-groupcov for 235u and 239pu, respectively. the relative standard deviation of 235u nubar below around 10 ev is approximately by 24 % higher for 56-groupcov. much larger difference can be observed for 239pu nubar. its rsd is about three times larger in 44-groupcov when compared to 56-groupcov. it should be also noted that its uncertainty value is lower when compared to 235u nubar. it is the reason for multiplication factor calculation uncertainty decrease during fuel depletion when adopting 56-groupcov. in the final figure 6, this comparison shows that the relative standard deviation of absorption in 238u is comparable for these two covariance libraries and it is below 5 % for all neutrons except fast neutrons. in addition to the different covariance data used for the two methodologies compared, uncertainties may be introduced into the broad-group cross-sections due to approximations in the grouping procedure. in [12], it is showed that the dominant uncertainty is generally with regard to the energy weighting function used to average the point-wise data within a broader groups. figure 5. 239pu nubar relative standard deviation by energy for reproduced from covariance libraries 44groupcov and 56groupcov. figure 6. 238u(n, γ) relative standard deviation by energy for covariance libraries 44groupcov and 56groupcov. 3. uncertainty analysis from geometry perturbation the uam benchmark model of pwr fuel rod presented in section 2.1 had its geometrical uncertain parameters of fuel radius, gap thickness, and clad thickness modeled in sampler. the methodology adopted and resulting propagated uncertainty to the neutron multiplication factor are presented. 11 daniel campolina, jan frybort acta polytechnica ctu proceedings 0 gwd/mtu 60 gwd/mtu name variable # samples kinf rsd(%) sd(pcm) kinf rsd(%) sd(pcm) case1 fuel_radius 300 1.40749 0.010 14 0.90494 0.002 2 case2 gap_thickness 300 1.40749 0.004 6 0.90494 0.000 0 case3 clad_thickness 300 1.40749 0.005 8 0.90494 0.001 1 case4 all together 300 1.40749 0.012 17 0.90494 0.003 3 table 4. variables sampled for each study case and results of uq with the contributors to uncertainty in kinf. 3.1. uncertain variables normal distribution of the geometrical parameters was assumed. the uam benchmark specifies values of standard deviations and there is also requirement to truncate the distribution at 3 standard deviations as lower cutoff and upper cutoff. table 3 presents the parameters considered in the sampling of variables. uq for each variable was calculated in order to perform the sensitivity study of the geometric parameters. the values of fuel radius (fuelr), gap outer radius (gapr) and cladding outer radius (cladr) as function of the sampled variables (tab. 3) are the following: • case1 (variable sampled: fuel_radius ) fuelr = fuel_radius gapr = fuel_radius+0.00955 cladr = fuel_radius + 0.00955 + 0.0673 • case2 (variable sampled: gap_thickness) fuelr = 0.46955 gapr = 0.46955 + gap_thickness cladr = 0.46955 + gap_thickness+0.0673 • case3 (variable sampled: clad_thickness) fuelr = 0.46955 gapr = 0.4791 cladr = 0.4791 + clad_thickness • case4 (variable sampled: all together) fuelr = fuel_radius gapr = fuel_radius + gap_thickness cladr = fuel_radius + gap_thickness + clad_thickness since the cladding outer radius (cladr) depends on uncertain variables values, the moderator/fuel volume ratio and the cladding density vary according to the sampled values. however, for being a neutron nonabsorbing material (zircaloy-4), the density change was disregarded. the fuel meat density (d), dependent on fuel radius for case 1 and case 4, is updated according to equation 1. this allows to keep the value of fuel mass constant when fuel radius sampled value is updated in the model geometry. the nominal density is worth 10.283 g/cm3 according to tab. 1. dnew[g/cm3] = 10.283 × ( 0.46955 fuel_radius )2 (1) 3.2. results the cases considered are presented in tab. 4, where the variable under study is given in the second column. the contribution to uncertainty in kinf are presented for 0 gwd/mtu and 60 gwd/mtu burnups. uncertainty in the fuel radius (case 1) is the biggest contributor to the overall calculation uncertainty due to the fact that its value directly influences fuel volume. the second in rank is the clad thickness represented by case 3. it was noticed that the effect of uncertainties in kinf is much more pronounced for the fresh fuel rather than for the final burnup condition. 4. conclusions uncertainty propagation analysis was performed for the burnup pin-cell exercise i-b of the uam benchmarks. the sampler code using scale 6.2 56groupcov covariance library had the results compared against participant-averaged result from the benchmark exercise i-b. the difference between propagated uncertainty (sd) in kinf calculated by sampler and the reference case (uam benchmark) was 52 %. such difference is expected since the codes used different versions of covariance libraries with different group structure. comparison of results for the fresh and spent fuel for the same covariance library was done for the sampler results alone, as there are no benchmark reference results. the observed 31 % difference in standard deviations is attributed to the fact that propagated uncertainty is mainly affected by nuclide composition that changes along the burnup. future research must include other nuclear libraries for comparison. when propagating geometrical uncertainties, the effect of uncertainty in fuel radius, clad and gap thickness was calculated. it was noted that the influence of geometric input uncertainties on kinf is more pronounced for the fresh than for the depleted fuel. this can be explained by the fact that fresh fuel contains higher density of fissile material and influences more the neutron population of the reactor. the sensitivity study also confirmed that uncertainty of fuel radius (case 1) is the biggest contributor to the overall calculation uncertainty since it influences directly fuel volume even though the total fuel mass is preserved. this study also confirmed that the contribution of manufacturing geometrical uncertainties to the prediction 12 vol. 14/2018 uncertainty propagation for lwr burnup benchmark of kinf is smaller than the contribution of cross-section uncertainties. acknowledgements the mobility of the brazilian co-author to department of nuclear reactors, czech technical university in prague has been possible with the erasmus mundus smart2 support (project reference: 552042-em-1-2014-1-fr-era mundus-ema2) coordinated by centrale supelec. references [1] e. b.t. rearden, m.a. jessee. scale code system version 6.2.1. tech. rep. ornl/tm-2005/39, oak ridge national laboratory, oak ridge, tennessee, 2016. [2] a. haldar, s. mahadevan. probability, reliability, and statistical methods in engineering design. john wiley, 2000. [3] k. ivanov, m. avramova, s. kamerow, et al. benchmark for uncertainty analysis in modeling (uam) for design, operation and safety analysis of lwrs, vol. 1. citeseer, 2013. [4] r. n. bratton, m. avramova, k. ivanov. oecd/nea benchmark for uncertainty analysis in modeling (uam) for lwrs–summary and discussion of neutronics cases (phase i). nuclear engineering and technology 46(3):313–342, 2014. doi:https://doi.org/10.5516/net.01.2014.710. [5] m. chadwick, m. herman, p. obložinskỳ, et al. endf/b-vii. 1 nuclear data for science and technology: cross sections, covariances, fission product yields and decay data. nuclear data sheets 112(12):2887–2996, 2011. doi:https://doi.org/10.1016/j.nds.2011.11.002. [6] l. fiorito, d. piedra, o. cabellos, c. diez. inventory calculation and nuclear data uncertainty propagation on light water reactor fuel using aleph-2 and scale 6.2. annals of nuclear energy 83:137–146, 2015. doi:https://doi.org/10.1016/j.anucene.2015.03.046. [7] w. wieselquist, t. zhu, a. vasiliev, h. ferroukhi. psi methodologies for nuclear data uncertainty propagation with casmo-5m and mcnpx: results for oecd/nea uam benchmark phase i. science and technology of nuclear installations 2013, 2013. doi:http://dx.doi.org/10.1155/2013/549793. [8] a. yankov, b. collins, m. klein, et al. a two-step approach to uncertainty quantification of core simulators. science and technology of nuclear installations 2012, 2012. doi:http://dx.doi.org/10.1155/2012/767096. [9] o. cabellos. presentation and discussion of the uam/exercise i-1b:"pin-cell burn-up benchmark" with the hybrid method. science and technology of nuclear installations 2013, 2013. doi:http://dx.doi.org/10.1155/2013/790206. [10] a. aures, f. bostelmann, m. hursin, o. leray. benchmarking and application of the state-of-the-art uncertainty analysis methods xsusa and shark-x. annals of nuclear energy 101:262–269, 2017. doi:https://doi.org/10.1016/j.anucene.2016.11.025. [11] e. fridman, j. leppänen. on the use of the serpent monte carlo code for few-group cross section generation. annals of nuclear energy 38(6):1399 – 1405, 2011. doi:https://doi.org/10.1016/j.anucene.2011.01.032. [12] z. ting. sampling-based nuclear data uncertainty quantification for continuous energy monte carlo codes. ph.d. thesis, école polytechnique féeérale de lausanne, 2015. doi:10.5075/epfl-thesis-6598. 13 http://dx.doi.org/https://doi.org/10.5516/net.01.2014.710 http://dx.doi.org/https://doi.org/10.1016/j.nds.2011.11.002 http://dx.doi.org/https://doi.org/10.1016/j.anucene.2015.03.046 http://dx.doi.org/http://dx.doi.org/10.1155/2013/549793 http://dx.doi.org/http://dx.doi.org/10.1155/2012/767096 http://dx.doi.org/http://dx.doi.org/10.1155/2013/790206 http://dx.doi.org/https://doi.org/10.1016/j.anucene.2016.11.025 http://dx.doi.org/https://doi.org/10.1016/j.anucene.2011.01.032 http://dx.doi.org/10.5075/epfl-thesis-6598 acta polytechnica ctu proceedings 14:36–41, 2018 1 introduction 2 uncertainty analysis from nuclear data 2.1 methodology 2.2 results 3 uncertainty analysis from geometry perturbation 3.1 uncertain variables 3.2 results 4 conclusions acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2018.15.0036 acta polytechnica ctu proceedings 15:36–40, 2018 © czech technical university in prague, 2018 available online at http://ojs.cvut.cz/ojs/index.php/app effect of viscous damping in numerical calculation of cable-membrane structures by the method of dynamic relaxation miloš huttner∗, petr fajman czech technical university in prague, faculty of civil engineering, thakurova 7, 166 29 prague 6, czech republic ∗ corresponding author: milos.huttner@fsv.cvut.cz abstract. the aim of this article is to assess the speed of convergence of numerical calculation of cable-membrane structures using dynamic relaxation method in the process of finding critical damping pre-calculated on undamped system. the procedure is tested on four different constructions in six numerical cases. the variety of examples is as large as possible to demonstrate the greatest versatility of the test procedure. the efficiency of the procedure is evaluated based on the number of iterations. keywords: cable-membrane structures, dynamic relaxation, critical damping, viscous damping. 1. introduction the design of cable-membrane structures is realized using numerical models of construction. execution of the numerical analysis is not easy. the shape of the structure, unlike conventional structures, is not known in advance. the initial equilibrium shape of the structure must be found during the phase called the form-finding process. the second phase is the static response process. both processes represent a geometrically non-linear case from a mathematical point of view, but each process has its own specifics. different matrix methods have been developed to address both processes (for example force density method [1], update reference strategy [2], transient stiffness method, etc.). the method of dynamic relaxation represents a very suitable alternative to matrix procedures [3]. in addition, the method can be easily used for both phases of the design of cablemembrane structures. consuming cpu time with more unknowns takes approximately linearly, so the method is very well applicable to models with a large number of degrees of freedom [4]. the dynamic relaxation method builds on the principle that the structure under free oscillation is directed to static equilibrium. the fictitious dynamic analysis is used to find static equilibrium. the artificial parameters (masses, a damping factor, a time interval) influence the speed of convergence. the convergence is the fastest if the damping (a viscous damping is considered) is critical [4]. for fast convergence, it is necessary to find the fundamental oscillation frequency, which is not easy for geometrically non-linear cases. one possible way is to use a pre-calculation with an undamped oscillation mode. the aim of this article is to assess the speed of convergence of numerical calculation of cable-membrane structures in the process of finding critical damping pre-calculated on undamped system. the procedure is tested on four different constructions in six numerical cases. both design processes (form-finding and static response) will be included. 2. methodology critical viscous damping coefficient can be found from [4]: c = 4πmf, (1) where c is viscous damping coefficient, m is mass (or vector of masses), f fundamental frequency of oscillation. the fundamental frequency of oscillation can be obtained pre-calculated on undamped system, with c set to zero. based on the coordinate displacement record, the fundamental oscillation frequency is determined by the formula [4]: f = 1 n × ∆t , (2) where ∆t is time interval, n is the number of iterations required to complete one cycle of oscillations in a fundamental mode. as can be seen from the equation 1, the critical damping factor is a multiple of the mass vector. this makes it easy to compare whether another damping coefficient is converging faster. this article investigates the determination of the damping coefficient based on the oscillation of the structure in the undamped mode (according to equation 2) with one cycle of oscillation. the procedure is a comparison with the otherwise set damping factor for a total of six computational cases (on four different structures). the efficiency of the process is evaluated based on the number of iterations. other parameters are considered as follows: the time interval is selected one second. the mass vector is determined based on the stiffness of each node and 36 http://dx.doi.org/10.14311/app.2018.15.0036 http://ojs.cvut.cz/ojs/index.php/app vol. 15/2018 effect of viscous damping in numerical calculation... is set for each node separately (which is one of the most effective methods, for example, see [5]). 3. testing there are a total of four reference examples, which are used together for a total of six cases. the variety of examples is as large as possible to demonstrate the greatest versatility of the test procedure. structures are modeled in 3d space and include the form-finding (ff) and static response (sr) processes. cables are numerically approximated using bar (ff and sr) and cable (sr) elements, see more [6]. membranes are approximated using isoparametric triangular elements (for ff and sr), see [3] or [7]. 3.1. example 1 the structure is taken from [6]. a scheme of the model structure can be seen in figure 1. the span l = 30.48 m. there have been many experiments on this real orthogonal cable net. thus, numerical results can be compared with real-measured values. cable elements are used to model cables. all cable segments have same cross-sectional areas 1.4645 × 10−4 m2 and young’s modulus 8.2737 × 1010 n/m2. the slack lengths of inner cables are 30.419 m, and the remaining cables are 31.76 m. each of the four internal joints 4, 5, 8 and 9 carries a downward concentrated load 35.56 kn. a small self weigh of 1.46 × 10−3 kn/m for cables is considered. the structure has 12 degrees of freedom. only the static response of the design is solved. figure 1. schema of example 1. perspective view. 3.2. example 2 the structure is taken from lewis [4]. the results can be compared for both processes (form-finding and static response). a structure schema is shown in figure 2. the symmetry of initial configuration about the x and y centre axes is maintained. the coordinates of node 29 are x = 12 m, y = 16 m and z = 3 m. bar elements are used to approximate the cables. the structure has 45 degrees of freedom. for the form-finding process, initial geometry is generated using a pre-tension force 90 kn in x direction, (except for the centre cable where it is 120 kn), a 30 kn in the y direction, plus a vertically point load 4.8 kn at all free nodes. for static response process the cross-sectional areas is assumed to be 350 mm2 in the x direction and 120 mm2 in the y direction. the elastic modulus is 160 kn/mm2. the structure is subjected to the further increase in load 2.0 kn at all free nodes. figure 2. schema of example 2. perspective view. 3.3. example 3 it is a fictitious membrane structure. a scheme of the model structure can be seen in figure 3. the structure is supported throughout the circuit and the dimensions are l = 10 m and h = 4 m. to generate mesh, triangular elements with a maximum edge size of 1.25 meters are used. thus, a model with 231 degrees of freedom is created. the model is tested for both design phases (ff and sr). for the ff process, a constant horizontal pretension σ = 2000 kn/m2 is used (the same for directions x and y). thickness of membrane is 0.001 m. for a static response, a vertical flat load fz = 500 n/m2 is considered. the membrane parameters are: the elastic modulus is 30 n/mm2, and the poisson’s ratio is 0.2. figure 3. schema of example 3. perspective view. 3.4. example 4 it is a fictitious cable-membrane structure that has the parameters of a real construction. a scheme of the model structure can be seen in figure 4. the structure is supported in nodes 1 to 6 and both top rings. the cables are around the perimeter of the structure. the dimensions of structure are l = 8 m and h = 3 m and the radius of the two top rings is 1 m. to generate mesh, triangular elements with a 37 miloš huttner, petr fajman acta polytechnica ctu proceedings maximum edge size of 0.7 meters are used. thus, a model with 1623 degrees of freedom is created. the model is tested only for form-finding process. for the ff process, a constant horizontal pretension σ = 1000 kn/m2 is used (the same for directions x and y). the thickness of membrane is 0.001 m. the horizontal pre-tension force 50 kn is considered in all cables. figure 4. schema of example 4. perspective view. 4. results all cases were tested in the author’s script for solving cable-membrane structures using the dynamic relaxation method created in matlab®. the numerical accuracy of all calculations was controlled by a residual limit force set at 0.0001 kn. 4.1. example 1 the numerical finite element model (in final equilibrium state) for the static response is shown in figure 5. fundamental frequency was estimated at f = 0.0189 hz, that corresponds to 0.237 multiple of the mass vector (α = 0.237). figure 5. example 1 (static response) in a final equilibrium state. the figure 6 shows the number of iterations for damping coefficient values, when c = αm. it has been tested for α in the range from 0.05 to 0.6. it can be seen that the critical damping coefficient (red square) was set very well and that the number of iterations is very minimal. 4.2. example 2 the numerical finite element model (in the initial and final state) for the form-finding process is shown in figure 7. fundamental frequency was estimated at figure 6. comparison of the number of iterations for the damping coefficient determined according to equations 1 and 2 (red square) with different damping coefficients for example 1 (sr). figure 7. example 2 (form-finding) in a final equilibrium state. f = 0.0926 hz, that corresponds to 1.163 multiple of the mass vector (α = 1.163). the figure 8 shows the number of iterations for damping coefficient values. it has been tested for α in the range from 0.8 to 1.6. it can be seen that the critical damping coefficient (red square) was set well and that the number of iterations is close to minimum. figure 8. comparison of the number of iterations for the damping coefficient determined according to equations 1 and 2 (red square) with different damping coefficients for example 2 (ff). fundamental frequency for static response was estimated at f = 0.0088 hz, that corresponds to 0.111 multiple of the mass vector (α = 0.111). the figure 9 shows the number of iterations for damping coefficient values. it has been tested for in the range from 0.04 to 0.22. it can be seen that the critical damping coefficient (red box) was set excellent 38 vol. 15/2018 effect of viscous damping in numerical calculation... and that the number of iterations is minimal. figure 9. comparison of the number of iterations for the damping coefficient determined according to equations 1 and 2 (red square) with different damping coefficients for example 2 (sr). 4.3. example 3 the numerical finite element model (in the initial and final state) for the form-finding process is shown in figure 10. fundamental frequency was estimated at f = 0.0151 hz, that corresponds to 0.190 multiple of the mass vector (α = 0.190). figure 10. example 3 (form-finding) in a final equilibrium state. figure 11. comparison of the number of iterations for the damping coefficient determined according to equations 1 and 2 (red square) with different damping coefficients for example 3 (ff). the figure 11 shows the number of iterations for damping coefficient values. it has been tested for in the range from 0.1 to 1.0. it can be seen that the critical damping coefficient (red square) was not set well and that the number of iterations is not close to minimum. fundamental frequency for static response was estimated at f = 0.0077 hz, that corresponds to 0.096 multiple of the mass vector (α = 0.096). the figure 12 shows the number of iterations for damping coefficient values. it has been tested for in the range from 0.04 to 0.22. it can be seen that the critical damping coefficient (red square) was set excellent and that the number of iterations is minimal. figure 12. comparison of the number of iterations for the damping coefficient determined according to 1 and 2 (red square) with different damping coefficients for example 3 (sr). 4.4. example 4 the numerical finite element model (in the initial and final state) for the form-finding process is shown in figure 13. fundamental frequency was estimated at f = 0.0417 hz, that corresponds to 0.523 multiple of the mass vector (α = 0.523). figure 13. comparison of the number of iterations for the damping coefficient determined according to 1 and 2 (red square) with different damping coefficients for example 4 (ff). the figure 14 shows the number of iterations for damping coefficient values. it has been tested for α in the range from 0.2 to 0.9. it can be seen that the critical damping coefficient (red box was set well and that the number of iterations is close to minimum. 5. conclusions this article investigated the determination of the damping coefficient based on oscillation of the struc39 miloš huttner, petr fajman acta polytechnica ctu proceedings figure 14. comparison of the number of iterations for the damping coefficient determined according to 1 and 2 (red square) with different damping coefficients for example 4 (ff). ture in the undamped mode with one cycle of oscillation. the procedure was a comparison with the otherwise set damping factor for a total of six computational cases (on four different structures). the efficiency of the process was evaluated based on the number of iterations. base on the results presented in previous section, it can be concluded that the proposed method of determining the dynamic relaxation damping coefficient is very suitable for cable-membrane structures. when setting a damping coefficient according to the proposed procedure, the minimum number of iterations (or near to minimum) was achieved in four of the six test cases. excellent results are achieved by the static response analysis procedure with a construction close to the final state. it has been shown that only one oscillation cycle can be used to determine the fundamental frequency. using multiple cycles would probably improve the frequency estimate, but the cpu time would increase. it could be tested in further studies. list of symbols c viscous damping coefficient [t s−1] m mass (or vector of masses) [t] f fundamental frequency of oscillation [hz] ∆t time interval [s] n the number of iterations required to complete one cycle of oscillations in a fundamental mode [–] l the span of the structure [m] h the height of the structure [m] α multiple of the mass vector [–] acknowledgements the results presented in this paper are outputs of the research project “sgs18/037/ohk1/1t/11development and application of advanced algorithms for numerical analysis and modelling in mechanics of structures and materials” supported by the czech technical university in prague. references [1] k. linkwitz, h. j. schek. a new method of analysis of prestressed cable networks and its use on the roofs of the olympic game facilities at munich. iabse, 1972. [2] e. r. bletzinger. a general finite element approach to the form finding of tensile structures by the updated reference strategy. int j space struct (14):131–146, 1999. [3] b. h. v. topping, p. iványi. computer aided design of cable membrane structures. saxe-coburg publications, 2007. [4] w. j. lewis. tension structures: form and behaviour. thomas telford, 2003. [5] m. huttner, j. maca, p. fajman. the efficiency of dynamic relaxation methods in static analysis of cable structures. advances in engineering software (89):28–35, 2015. [6] h. deng, q. f. jiang, a. s. k. kwan. shape finding of incomplete cable-strut assemblies containing slack and prestressed elements. computers and structures (83):21–22, 1767–1779, 2005. [7] m. r. barnes. form and stress engineering of tension structures. structural engineering review (6 (3-4)):175–202, 1994. 40 acta polytechnica ctu proceedings 15:36–40, 2018 1 introduction 2 methodology 3 testing 3.1 example 1 3.2 example 2 3.3 example 3 3.4 example 4 4 results 4.1 example 1 4.2 example 2 4.3 example 3 4.4 example 4 5 conclusions list of symbols acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2017.8.0024 acta polytechnica ctu proceedings 8:24–26, 2017 © czech technical university in prague, 2017 available online at http://ojs.cvut.cz/ojs/index.php/app functionalization of 3d fibrous scaffolds prepared using centrifugal spinning with liposomes as a simple drug delivery system michala rampichováa,∗, matej buzgoa, věra lukášováb, andrea míčkováa,b, karoliína vocetkováa,b, věra sovkováb, franco rustichellib, evžen amlera,b a uceeb, czech technical university in prague, bustehrad, czech republic b institute of experimental medicine of cas, prague, czech republic ∗ corresponding author: michala.rampichova@uceeb.cz abstract. 3d materials supporting cell adhesion, infiltration and proliferation are crucial for bone tissue engineering. to fulfill these expectations, it is beneficial to combine the 3d scaffold with drug delivery systems. release of the bioactive molecules directly in the site of the defect results in attraction of cells and their proliferation and differentiation in the required cell type. only properly differentiated cells produce proteins of extracellular matrix and can form new tissue. in the current study, we propose a simple drug delivery system composed of polycaprolactone fibers, prepared using centrifugal spinning, with adhered liposomes filled with platelet lysate. platelets contain wide range of growth factors and cytokines and can be used as their natural and autologous source. the scaffold was tested in vitro using mg-63 osteoblast-like cells. cell adhesion, proliferation and osteogenic differentiation were determined. three different concentrations of liposomes were used and compared to a scaffold with control liposomes filled only with pbs and to the plain polycaprolactone fibers. the results of the study showed that the platelet lysate stimulated cell proliferation. on the other hand, its effect on osteogenic differentiation was not proved. the combination of polymeric fibers with liposomes via simple adhesion was shown to be a promising scaffold for bone tissue engineering applications. in the future, it is necessary to focus on the stability of liposomes to prevent the burst release of the incorporated molecules. 1. introduction nowadays 3d materials supporting cell adhesion, infiltration and proliferation are needed in tissue engineering applications. these materials should serve not only as a scaffold, but also promote cell ingrowth and differentiation into the required cell type. therefore, it is beneficial to combine the scaffolding material with a drug delivery system. liposomes remain among the nanoparticles most widely used as carriers of active molecules [1]. nanofibers and microfibers are often used in tissue engineering, because they mimic the natural extracellular matrix (ecm) and enhance cell adhesion and proliferation [2]. centrifugal spinning uses centrifugal force to produce ultrafine fibers from melts and solutions [3], [4]. this method gives rise to fibers with submicron diameters and fluffy 3d structure. in the current study, pcl fibers prepared using the centrifugal spinning were combined with adhered liposomes. as a natural source of cytokines and growth factors the platelet lysate (pl) was used to boost the growth of mg-63 cells. 2. materials and methods 2.1. preparation of fibrous scaffold with adhered liposomes and its characterization fibrous meshes were prepared using centrifugal spinning device (cyclone 1000 l/m, fiberio, usa) from 40 % polycaprolactone (sigma aldrich) dissolved in chlorophorm:ethanol in volume ratio 9:1. orifice g30 and rotation speed 10000 rpm was used. unilamellar liposomes were prepared from l-α-phosphatidylcholine from egg yolk (sigmaaldrich) using the lipid film hydratation method followed by extrusion (1 µm pore size) [5]. liposomes were adhered to fibrous scaffolds for 2 hours and subsequently fibers were rinsed with pbs to wash out the non-adhered liposomes. five different samples were prepared: pcl fibers with maximal concentration of liposomes (25 mg/ml) (pcl/pl1); 4-fold diluted liposomes (pcl/pl2), and 10-fold diluted liposomes (pcl/pl3). plain pcl fibers (pcl) and pcl fibers with adhered liposomes contained pbs (pcl/pbs) were used as control samples. fibrous scaffolds with adhered liposomes were visualized using cryo-scanning electron microscopy (fei nova nanosem, cr) and size of liposomes was determined by mastersizer 3000. release of growth factors from liposomes with incor24 http://dx.doi.org/10.14311/app.2017.8.0024 http://ojs.cvut.cz/ojs/index.php/app vol. 8/2017 functionalization of 3d fibrous scaffolds figure 1. cryo-scanning electron microscopy of fibers prepared by centrifugal spinning techniques with adhered liposomes. detection of liposomes size distribution using dynamic light scattering (b) and detection of overall protein release (c). figure 2. mg-63 adhesion and proliferation on scaffolds measured using dna quantification (a) and cell metabolic activity measured using mts test (b). data were confirmed using confocal microscope. cell nuclei were stained using pi (red colour) and cell membranes using dioc6(3) (green). scale 200 µm, objective 10×. porated pl adhered on fibrous scaffolds was measured as a release of overall protein (quant-it protein assay kit, life technologies) and using elisa (tgf-β1, p-selectin; duoset®, r&d systems, usa). 2.2. in vitro test using mg-63 cells prepared samples were seeded with 3.1 × 104 mg63 cells/cm2. the metabolic activity of cells was measured using an mts assay (celltiter 96®aqueous one solution cell proliferation assay; promega). the proliferation of cells on the scaffold was measured from the amount of dna (quant-it™dsdna assay kit; life technologies). confocal microscopy (zeiss lsm duo) was used to visualize cell adhesion on the scaffolds. samples were stained using fluorescent probe 3,3’diethyloxacarbocyanine iodide (dioc6(3), invitrogen, 1 µg/ml in pbs) and propidium iodide (pi; 5 µg/ml in pbs). osteogenic differentiation was detected as synthesis and production of proteins typical for bone ecm, type i collagen and osteocalcin using real-time pcr (light cycler 480 ii; roche, switzerland) and immunofluorescence staining (osteocalcin ocg3, abcam, uk, af488 secondary antibody). results were evaluated statistically using one way anova. 3. results and discussion 3.1. liposome adhesion on fibers and release of protein content figure 1 shows adhesion of liposomes on the fibers. liposome size distribution measurement showed that 67 % of liposomes were in range 0.46 0.77 µm (fig 1b). the pattern of release was similar for tested proteins. there was visible burst release from liposomes after the first 15 minutes (fig 1c). the strongest release was detected in pcl/pl1 sample with the most liposomes. small release was visible on pcl/pl1 and pcl/pl2 samples 24 hours after liposome adhesion. the burst release of proteins was caused by low stability of the liposomes. 25 m. rampichová, m. buzgo, v. lukášová et al. acta polytechnica ctu proceedings 3.2. cell viability, proliferation and osteogenic differentiation on scaffolds cells adhered similarly on all scaffolds. cell proliferation was mostly visible on pcl/pl3 and pcl/pl2 respectively. after 14 days of cultivation, dna amount was higher on all samples containing thrombocyte lysate (pcl/pl1 3) compared to the sample with control liposomes with pbs (pcl/pbs) (fig 2a). cell viability was comparable for all samples (fig 2b). data were confirmed by confocal microscopy. cells adhered well on all samples and were uniformly distributed. cells proliferated till day 7, when confluent layers of cells were visible on pcl/pl1 3 (fig 2e-g). decrease in cell number was seen on day 14. detection of osteogenic markers was very low and did not show any differences between experimental and control samples (data not shown). positive influence of pl was shown in following days. interestingly, positive effect was more visible in samples with 4and 10-fold diluted liposomes (pcl/pl2 and pcl/pl3) and thus pl platelets contain a wide spectrum of growth factors and cytokines with different effect. in our study positive effect on cell proliferation was detected. on the other hand, no effect was shown on osteogenic differentiation. positive effect of pl was limited by its burst release from the liposomes. surface adhesion is simple and variable method of functionalization of the surface of fibers. one of the possibilities how to prolong the release is stabilization of liposomes using peg. another strategy is to incorporate bioactive molecules inside the fibers. up to date, only limited number of studies report using of forcespinning fibers as a drug delivery system [6]. 4. conclusion combination of 3d fibers prepared using centrifugal spinning and liposome adhesion can serve as simple and variable drug delivery system. in the future study we need to focus on liposome stability to prolong the release of incorporated molecules. acknowledgements this research was supported by the czech science foundation grant no. 15-15697s. authors thank to dr. benada from institute of microbiology of the cas for cryo-sem images. references [1] a. lowery, h. onishko, d. e. hallahan, z. han. tumor-targeted delivery of liposome-encapsulated doxorubicin by use of a peptide that selectively binds to irradiated tumors. journal of controlled release 150(1):117 – 124, 2011. doi:http://dx.doi.org/10.1016/j.jconrel.2010.11.006. [2] m. rampichová, j. chvojka, m. buzgo, et al. elastic three-dimensional poly (ε-caprolactone) nanofibre scaffold enhances migration, proliferation and osteogenic differentiation of mesenchymal stem cells. cell proliferation 46(1):23–37, 2013. doi:10.1111/cpr.12001. [3] a. m. loordhuswamy, v. r. krishnaswamy, p. s. korrapati, et al. fabrication of highly aligned fibrous scaffolds for tissue regeneration by centrifugal spinning technology. materials science and engineering: c 42:799 – 807, 2014. doi:http://dx.doi.org/10.1016/j.msec.2014.06.011. [4] m. rampichová, m. buzgo, j. chvojka, et al. cell penetration to nanofibrous scaffolds: forcespinning®, an alternative approach for fabricating 3d nanofibers. cell adhesion & migration 8(1):36 – 41, 2014. doi:10.4161/cam.27477. [5] l. d. mayer, m. b. bally, m. j. hope, p. r. cullis. techniques for encapsulating bioactive agents into liposomes. chemistry and physics of lipids 40(2):333 – 345, 1986. doi:http://dx.doi.org/10.1016/0009-3084(86)90077-0. [6] l. a. mary, t. senthilram, s. suganya, et al. centrifugal spun ultrafine fibrous web as a potential drug delivery vehicle. express polymer letters 7(3):238 – 248, 2013. doi:10.3144/expresspolymlett.2013.22. 26 http://dx.doi.org/http://dx.doi.org/10.1016/j.jconrel.2010.11.006 http://dx.doi.org/10.1111/cpr.12001 http://dx.doi.org/http://dx.doi.org/10.1016/j.msec.2014.06.011 http://dx.doi.org/10.4161/cam.27477 http://dx.doi.org/http://dx.doi.org/10.1016/0009-3084(86)90077-0 http://dx.doi.org/10.3144/expresspolymlett.2013.22 acta polytechnica ctu proceedings 8:24–26, 2017 1 introduction 2 materials and methods 2.1 preparation of fibrous scaffold with adhered liposomes and its characterization 2.2 in vitro test using mg-63 cells 3 results and discussion 3.1 liposome adhesion on fibers and release of protein content 3.2 cell viability, proliferation and osteogenic differentiation on scaffolds 4 conclusion acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2018.18.0032 acta polytechnica ctu proceedings 18:32–37, 2018 © czech technical university in prague, 2018 available online at http://ojs.cvut.cz/ojs/index.php/app mechanical model for human balancing on rolling balance board csenge a. molnara,b,∗, ambrus zeleia, tamas inspergera,b a mta-bme lendület human balancing research group, muegyetem rakpart 5, budapest, hungary b department of applied mechanics, budapest university of technology and economics, muegyetem rakpart 5, budapest, hungary ∗ corresponding author: csenge.molnar@mm.bme.hu abstract. a two-degree-of-freedom mechanical model was developed to analyze human balancing on rolling balance board in the frontal plane. the human nervous system is modeled as a proportionalderivative controller with constant feedback delay. the radius r of the wheels and the board distance h measured from the center of the wheel are adjustable parameters. simulation results using the mechanical model were compared with real balancing trials recorded by an optitrack motion capture system. the goal of the paper is to investigate whether the two-degree-of-freedom model is accurate enough to model the balancing task and to introduce a stabilometry parameter in order to characterize balancing skill in case of different set of r and h. the conclusion is that the angle of the upper body and the angle of the head also play an important role in the balancing process therefore a threeor four-degree-of-freedom model is more appropriate. keywords: balance board, human balancing, stability, stabilizability, stabilometry, time delay. 1. introduction nowadays, in the aging society, falls related to loss of balance are becoming the second most frequent reason for accidental death. therefore the analysis and the understanding of human balancing and the development of tools to prevent falling are more and more important tasks in biomechanical researches. from dynamics point of view, human balancing is the stabilization of the human body around an unstable equilibrium, namely, the upright posture. the stabilization process is controlled by the nervous system. processing the signals perceived by the sensory organs, determining and realizing the necessary muscle movement to maintain the balance require certain amount of time, therefore the control concept applied by the human nervous system can be modeled as a delayed feedback mechanism, where the feedback delay corresponds to the reaction time. there are many concepts to describe the control mechanism of the human nervous system, such as proportional-derivative (pd) feedback [1–4], proportional-derivative-acceleration (pda) feedback [5], clockand event-driven intermittent control [6–9], act-and-wait [10], drift-and-act control [11] and predictor feedback [2, 5, 12]. sensory dead zones and sampled feedback mechanism can also be involved into the model of human balancing to model the chaotic behavior of the balancing movements [13]. models for human balancing involves many uncertain components. besides the lack of knowledge of the exact feedback mechanism, the control gains and the value of the reaction time delay are all uncertain parameters that have to be identified by some fitting technique. the mechanical model of the human body also involves many uncertain parameters, i.e., the mass and the inertia of the body segments [14], and the passive stiffness and damping at the joints [6, 15]. when human subjects are balancing on a balance board, then the mechanical properties of the balance board can be determined with high accuracy, therefore its actual parameters can be introduced into the mechanical model. moreover, the parameters of the balance board applied in our research can be changed, thus the balancing performance at different conditions can be analyzed. most of the mechanical models for standing still involve a one-degree-of-freedom model, which is an inverted pendulum in the sagittal plane associated with ankle strategy [11, 16–18] and a four-bar mechanism in the frontal plane [19–21]. two-degree-of-freedom models also exist in the literature, e.g., the double inverted pendulum associated with the hip strategy [22]. balancing on a balance board in the frontal plane can be described by a two-degree-of-freedom model, since the angular position of the balance board plays an important role in the dynamics in addition to the position of the four-bar mechanism model of the human body. the goal of this paper is to determine whether the one-degree-of-freedom four-bar mechanism model of the human body in the frontal plane is sufficient for dynamic models of balancing on a rolling balance board. stabilizability in terms of the critical delay is compared to the balancing performance using the enveloping ellipse of the phase portrait as a stabilometry parameter. 32 http://dx.doi.org/10.14311/app.2018.18.0032 http://ojs.cvut.cz/ojs/index.php/app vol. 18/2018 example of an article with a long title figure 1. balance board, with interchangeable wheels and adjustable board heights. 2. balance board the balance board can be seen in fig. 1. two parameters can be changed: the radius r of the wheels and the distance h between the centre of the wheel and the board. the wheels were manufactured with radius of 50, 750, 100, 125, 150, 200 and 250 mm and parameter h can be adjusted by steps of 25 mm in case of each wheel. some of the wheels were manufactured with raised edge, which means, board distance h can take negative value. 3. balancing task the balancing task investigated in this paper is shown in fig. 2. the balancing subjects have to balance on the balance board in the frontal plane, with stretched legs, open eyes and clasped hands behind the back. they have to press their foot to the board, the relative motion between the board and the feet can be prevented this way. changing the wheel radius or the board distance has an important role in terms of stabilizability of the system. theoretical derivations and experimental results show that the smaller the wheel radius r and the smaller the board distance h, the more difficult the balancing task [23]. 4. mechanical model a two-degree-of-freedom mechanical model was created for the balancing task, where the human body is modeled as a four-bar mechanism, as can be seen in fig. 3. the legs are two homogeneous rigid bars and the trunk corresponds to a t-shaped rigid body connected to the legs by ideal pins. the feet are considered to be fixed to the board, and ankles behave as pins between the legs and the balance board. the generalized coordinates are the angle ϕ of the trunk relative to the balance board and the angle ϑ of the balance board relative to the environment. the control concept of the human nervous system is described as a time-delayed pd controller with a constant time delay τ , which corresponds to the reaction time of the balancing person. according to this assumption, the nervous system controls by signals proportional to the angular offset and velocity of the balance board and the human body. the control torque m is assumed to act between the hip and the legs as indicated in figure 2. balancing task, measurement setup. fig. 3. the passive stiffness sa and sh at the ankles and the hips is determined based on data from the literature [15, 24–27] assuming that both joints can be considered as spherical joints in case of small rotations. passive damping at the joints is neglected, therefore the equation of motion can be written as mq̈(t) + sq(t) = pq(t− τ) + dq̇(t− τ), (1) where m and s are the mass and the stiffness matrices, p = [ pϕ pϑ 0 0 ] , d = [ dϕ dϑ 0 0 ] , (2) are the matrices of the proportional and the derivative gains, respectively, and q(t) = [ ϕ(t) ϑ(t) ] (3) is the vector of generalized coordinates. mathematically, the equation (1) of motion is a system of linearized delayed differential equations. the stability analysis of the solution is performed using the semidiscretization method [28]. there exists a critical time delay τcrit for each combination of r and h, if the feedback delay is larger than τcrit then there are no pϕ, dϕ, pϑ, dϑ control gains associated with a 33 cs. a. molnar, a. zelei, t. insperger acta polytechnica ctu proceedings a d c b ϕ1 ϕ2 ϕ ϑ (3),mt , it (2),mr, ir (1),ml, il rolling ha h r x y z g m m sh sh sa sa mb, ib figure 3. two-degree-of-freedom mechanical model of balancing on a rolling balance board in the frontal plane. α β figure 4. definition of angles α and β. stable system. a stabilizability diagram can be constructed for each balancing subject, which shows the critical time delay as function of the balance board geometry (see fig. 8). contour lines show, that the critical time delay decreases for smaller r and h values, which indicates that the balancing task is more difficult. reference [23] contains a detailed description of the construction of the stabilizability diagram. 5. degrees of freedom of the human body model in order to validate the mechanical model, the balancing subject performed 60 s long balancing trials for a series of combinations of r and h. the arrows in figure 5. time history of angle α indicated in fig. 4 a). fig. 2 show the markers located on the human body and the balance board during the measurements. an optitrack motion capture system was used to record the 3d position of the markers. the sampling frequency was 120 hz. for a fixed wheel radius r, the halving method was used to determine the critical value of parameter h where the subject was not able to balance at least for 60 s. according to the mechanical model, the whole upper body was modeled as one rigid body. in order to validate this assumption, two angles were specified to measure the change in the posture of the upper body. angle α is the angle between the lines connecting the markers placed on the shoulders and on the hips (see fig. 4 a). angle β is the angle between the lines connecting the markers on the shoulders and the line connecting the midpoint of this line and the marker on the top of the head (see fig. 4 b). according to the rigid upper body assumption, angles α and β should not change during the balancing trials. the time history of angles α and β can be seen in fig. 5 and fig. 6, respectively. the mean value and the standard deviation are indicated by red and blue lines, respectively. the difference between the highest and lowest value of the angles are also indicated. it can be seen that the motion of the shoulders relative to the hip is larger (standard deviation of 4° and maximum difference of 13°) than the motion of the head with respect to the upper body (standard deviation of 3° and maximum difference of 9°). both variations are commensurable to the angles during the balancing trials (e.g., the angle of the legs with respect to the balance board). this observation implies that a more detailed multi-body model, which involves extra rigid bodies for the upper body and the head, is more appropriate. on the other hand, the introduction of extra rigid bodies in the model induces more uncertain parameters, such as the inertia of the upper body and the head, which may strongly affect the accuracy of the model. therefore in the following analysis, we use the rigid upper body model. 34 vol. 18/2018 example of an article with a long title figure 6. time history of angle β indicated in fig. 4 b). 6. stabilometry many different parameters can be found in the literature to characterize the performance during balancing tasks, such as mean power frequency, median frequency, frequency range power ratios, spectral power ap-ml ratio, largest amplitude and corresponding largest frequency, just to mention a few [29, 30]. in a previous work of the authors [23], the standard deviation of the balance board angle was used as a measure of balancing skill. in this paper, a stabilometry parameter based on the phase portrait of the balance board angle is introduced since the magnitude of the angle and angular velocity of the balance board are both related to the performance during balancing. the angle of the balance board relative to the environment is calculated from the position of the markers located on the left and right corner of the wheel (fig. 2) at each time step. the angular velocity is determined via discrete differentiation. an example for the corresponding phase portrait is shown in fig. 7 together with an enveloping ellipse. the semi-major axis a and b of the ellipse is calculated as the half of the difference between the maximum and minimum angle and angular velocity of the balance board, respectively. one can observe some parts on the phase portrait, where the density of the trajectories is lower, as shown by black in fig. 7. these part of the trajectory are related to the loss of concentration of the balancing subject for a very short time and appear as larger peaks in the time history of the balance board angle. before determining the ellipses, these regions caused by these temporary effect were eliminated. first, the normal probability density (npd) of the balance board angle was calculated. then, a threshold value of the npd was determined and the values of the balance board angle below this threshold were eliminated. a similar process was used to filter the angular velocity. the semi-axes and the area of the ellipse were calculated using the filtered angle and angular velocity. the result of the balancing performance by the subject can be seen in fig. 8. green circles refer to the r and h pairs, where the 60 s long balancing trial was figure 7. phase portrait of the balance board angle, data filtering, and the fitted ellipse. figure 8. stabilizability diagram. successful. the diameter of the circle is proportional to the area of the fitted ellipse. red circles indicates unstable trials. grayscale indicates the critical delay determined using the mechanical model. one can observe, that the calculated stabilizability and the measurement has the same tendency. the balancing task is easier in case of larger radius and larger board distance, which meets the personal experience and opinion of the balancing subject. 7. conclusions balancing performance by a human subject was compared to the critical reaction time delay obtained by the stabilizability analysis of the underlying mechanical model. unsuccessful balancing trials allows an estimation of the reaction time delay of the balancing subject. the grey scale in fig. 8 shows that if the reaction time is smaller than 0.85 s, then the subject is able to balance on the lowest set of the balance board with wheel radius r = 250 mm. comparison 35 cs. a. molnar, a. zelei, t. insperger acta polytechnica ctu proceedings of the value of the grey scale and the boundary between the successful and unsuccessful trials, which is approximately parallel to the contour lines of the stabilizability diagram, the reaction time is estimated to be 0.68 s. according to the literature, the reaction time associated with human posture is definitely lower then this value. this difference can be explained by the simplified mechanical model, i.e., the upper body was modeled as a rigid body. a multi-body model, which involves additional rigid bodies for the upper body and for the head may be more appropriate. another issue is that the feedback mechanism used in this model was a delayed pd control. in the literature, other types of control concepts are also often used to model human motor control. for instance, pda control [5] or predictor feedback [2, 5, 12] are also possible candidates for the control law of human balancing tasks. acknowledgements the research reported in this paper was supported by the únkp-17-2-i new national excellence program of the ministry of human capacities and by the higher education excellence program of the ministry of human capacities in the frame of biotechnology research area of budapest university of technology 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[23] c. a. molnar, a. zelei, t. insperger. human balancing on rolling balance board in the frontal plane. in 14th ifac workshop on time delay systems, pp. 1–6. budapest, hungary, 2018. [24] d. a. winter, a. e. patla, f. prince, et al. stiffness control of balance in quiet standing. journal of neurophysiology 80(3):1211–1221, 1998. doi:10.1152/jn.1998.80.3.1211. [25] y.-s. yoon, j. mansour. the passive elastic moment at the hip. journal of biomechanics 15(12):905–910, 1982. doi:10.1016/0021-9290(82)90008-2. [26] r. riener, t. edrich. identification of passive elastic joint moments in the lower extremities. journal of biomechanics 32(5):539–544, 1999. doi:10.1016/s0021-9290(99)00009-3. [27] a. silder, b. whittington, b. heiderscheit, d. g. thelen. identification of passive elastic joint moment–angle relationships in the lower extremity. journal of biomechanics 40(12):2628–2635, 2007. doi:10.1016/j.jbiomech.2006.12.017. [28] t. insperger, g. stépán. semi-discretization for timedelay systems: stability and engineering applications, vol. 178. springer science & business media, 2011. [29] g. nagymáté, z. orlovits, r. m. kiss. reliability analysis of a sensitive and independent stabilometry parameter set. plos one 13(4):e0195995, 2018. doi:10.1371/journal.pone.0195995. [30] g. nagymáté, r. m. kiss. parameter reduction in the frequency analysis of center of pressure in stabilometry. periodica polytechnica mechanical engineering 60(4):238–246, 2016. doi:10.3311/ppme.8999. 37 http://dx.doi.org/10.1016/j.jtbi.2012.06.019 http://dx.doi.org/10.1152/jn.1998.80.3.1211 http://dx.doi.org/10.1016/0021-9290(82)90008-2 http://dx.doi.org/10.1016/s0021-9290(99)00009-3 http://dx.doi.org/10.1016/j.jbiomech.2006.12.017 http://dx.doi.org/10.1371/journal.pone.0195995 http://dx.doi.org/10.3311/ppme.8999 acta polytechnica ctu proceedings 18:32–37, 2018 1 introduction 2 balance board 3 balancing task 4 mechanical model 5 degrees of freedom of the human body model 6 stabilometry 7 conclusions acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2018.18.0044 acta polytechnica ctu proceedings 18:44–47, 2018 © czech technical university in prague, 2018 available online at http://ojs.cvut.cz/ojs/index.php/app impact testing of ordnance gelatine under moderate strain rate conditions tomáš doktor∗, petr zlámal, jan šleichrt, tomáš fíla, daniel kytýř czech technical university in prague, faculty of transportation sciences, konviktská 20, praha 1, czech republic ∗ corresponding author: doktor@fd.cvut.cz abstract. an experimental study on energy absorption capabilities and strain rate sensitivity of ordnance gelatine was performed. strain energy density under quasi static compression and moderate strain rate impact tests was compared. in the study two types of material were tested, bulk ordnance gelatine and polymeric open-cell meshwork filled with ordnance gelatine. from the results a significant strain-rate effect was observed in terms of ultimate compressive strength and strain energy density. in comparison of the deformation behaviour under quasi static conditions and drop weight test the difference was very significant, however slight increase in both strength and strain energy density was observed even between different impact energies and velocities during the impact testing. the peak acceleration was significantly reduced in polymer meshwork filled by gelatine in comparison to the bulk gelatine. keywords: ordnance gelatine, strain energy density, impact testing. 1. introduction in the engingeering applications in the field of ballistic and blast protection, passive safety systems in vehicles (automotive, railway vehicles) the impact energy dissipation is among the crucial properties [1]. one of the possible solutions to achieve such capabilities is to utilise cellular solids in conjunction with strain rate sensitive filling [2]. in this study experimental investigation on strain rate sensitivity at moderate strain rates as well as the interaction of the cellular solid and viscous filling is described. as filling material ordnance gelatine was selected due to a flawless preparation procedure and availability. the ordnance (ballistic) gelatine is widely used as tissue simulant for assessment of damage of tissues induced by bullets [3]. however strain rate sensitivity under compressive loading was reported as well [4]. deformation response of ballistic gelatine blocks (plain and reinforced with polymeric meshwork) under moderate impact loading is described in this paper in terms of peak acceleration during the impact as well as total kinetic energy dissipation. 2. materials and methods 2.1. sample preparation for both impact and quasi-static testing two types of material were used, (i) bulk ordnance gelatine and (ii) open-cell polymeric meshwork filled with ordnance gelatine. the gelatine was prepared according to the reports of jussila [5]. as a base, 260 bloom beef gelatine (remi mb, ltd., czech republic) was used. the gelatine powder was poured into warmed water (temperature was 45 °c) in the second group the gelatine was reinforced with polymeric open-cell meshwork ft-s10fr (foam techniques, ltd, united kingdom) with tetrakaidecahedral cells, mean cell size 6 mm, mean strut thickness 0.6 mm. the dissolved gelatine after homogeneous mixing was poured into the meshwork. the samples from both batches (both pure and reinforced) were cured for 24 h in room temperature and subsequently stored for 24 h in refrigerator. after this curing the blocks were cut into samples of desired size. in the quasi static loading conditions the size of samples was 25 × 25 × 50 mm, and samples’ size for impact testing was approximately 60 × 50 × 50 mm. dimensions of the samples were limited by diameter of used loading plates. to ensure a better focus during optical observation captured face of each sample was sprayed granit paint to obtain random pattern. 2.2. quasi static tests the quasi static tests were performed using electromechanic uniaxial loading device instron 6530 (instron, inc., usa). the compressive tests were displacement driven with loading rate 3 mm · min−1, which corresponded to strain rate 0.001 s−1. maximum displacement was set to 30 mm. 2.3. impact tests instrumentation the moderate strain rate compression tests were performed by an in-house drop tower developed at ctu fts [6]. the drop weight is released by electromagnetic member and then guided on steel rods and induces the impact on a sample places at a stable plate. the drop-tower setup was instrumented by triaxial accelerometer (egcs3, te connectivity, ltd., 44 http://dx.doi.org/10.14311/app.2018.18.0044 http://ojs.cvut.cz/ojs/index.php/app vol. 18/2018 impact testing of ordnance gelatine figure 1. drop tower setup (1 motorized dropweight lift, 2 trigger inductive sensor, 3 guide rods, 4 electromagnetic release member, 5 drop-weight guide frame, 6 accelerometer, 7 drop-weight). usa) with loading capacity ±1000g and impact force transducer (200c20, pcb piezoelectronics, usa) with loading capacity 89 kn. both accelerometer and force transducer were connected to read out electronics ni9234 (national instruments, inc., usa) providing sampling rate 51200 samples per second. for the observation of deformation process a high speed camera (idt nx3, usa) was employed. the camera provided frame rate 2895 to 3310 fps at resolution 768 × 1312 px and the exposure time varied from 89 to 115 µs. for illumination of the loading scene pair of flash illuminators veritas constellation 60 were used. the impact test setup is depicted in figure 1 and figure 2. 2.4. impact test procedure the impact tests were carried out in three arrangements with different initial height and mass of the impactor. in each arrangement a pair of samples from the group a (bulk gelatine) as well as from group b (polymer meshwork filled with gelatin) were tested. the initial heights and impactor masses are listed in table 1 with corresponding impact energy, velocity and strain rate. the impact velocity was calculated using the initial heigt and gravity constant based on conservation of sum of kinetic and potential energy. figure 2. drop tower setup (1 specimen, 2 impactor, 3 flash-illumination, 4 high-speed camera). 0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 tr u e s tr e s s [ m p a ] true strain [-] stress strain diagrams at quasi static compression a, 0.001/s b, 0.001/s figure 3. stress-strain curves of quasi-static tests. 3. results from the quasi static loading test stress-strain curves which are depicted in figure 3 were calculated based on the measured force f and cross-head displacement u by formulae σeng = f a0 and εeng = u l0 , where a0 is specimen cross-section and l0 is the initial gauge length. the engingeering stress σeng and strain εeng were then converted to logarithmic strain ε and true stress σ using formulae ε = ln εeng and σ = σeng(1 + εeng). to evaluate the drop tests stress-strain diagrams were created from the measured acceleration and force. while the stress is measured directly by force transducer, strain calculation is based on double time integration of the vertical component of acceleration. 45 t. doktor, p. zlámal, j. šleichrt et al. acta polytechnica ctu proceedings arrangement initial height drop weight impact velocity impact energy [m] [kg] [ms-1] [j] 1 1.00 4.495 4.4 44.1 2 1.50 4.495 5.4 66.1 3 1.50 6.504 5.4 95.7 table 1. impact test parameters. 0 2 4 6 8 10 12 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 tr u e s tr e s s [ m p a ] true strain [-] stress strain diagrams group a 73/s, 44.1 j 73/s, 44.1 j 90/s, 66.1 j 90/s, 66.1 j 90/s, 95.7 j 90/s, 95.7 j figure 4. stress-strain curves of drop tests of group a (ordnance gelatine). 0 1 2 3 4 5 6 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 tr u e s tr e s s [ m p a ] true strain [-] stress strain diagrams group b 88/s, 44.1 j 88/s, 44.1 j 108/s, 66.1 j 108/s, 66.1 j 108/s, 95.7 j 108/s, 95.7 j figure 5. stress-strain curves of drop tests of group b (polymer meshwork filled with ordnance gelatine). comparison of the stress-strain curves is depicted in figure 5. for the assessment of dissipated impact energy strain energy density λ was computed using formula λ = ∫ εmax 0 σ dε the obtained values are presented in table 2. moreover for visual evaluation of the deformation behaviour the captured loading scene was used (selected series of loading scenes in distinct loading steps is depicted in figure 6). no. / strain energy peak group density [jcm-3] acceleration 1 / a1 1.04503 234.92g 2 / a1 1.00956 209.23g 3 / b1 0.57674 95.01g 4 / b1 0.74435 94.07g 5 / a2 1.12309 210.53g 6 / a2 0.98409 231.10g 7 / b2 0.55441 149.72g 8 / b2 0.79479 147.84g 9 / a3 1.42620 199.30g 10 / a3 1.69909 223.24g 11 / b3 1.22701 152.42g 12 / b3 1.24369 148.73g table 2. impact test results. 4. conclusions an experimental study on deformation behaviour of ordnance gelatine and polymeric open-cell meshwork. from the results a significant strain-rate effect was observed in terms of ultimate compressive strength and strain energy density. in comparison of the deformation behaviour under quasi static conditions and drop weight test the difference was very significant, however slight increase in both strength and strain energy density was observed even between different impact energies and velocities during the impact testing. the comparison between tested types of materials shows a significant reduction of the peak acceleration during the impact while the strain energy density decreased only slightly. this behaviour may be explained by the flow of the filling material thru the meshwork during the impact. the obtained results will provide a base for further modelling of the impact energy dissipation as well as for optimisation of the filling material used in cellular solids. acknowledgements the research was supported by the czech science foundation (research project no. 15-15480s) and by 46 vol. 18/2018 impact testing of ordnance gelatine figure 6. loading scene of selected sample (group a3) in several loading steps (time step between captured images is 0.89ms). czech technical university in prague (research projects sgs17/148/ohk2/2t/16 and sgs18/155/ohk2/2t/16). all the support is gratefully acknowledged. references [1] p. qiao, m. yang, f. bobaru. impact mechanics and high-energy absorbing materials: review. journal of aerospace engineering 21(4):235–248, 2008. doi:10.1061/(asce)0893-1321(2008)21:4(235). [2] t. doktor, p. zlamal, t. fila, et al. properties of polymer-filled aluminium foams under moderate strain-rate loading conditions. materiali in tehnologije 49(4):597–600, 2015. doi:10.17222/mit.2014.195. [3] j. breeze, m. midwinter, d. pope, et al. developmental framework to validate future designs of ballistic neck protection. british journal of oral and maxillofacial surgery 51(1):47–51, 2013. doi:10.1016/j.bjoms.2012.03.001. [4] j. kwon, g. subhash. compressive strain rate sensitivity of ballistic gelatin. journal of biomechanics 43(3):420–425, 2010. doi:10.1016/j.jbiomech.2009.10.008. [5] j. jussila. preparing ballistic gelatine review and proposal for a standard method. forensic science international 141(2-3):91–98, 2004. doi:10.1016/j.forsciint.2003.11.036. [6] o. jirousek, p. zlamal, p. koudelka, t. fila. development and validation of material models for closed-cell metal foam for impact simulation. civil-comp proceedings 106, 2014. 47 http://dx.doi.org/10.1061/(asce)0893-1321(2008)21:4(235) http://dx.doi.org/10.17222/mit.2014.195 http://dx.doi.org/10.1016/j.bjoms.2012.03.001 http://dx.doi.org/10.1016/j.jbiomech.2009.10.008 http://dx.doi.org/10.1016/j.forsciint.2003.11.036 acta polytechnica ctu proceedings 18:44–47, 2018 1 introduction 2 materials and methods 2.1 sample preparation 2.2 quasi static tests 2.3 impact tests instrumentation 2.4 impact test procedure 3 results 4 conclusions acknowledgements references 227 acta polytechnica ctu proceedings 1(1): 227–230, 2014 227 doi: 10.14311/app.2014.01.0227 the galactic center region imaged by veritas from 2010–2012 matthias beilicke1 for the veritas collaboration2 1department of physics and mcdonnell center for the space sciences, washington university, st. louis, mo, usa 2http://veritas.sao.arizona.edu/ corresponding author: beilicke@physics.wustl.edu abstract the galactic center has long been a region of interest for high-energy and very-high-energy observations. many potential sources of gev/tev γ-ray emission are located in this region, e.g. the accretion of matter onto the central black hole, cosmic rays from a nearby shell-type supernova remnant, or the annihilation of dark matter. the galactic center has been detected at mev/gev energies by egret and recently by fermi/lat. at tev energies, the galactic center was detected at the level of 4 standard deviations with the whipple 10m telescope and with one order of magnitude better sensitivity by h.e.s.s. and magic. we present the results from 3 years of veritas galactic center observations conducted at large zenith angles. the results are compared to astrophysical models. keywords: gamma-rays galactic center black hole non-thermal veritas. 1 introduction the center of our galaxy harbors a 4×106m� black hole (bh) coinciding with the strong radio source sgr a*. x-ray/mev/gev transients in this region are observed on a regular basis. various astrophysical sources located close to the galactic center (gc) may potentially be capable of accelerating particles to multi-tev energies, such as the supernova remnant sgr a east or a pulsar wind nebula [1]. furthermore, super-symmetric neutralinos χ are discussed as potential candidates for dark matter accumulating in the gc region and annihilating into γ-rays [2]. the resulting spectrum would have a cut-off near the neutralino mass mχ. assuming a certain dark matter density profile the expected γ-ray flux along the line-of-sight integral can be calculated as a function of mχ and the annihilation cross section [3] and can in turn be compared to measured upper limits. egret detected a mev/gev source 3eg j17462851 coincident with the gc position [4] and recently fermi/lat resolved several sources in the gc region [5], see fig. 3. however, uncertainties in the diffuse galactic background models and limited angular resolution at mev/gev energies make it difficult to study the morphologies of these sources. at gev/tev energies a detection from the direction of the gc was first reported in 2001/02 by the cangaroo ii collaboration with a steep energy spectrum dn/de ∝ e−4.6 at the level of 10% of the crab nebula flux [6]. shortly after, evidence at the level of 3.7 standard deviations (s.d.) was reported from the whipple 10 m collaboration [7]. the gc was finally confirmed as a gev/tev γ-ray source by the h.e.s.s. collaboration [8] (the position of the supernova remnant sgr a east could be excluded as the source of the γ-ray emission). the energy spectrum measured by h.e.s.s. is well described by a power-law dn/de ∝ e−2.1 with a cut-off at ∼15 tev. the h.e.s.s. observations revealed a diffuse gev/tev γ-ray component (dashed contour lines in fig. 3) which is aligned along the galactic plane and follows the structure of molecular clouds [9]; the emission is explained by an interaction of local cosmic rays (crs) with matter of the molecular clouds. the magic collaboration detected the gc (7 s. d.) in 2004/05 observations performed at large zenith angles (lza) [10], followed by a strong (> 10 standard deviations) veritas lza detection in 2010 [11]. 2 veritas observations of the galactic center gc observations due to its declination the gc can only be observed by veritas at lza (zenith angles 60 − 66 deg) – strongly decreasing the angular resolution and sensitivity. the use of the displacement parameter [12], between the center of gravity of the image and the shower position, has been used in the veritas event reconstruction which strongly improved the sensitivity for lza observations [11]. the performance and energy reconstruction have been confirmed on lza crab nebula data. the column density of the atmosphere changes with 1/ cos(z). in a conservative es227 http://dx.doi.org/10.14311/app.2014.01.0227 matthias beilicke timate, the systematic error in the energy/flux reconstruction can be expected to scale accordingly. more detailed studies are needed for an accurate estimate; for the gc observations we currently give a conservative value of a systematic error on the lza flux normalization of ∆φ/φ ' 0.4. the gc was observed by veritas in 2010–2012 for 46 hrs (good quality data, dead-time corrected) with an average energy threshold of ethr ' 2.5 tev. gc results the veritas sky map of the gc region is shown in fig. 3. an 18 s.d. excess is detected. no evidence for variability was found in the 3-year data. the energy spectrum is shown in fig. 1 and is found to be compatible with the spectra measured by whipple, h.e.s.s., and magic. since the large lza effective areas of the veritas observations compensate a shorter exposure of low-zenith observations, the statistical errors of the e > 2.5 tev data points are comparable or even smaller than those of the h.e.s.s. measurements. ] -1 s -2 d n /d e [e rg c m 2 e -1310 -1210 -1110 energy [ev] 1210 1310 v e r it a s ( 20 10 -2 01 2, p re lim .) galactic center veritas (gc) h.e.s.s. (gc) whipple (gc) magic (gc) figure 1: veritas energy spectrum measured from the direction of the gc (statistical errors only). also shown are bow ties representing the spectra measured by whipple [7], h.e.s.s. [8], and magic [10]. diffuse flux limit and dark-matter annihilation off-source observations were performed in a field located in the vicinity of the gc region (similar zenith angles and sky brightness) without a known tev γ-ray source. these observations are used to study the background acceptance throughout the field of view and allow the estimate of a diffuse γ-ray component surrounding the position of the gc. an upper limit of the diffuse γ-ray flux can in turn be compared with line-of-sight integrals along the density profile ∫ ρ2dl, in order to constrain the annihilation cross section for a particular dark matter model, dark matter particle mass and density profile ρ(r). due to its likely astrophysical origin the excess at the gc itself, as well as a region along the galactic plane, will be excluded from this analysis (work in progress). hadronic models hadronic acceleration models [13, 14] involve: (i) hadrons being accelerated in the bh vicinity (few tens of schwarzschild radii). (ii) the accelerated protons diffuse out into the interstellar medium where they (iii) produce neutral pions which decay into gev/tev γ-rays. linden et al. (2012) discuss the surrounding gas as proton target defining the morphology of the tev γ-ray emission [15]. changes in γ-ray flux in those models can be caused by changing conditions in the bh vicinity (e.g. accretion). the time scales of flux variations are ∼104 yr at mev/gev energies (old flares) and ∼10 yr at e > 10 tev (’new’ flares caused by recently injected high-energy particles) [13]. constraining the e > 10 tev spectral variability would serve as an important test for this class of models. ] -1 s -2 d n /d e [e rg c m 2 e -1310 -1210 -1110 -1010 energy [ev] 810 1110 1410 veritas (preliminary) galactic center veritas (gc) fermi (chernyakova et al., 2011) ballantyne et al. (2011) [hdr] chernyakova et al. (2011) [hdr] linden et al. (2012) [hdr] atoyan et al. (2004) [lep] figure 2: veritas energy spectrum compared to hadronic [13, 14, 15] and leptonic [16] emission models discussed for the gc source. the fermi/lat bow tie is taken from [13]. leptonic models atoyan et al. (2004) [16] discuss a bh plerion model in which a termination shock of a leptonic wind accelerates leptons to relativistic energies which in turn produce tev γ-rays via inverse compton scattering. the flux variability time scale in this model is on the order of tvar ∼100 yr. the hadronic and the leptonic models are shown together with the veritas/fermi data in fig. 2. the leptonic model clearly fails in explaining the flux in the mev/gev regime. however, this emission may well originate from a spatially different region or mechanism other than the tev γ-ray emission. the hadronic models can explain the sed by the superposition of different flare stages. future fermi/veritas flux correlation studies, as well as the measurement of the tev energy cut-off and limits on the e > 10 tev variability will serve as crucial inputs for the modeling. 3 summary and conclusion veritas is capable of detecting the gc within 3 hrs in observations conducted at zenith angles greater than 228 the galactic center region imaged by veritas from 2010–2012 figure 3: veritas sky map of the gc region (smoothed excess significances, ring background, scale truncated). the black contour lines indicate the gc and the supernova remnant g 0.9+0.1 as seen by h.e.s.s. [8]. the gray dashed lines indicate the h.e.s.s. diffuse emission along the galactic plane and from hess j1745-303 [9]. the position of hess j1741-302 is indicated, as well (circle); the flux/spectrum of this source make it very unlikely to be detected in veritas lza observations. the solid circles (cyan color) indicate the positions of the mev/gev sources taken from the second fermi/lat catalog [5]. 60 deg. the measured energy spectrum is found to be in agreement with earlier measurements by h.e.s.s., magic, and whipple. future observations to measure the cut-off energy in the spectrum and to determine limits on the flux variability at the highest energies will place constraints on emission models. the recently discovered giant molecular cloud heading towards the immediate vicinity of the gc bh [17] represents further motivation for future tev γ-ray monitoring of this region. an upper limit on diffuse γ-ray emission and, in consequence, a limit on the photon flux initiated by the annihilation of dark matter particles is work in progress. acknowledgement this research is supported by grants from the u.s. department of energy office of science, the u.s. national science foundation and the smithsonian institution, by nserc in canada, by science foundation ireland (sfi 10/rfp/ast2748) and by stfc in the u.k. we acknowledge the excellent work of the technical support staff at the fred lawrence whipple observatory and at the collaborating institutions in the construction and operation of the instrument. references [1] q.d. wang, f.j. lu, e.v. gotthelf, et al., mnras 367, 937 (2006). doi:10.1111/j.1365-2966.2006.09998.x [2] g. jungman, m. kamionkowski, & k. griest, phr 267, 195 (1996). 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[16] a. atoyan, & c.d. dermer, apj 617, l123 (2004). doi:10.1086/427390 [17] s. gillessen, r. genzel, t. fritz, et al., nature 481, 51 (2012). doi:10.1038/nature10652 230 http://dx.doi.org/10.1086/427390 http://dx.doi.org/10.1038/nature10652 introduction veritas observations of the galactic center summary and conclusion 146 acta polytechnica ctu proceedings 1(1): 146–150, 2014 146 doi: 10.14311/app.2014.01.0146 origin of x-ray spectral variation and the seemingly broad iron-line spectral feature in seyfert galaxies ken ebisawa1,2, naoki iso1,2, takehiro miyakawa3, hajime inoue1 1institute of space and astronautical science (isas), japan aerospace exploration agency (jaxa), 3-1-1 yoshinodai, chuo-ku, sagamihara, kanagawa 252-5210 2department of astronomy, graduate school of science, the university of tokyo, 7-3-1 hongo, bunkyo-ku, tokyo 113-0033 3tsukuba space center (tksc), japan aerospace exploration agency (jaxa), 2-1-1 sengen, tsukuba-shi, ibaraki 3058505, japan corresponding author: ebisawa@isas.jaxa.jp abstract we present systematic x-ray data analysis of the seyfert galaxies observed by suzaku to study origin of their hard x-ray (2 – 40 kev) variations. in particular, we examine if the “variable partial covering (vpc) model” proposed by miyakawa, ebisawa and inoue (2012), which was successful to explain spectral variations of mcg –6-30-15, is also valid for other seyfert galaxies or not. in this model, intrinsic x-ray luminosity of the agn is not significantly variable, and most observed flux and spectral variations are caused by change of the geometrical covering fraction of the extended x-ray source by ionized absorbing clouds in the line of sight. we found that the observed flux and spectral variations of 20 targets in addition to mcg–6-30-15 are successfully explained by the vpc model. the transmitted spectral component through the absorbing clouds has a characteristics spectral feature of the ionized iron k-edge, which is considered to be the origin of the seemingly broad iron-line feature commonly observed in seyfert galaxies. variation of the partial covering fraction of the constant x-ray luminosity source causes such an anti-correlation between the direct (non-obscured) component and the transmitted (obscured) component, that cancels their variations each other. the cancellation works most effectively at the energy band where intensities of the two components are the closest to each other, namely, just below the iron k-edge. this explains the significantly small fractional variations in the iron k-energy band, another well-known observational characteristic of seyfert galaxies. keywords: black holes active galactic nuclei accretion disk warm absorbers partial covering x-rays individual: mcg–6–30–15. 1 introduction recently, miyakawa, ebisawa and inoue (2012) (mei2012 hereafter) proposed a “variable partial covering (vpc) model ” to explain x-ray spectral variation of seyfert galaxies. in this model, the original xray luminosity of the agn is not significantly variable, and the apparent x-ray variation is primarily caused by variation of the geometrical covering fraction of the extended x-ray source by the intervening clouds having internal ionization structures. mei2012 applied the vpc model to suzaku observations of mcg-6-30-15, and successfully explained not only the small fractional variation in the iron energy band, but also the entire intensity and spectral variations in 1 – 40 kev (see also inoue, miyakawa and ebisawa 2011). it is interesting to examine if the vpc model is also valid for other seyfert galaxies, and intrinsic x-ray luminosities of the seyfert galaxies in general are significantly variable or not. in the present paper, we explore the suzaku archive to select seyfert galaxies which show similar x-ray spectral characteristics to mcg-6-30-15, and apply the vpc model to see if their x-ray intensity and spectral variations are explained by this model or not. 1.1 data selection we chose data from the suzaku public archive, as of 2011 october 5. our main purpose is to study x-ray intensity and spectral variations of the seyfert galaxies, which have similar spectral characteristics to mcg-630-15, in particular in the iron k-energy band. therefore, we selected only targets which are classified as seyfert galaxies, and known to show the seemingly broad iron k-line feature or a hint of that. 146 http://dx.doi.org/10.14311/app.2014.01.0146 origin of x-ray spectral variation and the seemingly broad iron-line spectral feature in seyfert galaxies to study spectral variations efficiently, we chose only the observations which satisfy the following conditions; (1) long enough total exposure time (> 60 ksec) for an observation, (2) bright enough that the total accumulate counts in an observation are more than 50,000 counts in 0.2 – 10 kev, and (3) the sources are variable more than 10% in 4 – 10 kev in an obseravtion. thus, we selected 27 observations for 25 targets (using 50 sequences in total). figure 1 shows two examples of the 0.2–12.0 kev light curves (ngc3516 and ngc4051) among the 27 observations. 2 data analysis and results 2.1 time-averaged spectra firstly, we analyze the time-averaged spectra of the 27 observations from the 25 sources. we apply the “3component model” proposed by mei2012. the model is represented as f = whwl (n1 +w2n2)p +rpn3 + ife, (1) where p is the intrinsic cut-off power-law spectrum, n1 and n2 are the direct power-law normalization and the absorbed power-law normalization, respectively. wh , wl, and w2 represents the transmissions due to highionized warm absorber, low-ionized warm absorber, and partial heavy absorber (nh > 1024 cm−2), respectively. each warm-absorber has two parameters, the hydrogen column-density, nh , and the ionization parameter, ξ, such that wh = exp(−σ(ξh)nh,h), wl = exp(−σ(ξl)nh,l) and w2 = exp(−σ(ξ2)nh,2), where σ(ξ) means the photo-absorption cross-section. r and n3 are the reflection albedo and reflection normalization by the neutral accretion disk, ife is a narrow iron kα emission line, respectively. the interstellar extinction is also included in the model fitting, but not explicitly shown in equation (1). for details of the spectral model, see mei2012. consequently, we found 22 time-averaged spectra out of 20 sources are successfully represented with the 3-component model. 2.2 intensity-sliced spectra next, we study spectral variations within the 22 observations (out of 20 sources), average spectra of which were successfully fitted by the 3-component model in the previous section. we examine if the ”variable partial covering (vpc) model” proposed by mei2012 may explain the observed spectral variations or not. to prepare, we briefly review the vpc model in the following: let’s define the “total normalization”, n , and the partial covering fraction, α, so that n = n1 +n2, (2) and n1 = (1 − α)n,n2 = αn. (3) detailed study of spectral variations of mcg-6-30-15 by mei2012 suggests that the low-ionized warm absorber, which is optically thin, is associated with the same heavy absorber causing the partial covering (see figure 12 in mei2012), so that wl ≈ 1 − σ(ξl)nh,l ≈ 1 − ασ(ξl)n (fixed) h,l ≈ (1 − α) + α exp(−σ(ξl)n (fixed) h,l ), (4) where n (fixed) h,l is the fixed amount of the column density of the low-ionized absorber associated with the absorbing cloud, presumably as an outer-envelope. consequently, the 3-component model (1) is rewritten as “double partial covering”, such that f = exp (−σ(ξh)nh,h)×( 1 − α+ α exp ( −σ(ξl)nfixed h,l )) × (1 − α+ α exp (−σ(ξ2)nh,2))pn +rpn3 + ife. (5) following mei2012, we create intensity-sliced spectra, and examine the spectral model (5). for each observation, we define four intensity ranges in the 0.2– 12.0 kev light curve so that the photon counts for each intensity range be approximately equal, and create the four intensity-sliced energy spectra. the red dotted horizontal lines in figure 1 indicate the intensity boundaries for each observation. we try to fit the four intensity-spectra in 2 – 40 kev simultaneously only making α variable. as a result, we successfully fitted the intensity-sliced spectra of the 20 observations of 18 sources only varying α. two examples are shown in figure 3. the rest two sources requires n to be slightly variable. 2.3 time-sliced spectra next, we see that time sequences of the energy spectra are also explained by the vpc model with timevarying partial covering fractions. in the xis lightcurves, we define boundaries of the time-sliced spectra every 2.3×104 sec, which corresponds to four suzaku orbital periods. the blue vertical dotted lines in figure 1 show the time-boundaries for each observation, and we made energy spectra from each time bin. numbers of the time-sliced spectra in a single observation are from 6 (4c 74.26) to 28 (ngc 4051). for each observation, a time-series of the spectra are fitted simultaneously with the vpc model in eq. (5). we first try to fit the spectra only allowing the partial covering fraction α to vary. if not successful (i.e., χ2 r > 1.2), the total normalization n is varied in addition. 147 ken ebisawa et al. ngc 3516 c ou nt r at e (s -1 ) time (104 s) 0 10 20 30 400. 5 1. 0 1. 5 2. 0 2. 5 ngc 4051 c ou nt r at e (s -1 ) time (105 s) 0 1 2 3 4 0 5 10 15 20 25 15 16 figure 1: xis light curves of ngc3516 and ngc4051 in 0.2–12.0 kev. the count rate is binned with 512 s. horizontal red-dotted lines show the count rate intervals with which the intensity-sliced spectra were made (section 2.2). vertical blue-dotted lines show the time intervals with which the time-sliced spectra were made (section 2.3). ngc 3516 in te ns ity ( s-1 k ev -1 ) 10 -1 10 -2 10 -3 slice 1 slice 2 slice 3 slice 4 energy (kev) χ 2 5 10 40 -3 0 3 ngc 4051 in te ns ity ( s-1 k ev -1 ) 1 10 -1 10 -2 10 -3 slice 1 slice 2 slice 3 slice 4 energy (kev) χ 2 5 10 40 -3 0 3 figure 2: model fitting results of the intensity-sliced xis and pin spectra for ngc3516 and ngc4051. for each observation, the four intensity-sliced spectra are made with approximately equal counts, and the four spectra are fitted simultaneously only varying the partial covering fraction, α. the data and the best-fit power-law model are in the upper panel, while the residuals are in the lower panel. as a result, 21 observations from 19 sources were explained only varying α. figure 3 shows variation of the observed counting rates in 0.2 – 12 kev and the partial covering fraction for two examples among the 21 observations. anti-correlation between the counting rate and the partial covering fraction is obvious, that means the apparent flux increase/decrease of these sources are caused by decrease/increase of the partial covering fraction of the constant luminosity x-ray sources. we notice variability in the optical and uv continuum and lines is often seen to be correlated with observed x-ray variability, while our model requires that the x-ray intrinsic luminosity hardly changes. this suggests that the osculation of the intrinsic x-ray source by the intervening absorbing clouds takes place just inbetween the central black hole and the optical/uv emitting regions, so that the optical/uv lights reflect the variable absorbed x-ray fluxes. 2.4 root mean square(rms) spectra finally, we calculate root mean square (rms) spectra for the 22 observations using the xis data in order to investigate time variation in the iron energy band. we use the same time-series of the spectra used in the previous section as well as the best-fit models. namely, rms spectra are calculated with a bin-width of 2.3×104 sec. we divided the time-sliced spectra into 15 energy band, and computed the rms spectra. we show two 148 origin of x-ray spectral variation and the seemingly broad iron-line spectral feature in seyfert galaxies ngc 3516 c ou nt r at e (s -1 ) time (104 s) c overing factor α 0 10 20 30 40 0. 5 1. 0 1. 5 2. 0 2. 5 0. 0 0.0 0.2 0.4 0.6 0.8 1.0 ngc 4051 c ou nt r at e (s -1 ) time (105 s) c overing factor α 0 1 2 3 4 0 5 10 15 15 16 0.0 0.2 0.4 0.6 0.8 1.0 figure 3: variations of the observed xis counting rates (0.2 – 12 kev; black, scale in left) and the partial covering fractions (red, scale in right) for ngc3516 and ngc4051. ngc 3516 v ar ia tio n a m pl itu de ( % ) energy (kev) 72 5 10 0 10 20 30 40 50 ngc 4051 v ar ia tio n a m pl itu de ( % ) energy (kev) 72 5 10 0 10 20 30 40 figure 4: rms (root-mean-square) spectra for ngc3516 and ngc4051. black points are calculated from the data, and red-histograms are calculated from the best-fit spectral models for the time-sequence spectra. examples of the 22 rms spectra in figure 4. the small variability of the iron line energy band is successfully explained by the vpc model. references [1] inoue, h., miyakawa, t. & ebisawa, k. 2011, pasj, 63, s669 [2] magdziarz, p., & zdziarski, a. a. 1995, mnras, 273, 837 [3] miller, l., turner, l. j., & reeves, j. n. 2010, mnras, 408, 1928 doi:10.1111/j.1365-2966.2010.17261.x [4] miyakawa, t., ebisawa, k. & inoue, h. 2012, pasj, 64, 140 [5] zoghbi, a. et al. 2010, mnras, 401, 2419 doi:10.1111/j.1365-2966.2009.15816.x discussion matteo guainazzi: (1) it has been recently proposed that the discovery of soft x-ray lags on time-scales consistent with reverberation from a few schwarzschild radii constitute a very strong evidence supporting relativistic disk reflection as a driver of xray variability. can your model reproduce them? (2) can your model put constrains on the location of the warm absorbing clouds? ken ebisawa: (1) we are aware of the recent reports that several seyfert galaxies indicate soft-lags, which some consider due to inner-disk reflection and relativistic reverberation. we have not explored yet how the observed soft-lags are explained in the scheme of our variable partial covering model. [note added in proof: 149 http://dx.doi.org/10.1111/j.1365-2966.2010.17261.x http://dx.doi.org/10.1111/j.1365-2966.2009.15816.x ken ebisawa et al. we noticed that the observed characteristic soft-lag is also consistent with reverberation caused by scattering x-rays passing through highly covering material, as pointed out by miller, turner and reeves (2010). this seems to be a likely mechanism, since our model intrinsically requires rather large covering fractions. however, we also notice that this interpretation does not explain the observed fe kα lags as pointed out by zoghbi et al. (2010).] (2) yes. from spectral variability, we estimate distance from the black hole and sizes of the warm absorbing clouds. please find miyakawa, ebisawa and inoue (2012) and inoue, miyakawa and ebisawa (2011). 150 introduction data selection data analysis and results time-averaged spectra intensity-sliced spectra time-sliced spectra root mean square(rms) spectra acta polytechnica ctu proceedings doi:10.14311/app.2019.25.0052 acta polytechnica ctu proceedings 25:52–57, 2019 © czech technical university in prague, 2019 available online at http://ojs.cvut.cz/ojs/index.php/app experimental investigation and simulation of 3d-printed lattice structures eva heiml∗, anna kalteis, zoltan major johannes kepler universität linz, institute of polymer product engineering, altenberger straße 69, 4040 linz, austria ∗ corresponding author: eva.heiml@jku.at abstract. lattice structures are currently of high interest, especially for lightweight design. they generally have better structural performance per weight than parts made of bulk material. with conventional manufacturing techniques they are difficult to produce, but with additive manufacturing (am) fabrication is feasible. to better understand their behaviour under various loading conditions two lattice structures in different configurations were observed. for each structure three different test specimens were designed and manufactured using selective laser sintering (sls). to investigate the mechanical performance under large deformations the specimens were made of a thermoplastic polyurethane (tpu), which shows a hyperelastic material behaviour. beside the experimental observations also finite element analyses (fea) were conducted to investigate the deformation behaviour in more detail. keywords: lattice structures, additive manufacturing, mechanical testing, finite element analysis. 1. introduction cellular structures are classified by their inner topology and are thus considered either as stochastic or as ordered. they can be further differentiated in open or closed cell types [1]. ordered materials generally have better mechanical properties, high surface area densities and lower pressure drops when compared to stochastic structures. the biggest disadvantage of stochastic cellular materials is the lack of design freedom. therefore, ordered structures, especially lattice structures, are of higher interest [2]. lattices have a trusslike structure with interconnected struts and nodes in a threedimensional space [3]. they consist of repeating unit cells, which have periodicity in three dimensions [1]. the differences in performance come from a different deformation behaviour. foams are goverend by cell wall bending, whereas lattice cells stretch and compress [2]. this stretch dominated behaviour means that the initial yield is followed by either plastic buckling or brittle collapse which leads to post yield softening and then at the densification strain the stress rises steeply [4]. ashby [4] has defined three design variables which describe the properties of cellular materials. the first variable is the material from which the structure is made. the second variable states that properties are different for varying cell topology and shape. and the third design variable is the relative density, which is the relation between the density of the cellular structure and that of the bulk material. the aim of this work presented here is to model and investigate the behaviour of two lattice structures, manufacturedwithsls, under tension and compression. the experimental data is compared to the results of the fea.a further objective is to investigate this behaviour when the configuration of the specimen is changed. 2. test specimen two lattice structures are chosen to be investigated more thoroughly. one is the octet structure, figure 1a, which consists of two regular tetrahedrons and one octahedron [5]. the second structure, figure 1b, consists of a hexagonal array which forms the horizontal planes that are connected by spatial struts [6]. the models are created in nx 12.0 (siemens plm software). to create the specimens the diameter of the trusses as well as the length of each unit cell are adapted. therefore, the overall dimension and the volume stay the same for each configuration. the overall dimensions were constrained by the building space of the sls-printer, which only allowed 60 mm in each direction and a minimum diameter of 1.2 mm. with three adjacent unit cells in each direction in space the basic lattice structure is created (octet 3 × 3 × 3 , hexagonal 3 × 3 × 3 ). to investigate the influence of increased number of unit cells another specimen with four cells is created (octet 4 × 4 × 4 , hexagonal 4 × 4 × 4 ). the third structure consists of five unit cells in two directions in space and three cells in the third one (octet 5 × 5 × 3 , hexagonal 5 × 5 × 3 ). to enable a uniform distribution of forces onto the specimen, 2.0 mm thick plates are added at the top and bottom of each structure. for the third specimen type these plates only cover an area of three times three unit cells, so that the forces apply on the same configuration as in the first specimen type. a summary of specimen data can be found in table 1. for each specimen information about the dimension of the unit cell, including the diameter of the trusses d, the overall dimension and the area where the load is applied is given. to make production easier and to reduce stress con52 http://dx.doi.org/10.14311/app.2019.25.0052 http://ojs.cvut.cz/ojs/index.php/app vol. 25/2019 investigation of 3d-printed lattice structures specimen unit cell dimension cube dimension load face area [mm] [mm] [ mm2] x y z d x y z octet 3 × 3 × 3 20 20 20 3 60 60 60 3600 octet 4 × 4 × 4 15 15 15 2.3 60 60 60 3600 octet 5 × 5 × 3 12 12 12 1.8 60 60 36 1296 hexagonal 3 × 3 × 3 11.5 11.5 19.91 3 34.5 34.5 59.75 1190.25 hexagonal 4 × 4 × 4 8.6 8.6 14.89 2.3 34.4 34.4 59.58 1183.36 hexagonal 5 × 5 × 3 6.9 6.9 11.95 1.8 34.5 34.5 35.83 428.49 table 1. specimen data (a) . octet structure (b) . hexagonal structure figure 1. lattice structures centrations radii are added at the strut connections. for the simulations they are removed, to enable a more regular mesh distribution. test runs showed, that the influence of the radii on the outcome of the simulation is neglectable. the lattice structures have very complex geometries, thus, the chosen manufacturing method is sls. this is a powder bed based process, where a laser beam selectively sinters material particles. this method is especially interesting for manufacturing lattice structures, because the unused powder can function as support material [7]. 3. mechanical testing 3.1. bulk material characterisation before the testing of the lattice structures started, a basic characterisation of the tpu was undertaken. for this, tension tests of iso 527-5b specimens at different test rates were carried out. the results for 0.1 mm · s−1 were then used to create the material model. results for the bulk material characterisation are depicted in figure 2. these results show the time dependence and the hyperelastic material behaviour. 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 strain (-) 0 2 4 6 8 s tr es s (m p a) 10 mm/s 1 mm/s 0.1 mm/s 0.001 mm/s figure 2. bulk material characterisation 3.2. lattice structure characterisation all compression and tension testing was carried out on a mts 852 test system at room temperature. the test speed of the machine was set to 0.1 mm · s−1. for compression tests the machine was set up in a way that the specimens rested on one compression plate while the other one applied the force from the top. for tension tests additional support structures, manufactured with fused deposition modelling (fdm) using acrylonitrile butadiene styrene (abs), were glued on the specimens. the clamps were then mounted on those support structures (figure 3). tension tests were conducted until either failure of the bonding layer or failure of the lattice occured. compression tests were stopped after reaching a certain strain. for strain evaluation digital image correlation (dic) was used. 53 e. heiml, a. kalteis, z. major acta polytechnica ctu proceedings (a) . compression test (b) . tension test figure 3. test setup 4. finite element analysis 4.1. material model generation the data from the bulk material tests at 0.1 mm · s−1 is used to generate a hyperelastic material model for the tpu, because the experiments were performed with this test rate. the true strain results together with the true stress are then used to create the material model in abaqus 6.14 (dassault systèmes). the ogden model (see [8]) at a strain energy potential of order 3 best describes the actual behaviour of the tpu and is therefore used in the simulation, see figure 4. 4.2. simulation methodology the fea is performed using abaqus as well. for compression tests rigid plates are added to the top and bottom of each structure. through these the boundary conditions are applied. a frictionless contact between the plates and the specimen is created. the deformation is the same as in the mechanical testing in order to compare the results of the simulations and experiments. for tension tests reference points above and below the support structures, connected via coupling, and kinematic constraints are used to apply the boundary conditions (figure 5). the support structures are included in the simulation, because they are made of abs, which has a low stiffness as figure 4. material model generation well. with linear tetrahedral elements a free mesh is created, on the bounding faces a mapped mesh is used. seed sizes are adapted for each structure, with at least four elements over the thickness. the strain-rate sensitivity of the bulk material is identified (see figure 2), this effect is neglected for material model used during fea. because of the time-independence of the material model it is possible to describe the test-rate of the experiments also with the explicit dynamic solver, which is used to improve computational efficiency, additionally mass scaling is utilised. since the generated material model is time independent the quasi-static tests can be represented with an explicit solver. to simulate the behaviour of the tpu the hyperelastic material model from figure 4 is implemented. both simulations are displacement controlled. 5. results 5.1. experiments the apparent tensile modulus and apparent compressive modulus is calculated with equation 1 in the standardised strain region (ε1 = 0.05%, ε2 = 0.25%). e = σ2 − σ1 ε2 − ε1 (1) the apparent tensile stress is evaluated at 20 % strain, since that is the highest strain where all structures can be compared. the apparent compressive plateau stress is evaluated for each structure as well (see figure 6). these two values are normalized with the load face area. a summary of the results can be found in table 2. the apparent tensile modulus increases for both structures when the number of unit cells is increased. the apparent compressive modulus shows the same behaviour for the octet structures. both the apparent tensile stress and the apparent compressive stress increase for a higher number of unit cells. the hexagonal 5 × 5 × 3 specimen shows the highest values, except for the apparent compressive modulus. 54 vol. 25/2019 investigation of 3d-printed lattice structures specimen apparent tensile modulus apparent tensile stress at 20 % strain apparent compressive modulus apparent compressive plateau stress [mpa] [mpa] [mpa] [mpa] octet 3 × 3 × 3 3.61 0.4614 1.31 0.35 octet 4 × 4 × 4 4.32 0.5042 0.54 0.43 octet 5 × 5 × 3 5.47 0.6640 0.11 0.47 hexagonal 3 × 3 × 3 6.74 0.7717 0.79 0.49 hexagonal 4 × 4 × 4 7.53 0.8755 1.81 0.65 hexagonal 5 × 5 × 3 15.51 1.5060 1.77 1.76 table 2. experimental results (a) . compression simulation (b) . tension simulation figure 5. simulation setup 5.2. simulation the experimental results are used to validate the simulations. the results of a range of selected structures are further discussed in this section. figure 7 displays the behaviour of the hexagonal 3 × 3 × 3 and hexagonal 4 × 4 × 4 specimens under tension. for small strains both configurations show a linear elastic behaviour. for a higher number of unit cells the slope of the curves is higher, both for experiment and simulation. especially for small displacements the results of experiment and simulation are in a good agreement. the problem of most tension tests was, that the 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 strain (-) 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 s tr es s (m p a) apparent plateau stress (a) . compression test 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 strain (-) 0.0 0.1 0.2 0.3 0.4 0.5 0.6 s tr es s (m p a) apparent tensile stress (b) . tension test figure 6. evaluation of test results bonding layer failed prior to the structure. only for the specimens with five unit cells in x and y direction failure of the material occured. figure 8 shows the results of the mechanical test and the simulation of the hexagonal structure shortly before the struts connecting the lattice structure with the support structure rip. until this point the experimental result and the result of the simulation are in a good agreement. depicted in figure 9 the struts start to rip at a displacement of around 7 mm and a force of 600 n. this 55 e. heiml, a. kalteis, z. major acta polytechnica ctu proceedings 0 5 10 15 20 25 30 35 displacement (mm) 0 500 1000 1500 2000 f or ce (n ) hexagonal 3x3x3 experiment hexagonal 3x3x3 simulation hexagonal 4x4x4 experiment hexagonal 4x4x4 simulation figure 7. tension tests of hexagonal 3 × 3 × 3 and hexagonal 4 × 4 × 4 (a) . finite element analysis (b) . mechanical test figure 8. tension test of hexagonal 5 × 5 × 3 is way less than what the other hexagonal structures endure, where the bonding layer failed at approximately 30 mm displacement and a force of 1400 n. the force acts on the same number of unit cells as on the hexagonal 3 × 3 × 3 specimen, but on a smaller area. for the same number of unit cells their size is an important factor for how much load they can bear. in figure 10 the results for compression tests of the octet 3 × 3 × 3 and the octet 4 × 4 × 4 specimens are depicted. for small displacements the results of the 0 2 4 6 8 10 12 displacement (mm) 0 200 400 600 800 1000 1200 f o rc e (n ) hexagonal 5x5x3 experiment hexagonal 5x5x3 simulation figure 9. tension test of hexagonal 5 × 5 × 3 mechanical tests show a linear behaviour and match with the simulation for both specimens. then again for a higher number of unit cells the force progression is higher. at a displacement of around 10mm a force plateau is reached and at a later point the densification starts, which again happens earlier for octet 4×4×4 . 0 10 20 30 40 50 displacement (mm) 0 5000 10000 15000 20000 25000 f or ce (n ) octet 3x3x3 experiment octet 3x3x3 simulation octet 4x4x4 experiment octet 4x4x4 simulation figure 10. compression tests of octet 3 × 3 × 3 and octet 4 × 4 × 4 6. conclusion this paper investigates the behaviour of two different lattice structures with three different configurations under tension and compression. as it is described by ashby [4] the geometry is a crucial factor for the behaviour of such structures. the specimens with three and four unit cells have the same volume for both structures respectively, thus the influence of the diameter on the apparent tensile modulus can be evaluated. as shown in figure 11, the modulus increases when the diameter decreases. it is assumed that stiffening is a result of internal constraint from adjacent unit cells, which increases 56 vol. 25/2019 investigation of 3d-printed lattice structures 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 diameter (mm) 2 3 4 5 6 7 8 9 a p p a re n t t en si le m o d u lu s (m p a) octet structure hexagonal structure figure 11. influence of the diameter on the apparent tensile modulus with the number of unit cells in the specimen. specimens with smaller and thus more unit cells show a stiffer response. for the validation of this assumption further configurations have to be investigated. in a first step only a simple time-independent material model is implemented, which will be extended to consider the time-dependence of the material. the model created to describe the lattice structures, when tension and compression is applied, is in a good agreement with the experimental data. since there is no failure criterion implemented, there are still some deviations, which can be resolved by expanding the model. with the method being validated by the experiments, it can be further developed for various loading conditions. acknowledgements the specimens for the work conducted were manufactured at bernstein innovation gmbh. the mechanical tests were performed in the ippe laboratory under the supervision of michael lackner. references [1] t. george. carbon fiber composite cellular structures, 2014. phd thesis, school of engineering and applied science, university of virginia. [2] d. rosen, s. r. johnston, m. reed. design of general lattice structures for lightweight and compliance applications 2019. [3] g. dong, y. tang, y. zhao. a survey of modeling of lattice structures fabricated by additive manufacturing. journal of mechanical design 139, 2017. doi:10.1115/1.4037305. [4] m. f. ahsby. cellular solids scaling of properties. in cellular ceramics: structure, manufacturing, properties and applications, chap. 1.1. wiley-vch verlag gmbh and co. kgaa, weinheim, 2005. doi:10.1002/3527606696.ch1a. [5] r. fuller. synergetic building construction, 1961. us patent 2,986,241. [6] j. aboudi, r. gilat. micromechanical analysis of lattice blocks. international journal of solids and structures 42:4372–4392, 2005. doi:10.1016/j.ijsolstr.2005.01.008. [7] w. tao, m. leu. design of lattice structure for additive manufacturing. pp. 325–332. 2016. doi:10.1109/isfa.2016.7790182. [8] r. ogden. large deformation isotropic elasticity: on the correlation of theory and experiments for incompressible rubber-like solids. proc roy soc lond ser a: math phys sci 326:556–585, 1992. doi:10.1098/rspa.1972.0096. 57 https://doi.org/10.1115/1.4037305 https://doi.org/10.1002/3527606696.ch1a https://doi.org/10.1016/j.ijsolstr.2005.01.008 https://doi.org/10.1109/isfa.2016.7790182 https://doi.org/10.1098/rspa.1972.0096 acta polytechnica ctu proceedings 25:52–57, 2019 1 introduction 2 test specimen 3 mechanical testing 3.1 bulk material characterisation 3.2 lattice structure characterisation 4 finite element analysis 4.1 material model generation 4.2 simulation methodology 5 results 5.1 experiments 5.2 simulation 6 conclusion acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2020.26.0107 acta polytechnica ctu proceedings 26:107–111, 2020 © czech technical university in prague, 2020 available online at https://ojs.cvut.cz/ojs/index.php/app mupif workflow editor and automatic code generator stanislav šulc∗, vít šmilauer, bořek patzák czech technical university in prague, faculty of civil engineering, department of mechanics, thákurova 7, 166 29 prague 6, czech republic ∗ corresponding author: stanislav.sulc@fsv.cvut.cz abstract. integrating applications or codes into mupif api model enables easy integration of such apis into any workflow representing complex multiphysical simulation. this concept of mupif also enables automatic code generation of the computational code for given workflow structure. this article describes a ’workflow generator’ tool for the code generation together with ’workflow editor’ graphical interface for interactive definition of the workflow structure and the inner data dependencies. the usage is explained on a thermo-mechanical simulation. keywords: mupif, workflow editor, code generator. 1. introduction complex physical tasks often need integration of very different models or solvers. since there exist models solving the particular tasks, it is a very effective approach to connect the existing software tools together to solve the complex tasks. a mupif platform [1] is a python [2] tool for such interconnection of several models together to compute coupled or linked multiphysical simulation. note that in our terminology a model can be some software, application or code. each model is covered in a python api derived from mupif class model, which has the following functions: __init__() # creates an instance of the class model. initialize() # initializes the model instance for a specific usecase. solvestep() # solves a computational step with defined length. terminate() # cleans all resources of the model instance. set() # sets inputs of the model, e.g. mupif field or property. get() # returns outputs of the model, e.g. mupif field or property. getcriticaltimestep() # returns maximum time step length these seven functions suffice for full control of a basic computation. the communication between models is realized with exchanging instances of mupif classes field or property. these instances can be obtained from a model by calling its get() method with appropriate parameters, and can be sent to a model by calling its set() method. both field and property instances keep their values, but they also have attributes describing their type, units and some additional info for unique identification. then, a property can be sent to a model and the model decides what to do with it. this concept creates environment for easy interconnection of even completely different codes, which can also be based on different programming languages. the value of a property is a tuple of any value types. a field has a tuple of nodal or cell values for each entity, according to the field type, while each value can be a tuple. a field is always linked with a mupif mesh, which gives the field space definition, interpolation and portability to tasks with different discretization. several models together create a workflow, which can be executed or added into another workflow, as it can behave like a model from outer scope, see fig. 1. the mupif class workflow is derived from class model and is extended with the following function: solve() # calls solvestep() until the target time is reached. which solves the whole simulation. a simplified python code of a simulation with two models can look like this: tm = thermal_model() mm = mechanical_model() tm.initialize(’input_t.in’, ...) mm.initialize(’input_m.in’, ...) while time<targettime: tm.solvestep(...) tf = tm.get(...) mm.set(tf) mm.solvestep(...) ... tm.terminate() mm.terminate() 2. mupif workflow generator the concept of mupif, especially the unified structure of the apis allows us to automate generation of the computational steering code when the simulation 107 https://doi.org/10.14311/app.2020.26.0107 https://ojs.cvut.cz/ojs/index.php/app s. šulc, v. šmilauer, b. patzák acta polytechnica ctu proceedings figure 1. example of a workflow structure. figure 2. workflow diagram of a thermo-mechanical simulation. workflow structure is defined. each api contains list of inputs and outputs which can be interconnected according to the data flow in the chain of models. this list also contains detailed description of each input or output. the mupif workflow generator project [3] provides python code environment to define the hierarchical structure of the simulation. it contains classes of blocks, where each block represents an item in the workflow diagram. there are blocks for following entities: mupif model, mupif property, mupif physical quantity, time loop and the whole workflow. there are also some other blocks and also a possibility of implementing a new block by user. workflow and time loop blocks can contain a sequence of another blocks. all blocks can have input and output data slots. in case of model block they represent the model inputs and outputs. in case of a property or field the block has one output slot representing its value. two data slots (output and input) can be connected, which represents the data exchange between models or setting of the input parameters. when all the blocks and the data dependencies are defined, the code can be generated. the mupif workflow code can be generated in two ways. the first option is that the workflow structure defines function solvestep(). in this case we call it a class workflow. the second option is that the workflow has a timeloop in itself and the structure defines the function solve(). in this case we call it an execution workflow. in case of a class workflow there is a possibility of setting some inputs or outputs, just like a model instance has some. it is realized using external data slots of the workflow. these slots are connected to some input or output slots inside the workflow, which transfers data from or to the outer scope, see this logic in fig. 1. this is further explained in section 4. the execution workflow cannot have such slots because the generated code behaves like a script to be run, not like a module to set some parameters and call its specific functions as in case of class workflow. 3. mupif workflow editor mupif workflow editor [4] is a graphical user interface of the workflow generator for easy definition of the simulation structure according to its moda [5] representation. each item in the workflow is represented as a block with list of input slots on the left and list of output slots on the right. these slots can be interactively linked to define the dependencies among 108 vol. 26/2020 mupif workflow editor and automatic code generator particular models, see fig. 4. the input slots can also be linked to some constant input data represented with blocks of mupif constant property, field or physical quantity. there is also a block to define a time-loop, a block for including custom code or ifelse block. when all the necessary dependencies are defined, the python code of the simulation can be generated and saved or executed instantly. the workflow editor provides saving of the workflow state into a json file to save the work and continue later. at the present time, it takes a part in the eu horizon 2020 project and thus it contains a list of all available involved models inside, but any model based on mupif model or workflow can be loaded into this tool from a python module. it automatically creates the structure and visual representation with the available data slots according to the inputs and outputs specification. 4. example of use the usage of the workflow editor and generator is described on a simple nonstationary thermo-mechanical simulation. this task is one of the mupif examples and can also be found in the workfloweditor github repository [4]. the geometry and boundary conditions are depicted in fig. 3. figure 3. schema of a thermo-mechanical task. the material parameters are: conductivity 1.0 w.m−1.k−1, capacity 1.0 j.kg−1.k−1, density 1.0 kg.m−2, young’s modulus e=30×109 pa, ν=0.25 and coefficient of thermal expansion 1.2×10−6 k−1. the mechanical response is caused only by the thermal expansion, there is no force load. the moda workflow representation of the task is depicted in fig. 2. 4.1. execution workflow we begin with the option of defining structure of the whole simulation in one execution workflow. the data flow from fig. 1 is realized in the workflow editor as shown in fig. 4. the thermal solver solves a nonstationary 2d thermal task on a rectangular domain. the boundary conditions on all edges can be set to the solver, see the thermal nonstat model inputs in fig. 4. the left and bottom edge temperature inputs are linked to the output of a block representing mupif constant temperature property of zero value. the top edge temperature is linked analogically to a block representing constant temperature of 10 °c. figure 4. execution workflow of the thermomechanical simulation. the output temperature field slot of the thermal task is linked to the input temperature field slot of the mechanical task, which yields in the following code: mm.set(tm.get(mupif.fid_temperature, ts.gettime())) 109 s. šulc, v. šmilauer, b. patzák acta polytechnica ctu proceedings where tm is the thermal model instance, mm is the mechanical model instance and ts is the computational time step. the geometry of both thermal and mechanical task is defined via input file for each task. the block blocktimeloop represents the simulation time loop and contains four model blocks. the last two blocks represent model named field export to vtk, which is a simple tool for saving given mupif field into a file. it must have been integrated into the mupif class model to enable such usage in the workflow editor. the mupif temperature and displacement fields are exported into files within each computational timestep. the first three blocks of the workflow represent mupif time physical quantity and define the run of the simulation time loop. 4.2. class + execution workflow we continue with the second option, which begins with definition of a class workflow with the thermal and mechanical models, see fig. 5. figure 5. class workflow of the thermo-mechanical simulation. the data slots are connected analogically, but the top edge temperature slot is linked to the external input data slot and thus this data requirement will be satisfied from outer scope. note that this input of the workflow is called ’external input data slot’ although inside the workflow its value is represented with an output data slot to be linked with another block’s input data slot. in case of external outputs it works analogically, see the two external output data slots, creating the outputs of this workflow. now we generate the class code and save it into a file. it contains a definition of a class derived from class workflow. then we create another workflow and insert a blocktimeloop into it, see fig. 6. figure 6. execution workflow of the thermomechanical simulation with thermo-mechanical workflow used as a model. this block contains the thermo-mechanical workflow as a model and illustrates the encapsulation of a complex simulation into a workflow which provides easier usage from outer scope. 4.3. task solution and results both approaches from section 4.1 and section 4.2 yield into an execution workflow for which we can generate the python code. it can be saved into a file or executed instantly from the workflow editor. the execution will produce the vtk [6] files, which can be easily visualized, see figs. 7 and 8. 110 vol. 26/2020 mupif workflow editor and automatic code generator figure 7. resulting temperature at time=10 s. figure 8. resulting displacement at time=10 s. 5. conclusions this article describes a graphical software tool for user-friendly definition of a multiphysical simulation structure and the following generating of a python code of the simulation. the usage of the workflow editor tool was explained on a thermo-mechanical task in two modes the first one is the whole task defined in one execution workflow. the second one is defining a class workflow to be imported into an execution workflow as a model. the example is available in the workflow editor github repository. in the future, a web-based version of the workflow editor will be created to provide easier access to this tool without any installation. acknowledgements we gratefully acknowledge financial support from eu horizon 2020 project, contract number: 721105. we also gratefully acknowledge the financial support from czech technical university in prague, grant number sgs19/032/ohk1/1t/11. references [1] mupif https://github.com/mupif/mupif, 2019. [2] python https://www.python.org/. [3] mupif workflow generator project repository. https://github.com/mupif/workflowgenerator, 2019. [4] mupif workflow editor project repository. https://github.com/mupif/workfloweditor, 2019. [5] the european materials modelling council moda, homepage. https://emmc.info/moda/. [6] vtk https://vtk.org/. 111 acta polytechnica ctu proceedings 26:107–111, 2020 1 introduction 2 mupif workflow generator 3 mupif workflow editor 4 example of use 4.1 execution workflow 4.2 class + execution workflow 4.3 task solution and results 5 conclusions acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2017.11.0049 acta polytechnica ctu proceedings 11:49–52, 2017 © czech technical university in prague, 2017 available online at http://ojs.cvut.cz/ojs/index.php/app factors influencing the fee for use of the railway infrastructure petr novák∗, václav cempírek jan perner transport faculty, studentská 95, pardubice, czech republic ∗ corresponding author: petr.novak2@student.upce.cz abstract. it is necessary to update the current system of levying charges for railway infrastructure to reflect the current market situation, to take into account the new transport strategy established by the european union, and to reflect legislative changes in relation to directive 2012/34/eu on creating a single european railway area. international trends aim to categorize tracks according to technical parameters and parameters based on shipment time, as well as many others. the existence of different opinions on the amount and way to charge fees for railway infrastructure have led the authors of this academic paper to create a plan for a system of charges to be derived from the current production factors as well as from a number of specific new factors used in countries neighboring the czech republic. keywords: prices, product factors, specific factors, transport routes, carriers. 1. introduction as defined in the white paper, "roadmap to a single european transport area – towards a competitive and resource efficient transport system," eu transport policy currently describes the point in time when the transport system’s users will cover all the costs they create. these costs include infrastructure costs (the development and maintenance of transport routes) as well as external ones – emphasized in the principle of "the user/polluter pays," which influences the environment and growing congestion. at the same time, the european commission (ec) has set requirements for lowering greenhouse gas emissions to around 20% of the 2008 level by 2030 and 60% of the 1990 level by 2050. these results should be attained by shifting 30% of road transport to railways by the year 2030 and up to 50% of such transport by 2050. the czech republic’s (cr) priority in transport policy, in accordance with directive 2012/34/eu of the european parliament and of the council of 21 november 2012 on creating a single european railway area, is implementing a competitive railway system and thereby equalizing conditions on the transport market. the extensive railway network in the cr is in direct contrast with the actuality of its fully operational lines, on which high-speed, passenger, and freight trains run and which are able to compete with road transport at roughly one fifth the rate. according to the pareto principle, 20% of the causes create 80% of the effects, which applies here in an exemplary way: the majority of railway system tracks have economic problems, because the revenues from transport do not cover the costs. 2. the current state of charging for railway infrastructure in the cr the amount to be charged is dependent on the distance traveled and the gross weight of the train, which the carrier reports to the infrastructure operator. these two parameters take into account the costs for transporting the train along the appropriate line (train-km – train kilometers) and determine the wear on the transport infrastructure caused by the train’s passage (gtk – gross ton-kilometer). charging for railway infrastructure is divided into two components. the first component used for charging is the actual use of the railway infrastructure; the second component is the fee for allocating capacity. 2.1. charging for railway infrastructure use in the cr the network statement and the ministry of finance’s current price bulletin provide annual updates on the prices for train passage on the nationwide and regional lines of the domestic railway infrastructure operated by the railway infrastructure administration (ria) in addition to the conditions for their use. these prices include not only the individual journey of a train along a track but also the organization of track-based modes of transportation, telecommunication connections between the railway operator’s employees and the train’s staff, publishing regulations, and transmitting information to carriers. the basic price is the price calculated according to a costing model using the unit prices set for passenger trains or freight trains. the basic price calculation has been set as the distance traveled by a given train along a category of track (le, lc, or lr) using data on the length and category of the sections (borders). 49 http://dx.doi.org/10.14311/app.2017.11.0049 http://ojs.cvut.cz/ojs/index.php/app petr novák, václav cempírek acta polytechnica ctu proceedings for calculating prices, the actual route covered by the train is decisive except in cases when the train is rerouted on account of a traffic disruption. the basic price is calculated according to equations 1, 2, and 3 below. cz = c1 + c2[czk] (1) where: • cz is the overall basic price for using railway infrastructure for the train’s journey [czk], • c1 is the price for using railway infrastructure for the train’s journey in the area of performance measured by the train kilometers traveled [czk], and • c2 is the price for using railway infrastructure for the train’s journey in the area of performance measured by the gross train kilometers traveled. c1 = s1e ∗ le + s1c ∗ lc + s1r ∗ lr[czk] (2) where: • s1 is the price for 1 km of a train journey (train-km) along e, c, or r category tracks [czk * km−1] and • le , lc , and lr are the distances traveled by a train along e, c, or r category tracks [km]. c2 = s2e ∗q∗le +s2c ∗q∗lc +s2r ∗q∗lr[czk] (3) where: • s2 is the price per 1000 gross ton kilometers transported along e, c, or r category tracks [czk*gtk−1 ], • q is one thousandth of the gross weight of the train in tons. the train’s weight is the sum of the weight of all the train cars including the weight of the passengers or freight [thousandths of gross tons], and • le , lc , and lr are the distances traveled by a train along e, c, or r category tracks [km]. the price c2 is calculated separately for each section of track traveled by the given train after a change in its weight (adding/subtracting train cars, a change in the type of train from a passenger train to a train with a trainset or the reverse). 2.2. charges for capacity allocation this fee is established in conjunction with: • the length of the time interval between when the request for railway infrastructure capacity allocation was submitted and the day that has been requested for its use, • the relationship between the submitted request for railway infrastructure capacity allocation and the deadline for composing the annual railway timetable (rt) or changes planned to the timetable, and • the difficulty of processing the request. included in the charge for allocating railway infrastructure capacity is thus compensation for the process of allocating line capacity, compensation for processing the timetable for the train assigned to the given carrier request (except for printing costs and distribution aids), compensation for the train’s operative deployment, and a fee for negotiating and executing the request on short notice. the price for allocating railway infrastructure capacity is calculated according to the equation: cp k = k1 + k2 ∗ l + k3 ∗ d[czk] (4) where: • cp k is the price for allocating transport infrastructure [czk], • k1 is the rate for processing and determining the rt and allocating transport infrastructure [czk], • k2 is the rate for the construction of the train route [czk*km−1], • l is the distance of the allocated route between the starting and ending point of the track on the railway network, where the ria fills the role of the line operator or allocator of capacity [km], • k3 is the daily rate for train route allocation [czk*day−1], and • d is the number of days of the journey for which the respective route has been allocated [days]. 3. international charges for railway infrastructure in all countries, the basic system for determining railway infrastructure prices is founded on a similar basis, i.e., according to train kilometers, gross ton kilometers, or using both these transport functions on a specific track. in specific countries, other parameters that modify the final price for using railway infrastructure have also been established. these often have decisive influence on the choice of transport, and it is in the interest of each infrastructure manager to be flexible when adjusting the charges for railway infrastructure use. using an extensive analysis of the fundamentals and procedures for calculating prices for using railway infrastructure employed by infrastructure managers who proceed in accordance with directive 2012/34/eu, the authors have formed the opinion that it is necessary to supplement the current method for calculating prices with a number of specific factors. 3.1. consolidating the prices for passenger and freight transport the motivation for consolidating the basic prices for passenger and freight transport is primarily to fairly reflect the costs included in the price for using railway infrastructure. the factor of passenger/freight 50 vol. 11/2017 factors influencing the fee transport only makes it possible to reflect transport’s negative effects in a general way (e.g., the degree of wear on the infrastructure, the use of transport line capacity, and limitations on the quality of services to other carriers). 3.2. combined transport the factor of combined transport should be maintained in the new price concept for railway infrastructure use. the reason for preserving it is that it is less costly for the infrastructure manager in comparison with other freight transport and does less to limit the quality of service for railway line use by other carriers if: • trains show a high level of transport regularity and speed and • there is relatively less wear on the infrastructure with respect to a smaller volume of train wagon repair. 3.3. shipping individual wagon loads the factor of individual, fully loaded wagons should be modified to more accurately reflect the areas where this type of transport is not sufficiently competitive from the ria’s perspective. the current method of conferring timetable advantage in a blanket manner should be developed so that support for this area focuses on the sections of the first and last miles. railway competitiveness is low for precisely these sections. 3.4. track categorization in the cr, railway tracks are divided into three categories: select nationwide tracks of european importance – e, other nationwide tracks – c, and regional tracks – r. this categorization has been enacted according to ria track routing, the tracks’ importance, and the scope of transport. in the authors’ opinion, this division does not take into account the actual quality of the tracks. the benefit of dividing the tracks into more categories is to take into account their actual quality from the perspective of maximum track speed. moreover, for tracks with low track speed, this would lower the price for using the railway infrastructure, which could lead to a higher level of transport performance. 3.5. infrastructure wear this specific key factor is used to allocate costs connected to infrastructure deterioration on individual trains in conjunction with european legislation. the reason for implementing this specific factor is to take into account the different levels of wear on railway infrastructure by individual trains; so far, this has been employed using the component of price derived from gross ton kilometers. a reflection of railway infrastructure deterioration should be maintained in the price for using railway infrastructure. 3.6. etcs equipping trains with the european train control system (etcs) is key for ensuring that european railways are interoperable. regulating this specific factor reflects the gradual growth in etcs use and the overall carrier costs for supplemental train equipment. regulating this specific factor within tariff policy should derive from the condition that it have a neutral effect on infrastructure managers. thus, the advantages for trains with etcs and disadvantages for trains without etcs should be balanced. 3.7. express trains priority is given to express trains in transit to the disadvantage of other types of trains. it is appropriate to reflect the difference in the quality of services for using the railway infrastructure for these trains by an additional fee on top of the basic price – primarily for passenger trains. the reason for implementing this factor is to take into account the higher quality of services for express trains using railway infrastructure. they have priority over other trains, and their journey is therefore faster and has less interruptions – without the necessity of stopping and avoiding other trains. 3.8. the traction of traction units this specific factor takes into account the cost to the public caused by transport. it is desirable to adequately regulate this factor in comparison with other modes of transportation and for it to simultaneously motivate carriers to lower the cost to the public. protecting the environment against the negative effects of transport is one of the ec’s key strategic goals. however, the authors believe that measures taken in this area for railway transport are not distinctly restrictive in nature – conversely, they tend to support their use. 3.9. reducing noise levels taking into account the noise levels of trains and other means of transport requires composing comprehensive methods to make it possible to quantify the public costs caused by individual modes of transport. for this reason, it is appropriate to implement the factor of "reducing noise levels" once there is a unified and time-tested european method that is able to take into account roadway noise levels. 3.10. the degree of track utilization the general differentiation of peak and off-peak periods makes it possible to optimize the use of railway infrastructure capacity. the factor of utilization is appropriate only for certain tracks, and it must always be established on an individual basis. support for optimizing railway infrastructure utilization aims to decrease the overloading of individual 51 petr novák, václav cempírek acta polytechnica ctu proceedings routes primarily during peak hours as well as to increase the use of tracks that currently have a distinct reserve capacity. international experience shows that it is not possible to influence certain effects of transport using universal tools; rather, it is necessary to react to them individually, e.g., by taking into account the operational peak and off-peak times for individual tracks. 3.11. a train’s minimum power density according to the authors, one specific factor that limits excessive consumption of railway infrastructure capacity is caused by insufficient traction unit performance and the consequently low speeds of certain trains. in turn, this lowers the risk of events occurring that block infrastructure capacity – caused by the gridlock of trains with insufficient traction units. 4. conclusion charging for roadway and railway infrastructure is a sensitive topic, because any type of change has effects on financing infrastructure. when looking for an optimal solution for charging fees, it should ensure the equalization of transport between branches and a competitive transport system; it is also necessary to take into account the policy of both the cr and the eu. analysis of the factors listed above will be the basis for establishing a new method for calculating railway infrastructure prices. for a comprehensive evaluation, it is necessary to ascertain the relevant information and the czech ministry of transport’s projected approach, which is tied to fulfilling the goals of the czech republic’s transportation policy and incorporating eu directives and regulations. in comparing the cr’s railway and roadway transport, where charges are levied for only 2.6% of the length of the roadway network and 98% of the railway network, it can be seen that there is obvious discrimination against railway transport in inter-branch competition. 52 acta polytechnica ctu proceedings 11:49–52, 2017 1 introduction 2 the current state of charging for railway infrastructure in the cr 2.1 charging for railway infrastructure use in the cr 2.2 charges for capacity allocation 3 international charges for railway infrastructure 3.1 consolidating the prices for passenger and freight transport 3.2 combined transport 3.3 shipping individual wagon loads 3.4 track categorization 3.5 infrastructure wear 3.6 etcs 3.7 express trains 3.8 the traction of traction units 3.9 reducing noise levels 3.10 the degree of track utilization 3.11 a train's minimum power density 4 conclusion acta polytechnica ctu proceedings doi:10.14311/app.2018.18.0015 acta polytechnica ctu proceedings 18:15–19, 2018 © czech technical university in prague, 2018 available online at http://ojs.cvut.cz/ojs/index.php/app multi-channel control system for in-situ laboratory loading devices václav radaa,b,∗, tomáš fílaa,b, petr zlámala,b, daniel kytýřa, petr koudelkab a czech academy of sciences, institute of theoretical and applied mechanics, prosecká 76, 190 00 prague 9, czech republic b czech technical university in prague, faculty of transportation sciences, konviktská 20, 110 00 prague 1, czech republic ∗ corresponding author: rada@itam.cas.cz abstract. in recent years, open-source applications have replaced proprietary software in many fields. especially open-source software tools based on linux operating system have wide range of utilization. in terms of cnc solutions, an open-source system linuxcnc can be used. however, the linuxcnc control software and the graphical user interface (gui) could be developed only on top of hardware abstraction layer. nevertheless, the linuxcnc community provided python interface, which allows for controlling cnc machine using python programming language, therefore whole control software can be developed in python. the paper focuses on a development of a multi-process control software mainly for in-house developed loading devices operated at our institute. the software tool is based on the linuxcnc python interface and qt framework, which gives the software an ability to be modular and effectively adapted for various devices. keywords: cnc, controlling, linuxcnc, python interface, python, parallel programming, qt, pyqt, qwt, pythonqwt, labjack. 1. introduction computer numerical control (cnc) is used for automation of a broad range of machines e. g. milling machines, lathes, 3d printers etc. cnc allows for very effective and precise manufacturing of parts with complex shape and has many other advantageous applications in various industrial fields [1]. there are many cnc software solutions, which vary in price, performance or closed-source commercial (siemens, fanuc, labview) and open-source solutions (linuxcnc, arduino primarily for hobby operation). our research group tends to use open-source solutions, because our needs are different from conventional industrial cnc applications [2]. therefore, some properties of the software has to be operationally modified according to our requirements. for this purpose, closed-source commercial cnc software is not suitable for use with our devices. in our applications, several types of actuators are used including stepper, servo-motors and linear voice coil actuator. purpose of the designs can be divided into three groups: • mechanical loading machines (e. g. in-situ loading devices for x-ray computed tomography) • positioning machines (e. g. optics and sample positioning) • sample preparation devices (e. g. automatic grinders) each machine type is equipped with common parts such as actuator, encoder, limit switches and with application specific equipment such as load cell, thermometer etc. the software package introduced in this paper allows for controlling of both the common and application specific equipment and user the interface can be tailored for the specific application owing to linuxcnc [3] python interface [4]. 2. approach in python, parallel programming [5] is a complex task, particularly due to python global interpreter lock (gil). gil prevents multiple threads to access interpreter internals at the same time by serializing the requests. python threads share all types of variables natively which makes them easy to use, but these threads cannot effectively bring any performance increase by utilizing multiple cpu cores. in order to force python interpreter to perform multiple tasks in parallel and utilize multiple cpu cores, separated python processes must be used, which allows for executing tasks of each process simultaneously. to demonstrate python multithreading and multiprocessing performance, a computational test of prime factorization was performed on 10 million integers. the testing scripts [6] was run on a 4-core, 8-thread (intel core i7-4790k @4.4 ghz) machine. the computation time results are shown in figure 1. the results confirm that python multiprocessing threads do not bring any performance increase. in fact, multiprocessing threads decrease the script performance substantially due to overhead caused by thread switches made by 15 http://dx.doi.org/10.14311/app.2018.18.0015 http://ojs.cvut.cz/ojs/index.php/app v. rada, t. fíla, p. zlámal et al. acta polytechnica ctu proceedings figure 1. comparison of multithreading and multiprocessing performance figure 2. inter-process communication scheme python interpreter. on the other hand, multiprocessing processes do increase the performance because of parallelism. however, sharing variables between python processes is in most cases very ineffective. mostly, it is more effective to implement some kind of inter-process communication. python supports multiprocessing library [7] which provides communication between processes using pipes or queues. python multiprocessing pipes are the simplest approach of multiprocessing communication in python. the pipes connect only two processes. one end of the pipe is used to send data by one process, the other end is used to receive the data by the other process. in order to provide connection between more than two processes, multiprocessing queues can be used, but multiprocessing pipes give more performance due to their simplicity. the control software consists of a multi-process core and graphical user interface (gui) process connected by multiprocessing pipes. the core includes four processes, stat poller, data logger, data keeper and command executor. its interconnection is shown in figure 2. the crucial process in the control software core is the stat poller. it is based on linuxcnc.stat() of linuxcnc python interface and extended by methods providing communication with sensors, output log file management etc. stat poller periodically updates machine status variables calling poll() method and obtains values from sensors. obtained data is sent to data logger, data keeper and to gui. data logger receives data from stat poller and saves it periodically to output file. data keeper keeps received data from stat poller in an array and sends the data to gui on request to show the data in a graph. the control software core contains also command executor process which is based on linuxcnc.command() of linuxcnc python interface. it provides execution of dynamically generated python string commands received from gui or stat poller using exec statement. 3. user interface the main objective of the gui is to not affect the control software core performance, so the user interface runs in a process separate from the control software core and is connected with it using multiprocessing pipes. all the pipes leading from control software core are connected to gui core, which handles communication between control software core and gui process itself. the user interface is aimed to be modular and straightforwardly adaptable for various experimental devices. each experimental device uses its specific linuxcnc initializing file, which contains data needed for control software startup. to take into consideration differences of particular devices, the initialization files were extended by specific entries. these entries define active axes of a device, axes equipped with a sensor (measurement axes), axes units etc. on control software startup, initializing file is loaded and user interface is dynamically generated considering specific entries in initializing file and individual experimental device needs. the user interface of the control software is developed using qt framework in cooperation with python binding pyqt [8]. the user interface is designed as a set of plugins (qwidgets), which contributes to straightforward adaptation to new experimental devices and helps to keep the user interface source code well structured. currently, various gui plugins were created to cover specific needs of laboratory devices. emergency stop button, power button etc. are elementary plugins and are common to all devices. other plugins allow for controlling by executing manual data input (mdi) commands based on g-code (rs-274), manual control of positioning axes of the device or sensors management. for proper sensors functionality initializing files of all supported sensors were created. on control software startup, these initializing files are loaded into software internal database. the files consist of numerical values important for appropriate operation of the sensor specified by calibration protocol or user requirement (measurement range, safe overload, amplification, sensitivity). after the system startup user specifies appropriate sensors which are mounted in particular experimental device using plugin for sensors choice. after the sensors are chosen, gui sends a command to control software core to start obtaining data from sensors periodically (e. g. force, temperature etc.). the control software core sends these data to gui and the data is shown in e. g. load cell plugin. all the data is periodically sent to data logger and saved to output 16 vol. 18/2018 multi-channel control system for in-situ laboratory loading devices figure 3. overall view of the control software user interface file. thus a plugin managing data logging was created. it enables the user to export output file either to a static config-given folder or using a save dialog interface. the plugin also enables creating new empty output file leaving the old data in temporary folder to prevent accidental data loss. the control software purpose is to reliably perform mechanical experiments. there was a special plugin created for controlling the displacement-driven and force-driven experiments. the control software provides a plugin allowing for plotting the data during experiment in a live graph. the live graph window is based on qwt (qt widgets for technical applications) library written in c++ and python binding for this library named pythonqwt . the qwt library was chosen for its performance to achieve higher live graph fps. the plugin enables to specify the amount of samples to plot. the refresh rate of the live graph decreases with increasing amount of samples. the optimal refresh rate is calculated automatically based on the number of samples using linear approximation. overall view of the control software user interface is shown in figure 3. 4. applications the modular design of the control software gives it an ability to control various laboratory devices separatedly or combine them in laboratory setups and control multiple devices as a single complex closed-loop controlled system [9]. moreover it is possible to integrate external procedures (e.g. auto-calibration [10]). functionality of the system is hereinafter demonstrated on the setup consisting of uni-axial loading device in arrangement for optical strain measurement where the camera is placed on triaxial table. the experimental procedure is in detail described in [11]. this is achieved by creating linuxcnc custom initializing file for each device and extending it with [rapo] section. the section name comes from the control software name raposoft (rada positioning software). the section contains specific additional parameters. for instance, the additional parameters of a triaxial motorized table are as follows: [rapo] axes\_active␣=␣x␣y␣z axes\_units␣=␣mm␣mm␣mm axes\_measurement␣= lj\_ains\_active␣= lj\_ains\_gain␣= lj\_ains\_invert␣= the motorized table is equipped with 3 active axes (x, y, z), all of the axes units are millimeters and it has no measurement (loading) axis, i. e. all of the axes are used only for positioning. devices with measurement (loading) axes included mostly use various sensors (typically load cells). parameters needed for sensors operation come with "lj" prefix, which refers to the labjack measurement cards (t7-pro, labjack corporation, usa) , used at our institute for establishing connection with the sensors using labjack.ljm library for python. usage of these parameters is demonstrated on uniaxial loading device. 17 v. rada, t. fíla, p. zlámal et al. acta polytechnica ctu proceedings [rapo] axes\_active␣=␣x axes\_units␣=␣um axes\_measurement␣=␣x lj\_ains\_active␣=␣0 lj\_ains\_gain␣=␣51 lj\_ains\_invert␣=␣1 the uniaxial loading device is equipped with a single active axis, the units are micrometers and the active axis is also measurement (loading) axis. the loading axis is equipped with a sensor (load cell) connected to labjack analog input no. 0. the labjack analog input signal can be amplified using ljtick-inamp amplifier and the amplification coefficient of the analog input is given by the gain parameter. the analog input signal can be inverted to set the appropriate load cell polarity while performing tension and compression tests. the motorized table and the uniaxial loading device can be used together as well. the parameters in initializing file look as follows: [rapo] axes\_active␣=␣x␣y␣z␣a axes\_units␣=␣mm␣mm␣mm␣um axes\_measurement␣=␣a lj\_ains\_active␣=␣0 lj\_ains\_gain␣=␣51 lj\_ains\_invert␣=␣1 the laboratory setup consists of 4 active axes in total, three of them are positioning axes of the table with millimeter units, one of them is measurement (loading) axis with micrometer units. parameters for the labjack analog inputs configuration are the same as for the single uniaxial loading device usage. the control software allows for controlling devices equipped with multiple sensors. this can be demonstrated on initializing file of a loading device developed at our institute used for four-point bending tests. the initializing file looks as follows: [rapo] axes\_active␣=␣x␣y axes\_units␣=␣um␣um axes\_measurement␣=␣x␣y lj\_ains\_active␣=␣0␣1 lj\_ains\_gain␣=␣51␣51 lj\_ains\_invert␣=␣1␣1 the device consists of 2 active axes, both of them are loading (measurement) axes with micrometer units. the device is equipped with 2 sensors (load cells) connected to labjack analog inputs no. 0 and no. 1. both of the analog inputs are amplified using coefficient 51 and their signal is inverted. 5. conclusions the presented control software allows for controlling laboratory devices developed at our institute, which are used for mechanical testing in different loading scenarios including time-dependent processes under controlled ambient conditions. it allows to control laboratory devices actuated by stepper, servo-motors, linear voice coil actuator etc. it provides support for various sensors operating on analog signal proportional to the applied excitation voltage (mv/v) e.g. loadcells or thermometers. moreover it is possible to integrate external closed-loop driven procedures. the modular design enables very effective adaptability and extendability which gives the control software capability to be used in a long-term way. acknowledgements the research has been supported by the european regional development fund in frame of the project kompetenzzentrum mechanobiologie (atcz133) in the interreg v-a austria czech republic programme and by operational programme research, development and education in project engineering applications of microworld physics (cz.02.1.01/0.0/0.0/16_019/0000766). references [1] p. smid. cnc programming handbook, third edition. industrial press, inc., 2007. [2] s. anderberg, t. beno, l. pejryd. energy and cost efficiency in cnc machining from a process planning perspective. in s. g. (ed.), sustainable manufacturing, pp. 393–398. springer, 2012. [3] the linuxcnc team. linuxcnc getting started guide. samurai media ltd., 2016. [4] linuxcnc python interface documantation. http://linuxcnc.org/docs/2.6/html/common/ python-interface.html. accessed: 2018-06-15. [5] g. zaccone. python parallel programming cookbook. packt publishing ltd., 2015. [6] e. bendersky. python parallelizing cpu-bound tasks with multiprocessing. https://eli.thegreenplace.net/2012/01/16/ python-parallelizing-cpu-bound-tasks-with-multiprocessing. accessed: 2018-07-03. [7] b. jones, d. beazley. python cookbook recipes for mastering python 3. o’reilly media, 2013. [8] m. summerfield. rapid gui programming with python and qt: the definitive guide to pyqt programming. prentice hall press, 1st edn., 2007. [9] d. kytyr, p. zlamal, p. koudelka, et al. deformation analysis of gellan-gum based bone scaffold using on-the-fly tomography. materials and design 134:400–417, 2017. doi:10.1016/j.matdes.2017.08.036. [10] m. vopalensky, d. vavrik, i. kumpova. optimization of the x-ray tube voltage with respect to the dynamical resolution in radiography and tomography. ndtnet journal 2018-02, 2018. 18 http://linuxcnc.org/docs/2.6/html/common/python-interface.html http://linuxcnc.org/docs/2.6/html/common/python-interface.html https://eli.thegreenplace.net/2012/01/16/python-parallelizing-cpu-bound-tasks-with-multiprocessing https://eli.thegreenplace.net/2012/01/16/python-parallelizing-cpu-bound-tasks-with-multiprocessing http://dx.doi.org/10.1016/j.matdes.2017.08.036 vol. 18/2018 multi-channel control system for in-situ laboratory loading devices [11] t. doktor, i. kumpova, m. sniechowski, et al. influence of printing and loading direction on mechanical response in 3d printed models of human trabecular bone. acta polytechnica ctu proceedings 18(10):24–27 2018. doi:10.14311/app.2018.18.0024. 19 http://dx.doi.org/10.14311/app.2018.18.0024 acta polytechnica ctu proceedings 18:15–19, 2018 1 introduction 2 approach 3 user interface 4 applications 5 conclusions acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2018.20.0005 acta polytechnica ctu proceedings 20:5–9, 2018 © czech technical university in prague, 2018 available online at http://ojs.cvut.cz/ojs/index.php/app btt & rflb the optimum set for steam turbine blades monitoring miroslav balda research and testing institute, tylova 46, 30100 plzeň, czech republic correspondence: balda@vzuplzen.cz abstract. btt, blade tip timing system, is a commercially available system generating files of precise times of blade tips when passing sensors attached in a machine stator. rflb, residual fatigue life of blades, is a postprocessor of those files evaluating estimates of fatigue lives of all blades fitted to the wheel. the set reduces a danger of unexpected blade failures. keywords: btt, rflb, fatigue, blades, maintenance. 1. introduction fatigue of materials is a very complicated phenomenon, which influences the quality of structures seriously. the bigger the structure is, the more important are the consequences of possible fatigue damages. the typical structures, in which any fatigue cracks should not occur, are turbomachines. any unexpected breakdown caused by the fatigue generates extreme losses of energy production. this is the reason why manufacturers of turbomachinery devote enormous care to a design of fatigue-save parts of a machine and to material testing. to common tests belong also fatigue tests of most exposed parts of the machine, blades of a turbine. the blades are working at rather complicated operational conditions like high radial stress from centrifugal forces and extreme velocity of wet steam turbulent flow exciting them to vibrations accompanied by alternating stresses. dynamic stresses at critical points of blades may initiate material damage, which results in a crack propagation when cumulated. since this phenomenon may end in a total breakdown of a blade followed by an unplanned shutdown of the machine, producers of turbomachines arm their products with systems, which are able to monitor blades at their critical places. 2. blade tip timing – btt the current monitoring systems used for that purpose are denoted as btt – blade tip timing systems because they are based on time measurement of blade tips when passing sensors (see fig. 1). a btt system consists of a set of sensors placed at the stator of the turbine above tips of blades of the selected wheel. outputs of ns sensors are connected to control inputs of their own counters of precise clock pulses. as soon as a tip of the vibrating blade passes a sensor, the sensor generates a pulse which initiates a transfer of the current content of the counter to its memory. there is still one sensor and the counter and its memory which counts pulses generated by a datum fixed at a shaft. at the end of the measurement, the contents of all memories are sent to ns + 1 files. the measured data, series of times, are processed offline into natural frequencies of blades and histograms of position magnitudes and other parameters of the measurement. 3. residual fatigue life of blades – rflb an application of a system rflb has in principle three main stages: • measurement of material properties, • modal analysis of the blade to be measured, • long time monitoring in situ in a power plant. all stages are very important for the final statement about the residual fatigue life of all measured blades. 3.1. material properties there are two material tests necessary for rflb system, the tensile test, and the high cycle fatigue test. the result of the tensile test is shown in the form of the diagram made by a testing device in fig.2. it may be accepted as a limiting case of the fatigue test during which the specimen has been broken at the half of the loading cycle. the high cycle fatigue (hcf) test is a different one. usually, a set of 8 10 specimens are loaded by harmonic processes of different amplitudes σam up to breaks after nam loading cycles [1]. the result of the test is a regression line to couples (nam, σam), called sn curve [2], which becomes a straight line when drawn in a diagram with axes lognam, log σam (see fig. 3). 3.2. modal analysis good information on the blade as an object of monitoring is very important. the detailed numerical analysis of the blade fitted in the wheel is made by a fem program. results of the analysis should contain the solution of the eigenvalue problem, e.g. natural 5 http://dx.doi.org/10.14311/app.2018.20.0005 http://ojs.cvut.cz/ojs/index.php/app miroslav balda acta polytechnica ctu proceedings figure 1. btt system with ns probes (sensors). frequencies and corresponding natural modes for a set of nodal diameter numbers. moreover, it is also important to have frequency response functions of circumferential deflection of a blade tip to stress tensors at critical places of the blade. 3.3. monitoring by the rflb the software system rflb complements the system btt by estimating fatigue lives of blades. it exploits the information contained in the btt output files for the purpose totally. such a complicated system, as the rflb is, should be split into modules fulfilling special tasks. the interactive matlab program rflb is built as a flexible one as possible with respect to events occurring in power plants. the system contains about 30 scripts and functions. the control module – rflb organizes, in a cooperation with the operator, the function of the system rflb. at first, the operator informs the module where to find the parameters of the blade, material, and results from the modal analysis. the operator may also interrupt the normal automatic run of the rflb when needed. there are many reasons for doing so. a typical situation occurs when the operator wants to look into the protocol or wants to plot the result of some past measurement. then, the program breaks the normal sequence of commands, fulfills the requirement of the operator, and continues just in the place, where has been interrupted. this behavior ensures the control module of the program. the module rflb initiates the run of the rflb with all new blades. it also enables to change some blades without a loss of continuity in the cumulation of damages of other blades. the same holds after a break in processing. the module btt is a driver for a continuous run of the system. it distributes tasks to specialized functions. the first of them is function getdata which reads the files generated by systems btt. there are served files from two btt systems, starman and hood. the function getdata is the only one which should be rebuilt if another btt system was applied for measurements. the reason for it is that the output files of systems btt are not standardized. the main role of function getdata is creating a vector composed of key-phasor times of once per revolution pulses from the red probe in figure 1 and a hyper-matrix (3d array) of times from all sensors and blade tips of the wheel. one vector of the 3d array is seen from figure 4. since the measured data are in the standard form, so the rest of the rflb system does not need any change. at the end of the function getdata, all-time series are fourier transformed into aliased fourier spectra by fast nieslony’s function rainflow [3]. the resulting fourier spectra are aliased in frequency due to the deep under-sampling of blade tip signals. frequency aliasing has to be eliminated by a spectrum reconstruction. this is implemented by function reconstr. it is well known that under-sampled signals defy reconstruction by normal way. fortunately, there is additional information at disposal. that is a set of natural frequencies obtained during the numerical 6 vol. 20/2018 btt & rflb the optimum set for steam turbine blades monitoring figure 2. material t671 tensile test. 0.5 1 10 100 1000 10000 100000 1e+06 1e+07 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500 1600 1700 number of cycles n a s tr es s am pl itu de σ a material t671 σ m =0 − woehler σ m 1 = 450 mpa σ m 2 = 600 mpa r m r 02 σ’ a0 σ’ a450 σ’ a600 σ c figure 3. t671 sn curves, σm = 0, 450, and 600 mpa. figure 4. vector of measured and reconstructed samples of one blade tip deflections. 7 miroslav balda acta polytechnica ctu proceedings figure 5. aliased dft of all blades in the wheel. modal analysis. the natural frequencies recalculated to the aliased ones serve for identification of evaluated peaks in dft. the frequency bands around dft peaks coinciding with aliased natural frequencies are moved on places corresponding real natural frequencies of the reconstructed spectra. in figure 5, there are positions of aliased natural frequencies shown in upper parts of the diagram by red ticks and order numbers. the upper frequency of the reconstructed fourier spectra is chosen by an order higher than the speed frequency, which is the sampling frequency fs, say, fs = 25 fs. the rest of the aliased spectra is spread within the reconstructed spectra under user wishes. the last stage of the reconstruction is the inverse fourier transform of the reconstructed spectra, what returns reconstructed signals in much higher resolution as seen in figure 4. if the moving of the spectrum bands was done with care, the reconstruction is successful, what is seen from figure 4. as soon as the reconstructed signals are ready, relative damages in critical places of the blades can be estimated by function damage. unfortunately, it is the most time-consuming operation, because of each blade signal has to undergo rain-flow analysis, what is a decomposition of each signal into full cycles, followed by a recalculation of time data into circumferential deviations and then to stress state in critical places on blades. for the purpose, the elements of the stress tensors obtained during modal analysis, are used for evaluating an effective (damaging) stress σd of kth loading cycle in the following form σ̃d,k,n = ∆tk × [ σ̃2 x + σ̃2 y + σ̃2 z (1) − 2µ (σ̃xσ̃y + σ̃yσ̃z + σ̃zσ̃x) + k2 c (τ̃2 xy + τ̃2 yz + τ̃2 zx) ]1/2 n , figure 6. relative damages of all blades. where kc =σc/τc ≈ √ 2(1 + µ) and symbols with tilde are dynamic components of a stress tensor in the critical point corresponding to nth mode of vibration. time-series of damaging stress course are decomposed into a set of full cycles and half cycles by a rainflow procedure getting thus stress amplitudes σak and mean stresses σmk needed for an estimation of elementary damage dk in kth cycle by a fatigue hypothesis. the most common one is the hypothesis of pålmgrenminer [4, 5] respecting an influence of σm [6]: dk,pm = 1/nam = 2 ( sa s′ a0 − sm )w , (2) where fictive amplitude s′ am replaces either σ′ am or 8 vol. 20/2018 btt & rflb the optimum set for steam turbine blades monitoring τ ′ am. it is evident from figure 3 that its values are given by the positions of intersections of a particular sn curve with the line nam = 0.5. for all family of sn curves holds that s′ am = s′ a0 − sm. this property of sn curves of steels saves time and money because it canceled the necessity of making expensive tests of prestressed materials. the system rflb cumulates elementary relative damages defined by equation (2), so it is able to yield information on the total relative damage d ≤ 1 of any blade of the wheel to the operator at any moment. in case, the damage d reached a chosen amount, the system issues an alarm message. residual fatigue life, defined as l = 1 −d, says how much time still remains to break down. 4. conclusions the article has shown the advantages gained by a combination of a hardware system btt with the software system rflb. installation of the set of both items in power plants for monitoring blades in potentially uncertain turbine stages may be very effective for the reliability of a machine because it diminishes a danger of unexpected breakdowns and enables to plan times of maintenance and a number of spare blades in store. list of symbols dam relative damage of a cycle with σa and σm d total relative damage fn natural frequency [hz] fs sampling frequency [hz] fs sampling frequency of reconstructed spectra [hz] fn nyquist frequency = fs/2 [hz] l relative residual fatigue life nam number of cycles to failure under σa and σm rm material strength [mpa] rp yield limit [mpa] s either σ or τ [mpa] t time [s] ts sampling period = 1/fs [s] w exponent of woehler’s curve µ poisson’s ratio σ stress in tension [mpa] σa amplitude of a stress cycle [mpa] σm mean stress of a stress cycle [mpa] σam amplitude of a cycle by mean stress [mpa] σ′ am intersection point of sn curve with n = 0.5 [mpa] τ stress in torsion [mpa] acknowledgements the author appreciates a kind support of his research from the technological agency of the czech republic via the center of competence no. te01020068, center of research and experimental development of reliable energy production references [1] j. chvojan: a test of material t671 high cycle fatigue (in czech), vzú plzeň, testing record no. 846/2003 [2] čsn 42 0368: fatigue testing of metals. statistical evaluation of fatigue test results of metals (in czech), czech normalizing institute, prague, 1973. [3] a. nieslony: rainflow counting algorithm. mathworks, file exchange, file no. 3026, 2003 [4] a. pålmgren: die lebensdauer von kugellagern, vdi zeitschrift (68), pp. 339-341, 1924. [5] m.a. miner: cumulative damage in fatigue, trans. asme, journal of applied mechanics, (12) pp. a159-a164, 1945. [6] m. balda: fatigue life assessment of prestressed material, engineering mechanics, c. fischer & j. náprstek eds., itam cas cz, proc. conf. engineering mechanics, svratka, may 14-17, 2018 9 acta polytechnica ctu proceedings 20:5–9, 2019 1 introduction 2 blade tip timing – btt 3 residual fatigue life of blades – rflb 3.1 material properties 3.2 modal analysis 3.3 monitoring by the rflb 4 conclusions list of symbols acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2018.18.0028 acta polytechnica ctu proceedings 18:28–31, 2018 © czech technical university in prague, 2018 available online at http://ojs.cvut.cz/ojs/index.php/app basic biomechanical characterization of polyurethane based artificial cancellous structures jan šleichrta,b,∗, daniel kytýřa, kateřina pithartováa,b, sascha senckc, david fürstd, andreas schrempfd a czech academy of sciences, institute of theoretical and applied mechanics prosecká 809/76, 190 00 prague 9, czech republic b czech technical university in prague, faculty of transportation sciences, konviktská 20, 110 00 prague 1, czech republic c university of applied sciences upper austria in wels, computed tomography research group, stelzhamerstrasse 23, 4600 wels, austria d university of applied sciences upper austria in linz, research group for surgical simulators linz, garnisonstrasse 21, 4020 linz, austria ∗ corresponding author: sleichrt@itam.cas.cz abstract. the main goal of this study is to validate elementary mechanical parameters of a newly designed open-cell foam. the purpouse for investigating artificial material is to approach the properties of the human bone in the case of its adequate replacement. investigated material can be also used as an artificial bone to train surgical procedures and to improve the skills of the surgeons. four sets of the foam with different chemical composition were subjected to an uniaxial quasi-static loading to describe basic mechanical behaviour of these samples. based on these experiments, the stress-strain diagrams were created as a comparative tool including calculation of the effective young’s modulus. the acquired knowledges will be used as input parameters of a follow-up study aimed at describing the morphology of presented structures and their response to mechanical experiments. a distortion effect of porosity on the results is not considered in this study. keywords: polyurethane foam, artificial bone, uniaxial loading, digital image correlation. 1. introduction open cell foams are very effective lightweight structures with impressive load-bearing function. therefore there is wide range of application for this type of material in modern engineered products. moreover, this spongy-like structure is very close to natural foam materials hence it meets the presumption for the usage in bioengineering [1]. in this case different types of polyurethane foam synthesized as artificial cancellous bone was investigated from biomechanical point of view. the goal of the project is to find suitable material for artificial vertebrae filling with similar mechanical and structural parameters to real vertebra body [2]. resulting product will serve during the training of medical doctors in spine surgery simulator developed at fhoö linz [3]. based on results of previous studies [4], four different foam were synthetized employing various additives in base polyurethane matter. similarly to the natural cancellous bone, artificial structure exhibit complex porous microstructure composed of irregular polyhedral cells [5] which is influenced by the additives. bone aging and disease could be simulated the same way [6, 7]. set of pilot compression tests were performed on batch of samples prepared from all material to evaluate basic mechanical parameters and to test loading range for in-house developed loading device which is planned to be used for 4d computed tomography testing procedure as a next step in this project. 2. experimental procedure 2.1. specimen description two-component polyurethane resin (kaupo, spaichingen, germany) was used as the base material. depending on a mineral filler, four types of polyurethane foam were created (listed in tab. 1). each type of foam was provided by the research group for surgical simulators linz (university of applied sciences upper austria in linz, austria). a small amount of highly-concentrated colour pigments was added to distinguish individual sets of samples (depicted in fig. 1). the same aditives (in detail listed in [4]) were used in all cases, e.g. 1 % of tap water as a blowing agent. all the foam samples were delivered in cylindrical shape with thin other shell and cut by a band saw to smaller rollers with a diameter d = 19.70 ± 0.14 mm and height h = 15.42 ± 0.42 mm. 28 http://dx.doi.org/10.14311/app.2018.18.0028 http://ojs.cvut.cz/ojs/index.php/app vol. 18/2018 biomechanical characterization of polyurethane structures filler bulk density [g/dm3] 50% calcium carbonate (caco3) 1% h2o 3% stabilizer 416.69 ± 21.85 50% calcium phosphate (3ca3(po4)·cao) 1% h2o 3% stabilizer 362.15 ± 17.16 30% barium sulphate (baso4) 1% h2o 3% stabilizer 381.45 ± 36.07 without filler 1% h2o 3% stabilizer 467.33 ± 59.74 table 1. average bulk density of the samples figure 1. four types of samples – color-coded according to the filler 2.2. experimental setup experimental setup consisted of an in-house developed uni-axial loading device and optical setup. force was measured by high-durability u9b series loadcell with nominal loading capacity 1 kn (hbm, germany). the axial movement of a clamp was performed by an electrical stepper motor with a ball screw. displacementdriven tests were performed with 10 µm/s loading rate for 10 mm displacement (corresponding to ≈ 50 % deformation). force and displacement data were recorded at a sample rate of 5 hz. experiments were controlled by the modified linuxcnc software running on a real-time kernel [8]. because of the need to evaluate the deformation optically, the samples were captured by high-resolution ccd camera manta g-504bmanta (allied vision technologies gmbh) equipped with a telecentric lens. the optical setup was placed on a 3-axis stage equipped with micrometric screws for precise positioning. all of the images were captured by unique timestamp to synchronize the force log. during the measurement, place of view had to be enlightened by external led light source kl 2500 (shott, germany). complete equipment is depicted in fig. 2. 2.3. digital image correlation high precision of the deformation measurement is a necessary step for proper evaluation of the strainstress diagram. one of the common and widely used method is digital correlation image (dic). this optifigure 2. 1 camera, 2 positionable table, 3 led source, 4 loading device, 5 place for specimen figure 3. definition of the subset and offset cal method is based on a specific small area tracking subset, defined by a variable m. the region is then searched for in the wider neighborhood defined by a variable offset (shown in fig. 3). a necessary condition is to ensure a granular coating on the surface of each sample. primarily random pattern allows a tracking of these areas during the experiment and calculation of the displacement, necessary to calculate the deformation. updated lucas-kanade algorithm was used for this purpose [9, 10]. 3. results based on the obtained force-displacement results, the stress-strain curves were calculated using total dimensions of the samples (fig. 4 to 7). these results show dependency of the ultimate stress on the used filler in polyurethane foam. the stress-strain curves are very similar when using calcium phospate and barium sulphate filler. higher values of the monitored quantities are reached by the calcium carbonate filler. the highest values of the ultimate-stress and young’s modulus are achieved when no filler is used but these results are distorted by a thin shell of a high stiffness on the samples. young’s modulus was calculated using linear regression applied on the elastic part of the stress–strain 29 j. šleichrt, d. kytýř, k. pithartová et al. acta polytechnica ctu proceedings 0 0.5 1 1.5 2 2.5 0 0.1 0.2 0.3 0.4 0.5 s tr es s [m p a] strain [-] samples a (50% calcium carbonate (caco3), 1% h2o and 3% stabilizer) a1, e = 51.357 mpa a3, e = 53.30 mpa a4, e = 46.49 mpa a5, e = 44.31 mpa figure 4. stress-strain curves of calcium carbonate filler 0 0.5 1 1.5 2 0 0.05 0.1 0.15 0.2 0.25 s tr es s [m p a] strain [-] samples b (50% calcium phosphate (3ca3(po4) . cao), 1% h2o and 3% stabilizer) b1, e = 19.43 mpa b2, e = 20.72 mpa b3, e = 23.50 mpa b4, e = 27.53 mpa b5, e = 25.70 mpa figure 5. stress-strain curves of calcium phosphate filler 0 0.5 1 1.5 2 0 0.05 0.1 0.15 0.2 0.25 s tr es s [m p a] strain [-] samples c (30% barium sulphate (bas04), 1% h2o and 3% stabilizer) c1, e = 37.891 mpa c2, e = 25.454 mpa c3, e = 24.953 mpa c4, e = 14.873 mpa c5, e = 21.867 mpa figure 6. stress-strain curves of barium sulphate filler diagrams. for the yield stress σy evaluation, christensen [11] second derivation criteria was used: σy = σ at ∣∣∣∣d2σ dε2 ∣∣∣∣ = max (1) for noise reduction significantly distorting resulting derived function rolling average filter computed from 0 1 2 3 4 5 0 0.05 0.1 0.15 0.2 0.25 s tr es s [m p a] strain [-] samples d (no filler, 1% h2o and 3% stabilizer) d1, e = 50.27 mpa d2, e = 54.43 mpa d3, e = 104.81 mpa d4, e = 107.51 mpa d5, e = 128.16 mpa figure 7. stress-strain curves of no filler five values as: σ′na = σ′n−2 + σ′n−1 + σ′n + σ′n+1 + σ′n+2 5 (2) results of the effective young’s modulus of each of the sample’s batch are listed in tab. 2. filler effective young’s modulus [mpa] 50% calcium carbonate (caco3) 1% h2o 3% stabilizer 48.63 ± 4.37 50% calcium phosphate (3ca3(po4)·cao) 1% h2o 3% stabilizer 23.35 ± 3.89 30% barium sulphate (baso4) 1% h2o 3% stabilizer 25.01 ± 8.35 without filler 1% h2o 3% stabilizer 89.04 ± 34.72 table 2. average of the effective young’s modulus of the samples 4. conclusions all samples were subjected to the uniaxial compression test. digital image correlatioon method was used for evaluating the displacement during the measuring. due to very precise synchronization between captured images of the sample’s surface and force logs the stress-strain diagrams could be calculated and effective young’s modulus was calculated for each of the sample. high dispersion of the young’s modulus is described by the standard deviation. this indicates that the porosity or wall thickness vary in the volume of the sample. further investigation of the material, especially from morphological point of view, is necessary. for this reason, other samples 30 vol. 18/2018 biomechanical characterization of polyurethane structures will be subjected to an internal structure study using more advanced methods as 4d computed tomography testing procedure. acknowledgements the research has been supported by operational programme research, development and education in project competitiveness boost of the centre of excellence in vysočina region (cz.02.2.69/0.0/0.0/16_027/0008475), by european regional development fund in frame of the project com3d-xct (atcz38) in the interreg v-a austria czech republic programme and by institutional support rvo: 68378297. references [1] k. l. calvert, k. p. trumble, t. j. webster, l. a. kirkpatrick. characterization of commercial rigid polyurethane foams used as bone analogs for implant testing. journal of materials science: materials in medicine 21(5):1453–1461, 2010. doi:10.1007/s10856-010-4024-6. [2] t. fila, d. kytyr, p. zlamal, et al. high-resolution time-lapse tomography of rat vertebrae during compressive loading: deformation response analysis. journal of instrumentation 9(5), 2014. doi:10.1088/1748-0221/9/05/c05054. [3] b. esterer, j. razenbock, m. hollensteiner, et al. development of artificial tissue-like structures for a hybrid epidural anesthesia simulator. 2016 38th annual international conference of the ieee engineering in medicine and biology society (embc) pp. 2099–2102, 2016. doi:10.1109/embc.2016.7591142. [4] d. fuerst, s. senck, m. hollensteiner, et al. characterization of synthetic foam structures used to manufacture artificial vertebral trabecular bone. materials science and engineering c 2017. [5] m. s. thompson, i. d. mccarthy, l. lidgren, l. ryd. compressive and shear properties of commercially available polyurethane foams. j biomech eng 125(5):732–734, 2003. doi:10.1115/1.1614820. [6] h. chen, s. shoumura, s. emura, y. bunai. regional variations of vertebral trabecular bone microstructure with age and gender. osteoporosis international 19(10):1473–1483, 2008. doi:10.1007/s00198-008-0593-3. [7] h. chen, x. zhou, h. fujita, et al. age-related changes in trabecular and cortical bone microstructure. international journal of endocrinology (id 213234):9, 2013. doi:10.1155/2013/213234. [8] v. rada, t. fila, p. zlamal, et al. multi-channel control system for in-situ laboratory loading devices. acta polytechnica ctu proceedings 18:15–19 2018. doi:10.14311/app.2018.18.0015. [9] b. lucas, t. kanade. an iterative image registration technique with an application to stereo vision. proceedings of the 7th international joint conference on artificial intelligence 2:674–679, 1981. [10] i. jandejsek, j. valach, d. vavrik. optimization and calibration of digital image correlation method. proccedings ean 2010 pp. 121–126, 2010. [11] r. m. christensen. observations on the definition of yield stress. acta mechanica 196(3):239–244, 2008. doi:10.1007/s00707-007-0478-0. 31 http://dx.doi.org/10.1007/s10856-010-4024-6 http://dx.doi.org/10.1088/1748-0221/9/05/c05054 http://dx.doi.org/10.1109/embc.2016.7591142 http://dx.doi.org/10.1115/1.1614820 http://dx.doi.org/10.1007/s00198-008-0593-3 http://dx.doi.org/10.1155/2013/213234 http://dx.doi.org/10.14311/app.2018.18.0015 http://dx.doi.org/10.1007/s00707-007-0478-0 acta polytechnica ctu proceedings 18:28–31, 2018 1 introduction 2 experimental procedure 2.1 specimen description 2.2 experimental setup 2.3 digital image correlation 3 results 4 conclusions acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2018.16.0011 acta polytechnica ctu proceedings 16:11–17, 2018 © czech technical university in prague, 2018 available online at http://ojs.cvut.cz/ojs/index.php/app analysis of the time course of a bridge abutments settlement luboš hruštinec slovak university of technology, faculty of civil engineering, department of geotechnics, radlinského 11, 810 05 bratislava, slovak republic correspondence: lubos.hrustinec@stuba.sk abstract. the paper deals with the time course of the bridge abutments settlement (consolidation of the subsoil) on the motorway d4 in stupava, slovakia. the bridge abutments are founded on an earth embankment 5.5 meters in height and a group of piles. over 6 years of geotechnical monitoring after the construction of bridge abutments, there were measured settlements from 102 to 106 mm. the measured settlement of intermediate bridge piers was only up to 16 mm. geotechnical calculations and analysis are focused on the comparison of the final settlements prognosis and its time course with the real measured values. keywords: bridge abutment, geotechnical calculations, settlement, consolidation. 1. introduction bridge objects no. 211-01 and 211-02 are situated at the crossroads “stupava” of the d4 motorway, which is bridging the d2 motorway in the direction from bratislava to stupava. a three-span bridge structure of total length 100.9 m and width 26.15 m was put into operation in 2010. bridge abutments are founded on piles in an embankment 5.5 m high. on the bridge supports, vertical deformations (settlements) have been continuously measured. in the time period from march 2010 to september 2016 (an overall period of 6.5 years) settlement of the internal piers was measured to be from 12.7 to 16.0 mm and settlement of the bridge abutments from 102 to 106 mm. such an uneven settlement (maximum angular deformation is ∆s / l = 0.0034) causes a significant increase of internal forces in the bearing elements of the bridge structure (especially in the bridge deck) due to the deformation load. in order to avoid failure of the bridge structure, an over-limit uneven settlement had to be eliminated by inserting compensating steel plates into the bridge bearing of the abutments. the paper deals with prognosis of the time course of the subsoil consolidation of the bridge abutments and the causes of their over-limit uneven settlement. this problem is solved in detail in [1]. 2. engineering and geological characteristics of the territory under study engineering and geological conditions of the studied territory and soil properties of the subsoil have been assessed on the basis of archive documentation [2]. the subsoil is formed by quaternary and neogene soils. quaternary soils are represented by deluvial, eolian, proluvial and fluvial sediments. neogene sediments are located in the whole area under quaternary sediments in the form of alternation of the clayey and sandy soils. in the location of bridge structure no. 211 the jm-5 to jm-8 exploration boreholes were conducted to a depth of 15 m. the layers of quaternary fluvial non-cohesive sediments occur in the form of sandy soils. in accordance with stn72 1001 [3] soils belong to classes: silty sand (s4-sm) and clayey sand (s5-sc). the thickness of the quaternary layer ranges from 0.7 m (borehole jm-5) up to 1.7 m below the surface (borehole jm-6). under the quaternary sandy soils there are neogene clay soils. the clays belong to certain classes: clay with medium plasticity (f6-cl) and clay with high plasticity (f8-ch). the consistency of the clay changes with the increasing depth, from firm to stiff. in the jm-8 borehole, the clay consistency is soft to firm. the groundwater level was found at 0.7 to 1.0 m below the surface. the above mentioned information and knowledge have been taken into account when determining the subsoil model and soil properties used in the geotechnical calculations. 3. determination of the subsoil models and soil properties for bridge abutments no. 1 and no. 4 the subsoil models were defined based on the evaluation of jm-5 and jm-8 boreholes, respectively. computational models of subsoil and bridge abutments used in the geotechnical calculations are shown in figure 1. the depth of the exploration boreholes (jm-5 and jm-8) was 15 m under the original terrain. geotechnical calculations showed that the deformation zone (ha) extends to a depth of 22.3 m below the terrain. based on the evaluation of the results of the geodetic measurements of the settlements and geotechnical calculations, it was assumed in the subsoil models that from the depth of 15 m to the depth of the deformation zone “ha” 11 http://dx.doi.org/10.14311/app.2018.16.0011 http://ojs.cvut.cz/ojs/index.php/app luboš hruštinec acta polytechnica ctu proceedings there are soils of class f8-ch with soft to firm consistency. the soil properties used in the geotechnical calculations are shown in table 1. 4. technical parameters of the bridge structure information about the technical parameters of the bridge structure were taken from [4]. the bridge loadbearing structure (bridge deck) is a monolithic structure made of pre-stressed concrete of class c35/45 with total height of 1.8 m. the three-span bridge beam has a range of fields 26.5 + 36.5 + 26.5 m. the total length of the bridge structure is 100.9 m (figure 2). in the transverse direction, the bridge consists of two separate structures. for the right traffic lane, the bridge is marked as no. 211-01 and for the left traffic lane no. 211-02 (figure 3). in the cross-section, two pre-stressed concrete beams are connected with a reinforced concrete slab (bridge deck). the total width in the transverse direction of the bridge is 13.75 m (structure no. 211-01), and 11 m (structure no. 21102). geometric boundary conditions of the abutment structures no. 1 and 4 (including embankment and backfill) for structures no. 211-01 and no. 211-02 have been taken from the project documentation[4]. bridge abutments are deep founded on piles and on the embankment (figure 2). the foundation consists of a group of piles with a diameter of 0.9 m, a length of 14 m (no. 211-01 group of 7 piles and no. 211-02 group of 6 piles). computational models of bridge abutments no. 1 and 4 are shown in figure 1. 5. determination of load states for geotechnical calculations the loads due to the bridge structures were taken from the static analysis mentioned in [1]. the total loads acting on the bridge abutments and subsoil are shown in table 2. in the geotechnical calculations, the most significant loads of the subsoil from the individual structures (embankment, abutment and the backfill behind the abutment) and the technological progress of the construction of bridge structures no. 211-01 and 211-02, were considered. three following load states were defined: • 1st load state (1st ls): construction of the embankment below the abutments. under the abutment no. 1 the height of the embankment is hn1 = 5.5 m, which causes a uniform plane load qn1 = γ × hn1 = 20 × 5.5 = 110 knm−2. under the abutment no. 4 the height of the embankment is hn4 = 4.8 m, which causes a uniform plane load qn4 = γ × hn4 = 20 × 4.8 = 96 knm−2. • 2nd load state (2nd ls): construction of piles, abutments and bridge decks (table 2). • 3rd load state (3rd ls): construction of a backfill behind the bridge abutments. behind the abutments the height of the backfill is hz = 4.3 m, which causes a load of size qz = γ × hz = 20 × 4.3 = 86 knm−2. from the defined load states it follows that the total load of the subsoil at the level of the terrain from embankments and backfills, causes the load of up to q1 = 196 knm−2 (for abutment no. 1) and q4 = 182 knm−2 (for abutment no. 4). the total vertical load acting locally below the abutments (at the foundation gap) has a size q211−01 = 220.9 knm−2 (for abutment no. 211-01) and q211−01 = 232.5 knm−2 (for abutment no. 211-01). from these values of load intensity it is obvious that the large embankment and backfill will have a significant impact on the final settlement of the abutments and cannot be neglected in the calculations. 6. results from geodetical measurements of the bridge supports settlement the vertical displacements (settlements) of the bridge supports have been continuously measured using the geodetic method of a very precise levelling. in the period from july 13, 2010 to september 3, 2016 (i.e. over 6 years), a total of 23 stage measurements have been performed. the detailed results of the geodetic measurements are given in [1]. graphical evaluation of the time course of settlement of supports no. 1 to 4 of the bridge structure no. 211-01 and no. 211-02 is shown in figure 4. the measured maximum settlements of the middle piers were 12.7 to 16.0 mm and the settlements of the edge bridge abutments were 102.0 to 106.0 mm. from the static analysis of a bridge structure, the maximum value of the settlement difference of the neighbouring supports ∆s = 20 mm [1] resulted. the measured maximum settlement difference of the neighbouring supports is ∆s = 106– 16 = 90 mm and maximal uneven settlement is ∆/l = 90/26500 = 0.0034. the above-mentioned overlimit uneven settlement causes a significant increase of the internal forces in the bridge bearing elements (especially in the bridge deck) due to the deformation load. to compensate for the over-limit uneven settlements of the bridge supports, the bridge deck structures at the place of abutments were lifted and steel washers with a thickness of 45 mm were inserted on november 15, 2010. 7. evaluation of geotechnical calculations the geotechnical calculations were focused on the determination of the rate of the final settlement and the time course of settlements of the bridge abutments no. 1 and 4. the calculations were performed in accordance with valid standards [5, 6]. the final settlement was calculated using the finite element method (planar task) and the time course of settlements was analyzed in terms of the theoretical assumptions of 12 vol. 16/2018 bridge abutments settlement geological period soil class-symbol consistency γ [kn/m3] ν [-] e [mpa] cv [m2·s−1] quaternary silt with low plasticity f5-ml firm 20.0 0.40 4.0 5.0e-06 silt sand s4-sm fine fraction is firm 18.0 0.30 8.0 7.5e-05 clayey sand s5-sc fine fraction is soft firm 18.5 0.35 8.0 3.8e-06 neogene clay with medium plasticity f6-ci soft to firm 21.0 0.40 3.0 5.4e-07 clay with high plasticity f8-ch soft to firm 20.5 0.42 2.0 2.2e-07 clay with high plasticity f8-ch firm 20.5 0.42 3.0 2.2e-07 clay with high plasticity f8-ch firm to stiff 20.5 0.42 4.0 2.2e-07 clay with high plasticity f8-ch stiff 20.5 0.42 6.0 2.2e-07 table 1. soil properties used in geotechnical calculations. figure 1. computational models for abutments no. 1 and 4 used in geotechnical calculations. 13 luboš hruštinec acta polytechnica ctu proceedings figure 2. longitudinal cross section of the bridge structure [4]. figure 3. cross section of the bridge structure [4]. bridge structure abutment type of load vz [kn] hx [kn] hz [kn] mx [knm] my [knm] 211-01 1 and 4 dead weight 7,870.5 0.0 0.0 0.0 2,245.0 211-02 1 and 4 dead weight 6,685.3 0.0 0.0 0.0 1,952.3 table 2. loads for the bridge abutments used in geotechnical calculations [4]. figure 4. the measured time course of settlements (si) of the bridge supports (piers and abutments) and settlements difference (∆si) of neighbouring supports. 14 vol. 16/2018 bridge abutments settlement figure 5. calculated time course of settlement of the bridge structure no. 211-01, abutment no. 1. bridge structure abutment calculated final settlement s measured settlement (3. 9. 2016) settlements (difference calc.-meas.)abutment embankment backfill completely (embankment, abutment, backfill) sa [mm] se [mm] sb [mm] sc [mm] sm [mm] ∆sc [mm] 211-01 1 26.65 92.00 35.45 154.10 102.00 52.10 4 34.45 86.60 40.70 161.75 106.00 55.75 211-02 1 28.30 93.15 36.20 157.65 104.70 52.95 4 32.90 85.30 38.25 156.45 104.00 52.45 table 3. comparison of calculated and measured settlements with the prognosis of further settlement ∆sc [mm]. one-dimensional consolidation [7]. geotechnical calculations yielded many quantitative and qualitative results. the detailed results of the calculations can be found in the author’s archive records. this article presents only some representative results of geotechnical calculations and their processing in a clear tabular and graphical form. graphical evaluation of the calculated time course of settlement of the individual structural parts (embankment, abutment, backfill, and the complete structure) of the abutment no. 1 (bridge no. 211-01) is shown in figure 5. the table comparison of the measured (as of september 3, 2016) and the calculated values of the final settlements of the bridge abutments with the prognosis of expected increase of settlement ∆s is given in table 3. the relative percentage comparison with the calculated final settlement is presented in table 4. the graphical evaluation of the comparison of the measured settlements and the calculated final settlements of bridge structure no. 211-01 for a period of 0 to 10 years is shown in figure 6. from the evaluation and analysis of the results of the geotechnical calculations and a comparison with the measured settlements, the following facts and knowledge follow: • calculated values of the final settlements (average values in the centre of abutments) range from 154.1 mm (bridge no. 211-01, abutment no. 1) to 161.75 mm (bridge no. 211-01, abutment no. 4). from the total value of the final settlements, the effects of loads due to the individual structural parts is the following: due to the embankment up to 59.7% (from 54.5 to 59.7%), due to the backfill behind the abutment it is up to 25.2% (from 23.0 to 25.2%) due to the subsoil loading by the abutment structure it is to 21.3% (from 17.3 to 21.3%). from the above mentioned it follows that the final value of the settlement is mainly affected by loading of the subsoil by the embankment and the backfill behind abutment. the final settlement caused by the embankment and backfill causes up to 82.7% (from 78.7 to 82.7%) of the total final settlements of the bridge abutments. the size of the calculated settlements is also significantly affected by the presence of clays soils with soft consistency in the bridge abutments subsoil. • the calculations results of the time course of settlements (under the assumed boundary conditions) showed that the subsoil consolidation will be finished (consolidation degree u = 100%) after 51.1 years (figure 5), or 80% degree of consolidation will be achieved after 10 years (figure 6). • from the comparison of the measured settlements (as of september 3, 2016) and the calculated final settlements, there is the result that the measured values reach up to 66.5% of the final settlements, i.e. approximately 2/3 of the final settlements value (table 4). the expected next settlements ∆s of the bridge abutments are in the range from 52.1 mm to 15 luboš hruštinec acta polytechnica ctu proceedings bridge structure abutment calculated final settlement s measured settlement (3. 9. 2016) settlements difference (calc.-meas.)abutment embankment backfill completely (embankment abutment, backfill) so [%] sn [%] sz [%] sc [%] sm [%] ∆sc [%] 211-01 1 17.3 59.7 23.0 100.0 66.2 33.8 4 21.3 53.5 25.2 100.0 65.5 34.5 211-02 1 18.0 59.1 23.0 100.0 66.4 33.6 4 21.0 54.5 24.0 100.0 66.5 33.5 table 4. the percentage comparison of calculated and measured settlements. figure 6. time course of settlement of the bridge structure no. 211-01, abutments no. 1 and 4 – comparison of measured and calculated values over a 10-year period. 55.75 mm (table 3), i.e. 33.5 to 34.5% of the final settlements (table 4). the above findings must be taken into account in the complex assessment of the reliability of the bridge structures no. 211-01 and no. 211-02. 8. conclusions based on the analysis and evaluation of the geotechnical calculations undertaken it can be stated that the over-limit settlements of the bridge abutments occurred mainly due to the following factors: • failures during the execution of the geotechnical survey (mainly insufficient depth of survey boreholes that did not reach the depth of the deformation zone; ending boreholes in cohesive soils of soft consistency; insufficient determination of geotechnical data valid for soils in the subsoil). • failures during the execution of the geotechnical calculations and design of the bridges abutments (especially neglecting the significant part of the loading of the subsoil due to the embankment and the backfill bridge abutments; neglecting the consolidation processes in the subsoil). to ensure the reliable operation of bridge objects no. 211-01 and no. 211-02, the following recommendations and proposed measures were formulated: • at regular intervals, to visually check the bridge structures and their load-bearing elements. during the bridge checking it is necessary to focus on deformation of the structures (including the embankment and the surrounding terrain), the origins of cracks and other disproportional phenomena (deformation of the terrain, inadequate deformations and rotation, etc.). • to perform ongoing geodetic measurements of vertical displacements (settlements) of bridge supports no. 1 to 4 (including internal piers no. 2 and no. 3) at a minimum of twice per year. • after each periodic measurement it is necessary to graphically evaluate the measured values of settlement and to compare them with the calculated values. if the measured settlement values are not 16 vol. 16/2018 bridge abutments settlement in accordance with the expected values, it is necessary to take measures to eliminate the uneven settlements and their effects on the reliability of the bridge structure. this involves following the recommendations of the static analysis of the bridge structure, such as e.g., inserting washers into the abutments bridge bearings. • if the measured settlements are higher than the expected ones, or they progressively rise, it is necessary to carry out an additional geotechnical survey to the depth of the deformation zone. • in case of another disproportional progressive increase of the settlements, it is necessary to take more complex measures, such as e.g., pre-injection of the subsoil using jet grouting technology. list of symbols γ unit weight with natural moisture [kn m−3] ν poissin ratio [–] edef modulus of deformation [mpa] cv coefficient of consolidation [m2 s−1] v y vertical force [kn] hx horizontal force in the x-axis direction [kn] hy horizontal force in the y-axis direction [kn] mx moment in the x-axis direction [knm] mx moment in the y-axis direction [knm] s settlement; vertical displacement [mm] ∆s settlement difference [mm] u degree of consolidation [%] acknowledgements the author is grateful for support from the grant agency vega of the slovak republic, project no. 1/0412/18. references [1] j. halovonik. assessment of stress in the bridges structures no. 211-01 and 211-02 – crossroad stupava. bratislava, 2016. [2] i. modlidba. crossroad stupava – south on the d2 motorway. a detailed engineering-geological survey. final report. terratest. bratislava, 2006. [3] stn en 72 1001: classification of soil and rock in engineering geology and geotechnics, 2010. [4] m. sloboda. crossroad stupava – south on the d2 motorway. documentation of actual construction. strabag – doprastav, 2011. [5] stn en 1997-1 eurocode 7: geotechnical design. part 1: general rules. 2005. [6] stn 73 1001: geotechnical structures. foundation, 2010. [7] j. jesenák. soil mechanics. stu bratrislava, 1994. 17 acta polytechnica ctu proceedings 16:11–17, 2018 1 introduction 2 engineering and geological characteristics of the territory under study 3 determination of the subsoil models and soil properties 4 technical parameters of the bridge structure 5 determination of load states for geotechnical calculations 6 results from geodetical measurements of the bridge supports settlement 7 evaluation of geotechnical calculations 8 conclusions list of symbols acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2017.11.0086 acta polytechnica ctu proceedings 11:86–90, 2017 © czech technical university in prague, 2017 available online at http://ojs.cvut.cz/ojs/index.php/app analysis of competitiveness of railway passanger transport between rakovník and prague rudolf vávra∗, vít janoš department of logistics and management of transport, faculty of transportation sciences, ctu in prague, czech republic ∗ corresponding author: vavrarud@fd.cvut.cz abstract. this paper deals with the issue of railway connection between rakovník and prague. initially, there is made an analysis of the transport supply and analysis of modal split. additionally, an operation concept is suggested in order to increase the attractiveness of the railway passenger transport, afterward, the transport possibilities, which can be accomplished by this approach, are evaluated. keywords: modal split, transport supply, logit model, operation concept, periodic timetable, travel time, public transport. 1. introduction the article deals with examining the competitiveness of the railway connection between prague and rakovník. rakovník (16081 inhabitants) is a town located in the western part of středočeský kraj (region of middle bohemia), outside the suburban area of prague, but some properties are similar to a suburban area – has strong daily transport relations due prague (according to the census from 2011 about 738 people daily). rakovník to prague does have good bus connection, but due to the existence of the railway connection in this session, and the advantages of rail transport through the continuous passage of agglomeration areas should address the issue of the attractiveness of this mode of transport. among solved railway lines are included (line numbers by railway timetable sždc): • 120 praha – kladno – rakovník, • 171 praha – beroun, • 173 praha – rudná u prahy – beroun, • 174 beroun – rakovník. the aim is to examine the competitiveness of rail passenger transport in relation rakovník – prague, using the current infrastructure, which includes: • an analysis of the modal split for the current state, • determination of the theoretical travel times possibilities of the train and comparing them with the travel times according to the current state, • proposal schematic operational concept in term of technological possibilities, • comparing travel times of created operational concept with travel times according to the current state and theoretically achievable travel times, • determination of the modal split for the proposed operational concept and measurement of the difference in the modal split of proposed operational concept from the current situation. 2. methods to determine the share of individual vehicles on the modal split is used logit model, namely a two-step nested logit model: pi = e µvi∑ j∈dp e µvj where pi means the probability of choice of transport mode i, e is euler’s number, dp is the set of all considered transport modes, µ means the elasticity of choice of transport mode on the basis of changes in generalized costs and vi means generalized costs expended for the journey by transport mean i. [1] nested logit model is phasing out the calculation in two phases. the first phase determines modal split between individual transport and public transport. the second phase examines then modal split of individual transport modes within public traffic. 2.1. first phase in the first phase it is necessary to determine the generalized cost of individual car transport and public transport. within public transport there is selected that transport mode of public transport, that is more favourable for the journey. generalized costs of individual car transport (viad) are calculated using the formula 1, where dkm is length of the journey (in km), niad are costs for 1 km of the journey, oiad is the average occupancy of the vehicle, diad čas is travel time (in hours), wčas is the perceived value of time (in czk/hour) and diad dost is an indicator of time availability in hours (includes attendance from origin of the journey to the car, time attendance from the car to final destination, time for finding a parking space, etc.). generalized costs of public transport (vv d) are calculated using the formula 2, where dv d čas is travel 86 http://dx.doi.org/10.14311/app.2017.11.0086 http://ojs.cvut.cz/ojs/index.php/app vol. 11/2017 analysis of competitiveness of railway passanger transport viad = dkm ∗ niad oiad +diad čas ∗wčas +diad dost ∗wčas (1) vv d = ( dv d čas ∗wčas +dv d dost ∗wčas + jv d + iv d,1/2 ∗wčas ) ∗ cv d−iad (2) time (in hours), wčas is the perceived value of time (in czk/hour), dv d dost is an indicator of time availability in hours (time attendance from origin of the journey to the stop/station of public transport, time attendance from the stop/station to final destination), jv d is fare (in czk), iv d,1/2 is half the interval between links (in hours) and cv d−iad is the calibration constant of mode choice. [1] the result of the first phase and are therefore values piad and pv d indicating the probability of the use of individual car traffic and public transport in the appropriate relation. 2.2. second phase in second phase there is necessary to determine generalized costs (vi) for relevant modes i of public transport. determination of these costs is performed according to the formula 3, where di čas is travel time (in hours) using transport mode i, wčas is the perceived value of time (in czk/hour), di dost is time availability of the transport mode i (in hours), ji is fare using transport mode i (in czk) and ii,1/2 is half the interval between links of the transport mode i (in hours). [1] results of second phase are the conditional probabilities pi∨v d of the use of transport mean i under the condition of using public transport. the probability pi of use of the transport mean is thus given by pi = pv d ∗ pi∨v d. 3. results and discussion 3.1. analysis of supply connection during the timetable 2015/2016 bus connection from rakovník to prague was during the timetable 2015/2016 accounted by lines 310610, 310612, 310613, which are operating between rakovník bus station and stop prague hradčanská and further by lines 310270, 310700, 310710, which are operating between rakovník bus station and stop prague zličín. average aggregate interval between individual links reaches during peak hours 10 minutes. shortest travel times between rakovník and hradčanská are about 55 to 75 min. the shortest travel times between rakovník and zličín are about 40 to 55 min. by train there is possible to go either using line 120 through kladno or using the lines 174 and 171 (eventually 174 and 173) through beroun. on the line 120 are operated in 120 minutes period fast trains praha – rakovník. connection rakovník to prague is complemented by a combination of fast regional trains praha – kladno (in 120 minutes period, between praha – kladno in leading with fast trains) and passenger trains kladno – rakovník (operated as the 120 minute period) with bond in the station kladno. the connection is also completed in peak hours of working days with one pair of fast regional trains rakovník – prague. these connections are operated in periodic timetable. on the line 174 are operated passenger trains beroun – rakovník, but without a fixed interval between links (i.e. not according periodic timetable). their average interval during peak hours of working days is around 60 minutes. in beroun is then formed a bond in direction prague. 3.2. analysis of modal split in timetable 2015/2016 for the analysis of the modal split between prague and rakovník were selected 3 relations. in rakovník is always considered the town centre. in prague, with respect to potential target for daily commuting, were selected these points: vítězné náměstí, anděl, václavské náměstí. to analyze the modal split was used nested logit model described in chapter 2. to determine values dkm and diad čas was used free route planner on website mapy.cz. the value niad is determined as the sum of the fuel cost per 1 km away (at an average consumption of 7 litres per 100 km) and other costs per 1 km of ride (related to the operation and acquisition of the vehicle). rates are taken from the decree no. 328/2014 coll., so niad = 6, 213 czk/km. for the average occupancy of the vehicle according to chmelík [1] was calculated with the value oiad = 2. for variable value diad dost is selected diad dost = 0, 25 h (5 minutes attendance from the journey origin to the vehicle, 5 minutes attendance from the car to the final destination and 5 minutes time on finding a parking space and parking). in the case of public transport is taken into account connection by bus and by train in the working day (on september wednesday as an average traffic day) in the direction from prague to rakovník, with the departure of the train / bus from prague in the time between 14:00 and 18:00. values di čas and ii,1/2 are based on supply of connections, with ii,1/2 = 2 ps , where ps means the number of considered connections in the solved time period. the value of the availability di dost of transport mean i is determined by the route planner and includes: 87 rudolf vávra, vít janoš acta polytechnica ctu proceedings vi = di čas ∗wčas +di dost ∗wčas + ji + ii,1/2 ∗wčas (3) figure 1. current modal split. • attendance of the reference point in prague to the appropriate stop/station of transport mean i , or to the stop of prague urban transport, • attendance from the bus station in rakovník, or from the railway station rakovník to the town centre, • additional reserve of 5 min. fare ji is determined from the carriers’ tariffs. for bus services is being considered at ordinary fare payment smart card, for rail services with custom fare in 25, at the prague urban transport then using annual coupon for an adult. perceived time value is calculated according to chmelík [1] as 0.24 multiple of the average gross hourly wages, while gross monthly wage czk 26480 and 168 working days in 2016 and 8 working hours per working day brings wčas = 37, 82857 czk/hour. values µ and cv d−iad were chosen after chmelík [1], therefore cv d−iad = 2, 2, µ = −0, 0031 for the first phase of the model and µ = −0, 0123 for second phase of the model. the resulting values for modal split for the current state shows figure 1. 3.3. determination of the theoretically achievable travel times theoretically achievable travel time was determined on the basis of completely free infrastructure capacity. there are considered no stops and therefore is neglected a number of restrictive conditions. to determine theoretically achievable rail travel time was used software fbs. theoretically achievable travel times were determined for followed relations: • praha masarykovo n. – rakovník, • praha-dejvice – rakovník, • praha hl.n. – rakovník, • praha-smíchov – rakovník. theoretically achievable travel times in these relations, as well as its comparison with the shortest travel time by train timetable 2015/2016, the shortest travel time by bus (or in combination with prague urban transport) and travel time by car, shows the graph in figure 3. 3.4. the proposal of schematic operational concept by the real technological possibilities despite the temporal lack of competitiveness of rail passenger services between prague – rakovník, which is evident from the graph in figure 3, was for solved lines created schematic operational concept by the real technological possibilities, that would have the effect of increasing the value of modal split compared to current situation. this can be achieved in the following ways: • shorting the travel times, • shorting the interval between links, • increasing number of direct rail links between rakovník and prague, • using principles of integrated periodic timetable (ipt). for creating the operational concept was used fbs. on the line 120 there is proposed hourly operation of fast trains between rakovník – prague, instead of the current two-hour interval. the line reaches the broader ipt node in minute 00 in rakovník. regional trains kladno – rakovník are kept in a two-hour interval with bond to passenger trains prague – kladno in kladno. on lines 171 and 174 is proposed introducing of fast regional trains prague – beroun – rakovník, which would be operated in 120 minutes interval. on the line 174 is also proposed to introduce two lines of trains, both kept in interval 120 minutes. ipt node in minute 00 in rakovník achieves, however, only one of them. on the 173 was taken into account the introduction of peak 20 minutes interval between prague smíchov and nučice station. for connection between rakovník – prague this line does not have much significance. the proposed operational concept (for peak hours) is displayed in the form of interval graphics in the figure 2. the graph in figure 4 shows similar information as the graph in figure 3, to which are added the shortest rail travel times from proposed operational concept. 3.5. determination of the modal split for the proposed operational concept for new proposed operational concept was calculated modal split again. it was used the same method of calculation as in section 3.2. 88 vol. 11/2017 analysis of competitiveness of railway passanger transport figure 2. interval graphic of the proposed operational concept. 89 rudolf vávra, vít janoš acta polytechnica ctu proceedings figure 3. comparison of theoretically achievable rail travel time. figure 4. comparison of theoretically achievable rail travel time, including shortest rail travel times from proposed operational concept. modal split for the proposed operational concept is shown in the graph in figure 5. the graph in figure 6 shows the modal shift of the proposed scheme compared to the level in force during the timetable 2015/2016. 4. conclusions the aim was to examine the competitiveness of rail passenger transport between rakovník and prague. of the share of modal split of the relevant transport modes in relation rakovník – prague, it was found, that the share of train is today in comparison with individual car traffic and the bus very low. then there were determined for the train relation rakovník – prague theoretical possibilities of achievable travel time, and it was found, that even when neglecting a number of restrictive conditions, the train could not be between prague and rakovník time-competitive, even compared to the bus, let alone compared to individual car transport. to calculate the modal-split was used nested-logit model. then was created an operational concept aimed at increasing the attractiveness of passenger rail transport in relation rakovník – prague. for the proposed operational concept has been found, how the train travel time could be closer to the theoretically achievable travel time. for timetable construction was used fbs. it was found, which transportation options may be achieve due to new operational concept and for this solution were determined new values of the modal split. after comparing new values with the previous (original) the results shows, that despite the proposed offer in passenger rail service between prague and figure 5. modal split in relations prague – rakovník using the proposed operational concept. figure 6. modal shift in relations prague – rakovník using the proposed operational concept. rakovník was not achieved appreciable improvement, and value of modal split for the train increased by only 0.42 %. the result of examination is that possibilities of contemporary rail infrastructure are so limited, that changes in timetable concept and offer of more connection does not allow significant improvements. so it is confirmed, that between rakovník and prague is not possible to achieve fundamental changes in the modal split without significant changes on to the rail infrastructure. references [1] j. chmelík. assessments of modal split in long-distance passenger transport [pdf]. review of economic perspectives – národohospodářský obzor, 2015. [cit. 2016-08-01]. 90 acta polytechnica ctu proceedings 11:86–90, 2017 1 introduction 2 methods 2.1 first phase 2.2 second phase 3 results and discussion 3.1 analysis of supply connection during the timetable 2015/2016 3.2 analysis of modal split in timetable 2015/2016 3.3 determination of the theoretically achievable travel times 3.4 the proposal of schematic operational concept by the real technological possibilities 3.5 determination of the modal split for the proposed operational concept 4 conclusions references acta polytechnica ctu proceedings doi:10.14311/app.2018.18.0048 acta polytechnica ctu proceedings 18:48–54, 2018 © czech technical university in prague, 2018 available online at http://ojs.cvut.cz/ojs/index.php/app modelling and simulation of lattice structures using various material models for polymeric materials kerstin lehner∗, anna kalteis, zoltan major institute of polymer product engineering, johannes kepler university linz, altenbergerstraße 69, 4040 linz, austria ∗ corresponding author: kerstin.lehner@jku.at abstract. based on lightweight design concepts, lattices are increasingly considered as internal structures. this work deals with the simulation of periodically constructed lattices to characterize their behaviour under different loadings considering various material models. a thermo-mechanical analysis was done, which is resulting in negative clte-values (coefficient of linear thermal expansion). simulations with linear-elastic behaviour were evaluated regarding the tensile, compression and shear modulus and the poisson’s ratio. some of the investigated structures behave auxetic. beside the linear elastic behaviour, also the hyper-elastic and visco-elastic behaviour of some structures were investigated. furthermore, elasto-plastic simulations were performed where the applied loading was biaxial. as a result the initial yield surfaces were presented. the individual rves (representative volume elements) can be utilized for different areas of application dependent on the used materials. keywords: periodic lattice structures, material models, auxetic behaviour, negative clte. 1. introduction based on lightweight design concepts, lattices are increasingly considered for additively manufactured components as internal structures in spite of classical bulk solid volumes. the structured construction of cellular materials has extremely advantageous properties for the lightweight design represented by a low material density combined with high stiffness and strength as well as an excellent damping behaviour [1]. dependent on the desired applications, various materials possessing different behavioural mechanisms are utilized for the design of components. the aim of the present work was to characterize and compare various modelled regular open cellular materials defined as lattice structures with applied periodic boundary conditions regarding different material models under various loading conditions. fig.1 depicts the task and procedure of the work performed. the results of the structures which are presented in this paper are highlighted. these results are chosen because they are the most informative ones. linear-elastic material behaviour is the simplest assumption of elasticity and occurs for small deformations [2]. thermoplastic elastomers behave hyper-elastically, which is characterized by a non-linear deformation behaviour and a unique stored strain energy density function depending on the current strain [3]. visco-elastic materials behave both viscous and elastic and show time-, temperatureand frequency-dependent characteristics [4]. elasto-plasticity describes polymeric materials, in which plastic deformation occurs when loaded beyond its limit of linear-elastic behaviour. in the next section the simulation approaches and results are presented and in the last section a conclusion for various lattice structures and different material models is drawn. 2. methodology in the following the methodology is described how the lattice structures are modelled and simulated. the structures are modelled in the software nx, which was developed by the company siemens. it is an interactive cad/cam/ cae-system. fig.2 shows the procedure of the modelling of the various structures represented by an octet structure. a matrix block is modelled by extrusion of a square with the dimensions 50 x 50 mm2. within this assembly space sketches are drawn along the desired geometry of the trusses. these are fully parametrised, which is when the size of the representative volume element (rve) is changed the inclusion size is automatically adapted. at these curves filled tubes are positioned, which are connected to each other. boolean operations were applied to first intersect the protruded inclusion parts from the rve and then subtract the inclusion from the matrix to get two individual parts. with these structures finite element simulations are performed in the software abaqus cae (simulia, dassault systemes). in order to model an rve in such a way that there is no influence of edge effects, which means that the rve has to be modelled as if being part of the continuous bulk material, commonly so called pbcs are used. using pbcs the single rve behaves as if being part of a continuum material [5]. therefore, perodic boundary conditions (pbcs) are implemented in abaqus cae by a python script, which was developed at the institute of polymer product engineering (ippe) by 48 http://dx.doi.org/10.14311/app.2018.18.0048 http://ojs.cvut.cz/ojs/index.php/app vol. 18/2018 modelling and simulation of lattice structures figure 1. task and procedure of the present work. dipl.-ing. dr. matei-constantin miron and dipl.ing. emil jacob pitz. the specific procedure and mechanism of this script can be found in [6]. 3. thermo-mechanical analysis thermal expansion is the tendency of matter to change in shape, area and volume in response to a change in temperature [7]. in the case of linear observation, the one-dimensional change in length is discussed. the strain in the direction of the length, which is equally to the relative expansion, divided by the change in temperature is called the material’s coefficient of linear thermal expansion (clte) and varies with temperature for most materials. the mathematical formula for the clte α is given in eq.1. ∆l is the change in length (∆l = l1 − l0), l0 is the initial length of the specimen and ∆t = t1 − t0 represents the temperature change. α = ∆l l0 · 1 ∆t (1) for clte-simulations predefined fields with a constant temperature rise of 10 k throughout the whole region are implemented. the reference points for the three directions in space, which are created by a script for periodic boundary conditions and mark the centres of the adjacent unit cells, are set free to permit expansion in all directions. the partitioning of structure 9 for the two-phase system is given in fig.3 with the outer part coloured grey and the inner part coloured purple. two different variations of structure 9 are investigated: structure 9a (tube diameter of 5mm) and structure 9d (tube diameter of 2.5mm). the used material data for the thermo-mechanical simulations are given in tab.1. material 1 has a lower e-modulus, a lower α and a higher ν than material 2. the resulting von mises stresses can be seen in fig.4 (top) for a material composition where the inner square is made material 1 material 2 e,mpa 1000 10000 ν, 0.45 0.2 α, 1 k 1e-6 1e-5 table 1. material data for clte-simulations [8]. of a stiffer material with a higher coefficient of linear thermal expansion (material 2) than the outer structural part. the maximum stresses occur at the connection points because of the hindered expansion of the inner square due to the lower expansion of the outer parts. fig.4 (bottom) shows the reversed construction where the inner square consists of material 1. greater stresses occur in the outer geometry part. when using material 1 for the outer structural part the cltes yield in negative values for the two variations of structure 9 and therefore contraction in z-direction (fig. 5, 1. and 2.). this effect is traced to a mechanical contraction normal to the plane in which the inner square consisting of material 2 lies resulting from the higher expansion of the inner material. this mechanical contraction overcomes the expansion due to the temperature increase and therefore yields to a negative clte in z-direction for this structure. compared to the literature results of [8], this lattice structure reveals nearly the same value of the thermal expansion in z-direction. when the outer structural part is made out of material 2 and the inner square consists of material 1 both investigated structures expand when increasing the temperature (fig. 5, 3. and 4.). for this material composition the highest clte-values are received in z-direction. 4. linear-elasticity linear-elasticity describes the material behaviour at small deformations [2]. most elastic materials exhibit 49 k. lehner, a. kalteis, z. major acta polytechnica ctu proceedings figure 2. modelling procedure represented by an octet structure. figure 3. partitioning of structure 9. a linear-elastic model, which is time-independent, and can be described by a linear relation between the stress σ and strain ε. linear-elastic behaviour for the lattice structures is characterized by the apparent lattice stiffness matrix wherefore displacement-controlled simulations of uniaxial tension (et ) and shearing (g) were performed in all directions to determine the entries of the stiffness matrix. for further investigations of the linear behaviour also simulations of uniaxial compression (ec) were done. the following input parameters are used for the linear-elastic simulations: e = 72000mpa, ν = 0.2. in fig.6 it can be seen that structure 6 shows the stiffest behaviour of all investigated structures in all directions. structure 3 has the lowest stiffness values related to tension and compression, but this rve shows a good applicability for purposes where shear loads are present. when comparing the different variations of structure 9 it can be seen that structure 9a (tube diameter of 5mm) results in the highest stiffness values whilst structure 9c (hollow tubes with a shell thickness of 0.5mm) figure 4. von mises stresses of a two-phase system caused by temperature increase. and structure 9d (tube diameter of 2.5mm) exhibit lower values, but also have the maxima in z-direction. special characteristics of the poisson’s ratio can be observed for structure 5 (fig.7 (top)), structure 8 (fig.7 (bottom left)) and structure 10 (fig.7 (bottom right)). in fig.7 these structures are illustrated and the von mises stress is depicted for different loading directions. structure 5 expands in y-direction when pulling in x-direction and vice versa, which is described by negative values for ν12 and ν21 (auxetic behaviour). with tension in z-direction this structure bears very high strains in xand y-direction. structure 8 shows an overall auxetic behaviour. due to the twisting mo50 vol. 18/2018 modelling and simulation of lattice structures figure 5. von mises stresses of a two-phase system caused by temperature increase. figure 6. comparison of the different structures regarding tensile (et 1), compression (ec1) and shear (g12) modulus. tion, structure 10 also behaves auxetic and therefore exhibits negative values for the poisson’s ratios. 5. hyper-elasticity hyper-elasticity represents a non-linear, isotropic and time-independent form of the elasticity. the relationship between an applied stress and the strain is linear for very small strains, but will reach a plateau in the stress-strain curve at a certain point [3]. the strain energy density function w is evaluated using the displacements resulting from the forcecontrolled simulations according to the law of neohookean solids, which is presented in eq.2 where c10 is a material constant and i1 is the first invariant of the stress tensor. the hyper-elastic material model (neo-hooke) was generated from experimental stress-strain-data resulted from uniaxial tensile tests of the bulk materials ninjaflex® (e = 6.5mpa, ν = 0.495) and tpu3 (e = 9.5mpa, ν = 0.495). the material constant c10 for the neo-hookean model figure 7. illustration of the auxetic behaviour of structure 5, 8 and 10. figure 8. comparison of w for the different lattice structures and for the two materials. has the value c10 = 0.78057 n mm2 for ninjaflex® and c10 = 1.56371 n mm2 for tpu3. wnh = c10 · (i1 − 3) (2) a bar chart of the w-values of the various relevant structures is given in fig.8. it can be seen that structure 10 reveals the highest values for w, which means that the applied force can be stored in the material as an energy to change the shape of the structure. therefore, this structure is the best fitting structure of all investigated rves for hyper-elastic characteristics. 6. visco-elasticity visco-elastic materials combine the characteristics of solids and liquids, and thus behave in part elastically and in part plastically. the effect of visco-elasticity is time-, temperatureand frequency-dependent. to determine the visco-elastic material behaviour two strainrates were evaluated. the simulations were performed 51 k. lehner, a. kalteis, z. major acta polytechnica ctu proceedings dynamic, implicit to allow strain-rate dependent calculations. the applied loadings were 0.5 mm s and 5 mm s for uniaxial tension. for the visco-elastic simulations experimental data from a dynamic-mechanical thermal analysis (dmta) were adapted into master curves with the help of the program viscoshift (ingenieurbuero herdy) and the prony parameters (tab.2) were calculated in the program viscodata (ingenieurbuero herdy). the directly calculated parameters ei,p rony, ki,p rony and τi,p rony are representing the young’s modulus, the compression modulus and the relaxation time, while the shear modulus gi,p rony has to be calculated by gi,p rony = ei,p rony 2 · (1 + ν) when assuming linear visco-elastic behaviour. the details of the exact implementation of the visco-elastic material behaviour into abaqus can be found in [9]. in order to be able to compare the results, these simulations were performed for structure 10 as well as for a multipurpose tensile specimen according to iso 527-1. in fig.9 it can be seen that an increased strain-rate leads to a stiffer behaviour for the investigated material independent of the structure. when comparing the two specimens the results show lower stress values for structure 10 (fig.9 (top)) than for the multi-purpose tensile specimen (fig.9 (bottom)), which represents the bulk material. this effect is traced to the low relative density of the lattice structure. 7. elasto-plasticity in real media, deformation is linearly elastic only up to a certain limit. if this limit is exceeded and the material is therefore loaded beyond its yield stress, in ductile materials plastic deformation occurs [8]. after reaching the yield point, which is the point on the stress-strain curve that indicates the limit of elastic behaviour and the beginning of plastic behaviour, the stressed body does not return to its original form when it gets unloaded. for the characterization of elastoplastic material behaviour the initial yield surfaces were determined. therefore simulations of biaxial loading conditions and coupled tension/compressionsimulations were necessary. the beginning of plasticity is assumed where a deviation of the experimental curve from linear behaviour can be detected to represent the real curve progression. when considering the physical yield point at approximately 32mpa the simulation model would be linear up to this point and would therefore not be suitable as approximation for the elasto-plastic behaviour. hence, the plastic strain and the yield stress for pp(h) (e = 1990mpa, ν = 0.42), which represent elasto-plastic material behaviour, are shown in fig.10 (yield point) and implemented in abaqus using mises-plasticity without consideration of triaxiality. in fig.11 the rves are pictured on the left side showing the locations of the beginning of yielding in red for biaxial tension in yand z-direction. the blue colour indicates elastic behaviour. on the right side the yield points for figure 9. illustration of visco-elastic behaviour of str. 10 (top) and iso-mps (bottom). the different load cases are connected with straight lines. these lines mark the border between elastic and plastic behaviour. if the stresses exceed this border, plasticity is present. additionally, the initial yield surfaces resulting from the yield criteria of von mises and tresca are plotted. the tresca criterion (eq.3) is marked with a dotted line and the von mises criterion (eq.4) is drawn as a dashed line. σt = max { 1 2 |σ1 − σ2|, 1 2 |σ2 − σ3|, 1 2 |σ3 − σ1| } (3) σ2 v = 1 2 · [(σ11 − σ22)2 + (σ22 − σ33)2 + (σ33 − σ11)2 + 6 · (σ2 23 + σ2 31 + σ2 12)] (4) the initial yielding of structure 3 (fig.11(top)) takes place in its centre and therefore possesses lower yield stresses than structure 4 (fig.11(top centre)), the centre of which is separated into more connection points. structure 8 (fig.11(bottom)) shows the highest stress values for the transition between elasticity and plasticity for biaxial loading with tension in one 52 vol. 18/2018 modelling and simulation of lattice structures ei,p rony,− gi,p rony,− ki,p rony,− τi,p rony, s 2.38 · 10−18 7.96 · 10−19 7.93 · 10−17 1.29 · 10−03 1.40 · 10−18 4.68 · 10−19 4.67 · 10−17 2.98 · 10−02 6.44 · 10−18 2.15 · 10−18 2.15 · 10−16 6.85 · 10−01 1.27 · 10−17 4.24 · 10−18 4.22 · 10−16 1.58 · 10+01 8.33 · 10−20 2.79 · 10−20 2.78 · 10−18 3.63 · 10+02 2.49 · 10−17 8.33 · 10−18 8.30 · 10−16 8.36 · 10+03 6.92 · 10−17 2.32 · 10−17 2.31 · 10−15 1.92 · 10+05 3.97 · 10−16 1.33 · 10−16 1.32 · 10−14 4.43 · 10+06 2.11 · 10−02 7.04 · 10−03 7.02 · 10−01 1.02 · 10+08 table 2. prony parameters: ninjaflex®. figure 10. experimental data for elasto-plastic material model: pp(h). direction and simultaneous compression in the other direction, whereas all non-auxetic structures have the highest yield stresses for loadings with the same sign. for structure 4 (fig.11(top centre)) the tresca yield criterion is very similar to the resulting yield surface and for structure 6 (fig.11(bottom centre)) the von mises criterion would be a good approximation. 8. conclusion performing a thermo-mechanical analysis simulations with partitioned inclusion geometries, which were applied with two different materials with various expansion behaviour, were conducted. the findings of [8] that a specific structural composition leads to a negative coefficient of linear thermal expansion could be validated for two-phase systems. regarding the parameters of the apparent lattice stiffness matrix, structure 6 was presented as the stiffest of all investigated structures in all directions using linear-elastic material behaviour. the lowest stiffness values related to tension and compression were detected for structure 3. negative values for the poisson’s ratio, which characterizes auxetic behaviour, could be observed for structure 5, structure 8 and structure 10. after performing force-controlled simulations with hyperelastic material behaviour the strain energy density function was calculated according to the law of neohookean solids. structure 10 revealed the highest values for w, which made this rve most suitable for large-strain non-linear elastic material behaviour. figure 11. initial yield surface: str. 3, 4, 6, 8. 53 k. lehner, a. kalteis, z. major acta polytechnica ctu proceedings visco-elastic simulations were performed using prony parameters of a generalized maxwell model to investigate the strain rate dependency of structure 10 compared to a multi-purpose tensile specimen. whilst an increased strain-rate led to a stiffer behaviour for the investigated material independent of the structure, structure 10 generally resulted in lower stress values than the bulk material. in order to represent the results of elasto-plastic material behaviour, initial yield surfaces were determined by applying biaxial loading conditions. in contrast to all investigated geometries, structure 2 and structure 3 exhibited the lowest yield stresses while structure 5 and structure 6 could withstand the highest stresses before yielding. all in all it was shown that the various lattice structures show different thermal and mechanical behaviour and therefore can be utilized for various purposes in different areas of application dependent on the used materials. references [1] k. drechsler, i. karb, r. kehrle, v. witzel. faserverbundstrukturen, leichtbau in natur und technik. themenheft forschung leichtbau pp. 14–21, 2007. [2] p. kelly. solid mechanics part i: an introduction to solid mechanics, chap. 6, pp. 145–153. 2013. [3] m. boyce, e. arruda. constitutive models of rubber elasticity: a review. rubber chemistry and technology 73(3):504–523, 2000. [4] m. blanter, i. golovin, h. neuhäuser, h. sinning. internal friction in metallic materials, vol. 90. springer-verlag berlin heidelberg, 2007. [5] v. d. nguyen. computational homogenization of cellular materials capturing micro-buckling, macro-localization and size effects. ph.d. thesis, universite de liege computational and multiscale mechanics of materials. 2014. [6] e. j. pitz. multiscale progressive damage modelling in continuously reinforced composite structures. master thesis, johannes kepler university linz, 2017. [7] p. a. tipler, g. mosca. physics for scientists and engineers, vol. 1. macmillan education, 2007. [8] j. aboudi, r. gilat. micromechanical analysis of lattice blocks. international journal of solids and structures 42(15):4372–4392, 2005. [9] m. machado, u. cakmak, i. kallai, z. major. thermomechanical viscoelastic analysis of woven-reinforced thermoplastic-matrix composites. composite structures 157:256–264, 2016. 54 acta polytechnica ctu proceedings 18:48–54, 2018 1 introduction 2 methodology 3 thermo-mechanical analysis 4 linear-elasticity 5 hyper-elasticity 6 visco-elasticity 7 elasto-plasticity 8 conclusion references acta polytechnica ctu proceedings doi:10.14311/app.2019.22.0072 acta polytechnica ctu proceedings 22:72–76, 2019 © czech technical university in prague, 2019 available online at http://ojs.cvut.cz/ojs/index.php/app electrical properties of fly ash geopolymer composites with graphite conductive admixtures cecílie mizerová∗, ivo kusák, pavel rovnaník brno university of technology, faculty of civil engineering, žižkova 17, 602 00 brno, czech republic ∗ corresponding author: mizerova.c@fce.vutbr.cz abstract. construction materials with increased electrical conductivity could be possibly used in health monitoring of structures (stress, deformation, damages), their maintenance or traffic monitoring. the aim of this study was the application of functional filler and its influence on the electrical properties of the alkali-activated fly ash matrix. the graphite powder was added to the reference material in the amount of 2–10%. besides the assessment of the critical amount of filler necessary to achieve a percolation threshold in the structure of the composite, the effect on the electrical properties of the matrix (resistance, capacitance, conductivity) was determined. the optimal amount of filler was also determined with respect to the changes in microstructure of the binder and its mechanical properties. keywords: fly ash, geopolymer, graphite, electrical properties, mechanical properties, microstructure. 1. introduction recently, one of the field in materials and structures research is the concept of advanced smart (or selfsensing) material. its composition design is based on the content of conductive fillers that enhance its electrical properties. for example, smart composite with higher electrical conductivity can be used for the assessment of structural health in terms of stress monitoring, localization of cracks and damages, humidity and temperature changes etc. [1]. there is a wide range of different functional fillers to use: steel fibres, carbon black, graphite powder, carbon fibres (cf), carbon nanotubes (cnf), graphene, nickel powder. so far, this approach has been studied predominantly in traditional cement concretes and composites technology. alkali-activated materials are binders derived from the reactions of raw materials with a high content of amorphous aluminosilicate phase with alkaline activator solution. alkaline activator (most often in form of alkali hydroxides and/or alkali silicates) is required for the appropriate conditions for the dissolution rate and the formation of new phases [2]. the first theoretical background of alkaline activation was developed by glukhovsky in 1959 [3]. the term “geopolymer” was firstly used by prof. davidovits and refers to a material formed by almost exclusively highly coordinated units of aluminosilicate structures forming regular polymeric three-dimensional structure [4]. compared to clay-based geopolymers, fly ash provides more favourable rheology of the fresh binder [5], strength of the hardened binder can be enhanced by thermal treatment. properties of alkali-activated binders are comparable or superior to portland cement binders [6]. fly ash geopolymers are generally more durable because of higher chemical and thermal resistance [7]. moreover, the geopolymer production using mostly secondary raw materials do not require an exploitation of natural resources and high energy consumption that significantly contribute to lower emissions of co2 compared to portland cement [8]. according to hanjitsuvan et al. [9, 10], the electrical conductivity of fly ash geopolymer matrix is affected by the naoh concentration in activator solution, the frequency spectrum and liquid activator/ash ratio (l/a). payakaniti [11] studied properties of geopolymer incorporated with cf stating the percolation threshold was reached by the geopolymer with 0.5% cf content, this value was increasing with growing l/a. this cf content was also responsible for the best mechanical performance. vaidya et al. [12] tested fly ash geopolymer concrete beams and cylinders with 0.4% cf while exposed to bending and compressive stress to assess the changes of electrical resistance. regarding the application of cnt in geopolymer concrete, saafi et al. [13] states that the optimal dosage is 0.5% which ensures the improvement of both electrical and mechanical performance, higher percentage leads to less effective dispersion and thus cluster formation within the matrix. in another study, the author used graphene as a conductive filler [14]. only 0.35% addition of graphene resulted in significant increase of strength and modulus of elasticity accompanied by lower porosity and 209% increase of conductivity. this paper is focused on the application of graphite powder as a conductive admixture in fly ash geopolymer mortars considering its impact on selected electrical properties, strength and changes in binder microstructure. 2. materials and methods the geopolymer binder was produced by mixing of fly ash (fa) with commercial sodium silicate solution with sio2/na2o = 1.6. the fly ash is a product of black coal combustion and its chemical composition is given in table 1. quartz sand with a maximum 72 http://dx.doi.org/10.14311/app.2019.22.0072 http://ojs.cvut.cz/ojs/index.php/app vol. 22/2019 electrical properties of fly ash geopolymer composites. . . sio2 al2o3 fe2o3 cao mgo stotal na2o k2o (%) 49.82 24.67 7.5 3.91 2.68 0.91 0.7 2.78 table 1. chemical composition of fly ash. ref g2 g4 g5 g6 g8 g10 fly ash (g) 350 350 350 350 350 350 350 sodium silicate (g) 280 280 280 280 280 280 280 graphite powder (g) 7 14 17.5 21 28 35 sand (g) 1050 1050 1050 1050 1050 1050 1050 2% triton x-100 (g) 7 14 17.5 21 28 35 1% lukosan (g) 3 6 7.5 7.5 7.5 7.5 water (g) 35 30 30 32 32 32 32 table 2. mix composition of fa geopolymer mortars with graphite powder. grain size of 2.5mm was used as aggregate. graphite powder pmm 11 was used as a conductive filler in 2, 4, 5, 6, 8 and 10% wt. of the fly ash and its dispersion within the matrix was supported by the addition of dispersing agent (2% triton x-100). defoaming agent (1% lukosan) was introduced to reduce the air content in fresh and hardened binder. the raw mix composition is presented in table 2. each of the mixes was produced following these steps: at first, the fly ash was mixed with water glass. then, the graphite powder and dispersing agent were added and homogenized with the binder and a small amount of water, if needed. thereafter, each fraction of aggregate (from fine to coarse) was added and mixed properly. the defoaming agent and remaining amout of water were added at the very end of the mixing procedure. the mortars were cast into prismatic moulds (40 × 40 × 160mm) and covered with a plastic sealant to avoid moisture loss. after 2 hours in the ambient conditions, the specimens were heated at 40 °c for 24 h. after demoulding, the hardened specimens were stored in the laboratory conditions (22 ± 2 °c, ϕ = 45 ± 5%) till the age of testing. the electrical and mechanical properties (compressive and flexural strength) were tested at the age of 56 days. the prepared prismatic samples were characterized by impedance spectroscopy in the range of 40 hz to 1 mhz using an agilent 33220a sinusoidal signal generator and an agilent 54645a dual-channel oscilloscope. the output voltage of the signal generator was 5.5v. the input values for the electrical capacity and the resistance of the oscilloscope were 13 pf and 1mω, respectively. in order to perform impedance analysis, the prismatic specimens were placed between parallel brass electrodes (30 × 100mm) so that a distance between electrodes was 40mm. conductivity measurements were performed in the range of 10 to 3510mhz. the microstructure and porosity of the geopolymers were evaluated using micromeritics poresizer 9310 and scanning electron microscope (sem) tescan mira3 xmu. 3. results and discussion electrical resistance of all samples decreased with higher frequency applied, as can be seen from fig. 1. the lowest resistance was observed in case of 10% graphite content (g10), at all frequencies. the differences in resistance curves are clearly visible up to 10 khz. above this frequency, the resistance of all samples fluctuate and the curves overlap. at 100hz, the resistance of reference sample was 159.80mω. the resistance of g5 dropped to 43.78mω and 0.85mω only at g10. changes of resistance and conductivity at chosen frequency are shown in fig. 2. while the resistance decreased continuously, we registered an exceptional increase in conductivity of specimen g10 from 0.38 to 6.31ms·m−1. these results suggest that geopolymer with 10% of graphite may have reached the percolation threshold. the capacitance of geopolymers with various graphite content as a function of frequency is shown in fig. 3. up to 6% of graphite, the changes in capacitance are comparable, but the capacitance of g8 and g10 shows the most notable increase over all frequencies. capacitance of all samples except g10 decreased with higher frequency. the capacitance of g10 first dropped from 222 to 122 pf (40–180hz), within the frequency 180–10 000hz it was about 125 pf and at higher frequency applied it was decreasing again. the changes of mechanical performance depending on the graphite content are displayed in fig. 4. the flexural strength was not negatively influenced by the graphite addition and fluctuated around 5mpa at all graphite concentrations. on the contrary, the compressive strength gradually decreased. while the reference sample without graphite reached 23.5mpa, the samples with 2% graphite content (g2) reached 17.7mpa and 10% graphite content (g10) 73 c. mizerová, i. kusák, p. rovnaník acta polytechnica ctu proceedings figure 1. variation in electrical resistance of fa geopolymers with different graphite content. figure 2. resistance (at 100hz) and conductivity (at 100mhz) of fa geopolymers with different graphite content. figure 3. variation in capacitance of fa geopolymers with different graphite content. 74 vol. 22/2019 electrical properties of fly ash geopolymer composites. . . figure 4. mechanical properties of fa geopolymers with different graphite content. figure 5. cumulative intruded volume of fa geopolymers with different graphite content. (a) . ref. (b) . g2. (c) . g5. (d) . g10. figure 6. sem images of ref, g2, g5 and g10 geopolymers. 75 c. mizerová, i. kusák, p. rovnaník acta polytechnica ctu proceedings only 7.8mpa, which is equal to 25% and 65% drop of strength, respectively. regarding the microstructure of the geopolymer binder, even 2% of graphite caused an increase in total porosity of the samples (fig. 5). while the geopolymers with a graphite content of 2–8% did not exhibit considerable differences, we observed a further increase in pore volume of g10 which is more than 60% higher than the reference sample. in fig. 6 we can see the nature of chosen binders in sem images. reference sample is characterized by homogeneous amorphous binder phase whereas number of pores and inhomogeneities increased with the amount of conductive filler. 4. conclusions the results mentioned above confirm that graphite powder can be applied to fly ash geopolymer matrix to enhance its electrical properties. higher graphite content resulted in reduced electrical resistance, especially in frequency up to 1 000hz. likewise, the samples with high graphite content exhibited an increase in capacitance. the best performance in terms of electrical properties was observed in case of geopolymer with 10% graphite content providing significant increase in conductivity, which may refer to achieving the percolation threshold. however, the presence of conductive filler in such concentration deteriorated the mechanical performance in compression by more than 65%; flexural strength of all samples remained constant regardless of the amount of graphite powder. despite the use of the defoaming agent, the microstructure of all geopolymers with graphite was characterized by higher pore volume which is in a good accordance with reduced compressive strength. we can conclude that graphite powder in fly ash geopolymer allows to develop a conductive network within the matrix but also causes generation of additional pores during mixing. the negative impact of graphite powder on the compressive strength is apparently attributed to a weak bond between the matrix and graphite particles because of nonpolar character of graphite surface. low toughness of graphite itself can further contribute to the strength loss at high concentrations. possible ways to reduce these impacts include more efficient defoaming or improved adhesion of graphite particles to the matrix by partial modification of its structure with hydrophilic substituents. acknowledgements this study has been financially supported by the but specific research project fast-j-18-5484 and by ministry of education, youth and sports under the ‘national sustainability programme i’ – the project no. lo1408 admas up. references [1] b. han, l. zhang, j. ou. smart and multifunctional concrete toward sustainable infrastructures, chap. self-sensing concrete, pp. 81–116. springer, singapore, 2017. doi:10.1007/978-981-10-4349-9_6. [2] c. shi, d. roy, p. krivenko. alkali-activated cements and concretes. crc press, 2003. doi:10.1201/9781482266900. [3] v. d. glukhovsky. soil silicates (gruntosilikaty). budivelnik publisher, kiev, 1959. [4] j. davidovits. properties of geopolymer cements. in first international conference on alkaline cements and concretes, pp. 131–149. 1994. [5] l. provis, j. s. van deventer. alkali activated materials. springer, 2014. doi:10.1007/978-94-007-7672-2. [6] p. duxson, a. fernández-jiménez, j. l. provis, et al. geopolymer technology: the current state of the art. journal of materials science 42(9):2917–2933, 2007. doi:10.1007/s10853-006-0637-z. [7] a. fernández-jiménez, i. garcía-lodeiro, a. palomo. durability of alkali-activated fly ash cementitious materials. journal of materials science 42(09):3055– 3065, 2007. doi:10.1007/s10853-006-0584-8. [8] k. p. mehta. reducing the environmental impact of concrete. concrete international 23(10):61–66, 2001. [9] s. hanjitsuwan, s. hunpratub, p. thongbai, et al. effects of naoh concentrations on physical and electrical properties of high calcium fly ash geopolymer paste. cement and concrete composites 45:9–14, 2014. doi:10.1016/j.cemconcomp.2013.09.012. [10] s. hanjitsuwan, p. chindaprasirt, k. pimraksa. electrical conductivity and dielectric property of fly ash geopolymer pastes. international journal of minerals, metallurgy, and materials 18(1):94–99, 2011. doi:10.1007/s12613-011-0406-0. [11] p. payakaniti, s. pinitsoontorn, p. thongbai, et al. electrical conductivity and compressive strength of carbon fiber reinforced fly ash geopolymeric composites. construction and building materials 135:164–176, 2017. doi:10.1016/j.conbuildmat.2016.12.198. [12] s. vaidya, e. n. allouche. strain sensing of carbon fiber reinforced geopolymer concrete. materials and structures 44(8):1467–1475, 2011. doi:10.1617/s11527-011-9711-3. [13] m. saafi, k. andrew, p. l. tang, et al. multifunctional properties of carbon nanotube/fly ash geopolymeric nanocomposites. construction and building materials 49:46–55, 2013. doi:10.1016/j.conbuildmat.2013.08.007. [14] m. saafi, l. tang, j. fung, et al. graphene/fly ash geopolymeric composites as self-sensing structural materials. smart materials and structures 23(6):065006, 2014. doi:10.1088/0964-1726/23/6/065006. 76 http://dx.doi.org/10.1007/978-981-10-4349-9_6 http://dx.doi.org/10.1201/9781482266900 http://dx.doi.org/10.1007/978-94-007-7672-2 http://dx.doi.org/10.1007/s10853-006-0637-z http://dx.doi.org/10.1007/s10853-006-0584-8 http://dx.doi.org/10.1016/j.cemconcomp.2013.09.012 http://dx.doi.org/10.1007/s12613-011-0406-0 http://dx.doi.org/10.1016/j.conbuildmat.2016.12.198 http://dx.doi.org/10.1617/s11527-011-9711-3 http://dx.doi.org/10.1016/j.conbuildmat.2013.08.007 http://dx.doi.org/10.1088/0964-1726/23/6/065006 acta polytechnica ctu proceedings 22:72–76, 2019 1 introduction 2 materials and methods 3 results and discussion 4 conclusions acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2018.15.0099 acta polytechnica ctu proceedings 15:99–103, 2018 © czech technical university in prague, 2018 available online at http://ojs.cvut.cz/ojs/index.php/app characterization of cement-based composite exposed to high temperatures via ultrasonic pulse method iva rozsypalová∗, michal vyhlídal, richard dvořák, tomáš majda, libor topolář, luboš pazdera, hana šimonová, zbyněk keršner brno university of technology, faculty of civil engineering, veveří 331/95, 602 00 brno, czech republic ∗ corresponding author: iva.rozsypalova@vutbr.cz abstract. in this paper, the attention is paid to the investigation of the influence of high temperature acting on specimens made from specially designed cement-based composite. the experimental programme was carried out on six sets of beam specimens with the dimensions of 20 × 40 × 200 mm. the specimens were loaded to a pre-set temperature of 100, 200, 400, 600, 800 and 1000 ◦c and then the temperature was kept for 60 minutes. when the temperature loading had been done, the specimens were left to cool down to the ambient temperature. after that, the ultrasonic pulse method was used to determine the degree of damage of temperature loaded specimens. the measured data obtained by this non-destructive method are in high correlation with values of informative compressive strength of the composite obtained after the temperature loading of specimens. keywords: fine-grained cement-based composite, glass spherical aggregate, high temperature, ultrasonic pulse method. 1. introduction cement-based composites belong to the frequently used building materials for a wide range of applications [1, 2]. structures and construction components utilizing this composite were commonly designed assuming normal service temperatures. however, high temperatures acting on cement-based composite – for example during the fire etc. – cause a wide range of physical and chemical processes, which result in changes in the structure of composite and thus affects its mechanical properties [3, 4]. according to the theory of fracture mechanics, cement-based composites, in this case fine-grained cement-based composite, show quasi-brittle behaviour. specimens made from these materials have the ability to carry the load even after the deviation from a linear branch of load–displacement diagram until the peak point and then the decrease of the loading force follows until the failure – see e.g. [5, 6]. one of the reasons for this behaviour could be the existence of the interfacial transition zone (itz). this well-known zone around aggregate particles of a few micrometers in size has the specific features. these are mainly higher porosity and calcium hydroxide contents compared to the bulk matrix [7–12]. the scope of this paper is to show the results of a pilot study which deals with the characterization of the damage degree of special cement-based composite specimens exposed to high temperatures via ultrasonic pulse method. 2. experimental programme – materials, specimens, methods three-part silicone moulds were made in plywood frame and subsequently used to produce the test specimens, see figure 1. the test specimens were manufactured from the special designed fine-grained cementbased composite. the fresh mixture was made using the spherical sodium-potassium glass aggregates with a diameter of 2 ± 0.2 mm, portland cement cem i 42.5 r (from cement plant mokrá) and water in a ratio of 3:1:0.35. the components were mixed in laboratory conditions using the standard mixer. after pouring and compaction of the fresh mixture, the moulds was sealed with a thin pe foil and placed in the stabilized laboratory conditions for 1 day, see figure 2. after demoulding the test specimens had been stored in a water bath until their testing. the experimental programme was carried out on six sets of specimens according to considered maximum temperature. always, three test specimens with the nominal dimensions of 20 × 40 × 200 mm were prepared per each testing set. a laboratory furnace classic type 5013 with 3 sides of heating coils was used to heat of test specimens. the temperature loading started at the laboratory temperature of 20 ◦c. the specimens were loaded with the temperature rate of 5 ◦c/min up to a pre-set temperature. the pre-set temperatures were 100 ◦c (reference temperature), 200 ◦c, 400 ◦c, 600 ◦c, 800 ◦c, and 1000 ◦c. the next part of the temperature–time curve was constant, i.e. the maximum temperature was kept for 60 minutes. after that, the specimens were left to cool down to the ambient temperature and had been placed in a room 99 http://dx.doi.org/10.14311/app.2018.15.0099 http://ojs.cvut.cz/ojs/index.php/app i. rozsypalová, m. vyhlídal, r. dvořák et al. acta polytechnica ctu proceedings figure 1. illustration of silicone moulds preparing. figure 2. illustration of manufacture of test specimens. with the temperature of 20 ◦c. all specimens were tested by ultrasonic pulse method before and after the temperature loading performed at the laboratory temperature. during the ultrasonic pulse method measurement, the specimens were excited by a sensor placed in the centre of side cross-section of specimen – transmitter t (see figure 3). impulses were generated by the pulse generator agilent 33220a. the acoustic emission system xedo was used to detect the waves. three piezoelectric sensors were placed on different locations of the specimen surface, see figure 3. the first sensor s1 was placed near the exciter on the perpendicular surface, the second one (s2) on the opposite surface to the transmitter near the edge and the third sensor (s3) was on the same surface as the first one but close to the opposite edge. the generated signal can be in either form of continuous noise or harmonic signals [13]. the evaluation of the signals response to the transmitted pulse in the tested object can be performed in two basic systems of analysis. in the first case, the resulting shift in relation to time is evaluated, and in the second case, the signals are converted to the frequency domain and a frequency analysis is performed. the results used in this paper are from the time domain. subsequently, all specimens were provided by an initial notch and selected specimens were tested in a three-point bending configuration. selected specimens will be tested at the centre of excellence telč, czech republic in the four-point bending configuration, and x-ray computed tomography of these specimens is also planned – specimens are bellow marked ct. analysis of results of these fracture tests are being prepared and they are not the matter of this paper. the compressive strength value was determined at the parts of specimens obtained after the three-point bending fracture tests were performed. the auxiliary loading device with two (upper and lower) square steel auxiliary platens was placed in the hydraulic testing machine. the specimens were placed between the auxiliary plates in such a way that the load was applied perpendicularly to the compaction direction, as can be seen in figure 4. the test was carried out in accordance with the principles of standards čsn en 196-1 [14] and bs 1881: part 119 [15]. increasing load was applied continuously until the failure when the loading force decrease. the maximum load applied to each specimen was recorded. the informative compressive strength value of material of the specimen was calculated by dividing the maximum load by the area of contact – nominal surface area 40 × 20 mm2. 100 vol. 15/2018 characterization of cement-based composite figure 3. illustration of the ultrasonic pulse method experimental set-up (transmitter t, sensors s1, s2, and s3). figure 4. the compression test of specimens: the auxiliary loading device with two auxiliary platen was placed in the hydraulic testing machine with selected specimens prepared for the test (left); specimen after testing. 3. results all measured data, results of the ultrasonic pulse method and informative compressive strengths for test specimens are shown in table 1. basic statistics – mean values and standard deviations – of corrected selected data, as well as coefficients of correlation are introduced in tables 2 and 3. the ratio between maximum amplitudes r21 and r31 is called signal attenuation at the constant length – numbers 1, 2 and 3 correspond to the sensors s1, s2, and s3, index “b” or “a” means measurement before or after temperature loading. the outcome of applied procedure is a proportion of ratio r of temperature loaded specimen (r21,a or r31,a) to ratio r of the non-degraded specimen (r21,b or r31,b) – labelled as r21 and r31, respectively. 4. conclusions after the exposure of cement-based composites to elevated temperatures their mechanical parameters are reduced [4]. therefore, the presented paper was focused on the evaluation of the damage degree of finegrained cement-based composite specimens exposed 101 i. rozsypalová, m. vyhlídal, r. dvořák et al. acta polytechnica ctu proceedings specimen bulk density γ temperature t ratio r21,b ratio r31,b ratio r21,a ratio r31,a ratio r21 ratio r31 compressive strength fc,i kg·m−3 ◦c – – – – – – mpa 01-100 ct 1912 100 0.0140 0.0276 0.0406 0.0545 2.900 1.975 – 02-100 1875 100 0.0190 0.0527 0.0089 0.0146 0.468 0.278 4.61 03-100 1847 100 0.0651 0.0425 0.0074 0.0113 0.114 0.266 3.58 04-100 1906 100 0.0154 0.0154 0.0098 0.0153 0.640 0.993 8.21 25 1925 100 0.0949 0.1391 0.0578 0.0916 0.609 0.659 6.03 26 1958 100 0.3462 0.4216 0.1154 0.1450 0.333 0.344 – 27 2009 100 0.2550 0.4041 0.0833 0.0816 0.327 0.202 8.44 05-200 ct 1912 200 0.0120 0.0289 0.0026 0.0111 0.217 0.384 – 06-200 1988 200 0.0156 0.0206 0.0042 0.0121 0.270 0.587 6.91 07-200 1956 200 0.0262 0.0296 0.0081 0.0180 0.309 0.607 4.68 08-200 1844 200 0.0255 0.0216 0.0072 0.0071 0.282 0.327 7.73 09-400 ct 1958 400 0.0166 0.0200 0.0037 0.0025 0.223 0.125 – 10-400 1993 400 0.2805 0.4328 0.0375 0.0609 0.134 0.141 4.42 11-400 2020 400 0.1404 0.1638 0.0166 0.0239 0.118 0.146 4.29 12-400 1975 400 0.1546 0.1421 0.0232 0.0240 0.150 0.169 5.21 13-600 ct 1951 600 0.0792 0.0748 – – – – – 14-600 1972 600 0.0799 0.0974 – – – – 1.88 15-600 1997 600 0.1051 0.1930 – – – – – 16-600 2017 600 0.1448 0.1586 – – – – 3.93 17-800 ct 1974 800 0.1554 0.1405 – – – – – 18-800 1942 800 0.1017 0.2213 – – – – – 19-800 1887 800 0.2204 0.1781 – – – – – 20-800 1899 800 0.0895 0.1810 – – – – – 21-1000 ct 1925 1000 0.0866 0.0901 1.5438 1.1155 17.82 12.38 – 22-1000 1953 1000 0.1186 0.1631 0.3696 0.4316 3.118 2.647 1.96 23-1000 1958 1000 0.1208 0.1438 0.6280 0.9794 5.200 6.809 1.67 24-1000 1987 1000 0.1259 0.1394 0.3037 0.3264 2.413 2.341 1.45 table 1. measured data, results of the ultrasonic pulse method and informative compressive strengths. temperature t bulk density γ ratio r21 ratio r31 compressive strength fc,i ◦c kg·m−3 – – mpa 100 1919 (53) 0.475 (0.148) 0.457 (0.308) 6.17 (2.15) 200 1925 (63) 0.270 (0.039) 0.476 (0.142) 6.44 (1.58) 400 1986 (26) 0.156 (0.460) 0.145 (0.018) 4.64 (0.50) 600 1984 (29) – – 2.91 (1.45) 800 1925 (40) – – – 1000 1956 (25) 3.577 (1.449) 3.933 (2.496) 1.69 (0.26) table 2. mean values (standard deviations) of selected data. to high temperatures via ultrasonic pulse method. resulting data from this method are in quite high correlation with loading temperature (coefficient of correlation was in range 0.63 to 0.92), as well as with compressive strength of composite determined after the temperature loading (coefficient of correlation was in range –0.59 to –0.91). the very high correlation is between ratios r21 and r31 (coefficient of correlation was in range 0.97 to 1.00) which means that it is sufficient placed the sensors on the surface in the case of a large structure. the strong relationship between compressive strength and temperature for this special cement-based composite with spherical glass aggregates (coefficient of correlation was in range 102 vol. 15/2018 characterization of cement-based composite γ t r21 r31 fc,i bulk density γ 1.00 temperature t 0.23 | 0.29 1.00 symm. ratio r21 –0.06 | 0.14 0.63 | 0.92 1.00 ratio r31 –0.03 | 0.12 0.70 | 0.92 0.97 | 1.00 1.00 compressive strength fc,i –0.19 | –0.63 –0.80 | –0.97 –0.66 | –0.91 –0.59 | –0.90 1.00 table 3. coefficients of correlation (dimensionless) of selected data: all values (table 1) | mean values (table 2). –0.97 to –0.80) was confirmed in this study. the ultrasonic pulse method as a non-destructive method seems to be well-suited for determining the damage degree caused by elevated temperature in case of this special cement-based fine-grained composite. list of symbols γ bulk density before temperature loading at temperatures higher than 100 ◦c fc,i informative compressive strength r21,a the ratio of the maximum amplitude from sensor s2 to the maximum amplitude from sensor s1 after temperature loading at temperatures higher than 100 ◦c r21,b the ratio of the maximum amplitude from sensor s2 to the maximum amplitude from sensor s1 before temperature loading at temperatures higher than 100 ◦c r31,a the ratio of the maximum amplitude from sensor s3 to the maximum amplitude from sensor s1 after temperature loading at temperatures higher than 100 ◦c r31,b the ratio of the maximum amplitude from sensor s3 to the maximum amplitude from sensor s1 before temperature loading at temperatures higher than 100 ◦c r21 r21 = r21,a/r21,b r31 r31 = r31,a/r31,b t temperature acknowledgements this outcome has been achieved with the financial support of the czech science foundation under project no. 16-18702s “amiri – aggregate-matrix-interface related issues in silicate-based composites”. besides the funding project, the controlled temperature loading of the test specimen was carried out under the thorough supervision of technician pavel kropáček at institute of physics, faculty of civil engineering, brno university of technology. references [1] p.-c. aïtcin, s. mindess. sustainability of concrete. modern concrete technology. spon press, new york, 2011. [2] a. m. neville. properties of concrete. pearson education limited, essex, 2011. [3] i. hager. behaviour of cement concrete at high temperature. bulletin of the polish academy of science 61(1):145–154, 2013. [4] s. lim. effects of elevated temperature exposure on cement based composite materials (doctoral dissertation), 2015. [5] b. l. karihaloo. fracture mechanics and structural concrete. concrete design and construction. longman scientific & technical, new york, 1995. [6] s. kumar, s. v. barai. concrete fracture models and applications. springer, berlin, 2011. [7] j. farran. contribution mineralogique a l’etude de l’adherence entre les constituants hydrates des ciments et les materiaux enrobes. revue des materiaux de construction (490–491,492):155–157,191–209, 1956. [8] rilem. interfacial transition zone in concrete. in rilem state-of-the-art report. e & fn spon, london, 1996. [9] rilem. second international conference on the interfacial transition zone in cementitious composites. e & fn spon, new york, 1998. [10] k. l. scrivener, a. k. crumbie, p. laugesen. the interfacial transition zone (itz) between cement paste and aggregate in concrete. interface science 12(4):411–421, 2014. [11] g. prokopski, j. halbiniak. interfacial transition zone in cementitious materials. cem concr res 30(4):579–583, 2004. [12] j. p. ollivier, j. c. maso, b. bourdette. interfacial transition zone in concrete. advn cem bas mat 2(1):30–38, 1995. [13] n. j. carino. the impact-echo method: an overview. in 2001 structures congress & exposition. 2001. doi:10.1061/40558(2001)15. [14] čsn en 196-1. 2016 methods of testing cement part 1: determination of strength. [15] b. 1881. testing concrete part 119 method for determination of compressive strength using portions of beams broken in flexure (equivalent cubemethod), 1983. 103 http://dx.doi.org/10.1061/40558(2001)15 acta polytechnica ctu proceedings 15:99–103, 2018 1 introduction 2 experimental programme – materials, specimens, methods 3 results 4 conclusions list of symbols acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2018.18.0024 acta polytechnica ctu proceedings 18:24–27, 2018 © czech technical university in prague, 2018 available online at http://ojs.cvut.cz/ojs/index.php/app influence of printing and loading direction on mechanical response in 3d printed models of human trabecular bone tomáš doktora,∗, ivana kumpováa, sebastian wrońskib, maciej śniechowskib, jacek tarasiukb, giancarlo fortec, daniel kytýřa a intitute of theoretical and applied mechanics of the cas, prosecká 76, 19000 prague 9, czech republic b agh university of science and technology, aleja adama mickiewicza 30, 30-059 kraków, poland c international clinical research center of st. anne’s university hospital brno, pekařská 53, 656 91 brno, czech republic ∗ corresponding author: doktor@itam.cas.cz abstract. the paper deals with investigation on directional variations of mechanical response in 3d printed models of human trabecular bone. sample of trabecular bone tissue was resected from human donor and 3d model was obtained by x-ray computed tomography. then a series of cubical samples was prepared by additive manufacturing technique and tested by uniaxial compression loading mode. mechanical response was compared in nine different combinations of direction of 3d printing and loading direction. the results show neglectible influence on the deformation response in elastic region (stiffness) and significant changes of the behaviour in plastic region (stress and strain at yield point, strain at full collapse). keywords: additive manufacturing, tomography, deformation response. 1. introduction in the regenerative medicine bone scaffolds serve to support the bone healing process [1, 2]. additive manufacturing provides possibilities for manufacturing of the scaffold structure with tailored properties. moreover, mechanical response and topological properties of additively manufactured stuctures may be investigated and optimised using experimental modeling [3, 4]. however, direction of the 3d printing related to the loading direction may influence the effective mechanical properties and therefore affect the reliability of such analyses. in order to investigate the influence of the printing direction a series of uniaxial loading tests was performed on additively manufactured models of the human trabecular bone. 2. materials and methods 2.1. sample preparation trabecular bone tissue was extracted from a cadaver human donor (male, 52 years, non pathological conditions). the extraction was performed using hollow drill using a low-speed drilling machine in direction coaxial with column femoris. diameter and length of the resected sample were 18 mm and 40 mm, respectively. the sample is depicted in figure 1 the resected tissue was then cleaned in ultrasonic cleaner using detergent solution in order to remove bone marrow. figure 1. trabecular bone sample resected from human caput femoris. scale bar 20 mm. 2.2. tomographic scanning and reconstruction the extracted sample was then tomographically scanned using toratom (unique patented tomographic facility placed in centre of excelence in telc, itam, czech republic) [5]. for the irradiation x-ray tube xwt 160 tchr (x-ray worx, gmbh, germany) was used with tube voltage 90 kv and target current 76µa. for the acquisition flat panel detector xrd 1622 ap 14 (perkin elmer, usa) was used. resolution of the detector was 2048 × 2048 px. the tomographic setup is depicted in figure 2. to achieve sufficient magnification, the following geometry of the tomographic setup was used: focus-detector distance was 1200 mm and focus-sample distance was 60 mm, which resulted in magnification 20 ×. duration time of the acquisition was 2000 ms per projection and resulting pixel size was 10µm. 24 http://dx.doi.org/10.14311/app.2018.18.0024 http://ojs.cvut.cz/ojs/index.php/app vol. 18/2018 influence of printing and loading direction in 3d printed models figure 2. toratom scanning setup. acquired tomographic projections were then recontructed using vg studio software tool (volume graphics, gmbh, germany). from the reconstructed matrix a cubical region of interest (roi) with size 1000 × 1000 × 1000 px3 was selected for 3d print. visualisation of the roi is depicted in figure 3. figure 3. selected region of interest in the reconstructed 3d matrix. scale bar 10 mm. 2.3. additive manufacturing subsequently, nine samples were manufactured using 3d printer, three in each group printed along different axes. the samples were prepared at additive manufacturing facility placed at agh university krakow. custom modified device b9creator (b9creations, llc, usa) was used together with base material vorex resin (madesolid, usa) with adition of photoabsorber sudan i (sigma aldrich, usa). cubic samples with edge length 10 mm were prepared (depicted in figure 4). resolution in the printed plane corresponded to pixel size 30×30µm and the layer thickness was 30µm as well. as typical thickness of human trabecula in the selected location varies in range 120 to 160µm [6] at least four voxels across the trabeculae thickness were present in the printed model. hence the 3d printer resolution was sufficient to preserve all features of the trabecular structure without scaling of the model. figure 4. additively manufactured trabecular bone structure. scale bar 10 mm. 2.4. uniaxial compression tests the additively manufactured samples were then subjected to uniaxial compression using a custom loading frame [7]. loading capacity od the loading frame was 2 kn. loading was provided by a stepper motor attached to a harmonic drive and controlled by an inhouse control software based on linuxcnc interface [8]. the setup was instrumented by force transducer (type u9b, hbm, germany) with loading capacity 1 kn. the loading setup is depicted in figure 5. displacement controlled compression tests with loading rate 30µm · s−1 were performed. final displacement corresponding to overall strain 0.55 was prescribed (entire available travel range of the loading frame was used to ensure description of both elastic region and plastic collapse of the cellular structure). however in several tests the loading was stopped at lower displacement due to the force drop which indicated complete failure of the structure. 2.5. strain measurement loading scene was captured by monochromatic ccd camera avt manta g504b (alied vision gmbh, germany) attached to a bi-telecentric lens tzcr072 (optoengineering, italy) at resolution 2452 × 2056 px at 2 fps. for camera read out in-house acquisition software [9] based on opencv library [10] was used. 25 t. doktor, i. kumpová, s. wroński et al. acta polytechnica ctu proceedings figure 5. custom loading setup for uniaxial compression tests. displacement of the loading supports was tracked by custom dic software tool [11] based on lucas-kanade trancking algorithm [12]. the overall strain was measured optically based on projections of the loading scene using digital image correlation algorithm (dic). due to the nearly transparent nature of base material of the samples the displacement tracking was not successfull at the specimens’ surface and dic was used for tracking of the loading supports only. 3. results from the calculated displacements and force records stress-strain diagrams were constructed and used for comparison among the tested groups. the obtained loading curves are depicted in figures 6, 7 and 8, where the samples were groupped with respect to the loading direction in order to provide comparison between different printing directions. 0 0.2 0.4 0.6 0.8 1 0 0.1 0.2 0.3 0.4 0.5 0 20 40 60 80 100 0 1 2 3 4 5 e n g . s tr e s s [ m p a ] l o a d in g f o rc e [ n ] eng. strain [-] displacement [mm] printed along x axis printed along y axis printed along z axis figure 6. loading curves of samples loaded along x direction. 0 0.2 0.4 0.6 0.8 1 0 0.1 0.2 0.3 0.4 0.5 0 20 40 60 80 100 0 1 2 3 4 5 e n g . s tr e s s [ m p a ] l o a d in g f o rc e [ n ] eng. strain [-] displacement [mm] printed along x axis printed along y axis printed along z axis figure 7. loading curves of samples loaded along y direction. 0 0.2 0.4 0.6 0.8 1 0 0.1 0.2 0.3 0.4 0.5 0 20 40 60 80 100 0 1 2 3 4 5 e n g . s tr e s s [ m p a ] l o a d in g f o rc e [ n ] eng. strain [-] displacement [mm] printed along x axis printed along y axis printed along z axis figure 8. loading curves of samples loaded along z direction. 26 vol. 18/2018 influence of printing and loading direction in 3d printed models 4. conclusions based on the uniaxial loading tests of different sets of 3d printed specimen of human trabecular bone influence of printing and loading direction on deformation behaviour was studied. compared to previous studies on deformation response of trabecular tissue (human femur [13] or rat vertebra [14]) the additively manufactured models exhibit similar behaviour up to the first plastic collapse in terms of continuous linear elastic region with smooth yield region. the mechanical response was compared in terms of effective young’s modulus, yield stress and strain and peak force at failure. stress-strain diagrams of the 3d printed models of trabecular bone were consistent in the groups with each loading direction in terms of stiffness (slope of the linear response). highest peak force was observed in samples loaded along the direction of 3d printing in all three groups. in several samples full collapse of the cellular structure occurred when the loading axis was perpendicular to the printing direction. acknowledgements the research has been supported by the european regional development fund in frame of the projects kompetenzzentrum mechanobiologie (atcz133) in the interreg v-a austria czech republic programme and by the institutional support of rvo: 68378297. this work was partially supported by the faculty of physics and applied computer science agh ust statutory tasks within subsidy of ministry of science and higher education. references [1] y. azizeh-mitra, h. m. enamul, p. r. g. s. v., u. nicholas. current strategies in multiphasic scaffold design for osteochondral tissue engineering: a review. journal of biomedical materials research part a 103(7):2460–2481, 2015. doi:10.1002/jbm.a.35356. [2] a. r. vaccaro, k. chiba, j. g. heller, et al. bone grafting alternatives in spinal surgery. the spine journal 2(3):206–215, 2002. doi:https://doi.org/10.1016/s1529-9430(02)00180-8. [3] p. koudelka, t. doktor, d. kytyr, et al. micromechanical properties of biocompatible materials for bone tissue engineering produced by direct 3d printing. key engineering materials 662:138–141, 2015. doi:10.4028/www.scientific.net/kem.662.138. [4] d. kytýř, t. doktor, o. jiroušek, et al. deformation behaviour of a natural-shaped bone scaffold. materiali in tehnologije 50(3):301–305, 2016. doi:10.17222/mit.2014.190. [5] i. kumpova, d. vavrik, t. fila, et al. high resolution micro-ct of low attenuating organic materials using large area photon-counting detector. journal of instrumentation 11(2), 2016. doi:10.1088/1748-0221/11/02/c02003. [6] a. nazarian, j. muller, d. zurakowski, et al. densitometric, morphometric and mechanical distributions in the human proximal femur. journal of biomechanics 40(11):2573–2579, 2007. doi:doi.org/10.1016/j.jbiomech.2006.11.022. [7] p. zlamal, o. jirousek, d. vavrik. a novel compression/tension device for investigation of trabecular bone failure using real-time micro-ct imaging. in 7th youth symposium on experimental solid mechanics, pp. 91–91. 2008. [8] v. rada, t. fila, p. zlamal, et al. multi-channel control system for in-situ laboratory loading devices. acta polytechnica ctu proceedings 18:15–19 2018. doi:10.14311/app.2018.18.0015. [9] t. doktor, d. kytyr, p. zlamal, et al. simulation of a three-point bending test on the isolated cell wall of aluminium foam. civil-comp proceedings 102, 2013. [10] bradski, gary. the opencv library, 2000. http://www.drdobbs.com/open-source/ the-opencv-library/184404319. [11] i. jandejsek, j. valach, d. vavrik. optimization and calibration of digital image correlation method. in proceedings of the 48th international scientific conference on experimental stress analysis, pp. 121–126. 2010. [12] b. d. lucas, t. kanade. iterative image registration technique with an application to stereo vision. vol. 2, pp. 674–679. 1981. [13] p. zlamal, o. jirousek, t. doktor, et al. compressive behaviour of trabecular tissue: finite element modelling and comparison using digital volume correlation. civil-comp proceedings 102, 2013. [14] t. fila, d. kytyr, p. zlamal, et al. high-resolution time-lapse tomography of rat vertebrae during compressive loading: deformation response analysis. journal of instrumentation 9(5), 2014. doi:10.1088/1748-0221/9/05/c05054. 27 http://dx.doi.org/10.1002/jbm.a.35356 http://dx.doi.org/https://doi.org/10.1016/s1529-9430(02)00180-8 http://dx.doi.org/10.4028/www.scientific.net/kem.662.138 http://dx.doi.org/10.17222/mit.2014.190 http://dx.doi.org/10.1088/1748-0221/11/02/c02003 http://dx.doi.org/doi.org/10.1016/j.jbiomech.2006.11.022 http://dx.doi.org/10.14311/app.2018.18.0015 http://www.drdobbs.com/open-source/the-opencv-library/184404319 http://www.drdobbs.com/open-source/the-opencv-library/184404319 http://dx.doi.org/10.1088/1748-0221/9/05/c05054 acta polytechnica ctu proceedings 18:24–27, 2018 1 introduction 2 materials and methods 2.1 sample preparation 2.2 tomographic scanning and reconstruction 2.3 additive manufacturing 2.4 uniaxial compression tests 2.5 strain measurement 3 results 4 conclusions acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2018.15.0094 acta polytechnica ctu proceedings 15:94–98, 2018 © czech technical university in prague, 2018 available online at http://ojs.cvut.cz/ojs/index.php/app microscopic and phase analysis of cement paste containing waste micronized marble powder zdeněk prošeka,b,∗, pavel tesáreka, jan trejbala a czech technical university in prague, faculty of civil engineering, thakurova 7, 166 29 prague 6, czech republic b czech technical university in prague, uceeb, trinecka 1024, 273 43 bustehrad, czech republic ∗ corresponding author: zdenek.prosek@fsv.cvut.cz abstract. the article focuses on the impact of waste marble powder on resulting cement composites. we investigate of influence waste marble powder on the hydration process and hence the resulting phase composition. the hydration process was investigated using calorimetry and phase composition of the resulting composite by electron microscopy. the results are compared with a reference sample composed of portland cement. keywords: micronized marble powder, marble sludge, cement composites, calorimetry, elemental analysis. 1. introduction the great demand for marble in export markets has led to increased his production. an important factor is the amount of mined stone, regardless of the quantity of waste. production lines for mining and stone processing produce two types of waste. the first one is in the form of crushed stone that arises by removing marble blocks from the line due to their bad shape or when the blocks are broken or cracked. the production of this waste is great, but its application is so effective in the form of aggregates and gravel to the concrete that it does not create any ecological risks. the second form of waste is marble sludge and dust, which occurs during the mining, crushing and cutting of marble blocks or during the final grinding and polishing process. the amount of waste produced is estimated, for example, for the egypt region at one million tonnes per year [1]. the amount of waste produced is not only associated with less advanced countries such as the middle east but also with europe, where, for example, in portugal, the amount of waste generated is estimated at 600 thousand tonnes per year [2]. an approximately 10 to 20% of the waste is further processed and used [3]. the most common use is in road infrastructure, as asphalt filler or underlayer [4]. the remaining unused waste is stored in the vicinity of the factories, regardless of their possible use, thus causing serious environmental problems. for these reasons, it is important to create an application of these wastes, which would result in improved safety and health protection by lowering dust particles in the air, lowering the cost of storage, transport, and waste handling, and generating potential revenue [5]. one option is to use waste as a partial substitute for portland cement, where recycling material could have a positive effect on resulting properties of cement composites. the resulting properties of the cement matrix are directly dependent on the composition. the resulting cement matrix is composed of a plurality of phases. the largest proportion of the phases has the c-s-h gel (hydrosilicate), which is furthermore formed by tobermorite, afwillite, xonotlite and other still unknown phases. furthermore, hydroaluminate, gypsum, ettringite, ferrite hydrate and portlandite (ch crystals) appear in the cement matrix [6]. they are several works that solve the possibility of replacing cement with marble sludge, and the result is two contradictory findings. the first statement is described in a study by demirel [7] at the firat technical university in turkey. he found using an electron beam microscope that when the marble sludge was added to the cement composite, the ch crystal size was changed. mixtures without marble sludge had a form of large ch crystals. while in a mixture with marble sludge had very small ch crystals and it was produced across all the matrix. this effect is explained by the fact that fine sludge grains act in the cement composite as nucleation centers for the growth of ch crystals and accelerate the hydration of silicate and aluminate phases [8]. the second claim is described in the work of aliabdo et al. [9]. they examined the morphology of the internal structures of the cement slurry with and without marble sludge by using a scanning electron microscope and they found that, is not distinctly different from each other, meaning that the marble sludge has no noticeable role during the hydration process and therefore only acts as an inert filler in the new product. with the development of the technique, new possibilities arise to confirm or rebut possible claims about the inclusion of marble sludge during hydration. one possibility is the treatment of marble sludge using high-speed milling [10], where the resulting micronized marble powder has a higher density and therefore a higher reactivity [11]. this recycling method has already been successfully used 94 http://dx.doi.org/10.14311/app.2018.15.0094 http://ojs.cvut.cz/ojs/index.php/app vol. 15/2018 microscopic and phase analysis ... figure 1. the image of micronized marble powder, magnified 100×, bse detector. for concrete recycling [12]. 2. materials and samples portland cement cem i 42.5 r was chosen as reference material from českomoravský beton, a.s. with a factory in radotín. it is a cement used for high strength concrete and mortars and plasters. the main ingredient is portland clinker with a proportion of 95 to 100 wt. %. the remaining amount of weight is the complementary components. the marble sludge that was created during the cutting and polishing of limestone marble was chosen for investigating the possibility of replacing cement with micronized mechanically activated marble powder. the marble sludge comes from an industrial area located on the west bank. the marble sludge was lumpy and contained big grains. the marble sludge was micronized on high-speed mills in collaboration with lavaris for improved workability, improved marble sludge properties, and increased surface area. the high-speed mill type sbd 600 with a power of 2×30kw with patented cutting edges was used for this purpose. two grinding modes and two types of grinding elements were used. in the first case, it was used element in the form of patented 400 mm diameter pins, and in the second case patented teeth with a diameter of 150 mm. in both cases, the grinding speed was set to 300 m/s. the result of high-speed grinding was micronized marble powder. two sets of samples were created for research. the first set was used for calorimetry testing, consisting of cement pastes with varying amounts of micronized marble powder. mixture m0 served as a reference and contained only cement. mixtures m5 to m50 contained portland cement and micronized marble powder in 5, 10, 15 and 50 wt. % (table 1). the water ratio, i.e., the weight of water to the total dry mass mixture, was 0.45. the second set was used for electron microscopy testing. two microscopic samples were created for the examination of the materials at the nano/micro level. the first sample was made by mixing micronized marble with epoxy resin and served to examine the size and shape of the grains and to analyze the recyclate itself. the second sample was made of a hardened cement paste which was made up of 50 wt. % portland cement and 50 wt. % recycled micronized marble powder. it was sample m50 from calorimetric measurements. before creating the cuts, it was necessary to fill the pore space with a suitable substance so as not to release the grains of materials during the preparation of the samples. the vacuum impregnation method for epoxy resin samples (epofix kit) was used for this purpose. the polished sections were prepared on struers’ tegramin machine. the samples were ground and polished in several steps to achieve the best surface quality of the samples. a different preparation procedure was performed for each sample due to different material. the first sample, the recycled micronized marble powder is homogeneous and does not react with water, so it is possible to use silica paper and water-based lubricant. in the first step, silica paper with a grain size of 220 grains/cm2 was used to remove the greatest unevenness after cutting. in the following steps, finer silica paper was used: 500 grains/cm2, 1200 grains/cm2, 2000 grains/cm2 and 4000 grains/cm2. each step lasted 2 minutes, and the pressure on the sample was 5 n. in the last step the samples were polished by hand using a 1/4 microns emulsion containing the nanodiamonds for 1 minute. the second sample is made up of several phases, so we used diamond plates that do not affect this inhomogeneity and does not occur relief in the grinding and polishing process. also, the cement reacts with water, so an alcohol-based lubricant has been used. md-piano 500 was used in the first step (corresponding 500 grain / cm2 silica paper roughness), and the next steps were md-piano 1200, md-piano 2000 and md-piano 4000. the grinding and polishing time was several hours because polishing and grinding were performed without pressure. finally, both samples were sputtered with a carbon layer at 30 nm to improve the required surface conductivity of the sample and improve the quality of the analysis. 3. experimental methods and results the development of hydration heat was measured to measure the effect of micronized marble powder on the hydration process of portland cement. measurement of the development of the hydration heat was performed on a tam air calorimeter. tam air is an isothermal calorimeter for accurately determining the heat flow (production and consumption) and the amount of heat produced. a total of 10 chambers were used for the measurements (two chambers for each mixture). the mixtures were tested for 7-day measurement of the development of heat flow at a constant temperature of 20 ◦c. the mixtures were stored in plastic closable containers which were described, weighed and eventually compacted before insertion into the calorimeter. each container contained approximately 25-38 g of the mixture. the output from the calorimetric measurement was the heat flow in w. 95 z. prošek, p. tesárek, j. trejbal acta polytechnica ctu proceedings mixtures replacing cement cement (cem i 42.5 r) [g] micronized marble powder [g] water [g] m0 0 wt. % 80 0 36 m5 5 wt. % 76 4 36 m10 10 wt. % 72 8 36 m15 15 wt. % 68 12 36 m50 50 wt. % 40 40 36 table 1. composition of the individual set/mixtures. element weight wt. [%] wt. standard deviation [%] atomic [%] c 16.93 0.33 30.06 o 32.29 0.50 43.04 mg 0.32 0.10 0.04 ca 50.47 0.41 26.86 total 100.00 100.00 table 2. evaluation of elemental analysis. figure 2. curve of heat evolution for tested materials. the results from the calorimetric measurement were converted to 1 g of cement based on the masses of the mixtures in the container. the feg sem merlin zeiss scanning electron microscope was used for microscopic structural and elemental analysis, which is located in the laboratory of electron microscopy and microanalysis at the university center of energy efficient buildings. qualitative and quantitative analysis of the chemical composition of the samples was performed using an x-ray microanalysis, namely an energy spectrometer (eds) by oxford instruments. the point id method was used in the first sample, where only selected areas (areas with marble limestone) were tested. the second sample was examined using line scans. the method best describes the possible effect of micronized marble powder on the final product. the microscope setting was such that the accelerating voltage was 10 kv, the current 1 na, the working table distance 8.5 mm, the measuring time of one point 50 µs and the resolution 1024 px. the composition of the recycled micronized marble powder can be determined from the results from the microscopic elemental analysis. figure 1. shows the areas under investigation, and the results from the microscopic elemental analysis are presented in table 2. the results are slightly distorted due to the need for sputtering the samples with a 30 nm carbon layer. it can be stated that about 99% of the waste material is made up of caco3 (calcite) and the remaining 1% is of minor constituents, the most important of which is mgco3. from the pictures taken during the microscopic analysis, it is clear that the grains consist 96 vol. 15/2018 microscopic and phase analysis ... figure 3. the image of cement paste containing micronized marble powder, magnified 500×. figure 4. concentration of calcium and silicon in line scan. of one phase, they have a sharp and oblong shape. the calorimetric measurement (figure 2.) shows the effect of micronized marble powder on the developing of hydration heat of the mixture. thanks to the conversion of the heat flow to 1 g of cement, the differences in the hydration process were highlighted. for mixtures with micronized marble powder, it can be observed that after 20 hours of mixing, there was a slight increase in the heat flow, probably due to the hydration of c3s. the difference in heat flow is apparent up to approximately three days (m50). further, the course of heat flow is similar, i.e., micronized marble powder has no significant effect on c2s hydration. this effect can be caused either by marble powder or the addition of finely ground particles. for this reason, we have proceeded to microscopic elemental analysis. figure 3 shows the examined area for which four line scans were created. a backscatter electron detector was used to display the phase contrast information. the result was similar for each scan, so one was chosen as representative, on which the results were described (figure 3). figure 4 shows the weight distribution of calcium and silicon and also their ratio (ca / si). emphasis was placed on these elements because cem i 42.5 r radotin has a high content of c3s (tricalcium silicate) and c2s (dicalcium silicate) and low content of c3a (tricalcium aluminate) and c4af (tetracalciumaluminoferite), i.e., the resulting product is mainly composed of these elements. at the point of occurrence of the c-s-h gel, that is, the phases resulting from the hydration and hydrolysis of c3s and c2s, the amount of calcium and silicon affect the resulting phases, where the greatest proportion of calcium has xonotlite and the smallest afwillit. overall, the identification and precise composition and structure of hydro silicates (hydro silicate gels) is difficult and therefore only the ca/si ratio is indicated. the resulting c-s-h gel can be chemically described 97 z. prošek, p. tesárek, j. trejbal acta polytechnica ctu proceedings as follows: xcao · ysio2 · zh2o, (1) where the high density c-s-h gel is in range: x = 0.5 − 1.5, y = 1, z = 0.5 − 2.5, and low density c-s-h gel x = 1.5 − 2, y = 1, z = 1 − 4. if we look at the weight of calcium (figure 4), we see that there is a greater concentration of calcium at the site of the grain. a large amount of calcium in the grain is due to its composition because it is mostly composed of calcium, i.e., caco3. ca/si ratio is increased up to a distance of about 50 microns from the grain, mainly due to the reduction in the amount of silicon, followed by a further 30 microns of gradual normalization of the values. from the results, it can be stated that another phase is preferred in the vicinity of the grains. this phase could be a low-density c-s-h gel. 4. conclusions this work focuses on the effect of micronized marble powder as a partial substitute for portland cement. the resulting cement pastes contained portland cement and micronized marble powder in 5, 10, 15 and 50 wt. %. based on the results, it can be concluded that: • the high-speed milling process generates grains that are angular and oblong. • calorimetry results showed that after 20 hours of mixing, there was a slight increase in heat flow caused by c3s hydration, the heat flow was the same, so the marble powder did not have a significant effect on the hydration of c2s. • from the results of the elemental analysis, it can be stated that the creation of one phase is preferred in the vicinity of the grains. this effect is probably because around the grain of marble is less free water needed to hydration of the cement. this phase could be a low-density c-s-h gel. the results of the work are in contradiction with the statement of demirel [7] and aliabdo [9], because marble powder during the hydration process is not completely inert, and smaller ch crystals do not appear in the surroundings of grains. in the future, the research will focus on confirming the theory of creating a low-density c-s-h gel around the grain of marble. for this reason, further work will be devoted to micromechanics around marble grains using nanoindentation. acknowledgements this outcome was supported by the czech technical university in prague under no. sgs16/201/ohk1/3t/11, by the czech science foundation research projects gacr 17-06771s and by the ministry of education, youth and sports within national sustainability programme i, project no. lo1605. the authors also thank the center for nanotechnology in civil engineering at the faculty of civil engineering, czech technical university in prague. references [1] a. mashaly, b. shalaby, m. el-hefnawi. characterization of the marble sludge of the shaq el thoaban industrial zone, egypt and its compatibility for various recycling applications. australian journal of basic and applied sciences 6(3):153–161, 2012. [2] l. lopes, r. martins. marbles from portugal. naturstein, online publication, naturalstone-online com 2012. [3] m. karaşahin, s. terzi. evaluation of marble waste dust in the mixture of asphaltic concrete. construction and building materials 21(3):616–620, 2007. [4] p. mehta, v. k. mehta. waste generation and minimization: a study of marble mines of rajsmand. international journal of informative & futuristic research 2(09):3049–3058, 2015. [5] m. el-gammal, m. ibrahim, e. badr, et al. health risk assessment of marble dust at marble workshops. nature and science 9(11):144–154, 2011. [6] t. c. powers. structure and physical properties of hardened portland cement paste. journal of the american ceramic society 41(1):1–6, 1958. [7] b. demirel. the effect of the using waste marble dust as fine sand on the mechanical properties of the concrete. international journal of physical sciences 5(9):1372–1380, 2010. [8] r. hela. příměsi do betonu / concrete admixtures . [9] a. a. aliabdo, a. e. m. a. elmoaty, e. m. auda. re-use of waste marble dust in the production of cement and concrete. construction and building materials 50:28–41, 2014. [10] z. prošek, a. osvald, j. topič, et al. influence of recycled concrete composition on its elastic stiffness. in key engineering materials, vol. 677, pp. 288–291. trans tech publ, 2016. [11] z. prošek, j. trejbal, j. topič, et al. utilization of the waste from the marble industry for application in transport infrastructure: mechanical properties of cement pastes. in iop conference series: materials science and engineering, vol. 236, p. 012092. iop publishing, 2017. [12] j. topič, z. prošek. properties and microstructure of cement paste including recycled concrete powder. acta polytechnica 57(1):49–57, 2017. 98 acta polytechnica ctu proceedings 15:94–98, 2018 1 introduction 2 materials and samples 3 experimental methods and results 4 conclusions acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2019.22.0088 acta polytechnica ctu proceedings 22:88–93, 2019 © czech technical university in prague, 2019 available online at http://ojs.cvut.cz/ojs/index.php/app recycling of construction waste using high-speed milling process: determination of waste concrete zdeněk prošeka,b,∗, pavel tesáreka, jan trejbala, tereza horováa a czech technical university in prague, faculty of civil engineering, thákurova 7, 166 29 prague, czech republic b czech technical university in prague, university centre for energy efficient buildings, třinecká 1024, 273 43 buštěhrad, czech republic ∗ corresponding author: zdenek.prosek@fsv.cvut.cz abstract. this paper deals with the use of high-speed milling process for recycling old concrete and direct determination of the potential of input waste. for this purpose, three different types of waste concrete were used: prefabricated railway sleeper, structural concrete of monolithic pillar and prefabricated drainage gutter. the paper directly examines the chemical and phase composition by xrf, x-ray diffraction (xrd), energy dispersive x-ray spectrometry (eds) and microscopic analysis, particle size distribution and ph of the recycled material. results of those analysis are used to select suitable recycled material. the suitability of choice is supported by mechanical tests of 28-day old cement pastes, where the compressive strength and dynamic modulus of elasticity are observed properties. specimens measuring 40 × 40 × 160mm are composed of 70 wt.% portland cement and rest is micronized concrete. in all cases, the results are compared with the reference material. keywords: recycling, concrete waste, cement composites, xrd, xrf, mechanical properties. 1. introduction construction produces the largest amount of waste from all sectors, for example in the eu, construction waste is almost 33% of the 2.5 billion tons of waste generated [1]. there is an effort to recycle this waste and reduce its storage on dumps according to the eu directive. nowadays, ca. 90% of waste is recycled and the remaining 10% is stored at the dumps [2]. concrete is the most used construction material and therefore sources of concrete recyclate are relatively large and easily accessible. however, due to different properties of old concrete, the production and consequently the quality of the recyclate differ greatly. the most important aspects that affect the final quality of recyclate are: the type of use of old concrete and thus the recipe (the quality of cement and aggregate), the age of concrete and the method of care (prefabricate, monolith). there are two main areas in the construction sector, from which the concrete recyclate is obtained, the road construction and the civil engineering. the highest level of recycling is in the field of road construction because the existing road construction can be processed directly on site using mobile recycling units and reused in the construction of a new one [3, 4]. in other cases, the old concrete is taken to a landfill or recycling facility. materials from recycling lines are most often used as coarse or fine aggregates in new concrete or in stabilizing layers [5, 6]. in this way, only coarse fractions are processed. in the case of a very fine fraction (grain size below 1mm), there is still no ideal solution to allow its use on a wider scale. several possible directions of application of this material are examined in cement composites [7–10] for example as thermally activated binder replacement [11], as mixtures for cement production [12] or as a part of geopolymers based binders [13]. due to high demands on energy consumption and co2 production the most sophisticated and environmentally friendly method is when very fine fraction of recycled concrete is used in its raw form. for this reason, the paper deals with the possibility of mechanical activation of concrete recyclate with high-speed milling. thus treated concrete may be used without other energy-intensive treatment or without auxiliary additives. mechanical activation of the recyclate occurs when hydrated cement paste is grinded and this reveal still unhydrated grains of clinker. this is primarily c2s which reacts in the long run. the amount of unhydrated clinker is directly dependent on the type of cement and the age of the concrete. in general, the amount in the cement matrix can be estimated between 10 to 20%, that 10 – 20wt.% of recyclate can form a binder in the future composite and the remaining amount will form a filler. recyclate not only reduces the amount of cement but also the amount of natural filler in future concrete [14, 15]. 2. materials and samples the research investigated three old types of concrete that were recycled to finely ground concrete powder using high-speed milling. the first finely ground concrete recyclate was obtained from the pb2 and the sb8 railway sleepers. these sleepers had been roughly crushed first and then metal components and 88 http://dx.doi.org/10.14311/app.2019.22.0088 http://ojs.cvut.cz/ojs/index.php/app vol. 22/2019 recycling of construction waste. . . reinforcement were separated. in this way, a coarse concrete recyclate, fraction 0 – 32mm, was obtained from which the fraction 0 – 16mm was separated and then milled. the second finely ground recyclate was obtained from concrete drainage gutter which did not contain reinforcement and therefore the individual damaged fittings were crushed and milled. just as in the case of sleepers, this is a prefabricated product with the difference that the concrete drainage gutter is considerably younger than sleepers that are from 10 to 50 years old and are made of aggregate and cement with worse properties. the last (third) material is finely ground concrete column. these columns are from monolithic structure, where waste was generated during the reconstruction of the existing hall of the canceled factory walter motors which was founded by josef walter in 1911. the recycling process was dealt with in this material and was therefore divided into two investigated materials. the recycling process was optimized and large amount of the original filler was removed for the column a, because high-speed milling was applied only to the fraction 0 – 1mm that originated after crushing coarse fraction of recyclate on small-volume wedge mill. the second column was the column b, which was also made of concrete recyclate fraction 0 – 16mm. these input materials were also investigated with reference portland cement cem i 42.5r from radotín area. from these materials the samples were additionally made for testing micromechanical properties. tested materials contained 70 wt.% of portland cement cem i 42.5r and 30 wt.% of finely ground concrete recyclate. water ratio was equal to 0.35. for each recyclate a kit was created which contained five specimens with dimensions 40 × 40 × 160mm. the second day after production, the specimens were demoulded and kept in 100% humidity, the temperature was 22 ± 1 °c during 28 days. 3. experimental methods to select the suitability of individual recyclates, a set of experimental methods were selected to describe the suitability of the material for its subsequent use. these methods include x-ray fluorescence analysis (xrf) for detecting the amount of inappropriate (foreign) substances, ph measurements for finding alkalinity of recyclates, granulometry for finding the effect of high-speed milling, xrd for determination of amount of clinker materials and electron microscopy. the suitability of selection was supported by mechanical tests of 28 days old cement pastes. x-ray fluorescence analysis (xrf) is a method based on secondary x-ray radiation and its output is a line spectrum in which the number of pulses per second is displayed for the respective wavelengths or respective energies. from this spectrum the weighting of individual elements is then calculated. elemental analysis by xrf spectrometer was performed on a spectro xepos (spectro cs s. r.) instrument. the ph was determined from aqueous extracts (in a weight ratio of 1:100) after 24 hours at room temperature (25 ± 1 °c) to ph meter (from monokrystaly a.s.). this type of ph meter is suitable for fast measurement with a deviation of about 0.2 degrees. to determine fraction of finely ground powder after the micromilling process and to determine the size of the medium grain a laser granulometer was used, the socalled dry method was used. for the determination of particle morphology and microscopic structural and elemental analysis a scanning electron microscope with the schottky cathode feg sem merlin zeiss was used, which is located in the laboratory of electron microscopy and microanalysis at the university center of energy efficient buildings. qualitative and quantitative analysis of the chemical composition of the samples was performed using an x-ray microanalysis, namely a oxford instruments energy spectrometer (eds). nondestructive resonance method was used to measure dynamic modules. the brüel & kjaer measuring set consists of the brüel & kjær type 3560-b-120, the brüel & kjær type 4519-003 acceleration sensor, the brüel & kjær type 8206 impact hammer and the control laptop. compressive strength was determined using a heckert, a model fp100 (hydraulic press). compressive strength was determined by a one-shot pressure test. testing was controlled by shifting at a constant velocity of 0.3mm/s. 4. results and discussion table 1 summarizes x-ray fluorescence analysis results. the table lists the elements, respectively. their oxides, which were present in a concentration above 1wt.%. the results compare individual samples of micronized recyclates with reference portland cement cem i 42.5r radotín. none of these recyclables contain excessive amounts of harmful substances. in columns a and b samples, there is a clear difference in the amount of si, which is the percentage of siliceous sand by about 8wt.%. the presented results also showed that samples contained increased content of iron which is caused by type of milling during which the gear wheel is worn. loi means immeasurable elements such as carbon and hydrogen. the most interesting part of the results is the amount of cao because it indicates the amount of unhydrated clinker and c-s-h gel. the best properties has the column a, the drainage gutter and the railway sleeper. in another measurement, ph was measured. both recyclates and reference cement were used as samples. the resulting ph of the reference cement was 12.6. the resulting values of the recyclates themselves range from 11.5 to 12.6 ph. the results show that none of the recycled material will reduce the alkalinity of the mixture and thus negatively affect the hydration. 89 z. prošek, p. tesárek, j. trejbal, t. horová acta polytechnica ctu proceedings oxides percent weight of individual oxides [%] cement sleeper gutter column a column b sio2 17.16 40.53 66.54 36.35 44.49 cao 66.67 21.09 22.18 23.80 16.37 al2o3 3.68 11.79 22.18 7.56 10.66 fe2o3 3.43 4.10 6.37 3.13 3.64 mgo 2.35 2.29 3.93 1.42 2.03 k2o 0.67 1.83 4.52 1.45 1.64 na2o 2.97 1.51 4.32 1.35 1.84 so3 4.96 1.21 1.06 1.57 1.19 loi 3.08 15.63 3.95 23.34 18.11 table 1. results of xrf analysis. figure 1. grain curves for cement and individual samples of recyclates. figure 1 compares typical granulometric curves of reference cement and individual recycled materials. recyclates from the gutter and the column a have a finer grain curve than reference cement (radotín). in contrast, sleepers and the column b are thicker than reference cement. it can be assumed that the recyclates from the gutter and column a will be more reactive whereby we can expect higher amount of unhydrated clinker grains. figure 2 shows a record of xrd analysis with individual peaks (extremes) belonging to the respective phases. the clinker minerals c3s and c2s have basically 5 strong peaks from 32.3 to 34.4 (figure 2), of which the peak at 32.65, 33.25 and 34.4 are c3s and c2s only, or are not significantly affected other phases. peak at 32.65 labeled “c3s and c2s” is basically the only one given by the c3s and c2s phases. to calculate the representation of individual clinker materials, a peak 32.65 was used because it is basically different and is composed only of these phases. for portland cement cem i 42.5r, the amount of clinker minerals c3s and c2s was 90% ± 5%. in the case of samples gutter and sleeper, the peak at 32.65 is not visible. it means that there is not more than 1% of c3s and c2s. in case of column a, peaks are visible at 35.65, 33.25 and 34.4 but is not visible at 33.25. the results showed that the column a and column b contain clinker grains. amount of clinker is larger for the sample from column a. namely, the sample from the column a contained approx. 6.3wt.% unhydrated clinker and the sample from column b 4.5wt.%. table 2 shows the results from an electron microscope analysis where 100 grains were measured for each material and elemental analysis was performed for each grain and the grain size was measured. from the elemental analysis, phases were subsequently derived by stoichiometry. high speed milling produses grains in one direction, so the shape index was created, therefore the ratio of the widest part of the grain to the narrowest. results showed that the aggregate formed from quartz provides a larger grain size, moreover, grains during milling are not so unidirectional. the results of the previous five analysis showed the most suitable material for recycling the column a, the column with a larger part of the aggregate removed. to confirm this assumption, mechanical tests were carried out. results of these tests are shown in figure 3. the results of the compressive strength 90 vol. 22/2019 recycling of construction waste. . . figure 2. phases of spectra from xrd analysis. materials phase fraction average minimum grain size average maximum grain size shape index [µm] [µm] [µm] sleeper aggregate 5 210 41.4 61.3 1.5 c-s-h gel + clinker 3 250 29.1 46.5 2.0 portlandite 2 15 2.3 5.2 2.1 gutter aggregate 3 270 40.3 68.9 1.6 c-s-h gel + clinker 2 220 31.8 53.3 2.0 portlandite 7 25 9.7 15.2 1.5 column a aggregate 5 50 16.4 24.5 1.6 c-s-h gel + clinker 3 30 10.2 14.1 1.9 portlandite 5 15 5.1 10.3 2.0 column b aggregate 2 40 14.8 22.4 1.3 c-s-h gel + clinker 2 50 15.4 20.2 1.9 portlandite 5 30 10.2 30.3 1.8 table 2. results of eds analysis. 91 z. prošek, p. tesárek, j. trejbal, t. horová acta polytechnica ctu proceedings figure 3. mechanical properties of tested composites, compressive strength – left, dynamic modulus of elasticity right. and the dynamic modulus of elasticity confirm the previous assumptions and that the recyclable is best seen as column a, which reached a strength of 85.37± 5.68mpa, ie by 7mpa less than reference cement and 8mpa more than other recyclates. 5. conclusion this work deals with the use of high speed grinding for recycling of old concrete and the direct determination of the potential of the input waste. four different materials were tested and the results show that: • none of the recyclates contained excessive amount of harmful substances, • ph of the recyclates has a similar value to the reference mixture and thanks to that none of the recyclates significantly influences ph of the hydration process, • granulometry has shown that aggregate has a major effect on the resulting granulometry because the gutter, which contained worse aggregate and the column a, where most of aggregate has been removed, have reached the required granulation for activation, • xrd analysis showed that the largest amount of unhydrated clinker and thus the bigger potential for activation had recyclate from the monolithic structure of the column, • results of the mechanical tests were confirmed by the previous analysis and that the recyclate from the column a appeared to be the most appropriate because it reached 10% stronger than the other materials. in the future, the research will focus on other concrete waste, which will be pre-recycled. acknowledgements this was financially supported by faculty of civil engineering, czech technical university in prague (sgs project sgs16/201/ohk1/3t/11) and by grant agency of the czech republic (gačr 17-06771s). references [1] eurostat. waste statistics. http://epp.eurostat.ec.europa.eu/. accsessed: 2018-09-27. [2] european parliament and council of the european union. directive 2008/98/ec of the european parliament and of the council. https://eur-lex.europa.eu/legal-content/cs/txt/ pdf/?uri=celex:32008l0098&from=lv. accessed: 2018-09-27. [3] j. r. jimenez, j. ayuso, f. agrela, et al. utilisation of unbound recycled aggregates from selected cdw in unpaved rural roads. resources, conservation and recycling 58:88–97, 2012. [4] a. pasandín, i. pérez. overview of bituminous mixtures made with recycled concrete aggregates. construction and building materials 74:151–161, 2015. [5] f. rodrigues, m. t. carvalho, l. evangelista, j. de brito. physical-chemical and mineralogical characterization of fine aggregates from construction and demolition waste recycling plants. journal of cleaner production 52:438–445, 2013. [6] j. xiao, w. li, y. fan, x. huang. an overview of study on recycled aggregate concrete in china (1996-2011). construction and building materials 31:364–383, 2012. [7] j. schoon, k. de buysser, i. van driessche, n. de belie. fines extracted from recycled concrete as alternative raw material for portland cement clinker production. cement and concrete composites 58:70–80, 2015. [8] z. shui, d. xuan, w. chen, et al. cementitious characteristics of hydrated cement paste subjected to various dehydration temperatures. construction and building materials 23:531–537, 2009. [9] y. j. kim, y. w. choi. utilization of waste concrete powder as a substitution material for cement. construction and building materials 30:500–504, 2012. [10] f. tomosawa, t. noguchi. integrated design and environmental issues in concrete technology, chap. towards completely recyclable concrete, p. 263–272. e & fn spon, london, 1996. 92 http://epp.eurostat.ec.europa.eu/ https://eur-lex.europa.eu/legal-content/cs/txt/pdf/?uri=celex:32008l0098&from=lv https://eur-lex.europa.eu/legal-content/cs/txt/pdf/?uri=celex:32008l0098&from=lv vol. 22/2019 recycling of construction waste. . . [11] z. shui, d. xuan, h. wan, b. cao. rehydration reactivity of recycled mortar from concrete waste experienced to thermal treatment. construction and building materials 22(8):1723–1729, 2008. [12] j. schoon, l. van der heyden, p. eloy, et al. waste fibrecement: an interesting alternative raw material for a sustainable portland clinker production. construction and building materials 36:391–403, 2012. [13] s. ahmari, x. ren, v. toufigh, l. zhang. production of geopolymeric binder from blended waste concrete powder and fly ash. construction and building materials 35:718–729, 2012. [14] j. topič, z. prošek, j. fládr, p. tesarek. influence of fineness of recycled concrete powder on the heat evolution and influence of its amount on mechanical-physical properties of cement paste. waste forum 2:268–274, 2018. [15] z. prošek, j. topič, j. ďureje, j. trejbal. comparison between the impact of micronized waste concrete and marble additions on mechanical properties of cementitious pastes. waste forum 2:262–267, 2018. 93 acta polytechnica ctu proceedings 22:88–93, 2019 1 introduction 2 materials and samples 3 experimental methods 4 results and discussion 5 conclusion acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2017.12.0042 acta polytechnica ctu proceedings 12:42–49, 2017 © czech technical university in prague, 2017 available online at http://ojs.cvut.cz/ojs/index.php/app influence of hmi ergonomy on drivers in cooperative systems area tomáš jizba czech technical university in prague, faculty of transportation sciences, konviktská 20, prague 1, czech correspondence: jizbatom@fd.cvut.cz abstract. modern vehicles are getting smarter and utilize more and more the advantages of advanced driver assistance systems (adas). deployment of upcoming technologies, such as cooperative systems (v2x), will most likely be the key step towards a significant reduction of accidents across the globe. some of these systems extend driver’s field of vision, so the driver can be forewarned against a wide range of threats. unlike technology, the human processing capacity has remained almost unchanged over centuries. therefore, it is necessary to bear in mind that drivers have restricted capabilities to process multiple warnings. in this context, an important question arises: how can be v2x warnings integrated into the human machine interface (hmi) of vehicles, and what warning policy is needed to ensure high usability, acceptance, efficiency and understanding of such a warning interface from the driver’s point of view. using a human centered design approach, two concepts of visual driver-vehicle interface for v2x warnings were developed and evaluated. one of those interfaces was based on a 1-stage warning policy and the second one utilized a dynamic 3-stage warning approach. the research revealed that drivers prefer to have dynamic warnings that correspond with the level of danger to which they are exposed, rather than only one warning displayed in a situation of imminent danger. however, with regards to overall usability score and workload measurements, results showed that the 3-stage system performed worse in comparison with the 1-stage system. keywords: v2x, warning stages, gui, adas, cooperative systems, hcd, usability, acceptability, ux. 1. introduction the internet, as a powerful medium, influences our lives more than ever before and is starting to be present in distinct areas of human interest. one of these areas is transportation, where vehicles are no longer considered to be isolated units, but they are seen as a part of communication chain. in this sense, one can see transportation as one of the biggest representative of the so-called internet of things (iot), where particular mobile nodes (vehicles) are capable of regularly exchanging their status via a wireless network. related technologies are to be integrated within new vehicles to improve safety, comfort, efficiency and overall experience by communicating with traffic flow and infrastructure. according to long-term predictions it is expected that 75% of the estimated 92 million cars shipped globally in 2020 will be fitted with internet-connection hardware (smart obu)[1]. these connected vehicles bring brand new opportunities for suppliers, oems and developers, but it is always necessary to bear in mind that the vehicle and traffic environment are very specific in terms of safety requirements. especially during hmi design development,safety issues play an important role and markedly influence the final look of intended systems or applications. 1.1. the human in a connected automotive world newly developed comfort, safety and infotainment systems in vehicles should help us to perform various tasks more safely, efficiently and with a higher level of comfort. on the other hand, each newly developed system can also increase driver’s learning effort to understand its usage and negatively influence his cognitive resources. the authors in [2] stated that the human brain is limited and not capable of performing several tasks at the same time and with the same quality level. thus, the increased number of information resources in vehicles decreases driver’s performance significantly and distracts him from his primary driving task. driver distraction can be defined differently (see [3]), but always implies diversion of the driver’s attention away from activities critical for safe driving. this diversion can be both physical (i.e., eyes off the road) and/or mental (i.e., mental overloading). therefore, the delivery and form of information should cause as little diversion from the primary task as possible in order to prevent hazardous situations. this fact is especially valid for safety-related information based on communication among vehicles (v2v) and among vehicles and the infrastructure (v2i), i.e. for cooperative systems (v2x), on which is the focus of this paper. 42 http://dx.doi.org/10.14311/app.2017.12.0042 http://ojs.cvut.cz/ojs/index.php/app vol. 12/2017 influence of hmi ergonomy on drivers in cooperative systems area 1.2. safety via vehicle-to-x technology in spite a decreasing trend in recent years, the number of fatalities still represents a significant economical loss for society as a whole. in terms of accidents, lack of driver awareness and attention has been identified as the leading cause of car accidents with an estimated percentage of 26% to 56%. nearly 90% of severe accidents could be positively influenced through an advanced driver assistance system (adas)[4]. these systems integrate safety features and technologies designed to avoid collisions and accidents by alerting the driver to a potentially dangerous situation, or directly taking over control of the vehicle in case of such a situation. in upcoming years, the adas should utilize the available wireless connectivity of v2x to enhance passenger safety via mutual exchange of cooperative messages. regarding the v2x, these can be divided into several categories. one of those categories distinguishes v2x systems based on so-called warning stages, i.e. based on levels of warning that they provide to drivers. in general, one-stage warning systems provide drivers with only so called imminent crash warning1 (icws), whereas two-stage warning systems provides icws plus so called cautionary crash warnings2 (ccws). finally, a multi-stage warning system3 is based on a continuous warning strategy which means that the driver is regularly informed about the state of given event/danger[5]. the question arises, how the driver handles several information outputs from more-stage warning systems (i.e., two and more stage systems) which are forwarded to him/her in short time proximity. one can expect that in certain situations, drivers will be facing multiple warnings at the same time. since the amount of a driver’s attentional resources is limited, it is necessary to find a suitable way how to present these gathered warnings in order not to overwhelm the driver. nowadays, visual displays represent the prevalent interface when it comes to warning of the driver inside the vehicle. hence, it is essential to investigate how drivers react upon cumulated visual stimuli (i.e., the number of different warnings and information presented on a display) in different traffic situations. 1.3. goals and objectives of this work within this work, two types of warning systems were tested, one on the basis of one-stage warning policy (see figure 1) and the second one on the basis of multi-stage warning policy comprising three warning stages (see figure 2). human factors literature ([5],[6] or [7]) and several conducted studies (e.g. [8], [9] 1require immediate corrective action within 2 seconds, otherwise an event becomes unavoidable. 2require immediate attention and possible corrective action within 2 to 10 seconds, otherwise they can escalate to icws. 3for instance a five-stage system may comprise levels such as: 1) no vehicle detected, 2) vehicle detected, 3) caution, 4) approaching imminent, and 5) imminent. or [10]) suggest that multi-stage warning systems enhance a driver’s performance over a 1-stage system[11]. however, there is currently a lack of available data regarding visual distraction, workload, and driver comfort in order to favor multi-stage warning systems over 1-stage systems. therefore, the main goal of this work was to assess the impact of these two visual warning interfaces on a defined user group concerning acceptability, usability and workload. figure 1. one-stage warning gui. figure 2. three-stage warning gui. 2. implementation and testing the proposed systems were tested within an artificial environment, which should substitute the real traffic environment. the test in the real environment may be quite dangerous with respect to the intended purpose of the system. therefore, a simulator study was chosen for the final evaluation of the systems and their mutual comparison. 2.1. simulator description the experiment was conducted on the ctu’s highfidelity driving simulator (see figure 3). the participants were seated in a škoda octavia ii mock-up 43 tomáš jizba acta polytechnica ctu proceedings that was placed on a static base, and surrounded by 3 wide-screens on which the road and traffic environment was projected. the mock-up was fully equipped with a roof, mirrors, a passenger seat and original controls, such as pedals, a steering wheel, dashboard (speedometer), and automatic transmission. sounds of the engine and blowing wind were simulated as well by using an akai sound system with 7 loudspeakers situated in and around the mock-up. the experiment supervisor was seated closely behind the mock-up. there he had access to a control computer (mac, intel i5 2.7 ghz processor, 8gb, intel iris graphic card) which was connected to an in-vehicle display (7" ios tablet) by means of a lightning / usb cable. he could observe the projected environment, participant’s behavior, and additionally communicate with the participant during the sessions. three pcs (intel i7 processor, nvidia quadro k4200 graphic card), matlab, zmodeler and rhino software were used for simulation purposes. the pcs generated real-time images with a refresh frequency of 60 hz, and sent them to the three full hd (1920x1080) projectors. the projectors projected the images on a screen in front of the mock-up and on the side screens that were inclined to the front screen at 90°. figure 3. škoda octavia ii mock-up. as a centralized source of visual and auditory warnings a nomadic device in the form of a 7" tablet with a high-resolution display (2048 x 1536 px) was used. the tablet was situated above the middle console (see figure 4) in a way that its position fulfilled recommendations from esop and jama, and simultaneously its presence did not obstruct driver’s field of vision. moreover, research [12] suggests this position to be optimal in terms of reaction times, duration of lane departures, perceived workload and eye-movement latencies to presented stimuli. 2.2. scenario elaboration for the impact assessment, a highway scenario containing several potentially dangerous events was created. the impacts were studied during conditions of normal visibility on the highway which consisted of 2x2 traffic lanes (see figure 5) with approximately 10% of the figure 4. the tablet device within the cockpit. traffic volume in the right lane and it contained passenger vehicles. the lanes were mainly straight with a few gentle curves. each participant encountered two traffic jams during experimental trials. these traffic jams were approximately 3000 meters apart from each other and their regulation was based on the vehicles speed. for example, vehicles had to slow down when approaching the jam (i.e., 50 km/h at the tail), drive in a stop-and-go mode in the jam (i.e., 0-50 km/h) and accelerate when leaving the jam (i.e., towards 100 km/h). at the beginning of each traffic jam emerged temporary heavy braking vehicles, which created another dangerous event for participants. besides traffic jams, the scenario contained one place with a shallow puddle (7x7 meters) simulating a slippery road across both lanes (see figure 5), and one place with an immobilized vehicle on the edge of the right lane. figure 5. highway simulation with dangerous events. with respect to the ego vehicle, these dangerous events changed their urgencies. since some of them only occurred temporarily, the triggered warnings differed in the 1-stage warning system compared to the 3-stage warning system. the reason was the warning policy which differed in both systems. whilst the 3-stage warning system could display all three levels of warnings, the 1-stage warning system displayed only the most imminent warning in the given time, i.e. 44 vol. 12/2017 influence of hmi ergonomy on drivers in cooperative systems area warning stage visual cue timing (ttc) 1st > 13s 2nd 3s < ttc ≤ 13s 3rd ≤ 3s table 1. ttc threshold for particular warning stages. characteristics participants male 12 (66.7%) female 6 (33.3%) age (18-30 years) 8 (44.4%) age (31+) 10 (55.6%) driving license (≤ 6 years) 9 (50%) driving license (≥ 7 years) 9 (50%) smartphone while driving 15 (83.3%) table 2. personal characteristics of participants (n = 18). only the high-level warning with the highest urgency. every new warning or new state4 of the warning was accompanied with an auditory cue in the form of a sound beep. this was supposed to direct driver’s attention towards the in-vehicle display, since the visual warning itself is not omnidirectional. the timing of warnings was based on the time-to-collision (ttc) parameter [13] that determined individual warning stages (see table 1). 2.3. participants an procedure a total of 18 participants took part in the driving simulator experiment. the average age of the participants was 32.5 years (sd=10,05 years). table 2 shows some personal characteristics of the participants. the main experiment consisted of one training session and three experimental sessions, with a total duration of approximately 1 h. in the first experimental session, participants were informed that they will drive through a highway test track without any assistance system (i.e., without any secondary task). the participants were asked to drive in the right lane and maintain driving speed 100 kph unless they would be forced by the environment to change this speed. they were also told to brake only if necessary. after driving for about 10 minutes through the highway test track and responding to 8 dangerous events, the participants arrived at the tail of the second congestion where the simulation ended. at the end of the first session, participants were asked to evaluate their experienced effort (rsme scale [14]) while driving on the test track. 4 new state of the warning implies transition from, e.g. the low-level warning to the mid-level warning, or from the mid-level warning to the high-level one. before the second and third experimental sessions, the participants were given pre-measurement van der laan questionnaires [15] in order to assess primary acceptability of both systems (1-stage and 3-stage). after the completion, an eye-tracker calibration followed using a head-mounted eye-tracking device smi iview x hed. once the calibration process was done properly, one could obtain an accurate estimate of the gaze position (gaze pointer) within the scene, together with particular durations. in this work, five glance characteristics were measured, namely: mean glance duration, total number of glances, total glance time at the in-vehicle device, maximum glance duration, and standard deviation of glance durations (i.e., variability of glance durations). regarding the driving instructions (i.e., speed, etc.), these remained the same as in the first session. dualtask sessions were randomly reversed for each participant, so that some participants started with the 1stage warning system and then proceeded with 3-stage system or vice versa. after each ride there again followed the questionnaire with the subjective workload assessment (rsme). when the participants finished both experiment sessions, the experiment supervisor gave them post-measurement van der laan questionnaires concerning systems’ after-acceptability. after filling out these questionnaires, the participants were additionally interviewed regarding usability (sus) by using 5-point likert scales. the whole experiment was finished by a free discussion with the participant. 3. results elaboration 3.1. workload assessment 3.1.1. glance behavior out of 18 tested participants in this experiment, 16 provided adequate eye-movement data. in the table 3 one can find mean and standard deviation for each glance characteristic in relation to the warning system (1-stage/3-stage). furthermore, a paired-samples ttest was chosen to test significant differences between both systems. therefore, the table also contains t values, degrees of freedom, p values, and effect sizes for each pair (if significant difference). there were four statistically different pairs of glance characteristics. it was no surprise that there were statistically significant differences in the total glance time (t(15) = -12,974, p < .001) and the number of glances (t(15) = -15,179, p < .001) between both systems. the drivers in the 3-stage trial were confronted with a lot more warnings compared to the 1-stage trial, which resulted in a higher division of attention (see the total glance time in the table 3). besides, warnings in the 3-stage system occupied the display much longer, which tempted drivers to look at the in-vehicle display more often. in any case, the drivers spent a significantly higher portion of time observing the in-vehicle display with the active 3-stage warning system (19.551 ± 5.722 s) compared to the 1-stage 45 tomáš jizba acta polytechnica ctu proceedings 1-stage µ(σ) 3-stage µ(σ) t df p(two tailed) effect size (cohen’s d) mean glance duration .943 (.231) s .939 (.125) s .130 15 .899 – total glance time 3.499 (1.041) s 19.551 (5.722) s -12.974 15 <.001 3.24 (large effect) number of glances 3.75 (.93) 20.56 (4.72) -15.179 15 <.001 3.79 (large effect) maximum glance duration 1.103 (.227) s 1.413 (.165) s -11.654 15 <.001 2.91 (large effect) standard deviation of glance durations .149 (.082) s .276 (.035) s -5.467 15 <.001 1.37 (large effect) table 3. paired-samples t-test – glance characteristics. warning system (3.499 ± 1.041 s). apparently, the higher total glance time was conditioned by a larger number of glances on the display (20.56 ± 4.72) in comparison with the 1-stage system (3.75 ± 0.93). a higher number of visual stimuli on the display probably caused a significantly higher maximum glance duration (1.413 ± 0.165 s) while driving with the 3-stage system compared to the 1-stage one (1.103 ± 0.227 s). the 3-stage warning interface resulted in a more complex and more time demanding visual search task compared to the 1-stage warning system. especially in the situation, where the drivers were confronted with multiple warnings at the same time, their ability to scan the 3-stage warning interface decreased, resulting in higher glance durations. another reason behind the higher maximum glance duration was the complexity of testing scenario. the low-level congestion warnings were triggered on straight sections of the highway with perfect visibility, so some drivers spent a longer time observing the interface without jeopardizing their safety. the last statistically significant difference was found for standard deviation of glance durations (t(15) = -5,467, p < .001). as one can see in the table 3, the 3-stage warning system led to a higher dispersion (variability) of glance durations (.276 ± 0.035 s) compared to the 1-stage warning system (.149 ± 0.082 s). while using the 3-stage system, drivers tended to alternate their glance behaviour. this was done by alternating saccadic movements with a longer length of fixation and saccadic movements with a shorter length of fixation. this glance behaviour and glance characteristic, respectively, may most likely imply a higher workload of drivers using the 3-stage system. 3.1.2. subjective measurement – rsme scale in terms of subjective workload, rsme scores (figure 6) revealed that drivers perceived the highest effort (47.67 ± 22.02) while driving with the 3-stage system. less effort drivers perceived in case of the 1-stage system (35.33 ± 15.87), and they felt the least burdened when they were driving without any system (28.83 ± 13.99). paired samples t-test showed statistically significant difference in means for all three combinations (see table 4). figure 6. rsme scores. these findings correlate with the glance behaviour of drivers (3.1.13.1.1), so the drivers’ attentional resources were more burdened while driving with the 3-stage warning system than with the 1-stage system. in case of the 1-stage system, the perceived effort can be qualified as a little, whereas some effort or rather a lot of effort was perceived during the interaction with the 3-stage system. 3.2. usability results the participants were given instructions to mark a response to every statement within the sus, and were asked not to spend too much time on any one statement. after the comparison of both means of sus scores, one can conclude that the 1-stage system was perceived significantly more usable than the 3stage system (see table 5). regardless of the lower score of the 3-stage system (76.11 ± 7.96) compared to the 1-stage system (84.31 ± 5.54), both systems can be declared as usable. if the scores would be transferred to the adjective scale (see the y axis in the figure 7), the usability of the 46 vol. 12/2017 influence of hmi ergonomy on drivers in cooperative systems area baseline µ(σ) 1-stage µ(σ) 3-stage µ(σ) t df p(two tailed) effect size (glass δ) pair 1 1-stage system vs. 3stage system – 35.33 (15.87) 47.67 (22.05) -2.345 17 .031 .655(medium effect) pair 2 baseline vs. 1-stage system 28.83 (13.99) 35.33 (15.87) – -3.472 17 .003 .46(medium effect) pair 3 baseline vs. 3-stage system 28.83 (13.99) – 47.67 (22.05) -3.195 17 .005 1.35(large effect) table 4. paired-samples t-test – rsme. 1-stage µ(σ) 3-stage µ(σ) t df p(two tailed) effect size (cohen’s d) sus scores 84.31 (5.54) 76.11 (7.96) 3.615 17 .002 2.10 (large effect) table 5. paired-samples t-test – sus. figure 7. sus scores. 1-stage system would be evaluated almost as an excellent, whereas the usability of the 3-stage system would be marked as good. the reason behind such a significant difference in the usability scores is most likely a much simpler interface of the 1-stage system with substantially reduced functionality (e.g., only one warning per screen, no transitions of warnings, no position cue, etc.). 3.3. acceptability – van der laan scores in table 6 the results from pre-trial6 and post trial7 van der laan scales are shown. the table contains mean usefulness and satisfaction scores for both systems together with their standard deviations. as it is apparent, both systems were rated equally in terms of pre-trial usefulness, and both suffered a certain decline in this score after the trials. in case of the 1-stage warning system, this decline was found 6the participants had no experience with systems to be tested. the experiment leader only explained them the differences between these two systems and their goals. 7after the experience with the systems. to be statistically significant with t(17) = 2.486, p = .024 and d = .59 (medium effect). in terms of the satisfaction score, the participants showed negative trend after the trial with the 3-stage warning system, but with no statistical significance (t(17) = 1.256, p = .226). even though the participants had higher expectations before using the systems, the overall attitude towards both systems seems to be positive (see figure 8). figure 8. post van der laan scores. apparently, the participants felt slightly more satisfied while driving with the 1-stage warning system. on the other hand, the 3-stage warning system was rated as more useful. nevertheless, these differences were not found to be statistically significant. 3.4. final decision at the end of the whole experiment, the participants were asked to make a final decision, i.e. which of these systems they would like to have in their cars. the results (figure 9) showed that 66.7% (12) of participants would rather use the 3-stage warning system, whereas the rest 33.3% (6) of them would prefer the 1-stage system. 47 tomáš jizba acta polytechnica ctu proceedings before measurement after measurement system usefulness satisfaction usefulness satisfaction 1-stage 1.356 (.448) .653 (.412) .956 (.525) .653 (.543) 3-stage 1.356 (.596) .708 (.502) 1.133 (.672) .458 (.729) table 6. van der laan – pre-and post scores figure 9. final decision chart. 4. conclusion off-road glances (or glances to in-vehicle devices) are of particular concern for evaluating the potential for distraction from hmis. glance characteristics, such as longer glance durations and/or a higher number of glances at an in-vehicle gui may indicate its unintelligible design for the user resulting in higher investments of his/her attentional resources and in higher workload in general. since the resources could otherwise be solely used for the primary task, their exploitation must be time effective. this work tried to evaluate two kinds of warning systems which were based on different warning strategies. despite the more complex visual search task in the 3-stage warning system, results showed that there was no single glance to the display that was longer than 2 s threshold (see [16–18]) made by any of the 16 participants. only one participant experienced glance duration longer than 1.6 s (1.68 s). both systems, therefore, can be considered to be safe in terms of glance behaviour. in spite of the higher perceived workload and the lower usability score, the drivers would probably equip their vehicles with the 3-stage warning system. however, further studies should be conducted to assess driving performance (e.g., braking reaction time, lane deviations, etc.) of drivers while interacting with this system. an optimal solution seems to be to switch between both systems depending on the current workload of the driver. in case of the higher workload, the 1-stage system would be active, whereas in less attention demanding conditions the 3-stage system would warn the driver continuously. this could be done via so called workload manager [19] that monitors the environment (e.g., lane parameters, weather, etc.) as well as the driver state (e.g., eye-blinks, heart rate, etc.), and makes decision about when information should be presented to the driver and in what form. such an alternating policy may best address driver’s processing bottlenecks and may lead to an efficient human-system interaction without any impairment of road safety. 5. recommendation for future research it is clear that the aforementioned glance characteristics could differ in more complex scenarios with respect to the road type, road curvature, position of the in-vehicle device, environmental conditions (e.g., fog), surrounding stimuli (e.g., more stimuli in cities), and/or during additional task(s) at hand (i.e., during higher distraction). within this study, the conditions were very convenient in terms of potential workload. hence, additional experiments should be conducted to prove the designs under different conditions including the real world conditions. one could also include a 2-stage warning system for comparison, i.e. a system with solely mid-level and high-level warnings. many participants stated that the 1-stage system was useless in certain situations, because the dangerous event was visible in advance. they also pointed out that low-level warnings within the 3-stage system were showed too soon before the dangerous events. this may lead to so called "cry wolf" effect [20], which could harm the credibility of the whole 3-stage warning system. the 2-stage warning system could therefore represent a suitable compromise between these two systems. in addition, deployment of so-called likelihood alarm display [21] could significantly enhance the credibility of multi-stage systems by providing likelihood of event presence. the study also revealed that for half of participants the number of warnings was too high in certain situations, i.e. the screen was too cluttered. therefore, an additional study should find out how many warnings should be presented per screen to increase situation awareness. finally, the warning modalities and their combinations, respectively, should be further examined. in particular, the low-level warnings could benefit from a haptic feedback, which could reduce the startle effect from sound cues and simultaneously supplement non-omnidirectional visual warnings. 48 vol. 12/2017 influence of hmi ergonomy on drivers in cooperative systems area acknowledgements this study was conducted as a part of other studies included in a double degree programme named intelligent transport systems at the ctu in prague and the fh technikum wien. the experimental part of the work was carried out at the department of vehicle technology at the ctu, so i would like to thanks all the staff from this department who enabled me to use their simulator and equipment. special thanks goes to mr. jan válek, without whom it would not have been possible to carry out any experiment. references [1] greenough, j. (2015) ’the connected car is creating a massive new business opportunity for auto, tech, and telecom companies’ business insider [online], available: http://www.businessinsider.com/connected-carforecasts-top-manufacturers-2015-2 [accessed 8 apr 2016]. [2] regan, m. a., lee, r. b. and gladstones, w. h (1989) ’division of attention: the single-channel hypothesis revisited’, journal of experimental psychology: human experimental psychology, 41(a), 1-17.. [3] young, r. a. (2012) a tabulation of driver distraction definitions, m.a. thesis, wayne state university of medicine. [4] rami, z. (2014) integrative warning concept for multiple driver assistance systems, dissertation work (phd), university of wãijrzburg. [5] campbell, j., richard, ch., brown, j., mccallum, m. (2007) ’crash warning system interfaces: human factors insights and lessons learned’, report no. hs 810 697, washington, d.c., usa: the national highway traffic safety administration (nhtsa). [6] lee, j.d., hoffman, j.d. and hayes, e. (2004) ’collision warning design to mitigate driver distraction’, accepted for sigchi conference on human factors in computing systems, april. [7] lenne, m., triggs, t. (2009) ’warning drivers of approaching hazards: the importance of location clues and multi-sensory cues’, the journal of human factors, security and safety, 203-211. [8] stanton, n., lew, r., boyle, n., hope, r. j., dyre, b. p. and bustamante, e. a. (2011) ’an implementation of a graded deceleration display in brake light warning systems’, proceedings of the human factors and ergonomics society annual meeting, vol. 1, 1573-1577. [9] mcgehee, d.v., brown, t. l., lee, j. d. and wilson, t. b. (2002) ’effect of warning timing on collision avoidance behavior in a stationary lead vehicle scenario’, the journal of the transportation research board, 1803, 1-6. [10] lenne, m., triggs, t. 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(1985) ’the construction of a scale to measure subjective effort’, delft, holland: department of philosophy and social sciences, delft university of technology. [15] van der laan, j. d., heino, a. and de waard, d. (1997) ’a simple procedure for the assessment of acceptance of advanced transport telematics’, transportation research part c, 5(1), 1-10. [16] rockwell, t. h. (1988) ’spare visual capacity in driving – revisited’, in: gale, a.g., freeman, m. h., haslegrave, c. m., smith, p. and taylor, s. p. (eds.) vision on vehicles ii, 317-324, amsterdam: north holland. [17] wierwille, w. w. (1993) ’visual and manual demands of in-car controls and displays’, london: taylor and francis. [18] tijerina, l., barickman, f. s. and mazzae, e. n. (2004) ’driver eye glance behavior during car following’, report no. hs 809 723, washington, d.c., usa: the national highway traffic safety administration (nhtsa). [19] green, p. (2004) ’driver distraction, telematics design, and workload managers: safety issues and solutions’, report no. 2004-21-0022, the university of michigan transportation research institute (umtri), ann arbor, mi. [20] breznitz, s. (1984) ’cry wolf: the psychology of false alarms’, hillsdale, n.j.: lawrence erlbaum associates. [21] sorkin, r. d., kantowitz, b. h. and kantowitz, s. c. (1988) ’likelihood alarm displays’, the journal of human factors, 30, 445-459. 49 acta polytechnica ctu proceedings 12:42–49, 2017 1 introduction 1.1 the human in a connected automotive world 1.2 safety via vehicle-to-x technology 1.3 goals and objectives of this work 2 implementation and testing 2.1 simulator description 2.2 scenario elaboration 2.3 participants an procedure 3 results elaboration 3.1 workload assessment 3.1.1 glance behavior 3.1.2 subjective measurement – rsme scale 3.2 usability results 3.3 acceptability – van der laan scores 3.4 final decision 4 conclusion 5 recommendation for future research acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2021.31.0010 acta polytechnica ctu proceedings 31:10–17, 2021 © 2021 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague research tools aplicable in designing of high-speed and high-power rail vehicles vojtěch dybala czech technical university in prague, faculty of mechanical engineering, department of automotive, combustion engines and railway engineering, technická 4, prague, czech republic correspondence: vojtech.dybala@fs.cvut.cz abstract. the importance of the railway transport of both goods and passengers continuously grows as it is in many points of view more ecological and economical solution in comparison with road transport. just the importance has been supporting efforts to design more powerful and faster trains reaching traction powers more than 1.6 mw per a wheel-set or service top speed much more than 300 km/h till these days. to reach design which has enabled such a kind of performances it was necessary to research dynamic behaviour of railway vehicles. both via laboratory measurements and simulations. the laboratory experiments have been carried out on a specially designed laboratory equipment called roller rigs. a laboratory equipped by roller rig for testing of the railway vehicle dynamic behaviour has been built at the faculty of mechanical engineering at ctu in prague, also within phd study programs and sgs grants. another powerful tool within research activities is a simulation. kind of a such simulation will be presented by this contribution with the focus on the torsion dynamics of high-power fully-suspended drive of a railway vehicle, which has been developed also by phd students under financial support of the faculty of mechanical engineering and related grants. keywords: railway vehicle, dynamic, roller rig, rollers, drive, torsion dynamics. 1. introduction in the railway vehicle’s research has been used different tools. these are calculations and simulations carried out via specialized software, measurements on real vehicles on real tracks and measurements carried out in laboratories on roller rigs. the importance of roller rigs comes just from laboratory conditions and its advantages. the possibility to define exact conditions of measurements, ability to reproduce measurements, research of dangerous phenomena and low costs against measurements on real vehicles on real tracks. all of this caused from the 2nd half of the 20th century till these days an expansion of roller rigs in connection with the need to research dynamic behaviour of railway vehicles. whereas roller rigs were built for base research there has been also a need to do an applied research on the engineering level via simulations in the industry. for these purposes various software has been developed and used. the university setting enables to apply both approaches for phd students, however its limited in comparison with industry companies and specialized research centres. nevertheless, applicability of roller rigs and simulation results of torsion oscillations and their excitation were achieved. 2. roller rigs the roller rig is a device, which consists of three main parts. these are a frame or an external construction of the roller rig, rollers and a propulsion system of rollers with related components as e.g. couplings and shafts. the most important are the rollers as these components shall simulate rails as a part of the railway infrastructure. the rollers simulating endless rails can be of two basic design types, as applied for roller rigs at ctu in prague: • disc type rollers – figure 1 left, • drum type rollers – figure 1 right, which is a prototype roller from the historical point of view roller rigs touches the remote past. for example [1], figure 2 presents the roller rig from swindon in england specified to measure a traction power of steam locomotives. another example of a roller rig, which was built in the real-life scale, can be the workplace created in 1977 by deutsche bahn ag in munich, see [1, 2] and figure 3. this research workplace was dedicated for the research and development of the first generation of ice high-speed train. this device enabled to research train dynamic behaviour on its limits under different conditions – speeding in the straight track, acceleration and braking behaviour or ride on a track with deviated geometry etc. except of these big real-life scale devices reduced scale devices have been created. in this area the scaled roller rigs, which has been built at the faculty of mechanical engineering at ctu in prague, see figure 4 and figure 5, can be taken as examples. the first roller rig has been used for the education and its abilities and continuous development were presented [3, 4]. student’s measurements are focused on the transverse dynamic of railway bogie movements within the education. 10 https://doi.org/10.14311/app.2021.31.0010 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 31/2021 designing of high-speed and high-power rail vehicles figure 1. roller rigs – faculty of mechanical engineering at ctu in prague. 1 figure 2. the roller rig in swindon [1]. the second roller rig has been under construction and without a practical used till these days. its purpose is generally oriented into torsion dynamics and for first attempts to measure the adhesion phenomena in the wheel-roller contact as a simulation of the wheel-rail contact, which we know from numbers of measurements. as a good example can be used results presented in [5] and figure 6. the basic idea for the supposed measurements of the adhesion coefficient comes from the picture of forces, which act in the wheel-rail contact as presented in [7] and figure 7 and the knowledge, that the adhesion coefficient is a function of a wheel slip. according to the equation (1) the tangential force t1, which is an equivalent of the wheel torque and vertical wheel force q will be measured and adhesion coefficient µ will be calculated. t1 = qµ (1) as the adhesion coefficient is a function of the wheel slip, the slip itself will be calculated based on the difference in rotation speed of a wheel-set and a roller, which will be measured. on the real vehicle the slip is defined based on the difference of the vehicle speed and real rotation speed of a wheel-set. this fact is defined by the equation (2) and its deduction in [7] and figure 8. s = rkωk − v v (2) 3. torsion oscillations research within a generally defined task to research transition and electromechanical phenomena in traction drives of railway vehicles, the phenomenon of torsion oscillations has been researched as well. the torsion oscillation problems were studied also in the past, which is documented in [5]. that contribution presents results from simulations of the torsion dynamics for the mathematical model of the experimental roller rig with different types of wheelset drives. this contribution deals with torsion dynamics and oscillations, which are excited by harmonic components of the traction motor electromagnetic torque. the intended simulation model, which was created in matlab simulink, utilizes the idea, that the railway vehicle can be simplified into three basic separate sub-models from the perspective of the drive system. these are the electrical part, the mechanical part and the wheel-rail contact part, as shown in [8] and figure 9. fundamentals and function descriptions of the model as a whole were published in [7–9] and therefore no attention will be payed to them. the intention of this contribution is to present results of simulations focused on the oscillations. the phenomenon of torsion oscillations has been researched via a mathematical model which is equivalent to a fully-sprung mechanical drive of a locomotive, see [7] and figure 10. for the simulation model the drive has been reduced into a schematic torsion system as presented in figure 11. particular rotation masses, defined by their mass of inertia jx and torsion springs between them, defined 11 vojtěch dybala acta polytechnica ctu proceedings figure 3. the roller rig built by db-ag in munich [1, 2]. figure 4. the roller rig – faculty of mechanical engineering at ctu in prague. by their torsion stiffness kx are mathematically represented within the simulation model like in [7] and figure 12. the electromagnetic torque m produced by the asynchronous motor model, which accelerates whole model of the railway vehicle, is the source of excitation of the oscillations in the torsion system, because it consists of a nominal value and superimposed harmonic components. the course of this rippled torque within a simulation can be seen in figure 13. fast fourier transformation (fft) of the electromagnetic torque in figure 14 revealed harmonic components with typical frequencies. these harmonic components interact with the mechanical system in such a way, that they excite the oscillations. simulations proved, that excited oscillations can reach significant magnitudes for oscillation frequencies, which are the natural frequencies of the mechanical system. as in this simulation case dominant oscillations for frequencies around 2400 hz appeared in figure 5. the roller rig – faculty of mechanical engineering at ctu in prague. figure 6. comparison of measured and calculated coefficient of adhesion [6]. fft in gear’s acceleration torques, the example for the gearwheel acceleration torque course is shown in figure 15 and figure 16. table 2 shows that the magnitude of harmonic 12 vol. 31/2021 designing of high-speed and high-power rail vehicles harmonic frequency [hz] harmonic component magnitude in relation to nominal value of signal [%] phenomenon description 137 106 3rd multiple of 1st harmonic of motor supply voltage 800 and its multiples 10 to 55 even and odd multiples of inverter switching frequency +/137 0 to 8 side bands of switching frequency multiples table 1. harmonic frequencies of electromagnetic torque – overview and description. harmonic frequency [hz] magnitude of harmonic component in relation to nominal value of signal [%] phenomenon description 2400 105 3rd multiple of inverter switching frequency 2401 185 7th natural frequency of the mechanical system table 2. harmonic frequencies of the gearwheel accelerating torque – overview and description. figure 7. wheel-rail contact forces [7]. components of the rippled torque can be significantly higher than nominal value of the torque. in presented case 85% higher for the frequency of 2401 hz. this behavior is caused by the fact, that the 3rd multiple of the inverter switching frequency 2400 hz contained in the electromagnetic torque, which excites system’s oscillation, is close to 7th natural frequency of the figure 8. wheel slip deduction [7]. figure 9. visualization of fundamental model parts [8]. mechanical system ≈ 2401 hz. 4. conclusions with respect to the roller rigs as research and education tools at the faculty of mechanical engineering, ctu in prague the intention is to build the roller rig, which will enable to realize measurements of adhesion 13 vojtěch dybala acta polytechnica ctu proceedings figure 10. locomotive fully-sprung drive – cross section [7]. figure 11. schematic torsion system. 14 vol. 31/2021 designing of high-speed and high-power rail vehicles figure 12. mass and spring mathematical representation [7]. coefficient in the first step and afterwards possibly extend laboratory experiments for students. regarding nowadays results from the research oriented towards the torsion oscillations in the railway vehicle traction drives we can say, that these oscillations with significantly high magnitudes can be observed within torques loading particular components of the traction drive even in steady states, not only during transition phenomena. however, such observed torque ripple does not lead to the overloading of these components with respect to their strength, it will have an impact on the damage cumulation and thus on their life parameters. this idea is heading this research to be focused on the investigation of sensitivity of different traction drive’s design arrangement on the torsion oscillations and possibilities how to reduce them by means of utilization of modified standard components as couplings or gears. 5. list of symbols fx coefficient of friction [-], t1 tangential force [n], q vertical wheel force [n], µ coefficient of adhesion [-], s wheel slip [%], rk wheel radius [m], ωk angular speed of a wheel [rad/s], ν vehicle velocity [m/s], jx mass of inertia [kg·m2], kx torsion stiffness [n·m·rad−1], ϕx angle [rad], r1 pinion rolling radius [m], r2 gearwheel rolling radius [m], i gear ratio [-], m motor torque, elmag. [n·m], ω1,ω2 angular speed [rad·s−1], m1,m2 torque [n·m]. acknowledgements this research has been realized using the support of the technology agency of the czech republic, programme national competence centres, project #tn01000026 josef bozek national center of competence for surface transport vehicles and the support of the czech technical university and the related grant – no. sgs20/120/ohk2/2t/12. this support is gratefully acknowledged. references [1] n. bosso, a. gugliotta, a. soma. simulation of narrow gauge railway vehicles and experimental validation by mean of scale tests on roller rig, 2008. online, issn 15729648, https://doi.org/10.1007/s11012-008-9128-4. [2] a. jaschinski, h. chollet, s. iwnicki. the application of roller rigs to railway vehicle dynamics. vehicle system dynamics 31, amsterdam, 1999, https://doi.org/10.1076/vesd.31.5.345.8360. [3] s. myamlin, j. kalivoda, l. neduzha. testing of railway vehicles using roller rigs. procedia engineering 2017, 2017, issn 1877-7058, https://doi.org/10.1016/j.proeng.2017.04.439. [4] j. kalivoda, p. bauer. mechatronic bogie for roller rig tests. the dynamics of vehicles on roads and tracks – proceedings of the 24th symposium of the international association for vehicle system dynamics, 2016, https://doi.org/10.1201/b21185-96. [5] j. kolář, m. dub. matematické modelování torzní dynamiky pohonu dvojkolí. in současné problémy v kolejových vozidlech 2017: xxiii. konference s mezinárodní účastí, sborník příspěvků, pardubice, 2017, p. 195-204, isbn 978-80-7560-085-1. [6] o. polach. creep forces in simulations of traction vehicles running on adhesion limit. 6th international conference on contact mechanics and wear of rail/wheel systems, göthenburg, june 10-13, 2003. [7] v. dybala, m. libenský, b. šulc, c. oswald. slip and adhesion in a railway wheelset simulink model proposed for detection driving conditions via neural networks. sborník vědeckých prací vysoké školy báňské – technické univerzity ostrava, řada strojní, ostrava, 2018, issn 1804-0993 (online), https://doi.org/10.22223/tr.2018-1. [8] v. dybala. simulace zvlnění elektromagnetického momentu asynchronního trakčního motoru lokomotivy napájeného ze střídače. konference studentské tvůrčí činnosti 2019, sborník konference, praha, 2019, isbn 978-80-01-06564-8. [9] v. dybala. vliv regulace na frekvenční složení fázových proudů a elektromagnetického momentu při simulování napájení asynchronního motoru ze střídače. súčasné problémy v koľajových vozidlách – prorail, 2019. diel i, žilina, 2019, isbn 978-80-89276-58-5. 15 https://doi.org/{10.1007/s11012-008-9128-4} https://doi.org/{10.1076/vesd.31.5.345.8360} https://doi.org/{10.1016/j.proeng.2017.04.439} https://doi.org/{10.1201/b21185-96} https://doi.org/{10.22223/tr.2018-1} vojtěch dybala acta polytechnica ctu proceedings figure 13. rotor speed and rippled torque of asynchronous motor. figure 14. fft analysis of the electromagnetic torque. 16 vol. 31/2021 designing of high-speed and high-power rail vehicles figure 15. course of gearwheel accelerating torque. figure 16. fft analysis of the gearwheel accelerating torque. 17 acta polytechnica ctu proceedings 31:10–17, 2021 1 introduction 2 roller rigs 3 torsion oscillations research 4 conclusions 5 list of symbols acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2018.15.0057 acta polytechnica ctu proceedings 15:57–62, 2018 © czech technical university in prague, 2018 available online at http://ojs.cvut.cz/ojs/index.php/app molecular statics simulation of nanoindentation using adaptive quasicontinuum method karel mikeša,∗, ondřej rokošb, ron h. j. peerlingsb a czech technical university in prague, thákurova 7, 166 29 prague 6, czech republic b eindhoven university of technology, p.o. box 513, 5600 mb, eindhoven, the netherlands ∗ corresponding author: mikes.karel.1@fsv.cvut.cz abstract. in this work, molecular statics is used to model a nanoindentation test on a two-dimensional hexagonal lattice. to this end, the quasicontinuum (qc) method with adaptive propagation of the fully resolved domain is used to reduce the computational cost required by the full atomistic model. three different adaptive mesh refinement criteria are introduced and tested, based on: (i) the zienkiewicz– zhu criterion (used for the deformation gradient), (ii) local atoms’ site energy, and (iii) local lattice disregistry. accuracy and efficiency of individual refinement schemes are compared against the full atomistic model and obtained results are discussed. keywords: molecular statics, quasicontinuum method, adaptivity, nanoindentation. 1. introduction nanoindentation is a commonly used testing procedure applied to small volumes of materials for measuring their micromechanical properties. typically, a hard tip (i.e. indenter) with known mechanical properties is pressed into an examined sample of unknown mechanical properties. loading force and penetration depth of the indenter are recorded during the loading and unloading stages, providing a basis for the estimation of the unknown mechanical properties. numerical models are typically used as a tool for better understanding the underlying phenomena, and to obtain detailed information about local mechanisms occurring below the indenter tip (such as dislocation nucleation, propagation, and interaction), which directly influence measured reaction force. to this end, both the indenter and specimen are typically modelled at the atomistic level using molecular statics or molecular dynamics, entailing high computational costs when realistic configurations and dimensions are used. the quasicontinuum (qc) method (cf. e.g. [1]) is employed to simplify the full atomistic model, to reduce the associated computational costs, and to allow for modelling of realistic situations. this paper focuses on the predictive abilities of an adaptive qc methodology (recalled in section 3) in combination with three types of error indicators/estimators for local mesh refinement compared against the full atomistic simulations. in particular, (i) the zienkiewicz–zhu error estimator (used for the deformation gradient), (ii) an indicator based on local atoms’ site energy, and (iii) an estimator based on local disregistry profiles are tested for a simple twodimensional indentation test. the individual definitions are outlined in section 3.3, whereas the accuracy and associated computational costs are discussed in section 4. the paper closes with conclusions and recommendations in section 5. 2. full atomistic model atomistic models based on molecular statics are characterized by an underlying lattice in combination with an interatomic potential. in this work, a twodimensional hexagonal lattice with lattice spacing d0 is used, as shown in fig. 1. individual atom interactions are described by the lennard–jones (lj) potential, defined as φαβ(rαβ) = ε [( rm rαβ )12 − 2 ( rm rαβ )6 ] , (1) where rαβ = ‖rβ−rα‖`2 denotes the distance between two atoms α and β, rm denotes the distance at which the interaction energy reaches its minimum, and ε is the energy well depth. the total potential energy associated with the entire atomic structure is computed as a sum over all interactions, i.e. e(r) = 1 2 natm∑ α,β; α 6=β φαβ(rαβ), (2) where natm represents the number of atoms, and r is a column storing their positions. because evaluation of the interatomic potential for all pairwise combinations is computationally expensive, and because long-distance interactions have negligible contributions to the total potential energy, a cut-off radius rcut is considered [1], beyond which interactions are neglected. such a simplification introduces a discontinuity of φαβ at rcut, which is removed by subtracting a linear function to assure zero value and zero slope of φαβ at rcut. as sketched in fig. 1, a 57 http://dx.doi.org/10.14311/app.2018.15.0057 http://ojs.cvut.cz/ojs/index.php/app k. mikeš, o. rokoš, r. h. j. peerlings acta polytechnica ctu proceedings d0 d0 θ = 120◦ rcut = 2.5d0 h0 = d0 √ 3/2 figure 1. a geometry of hexagonal lattice and corresponding cut-off radius (dashed line). cut-off radius rcut = 2.5 d0 is employed to provide next-to-nearest interactions. in order to find a stress-free configuration, an initial relaxation is carried out on an ideal periodic lattice with spacing d0 = rm, which results in a reduced lattice spacing d0 = 0.9917496 rm used for constructing the initial system. the geometry of employed indentation test is sketched in fig. 2. the specimen domain is of the size 128d0 × 128h0, contains 16, 862 atoms, and considers atoms near the bottom and both vertical edges as fixed, whereas the top edge is a free surface. the flat indenter is modelled at the atomistic level using the same hexagonal lattice as used for the specimen, but having infinite stiffness. its geometry is specified through a width 11d0 at the tip and two surfaces inclined by 60◦, as shown in fig. 3 (left). the positions of all indenter atoms are prescribed in 80 uniform loading, and 80 uniform unloading steps, achieving the maximum indentation depth 8d0. the interaction strength between atoms of the indenter and the tested material is reduced by a factor of 0.55 (compared to the atoms of the tested material) to prevent tearing of the indented specimen during the unloading stage. the total potential energy of the atomistic system e(r) is minimized at each time step using the trust-region algorithm; for further details see e.g. [2]. 3. quasicontinuum method the quasicontinuum (qc) method is a concurrent multiscale technique introduced in [3]. the key idea consists in combining the accurate but expensive atomistic description only in regions of high interest with a cheap continuum approximation elsewhere. the specimen domain therefore is divided into two parts: (i) the fully-resolved region, in which the full nonlocal atomistic model is used, seamlessly coupled with (ii) a coarse-grained continuum region, in which interpolation through triangular elements along with an efficient summation scheme is introduced. x y 12 8h 0 128d0 figure 2. a sketch of the tested setup: specimen being indented (grey), and indenter (black). figure 3. detail of indenter area at the beginning of the loading process (left), and after unloading (right). 3.1. interpolation the first step in the qc reduction is interpolation, which introduces the so-called repatoms through which the kinematic behaviour of the entire system is reconstructed according to r = φrrep, (3) where rrep is a column storing positions of all repatoms, interpolated through an interpolation matrix φ associated with the adopted triangulation. the number of repatoms is typically much smaller compared to the number of all atoms, reducing thus the computational effort required. two example triangulations associated with different sizes of the fullyresolved regions are shown in fig. 4. 3.2. summation in the second qc reduction step, a so-called summation rule is introduced to avoid the necessity of visiting all atoms when assembling the total potential energy in eq. (2). to this end, the site energies of all atoms situated inside a triangular element are approximated by the energy of only a few, or even one sampling atom and its corresponding weight factor wα, i.e. e = nsampatm∑ α wαφα. (4) 58 vol. 15/2018 molecular statics simulation of nanoindentation 0 20 40 60 80 100 120 140 -60 -40 -20 0 20 40 60 0 20 40 60 80 100 120 140 -60 -40 -20 0 20 40 60 figure 4. initial triangulation with small (left) and large (right) fully-resolved region. repatoms are shown as black dots, interpolation elements as blue triangles, sampling atoms as red dots, and remaining atoms as grey dots. in eq. (4), φα is the site energy of a sampling atom α, defined as φα = 1 2 natm∑ β; α6=β rαβ<rcut φαβ(rαβ). (5) although multiple possibilities are available in the literature to select sampling atoms and their corresponding weight factors, the central summation rule introduced in [4] is used hereafter (cf. fig. 4). 3.3. adaptivity the area of high interest (i.e. the fully-resolved region and hence also the associated triangulation) can adaptively evolve at each time increment or iteration to accommodate dislocation movements, while retaining qc efficiency. to this end, the fully-resolved region is sequentially updated as follows. at each time increment, the system is equilibrated for a fixed fully-resolved region. using a selected refinement criterion (detailed below), the triangulation is checked and refined if required. any additional atoms are added as repatoms, the interpolation mesh is updated, and equilibrium is restored. such a procedure is repeated until the mesh refinement criterion is satisfied for all elements, proceeding subsequently to a new load increment. in total three refinement criteria are tested: (i) the zienkiewicz–zhu error estimator (zz), as introduced in [5], used for the deformation gradient. that is, projection of the deformation gradient is used to estimate the local error inside each element. a threshold value zztr is specified to determine elements to be refined, and atoms located in those elements to become repatoms. (ii) the error indicator based on a local energy criterion uses the local atoms’ site energy to determine elements to be refined. first, the site energy of each sampling atom is tested for the following condition φα ≥ etr, (6) where etr is a selected critical energy threshold. sampling atoms satisfying the condition of eq. (6), as well as their neighbours within radius rref , are labelled as critical atoms. interpolation elements that contain at least one critical atom are refined, i.e. all atoms in such elements are added as repatoms. (iii) the error indicator based on disregistry profile works in a similar way as the local energy criterion. the only difference consists in the condition for the selection of critical atoms, now specified as natm∑ β; α6=β rαβ<rcut ||rβ − rαt ||`2 ≥ dtr, (7) where dtr is a selected disregistry threshold, and ||rβ − rαt ||`2 denotes the distance of atom β from the closest position of its theoretical neighbouring atom α considered in the reference configuration. the positions of either all eighteen nearestand next-to-nearest-neighbour atoms or just the six nearest-neighbour atoms can be used to evaluate the condition of eq. (7). all atoms within radius rref are again labelled as critical atoms. 4. results numerical simulations of the nanoindentation test, as described in section 2, are carried out to analyse the adaptive propagation of the fully-refined region associated with movements of individual dislocations. in the full atomistic model, initially all atoms are displaced elastically until a critical penetration depth is reached, which triggers nucleation of four dislocations positioned symmetrically (due to the symmetry of the problem) under both indenter edges. upon further loading, these dislocations propagate along preferred lattice directions until reaching the boundary of the specimen. during unloading, however, when the last symmetric dislocation pair annihilates, one direction is preferred due to numerical round-off errors, resulting in a nonsymmetric final shape of the indent (see fig. 3 (right)). results of the full simulation are used as the reference solution against which the qc simulations are compared, and are summarized along with the corresponding performance in table 1. the force–displacement diagram is shown in fig. 6. in qc simulations, movements of individual dislocations can be captured properly only inside the fully-resolved region. the adaptive algorithms introduced in section 3.3 are used to this end, providing results summarized in fig. 5 for the zienkiewicz–zhu error estimator and loading steps number 28 (penetration of the first symmetric dislocation pair outside of the original area of high interest), 39 (one step before nucleation of the second symmetric dislocation pair), 55 (one step before the nucleation of the third symmetric dislocation pair), and 80 (maximum penetration depth). the following cases for error estimators/indicators in qc methodology have been tested: 59 k. mikeš, o. rokoš, r. h. j. peerlings acta polytechnica ctu proceedings figure 5. results corresponding to qc simulations with zienkiewicz–zhu mesh refinement criterion. repatoms in different loading steps are shown, corresponding to: loading step number 28 (top left), 39 (top right), 55 (bottom left), and 80 (bottom right). the colour of individual atoms corresponds to the local disregistry level. dashed lines indicate the reference specimen shape. initial/final iterations mesh time repatoms iterations full solution 16862/16862 4980 1.00 (1:26:31) qc fix small 952/952 3549 0.061 qf fix large 1900/1900 3560 0.099 qc zz 952/6224 8957 242 0.772 qc dis 952/8143 7536 149 0.751 table 1. numerical performance corresponding to individual computational models. 60 vol. 15/2018 molecular statics simulation of nanoindentation figure 6. normalized force–displacement diagrams corresponding to the different computational models. figure 7. the number of repatoms relative to the number of all atoms as a function of load increment for the different computational models. (1) two simulations with fixed fully-resolved regions of different sizes, see fig. 4, referred to as qc small and qc large; (2) one simulation with an adaptive area of high interest using the zz criterion with zztr = 0.015, referred to as qc zz; (3) one simulation with an adaptive area of high interest based on local energy with refinement radius rref = 5d0 and threshold value corresponding to 6% change of the initial potential energy (for a typical internal atom this value corresponds to etr = −2.95); and (4) one simulation with adaptive area of high interest based on lattice disregistry with threshold value dtr = 0.5d0, the nearest-neighbour option, and refinement radius rref = 5d0, referred to as qc dis. the simplest qc simulation with a small fixed fullyresolved area (qc small) provides a significant speedup of a factor of 16 compared to the full atomistic model. the initial elastic behaviour and nucleation of the first symmetric dislocation pair is captured accurately, cf. fig. 6, whereas in later stages (i.e. once the first symmetric dislocation pair reaches the boundary of the fully-resolved region), the resulting behaviour becomes overly stiff. this is clearly visible in the corresponding force–displacement diagram. a partial improvement can be achieved by enlarging the fully-resolved qc region (qc large), which, nevertheless, suffers from the same shortcomings. the overestimation of the force–displacement diagram is less significant, capturing more accurately the unloading branch. the shapes of local force peaks do not correspond, however, to the exact solution due to the obstructed dislocation movements. the achieved speed-up is approximately of the order of 10. clearly, only the adaptive schemes are able to capture properly the dislocation movement under the tip of the indenter. note that all adaptive qc examples are initialized with the fully-resolved area of the same size as used for the qc small simulation. the zz criterion detects and refines elements near individual dislocations, allowing for their propagation throughout the entire specimen. this is reflected in the corresponding force-displacement curve in fig. 6, where a significant improvement in accuracy compared to the approaches with fixed meshes can be observed. finally, the local lattice deflection criterion is used, which provides (in the case considered) practically the same results as the energy criterion (not shown). generally, both criteria are not equivalent and differences can be observed in examples with different type of atoms, where the energy criterion is more robust at the interface. however, in case of a single crystal with one type of atom, for each value of disregistry threshold a corresponding value of energy threshold that provides virtually the same results can be found. compared to the zz condition, local lattice deflection provides similar accuracy in the resulting reaction force, while requiring a slightly higher number of repatoms (cf. fig. 7). in spite of this fact, its overall performance is slightly faster compared to zz (cf. tab. 1), explained by the fewer number of mesh iterations needed. unlike zz, the indicator based on the local disregistry provides two parameters, dtr and rref , for controlling the mesh evolution. whereas dtr locates the position inside the dislocation core region (see colours in fig. 5), rref reflects the size of the fully-resolved region considered around that position. large values of rref therefore provide fast propagation in the direction of the dislocation movement, but at the same time may require too many repatoms in the perpendicular direction. the value rref = 5d0 proved to provide a good balance. the performance of all qc approaches is summarized in tab. 1, where the number of repatoms, required number of solver and mesh iterations, and computational times are reported. all adaptive qc approaches provide accurate descriptions during the loading as well as unloading stages, resulting in acceptable estimates of the corresponding reaction force. although the obtained gain in computation time is relatively small (about 25% compared to the full simulation), a more substantial speed-up is expected for 61 k. mikeš, o. rokoš, r. h. j. peerlings acta polytechnica ctu proceedings simulations with larger domains. 5. conclusions in this contribution, a quasicontinuum (qc) method with different mesh refinement strategies has been compared against the full molecular statics simulation for the case of a two-dimensional nanoindentation test. it has been shown that all three tested adaptive qc approaches provide acceptable agreement with the underlying full atomistic simulation. two adaptive approaches (based on local energy and lattice disregistry) provide equivalent results compared to the widely used zienkiewicz–zhu error estimator, while performing slightly faster due to fewer mesh iterations. overall, adaptive qc approaches reduce computational time by approximately 25% compared to the full simulation. for large domains, more significant speed-ups with similar accuracy can be expected. list of symbols d0 horizontal lattice spacing h0 vertical lattice spacing φαβ lennard–jones (lj) potential ε energy parameter of lj potential rm distance parameter of lj potential rαβ distance between two atoms α and β rcut cut-off radius e potential energy natm number of all atoms nsampatm number of sampling atoms r atom positions rrep repatom positions φ interpolation matrix φα site energy of an atom α wα weight factor etr energy threshold dtr disregistry threshold acknowledgements financial support of this research from the grant agency of the czech technical university in prague (sgs18/037/ohk1/1t/11), and from the czech science foundation (gačr 17-04150j) is gratefully acknowledged. references [1] e. b. tadmor, r. e. miller. modeling materials: continuum, atomistic and multiscale techniques. cambridge university press, 2011. [2] a. r. conn, n. i. gould, p. l. toint. trust region methods, vol. 1. siam, 2000. [3] e. b. tadmor, m. ortiz, r. phillips. quasicontinuum analysis of defects in solids. philosophical magazine a 73(6):1529–1563, 1996. [4] l. a. a. beex, r. h. j. peerlings, m. g. d. geers. central summation in the quasicontinuum method. journal of the mechanics and physics of solids 70:242–261, 2014. [5] o. c. zienkiewicz, j. z. zhu. the superconvergent patch recovery and a posteriori error estimates. part 2: error estimates and adaptivity. international journal for numerical methods in engineering 33(7):1365–1382, 1992. 62 acta polytechnica ctu proceedings 15:57–62, 2018 1 introduction 2 full atomistic model 3 quasicontinuum method 3.1 interpolation 3.2 summation 3.3 adaptivity 4 results 5 conclusions list of symbols acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2020.26.0112 acta polytechnica ctu proceedings 26:112–116, 2020 © czech technical university in prague, 2020 available online at https://ojs.cvut.cz/ojs/index.php/app mechanical and image analysis of adhesion between mineral aggregate and bituminous binder jan trejbal∗, tereza valentová, václav nežerka, petr semerák czech technical university in prague, faculty of civil engineering, department of mechanics, thákurova 7, 166 29 prague 6, czech republic ∗ corresponding author: jan.trejbal@fsv.cvut.cz abstract. asphalt mixtures often fail due to poor interaction between mineral aggregate and bituminous binder. therefore, many efforts are being made on adhesion improvement between the two materials. in this work, paving grade bitumen 50/70 was doped with two types of adhesion promoters. asphalt mixtures composed of crushed aggregate brant coated by binder were made and exposed to stripping water. then, they were subjected to visual and digital image analysis aiming to quantify aggregate residual bitumen-coated areas. besides, two cylindrical samples of aggregate were bonded together by a thin film of bitumen doped with adhesion promoters. after solidification of binder, force needed for separation of the two parts was measured. it was shown that residual bitumen-coated areas were increased by 20-30% by adding adhesion promoters into binder. mechanical adhesion of thus modified binder to aggregate was increased by up to twice. keywords: bituminous binder, adhesion, adhesion promoters, pull-off tests, image analysis. 1. introduction asphalt mixtures used in construction of road pavements are composite materials consisting of two main phases: mineral aggregate and bituminous binder. adhesion between them is the key parameter which is responsible for road mechanical properties as well as lifetime. if adhesion is insufficient, the two materials can be disintegrated. consequently, pavement structure is thus damaged with potholes [1]. physicochemical interaction between binder and aggregate is complex process based on adhesive bonds. these are influenced by bitumen chemical composition (especially presence of polar components), temperature of materials used, and surface properties of aggregate [2, 3]. it has to be also taken into account that adhesion between binder and aggregate, alongside the parameters mentioned above, is strongly influenced by water. most of aggregates used for production of road pavements are composed of hydrophilic acid-based minerals like granite and quartz. these exhibit strong affinity for water, higher than for bitumen [4, 5]. given the fact that aggregate of the most frequented hot-mix asphalt mixtures is during production of asphalt mixtures dried and preheated to high temperatures often exceeding 150 °c [6, 7], the presence of internal water (water contained in row aggregate) can be practically excluded. however, the exposition to external water, ie., water entering pavements because of poor drainage or highly wet subgrade is unavoidable. consequently, bitumen is thus replaced by water due to its low interaction with aggregate, followed by stripping effect – an adhesion failure between bituminous material and aggregate surface [4, 5]. the whole process of so-called asphalt moisture damaged is illustrated in figure 1. the figure a) shows theoretical ideal state: aggregate is perfectly coated by thin film of bitumen and is thus protected against water penetration. however, the real state is depicted in figure b) where the continuous layer of bitumen is damaged due to either poor coating process or abrasion against other aggregate particles. because of traffic loads and phenomenons described earlier, water gradually penetrates the aggregate through the bitumen coat defect and thus separates the bitumen thin film from aggregate surface [8, 9]. adhesion lost a) b) c) aggregatebitumen coat water figure 1. aggregate coated by bitumen thin film, a) ideal state, b) real state, and c) stripping due to water. to avoid stripping, different types of adhesion promoters at the form of liquid additives can be added into bituminous binder. the role of such chemical surfactants is to maximize the formation of intermolecular interactions between bitumen and aggregate surface [4, 5] and to reduce aggregate hydrophilicity. these are usually based on alkylsilanes, phospho112 https://doi.org/10.14311/app.2020.26.0112 https://ojs.cvut.cz/ojs/index.php/app vol. 26/2020 adhesion between mineral aggregate and bituminous binder mark binder adhesion promoter admixture amount [wt.%] r 50/70 0 m1 50/70 unsaturated fatty acids with diethanolamine 0.2 m2 50/70 phosphorus-based 0.3 table 1. summarization of three variants of bituminous binder 50/70 used. rus, chemical surfactants comprising amines, or unsaturated fatty acids combined with diethanolamine. their amount differed from 0.2 to 0.4% of binder weight. the choice of a suitable additive depends on the origin of used aggregate, the type of binder, and temperature of asphalt mixture preparation [10, 11]. with using of adhesion promoters, a new question about their efficiency arises: how do they improve adhesion between the two materials? generally speaking, two evaluating basic methods are known [12]: (i) loose asphalt mixture tests and (ii) mechanical analysis. the first of them is based on subjective visual or digital image analysis [13, 14]. asphalt mixture is made according to relevant technical standard [6, 15] and then exposed to hot stripping water or abrasion for certain period. after that, the stripped aggregate areas are quantified and assessed. these methods are indirect and it is therefore clear that adhesion between the two materials can not be quantified from the mechanical point of view. the second mentioned method – mechanical analysis – is then divided into two subcategories as indirect and direct. indirect testing is based on mechanical behavior evaluation of asphalt mixture specimens. in recent technical practice, we are talking primarily about tensile strength tests of prismatic or cylindrical asphalt mixture specimens. the durability and resistance of compacted specimens is checked before and after water exposition [16]. it is obvious that the indirect testing can be influenced by many variable factors, for example by mixture inhomogeneity, aggregate shape, etc. the all shortcomings listed above can be overcome using the direct mechanical adhesion analysis. aggregate-bitumen bonds can be determined by means of peel or pull-off tests. in case of the first mentioned, polymeric or metal belt is bonded using bitumen as the adhesive thickness-controlled layer on preheated prismatic sample of aggregate. after cooling of the specimen and solidification of bitumen, the belt is peeled off under angle of 90° (to aggregate surface plane), while the fracture development is observed [17, 18]. pull-of tests are based on the same general principles. thin layer of bitumen is sandwiched between two cylindrical aggregate samples. one part of aggregate is after technological pause pulled off, aiming to evaluate interfacial bond strength between the two materials [18]. such a method seems to be the most effective of all mentioned, because it combines only two main materials and provides the most valuable data from the perspective of interface interaction. although the pull-off method is not widespread in the czech republic, we decided to use it in order to determine bond strength between mineral aggregate and differently modified bituminous binder. to extend such obtained results, we made asphalt mixtures and subjected them to standard loose asphalt mixture tests. 2. materials 2.1. aggregate and bituminous binder for purposes of this study, aggregate from brant quarry was chosen. it is composed primarily of granite porphyry, or porphyry microgranite (quartz, potassium feldspar, and mica rocks), therefore it is hydrophilic and thus more susceptible to water damage, with poor adhesiveness to binder and elevated potential of stripping failures. its surface can be characterized as porous and its composition as relatively homogeneous. paving grade bitumen 50/70 was used, standardized by [19]. 2.2. adhesion promoters to ensure stronger adhesive bonds between both materials, two types of liquid-form chemical-based additives were used. these were applied at the trace amount of 0.2 and 0.3wt.% of bitumen according to recommendations of their manufacturers. reference and two modified binder variants were used, as summarized in table 1. in the case of mixture marked as m1, reactive products of unsaturated fatty acids with diethanolamine were used. the additive can be characterized as brown viscous fluid, which is recovered by condensation of unsaturated fatty acids at high temperature with disposal of water. the acids are dissoluble in ethanol and acetone and can be emulsified in water. in the bituminous binders, such additives are soluble at standard blending/mixing temperatures. the additive according to the producer shall be resistant to long-term heating (150 hours) at 150 to 160 °c without any changes in its activity. the second mixture, m2, was doped with a new generation of phosphorus-based adhesion promoters. this type is specific due to versatile application and improved thermal stability allowing to resist higher temperatures. 2.3. sample preparation cylindrical specimens having 42±0.2mm in diameter were drilled from block of aggregate using table grill equipped with diamond tipped tile drill bit. then, such obtained samples were cut off to 25mm length using saw struers secotom-15 with diamond cutting 113 j. trejbal, t. valentová, v. nežerka, p. semerák acta polytechnica ctu proceedings blade to ensure perfectly planar cut, perpendicular to longitudinal axis of the specimens. two thus obtained specimens were then preheated to 150±5 °c and sandwiched with 1mm thick layer of the bitumen with the same temperature. the thickness of the bitumen layer was ensured by applying its properly weighed amount. after that, the sandwiched specimens were tempered to 15±1 °c. according to [6], crushed aggregate of fraction 8/16mm (in sum ca. 300 g, temperature 160±5 °c) was coated by the bituminous binder (12±0.3 g, temperature 170±5 °c). after 24 hours of samples storage, these were submerged into stripping water having 60 °c for following 60±5 minutes. then, they were subjected to visual and digital image analysis as described in separate paragraph below. 3. experimental methods 3.1. pull-off tests sandwiched cylindrical specimens, 5 pieces for each mixture, were fixed to clamping chuck composed from two mirror half circles. a space between the chucks and testing specimens was filled with rubber seal in order to ensure strong adhesion between them. then, thus fixed specimen was placed to steel cage ensuring its direct guiding during loading. the cage is movable in the direction of specimen axis. it is equipped with tensile sticks from the both sides. these are used for fixing the whole construction in a loading frame, as captured in figure 2. sandwiched specimens were loaded through the construction by tension using a loading frame mts alliance rt/30. the experiment was displacement controlled at the constant rate of 0.5mm/min. figure 2. construction for pull-off tests fixed in loading frame. in the middle, there is a sandwiched specimen placed. 3.2. visual and image analysis following [6], the conventional visual analysis was based on subjective assessment of asphalt mixtures by two evaluators, who classified the rate of stripped areas of aggregate. in order to minimize "human factor" and inaccurate evaluators assessment, semi-automatic digital image analysis was employed. asphalt mixtures were captured using dslr camera canon eos 6d (full-frame sensor) equipped with lens canon ef 100mm f/2.8l macro is usm. such device allowed to capture even insignificant details being necessary for proper assessment. the samples were lighted by daylight only. consequently, thus obtained images were subjected to evaluating software enabling entropy-based image segmentation. thanks to local entropy calculation, the software is capable to assess roughness of the texture. the areas with high entropy are considered to belong to aggregates while low entropy areas represent the bituminous matrix [14]. 4. results and discussion 4.1. pull-off tests force as a function of displacement obtained during pull-off testing is for all three types of sandwiched specimens shown in figure 3. all curves were calculated as arithmetic mean from 5 independent measurements. a significant difference was detected in comparison of specimens with modified and reference bitumen. both modified specimens (m1 and m2) showed bonding strength approximately twice as higher as those containing reference binder (r). moreover, reference samples showed perceptibly less gradual increase in recorded force in initial stages of loading. comparing both samples sandwiched with doped bitumen to each other and considering measurement deviations, it can not be concluded which adhesion promoter is better. 4.2. visual and image analysis visual analysis of two independent operators revealed that bitumen surface coated area reached on 60% only in the case of the reference mixture (r), with belonging verbal classification as "unsatisfactory" according to [6]. unlike both modified mixtures (m1 and m2) exhibited the area about 80% – "satisfactory" classification (see table 2). entropy-based image segmentation, however, provided quite different results than visual analysis. concretely, reference mixture (r) exhibited 67.1% of coated areas, while m1 and m2 94.7 and 96.0%, respectively (table 2). given the fact that the semiautomatic analysis is considered to be more accurate than the subjective one, such obtained results are assumed to be more valuable and conclusive. moreover, as it is shown in figure 4, the green colored regions, marking the coated areas, prove that the amount of stripped areas is negligible. regardless to differences between results from both methods, it can be concluded that adhesion promoters significantly contributed for improving in adhesion between mineral aggregate and bituminous binder. 114 vol. 26/2020 adhesion between mineral aggregate and bituminous binder fo rc e [n ] displacement [mm] 0 500 100 200 300 400 0 2.01.51.00.5 r m1 m2 figure 3. force as a function of displacement revealed during pull-off experiment. mark coated area by classification by coated area by classification by visual analysis (%) visual analysis dic analysis (%) dic analysis r 60 d-e: unsatisfactory 67.1 d: unsatisfactory m1 80 c: satisfactory 94.7 a: good m2 80 c: satisfactory 96.0 a: good table 2. summarization of binder coated area results obtained by both visual and digital image (dic) analysis. figure 4. images of aggregate coated by binder (top) and highlighted coated areas as a result form dic analysis (bottom). 115 j. trejbal, t. valentová, v. nežerka, p. semerák acta polytechnica ctu proceedings 5. conclusions adhesion between paving grade bitumen 50/70 and mineral aggregate from brant quarry was analyzed by means of visual, digital image, and mechanical analysis. in order to increase an interaction between the two materials, bitumen was doped with adhesion promoters based on unsaturated fatty acids with diethanolamine and phosphorus-based at the amount of 0.2 and 0.3wt.% of binder. asphalt mixtures composed from crushed aggregate (8-16mm) coated by binder were exposed to stripping water. consequently, areas coated by binder were quantified. then, the adhesion was analyzed mechanically using pull-off tests, when two cylindrical parts of aggregate were sandwiched with 1mm layer of bitumen binder and consequently pulled-off. it was shown that adhesion between the two materials was increased using adhesion promoters by ca. 20 and 30% according to visual and digital image analysis, respectively. pull-off tests revealed that mechanical adhesion between aggregate and modified bitumen was increased by up to twice. acknowledgements this work was supported by the czech science foundation (research project no. 18-13830s) and by the czech technical university (no. sgs19/148/ohk1/3t/11) and the support is gratefully acknowledged. special thanks to tereza horová for editing assistance and to dr. petr bittnar for helping with experimental testing. references [1] n. ahmad. 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[19] čsn en 12591: bitumen and bituminous binders – specifications for paving grade bitumens, praha: czech office for standards, metrology and testing, 2009. 116 https://doi.org/10.1016/j.cis.2008.08.011 https://doi.org/10.3390/app7050524 https://doi.org/10.1016/j.conbuildmat.2018.03.136 https://doi.org/10.1080/14680629.2017.1304265 https://doi.org/10.3846/1392-3730.2009.15.169-179 https://doi.org/10.1016/j.conbuildmat.2019.01.167 https://doi.org/10.1016/j.conbuildmat.2012.08.033 https://doi.org/doi:10.14311/app.2018.15.0126 https://doi.org/10.1016/j.conbuildmat.2014.04.012 https://doi.org/doi:10.14311/app.2019.21.0038 https://doi.org/10.1080/14680629.2019.1605304 https://doi.org/10.1080/14680629.2010.9690311 https://doi.org/10.1016/j.ijadhadh.2014.05.009 https://doi.org/10.1016/j.ijadhadh.2015.06.012 acta polytechnica ctu proceedings 26:112–116, 2020 1 introduction 2 materials 2.1 aggregate and bituminous binder 2.2 adhesion promoters 2.3 sample preparation 3 experimental methods 3.1 pull-off tests 3.2 visual and image analysis 4 results and discussion 4.1 pull-off tests 4.2 visual and image analysis 5 conclusions acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2018.16.0025 acta polytechnica ctu proceedings 16:25–31, 2018 © czech technical university in prague, 2018 available online at http://ojs.cvut.cz/ojs/index.php/app analysis of remedial actions designed to stabilize a highway cut in a landslide area peter turček, monika súľovská, miloslav kopecký, jakub panuška∗ slovak university of technology, faculty of civil engineering, department of geotechnics, radlinského 11, 810 05 bratislava, slovak republic ∗ corresponding author: jakub.panuska@stuba.sk abstract. the carpathian flysch is widely recognized as a geological environment prone to slope movements. this fragility in such an environment is due to its complicated geological and hydrogeological conditions. weakened slip surfaces (developed in clay like material), in combination with present or past tectonic activity and difficult hydrogeological conditions with aquifers under pressure, are the main causes of slope movements. the article proposes to analyze the design of remedial action in a landslide area near žilina. slip surfaces and water conditions are assumed regarding present geotechnical monitoring. deep cut with two rows of pile walls is analyzed by the limit equilibrium method and finite element analysis. interaction between two walls and the effect of an amount of volume of sliding mass on resultant earth pressures and anchor forces were studied. the results from finite element analysis are compared with analytical calculations and the differences are discussed. keywords: carpathian flysch, landslide stabilization, anchored pile wall. 1. introduction the environment of the carpathian flysch creates complicated geological conditions. often, layering of coarse permeable layers (sandstone, conglomerate which are also aquifers) with high strength, and in some places weathered soil like claystone in combination with tectonics, presents positive conditions for development of slope movements in the form of landslides and block deformations. such movements often threaten manmade structures [1, 2]. highway d1 near žilina in part lietavská lúčka – višňové passes through this landslide-prone area in the carpathian flysch. bridges, deep cuts and high embankments are constructed in this unstable relief. new slip surfaces and landslide areas were found in the locations of projected cuts during preparation phase. these instabilities were measured by recently installed inclinometers. former designs, based on a paucity of information, had then to be revised. additional information about landslide activity, depth of slip surfaces and the geological environment were collected concurrently with the design of remedial actions. this information was systematically reviewed during the design of retaining structures. in most of the cases new findings resulted in significant modifications to the former design. the finite element method (fem) was used to study interaction effects between two walls and the effect of an amount of sliding soil mass on resultant earth pressures and anchor forces essential for the structural design of elements in the whole system. 2. geological conditions on site the highway in the presented section passes through an area which consists of partially masked older and also recent younger significant landslides. some block type deformations were also found near an adjacent bridge. formations of landslides were affected by tectonic activity in scarp parts and by erosion activity of a local stream in the toe. at the studied site the paleogene saddle is covered by deluvial clays of low to intermediate plasticity. the clays are classified as firm to stiff. paleogene formation made of claystone, sandstone and conglomerate is weathered to a depth of 15 m. tectonically fractured material contains the clay mineral – smectite. several aquifers with pressurized water were found. these are usually connected to tectonics or layers of conglomerate or sandstone. due to these complicated conditions the ground water level (gwl) was hit at different depths. the stable gwl was in 1999 found to be in depth 1.3 m below terrain with sometimes outflows from the borehole. these facts were confirmed during geotechnical monitoring (gtm). outflow on terrain was observed in some open pipe piezometers. these findings confirmed also the pressurized character of gwl and indicate the necessity for installation of horizontal drainage boreholes (hov). the effect of hov on slopes stability was also evaluated by initial calculations which considered a lowering of gwl. the scope of this article analyzes the design of cut support by a retaining wall 0.834–1.274 km. the area was divided into two parts, one virgin not affected by landslides and the second one in a landslide prone area. soil / rock parameters used in calculations (table 1 and table 2) were set according to broad geological studies which were done from 1999 [3–5] to 2016 and 25 http://dx.doi.org/10.14311/app.2018.16.0025 http://ojs.cvut.cz/ojs/index.php/app p. turček, m. súľovská, m. kopecký, j. panuška acta polytechnica ctu proceedings of comparable experience. the coordination of the site is in figure 1 with the part marked for calculation. figure 2 represents the geotechnical model put forward in this article. deeper movements were found which were not in accordance with information from the geological survey. these movements were found at depths of 12 m and were observed during drilling of hov. according to the geological survey the layer of conglomerates with pressurized water were probably hit. it is assumed that measured deformations of a magnitude of 10 mm in adjacent ink – 17 were caused by sudden decrease of water pressure in the aquifer. this resulted in immediate increase of effective pressure together with drag flow which activated movement. these movements were stabilized but activation of residual parameters of shear strength on the slip surface was assumed for the calculations. deformations of max. 40 mm in depths from 2 to 9 m were measured by inclinometers installed in the studied area. 3. designed remedial actions the maximum depth of cut is projected as 17 m. former designs were recalculated according to present measurements and findings. geo5 software module “sheeting check” was used to verify stability of the anchored pile wall. overall slope stability was verified in module “slope stability” with circular (bishop) and polygonal (sarma) slip surface. this paper analyses a cut of depth 15.35 m at 1.150–1.175 km (figure 3) where the deepest movements were observed. the rest of the calculations follow similar assumptions and principles as those introduced at km 1.150–1.175. the retaining wall was designed with increased pressure from creeping landslide [6]. the increased creeping pressure ecr is calculated as: ecr = m(ϕ)γ h 2 2 cos 2ϕ (1) where m(ϕ) is a multiplication factor for creeping pressure and depends on friction angle ϕ and flexibility of structure (herein considered as 1.1), γ is soil unit weight and h is the height of sliding mass. equation (1) is applicable if inclination of slip surfaces is more or less parallel to the surface of terrain, ϕ=β (where β is terrain inclination). peak shear strength parameters were used after stabilization of the sliding mass due to the idea that old slip surfaces were stabilized by a retaining structure which was designed to withstand these actions. thus expected movements were considered in stable layers. one of the assumptions was that former slip surfaces with residual shear strength affect minimally newly developed ones under a retaining wall. calculations considered also construction phases. the deepest excavation was considered 1 m below future road layers. žilina is in a seismically active region and thus seismic actions were considered for design of retaining wall and slope stability analysis. these calculations were performed according to mononobe–okabe theory with horizontal kh and vertical kv (up and down) factors of acceleration. a surcharge of 20 kpa was considered at the bottom of the excavation due to construction of road layers. a complicated situation is also described by number of evaluated sections along a 440 length of cut. nine characteristic sections were considered with different geotechnical models and depth of slip surfaces. the design of remedial actions reflects different boundary conditions. the most critical section (figure 3) will be shown as an example. slip surface at depth of 12 m was identified according to measurements on ink – 17. the ground water level before constructions reached a maximum level 0.5 m below terrain. during optimization of solution a cut with a depth of 15.35 m stabilized by one row of pile wall seemed to be not a relevant and feasible solution. the upper part of the pile wall will be constructed after an excavation of 0.5–1.0 m. piles will be constructed of concrete c30/37 with diameter of 900 mm, length 16 m with spacing 1.3 m. anchors in position 0.5 m below the pile head will be installed after a subsequent excavation of 1.5 m. anchors were designed with total length 32 m (26 m free length, 6 m fixed length), inclined 30◦ from the horizontal with a spacing 1.3 m. pre-stressing force was set to 400 kn. the fixed part has to be placed in position below the slip surface in an intact claystone environment and thus a steep inclination of 30◦ with long free length 26 m was designed. increased pressure from the creeping landslide was calculated according to [6]. additional force 54.62 kn/m uniformly distributed up to a depth of 10 m was set as an additional load to earth pressures behind the wall. earth pressures were solved by geo5 software according to the method of dependent pressures. the bottom wall was modelled as a single wall with surcharge (the effect of the upper part). the cut at the upper wall has a depth of 4.5 m. a bench with width of 5 m was designed between the upper and bottom wall due to maintenance and decrease of earth pressures acting on the bottom wall. the bottom wall is designed as a pile wall constructed of concrete c30/37 with pile diameter 900 mm, length 18 m and spacing 1.3 m. two rows of anchors were designed due to excavation to depth 9.1 m. positions of anchors are at depths 1.0 and 5.5 m below the pile head. anchors were designed with total length 32 m (23 m free length, 8 m fixed length), inclined 30◦ from horizontal with spacing 1.3 m. pre-stressing force was set to 450 kn. the water level is assumed to decrease with excavation in combination with horizontal drainage boreholes. permanent horizontal drains will be installed to achieve efficient draining of the environment behind the wall. this effect was observed on different constructions in similar geological conditions. difference of water levels behind and in front of the wall was 26 vol. 16/2018 remedial actions soil / rock γ γsat ϕ’ c’ δ ν edef [kn/m3] [kn/m3] [◦] [kpa] [◦] [-] [mpa] deluvial clay / debris 20.0 21.0 17.0 15.0 8.5 0.4 5 weathered claystone 20.5 21.5 22.0 16.0 11.0 0.35 20 partially weathered claystone 22.0 23.0 22.0 30.0 11.0 0.3 60 tectonically disturbed claystone 21.2 21.5 20.0 16.0 9.5 0.4 20 table 1. soil/rock parameters used in calculations. soil / rock e50 eoed eur νur m [mpa] [mpa] [mpa] [mpa] [-] [-] deluvial clay / debris 5.0 5.0 15.0 0.25 0.6 weathered claystone 20.0 20.0 60.0 0.2 0.6 partially weathered claystone 60.0 60.0 180.0 0.2 0.6 tectonically disturbed claystone 20.0 20.0 60.0 0.2 0.6 table 2. soil/rock parameters used in fem calculations. figure 1. landslides map with inclinometers (red circles) and open-pipe piezometers (blue circles). figure 2. geotechnical section 2 through ink-17. 27 p. turček, m. súľovská, m. kopecký, j. panuška acta polytechnica ctu proceedings figure 3. geotechnical model at km 1.150–1.175. assumed as 3 m during excavations. this assumption has to be verified during excavation through measurements in adjacent open pipe piezometers due to the presence of sandstone and conglomerate lenses with pressurized water. characteristic pull out resistance was set to 150 kn/m of the fixed part. this value was measured during load tests of the anchors [7]. 4. finite element analysis three sub-tasks were analyzed to verify assumptions made by the authors and to verify the safety of the design. the first task is interaction between the upper and bottom walls. the second task is verification of the assumption that old active slip surfaces will be successfully stabilized and thus peak shear strength parameters can be used because newly developed slips surfaces will develop in an undisturbed environment. thus residual parameters on old slip surfaces and also old active slip surfaces affect minimally the resulting safety factor. the third task was to analyze the effect of activation of multiple sliding masses (sm). activation of three possible sm was considered. the geotechnical model used in the fem calculation (and also for limit equilibrium slope stability) is in figure 4. finite element software plaxis was used for modelling of walls interaction. the hardening soil material model (hsm) was used because of its capability to model unloading/reloading stiffness and thus model deformations after excavation more realistically. this model also includes shear and volumetric hardening with increasing strains [8]. choice of input stiffness parameters was based on statistical processing of in-situ pressure meter tests [5], laboratory tests (oedometer) [5] and comparable experience. parameters for fem calculation are in table 2. parameter m for modeling stress dependency of stiffness was chosen rather arbitrarily. angle of dilation ψ was assumed as 0◦. due to insufficient information on deformation parameters the fem model was used just to verify the mechanism of walls interaction. due to insufficient information on deformation parameters the fem model was used just to verify the mechanism of walls interaction. residual parameters at the given site were back calculated as ϕ(r)=12◦ and cr=0 kpa [9]. these parameters were applied to a discrete zone of slip surface (1 m) in the fem calculation. residual parameters at different slips surfaces were activated at different stages of calculation to consider the effect of an amount of sliding mass. excavation stages were considered as they are expected to be performed during constructions works. 5. analysis of the results horizontal earth pressures σh and effective geostatic/vertical stress σv behind the wall and in front of the wall were extracted from the fem calculation from elements adjacent to the structure. total force fi from acting horizontal pressure σhi was obtained by the integration of pressure over the length of the structure l [10]: fi = ∫ l 0 σhidl (2) maximum/limiting lateral earth pressures were calculated from vertical effective stresses obtained by the fem calculation. limiting earth pressures (σa – active case and σp passive case) for cohesive soil were 28 vol. 16/2018 remedial actions figure 4. geotechnical model at km 1.150–1.175 with three slip surfaces expected. calculated as: σa = σvka − 2c′ √ ka (3) σp = σvkp + 2c′ √ kp (4) where ka and kp are coefficients of active and passive pressures respectively according to coulomb with due consideration of terrain inclination and soil / wall friction. horizontal effective stresses for one activated sliding mass (1sm) are shown in figure 5a with additional load (ecr = 546.2 kn – independent of sliding volume activated) in geo5. it is shown that the magnitudes of the stresses from the fem calculation in comparison with stresses from geo5 (ecr + earth pressures) are almost the same. the difference in resultant force after integration along the depth of the wall is approximately 50 kn/m. horizontal effective stresses acting on the bottom wall are shown in figure 5b. a small increase in stresses acting on the wall is shown by fem calculation. difference in pressures for the bottom wall is higher than in the case of the upper wall. the difference is 160 kn/m with the fem model (higher stress for fem) for the active case and 300 kn/m with the fem model (higher stress for fem) for the passive case. the retaining structure will be able to mobilize additional passive resistance and resist and withstand even the increased forces found in the fem model. additional mobilization of anchors is also possible. a difference in anchor forces is almost 150 kn (max. force 633.63 kn – fem in case of 3sm activated). this value is slightly above the resistance due to max. pull out resistance 605.32 kn with partial safety factor γ = 2. passive and active limit states behind the structure showed that resultant pressure is in between the active and passive case. this seems to be obvious while pre-stressing forces are applied and act in opposite direction to the sliding mass. it is evident that in both cases, upper wall and bottom wall, the closest places to the passive case are those where the anchors are placed. if additional force ecr was added in geo5 enough deformation was achieved to mobilize active pressure. creeping force ecr is about 250 kn/m higher than the difference between the phase with no sm activated and three sm activated which is 292.38 kn (table 3). time effects and creep were not modeled in fem calculations. results for the different sm (up to three sliding masses – 3sm) activated are in figure 6 and in table 3. as is shown forces in all anchors, and also earth pressures behind and in-front of the wall, are increasing with increasing activation of sliding masses. increase in pressure is predominantly related to the zone near the slip surface. the increase in driving forces/pressures results in an increase in anchor forces and the mobilization of more passive resistance. these factors also influence internal forces and also deformations but these are yet to be analyzed. the change in the shape of pressure diagrams is not so dramatic. the upper wall is most affected by activation of multiple sliding masses. an increase of resultant forces in front of the structure is around 200 kn for the upper wall. forces behind the structure are increased above 300 kn and forces in the anchors around 150 kn. resultant force in the front of the wall was almost unaffected by activation of multiple sliding masses. forces behind the wall were increased as a result of additional mobilization of pressures in front of the upper wall. this also leads to an additional increase of forces in anchors. the sum of horizontal forces acting on the wall was calculated as:∑ f = (fanchor−1 + fanchor−2) cosα+ fin−front − fbehind (5) where fin−front is the resultant force in front of the structure obtained as integration of σin−front over length of structure l, fbehind is resultant force behind the structure, fanchor−1 and fanchor−2 are forces in the first or second row of anchors and fs is the overall factor of safety after full excavation. as is shown eq. (5) led to a positive result which means 29 p. turček, m. súľovská, m. kopecký, j. panuška acta polytechnica ctu proceedings figure 5. horizontal effective stresses for one sliding mass activated a) upper wall; b) bottom wall. figure 6. horizontal effective stresses for multiple sliding masses (sm) activated a) upper wall; b) bottom wall. case fin−front fbehind fanchor−1 fanchor−2 ∑ fanchor−1 fs [kn] [kn] [kn] [kn] [kn] [-] 0 sm upper wall 1438.71 1784.57 480.92 70.63 1.35 0 sm bottom wall 1888.38 2386.94 553.57 599.48 500.01 1.35 1 sm upper wall 1576.61 1935.44 514.57 86.80 1.347 1 sm bottom wall 1876.11 2384.93 574.18 595.60 504.24 1.347 2 sm upper wall 1588.54 1955.27 518.02 81.89 1.355 2 sm bottom wall 1842.98 2651.77 574.58 558.31 172.32 1.355 3 sm upper wall 1642.00 2076.95 633.63 113.79 1.34 3 sm bottom wall 1871.58 2698.28 596.31 562.98 177.27 1.34 table 3. results of fem analysis. 30 vol. 16/2018 remedial actions that passive forces are higher than triggering and thus stability can be achieved. the difference between the safety factor for geo5 and the fem calculation is negligible ∆fs = 0.023. overall slope stability was verified in the first model with peak shear strength parameters because it is assumed that old slip planes are stabilized by pile walls which are designed to withstand these actions (an increase of load on the wall due to creeping landslide). residual parameters in verification model 2 were set in the scarp area in case that newly developed movements will use present weak planes. the difference between these two models was ∆fs = 0.05 (bishop) and 0.07 (sarma), which is a negligible difference. this tendency was also shown by the fem calculations (table 3) which in general lead to the same safety factor as in the case of limit equilibrium methods. 6. conclusions the article presents an analysis of the remedial actions designed to secure a future highway cut. the effect of multiple sliding masses activated and interaction of walls were considered. analysis showed that an increasing volume of moving mass results in a slight increase of forces pressure acting on the upper wall. the bottom wall was in general affected almost in the same way due to a slight increase in anchor forces necessary to provide enough support for the toe of the upper wall. the analytical solution provided a higher resultant force from creeping mass than modeled by fem. time effects and creeping behavior of sliding mass were not considered, which perhaps explains that observation. it is always beneficial to use analytical solutions and fem calculation to observe possible mechanisms. a critical review with the pros and cons of both approaches has to be considered. parametric study and possibly sensitivity analysis (which was done only partially in the presented article) should be carried out to observe different effects. the possibility of progressive activation has to be verified through the conducted geotechnical monitoring. forces in the anchors and slope movements have to be measured and the fem model has to be calibrated. assumptions about the stabilization of recent sliding mass and development of new slip surfaces in a virgin environment were confirmed and have to be proved during construction. geotechnical monitoring is one of the essential steps as a part of the observational method in such a complicated environment as the carpathian flysch. the obtained data will be used to carefully calibrate the model and to propose an optimized solution in future designs for the carpathian flysch. acknowledgements the paper is a part of results from research project supported by slovak agency vega no. 1/0882/16 “influence of boundary conditions on limit states of geotechnical structures”. references [1] m. kopecký, m. ondrášik, j. frankovská, d. antolová. landslides on the slopes of water reservoirs in the environment of carpathian flysch. in proceedings of the 13th international symposium water management and hydraulic engineering. nakladateľstvo stu bratislava, 2013. [2] m. kopecký, m. ondrášik. landslides on the route of the designed highway d1 in the area of inner carpathian flysch belt. in proceedings of the 14th danube-european conference on geotechnical engineering. slovak university of technology, 2010. [3] a. matejček, e. fussgänger. geological documentation of survey boreholes; geofos s. r. o., žilina; d1 lietavská lúčka – višňové, 06/1999. [4] a. matejček, e. fussgänger. geological documentation of archived survey boreholes; geofos s. r. o., žilina; d1 lietavská lúčka – višňové, 06/1999. [5] a. matejček, e. fussgänger, m. panek, et al. final report of a detailed engineering geological and hydrogeological survey; geofos s. r. o., žilina; d1 lietavská lúčka – višňové, 06/1999. [6] h. brandl. retaining structures for rock masses. butterworth-heinemann, oxford, 1994. doi:https://doi.org/10.1016/c2013-0-01244-3. [7] p. turček, j. panuška. design and confirmation of anchors at highway d1 on the section lietavská lúčka – višňové, 06/2017. [8] t. schanz, p. vermeer, p. bonnier. the hardening soil model: formulation and verification. in proceedings of the international symposium beyond 2000 in computational geotechnics. a.a. bakema publishers, usa, 1999. [9] m. súľovská, m. kopecký, j. panuška. stabilization of landslide area for foundations of a highway bridge. in proceedings of the 16th international multidisciplinary scientific geoconference. sgem, sofia, 2016. [10] z. chunmei, s. wei, c. j.van.w. comparing two methods to estimate lateral force acting on stabilizing piles for a landslide in the three gorges reservoir, china. engineering geology 173:41–53, 2014. doi:https://doi.org/10.1016/j.enggeo.2014.02.004. 31 http://dx.doi.org/https://doi.org/10.1016/c2013-0-01244-3 http://dx.doi.org/https://doi.org/10.1016/j.enggeo.2014.02.004 acta polytechnica ctu proceedings 16:25–31, 2018 1 introduction 2 geological conditions on site 3 designed remedial actions 4 finite element analysis 5 analysis of the results 6 conclusions acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2018.15.0046 acta polytechnica ctu proceedings 15:46–50, 2018 © czech technical university in prague, 2018 available online at http://ojs.cvut.cz/ojs/index.php/app application of the modified magnetoelastic method and an analysis of the magnetic field tomáš kliera, tomáš míčkaa, michal polákb,∗, tomáš plachýb, milan hedbávnýc, roman jelínekc, filip bláhab,d a pontex, spol. s r. o., bezová 1658, 147 14 prague 4, czech republic b department of mechanics, faculty of civil engineering, czech technical university in prague, thakurova 7, 166 29 prague 6, czech republic c freyssinet cz a. s., zápy 267, 250 01 brandýs nad labem, czech republic d department of concrete and masonry structures, faculty of civil engineering, czech technical university in prague, thákurova 7, 166 29 prague 6, czech republic ∗ corresponding author: polak@fsv.cvut.cz abstract. in the technical praxis there is very often a need of axial force determination in the important structural elements of a building during its construction or operational state. the paper presents practical application of the new approach based on the magnetoelastic principle, especially aimed on experimental evaluation of the axial forces in the prestressed reinforcement on prestressed concrete structures. described approach is usable not only for newly built structures but in particular for existing ones. the experiment was realized on a prestressed concrete beam dismantled from a bridge that was put out of operation. the influence of a simulated defect of the reinforcement on its magnetoelastic properties has also been investigated. during result evaluation, the knowledge based on the theoretical analysis of the electromagnetic field was used. keywords: concrete, tensile force, magnetoelastic method, prestressed strand, prestressed cable, prestressed wire, elastomagnetic sensor. 1. introduction in civil engineering practice, five experimental techniques are usually used for evaluation of axial tensile forces, the direct measurement of the force by a preinstalled load cell, the approach based on a strain measurement with strain gauges, the vibration frequency method [1, 2], the force determination in a flexible structural element based on the relation between the transverse force and the caused transverse displacement [3, 4], and the magnetoelastic method [5– 10]. all mentioned methods have their advantages and disadvantages that are discussed in more detail in the reference [7] and [8]. only three of them are usable for new experiment on an existing structure that is in service. the first one is the vibration frequency method that is applicable only to structural elements with relatively long free vibrating length. the uncertainty of the specified force is moreover significantly influenced by the element bending stiffness and by the element boundary conditions especially if they are complicated and vague [1, 2]. the second one is method based on the relation between the transverse force and the transverse element displacement that is usable only for significantly flexible elements with a relatively long free length. due to the transverse displacement the tensile force in the considerably short element during the experiment substantially increases even in the range of tens of percent [3, 4]. the third one is new approach based on the magnetoelastic method [5–7]. according to our opinion, the new approach is the most suitable procedure for tensile stress evaluation in prestressed elements on existing structures that are in service. the basic aim of the project is to research and develop a new approach based on the “magnetoelastic” physical principle. the approach is designed for the measurement of axial forces in structural elements made of ferromagnetic materials (in steel prestressed rods, wires and cables) on existing or newly constructed building structures. the research solved by our team seeks to focus primarily on solving the task of “determining the actual size of the force in a prestressed concrete element on an existing structure” which is very common in technical practice but is often not resolvable by any other method used nowadays. this paper presents one of the typical problems that have been solved so far [8–10]. in the conclusion, the properties and risks of this approach are summarized. 2. theory – physical principle elastomagnetic resp. magnetoelastic principle was explained in more detail in the sources [5, 6]. here are briefly summarized the basic facts: • the mechanical state and magnetic properties of the ferromagnetic material interact. 46 http://dx.doi.org/10.14311/app.2018.15.0046 http://ojs.cvut.cz/ojs/index.php/app vol. 15/2018 application of the modified magnetoelastic method... • the magnetic properties of the particular material define the relationship between the magnetic field shape described by the magnetic field intensity “h” [a.m−1] and the shape of the magnetic induction field “b” [t] (also named the magnetic flux density). for non-ferromagnetic materials the dependence is elementary, while the dependence for ferromagnetic ones is very complex, influenced by a number of parameters (temperature, mechanical stress, history of magnetic and mechanical stress for example). • the magnetic field intensity “h” can be monitored locally, for example, by hall sensors. the field of magnetic induction “b” may be monitored by means of a coil that integrates the normal component of the magnetic induction field over the area of individual coil turns. 3. theory – physical principle figure 1 shows a diagram of a fully equipped elastomagnetic sensor (hereinafter em sensor). fundamental sensor components are a controlled magnetic field source (the primary coil in this case), a sensor of magnetic field intensity “h” in a measured crosssection (the system of hall’s sensors, the secondary coil 2), a sensor of magnetic flux that is closely related to the magnetic induction “b” in the measured section (the secondary coil 1) and em sensor protection against magnetic influences from its surroundings (steel shield). figure 1. diagram of a fully equipped elastomagnetic sensor (em sensor). the function and principle of hall´s sensors were explained in more detail in the reference [6]. 3.1. maximal configuration the reader of this paper certainly assumes that a fully featured sensor offers the greatest possibilities to increase accuracy and reduce uncertainties in determining the axial force in the measured prestressed element. but the reader will also not be surprised that the fully equipped sensor is spatially larger, that may limit its applicability, and also more complicated. this complexity affects its higher production time, the higher usage requirements for this sensor in civil engineering practise and as well as the higher demands on the equipment of the used measuring system. 3.2. minimal configuration here is described an opposite extreme of the em sensor – the minimally featured one. the minimalist em sensor consists of a primary coil and a secondary coil 1 only. in this case, the intensity of the magnetic field “h” is determined indirectly from a completely different physical quantity, from the current flows through the primary coil. the greatest risk of using such sensor results from this approach. any change in the magnetic surroundings around the sensor (a removal of a massive steel falsework after concrete hardening for example) causes completely “silently” the substantial or fatal changes in sensor parameters. the sensor’s steel shielding reduces the range of this effect, the level of this reduction depends on the quality of the em sensor shielding, nevertheless it is never one hundred percent. for example, the minimum sensor configuration is completely unusable when applied to steel-fibre concrete. the em sensors and their appropriate equipment that are standardly used at present [8–10] evaluate measured prestressed forces in a relatively simple way. however, these standard em sensors, that have hitherto been used in civil engineering practice, are composed from the primary and secondary coils only and that is the minimal possible configuration of the em sensor as it is mentioned in this chapter. 3.3. measurement of magnetic field intensity if we are not satisfied with the indirect measurement of the magnetic field intensity “h” described in the previous paragraph, it is possible to use the hall sensors for this purpose. well-calibrated hall sensors allow very accurate determination of “h” in close proximity to the studied object, for example even in the gaps between the strands or between the prestressed wires in the experimentally investigated prestressed cable, and they are particularly advantageous when the observed magnetic field is asymmetrical. one disadvantage of this em sensor configuration is the fact that modern hall sensor are integrated semiconductor circuits that are not, in principle, reliable for long-term use in exposed surroundings. an alternative to hall sensors is to add a secondary coil 2 to the sensor construction. this coil, as well as the concentrically located secondary coil 1, observes a change in the magnetic flow passing through its turns. the mutual difference between magnetic flows measured by the coil 2 and the coil 1 indicates a change in flow passing through the annulus between turns of both secondary coils. in this space there are only non-ferromagnetic materials and therefore the magnetic flux can be readily recalculated to the average normal component of the magnetic field intensity “h” 47 t. klier, t. míčka, m. polák et al. acta polytechnica ctu proceedings in this annulus. the disadvantage of this sensor arrangement is that the intensity of the magnetic field is determined in comparison with the hall sensors at a greater distance from the studied prestressed elements and also that the secondary coil 2 increases the dimension of the sensor transverse cross-section. on the other hand, the advantage of this em sensor configuration is that the output of the em sensor is integrated from the larger area and therefore it is more resistant to the effects of local anomalies in the magnetic field and the application of secondary coil 2 also allows to not use the hall probes within the em sensor and it positively affects the reliability of the em sensor in its long-term use. 3.4. optimal configuration of the em sensor for existing structures the so far gained experience by em sensor applications on existing structures shows that it seems optimal to assemble an em sensor from all the parts shown in figure 1. its complete configuration allows the best available calibration of an em sensor under specific conditions. if the long-term use of the retrofitted em sensors is assumed, then it is possible to dismantle the hall probes from the em sensor after its initial calibration, and then process only the outputs of a pair of secondary coils. otherwise, for a one-off experiment, it is possible to omit the secondary coil 2 installation because there is no use for it. 4. experiment on a real structure the experiment was carried out on the prefabricated prestressed concrete beam of the type ka-67, which was dismantled from a bridge put out of service and which did not serve the original purpose for years. thanks to this fact, the experiment could be done without minimizing damage of the concrete part of the beam. it was also possible to carry out test in which an influence of a simulated defect of the studied cable on its magnetoelastic properties has been investigated. the force in the prestressed steel cable was intentionally released by means of the gradual cut off prestressed wires forming cable. this experiment was focused on the following problematic areas: verification of real spatial requirements for an em sensor additional installation on a prestressed cable, optimization of the geometric arrangement of the em sensor to real conditions and theoretical modelling of the electromagnetic field of the sensor taking into account the general geometric arrangement of the measured cross-section of the studied prestressed reinforcement. the theoretical model created in the software ansys maxwell 3d that corresponds to the experiment arrangement and conditions is shown in figure 5. the design of the applied em sensor has been optimized for the particular conditions of one-off experiment realized on the “small” in-situ cable, see figure 2. the production of the secondary coil in-situ. figure 3. the holder for the set of four hall sensors with the embedded secondary coil 1 positioned on the studied cable. figure 2. a cable composed of only a few prestressed wires allowed the use of a portable primary coil to emit a sufficiently intense electromagnetic field, see figure 2. only the secondary coil 1 was produced in-situ, see figure 1 and figure 3. the set of hall sensors located on the holder stabilizing the ideal mutual position of all sensor parts relative to each other was also demountable and portable. the steel shield with regard to detailed magnetic field monitoring in the measured cable cross-section by four hall probes was not necessary. the condition of successful application of the elastomagnetic measuring method is the detailed knowledge of the composition of the electromagnetic field generated by the primary coil in the measured cross-section of the studied prestressed element. based on the analysis of the results obtained from the theoretical model (see figure 3), it is possible to create a functional relationship between the real measured variables and the variables necessary for the evaluation of the solved prestressing force or the stress in the studied prestressed reinforcement. for example, we need to know the average value of the magnetic field intensity in the 48 vol. 15/2018 application of the modified magnetoelastic method... figure 4. the complete em sensor on the cable. figure 5. the theoretical model of the em sensor positioned on the studied prestressed cable – only half of the model visualization. cross-section of the studied prestressed element, but we can only measure it locally at the positions of hall sensors. in figure 7 there is a graphical representation of the results obtained during the realized experiment. the red lines with the “+” node symbols show the measured values of the average stress in the cable. they differ by the evaluation curve used for determination of the results. davg is the mean curve obtained from available results evaluated by previously realized experiments carried out for three different types of prestressed wires [6, 7] and d1 is the most appropriate evaluation curve obtained for the prestressed wire most similar to that used in the tested structural element. the black and blue curves show the computed values of the average stress in the studied cable according to different computational models of the prestressed figure 6. the theoretical model of the em sensor with the studied prestressed cable – magnetic field intensity in the cross-section of the secondary coil 1. figure 7. 7 development of the axial tensile force in the cable during the gradual prestressed wires cut off. concrete beam. the black curve corresponds to a model where zero deformation of concrete is assumed during the gradual cutting off of the wires. the blue thin curve corresponds to the assumption of uniform elastic deformation in concrete. in addition, the blue thick curve includes a local change in the field of deformation due to the demolition work performed to allow the installation of the em sensor. 49 t. klier, t. míčka, m. polák et al. acta polytechnica ctu proceedings the very good match between the average stresses measured and evaluated according to the curve d1 (thick red curve in figure 7) and the non-linear static model of the observed structural element (thick blue curve in figure 7) is obvious. the more significant differences in these two curves occurred only in the area where almost all cable wires are cut off. the deviation between these two curves is probably caused by different stresses than axial. in the case of the described experiment, it is probably due to the weight of the primary coil acting in the transverse direction on the remaining uncut wires. 5. conclusions the presented experiment confirmed the applicability of the developed new measuring approach based on the “magnetoelastic” physical principle to real existing structures. accuracy and uncertainty of results obtained by this approach are significantly affected, as other methods, by the quality of the experiment realization, but also by the quality of the used evaluation curve. obviously, the highest quality of the evaluation curve can be obtained from a prestressed reinforcement sample taken directly from the investigated structure, but in most real cases it is difficult or totally impossible. therefore, it is important to gradually create the widest possible database of evaluation curves for prestressed elements commonly used in the building industry, both today and in the past. acknowledgements the results presented in this paper are outputs of the research project fv 30457 “utilization of a magnetoelastic method for increasing the reliability and durability of existing and newly built prestressed concrete structures” supported by ministry of industry and trade of the czech republic. references [1] m. polák, t. plachý. determination of forces in roof cables at administrative center amazon court. procedia engineering 48:578–582, 2012. [2] m. polák, t. plachý. experimental evaluation of tensile forces in short steel rods. applied mechanics and materials 732:333–336, 2015. [3] p. fajman, m. polák. measurement of structural cable of membranes. in proceedings of the 50th annual conference on experimental stress analysis, vol. 50, pp. 61–64. 2012. [4] p. fajman, m. polák, j. máca, t. plachý. the experimental observation of the prestress forces in the structural elements of a tension fabric structure. applied mechanics and materials 486:189–194, 2014. [5] t. klier, t. míčka, m. polák, et al. modern method for measurement of the axial forces in elements of building structures. in proceedings of the conference betonářské dny 2016, vol. 23. čbs, 2016. [6] t. klier, t. míčka, t. plachý, et al. the verification of a new approach to the experimental estimation of tensile forces in prestressed structural elements by method based on the magnetoelastic principle. matec web of conferences 107(00015):1–6, 2017. [7] t. klier, t. míčka, m. polák, et al. the in situ application of a new approach to experimental estimation of tensile forces in prestressed structural elements by method based on the magnetoelastic principle. in proceedings of the 55th conference on experimental stress analysis 2017, vol. 55, pp. 122–132. technical university of košice, 2017. [8] a. m. sarmento, a. lage, e. caetano, j. figueiras. stress measurement and material defect detection in steel strands by magneto elastic effect. comparison with other non-destructive measurement techniques. in proceedings of the 6th international conference on bridge maintenance, safety and management iabmas 2012, pp. 914–921. iabmas, 2012. [9] m. chandoga, p. fabo, a. jaroševič. measurement of forces in the cable stays of the apollo bridge. in proceedings of the 2nd fib congress, pp. 674–675. fib, 2006. [10] h.-j. wichmann, a. holst, h. budelmann. magnetoelastic stress measurement and material defect detection in prestressed tendons using coil sensors. in proceedings of 7th international symposium on nondestructive testing in civil engineering ndtceš09, p. 6. laboratoire central des ponts et chausses, 2009. 50 acta polytechnica ctu proceedings 15:46–50, 2018 1 introduction 2 theory – physical principle 3 theory – physical principle 3.1 maximal configuration 3.2 minimal configuration 3.3 measurement of magnetic field intensity 3.4 optimal configuration of the em sensor for existing structures 4 experiment on a real structure 5 conclusions acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2019.25.0073 acta polytechnica ctu proceedings 25:73–78, 2019 © czech technical university in prague, 2019 available online at http://ojs.cvut.cz/ojs/index.php/app mechanical and structural properties of collagen nanofribrous layers under simulated body conditions jitka říhováa,∗, tomáš suchýb, lucie vištejnovác, lukáš hornýa, monika šupováb a czech technical university in prague, faculty of mechanical engineering, department of mechanics, biomechanics and mechatronics, technická 4, 166 07 prague 6, czech republic b czech academy of sciences, institute of rock structure and mechanics, v holešovičkách 94/41, 182 09 prague 8, czech republic c charles university, medical faculty in pilsen, biomedical center, alej svobody 1655/76, 323 00 pilsen, czech republic ∗ corresponding author: jitka.rihova@fs.cvut.cz abstract. the theme of this paper is the analysis of mechanical and structural properties of nanofibrous col under simulated body conditions and in the presence of osteoblasts and dermal fibroblasts. col were prepared by electrostatic spinning of 8 wt% collagen type i dispersion with 8 wt% (to col) of peg in phosphate buffer/ethanol solution (1/1 vol). the stability of col was enhanced by means of cross-linking with edc and nhs at a molar ratio of 4:1. col were exposed in culture medium for 21 days and human saos-2 human dermal fibroblasts and osteoblasts were cultured therein for 21 days as well. the cell culture on col was assessed by fluorescence microscopy and metabolic activity. then the metabolic activity of both cell types grown on col and ps were measured after 1, 7, 14 and 21 days using the alamar blue assay method. mechanical properties were determined using an tensile test. the influence of the cell activity on secondary structure of col was verified by ir spectroscopy. furthermore, the influence of cells on col was evaluated by sem. keywords: collagen, fibroblasts, osteoblasts, mechanical properties, structural properties, biodegradation. 1. introduction part of the tissue engineering is research and development of newmaterials whichwill be used in biomedicine. one of the directions is the development of biocompatible nanofibers which can be prepared, for example, from polymers. these nanostructures can provide temporary mechanical and structural support for cells and help restore damaged tissue. current research in the field are concentrated mostly on materials that are completely resorbable in the human body over time and their properties can simulate the function of the original tissue. understanding the factors that influence their degradation is crucial for the development of polymer degradable systems, especially in terms of degradation kinetics, changes in mechanical properties, and identification of degradation products [1, 2]. the term biodegradation has been known for degradation occurring in biological environment. in the context of biomedical applications, biodegradation can be defined as the gradual decay of a material driven by a specific biological activity [3, 4]. changes in the physico-chemical properties of materials that are exposed to body fluids are the result of chemical, physical, mechanical, and biological interactions between the material and the environment [5, 6]. 2. material and methods 2.1. nanolayer preparation collagen nanolayers (col) were prepared by electrostatic spinning (4spin, contipro, czech republic). the first step in the preparation was to mix the desired amount of collagen type i (calf skin, vup medical, czech republic) with polyethylene oxide (mw 900,000, sigma-aldrich, germany) and phosphate buffer (pbs, sigma-aldrich, germany). this mixture was incubated at 37 ◦c for 3 days. the hydrolysed collagen must be homogenized (10000 rpm, t25, ultra-turrax, ika, germany) to form a collagen dispersion, to which ethanol (penta, czech republic) is added. the electrospinning solution is prepared as 8 % w/w. collagen in a mixture of phosphate buffer and ethanol (1:1 by volume). the proportion of polyethylene oxide, as a auxiliary polymer which was completely washed after crosslinking of the nanofiller, was 8 % by weight (to collagen). after that col had to be cross-linked, which helps to increase its stability. the crosslinking was performed chemically with 95 % ethanol (penta, czech republic) with n-(3-dimethylaminopropyl)n-ethylcarbodiimide hydrochloride (edc, sigmaaldrich, germany) and n-hydroxysuccinimide (nhs, 73 http://dx.doi.org/10.14311/app.2019.25.0073 http://ojs.cvut.cz/ojs/index.php/app j. říhová, t. suchý, l. vištejnová et al. acta polytechnica ctu proceedings sigma-aldrich, germany). the edc / nhs molar ratio was 4:1. in this solution, the nanoparticles were soaked for 24 hours at 37 ◦c. finally, the col is rinsed with sodium phosphate (penta, czech republic) three times for 20 minutes. subsequently, the col were twice rinsed in distilled water, which is important for the complete washing of the polyethylene oxide. finally, washed col were frozen (−30 ◦c) and dried in a lyophilizer (virtris benchtop, usa) [7]. 2.2. exposure in culture medium and in the presence of cells for the exposure of col in simulated body conditions, dermal fibroblasts and osteoblasts (human line saos-2) cultivated on the surface of the layers were used. both cell types were first cultivated on 75 cm2 polystyrene culture bottles (techno plastic products, switzerland) in culture medium. the culture medium contained dulbecco’s modified eagle medium (dmem, thermo fisher scientific, usa), fetal bovine serum (fbs, thermo fisher scientific, usa), penicillin (100 u/ml), streptomycin (0.1 mg/ml), lglutamine, and 1 % non-essential amino acid (biosera, usa). cultivation took place at 37 ◦c and 5 % co2. on the day of the experiment, the dermal fibroblasts were sedimented with trypsin/edta and counted using a bürker cell. for fluorescence microscopy and for measuring metabolic activity, col were prepared in the form of a 0.3 cm circle, rinsed for 20 min in 1 % ethanol and 10 min in pbs. the samples were then placed in a 96 polystyrene panel (techno plastic products, switzerland) and immersed in 150µm of culture medium until they were seeded with cells. for mechanical tests, 9 × 50 mm strips were rinsed for 20 minutes in 70 % ethanol and 10 minutes in pbs. 20 × 50 mm (spl life sciences, korea) culture chambers were each placed in two pieces and immersed in 4 ml of culture medium until they were fitted with cells. the dermal fibroblasts were diluted to a culture medium at a concentration of 30, 000 cells/ml and loaded onto collagen nanoparticles at a deposition density of 15, 000 cells/cm2 and a volume of 150µl. the osteoblasts were diluted to a culture medium at 30, 000 cells/ml and loaded onto collagen materials at a loading density of 15, 000 cells/cm2 and a volume of 150µl. both cell types were cultured for 21 days, when the measurements ranged on days 1, 7, 14 and 21. the culture medium was changed three times a week. 2.3. biological tests the cell culture on col were assessed by fluorescence microscopy and metabolic activity. first, the proper seeding by both cell types was evaluated. dermal fibroblasts and osteoblasts were dyed at 1, 7, 14 and 21 days after the start of the experiment. at each time, both cell types were cultured on col and on polystyrene (ps) stained with a 1µg/ml solution of calcein am (thermo fisher scientific, usa) in culture medium for 30 min at 37 ◦c and 5 % co2. after the staining period, the samples were rinsed with pbs and transferred to the cover glass. clear fluorescence images of both types of cells cultured on col and ps were acquired using the olympus ix83 (olympus, switzerland) microscope at 488 nm excitation and 528 nm emission. images were processed using imagej (national institute of health, bethesda, usa). the metabolic activity of dermal fibroblasts and osteoblasts grown on col and ps were measured at 1, 7, 14 and 21 days after the start of the experiment using the alamar blue assay method. from each sample, culture medium was aspirated and 150µl of new culture medium containing 10× diluted alamar blue (thermofisher scientific, usa) was added. samples were incubated for 2 hours at 37 ◦c and 5 % co2. subsequently, 100µl of culture medium was pipetted into a 96 plate (nunc, denmark) and the fluorescence of the solution at 530 nm excitation and 590 nm emission was measured using a synergy ht (biotek, usa) spectrophotometer. from the measured values, the fluorescence of the culture medium itself was read and the values were expressed as the fluorescence ratio at day 7, 14 and 21 on fluorescence. values thus reflect the change in the metabolic activity of cultured cells on col and on ps over time. the experiment was performed in 6 replicates. 2.4. mechanical tests changes in mechanical properties were measured by a uniaxial tensile test in which the initial modulus of elasticity in the first and the last (fifth) load cycle and the ultimate tensile strength of the collagen layer were evaluated. the tensile test was carried out in the laboratory of biomechanics at the ctu in prague. measurements were carried out on a multi-axis test machine (zwick/roell, germany) with force sensors u9b by hbm with a range of ± 25 n and ± 250 n equipped with a non-contact optical extensometer. this extensometer, using the contrast marks on the samples, records the course of deformation during loading. the data was recorded at a frequency of 20 hz and stored on a computer. the position of the marks, the position of the clamping jaws and the force were measured. mechanical tests were performed for groups of 8 samples in medium, with osteoblasts and fibroblasts for each time interval. because of viscoelastic behavior of hydrated col, which also depends on the previous deformation, it is necessary to load and relieve the sample several times before performing the inter-strength test. estimation of the initial modulus of elasticity occurred both for the first load cycle, thanks to which we can determine the effect of dermal fibroblasts and osteoblasts on the surface layers of the material, as well as the last load cycle from which the effect of cells on the inner structure of the material can be estimated. from the measured data, graphs of stress σ strain ε relationships were plotted. the values for these charts were calculated 74 vol. 25/2019 mechanical and structural properties of collagen nanofribrous from: σ = f s [mpa] , (1) ε = ∆l l0 [−] , (2) where f is the force measured by the force sensors [n], s is the cross sectional area of the unloaded sample [mm2] (we consider the rectangular cross-section), ∆l is the elongation of the sample determined by the change of the contrast marks on the samples [mm] and l0 is the initial length of the sample [mm]. 2.5. structural changes structural changes were monitored by infrared spectroscopy and scanning electron microscopy. secondary structure of col was studied by infrared spectrometry (ftir), which allows study of its changes (denaturation) after various processes (isolation, crosslinking, sterilization). the aim of this analysis was to verify the possible effect of cell activity as well as exposure in the culture medium to the secondary collagen structure. the structure of col was studied using protégé 460 e.s.p. (thermo nicolet instruments, usa) by a method of attenuated total reflection atr (gladiatr, pike technologies, usa) with a diamond crystal. the resulting spectra were obtained from 128 scans with a resolution of 4 cm−1. belt areas (integral absorbance) were determined using omnic 7 software, which was also used for the deconvolution procedure of the amide bands i. the input data for band separation (number, position, half width, relative intensity and shape) were obtained using the fourier self-deconvolution procedure and further specified by the fitting procedure using the gaussian profile function. the individual col were measured at 10 different sites and the values of the waist areas after deconvolution were statistically evaluated. a sem analysis was used to basically assess the surface of collagen layers quanta 450 electron microscope (fei, usa). to assess the effect of cultivation on the surface of col the images (magnification 500× and 25, 000×) were taken at time intervals on day 1, day 7, day 14, and day 21. another reason of the sem analysis was to determine the fiber diameter before and after the crosslinking. to evaluate these data, the diameter of the fibers was evaluated manually by measuring their diameter (at magnification of 25, 000×). all samples were sputter-coated with a thin layer of gold (emissions k550x, quorum technologies, uk) prior to scanning. 3. results 3.1. biological test after the first day of culture, both dermal fibroblasts and osteoblasts were similar to the rounded shape of the cell on ps and on the col (see fig. 1). in the following days of cultivation (7th, 14th, 21st day) dermal fibroblasts on ps showed a typical elongated cell shape and covered most of the area planted. most cells were oriented in the same direction. in contrast, dermal fibroblasts on col did not show a one-way arrangement. the cells had a typical shape with elongated protrusions throughout the col space indicating their physiological comfort. on the 7th, 14th, and 21st days of cultivation, osteoblasts on ps showed a typical cuboid shape and covered most of the area planted. osteoblasts on col were more rounded, and the cells grew on multiple layers (see fig. 1). figure 1. dermal fibroblasts and osteoblasts cultivated on polystyrene and collagen were stained with calcein am solution, green color signifies metabolically active cells (scale 100µm). compared to the first day, the metabolic activity of dermal fibroblasts on the col was 9 times higher (8.8±2.5) after 7 days of culture, and 5 times higher on ps (4.5 ± 0.6). this difference between col and ps was statistically significant (p < 0.05). after 14 and 21 days of culture, the metabolic activity of these cells on the col was 6 times higher (5.±0.7 and 5.9±1.4). on ps after 14 days 6 times and 21 days 5 times higher (5.6 ± 0.7 and 5.3 ± 0.4) versus the first day. these differences between col and ps were not statistically significant (p < 0.05) (see fig. 2). metabolic activity of osteoblasts was 10x higher (9.7 ± 2.9) on the col after 7 days of cultivation, and 11 × higher (11.0±1.6) on the ps compared to the first day. this difference was not statistically significant (p < 0.05). after 14 days of cultivation, osteoblastic metabolic activity on col was 7 times higher (7.3 ± 0.5) and on ps nearly 13 times higher (12.7±1.8) versus day 1. after 21 days, their metabolic activity on col 10 × higher (10.2 ± 3.5) and on ps nearly 16 times higher (15.8 ± 0.9). differences between col and ps were statistically significant (p < 0.05) at both time points (see fig. 2). 75 j. říhová, t. suchý, l. vištejnová et al. acta polytechnica ctu proceedings figure 2. metabolic activity of dermal fibroblasts and osteoblasts grown on col and ps, * indicates statistically significant differences (p = 0.05, t-test) 3.2. mechanical properties from the measured data on samples exposed to medium without or with both cell types (n = 8) on the 1st, 7th, 14th and 21st days, stress-strain relationships were plotted in figs. 3 4. from the measured data, the initial modulus of elasticity was obtained as the slope of the tangent made to a stress-strain relationship on the initial linear part in the first and the last cycle. the ultimate tensile strength was determined as the maximum observed value of the nominal stress during the last loading cycle sample. the samples were labeled as follows: m for samples exposed only in medium, f for samples with dermal fibroblasts and s for samples with osteoblasts. the values of the initial elastic moduli in the first load cycle were not different for the samples exposed on the 1st and 21st day (see fig. 5). in both dermal fibroblasts and osteoblasts (see fig. 5), the values of the initial modulus of elasticity in the first loading cycle were decreased by 16 % and 36 %. a statistically significant difference in the last load cycle was observed only in osteoblast specimens (see fig. 5). for samples in the medium and fibroblast samples (see fig. 5), the difference in values was not statistically significant. for samples exposed in the medium, fibroblast samples and osteoblast samples, a statistically significant increase in the strength limit was recorded, by 12 % (see fig. 6). 3.3. structural properties 3.3.1. infrared spectroscopy collagenous materials were comparable from a spectroscopic point of view. the quartet of the ∼ 1205, 1240, 1280 (amide iii) and 1340 cm−1 quartets figure 3. stress-strain graphs for the first and last load cycles. for specimens exposed in 1. (m1) and 21 (m21), samples with fibroblasts (f1) and day 21 (f21) and samples with osteoblasts (s1) and day 21 (s21). figure 4. stress-strain graphs for the first and last load cycles. for specimens exposed in 1. (m1) and 21 (m21), samples with fibroblasts (f1) and day 21 (f21) and samples with osteoblasts (s1) and day 21 (s21). were a clear proof of the existence of a helical structure, in other words, the natural collagen structure was maintained at the appropriate level during the preparation of the layers by electrostatic spinning, to its fundamental violation. spectral decomposition of the amide band i made it possible to determine the ratio of the helical portion of collagen represented by the 1660 cm−1 band to the denatured structures based 76 vol. 25/2019 mechanical and structural properties of collagen nanofribrous figure 5. box plot comparing the initial tensile modulus in the first and the last cycle 1. day (m1) and 21. (m21) day of samples exposed in medium, 1. (f1) and 21. day (f21) of fibroblast, 1. (s1) and 21 (s21) of osteoblast samples, * indicates statistically significant differences (mann-whitney, 0.05) figure 6. box plot comparing the tensile strength limit of 1 (m1) and 21 (m21) of the day of samples exposed in medium, f1 and f21 of fibroblast samples, 1 (s1) and 21. day (s21) of osteoblast specimens, * indicates statistically significant differences (mannwhitney, 0.05) on the 1630 cm−1 bands and the 1615 cm−1 band. the 1660/(1630 + 1615) ratios were determined from measurements of 10 different sites of the collagen layers exposed to the culture medium (m1 and m21) and of the layers exposed to the cells. fibroblasts (f1 and f21) and osteoblasts (s1 and s21). the obtained data were statistically evaluated and compared. as can be seen, only the s1 and s21 pairs are statistically significant (see fig. 7). 3.3.2. electron microscopy the average fiber diameter of the sample before crosslinking was 261±49 nm. the average fiber diameter of the sample after crosslinking was 260 ± 35 nm. as can be seen from fig. 8 the fiber diameters before and after figure 7. box plot comparing ratio of helical and denatured areas of collagen (1660/(1630 + 1615)) 1. day (m1) and 21. (m21) day of samples exposed in medium, 1. (f1) and 21. day (f21) of fibroblast, 1. (s1) and 21 (s21) of osteoblast samples the cross-linking were not statistically significantly different. the mean fiber diameter of the sample did not changed during the exposure in the medium (202 ± 30 nm on the first day and 196±26 nm on the last day). the mean fiber diameter of the fibroblast samples after cross-linking the first day was 187 ± 33 nm, the last day 184 ± 32 nm. for osteoblast samples the mean fiber diameter of the after crosslinking was 198 ± 27 on the first day and 198 ± 25 on the last day. from fig. 9 it was obvious that the fiber diameter values were not statistically significant in the case of exposed samples. figure 8. a box plot comparing the fiber diameter before and after the crosslinking. fiber diameters are not statistically significant different. 4. conclusion during this work it was found that both fibroblasts and osteoblasts can influence the mechanical and structural properties of collagen layers by their activities. the activity of the cells has been demonstrated especially in the case of osteoblasts. in their presence there was a reduction in the modulus of tensile elasticity, which manifested itself in both the first and last load cycles. in the case of fibroblasts, this phenomenon was observed only in the first cycle, ie in a situation where the mechanical properties are more affected by the properties of the surface layers. in the case of an increase in the tensile strength after a three-week exposure, it was not shown that it was due to cell 77 j. říhová, t. suchý, l. vištejnová et al. acta polytechnica ctu proceedings figure 9. a box plot comparing the fiber diameter after first (m1) and 21 (m21) in medium, 1. (f1) and 21. (f21) days of fibroblast culture and 1. (s1) and 21. (f21) days of culture of fibroblasts and 1. (s1) and 21. (s21) days of cultivation with osteoblasts. fiber diameters are not statistically significant different. activity, since higher strengths were also achieved in samples exposed in the cell-free medium itself. higher activity of osteoblasts was further demonstrated by infrared spectroscopy, which showed an increase in the ratio of helical collagen to denatured structures. the presence of cells did not affect the diameter of the fibers of the collagen nanoparticle. fibroblasts and osteoblasts were succesfully plated and cultured on col. changes in the initial modulus of elasticity were observed most in the first load cycle, where both the fibroblast and the osteoblast samples showed statistically significant differences between the 1st and 21st day of the cultivation. for the last load cycle, statistically significant changes between 1st and 21st day of cultivation were observed only in samples with osteoblasts. on the other hand, ultimate tensile strenght increased in both cell culture media as well as in layers with cultured cells, thus the changes in ultimate tensile strength cannot be addressed only to cell activity. the infrared spectroscopy revealed the changes in helical structure of col only in osteoblast specimens. fiber diameters changes which were measured from images from electron microscopy were not observed. fibroblasts and osteoblasts can influence mechanical and structural properties of collagen nanoparticles. identification of cellular mechanisms responsible for the mechanical changes will be the aim of following research focused on the analysis of enzymatic activity and remodelation procedures of dermal fibroblasts and osteoblasts. acknowledgements the study was supported by project no. cz.02.1.01/0.0/0.0/16_019/0000787 "fighting infectious diseases", awarded by the ministry of education, youth and sports of the czech republic, financed from the european regional development fund. references [1] k. jahan, m. tabrizian. composite biopolymers for bone regeneration enhancement in bony defects. biomaterials science 4(1):25–39, 2016. doi:10.1039/c5bm00163c. [2] f. o’brien. biomaterials & scaffolds for tissue engineering. materials today 14(3):88–95, 2011. doi:10.1016/s1369-7021(11)70058-x. [3] h. azevedo, r. reis. understanding the enzymatic degradation of biodegradable polymers and strategies to control their degradation rate. crc press, boca raton, 2005. doi:0.1201/9780203491232.ch12. [4] l. ruiz-rubio, l. perez-alvarez, j. vilas-vilela. biodegradable shape-memory polymers. advanced structured materials 115:219–236, 2020. doi:10.1007/978-981-13-8574-2_10. [5] d. williams, s. zhong. biodeterioration/biodegradation of polymeric medical devices in situ. international biodeterioration and biodegradation 34(2):95–130, 1994. doi:10.1016/0964-8305(94)90002-7. [6] d. kytyr, p. zlamal, p. koudelka, et al. deformation analysis of gellan-gum based bone scaffold using on-the-fly tomography. materials and design 134:400–417, 2017. doi:10.1016/j.matdes.2017.08.036. [7] t. suchy, m. supova, e. klapkova, et al. the release kinetics, antimicrobial activity and cytocompatibility of differently prepared collagen/hydroxyapatite/vancomycin layers: microstructure vs. nanostructure. european journal of pharmaceutical sciences 100:219–229, 2017. doi:10.1016/j.ejps.2017.01.032. 78 https://doi.org/10.1039/c5bm00163c https://doi.org/10.1016/s1369-7021(11)70058-x https://doi.org/0.1201/9780203491232.ch12 https://doi.org/10.1007/978-981-13-8574-2_10 https://doi.org/10.1016/0964-8305(94)90002-7 https://doi.org/10.1016/j.matdes.2017.08.036 https://doi.org/10.1016/j.ejps.2017.01.032 acta polytechnica ctu proceedings 25:73–78, 2019 1 introduction 2 material and methods 2.1 nanolayer preparation 2.2 exposure in culture medium and in the presence of cells 2.3 biological tests 2.4 mechanical tests 2.5 structural changes 3 results 3.1 biological test 3.2 mechanical properties 3.3 structural properties 3.3.1 infrared spectroscopy 3.3.2 electron microscopy 4 conclusion acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2018.14.0049 acta polytechnica ctu proceedings 14:49–55, 2018 © czech technical university in prague, 2018 available online at http://ojs.cvut.cz/ojs/index.php/app heat transfer modelling of two-phase flow in isolated channel daniel vlček∗, vojtěch caha, martin ševeček kjr fjfi čvut, v holešovičkách 2, praha 8, czech republic ∗ corresponding author: daniel.vlcek1@gmail.com abstract. this paper deals with post-chf (critical heat flux) heat transfer with the focus on different regimes of film boiling. the new thermal-hydraulic code tube 2.0 is presented. this code uses the equation of energy conservation and predefined correlations to establish wall temperature, the departure of nucleate boiling ratio as well as other parameters of cooling in a simple geometry an isolated channel. with experimental data of inverted annular film boiling from stewart, the best-performing correlation for calculation of post-chf heat transfer in the channel was determined. finally, the new presented code tube 2.0 and subchannel code subcal owned by chemcomex a.s. are compared using results of various experiments conducted by becker. data from stewart could not be used because of inability to predict the onset of boiling crisis with several correlations. keywords: post-chf, heat transfer, modelling and simulation, two-phase flow, correlations, experimental data. 1. introduction since 1978 when the “hot patch” technique was used by groeneveld (1978) to arrest the quench front and establish stationary film boiling in a heated tube, an increasing amount of steady-state film boiling heat transfer data has been obtained in various laboratories all over the world. at first, the refrigerants were used as a medium. later, the first experiments with water were performed by fung [1] at atmospheric pressure or by stewart at a wider range of parameters [2]. the simplest test-section used for these types of experiments is a single-tube geometry [3]. although a lot of experiments with hot patch were conducted, there is lack of film boiling heat transfer correlations in these specific conditions. during the conditions, in which the hot patch experiments are performed, the inverted annular film boiling (iafb) mostly occurs and as it is shown further, it can not be considered as dispersed flow film boiling (dffb). this heat transfer regime generally occurs at low qualities and sub-cooled conditions at void fractions less than 0.5. low quality here goes hand in hand with high heat flux. the main characteristics of iafb are the low heat transfer coefficients due to vapor film covering the channel wall. the phase distribution during iafb consists of a continuous liquid core at the center of the channel surrounded by a vapor blanket covering the heated surface as it is shown in the fig. 1 where the iafb regime is schematically presented in the right tube. the development of phases is clearly different in these regimes and the same applies for the approach to estimating heat transfer coefficient [4]. heat transfer by radiation from the wall to the liquid is important when the wall temperatures are greater than 600 ℃, yet the most important change from dffb regime is dealing with the heat transfer through vapor film. the vapor film is assumed to have a higher velocity than the liquid core, and the vapor velocity increases together with the vapor layer thickness. this is due to the low viscosity and low density of the vapor, which enables the vapor flow to experience a higher acceleration than the dense core flow [3]. 2. comparison of correlations in order to compare several correlations for postchf heat transfer coefficient, new computational code tube 2.0 was created. before introducing the code, the experimental data and available corelations will be described first. 2.1. stewart experiments for the purpose of the comparison, the stewart’s [2] film boiling experimental data with the focus on iafb regime were chosen. the experiments were conducted on mr-1 flare loop in chalk river laboratories. the test section was made of 207.1 cm long inconel 600 tube with inner diameter 0.8928 cm. indirect method of 10.1 cm long hot patch was used here and the loop was capable of providing pressure in range 2.0–10.0 mpa and mass flux in range 120– 2750 kg.m−2.s−1. wall temperatures were measured with twenty thermocouples spread out over the whole test section, however, the inlet section was covered more densely. experiments were divided into 51 runs with different inlet subcooling, mass flux, and pressure. heat flux was slowly changed during each run in order to obtain minimum film boiling temperature data. so the heat flux was relatively low for estimation of chf as it is stated further. 49 http://dx.doi.org/10.14311/app.2018.14.0049 http://ojs.cvut.cz/ojs/index.php/app d. vlček, v. caha, m. ševeček acta polytechnica ctu proceedings figure 1. the difference between iafb (right) and dffb (left) regime inducted by different heat flux [4]. 2.2. tube 2.0 code the tube 2.0 [5] code further develops the code tube created at department of nuclear reactors, ctu in prague. tube 2.0 is based on the equation of energy conservation and predefined correlations to establish wall temperature, the departure of nucleate boiling ratio as well as other parameters of cooling in a simple geometry an isolated channel. it allows calculating with two-phase flow using the homogeneous equivalent model with respect to thermal non-equilibrium. this code exceeds the complex programmes with its easy manipulation and with a possibility to define own correlations whether it is for predicting of boiling crisis, for computation heat transfer coefficient in postcorrelation regime dougall-rohsenow [6] dffb groeneveld-delorme [7] dffb condie-bengston [8] dffb groeneveld’s tables [9] dffb/iafb bromley [10] iafb berenson [11] iafb breen-westwater [12] iafb mosaad-johannsen [3] iafb table 1. compared post critical correlations in tube 2.0. correlation range of p [mpa] biasi [14] 0.27-14.0 bowring [14] 0.2-19.0 zuber [14] no limits smolin [14] 3.0-10.0 pg-t [15] 0.3-18.7 okbm bezrukov [15] 9.7-18.0 chf lookup tables [16] 0.1-20.0 table 2. used correlations for predicting chf. chf regime etc. it was written in fortran language which was chosen for its short computation time. around 15 post-chf heat transfer correlations were defined in tube 2.0, however, most of them are meant for dffb regime. the chosen correlations for comparison are shown in tab. 1. on the basis of previous work of vlček [13] the best performing correlations of dffb regime that could be used for low-quality data were chosen. tables from groeneveld cover a wide range of application. the equations from berenson and breen-westwater are derived from bromley’s and finally, correlation from mosaad and johannsen was determined partly on basis of stewart’s data and should be one of the best correlations for iafb regimes according to mosaad [3]. the stewart’s experimental data are post critical so the computation code must predict the onset of boiling crisis before it starts to use post critical correlations. for that purpose, the correlations shown in the tab. 2 were chosen. the selection of equations was broad enough to cover all potential inaccuracies caused by approaching lateral parts of ranges. in these parts is the precision of predicting significantly worse. however, even with this wide selection it was not possible to achieve post-critical regime. the onset had to be set manually into two-thirds of hot patch section according to stewart’s experimental measurements of temperature drops. the problems with estimation of chf were probably caused by very low qualities and also by lowering heat flux in order to obtain minimum film boiling data. however, these results were unexpected and show that there is a need for new predicting correlations of chf for iafb regime because these conditions may occur during the rewetting phase of loca. 50 vol. 14/2018 heat transfer modelling in tube correlation avg [%] std [%] condie-bengston 0.31 0.21 groeneveld 5.7 [14] 0.12 0.10 groeneveld 5.9 [14] 0.13 0.11 groeneveld-delorme 0.11 0.05 groeneveld’s tab. 0.18 0.14 table 3. the statistical summary of comparison of tube 2.0 and subcal with the use of 12202 points from becker’s experimental database. 2.3. validation of tube 2.0 at the beginning of the validation process of the new computational code tube 2.0, the experimental data from stewart could not be used, so the data from becker [17] were selected instead. the experiments were conducted in stockholm at the royal institute of technology and the database provides more than 15,000 experimental data points of post-critical heat transfer in 7 m long tube with variation of three diameters from 1 to 2.47 cm with pressure range 3– 20.5 mpa, mass flux range 500–3000 kg·m−2·s−1 and quality from -0.03 to 1.66. the boiling regime was dffb so there was no need for iafb correlations. 3. results 3.1. validation in the first part of validation process code-to-code comparison, complex code subcal [15] owned and developed by chemcomex a.s. was chosen as the reference code. it is a subchannel code designed for computation of thermal-hydraulic safety margins for vver reactors by solving equations of energy and momentum conservation. as was mentioned above, the experimental database from stewart had to be replaced by becker’s data. not even the subcal code was able to predict the onset of critical heat flux on stewart’s data. the becker’s experiments were analyzed with the codes by using several dffb correlations as it is shown in the tab. 3. the deviations are minimal and these results led to the first part of the validation of the code tube 2.0. the small deviations in the final wall temperatures were probably caused by the different method of obtaining the convergence error. to completely validate the code tube 2.0 another experimental or benchmark data should be used. 3.2. comparison of correlations as was mentioned above, the stewart’s data together with the code tube 2.0 led to a selection of best performing post critical correlation. as predicted, the dffb equations significantly under/overestimated the experimental data. it is demonstrated in the fig. 2 which shows calculated data on the x axis and experimental data on the y axis. the full black line denotes 0% deviation and green, resp. red belongs to 25%, resp. 50% deviation. the top figure 2. comparison of experimental and calculated values of wall temperature with use of dffb correlations. graph represents correlation from groeneveld and delorme which often shows good agreement with dffb experimental data. however, it significantly overestimates the data. on the other hand, the equation of condie and bengston show better agreement with the experimental data, but there is an important underestimation of the data which is inapplicable for safety analyses where the conservative approach is desired. as presumed, these correlations cannot be used for iafb regime. most of them were derived from dittusboelter relationship considering only convective heat transfer, however, the conductive effect and radiation are neglected. correlation dougall-rohsenow was similar to groeneveld-delorme which also significantly overestimates the data. further analysis showed that these correlations improve results with higher pressures together with mass fluxes. it is shown in the fig. 3 where only the data from run with highest pressure (9 mpa) and highest mass flux (2750 kg·m−2·s−1) were selected. it is probably caused by the development of flow regime from iafb to dffb with modification of inlet parameters. mosaad [3] suggests to use weber 51 d. vlček, v. caha, m. ševeček acta polytechnica ctu proceedings figure 3. comparison of experimental and calculated values of wall temperature with selected data from run with highest pressure and mass flux. number, see eq. (1), for determining the regime in question. we = ρv · (uv − ul)2 · (dh − 2δ) σ (1) if the weber number is below wecr = 15, it is inverted annular film boiling regime and the opposite stands for dispersed flow film boiling. here is important to note that different literature can provide greater or smaller value of wecr. for the runs with pressure above 8 mpa, weber number always exceeded specified value. the groeneveld’s tables (see the top part of fig. 5) gave the best result and were selected as best performing correlation. the points are uniformly arranged across the center line. there were not observed pressure or inlet subcooling effects on heat transfer coefficient, however with reducing the data set for runs with the smallest mass flux the accuracy increased. the tables were created out of more than 30000 values from several experiments. groeneveld collected both iafb and dffb experimental data and extrapolated tables from it. the outcome of this is a set of tables perfectly capable of predicting post critical heat transfer coefficient by linear interpolation among table values. however, due to the small number of considered experiments featuring low mass fluxes, accuracy for such conditions is limited, see fig. 4. finally, the iafb correlations are presented. as stated before, all three examined correlations (bromley, berenson, breen, and westwater) show significant similarities; this is also the case for calculation results. the best of them was the berenson’s correlation which is shown in the middle part of fig. 5. it has been observed that with removing values from thermocouples placed in the outlet part of the test section, the accuracy increases. it is probably caused by unstable vapor film close to the end of the test section that has arisen by intensifying the turbulence figure 4. comparison of experimental and calculated values of wall temperature with reduced data base for the values of smallest mass flux. effect. furthermore, outlet section is characterized by highest qualities which could in some cases lead to a change of film boiling regime. it was also observed that with the lowest mass fluxes, the correlation’s precision deteriorated. mosaad extrapolated his original correlation [18] to greater ranges of pressure and mass flux and claims that the extrapolated correlation has a minimal deviation from stewart’s reduced data. however, these results were not confirmed using the tube 2.0 code. probably there was a mistake in the paper [3] in definitions of the equations or some of them were missing. the data are uniform but the correlation significantly overestimates the data (see the bottom part of fig. 5). with low mass fluxes and low subcooling the results got slightly better. all of the mentioned data were statistically processed and the results of average and standard deviations together with the number of points are shown in the tab. 4. the statistics of reduced data is demonstrated in the tab. 5 where the improvements, resp. deterioration are highlighted with green, resp. red color. 4. conclusions the new computation code tube 2.0 was presented. one step of the complex validation process (code-tocode comparison) was done using becker’s experimental data and subchannel code subcal. the results of five correlations are shown in tab. 3 with minimal deviations. the deviations originate probably in different convergence methods that were used. it was shown that for simple geometry an isolated channel, it is not necessary to use complex codes designed for thermal-hydraulic subchannel analysis of larger geometries. the use of simple and faster codes as tube 2.0 is satisfactory. 52 vol. 14/2018 heat transfer modelling in tube correlation avg [%] std [%] n [-] dougall-rohsenow 53.12 71.46 25897 groeneveld-delorme 146.04 147.12 22929 condie-bengston -29.32 12.07 25895 groeneveld’s tab. 1.33 10.56 25897 bromley 4.06 19.72 25897 berenson 1.56 17.95 25897 breen-westwater 10.64 19.98 25897 mosaad-johannsen 75.02 25.19 15569 table 4. overall statistic analysis of correlations compared with stewart’s experimental data. correlation reduction avg [%] std [%] n [-] berenson without last tcs 6.60 16.76 10904 dougall-rohsenow ↑ p = 9 mpa 7.49 33.66 4161 dougall-rohsenow ↑ g = 2750 kg.m−2.s−1 11.69 7.66 2565 groeneveld-delorme ↑ p, ↑ g, ↓ h 6.82 13.70 1368 groeneveld’s tab. ↓ g = 120 kg.m−2.s−1 22.47 15.44 1501 groeneveld’s tab. ↑ g = 2750 kg.m−2.s−1 2.31 7.36 2565 groeneveld’s tab. g = 215− 2750 kg.m−2.s−1 0.0003 0.09 1368 mosaad-johannsen ↓ g = 120 kg.m−2.s−1 40.14 19.43 1330 berenson ↓ g = 120 kg.m−2.s−1 25.79 13.41 1501 mosaad-johannsen ↓ h = 50 kj.kg−1 64.70 23.60 5386 condie-bengston ↑ h = 255 kj.kg−1 -16.56 8.12 3230 berenson ↑ h = 255 kj.kg−1 15.85 22.80 3230 table 5. statistic analysis of correlations compared with stewart’s experimental data with specific reductions. green colour stands for improvement and red for deterioration. after code-to-code comparison of tube 2.0, the best performing correlation for inverted annular film boiling regime was found. extensive analysis was done on basis of stewart’s data. an interesting phenomenon was observed. not even one correlation, see tab. 2 was able to predict the onset of the boiling crisis. for that reason, the stewart’s experiments were not used for the part of the validation process of tube 2.0. it was probably caused by low pressure and mass flux in the presence of low quality. in these conditions correlations almost reached their declared limit of validity. although the main reason was probably due to the determination of experimental data. the data served for finding minimum film boiling data, so from the beginning of the crisis, the heat flux has been reduced. most of the correlations for departure from nucleate boiling ratio are controlled by heat flux. for safety analysis (loca etc.) it is then important to find correlations that are capable of predicting the boiling crisis. in order to validate the tube 2.0 code more experimental or benchmark data should be used. after manual setting of onset of film boiling, the comparison of several correlations, see tab. 1, has been presented. it has been found that correlations determined for dispersed annular film boiling regime, such as dougall-rohsenow and groeneveld-delorme, were strongly overestimating experimental data, see fig. 2. correlation of condie and bengston was the only one underestimating the data. this is inappropriate for safety analysis where the conservative approach is desired. although, in conditions with high pressure and mass flux increase in accuracy was observed, see fig. 3. this was probably caused by the transition to dffb regime which was confirmed by comparing the weber number, see eq. (1), as it is recommended by mosaad [3]. the equations for inverted annular film boiling regime bromley, berenson, breen-westwater gave almost the same results. the distribution was more uniform and the berenson’s correlation featured the lowest deviation, see fig. 5. these correlations were designed for iafb regime. however the best results were reached with tables from groeneveld, see fig. 5. only observed weakness for the tables was low mass flux, see fig. 4. it is due to the lack of performed experiments with low flow. nevertheless, the tables could be used and are precise both with dffb and iafb regime. the last examined correlation mosaad-johannsen was supposed to be the most accurate, however large deviations from the experimental data were observed. uniform bias was clearly visible in the predicted data (see the bottom part of fig. 5) and this caused significant overestimation of the experimental data. mosaad [3] shows that this correlation is perfect for iafb data and moreover, it was partly derived from stewart’s data. the only source of this correlation was in mosaad’s paper so it should be applied with caution until its validity is verified. the overall statistic analysis is shown in the tab. 4 53 d. vlček, v. caha, m. ševeček acta polytechnica ctu proceedings figure 5. comparison of experimental and calculated values of wall temperature with use of lookup tables and iafb correlations. and the analysis with reduced data by pressure, mass flux, resp. subcooling is shown in tab. 5. more complex analysis of the code tube 2.0 and comparison of correlations can be found in the literature [19]. list of symbols avg average deviation chf critical heat flux dffb dispersed flow film boiling iafb inverted annular film boiling std standard deviation tc thermocouple δ film thickness [m] ρ density [kg m−3] σ surface tension [n m−1] dh hydraulic diameter [m] g mass flux [kg m−2 s−1] h inlet subcooling [kj kg−1] n number of points [–] p pressure [pa] u velocity [m s−1] we weber’s number [–] cr index related to the critical value of we v index related to the vapour l index related to the liquid acknowledgements the support from student grant contest from czech technical university with code sgs16/252/ohk4/3t/14 is gratefully acknowledged. many thanks belong to my supervisor ing. jiří čížek and my consultants ing. martin ševeček and ing. dušan kobylka, ph.d who provided insight and expertise that greatly assisted the research. i would also like to thank bc. lubomír bureš for his constructive criticism and friendly advice during the project work. references [1] k. k. fung. subcooled and low quality film boiling of water in vertical flow at atmospheric pressure. ph.d. thesis. university of ottawa, ottawa, 1981. [2] j. c. stewart. low quality film boiling at intermediate and elevated pressures. master thesis. university of ottawa, ottawa, kanada, 1981. [3] m. mosaad, k. johansen. an improved correlation for subcooled and low quality film boiling heat transfer of water at pressures from 0.1 to 8 mpa. nuclear engineering and design vol. 135(issue 3):355–366, 1992. doi:10.1016/0029-5493(92)90202-7. [4] m. a. e. nakla. experimental and analytical study of inverted annular flow film boiling heat transfer in a vertical tube using r-134a, 2007. [5] v. caha, d. vlcek. tube 2.0-postchf users guide. ctu fnspe dnr, prague, 2017. [6] r. l. dougall, w. m. rohsenow. film boiling on the inside of vertical tubes with upward flow of the fluid at low qualities. massachusetts institute of technology, usa, 1st edn., 1963. 54 http://dx.doi.org/10.1016/0029-5493(92)90202-7 vol. 14/2018 heat transfer modelling in tube [7] d. groenvled, g. delorme. prediction of thermal non-equilibrium in the post-dryout regime. nuclear engineering and design 36(1):17–26, 1976. doi:10.1016/0029-5493(76)90138-2. [8] k. g. condie, s. j. bengston, s. l. richlen. post-chf heat transfer data analysis, comparison and correlation, nepublikováno. [9] d. groenveld, l. leung, a. vasić, et al. a look-up table for fully developed film-boiling heat transfer. nuclear engineering and design 225(1):83–97, 2003. doi:10.1016/s0029-5493(03)00149-3. [10] l. a. bromley. heat transfer in stable film boiling. u.s. atomic energy commission, oak ridge, tennessee, 6th edn., 1949. [11] p. j. berenson. film-boiling heat transfer from a horizontal surface. journal of heat transfer 83(3):351–356, 1961. doi:10.1115/1.3682280. [12] z. li. inverted annular film boiling model analysis. master thesis. the pennsylvania state university, usa, 2013. [13] d. vlcek. heat transfer in pressurized water reactors fuel rod bundles at high void fraction. bachelor thesis. ctu fnspe dnr, prague, 2016. [14] n. e. todreas, m. s. kazimi. nuclear systems i. hemisphere publ. comp, new york [etc.], 2nd edn., 1989. [15] j. cizek, j. krejci. subcal-ete: dokumentace programu. chemcomex a.s., praha 10, 1st edn., 2010. [16] d. groenveld, j. shan, a. vasić, et al. the 2006 chf look-up table. nuclear engineering and design 237(15-17):1909–1922, 2007. doi:10.1016/j.nucengdes.2007.02.014. [17] k. m. becker, c. h. ling, s. hedberg, g. strand. an experimental investigation of post dryout heat transfer. royal institute of technology, stockholm, 1st edn., 1983. [18] m. mosaad, k. johanssen. experimental study of steady-state film boiling heat transfer of subcooled water flowing upwards in a vertical tube. experimental thermal and fluid science vol. 2(issue 4):477–493, 1989. doi:10.1016/0894-1777(89)90009-5. [19] d. vlcek. heat transfer modelling of pwr fuel rods bundle during a two-phase flow. research task. ctu fnspe dnr, prague, 2016. 55 http://dx.doi.org/10.1016/0029-5493(76)90138-2 http://dx.doi.org/10.1016/s0029-5493(03)00149-3 http://dx.doi.org/10.1115/1.3682280 http://dx.doi.org/10.1016/j.nucengdes.2007.02.014 http://dx.doi.org/10.1016/0894-1777(89)90009-5 acta polytechnica ctu proceedings 14:49–55, 2018 1 introduction 2 comparison of correlations 2.1 stewart experiments 2.2 tube 2.0 code 2.3 validation of tube 2.0 3 results 3.1 validation 3.2 comparison of correlations 4 conclusions list of symbols acknowledgements references 257 acta polytechnica ctu proceedings 2(1): 257–260, 2015 257 doi: 10.14311/app.2015.02.0257 optical low resolution spectroscopic observations of t pyx during the early phase of 2011 outburst a. arai1,2, m. isogai2,3, m. yamanaka4, h. akitaya5, m. uemura5 1center for astronomy, university of hyogo 2koyama astronomical observatory, kyoto sangyo university 3national astronomical observatory of japan 4kwasan observatory, kyoto university 5higashi-hiroshima astronomical observatory, hiroshima university corresponding author: arai@nhao.jp abstract we report on the results of our low resolution spectroscopic observations during the 2011 outburst of the recurrent nova t pyx. our observations were performed from 0.19 days to 34 days after the eruption discovered by m. linnolt. we found wolf-rayet like features in our spectrum during the initial rising phase on t = 0.19 d. following spectral developments are consistent with previous works. we discuss that the early phase of t pyx is divided into three stages, a short lived wr-like stage, he/n stage and fe ii stage. keywords: cataclysmic variables optical spectroscopy individual: t pyx. 1 introduction the initial rising phase of novae is a very intriguing phase in the point of view of thermonuclear runaway and development of spectral features. generally, observations are very poor in the initial rising phase, because it is very rare that novae are discovered during their initial rising phase. especially, spectroscopic observations lack at these very early epochs. t pyx is one of the recurrent novae. its previous outbursts were observed in 1890, 1902, 1920, 1944, and 1967 (schaefer et al. 2010). low resolution spectroscopic observations in past outbursts were performed around their visual maximum (e.g., adams and joy 1920, joy 1945, and catchpole et al. 1969). recurrent novae (rne) are semi-detached binary systems consisting of a massive white dwarf with accretion material from a companion star (at typical mass transfer rate ' 10−7 m� yr−1). they are the prime candidates for being type ia supernovae (sne) progenitor. recurrent novae are classified into three types, the u sco-, the rs ophand the t pyx-types, based on the light curves and spectroscopic features (anupama et al. 2008). but, the light curve of t pyx in the early phase is more similar to slow rising classical novae, than the u scoor the rs oph-type rne. in past outbursts, t pyx showed a rapid rising to v ∼ 8, followed by a pre-maximum halt and a further slow rising to the maximum light (v ∼ 6.5) for ∼ 20 days with variations of ∆v ∼ 0.2. the sixth outburst of t pyx was discovered at visual magnitude 13.0 on ut 2011 april 14.2931 (= jd 2455665.7931, t = 0 d) by m. linnolt (waagan et al. 2011). after the discovery, many observers carried out follow up observations. the study in the early spectroscopic observations and photometry revealed that t pyx showed rare hybrid spectral classification types (williams 1992), he/n fe ii he/n transition (e.g. izzo et al. 2012, imamura and tanabe 2012, surina et al. 2014). following prompt target of opportunity observations were also performed in x-ray and radio just after the discovery of the outburst (e.g., kuulkers et al. 2011, nordsieck and shara 2011, chomiuk et al. 2011). here, we report our spectroscopic results of t pyx from the very early phase. in this paper, we focused on the wolf-rayet feature in the initial rising phase on t = 0.19 d (2011 april 14.48) and following developments of spectral features from t = 2 d to t = 34 d. 2 observations our low dispersion optical spectroscopic observations of t pyx were performed during 13 nights from 2011 april 14 to may 18 at two sites. one site is koyama astronomical observatory (kyoto sangyo university). we used the araki telescope with losa/f2 spectrograph (shinnaka et al. 2013). the wavelength coverage 257 http://dx.doi.org/10.14311/app.2015.02.0257 a. arai et al. is 4000–8000 å and the spectral resolution is r ∼ 580 at 6563 å. another one is higashi-hiroshima astronomical observatory (hiroshima university). we use the kanata telescope with howpol (kawabata et al. 2008). the wavelength coverage is 4200–9000 å and the spectral resolution of r ∼ 400 at 6563 å. the v-band light curve and epochs of our spectroscopic observations are shown in the panel (a) of figure 1. -800 -700 -600 -500 -400 -300 -200 -100 0 0 5 10 15 20 25 30 35 40 e .w . [ a ng st ro m s] day from the discovery (c) 400 600 800 1000 1200 1400 1600 1800 f w h m [k m s -1 ] (b) hα hβ oi 7774 feii 5169 hei 5876 5 6 7 8 9 10 11 12 v -b an d m ag ni tu de s (a) figure 1: (a) the v-band light curve of the early phase of t pyx 2011 quoted from aavso database. short tics denote epochs of our observations. (b) the fwhms of prominent emission lines. (c) the equivalent widths for the same lines in the panel (b). 3 results and discussion 3.1 wolf-rayet like features in the initial rising phase on t = 0.19d figure 2 shows our first spectrum on ut 2011 april 14.48 (t = 0.19 d), just 4.4 hours after the discovery by m. linnolt. the spectrum exhibits many highly excited emission lines, he ii, c iv, n iii, n iv, on this spectrum. the spectral features are reminiscent of wn type wolf-rayet (wr) stars (crowther 2007 and references therein). we identified these emission lines with reference to spectral atlases of wolf-rayet stars (hamann 1995, conti 1990). the strong emission at c vi (λ5802) is, however, observed in wc type rather than wn type. in wolf-rayet stars, emission lines of highly excitation levels for c, n or o are often observed as strong broaded emission lines (fwhm = 800 2000 km s−1). in generally, the stellar temperature for wr stars is around 5× 104 – 105 k, and wr winds are accelerated by line driven pressure (crowther 2007). in the ordinary picture of novae, during the initial rising phase, shells / a photosphere is expanding. absorption components are expected to be detected for the balmer and fe ii, o i, or he i transition. t pyx showed, however, no significant p-cygni profiles at the expanding velocities of highly excited emission lines (fwhm ∼ 1100 km s −1) on t = 0.19 d. this implies that the envelope of wd would be optically thin on the moment, that is, the optically thick wind (kato 1994) would be still weak on that time. the t pyx binary system has an orbital period of 1.83 h and a mass ratio of 0.2±0.03 (uthas et al. 2010). such compact binary system has a dwarf companion, suggesting that the amount of surrounding gas supplied by the companion star of t pyx is much less than the rs oph-type rne, which show highly excited emission by shock (e.g., [fe vii]). hence, the wolf-rayet like spectral features of t pyx would be originated in the nova envelope, not by shocks with the circumbinary gas. at this time, wolf-rayet like spectral features are reported for a few novae. in the pu vul (1979), wn type features are discovered in optical and uv during its decline phase (tomov et al. 1991, sion et al. 1993). in the spectra of ag peg, rr tel and rt ser, wr-like emission lines appeared after the visual maximum (kenyon 1986, tomov et al. 1991 and references therein). our result is the first case of wr-like spectral features in the rising phase of novae. 3.2 developments after t = 2d such wr-like spectral features disappeared in our spectra from t = 2 d as shown in figure 3. no wr-like lines were reported after t = 0.8 – 1 d (shore et al. 2011, izzo et al., 2012, surina et al. 2014). the brightness of t pyx on t = 2 d was v = 8, in the pre-maximum halt as shown in the panel (a) of figure 1. on t = 2 d, hα, hβ, o i, he i and n ii have been strengthen, while emission lines of he ii, c iv and n iv disappeared. this result indicates that the spectral type of t pyx had changed from wr-like type to he/n type. the expanding velocity estimated from blue-shifted absorption minimum of p-cygni profiles of hβ is about 1400 km s−1. 258 optical low resolution spectroscopic observations of t pyx during the early phase of 2011 outburst 1.0 2.0 3.0 4.0 5.0 6.0 4000 4500 5000 5500 6000 6500 7000 7500 n or m al iz ed fl ux wavelength (å) h δ/ h e ii + n iv h e ii 41 99 h γ n ii i 4 51 5 h e ii 45 41 + c iv n v 4 60 3 + 4 61 8 n ii i 4 65 0 h e ii 46 86 h β o v 4 94 0 or h e i 4 92 2, f e ii 49 24 , f e v ii 49 40 f e ii 55 81 + 5 58 2 + 5 59 0 c iv 5 80 2 h α h e ii 66 83 n iv 7 10 9− 71 23 h e ii 71 77 ⊕ ⊕ figure 2: first spectrum of t pyx on t = 0.19 d. the highly excited emissions were detected likely to wolf-rayet stars. cross circles denote positions of telluric absorption bands. 0 5 10 15 20 25 4000 4500 5000 5500 6000 6500 7000 7500 8000 n or m al iz ed fl ux wavelength (å) h α h β h γ h δ fe ii h ei h ei /n ii h ei h ei h ei o i t=0.19 kao t=2 hhao t=3 kao, hhao t=5 kao t=6 kao t=7 kao t=9 hhao t=11 kao t=12 hhao t=18 kao t=29 kao, hhao t=33 kao t=34 kao figure 3: low resolution spectra of t pyx from t = 0.19 – 34 d. observations were carried out at koyama astronomical observatory (kao) and higashi-hiroshima astronomical observatory (hhao). we identified the balmer, he i, n ii, fe ii multiplets, o i, na i lines. the developments of fwhms and equivalent widths of emission components of these lines are shown in the panel (b) and (c) of figure 1, respectively. the fwhm of hα (and also hβ) measured in our spectra (the panel (b) of figure 1) present a rapid increase from 1100 km s−1 to 1600 km s−1 during t = 0.19 – 2 d. surina et al. (2014) reported faster velocity (vej = 4000 km s−1) on t = 0.8 d. after this event, the fwhm of hα decreased gradually to ∼ 1000 km s−1 until t = 12 d in parallel with the increasing of their equivalent widths from -70 å to -320 å, as shown in the panel(c) of figure 1. fwhms of other emission lines also showed similar trend, and their equivalent widths decreased to the minimum around t = 20 d (e.w. of hα = -200 å). from t = 12 d to t = 34 d, fwhms of all lines increased again, from ∼ 1000 km s−1 to ∼ 1300 km s−1 for hα, hβ and o i (λ7774). hβ, n ii (λ5679), o i (λ7774) are accompanied with p-cygni profiles clearly. t pyx showed a he/n-type spectra from t = 2 d to t = 6 d, during pre-maximum halt. decrease rates of equivalent widths of he i and n ii are similar to the one observed in the balmer lines at the same epoch. on the other hand, fe ii emission lines emerged aound t = 7 d as seen in the panel (c) in figure 1. the equivalent widths of fe ii multiplets(42, 49) were growing gradually from t = 7 d to t = 34 d. in this epoch, increasing trend of fwhm in fe ii and o i were also observed as seen in hα and hβ. on t = 34 d, the equivalent width of fe ii and o i were slightly decreased due to the growth of continuum light in this epoch. these results indicate that the strong optically thick wind were gradually developing and fe ii type features replace a he/n features around t = 7 d, consisting with surina et al. (2014). after the entering into the fe ii type, hβ, n ii (λ5679), o 259 a. arai et al. i (λ7774) are accompanied with clear p-cygni profiles. these spectral developments are also reported and discussed in detail in shore et al. (2011), izzo et al. (2012), imamura and tanabe (2012), and surina et al. (2014). as pointed out by imamura and tanabe (2012), a similar hybrid behavior during the pre-maximum halt phase was also observed in only a slow developing nova, v5558 sgr (tanaka et al. 2011). t pyx reveals intriguing spectral transition of spectral type of nova at more earlier stage than that observed in v5558 sgr. the hybrid (he/n fe ii) transition would be a common behavior in slow rising novae. very likely, the early observed wr-like features may be also a common evolution just after the explosion for slow rising novae. based on our results and combination with previous studies, we summarize that the development during the very early phase of the 2011 outburst of t pyx is composed of three stages: (1) the wr-like stage, which shows short lived (duration < 1 d) wr-like spectral line features, (2) the he/n stage (several days), and (3) the fe ii-type stage. acknowledgement we thank taichi kato (kyoto university) for valuable comments on our observational data. this research was supported(, in part,) by a grant from the hayakawa satio fund awarded by the astronomical society of japan, and optical & near-infrared astronomy inter-university cooperation program, supported by the mext of japan. references [1] adams, w. s. and joy, a. h. 1922, publications of the american astronomical society, 4, 139 [2] anupama, g. c., evans, a., bode, m. f., o’brien, t. j. and darnley, m. j. 2008, astronomical society of the pacific conference series, 40, 31 [3] catchpole, r. m. 1969, mnras, 142, 119 doi:10.1093/mnras/142.1.119 [4] chomiuk, l., et al. 2011, the astronomer’s telegram, 3318 [5] conti, p. s. and massey, p. and vreux, j.-m. 1990, apj, 354, 359 doi:10.1086/168694 [6] crowther, p. a. 2007, ara&a, 45, 177 doi:10.1146/annurev.astro.45.051806.110615 [7] hamann, w.-r. and koesterke, l. and wessolowski, u. 1995, a&as, 113, 459 [8] imamura, k. and tanabe, k. 2012, pasj, 64, l9 [9] izzo, l., et al. 2012, mmsai, 83, 830 [10] joy, a. h. 1945, pasp, 57, 171 doi:10.1086/125711 [11] kato, m. and hachisu, i. 1994, apj, 437, 802 doi:10.1086/175041 [12] kawabata, k. s., et al. 2008, spie, 7014, 4 [13] nordsieck, k. h. and shara, m. 2011, the astronomer’s telegram, 3289, 1 [14] kenyon, s. j. 1986, aj, 91, 563 [15] kuulkers, e, et al. 2011, the astronomer’s telegram, 3285, 1 [16] schaefer, b. e., pagnotta, a. and shara, m. m. 2010, apj, 708, 381 doi:10.1088/0004-637x/708/1/381 [17] shinnaka, y., et al. 2013, icarus, 222, 734 doi:10.1016/j.icarus.2012.08.001 [18] shore, s. n., augusteijn, t., ederoclite, a. and uthas, h. 2011, a&a, 533, 8 [19] sion, m. edward et al. 1993, aj, 106, 2118 [20] surina, f., et al. 2014, aj, 147, 107 [21] tanaka, j., et al. 2011, pasj, 63, 911 [22] tomov, t., et al. 1991, mnras, 252, 31 doi:10.1093/mnras/252.1.31p [23] uthas, h. and knigge, c. and steeghs, d. 2010, mnras, 409, 237 doi:10.1111/j.1365-2966.2010.17046.x [24] waagan, e., et al. 2011, cbet, 2700, 1 [25] williams, r. e. 1992, aj, 104, 725 260 http://dx.doi.org/10.1093/mnras/142.1.119 http://dx.doi.org/10.1086/168694 http://dx.doi.org/10.1146/annurev.astro.45.051806.110615 http://dx.doi.org/10.1086/125711 http://dx.doi.org/10.1086/175041 http://dx.doi.org/10.1088/0004-637x/708/1/381 http://dx.doi.org/10.1016/j.icarus.2012.08.001 http://dx.doi.org/10.1093/mnras/252.1.31p http://dx.doi.org/10.1111/j.1365-2966.2010.17046.x introduction observations results and discussion wolf-rayet like features in the initial rising phase on t = 0.19d developments after t = 2d acta polytechnica ctu proceedings doi:10.14311/app.2018.18.0072 acta polytechnica ctu proceedings 18:72–76, 2018 © czech technical university in prague, 2018 available online at http://ojs.cvut.cz/ojs/index.php/app testing of hybrid nickel-polyurethane foams at high strain-rates using hopkinson bar and digital image correlation marcel adornaa,∗, petr zlámal a, tomáš fílaa, jan faltaa, markus feltenb, michael friesb, anne jungb a czech technical university in prague, faculty of transportation sciences, department of mechanics and materials, konviktská 20, 120 00 prague 1, czech republic b universität des saarlandes, institute of applied mechanics, campus a4.2, saarbrücken, 66123, germany ∗ corresponding author: adorna@fd.cvut.cz abstract. in this paper split hopkinson pressure bar (shpb) was used for dynamic testing of nickel coated polyurethane hybrid foams. the foams were manufactured by electrodeposition of a nickel coating on the standard open-cell polyurethane foam. high strength aluminium alloy bars instrumented with foil strain-gauges were used for dynamic loading of the specimens. experiments were observed using a high-speed camera with frame-rate set to approx. 100-150 kfps. precise synchronisation of the high-speed camera and the strain-gauge record was achieved using a through-beam photoelectric sensor. dynamic equilibrium in the specimen was achieved in all measurements. digital image correlation technique (dic) was used to evaluate in-plane displacements and deformations of the samples. specimens of two different dimensions were tested to investigate the collapse of the foam structure under high-speed loading at the specific strain-rate and strain. keywords: shpb, hybrid foam, dic, strain-gauge measurement. 1. introduction in recent years, we have seen increasing interest in the use of light metal foams in the automotive, aerospace and other applications. this is mainly due to weight reduction, while maintaining load and strength of the construction. another advantage is the ability to achieve better energy absorption capabilities and suitable acoustic damping properties [1]. for design and optimization in the aforementioned applications detailed description of the material properties, particularly under dynamic loads, is necessary. metal foams and similar soft materials in general exhibit a nonstandard deformation behaviour in comparison with conventional materials (e. g. metals) [2], making it difficult to adopt standard measurement techniques for their high strain-rate characteristics and complex deformation behaviour [3–6]. this work is focused on the investigation of the deformation behaviour of nickel coated polyurethane hybrid foam using an innovative shpb device in combination with dic. the presented experimental data describes the course of the foam structure collapse and extends the knowledge of deformation mechanism of hybrid nickel-polyurethane foams, where a coating of nano-crystalline nickel is deposited on cheap open-cell polyurethane foams. 2. materials and methods 2.1. sample preparation polyurethane (pu) foams with a pore size 20 ppi (pores per inch) from jumpinoo, spenge, germany was used. two types of cylindrical specimens with diameter of approx. 20mm and height approx. 20mm (long specimens) and approx. 10mm (short specimens) were cut by hot wire cutting from the foam plates. since pu foams are not electrically conductive, the foams must be rendered electrically conductive by dip coating in a graphite lacquer (crc kontakt chemie, iffezheim, germany) to be able to coat the foams by electrodeposition. the electrodepostion of nanocrystalline nickel (ni) on the carbon coated pu foams was carried out using a commercial ni sulfamate electrolyte with a ni content of 110 g/l (enthone gmbh, langenfeld, germany) at an electrolyte temperature 50 ◦c and ph 3.8. in order to guarantee for a nearly homogeneous coating thickness all over in the foam, the foam was centered as cathode in a double-walled cube filled with ni pellets (a.m.p.e.r.e. gmbh, dietzenbach, germany) acting as sacrificial anode. the electroplating was performed at current density 1.5ma/cm3 until a theoretical coating thickness of 75µm ni was deposited. for more details, it is referred to the literature [7–9]. the specimen of hybrid foam in the loading device is shown in fig. 1. 2.2. digital image correlation the deformation of the sample during the test was recorded using a high-speed camera. high-speed imaging enables to capture the deformation process of shpb tests in suitable quality (sufficient number of images with small changes between them) for the strain assessment. based on the captured images, changes 72 http://dx.doi.org/10.14311/app.2018.18.0072 http://ojs.cvut.cz/ojs/index.php/app vol. 18/2018 testing of hybrid ni/pu foams using shpb and dic figure 1. ni/pu foam specimen in the shpb experimental setup of the sample structure can be evaluated, using digital image correlation (dic) technique. nowadays, dic technique is a well established method for the assessment of strain in the complex structure using tracking of virtual pattern selected in the sequence of images where the displacements are to be evaluated. each point of the virtual pattern is subsequently tracked through to each pair of images (reference and deformed) in the series of loading states. the computation implementation of the tracking algorithm is based on subsets of pixels which are defined as a square shaped area with defined pixel dimensions formed around the individual tracked point (fig. 2). the size of the subset area has to be suitably chosen with respect to computational cost and traceability. as the specimen is deformed, the reference square pattern is also deformed (combination of translational, rotational and shear deformations) in the image sequence. the position of the deformed subset in the image is calculated based on the extreme of correlation coefficient calculated using the selected criterion. in this work, the algorithm derived from the lucaskanade tracking procedure [10] and implemented in a custom dic tool and the matlab toolkit [11] was used. in this tool, the correlation coefficients were estimated during the two-step procedure, first at pixel level and second at sub-pixel level to determine the values of displacement more accurately. at the pixel level, the sum-squared difference (ssd) criterion was used whereas the lucas-kanade algorithm based on the zero-normalised ssd (znssd) criterion was used at the sub-pixel level. then, strain fields could be calculated from resulting displacements in two dimensions of each point (output of dic algorithm) and mapped to the original structure to describe the inplane deformation behaviour of the sample. 2.3. split hopkinson pressure bar testing of the hybrid foams was performed using a modified kolsky shpb setup. diameter of the measurement bars and the striker bar was 20mm. all bars were made of high-strength aluminium alloy (enaw-7075). the striker was propelled using a gas-gun with 16 bar maximum pressure. the gas-gun system was composed of the following parts: a steel barrel with the maximum stroke 2500mm, a high-flow fast figure 2. parameters of the digital image correlation points , parameter m represents the dimension of subset area release solenoid valve (366531, parker, usa), a 20 l air reservoir equipped with pressure gauge and other peripherals (compressor, piping etc.). length of the both incident and transmission bar was 1600mm. the bars were supported by a set of a low friction polymerliner slide bearings (drylin fjum, igus, germany) mounted in a custom stainless steel housing. the striker bar had length 500mm. the surfaces of the bars were precisely finished to maintain a tight diameter tolerance for smooth motion in the bearings without notable clearance. the axis of the bars was carefully set to minimize friction and to maintain straightness of the shpb setup. geometrical alignment is very important for the minimization of the undesirable effects (increased friction, bending of the bars etc.) affecting the measurement accuracy. the impact faces of the bars were precisely ground and were set to maintain plan-parallelism between the faces better than 0.05mm (measured by feeler gauge). an overview of the experimental setup is shown in fig. 3. 2.4. instrumentation the strain wave passing through the incident and transmission bar was measured at two measurement points (mps) using foil strain gauges (3/120 ly61, hbm, germany) with 3mm active length. the strain gauges were arranged to wheatstone half-bridge circuit to double the output signal in comparison to single gauge and to eliminate the potential influence of bending of the bars. on each measuring bar (1600mm in length) one mp was set at a distance of 800mm from each bar end. the foil strain gauges were selected for this application due to their linearity and range of possible strain measurement (up to 50,000µε). the disadvantage of foil gauges is the need to amplify the signal before digitization. this disadvantage can be solved by using semiconductor strain gauges but their nonlinear characteristic and low measurement range (up to 2,500µε) makes them unsuitable for our application. each strain gauge was bonded on the bar 73 m. adorna, p. zlámal, t. fíla et al. acta polytechnica ctu proceedings figure 3. shpb experimental setup overview with a single component low-viscosity cyanoacrylate adhesive (z70, hbm, germany) and cured for at least 12 hours to ensure maximum strength of the bond. to reduce the noise in the measurement chain (maximize to signal-to-noise ratio of the output signal) the powering of the mps was performed using custom battery units with an excitation voltage 4v. mps signal output were amplified using active differential low noise amplifier (el-lna-2, elsys ag, switzerland) with gain 100 and sampled using high speed 16-bit digitizers (pci-9826h, adlink technology, inc., taiwan) with a maximal 20 mhz sample rate and triggered by signal of through-beam photoelectric sensor (fs/fe 10-rl-ps-e4, sensopart, germany) installed on the barrel of the shpb. the signal from the through-beam photoelectric sensor is also used for the high-speed camera triggering and time synchronization of the strain gauges signal with the captured image sequence. for accurate dic evaluation it is crucial to capture the images in sufficient quality, i.e, properly illuminated and with satisfactory frame rate to ensure a sufficiently small structure changes between the images. thus, the deformation of the sample during the shpb test was observed using a high-speed camera (fastcam sa5, photron, japan) with a selected region of interest (rio) with pixel resolution 256 × 216. the selected roi enabled to reach a frame rate of approximately 105 kfps and 4 pixels resolution per strut thickness of hybrid foam. illumination of the sample surface was performed using a pair of high intensity led lights (constellation 60, veritas, usa). data acquisition was employed using a custom virtual instrument designed in labview environment (national instruments, usa) with tdms binary file output format also readable in the matlab toolkit for further analysis. 3. results 3.1. digital image correlation results two sets of specimen with nominal diameter 20mm and nominal length 10mm (short samples) and 20mm (long samples) were measured using shpb at two different strain-rates. in the paper, the individual strainrates are designated low and high. this designation is relative to the sample. strain-rate achieved during experiments depends not only on striker velocity, but also lenght of tested sample (short samples low strain rate ca. 1500 s−1 , high strain rate ca. 5000 s−1 , long samples low strain rate ca. 1400 s−1, high strain rate ca. 2200 s−1). an identical grid of 22 × 14 correlation points was used to cover the area of the specimen and the end parts of the measuring bars on the high-speed camera images. the position of the grid points was tracked using dic and new positions of the grid were identified in the whole sequence of the captured high-speed camera images which were converted from raw format to 8-bit png by a lossless compression algorithm and were not subjected to any additional preprocessing. in all cases, digital image correlation was successful, the mean correlation coefficient was higher than 90% for all measurements. in fig. 4 median value of the correlation coefficient for three representative experiments: short sample at low strain-rate (test 270), short sample at high strain-rate (test 285), long sample at high strain-rate (test 317) is plotted against strain evaluated using dic. it demonstrates that the trend of all three curves is the same and that the dic results for short samples can be considered reliable up to approx. 0.3 overall strain. in this strain range, the strains evaluated using dic are in very good agreement with the strain-gage measurements. in case of the long samples, the median correlation coefficient exhibits similar trend with dramatic drop of its value for strain higher than 0.2. in strain higher than 0.2 − 0.3, the results provided by dic started to be distorted and unreliable. this phenomena is represented by the drop and the oscillations of the correlation coefficient curves (see green and blue curve in fig. 4) and significant differences in stress-strain curves evaluated using dic and straingauge at strain higher than 0.2 − 0.3 (see fig. 6 -7). evolution of the full-field in-plane strain of the foam structure evaluated using dic is shown in fig. 8 for three selected representative cases. loss of correlation 74 vol. 18/2018 testing of hybrid ni/pu foams using shpb and dic was caused by the densification of the foam structure changing significantly the original structure of the foam. another reason was that diameter of the specimen during compression started to be higher than the diameter of the bars. strain was therefore localized in the central part of the specimen. this distortion caused by the increase of the diameter was even more significant for longer samples (fig. 8 bottom). the dynamic equilirium was achieved in all experiments as the samples were deformed uniformly on both ends in short time after the wave impact on the specimen(see fig. 8 second column of the images). in fig. 8 it is also shown, that hybrid foam deformation occurs locally in areas where the structure is the weakest and collapse of the structure occurs on this plane. figure 4. representative median value of the correlation coefficient plotted against strain for three selected experiments (test 270 short sample at low strain rate, test 285 short sample at high strain rate, test 317 long sample at high strain rate) 3.2. dic and strain-gauge measurement results comparison stress-strain curves with strain derived using dic were evaluated for each tested sample and compared with the strain-gauge curves. a comparison for three measurements (corresponding with full-field dic analysis shown in fig. 8) are shown in fig. 5 to fig. 7 (green lines used for dic curve, orange lines for strain-gauge results). in all cases, stress-strain curves with strain derived using dic are very close to the strain-gauge curves up to 0.3 strain for the short specimens and 0.2 for the long specimens. differences of the dic curves to strain-gauge curves after 20-30% deformation are caused by the fact that sample size exceeds the measuring bars diameter, resulting in a loss of correlation points in parts outside the monitored area. moreover, significant deformation of the sample also leads to a loss of correlation points due to the collapse of the structure and shift of individual struts out of high-speed camera focus. 4. conclusion shpb experimental device was successfully used for the dynamic compression of the ni/pu hybrid foam. in-plane strain was successfully evaluated from the figure 5. comparison of stress-strain diagrams with strain evaluated using strain-gage and dic, experiment no. 270 figure 6. comparison of stress-strain diagrams with strain evaluated using strain-gage and dic, experiment no. 285 figure 7. comparison of stress-strain diagrams with strain evaluated using strain-gage and dic, experiment no. 317 high-speed camera images using digital image correlation technique. dic results were found to be in a very good agreement with strain-gauge measurements up to 30% of deformation for the short samples and 20% of deformation for the long samples. dynamic equilibrium was achieved in all measurements as the specimens were deformed uniformly on both ends in short time after the wave impact on the specimen. it was shown that both shpb and dic are suitable tools for advanced characterization of the hybrid foam under impact loading. 75 m. adorna, p. zlámal, t. fíla et al. acta polytechnica ctu proceedings figure 8. full-field dic analysis: in-plane longitudinal strain top row: experiment no. 270, vs = 22.1 ms−1 (low strain-rate, short sample), d = 22.4 mm, l = 10.1 mm middle row: experiment no. 285, vs = 38.1 ms−1 (high strain-rate, short sample), d = 21.9 mm, l = 7.5 mm bottom row: experiment no. 317, vs = 43.7 ms−1 (high strain-rate, long sample), d = 21.8 mm, l = 18.6 mm vs [ms−1] striker velocity, d [mm] sample diameter, l [mm] sample length acknowledgements the research was supported by the czech science foundation (project no. 15-15480s) and the internal grants of the czech technical university in prague (projects no. sgs17/148/ohk2/2t/16 and no. sgs18/153/ohk2/2t/16)). all the financial support is gratefully acknowledged. references [1] t. fila, p. zlamal, o. jirousek, et al. impact testing of polymer-filled auxetics using split hopkinson pressure bar. advanced engineering materials 19(10), 2017. doi:10.1002/adem.201700076. 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[7] a. jung, h. natter, s. diebels, et al. nanonickel coated aluminum foam for enhanced impact energy absorption. advanced engineering materials 13(1–2):23–28, 2011. doi:10.1002/adem.201000190. [8] a. jung, k. m. r., e. lach, et al. hybrid metal foams: mechanical testing and determination of mass flow limitations during electroplating. international journal of material science 2(4):97–107, 2012. doi:10.1002/adem.201000190. [9] a. jung, s. diebels. synthesis and mechanical properties of novel ni/pu hybrid foams: a new economic composite material for energy absorbers. advanced engineering materials 18(4):532–541, 2015. doi:10.1002/adem.201500405. [10] b. d. lucas, t. kanade. an iterative image registration technique with an application to stereo vision. in proceedings of imaging understanding workshop, pp. 674–679. 1981. [11] i. jandejsek, j. valach, d. vavrik. optimization and calibration of digital image correlation method. pp. 131–136. 2010. 76 http://dx.doi.org/10.1002/adem.201700076 http://dx.doi.org/10.1007/s11340-008-9145-1 http://dx.doi.org/10.1016/j.msea.2014.10.026 http://dx.doi.org/10.1051/epjconf/20159404030 http://dx.doi.org/10.1016/s0921-5093(00)01219-3 http://dx.doi.org/10.1016/s0734-743x(99)00153-0 http://dx.doi.org/10.1002/adem.201000190 http://dx.doi.org/10.1002/adem.201000190 http://dx.doi.org/10.1002/adem.201500405 acta polytechnica ctu proceedings 18:72–76, 2018 1 introduction 2 materials and methods 2.1 sample preparation 2.2 digital image correlation 2.3 split hopkinson pressure bar 2.4 instrumentation 3 results 3.1 digital image correlation results 3.2 dic and strain-gauge measurement results comparison 4 conclusion acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2017.13.0071 acta polytechnica ctu proceedings 13:71–74, 2017 © czech technical university in prague, 2017 available online at http://ojs.cvut.cz/ojs/index.php/app using homogenization and nanoindentation for microfibril angle determination of spruce lucie kucíkováa,∗, vladimír hrbeka,b, jan vorela, michal šejnohaa a czech technical university in prague, faculty of civil engineering, thákurova 7, 166 29 prague 6, czech republic b institute of theoretical and applied mechanics as cr, v. v. i., prosecká 809/76, 190 00 prague 9, czech republic ∗ corresponding author: lucie.kucikova@fsv.cvut.cz abstract. this paper is concerned with the evaluation of microfibril angle of spruce. the microfibril angle is defined as an inclination of microfibrils from the longitudinal axis, the direction of lumens. it is well known and further supported by the present study that the microfibril angle has a great influence on the final mechanical properties of wood. this angle could be measured either directly using, e.g. polarisation microscopy, x-ray diffraction, infrared spectroscopy, or indirectly, as used in this study, by employing the nanoindentation measurements. therein, the measured indentation modulus is compared with that obtained numerically using the anisotropic theory of indentation. the latter one depends on the entire stiffness matrix derived through homogenization and the searched microfibril angle. in view of the cell wall microstructure, the effective cell properties were found using the two-step micromechanical homogenization adopting both the self-consistent and mori-tanaka methods. keywords: spruce, nanoindentation, microfibril angle, homogenization. 1. introduction wood is an organic material with highly irregular microstructure on multiple scales. wood is basically divided into softwood and hardwood. in this paper, the norway spruce (picea abies), which belongs to softwood (conifers), was examined. tree stem divides into several components, where the xylem (wood mass) is covering the most part and is the main object of interest. a layered structure appears in the xylem, where earlywood is changing with latewood. one layer of the earlywood and one of the latewood make up the annual ring together. for earlywood, thinner cell walls and larger lumens (ellipsoidal cavity inside the cell) are typical, whereas thicker walls and very thin lumens are typical for latewood. because of this difference, both layers are examined separately. the basic wood constituents are tracheids comprising approximately 94% of the total volume [1]. tracheids are long closed tube-like cells, with almost rectangular cross section, converging on their tips, oriented mainly longitudinally. therefore, the cell wall is the main supporting constituent. the lumens are considered as voids, which are mainly used for transport of water and nutrients. the cell wall could be considered as multilayer laminate, where each lamella corresponds to one layer. furthermore, each layer could be described as a fiber composite. individual layers are basically divided into the warty layer, the secondary wall, the primary wall and middle lamella (sometimes called intercellular layer), described outwards. for more details about ultrastructure of the cell wall see [1–3]. the major part of the cell wall is covered by the middle layer of the secondary wall (s2 layer). the main load-bearing part of the cell wall is the cellulose in the form of microfibrils. microfibrils run helically along the cell wall and the inclination from the longitudinal axis is defined as the microfibril angle (mfa). it is confirmed by many studies that mfa has a great impact on final mechanical properties of the wood. the mfa can be measured directly, see [4]. another possible approach, introduced in this paper, is mfa estimation using comparison between data from the up-scale homogenization and the results of nanoindentation of the cell wall. 2. nanoindentation nanoindentation is nowadays the most commonly applied measurement method to determine material properties on micro and nano scale. this method is based on the contact mechanics with limitations for some types of material responses e.g. viscoelastic solids. the method was developed for measuring hardness and elastic modulus from load-displacement data, obtained during one cycle of loading and unloading by using sharp, self-similar indenters, such as the berkovich triangular pyramid. the basic principle of this method is pushing the small hard tip with known geometry and material properties into a material with unknown properties, with simultaneous recording of the load and indentation depth relative to the initial undeformed surface. the basic output of the nanoindentation measurement is the load-displacement curve, depicted in figure 1, where pmax is the maximum 71 http://dx.doi.org/10.14311/app.2017.13.0071 http://ojs.cvut.cz/ojs/index.php/app l. kucíková, v. hrbek, j. vorel, m. šejnoha acta polytechnica ctu proceedings figure 1. load-displacement curve [5]. figure 2. surface scan (25×25 µm) of earlywood before (left) and after (right) nanoindentation. load, hmax is the maximum displacement, s = dp/dh is the elastic unloading stiffness defined as the slope of the upper part of the unloading curve and hf is the final depth – the permanent penetration depth after the indenter is fully unloaded. in this model, the elastic and plastic deformation during loading is assumed, whereas during unloading only elastic displacements are recovered. for more information about nanoindentation and its principles see [5]. small samples of the norway spruce were embedded in an epoxy resin to stabilize the porous microstructure of the wood. next, they were cut into slices, grounded and polished to obtain as smooth surface as possible. roughness of the sample surface significantly affects the final results of nanoindentation. the measurement was performed on a hysitron tl750 equipped with berkovich diamond indenter. the indents were located to the s2 layer of various cells, with different age, position in a stem both longitudinal and horizontal etc., using click script device. due to the viscoelastic response of wood, the trapezoidal load function with holding part was used. after the measurement, it was necessary to control location of the indents, using surface scans, and exclude wrong positions. the example of the surface scan is shown in figure 2, where the indents on the bottom part are evidently on the wrong position. also curves with wrong shape, caused by surrounder [gpa] coeff. of variation earlywood 12.88 0.18 latewood 13.23 0.19 overall 13.06 0.19 table 1. indentation moduli. ing conditions (temperature, vibrations etc.), surface roughness etc., should be excluded too. after disregarding all wrong load displacement curves, data evaluation can be performed. the standard output of the measurement comprises indentation modulus er, projected area a, maximum load pmax, contact depth hc etc. these values are computed automatically by software implemented in the measurement device. in this case the samples were prepared in a way the indentation direction is practically identical to the longitudinal axis of the cell. from that, the indentation modulus er corresponds to the axial modulus of the cell wall. the normal distribution was used for the data evaluation, where the final number of data is 761 for earlywood, 738 for latewood and 1499 altogether. the resulting means of the indentation moduli are summarized in table 1. clearly, the difference between earlywood and latewood properties is insignificant as was also suggested in [6]. 3. homogenization wood is generally anisotropic material with more or less random microstructure displayed on several scales. this fact promotes micromechanical homogenization as a suitable approach to obtain mechanical properties of the cell wall. for our purpose, only two-scale homogenization was used, following method described in [7]. the homogenization starts at the level called polymer network, with a characteristic length of a representative volume element (rve) in the range of 8–20 nm. two variants were used for the first step. the first one comprises two phases (lignin and hemicellulose), neglecting extractives and considering the dried state of the wood (without water). whereas the second one comprises three phases (lignin, hemicellulose and water+extractives). it is considered that extractives are dissolved in a water and from that it is considered as one phase. in this case the moisture content was set to 0.1, temperature 20°c, relative humidity 0.5 and dry wood density 430 kg/m3. according to authors, all phases are treated on the same footing, from that the self-consistent method seems to be a suitable method. lignin was set as a matrix and the rest of the phases as spherical inclusions. all of the phases are considered as isotropic materials. the input values of all phases and corresponding volume fractions were taken from [7]. the second step provides mechanical properties of the cell wall, which are important for the mfa computation. the cell wall thickness is about 1.5 µm 72 vol. 13/2017 microfibril angle determination of spruce variant 1 variant 2 [gpa] ea et ea et polymer network 6.89 6.89 3.59 3.59 cell wall 44.26 9.34 36.21 5.33 table 2. axial and transverse young moduli of cell wall from two-step micromechanical homogenization. figure 3. angle dependency of the indentation modulus. and 5 µm in case of earlywood and latewood, respectively [1]. the computational procedure is the same for both variants. this step comprises three phases: polymer network, which is set as a matrix, crystalline and amorphous cellulose, both considered as cylindrical inclusions. all inclusions are parallel to the longitudinal axis, which corresponds to zero microfibril angle. following the cell wall fibrous composition, the mori-tanaka method appears to be a suitable choice for this level. both the self-consistent and mori-tanaka micromechanical models are further described in [8]. the results for both variants are summarized in table 2. it is evident that the difference caused by neglecting extractives and water is significant. 4. mfa evaluation this section offers a simplified approach to the derivation of mfa by combining anisotropic theory of nanoindentation and classical micromechanics described in the previous section. by comparing the results of nanoindentation in table 1 and the theoretical predictions from micromechanical homogenization in table 2, although not fully compatible, it becomes clear that the homogenization provides considerably higher values. this discrepancy is caused, among other factors, by the microfibril angle, which affects the final value of longitudinal modulus. in [9] the authors describe the longitudinal modulus ea as a function of mfa and introduce the loading angle, which depends on mfa. as it was mentioned in section 3, the resulting effective matrix is defined for zero microfibril angle. to obtain the effective properties at cell wall level assuming a non-zero mfa, the matrix transformation must be performed. adopting the anisotropic variant 1 variant 2 mfa [°] cov mfa [°] cov earlywood 41.34 0.32 19.01 0.49 latewood 39.69 0.36 19.10 0.48 overall 40.53 0.34 19.05 0.49 table 3. mean values of the mfa. (cov = coefficient of variation). theory of indention presented in [10] we obtain the indentation modulus as a function of the rotated homogenized stiffness matrix to be compared with that measured by nanoindentation. matching these two moduli allows us to extract the searched mfa, see also [11, 12] for further reference. the variation of indentation modulus as a function on mfa is plotted in figure 3. this approach allows us to compare data from nanoindentation measurement and rotated values from homogenization. finding the match of the longitudinal moduli from both methods ehom and eindent, respectively, provides an estimation of the microfibril angle. the resulting mean values of the microfibril angle are summarized in table 3. the results of variant 1 are higher than values of variant 2. the mfa computed with input data of variant 2 is in the range given in literature. for example in [2] author states a range of the mfa of the middle layer about 0° – 30°. 5. conclusions this paper was concerned with several specific topics of the material analysis of wood elastic response. firstly, the direct measurement method on micro scale was introduced. thanks to the nanoindentation, we are able to measure the mechanical properties of the cell wall. nevertheless, the method is still under development and several factors are not known yet. unfortunately, the wood is heterogeneous and very variable material. the material properties are changing with growth conditions in case of the living tree and even when the wood is in the form of the timber, they differ with temperature and humidity. this makes the measurement even more difficult. possibly the main task is the sample preparation. moisture absorption and desorption of the wood causes great problems during preparation. also its softness complicates cutting and the thermal degradation limits the oven drying. even the evaluation of the measured data is still under development, especially in case of viscoelastic materials, to which the wood belongs. despite all these factors, we consider that the nanoindentation gives more or less good approximation of the cell wall mechanical properties. the mean values of the indentation modulus are summarized in table 1. in this paper the evaluation of the measured data for an anisotropic half space was employed [10]. another method introduced in this paper is the micromechanical homogenization. following [7] the 73 l. kucíková, v. hrbek, j. vorel, m. šejnoha acta polytechnica ctu proceedings two-step homogenization procedure was performed to obtain effective mechanical properties of the cell wall. there are two variants of the computation. the first one is neglecting the extractives and considers the dry state, whilst the second one includes extractives and initial conditions are set to: moisture content 0.1, temperature 20°c, dry wood density 430 kg/m3. all input arguments were taken from literature [7]. two computational methods were used – the selfconsistent and the mori-tanaka method. for the first step – polymer network comprising two or three phases (depends on the variant) the self-consistent method was employed. due to the fibrous nature of the cell wall, the mori-tanaka method was chosen as a suitable approach for the second step (cell wall). in this case the layered structure of the cell wall was neglected and the resulting properties written in table 2 correspond to the s2 layer of the cell wall. among other factors, the microfibril angle influences the mechanical properties of the cell wall. except for the various methods of the direct measurement [4], the computational approach could be performed. considering the nanoindentation data as real properties of the cell wall with yet unknown microfibril angle and results from homogenization as the real properties with zero mfa, the angle could be computed as only variable using the comparison of nanoindentation data and rotated values from the homogenization. the mfa-indentation modulus dependency is depicted on figure 3 and the final values of mfa are summarized in table 3. the values of the variant 2 correspond to the data given in literature [2]. there are still many inaccuracies in this method of the mfa determination. very helpful could be the verification of the model using the direct measurement of the microfibril angle on the same samples. due to the visco-elastic response of the wood, the different type of the loading during nanoindentation should be performed. even the sample preparation can be improved, using the microtome cutting rather than grinding and polishing. all these improvements and even more are the task of the future research. acknowledgements the financial support provided by the gačr grant no. 1510354s and by the czech technical university in prague within sgs project with the application registered under the no. sgs17/168/ohk1/3t/11 is gratefully acknowledged. references [1] p. o. kettunen. wood: structure and properties. trans tech publications ltd, uetikon-zuerich, 2006. [2] h. e. desch, j. m. dinwoodie. timber: structure, properties, conversion and use. 7th. macmillan press, hampshire, 1996. [3] m. fujita, h. harada. ultrastructure and formation of wood cell wall. in d. n.-s. hon, n. shiraishi (eds.), wood and cellulosic chemistry, pp. 1–49. marcel dekker, inc., new york, 2001. [4] l. donaldson. microfibril angle: measurement, variation and relationships a review. iawa journal 29(4):345–386, 2008. [5] w. oliver, g. pharr. measurement of hardness and elastic modulus by instrumented indentation: advances in understanding and refinements to methodology. journal of materials research 19(1):3–20, jan 2004. [6] l. melzerová, l. kucíková, t. janda, m. šejnoha. estimation of orthotropic mechanical properties of wood based on non-destructive testing. wood research 61(6):861–870, 2016. [7] k. hofstetter, c. hellmich, j. eberhardsteiner. development and experimental validation of a continuum micromechanics model for the elasticity of wood. european journal of mechanics a/solids 24(6):1030–1053, 2005. [8] m. šejnoha, j. zeman. micromechanics in practice. wit press, southampton, boston, 2013. [9] w. gindl, t. schöberl. the significance of the elastic modulus of wood cell walls obtained from nanoindentation measurements. composites part a: applied science and manufacturing 35(11):1345–1349, 2004. [10] j. vlassak, m. ciavarella, j. barber, x. wang. the indentation modulus of elastically anisotropic materials for indenters of arbitrary shape. journal of the mechanics and physics of solids 51(9):1707–1721, 2003. [11] a. jäger, t. bader, k. hofstetter, j. eberhardsteiner. the relation between indentation modulus, microfibril angle, and elastic properties of wood cell walls. composites part a: applied science and manufacturing 42(6):677–685, 2011. [12] m. šejnoha, t. janda, j. vorel, et al. combining homogenization, indentation and bayesian inference in estimating the microfibril angle of spruce. procedia engineering 190:310–317, 2017. 74 acta polytechnica ctu proceedings 13:72–75, 2017 1 introduction 2 nanoindentation 3 homogenization 4 mfa evaluation 5 conclusions acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2018.20.0010 acta polytechnica ctu proceedings 20:10–15, 2018 © czech technical university in prague, 2018 available online at http://ojs.cvut.cz/ojs/index.php/app an experimental study of the coarse droplets formation ondřej bartoš∗, lucie měšťanová czech technical university in prague, technická 4, prague 6, czechia ∗ corresponding author: ondrej.bartos@fs.cvut.cz abstract. the paper introduces the first results from a new steam/air wind tunnel built in order to study the coarse water droplets in steam turbines and their formation. the wind tunnel was designed with the aim to have a testing facility with similar flow conditions for the study of liquid films as found in steam turbines. the tunnel is equipped with common devices for the measurement of the flow and for the analysis of the diameter distribution of the droplets. two measurements methods were used for the measurement of the droplets size, photogrammetry and light scattering. the liquid film is artificially produced on an airfoil placed in the nozzle and liquid is pumped by a dosing pump on the surface of the airfoil. coarse droplets formed from the liquid films have a negative effect on the reliability and efficiency of the turbines due to the erosion and corrosion by the droplets impact on the leading edges of the blades. keywords: steam turbine, coarse droplets, liquid films. 1. introduction the atomization of liquid is a widely studied problem for spray nozzles and in the field of aerosol research. a similar phenomenon is known in steam turbines but mostly with undesirable effects, coarse droplets have a negative effect on the reliability and efficiency of steam turbines. the study of the formation and evolution of water droplets in turbines at high velocity plays an important role in the continuous effort to enhance energy production and decrease its environmental impact. there are other applications where liquid films are present on a moving body, for example on the trailing edge of wings, propellers and blades and the result of this study could be useful. nevertheless, due to the wide range of applications the project is focused on the conditions in steam turbines. in the long term the authors of the paper are concerned with experimental research into the liquid phase structure in steam turbines [1]. two groups of water droplets are present in steam turbines. the nucleation of the droplets and their subsequent growth through condensation are responsible for the presence of very fine droplets. the expected range of fine droplets is from 50 nm to 800 nm. coarse droplets are formed by the disruption of water films on the blades and casing surfaces in the low-pressure turbine stages. the present knowledge of the coarse water flow field and the size distribution of the droplets in turbines is still insufficient. the range of the diameters of the coarse droplets is very wide, from a few micrometers to one millimeter. the coarse droplets are responsible for some unfavorable effects like additional energy losses and a reduction in the lifetime of the blades (fig. 1). a better understanding of the formation of coarse droplets and their properties may provide future enhancements for the steam turbine and other similar applications. previous measurements of coarse droplets directly in the steam turbine made it necessary to build an experimental tunnel for the study of the formation of the droplets and their properties [2] in the laboratory. the break up of liquid film in steam turbines is also connected with another interesting effect known as “charge separation”. the electrostatic charge in steam turbines is mainly connected with the existence of an electrostatic potential between the stator and the rotor of the turbine. the potential can cause bearings damage due to a short circuit of the current and the subsequent operational failure of the turbines. the first to describe the problem was t. sauer [3], who measured the current between the rotor and the ground for different load and operational states. he expected that the origin of the electrical current arose from the similar phenomena described by lenard [4], who had observed that small droplets in the vicinity of waterfalls have an electric charge. sauer expected that the current was dependent on the turbine load and chemical properties of the steam and he expected that the positive current was connected with the erosion of the blades. the study of this electrostatic phenomena is the second main goal for the new wind tunnel. this note is added for the purpose to mention such the effect which can be connected with the coarse droplets formation and its possible influence on the erosion and corrosion due to the droplets discharge. 2. description of the new tunnel the motivation for building the new wind tunnel arises from the previous measurement [5] of coarse droplets in the steam turbine under operational conditions. the measurement was performed in three power stations in czechia. in all steam turbines the amount 10 http://dx.doi.org/10.14311/app.2018.20.0010 http://ojs.cvut.cz/ojs/index.php/app vol. 20/2018 coarse droplets figure 1. extremely damaged turbine blades. figure 2. new wind tunnel test section. of coarse droplets detected was very low due significantly lower number density in compare with the fine droplets and short exposure time of the light source. a photogrammetric method was implemented in the probe of the diameter 50 mm for the measurement in the steam turbine under operation. the diameter of the captured droplet images was around 60 µm. the diameter of the coarse droplets according to the weber number (eq. 1) was expected smaller [6]. due to difficulties connected with taking measurements in an operating steam turbine, the new wind tunnel was designed and manufactured for the analysis of the formation of coarse droplets from liquid films. the goal of the design was to model the conditions in the steam turbine as closely as possible together while maintaining the advantages of working in the laboratory. the tunnel is designed in the shape of a typical converging-diverging nozzle (fig. 2), but the planned operational regime is subsonic or transonic. an airfoil naca0008 is placed 50 mm behind the nozzle throat. the airfoil simulates a blade in a turbine and it is possible to replace it or even remove it. on the airfoil is a groove for supplying liquid on the surface. the tunnel is equipped with four large windows to provide good optical access for the measurement. liquid is pumped to the airfoil through a dosing pump with a flow from 1 µl/min to 500 µl/min. this range is figure 3. measurement schema of the nozzle. sufficient for the simulation of different liquid films on the airfoil. it is possible to operate the tunnel with steam in continuous flow mode or with compressed air for periodic measurements. the time duration of the measurement in the wind tunnel with air depends on the pressure in the compressor storage vessel and it is for air in range 15 to 25 s. the pressure and temperature of the entire measurement sequence is acquired by the ni scxi measurement system for the description of the flow condition for each measurement. mass flow is determined by the stagnation conditions in the settling chamber and the flow blockage in the nozzle throat. the schema of the optical and flow field measurement is shown in fig. 3. 3. instrumentation and measurement the expected diameter of coarse droplets in steam turbines is in the range between 1 to 500 µm a similar range is expected for the nozzle as well. optical measurement of the size distribution of the droplets is commonly used. nevertheless, the size range of the diameters of the droplets can hardly be measured with one single optical technique. for that reason the new wind tunnel is equipped with two optical methods, photogrammetry and light scattering. with sufficient overlaps, these methods can cover most of the range. photogrammetry is a method for determining the object properties in a photographic image. the theoretical resolution of the method is determined by the pixel pitch of the camera and the resolution of the lens. the size of the pixel pitch usually ranges from 2 to 6 µm or common digital cameras. the magnification of the lens is limited by the minimum exposure time of the camera when a global shutter is used. higher magnification can provide better resolution for smaller droplets, but the displacement of the droplets may then be greater than the size of the observed volume when the picture is taken. the velocity of droplets can reach more than 200 m/s in a turbine and that rate may be even higher in the nozzle. the velocities of the droplets and the minimum exposure time define the size of the observed volume. the entire observed volume must be caught on the camera sensor and this limits the magnification of the lens. this means that each droplet is represented on the picture by a line. 11 ondřej bartoš, lucie měšťanová acta polytechnica ctu proceedings figure 4. droplets formation on the trailing edge of the profile in the wind tunnel. figure 5. the spraytec position on the wind tunnel. figure 4 shows an example of back-side illumination of droplet paths captured in the laboratory. the length of the path and the exposure time allows for computation of the velocity of the droplet and determines the expected direction of the droplet’s movement. but it is necessary to keep in mind that the view of the motion is two-dimensional. this means that components of the velocity can only be viewed perpendicular to the axis of the camera. the depth of the observed volume is determined by the depth of field of the lens. a telecentric lens is more advantageous because the image size is independent of the distance from the lens. calibration is required for the correct pixel/dimension ratio. a light source with the duration of the flash ranging from 5-100µs is used. the second method for measuring the diameter of the droplets is based on light scattering. the analyzer used is an industrial device called spraytec produced by malvern instruments. the results are achieved through the use of laser light scattering. the placement of spraytec in the wind tunnel is shown in fig. 5. 4. data processing after acquiring the first measurements with the photogrammetric method and light scattering, the following knowledge was obtained. an example of the application of the photogrammetry method can be seen in fig. 4. in the picture one can see the break up of the liquid film from the airfoil. two groups of droplets are formed from the airfoil. in the first stage bigger droplets are formed but later these break up into smaller droplets. sometimes bigger droplets remain in the flow. the measurement stagnation condition was 3 bar and 295 k. the smaller droplets are under the detection limit of the photogrammetry method and it wasn’t possible to measure their size distribution function. the result of the analysis of the bigger droplets: mean diameter of the distributions is around 68 µm, sauter mean diameter 181 µm. for the measurement with the spraytec system similar conditions were used: 3 bar, 295 k. the flow of the demineralized water used for the atomization was 5 µl/min. the results of the volume distribution are presented in fig. 6. all 27 measured volume distributions have 2 significant mods with respect of the diameter. the first maximum (small droplets) was for the diameter around 10 µm and bigger droplets had a mod around 600 µm. the velocity of air before the nozzle throat was quasiconstant during the measurement at 250 m/s which corresponds with ma = 0.8 for stagnant conditions and static pressure as shown on fig. 7. 5. results a good knowledge of the flow field in the wind tunnel is necessary for the understanding of the droplets formation. the simultaneous measurement of the flow parameters during the droplets break up on the trailing edge of the aerofoil are logged, the measurement of the stagnation parameters in the settling chamber and static pressure along the nozzle profile. the velocity profile in the test section was measured at the middle of the second optical window at position of 244 mm (distance/throat is 16) down the nozzle throat. the measurement was done with both a pitot probe and a probe for the measurement of the static pressure. the probes moved along the high of the channel at 4 mm increments. the velocity profile is shown on the fig. 8 for three different time positions after the beginning of the expansion. the total time for the measurement with the air takes 15 sec, for this reason we use three time position to see the velocity profile behaviors. the time position was 3, 5 and 8 seconds after opening the valve. the velocity profiles indicate high velocity at the measurement position and no significant flow separation on the walls. for the assessment of the droplets formed by the gas flow atomization a weber number is usually used. it is a dimensionless number which respects the flow inertia and surface tension of the formed droplets. the weber number can be written as, we = ρgw 2 rd σ (1) 12 vol. 20/2018 coarse droplets 0 10 20 30 40 50 1 10 100 1000 vo lu m e  fr ac tio n  [% ] droplet diameter [m‐6] figure 6. an example of the distribution function (volume fraction) acquired by spraytec. the figure shows volume fraction distributions for 27 different measurement. two significant droplet diameter mods are clearly visible for all of the measurements. 0 2 4 6 8 10 12 14 16 0 50 100 150 200 250 300 0 2 4 6 8 10 12 14 w eb er  n um be r [ ‐] pr es su re  [k pa ],  ve lo ci ty  [m /s ] measurement time [s] static pressure velocity total pressure we figure 7. profile of the velocity and pressures during the measurement. where ρg is the vapour density, σ is the surface tension of water, wr is the relative velocity between the vapour and droplets, and d is the diameter of the droplets. in previous works [6, 7] a critical weber number was found which indicates the biggest droplet formed by the gas liquid interaction for a higher mach number of approximately 40. the required data about the diameter of the droplets disagrees with the commonly used estimation with the weber number. in general, the disagreement increases with the velocity. in this work new knowledge about the bi-modal distribution of the formed droplets was used for additional data processing. the data about the size distribution function was divided to two groups and for each of these the sauter diameter was computed. the edge diameter is 60 µm. on the fig. 9 the time dependence of the sauter diameter of both groups acquired from the measurement with spraytec can be observed. in the case of this measurement, the weber number for the first group of smaller droplets is we around 5-10 and for second group of the bigger droplets the mod is around we = 500. 6. conclusions the goal of this paper is to introduce a new facility and method of measurement, therefore only preliminary data from the measurements is available now. systematic study of the phenomena will continue and the new tunnel has proved to be very promising for further research. the explanation and proof of the bi-modal droplet size distribution can provide new insight for the design of turbine blades. 13 ondřej bartoš, lucie měšťanová acta polytechnica ctu proceedings 0 50 100 150 200 250 300 350 0 10 20 30 40 v e lo ci ty [ m /s ] heigth of the nozzle channel [mm] 3 sec 5 sec 8 sec figure 8. profile of the velocity in the nozzle channel. the profile was processed for the three time position after the measurement sequence start. 0 2 4 6 8 10 12 14 16 0 2 4 6 8 10 d 3 2 [m ‐6 ] time [s] figure 9. three arbitrary time lines of the sauter diameter. list of symbols d droplet diameter [m] w velocity [m/s] d32 sauter mean diameter [m] %g gas density [kg m−3] σ surface tension [n/m] ma mach number re reynold’s number we weber number acknowledgements we gratefully acknowledge the support by the grant agency of the czech technical university in prague, grant no. sgs 13/18 and grant no. 16-20123s of the grant agency of the czech republic. references [1] o. bartoš, m. kolovratník. adjustment of the optical probe for the measurement in the steam turbines. in experimental fluid mechanics 2009 – conference proceedings. liberec, czech republic, 2009. [2] o. bartoš, x. cai, m. kolovratník. a detection of the coarse water droplets in steam turbines. in epj web of conferences, vol. 67. 2014. [3] t. sauer. electrostatic charging of rotors. die wärme 59(35):575–576, 1936. [4] p. lenard. ueber die electricität der wasserfälle. annalen der physik 282(8):584–636, 1892. doi:10.1002/andp.18922820805. [5] kolovratník, michal, bartoš, ondřej. ctu optical probes for liquid phase detection in the 1000 mw steam turbine. epj web of conferences 92:02035, 2015. doi:10.1051/epjconf/20159202035. 14 http://dx.doi.org/10.1002/andp.18922820805 http://dx.doi.org/10.1051/epjconf/20159202035 vol. 20/2018 coarse droplets [6] j. valha. proudění mokré páry a její erozivní účinky v průtočné části parní turbíny. in svuss, sborník referátů. 1978. [7] m. j. moore, c. h. sieverding. two-phase steam flow in turbines and separators. mcgraw-hill, 1976. 15 acta polytechnica ctu proceedings 20:10–15, 2018 1 introduction 2 description of the new tunnel 3 instrumentation and measurement 4 data processing 5 results 6 conclusions list of symbols acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2019.25.0064 acta polytechnica ctu proceedings 25:64–67, 2019 © czech technical university in prague, 2019 available online at http://ojs.cvut.cz/ojs/index.php/app inverse computational determination of johnson-cook parameters using the shpb test apparatus anja mauko∗, branko nečemer, zoran ren university of maribor, faculty of mechanical engineering, smetanova ul. 17, 2000 maribor, slovenia ∗ corresponding author: anja.mauko@um.si abstract. the paper describes determination of the material parameters of the johnson-cook constitutive model of steel s235 jr sample material by applying the inverse computational methodology using the digital twin model of the shpb. a quasi-static tensile testing of bulk material was conducted first to determine the base material parameters. this was followed by dynamic impact testing at two different strain rates using the shpb. a digital twin computational model was built next in the ls-dyna explicit finite element system to carry out the necessary computer simulations of the shpb test. the inverse determination of strain hardening material parameter of johnson-cook model was done by using the nelder-mead simplex optimisation by comparing the measured and computed stress to time signals on incident and transmission bars. the obtained johnson-cook material parameters much better describe the sample material behaviour at very high strain-rates in computational simulations, if compared to the parameters derived by the classic, one-dimensional wave propagation hopkinson procedure. keywords: shpb test apparatus, johnson-cook constitutive model, inverse determination, neldermead optimization, computer simulations. 1. introduction many studies have been carried out in the past to understand and thus enable predicting the material response under dynamic impact loading. the johnsoncook (jc) constitutive model is most commonly used to describe the material strain-rate dependency in dynamic computer simulations. this paper describes an inverse computational method used to determine the strain-rate dependent parameters of the jc constitutive model for steel s235 jr [1]. the base quasi-static constitutive parameters were determined by classic quasi-static tensile testing. the strain-rate dependent material behaviour at different deformation rates was determined by using the split hopkinson pressure bar (shpb) test apparatus [2]. the jc model was then validated by digital twin simulations of the shpb. 2. materials and methods 2.1. quasi-static tensile testing the quasi-static tensile tests were carried out on instron 1255 tensile device equipped with the aramis system for true stress true strain determination. the testing samples were produced from steel s235 jr drawn wire with diameter of 6 mm. the testing was conducted at room temperature t = 23 ◦ and cross-head velocity of 0.5 mm · min−1, which corresponded to specific strain-rate ε̇0 = 3.06 · 10−2 s−1. the obtained results in terms of young’s modulus, yield stress and tensile strength are listed in table 1. 2.2. dynamic impact shpb testing the impact tests were performed on the shpb test apparatus, shown in figure 1. the shpb set-up consist of three main parts: the loading device, bar components and data acquisition and recording system [3, 4]. the role of loading device is to accelerate the striker to impact the incident bar, wherein the stress wave is produced. the nitrogen was used as the acceleration gas. the length of the striker is crucial for the amount of the energy produced by the impact. according to preliminary calculations, a 350 mm length of the striker was found to be sufficient.two laser beam sensors, located directly before the impact of the striker and the incident bar were used to measure strikers velocity. the second part of the shpb apparatus consists of the incident and the transmission bar with the sample sandwiched between them. the incident and transmission bar were each 1500 mm long with diameter of 20 mm. both bars and striker were made from the titanium alloy ti6al4v. the stress-strain waves were measured with the symmetrically attached foil strain-gauges on both bars with maximal sampling frequency of 5 mhz. strain-gauges, arranged in wheatstone quartal-bridge, had 2 mm of active length and resistivity 120 ω.the strain gauges signal was amplified by the fe-h79-ta fylde amplifier with gain of 100. the low-pass butterworth filter was used to filter-out the high frequency noise recorded during the experiments. the s235 jr steel samples were 3 mm long and of 6 mm diameter. experimental results at two different strain-rates were required for determination of the jc 64 http://dx.doi.org/10.14311/app.2019.25.0064 http://ojs.cvut.cz/ojs/index.php/app vol. 25/2019 inverse computational determination of the jc parameters using shpb young’s modulus yield stress tensile strength e [mpa] rp0.2 [mpa] σm [mpa] 192 046 635 951 table 1. material parameters obtained at quasi-static experiments. figure 1. shpb test apparatus at the university of maribor. figure 2. comparison between filtered and unfiltered wave signal. parameters. the first experiment was conducted at the striker speed of v1 = 17.05 m · s−1. the second experiment was conducted at the striker speed of v2 = 15.44 m · s−1. figure 5 shows the results of both experiments in terms of stress signals recorded at strain gauges and the achieved strain-rate. 2.3. johnson-cook material model the johnson-cook constitutive model allows to describe the effect of an isotropic kinematic hardening, strain-rate and temperature dependence of the material under the impact loading. it also considers the softening of the material that can occur due to adiabatic heating at higher deformation rates. one of the advantages of the jc model is its versatile use for different materials. the basic equation of the johnson-cook constitutive model determines a current yield stress of the material as [5]: σ = ( a+b · εnp ) · ( 1 + c · ln ( ε̇p ε̇0 )) · ( 1 − ( t − tr tm − tr )m) , (1) where a, b, c, n and m are model parameters, εp is the effective plastic strain, ε̇p ε̇0 is the normalized plastic strain-rate, t is material temperature, tr is the room temperature and tm is the melting temperature. since all the experiments were conducted at room temperature, the equation 1 has been simplified to: σ = ( a+b · εnp ) · ( 1 + c · ln ( ε̇p ε̇0 )) . (2) the same simplification of the jc model has already been applied in articles [6] and [7], on the basis of which the temperature effect was neglected in this research. parameter a represents the quasi-static yield stress at room temperature as a = rp0.2. the isotropic static hardening is captured by the parameters b and n, which were determined on the base of the quasi-static response of the material, using equations given by sobolev and radchenko [5]. to determine the parameter of the dynamic hardening of material c it was necessary to perform dynamic impact tests at two different strain rates and to apply the inverse computational methodology as described in the following chapters. it was previously determined that the application of classic, one-dimensional wave propagation hopkinson procedure returns false dynamic hardening material parameter c, therefore inverse computational determination of parameter c was proposed. the final obtained values of the jc material parameters are listed in table 2. 2.4. digital twin model of the shpb a digital twin model of the used shpb test apparatus was built in the ls-dyna explicit finite element computer simulation system. the dimensions and parameters of the model were identical to the actual values observed on the shpb test apparatus that was used to perform the experiments. 65 a. mauko, b. nečemer, z. ren acta polytechnica ctu proceedings figure 3. the experimental stress signals recorded at strain gauges and the achieved strain-rate obtained at two different striker velocities on shpb test apparatus. top row: strikers velocity v1 = 17.05 m · s−1, from left to right stress to time, stress to strain, strain rate to time diagram. bottom row: strikers velocity v2 = 15.44 m · s−1, from left to right stress to time, stress to strain, strain rate to time diagram. only the basic components of the shpb test apparatus were modelled, i.e the striker, incident and transmission bar and the sample between the bars. due to double symmetry of the problem only a quarter of the overall model was used in simulations to reduce the simulation time, figure 4. the quarter model was validated by comparing its results with the results of the the full computational model of shpb test apparatus. a linear-elastic isotropic constitutive material model was used for the striker, incident and transmission bar. the johnson-cook constitutive material model was used to define the material properties of a specimen to incorporate the dynamic response of the material, with the model parameters a, b, n and c, table 2. the following contact conditions were defined: striker-incident bar, incident bar-sample, sample-transmission bar. all described contact were automatic surface to surface contacts. a dynamic friction coefficient of 0.1 was considered in the contacts between the incident bar-sample and sampletransmission bar, as it is proposed for the steel to titanium greased contact. at the striker-incident bar contact, a dynamic friction coefficient of 0 was defined. linear hexagonal finite elements of type c3d8r were used to spatially discretise the bars and the sample. the size of the finite elements was determined with the convergency analysis. the final mesh contained 12.206 finite elements. the last step in creating a digital twin model was determination of boundary conditions. the shpb model was embedded in such a way that only the movement in axial direction was enabled (z-direction, shown in figure 4). the initial striker speed was equal to the speed of the striker observed in the experimental figure 4. digital twin model of the shpb test apparatus. testing on the shpb test apparatus. 2.5. inverse computational determination of parameter c dynamical hardening parameter c was determined from the experimental results recorded at two different striker velocities by applying the inverse computational method using the optimization software optimax, featuring the nelder-mead simplex method [8]. the value of parameter c was optimised to achieve the best fit of the digital twin computational results of stress signals with the experimental results. the best fit was at relative mean error (rme) of 10 %, determined using equation 3, were σ1,i represents longitudinal experimental stress and σ2,i longitudinal numerical stress. the final optimised value of parameter c is listed in table 2. rme = √ 1 n ∑n i=1 (σ1,i − σ2,i)2 100 (3) 3. results the final optimized value of parameter c was 0.31 [-]. the comparison of digital twin and experimental stress to time signals using the final jc material 66 vol. 25/2019 inverse computational determination of the jc parameters using shpb figure 5. comparison of experimental and numerical results. on the left figure is presented comparison between experimental and numerical results for strikers velocity v1 = 17.05 m · s−1. on the left figure is presented comparison between experimental and numerical results for strikers velocity v2 = 15.44 m · s−1. parameters shows deviation of the reflected waves at 10.65 % and for the transmitted waves at 27.89 % at the striker velocity of v1 = 17.05 m · s−1, figure 5. at the striker velocity v2 = 15.44 m · s−1 the deviation between reflected waves was 12.75 % and for the transmitted waves 20.08 %. the average deviation was 17 %, which was acceptable for the purpose of the first study. to improve the final results, the future work will be focused on the strain-rate dependency of parameter c. jc material parameters s235 jr steel a, [mpa] 635 b, [mpa] 314 n, [-] 0.75634 c, [mpa] 0.31 table 2. final values of jc material parameters. 4. conclusion the paper describes the procedure for determining the material parameters of the johnson-cook constitutive model of steel s235 jr sample material by applying the inverse computational methodology using the digital twin model of the split hopkinson pressure bar test apparatus. a classic quasi-static tensile testing of sample steel material was conducted first to determine the base material parameters, i.e. the young’s modulus, the yield stress and the tensile strength. this was followed by dynamic impact testing at two different strain-rates using the split hopkinson pressure bar test apparatus. a digital twin computational model was built next in the ls-dyna explicit finite element system to carry out the necessary computer simulations of the shpb test. the inverse determination of strain hardening material parameter of johnson-cook constitutive model was done by using the nelder-mead simplex optimisation method by comparing the measured and computed stress to time signals on incident and transmission bars. the obtained johnson-cook material parameters much better describe the sample material behaviour at very high strain rates in computational simulations, if compared to the parameters derived by the classic, one-dimensional wave propagation hopkinson procedure. however, a substantial deviation of results between experiments and digital twin was observed which necessitates further, more detailed research. acknowledgements the authors acknowledge the financial support from the slovenian research agency (research core funding no. p20063). references [1] a. mauko. inverzna numerična določitev parametrov johnson-cook konstitutivnega modela jekla z uporabo shpb preizkuševališča 2018. https: //dk.um.si/dokument.php?id=129259&lang=eng. [2] w. chen, b. song. split hopkinson (kolsky) bar design, testing and application, vol. 39. 2008. [3] l. adanič, b. nečemer, m. frankovič. konstruiranje, gradnja in umerjanje shpb preizkuševališča 2015. https: //dk.um.si/dokument.php?id=80745&lang=eng. [4] b. nečemer. numerično modeliranje in validacija shpb-preizkuševališča 2017. https: //dk.um.si/dokument.php?id=116571&lang=eng. [5] a. sobolev, m. radchenko. use of johnson-cook plasticity model for numerical simulations of the snf shipping cask drop tests. nuclear energy and technology 2(4):272–276, 2016. doi:10.1016/j.nucet.2016.11.014. [6] d. n. zhang, q. q. shangguan, c. j. xie, f. liu. a modified johnson-cook model of dynamic tensile behaviors for 7075-t6 aluminum alloy. journal of alloys and compounds 2015. doi:10.1016/j.jallcom.2014.09.002. [7] z. xie, y. guan, j. lin, et al. constitutive model of 6063 aluminum alloy under the ultrasonic vibration upsetting based on johnson-cook model. ultrasonics 96:1–9, 2019. doi:10.1016/j.ultras.2019.03.017. [8] m. borovinšek. optimax user’s manual v0.6.3, 2017. http://lace.fs.uni-mb.si/wordpress/ borovinsek/wp-content/uploads/sites/6/2017/01/ optimax-users-manual-v0.6.3.pdf. 67 https://dk.um.si/dokument.php?id=129259&lang=eng https://dk.um.si/dokument.php?id=129259&lang=eng https://dk.um.si/dokument.php?id=80745&lang=eng https://dk.um.si/dokument.php?id=80745&lang=eng https://dk.um.si/dokument.php?id=116571&lang=eng https://dk.um.si/dokument.php?id=116571&lang=eng https://doi.org/10.1016/j.nucet.2016.11.014 https://doi.org/10.1016/j.jallcom.2014.09.002 https://doi.org/10.1016/j.ultras.2019.03.017 http://lace.fs.uni-mb.si/wordpress/borovinsek/wp-content/uploads/sites/6/2017/01/optimax-users-manual-v0.6.3.pdf http://lace.fs.uni-mb.si/wordpress/borovinsek/wp-content/uploads/sites/6/2017/01/optimax-users-manual-v0.6.3.pdf http://lace.fs.uni-mb.si/wordpress/borovinsek/wp-content/uploads/sites/6/2017/01/optimax-users-manual-v0.6.3.pdf acta polytechnica ctu proceedings 25:64–67, 2019 1 introduction 2 materials and methods 2.1 quasi-static tensile testing 2.2 dynamic impact shpb testing 2.3 johnson-cook material model 2.4 digital twin model of the shpb 2.5 inverse computational determination of parameter c 3 results 4 conclusion acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2018.15.0120 acta polytechnica ctu proceedings 15:120–125, 2018 © czech technical university in prague, 2018 available online at http://ojs.cvut.cz/ojs/index.php/app validation of linked simulation for fire-exposed concrete structures stanislav šulca,∗, vít šmilauera, františek waldb a department of mechanics, faculty of civil engineering, ctu in prague, thakurova 7, 166 29 prague 6, czech republic b department of steel and timber structures, faculty of civil engineering, ctu in prague, thakurova 7, 166 29 prague 6, czech republic ∗ corresponding author: stanislav.sulc@fsv.cvut.cz abstract. this article presents linked computational approach for fire simulation and its effects on structure using adiabatic surface temperature. the simulation solves a weakly-linked problem, consisting of computational fluid dynamics (cfd), heat transport and mechanical model. the temperature field from the cfd creates cauchy and radiative boundary conditions for the thermal model. the temperature field from an element is passed further to the mechanical model, which induces thermal strain and modifies material parameters. this article also brings a validation of the linked simulation, based on experiment with a concrete block exposed to fire in a furnace. the material model uses standard material properties given in eurocode 2 en 1992-1-2. keywords: thermo-mechanical model, virtual furnace, computational fluid dynamics, multiphysics, fire resistance. 1. introduction structural fire assessment creates essential part in design and maintenance of steel, concrete or timber structures. expensive empirical tests carried out in furnaces with gas burners present traditional approach. this article describes a software tool for prediction of the thermo-mechanical response of a structure exposed to fire. it extends our previously published concept [1]. we created a python library, which interconnects codes solving the particular physical phenomena (cfd, thermal and mechanical task). the issue of cfd is solved with the fire dynamics simulator software (fds) [2], developed at nist. we created a standardized api for governing the computation and exporting the data, which brought some changes in the structure of the code. this computation provides the temperature fields for the following thermal analysis. our fds computations have already been validated against experiment by cábová et al. [3]. the thermo-mechanical analysis of the element takes place in oofem [4] software. the thermal analysis uses adiabatic surface temperature (ast) field from fds, which defines both cauchy and radiative boundary conditions on the element’s surfaces. ast is further explained in sec. 1.1. then, the solution of the thermal analysis passes resulting temperature field to the mechanical analysis, which induces thermal strain and modifies the material parameters as yield strength and elastic modulus. interfacing fds and oofem is realized using mupif [5] python library, which provides useful tools such as exporting data to vtu format, parallel computations and advanced handling with data fields and meshes. both the fds fortran and oofem c++ codes were compiled as shared libraries and imported into the python code. the top-level steering script controls execution of both codes, data synchronization etc. fig. 1 shows the data flow using the moda diagram [6]. 1.1. adiabatic surface temperature convection and radiation present dominant boundary conditions for a structure exposed to fire. this could be facilitated by ast, a concept brought by wickström [7]. the total heat flux to the surface of an element reads qtot = qcon + qrad (1) convection and radiation terms are further expressed as qcon = h(tg,cfd − ttm) (2) qrad = ε(rinc,cfd − σt 4 tm) (3) where h is the heat transfer coefficient, σ is the stefanboltzmann constant, ε is the emissivity of the exposed surface, ttm is the temperature of a surface in the thermal model in oofem, tg,cfd is the gas temperature in the cfd model in fds and rinc,cfd is the so called incidental radiation. it is the sum of all radiative sources in fds rinc,cfd = ∑ εifiσt 4 inc,i (4) where tinc,i stands for the temperature of each flame or hot surface, εi is its emissivity and fi is the view factor between the source and the oriented surface 120 http://dx.doi.org/10.14311/app.2018.15.0120 http://ojs.cvut.cz/ojs/index.php/app vol. 15/2018 validation of linked simulation for fire-exposed concrete structures figure 1. data workflow diagram. of the structure. fds can automatically obtain the value of rinc,cfd. fds software computes ast (tast). it corresponds to the temperature of a perfect insulator’s surface with zero conduction, see fig. 2. the heat flux to this surface is set to zero by the following equation 0 = ε ( rinc,cfd − σt 4 ast ) + h (tg,cfd − tast) (5) figure 2. plate thermometer for ast measurement. the total heat flux from the fire model to the thermal model reads as qtot = ε ( rinc,cfd − σt 4 tm ) + h (tg,cfd − ttm) (6) and subtracting eqs. 5 and 6 leads to qtot = εσ ( t 4 ast − t 4 t m ) + h (tast − tt m ) (7) according to eq. 7, passing a single field tast suffices to link cfd with the thermo-mechanical task. 2. validation experiment with a concrete block we present a validation based on a concrete block (0.35 m×0.15×0.2 m) exposed to fire in a horizontal furnace, placed below the its ceiling. the experiment also involved concrete column elements, which were a part of another research project. concrete class c30/37 with cement’s amount of 370 kg/m3 constituted the block. in order to avoid spalling of the surface, the concrete contained 1.5 kg/m3 of pp fibers. indeed, no spalling had occurred as evident from fig. 3. figure 3. concrete block after the fire exposure. the furnace dimensions are: length=4.00 m, width=2.95 m and height=2.57 m. see the horizontal and vertical sections in figs. 5 and 6. the experiment was realized in the fire laboratory of pavus a.s. the furnace was heated by 8 natural gas burners. flue gas exhaust system was performed using a frequency fan 121 s. šulc, v. šmilauer, f. wald acta polytechnica ctu proceedings placed in a conduit which was connected to the opening (500 mm x 800 mm) in the floor of the furnace. the temperature in the furnace followed the standard temperature curve [8] t (t) = 345 · log ( 2 15 t+ 1 ) + 20, (8) where t is in seconds and t in degree celsius. the curve is displayed in fig. 4 with bounds ±100◦c according to recommendation [9]. heat power of gas figure 4. standard temperature curve with tolerances for fire resistance testing. burners followed the curve from fig. 7 with the maximum output of 258 kw. fds used this curve for explicit control of the burners. the experiment lasted 120 minutes and the initial temperature of the whole furnace was 10◦c. figure 5. horizontal section of the furnace. 2.1. simulation the fds task used global mesh consisting of 36 × 36×24 elements, see fig. 9. the fds simulation took 72 hours on 4 cpu cores. regular mesh in oofem contained 14 × 6 × 10 quadratic brick elements, see fig. 10. the adiabatic surface temperature was exported from points around the beam from fds in periods of 2 s, which is the computational step length in oofem. we computed the thermal response for the validation purpose and the mechanical response to show the prediction of the mechanical behavior. figure 6. vertical section of the furnace. figure 7. total power of gas burners. figure 8. cross-section of the concrete block with positions of the thermal sensors. we used standard thermal material properties from eurocode 2. fig. 11 presents thermal conductivity and fig. 12 presents thermal capacity of concrete. the mechanical material properties are: young’s modulus e=30 gpa, density d=2400 kgm−3, ν=0.25, α = 12 × 10−6 k−1, tensile strength is 2.1 mpa and fracture energy is 250 jm−2. the emissivity of concrete surface was set to 0.85 and the heat transfer coefficient was set to zero. it has almost no impact on the the results, see more in sec. 2.3. 2.2. results visualization of temperature field provides fig. 13 at the time of 7200 s. the mechanical task computes corresponding stresses; an example of σxx field from the isotropic damage model is presented in fig. 14. corresponding scalar damage variable field in shown 122 vol. 15/2018 validation of linked simulation for fire-exposed concrete structures figure 9. meshing in the fds model, horizontal and vertical cut through the block. figure 10. oofem mesh with ast points. figure 11. thermal conductivity of concrete. in fig. 15, while fig. 16 presents crack width with the maximum value of 4.8 × 10−4 m. unfortunately, the crack width remained without validation. figure 12. thermal capacity of concrete. figure 13. temperature at time=7200 s. figure 14. stress σx at time=7200 s. the temperatures inside the concrete block were measured in a section, which goes through the center of the block. positions of the sensors are displayed in fig. 8. the results in figs. 17, 18, 19 and 20 present very satisfying match of the experimental data with the computed temperatures. we have also analyzed the influence of the emissivity and heat transfer coefficient on the results, which is negligible. for this task, the material parameters are significantly dominant. 123 s. šulc, v. šmilauer, f. wald acta polytechnica ctu proceedings figure 15. damage scalar parameter at time=7200 s. figure 16. crack width at time=7200 s. figure 17. temperature in #1. figure 18. temperature in #2. figure 19. temperature in #3. figure 20. temperature in #4. 2.3. sensitivity to heat transfer coefficient and emissivity we have checked the sensitivity of the results to setting of the heat transfer coefficient end the emissivity coefficient. fig. 21 displays the dependency on emissivity and fig. 22 displays the dependency on heat transfer coefficient. both these graphs show that this figure 21. temperatures in #1 for several values of the emissivity coefficient ε with no convection. figure 22. temperatures in #1 for several values of the heat transfer coefficient h with ε = 0.95. 124 vol. 15/2018 validation of linked simulation for fire-exposed concrete structures setting of the simulation makes almost no difference in the results and justifies the values we used for the validation. 3. conclusions this article presents new software tool and its general idea, which is interconnecting of existing softwares and using them to solve complex multi-physical problems. the softwares fds and oofem were glued together with a python library based on python tool mupif. the validation based on experiment with a concrete block exposed to fire in a furnace confirms relevancy and good accuracy of the developed software tool’s computations. acknowledgements we gratefully acknowledge the financial support from czech technical university in prague (grant sgs18/037/ohk1/1t/11) and czech science foundation (grant 16-18448s “coupled model of structural standard fire test”). references [1] s. šulc, v. šmilauer, f. wald. presentation and paper "coupled simulation for fire-exposed structures using cfd and thermo-mechanical models" at nmm2017, prague, czech republic, 2017. [2] k. mcgrattan, s. hostikka, r. mcdermott, et al. fire dynamics simulator user’s guide. nist special publication 1019, sixth edition, national institute of standards and technology, 2017. doi:10.6028/nist.sp.1019. [3] k. cábová, n. lišková, p. novotná, et al. presentation and paper "modelling of standard fire test" at engineering mechanics 2017, 15th 18th of may 2017, svratka, czech republic, 2017. [4] b. patzák. oofem an object-oriented simulation tool for advanced modeling of materials and structures. acta polytechnica 52(6):59–66, 2012. [5] b. patzák, d. rypl, j. kruis. mupif a distributed multi-physics integration tool. advances in engineering software 60-61:89 – 97, 2013. doi:10.1016/j.advengsoft.2012.09.005. [6] the european materials modelling council moda, homepage. url: https://emmc.info/moda/. [7] u. wickström, d. duthinh, k. mcgrattan. adiabatic surface temperature for calculating heat transfer to fire exposed structures. in proceedings of the eleventh international interflam conferrence. inter science communications limited, london, 2007. [8] en 1991-1-2: eurocode 1: actions on structures part 1-2: general actions actions on structures exposed to fire, 2002. [9] en 1363-1 fire resistance tests part 1: general requirements, 2012. 125 http://dx.doi.org/10.6028/nist.sp.1019 http://dx.doi.org/10.1016/j.advengsoft.2012.09.005 acta polytechnica ctu proceedings 15:120–125, 2018 1 introduction 1.1 adiabatic surface temperature 2 validation experiment with a concrete block 2.1 simulation 2.2 results 2.3 sensitivity to heat transfer coefficient and emissivity 3 conclusions acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2018.19.0001 acta polytechnica ctu proceedings 19:1–6, 2018 © czech technical university in prague, 2018 available online at http://ojs.cvut.cz/ojs/index.php/app mixed core subchannel model for subcal and coupling with andrea vojtěch cahaa,b,∗, jiří čížekb a department of nuclear reactors, fnspe, ctu in prague, v holešovičkách 2, prague, czech republic b chemcomex, a.s., elišky přemyslovny 379, prague, czech republic ∗ corresponding author: vojtech.caha@fjfi.cvut.cz abstract. this paper presents the results of an analysis of flow distribution in vver-1000 mixed core consisting of fuel assemblies with non-identical spacing grids. the calculation was carried out using the modified subchannel code subcal-az which allows to calculate 3d thermal-hydraulic characteristics of the coolant flow in the full core subchannel model coupled with the neutron-physical code andrea. this full core subchannel model was created in three variants depending on the andrea calculations. the first variant (homogeneous core) consisted of 163 hydraulically identical fuel assemblies tvsa-t mod.2, whereas the other variants (mixed cores) consisted of fuel assemblies tvsa-t mod.0, mod.1 and mod.2. these fuel assemblies mainly differ in types, number and axial coordinate of spacing grids and also in diameter of guide tubes. the influence of mixed core to flow distribution was obtained by comparing these variants. keywords: subchannel analysis, subcal, mixed core, code coupling, vver-1000. 1. introduction when a reactor core consists of more than one type of fuel assembly, the flow redistributions due to differences in fuel assembly pressure drops may be introduced. the crossflow, as well as the axial flow, results in flow-induced coolant temperature changes and thus may influence a heat generation in a fuel rod. the mixed core can also cause that a flow penalty may be applied with respect to the homogeneous reference core in detail stand-alone fuel assembly subchannel calculation where lateral flow and turbulent mixing between fuel assemblies is not considered. an important aspect of core performance evaluation is the ability to predict interchange transfer of mass and energy. two mechanisms are identified: crossflow and thermal mixing. crossflow involves net transfer of the coolant (both mass and energy) between assemblies or subchannels. thermal mixing is energy exchange due to a radial enthalpy gradient with no associated net mass transfer. modified subchannel thermal-hydraulic code such as subcal-az [1], [2] can be used to account for the effects of crossflow in fuel assemblies and reactor cores due to hydraulic mismatch or power gradient, and perform the more accurate coolant temperature or density distribution. more accurate results can be used as input for neutron-physical codes to improve their results or licensing applications for example. the thermal-hydraulic subchannel code subcal is developed in chemcomex and used in the nuclear industry in czech republic for vver core design, licensing applications and safety assessments of fuel cycles [3]. subcal was developed based on the cobra code series. similar to other subchannel codes currently used for pwr and vver analyses, subcal predicts the three-dimensional velocity, pressure, and thermal energy fields and fuel rod temperatures for singleand two-phase flow in pwr or vver. subcal solves the finite difference equations for mass, energy, axial and lateral momentum conservation for an interconnected array of channels, assuming incompressible and thermally expandable homogeneous coolant flow. although the formulation is homogeneous, empirical models are incorporated into the code to account for subcooled boiling and vapor/liquid slip in two-phase flow. the subcal code was validated and standardized by sújb (nuclear regulatory body of the czech republic) in 2018 for tvsa-t mod.0/1/2 and lta fuel (westinghouse vver-1000 fuel). the andrea macro code [4] is a program developed in újv řež since 2005. it serves for calculations of reload design and safety analyses of vver reactors. the program is also standardized by sújb for analyses of reactors vver-1000. the program solves a two-group diffusion equation with help of contemporary nodal method which ensures sufficient accuracy and speed of calculations. it enables to work with an arbitrary axial nodalization, the calculations can be carried out in one sixth of a reactor core or in a full core manner. 2. core subchannel model the rough subchannel code subcal-az for the entire vver-1000 core allows the modeling of all 163 assemblies following the neutron-physical calculation by the andrea. the subchannel code subcal is used to calculate local coolant parameters in individual assemblies and axial nodes. single and also mixed 1 http://dx.doi.org/10.14311/app.2018.19.0001 http://ojs.cvut.cz/ojs/index.php/app vojtěch caha, jiří čížek acta polytechnica ctu proceedings core fuel cycles can be evaluated. the paper describes a mixed core model composed of tvsa-t mod.0, mod.1 and mod.2 fuel assemblies. following the offline coupling of the subcal-az and andrea codes, three cycles calculations were performed by iterative approach. after two to three iterations, the power and temperature distributions have stabilized. the full core subchannel model was created by connecting 163 separate tvsa-t fuel assemblies where the subchannels of these fuel assemblies were merged together and hence the new types of subchannels were created, i.e. one subchannel represents one fuel assembly. the schematic drawing of the model is shown in figure 1. the main characteristics of the full core model are listed in table 1. the axial length and nodalization were taken from previous neutronphysical calculation of power distribution by the code andrea [5]. a non-uniform axial nodalization into 204 nodes was used at total length of 3730 mm, thus each node was within range 10.8 to 19.2 mm. for the purposes of assessing the influence of the mixed core on the coolant flow and temperature distribution, three variants of the core load were created. the first variant consisted of 163 hydraulically identical fuel assemblies tvsa-t mod.2 (homogeneous core), while the other variants (mixed core) were composed of 48 and 97 fuel assemblies tvsa-t mod.2 and 115 and 66 fuel assemblies tvsa-t mod.0/1 (figure 2 and figure 3). the fuel assemblies tvsa-t mod.1 and mod.2 differ, for the purpose of subchannel code calculation, in particular in the number and types of spacing grids and the diameter of the guide tubes. there are also other differences but these do not affect subchannnel calculation. all types of spacing grids are shown in table 2 and their coefficients of resistance can be found in [2]. different types of spacing grids for tvsa-t mod.1 and mod.2 fuel assemblies are associated with different turbulent mixing coefficients values. for mod.2 fuel assemblies, the value of the turbulent mixing coefficient β1 is given along the entire axial length, while mod.2 fuel assembly is divided into two axial parts. a value of β2 is taken into account up to the first mixing grid (type 1 – approx. 2500 mm), whereas the second part has a turbulent mixing coefficient equal to β3 > β2. all turbulent mixing coefficients were carefully set up on the basis of mixing experiments for stand-alone fuel assemblies [6]. at the interface between mod.1 and mod.2 fuel assemblies, the mean value is considered. the axial geometry of spacing grids for both types of fuel assemblies is shown in figure 4. 3. subcal-az and coupling with andrea subcal-az is a computational code consisting of the subchannel code subcal and python structure designed to automatically process data from the neutronphysical code andrea output files and transform them into input files of the subcal code. thus, parameter number [-] subchannels 163 boundaries 444 rods 163 axial nodes 204 table 1. core subchannel model. type tvsa-t t1 mod.2 t2 mod.2 t3 mod.2 t4 mod.1 t5 mod.1 t4 mod.0 t5 mod.0 table 2. types of spacing grids. described computational complex subcal-az is the thermal-hydraulic part of the off-line coupling of the subcal and andrea codes. all necessary parameters such as axial nodalization, core map (position of fuel assemblies in the core), power distribution or regime parameters are assumed from andrea output files. in comparison with the neutron-physical code andrea the subchannel calculation requires finer axial nodalization due to rapid changes of flow behind spacer grids, hence it was considered four times finer axial nodalization (andrea – 51 axial nodes, subcal – 204 axial nodes). the power distribution is loaded from the andrea output files as node pin power and merged to node assembly power and then transformed to subcal numbering and saved to input files. another regime parameters, which are also taken over, are system pressure, inlet temperature or inlet flow. after subcal calculation, in addition to standard output files, a subcal-andrea file containing matrices of selected parameters (linear power, mass flow, coolant temperature and coolant density) prepared for input to the andrea computation is created. 4. results there were evaluated three variants (cycles) with different number of tvsa-t mod.2 fuel assemblies to obtain flow distribution in the core. the most important parameter which was observed is the average mass flow [kg/s] in fuel assemblies with respect to the type of fuel assembly. this parameter could be later used for more accurate subchannel calculations of stand-alone fuel assemblies in mixed core as inlet flow. figures 5, 6 and 7 show axial dependence of 2 vol. 19/2018 mixed core subchannel model for subcal and coupling figure 1. core subchannel model with subchannel (fuel assemblies) numbering. figure 2. mixed core u2c16. [5] mass flow in several fuel assemblies and axial position of spacer grids and its resistance coefficients. the fuel assemblies (subchannels) were chosen as a representative set for given fuel cycle the sets differ because of different fuel assemblies in the core positions, see figures 1, 2 and 3. the flow rate in homogeneous core (figure 5) depends mostly on power of fuel assembly and its position in the core, the average flow rate is for all fuel assemblies almost the same. axial changes of flow rate are gradual and in narrow range. whereas, the average flow rate in mixed cores strongly depends also on the type of fuel assembly, figure 3. mixed core u2c17. [5] their resistance coefficient and flow area. axial dependence of the flow rate is also influenced by position and type of spacer grids in fuel assembly itself and neighboring fuel assemblies. we can observe rapid changes of flow rate immediately behind spacer grids induced by lateral flow. the decrease in average flow rate is the most significant in fuel assemblies with higher resistance coefficient surrounded by more fuel assemblies of other type. this decrease could reach up to 4 % in calculated fuel cycles. the manufacturer of the fuel assemblies (tvel) provides a simple formula (equation 1) to estimate 3 vojtěch caha, jiří čížek acta polytechnica ctu proceedings figure 4. axial geometry of tvsa-t mod.1/2. the change of flow rate in mixed cores. it depends only on number of two types of fuel assemblies and their resistance coefficients and flow areas. subchannel calculations found a good agreement of average flow rate in given type of fuel assembly and flow reduction from equation 1, [2]. gmod.1 = nmod.1 +nmod.2 nmod.1 +nmod.2 smod.2 smod.1 √ ξmod.1 ξmod.2 (1) where: gmod.1 = relative flow rate in tvsa-t mod.1 nmod.1 = number of tvsa-t mod.1 in the core nmod.2 = number of tvsa-t mod.2 in the core smod.1 = flow area of tvsa-t mod.1 smod.2 = flow area of tvsa-t mod.2 ξmod.1 = resistance coefficient of tvsa-t mod.1 ξmod.2 = resistance coefficient of tvsa-t mod.2 iteration error [%] time [ddd:hh:mm:ss] 1-2 2-3 3-4 000:00:00:00 1.045 0.068 0.005 002:00:00:01 1.035 0.069 0.006 005:00:00:02 0.964 0.069 0.006 010:00:00:03 0.866 0.056 0.006 030:00:00:04 0.863 0.053 0.004 050:00:00:05 0.844 0.067 0.003 070:00:00:06 0.777 0.061 0.004 090:00:00:07 0.710 0.052 0.003 110:00:00:08 0.689 0.045 0.002 130:00:00:09 0.704 0.043 0.002 150:00:00:10 0.707 0.044 0.002 170:00:00:11 0.711 0.044 0.002 190:00:00:12 0.576 0.043 0.002 210:00:00:13 0.502 0.026 0.002 230:00:00:14 0.373 0.018 0.002 250:00:00:15 0.333 0.042 0.002 270:00:00:16 0.304 0.037 0.007 290:00:00:17 0.297 0.034 0.005 310:00:00:18 0.284 0.034 0.004 317:00:00:19 0.310 0.034 0.004 347:00:00:20 0.000 0.000 0.000 table 3. linear power assembly node [w/cm] iteration error u2c17. the second goal of this work was to provide an off-line coupling of the subchannel code subcal and the neutron-physical code andrea and replace simple model of isolated channels for evaluation of moderator/coolant temperature or density. the most significant changes were observed in power distribution and an example of iteration error is shown in table 3 for several states during one year fuel cycle. all calculations showed that three or four iterations are needed to achieve final results. 5. conclusions whereas the andrea code contains a th module that calculates coolant heating only for fuel assemblies by isolated channel method, where pressure or turbulent lateral flow between subchannels (fuel assemblies) is not considered, the calculations were performed with the subcal-az code, which involves both pressure and lateral flow. the off-line coupling of subcal and andrea allows to evaluate the methodical error of the isolated channel method. there were performed three different core loads – mixed and homogeneous cores. the andrea code calculated the nodal pin power distribution, while the subchannel code subcal provided local flow, temperature and density of coolant in each subchannel and axial node. the temperature and density distribution from the subcal was again used in the andrea to calculate power distribu4 vol. 19/2018 mixed core subchannel model for subcal and coupling figure 5. mass flow in homogeneous core 163 tvsa-t mod.2. figure 6. mass flow in u2c16 48 tvsa-t mod.2. 5 vojtěch caha, jiří čížek acta polytechnica ctu proceedings figure 7. mass flow in u2c16 97 tvsa-t mod.2. tion. after three or four iterations, the power and temperature distribution have stabilized. in conclusion, th calculations using the andrea isolated channel method do not affect the power and temperature distribution for homogeneous cores. on the contrary, the mixed core power distribution and axial offset are significantly influenced by used method. [7] the error of linear power node calculation caused by neglecting of lateral flow and turbulent mixing (isolated channel method) could reach more than 1 % in mixed cores. the subchannel calcultaion of flow rate in fuel assemblies is important part of new type fuel assemblies licensing loaded in mixed cores and safety assesment of fuel cycles. references [1] j. čížek, v. caha. subcal-ete dokumentace programu, rev. 1, 108064.00.093.857. chemcomex, 2018. [2] j. čížek. stanovení průtoku v palivových souborech směsné az složené z tvsa-t mod.0/1 a lta, rev. 0, újv-orf-2018-005. újv řež, 2018. [3] j. čížek, v. caha, j. krejčí. 01-th metodika bezpečnostního hodnocení palivových vsázek, rev. 4, 108368.171437. chemcomex, 2017. [4] j. hejzlar, r. vočka. kvalifikace výpočetního programu andrea v2.2, rev. 0, ujv-z-4484. újv řež, 2016. [5] j. hejzlar. andrea mixed core calculation: cycles u2c16 and u2c17. újv řež, 2018. [6] v. caha, j. čížek. subcal-ete koeficient turbulentního míšení: tvsa-t mod.2, rev. 0, 00.432.179. chemcomex, 2016. [7] j. hejzlar. propojení programů andrea a subcal, rev. 0, újv-orf-2018-017. újv řež, 2018. 6 acta polytechnica ctu proceedings 19:2–7, 2018 1 introduction 2 core subchannel model 3 subcal-az and coupling with andrea 4 results 5 conclusions references acta polytechnica ctu proceedings doi:10.14311/app.2018.19.0007 acta polytechnica ctu proceedings 19:7–13, 2018 © czech technical university in prague, 2018 available online at http://ojs.cvut.cz/ojs/index.php/app scale and serpent two-group cross-section data generation ondřej nováka,∗, jan frýborta, lubomír sklenkaa, jan rataja, ondřej chválab a czech technical university in prague, faculty of nuclear sciences and physical engineering, department of nuclear reactors, prague, czech republic b university of tennessee at knoxville, department of nuclear engineering, tennessee, usa ∗ corresponding author: ondrej.novak2@fjfi.cvut.cz abstract. in this work, vver fuel assembly is calculated using two different codes serpent and scale. multiplication factor and several two-group macroscopic parameters including scattering cross-section, fission cross-section, total cross-section and diffusion coefficient were studied. in addition to this, fuel isotopic composition dependency on burnup was calculated as a part of this study. obtained data are compared and differences are discussed. both codes provides similar results for all studied parameters. keywords: vver, serpent, vvantage-6, fuel cycle, burnup, scale, temelin nuclear power plant, depletion calculation, homogenized cross-sections. 1. introduction the crucial part of nuclear safety is based on results obtained by calculation. to ensure accurate results, codes are compared with experimental data (benchmarking) and also among each other [8], [6]. in this work, vver fuel assembly depletion is calculated using two different codes monte-carlo code serpent and deterministic sequence triton from scale package. multiplication factor and several two-group macroscopic parameters including scattering cross-section, fission cross-section, total crosssection and diffusion coefficient are calculated and compared. in addition to that, fuel isotopic composition dependency on burnup were calculated. all values are calculated during the fuel depletion in order to cover the whole fuel cycle, which means burnup up to 60.5 mwd/kgu. in addition, several serpent memory optimization settings and their impact on macroscopic data generation were tested. the triton calculation sequence from the scale package [5] represents the traditional approach to fuel depletion calculations and few-group data generation. it is based on deterministic code newt producing data for the well–established origen depletion code. serpent code [9] was selected due to the fact that the code is designed directly for reactor core calculations combining monte-carlo based transport calculations and advanced cram fuel depletion method [10]. serpent and scale codes allow homogenized fewgroup cross-sections and other nuclear constants describing fuel assembly behavior to be produced. these results can be later used in full-core calculations. both codes use different approach to obtain homogenized cross-section and other nuclear data. therefore, comparison provides valuable information about the methods used. scale code has a longer history in this field of application compared to serpent code, thus scale code was selected as the reference code for this study. firstly, the model and the calculation itself is described, then results are presented and discussed. 2. fuel assembly description for this particular study, fuel assembly model was created based on vvantage-6 fuel design [2] (see figure 1). vvantage-6 fuel was developed by westinghouse company for vver-1000 reactors. therefore, it features hexagonal fuel lattice. this fuel type was used in the temelin nuclear power plant reactor for the first 10 years of operation. the fuel assembly without ifba absorber was selected for this study. assembly parameters are listed in the table 1. all fuel pins are identical, with flat 3% enrichment. temperatures used in this study are listed in the table 2. 3. calculation setup two dimensional models of the fuel assembly in serpent code (version 2.1.27) and scale code (version 6.2.1) were created based on the vvantage-6 design specified in table 1. specific power 35.54 w/ghm was used in both calculations. b1-leakage corrected critical spectrum was applied in both calculations as well. in serpent calculation, b1 correction used 238 group energy structure (scale238 [5]). fast group (group 1) was above 0.625 ev and thermal group (group 2) below that for both codes. serpent calculation were conducted with 10000 neutrons per cycle, 400 active cycles and 50 inactive cycles. input cross-section data for serpent calculations were obtained from endf/b-vii.1 library [7]. 7 http://dx.doi.org/10.14311/app.2018.19.0007 http://ojs.cvut.cz/ojs/index.php/app o. novák, j. frýbort, l. sklenka et al. acta polytechnica ctu proceedings figure 1. vvantage-6 fuel assembly [11]. parameter vvantage 6 pin cladding outer diameter [mm] 9.144 pin cladding inner diameter [mm] 8.001 pin cladding thickness [mm] 0.5715 pin cladding material zr-4 (303) fuel pellet outer diameter [mm] 7.844 number of fuel pins in fa 312 pin pitch [mm] 12.75 number of instrumentation tubes 1 tube material zr-4 (303) inner tube diameter [mm] 12.6 outer tube diameter[mm] 11.0 tube thickness [mm] 0.8 number of guide tubes 18 tube material zr-4 (303) outer tube diameter over the absorber area [mm] 12.6 inner tube diameter over the absorber area [mm] 11.0 tube thickness over the absorber area [mm] 0.8 outer tube diameter in the absorber area [mm] 10.41 inner tube diameter in the absorber area [mm] 8.6 tube thickness in the absorber area [mm] 0.905 total assembly weight [kg] 766 uo2 density [g/cm3] 10.412 zr-4 density [g/cm3] 6.53 table 1. fuel assembly parameter summary [3]. 8 vol. 19/2018 scale and serpent two-group cross-section data generation fuel temperature 1005 k soluble boron concentration 525 ppm (3 g/kg) coolant density 0.7169 g/cm3 coolant temperature 578 k table 2. calculation parameters. scale code used the default detailed endf/b-vii.1 252 group library. reflective boundary conditions resulted in infinite lattice calculation for both codes. serpent allows automatic division of fuel pins into radial regions for more realistic depletion calculations. all fuel pins were divided into 5 radial regions and each region was burned separately in this study. in addition, there were unique materials for all fuel pins in the fuel assembly, thus their material compositions were treated separately. scale calculation used 60◦ symmetry to limit the number of depleted materials. fuel pins in symmetrical positions were considered as identical material. no radial regions in fuel pins were defined as there is no burnable absorber. scale and serpent calculation used 51 burn steps with final burnup at 60.5 mwd/kgu1. seven steps were between 0 mwd/kgu to 1, followed by 44 steps with approximately 1.3 mwd/kgu distance. additional 231 isotopes were added in scale burnup calculation using addnux=3 parameter. serpent used the default isotope inventory setting. 4. results the following parameters were studied: multiplication factor, scattering cross-section (from first group to the second one). in addition to these, fission cross-section, total cross-section and diffusion coefficient, all for two groups. all these parameters were studied for different serpent memory optimization setup. nominal optimization (number 4) was used to produce results which are compared with scale values. 4.1. multiplication factor multiplication factor results are plotted in figure 2a. in order to better show the differences, absolute differences between scale and serpent results were calculated and plotted in figure 2b (scale is the reference one). statistical uncertainty in serpent results is only 22 pcm. uncertainty introduced by nuclear data may reach several hundreds of pcm. however, serpent and scale calculation used nuclear data from an identical library (endf/b-vii.1). scale calculation used preprocessed data in 252 group library, serpent used continuous version. serpent values are higher compared to scale results for boc and eoc. values around 30 mwd/kgu are similar for both codes. initial multiplication factors differ more than 900 pcm from scale results. difference change to 0 160 mwd/kgu is considered as maximal burnup value for currently fuel used. (a). (b). figure 2. multiplication factor from scale and serpent calculation (a) and scale difference from serpent results (b). pcm reached around 30 mwd/kgu. eoc difference reached almost 600 pcm. 4.2. total cross-section total cross-sections for both codes are in good agreement. values are plotted in figure 3a for the fast group and in figure 3b for the thermal group. the serpent code initially calculates the total cross-section by about 0.4 % greater than scale. the value gradually increase during the course of fuel depletion. at burnup level about 20 mwd/kgu the scale value gets greater and the final difference is 0.4 %. results for the thermal group results are almost identical. except for the initial limited period, the differences are limited to 0.1 %. serpent value relative uncertainty is only 0.015% for thermal group and 0.014% for fast group. 4.3. diffusion coefficient the diffusion coefficient is an important macroscopic parameter. diffusion coefficient was calculated for both groups. the coefficient values are plotted in 9 o. novák, j. frýbort, l. sklenka et al. acta polytechnica ctu proceedings (a). (b). figure 3. total cross-section dependency on burnup for fast group (a) and for thermal group (b). figures 4a and 4b. fast group coefficient obtained by scale calculation is higher compared to serpent results. thermal group results are opposite, serpent values are higher compared to scale results. difference for fast group starts approximately at 10% at boc, slowly decreasing and reaching only 3% at eoc. slow group values differs by −7% at boc, drop to −8.6% where it remains till the end. the difference observed in both groups is probably rooted in code structure (stochastic vs deterministic) and in nuclear data difference. serpent value relative uncertainty is only 0.025% for thermal group and 0.021% for fast group. 4.4. fission cross-section fission cross-sections for two energy groups are plotted in figures 5a and 5b. discrepancies from serpent results are plotted in figures 6a (fast group) and 6b (thermal group). fission cross-section for thermal group behaves similarly to the multiplication factor. scale cross-section increases in the first part of fuel cycle, which probably results in decrease of keff difference. maximum is observed similar to minimum (a). (b). figure 4. diffusion coefficient dependency on burnup for fast group (a) and for thermal group (b). of keff difference around 30 mwd/kgu. serpent value relative uncertainty is only 0.038% for thermal group and 0.053% for fast group. 4.5. scattering cross-sections scattering cross-section from fast group to thermal group is plotted in figure 7. both results have similar behavior. serpent values differ from 0.7% to 1% in discrepancy. serpent value relative uncertainty is only 0.014%. 4.6. isotope concentrations number densities of dominant actinides 235u, 238u, and 239pu during fuel burnup were calculated to support understanding of fuel behavior. results are printed in figures 8a, 8b and 8c (including difference from scale results). the observed isotopic changes are consistent but there are differences that can explain differences in the few-group cross-sections. it can be seen that serpent calculates faster 235u depletion than scale and also 239pu number density is lower for the majority of burnup levels. this con10 vol. 19/2018 scale and serpent two-group cross-section data generation (a). (b). figure 5. fission cross-section dependency on burnup for fast group (a) and for thermal group (b). tributes to different and total cross-sections calculated by these two codes. 4.7. memory optimization option in serpent code serpent 2 allows modify calculation in the ram requirements using memory optimization. default mode 4 is recommended for cross-section and other nuclear data generation and has the highest memory requirements. modes 3, 2 and 1 have lower memory requirements and extend calculation duration. more differences are described in serpent forum [1]. in order to compare memory optimization, multiplication factor and other nuclear data were calculated with different optimization. results obtained are similar for all cases and the difference is under statistical uncertainty. the difference for multiplication factors are smaller than 70 pcm which within three standard deviations of the calculation. 5. conclusion monte carlo code serpent and triton calculation sequence from the scale package were used to solve (a). (b). figure 6. scale fission cross-section discrepancy to serpent values for fast group (a) and for thermal group (b). the identical task: calculations of multiplication factor and several two-group macroscopic parameters including scattering cross-section, fission cross-section, total cross-section and diffusion coefficient during depletion of a vvantage-6 fuel assembly. in addition, fuel isotopic composition evolution of selected nuclides was determined. both codes use different methods for solving this neutron transport and depletion coupled task. the results obtained were compared and differences were discussed for each studied parameter. the multiplication factor obtained by both codes are compared in the figure 2a and the differences are plotted in 2b. the initial multiplication factors determined by serpent differs by more than 900 pcm from the scale code results, then it decreased to 0 pcm at around 30 mwd/kgu, however the eoc difference reached almost 600 pcm. total cross-section for both codes is in a good agreement. values are plotted in figure 3a for the fast group and in figure 3b for the thermal group. the relative difference is between ±0.4 %. 11 o. novák, j. frýbort, l. sklenka et al. acta polytechnica ctu proceedings figure 7. scattering cross-section from fast group to thermal group, dependency on burnup. the next studied parameter was the diffusion coefficient. results for the fast group are shown in figure 4a and for the thermal group in 4b. higher differences can be seen for this parameter. they start for the fast group at approximately 10% for boc, slowly decreasing and reaching only 3% at eoc. the thermal group values differs by −7 % at boc, than dropping to −8.6 % where it remains till the end of fuel depletion. the fission cross-section results are plotted in figure 5a for the fast group and in in figure 5b for the thermal group. differences between codes shown in figures 6a and 6b have similar trends and serpent results are consistently lower. the last macroscopic parameter studied was the scattering cross-section. obtained values are shown in figure 7. both codes determined similar burnup dependence with serpent values being different from the scale results by 0.7% to 1%. in addition to macroscopic cross-sections, several isotopic concentration were calculated and studied as well. number densities of dominant actinides 235u, 238u, and 239pu during fuel burnup were calculated. results (including serpent differences from the scale results) can be seen in figures 8a, 8b and 8c. isotopic changes are mostly in agreement, however, serpent calculates faster 235u depletion than scale. moreover, 239pu number density is lower for majority of burnup levels. this contributes to above observed differences of fission cross-section between these two codes. the memory optimization option in serpent code was studied as a part of this study. all four memory optimization were tested. results for all cases were similar and therefore it can be stated that different memory optimization do not affect the results. only the memory requirements and required computational time is affected. in summary, serpent and scale code were compared for burnup calculation and macroscopic crosssection data generation for vvantage-6 fuel assembly. both codes provided mostly similar results. some (a). (b). (c). figure 8. isotope concentration of (a) 235u, (b) 238u and (c) 239pu for both codes and difference between them. differences were however observed in isotopic compositions. it is also reflected in the relevant macroscopic cross-sections. acknowledgements the paper was supported by the project “strengthening and development of research at czech technical uni12 vol. 19/2018 scale and serpent two-group cross-section data generation versity in prague with the use of research infrastructure vr-1 training reactor for research activities” (reg. nr. cz.02.1.010.00.016_0130001790) supported by operational programme research, development and education co-financed from european structural and investment funds and from state budget of the czech republic. support from iaea fellowship number c6/czr/14007 was used in this work. references [1] discussion forum for serpent users [online]. 2007 [accessed on 2017-04-15]. available at: http://ttuki.vtt.fi/serpent/index.php [2] j.r. secker, r.w. miller, l.t. mayhue, r.n. milanova: "advanced design and operating strategies to enhance the safety, operability, and efficiency of vver-1000 reactors." nuclear science and engineering, 121 (1995) 142-145. [3] o. huml aplikace kódů mcnp/x pro výpočet vyhoření a jeho vlivu na neutronově fyzikální charakteristiky reaktorových systémů, neutron physics and burnup calculation of nuclear reactor systems by mcnp/x code . prague, 2014. disertation (ph.d.). czech technical university in prague, faculty of nuclear sciences and physical engineering, department of nuclear reactors (available in czech only). [4] o. novák. study of vver 1000 fuel cycle using nestle and origen codes . prague, 2016. diploma thesis (ing.). czech technical university in prague, faculty of nuclear sciences and physical engineering, department of nuclear reactors. available at: https://www.academia.edu/32855663/study_of_vver_1000_fuel_cycle_using_nestle_and_origen_codes [5] b.t. rearden and m.a. jessee. scale code system, ornl/tm-2005/39, version 6.2.1, oak ridge national laboratory, oak ridge, tennessee (2016). available from radiation safety information computational center as ccc-834. [6] e. fridman, j. leppänen, "on the use of the serpent monte carlo code for few-group cross section generation," annals of nuclear energy, 2011, 38(6), 1399-1405. doi: 10.1016/j.anucene.2011.01.032. issn 0306-4549. [7] m.b. chadwick, m. herman, p. oblozinsky, et al.: "endf/b-vii.1 nuclear data for science and technology: cross sections, covariances, fission product yields and decay data." nuclear data sheets, 2011, 112(12). doi: 10.1016/j.nds.2011.11.002. 2887-2996. [8] t. lötsch, "fuel assembly burnup calculations for vver fuel assemblies with the monte carlo code serpent." kerntechnik, 2014, 79(4), 295-302 [cit. 2017-02-16]. doi: 10.3139/124.110455. issn 0932-3902. [9] j. leppänen, m. pusa, t. viitanen, v. valtavirta, t. kaltiaisenaho: "the serpent monte carlo code: status, development and applications in 2013," ann. nucl. energy, 82 (2015) 142-150. [10] j. leppänen, a. isotalo: "burnup calculation methodology in the serpent 2 monte carlo code." in proc. physor 2012, knoxville, tn, apr. 15-20, 2012. [11] mecir, v. "temelin npp fuel experience". 7 international conference on wwer fuel performance, modelling and experimental support, bulgaria, 2008. 13 acta polytechnica ctu proceedings 19:7–13, 2018 1 introduction 2 fuel assembly description 3 calculation setup 4 results 4.1 multiplication factor 4.2 total cross-section 4.3 diffusion coefficient 4.4 fission cross-section 4.5 scattering cross-sections 4.6 isotope concentrations 4.7 memory optimization option in serpent code 5 conclusion acknowledgements references references acta polytechnica doi:10.14311/app.2020.27.0084 acta polytechnica 27(0):84–89, 2020 © czech technical university in prague, 2020 available online at http://ojs.cvut.cz/ojs/index.php/app distribution of hydration products in the microstructure of cement pastes michal hlobil czech academy of sciences, institute of theoretical and applied mechanics, prosecká 809/76, 19000 prague, czech republic correspondence: hlobil@itam.cas.cz abstract. this case study focuses on the quantification of the amorphous hydrate distribution in the microstructure of hardened cement paste. microtomographic scans of the hardenend cement paste were thresholded based on histogram image analysis combined with microstructural composition obtained from cemhyd3d hydration model, to separate unhydrated cement grains, crystalline and amorphous hydrates, and capillary pores. the observed spatial distribution of the amorphous hydrate exhibited a strong spatial gradient as the amorphous gel tended to concentrate around dissolving cement grains rather than precipitate uniformly in the available space. a comparative numerical study was carried out to highlight the effect of the spatially (non)uniform hydrate distribution on the compressive strength of the hardened cement paste. keywords: cement paste, hydration products, microstructure, modelling. 1. introduction upon contact with water, the clinker and gypsum grains present in portland cement start to dissolve, releasing ions into the solution. upon saturation, hydration products (i.e. hydrates) begin to precipitate on their surface. with progressing cement dissolution, the hydrates fully cover the grains and form dense reaction rims. these then densify with time, gradually expand into the available pore space, and eventually interconnect each other to form a solid internal network. hydrates in portland cement pastes consist primarily of the amorphous calcium-(aluminate)-silicatehydrate (c-(a)-s-h) gel as well as other crystalline hydrates, namely portlandite (ch), monosulfate (afm), ettringite (aft), and others, depending on the mineral cement composition [1]. the amorphous c-s-h gel occupies the largest volume fraction in the hydrated cement paste microstructure, and is therefore largely responsible for the engineering properties of the material. the distribution of the amorphous gel in space depends on several factors, namely on the particle size distribution of the cement (clinker) used and consequently on its fineness, on the spacing of cement grains in the mixture governed by the water-to-cement ratio, and on the mineralogical cement composition. the microstructure of cementitious composites can nowadays be commonly visualized with a scanning electron microscope (sem), in particular using the backscattered electron mode (bse), provided a suitable specimen cross-section is prepared [2, 3]. in particular, the specimen must be cut, polished and carbon/gold coated for greater viewing sharpness, which adds to a rather laborious preparation [4]. recent developments of in the field of computed microtomography allow to analyze specimens not only without any prior surface treatment, but also allow to see oxides [wt %] st. dev cao 65.04 0.43 sio2 20.60 0.25 al2o3 5.60 0.24 mgo 1.47 0.10 na2o 0.10 0.02 k2o 0.61 0.02 so3 2.31 0.07 p2o5 0.10 0.01 fe2o3 2.49 0.06 table 1. chemical composition of cement id 133 determined by edx. into the specimen, in particular to reconstruct the whole 3d microstructure [5]. this opens the door for the characterization of volume-based characteristics of the microstructure, such as percolation or spatial distribution of phases. 2. materials and methods the cement used in this study was produced in the astm cement and concrete reference laboratory (nist), usa, under the id 133, and can be characterized as a type i/ii ordinary portland cement with a chemical composition listed in table 1. gypsum was added as 5.4 % on a volume basis. the cement is ground to a blaine fineness of about 350 m2/kg and features a cumulative cement particle size distribution as shown in figure 1. fresh cement paste was prepared by mixing conditioned distilled water and cement in a mass ratio of 0.45. subsequently, the paste was poured into molds and kept in saturated conditions until testing. 84 http://dx.doi.org/10.14311/app.2020.27.0084 http://ojs.cvut.cz/ojs/index.php/app vol. 27/2020 distribution of hydrates in the microstructure of cement pastes 0 0.2 0.4 0.6 0.8 1 0 10 20 30 40 50 60 70 80 blaine fin eness ≈ 350 m 2 /kg w e ig h t fr a c ti o n [ − ] particle size [µm] id 133 figure 1. discretized particle size distribution of cement id 133. 2.1. tomographic scanning of hardened cement paste paste specimens were demolded and scanned using a microtomograph at the european synchrotron radiation facility near grenoble, france, to obtain a three dimensional reconstruction of the cement paste microstructure at selected times. the size of the scanned volume is 1024 × 1024 × 512 volume elements (voxels) with a resolution of 0.95 µm/voxel which provides an effective microstructural volume of 972×972×486 µm3. the signal from the tomograph is converted into a greyscale monochrome image with color intensities proportional to the atomic number of the element encountered in the particular voxel, ranging from full black to bright white similar to the traditional sem/bse imaging. the 3d tomographic image captures the full microstructural features, namely the volumetric distribution of the individual material phases. slicing the full 3d image into individual slices allows to directly visually compare the microstructure with the traditional 2d greyscale sem/bse images. in particular, clinker minerals and gypsum appear as bright areas which are embedded into a mid-tone grey bulk matrix consisting from a mixture of hydration products, and lastly dark grey-to-black areas correspond to capillary porosity, see figure 2. arbitrary segmentation of material phases from the 2d image slices is not straightforward since the 3d volume generally contains a non-negligible noise, and the distinction between hydration products and capillary porosity is challenging. 2.2. modeling of microstructural development since a quantitative microstructural analysis of the investigated hardened cement paste was not available, a simulation of the cement hydration and microstructure development was carried out using the cemhyd3d software package developed by d. bentz at nist, usa [6]. in its third revision [7], cemhyd3d attempts to reconstruct the microstructure of the investigated system by generating a 3d representation figure 2. cropped 2d slice of the tomographic data showing the microstructural features of a hardened cement paste; image size 158 × 158 pixels capturing an area 150 × 150 µm2. of cement particles in water, while matching the particle size distribution of the real cement along with the individual phase volume and area fractions. periodic boundary conditions are respected during the generation of the 3d volume containing cement particles to omit the local segregation phenomena related to the wall-effect, e.g. near the outer specimen surface, or near the aggregates where itz forms [8]. the model accounts for the chemical reactions of the major cement minerals (alite, belite, aluminate, and ferite phases) along with calcium sulfate phases (an-/hemi/di-hydrate). implemented chemical reactions follow the described stoichiometry [9]. computationally, the hydration reactions are implemented as a series of cellular automata-like rules, describing in repeated cycles the dissolution, diffusion, and interaction of diffusing species with clinker and gypsum particles. a posteriori, calibration of hydration kinetics is necessary for mapping the calculated results to real time. this is achieved by fitting the calculated results of the heat evolution into a parabolic dispersion model via time = t0 + β cycle2, (1) where t0 stands for the induction time and β is a fitting parameter, which has to be calibrated from experiments. for the present cement id 133 studied, t0 = 0 as the induction period is implicitly accounted for since the second version of this hydration model [10] by adjusting the early-dissolution probabilities for all clinker phases, which are made proportional to the square of the amount of c-s-h produced, and β = 0.0003 as identified from isocalorimetric measurements carried out in nist. 85 michal hlobil acta polytechnica 0 20 40 60 80 100 0 0.2 0.4 0.6 0.8 1 v o lu m e f ra c ti o n [ % ] degree of clinker hydration ξclin [−] t= 4 .5 h t= 1 1 h t= 1 2 4 h cement crystal. hydrates amorphous hydrate capillary pores figure 3. the volume fraction evolution of microstructural phases in cement id 133, cured at 20 °c in saturated conditions, as simulated by cemhyd3d. figure 4. partially hydrated artificial microstructure from cemhyd3d; one slice showing a cross-section 100 × 100 µm with a resolution 1pixel (voxel)=1 µm; color map corresponds to figure 3. 3. results the microstructural evolution as quantified by simulating the hydration of an rve of cement paste with a side equal to 100 µm using cemhyd3d is shown in figure 3. to simplify the microstructural characterization, all unhydrated clinker phases together with all calcium sulfate forms (including secondary gypsum formed) are merged together into the phase labeled as “cement”. similar simplification was done also for crystalline hydrates which include portlandite, ettringite, monosulfate, and iron hydroxide. the amorphous hydrate then consists of the calcium silicate and calcium aluminate hydrates. a single 2d slice of the modeled microstructure from cemhyd3d is then shown in figure 4 with a color map matching figure 3. combining histogram image analysis with known (calculated) volume fractions of material phases opens door for accurate thresholding of the tomographic figure 5. thresholded 2d slice of the tomographic data separating cement (clinker+calcium sulfate phases), amorphous and crystalline hydrates, and capillary porosity; image size 158 × 158 pixels capturing an area 150 × 150 µm2; color map corresponds to figure 3. slices, as described next. first, a 2d slice from the tomographic data is extracted and cropped to 158 × 158pixels capturing an area of 150 × 150 µm to reduce computational time for analysis and yet cover a sufficiently large area for a representative analysis. since the raw tomographic data contains noise, a suitable smoothing image filter is applied to reduce the image noise. subsequently, a histogram of the cropped image is produced, quantifying the amount of pixels of a given grey shade in 8 bit color depth, i.e. 255 colors, ranging from 0 (representing black) to 255 (white). converting the histogram into its cumulative form, and normalizing it with respect to the total amount of pixels in the image allows to keep the abscissa values and renumber the ordinate from 0 to 1. in such form, the cumulative histogram can be used to identify two threshold values for porosity and cement particles by comparing the calculated volume fractions from cemhyd3d with the ordinate values, pointing to the sought threshold values on the abscissa. the greyscale values lying in between these limits are attributed to the hydration products. separation of the hydrate phase into the crystalline and amorphous parts is not possible using histogram analysis, since the phase contrast manifesting as similar grey shade is too low. however, the formation of crystalline hydrates within the microstructure is random as they tend to form both on the surface of cement particles as well as nucleate within the amorphous hydrate [11] consequently, the crystalline hydrates were randomly distributed within the hydration products following their volume fraction, see figure 5 for final result. 86 vol. 27/2020 distribution of hydrates in the microstructure of cement pastes 0 500 1000 1500 2000 2500 3000 0 5 10 15 20 25 30 35 p ix e l c o u n t [− ] minimum distance from cement grain [µm] t=124 h t=11 h t=4.4 h figure 6. quantification of the spatial distribution of amorphous hydrates as a function of the minimum distance from the cement grain. 3.1. spatial distribution of amorphous hydrates quantification of the spatial distribution of the amorphous hydrate gel from the thresholded tomographic image is performed by measuring the distance between the given amorphous hydrate pixel and the closest cement particle. counting such minimum distances for all hydrate pixels in the image and binning the results shows a rather strongly non-uniform hydrate distribution in the cement paste microstructure, see figure 6. in the early stages of hydration (4.5 and 11 hours), the amorphous hydrates form dominantly around cement grains which can be seen as the distance from the grain is rather short, i.e. only single units of micrometers. with progressing hydration, cement grains dissolve and create additional space for new hydrates to form. however, even then the hydrates tend to precipitate in rings, minimizing the distance to the nearest cement grain, and do not precipitate randomly in space. 3.2. prediction of cement paste compressive strength local spatial distribution of hydration products directly affects the macroscopic mechanical properties of cement paste, in particular the compressive strength. two case studies will be considered to show the impact of the hydrate distribution, namely i) the hydrate density distribution exhibiting a strong spatial gradient as observed in thresholded micrographs, see figure 5, and ii) an uniform hydrate distribution. in both cases, the cement grain locations as well as volume fractions of all phases remain identical. the only difference stems from the fact, that in the second case, the hydrates are smeared uniformly throughout the microstructure, which effectively changes the capillary pore size system, compare figures 5 and 7. a micromechanical model based on finite element homogenization is formulated for both cases in such a way that the finite element mesh is generated over the thresholded bitmap micrograph using quadrilateral elements in place of image pixels. local mechanical properties of the identified phases, e.g. of clinker, figure 7. artificially created microstructure based on figure 5, featuring an enforced uniform distribution of hydrates in space; image size 158×158 pixels capturing an area of 150 × 150 µm2; color map corresponds to figure 3. phase e ν ft gf ref [gpa] [–] [mpa] [j/m2] clinker 139.9 0.30 – – [13] amorph. hyd. 24.4 0.24 264.1 4.4 [12] crystal. hyd. 42.3 0.32 – – [14] porosity 0.001 0.49 – – [15] table 2. material properties obtained from nanoindentation. of amorphous and crystalline hydration products are obtained by nanoindentation measurements [12], see table 2. 0 20 40 60 80 100 0 0.002 0.004 0.006 0.008 0.01 0.012 0.014 c o m p re s s iv e s tr e n g th [ m p a ] compressive strain [−] gradient distribution uniform distribution figure 8. results of compressive strength simulation for both hydrate spatial distributions (gradient and uniform) for virtual microstructures of cement id 133 at 124 hrs. two distinct material models are assigned to the individual phases. a simple linear elastic constitutive behavior is used for cement, crystalline hydrates, and 87 michal hlobil acta polytechnica porosity, whereas the amorphous hydrates are modeled using a nonlinear fracture mechanics approach combined with an isotropic damage model for tensile failure, originally described in [16]. the virtual model of the microstructure features periodic boundary conditions in the vertical direction along with supported nodes to prevent rigid body translation and rotation. the problem is solved in the plane stress domain using a nonlinear static analysis carried out with the newton-raphson iterative solver by applying a predefined eigenstrain load increment until failure. results of the microstructural simulation are shown in figure 8. the microstructure based on the thresholded tomographic data exhibits a peak compressive strength of 13.7 mpa, whereas the virtual microstructure exhibiting an uniform distribution of hydrates fails at 78.2 mpa. such an increase of 470% was achieved by changing the hydrate spatial density distribution while keeping the volume fractions of all phases identical. 4. conclusions combining state-of-the-art microtomography to obtain a 3d microstructure with the quantification of volume phases using a hydration model cemhyd3d allowed to view, threshold and separate the cement grains, crystalline and amorphous products, and capillary pores in a representative volume of hardended cement paste with initial water-to-cement ratio of 0.45. focusing on the amorphous hydrate phase, its distribution was measured by determining the minimum distances of each hydrate pixel to the closest cement grain. results of this study indicate a non-uniform distribution with a strong gradient in space. in the early stage of hydration, the hydrates precipitate dominantly around cement grains. with ongoing hydration the distances between cement grains increase as these dissolve, however even then the hydrates tend to minimize the necessary distance and precipitate in rings rather than randomly in space. numerical modeling showed that enforcing an uniform hydrate distribution on the same microstructure (same by means of identical volume fractions of all phases and identical placement of unhydrated cement grains) resulted in a significant increase of compressive strength. this stems from the fact that the uniform hydrate distribution resulted in an alternation of the capillary pore shape and an effective pore size decrease, which consequently manifested as an increase of hardened cement paste compressive strength. acknowledgements the author gratefully acknowledges support from the czech science foundation project 19-25163y. references [1] p. termkhajornkit, q. h. vu, r. barbarulo, et al. dependence of compressive strength on phase assemblage in cement pastes: beyond gel–space ratio experimental evidence and micromechanical modeling. cement and concrete research 56:1–11, 2014. doi:10.1016/j.cemconres.2013.10.007. [2] s. diamond. the microstructure of cement paste and concrete a visual primer. cement and concrete composites 26(8):919–933, 2004. doi:10.1016/j.cemconcomp.2004.02.028. [3] k. scrivener. backscattered electron imaging of cementitious microstructures: understanding and quantification. cement and concrete composites 26(8):935–945, 2004. doi:10.1016/j.cemconcomp.2004.02.029. [4] k. l. scrivener, r. snellings, b. lothenbach. a practical guide to microstructural analysis of cementitious materials. crc press, 2016. [5] e. gallucci, k. scrivener, a. groso, et al. 3d experimental investigation of the microstructure of cement pastes using synchrotron x-ray microtomography (µct). cement and concrete research 2007. [6] d. p. bentz. a three-dimensional cement hydration and microstructure program. i. hydration rate, heat of hydration and chemical shrinkage. tech. rep., nist building and fire research laboratory, gaithersburg, maryland, 1995. [7] d. p. bentz. cemhyd3d: a three-dimensional cement hydration and microstructure development modeling package. version 3.0. tech. rep., nist building and fire research laboratory, gaithersburg, maryland, 2005. [8] k. l. scrivener, a. k. crumbie, p. laugesen. the interfacial transition zone (itz) between cement paste and aggregate in concrete. interface science 12(4):411–421, 2004. [9] h. f. w. taylor. cement chemistry. thomas telford publishing, 2nd edn., 1997. [10] d. p. bentz. cemhyd3d: a three-dimensional cement hydration and microstructure development modeling package. version 2.0. tech. rep., nist building and fire research laboratory, gaithersburg, maryland, 2000. [11] f.-j. ulm, m. vandamme, h. jennings, et al. does microstructure matter for statistical nanoindentation techniques? cement and concrete composites 32(1):92–99, 2010. [12] j. němeček, v. králík, v. šmilauer, et al. tensile strength of hydrated cement paste phases assessed by micro-bending tests and nanoindentation. cement and concrete composites 73:164–173, 2016. doi:10.1016/j.cemconcomp.2016.07.010. [13] k. velez, s. maximilien, d. damidot, et al. determination by nanoindentation of elastic modulus and hardness of pure constituents of portland cement clinker. cement and concrete research 31(4):555–561, 2001. doi:10.1016/s0008-8846(00)00505-6. [14] c.-j. haecker, e. j. garboczi, j. w. bullard, et al. modeling the linear elastic properties of portland cement paste. cement and concrete research 35(10):1948– 1960, 2005. doi:10.1016/j.cemconres.2005.05.001. 88 https://doi.org/10.1016/j.cemconres.2013.10.007 https://doi.org/10.1016/j.cemconcomp.2004.02.028 https://doi.org/10.1016/j.cemconcomp.2004.02.029 https://doi.org/10.1016/j.cemconcomp.2016.07.010 https://doi.org/10.1016/s0008-8846(00)00505-6 https://doi.org/10.1016/j.cemconres.2005.05.001 vol. 27/2020 distribution of hydrates in the microstructure of cement pastes [15] v. šmilauer, z. bittnar. microstructure-based micromechanical prediction of elastic properties in hydrating cement paste. cement and concrete research 36(9):1708–1718, 2006. doi:10.1016/j.cemconres.2006.05.014. [16] m. hlobil, v. šmilauer, g. chanvillard. micromechanical multiscale fracture model for compressive strength of blended cement pastes. cement and concrete research 83:188–202, 2016. 89 https://doi.org/10.1016/j.cemconres.2006.05.014 acta polytechnica 27(0):84–89, 2020 1 introduction 2 materials and methods 2.1 tomographic scanning of hardened cement paste 2.2 modeling of microstructural development 3 results 3.1 spatial distribution of amorphous hydrates 3.2 prediction of cement paste compressive strength 4 conclusions acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2018.16.0001 acta polytechnica ctu proceedings 16:1–5, 2018 © czech technical university in prague, 2018 available online at http://ojs.cvut.cz/ojs/index.php/app proposal for securing a rock massif near beroun railway station jiří barták czech technical university in prague, faculty of civil engineering, department of geotechnics, thákurova 7, 166 29 prague, czech republic correspondence: bartakj@fsv.cvut.cz abstract. the proposal concerns the securing of a 100 m long section of the rock slope along the lead track at beroun railway station, where a relatively massive rock block collapsed in june 2016. the rock slope is about 30 m high and its character is mainly determined by the orientation of several discontinuity systems. it was proposed for the most dangerous discontinuities that monitoring elements be used for measuring deformations at least for half a year. dynamic barriers and protective fences were proposed for the most endangered parts of the slope. keywords: rock slope, rock fall, climbing inspection, dynamic barrier. 1. basic geotechnical characteristics the geotechnical characteristics relate to a circa 100 m long section of the rock slope along the lead track at beroun railway station (see figure 1). the rock slope in the particular area is very steep, accessible only by using climbing techniques. the abovementioned lead track runs closely along the base of the slope. that this track be open to use is very important for the organisation and smooth running of station operations. the rock slope dealt with by the design is flat on the top, then it is slightly sloping, passing into a steep rock half-cutting, which was formed historically by river erosion, and recently by the development of the rail track. the rock slope in the location of the rock block collapse is about 30 m high and the height decreases toward the south-west. the current character of the slope surface is given mainly by the orientation of several discontinuity systems. there are local small ridges there as well as ledges and sheer parts in some locations even overhanging. modest vegetation cover is evident in the steep part of the rock slope. it depends on mainly fissure systems for rooting. this process leads to widening of the fissures and long-term weathering, thus negatively affecting the partial surface stability. in the top parts, the vegetation passes into relatively continuous selfseeding vegetation. the rock massif is made up of volcanic rock types of the palaeozoic age – granulites, altered basalts and their tuffs. these rock types are very hard, resistant to weathering. the rock wall has a general sw – ne trend and four fundamental fissure systems can be distinguished in it (see figure 2). fissures (a), trending roughly parallel to the slope base, with the maximum slope line running down the slope, are the least favourable. fissures (b) dip into the slope and are perpendicular to the system (a), thus creating potentially unstable blocks. the fissure systems (c) and (d) cut through the previous systems at angles, creating characteristic crests and reducing the sizes of the potentially unstable blocks. the discontinuity surfaces are relatively smooth or maybe in some cases, moderately undulated. 2. existing condition and remedial action an improving and rehabilitation action, consisted in the execution of a survey, involving the removal of rock debris, some self-seeding bushes and small weathered parts of the rock, was taken in the location of concern approximately five years ago. a block of loosened rock fell down to the space of the lead track at the beginning of june 2016. it made the operation of the track impossible. the liquidation of the extremely strong rock block, hard to drill and therefore difficult to disintegrate, was very exacting (see figure 3). the slope deformation, which happened obviously for the first time in the location of concern, can be characterised as displacement along predetermined discontinuity surfaces, in this particular case along the surfaces (c) and (d), with the block size limited by the discontinuity surface (b) – see figure 4. it is in substance, the last phase of a long-term creeping movement which, under certain conditions, unexpectedly develops into a rock fall. the usual conditioning factors of the creep and unexpected change in the rate of the movement of the rock blocks comprise the geomorphological disposition of the slope, weathering, the effect of surface water as well as groundwater, and the influence of vegetation and anthropogenic actions. in the majority of cases, this includes general assessment of the location. it is a concurrent action of several effects slowly reducing the shear strength on discontinuity surfaces that triggers the above-mentioned 1 http://dx.doi.org/10.14311/app.2018.16.0001 http://ojs.cvut.cz/ojs/index.php/app jiří barták acta polytechnica ctu proceedings figure 1. layout of the location of concern. figure 2. discontinuity systems on the rock wall being monitored. creeping movement. over a long time or also in the case of a sudden change in external conditions, the shear strength on discontinuity surfaces is exceeded by the tangential component of the dead weight of the loosened part of the rock massif, which then in fact becomes the immediate cause of the rock fall. a climbing inspection of the rock slope [1] was conducted after the fall of the rock block on the lead track. the inspection lay in removing rock debris and loose rock fragments or small rock blocks and liquidation of a part of the self-seeding vegetation. this work ensures safer operation along the lead track as well as the possibility for relatively safe execution of other rehabilitation work in the future. detailed photographic documentation allowing for tipping the locations on the rock massif fissures and dislocations recommendable for the installation of mechanical deformation monitoring elements was provided during the inspection. the monitoring elements need to be installed on about 10 dangerous discontinuities without any unnecessary delay. they should be observed at around half-year intervals always before and after a climatic change – at the end of autumn and at the beginning of spring. contingent smaller-extent cleaning of the wall and inspection of the wall condition will be carried out concurrently with the unavoidable climbing checks of the measuring instruments, so that the possibility of the loss of stability of some of the larger rock block can be assessed. increased attention has to be devoted to areas no. 1 and 2 (see figure 5), where the danger of loosening rock blocks is greatest. the inspection of the wall and the photographic documentation signal the possibility of some blocks losing their stability in the above-mentioned areas. 2 vol. 16/2018 rock massif figure 3. fallen rock block after partial disintegration. figure 4. the location of a loosening of the fallen rock block. figure 5. dangerous areas of the rock wall. 3 jiří barták acta polytechnica ctu proceedings figure 6. dynamic barrier – schematic view and cross section. figure 7. detail of a loop-type breaking element (energy absorber) of the dynamic barrier. figure 8. protective fence – schematic cross section. 4 vol. 16/2018 rock massif figure 9. locations for the placement of dynamic barriers and the protective fence in the endangered areas of the rock wall. 3. proposal for rehabilitation measures taking into consideration the exposed section of the rock slope above the lead track, the author of the assessment contemplated carrying out a design for rehabilitation measures which would reliably ensure operational safety in the railway yard. it would comprise a dynamic barrier under the particular area no. 1, possibly even above the area no. 2, and a protective fence or another type of a solid barrier under this area no. 2. dynamic barriers are structures tested experimentally as well as in practice, which are capable of restraining a falling rock block with the maximum kinetic energy of up to 5,000 kj. this capability is provided by the combined action of a net with a metal/cable mesh of rings and a system of binding cable with a system of posts, which involves a multiple anchoring to the rock substrate in the directions transverse, longitudinal and perpendicular to the slope. the cable anchoring elements are equipped with an efficient breaking system (energy absorbers of various types). materials concerning the rehabilitation of unstable rock massifs provided by [2]. the diagram of the breaking system of a dynamic barrier is presented in figures 6 and 7, and protective fence in figure 8. it will be suitable to install the dynamic barriers on the rock slope of concern roughly at one third of the height of the wall under the area no. 1 and possibly above the area no. 2, approximately in the middle of the wall height. the barriers will be 2 m up to 2.5 m high. the locations for the placement of the dynamic barriers and protective fence are schematically indicated in figure 9. all of presented figures were captured during climbing research by [3]. 4. conclusions the rock slope located south-east of beroun railway station can be assessed as an endangered location, which poses a potential and rather hard to predict threat to the existing lead track. it was therefore recommended that simple mechanical monitoring elements be installed for monitoring the deformations on dangerous discontinuities. the monitoring of deformations on selected fissures, combined with the inspection and basic cleaning of the wall, possibly with tipping of the endangered blocks which will have to be removed from the wall, should be carried out at half-year intervals at a minimum. it will be necessary in the case of more pronounced manifestations of deformations to consider the implementation of more radical rehabilitation measures and structures, in particular the installation of dynamic barriers and protective fences. an active approach of the railway infrastructure administration which is a state organisation, is considered desirable. references [1] inspection. inspection of the location dated 9th june 2016. [2] materials concerning the rehabilitation of unstable rock massifs provided by geobrugg ag, strix a. s., maccafferi s. r. o. [3] j. barták, oremo. climbing research and photographic documentation of the location of concern carried out by oremo and barták j., 02/2011. 5 acta polytechnica ctu proceedings 16:1–5, 2018 1 basic geotechnical characteristics 2 existing condition and remedial action 3 proposal for rehabilitation measures 4 conclusions references acta polytechnica ctu proceedings doi:10.14311/app.2019.25.0058 acta polytechnica ctu proceedings 25:58–63, 2019 © czech technical university in prague, 2019 available online at http://ojs.cvut.cz/ojs/index.php/app x-ray micro-tomography characterization of voids caused by three-point bending in selected alkali-activated aluminosilicate composite ivana kumpováa,b,∗, iva rozsypalováa, zbyněk keršnera, pavla rovnaníkováa, michal vopálenskýb a brno university of technology, faculty of civil engineering, veveří 331/95, 602 00 brno, czech republic b czech academy of sciences, institute of theoretical and applied mechanics, prosecká 809/76, 190 00 prague, czech republic ∗ corresponding author: ivana.kumpova@vutbr.cz abstract. this paper deals with the pilot characterization of a special alkali-activated aluminosilicate composite composed of waste brick powder, brick rubble and a solution of potassium water glass. fracture tests were conducted on the specimens via three-point bending and fracture parameters were evaluated. selected specimen was investigated using micro-tomography to supplement the results with visual information about the inner structure of this newly designed material before and after the mechanical loading. tomographic measurements and image processing were conducted for a qualitative and quantitative assessment of changes in the internal structure with an emphasis on the calculation of porosimetric parameters and visualization of the fracture surface. fractal dimension of fracture surface was estimated. keywords: alkali-activated aluminosilicate composite, fracture surface, x-ray micro-tomography, 3d porosity evaluation, fractal dimension. 1. introduction a thorough understanding of the initialization process and the formation of cracks and their propagation in various materials is essential in many engineering fields and applications, as one of the crucial safety and economic factors for the preservation of existing and development of new construction [1–3]. although macro cracks affect the behavior of the structures on a macroscopic scale, critical micro cracking processes leading to structural failure occur in the structure of the material at the microscopic scale. advanced building materials are commonly characterized only by values of the basic mechanical fracture properties, usually determined by evaluation of quasi-static fracture experiments (e.g. bulk density, modulus of elasticity, effective fracture toughness, effective toughness, and specific fracture energy) and it is difficult to quantify, model and predict their behavior [4–6]. current industrial computed tomography (ct) is used to obtain visual information about the state and the spatial arrangement of the internal structure of the investigated material. using this non-destructive method, the same sample (test specimen) can be scanned repeatedly and observed for changes in its structure occurred as a result of the time-dependent process (maturation, aging, loading with various chemical and physical influences). the method is also perceived as a promising tool to supplement the standard quasi-static fracture experiments with image information related to the spatial distribution of cracks and voids in different phases of failure [7–12]. results can be used also to validate existing models and predictions. advanced digital image processing of tomographically acquired 3d virtual models enables to quantify the representation, spatial orientation and size of individual phases with respect to the achieved resolution. quantitative description of individual structures and phases in the obtained 3d tomographic model is based on the thresholding of the voxel values and their subsequent registration. it is necessary to realize that the values of voxels forming the 3d image information is given by a huge range of variables, starting with the parameters of the examined object (e.g. thickness, density, proton number of atoms in individual structures, etc.) through the parameters of the x-ray radiation (e.g. accelerating voltage, efficiency of energy transformation to photon radiation, filtration, etc.) and used detector (e.g. method of the transmitted radiation detection and its conversion to a digital signal, size of pixel matrix, point spread function, sensibility, etc.) to parameters of resulting 3d matrix (e.g. method of reconstruction, image correction, data type, etc.). for this reason, it is generally not possible to determine exactly the threshold for the separation of structures of interest (e.g. voids, fibers, inclusions, etc.) from other present structures. it can be determined more accurately only under the assumptions that comparative measurement of the standard of the material under investigation with known parameters is available. however, this is not common in the case of tomographic research of building materials and thus 58 http://dx.doi.org/10.14311/app.2019.25.0058 http://ojs.cvut.cz/ojs/index.php/app vol. 25/2019 µct characterization of voids caused by three-point bending the threshold setting is dependent on the operator’s decision. in such case, the results may be affected by a relatively large error and should be checked / paired using other methods. 2. material and methods 2.1. specimen and mechanical properties evaluation powdered waste ceramics as precursors for the preparation of alkali-activated aluminosilicate composites have not been much studied in terms of fracture mechanics in the past [13, 14] but they have good prerequisites for future application as environmentally friendly substitutes of cement-based composites. the pilot characterization of special alkali-activated aluminosilicate composite (aaac) material composed of waste brick powder (grain size range 0 − 1 mm), brick rubble (grain size range 0 − 4 mm) and solution of potassium water glass with silicate modulus ms = 0 − 4 mm was intended. specimens of nominal dimensions of 40 × 40 × 160 mm) were provided with an initial edge notch in the middle of the span (the longest dimension) extending to 1/3 of the specimen's depth as seen in figure 1. figure 1. illustration of fracture test in three-point bending (left), selected specimens after the fracture tests in three-point bending (top right), and specimen after test in compression (bottom right). fracture tests were conducted on specimens via three-point bending according to [15] with a supports span of 140 mm. the 28-days basic mechanical fracture parameters (mean value, coefficient of variation) – bulk density (1911 kg·,m−3, 0.9 %), modulus of elasticity (4.64 gpa, 18.6 %), effective fracture toughness (0.374 mpa · m 1 2 , 7.6 %), effective toughness (30.6 n · m−1, 10.1 %), and specific fracture energy (45.7 j · m−2, 5.0 %) – of the proposed material were obtained from the tests performed on the set of seven test specimens [16]. fracture parameters were evaluated from load versus deflection diagrams using effective crack model [15] and work-of-fracture method [17]. the informative compressive strength value (30.6 mpa, 6.3 %) was determined on the specimen's fragments after the fracture tests were completed [18]. photos in figure 1 illustrate mentioned aaac specimens, fracture test, as well as test in compression of parts of specimens after three-point bending test. further mechanical fracture characterization of this aaac is beyond the scope of this article. 2.2. x-ray computed tomography before and after the fracture test in three-point bending, one of the specimens was investigated using microtomography to supplement the results with the visual information about the inner structure of the proposed material. data acquisition for x-ray ct measurements was performed using patented twinned orthogonal adjustable tomograph (toratom, ep 2835631 b1) depicted in figure 2. the specimen was tomographed by one pair of the x-ray source–detector. the micro-focus x-ray tube (xwt-240-tchr, x-ray worx, germany) operating at a voltage of 210 kv, current on the target of 157µa and a power of 33 w was used for both ct scans. gadox flat panel (xrd-1622ap-14, perkin elmer, usa) with an active area size of 409.6 × 409.6 mm, a matrix of 2048 × 2048 px and a resolution of 200µm per pixel operating at a capacity of 0.5 pf was used as an imaging detector. figure 2. toratom tomography system: active damped anti-vibration table with high-precision cnc positioning system (1) holds two imaging pairs of the x-ray tube (2, 3) and detector (4, 5) in an orthogonal arrangement with shared rotary stage (6) accommodating the investigated specimen. by adjusting the assembly to the x-ray spotdetector distance of 1149.90 mm and the x-ray spotsample distance of 174.26 mm, a geometrical magnification of approximately 6.6 × was achieved, leading to a voxel size (resolution) of 30.3µm. the geometrical parameters were chosen in order to obtain the best possible resolution with respect to the size of the detector area and the size of the area of interest on the object being examined, which was determined as a cube with 40mm edge centered in the area of the initiation notch. for the correction of acquired projections standard "dark field" and "open beam" (ffc) corrections were used. ffc data was averaged over 100 59 i. kumpová, i. rozsypalová, z. keršner et al. acta polytechnica ctu proceedings images with an acquisition time of 1200 ms. a total of 2880 projections were taken for each tomography with an acquisition time of 1200 ms. the resulting 3d models were calculated by the filtered back projection method using vg studio max 3.2 software (volume graphics gmbh, germany). 2.3. porosity analysis for the porosity and voids analysis, the threshold algorithm embedded in the porosity / inclusion analysis module of the vg studio max 3.2 software (volume graphics gmbh, germany) was used. based on the three dimensional analysis of dataset binarized according the selected threshold the algorithm finds all structures below/above this threshold and provides many parameters for each such structure. during the void analysis each voxel is considered a defect if the grey value is below the specified threshold. the threshold was determined by the results of the automatic surface determination procedure based on the histogram. the result is the material boundary defined by one gray value globally applied to the object serving as a threshold: brighter areas are considered as a material, darker areas are considered as a background (air). the surface determination calculates the material boundary in sub-voxel accuracy by trilinear interpolation. to avoid registration of the noise particles, porosity analysis result was filtered so that only voids bigger than 8 voxels (0.22 · 10−3 mm3) were taken into account. according to the number of voids and their volume, partial and total porosity ratio was calculated. for each detected void, the surface area, volume and number of voxels it consists of was monitored. attention was also paid to sphericity, which is a measure for the ratio between the surface area of a sphere with the same volume as the void and the surface of the void itself. 2.4. fractal dimension estimation fracture surface (magistral crack) registered during void analysis was extracted in a form of 3d binary image and its fractal dimension d was estimated by applying the box-counting algorithm embedded in bonej plugin of the free open source software imagej [19, 20]. in this algorithm – according to the equation 1 – grids of diminishing size are scanned over the 3d image and the number of boxes n containing at least one foreground voxel (part of investigated structure) is counted. as the box sizem decreases and the grid becomes finer, the proportion of foreground boxes increases in a fractal structure. d = log(m) − log(n) (1) the box counting algorithm produces a pair of log(m), − log(n) values for each iteration it runs. these pairs are passed to a curve-fitting algorithm, which returns the slope of the linear function which describes them (regression fit). the coefficient of this slope is the fractal dimension value. the box counting algorithm was applied with starting box size of 232 px and box scaling factor of 2. four grid translations were performed to find the optimal covering. 3. results and discussion the tomographic investigation of the alkali-activated aluminosilicate composite was focused mainly on the determination of the resulting fracture surface from acquired 3d tomographic models. attention was also paid to the porosimetric parameters or more accurately parameters of voids including both pores and cracks and other defects. the acquired ct reconstructions were spatially aligned to each other so that the individual tomographic cross sections of the sample before and after loading corresponds. then the region of interest (roi) shown in figure 3 was subsequently selected from both models, large enough not to lose its meaningful value, however not including the areas of the surrounding air and initiating notch, which would interfere with further processing. dimensions of the selected roi were 39.82 × 26.58 × 39.42 mm. figure 3. visualization of the obtained 3d model of the scanned area. part indicated by the red color specifies the volume used for the determination of porosimetric parameters and further investigation of the crack spatial distribution. a differential model was calculated to visualize and highlight the differences in the internal structure of the specimen caused by fracture. an example cross sections are shown in figure 6 and figure 7. 3d spatial model of the magistral crack registered during void analysis was extracted. from the results shown in figure 8, the macro-crack fractal dimension of 2.154 was estimated, which is typical value for concrete. further selected porosimetric results are summarized in the table in figure 4, and dependence of number of pores on their size and their contribution to total porosity is graphically expressed in figure 5. several conclusions can be estimated. a newly formed macro crack with a volume of 45.66 mm3 and a surface of 1500 mm2 has been revealed. its projected length in x, y, z directions was estimated to 39.44 mm, 17.91 mm and 7.05 mm respectively. area of the crack 60 vol. 25/2019 µct characterization of voids caused by three-point bending figure 4. table summarizing selected porosimetric parameters. figure 5. graph of the porosimetric results. bars in the graph represent histogram of the pore distribution within the investigated roi before and after the three-point bending fracture test. curves on the secondary axis represent the cumulative porosity at each pore volume interval. projected in xy, xz, yz plane (for orientation see figure 6 and figure 7) was calculated to 320.00 mm2, 82.06 mm2 and 41.55 mm2 respectively. after the loading, the formation of new voids and the increase of the volume of the existing ones can be observed, which is caused by the thinning and cracking of the material. total porosity of the roi after loading increased by 0.2 % with the macro crack contribution of 0.11 %. an increase of the average void's surface and decrease of the average sphericity ratio indicates the formation of the micro cracks. in the case of material with compact not interconnected pores, as aaac presented, these values can serve very well to separate the results of the void analysis on pores (high sphericity) and cracks (low sphericity) and further investigation of one group only. 4. conclusions powdered waste ceramics as precursors for the preparation of selected alkali-activated aluminosilicate composite were studied in this paper. after the previously determined mechanical fracture parameters of this material, attention was focused on the use of advanced tomographic method to analyze the porosity and to determine the fractal dimension of the projected fracture surface of selected specimen after fracture test in three-point bending. acknowledgements this outcome has been achieved within the framework of the project of the specific university research of the brno university of technology no. fast-j-19-6084 and with the financial support of the czech science foundation un61 i. kumpová, i. rozsypalová, z. keršner et al. acta polytechnica ctu proceedings figure 6. visualization of tomographic cross-sections taken in the same place parallel to the direction of the crack propagation (xy plane). 1) visualization of the cross-section before three-point bending; 2) visualization of the cross-section after three point bending; 3) plane representing the position of the visualized cross-sections (orientation of the volume corresponds to figure 3); 4) differential image with highlighted differences (cracks). figure 7. visualization of tomographic cross-sections taken in the same place perpendicular to the direction of the crack propagation (xz plane). 1) visualization of the cross-section before three-point bending; 2) visualization of the cross-section after three point bending; 3) plane representing the position of the visualized cross-sections (orientation of the volume corresponds to figure 3); 4) differential image with highlighted differences (cracks). der project no 19-01982s alkali-activated aluminosilicate composites based on ceramic precursors. references [1] k. germaniuk, t. gajda, a. sakowski, et al. bridge structures cracks-what made that phenomena so common? transportation research procedia 14:4030–4039, 2016. doi:10.1016/j.trpro.2016.05.500. figure 8. visualization of the extracted fracture surface with the table and graph of the values obtained using box counting algorithm. 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[11] i. kumpova, t. fila, d. vavrik, z. kersner. x-ray dynamic observation of the evolution of the fracture process zone in a quasi-brittle specimen. journal of instrumentation 10(8):c08004, 2015. doi:10.1088/1748-0221/10/08/c08004. [12] i. kumpova, m. vopalensky, t. fila, et al. on-the-fly fast x-ray tomography using a cdte pixelated detector application in mechanical testing. ieee transactions on nuclear science 65(12):2870–2876, 2018. doi:10.1109/tns.2018.2873830. [13] p. chindaprasirt, p. paisitsrisawat, u. rattanasak. strength and resistance to sulfate and sulfuric acid of ground fluidized bed combustion fly ash–silica fume alkali-activated composite. advanced powder technology 25(3):1087–1093, 2014. doi:10.1016/j.apt.2014.02.007. [14] d. bondar, c. j. lynsdale, n. b. milestone, et al. effect of type, form, and dosage of activators on strength of alkali-activated natural pozzolans. cement and concrete composites 33(2):251–260, 2011. doi:10.1016/j.cemconcomp.2010.10.021. [15] b. l. karihaloo. fracture mechanics and structural concrete. longman scientific & technical, 1995. isbn: 978-0582215825. [16] i. rozsypalova, i. kumpova, t. majda, et al. basic mechanical fracture characterization of a selected alkali-activated aluminosilicate composite. in euromat 2019 abstracts, p. 2157. swedish society for materials technology, 2019. [17] rilem tc fmc 1: determination of the fracture energy of mortar and concrete by means of three-point bend tests on notched beams. materials and structures 18(4):287–290, 1985. doi:10.1007/bf02472918. [18] british standards institution. en 13892-2:2002 (e): methods of test for screed materials–part 2: determination of flexural and compressive strength, 2002. [19] m. doube, m. m. kłosowski, i. arganda-carreras, et al. bonej: free and extensible bone image analysis in imagej. bone 47(6):1076–1079, 2010. doi:10.1016/j.bone.2010.08.023. [20] n. l. fazzalari, i. h. parkinson. fractal dimension and architecture of trabecular bone. the journal of pathology 178:100–105, 1996. doi:10.1002/(sici)10969896(199601)178:1<100::aid-path429>3.0.co;2-k. 63 https://doi.org/10.1016/j.proeng.2017.05.233 https://doi.org/10.1016/j.nima.2010.06.162 https://doi.org/10.1088/1748-0221/10/08/c08004 https://doi.org/10.1109/tns.2018.2873830 https://doi.org/10.1016/j.apt.2014.02.007 https://doi.org/10.1016/j.cemconcomp.2010.10.021 https://doi.org/10.1007/bf02472918 https://doi.org/10.1016/j.bone.2010.08.023 https://doi.org/10.1002/(sici)1096-9896(199601)178:1<100::aid-path429>3.0.co;2-k https://doi.org/10.1002/(sici)1096-9896(199601)178:1<100::aid-path429>3.0.co;2-k acta polytechnica ctu proceedings 25:58–63, 2019 1 introduction 2 material and methods 2.1 specimen and mechanical properties evaluation 2.2 x-ray computed tomography 2.3 porosity analysis 2.4 fractal dimension estimation 3 results and discussion 4 conclusions acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2020.26.0144 acta polytechnica ctu proceedings 26:144–148, 2020 © czech technical university in prague, 2020 available online at https://ojs.cvut.cz/ojs/index.php/app pre-processing of additive manufacturing input files for numerical simulation jan voříšek∗, bořek patzák czech technical university in prague, faculty of civil engineering, department of mechanics, thákurova 7, 166 29 prague 6, czech republic ∗ corresponding author: jan.vorisek@fsv.cvut.cz abstract. in this contribution, we present the concept of a 3d printer software emulator facilitating the creation of a spatial finite element mesh suitable for the printing process simulation. the concept is based on gradual processing of a native 3d printer input file (in a g-code format). this file contains a complete description of manufacturing process consisting of series of individual commands interpreted by a printer. the effect of each command needs to be precisely evaluated to obtain the position of the printer head and the volume of the deposited material in any given time. the calculation is performed in the same way as in the marlin printer firmware using the trapezoidal motion curves and a command buffer. to represent a computational model, a discrete voxel model with variable edge length and time discretization is used. the volume of deposited material is calculated for each voxel as a function of time. the resulting model is suitable for numerical analysis of the printing process. keywords: additive manufacturing, 3d printing, 3d printer emulator. 1. introduction additive manufacturing emerged as an innovative manufacturing process in the field of rapid prototyping. it is based on incremental deposition or sintering of the material in thin layers. wide range of materials, including plastic, metal or even concrete, can be used to produce spatial objects of complex geometries and shapes (see figure 1). the objects can be designed using currently available cad software. printing a prototype is often more affordable than making one by hand, and certainly less costly than contracting a manufacturer to do it by conventional manufacturing, especially in the fields where low quantities of parts with a high degree of customization and complexity play the critical role. an extensive overview of the additive manufacturing methods is available in [1]. additive manufacturing consists of several independent sub-processes. everything begins with a 3d model which, in order to be printed, must be processed using specialised software called slicer. the slicing process produces a sequence of commands to control the movement and operation of the machine stored in a g-code file line by line. this file describes the complete processing (it is directly interpreted by the printer) and is used as an input for the presented pre-processing tool. visualization of the movement commands can be seen in figure 2 and figure 4. in recent years, there has been an increasing interest in the simulation of the additive manufacturing process. parts produced primarily with extrusion of molten thermoplastics often do not reach designed dimensions due to the cracking and warping induced by large temperature gradients [2, 3]. such issues are also figure 1. visualization of the geometry of a printed object in cad. figure 2. visualization of the movement commands contained in the input file (single layer). moves with material extrusion are drawn with blue color, moves without extrusion with black color. 144 https://doi.org/10.14311/app.2020.26.0144 https://ojs.cvut.cz/ojs/index.php/app vol. 26/2020 pre-processing of additive manufacturing input files present in conventional manufacturing processes like injection moulding where accurate numerical simulation allows to mitigate such issues by optimising the object geometry or the production process itself [4]. numerical simulation of the manufacturing process is indispensable to produce tailored, defect-free and optimised products. due to the complexity of the process, the coupled transient thermo-mechanical analysis is needed for reliable predictions. some research of the additive manufacturing has been already conducted using the finite difference method [3]. in this contribution we assume that the finite element method (fem) will be used to solve the problem. our goal is to develop a software tool able to automatically prepare inputs for the fem model (domain discretization, boundary conditions, etc.) straight from the additive manufacturing input files. not only should our tool generate a suitable 3d mesh, but it should also provide the history of the infill percentage of the single mesh elements. by knowing the infill percentage, we will be able to consider the two-phase characteristics of the voxels (thermoplastics and air). the process is, in a sense, an inverse process of slicing as it produces the simplified (voxelized) version of the original object. several g-code processing utilities are available online [5, 6]. these tools provide print time estimation and filament consumption. however, they only consider the printer moves as vectors, and thus constructing the voxel model with the material extrusion history is not possible. our solution extends the idea of the movement commands into the 3d space considering each of them as a block of material with a finite volume. 2. the model finite element model can generally consist of elements of various shapes. we have chosen a discrete voxel model with variable edge length along each axis due to its simplicity. assuming all the edge lengths equal leaves us with a simple cube. this simplification will allow the precomputation of shape function values and their derivatives as they are same for all the elements. on top of that, such a hexahedral element is commonly used in the fem software packages. 3. g-code pre-processing the first step into the simulation of additive manufacturing is to pre-process the raw input files. these files contain a line-separated set of instructions for the 3d printer to follow. a complete overview of all the available g-codes is available online [7]. only a small subset of the commands, which controls the deposition of the material, is considered. processing of the printer input file begins with determining the duration of individual commands and particularly of printer head moves. this step is performed only once and results can be reused to generate simulation inputs with different resolution. 3.1. duration of moves it is essential to capture the real printing speed in the sequence of point-to-point moves. the target printer speed is defined in the g-code file for each move, yet the actual motion profile is to be decided by the individual printer. constant acceleration is the most used planning strategy resulting in a trapezoidal velocity profile due to its mathematical simplicity and limited computing power of the 3d printers microprocessors. the printer stores a limited number of moves, usually 16, in a buffer and recalculates the acceleration profile allowing the printer to be able to stop after the last move, if needed. the typical printing speed of one print layer can be observed in figure 3. maximal accelerations and velocities must be respected in the direction of each axis. these limits can be taken from the 3d printer specification or the source code of the printer controller firmware (marlin, reprap). duration of the movements can be determined using the calculated velocity and known distance. by gradually summing up the duration of individual commands, we can obtain the current time for any given print head position. this information is essential for numerical simulation as we can reverse this procedure to calculate the position of the print head for any given time (time step). figure 3. visualization of the printer head speed during processing of a single layer assuming the trapezoidal velocity profile. 3.2. reconstructing the extrusion width extrusion width is essential for calculating the intersection of the extrusion volume and the individual voxels to determine the amount of material deposition inside each voxel. it is difficult to define the exact shape of the extrusion cross-section. therefore slicers use various simplified shapes for their internal calculations, most notably a rectangle with curved sides (fig. 6) or a simple rectangle [8]. 145 jan voříšek, bořek patzák acta polytechnica ctu proceedings figure 4. g-code visualization of the first layer of a 20 × 20 × 20mm cube. figure 5. activated voxel elements of a first layer of a 20×20×20mm cube including volume assignment. extrusion volume shape is automatically set in the slicing software based on the nozzle diameter or manually input by the user. however, the information is not present in the exported g-code file, and so it must be recalculated using known variables. to make the calculation slicer-independent, we assume a rectangular cross-section of the deposition. the extruded volume can be calculated as vin = wihili (1) where wi is extrusion width, hi is extrusion volume height and li is extrusion volume length (see figure 6). it must be equal to the volume of material coming into the extruder given as vin = πd2 4 ∆e (2) where d is nozzle diameter and ∆e is length of extruded material. the unknown extrusion width is then expressed as figure 6. extrusion width geometry used to produce g-code in slic3r software [8] wi = πd2 4hili ∆e (3) the simplified calculation produces a slightly narrower extrusion width than the original slic3r value assuming oval-shaped sides of the cross-section. the difference is negligible and will be not be taken into account in any further calculations. for slicers assuming rectangular extrusion cross-sections, the calculated width will match the original value. extrusion width calculation can be omitted for additive manufacturing processes based on the laser beam sintering of material such as stereolitography. in such case the width is defined by the beam diameter. 4. voxel activation and filling the voxel-activation and the extruded volume assignment is performed for each combination of time-step length and voxel-size. see figure 5 for an example of a single processed print layer. 4.1. the principle single depositions, represented by block geometry, are to be decomposed (projected) into the individual voxel contributions. calculating the intersection of two generally oriented blocks would be very computationheavy. fortunately, the deposition of the material occurs in horizontal planes, and so the intersection can be decomposed into two much simpler processes. the first process is the so called horizontal decomposition, where extrusion volume (represented as arbitrarily oriented rectangle is decomposed to individual voxel column contributions (rasterization). in the second step (vertical decomposition), the extruded volume is distributed to individual voxels in a column according to the height and position of extruded material volume. these two subprocesses together provide history of extruded material volume in each voxel. 146 vol. 26/2020 pre-processing of additive manufacturing input files figure 7. activation sequence of the individual voxel elements. the colour of each voxel corresponds to its infill percentage (vof). the centre of the print consists mostly of filled elements shown in red colour while blue elements near the surface of the printed part are almost empty. 4.2. optimization the bounding box of the depositions is calculated to save computer resources, so only the voxels contained within the bounding box are considered for the intersection calculation. furthermore, it is possible to use bresenham’s line algorithm to only mark the affected voxels by doing only integer math. 4.3. issues some issues emerged after implementing the abovedescribed process. slicers usually produce overlapping extrusions (see figure 8) to improve the bonding of the parallel extrusions. the resulting infill history contained over-filled voxels (volume over 100%). the issue is also caused by considering the depositions as non-continuous (see figure 9). the second issue can be eliminated by considering the intersection of successive depositions. the overlapping volume is rotated by 180 degrees and moved into the affected voxels as can be seen in figure 9. figure 8. overlap of the two concurrent depositions (overlapping volume marked with red). figure 9. overlap of the two non-straight depositions (overlapping volume marked with red). the red volume must be redistributed into the green area. the voxel volume is checked after each volume difference assignment, and the surplus volume (over 100%; if applicable) is redistributed into the eight horizontal neighbouring elements to keep the maximum volume of each voxel at 100%. 5. conclusions prototype g-code pre-processor and voxel-model generator were implemented in javascript based on the previously formulated methodology. several realworld files were processed using the tool. example of complex geometry can be seen in figure 7. so far, it appears that it is possible to create voxelbased geometry. the developed tool enables to observe the time sequence of element activation immediately after the input file analysis. for example the model in the figure 7 was processed on a generic consumer laptop in one thread using a 1 × 1 × 1 m voxel grid in 2.6 seconds. it is desirable to perform 3d numerical analysis of the additive manufacturing process using the knowledge of element activation sequence and the vof distribution over time. 6. future work our main goal is to create a robust, user-friendly tool for the numerical analysis of additive manufacturing and its optimization. the tool should be able to take the g-code input file, process it to obtain mesh with element infill history and finally, run fem analysis and post-process the results. this paper solves the preprocessing part as the first step in an overall complex process. for proper numerical simulation, it is desired to identify the thermal, mechanical, as well as processrelated properties of the materials involved. we foresee the formulation of suitable constitutive material models and development of suitable adaptive fe solver. furthermore, we also expect to conduct lab-scale measurements to determine constitutive parameters and develop and calibrate suitable material models. the idea of gradual voxel activation could be extended from to the activation of generally shaped elements of a finite element mesh. the elements generally do not need to form any regular grid. therefore current advanced mesh generators could be used to discretize printed object. such elements can be gradually activated and filled in a same principle as the voxels. however, we expect the intersection of the extrusion block and a general element to be very computationally expensive. 147 jan voříšek, bořek patzák acta polytechnica ctu proceedings acknowledgements the financial support of this research by the grant agency of the czech technical university in prague (sgs project no. sgs19/032/ohk1/1t/11) is gratefully acknowledged. references [1] h. bikas, p. stavropoulos, g. chryssolouris. additive manufacturing methods and modelling approaches: a critical review. the international journal of advanced manufacturing technology 83(1-4):389–405, 2015. doi:10.1007/s00170-015-7576-2. 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[8] g. hodgson, a. ranellucci, j. moe. slic3r manual — flow math. https://manual.slic3r.org/advanced/flow-math. accessed: 2019-07-15. 148 https://doi.org/10.1007/s00170-015-7576-2 https://doi.org/10.1243/09544054jem990 https://doi.org/10.1007/s11837-019-03338-6 https://github.com/hudbrog/gcodeviewer http://www.gcodeanalyser.com/ https://reprap.org/wiki/g-code https://manual.slic3r.org/advanced/flow-math acta polytechnica ctu proceedings 26:145–149, 2020 1 introduction 2 the model 3 g-code pre-processing 3.1 duration of moves 3.2 reconstructing the extrusion width 4 voxel activation and filling 4.1 the principle 4.2 optimization 4.3 issues 5 conclusions 6 future work acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2018.14.0014 acta polytechnica ctu proceedings 14:14–20, 2018 © czech technical university in prague, 2018 available online at http://ojs.cvut.cz/ojs/index.php/app calculation uncertainties in spent fuel inventory determination tomas czakoja,∗, jan fryborta, martin loveckyb a czech technical university in prague, fnspe, dnr, v holesovickach 2, 18000 prague, czech republic b skoda js a.s., orlik 266/15, 31600 bolevec, plzen, czech republic ∗ corresponding author: tomas.czakoj@gmail.com abstract. in the depletion calculation of the nuclear fuel, the uncertainty is of utmost importance, as it affects the uncertainty of the subsequent calculation, when the calculated composition is used. the calculations are even more important when they are safety related, e. g., when determining the reactivity or emissions of radioactivity to the environment. this work covers the depletion model of gd-2m+ fuel which was developed in origen-arp/triton calculation sequences, both being parts of a scale 6.2.1 package. the uncertainties of the respective calculation model were determined by comparison with experimental data for both calculation sequences. the effects of operational and manufacturing uncertainties on concentration of the most important nuclides using triton depletion model of gd-2m+ fuel were determined. the effect of respective uncertainties manifested in changes of composition on the multiplication factor using monte-carlo sequence keno-vi were also specified. keywords: uncertainties, nuclear, fuel, calculation, depletion. 1. introduction the awareness of the uncertainty of a calculation is very important, especially in nuclear reactor applications. inaccurate determination of spent fuel inventory can result in higher decay power, higher emissions or by increased reactivity which can be very dangerous. this work estimates the accuracy of the calculation performed by comparison of experimentally determined spent nuclear fuel inventory with the calculated one. the study of influence of variation in some operational and manufacturing input aspects, such as temperature, power level or enrichment, was conducted. the respective study focuses on the spent fuel inventory determination and calculation of its multiplication factor. 2. calculation methods all calculations described in this paper were made in scale code system version 6.2.1 [1]. the depletion was simulated in triton and origen-arp sequence, the determination of the reactivity important nuclides was carried out in tsunami-2d sequence and the determination of the multiplication factor was obtained with the use of the keno-vi stochastic sequence. both depletion and sensitivity calculations used a multigroup version of endf/b vii.1 nuclear data library, keno-vi simulation used continuous energy version of the aforementioned library [1, 2]. 2.1. depletion model two depletion models of gd-2m+ fuel were created. the fuel used in the depletion models is currently used in vver-440 reactors in dukovany power plant. this fuel assembly contains fuel pins without a burnable figure 1. enrichment of each fuel pin in the gd2m+ fuel assembly. numbers mean enrichment in weight fractions and the hexagon with brown color shows position of fuel pin with burnable absorber. [3]. absorber which does not have a central hole in the fuel pellets. only exception is six fuel pins with gd2o3 which contain a central hole [3]. figure 1 shows the enrichment of each fuel pin in the respective fuel assembly, as well as the position of central tube (ct) and fuel pin with burnable absorber gd2o3. the numbers in figure 1 mean enrichment in weight fractions and the hexagon with brown color shows position of fuel pin with the burnable absorber [3]. the first depletion model was created in triton sequence. the detailed model included the description of geometry, composition, power history and several other important parameters affecting the accuracy of 14 http://dx.doi.org/10.14311/app.2018.14.0014 http://ojs.cvut.cz/ojs/index.php/app vol. 14/2018 calculation uncertainties in spent fuel inventory determination the calculation. this model contained an infinite array of the fuel assemblies with b1 buckling, where the assumed spectrum is identical to the critical one [4]. the depletion model used five cycles with a duration of 330 days with the specific power set to 30.535 w g−1 hm and the shutdown between cycles of 35 days. the time step between depletion was 1.2 days for first 6 days of cycle, next 24 days used 6 days long time step and following 300 days used 20 days long time step. the depletion model was originally set to maximum accuracy and after the creation of the model, many optimization calculations were conducted. each calculation used the basic model with one modified parameter thus quantified the effect of selected reduction. in case the effect was negligible, the reduction was implemented in the final model. the optimized final model was twice faster than the basic one and the accuracy of the results was not affected [1]. the optimized parameters were time steps or legendre order in the transport theory. next optimized parameter was addnux, which adds trace quantities of certain groups of nuclides to the inventories of depletion materials which causes better tracking of these nuclides during the calculation. last optimized parameter was assign. this parameter groups depletion materials. each material of the respective group is independently depleted, but the calculation uses average cross section. the second depletion model was made in origenarp sequence. this sequence, together with the model, is very fast and simple, however it requires a data file from the previous triton calculation. the results are similar to results from slow detailed triton calculation, if an appropriate data file is used. the origen-arp model contained only a homogeneous composition and the power history, no other parameters are required [1]. 2.2. comparison with experimental data because there were no avaliable experimental data of the spent gd-2m+ fuel, it was nessesary to use the data of another fuel type. the data used for the comparison were retrieved from project istc # 3958. the goal of the project was to obtain data from the spent nuclear fuel for description of its composition [5]. the fuel assembly used in the istc project differed from the gd-2m+ fuel assembly in several aspects. the first difference was in enrichment because istc fuel assembly contained fuel pins with constant enrichment. next difference was in the geometry because istc fuel contained central hole and also had moderately different dimensions. because of these distinctions, the istc fuel assembly contained smaller amount of uranium than the gd-2m+ fuel assembly. another significant difference was the absence of any burnable absorber in istc fuel. all these diferences are listed in table 1 [3, 5, 6]. gd-2m+ istc #3958 fuel enrichment 3.6/4.0/4.6 % 4.4 % outer fuel diameter 0.78/0.76 cm 0.757 cm inner fuel diameter 0/0.12 cm 0.12 cm inner cladding radius 0.793/0.773 cm 0.772 cm burnable absorber gd2o3 table 1. differences between gd-2m+ and istc fuel 157gd 156gd 158gd 159tb 155gd 161dy 154gd 239pu 238u 241pu 240pu 135xe 103rh 143nd 133cs 237np 149sm 131xe 242pu 236u 99tc 155eu 153eu 151sm 154eu 152sm 147pm 145nd 243am 109ag 101ru 95mo 238pu 148mpm 150sm 105pd 147sm 134cs 105rh 141pr 139la 241am 235u 234u 16o 242cm* 244cm* 108pd* table 2. nuclides with big impact on the reactivity the istc project # 3958 published information about 12 samples which came from the above mentioned fuel assembly. it includes information about burnup, location of samples in the fuel assembly and also the content of certain nuclides in each sample. based on the published information, two depletion models describing depletion of istc fuel were made. one model was made in the triton sequence and one was made in the origen-arp sequence. the model for comparison was similar to the model described in previous section, only the geometry and composition were modified for description of the fuel assembly from the project [5, 6]. 2.3. influence of uncertainties 2.3.1. effect on the spent fuel inventory study of the influence of uncertainties was performed on changes in a content of group of nuclides in the spent fuel inventory. these nuclides were determined using the tsunami sensitivity calculation which determined that said nuclides affect reactivity considerably. the sensitivity calculation used spent fuel inventory determined by the model described in the section 2.1. the inventory was loaded into the tsunami model, which was obtained from the depletion model by modification of depletion parameters to sensitivity control parameters. determined nuclides are listed in table 2. nuclides listed in this table had the minimal sensitivity of 10−4 in at least one material. three nuclides marked with the star had smaller sensitivity, however they were accounted for because of their significant inaccuracy determined by the comparison with experimental data. effect of the uncertainties on the spent fuel inventory was studied by comparison of the inventory calculated without any applied uncertainty and the inventory calculated with one arbitrary uncertainty applied. uncertainties used in this comparison include changes of parameters describing the operation of the reactor, fuel assembly geometry or composition. 15 t. czakoj, j. frybort, m. lovecky acta polytechnica ctu proceedings quantity nominal lower higher moderator temperature 555 k 541 k 573 k fuel temperature 885 k 785 k 985 k boron content 524 ppm 0 ppm 1049 ppm power level 30.535 g cm−3 27.4815 g cm−3 33.5885 g cm−3 table 3. operational uncertainties uncertainty nominal lower higher absorber content (wt) 3.35 % 3.2 % 3.5 % uranium content 1230/1141 g 1213/1124 g 1247/1158 g enrichment (wt) 3.6/4/4.6 % 3.55/3.95/4.55 % 3.65/4.05/4.65 % outer fuel diameter 0.78/0.76 cm 0.777/0.757 cm inner fuel diameter 0/0.12 cm 0/0.09 cm outer cladding radius 0.91 cm 0.906 cm 0.914 cm inner cladding radius 0.793/0.773 cm 0.799/0.779 cm table 4. material uncertainties element content [%] element content [%] n 2,9e-03 pb 8,0e-04 f 2,2e-02 zn 1,1e-03 na 5,2e-03 cu 1,1e-03 mg 7,0e-03 ni 1,4e-02 si 7,1e-03 co 2,0e-05 cr 3,0e-03 fe 1,6e-02 mn 2,0e-04 table 5. content of impurities [7] complete list of the used uncertainties is presented in tables 3, 4 and 5. all the calculated inventories had equal burnup of 50.4 mwd kg−1 hm, which corresponds to the burnup of spent fuel after five years depletion in the nuclear reactor. because of the preservation of the burnup, cases with lower or higher power level were depleted for longer or shorter time, respectively. 2.3.2. effect on the multiplication factor effect of the uncertainty on the multiplication factor was evaluated by determination of the multiplication factor of spent fuel. the composition of this fuel was calculated in previous part. the evaluation was made in keno-vi 3d stochastic sequence. the model was created by loading of the composition to the gd-2m+ depletion model and its modification for keno-vi sequence. the calculation used 2000 neutron generations with 50000 neutrons per generation and first 50 generations being inactive. the calculation was automatically terminated once the stochastic deviation decreased below 0.00020 because of the limited available computation time. 3. results and discussion 3.1. comparison with experimental data calculated content of selected nuclides in each sample was compared to the experimentally determined content. this comparison was created as a fraction of computed and experimental data, marked as c/e. an arithmetic mean of c/e for every nuclide is in table 6 and in figure 2. as presented in table 6, the results are relatively accurate for most of the elements, such as uranium, cesium or neodymium, but for some elements the inaccuracies are larger, such as for 108pd or 151eu. at the moment, there are no clear reasons for these inaccuracies and more detailed research for clarification of these inaccuracies is needed. 3.2. influence of uncertainties 3.2.1. effect on spent fuel inventory figure 3 depicts changes in the spent fuel inventory after the modification of moderator temperature. results show that the biggest changes in the content are for the isotopes with high cross section in thermal energies. this phenomenon is caused by the worsened moderation, which leads to the worsened absorption in thermal part of the spectrum. changes in the spent fuel inventory after modification of fuel temperature are presented in figure 4. the effect of change of fuel temperature is smaller in comparison with the effect of change of moderator temperature, but is important to take account that change in fuel temperature is usually many times higher than changes in moderator temperature. the change in inventory after the modification of fuel temperature is mainly caused by change of doppler effect. higher temperature decreases the maximum of the resonance, which results in higher resonance absorption. higher absorption at isotope of 238u then leads to higher production of actinides such as plutonium [8]. figure 5 shows results for changes of boron content in the moderator. it is obvious that higher concentration of boron leads to higher content of gadolinium in the spent fuel. this is caused by the worsened moderation which results in lower absorption in thermal part of the spectrum. 16 vol. 14/2018 calculation uncertainties in spent fuel inventory determination c/e c/e c/e nuclide triton origen-arp nuclide triton origen-arp nuclide triton origen-arp 234u 1.03 ± 0.09 1.03 ± 0.09 242cm 1.23 ± 0.14 1.24 ± 0.14 149sm 0.80 ± 0.67 0.80 ± 0.67 235u 0.96 ± 0.00 0.96 ± 0.00 244cm 1.23 ± 0.05 1.23 ± 0.05 150sm 1.07 ± 0.43 1.07 ± 0.43 236u 0.95 ± 0.00 0.95 ± 0.00 241am 1.17 ± 0.01 1.17 ± 0.01 151sm 1.09 ± 0.45 1.09 ± 0.45 238u 0.99 ± 0.00 0.99 ± 0.00 243am 0.91 ± 0.03 0.91 ± 0.03 152sm 1.05 ± 0.37 1.04 ± 0.37 238pu 1.19 ± 0.01 1.20 ± 0.01 133cs 0.99 ± 0.01 0.99 ± 0.01 155gd 1.13 ± 0.15 1.13 ± 0.15 239pu 1.23 ± 0.00 1.23 ± 0.00 134cs 0.94 ± 0.01 0.94 ± 0.01 95mo 0.49 ± 0.15 0.49 ± 0.15 240pu 1.16 ± 0.00 1.16 ± 0.00 135cs 1.15 ± 0.01 1.10 ± 0.01 105pd 1.13 ± 0.62 1.13 ± 0.62 241pu 1.20 ± 0.00 1.20 ± 0.00 137cs 0.99 ± 0.00 0.99 ± 0.00 108pd 2.67 ± 3.73 2.67 ± 3.73 242pu 1.14 ± 0.01 1.14 ± 0.01 151eu 0.39 ± 0.09 0.39 ± 0.09 109ag 0.70 ± 0.11 0.70 ± 0.11 143nd 1.05 ± 0.00 1.05 ± 0.00 153eu 1.12 ± 0.73 1.12 ± 0.73 101ru 1.06 ± 0.40 1.06 ± 0.41 145nd 1.02 ± 0.00 1.02 ± 0.00 154eu 1.22 ± 0.94 1.22 ± 0.94 237np 0.79 ± 0.01 0.79 ± 0.01 146nd 1.01 ± 0.00 1.01 ± 0.00 155eu 1.09 ± 0.68 1.09 ± 0.67 144ce 0.99 ± 0.15 0.99 ± 0.15 148nd 1.01 ± 0.00 1.01 ± 0.00 147sm 1.03 ± 0.36 1.03 ± 0.36 table 6. comparison of calculated and experimental data figure 2. comparison of calculated and experimental data figure 3. effect of change in moderator temperature on composition. similar observation can be seen in the content of samarium. both its isotopes 149sm and 151sm have a high microscopic cross section for absorption in thermal energies. similarly to gadolinium, higher boron figure 4. effect of change in fuel temperature on composition. content worsens moderation which causes lower absorption in samarium isotopes [2]. results for different power levels are shown in figure 6. the power level change affects only few isotopes, content of most of the isotopes remains almost un17 t. czakoj, j. frybort, m. lovecky acta polytechnica ctu proceedings figure 5. effect of change in boron content on composition. figure 6. effect of change in power level on composition. changed. these changes are mainly caused by longer depletion time, which allows for a higher accumulation of certain isotopes in the spent fuel, caused by the decay of their parent nuclides. another reason for the changes in content is connected to the higher production of short-lived fission products and its daughter isotopes for higher power levels, such as 135xe or 105rh [9]. figure 7 shows results of impact of the variation in the gd2o3 content in spent fuel. this variation affects mainly the gadolinium content in spent fuel inventory, yet it also leads to increased content of 161dy and 159tb isotopes, due to them originating from gadolinium according to equations (1) (3) [10]. effect on content of any other nuclides is negligible. 158gd + n→ 159gd β− −−−−−→ 18,479h 159tb (1) 159tb + n→ 160tb  β− −−−→ 72,3d 160dy + n→ 161dy +n→ 161tb β− −−−−→ 6,906d 161dy (2) 160gd+n→ 161gd β− −−−−−→ 3,66min 161tb β− −−−−→ 6,906d 161dy (3) figure 8 shows results of impact of change in the uranium content. as expected, the higher initial content of uranium leads to higher final content of all figure 7. effect of change in gd2o3 content on composition. figure 8. effect of change in uranium content on composition. investigated nuclides, however, the difference between content of various nuclides is visible. the results of variations in the enrichment are depicted in figure 9. the changes in uranium isotopes content were expected because their initial content was also modified together with the enrichment [11]. variations in content of isotopes 242am, 244cm or 242pu are also observed. their content in spent fuel is higher with lower enrichment. this phenomenon is caused by their production from the 238u because the initial content of this isotope also varies with enrichment. another effect of change in enrichment is also connected to modification of neutron spectrum because lower content of 238u leads to lower parasitic absorption. figures 10 and 11 show results for changes in fuel and cladding radii. the impact of fuel radius change is negligible, which is probably caused by modification of fuel density made with the objective of preservation of fuel content. bigger effects observed for fuel pins without gadolinium are obviously caused by their higher number in the fuel assembly. effect of modification of outer cladding radius is higher, which is caused by change of the amount of moderator near to the fuel pin. this change leads to better (or worse) moderation and subsequently to a modification of the neutron spectrum. effect of variation of inner cladding radius is negligible because it only changes the amount of cladding material, which has only small influence on neutrons. 18 vol. 14/2018 calculation uncertainties in spent fuel inventory determination figure 9. effect of change in enrichment on composition. figure 10. effect of change in fuel radius figure 12 shows the effect of impurities in fuel. this effect is both positive and negative and for most applications, it can be considered negligible. nevertheless it is possible to find some cases, in which the impurities can be important. for example, content of nitrogen can lead to production of radioactive 14c, which may have impact on the overall activity [12]. 3.2.2. effect on multiplication factor effect of the operational uncertainties was tested only with investigated uncertainty included in the depletion simulation. this means that the determination of multiplication factor is not influenced by presence of the uncertainty, but it is influenced only by change in inventory. results of this calculation can by seen in figure 13. the biggest impact definitely comes from the change of boron content in the moderator. much smaller, but also important is the effect of change in the moderator temperature. other uncertainties are of smaller importance. both this effects are mainly caused by modification of content of nuclides with high cross section for absorption such as 155gd, 157gd, 149sm and 151sm which negatively influence neutron balance [2]. figure 14 shows the influence of the material uncertainties on the multiplication factor. this influence has smaller importance than the operational uncertainties. the most important uncertainty is initial enrichment of the fuel, which will be mainly caused by change in final content of fissile isotope 235u. second figure 11. effect of change in cladding radius figure 12. effect of impurities in the fuel biggest effects comes from different initial uranium content, which can probably mainly assigned to change in boron content together with changed content of fissile isotopes of plutonium and uranium. figure 15 shows impact of changes in geometry. these uncertainties have only small importance and the most significant is the change in the outer diameter of fuel with gadolinium absorber together with the change in its inner cladding diameter. 4. conclusions two depletion models of gd-2m+ fuel were created and the triton model was optimized for faster calculation. this model was used for the study of influence of the operational and manufacturing uncertainties. comparison of computed and experimental data was conducted for both used depletion sequences. no important difference in results from both sequence was found. obtained results were relatively accurate, but few nuclides, such as 108pd or 151eu, with inaccurate determination were found. the dependence between the inaccuracy of 108pd or 151eu and between the group of tested uncertainties was not found. on the contrary, the sensitivity of 242cm and 244cm content to the boron content in the moderator and to the moderator temperature was found, which is caused by modification of neutron spectrum. the influence of uncertainties on the spent fuel inventory and the multiplication factor was investigated. the determination of the multiplication factors has 19 t. czakoj, j. frybort, m. lovecky acta polytechnica ctu proceedings figure 13. effect of operational uncertainties on multiplication factor only with uncertainty included in fuel depletion figure 14. effect of material uncertainties on multiplication factor allowed for a comparison of importance of each uncertainty. it was found that most important are the operational uncertainties, especially boron content in the moderator and the moderator temperature. although fuel temperature change seems to be much low important than the previously mentioned, it is important to remember that fuel temperature changes are much high than the moderator temperature changes. importance of other investigated operational uncertainties is much lower than the importance of the previous. furthermore, it was discovered that in order to apply the conservative approach, it is necessary to set up lower boron content and higher temperature of moderator. this approach leads in faster depletion of gadolinium burnable absorber which leads in lower absorption. on the contrary, higher fuel temperature leads in higher gadolinium content, but this effect is prevailed by higher production of fissile isotopes of plutonium together with lower 235u depletion. due to this fact, higher fuel temperature is more conservative. effect of all manufacturing uncertainties is lower than the most important operational uncertainties. the most important is correct set up of the enrichment and the uranium content in fuel. it is conservative to overestimate both values. references [1] b. t. rearden, m. a. jessee. scale code system 6.2.1. tech. rep., 2016. doi:10.2172/1326509. figure 15. effect of uncertainties of geometry on multiplication factor [2] m. chadwick, m. herman, p. obložinský, et al. endf/b-vii.1 nuclear data for science and technology: cross sections, covariances, fission product yields and decay data. nuclear data sheets 112(12):2887 – 2996, 2011. special issue on endf/b-vii.1 library, doi:10.1016/j.nds.2011.11.002. [3] p. mikolas, z. fakezasova, m. sasek, et al. zmena casti ppbz pro zavedeni paliva gd-2m+edu: 4.3 jaderne charakteristiky, 2013. [4] j. leppanen, m. pusa, e. fridman. overview of methodology for spatial homogenization in the serpent 2 monte carlo code. annals of nuclear energy 96(supplement c):126 – 136, 2016. doi:https://doi.org/10.1016/j.anucene.2016.06.007. [5] s. pavlov. investigation of the vver-440 fuel burnup and nuclide composition for the use in burnup credit-related tasks (buc-ii) final scientific technical report on task 5, dimitrovgrad. istc project #3958p, dimitrovgrad, 2011. [6] l. markova, f. havluj. specification of simplified benchmark based on #3958 istc pie. nuclear research institute at rez, czech republic, 2011. [7] l. pajo. uo2 fuel pellet impurities, pellet surface roughness and n (18o)/n(16o) ratios, applied to nuclear forensic science. ph.d. thesis, university of helsinki, faculty of science, department of chemistry, 2011. [8] j. r. lamarsh. introduction to nuclear engineering, 2nd edition. addison-wesley, 1983. [9] b. hermansky. dynamika jadernych reaktoru : docasna vysokoskolska ucebnice pro studenty obor jaderne inzenyrstvi. ceskoslovenska redakce vn mon, praha, 1st edn., 1987. vydavatel: csr. ministerstvo skolstvi, mladeze a telovychovy. [10] y. tanoue, t. yokoyama, m. ozawa. resource evaluation of heavy rare earth derived from the spent gd2o3 burnable poison in lwrs. journal of energy and power engineering 10(4), 2016. doi:10.17265/1934-8975/2016.04.004. [11] s. cerne, o. hermann, r. westfall. reactivity and isotopic composition of spent pwr (pressurized-waterreactor) fuel as a function of initial enrichment, burnup, and cooling time. tech. rep., 1987. doi:10.2172/5561567. [12] m.-s. yim, f. caron. life cycle and management of carbon-14 from nuclear power generation. progress in nuclear energy 48(1):2 – 36, 2006. doi:https://doi.org/10.1016/j.pnucene.2005.04.002. 20 http://dx.doi.org/10.2172/1326509 http://dx.doi.org/10.1016/j.nds.2011.11.002 http://dx.doi.org/https://doi.org/10.1016/j.anucene.2016.06.007 http://dx.doi.org/10.17265/1934-8975/2016.04.004 http://dx.doi.org/10.2172/5561567 http://dx.doi.org/https://doi.org/10.1016/j.pnucene.2005.04.002 acta polytechnica ctu proceedings 14:8–14, 2018 1 introduction 2 calculation methods 2.1 depletion model 2.2 comparison with experimental data 2.3 influence of uncertainties 2.3.1 effect on the spent fuel inventory 2.3.2 effect on the multiplication factor 3 results and discussion 3.1 comparison with experimental data 3.2 influence of uncertainties 3.2.1 effect on spent fuel inventory 3.2.2 effect on multiplication factor 4 conclusions references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2021.31.0018 acta polytechnica ctu proceedings 31:18–26, 2021 © 2021 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague possibilities of high-speed railway turnout data description adam hlubuček czech technical university in prague, faculty of transportation sciences, konviktská 20, prague, czech republic correspondence: hlubuada@fd.cvut.cz abstract. this paper aims to summarize possibilities how to create data models of high-speed railway turnouts. the turnouts designed for high-speed operation require specific geometric solution. as the uic railtopomodel is considered an international recommendation in the field of data modeling of railway infrastructure, this issue is assessed in terms of models based on its principles. solving this problem can affect the future development of the multipurpose railway infrastructure model gradually emerging at the ctu railway laboratory in prague using the railtopomodel principles. whereas the railtopomodel itself does not define any specific types of entities, the railml® 3.1 specifications are also used for assessment purposes. turnouts are viewed both in terms of topology and in terms of functional infrastructure. in the final sections, recommendations are given on how to deal with the problems found, e. g. in terms of implementation into the multipurpose railway infrastructure model. keywords: high-speed turnout, railway infrastructure, railtopomodel, railml. 1. introduction a railway turnout is a very complex technical equipment consisting of many components. it allows the vehicle movement to be transferred between two continuing tracks. conventional turnouts can be simply described by a not very large set of parameters, as the radius of each of their branches has a constant value. in order to increase speed in the branching direction, the high-speed turnouts are designed using a more complex geometric solution. this is because they require precise track geometry adapted to limit the high dynamic forces generated by the passage of a vehicle. [1, 2] in order to fulfill multiple needs of many specific process information systems (for example automatic train control systems, automatic train operation systems and train scheduling applications), an exact and unambiguous data description of railway infrastructure is necessary. detailed knowledge of the track geometry parameters is especially essential to calculate the static speed profile, which defines the allowed speed for certain types of railway vehicles all along the route. the ctu railway laboratory in prague focuses to support development of the above-mentioned technologies. [3] among others, the multipurpose railway infrastructure model is being developed at the workplace. the data model is designed to create the infrastructure core database, which structure should enable to meet the needs of related applications and allow to export the data to standardized formats (for example railml® 3). this paper proposes several possible ways how to implement the high-speed turnouts to the data model. 2. selected specifics of high-speed turnouts maximum allowed running speed of a railway vehicle when passing a horizontal curve is limited by the value of unbalanced lateral acceleration (caused by superelevation deficiency) and its rate of change with respect to time. abrupt change of superelevation deficiency and the rate of change of superelevation deficiency as a function of time are often used to express these characteristics. if no superelevation is applied to a segment of a track (or to a branch of a turnout) the speed essentially depends on the radius of curvature and the way it changes. [4] in order to avoid abrupt change of superelevation deficiency, transition curves are usually provided between a straight and curved segment of a track. their purpose is to change the radius from infinity at the straight segment of the track to the radius of the following circular arc gradually, i. e. without abrupt change of curvature. transition curves can be used to join two circular arcs with different radius of curvature as well. nowadays, clothoids are mostly adopted to provide transition curves. the clothoid is a curve with a linear variation of curvature. in the case of a superelevated arc, gradient due to superelevation (also called as superelevation ramp) is often applied within the adjacent transition curve. [5] the most of ordinary turnouts are designed without using transition curves and without superelevation. it causes relatively high values of abrupt change of su18 https://doi.org/10.14311/app.2021.31.0018 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 31/2021 possibilities of high-speed railway turnout data description figure 1. the course of curvature in the turning branch of the j60-1:33,5-8000/4000/14000-phs turnout perelevation deficiency affecting the passing vehicle. in order to reduce lateral forces between wheel and rail while allowing high-speed train operation, it is necessary to adjust the turnout geometric parameters accordingly. high running speed requires high value of radius which significantly affects the shape and size of a turnout. therefore, slim turnouts adapted for high-speed operation usually reach the designed length of several hundred meters. the radius of the branch arc typically reaches a value of several thousand meters also in the turning direction, in such cases. for this reason, long switch rails are necessary for vehicle to be able to turn onto another track. these switch rails often require several point machines to be handled, then. an additional point machine may be required to manipulate the movable parts of the frog. [6] in some cases, the geometry of a high-speed turnout branch is optimized by the means of transition curves application. for example the turnout type designated as j60-1:33,5-8000/4000/14000-phs (which is the first turnout with unstable curvature developed in the czech republic) is designed using two clothoid curves in the turning branch. turning away from the straight direction, the branch begins at the radius of curvature of 8000m, gradually changing to 4000m (at the length of 38m), which is the radius of the following circular arc (of the length of 40m), followed by the second transition curve (54m long, in this case) ensuring change the radius of curvature to 14 000m. the course of curvature in the turning branch of this turnout is shown in the figure 1. continuation of this clothoid can ensure that the inflection point is reached at an appropriate distance so that it allows the construction of a simple crossover assembled from these turnouts with axis spacing of 4,75m. [7] 3. railway infrastructure data description based on the uic railtopomodel the railtopomodel initiative was introduced in 2013 [8] in order to create a common generic standard as regards the railway infrastructure data modeling. in 2016, the railtopomodel was released as uic international railway solution 30100 [9]. the uic railtopomodel [10] represents a topological model of railway infrastructure based on a „connexity graph“. the whole model consists of several subsystems, as they follow: • base • topology • positioning • netentity • location [11] the model makes it possible to describe the railway infrastructure at several different levels of detail. socalled net elements and net relations are used to create the topology of a network. individual net elements can be grouped in order to express an element at a less detailed level. for functional reasons, the most detailed level should be defined with the use of linear elements, which can represent, for example, tracks or line sections (depending on the intended purpose of the specific model use case). it is possible to express certain position within a net element using intrinsic coordinate. this coordinate can take value from 0 to 19 adam hlubuček acta polytechnica ctu proceedings figure 2. the base, network and topology modules of the multipurpose railway infrastructure model 1, whereas 0 means the beginning and 1 indicates the end of a linear net element. two net elements can be joined only at the position of 0 or 1, which is realized by the means of net relations. the navigability attribute defines whether is enabled to pass through the given net relation or not (resp. in which direction). the railtopomodel also allows to define several positioning systems of two types. it is possible to create both linear and geometric positioning systems. linear and geometric coordinates belonging to these positioning systems can be assigned to individual intrinsic coordinates of net elements as well as to appropriate locations of net entities. net entities themselves represent different types of railway infrastructure objects and characteristics which can be localized to the topological layer. the railtopomodel itself do not define any specific types of net entities. these types can be introduced as individual extensions to the model. depending on the nature of localized object or property (eventually level of detail), the net entities can be placed using spot, linear and area type of location. a location of a net entity can be created based on both intrinsic and external (linear or geometric) coordinates. several different locations for the same net entity instance are enabled. linear and area locations can cover several (sections of) net elements which is provided by the associated net element class. in some cases, as regards these locations, it is essential to specify values of the keeps orientation and application direction attributes. [9, 11] currently, the most advanced use case of the railtopomodel approach is apparently the railml® 3 data format, [12] as mentioned in more detail further. 20 vol. 31/2021 possibilities of high-speed railway turnout data description figure 3. the positioning system, net entity and location modules of the multipurpose railway infrastructure model 4. multipurpose railway infrastructure model development in order to deploy infrastructure data repository for several key projects dealing with spatial aspects of the railway domain, the ctu railway laboratory in prague decided to implement the core model, largely based on the railtopomodel principles. the multipurpose railway infrastructure model is a relational data model which directly allows it to be used as a database meeting mysql standards. at the same time, however, it reflects an object-oriented approach to railway data modeling. each type of data object is divided into several tables connected by identifying relations representing successive specializations of the particular object types. by simply joining such interconnected tables, a comprehensive overview of individual specialized objects can be obtained. in addition, this approach allows to easily add, remove and change the attributes common for several different types of objects. whenever possible, the relational model tables reflect the railtopomodel classes in terms of naming, attributes and connections. the base, network and topology modules (as can be seen in the figure 2) basically correspond to the railtopomodel layout, from this point of view. for functional purposes, the cardinality of n:n has been introduced, as regards some relations. in such cases, appropriate interconnection tables are added to the model to express the assignment. in the case of more substantial functional changes, new names of the tables are used in order to avoid confusion. 21 adam hlubuček acta polytechnica ctu proceedings figure 4. the visualization module of the multipurpose railway infrastructure model such changes are primarily about implementation of the associated coordinate and associated net element classes converted to the associated point and associated section tables and adjustment of the concept of the net entity locations which distinguishes between physical and functional (associated) location of net entities. for technical reasons, the mechanism of assigning linear a geometric positioning system coordinates to associated points and physical locations has also been modified. these issues are related to the positioning system, net entity and location modules shown in the figure 3. inspired by the railml® 3.1 specifications, [12] the concept of net entity templates has been added to the the multipurpose railway infrastructure model. the net entity table has been adapted for this purpose. the table is located at the interface between the core and use case-specific part of the model. the model has also implemented the visualization module introduced by the railml® 3.1 data format. in addition to linear and geometric positioning systems, it also allows a user to create screen positioning systems and visualization objects. the visualization module implementation can be seen in the figure 4. the use case-specific part includes concrete types of net entities representing various objects and properties which can be localized to the topological layer. the relevant data classes are specializations of the net entity class allocated within the core model at the interface with the use case-specific part. in order to ensure feasible data export and import, it is possible to design these classes and their attributes taking into account the structure of the railml® 3 exchange format. it is appropriate to focus on the geometry, functional infrastructure and protectively physical assets (not yet defined in version 3.1) substructures of the infrastructure subschema. 5. railml® 3 infrastructure subschema railml® [13] is an open source xml-based data format used in the railway branch. the railway markup langue was originally designed to enable data exchange in the scope of timetabling. later on, the specifications [14] were extended to the subsystems of rolling stocks, infrastructure and interlocking. in recent years, attention has been focused especially on the third version development. as regards the infrastructure schema, railml® 3 is essentially based on the railtopomodel principles, whereas the implemented core model is extended to specific aspects mainly related to the net entity subsystem. the first public version of railml® 3 was released on 19 february 2019. [12] noting the fact that railml® 3 respects the use case-oriented approach, it can be said that railml® 3.1 takes into account the following use cases: • network statement • schematic track plan • interlocking module engineering data • simulation [15] each of the above listed use cases is related to either the infrastructure or the interlocking schema or both. depending on further implementation of other use cases, the particular schemas will be complemented with new aspects, especially new specific types of net entities and their attributes. regarding the railml® 3 file structure, the infrastructure schema is represented by the infrastructure element contained in the top-level railml element. the infrastructure element can contain several other subelements representing particular thematic views, as stated below: • topology • geometry • functional infrastructure • physical infrastructure • infrastructure states • infrastructurevisualizations [12] the allowable structure has been gradually evolving during the the railml® 3 development and was established as described further. the topology module is 22 vol. 31/2021 possibilities of high-speed railway turnout data description essentially based on the railtopomodel topology subsystem. the corresponding topology element includes container elements for net elements, net relations and networks. their subelements represent specific instances of the respective types of topological objects. all available information about net entities is contained in the geometry and functionalinfrastructure elements, as the physicalinfrastructure element does not yet have internal structure defined. these modules also use the pattern of container elements with nested specific instances of net entities of its type. the geometry module consists of horizontal curves, gradient curves and geometry points. these track geometry objects are specializations of the net entity class as defined in the railtopomodel specification. therefore, they can be localized to the topology layer and using coordinates of positioning systems. another group of net entities introduced by the railml® 3.1 infrastructure schema is represented by the functional infrastructure module containing various railway infrastructure facilities and characteristic (e. g. lines, operation points, tracks, switches, crossings, signals, platforms, speeds and track gauges) from a functional point of view. that makes it probably the most complex part of so far evolved railml® 3. next to the functional infrastructure, the module of physical facilities (in the sense of assets) should be developed at a later date, depending on the upcoming use cases. the physicalinfrastructure element does not yet have internal structure defined. furthermore, railml® 3.1 is able to express infrastructure states in relation to individual elements. that makes it more powerful tool than the current version of railtopomodel which can only define one period of validity, as regards selected objects. the last of the railml® 3.1 infrastructure modules deals with infrastructure visualizations. this module enables the infrastructure to be visualized by making it possible to display various objects on the screen using screen coordinates. [12] 6. possible representation of turnouts using the railtopomodel and railml® 3.1 modeling principles a turnout – as the subject of data description – can be viewed from several different points of view. as regards the railtopomodel and railml® 3 methodics, we can distinguish between the topological and the net entity approach. concerning the railml® 3.1 version, it can be described either as part of a functional infrastructure or as an interlocking device. prospectively, the physical infrastructure perspective should also be implemented. [9, 12] 6.1. topological approach regarding the topological point of view, an ordinary turnout can be modeled using three interconnected linear net elements representing the tracks adjacent to the turnout. there is no net element to represent the turnout itself, at this level of detail. the turnout is only implicitly expressed from the functional perspective with the use of net relations connecting the net elements (see the subfigure a) of the figure 5). each net relations connects two net elements in the roles a and b. each net element can be connected at the point with an intrinsic coordinate of the value of 0 or 1. for each net relation, the attribute of navigability must be defined. the navigability attribute can reach the value of ab, ba, both or none expressing the direction in which the net relation is navigable. [9, 12] for modeling the ordinary turnout as described above, two net relations with navigability of the value of both and one net relation with navigability of the value of none are used. it is also possible to express the turnout as a separate non-linear net element connected to three linear net elements. this kind of description can facilitate the localization process of the net entity expressing the appropriate technical equipment of the turnout. nevertheless, there is no possibility to express the navigability attribute in an appropriate manner, as it cannot be distinguished in which direction to continue (or not) after reaching the non-linear net element (see the subfigure b) of the figure 5). the possible solution is to consider the turnout as a linear net element (see the subfigure c) of the figure 5) or to proceed to an ever more detailed level in order to distinguish the individual branches of the turnout to be considered as two separate net elements (see the subfigure d) of the figure 5). the approach of two separate branches allows to easily model directional conditions and other detailed features related to the turnout (especially in the case it is not desirable to express them for the switch as a whole). nevertheless, the necessary number of net elements and net relations increases. in general, the railtopomodel net elements are supposed to be dimensionless. [9] nevertheless, in terms of some use cases, e. g. as regards the railml® 3 format, the need to introduce the length attribute for the net element instances has arisen. [12] in order to assign the value of the length attribute to each net element correctly and precisely, it is desirable to disintegrate the turnout non-linear net element to three separated linear net elements interconnected at one specific point (see the subfigure e) of the figure 5). the internal topology of every single turnout is similar to the first said topological example, then. in addition, there are several possible interpretations of how his model could be explained. the specific point of interconnection may represent the point of the frog, the intersection of the continuous and branching track axis, the point of the track axis at 23 adam hlubuček acta polytechnica ctu proceedings figure 5. possible expressions of an ordinary turnout using net elements and net relations the level of the tip of tongue etc. each of these specific representations has its significance for a particular use case and affects in which way the attribute values are assigned to particular net elements. in particular, this approach makes it possible to create predefined and easily repeatable patterns of internal topological representation of turnouts. 6.2. net entity approach while the topological point of view allows to express turnouts using net elements and net relations, the perspective of net entities makes it possible to describe them with factual parameters and to locate them to the topological layer and using external coordinates. as already mentioned, the railtopomodel specifications itself state no specific types of net entities, as this is a matter of particular use cases. [9] regarding the railml® 3.1 infrastructure schema, there is a switchis complex type defined in order to describe a turnout from the functional perspective. it serves to express the permissible structure of individual switchis elements, nested in the switchesis container element, being a part of the functional infrastructure view. together with the crossing, bufferstop and border complex types, the switchis complex type is one of the specializations of the tracknode complex type derived from the functionalinfrastructureentity complex type. further, this is a specialization of the entityis complex type which is derived from the complex type based on the locatednetentity railtopomodel class. [12] the “is” substring was added to the designation of the complex type in order to distinguish between the infrastructure and interlocking net entity. for this time, we focus on the switch entity as part of the functional infrastructure view, accordingly. the switchis complex type is designed to express all applicable types of turnouts, i. e. including the slip turnouts. it may have the following attributes defined: • basedontemplate • belongstoparent • branchcourse • continuecourse • type [12] the type attribute allows to distinguish between the ordinary switch, outside curved switch, inside curved switch, single switch crossing and double switch crossing constructions. the basedontemplate and belongstoparent attributes are intended to contain a reference to other elements of the switchis complex type (in the role of the template or parent in relation to the described instance). the branchcourse and continuecourse attributes can take the value of left or right expressing the direction of the particular branches. 24 vol. 31/2021 possibilities of high-speed railway turnout data description these turnout branches can be described as separate elements nested in the switchis element. the leftbranch and rightbranch elements are relevant for an ordinary (resp. outside curved or inside curved) switch. the straightbranch and turningbranch elements can be used up to twice within any switchis instance expressing either of the switch crossing types. all the above-mentioned elements expressing any of the turnout branches are of the switchcrossingbranch complex type. [12] for this reason, they can be described by the following attributes: • branchingspeed • joiningspeed • length • netrelationref • radius [12] the branchingspeed and joiningspeed attributes indicate the permitted speed of the described turnout branch in kilometers per hour with respect to the direction of travel (which may be relevant, for example, in the case of some types of ordinary turnouts). the length attribute expresses the length of the turnout branch in meters. the netrelationref attribute allows the turnout branch to be assigned to one of the net relations from the topological layer. finally, the radius attribute means the radius of the arc of the relevant turnout branch expressed in meters. alternatively, the directional conditions of turnout branches can be modeled independently of the relevant switchis elements, using the horizontalcurve complex type from the geometry view. the net entity representing a horizontal curve can be described by the curvetype, azimuth, deltaazimuth, radius and length attributes. a similar approach can also be used to describe the permitted speed in the switch branch. the speed can be expressed using the speedsection elements with the possibility of nesting an unlimited amount of elements referring to related speed profiles. next to the maximum speed, the speedsection complex type makes it possible to express, if the described speed limit is temporary and signaled. [12] 7. assessment of the usability of the railtopomodel and railml® 3.1 modeling principles for the needs of high-speed turnouts description as regards the topological point of view, there is no difference between high-speed turnout and conventional railway turnout, if it is an ordinary turnout. depending on the desired level of detail and functional requirements, any of the above-mentioned approaches to modeling the railway network topology can be useful. if necessary, the branches of high-speed turnouts can be modeled in even more detailed level, which the approach of net elements connected by net relations seamlessly allows. these net elements of more detailed levels can be standardly aggregated in order to create the less detailed ones. the only possible difference from the current state of the railtopomodel and railml® 3.1 data format may lie in the fact that new values of the descriptionlevel attribute should be introduced if the more detailed topological description needed. only the macro, meso and micro values are currently available, while the macro and meso levels are not sufficiently detailed to show turnouts. although the irs 30100 document [9] also outlines the nano level, it has not been clearly specified and included among the usual levels yet. expressing high-speed turnouts as net entities (if we focus on the functional infrastructure view) is more problematic. the railtopomodel itself is not in charge to express so specific types of objects such as turnouts at all. [9] although the railml ® 3.1 format provides a relatively extensive and sophisticated apparatus for describing the basic characteristics of railway turnout, [12] it is still not sufficient to express all the geometric properties of high-speed turnouts properly. some turnouts designed for high-speed operation, as seen in the the j60-1:33,5-8000/4000/14000phs turnout type example, have a relatively complicated geometric solution of the branches. the single radius attribute is far from sufficient to describe all directional conditions. it would be necessary to define several new attributes to express required properties of the above-mentioned turnout type. its turning branch consist of three different curves, two of which are transition curves with unstable curvature. although such parameterization of directional conditions would be feasible, it would never be able to cover all possible geometric solutions of turnout branches (including those that may be developed in the future). many of these parameters would be completely unnecessary when describing most other turnouts. a more progressive approach is to extend the allowable internal structure of the switchcrossingbranch complex type elements by introducing nested elements expressing subsections of a turnout branch. each such subsection could represent a segment describable by one curve type. such segments could be described by similar attributes as the horizontalcurve elements. these elements make it possible to distinguish between different curve types, such as straight, arc and clothoid. [12] nevertheless, a significant disadvantage lies is the fact that it is not possible to assign the radius of the arc at different positions of the horizontal curve directly. this problem arises especially when describing transitions curves. for this reason, it would be appropriate at least to replace the radius attribute of the horizontalcurve complex type with two similar attributes expressing the radius at the start point and at the end point of the described curve. 25 adam hlubuček acta polytechnica ctu proceedings another solution for one of the railml® 3 upcoming subversions is not to describe turnouts in terms of geometry within the switchis elements at all and use the description of directional conditions exclusively through the horizontalcurve elements instead. this solution would require modifying the horizontal curve description structure similar to the previous proposal. in addition, it would be appropriate to provide the possibility to assign individual horizontal curve to a specific turnout branch. as another point, the turnout branch entity itself could only be described using an aggregated data item of minimal radius, as this is critical in many respects. in cases where this would be sufficient, such a value could be used as the only descriptive aspect of the geometry, if so described. it is always a matter of a specific use case as accurate and detailed description is required. 8. conclusions this article provided insight into the specifics of highspeed railway turnouts and the possibilities of their data modeling. when compared with the principles of the railtopomodel and railml ® 3.1 data format, it was found that there is no significant difference between a conventional ordinary turnout data model and a possible high-speed turnout model, as regards the topological point of view. nevertheless, the functional infrastructure interpretation of a turnout – as offered by the railml ® 3.1 infrastructure model – is insufficient, in this respect. some high-speed turnouts require the geometry to be modeled in a much more precise way. if it is decided to include such turnouts in the multipurpose railway infrastructure model, there are several ways to implement it. we can either increase the number of attributes or extend the internal structure of the turnout data description or solve the geometry of the turnout separately and reference it using external links. these approaches would allow an accurate and detailed description of a high-speed turnout. it is also possible to use only an aggregated data item expressing the value of the minimum radius in each turnout branch instead of the detailed description of its geometry. such a simplified approach is appropriate especially in less demanding use cases. references [1] m. hamadache, s. dutta, o. olaby, et al. on the fault detection and diagnosis of railway switch and crossing systems: an overview. applied sciences 9, 2019. https://doi.org/10.3390/app9235129. [2] m. hamarat, s. kaewunruen, m. papaelias. a life-cycle cost analysis of railway turnouts exposed to climate uncertainties. iop conference series: materials science and engineering 471, 2019. https://doi.org/10.1088/1757-899x/471/6/062026. [3] m. leso, d. kamenický, p. koutecký, a. hlubuček. dopravní sál fd – železniční laboratoř pro výuku i výzkum. vts správy železnic 1, 2019. [4] m. kapitsa, d. bobyr, a. desiak. determining permissible speed of tilting train in curved track. science and transport progress bulletin of dnipropetrovsk national university of railway transport 70: 29–40, 2017. https://doi.org/10.15802/stp2017/109537. [5] m. lindahl. track geometry for high-speed railways. tech. rep., royal institute of technology, stockholm, 2001. [6] h. ossberger. modern turnout technology for high speed [online]. https://www.unece.org/fileadmin/ dam/trans/doc/2016/ter/heinz_ossberger_modern_ turnout_technology.pdf. [7] l. raif, b. puda, j. havlík, m. smolka. design of high-speed turnouts and crossings. iop conference series: materials science and engineering 236, 2017. https://doi.org/10.1088/1757-899x/236/1/012044. [8] uic. feasibility study uic railtopomodel and data exchange format [pdf], 2013. http://www.railtopomodel.org/files/download/ railtopomodel/270913_trafit_ finalreportfeasibilitystudyrailtopomodel.pdf. [9] uic. railtopomodel v1.0 [pdf], 2016. http://www.railtopomodel.org/en/download/ irs30100-apr16-7594bca1524e14224d0.html?file= files/download/railtopomodel/180416_uic_ irs30100.pdf. [10] railtopomodel. railtopomodel [online], ©2020. http://www.railtopomodel.org. [11] uic. railtopomodel v1.1 [online], 2017. https://uic.org/rail-system/railtopomodel. [12] railml. railml schema version 3.1 [xsd], 2019. [13] railml. railml [online], ©2002–2020. https://www.railml.org. [14] railml. the railml subschemas [online], ©2002–2020. https://www.railml.org/en/user/subschemes.html. [15] railml. use cases [online], ©2002–2020. https://www.railml.org/en/user/use-cases.html. 26 https://doi.org/10.3390/app9235129 https://doi.org/10.1088/1757-899x/471/6/062026 https://doi.org/10.15802/stp2017/109537 https://www.unece.org/fileadmin/dam/trans/doc/2016/ter/heinz_ossberger_modern_turnout_technology.pdf https://www.unece.org/fileadmin/dam/trans/doc/2016/ter/heinz_ossberger_modern_turnout_technology.pdf https://www.unece.org/fileadmin/dam/trans/doc/2016/ter/heinz_ossberger_modern_turnout_technology.pdf https://doi.org/10.1088/1757-899x/236/1/012044 http://www.railtopomodel.org/files/download/railtopomodel/270913_trafit_finalreportfeasibilitystudyrailtopomodel.pdf http://www.railtopomodel.org/files/download/railtopomodel/270913_trafit_finalreportfeasibilitystudyrailtopomodel.pdf http://www.railtopomodel.org/files/download/railtopomodel/270913_trafit_finalreportfeasibilitystudyrailtopomodel.pdf http://www.railtopomodel.org/en/download/irs30100-apr16-7594bca1524e14224d0.html?file=files/download/railtopomodel/180416_uic_irs30100.pdf http://www.railtopomodel.org/en/download/irs30100-apr16-7594bca1524e14224d0.html?file=files/download/railtopomodel/180416_uic_irs30100.pdf http://www.railtopomodel.org/en/download/irs30100-apr16-7594bca1524e14224d0.html?file=files/download/railtopomodel/180416_uic_irs30100.pdf http://www.railtopomodel.org/en/download/irs30100-apr16-7594bca1524e14224d0.html?file=files/download/railtopomodel/180416_uic_irs30100.pdf http://www.railtopomodel.org https://uic.org/rail-system/railtopomodel https://www.railml.org https://www.railml.org/en/user/subschemes.html https://www.railml.org/en/user/use-cases.html acta polytechnica ctu proceedings 31:18–26, 2021 1 introduction 2 selected specifics of high-speed turnouts 3 railway infrastructure data description based on the uic railtopomodel 4 multipurpose railway infrastructure model development 5 railml® 3 infrastructure subschema 6 possible representation of turnouts using the railtopomodel and railml® 3.1 modeling principles 6.1 topological approach 6.2 net entity approach 7 assessment of the usability of the railtopomodel and railml® 3.1 modeling principles for the needs of high-speed turnouts description 8 conclusions references acta polytechnica ctu proceedings doi:10.14311/app.2018.15.0051 acta polytechnica ctu proceedings 15:51–56, 2018 © czech technical university in prague, 2018 available online at http://ojs.cvut.cz/ojs/index.php/app determination of hygroexpansion coefficients of wood using analytical and numerical homogenization lucie kucíková, michal šejnoha∗, jan vorel czech technical university in prague, faculty of civil engineering, thákurova 7, 166 29 prague 6, czech republic ∗ corresponding author: sejnom@fsv.cvut.cz abstract. this paper is concerned with the determination of hygroexpansion coefficients of wood. wood is a naturally hygroscopic material, attracting moisture from its surroundings. its dimensions change depending on the actual moisture content. this dependence is characterized by the coefficient of hygroexpansion. for complex microstructures, this coefficient can be determined, similarly to the coefficient of thermal expansion, either from analytical or numerical homogenization. the homogenization procedure, used in this paper, comprises several steps corresponding to the wood structural composition. accuracy of this approach is governed by correctness of the chosen input parameters. one of the most important parameters are the volume fractions of the earlywood and latewood. ascertained dependence of the earlywood and latewood volume fractions on dry wood density is also presented. keywords: wood, homogenization, hygroexpansion coefficient. 1. introduction environmentally friendly materials are getting to the forefront of global interest and one of them is also wood. the wood is a natural cellular material and as such its properties change depending on growth site and species. spruce wood, examined in this paper, belongs to conifers and therefore consists mainly of tracheids [1], long closed tube-like cells with cavity inside (called lumen). there are two types of tracheids. the first one is characterized by almost square crosssection and thin cell wall, and it forms the earlywood part of the growth ring. the second one forms the latewood part of the growth ring. compared to the previous one, its cross-section is almost half in width in the radial direction and the cell wall is much thicker. generally, a multilayered cell wall structure is simplified to three layers [2]: m layer (middle lamella, primary wall and outer layer of secondary wall s1), s2 layer (middle layer of secondary wall), and s3 layer (inner layer of secondary wall). each layer is composed of four basic elements: lignin, hemicellulose, cellulose (crystalline and amorphous) and extractives. due to their low volume fraction, the extractives are often neglected, as was also the case in the present study. the cellulose and hemicellulose are in the form of microfibrils, bonded together by lignin [3]. the microfibrils wound helically along the cell and their deviation from a longitudinal axis is called a microfibril angle (mfa). the microfibril angle generally changes within the cell wall, but for simplicity it is assumed constant in individual layers. clearly, it is one of the main factors influencing the wood properties, see e.g. [4] wood is a naturally hygroscopic material, i.e. it attracts moisture from surrounding atmosphere. the water is present in wood in two forms: free water (i.e. liquid water or water vapor in pores) and bound water (i.e. liquid water in the cell wall). the amount of water present in wood is characterized by the moisture content (mc), defined as the ratio of the mass of water to the mass of oven-dried material. moisture content at which the cell walls are fully saturated and with no free water in the lumens is called a fiber saturation point (fsp). the average value of fsp is about 30% [5]. interestingly, several definitions of fiber saturation point can be found in literature [6]. below this point, the major part of dimensional changes occurs in the form of shrinkage (contraction) or swelling (expansion). the resulting moisture strain is linked to the change in moisture content through the hygroexpansion coefficient αh [-]. estimating this coefficient at the level of wood is the principal objective of this paper. to that end, several homogenization techniques combining classical micromechanics and unit cell simulations will be adopted. these are briefly outlined in section 3 preceded in section 2 by microstructure quantification, both in terms of material composition and geometry. the resulting predictions are presented in section 4 followed by the summary of essential observations in section 5. 2. materials special attention should be paid to the selection of input parameters used in homogenization. cell wall constituents are very small particles, which are difficult to measure. in this paper, properties presented 51 http://dx.doi.org/10.14311/app.2018.15.0051 http://ojs.cvut.cz/ojs/index.php/app l. kucíková, m. šejnoha, j. vorel acta polytechnica ctu proceedings e11 e22 = e33 g12 = g13 ν21 = ν31 ν22 = ν33 αh,11 αh,22 = αh,33 [gpa] [gpa] [gpa] [-] [-] [-] [-] cellulose 150.00 17.50 4.50 0.01 0.50 0.000 0.000 hemicellulose 16.00 3.50 1.50 0.10 0.40 0.000 1.368 lignin 2.75 2.75 1.03 0.33 0.33 0.351 0.351 table 1. phase properties at cell wall level [2]. in [2] were taken as the most relevant and used for computations. properties of individual cell wall constituents are summarized in table 11. besides the properties, it is necessary to know the volume fractions of individual constituents within each cell wall layer. these are stated in table 2. in each layer the hemicellulose and cellulose are deviated by a different microfibril angle. the values of mfa in m and s3 layers were taken from [2]. the mfa of s2 layer was computed by comparing the values found from homogenization and nanoindentation measurements, see [7]. layer m s2 s3 fcellulose 0.20 0.49 0.49 fhemicellulose 0.15 0.27 0.27 flignin 0.65 0.24 0.24 mfa [°] 45.00 32.53 75.00 table 2. volume fraction of cell wall constituents and mfa of individual cell wall layers [2]. nanoindentation is a nano-/micro-scale measurement technique based on contact mechanics, usually used to determine the material hardness and elastic modulus. the method grounds on pushing a small hard tip with known geometry and properties to the tested material while simultaneously recording the applied load and the measured displacement. herein, only the s2 layer, as the principal carrier of the cell wall stiffness, was indented using the berkovich indenter. a certain hold period at the level of maximum force was considered within each loading cycle to eliminate the viscoelastic response of the material. the indentation moduli were then estimated from the unloading part of the loading diagram, see e.g. [8], [9] for more details. according to the theory of anisotropic indentation presented in [10], the indentation modulus can also be defined in terms of the homogenized stiffness matrix pertinent to the s2 layer while taking into account the microfibril angle. note that the effective stiffness matrix in the local coordinate system can be acquired for example from eq. (5) adopting the material data from table 1 and the volume fractions from table 2. the stiffness matrix entering the theoretical expression of indentation modulus further calls for transformation 1axis-1 is oriented along longitudinal (microfibril) direction, axes-22,33 along transverse direction. into the mfa coordinate system. the actual mfa is determined by matching the measured and theoretically calculated indentation moduli as presented, e.g. in [11, 12]. the resulting mfa for s2 layer is listed in table 2. the distribution of indentation modulus as a function of mfa and the probability density function of mfa derived from a large set of indents are plotted in fig. 1 for illustration. (a) (b) figure 1. a) variation of indentation modulus as a function of mfa, b) probability density function of mfa however, a word of caution is in place when adopting the results from nanoindentation measurements directly without any adjustment for potential experimental error. as demonstrated in [13] such a step may significantly underestimate the resulting effective stiffness when compared, e.g. to macroscopic measurements of the effective longitudinal elastic modulus. consequently, the predicted coefficients of hygroexpansion may also be highly inaccurate. to address this issue we provide, apart from the measured s2 layer mfa=32.53◦, the results for mfa=10◦, 15◦, 20◦ for the sake of illustration. the numerical part of homogenization requires the formulation of a periodic unit cell (puc) representing the material microstructure. altogether, three types of pucs were developed to represent the earlywood, latewood, and equivalent wood, while accounting for real microstructural dimensions. the equivalent wood is a combination of both the earlywood and latewood and was created such as to reflect the volume fraction of lumens of the two types of wood within a single growth ring. apart from that, the volume fractions of individual layers building the cell wall are needed. herein, we adopt the particular values from [2]. to get the volume fraction of lumens corresponding to an equivalent wood requires the knowledge of the volume fractions of lumens of the earlywood and latewood as well as the volume fraction of earlywood and latewood at the level of growth rings. a potential 52 vol. 15/2018 determination of hygroexpansion coefficients method of attack to derive these quantities is provided by image analysis. color and binary images of the earlywood of spruce are presented in figs. 2(a,b) for illustration. (a) (b) (c) figure 2. a) original gray-scale image of earlywood, b) transformed binary image of earlywood, c) images of growth rings. these images were taken at several locations within several growth rings to increase reliability of the estimated volume fractions, at least on the average sense. the variability of microstructure over the length of the growth ring as well as the transition zone (a gradual change between earlywood and latewood) were, therefore, neglected. fig. 2(c) then offers similar images at the level of growth rings. considering a constant width of the latewood part equal to 0.2 mm, see also [2], these images allow for obtaining the dependence of the earlywood volume fraction on dry wood density plotted in fig. 3. figure 3. dependence of the earlywood volume fraction on dry wood density [kg/m3]. thus, given the dry wood density ρdry [kg/m3] we can estimate the volume fraction of earlywood few directly from the fitting equation2 few = 5 · 10−6 · ρ2 dry − 5 · 10−3 · ρdry + 2.093, (1) without passing through a tedious measurement process. the volume fraction of latewood is then simply given by flw = 1− few . (2) it is now possible to define the equivalent wood as basic weighted average of both the earlywood (ew) and latewood (lw). the volume fraction of lumens of equivalent wood flum is therefore flum = few · flum,ew + flw · flum,lw , (3) where flum,ew , flum,lw are the volume fractions of lumens of earlywood and latewood, respectively. it is now easy to get the desired dimensions of the wood cells. figure 4. periodic unit cells of earlywood, latewood, and equivalent wood. the resulting periodic unit cells are shown in fig. 4. when constructing these unit cells we adopted the inner dimensions of lumens, i.e. the dimensions of the inner rectangle of the earlywood and latewood pucs, from [1]. considering the volume fractions of lumens as flum,ew = 0.69 for the earlywood, flum,lw = 0.12 for the latewood [7], and dry wood density ρdry=390 kg/m3 allowed us to calculate the cell wall thickness. the volume fractions of individual layers in the cell wall rendered its final subdivision distinguishing again m layer (blue, b), s2 layer (red, r), and s3 layer (green, g). the actual thicknesses are provided in the left bottom corner of the picture for the sake of completeness. the dimensions of the equivalent wood (eqw) were derived in analogy with eq. (3) as the weighted average of the dimensions of earlywood and latewood, respectively. the actual arrangement of the unit cells in fig. 4 may range from perfectly regular to hexagonal array. with reference to fig. 2 the latter one is considered in the present study. the discretized transverse crosssection of the 3d unit cells are plotted in fig. 5. some improvements in the prediction of effective properties 2point out that this simple quadratic equation is only applicable for the plotted range of dry wood density (ρdry ∈ (350−500) [kg/m3]). 53 l. kucíková, m. šejnoha, j. vorel acta polytechnica ctu proceedings can be expected if addressing the microstructure variability more closely [2]. in the light of the present approach this would call for so called statistically equivalent periodic unit cell, which is more refined and matches the actual microstructure in view of twopoint correlation function [14]. (a) (b) figure 5. geometry and plane view of discretized pucs: a) earlywood, b) latewood. 3. methods wood is often treated as a natural composite. its structural composition leads to application of hierarchical homogenization, starting from the level of cell wall constituents up to the level of the solid wood. in this case, a combination of analytical and numerical homogenization is used to determine the effective hygroexpansion coefficients. however, according to the formula for the overall strain ε = εe + εh, εh = αhu, (4) where εe denotes the elastic strain, εh the hygroexpansion strain, αh the vector of hygroexpansion coefficients3 and u the change of moisture content, the effective hygroexpansion coefficients have to be computed simultaneously with the effective mechanical properties. the computation comprises several steps. the first step corresponds to the level of cell wall layers. each layer (m, s2, and s3) is composed of three phases, where lignin is considered as a matrix phase and hemicellulose and cellulose as circular cylindrical inclusions. properties of individual phases are stated in table 1 and their volume fractions within each layer appear in table 2. using the mori-tanaka method, one of the most popular analytical methods, we arrive at the estimates of the effective stiffness matrix lmt in the form lmt = l0+ ( n−1∑ r=1 fr(lr − l0)adill r ) ( f0i + n−1∑ r=1 fradill r )−1 , (5) 3note that αh is assumed constant independent of moisture. experimental measurements are currently under way to support this assumption. where l0 and lr are the stiffness matrices of the matrix phase (lignin) and inclusions (hemicellulose, cellulose), i is the second order identity tensor, f0 and fr are the volume fractions of corresponding phases, and adill r is the partial strain concentration factor defined as adill r = ( i + srl−1 0 (lr − l0) )−1 , (6) where sr is the eshelby tensor depending on the shape of the inclusion. the effective hygroexpansion coefficients are computed using the levin formula as αhom = n−1∑ r=0 fr(bmt r )tαr, (7) where αr denotes the vector of hygroexpansion coefficients of individual phases, bmt r is the stress concentration factor derived again from the mori-tanaka method. for individual phases we get bmt 0 = [f0i + n−1∑ r=1 frbdill r ]−1, bmt r = bdill r bmt 0 , r = 1, . . . , n − 1, bdill r = [i + l0(i− sr)(l−1 r − l−1 0 )]−1. (8) although analytical homogenization may still be exploited even above the cell wall level, we abandon this approach and turn our attention to the periodic unit cell representation of cellular microstructure, recall fig. 5. this approach builds upon the application of the hill lemma 〈δε(x)tσ(x)〉 = δet σ, (9) within the framework of the 1st order homogenization theory. the angle brackets in eq. (9) denote the volume averaging. bearing in mind that the evaluation of effective hygroexpansion coefficients is identical to that of thermal expansion, we follow [14] and consider the stress based formulation. we begin with the local constitutive law written as σ(x) = l(x)(ε(x)−αh(x)u), (10) where l is the local stiffness matrix, αh is the vector comprising hygroexpansion coefficients and u denotes the moisture content change. within the 1st order homogenization the local strain ε(x) is split into two parts ε(x) = e + ε∗(x), (11) where e is a macroscopic part constant over the puc and ε∗(x) is a fluctuation part, which has to disappear upon volume averaging. this can be achieved, e.g. through the application of periodic boundary conditions [14] in the solution of the discretized system of governing equations (13). the fem discretization provides the local strain in the form ε(x) = e + b(x)r, (12) 54 vol. 15/2018 determination of hygroexpansion coefficients e11 e22 e33 g23 g13 g12 ν21 ν31 ν32 α11 α22 α33 [gpa] [gpa] [gpa] [gpa] [gpa] [gpa] [-] [-] [-] [1/k] [1/k] [1/k] m layer 34.24 4.62 3.96 1.32 1.32 1.60 0.03 0.04 0.41 0.0246 0.3920 0.5410 s2,3 layer 78.72 7.15 6.01 2.05 2.05 2.46 0.02 0.02 0.41 0.0078 0.3247 0.5089 table 3. effective properties at cell wall level from analytical homogenization. where r represents the nodal displacements of the fluctuation displacement field u∗. substituting eq. (12), combined with eq. (10), back into eq. (9) and collecting terms associated with δr and δe finally gives the aforementioned discretized system of algebraic equations in the form[ ∫ ω l dω ∫ ω l b dω∫ ω bt l dω ∫ ω bt l b dω ]{ e r } ={ ως + ∫ ω lαhudω∫ ω bt lαhudω } , (13) with the macroscopic strain e and the nodal fluctuation displacements r as unknown quantities. solving the system (13), while setting the macroscopic stress σ = 0 and the moisture change u = 1, provides the effective hygroexpansion coefficients as components of the macroscopic straine. this step takes into account the microfibril angle as a deviation of inclusions from the longitudinal axis. thus for each segment along the perimeter of the cell wall a proper transformation of coordinates from local to global coordinate system is needed [2]. if using the puc corresponding to equivalent wood we get the effective coefficients of hygroexpansion directly at the level of the solid wood. on the contrary, employing the unit cells pertinent to the earlywood and latewood requires an additional homogenization step, where the corresponding effective stiffness matrices are also needed. this can be achieved by setting u = 0 in eq. (13) and load the unit cell in turn by each of the six components of σ set equal to one to get individual columns of the 6× 6 compliance matrix mhom as corresponding strain volume averages. the required effective stiffness matrix lhom then follows from simple matrix inversion. once having the effective stiffnesses of the earlywood and latewood we may proceed with the derivation of αhom at the level of solid wood. in such a case one may accept the similarity between the structure of growth rings and a laminate and adopt classical lamination theory, see e.g. [15]. nevertheless, we proceeded in the footsteps of the previous calculations and performed fem homogenization of a two layer puc where individual layers represent the earlywood and latewood, respectively. 4. results evaluation of effective coefficients of hygroexpansion of wood required several computational steps on individual scales. the homogenized elastic stiffnesses and coefficients of hygroexpansion of individual layers of the cell wall were determined first using the mori-tanaka method, particularly eqs. (5) and (7). the resulting effective properties assuming mfa=0 in all layers are stored in table 34. as these refer to the level of cell wall, they are the same for earlywood, latewood and equivalent wood. 0 10 20 30 40 50 mfa [deg] 0 5 10 15 20 e la s ti c m o d u li [g p a ] e l e t e re l = 14 gpa e l = 6 gpa 15 32.53 (a) 0 10 20 30 40 50 mfa [deg] 0 0,1 0,2 0,3 0,4 α [ − ] α h,l α h,t α h,r (b) figure 6. variation of selected homogenized properties as a function of mfa: a) elastic moduli, b) coefficients of hygroexpansion. the effective coefficients of hygroexpansion of earlywood, latewood and equivalent wood followed directly from the application of eq. (13) at the level of lumens. therein, the properties from table 3 were properly adjusted to account for the variation of mfa in individual cell wall layers, recall table 2. when adopting the earlywood and latewood pucs in fig. 5 we exercised eq. (13) one more time at the level of growth rings on a laminate-like puc. for illustration we plot the variation of the selected homogenized moduli and 4axis-1 is oriented in longitudinal direction, axis-2 in circumferential direction, axis-3 in cell wall thickness direction. 55 l. kucíková, m. šejnoha, j. vorel acta polytechnica ctu proceedings coefficients of hygroexpansion as a function of mfa5. more specifically, the mfa in s2 layer was varied from 0◦ to 50◦ while keeping the mfa values from table 2 for the m and s3 layers. based on the results from simple tensile tests the expected mfa would range from 10◦ to 20◦. as seen in fig. 6(a) the value of mfa=32.53◦ would greatly underestimate the wood stiffness. thus to compare the resulting predictions of the homogenized coefficients of hygroexpansion provided by the 3 step (cell wall level, level of lumens, level of annual rings) and 2 step (cell wall level, level of lumens assuming equivalent wood) homogenization we set mfa=15◦. the corresponding results are available in table 4. comparing the last two rows it can be seen that the equivalent wood puc can safely be used in homogenization analyses thus skipping the last homogenization step at the level of annual rings. αh,l αh,t αh,r earlywood 0.002 0.34 0.21 latewood -0.003 0.32 0.39 solid wood 0.001 0.33 0.23 equivalent wood 0.001 0.34 0.23 table 4. effective coefficients of hygroexpansion of wood from numerical homogenization (mfa=15◦). 5. conclusions in this paper the mori-tanaka micromechanical scheme was effectively combined with numerical simulations based on the 1st order homogenization theory. while the mori-tanaka method provided the effective properties of individual cell wall layers, numerical homogenization was adopted at the level of tracheids and annual rings. in addition, an equivalent wood was introduced in order to avoid the last homogenization step at the level of annual rings, which is needed when the earlywood and latewood are considered independently. this will prove useful when modeling the evaluation of moisture content in wood through the application of multi-fickian moisture transport model [16, 17]. as seen in table 4, the results for the solid wood and equivalent wood are very similar. this clearly promotes the applicability of equivalent wood and thus the derivation of effective properties through a single homogenization step above the cell wall level. inadequacy of using mfa obtained directly from nanoindentation was also mentioned. acknowledgements the financial support provided by the gačr grant no. 1805791s and by the czech technical university in prague within sgs project with the application registered under the no. sgs17/168/ohk1/3t/11 is gratefully acknowledged. 5l stands for longitudinal (lumen) direction, t for tangential direction, r for radial direction. references [1] p. o. kettunen. wood: structure and properties. trans tech publications ltd, uetikon-zuerich, 2006. [2] k. persson. micromechanical modelling of wood and fibre properties. ph.d. thesis, lund university, 2000. [3] h. e. desch, j. m. dinwoodie. timber: structure, properties, conversion and use. 7th. macmillan press, hampshire, 1996. [4] l. donaldson. microfibril angle: measurement, variation and relationships–a review. iawa journal 29(4):345–386, 2008. [5] r. j. ross, et al. wood handbook: wood as an engineering material. usda forest service, forest products laboratory, general technical report fpl-gtr-190, 2010: 509 p 1 v 190, 2010. [6] e. t. engelund, l. g. thygesen, s. svensson, c. a. hill. a critical discussion of the physics of wood–water interactions. wood science and technology 47(1):141–161, 2013. [7] l. kucíková. micromechanical study of spruce wood. master’s thesis, czech technical university in prague, 2018. [8] w. c. oliver, g. m. pharr. measurement of hardness and elastic modulus by instrumented indentation: advances in understanding and refinements to methodology. journal of materials research 19(1):3–20, 2004. [9] m. l. oyen, r. f. cook. a practical guide for analysis of nanoindentation data. journal of the mechanical behavior of biomedical materials 2(4):396–407, 2009. [10] j. vlassak, m. ciavarella, j. barber, x. wang. the indentation modulus of elastically anisotropic materials for indenters of arbitrary shape. journal of the mechanics and physics of solids 51(9):1707–1721, 2003. [11] a. jäger, t. bader, k. hofstetter, j. eberhardsteiner. the relation between indentation modulus, microfibril angle, and elastic properties of wood cell walls. composites part a: applied science and manufacturing 42(6):677–685, 2011. [12] m. šejnoha, l. kucíková, j. vorel, et al. comparing nano and macroindentation in search for microfibril angle in spruce. internationl journal of computational methods and experimental measurements 5(2):135–143, 2017. [13] t. janda, l. kucíková, j. vorel, et al. bayesian inference as a tool for improving the prediction of effective elastic properties of wood. advances in engineering software 2018. submitted. [14] m. šejnoha, j. zeman. micromechanics in practice. wit press, southampton, boston, 2013. [15] g. w. milton. the theory of composites. cambridge monographs on applied and computational mathematics. cambridge university press, 2002. doi:10.1017/cbo9780511613357. [16] k. krabbenhoft, l. damkilde. a model for non-fickian moisture transfer in wood. materials and structures 37(9):615–622, 2004. [17] h. l. frandsen, l. damkilde, s. svensson. a revised multi-fickian moisture transport model to describe non-fickian effects in wood. holzforschung 61(5):563–572, 2007. 56 http://dx.doi.org/10.1017/cbo9780511613357 acta polytechnica ctu proceedings 15:51–56, 2018 1 introduction 2 materials 3 methods 4 results 5 conclusions acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2019.22.0067 acta polytechnica ctu proceedings 22:67–71, 2019 © czech technical university in prague, 2019 available online at http://ojs.cvut.cz/ojs/index.php/app verification of the basic properties of recycled and natural aggregates karel mikulica∗, iveta hájková brno university of technology, faculty of civil engineering, veveří 331/95, 602 00 brno, czech republic ∗ corresponding author: mikulica.k@fce.vutbr.cz abstract. in the future, it is planned to use up to 50% of construction and demolition waste (c&dw) for the production of new building structures. this leads us to think about how we can use recycled concrete aggregate (rca) as a substitute for natural aggregate (na) in concrete mixtures. this is why we compare the two typical representatives of recycled aggregates with a representative of natural aggregates. as a representative of recycled aggregates, we chose pure concrete recycled from the cutting of concrete and mixed recyclate from the demolition of the apartment building. as a representative of natural stone, we chose the extracted aggregate. keywords: construction and demolition waste (c&dw), recycled concrete aggregate (rca), natural aggregate (na), stone testing. 1. introduction aggregates form the supporting framework of the concrete mixture, occupying almost 80 % of the volume in the concrete. because of this, the issues of sustainability of construction and recycling come to the subconscious. although there is currently a very rapid development in the field of building materials, concrete remains a building that has one of the widest applications in the construction industry. the composition of the concrete can be very varied depending on the filler, additives or additives used. however, the filler function always forms aggregates. depending on the properties of the structures, it is possible to modify the concrete composition to meet individual requirements, eg strength, compactness, resistance to the influences. one of the requirements may also be its subsequent recycling. concrete as a composite material consists of two components, aggregate and cement slurry. however, the third component, which is a layer at the interface of the cement slurry and aggregate, is thin, about 10 50microns, and we call it the transit zone. it is formed by the fact that the grains of cement are distant from the grain surface of the aggregate and thus the grain is covered only with a layer of water and a small amount of cement particles. the effort of cement to deviate from the aggregate is also supported by the homogenization process in the mixer. [1] plain concrete from recycled concrete does not reach the original concrete properties. recycled concrete often has less strength and more creep and shrinkage than the original concrete. improvement of these properties can occur with the use of epoxy dispersions without the presence of a solvent. this improves the adhesive properties of cement bonding cement recyclates and, if the dispersions are applied to dry recycled material, have an effect on reducing the water absorption value. the characteristics of concrete from recycled aggregate are affected by impurities that contain recycled aggregate. using the dirt will be transferred to a new material or construction. these are impurities in the form of clay, glass, plaster, plastics, textiles, papers, etc. the importance of dirt and their percentage of recycled concrete is not negligible, reflecting the quality of the newly developed material. [2] the use of modified construction and demolition waste (c&dw) as a partial substitute for natural aggregates (na) saves not only natural resources, but at the same time contributes to the reduction of the amount of construction waste in landfills. reasons for using recycled aggregate (rca), however, are not only these stated benefits that correspond to the objectives of sustainable construction, but also its cost and affordability, which is more favorable in many states than natural aggregates. the disadvantage of recycling, however, is that secondary material no longer achieves the precise properties of the primary raw material, in our case, natural aggregate. methods of testing, sorting and classification are covered by european standards en 12620 + a1 aggregates for concrete [3] and en 933-11 testing of the geometrical properties of aggregate classification of components of gross recycled aggregate. [4] therefore, it is not used to a greater extent. an important aspect in the production of recycled aggregate is also the economic aspect, it is important that the production of recycled aggregate is not more expensive than the extraction of natural raw materials, thus the importance of recycling would be lost. because the durability of the concrete is limited, we need to anticipate its decay and subsequent demolition as soon as a new concrete structure is built. therefore, we should think about the solution of the concrete afterwards when it stops performing its function. 67 http://dx.doi.org/10.14311/app.2019.22.0067 http://ojs.cvut.cz/ojs/index.php/app karel mikulica, iveta hájková acta polytechnica ctu proceedings it is, however, appropriate to say that rca has not yet been widely used in the construction industry due to the diversity of individual recycled concrete mixtures. for this reason, several physico-mechanical properties such as bulk density, porosity, water absorption, frost resistance, reactivity of unhydrated cement in rca, leachability of pollutants, purity of rca itself, alkaline silicate reaction, etc. are observed in rca. [5] 2. materials and methods to verify the basic properties of recycled aggregates, we chose two types of slightly different concrete recyclates. rca 1 is the concrete recycled from the cutting of the outdoor concrete surface. on the other hand, rca 2 comes from the demolition of a residential building and contains a wide range of different materials from the concrete ceramic shards, glass pieces. as a reference natural stone for our needs was selected directly from the water extracted rock. in order to achieve the same input conditions for all the aggregates compared, we chose the gravel fraction of 0 16mm for testing. we subjected to the tests and measurements described below. grain. determination of granularity, or screening of aggregates, is one of the basic tests performed on aggregates according to en 933-1 testing for particle size distribution sieving method. [6] the principle is to sort and separate the material by means of a set of sieve into several fractions with a descending size of holes. the result is graphical representation of the representation of individual fractions in the aggregate sample. the sample size of the test specimen depends on the maximum grain size d. in our case, we used a fraction of 0 32mm for all tested aggregates. at the same time, we determined the percentage of fine particles < 0.063mm in the form of nozzles. density. the rca bulk density depends on the residual mortar. since the residual mortar has a lower density than aggregate, the bulk density of the recycled aggregate is lower than natural aggregate. its value is from 2100 2500 kg/m3, natural aggregate has a bulk density of about 2400 2900 kg/m3. other dependencies are grain size. the smaller the grain, the lower the bulk density, but also the strength of the concrete. bulk density has an effect on water absorption, resistance to chrl. the smaller the concrete is more absorbent and less resistance to chrl. [5] testing of the bulk density of aggregates as described in en 1097-6 tests for mechanical and physical properties of aggregates part 6: determination of particle density and water absorption. [7] suffocation. the surface of the recycled aggregate is dusty and cracked, causing bumps and crushing when recycling concrete slabs. the cement is more porous and more absorbent in the area of the transit zone, leading to the formation of larger ca(oh)2 particles and thus the increased permeability of the concrete. [5] therefore, we tested 24 hours of water absorption on the samples according to en 1097-6 tests for mechanical and physical properties of aggregates part 6: determination of particle density and water absorption. [7] shape index. this test is determined only for rough aggregate according to en 933-4 tests for geometrical properties of aggregates part 4: determination of particle shape shape index. [8] principle is the distribution of individual grains according to the ratio of their length to thickness (l/e), where the shape index is calculated as a grain fraction with a ratio greater than 3. bulk density. this is the mass of the aggregate unit of aggregate with cavities and pores, including spaces and between grains. we distinguish bulk density of loose and shattered aggregate. for this post, we tested the first property we determined according to en 1097-3 tests for mechanical and physical properties of aggregates part 3: determination of loose bulk density and voids. [9] 3. results this part of the paper summarizes the measured results of our tested samples of two recycled concrete aggregates (rca 1 and rca 2) with one reference mined aggregate (na reference). the results are written in the enclosed tables. in the first step, we first determined the percentage of fine particles, when this test went freely to the screening of the test samples. this basis specification is introduced in table 1. the detailed particle size distribution of studied materials in shown in table 2. resulted gradation curve is subsequently depicted in figure 1. a further test was the comparison of volumetric weights to avoid variations due to the different moisture content of the aggregates, we determined the bulk density on samples dried to a steady state. the results obtained are summarized in table 3. we used samples from the bulk density test to determine the 24-hour water absorption so that the conditions for all samples were the same. the results of water absorption test are shown in table 4. the shape index was determined on a limited fraction of 4 8 and 8 16mm using a special sliding scale with a ratio of 1:3. the result is an l/e ratio where grains with a greater than 3 ratio are not suitable for use in the concrete mixture due to the unevenness of the material, which could occur during compaction of the concrete mixture to form caves or to break these non-conforming grains. the detailed results are introduced in table 5. the final test was to determine the bulk density of loose aggregate using a standard vessel of known volume, in our case 10 liters. obtained results of single studied aggregates are well visible in table 6. 68 vol. 22/2019 verification of the basic properties of recycled and natural aggregates sample identification percentage of fine particles (< 63µm) [%] reference na 0.23 rca 1 0.45 rca 2 1.38 table 1. determination of percentage of fine particles. mesh sizes [mm] total fall [%] reference na rca 1 rca 2 0.063 0.0 0.0 0.0 0.125 0.1 0.2 1.6 0.25 0.6 0.7 5.2 0.5 2.9 1.4 15.1 1 6.2 2.0 25.9 2 9.4 2.3 34.4 4 14.3 2.5 44.2 5.6 23.3 3.1 51.1 8 50.9 14.7 58.4 11.2 76.8 58.2 65.3 16 98.1 97.7 71.7 22.4 100 100 76.1 31.5 100 100 85.6 table 2. the results of the screening analysis of aggregates. sample identification faction [mm] density of the dried sample ρrd [kg/m3] reference na 0 – 4 2630 4 16 2580 rca 1 0 – 4 2080 4 16 2270 rca 2 0 – 4 2240 4 16 2230 table 3. determination of bulk density in the dried state. figure 1. graphical determination of the results of screen analysis of aggregates. 69 karel mikulica, iveta hájková acta polytechnica ctu proceedings sample identification faction [mm] weariness after 24 hours wa24 [%] reference na 0 – 4 1.2 4 16 0.75 rca 1 0 – 4 9.5 4 16 5.7 rca 2 0 – 4 5.9 4 16 5.3 table 4. determination of 24 hours water absorption. sample identification faction [mm] shape index si reference na 4 8 10 8 16 13 rca 1 4 8 10 8 16 4 rca 2 4 8 6 8 16 9 table 5. determination of the shape index. sample identification faction [mm] the bulk density of the loose sample [kg/m3] reference na 0 16 1580 rca 1 0 16 1120 rca 2 0 16 1550 table 6. determination of bulk density of loose aggregate. 4. conclusion the largest proportion of fine particles and a significant fraction of particles below 4mm had rca 2, the resulting value being about 3 times higher than for rca 1 and even 6 times higher than for na reference. the screen analysis itself showed that rca 2 contains almost 45% of grains below 4mm, whereas rca 1 has only 2.5%. this result was probably due to the different jaw setting of the crushing machine. determination of bulk density has shown that both recycled concrete aggregates have a similar bulk density of approximately 2250 kg/m3, ie about 350 kg/m3 lower than the reference na 2600 kg/m3. 24 hours of water absorption has highlighted the most common problem in using recycled concrete aggregates, ie water absorption. this is about 5 times higher for recycled concrete and for rca 1 fraction 0 4mm even 9 times higher than for reference na (about 1%). as a result, a higher proportion of cement dust may be present in the fraction. when evaluating the shape index test best suited to 8 16mm rca 2 fractions with 4, we can see why the lowest bulk density of 1120 kg/m3 was measured for rca 1 compared to approximately 1550 kg/m3 for reference na and rca 2 . acknowledgements this work was financially supported by project fast-j18-5554 “possibilities of using recycled concrete for reuse in construction”. references [1] a. shah, b. r. chughtai, m. m. ghumman, et al. mechanical properties of recycled aggregates concrete, concrete engineering. https:// www.engineeringcivil.com/mechanical-propertiesof-recycled-aggregates-concrete.html. [2] t. pavlů. zkoušení a vlastnosti recyklovaného kameniva pro použití do betonu, stavba.tzb-info.cz 10265. https://stavba.tzb-info.cz/beton-malty-omitky/ 10265-zkouseni-a-vlastnosti-recyklovanehokameniva-pro-pouziti-do-betonu, 2013. [3] en 12620+a1 aggregates for concrete. european standard, european commitee for standartization. [4] en 933-11 testing of the geometrical properties of aggregate classification of components of gross recycled aggregate. european standard, european commitee for standartization. [5] e. skriňáková. studium možností využití kameniva z recyklovaných betonů pro výrobu konstrukčních betonů. bachelor thesis, vysoké učení technické v brně, fakulta stavební, ústav technologie stavebních hmot a dílců, brno, 2014. [6] en 933-1 tests for geometrical properties of aggregates part 1: determination of particle size 70 https://www.engineeringcivil.com/mechanical-properties-of-recycled-aggregates-concrete.html https://www.engineeringcivil.com/mechanical-properties-of-recycled-aggregates-concrete.html https://www.engineeringcivil.com/mechanical-properties-of-recycled-aggregates-concrete.html https://stavba.tzb-info.cz/beton-malty-omitky/10265-zkouseni-a-vlastnosti-recyklovaneho-kameniva-pro-pouziti-do-betonu https://stavba.tzb-info.cz/beton-malty-omitky/10265-zkouseni-a-vlastnosti-recyklovaneho-kameniva-pro-pouziti-do-betonu https://stavba.tzb-info.cz/beton-malty-omitky/10265-zkouseni-a-vlastnosti-recyklovaneho-kameniva-pro-pouziti-do-betonu vol. 22/2019 verification of the basic properties of recycled and natural aggregates distribution sieving method. european standard, european commitee for standartization. [7] en 1097-6 tests for mechanical and physical properties of aggregates part 6: determination of particle density and water absorption. european standard, european commitee for standartization. [8] en 933-4 tests for geometrical properties of aggregates part 4: determination of particle shape shape index. european standard, european commitee for standartization. [9] en 1097-3 tests for mechanical and physical properties of aggregates part 3: determination of loose bulk density and voids. european standard, european commitee for standartization. 71 acta polytechnica ctu proceedings 22:67–71, 2019 1 introduction 2 materials and methods 3 results 4 conclusion acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2017.7.0001 acta polytechnica ctu proceedings 7:1–6, 2017 © czech technical university in prague, 2017 available online at http://ojs.cvut.cz/ojs/index.php/app shearing tests applied to pantographic structures gregor ganzoscha,∗, francesco dell’isolab, emilio turcoc, tomasz lekszyckid, wolfgang h. müllera a tu berlin, chair of continuum mechanics and materials theory, einsteinufer 5, berlin, germany b la sapienza university, dipartimento di ingegneria strutturale e geotecnica, via eudossiana 18, rome, italy c uniss, dipartimento di architettura, design e urabanistica, via garibaldi 35, alghero, italy d pan warszawa, instytut podstawowych problemów techniki, ul. pawinskiego 5b, warszawa, poland ∗ corresponding author: ganzosch@tu-berlin.de abstract. with the advancements in 3d printing technology, rapid manufacturing of fabric materials with complex geometries became possible. by exploiting this technique, different materials with different structures have been developed in the recent past with the objective of making generalized continuum theories useful for technological applications. so-called pantographic structures are introduced: inextensible fibers are printed in two arrays orthogonal to each other in parallel planes. these superimposed planes are inter-connected by elastic cylinders. five differently-sized samples were subjected to shear-like loading while their deformation response was analyzed. results show that deformation behavior is strong non-linear for all samples. furthermore, all samples were capable to resist considerable external shear loads without leading to complete failure of the whole structure. this extraordinary behavior makes these structures attractive to serve as an extremely tough metamaterial. keywords: pantograpghic structures, shearing tests, generalized continuum theories. 1. introduction micro-electro-mechanical-systems (mems) are rapidly becoming smaller. the miniaturization of these systems requires new constitutive equations for describing their mechanical and material behavior (e.g., the deformation pattern of accelerometer sensors in cars). in [1] it was shown that the elastic modulus of epoxy increases if the outer dimensions of the specimen become smaller. the reason for this behavior is hidden in an internal substructure. such a phenomenon is a.k.a. size effect and it characterizes the departure of mechanical behavior from classical continuum mechanics when changing the outer dimension. however, this effect is not limited to the microscopic world. in the literature size effects of different materials of all length scales can be found, starting from investigations on block masonry in the meter-scale [17], continuing with experiments on polymer foams in the millimeter range [10], on to studies on the micrometer-scale in silicon beams [5], and ending finally with bending experiments of nanowires on the nanometer-scale [18]. another example of intrinsic length scales in complex macroscopic specimens are so-called pantographic structures (see fig. 1), which were developed and intensively studied by dell’isola and his team in [2]. here the length and elastic properties of the constituting trusses in combination with the stiffness of their joints form the internal platform on which their macroscopic deformation behavior relies. such intrinsic length scale parameters are the true reason behind what is phenomenologically called size effect. classical elasticity theory of continuum mechanics is not able to explain this kind of material behavior because it does not contain an intrinsic length scale. this is why generalized continuum theories must be used to overcome such limits. new material parameters show up during the development of the corresponding constitutive equations. therefore experiments have to be performed, by means of which these new and unknown parameters can be determined. figure 1. example of a pantographic structure developed by [2]. in recent years, accompanied by the advancements in 3d printing technology, new experimental techniques have been developed with the objective of making generalized continuum theories useful in technology [1–3, 5, 8]. different experimental set-ups (microscopic as well as macroscopic) when using different materials with complex structures (e.g., static and dynamic bending tests on foams) were developed [1, 2, 5, 8–10, 14]. an experiment was performed on 1 http://dx.doi.org/10.14311/app.2017.7.0001 http://ojs.cvut.cz/ojs/index.php/app g. ganzosch, f. dell’isola, e. turco et al. acta polytechnica ctu proceedings so-called pantographic structures by [2]. two families of inextensible fibers, which can also be treated as beams, are 3d-printed in two arrays orthogonal to each other in parallel planes. these are then superimposed. the parallel planes are inter-connected by cylindrical elastic pivots of different diameters and heights. based on a second gradient continuum model [2–4, 11–15], the deformation behavior of the samples was assessed in numerical simulations during an extension test. non-linear models with specific intrinsic parameters led to non-classical behavior in the simulation [2]. based on these results, real macroscopic tensile tests were designed and performed, resulting in an elongation along the direction of the shorter sides of the pantographic samples: figure 2. results of a tensile test applied to a pantographic structure with a pivot height of 0.5 mm and a pivot diameter of 0.9 mm from [2]. force over displacement is plotted in fig. 2. the failure of the pantographic lattice is visualized by local minima, marked by b), c), d) and e) in fig. 2. each minimum relates to fracture of a beam or a pivot. surprisingly after one failure within the structure, which was subjected to external loading, the specimen recovers and is able to carry even higher loads than before. the objective of this work is to find out in experiments as to whether the pantographic structures show the same non-linear behavior in shearing tests. 2. material and methods because of their complex periodic structure, the pantographic samples considered for the investigations in shearing tests were 3d-printed. polyamide powder was used as raw material. 3d-models were generated by using a commercial cad software and saving them in stl format file, which was then used as input for the 3d printer. five variations of specimens with different inner geometries but equal material densities were investigated. the pantographic structure consists of rectangular beams and cylindrical pivots. a schematic shows different beam and pivot parameters in fig. 3. details can be found in tab. 1. the inner parameters of the pivots are kept constant (except for the diameter of sample e with d = 3.00 mm, see figure 3. schematic cross-section of a periodic cell of a pantographic structure. sample w hbeam d hpivot a 1.00 mm 1.00 mm 0.90 mm 1.00 mm b 1.60 mm 1.00 mm 0.90 mm 1.00 mm c 1.60 mm 1.60 mm 0.90 mm 1.00 mm d 2.25 mm 1.60 mm 0.90 mm 1.00 mm e 2.25 mm 1.60 mm 0.90 mm 3.00 mm table 1. overview of five specimens with different beam and pivot parameters. tab. 1), so that investigations of pantographic structures will focus on different beam parameters and their influence on the deformation behavior during shear tests. the outer dimension of every specimen is kept constant (length= 204 mm, width= 70 mm). an overview of the whole experimental setup used for the shear tests on planar pantographic sheets is shown in fig. 4. figure 4. experimental setup during shear deformation: camera and illumination devices on the left, pantographic sample mounted in an mts tytron 250 testing machine in the middle, interface of control software on the right. an mts tytron 250 testing-device controlled by the software stationsmanager v 3.14 was used during the shearing tests. applied force was measured by a load cell attached to the device, which is able to record axial forces in a range of n = ±250 n. the displacement was imposed horizontally on the top of the specimen with a loading rate of v = 15 mm/min. the experiments were displacement-controlled using a dc-linear motor in combination with a spindle. displacement was measured and monitored by a deviceown encoder unit. almost frictionless movement was 2 vol. 7/2017 (a) . mounted sample a before deformation. (b) . deformed sample a directly before first failure occurs. (c) . sample a after complete failure. figure 5. shear deformation of sample a. achieved by using an air-film-bearing. external vibration was avoided by using a massive substructure and by arranging the system horizontally. additionally to force-displacement recording, pictures were taken (0.25 pictures/second) by means of a commercial canon eos 1000d camera with a resolution of 4272 × 2848 pixels. free software imagej 1.50i was used to evaluate image data. open source software scidavis 1.14 was used for preparation, manipulation, and evaluation of acquired machine data. fig. 5 shows the deformation of sample a before, during, and after the shearing test. 3. results results of all five samples are visualized in fig. 6 with the help of a force-displacement diagram. local minima in fig. 6 show a failure of a beam or a pivot. it can be recognized that all samples show different and pronounced non-linear behavior. in order to visualize the results for a discussion, the graph of each sample is plotted separately, starting with sample a in fig. 7, and ending with sample e in fig. 11. in sample a the first failure occurs at fa = 26.0 n. figure 6. raw-data of all samples; force f is plotted over displacement x. figure 7. raw-data of sample a; force f is plotted over displacement x. figure 8. raw-data of sample b; force f is plotted over displacement x. surprisingly, samples b (fb = 46.9 n) and c (fc = 50.5 n) are able to resist the highest loads. sample d resists until fd = 23.9 n. sample e shows the lowest strength and failure occurs at about fe = 16.8 n. in sample a and b, a beam failure occurs on the lower left corner (shown for sample a in fig. 12). in contrast to the lower right corner, where just low deformation occurs, the beams at the lower left corner experience a large elongation between the pivots. in 3 g. ganzosch, f. dell’isola, e. turco et al. acta polytechnica ctu proceedings figure 9. raw-data of sample c; force f is plotted over displacement x. figure 10. raw-data of sample d; force f is plotted over displacement x. figure 11. raw-data of sample e; force f is plotted over displacement x. this region, bending energy of fibers is concentrated leading to failure of a beam. first failure of sample c, d, and e is quite different (shown for sample c in fig. 13). obviously, bending energy of fibers is so high in the middle-left that pivots in this region (marked by the circle in fig. 13) start to separate from the beam structure. notice, that the variation of inner parameters leads to different deformation and rupture behavior. the influence of inner parameters on the material behavior of the specimen will be described and discussed. figure 12. deformed sample a directly after first failure occurs, region marked by the circle. figure 13. deformed sample c directly after first failure occurs, region marked by the circle. by enlarging the width of the beams in the pantographic structure from sample a (w = 1.00 mm) to sample b (w = 1.60 mm), the resistance to shear deformation increases by almost 80%. this enlargement has a very high influence on the strengthening of this structure. by enlarging the height of the beams from sample b (hbeam = 1.00 mm) to sample c (hbeam = 1.60 mm), the resistance to shear deformation increases by just 7.7%. this is a much smaller influence compared the one from sample a to b mentioned before. but still sample c is able to carry the highest shear load until the first failure occurs (fc = 50.5 n). by enlarging the width of the beams from sample c (w = 1.60 mm) to sample d (w = 2.25 mm), the resistance to shear deformation decreases by 52.7%. 4 vol. 7/2017 this observation is contrary to our assumptions and indicates an upper limit value for the width of beams between 1.60 mm ≤ w < 2.25mm. finally, by enlarging the height of the pivots from sample d (hpivot = 1.00 mm) to sample e (hpivot = 3.00 mm), the resistance to shear deformation decreases by 29.7%. this can be explained by the higher lever arm between the beams resulting in a high torque which then leads to failure. notice that complete failure for samples a, d, and e occurs at a deformation of about x = 190 mm. absolute structural failure of the flexible sample b and c could not be investigated due to movement-limitations of machine-axis. this is also the reason for unloading curves in sample b and c, which occurred during the removal procedure of the samples from testing device. interestingly, sample a, b, and c show a strong non-linear increase of their slopes before first failure occurs. this is not the case for sample d and e with higher values for the width of beams (w = 2.25 mm). furthermore, samples b, d, and e are able to carry even higher loads after first failure. because of the complex geometry, the beams reorganize themselves resulting in a higher resistance to shear loading. this behavior was also observed during tensile tests for the case of uni-axial loading as afore-mentioned in [2]. sample xm [mm] xi [±1 mm] 4x [±1 mm] a 143.96 142.47 1.49 (1.04%) b 142.91 141.50 1.41 (0.99%) c 145.71 145.29 0.42 (0.29%) d 122.96 122.13 0.83 (0.68%) e 123.77 123.49 0.28 (0.23%) table 2. listed values of displacement based on image data (xi) and based on machine data (xm) at the time when first breakage of the specimen occurred. difference of displacement between machine data and image data (4x) is presented in the last column. by comparing image data with machine data, no significant difference in displacement was observed (see tab. 2). sample a shows the highest difference between displacement obtained from machine data and main displacement obtained from image data(4xa = 1.04%), sample e shows the lowest one (4xe = 0.23%). note that displacements captured by the camera are smaller than the ones obtained from the testing device. we assume, that one reason for this observation is the compliance (inverse of stiffness) of the testing device and its structure: deformation of clamping jaws and mounting support influences the real displacement. another reason is based on the uncertainty of image data: pictures were taken every 4 seconds (as described in sect. 2), in which the loading-axis was able to travel up to 1 mm (error of ±1 mm in second column of tab. 2, xi). this results in a discrepancy between real and measured displacement. furthermore it has to be mentioned that image data was just able to capture the deformation behavior in a two dimensional plane. but during the experiments it was observed that the structure was also deforming in the third direction (out-of-plane). this buckling-like behavior varied strongly for each sample. highest out-of-plane-movements were observed in experiments involving sample a and b (up to 19 mm). sample c, d, and e showed very weak out-of-plane-movements or even none. the investigation of this effect is strongly recommended in further research projects. manufacturing process (rapid prototyping) may have a big influence on the material behavior as well (e.g., direction of printing or thickness of layer) and should also be part of future investigations. notice, that each sample was just measured once. to increase the quality of results, each experiment should be repeated under exactly the same experimental conditions. acquired data should then be compared among each other. 4. conclusions deformation behavior of various pantographic structures with different inner parameters was investigated in shearing tests. we showed that the variation of inner parameters (beam and pivot dimensions) have a deep impact on the material behavior. all samples behaved non-linearly and were capable to resist considerable external shear loads without leading to complete failure of the whole structure. because of the complex geometry beams and pivots reorganize themselves resulting in an higher resistance to outer load, so that even higher loads can be carried after failure (see sample b, d, and e). an aim for further investigations should be to provide a better predictability of structural failure behavior by manipulating the inner parameters in a way that all force-peaks should be at one level. this would make the deformation behavior of the structure more predictable and would contribute to make these structures available for application in technology. real experiments are quite involved. thus, the obtained results shall be used to develop further simulation models as shown in [3] or [14, 15]. it is also possible to investigate the influence of inner parameters on the material behavior in a numerical analysis [14, 16]. however, these models are based on many simplifications and restrictions so that a true validation would still be challenging and should be investigated further. as afore-mentioned in sect. 3, investigated lattices were just examined in context with two-dimensional planar structures (neglect of twist or buckling). other forms, such as cylinders or rectangles, would contribute to three-dimensional stability and would help to make these structures useful for technology. in summary one may say that the combination of strength and lightness makes these structures attractive to serve as an extremely tough metamaterial (e.g., airplane frame parts or artificial bone scaffolds). 5 g. ganzosch, f. dell’isola, e. turco et al. acta polytechnica ctu proceedings list of symbols f force [n] x displacement [mm] xm displacement based on machine data [mm] xi displacement based on image data [mm] 4x difference between xm and xi [mm] w width [mm] d diameter [mm] hbeam beam-height [mm] hpivot pivot-height [mm] v velocity [mm] references [1] liebold, c., müller, w.h. applications of higher-order continua to size effects in bending: theory and recent experimental results. generalized continua as models for classical and advanced materials, springer international publishing, pp. 237-260, 2016 [2] dell’isola, f., lekszycki, t., pawlikowski, m., grygoruk, r., greco, l. designing a light fabric metamaterial being highly macroscopically tough under directional extension: first experimental evidence. zeitschrift für angewandte mathematik und physik, zamp, 66(6), pp. 3473-3498, 2015 [3] dell’isola, f.,della corte, a., giorgio, i., scerrato, d. pantographic 2d sheets: discussion of some numerical investigations and potential applications. international journal of non-linear mechanics, elsevier, 80, pp. 200-208, 2016 [4] placidi, l., andreaus, u., della corte, a., lekszycki, t. gedanken experiments for the determination of two-dimensional linear second gradient elasticity coefficients. zeitschrift für angewandte mathematik und physik, zamp, 66(6), pp. 3699-3725, 2015 [5] liebold, c. größeneffekt in der elastizität. dissertation, technische universität berlin, germany, 2015 [6] liebold, c., müller, w.h. comparison of gradient elasticity models for the bending of micromaterials. computational materials science. elsevier, pp. 52-61, 2016 [7] liebold, c., müller, w.h. measuring material coefficients of higher gradient elasticity by using afm techniques and raman-spectroscopy. generalized continua as models for materials, springer-verlag berlin heidelberg, pp. 255-271, 2013 [8] ganzosch, g., müller, w.h. experimental techniques applied to generalized continuum theories a state-of-the-art report. proceedings of the xliv summer school-conference. 44, pp. 149-161, 2016 [9] lam, d. c. c., yang, f., chong a. c. m., wang, j., tong, p. experiments and theory in strain gradient elasticity. journal of the mechanics and physics of solids. 51(8), pp. 1477-1508, 2013 [10] lakes, r., drugan, w.j. bending of a cosserat elastic bar of square cross section: theory and experiment. journal of applied mechanics, 82(9), pp. 2-8, 2015 [11] dell’isola, f., dagostino, m. v., madeo, a., boisse, p., steigmann, d. minimization of shear energy in two dimensional continua with two orthogonal families of inextensible fibers: the case of standard bias extension test. journal of elasticity, 122(2), pp. 131-155, 2016 [12] pipkin, a.c. plane traction problems for inextensible networks. q.j. mech. appl. math., 34(4), pp. 415-429, 1981 [13] rivlin, r.s. plane strain of a net formed by inextensible cords. collected papers of rs rivlin, springer berlin, pp. 511-534, 1997 [14] turco, e., dell’isola, f., rizzi, n.l., grygoruk, r., müller, w.h., liebold, c. fiber rupture in sheared planar pantographic sheets: numerical and experimental evidence. mechanics research communications, elsevier, 76, pp. 86-90, 2016 [15] turco, e., dell’isola, f., cazzani, a., rizzi, n.l. hencky-type discrete model for pantographic structures: numerical comparison with second gradient continuum models. zeitschrift für angewandte mathematik und physik, zamp, 67(85), pp. 01-28, 2016 [16] abali, b.e., müller, w.h., eremeyev, v.a. strain gradient elasticity with geometric nonlinearities and its computational evaluation mechanics of advanced materials and modern processes, 1(1), pp. 01-11, 2015 [17] pau, a., trovalusci, p. block masonry as equivalent micropolar continua: the role of relative rotations. acta mechanica, 223(7), pp. 1455-1471, 2012 [18] jing, g.y., duan, h.l., sun, x.m., zhang, z.s., xu, j., li, y.d., wang, j.x., yu, d.p. surface effects on elastic properties of silver nanowires: contact atomicforce microscopy. physical review b, 73, pp. 01-06, 2006 6 acta polytechnica ctu proceedings 7:1–6, 2017 1 introduction 2 material and methods 3 results 4 conclusions list of symbols references acta polytechnica ctu proceedings doi:10.14311/app.2020.29.0006 acta polytechnica ctu proceedings 29:6–10, 2020 © czech technical university in prague, 2020 available online at http://ojs.cvut.cz/ojs/index.php/app landslide monitoring in a built-up area petr černocha,b,∗, jiří košťálb,c, adam podojilc a čez energetické produkty, s.r.o. komenského 534, 253 01, hostivice, czech republic b czech technical university in prague, faculty of civil engineering, department of geotechnics, thákurova 7, 166 29 praha 6, czech republic c inset, s.r.o., lucemburská 7, 130 00, praha 3, czech republic ∗ corresponding author: cernoch.petr@inset.com abstract. the article deals with the evaluation and comparison of measured data from geotechnical monitoring of a gabion wall and an adjacent slope. in 2015, new anomalous phenomena related to deformations and a shift in the area of previously activated landslide were recorded on some monitoring elements in the area of the gabion wall. measurements carried out in previous stages had revealed the re activation of landslides, probably due to a significant rise in groundwater levels, whose fluctuation has been up to 3 m. a proper evaluation of the obtained results cannot be quantified without the determination of warning status or without a stability calculation. only the trend and magnitude of the measured values can be evaluated. keywords: monitoring, slope movements, slope stability, warning status. 1. introduction the development and expansion of cities comes up against spatial limits, making it necessary to find a way to use less suitable areas within the city which were, for many possible reasons, omitted at earlier stages of development. these are often areas with an unfavourable terrain morphology requiring expensive and time-consuming landscaping or they have a complicated geological development creating complex foundation conditions for newly emerging structures. furthermore, these can be localities prone to geodynamic phenomena, or perhaps they are areas with shallow groundwater. during the construction of utility lines in unfavourable geological conditions, it is essential to perform long-term and comprehensive geotechnical monitoring. geotechnical monitoring has a basic function in any system of geotechnical risk management during construction and involving engineering and utility work, the aim of which is to monitor all the effects induced in the environment that could have an adverse influence on the construction. in localities susceptible to a loss of stability, it is necessary to monitor the speed of slope movements, changes in slope geometry and changes in the mechanical and physical properties in the landslide zone, or indeed the effectiveness of the stabilization measures used. in this particular case, the monitoring focuses on a sloping area in the municipality of turnov which has complex geological and hydrogeological conditions as described in the following chapter, and which impose higher demands on the construction project of a department store with an adjacent car park that is implemented. a need arose to stabilize the height difference of the store and the car park against the adjacent slope and this also involved a need to address appropriate drainage of the gabion wall. 2. on the structure concerned 2.1. a brief description of the natural conditions in the area of the structure the slope above the store that was monitored, and stabilized by a gabion wall, is situated near the centre of the town turnov in a commercial zone near the i/35 road. the gradient of the slope that was monitored is approximately 20° towards the gabion wall. the gabion wall overcomes the height difference between the slope and the store car park (fig. 1) and ensures the stability of the slope. the area of interest is characterized by very complex geological conditions, which are significantly impacted by the geomorphological development of the area. during the quaternary period, a local watercourse flowing south of the site created an erosive slope with exposed sandy marls and with considerable surface weathering. a relic of the erosive marl terrace of the stream has been preserved at the top of the slope. the southern side of the slope was subsequently flooded with a thick discordant loess drift. the following elements can be discerned from archive probes: • the subsoil of the locality is formed by weathered sandy silt stones of the cretaceous era, with a character of sandy-clay to clay with low plasticity and including a gradual transition to weathered and then to semi-rock marls of the r6 class. 6 http://dx.doi.org/10.14311/app.2020.29.0006 http://ojs.cvut.cz/ojs/index.php/app vol. 29/2020 landslide monitoring in a built-up area figure 1. the slope above the gabion support wall, a car park with a shop (right) • loess drifts formed by decalcified loess clay with a soft to solid consistency of the character of sandy clay to clay with low plasticity are situated on the subsoil. in several places, gravel clay run-offs were identified. • the upper layer of the soil horizon is composed of an upper layer of humus and a lower layer formed by sandy clays. the hydrogeological conditions in the area of interest were conditioned by the geological structure and the articulated morphology of the terrain. the groundwater level (gwl) is about 5 to 8 m below the ground, with a verified fluctuation of up to 3 m. because of the morphology of the area, the groundwater supply would presumably be provided by atmospheric precipitation, which infiltrates by gravity through lowpermeable sandy clays (f3 ms) and locally developed moderately permeable gravel clays (f1 mg to g4 gm) to the practically impermeable subsoil formed by underlying silt stones (weathered marls on f6 ci). in periods of high precipitation, the loess clay is saturated, which leads to a change in its consistency, making it soft. the occurrence of local springs which can instigate the formation of slope movements can be expected on the slope [1]. the region concerned is characterized by low precipitation and relatively high air temperatures. the climate is warm, with a short and moderately warm spring and a short and moderately warm autumn. the winter is slightly warm, very dry and short, with a short duration of snow cover. the summer is long and warm. 2.2. the slope failure monitoring system the store concerned and the adjacent car park are cut into the adjacent slope on the north side, and the gabion wall overcomes the height difference. during the construction of the gabion wall in 2005, a landslide was initiated on the slope above the car park, and which was subsequently reclaimed. because of this extraordinary event, the monitoring system of the store under construction and the gabion wall were completed with some elements, the measurement of which was intended to verify and control the stabilization of the reclaimed landslide area (fig. 2). inset ltd. provided continuous monitoring of the gabion support wall and the adjacent slope from april 2007 to june 2018 with a measurement frequency of four times a year. slope shifts were monitored using a system of fixed geodetic points located on the slope above the gabion wall, all measured by an accurate gps station and a network of geodetic points located directly on the surface of the gabion wall aimed at the total station. horizontal deformations in the depth profile of the slope were monitored by inclinometric boreholes. the groundwater level was measured and read in observation probes. the measured values showed several years of stabilization in development trends, with oscillating movements in the range of ± 15 mm. the area of the locality of interest was assessed as stabilized for many years. 2.3. reactivation of slope movements the results of the monitoring over the years 2005-2014 indicated stable values for the movements. in 2015, slope movements were probably reactivated. on the ground, the formation of bumps – typical of slope movements – was observed at a place where there was significant water saturation [2]. two inclinometric boreholes (located in the line of the historical landslide in a row) caught accelerating horizontal deformations documenting the formation of a sliding surface at a depth of 6 m below the ground (fig. 3) with a shift along the slope line by up to 5 7 p. černoch, j. košťál, a. podojil acta polytechnica ctu proceedings figure 2. monitoring system situation. p11-62: geodetic points, j102a-106a: ik boreholes, sp1 2: observation piezometers. figure 3. development of inclinometric measurements over the years 2014-2018 at the borehole j102a mm on a quarterly basis, with an initial increase in speed. the observed deformations (shifting in the direction of the store) on the gabion wall were on the scale of millimetres. the development of the gwl in the observation piezometers indicated the formation of a local aquifer in the area of an earlier landslide and monitored slope movements, and probably the area with developed permeable gravel sediments. the observation probes recorded a significant oscillation of the gwl by up to 3 m, as well as significant differences in the gwl in individual probes, with a higher level in the area of the monitored landslide while probe sp1 indicated a long-term gwl at a level of about 5 m above sea level, compared to probe sp2, which indicated gwl at about 8 m below the terrain. in the area of the landslide movements a gwl supply is expected, as well as the permanently abundant spring which kept the local aquifer saturated even in periods with lower than average precipitation, the gwl in probe sp1 showing no significant correlation with precipitation. the slope mass movements probably re-occurred as a result of an increase in the groundwater level in the area of probe sp1 (the area of the former landslide and a permanently water-saturated site) which led to increased pore pressures on the previously formed sliding surfaces. the landslide processes were linked to areas of weakened zones with a residual soil strength, which were very susceptible to changes in load conditions and hydraulic conditions [3]. the movements that were detected probably did not indicate the depletion of the stability capacity in the supporting structure but functioned as a sufficient warning for the timely completion of the reclamation and stabilization measures adopted in the slope and the gabion wall. in order to stabilize the landslide movements it was critical to reduce the gwl to below the level of the previously formed sliding surface. after the initial acceleration of the slope movements up to 1 cm/year, the rate of sliding has slowed to a gentler pace since 2016 (1 mm/year – the speed of the movement of the slope equal to creeping). 2.4. proposed solutions to stabilization of slope movements on the basis of the monitoring of the active slope movement, which was assessed as risky as a result of the 8 vol. 29/2020 landslide monitoring in a built-up area formation of a landslide endangering the stability of the gabion wall, the monitoring supplier recommended adopting adequate measures to lower the groundwater level. the objective of the gwl reduction was to "arrest" it below the level of the sliding surface which was identified at a depth of 6 metres below the ground in the vicinity of the borehole j102a and approx. 3 metres below the ground in the vicinity of j103a. as part of the building conversions – a remodelling of the store and an increase in the parking capacity – the investor proceeded to enhance the gabion wall with horizontal drainage boreholes to drain groundwater from the more permeable soils and to put in place a drainage groove in the area of the slope movements for effective drainage of surface water in the adjacent slope to the existing drainage system of the store and the car park [4]. the conversions were further supplemented with six-strand ground anchors driven through the gabion wall, extending beyond the localized sliding surface in order to increase the stability of the gabion wall. the aforementioned reclamation measures were implemented in 2018. the ongoing monitoring has shown a positive effect of the building conversions on the overall stability of the slope, including the gabion wall [4]. nevertheless, it is still vital to continue the monitoring. 3. proposed warning status and measures no opinions or recommendations for monitoring on the construction site concerned were determined by the designer, and no warning status was determined either. with no warning status determined and no stability calculation (including its verification) carried out, it was impossible to quantify the actual seriousness of the results obtained. on the basis of a professional geotechnical assessment, it was possible to evaluate the trend and size of the measured values with respect to the structural design and geometry of the supporting wall. on the basis of the results of the long-term comprehensive monitoring and expert assessment of the development of the measured values, the following warning statuses were proposed, depending on the magnitudes of the horizontal deformations of the supporting wall and the slope at the site of the landslide movements and the development of the gwl. 3.1. definition of warning status a) development of the groundwater level in the locality. the swelling of the groundwater level has a negative effect on the stability of the slope masses above the sliding surface and could lead to disturbance of the stability of the gabion wall. the development of the gwl is monitored by the observation piezometers. warning status degree detected condition 0 − no shift or shifts of an oscillating nature; 1 − shifts of a creep nature with a decreasing gradient; 2 − shifts of a creep nature with a constant gradient in millimetres per year; 3 − shifts of a creep nature with an increasing gradient indicating the beginning of a landslide (cm/year). b) stability of the slope and supporting gabion wall. the stability of the slope and the gabion wall is monitored by inclinometric probes and a network of geodetic points located in the slope and on the gabion wall. the measurements are aimed at capturing changes in the speed of the slope movements and possible manifestations of displacements of the terrain, as well as deformation of the gabion wall. warning status degree detected condition 0 − no measures required; 1 − compare with the development of levels in the observation boreholes; 2 − perform stability and deformation recalculation (fem); 3 − design stabilization measures. 4. discussion concerning the results of the monitoring geotechnical monitoring is an essential safety element in the implementation and operation of structures in complex geological conditions, including geodynamic phenomena in urban built-up areas. in this context, a landslide was monitored which exhibited reactivation on a long-term basis. however, without precisely delineated specifics and warning statuses, it is difficult to define the risk clearly and identify the necessary measures that would be acceptable and effective for the investor. despite the absence of warning statuses, measures stabilizing the initial landslide movements were proposed immediately after the emergency all based on the measurement results. for the time being, the following measurements show the effectiveness of the building conversions – the gwl level has been reduced and the slope movements have slowed down. to verify the effectiveness of the reclaiming adjustments, it will be appropriate to carry out stability deformation calculations (fem) with subsequent verification based on the data obtained from ongoing monitoring on a continuous basis. 9 p. černoch, j. košťál, a. podojil acta polytechnica ctu proceedings 5. conclusion landslide localities in built-up areas require long-term and comprehensive monitoring. the designer did not issue any opinions, recommendations, or warning statuses for the construction of the store concerned. the determination of warning statuses facilitates the evaluation of geotechnical monitoring regarding the specifics of a construction and clearly determines the measures that must be taken when declaring the appropriate level of warning status. as the emergency occurred (the landslide started again), it was necessary to propose stabilization measures and to establish warning statuses based on long-term geotechnical monitoring. with the warning statuses introduced for the given construction, the evaluation of the measured values and the subsequent discussion with the designer and the investor on the variants of the remediation measures, all became much easier. references [1] p. černoch, j. košťál. geotechnical risks of foundation of warehouses built on brownfields. ernst & sohn, verlag für architektur und technische wissenschaften gmbh & co. kg, berlin, germany. 2018. [2] j. košťál. landslide monitoring, safety measurements, 2007 – 2018, ms. inset s.r.o.; prague, the czech republic. 2018. [3] p. černoch, j. košťál. brownfields utilization and assessment of foundation construction. xvi danube european conference on geotechnical engineering; 07-09 june 2018, skopje, r. macedonia; paper no. 20170407. [4] a. podojil. assessment of the influence of the object stabilization of the retaining wall on the groundwater level, ms. inset s.r.o., prague, the czech republic. 2018. 10 acta polytechnica ctu proceedings 29:6–10, 2020 1 introduction 2 on the structure concerned 2.1 a brief description of the natural conditions in the area of the structure 2.2 the slope failure monitoring system 2.3 reactivation of slope movements 2.4 proposed solutions to stabilization of slope movements 3 proposed warning status and measures 3.1 definition of warning status 4 discussion concerning the results of the monitoring 5 conclusion references acta polytechnica ctu proceedings doi:10.14311/app.2018.18.0055 acta polytechnica ctu proceedings 18:55–60, 2018 © czech technical university in prague, 2018 available online at http://ojs.cvut.cz/ojs/index.php/app utilization of image and signal processing techniques for assessment of built heritage condition petr koudelka∗, veronika koudelková, tomáš doktor, ivana kumpová, daniel kytýř, jaroslav valach czech academy of sciences, institute of theoretical and applied mechanics, prosecká 809/76, 190 00 prague 9, czech republic ∗ corresponding author: koudelkap@itam.cas.cz abstract. historical buildings represent invaluable heritage from the past and therefore their protection is a very important task. assessment of their condition must not cause damage accumulation, thus the least possible volume removed from the structure is essential. as many historical buildings in the czech republic are built using sandstone that can be considered as a typical heterogeneous system, statistical signal processing is a promising approach for determination of the representative volume element (rve) dimensions. such calculations can be carried out on the domain of logical arrays representing binary images of the materials microstructure. this paper deals with processing of image data obtained using sem-bse and high resolution flatbed scanner for determination of rve dimensions. advanced image processing techniques are employed and results from calculation using grayscale data are presented and compared with results calculated on the basis of color input images. keywords: heterogeneous systems, natural stones, image processing, k-means clustering, representative volume element. 1. introduction conservation, rehabilitation and strengthening of the built heritage are all claimed by modern societies requiring development of appropriate intervention techniques [1]. in order to facilitate a proper maintenance, historic buildings’ condition has to be assessed on regular basis. assessment itself must not cause damage accumulation and has to be carried out with caution [2]. therefore, in the case of material parameters required for evaluation, the least possible volume of removed sample is essential. the extracted specimen should represent the material at the level of a constructional element and thus be suitable for investigation of material properties leading to determination of representative volume element (rve). there are several methods for quantification of porosity and/or volume fractions of constituting phases ranging from radiography imaging to experimental methods such as mercury porosimetry. among available approaches, image processing routines offer balanced ratio between equipment or time demands and reliability, eventually reproducibility of results. in this paper, possibility of utilization of image processing techniques for consequent spectral analysis of the specimens’ microstructures is demonstrated on a set of natural stone samples commonly used in the czech historical buildings. 2. materials in the czech republic, many historical buildings have been built using different types of sedimentary rocks. in the prague region, this involves particularly opuka stone and different types of sandstone gathered at various locations in the area. in many of the most famous prague monuments including the charles bridge, sandstones of the following types were used for both architectural and decorative purposes: (1.) petřín sandstone hard sandstone with ferruginous matrix from petřín quarry is a dark brown quartz-rich stone with claystone fragments as addendum (see figure 1). the average value of bulk density is 1930 kg/m3 and porosity is 29.2 % [3]. (2.) nehvizdy sandstone middle grained porous sandstone from nehvizdy quarry has horizontal layering conditioned by alternation of middle grained laminas and fine grained laminas (see figure 2). main component of both clasts and matrix is sio2. the average value of bulk density is 1940 kg/m3 and porosity is 24.33 % [4]. (3.) sandstone from the charles bridge light pinkishgrey hard porous arkose (type of sandstone with more than 25 % of the feldspar grains) is coarse grained sandstone from žehrovice (see figure 3. the average value of bulk density is 2030 kg/m3 and porosity is 25.16 % [3]. 3. methods from the microstructural point of view, all of these materials can be described as multi-phase heterogeneous systems (composites) suitable for analysis using 55 http://dx.doi.org/10.14311/app.2018.18.0055 http://ojs.cvut.cz/ojs/index.php/app p. koudelka, v. koudelková, t. doktor et al. acta polytechnica ctu proceedings figure 1. petřín sandstone sample captured using high-resolution flatbed scanner. figure 2. nehvizdy sandstone sample captured using high-resolution flatbed scanner. figure 3. žehrovice sandstone sample captured using high-resolution flatbed scanner. statistical signal processing methods [5, 6]. the constituent phases are in this case harder particles, binder (matrix) and pores. in order to utilize grayscale-based statistical signal processing algorithms for rve dimensions assessment, acquired images have to be properly segmented to set of logical arrays, where every array contains structural information related to only one of constituent phases. 3.1. image acquisition there are numerous methods and devices suitable for acquisition of images in sufficient quality and resolution for precise structural information extraction. according to application of two different segmentation techniques presented in this paper two different data sources were selected: (1.) sem mira ii lmu (tescan, cz) sem device equipped with the back-scattered electron (bse) detector. the scanning was performed with identical operating parameters to guarantee the same acquisition conditions. samples were polished and coated by a thin layer of carbon (thickness approx. 15 nm) prior to scanning to increase surface conductivity for sem observations. the device produces 64 dpi 8-bit grayscale images (see figure 4). (2.) high resolution flatbed scanner to compare results from sem imaging with data produced with commonly available device, images of other samples were captured using high resolution flatbed scanner epson perfection v350 (seiko epson corp., japan) with resolution 6400 dpi, 3.75µm pixel size and 16bit srgb output images (figures 1, 2, and 3). 3.2. image processing several techniques have been developed for image segmentation ranging from general purpose algorithms to highly specialized single-purpose routines for specific applications. however, for successful and effective image segmentation, proper technique has to be always combined with domain’s specific knowledge. generally, image segmentation procedure encounters: i) thresholding, ii) clustering, iii) compression-based methods, iv) histogram-based methods, v) edge detection, vi) region-growing methods, vii) watershed transformation, and viii) multi-scale segmentation. selection of segmentation algorithm or their combination depends not only on domain’s properties, but also on character of the input data. 3.2.1. segmentation of grayscale image data it has been shown that for cellular metals thresholding based segmentation on grey-scale image is sufficient enough for reliable predictions of rve dimensions and numerical homogenization routines [6]. however for natural stones results of simple thresholding may not be satisfactory and more complex routines have to be applied (see figure 5). in this paper the following briefly characterized methods have been used on grayscale image data captured in sem: (1.) gradient magnitude calculation here multidimensional filter with sobel operator is used for intensity gradient magnitude calculation to emphasise borders of objects in the image. (2.) morphological techniques these operations are based on dependence of a value of given pixel on values of pixels in defined neighbourhood characterized by considered structuring element. there are two general types of morphological procedures opening and closing. opening is erosion (setting the value of each pixel to the minimum value of the pixels 56 vol. 18/2018 image and signal processing for assessment of heritage condition figure 4. arkose from the charles bridge (left) and petřín sandstone (right) captured using bse detector in sem. in the neighbourhood) followed by dilation (setting the value of each pixel to the maximum value of the pixels in the neighbourhood), whereas closing is dilation followed by erosion [7]. more precise morphological technique is the morphological reconstruction. here, first steps in both opening and closing of the image are followed by image reconstruction, which is conceptually repeated dilation between marker and mask image. processing of images using reconstruction is based on connectivity concept rather than a structuring element and the routine repeats until object stability is reached, i.e. mask image does not change with increasing iterations. (3.) thresholding the simplest segmentation method based on a threshold value to convert a greyscale image into a binary image [7]. it is based on the premise that distinct objects in the image have different levels of brightness and that the value of brightness is the same/similar in pixels of a continuous object. in this method, determination of threshold value depends on domain’s character and has significant influence on segmentation results. when threshold value is defined globally from the whole image, its initial estimation can be performed using otsu’s method [8], which is histogram shape-based image thresholding algorithm. possible problems arising from illumination inhomogeneity may be overcome by local thresholding. instead of having a single global threshold value, the algorithm allows threshold itself to be smoothly varied across the image. the initial value can be determined by various approaches (median value of selected neighbourhood, bernsen method, mid-gray method, niblack’s, sauvola’s method, etc.). (4.) watershed transformation for the purpose of watershed transformation, grayscale image is considered as a topographic relief defined by intensities of individual pixels. principle of this transformation is directly derived from behaviour of water drops falling on a topographic relief. water drop follows (flows along) gradient of a path to reach a local minimum (minimum altitude in an area enclosed by a topographical border). then, watershed of a relief corresponds to the limits of the adjacent catchment basins of the drops of water [9, 10]. (5.) edge detection segmentation of the image can be also performed using edge detection to identify individual elements in the image (in this case individual particles of stones) with consequent application of methods mentioned hereinbefore. many of edge detection methods (e.g. canny [11]) are based on intensity gradient magnitude calculation, when edges are detected at locations, where the gradient is maximal. gradient calculation is followed by non-maximum suppression. image is searched to determine if the gradient magnitude assumes a local maximum in the gradient direction producing logical array of edges referred to as thin edges. then a two-level thresholding with hysteresis has to be applied in order to filter isolated pixels representing isolated local maxima. 3.2.2. segmentation of colour image data particularly when sem imaging device is not available for image acquisition, one can be interested in possibility of segmentation of images captured by general purpose devices that are commonly available. this approach is in this paper shown on colour image segmentation of images captured by high resolution flatbed scanner. all image segmentation methods can be applied, when full colour image is simply converted into monochromatic form at the disadvantage of loss of information during the conversion. however, on the colour images, clustering approach is promising even though computational requirements may be high according to size of input data. here, k-means clustering is a common method of cluster analysis. to 57 p. koudelka, v. koudelková, t. doktor et al. acta polytechnica ctu proceedings figure 5. result of improper segmentation in grey intensity space leading to substantial loss of information with upscaled detail of over-segmented structure. separate objects in analysed space and aggregate them into groups (clusters), k-means clustering treats each object as having a unique location in space [12]. partitions are found such that objects within each cluster are as close to each other as possible, and as far from objects in other clusters as possible. this process can be noted as minimizing within-cluster sum of squares (objective function) according to argmin k∑ i=1 ∑ xj∈si ‖ xj − µi ‖2, (1) where µi is the mean of points in si (cluster centre). in the demonstrated application, input parameter for the calculation is number of phases constituting the material that has to be a priori known. this number of phases then corresponds to number of clusters to be generated. according to algorithmization used in this work, image representation of the investigated material has to be transformed into cielab colour space derived from cie xyz tristimulus values. this space consists of the luminosity layer l, the chromaticity-layer a, indicating, where colour falls along the red-green axis, and the chromaticity-layer b indicating, where the colour falls along the blue-yellow axis (similarly to the human vision). all of the colour information is contained in the a and b layers and the difference between two colours can be measured using the euclidean distance metric. since the colour information exists only in the a∗ b space, objects for clustering are then pixels with a and b values. 3.3. rve assessment utilization of statistical signal processing techniques on determination of typical structural periods in cellular metals was shown in [13], whereas application on natural stones was presented in [6]. the method is based on spectral analysis of a segmented binary image, where large peaks in the power spectrum density (psd) function indicate typical structural periods of the material’s microstructure and hence the rve dimensions. all calculations are created on the domain of spatial path variable ϕ (s) for the material’s phase function leading to the following formulation of autocorrelation function r∗xx (σ) and its corresponding psd function s∗xx (fs) r∗xx (σ) = lim s→∞ 1 s ∫ ls 0 φ (s)φ (s+ σ) ds, (2) s∗xx (fs) = ∫ ∞ −∞ r∗xx (σ) e−2πiσdσ, (3) where ls is the one-dimensional path length and fs is the structural frequency in m−1, which is reciprocal of the searched structural period. spectral estimation itself has been performed using periodogram denoted by the equation s∗xx (fs) = 1 ls |ξ (fs, ls)|2 , (4) where ξ (fs, ls) is the discrete fourier transform (dft) of the phase function ϕ (s) given by ξ (fs, ls) = ∫ ls 0 ϕ (s) e−2πifssds. (5) 4. results hereinbefore characterized methods and tools have been applied on sem bse and optical scanner images in order to develop image processing procedures for phase volume fraction identification in natural stone materials as well as to show their applicability on pre-processing of image data for spectral analysis. concerning the sem grayscale data, edge detection, watershed transformation and pure gradient magnitude algorithms proved unsuitable for analysis of materials with such a kind of microstructure. edge detection approach is in this case significantly computationally demanding and fails due to indistinct grain boundaries and in some cases due to low contrast between the neighbouring elements (grains). watershed transformation was applied in the form of meyer implementation. when the captured images are seen as a topographic relief, it is evident that watershed algorithm is not in this case applicable. from topographic point of view, the images are in major part constituted by plateaus rather than valleys. this implies that watershed may be used only to identify pores, but still, due to complex and stochastic connectivity between grains of individual phases, the watershed algorithm fails with almost no watershed lines created, substantial oversegmentation and loss of information as a result of numerical errors. consequently, marker-controlled watershed was tested with set of markers pre-defined by morphological operations. 58 vol. 18/2018 image and signal processing for assessment of heritage condition however, even this effort led to the same results. as a demonstration, pure gradient magnitude calculation was performed resulting in boundaries between grains generated together with noise and spurious elements, where grains are constituted of material with mixtures. any algorithm applied on these results fails in oversegmentation showing that on such a data additional processing has to be carried out. hence, the following proposed image processing method for quantification of individual phase’s volume fractions was developed and implemented in matlab environment. visual inspection of images captured in sem shows at high magnification that considerable noise is present already during acquisition. for this reason, the images are firstly manipulated to remove noise. among available tools, adaptive filtering using wiener filter [14] was applied to preserve edges and possible high frequency parts of microstructure. then, local variances in intensities (inside grains) were corrected by morphological reconstruction procedure comprising both openingand closing-by-reconstruction processing. in the next step, custom created thresholding routine to extract information about the grey phase (quartz particles) was applied. in this moment, visual inspection is necessary to set upper and lower intensity limit of the thresholding routine. these parameters cannot be determined automatically due to two reasons: the images exhibit inhomogeneous intensity represented by different intensity of selected grains of the same material (that should be the same thanks to bse detector) caused by the morphological reconstruction and it is necessary to extract information about only one phase implying that two level threshold is necessary. to segment into black phase (pores filled with epoxy resin) simple global threshold can be used again preferably with manually selected threshold value after visual inspection. resulting binary images are repeatedly dilated and eroded with various structural elements to smooth the microstructure and to avoid isolated elements. third phase (feldspar) was identified by subtraction of quartz and porous areas from the original image. in the last step, volume fractions were evaluated by summations of logical arrays of relevant phases. segmentation to quartz particles is shown in figure 6. the same effort was made to identify the constituent phases in the colour image data. images that were cropped to include only homogeneous area without oblique borders were in the first step converted from srgb space to the lab space suitable for cluster analysis. k-means clustering was performed with algorithm pre-set to find 3 clusters corresponding to 3 phases present in the samples. resulting 3 images of clusters were then re-converted to rgb space and transformed to binary ones for volume fractions’ computation. moreover when the clusters were visually inspected it has been found out that such an analysis is an applicable approach for identification of dominating structural directions in the material infigure 6. output from segmentation procedure showing image suitable for structural analysis with highlighted detail of microstructure. dicating overall planes of layer formations originated during material’s sedimentation and compaction (see figure 7). figure 7. microstructure subjected to cluster analysis with depicted dominating planes (yellow lines). volume fractions determined in all the calculations were compared to results from mercury porosimetry measurements. it has been ascertained that volume fractions from image computations are generally lower due to inability of imaging devices to capture micropores in the material that significantly contribute to overall porosity. sem bse images proved to be preferable for image processing and phases identification as the image data are derived from atomic composition of the phases rather than their appearance in optical spectrum. as the developed image analysis routines are superior to commonly used methods, the signal 59 p. koudelka, v. koudelková, t. doktor et al. acta polytechnica ctu proceedings processing approach to rve dimensions calculation then yields more reliable results. furthermore, it enables rve dimensions to be calculated not only with respect to porosity (or its distribution), but also according to distribution of individual phases present in the materials. 5. conclusion microstructures of heterogeneous materials extracted from historic buildings have been analysed to identify volume fractions of individual phases present in the materials, to calculate porosity and to demonstrate utilization of signal processing routines for rve dimensions calculation. advanced image processing methods have been applied in order to obtain high quality data for consequent signal processing analysis. input images were captured using sem equipped with bse detector producing grayscale images and using high resolution flatbed scanner producing full colour srgb images. according to colour space, different image segmentation algorithms have been developed. obtained results have been compared to experimentally determined porosities assessed using mercury porosimetry [3, 4]. it has been found out that optical methods are suitable for volume fractions calculation of constituting phases. in contrary, optical porosity determination gives lower values as optical methods are not able to distinguish micropores that are significantly contributing to overall porosity. k-means clustering method applied on colour data is a promising approach for identification of dominating structural directions. employment of advanced image segmentation routines yields more reliable data in consequent spectral analysis calculating rve dimensions. this maximizes probability that specimen with such dimensions extracted from historical building represents the material at the level of constructional element and leads minimal intervention to the built heritage structures. however, given the stochastic nature of their microstructure, large number of samples has to be investigated for every type of stone to obtain reasonable statistics of results. acknowledgements this work was supported by the czech science foundation (project no. p105/12/g059) and by rvo: 68378297. references [1] m. drdacky, z. slizkova. failure and repair in cultural heritage conservation. pp. 607–616. 2010. [2] m. drdacky, j. lesak. non-invasive survey of detachment of historic rendering. vol. 2, pp. 591–597. 2006. [3] z. slizkova, j. bryscejn, e. cechova, et al. technologicka priprava restaurovani pole xi karlova mostu v praze. itam as cr, v.v.i., 2010. [4] h. hasnikova. testing mechanical properties of natural stones used as a building material. engineering mechanics 20(5):427–435, 2013. [5] m. drdacky. non-standard testing of mechanical characteristics of historic mortars. international journal of architectural heritage 5(4-5):383–394, 2011. doi:10.1080/15583051003717788. [6] p. koudelka, d. kytyr, v. petranova, et al. material testing of natural stones used in historical buildings based on scanning electron microscopy and nanoindentation. key engineering materials 586:186–189, 2014. doi:10.4028/www.scientific.net/kem.586.186. [7] r. gonzales, r. woods. digital image processing. prentice hall ptr, 2007. [8] n. otsu. a threshold selection method from gray-level histograms. ieee transactions on systems, man, and cybernetics 9(1):62–66, 1979. doi:10.1109/tsmc.1979.4310076. [9] s. beucher, c. lantuejoul. use of watersheds in contour detection. 1979. [10] f. meyer. a watershed algorithm progressively unveiling its optimality. lecture notes in computer science (including subseries lecture notes in artificial intelligence and lecture notes in bioinformatics) 9082:717–728, 2015. doi:10.1007/978-3-319-18720-4_60. [11] j. canny. a computational approach to edge detection. ieee transactions on pattern analysis and machine intelligence pami-8(6):679–698, 1986. doi:10.1109/tpami.1986.4767851. [12] j. macqueen. some methods for classification and analysis of multivariate observations. in in 5-th berkeley symposium on mathematical statistics and probability, pp. 281–297. 1967. [13] p. koudelka, t. doktor, j. valach, et al. effective elastic moduli of closed-cell aluminium foams homogenization method. pp. 244–250. 2012. [14] j. lim. two-dimensional signal and image processing. third edition. prentice hall ptr, 1990. 60 http://dx.doi.org/10.1080/15583051003717788 http://dx.doi.org/10.4028/www.scientific.net/kem.586.186 http://dx.doi.org/10.1109/tsmc.1979.4310076 http://dx.doi.org/10.1007/978-3-319-18720-4_60 http://dx.doi.org/10.1109/tpami.1986.4767851 acta polytechnica ctu proceedings 18:55–60, 2018 1 introduction 2 materials 3 methods 3.1 image acquisition 3.2 image processing 3.2.1 segmentation of grayscale image data 3.2.2 segmentation of colour image data 3.3 rve assessment 4 results 5 conclusion acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2021.31.0005 acta polytechnica ctu proceedings 31:5–9, 2021 © 2021 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague high-speed rail network and periodic timetable: a comparative analysis of operational concepts michal drábek∗, dominik mazel, jiří pospíšil czech technical university in prague, faculty of transportation sciences, department of logistics and management of transport, horská 3, 128 03 prague 2, czech republic ∗ corresponding author: xdrabek@fd.cvut.cz abstract. this paper compares chosen european high-speed railway (hs) networks in terms of their offer of hs passenger service. the criteria chosen for comparison are network topology, degree of service periodicity and degree of coordination between subsequent services. only services with hs trains are taken into account. as a result, each examined network is classified according to prevailing approach to passenger service – either line/service (ls) approach, where transfer connections are in general not anticipated, or network (n) approach, with regular (mostly periodic) public transport lines and periodic transfer connections between them. the comparative analysis has shown that geography had crucial impact not only on national (or regional) hs line network, but on the hs operational concept as well. on trunk hs lines, which connect most populated agglomerations in particular country, there is always – at least during peak times – some form of periodic service, despite compulsory seat reservation (except state-owned carriers in austria and germany). half of analyzed networks can be characterized by n approach – at least on trunk hs lines or within central "core" part of hs network. for czech hs network, authors recommend to define a core network with application of integrated periodic timetable. keywords: high-speed railway, operational concept, passenger service, periodicity. 1. introduction high-speed (hs) railway service network is a specific form of long-distance railway network. the reasons are longer average trip distance (due to higher commercial speed) and higher planned load factor of hs trains (due to rolling stock and infrastructure cost). hs railway very often beats car even in door-to-door travel time, which leads to unprecedented modal shift towards railway. for estimation of modal-split based on comparation of travel time, see for instance vávra and janoš [1]. either this network is more complex – mostly in the case of state-owned carrier(s), or more simple. two approaches for design of hs operational concept can be distinguished: • line/service (ls) approach • network (n) approach ls approach focuses on origin-destination (od) pairs and times with the highest demand. these od pairs are mostly radial, i.e. to/from central agglomeration. transit trips through this centre are perceived as secondary, i.e. there can be a connection in the central station, but transfer time can be very high, as timetable was not adjusted for this requirement. n approach is based on services which operate in regular public transport lines. all services run through (at least mostly) equal route, with some degree of periodicity (either more service pairs daily, or with some period – at least some services, typically during peak times in relevant direction). periodic transfer connections between these public transport lines are anticipated. the extremely periodic concept is integrated periodic timetable (ipt, taktfahrplan). for more detailed explanation, see for instance weidmann [2]. the aim of this paper is to compare chosen national hs networks in terms of their topology and degree of periodicity of hs passenger services. finally, each network operation will be assigned to (predominant) ls or n approach. 2. methodology and scope of the analysis data on european hs railway network were taken from uic high speed rail 2018 publication [3]. the timetable data, as of 2019/2020 railway timetable, were gathered from timetable websites or online publications of particular carriers [4–14]. for each analyzed national or regional network, following criteria were chosen: • network topology . one or two principal hs lines . star (one center and radial hs lines) 5 https://doi.org/10.14311/app.2021.31.0005 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en m. drábek, d. mazel, j. pospíšil acta polytechnica ctu proceedings . raster network • prevailing degree of periodicity . no periodic service . periodic service in peaks only . partially periodic service on trunk routes . almost fully periodic – with some minor irregularities . fully periodic (with possible additional peak service) following hs networks were analyzed: • austria (öbb and westbahn) • germany (ice) • thalys (belgium, netherlands, france and germany) • france (tgv incl. oui) • italy (fs – frecce trains and .italo) • spain 3. results of the analysis 3.1. austria two principal hs corridors are planned in austria. the first one, western railway (westbahn) is partially in operation – from vienna to linz, with a short section westwards. in this section, there is mostly a parallel hs line to a conventional line, with frequent connecting junctions. further sections are in construction or planned – new lower inn valley railway to innsbruck and brenner base tunnel to italy. the next corridor, currently in construction, goes southwestwards from vienna through graz and klagenfurt. despite axis nature of these hs corridors, the operation there is fully integrated into national integrated periodic timetable [4]. on the only austrian hs line in operation, there are three hs carriers. all operate in zero-symmetric periodic timetables. state-owned öbb-personenverkehr ag operates railjet network, with regular periodic service. the first railjet axis, prague – brno – vienna – graz, is operated in 2-hour period. there are periodic transfer connections with railjet to/from western railway. however, this public transport line does not currently use any hs infrastructure [5]. on the western railway, there operate even two segments of hourly raijlet services between vienna and salzburg [6]. the lower segment fulfills function of intercity service, but uses the hs line from vienna to tullnerfeld junction only. the higher segment, named railjet xpress, serves only vienna, st. pölten, linz and salzburg. from vienna, both segments depart interposed in common 30-min period, but in salzburg they meet each other around minute zero. so, periodic transfer connections are enabled. each 2-hours, an unit from railjet xpress proceeds directly to munich. from salzburg westwards, there is hourly intercitylike service, mostly operated by railjet xpress, but occasionally by railjet, eurocity etc. similar kind of irregularities occurs from budapest to vienna, where mostly railjet xpress services operate, proceeding directly westwards. the service period from/to budapest varies from 30 min to 2 hours. second hs carrier is german state-owned deutsche bahn (db), with intercity-express (ice) services to frankfurt, mostly with 2-hour service period. last hs carrier is (mostly) privately-owned company westbahn (not to be confused with western railway), which operates in mostly 2-hour period between vienna and salzburg [7]. austrian hs operation can be classified as periodic, with few irregularities, and n approach. 3.2. germany german hs railway network creates a raster, in accordance with polycentric settlement of the country. on the main (trunk) routes, there is mostly periodic offer of ice services. basic service period of a long-distance public transport line is usually 2 hours. there are many minor deviations in departure or arrival minutes, or in presence of intermediate stops of secondary significance (i.e. cities with population circa 100,000). another deviations occur in days of service operation. for some services, there are alternative destinations, which may result in a gap in periodic service during some part of the line. there are some cases of interpositions into half period on trunk routes (and even in the case of alternative routes, but same od-pair, e.g. nuremberg – hamburg via hannover or via erfurt). on particular sections between major cities (which are usually principal railway nodes as well), one line of hs trains serves as "lower segment", serving middle-sized cities. other lines of hs trains run through without stop [8]. german hs operation can be considered predominantly periodic, with n approach, albeit with many minor irregularities. 3.3. thalys thalys is a joint venture of french sncf (majority owner) and belgian nmbs/sncb state-owned rail carriers. although predominantly periodic, thalys services are not interconnected at all. the reason is probably comparably short travel distances, so any transfer would disturb the passengers. the service period is mostly 1 or 2 hours (peak), between paris and brussels even 30 min during peaks. from brussels, there are 3 hs lines: to paris nord station, to cologne and to amsterdam. from amsterdam, 13 pairs of services run to brussels, which include 10 pairs that proceed to paris. the period is 1 hour, but there is a 3 hours peak gap from amsterdam. from cologne, 4 pairs of services run to brussels, but there are more additional german ice hs services that make together 1 hour period (with 2 pairs of services left out). from paris, there is 30 min service period during both morning and afternoon 6 vol. 31/2021 high-speed rail network and periodic timetable peaks, but there is no 1 hour period kept all day long. peaks, there are 1.5 hour gaps between subsequent services [9]. thus, despite periodic services (with many irregularities), thalys network is characterized with ls approach. 3.4. france french geography is characterized by strong dominance of île-de-france (paris agglomeration), where one out of six french inhabitants lives, and by transport network converging to paris. hs network is no exception. from four paris dead-end stations, a four-pointed star network (as of 2020) runs out. farther away, some hs railway lines (lgv) branch to various destinations. this network is completed by a hs bypass line, lying eastwards from paris, with stops at roissy – charles de gaulle airport and marne-la-vallée-chessy. french hs service offer is mainly focused on direct connection of paris with as many cities as possible – for smaller cities, there can be circa 2 to 4 pairs of train à grande vitesse (tgv) services per day. this policy leads to more complex branching of tgv routes – much beyond branching of hs railway lines. sometimes there run two coupled tgv units from paris, and from some node station they proceed to different destinations. by nature, such policy makes periodic service offer hardly possible. moreover, even on trunk sections of hs lines, common for more end destinations (e.g. to lyon or to strasbourg), there is almost no periodicity of service – only some groups of subsequent services during afternoon peak from paris (or during morning peak to paris). there are two more factors that increase complexity and ambiguity of tgv service concept. the first one is phenomenon of stations, located directly on a hs line. in some cases, such station lies "in the fields", located between more comparably populated cities. otherwise, this station serves a large city, but lies few to circa 20 km remote. so, there is another station in the city centre. in most such cases, there are both tgv services which serve the station on a hs line, and the others, which run to the centre. the second factor is phenomenon of "tgv intersecteurs" services, which stop on bypass lgv, but not in paris itself [10, 11]. french hs operations can be considered as practically not periodic, with ls approach. 3.5. italy italian hs network consists of three north-south corridors, which merge together in bologna. then, a single hs line (including direttissima hs line, whose construction started as first one in europe) proceeds via rome to naples and salerno. from naples, there is a mostly completed hs branch to adriatic coast (to foggia and bari). moreover, in sicily, there are two partially complete coastal hs lines from messina to palermo and to catania. at present, they are served by conventional trains only. in northern italy, there is an east-west corridor from venice to turin. on the main (trunk) routes, there is mostly periodic offer of frecce (literally "arrows") hs trains, but, contrary to germany, these trains do not always operate in regular public transport lines. the service period of these trains, operated by state-owned fs (trenitalia) carrier is 1 or 2 hours. the frecce offer concept consists of three products, which differ in type of trainset: • red arrow ("frecciarossa") stands for hs train with maximum speed 300 km/h • silver arrow ("frecciargento") stands for tilting hs trains with maximum speed 250 km/h • white arrow ("frecciabianca") stands for classical trainsets with maximum speed 200 km/h since majority of italian hs lines are designed for 300 km/h, there are obviously services that cannot use this maximum speed, because of considerably lower maximum trainset speed. according to actual timetable [12] this situation occurs few times a day, and there is no lengthening of regular running times. in general, such heterogeneity of train paths leads to substantial drop in railway capacity utilization – see for instance janoš and kříž [15]. the competitor – nuovo trasporto viaggiatori (ntv), with services labeled as ".italo" – operates public transport lines between milan and venice, and on three principal hs lines, further together to rome and naples, with few pairs of services proceeding to salerno. single pairs of services proceed directly to further chosen cities in northern italy. .italo services depart in general at regular, periodic times. there are five public transport lines (milan – venice, 3 hs corridors), with few lengthenings northwards. from milan to rome, besides line turin – rome (– naples) there is in addition a non-stop line between these two cities. both lines operate hourly, with few service pairs left out. another public transport lines operate in periods varying from 1 to 3 hours during the day, sometimes with even longer off-peak gap. transfer connections are announced, but not anticipated in planning – the transfer time can reach up to 1.5 hours [13]. as a conclusion, hs passenger service in italy (both state and private carriers) can be classified as predominantly periodic (at least on trunk routes), with n approach (state carrier fs), since there are periodic transfers to non-hs services with transfer time of 30 min or less. 3.6. spain spanish hs railway network follows the country’s specific – mostly mountainous – geography. besides madrid agglomeration and few middle-sized agglomerations, vast majority of population lives in coastal areas or in major valleys, where most of the largest 7 m. drábek, d. mazel, j. pospíšil acta polytechnica ctu proceedings spanish cities are located. contrary to conventional network with wide (iberian) gauge, hs network is constructed with standard gauge. the hs railway network is star-shaped, with two central stations in madrid. chamartín station is a terminus of hs lines to northern and western coast. atocha station is a terminus of remaining hs network (eastern and southern coast and some inland destinations like toledo and granada). there are two main segments of hs service – ave, and avant with alvia [14]. ave is a distinctly hs service with operating speed up to 300 km/h. avant and alvia are lower hs segments with operating speed up to 250 km/h. while avant serves middle distances on hs lines, alvia trains are equipped with gauge-changing device (for some services, talgo trainsets are deployed). thus, alvia trains offer direct services far beyond hs network (mostly to regional centres in coast areas – mainly northwards from madrid). since many destinations and lower frequency (few service pairs daily), there is no regular period of alvia services. in some cases, two alvia trains (or two other hs trains) run coupled, at least during part of their route. spanish hs network is, in general, characterized by non-periodic service, and transfer connections are not anticipated. however, on trunk routes (from madrid to barcelona, sevilla, malaga and valencia) and for some shorter distances (e.g. madrid – toledo), there is periodic service at least during peak times. the period is mostly 1 hour, with almost all-day 30 min period between madrid and barcelona, with additional peak services. there are even few direct hs services between barcelona and sevilla, and euromed coastal hs services from barcelona to valencia. the periodic service is not without various irregularities – either different stopping pattern for each service, or some 1.5 hour gaps between services. so, hs trains to toledo depart from madrid hourly, mostly in the minute 50, but in off-peak time in the minute 20. as a conclusion, spanish hs passenger service can be characterized as partially periodic, with ls approach. 3.7. structured comparison of analyzed networks structured comparison of analyzed networks and hs operational concepts is displayed in table 1. 4. conclusions and recommendations the comparative analysis has shown that geography had crucial impact not only on national (or regional) hs line network, but on the hs operational concept as well. if there is a clearly dominant central agglomeration, the hs passenger service is usually driven by ls approach. the only exception is austria. on trunk hs lines, which connect most populated agglomerations in particular country, there is always at least during peak times some form of periodic service, despite compulsory seat reservation (except state-owned carriers in austria and germany). half of analyzed networks can be characterized by n approach at least on trunk hs lines or within central "core" part of hs network. on the other hand, because hs trains are very expensive, the carriers (even state-owned ones) focus on operational efficiency, at the expense of timetable simplicity and regularity. a notorious example is a 1.5-hour gap between subsequent services. czech republic stands before strategic decisions on high-speed operational concept, since planning of hs lines is already almost in the beginning. the authors recommend to define core part of hs network with application of integrated periodic timetable, and operation without compulsory seat reservation for at least part of a train. open access services should be enabled in such way that they would not disturb this concept. acknowledgements this paper is a partial output of research project ck01000004 "efficient operational concept for rapid services", which is co-financed from the state budget by the technology agency of the czech republic under the transportation 2020+ (doprava 2020+) programme – http://www.tacr.cz, and by ctu in prague, faculty of transportation sciences. the authors hereby acknowledge both sponsors for funding for this project. references [1] r. vávra, v. janoš. analysis of competitiveness of railway passenger transport between rakovník and prague, 2017. acta polytechnica ctu proceedings 11:86–90, https://doi.org/10.14311/app.2017.11.0086. [2] u. weidmann. system-und netzplanung. band 1.1. system-und netzplanung des personenverkehrs, may 2008. ivt eth zürich, zürich. [3] uic. uic high speed rail 2018. [2020-03-22], https://uic.org/img/pdf/ic_high_speed_2018_ ph08_web.pdf. [4] öbb-infrastruktur ag. schienennetznutzungsbedingungen 2020 der öbb-infrastruktur ag. [2020-03-22], https://infrastruktur.oebb.at/de/ geschaeftspartner/schienennetz/snnb/snnb-2020/ schienennetz-nutzungsbedingungen-2020.pdf. [5] české dráhy. jízdní řád 2020, dálkové vlaky, ex3, r9, r19. [2020-03-22], https://www.cd.cz/images/cdosn/ trains/ex3-r9-r19-2020.pdf. [6] öbb kundenservice. schnell und komfortabel richtung westen. [2020-03-22], https://www.oebb.at/dam/jcr: 21c69cd4-78f6-472e-a907-637f9ee11a08/ folder-go-west.pdf. 8 http://www.tacr.cz https://doi.org/10.14311/app.2017.11.0086 https://uic.org/img/pdf/ic_high_speed_2018_ph08_web.pdf. https://uic.org/img/pdf/ic_high_speed_2018_ph08_web.pdf. https://infrastruktur.oebb.at/de/geschaeftspartner/schienennetz/snnb/snnb-2020/schienennetz-nutzungsbedingungen-2020.pdf https://infrastruktur.oebb.at/de/geschaeftspartner/schienennetz/snnb/snnb-2020/schienennetz-nutzungsbedingungen-2020.pdf https://infrastruktur.oebb.at/de/geschaeftspartner/schienennetz/snnb/snnb-2020/schienennetz-nutzungsbedingungen-2020.pdf https://www.cd.cz/images/cdosn/trains/ex3-r9-r19-2020.pdf https://www.cd.cz/images/cdosn/trains/ex3-r9-r19-2020.pdf https://www.oebb.at/dam/jcr:21c69cd4-78f6-472e-a907-637f9ee11a08/folder-go-west.pdf https://www.oebb.at/dam/jcr:21c69cd4-78f6-472e-a907-637f9ee11a08/folder-go-west.pdf https://www.oebb.at/dam/jcr:21c69cd4-78f6-472e-a907-637f9ee11a08/folder-go-west.pdf vol. 31/2021 high-speed rail network and periodic timetable hs network topology degree of periodicity hs oper. approach austria 1 principal route fully periodic n germany raster almost fully periodic n thalys star periodic ls france star + branching periodic in peaks only ls italy raster + 1 principal route + branching periodic (trunk routes) n spain star+branching+raster part. periodic (trunk routes) ls table 1. structured comparison of networks and hs operational concepts. [7] westbahn management gmbh. where do you want to travel? [2020-03-22], http://westbahn.at. [8] db vertrieb gmbh. die bahn. timetable & booking. [2020-03-22], http://www.bahn.com. [9] thalys. book your next trip. [2020-03-22], http://www.thalys.com. [10] sncf. itinerary. [2020-03-22], http://www.sncf.com. [11] oui.sncf. votre fiche horaire. [2020-03-22], https: //www.oui.sncf/services-train/fiches-horaires. [12] trenitalia. gruppo ferrovie dello stato italiane. f. dello stato italiane. [2020-03-22], http://www.trenitalia.com. [13] .italo. nuovo trasporto viaggiatori. where are you going? [2020-03-22], http://www.italotrento.it. [14] renfe. [2020-03-22], http://www.renfe.com. [15] v. janoš, m. kříž. infrastructure parameters affecting capacity of railways in ten-t, 2016. acta polytechnica ctu proceedings 5:22–25, https://doi.org/10.14311/app.2016.5.0022. 9 http://westbahn.at http://www.bahn.com http://www.thalys.com http://www.sncf.com https://www.oui.sncf/services-train/fiches-horaires https://www.oui.sncf/services-train/fiches-horaires http://www.trenitalia.com http://www.italotrento.it http://www.renfe.com https://doi.org/10.14311/app.2016.5.0022 acta polytechnica ctu proceedings 31:5–9, 2021 1 introduction 2 methodology and scope of the analysis 3 results of the analysis 3.1 austria 3.2 germany 3.3 thalys 3.4 france 3.5 italy 3.6 spain 3.7 structured comparison of analyzed networks 4 conclusions and recommendations acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2018.15.0012 acta polytechnica ctu proceedings 15:12–19, 2018 © czech technical university in prague, 2018 available online at http://ojs.cvut.cz/ojs/index.php/app comparison of drying shrinkage and drying creep kinetics in concrete lenka dohnalová∗, petr havlásek department of mechanics, faculty of civil engineering, czech technical university in prague, thákurova 7, 166 29 prague 6, czech republic ∗ corresponding author: lenka.dohnalova@fsv.cvut.cz abstract. the aim of this paper is to show and compare the time evolution of drying shrinkage and drying creep in concrete from three different perspectives. the first one analyzes the basic equations defined in the most common design codes and prediction models for the description of the long-term behavior of concrete (aci 209, ec2, model code 2010, b3, b4). next, the evolution of drying creep and shrinkage is examined by processing suitable experimental data available from the database developed at the northwestern university. finally, the last point of view investigates the results obtained from the finite element simulations employing the material point approach, in particular, the material model based on the microprestress-solidification theory. keywords: drying creep, shrinkage, concrete, design code, prediction model. 1. introduction concrete drying is accompanied by its gradual contractive volume changes referred to as shrinkage and additional compliance called drying creep or the pickett effect. on contrary to basic creep (i.e. creep of hygrally sealed specimens at room temperature), both drying shrinkage and drying creep are believed to be bounded. the complex interplay between shrinkage and creep determines the evolution of stresses and so its thorough analysis can help to identify potential cracking and thus can contribute to estimation of the lifetime of the concrete structure. this is the case for members restrained either externally (statically indeterminate structures) or internally (nonuniform stress field generated by the steep gradient of the profile of relative humidity) but very often by the combination of both. this paper summarizes and extends the final thesis of the first author [1]. the first part of this paper compares the time evolution of drying creep and drying shrinkage according to the most common design codes and prediction models. these models operate on the cross-sectional level and thus they provide the estimated average values of strain and compliance which are assumed to be uniform across the cross-section. nowadays, most of the prediction models use as the input parameters the recipe of the concrete mixture, the 28-day compressive strength f c, and the duration of curing (or the onset of drying). in order to estimate the rate of drying (and so the rate of shrinkage and drying creep), it is necessary to provide the model with the size (usually by the quantity called “equivalent thickness” computed as the ratio of volume to drying surface) and the shape of the cross-section. the second part of the paper examines and processes the available experimental data gathered in the database developed and maintained at the northwestern university (nu) [2]. the next part compares the drying shrinkage and drying creep kinetics obtained from the results of the finite element simulations. these simulations operate on the material point level which implicates the non-uniform stress field. if such model is correctly calibrated, it is applicable not only for the analysis of complex and creep-sensitive structures, but also for the assessment of potential surface or internal cracking stemming from the severe drying conditions or excessive temperature generated by cement hydration in massive structures. the results are compared and discussed in the last part of the paper. 2. design codes and prediction models two design codes (aci 209.2r–08 [3] and eurocode 2 [4]) and three prediction models for the long–term behavior of concrete (b3 model [5], b4 model [6] and fib model code 2010 [7]) were selected for a detailed analysis of time evolution of drying creep and drying shrinkage. the investigated prediction models split concrete creep additively into basic creep and drying creep components allowing to evaluate drying creep directly. on the other hand, the analyzed design codes (aci 209, eurocode 2) provide only the formula for the total compliance; therefore, the drying creep compliance was obtained by subtracting the compliance at sealed conditions from the total compliance corresponding to the drying conditions. the total shrinkage is usually considered as the sum of autogenous and drying shrinkage. the b3 model neglects the autogenous shrinkage, for this reason the drying shrinkage is set equal to the total shrinkage. 12 http://dx.doi.org/10.14311/app.2018.15.0012 http://ojs.cvut.cz/ojs/index.php/app vol. 15/2018 comparison of drying shrinkage and drying creep kinetics in concrete the aci 209 does not neglect the autogenous shrinkage but provides only the formula for total shrinkage which is nil at 100% relative humidity. therefore, similarly to the b3 model, the total shrinkage is taken as the drying shrinkage. even though the individual models use different functions to define the time evolution of drying creep and shrinkage, the shape of the resulting curves is very similar. bigger differences are observed in the final values but these are not in the scope of the present paper. next, we present a brief description of the design and prediction models used in this study. throughout the different models εsh,d stands for the drying shrinkage and henv for the ambient relative humidity (expressed as decimal). the expressions for the drying creep compliance jd are for simplicity presented with t′ = t0, i.e. with the age at loading t′ coinciding with the onset of drying t0. the duration of loading/drying is denoted as t̂ = t− t′ = t− t0. in some models the evolution of drying is derived from v/s which is the ratio of the volume of the concrete member to its surface exposed to the ambient relative humidity. b3 model was developed by bažant and co–workers at the northwestern university and uses, similarly to the other prediction models, empirical formulae to determine the time development of creep and shrinkage. the parameters in the model can be estimated based on the composition of the concrete mixture, the mean compressive strength and in the case of drying on the ambient relative humidity and on the size and shape of the concrete member. the autogenous shrinkage is neglected in the model and so the range of applicability is limited to the water-to-cement ratio above 0.35. the drying creep compliance and drying shrinkage are defined by the following two equations jd(t̂) = q5 e8 ( (1−henv)·tanh √ t̂ τsh −1 ) − 1 0.5 (1) εsh(t̂) = −ε∞sh · kh · tanh √ t̂ τsh (2) here, τsh is the shrinkage half–time depending on the size and shape of the member, ε∞sh is the absolute value of the ultimate drying shrinkage at henv = 0, kh is a humidity-dependent factor, and q5 is an adjustable parameter. b4 model builds on the previous b3 model and uses more input parameters of the concrete mixture (e.g. aggregate type, additives and admixtures) as well as more elaborate formulae for the prediction of the model parameters. nevertheless, the structure of basic equations of the b4 model is very similar to its predecessor. the autogenous shrinkage and drying shrinkage are treated in the b4 model separately. fib model code 2010 the next two equations present a simplified form of the drying creep compliance and shrinkage functions jd(t̂, t′) = 1 e28 β∗dc [ t̂ βd + t̂ ] √ t′ 3.5+ √ 5.29 t′ (3) εsh,d(t̂) = εsh,d0 βrh [ t̂ 0.035d2 + t̂ ]0.5 (4) where e28 is the modulus of elasticity at the age of 28 days, d is the effective cross section thickness (twice the volume-surface ratio), βd is a size dependence factor, βrh is a humidity dependence factor, and β∗dc and εsh,d0 are model parameters depending on compressive strength, relative humidity and the age at loading. eurocode 2 does not specify an explicit formula for the evaluation of the compliance function j from the creep coefficient ϕ, the conversion was done using a modified expression from model code 1990 [8]. j(t̂, t′) = 1 ecm(t′) + ϕ 1.05ecm(28) [ t̂ βh + t̂ ]0.3 (5) εsh,d(t̂) = kd · εsh0 · t̂ t̂+ 0.04 ·d1.5 (6) where ecm(t′) is the secant modulus of elasticity of concrete at the age of loading, βh is a parameter depending on relative humidity and member size, kd · εsh0 expresses the ultimate value of drying shrinkage. aci 209.2r–08 on contrary to ec2, the american standard determines the compliance function from the creep coefficient and the secant modulus of elasticity at the age at loading ecm(t′) j(t̂, t′) = 1 ecm(t′) · [ 1 + t̂ψ d+ t̂ψ · ϕu ] (7) εsh(t̂) = εshu · t̂α f + t̂α (8) where d, ψ, f , and α are parameters, and ϕu and εshu are the ultimate creep coefficient and the ultimate shrinkage, respectively, both influenced by the ambient relative humidity. there are two distinct options how to select the model parameters defining the time evolution of drying creep and shrinkage. the first one, as simplified in [3] from the former version of the code [9], is to use the average and fixed values of the parameters, then ψ = 0.6, α = 1.0 and in general d 6= f . the second possibility, which reflects the size and shape of the structural member, is to set the exponents α = ψ = 1.0, and d = f which are determined from v/s of the drying member. 13 lenka dohnalová, petr havlásek acta polytechnica ctu proceedings basic parameters f c ecm28 t0 t′ c w/c a/c cem. henv 2v/s mpa gpa day day kgm−3 type mm example from aci [3] 33.3 28.2 7 7 409 0.50 4.23 i 0.7 200 experiment troxell [10] 16.5 na 28 28 320 0.59 5.67 i 0.5/0.7 45 experiment bryant [11] 50.1 29.8 8 8 390 0.47 5.09 i 0.6 150/75 ∗) experiment keeton [12] 45 25.9 8 8 452 0.46 3.74 iii 0.2/0.5 65 ∗) 150 mm slab, 75 mm prism table 1. details of the experimental setup in the creep and shrinkage experiments. 10-1 100 101 102 103 104 0 10 20 30 40 10-1 100 101 102 103 104 0 100 200 300 400 500 600 figure 1. drying creep compliance curves (top) and drying shrinkage curves (bottom) given by the analyzed design codes and prediction models. the time evolution of drying creep and shrinkage shown in fig. 1 was obtained with the parameters computed for the concrete mixture and the specimen properties defined in appendix c of aci 209 [3]. the parameters are listed in tab. 1. the specimen is considered as a slab with 200mm thickness and moist cured for 7 days. the aggregate type used in the b4 model is the quartzite. the curves in fig. 1 are evaluated for the duration of 100 years which is the typical design period of most of the bridges and engineering structures. both figures show the horizontal time axis in the logarithmic scale which is more suitable for processes with initially very high rate and which gradually decreases. fig. 2 presents the time evolution of normalized dry100 102 104 0 0.5 1 100 102 104 0 0.5 1 100 102 104 0 0.5 1 100 102 104 0 0.5 1 figure 2. time evolution of normalized drying creep and shrinkage according to the selected design codes and prediction models. the functions are normalized with respect to the ultimate value. decreasing drying creep from ec2 model is physically unjustified and stems from the model formulation. ing creep, (j̄d, solid red lines) and drying shrinkage (ε̄sh,d, dashed blue lines) for the selected design codes and prediction models. the evolution is normalized with respect to the ultimate value. the presented response of the models is based on the input parameters specified in the first line of tab. 1. naturally, the curves will be different for a modified composition, dimensions or other conditions, but the dominant trends will prevail provided that the ambient relative humidity is kept constant (see the first part in section 5). a different way of comparison is shown in fig. 3 where the normalized drying creep is plotted against normalized drying shrinkage. however, here, the real kinetics of both phenomena is not reflected. it must be noted that in the present example (shown in figs. 1–3), the aci model uses the recommended fixed values of parameters (i.e. the model is size independent). 14 vol. 15/2018 comparison of drying shrinkage and drying creep kinetics in concrete 0 0.2 0.4 0.6 0.8 1 0 0.2 0.4 0.6 0.8 1 figure 3. comparison of normalized drying shrinkage and normalized drying creep evolution according to the selected design codes and prediction models. normalization is done with respect to the ultimate value. 3. experimental data the freely available experimental database developed at the northwestern university [2] was selected as the source of suitable experimental evidence of drying shrinkage and drying creep. some experimental data comprised in this database were used for the calibration of the models presented in the previous section, and so it can be expected that the processed experimental data will very likely resemble the curves in figs. 2 and 3. the database contains 61,930 values measured on 3,308 specimens (1,439 for creep and 1,869 for shrinkage). these values come from 362 experimental surveys (172 creep, 190 shrinkage). some experimental surveys analyzed more than one concrete composition; in total it was 378 in the case of creep and 652 different compositions in the case of shrinkage. only 68 experimental surveys are relevant to this study because only those used the same recipe of the concrete mixture for both creep and shrinkage specimens. for the present purpose it was necessary to further narrow the selection only to those experiments which tested simultaneously drying shrinkage on unloaded specimens, basic creep, and total creep matter. additionally, the loading in the creep experiments had to be applied simultaneously with the beginning of drying of both loaded and unloaded specimens. the database contains only very few surveys which provide the information on autogenous shrinkage and sometimes it is difficult to distinguish between experiments carried out at sealed conditions and experiments where the specimens are immersed in water or exposed to very high relative humidity and thus exhibit swelling. in order to eliminate this deficiency an additional criteria were set on the value of water-to-cement ratio (minimum value w/c = 0.4) and the compressive strength (maximum value f c = 60mpa), then the autogenous shrinkage can be treated as negligible compared to the magnitude of the drying shrinkage. only 97 sets from 9 experimental surveys met the aforementioned criteria. unfortunately, it was necessary to further reduce the selection because of incorrect or missing pieces of information, or too short duration of the experiment. the duration was judged as sufficient if the rate of drying creep and shrinkage was minimal at the end of the experiment. this was satisfied only in 6 experimental surveys (with 71 specimens) out of which 3 representative are partially presented in this paper. the value of drying creep was determined as the difference between total and basic creep; when needed (in the case of different times of measurement), the value was computed using linear interpolation between two adjacent measurements. the drying creep and shrinkage was afterwards normalized with respect to the last value attained. the suitable experiments are summarized in the following paragraphs. experiments of troxell [10] adopted in this study are part of an extensive experimental survey focused on the influence of different aspects on the development of creep and shrinkage. the experiment started in 1928 and its duration exceeded 20 years. the data used in this study come from the experimental series #3 which examined the influence of relative humidity; the specimens were cylinders with 102mm diameter and 356mm height. the basic properties and the composition of the concrete mixture are specified in tab. 1. the nu database does not contain data for t̂ < 12 days and thus completely misses the initial evolution of creep and shrinkage. two triplets of specimens were suitable for the comparison of drying shrinkage and drying creep kinetics, one for 50%, the second one for 70% relative humidity. the results are shown in fig. 4. experiments of bryant [11] was initially focused on study of creep and shrinkage in bridge structures. only specimens with dimensions 150× 150× 600mm were suitable for the present study because only this size was loaded at the age when the drying began which was not the case of the remaining sizes. the specimens should have realistically represented structural members slabs and pillars, to achieve this, one set of specimens had both ends and two opposite lateral sides hygrally sealed (specimens referred to as “slab”) which resulted in unidirectional drying. the other set was sealed only at the ends and thus the specimens were drying evenly from all lateral sides, this set is denoted as “prism”. 15 lenka dohnalová, petr havlásek acta polytechnica ctu proceedings 0 0.2 0.4 0.6 0.8 1 0 0.2 0.4 0.6 0.8 1 figure 4. comparison of normalized drying shrinkage and normalized drying creep evaluated from the experiments of troxell and bryant. only the points represent the experimental data. at the age of 8 days all specimens were transferred to the environment with 60 % relative humidity. unfortunately, during the experiment the aluminum foil used for sealing has corroded and especially the entirely sealed specimens started unintentionally drying resulting into an increase in creep or shrinkage rate. for this reason the autogenous shrinkage was neglected and the total shrinkage was taken as the drying shrinkage. at the later stage of the experiment, the measurements of the basic creep (spoiled by the sealing leak) partially influenced the determination of drying creep. the processed results are for both groups (slabs and prisms) shown in fig. 4. experiments of keeton [12] the suitable experimental data come from the part of research studying the influence of ambient relative humidity, specimen size, and the load level on creep and shrinkage. the research started in 1957 and lasted for 897 days. the data are not from the nu database but instead from the original paper (and are different). in several cases the increments of basic creep are larger than the increments of total creep. this was the problem especially of smaller specimens with lower level of loading and obviously specimens exposed to higher of relative humidity. additionally, both loaded and unloaded companion specimens were not sealed but were subjected to 100% relative humidity. owing to the aforementioned reasons the smaller specimens, the specimens with low loading level and specimens drying at 75% relative humidity are omitted from the present analysis. fig. 5 shows processed results obtained from experiments on cylinders 152× 457mm drying at 20% and 50% relative humidity and loaded at 30% and 40% of 0 0.2 0.4 0.6 0.8 1 0 0.2 0.4 0.6 0.8 1 figure 5. comparison of normalized drying shrinkage and normalized drying creep evaluated from the experiments of keeton at 20% and 50% relative humidity and two levels of loading. f ′c is the compressive strength at the age of loading. points are the experimental data. the compressive strength at the age of loading, f ′c. in this figure, the higher initial level of shrinkage stems from the first measurement in which the total creep was smaller than the basic creep. 4. data from finite elements simulations except for the prismatic specimens 150×150×600mm shown in the previous section of this study, the bryant’s experimental survey comprises the information on shrinkage and drying creep measured on geometrically specimens ranging from 100×100×400mm to 400× 400× 1600mm prepared from a single concrete batch. for this reason this experiment was in [13] and [14] selected as an ideal opportunity for verification of the material model based on microprestresssolidification theory (mps) [15]. it turned out that the original mps gives the opposite size effect on drying creep than the experiment and for this reason the mps model was modified [13, 14]. here we present the comparison of drying creep and shrinkage for both approaches. hereafter, the modified version of mps model is referred to as “mps improved”. the presented data use the processed results obtained from the finite element simulations carried out in the open source fe package [16]. the problem was analyzed as one-way coupled with the structural problem depending on the moisture transport according to the bažand and najjar model [17] for diffusion of water vapor. in order to obtain the shrinkage and drying creep kinetics, it was necessary perform, similarly to the real experiment, 3 analy16 vol. 15/2018 comparison of drying shrinkage and drying creep kinetics in concrete 100 102 104 0 0.5 1 100 102 104 0 0.5 1 100 102 104 0 0.5 1 100 102 104 0 0.5 1 figure 6. the processed results of fe simulations of bryant’s experiment showing the time evolution of normalized drying creep and shrinkage. the top two figures are for 150mm slabs bottom two figures are for 150 × 150mm prism. the curves are normalized with respect to the ultimate value. ses: basic creep, drying creep and shrinkage. the values of the material parameters in the models were selected in order to obtain a reasonably accurate fit of all specimens (“prisms” and “slabs” of different sizes), therefore, the agreement of the simulation of 150mm specimen with the experimental data cannot be expected to be satisfactory. the normalized drying creep and shrinkage is for the original and improved mps model and the two geometries shown in fig. 6. 5. discussion in general, according to the prediction models and design codes, the time evolution of drying creep and drying shrinkage depends dominantly on the ambient relative humidity and on the size of the concrete member, the influence of other factors is not so significant. when evaluating the differences in kinetics of drying creep and drying shrinkage, each model is sensitive only to certain input parameters. in the case of the b3 model, the only factor which affects the relationship between drying creep and drying shrinkage is the ambient relative humidity. at low relative humidity, the drying creep is delayed behind shrinkage, but on contrary to this, at high levels (henv > 0.8), the delay is in shrinkage. however, the difference is in both cases relatively small, see fig. 7. on the other hand, the fib mc2010 is completely insensitive to henv. the kinetics is influenced dominantly by the size of the member (its v/s), see fig. 8. the larger the size the more is the curve in fig. 2 bent downwards and the bigger is the delay of shrinkage behind drying creep. this is caused by the fact that with decreasing v/s the drying shrinkage becomes 0 0.2 0.4 0.6 0.8 1 0 0.2 0.4 0.6 0.8 1 figure 7. the influence of different levels of ambient relative humidity on the drying creep vs. drying shrinkage kinetics according to the b3 model. the remaining parameters are from the first line in tab. 1. 0 0.2 0.4 0.6 0.8 1 0 0.2 0.4 0.6 0.8 1 figure 8. the influence of different v/s ratios on the drying creep vs. drying shrinkage kinetics according to the fib mc 2010 model. the remaining parameters are from the first line in tab. 1 more accelerated than the drying creep. to certain extent the kinetics is also influenced by the onset of drying, but for reasonable values the differences are not substantial. the eurocode 2 predicts, except for extreme combinations of parameters, delay in shrinkage behind drying creep. the model is sensitive both to v/s a and henv. for smaller specimens and higher relative humidity the drying creep curve becomes more non-monotonic. 17 lenka dohnalová, petr havlásek acta polytechnica ctu proceedings 0 0.2 0.4 0.6 0.8 1 0 0.2 0.4 0.6 0.8 1 figure 9. the comparison of the experimental data, prediction models and fe simulations of the bryant’s experiment, 2v/s = 150 mm (slab). aci 209 is the exception to the other presented models. for first approach with the average recommended values (according to [9]) the kinetics of drying creep and shrinkage remains constant, with an initial small delay of shrinkage behind drying creep. according to the second approach, which sets equal the exponents α and ψ as well as the parameters d and f , the kinetics of drying creep and drying shrinkage become identical (i.e. the function would be a diagonal in fig. 3). as presented in figs. 4 and 5, the selected suitable experimental data give the opposite trend compared the prediction models (except for b3/4 at lower relative humidity and aci at the later stage of drying). this means that in the experiments the drying shrinkage is a little bit ahead of the drying creep. the points in these figures correspond to the experimental data which are interpolated by the curves of the same color. two issues have to be pointed out. first, the resulting shape of the curve in the normalized plot shrinkage vs. drying creep depends on the duration of the experiment. insufficiently long experiments can lead to misleading conclusions. second, as demonstrated in fig. 5, the readings should be documented more frequently in the initial phase of drying and the specimens should be instrumented as soon as possible. in the case of keeton’s experiment, the first displayed data point corresponds to the drying duration 7 days and the shrinkage strain reaches almost 40 % of the last measured value (after approx. 3 years). (the first data points for t̂ = 1 day which would have yielded negative jd is omitted here.) according to the computations with the original version of the material model based on the microprestresssolidification theory, the drying creep evolves in an 0 0.2 0.4 0.6 0.8 1 0 0.2 0.4 0.6 0.8 1 figure 10. the comparison of the experimental data, prediction models and fe simulations of the bryant’s experiment, 2v/s = 75 mm (prism). unexpected fashion. first, as shown in the two left graphs in fig. 6 and in figs. 9 and 10, the drying creep evolves initially faster than drying shrinkage, then, suddenly, the creep stops and after certain time it continues with a considerable delay after shrinkage. this effect is more pronounced in the case of prismatic shape which dries more rapidly. the response of the improved mps model is more promising, see the two right graphs in fig.6. similarly to the cross-sectional approaches, it gives a small initial delay of shrinkage after drying creep, afterwards, the relationship is almost linear. however, the initial delay is reversed compared to the analyzed experimental data. very probably, in the case of the models working on the cross-sectional approach, the differences of the shrinkage and drying creep kinetics origin from their optimization and calibration procedure. very likely the models were calibrated against the entire database with evenly distributed weights and without emphasizing that some specimens are companions belonging to the same experimental survey. 6. conclusions the presented study analyzed the time evolution of drying shrinkage εsh,d and drying creep jd from three different perspectives: the most common design and prediction models, experimental data measured on laboratory concrete specimens, and the finite element simulations. the following conclusions can be drawn: • even though the design codes and prediction models are calibrated on a very similar experimental database, jd and εsh,d are described in each model 18 vol. 15/2018 comparison of drying shrinkage and drying creep kinetics in concrete differently and their kinetics is different. additionally, in each model, the relationship between the jd and εsh,d depends on different factors. three of the analyzed models, b3, b4, and aci 209, give for the standard conditions comparable kinetics of these processes. however, in the case of fib mc2010 and ec2 the delay of εsh,d behind jd is substantial. according to ec2, jd is a non-monotonic function. • a closer inspection of the experimental database which contains a vast amount of data revealed that only very few experiments fulfill the needs of the present research. unfortunately, most of the experiments does not last sufficiently long or certain essential pieces of information are missing. the analyzed data indicate that the jd is initially slightly delayed behind εsh,d, which means that the trend is exactly opposite compared to the prediction models. • the improved mps model gives more realistic kinetics of jd and εsh,d compared to its predecessor which unexpectedly gives a significant decrease in drying creep rate in the middle of drying. both models predict, similarly to the cross-sectional models, an initial small delay of εsh,d behind jd. • sufficiently long experiments on normal-strength concrete are vital for the development and proper calibration of the material models for creep and concrete, however, they are very scarce. acknowledgements the authors gratefully acknowledge financial support from the grant agency of the czech technical university in prague, project number sgs18/037/ohk1/1t/11. the numerical analyses have been performed with oofem, an open-source object-oriented finite element program [16, 18]. references [1] l. dohnalová. comparison of shrinkage and drying creep kinetics of concrete, bc. thesis. czech technical university in prague, 2018. [2] m. hubler, r. wendner, z. bažant. comprehensive database for concrete creep and shrinkage: analysis and recommendations for testing and recording. aci 112:547–558, 2015. [3] aci committee 209. guide for modeling and calculating shrinkage and creep in hardened concrete. aci, 2008. [4] český normalizační institut. eurokód 2: navrhování betonových konstrukcí část 1-1: obecná pravidla a pravidla pro pozemní stavby. český normalizační institut, 1996. [5] z. bažant, s. baweja. creep and shrinkage prediction model for analysis and design of concrete structures: model b3. adam neville symposium: creep and shrinkage structural design effects 2000. [6] rilem technical committee tc-242-mdc (z.p. bažant, chair). model b4 for creep, drying shrinkage and autogenous shrinkage of normal and high-strength concretes with multi-decade applicability (rilem technical committee tc-242-mdc multi-decade creep and shrinkage of concrete: material model and structural analysis). materials and structures 48:753–770, 2015. [7] fédération internationale du béton. model code 2010. no. vol. 65 in fib bulletin. international federation for structural concrete (fib), 2012. [8] comité euro–international du béton (ceb). ceb–fip model code 1990: design code. t. telford, london, 1993. [9] a. c. 209. prediction of creep, shrinkage, and temperature effects in concrete structures. american concrete institute, 1997. [10] g. troxel, j. raphael, r. davis. long-time creep and shrinkage tests of plain and reinforced concrete. proc astm 58 pp. 1101–1120, 1958. [11] a. h. bryant, c. vadhanavikkit. creep, shrinkage-size, and age at loading effects. aci materials journal 84:117–123, 1987. [12] keeton. study of creep in concrete, phase 1 (i-beam). u. s. naval civil engineering laboratory, port hueneme, california, 1965. [13] p. havlásek. creep and shrinkage of concrete subjected to variable environmental conditions, phd. thesis. czech technical university in prague, 2014. [14] z. bažant, p. havlásek, m. jirásek. microprestress-solidification theory: modeling of size effect on drying creep. in n. bicanic, h. mang, g. meschke, r. de borst (eds.), computational modelling of concrete structures, pp. 749–758. crc press/balkema, eh leiden, the netherlands, 2014. [15] z. p. bažant, a. p. hauggaard, s. baweja, f. j. ulm. microprestress solidification theory for concrete creep. i: aging and drying effects. journal of engineering mechanics 123:1188–1194, 1997. [16] b. patzák. oofem home page, 2000. http://www.oofem.org. [17] z. p. bažant, l. j. najjar. nonlinear water diffusion in nonsaturated concrete. materials and structures 5:3–20, 1972. doi:10.1007/bf02479073. [18] b. patzák. oofem an object-oriented simulation tool for advanced modeling of materials and structures. acta polytechnica 52(6):59–66, 2012. 19 http://dx.doi.org/10.1007/bf02479073 acta polytechnica ctu proceedings 15:12–19, 2018 1 introduction 2 design codes and prediction models 3 experimental data 4 data from finite elements simulations 5 discussion 6 conclusions acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2018.18.0038 acta polytechnica ctu proceedings 18:38–43, 2018 © czech technical university in prague, 2018 available online at http://ojs.cvut.cz/ojs/index.php/app numerical modelling of compressive characteristics of auxetic structures michaela neuhäuserová∗, petr koudelka czech technical university in prague, faculty of transportation sciences, department of mechanics and materials, konviktská 20, 110 00 prague 1, czech republic ∗ corresponding author: neuhauserova@fd.cvut.cz abstract. the paper is focused on numerical analysis of mechanical behaviour of auxetic structures with re-entrant tetrakaidecahedral unit cell subjected to uni-axial quasi-static compression. the mechanical behaviour was evaluated inversely with respect to selected geometrical parameters of the unit cell and two different loading modes. finite element method was used for the numerical analysis of the problem. a set of fully parametric tools has been developed, which enabled automated execution and evaluation of virtual experiments. from results of the simulations, young’s modulus, the characteristics of the poisson’s ratio function, and the deformation energy density were estimated. the relation between these characteristics and geometry of the unit cell, particularly the re-entrant angle and the relative density, was evaluated. results of the numerical simulations for the unit cell and representative volume element of its three-dimensional periodic assembly are presented. keywords: auxetic structures, re-entrant tetrakaidecahedron, finite element method, uni-axial loading, quasi-static compression. 1. introduction 1.1. background porous solids, particularly metal foams, are nowadays often utilized in various kinds of applications due to their advantageous mechanical properties. for example, their relatively low specific weight and thus high specific strength, make them suitable for applications requiring mass reduction and simultaneous high energy absorption [1]. however, for constructions subjected to extreme ways of loading such as high strain-rate impacts or blasts, compressive properties of conventional metal foams are insufficient [2]. thus, alternative materials are sought and investigated. one of possible approaches is development of optimized auxetic structures [3]. 1.2. auxetic structures auxetic materials are materials with negative poisson’s ratio. when stretched laterally, they get thicker in transverse direction and vice versa. due to this unconventional deformation behaviour, auxetics have unique mechanical properties including the ability to absorb relatively high amount of deformation energy [4]. there exist only a few known natural auxetic materials, majority of them are produced artificially based on geometric models with deformation behaviour analytically described in closed form [5] . modern production methods such as additive manufacturing allows precise production of structures with controlled geometry. therefore, these methods are used for production of auxetic materials based on known geometric models [6]. 1.2.1. re-entrant tetrakaidecahedron auxetic structure with unit cell in shape of re-entrant tetrakaidecahedron was first introduced by choi and lakes in 1995 [7]. in their study, they presented analytical model of mechanical behaviour of the unit cell as well as simple evaluation experiment for irregular assembly of the unit cells. both the analytical and the experimental data provided in the study indicated auxetic character of the structure with reentrant tetrakaidecahedral unit cells. the geometry of the auxetic unit cell is a modification of conventional tetrakaidecahedron with square faces protruded in the inner space of the cell. schemes of both, conventional and re-entrant tetrakaidecahedrons can be seen in figure 1. figure 1. geometry of conventional tetrakaidecahedron (left) and re-entrant tetrakaidecahedron (right) [7] this geometry of the unit cell was for this study selected due to its spatial symmetry, which determines potential of auxetic structure created by threedimensional periodic assembly of this unit cell to be mechanically orthotropic [8]. therefore, the structure can exhibit auxetic character in three orthogonal di38 http://dx.doi.org/10.14311/app.2018.18.0038 http://ojs.cvut.cz/ojs/index.php/app vol. 18/2018 rections in contrast to other auxetics, which in most cases exhibit negative poisson‘s ratio only in one or two orthogonal directions [5]. mechanical behaviour of the re-entrat tetrakaidecahedral unit cell has been described by choi and lakes in [7]. the model is based on geometrical aspects of deformations occurring in the structure if square faces of the cell is loaded. the equations 1 and 2 present the analytical model for elastic and for plastic deformations [7]. as can be seen from the equations, the model is based on the angles of the protruded ribs in the structure. the figure 2 illustrates the re-entrant angle ϕ. the angle θ represents the angle by which the rib jk rotate around the point k during larger deformations. νel = − sin (ϕ− π/4) cos (ϕ− π/4) (1) νpl = −cos (ϕ− π/4 − θ) − cos (ϕ− π/4) sin (ϕ− π/4 − θ) − sin (ϕ− π/4) (2) l k y x j ϕ π 2 − ϕ figure 2. one quater of cross section of the re-etrant tetrakaidecahedral unit cell 1.3. aim the main aim of the study is to perform numerical analysis of microstructural characteristcs to inversely evaluate effective mechanical properties of auxetic structures with re-entrant tetrakaidecahedral unit cell. for this reason, simulated uni-axial quasi-static compression testing of the structures was performed using the ansys finite element (fe) software. 2. numerical analysis to perform the numerical analysis, a set of fully parametric tools has been developed in ansys software and automated execution and evaluation of virtual experiments have been implemented. 2.1. geometric models the numerical simulations were performed using two different geometric models of the structure. the unit cell of the re-entrant tetrakaidecahedron itself as well as a representative volume element (rve) of the structure created by three-dimensional periodic assembly of the unit cells were analysed. the analysis of the unit cell itself was performed foremost for the comparison with the analytical model as well as for acquiring the knowledge of deformation processes on the elementary level of the structure. however, as can be seen in the results section, the results obtained from analysis of rve of the structure are rather predicative in terms of practical utilization of such a structure. the dimensions of the unit cell itself were 2.3 × 2.3 × 2.3mm since the dimension of a unit beam in the structure (one side of the rectangular face) is 1mm. the size of the rve was estimated based on analysis of young‘s modulus of structures created by increasing number of cells. for lower numbers, the value of young‘s modulus significantly varied. the higher number of cells was, the young’s modulus of the structure rather converged to a specific value and lowest number of cells for which the value converged was 12 along each axis. therefore, the rve contained 12 unit cell along each axis yielding 1728 unit cells in total. the dimensions of the rve were 27.6 × 27.6 × 27.6mm. the geometric models were generated with variable parameters such as the size of the re-entrant angle and the relative density of the structure. the size of the re-entrant angle determines the rate of protrusion of rectangular faces into the inner space of the unit cell (the larger the angle is, the more the cell is enclosed). the relative density is given by the volumetric ratio of solid mass in the structure and in the models, it is varied by changing the thickness of beams creating the structure. in the figure 3 can be seen how different values of these parameters affect the overall design of the unit cell. figure 3. top: the affect of increasing re-entrant angle on the design of the unit cell (from left to right). bottom: the affect of increasing relative density on the design of the unit cell (from left to right) 2.2. material model construction material was simulated with isotropic, linear elastic material model. its parameters, particularly young’s modulus and poisson’s ratio, were set according to values corresponding to powder stainless steel ss 316l-0407 by renishaw. the values of the parameters are noted in table 1. this material was selected since it is commonly used for selective laser sintering (additive manufacturing method) and based on the results of the study, real samples of the structure are about to be produced using this specific material. 39 michaela neuhäuserová, petr koudelka acta polytechnica ctu proceedings parameter value young’s modulus 190 gpa poisson’s ratio 0.27 table 1. parameters of material model 2.3. finite element mesh and numerical boundary conditions in order to perform the numerical analysis, the geometric models were further transformed to the mesh of finite 3d beam elements with linear shape function and six degrees of freedom at each node. boundary conditions were specified at selected nodes of the models. all degrees of freedom were constrained at all nodes with maximal coordinate in one selected spatial direction forming one support in the virtual compressive experiment. in terms of loading, there were two different loading modes analysed. in the first mode, the loads were applied on the nodes in the vertices of square faces of the unit cell. in the second mode, the loads were applied on the nodes in the vertices of the triangular faces. schemas with highlighted nodes with specified boundary conditions in both of the loading modes can be seen in figure 4. the loading procedure was displacement driven in both cases. in order to evaluate young‘s modulus, the models were loaded in one load step up to 2% of deformation. the other parameters were estimated from loading in multiple load steps. the model of rve was loaded in five load steps to the total deformation of 5% and the model of the unit cell was loaded in 30 load steps to the total deformation of 10%. figure 4. boundary conditions in first loading mode (left) and in the second loading mode (right). the first of the loading modes was analysed especially because the analytical model of the unit cell’s deformation behaviour is based on this mode [7]. however, the second mode rather corresponds to the way in which the loads would be transferred into the structure during a real compression testing. 3. results the results of the numerical simulations has been utilized for inverse estimation of selected material characteristics of the structures. particularly, young’s modulus, or relative young’s modulus (the ratio of young’s modulus of the structure and young’s modulus of the construction material), poisson’s ratio and the deformation energy density were evaluated with respect to specific geometrical parameters of the unit cell, such as the re-entrant angle and the relative density of the structure. moreover, the mechanical behaviour of the structure was analysed with respect to two different loading modes. 3.1. relative young’s modulus with respect to relative density the dependency of relative young’s modulus on relative density of the structure rve with re-entrant angle 70◦ was analysed in both loading modes. as can be seen at the resulting diagrams in figure 5, the relative young’s modulus is in both cases proportional to the relative density of the structure and the curves have very similar trend for both loading modes. figure 5. relative young’s modulus of rve of the structure plotted against its relative density the acquired diagrams have similar trend to such diagrams for conventional porous solids [2]. however, the values are lower than values estimated for conventional foams with open cells e.g. gibson-ashby cells analysed in [9]. this trend corresponds to the assumption that auxetic structures have lower stiffness and thus also lower value of young’s modulus or relative young’s modulus than conventional porous solids [10]. 3.2. poisson’s ratio of the structure with respect to the size of re-entrant angle then the dependency of poisson’s ratio on the size of re-entrant angle was analysed on the model of rve with relative density equal to 0.02. as can be seen on resulting diagrams in figure 6, the values of poisson’s ratio are inversely proportional to the size of re-entrant angle in both loading modes. as can be also seen, for most of the geometries the value of poisson’s ratio was not negative within analysed deformation range. nevertheless, the trend of the resulting diagram is in all cases descending and thus it is likely that negative values could be obtained at larger deformations. it was not possible to perform the simulations of larger deformations due to the simplicity of the model which is used in this study. this model would not 40 vol. 18/2018 consider contacts of the parts during larger deformations. in order to simulate such tasks, the current model needs to be further modified. figure 6. the poisson’s ratio function (strain dependent poisson’s ratio) of rve with respect to the size of re-entrant angle in first loading mode (top) and second loading mode (bottom). 3.3. poisson’s ratio of the structure with respect to its relative density the poisson’s ratio function (strain dependent poisson’s ratio) for the rve of the structure with reentrant angle of size 70◦ was also analysed with respect to relative density of the structure. in figure 7 are depicted resulting diagrams. figure 7. the poisson’s ratio function of rve with respect to the relative density of the structure it can be seen that the values of poisson’s ratio are directly proportional to relative density of the structure. this phenomenon occurs as a result of filling the total volume of the structure with the construction material. thus, the range of deformations within the structure is limited. 3.4. deformation energy density the deformation energy density λ is a mechanical characteristic representing the ability of material to absorb deformation energy and its value is given by the area under the stress-strain curve. the results obtained for rve with relative density of 0.02 with respect to different re-entrant angles are denoted in the table below. the values of λ1 corresponds to the deformation energy density from the first loading mode (loading of rectangular faces) and the λ2 denotes the values for the second loading mode (loading of triangular faces). re-entrant angle ϕ λ1[j/m3] λ2[j/m3] 60◦ 6732 118 65◦ 5973 91 70◦ 5245 73 75◦ 4660 56 80◦ 4119 49 table 2. deformation energy density of rve with respect to re-entrant angle size the values of deformation energy density are inversely proportional to the size of re-entrant angle for both loading modes. moreover, there is a significant difference between the values with respect to the loading mode. as has been observed from the models of the structure after deformation, the mechanical response of rve differs for the loading modes. when the loads are applied on the vertices in rectangular faces, the structure "densifies" in the area where the loads are applied. in the other hand, when the vertices of triangular faces are loaded, the deformation is distributed gradually throughout the structure and displacements are spread layer by layer. therefore, the energy accumulated in the structure is higher in the first case. similarly to conventional porous solids, auxetic structures have the potential to absorb relatively high amount of deformation energy especially due to significant stress plateau, which occurs after yield point [2]. however, the simulations in this study were performed only in a limited deformation range with only a linearelastic material model and thus this phenomenon can not be observed. therefore, further investigation of larger deformation ranges on modified model of rve is necessary in order to fully evaluate the ability of this structure to absorb deformation energy. 41 michaela neuhäuserová, petr koudelka acta polytechnica ctu proceedings 3.5. evaluation of the analytical model as has already been mentioned, the analytical model of auxetic unit cell in shape of re-entrant tetrakaidecahedron was first presented by choi and lakes in 1995 [7]. since then, the literature does not provide any further studies concerning such a unit cell or any structure based on this unit cell. the comparison of the poisson’s ratio function estimated on the basis of the analytical model presented in [7] and the results obtained from the finite element analysis performed in this study can be seen in figure 8. the analytical model is defined with load applied at the square face of the cell. therefore, the results of numerical analysis of the first loading mode were used for the comparison. it is obvious that in both cases, according to analytical model as well as finite element model, the poisson’s ratio of the structure is inversely proportional to the size of the re-entrant angle. numerical results varies from the analytical significantly in the initial deformation zone up to approximately 4% of deformation. nevertheless, for larger deformations the values of poisson’s ratio estimated numerically are similar to the analytical values. given differences can be given by the nature of the analytical model, which is based only on the size of the re-entrant angle and does not assume any other parameters which can affect the mechanical behaviour of the structure. figure 8. comparison of poisson’s ratio functions obtained from analytical model [7] and from the finite element analysis performed in this study 4. conclusions this study was focused on numerical analysis of mechanical behaviour of auxetic structure with unit cell in shape of re-entrant tetrakaidecahedron. simulated mechanical behaviour of the representative volume element of the structure as well as the unit cell itself were analysed. selected mechanical properties, such as young‘s modulus, poisson‘s ratio and the deformation energy density were evaluated with respect to specific geometrical parameters of the unit cell, such as the re-entrant angle and the relative density of the structure. moreover, the mechanical behaviour of the structure was analysed with respect to two different loading modes. the results from the analysis of the unit cell were used for evaluation of analytical model of the cell. the values of poisson‘s ratio obtained from the numerical simulations of deformations larger than 4% were similar to the analytical values and hereby the auxetic potential of such a geometry was numerically verified. however, further investigation of the structure is necessary in order to fully verify and maybe modify the analytical model due to the differences in the poisson’s ratio functions obtained from the analytical model and from the numerical simulations. e.g. verification experiments or more complex numerical analyses should be considered. the results obtained from the analysis of the rve were used to evaluate effective properties of periodical assembly of unit cells. the knowledge of the relation between the mechanical properties of the structure and its geometric parametes can be further utilized in development of optimization procedures [11]. in such a process, optimal geometry of the structure corresponding to required mechanical properties can be proposed. moreover, the unit cell geometry can be further modified in order to obtain deformation behaviour corresponding to loading of square faces across the whole periodic assembly of the cells. this can be achieved e.g. by adding stems between the square faces of the neighbouring unit cells. thus, the cells would not be connected through the vertices of triangular faces but through the stems between the square faces and so the deformation would spread throughout the structure in a different manner. in a future research, the optimized micro-structural models will be used for production of samples for experimental verification of the numerical results during a series of validation experiments. acknowledgements the research has been supported by project inafym within operational programme research, development and education (project no. cz.02.1.01/0.0/0.0/16_019/0000766). references [1] y. sugimura, j. meyer, m. y. he, et al. on the mechanical performance of closed cell al alloy foams. acta materialia 1997. [2] p. koudelka, o. jiroušek, t. fíla, t. doktor. compressive properties of auxetic structures produced with direct 3d printing. materiali in tehnologije 2016. [3] t. doktor, p. koudelka, t. fíla, o. jiroušek. finite element based structural optimization of auxetic structures. civil-comp proceedings 2015. [4] r. s. lakes. foam structures with a negative poisson’s ratio. science 1987. [5] k. k. saxena, r. das, e. p. calius. three decades of auxetics research – materials with negative poisson’s ratio: a review. advanced engineering materials 2016. 42 vol. 18/2018 [6] r. critchley, i. corni, j. wharton, et al. the preparation of auxetic foams by three-dimensional printing and their characteristics. advanced engineering materials 2013. [7] j. b. choi, r. s. lakes. nonlinear analysis of the poisson‘s ratio of negative poisson‘s ratio foams. journal of composite materials 1995. [8] x. hou, v. v. silberschmidt. metamaterials with negative poisson‘s ratio: a review of mechanical properties and deformation mechanism. springer, cham, 2015. [9] p. koudelka, o. jiroušek, j. valach. determination of mechanical properties of metal foams using microstructural models. 2011. [10] x.-t. wang, b. wang, z.-h. wen, l. ma. fabrication and mechanical properties of cfrp composite three-dimensional double-arrow-head auxetic structures. composites science and technology 2018. [11] p. koudelka, p. zlámal, d. kytýř, et al. on the modelling of the compressive behaviour of metal foams: a comparison of discretization schemes. proceedings of the fourteenth international conference on civil, structural and environmental engineering computing 2013. 43 acta polytechnica ctu proceedings 18:38–43, 2018 1 introduction 1.1 background 1.2 auxetic structures 1.2.1 re-entrant tetrakaidecahedron 1.3 aim 2 numerical analysis 2.1 geometric models 2.2 material model 2.3 finite element mesh and numerical boundary conditions 3 results 3.1 relative young's modulus with respect to relative density 3.2 poisson's ratio of the structure with respect to the size of re-entrant angle 3.3 poisson's ratio of the structure with respect to its relative density 3.4 deformation energy density 3.5 evaluation of the analytical model 4 conclusions acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2019.25.0068 acta polytechnica ctu proceedings 25:68–72, 2019 © czech technical university in prague, 2019 available online at http://ojs.cvut.cz/ojs/index.php/app strain-rate and printing direction dependency of compressive behaviour of 3d printed stainless steel 316l michaela neuhäuserová∗, petr koudelka, jan falta, marcel adorna, tomáš fíla, petr zlámal czech technical university in prague, faculty of transportation sciences, department of mechanics and materials, na florenci 25, 110 00 prague 1 ∗ corresponding author: neuhauserova@fd.cvut.cz abstract. the paper is focused on evaluation of the relation between mechanical properties of 3d printed stainless steel 316l-0407 and printing direction (i.e. the orientation of the part which is being printed in the manufacturing device) subjected to compressive loading at different strain-rates. in order to evaluate the strain rate dependency of the 3d printed material’s compressive characteristics, dynamic and quasi-static experiments were performed. three sets of bulk specimens were produced, each having a different printing orientation with respect to the powder bed plane (vertical, horizontal and tilted). to assess the deformation behaviour of the 3d printed material, compressive stress-strain diagrams and compressive yield strength and tangent modulus were evaluated. keywords: 3d printing, selective laser sintering, stainless steel 316l, printing direction, split hopkinson pressure bar. 1. introduction selective laser sintering (sls) is an additive manufacturing/3d printing method based on sintering of powdered material (e.g. metal or alloy powder) into a solid body using a precisely focused laser beam with a specific power. due to its principle, it is a viable procedure for production of parts with a complex geometry that would not be easy or even possible to produce by conventional manufacturing procedures [1]. using this method, required part is directly sintered according to its cad model. to be able to reliably predict mechanical properties of the parts produced in such a way, it is necessary to know the characteristics of the sintered material itself. such a knowledge enables to tune the material model for numerical simulations which allows to obtain more reliable results of computational analysis and to optimize the design of 3d printed parts [2]. in general, there are a lot of parameters affecting the quality of the final material produced by sls, such as laser power, printed layer thickness, granularity of the powdered material or the orientation of the part during its production [3]. this study is focused on the dependency of mechanical behaviour of sls printed austenitic steel 316l-0407 on the printing direction (i.e. the orientation of the printed layers of the material with respect to the powder bed plane). moreover, the presented study aims also on evaluation of the strain-rate dependency of mechanical characteristics of the material, which is necessary for designing parts tailored to their applications. 2. materials and methods experimental study presented in this paper consisted of two types of experiments, quasi-static and dynamic, which were performed with the sls printed bulk specimens of austenitic steel 316l-0407. in order to assess the strain-rate sensitivity as well as the effect of printing direction on the compressive behaviour of the investigated material, three different sets of specimens, each with different printing orientation, were tested at various strain-rates. the quasi-static experiments were performed using electro-mechanical loading device and dynamic experiments were conducted using split hopkinson pressure bar (shpb) experimental apparatus [4]. 2.1. specimens all the specimens were designed using a parametric modeller and produced by renishaw am250 3d printing device. the most important parameters of the sls procedure used for the specimens production are listed in tab. 1. the mean density of the sls printed material (measured of reference cylinder) was 7.52 ± 0.17 g/cm3. according to the material sheet provided by renishaw [5], the density of wrought material is 7.99 g/cm3. thus, it can be assumed there is a 6% porosity in the material produced by the sls method, which corresponds to the difference in the values of density. the design of the specimens differed for quasi-static and dynamic experiments due to the limits of the experimental devices used for their testing. the particular designs are thus described in the following subsections. 68 http://dx.doi.org/10.14311/app.2019.25.0068 http://ojs.cvut.cz/ojs/index.php/app vol. 25/2019 strain-rate and printing direction dependency of 3d printed 316l parameter value scanning strategy chessboard laser power max 200 w powdered material particle size 15-45 µm layer thickness 50 µm table 1. parameters of sls procedure 2.1.1. specimens for quasi-static experiments the specimens for quasi-static experiments had cylindrical shape with diameter of 5 mm and height of 10 mm. therefore, the height to diameter ratio was 2. the specimens were printed with three different orientations vertically, at the angle of 0°, horizontally, at the angle of 90° and tilted,at the angle of 45° in every case related to the powder bed plane (see fig. 1). figure 1. orientation of the specimens durig production: a) vertical, b) horizontal and c) tilted. 2.1.2. specimens for dynamic experiments "dog bone" shaped specimens with overall dimensions of 18 × 16 mm were used for dynamic experiments. the particular design of the specimens for dynamic tests (see fig. 2) was selected due to the parameters of the used shpb apparatus. the contact faces of the specimens needed to have a wider diameter in order to achieve similar material impedance as has the material of the bars. in this regard, the deformation wave is not reflected at the bar-specimen interference but is transmitted into the specimen without any significant loss. however, the center part of the specimen needed to be thinner in order to achieve a sufficient deformation during the dynamic experiments. the specimens were also 3d printed with the same three different orientations with respect to the powder bed plane (see fig. 3). figure 2. dimensions of dog bone shaped specimen used for dynamic experiments. figure 3. orientation of the specimens durig production: a) vertical, b) horizontal and c) tilted. 2.2. experimental setup to assess the strain-rate dependency of mechanical behaviour of the investigated material, two different loading devices were used depending on the type of the experiments. each of the experimental setups is described in following subsections. 2.2.1. quasi-static experiments electro-mechanical loading device 3382 (instron, usa) was used for the quasi-static experiments. the loading procedure was displacement controlled with the loading velocity of 1 mm/min which yielded in a strain rate of approx. 0.002 s−1. applied force was measured using a 100 kn load cell. 2.2.2. dynamic experiments the dynamic experiments were conducted using shpb apparatus equipped with high-strength aluminium alloy bars (en-aw-7075) having a diameter of 20 mm. depending on the required strain-rate, two different striker bars were used in the experiments: striker bar with the length of 500 mm for higher strain-rate and of 650 mm for lower strain-rate. using the striker bars of different lenghts enables to achieve the same deformation range at both the strain-rates. the incident and the transmission bar had each the length of 1600 mm. foil strain gauges 3/120 ly61 (hbm, germany) with active length of 3 mm wired in the wheatstone half-bridge arrangement were used for the instrumentation of the bars. in order to reduce the wave dispersion effects, the pulse-shaping technique was engaged using cylindrical soft copper shapers with diameter of 7 mm and thickness of 1 mm. described experimental setup can be seen in fig. 4. figure 4. overview of the shpb apparatus used for dynamic experiments. 69 m. neuhäuserová, p. koudelka, j. falta et al. acta polytechnica ctu proceedings 3. results for each printing orientation and each strain-rate, three experiments were performed yielding in 9 quasistatic and 18 dynamic experiments in total. compressive behaviour of the investigated material under quasi-static loading conditions is expressed by the mean stress-strain curves in fig. 5. note, that the values of strain were evaluated based on the cross-head displacement. each curve in the above-mentioned figure corresponds to a different printing orientation. the diagram also shows the standard deviation of each of the curves. the deformation behaviour is very similar up to the strain of 0.2. for the higher strain values, changes in the slope of the curves occur as the material was densifying and the specimens were bulging out in a form of a barrrel. at that the slope of the curve differ for each of the printing direction, too. the densification effects are apparent for all the specimens no matter the printing orientation. however, for the specimens printed horizontally are these effects a little more significant as the mean stress for these specimens reaches the highest values. figure 5. mean stress-strain curves evaluated from quasi-static experiments compressive yield strength and tangent modulus at the beginning of the plastic region were evaluated from the quasi-static experiments. the mean values of these characteristics for each of the printing directions are together with the standard deviations listed in tab. 2. the yield strenght evaluated for the specimens printed with vertical direction has the same value (considering the standard deviation) as the value listed in the material sheet [5]. however, for the vertical printing direction the value estimated from the experiments is smaller by approx. 25 % than the value presented in the same material sheet. for the purpose of comparison, the quasi-static compressive response of dog bone shaped specimens was also evaluated. the stress-strain curves for these specimens together with the diagrams for the cylindrical specimens can be seen in fig. 6. printing direction σy [mpa] et[gpa] 0° 415 ± 34 2.48 ± 0.13 90° 496 ± 33 2.23 ± 0.11 45 ° 443 ± 4 2.32 ± 0.12 table 2. mean values and standard deviation of young’s modulus and yield strength evaluated from quasi-static experiments the compressive response of the dog bone shaped specimen corresponds well to the behaviour of the cylindrical specimens taking into account the standard deviation of the results. nevertherless, there is one significant difference. for the cylindrical specimes, there are noticeable changes in the slopes of the stress-stain curves around the strain of 0.5 but for the dog bone shaped specimens the slopes remain invariable. this indicates that the densification and barreling effects are not so significant for the dog bone shaped. figure 6. mean stress-strain curves for cylindrical and dog bone shaped specimens evaluated from quasistatic experiments the difference between quasi-static and dynamic compressive response of the material arises from the stress-strain curves in fig. 7, where all the mean stress-strain and strain-rate-strain curves are shown. the strain-rate-strain diagrams show that similar strain-rates were achieved in all dynamic experiments. the higher strain rate was approx. 6000 s−1 and the lower was approx. 2000 s−1. as can bee seen from the stress-strain curves, higher values of stress were achieved during the dynamic compression compared to the results of the quasi-static experiments. in other words, the material exhibits a higher strength at dynamic loading conditions. the particular stress-strain diagrams supplemented by the standard deviation plots for each of the 70 vol. 25/2019 strain-rate and printing direction dependency of 3d printed 316l figure 7. mean stress-strain and strain-rate-strain curves evaluated from quasi-static and dynamic experiments with dog bone shaped specimens printing orientations can be seen in figs 8, 9 and 10. the dependency of the strain-rate sensitivity and the printing direction of the material is noticable when comparing these diagrams. the specimens with horizontal printing orientation embodied the most significant differences (on average 28%) between the stresses achieved during the quasi-static and the dynamic loading. the smallest differences (on average 14%) in the values of stress concentrated in the material during the quasi-static and the dynamic experiments were noted for the tilted printing orientation. the average difference of the stress values evaluated from quasi-static and dynamic testing of vertically printed specimens was 21%. figure 8. mean results for the specimens printed at the angle of 0° 4. conclusions experimental study presented in this paper revealed a strain-rate sensitivity of mechanical response of sls printed austenitic stainless steel 316l. a significant increase (up to 28% on average) in evaluated stress values occured for dynamic experiments compared to the figure 9. mean results for the specimens printed at the angle of 90° figure 10. mean results for the specimens printed at the angle of 45° values obtained from quasi-static experiments. moreover, the effect of printing direction of the material on its mechanical response was studied. at quasi-static loading conditions, only a minor effect of this characteristic of sls printed material was noticed. the specimens printed horizontally exhibited an increased compressive strength compared to the other investigated printing directions. more significant effect of printing direction occured during the dynamic experiments. the specimens printed horizontally exhibited the most strain-rate sensitive mechanical behaviour as the stress values evaluated from quasi-static and dynamic experiments differed by 28% on average. the average difference in the stress values evaluated for the other investigated printing directions, vertical and tilted, were 21% and 14% respectively. all the evaluated dependencies should be considered when designing any part produced using the sls production method. especially the knowledge of the strain-rate sensitivity can be conveniently used when designing parts exposed to dynamic loading conditions. 71 m. neuhäuserová, p. koudelka, j. falta et al. acta polytechnica ctu proceedings acknowledgements the research was supported by the czech science foundation (project no. 19-23675s) and the internal grants of the czech technical university in prague (projects no. sgs19/123/ohk2/2t/16). all the financial support is gratefully acknowledged. references [1] n. ahmed. direct metal fabrication in rapid prototyping: a review. journal of manufacturing processes 42:167 – 191, 2019. doi:10.1016/j.jmapro.2019.05.001. [2] b. song, e. nishida, b. sanborn, et al. compressive and tensile stress-strain responses of additively manufactured (am) 304l stainless steel at high strain rates. journal of dynamic behavior of materials 3, 2017. doi:10.1007/s40870-017-0122-6. [3] n. li, s. huang, g. zhang, et al. progress in additive manufacturing on new materials: a review. journal of materials science and technology 35(2):242 – 269, 2019. recent advances in additive manufacturing of metals and alloys, doi:10.1016/j.jmst.2018.09.002. [4] t. fíla, p. zlámal, o. jirousek, et al. impact testing of polymer-filled auxetics using split hopkinson pressure bar. advanced engineering materials 19(10):1700076, 2017. doi:10.1002/adem.201700076. [5] renishaw. data sheet: ss 316l-0407 powder for additive manufacturing: ampd am 3d printing metal powder am250. https://resources.renishaw.com/en/ details/data-sheet-ss-316l-0407-powder-foradditive-manufacturing--90802, 2017. 72 https://doi.org/10.1016/j.jmapro.2019.05.001 https://doi.org/10.1007/s40870-017-0122-6 https://doi.org/10.1016/j.jmst.2018.09.002 https://doi.org/10.1002/adem.201700076 https://resources.renishaw.com/en/details/data-sheet-ss-316l-0407-powder-for-additive-manufacturing--90802 https://resources.renishaw.com/en/details/data-sheet-ss-316l-0407-powder-for-additive-manufacturing--90802 https://resources.renishaw.com/en/details/data-sheet-ss-316l-0407-powder-for-additive-manufacturing--90802 acta polytechnica ctu proceedings 25:68–72, 2019 1 introduction 2 materials and methods 2.1 specimens 2.1.1 specimens for quasi-static experiments 2.1.2 specimens for dynamic experiments 2.2 experimental setup 2.2.1 quasi-static experiments 2.2.2 dynamic experiments 3 results 4 conclusions acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2019.22.0062 acta polytechnica ctu proceedings 22:62–66, 2019 © czech technical university in prague, 2019 available online at http://ojs.cvut.cz/ojs/index.php/app influence of corundum as coarse aggregate in high-performance concrete on projectile impact resistance michal mára∗, zdeňka říhová, markéta kočová czech technical university in prague, faculty of civil engineering, thákurova 7, 166 29 prague 6, czech republic ∗ corresponding author: michal.mara@fsv.cvut.cz abstract. the aim of this paper is focused on the influence of corundum as coarse aggregate in high-performance concrete on projectile impact resistance in terms of depth of penetration and size of the crater. based on the experimental programme, samples of high-performance steel-fibre-reinforced concrete with different types and quantities of coarse aggregates were designed and produced, to be subjected to impact loading in the form of projectile impact. damaged samples were scanned using a 3d scanner and their surface was evaluated. the goal of the paper was to find the optimum content of coarse aggregate in the mixture in order to improve its resistance against projectile impact. keywords: high-performance concrete, hpc, coarse aggregate, corundum, projectile impact, depth of penetration, crater surface, 3d scanning. 1. introduction in recent years, great emphasis is placed on durability, strength and safety of concrete structures, especially civil and military structures, which could be the target of deliberate or accidental shock loads. these structures, including objects of strategic importance, such as government buildings, reinforcements, defence shelters or other protective structures, must be resistant against this type of loads. these elements and their material are designed to withstand the possible extreme loads that can be caused by projectiles, fragments, explosions, cars or aircraft impact, earthquakes or terrorist attacks. one of the materials that have a high potential to use for these structures is high-performance concrete with a dispersed steel reinforcement (hpsfrc). this research is focused on improving the resistance of hpsfrc slabs to projectile impact, with aiming to improve the properties while maintaining the thickness and weight of the slabs for easy handling. from previous studies [1–3] it is known that the increasing amounts of steel fibres over a certain limit worsens the workability of fresh concrete and lowers of compressive strength and modulus. for this reason, this work is focused on improving hpsfrc by adding coarse aggregate. concrete resistance against projectile impact depends on many aspects. improved resistance against impact loads with respect to penetration depth and crater diameter can be achieved by reducing water/cement ratio and increasing compressive strength of concrete. the presence of dispersed steel fibres then decreases the diameter of the crater. for these reasons, the use of hpsfrc is the ideal solution for building structures that could be potentially exposed to impact loads. it is known that the conventional fibre-reinforced concrete (frc) with normal strength matrix has a high energy absorption capacity when hit by a projectile. however, studies show that hpsfrc has a much greater energy absorption capacity in both quasi-static and dynamic loading [4]. the resistance of the concrete against the impact of the projectile is also affected by the size, strength and hardness of the added aggregate. in order to achieve the highest compressive strength, the presence of coarse aggregate in hpsfrc is usually reduced or even eliminated in order to achieve the required homogeneity and density of the mixture. the antipenetration capabilities of coarse aggregate have been therefore almost neglected. however, in the projectile penetration experiments performed by zhang et al. [5], langberg and markeset [6] or wu et al. [7], they found that the penetration depth of the projectile does not decrease as long as the compressive strength of the concrete reaches a certain boundary. the optimal compressive strength for hpsfrc, from which the protective structures will be constructed, has been designed to be 90-150mpa in terms of comprehensive considerations of effective protection and cost of production [4]. 2. material and mixtures as a coarse material for hpsfrc, an artificial white corundum was chosen (figure 1), mainly because of its very high hardness (mohs hardness scale number 9) and its impact resistance. an experimental study with uhpsfrc with added corundum as coarse aggregate (uhp-casfrc) was carried out by wu [4], who focused on uhp-casfrc samples with different contents and different corundum fractions. the sam62 http://dx.doi.org/10.14311/app.2019.22.0062 http://ojs.cvut.cz/ojs/index.php/app vol. 22/2019 influence of corundum as coarse aggregate. . . figure 1. corundum, fraction 3-5mm. ples were tested by a projectile with impact velocity in the range of 510-850m/s, an average mass of 341.2 g and a diameter of 25.3mm. from the results, it was concluded that when the aggregate grain size increases, the concrete achieves higher compressive strengths. when the fraction was changed from 5-20mm to 3545mm, the compressive strength increased by 18%. in terms of depth of penetration (dop) of the projectile, it can be stated that when the aggregate grain size is increased from 0.5d (where d is the diameter of the projectile) to 1.5d and 2.5d its value decreases gradually. with increasing grain size of filler, the probability of a direct hit of coarse aggregate particles increases. therefore, it is possible in this way greatly enhance impact resistance of uhpsfrc, if coarse aggregate with high hardness and strength is used. the surface of the crater after the impact of the projectile decreases as the grain size of the added coarse aggregate increases. slightly better results are also obtained when the content of corundum in the mixture is increased. the average surface area of the crater with 45% corundum content was 12.5% smaller compared to the 30% corundum content [4]. within the experimental program, six types of samples were proposed one reference mixture and five other mixtures with a variable percentage of corundum (10%, 15%, 20%, 22.5% and 30%). all slabs were made in a thickness of 50mm and a size of 400 × 300mm. the steel fibres were 13mm in length and 0.15mm in diameter. the amount of added fibres for all samples was 1.5% by volume of the resulting composite. a dry prefabricated mixture of a multifunctional silicate composite [8] was used, to which the corundum was added. this prefabricated mixture comprises a cement matrix which forms a ground cement clinker with a specific surface area of more than 150m2/kg. the dry prefabricated mixture contains the high-range water reducer, a defoaming agent, silica-fume, silica powder and fine sands with a maximal grain size of 1.2mm. the number of individual components can be seen in table 1. the water to cement ratio of this mixture was 0.3. two kinds of samples were made with the prefabricated mixture with 10% and 15% content of corundum from the total volume of the composite. the corundum was used in a 3-5mm fraction; its figure 2. capture the crater by using a 3d scanner. bulk density was 3850 kg/m3. the producer did not provide us with the granulometry of this fraction. for further corundum samples, a custom mixture was designed to meet optimized hpsfrc granulometry. as well as prefabricated mixture it contained silica powder, microsilica, a superplasticizer and a defoaming agent. the composition of this mixture can be seen in table 2. in this recipe, corundum replaced fine aggregates in 15%, 22.5% and 30% of the total aggregates content. the mixture uses a cement class 52.5, the water-to-cement ratio was designed to 0.3. 3. experimental programme besides the slabs, three beams of 160 × 40 × 40mm from each mixture were made. for these samples was performed a three-point test for flexural strength. average strengths and bulk densities of materials are shown in table 3. the table is supplemented with the average values of hpsfrc samples made from a dry prefabricated mixture of multifunctional silicate composite without added coarse aggregate. prepared slabs of 400×300×50mm were subjected to a projectile impact 7.62 × 39 in the shooting range. it is a 7.62mm full-metal jacket with a soft lead core. the weight of this projectile was 8.04 g, the velocity of the projectile is around 710 m/s. in previous experiments, uhpsfrc slabs 50mm thick without added coarse aggregate stopped this type of projectile, so it is assumed that the plates with coarse aggregate should also resist this type the projectile [9]. evaluation of the results was carried out by using the 3d scanner david, which employs two-picture photogrammetry (stereophotogrammetry). for each sample, the crater on the front side was captured using this 3d scanner (figure 2). the scanner was placed at a suitable distance and the correct resolution was set and the surface image of the board was scanned (figure 3). to capture the entire surface of the crater were taken pictures from four different angles. these four pictures were putted together by software david and created one final 3d picture of crater. the resulting surface was exported in .obj format and subsequently processed in autocad civil 3d software. then, in this program, from the files that 63 m. mára, z. říhová, m. kočová acta polytechnica ctu proceedings mass proportions component 1 binder 0.1 active form sio2, so called microsilica 0.25 silica powder (fraction d50 around 6µm) 1.6 silica sand with smooth granulometry (0.1 1.2mm medium size grain d50 – around 500µm) 0.01 high range water reducer (superplasticizer) 0.001 defoaming agents table 1. components of the prefabricated mixture – proportions by weight. composition hpsfrc corundum corundum corundum corundum corundum 10% 15% 20% 22.5% 30% cement 1 additives 0.4 water 0.3 hrwr 0.025 fine sands 1.76 1.59 1.50 1.42 1.36 1.23 corundum 0 0.17 0.26 0.34 0.40 0.53 table 2. composition of high-performance concrete with corundum. figure 3. picture the surface of the board. figure 4. picture of crater surface in autocad civil 3d using point cloud. contained the 3d coordinates of the scanned craters, it created a point cloud (figure 4). the software then created a 3d surface and was able to measure the area of the crater (3d surface), diameter of the created crater and the penetration depth of the projectile. 4. results and discussion the resulting values of diameter and depth of penetration of the surface craters were obtained using autocad civil 3d are shown in tab. 4. the following graph (figure 5) shows that adding 10% and 15% corundum aggregates leads to a decreasing penetration depth of 23.4mm to 20.7mm and at a further increase of corundum content the dop even decrease slightly to 19.7mm. for samples of designed hpsfrc mixture, with increasing amount of corundum is achieved to reduce a depth of penetration from 23.4mm to 19.7mm. the chart in figure 5 shows the dependence of the depth of penetration on the amount of corundum added. in general, it can be stated, that with the increasing amount of aggregate, the depth of penetration decreases, but the optimal composition of the mixture must be still found. if the grain curve is not filled with sufficient fine grain aggregate, it will occur to microcracks of the composite due to insufficient compaction of the mixture. the next chart (figure 6) shows the dependence of the diameter of the crater on the amount of corundum added. with the higher content of coarse aggregate in hpsfrc, the diameter of the crater formed by the impact of a projectile is decreasing. in the prefabricated mixture, when the corundum is increased from 10% to 20%, the crater diameter is reduced from 81mm to 68.2mm. on the contrary, when com64 vol. 22/2019 influence of corundum as coarse aggregate. . . mixture corundum compressive strength tensile strength density [kg/m3] [mpa] [mpa] [kg/m3] hpsfrc 0 158.0 25.60 2373 corundum 10% 119.5 164.6 30.19 2293 corundum 15% 181.1 164.2 30.86 2400 corundum 20% 239.0 173.3 28.16 2433 corundum 22.5% 296.4 163.5 31.25 2391 corundum 30% 375.0 170.4 26.40 2468 table 3. mechanical properties of high-performance concrete with corundum. samples dop diameter 2d surface 3d surface [mm] [mm] [mm2] [mm2] corundum 10% 20.7 81.0 5158 43095 corundum 15% 23.4 77.2 4688 39619 corundum 20% 19.7 68.2 3710 47967 corundum 22.5% 20.7 75.6 4506 40107 corundum 30% 19.7 66.7 3506 28384 table 4. crater dimensions of uhpfrc after projectile impact. figure 5. dependence of depth penetration on corundum content. figure 6. dependence of crater diameter on corundum content. 65 m. mára, z. říhová, m. kočová acta polytechnica ctu proceedings paring the mixture with 22.5% and 20% corundum, the crater diameter drops from 75.6mm to 66.7mm. other variations in the crater diameter are due to the diversity between prefabricated and a custom mixture. both mixtures have different amount of fine sands and corundum aggregate. that’s why the corundum sample has a 22.5% larger crater diameter than corundum 20%, even if there is coarser aggregate. 5. conclusions in terms of resistance to impact of the projectile, it has been found the addition of coarse corundum aggregate to high-performance steel-fibre-reinforced concrete (hpsfrc) has a positive effect on its impact resistance. the depth of penetration, as well as the diameter of the crater, depends on the compressive strength of the composite and the amount of aggregate added. the higher the compressive strength of the material and the coarser aggregate added to the hpsfrc, the better the impedance properties the material proves. these properties also depend on the quality of the added aggregate, especially on its strength and hardness. coarse aggregates have shown great potential to improve the resistance of hpsfrc, as the addition of a higher amount of corundum has improved the hpfsrc impact behaviour. the aim of this article was not to compare the benefits of corundum depending on the price. however, as literature shows, in larger structures the higher corundum price is defended in case of a significant improvement in the resistance to the projectile impact. acknowledgements this paper was prepared as a part of research supported by the ctu in prague [project no. sgs18/057/ohk1/1t/11]. the authors gratefully acknowledge the financial support from the ministry of industry of the czech republic [project no. fv10547]. references [1] p. máca, r. sovják, p. konvalinka. mix design of uhpfrc and its response to projectile impact. international journal of impact engineering 63:158–163, 2014. doi:10.1016/j.ijimpeng.2013.08.003. [2] v. corinaldesi, g. moriconi. mechanical and thermal evaluation of ultra high performance fiber reinforced concretes for engineering applications. construction and building materials 26:289–294, 2012. doi:10.1016/j.conbuildmat.2011.06.023. [3] r. yu, p. spiesz, h. brouwers. mix design and properties assessment of ultra-high performance fibre reinforced concrete (uhpfrc). cement and concrete research 56:29–39, 2014. doi:10.1016/j.cemconres.2013.11.002. [4] h. wu, q. fang, j. gong, et al. projectile impact resistance of corundum aggregated uhp-sfrc. international journal of impact engineering 84:38–53, 2015. doi:10.1016/j.ijimpeng.2015.05.007. [5] m.-h. zhang, v. shim, g. lu, c. chew. resistance of high-strength concrete to projectile impact. international journal of impact engineering 31:825–841, 2005. doi:10.1016/j.ijimpeng.2004.04.009. [6] h. langberg, g. markeset. high performance concrete-penetration resistance and material development. in proceedings of the ninth international symposium on interaction of the effects of munitions with structures, pp. 933–941. 1999. [7] h. wu, q. fang, x. chen, et al. projectile penetration of ultra-high performance cement based composites at 510–1320m/s. construction and building materials 74:188–200, 2015. doi:10.1016/j.conbuildmat.2014.10.041. [8] k. kolář, z. bažantová, p. konvalinka. suchá prefabrikovaná směs silikátového kompozitu. patent, úřad průmyslového vlastnictví, 2015. [9] t. vavřiník. odolnost vysokohodnotného betonu proti nárazu projektilu. bachelor thesis, fakulta stavební, čvut v praze, 2012. 66 http://dx.doi.org/10.1016/j.ijimpeng.2013.08.003 http://dx.doi.org/10.1016/j.conbuildmat.2011.06.023 http://dx.doi.org/10.1016/j.cemconres.2013.11.002 http://dx.doi.org/10.1016/j.ijimpeng.2015.05.007 http://dx.doi.org/10.1016/j.ijimpeng.2004.04.009 http://dx.doi.org/10.1016/j.conbuildmat.2014.10.041 acta polytechnica ctu proceedings 22:62–66, 2019 1 introduction 2 material and mixtures 3 experimental programme 4 results and discussion 5 conclusions acknowledgements references doi:10.14311/app.2021.30.0131 acta polytechnica ctu proceedings 30:131–134, 2021 © czech technical university in prague, 2021 available online at http://ojs.cvut.cz/ojs/index.php/app modelling of time-dependent behaviour of particulate thermoset polymers jan vozáb∗, jan vorel czech technical university in prague, faculty of civil engineering, department of mechanics, thákurova 7, 166 29 prague 6, czech republic ∗ corresponding author: jan.vozab@fsv.cvut.cz abstract. a preliminary study of a numerical model describing the behaviour of polymer-based composites is presented. the numerical model consists of three main parts. the first is the microplane m4 model, which is the main part of the model and is used to simulate elastoplastic behaviour and damage. the second part consists of a generalized maxwell model, which adds the effect of linear creep of the material to the calculation. the last part is a free volume model that extends the linear creep to the nonlinear creep. the creep is calculated on the deviatoric part of the normal stress of each microplane, which allows the model to capture the polymer behaviour adequately without adjusting the free volume of the model. keywords: creep, free volume, microplane, thermosets, viscoelasticity. 1. introduction many different materials are used in building constructions. one specific type that is increasingly used is epoxies. these polymers are often used to form different types of composites, as universal adhesives, rigid foams, or as structural adhesives [1]. like many polymeric materials, they are subject to nonlinear creep, which is associated with susceptibility to changes in temperature and humidity [2–4]. to properly design components made of this material, the appropriate numerical tool is essential. many models, describing broad nonlinear time-dependent responses of these material types, have been developed [5, 6]. by combining these approaches with the generally accepted material model for mechanical loading, the overall material response can be adequately described. in this paper, we introduce the novel approach combining a microplane material model m4 [7, 8], which is enhanced with the free volume approach on the microplane level to capture the time-dependent behaviour [5]. 2. numerical model simulating nonlinear behaviour is challenging because no nonlinear theory has been introduced that can describe all materials. however, many models have been developed that describe broad nonlinear responses [5, 6]. by combining these approaches, it is then possible to describe the behaviour of these materials. the numerical model described in this paper consists of three main parts. the microplane material model m4 [7, 8] with improvements presented in [9] is used as a fundamental part. the maxwell chain on the microplane level is used to capture the creep behaviour [10]. the additional free volume approach [5] allows for describing nonlinear viscoelastic behaviour under small to moderate strains. 2.1. microplane model m4 the microplane material model m4 [7] is the fourth version of the microplane model introduced by bažant [11, 12]. this constitutive model is characterized by the relationships between stresses and strains projected onto a surface of a microplane, which is oriented by its own normal vector. the basic idea is that the strain vector −→εn on the microplane is a projection of a strain tensor ε. then the normal strain on the microplane have the form εn = nijεij . (1) similarly, the shear components, which are characterized in the direction m and l, are given by orthogonal vectors, −→m and −→ l , take the form εm = mijεij ; εl = lijεij . (2) in which mij = (minj) + (mjni)/2 and lij = (linj + ljni)/2 [13, 14]. all three components can be seen in fig.1(a). the static equivalence is computed approximately by the principle of virtual work written for the surface ω of a unit hemisphere 2π 3 σijδεij = ! ω (σn δεn + σlδεl + σm δεm )dω. (3) this equation represents that the virtual work of macro-stresses within a unit sphere must be equal to the virtual work of micro-stresses regarded as the tractions on the surface of the sphere [7]. substituting δεn = nijδεij , δεm = mijδεij and δεl = lijδεij , and noting that the last variational equation must 131 http://dx.doi.org/10.14311/app.2021.30.0131 http://ojs.cvut.cz/ojs/index.php/app jan vozáb, jan vorel acta polytechnica ctu proceedings x 3 x 2x 1 (a) e 2 e 1 e 3 e m e l e n e t e (b) figure 1. microplane model: (a) distribution of integration points (microplane normals) system of 21 microplanes per hemisphere; (b) microplane strain components include any variation δεij , the result is the following basic equilibrium equation [12]: σij = 3 2π ! ω sijdω ≈ 6 nm" µ=1 wµs (µ) ij , (4) with sij = σn nij + σm mij + σllij . (5) this integral is an approximation by an optimal gaussian integration formula for a spherical surface, as you can see in fig.1(b). this surface is representing a weighted sum over the microplanes. in the finite element algorithms, this integral has to be computed at each integration point of each finite element in each time step [7]. the most generalized constitutive relations on the microplane level have the forms σn (t) = f t τ=0 [εn (τ), εm (τ), εl(τ)] , (6) σl(t) = gt τ=0 [εn (τ), εm (τ), εl(τ)] , (7) σm (t) = ht τ=0 [εn (τ), εm (τ), εl(τ)] , (8) where f , l and h are functionals of the microplane strains in time t. volumetric/deviatoric decomposition of the normal strain component [15] needs to be employed to capture the poisson ratio of polymers adequately εd = εn − εv; εv = εkk/3, (9) where εv stands for the volumetric strain and d denotes the deviatoric component. 2.2. maxwell chain the formulation of the microplane model m4 briefly described in the previous section is extended to incorporate time-dependent behaviour of epoxies. approach similar to [10] is utilized. creep is developed at every integration point, i.e. on each microplane of every integration point. on the level of microplanes, creep is implemented as linearly viscoelastic. the behavior of each maxwell element (µ), where a spring and a dashpot are connected in series, is expressed as ε̇µ = σ̇µ eµ + σµ ηµ , (10) where eµ is the elastic modulus and ηµ is the viscosity of the element. in the proposed model, the viscosity is assumed only for the deviatoric part of the normal strain component on each microplane separately. thus, according to the approximation by the generalized maxwell model, the equation for calculation of deviatoric stress predictor in the incremental form reads σve d = σi d + e′′ d(εd − εi d) − ∆σ′′ d = = σi d + e′′ d(∆εd − ∆ε′′ d), (11) where i means the initial value in the time step ∆t and ve stands for the viscoelastic stress increment, which is the generalization of the elastic stress increment. variables labeled by ′′ are calculated according to the equations e′′ d = n" µ=1 1 − e−∆t/τµ ∆t/τµ ed,µ, (12) ∆σ′′ d = n" µ=1 (1 − e−∆t/τµ)σi d,µ, (13) ∆ε′′ d = ∆σ′′ d/e′′ d, (14) where n is the number of maxwell elements and τµ = ηµ/ed,µ relaxation time of the µ-th element. if a constant poisson’s ratio ν is assumed ed,µ = 5eµ (2 + 3γ)(1 + ν) , (15) where γ is a parameter that may be chosen or can be optimized so as to match the given test data best (this value is set to 1 in this study) [16]. note that remaining constitutive relations remain unchanged and the moduli are updated based on e′′ = e′′ d(2+3γ)(1+ν). 2.3. free volume the last extension of our model is a free volume approach to extend the viscoelastic behaviour to a nonlinear scale, which is more convenient for the material. the description of this approach can be found 132 vol. 30/2021 modelling of particulate thermoset polymers figure 2. reological model of the maxwell chain. τµ eµ [mpa] 1 10 2.6174 2 102 107.5458 3 103 455.0239 4 104 933.0051 5 105 562.4520 6 106 273.8973 7 107 198.4917 8 108 135.9774 9 109 41.7351 10 1010 4.0175 11 1011 4.7209 12 1012 -0.0819 13 ∞ 39.9691 table 1. used values of maxwell chain. values converted from [5, tables 1 and 2]. in [5]. in the employed approach, the dilation is introduced into the time scale, which results in a nonlinear system of equations. note that for the case of infinitesimal dilatation, the nonlinear free volume theory reverts to linear viscoelasticity. in general, temperature (t ), moisture content (c) and mechanical dilatation (θ) influence the time scale of the material in a similar manner such that a = a(t, c, θ). (16) the shift factor a is connected to the free volume as suggested in [17, 18] log10 a = b 2.303 # 1 f − 1 f0 $ , (17) where f is fractional free volume, f0 represent the fractional free volume at a reference state. the coupling of the free volume to material parameters that affect its macroscopic volume can be expressed using a linear dependence f = f0 + αv∆t + βv∆c + δθ, (18) where αv is the volumetric thermal expansion of the free volume, βv represent the volumetric expansion due to a change in moisture content and δ relates the change if the free volume due to mechanical volume changes of the polymer. combining eq. (17) with eq. (18) we get log a = −b 2.303f0 # αv∆t + βv∆c + δθ f0 + αv∆t + βv∆c + δθ $ . (19) this equation includes the influence of all essential properties related to the development of the volume changes. the influence of the shift factor on the stress response is through convolution, or superposition, integrals relating the time dependent stress response to the strain history [5] by σd = ! t 0 e′′ d(t′ − τ ′)∂εd ∂τ dτ, (20) where the kernel takes the form t′ − τ ′ = ! t τ dξ a[t (ξ), c(ξ), θξ] (21) to account for the temperature, moisture and dilatation histories. note that in [5], the modified free volume approach is utilized to simulate shear-dominated loading scenarios with little or no dilatation. such an approach is not needed in the current formulation since the creep behaviour is defined on microplanes. parameter value b 0.5 f0 0.1 δ 1 αv 2.857 × 10−3 °c−1 t0 55 °c table 2. used material parameters. values taken from [5]. 3. results in this phase of the development process, we used a single-point calculation to verify the numerical model using [19]. the values for the maxwell chain from table 1 and the material parameters from table 2 were used in the simulation. the element was loaded by pure shear deformation. fig. 3 shows the force vs. displacement diagram. the blue line represents the elastic loading of the element, and it represents the upper bound for the presented results. the red line shows the elastic loading of the material, including the effect of the maxwell chain. as expected, creep affects the magnitude of the reaction, and therefore the curve has a smaller slope. the last green line represents the investigated microplane model. initially, it has the same inclination as the elastic model with the maxwell chain, but as the deformation increases, the response deviates. note that for this study, the microplane model behaviour is assumed to be purely elastic. 133 jan vozáb, jan vorel acta polytechnica ctu proceedings figure 3. results from finite element analysis: cube with an edge 1 cm, loading pure shear. 4. conclusions this paper presented a new model that can be used to describe the nonlinear creep of thermosets, which is composed of three parts: (i) the microplane model m4, which calculates elastoplasticity and damage; (ii) the maxwell chain model, which adds the effect of linear creep to the calculation; (iii) the free volume model, which extends creep to a nonlinear scale. in fig.3, we can see that according to preliminary results, the model behaves as expected because the effect of creep and softening is noticeable under shear load. at the same time, it is necessary to test the model on a larger scale at different load cases to ensure its accuracy. the arcan experiment presented in [5] would be appropriate. furthermore, it is necessary to test the model under a combination of different temperatures and humidity for verification of the free volume model parameters. if the accuracy of the model is still not sufficient, it could be caused by curing of the material which could be additionally connected to our numerical model. acknowledgements the financial support provided by the gačr grant no. 19-15666s and by the czech technical university in prague within sgs project with the application registered under the no. sgs20/155/ohk1/3t/11 is gratefully acknowledged. references [1] m. a. meyers, k. k. chawla. mechanical behavior of materials. cambridge university press, 2nd edn., 2008. [2] j.-p. pascault, h. sautereau, j. verdu, r. j. williams. thermosetting polymers, vol. 64. crc press, 2002. [3] r. f. landel, l. e. nielsen. mechanical properties of polymers and composites. crc press, 1993. [4] p. silva, t. valente, m. azenha, et al. viscoelastic response of an epoxy adhesive for construction since its early ages: experiments and modelling. composites part b: engineering 116:266–277, 2017. [5] c. f. popelar, k. m. liechti. multiaxial nonlinear viscoelastic characterization and modeling of a structural adhesive. journal of engineering materials and technology 119(3):205–210, 1997. https://asmedigitalcollection.asme.org/ materialstechnology/article-pdf/119/3/205/ 5681124/205_1.pdf doi:10.1115/1.2812245. [6] g. cusatis, d. pelessone, a. mencarelli. lattice discrete particle model (ldpm) for failure behavior of concrete. i: theory. cement and concrete composites 33(9):881–890, 2011. [7] z. p. bažant, f. c. caner, i. carol, et al. microplane model m4 for concrete. i: formulation with work-conjugate deviatoric stress. journal of engineering mechanics 126(9):944–953, 2000. [8] f. c. caner, z. p. bažant. microplane model m4 for concrete. ii: algorithm and calibration. journal of engineering mechanics 126(9):954–961, 2000. [9] g. di luzio. a symmetric over-nonlocal microplane model m4 for fracture in concrete. international journal of solids and structures 44(13):4418–4441, 2007. [10] z. p. bažant, f. c. caner, m. d. adley, s. a. akers. fracturing rate effect and creep in microplane model for dynamics. journal of engineering mechanics 126(9):962–970, 2000. [11] z. bažant. microplane model for strain-controlled inelastic behaviour, chapter 3. mechanics of engineering materials, chapter 3 1984. [12] z. p. bažant, p. g. gambarova. crack shear in concrete: crack band microplane model. journal of structural engineering 110(9):2015–2035, 1984. [13] z. p. bažant, p. c. prat. microplane model for brittle-plastic material: i. theory. journal of engineering mechanics 114(10):1672–1688, 1988. [14] z. p. bažant, p. c. prat. microplane model for brittle-plastic material: ii. verification. journal of engineering mechanics 114(10):1689–1702, 1988. [15] i. carol, z. bažant. damage and plasticity in microplane theory. international journal of solids and structures 34(29):3807–3835, 1997. [16] z. bažant, y. xiang, p. prat. microplane model for concrete. i: stress-strain boundaries and finite strain. journal of engineering mechanics 122(3):245–254, 1996. [17] a. k. doolittle. studies in newtonian flow. ii. the dependence of the viscosity of liquids on freeâăřspace. journal of applied physics 22(12):1471–1475, 1951. https://doi.org/10.1063/1.1699894 doi:10.1063/1.1699894. [18] a. k. doolittle, d. b. doolittle. studies in newtonian flow. v. further verification of the freeâăřspace viscosity equation. journal of applied physics 28(8):901–905, 1957. https://doi.org/10.1063/1.1722884 doi:10.1063/1.1722884. [19] es3. mars modeling and analysis of the response structures. 134 https://asmedigitalcollection.asme.org/materialstechnology/article-pdf/119/3/205/5681124/205_1.pdf https://doi.org/10.1115/1.2812245 https://doi.org/10.1063/1.1699894 https://doi.org/10.1063/1.1699894 https://doi.org/10.1063/1.1722884 https://doi.org/10.1063/1.1722884 acta polytechnica ctu proceedings doi:10.14311/app.2018.15.0131 acta polytechnica ctu proceedings 15:131–136, 2018 © czech technical university in prague, 2018 available online at http://ojs.cvut.cz/ojs/index.php/app strength of composite yarn under biaxial loading soňa valentováa,∗, vladimír hrbeka,b, jan vorela, michal šejnohaa a czech technical university in prague, faculty of civil engineering, thákurova 7, 166 29 prague 6, czech republic b institute of theoretical and applied mechanics as cr, v.v.i., prosecká 76, 190 00 prague 9, czech republic ∗ corresponding author: sona.valentova@fsv.cvut.cz abstract. the present paper is concerned with two types of periodic unit cell of composite yarn with different geometry but the same material properties. their macroscopic response under tensile and compressive loading in the transverse direction and their combination are plotted in the graphs. based on stress-strain curves the failure envelopes are constructed. a simple maximum stress criterion and linear softening law is used in the adopted progressive damage analysis to outline the softening part of stress-strain diagrams. finally the impact of selected representative volume element is observed through the comparison of results gained for both designed periodic unit cells in the microlevel, meaning the level of yarn. keywords: composite yarn, periodic unit cell, strength, damage analysis, failure envelope. 1. introduction demands on material properties increase as many different industry branches develop. therefore new materials, like textile composites become popular, especially for their variability, that consist for example in type of material used for fibers and matrix, type of weave, volume fraction of individual phases, curing temperature and so on. every mentioned factor influences the final macroscopic properties, so one of their big advantage is the possibility to adapt their final macroscopic properties. for their high specific strength, stiffness and temperature resistance they find use for example in automobile or aerospace industry. one of the most typical reinforcement are glass fibers, which belong to the most price favorable fibers and also have quite high modulus and strength. next commonly used fibers are carbon and basalt fibers. these are better in terms of mechanical properties. nowadays carbon fibers are still more often substituted by the basalt fibers thanks to their similar mechanical properties and lower price. for the possibility an extensive and mainly safe use of such material the knowledge of their behavior under different loading cases for both, in the elastic part and also after the damage occur, is necessary. as the experiments are complicated not only in terms of feasibility, but also from the time and financial point of view, some effective tool to obtain their macroscopic response numerically is desirable. the structure of braided composite is very complex, thus the appropriate simplification provided through the determination of representative volume element and subsequently construction of periodic unit cell, substituting its structure is important for the numerical analysis. in this paper, we adopt two types of periodic unit cell in the micro-scale, meaning the level of yarn, with attention concerned to the evaluation of macroscopic strength for unidirectional fiber composite under tensile and compressive on-axis loading in the transverse direction, see fig. 3 and fig. 4, and using their combination the failure envelope expressing the effective strengths in arbitrary transverse direction could be draw up. in case of textile composite, see the cross section and 3d periodic unit cell of such a composite in fig. 1, for obtaining its macroscopic properties, results from analysis introduced in this work will be applied to the meso-scale level where the effective properties of yarns are needful. moreover, the influence of elected periodic unit cell of yarn is investigated via the evaluation of effective elastic properties with the hand of finite element method subsequently compared with elastic properties gained by the mori-tanaka averaging scheme. 2. periodic unit cell of yarn design of periodic unit cell (puc) of yarn is based on the image analysis carried out on the cross sections of yarns, see fig. 2a, consisting of determination of geometrical parameters and then evaluation of volume fractions of individual phases. based on the knowledge of the inner geometry a representative volume element (rve) is determined, for more details see [1]. typical and the most simplified puc of yarn dealing with the idealized hexagonal distribution of fibers was designed, can be seen in fig. 3. moreover, 4-fiber puc of yarn with more complex rve was created, see fig. 4, to be able to verify the adequacy of designed puc. 131 http://dx.doi.org/10.14311/app.2018.15.0131 http://ojs.cvut.cz/ojs/index.php/app s. valentová, v. hrbek, j. vorel, m. šejnoha acta polytechnica ctu proceedings (a). (b). figure 1. (a) cross section of braided composite, (b) 3d-sepuc of composite. (a). (b). figure 2. (a) cross section of yarns with coordinate system, (b) example of mesh of yarns in composite. 3. failure criteria and damage model when loading of periodic unit cell representing yarn of braided composite we can distinguish two types of failure modes: • fiber failure mode • matrix failure mode in contrast with the direct stiffness reduction method, which simply assign zero stiffness to the integration point of mesh element, where the strength is reached, in adopted progressive damage model proposed by fang et al. [2, 3] lower individual elements of the stiffness matrix according to the magnitude of loading to reflect a material degradation with the evolution of cracks. then we can observe the macroscopic response also after the strength is exceeded. damage evolution is expressed as dj = χjfeq (χjeq − χjieq) χjeq(χ jf eq − χjieq) , j = 1t, 2t, 3t, 1c, 2c, 3c (1) in eq. 1 χjieq and χjfeq represents the equivalent displacement at the onset of failure and the equivalent displacement at the full damage, respectively.χjeq expresses equivalent displacement in the current loading step. the initial value is defined from the first load increment as χjieq = χjeq√ fj (2) where fj is j th failure criterion. as a simple maximum stress criterion is adopted in present work, following condition must be fulfilled to start developing the damage after the crack initializes. σii sij >= 1, i = 1, 2, 3; j = t,c (3) figure 3. 2-fiber puc puc-1. figure 4. 4-fiber puc puc-2. to define the displacement at the full damage in the nonlinear constitutive model, the fracture energy as the principal material property is indispensable. it can be written as∫ wf c 0 σdwc = ls ∫ fd 0 (ε− εel)dσ = ls ∫ εf 0 σdε (4) the fracture energy is graphically expressed in fig. 5. (a). (b). figure 5. (a) stress-crack opening displacement diagram with linear softening law, (b) stress-strain diagram with softening. 4. numerical experiment numerical experiment was carried out on a basalt fiber composite with pyrolized polysiloxane matrix. as the material is heterogeneous, several indentation measurements were performed on both phases of the available sample, fibers and matrix. results from these measurements are listed in tab. 1. average results of elastic material properties partially taken from the experiment, partially from literature, see [4] and partially set according to the analogy with the carbon fibers, are summarized in tab. 1. young’s modulus was derived from indentation results based on contact mechanics principles of two colliding bodies, as described in [5, 6]. these results have already been presented in [7]. principles of the nanoindantation method are described in [5]. effective elastic properties of investigated composite summarized in tab. 3 were also investigated in order to 132 vol. 15/2018 strength of composite measurement red. mod. young’s mod. hardness contact depth number er [gpa] e [gpa] h [gpa] hc [nm] basalt fiber – longitudinal direction 1 67.81 ± 8.09 67.72 ± 7.68 5.27 ± 0.56 191.92 ± 6.55 2 75.95 ± 6.55 75.66 ± 6.21 5.95 ± 0.88 106.09 ± 8.91 3 76.85 ± 7.75 76.63 ± 7.35 5.71 ± 0.76 108.33 ± 7.98 4 66.17 ± 10.44 66.08 ± 9.93 5.49 ± 0.75 161.55 ± 12.22 5 71.76 ± 7.30 71.15 ± 6.92 5.79 ± 0.38 155.99 ± 5.62 6 61.12 ± 6.68 60.85 ± 6.33 5.28 ± 0.74 239.19 ± 18.71 average 69.94 ± 7.80 69.68 ± 7.40 5.58 ± 0.68 160.51 ± 10.00 basalt fiber – transverse direction 1 74.09 ± 6.22 73.65 ± 6.22 5.88 ± 1.17 107.60 ± 11.80 2 75.98 ± 3.79 75.69 ± 3.58 5.94 ± 0.54 105.58 ± 5.57 3 57.31 ± 3.58 56.85 ± 3.38 5.00 ± 0.55 116.33 ± 7.25 4 55.34 ± 10.55 54.80 ± 10.03 4.45 ± 1.16 126.64 ± 17.08 5 68.77 ± 8.09 68.65 ± 7.68 5.15 ± 0.54 114.39 ± 6.80 6 59.61 ± 11.70 59.26 ± 9.44 5.29 ± 1.22 117.95 ± 15.13 average 65.18 ± 7.32 64.82 ± 7.03 5.29 ± 1.19 114.52 ± 14.15 pyrolyzed polysiloxane matrix 1 67.57 ± 3.82 67.47 ± 3.61 7.29 ± 0.96 56.64 ± 4.25 2 56.45 ± 3.77 55.95 ± 3.56 5.44 ± 0.44 110.65 ± 4.90 3 60.20 ± 5.09 59.88 ± 4.82 6.01 ± 0.54 104.88 ± 5.21 4 56.70 ± 8.53 56.22 ± 8.10 5.63 ± 1.02 109.81 ± 10.39 5 58.39 ± 7.29 57.98 ± 6.91 5.68 ± 0.99 109.45 ± 11.72 6 55.82 ± 5.00 55.30 ± 4.73 6.23 ± 1.08 103.85 ± 10.21 average 59.19 ± 5.58 58.83 ± 5.29 6.05 ± 0.84 99.21 ± 7.78 table 1. indentation results of individual material phases. material e g ν gf st sc [gpa] [gpa] [–] [n/mm] [gpa] [gpa] fiber – longitudinal direction 69.68 28.10 0.40 2 –1.5 fiber – transverse direction 64.82 26.14 0.24 0.5 –1.5 pyrolyzed polysiloxane matrix 58.83 23.72 0.24 0.001 0.08 –0.16 table 2. considered parameters of individual materials. verify the adequacy of selected representative volume elements. they were obtained by the micromechanical mori-tanaka averaging scheme, which builds upon the knowledge of the shape and orientation of the reinforcement, material properties and volume fraction of individual phases. next, they were gained with the help of more complex first order homogenization approach, but on the other hand more numerical and time demanding method with the need of construction of puc. the elastic properties provided by the finite element method for two designed pucs illustrated in fig. 3 and fig. 4 agrees well with the mori-tanaka predictions. the main part of presented paper is numerical evaluation of strength of mentioned composite. such an analysis was performed in the transverse direction of both designed pucs. at first both the on-axis tensile and compressive loading and then their combinations necessary for construction of the failure envelope were applied. the failure envelope plotted in the fig. 10 illustrates the strength in the transverse direction of yarn. the shape perfectly corresponds to our presumption leading from the adopted simple maximum stress criterion mentioned above in eq. 3. the maximum and minimum values of reached effective stresses are marked and provided in the graphs for the on-axis loading directions. further and the last output of this study is evaluation of the nonlinear macroscopic response of the composite. for this purpose, stress-strain curves for both pucs were generated (fig. 6 to fig. 9). one for each on-axis transverse direction for tensile and compressive loading increments. 133 s. valentová, v. hrbek, j. vorel, m. šejnoha acta polytechnica ctu proceedings model type e11 e22 e33 g23 g31 g12 cf [gpa] [gpa] [gpa] [gpa] [gpa] [gpa] [–] sepuc-1 fem 62.96 62.97 65.86 26.46 26.48 23.5 0.62 sepuc-2 fem 62.98 62.96 65.86 26.48 26.46 23.5 0.62 mori-tanaka method 62.97 62.97 65.88 26.47 26.47 23.5 0.62 table 3. effective elastic moduli. figure 6. stress-strain curves for on-axis tensile loading in the x-direction. figure 7. stress-strain curves for on-axis tensile loading in the y-direction. evidently the macroscopic response gained for both pucs gives almost the same results not only in the elastic part but also for the nonlinear behavior with just negligible differences. 5. conclusion the presented paper was concerned with two types of periodic unit cell of composite yarn with different representative volume element but the same material properties. at first their effective elastic properties were obtained both by the mori-tanaka averaging scheme and finite element method. based upon gained figure 8. stress-strain curves for on-axis compressive loading in the x-direction. figure 9. stress-strain curves for on-axis compressive loading in the y-direction. results the adequacy of the designed puc could be verified. in the next step the macroscopic elastic and nonlinear response for tensile and compressive loading in the transverse on-axis direction were investigated and plotted in the diagrams. on the basis of the presented methodology we can observe macroscopic behavior of composite after cracks initialize and damage starts to develop. a simple maximum stress criterion and linear softening law was adopted in this study to outline the nonlinear part of stress-strain diagrams. based on the stress-strain curves derived from the biaxial loading, the failure envelopes were constructed. re134 vol. 15/2018 strength of composite figure 10. failure envelope for biaxial loading in the transverse direction. alized computational analysis shows the macroscopic strength of investigated composite for the input parameters summarized in tab. 3. differences in failure envelopes (fig. 10) and in stress-strain curves raised for both examined pucs (fig. 6 to fig. 9) are almost imperceptible and could be caused by the unequal generated mesh density. consequently we can afford to consider the disparities such insignificant that the simplification of yarn through the designed puc is perfectly acceptable. the model was tested on mentioned composite but with incomplete knowledge of needed input material parameters. in the future work we will pay our attention to another numerical and nanoindentation experiments to get necessary parameters to accomplish more accurate analysis on micro-scale and continue with numerical predictions of composite with more complex structure, e.g. illustrated in fig. 1. list of symbols ν poisson ratio [–] er reduced modulus [gpa] e young’s modulus [gpa] h hardness [gpa] hc contact depth [nm] g shear modulus [gpa] gf fracture energy [n/mm] st critical strength in tension [gpa] sc critical strength in compression [gpa] eij homogenized elastic modulus [gpa] gij homogenized shear modulus [gpa] dj damage parameter [–] χeq equivalent displacement [m] σij stress [gpa] εij strain [–] acknowledgements the financial support provided by the sgs project with the application registered under the no. sgs18/037/ohk1/1t/11 is gratefully acknowledged. references [1] j. zeman, m. šejnoha. from random microstructures to representative volume elements. modelling and simulation in materials science and engineering 15(4):325–335, 2007. doi:10.1088/0965-0393/15/4/s01. [2] g. d. fang, j. liang, q. lu. investigation on the compressive properties of the three dimensional four-directional braided composites. composite structures 93(2):392–405, 20011. doi:10.1016/j.compstruct.2010.09.002. [3] g. d. fang, j. liang, b. l. wang. progressive damage and nonlinear analysis of 3d four-directional braided composites under unidirectional tension. composite structures 89(1):126–133, 20009. doi:10.1016/j.compstruct.2008.07.016. 135 http://dx.doi.org/10.1088/0965-0393/15/4/s01 http://dx.doi.org/10.1016/j.compstruct.2010.09.002 http://dx.doi.org/10.1016/j.compstruct.2008.07.016 s. valentová, v. hrbek, j. vorel, m. šejnoha acta polytechnica ctu proceedings [4] m. černý, p. glogar, z. sucharda. mechanical properties of basalt fiber reinforced composites prepared by partial pyrolysis of a polymer precursor. journal of composite materials 43(9):1109–1120, 2009. doi:10.1177/0021998308097732. [5] w. c. oliver, g. m. pharr. measurement of hardness and elastic modulus by instrumented indentation: advances in understanding and refinements to methodology. journal of material research 19(1):3–20, 2004. doi:10.1557/jmr.2004.0002. [6] h. hertz. on the contact of rigid elastic solids. hertz’s miscellaneous papers p. chap. 6, 1896. original: über die berührung fester elastischer körper, journal für die reine und angewandte mathematik, 92: 156–171, 1881. [7] s. valentová, v. hrbek, m. šejnoha. evaluation of effective properties of basalt textile reinforced ceramic matrix composite. acta polytechnica ctu proceedings 13:142–147, 2017. doi:10.14311/app.2017.13.0142. 136 http://dx.doi.org/10.1177/0021998308097732 http://dx.doi.org/10.1557/jmr.2004.0002 http://dx.doi.org/10.14311/app.2017.13.0142 acta polytechnica ctu proceedings 15:131–136, 2018 1 introduction 2 periodic unit cell of yarn 3 failure criteria and damage model 4 numerical experiment 5 conclusion list of symbols acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2018.14.0034 acta polytechnica ctu proceedings 14:34–41, 2018 © czech technical university in prague, 2018 available online at http://ojs.cvut.cz/ojs/index.php/app coupled simulation of gas cooled fast reactor fuel assembly with nestle code system filip osusky∗, stefan cerba, jakub luley, branislav vrban, jan hascik institute of nuclear and physical engineering, faculty of electrical engineering and information technology, slovak university of technology in bratislava, ilkovicova 3, bratislava, slovakia ∗ corresponding author: filip.osusky@stuba.sk abstract. the paper is focused on coupled calculation of the gas cooled fast reactor. the proper modelling of coupled neutronics and thermal-hydraulics is the corner stone for future safety assessment of the control and emergency systems. nowadays, the system and channel thermal-hydraulic codes are accepted by the national regulatory authorities in european union for license purposes, therefore the code nestle was used for the simulation. the nestle code is a coupled multigroup neutron diffusion code with thermal-hydraulic sub-channel code. in the paper, the validation of nestle code 5.2.1 installation is presented. the processing of fuel assembly homogeneous parametric cross-section library for nestle code simulation is made by the sequence triton of scale code package system. the simulated case in the nestle code is one fuel assembly of gfr2400 concept with reflective boundary condition in radial direction and zero flux boundary condition in axial direction. the results of coupled calculation are presented and are consistent with the gfr2400 study of the gofastr project. keywords: coupled calculation, fast reactor, gfr 2400, nestle, neutronics, thermal-hydraulics, thermal feedback, validation. 1. introduction nowadays, the nuclear reactors are an appropriate option to decrease co2 production as the source with high energy capacity. however, the amount of fissionable 235u resources in the world are decreasing and, according to study [1], the entire conventional resources are sufficient for 240 years. together with public pressure to reduce long-lived radiological nuclides, there is necessity to introduce new alternative energy sources, which are capable to replace commercial thermal reactors in the future. therefore, the generation-iv international forum (gif) [2] identified perspective conceptual designs of fast reactor systems including the gas cooled fast reactor (hereinafter gfr2400). however, multiple studies raise questions about the gfr2400 technical feasibility [3] due to safety reasons. moreover, according to study [4] the recriticality issues may occur for fast reactor concepts and therefore the proper modelling of coupled neutronics with thermal-hydraulics is necessary to avoid these type of scenarios and for estimation of conceptual designs safety. the institute of nuclear and physical engineering (inpe) was involved in the gofastr project [5] that was focused on development of gfr2400 design and inpe was also participating in allegro project which is oriented in research and development of gfr2400 demonstration unit. the paper is focused on the coupled calculation of the gfr2400 fuel assembly by the nestle code system. the nestle code is described in section 2 together with the validation of installation. section 3 describes the methodology of the coupled calculation and includes part about processing of appropriate parametric multigroup macroscopic cross-section library by scale code system, which is necessary for the calculation by the nestle code. the geometry and material properties of gfr2400 are presented in section 4. the results of the scale and nestle calculations are shown in the last section 5 and are compared with the gfr2400 reference study [6] performed during the gofastr project [5]. 2. nestle code 2.1. nestle code 5.2.1 description the main advantage of the nodal code nestle is the already implemented internally coupled neutronic and thermal-hydraulic iterative calculation path. the nestle code solves 2 or 4 group neutron diffusion equation utilizing the nodal expansion method [7]. −∇ ·dg∇φg + σtgφg = g∑ g′=1 σsgg′φg′+ χg k g∑ g′=1 νg′σfg′φg′ (1) where dg is diffusion coefficient [cm], φg represents the neutron flux [cm−2·sec−1], σtg stands for total macroscopic cross-section [cm−1], σsgg′ for in-group 34 http://dx.doi.org/10.14311/app.2018.14.0034 http://ojs.cvut.cz/ojs/index.php/app vol. 14/2018 simulation of reactor fuel assembly with nestle code system scattering cross-section [cm−1], χg is the fission neutrons yield, k represents the multiplication factor (or fundamental eigenvalue), νg stands for average number of neutrons per fission and σfg represents the macroscopic fission cross-section [cm−1]. to calculate the above-mentioned diffusion equation, it is necessary to process appropriate parametric multigroup macroscopic cross-sections, according to the following equation: σ̂xg = a1xg + 2∑ n=1 a(n+1)xg (∆%c)n + a4xg ∆tc+ a5xg ∆(tfeff )1/2 + 3∑ n=1 a(n+5)xg (∆nsp)n (2) where σ̂xg represents the macroscopic cross-section for particular reaction type x and in the energy group g, ajxg are expansion coefficients of modified power series, ∆%c is the deviation from the reference coolant density [g·cm−3], ∆tc stands for deviation from the reference coolant temperature [ °f], ∆(tfeff )1/2 is the change in the square root of the effective fuel temperature in [ °f] from the reference condition and ∆nsp represents the deviation from the reference soluble poison concentration [cm−3·10−24] [7]. more details could be found also in the previous study [8]. 2.2. validation of installation the first version of the nestle code was developed at north carolina state university beginning in the late 1980’s and the latest version of the nestle code 5.2.1, which is available at nea data bank [9], was made in 2003. the license of newer developer version of the nestle code was not obtained by the inpe and therefore the whole calculation presented in the paper was performed on the version 5.2.1. the nestle code 5.2.1 was tested for sun computers with aix operating system based on powerpc (ppc) processor architecture with xlf fortran 77 compiler in 2004. therefore, the correct installation was difficult and the virtualization of ppc architecture (on ubuntu system 14.04.5 [10]) was necessary to perform for the successful nestle compilation by an updated version of xl fortran compiler (v. 15.1.5) [11]. the nestle calculation is a little bit inconvenient on virtualized ppc machine due to the more than 2 orders of magnitude higher computational time. based on the successful installation, the original code was slightly modified to comply with gfortran compiler, which is available on multiple architecture platforms. the results of the validation of installation by the benchmarks included in the installation package of the nestle code are shown in table 1 for the version compiled by gfortran compiler on ubuntu system 14.04.5 with amd64 architecture. the results shown in table 1 are identical for the platforms with different architecture. the difference in results of keff is less than 1 pcm for almost all cases. the deviation of the calculated coolant and fuel temperatures from the benchmark values is insignificant (less than 10−1 °f). however, the calculated results of test case number 10 achieves higher deviation from the benchmark. the testcase was prepared for calculation of the boron acid concentration in the different fuel cycles of the core and obviously, at the end of the cycle (eoc), the boron concentration should be equal to 0. the calculated boron concentration in testcase number 10 is 1.7637 ppm (parts per million) and this difference should be discussed with the developer team in tennessee in the future. the next section describes the calculation process and the processing of macroscopic cross-section library for nestle code. 3. calculation methodology calculation scheme is presented in fig. 1. triton sequence of the scale code system [12] is used to process appropriate parametric homogenized multigroup macroscopic cross-section library for the nestle code. the modelled states of the nuclear system, referred as branches, are describing the deviation from the reference state of the nuclear system according to eq. 2 (i.e. difference in coolant density, coolant temperature and square root of effective fuel temperature for the case of gfr2400). the above mentioned branches estimate the response of the nuclear system induced by the change of thermal-hydraulic parameters in the system. the triton sequence calls the deterministic multigroup code newt, which estimates the neutron flux in the investigated system and process homogenized collapsed and weighted macroscopic cross-sections according to the calculated flux. a simple script was developed at inpe to prepare appropriate approximations for nestle code according to the provided cross-section library made by the triton sequence. initially, the nestle code calculates the neutron flux distribution in the system. according to the calculated fission reaction rates, the thermal-hydraulic sub-channel calculation estimates the temperature distribution and density of the coolant in the system. according to these parameters, the macroscopic crosssection library is updated for next iteration step of the neutronic calculation within the nestle code. this process repeats until the calculation reaches the initial stopping criteria of the calculation. if the convergence of the solution is not achieved, it is necessary to update the input data and the conditions for both scale and nestle calculations. the next section describes the investigated case, which is the fuel assembly based on the gfr2400 model. 35 f. osusky, s. cerba, j. luley et al. acta polytechnica ctu proceedings test case number calculated value of keff benchmark value of keff test case number calculated value of keff benchmark value of keff 1 1.000366 1.000364 14 1.000246 1.000245 2 1.000364 1.000364 15 0.989215 0.989215 3 1.000365 1.000356 16 1.000004 1.000000 4 0.999968 0.999968 17 1.006454 1.006454 5 1.000365 1.000363 18 1.009920 1.009920 6 1.122087 1.122004 19 1.011475 1.011475 7 1.121459 1.121373 20 0.977857 0.977858 8 1.121039 1.121117 21 1.005640 1.005640 9 1.200129 1.200129 22 0.999981 0.999982 10 0.999902 1.000088 23 0.999936 0.999936 11 0.979755 0.979755 24 0.999997 0.999997 12 0.989589 0.989589 25 0.999993 0.999992 13 1.088344 1.088343 26 0.999993 0.999994 table 1. validation of the nestle code installation compiled by gfortran compiler. figure 1. calculation iteration scheme of the simulation. 4. description of investigated case the current experience of the gas cooled reactor technology has more than 1000 reactor years of gas thermal reactor operation in europe [13]. moreover, slovakia operated the gas cooled thermal reactor a1 until mid-1980’s and is one of the possible countries to build the gfr2400 demonstration unit (referred as allegro). therefore, the research institutes in slovakia are involved in the research and development of gas cooled fast reactor technology and the paper focuses on gfr2400 concept. the power of the gfr2400 reactor concept is 2400 mwth and the planned efficiency of the indirect brayton cycle is 45%. the used coolant is helium at 7 mpa pressure and the heat transfer is provided by three blowers with intermediate heat exchangers. the mass flow rate of the coolant inside of the fuel assemblies is 1213 kg·s−1 and the bypass flow rate of the coolant between the fuel assemblies is 60 kg·s−1. the high coolant volume fraction is characteristic for this concept (43 v/vtot%). the basic thermal-hydraulic parameters are shown in table 2 and these parameters are described in more detail in previous studies [8, 14] and are accompanied with the material composition. the cross sectional view of the gfr2400 core is shown in fig. 2. the chosen fuel material is uranium and plutonium carbide (hereinafter u-puc) and one from the u-puc benefits are appropriate thermal properties for the cores with high temperature gradient, such as gfr2400. the core is mixed from two zones with different enrichment: the inner core, with lower 36 vol. 14/2018 simulation of reactor fuel assembly with nestle code system parameter value parameter value thermal power [mw] 2400 primary coolant he primary pressure [mpa] 7 pressure drop in core [mpa] 0.143 mass flow rate [kg·s−1] 1213 bypass flow rate [kg·s−1] 60 core inlet temp. [ °c] 400 core outlet temp. [ °c] 780 table 2. basic thermal-hydraulic parameters [15]. figure 2. cross sectional view of the gfr 2400 core [15]. volumetric enrichment of puc 14.12%, and the outer core, with higher volumetric enrichment of puc of 17.65%. the purpose of two fuel zones with different enrichment is to achieve uniformly distributed neutron flux in radial direction of the core. however, the calculation will be performed with arithmetic average volumetric enrichment of plutonium isotopes 15.84% in the carbide fuel (table 3). the cross sectional view of the fuel pin is shown in fig. 3. the middle part in the fig. 3. represents the cut of u-puc fuel pellet. the gap between the fuel pellet and internal liner of cladding is filled with helium. the cladding is made from mixed structure of sicf/sic and this material is chosen thanks to its stable behaviour during high temperature gradients. however, the sic can be easily penetrated by fission gases and therefore, the special internal liner was introduced, made in the form of w14re. the w14re liner improves the fission gas retention of the fuel pin and moreover, prevents the carbonization of the cladding due to fuel-cladding interaction during the fuel cycle. the homogenization of the gap, internal liner and cladding with external liner was performed to achieve faster and stable convergence of deterministic calculation made by newt code during the processing of macroscopic cross-section library. the description of the homogenization process could be found in the fuel density [g·cm−3] 10.9002 %v in puc 15.84 %w in puc 15.81 %v in uc 84.16 %w in uc 84.19 %w pu-238 0.4065 %w pu-239 8.4314 %w pu-240 3.8995 %w pu-241 1.1142 %w pu-242 1.0991 %w am-241 0.1054 %w u-235 0.5770 %w u-238 79.5684 %w c-nat 4.7984 table 3. fuel material composition [15]. figure 3. cross sectional view of the gfr 2400 fuel pin [15]. 37 f. osusky, s. cerba, j. luley et al. acta polytechnica ctu proceedings homogenized cladding [g·cm−3] 2.9579 %w w-182 4.9824 %w w-183 2.6942 %w w-184 5.7989 %w w-186 5.4380 %w re-185 1.9861 %w re-187 3.3588 %w si-28 48.7308 %w si-29 2.5611 %w si-30 1.7492 %w he-04 0.0168 %w c-nat 22.6838 table 4. homogenized cladding material composition [16]. figure 4. cross sectional view of the gfr 2400 fuel assembly [15]. previous study [16] and the results of the fuel pin homogenization is shown in table 4. the gfr2400 core consists of 516 fuel assemblies and the height of the core part is 1.65 m. each fuel assembly consists of 217 fuel pins and the cross sectional view of the fuel assembly is shown in fig. 4. 5. results 5.1. scale calculation as mentioned above, the triton sequence was used for processing of the parametric macroscopic crosssection library for the nestle code. the mesh generated for the calculation is shown in fig. 5. figure 5. mesh of the investigated case (the orthogonal mesh of the fuel pin is 4x4 and the mesh for whole fuel assembly is 40x40). the standard scale cross-section library, referred as v7-252, was used and is based on endf/b-vii.1 evaluated data. the 252 neutron group library was developed to adequately capture spectral and temperature effects important for reactor systems [12] and therefore was used for the branch calculation by the triton sequence. white boundary condition was considered during the calculation and the convergence criterion was set to the particular mixtures and for the eigenvalue (εkeff < 10−5). the calculations have converged for each branch calculation (inner and outer convergence was established). the reference conditions of nuclear system were the following: temperature 973.15 °c of the fuel; temperature 673.15 °c of the coolant; and the coolant density 4.9439 kg·m−3. the branch calculation was performed for the temperature range of the fuel until 1773.15 °c with 50 °c step. the coolant temperature range was up to 1473.15 °c with the same step (50 °c). the coolant density change corresponded to the coolant temperature change, which was mentioned above. the estimated multiplication factor was 1.21519 by the deterministic newt calculation for the reference case. the triton sequence successfully process the parametric homogenized multigroup macroscopic cross-section library in raw binary data-format. automatization script was used to prepare the approximations to the nestle code and the results are shown in next sub-section. 5.2. nestle calculation the boundary conditions of nestle calculation were set to reflective in radial direction and in axial direction to zero flux. the total flowrate of the coolant was 2.351 kg·s−1 for one fuel assembly and the reference power generation was 367.352 mw·m−3 for the fuel. the fuel assembly results of the nestle calculation are compared with results of the previous studies [8, 14] and with the reference gfr2400 38 vol. 14/2018 simulation of reactor fuel assembly with nestle code system figure 6. results of the coolant axial temperature distribution. figure 7. results of axial power distribution. figure 8. results of the fuel axial temperature distribution. 39 f. osusky, s. cerba, j. luley et al. acta polytechnica ctu proceedings study [6]. in fig. 6, the results of axial coolant temperature distribution are compared between the nestle calculation, cfd calculation made by the flowvision code [8, 17] and between the gfr2400 study [6]. the nestle calculation results show lower temperatures in the axial direction of the core than the results obtained by the cfd simulation and gfr2400 study. in case of the cfd simulation and the gfr2400 study, the modelled case was one pin with periodical/symmetrical boundary condition, which led to smaller cross-sectional area where the coolant flows (the cross sectional area is approximately 108.57 cm2) than in the case of one full fuel assembly (111.09 cm2). therefore, the effectiveness of the heat removal is higher in the nestle calculation case, what resulted in lower temperature at the outlet, compared to the cfd simulation and the gfr2400 study. results of the axial power distribution are compared in fig. 7. the scale keno vi three-dimensional calculation was performed in the previous study [14]. the scale model was divided into 10 zones in axial direction and the model was externally coupled with cfd flowvision calculation. the standard scale cross-section library, referred as v7-238, was used in this case and is based on endf/b-vii.0 evaluated data. the investigated case of the scale-flowvision calculation was one fuel pin with reflective boundary condition in radial direction (symmetrical boundary condition in the case of cfd simulation) and zero flux boundary condition in axial direction (pressure outlet in the case of cfd simulation). neutronic material parameters of each zone were directly influenced by the cfd simulation. the axial reflectors below and above the core were not considered for the nestle calculation and for the scale keno vi calculation. therefore, the results of the simulated cases made by nestle and scale codes (fig. 7) are not consistent with gfr2400 study [6], where the axial reflectors were considered. the application of the axial reflectors results in the higher fission reaction rate in the axial boundary regions than for the case without application of the axial reflectors. the results of axial fuel temperature are shown in fig. 8. and are reflecting the calculated axial power distribution. there is good agreement of the calculated cases by nestle and cfd simulations. an additional simulation was performed by the newt code, where the temperatures and densities were averaged, according to the performed nestle calculation. the result of the multiplication factor was 1.21401. the calculated multiplication factor of nestle code was 1.19661 and the difference from the newt simulation could have been caused by different approach of the deterministic-transport (typical for the newt code) and nodal-diffusion calculation (typical for the nestle code) codes. 6. conclusion the validation of the nestle code system installation is described in the paper. the results of the test cases were consistent with the benchmark results except one. in the test case number 10, the calculated boron acid concentration at the end of the fuel cycle was not equal to 0, what resulted in the bias of the multiplication factor for this test case. this behaviour will be discussed in the future with the developer team in tennessee. the coupled simulation of the gfr2400 fuel assembly was performed by the nestle code and was compared with previous studies and with the reference gfr2400 study from the gofastr project. the results made by the nestle code are in good agreement with cfd simulations made by the flowvision code. the axial reflectors above and below the core were not considered in the nestle simulation and this modelling approach resulted in the decrease of the power at the top and the bottom of the fuel assembly in comparison with gfr2400 study, where these reflectors were included in the model. the technical feasibility of the gfr2400 design raises questions and therefore the future activities with benchmarking are needed for this concept. the main reason is that the material composition of the design is quite unique and the appropriate benchmark does not exist for the gfr2400 core. acknowledgements this work was financially supported by stu grant scheme for support of young researchers hopone no. 1323. references [1] nuclear energy agency and the international atomic energy agency. uranium 2016: resources, production and demand, 2016. https://www.oecd-nea.org/ndd/ pubs/2016/7301-uranium-2016.pdf. [2] t. abram. generation-iv nuclear power: a review of the state of the science. energy policy 36:4323–4330, 2008. [3] g. locatelli. generation iv nuclear reactors: current status and future prospects. energy policy 61:1503–1520, 2013. [4] h. ninokata. a study on recriticality characteristics of fast reactors in pursuit of recriticality-accident-free concepts. progress in nuclear energy 29:387–393, 1995. [5] european commission. gofastr : european gas cooled fast reactor. [2017-11-30], http: //cordis.europa.eu/project/rcn/96860_en.html. [6] european commission. gofastr : gfr2400 core neutronics and thermal-hydraulics characterization, 2012. work package 1.1: gfr conceptual design, http://www.janleenkloosterman.nl/reports/ gofastr_d1111_201203.pdf. [7] electric power research center, north carolina state university. nestle version 5.2.1: few-group neutron diffusion equation solver utilizing the nodal expansion method for eigenvalue, adjoint, fixed-source steady-state and transient problems, 2003. 40 https://www.oecd-nea.org/ndd/pubs/2016/7301-uranium-2016.pdf https://www.oecd-nea.org/ndd/pubs/2016/7301-uranium-2016.pdf http://cordis.europa.eu/project/rcn/96860_en.html http://cordis.europa.eu/project/rcn/96860_en.html http://www.janleenkloosterman.nl/reports/gofastr_d1111_201203.pdf http://www.janleenkloosterman.nl/reports/gofastr_d1111_201203.pdf vol. 14/2018 simulation of reactor fuel assembly with nestle code system [8] f. osusky. test case specifications for coupled neutronics-thermal hydraulics calculation of gas-cooled fast reactor. journal of physics: conference series 781(1):10, 2016. http: //iopscience.iop.org/1742-6596/781/1/012038. [9] nuclear energy agency. ccc-0641 nestle 5.2.1. [2017-11-30], http://www.oecd-nea.org/tools/ abstract/detail/ccc-0641/. [10] ubuntu. ubuntu-14.04.5-server-ppc64el. [2017-11-30], http: //cdimage.ubuntu.com/releases/14.04/release/. [11] ibm. getting started with xl fortran for little endian distributions. version 15.1.5. [12] oak ridge national laboratory. scale code system, 2016. version 6.2.1. [13] r. stainsby. gas cooled fast reactor research in europe. nuclear engineering and design 241:3481–3489, 2011. [14] f. osusky. definition of the thermal-hydraulic model of the gas-cooled fast reactor, 2017. in nuclear 2017: 10th international conference on sustainable development through nuclear research and education. pitesti, romania. may 24-26, 2017. pitesti: raten icn pitesti. issn 2066-2955. [15] z. perko. core neutronics characterization of the gfr2400 gas cooled fast reactor. progress in nuclear energy 83:460–481, 2015. [16] f. osusky. macroscopic cross-section processing for nestle code system, 2017. in apcom 2017 : proceedings of 23rd international conference on applied physics of condensed matter. strbske pleso, slovak republic, june 12-14, 2017. vol. 1. bratislava: publisher spektrum stu. isbn 978-80-227-4699-1. [17] capvidia. flow v ision: version 3.09.04, 2015. installation and setup guide. 41 http://iopscience.iop.org/1742-6596/781/1/012038 http://iopscience.iop.org/1742-6596/781/1/012038 http://www.oecd-nea.org/tools/abstract/detail/ccc-0641/ http://www.oecd-nea.org/tools/abstract/detail/ccc-0641/ http://cdimage.ubuntu.com/releases/14.04/release/ http://cdimage.ubuntu.com/releases/14.04/release/ acta polytechnica ctu proceedings 14:28–35, 2018 1 introduction 2 nestle code 2.1 nestle code 5.2.1 description 2.2 validation of installation 3 calculation methodology 4 description of investigated case 5 results 5.1 scale calculation 5.2 nestle calculation 6 conclusion acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2021.31.0053 acta polytechnica ctu proceedings 31:53–57, 2021 © 2021 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague railway superstructure for high-speed rails lukáš týfaa,∗, jan kruntoráda, ondřej tomaschkob a czech technical university in prague, faculty of transportation sciences, department of transportation systems, horská 3, 128 03 prague 2, czech republic b db projekt stuttgart-ulm gmbh, räpplenstraße 17, 70191 stuttgart, germany ∗ corresponding author: tyfa@fd.cvut.cz abstract. the construction of the track is solved as a conventional (with track ballast) or as a ballastless track (without track ballast). when deciding on the choice of the most suitable track design for high-speed rail, it is necessary to emphasize that with increasing line speed, the requirements for the quality of the track geometry increase, including the size of the deviations of individual geometric parameters of the track from the projected values. when assessing the choice of track structure – especially for high-speed rails – it is therefore necessary and correct to assess the total costs (investment and operational) for a sufficiently long time. db netz ag’s present experience with a ballastless track since its first implementation in 1972 in rheda station, for almost 50 years, shows that the required lifetime of 60 years has its real justification. in addition, current research and development in the field of ballastless track is moving towards extending the life of ballastless track well beyond this limit. the paper is focused on the comparison a ballasted and ballastless track construction. it deals in more detail with evaluation of the advantages and disadvantages of a ballastless track construction. the main objective is to assess the use of ballastless track structure for the construction of high-speed lines. keywords: track construction, track structure, track ballast, ballastless track, high-speed rail, investment and operational costs. 1. ballastless track characteristics the construction of the track is solved as a conventional (with track ballast) or as a ballastless track (without track ballast). in conventional track construction, the ballast bed performs the function of transmitting force effects from the rail supports to the track substructure and serves to make the railway superstructure more elastic (attenuates dynamic forces from moving vehicles); at the same time, it resists the displacement of the track skeleton in the transverse and longitudinal direction, ensures the drainage of rainwater from the track and enables the directional and height adjustment of the track geometry. in the case of a ballastless track, the ballast bed is replaced by cement concrete or asphalt concrete, and thus the load transfer function of the rolling stock is taken over by the load-bearing concrete layer [1]. the missing elasticity of the ballast bed is replaced by additional elastic elements in the rail fastening, or in a loadbearing slab of a ballastless track (elastic layer for concreted transverse sleepers or slabs). ballastless track is used worldwide on all types of railways (included railway lines as well as city railways systems – trams systems and underground systems) and there are many types of this structure about concerning their manufacturers and suitability for use – in particular, the ballastless track types suitable for urban railways with a cover that road vehicles can drive on, and the ballastless track types with the open superstructure (without the cover), which can be supplemented with various elements to eliminate the noise propagation (absorbers). furthermore, specific types of a ballastless track can be divided according to the principle of rail mounting, according to the degree of use of prefabricated elements, and according to the system of construction and use of transverse sleepers [2]. the decision to use either the conventional track construction or the ballastless track applies not only to the railway superstructure but is also reflected in the requirements for the track substructure – when choosing a ballastless track, significantly higher demands are placed on its stability [3]. when deciding on the choice of the most suitable track construction for high-speed rails, it is necessary to emphasize that with increasing line speed, the requirements for the quality of the track geometry increase, including the size of the deviations of individual geometric parameters of the track from the projected values. at the same time, a stable geometric position of the track with minimal deviation is a condition for smooth running of rolling stock, for low wear of the railway superstructure and wheels of vehicles, for low noise and vibration emissions. with the same track construction and at the same time increasing line speed, the costs of track maintenance and demand on possession activity increase. track possessions are not only highly unattractive for railway carriers and their customers, but also reduce the railway operator’s revenues from the railway use fee, and they are difficult in organizing and providing a 53 https://doi.org/10.14311/app.2021.31.0053 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en l. týfa, j. kruntorád, o. tomaschko acta polytechnica ctu proceedings high level of safety and require the deployment of costly track machinery with specially qualified personnel. higher maintenance costs are negatively reflected in the overall economic evaluation of the construction work during its entire life, and frequent maintenance can paradoxically shorten the life of some materials (ballast bed). 2. advantages and disadvantages of a ballastless track advantages of a ballastless track compared to a conventional track construction: • significantly lower maintenance requirements (only replacement of worn rails or rail fastening elements; no tamping, no gravel cleaning), and therefore very low operating costs, no operation disruption, and minimal track possession. • longer service life (stated at least 50 years, in germany the internal regulation of db netz ag requires 60 years). during the planned lifetime of the ballastless track, the ballast bed in the conventional track structure is established 2-3 times, because simple maintenance is no longer effective, which means high costs and extensive track possessions with significant complications (see above). • lower construction height, which is a great advantage for track substructure – especially in tunnels it leads to a smaller stope, which significantly saves investment costs; and bridges can be designed with a lower construction height, which can be used for the easier design of track level or increase the bridge span (e.g. increase underpass height on the road). the lower construction height of the ballastless track can be used for additional line electrification in existing tunnels. • a lower weight of the structure at a lower construction height, which is reflected in a lower load on track substructure (especially bridge structures). • smaller land take, as it is not necessary to widen the sub-ballast surface in the curve. even with a maximum cant of 180 mm, the sub-ballast surface has a constant width of 12.10 m at db netz ag (at a track-center distance of 4.50 m). • possibility to use a higher cant and its deficiency, which allows the design of smaller curve radius for a given line speed, and thus facilitates the routing the track through the terrain. the smaller curve radius together with the constant width of the sub-ballast surface make the possibility to route a new highspeed rail parallelly with an existing motorway (e.g. high-speed rail stuttgart – ulm or nürnberg – ingolstadt), creating a single transport corridor. when designing a high-speed rail using ballastless track, which allows achieving the required line speed with smaller curve radius than the conventional track construction, and with high gradient, it is possible to achieve a reduction in the share of bridges and tunnels, which has a significant positive economic impact not only on investment costs but also on operational costs (significantly lower subsequent maintenance of the track). • high transverse and longitudinal stability of the track, which is especially important for the stability of the continuous welded rail. • in the event of an emergency, a more convenient escape route for passengers, which allows faster evacuation, which is essential for tunnels. however, it is still necessary to assess the possibility of driving the ballastless track by rescue and evacuation road vehicles. for example, in germany, for this reason, special slabs must be laid between the rails. this adjustment of the conventional track construction with a rail bed would need the maintenance of the track geometry, removal, and re-laying of special slabs, which is uneconomical and time-consuming. it is not necessary to disassemble these special slabs for the ballastless track due to inspection of rail fastening, rail replacement, or rail grinding. • almost dust-free operation. • non-restricted use of eddy current brakes, which only applies if the track is used by vehicles equipped with these types of brakes. • there is no risk of damage caused by flying gravel from the railway bed, during which the individual grains of ballast are sucked by passing rolling stock and thrown out of the track. more precisely, no additional measures are required in the track construction to prevent this dangerous phenomenon (sealing of gravel in the track bed). flying gravel not only destroys rail vehicles, but also damages the entire structure of the railway superstructure (sleepers and rail fasteners are most often damaged, but the rails are also damaged). • there is no need to use very harmful chemicals based on herbicides and pesticides to destroy vegetation in the track area. disadvantages of a ballastless track compared to a conventional track construction: • higher investment costs. the multiplicity varies significantly, especially according to countries and their different prices of raw material inputs, labour costs and machinery costs, as well as according to the scope of building of a ballastless track. in the conditions of the czech republic, the multiplicity is expected to be approximately at the level of two to three times the investment costs of a track with a ballast bed. however, in the construction of a new high-speed rail, the investment costs of railway superstructure rarely exceed 10% of the total costs, so the more expensive ballastless track will not increase the price of the whole construction many times over. besides, experience from db netz ag’s 54 vol. 31/2021 railway superstructure for high-speed rails figure 1. ballastless track type rheda 2000 before casting (by concrete), photography author: db projekt stuttgart-ulm gmbh/ondřej tomaschko investment projects shows that the cost of ballastless track construction in a tunnel is lower than the cost of building a conventional track structure with a ballast bed and with a sub-ballast vibration damping mat. • necessary high demands on the accuracy and quality of construction – but the same as with other cement concrete or asphalt concrete constructions. the track geometry cannot be significantly corrected, however, the ballastless track fastening systems are designed to allow the rail to be moved vertically and transversely in the unit order of a few tens of millimetres (e.g. vossloh’s 300 series rail fastening system allows additional height control -4/+56 mm, exceptionally up to -4/+76 mm, and directional control ±16 mm). even with conventional track construction, it is not possible to significantly change its position (especially in the case of continuous welded rail). • slower construction, although currently even monolithic types of the ballastless track (e.g. type rheda 2000) are set up with the use of machine laying (finishers), when it is possible to achieve an output of up to 250 m of established track per day (in two shifts), or more precisely twice for the hydraulically bound layer under the top concrete slab (hgt). it is susceptible to track substructure stability (settlement), therefore its establishment on bridges and tunnels is simpler, thus it is more frequent (even with the use of other advantages mentioned above) – on the earthwork, multi-layer load-bearing systems are required. for example, in germany, a permit for the construction of a ballastless track is issued only when the settlement of a maximum of 15 mm is confirmed from the performed measurements and subsequent calculations for the entire planned ballastless track service life, that means within 60 years. • higher noise emissions, which can be eliminated by adding elements of active noise protection (absorbers, rail damper, etc.); however, these elements increase investment and operating costs. high noise emissions from moving trains must also be solved in the conventional track construction – especially at very high speeds (from approx. 250 km/h) the dominant is aerodynamic noise, which intensity is influenced exclusively by the running train (depends on train speed, shape, and its external surfaces). 55 l. týfa, j. kruntorád, o. tomaschko acta polytechnica ctu proceedings figure 2. machine laying by finisher of the hydraulically bound layer under the top concrete slab (hgt), photography author: db projekt stuttgart-ulm gmbh/ondřej tomaschko • in the event of structural failure (derailment, settlement of the foundation, etc.), extensive and expensive repairs are required. however, in the case of the consequences of train operation (derailment), it usually takes less time than removing vehicles and investigating an emergency on-site. also, the ballastless track establishment includes the concept of its repair and rehabilitation. most types of ballastless track structures can be put into temporary operation in a relatively short time after an extraordinary event or the occurrence of a defect, and the subsequent adjustment to the original state can then be carried out in the planned track possession. • more time-consuming and financially demanding removal of its construction and subsequent recycling of the obtained material at the end of ballastless track service life. 3. other factors influencing the decision on the choice of type of railway construction in addition to the requirement for a long service life and at the same time low maintenance costs, the track structure must be designed in such a way that it can easily withstand the effects of frost and its composition does not show any deformation. with the conventional track design with a ballast bed, the process of increased wear of the individual grains of gravel begins when the forced oscillation frequency approaches 30 hz. with an axle distance in the bogie of 2.50 m and an ideal wheel-rail contact, this situation occurs at a train speed of approx. 270 km/h. other factors, such as surface unevenness of the wheel or defects in the rails, also contribute to achieving this frequency. with increasing train speeds and frequencies, increasing vibration amplitudes and higher dynamic stresses, the intervals for maintenance 56 vol. 31/2021 railway superstructure for high-speed rails of the railway superstructure must be shortened [4]. from the point of view of maintaining the planned operation, maintenance is only possible in regular possession during the night, which can only take a few hours for operational reasons. another factor that affects the stress in the railway superstructure is the stiffness of the entire track structure of ballast track construction. the stiffer the construction system, the more the ballast bed is stressed. especially on bridges and in tunnels, which are abundant on high-speed rails, the stiffness of the system is higher due to the hard base (bridge loadbearing structure, concrete slab in the tunnel) than of the track on earthworks, and thus the ballast bed is significantly more stressed. the lifetime period of the ballast bed is improved or extended by suitable steps, such as the use of transverse sleepers with a larger loading area, a more elastic rail fastening system, the placement of sleepers with under sleeper pads or the placement of sub-ballast vibration damping mats [5]. experience shows that the adjustment of the track geometry at a line speed above 250 km/h must be done after a passing a load of about 100 million gross tons. at db netz ag, therefore, since the mid-1990s, a ballastless track has been prescribed for all new constructions as a type of railway track structure, which also applies to all newly built tunnels, regardless of the operating load. 4. conclusion the economic evaluation of the construction work is a fundamental criterion that decides about its realization, or more precisely about its scope. in this assessment, it is necessary to correctly set the payback period of the investment, which sufficiently considers the lifetime period of the main components of the construction, as well as the costs of maintenance and possession activities. when considering life cycle costs (lcc), investment costs make up only a part of the total costs and it turns out that maintenance costs often far outweigh investment costs. db netz ag’s present experience with a ballastless track since its first implementation in rheda railway station in 1972, for almost 50 years, shows that the required lifetime period of 60 years has its real justification. in addition, current research and development in the field of ballastless track is moving towards extending the lifetime period of ballastless track well beyond this limit. when assessing the choice of track structure – especially for high-speed rails – it is therefore necessary and correct to assess the total costs (investment and operational) for a sufficiently long time. this number of years should ideally be equal to the value of the least common multiple of the lifetime period of the ballastless track and the lifetime period of the conventional track structure with ballast bed, because in this case the residual value of both variants will be zero at the end of the payback period and in addition, it will be possible to easily include in the total costs the costs of the track removal. references [1] sždc s9. pevná jízdní dráha. praha: správa železniční dopravní cesty, s. o., 2012, 52 s. [2] o. plášek, et al. železniční stavby: železniční spodek a svršek. vyd. 1. brno: cerm, 2004. 291 s. isbn 80-214-2620-9. [3] b. lichtberger. track compendium: formation, permanent way, maitenance, economics. 1st ed. hamburk: eurailpress, ©2005. 634 s. isbn 3-7771-0320-9. [4] s. freudenstein, et al. feste fahrbahn in betonbauweise. betonkalender 2015. [online]. d-69451 weinheim, germany: wiley-vch verlag, 2014, 2015-0123, pp. 529-584 [cit. 2016-08-31]. isbn 978-3-4336-03406, https://doi.org/10.1002/9783433603406.ch10. [5] e. darr, w. fiebig. feste fahrbahn: konstruktion und bauarten für eisenbahn und strassenbahn. hamburg: eurailpress, ©2006. 270 s. isbn 3-8266-1485-2. 57 https://doi.org/10.1002/9783433603406.ch10 acta polytechnica ctu proceedings 31:53–57, 2021 1 ballastless track characteristics 2 advantages and disadvantages of a ballastless track 3 other factors influencing the decision on the choice of type of railway construction 4 conclusion references acta polytechnica ctu proceedings doi:10.14311/app.2018.15.0088 acta polytechnica ctu proceedings 15:88–93, 2018 © czech technical university in prague, 2018 available online at http://ojs.cvut.cz/ojs/index.php/app modeling of underground structures subjected to earthquake by combining 1d free-field and 2d pseudo-static analyses tereza poklopová, veronika pavelcová, tomáš janda, michal šejnoha∗ czech technical university in prague, faculty of civil engineering, thákurova 7, 166 29 prague 6, czech republic ∗ corresponding author: sejnom@fsv.cvut.cz abstract. the paper deals with the evaluation of tunnel construction subjected to earthquake using a pseudostatic analysis combined with the finite element method. the initial stage of calculation is concerned with the design of computational model and subsequent analysis of the actual excavation process. the case study of the response of a selected construction to earthquake follows next. to that end, the so called 1d free filed dynamic analysis is performed first to generate appropriate loading conditions in terms of a layered-wise constant shear strain. therein, two particular boundary conditions, termed the fixed and absorbing boundary, are examined. the corresponding loading conditions are finally introduced in a 2d plane strain analysis to yield the internal forces developed in the tunnel lining. the results clearly show inadequacy of the fixed boundary and promote the use of absorbing boundary conditions for the present soil profile. keywords: earthquake, shear weave, accelerogram, finite element method, free-field, pseudostatic, absorbing boundary, fixed boundary, lining forces. 1. introduction design of underground structures loaded by earthquake is typically based on analytical solutions utilizing pseudostatic methods. this may yield less accurate or rather conservative results. in addition, adopting simplified methods generally calls for a number of restrictions such as the assumption of homogeneous elastic medium and a circular or rectangular frame structure representing the tunnel. if these assumptions apply, one may easily follow approaches described, e.g. in [1, 2]. the effect of earthquake is then simply represented by a static load due to shear strain derived from a velocity of the propagating shear wave and the associated maximum velocity of vibrating soil grains. this causes, e.g. ovaling or racking of a tunnel cross-section, which in turn serves as a basis for the stress analysis of the tunnel lining [2–4]. in this step, the soil-structure interaction is, however, neglected. overcoming the above restrictions offers an application of the finite element method [5, 6]. from the reliability of structural design point of view it seems most appropriate the use of a fully dynamic analysis [7, 8]. nevertheless, such an approach typically suffers from the lack of input parameters, notwithstanding the computational complexity. a suitable alternative, at least in the stage of a preliminary design, is then seen in exploitation of the previously mentioned pseudostatic method built upon the use of fem and one-dimensional (1d) fully dynamic free field analysis generating the load, see e.g. [3, 4]. this computational strategy will also be be examined in this paper with several recommendations for its practical application. we open the discussion is section 2 by briefly introducing the assumed computational model and general steps preceding the pseudostatic analysis. the basic principles of free field analysis (ffa) will be presented next in section 3. the actual pseudostatic analysis with emphases on the application of various types of boundary conditions is outlined in section 4. the essential findings are finally summarized in section 5. 2. calculation steps to introduce the subject we first present two computational models of a circular tunnel built in a soft soil about 10m above the base rock. the material parameters of the two layers are listed in table 1. while the response of clayey soil is assumed to be well represented by the mohr-coulomb (mc) material model, the limestone layer is considered as elastic. in [4] the material parameters of the mc model, not provided by the laboratory measurements including the increasing magnitude of the young modulus with depth, were tuned to render the resulting distribution of the lining internal forces at the end of excavation similar to those provided by the cam clay model. 2.1. excavation step selection of geometrical model no doubt that the response of soil subjected to earthquake depends on the current state of stress generated 88 http://dx.doi.org/10.14311/app.2018.15.0088 http://ojs.cvut.cz/ojs/index.php/app vol. 15/2018 modeling of underground structures subjected to earthquake soil layer e [mpa] ν [-] c [kpa] ϕ [◦] γ [kn/m3] clayey soil 15-40 0.4 10 25 19.6 limestone rock 630 0.3 21.7 table 1. material parameters of subsoil layers. element size number of settlement [mm] lining forces plastic strain [m] elements terrain crest invert nmax [kn/m] mmax [knm/m] [%] 0.25-2 4840 16.4 53.5 -26.8 952.8 129.8 0.91 0.5-4 2322 16.6 53.5 -26.3 958.4 135.5 0.85 0.5-8 1700 16.9 54.2 -26.6 948.6 138.8 0.86 1 8 1172 16.7 52.8 -24.7 935.2 165.6 0.59 table 2. evolution of settlement and lining internal forces as a function of mesh density. by the preceding actions. here we limit our attention to the effect of excavation only. while the actual tunnel was built using the tunnel boring machine, the modeling of the excavation process exploited the classical convergence confinement method (cc). to bring it closer to the, perhaps more suitable, volume loss method, the excavation sequence was proposed such as to generate final terrain settlement less than 2 cm. to that end, two geometrical models in fig. 2 were created to address the influence of the rock layer in static analysis on the one hand, and, on the other hand, to allow for considering two particular boundary conditions used in the dynamic analysis discussed next in section 3. (a) (b) figure 1. geometrical model: a) model-1 one layer, b) model-2 two layers. the excavation analysis was carried out in three consecutive steps to obtain the initial distribution of internal forces, see [4] for details. the evolution of settlement together with the evolution of internal forces as a function of the mesh density were adopted in search for the optimal mesh. the results presented in table 2, see also fig. 2 for illustration, promoted the mesh with 6-node triangular elements of the element size ranging from 0.5 m to 8 m for the pseudostatic analysis. point out that both models in fig. 1 provided essentially the same estimates of the initial distribution of internal forces thus giving seemingly the preference to less computationally demanding model-1. 2-0.254-0.58-0.5 8-1 0 20 40 60 80 100 120 140 160 180 0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 5500 m m ax (k n m /m ) n elements figure 2. evolution of lining moment as a function of mesh density. 2.2. pseudostatic analysis loading conditions as mentioned in the introductory part, the pseudostatic analysis considers the kinematic boundary conditions to generate the layer-wise uniform shear strain corresponding to free field conditions, the state prior to any construction. in case of a single wave propagating through a homogeneous material the free-field shear strain is provided by γ = vs cs , (1) where cs = √ g/ρ is the velocity of the shear wave and vs is the velocity of the soil particle; ρ,g stand for the soil density and elastic shear modulus, respectively. the particle velocity can be estimated based on the location and magnitude of earthquake, type of soil and the depth of tunnel [2]. however, the above assumptions are generally not met particularly if assuming a record of a real earthquake such as the one in fig. 3. it will be shown in the next section that in such a case it is more appropriate to derive the shear strain directly from a fully dynamic analysis of a 1d free field column. 3. free field dynamic analysis if a geotechnical construction can be analyzed in a two-dimensional (2d) environment under plane-strain conditions, the free field dynamic analysis reduces to the solution of a 1d column problem, see fig. 4(a), 89 t. poklopová, v. pavelcová, t. janda, m. šejnoha acta polytechnica ctu proceedings 0 5 10 15 20 25 30 35 40 45 50 a (m /s 2 ) t (s) 0.5 -0.5 -1.5 -2.5 1.5 2.5 (a) 0 5 10 15 20 25 30 35 40 45 50 v ( m /s ) t (s) 0.3 0.1 -0.1 -0.3 -0.5 0.5 (b) figure 3. adopted earthquake records: a) acceleration, b) velocity. having the same soil profile as the actual 2d computational model and being subjected to the same loading and boundary conditions. as it might not always be fulfilled by the 2d model, the free field conditions assume parallel and homogeneous layers in the horizontal direction. fixed b.c. 𝑢(𝑦, 𝑡) = 𝑢(0, 𝑡) + 𝑢𝑅(𝑦, 𝑡) 𝑢𝐼 𝑢𝑂 𝑢𝑅(0, 𝑡) = 0 𝑢𝐼0 = 𝑢𝐼 + 𝑢𝑂 𝑦 absorbing b.c. 𝑢(𝑦, 𝑡) = 𝑢𝐼0(𝑡) + 𝑢𝑅(𝑦, 𝑡) 𝑢𝐼0 𝑢𝑂 𝑢𝑂(0, 𝑡) = 𝑢𝑅(0, 𝑡) 𝜌𝑐𝑠(𝑢�̇�(0, 𝑡) − 𝑢𝐼0̇ (𝑡)) (a) (b) rock outcrop motion on terrain: 𝑢(𝑦, 𝑡) = 2𝑢𝑢 𝑢𝑢 𝑢𝑑 𝑢𝑢 = 𝑢𝑑 𝑢𝐼0 = 2𝑢𝑢 fixed b.c. absorbing b.c. 𝑢𝑑 𝑢𝐼0 = 𝑢𝑢 (c) figure 4. a) free field column with fixed boundary, b) free field column with absorbing boundary, c) loading and boundary conditions. turning now our attention to boundary conditions, two scenarios may occur. first, the soft soil layer containing a construction might be separated by an infinitely stiff base rock. the computational model is then truncated at the interface of the two layers and the resulting boundary is termed the fixed boundary. such a model appears in fig. 4(a) and corresponds to the 2d model in fig. 1(a). however, we often encounter a situation where the model must be truncated within a layer. such a boundary, depicted in fig. 4(b), requires the outgoing wave uo to be transmitted across this boundary. because the layer, containing the boundary, is assumed to be infinitely long, this requirement is essentially the same as if absorbing this wave at the boundary so it cannot be reflected back to the analyzed domain. this boundary will therefore be termed the absorbing boundary and will be linked to the 2d model in fig. 1(b). introduction of these two types of boundary conditions into the fem analysis is outlined in the next section. as for loading, we consider a real acceleration in fig. 3 in the form of a rock outcrop motion, see fig. 4, where uu, ud correspond to the upward and downward traveling waves, respectively. it turns out, see also the next section, that in case of fixed boundary conditions the total motion1 will be adopted when employing the fixed boundary, whereas only one half of the total motion2, corresponding to the incoming wave ui , will be prescribed in case of absorbing boundary. 3.1. theoretical background we begin by writing the total displacement field u(y, t) = uu(y, t) + ud(y, t) = ui(y, t) + uo(y, t) = ui0(t) + ur(y, t), (2) where ui0(t) = u(0, t) is the displacement field prescribed at the bottom boundary at y = 0, and ur(y, t) = {ur(y, t), vr(y, t)} is the searched relative displacement vector. going back to fig. 1(a) it becomes clear that in case of fixed boundary it holds ur(0, t) = 0 at the bottom boundary. (3) the fixed boundary does not therefore require any special treatment. so we focus now on absorbing boundary and briefly describe the derivation of the absorbing boundary condition presented in fig. 4(b). for more details the interested reader is referred to [9]. examining eq. (2) suggests u ′ o(y, t) = u ′ r(y, t)− u ′ i(y, t), (4) where (f ′) stands for the space derivative with respect to y. as shown in [9, 10] we may further write u ′ o(y, t) = 1 cs u̇o(y, t) (5) u ′ i(y, t) = − 1 cs u̇i(y, t), (6) 1the actual magnitude of the record being equal to some of the incoming and outgoing waves as the outgoing wave is reflected back to the domain. 2this assumption is valid for the case study in fig. 4(b). 90 vol. 15/2018 modeling of underground structures subjected to earthquake where (ḟ) represents the time derivative. substituting eqs. (5) and (6) into eq. (4) gives for y = 0, see also fig. 4(b), u ′ o(0, t) = u ′ r(0, t) + 1 cs u̇i0(t) = 1 cs u̇r(0, t). (7) considering the shear wave only, eq. (7) combined with the hook law provides the static boundary condition in the form τ(0, t) = gu ′ r(0, t) = ρcs (u̇r(0, t)− u̇i0(t)) (8) to be satisfied at the bottom boundary. note that this condition corresponds to a viscous damper with the viscosity η = ρcs introduced at y = 0, recall fig. 4(b). in the absence of material damping the principal of virtual work reads δüt rmür + δut rkur + δu̇t rcu̇r|y=0 = −δüt rmüi0 + δu̇t rcu̇i0|y=0. (9) where m,k represent the mass and stiffness matrices and the damping matrix c, active at the bottom boundary only, is provided by c = [ ρcs 0 0 ρcp ] , (10) where cp = √ eoed/ρ is the pressure wave velocity with eoed being the oedometric modulus. the discretized system of governing equations of motion resulting from eq. (9) is typically solved with the help of newmark method [5]. 3.2. loading conditions shear strain the time variation of the shear strain u ′(y, t) was found by employing eq. (9). to that end, the acceleration üi0 and velocity u̇i0 records in fig. 3 were adopted. note that in case of fixed boundary the damping term in eq. (9) drops out. it is also worth mentioning that eq. (8) was derived assuming the elastic behavior. in this regard, the elastic moduli g,eoed corresponding to an unloading/reloading modulus should be employed. the deformation modulus for the clayey soil in table 1 was therefore replaced by a dynamic modulus and set equal to 80 mpa for the entire layer. in the light of subsequent analysis the maximum values of the shear strain u′(y, t) developed along the vertical direction of the analyzed domain during the duration of earthquake were collected. a graphical representation for the two types of boundary conditions appears in fig. 5. the corresponding particle velocity, to be potentially introduced in eq. (1), is plotted for the sake of completeness. we immediately observe that the maximum shear strain is in case of fixed boundary considerably bigger in comparison to absorbing boundary. this result, attributed to the interference of the incoming wave and the one being reflected back on the fixed boundary (infinitely stiff base rock) will be analyzed in more details in the next section. the resulting kinematic boundary conditions for the two models are plotted in fig. 6. model-1 fixed b.c. -49 -39 -29 -19 -9 z (m ) γmax (-) γmean = 0.023 0.0 0.01 0.02 0.03 0.04 0.05 -49 -39 -29 -19 -9 z (m ) vs,max (m/s) vs,mean = 2.802 m/s 0.0 1.0 2.0 3.0 4.0 5.0 tunnel axis (a) model-2 -59 -49 -39 -29 -19 -9 z (m ) γmax (-) γmean= 3.541∙10-3 γmean= 0.630∙10-3 0.0 0.002 0.004 0.006 0.008 -59 -49 -39 -29 -19 -9 z (m ) vs,max (m/s) vs,mean= 0.335 m/s vs,mean= 0.549 m/s 0.00 0.20 0.40 0.60 0.80 1.00 tunnel axis absorbing b.c. (b) figure 5. distribution of maximum shear velocity and shear strain: a) fixed boundary, b) absorbing boundary. 4. pseudostatic analysis this section reports on the application of simplified pseudostatic analysis to two geometrical models in fig. 1 being linked two types boundary conditions described in the previous section. the pseudostatic analysis represents a new calculation stage in which the analyzed domain is loaded by the prescribed displacements compatible with the average shear strain provided by the free field analysis, see fig. 6. γ = 0.023 1 127.0 mm γ = 0,0035 γ = 0.00063 179.8 mm 6.3 mm γ = 0.0035 (a) (b) figure 6. kinematic loading conditions: a) fixed boundary, b) absorbing boundary. the value of the maximum displacement associated with the fixed boundary immediately suggests inadequacy of this type of boundary condition in the present analysis. herein, the young modulus of the base rock equal to 630 mpa, recall table 1, is far from infinity particularly if compared to the value of 80 mpa adopted for the clayey soil. this can be supported by calculating the value of particle velocity following 91 t. poklopová, v. pavelcová, t. janda, m. šejnoha acta polytechnica ctu proceedings the procedure outlined in [2]. in our particular case, see [4] for details, the particle velocity vs would be approximately equal to 0.99 m/s. this is about three times bigger than the average velocity reported in fig. 5(a) for the fixed boundary and comparable with the one found for the absorbing boundary in fig. 5(b). the suitability of using the absorbing boundary conditions will further be supported by inspecting the values of lining internal forces in the next section. but before proceeding, a word of caution is in place as to the application of material models in pseudostatic analysis. this becomes clear by examining the results plotted in fig. 7. these were derived by considering the computational model-1 with only one layer of soil free of any tunnel (free field conditions) and loaded by the prescribed constant shear strain γ = 0.023, recall fig. 5(a). to be consistent with the free field conditions the same value of shear strain should be reproduced at every point within the domain3. this is certainly not true in the first case, see figs. 7(a,b) assuming the soil with a variable stiffness and its response driven by the mc material model. as expected such a loading may generated an unacceptable plastic strains at the vicinity of terrain surface where the compressive stresses are close to zero. thus the nonlinear constitutive model, if at all, should only be applied in the vicinity of the tunnel. as further seen in 7(c) the use of elastic model while still keeping the variable stiffness assumed in excavation analysis does not provide an acceptable result. as evident from fig. 7(d), that is only found if considering a truly homogeneous (one modulus for the entire layer) elastic soil, thus the material setting used previously in ffa. (a) (b) (c) (d) figure 7. a) total shear strain variable stiffness and mc model, b) equivalent plastic strain variable stiffness and mc model, c) total shear strain variable stiffness and elastic model, d) total shear strain constant stiffness with the layer and elastic model. such conditions then yield the expected distribution of shear strains for the two computational models 3this is not exactly true as the static boundary condition requires the shear stress and thus also the shear strain be zero at the terrain boundary [9]. as evident from fig. 8. clearly, the perturbation caused by the presence of tunnel is confined to its close vicinity. γmax = 4.04% γmax = 0.64% (a) (b) figure 8. distribution of shear strain: a) model1, loading from fig. 6(a), b) model-2, loading from fig. 6(b). 4.1. lining forces due to earthquake to further appreciate the differences in the response provided by the two types of boundary conditions we examine the resulting values of the lining internal forces at the end of the pseudostatic analysis. for illustration we plot in fig. 10(a) the distribution of moments found at the end of excavation step, thus prior to the introduction of surcharge due to earthquake. the impact of earthquake, in the light of pseudostatic analysis, is clearly visible in figs. 10(b,c). -1500 -1000 -500 0 500 1000 1500 0 π/4 π/2 3π/4 π 5π/4 3π/2 7π/4 2π m ( k n m /m ) ϕ (rad) end of excavation after pseudostatic analysis 1380.8 1393.8 1384.2 1393.6 65.0 90.4 64.8 90.3 90.3 (a) -300 -200 -100 0 100 200 300 0 π/4 π/2 3π/4 π 5π/4 3π/2 7π/4 2π m ( k n m /m ) ϕ (rad) end of excavation after pseudostatic analysis 229.5 231.3 231.9 230.2 90.3 65.0 90.4 90.3 64.8 (b) figure 9. distribution of moments prior to and after running the pseudostatic analysis: a) fixed boundary, b) absorbing boundary. even more clear illustration of the influence of the selected type of boundary is seen in figs. 9 showing variation of moments as a function of the circumferential angle. the use of fixed boundary conditions is clearly incorrect as the maximum moment in fig. 9(a) exceeds 10 times the value of the moment at the end of excavation step. the normal force, not shown, becomes even positive at some locations with the largest value over 1600 kn/m. given the depth of the tunnel, such 92 vol. 15/2018 modeling of underground structures subjected to earthquake -90,4 6 4 ,8 -90,3 90,3 -6 5 ,0 90,4 -85,8 -1384,2 13 93 ,6 -90,5 90,5 1380,8 -1 39 3, 8 85,8 -90,4 -231,9 23 0, 2 -88,8 88,8 229,5 -2 31 ,3 90,4 (a) (b) (c) figure 10. distribution of moments: a) at the end of excavation step, b) at the end of pseudostatic analysis fixed boundary, c) at the end of pseudostatic analysis absorbing boundary. a result is highly improbable. much reasonable results are achieved with the use of absorbing boundary as displayed in fig. 9(b) completely eliminating the positive normal forces, see [4] for further details. the resulting distributions of moment increments caused by earthquake and pertinent to two types of boundary conditions are finally compared in fig. 11 further highlighting the inadequacy of the fixed boundary conditions, at least for the present geometrical and material settings. -1500 -1000 -500 0 500 1000 1500 0 π/4 π/2 3π/4 π 5π/4 3π/2 7π/4 2π δ m ( k n m /m ) ϕ (rad) model-1 model-2 216.1 221.4 215.8 216.6 1385.5 1388.5 1383.3 1384.0 figure 11. distribution of moments comparing the impact of fixed and absorbing boundary conditions. 5. conclusions the present paper was concerned with the application of pseudostatic analysis to address the effect of earthquake on the evolution of internal forces within the lining of a circular tunnel. unlike in most practical approaches the loading conditions for the final 2d analysis were found from a fully dynamic analysis of a 1d free field column problem subjected to a real earthquake. in this regard, two types of boundary conditions prescribed at the bottom boundary were examined. while fixed boundary is typically adopted in cases when a soft soil layer is supported by an infinitely stiff base rock, the absorbing boundary must be introduced if the depth of computation model appears within a layer, regardless whether soft or stiff. such a boundary condition then allows for completely absorbing the incoming wave so it is not reflected back into to the analyzed domain as in case of fixed boundary. unless the based rock is truly stiff, the fixed boundary should be avoided, as it leads, unlike the absorbing boundary, to unacceptable results. this was shown herein for one particular example. acknowledgements the support provided by the sgs project no. sgs18/037/ohk1/1t/11 and the tačr project no. te01020168 is gratefully acknowledged. references [1] j. penzien. seismically induced racking of tunnel linings. earthquake engineering and structural dynamics 29:683–691, 2000. [2] y. hashash, j. hook, b. schmidt, j. yao. seismic design and analysis of underground structures. tunneling and undergrounding space technology 16:247–293, 2001. [3] d. kučera. using finite element method in practical design of geotechnical structure under seismic loading. master’s thesis, czech technical university in prague, faculty of civil engineering, 2017. in czech. [4] t. poklopová. evaluation of real underground structure subjected to earthquake pseudostatic analysis. bachalor’s thesis, czech technical university in prague, faculty of civil engineering, 2018. in czech. [5] o. řeřicha, z. bittnar. finite element method in dynamic analysis of structures. nakladatelství technické literatury, 1981. in czech. [6] z. bittnar, j. šejnoha. numerical methods in structural engineering. asce press, 1996. [7] v. pavelcová. evaluation of real underground structure subjected to earthquake fully dynamic analysis. bachalor’s thesis, czech technical university in prague, faculty of civil engineering, 2018. in czech. [8] v. pavelcová, t. poklopová, t. janda, m. šejnoha. the influence of boundary conditions on the response of underground structures subjected to earthquake. acta polytechnica 2018. accepted. [9] o. zinkiewicz, n. bicanic, f. shen. generalized smith boundary a transmitting boundary for dynamic computations, vol. 207. innme swansea, 1986. [10] d. kučera. posouzení geotechnické konstrukce na seismické zatížení metodou konečných prvku. master’s thesis, czech technical university in prague, faculty of civil engineering, 2017. 93 acta polytechnica ctu proceedings 15:88–93, 2018 1 introduction 2 calculation steps 2.1 excavation step selection of geometrical model 2.2 pseudostatic analysis loading conditions 3 free field dynamic analysis 3.1 theoretical background 3.2 loading conditions shear strain 4 pseudostatic analysis 4.1 lining forces due to earthquake 5 conclusions acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2018.19.0036 acta polytechnica ctu proceedings 19:36–45, 2018 © czech technical university in prague, 2018 available online at http://ojs.cvut.cz/ojs/index.php/app residual heat power removal from spent nuclear fuel during dry and wet storage daniel vlček department of nuclear reactors, faculty of nuclear sciences and physical engineering, czech technical university in prague, v holešovičkách 2, praha 8, czech republic correspondence: daniel.vlcek1@gmail.com abstract. this project deals with the thermal analyses of the wet and dry storages of the spent nuclear fuel. the dry spent fuel storage sub-channel code cobra-sfs has been used in order to calculate the temperature field. in this code, the new model of residual heat removal was created for the skoda 1000/19 cask where the spent nuclear fuel tvsa-t type from npp temelin will be stored. the object of calculations was to obtain the inside temperatures under maximum loads. after that, the results were compared to the requirements of the local regulatory body. because of the absence of experimental data, the validation of the created computational models could not be accomplished. however, according to the verification scheme of the cobra-sfs authors, the verification of the new models was implemented. keywords: spent nuclear fuel, residual heat removal, dry storages, tvsa-t fuel, thermal analysis, subchannel analysis. 1. introduction a steel cask is a multifunctional system for storing and for transporting spent fuel. as other options, it has to guarantee the cooling of fuel assemblies. this requirement is of primary importance to fuel integrity preservation and, from the regulatory point of view, it is fulfilled if cladding temperature is below the prescribed limit of 350 ℃ in the czech republic which is in agreement with iaea safety guides [1]. in this frame, the thermo-fluid dynamics of spent fuel storage casks are a key aspect of safety analyses. in the 80s, thermal analyses were carried out for low burnup fuel (around 30 mwd/kgu) stored in casks as castor (wheeler et al. [2]; rector et al. [3]), tn24p (mckinnon et al. [4]), and rea-2023 (mckinnon et al.[5]). the thermal analyses were performed with vacuum, nitrogen, and helium backfill environments in both vertical and horizontal cask orientations. at the time, thermal calculations were conducted with the subchannel code cobra-sfs, which is an extension of cobra (thermal-hydraulic analysis computer code). cobra-sfs retains all the important features of the cobra codes for single-phase analysis, and extends the range of application to problems with two-dimensional radiative and three-dimensional conductive heat transfer to allow analyses of multiassembly spent-fuel storage and transportation systems (currently the newest version cycle 4 came out [6]). the code allows for steady-state threedimensional modeling and uses a control-volume approach to solve the mass, momentum and energy conservation equations to determine pressure, temperature and velocity fields, under forced and natural convection heat transfer conditions, in both steadystate and transients. the steady-state form of the figure 1. horizontal cut of the model skoda 1000/19 storage cask. differential equations is solved in each of the three orthogonal coordinate directions as if they were three independent one-dimensional problems. 36 http://dx.doi.org/10.14311/app.2018.19.0036 http://ojs.cvut.cz/ojs/index.php/app vol. 19/2018 heat removal from spent nuclear fuel nowadays, high burnup of spent fuels (< 60 gwd/tu) as well as proliferation of dry cask storage due to in-pool storage limitations, have made it necessary to update thermal analyses addressing current conditions and cask designs. in addition, as most countries have not made a decision concerning the last stage of the nuclear fuel cycle yet, an extension of thermal studies to time spans up to 300 years seems recommendable [7]. in present cask analyses, thermal calculations are often carried out with computational fluid dynamics (cfd) 3d codes, so that an accurate insight into the thermal response of the cask is achieved. the hypothesis and approximations taken into account are an important part of the methodology applied. a proper procedure is fundamental to obtain a sound thermalfluid dynamic characterization of the simulated system. however, a commitment between a sound characterization and the computational cost should be considered [8]. models in cobra-sfs have never been validated for eastern type fuel with triangular geometry. although the authors of the code guarantee errorless function, the correct application brings problems that are not discussed in the available literature. 2. modeling the storage cask the main object of this work was to obtain thermal distribution of the skoda 1000/19 storage cask built in czech republic in the sjs company. it has been developed from the castor 1000/19 produced by the german gns [9]. the dimension parameters are classified and therefore could not be specified here. firstly, a simplified model has been created for the vertical position used for storing. horizontal cut of this model, see fig. 1, can be divided in three parts top, middle and bottom structures. in the centre part the channel region is located and will be discussed in detail. in order to calculate reverse flow in the sub-channel model, cobra-sfs (cycle 4) [6] enables to use the plenum model which represents top and bottom structures. the channel inlet and outlet conditions are then calculated in the code as required to satisfy overall mass conservation in the system and any specified assembly pressure drop boundary conditions. the plenum model assumes a one-dimensional node for the fluid, but the conduction heat transfer paths in the plenum regions can be radially and axially non-uniform. it is also possible to include heat losses through the sides of the plenum region, as well as through the top or bottom. natural circulation is created in the cask with helium backfill at approximately 40 kpa. therefore the helium data could be derived from lit. [10]. the solid structure of cask is made out of steel (blue in the fig. 1) and the main part of cast iron (grey). for the purpose of moderating neutrons the polyethylene plates and rods have been used (yellow). the material properties data were obtained from matweb [11]. figure 2. lateral section of the model skoda 1000/19 storage cask with highlighted assemblies (top) and all subchannels (bottom). fuel assemblies are held in the channel region by a carry grid built from a combination of borated and clean aluminium alloys to maintain the system below criticality. there are 19 positions for fuel assemblies in the cask and thanks to the symmetry the model could have been simplified to one sixth, cf. fig. 2. in the cobra-sfs code it is necessary to split the model into channels, slab parts, rods and surroundings in the form of plena. it can be also seen 18 isolated areas assemblies which are not connected to each other in lateral direction in the top fig. 2. the whole model is divided into 48 axial nodes each consists of 256 sub-channels, see bottom fig. 2, and 90 slab structures. 37 daniel vlček acta polytechnica ctu proceedings 2.1. boundary conditions the boundary conditions are key input parameters used for calculation of temperature field. for this analysis the residual heat power of fuel assemblies and surrounding temperature have been used. the temperature was set to 38 ℃. the value of heat power which is generated by fuel assembly was derived from the potential maximum load of storage cask 21.68 kw. the input data of residual heat was then divided into three schema. homogeneous option (scheme a) with 1141 w per fuel assembly and two heterogeneous with maximum 1334 w in the centre (scheme b) and the last (scheme c) has power peak 1264 w in the second row. the company sjs uses these schemes as well, however at the end the author did not obtained the data from their calculations. despite this the three schemes remained the same, but the analysis could not be validated or at least calibrated. 2.2. tvsa-t fuel after the residual heat power was set, the axial and radial heat profile had to be maintained for the tvsat fuel. the radial profile was set to be simply homogeneous, however the axial profile was calculated using a module of scale 6.2.2 called triton that combines the transport code newt for nuclear data preparation for the computational procedure with the depletion code origen. the results of axial profile can be seen for different enrichment and burn-up in fig. 3. the value of 4.4 % and 50 mwd/kgu were chosen and the time dependence on residual heat power was estimated as is presented in the fig. 3. the homogeneous radial profile serves as first approximation. it is not ideal, however it was easier to spot possible errors in the input file. for the future work would be necessary to add radial profile. it would affect the temperature peaks described below. the parameters of tvsa-t fuel are summarized in the table 3 below. the fuel assemblies have been simplified by lumping together the fuel rods as well as the subchannel around them. the simplification was made due to computation time especially in the peripheral parts. the number of fuel rods has been reduced to 115 from 331 and the number of sub-channels from 720 to 78. because of the symmetry, the assemblies were not the same. in the input file of cobra was necessary to create them separately as well as in the input file of radgen as can be seen in the fig. 2. 2.3. heat transfer and outlet conditions heat transfer is realized from the fuel assemblies by the form of conduction, convection and radiation. the contact of fluid with fuel rods is described by the nusselt number which is equal to 3.66 for the laminar flow and for the turbulent flow the dittusboelter equation can be used. however the turbulent figure 3. axial profile (top) and dependence of heat power on time (bottom) of tvsa-t fuel. flow practically did not occur due to low velocities of helium inside the cask. the pressure loss was derived with the use of grid factor equal to 100/re [12]. axial heat transfer in the fluid was neglected and the thermal resistance between adjacent nodes was used in order to obtain axial and radial conductive heat transfer. for the radiation heat transfer it was necessary to estimate radiation view factors of black/grey bodies. the radgen code, a standalone module of cobrasfs, was used for the fuel assemblies. view factors of the empty ones were defined using hottel’s method together with the cox string correlation [13]. after the calculation of view factors was done, the emissivities of surfaces had to be set. literature advises to use a value of emissivity equal to 0.8 for oxidized zirconium surface, 0.6 for aluminium surfaces and 0.4 for the steel ones [14]. closure of the set of equations was realized by adding the outlet conditions. the first was the outlet temperature, in this case uniform in axial direction. the second very important part was to set the outlet conditions of the cask and the two plena. the correct way for obtaining thermal resistivity of these parts is measuring it, however authors of cobra-sfs [5] recommend a theoretical approach where the heat flux at the sides is calculated as follows. the most universal equation for the heat transfer is (regarding 38 vol. 19/2018 heat removal from spent nuclear fuel figure 4. verification step 3 (top) with the temp. of all slab structures and step 4 (bottom) with pairs of selected fuel rods in assembly 3 vs. 4. radiation heat transfer): q = c1 · [c2 (ti − ti+1)]c3 · (ti − ti+1) , (1) nevertheless this relation is often overly complex. the maximal simplification is achieved in the case of the connection of two solid structures conduction connection. the coefficients are set as: c1 = λ l , (2) where λ is thermal conductivity and l is the width of connection. c2 = 1, c3 = 0. for the peripheral parts of cask which are in contact with fluid it is necessary to use a different approach to include natural convection. usually the approach with a nusselt number is chosen: nu = α · l λ = c1 · (gr pr)c2 , (3) where the constants c1,2 depend on the choice of a natural convection correlation and the solution leads to these equations: c1 = c1 · ( λ l ) ; c2 = g · γ · l3 · ρ2 · cp η · λ ; c3 = c2. (4) this approach of setting the outlet conditions can be used as the first estimation of heat flux through the cask. other option can be setting the coefficients as stated above and then change them preserving the ratios between them in order to achieve desired temperature. the last option can be setting a different value of nusselt number for laminar flow, which was set to 3.66 (experimental data from [5]) and bias the rod wall temperatures. 39 daniel vlček acta polytechnica ctu proceedings figure 5. verification step 3 (top) and 4 (bottom) with the temperature differences of absorption rods. 3. verification having successfully developed an input file for the cobra-sfs code, the next task is verifying that the model correctly represents the system being modelled. the input error checking in the code will readily capture actual mistakes in the input stream, but it will not generally capture errors in the consistency of the input. long experience [6] with this process has shown that the following steps can reliably uncover most, if not all, such errors in consistency of input. (1.) running the model with zero decay heat generation, uniform temperature specified on all boundaries, omitting thermal radiation modeling (2.) running the model with zero decay heat generation, uniform temperature specified on all boundaries, add in thermal radiation modeling in the fuel assemblies (3.) same conditions as step 2, but expand group radg to include user-defined black body view factors in open assemblies, in group radg (4.) run the model with all thermal radiation modeling included and uniform decay heat generation rate in all assemblies with fuel rods after each step the symmetry and consistency of boundary temperature must be compared. for better illustration, the temperature differences were plotted into bar charts. during the first three steps there was no heat generation. after elimination of the occurred errors the figures from these steps were exactly the same. the difference appeared at the last step. in the fig. 4 on the left the temperatures of the slab structures are plotted. it seems that some errors could not have been erased however the temperature difference is about 0.4 ℃. the same fact can be observed in the fig. 5, resp. 6, where the temperatures of absorption 40 vol. 19/2018 heat removal from spent nuclear fuel figure 6. verification step 3 (top) and 4 (bottom) with the temperature differences of peripheral basket structures. rods (aluminium structures around fuel assemblies), resp. peripheral parts of the basket are plotted. when the heat generation was included the differences went higher, however maximal value of the difference was 0.65. in the fig. 4 on the right the symmetry of fuel rods between assembly 3 and 4 is presented. the maximal temperature difference was also lower than 1 ℃ therefore the model has been declared verified. the validation of this model could not be done, because the data from sjs were not from commercial purposes available also the accessible literature does not provide any data from measurements of skoda or castor 1000/19. the sjs used cfd approach with the ansys code. 4. results 4.1. maximum temperature the main object of the computation is to calculate the temperature in the most heated rod in the whole cask. from the regulatory point of view, the cask can be authorized if cladding temperature is below the prescribed limit of 350 ℃ [1]. in the fig. 7 it can be seen that the limit is fulfilled for all loading schema with maximal heat power of 21.68 kw. the relative distance is measured for the whole fuel assembly and in the figure the position of fuel in the assembly together with axial profile can be clearly observed. the maximum temperature is located in the centre of middle fuel assembly in the case of loading schemes a and b. in the case of the loading scheme c the maximum lies in the first row (see assembly 1 in fig. 2). temperatures are specified in the tab. 1. 41 daniel vlček acta polytechnica ctu proceedings loading temperature [℃] scheme a 306.7 scheme b 316.5 scheme c 311.1 table 1. summary of maximal cladding temperatures during different loads. the maximum lies 2149 mm from the bottom of the cask (4550 mm in total length). 4.2. radial distribution of temperatures radial distribution of the slab structure temperatures is shown in the fig. 8. there three rows of fuel assemblies can be seen together with solid nodes in the cask. on surface the temperature should not exceed the limit of 85 ℃ as far as the npp ete requires. this requirement was almost fulfilled with temperature 88 ℃. however the results cover the highest possible load and the calculations slightly overpredict the actual data. the radial profile should be flattened however in the figure only the solid structures are plotted. if the temperatures of surrounding sub-channels were also plotted, the profile would be more balanced especially between fuel assemblies. the decrease of temperature at the peripheral cask structure is a result of conductive heat transfer with low thermal resistance. in the figure the maximum under the scheme c can also be discerned to be located in the first row instead of the centre of cask. 4.3. time dependence on maximal temperature the last studied parameter in detail is the axial dependence of cladding temperature on time. the effect of axial distribution is mentioned above. the development of heat power in time is presented in the bottom part of fig. 3. the data from here were the input parameters and led to an axial temperature evolution, see fig. 9. as a result of radioactive decay the heat power decreases together with the maximal temperature, however even after 50 years the temperatures inside the cask reach 150 ℃. 4.4. flow rate the flow rate inside the cask has been changing from the initial guess by iterative solution to its actual value on the basis of natural circulation. maximum of the flow was obviously heading upwards in the centre of the cask (considering the whole assemblies), where the highest temperatures occurred (except the loading scheme c), and it was constantly decreasing in the assemblies 1, 3, 4, 2. even in some peripheral sub-channels of these assemblies the flow direction has changed. the flow rate was completely downwards in the peripheral assembly no. 18. mean values of flow rates are presented scheme a b c assembly 1 5.53e-6 6.35e-6 7.94e-6 assembly 2 4.54e-7 -6.35e-7 -5.44e-7 assembly 3 1.04e-6 9.98e-7 6.80e-7 assembly 4 1.09e-6 9.98e-7 6.80e-7 assembly 5 1.36e-6 1.77e-6 7.71e-7 assembly 18 -9.53e-6 -9.53e-6 -9.53e-6 table 2. a summary of flow rates [kg/s] in different assemblies and loads in table 2. the total flow direction has switched in assembly 2 in schemes b and c. it was probably due to the fact that the peripheral assembly 18 could not transfer more fluid and therefore a part of the flow has moved to different assemblies. 5. conclusions simplified model of the multifunctional storage cask skoda 1000/19 containing tvsa-t fuel assemblies has been created. the input file for the code cobrasfs has been prepared together with input data from code radgen providing radiation view factors that are necessary for the radiation heat transfer. in the model the inner carrying parts were modelled in detail, however at the peripheral parts, especially at top and bottom of the cask many approximations were used. to complete the model material properties, boundary conditions were added. the initial parameters of the spent fuel were obtained from the origen code. the boundary conditions were estimated with the use of the methodology of the cobra code creators. the important part connected with creating a new model is its verification. here the four-steps-long verification process is carried out. the solution consists of three steps with no heat power generation and the last step with it already included. this procedure should cover most of the potential errors made during creating the input file. in the output data the undesirable temperature peaks are searched for together with asymmetries in the results. in the first three steps, the temperature should be equal to the surrounding temperature. seeing that the temperature differences in observed symmetry have not surpassed 1 ℃ even in the last step, the model has been declared verified. the validation or at least calibration could not be done since the data are from the commercial purposes classified. the main object of this work was to find out whether the cobra-sfs code can predict temperature field in a storage system with triangular geometry even though it has not been validated for that. it is shown that it is possible, however the cobra manual has not clarified many of the potential questions and it depends strongly on the researcher, how the problems will be solved. the results are discussed in the last part of this paper. it shows that the safety guide recommendation 42 vol. 19/2018 heat removal from spent nuclear fuel figure 7. axial dependence of cladding temperature of the most heated rod during different loads in the skoda 1000/19 storage cask. figure 8. radial dependence of temperature in the skoda 1000/19 storage cask. for the maximum cladding temperature of 350 ℃ is fulfilled and the axial distribution of maximal temperature of the most heated rod is presented in the fig. 7 for three different load schema. in the fig. 8 the temperatures are at the level where the maximum occurs plotted in radial direction. the last displayed data are plotted in the fig. 9, where the dependence of heat power on time is shown. it can be clearly seen that even after 50 years the maximal temperatures are still high. as a result of this it is not necessary only to ensure temperature limits but also to preserve fuel integrity. finally the flow rates of all assemblies has been compared and summarized into table 2. the interesting thing happened in assembly 2 in schemes b and c, where the total flow direction has switched from upwards to downwards. it was probably caused by the fact that the peripheral assembly 18 could not transfer more fluid and therefore a part of the flow has moved to different assemblies. the work in this paper confirms that the spent nuclear fuel from vver reactors can be characterized using cobra-sfs code in a way similar to the fuel from western types of reactors. the simplified model has been created however there is a lot of possibilities for future work including changes in the cobra code or creation of a more detailed model of the storage cask. including different loads together with heterogeneous radial power is a possibility as well. 43 daniel vlček acta polytechnica ctu proceedings figure 9. axial dependence of cladding temperature of the most heated rod on time during load scheme a in the skoda 1000/19 storage cask. fuel pellet: inner diameter [mm] 1.2 outer diameter [mm] 7.6 pellet height [mm] 9-12 pellet density [g.cm−3] 10.4–10.7 thermal conductivity [w/m.k] 5 heat capacity [kj/kg.k] 0.2470 αgap [w/m2.k] 16000 fuel rod: inner cladding diameter [mm] 7.73 outer cladding diameter [mm] 9.1 rod length [mm] 3925 rod active length [mm] 3680 cladding material zr-4; e-110 fuel assembly: average enrichment [%] <5 burnable absorber gd2o3 pitch [mm] 12.75 number of spacer grid 8 spacer grid material e-110; steel guide tube inner diameter [mm] 10.9 guide tube outer diameter [mm] 12.6 guide tube material e-635 fuel assembly width [mm] 236 assembly length [mm] 4570 linear power [kw.m−1] 16.0 max. linear power [kw.m−1] 44.8 table 3. the main parameters of tvsa-t fuel [15]. list of symbols α heat transfer coefficient [w m−2 k−1] η dynamic viscosity [pa s] γ isobaric thermal expansion coefficient [k−1] λ thermal conductivity [w m−1 k−1] ρ density [kg m−3] cp isobaric heat capacity [j kg−1 k−1] g gravitational acceleration [m s−2] l characteristic length [m] q heat flux [w m−2] c, c coefficients [–] t temperature [k] gr grashof’s number [–] nu nusselt’s number [–] pr prandtl’s number [–] re reynolds’s number [–] acknowledgements the support from student grant contest from czech technical university with code sgs16/252/ohk4/3t/14 is gratefully acknowledged. many thanks belong to my supervisor ing. jiří čížek and my consultants ing. martin ševeček and ing. dušan kobylka, ph.d who provided insight and expertise that greatly assisted the research. i would also like to thank ing. lubomír bureš for his constructive criticism and friendly advice during the project work. references [1] spent nuclear fuel storage in standalone nuclear systems. safety guide bn-02.2. sujb, praha, 1st edn., 2010. [2] c. wheeler, r. mccann, n. lombardo, et al. hydra and cobra-sfs temperature calculations for castor-ic, rea-2023, castor-v/ 21, and tn-24p cask. third 44 vol. 19/2018 heat removal from spent nuclear fuel international spent fuel storage technology symposium: workshop 1(1):77–98, 1986. [3] d. rector, j. cuta, n. lombardo. cobra-sfs thermal-hydraulic analysis of the castor-1c and rea 2023 bwr storage casks containing consolidated spent fuel. pacific northwest laboratory, richland, washington, 1st edn., 1986. [4] m. mckinnon, j. creer, t. michener, et al. the tn24p pwr spent fuel storage cask: testing and analysis. electric power research institute, palo alto, california, 1st edn., 1987. [5] t. e. michener, j. m. cuta, d. r. rector, et al. cobra-sfs: thermal-hydraulic analysis code for spent fuel storage and transportation casks. battelle: pacific northwest national laboratory, richland, washington 99352, 3rd edn., 1995. [6] t. e. michener, j. m. cuta, d. r. rector, h. e. adkins. cobra-sfs: thermal-hydraulic analysis code for spent fuel storage and transportation casks. battelle, pacific northwest national laboratory, richland, washington 99352, 1st edn., 2015. [7] d. vlcek. residual heat power removal from spent nuclear fuel during dry and wet storage. master thesis. dnr fnspe ctu, prague, 2018. [8] l. e. herranz, j. penalava, f. feria. cfd analysis of a cask for spent fuel dry storage. annals of nuclear energy 2015(76):54–62, 2015. doi:10.1016/j.anucene.2014.09.032. [9] t. funke, p. brich. castor 1000/19: transport and storage cask for spent nuclear fuel from npp temelin. gns, cez, 1st edn., 2008. [10] r. d. mccarty. thermophysical properties of helium-4 from 2 to 1500 k with pressures to 1000 atmospheres. national bureau of standards, boulder, colorado, 1st edn., 1972. [11] matweb. available from 2018-05-07. automation creations, 1990. [12] m. sevecek. spent nuclear fuel behavior simulation during dry long-term storage in the storage and transportation casks castor. master thesis. dnr fnspe ctu, prague, 2014. [13] r. l. cox. radiative heat transfer in arrays of parallel cylinders. tennessee univ., knoxville, 1976. [14] h. wang, j. k. mccoy, d. stahl. spent nuclear fuel effective thermal conductivity report. u.s. department of energy, las vegas, 1st edn., 1996. [15] l. sklenka. provozní reaktorová fyzika. vydavatelství čvut, praha, 1st edn., 2001. 45 http://dx.doi.org/10.1016/j.anucene.2014.09.032 acta polytechnica ctu proceedings 19:36–45, 2018 1 introduction 2 modeling the storage cask 2.1 boundary conditions 2.2 tvsa-t fuel 2.3 heat transfer and outlet conditions 3 verification 4 results 4.1 maximum temperature 4.2 radial distribution of temperatures 4.3 time dependence on maximal temperature 4.4 flow rate 5 conclusions list of symbols acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2016.3.0007 acta polytechnica ctu proceedings 3:7–10, 2016 © czech technical university in prague, 2016 available online at http://ojs.cvut.cz/ojs/index.php/app semi-active damping performance of iron particle filled silicone rubber florian dirisamera,∗, umut cakmaka, edmund marthb, zoltan majora a institute of polymer product engineering, johannes keppler university linz, altenbergstraße 69,4040 linz, austria b institute of power electronics and electrical drives, johannes keppler university linz,altenbergstraße 69, 4040 linz, austria ∗ corresponding author: florian.dirisamer@jku.at abstract. the aim of this work was to design, produce and evaluate a demonstrator to visualize the magneto-induced damping behaviour of materials. in contrast to standard materials, the damping coefficient of iron particle filled silicone rubbers can be controlled by a semi-active magnetic field. this field effect should be characterized in order to evaluate the suitability of these magnetorheological silicone elastomers for the use in different configurations and applications. keywords: semi-active damping, magnetorheological silicone elastomer, damper demonstrator. 1. introduction all products with moving or rotating parts exhibit vibrations in operating state. for most applications these vibrations are unwanted and proper vibration isolation or absorption devices and materials are required to ensure safety and serviceability. depending on the application (loading, severity of a failure, environmental and operating conditions), vibrations can be reduced by passive (e.g., rubber dampers), semi-active (i.e., adjusting the damping behavior by an external physical field) or active damping (i.e., closed-loop feedback control by inducing counter vibration, among others) [1, 2] a number of studies [3, 4] have been conducted to understand the magneto-mechanical behavior of silicone rubber filled with magnetically polarizable particles. by applying an external magnetic field, the mechanical behavior is rapidly, continuously and can be reversibly changed. in this work, we investigate the semi-active damping performance of a magnetorheological silicone elastomer. a demonstrator was designed to study the damping performance in varying operating states. the magnetic field is induced directly by the demonstrators electromagnet. 2. concept and approach in order to be able to study as many different o-ring (cross-section) geometries and material formulations as possible and to achieve accurate results, different demonstrator designs were considered. to finally determine the best solution for this task, a so called creative method based morphologic box was used in the conceptual design. for this, the construction of the damper demonstrator was split into three main parts: • steel case, including an electromagnet • motor unit • damping unit figure 1. flux density plot of the demonstrator magnet with a coil current density of 10 a mm−2. 2.1. simulation a finite element (fe) magnetostatic simulation based on matlab and the open source "finite element method magnetics" (femm) software was performed with different geometries of the steel case, since it serves as the iron core of the electromagnet. as the part is rotationally symmetric, it could be represented as a 2d axissymmetric problem. the design was optimized in terms of a space-saving solution and a strong homogeneous field in the air gap, where the damping unit is located. figure 1 illustrates the simulation result of the final selected geometry. 2.2. construction and manufacturing after designing each of the three main demonstrator parts separately, defining the connections between them the construction could be done in detail. to generate the vibrations, a dc-motor (2342 s018 cr, faulhaber) 7 http://dx.doi.org/10.14311/app.2016.3.0007 http://ojs.cvut.cz/ojs/index.php/app f. dirisamer, u. cakmak, e. marth, z. major acta polytechnica ctu proceedings figure 2. constructed assembly of the damper demonstrator (left) and damper demonstrator in section view with labelled components(right) material shore-a hardness tangoblack+ 27 dm 9860 60 dm 9895 95 table 1. non-magnetoactive materials used for o-rings with eccentric weights was used. figure 2 shows the final construction of the damper demonstrator and its section view. the motor-mount, the connection-element, some of the tested o-rings and a mold for magnetorheological silicone elastomers were manufactured via a 3d-printing method using photopolymers (objet connex 350), the steel case via cnc-turning and -milling and the coil via winding around a zylinder. 3. manufacturing of the o-rings in order to show the difference between the damping behaviour of unfilled and iron particle filled silicone rubbers within the magnetic field, o-rings made of different unfilled elastomers and with different contents of iron particles within the silicone rubber were moulded. these materials were glued onto a steel plate to attach them to the steel case and on the connection-element to link it to the motor unit. 3.1. non-magnetoactive different photopolymers for 3d-printing (stratasys ltd.) were selected as non-magnetoactive materials for the printed o-rings. these materials are one pure tangoblack+ and two digital materials, mixtures of verowhite and tangoblack+ (table 1). 3.2. magnetoactive the magnetorheological elastomers were produced from polydimethylsiloxane (pdms) and mixed with iron particles in the ratios 1:5, 1:10, 1:15 and 1:20. a mold (figure 3) for manufacturing the o-rings was 3d-printed. figure 3. mold in section view left side with closed outer parts and centerpiece, right side with removed outer parts and collapsed center part. figure 4. damping unit with labeled components connection element (1), damping element (2), steel plate (3). to simplify the demolding, the outer parts can be removed and the centerpiece can be collapsed. the iron powder was dispersed in the pdms matrix, vacuum was applied to degas the mixture, which was then filled into the mold and finally was cured in the oven for 24 hours at 60◦c . afterwards these rings were demolded and finally glued to the steel plate and the connection-element (figure 4). 4. test setup to calculate the damping-performance, the acceleration forces in each direction are recorded via an arduino based acceleration sensor, which is capable of measuring in x, y and z-direction as well as the rotations around these axes. this sensor is placed on the motor unit 8 vol. 3/2016 damper demonstrator figure 5. workflow of the test setup figure 6. test setup. and connected to an arduino mega 2560 board, which communicates with the pc, on which a c++ program evaluates the acceleration data to calculate the damping coefficient. the rotational speed of the dc-motor and the magnetic field caused by the coil is driven by two laboratory power supplies. the workflow is shown in figure 5 and the test setup is shown in figure 6. the equation for calculating the damping coefficient is given below. d (ω,b) = √ f 2 e · ω4 − â2 (b) ·m2 d · (ω2 − ω2 0)2 2 ·md · â (b) · ω (1) the parameters f0, ω0 and ω are calculated as follows: fe = me · ω2 · re (2) ω0 = √ kr md (3) kr = e 2 · (1 + ν) · π ·dm · b h (4) ω = 2 · π · n (5) the symbols and their physical meanings are listed at the end of this paper. 5. results while changing many different configurations of rotational speed of the vibrating motor and the magnetic field of the coil within the damping element, the variation of the acceleration due to the magnetic field for each damping element could be measured and the damping coefficient (equation 1) could be calculated. the results are visualized as nominal accelerations, where a decreasing acceleration lead to an increasing acceleration. the rotational speed of the vibrating motor was changed from 0rpm to 8000rpm in 18 steps and the magnetic field within the coil was changed from 0t to 0.72t in 8 steps. 5.1. non-magnetoactive non-magnetoactive materials should not show any influence of the damping coefficient on an incresing coil current. therefore, the nominal acceleration should be constant. figure 7 show the result of the nonmagnetoactive with an increasing nominal acceleration which indicates an decreasing damping coefficient. this result can be explained via the magnetic field within the steel case, which influence the movement of the dc-motor. 5.2. magnetoactive the prediction for a magnetorheologic material is, that the damping coefficient will increase with an increasing magnetic field within the damping element. therefore, the acceleration has to decrease. figure 8 shows the result for the iron particle filled silicon rubber with an relation of 1:20 at a rotational speed of 2700rpm. the decreasing nominal acceleration for an increasing coil current is clearly to see, apart from the acceleration at icoil = 7a, which can be explained by the same connection as the standard elastomers, the effect of the magnetic field on the movement of the dc-motor. 6. conclusions this paper compares the influence of the magnetic field on the damping coefficient of magnetorheologic and non-magnetorheologic materials. therefore, this demonstrator was designed to visualize these effects. in general, the result of an increasing damping coefficient for magnetorheologic materials with an increasing magnetic field within the damping element are shown. however these predicted progresses are superimposed by a second progress, which can be interpreted as the influence of the magnetic field within the center of the steel case, which affects the movement of the dc-motor. as a result, the damping coefficient of the non-magnetorheologic material seems to decrease, which is not possible for that material. 9 f. dirisamer, u. cakmak, e. marth, z. major acta polytechnica ctu proceedings figure 7. results standard material. figure 8. results iron particle filled silicone rubber. list of symbols â maximum acceleration [m s−2] b magnetic field [t] b width damping element [m] d damping coefficient [n s m−1] dm mean diameter damping element [m] e young’s modulus [mpa] fe force of eccentric mass [n] h height diameter damping element [m] kr spring constant [n m−1] md mass damping and motor unit [kg] me mass eccentric mass [kg] ν poisson’s ratio [–] n rotational speed [s−1] re distance center of mass to axis of rotation [m] ω angular frequency [rad s−1] ω0 characteristic angular frequency [rad s−1] acknowledgements the research for this paper was performed within the framework of the regio13 project, with the contribution of the linz center of mechatronics gmbh (lcm) in austria. regio13 is funded by the state government of upper austria (wi-225867-2012). references [1] d. sciulli. dynamics and control for vibration isolation design 1997. [2] u. çakmak, f. hiptmair, z. major. applicability of elastomer time-dependent behavior in dynamic mechanical damping systems. mechanics of time-dependent materials 18(1):139–151, 2014. doi:10.1007/s11043-013-9219-z. [3] m. lokander, b. stenberg. performance of isotropic magnetorheological rubber materials. polymer testing 22(3):245–251, 2003. doi:10.1016/s0142-9418(02)00043-0. [4] z. major, b. schrittesser, g. filipcsei. characterisation of dynamic mechanical behaviour of magnetoelastomers. plastics, rubber and composites 38(8):313–320, 2009. doi:10.1179/146580109x12473409436986. 10 http://dx.doi.org/10.1007/s11043-013-9219-z http://dx.doi.org/10.1016/s0142-9418(02)00043-0 http://dx.doi.org/10.1179/146580109x12473409436986 acta polytechnica ctu proceedings 3:7–10, 2016 1 introduction 2 concept and approach 2.1 simulation 2.2 construction and manufacturing 3 manufacturing of the o-rings 3.1 non-magnetoactive 3.2 magnetoactive 4 test setup 5 results 5.1 non-magnetoactive 5.2 magnetoactive 6 conclusions list of symbols acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2018.18.0061 acta polytechnica ctu proceedings 18:61–65, 2018 © czech technical university in prague, 2018 available online at http://ojs.cvut.cz/ojs/index.php/app modelling and simulation of rocking suitcases hanna zs. horvatha,∗, denes takacsa,b a department of applied mechanics, budapest university of technology and economics, budapest, hungary b mta-bme research group on dynamics of machines and vehicles, budapest, hungary ∗ corresponding author: horvath.hanna.zsofia@gmail.com abstract. the rocking motion of two-wheeled suitcases is investigated. a rigid body mechanical model of the suitcase is constructed. all of the possible motion states (both wheels on the ground, one wheel on the ground, none of the wheels on the ground) are taken into account. the switching between the motion states is accomplished by a simple impact model. the motion of the suitcase is investigated through numerical simulations, furthermore the domain of the attraction of the stable rectilinear motion is identified. the model is partly validated by experiments. keywords: stability, suitcase, non-smooth system, non-linear system. 1. introduction the rocking motion of two-wheeled suitcases is an annoying problem since it makes the use of suitcases uncomfortable in the everyday life. large enough perturbations of the rolling suitcase can lead to the rocking motion, when the suitcase jumps from one of its wheels to the other, and in crucial cases the suitcase overturns. the topic is researched even nowadays (see [1]), although papers focusing on rocking suitcases were already published in the 90’s (see [2–4]). the interest of researchers is supported by the fact that similar stability problem of trailers exists in vehicle dynamics (see, for example, [5]). from mechanical point of view, the analysis of the rocking suitcase problem is very complex. to begin with, the system is non-smooth since different equations describe the different motion states. additionally, in case of contacting wheel, the spatial motion of the suitcase is influenced by the kinematic constraints, which makes the derivation of the equation of motion more complicated. due to this complexity, former studies on the field do not take into account all of the motion states of the suitcase, namely, the case when none of the wheels has contact with the ground is neglected. in this study, a mechanical model of the rocking suitcase is constructed in which all motion states are considered. experiments are also shown that confirm the presence of these motion states. the contents of the paper is the following. the simplified mechanical model is introduced in sec. 2. the different motion states together with the corresponding number of the generalized coordinates are summarized in sec. 3. the motion of the system is analysed through numerical simulations in sec. 4. based on these simulations, experiments are carried out in sec. 5. the comparison of the simulation and the measurement results can be found in sec. 6. 2. mechanical model the simplified mechanical model is depicted in fig.1. the suitcase is pulled in the x direction with constant speed v by the ball-joint at a. the body of the suitcase is assumed to be rigid and it is modelled by means of massless rods and by a lumped mass m at the center of mass c of the suitcase. the position of the lumped mass is characterized with parameters e and f . the length and the width of the suitcase are described by the parameters l and 2b, respectively, while h is the distance between the towing point a and the ground. the radius of the wheel (r) is assumed to be zero, which is a suitable approximation in our simplified model and it is also used in several studies (see, for example, [1]). on the one hand, the radius of the wheel is relatively small to the other dimensions of the suitcase and trajectory of the wheel center points and the paths of the contact points are very close to each other. on the other hand, the gyroscopic effect acting on the wheels is negligible since the mass moment of inertia of the wheels and the rotational speed are very small. 3. motion states one can differentiate four different motion states of the suitcase: both wheels have contact with the ground, one wheel (right or left wheel) is on the ground, none of the wheels is on the ground. see fig. 2, where the motion states are given together with the transitions between them. the different motion states correspond to 1, 3 and 6 dimensional state spaces, respectively. when both wheels are on the ground, the suitcase makes in-plane motion and the equations of motion can be easily derived from the kinematic constraint of rolling. the position of the suitcase is given by the deflection angle ψ (see in fig.1), and the motion can be described in a one-dimensional phase space. having only one wheel on the ground is the most intricate case, since the motion is spatial, while the 61 http://dx.doi.org/10.14311/app.2018.18.0061 http://ojs.cvut.cz/ojs/index.php/app hanna zs. horvath, denes takacs acta polytechnica ctu proceedings figure 1. the simplified mechanical model of a towed two-wheeled suitcase. the generalized coordinates for the right wheel rolling case. figure 2. the motion states with the corresponding dimensions of the state spaces, and the possible transitions between them. kinematic constraint of rolling has to be considered. thus, for example, the equations of motion can be derived with the so-called routh–voss-equation. in this case, the governing equations lead to a threedimensional phase space given by the tilting angle β, the angular velocity ω = β̇ and deflection angle ψ (see fig. 1). if none of the wheels have contact with the ground, the motion is spatial but the system simplifies to a spatial mathematical pendulum with an extra rotational degree of freedom about the axis determined by the points a and c. namely, a six-dimensional state space governs the motion. to make switches between the motion states, the detachments of the wheels from the ground and the impact between the wheels and the ground have to be handled. namely, if the normal force at one of the wheels that has contact with the ground becomes zero, the wheel can leave the ground, and another motion state will govern the further motion of the suitcase. similarly, if one of the non-contacting wheel touches the ground with non-zero normal velocity, impact happens between the wheel and ground. to describe the impact, a complex impact model has to be applied since classical rigid body impact models could not be used to describe the practically observed transitions. here, we use a spring and a damper as a simple contact element to model the impact of the wheel. the deformations of the springs at the left and the right wheels can be described by the vertical positions zl and zr of the wheel centre points. if one of the wheels is on the ground then the contact spring is compressed and the vertical coordinate of the wheel is a small negative value. this small vertical displacement of the wheel centre point is negligible relative to the large scale motion of the suitcase. moreover, since the dynamic of the impact is much faster than the position change of the body, the simulations of the impact can be carried out in another time scale. this method is often used for such multi-scale systems. 4. numerical simulations the governing equations and the impact model were implemented in computer code. the implemented equations of motion were rewritten into a system of first order ordinary differential equations. fourth order runge–kutta method was used in these numerical simulations with fixed time step. since the system is non-smooth and all motion states had different number of degrees of freedom, and consequently different generalized coordinates and generalized velocities, the coupling between the motion states is a complex task. see fig. 2, where the transitions can be seen with all the possible scenarios. simulations were run for different initial conditions (β(t = 0) = β0, ω(t = 0) = ω0 and ψ(t = 0) = ψ0 values) given for the left wheel rolling case and for various parameters (e.g. towing velocity v, geometrical parameters l, h, e and f). in our study, a motion is called stable if the simulation ends in the motion state in which both wheels are on the ground. otherwise, the motion is called unstable, namely, the suitcase overturns. the time histories for a stable motion can be seen in fig. 3. the suitcase rolls on its left wheel (panels in the first row), then the right wheel has an impact meanwhile the left wheel is already on the ground (panels in the second row show the deformations zl and zr of the left and right wheels, respectively), at 62 vol. 18/2018 modelling and simulation of rocking suitcases figure 3. time histories for the initial conditions: β0 = −0.2 [rad], ω0 = 0 [rad/s], ψ0 = 0.1 [rad], and for parameter values: l = 0.58 m, h = 0.553 m, b = 0.16 m, e = 0.3 m, f = 0.1 m, m = 5 kg, v = 1 m/s. last both wheels have contact with the ground (panel in the third row). one can also draw the basis of attraction of the rectilinear motion by means of these numerical simulations. in fig. 4, the results of a simplified mechanical model are shown, where the fourth motion state (no wheels on the ground) was not considered. in the figure, one figure 4. the basis of attraction of the rectilinear motion. the parameter values are the following: l = 1 m, h = 0.6 m, b = 0.3 m, m = 10 kg. different colours represent different basis of attraction considering different f values. can observe the attractive and the repelling regions, i.e. below and above a plotted line, respectively. other figures could be depicted considering different geometrical parameter (e, b, h or l) values. if the value of b or l is increased, the attractive region grows as well. in case of increasing e, f or h, the repelling region grows. due to the non-linearity of the system, there exists such towing velocities in case of a certain f value, that for specific initial tilting angles β0 can lead to straight rolling, while a reduced initial angle (smaller perturbation) generates unstable behaviour and the suitcase rolls over, see the blue loop in fig. 4. such phenomenon can be caused by the presence of unstable limit cycles, see [6]. similar behaviour was also observed in our experiments. 5. measurements for the measurements, a model-based experimental setup was built and was placed on a treadmill while the towing point a was fixed with a ball-joint. the velocity of the treadmill can be varied between 0.5 km/h and 18 km/h. the measurement setup can be seen in fig. 5. figure 5. the measurement setup. the parameter values: l = 0.58 m, b = 0.16 m, m = 5 kg. parameters h, e and f and the velocity v can be varied. measurements showed that parameters e and f play a huge role in the stability of two-wheeled suitcases. two types of measurements were carried out in order to validate the results of the numerical simulations. during the „classical” measurement, the towed structure was perturbed, namely an obstacle was placed onto the treadmill at one of the wheels. the stability of the system could only be determined on statistical basis (out of ten measurements, how many where stable and how many were unstable), since the identical perturbation could not be guaranteed during the measurement. based on these measurements, we could conclude that parameters e and f play a huge role in the stability of two-wheeled suitcases. the domain of attraction of the rectilinear motion is large even in case of large velocities when the values of parameters e and f are small. measurements with optitrack motion capture system were also carried out. four markers were placed on the suitcase (at the towing point a, at the center of mass c and at the left (l) and the right (r) wheels). an obstacle was placed before the left wheel. four cameras recorded the positions of the markers with respect to time and as a result, the spatial motion of the suitcase was captured. since this kind of measurement provides us information about the 63 hanna zs. horvath, denes takacs acta polytechnica ctu proceedings markers’ positions, velocities and accelerations can be computed. as a result, important information can be gained about the level of perturbations with the evaluation of these measurements. therefore, a more sophisticated identification of the basis of attraction can be carried out. snapshots about the measurements is shown in fig. 6, where the rocking motion can be also observed. the suitcase was perturbed by an obstacle before its left wheel, as a result, its left wheel leaves the ground (1). the suitcase rocks from its left wheel (2) to its right wheel (3). there are time instances when none of the wheels touches the ground (4). after this "flying" motion, the body has an impact with the ground (5), then it overturns (6). figure 6. snapshots about the measurement on the treadmill. the suitcase starts to rock and at the end, it overturns. a stable motion is illustrated in fig. 7, where the position of the markers are plotted for the measured time interval. it can be seen that the suitcase was perturbed at the left wheel, therefore it jumped from the ground and after falling back it had several impacts with the ground. the maximum of the left wheel’s vertical position was about 12 mm, while this value was 8 mm for the right wheel. as it was formally mentioned in sec. 3, the negative values of the vertical positions of the wheels refer to the elastic deformations of the wheels. after some impacts, the rocking motion of the body dies away, the system stabilizes. an unstable motion is depicted in fig. 8. the suitcase rocks from one wheel to the other and the amplitudes grow with respect to time. as a result, the suitcase overturns. 6. comparison the validation of the model can be done by comparing the simulation results to the measurement ones. here, we only give a comparison for a specific parameter setup of the suitcase, see fig. 9. the parameter values of the experiment were: l = 0.58 m, b = 0.16 m, e = 0.135 m, f = 0.06 m, m = 7.2 kg, h = 0.553 m. figure 7. example from an optitrack measurement, stable motion. the paths of the markers can be seen on the left figure. zl and zr denote the vertical displacement (with respect to time) of the left and the right wheel, respectively. figure 8. example from an optitrack measurement, unstable motion. the paths of the markers can be seen on the left figure. zl and zr denote the vertical displacement (with respect to time) of the left and the right wheel, respectively. in the figure, the result of the simulation is plotted with blue line, the attractive and the repelling regions with respect to the initial angle β0 can be seen below and above this line. the measurement results are plotted with red dots. the secondary axis at the right hand side of the diagram shows the diameter d0 of the cylindrical obstacle, which were used to the initiate the rocking motion of the suitcase. in the figure, the diameter is considered to be proportional with the initial tilting angle β0. as it can be seen in the figure, the results of the simulations and the measurements are qualitatively the same. the attractive domain is smaller for larger speeds. of course, some quantitative difference can be identified in the figure. one of the reason of this difference is that we assumed here that the diameters of the obstacles are linearly proportional with the initial tilting angles. as mentioned before, this assumption is not perfect, and a more sophisticated comparison 64 vol. 18/2018 modelling and simulation of rocking suitcases figure 9. comparison of the results of the numerical simulation and measurements. the simulation result is depicted with the blue line, while the measurement points are the red dots. of the simulation and measurement results could be done with the evaluation of optitrack measurements, which is the task of the future work. 7. conclusions a simplified mechanical model of two-wheeled rocking suitcases was constructed. a summary about the equations of motion of the four different motion states were given, and also a simple impact model was presented. through numerical simulations important information about the non-linear behaviour of the suitcase was identified. an experimental rig was shown by which the numerical results were partly validated, namely, it was shown that the simulation results are qualitatively correct. further research on the mechanical model and on the experiment can provide basic rules that can help the design of suitcases having better stability properties. 8. acknowledgements the publication of the work reported herein has been supported by etdb at bme. the authors thank laszlo bencsik for his contribution in the use of the optitrack measurement system. references [1] facchini, g., sekimoto, k., courrech du pont, s.: the rolling suitcase instability: a coupling between translation and rotation. in: royal society, 473(2202). 2017. [2] suherman, s., plaut, r.h., watson, l.t., thompson, s.: effect of human response time on rocking instability of a two-wheeled suitcase. in: journal of sound and vibration, 207 (5). 1997. 617–625. p. [3] o’reilly, o. m. and varadi, p. c.: a travelers woes: some perspectives from dynamical systems, iutam symposium on new applications of nonlinear and chaotic dynamics in mechanics (ed.:f. c. moon), kluwer ac. publ., dordrecht, pp. 397–406. 1999. [4] plaut, r. h.: rocking instability of a pulled suitcase with two wheels, acta mechanica 117, 165–179. 1996. [5] troger, h. and zeman, k.: a nonlinear-analysis of the generic types of loss of stability of the steady-state motion of a tractor-semitrailer, vehicle system dynamics 13(4), 161–172. 1984. [6] stepan, g.: delay, nonlinear oscillations and shimmying wheels. in: proceedings of symposium chaos, kluwer ac. publ., dordrecht, pp. 373–386. 1997. 65 acta polytechnica ctu proceedings 18:61–65, 2018 1 introduction 2 mechanical model 3 motion states 4 numerical simulations 5 measurements 6 comparison 7 conclusions 8 acknowledgements references acta polytechnica doi:10.14311/app.2020.27.0067 acta polytechnica 27(0):67–72, 2020 © czech technical university in prague, 2020 available online at http://ojs.cvut.cz/ojs/index.php/app on the design of coating system using nanomechanical experiments david merciera,∗, claes fredrikssonb a ansys granta education division, tour part-dieu 129 rue servient, lyon, 69003 france b ansys granta education division, 62 clifton road, cambridge, cb1 7eg united kingdom ∗ corresponding author: david.mercier@ansys.com abstract. the aim of this paper is to identify performance indices for the design of coating systems, using results from nanomechanical tests. the use of nanoindentation, nanoscratch and pin-on-disk experiments applied to coatings for mechanical and tribological property estimation is presented. then, multiple performance indices, based on experimental and numerical results found in the litterature, are listed in order to help the material selection and optimization of durability and performance of coatings. keywords: coating, fracture, material selection, nanoindentation, performance index, scratch. 1. introduction the use of coatings to improve surface properties of components is constantly increasing [1, 2] and the significance of material selection in coating systems design is well recognized [3–5]. the definition of the material combined with the coating structure is a key step in the design process, particularly in the field of surface protection from damage in mechanical systems [6]. in this context, the multiscale mechanical stability of coatings on substrates can be characterized using nanomechanical experiments [7–12]. the knowledge of the mechanical and tribological behavior of the coating, the coating-substrate interface and the substrate, enables the construction of selection maps (material property or failure mechanism) [3, 13, 14]. ashby’s methodology with specific performance indices, as implemented in the granta selector software [8, 15–17], can be employed for this purpose, see figure 1. selection maps help engineer coatings by linking coating performance with its structure, material property and the coating process (elaboration and surface treatment). a summary of mechanical and tribological properties extracted from experimental results, based on nanoindentation, nanoscratch and pin-on-disk techniques, is given in table 1, below. these experiments are often used to promote the analysis of load (f) or contact stiffness (s) vs. vertical displacement (h) or horizontal displacement (d) curves, which can be coupled with corresponding microscopic observations of residual indents or grooves. schemes of residual imprints are given for each experiment with the description of geometrical parameters used in the equations. the indices n, t, ind, crit, max, c, s, int and r in equations 1 to 13, refer, respectively, to the normal and tangential axis to the coating surface, the indenter properties, the critical event (crack formation, dislocation nucleation, etc.) associated with a pop-in or a jump on the load-displacement curve, to a maximum parameter value, the coating, the substrate, the coating-substrate interface and to the residual value. e’ in equation 1 represents the reduced elastic modulus with both indenter and material elastic contributions. knowing poisson’s ratio, ν, of the indented material and elastic properties of the indenter, it is possible to estimate the elastic modulus of the tested material using equation 2. alpha (α) in equation 5 is an empirical constant, usually taken to be about 0.016, based on a fit to experimental data. finally, the coating thickness in equations 7 to 9 is denoted t and the residual stress of the material used in equation 9, is referred to as σr . based on the combination of these materials properties, performance indices (pis) can be established to characterize the performance of a material in a given application. a non-exhaustive list obtained from the literature is proposed, see table 2, below. within table 2, fcrit in equation 24 corresponds to the critical load estimated during a scratch test, when the penetrator is touching the substrate surface and the coating no longer plays its protective role. these criteria, defined as a function of the mechanical surface solicitation, tend to describe a specific resistance of the coating system. based on the pis in table 2, some examples of material property maps and failure mechanism maps have been created for different case studies. on one hand, property maps are usually quantitative and directly plotted using experimental results given in each reference papers, and on the other hand failure mechanisms maps are qualitative and based on a correlation between the observed failure mechanisms trend and the level of different properties or pis. these results are presented and discussed in section 2 below. 67 http://dx.doi.org/10.14311/app.2020.27.0067 http://ojs.cvut.cz/ojs/index.php/app david mercier, claes fredriksson acta polytechnica figure 1. properties required for selection maps creation to support coating systems design. 2. results and discussion 2.1. the case of homogeneous coatings the first example of material coating selection is about substrate surface durability and improvement by elaborating sol-gel coatings on glass and steel samples [22]. for these sol-gel coatings, the effective elastic moduli as well as the hardness values, along the indentation depth, were estimated using equations 1 to 3. the effect of substrate, of the presence of nanoparticles (nps) and of annealing at higher temperature were studied. then, the mechanical failure modes of solgel coatings were investigated using both berkovich nanoindentation and nanoscratch technique. microscopic observations of residual imprints and residual grooves show that both exhibit chipping in the case of thick coatings, especially on glass substrate, and no dramatic failure for thin coatings applied on both substrates. it is found that the mechanical properties of the sol-gel and the mechanical stability of the coatings on substrates are influenced dramatically by the presence of nanoparticles and the thermal treatment. the interfacial fracture toughness of sol-gel coatings on substrate was estimated using equations 9 to 11. material property and failure mechanism maps based on experimental results were then created using performance indices from table 2. these can subsequently guide the sol gel coating selection, see figure 5. it appears that annealed sol-gel coating containing nps are the best candidates in this case, offering mechanical protection with good adhesion. 2.2. the case of composite coatings in the second example, a particle reinforced metal matrix composites (prmcc) coating, deposited on a steel substrate is studied for its protection against corrosion and for its good wear resistance. these are two properties of primary importance for mechanical applications [9, 23]. a thick nickel coating containing micrometric silicon carbide (sic) particles was electro-deposited on a steel substrate from a suspension of sic in a watts-type ni-plating bath. the texture and microstructure of the coating were examined using electron backscatter diffraction combined with energy-dispersive spectroscopy for phase differentiation. the interfacial strength between the ni matrix and the sic particles was investigated by nanoscratch experiments. grid nanoindentation technique coupled with an image correlation-based targeted indentation analysis and a statistical data treatment were employed. the mechanical properties of the composite were quantified, and the mechanical properties of each phase were extracted, using equations 1 to 3. finally, empirically knowing the material property of the prmmc coating (light blue bubble on figure 6a) and of the raw reference materials (ni and sic data from [15]), it is possible to create an ashby map and a failure mechanism map for this example (figure 6). the green, dark blue and small orange dots on the ashby map are obtained using voigt, reuss and voigt-reuss laws of mixture, with reference material property. the conclusion of this example is that prmmc have very interesting mechanical properties arising from the combination of a relatively ductile matrix (less fracture) with harder reinforcing particles (less wear). 2.3. the case of multilayer coatings the third case study is based on the development of multilayer structures with diamond-like carbon (dlc) coating on the top surface to protect steel substrates from wear [8]. in this framework, graded compositions based on cr/crn/(cr,n)-dlc and cr/cr-dlc coatings were used to progressively increase the coating hardness and to improve the adhesion of the dlc layer. the mechanical and tribological properties of these multilayer structures were obtained by nanoindentation, nanoscratch, and ball-on-disk tribometer. using performance indices based on a mix of adhesion, mechanical and tribological properties, material property and failure mechanism maps were proposed (figure 7). based on these results, the combination of a thick dlc layer with a cr base is a promising solution for mechanical and wear protection, in the case of steel substrates. 68 vol. 27/2020 nanomechanical experiments: design of coating system table 1. summary of nanomechanical experiments and expected results for mechanical properties, adhesion and tribological property calculations. csm is for continuous stiffness measurement. 69 david mercier, claes fredriksson acta polytechnica mechanical surface solicitation criterion definition pi definition eq. ref. blunt indentation yielding resistance h3 e2 (14) [15] cracking resistance k2 ic − ( 1− ν2) e (15) [15] blunt scratch yielding resistance h3 e2 (16) [15] cracking resistance k3 ic e2 (1− 2ν)3 (17) [15] sharp indentation yielding resistance h (18) [15] cracking resistance k4 ic h3 (19) [15] indentation and scratch "hard-film-on-soft-substrate" yielding resistance e ′1/3 c e ′1/3 s √ σe,s (20) [16] cracking resistance k 2/3 ic (21) [16] sliding contact wear resistance "elastic strain to failure" h e (22) [11, 18] indentation or scratch chipping resistance kint,γ or u (23) [19] sliding contact scratch resistance fcrit µ (24) [8] sliding contact wear resistance k (25) [20, 21] table 2. non-exhaustive list of performance indices for coating system design. figure 5. a) material property map and b) failure mechanism map for sol gel coating. 3. conclusions this paper presents mechanical and tribological design considerations of different coating systems and investigates nanomechanical techniques and performance indices bases methods for their assessment. through different case studies, the use of nanoindentation, nanoscratch and pin-on-disk techniques are presented for the characterization of mechanical stability for several coated substrates. the material selection is based on a combination of experimental results with material property and failure mechanism maps. finally, the knowledge of material (substrate and coating), of the interface and of the surface coating property, coupled with failure mechanism analysis is a keystone in the selection process for coatings design application. 70 vol. 27/2020 nanomechanical experiments: design of coating system figure 6. a) material property map and b) failure mechanism map for prmmc coating selection. figure 7. a) material property map and b) failure mechanism map for dlc multilayer coating. acknowledgements this work was partially performed under the support of the beware fellowships programs 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[23] d. mercier, j.-f. vanhumbeeck, m. caruso, x. vanden eynde. caracterisation mecanique par nanoindentation d’un revetement composite a matrice nickel electrodepose. materiaux & techniques 105:106, 2017. doi:10.1051/mattech/2017014. 72 https://doi.org/10.1016/j.matdes.2019.107827 https://doi.org/10.1016/j.matdes.2019.107762 https://doi.org/10.1016/s0043-1648(00)00488-9 https://doi.org/10.1016/s0927-796x(01)00040-7 https://doi.org/10.1007/s10971-019-05196-9 https://doi.org/10.1051/mattech/2017014 acta polytechnica 27(0):67–72, 2020 1 introduction 2 results and discussion 2.1 the case of homogeneous coatings 2.2 the case of composite coatings 2.3 the case of multilayer coatings 3 conclusions acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2019.22.0083 acta polytechnica ctu proceedings 22:83–87, 2019 © czech technical university in prague, 2019 available online at http://ojs.cvut.cz/ojs/index.php/app the use of lightweight aggregate in preparation of thermal insulation lime-based renders jaroslav pokorný∗, milena pavlíková, zbyšek pavlík czech technical university in prague, faculty of civil engineering, thákurova 7, 166 29 prague 6, czech republic ∗ corresponding author: jaroslav.pokorny@fsv.cvut.cz abstract. lime-based renders are common part of historical or older buildings that don’t always provide a comfortable inside climate due to the problems with high thermal losses. thermal insulation renders may possibly retrofit or replace original render layers and mitigate negative impacts of harmful external climate. in this respect, determination of basic structural, mechanical and thermal properties of lime-based renders containing various amount of perlite that was used as partial silica sand replacement is presented in the paper. experimental tests performed for 28 days high relative humidity-cured samples showed significant decrease in bulk density and apparent density for renders with incorporated perlite compared to reference render mix composed of silica sand-based aggregate only. accordingly, porosity of perlite mortars was significantly higher, what led to the lower thermal conductivity values in comparison with reference render. although the mechanical resistance of perlite-modified renders was lower than that of reference material, it was still sufficient for their use as thermal insulation layer compatible with older construction materials. based on the obtained data it can be concluded, the analysed hydrated lime-based plasters with perlite admixture can be considered as promising materials for buildings refurbishment. keywords: hydrated lime, perlite, renders, thermophysical properties. 1. introduction from historical point of view, rendering mortars have always been considered and applied as walls and building facades layers mainly serving to the masonry protection against weathering causing its gradual decay. nevertheless, their decorative aspects were also of considerable importance to achieve aesthetical perfection [1, 2]. aerial lime plasters and mortars belong among the traditional building materials used for masonry and facades until late of the 19th century. lime was very often applied binder due to its ability to add plasticity and good workability to mortar mixture [3]. however, setting and hardening of pure lime mortars is a slow process because of its dependence mainly on reaction of calcium hydroxide with atmospheric carbon dioxide as shown in eq.(1) ca(oh)2 + co2 −→ caco3 +h2o. (1) on the other hand, the specific porous structure of lime mortars with high open porosity, usually ranging from 22 to 35%, allows to pass a certain amount of moisture present in the masonry and thus keep the structure breathable [1, 4]. despite the protection and decorative aspects of traditional renders, their thermal conductivity is high compared to the en 998-1 definition of a thermal insulation render (λ ≤ 0.20 w/mk) and varies typically in dry condition from 0.12 to 0.80w/mk [2, 5]. the energy retrofitting of existing buildings should be considered as a reasonable approach to reduce the overall building energy consumption [6]. as presented by barbero-barrera, facades are, among the thermal envelope components, the constructive system with the highest impact energy demand of buildings [7]. most of buildings are made up of masonry with traditional renders which can be replaced or retrofit by thermal insulation renders made in the same way as those traditional. in production of thermal insulation renders, highly preferred possibility is adding of various types of mineral lightweight aggregates, such as vermiculite, zeolite, diatomite, perlite and pumice [6]. in this connection, abidi et al. [8] tested plasters on gypsum basis containing perlite and vermiculite. experimental results showed decrease of thermal conductivity up to 0.16w/mk from initial value 0.50w/mk. corbanaro et al. [9] presented similar observation for mortars containing natural hydraulic lime, portland cement and perlite. with regarding to the number of performed studies aimed at currently used renders, the influence of silica sand substitution with perlite on properties of traditional lime-based render is examined in the paper. performed experiments pointed to the significant lightening of modified renders accompanied with improvement of their thermal insulation function. 83 http://dx.doi.org/10.14311/app.2019.22.0083 http://ojs.cvut.cz/ojs/index.php/app j. pokorný, m. pavlíková, z. pavlík acta polytechnica ctu proceedings material specific surface bulk density apparent density d10 d50 d90 [m2/kg] [kg/m3] [kg/m3] µm hydrated lime 2 211 ± 25 233 ± 12 2 210 ± 11 0.8 4.2 50.3 table 1. characteristics of lime hydrate cl 90-s. substance content in kg/m3 m-r mp 25 mp 50 mp 75 mp 100 hydrated lime 326.1 336.5 364.6 384.6 408.1 sand 0.0/0.5 434.5 336.5 243.1 128.3 sand 0.5/1.0 434.5 336.5 243.1 128.3 sand 1.0/2.0 434.5 336.5 243.1 128.3 perlite ex 100 2.5 5.3 8.5 11.1 perlite pb 150 41.7 89.2 141.2 187.7 water 391.3 319.7 266.0 200.0 134.3 table 2. proportion of mortar mixes. 2. experimental 2.1. studied materials and samples preparation as a fundamental binder component of mortar mixes, hydrated lime cl 90-s produced in factory vápenka čertovy chody inc. (lhoist s.a. group), czech republic, was used. hydrated lime disposes very high purity and meets requirements of the standard en 459-1 [10]. basic properties and particle size distribution data of cl 90-s are summarized in table 1. a silica sand (filtrační písky ltd., czech republic) composed of three fractions 0.0/0.5; 0.5/1.0 and 1.0/2.0mm combined together in mass ratio 1/1/1, was applied as reference aggregate. in modified render mixes, sand was partially or fully substituted with two commercially available types of perlite, ex 100 and pb 150, mixed together in volume ratio 1/4. five types of mortar batches were prepared. silica aggregate in reference mix m-r was replaced by lightweight aggregate in the amount of 25; 50; 75 and 100% by volume. the dosage of batch water corresponded to the value of spreading 160 × 160 ± 5mm that was kept constant for all renders. before adding, perlite mix was left in plastic box filled by water for 24 hours. composition of particular mortar mixes is expressed in table 2. from fresh render mixes, prisms having dimensions 40× 40× 160mm and cubes with side of 70mm were casted. in samples casting, the filled moulds were compacted by 5 hits on laboratory table. after 48 hours, the particular samples were unmoulded and stored for 26 days in conditions with high relative humidity of approx. 98% and at temperature of 20± 1 °c. 2.2. particle size distribution particle size distribution of hydrated lime was accessed using a laser diffraction particle size analyser analysette 22 nanotec (fritsch) equipped with a red and green laser allowing to detect particles in diameter range from 0.08 to 2 000µm. 2.3. sieving analysis determination of particle size distribution of aggregates was performed according to the standard en 933-1 [11]. at first, aggregate sample was dried at 105 ± 5 °c to constant mass. then, after natural cooling, the sample was placed into series of sieves with mesh size 2.0; 1.0; 0.5; 0.25; 0.125 and 0.063mm respectively, and mechanically shaken by vibratory sieve shaker retsch as 200. 2.4. pozzolanic activity and specific surface area pozzolanic activity was studied for a finer type of perlite only. for its asseesment, modified chapelle test according to the french norm nf p 18-513 was performed [12, 13]. the method is based on the reaction of 1 g of tested powder material with 2 g cao in water [13]. the value of aggregate specific surface was determined according to the standard en 196-6 with the use of blaine apparatus [13, 14]. 2.5. basic physical properties studied renders were characterized by their basic structural properties such as bulk density, apparent density and total open porosity. the bulk density was measured according to the standard en 1015-10 [15] by weighing sample dry mass and determination of its volume by a digital calliper [16]. the apparent density was accessed using automatic helium pycnometer pycnomatic atc (porotec). the total open porosity was calculated on the basis of knowledge of the bulk and apparent density values. the relative expanded uncertainty of applied testing method was 5% [16]. 84 vol. 22/2019 the use of lightweight aggregate. . . figure 1. sieve analysis of perlite and sand mix. 2.6. mechanical properties among the mechanical properties, compressive strength, flexural strength and dynamic young’s modulus were examined. the strength tests were performed according to the standard en 1015-11 [17]. the flexural strength was tested in a three point bending arrangement on standard prisms having dimensions of 40 × 40 × 160mm. the broken halves from the flexural strength tests were then used for determination of the compressive strength, whereas the loading area was 40 × 40mm [18]. the relative expanded uncertainty of the both strength tests was 1.4%. the dynamic young’s modulus was measured by the pulse ultrasonic method (astm c597 – 16), using a dio 562 device (starman’s electronics) [19]. the expanded uncertainty of the dynamic young’s modulus determination was 2%. 2.7. thermal properties the measurement of thermal parameters was conducted on a transient impulse method principle using device isomet 2114 (applied precision) equipped with surface circular probe [16]. the dried plaster samples in the form of cubes with side dimension of 70mm were used for the measurement [16]. additionally, thermophysical parameters of perlite mix, and silica aggregate in dependence on compaction time were tested. for the thermal conductivity (λ) in the range 0.05–2.0w/mk was the measuring accuracy 10% of reading. for the volumetric heat capacity (c) in the range 4.0×104−4.0×106 j/m3k, the measuring accuracy was 15% of reading [16]. 3. results and discussion material characteristics of applied aggregates are given in table 3. the reason for combination of two types of perlite was on the one hand to add more fine powder and gain smooth particle size distribution curve, as indicates figure 1. on the other hand, perlite ex 100 (finer than pb 100) showed pozzolanic activity that could potentially support creation of a denser composite structure due to formation of hydrates of c-s-h type [20]. thermophysical parameters of perlite mix, as well as silica aggregate in dependence on compaction time, are given in table 4. from obtained data is clearly visible the high thermal conductivity of silica aggregate that is approx. 7.5 times higher compared to perlite. accordingly, the volumetric heat capacity of silica aggregates was approx. 9 times higher than data obtained for perlite mix. with prolongation of compaction time, both the thermal conductivity and volumetric capacity increased as a consequence of minimization of air gaps between neighbouring aggregate grains. the procedure of this test simulated aggregate behaviour during mix casting and compaction. there is evident, after 20 s (sand) or 30 s (perlite mix) the values of the tested thermophysical parameters were practically stable. the basic structural properties of studied plasters are shown in table 5. perlite incorporation resulted in a significant decrease in renders bulk and apparent densities. accordingly, the increase in the total open porosity was quite apparent. these findings we assign to the low apparent density of perlite and porosity of embedded perlite particles. the total substitution of sand with perlite mix resulted in the one-third unit weight of render mp 100 compared to that measured for reference render. in general, data on structural properties were promising in the sense of improvement thermal insulation performance of perlite-modified renders because high porosity significantly retards heat transport due to the low thermal conductivity of dry air. the mechanical resistance data measured on 28 days cured samples is summarized in table 6. as expected, with increasing dosage of lightweight aggregate in render composition and thus with increase in renders porosity, gradual decline in all tested mechanical parameters was observed. the above mentioned assumption of contribution of pozzolanic activity of perlite ex 100 to creation of more-dense render structure was not confirmed, probably due to the low content of this pozzolana active material in the mix and limitation of contact of its particles with lime due to the presence of other fine and coarser aggregate particles. quantitatively, the highest drop in both compressive and flexural strengths under 0.5mpa exhibited render mp 100, what is in agreement with its highest porosity. thermophysical properties measured on dried hardened cubic samples are given in table 7. silica sand undeniably caused high thermal conductivity of reference render exceeding 1.3w/mk. on the other hand, influence of porous lightweight aggregate on improvement of thermal insulation function of analysed materials was quite apparent. the higher the perlite content in the render mix, the lower the thermal conductivity of the sample. in fact, the total substitution of silica sand by perlite in tested render mixes resulted in the thermal conductivity even eleven times lower 85 j. pokorný, m. pavlíková, z. pavlík acta polytechnica ctu proceedings material bulk density apparent density pozzolanic activity [kg/m3] [kg/m3] [mgca(oh)2/1 g] perlite ex 100 (0.0/0.5) 55 ± 5 651 ± 5 959 perlite pb 150 (0.0/2.0) 233 ± 12 572 ± 4 sand 0.0/0.5 1 478 ± 40 2 654 ± 13 sand 0.5/1.0 1 505 ± 45 2 648 ± 13 sand 1.0/2.0 1 568 ± 50 2 640 ±12 table 3. material parameters of used aggregates. time [s] sand mix | perlite mix bulk density λ c× 106 bulk density λ c× 106 [kg/m3] [w/mk] [j/m3k] [kg/m3] [w/mk] [j/m3k] 0 1 657 ± 25 0.41 ± 0.4 1.57 ± 0.24 125 ± 2 0.05 ± 0.01 0.17 ± 0.02 10 1 910 ± 28 0.56 ± 0.6 1.68 ± 0.25 147 ± 3 0.06 ± 0.01 0.19 ± 0.03 20 1 916 ± 29 0.58 ± 0.6 1.68 ± 0.25 151 ± 3 0.06 ± 0.01 0.20 ± 0.03 30 1 922 ± 22 0.58 ± 0.6 1.68 ± 0.25 153 ± 3 0.06 ± 0.01 0.22 ± 0.03 60 1 927 ± 22 0.58 ± 0.6 1.69 ± 0.26 153 ± 3 0.06 ± 0.01 0.22 ± 0.03 table 4. thermophysical properties of sand and perlite aggregate in dependence on compaction time. material bulk density apparent density total open porosity [kg/m3] [kg/m3] [%] m-r 1 756 ± 26 2 598 ± 13 32.4 ± 1.6 mp 25 1 514 ± 21 2 389 ± 11 36.6 ± 1.8 mp 50 1 216 ± 18 2 101 ± 11 42.1 ± 2.0 mp 75 945 ± 13 1 835 ± 9 48.5 ± 2.3 mp 100 617 ± 9 1 378 ± 7 55.6 ± 2.8 table 5. basic structural properties of studied renders. material flexural strength compressive strength young’s modulus [mpa] [mpa] [gpa] m-r 1.1 ± 0.1 1.3 ± 0.1 4.4 ± 0.1 mp 25 0.8 ± 0.1 1.0 ± 0.1 3.3 ± 0.1 mp 50 0.6 ± 0.1 0.9 ± 0.1 1.8 ± 0.1 mp 75 0.5 ± 0.1 0.8 ± 0.1 1.4 ± 0.1 mp 100 0.4 ± 0.1 0.4 ± 0.1 0.7 ± 0.1 table 6. mechanical parameters of studied renders. material thermal conductivity thermal diffusivity volumetric heat capacity [w/mk] ×10−6 [m2/s] ×106 [j/m3k] m-r 1.34 ± 0.13 0.87 ± 0.08 1.58 ± 0.24 mp 25 0.86 ± 0.08 0.55 ± 0.06 1.53 ± 0.22 mp 50 0.48 ± 0.04 0.32 ± 0.02 1.49 ± 0.22 mp 75 0.26 ± 0.03 0.21 ± 0.02 1.42 ± 0.21 mp 100 0.12 ± 0.01 0.19 ± 0.01 0.60 ± 0.09 table 7. thermophysical parameters of studied renders. 86 vol. 22/2019 the use of lightweight aggregate. . . than that of the reference render. on the contrary, the low bulk density of perlite-modified renders reduced their heat storage capacity, as show gradually decreasing volumetric heat capacity values. 4. summary the effect of silica sand substitution by lightweight aggregate on properties of lime-based renders was studied in the paper. due to the low weight of incorporated perlite mix, the studied renders were markedly lightened. despite pozzolanic activity of perlite ex 100, there was recorded decrease in renders mechanical resistance with increasing perlite dosage in render mix. on the other hand, the high porosity of renders resulted in the reduction of heat transport parameters when the measured thermal conductivity of perlitemodified specimens was even eleven times lower than that of the traditional lime-based material. based on the performed tests and 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[20] m. pavlíková, l. zemanová, m. záleská, et al. ternary blended binder for production of a novel type of lightweight repair mortarn. materials 12(6), 2019. doi:10.3390/ma12060996. 87 http://dx.doi.org/10.1016/j.conbuildmat.2011.10.059 http://dx.doi.org/10.1016/j.conbuildmat.2017.10.115 http://dx.doi.org/10.1016/j.conbuildmat.2014.09.010 http://dx.doi.org/10.1016/j.cemconcomp.2018.03.005 http://dx.doi.org/10.1016/j.enbuild.2018.08.017 http://dx.doi.org/10.1016/j.enbuild.2014.02.065 http://dx.doi.org/10.1016/j.compositesb.2014.07.030 http://dx.doi.org/10.1016/j.egypro.2015.11.034 http://dx.doi.org/10.1063/1.5043878 http://dx.doi.org/10.1063/1.4994510 http://dx.doi.org/10.1016/j.wasman.2018.09.004 http://dx.doi.org/10.4028/www.scientific.net/kem.722.52 http://dx.doi.org/10.3390/ma12060996 acta polytechnica ctu proceedings 22:83–87, 2019 1 introduction 2 experimental 2.1 studied materials and samples preparation 2.2 particle size distribution 2.3 sieving analysis 2.4 pozzolanic activity and specific surface area 2.5 basic physical properties 2.6 mechanical properties 2.7 thermal properties 3 results and discussion 4 summary acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2018.18.0077 acta polytechnica ctu proceedings 18:77–81, 2018 © czech technical university in prague, 2018 available online at http://ojs.cvut.cz/ojs/index.php/app preprocessing of hopkinson bar experiment data: filter analysis marcel adorna∗, jan falta, tomáš fíla, petr zlámal czech technical university in prague, faculty of transportation sciences, department of mechanics and materials, konviktská 20, 120 00 prague 1, czech republic ∗ corresponding author: adorna@fd.cvut.cz abstract. this work presents a data preprocessing procedure for signal acquired during high strain-rate loading using a custom split hopkinson pressure bar (shpb). before the evaluation of the experimental data, preprocessing of the measured signals including application of suitable digital or analog filter needs to be performed. our department mainly focuses on measurements performed on advanced materials (e.g. materials with predefined structures or hybrid foams). for such measurements, it is essential to perform data preprocessing and apply suitable filter, to be able to appropriately determine deformation pulses on the measuring bars. this paper focuses foremost on spectral analysis of the measured signals, and design of optimal method of data filtering. data from several different shpb experiments were processed and results of different filtering methods are shown in this paper. parameters of the best performing filter were optimized and shown to be universal for wide range of shpb measurements. keywords: shpb, filter analysis, strain-gage measurements, chebyshev filter. 1. introduction recent industrial development shows an increase in industrial applications of advanced materials and structures. for example, in transportation industry complex materials providing high absorption of impact energy together with low density are required. materials with predefined structures produced using various additive manufacturing methods (e. g. materials with auxetic behaviour) or hybrid foams (e. g. polymeric core with metal coating) show great potential in various energy absorption applications [1]. by using these types of materials, it is possible to significantly improve efficiency and safety of constructions or vehicles. however, proper description of their complex structural and mechanical properties might be very challenging due to their complicated structures, especially in case of high strain-rate loading [2]. therefore, it is essential to not only perform precise high velocity material tests but also develop advanced methods that would enable reliable evaluation of such experimental data. split hopkinson pressure bar (shpb) technique is well established method for investigation of deformation behaviour of conventional materials (e. g. bulk metallic materials) under high strain-rate loading. for experiments with soft materials and materials with complex structures, a modified shpb (usually with in-house design and application tailored properties) together with custom evaluation software is required [3]. for cellular materials, it is shown that precise application of suitable filter is essential, and relevance of the evaluated results depends on proper pre-processing of the experimental data. experimental setup might differ in terms of used measurement bars (usually alluminum or pmma bars are used), length of used striker (up to 500 mm), impact velocity of used striker (up to 50 ms−1), number and type of strain gauges (both foil and semiconductor strain gauges are used) and their position on measuring bars. pre-processing procedure may therefore differ according to the used experimental setup and different type of filter (e.g. butterworth filter, chebyshev filter etc.) might suit specific experimental data. measured signals from shpb experiments are subject to high frequency noise and contain various peak values (e. g. valve release interference peak). due to this, it is important to apply a suitable filter to process the data before its further evaluation. however, proper filter analysis and spectral analysis of measured signals is needed. in this paper, signals from several shpb measurements using different stiker lengths and impact velocities of strikers were analysed. optimal filter was found, one which best suits very specific requirements of shpb data, such as filtering only identified noise frequencies while not affecting properties of measured elastic waves which would lead to unprecise experimental results. 2. materials and methods 2.1. split hopkinson pressure bar basic scheme of split hopkinson presure bar (shpb) is shown in fig. 1. all experiments were performed using a custom kolsky shpb setup (overview shown in fig. 2). both bars and the striker have the same diameter 20 mm. high-strength aluminium alloy (enaw 7075) was used as a material of the bars. the striker was accelerated using a gas-gun system with 16 bar maximum pressure. the incident and the trans77 http://dx.doi.org/10.14311/app.2018.18.0077 http://ojs.cvut.cz/ojs/index.php/app marcel adorna, jan falta, tomáš fíla, petr zlámal acta polytechnica ctu proceedings striker incident bar specimen transmission bar mp1 mp2 incident pulse re ected pulse transmission pulse wheatstone bridge circuit low-noise di erential ampli er pc labview tdms le figure 1. basic scheme of shpb transmission bar safety shield incident bar damper measuring point gas gun laser through-beam photoelectric sensor control station (pc) figure 2. loading device overview mission bar had the same length 1600 mm and were guided and supported by set of a low-friction polymerliner slide bearings (drylin fjum, igus, germany) with a custom stainless steel housing. all surfaces of both bars and the striker were carefully grounded and polished to ensure a smooth motion of the bars in the bearings and achieve plan-parallerism of the bars faces. 2.2. strain gauge measurement elastic strain waves were measured on two measuring points equipped with foil strain gauges in wheatstone half-bridge arrangement during the test. active differential low noise amplifier (el-lna-2, elsys ag, switzerland) with gain 100 was used to amplify strain gauge signal before the digitization. the strain gauge signal was sampled using 16-bit digitizer (pci-9826h, adlink technology, inc., taiwan) with maximal 20 mhz sample rate to maximize signal-to-noise ratio of measured signals. however, all signals were subject to the high frequency noise as well as white gaussian noise of different amlitudes and contained various peak values (e. g. valve release interference peak) which were not significant for the experiment evaluation. 2.3. auxetic structures filter analysis and optimisation was performed on the measured signals from experiments performed with a group of so called auxetic materials with predefined periodic structure. these materials exhibit in-plane negative strain-dependent poisson’s ratio. selective laser sintering (sls) method was used for production of the samples and manufacturing was performed using (a) (b) figure 3. 2d missing-rib auxetic structure the am250 system (renishaw, uk) by sintering the 316l-0407 powdered austenitic steel [4]. structure with missing-rib unit-cell geometry (i.e. 2d missingrib) is shown in fig. 3. resulting optimized filter is however to be universal for all measurements of cellular materials (with similar material properties as auxetics) preformed using our aluminium shpb bar setup. 3. measured signal analysis analysis of the measured signals was performed using matlab environment (mathworks inc.). spectral analysis of signals was carried out using fast fourier transform. three different types of filters implemented in matab were tested and evaluation of filter performance was carried out to select the best filter for the chosen experimental setup. an exemplary elastic wave signal with applied filter is shown in fig. 4. figure 4. elastic wave signal with applied filter periodograms of measured signals were obtained using matlab tool to investigate main noise frequencies present in the signal. note that due to used sampling frequency of 20 mhz, axes of periodograms obtained using matlab function do not directly correspond with noise frequencies, but can be easily derived from it. influence of boundary conditions of the experiment (impact velocity, striker length etc.) on measured signals and its noise are investigated. 3.1. measuring bars signals periodograms of measured signals on incident and transmission bar are shown in fig. 5. it is shown, that for both incident and transmission signal, the main 78 vol. 18/2018 preprocessing of shpb experiment data: filter analysis noise frequencies were the same, which suggests that noise properties were also the same and there were no additional vibrations caused by the bars contact. it is also shown, that the main noise frequencies were of singular character and could be attenuated using a specific notch filter. in the range from 2 mhz to 4 mhz all frequencies also showed noisy character, which could be attenuated using band-stop filter. figure 5. periodogram of incident and transmission signal: ls = 500 mm (long striker), vs = 20.85 ms−1 (low striker velocity) 3.2. striker length periodograms of representative measured signals from experiments performed using short and long striker are shown in fig. 6. main noise frequencies were again almost the same and it was shown, that signal noise is independent on length of used striker during shpb experiment. figure 6. periodogram of incident signal: ls = 500 mm (long striker), vs = 24.05 ms−1 (low striker velocity) and ls = 100 mm (short striker), vs = 19.32 ms−1 (low striker velocity) 3.3. striker velocity experiments with wide range of striker velocities were performed to determine whether measured signals and its noise depends on striker impact velocity. periodogram of measured signals from experiments performed using high and low striker velocity are shown on fig 7. it is shown, that main noise frequencies are again almost identical and therefore it can be assumed that properties of measured noise are independent on striker velocity. figure 7. periodogram of incident signal: ls = 500 mm (long striker), vs = 45.21 ms−1 (high striker velocity) and ls = 500 mm (long striker), vs = 21.03 ms−1 (low striker velocity) 4. filter analysis filter analysis and evaluation of filter performance was carried out to select the best and universal filter for the used shpb setup. main goal of data pre-processing was to get rid of high frequency noise while preserving measured signals undeformed and thus minimize of the negative effect of filtration on experimental results. both low-pass butterworth analog filter and digital savitzky-golay filter were being used for shpb signals filtering. these two filters and their performance were compared with designed optimal filter based on inverse chebyshev procedure implemented in matlab. 4.1. butterworth filter low-pass butterwoth filter was used in previous studies performed on our department due to its maximally flat frequency response in both passband and stopband. hovewer, in order to filter signals in a satisfying manner, stopband of butterworth filter and its corresponding attenuation deformed measured signal too much and important parts of the experimental data were often lost. 4.2. savitzky-golay digital filter the savitzky-golay filters are usually used for data smoothing and denoising in the spectral analysis, signal processing, image processing field etc. its basic principle is based on the least square error fitting between original data and polynomial function with predefined degree inside a moving window [5, 6]. initial setting of these two parameters thus have an immense influence on smoothing procedure and its outcome. this also means that these predefined parameters correspond with signal-to-noise ratio of measured signals (see also [7]) and therefore it is necessary to redefine these parameters separately for each measurement in order to smooth the data in similar way. 79 marcel adorna, jan falta, tomáš fíla, petr zlámal acta polytechnica ctu proceedings 8.4 8.6 8.8 9 9.2 9.4 time [s] ×10 -3 -1.5 -1 -0.5 0 0.5 1 1.5 2 v o lt a g e [ v ] original signal butterworth low-pass filter savitzky-golay filter optimized chebyshev filter 8.55 8.6 8.65 time [s] ×10 -3 1.5 1.55 1.6 1.65 1.7 1.75 1.8 1.85 1.9 v o lt a g e [ v ] detail a detail a figure 8. comparison between different filters for incident signal: ls = 500 mm (long striker), vs = 45.36 ms−1 (high striker velocity) 0 0.5 1 1.5 2 2.5 time [s] ×10 -4 -1 0 1 2 3 4 5 f o rc e [ n ] ×10 4force equilibrium without filter 0 0.5 1 1.5 2 2.5 time [s] ×10 -4 -1 0 1 2 3 4 5 f o rc e [ n ] ×10 4force equilibrium with sg filter force incident bar-specimen force specimen-transmission bar 0 0.5 1 1.5 2 2.5 time [s] ×10 -4 -1 0 1 2 3 4 5 f o rc e [ n ] ×10 4force equilibrium with optimal filter figure 9. comparison of force equilibrium obtained without signal filtering and with two different filters: ls = 500 mm (long striker), vs = 45.36 ms−1 (high striker velocity) in fig. 10 an effect of optimized filter application on strain equilibrium is shown. it was shown that optimized filter effect on experimental results was essential. smoothing effect of both butterworth and sg filter is not sufficient for evaluation of experiment equilibrium. it was also shown, that optimized filter is universal for all shpb experiment signals. this filter was previously used mainly for its simple implementation and input parameter modification. however it is not possible to derive an optimal solution for wide range of shpb experiments, because its input parameters can not be directly linked with spectral analysis of signal and thus can deform it or filter out important information from it (e.g. mechanical oscillations in the measurement bars). 4.3. chebyshev filter (type 2) combination of three inverse chebyshev band stop signal processing filters was used for optimization of shpb signals filtration process. this type of filter was chosen mainly because of its maximally flat response in pass band and equiripple in stop band. this property could be used to design two notch filters (filters with very narrow stop band range) to attenuate two most important noise frequencies present in all measured signals shown in previous chapter. there are other less important singular noise frequencies present in measured signals, as can be seen in fig. 5 7, but adding another component to designed filter did not improve the overall outcome of filtration. those two frequencies were identified to be f1 = 639 khz and f2 = 980 khz. for both frequencies attenuation of as = 70 db, filter order was 2. the third part of designed filter was designed again as an inverse chebyshev filter band stop in range from 2 mhz to 4 mhz with attenuation of as = 20 db and same filter order. it is shown, that this optimized filter fits experimental data in peak values and also visually provides the best results compared to other tested filters (fig. 8, detail a). 5. results evaluation of experimental shpb data was performed both with and without pre-processing of signals using optimized filter. for comparison low-pass butterworth filter with 250 khz passband and attenuation of 80 db (which effectively attenuates lower frequencies without deforming measured signal, but does not smooth signals in acceptable fashion) and savitzky-golay fil80 vol. 18/2018 preprocessing of shpb experiment data: filter analysis 0 1 2 3 time [s] ×10 -4 -3 -2 -1 0 1 2 3 s tr a in [ -] ×10 -3 strains equilibrium without filter 0 1 2 3 time [s] ×10 -4 -3 -2 -1 0 1 2 3 s tr a in [ -] ×10 -3 strains equilibrium with butterworth filter incident wave reflected wave transmission wave trans-ref inc+ref 0 1 2 3 time [s] ×10 -4 -3 -2 -1 0 1 2 3 s tr a in [ -] ×10 -3 strains equilibrium with optimized filter figure 10. comparison of strain equilibrium obtained without signal filtering and with two different filters: ls = 500 mm (long striker), vs = 24.05 ms−1 (high striker velocity) ter with polynomial degree 2, frame width 99 (low polynomia degree was chosen over higher polynomial, see [6] and [8]). in fig. 9 an effect of optimized filter application on force equilibrium as one of the main results of shpb test evaluation is shown. 6. conclusion analysis of measured shpb signals was performed. it was shown that noise properties of shpb signals were independent on the length of used striker and its impact velocity. it was also shown that noise properties are the same for both incident and transmission bar. random noise present in the measured signals was therefore identified using spectral analysis. this noise (consisting of several singular noise frequencies) was identified to be only of the electric character. to filter the noise, optimal filter which minimized deformation of signal spectrum was designed based on inverse chebyshev filtering procedure. designed filter performance was compared with two different filtering methods which were both previously being used for shpb signals pre-processing. results of several experiments with and without applied filter were shown. to conclude, the optimized filtration method for pre-processing of shpb data was introduced. the method was suitable for automated data filtering of signal from aluminum bars low impedance materials sls manufactured auxetic lattice. acknowledgements the research was supported by the czech science foundation (project no. 15-15480s) and by grant agency of the ctu in prague (grants no. sgs18/153/ohk2/2t/16 and sgs17/148/ohk2/2t/16). all the financial support is gratefully acknowledged. references [1] a. jung, s. diebels. microstructural characterisation and experimental determination of a multiaxial yield surface for open-cell aluminium foams. materials and design 131:252–264, 2017. doi:10.1016/j.matdes.2017.06.017. [2] t. fila, p. zlamal, o. jirousek, et al. impact testing of polymer-filled auxetics using split hopkinson pressure bar. advanced engineering materials 19(10), 2017. doi:10.1002/adem.201700076. [3] g. sunny, f. yuan, v. prakash, j. lewandowski. design of inserts for split-hopkinson pressure bar testing of low strain-to-failure materials. experimental mechanics 49(4):479–490, 2009. doi:10.1007/s11340-008-9145-1. [4] p. koudelka, o. jirousek, t. fila, t. doktor. compressive properties of auxetic structures produced with direct 3d printing. materiali in tehnologije 50(3):311–317, 2016. doi:10.17222/mit.2014.204. [5] a.-x. zhao, x.-j. tang, z.-h. zhang, j.-h. liu. the parameters optimization selection of savitzky-golay filter and its application in smoothing pretreatment for ftir spectra. pp. 516–521. 2014. doi:10.1109/iciea.2014.6931218. [6] b. zimmermann, a. kohler. optimizing savitzky-golay parameters for improving spectral resolution and quantification in infrared spectroscopy. applied spectroscopy 67(8):892–902, 2013. doi:10.1366/12-06723. [7] m. jakubowska, w. kubiak. adaptive-degree polynomial filter for voltammetric signals. analytica chimica acta 512(2):241–250, 2004. doi:10.1016/j.aca.2004.03.007. [8] p. barak. smoothing and differentiation by an adaptive-degree polynomial filter. analytical chemistry 67(17):2758–2762, 1995. doi:10.1021/ac00113a006. 81 http://dx.doi.org/10.1016/j.matdes.2017.06.017 http://dx.doi.org/10.1002/adem.201700076 http://dx.doi.org/10.1007/s11340-008-9145-1 http://dx.doi.org/10.17222/mit.2014.204 http://dx.doi.org/10.1109/iciea.2014.6931218 http://dx.doi.org/10.1366/12-06723 http://dx.doi.org/10.1016/j.aca.2004.03.007 http://dx.doi.org/10.1021/ac00113a006 acta polytechnica ctu proceedings 18:77–81, 2018 1 introduction 2 materials and methods 2.1 split hopkinson pressure bar 2.2 strain gauge measurement 2.3 auxetic structures 3 measured signal analysis 3.1 measuring bars signals 3.2 striker length 3.3 striker velocity 4 filter analysis 4.1 butterworth filter 4.2 savitzky-golay digital filter 4.3 chebyshev filter (type 2) 5 results 6 conclusion acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2018.18.0066 acta polytechnica ctu proceedings 18:66–71, 2018 © czech technical university in prague, 2018 available online at http://ojs.cvut.cz/ojs/index.php/app experimental investigation of the failure behaviour of polypropylene compounds for instrumented puncture tests florian kiehasa,∗, anna kalteisa, michael jerabekb, zoltán majora a institute for polymer product engineering, altenberger straße 69, 4040 linz, austria b borealis polyolefine gmbh, sankt-peter-straße 25, 4020 linz, austria ∗ corresponding author: florian.kiehas@jku.at abstract. instrumented puncture tests according to iso6603-2 and astmd3763 were executed for five different polypropylene compounds (talcum-, glass fibreand elastomer modified) with specimen thicknesses ranging from 1mm to 4mm. over 1500 puncture tests were executed at the impact & long-term behaviour laboratory of the company borealisr in linz. this serves as strong foundation for statistical evaluations of the ductile/brittle transition temperature. for different materials and ductile/brittle transition determination methods, similar trends have been observed, which were characterized by introducing shift factors. keywords: polypropylene, compound, puncture, mechanical testing, ductile/brittle transition. 1. introduction for polymers, the energy absorption capacity strongly depends on the morphology of the plastic and the experimental environment. in the case of impact or puncture tests, temperature and strain rate variations have a significant influence on the failure behaviour: almost all unoriented plastics exhibit brittle behaviour at low temperatures as well as at high strain rates [1]. mechanical measurements of different puncture test standards result in distinct force-deflection curves, which in turn give different transitions from ductile to brittle. furthermore, there is no concise method for the calculation of an exact transition temperature. it is assumed to be somewhere in the transition regime, a temperature range where sudden drops in energy absorption capacity indicate a change of impact behaviour from ductile to brittle. first, a precise method for the calculation has to be introduced. then, the difference of the transition as a function of specimen thickness, temperature, test standard and other parameters can be determined. 2. experimental investigation 2.1. test parameters the test setup and parameters of iso6603-2 and astmd3763 vary greatly. the main difference is the strikerand support ring geometry. for this reference analysis, tests for both standards were executed clamped, lubricated and at a speed of 4400mm/s to allow for comparison. therefore, the astmd3763 was slightly modified. a summary of all parameters can be looked up in table 1 and figure 1 shows the different geometries of each test standard. a more detailed description of each test standard can be looked up in literature [2, 3]. iso modified* astm specimen dimensions 60×60 mm ∅102 mm specimen thickness 1,2,3 mm 1,2,3,4mm* striker diameter 20.0 mm 12.7 mm support ring diameter 40.0 mm 76.0 mm test speed 4400 mm/s 4400mm/s* clamped yes yes lubricated yes yes* table 1. selected test conditions and parameters. figure 1. striker and support ring geometries iso (left) and astm (right). 2.2. materials and test schedule an extensive test program with a total number of 1586 specimens was carried out. the workload was divided between two machines: a servohydraulic roell/amsler high speed and a gravitative (spring driven) ceast fractovis plus. the comparability of measurement data obtained by both machines is documented by [4]. five different polypropylene grades developed and produced by the company borealisr were selected for this analysis. three materials are in-reactor made heterophasic copolymers, which are different in rubberand matrix design (elastomer modified polypropylene m1 to m3). m5 is also an in-reactor made heterophasic copolymer filled with talc. m4 is a glass filled polypropylene. 66 http://dx.doi.org/10.14311/app.2018.18.0066 http://ojs.cvut.cz/ojs/index.php/app vol. 18/2018 investigation of pp-compounds for instrumented puncture tests four specimen thicknesses from 1mm to 4mm were tested with the exception of 4mm iso6603-2 specimens, for which no injection moulding tool was available for manufacturing. 1mm thick glass-fibre reinforced iso plaques were also not available. examined conditioning temperatures range from -70 ◦c to 23 ◦c. eight temperature levels with five repetitions respectively were executed for each material, test standard and specimen thickness. in certain cases, additional measurements with three repetitions were carried out in the transition regime. altogether, there are 34 test series with descriptions following table 2: polypropylene compound test standard thickness m1(elastomer modified pp) iso(iso 6603-2) t1(1mm) m2(elastomer modified pp) astm(astmd3763) t2(2mm) m3(elastomer modified pp) t3(3mm) m4(glass fibre reinforced pp) t4(4mm) m5(talcum reinforced pp) table 2. nomenclature of test series. for example, m4astmt3 represents a test series of material m4 with 3mm thick specimens tested under the modified astmd3763 standard conditions. 2.3. data evaluation for both test standards, the failure criterion is 50% force drop after peak load fm. the corresponding puncture energy ep is obtained by integrating the force up to the deflection at puncture sp. the transition from elastic to elastic-plastic material behaviour was described with the introduction of a yield equivalent point (sy/fy)[5, 6]. the initial slope klin (corresponding to the relative stiffness [3]) of the curve can be approximated through the specification of another characteristic data point (sy0/fy0), which is necessary to avoid deviations caused by discontinuities at the start of impact. these points are defined as having the largest orthogonal distance from the direct connection line between origin (0/0) and peak load (sm/fm). thus, the energy em expended up to the maximum load can be split into an elastic part eel and a plastic part epl. the graphical determination of the starting and end point of the initial slope, as well as the geometric interpretation of the plastic and elastic energy contributions are illustrated in figure 2. following characteristic values are examined: • maximum force/deflection/energy fm/sm/em • puncture force/deflection/energy fp/sp/ep • initial force/deflection fy0/sy0 • yield equivalent force/deflection fy/sy • elastic/inelastic energy portions eel/epl • relative stiffness klin • width of plastic plateau ∆sw klin = fy − fy0 sy − sy0 (1) eel = f 2 m 2klin (2) epl = em − eel (3) figure 2. evaluation of force-deflection data. two characteristic energy ratios have been introduced to describe the elastic properties as well as the failure behaviour of force-deflection data: the elastic energy ratio rel and the ductility index [7] rduc are specified in the following equations: rel = epl + eel eel (4) rduc = ep − em ep (5) 2.3.1. curve characterization the iso 6603-2 curve type evaluation is a common method to characterize the force/deflection data obtained by puncture test ecperiments. it provides four typical types of curve progression that can usually be observed during data evaluation. in general, these range from ductile to brittle and focus on information like yielding, crack initiation and crack propagation [2]. obtained data has been divided into these four categories, which are compiled in figure 3. the determination of the ductile/brittle transition by means of curve type characterization is described in section 2.4.5. 2.3.2. optical characterization the different types of fracture appearance are taken from a company-internal standard developed by general motors corporation. it grades fracture appearances from 1 to 11 and covers almost all kinds of break that may occur during impact testing. a subdivision ranging from ductile, to semi-ductile, up to brittle can be made. illustrations of the individual fracture types are depicted in figure 4. the optical characterization was utilized to determine the ductile/brittle transition as well, which is described in section 2.4.6. 67 f. kiehas, a. kalteis, m. jerabek, z. major acta polytechnica ctu proceedings figure 3. curve types: yd yielding with deep drawing (top left), ys yielding with stable crack (top right), yu yielding with unstable crack (bottom left), ny no yielding (bottom right) [2]. 1 2 3 4 5 6 7 8 9 10 11 figure 4. fracture appearances: from ductile (1 5), to ductile-fragile (6 7), to fragile-ductile (8 9), up to fragile (10 11). 2.4. ductile/brittle transition temperature various methods for the determination of the ductile/brittle transition temperature have been proposed in literature. for impact tests with steep inclines in the transition regime, not only the tangent formation of the upperand lower shelf impact strengths [8, 9] but also optical investigations of the fracture appearance was investigated [10]. in [11] the inflection point of the temperature-dependent course was determined. likewise, the penetration energy was proposed for exploration. for this reference analysis, six different methods for the determination of the ductile/brittle transition temperature have been introduced and will be listed in the following subsections. it should be pointed out, that values are interpolated between data points. figure 5 depicts graphic representations for the determination of each transition temperature. 2.4.1. puncture energy the arithmetic mean of the local minimum and maximum puncture energy ep is calculated and the transition is assumed at the corresponding condition temfigure 5. determination of transition temperatures. perature te . 2.4.2. elastic energy ratio the peak load energy em is compared to the elastic energy eel. it should be pointed out that the minimum value for this energy ratio is 1 when the plastic portion disappears. this case corresponds to absolutely no yielding before peak load with the corresponding curve type ny no yielding. threshold values of 1.4, 1.2 and 1.0 were examined, with trat 1.2 giving the most consistent results. 2.4.3. ductility index in contrast to the elastic energy ratio rel, the ductility index rduc is only obtained from information of the force deflection data after peak load. therefore, yielding before peak load can not be detected with this method and is neglected. a ductility index of 0 implies, that the peak load is directly followed by an unstable crack, and consequently brittle fracture. threshold values of 40%, 30% and 5% were examined, with tduc 30 giving the most consistent results. 2.4.4. deflection for completely brittle fractures, all characteristic events of a puncture test happen in a narrow time frame. the deflection difference between sm, sp and stotal (as well as sy when there is no plastic plateau) becomes very small. t s is the temperature where sm and sp converge. 2.4.5. curve characterization this method for the ductile/brittle transition temperature determination takes advantage of the elaborate iso6603-2 curve characterization introduced in section 2.3.1. the curve types usually occur in the sequence of yd, ys, yu and ny from high to low 68 vol. 18/2018 investigation of pp-compounds for instrumented puncture tests temperatures. an attempt was made to describe the ductile/brittle behaviour by looking out for the transitions from yu yielding with unstable crack (bottom left) to ny no yielding. this leads to two specific temperatures ty u and tny , which are triggered at the first appearance of their respective curve type. 2.4.6. optical characterization tgm corresponds to the test temperature, where a fragile fracture type occurs for the first time. 2.5. overview transition temperatures a compilation of all calculated transition temperatures is given in table 3. with the exception of the glass fibre reinforced material m4, all materials follow similar trends between test standards and specimen thicknesses. reproducibility, consistency and comparability are the main factors that need to be considered when evaluating the transition temperatures. at the same time, the calculations involved should be as simple as possible. tduc 30 shows the most inconsistent results because it is sensitive to noise of the force-deflection data after crack initiation. despite occasional outliers, the iso characterization has proven itself to be an adequate representation of the transition regime with most of the other transition temperatures lying between the boundary values of ty u and tny . for thin specimens, tny and ty u stretch over a wide temperature range, which gets more narrow for thicker specimens. in comparison to all the other categories, the fracture appearance usually gives much higher transition temperatures tgm , because fractures are classified as brittle as soon as crack initiation is observed disregarding any plasticity that might be present before. 2.6. transition shift factors since the trends of all categories are strikingly similar as can be seen in table 3, the shift factors are contemplated to be universally applicable for all transition temperatures. however, the impact behaviour of glass fibreand talcum reinforced pp-compounds is vastly different from the other tested materials. consequently, only elastomer modified pp-grades (m1, m2, m3) as well as the most consistent transition temperatures (te , trat 1.2 , t s) are used to determine the shift factors. for data extrapolation, 1mm iso test series are chosen as initial reference values, since 1mm test series show the least standard deviations and iso is generally more consistent than astm. this procedure is illustrated in table 4. finally, these shift factors are summarized in average values α, β, γ, δ, ε and ζ, which are listed in table 5. it should be pointed out, that the transition from ductile to brittle was not fully covered for the material m2, even at -70 ◦c, which is the reason why t s values for m2isot2 and m2isot3 are assumed to be -70 ◦c. t e t rat 1.2 t s t duc 30 tny ty u tgm m1isot1 -47 -50 -49 -35 -52 -30 -30 m1isot1 -57 -59 -57 -53 -58 -52 -50 m1isot3 -56 -56 -58 -55 -60 -56 -56 m1astmt1 -40 -29 -30 18 -40 -23 -30 m1astmt2 -47 -45 -50 -41 -50 -40 50 m1astmt3 -43 -43 -50 -36 -50 -50 -40 m1astmt4 -45 -45 -50 -41 -50 -50 -50 m2isot1 -50 -55 -50 -40 0 -20 m2isot2 -63 -66 -70 -56 -30 -30 m2isot3 -64 -65 -70 -40 -30 m2astmt1 -40 -36 -37 7 23 -20 m2astmt2 -55 -55 -60 -41 -40 -20 m2astmt3 -52 -50 -60 -43 -50 m2astmt4 -54 -60 -22 -30 -30 m3isot1 -19 -20 -17 6 -30 -10 -20 m3isot2 -28 -29 -30 -8 -40 -10 -30 m3isot3 -26 -27 -25 -17 -34 -20 -30 m3astmt1 -23 -3 -10 16 -20 23 -10 m3astmt2 -26 -14 -19 5 -20 0 -10 m3astmt3 -26 -13 -19 7 -20 -10 -10 m3astmt4 -24 -28 -6 -30 -10 -20 m4isot2 -18 m4isot3 -7 -7 m4astmt1 -17 -17 -11 m4astmt2 -7 -27 -52 -27 m4astmt3 -6 -14 -5 -1 m4astmt4 5 -59 -22 6 m5isot1 -35 -35 -27 -4 -36 -10 -30 m5isot2 -36 -36 -36 -23 -38 -30 -40 m5isot3 -39 -36 -21 -50 -20 -40 m5astmt1 -35 -25 -21 11 -30 23 -30 m5astmt2 -35 -34 -30 -17 -40 -20 -30 m5astmt3 -34 -30 -39 -14 -40 -20 -30 m5astmt4 -27 -39 -12 -40 -20 -20 table 3. overview of all transition temperatures (all values in celsius); nomenclature e.g. m3isot1: material m3, iso test standard and 1mm thick specimens. t e t rat 1.2 t s m1isot1 226 = x 223 = x 224 = x m1isot1 216 = x · 0.955 214 = x · 0.959 216 = x · 0.964 m1isot3 217 = x · 0.960 217 = x · 0.973 215 = x · 0.959 m1astmt1 233 = x · 1.030 244 = x · 1.094 243 = x · 1.084 m1astmt2 226 = x · 1.000 228 = x · 1.022 223 = x · 0.995 m1astmt3 230 = x · 1.017 230 = x · 1.031 223 = x · 0.995 m1astmt4 228 = x · 1.008 228 = x · 1.022 223 = x · 0.995 m2isot1 223 = x 218 = x 223 = x m2isot2 210 = x · 0.941 207 = x · 0.949 208 = x · 0.932 m2isot3 209 = x · 0.937 207 = x · 0.954 208 = x · 0.932 m2astmt1 233 = x · 1.044 237 = x · 1.087 236 = x · 1.058 m2astmt2 218 = x · 0.977 218 = x · 1.000 213 = x · 0.955 m2astmt3 221 = x · 0.991 223 = x · 1.022 213 = x · 0.955 m2astmt4 219 = x · 0.982 213 = x · 0.955 m3isot1 254 = x 253 = x 256 = x m3isot2 245 = x · 0.964 244 = x · 0.964 243 = x · 0.949 m3isot3 247 = x · 0.972 246 = x · 0.972 248 = x · 0.968 m3astmt1 250 = x · 0.984 270 = x · 1.067 263 = x · 1.027 m3astmt2 247 = x · 0.972 259 = x · 1.023 254 = x · 0.992 m3astmt3 247 = x · 0.972 260 = x · 1.027 254 = x · 0.992 m3astmt4 249 = x · 0.980 245 = x · 0.957 table 4. determination of individual shift factors (all values in kelvin). avg. std. α 0.953 0.011 iso1mm to iso2mm β 0.958 0.015 iso1mm to iso3mm γ 1.053 0.035 iso1mm to astm1mm δ 0.993 0.022 iso1mm to astm2mm ε 1.000 0.026 iso1mm to astm3mm ζ 1.005 0.038 iso1mm to astm4mm table 5. average kelvin temperature shift factors α, β, γ, δ, ε and ζ and their standard deviations. the standard deviation of the shift factors listed in table 5 become greater for thicker specimens. the predictions from 1mm iso to thicker iso series shows accurate predictions, while extrapolations from iso to astm generally hold higher deviations. figure 6 compares the predictions of the average shift factors with the results of the measured data. 69 f. kiehas, a. kalteis, m. jerabek, z. major acta polytechnica ctu proceedings (a) m1 (b) m2 (c) m3 figure 6. shift factor predictions versus reality: the trend between iso 1mm (black) and thicker specimens (purple) can be predicted (blue) with temperature shift factors α, β, γ, δ, ε and ζ. 3. conclusions the experimental investigations show that transition temperatures for elastomer modified materials hold similar trends between test standards and specimen thicknesses. temperature shift factors can be established to allow for the prediction of transition temperatures for elastomer filled pp at different testing conditions. while the data extrapolation is more consistent for iso6603-2 than astmd3763, satisfying values for all tested materials and conditions can be achieved. this may drastically reduce testing time, effort and costs and allow for comparison of data obtained by different test standards. list of symbols fm peak load [kn] sm deflection at peak load [mm] em energy up to peak load [j] fp puncture force [kn] sp puncture deflection [mm] ep puncture energy [j] fy0 initial force [kn] sy0 initial deflection [mm] fy yield equivalent force [kn] sy yield equivalent deflection [mm] eel elastic energy portion [j] epl plastic energy portion [j] klin relative stiffness [n/mm] ∆sw width of plastic plateau [mm] rel elastic energy ratio [–] rduc ductility index [–] stotal total deflection [mm] y d yielding with deep drawing y s yielding with stable crack growth y u yielding with unstable crack growth ny no yielding t temperature [◦c] te transition temperature; puncture energy [◦c] trat 1.2 transition temperature; elastic energy ratio [◦c] tduc 30 transition temperature; ductility index [◦c] tgm transition temperature; optical [◦c] tny transition temperature; curve type ny [◦c] ty u transition temperature; curve type yu [◦c] α, β, γ, δ, ε, ζ average shift factors [–] acknowledgements this research was supported by borealis polyolefine gmbh (linz, a.) references [1] e. baur, t. a. osswald, n. rudolph. saechtling kunststoff taschenbuch. 31st printing. carl hanser verlag, 2013. [2] plastics -determination of puncture impact behaviour of rigid plastics part 2: instrumented impact testing. iso 6603-2, international organization for standardization, geneva, switzerland, 2000. [3] astm d3763-15, standard test method for high speed puncture properties of plastics using load and displacement sensors. astm international 2015. doi:10.1520/d3763-15. [4] d. huber. temperaturgeführte durchstoßprüfung von kunststoffen vergleich von durchstoßprüfmaschinen, bachelor of science, montanuniversität leoben, 2009. [5] m. böhning, u. niebergall, a. adam, w. stark. influence of biodiesel sorption on temperature-dependent impact properties of polyethylene. polymer testing 40:133 – 142, 2014. doi:10.1016/j.polymertesting.2014.09.001. [6] v. hristov, r. lach, w. grellmann. impact fracture behavior of modified polypropylene/wood fiber composites. polymer testing 23(5):581 – 589, 2004. doi:10.1016/j.polymertesting.2003.10.011. [7] a. pavan, j. williams. fracture of polymers, composites and adhesives. volume 27. elsevier science, 2000. [8] a. van der wal, a. wal, j. mulder, r. gaymans. fracture of polypropylene: 2. the effect of the crystallinity. polymer 39(22):5477–, 1998. doi:10.1016/s0032-3861(97)10279-8. [9] a. wal, a. van der wal, j. mulder, et al. polypropylenerubber blends:1. the effect of matrix properties on the impact behaviour. polymer 39(26):6781–, 1998. doi:10.1016/s0032-3861(98)00170-0. [10] a. van der wal, a. wal, r. gaymans. polypropylene-rubber blends 5: deformation mechanism during fracture. polymer 1999(40):6067–6067, 1999. doi:10.1016/s0032-3861(99)00216-5. 70 http://dx.doi.org/10.1520/d3763-15 http://dx.doi.org/10.1016/j.polymertesting.2014.09.001 http://dx.doi.org/10.1016/j.polymertesting.2003.10.011 http://dx.doi.org/10.1016/s0032-3861(97)10279-8 http://dx.doi.org/10.1016/s0032-3861(98)00170-0 http://dx.doi.org/10.1016/s0032-3861(99)00216-5 vol. 18/2018 investigation of pp-compounds for instrumented puncture tests [11] c. grein, k. bernreitner, a. hauer, et al. impact modified isotatic polypropylene with controlled rubber intrinsic viscosities: some new aspects about morphology and fracture. journal of applied polymer science 87(10):1702–1712. https://onlinelibrary. wiley.com/doi/pdf/10.1002/app.11696 doi:10.1002/app.11696. 71 https://onlinelibrary.wiley.com/doi/pdf/10.1002/app.11696 https://onlinelibrary.wiley.com/doi/pdf/10.1002/app.11696 http://dx.doi.org/10.1002/app.11696 acta polytechnica ctu proceedings 18:66–71, 2018 1 introduction 2 experimental investigation 2.1 test parameters 2.2 materials and test schedule 2.3 data evaluation 2.3.1 curve characterization 2.3.2 optical characterization 2.4 ductile/brittle transition temperature 2.4.1 puncture energy 2.4.2 elastic energy ratio 2.4.3 ductility index 2.4.4 deflection 2.4.5 curve characterization 2.4.6 optical characterization 2.5 overview transition temperatures 2.6 transition shift factors 3 conclusions list of symbols acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2020.26.0056 acta polytechnica ctu proceedings 26:56–63, 2020 © czech technical university in prague, 2020 available online at https://ojs.cvut.cz/ojs/index.php/app localization analysis of an orthotropic multi-surface plasticity model under uniaxial stress claudio pagani∗, milan jirásek, martin horák czech technical university in prague, faculty of civil engineering, department of mechanics, thákurova 7, 166 29 prague 6, czech republic ∗ corresponding author: claudio.pagani@fsv.cvut.cz abstract. numerical simulations of masonry structures are often based on continuum macromodelling approaches that need constitutive laws able to phenomenologically reproduce the behavior of the material. to capture the deformation process up to failure, appropriate softening laws are needed to take into account the contraction of the yield stress domain caused by cracking and crushing. it is well known that softening may lead to localization of inelastic strain. this paper focuses on localization analysis of an orthotropic macro-scale model in the framework of multi-surface plasticity, which describes the in-plane behavior of masonry structures. preliminary results reported in this short paper are limited to uniaxial stress states. analytical localization conditions are first derived for uniaxial stress states with principal axes aligned with the material axes of orthotropy. then, localization analysis is extended to an arbitrary angle between the principal stress axes and the axes of orthotropy. keywords: multi-surface plasticity, localization, weak discontinuity, masonry. 1. introduction masonry structures are characterized by two different failure modes, the in-plane and out-of-plane mechanisms. regarding the study of in-plane mechanisms, different strategies could be adopted for numerical simulations of the behavior of masonry structures. among others, continuum macro-modeling is a frequently used approach, in which masonry structures are treated as homogenized continua and, in general, discretized by the finite element method. this kind of modelling needs the choice of a constitutive model, which could be formulated within the framework of nonlinear elasticity, plasticity, damage mechanics, smeared crack models, or their combinations, and its aim is to phenomenologically reproduce the average behavior of masonry. this paper is focused on a study of an orthotropic model in the framework of multisurface plasticity, with the yield function described as the composition of two different surfaces for the tensile and the compressive behavior [1, 2]. both failure surfaces evolve according to hardening/softening laws that may lead to localization of plastic strain. conditions for the onset of localization depend on many factors, such as the stress state, the destabilizing effects of the non-associated flow rule in plasticity, or, in this particular case, the orthotropy of the elastic stress-strain law. in this paper, the localization analysis of the presented macro-model is carried out under the condition of uniaxial stress in tension and in compression. conditions for the onset of localization are derived and preferential directions of the emerging weak discontinuity are evaluated. 2. model description this paper deals with an advanced model for the in-plane failure mechanisms of masonry structures proposed by lourenço [1, 2], which takes into account the anisotropic structure of masonry and is formulated in the framework of multisurface plasticity. the analysis can be reduced to two dimensions when plane-stress conditions are assumed. in the following description, the cartesian axes x and y are considered aligned with the material axes, which, for regular periodic masonry, are the horizontal and vertical ones (bed and head mortar joints, respectively). in order to avoid pathological mesh sensitivity of the results to the element size, model parameters that affect the stress-strain diagrams in tension and compression are adjusted by means of the equivalent length le. in the original paper, the crack band size was related to the area of the elements: le = αh √ ae (1) where αh is 1 or √ 2 for quadratic and linear elements, respectively. this rule, which has no convincing justification, may induce a large error for elongated elements and even for square elements since the crack band size depends also on the orientation with respect to the element sides. in this paper, the projection method is considered, i.e., the effective crack band size is estimated by projecting the element onto the direction perpendicular to the estimated crack band direction [3]. the model requires in total 16 parameters, which are reported in table 1, and are divided into groups governing orthotropic elasticity, tensile and compres56 https://doi.org/10.14311/app.2020.26.0056 https://ojs.cvut.cz/ojs/index.php/app vol. 26/2020 localization analysis of an orthotropic multi-surface plasticity model sive plastic behavior. the meaning of the symbols is described in the following sections. 2.1. orthotropic elasticity the linear elastic stress-strain relation under planestress conditions can be written as σ = deε (2) where stress and strain components are stored in column matrices using the voigt notation: σ =  σx σy σxy  , ε =  εx εy γxy  (3) the elastic stiffness matrix for an orthotropic material reads de = 1 ζ  ex νyxex 0 νxyey ey 0 0 0 ζgxy  (4) where ζ = 1− νxyνyx is an auxiliary parameter. the elastic behavior is characterized by four parameters: ex, ey, νxy, and gxy. the other in-plane poisson’s ratio is determined from symmetry and is given by νyx = νxyeyy/exx. the plane stress condition (σz = 0) leads to the following expression for the out-of-plane normal strain: εz = −1 ζ [(νxz + νyzνxy)εx + (νxzνyx + νyz)εy] (5) hence, additional two poisson’s ratios (νxz and νyz) have to be known if one is interested in computing the out-of-plane strain. 2.2. plastic behavior in tension an orthotropic rankine type yield surface is considered for the tensile behavior, with orthotropic softening controlled by a single scalar quantity. the corresponding yield function is defined as ft(σ, κt) = (σx − σt,x(κt)) + (σy − σt,y(κt)) 2 + + √( (σx − σt,x(κt))− (σy − σt,y(κt)) 2 )2 + ατ2 xy (6) degradation of the yield stress is governed by a single scalar internal variable κt. the softening laws σt,i(κt) = ft,i exp ( − left,i gft,i κt ) , i ∈ {x, y} (7) describe the dependence of the equivalent tensile yield stresses in directions x and y in the form of exponential softening and can be collected into the column matrix qt =  σt,x(κt) σt,y(κt) 0  (8) parameters ft,i and gft,i, i ∈ {x, y}, are the tensile strength and the fracture energy in tension corresponding to the i-axis, respectively. to avoid snap-back at the constitutive level, the value of the tensile strength is reduced if the elements are too large, according to the rule ft,i = √ gft,iei le if le > gft,iei f2 t,i (9) parameter α = ft,xft,y τ2 u,t (10) controls the shear stress contribution to tensile failure, where τu,t is the shear strength in tension. the flow rule is non-associated and is derived from the plastic potential gt(σ, κt) = (σx − σt,x(κt)) + (σy − σt,y(κt)) 2 + + √( (σx − σt,x(κt))− (σy − σt,y(κt)) 2 )2 + τ2 xy (11) the rate of the softening parameter is considered equal to the rate of the maximum principal plastic strain: κ̇t = ε̇p1 = ε̇px + ε̇py 2 + 1 2 √( ε̇px − ε̇py ) + ( γ̇pxy )2 (12) it is assumed that the hardening regimes in tension and compression are uncoupled, i.e., each of the surfaces is associated with its own hardening variable that grows only if the surface is active. under this assumption, the expression of κ̇t reduces to κ̇t = λ̇t (13) where λt is the plastic multiplier associated with the tensile yield surface. in conclusion, there are five model parameters controlling the tensile behavior: two strengths, ft,x and ft,y, two fracture energies, gft,x and gft,y and the parameter α that governs the coupling of shear with the normal stresses. 2.3. plastic behavior in compression the adopted failure surface in compression corresponds to a hill-type yield function, fc = σ2 x σ2 c,x(κc) + βσxσy + γτ2 xy σc,x(κc)σc,y(κc) + σ2 y σ2 c,y(κc) −1 (14) where β is a parameter that controls the coupling between the normal stresses. parameter γ controls the shear stress contribution to failure and is given by γ = fc,xfc,y τ2 u,c (15) 57 c. pagani, m. jirásek, m. horák acta polytechnica ctu proceedings where fm,x and fm,y are the values of uniaxial compression strength in directions x and y, and τu,c is the shear strength in compression. hardening and softening are controlled by a strainlike internal variable κc, defined by the rate equation κ̇c = 1√ σc,x(κc)σc,y(κc) σt ε̇p (16) which, under the assumption of uncoupled hardening regimes in tension and compression, reduces to κ̇c = λ̇c (17) where λc is the plastic multiplier associated with the compressive yield surface. the values of current compressive yield stress in directions x and y, σc,x and σc,y, are evaluated from κc using the hardening-softening law (figure 1) σc,i =  σi,i + (σp,i − σi,i) √ 2κc κp − κ2 c κ2 p κc ≤ κp σp,i + (σm,i − σp,i) ( κc−κp κm,i−κp )2 κp < κc ≤ κm,i σr,i + (σm,i − σr,i) e 2m(κc−κm) κm−κp κm,i < κc (18) where parameter m = σm,i − σp σm,i − σr,i (19) corresponds to parabolic hardening followed by parabolic and later exponential softening. this law contains strain-like parameters κp, which represents the value of κc at the peak, i.e., at transition from hardening to softening, and κm,x and κm,y, which correspond to the transition from parabolic to exponential softening, and also stress-like parameters σi,x and σi,y, which are the initial values of yield stress in directions x and y, σp,x and σp,y, which are the peak values of yield stress, σm,x and σm,y, which are the values of yield stress at transition from parabolic to exponential softening, and σr,x and σr,y, which are the values of residual yield stress. figure 1. hardening/softening law for the compressive yield stress in absence of specific experimental tests, the characteristic yield stress values can be set to σp,i = fc,i σi,i = 1 3fc,i (20) σm,i = 1 2fc,i σr,i = 1 10fc,i (21) the value of the equivalent plastic strain at peak, κp, is considered to be an additional parameter of the model. the value of κm,i depends on the estimated crack band width le (related to the finite element mesh) and is given by κm,i = 75 67 gfc,i lefc,i + κp (22) where gfc,i is the fracture energy in compression corresponding to the i-axis. in order to avoid snapback at the constitutive level, the compressive strength is reduced if the elements are too large, according to the formula fc,i = √ 75 67 gfc,iei le if κm,i < fc,i ei + κp (23) the values of current compressive yield stress in directions x and y could be cast into the column matrix qc =  σc,x(κc) σc,y(κc) 0  (24) an associated flow rule for compression regime is used, which means that the rate of plastic strain is given by ε̇p = λ̇c ∂fc(σ, κc) ∂σ (25) in summary, six model parameters are used for description of the inelastic behavior in compression: compressive strengths fc,x and fc,y, fracture energies gfc,x and gfc,y, coupling parameter between normal stresses in two directions, β, and the equivalent plastic strain value corresponding to the peak, κp. 3. localization analysis in plasticity suppose that plastic loading takes place with only one yield surface activated, and let i be the index of the active surface ( i ∈ {t, c}). in the following derivation, a comma denotes a partial derivative with respect to the variable following the comma and no sum over repeated indices is considered. the basic equations characterizing the flow theory of plasticity include the elastic-plastic split, ε = εe + εp (26) the stress-strain law for the elastic part (2), the yield condition fi(σ, qi) = 0 (27) 58 vol. 26/2020 localization analysis of an orthotropic multi-surface plasticity model the flow rule, ε̇p = λ̇igi,σ (28) the definition of hardening/softening variable in the rate form, κ̇i = λ̇i (29) the dependence of the yield strength on the hardening variable qi = hi(κi) (30) and the loading/unloading conditions fi ≤ 0, λ̇i ≥ 0, λ̇ifi = 0 (31) during plastic flow, the yield function must remain equal to zero, and so the rate of its change is also zero. this consideration leads to the consistency condition, λ̇iḟi = 0 (32) from which the elasto-plastic stiffness matrix can be computed. the resulting formula reads dep = de − de : gi,σ ⊗ fi,σ : de fi,σ : de : gi,σ − fi,qi ·hi = = de − 1 h de : gi,σ ⊗ fi,σ : de (33) where h = fi,σ : de : gi,σ − fi,qi ·hi (34) and hi = qi,κi (35) the condition of incipient weak discontinuity, as inspired by the early works of hadamard [4] and hill [5] and developed, among others, for plasticity [6, 7] and for damage [8, 9], is given by (n ·dep · n) ·m = 0 (36) where n is the normal to the discontinuity surface, and qep = n ·dep · n (37) is the elasto-plastic acoustic tensor. according to (36), this tensor is singular at the onset of localization. the eigenvector m corresponding to the zero eigenvalue is called the polarization vector. the elasto-plastic acoustic tensor can be expressed as qep = n ·dep · n = = n ·de · n− 1 h n ·de : gi,σ ⊗ fi,σ : de · n (38) which can be recast in a simpler form qep = qe − 1 h b⊗ a (39) where qe = n ·de · n (40) a = n ·de : fi,σ (41) b = n ·de : gi,σ (42) after some calculations, by imposing singularity of the elastoplastic acoustic tensor, the condition of an incipient weak discontinuity (36) becomes − fi,qi ·hi = a ·q−1 e · b− fi,σ : de : gi,σ (43) since we are in plane-stress conditions and considering the plane of symmetry perpendicular to the z-axis, one can restrict attention to normals n for which n =  n1 n2 0  (44) under the normalizing constraint n2 1 +n2 2 = 1. considering major and minor symmetries of the orthotropic elastic stiffness tensor and the plane-stress condition, the quantities entering in (43) can be simplified. namely, the orthotropic linear elastic acoustic tensor is given by qe = [ d1111n 2 1 +d1212n 2 2 (d1122 +d1212)n1n2 (d1122 +d1212)n1n2 d1212n 2 1 +d2222n 2 2 ] (45) where d1111 = exx 1− νxyνyx (46) d2222 = eyy 1− νxyνyx (47) d1122 = νyxexx 1− νxyνyx (48) d1212 = gxy (49) other important quantities are evaluated as fi,σ : de : gi,σ = d1111f 11 i,σg 11 i,σ+ +d1122(f11 i,σg 22 i,σ + f22 i,σg 11 i,σ)+ + 4d1212f 12 i,σg 12 i,σ +d2222f 22 i,σg 22 i,σ (50) a = [ (d1111f 11 i,σ +d1122f 22 i,σ)n1 + 2d1212f 12 i,σn2 2d1212f 12 i,σn1 + (d1122f 11 i,σ +d2222f 22 i,σ)n2 ] (51) b = [ (d1111g 11 i,σ +d1122g 22 i,σ)n1 + 2d1212g 12 i,σn2 2d1212g 12 i,σn1 + (d1122g 11 i,σ +d2222g 22 i,σ)n2 ] (52) where f iji,σ and giji,σ are the ij components of the derivative of the yielding function and the plastic potential, respectively, with respect to stress tensor. 4. localization analysis for uniaxial stress let us consider the case of uniaxial stress (figure 2) whose direction is inclined by an angle φ with respect to the x-axis that, together with the y-axis, forms the pair of orthotropy axes. let θ be the angle that the generic normal n to the crack surface forms with the material x-axis, so that it can be expressed as n =  cos θ sin θ 0  (53) 59 c. pagani, m. jirásek, m. horák acta polytechnica ctu proceedings the stress tensor matrix in the oxy reference frame corresponding to uniaxial tension of magnitude σ̄ and rotated by a generic angle φ with respect to the x-axis is given by σ(σ̄, φ) = qσ̂qt (54) where σ̂ = [ σ̄ 0 0 0 ] q = [ cosφ − sinφ sinφ cosφ ] (55) the value σ̄ should be the one that triggers localization for the given stress angle φ, so it is the value corresponding to the onset of plastic flow and is determined by the condition fi = 0 where fi is yield function given by (6) or (14) for tension or compression, respectively. subsequently, one can substitute the determined value of σ̄ for a given φ into the right-hand side of (43) and obtain an expression depending only on the angle θ. the localization angle θloc is determined as the one that maximizes the right-hand side of (43) and the corresponding value is the critical hardening modulus (hcrit = h(θloc)). the polarization vector m, the eigenvector of the elastoplastic acoustic tensor corresponding to the zero eigenvalue, characterizes the failure mode, ranging from tensile splitting (mode i) with m = n to shear slip (mode ii) with m perpendicular to n. figure 2. localization (n) and polarization (m) vectors for uniaxial stress σ̄ inclined by φ with respect the material x-axis in the case of uniaxial tension and compression along the x or y-axis, the expressions of the critical hardening moduli can be found in a closed form. first, it is useful to define the quantity ξ, common to all the expressions: ξ(θ) = − ( sin4 θ exx + cos4 θ eyy + +exx − 2gxyνxy exxgxy sin2 θ cos2 θ )−1 (56) t in the isotropic case, the previous expression reduces to ξ = −e (57) where e is the young modulus. subsequently, indicating by subscript t or c for tensile or compressive uniaxial stress and by subscript x or y its direction, the expressions of the critical hardening moduli are given by ht,x(θ) = ξ(θ) sin4 θ (58) ht,y(θ) = ξ(θ) cos4 θ (59) hc,x(θ) = ξ(θ) ( βµ 2 cos2 θ + 1 µ sin2 θ )2 (60) hc,y(θ) = ξ(θ) ( µ cos2 θ + β 2µ sin2 θ )2 (61) where µ = √ fcx/fcy. all the critical hardening moduli have the maximum value equal to zero, so localization starts at the onset of plastic flow for tension (since, under tension, abrupt softening occurs) and at the peak of the compressive yield stress for compression. in the case of uniaxial tension (figures 3-4), normal n is aligned with the stress, in fact θ = φ. in the case of uniaxial compression (figures 5-6), for the chosen set of mechanical parameters, the expressions of the critical hardening moduli exhibit two peaks corresponding to two different possible cracks that are symmetric with respect to the compressive stress axis. figure 3. h(θ) for uniaxial tension in x-direction figure 4. h(θ) for uniaxial tension in y-direction the localization analysis is then extended to the generic case of uniaxial tension or compression inclined by an angle φ with respect to the x-axis, with φ ∈ [0, π/2]. the mechanical parameters used are the same chosen by the author of the model for elementary tests under uniaxial stress [1, 2] and are reported in table 1. 60 vol. 26/2020 localization analysis of an orthotropic multi-surface plasticity model elastic parameters tensile parameters exx eyy νxy gxy ft,x ft,y gt,x gt,y α [gpa] [gpa] [-] [gpa] [mpa] [mpa] [n/m] [n/m] [-] 10.0 5.0 0.20 3.0 1.0 0.5 20 10 1.0 compressive parameters fc,x fc,y gc,x gc,y β γ κp [mpa] [mpa] [n/m] [n/m] [-] [-] [-] 10.0 5.0 5000 2500 -1.0 3.0 5·10−4 table 1. mechanical parameters considered for localization analysis figure 5. h(θ) for uniaxial compression in x-direction figure 6. h(θ) for uniaxial compression in y-direction the critical hardening moduli, except for the particular case of uniaxial tension along the material axes, always exhibit two maxima that correspond to the formation of two potential weak discontinuity surfaces. the solid and dashed lines represented in figures 7 to 12 correspond to the aforementioned two solutions. the dependence of the localization angles on the uniaxial stress angle φ for tension is represented in figure 7, while the difference between the localization angles and the tension angle is shown in figure 8. the variation of the inner product n ·m between the normal to crack surface and the polarization vector is represented in figure 9, which shows that for φ = {0, π/2} its value is equal to 1 (tensile splitting), whereas for intermediate values of φ the inner product is lower, which corresponds to a misalignment between n and m. it is worth noting that the two graphs are coincident since the polarization vectorm1 corresponding to the normal n1 coincides with the other normal n2 and vice versa. figure 7. dependence of θloc on φ for uniaxial tension figure 8. difference between θloc and φ as a function of φ for uniaxial tension figure 9. dependence of n · m on φ for uniaxial tension in the case of uniaxial compression, the dependence of the localization angles on the stress angle is reported in figure 10). from figure 11 one can see that the differences between the localization angles and the stress angle are almost constant and close to the values π/4 and 3π/4. figure 12 shows that the graphs of 61 c. pagani, m. jirásek, m. horák acta polytechnica ctu proceedings figure 10. dependence of θloc on φ for uniaxial compression figure 11. difference between θloc and φ as a function of φ under uniaxial compression the inner products n ·m corresponding to the two crack surfaces coincide as for the tensile behavior and that the polarization vector is orthogonal to the corresponding normal (shear slip) for φ = 0, whereas for different values of φ the inner product increases up to its maximum value for φ = π/2. 5. conclusions localization analysis under uniaxial tensile and compressive stress has been carried out. analytical results in the particular cases of tension or compression aligned with the material principal axes have been obtained and it has been shown that, for tensile stress, the weak discontinuity surface will form in the direction orthogonal to the stress direction while, in the case of compression, two different discontinuity surfaces, symmetric with respect to the stress direction, could develop. the analysis has been extended by varying the angle that the stress forms with the material axes. it has been shown that for both tension and compression two localization angles can be found and some observations related to the dependence of the localization angles and the polarization vectors on the stress angle have been presented. for all the cases described here, the localization condition is for the first time satisfied at the peak of the stress-strain diagram, i.e., the critical plastic modulus associated with the most favorable direction of a potential discontinuity is zero. for this reason, the results are not affected by the size of finite elements. the adjustment of the softening laws dependent on figure 12. dependence of n ·m on φ for uniaxial compression the element size according to (7) or (22) affects only the post-localization stage of response. list of symbols α parameter that controls the shear stress contribution to tensile failure β parameter that couples the normal stresses in the compressive failure surface γ parameter that controls the shear stress contribution to compressive failure le crack bandwidth h hardening modulus hcrit critical hardening modulus κp value of the compressive hardening parameter at peak fi,σ gradient of the i-th yield function with respect to the stress tensor gi,σ gradient of the i-th plastic potential with respect to the stress tensor n normal to the discontinuity surface m polarization vector φ uniaxial stress angle with respect to the x-axis θ angle that the generic normal to the discontinuity surface forms with the x-axis θloc localization angle acknowledgements this research has been financially supported by the european regional development fund through the center of advanced applied sciences at the czech technical university in prague (project no. cz.02.1.01/0.0/0.0/16_019/0000778). references [1] p. lourenço. an orthotropic continuum model for the analysis of masonry structures. tech. rep., university of delft, delft, holland, 1995. [2] p. lourenço. an anisotropic macro-model for masonry plates and shells: implementation and validation. ph.d. thesis, delft university of technology, faculty of civil engineering, 1997. [3] m. jirásek, m. bauer. numerical aspects of the crack band approach. computers and structures 2012. doi:10.1016/j.compstruc.2012.06.006. 62 https://doi.org/10.1016/j.compstruc.2012.06.006 vol. 26/2020 localization analysis of an orthotropic multi-surface plasticity model [4] j. hadamard. leçons sur la propagation des ondes. librairie scientifique a. hermann et fils, paris, 1903. [5] r. hill. a general theory of uniqueness and stability in elastic-plastic solids. journal of the mechanics and physics of solids 6(3):236–249, 1958. [6] n. s. ottosen, k. runesson. properties of discontinuous bifurcation solutions in elasto-plasticity. international journal of solids and structures 27(4):401–421, 1991. [7] j. w. rudnicki, j. rice. conditions for the localization of deformation in pressure-sensitive dilatant materials. journal of the mechanics and physics of solids 23(6):371–394, 1975. [8] m. jirásek. mathematical analysis of strain localization. revue européenne de génie civil 11(7-8):977–991, 2007. [9] m. jirásek, m. horák. localization properties of damage models. in computational modelling of concrete structures, pp. 327–336. crc press, 2010. 63 acta polytechnica ctu proceedings 26:56–63, 2020 1 introduction 2 model description 2.1 orthotropic elasticity 2.2 plastic behavior in tension 2.3 plastic behavior in compression 3 localization analysis in plasticity 4 localization analysis for uniaxial stress 5 conclusions list of symbols acknowledgements references acta polytechnica doi:10.14311/app.2020.27.0001 acta polytechnica 27(0):1–5, 2020 © czech technical university in prague, 2020 available online at http://ojs.cvut.cz/ojs/index.php/app compression of ecaped titanium micro-pillars for two principal orientations david vokoun∗, jan maňák, karel tesař, stanislav habr czech academy of science, institute of physics, na slovance 1999/2, prague, czech republic ∗ corresponding author: vokoun@fzu.cz abstract. the thermomechanical processing by equal-channel angular pressing (ecap) is used for certain metals and alloys in order to make their structure fine and to increase material strength. in the previous study done at our institute, grade 2 titanium was successfully processed using four consecutive route a passes via a 90 ◦ ecap die with high backpressure at room temperature. orientation dependence of compressive and tensile loading of ecaped titanium samples was demonstrated at macro-scale. however, scarce attention has been paid so far to the mechanical behavior of ecaped titanium samples at micro-scale. in the present study, compression experiments on titanium micropillars, fabricated using focused ion beam, are carried out for two main directions in respect to preceding ecap pressing (insert and extrusion directions). the purpose of this study is to discuss the orientation dependence of mechanical response during compression of the as-ecaped titanium micro-pillars. keywords: ecap, micro-pillar, titanium. 1. introduction titanium is a light-weight material with hexagonal structure at room temperature. due to high specific strength (strength-to-weight ratio), titanium and titanium based alloys are used in carand aerospaceindustries. in addition, titanium and its oxides are biocompatible. sound biocompatibility together with high resistance to the onset of localized corrosion make titanium a good candidate for biomedical applications. furthermore, fabrication of metalic micro-parts has been receiving much attention in recent years due to the rapid development of micro-electro-mechanical systems (mems). therefore, understanding of mechanical properties of various metalic materials including titanium at micrometric level is of importance. it only makes sense to produce and mechanically test microparts which are either monocrystalline with a fixed relation between the crystallographic and geometric axes [1] or which contain submicron grains (grains which are small enough to guarantee reproducibility of mechanical tests carried out on arbitrary two microsamples of identical dimensions). however, grains of titanium samples produced in a conventional way are large and their refinement requires special methods. the small tensile strength of titanium (commercially pure, grade 2 titanium, has tensile strength only 345 mpa [2]) can be increased either through alloying [3, 4] (a high chance of decreasing or losing biocompatibility), or through severe plastic deformation (spd) processes used for grain refinement. the increase of tensile strength in various metals and alloys through ecap (equal channel angular pressing) and other spd methods is generally known, e.g. the authors of [5] showed an increase of tensile strength of ecaped commercially pure (cp) titanium due to the ecap process up to 1000 mpa (true stress). ecap is a wide-spread spd technique [6] used also for titanium and titanium based alloys. during an ecap procedure, samples (billets) are heavily deformed with strains close to or exceeding 100% (depending on the used die geometry and the number of passes) due to repeated pressing a metalic billet through a system of two channels, axes of which intersect under an abrupt angle, usually 120◦ or 90◦. a great number of studies, so far published, describe preparation of ecaped titanium samples and evaluate microstructure, various physical and mainly mechanical properties of ecaped titanium samples in dependence on the ecap parameters [5, 7–11]. the key ecap parameters are related first to the orientation of a billet upon the repeated insertion in a die in respect to the previous passing of the billet through the die (routes a, ba, bc, and c [6]). the ecap parameters are also related to the die geometry and the billet temperature. angle ϕ (channel angle) is the abrupt angle between the two die channels and angle ψ (corner angle) represents the outer arc of curvature where the two channels intersect [6]. the other ecap parameters are e.g. number of the passes through a die, pressing speed, values of backpressure, pressing temperature, etc. room-temperature ecap processing is not so common for titanium although it brings the advantage of enhanced grain refinement due to the suppressed grain structure recovery. room-temperature ecap processing using channel angle 90◦ and less with cp grade 2 titanium is still challenging [5] due to material cracking after the first or second pass of the material through the die. in ref. [5], room-temperature ecap processing is described and later used in our study in exactly the same way with the same titanium alloy (identical chemical composition). 1 http://dx.doi.org/10.14311/app.2020.27.0001 http://ojs.cvut.cz/ojs/index.php/app d. vokoun, j. maňák , k. tesař, s. habr acta polytechnica figure 1. scheme of the ecap die with the coordinate system. the abbreviations id, ed and td represent the insert, extrusion and transverse directions, respectively. figure 2. the sem image of a micro-pillar (a) and the diamond tip approaching one of the fabricated micro-pillars (b). the purpose of the present work is to extend the previous work [5] in relation to compression experiments at room temperature. to our knowledge, so far no work on compression of ecaped titanium micropillars has been done. however, mechanical testing on micro-pillars of other ecaped metals and alloys has been performed recently [12, 13]. the advantage of our work is the availability of compressive tests on ecaped titanium at both macroand micro-scale and the possibility of comparison. 2. samples a cube, 3 mm × 3 mm × 3 mm, made of ecaped titanium was cut out (using wire electro-discharge machining) in such a way that each cube face was perpendicular to one of the principal directions insert direction (id), extrusion direction (ed) and transverse direction (td) (see fig. 1). the micro-pillars, each in the shape of a rectangular prism with a square base (see fig. 2a), were micro-fabricated and observed using an fei quanta 3d dual-beam sem/fib (scanning electron microscope/focused ion beam) system (fei, hillsboro, usa) with a ga+ ion source operated at 30 kev with various currents using automated milling script. each micro-pillars’ face was again perpendicular to one of the principal directions (id, ed, and td). as for dimensions of the micro-pillars, their height and width were approximately 8.5 µm and 3.5 µm, respectively. the micro-pillars were of two types denoted as id and ed micro-pillars. id {ed} micro-pillars had their long axis parallel to id {ed} direction, respectively. no micro-samples with pillar’s long axis parallel to td direction were prepared since previous studies showed almost identical mechanical characteristics in id and td directions. it was checked that each micro-pillar contained grains small enough not to limit reproducibility of the following compression tests. there were three id and four ed micro-pillars fabricated; together, seven micro-pillars. for each micropillar just one force-displacement curve was measured and converted into stress-strain curve. however not all 2 vol. 27/2020 ecaped titanium micro-pillars compression stress-strain data were used. most stress-strain curves were excluded because of a misalignment during the compression. 3. methods the ecap die with a channels’ diameter of 11 mm and the channel and corner angles equal to 90◦ and 0◦, respectively, was used. back pressure regulated between 270 mpa and 590 mpa to maintain an average plunger pressure of 2800 mpa was applied during the ecap process. the inserted titanium rod-like sample, 35 mm long, was encapsulated in a lubricant-treated thin steel container in order to eliminate frictional forces. the details of the ecap process can be found in [5]. the ecap process was carried out at room temperature using route a. the number of passes of the billet through the die was four. the in situ compression tests were performed at room temperature using a hysitron pi 85 sem picoindenter (hysitron, minneapolis, usa) with a 20 µm flat-punch diamond tip. figure 2b shows the diamond tip approaching one of the fabricated micro-pillars. the true strain rate was around 5.2 × 10−4 s−1. the micro-pillars were compressed up to about 10 %. a special care was given to the alignment of the loading and micro-pillars’ long axes during loading. the loading data with an obvious difference in young’s moduli at the onset of loading and unloading were excluded. the grain morphology at the micro-pillar longitudinal sections were acquired using the fei quanta 3d equipped with an edax hikari ebsd (electron backscatter diffraction) detector (edax, new jersey, usa) and analyzed with a help of edax oim analysis v8 software. 4. results and discussion the grain morphology of the micro-pillars (not shown here) corresponds to the micro-structure of the ecaped titanium samples studied in [5]. herein, it is worth mentioning that the mean grain size determined in [5] using an x-ray diffraction (xrd) (crystallite size calculated from line broadening of the xrd peaks) was 50 nm and the prevailing grain size determined in [5] using a transmission electron microscope was in the range 50 nm – 300 nm. the ebsd map of an ecaped ti sample processed identically as the present sample can be found in [14]. as of the present micro-samples, there are regions of recrystallized grains with a mean grain size of 200 nm and regions of non-recrystallized grains. the grain size of the largest non-recrystallized grains does not exceed 3 µm. the micro-samples contain only a few large grains and these grains still contain regions with various crystallographic orientations. in addition, an arbitrary micro-pillar cross-section area (3.5 µm × 3.5 µm) contains at least five grains. low-angle grain boundaries prevail among the noncrystallized grains. furthermore, it can be assumed that high deformation energy is accumulated in the regions of non-recrystallized grains. figure 3 shows the diagram of true compression stress versus true strain for id and ed micro-pillars, id and ed macrosamples and titanium bulk sample. in fig. 3, the stress-strain data belonging to compression loading of ecaped titanium samples denoted as id and ed macro-samples and titanium macro-sample with no ecap treatment denoted as ti bulk (coarse-grained sample) were taken from our previous study [5]. the id, ed and ti-bulk macro-samples had a shape of a cylinder with a diameter of 3 mm, a height of 6 mm and their rotational axes along id, ed and ld directions, respectively. all the samples were mechanically tested at room temperature. in [5], it is shown that at macroscopic level, mechanical properties of ecaped titanium are anisotropic. the maximum compression stress in id direction is about 24 % higher than the maximum compression stress in ed direction (see fig. 3). figure 3. the diagram true compression stress versus true strain for id and ed micro-pillars, id and ed macro-samples and titanium bulk sample. at microscopic level, in case of id and ed micropillars, maximum stress in id micro-pillar is still greater than maximum stress in ed micro-pillar but the stress ratio, (σid)max/(σed)max, is close to 1. there is a question whether the micro-samples are less anisotropic (in compression) than the macro-samples. however, at this stage of our study, it is not possible to make any conclusion. stress-strain data for more micro-pillars are necessary in further research. if it turns out that there really is less compression anisotropy in the micro-pillars compared to the macrosamples, then a potential explanation might be as follows: the effect of anisotropy in compression, i.e. yield strength depends on the compression direction, is likely due to anisotropic distribution of dislocations in the ecaped samples. if the size of the ti ecaped samples decreases, as in the present study, to units of microns, then the plastic deformation (during compression) is associated with the creation and movement of newly formed dislocations, rather than with motion and interactions of already existing dislocations (as it is in bulk samples) [15]. this means there is a weaker 3 d. vokoun, j. maňák , k. tesař, s. habr acta polytechnica effect of the existing anisotropic distribution of dislocations. this might be a reason for the potentially less significant effect of compression anisotropy in the micro-pillars, compared to the macro-samples. another distinct phenomenon in fig. 3 is the samplesize effect, obvious when comparing the true stress true strain curves belonging to the micro-pillars and the ecaped titanium macro-samples. it is assumed that the mean grain size of the compared samples is approximately identical. xu et al. [16] observed the sample-size effect for pure aluminum in the form of softening behavior (negative work-hardening rates) and a non-significant yield stress change with a reduction of the specimen size. however, for the fixed small grain sizes, the authors did not test sample dimensions for which the sample-size effect is significant i.e. when the sample size decreases to dimensions comparable to about ten times the average grain size. in addition, the crystal structure of aluminum is cubic whereas the crystal structure of titanium is hexagonal at room temperature. in the present study, both yield stress and the work-hardening rate increased remarkably with the reduction of sample dimensions to only units of microns. these observations are perfectly consistent with compression tests done on ecaped mg-5 wt.% al micro-fabricated into a shape of micro-pillars [17]. the sample-size effect observed in the present study is based on the dislocation starvation theory [15] and the effect of grains close to the sample’s surface since the volume ratio of surface grains increases with the sample size decrease. the theoretical background of the observed sample-size effect might be found also in [18]. 5. conclusions ecaped titanium (grade 2) is successfully reproduced at room temperature (route a, four passes) using an ecap-back-pressure die with a channels’ diameter of 11 mm and the channel and corner angles equal to 90◦ and 0◦, respectively. a concern was raised that ecaped titanium micro-samples in compression might be less anisotropic than the macrosamples. further study is necessary to clarify this concern. furthermore, comparing the compression stress – strain curves for ecaped titanium microsamples and macro-samples, both yield stress and the work-hardening rate increase remarkably with the reduction of sample dimensions. acknowledgements support of the work under the project ga17-05360s is acknowledged. references [1] j. maňák, d. vokoun. microbending experiments on pure magnesium with nonbasal slip orientation. materials 11(8):1434, 2018. doi:10.3390/ma11081434. 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[18] v. kafka, d. vokoun. nanowires: diameter-dependent strength criterion. iop conference series: materials science and engineering 504:012110, 2019. doi:10.1088/1757-899x/504/1/012110. 5 https://doi.org/10.1016/j.msea.2007.12.038 https://doi.org/10.1016/j.msea.2015.08.060 https://doi.org/10.1109/inec.2010.5424788 https://doi.org/10.1088/1757-899x/504/1/012110 acta polytechnica 27(0):1–5, 2020 1 introduction 2 samples 3 methods 4 results and discussion 5 conclusions acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2017.13.0102 acta polytechnica ctu proceedings 13:102–108, 2017 © czech technical university in prague, 2017 available online at http://ojs.cvut.cz/ojs/index.php/app wang tile size in terms of circular particle dynamics david šedlbauer czech technical university in prague, faculty of civil engineering, department of mechanics, thákurova 7, 166 29 prague 6, czech republic correspondence: david.sedlbauer@fsv.cvut.cz abstract. the main advantage of the wang tiling concept for material engineering is ability to create large material domains with a relatively small set of tiles. such idea allows both a reduction of computational demands and preserving heterogeneity of a reconstructed media in comparison with traditional cell concepts. this work is dealing with a random heterogeneous material composed of monodisperse circular hard particles within a matrix. the wang tile sets are generated via algorithm with molecular dynamics and adaptive boundaries approach. even though previous works proved usefulness of the wang tiling for material reconstruction, still plenty of questions remain unanswered. in here we would like to provide simulations with emphasis on the overall particle distribution and the ratio of hard disc number to tile size. the results and discussion should followers help with settings of both tile generations and the tiling algorithms when creating samples of various degree of heterogeneity. keywords: wang tiling, random heterogeneous material, tile size, adaptive boundaries. 1. introduction despite the exponentially increasing computing power, simulations and modelling of random heterogeneous materials on microscale have been performed mostly using the periodical unit cells (pucs) forming a representative sample. unfortunately the nature of the puc is in contradiction with the terms random and heterogeneity typicaly characterizing investigated material. the main idea of the wang tiling [1] is to stack large (infinite) aperiodic plane via relatively small set of tiles. within last few years a group of researches has been dealing with the wang tiling approach as a tool for random heterogeneous material modelling. there is no need to stack strictly aperiodic tiling for usage in material engineering, therefore stochastic cshd tiling algorithm has been utilized [2]. in general, algorithms for generation of the tile sets with circular particles can be divided into two groups based on the nature of a particle positioning. the pilot work [3] linking the wang tile principles with material engineering, utilized static approach, whereas in this paper a process based on the molecular dynamics is utilized. the sets for tiling shown on the following figure 1 were generated with the same algorithm. the first tiling is stacked of the set, where each tile includes only one particle. the second tiling is composed of the set, where size of each tile is twice bigger than in the first case. therefore tile contains more particles, but the overall volume fraction remains the same for both tilings. the last tiling is made of the set with 36 particles per tile, but finial domain exhibits the same fraction as well. figure 1. examples of tilings with: 1 inclusion/tile (30×30 tiles), 4 inclusions/tile (15×15 tiles), 36 inclusions/tile (5×5 tiles); wang tiling set of 8 tiles over 4 codes. 102 http://dx.doi.org/10.14311/app.2017.13.0102 http://ojs.cvut.cz/ojs/index.php/app vol. 13/2017 wang tile size in terms of circular particle dynamics it is obvious, that the first simulation represents s arrangement, whereas the second and the third tiling form rather r arrangement (particles randomly over whole area) [4]. one can also easily observe differences in the degree of heterogeneity, where in the first sample there are visible interferences and the structure is closer to the ordered system in comparison with the last tiling with obvious particles clusters. therefore the main goal of the presented contribution is to provide some simulations and build up recommendations for particle-tile size relation with emphasis on overall arrangement and different states of heterogeneity/ordering. these suggestions should help followers with the settings of a generation algorithm when tiling a material model with required geometrical properties. the contribution is organized as follows. the next chapter briefly presents algorithm for the wang tile set generation. the beginning of the third part is devoted to tile size dimensions in terms of various packing states. the last but one subchapter presents simulations and include the discussion of obtained results. in the conclusion the summary of the paper with possible ways of the future work is presented. 2. algorithm for tile generation the basis of the wang tiling is represented by set of building blocks, wang tiles. these can be seen originally as squares with codes on edges, which enables to keep compatibility of neighbours in the final tiling. during years there were works attempting to find the minimal set of tiles for aperiodic tiling. the last known minimal aperiodic set of tiles consists of eleven pieces over four colours (codes) [5]. since for material engineering there is no need to keep strictly aperiodic tiling, we utilize stochastic cshd algorithm [2]. considering four different possible codes on edges, the minimal set consists of eight tiles. for the group of materials composed of circular hard particles within a matrix an algorithm inspired by molecular dynamics has been successfully utilized, where particles move and collide during reconstruction process until the stopping criterion is reached. the similar principle (modified lubachevskystillinger [6]) utilizes the algorithm for tile set generation. in the begging of the process, the particles zero initial radii are randomly thrown into each square tile and assigned with random velocity vectors. the particles (inclusions) grow and collide with walls and with each other during the motion until they have reached desired diameter and arrangement. the description of algorithm can be found in [7] together with recomanded optimized particle velocities. one of the main problems, the algorithm has to deal with, is the compatibility on tile edges/corners. if there is no trespassed particles, wang tiles are reduced only to separated systems with noticeable borders in final tiling. in order to avoid such an artificial artefact a concept of adaptive boundaries was developed [8]. in here the tile edges represents borders for figure 2. principle of adaptive boundaries. particles centres instead of particle surfaces. thus inclusions in the master tile are affected with a motion of particles in possible tile neighbours. therefore the process leads on one hand to higher computational demands connected with higher number of collisions between particles, on the other hand there is a system with adaptive boundaries reducing unwanted artificial periodicity. an example of generated tile set with highlighted adaptive boundaries is in figure 2. 3. simulations and discussion in this chapter simulations are presented ilustrating the tile generation together with tiled samples, where we would like to point out the problems user has to deal with and discuss possible solutions. 3.1. tile size and particle packing in the concept of the wang tiling every tile used to be of unit size. but for modelling, the base block has to have a size of certain value or at least to be in the certain ratio with other structural dimensions. the determination of the tile size can be divided into two consecutive steps. in the first one there is need to define minimal geometry demands, in order to be able to reach various states of packings. within the second step the size of tiles as well as tiling is compared to the characteristic structural sizes of a modelled sample or/and modified with emphasis on overall required material properties. this contribution deals mostly with the first condition. as mentioned in the problem definition, each wang tile could have different number of particles. such an assumption has to be taken into account for both full tiling and materials with lower packing density. the very first thing in a definition of the tile size should be identification of possible packing states. when the required material is fully packed or domain should have clusters of particles, it is necessary to cogitate the close dense packing [9]. since the dynamic algorithm involves adaptive boundaries, it is possible to achieve the maximal hexagonal dense packing of equal-sized inclusions with the volume fraction of 0.9069. nevertheless in order to be on the safe side with a definition of square tile dimensions, reader is referred to the densest packing of equal-sized disks in a 103 david šedlbauer acta polytechnica ctu proceedings case no. inclusions/tile tile size tiling 1 1 20 30×30 2 9 60 10×10 3 36 120 5×5 table 1. monodisperse tiling – settings. square volume [10, 11]. it has to be noticed that such an assembly leads to particle periodic artefacts and ordered homogenized samples, which are far away from the term of random or heterogeneous. for the investigated materials more common are so-called jammed states (mechanical stable states). the most disordered jammed state used to be called maximally random jammed state, which can replace the term random close packing. for more information regarding densest packing states the reader is referred to [9, 12], since in our simulation we will focused on tiling with lower particle fraction and higher degree of freedom. 3.2. tiles with the same number of equal-size inclusions in general it is better to have the same volume fraction in every tile. with this assumption the probability of accepting a next tile in tiling should be of value 0.5 in every step of the algorithm, assuring the material volume fraction. the term volume fraction, or in this case packing fraction, is defined as ratio of the sum of particle areas to overall space. the dimension of space is relative to tile size, therefore the number of particles within a tile defines (with respect to the required packing fraction) the size of each tile. in the following examples, where different number of particles in tiles are used, we will compare how this setting affects tiled model in terms of computer demands and simplified basic statistical description. for all analysed cases the packing fraction is 0.442. we want to show how different tiling could be produced with various tile size, therefore the packing fraction is set well below the fraction for maximal random dense packing in order to preserve the degree of freedom for particles. for clarity the size of a final tiling is 600×600 pixels/microns. there are three examples where the tile size increases gradually. in the first case we have only one inclusion in every tile. since the size of tile is 20 pixels, the final tiling consists of 30×30 tiles. in the tab. 1 the summary of investigated cases with settings definition is presented. in the figures 3 4 5 generated sets and examples of tilings with pore division and appropriate pore size distribution are shown [13]. (a). (b). (c). figure 3. the first setings – (a) generated set, (b) tiling with pores, (c) pore size distribution. in the first case, the tiling exhibits similarity to the material with lattice. the particles in every tile have limited freedom given by the tile size-particle radius ratio which is 0.375. thus the pore size histogram is quite narrow without existence of extra large or small pores. such setting is valid only for a reconstruction of domain with high-ordered particle system or with periodic arrangement. in the second investigated case, the obviously different arrangement of particles without large visible periodic artefacts is obtained. the diagram exhibits wide range of pore distribution, which was achieved due to more space for particle motion. nevertheless there is still very predominate pore size indicating lower level of disordering. despite the fact of wider pore distribution, the local particle clusters occured, caused by small number of particles within a tile and the nature of the stochastic tiling. 104 vol. 13/2017 wang tile size in terms of circular particle dynamics figure 4. the second setings – generated set, tiling with pores, pore size distribution. in the last tiling, where each tile includes 36 particles, we gain the system with pore distribution near to the normal distribution. this confirms the claim that the larger wang tile the more heterogenous is reconstructed material. moreover, the results from the two point probability function [9] of tiling with 36 particles exhibit reduction of unwanted secondary peaks, which indicates the decrease of periodicity, especially in comparison to the tiling composed with the puc system 6. please note that these figures were obtained as average from ten different tilings with the same size and the same number of tiles. the resolution (pixels/tile) was decreased to maintain the clarity of figures. nevertheless the definition of maximal or sufficient tile size should be in accordance with required mechanical properties which needs further statistical description and investigation. but one has to have in mind the increasing computational demands. figure 5. the third setings – generated set, tiling with pores, pore size distribution. figure 6. two point probability function:36 inclusions/tile – top puc tiling, bottom wang tilling. 105 david šedlbauer acta polytechnica ctu proceedings incl. per tile frac. bound collisions particle collisions summary collisions 1 0.049 39 4 43 2 0.098 72 29 101 3 0.147 96 78 174 4 0.196 128 137 265 5 0.245 151 248 399 6 0.295 180 441 621 7 0.344 212 574 786 8 0.393 243 829 1072 table 2. computer demands – number of collisions. inclusions per tile bound collisions particle collisions summary collisions 1 46 119 165 4 117 352 469 36 950 7683 8633 table 3. computer demands – number of collisions of representatives despite the author’s best effort, the whole process definitely lacks internal optimization of the code to allow comparison in terms of the computer demands. nevertheless, the computationally most demanding task is in a correlation with number of collisions. after collision of any particle with wall or another particle, velocity vectors have to be recalculated. the table 2 presents summary of number of collisions for increasing number of particles within a tile of the same size. it has to be noted that these values are averaged over ten measurements. there is obvious interference between number of wall rebounds and particle collisions. with packing fraction around 0.2 the number of both collision types is approximately equal. for lower fraction there is much more free space and particles do not colide so often. with higher fraction the situation is reversed. in table 3 number of collisions are compared for the three representative cases. since the packing fraction was 0.442, there is a higher probability of particleparticle collisions then for rebounds from walls. if we consider the first case as a reference one, then the performed representative results together with internal checks indicate following: the number of particleparticle collisions grow approximately with the same rate as tile size till the tiled area is ten times larger compared with the reference tile. the algorithm for particle motion begins with randomized velocity vectors. if we want to regulate computer demands, several possibilities exist. the easiest one is to generate wang tiles of a small size, but with such settings the tiling exhibits periodicity artefacts. moreover the final reconstructed material sample of the same size requires tiling with higher number of tiles. therefore savings on tile generation can be consumed on tiling algorithm. another possibility is to modify algorithm for tile generation, where velocity vectors will be optimized and more reduced. the similar approach has already been used in optimization of tiles with border areas [7]. there is also an option based on the semi-dynamic algorithm, where in the first step the particles are placed in a tile with certain initial radius. but in this process computer power will be used for initial arrangement of particles, thus savings are questionable. 3.3. tiles with different number of equal-size inclusions the wang tiling set, where each tile includes the same number of particles, fits either domains with lattices or materials with lower degree of heterogeneity [14]. if the reconstructed material sample should have some particle clusters or high degree of heterogeneity, the best way is to utilize the wang tiling with different number of particles in tiles. nevertheless with different tile particle packing other issues arise. the most critical one is dsign of tiling algorithm in terms of prescribed overall volume fraction. when the set includes very varying tiles and tiling is made of relatively small number of tiles, the tiling algorithm cannot assure required volume fraction because of the uniform acceptance probability. this problem can be solved with observation of tiling process, where acceptance probability varies over tile set. after a certain part of the wang tiling is complete, the temporary packing fraction can be recalculated and tiles will be assigned with an updated acceptance probability. another solution might be to generate a new set with similar distribution of particles over tiles. the last but not least option is to transfer particles from tile to tile within the tiling algorithm. following examples show tiling with very individual particle packing in each tile. it has to be noticed that these examples were made without any improvement in tiling algorithm. thus the final pattern for each case exhibits different overall particle fraction. obviously we gain in both cases samples with high degree of heterogeneity. this statement is verified using visual check and the pore distribution histogram with representation of both small and large pores. occurrence of the large pores is caused by tiles with minimal particle fraction and another phenomenon – empty tiles. due to the usage of adaptive boundaries approach, empty tile should not be considered as a periodic cell. however such tile includes an area where any particle from tiling can never get to. one empty tile in the set is suitable for tiling diversity. more empty tiles result in redundancy of full basic wang tile set. but existence of the same tiles in a set also offers an opportunity to increase acceptance probability 106 vol. 13/2017 wang tile size in terms of circular particle dynamics figure 7. artifitial tiling 10×10 from basic set including 10, 8, 6, 4, 2, 1, 1, 0 equal-sized discs. of tiles without any intervention in tiling algorithm. the similar postulate is valid for expanded tile set as well as identical tiles with particles within. 4. conclusions the concept of wang tiling has already been rooted in the family of approaches, which are suitable for random heterogeneous material modelling. it brings possibilities of aperiodic tiling of large material domains with a relative small set of basic tiles. in this contribution we were dealing with a random heterogeneous domain composed of equal-sized hard discs within a matrix. the basic set for tiling of such artificial material sample was obtained via algorithm based on molecular dynamic and adaptive boundaries approaches. even though this type of generation exhibits potential, there has been so far modelled only general samples without tile size investigation. therefore we focused in this contribution on specific types figure 8. artifitial tiling 10×10 from basic set including 8, 8, 8, 8, 0, 0, 0, 0 equal-sized discs. of material domains with various particle configurations and volume fractions. the final tilings were constructed using the stochastic cshd algorithm. in the first part of simulations, generated wang tile set consists of tiles with the same particle fraction. the second part presented analysis of tiling with tiles of large differences in number of particles inside. based on presented examples of illustrative tilings, one can show, how the size of tiles (and appropriate number of particles within) affects possibilities of particle arrangement and ordering of the obtained sample. these tests proved the variability of wang tiling in connection with the tile generator based on molecular dynamics. concurrently the discussion over occurring arrangement could help followers with settings of both tiling algorithm and the generator. the investigated material contained only monodisperse hard particles. but the wang tile approach and the described generator are suitable for a wide range of 107 david šedlbauer acta polytechnica ctu proceedings material domains. therefore future work will focus on other material models with various particle numbers and shapes. within the next steps we will also integrate more statistical descriptions in order to quantify and compare material characteristics. finally, whole process will be extended to 3d. acknowledgements the financial support of the grant agency of the czech technical university in prague, grant no. sgs17/042/ohk1/1t/11, is gratefully acknowledged. references [1] h. wang. proving theorems by pattern recognition ll. bell labs technical journal 40(1):1–40, 1961. doi:10.1002/j.1538-7305.1961.tb03975.x. [2] m. f. cohen. wang tiles for image and texture generation. second printing. association for computing machinery, 2003. [3] j. novák. microstructural enrichment functions based on stochastic wang tilings, 2013. [4] t. müllner. characterization of microscopic disorder in reconstructed porous materials and assessment of mass transport-relevant structural descriptors. new journal of chemistry 40(5):3993–4015, 2016. doi:10.1039/c5nj03346b. [5] e. jeandel. an aperiodic set of 11 wang tiles. arxiv preprint arxiv p. 1506.06492, 2015. [6] b. d. lubachevsky. geometric properties of random disk packings. journal of statistical physics 60(5-6):561–583, 1990. doi:10.1007/bf01025983. [7] d. šedlbauer. optimized wang tiles generation for heterogeneous materials modelling. applied mechanics and materials pp. 183–190, 2016. doi:10.4028/www.scientific.net/amm.825.183. [8] d. šedlbauer. adaptive boundaries of wang tiles for heterogeneous material modelling. advanced materials research pp. 159–166, 2017. doi:10.4028/www.scientific.net/amr.1144.159. [9] s. torquato. random heterogeneous materials: microstructure and macroscopic properties. springer science & business mediat, 2013. [10] p. szabó. packing up to 200 equal circles in a square. models and algorithms for global optimization optimization and its applications 2007. doi:10.1007/978-0-387-36721-7_9. [11] k. w. desmond. random close packing of disks and spheres in confined geometries. physical review e 80(5):051305, 2009. doi:10.1103/physreve.80.051305. [12] y. jiao. characterization of the structure of heterogeneous materials and particle packings. princeton university, 2010. [13] a. rabbani. an automated simple algorithm for realistic pore network extraction from micro-tomography images. journal of petroleum science and engineering 123:164–171, 2014. doi:10.1016/j.petrol.2014.08.020. [14] i. schenker. quantification of the heterogeneity of particle packings. physical review e 80(2):021302, 2009. doi:10.1103/physreve.80.021302. 108 http://dx.doi.org/10.1002/j.1538-7305.1961.tb03975.x http://dx.doi.org/10.1039/c5nj03346b http://dx.doi.org/10.1007/bf01025983 http://dx.doi.org/10.4028/www.scientific.net/amm.825.183 http://dx.doi.org/10.4028/www.scientific.net/amr.1144.159 http://dx.doi.org/10.1007/978-0-387-36721-7_9 http://dx.doi.org/10.1103/physreve.80.051305 http://dx.doi.org/10.1016/j.petrol.2014.08.020 http://dx.doi.org/10.1103/physreve.80.021302 acta polytechnica ctu proceedings 13:103–109, 2017 1 introduction 2 algorithm for tile generation 3 simulations and discussion 3.1 tile size and particle packing 3.2 tiles with the same number of equal-size inclusions 3.3 tiles with different number of equal-size inclusions 4 conclusions acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2018.15.0109 acta polytechnica ctu proceedings 15:109–113, 2018 © czech technical university in prague, 2018 available online at http://ojs.cvut.cz/ojs/index.php/app constitutive model for timber fracture used for fe simulation of lvl arch eliška šmídová∗, petr kabele czech technical university in prague, faculty of civil engineering, thákurova 7, 166 29 prague 6, czech republic ∗ corresponding author: eliska.smidova@fsv.cvut.cz abstract. non-linear finite element simulation of four-point bending test of a small-size arch is described in this paper. the arch of 780mm span is made of yellow poplar (liriodendron tulipifera) laminated veneer lumber (lvl) and it is manufactured with a through crack parallel to fibers in the middle of its crown. 2d homogeneous orthotropic constitutive model of tensile and shear fracture in timber that has been recently developed and implemented into atena® finite element software by the authors is used for the numerical calculation. both standard and compact tension (ct) test results are used for the material model calibration. results show that the model successfully reproduces the increasing part of the load-displacement response. furthermore, the model can capture the most distinctive features of the crack pattern. keywords: timber, 2d constitutive model, orthotropic elasticity, orthotropic tsai-hill failure criterion, fixed smeared crack model, traction-separation law, compact tension test. 1. introduction concerning environmental impact, energy efficiency and handling costs, timber and engineered wood products represent a promising alternative to the most frequently used structural materials such as concrete or masonry. the wood-derived products of laminated veneer lumber (lvl), cross-laminated timber (clt) or glue-laminated timber (glt) provide increased strengths and stiffness compared to solid wood. timber and its engineered products are typical for orthotropic behavior. sawn timber has three mutually perpendicular axes in which we distinguish independent mechanical properties: longitudinal (parallel to grain), radial and tangential. let us note that properties, such as tensile strength and modulus of elasticity, related to the direction parallel with fibers substantially exceed those related to the other directions [1]. natural variability in material properties of timber has been addressed by some authors [2, 3]. this material feature is often tackled by weibull strength theory [4]. exposed to tensile and shear loading, timber exhibits linear elastic behavior up to failure that is followed by brittle fracture. in contrast, compression in timber leads to a ductile response [5]. various failure criteria have been developed for timber [6, 7] out of which a commonly applied one corresponds to a tsai-hill formulation [8]. being a complex biological structure, timber contains natural imperfections such as knots, pitch pockets, reaction wood or cross grain. because such inhomogeneities have a negative impact on the mechanical properties, structural timber, especially the engineered one, is usually carefully selected and processed to provide an improved structural behavior [1]. influence of some of these weaknesses (e.g., distribution of knots and finger joints) to strength and failure pattern of lvl beams is tackled in [9], among others. two-dimensional and three-dimensional phenomenological constitutive models capturing timber behavior under multi-axial stress state are based on elasticity [10], classical plasticity [11], multisurface plasticity [12], smeared crack concept [5, 13], smeared fictitious reinforcing fibers [14] and interface schemes [15–17]. 2. constitutive model for non-linear finite element simulation of 4pbt of yellow poplar (liriodendron tulipifera) laminated veneer lumber (lvl) arch, we use the 2d homogeneous orthotropic constitutive model of tensile and shear fracture in timber modified for available input data. the model has been recently developed and implemented into atena® finite element software [18] by the authors [13, 19]. 2.1. model properties let us review the properties of the modified constitutive model. it captures: • material orthotropy for small deformations in both linear and non-linear range • elastic behavior until the anisotropic failure criterion is fulfilled • inelastic behavior after failure criterion is satisfied • post-failure response for cracks along the grain • unloading/reloading behavior the post-cracking response is treated by the fixed smeared crack model [20] where reinhardt-hordijk function is used [21]. 109 http://dx.doi.org/10.14311/app.2018.15.0109 http://ojs.cvut.cz/ojs/index.php/app eliška šmídová, petr kabele acta polytechnica ctu proceedings 2.2. model components the properties of the model described in section 2.1 are implemented in the form of (i) failure criterion and (ii) cohesive law. 2.2.1. failure criterion with regard to available input data from literature, we use the most frequently used failure theory for wood. it was proposed by norris [8] and it is also referred to as the tsai-hill criterion. in a 2d stress state, it has the form of equation eq. 1. ( σx ftx )2 − σxσy ftxfty + ( σy fty )2 + ( τxy fs )2 = 1 (1) 2.2.2. cohesive law cohesive (traction-separation) law relates crack surface normal traction tn to relative crack normal displacement δn with respect to normal crack direction (n). the cohesive law is expressed by eq. 2 as follows: tn(δn) =  fty (( 1 + ( c1δn δn,crit )3 ) exp ( c2δn δn,crit ) − exp (−c2) ( 1 + c3 1 ) δn δn,crit ) for δn ∈ [0, δn,crit) 0 for δn > δn,crit (2) 3. calibration of the model the model requires in total ten input parameters. based on data available in the literature, we obtained material input parameters for yellow poplar lvl both directly from standard tests and employing inverse analysis of traction-separation relationship from compact tension (ct) test (figure 1). representing first part of input parameters for fe simulation of lvl arch, selected standard tensile and shear experimental results [1, 17] are shown in table 1. within experimental test campaign of yellow poplar lvl arch available in literature [22], only two load vs. crack mouth opening displacement (cmod) responses of ct specimens cut from the arch base and three load vs. displacement responses of ct specimens cut from the arch crown were reported. it is important to note, that displacement of the latter curves is not relevant being biased by the stiffness of the load-cell. thus, we estimated the parameters defining cohesive law in crack-normal direction, i.e. δn,crit, fty, c1, c2, in two steps. first, for each of two responses of ct specimens from the arch base, we get the average of four tractionseparation curves using sequentially linear approach to inverse analysis including multi-pass enhancement [23]. consequently, we find the best fit of the cohesive law eq. 2 for the average curve. similarly, we find the best fit for the average of all eight traction-separation curves. the overview of cohesive law parameters figure 1. compact tension (ct) test set up: w = 46mm, a = 20mm and thickness is 11mm. ey ex ftx fs gxy νxy [mpa] [mpa] [mpa] [mpa] [mpa] [-] 450 14600 110 8.2 899 0.32 table 1. average values of input data for yellow poplar lvl from standard tests [1, 17]. calculated for ct specimens from the arch base is shown in table 2 together with r2 statistics that witness a good fit of corresponding responses from finite element simulation and experiment. second, for each of three values of maximum load from response of ct specimens sawn from the arch crown, we estimate tensile strength parameter fty considering remaining parameters of {c1, c2, δn,crit} to be equal to those calculated in step one for ct specimens from arch base. for this purpose, we apply the trial-and-error method to fe inverse analysis. a considerable difference between maximum loads applied to ct specimens cut from the arch base and crown can be observed. therefore, we use the estimations of cohesive law parameters calculated for the arch crown (table 3) expecting them to be more representative for fe simulation of the crack pattern spreading across the crown. the responses (load vs. cmod) and traction-separation relationships (stress vs. cod) from (i) experiments, (ii) sequentially linear or (iii) trial-and-error approach to inverse analysis are displayed in figure 2 and figure 3, respectively. 4. fe simulation of 4pbt of the arch the subject of the fe simulation is a four-point bending test (4pbt) of yellow poplar lvl arch with a through pre-crack parallel with grain in the middle of its crown [22, 24]. teflon paper was placed between two laminations preventing them from being glued. the arch has the span of 780mm, cross-section height and width of 57mm and 25mm, respectively, and lamina thickness of 3.2mm. the overall test setup is depicted in figure 4. we generated regular 2d fe mesh composed of approx. 3.2mm by 3.2mm quadrilaterals aligned with 110 vol. 15/2018 constitutive model for timber fracture response δn,crit fty c1 c2 r2 [mm] [mpa] [-] [-] [-] max 1.41 15.90 4.10 7.90 0.93 avg 1.07 14.00 4.10 8.60 − min 0.57 14.00 4.10 10.00 0.96 table 2. parameters of cohesive law calculated by sequentially linear approach to inverse analysis [23] of load vs. cmod responses of ct specimens cut from arch base. {δn,crit, c1, c2} fmaxty favgty fminty {[mm], [−], [−]} [mpa] [mpa] [mpa] {1.41, 4.10, 7.90} 2.60(#1) 2.10(#2) 1.60 {1.07, 4.10, 8.60} 2.90(#3) 2.30(#4) 1.80 {0.57, 4.10, 10.00} 3.90 3.00 2.30 table 3. parameters of cohesive law estimated by trial-and-error approach to inverse analysis of max. load responses of ct specimens cut from arch crown. the arch outline and lamina. we assigned fiber orientation to each fe as an input material property. as the proposed material model treats post-cracking behavior by the smeared crack approach, we modeled the pre-crack as a missing row of fe elements. let us note, that in this way we get the same results as for the pre-crack modeled as a row of fes with an unloaded smeared crack at the beginning of a numerical simulation. we ran fe simulations of lvl arch for each of 9 estimates of input parameters defining the traction-separation relationship for the arch crown table 3. 5. results and discussion we present four most relevant results are presented in figure 5 in the form of load vs. midspan deflection. out of them, we calculated the best response for sim. (#1) (i.e., maximum estimated critical crack opening displacement δn,crit = 1.41mm and tensile strength fmaxty = 2.6mpa). regarding significant variability of timber material parameters, the results show that the model can reproduce the increasing part of the load-displacement response well. in order to obtain statistically more relevant results, an experimental campaign containing numerous specimens for each type of test needs to be conducted. comparison of crack pattern across arch crown from experiment (figure 6) and fe simulation (figure 7) shows that the material model can capture both the crack propagation from the pre-crack tips (primary crack) and crack initiation of the crack below the precrack (secondary crack). however, the model neglects initiation of the crack above the pre-crack (tertiary crack). the short crack spacing of only twice the lamina thickness between primary and tertiary crack can cause that stored energy is released by opening figure 2. load vs. cmod responses of ct specimens cut from arch base and crown: (i) experimental (exp.), (ii) numerically simulated by means of sequentially linear approach to inverse analysis (sim.) and (iii) estimated by trial-and-error approach to inverse analysis (estim.). and spreading of the primary crack instead of the adjacent tertiary crack initiation and growth. in the end, we can compare the results for yellow poplar lvl arch from smeared crack model to the results calculated by interface scheme [22, 24]. to this end, let us briefly describe the latter approach first. interface elements were placed between the lamina and the initial crack was modeled as a gap of finite length separating the arch layers. interface behavior was governed by (i) modulus of elasticity and (ii) maximum tensile strength, both in the direction perpendicular to fibers, and (iii) fracture toughness acquired from compact tension specimens cut from arch crown [16]. the response obtained by the interface scheme is shown in figure 5 as sim. interf. this curve represents a weak reproduction of the response compared to sim. (#1) to sim. (#4). on the other hand, the interface approach can capture both the primary crack spreading and the secondary crack initiation and growth. discrepancies between the two approaches can be attributed to: (1.) significant variability of timber material parameters that needs to be handled appropriately. (2.) specific acquisition of the most relevant input parameters that control the crack initiation and post-cracking behavior. 111 eliška šmídová, petr kabele acta polytechnica ctu proceedings figure 3. traction-separation relationships (stress vs. cod) of ct specimens cut from arch base and crown: (i) experimental (exp.), (ii) numerically simulated by means of sequentially linear approach to inverse analysis (sim.) and (iii) estimated by trial-and-error approach to inverse analysis (estim.). figure 4. scheme of 4pbt of yellow poplar lvl arch with a through pre-crack in its crown. 6. conclusion we used 2d homogeneous orthotropic constitutive model of tensile and shear fracture in timber for fe simulation of small-size yellow poplar lvl arch subjected to four-point bending. we presented the main features of the model. input data were obtained both (i) directly from standard tensile and shear tests and (ii) by applying inverse analysis to compact tension (ct) test results for yellow poplar laminated veneer lumber (lvl). for the limited availability of relevant experimental data from ct test results, we calculated the estimation of parameters that define traction-separation relation. numerical simulations of yellow poplar lvl arch confirm that the applied constitutive model can capture both the increasing part of the load-displacement response and pre-crack spreading together with the initiation of a secondary crack. we compared the results obtained by utilizing the proposed model to those calculated by interface approach. both models can predict primary and secondary crack initiation figure 5. load vs. midspan deflection response of 4pbt of lvl arch obtained from experiments (exp.) and fe simulations (sim.). i i.i. ii. iii. figure 6. arch crown cracking (primary i., secondary ii. and tertiary iii.) during experiment at two load levels. and development well. on the other hand, the interface model yields a weak load vs. midspan deflection response. considerable variability of timber material characteristics requires both statistically significant results for each type of experiment and an appropriate statistical approach to obtain relevant results from numerical models. list of symbols c1, c2 material parameters controlling the shape of the softening curve [–] δn crack opening displacement [mm] δn,crit critical value of crack opening displacement [mm] ex modulus of elasticity along the grain [mpa] 112 vol. 15/2018 constitutive model for timber fracture figure 7. arch crown cracking in fe simulation: primary and secondary crack. ey modulus of elasticity across the grain [mpa] ftx tensile strength parallel with grain [mpa] fty tensile strength perpendicular to grain [mpa] fs shear strength [mpa] gxy shear modulus [mpa] νxy poisson’s ratio [–] r2 coefficient of determination [–] σx normal stress parallel with grain [mpa] σy normal stress perpendicular to grain [mpa] τxy shear stress [mpa] acknowledgements the support by the gačr grant no. 18-05791s is gratefully acknowledged. references [1] wood handbook wood as an engineering material. forest product laboratory, 2010. [2] t. tannert, t. vallee, s. franke, p. quenneville. comparison of test methods to determine weibull parameters for wood. world 15:19, 2012. [3] l. melzerova, m. sejnoha, p. klapalek. bending tests and simulations of glt beams. applied mechanics and materials 611:54–59, 2014. doi:10.4028/www.scientific.net/amm.611.54. [4] w. weibull. a statistical theory of strength of materials. in proceedings of the royal swedish institute. 1939. [5] s. holmberg, k. persson, h. petersson. nonlinear mechanical behaviour and analysis of wood and fibre materials. computers & structures 72(4):459–480, 1999. [6] b. kasal, r. j. leichti. state of the art in multiaxial phenomenological failure criteria for wood members. progress in structural engineering and materials 7(1):3–13, 2005. doi:10.1002/pse.185. [7] e. smidova, p. kabele. comparison of failure criteria for wood in tensile-shear stress state. acta polytechnica ctu proceedings 13:115, 2017. doi:10.14311/app.2017.13.0115. [8] c. b. norris. strength of orthotropic materials subjected to combined stresses, 1962. [9] l. melzerova, m. sejnoha. influence of distribution of finger joints and timber flaws on the damage evolution of laminated glued timber beams during four point bending. in wcte 2016. vienna, austria, 2016. [10] n. khorsandnia, h. r. valipour, k. crews. nonlinear finite element analysis of timber beams and joints using the layered approach and hypoelastic constitutive law. engineering structures 46:606–614, 2013. doi:10.1016/j.engstruct.2012.08.017. [11] b.-h. xu, a. bouchair, m. taazount, p. racher. numerical simulation of embedding strength of glued laminated timber for dowel-type fasteners. journal of wood science 59(1):17–23, 2013. doi:10.1007/s10086-012-1296-0. [12] p. mackenzie-helnwein, j. eberhardsteiner, h. a. mang. a multi-surface plasticity model for clear wood and its application to the finite element analysis of structural details. computational mechanics 31(1-2):204–218, 2003. doi:10.1007/s00466-003-0423-6. [13] e. smidova, p. kabele. constitutive model for timber fracture under tensile and shear loads. applied mechanics and materials 784:137–144, 2015. doi:10.4028/www.scientific.net/amm.784.137. [14] h. r. valipour, k. crews. efficient finite element modelling of timber beams strengthened with bonded fibre reinforced polymers. construction and building materials 25(8):3291–3300, 2011. doi:10.1016/j.conbuildmat.2011.03.017. [15] x. p. xu, a. needleman. continuum modelling of interfacial decohesion. solid state phenomena 35-36:287–302, 1993. doi:10.4028/www.scientific.net/ssp.35-36.287. [16] x.-p. xu, a. needleman. numerical simulations of fast crack growth in brittle solids. journal of the mechanics and physics of solids 42(9):1397–1434, 1994. doi:10.1016/0022-5096(94)90003-5. [17] b. kasal, r. blass. experimental and analytical investigation of crack development in composite reinforced laminated arch. materials and structures 46(1-2):173–180, 2013. doi:10.1617/s11527-012-9892-4. [18] j. cervenka, l. jendele, v. cervenka. atena program documentation, part 1, theory, 2008. [19] e. bartunkova. constitutive model of timber. ph.d. thesis, czech technical university in prague, 2013. [20] a. hillerborg, m. modeãľr, p.-e. petersson. analysis of crack formation and crack growth in concrete by means of fracture mechanics and finite elements. cement and concrete research 6(6):773–782, 1976. [21] h. reinhardt, h. cornelissen, d. hordijk. tensile tests and failure analysis of concrete. journal of structural engineering 112(11):2462–2477, 1986. [22] r. a. blass. laminated wooden arches reinforced with glass fiber rods. ph.d. thesis, the pennsylvania state university, 2011. [23] j. vorel, p. kabele. inverse analysis of traction-separation relationship based on sequentially linear approach. arxiv:180208040 [cs] 2018. arxiv: 1802.08040. [24] b. kasal, r. blass. experimental and analytical investigation of crack development in composite reinforced laminated arch. materials and structures 46(1-2):173–180, 2013. doi:10.1617/s11527-012-9892-4. 113 http://dx.doi.org/10.4028/www.scientific.net/amm.611.54 http://dx.doi.org/10.1002/pse.185 http://dx.doi.org/10.14311/app.2017.13.0115 http://dx.doi.org/10.1016/j.engstruct.2012.08.017 http://dx.doi.org/10.1007/s10086-012-1296-0 http://dx.doi.org/10.1007/s00466-003-0423-6 http://dx.doi.org/10.4028/www.scientific.net/amm.784.137 http://dx.doi.org/10.1016/j.conbuildmat.2011.03.017 http://dx.doi.org/10.4028/www.scientific.net/ssp.35-36.287 http://dx.doi.org/10.1016/0022-5096(94)90003-5 http://dx.doi.org/10.1617/s11527-012-9892-4 http://dx.doi.org/10.1617/s11527-012-9892-4 acta polytechnica ctu proceedings 15:109–113, 2018 1 introduction 2 constitutive model 2.1 model properties 2.2 model components 2.2.1 failure criterion 2.2.2 cohesive law 3 calibration of the model 4 fe simulation of 4pbt of the arch 5 results and discussion 6 conclusion list of symbols acknowledgements references tm-2018.pdf acta polytechnica ctu proceedings doi:10.14311/app.2020.26.0039 acta polytechnica ctu proceedings 26:1–6, 2020 © czech technical university in prague, 2020 available online at https://ojs.cvut.cz/ojs/index.php/app modelling of crack propagation: comparison of discrete lattice system and cohesive zone model karel mikeša,∗, franz bormannb, ondřej rokoša,b, ron h.j. peerlingsb a czech technical university in prague, faculty of civil engineering, department of mechanics, thákurova 7, 166 29 prague 6, czech republic b eindhoven university of technology, department of mechanical engineering, p.o. box 513, 5600 mb, eindhoven, the netherlands ∗ corresponding author: karel.mikes@cvut.cz abstract. lattice models are often used to analyze materials with discrete micro-structures mainly due to their ability to accurately reflect behaviour of individual fibres or struts and capture macroscopic phenomena such as crack initiation, propagation, or branching. due to the excessive number of discrete interactions, however, such models are often computationally expensive or even intractable for realistic problem dimensions. simplifications therefore need to be adopted, which allow for efficient yet accurate modelling of engineering applications. for crack propagation modelling, the underlying discrete microstructure is typically replaced with an effective continuum, whereas the crack is inserted as an infinitely thin cohesive zone with a specific traction-separation law. in this work, the accuracy and efficiency of such an effective cohesive zone model is evaluated against the full lattice representation for an example of crack propagation in a three-point bending test. the variational formulation of both models is provided, and obtained results are compared for brittle and ductile behaviour of the underlying lattice in terms of force-displacement curves, crack opening diagrams, and crack length evolutions. the influence of the thickness of the process zone, which is present in the full lattice model but neglected in the effective cohesive zone model, is studied in detail. keywords: lattice model, damage, finite element method, cohesive zone model, three-point-bending test, crack propagation. 1. introduction lattice models with dissipative interactions are often used for modelling of materials with discrete microstructures such as textile, paper, 3d printed materials, or concrete; see e.g. [1–3]. the advantage of lattice models lies in their ability to capture microstructural interactions individually, thus representing the geometry of the underlying micro-structure as well as the associated constitutive laws accurately and naturally. because non-local behaviour is important for mechanisms occurring in the process zone at the crack tip, discrete models are suitable for problems such as damage localization, crack propagation, and branching. lattice models suffer, on the other hand, from high computational and memory requirements, necessitating simplifications especially in cases in which multiple evaluations are required, such as optimization, material parameter identification, or stochastic simulations. although multiple options for building effective models exist, in this contribution we focus on a continuum approximation of the underlying microstructure with an embedded cohesive zone model. crack propagation and its cohesive properties are described in such an approach by a suitable traction-separation law, whereas the bulk behavior of the continuum may often be elastic. the aim of this paper is detail comparison of crack propagation for discrete lattice model and continuous cohesive zone approach. the accuracy and performance of cohesive zone model predictions are assessed on the example of crack propagation in a three-pointbending test, as shown in figure 1, by comparison with the exact underlying full lattice model. 2. full lattice model an admissible configuration of a rate-independent system of interest is specified by a state variable q = (r, z), where r stores the kinematic variables (displacement of nodes) and z the internal variables (damage in bonds). the energetic solution q is determined using the following incremental minimization problem at time tk q(tk) ∈ arg min q̂∈q πk(q̂; q(tk−1)), k = 1, . . . , nt , (1) with an initial condition q(0) = q0, where q denotes the space of all admissible configurations, and πk is an incremental energy defined as πk(q; q(tk−1)) = e(tk, q) +d(z, z(tk−1)). (2) the incremental energy consists of the potential (gibbs type) energy e and the dissipation distance d. 39 https://doi.org/10.14311/app.2020.26.0039 https://ojs.cvut.cz/ojs/index.php/app k. mikeš, f. bormann, o. rokoš, r.h.j. peerlings acta polytechnica ctu proceedings l h f ~ex ~ey ωa figure 1. geometry and boundary conditions of the considered three-point-bending test with a close-up of the full lattice representation. the potential energy reads e(tk, q) = v(q)− ft ext(tk)r, (3) where v is the internal free energy and f ext is a vector of external loads. the dissipation distance d(z2, z1) measures the minimum dissipation by a continuous transition between two consecutive states z1 and z2; for further details see [4] (section 3.2). it can be written as d(z2, z1) = 1 2 ∑ α,β∈bα dαβ(z2, z1), (4) where bα denotes the set of nodes connected with node α. dαβ is the dissipation distance for a single bond αβ, defined as follows dαβ(z2, z1) = { dαβ(ω2)−dαβ(ω1) if ω2 ≥ ω1 +∞ otherwise, (5) where dαβ(ω) reflects the amount of energy dissipated in that bond during a unidirectional damage process up to a given damage level ω. in each time step, the minimization problem of eq. (1) is solved to obtain a local minimum that satisfies the energy balance v(q(tk))− v(q(0)) + vard(q, tk) =wext(q, tk), (6) which equates the internally stored energy v(q) − v(q0) plus the dissipated energy vard(q, tk) = k∑ i=1 d(z(ti), z(ti−1)), (7) with the work performed by the external forces wext(q, tk) = k∑ i=1 1 2 [f(ti) + f(ti−1)]t[r(ti)− r(ti−1)]. (8) unless stated otherwise, the same material model is considered for each lattice bond. in undamaged state, it is defined by pair potential φαβ(lαβ) = 1 2e0a0l αβ 0 ( ε(lαβ) )2 , (9) where e0 is the young’s modulus and a0 the crosssectional area of the bond. l0 denotes the initial length parameter ε0 εf e0 a0 brittle 0.005 10ε0 1/ε0 1 ductile 0.005 100ε0 1/ε0 1 table 1. parameters of the lattice model. of the bond and ε(lαβ) = (lαβ − l0)/l0 is the bond strain. the internal energy is defined as v(r, z) = 1 2 ∑ α,β∈bα [ (1− ωαβ)φαβ(lαβ) ] . (10) the elastic energy of a bond φαβ is reduced by the damage variable ωαβ(ε), defined as ωαβ(ε) = { 1− ε0 exp ( − ε−ε0 εf ) if ε ≥ ε0 0 if ε < ε0, (11) where ε0 is the limit elastic strain, and the parameter εf characterizes the softening branch of the associated stress-strain diagram. the particular choice of material and geometric parameters corresponding to the lattice model are specified in table 1. for large lattice structures, the simulation must be controlled by the increment of dissipation, cf. e.g. [5], because both the (typically used) crack mouth opening displacement control (cmod) as well as crack tip displacement opening control (ctod) turned out to be insufficient to keep the energy balance of eq. (6) satisfied. 3. cohesive zone model the lattice problem as specified in figure 1 is approximated with an effective continuous finite element cohesive zone model (fe-cz). for that purpose, the following assumptions are adopted: (1.) only one crack is initiated; (2.) the crack trajectory is a straight line and the crack propagates vertically in the middle of the beam; (3.) all damage processes take place in the predefined crack path, whereas the rest of the domain remains elastic. 40 vol. 26/2020 comparison of discrete lattice system and cohesive zone model ωa r (elastic region) l h f γcz ωa l (elastic region) cohesive zone with traction separation law figure 2. geometry and boundary conditions of the finite element cohesive zone model (fe-cz) subjected to a three-point bending test: elastic domain (grey) and cohesive zone (dashed line). 3.1. model description the crack is represented as one cohesive zone, γcz, normal to the ~ex direction, which splits the considered domain ω into two symmetric parts ωl and ωr: γcz = { ~x ∈ r2 : x = xcz = 0, |y| ≤ h/2 } , ωl = { ~x ∈ r2 : −l/2 ≤ x < xcz, |y| ≤ h/2 } , ωr = { ~x ∈ r2 : xcz < x ≤ l/2, |y| ≤ h/2 } , ω = ωl ∪ ωr, (12) where xcz = 0 denotes the horizontal position of the vertical cohesive zone, cf. figure 2. the total free energy ψ of the fe-cz model consists of two contributions, i.e. ψ = ∫ ω\γcz ψe dω + ∫ γcz ψcz dγcz, (13) where ψe is the elastic strain energy density ψe = 1 2 ε : c : ε, (14) corresponding to a linear elastic isotropic material under plane strain assumptions, described through a fourth-order constitutive tensor c, even though the full lattice is not isotropic. in eq. (14), the elastic strain tensor is expressed as the symmetric part of the displacement gradient ~∇~u, i.e. ε = 1 2 ( ~∇~u+ ( ~∇~u )t ) . (15) the cohesive zone potential ψcz is a function of the displacement jump ~∆ = j~uk = ~ur − ~ul (16) between the two sub-domains ωr and ωl. due to the symmetry of the problem, the tangential component of the jump ∆t vanishes along the cohesive interface, i.e. ~ul y = ~ur y , on γcz, (17) meaning that the cohesive zone potential ψcz becomes a function of the normal crack opening ∆n only. the constitutive parameters of both models, i.e. c and ψcz(∆n), are fitted in section 3.2. for the solution, in each loading step the resulting total potential energy of eq. (13) is minimized with respect to the degrees of freedom associated with a discretized displacement field of the continuum model. in order to handle brittle behaviour and snap-back, cmod indirect control is used. 3.2. material parameter calibration both effective models, i.e. the cohesive zone potential ψcz and elasticity stiffness tensor c, are identified directly from the properties of the underlying lattice by homogenization. for that purpose, a single periodic unit cell is considered, as shown in fig. 3. note that the cross-sectional areas of the horizontal and vertical bounding links are reduced by a factor of 0.5 to account for periodicity. l0 l0 ∆n ∆n figure 3. the geometry of unit lattice used for calibration of elastic properties and traction-separation law. by homogenization, the effective stiffness tensor can be assembled and compared with the plane strain formula for hooke’s law to obtain the elastic coefficients. in particular, the effective young’s modulus reads eiso = 5 6 e0a0 l0 (1 + √ 2/2), (18) whereas poisson’s ratio ν is fixed at ν = 1/4 for the given lattice geometry; for more details see e.g. [6, appendix a]. 41 k. mikeš, f. bormann, o. rokoš, r.h.j. peerlings acta polytechnica ctu proceedings figure 4. normalized traction as a function of normalized opening for the two parameter sets considered, cf. tab. 1. figure 5. the damage (red) in bonds computed with full lattice model: evolution of the process zone one step before crack initiation (top) and final process zone and fully developed crack path in the last step of simulation (bottom). instead of the cohesive zone potential ψcz, the corresponding traction, t , defined as t (∆n) = dψcz(∆n) d∆n , (19) is evaluated numerically from the response of the periodic unit cell subjected to a prescribed displacement ∆n with the boundary conditions according to figure 3. the computed values of tractions corresponding to the two different parameter sets of tab. 1 are plotted in figure 4. the cohesive zone potential ψcz can be obtained from these tractions by numerical integration. but effectively, for the practical implementation, the values of ψcz are not needed because the problem is solved for the force balance by the standard newton algorithm. 4. results in this section, the full lattice model defined in section 2 (referred to as full lattice), and finite element cohesive zone model defined in section 3 (referred to as fe-cz ), are compared on a three-point-bending test of a rectangular specimen of dimensions 2048l0×256l0, for the brittle and ductile lattice material settings (recall table 1 and figure 4). for the full lattice model, individual bonds start to damage in a relatively ductile manner, resulting in a considerable process zone, featuring multiple possible crack paths, cf. figure 5 (top). after localization, a full (central) crack develops across the entire height of the specimen, cf. figure 5 (bottom). the force-displacement responses are compared in figure 6. the fe-cz is able to exactly capture the initial elastic stiffness. however, the peak force is significantly underestimated by fe-cz , for both materials. the relative peak force error (of fe-cz in comparison with full lattice) is 23.3 % for the brittle and even 33.4 % for the ductile material. this is caused by the fact that for the full lattice a large amount of inelastic strain along the bottom edge is accumulated in the process zone. this complex behavior cannot be captured with one cohesive zone and otherwise elastic behavior naturally results in significant deviations in the fe-cz model. the inelastic process zone in the full lattice unloads the crack mouth (compared to fe-cz ) and, therefore, higher loading force is needed to initiate the crack. to verify this, we limit the process zone in the lattice model in a further simulation (referred to as elastic lattice). in this model, the damage can evolve only in a narrow band of width 2l0 positioned along the vertical axis of the symmetry, whereas the remaining lattice is considered as purely elastic. to this end, the critical strain parameter is modified as εi0 = { ε0 if |xi − xcz| ≤ l0 ∞ othervise (20) 42 vol. 26/2020 comparison of discrete lattice system and cohesive zone model figure 6. normalized force–displacement diagrams for a brittle (left) and a ductile (right) lattice. the results of elastic lattice correspond to the lattice where the damage can evolve only in a narrow band of width 2l0, whereas the remaining lattice is elastic. figure 7. normalized opening profiles for fixed pre-peak loading force f/(e0a0ε0) = 56 (left) and for post-peak loading force f/(e0a0ε0) = 14 (right) computed for the ductile material setting. figure 8. normalized crack length as a function of crack mouth opening displacement (cmod) computed for the ductile material setting. where xi − xcz is the horizontal distance between the center of the i-th bond and the symmetry axis. for all considered aspects, the fe-cz model provides very similar results to the elastic lattice, where the influence of the process zone is removed. it reveals that the cohesive zone in fe-cz is able to exactly capture the damage properties of the underlying lattice and the error in the peak force is produced mainly by the missing process zone. this can also be observed in the difference of preand post-peak opening profiles, corresponding to a fixed loading force, as shown in figure 7 for two magnitudes of the applied force. here we clearly see that the fe-cz and elastic lattice (both with a narrow process zone) tend to show significantly larger crack mouth openings before crack localization. once the crack localizes and opens, however, all models provide very accurate results. also the kinematics of crack propagation, depicted in figure 8, is accurately captured by both the elastic lattice and fe-cz . the fe-cz (compared to the full lattice model in computing time) provides, however, a substantial speed-up of the order of 50. 5. conclusion in this contribution, a cohesive-zone finite element approach to modelling of crack propagation in materials with discrete microstructures has been compared against the full lattice model that features phenomena observed in real materials, such as non-local behaviour, large deformations and rotations of individual struts, and distributed cracking. it has been shown that the cohesive-zone model achieves a substantial speed up, while being sufficiently accurate in terms of kinematic quantities such as overall displacement, crack length, or crack opening profile. on the other hand, the conjugate quantities, 43 k. mikeš, f. bormann, o. rokoš, r.h.j. peerlings acta polytechnica ctu proceedings such as the peak force, are significantly underestimated due to the missing process zone which results in a significantly more ductile response of the considered lattice system. with increasing ductility of the lattice, the accuracy further decreases. the presented results have also shown that in order to correct for this deficiency, more ductile behaviour should be accounted for, e.g., by adding multiple (parallel) cohesive zones that cover the size of the main part of the process zone. however, the spatial positioning and orientation of such secondary cohesive zones is not known and needs to be based on prior knowledge. in the most general case, every inter-element interface can be considered as a cohesive zone, which might significantly complicate the entire procedure, introduce a dependence on the discretization (finite element size and orientation), and cause significant computational overhead. let us note that, apart from the homogenization procedures used in this paper, the effective cohesive-zone constitutive parameters can be obtained by fitting directly the overall macroscopic force–displacement curves. although this might improve the macroscopic accuracy (in terms of force–displacement curve) achieved by the cohesive-zone model, such a phenomenological approach would not directly represent the underlying microstructure. this would in turn mean that different geometries would require independent identifications and that the accuracy in kinematic quantities (such as crack opening profile) might be decreased. the cohesive zone model provides an efficient and effective tool for the description of crack propagation in discrete systems, although care must be taken to position the cohesive zone interface properly to cover not only the crack trajectories, but also phenomena important for given problem. list of symbols q state variable r kinematic variable z internal variable t parametrization time nt number of time steps q space of all admissible configurations πk incremental energy e potential energy d dissipation distance v internal free energy vard dissipated energy f ext vector of external loads α, β node indices bα nearest-neighbour nodes to α ωαβ damage level in a bond αβ dαβ energy dissipated in bond αβ wext work performed by the external forces φ pair potential e0 young’s modulus of individual bonds a0 cross-sectional area l length of deformed bond l0 initial lattice spacing ε, ε0 bond strain, critical bond strain εf damage softening parameter ω elastic domain γcz cohesive zone ψ total free energy ψcz cohesive zone potential ψe elastic strain energy density ε elastic strain tensor σ elastic stress tensor c elastic stiffness tensor ~u displacement field ∆n,∆t normal/tangential jump in displacements eiso effective young’s modulus ν effective poison’s ratio f magnitude of the point loading force u displacement under the point loading force x, y cartesian coordinates acknowledgements financial support of this work from the grant agency of the czech technical university in prague (sgs) under project no. 19/032/ohk1/1t/11 and from the czech science foundation (gačr) under project no. 17-04150j is gratefully acknowledged. references [1] l. beex, c. verberne, r. peerlings. experimental identification of a lattice model for woven fabrics: application to electronic textile. composites part a: applied science and manufacturing 48:82–92, 2013. doi:10.1016/j.compositesa.2012.12.014. 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[6] k. mikeš, m. jirásek. quasicontinuum method extended to irregular lattices. computers & structures 192:50–70, 2017. doi:10.1016/j.compstruc.2017.07.002. 44 https://doi.org/10.1016/j.compositesa.2012.12.014 https://doi.org/10.1016/j.ijsolstr.2014.12.006 https://doi.org/10.1061/(asce)0733-9399(2003)129:12(1439) https://doi.org/10.1007/978-1-4939-2706-7 https://doi.org/10.1002/nme.2447 https://doi.org/10.1016/j.compstruc.2017.07.002 acta polytechnica ctu proceedings 26:1–6, 2020 1 introduction 2 full lattice model 3 cohesive zone model 3.1 model description 3.2 material parameter calibration 4 results 5 conclusion list of symbols acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2017.13.0097 acta polytechnica ctu proceedings 13:97–101, 2017 © czech technical university in prague, 2017 available online at http://ojs.cvut.cz/ojs/index.php/app geometrical optimization of dental implants with regard to osseointegration luboš řehounek∗, františek denk, aleš jíra czech technical university in prague, faculty of civil engineering, thákurova 7, 166 29 prague 6, czech republic ∗ corresponding author: lubos.rehounek@fsv.cvut.cz abstract. a newly developed reference dental implant specimen type was subjected to numerical simulations of osseointegration. the goal of these tests was to optimize the geometry of the implant so as to reduce local stress concentrations and provide a better flow of stress through the whole implant body. conditions for osseointegration were considered when evaluating the anchoring system of the implant in regard to its placement in the human cancellous and cortical bone. numerical simulations showed that stress concentrations occur mostly in the upper cylindrical part of the implant. by increasing the width of this cylindrical part, we were able to reduce the maximum values of von-mises stress by 20 %. keywords: dental implant, optimization, trabecular, numerical, fem. 1. introduction the main shortcoming of commonly manufactured dental implants is their predisposition towards early failure [1–3]. this unwanted factor can be attributed to uneven distribution of stresses in the implant, periimplant area and bone [4, 5]. our main goal in developing new implants was to reduce stress concentrations occuring mainly in the upper cylindrical part of the implant, just below the abutment, while preserving the osseointegration benefits of the implant. the tested implant has been patented in the czech republic [6]. the numerical analysis describes the mechanical behavior of the implant in regard to mechanical loads and the corresponding biomechanical stablity in the bone tissue. the tests focus on two aspects – mechanical behavior of the implant under a prescribed mechanical load and osseointegration of the implant inside the human mandible. the first part of the analysis focuses on the osseointegration of the implant in human mandible. the aim of the second part of the tests was to determine the ultimate bearing capacity of the dental implant stem according to the čsn en iso 14081 – “stomatology – implants – dynamic fatigue testing of intraosseous dental implants” standard. this paper will only focus on the first part of the tests (osseointegration) and leave the second part (mechanical tests) for further studies, as some results are already available, but additional simulations are still needed. the analysis provides an overview of stress and deformation relations of the tested implant stem in a prescribed environment in all of its parts. this is simulated by embedding the implant stem in a block of human bone (fig. 2) and subsequently evaluating the stress and deformation relations at surface areas of the stem and at the interfaces of the stem and bone tissue. numerical simulations were done in the ansys apdl system by incorporating previously created 3d geometries (fig. 1) into the environment of the software. the geometry of the implant stem was created in detail at the surface areas of the implant. the internal part of the stem has been created with less detail (i.e.: without the screw-thread). 2. numerical testing of osseointegration for the purpose on numerical analysis in ansys apdl, we created an environment where the dental implant is fixed into the bone tissue of human mandible. the implant consists of the intraosseous stem and the implant superstructure, which represents the abutment and the crown. the connection of individual volumetric bodies is attained using rigid constraints generated by the volumetric “vglue” operations. the mesh of the model consists of quadratic tetrahedrons type solid 187 generated in the “smart size” mode. the material of all components of the dental implant is the titanium alloy ti-6al-4v. the stem of the implant is fixed into a volumetric model of human mandible, situated according to real implantation procedures. the geometrical orientation of the ribs was arranged in the least favorable position (in regard to the results of mechanical tests). the exact geometry of the bone was generated by boolean operations (subtracting the full body volume of the stem from the body of the bone). the connection between contact surfaces of the stem and the bone was attained by volumetric “vglue” operations. the numerical model of the mandible was then subsequently remeshed according to the new boundary conditions (fig. 2). the front faces of the mandibles have symmetric 97 http://dx.doi.org/10.14311/app.2017.13.0097 http://ojs.cvut.cz/ojs/index.php/app l. řehounek, f. denk, a. jíra acta polytechnica ctu proceedings figure 1. 3d model of the designed implant 0001 (ansys apdl): whole 3d model (left), longitudinal section (middle) and fem mesh (right). figure 2. numerical model of the mandible block which supports the intraosseous part of the implant stem – volumetric model (left), fem mesh (middle) and total deformation isolines (right). boundary conditions and the lower strip of nodes is set to have zero displacement value. physiological load is represented by a single slant force applied to the central point of the upper base of the superstructure. the force magnitude is f=130 n. the direction of the force is defined by contribution of individual force components according to the total load imposed on a single tooth. the magnitude of normal force is 125 n and tangent force is 35 n (fx =25 n, fy =25 n, fz =125 n). 3. optimization of the cylindrical part the following optimization will assume that the small strip of the cylindrical stem forming the upper structure of the stem is always the area where values of stress concentrate. these extremities are caused figure 3. numerical analysis of the reference implant 0001: displacement – umax =25 µm (left), equivalent von mises stress – σeqv,max =91.7 mpa (middle and right). 98 vol. 13/2017 geometrical optimization of dental implants figure 4. optimization of the geometrical shape in the upper cylindrical part: reference model 0001a (cyl. width 0.50 mm) (left), model 0001b (cyl. width 1.25 mm) (middle) and model 0001c (cyl. width 3.00 mm) (right). figure 5. numerical analysis of the reference implant 0001b: displacement – umax =25 µm (left), equivalent von mises stress – σeqv,max,1 =97.1 mpa, σeqv,max,1 =73.2 mpa (middle and right). by complex geometrical shapes and also a relatively small width of the cylindrical strip. the goal of the optimization will be to effectively modify the geometrical shape of the implant in the area of fixation of the stem into cortical bone of the human mandible. also, the overall stability of the intraosseous part must not be compromised. two new variants have been tested – variant 0001b and 0001c. the modification represents increasing of the width of the cylindrical part. the width of the cylindrical part of the reference implant 0001a is 0.50 mm. the model 0001b had the width increased up to 1.25 mm and model 0001c up to 3.00 mm. identical simulations were run on all of the mentioned variants and the outcomes of the numerical analyses are shown in fig. 3, fig. 5 and fig. 6. on the basis of equivalent von-mises stress disfigure 6. numerical analysis of the reference implant 0001c: displacement – umax =25 µm (left), equivalent von mises stress σeqv,max =73.6 mpa (middle and right). tribution at the surface areas of the imlpant stem, we can observe that the widening of the cylindrical strip of the upper part without any other geometrical modification positively affects the stress distribution by lowering extremities. the variant 0001b (cylindrical part 1.25 mm) has a mean value of σeqv,avg =61.6 mpa and the stress flow is distributed over a larger area. there is an extreme of σeqv,max =73.2 mpa. there is also another extreme of σeqv,max =97.1 mpa spread over a very small area situated in the edge of the cylindrical part. the system of vertical ribs is situated in both cortical and cancellous bone. the variant 0001c (cylindrical part 3.00 mm) has a maximum value of σeqv,max =73.6 mpa, but the system of vertical ribs is situated now only in cancellous bone. 99 l. řehounek, f. denk, a. jíra acta polytechnica ctu proceedings implant type osseointegrated in bone tissue of mandible displacement of the implant and bone tissue equivalent von-mises stress optimization influence ut [µm] σeqv,max [mpa] [%] implant 0001a 25 91.70 100.00 implant 0001b 25 73.20 79.80 implant 0001c 25 73.60 80.00 table 1. overview of the results provided by geometrical modification. reference implant 0001a and modified implants 0001b and 0001c. figure 7. two stl files showing the final geometrical solution of the implant (trabecular outer shell second from left) (left), the final implant model (middle) and homogenized layer of the outer trabecular structure for easier simulations (right). 4. conclusions 4.1. results the results of the numerical simulations show that by increasing the upper cylindrical portion of the stem, we can improve the distribution of stresses at the surface areas of the implant and also reduce extreme values of stress. the variant 0001b showed that by increasing the width of the cylindrical part by 0.75 mm (from 0.50 mm to 1.25 mm), we can reduce stresses by approximately 20 % and achieve a more even distribution. there is, however, an extreme value of 97.1 mpa situated at the edge of the cylindrical part over a very small area. this extreme is caused by discontinuities in geometry (sudden change of curvature), which will be accounted for by rounding the edge of the cylinder by a radius of at least 0.1 mm during the production process. biomechanical stability is achieved by situating the vertical ribs of the implant inside the cortical bone of the mandible. the variant 0001c (width from 0.75 mm to 3.00 mm, 2.50 mm increase) also showed a 20 % reduction in values of stress. the cylindrical part of the implant does not permeate through the whole layer of cortical bone, so the distribution of stress is more even and there are no larger extremes in values of stress. this geometrical solution, however, might not provide sufficient biomechanical stability as the vertical ribs, which are supposed to anchor the implant in the bone, are only situated in cancellous bone. considering the biomechanical implications of the presented results, we evaluated the variant 0001b with a 1.25 mm wide cylindrical strip as the most efficient solution, which reduces extreme values of stress as well as provides sufficient stability inside thevbone tissue. 4.2. future prospects the chosen variant 0001b has good mechanical stability and stress distribution and was chosen as the variant for further development. with the shape solution of the implant stem complete, the next step in the research will be to create a numerical model of a composite implant (fig. 8). this implant will comprise of both homogeneous and trabecular portions. trabecular structure has been discussed at length in one of other author’s works [7]. the implant will maintain its homogeneous stem. the outer shell with the stabilizing ribs will, however, be comprised exclusively of the trabecular structure. further simulations will be made and multiple variants of different layer thickness analysed in order to determine which shape solution is the best for osseointegration, mechanical stability and machanical bearing capacity. the approach towards modelling the trabecular structure in future research will be focused on sustainable, repeatable analyses. we have already found this 100 vol. 13/2017 geometrical optimization of dental implants figure 8. whole implant body of the implant for further research. implant has a homogeneous stem and an outer layer comprising of the trabecular structure. the implant has been patented in czech republic [6]. to only be possible by making a virtual homogenized material (fig. 7) with specific curve-fitted properties determined by experiment [7]. this approach allows us to apply previously determined mechanical properties of the trabecular structure onto a homogeneous material that is easy to analyze in numerical environment. by running a few simple analyses, we found this to be the optimal approach as it is not very demanding in computation times and it maintains sufficient precision. by manipulating the shape of the outer trabecular structure, we will then be able to determine which variant is optimal in regard to reduction of stress shielding and mechanical bearing capacity. acknowledgements the financial support by the faculty of civil engineering, czech technical university in prague (sgs project no. sgs17/168/ohk1/3t/11) is gratefully acknowledged. references [1] l. le guéhennec, a. soueidan, p. layrolle, y. amouriq. surface treatments of titanium dental implants for rapid osseointegration. dental materials 23(7):844–854, 2007. doi:10.1016/j.dental.2006.06.025. [2] r. osman, m. swain. a critical review of dental implant materials with an emphasis on titanium versus zirconia. materials 8(3):932–958, 2015. doi:10.3390/ma8030932. [3] t. jemt, m. olsson, f. renouard, et al. early implant failures related to individual surgeons: an analysis covering 11,074 operations performed during 28 years. clinical implant dentistry and related research 18(5):861–872, 2016. doi:10.1111/cid.12379. [4] m. i. z. ridzwan, s. shuib, a. y. hassan, et al. problem of stress shielding and improvement to the hip implant designs: a review, 2007. doi:10.3923/jms.2007.460.467. [5] m. long, h. j. rack. titanium alloys in total joint replacement–a materials science perspective. biomaterials 19(18):1621–1639, 1998. doi:10.1016/s0142-9612(97)00146-4. [6] f. denk, f. denk, a. jíra. a dental implant shaft, czech republic, 306457, 2017. [7] l. řehounek, a. jíra. experimental and numerical analyses of a 3d-printed titanium trabecular dental implant. acta polytechnica 57(3):218–228, 2017. doi:10.14311/ap.2017.57.0218. 101 http://dx.doi.org/10.1016/j.dental.2006.06.025 http://dx.doi.org/10.3390/ma8030932 http://dx.doi.org/10.1111/cid.12379 http://dx.doi.org/10.3923/jms.2007.460.467 http://dx.doi.org/10.1016/s0142-9612(97)00146-4 http://dx.doi.org/10.14311/ap.2017.57.0218 acta polytechnica ctu proceedings 13:98–102, 2017 1 introduction 2 numerical testing of osseointegration 3 optimization of the cylindrical part 4 conclusions 4.1 results 4.2 future prospects acknowledgements references acta polytechnica doi:10.14311/app.2020.27.0073 acta polytechnica 27(0):73–78, 2020 © czech technical university in prague, 2020 available online at http://ojs.cvut.cz/ojs/index.php/app evaluation of internal cohesion of multiphase plasma-sprayed coatings by cavitation test: feasibility study radek mušáleka,∗, emanuele nardozzab, tomáš tesařa,c, jan medřickýa,c a czech academy of sciences, institute of plasma physics, department of materials engineering, za slovankou 1782/3, 182 00 prague 8, czech republic b university of modena and reggio emilia, faculty of materials engineering, via pietro vivarelli 10, modena, italy c czech technical university in prague, faculty of nuclear sciences and physical engineering, trojanova 13, 120 00 prague, czech republic ∗ corresponding author: musalek@ipp.cas.cz abstract. mechanical characterization of plasma-sprayed coatings at microscopic level represents a major challenge due to the presence of numerous inherent microstructural features such as cracks, pores, or splat boundaries, which complicate coatings characterization by conventional testing methods. need for reliable testing of structural integrity of newly developed multiphase plasma-sprayed coatings introduced even more complexity to the testing. in this study, applicability of indirect vibratory cavitation test (adapted from astm g32 standard) for such testing was evaluated. three plasmasprayed coatings having distinctive microstructures were tested: i) conventional alumina coating deposited from coarse powder, ii) hybrid coating deposited by co-spraying of coarse alumina powder and fine yttria-stabilized zirconia (ysz) suspension, and iii) compact alumina coating deposited from fine ethanol-based suspension. differences in the coatings internal cohesion were reflected in different failure mechanisms observed within the cavitation crater by scanning electron microscopy and mean erosion rates being i) 280 µm/hour, ii) 97 µm/hour and iii) 14 µm/hour, respectively. keywords: cavitation damage, cohesion, failure analysis, plasma spray coatings. 1. introduction plasma-sprayed coatings belong to the family of thermal sprayed materials and are used in numerous applications, typically to protect the substrate material from the aggressive environments. thermal barrier coatings or wear-resistant coatings may be listed as typical examples. they often contain numerous voids (pores and cracks) which are desirable from the functional point of view (e.g. providing strain tolerance or decreasing thermal conductivity) but together with rather low coating thickness make evaluation of coatings mechanical properties quite challenging. hardness testing may serve as a good illustration [1]. for high loads, indent size may be higher than the available coating thickness or lead to excessive coating cracking. on the other hand, for small loads, hardness values may be not representative of the whole coating microstructure as the loaded volume contains only individual splats. this applies for both conventional and instrumented indentation. another example can be adhesion/cohesion testing of the coatings by so called "pull-test" [2], where the coating is glued to the dummy counter-part and loaded across the interface until failure. this test is widely used for thermally sprayed coatings, but may provide highly unreliable results for coatings which are porous, thin, or have adhesion/cohesion higher than strength of the available glue (typically 70-80 mpa) [3]. unfortunately, it is quite common that novel coatings meet at least one of the above mentioned criteria. it is therefore desirable to seek for new types of tests which may be applied for plasma-sprayed coatings and provide measure of the coatings durability (integrity) reflecting internal coating cohesion and/or adhesion to the substrate. such test should be highly repeatable, representative for the whole coating, economical, and easy to perform on samples with simple geometry with coatings having as-sprayed as well as finished surface. it should also mimic loading mode in some typical coating application. recently, vibratory cavitation test emerged as a potential candidate [4–6]. it may be relatively easily adapted and simulates conditions in applications where the cavitation occurs, such as hydraulic turbines, pumps, steering rudders, etc. [7, 8]. sample (in this case a coated part) is immersed into appropriate liquid and its surface is repeatedly exposed to aggressive pressure waves generated by collapse of cavitation bubbles. in the "direct cavitation setup", cavitation is induced by high-frequency oscillations of the sample itself. for testing of coated samples, "indirect cavitation setup" (also denoted as "alternative setup") is more suitable. in this setup, 73 http://dx.doi.org/10.14311/app.2020.27.0073 http://ojs.cvut.cz/ojs/index.php/app r. mušálek, e. nardozza, t. tesař, j. medřický acta polytechnica coating feedstock type fd sd fr ct (mm) (mm) (kg/h) (µm) aw24 al2o3 powder / 55 / 380 8.8 / 327 ± 16 sr086 al2o3 powder / ysz suspension 35 / 20 130 7.0 / 6.7 771 ± 10 sr119 / al2o3 suspension / 25 100 / 6.0 167 ± 10 note: fd feeding distance, sd spraying distance, fr feed rate, ct coating thickness. table 1. spraying conditions for powder/suspension. exchangeable tip made of durable material is placed over the sample and its vibrations form stream of cavitation bubbles which erode the sample surface (figure 1a). as described by astm g32 standard [9], durability of tested materials may be then evaluated in terms of their mass loss against exposure time or some other characteristics derived from this curve. for testing of plasma-sprayed coatings, it is also interesting that individual cavitation bubbles erode the tested coating at microscopic scale (reflecting the coatings microstructure), but at the same time, their number is high enough to provide representative coatings characteristics. figure 1. a) schematic of the indirect cavitation test for a coated sample. b) geometry of used horn tip red area (115.7 mm2) is closest to the sample and emitting cavitation bubbles. aim of this study was to test applicability and sensitivity of the vibratory cavitation test for evaluation of novel multiphase plasma-sprayed coatings. test was applied on three ceramic coatings which were expected to have greatly different resistance against cavitation. 2. experimentals three coatings were prepared on grit-blasted steel coupons (20 × 30 × 2.5 mm3) by hybrid waterstabilized plasma (wsp-h) torch wsp-h 500 (projectsoft hk, a.s., czechia). coating aw24 was a conventional al2o3 coating sprayed from coarse dry powder, coating sr086 was experimental multiphase coating prepared by "hybrid" co-spraying of coarse dry al2o3 and fine ysz (yttria-stabilized zirconia) suspension, and coating sr119 was dense al2o3 coating deposited from fine suspension. materials used for spraying were surprex aw24 powder (fujimi inc., japan, granulometry -75 + 38 µm), 25% ysz suspension in ethanol (treibacher industrie ag, austria, mean particle size ∼0.5 µm) and ethanol-based 10% suspension of al2o3 (allied high tech products inc., usa, mean particle size ∼0.3 µm). principal deposition parameters are listed in table 1. for details, see [10–12]. scanning electron microscope (sem) evo ma 15 (carl zeiss smt, germany) was used for structural observations using back-scattered electron detection mode. cross-sections and free-surfaces of the coatings (figure 2) show substantial differences in the coatings microstructures. coating aw24 sprayed from coarse powder showed conventional lamellar microstructure with large alumina splats and numerous intersplat and intrasplat cracks and pores. hybrid coating sr086 consisted of large alumina splats originating from coarse powder interconnected on their surface by miniature ysz splats originating from fine suspension. coating sr119 sprayed from fine suspension showed densely packed splats. their reduced size effectively suppressed formation of intrasplat cracks, which are typical for splats sprayed from coarse powders due to development of quenching stresses [13, 14] compare aw24 and sr119 in fig. 2. cavitation test was carried out according to the modified astm g32-16 test in "indirect cavitation" setup [9]. the horn tip made of durable titanium alloy was placed over the sample and its vibration formed stream of cavitation bubbles which eroded the sample. all samples were tested in as-sprayed condition, i.e. without any surface polishing. samples were weighted, immersed in distilled water (25 ± 2 °c) and put under the horn tip so that the spacing between the horn tip and the sample surface was 0.6 mm. formation of cavitation bubbles was imposed by vibration of the horn tip with frequency of f = 20 khz and peak-to-peak amplitude apeak-peak = 50 µm. after preselected time interval, samples were taken out, rinsed and dried with hot air, and weighted again. the whole procedure was repeated several times until considerable revelation of the substrate. astm g32-16 standard prescribes shape of the horn tip as circular (∅15.9 mm, area 198.6 mm2). in this test, tip geometry was modified (cropped circle ∅13 mm, area 115.7 mm2 see figure 1b) to fit the whole eroded area within the coated surface. in order to compensate for difference in tip geometry, erosion rates (in g/hour) were normalized by coating density and horn tip area providing mean erosion rate (in µm/hour). 74 vol. 27/2020 multiphase plasma-sprayed coatings internal cohesion figure 2. coatings microstructure. cross-section (left) and free-surface (right). bright spots in sr086 coating are ysz splats. sem. figure 3. a) cumulative erosion vs time curves. filled points were used for linear fit. b) erosion rates (blue) and coating microhardness values (yellow). 3. results and discussion obtained experimental curves of cumulative mass loss versus cumulative exposure time are compared for all three coatings in figure 3a. none of the coatings showed considerable incubation period and, in all cases, the coating weight loss closely followed linear trend in time. for the hybrid coating sr086, high number of measurements enabled detection of minor acceleration stage at the beginning of the test, but the erosion rate stabilized soon. at the terminal stage of the test, substrate started to be revealed for all coatings (figure 4), so data points only from the central part of the experimental curves were fitted by linear dependency. this enabled to quantify the coatings durability in terms of erosion rate in g/hour (figure 3a) which could be with knowledge of theoretical coating density and area of horn tip transformed into mean thickness loss in µm per hour (figure 3b). from the obtained erosion rates and appearance of cavitation craters (figure 4 to 6), following conclusions may be stated: • as expected, coating aw24 with rather loose microstructure showed the lowest cavitation resistance with erosion rates reaching 118 mg/hour or 280 µm/hour. relatively low adhesion of the coating to the substrate was at the end of the test reflected also on large-scale revelation of the substrate under the cavitation crater. coating showed tendency to fail by initial formation of wide cavitation pits which was soon followed by coating detachment in large crumbled pieces. • for coating sr086, different deposition conditions (in particular lower spraying distance) and addition of secondary phase promoted mutual bonding of the splats which led to decrease of cavitation rate to 43 mg/hour or 97 µm/hour. also, failure mode of the coating was different as it showed tendency to fail by exfoliation which formed noticeable steps on the sides of the cavitation crater. these possibly followed the interfaces generated during the coating deposition between sub-layers formed by consecutive spraying passes. • for coating sr119, cavitation rate further dropped to just about 6 mg/hour or 14 µm/hour. these low values confirmed high structural integrity of the coating. it may be noted that the same coating showed in "pull-test" (astm c633) extreme tensile adhesion/cohesion strength of about 51 mpa [10]. high cohesion of the coating was reflected also in its different failure mode, as the cavitation crater was formed by numerous narrow pits which only slowly propagated into the coating. in figure 3b, mean erosion rates may be related also to the coatings microhardness values. as expected, the most compact coating sr119 showed the highest hardness whereas the multiphase nature of hybrid coating was reflected in high scatter of microhardness values. it may be therefore concluded that cavitation test seems to convincingly reflect differences in the internal microstructure of the plasma-sprayed coatings and in particular their internal cohesion. qualitatively different response of the coatings to 75 r. mušálek, e. nardozza, t. tesař, j. medřický acta polytechnica figure 4. eroded samples (overview) at the end of the test. total exposure time in minutes. figure 5. rim of cavitation crater after the test. bright spots are the revealed substrate. sem. figure 6. cross-sections through cavitation crater after the test. light microscopy. the cavitation loading may be observed also from micrographs of the damaged free-surfaces and their cross-sections (figure 7). in case of aw24 coating, numerous internal voids (intrasplat cracks in particular) and weak bonding between splats promoted crushing of the splats and their mutual debonding. in case of sr086 coating, the failure was possibly delayed by improved bonding of splats due to presence of secondary ysz phase. good bonding between alumina and ysz may be assumed from the persevering presence of numerous miniature ysz splats on the surfaces of heavily eroded alumina splats. however, when the cavitation damage reached weakened interpass (sub-layer) interfaces or areas with locally deficient ysz phase, in-plane failure was promoted leading to detachment of the coating in large platelets and eventually formation of noticeable steps on the sides of the cavitation crater. on the eroded surface of sr119 coating, heavily crushed miniature splats could be observed which corresponds to high exposure time, but no large-scale damage of the coating microstructure was observed deeper in the coating. as apparent from figures 4 to 6, when the cavitation damage reached the substrate, cavitation crater had tendency to widen rather than to penetrate deeper into the substrate. substrate below the coatings was thus effectively exposed clearly revealing morphology of its originally grit-blasted surface which could be easily identified from the presence of embedded sharpedged grit-blasting particles (figure 8a). in several isolated locations, formation of shallow pits penetrating the substrate was also detected (figure 8b). signs of plastic deformation and attack of grain boundaries could be observed at the bottom of these pits. nevertheless, negligible loss of the substrate material explains why the apparent linearity of the mass loss curves was not compromised even at the terminal stage of the cavitation test when the substrate started to be exposed. 76 vol. 27/2020 multiphase plasma-sprayed coatings internal cohesion figure 7. coating damaged by cavitation. free-surface (left) and cross-section (right). sem. figure 8. a) exposure of the original grit-blasted substrate surface at the bottom of the cavitation crater (coating aw24). b) cavitation pit penetrating substrate (coating sr119). free-surface (left) and cross-section (right). sem. 77 r. mušálek, e. nardozza, t. tesař, j. medřický acta polytechnica 4. conclusions indirect vibratory cavitation test adapted from astm g32 standard was proven to be potentially valuable for screening of novel plasma-sprayed coatings deposited by hybrid water-stabilized plasma torch. test could be easily carried out on as-sprayed samples with various thicknesses and without finishing (i.e. polishing) of the surfaces. cavitation test reflected well the differences in the coatings microstructures. testing confirmed excellent durability of dense alumina coating deposited from ethanol-based suspension and also indicated that introduction of secondary miniature ysz phase in so-called hybrid coatings may, together with short spraying distance, improve internal cohesion of the plasma-sprayed alumina. acknowledgements financial support provided through czech science foundation project 19-10246s "deposition mechanisms and properties of multiphase plasma sprayed coatings prepared with liquid feedstocks" is gratefully acknowledged. references [1] j. nohava, r. musalek, j. matejicek, m. vilemova. a contribution to understanding the results of instrumented indentation on thermal spray coatings case study on al2o3 and stainless steel. surface and coatings technology 240:243 – 249, 2014. doi:10.1016/j.surfcoat.2013.12.033. [2] standard test method for adhesion or cohesion strength of thermal spray coatings, american society for testing and materials (astm), c633 13, 2017. doi:10.1520/c0633-13r17. [3] r. musalek, v. pejchal, m. vilemova, j. matejicek. multiple-approach evaluation of wsp coatings adhesion/cohesion strength. journal of thermal spray technology 22:221–232, 2013. doi:10.1007/s11666-012-9850-2. [4] v. matikainen, s. r. peregrina, n. ojala, et al. erosion wear performance of wc-10co4cr and cr3c225nicr coatings sprayed with high-velocity thermal spray processes. surface and coatings technology 370:196 – 212, 2019. doi:10.1016/j.surfcoat.2019.04.067. [5] v. matikainen, k. niemi, h. koivuluoto, p. vuoristo. abrasion, erosion and cavitation erosion wear properties of thermally sprayed alumina based coatings. coatings 4:18–36, 2014. doi:10.3390/coatings4010018. [6] j. kiilakoski, j. puranen, e. heinonen, et al. characterization of powder-precursor hvof-sprayed al2o3-ysz/zro2 coatings. journal of thermal spray technology 28:98–107, 2019. doi:10.1007/s11666-018-0816-x. [7] g. l. chahine, j.-p. franc, a. karimi. laboratory testing methods of cavitation erosion, pp. 21–35. springer netherlands, dordrecht, 2014. doi:10.1007/978-94-017-8539-6_2. [8] h.-b. lee, c.-s. park, s.-m. son, et al. combatting rudder erosion with cavitation-resistant coating. journal of protective coatings & linings 32(3):38–41, 2015. [9] astm g32 16 standard test method for cavitation erosion using vibratory apparatus, 2016. doi:10.1520/g0032-16. [10] t. tesar, r. musalek, j. medricky, et al. development of suspension plasma sprayed alumina coatings with high enthalpy plasma torch. surface and coatings technology 325:277 – 288, 2017. doi:10.1016/j.surfcoat.2017.06.039. [11] r. musalek, j. medricky, t. tesar, et al. suspensions plasma spraying of ceramics with hybrid water-stabilized plasma technology. journal of thermal spray technology 26:37–46, 2017. doi:10.1007/s11666-016-0493-6. [12] j. medricky, r. musalek, m. janata, et al. cost-effective plasma spraying for large-scale applications. in: itsc 2018 proc. int. therm. spray conf., orlando, florida, pp. 683–689. 2018. [13] s. kuroda, t. clyne. the quenching stress in thermally sprayed coatings. thin solid films 200(1):49 – 66, 1991. doi:10.1016/0040-6090(91)90029-w. [14] r. musalek, j. medricky, t. tesar, et al. controlling microstructure of yttria-stabilized zirconia prepared from suspensions and solutions by plasma spraying with high feed rates. journal of thermal spray technology 26(8):1787–1803, 2017. doi:10.1007/s11666-017-0622-x. 78 https://doi.org/10.1016/j.surfcoat.2013.12.033 https://doi.org/10.1520/c0633-13r17 https://doi.org/10.1007/s11666-012-9850-2 https://doi.org/10.1016/j.surfcoat.2019.04.067 https://doi.org/10.3390/coatings4010018 https://doi.org/10.1007/s11666-018-0816-x https://doi.org/10.1007/978-94-017-8539-6_2 https://doi.org/10.1520/g0032-16 https://doi.org/10.1016/j.surfcoat.2017.06.039 https://doi.org/10.1007/s11666-016-0493-6 https://doi.org/10.1016/0040-6090(91)90029-w https://doi.org/10.1007/s11666-017-0622-x acta polytechnica 27(0):73–78, 2020 1 introduction 2 experimentals 3 results and discussion 4 conclusions acknowledgements references doi:10.14311/app.2020.28.0008 acta polytechnica ctu proceedings 28:8–14, 2020 © czech technical university in prague, 2020 available online at http://ojs.cvut.cz/ojs/index.php/app high-temperature oxidation of chrome-nickel alloy adéla chalupováa,c,∗, martin steinbrückb, mirco grosseb, jakub krejčíc, martin ševečeka a czech technical university in prague, faculty of nuclear sciences and physical engineering, department of nuclear reactors, v holešovickách 2, 180 00 prague 8, czech republic b karlsruhe institute of technology, hermann-von-helmholtz-platz 1, 76344 eggenstein-leopoldshafen, germany c ujp praha a.s., nad kamínkou 1345, praha-zbraslav, 15610, czech republic ∗ corresponding author: chalupova@ujp.cz abstract. the investigations in this paper deal with the cr-ni alloy. the material has been recently proposed as a potential atf concept, primarily due to its behaviour under high-temperature oxidation. a set of experiments to determine the melting point and describe the oxidation kinetics of the cr-ni alloy were performed in karlsruhe institute of technology. presented results reveal its superb oxidation resistance comparing to zirconium alloys. therefore, the alloy has a great potential for nuclear applications. keywords: atf, cr-ni, fuel cladding, high-temperature oxidation, loca, nuclear fuel. 1. introduction zr-based cladding has been used in light water reactors for many decades. it has been chosen for nuclear applications due to its low thermal neutron capture cross-section together with good corrosion behaviour under normal operation conditions. yet, in high-temperature steam, such as might occur under severe accident conditions, a rapid increase in oxidation kinetics is observed. in response to the events in three mile island (1979) and more recently in fukushima dai-ichi (2011), the development and deployment of advanced accident tolerant fuel (atf) became of worldwide interest [1]. one of the possible approaches to achieve higher safety during severe accident scenarios is to replace the current zirconium alloys with new materials with higher oxidation resistance and lower heat and hydrogen production. an alloy similar to 42xhm is considered as a promising material in the development of atf. the 42xhm alloy is highly resistant to water vapour and it almost does not undergo to low-temperature embrittlement. the alloy has been already used as the structural material of absorbing rods in wwer-1000 reactors and as the fuel cladding in nuclear-powered ships and submarines. the previous experiments on 42xhm alloy were presented in [2], [3]. nevertheless, in comparison to the zr-based cladding, the cr-ni has a large thermal neutron absorption cross-section. several solutions to counterbalance the absorption have been proposed. the strength of the alloy allows thinner cladding; hence the pellets can be slightly larger to give the same cold gap width in the rod. still, even with a thinner cladding wall, it appears necessary to increase the fuel enrichment beyond the current 5% limit. another way to compensate for the high thermal neutron absorption is the implementation of high-density fuel instead of uo2 ceramics. to increase the capacity of uranium, it has been proposed to use heavily doped uranium-molybdenum fuel as well as modified uranium silicide fuel or a dispersion type fuel [4]. the melting point and high-temperature oxidation of the cr-ni alloy were studied and the results are presented. 2. methods 2.1. material the chemical composition of the cr-ni alloy used for these experiments is summarized in table 1. segments were cut from non-irradiated longer tubes, deburred, ground, and cleaned in acetone. the ultrasonically cleaned specimen was measured using an analytical balance with a resolution of 0.0001 g and a calliper with a resolution of 0.01 mm. as the deformation is not isotropic, the length was measured at 3 locations of the specimen and the final length is the average of these three measurements. the geometry of the tested specimens was cylindrical with a height of 20 mm, an outer diameter of 8.3 mm, and an inner diameter of 7.1 mm. before each experiment, the specimen was weighed in a balance with a resolution of 0.1 mg to determine the mass gain during the exposition and to compare it with data obtained from tg. 8 http://dx.doi.org/10.14311/app.2020.28.0008 http://ojs.cvut.cz/ojs/index.php/app vol. 28/2020 high-temperature oxidation of chrome-nickel alloy element cr mo ti ni wt.% 42 1-1.5 0.25 balance table 1. chemical composition of cr-ni alloy 2.2. differential thermal analysis differential thermal analysis (dta) is a thermoanalytic technique to analyse a change in properties of a sample, i.e., melting, decomposition, or changes in the crystal structure. in dta, the temperature of a sample is compared with the reference material. the reactions are typically graphed from an arbitrary baseline as a function of temperature. exothermic reactions per definition show upward curves and endothermic reactions downward curves from the baseline. the dta analysis was performed to determine the accurate melting point of the cr-ni alloy. thermal balance, particularly a simultaneous thermal analysis facility, sta 449 f3 jupiter eco from netzch company, allowing the measurement of mass changes and thermal effects, was used to perform the differential thermal analysis. contrary to most stas, the balance does not use the external thermostat and the temperature of the chamber is regulated electronically. the sample of cr-ni alloy and the reference alumina powder (al2o3) were weighted and placed symmetrically in the sta facility. the furnace was programmed to heat up to 1450°c (30 k/min) and subsequently to cool down to 100°c (100 k/min). during the process, a differential thermocouple was set to detect the temperature difference between the sample and the reference powder. the experiment was carried out in argon atmosphere to prevent oxidation reactions. 2.3. thermo-gravimetric experiment the most common method of measuring oxidation rate is the observation of oxide accumulation in time. the gravimetric measurements in this study were performed continuously. the experimental facility consisted of a thermal balance sta 449 f3 jupiter from netzsch company, the steam generator from bronkhorst company, and a quadrupole mass spectrometer netzsch qms 403 c. the facility sta 449 f3 jupiter presents the older configuration compared to the sta mentioned before. the weighting chamber’s temperature is regulated by an external thermostat. the measured specimen was placed into the furnace. an alumina plate was used to support the specimen, as shown in figure 1. first, the furnace was sealed and purged with argon for 10 minutes to remove the atmospheric gases. all experiments were performed with a flow rate of pure steam of 2 g/h. additionally, 50 ml/min of argon was injected in the balance volume as a protective and carrier gas. considering the given flow rates and the molar volume figure 1. specimen before exposition under high temperature steam oxidation 0 200 400 600 800 1000 1200 1400 0 10000 20000 30000 40000 50000 60000 70000 80000 90000 te m pe ra tu re [° c] time [s] → 20 hours ↘ 30 k/min ar: 50 ml/s h2o: 2 g/h ar: 50 ml/s ar: 50 ml/s ↗ 10 k/min figure 2. isothermal experiment of argon 22.4 dm3, the resulting steam molar concentration in the fluid at the outlet was 45%. due to the special construction of the furnace, the steam concentration of the sample was over 95%. oxidation occurred on both sides. generally, two varieties of the experiment were performed – isothermal tests with 20 hours annealing time and a transient test with a heating rate of 5 – 10 °c/min. to determine the oxidation kinetics of the cr-ni alloy, a set of isothermal experiments was performed. the course of an experiment is shown in figure 2. the test was initiated at a temperature of 90 °c. the specimen was subsequently heated up to the desired temperature with a heating rate of 10 k/min. the 10-minute long isothermal phase followed to homogenize the temperature and to adjust the vapour generation. the subsequent temperature plateau with vapour supply took 20 hours. finally, the specimen 9 a. chalupová, m. steinbrück, m. grosse et al. acta polytechnica ctu proceedings 0 200 400 600 800 1000 1200 1400 0 2000 4000 6000 8000 10000 12000 14000 16000 te m pe ra tu re [° c] time [s] ↗ 5 k/min ↘ 30 k/min ↗ 10 k/min → 10 min ar: 50 ml/s ar: 50 ml/s h2o: 2 g/har: 50 ml/s figure 3. the transient experiment was cooled down to 100 °c with the cooling rate of 5 k/min. the isothermal experiments were performed at the temperatures of 700 °c, 800 °c, 900 °c, 1000 °c, 1100 °c and 1200 °c. to confirm the change in kinetics, the transient experiment was performed. the course of the experiment is summarized in figure 3. the test was initiated at 90 °c. subsequently, the specimen was heated with the heating rate of 10 k/min up to 500 °c. the isothermal period at the temperature of 500 °c followed for 10 minutes. afterwards, the constant steam flow of 2 g/h was initiated, and the specimen continued to heat up to 1240 °c with the heating rate of 5 k/min. finally, the specimen was cooled down to the temperature of 100 °c with the cooling rate of 30 k/min in argon. 2.4. post-test examination the thermogravimetric analysis and mass spectroscopy were measured online during the experiment. eventually, the obtained data were analysed and the expected corrosion rate was calculated based on the specific oxidation behaviour, by means of parabolic or linear kinetics. the specimens were visually analysed to detect any relevant changes in appearance, i.e., the colour of the oxide, macroscopic dissolution, cracking, or spalling. to perform further analysis, it was necessary to cut the specimens into several rings. the metallographic cross-sections were prepared using the standard polishing procedure and metallographic evaluation followed. the thickness of the oxide scale and the wall were measured. additionally, the specimen’s surface was treated with au before scanning electron microscopy (sem) analysis. a sem is an electron microscopy technique that produces images of a sample by scanning the surface with a focused beam of electrons. the electrons interact with atoms in the sample, producing various signals that contain information about the surface topography and composition of the sample. the received signals are collected and examined. specific xray energies or wavelengths are selected and counted figure 4. the determined melting point and the solidification point of cr-ni alloy via energy-dispersive x-ray spectroscopy (eds) for elemental analyses. the surface and under-surface morphologies of crni alloy after oxidation at 1200 °c in steam for 20 hours were analysed using sem/eds. chemical elements present in the specimen were determined qualitatively. 3. results 3.1. the melting point determination during dta, the transition temperatures were determined. the dta graph in figure 4 visualizes dta in relation to temperature. the nature of the reaction was derived from the peak orientation. the melting causes an endothermic reaction, the onset of the signal was determined at 1356.7 °c. the solidification causes an exothermic reaction. according to the graph, it was initiated at a lower temperature of 1355.8 °c. 3.2. high-temperature oxidation during oxidation, the water molecules are adsorbed on the surface of the metal, they dissociate, and the oxygen diffuses into the metal. after reaching the solubility limit of oxygen in the metal, the oxide scale is formed and thickens at the expense of the metal. generally, the rate of thickening determines the oxidation kinetics [5] [6]. the kinetics of oxidation can be described by equation (1). wg s = km · tn (1) where wg is the weight gain of the specimen, s is the surface of the sample, km is the rate constant, and t is the oxidation time of the isothermal experiment. the weight gain per unit area in relation to time for isothermal experiments in steam at various temperatures is shown in figure 5. there are two major trends. the weight gain of the alloy under oxidation 10 vol. 28/2020 high-temperature oxidation of chrome-nickel alloy figure 5. oxidation kinetics of cr-ni at 700 °c, 800 °c, 900 °c, 1000 °c, 1100 °c and 1200 °c in steam. the dot curves indicate a fitting relationship between weight gain over an area unit and oxidation time of 20 hours temperature [°c] time exponent n 700 1.03369 800 0.92483 900 0.82321 1000 0.47945 1100 0.33144 1200 0.41488 table 2. time exponents of the oxidation kinetics of cr-ni alloy at various temperatures temperatures between 700 °c and 900 °c is rather linear. above 1000 °c the curve is more likely parabolic, yet not precisely. the curves in figure 5 show the fitting results of weight gain per unit area in relation to time. time exponents from (1) are summarized in table 2 for all experimental temperatures. figure 6 shows the hydrogen concentration in the off-gas during the isothermal experiment at 1200 °c as a function of time. considering that the steam molar concentration in the fluid is 45%, the hydrogen release rates measured by the mass spectrometer are proportional to wg/s and in good correspondence to the calculation. at the early stage of oxidation, the hydrogen release is increased due to the faster oxidation rate. the subsequent stabilization in oxidation rate can be attributed to the protective oxide scale formation. figure 7 presents the general posttest appearance after isothermal oxidation in steam at 700 °c, 800 °c, 900 °c, 1000 °c, 1100 °c and 1200 °c for 20 hours and after oxidation at a temperature transient with the maximum temperature of, 1250 °c. all specimens showed a compact structure. they differ in colour and degree of degradation. apparently, the oxide scale changes from green monotone grey to monochrome grey with a visible pattern. despite the compact structure of the specimens, at temperatures figure 6. hydrogen content in time during the isothermal experiment at the temperature of 1200 °c figure 7. post-test appearance of cr-ni after oxidation at temperature transient for 150 minutes and after oxidation in steam at 6 different temperatures: 700 °c, 800 °c, 900 °c, 1000 °c, 1100 °c, 1200 °c for 20 hours of 1100 °c and 1200 °c, a fine powder falling from the specimens’ surface to the plate crucible was observed. the metallographic image in figure 8 gives a closer view of the structure of the specimens. at a first glance, the specimens show discontinuous porous oxide scale. thus, it appears unprotective. on the contrary, the oxidation kinetics remained in between parabolic and cubic (n=0.33-0.48) with no evidence of 11 a. chalupová, m. steinbrück, m. grosse et al. acta polytechnica ctu proceedings figure 8. microstructure of specimen cross sections after isothermal oxidation at 1100 °c in steam for 20 hours kinetics transition. a possible explanation could be the delamination of the oxide scale during the cooldown period and breaking of the hard oxide during the metallographic preparation of the samples. this fact is supported by the presence of fine powder in the tg balance volume after the experiment. micrographs in figure 9 to figure 11 present sem analysis of the surface and under-surface morphologies of the cr-ni alloy after oxidation at 1200 °c in steam for 20 hours with corresponding eds measurements. the sem/eds image in figure 9 identified the discontinuous surface layer of cr2o3. the carbon and au impurities are detected due to the polishing procedure and surface treatment before sem/eds. eds mapping in figure 10 shows ni-cr morphology in the specimen’s wall. isolated ti-enriched islands were occasionally observed by eds, as shown in figure 11. 4. discussion the melting point of the cr-ni alloy was determined using the differential thermal analysis to 1356.6 °c. according to the ni-cr phase diagram [7], the melting point of nickel and chromium at the pressure of 101325 pa is 1390 °c, regarding the content of chromium 42%. the phase transformation of the real alloy is influenced by the heating rate and its composition. in comparison, the melting point of zirconium alloy reaches 1850 °c. however, since the exothermic oxidation rate of ni-cr alloy is far more moderate than in the case of zirconium cladding, the melting temperature is expected to be reached later in a typical accident scenario, hence the coping time is increased. consequently, the lower melting point may not be a limiting factor for atf requirements. figure 9. sem micrograph of oxide scale formed on the surface of cr-ni after oxidation at 1200 °c in steam for 20 hours accompanied with eds measurement of the marked point the oxidation kinetics of the cr-ni alloy were studied in the temperature range from 700 °c up to 1200 °c. the alloy exhibited rather linear kinetics after oxidation in steam at temperatures 700 °c and 800 °c. at temperatures from 900 °c to 1200 °c the oxidation rate of the alloy starts to be controlled by diffusion and thus it resembles the parabolic kinetics. it was confirmed that the cr-ni has very low weight gain, i.e., wg/s from equation (1), under high-temperature oxidation. in view of this fact, a very high precision of the experiment is desirable. figure 12 shows the arrhenius plot for various proposed atf designs (cr-ni. fecral, inconel 617) in comparison to standard zr-based cladding materials (zr1%nb, zircaloy-4) and cathcard-pawel correlation. on the whole, all potential atf materials outperform zr-based alloys. it needs to be mentioned 12 vol. 28/2020 high-temperature oxidation of chrome-nickel alloy temperature range [°c] a [mg.dm−2.s1/2] b [k] 700-900 0.2 -6951.9 1000-1200 3339.8 -15468.0 table 3. calculated parameters a and b from arrhenius equation for oxidation kinetics of cr-ni alloy figure 10. sem micrograph of the wall beneath the oxide scale of cr-ni after oxidation at 1200 °c in steam for 20 hours accompanied with eds measurement of the marked point figure 11. sem micrograph of the ti grain formed beneath the oxide scale of cr-ni after oxidation at 1200 °c in steam for 20 hours accompanied with eds measurement of the marked point that the correlation for cr-ni alloy is not highly accurate, given that its kinetics does not correspond to the parabolic law, see table 2. to determine the activation energy from experiments at isothermal temperatures, the arrhenius equation (2) has been used. k = a · e e r·t = a · e− b t (2) where k is the rate constant, a is the frequency factor or preexponential factor, e is the activation energy for the reaction, r is the gas constant, and t is the absolute temperature. therefore, the oxidation rate is a function of reaction temperature and examined material. figure 12. arrhenius-plot of the estimated oxidation rate constants for various alloys, i.e. zr1%nb, zircaloy-4, cr-ni, fecral in steam [8] the parameters a and b were estimated considering parabolic kinetics. results are summarized for two temperature ranges in table 3. 5. conclusions the presented results show a better resistance of crni alloy compared to zirconium alloys during hightemperature oxidation in steam. all visual evaluation, microstructure analysis, and weight gain, together with the derived oxidation kinetics parameters confirmed its stability under severe accident conditions. these conclusions confirm that cr-ni is a promising atf material candidate. the main disadvantage of the the alloy is the high cross-section of thermal neutrons. accordingly, a few possible ideas on how to improve the alloy’s neutron performance were introduced. it has been declared that the cr-ni alloy (42xhm) allows a thinner wall of the cladding compared to zirconium. however, sufficient compensation for the high thermal neutron absorption requires a substantial increase in fuel enrichment [4]. the alloy is often associated with fourth-generation reactors. an implementation of high-density fuel instead of uranium dioxide pellets increases the capacity of uranium, hence the usage of the oxidation-resistant cr-ni alloy as a cladding material could be a possible way to achieve a highly efficient and essentially safe fuel system. acknowledgements the authors thank the whole group of high-temperature materials chemistry (namely: p. severoloh, ulrike stegmaier, jürgen moch) for sample preparation and posttest evaluation. this research was held during the internship, financially supported through the enen+ project, that has 13 a. chalupová, m. steinbrück, m. grosse et al. acta polytechnica ctu proceedings received funding from the euratom research and training work programme 2016 – 2017 – 1 #755576. references [1] k. a. terrani. accident tolerant fuel cladding development: promise, status, and challenges. journal of nuclear materials 501, 2018. doi:10.1016/j.jnucmat.2017.12.043. [2] a. v. vatulin, v. p. kondrat’ev, v. n. rechitskii, m. i. solonin. corrosion and radiation resistance of “bochvaloy” nickel-chromium alloy 46(11):469–473. number: 11 reporter: metal science and heat treatment, doi:10.1007/s11041-005-0004-8. [3] b. gurovich, a. frolov, i. fedotov. improved evaluation of ring tensile test ductility applied to neutron irradiated 42xnm tubes in the temperature range of (500–1100)°c. nuclear engineering and technology 52(6):1213 – 1221, 2020. doi:10.1016/j.net.2019.11.019. [4] a. a. savchenko, m. leontieva-smirnova, g. kulakov, et al. peculiarities of stainless steel application as atf in vver’s. 2018. [5] d. j. young. chapter 1 the nature of high temperature oxidation. in d. j. young (ed.), high temperature oxidation and corrosion of metals (second edition), pp. 1 – 30. elsevier, second edition edn., 2016. doi:10.1016/b978-0-08-100101-1.00001-7. [6] d. j. young. chapter 3 oxidation of pure metals. in d. j. young (ed.), high temperature oxidation and corrosion of metals (second edition), pp. 85 – 144. elsevier, second edition edn., 2016. doi:10.1016/b978-0-08-100101-1.00003-0. [7] c. t. inc. nickel-chromium (ni-cr) phase diagram. http://www.calphad.com/nickel-chromium.html. [8] m. steinbrück, n. vér, m. große. oxidation of advanced zirconium cladding alloys in steam at temperatures in the range of 600–1200 °c. oxidation of metals 76:215–232, 2011. 14 https://doi.org/10.1016/j.jnucmat.2017.12.043 https://doi.org/10.1007/s11041-005-0004-8 https://doi.org/10.1016/j.net.2019.11.019 https://doi.org/10.1016/b978-0-08-100101-1.00001-7 https://doi.org/10.1016/b978-0-08-100101-1.00003-0 http://www.calphad.com/nickel-chromium.html acta polytechnica ctu proceedings doi:10.14311/app.2018.19.0014 acta polytechnica ctu proceedings 19:14–21, 2018 © czech technical university in prague, 2018 available online at http://ojs.cvut.cz/ojs/index.php/app advanced homogenization methods for pressurized water reactors pavel suk department of nuclear reactors, faculty of nuclear science and physical engineering, czech technical university in prague, v holešovičkách 2, 180 00 prague 8, czech republic correspondence: sukpave2@fjfi.cvut.cz abstract. macroscopic cross section generation is key part of core calculation. commonly, the data are prepared independently without a knowledge of fuel loading pattern. the fuel assemblies are simulated in infinite lattice (with mirror boundary conditions). rehomogenization method is based on combination of actual neutron flux in fuel assembly with macroscopic data from infinite lattice. rehomogenization method was implemented into the macrocode andrea and tested on a reference cases. cases consist of fuel cases, cases with strong absorber, cases with absorption rods, or cases with reflector assemblies. testing method is based on a comparisons of homogenized and rehomogenized macroscopic cross sections and later on a comparisons of relative power of each fuel assembly. above that there is comparison of eigenvalue. keywords: homogenization, rehomogenization, full core calculation, andrea, helios, macroscopic data preparation, neutron flux. 1. introduction and main motivation calculation of main neutronic data for power nuclear reactors is complicated multi-level process. despite steadily rising computational power of computers, there is no way to calculate real fuel loading pattern using accurate deterministic or stochastic codes. this problematic is divided in two levels: preparation of macroscopic data and calculation real core with macrocodes with diffusion or simplified transport solution. the data preparation process for fuel assemblies is provided by deterministic or stochastic codes (microcodes). deterministic codes solve transport equation (the scale newt [1], the helios [2]) and the stochastic codes (the serpent [3]) simulate the batch of particles. the data for macrocodes are prepared with simulation of identical fuel assemblies in infinite lattice. this approach is called standard homogenization method. the method considered some simplification which are not fulfilled in the real fuel loading pattern. the simplifications are: • zero neutron escape over the boundary of fuel assembly, • symmetrical distribution of neutron flux inside fuel assembly, • energy spectrum of neutron during data preparation process is very different from real energy spectrum. comparison of neutron flux inside fuel assembly is illustrated in figure 1. fuel assembly is simulated in infinite lattice (left) and next to the strong absorber (right). the figure clearly shows that in simulated fuel assembly next to the strong absorber is significant neutron flux tilt, which can’t be considered by standard homogenization method. the solution of unfulfilled assumptions (symetrical distribution of neutron flux inside fuel assembly and zero escape over the boundary of fuel assembly) of standard data preparation process can be found with using rehomogenization method. the rehomogenization method is based on method cited in article of aldo dall’osso [4], who tested advanced spatial rehomogenization. this method is based on a actual distribution of neutron flux inside fuel assembly during the full core calculation. the data prepared by this method should better reflect spatial dependence of neutron flux during the data preparation process and the neutron escape from fuel assembly. since the rehomogenization method depend on fuel pattern, it must be implemented in macrocode, where are the data continuously edited with respect to calculation of main neutron-physical characteristics. 2. rehomogenization theory the rehomogenization method can be divided into two parts. in the first part, the neutron flux in the fuel assembly is calculated. in the second part, the macroscopic data are prepared with actual neutron flux. since the main intention of the method is to ensure more realistic data in macrocode, there is need to use data provided by macrocode itself during the calculation. the interface and mean neutron flux values were used for the neutron flux distribution calculation inside fuel assembly, because the macrocode andrea calculate them. neutron fluxes on the interfaces of fuel assembly (interfaces e, ne, nw, w, sw, se) and mean value of neutron flux in fuel assembly were 14 http://dx.doi.org/10.14311/app.2018.19.0014 http://ojs.cvut.cz/ojs/index.php/app vol. 19/2018 advanced homogenization methods for pressurized water reactors figure 1. distribution of neutron flux in fuel assembly simulated in infinite lattice (left) and behind strong absorber (right). used as input data for calculation of neutron flux distribution. 2.1. interpolation of neutron flux calculation of the neutron flux distribution can be done by two dimension interpolation function. in terms of the simple approach of interpolation, the deviation of the neutron flux was interpolated using the following polynomial functions: φ(x, y)−φ = c1 · x+ c2 · y+ +c3 · (x2 + y2 − c) + c4 · (x2 − y2)+ +c5 · x · y. (1) constants c1 to c5 are interpolation constants, constant c is orthogonalization constant. to consider local deformation in neutron flux (for example by cell without fuel), the interpolated neutron flux for each cell with coordinates (xb, yb) were multiplied by factor φ∞ b φ∞ , where φ∞b is neutron flux of cell b in infinite lattice and φ∞ is mean value of neutron flux in fuel assembly simulated in infinite lattice. comparison of interpolated neutron flux with reference values obtained by simulation of case in deterministic code shows that neutron flux cell data does not agree with real distribution of neutron flux in fuel assembly. it turns out that problem is in complete solution of diffusion equation. interpolation using polynomial function can express only particular solution of diffusion equation, but it can not approximate homogeneous part of solution. 2.2. reconstruction of neutron flux the reconstruction method of neutron flux is more complicated approach in comparison with the interpolation method. the reconstruction is based on solution of two groups diffusion equation in fuel assembly: ∇2~φ = ( σa1−νσf1/kef d1 −νσf2/kef+σg2g1 d1 −σg1g2 d2 σa2 d2 ) ~φ, (2) where the quantities in equation are: • σa1, σa2 macroscopic cross sections for neutron absorption in energetic groups 1 and 2, • νσf1, νσf2 macroscopic cross sections for fission in both groups multiplied by average number of neutron from fission, • d1, d2 diffusion coefficient, • σg1g2, σg2g1 scattering cross section from first energy group to second and conversely, • ~φ is vector of scalar neutron flux in both groups, multiplied equation 2 by vector ~v, the diffusion equation can be rewritten to form: ~v · ∇2~φ = ~v ·m~φ = mt (~v~φ) (3) if is vector ~v chosen as an eigenvector of matrix mt , eigen to eigenvalue λg, for its component applies: vg = (m21,m11 − λg) (4) equation 3 can be rewritten as two simple wave equations: ∇2ξg − λgξg = 0, (5) where g takes value 1 and 2. using vector form, the system can be written as: ~ξ = r~φ, (6) 15 pavel suk acta polytechnica ctu proceedings where matrix r is: r = ( σa1−νσf1/kef d1 −νσf2/kef+σg2g1 d1 −σg1g2 d2 σa2 d2 ) . (7) the goal of this method is to obtain quantity ξg, from which neutron flux can be obtained by using inverse matrix r in both energy groups. general solution of wave equation 5 is: ξg(~r, φ) = ∫ ag(α)cn (√ |λg|~r cos(φ− α) ) dα + ∫ bg(α)sn (√ |λg|~r cos(φ− α) ) dα, (8) where ag(α) and bg(α) are constants dependent on interface of fuel assembly and function sn and cn are defined according to value of eigenvalue λg, see table 1. general function real function condition sn(x) sin(x) λg < 0 sinh(x) λg > 0 cn(x) cos(x) λg < 0 cosh(x) λg > 0 table 1. function sn and cn for smooth neutron flux profile reconstruction given that, the interfaces of the fuel assembly are on discrete coordinates and discrete angles. the equation 8 can be rewritten as: ξg(~r, φ) = 2∑ i=0 ag,icn (√ |λg|~r cos(φ− iπ3 ) ) + 2∑ i=0 bg,isn (√ |λg|~r cos(φ− iπ3 ) ) , (9) where constants ag,i and bg,i are determined by known mean value of neutron flux on each interface of fuel assembly. by express of mean value of quantity ξg from equation 9, it could be obtained for each fuel assembly interface set of equation for k = 1, 2, ..., 6 as: ξg(k) = 2∑ i=0 ag,iai,k + 2∑ i=0 bg,ibi,k, (10) where ai,k and bi,k are average values of chosen function over fuel assembly interface. the equation system can be solved due to known values ξg(k) on fuel assembly interface, from which can be obtained constants ag,i and bg,i. based on known constants, course of quantity ξg(~r, φ) can be determined in all fuel assemblies. thanks to the knowledge ξg(~r, φ), equation 6 can be solved. it is possible to calculate neutron flux in elementary volume of coordinates ~r and φ from the solution of equation 6. [5] for more accurate reconstruction of neutron flux, there are included values of neutron flux in assembly corners. equation system 9 is replaced by: ξg(~r, φ) = 5∑ i=0 ag,icn (√ |λg|~r cos(φ− iπ6 ) ) + 5∑ i=0 bg,isn (√ |λg|~r cos(φ− iπ6 ) ) . (11) 2.3. reconstructed neutron flux application to macroscopic cross section values the rehomogenization is data preparation process based on more accurate distribution of neutron flux in fuel assembly. the data preparation process is based on the flux-weighted volume method (fwv): σg = ∑ h∈g ∑ i viσi,hφi,h∑ h∈g ∑ i viφi,h , (12) where i indicates spatial discrete cells with different materials, vi are volumes of these cells and σi,h are macroscopic cross sections of cell i and energy h, which is from interval g = (eg−1, eg) and φi,h is neutron flux in cell i and energy h. it has been proven that the identically assembly data as the data homogenized from macroscopic cross sections for each cell can be prepared by fwv. using the reconstructed neutron flux and cell macroscopic cross sections from infinite lattice, the new data for fuel assemblies were obtained thank to equation 12. 3. method testing the method testing was performed based on data comparison with reference model case calculated with accurate deterministic code helios. these cases could be divided as: • fuel cases, • cases with strong absorber, • cases with regulation rods, • cases with reflector assembly. test tasks are divided to two independent categories. the method was firstly tested using comparison with accurate data for neutron flux distribution calculation in fuel assembly test of the method. these data were found by simulating reference case in the microcode helios. in the second step, the neutron flux distribution was found using data provided by diffusion approach practice test. the flowchart of the data flow is in figure 3. the orange arrows show the data flow during the test of the method. the green arrows show the data flow during the practice test. the data from flux interpolation test had high deviation and due to it, they were not used for the rehomogenization process. if the rehomogenized data are in better agreement with reference data and their change compared to the solution of infinite lattice in absolute value is more 16 vol. 19/2018 advanced homogenization methods for pressurized water reactors a1 a2 a3 a4 figure 2. illustration of larger case with caption of each fuel assembly. figure 3. the flowchart of the both testing method. 17 pavel suk acta polytechnica ctu proceedings figure 4. deviations of smooth profile neutron flux reconstructions for a1 from accurate solution with using andrea reconstruction process in case a with data from helios. than 10%, than the data are with green colour. if the rehomogenized data are in wore agreement with reference data and their change compared to the solution of infinite lattice in absolute value is more than 10%, than the data are with red colour. the results with lower absolute change than 10% are with black colour. 3.1. test of method this part of testing is focused on analysis of all the benefits of the rehomogenization method in case of accurate input parameters for calculation of smooth neutron flux profile inside fuel assembly. the monitored parameters were the neutron flux distribution and the macroscopic cross section. moreover the eigenvalue was calculated from macroscopic cross section using two groups diffusion equation in infinite lattice. 3.1.1. case a the case a consists of fuel assembly a1-a13a, a2a40a6 [6]. fuel assembly a13a is situated in the middle of the case, around its are 6 fuel assemblies a40a6. the whole structure is simulated inside infinite lattice. it is quite realistic model of fuel assemblies distribution in a reactor core. results of deviation of smooth profile neutron flux reconstruction with accurate helios data for each fuel assembly and energy group are in figures 4 and 5. figures 4 and 5 show good compliance of smooth profile neutron flux, obtained by reconstruction process, with reference calculation via helios. the macroscopic cross sections prepared using reference calculation in case together with deviation of infinite lattice calculation, respectively rehomogenized solution from values prepared in case are in the table 2. deviations were calculated using equation: ∆ref,x = σref − σx σref · 100, (13) where quantity x means reconstructed macroscopic figure 5. deviations of smooth profile neutron flux reconstructions for a2 from accurate solution with using andrea reconstruction process in case a with data from helios. type σref(cm−1) ∆ref,∞(%) ∆ref,rec(%) fuel assembly a1 a13a σa,g1 8.07e-03 -0.748 -0.580 σa,g2 5.64e-02 -0.759 -1.113 dg1 1.32e+00 -2.445 -2.454 dg2 4.09e-01 4.562 4.410 νσf,g1 3.95e-03 -1.684 -1.482 νσf,g2 6.13e-02 -0.640 -1.287 κσf,g1 4.99e-14 -1.607 -1.405 κσf,g2 8.15e-13 -0.640 -1.286 σg1g2 1.86e-02 -2.448 -2.570 σg2g1 1.01e-03 9.454 9.505 fuel assembly a2-a40a6 σa,g1 9.74e-03 0.097 -0.117 σa,g2 1.07e-01 0.106 -0.088 dg1 1.34e+00 0.285 0.301 dg2 3.99e-01 -1.117 -1.127 νσf,g1 7.43e-03 0.128 -0.103 νσf,g2 1.60e-01 0.132 -0.125 κσf,g1 9.59e-14 0.125 -0.107 κσf,g2 2.12e-12 0.132 -0.125 σg1g2 1.64e-02 0.231 0.320 σg2g1 1.64e-03 -1.144 -1.136 table 2. macroscopic cross sections prepared using simulation in reference case (ref) for case a and deviations solution in infinite lattice, respectively rehomogenized cross sections. cross sections or cross sections prepared in infinite lattice calculation. besides macroscopic cross sections, eigenvalues calculated from two group diffusion equation and rehomogenized data were analysed. the values are provided in table 3. eigenvalue provides additional integral quantity, decisive about suitability of rehomogenization method of macroscopic cross sections. 18 vol. 19/2018 advanced homogenization methods for pressurized water reactors fa k∞ref ∆k∞ref,∞ ∆k∞ref,rec a1 0.903885 -282 pcm -533 pcm a2 1.218033 59 pcm 47 pcm table 3. eigenvalues calculated from macroscopic cross section of case a using two group diffusion equation. figure 6. deviations of smooth profile neutron flux reconstructions for a2 from accurate solution with using andrea reconstruction process in case f with data from helios. 3.1.2. case f case f is larger case, in which was central fuel assembly replaced by model of hrk regulation assembly. model with hrk was chosen as a real problem with higher neutron absorption than in model with regulation cluster. regulation by hrk assembly is used for instance in reactor wwer-440, where is fuel assembly replaced by absorbing part. interlacing of reconstructed neutron flux for fuel assembly adjoining to hrk assembly (a2) is in figure 6. the neutron flux deviation are on the level of -5 to 3% in case of interface next to the hrk assembly. the neutron flux was reconstructed for other fuel assemblies with deviations around 1% in comparison with accurate solution from helios. 3.1.3. case g within the complete description of possible use of rehomogenization method, there was also simulated situation with reflector assembly. central assembly was replaced by part of wwer-1000 reflector assembly, the rest three fuel assemblies were a40a6. the figure 7 shows comparison of reconstructed neutron flux with accurate solution obtained by simulation in helios for fuel assembly next to the reflector assembly. maximal deviations in the reconstruction are in the thermal group near the reflector interface. the deviations were up to 7%. 3.2. practice test in the second step there were used boundary conditions calculated via andrea macrocode for neutron flux distribution calculation. under this subsection type σref(cm−1) ∆ref,∞(%) ∆ref,rec(%) σa,g1 9.74e-03 0.119 -0.088 σa,g2 1.07e-01 -0.257 -0.572 dg1 1.34e+00 0.418 0.433 dg2 4.05e-01 0.372 0.339 νσf,g1 7.43e-03 0.116 -0.112 νσf,g2 1.59e-01 -0.089 -0.376 κσf,g1 9.59e-14 0.119 -0.110 κσf,g2 2.12e-12 -0.089 -0.376 σs,g1 1.64e-02 -0.146 -0.059 σs,g2 1.67e-03 0.400 0.351 table 4. macroscopic cross sections prepared using simulation in reference case (ref) for case f and deviations solution in infinite lattice, respectively rehomogenized cross sections. k∞ref ∆k∞ref,∞ ∆k∞ref,rec 1.218522 108 pcm 177 pcm table 5. eigenvalues calculated from macroscopic cross section of case f using two group diffusion equation. figure 7. deviations of smooth profile neutron flux reconstructions for a2 from accurate solution with using andrea reconstruction process in case g with data from helios. type σref(cm−1) ∆ref,∞(%) ∆ref,rec(%) σa,g1 9.78e-03 0.540 0.335 σa,g2 1.07e-01 -0.005 -0.325 dg1 1.34e+00 0.256 0.271 dg2 4.03e-01 -0.210 -0.245 νσf,g1 7.44e-03 0.196 -0.032 νσf,g2 1.59e-01 -0.038 -0.328 κσf,g1 9.60e-14 0.238 0.010 κσf,g2 2.12e-12 -0.038 -0.328 σs,g1 1.65e-02 0.702 0.787 σs,g2 1.67e-03 0.939 0.887 table 6. macroscopic cross sections prepared using simulation in reference case (ref) for case g and deviations solution in infinite lattice, respectively rehomogenized cross sections. 19 pavel suk acta polytechnica ctu proceedings k∞ref ∆k∞ref,∞ ∆k∞ref,rec 1.216401 -104 pcm -33 pcm table 7. eigenvalues calculated from macroscopic cross section of case g using two group diffusion equation. type σref(cm−1) ∆ref,∞(%) ∆ref, rec(%) σa,g1 9.74e-03 2.500 2.507 σa,g2 1.07e-01 10.100 10.171 σf,g1 2.93e-03 0.127 0.127 σf,g2 6.56e-02 0.131 0.134 σs,g1g1 5.23e-01 -0.018 -0.018 σs,g1g2 1.64e-02 0.232 0.232 σs,g2g1 1.64e-03 -1.140 -1.143 σs,g2g2 1.25e+00 0.111 0.112 dg1 1.34e+00 0.285 0.285 dg2 3.99e-01 -1.120 -1.117 table 8. σ homogenized in reference case (ref) and deviation infinite and reconstructed cross sections for case a with boundary conditions from andrea. the neutron fluxes on the interfaces of fuel assemblies, macroscopic cross sections and in conclusion eigenvalues are compared at first. the eigenvalues in this approach were calculated via macrocode andrea. the neutron leakage from the system was calculated using the helios microcode (using diffusion coefficient and buckling factor). this neutron leakage from the system was installed to macrocode andrea. the eigenvalue could be reached as keff = 1 with data from case model. the deviation of eigenvalue from 1 is given by deviations in macroscopic cross sections from reference solution. the deviation of eigenvalue from criticality is in this case integral parameter of rehomogenization. case a the macroscopic cross sections for fuel assembly a2 in case a are compared in the table 8. the deviation of eigenvalue from criticality increased from 20 pcm to 30 pcm in this configuration with using rehomogenized cross section. case f the compared macroscopic cross sections for fuel assembly a2 in case f are in the table 9. the deviation of eigenvalue from criticality decreased from 314 pcm to 123 pcm in this configuration with using rehomogenized cross section. case g the macroscopic cross sections for fuel assembly a2 in case g are compared in the table 10. the deviation of eigenvalue from criticality decreased from 99 pcm to 46 pcm in this configuration with using rehomogenized cross section. type σref(cm−1) ∆ref,∞(%) ∆ref, rec(%) σa,g1 9.74e-03 2.497 2.507 σa,g2 1.07e-01 10.104 10.171 σf,g1 2.93e-03 0.127 0.127 σf,g2 6.56e-02 0.131 0.134 σs,g1g1 5.23e-01 -0.018 -0.018 σs,g1g2 1.64e-02 0.232 0.232 σs,g2g1 1.64e-03 -1.143 -1.143 σs,g2g2 1.25e+00 0.111 0.112 dg1 1.34e+00 0.285 0.285 dg2 3.99e-01 -1.116 -1.117 table 9. σ homogenized in reference case (ref) and deviation infinite and reconstructed cross sections for case f with boundary conditions from andrea. type σref(cm−1) ∆ref,∞(%) ∆ref, rec(%) σa,g1 9.77e-03 2.783 2.794 σa,g2 1.07e-01 9.725 9.789 σf,g1 2.93e-03 0.141 0.141 σf,g2 6.54e-02 -0.126 -0.123 σs,g1g1 5.23e-01 0.091 0.091 σs,g1g2 1.65e-02 0.475 0.475 σs,g2g1 1.67e-03 0.678 0.678 σs,g2g2 1.25e+00 -0.343 -0.342 dg1 1.34e+00 0.371 0.371 dg2 4.03e-01 -0.081 -0.081 table 10. σ homogenized in reference case (ref) and deviation infinite and reconstructed cross sections for case g with boundary conditions from andrea. 3.3. results evaluation the benefits from rehomogenization process with the reference neutron flux are not too noticeable and macroscopic cross sections change a little. this fact was found during the analysis based on the accurate data from the transport code helios. total influence of rehomogenization process showed up beneficial in case a case g. the macroscopic cross section for all fuel assemblies were apparently much more dependent on the neutron spectra during the data preparation process, than on the distribution of neutron flux. unfortunately the spectral homogenization process could not be solved without knowledge of loading pattern. deviations of neutron flux on the interfaces of fuel assembly between accurate solution from transport code and diffusion approach are up to 15%. the macroscopic cross sections report the same behaviour as in the case of the accurate boundary values despite such deviations in boundary conditions for reconstruction. oppositely to the test of method task, there were registered benefits in case of fuel assembly next to the hrk assembly and next to the reflector assembly, but no benefits in case consisted from two fuel assemblies. the neutron flux tilt is apparently too low to 20 vol. 19/2018 advanced homogenization methods for pressurized water reactors bring better results because too high deviations in the boundary neutron fluxes and due to there is no way to prepare better macroscopic data for these problem. 4. conclusion the main goal of study is to analyse the benefits, which the method can bring to the full core calculation. new sets of macroscopic data were calculated via fwv with reconstructed neutron flux and macroscopic cross sections (calculated in infinite lattice) for each cell. the whole method was tested with input data for reconstruction process obtained by simulation in helios. the general trend of the rehomogenized data was not found and due to there is no way to find total influence on the calculation. results of eigenvalue show that rehomogenization can bring better results in case a and case g. in the second step, the method was tested with neutron flux boundary condition calculated by diffusion code andrea. benefits of this approach were minimal, because the cross sections were still the same, but total benefit was positive. eigenvalues with rehomogenized macroscopic cross section were relatively identical to eigenvalue from reference case. during this study was found that neutron flux inside fuel assemblies can not be approximated by easy polynomial functions, but there is need to use reconstruction method based on a solving diffusion equation. despite the correctly reconstructed neutron flux shape, the macroscopic cross sections do not change significantly and this change was not essential to calculation via andrea. this situation led to the question if is possible to separate spatial and spectral homogenization process. probably this effect can be analysed by performing rehomogenization process using more neutron groups. list of symbols φ(x, y) neutron flux in the cell with coordinates x and y [cm−2s−1] φ mean value of neutron flux in fuel assembly [cm−2s−1] ~φ vector of grouped scalar neutron fluxes [cm−2s−1] σig macroscopic cross section of reaction i and energy group g [cm−1] ν neutron fission yield [–] κ energy released during fission process [–] keff effective eigenvalue [–] dg diffusion coefficient of group g [cm] vi volume of cell i [cm3] k∞ eigenvalue calculated using two group diffusion equation [–] ∆k∞min,x deviation of eigenvalue of calculation x calculated with two group diffusion equation from reference calculation [pcm] a index for absorption [–] f index for fission [–] g1g2 index for scatter from group 1 to group 2 [–] g2g1 index for scatter from group 2 to group 1 [–] min index for reference calculation in case [–] ∞ index for calculation in infinite lattice [–] rec index for calculation from rehomogenized data [–] references [1] m. a. jessee, m. d. dehart. newt: a new transport algorithm for two-dimensional discreteordinates analysis in non-orthogonal geometries. ornl/tm-2005/39, 2016. [2] c. wemple, h.-n. gheorghiu, r. stamm’ler, e. villarino. the helios-2 lattice physics code. https://www.studsvik.com/sharepointfiles/the% 20helios-2%20lattice%20physics%20code.pdf. [3] j. leppänen. serpent a continous-energy monte carlo reactor physics burnup calculation code. vtt technical research centre of finland, 2015. [4] a. dall’osso. a spatial rehomogenization method in nodal calculations. science direct, 2006. https://www.researchgate.net/publication/ 245135037_a_spatial_rehomogenization_method_in_ nodal_calculations. [5] f. havlůj, r. vočka. andrea developer documentation. nuclear research institute at řež, 2005-2013. [6] f. havlůj, j. hejzlar, r. vočka. andrea data libraries, vver-1000/tvsa-t. nuclear research institute at řež, 2016-. 21 https://www.studsvik.com/sharepointfiles/the%20helios-2%20lattice%20physics%20code.pdf https://www.studsvik.com/sharepointfiles/the%20helios-2%20lattice%20physics%20code.pdf https://www.researchgate.net/publication/245135037_a_spatial_rehomogenization_method_in_nodal_calculations https://www.researchgate.net/publication/245135037_a_spatial_rehomogenization_method_in_nodal_calculations https://www.researchgate.net/publication/245135037_a_spatial_rehomogenization_method_in_nodal_calculations acta polytechnica ctu proceedings 19:14–21, 2018 1 introduction and main motivation 2 rehomogenization theory 2.1 interpolation of neutron flux 2.2 reconstruction of neutron flux 2.3 reconstructed neutron flux application to macroscopic cross section values 3 method testing 3.1 test of method 3.1.1 case a 3.1.2 case f 3.1.3 case g 3.2 practice test 3.3 results evaluation 4 conclusion list of symbols references acta polytechnica ctu proceedings doi:10.14311/app.2018.19.0046 acta polytechnica ctu proceedings 19:46–52, 2018 © czech technical university in prague, 2018 available online at http://ojs.cvut.cz/ojs/index.php/app selection of reference value of longitudinal to transversal ultrasonic waves velocity ratio on pressure purpose steels for detection of creep tomáš zavadila,b a advanced technology group, s.r.o., toužimská 771, 199 02 prague, czech republic b department of materials, faculty of nuclear sciences and physical engineering, czech technical university in prague, trojanova 13, 120 01 prague, czech republic correspondence: zavadilt@atg.cz abstract. common degradation process for industrial equipment in power and petrochemical industry is creep. the ultrasonic waves velocities and their ratio can provide feedback on stress-strain conditions generated by heat. selection of potentially critically affected parts of pressure equipment can be theoretically accessed by their means. selection of such critical parts or areas is done by comparison with reference benchmark value. setting the standard reference value for conventional pressure purpose steel is crucial so it can be later used as a benchmark for selection of critical areas on equipment under operation. the article proposes a benchmark value to be utilized for p265gh pressure purpose steel and steels of similar grade. keywords: residual stress, non-destructive testing, ultrasonic testing, creep. 1. introduction creep is one of the main phenomena responsible for the failure of plant pressure equipment operating at elevated temperatures. creep is associated with the synergistic effect of changes of microstructure and strain accumulation, which leads to nucleation, growth and coalescence of microvoids, and subsequently to component failure. extending the service life of major components is crucial in high temperature plant applications. the life expectancy is based on the ability of the material to retain its high-temperature creep strength for period of at least twice the projected design life. development of new methods that shall be able to assess the material condition is therefore necessary [1]. various non-destructive testing techniques have been developed to detect the creep damage. techniques range from acoustic emission, infrared thermography, ultrasonic attenuation and velocity measurements, acoustic harmonic measurements up to eddy currents [2, 3]. acousto-elastic and acousto-plastic effects are responding to stress in elastically deformed body and to residual stresses created by macroscopic deformation. most of residual stresses are induced by plastic strain. creep is a long-time plastic deformation of equipment subject to heat and pressure, therefore acoustic measurements of residual stresses are suitable candidate for the proposed application for detection of creep degradation process. in order to measure the response to creep degradation process it is necessary to measure the stress values itself, or compare individual measurements suspected from creep to a reference benchmark value for unaffected material. the following article is dealing with setting of this reference benchmark value. 2. stress measurement by conventional ultrasonic testing the ultrasonic waves velocity is a function of stress σ as described by obraz [4]. example of a simple uniaxial stress measurement in a bar may be written as follows: σ = δt− (βt + αt )∆t βσ + 1 ε (1) where δt = ∆t/t0 is fractional change in time of flight (tof), αt and βt are coefficients of heat expansion, ∆t is change of temperature and βσ is acoustoelastic coefficient and ε is the fractional change in length. if the ∆t → 0 (there is no significant temperature change), the formula can be simplified as: σ = δt βσ + 1 ε (2) we may use the tof of either longitudinal ultrasonic waves or transversal waves. putting the tof values to a ratio, the situation further transforms to: σ = t0t t0l − tt tl (βσl + 1 ε )[ t0t t0l + tt tl ] (3) where t0t /t0l is tof ratio on tested part before applied loading and tt /tl after the applied loading. this formula, however, is difficult to use during inservice inspections for the following reasons: 46 http://dx.doi.org/10.14311/app.2018.19.0046 http://ojs.cvut.cz/ojs/index.php/app vol. 19/2018 selection of ultrasonic waves velocities for detection of creep (1.) the equipment’s original state is not available for comparison. (2.) the fractional change of length ε cannot be calculated as the precise dimensions may not be known. (3.) the stress in operated equipment is rather multiaxial than uniaxial, especially when considering creep-induced strengthening. therefore despite the ultrasonic waves velocity (and respective tof) are changed due to the induced residual stress, it cannot be measured directly. this implies that precise measurement of stress σ cannot be used and ordinal means of evaluation (i.e. comparison of tested area with (a set of) reference samples) for detection of critical areas is the only option. the ultrasonic waves velocities for generally anisotropic material of the tested part are given (as proposed by according to schneder and goebbels [5]) as follows: czz = √ λ+ 2µ+ (2l + λ)ξ + (4m+ 4λ+ 10µ)εz ρ czx = √ µ+ (λ+m)ξ + 4µεz + 2µεx − 0.5nεy ρ czy = √ µ+ (λ+m)ξ + 4µεz + 2µεy − 0.5nεx ρ (4) where cij is ultrasonic waves velocity in plain ij, ρ is density, εi is strain in the given axis, ξ = εx + εy + εz, λ and µ are lamé’s constants, and l, m, n are murnaghan’s constants. young modulus e and shear modulus g are linked with lamé’s constants as described in the formula below: e = ν(3λ+ 2µ) λ+ µ ; g = µ (5) where ν is poisson’s ratio. assuming the tested volume of the tested part has isotropic behavior (i.e. the material properties are not changing with directions) and using the abovementioned formula (5), the equations for longitudinal and transversal waves velocity (4) may be then further simplified to the following: cl = √ e ρ 1− ν (1 + ν)(1− 2ν) ct = √ g ρ = √ e ρ 1 2(1 + ν) (6) where e is young modulus, g is shear modulus, ρ is density and ν is poisson’s ratio. the common observation is that the longitudinal waves velocity cl increases with applied load in elastic region while the transversal waves velocity decreases. as was experimentally confirmed by the team from det norske veritas [6], the increase of velocity of the longitudinal waves is limited and when it reaches a critical level of strain, it starts to decrease again. similar result was observed also for transversal waves by kobayashi [7] where the velocity decreases in elastic region, while in the plastic region it again increases. the behavior was described as being a consequence of anisotropy in elastic properties and inhomogeneous localization of plastic strain [8, 9]. such may include creation of point defects, slip bands and yield vertex which is responsible for degradation of elastic modulus. the inverse behavior of longitudinal and transversal waves’ velocity may be used advantageously when observed as a ratio of both values, i.e. cl/ct (also l/t ratio, dimensionless). the problem can be then transformed to a single-variable equation (only variable is the poisson’s ratio) with monotone-increasing features: cl ct = √ e ρ 1− ν (1 + ν)(1− 2ν) √ ρ g = √ 2(1− ν) 1− 2ν = f(ν) (7) this can help to determine the current stress/strain relation as was observed by kumar et al. [10] in their article about correlation of transversal (and longitudinal) wave velocity and poisson’s ratio. young and shear modulus tend to vary in the same direction in any metal material. this implies that the variation of the poisson’s ratio is dependent only on how much each of the moduli will be affected. let’s assume the range of poisson’s ratio is practically bound to 0 < ν < 0.5 (for isotropic solids) [11]. the higher the ratio the less volume change during deformation, where ν = 0.5 means no volume change. plot of the abovementioned part of the equation (7) is on fig. 1. poisson’s ratio as ν = 0.25 ÷ 0.30 is a common value for conventional structural steels and similar, so this provides preliminary expected range of l/t ratio values of cl/ct = 1.73÷ 1.87. 3. effect of creep on l/t ratio values ongoing research of the author [13] performed measurements on collapsed membrane wall from p265gh pressure purpose steel that was subject to creep rupture. the observed results concluded the it was shown that the l/t ratio is sensitive to plastic deformation due to strengthening mechanisms and is able to detect areas exposed to tension or compression due to bending as well as plastic deformation due to combined effect of internal pressure and heat. the values of l/t ratio in this experiment proven to be in range of 1.80 ÷ 1.85 in measurement spots far from the area affected by creep. the values were growing from the reference value towards its maximum of 2.086 located on the crack edge from all directions. the affected area of elevated l/t ratio values was < 50 mm from the crack edge. the key results are shown in fig. 2. from the fig. 2 it can be assumed that the 1.85 might be the benchmark reference value for p265gh 47 tomáš zavadil acta polytechnica ctu proceedings figure 1. cl/ct as a function of the poisson’s ratio ν for isotropic continuum. the selected area of values common for structural steels (ν = 0.25 ÷ 0.30) provides expected range of cl/ct = 1.73 ÷ 1.87 [12]. figure 2. sample subject to creep rupture with measured values of l/t ratio along the planes r1 (blue), r2 (red) and r3 (green). connecting lines in the charts do not provide any physical relation or a trend, they are for better orientation only [13]. grade pressure purpose steel. if the premise is accepted, the elevated values of 2.086 were above the 3s (3 standard deviations) of the assumed benchmark value and therefore the elevated values should be considered as relevant and referring about non-standard stress-strain conditions, potentially caused by a degradation process as creep (the type degradation process is assumed due to knowledge of the history of operation of the particular tested part). the following part of the article focuses on finding a standard benchmark value for unaffected material (i.e. non-degraded) and therefore proving the abovementioned assumption is justifiable. 4. experimental setup the experiment intends to find expected value that can be used as a standard global benchmark value of the l/t ratio measured on pressure purpose steel that is not subject to long-time processes causing high residual stresses. a set of steels with similar chemical composition (to each other as well as to the original p265gh steel) and different levels of heat treatment have been tested and mutually compared. the each of the following: (1.) expected value and standard deviation of each of the steels, (2.) value trend based on chemical composition and heat treatment, (3.) and range of values for the whole steel group, shall then help determining what are the benchmark l/t ratio values as well as what are the values that might be already considered as caused by severe degradation processes. if the values will be in controlled range for unaffected steel (i.e. standard deviation of l/t ratio values for samples with the same carbon content but various heat treatment shall be low), it can be assumed 48 vol. 19/2018 selection of ultrasonic waves velocities for detection of creep that the l/t ratio has materialand heat-treatmentspecific values. that would imply a significant change of material properties due to long exposure to heat is measurable, and clearly distinguishable from unaffected material. for this experiment, the level of l/t ratio considered as out of standard value range shall be defined as: ∆ cl ct = ∆ cl ct (σ) > 3s; σ � σ0 (8) or more precisely: ∆ cl ct (σ) > 3 ·maxj{sj} = = 3·maxj √√√√ 1 n − 1 nj∑ i=0 ( cl cti − 〈 cl ct 〉j)2; σ � σ0 (9) where sj is the standard deviation of the steel j, nj is the number of measurements for the benchmark value of steel j, 〈cl/ct 〉 is the average mean value of the all steel types, σ0 a generally unknown stress level of steel that is not subject to longtime exposure to heat and σ is a stress of affected (exposed, degraded) volume. the result shall be compared with the expected result acc. to fig. 1, where it should fit the expected range of values 1.73÷ 1.87. 4.1. samples samples subject to the experiment were short bars with a length of 20 mm and various diameter up to 20 mm. samples were from three types of standard structural steel (c16e/1.1148, c35/1.0501, c55/1.0535) heat treated in a laboratory conditions for 2 quenching temperatures tq (850 °c and 950 °c) and 4 various tempering temperatures tt (400 °c, 500 °c, 600 °c, and 700 °c) for one hour. the samples were in totally 24 sets and each set of samples had between 4 and 5 samples. the selected steel types were similar in the chemical composition to the conventional p265gh pressure purpose steel [12]. the closest steel (by chemical composition) to p265gh was c16e, other steels had mainly higher carbon content, which assumes different (higher) standard values for the l/t ratio. the heat-treatment processings were selected to simulate maximal structural changes that are not caused by extreme degradation processes (i.e. mainly phase transform and recrystallization effects). 4.2. equipment measurement was performed on sonatest sitescan 250s by ultrasonic pulse-echo technique. the standard ultrasonic testing (ut) equipment was selected on purpose to 1) simulate standard in-service inspection options of measurement and 2) to fit the measurement with analysis done on collapsed membrane wall in experiment briefly described in chapter 3. the time of flight (tof) was measured directly on the device and it was recalculated to ultrasonic waves’ velocity. conventional direct ut probes sonatest rdt2550 with peak frequency 4.64 mhz for longitudinal waves and panametrics v155 with peak frequency of 4.25 mhz for transversal waves were used. measurement areas were flat, with smooth surface and without any corrosion products that may affect the measurement. the precise thickness was measured by a micrometer. 5. results general the measured values were well controlled with l/t ratio in the range of cl/ct = 1.812 ÷ 1.835. the average l/t ratio was 〈cl/ct 〉 = 1.82. the maximal nominal value as well as standard deviation s for the measurements was maxj{sj} = 0.005 for j equal to steel c55 quenched on tq = 950 °c, which was fully phase transformed to martensitic structure. steel c16e steel c16e/1.1148 is by chemical composition closest to the p265gh steel. compared to the other groups it has the lowest amount of carbon c and higher average of manganese mn. values of the l/t ratio for this steel have the average of 〈cl/ct 〉c16e|850 = 1.816± 0.001 for group quenched on tq = 850 °c and 〈cl/ct 〉c16e|950 = 1.819±0.001 for group quenched on tq = 950 °c. the values are virtually almost constant with changing tempering temperature tt . steel c35 steel c35/1.0501 has the average amount of carbon c and lower average of manganese mn. values of the l/t ratio have the average of 〈cl/ct 〉35|850 = 1.816 ± 0.002 for group quenched on tq = 850 °c and 〈cl/ct 〉35|950 = 1.819± 0.003 for group quenched on tq = 950 °c. the values have monotonously decreasing trend with increasing tempering temperature tt . steel c55 steel c55/1.0535 compared to the other groups has the maximal amount of carbon c and lower average of manganese mn. values of the l/t ratio have the average of 〈cl/ct 〉c55|850 = 1.821 ± 0.003 for group quenched on tq = 850 °c and 〈cl/ct 〉c55|950 = 1.826 ± 0.005 for group quenched on tq = 950 °c. the values have monotonously decreasing trend with increasing tempering temperature tt . chemical composition, tempering and quenching effect general trends were observable on the charts of measured values. the measured l/t ratio values were: • the higher the higher is the concentration of carbon c (see fig. 5), • the higher the lower is the tempering temperature tt (see fig. 4), 49 tomáš zavadil acta polytechnica ctu proceedings steel type c [%] mn [%] si [%] cr [%] ni [%] cu [%] p [%] s [%] c16e/1.1148 > 0.13 > 0.60 > 0.15 < 0.25 < 0.30 < 0.30 < 0.04 < 0.04(čsn 412020) < 0.20 < 0.90 < 0.40 c35/1.0501 > 0.30 > 0.50 > 0.15 < 0.25 < 0.30 < 0.30 < 0.04 < 0.04(čsn 412040) < 0.40 < 0.80 < 0.40 c55/1.0535 > 0.52 > 0.50 > 0.15 < 0.25 < 0.30 < 0.30 < 0.04 < 0.04(čsn 412060) < 0.60 < 0.80 < 0.40 table 1. chemical composition of the steel group used for test samples (composition acc. to czech standards čsn 412020, čsn 412040 and čsn 412060). figure 3. measured l/t ratio values grouped by steel type. c16e (blue), c35 (red) and c55 (green). values for quenching temperatures tq = 850 °c (circle) tend to be in all cases lower than for tq = 950 °c (triangle). • higher for higher quenching temperature tq (see fig. 3). for quenching temperature of tq = 850 °c the values across the chemical composition range shown smaller standard deviation (max s = 0.042%) and across the tempering temperature higher standard deviation (max s = 0.163%). for the quenching temperature tq = 950 °c the values increased to 0.054% for tq = 850 °c and 0.243% for tq = 950 °c respectively. 6. discussion values of the l/t ratio during the experiment on conventional structural steels c16e, c35 and c55 were well controlled within range of 1.812 ÷ 1.835. these steels (especially c16e) were selected for their similarity with to p265gh conventional pressure purpose steel. minimal values were observed in general for low carbon content and low quenching temperatures. the values were changing by heat-induced structural changes and induced phase transformation. that is logical, because they may be a source of a creation and/or change of residual stress. such can be measured by the ultrasonic waves velocity changes or changes of its ratio as explained in chapter 2. a simple relation between the l/t ratio and e.g. the heat treatment temperature however may not be drawn. the reason is that a minimal temperature may be needed to introduce change of stress-strain conditions (i.e. by phase transform etc.). it is expected the l/t ratio as a function of temperature may be only semi-continuous and may include occasional sudden leaps. this phenomena can be demonstrated on samples with higher carbon content and nominal quenching temperature (c55, tq = 950 °c). higher quenching temperature well above ac3 ensures full phase transformation. higher carbon content opens the ttt (timetemperature-transformation) diagram and pushes the bs (bainite start) curve to longer times. original α–fe is transformed to γ–fe and then back with creation of martensitic or martensitic-bainitic structure. low tempering temperature does not break the created martensitic and/or bainitic structure and has minimal impact. because the martensitic phase transformation is accompanied with applied stress from the lattice, the highest l/t ratio values can be found there (〈cl/ct 〉c55|950 = 1.833± 0.002). other side of the spectra of provided samples represents steel c16e, which has the lowest carbon content and is quenched on temperature tq = 850 °c, i.e. under ac3 curve. it can be expected to observe lower l/t ratio numbers. the phase transformation is only partial (a1 < tq < ac3) and low carbon content pushes the ttt diagram closer to short times, which makes the cooling speed too slow to create purely martensitic structure. additional tempering, especially on higher temperatures, i.e. 600 °c and more tends to create sorbitic structure. such process re50 vol. 19/2018 selection of ultrasonic waves velocities for detection of creep figure 4. measured l/t ratio values grouped by quenching temperatures. c16e (blue) had in general the lowest l/t ratio values, c35 (red) average and c55 (green) the highest for both quenching temperatures tq = 850 °c (circle) and tq = 950 °c (triangle). figure 5. measured l/t ratio values grouped by tempering temperatures. the l/t ratio values decreased for with increasing tempering temperature for all, c16e (max c = 0.2%), c35 (max c = 0.4%) and c55 (max c = 0.6%) and quenching temperatures tq = 850 °c (left) and tq = 950 °c (right). lieves the residual stresses and the l/t ratio is in range of 〈cl/ct 〉c16e|850 = 1.816± 0.001. all other measurements were in between of those two extremes. the experimental results fitted to the calculated range of cl/ct = 1.73÷ 1.87 in chapter 2 for structural steels as was originally expected. the c16e had the most similar chemical composition of the test samples to p265gh conventional pressure purpose steel. this steel is used for production of e.g. membrane walls and first level superheaters. the first approximation of expected value for the p265gh steel unaffected by large-scale heat effect or creep process should be close to the value of this (c16e) steel. expected range acc. to equation (9) and the reference (benchmark) value can be calculated as follows: 〈 cl ct 〉p265|ref = 〈 cl ct 〉j± ± 3 ·maxj √√√√ 1 n − 1 nj∑ i=0 ( cl cti − 〈 cl ct 〉j )2 (10) where j is referring to the set of samples of c16e with tq = 850 °c. if the value are filled: 〈 cl ct 〉p265|ref = 1.817±3·0.004 = 1.817±0.012 (11) where 〈 cl ct 〉p265|ref is the reference (benchmark) value range for unaffected p265gh steel. notice that the 3s is only 0.7% of the average value. this means the measurement of unaffected steel should be very precise and easily separable from measurements from affected material. the samples subject to severe strengthening shall have values significantly higher than the maximal reference value range. this means the values expected for affected material should fulfill the formula 〈 cl ct 〉p265|test � 〈 cl ct 〉maxp265|ref = 1.829. measurements performed on collapsed membrane wall from p265gh pressure purpose steel as described in chapter 3 found maximal values of l/t ratio peaking to 〈 cl ct 〉maxp265|test = 2.086 close to the crack tip as visible on fig. 2 [13]. that is a value 14% higher than the reference value (it is also 67 times the standard deviation sc16e|850, see formula (10)). it is therefore obvious that severe changes of stress-strain conditions can be clearly separable from the provided reference 51 tomáš zavadil acta polytechnica ctu proceedings value. such changes may be caused by creep, but also other processes, that impact local stress level in the material. 7. conclusion the goal of this article was to find expected benchmark reference value of ratio of longitudinal and transversal ultrasonic waves velocities (l/t ratio) for detection of creep degradation process on conventional non-alloyed pressure purpose steel (e.g. p265gh). the article demonstrates that the l/t ratio is usable method of detection of creep-induced strengthening be measuring induced stress. the performed experiment on three types of structural steel (c16e, c35 and c55) found that the values of l/t ratio are well controlled around the value of 1.82 (dimensionless) for material not subject to degradation and do not drop below 1.812 or raise above 1.835. this is in agreement with computed expected results with range of cl/ct = 1.73÷ 1.87 for steels with poisson’s ratio ν = 0.25÷ 0.30. similarity of c16e structural steel with p265gh pressure purpose steel (steel used for production of membrane walls, first level superheaters etc.) was used to calculate a first level approximation of reference benchmark value range for non-degraded p265gh steel with value 〈 cl ct 〉p265|ref = 1.817±0.012. due to the strengthening mechanisms the upper bond of the reference value 〈 cl ct 〉maxp265|ref = 1.829 is a benchmark for comparison with potentially degraded material. comparison with results measured on collapsed membrane wall from p265gh subject to creep rupture also confirmed the reference value being accurate. it was observed that creep-induced strengthening is both measurable and clearly distinguishable from the reference value, with peak value 〈 cl ct 〉maxp265|test = 2.086. in conclusion the article proved there is a materialand heat-treatment-specific reference benchmark value that can be used for detection of structural changes of the material through change of its stress-strain conditions. the evaluation of degradation process by this means is expected to be ordinal (comparison with a set of reference samples). such application may be used in case of e.g. on-site rla inspections. continuation of the research will now focus on simplification of the measurement process for on-site inspections and verification of ability to detect the degradation process in its early stages. acknowledgements this article is based on author’s bachelor’s degree thesis and is a part of ongoing postgraduate studies on czech technical university in prague, faculty of nuclear sciences and physical engineering. valuable discussions with mr. jaroslav pitter (materials and corrosion specialist, thesis advisor), colleagues from the atg, ltd., mr. petr žbánek (ut l3), mr. jiří blahušek (ut l3), mr. jan švub (rla inspector) and the professor jiří kunz from the department of materials, faculty of nuclear sciences and physical engineering, ctu in prague, are gratefully acknowledged. this article is a derivative based on a presentations held on the student conference šimáně 2018 and 57th bindt conference in birmingham including respective books of proceedings. references [1] v. sklenička, k. kuchařová, m. svoboda, et al. long-term creep behavior of 9–12%cr power plant steels. materials characterization 51(1):35 – 48, 2003. doi:10.1016/j.matchar.2003.09.012. [2] s. baby, b. n. kowmudi, c. omprakash, et al. creep damage assessment in titanium alloy using a nonlinear ultrasonic technique. scripta materialia 59(8):818 – 821, 2008. doi:10.1016/j.scriptamat.2008.06.028. [3] g. sposito, c. ward, p. cawley, et al. a review of nondestructive techniques for the detection of creep damage in power plant steels. ndt&e international 43(7):555 – 567, 2010. doi:10.1016/j.ndteint.2010.05.012. [4] j. obraz. zkoušení materiálu ultrazvukem. sntl. [5] e. schneider, v. goebbels. vdi-berichte no. 439. [6] s. eldevik, a. a. f. olsen, p. lunde. sound velocity change owing to the acoust-elastic/plastic effect in steel measured using acoustic resonance technology (art). in scandinavian symposium on physical acoustics, geilo, jan 29th feb 1st. [7] m. kobayashi. acoustoelastic theory for plastically deformed solids. jsme international journal ser 1, solid mechanics, strength of materials 33:310–318, 1990. doi:10.1299/jsmea1988.33.3_310. [8] m. kobayashi. ultrasonic nondestructive evaluation of microstructural changes of solid materials under plastic deformation–part i. theory. international journal of plasticity 14(6):511 – 522, 1998. doi:10.1016/s0749-6419(98)00005-9. [9] m. kobayashi. analysis of deformation localization based on proposed theory of ultrasonic wave velocity propagating in plastically deformed solids. international journal of plasticity 26(1):107 – 125, 2010. doi:10.1016/j.ijplas.2009.05.004. [10] a. kumar, t. jayakumar, b. raj, k. ray. correlation between ultrasonic shear wave velocity and poisson’s ratio for isotropic solid materials. acta materialia 51(8):2417 – 2426, 2003. doi:10.1016/s1359-6454(03)00054-5. [11] h. h. hilton, s. yi. the significance of (an)isotropic viscoelastic poisson ratio stress and time dependencies. international journal of solids and structures 35(23):3081 – 3095, 1998. doi:10.1016/s0020-7683(97)00357-0. [12] t. zavadil. ndt assessment of microstructure changes of materials after heat treatment, 2009. bachelor’s thesis, ctu in prague. [13] t. zavadil. the use of ndt methods for detection of microstructure changes of steels when performing residual life assessment of industrial equipment, 2012. master’s thesis, ctu in prague. 52 http://dx.doi.org/10.1016/j.matchar.2003.09.012 http://dx.doi.org/10.1016/j.scriptamat.2008.06.028 http://dx.doi.org/10.1016/j.ndteint.2010.05.012 http://dx.doi.org/10.1299/jsmea1988.33.3_310 http://dx.doi.org/10.1016/s0749-6419(98)00005-9 http://dx.doi.org/10.1016/j.ijplas.2009.05.004 http://dx.doi.org/10.1016/s1359-6454(03)00054-5 http://dx.doi.org/10.1016/s0020-7683(97)00357-0 acta polytechnica ctu proceedings 19:46–52, 2018 1 introduction 2 stress measurement by conventional ultrasonic testing 3 effect of creep on l/t ratio values 4 experimental setup 4.1 samples 4.2 equipment 5 results 6 discussion 7 conclusion acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2018.18.0006 acta polytechnica ctu proceedings 18:6–9, 2018 © czech technical university in prague, 2018 available online at http://ojs.cvut.cz/ojs/index.php/app optimization of decompressive craniectomy based on finite element simulations mate hazaya,∗, bernadett bakosa, peter jozsef tothb, andras bukib, imre bojtara a department of structural mechanics, budapest university of technology and economics, muegyetem rkp. 3, budapest, hungary b department of neurosurgery, university of pecs, ret str. 2, pecs, hungary ∗ corresponding author: hazay.mate@epito.bme.hu abstract. decompressive craniectomy (dc) is a neurosurgical procedure which is often applied to decrease the intracranial pressure (icp), even if its optimal execution in terms of the size and location of the skull opening is not known. the current research focuses on dc from a biomechanical perspective. a finite element (fe) modelling strategy is applied where patient-specific head models are developed. these numerical models are used to perform virtual experiments where dc is simulated several times with skull openings having different size and location. during the simulations icp, stress and strain distributions in the brain tissue are monitored in the function of the skull opening details. in the knowledge of these objective functions suggestions could be made regarding the applied optimization procedure which can lead to the identification of optimal dc scenarios in the future. keywords: intracranial pressure, numerical simulations, patient-specific head models, decompressive craniectomy. 1. introduction traumatic brain injuries (tbis) have a devastating global epidemiological importance [1]. after a tbi the swelling of the brain tissue may occur which can lead to the increment of the intracranial pressure (icp) [2]. increased icp can cause secondary injury which can lead to serious neurological damages and death, therefore an important goal of the treatment procedure is the reduction of the pressure [2]. in order to decrease icp, the so-called decompressive craniectomy (dc) neurosurgical operation can be performed, where a portion of the skull is removed and the underlying dura mater is opened to allow a bulging deformation of the parenchyma [3]. due to the expansion of the brain tissue, the icp decreases, however, axons may be subjected to extreme stretching which can be responsible for the unfavourable outcome which was observed after patients had been treated with dc [4]. currently, the effective application of dc is challenging since its optimal execution in terms of the size and the location of the skull opening is controversial [5]. our current research focuses on dc from a biomechanical perspective. our main goal is to analyse the biomechanical behaviour of the soft and highly deformable brain tissue in the function of the size and location of the skull opening. in the knowledge of the pressure reduction and strain distributions suggestions can be made about the optimization procedure which could be applied in the future in order to identify optimal dc scenarios, where significant pressure reduction could be accomplished without the excessive stretches of the brain tissue. 2. methods in order to analyse the mechanical response of the brain, a finite element (fe) modelling strategy is applied which includes the development of head models with patient-specific geometry. its preparation starts with nonlinear medical image registration where the ct scans of a healthy volunteer are morphed to the preoperative ct scans of the patient based on the patient’s cranial shape [6] using b-splines [7] in 3d slicer environment [8]. in the resulting ct scans the cranial shape approximately agrees with the patient’s cranial shape, but the shape of the intracranial anatomic parts represent an average healthy condition [9]. on these scans the different anatomic parts are distinguished by segmentation performed in 3d slicer using threshold based semi-automatic method. after the pixelwise segmentation, the obtained surface meshes are smoothed and corrected by vcg reconstruction [10] in meshlab [11]. smoothed surface models are converted to 3d cad geometry in ansys® spaceclaim (release 18.0, ansys, inc., pa, usa) and the obtained geometry is imported to ansys® workbench (release 18.0, ansys, inc., pa, usa) where the finite element models are developed. similarly to previous research [12], the geometry includes the skull, dura mater, brain tissue, outer cerebrospinal fluid (csf) space, lateral ventricles and the falx cerebri. however, the outer csf space and the ventricles are represented as gaps in the geometry, while the supporting effect of the csf on the surface of the parenchyma is taken into account by (i) a pressure load with 5 mmhg intensity (approximating an initial healthy state) and (ii) an 6 http://dx.doi.org/10.14311/app.2018.18.0006 http://ojs.cvut.cz/ojs/index.php/app vol. 18/2018 optimization of decompressive craniectomy based on fe simulations figure 1. obtained displacements at the main steps of the fe analysis. figure 2. obtained icp at the main steps of the fe analysis. elastic spring support (modelling the increased pressure at the surfaces due to swelling). in the model the brain tissue is considered as a homogeneous, isotropic domain having a hyperelastic 2nd-order ogden material model which was applied in previous research [13], while the skull and dura is considered as a linear elastic continuum [9]. in order to separate fe mesh generation from the unimportant geometric features, a virtual topology is constructed in ansys by generating virtual cells. in case of this highly irregular geometry, the fe mesh is created by the so-called patch independent algorithm of the ansys® workbench. as a result, a fe mesh is obtained with good quality tetrahedron elements for the skull and brain tissue and triangular elements for the falx cerebri. accordingly, the total number of elements in the models is between 100000-120000. during the analysis each node of the skull is constrained against translational displacements, while the brain tissue is supported by the above-mentioned weak springs representing the effect of csf, and a frictionless contact is established between the brain and skull and between the brain and falx. displacement and icp distributions obtained at the main steps of the fe analysis are illustrated in fig. 1 and fig. 2. the simulations start from an initial (unloaded) reference state, then the boundaries of the brain tissue are subjected to the above-mentioned 5 mmhg pressure in order to reach an approximate healthy state (fig. 1.a, fig. 2.a). afterwards, the brain swelling due to edema is simulated by an artificial thermal expansion in order to reach the preoperative (i.e. injured) state (fig. 1.b, fig. 2.b). finally, the skull opening is performed by prescribed displacements of the bone flap and the corresponding bulging displacement is reached at the postoperative state (fig. 1.c, fig. 2.c). in order to simulate that certain magnitude of csf flows out during the operation, near the craniectomy the 5 mmhg initial pressure and the spring elements are killed at the time of the skull opening. these models are calibrated and validated against clinical measurements by reconstructing real-life dc cases where the observed (i.e. measured at the clinic) and predicted (i.e. obtained by fe simulations) icp and deformations are compared on a qualitative and quantitative manner [9]. during the calibration the magnitude of the fictive thermal loading is determined based on the equality of the observed and predicted icps near the lateral ventricles at the preoperative state. results of the validation procedure are detailed elsewhere [9]. in this work two different models (one with unilateral and the other one with bifrontal skull opening) were applied to perform virtual experiments where dc is simulated several times with different sized skull openings. following this procedure, it is monitored how the biomechanical behaviour of the brain tissue depends on the size and location of the skull opening. 3. results some fundamental aspects regarding the biomechanical behaviour of the brain are shown in fig. 3 in terms of displacements (left), shear strains (middle) and principal strain (right). it can be seen that even for a relatively low (18 mmhg) preoperative pressure the brain tissue suffers relatively large bulging deformations (fig. 3.a). due to these excessive deformations the soft tissue is subjected to two different loading phenomena. firstly, a relatively large shear zone occurs (i) along the perimeter of the skull opening and (ii) between the ventricles and the edges of the skull opening (fig. 3.b). moreover, a relatively large volume of the brain tissue is under tension (fig. 3.c), although 7 m. hazay, b. bakos, pj. toth et al. acta polytechnica ctu proceedings figure 3. biomechanical behavior of the brain tissue during dc. figure 4. geometry and fe results in case of different dc scenarios. figure 5. icp reduction in the function of skull opening area. figure 6. tendencies of damaged brain volume in case of different skull opening area. 8 vol. 18/2018 optimization of decompressive craniectomy based on fe simulations it is expected that even minimal tensile strains can cause neurological damage if they maintained for a long duration such as in the postoperative stage [6]. based on these observations, besides icp and total displacements, the shear strains and the maximum principal strains were continuously monitored during the simulation of different dc scenarios. regarding these parameters, some illustrative results obtained by fe simulations in case of different skull openings are shown in fig. 4. based on the results of virtual experiments the pressure reduction seems to have an approximate linear dependence on the skull opening area as it is shown in fig. 5. tendencies of the damaged relative brain volume in case of assumed strain thresholds are shown in fig. 6 due to maximum principal strains (fig. 6.a) and shear strains (fig. 6.b). in fig. 5 and fig. 6 multiple objective functions can be seen which should be minimized. since these are monotonic competitive functions the so-called paretofront (i.e. the set of solutions which are optimal in a sense that one objective function (e.g. in fig. 6) can take a lower value if and only if the other objective function (in fig. 5) takes a larger value) consists of infinitely many points. 4. discussion in this work numerical simulations of decompressive craniectomy were performed with different sized unilateral and bifrontal skull openings, in order to analyse the mechanical behaviour of the brain tissue in the function of the skull opening details. it was observed that the pressure reduction depends on the area of the skull opening in an approximately linear way (fig. 5). however, as the area of the skull opening increases the damaged brain volume due to maximum principal and maximum shear strains increases as well (fig. 6). these multiple monotonic objective functions lead to infinitely many possible optimal solutions with the pareto-type optimization. therefore, it is suggested to define a target icp value as a constraint and determine the optimal size of the skull opening based on the skull opening area versus icp objective function (fig. 5). in this way it could be avoided to apply a larger skull opening than the necessary which would cause unnecessarily large deformations and damage of the brain tissue. acknowledgements this research was supported by the unkp-17-3-1 new excellence program and the bme-biotechnology fikp grant (bme fikp-bio) of the ministry of human capacities. references [1] t. frieden, d. houry, g. baldwin. report to congress on traumatic brain injury in the united states: epidemiology and rehabilitations, 2010. [2] s. badri, j. chen, j. barber, et al. mortality and long-term functional outcome associated with intracranial pressure after traumatic brain injury. intensive care medicine 38(11):1800–1809, 2012. doi:10.1007/s00134-012-2655-4. [3] i. timofeev, m. czosnyka, j. nortje, et al. effect of decompressive craniectomy on intracranial pressure and cerebrospinal compensation following traumatic brain injury. journal of neurosurgery 108(1):66–73, 2008. doi:10.3171/jns/2008/108/01/0066. [4] d. cooper, j. rosenfeld, l. murray, et al. decompressive craniectomy in diffuse traumatic brain injury. new england journal of medicine 364(16):1493– 1502, 2011. doi:10.1056/nejmoa1102077. [5] a. kolias, p. kirkpatrick, p. hutchinson. decompressive craniectomy: past, present and future. new england journal of medicine 364(16):1493–1502, 2011. doi:10.1038/nrneurol.2013.106. [6] h. von holst, x. li, s. kleiven. increased strain levels and water content in brain tissue after decompressive craniotomy. acta neurochirurgica 154(9):1583–1593, 2012. doi:10.1007/s00701-012-1393-2. [7] s. gu, x. meng, f. sciurba, et al. bidirectional elastic image registration using b-spline affine transformation. computerized medical imaging and graphics 38(4):306– 314, 2014. doi:10.1016/j.compmedimag.2014.01.002. [8] s. pieper, m. halle, r. kikinis. 3d slicer. proceedings of the 2nd ieee international symposium on biomedical imaging: nano to macro, 2004 apr 15-18, arlington, va, usa 1:632–635, 2004. [9] m. hazay, a. varga, e. nagy, et al. finite element reconstruction of decompressive craniectomy. biomechanica hungarica -(-):in press, 2018. [10] m. nabil, m. betro, m. metwally. 3d reconstruction of ancient egyptian rock-cut tombs: the case of m.i.d.a.n.05. in the proceedings of the xxiv international cipa symposium pp. 443–447, 2013. 2013 sep 2-6, strasbourg, france. [11] p. cignoni, m. callieri, m. corsini, et al. meshlab: an open-source mesh processing tool. in the proceedings of the 6th eurographics italian chapter conference pp. 129–136, 2008. 2018 jul 2-4, salermo, italy. [12] x. li, h. von holst. finite element modeling of decompressive craniectomy (dc) and its clinical validation. advances in biomedical science and engineering 2(1):1–9, 2015. doi:10.15764/abse.2015.01001. [13] t. fletcher, b. wirthl, a. kolias, et al. modelling of brain deformation after decompressive craniectomy. annals of biomedical engineering 44(12):3495–3509, 2016. doi:10.1007/s10439-016-1666-7. 9 http://dx.doi.org/10.1007/s00134-012-2655-4 http://dx.doi.org/10.3171/jns/2008/108/01/0066 http://dx.doi.org/10.1056/nejmoa1102077 http://dx.doi.org/10.1038/nrneurol.2013.106 http://dx.doi.org/10.1007/s00701-012-1393-2 http://dx.doi.org/10.1016/j.compmedimag.2014.01.002 http://dx.doi.org/10.15764/abse.2015.01001 http://dx.doi.org/10.1007/s10439-016-1666-7 acta polytechnica ctu proceedings 18:6–9, 2018 1 introduction 2 methods 3 results 4 discussion acknowledgements references doi:10.14311/app.2021.30.0114 acta polytechnica ctu proceedings 30:114–120, 2021 © czech technical university in prague, 2021 available online at http://ojs.cvut.cz/ojs/index.php/app application of the mori-tanaka method to describe the rate-dependent behavior of unidirectional fibrous composites soňa valentováa, michal šejnohaa,∗, jan vorela, radek sedláčekb, pavel padevěta a czech technical university in prague, faculty of civil engineering, department of mechanics, thákurova 7, 166 29 prague 6, czech republic b czech technical university in prague, division of biomechanics, faculty of mechanical engineering, technická 4, 160 00 prague, czech republic ∗ corresponding author: sejnom@fsv.cvut.cz abstract. this paper examines the possibility of using the mori-tanaka micromechanical model describe the rate dependent behavior of the polymer matrix based fibrous composites. the generalized leonov model is adopted to capture the time and rate dependent character of the selected matrix, while fibers are assumed elastic. the performance of the mori-tanaka method is tested against the finite element simulations carried out in the framework of first-order homogenization. for simplicity, the periodic hexagonal array model is chosen to represent the microstructural arrangement of fibers in the yarn cross-section. to match the predictions provided by the two approaches a suitable modification to the original mori-tanaka method is proposed. an extensive parametric study is presented to illustrate a considerable improvement of the predictive capability of the modified mori-tanaka method. keywords: fibrous composites, generalized leonov model, homogenization, maxwell chain model, mori-tanaka method, periodic unit cell, viscoelasticity. 1. introduction unidirectional fiber-reinforced composites have already found a way into a variety of industrial fields. for such reasons as low weight, good thermal properties, high corrosion and chemical resistance, high longitudinal stiffness and strength, lower economic and environmental cost compared to other widely used materials, their application appears in aerospace, automotive, marine, and various construction sectors. in order to take full advantage of the benefits the composite material offers, a suitable computational model predicting its macroscopic response including the rate dependent behavior due to the viscous character of the matrix phase is necessary. despite a random nature of the distribution of fibers in the matrix, easily visible from images of material cross sections, a periodic hexagonal arrangement (pha) model [1] is adopted, because for high volume fraction of fibers it has been suggested as sufficiently accurate representation of a statistically homogeneous microstructure [2]. moreover, when limiting attention to elasticity the mori-tanaka (mt) method [3] delivers the macroscopic response comparable to finite element (fem) simulations assuming the pha model [4]. unfortunately, when moving beyond elasticity the original mori-tanaka scheme appears too stiff [5] and some action is needed to bring the predictions closer to those provided by fem [6, 7]. in the present study, we build upon the approach proposed in [8] for asphalts. however, their original formulations requires some modification to suit the present material system. while attention is primarily accorded to testing the mori-tanaka method as a very efficient substitute for a highly expensive finite element method in large scale simulation of textile composites [9], the material model representing the epoxy matrix is mentioned only briefly. further details on extensive experimental program to calibrate the leonov model [7, 8, 10] can be found in [11]. the paper is organized as follows. section 2 introduces the material system examined herein. next, section 3 shortly describes the leonov viscoelastic model adopted for the epoxy matrix. for the sake of convenience we briefly summarize the main results of the experimental program needed to calibrate the constitutive model. in particular, creep tests at different stress levels and rate dependent uniaxial tensile strain controlled experiments are described. finally, section 4 outlines two types of modifications of the mori-tanaka method accompanied by an extensive parametric study to examine their potential. the principal achievements are then summarized in section 5. 2. examined composite system following our recent study on the mori-tanaka method [5] we again consider a unidirectional fibrous 114 http://dx.doi.org/10.14311/app.2021.30.0114 http://ojs.cvut.cz/ojs/index.php/app vol. 30/2021 viscoelastic modeling of unidirectional fibrous composites figure 1. cross section of examined fibrous composite. (a) (b) figure 2. a) mts alliance 30kn, b) polymer dog bone specimens. ea et ga gt νa cf [gpa] [gpa] [gpa] [gpa] [-] [-] carbon fibers 294 13 12 5 0.24 0.56 table 1. material properties of carbon fibers and its volume fraction in yarn. composite system made of elastic carbon fibers and nonlinear viscoelastic polymer matrix. a representative cross-section of the yarn and its binary form is displayed in the fig. 1. an image analysis was carried out to acquire the volume fraction of the fibers, see table 1 together with material properties taken from [9]. because fibers are transversely isotropic and the matrix is isotropic, the overall macroscopic response is expected to be also transversely isotropic. this is precisely what both the mt method and the pha model would predict in case of elastic response. while the mt method keeps this property even beyond elasticity, the pha model may yield the overall behavior orthotropic [5]. thus the application of the mt methods calls for caution when applied to real material systems. 3. generalized leonov model the generalized leonov model is adopted here to describe the nonlinear viscoelastic behavior of the epoxy matrix. the formulation assumes the bulk response be linearly elastic σm = kεv. (1) the viscoelastic response is thus driven solely by the deviatoric stress and strain components. for the maxwell chain model the corresponding constitutive equations read dsij dt = m! µ=1 2gµ(deij dt − dep,µ ij dt ), (2) sij = m! µ=1 sµ ij . (3) 115 s. valentová, m. šejnoha, j. vorel et al. acta polytechnica ctu proceedings figure 3. strain rate dependent uniaxial tensile tests. figure 4. eyring plot. the creep strain increment follows from the eyring flow model as dep dt = 1 2a sinh(τ/τ0) , (4) where a and τ0 are model parameters determined experimentally from either uniaxial [10, 12] or torsional tests [8] performed at different strain rates. for example, rewriting eq. (4) for a uniaxial tension we get σy = τ0 √ 3 ln(2a √ 3) + τ0 √ 3 ln ε, (5) which upon substituting the experimentally observed yield, or the maximum, tensile stress σy allows us to determine the necessary material parameters, see figures 3 and 4 and the next section. 3.1. strain rate dependent tensile tests six dog bone specimens in fig. 2(b) made of 285/500 aero havel epoxy resin were loaded in simple tension in the displacement controlled regime at a specific strain rate until failure using the mts alliance 30kn electromechanical testing machine equipped with 30kn load cell, see fig. 2(a). the resulting stress strain diagrams in fig. 3 confirm the rate dependent response of the epoxy matrix. replotting the yield stress as a function of the applied strain rate in a logarithmic scale and applying simple linear regression gives the material parameters of the eyring flow model in eq. (5). figure 5. creep experiments at various stress levels. figure 6. approximation of master curve. 3.2. creep tests the maxwell chain model requires calibration of the creep relaxation function typically found using the laplace transform of the creep compliance function. one-dimensional format of these functions can be written as j(t) = m! µ=1 jµ " 1 − exp # − t τµaσ(t) $% , (6) r(t) = m! µ=1 eµ exp # − t θµaσ(t) $ . (7) where τµ and θµ are the selected retardation and relaxation times, respectively, and aσ is the stress shift factor to define a nonlinear viscosity of a dashpot element in, e.g. the kelvin unit ηµ(τeq) = τµjµaσ(τeq), τeq = & 1/2sijsij . the compliances jµ can be derived by minimizing the least square difference of j (t) given by eq. (6) and experimentally constructed master curve for the selected set of retardation times, while exploiting the time-stress superposition principle. note that τ1 should be sufficiently small to represent in the limit τ → 0 a solid material. theoretically, τ1 = 0.001 might not satisfy this requirement but the resulting series validated in [11] proved satisfactory. to that end, a series of creep tests at different stress levels is performed first. the creep curves in fig. 5 are 116 vol. 30/2021 viscoelastic modeling of unidirectional fibrous composites (a) (b) figure 7. a) periodic unit cell (puc) with hexagonal arrangement of fibers in yarns, b) mesh of puc. µ τµ [s] jµ [mpa−1] θµ [mpa·s] eµ [mpa] 1 0.001 2.606512×10−4 9.927397×10−3 2.787166×101 2 0.01 1.905071×10−6 9.966502×10−2 1.278184×101 3 0.1 8.808431×10−7 9.815126×10−1 7.056602×101 4 1 4.934025×10−6 9.543319×10+0 1.711529×102 5 10 1.276165×10−5 9.344254×10+1 2.334448×101 6 100 1.969419×10−5 9.580883×10+2 1.418353×102 7 1000 1.290521×10−5 8.275395×10+3 5.659977×102 8 10000 6.291266×10−5 9.647045×10+4 1.586346×102 9 100000 7.887707×10−6 2.005373×10+5 1.944645×103 10 1000000 1.577867×10−3 4.168654×10+5 5.096147×102 table 2. parameters of maxwell chain model. then horizontally shifted to get the master curve for the selected reference stress. the experimental points as well as the resulting polynomial representation are plotted in fig. 6. ten maxwell units in parallel were considered in the present study to represent the maxwell chain model. the resulting parameters are listed in table 2. the shear moduli in eq. (2) are obtained from eµ for the selected poisson ratio set equal to 0.39. further details including validation of the proposed constitutive model are available in [11]. 4. homogenization as mentioned in the introductory part, we consider the predictions provided by fem as a benchmark to test the performance of the mt method. owing to a space limitation we present, with reference to fem simulations, the pha computational model only, see fig. 7, and refer the reader for details on theoretical grounds of the first-order homogenization to, e.g. [4, 13, and references therein]. 4.1. mori-tanaka method: standard formulation the mt method, however, deserves more attention. to begin with, we write the increments of local stress averages in individual phases as ∆σf = lf ∆εf , ∆σm = 'lm(∆εm − ∆µm), (8) where lr, r = f, m1, is the phase material stiffness matrix and 'lm represents the dependence on the current viscoelastic modulus. the local strain increments written in terms of the prescribed macroscopic strain increment ∆e and the increment of the creep strain ∆µm developed in the matrix follow from dvorak’s transformation field analysis, see [14], in the form ∆εf = 'af ∆e + 'dfm∆µm, (9) ∆εm = 'am∆e + 'dmm∆µm. (10) where 'ar and 'drm are the mechanical strain localization factors and strain transformation influence functions, respectively. in the light of the mt method [3] they can be expressed as 'am = ( cmi + cf 'tf )−1 , 'af = 'tf 'am, (11) 'drm = * i − 'ar + * 'lm − lf +−1 'lm. (12) the partial strain concentration factor 'tf follows from the eshelby transformation field analysis and in general it is a function the shape and orientation of the fiber and instantaneous properties of the matrix. the principal drawback of the mt method is evident from fig. 8 comparing the fem simulations with 1subscripts f, m stand for the fiber and matrix phase, respectively. 117 s. valentová, m. šejnoha, j. vorel et al. acta polytechnica ctu proceedings figure 8. stress-strain diagrams for various uniaxial tensile strain rate loading. the mt results for various strain rates of the applied in-plane shear strain. it is reasonable to attribute the observed stiff response of the mt method to its inability to represent localization of inelastic strain since dealing with phase volume averages only. 4.2. mori-tanaka method: modified formulation owing to the previous results we propose a simple modification to match the predictions provided by both mt method and fem. being inspired by [8, 12] we attempt to describe the stress redistribution due to formation of shear bands by the reduction of stress taken by the fibers through a damage like parameter ω and write the fiber stress increment as ∆σf = (1 − ω)lf ∆εf . (13) two particular formulations describing the evolution of ω are investigated. the one suggested in [12] reads ω = 1 − " τ t eq nτ0 /sinh( τ t eq nτ0 ) %m , (14) where m, n are the model parameters and the equivalent stress τeq is given by τeq = , 1 2sijsij . (15) the influence of parameters n and m is illustrated in fig. 9 suggesting a rather similar effect of the two parameters both in terms of stiffness and overall stress reduction. thus tuning only one of them appears sufficient in adjusting the macroscopic response. however, as seen in fig. 10, plotting the fem and mt estimates of the macroscopic response for the applied rate of macroscopic in-plane shear strain ėxy = 1 × 10−4 s−1 shows that such a formulation does not provide satisfactory results. therefore, we propose a new formulation written as ω = n " 1 − # τ t eq − t τ0 /sinh( τ t eq − t τ0 ) $%m , (16) (a) (b) figure 9. influence of parameters m, n in describing the evolution of ω according to eq. (14): (a) influence of parameter m , n=1, (b) influence of parameter n , m=1. figure 10. comparison of fem and mt predictions with ω given by eq. (14). where m, n, t are again the model parameters, which, unlike to eq. (14), provide more freedom in the model calibration. the way they control the evolution of ω is evident from fig. 11. while parameter m adjusts the rate of evolution of ω with τeq, the other two parameters n, t merely shift the damage curve horizontally and 118 vol. 30/2021 viscoelastic modeling of unidirectional fibrous composites (a) (b) (c) figure 11. influence of parameters m, n in describing the evolution of ω according to eq. (16): (a) influence of parameter m , n=1, t =0; (b) influence of parameter n , m=1, t =0; (c) influence of parameter t , m=n=1. vertically. thus n controls the minimum stress the fibers may take and t serves to delay the onset of damage evolution. note that these two parameters are subject to some limitations, particularly n = 〈0, 1〉, (17) and if τeq < t then ω = 0, (18) figure 12. comparison of fem and mt predictions with ω given by eq. (16). if τeq > t then ω follows from eq. (16). (19) figure 12, showing again the macroscopic stressstrain curve for the applied shear strain rate ėxy = 1 × 10−4, promotes this new formulation as almost a perfect match between fem and mt predictions can be obtained with properly adjusted parameters m, n, t in eq. (16). 5. conclusions the main goal of this study was to confirm suitability of the mori-tanaka method to replace demanding finite element simulations in the prediction of the macroscopic response of fibrous composites. this would prove advantageous particularly in a multiscale analysis of textile composites where the response at the level of yarn must be solved independently from the textile ply. to that end, two modifications to the original mt formulation were tested with particular attention devoted to the nonlinear viscoelastic response of the matrix phase governed by the generalized leonov model. both modifications attempt to reduce the stress taken by the fibers by introducing a damage like parameter ω in the constitutive equation for the fiber phase. the first modification originated from the work of valenta et al. [8, 12]. while successful for asphalt mixtures it proved insufficient in the present study. therefore, a new modification was proposed to allow for more freedom in the calibration of the evolution law for damage parameter ω. the results clearly show a superiority of this new formulation allowing us to receive a very good agreement between fem and mt predictions. this also promotes the mt method as a perfect tool for the prediction of macroscopic response of two-phase composites made of aligned fibers embedded into a polymer matrix. acknowledgements the financial support provided by the gačr grant no. 19-15666s and by the czech technical university in 119 s. valentová, m. šejnoha, j. vorel et al. acta polytechnica ctu proceedings prague within sgs project with the application registered under the no. sgs20/038/ohk1/1t/11 is gratefully acknowledged. references [1] j. l. teplý, g. j. dvorak. bound on overall instantaneous properties of elastic-plastic composites. journal of the mechanics and physics of solids 36(1):29–58, 1988. [2] j. zeman, m. šejnoha. numerical evaluation of effective properties of graphite fiber tow impregnated by polymer matrix. journal of the mechanics and physics of solids 49(1):69–90, 2001. [3] y. benveniste. a new approach to the application of mori-tanaka theory in composite materials. mechanics of materials 6:147–157, 1987. [4] m. šejnoha, j. zeman. micromechanics in practice. wit press, southampton, boston, 2013. [5] s. valentová, m. šejnoha, j. vorel. comparing moritanaka method and first-order homogenization scheme in the viscoelastic modeling of unidirectional fibrous composites. acta polytechnica ctu proceedings 26:133–138, 2020. doi:10.14311/app.2020.26.0133. [6] r. masson, m. bornert, p. suquet, a. zaoui. an affine formulation for the prediction of the effective properties of nonlinear composites and polycrystals. journal of the mechanics and physics of solids 48:1203–1227, 2000. [7] m. šejnoha, r. valenta, j. zeman. nonlinear viscoelastic analysis of statistically homogeneous random composites. international journal for multiscale computational engineering 2(4):645–673, 2004. [8] r. valenta, m. šejnoha, j. zeman. macroscopic constitutive law for mastic asphalt mixtures from multiscale modeling. international journal for multiscale computational engineering 8(1):131–149, 2010. [9] j. vorel, e. grippon, m. šejnoha. effective thermoelastic properties of polysiloxane matrix based plain weave textile composites. international journal for multiscale computational engineering 13(3):181–200, 2015. [10] t. a. tervoort. constitutive modeling of polymer glasses: finite, nonlinear visocelastic behaviour of polycarbonate. ph.d. thesis, eindhoven university of technology, eindhoven, 1996. [11] m. šejnoha, s. valentová, j. vorel, et al. modeling of viscoelastic response of unidirectional fibrous composites made of basalt and carbon fibers. in w. p. de wilde, s. hernandes (eds.), 10th international conference on high performance and optimum design of structures and materials. wit press elibrary, https://www.witpress.com/elibrary, 2020. [12] r. valenta, m. šejnoha. hierarchical modeling of mastic asphalt in layered road structures based on the mori-tanaka method. acta polytechnica 52(6):48–58, 2012. [13] s. valentová, v. hrbek, j. vorel, m. šejnoha. strength of composite yarn under biaxial loading. acta polytechnica ctu proceedings 15:131–136, 2018. [14] g. j. dvorak, y. benveniste. on transformation strains and uniform fields in multiphase elastic media. proceedings of the royal society of london series a mathematical, physical and engineering sciences 437(1900):291–310, 1992. 120 https://doi.org/10.14311/app.2020.26.0133 acta polytechnica ctu proceedings doi:10.14311/app.2019.22.0107 acta polytechnica ctu proceedings 22:107–112, 2019 © czech technical university in prague, 2019 available online at http://ojs.cvut.cz/ojs/index.php/app chloride resistance of nano-particle repaired concrete rostislav šulc∗, michal himmel, jiří němeček czech technical university in prague, faculty of civil engineering, department of construction technology, thákurova 7/2077, 166 29 prague 6, czech republic ∗ corresponding author: rostislav.sulc@fsv.cvut.cz abstract. this paper describes the results of the experimentally modeled chloride ion penetration into different quality concrete structures with fully developed carbonation. parts of concrete samples were further treated, to increase the resistance of concrete to chloride ion penetration, with a colloidal solution of nano-silica or solution of sodium methyl siliconate. measurement of chloride profiles was performed on samples subjected to free diffusion of cl− ions and on samples subjected to accelerated tests. the chloride ion profile in the samples was determined in layers of 5mm thickness up to a total of 45mm. [1, 2] keywords: chloride resistance, nano-particle, nano-silica, concrete resistance, chloride accelerated test, natural diffusion. 1. introduction the motivation for this experiment is to verify the possibilities of using nano-sized particles to close the porous structure of the concrete. therefore, a colloidal solution nano-silica (ns) or 10% solution of sodium methyl siliconate (sms) were used for post-mix treatment of concrete. the solid particles of the solution fill up and clog the open pores that are entering points for salt molecules into the concrete. such a closed structure should better withstand aggressive salt water and thereby increase the resistance of concrete to counteract the action of chlorides dissolved in water. the result of this experiment is to provide information on the amount of chlorides penetrating into the depth of the concrete and to limit the possibility of reducing the amount of chlorides entering the concrete structure by closing the microstructure of the concrete. for this test were the used mixtures of concrete also fully carbonated. [1–4] 2. materials and mix proportions concrete specimens were made from portland cement (cem i 42.5 r location mokrá) as a binder and sand and natural crushed aggregate from zbraslav location were used as aggregate. no other admixtures or additives were used. in table 1 is shown mixture composition per 1m3. for this experiment was prepared mixture so called “normal concrete” (no2a) quality concrete with predicated compressive strength over 40mpa. second was used so called “low quality concrete” with compressive strength around 20mpa. concrete was mixed for 5minutes in laboratory concrete mixer, subsequently was mixture cast into cylindrical and cubic moulds and vibrated. the samples were unmoulded and submerged into water bath after 48 hours for mixing. testing cubes of dimensions 150×150×150mm were used for compressive strength test and from cylinders with 100mm in diameter and height of 200mm were prepared test samples with a diameter of 100mm and a height of 50mm for the chloride migration tests. measured compressive strength of testing concrete samples was 48.9mpa for concrete no2a. this value was determined as the average of three test specimens. the average bulk density of the concrete was 2351 kg·m−3. this value was also determined as the average of three test specimens. for concrete lo2 these numbers are: 20.5mpa and 2230 kg·m−3. after cutting of samples was done, the carbonation process began. samples were left in storage box with moisture around 65% and slow income of gas carbon dioxide co2. until the phenolphthalein test indicated that the samples were fully carbonated. 3. used methods 3.1. vacuum penetration all samples were stored in desiccator in vacuum for minimum of 2 hours. after this was done, distilled water was poured in, so the samples were fully submerged, and after than vacuum was done again in desiccator. thus, the samples were penetrated with water in vacuum for another minimum of 2 hours. [4] 3.2. natural chloride penetration the samples were coated on the underside and the sides with the epoxy adhesive. than the samples were placed in the mold base for the test of natural diffusion. subsequently, vacuum penetration as described above, was performed on the samples. next, a collar was enclosed in the mold and the samples were sealed from all sides except the upper surface. then the samples were submerged in 3% nacl solution and, 107 http://dx.doi.org/10.14311/app.2019.22.0107 http://ojs.cvut.cz/ojs/index.php/app r. šulc, m. himmel, j. němeček acta polytechnica ctu proceedings binder aggregate water ratio cem i-42.5r 0-4mm 4-8mm 8-16mm water/binder (kg/m3) no2a 436.4 872.7 581.8 290.9 186.0 lo2 261.8 1150 581.8 291 210.2 table 1. definition of concrete composition. silica [wt.%] 50 ph 9.5 viscosity [cp] 8 density [g·cm−3] 1.4 table 2. typical properties of levasil cb8 [5]. cas no. 16589-43-8 einecs no. 240-648-3 formula ch6o3si·xna flash point >100 °c color and appearance colorless clear liquid solid contents 30% silicone solids contents 19% ph value 12-13 density 25/25 °c 1.25 table 3. typical properties of sodium methyl siliconate. top of mold was sealed to eliminate evaporation, and kept in constant laboratory temperature (22 ± 2 °c) for approximately 150 days. after these workflows were done, the samples were dried in heat chamber at 55 °c for at least 7 days. 3.3. protection by dispersion of colloidal silica same as written above (accelerated migration) with exception of using nanoparticle solutions to the side of cathode. these solutions were levasil cb8 (characteristics of levasil cb 8 are shown in table 2) and sodium methyl siliconate (characteristics are shown in table 3). [4] 3.4. nano-silica levasil cb8 colloidal solution was used in this experiment. levasil cb8 is an alkaline, aqueous dispersion of colloidal silica that is approximately 50% solids by weight. the silica dispersion is sodium stabilized and the amorphous silica particles carry a negative surface charge. the sio2 particles are discrete, have a smooth, spherical shape, and are present in a wide particle size distribution. the physical appearance of the dispersion is a white liquid, slightly more viscous than water. [5] 3.5. sodium methyl siliconate this solution is commonly used as hydrophobic protective layer of surfaces of natural stone, like sandstone, limestone etc. in our experiment the solution was diluted to 10% of solid contents. 3.6. natural chloride penetration after protection by dispersion of colloidal silica samples prepared according to the procedure described under protection by dispersion of colloidal silica were used for this experiment. the penetrated samples were then subjected to the procedure described in natural chloride penetration. the side of the sample, which was exposed to colloidal silicon solution, was then exposed to a solution of 3% nacl. of course, samples were kept in constant laboratory temperature (22 ± 2 °c) for approximately 150 days. after that the samples were dried in heat chamber at 55 °c for at least 7 days. 3.7. electro migration (accelerated migration) after protection by dispersion of colloidal silica test started same way as chloride electro migration. the surface of test sample, which was covered with a protective layer, was putted into this chamber with solution of 3% nacl. into the chamber on the other side of the sample was solution of 0.3 m na(oh) added. again, electrolytes were connected (to the anode side with 0.3molar naoh solution and to the cathode side with 3% nacl solution) to a power source with a constant voltage of 20v for 48 hours. [4] 3.8. description of chloride profile measurement on the prepared samples, the chloride profile and the amount of chlorides in the given concrete layer were measured. the chloride concentration was analysed in 5mm depth-steps. it means, that the concrete specimens were drilled in 5mm steps and the powder from drilling was collected in amount of 1.5 g. the samples contained material from a depth of 0-5mm; 5-10mm; 10-15mm; 15-20mm, 2025mm; 25-30mm; 30-35mm; 35-40mm; 40-45mm. the results show the values of “average” depth, i.e. 2.5mm; 7.5mm; 12.5mm; 17.5mm; 22.5mm; 27.5mm; 32.5mm; 37.5mm; 42.5mm. the scheme of the compared experiments is shown in table 4. [4] approximately 10ml of 20% hno3 was added to the collected 1.5 g powder samples and the sample was shaken carefully for approx. for 1 hour and then let to stabilize for approx. 24 hours. [4] 108 vol. 22/2019 chloride resistance of nano-particle repaired concrete figure 1. electro migration chamber. samples no2a with ns protection natural penetration accelerated migration with sms protection natural penetration accelerated migration without protection natural penetration samples lo2 with ns protection natural penetration accelerated migration with sms protection natural penetration accelerated migration without protection natural penetration table 4. scheme of experiments. measurement of the concentration of chloride ions in the leaching solution was performed using the ion selective electrode and reference mercury sulphate electrodes scanlab system. prior to each measurement, the ion-selective electrode was calibrated in concentrated kcl solution. a total of 6 calibration solutions of 10-4 were prepared with concentration of 10−3; 3 × 10−3; 10−2; 3 × 10−2; 10−1. the calibration curve was set by breaking the values measured in the calibration solutions with a determination coefficient higher than 0.999. [4] 4. results and discussion all resulting values in all figures are the average values of the measurements made on 3 independent samples. figure 2 shows an electrical current waveform pattern in accelerated migration of chloride ion samples over a period of 48 hours. from the current flow in samples protected by dispersion of colloidal silica, it is obvious that the conductivity of the colloidal solution is lower, but also that the colloidal silicon particles in the solution have a truly negative charge. it is clear from this that the amount of chloride ion transferred was less with protection of levasil cb8. as above in figure 2 with the difference of used concrete. figure 3 shows “normal quality concrete”. with this concrete the difference between used protective layer is extremely small. figure 4 shows the chloride profile in normal quality samples after procedure of natural chloride penetration. in comparison to unprotected samples, protection of levasil cb8 shows zero level of protection, even the protected samples show higher concentrations of chloride ions that unprotected ones. the real question is why sms protected samples shows this kind of graph and not the hyperbole as was expected. figure 5 shows the concentration of chloride ions in the samples of low quality concrete. these samples were subjected to protection of colloidal silica and natural chloride penetration. all of those results the course of the chloride profile corresponds to the total periodic hyperbole that was expected for this type of test. unlike the figure 4 results we can say that the solution of sms works to some degree. absolute results show larger numbers that normal quality concrete but that was quite the point of those mixtures. figure 6 shows the chloride profile in normal quality samples with protective layer of colloidal silica. in comparison there are samples after natural diffusion and accelerated migration tests. solution of sodium methyl siliconate shows better results with protection. in figure 7 is shown the concentration of chloride ions in the samples of low quality concrete. samples subjected to chloride electro migration show significantly lower chloride ion values than naturally chloride penetration samples. the course of the chloride electro migration profile in the chloride electro migra109 r. šulc, m. himmel, j. němeček acta polytechnica ctu proceedings figure 2. the course of the electrical current in the electrolyte during the preparation of the samples. figure 3. the course of the electrical current in the electrolyte during the preparation of the samples. figure 4. concentration of chloride ions normal quality concrete, natural diffusion. 110 vol. 22/2019 chloride resistance of nano-particle repaired concrete figure 5. concentration of chloride ions low quality concrete, natural diffusion. figure 6. concentration of chloride ions normal quality concrete, natural diffusion. 111 r. šulc, m. himmel, j. němeček acta polytechnica ctu proceedings figure 7. concentration of chloride ions low quality concrete, natural diffusion. tion sample suggests that only a very small amount of chloride ions were penetrated into the sample compared to natural chloride penetration. 5. summary the results presented in this paper show the possibility of protection of concrete structures against the effect of chlorides dissolved in water. many experiments have been carried out, which have demonstrated the possibility of simulating the acceleration of the chloride ion penetration into the concrete structure. further experiments were performed showing the possibility of penetration of the concrete structure of micro particles. the results here are to be further checked on real-world constructions. the main points can be summarized as follows: (1.) the chloride ion penetration into concrete can be simulated by passing the electric current through the electrolyte. (2.) the simulated phenomenon is highly dependent on boundary conditions, in particular the strength of the electric current, effect of the time of the electric field and the concentration of the electrolyte solution. (3.) the dispersion of colloidal silica can be penetrated by means of an electric current into concrete samples. (4.) dispersion of colloidal silica can have positive results when closing the porous concrete structure. (5.) dispersion of colloidal silica can reduce the amount of chloride ions dissolved in water when they penetrate into the concrete structure. acknowledgements financial support of the czech science foundation (project 17-05360s) and the grant agency of the czech technical university in prague (sgs16/135/ohk1/2t/11) is gratefully acknowledged. references [1] r. šulc, m. himmel, j. němeček. chloride resistance of nanoparticle repaired concrete measured by accelerated tests. in special concrete and composites 2017, vol. 760 of key engineering materials, pp. 91–95. trans tech publications, 2018. doi:10.4028/www.scientific.net/kem.760.91. [2] p. hlaváček, j. němeček. accelerated chloride migration tests in concrete. in engineering mechanics 2016 book of full texts, pp. 1–4. institute of thermomechanics, as cr, v.v.i., prague, 2016. [3] j. němeček, p. hlaváček, m. kouřil, t. jamborová. electrochemical injection of nanoparticles into concrete. in proceedings of the rilem conference. materials, systems and structures in civil engineering 2016, segment on electrochemistry in civil engineering, pp. 81–87. rilem publications s.a.r.l., prague, 2016. [4] j. němeček, y. xi. nanotechnology in construction, chap. electrochemical injection of nanoparticles into existing reinforced concrete structures, pp. 213–218. springer, new york, 2015. [5] n. akzonobel. levasil cb8 colloidalsilica, data sheet. https://colloidalsilica.akzonobel.com/ construction/concrete/. 112 http://dx.doi.org/10.4028/www.scientific.net/kem.760.91 https://colloidalsilica.akzonobel.com/construction/concrete/ https://colloidalsilica.akzonobel.com/construction/concrete/ acta polytechnica ctu proceedings 22:107–112, 2019 1 introduction 2 materials and mix proportions 3 used methods 3.1 vacuum penetration 3.2 natural chloride penetration 3.3 protection by dispersion of colloidal silica 3.4 nano-silica 3.5 sodium methyl siliconate 3.6 natural chloride penetration after protection by dispersion of colloidal silica 3.7 electro migration (accelerated migration) after protection by dispersion of colloidal silica 3.8 description of chloride profile measurement 4 results and discussion 5 summary acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2019.22.0077 acta polytechnica ctu proceedings 22:77–82, 2019 © czech technical university in prague, 2019 available online at http://ojs.cvut.cz/ojs/index.php/app analysis of corrosion products from ww2 concrete bunkers in czechia michal nývlt∗, jiří pazderka czech technical university in prague, faulty of civil engineering, thákurova 7/2077, 166 29 prague, czech republic ∗ corresponding author: michal.nyvlt@fsv.cvut.cz abstract. the paper deals with analysis of corrosion products from historic concrete bunkers which were built in the 1930’s. most of these objects evince numerous failures due to water penetration into the concrete structures. two heavy bunkers became the target of research which was focused on chemical and biological analysis of corrosion products from concrete structures. the results showed that concrete used for two analyzed fortifications successfully resisted the long-lasting effects of chemical corrosion as well as bio-corrosion (this corrosion causes only aesthetic defects). keywords: corrosion, concrete, bunker, ww2, failures. 1. introduction just before ww2, in the 1930’s, a lot of concrete fortifications was built in the czechoslovakia (czechia today). after 80 years, most of these military objects evince numerous failures due to water penetration into the concrete structures. the major part of these objects consists of light infantry bunkers (type 36 and type 37) but along the nord border of czechia, there were constructed 260 heavy bunkers following the example of the french maginot line. they were large two floored buildings with strengthening ballistic protection made of heavily reinforced concrete. two of these objects became the target for subsequent research which was focused on analysis of corrosion products from concrete structures. 2. experimental program for the purpose of the work, 10 samples were taken. 5 for salinity and 5 for localization of mold. the probes were carried out using a disinfected spatula by scraping off the surface of the concrete walls. the sampling sites were selected to map the chemical and biological damage of two object heavy concrete fortifications. these are bunkers no. t-s-20 and no. t-s-19 (figure 1, figure 2). 3. materials and method a total of 5 samples of concrete from two heavy fortification buildings were tested. most inorganic compounds are water-soluble due to their ionic structure. it is possible to detect the contents of these by using an aqueous solution. the salinity evaluation was started by weighing 2 g of the crushed sample into the erlenmeyer flasks. after addition of 100ml of distilled water, the flasks were transferred to an electric cooker, where they heated to boiling point and then subjected to ultrasound. this step resulted in the fragmentation of individual parts (figure 3). the prepared solution was sedimented for 24 hours. the portion of the sample without haze and sediment was removed using a pipette. the prepared samples were then placed in clean glass caps and prepared for further examination (figure 4). the merck uv-vis photometer spectrogant pharo 300 was used to evaluate all methods. 3.1. ph measuring the first step of experiment was made an indicative measurement of ph using analytical test strips. the reason for this was to determine the range of measurements for further use. in case of too high concentrations it would not be possible to obtain the results from the photometer. for samples 2 and 5, it was necessary to dilute with 1:10 distilled water. using a pipette, 1ml of pre-prepared sample and 3 drops of universal indicators were added to the tube. after shaking and terminating the reaction, the sample changed color. on the basis of the colorimetric reaction, the approximate ph was determined (figure 5, figure 6). 3.2. measurement of chloride salts cl− chloride ions react with mercury thiocyanate to form slightly dissociated mercuric chloride [1]. loose thiocyanate is reacted with ferric ions in a red ferric thiocyanate which was determined photometrically [1]. 5ml of the treated sample was measured using a pipette. additionally, 2.5ml of cl−1 reagent and 0.5ml of cl−2 reagent were added. after thorough shaking and settling of the sample (1minute), the sample was evaluated. the evaluation was carried out in the spectrogant pharo 300 uv-vis photometer. 3.3. measurement of ammonia nh+ 4 ammoniacal nitrogen (nh4-n) occurs partly in the form of ammonium ions and partly as ammonia [1]. 77 http://dx.doi.org/10.14311/app.2019.22.0077 http://ojs.cvut.cz/ojs/index.php/app michal nývlt, jiří pazderka acta polytechnica ctu proceedings figure 1. sampling site: bunker no. t-s-20. figure 2. sampling site: bunker no. t-s-19. figure 3. the samples are exposed to ultrasound. figure 4. inserting the samples into clean glass cap. figure 5. samples with added ph indicator. figure 6. colorimetric evaluation of ph by color and scale. 78 vol. 22/2019 analysis of corrosion products from ww2 concrete bunkers. . . among these forms there is a ph-dependent equilibrium [1]. in strongly alkaline solutions, nh4-n is present almost exclusively as ammonia which reacts with hypochlorites to form monochloramine [1]. monochloramine is further reacted with thymol to give blue indophenol, the concentration of which is then determined photometrically [1]. a reagent is added to the 5ml treated sample. 0.6ml of nh4-1 reagent and 0.06ml of nh4-2 reagent are used. after shaking, the tube was left for 5 minutes and then 4 drops of nh4-3 were added. the evaluation was carried out in the spectrogant pharo 300 uv-vis photometer. 3.4. measurement of sulphate salts so−2 4 sodium ions react with barium iodate, iodine ions are released [1]. they oxidize to form a brown-red compound that is photometrically determined [1]. in this assay method, 2.5ml of the conditioned sample was metered into the prepared tubes. experimental ph measurements for samples 2 and 5 found to be ph higher than 10. for this reason, these samples were diluted 1:10 with distilled water. subsequently, 2 drops of so−1 4 reagent and 0.06ml of so−2 4 reagent were added to every tubes with samples. after shaking, the tubes were heated to 40 °c for about 5minutes. then, 2.5ml of so−3 4 reagent was added and a sample was filtered through the filter paper into an empty tube. four drops of so−4 4 were added to the filtered solution followed by further heating to 40 °c. after 7minutes, the sample could be evaluated in a spectrogant pharo 300 uv-vis photometer. 3.5. measurement of nitrate salts no− 3 in concentrated sulfuric acid, the nitrate ions react with the benzoic acid derivative to form the red colored nitro compound whose concentrations are determined photometrically [1]. prior to the start of the test, a solution of 0.06ml of no−1 3 reagent and ml of no−2 3 reagent was required to be prepared. after shaking the vials and completely dissolving the reagents, add 1.5ml of the conditioned sample. subsequently, the samples were sedimented. sedimentation samples could be evaluated in a spectrogant pharo 300 uv-vis photometer. 3.6. microbiology and mold within the experiment, it was also interesting to deal with biological damage to objects. biological damage can be investigated in several ways, for the purpose of the work, it was most appropriate to examine the samples using a microscope. sterile instruments were picked up in trace amounts of samples and seeded into the broth with agar solution in prepared petri dishes. agar preparation was carried out in 200ml erlenmeyer flasks. for 200ml of solution it is necessary to add 3.8 g of agar and 5 g malt broth. all ingredients figure 7. heating the agar solution on an electric cooker. figure 8. used sterilization device. were mixed and heated with 200ml of distilled water (figure 7). the prepared agar solution was sterilized in an autoclave for exactly 20minutes at 120 °c and pressure 100 kpa (figure 8). to the clean, sterile petri dishes, about 3mm thick solution was poured and quickly covered to prevent contamination from outside. it is then possible to transfer specimens to the solid solution. samples were cultured in a thermostat at 22 ± 3 °c for 2weeks. the mold appeared in all 5 samples (figure 9, figure 10). the mold sample was transferred to the glass slide. then it was folded over the glass cover and viewed under a microscope with magnification of ×40 and ×100 (figure 11). after consultation with the microbiologist, 6 species of mold were identified and identified. alternaria alternata, fusarium culmorum, trichoderma viride, cladosporium herbarum, phoma eupyrena and penicillium (figure 12 – figure 14). all species found are naturally occurring molds that normally would not have a significant impact on the durability of the concrete structures in question. 79 michal nývlt, jiří pazderka acta polytechnica ctu proceedings degree of salvation of masonry salt content in mg/g and percentage by weight chloride content nitrate content sulphate content [mg/g] [% by weight] [mg/g] [% by weight] [mg/g] [% by weight] low values < 0.75 < 0.075 < 1.0 < 0.1 < 5.0 < 0.5 increased values 0.75 – 2.0 0.075 – 0.20 1.0 – 2.5 0.1 – 0.25 5.0 20 0.5 – 2.0 high values 2.0 – 5.0 0.20 – 0.50 2.5 – 5.0 0.25 – 0.50 20 50 2.0 – 5.0 very high values > 5.0 > 0.50 > 5.0 > 0.50 > 50 > 5.0 table 1. table from the czech technical standard čsn p 73 0610. sample identification weight ph chloride content ammonia content nitrate content sulphate content g] [-] [mg/g] [mg/g] [mg/g] [mg/g] 1 2.1 8.5 < 0.53 0 0.48 7.1 2 2.0 10 0 0 0.42 301 3 1.9 8.5 < 0.48 0 1.178 6.7 4 2.0 8.5 0 0 0.44 7 5 2.0 10 < 0.42 0 0.8 35.41 1 experimental ph measurements for samples 2 and 5 found to be ph higher than 10. for this reason, these samples were diluted 1:10 with distilled water. table 2. salinity measurement. figure 9. mold on sample number 1. figure 10. mold on sample number 2. figure 11. observation of mold samples by microscope. figure 12. alternaria alternata. 80 vol. 22/2019 analysis of corrosion products from ww2 concrete bunkers. . . sample identification chlorides ammonia nitrates sulphates 1 low values low values increased 2 low values low values high values 3 low values increased values increased values 4 low values low values increased values 5 low values low values very high values table 3. salinity measurement. figure 13. cladosporium herbarum. figure 14. penicillium. 4. results and discusion a spectrogant pharo 300 uv-vis photometer gives the result in mg/l, so it is necessary to make a conversion to mg/g. the following table reads the converted values to mg/g. the ph value ranges from 8.5 to 10, which is common with this old concrete. it can be determined that the water penetrating from the subsoil and the rainwater are not strongly acidic. salinity sample survey shows very low levels of saturation with chlorides and nitrates. the presence of chemicals was most evident in sample number 2 (interior of the basement of heavy fortification model no. 20) and sample number 5 (the lower part of the wall in the exterior of heavy fortification model no. 20). sulphates at this point are most likely to enter the concrete because of the lack of waterproofing is missing on the walls and floor. knowledge of chemical content in concrete is very important for the future design of remediation measure (such as using crystalline materials [2–4]). knowing the amount of sulphates is important because of the aggressiveness assessment of lime and cement mortars and plasters. it also influences the possible use of grouts of the same composition. another part of the experiment was to determine the presence of mold. a total of 6 common types of mold that have been found should not affect any remediation. 5. conclusion the concrete used for two analyzed czech ww2 fortifications successfully resisted the long-lasting effects of chemical corrosion as well as bio-corrosion. the chemical corrosion causes only aesthetic defects. this conclusion applies only to mechanically preserved objects that have not been damaged by military tests (cannons shooting) or the looting of raw materials from their construction. in addition, the findings open the way for improved rehabilitation methods, which methodology is the subject of further research conducted by the authors. acknowledgements this article was created with the support of the project naki ii dg18p02ovv063. references [1] merck kgaa. http://www.merckmillipore.com/cz/cs, 2018. accessed: 16 july 2018. [2] m. j. al-kheetan, m. m. rahman, d. a. chamberlain. a novel approach of introducing crystalline protection material and curing agent in fresh concrete for enhancing hydrophobicity. construction and building materials 160:644–652, 2018. doi:10.1016/j.conbuildmat.2017.11.108. [3] p. reiterman, j. pazderka. crystalline coating and its influence on the water transport in concrete. advances in civil engineering 2513514, 2016. doi:10.1155/2016/2513514. 81 http://www.merckmillipore.com/cz/cs http://dx.doi.org/10.1016/j.conbuildmat.2017.11.108 http://dx.doi.org/10.1155/2016/2513514 michal nývlt, jiří pazderka acta polytechnica ctu proceedings [4] j. pazderka, e. hájková. crystalline admixtures and their effect on selected properties of concrete. acta polytechnica 56(4):306–311, 2016. doi:10.14311/ap.2016.56.0306. 82 http://dx.doi.org/10.14311/ap.2016.56.0306 acta polytechnica ctu proceedings 22:77–82, 2019 1 introduction 2 experimental program 3 materials and method 3.1 ph measuring 3.2 measurement of chloride salts cl 3.3 measurement of ammonia nh4+ 3.4 measurement of sulphate salts so4-2 3.5 measurement of nitrate salts no3 3.6 microbiology and mold 4 results and discusion 5 conclusion acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2018.18.0020 acta polytechnica ctu proceedings 18:20–23, 2018 © czech technical university in prague, 2018 available online at http://ojs.cvut.cz/ojs/index.php/app instrumentation of four-point bending test during 4d computed tomography daniel kytýřa,∗, tomáš fílaa, petr koudelkaa, ivana kumpováa, michal vopálenskýa, leona vavrob, martin vavrob a czech academy of sciences, institute of theoretical and applied mechanics prosecká 809/76, 190 00 prague 9, czech republic b czech academy of sciences, institute of geonics studentská 1768, 708 00 ostrava-poruba, czech republic ∗ corresponding author: kytyr@itam.cas.cz abstract. high-resolution time-lapse micro-focus x-ray computed tomography is an effective method for investigation of deformation processes on volumetric basis including fracture propagation characteristics of non-homogeneous materials subjected to mechanical loading. this experimental method requires implementation of specifically designed loading devices to x-ray imaging setups. in case of bending tests, our background research showed that no commercial solution allowing for reliable investigation of so called fracture process zone in quasi-brittle materials is currently available. thus, this paper is focused on description of recently developed in-situ four-point bending loading device and its instrumentation for testing of quasi-brittle materials. proof of concept together with the pilot experiments were successfully performed in a ct scanner toratom. based on results of the pilot experiments, we demonstrate that crack development and propagation in a quasi-brittle material can be successfully observed in 3d using high resolution 4d micro-ct under loading. keywords: in-situ loading device, four-point bending, 4d micro-ct, crack propagation. 1. introduction proper characterization of damage processes in nonhomogeneous materials requires development of advanced experimental procedures and instrumentation of testing devices. one of currently investigated problems is evaluation of the so called fracture process zone (fpz) [1] together with crack propagation in quasi-brittle building materials [2], e.g. sandstones or man-made cementitious composites. for this purpose, notched samples are typically submitted to flexural loading in standard loading frames. in case of optical or thermographical measurement, basic information about crack propagation can be obtained, but information about deformation processes and fpz evolving within volume of the tested sample stays uncovered. for that reason, radiographical observation of the loading procedure capable of revealing microstructural variations in 3d is one of the most efficient methods [3]. to obtain relevant information about deformation and fracture behaviour of investigated quasi-brittle material, development of complex experimental procedure including design of a specialised loading device was necessary. we proposed a novel approach to four-point bending experiments during high-resolution micro-ct scans, which is based on vertical orientation of the investigated specimen, whose axis of rotations is identical with rotational axis of the ct scanner. this arrangement significantly reduces fluctuation of x-ray attenuation during the imaging and thanks to smaller source-object distance higher resolution of the reconstructed 3d images can be achieved during the experimental procedure. aside from technical details of the experimental device, proof of concept together with the preliminary results are presented in this paper. 2. fracture of quasi-brittle material fracture of quasi-brittle materials is accompanied by formation and evolution of an inelastic zone around the propagating crack tip. this area referred to as the fpz, which starts to appear at ≈ 75 % of ultimate stress is responsible for the non-linear propagation of the crack in quasi-brittle materials [4]. typical deformation response to the loading of the quasi-brittle material together with scheme of fpz is depicted in fig. 1. in this zone, the damage on several scale levels of material microstructure can be detected. since the fpz is a non-linear zone of damage in front of the macroscopic crack propagating through the material, its influence on overall damage resistance of quasi-brittle structural elements has to be treated with respect to ratio of fpz size to size of the assessed structural element. it has already been shown that size of the fpz may be comparable to the material’s basic constituents (e.g. the aggregates and voids) or it can be greater in extent by up to two orders of magnitude [5]. here, x-ray micro-ct measurement plays crucial role in detection of the crack tip and fpz propagation in the materials’ microstructure. 20 http://dx.doi.org/10.14311/app.2018.18.0020 http://ojs.cvut.cz/ojs/index.php/app vol. 18/2018 instrumentation of 4pb test during 4d ct crack tip fpz f o rc e displacement figure 1. typical f–d curve of a quasi-brittle material subjected to flexural loading (left), schematic propagation of fpz (right) 3. instrumentation since 2013, the modular system for x-ray based observation of time-dependent processes in wide range of structural and functional materials has been continually developed to fulfill recent challenges of experimental mechanics and to keep all technologies of the system up-to-date. the system consists of the patented toratom imaging device (european patent no. ep2835631) and various loading device for specific experimental purposes. 3.1. x-ray imaging device in the current configuration, the device consists of two modular perpendicular imaging chains, which enables dual-source/dual-energy radiographical and tomographical experiments. wide range of its applications is given by various x-ray sources and detectors currently available. the first xwt 160 tchr (x-ray worx, germany) 160 kev microfocus transmission tube with 1µm focal spot at target power of 3 w can be used for detailed analysis of materials with complex microstructure. the second high power xwt 240 se (x-ray worx, germany) 280 w reflection tube allows to analyse physically large samples or samples of materials with high attenuation of x-rays. furthermore, it is possible to use different types of radiation imaging detectors scintillating large-area flat panel detector, or various semiconductor pixelated detectors of the medipix family [6, 7] with theoretically unlimited dynamic range, including the large area detector composed of 10 × 10 timepix 2 assembly [8], fast devices with maximum possible frame rate of 200 fps without compression in two-row/twocolumn configuration of timepix chips [9], and also spectroscopic detectors. for the analysis presented in this paper, a combination of the xwt 160 source and xrd1622 (perkinelmer, usa) scintillating large-area flat panel detector was used. 3.2. image correction procedure it is a well known fact that the raw radiograms cannot be usually used for a high-resolution ct reconstruction procedure with sufficient quality of the reconstructed 3d image as they may suffer from stationary noise caused by the inhomogeneous characteristics of individual pixels, inhomogeneity of the background brightness caused by the intensity profile of the x-ray beam, and abnormal response of individual pixels (so called bad pixels). these effects then form different types of artifacts in the 3d reconstruction. for their elimination, the flat field correction [10] with equalization of the images and bad pixel correction were applied. these corrections were performed using the torast in-house developed python software (for a usage example see [11]). all the functions are available in a batch mode, allowing a comfortable processing of whole sets of projection images. new features are currently being integrated, such as the compensation of focal spot movement, elimination of residual gradients in the flat-field corrected images). in the future, the software will be integrated with the control software of the toratom device to enable advanced acquisition methods. 3.3. loading device the concept for 4d micro-ct of the bending experiments was developed based on experience from measurements presented in [12, 13], where conventional three-point bending tests (see fig. 2 left) were employed. although it was possible to obtain quantification of the crack-tip propagation processes, the horizontal orientation of the sample perpendicular to the rotational axis of the ct scanner together with the loading supports covering parts of the radiograms caused limitations in terms of contrast and resolutions. loading rotation axis loading loading rotation axis four−point bending test three−point bending test analysis region analysis region figure 2. basic principles of the loading scenarios these disadvantages were overcome by employment of four-point bending setup, where the sample is oriented vertically with its longitudinal axis identical with the rotational axis of the ct scanner (see fig. 2 right). the newly developed device (czech national patent no. pv2018-28 pending) is composed of three main 21 d. kytýř, t. fíla, p. koudelka et al. acta polytechnica ctu proceedings figure 3. components of the 4pb loading device components: a pair of motorized loading units with integrated movable outer supports, a pair of a stationary inner supports, and the cylindrical load bearing frame housing the loaded specimen together with the components of the loading device. the design of the device in presented in fig. 3. the loading device is during the radiographically observed loading sequence oriented vertically and fixed on rotary stage of the ct scanner. to ensure high contrast in the acquired radiograms, the part of the loading frame located inbetween the inner supports was manufactured from carbon fiber composite (t700s carbon fibers, mtm57 series epoxy resin) with low attenuation of x-rays with the nominal thickness 1.95 mm. the outer movable supports are integrated with driving units with precision captive stepper linear actuators (23-2210, koco motion dings, usa) and precision linear guideways (mgw12, hiwin, japan). both driving units are independent and equipped with linear encoders with resolution 1µm (lm10, renishaw, uk) for highly precise positioning. each driving unit is also equipped with individual load cell (lcm300, futek, usa) for force measurements on the outer supports. the maximum load capacity of the device is 1500 n per single support. position accuracy and repeatability in single-direction loading is better than 10µm with sensitivity better than 1µm. 3.4. experimental procedure the batches of specimens in shape of a cylinder with diameter d = 36 mm and length l = 195 mm were drilled out from the godula sandstone, kocbeře sandstone, and mšené sandstone that all represent traditional building and decorative natural stones widely used in masonry for about a thousand years [14]. the core drilling was carried out parallel to the sandstone bedding planes. in central part of the sample, a chevron notch was prepared using a circular diamond blade. the specimen was subjected to four-point bending loading sequence performed as a force-driven experiment to ensure uniform load distribution on the supports (outer supports span lout = 180 mm, inner supports span lin = 150 mm). experiments were controlled by the modified linuxcnc software running on a real-time kernel [15]. during the experiment, displacement was interrupted at six discrete loading steps, where radiographical imaging was performed. in the loading sequence, one load-step was performed during the material’s hardening phase, one load-step at the ultimate-stress point (peak force reached was approx. 325 n), and four load-steps were performed during the post-peak softening phase. the reconstructed image data from the notch region with resolution better than 20µm showing evolution of the crack and the related changes in microstructure are depicted in the fig. 4. figure 4. visualization of damaged sample in three perpendicular planes based on the reconstructed microct data corresponding to displacement 320 µm 4. conclusions proof of concept based on the pilot experiments was successfully performed in a micro-ct scanner. based on these experiments, it was shown that crack development and propagation in a quasi-brittle material can be observed in 3d using in-situ loading procedure and high resolution 4d micro-ct imaging. arrangement of the new experimental setup together with higher stiffness of the loading device allows to interrupt the loading procedure at any point of the material’s f–d curve without the risk of sudden collapse of the sample. concept of vertically oriented sample also allows to increase signal to noise ratio in the acquired projections to achieve higher contrast in the reconstructed 3d images. 22 vol. 18/2018 instrumentation of 4pb test during 4d ct acknowledgements the research has been supported by the european regional development fund in frame of the project com3dxct (atcz38) in the interreg v-a austria czech republic programme and by operational programme research, development and education in project inafym engineering applications of microworld physics (cz.02.1.01/0.0/0.0/16_019/0000766). references [1] j. klon, v. vesely. modelling of size and shape of damage zone in quasi-brittle notched specimens – analytical approach based on fracture-mechanical evaluation of loading curves. frattura ed integrita strutturale 11(39):17–28, 2017. doi:10.3221/igf-esis.39.03. [2] v. vesely, z. kersner, i. merta. quasi-brittle behaviour of composites as a key to generalized understanding of material structure. procedia engineering 190:126–133, 2017. doi:10.1016/j.proeng.2017.05.317. [3] l. skarzynski, j. suchorzewski. mechanical and fracture properties of concrete reinforced with recycled and industrial steel fibers using digital image correlation technique and x-ray micro computed tomography. construction and building materials 183:283–299, 2018. doi:10.1016/j.conbuildmat.2018.06.182. [4] t. anderson. fracture mechanics: fundamentals and applications. third edition. crr press, 2004. [5] v. vesely, z. kersner, j. nemecek, et al. estimation of fracture process zone extent in cementitious composites. chemicke listy 104(15 spec):s382–s385, 2010. [6] j. jakubek. data processing and image reconstruction methods for pixel detectors. nuclear instruments and methods in physics research section a: accelerators, spectrometers, detectors and associated equipment 576(1):223–234, 2007. doi:http://dx.doi.org/10.1016/j.nima.2007.01.157. [7] e. gimenez, v. astromskas, i. horswell, et al. development of a schottky cdte medipix3rx hybrid photon counting detector with spatial and energy resolving capabilities. nuclear instruments and methods in physics research, section a: accelerators, spectrometers, detectors and associated equipment 824:101–103, 2016. doi:10.1016/j.nima.2015.10.092. [8] i. kumpova, d. vavrik, t. fila, et al. high resolution micro-ct of low attenuating organic materials using large area photon-counting detector. journal of instrumentation 11(2), 2016. doi:10.1088/1748-0221/11/02/c02003. [9] d. kytyr, p. zlamal, p. koudelka, et al. deformation analysis of gellan-gum based bone scaffold using on-the-fly tomography. materials and design 134:400–417, 2017. doi:10.1016/j.matdes.2017.08.036. [10] j. rowlands, y. j. flat panel detectors for digital radiography. in handbook of medical imaging, vol. 1: physics and psychophysics. spie press, 2000. [11] m. vopalensky, d. vavrik, i. kumpova. optimization of the x-ray tube voltage with respect to the dynamical resolution in radiography and tomography. ndtnet journal 2018-02, 2018. [12] l. vavro, k. soucek, d. kytyr, et al. visualization of the evolution of the fracture process zone in sandstone by transmission computed radiography. procedia engineering 191:689–696, 2017. doi:10.1016/j.proeng.2017.05.233. [13] i. kumpova, m. vopalensky, t. fila, et al. on-the-fly fast x-ray tomography inspection of the quasi-brittle material three point bending test. 2016 ieee nuclear science symposium, medical imaging conference and room-temperature semiconductor detector workshop 2017-january, 2017. doi:10.1109/nssmic.2016.8069950. [14] m. vavro, l. vavro, p. martinec, k. soucek. properties, durability and use of glauconitic godula sandstones: a relatively less known building stone of the czech republic and poland. environmental earth sciences 75(22), 2016. doi:10.1007/s12665-016-6248-3. [15] v. rada, t. fila, p. zlamal, et al. multi-channel control system for in-situ laboratory loading devices. acta polytechnica proceedings ctu proceedings 18:15–19 2018. doi:10.14311/app.2018.18.0015. 23 http://dx.doi.org/10.3221/igf-esis.39.03 http://dx.doi.org/10.1016/j.proeng.2017.05.317 http://dx.doi.org/10.1016/j.conbuildmat.2018.06.182 http://dx.doi.org/http://dx.doi.org/10.1016/j.nima.2007.01.157 http://dx.doi.org/10.1016/j.nima.2015.10.092 http://dx.doi.org/10.1088/1748-0221/11/02/c02003 http://dx.doi.org/10.1016/j.matdes.2017.08.036 http://dx.doi.org/10.1016/j.proeng.2017.05.233 http://dx.doi.org/10.1109/nssmic.2016.8069950 http://dx.doi.org/10.1007/s12665-016-6248-3 http://dx.doi.org/10.14311/app.2018.18.0015 acta polytechnica ctu proceedings 18:20–23, 2018 1 introduction 2 fracture of quasi-brittle material 3 instrumentation 3.1 x-ray imaging device 3.2 image correction procedure 3.3 loading device 3.4 experimental procedure 4 conclusions acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2021.31.0001 acta polytechnica ctu proceedings 31:1–4, 2021 © 2021 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague energy saving in rail transport jan antoš czech technical university in prague, faculty of transportation sciences, horská 3, 128 03 prague 2, czech republic correspondence: antosja2@fd.cvut.cz abstract. aim of this article is to introduce and present advantages and possibilities of using simulation tools to calculate energy flows of tractive vehicle in all phases of its ride. the focus is on the braking phase, especially on the braking energy recuperation that offers the highest energy saving potential. energetic simulation was carried out as a part of a current railway reconstruction study, which is a good example of possible energetic calculation use. energetic calculation in the simulation are performed using so called "exact method" developed at the faculty of transportation. the advantage of the method is that it provides a high degree of accuracy. due to the simulation calculation being performed in real-time, the user can enter the calculation even during the simulation. crucial part of every simulation are quality input data, such as vehicle braking characteristic (including all working points below the envelope line together with the efficiency characteristics of the power train parts (engine, transmission, convertor and others)). also, sufficiently precise (with a precision of 1 meter) railway track details (detailed geometrical and other information on the current and reconstructed railway track or on the railway track proposal. to present savings during the ride, the railway track section on the track montreál-sherbrooke was chosen. on this track, (especially electrification and partial reconstruction of the superstructure) is planned. reconstruction level will be dependant on the local economical and social conditions. however, in the most optimistic case, total electrification of the track will be undertaken, or lower level of the reconstruction when the vehicles that have very similar characteristics. the railway track section is located between the cities of montreál and sherbrooke, that are 160 km apart. to compare the current, and possible future, state, comparison of energy consumption of the vehicle was conducted on the same track with the same geometrical characteristics. we can assume that the overall energy intensity of the ride on the newly reconstructed track (eg. with greater radius of the arches) will be lower than the overall energy intensity of the ride on the track before reconstruction. current data on energetic consumption are not available, therefore they were obtained by simulation and verified by calculations and considered sufficiently accurate. presumably, the input simulation parameters required for the simulation will not always be delivered in the sufficient quality or will be partly missing completely. in these cases they will be substituted by various algorithms, that can be machine-processed and that are part of the simulation calculations. one of the crucial algorithms is so called "moment efficiency". its machine processing is not entirely developed and therefore, this simulation will be conducted without its usage. for financial reasons, simulation calculation was verified only by the theoretical physical calculation method. based on several partial results, emergent computing tool is functional and sufficiently accurate. the simulation program is supposed to be experimentally verified after its finishing. the outcomes presented in this article are calculated by simulated computation and were not confronted with the real situation in any way. however, relevant calculation algorithms, processes and theoretical background of the simulation were already verified on the older software versions. therefore, the simulated outcomes are supposed to be at least sufficiently accurate to provide preliminary study for the railway reconstruction. presented calculations were executed by using author’s simulation software. part of the methodology was taken from the previous, already verified, versions of the software for vehicle ride simulation. keywords: energetic simulation, consumption, efficiency, montreál. 1 https://doi.org/10.14311/app.2021.31.0001 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en jan antoš acta polytechnica ctu proceedings 1. introduction there are two main ways to achieve energy saving in rail transport. we can achieve energy savings by complete change of the vehicle type (its propulsion) or the infrastructure. moreover, we can achieve energy savings by improving either vehicle type or the infrastructure. to solve both versions successfully, energetic, computer-processed proposition of the relevant transport cases (transport energetic simulation) could help. in this work, i am dealing with the vehicle energy. therefore, the vehicle operation (not infrastructure) will be analysed from energetic aspect in this article. to do so, energetic simulation of the vehicle ride on the specific railway section was conducted. energetic simulation was carried out as a part of a current railway reconstruction study, which is a good example of possible energetic calculation use. this energetic simulation was part of the railway track montreál-sherbrooke reconstruction study. in this study, energy aspect was not the main parameter. therefore, simulation inputs are partly less quality than are required by purely energetic simulation. for given purpose, the railway track description is simplified. however, to compare the outcomes, the same input data (track parameters etc.) were so the inaccuracy is negligible. in general, the more accurate input data, the more accurate simulation outcomes. therefore, it is crucial to ensure that the input data used for the following characteristics is as detailed and comprehensive as possible: • vehicle (tractive and braking capacity of the vehicle, weight, etc.), • railway track and external conditions in its surrounding (meteorological conditions, temperature, humidity etc.). to demonstrate this energetic simulation, one of widespread locomotives from series 363, or more precisely its modernised successor 363.5, was chosen. this locomotive is capable of electric energy recuperation, in other words the possibility to export electric energy back to the grid or to other use. using these two (almost the same) locomotives will be demonstrated on the example of the ride on the track between montreál – sherbrooke with the same encumbrance (the same number of equally loaded carriages). the vehicle chosen for the simulation is not to be used on the track. however, it was agreed that it is more than sufficient for the demonstration of the possible energetic savings needs. 2. simulation of energy masses on track montreál-sherbrooke assignment 2.1. information on the vehicle, exact calculation method following values were used in the simulation: figure 1. locomotive series 363, source: author. • locomotive series 363 and 363.5 (weight 89 tons) see in figure 1, • encumbrance: 5 carriages with the total weight 270 tons, • length of the train 157 m, • calculated maximum change of the tensile power and braking power: 39 n/kn, • rotating mass coefficients for the locomotive and carriages: 1,2 and 1,05, • constants to determine resistance to rolling of the locomotive: a = 1,05; b = 0,0104; c = 0,00025, • constants to determine resistance to rolling of the carriages: a = 0,98; b = 0,0104; c = 0,00021. other inputs for the simulation are detailed tensile and power vehicle characteristics (vehicle series 363) and braking characteristic of the same vehicle (based on the manufacturer’s data). unfortunately, this braking characteristic includes only "envelope" (values only for 100%) of the relative ride, other lower grades of the relative ride in the simulation were obtained by calculation. 2.2. detailed information on the railway line and external conditions the simulation as such is ready for entering all elements that can be part of the particular railway section (arches, rail-switches, tunnels etc.). for the above mentioned reasons, the real state of the railways is simplified. in the calculations, approximately 5% error is used compared to the state with more precisely entered inputs, which is satisfactory considering task assignment with the railway track section with route length almost 160 km. this approach was taken to avoid unnecessary complexity of the task. given conditions: • there are no elevation changes on the track (ascent, descent), which is in accordance with the real state as the railway track is on the level ground, 2 vol. 31/2021 energy saving in rail transport • the railway track was assigned as a straight track, with no bends. this assumption does not correspond with the real situation and this difference will cause possible 5-10% deviation from the precise calculation, • other parts of the track (rail-switches, tunnels,. . . ) also were not assigned. however, there are no tunnels on the real track and the inaccuracy is therefore very small, • nevertheless, speed restriction on the railway track were assigned (both technical and legal) see figure 2. the most important is the maximum speed limit. considering that we are dealing with a side railway track, it is not supposed to be, not even after the reconstruction, grade separated with all roads. the speed limit for this type of railway track i canada is 85 miles per hour, (roughly 137 km/h). moreover, there are four stations along the railway line, that the train calls at. see table 1. stationing [km] station name 0 montreal cent. station 39, 642 st. jean station 60, 744 farnham station 88, 900 bromont station 133, 783 magog station 158, 753 sherbrooke station table 1. station on the line montreál-sherbrooke, source: author. considering environmental conditions (eg. temperature, air humidity and density, wind speed and direction) is vital part of the simulation. within the scope of the simulation, simplified conditions were considered, such as normal temperature (20°c), dry real heads (air humidity close to zero) and weather conditions near zero. 3. course of the simulation one of the main criteria for this study was the shortest running time, not the energy consumption. therefore, the type of ride set up for the simulation was the one with the shortest running time under the given circumstances. to achieve shortest running time, the train ride consist of the following stages only: • maximum possible acceleration of a train (thus always with 100% relative thrust) to reach the maximum allowedspeed, • riding at a constant speed (the maximum allowedspeed), • maximum possible deceleration (the brake action with constant moderation with maximal value −1m.s−2 was chosen) to the given next lower allowed speed, or to stop vehicle completely. this type of ride does not include any coating. it is crucial to stress that this does not often occur in real life. simulation of two train rides were conducted. both cases have the same simulation assignment (including the same type of ride to achieve shortest running time) to enable their comparison. the first ride was simulated with the locomotive series 363 (older type) without energy recuperation capability. the second ride simulation was conducted with the reconstructed locomotive series 363.5, that is capable of recuperating of energy. outcomes of these two simulations can be seen on the following figures 3, 4 and 5. figure 3. wattage on the collector for the nonrecuperating vehicle, source: author. figure 4. wattage on the collector for the recuperating vehicle, source: author. 3 jan antoš acta polytechnica ctu proceedings figure 2. speed profile of the line montreál-sherbrooke, source: author. vehicle without recuperation [kwh] vehicle with recuperation [kwh] total energy consumption 2254 2034 table 2. simulated energy consumption on the railway track montreál-sherbrooke, source: author. figure 5. cumulative energy consumption for recuperated and no-recuperated, source: author. total energy consumption of the single train ride in the same direction montreál-sherbrooke (see figure 5), once with the locomotive 363 and the other time with the locomotion 363.5, can be found in the table 2. total electric energy savings of the train with recuperation in comparison with the same train without recuperation on this track is 2254 kwh 2034 kwh = 220 kwh. running time, the shortest one under these conditions, was 5275 seconds, considering no staying at the stops. 4. final energy savings within one day operation on the railway track montreál-sherbrooke presumed interval on this single-track railway is 2 hours (assuming it will not be converted into doubletrack railway during the reconstruction). it makes roughly 10 trains a day in each direction, therefore 20 trains a day altogether. naturally, it is just an estimated average value, results accuracy will therefore be only ordinal. the situation is intentionally simplified, therefore it would not make sense to simulate all 20 rides under different conditions (various train, various external conditions etc.). total energy savings within one day was obtained by simple multiplication energetic savings from one ride by expected number of rides per day. with the savings of 220 kwh from one ride, the total saving from 20 rides a day will be 4400 kwh. 5. conclusion overall daily energy savings are roughly 4400 kwh. with the energy prices 3 czk/kwh, the savings are roughly 13200 czk/day. however this number may seem rather high, it is important to stress that railways operators (especially in the czech republic) cannot work with the gained surplus energy. as a result, majority of the excess energy is wasted in train braking resistors and the only use therefore is reduced wear on parts of brakes otherwise subjected to physical stress. it is necessary to emphasise that these savings could be made only if the gained energy (or at least its major part) was used efficiently. references [1] opava, j.: exact calculations of traction energy quantities: conference 20 years of faculty of transportation sciences. čvut, fakulta dopravní, praha, 2013. 4 acta polytechnica ctu proceedings 31:1–4, 2021 1 introduction 2 simulation of energy masses on track montreál-sherbrooke assignment 2.1 information on the vehicle, exact calculation method 2.2 detailed information on the railway line and external conditions 3 course of the simulation 4 final energy savings within one day operation on the railway track montreál-sherbrooke 5 conclusion references doi:10.14311/app.2021.30.0121 acta polytechnica ctu proceedings 30:121–125, 2021 © czech technical university in prague, 2021 available online at http://ojs.cvut.cz/ojs/index.php/app thermo-mechanical model for concrete pavement jakub veselý∗, vít šmilauer czech technical university in prague, faculty of civil engineering, department of mechanics, thákurova 7, 166 29 prague 6, czech republic ∗ corresponding author: jakub.vesely.2@fsv.cvut.cz abstract. this paper describes a numerical thermo-mechanical model for concrete pavement, implemented in oofem software. the thermal part is a heat transfer problem with appropriate initial and boundary conditions (sun irradiation, radiation and convection), calibrated from experimental data. heat release from cement hydration is also included, calibrated for commonly used cements to demonstrate the difference that can be achieved with the binder selection. the mechanical part of the problem is composed of a 3d elastic concrete slab, subsoil winkler-pasternak elements and 1d interface elements, allowing separation in tension. the winkler-pasternak constants c1 and c2 were firstly determined from tp170 document and refined later from static load tests on the highway. the model validates well temperature field, static load test and provide several useful insight such as feasible time for summer casting, stress/strain fields and slab separation from the base. keywords: concrete pavement, heat transfer, thermo-mechanical analysis. 1. introduction durability of concrete pavements presents important research topic in the world and in the czech republic. recent developments in concrete pavement construction made construction process faster, but in several cases at the expense of durability. currently adopted design approaches still heavily relies on westergaard’s theory, resulting in dominant wheel load over temperature effects [1]. however, only linear temperature profile is presumed, but in reality the thermal effects are too general, unable to be captured with classical beam/slab theory. instead, numerical approaches are used [2]. there are several indications of decreased durability of concrete pavements. one of them lies in choosing suitable binder, where hydration kinetics and microdefects take place [3]. to demonstrate this decisive factor for durability, an experimental pilot section on a newly built part of the d1 highway was carried out, using slag-blended binder with slow hydration kinetics[4]. the monitoring of pavement is ongoing since the realization phase on one selected representative slab. there are several measured quantities, such as air temperature, sun irradiation, temperature of concrete and strains in several points in the slab. several months after the realization, the static load test took the place. the 3d thermo-mechanical model stems from this slab monitoring campaign and was calibrated. the model is implemented in oofem software [5, 6]. the thermal task is solved first and the resulting temperature field enters into the computation of mechanical task. the boundary conditions for the thermal task are derived from measured data (air temperature, sun irradiation and radiation) and are approximated into simple functions. elastic material properties are used to describe the mechanical task, with commonly used elastic parameters for concrete. the soil is described with winkler-pasternak elements, the two parameters c1 and c2 are determined from comparison with 3d continuum model and with elastic properties from tp170 [1] and from static load test results. 2. concrete slab and its monitoring a new section of d1 highway, the key infrastructure construction in the czech republic, is situated in central moravia between cities lipník nad bečvou and přerov. a slow-hardening slag-blended cement was used on approximately 7 km length and a single slab instrumented with strain and temperature gauges. the dimensions of the slab are 5000 mm in length, 3750 mm in width and 290 mm in thickness, these dimensions are commonly used. concrete pavement is constructed using two layer concrete casting, the bottom layer is 240 mm thick the top one attains 50 mm. concrete for both layers differs, the upper layer contain more binder (+50 kg m−3) and smaller aggregates. concrete slab rests on compacted base course, such as mzk (compacted gravel with defined moisture) or šda (vibrated gravel). there are 18 measuring points in the slab, instrumented with strain and temperature gauges (fig. 2). they are placed in 6 different positions (4 close to the middle of every edge and 2 in the middle of the slab) and at 3 height levels (50 mm, 140 mm and 240 mm from the top surface of the slab). they are situated in a safe distance from the joints, dowel bars and anchors (1). 121 http://dx.doi.org/10.14311/app.2021.30.0121 http://ojs.cvut.cz/ojs/index.php/app jakub veselý, vít šmilauer acta polytechnica ctu proceedings figure 1. plan view of the instrumented slab. figure 2. measuring points in the concrete slab. 3. thermal model the thermal model is implemented in oofem software and solves heat transport: − ∇t q(x) + q(x, t) = ρ(x)cv(x)∂t (x, t) ∂t (1) where q is the heat power from hydration and t is the thermal field. material properties, such as thermal capacity and conductivity are defined. the quadratic 3d elements are used in this model to describe well rather parabolic profile across slab’s thickness. geometry and mesh were created in salome preprocessor, see figure 3. 3.1. influence of binder selection for heat generation one of the possible methods for decreasing the heat liberation is replacing the commonly used ordinary portland cement (cem i 42,5r) with different binders. there are several alternatives to be chosen, for example using lower cement class cem i 32,5r or blended slag system. the differences in maximal temperatures can be seen in figure 4, showing 3◦c difference during hot summer days. figure 3. mesh for the thermal model. figure 4. temperature evolution on the top of the slab, differences in used binders. 3.2. optimal casting time another possible reduction of maximum temperature happens due to appropriate time of casing. usually, casting happens between 6-12 am (in summer), as shown in figure 5, but this is not the optimum since hydration power adds with sun irradiation. casting between 1-10 pm shows much better performance, when the peak of hydration power separates from sun irradiation. figure 5. temperature on the top of the slab generated from hydration with different casting times. 4. mechanical model the mechanical task operates on 3d elastic concrete slab with quadratic brick elements. the foundation system is simplified using winkler-pasternak elements, with c1 and c2 constants. the w-p finite elements are connected to the slab with 1d interface elements with different properties in compression and tension, allowing separation from the base. 4.1. model parameters elastic material properties are considered in a standard way; concrete e = 32500 mpa, ν = 0.2, 122 vol. 30/2021 thermo-mechanical model for concrete pavement figure 6. mesh for the mechanical model. figure 7. nodal displacement in vertical direction for 3d continuum model. figure 8. nodal displacement in z direction for w-p model. α = 10−6 1/◦c. the winkler-pasternak model has c1 =70.0 mnm−3 c2 = 60.0 mnm−1. geometry and mesh is created in salome preprocessor, see figure 6. interface material has low shear stiffness (ks = 0.01 mn/m), high stiffness in compression (kn = 2000 mn/m) while in tension only 0.2 mn/m. 4.2. calibrating winkler-pasternak constants w-p elements save significant degrees of freedom and computation time. it is cumbersome to determine the two elastic constants c1 and c2, in this case the blind prediction is done based on the 3d continuum model and then a correction was made, based on the static load tests carried out with two heavy trucks. in the blind prediction with 3d continuum model in figure 7, we use elastic parameters that are given by tp170 [1]. the foundation system consists of three figure 9. comparison of deformed shape. figure 10. trucks positioning for static load test. materials mzk (compacted gravel with defined moisture), šda (vibrated gravel) and soil pi. we expect that the soil properties meet the pi requirements, based on used combination of foundation system, that is taken from pavement catalogue, that can be found in tp170 and is commonly used for pavement design in the czech republic. for mzk we use these elastic properties e = 600 mpa, ν = 0.25, for šda e = 400 mpa, ν = 0.3 and for pi soil e = 120 mpa, ν = 0.325. the calibrated parameters were found as c1 = 34.6 mnm−3 c2 = 50.0 mnm−1, based on iterative comparison of deformed shape in figure 9, 3d continuum model in figure 7 and w-p model in figure 8, 4.3. static truck load test the static truck load test on the experimental slab took place apr 29, 2019. the loading was realised with 2 trucks, each with 4 axles, the weight of trucks was 37.08 t and 38.42 t. the goal of the static load test was to induce the concave shape, with the truck wheels loading the edges of the slab and the convex shape with trucks loading the centre of the slab. this was reconstructed with mechanical model, with distributed local load representing the truck wheels. new w-p constants were determined, based on comparison with the strains from mechanical model. we obtained c1 =70 mnm−3 and c2 = 60.0 mnm−1, so our presumption was not far. as shown in figures 11 and 12 for concave and convex deformed shapes, we have quite a good fit in six different measuring positions and the top gages, considering several facts that are influencing the behavior of the slab, such as interaction of neighboring slabs and the fact 123 jakub veselý, vít šmilauer acta polytechnica ctu proceedings figure 11. concave deformed shape simulation (top) and strain validation in 6 different measuring points in the top height level (bottom). that the foundation system is not perfectly planar. 5. thermo-mechanical coupled task the idea of thermo-mechanical model is to solve the two task in the staggered manner, solving the thermal part first and in every step export the temperature field to the mechanical task, where it appears as a loading. it is necessary to set the coefficient of linear thermal expansion α. there are 2 important scenarios that can be simulated using this model, the influence of initial casting for strain and the behavior of the concrete pavement under environmental loading. the thermal model starts 6 hours earlier to ensure, that the temperature field is not influenced with the initial temperature set for every used layer. the mechanical task starts to interact with the temperature field exported from the thermal task. the thermal loading derive typical concave (fig.13) and convex (fig.14) deformation shapes. it appears that the thermal load induces higher strains [7] (and consequently stresses) than the wheel load for concrete pavement. it is clear that non-linear temperature profile is too far from commonly prescribed linear gradient used in pavement analysis, leading to unacceptable simplifications. 6. future model development the weakly-coupled thermo-mechanical model is mature for the prediction of actual behavior of concrete figure 12. convex deformed shape simulation (top) and strain validation in 6 different measuring points in the top height level (bottom). figure 13. positive temperature gradient causing concave deformed shape. figure 14. negative temperature gradient causing convex deformed shape. 124 vol. 30/2021 thermo-mechanical model for concrete pavement pavements and to pinpoint critical aspects influencing the service life. the models allow incorporation of moisture transport, creep and shrinkage phenomena, providing even realistic stresses. they may enter into fatigue calculation, covering combination of thermal and traffic loading. 7. conclusion this paper deals with thermo-mechanical numerical model for concrete pavement with corresponding validations. there are several identified conditions that influence achieved maximal temperature. the solution for mechanical task using winklerpasternak soil elements is presented, with possible method how to determine the c1 and c2 constants. they were first predicted using 3d half-space approach and then validated and corrected with static truck load test. future directions for possible development of the modelling is also presented. acknowledgements we gratefully acknowledge the financial support from the czech technical university in prague, the grant sgs20/107/ohk1/2t/11 and from technology agency of the czech republic under project th03020404. references [1] j. kudrna, m. varaus, f. luxemburg, l. vébr, i. racek, j. fiedler and a. arťušenko. navrhování vozovek pozemních komunikací, tp170. tech. rep., ministerstvo dopravy, 2004. [2] a. ioannides. concrete pavement analysis: the first eighty years. international journal of pavement engineering 7(4):233–249, 2006. [3] r. burrows. the visible and invisible cracking of concrete. americal concrete institute, 1998. [4] g. de schutter, l. taerwe. degree of hydrationbased description of mechanical properties of early age concrete. materials and structures 29(7):335–344, 1996. [5] b. patzák. oofem an object-oriented simulation tool for advanced modeling of materials and structures. acta polytechnica 52(6):59–66, 2012. [6] b. patzák, et al. object oriented finite element method oofem, since 1993. http://www.oofem.org. [7] s. kim, h. ceylan, k. gopalakrishnan. finite element modeling of environmental effects on rigid pavement deformation. frontiers of structural and civil engineering 8(2):101–114, 2014. 125 acta polytechnica ctu proceedings doi:10.14311/app.2019.25.0083 acta polytechnica ctu proceedings 25:83–88, 2019 © czech technical university in prague, 2019 available online at http://ojs.cvut.cz/ojs/index.php/app feasibility study of micro-lattice structures by multiphoton lithography markus wimmer∗, zoltan major johannes kepler university linz, institute of polymer product engineering, altenberger strasse 69, 4040 linz, austria ∗ corresponding author: markus.wimmer@outlook.at abstract. the paper describes the possibilities of additive manufacturing with multiphoton lithography. the basis of this technology is that a laser beam (with a certain wavelength) is fired into the mixture of a monomer and a photo-initiator. when the energy of the laser is high enough, the latter acts as a catalyser for the polymerization of the monomer compound. this study focuses on the influences of certain parameters of the multiphoton lithography process. one of the important aspects is the choice of the solvent for the post processing. in sequence to the solvent problem, the influence of the layer height is examined. furthermore the limits and possibilities of the setup in use are investigated. as an example the differences in fabrication with the laser firing with "constant frequency" and "constant density" were subject of this investigation. the second goal of the study was to compare three different structures consisting of periodically repeating elements, scaled in size and number of elements per side. keywords: multiphoton lithography, lattice structures. 1. introduction the technology used for this research is "femtosecond laser multiphoton polymerization". it is an auspicious technology that can be used for the fabrication of micro structures in fields like bio medicine (cellgrowing), or photonics (photonics crystals) [1]. to generate a 3d nano structure with this system, a mixture of a photo initiator and a monomer is required. a laser with a certain wavelength (in this case 1028nm) is firing in this mixture and the energy inserted cracks open the photo initiator chains. the so formed radicals start the process of polymerization by cross linking the surrounding monomer. this effect occurs locally around the focus point of the laser. figure 1. schematic: multiphoton lithography [2] to move from a dot to a defined 3d structure, the desired geometry has to be prepared by a slicer. the slicer disassembles the model into a number of slices with constant height. a smaller height results in a better resolution. the downside of small layer heights is, that the production time is proportional to the height. half the layer-thickness results in a twice as long production time. another problem could be that the contact area between the layers might be too small which figure 2. common monomers [3] causes the mechanical properties to suffer, or in the worst case, the structure collapses. the sliced model is than written by the laser. therefore it moves along the area of each slice and polymerizes the material around. this continues for each from the slicer prepared layers. all the layers combined give the part its shape (figure 1 (a)). the so created model is still covered in unpolymerized monomer(figure 1 (b)). to get rid of the monomer drop around it, the probe has to get developed. this means, that the sample slide gets washed with ethanol or acetone. due to this, the unused monomer gets washed away from the polymerized structure and the written part stays on the sample-slide (figure 1(c)). there is a variety of resins available to use for this process. there is a variety of resins (consisting of a photoinitiator and a monomer) available to use for this process. some examples are shown in figure 2. the monomer in use for this investigation is peta. 83 http://dx.doi.org/10.14311/app.2019.25.0083 http://ojs.cvut.cz/ojs/index.php/app markus wimmer, zoltan major acta polytechnica ctu proceedings 2. examined influences the following images were taken on the scanning electron microscope "philips sem-525m". the samples are prepared by sputtering (applying a thin layer of gold dust to the structures to ensure a conductive connection between probe and stage of the microscope). the sputter coater in use is the model "scd 005 cool sputter coater" from bal-tec. 2.1. solvents the first aspect to analyze is the influence of the solvent used for the development process. ethanol and acetone are the two options. the structures designed consisted of 4 separate clusters with 5 micro cells in each direction in plane and one micro cell in height (structures were designed by kerstin lehner(jku) in her master thesis). four of those clusters are stitched together to generate the structure shown here. as can be seen in figure 3, the structure developed with ethanol suffered heavy damage during the developing process. therefore the following samples were all developed with acetone. figure 3. structure developed with ethanol figure 4. structure developed with acetone 2.2. layer height as in most additive manufacturing processes, the layer height is expected to have a major influence on the results of the samples as well as on the fabrication time. a small layer height leads to a more defined structure normal to the sample slide (z direction). the fabrication time is directly influenced by the layer height. therefore a twice as large layer height results in half the time needed for the production. the samples displayed in figures 5 & 6 show the expected impact of an increased layer height. the structure gets bulkier and has a reduced grade of details. figure 5. structure with 90nm layer height figure 6. structure with 200nm layer height 2.3. constant density the following samples are not written with the standard "constant frequency" mode but instead with "constant density". the theory behind this is, that with a constant frequency, the pulse of the laser is a sole function of the time, not considering the acceleration 84 vol. 25/2019 micro-lattice structures by multiphoton lithography of the stage or galvoscanner. therefore the distance between the impacts of the photons varies depending on the speed of the galvoscanner and the stage. the galvoscanner (figure 7) is basically a fast moving mirror that directs the laser with a multiple of the stages velocity and acceleration but with a limited field of view (operating area). figure 7. galvoscanner [4] in the "constant density" mode or pso (position synchronized output) the pulse is not dependent on the time, but on the position of the laser. therefore the frequency increases with the speed of the stage or galvoscanner and guarantees a constant distance between the pulses, even in time of acceleration. to improve the stability of the structure, there is also the option to use a "pulse burst". that means that every time the laser gets triggered, multiple pulses are fired. those are still dependent on the position. this should prevent an overlapping of pulses during low speeds. overlapping can lead to so called "micro explosions" which can damage the surrounding structures.[5] with the increase of the stageor galvospeed, the pulse density has to be reduced in order to stay beneath the maximum possible frequency of 920 khz which is the maximum the machine is capable of. the results of the sample look very disappointing. instead of an improvement due to the constant allocation of the pulses, the structure seems to be consisting of horizontal rods with barely any cohesion at all. the unusual shape of the layers might be connected to the pulse burst in use. those two in rapid frequency fired pulses might result in those patterns. the fact that they are all oriented in the same direction supports this theory. 2.4. maximising structure size another point of interest was to figure out which fabrication size is possible in a reasonable time. due to extrapolating of the writing time of smaller parts, a 35x35x35 micro cell large cube is the maximum that is possible in a time frame of 12 hours. this represents a side length of 0.28mm, therefore, the layer height is increased to the maximum that was tested so far (800nm). figure 8. density and frequency graph (a.u. stands for arbitrary unit)[5] figure 9. constant density the galvoscanner is used in the way that the stage positioned in the center of a 5x5 cluster and the galvoscanner writes the micro cells. as a result of the faster acceleration and velocity limit of the galvoscanner, this reduces the production time per cluster to 1/15 in comparison to stage-only writing. due to the limited operating radius of the galvoscanner, the stage has to re-position after each cluster to the center of the next one. the structure, as seen in figures 10 to 13, got only slight geometric shortcomings. as seen in figure 12 ,there is a stitching error between the 5x5 clusters. another anomaly are the shining white spots between some of the micro cells. a possible explanation for the existence of these spots is that there is unpolymerized peta stuck in the holes between the structures. the radiation from the electron microscope might have stimulated it and that is the reason for the glowing. in figure 13 there is also a piece of dust that got into the sample and now sticks on the side. 85 markus wimmer, zoltan major acta polytechnica ctu proceedings figure 10. structure with 35x35x35 microcells overview (0.28mm side length) figure 11. structure with 35x35x35 microcells closeup (0.28mm side length) 2.5. variation of micro cell structures in this part of the research, three different micro-cells (figure 14) are compared in two different ways. 2.5.1. quantity scaling at first the elements got scaled in number with a ratio of 1:2:4. this ratio results 5, 10 and 20 micro cells per side. the inspection of the results shows the already known stitching problem. the most plausible reason behind this is a slight inaccuracy in the stage and/or galvoscanner movement. that means that in the medium and large construct, every five micro cells there is less distance to the next micro cell than there should be. next in line is the octet structure. while the external dimensions still remain a cube, the interior increases in complexity. the standard size of a micro cell of this type is 10 microns and a cluster consists of 3x3 micro cells. therefore, the side lengths vary from 30 microns to 120 microns which represents three, six and twelve microns cells per side. as there can be seen in figure 15, the base structure is not even remotely preserved. the structures with more elements show the same results. figure 12. structure with 35x35x35 micro cells, detail 1 figure 13. structure with 35x35x35 micro cells, detail 2 the last structure in comparison is the hex structure (figure 16). the characteristic of this one is, that it got different dimensions in length and width. therefore, the base area of the result this time is not a square. in this iteration the dimensions of one micro cell are 15 microns x 17.32 microns. a cluster consists of three of those micro cells in each direction. the overview figure 16 show a decent quality regarding the shape and contour. the closeup on the medium structure in figure 17 visualizes that there is a problem with the connections within the structure. this error is reproducible and seems to be limited to big structures. therefore it is evident that this is in connection to a software issue. 2.5.2. dimension scaling in the second part of the scaling process, the side lengths of the micro cells get changed. the ratio remains the same with 1:2:4. the resulting dimensions therefore are 4, 8 and 16 microns per side for the hex structure. for the octet structure the resulting side length are 5, 10 and 20 microns. the hex structure results in micro cell dimensions of 7.5 microns x 8.66 microns up to 30 microns x 34.64 microns. the structure in figure 18 is the smallest of the three. even 86 vol. 25/2019 micro-lattice structures by multiphoton lithography figure 14. lattice structure base elements: nonlinear thrust structure, octet structuer, hex structure (left to right) figure 15. quantity scale: octet structure though the details are noticeably worse, the result is acceptable in respect to the dimensions. reasons for this might be that the microscope can not deliver a better resolution in this dimension or the writing accuracy is at its limit. but even through the blur the basic shape of the micro cell seems to be preserved. furthermore the structure did not collapse and still shows the significant holes. the samples of the octet structure show the same result as the one with the nonlinear thrust structure. the grade of detail of the small structure makes it blurry and unrecognisable (similar to figure 15 that’s twice the size of the structure in question). scaled to 16 microns per element the details of the pattern increases drastically (figure 19). the glowing in the middle indicates, that there is still unpolymerized monomer inside of the structure that was not removed during the developing process. the first impression of the hex results is quite satisfying. all of the structures still got recognizable micro cell shapes and none of them collapsed. the smallest one (figure 20) shows the already known resolution boundary of the laser with the used settings and in consequence got quite bulky. while the medium sized construct does not show any anomalies, the big structure with 30 microns x 34.64 microns element size shows the already known stitching problem. a close look reveals a line between the three clusters alongside one axis (bottom left to top right). figure 16. quantity scale: hex structure figure 17. quantity scale: hex structure: medium detail 3. conclusion this paper investigates the influences of manufacturing parameters on multiphoton lithography. the parameters in focus are the solvent and the height of the layers. the effect of ethanol (figure 3) and acetone (figure 4) as developing fluid demonstrates that acetone is the superior choice for this application. the influence of the layer height behaves similarly like in other additive manufacturing methods. small layer heights result in a more detailed geometry with the drawback of long fabrication times (figure 5). high layers on the other hand decrease the time needed for production but the structure gets more bulky (figure 6). in order to produce reasonably fast and big structures, layer heights in the scale of 800 nm turned out to be ideal. for more complex structures a lower layer height is appropriate. the tests with the "constant density" render this option useless. but the theory behind this method is promising and might turn out important for the future. for the future i expect this technology to get essential for the field of bio medicine. there it could be used to generate the environment for cell growing. 87 markus wimmer, zoltan major acta polytechnica ctu proceedings figure 18. dimension scale: nonlinear thrust structure (4 microns element size) figure 19. dimension scale: octet structure (16 microns element size) acknowledgements this work was supported by fh-prof. di dr. jaroslaw jacak (upper austrian university of applied sciences), boris buchroithner (institute of applied physics johannes kepler university) and valdemaras juzumas (workshop of photonics). a special thanks to univ.-prof. di dr. werner baumgartner from the institute of biomedical mechatronic on the johannes kepler university for providing the sem. references [1] e. stankevičius, m. malinauskas, m. gedvilas, et al. fabrication of periodic micro-structures by multi-photon polymerization using femtosecond laser and four-beam interference. materials science 17, 2011. doi:10.5755/j01.ms.17.3.587. figure 20. dimension scale: hex structure (7.5 microns x 8.66 microns side length per element) figure 21. dimension scale: hex structure (30 microns x 34.64 microns side length per element) [2] workshop of photonics. mpp brosiura, 2018. https://wophotonics.com/wp-content/uploads/ 2015/11/mpp-brosiura.pdf. [3] a.ovsianikov, j.stampfl, r.liska. multiphoton lithography techniques, materials and applications. wiley-vch, 2017. doi:10.1002/9783527682676. [4] nutfield technology. nutfield-tech-sp-30-galvanometer, 2018. http://nutfieldtech.com/wp-content/uploads/ 2016/07/nutfield-tech-sp-30-galvanometer.jpg. [5] workshop of photonics. sca manual, 2018. 88 https://doi.org/10.5755/j01.ms.17.3.587 https://wophotonics.com/wp-content/uploads/2015/11/mpp-brosiura.pdf https://wophotonics.com/wp-content/uploads/2015/11/mpp-brosiura.pdf https://doi.org/10.1002/9783527682676 http://nutfieldtech.com/wp-content/uploads/2016/07/nutfield-tech-sp-30-galvanometer.jpg http://nutfieldtech.com/wp-content/uploads/2016/07/nutfield-tech-sp-30-galvanometer.jpg acta polytechnica ctu proceedings 25:83–88, 2019 1 introduction 2 examined influences 2.1 solvents 2.2 layer height 2.3 constant density 2.4 maximising structure size 2.5 variation of micro cell structures 2.5.1 quantity scaling 2.5.2 dimension scaling 3 conclusion acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2018.14.0021 acta polytechnica ctu proceedings 14:21–26, 2018 © czech technical university in prague, 2018 available online at http://ojs.cvut.cz/ojs/index.php/app ultra-low intensity proton beams for radiation response related experiments at the u-120m cyclotron tomas matlochaa,b,∗, filip krizeka a nuclear physics institute of the cas, p.r.i. rez 130, 250 68 rez, czech republic b czech technical university in prague, faculty of nuclear science and physical engineering, brehova 7, 115 19 praha 1, czech republic ∗ corresponding author: matlocha@ujf.cas.cz abstract. the u-120m cyclotron at the nuclear physics institute (npi) of the czech academy of sciences in rez is used for radiation hardness tests of electronics for high-energy physics experiments. these tests are usually carried out with proton fluxes of the order of 105–109 proton · cm−2 · s−1. some tests done for the upgrade of the inner tracking system of the alice experiment at cern, however, required proton beam intensities several orders of magnitude lower. this paper presents a method which has been developed to achieve the proton beam flux of the order of 1 proton · cm−2 · s−1. the method is mainly based on reduction of the discharge current in the cyclotron internal penning type ion source. influence of this new operation mode on the lifetime of ion source cathodes is discussed. keywords: ultra-low intensity proton flux, penning ion source, negative hydrogen yield resonance, cyclotron irradiation facility, radiation hardness tests. 1. introduction the npi cyclotron u-120m [1] is a four-sector isochronous cyclotron which was developed in the joint institute of nuclear research in dubna (russia) and installed in the npi in 1977. since then it has been continuously upgraded [2]. the cyclotron is tunable in a frequency range from 10 to 27mhz and provides proton beams with energies ranging from 6 to 37mev. the cyclotron can also accelerate ions with the mass to charge ratio up to m/q = 2 to the energies up to 10mev per nucleon. an overview of the available ion beams together with their maximal energies and target beam intensities is presented in tab. 1. the uncertainty in beam energy for protons at 35mev is ± 0.4mev. the cyclotron u-120m is mainly used for production of medical radioisotopes and for astrophysical and radio-biological experiments. in recent years, it is utilized also by experiments that test radiation hardness of various electronic components of highenergy physics detectors. the continuous upgrade of all machine technology systems allowed to reach high reliability and very good reproducibility of the beam parameters between different irradiation campaigns. this is especially important for our cooperation on the radiation hardness tests of electronics for the upgrade of the inner tracking system (its) detector of the alice experiment at cern [3]. these tests require stable proton beams with fluxes below 109 proton · cm−2 · s−1. a new method to provide proton beams with such parameters had to be developed. a detailed methodology overview of the radiation hardness tests done for the alice its upgrade project figure 1. the cyclotron u-120m. together with a description of the corresponding beam and dose monitoring system can be found in [4]. 2. beam monitoring and measurement in the tests of radiation hardness done at the u-120m cyclotron facility, intensity of the proton beam is monitored 130 cm from the cyclotron negative mode beamline exit window using an ionization chamber farmer 30010 from ptw-freiburg [5]. the chamber is connected to a unidos e universal dosemeter. the chamber allows to measure the proton flux in the range 102–109 proton · cm−2 · s−1. to monitor, the fluxes below 104 proton · cm−2 · s−1 a timepix device [6–8] is used. the timepix is a hybrid silicon pixel detector. the timepix sensor has 256 × 256 pixels, an active area of 2 cm2, and a thickness of 300µm, see 21 http://dx.doi.org/10.14311/app.2018.14.0021 http://ojs.cvut.cz/ojs/index.php/app tomas matlocha, filip krizek acta polytechnica ctu proceedings h− / h+ d− / d+ 4he2+ / 3he2+ intensity internal [µa] – / 200 – / 100 40 / 20 intensity external [µa] 50 / 5 35 / 5 5 / 2 energy [mev] 6–37 11–20 4–40 / 3–55 table 1. energies and intensities of the available ion beams at the u-120m cyclotron irradiation facility. figure 2. detail of the timepix sensor [6]. figure 3. irradiation setup with the farmer ptw 30010 ionization chamber and the timepix device. fig. 2. the mean energy deposited by one 30mev proton in the sensor is about 1mev. details about the ionization chamber calibration and the measurement setup can be found in [4]. a photo of the irradiation setup is shown in fig. 3. 3. u-120m ion source the ion source is an internal cold cathode penning type ion source (pig). the ion source body is made of a tungsten-copper composite. the cathodes are made of tantalum due to its low sputter rates and high lifetime [9]. the tantalum cathodes yield also the highest gain of negative hydrogen ions when compared to other previously tested materials, such as molybdenum or tungsten. the ion source works in a dc arc mode and is capable to deliver beams up to 700µa dc equivalent for h− and 2ma dc eq. for protons. a lifetime of the ion source for hydrogen and deuterium operation is several hundreds of beam hours. the maximal recorded lifetime for the h− regime is about 700 beam hours. the maintenance period is kept at 250 beam hours for reliable operation, since a failure of the ion source after about 300 beam hours can occur without any previous warning signs. this preventive maintenance thus reduces the number of unplanned cyclotron shut downs. the lifetime of the ion source for the helium operation is ≈ 70 beam hours for 4he2+ ions and ≈ 30 beam hours for 3he2+. regulation capabilities of the ion source are described in the next section. a scheme of the u-120m ion source is shown in fig. 4, more details can be found in [10]. 4. beam intensity regulation in the negative mode of the u-120m cyclotron, the highest proton beam current that can be used to irradiate the target is about 50µa. this corresponds to the total flux of 3.1 × 1014 proton · s−1. such a high current can be used to irradiate passive devices to produce material damage but it is too high for radiation testing of electronics like silicon sensors or field programmable gate arrays. typical fluxes that are used for the electronics tests at the u-120m cyclotron facility are in the range 103–109 proton · cm−2 · s−1. lowering the proton beam flux from its maximal intensity by two or three orders of magnitude can be achieved by increasing the gas pressure in the ion source and lowering of the cyclotron beam filling factor [4]. achieving even lower fluxes requires to combine several other methods to ensure that the delivered beam is stable. a significant difficulty for the ultra-low beam tuning process is the fact that the cyclotron internal beam current monitors have poor resolution and are not able to measure the beam current below the total 22 vol. 14/2018 ultra-low intensity proton beams at the u-120m cyclotron figure 4. scheme of the u-120m cyclotron pig ion source. the number coding denotes the tungstencopper body of the ion source (1), the anticathode (2), the extraction slit (3), and the cathode (4). flux of ≈ 108 proton · s−1. from this reason, the machine is first tuned properly with higher internal beam intensities for which the internal measurement is possible. subsequently, the beam is extracted to the experimental site where the beam is focused to the desired beam spot. finally, the beam intensity is down-regulated to the requested value based on the measurement provided by the ionization chamber. the usual methods used for lowering the beam intensity at the u-120m cyclotron are as follows: • reducing the filling factor of the cyclotron beam, • worsening the vacuum quality in the cyclotron vacuum chamber, • increasing the gas pressure in the ion source, • shifting the horizontal position of the ion source extraction slit with respect to the slit of the puller electrode on the dee, • lowering the arc current in the pig ion source, • stopping a part of the beam on an integral beam probe at the final beam orbit in the vacuum chamber, • collimating the extracted beam on a vertical input slit of the beamline. more detailed description of the above mentioned methods can be found below. 4.1. u-120m radio frequency structure the cyclotron filling factor is regulated by a pulsewidth modulation (pwm) of the main cyclotron carrier radio frequency (rf) 10–27mhz with a 150hz macropulsed signal. the minimal duty cycle of the modulating pulse is 3%, which corresponds to about 200µs. this time is the minimal time necessary for a sufficient excitation of the cyclotron resonator. the maximal duty cycle of the macropulsed signal depends on the cyclotron rf. it is 70% for the 10mhz rf, but only 25% for the 27mhz rf. the reason for the lower pwm range at higher rf is a sparking protection of the rf system. at present, the regulation step of the duty cycle is 1%. a modification of the modulator device to allow regulation step below 0.1% is in progress. 4.2. regulation by vacuum quality and ion source gas the vacuum in the cyclotron chamber is maintained by three diffusion pumps. if some of them are switched off vacuum quality worsens. the internal beam intensity then gets lower by about one order of magnitude. the operational margin of the beam regulation via changing the ion source hydrogen gas pressure is relatively limited. highly non-optimal gas pressure leads to high ion source output instabilities. the ion source pressure regulation can lower the beam intensity by up to two orders of magnitude. 4.3. ion source shift the beam current can be effectively regulated by two or three orders of magnitude by shifting the horizontal position of the ion source extraction slit with respect to the slit of the extraction electrode on the dee, the so-called puller. the optimal position of the ion source is found by minimizing of the beam loses on the puller during the first beam orbit. when the ion source extraction slit is shifted from its optimal position, a part of the beam is stopped on the puller. a planar view of the cyclotron central region with the ion source shifted with respect to the puller is shown in fig. 5. however, note that stopping a very intense beam on the puller warms up the electrode. consequently, there can be thermally stimulated emission of electrons to the cyclotron vacuum chamber which leads to sparking. thus for stable operation, it is necessary to pre-regulate the beam intensity extracted by the puller from the ion source. 4.4. ion source discharge current regulation in routine operation, the arc current is kept near 1a which ensures stable operation. when the arc current is below 0.5a and over 1.4a the ion source can became unstable, especially in the second half of its cathode lifetime. in order to find a way how to reduce the proton flux, the ion source was studied at the test bench with the goal to investigate whether the proton flux can be regulated by the ion source discharge current. in these measurements, the ion source was operated in a pulse mode. the voltage pulses had a length of a few tens of millisecond. it was observed that the yield of negative hydrogen ions is maximal at the beginning of the pulse. a few milliseconds after 23 tomas matlocha, filip krizek acta polytechnica ctu proceedings figure 5. planar view showing the shifted extraction slit (2) of the ion source (1) with respect to the puller (3) on the dee (4). a part of the beam (5) is stopped on the puller during the first orbit. simulated in durycnm4 [11]. the ignition, the intensity of ions decreased as the cathode warmed up. however, operation of the ion source in the pulse mode turned out to be problematic, because the ignition process in the pig ion source was unstable. with 10ms long pulses and a repetition rate up to 10 hz, it was noted that the ignition got delayed with respect to the beginning of the high voltage pulse. a part of the pulses did not start properly, and the moment of the ignition of the ion source discharge was randomly floating in the first millisecond. the problem with the delayed ignition was not solved even when rising the ignition voltage up to 3000v from its nominal value 1500–2000v. in addition, a part of the pulses, which started properly at the beginning of the high voltage pulse, exhibited significant ringing, i.e. the ion source discharge current highly oscillated at the beginning of the pulse until the arc got stabilized. both issues were strongly suppressed when we introduced a small, residual constant discharge current of 5ma. the pulse operation of our pig ion source then became stable with rectangular discharge current pulses. a deeper investigation of the ion source parameters at the test bench showed that the ion source discharge current in the milliampere region is stable, if the cathodes are new or in a very good condition. the ion source pulsed mode was finally not utilized for the cyclotron operation, but the regime with the small residual currents has found its use in experiments that require a low flux of protons. 4.4.1. the minimal ion source discharge current the minimal stable current of the pig ion source discharge measured at the test bench was about 2ma. for lower discharge currents, the arc switched off. when the measurement of the lowest possible discharge current was repeated at the cyclotron with a)µcurrent ( 1 10 210 3 10 410 io n s o u rc e a n o d e v o lt a g e ( v ) 0 100 200 300 400 500 600 700 figure 6. current-voltage characteristic of the ion source for small discharge currents. a)µcurrent ( 1 10 210 310 410 ) -1 t ot al fl ux ( pr ot on s 510 610 710 810 910 1010 figure 7. total cyclotron flux as a function of the ion source arc current. figure taken from [12]. rf being switched on, it was observed that the ion source discharge current did not have any practical lower limit. let us point out that usually, the pig ion source needs some minimal voltage, typically several hundreds volts, to ignite. in the measurement done at the cyclotron, the discharge current started to increase almost linearly as the anode voltage exceeded ≈ 10v, see the measured current-voltage characteristic in fig. 6. in the range 0–3.5µa, the ion source behaved as a pure resistive element. figure 7 shows the measured total proton flux as a function of the ion source discharge current in the range 1–1000µa. when switching the ion source off, the cyclotron still provided a residual total proton flux of about 5 × 104 proton · s−1, see fig. 8. this residual proton flux can be explained based on the following considerations. the distance between the ion source and the puller is about 3mm and the amplitude of rf voltage is 34 kv. as a result there is a strong electric field (about 10mv/m) between the puller electrode of the dee and the grounded ion source. this strong electric field stimulates an emission of electrons from the puller surface. the electrons get accelerated and ionize the neutral gas leaking from the ion source. this effect probably causes the primary ionization. 24 vol. 14/2018 ultra-low intensity proton beams at the u-120m cyclotron a)µcurrent ( 0 0.5 1 1.5 2 2.5 3 ) -1 t ot al fl ux ( pr ot on s 410 510 figure 8. total proton flux as a function of the ion source discharge current. the zero current value correspods to the ion source switched off. figure taken from [12]. the produced space charge partly leaks back into the ion source and causes its almost linear behavior in the range of anode current 0–10µa. this operation mode of the ion source was successfully utilized for the low proton flux beam experiments at the npi irradiation facility. the flux of h− as a function of the ion source discharge current exhibits also a pronounced narrow resonant structure at 3–5µa, see fig. 7. the resonance is clearly visible also in fig. 9 which shows the ion source efficiency versus the ion source current. the ion source efficiency is defined as the number of h− ions produced per unit ion source discharge power. this resonance occurs in all conducted experiments with various magnetic field and different ion source gas pressure. the amplitude of the resonance depends on the ion source gas pressure and its position slightly shifts when the cyclotron magnetic field is changed. for the experiments which require a low intensity proton beam, it is necessary to avoid operation in this resonance region, as there the flux varies considerably with any subtle change of the ion source discharge current. to our knowledge, the observed resonance is not described in the literature and possibly may find use in the fields where an energetic efficiency is important, e.g. in ion thrusters. 4.5. the cathodes embrittlement our experience gained from the operation of the ion source in the regime with very low discharge currents shows that the cathodes become very fragile and after several tens of beam hours fall apart. this hydrogen embrittlement phenomenon is well known [13]. the low discharge current regime, however accelerates this process significantly. the cathodes get much more fragile when running 50 hours with 500µa ion source discharge current than after running several hundreds beam hours with 1a. the lower cathodes temperature caused by the low discharge current, probably do not allow the hydrogen to be properly annealed. until the a)µcurrent ( 1 10 210 3 10 410 w ) µ / ­ is e ff ic ie n c y ( h 0 2 4 6 8 10 12 14 16 18 6 10× figure 9. negative hydrogen yield efficiency versus ion source discharge current. figure 10. disintegrated cathodes after embrittlement by a hydrogen diffusion into tantalum. embrittlement issue will be solved, the cathodes have to be replaced after several tens of beam hours, to prevent that they fall apart and cause the cyclotron unplanned shutdown. a photo of the cathodes that got broken after 80 beam hours is shown in fig. 10. 5. the minimal beam intensity with the current experience, the proton fluxes in the range 103–109 proton · cm−2 · s−1 can be achieved in relatively short time (≈ 5min) using just the basic tune of the cyclotron and the beamline elements following the settings from previous runs. then the output proton flux can be easily controlled by the ion source discharge current. on the other hand, tuning of the fluxes below 103 proton · cm−2 · s−1 requires more time (≈ 30min). it is necessary to combine all of the above mentioned intensity lowering techniques with a special care given to the stability of the beam. figure 11 illustrates density of protons on the timepix sensor for fluxes in the range 1–103 proton · cm−2 · s−1. after a proper tune, the flux can be continuously regulated over the whole range only by the ion source discharge current. the minimal possible flux reachable at our irradiation facility was estimated to be about 0.05 proton · cm−2 · s−1. the stability of intensity for the fluxes over 103 proton · cm−2 · s−1 is better than 10% [4]. using the proper timepix settings, the timepix 25 tomas matlocha, filip krizek acta polytechnica ctu proceedings pixel # 0 50 100 150 200 250 p ix el # 0 50 100 150 200 250 co un ts 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 pixel # 0 50 100 150 200 250 p ix el # 0 50 100 150 200 250 co un ts 0 0.5 1 1.5 2 2.5 3 3.5 4 pixel # 0 50 100 150 200 250 p ix el # 0 50 100 150 200 250 co un ts 0 1 2 3 4 5 6 7 8 figure 11. the ultra-low proton fluxes at the u-120m cyclotron for various ion source discharge currents. timepix collection time 30 seconds, bias 5v. left: the ion source off, flux ∼ 0.7 proton · cm−2 · s−1. middle: the discharge current 60µa, flux ∼ 60 proton · cm−2 · s−1. right: the discharge current 200µa, flux ∼ 420 proton · cm−2 · s−1. figure taken from [12]. detector counts individual protons with 100% efficiency. the measurement of the average proton flux with timepix assumes that the 1ms long time frames are distributed uniformly in time. the uncertainty in the proton flux is then given by the uncertainty in proton counting. since proton cluster merging is small for ultra low fluxes the estimated uncertainty is better than 5%. 6. conclusion the u-120m cyclotron irradiation facility is able to provide ultra-low intensity proton beams which are suitable for experiments that study response of a detector to individual particles. the minimal reachable intensity of the proton flux that was reached was well below 1 proton·cm−2·s−1. the proton flux can be continuously regulated from 10−1 to 1014 proton · cm−2 · s−1. good stability of the beam parameters is guaranteed for fluxes greater than ≈ 103 proton · cm−2 · s−1. 7. acknowledgement the authors thank the institute of experimental and applied physics, czech technical university in prague for providing the timepix device. this work was supported by the ministry of education, youth and sports of the czech republic projects lm2015056 (canam), lm2015058 and ltt17018. references [1] nuclear physics institute of the cas, czech republic. isochronous cyclotron u-120m. http://accs.ujf.cas.cz. [2] m. cihak, j. lacman, j. stursa . new control system for the isochronous cyclotron u-120m. proceedings of the 15th international conference on cyclotrons and their applications 1998. http://accelconf.web.cern. ch/accelconf/c98/papers/h-05.pdf. [3] b. abelev ( the alice collaboration). upgrade of the alice experiment: letter of intent. journal of physics g: nuclear and particle physics 41, 2014. doi:10.1088/0954-3899/41/8/087001. [4] f. krizek et al. . cyclotron based irradiation setup for alice inner silicon tracker upgrade preprint. nuclear inst and methods in physics research, a, nima-d-17-00930 2017. [5] ptw freiburg, germany. ptw farmer ionization chambers. http://www.ptw.de/farmer_chambers0.html. [6] x. llopart, r. ballabriga, m. campbell, l. tlustos, w. wong . timepix,a 65k programmable pixel readout chip for arrival time, energy and/or photon counting measurements. nucl instrum methods phys res a 581:485–494, 2007. doi:10.1016/j.nima.2007.08.079. [7] t. holy, j. jakubek, s. pospisil, j. uher, d. vavrik, z. vykydal . data acquisition and processing software package for medipix2. nucl instrum methods phys res a 563:254–258, 2006. doi:10.1016/j.nima.2006.01.122. [8] z. vykydal, j. jakubek, s. pospisil. usb interface for medipix2 pixel device enabling energy and position-sensitive detection of heavy charged particles. nucl instrum methods phys res a 563:112–115, 2006. doi:10.1016/j.nima.2006.01.114. [9] s. busold, h. rocken, j. m. schippers, a. s. partowidjojo . recent ion source developments for varians probeam cyclotron. proceedings cyclotrons 2016 pp. 316–318, 2016. doi:10.18429/jacow-cyclotrons2016-thp06. [10] t.matlocha. ion source for a single particle accelerator. acta polytechnica ctu proceedings 4:50–55, 2016. https://ojs.cvut.cz/ojs/index.php/app/article/ view/3743/3808 doi:10.14311/ap.2016.4.0050. [11] milan cihak, jan stursa, ondrej lebeda. beam dynamic simulation in the isochronous cyclotron u-120m. cyclotrons and their applications, eighteenth international conference pp. 385–387, 2007. http://accelconf.web.cern.ch/accelconf/c07/ papers/385.pdf. [12] t.matlocha. modification of a classical penning ion source operating mode for sub-femtoampere beams at the u-120m cyclotron in review. aip proceedings of 17th international conference on ion source 2017. [13] a. baumel, p. drodten, e. heitz, r. bender. tantalum, niobium and their alloys corrosion handbook, 2008. doi:10.1002/9783527610433.chb02140. 26 http://accs.ujf.cas.cz http://accelconf.web.cern.ch/accelconf/c98/papers/h-05.pdf http://accelconf.web.cern.ch/accelconf/c98/papers/h-05.pdf http://dx.doi.org/10.1088/0954-3899/41/8/087001 http://www.ptw.de/farmer_chambers0.html http://dx.doi.org/10.1016/j.nima.2007.08.079 http://dx.doi.org/10.1016/j.nima.2006.01.122 http://dx.doi.org/10.1016/j.nima.2006.01.114 http://dx.doi.org/10.18429/jacow-cyclotrons2016-thp06 https://ojs.cvut.cz/ojs/index.php/app/article/view/3743/3808 https://ojs.cvut.cz/ojs/index.php/app/article/view/3743/3808 http://dx.doi.org/10.14311/ap.2016.4.0050 http://accelconf.web.cern.ch/accelconf/c07/papers/385.pdf http://accelconf.web.cern.ch/accelconf/c07/papers/385.pdf http://dx.doi.org/10.1002/9783527610433.chb02140 acta polytechnica ctu proceedings 14:15–20, 2018 1 introduction 2 beam monitoring and measurement 3 u-120m ion source 4 beam intensity regulation 4.1 u-120m radio frequency structure 4.2 regulation by vacuum quality and ion source gas 4.3 ion source shift 4.4 ion source discharge current regulation 4.4.1 the minimal ion source discharge current 4.5 the cathodes embrittlement 5 the minimal beam intensity 6 conclusion 7 acknowledgement references acta polytechnica doi:10.14311/app.2020.27.0048 acta polytechnica 27(0):48–52, 2020 © czech technical university in prague, 2020 available online at http://ojs.cvut.cz/ojs/index.php/app comparison of mo coating deposited by twas and aps sinplexpro thermal spray techniques šárka houdkováa,∗, jakub antoša, petra šulcováa, kateřina lencováa, jan hajšmanb, karolína burdováb, josef duliškoviča a výzkumný a zkušební ústav plzeň, tylova 1581/46, 301 00 plzeň, czech republic b západočeská univerzita v plzni, fakulta strojní, univerzitní 2732/8, 301 00 plzeň, czech republic ∗ corresponding author: houdkova@vzuplzen.cz abstract. the wear resistant mo coatings were deposited using smartarc electric arc wire spray system and advanced sinplexpro high throughput atmospheric plasma spray gun. the coatings were analyzed in terms of microstructure, mechanical properties and tribological characteristics, including wear rate and wear mechanism. it was shown that despite of the differences in deposition process principles, the properties of the coatings don’t differ with respect to their functional properties. both coatings reached comparable hardness, microhardness, wear resistance and coefficient of friction. the economic aspects remain the main argument for the recommendation of the most suitable thermal spray deposition technique for the application of molybdenum coatings. keywords: aps, astm g-133, coating, cof, mo, nanoindentation, sliding thermal spray, twas, wear. 1. introduction thermally sprayed mo coatings are widely used as surface treatment offering high wear and scuffing resistance. due to their superior tribological behavior they are applied especially in sliding contact [1, 2]. to deposit the mo-based coating, various thermal spray techniques can be utilized. as the coating’s microstructure is dependent on the used thermal spray method the differences in the wear resistance and friction behavior can be also expected [3]. the atmospheric plasma spraying (aps) is the most widespread technique for thick coating deposition providing the possibility to spray a wide range of materials, including metals, hardmetals or ceramics with various functionalities, it is also usually utilized to deposit the wear and scuffing resistant mo coatings. in addition to the examination of coatings sliding properties [2, 4, 5], the spraying of mo was also used for studying the process-structure-property relationships and for creating so called "process maps" of thermal spraying [6, 7]. as various studies were previously published on available aps systems, there is a lack of information concerning the recently developed sinplexpro high throughput atmospheric plasma spray gun. the twin wire electric arc spraying (twas) is a technique utilizing the material in the form of wires. the principle of the twas limited its use on the materials that are electrically conductive. that is why it is used primarily for spraying of fe and ni-based alloys, even though the wires with more complex compositions are also available [8]. the lower operating costs, as well as lower price of feedstock material in the form of wire compared to powder, can make the twas technique desirable alternative to plasma spraying if it produces the mo coatings with sufficient properties. in the paper, the comparison of mo coatings, sprayed by aps sinplexpro plasma torch and smartarc electric arc wire spray system is done to evaluate the differences in the coatings microstructure, properties and deposition efficiency. the aim is to recommend the most suitable spraying system for the application of mo coatings on the sliding surface of piston rings for the automotive industry. 2. experimental feedstock material: two kinds of feedstock material were used: (i) the pure (99.55%+) molybdenum powder amdry 313x from oerlicon metco, agglomerated and densified, with (45-75 µm) size range for aps spraying and (ii) the pure molybdenum (99.9%+) wire w400.1 from flame spray technologies, 1.6 mm diameter. spraying: the sinplexprotm plasma torch and the smartarctm twas spraying system, both from oerlicon metco, were used for spraying onto grid blasted (al2o3, 0.8-1 mm grain size) carbon steel substrates (45x25x5 mm). the spraying parameters were previously optimized for reaching the highest coating quality. to evaluate the deposition efficiency, the consumption of feedstock material was measured during spaying of 1 mm thick coating onto the prototype part piston ring spine ∅ 120 mm, 280 mm long. microstructure evaluation: the scanning electron microscope (sem) evo ma25 from zeiss with lab6 thermal filament and equipped by edx detector sdd 48 http://dx.doi.org/10.14311/app.2020.27.0048 http://ojs.cvut.cz/ojs/index.php/app vol. 27/2020 mo coating: twas vs. aps sinplexpro figure 1. the sem of mo coatings sprayed by a) aps and b) twas technique figure 2. the coatings microhardness depth profile x-max 20 oxford instruments was used to evaluate the microstructure of the coating on the crosssections. the phase compositions were analysed by x-ray diffraction (xrd), using d8 discover diffractometer with 1d detector and cokα radiation. the obtained diffraction patterns were subjected to quantitative rietveld analysis performed in topas 4.2 [9] in ipp ascr. coating properties evaluation: the adhesion tests were realized in accordance with astm c633-13 [10]. for each value, at least 3 measurements were done. surface roughness was measured in accordance with en iso 4288 [11]. for each value, at least 3 measurements were done. the surface hardness hr 15n was measured on the gently ground surface of coatings. for each value, at least 5 measurements were done. microhardness hv0.3 was measured in the middle of coatings cross-sections. for each value, at least 7 measurements were done. microhardness depth profile hv0.1 was measured on the coating’s cross sections in 0.05 µm steps. for each value, at least 3 measurements were carried out in one depth. the instrumented hardness was measured in rti uwb using the nanotest vantage testing device from micro materials company, equipped with berkovich indenter, by 5 n testing load, 200 mns−1 loading rate, 10 s dwell period at maximum load, 20 indents were performed 10 µm below the surface and 20 indents 100 µm above the interface. the sliding wear resistance was measured by ball-onflat test, in accordance with astm g-133 [12], using 6 mm diameter. cr-steel ball counterpart; 25 n load, 50 hz frequency; 1000 s test duration. during the test, the coefficient of friction was recorded. the coatings sliding wear resistance k [mm3/nm] was determined from the wear tracks profile measurements. the average value of three astm g-133 tests on each sample is reported. 3. results and discussion the microstructure of the aps and twas sprayed coatings is shown in figure 1. in both cases, the porosity, as well as cracks of the individual particles (splats), can be observed. the size of the splats is lower in the case of aps coating, resulting from the fine powder compared to the splats originated from the molten wire tips. both coatings have a variable microstructure across the cross-sections. less porosity and inter-splat decohesion was observed in the lower part of the coatings, compared to the upper part. this phenomenon is still unclear, nevertheless, its intensity is dependent on the spraying parameters of each technology, related 49 š. houdková, j. antoš, p. šulcová et al. acta polytechnica figure 3. coefficient of friction in dependence on the time of the test for a) aps and b) twas sprayed coatings mo coating hit (gpa) eit (gpa) hit (gpa) eit (gpa) upper layer upper layer lower layer lower layer aps 3.76 ± 0.43 138.6 ± 8.6 3.39 ± 0.33 103.7 ± 10.0 twas 3.31 ± 0.69 83.3 ± 8.4 2.98 ± 0.46 66.4 ± 8.5 table 1. results of instrumented indentation. probably to the temperature and velocity of the particles, as well as to intensity and frequency of cooling during spraying. the amount of energy, transferred to the coated sample can be responsible for coatings densification. simultaneously, the influence of transferred heat and impact energy can be connected with changes of substrate hv0.1 microhardness (see fig. 2). the phase composition was evaluated by xrd. both coatings composed of mo, accompanied by a small amount of moo2. according to the rietveld quantitative analyses, its amount reached 0.76 wt.% for aps coating and 2.44 wt.% for twas coating. the thicknesses of the coatings were measured 0.48 ± 0.02 mm and 0.77 ± 0.01 mm for aps and twas coating resp. as the number of passes necessary to reach the thickness was 10 and 6, the thickness per pass was 48 µm and 128 µm for aps and twas coating resp. the consumption of feedstock material was measured during spraying the piston ring spine (∅120 mm-280 mm long). to spray 1 mm thick coating, the 0.2 kg of powder was consumed during aps spraying, while 1.83 kg of wire was needed for twas spraying. the adhesion of the coatings was 37.4 ± 5 mpa for aps and 32 ± 5 mpa for twas coating. these values are lower than values, found in ref [3]. coating surface roughness was 8.62 ± 0.27 µm ra and 56.85 ± 3.80 µm rz for aps and 18.89 ± 0.92 µm ra and 114.52 ± 4.01 µmrz for twas coating. while the roughness of aps coating is in agreement with ref. [3], the ra of twas coating is much higher in [3] (18.89 vs. 9.81 ra). the surface roughness is connected with the spreading of splats after the droplets impact and can be considered as a parameter of coatings quality. surface hardness hr 15n reached comparable values (67.4 ± 1.4 and 67.8 ± 1.5 for aps and twas coating resp.). the surface hardness value involved besides the property of the material, also the contribution from present pores, cracks and intersplat boundaries. the microhardness hv0.3 of the coatings, measured on the coating’s cross-sections was slightly lower for aps coating compared to twas (327 ± 20 and 387 ± 37 for aps and twas coating resp). the presence of a higher amount of hard oxide inclusions can be responsible for higher microhardness of twas coating. the overall hv0.3 values are in agreement with the microhardness measured in ref. [1, 3, 4]. to evaluate the variation of coating properties across the cross-sections, the microhardness depth profile was done. as can be seen from figure 2, there is not a significant difference between aps and twas coating. in both cases, the hardness increases towards the surface. such observation seems to be in contradiction with the assumption, that the less porous microstructure close to the coating-substrate boundary will be harder than the porous layer in the upper part of the coating. to confirm the microhardness results, the instrumented indentation measurement was done on both coatings in the upper porous and lower dense layer of each coating. the load of 5 n was used for indentation to include not only the inner splats properties but also the influence of porosity and intersplats cohesive strength. the measured values of hit and eit are summarized in the table 1. even though high scatter of the measured values given by heterogeneous microstructures of both coat50 vol. 27/2020 mo coating: twas vs. aps sinplexpro figure 4. wear mechanism for aps (a,b,c) and twas (d.e.f) sprayed coatings ings, it can be observed that the lower dense close-tointerface layer has a lower hardness and lower modulus of elasticity, compared to a higher, more porous layer close to surface. this observation is in agreement with the hv0.1 depth profile measurement. during the ball-on-flat linear oscillating test, the coefficient of friction (cof) was recorded and can be seen in figure 3. similar behavior of both types of coatings can be seen. the continuous slow increase was followed by a sudden drop of cof value in 400600 s of the test. after, the constant cof value of 0.7 was recorded till the end of the test. the sudden decrease of cof is usually connected with the creation of stable tribolayer in the wear track. also, the transfer of worn coating material onto the surface of the counterpart can be another reason for cof decrease, however, it was not analyzed during this study. the wear resistance of both types of coatings is very similar. for aps sprayed coating, wear coefficient k was 1.91*10-4 ± 1.9*10-5 mm3/nm, while for twas coating 2.06*10-4 ± 2.8*10-5 mm3/nm. for comparison, the wear coefficient of aisi 316l steel is 2.74 ± 1.37. in [2] the reported wear rate was much lower compared to the results of samples evaluated in this study. the differences in wear tests (astm g-99 vs astm g-133) and particularly in used loads (5n vs 25n) can play a significant role in the wear rate. the wear mechanism can be seen in figure 4. for both types of coatings, the delamination of the parts or whole splats was identified (fig. 4b,e) as the main mechanism responsible for wear. except of delam51 š. houdková, j. antoš, p. šulcová et al. acta polytechnica ination, ploughing takes place. in the wear track, the tribo-layer was locally identified (fig. 4c,f). the tribo-layer usually consist of deformed coating wear debris, transferred counterpart material and oxides, originated during sliding [13]. the edx analyses show the presence of fe (ca. 3-4 wt.%) and cr (ca. 1-1.5 wt.%) in the wear track, originated from the counterpart, together with 11-14 wt.% of oxygen confirming the assumption of tribo-oxidation. there are no differences in the wear mechanism between aps and twas sprayed coating, except the size of delaminated areas. they are bigger in the case of twas coating, which is connected with bigger splats dimensions (see fig. 1). the bigger size of delaminated splats is than responsible for slightly higher wear volume. 4. conclusions despite of the differences in deposition process principles, the properties of the coatings don’t differ with respect to their functional properties. both coatings reached comparable hardness, microhardness, wear resistance and coefficient of friction. wear mechanisms also don’t differ splat delamination and ploughing are the main observed mechanism responsible for wear. in the case of twas, the size of delaminated areas is bigger in consequence of the bigger size of original splats. considering the economic aspects, the significantly lower deposition efficiency of twas technology and longer time of deposition makes the twas 70 % more expensive than aps sinplexpro for spraying of a reference component, despite of more than 2 times higher unit price of mo powder compared to wire. acknowledgements the paper has originated in the framework of the project of technology agency of the czech republic no. tj02000135. references [1] j. ahn, b. hwang, s. lee. improvement of wear resistance of plasma-sprayed molybdenum blend coatings. journal of thermal spray technology 14(2):251–257, 2005. doi:10.1361/10599630523827. 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[13] g. bolelli, b. bonferroni, j. laurila, et al. micromechanical properties and sliding wear behaviour of hvof-sprayed fe-based alloy coatings. wear 276-277:29 – 47, 2012. doi:10.1016/j.wear.2011.12.001. 52 https://doi.org/10.1361/10599630523827 https://doi.org/10.1016/j.wear.2007.01.018 https://doi.org/10.1007/s11666-018-0706-2 https://doi.org/10.1016/s0043-1648(98)00395-0 https://doi.org/10.1016/s0043-1648(01)00842-0 https://doi.org/10.1016/s0921-5093(02)00642-1 https://doi.org/10.1016/j.msea.2005.04.056 https://doi.org/10.1107/s0021889869006558 https://doi.org/10.1016/j.wear.2011.12.001 acta polytechnica 27(0):48–52, 2020 1 introduction 2 experimental 3 results and discussion 4 conclusions acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2019.23.0009 acta polytechnica ctu proceedings 23:9–13, 2019 © czech technical university in prague, 2019 available online at https://ojs.cvut.cz/ojs/index.php/app reconstruction of negrelli’s viaduct from a geotechnical engineering point of view linda černá vydrováa,b a czech technical university in prague, faculty of civil engineering, department of geotechnics, thákurova 7, 166 29 prague, czech republic b hochtief, a. s., division traffic infrastructure, plzeňská 16, 150 00 prague, czech republic correspondence: linda.cernavydrova@hochtief.cz abstract. the reconstruction project deals with the unsatisfactory condition of the bridge construction, involving the railway superstructure with its signalling, communication and power equipment as well as the overhead lines all in an integrated manner. the article presents the contractor’s view of this unique project and the initial experience gained during the first year of the construction. emphasis is laid on geological conditions and on geotechnics, especially as they relate to work on the special foundation which constitute a significant part of the reconstruction. keywords: negrelli’s viaduct, bridge structure, subsoil, load capacity of arches, repair, special foundation. 1. basic background information on the project the owner builder is správa železniční dopravní cesty, státní organizace (the railway infrastructure administration authority, a state organization). the funding is provided by the connecting europe facility (cef) programme of the eu; the project is co-funded at the national level by státní fond dopravní infrastruktury (the state fund for transport infrastructure). the general designer is the company sudop praha a. s. reconstruction of the bridge is performed by a joint venture of the companies hochtief cz a. s. (division of traffic infrastructure,), strabag rail a. s. and avers, s. r. o. the construction work has been carried out during a complete closure of the railway on the bridge starting july 2017. the railway traffic will be resumed in january 2020. the finishing work will continue until june 2020. reconstruction of the negrelli’s viaduct (figure 1) is closely linked to the planned construction project named upgrade of railway line praha–kladno, which the viaduct reconstruction will make possible in the future [1–3]. 2. brief description of the project the negrelli’s viaduct is situated on the railway track section praha masarykovo nádraží – praha bubny that forms part of the railway line from praha masarykovo nádraží to děčín hl. n. (route segment 0801) and praha masarykovo nádraží hrabovka – praha masarykovo nádraží karlín (route segment 1505). it was opened to rail transport in may 1850. in 1875, it was extended by a "connecting viaduct" for the railway line hrabovka – karlín. both these parts of the railway line are integrated in a countrywide railway network owned by the czech republic, figure 1. bird’s-eye view of the negrelli’s viaduct during the reconstruction. represented by správa železniční dopravní cesty, s. o. (the railway infrastructure administration authority); the rail transport operator is české dráhy, a. s. (czech railways). the bridge, together with the signal box no. 249 is on the list of cultural heritage buildings [1–3]. 3. purpose and preparation of the reconstruction the main reason for the reconstruction of the bridge was the poor technical condition of the arches, see figure 2. due to defects in the waterproofing and drainage systems that resulted in water leaks into the structure, the arches have suffered a number of flaws and thus required repairs of the masonry or replacement of the masonry components. it has proved necessary to replace and renew the drainage system of the bridge, to strengthen the foundation of the selected 9 https://doi.org/10.14311/app.2019.23.0009 https://ojs.cvut.cz/ojs/index.php/app linda černá vydrová acta polytechnica ctu proceedings figure 2. view of the exposed sandstone arches of the viaduct. piers and to reinforce the masonry with grouting. in some cases, considering the extent of damage to the masonry, it is necessary to dismantle the arches completely, to replace the worn-out stone blocks by new ones and then to re-mason the arches and the piers as well, if this also is necessary. the purpose of the project is to ensure that the obligatory parameters that apply to upgraded railway lines will be met. that means, in particular, the structure clearance gc, the load capacity class d4, adjustments of the track geometry to remove the local speed limits to secure adequate capacity of the railroad, also to comply with the health limits regarding noise and vibrations, and to replace the inadequate structures and equipment. 4. geological and geotechnical conditions the location in question is at the alluvial plain of the vltava river. the terrain has been affected by anthropogenic activity to the maximum extent. it is an area that used to be behind the historic walls of the city of prague. on the right bank, there were three tributaries of the vltava river, but these are now filled up. the ground has been shaped by embankments that are up 6m thick. the bedrock is made up of rocks from the prague ordovician period. on the right bank of the vltava river, there are the šárka and bohdalec formations that turn into the záhořany formations in approaching the vltava river. northward, at the island rohanský ostrov, the bedrock changes to the vinice formation. underneath the riverbed, there are also letná formations. all these formations belong to the upper paleozoic era, the beroun phase. these formations can be characterized as a sequence of furrowed dark grey siltites, silty shales, clay shales and clay stones. the superincumbent formations in the given area are represented especially by the typical pleistocene fluvial sediment drifts covered by holocene alluvia and fills. the terraced sediments of the vltava river consist of sands with clay. at deeper levels, the sediments are substituted by sands and gravel sands. at the base, the sediment is often of coarse gravel or boulder [1–3]. 4.1. hydrogeological conditions the occurrence of groundwater in the given location is associated particularly with sand and gravel sand terraced areas that are porous and easily permeable. in those, a continuous groundwater level is created, the depth of which depends on the water level of the vltava river. the ordovician rock bed is poor in the groundwater. the unweathered shales are not very permeable; if weathered, they have the nature of clay soil that is not easily permeable. the groundwater in the ordovician shales has a predominantly sulphate aggressivity and the highest degree of aggressivity is in the bohdalec formation [1]. 4.2. tectonic conditions at the place where negrelli’s viaduct starts (on the karlín side at the foot of the vítkov hill), there is a significant tectonic line – the prague fault. this tectonic fracture causes a substantial decrease in the strength of the adjacent rocks. the prague fault is on the north side merging with a massive fault zone. it exists as a wide zone of fractures consisting of a number of parallel faults [1]. 5. brief description of the reconstruction even though large-scale surveys were carried out, it was not possible to rule out beforehand that after having removed the upper courses on the stonework, and having exposed the underside of the arches, or even after the areas not accessible at the time of preparation of the design would have been made accessible, so that it would then have to be decided – during the actual construction itself – in favour of a change in the extent of the repair works, replacement of the stones or indeed rebuilding the arches. the viaduct is reconstructed in its entire length, which is 1,413m. all 100 arches will be repaired. 8 of them span over two tributaries of the vltava river, 5 bridge structures run across roads, 2 bridge structures that were built later will be replaced and 14 arches completely rebuilt. a strict and clear number coding system has been created for the stones (applied in photogrammetric measuring), which facilitates an unmistakable identification of the stones even if they have to be dismantled and then placed back into the arches. given the fact that the bridge is on the list of cultural heritage buildings, a restoration survey and detailed diagnostics of the individual stonemasonry components represent almost a third of the total scope of the work. based on the diagnosis, there is then a decision on the further method and progress of the repair works. 10 vol. 23/2019 negrelli’s viaduct reconstruction at the beginning of the reconstruction, the built-in structures and extensions of the bridge were removed. further, the railway superstructure was taken down, the overhead lines removed and the track ballast excavated. on the supporting brackets on the side of bridge or on the cable trays, a 22 kv power cable of predi a. s. was temporarily suspended – it had been installed on the bridge but had impeded the work to be performed. contrary to the assumptions based on the design documents, no slab foundation was found. it had been substituted by a layer of gravel, which had to be removed, processed and placed back, to minimize the risk of point load on the arches when machines move on the bridge. given the condition of the exposed piers, it was designed to grout the pier shafts in order to secure adequate load capacity and to eliminate the voids in the masonry. further, the backfills in the arches have been removed, the arches exposed, a survey carried out and the underside of the arches repaired. before that, the arches had to be propped with centerings on the underside. the undersides of the arches will then be waterproofed and the arches filled with pervious concrete. on the arch fills, a new reinforced concrete slab with an insulation system has been designed. the slab spreads the load onto the repaired arches. the parapet and handrails will be new. installation of the railway superstructure will follow and so too will fitting of the railway equipment and placement of a raceway and other pipe sleeves into the track ballast. then the overhead lines will be installed. concurrently with commencement of work on the deck, cleaning of the face sides of the arches from the ground level was started and the masonry components were diagnosed. right after that, the subsoil was consolidated by jet grouting. in order to repair the substructure, in one bed of the vltava river, double sheet pile caissons are being currently built. in the other riverbed, repair works to the pier shafts are underway – and due to the circumstances, there is required the assistance of divers. after a complete repair of the substructure, landscaping and finishing work will follow [1–3]. 6. special foundation work at the end of the 19th century until the 19th century, more substantial settlements of structures were prevented by founding the buildings on compressible soil on timber piles or more often, on horizontal timber grids or on a layer of sand no less than 1m thick thus spreading the weight of a building over a larger area. soft soil was also compacted by pole ramming. the massive piers of negrelli’s viaduct were founded on robust timber grids or timber piles or right on the rock itself if the foundation bottom hit it. the foundation of buildings in water was a tough challenge to surmount in the middle ages because the only way to pump water was to bring it out in buckets or to pump it using a treadwheel with buckets powered by humans or horses. it was not uncommon for this to engage 300 people, but it could hardly be a substitute for a contemporary 30 kw engine. for the caissons, sheet piles were used, and also balks of timber with a wedge-shaped point that was plated. a couple of those tips, sheet piles and piles were found during excavations of material from the future caissons. the sheet-pile walls also consisted of guide piles of a circular section bearing the ties. in between the ties, the individual sheet piles were then rammed. due to unevenness of the timber and inaccuracy of the ramming, gaps were created between the sheet piles which were sealed with a tow and hard wedges. even though a willing and cheap labour force was available for the heavy manual work, for founding the piers of negrelli’s viaduct, steam engines were used as well – in particular, to pump the water from the caissons. the steam engines were housed in wooden sheds and were connected with bucket pumps or screw pumps; each of them had a performance of about 12 k. in the press of these days it is mentioned that the steam engines could power also lime mills and rams. however, these were not conventional steam engines taking the form of a compact traction engine as known in later years. more specific information about these machines is unfortunately missing. the stones for the bridge construction were transported on the river; for their handling on the site, trolleys were used, which were operated manually [4]. 7. special foundation works 7.1. grouting of the pier subsoil – single-phase jet grouting m1 the selected piers were underpinned with jet grouted columns. the pilot bores through the existing foundation masonry of the bridge piers were driven using a full-face percussion rotary drilling method (submersible hammer) with a drill hole diameter of 150mm with an air flush; a hydraulic drilling rig on a tracked undercarriage msv 741/20 (producer: zs, a. s.) and a compressor atlas copco xahs were also used, as depicted in figure 3. the drill holes for the ti columns and the subsequent ti m1 (ascending method) were driven using a full-face rotary drilling method with a drill hole diameter of 140mm to 185mm and with a combination of water and cement flush. hydraulic drilling rigs were used – hbm 12k/hy-zs (producer: hausherr, germany, and customized by zs, a. s.) and a drilling rig jano hvs 482 on a tracked undercarriage and in confined spaces, a hydraulic drilling rig on a tracked undercarriage msv 741/20. the grout was produced at an automated production centre twm 30, the cement was batched from a pressure container 11 linda černá vydrová acta polytechnica ctu proceedings figure 3. special foundation works at the former custom house. 56m3. for ti m1, a high-pressure pump tw 600 was used [5]. 7.2. conventional grouting of the pier shafts with respect to the expected porosity of the pier fillings, in order to increase their load capacity, lowpressure grouting was designed. the drill holes for fitting a pvc bushing were driven using a double rotary drilling method with a drill hole diameter of 203mm with an air flush; a hydraulic drilling rig on a tracked undercarriage jano-7 hvs 6187 and hbr 504 ftw and a compressor atlas copco xrvs were used. the drill holes for the grouting were driven using a full-face percussion rotary drilling method (submersible hammer) with a drill hole diameter of 125mm to 152mm with an air flush. a hydraulic drilling rig on a tracked undercarriage jano-7 hvs 6187 and hbr 504 ftw and a compressor atlas copco xrvs were used. for dust control during the drilling, a foam deduster was used. for the grouting, an injection pump haponic 4/52, an activation mixer akc 025 and a mixer truck dm 025 were all used. an injection level in a drill hole was defined with a simple expandable oburator ring. 7.3. grounding micropiles in the selected piers, grounding micropiles were installed. the drill holes for the micropiles were driven using a double rotary drilling method with a drill hole diameter of 140mm to 185mm with an air flush. a hydraulic drilling rig on a tracked undercarriage jano-7 hvs 6187 and hbr 504 ftw and a compressor atlas copco xrvs were used. for dust control during the drilling, a foam de-duster was used. for the grouting, an injection pump haponic 4/52, an activation mixer akc 025 and a mixer truck dm 025 were all used. the injection levels were defined in the steel rod of the micropiles using a double expandable oburator ring. 7.4. principle of the designed caissons the space around the piers that was needed for their repair was designed to be supported by building double sheet pile caissons. since the caissons are quite narrow in relation to their depth of 5.0 to 5.6m, spacing of the sheet piles is 2.2m and the caissons are, at the crest, rested on the pier masonry. the maximum excavation along a pier is to the rock bed level, which is in practical terms to the sheet pile footing. the caisson structure consists of two parallel sheet pile walls cross-connected with rods. the spacing of the rods leading from the bottom level to the crest of the opposite wall is, in the case of the outside wall, 2.4m and in the case of the inner wall to be excavated to the footing, 1.2m. the inner sheet pile wall footing is moreover supported with tie bars driven into the rock bed at a spacing of 1.2m. the space between the sheet piles is filled up. to prevent water leaks under the sheet pile footings in case the footing is not embedded in the impermeable bed, e.g. if the rock bed surface is not even, or if there are boulders at the gravel base, every other sheet pile of the water-facing wall is fitted with an injection pipe allowing additional sealing. in order that the drilling rig could travel to the caisson crest level, it was designed to widen the crest towards the pier along the inner sheet pile wall by about 130 cm. the platform consists of two sheet piles fitted on steel braces. 8. conclusion the location of the viaduct has been often criticized since the end of the 19th century when this railway construction became an obstacle to traffic as the city developed. it has also been viewed as an unwelcome intervention into the city space, suggesting that the whole route should have been designed much more to the east. at the time of the construction, however, the cheap land in the countryside were the most convenient for the railway and this fact influenced the location of the viaduct and its route. just like the construction itself, its designer was not duly appreciated at the time either. this is readily apparent from the name of the bridge as well – for a hundred years it was called generally "viaduct of společnost státní dráhy" i.e. state railway company or later "the railway viaduct from karlín to holešovice". it was named after its creator only from the 1950s. to this day, the construction is regarded by prague citizens as a hideous and bothersome structure that, in spite of all reconstructions and subsequent enlargements for a lighter easement, still complicates traffic conditions. hardly anyone realizes how old this bridge structure really is-and how beautiful it is. last but not least, thanks to the reconstruction, a significant cultural heritage transport engineering building will be preserved. 12 vol. 23/2019 negrelli’s viaduct reconstruction references [1] sudop. sudop praha, a. s., rekonstrukce negrelliho viaduktu, doplňkový stavebně technický a ig průzkum (survey documents), souhrnná zpráva (summary report). 07/2014. [2] t. martínek. rekonstrukce negrelliho viaduktu. in proceedings of 23rd international symposium mosty/bridges 2018. sekurkon, brno, 2018. [3] l. černá vydrová. rekonstrukce negrelliho viaduktu. stavebnictví 8:32–38, 2018. [4] p. bažant. metody zakládání staveb. second printing. academia, československá akademie věd, 1973. [5] z. bauer. stavební stroje firmy lanna. corona, prague, 2005. 13 acta polytechnica ctu proceedings 23:9–13, 2019 1 basic background information on the project 2 brief description of the project 3 purpose and preparation of the reconstruction 4 geological and geotechnical conditions 4.1 hydrogeological conditions 4.2 tectonic conditions 5 brief description of the reconstruction 6 special foundation work at the end of the 19th century 7 special foundation works 7.1 grouting of the pier subsoil – single-phase jet grouting m1 7.2 conventional grouting of the pier shafts 7.3 grounding micropiles 7.4 principle of the designed caissons 8 conclusion references acta polytechnica ctu proceedings doi:10.14311/app.2018.15.0104 acta polytechnica ctu proceedings 15:104–108, 2018 © czech technical university in prague, 2018 available online at http://ojs.cvut.cz/ojs/index.php/app geometry and mechanical properties of a 3d-printed titanium microstructure luboš řehounek∗, petra hájková, petr vakrčka, aleš jíra czech technical university in prague, faculty of civil engineering, thákurova 7, 166 29 prague 6, czech republic ∗ corresponding author: lubos.rehounek@fsv.cvut.cz abstract. construction applications sometimes require use of a material other than construction steel or concrete – mainly in cases, where strength to weight ratio needs to be considered. a suitable solution to this problem are structures manufactured using the 3d printing process, as they have a very good strength to weight ratio (i.e.: ti-6al-4v – σult = 900 mpa and ρ = 4500 kg/m3). trabecular structures are porous structures with local material characteristics identical to their commonly manufactured counterparts, but due to their geometry, they have different global mechanical properties and are suited for special applications. we designed and manufactured six variants of these structures and subjected them to uniaxial compression tests, nanoindentation tests and subsequently evaluated their differences and elastic moduli. the values of global moduli e are in the range of 2.55 gpa – 3.55 gpa for all specimens. keywords: trabecular, porous, 3d printing, young’s modulus, nanoindentation. 1. introduction the trabecular structure functions as an alternative to conventional, homogeneous cross-sections of implants. generally, it represents a 3d system of beams of certain length and composition embedded in a 3d matrix. in our case, they have an equal length and are represented by truss-like beams (fig. 2). trabecular (porous) structures have many benefits as they can be used in applications where conventional structures are not sufficient. among their many benefits is mainly their shapeability as the final shape only depends on the design. they can generally be used in applications where it is desirable to either provide a porous material due to the nature of the application or where reduced global modulus is needed [1]. trabecular structures have, usually, lower moduli than conventional, homogeneous structures. this stems from the fact that they have a porous composition with a reduced cross-section (fig. 2). they can also be used for the development of alternative geometrical solutions of stems of dental implants [2]. trabecular structures are usually produced by 3d printing. printing 3d metal structures is a process that falls under the category of additive manufacturing (am). additive manufacturing has many variants; our specific application used the technology of 3d printing of metals. this part is usually done using a computer-aided design (cad) environment. upon its completion, an stl model file is divided into thin cross sections [3, 4] and sent to the 3d printer to be processed. up to this point, the process is similar to the common layer-by-layer 3d-printing of plastic. what differentiates the process of printing metals from the standard technology is using a laser beam to melt down a layer of metal powder. during each cycle, the coater applies a thin layer of powder, which is processed by a laser at a pre-set melting point in a pre-determined order [4]. this process solidifies the loose powder into a 3d-layered object. 3d printing is a very modern and perspective method in manufacturing of implants. it allows for very complex structures, which would not have been conceivable with traditional metalworking. it is also beneficial due to its ability to create small batches of products, thus being a more flexible way of manufacturing. on the other hand, 3d printing still has its disadvantages, mainly heat dissipation problems and uneven material processing. from our own research, it is already known to us that uneven heat dissipation can cause cracks at the interface of the homogeneous and trabecular cross-sections. dislocations of individual beams of the structures are also not uncommon and can cause wide impairments in the outcomes of the mechanical tests, rendering them effectively useless. these problems can, however, be solved by proper preparation of the 3d model, the printer assembly and by microscopic observation of the final structures. 2. materials and methods 2.1. description of test specimens the trabecular structures were manufactured by means of selective laser melting (slm) in an argon atmosphere of purity of 4.6. the material used for 3d printing was ti-6al-4v in the form of metal powder. the powder is named rematitan (concept laser). after the 3d printing, the specimens were heat-treated to eliminate inhomogeneities which are created during the 3d printing process. the treatment 104 http://dx.doi.org/10.14311/app.2018.15.0104 http://ojs.cvut.cz/ojs/index.php/app vol. 15/2018 properties of 3d-printed titanium figure 1. basic element cells for creation of the test specimens. figure 2. real manufactured specimens a-f used for the uniaxial compression test. was done in a vacuum furnace by gradual heating up to 840° c and subsequent quenching to 500° c. the geometrical solution of the trabecular structures was designed considering the optimal pore size for bone ingrowth. the desired pore size of 350-550 µm was achieved by using basic element cells labeled as diamond, dode thick and rhombic dodecahedron (fig. 1). every basic element structure was manufactured in two variants of different density. a total od 21 test specimens was created. every specimen comprises of a trabecular middle section (dimensions 14×14×14 mm) and upper and lower homogeneous base (1 mm thick, fig. 2). the composition of the specimens is listed in tab. 1. specimens from the a and c series are designed using the diamond element cell with density of 18 (a) and 14 (c) element cells per the width of the specimen. specimens from the b and d series are designed using the dode thick elements with density of 14 (b) and 11.5 (d) element cells per the width of the specimen. series structure type density a diamond 18 b dode thick 14 c diamond 14 d dode thick 11.5 e rhombic dodecahedron 11.5 f rhombic dodecahedron 9.5 table 1. structure type of individual specimens and their density (basic element count). figure 3. a specimen from the b series before loading (left) and during loading (right). element cells rhombic dodecahedron were used to design specimens from series e and f with density of 11.5 (e) and 9.5 (f) per the width of the specimen. 2.2. macromechanical analysis macromechanical analysis of the trabecular structures was carried out by a compression test. the compression test was performed in accordance with iso 13314:2011 compression test for porous and cellular metals [5]. the test was performed on the mts alliance rt-30 machine (ctu, czech republic) at room temperature. the loading direction for all structures was parallel to the building direction (perpendicular to the layers fabricated during 3d printing). respecting the prescribed loading direction was important, as the trabecular structures are orthotropic due to character of 3d printing. the crosshead speed was set to 1.00 mm·min-1. the mechanical properties were determined from the stress-strain curves (fig. 4). elastic modulus in compression e was calculated from the slope of the stress-strain curve in the linear elastic area in accordance with the iso 13314 standard [5]. as the calculated elastic modulus represents the global parameter of the trabecular structure with pores, it has to differ (be lower) from the reduced modulus of the alloy itself. evaluation of experimental data was done as described on fig. 5. it is important to distinguish between the modulus of the material and modulus of the entire structure, as these two values vary dramatically. for evaluation of young’s modulus e, we used the linear parts of the stress-strain curves of individual specimens obtained from the compression tests. determination of the range of the loading curve is described 105 l. řehounek, p. hájková, p. vakrčka, a. jíra acta polytechnica ctu proceedings . figure 4. stress-strain curves obtained from the compression test. curves shown belong to the f specimen group. note the method used to determine global e described in fig. 5. figure 5. stress-strain curve to determine the characteristic values from compression testing of porous and cellular metals [5]. grayed out area in the graph represents the plateau stress, from which the mean value of σpl is calculated. the plateau stress is a mean value of stress calculated from stress values in between the range of strain e = 20% and e = 30%. σ70 and σ20 then represent a range of values for caluculation of young’s modulus of the entire structure by the least squares method. in [5] as well as fig. 5. the final values of e for each specimen are listed in tab. 2. 2.3. micromechanical analysis the micromechanical analysis was performed on the csm instruments nanoindenter at the laboratory of ctu, faculty of civil engineering, prague. the specimens were embedded in epoxy resin, cut in transversal sections on a slow-rotating cutter and subsequently ground and polished to the desired level of quality with sandpapers of roughness of 1200–4000. to eliminate the surface tension and shear stiffness inside the atomic structure of the material, we used the method of directed force with cyclic loading. the loading force reached a value range of 10–70 mn and was applied in 7 cycles with a loading/unloading speed of 180 mn/min. the micromechanical analysis proved the micromechanical properties dependent on the depth of the indent (magnitude of the loading force). from a contact depth of hc 470 nm (corresponding loading force 30 mn), the trend of er (reduced modulus) and hit (hardness) is constant. we can, therefore, assume the value of 30 mn as a basic value of loading force. values of micromechanical properties that correspond with this value are er = 118 gpa and hit = 5.19 gpa. for comparison, we also performed additional micromechanical tests on conventionally machined specimens, which turned out to be almost identical with values of er = 118 gpa and hit = 4.58 gpa. the tests on these specimens were carried out in the same way and environment as the tests of 3d-printed specimens. 3. results the results of the macromechanical compression tests are listed in tab. 2. series b, c and f were measured and evaluated successfully, but series a, d and e encountered problems during the compression test. the first 3 specimens from series a did not successfully 106 vol. 15/2018 properties of 3d-printed titanium series modulus of individual specimens [gpa] mean modulus of a series [gpa] a1 exceeded max. machine load n/aa2 exceeded max. machine load a3 exceeded max. machine load a4 2.69 b1 2.74 2.78b2 2.99 b3 2.77 b4 2.63 c1 3.47 3.46c2 3.50 c3 3.41 c4 3.48 d1 exceeded max. machine load n/ad2 exceeded max. machine load d3 exceeded max. machine load e1 exceeded max. machine load n/ae2 exceeded max. machine load e3 exceeded max. machine load f1 2.55 2.58f2 2.62 f3 2.58 table 2. values of all young’s moduli calculated from individual series of specimens according to iso 13314 [5]. some series of experiments exceeded the maximum value of appliable load, more on this topic in section conclusions. figure 6. the trend of both measured micromechanical properties of porous specimens manufactured from a ti-6al-4v metal powder. the trend proved to be dependent on the depth of the indent. reach a strain of 0.2 and therefore it was not possible to determine the values of moduli according to the used method [5] as it is required to calculate the plateau stress as a mean value of stress in the strain range of 0.2–0.3. the problem arose as the machine prematurely reached its maximum value of load it can impose on the specimen. one specimen was successfully measured and is listed in tab. 2. series d and e 107 l. řehounek, p. hájková, p. vakrčka, a. jíra acta polytechnica ctu proceedings encountered the same problem, but on a rather larger scale as the specimens were so stiff they did not even reach a strain of 0.1 before reaching the maximum machine load and ending the experiment prematurely. 4. conclusions three out of a total 6 series of specimens failed the test on the mts alliance rt-30 machine as their geometrical solution provided a modulus so great it could not be measured. this shortcoming, although unfortunate, could not be accounted for beforehand as the geometrical solutions of the trabecular structures and their shape were developed with no prior expectations and knowledge of the total load needed to fully collapse the structures. the failure to measure the modulus stems from the fact that the experiment needs to be carried out at least to the range of 0.3 strain to meet the criteria of the used standard [5]. three series of specimens were measured successfully and with good conformity amongst specimens of a given series (tab. 2). the relation between their geometrical solution and values of moduli e (fig. 2 and tab. 2) can be taken into account in future development of a structure with specific requirements for its value of global modulus. the micromechanical analysis also proved that 3dprinted specimens have almost same mechanical properties on the micro level as specimens manufactured by conventional machining with hardness hit being slightly higher (12 % increase in 3d-printed specimens). this fact is probably caused by different heat treatment processes which are needed in case of 3dprinted specimens. acknowledgements the financial support provided by the technology agency of czech republic, project n.tj01000328 and by the faculty of civil engineering, ctu, prague, project n.sgs17/168/ohk1/3t/11 is gratefully acknowledged. references [1] t. hasegawa, s. kawabata, d. takeda, et al. survival of brånemark system mk iii implants and analysis of risk factors associated with implant failure. international journal of oral and maxillofacial surgery 46(2):267–273, 2016. doi:10.1016/j.ijom.2016.10.014. [2] l. řehounek, f. denk, a. jíra. numerical simulations of static strength of novel dental implants. acta polytechnica 58(4):259, 2018. doi:10.14311/ap.2018.58.0259. [3] f. e. wiria, j. y. m. shyan, p. n. lim, et al. printing of titanium implant prototype. materials and design 31(suppl. 1):s101–s105, 2010. doi:10.1016/j.matdes.2009.12.050. [4] a. el-hajje, e. c. kolos, j. k. wang, et al. physical and mechanical characterisation of 3d-printed porous titanium for biomedical applications. journal of materials science: materials in medicine 25(11):2471– 2480, 2014. doi:10.1007/s10856-014-5277-2. [5] i. standard. iso 13314 mechanical testing of metals, ductility testing, compression test for porous and cellular metals. reference number iso 13314(13314):1–7, 2011. doi:iso 13314:2011. 108 http://dx.doi.org/10.1016/j.ijom.2016.10.014 http://dx.doi.org/10.14311/ap.2018.58.0259 http://dx.doi.org/10.1016/j.matdes.2009.12.050 http://dx.doi.org/10.1007/s10856-014-5277-2 http://dx.doi.org/iso 13314:2011 acta polytechnica ctu proceedings 15:104–108, 2018 1 introduction 2 materials and methods 2.1 description of test specimens 2.2 macromechanical analysis 2.3 micromechanical analysis 3 results 4 conclusions acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2018.14.0027 acta polytechnica ctu proceedings 14:27–33, 2018 © czech technical university in prague, 2018 available online at http://ojs.cvut.cz/ojs/index.php/app neutronic analysis of the candidate multi-layer cladding materials with enhanced accident tolerance for wwer reactors ondřej nováka,∗, martin ševečeka,b a department of nuclear reactors, faculty of nuclear sciences and physical engineering, czech technical university in prague, prague, czech republic, v holešovičkách 2, prague 8, czech republic b department of nuclear science and eng., massachusetts institute of technology, cambridge, usa ∗ corresponding author: ondrej.novak2@fjfi.cvut.cz abstract. the paper summarizes preliminary results of neutronic analysis of candidate atf cladding materials that are under development at the ctu in prague. to evaluate basic neutronics-related characteristics the serpent code was used which is a monte-carlo based simulation tool. a model of wwer fuel was developed and basic neutronic analysis performed. all coating materials entail certain reactivity penalty compared to reference uncoated cladding that was quantified. the coating of cladding affects also other neutron-physical parameters of cores that modify the performance of the wwer reactors that are discussed. keywords: wwer, nuclear fuel, cladding, accident tolerant fuel, neutronics, serpent. 1. introduction new types of nuclear fuels for light water reactors with enhanced accident tolerance have been recently researched around the world. the concepts considered as future accident tolerant fuels (atf) include a development of completely new fuel pellets (nitrides, silicides, fcm)[1], cladding materials (fecral, sic, mo-alloys), non-fuel components or slight modifications of current fuel system [2–6]. the modification of current fuel system is considered as a near-term concept which can be developed and adopted by the industry in less than 10 years [7]. most of the atf concepts entails a certain reactivity penalty that has to be precisely determined during the development due to economical and operational concerns. many of the recent papers and reports focused on neutronic performance of atf fuel concepts for traditional pwr and bwr reactors [8–11]. however, the atf fuels can and should be employed also in the wwer reactors that are in operation in the czech republic, bulgaria, slovakia, hungary, finland, ukraine, india, and russia. the wwer reactors perform similarly to pwr reactors in the accidental and transient conditions and therefore most of the material and thermal testing performed in pwr conditions is valid also for wwer reactors [12]. however, the wwer reactors have their unique specifics including hexagonal fuel assembly geometry, fuel assembly shroud, different coolant chemistry, different materials or harder neutron spectrum. these specific aspects affect in particular neutron-physical performance of the fuel. for that reason, models of neutronics performance of the reactor wwer-1000 were developed in the serpent code, which is a well-known monte-carlo figure 1. sem micrograph of cr cold-spray coated zircaloy-4 cladding with nonuniform coating thickness. the top surface can be finished by polishing but the interface will remain nonhomogeneous due to high hardness of cr particles. based code, developed in the vtt, finland [13]. this model was used to analyze the performance of different multi-component claddings considered as potential candidates for the atf development. some of the concepts have been developed at the ctu [14] or mit [15] and some are considered as alternative options and will be investigated in future research. all studied concepts are based on the zircaloy-4 alloy substrate (isotopic material composition is noted in table 2). this alloy serves as a substrate on which different coating materials are applied. the coating materials include chromium nitride, chromium, zirconium silicate, fecral or molybdenum. an example of zircaloy-4 cladding coated by pure chromium using 27 http://dx.doi.org/10.14311/app.2018.14.0027 http://ojs.cvut.cz/ojs/index.php/app ondřej novák, martin ševeček acta polytechnica ctu proceedings figure 2. sem micrograph of e110 cladding coated using pvd with crn after high temperature steam oxidation at 1200◦c for 9 minutes. even after oxidation the coating is homogeneous and its thickness can be easily controlled. cold spray process is shown in figure 1, other concept is a cladding coated by physical vapor deposition (pvd) method with crn. this coating is under testing at ctu in prague and an example of crn coated cladding exposed to 1200◦c steam fro 9 minutes is shown in figure 2. as can be seen from both figure, different deposition technique produce coatings with different characteristics. cold-spray coatings are more non-uniform and thicker. pvd coatings are generally thinner because the deposition is very slow. pvd can however produce very homogenous high-quality coatings, parameters of which can be easily controlled. reactivity penalties and the effect of different coatings on cycle length has been studied and detailed studies are still on going. changes of neutron spectra caused by deposited cladding coatings compared to reference uncoated case are also investigated as well as effects of coatings on change of reactivity coefficients. different thicknesses of coating materials are studied and simple economic penalties for each material presented. by changing the neutron spectrum in the core, the relative fission power distribution or pu-239 and other actinides production might change for a particular concept that has been also included in the study. all calculations and their results are directly compared to the uncoated reference zircaloy-4 cladding alloy that has been by the industry for decades to clearly show potential benefits and negatives of the particular concept. 2. model and methodology in order to obtain requested neutronics performance parameters, single 2d fuel pin model was developed using the serpent code (version 2.1.26). wwer fuel pin model is based on real wwer-1000 fuel geometry. model of the fuel pin is shown in figure 2. the boundary conditions were set to reflective. fuel pin parameters and materials used are noted in table 1. the parameter value outer cladding radius [mm] 4.572 inner cladding radius [mm] 4.0005 cladding thickness [mm] 0.5715 cladding material zircaloy-4 fuel pellet outer radius [mm] 3.922 pin pitch [mm] 12.75 uo2 density [g/cm3] 10.412 uo2 enrichment [wt.% ] 3.8 zry-4 density [g/cm3] 8.00 table 1. wwer-1000 fuel pin parameters used in the simulation. isotope weight fraction [-] 90zr 0.49717 91zr 0.10963 92zr 0.16941 94zr 0.17542 96zr 0.028864 120sn 0.015 56fe 0.002 table 2. zircaloy-4 isotopic composition. fuel enrichment was optimized for better performance of the monte carlo calculation. both transport and burnup calculations were used for studied materials. to obtain accurate results also for high burnup value, high number of burn steps1 and high number of neutron particles were simulated in many cycles2. moreover, fuel pellet and cladding were divided into 5 radial regions that were burned separately to enhance depletion accuracy. b1 fundamental mode calculation was applied as well. the newest endf/b-vii.1 library was used as a source of all nuclear data. power density 0.03554 kw/g was used for all calculations. main calculation parameters (temperature, boric acid concentration) simulated representative conditions of the wwer-1000 reactor core. these calculation parameters are summarized in table 1. boric acid concentration is based on the middle cycle value, temperatures refer to average material temperature under nominal conditions (at the full reactor power). 3. studied materials several promising coating materials were tested in this neutron analysis study. the choice of coating materials is based on evaluation metrics for accident tolerant fuel concepts developed specifically for the wwer reactors [16]. considered materials include namely: crn, cr2n, cr, zrsi2, sic, ti, fecr10al6, 1following burnup steps were used: 0.0001, 0.09, 0.26, 0.5, 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, 6, 7, 8, 9, 10, 11, 12, 14, 16, 18, 20, 22, 24, 26, 28, 30, 32, 34, 36, 38, 40, 42, 44, 47, 50, 53, 56, 60, 63, 70 2400 000 neutrons per cycle, 400 active cycles and 50 inactive cycles 28 vol. 14/2018 neutronic analysis of the candidate multi-layer cladding materials figure 3. fuel pin model from the serpent code. the cladding and pellet are divided into several concentric rings to allow high precision of the simulation of isotopic changes. mo. other concepts that are under development elsewhere [17] are also considered, however results are not presented here: tin, tialn, alcrn, ti2alc, ti–si–c, zr–al–c. most promising concepts (that are currently under development at ctu in prague) were calculated with different coating thicknesses and with full burnup calculations. calculation regime for each material is noted in table 5. this selection resulted from very high computational requirements, especially the burnup calculation is extremely time consuming. all considered coating thicknesses used are noted in table 4. the thicknesses of coatings considered are based on experience with high-temperature testing of respective concepts. the protective nature of coatings with thickness of 5 microns and lower is very limited [14]. moreover, some deposition techniques, namely cold-spray process, produce non-uniform coatings and it is therefore necessary to consider higher thicknesses in order to ensure uniform layer of coating. an example of cold spray-coated sample is shown in figure 1. the maximal thickness is based on potential coating stability and also economic consequences. it should be noted, that the optimal thickness of the coating is between 15-60 microns from the perspective of hightemperature testing. because each coating entails certain reactivity penalty its thickness should be reduced. the thickness of the final concept will be based on compromise between oxidation resistance and economical limitations. the coating layer generally negatively impacts multiplication factor resulting in shortening of fuel cycle which has negative economic consequences on whole power plant. the decrease in effective power days is not easy to predict using only fuel pin calculation, due to the general influences of fuel handling and core design, different fuel pin positions in fuel assemblies and reactor operation history. however, relative impact of parameter value fuel temperature 1005 k soluble boron concentration 525 ppm (3g/kg) coolant density 0.7169 g/cm3 number of radial pin regions 5 coolant temperature 578 k table 3. stationary parameters of the serpent calculation chosen based on standard wwer-1000 operational characteristics. coatings on fuel cycle length can be estimated. the methodology used is based on the multiplication factor for final burnup value (70 mwd/kgu ) in case of fuel pin without coating. then from the results for fuel pin with coating, burnup point with similar multiplication factor is estimated. difference between estimated value and the final burnup value is calculated and is used as factor describing the decrease in fuel cycle length. the breakpoint value depends mostly on fuel costs (especially additional enrichment costs). final decision of economical acceptance should be based on deeper economical study and will be presented in future. 4. results as was previously mentioned, several coating materials were simulated (crn, cr, zrsi2, sic, ti,. . . ) most of these materials underwent precise burnup calculation. comparison of all calculated cases for zero burnup is noted in the table 6. the column difference shows difference in reactivity in pcm between the reference case without coating material and coated concept. as it is shown in the table, decrease in multiplication factor was observed for all materials and the absolute value of the decrease is closely linked to the higher absorption cross section of coating materials. decrease in multiplication factor results in shorter fuel cycle which has direct impact on power plant profit. the 1% decrease in fuel cycle length can be considered as acceptable when consequently increasing safety of the power plant. moreover, 1% decrease in fuel cycle length offers sufficient thickness of coating material so that coating material can provide requested safety enhancement. some materials resulted in only small decrease in keff , which means that impact on fuel cycle length is not crucial. however, molybdenum coating has significant impact on fuel cycle length. all materials, except mo, has decrease in fuel cycle length around 1% for 20 µm thick coating. for 50 µm thick coating is the decrease above 2%. molybdenum reaches 4% for 50 µm. the best material with the lowest fuel cycle length influence can be considered zrsi2. this coating material even increased multiplication factor for high burnup values when compared to reference case (see the figure 5). the data summarized in table 6 are shown also in figure 3. the decrease in reactivity corresponds 29 ondřej novák, martin ševeček acta polytechnica ctu proceedings outer cladding radius [mm] coating thickness [µm] 4.572 0 4.578 6 4.581 9 4.587 15 4.592 20 4.602 30 4.622 50 4.672 100 table 4. coating thicknesses used in calculations and corresponding outer cladding radius. the inner cladding radius is constant. figure 4. relative reactivity penalty for considered coated claddings with various thicknesses material burnup coating thicknesses crn yes 6, 9, 15, 20, 30, 50, 70,100 cr yes 15 ,30, 50, 70, 100 sic yes 15, 20, 100 ti no 9, 15, 20, 30, 50, 70, 100 zrsi2 yes 15, 20, 30, 50, 70, 100 cr2n no 20, 30, 50, 70,100 mo no 20, 30, 50, 70,100 table 5. coating material and tested parameters. clearly to absorption neutron cross section. the penalty of coatings also increases with thickness of coatings as expected. the changes in neutron flux for considered materials are shown in figure 6. decrease in the thermal part of neutron spectrum was observed. this change may lead to changes of reactivity coefficients and effective cross sections of materials in the core. this decrease suggest the decrease in reactivity of whole system described above. on the other hand, an increase in neutron flux in epithermal and fast neutron spectrum affect isotopic composition of materials due to different effective absorption. this results need however further confirmation and additional full core calculations. the change of reactivity coefficients suggest that core behavior in reactivity-related events (ria, mslb) will differ from the reference case. adfigure 5. multiplication factor dependency on burnup value for 70µm coating ditionally, different neutron spectrum in the core will lead to different material composition including minor actinides that are crucial from the point of reactor operation. isotopic changes of coating materials were mostly dependent on neutron capture probabilities of involved isotopes in coating. isotopic composition of pellets differs due to changes in neutron spectrum described above. as an example the production of pu-239 dur30 vol. 14/2018 neutronic analysis of the candidate multi-layer cladding materials coating material coating thickness [µm] outer cladding radius [mm] keff [-] keff uncertainty difference [pcm] reference case 0 4.572 1.28949 3.60e-05 0 cr 15 4.587 1.28424 3.90e-05 -409 cr 30 4.602 1.27897 3.50e-05 -823 cr 50 4.622 1.27209 3.70e-05 -1368 cr 70 4.642 1.26513 4.00e-05 -1925 cr 100 4.672 1.25482 3.80e-05 -2763 cr2n 20 4.592 1.28163 3.50e-05 -613 cr2n 30 4.602 1.27777 3.70e-05 -917 cr2n 50 4.622 1.27005 3.60e-05 -1531 cr2n 70 4.642 1.26224 4.00e-05 -2159 cr2n 100 4.672 1.25073 4.10e-05 -3099 crn 6 4.578 1.28357 3.60e-05 -461 crn 9 4.581 1.28592 3.70e-05 -278 crn 15 4.587 1.28707 3.70e-05 -188 crn 20 4.592 1.28179 3.80e-05 -601 crn 30 4.602 1.27783 3.60e-05 -912 crn 50 4.622 1.27005 3.80e-05 -1531 crn 70 4.642 1.26233 3.80e-05 -2152 crn 100 4.672 1.25083 3.60e-05 -3091 mo 20 4.592 1.27511 3.80e-05 -1128 mo 30 4.602 1.26880 3.60e-05 -1631 mo 50 4.622 1.25727 3.90e-05 -2563 mo 70 4.642 1.24646 4.00e-05 -3452 mo 100 4.672 1.23096 4.10e-05 -4755 sic 15 4.587 1.28850 3.70e-05 -77 sic 20 4.592 1.28817 3.80e-05 -102 sic 100 4.672 1.28237 3.60e-05 -555 ti 9 4.581 1.28814 3.70e-05 -105 ti 15 4.587 1.28725 3.70e-05 -174 ti 20 4.592 1.28641 3.80e-05 -239 ti 30 4.602 1.28481 3.90e-05 -364 ti 50 4.622 1.28170 3.70e-05 -608 ti 70 4.642 1.27846 3.70e-05 -863 ti 100 4.672 1.27391 3.60e-05 -1223 zrsi2 15 4.587 1.28832 3.80e-05 -91 zrsi2 20 4.592 1.28793 3.70e-05 -121 zrsi2 30 4.602 1.28711 3.60e-05 -185 zrsi2 50 4.622 1.28542 3.90e-05 -317 zrsi2 70 4.642 1.28384 4.00e-05 -440 zrsi2 100 4.672 1.28123 3.90e-05 -645 table 6. multiplication factors for different coating materials and thicknesses. 31 ondřej novák, martin ševeček acta polytechnica ctu proceedings figure 6. flux difference between the reference case (cladding without any coating) and claddings with fully dense 15 microns thick coatings. figure 7. change in plutonium-239 concentration for different coating materials ing burnup is shown in figure 7 for different cladding materials. the effect increases with increasing burnup as expected. different breeding ratio and change in reactivity coefficients affect also other reactor performance parameters as axial offset, radial peaking, material irradiation damage or performance during accidents e.g. ria[10]. for that reason, full core calculations need to be performed before concluding the evaluation of different atf concepts including the precise definition of effective cross sections. 5. conclusion vast selection of coating materials considered for atf cladding was studied in area of neutronic performance of a wwer reactor. impact of coating layer to multiplication factor and reactivity during whole fuel cycle was studied. in addition, changes of material itself caused by neutron interaction were calculated and discussed. moreover, neutron flux changes were studied as well. multiplication factor in all cases decreased due to higher neutron absorption of coating materials. this causes reducing of cycle length having economic consequences for utilities. for that reason, the thickness of the coatings need to be optimized not only from the perspective of safety enhancement but also from economical perspectives. by using coated claddings, the neutron spectrum in a reactor will change. this brings new questions related to operation of a reactor and its safety. detailed full-core calculations need to be performed in order to precisely quantify the effects of particular coatings. however, this study reveals only neutronics-related performance of the atf claddings. to conclude and select the most appropriate coating material enhancing the accident tolerance of wwer reactors, other studies from different field must be included. acknowledgements this work was supported by the grant agency of the czech technical university in prague, grant no. sgs16/252/ohk4/3t/14 and iaea crp 21065 “development and testing of coated fuel cladding for vver reactors with enhanced accident tolerance”. 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[17] c. tang, m. stueber, h. j. seifert, m. steinbrueck. protective coatings on zirconium-based alloys as accidenttolerant fuel (atf) claddings. corrosion reviews 35(3):141–165, 2017. doi:10.1515/corrrev-2017-0010. 33 http://dx.doi.org/10.1016/j.anucene.2016.09.033 http://dx.doi.org/10.13182/nt14-22 http://dx.doi.org/10.1016/j.net.2017.12.011 http://dx.doi.org/10.14311/ap.2016.4.0089 http://dx.doi.org/10.1515/corrrev-2017-0010 acta polytechnica ctu proceedings 14:21–27, 2018 1 introduction 2 model and methodology 3 studied materials 4 results 5 conclusion acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2019.22.0099 acta polytechnica ctu proceedings 22:99–106, 2019 © czech technical university in prague, 2019 available online at http://ojs.cvut.cz/ojs/index.php/app the physical properties of cementitious composites for applications in a high-temperature energy storage device stanislav šťastník∗, lenka bodnárová brno university of technology, faculty of civil engineering, veveří 95, 602 00 brno, czech republic ∗ corresponding author: stastnik.s@fce.vutbr.cz abstract. the article was focused on experimental verification of the identification of thermal characteristics of high-performance cementitious composites at elevated temperatures. the purpose of the research was to verify the properties, and consequently the suitability of these special cement composites for the production of a heat storage device for the long-term accumulation of thermal energy. the theoretical prerequisites for the identification of thermal characteristics and the practical procedure of experimental verification of the characteristics were presented. a modified procedure for the determination of temperature conductivity α by the hot wire method was suggested. subsequently, the experimental verification and the results of the measured thermal characteristics were listed – and the temperature conductivity α, the thermal conductivity λ [w/m·k], the volumetric heat capacity and the specific heat capacity c [j/kg·k] by the modified method were determined. keywords: cementitious composite, thermal conductivity, volume heat capacity, specific heat capacity, carbon nanotube. 1. introduction continuous growth in greenhouse gas emissions and fuel price increases are key drivers for the more efficient use of renewable energy sources. renewable sources have a practically limitless lifespan as they consist of solar energy, wind energy, geothermal energy, hydropower and tidal energy. the most effective source of energy is direct sunlight, and solar energy is used by photovoltaic power plants. the current problem is the realisation of the solar energy collection and its storage in a heat storage device for long-term accumulation of thermal energy. at present, the use of renewable energy sources is realised only in so-called island systems. the proposed heat storage system consists of a high-performance cementitious composite heat storage device and a thermal insulating cover. the aim of this system is to accumulate and maintain the temperature range within the required limits, so that it depends upon its thermal accumulation characteristics. these thermal accumulation characteristics are determined by physical thermal characteristics including, in particular, the specific heat capacity and thermal conductivity. an important role in terms of heat accumulation is also played by the bulk density. the working temperature of the solar accumulating storage device ranges from 250 °c to 800 °c. the temperature level is dependent not only on the supply and pumping of energy, but on the insulation of the accumulating storage device. for proper functioning and long-term durability, the choice of a suitable material for the accumulating storage device is crucial. one option is to construct an accumulating storage device from concrete. resistance during exposure to high temperatures is very important. another valid aspect is thermal conductivity. the higher the thermal conductivity of the storage device material, the faster the accumulation progress. the addition of carbon nanoparticles can positively affect the durability of the cement composites as well as the thermal conductivity of the composites. for example, [1–3] lists the results of the thermal conductivity measurement of cementitious nanocomposites with carbon nanotubes, and [4] lists the changes in thermal conductivity of polymer matrix composites. these new findings have been taken into account when designing the structure of experimentally tested high-performance cementitious composites. within the research work, the requirements for the thermal characteristics of the cement composite were defined. subsequent experimental works consisted in identifying the thermal characteristics of selected cementitious composites at elevated temperatures. 2. prerequisites for identifying thermal characteristics in order to estimate the effects of heat-stressed building structures, it is necessary to know the values of the basic thermal characteristics of the materials used. the classical evolutionary fourier equation represents non-stationary heat transfer. to study the temperature distribution of a given structure, it is necessary to know the thermal conductivity values, λ, if the process is stationary. it is also necessary to know the specific heat capacity c related to the density of the material ρ. the formulation of the fourier equation 99 http://dx.doi.org/10.14311/app.2019.22.0099 http://ojs.cvut.cz/ojs/index.php/app stanislav šťastník, lenka bodnárová acta polytechnica ctu proceedings in cartesian coordinates is as follows: ∂t ∂t = λ ρ · c ( ∂2t ∂x2 + ∂2t ∂y2 + ∂2t ∂z2 ) + qz ρ · c (1) after transforming the relation (1) into the cylindrical coordinates in the two-dimensional space, we obtain an equation: dt dt = α ( ∂2t ∂r2 + 1 r ∂t ∂r + 1 r2 ∂2t ∂ϕ2 ) + qz ρ · c (2) a suitable method for measuring refractories in the temperature range from 20 °c to 800 °c is the hot wire method. this method allows computational identification of thermal conductivity and heat capacity at elevated temperatures. material identification by the heating wire method uses the following assumptions: (1.) the heating wire is considered to be thin and its thermal and temperature conductivity can be neglected; (2.) the material sample can be assumed to be a long rotary cylinder with a large radius r, in whose axis lies the heating wire; (3.) the heating wire, at the time t = 0, supplies the constant heat output qw [w·m−1] to the unit of length, so that the power qwl [w] can be related to the surface area of the cylinder with content 2πrl, where r is the distance of all points from the heating wire with consideration of the previous assumptions for the limit transition r → 0+; (4.) the material is considered homogeneous and isotropic with the scalar characteristics of λ, and α; (5.) the characteristics λ and α can be at a given temperature level and can be considered as independent of the temperature of the sample t (r, t) for any radius r ∈ r+ and time (the set r+ is considered as a set of positive real numbers); (6.) the ambient temperature of the sample is unchanged during the measurement. under these assumptions, it is possible to use a classical analytical relation (2) for the distribution of temperature in the measured sample [5]. as a result, the ambient temperature of the heating wire at the distance r in time t reaches the following values: t (r, t) = t0 + q 4πλei ( r2 4αt ) ∀(r, t) ∈ r+×r+ (3) in which ei represents an exponential integral ei(β) = ∫ ∞ β exp(−u) u du (4) ei is defined for any number β ∈ r+. therefore, in accordance with (3), the following equation is applicable: t (r, t) = t (r, ta)− q 4πλ ( ei ( r2 4αt ) − ei ( r2 4αta )) ∀(r, t) ∈ r+ ×r+ (5) it is obvious that the temperature t (r, t) from equation (5) conforms to the evolutionary partial heat equation without internal sources for the axially symmetrical task given by the correlated relation rκ ∂t ∂t = ∂ ∂r ( λ ∂t (r, t) ∂r ) ∀(r, t) ∈ r+ ×r+ (6) with the initial condition at time t→ 0+ lim t→0+ t (r, t) = t0 ∀r ∈ r+ (7) the boundary condition of the dirichlet type lim r→∞ t (r, t) = t0 ∀t ∈ r+ (8) and the neumann type boundary condition lim r→0+ −λ∂t (r,t) ∂r q 2πr = 1 ∀t ∈ r+ (9) in condition (9), the denominator is the fourier heat flux. the derivation of equation (10) from the standard heat distribution equation in cartesian coordinates is conducted in [6] chapter 9. the author also discussed this issue in [7] chapter 3. the thermal conductivity λ represents the density of the heat flow divided by the temperature gradient. the derived physical quantity is the thermal conductivity α, which is formulated by relation α = λ κ . 3. determination of thermal conductivity by the method of heating wire (iso 8894-1) experimental determination of thermal conductivity is carried out according to iso 8894-1 [8], because the procedure allows the determination of the thermal conductivity coefficient λ by the dynamic method for dense materials, with a thermal conductivity of up to 25w·m−1·k−1. the temperature rise time of the heating wire is measured as a result of the propagation of the thermal wave induced by the supplied constant power through the line conductor, which is stored inside the monitored sample of the building material. the sample is placed in a heated space, whereby the condition for the evaluation of the physical quantities is the stability of the ambient temperature at a tolerance of 10k. the heating wire is made of a material called kanthal, with a length equal to the length of the test specimen, and the ends of the heating wire are connected to a supply voltage and a digital multimeter. a stabilised power supply, whose voltage and current supply the corresponding heat output to the heating wire, may not fluctuate by more than 2% during the measurement. the required heat capacity of the heating wire must be greater than 250w·m−1. the measuring thermocouples are of a ni/nicr design. test procedure: the heating wire and measuring thermocouples are placed between the upper and lower 100 vol. 22/2019 the physical properties of cementitious composites. . . part of the test body, and thermocouple 2 is placed in a groove in the centre of the contact edge of the fitting before eventually being sealed. thermocouple 1 is placed into the groove in the test material perpendicular to the heating wire. the test unit is then placed in the furnace. due to the uniform heating in the furnace, the test unit is placed on mats made of a similar material to the test sample. the heating wire is connected to a stabilised dc source. if the conditions given by the relevant standard (iso 8894-1, see point a) to f)) are met, it is possible to connect the current from the stabilised source and gradually, at certain time intervals, to measure temperatures on both thermocouples. the effect of the heating power must be carried out during the time t determined for the assumed λ, ρ and c values according to iso 8894-1. after this time, the heating current is interrupted and the recording of temperature course is terminated. once the temperature between the two thermocouples has stabilised, the measurement can be repeated under the same conditions. [8] the evaluation of the results is taken from the sequence of temperature difference quotients at the current point in time ti and its double. the temperature differences are calculated to the start-up temperature of the power supply. thermal conductivity is calculated based on the following relationship: λ = p 4πl −ei ( −r2 4αt ) ∆θ(t) (10) p = u · i (11) in the above relation, ei is the exponential integral given by the dependence (9) in which the expression is formulated as follows: u = −r 2 4αt (12) the measured temperatures are tabulated at the same time intervals. when evaluating the results, the calculation is performed by using the ratio of differences of the measured temperatures at time t and also at double time 2t. the ratio of the measured differences is designated by the symbol z: z = ∆θ(2t) ∆θ(t) (13) the value of the exponential integral ei is a function of this variable and is tabulated in iso 8894-1 [8]. in the measurement table, the z values are calculated for each time point to which the corresponding integral is assigned. in each i-line, according to relation (16), the thermal conductivity λi is calculated. the resulting thermal conductivity is evaluated as the average of all λi. λ = ∑n i=1 λi n (14) due to the fact that the number of measured values, and therefore, the length of measuring time intervals are limited because of the calculation slowness, the accuracy of the evaluation is reduced. 4. experimental part 4.1. procedure for determining the thermal conductivity and heat capacity of the material by a modified hot wire method suppose we have now experimentally measured t ∗(r, t) and t (r, t) temperature values expressed analytically on the set r+ ×r+. we now introduce the function f (λ, κ) so that the following is valid: f (λ, κ) = 1 2(t − t ∗, t − t ∗) (15) the formula in brackets (.,.) denotes the scalar product in the lebesgue space of functions integrable in the square on the setr+×r+ with a suitably chosen weight w(r, t). the minimum function (15) clearly corresponds to the κ and λ values. if we specifically know the temperature values for radii n{r1 . . . rn} and times m{t1 . . . tm}, the mentioned integral scalar product for any admissible functions ϕ and ψ can be defined by the relation: (ϕ,ψ) = ∫∫ ∞ 0 w(r, t)ϕ(r, t)ψ(r, t) dr dt (16) for positive discrete weight wij properties: n∑ i=1 m∑ j=1 wij = 1 (17) and the dirac two-dimensional measure: w(r, t) = n∑ i=1 m∑ j=1 δ(r − ri, t− tj) · wij (18) then f is generally a nonlinear function of two variables κ and λ. in the calculation (15) n = 1 and w11 = w12 = . . . w1m = 1/m are selected. if, in the definition (15), a variable α = q/4πλ is considered instead of the variable λ, then it is based on (18):+( ln t ta , ln t ta ) α = 1 2 ( t ∗ − t ∗a , ln t ta ) (19) the values with the asterisk in the formula (19) represent experimentally measured data. from this, it is possible to directly calculate the thermal conductivity value α. for simplicity, let us consider that we want to determine the value α from another independent experiment. from the previous, it differs only by measuring it at a different distance from the line power source, so r1 = d. instead of variable a, we can now 101 stanislav šťastník, lenka bodnárová acta polytechnica ctu proceedings consider function f as a function of the new variable b = r2 1 4κ . then equation (5) has the following form: t (r1, t) = t (r1, ta) + κ ( ei ( b t ) − ei ( b ta )) ∀t ∈ r+ (20) if we mark the first and second derivatives of the function f with apostrophe, then we gradually get the values necessary for the subsequent calculations. the basic function f has the following form: f (λ, b) = 1 2(t − t ∗, t − t ∗) (21) the first derivation: f ′ = (t ′, t − t ∗) (22) the second derivation: f ′′ = (t ′, t ′) + (t ′′, t − t ∗) (23) if the first and second derivations of function (15) are known, then it is possible to apply the iterative newton method of tangents in the form: f ′′(b− b∼) = f ′ (24) the wave line above variable b indicates a refined parameter value. generalising this approach is therefore the choice n = 2 with the condition of approaching the distance r1 → 0 from the right and r2 = d. additionally, the procedure may be modified by the levenberg-marquardt technique according to [6] in chapter 10b. if we want to formally observe the standardised method according to iso 8894-1, we can evaluate the thermal conductivity for r1 → 0+, where it is not necessary to know the κ value. it would then be possible to indirectly detect the value b by the iterative process and afterwards set parameter κ from this value. in the research work, the calculation of λ and κ is verified by experiment, which applies the modified heating wire method. the difference between this method and the classical method according to iso 8894 is, on the one hand, in the method of supplying heat energy through the power that is realised by the heating wire and then the subsequent cooling of the sample after the interruption of the supply; and on the other hand, in the distribution of thermocouples for measuring the temperature of the gradual thermal wave. the process is time-limited by both heating and cooling time (both time intervals are the same). 4.2. arrangement of an experimental assembly for the procedure of determining the thermal conductivity by the hot wire method the arrangement of samples and thermocouples for the experiment differed from the arrangement for measurement by the λ method according to iso 8894-1 [8]. figure 1. the sample arrangement when determining the thermal conductivity λ and thermal capacity c by the hot wire method. the first measuring thermocouple lie in close proximity to the heating wire, usually at a distance of 3mm, and the other thermocouple lies parallel to the heating wire at a distance of 15mm. the schematic arrangement of the measured block is shown in figure 1. the process of the temperature and measurement setting is controlled by the computer. 5. results and discussion 5.1. summary of knowledge about materials for realisation of sensible storage devices the objective of the experiment was to prove the properties of silicate materials in terms of their use in the realisation of sensible high-capacity and hightemperature storage containers. for this purpose, samples of cementitious composites resistant to temperatures above 800 °c were selected for their realisation. indicators of the suitability for use of the chosen materials were the physical values of the thermal conductivity λ and the specific heat capacity c. a suitable storage container must have such properties to ensure the required energy use. the storage container is charged by insolation of the collector during the climatic year. a validated sensible energy storage device can be used as a power supply for a family home and has been designed for minimal temperature loss. the input data were density, specific heat capacity, immediate power output over a one-year period, storage container size and daily consumption. the capacity of the storage container and its dynamic behaviour depends, inter alia, mainly on the value of the specific heat capacity of the building material and its density. another important factor of energy accumulation is its thermal conductivity. the higher the thermal conductivity of the container’s building material, the faster the accumulation progress. the working temperatures of the storage container vary between 200 °c and 500 °c. in this area, we have to look for values with a favourable specific heat capacity and thermal conductivity. cementitious composites of various compositions were tested. five variants of cement composites (ca1, ca2, cc1, cc2, cq) were tested. the matrix of 102 vol. 22/2019 the physical properties of cementitious composites. . . figure 2. temperature changes t [°c] in the sample of cementitious composite cc1 at 200 °c depending on time t [s]. figure 3. the iterative convergence process of λ and κ. the ca1 and ca2 composites was made from aluminous cement, composite matrix cc1, cc2 was based on portland cement and sample cq was magnesium based composite. figure 2 shows the change of temperatures t in the sample of the cementitious composite at 200 °c with respect to time t, and figure 3 shows the convergence of iterations λ and κ for the sample of the cementitious composite. the measurement results of the selected cementitious composites are presented in charts 4 and 5. the measurement results of the cementitious composites with different composition are presented on the diagrams in figures 6 and 7. 103 stanislav šťastník, lenka bodnárová acta polytechnica ctu proceedings figure 4. the course of the change in thermal conductivity [w/m·k] and volume heat capacity [j/m3·k] at the temperature for cementitious composite cc1 depending on ambient temperature [°c]. figure 5. change in specific heat capacity c [j/kgcdotk] for cementitious composite cc1 depending on ambient temperature [°c]. 6. conclusion for the intended application of the high-performance cementitious composite for the heat accumulating storage device, it is necessary to achieve the highest possible thermal conductivity and heat capacity. the thermal conductivity value is related to the rate of equalisation of the temperature differences in the storage device volume, which is the decisive parameter for charging and discharging of the high-temperature heat storage device. the heat capacity value of the storage device core material determines the total heat content, and is a decisive parameter in determining its geometric volume. thermal identification over a wide range of operating temperatures is necessary to verify the thermal behaviour of the various cementitious composites under the real conditions of incorporation into the core of the high-temperature heat storage device. the best results were obtained with materials with a high magnesium oxide content. these results, however, are not presented in the comparisons in figures 6 and 7 because this material is incomparable, in terms of price, with the tested high-performance cementitious composites, and its price does not currently allow its incorporation into the real-world construction of the high-temperature heat storage device. by comparing the thermal conductivity and thermal capacity values in the temperature range of the operating temperatures of 20 °c to 800 °c, it appears to be the most suitable cementitious composite with the designation cc1, whose favourable physical properties are also related to its density. the useful physical properties of the final sensible storage device made of cc1 material are closely related to the use of the thermal insulating layers in the packaging construction, the overall needed size of the storage device, and its operating states. the specific 104 vol. 22/2019 the physical properties of cementitious composites. . . figure 6. change in thermal conductivity depending on the ambient temperature for the monitored cementitious composites. figure 7. change in heat capacity depending on the ambient temperature for monitored cementitious composites. design and arrangement of the high-temperature heat storage device will depend on the thermal balance of the energy flows, their time distribution, etc. however, due to the favourable properties of the storage device core material, it provides the most favourable physical characteristics. acknowledgements this outcome has been achieved with the financial support of project fast-s-18-5410, supported by brno university of technology. references [1] m. hassanzadeh-aghdam, r. ansari, m. mahmoodi, et al. a comprehensive study on thermal conductivities of wavy carbon nanotube-reinforced cementitious nanocomposites. cement and concrete composites 90:108–118, 2018. doi:10.1016/j.cemconcomp.2017.09.021. [2] d. exarchos, p. dalla, i. tragazikis, t. matikas. the effect of cnts reinforcement on thermal and electrical properties of cement-based materials. in smart sensor phenomena, technology, networks, and systems integration 2015, vol. 9436, pp. 94360e–1. 2015. 105 http://dx.doi.org/10.1016/j.cemconcomp.2017.09.021 stanislav šťastník, lenka bodnárová acta polytechnica ctu proceedings [3] a. l. pisello, a. d’alessandro, s. sambuco, et al. multipurpose experimental characterization of smart nanocomposite cement-based materials for thermalenergy efficiency and strain-sensing capability. solar energy materials and solar cells 161:77–88, 2017. [4] c. huang, x. qian, r. yang. thermal conductivity of polymers and polymer nanocomposites. materials science and engineering r-reports 132:1–22, 2018. doi:10.1016/j.mser.2018.06.002. [5] h. s. carslaw, j. c. jaeger. conduction of heat in solids. clarendon press oxford, 1959. [6] p. bochev, m. gunzburger. least-squares finite element methods. springer, new york, 2009. [7] l. dohnalová, i. trhlíková, o. zmeškal. temperature dependence of thermal parameters of building materials. in proceedings of thermophysics in podkylava, pp. 194–207. bratislava, 2013. [8] iso 8894-1 refractory materials determination of thermal conductivity part 1: hot-wire methods (cross-array and resistance thermometer). standard, european committee for standardization, 2011. 106 http://dx.doi.org/10.1016/j.mser.2018.06.002 acta polytechnica ctu proceedings 22:99–106, 2019 1 introduction 2 prerequisites for identifying thermal characteristics 3 determination of thermal conductivity by the method of heating wire (iso 8894-1) 4 experimental part 4.1 procedure for determining the thermal conductivity and heat capacity of the material by a modified hot wire method 4.2 arrangement of an experimental assembly for the procedure of determining the thermal conductivity by the hot wire method 5 results and discussion 5.1 summary of knowledge about materials for realisation of sensible storage devices 6 conclusion acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2018.19.0022 acta polytechnica ctu proceedings 19:22–29, 2018 © czech technical university in prague, 2018 available online at http://ojs.cvut.cz/ojs/index.php/app study of thermal creep of coated cladding materials martin ševečeka,b,∗, koroush shirvanb, ronald g. ballingerb a department of nuclear reactors, faculty of nuclear sciences and physical engineering, czech technical university in prague, v holešovičkách 2, prague 8, czech republic b department of nuclear science and engineering, massachusetts institute of technology, 77 massachusetts ave, cambridge, ma, usa ∗ corresponding author: martin.sevecek@fjfi.cvut.cz abstract. coated cladding materials are considered as a near-term concept for so-called accident tolerant fuel materials. their behavior in accidental conditions is mainly studied around the world, however, they need to survive in-reactor normal operation before reaching theoretical accidental conditions. an out-of-pile experiment was designed to study accelerated inward thermal creep and outward displacement of a cladding that occurs after pellet-cladding interaction. four different materials deposited by the cold-spray technique were studied as well as the reference uncoated one. the results confirm theoretical prediction that due to a mismatch between the fundamental physical properties of each layer, high stresses will build up and the plastic strains expected will lead to coating cracking. keywords: accident tolerant fuel, fuel cladding, creep, coating. 1. introduction zirconium alloys have been used as a fuel cladding by the nuclear industry for decades in light water reactors (lwrs) as well as other reactor designs such as candu or rbmk. despite the widespread use of zirconium alloys in nuclear applications, relatively little creep data exists for these materials. creep is slow timeand temperature-dependent strain that occurs in a material under load at higher temperatures. in a creep test, a constant load is applied to a specimen maintained at a constant temperature. the strain is then measured over a period of time. primary creep is a period of primarily transient creep with decreasing creep rate due to work hardening of the material. then secondary creep is a period with roughly constant creep or strain rate due to which is also referred as steady-state creep due to balance between work hardening and annealing. if the cross-sectional area reduces enough, tertiary creep occurs. the creep rate increases due to necking and increased local stress and eventually results in fracture as shown in fig. 1. irradiation creep is the additional strain that occurs as a result of irradiation and stress in material that needs to be considered in nuclear applications [1]. the presented experiment was performed ouf-of-pile therefore only thermal creep was studied. at the beginning of the nominal lwr operation, the cladding is subjected to the differential pressure (coolant pressure minus internal gas pressure) and creeps down in compression due to this loading [3]. progressively, the pellet-cladding gap closes under the combined effect of the cladding creep and the pellet swelling induced by fission product accumulation. figure 1. typical creep curve for materials under load at elevated temperature [2]. while the gap closes, the cladding, initially loaded in compression, progressively gets in contact with the pellets and becomes loaded in tension [4]. the tensile stress increases slowly up to about 20 to 30 mpa, which corresponds to an equilibrium value, according to the cladding creep rate and the pellet swelling rate.[5, 6] an experimental setup was designed and built to simulate this in-reactor behavior of cladding materials out-of-pile. the nuclear fuel cladding can be loaded in compression or tension between 0 350 mpa at the temperature between 20-320◦c. the samples were partially instrumented and some of the parameters were evaluated outside of the autoclave after a certain testing period. 22 http://dx.doi.org/10.14311/app.2018.19.0022 http://ojs.cvut.cz/ojs/index.php/app vol. 19/2018 thermal creep of coated cladding (a) . zry4 (b) . cr (c) . fecral (d) . mo (e) . fecral+mo figure 2. pressure tubes for creep experiments in as-coated state. cr-coated tube is fully coated including the welds and the end cap, the other coatings cover linear part of tubes only. the main objective of this experiment is to study so-called accident tolerant fuels [7]. there are many concepts of atf under development all around the world and one of the most promising near-term design is the multicomponent cladding [8–11]. zr alloys still serve as the main structural component on which protective coatings are applied. these coatings have slightly different physical behavior than the substrate (e.g. thermal expansion, swelling) which induces high stresses into the cladding and results in high plastic strains [12]. additionally, the coatings impose high stresses originating from the particular deposition process used. there is a concern that the coating or cladding might be disrupted when these loads are combined with other in-reactor strains such as creep described above. therefore, the presented experiment emulates the strains typical of in-reactor conditions with advanced cladding materials and its response is studied. 2. materials and methods standard commercially available zircaloy-4 was used as a substrate for all tests. the composition is as follows: 1.32-sn; 0.21-fe; 0.11-cr; 0.13-o; bal-zr. the substrate was surface polished in ethanol lubricant with 320 grit sic abrasive paper before deposition of coatings, degreased and cleaned in di water, ethanol and acetone in agreement with [13]. four different coating materials were deposited by the cold spray technique – pure cr (deposited by army labs), fecral – low alloy (la); pure mo; and fecral+mo multilayer (deposited by university of wisconsinmadison). the as-coated tubes are shown in fig. 2. uncoated zircaloy-4 was also studied as the reference case. the reference uncoated zry-4 pressure tube and the as-coated samples are shown in fig. 2. the standard zry-4 tubes were welded with an end cap on one end and with a thinner zry-4 tube at the other end. the as-prepared samples were then pressurized by air at room temperature up to 30 mpa to check the quality of welds. the zry-4 samples were then coated using the cold spray technique with pure chromium, molybdenum, fecral low alloy and multilayer of fecral+mo. the leakage test was then repeated using air. samples were pressurized by air at room temperature up to 11 mpa for 5 minutes with no pressure decrease. then he leakage test was performed with the base pressure of 0.013332 pa with the leak rate 1.3e-8 pa*liter/second and no indications. there was a concern that the welding can be partially disrupted by cold spraying but it was not confirmed. a detail of the over-coated weld with cr is shown in fig. 3. two fecral la coated tubes (pt-12 and pt-13) were tested. the tubes have been polished after deposition with 600 grit sic paper and the surface roughness 23 m. ševeček, k. shirvan, r. g. ballinger acta polytechnica ctu proceedings figure 3. cr cs coated pressure tube detail of the bottom weld. is in the range 250 to 320 nm. the thickness of the coatings is 0.097±0.006 µm. the fecral low alloy composition in weight percentage is fe-10cr-6al. two pressure tubes identified as pt-17 and pt-18 were coated with mo and then over-coated with fecral la. the linear length of the tubes is coated but the curved end caps are not coated. because the surface roughness plays a role in mechanical and corrosion behavior, the entire sample surface was polished with 600 grit sic paper. the surface roughness is in the range of 250 to 360 nm and the average thickness of the coating system is 110±10 µm. some roughness remained after polishing on both samples. similar to the other mo/fecral samples, uniform deposition of the fecral la on top of the as-deposited mo was difficult. this resulted in some thinner regions of fecral. to ensure that the fecral coating is not breached, which would expose mo inter-layer or zr substrate, polishing was stopped wherever fecral layer was less than or equal to 50 µm. two pts (pt-15 and pt-16) were coated with mo. only the linear length of the tube is coated and curved end caps are not coated. the tubes have not been polished as it might breach the coating due to its minimal thickness. the thickness of the coatings is about 27 µm as measured by a micrometer (including surface roughness). pure molybdenum was deposited on the substrate as confirmed by xrd after deposition. the x-ray diffraction pattern of the mo coating (not shown here) showed perfect agreement with mo reference with no extraneous peaks originating from oxides or other impurities. pressure tubes coated with cr were prepared using vrc gen iii system at arl, ma. the detailed deposition parameters can be found in [8]. 2.1. cold spray technique all tested samples were coated using the cold spray process (cs). it involves the acceleration of micronsized particles in the form of powders that are carried in a high pressure and sometimes heated gas stream in the solid state toward a suitable substrate. during the figure 4. schematic representation of cold spray process [14]. interaction with the substrate, the particle undergoes tremendous plastic deformation. upon impact, the plastic deformation disrupts and breaks down surface oxide layers on both the powder and substrate leading to a metallurgical bond, as well as mechanical interlocking. since the feedstock powder is deposited in the solid state, the microstructure is retained after deposition with the exception of dynamic recrystallization due to high strain levels. a schematic representation of the cs is shown in fig. 4. 3. experimental setup the simplified schematics of the experimental setup is shown in fig. 5. two independent heaters are placed onto the autoclave designed for pressure up to 24.13 mpa (3500 psi) and thermally insulated from outside. they are controlled by the in-house designed controller that uses two thermocouples and two pressure gauges. four proportional-integral-derivative (pid) controllers are interconnected to ensure that the parameters are within allowable limits and define the temperature of the preheater and autoclave by providing power to heaters. the water pressure is controlled by high-pressure pump which will run throughout each experiment on one side and by the back pressure regulator on the other side. the autoclave pressure for the experiments was between 13.4 14 mpa depending on the particular test. instrument leads as well as the pressure tubes exit the autoclave through an appropriately configured conax fittings. a data acquisition system records temperature from four thermocouples. two are inserted into the autoclave at different axial positions, one measures and controls the preheater and the last checks the temperature at the exit of the high-pressure part of the loop for safety reasons. the system collects also pressure from three pressure transducers. one is installed at the high-pressure pump outlet and controls the high pressure inside the autoclave, one is connected to the tubing on the low-pressure section of the water loop and the third is connected to the inert gas inlet and controls the ar pressure inside cladding tubes. omega pressure gauge and omega dp41-e unit were used to measure argon pressure inside the gas line. agilent 34970a was used as a data acquisition unit and also as an ad converter. the data measured were 24 vol. 19/2018 thermal creep of coated cladding coolant pressurized h2o temperature [°c] 280 – 320 system pressure [mpa] 12.4 – 14.47 tube internal pressure [mpa] 0.1 – 41.0 coolant ph (at 25 ◦c) 6.6 – 7.3 inlet conductivity [ms/cm] 0.01 – 0.05 outlet conductivity [ms/cm] 0.03 – 0.1 ox. concentration [sat%] 0.39 – 0.4 table 1. experimental parameters for autoclave coolant. figure 5. schematics of the experimental setup. then stored in a computer. thornton multiparameter 770max unit was used to collect inlet and outlet water conductivity, ph and oxygen concentration. matheson hydrogen generator nm series ii was used to generate hydrogen and bubble it into the di water tank where the partial pressure of the gases was measured. water conductivity meters, ph meter, and oxygen meter were used to control the chemistry inside the system. additionally, hydrogen was bubbled into the water tank to reduce corrosion by reducing electrochemical potential. hydrogen concentration was not measured but it can be calculated from the partial hydrogen pressure inside the water tank using henry’s law considering that the di water inside the tank is saturated. the gas line interconnecting the pressure tubes with ar gas tank was first under atmospheric pressure which results in the downward creep of the cladding (compression) due to the differential pressure inside the autoclave. later the gas line was pressurized by argon up to 41.36 mpa (6000 psi) to simulate the outward creep (tension). figure 6. photo of the autoclaves, gas system and water loop. the picture of the experimental setup is shown in fig. 6. the second autoclave can be interconnected and allows to test more samples at the same time. the autoclaves are dynamic and allow online control of the chemistry as well as purification of the di water. small leakages were found in the gas line that pressurizes pressure samples during the test e.g. in fig. 12 (green line). for that reason, the load was slowly decreasing. later the tubing was replaced and lower loads were applied with no leakages. 4. results due to the limited length of this paper, only results for fecral+mo samples (pt-17, pt-18) are shown. three consequent test runs were performed. during the first run, the pressure tubes were under atmospheric pressure and the autoclave pressure was around 13.5 mpa resulting in fast down creep. because the outer surface of the pressure tubes is exposed to the standard hwc environment [15] the pressure tube corrodes. for that reason, the chemistry of the environment was monitored. the temperature of the system was between 305-315 ◦c and hydrogen was bubbled into the di water tank to reduce oxygen concentration and the electrochemical potential. the oxygen concentration during run 1 is shown in fig. 9. the fluctuations are caused by periodic operation of the hydrogen bubbler but the concentrations are generally very low. water ph slowly decreases during all runs between 7.2 down to 6.7 at the end of the test. the water conductivity was measured at the inlet and outlet of the system. a filter is placed between them to clean the water and keep its properties during the test. the rapid increase of the outlet conductivity at the beginning of the test is probably caused by rapid corrosion of mo. mo quickly corrodes and creates volatile oxide in hwc environment. mo should be fully overcoated by fecral but it seems that some parts have defected or some residual mo was left on the outer surface. the corrosion reduced after about 20 hours as shown in fig. 8. 25 m. ševeček, k. shirvan, r. g. ballinger acta polytechnica ctu proceedings figure 7. fecral+mo run 1 autoclave temperature and pressure. figure 8. fecral+mo run 1 water inlet and outlet conductivity and ph. figure 9. fecral+mo run 1 oxygen concentration. (a) . top (b) . overcoat (c) . bottom figure 10. visual inspection of fecral+mo pressure tubes after run 1 compression. figure 11. fecral+mo run 2 autoclave temperature and pressure, inner tube pressure. the run 1 when pressure tubes were under compression lasted 245 hours. the system was then cooled and depressurized and samples visually examined. the samples were then placed back into the autoclave and run 2 was initiated. the tubes were first pressurized by ar from the gas line and the system was filled with water and pressurized. after cleaning of the loop the heatup of the autoclave started. the leakage of ar in the gas line was discovered but the test continued for 120 hours. as can be seen from fig. 12 the water conductivity increased rapidly at the beginning after loading the samples with the inner load. the decrease of conductivity is much slower in comparison with run 1 which suggests that the fecral coating cracked due to excessive loads and exposed mo coating. run 2 was interrupted due to leakages in the gas line. samples were visually inspected and then placed back for run 3 for another 190 hours. the leakage of ar was repaired so that the load can be constant during the run. the inner pressure reached 28 mpa. the autoclave temperature and pressure were similar to previous runs. as shown in fig. 16, there was a sudden increase of water conductivity after 140 hours. it was explained by a crack in the 26 vol. 19/2018 thermal creep of coated cladding figure 12. fecral+mo run 2 water inlet and outlet conductivity and ph. figure 13. fecral+mo run 2 oxygen concentration. (a) . top (b) . overcoat (c) . bottom figure 14. visual inspection of fecral+mo pressure tubes after run 1 and run 2 tension. figure 15. fecral+mo run 3 autoclave temperature and pressure, inner tube pressure. figure 16. fecral+mo run 3 water inlet and outlet conductivity and ph. coating on pt-18 that was later identified. this crack was much bigger in comparison with the micro-cracks observable under a microscope. visual inspection of the samples showed different axial corrosion behavior. most of the sections of the samples formed dark red iron oxide similar to fecral pt-12 and pt-13 where no interlayer was applied. however, some parts of the sample were dark grey and black. it seems that the fecral did not fully protect mo inter-layer in these regions. mo interacted with fecral and slightly corroded due to the cracks formed in the outer coating. this can be observed in fig. 18. the darker regions are visible where the fecral was thinner (top of the tube) and where the inner load was the highest (lower third of the pressure tube). 5. discussion and conclusions the coated cladding is a very promising candidate as an accident tolerant fuel material. it can be developed in a reasonably short time and it is similar to the current fuel system which will result in easier licensing and less challenging adoption of the technology by the industry. however, there are still many unresolved issues related to the material behavior in-reactor. this includes coated cladding creep and periodic strains that occur during normal operation. an experimen27 m. ševeček, k. shirvan, r. g. ballinger acta polytechnica ctu proceedings figure 17. fecral+mo run 3 oxygen concentration. (a) . top (b) . overcoat (c) . bottom figure 18. visual inspection of fecral+mo pressure tubes after run 1, run 2 and run 3 tension. tal system was designed and build to test thermal creep of these materials out-of-pile with consequent corrosion of the material in hwc environment. the cladding first creeps down as a result of differential pressure and then the pellet-cladding interaction is simulated using high ar pressure that leads also to outward creep. five different materials were studied (zry-4, zry+fecral, zry+mo, zry+cr, zry+mo+fecral) and results of the last multicomponent concept presented. there is a mismatch between fundamental physical parameters between each of the coatings and substrate (e.g. thermal expansion, creep rate, swelling). the differences will lead to high stresses in the material and strains during the long-term operation and they can even jeopardize cladding integrity. as shown in fig. 19 the strains of the cladding material lead to cracking of the coating layers. it should be noted that the thicknesses of the studied materials are higher than expected for the final concepts, therefore, the related stresses are higher as well. for example, the fecral+mo cladding includes 25 µm mo layer with about 150 µm layer of fecral as shown in fig. 20. the multicomponent cladding with two coating layers brings more challenges due to more interacting layers. figure 19. sem micrograph of the pt17 sample after the test (all three runs). the cracks in both fecral and mo coating are visible. there is no spallation visible but the bonding between fecral and mo was reduced. the sem micrographs show that cracks first initiated at the mo/fecral interface. molybdenum is harder and more brittle than both zry-4 and fecral. it cannot accommodate as much strain as the other two materials, therefore, the crack initiation starts here. the results showing inlet and outlet water conductivity during the test are directly related to corrosion of the samples. unfortunately, it cannot be distinguished which of the samples placed inside the autoclave at the same time caused the observed effects. separate corrosion tests of mo-coated samples showed rapid corrosion and volatility of the pure mo coating in hwc and nwc environment. based on the testing of pure mo and fecral it can be distinguished when mo was exposed to the environment. there was an increase at the beginning of the first run which is probably related to the defects in the coating from fabrication. second increase was observed during the second run when probably most of the cracking in coatings happened. the last increase was after 140 hours of the third run and is related to a single failure in one of the samples. energy-dispersive x-ray spectroscopy was performed to study the elemental composition at the cross-section of the samples. the eds line scan did not confirm the diffusion and mixing of the materials. there are bonding layers at the interfaces between each layer that were challenged by the test where neighboring materials are partially mixed. eds is however not very sensitive to low concentrations and more detailed study using wds is needed. the esd also confirms the composition of each layer and their thickness as shown in fig. 20. coated cladding materials have been widely tested in accidental conditions where they can bring additional benefits. however, to utilize them in commercial reactors they also need to perform well during normal operation. thermal inward creep and outward 28 vol. 19/2018 thermal creep of coated cladding figure 20. eds linescan with sem micrograph of the pt17 sample. the thickness of the mo interlayer is around 25 µm and fecral la coating is about 150 µm thick. high number of cracks is visible in both mo and fecral. displacement of cladding caused by pellet-cladding interaction are the standard behavior of the zr-based fuel system. as was shown, these standard conditions might challenge multicomponent cladding mainly due to mismatch of the fundamental physical properties of the applied layers. the tests were performed outof-pile and it is expected that in-pile testing with irradiation creep, swelling or growth will challenge the multicomponent cladding even more. acknowledgements the support for this work was provided by us department of energy integrated research project grant: dene0008416. chrome coated samples were provided by victor champagne (arl), matt siopis (utrc) and aaron nardi (utrc). the fecral and mo coated samples were coated by irp team at university of wisconsin-madison. references [1] j. p. foster, m. a. mcgrath. in-reactor creep behavior of zircaloy-2. tech. rep., american nuclear society ans, la grange park (united states), 2007. [2] k. l. murty, s. gollapudi, k. ramaswamy, et al. 3 creep deformation of materials in light water reactors (lwrs). in k. l. murty (ed.), materials ageing and degradation in light water reactors, woodhead publishing series in energy, pp. 81–148. woodhead publishing, 2013. doi:10.1533/9780857097453.1.81. [3] h. maki, t. hara. out-of-reactor study on external-pressure creep of zircaloy-2 fuel cladding tubes. journal of nuclear science and technology 12(1):43–52, 1975. doi:10.1080/18811248.1975.9733067. [4] b. g. kim, j. l. rempe, d. l. knudson, et al. in-situ creep testing capability development for advanced test reactor. tech. rep. inl/ext-10-17779, idaho national laboratory (inl), 2010. doi:10.2172/989906. [5] j. brochard, f. bentejac, n. hourdequin, et al. modelling of pellet cladding interaction in pwr fuel p. 8. [6] d. r. olander. fundamental aspects of nuclear reactor fuel elements: solutions to problems. tech. rep., california univ., berkeley (usa). dept. of nuclear engineering, 1976. [7] k. a. terrani. accident tolerant fuel cladding development: promise, status, and challenges. journal of nuclear materials 501:13–30, 2018. doi:10.1016/j.jnucmat.2017.12.043. [8] m. ševeček, a. gurgen, a. seshadri, et al. development of cr cold spray–coated fuel cladding with enhanced accident tolerance. nuclear engineering and technology 50(2):229–236, 2018. doi:10.1016/j.net.2017.12.011. [9] j. krejci, m. sevecek, l. cvrcek. development of chromium and chromium nitride coated cladding for vver reactors. 2017 wrfpm . [10] c. tang, m. stueber, h. j. seifert, m. steinbrueck. protective coatings on zirconium-based alloys as accidenttolerant fuel (atf) claddings. corrosion reviews 35(3):141–165, 2017. doi:10.1515/corrrev-2017-0010. [11] m. ševeček, j. kubáň, m. valach, r. škoda. development of high thermal conductivity uo2–th heterogeneous fuel. progress in nuclear energy 108:489–496, 2018. doi:10.1016/j.pnucene.2018.06.012. [12] m. wagih, b. spencer, j. hales, k. shirvan. fuel performance of chromium-coated zirconium alloy and silicon carbide accident tolerant fuel claddings. annals of nuclear energy 120:304–318, 2018. doi:10.1016/j.anucene.2018.06.001. [13] astm international, west conshohocken, pa. astm b614-16 standard practice for descaling and cleaning zirconium and zirconium alloy surfaces 2016. [14] b. maier, h. yeom, g. johnson, et al. development of cold spray coatings for accident-tolerant fuel cladding in light water reactors. jom 2017. doi:10.1007/s11837-017-2643-9. [15] p. scott, p. combrade. corrosion in pressurized water reactors. corrosion: environments and industries 13 c:362–385, 2006. 29 http://dx.doi.org/10.1533/9780857097453.1.81 http://dx.doi.org/10.1080/18811248.1975.9733067 http://dx.doi.org/10.2172/989906 http://dx.doi.org/10.1016/j.jnucmat.2017.12.043 http://dx.doi.org/10.1016/j.net.2017.12.011 http://dx.doi.org/10.1515/corrrev-2017-0010 http://dx.doi.org/10.1016/j.pnucene.2018.06.012 http://dx.doi.org/10.1016/j.anucene.2018.06.001 http://dx.doi.org/10.1007/s11837-017-2643-9 acta polytechnica ctu proceedings 19:22–29, 2018 1 introduction 2 materials and methods 2.1 cold spray technique 3 experimental setup 4 results 5 discussion and conclusions acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2018.15.0137 acta polytechnica ctu proceedings 15:137–141, 2018 © czech technical university in prague, 2018 available online at http://ojs.cvut.cz/ojs/index.php/app which fans are the biggest acceleration of the grandstand? martin verner czech technical university in prague, faculty of civil engineering, thakurova 7, 166 29 prague 6, czech republic correspondence: martin.verner@fsv.cvut.cz abstract. the fans of ac sparta prague and sk slavie prague have been arguing about the fact whose fans support their team better since the end of 20th century. there have been made many experiments on the intensity of vibrations at football stadiums during matches. altogether 11 matches were watched. frequencies caused by the fans leadinng to vibrations and the size of those vibrations were provided from the results. based on those facts the question about the bigger intensity of fans‘ support was answered. keywords: vibrations, grandstands, spectators‘ behaviour, stadium, czech first league. 1. introduction people like entertainment since ancient greece. the most popular leasure time activity was theater and sport matches, especially olympic games. roman emperors tried to keep common people in peaceful mood through games and bread. this was renewed in 19th century when a new sport appeared – football. even austria-hungary favoured football clubs. there were two football clubs in prague. ac kralovske vinohrady was founded in 1891. it was renamed to ac sparta in 1894. the academic cyclistic club slavie was founded in 1892. the club was renamed to sport club slavie in 1895. those two clubs became sport rivals from the very beginning. the fans of each club claim that they support their team in the best and most intense way and so that they provide the players with power to win the match. the fans say that they can cheer for their team in such a way that they could break down the stadium. the question is how to judge the cheering intensity in an objective way. does the number of fans make the difference? or is it the sound volume? or the vibrations intensity? this article compares the fans‘ behaviour with the vibrations intensity made at the stadiums sk slavie prague and ac sparta prague [1–6]. 2. circumstances influencing vabrations intensity 2.1. stadiums construction vibrations intensity depends on several factors. first of all on the construction type itself. sk slavie prague plays its home matches at eden stadium, which was opened in 2008. the stadium is built on the place of original slavie stadium where used to be a pond. it is a clasical oval stadium for approx. 20,000 spectators. the construction is made of reinforced concrete parts with steel rooftop. the figure 1. platform construction: a) eden stadium – the overall view [5], b) eden stadium detail [7]. platforms are made of a system of concrete crossbeams (plain crossbeams) on which reinforced concrete l-shaped panels are laid. the panels create an underpass in spectators front and there are seats for the spectators as well (figure 1). ac sparta prague plays at letna stadium, which was reconstructed in 1994. the stadium is divided in two parts. east (main) platform was built in 1937 as a reinforced concrete construction. the other parts of the stadium are made of steelconcrete construction 137 http://dx.doi.org/10.14311/app.2018.15.0137 http://ojs.cvut.cz/ojs/index.php/app martin verner acta polytechnica ctu proceedings figure 2. platform construction: a) letna stadium – the overall view [8], b) letna stadium detail [1]. and they are two storeyed. the second storey is made of steel crossbeams (a carrier beam with overhanging end) on which precast reinforced concrete l-shaped panels are laid. the roof is made of a steel framed structure (figure 2). it results from the text above that the platforms are similar but each of them is completely different from the dynamics point of view. eden stadium shows bigger vibrations intensity locally – on the concrete panels while letna stadium tends to show more intense vibrations of the concole part of the platform as a whole. 2.2. fans‘ behaviour 2.2.1. ultras the response of the most active fans was observed on the stadiums. these fans meet in a particular sector of stadium and are called “ultras”. “ultras” are the type of fans who ultra-fanatically support their football club. the mark of their behavior is using of flares (primarily as part of choreography), loud group cheering and creationing of giant banners in football stadiums that serve to create a stormy atmosphere on matches. the behavior is used to encourage their own team, intimidate opponents and fans of their opponents. the other major features include the inclination to ultra-right and racism. “ultras” are located on the north granstand on the stadium in eden (3). fans use pyrotechnics to a lesser extent and are among the quieter fans in the czech football league. letna‘s “ultras” (4) are the exact opposite type of fans. they are one of the most problematic fans in the czech republic. they use pyrotechnics a lot, they are fighting and the club itself is often fined for their behavior (pyrotechnics are prohibited in the stadiums in the czech republic) and their space is closed during the match at least once a season. figure 3. ultras at eden stadium [9]. figure 4. ultras at letna stadium [10]. 2.2.2. fan behavior from a dynamic response view another factor influencing the vibrations intensity is the fans themselves. the basic parametre concerning the fans is the number of them. then it is their behavoiour. their behaviour can be divided into two categories – the behaviour in general and the behaviour on the platform. the czech spectator is very specific. when the team plays well, the fans cheer with higher intensity and attend the stadium in bigger quantitites. in case the club goes through worse time, the number of the spectators and the intensity of their support decline. the behaviour on the platform itself can be divided into several categories. there are two types of relevant jumping – type 2 and type 3 for our needs of comparing vibrations. the jumping type 2 is defined as follows: the spectators stand on the platform. the whole rows of spectators catch each other‘s shoulders under the direction and start jumping on the spot. it leads to a quick synchronization of the spectators and, according to the results of the experiments, to the most intense construction stimulation. the jumping type 3 is defined as follows: likewise type 2 the fans stand on the platform and catch each other‘s shoulders. they start jumping. type 2 differs 138 vol. 15/2018 which fans are the biggest acceleration of the grandstand? from type 1, they do not move just up and down and also horizontally. from the dynamics point of view the position of the fans on the platform is crucial. slavie fans stand on concrete panels. sparta fans are on the second storey of the platform on the concole part of the beam [1–6]. 2.3. measurement way 2.3.1. monitoring the spectators the entire ultras‘s sector was monitored using a video camera at both stadiums. the camera was located at a sufficient distance from the sector due to security of cameraman. fans do not like when someone shoots them. the camera was approximately 100 meters from the sector at the stadium in eden. letna was located at 85 m. 2.3.2. monitoring the acceleration of the grandstand the vibrations intensity was measured by acceleration sensors (bruel and kear type 4507 b005). there were used four sensors in total. three of them entered vertical acceleration and one of them entered horizontal acceleration. at eden the sensors were placed onto a concrete panel (vertical + horizontal) and onto neighbouring crossbeams (figure 5) at letna only the vibrations of console steel beams were tracked. from the technical point of view it was not possible to place the sensors onto concrete panels safely [1–6]. figure 5. sensors placement: a) letna stadium [1], b) eden stadium [4]. 3. observation period three experiments were observing the platform vibrations intensity during football match. the first measurement was done at letna in 2013 followed by measurements at letna in 2015 and at eden in 2016. 3.1. letna stadium 2013 the observation was done during two football matches at the end of the autumn part of the 2013/2014 season. it was at that time when ac sparta prague won the autumn. there was not a camera watching the behavior of individual spectators. at the same time, only the vertical vibrations of the platform were recorded (there was no horizontal sensor). the fans managed to drive an acceleration of 1.96 m·s−2 at drive frequency 2.35 hz (figure 6 and 7). the acceleration was recorded after the end of the second match when the fans celebrated the victory in the middle of the competition. it resulted from the experiment that it should have been caused by jumping of the entire rows vertically [1–3, 6] figure 6. maximum platform acceleration in 2013 [2]. figure 7. frequency at letna in 2013 [6]. 3.2. letna stadium 2015 the second observation was taken in the spring part of the competition of 2014/2015 season. in total 5 matches were observed. in contrast to the previous experiment a horizontal acceleration sensor was added. there was also a camera used to watch the fans‘ behaviour. the season was not successful for sparta at 139 martin verner acta polytechnica ctu proceedings all. as the end of the competition was approaching the placement of the club was clear. gradually the number of spectators was decreasing and the level of vibrations was dropping down as well. the biggest vertical acceleration (1.96 m·s−2 at drive frequency of 2.5 hz – figure 8) was observed during the third observed match. the video record showed that it was caused by jumping of individual rows vertically [1–3, 6] figure 8. maximum platform acceleration in 2015 [6]. 3.3. eden stadium the last football matches observation was taken at eden. again, the spring part of the competition of 2015/2016 season was observed. four matches were specific. the experiment was done at letna as well as the previous ones. slavie was going through a financial crisis at that time. right before the start of the competition a new investor came to the club who tried to stabilize the club. the visit rate dropped down significantly after the firts match. the biggets vibrations were recorded during the first match (2.84 m·s−2 at drive frequency of 2.17 hz – figure 9 and 10). it was caused by jumping of the vertical rows. second biggest vibrations were, again, recorded during the first match (2.35 m·s−2 at drive frequency of 2.18 hz). this was measured during jumping of individual rows horizontally [4, 5] figure 9. maximum platform acceleration in 2016 [4]. figure 10. frequency at eden in 2016 [5]. 4. conclusions when considering only the absolute strenght of vibrations the most active fans are those of sk slavie. as mentioned in chapter 2. the sensors at letna were not laid in concrete panel. unlike eden the vibrations of the entire platform were recorded resulting in the fact that the ac sparta fans could be the most active ones. it is not good to compare the results just through the vibrations level because the constructions inluded in the experiments were of different dynamic features. the only way to compare the activity is though the drive frequency caused by the fans. when having a look at the drive frequencies and material composition of the platforms (vibration dampening capacity influence) it is clear that the fans drove with the same intensity. maximum drive frequncy was very similar during both matches. the answer to the question asked at the beginning is that the fans – from the dymacics point of view – support their teams with similar intensity. acknowledgements the research was financial supported by the faculty of civil engineering at ctu in prague (sgs project no.16/201ohk1/3t/11) and the czech science foundation project no. ga15-15728s. references [1] m. verner, t. plachy, m. polak. an experiment focused on fans behaviour and induced grandstand vibrations during a football match. trends in statics and dynamics of constructions ii zãijrich: trans tech publications offices ltd pp. 75–78, 2016. [2] m. verner, m. polak, t. plachy. an experimental analysis of grandstand vibrations caused by crowd of spectators during two football matches. aplied methods of the analysis of static and dynamic loads of structures and machines ii zurich: trans tech publications ltd pp. 312–315, 2016. [3] v. martin. behaviour of fans during the football match. modern methods of experimental and computational investigations in area of construction pfaffikon: trans tech publications inc pp. 77–80, 2016. 140 vol. 15/2018 which fans are the biggest acceleration of the grandstand? [4] t. plachy, m. polak, m. verner. comparison of the fans behaviour and the induced grandstand vibrations on two different football stadia. dynamics of civil engineering and transport structures and wind engineering (dyn-wind’2017) 2016. [5] t. plachy, m. polak, m. verner. an experimental study focused on spectators’ behavior and induced vibrations of a reinforced concrete grandstand on a football stadium. procedia engineering oxford: elsevier ltd) pp. 2871–2876, 2017. [6] m. polak, t. plachy, m. verner. a comparison of spectators induced grandstand vibrations and fans behavior during two football matches between the same teams. 54th international conference on experimental stress analysis 2016 p. 6, 2016. [7] konstrukce media, s.r.o. stadion eden postup praci 2005. http://www.konstrukce.cz/clanek/ stadion-eden-postup-praci/. [8] empresa media, a.s. zemrel architekt mandel, kterz navrhl stadion sparty 2011. https: //www.tyden.cz/rubriky/lide/ceske-celebrity/ zemrel-architekt-mandel-ktery-navrhl-stadion-sparty_ 214905.html. [9] mafra, a.s. ultras eden. choreo slavistickych fanousku behem derby. https://fotbal.idnes.cz/foto.aspx?r=fotbal& foto1=tof6a5d61_183459_1597445.jpgl. [10] mafra, a.s. sparta vs. lazio, plne tribuny, novinari z nemecka a hsote bez treninku. https://fotbal.idnes.cz/ sparta-lazio-rim-osmifinale-evropske-ligy-fi8-/ fot_pohary.aspx?c=a160309_150622_fot_pohary_ min. 141 http://www.konstrukce.cz/clanek/stadion-eden-postup-praci/ http://www.konstrukce.cz/clanek/stadion-eden-postup-praci/ https://www.tyden.cz/rubriky/lide/ceske-celebrity/zemrel-architekt-mandel-ktery-navrhl-stadion-sparty_214905.html https://www.tyden.cz/rubriky/lide/ceske-celebrity/zemrel-architekt-mandel-ktery-navrhl-stadion-sparty_214905.html https://www.tyden.cz/rubriky/lide/ceske-celebrity/zemrel-architekt-mandel-ktery-navrhl-stadion-sparty_214905.html https://www.tyden.cz/rubriky/lide/ceske-celebrity/zemrel-architekt-mandel-ktery-navrhl-stadion-sparty_214905.html https://fotbal.idnes.cz/foto.aspx?r=fotbal&foto1=tof6a5d61_183459_1597445.jpgl https://fotbal.idnes.cz/foto.aspx?r=fotbal&foto1=tof6a5d61_183459_1597445.jpgl https://fotbal.idnes.cz/sparta-lazio-rim-osmifinale-evropske-ligy-fi8-/fot_pohary.aspx?c=a160309_150622_fot_pohary_min https://fotbal.idnes.cz/sparta-lazio-rim-osmifinale-evropske-ligy-fi8-/fot_pohary.aspx?c=a160309_150622_fot_pohary_min https://fotbal.idnes.cz/sparta-lazio-rim-osmifinale-evropske-ligy-fi8-/fot_pohary.aspx?c=a160309_150622_fot_pohary_min https://fotbal.idnes.cz/sparta-lazio-rim-osmifinale-evropske-ligy-fi8-/fot_pohary.aspx?c=a160309_150622_fot_pohary_min acta polytechnica ctu proceedings 15:137–141, 2018 1 introduction 2 circumstances influencing vabrations intensity 2.1 stadiums construction 2.2 fans` behaviour 2.2.1 ultras 2.2.2 fan behavior from a dynamic response view 2.3 measurement way 2.3.1 monitoring the spectators 2.3.2 monitoring the acceleration of the grandstand 3 observation period 3.1 letna stadium 2013 3.2 letna stadium 2015 3.3 eden stadium 4 conclusions acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2019.24.0001 acta polytechnica ctu proceedings 24:1–8, 2019 © czech technical university in prague, 2019 available online at http://ojs.cvut.cz/ojs/index.php/app high-temperature steam and air oxidation of chromium-coated optimized zirlo™ gauthier bourdona,b,∗, martin ševečeka, jakub krejčíc,a, ladislav cvrčekd a czech technical university in prague, faculty of nuclear sciences and physical engineering, břehová 7, 115 19 prague 1, czech republic b grenoble-inp, phelma, 3 parvis louis néel, 38016 grenoble, france c ujp praha a.s., nad kamínkou 1345, 156 10 prague – zbraslav, czech republic d czech technical university in prague, faculty of mechanical engineering, technická 4, 166 07 prague 6, czech republic ∗ corresponding author: gauthier.bourdon@grenoble-inp.org abstract. the presented study focuses on cr-coated cladding material and its oxidation performance in high-temperature steam and air. as the substrate material, optimized zirlo™ was used which was coated by chromium using unbalanced magnetron sputtering. the experimental methods are described and the presented results focus mainly on weight gain and oxidation kinetic evaluation. the experiments were carried out in the high-temperature range between 1000 and 1300 °c. the test conditions focus mainly on the air oxidation tests that are relevant for some of severe accidents with air ingress and have not yet been studied in detail and are not available in literature. these data are supported with steam oxidation data to directly compare the kinetics in two different oxidizing environments. keywords: cladding material, accident tolerant fuel, steam oxidation, air oxidation. 1. introduction since the fukushima-daiichi accident in 2011, new nuclear fuel and cladding materials have been investigated and developed to further enhance safety of currently operated nuclear power plants. generally, they are called accident tolerant or advanced technology fuels. the main objective of these advanced materials is to enhance the accidental tolerance in case of design basis accidents (dbas) and design extension conditions. as one of the atf candidates, new coated cladding materials are being studied [1, 2]. the objectives of these new cladding materials include prevention of a degradation and failure of claddings due to excessive oxidation during postulated dbas such as loss of coolant accident (loca) or more severe ones with air ingress. indeed, when there is an accident with air ingress in a reactor core, a spent fuel pool [3] or in transportation cask, cladding strongly oxidizes in air or steam/air mixture. this causes release of heat as well as acceleration of cladding degradation and progression of accident scenario. the application of protective coatings was proposed to protect the zr-based alloys under high-temperature accidental conditions and to prevent breakaway oxidation. many coating materials have been studied so far, but the most promising seems to be material coated with pure chromium or cr-based materials. further development of these materials is motivated by the fact that although current alloys are very efficient in terms of oxidation at normal operating conditions and have good mechanical properties, during accidents as described above, they undergo a strong oxidation which results in degradation of their mechanical properties. in addition, there is a lack of high-quality experimental data on these new materials that will allow a comparison of their performance and also development of new correlations that can be implemented into simulation tools that are used for evaluation or licensing. 2. materials 2.1. substrate all of the tests presented in this paper were done either with uncoated as-received optimized zirlo™ or cr-coated optimized zirlo™. this alloy has been developed by westinghouse electric company and has been utilized in light water reactors (lwrs) worldwide since 2000 and also in wwer reactors that are in operation in the czech republic. according to [4], the composition of optimized zirlo™ with partially recrystallized annealed microstructure (prxa) is shown in table 1. all uncoated samples are 30 mm high cylinders with an outer diameter of 9.14 mm and inner diameter is 8 mm. two holes have been drilled in the top part of the sample in order to be able to hang them during the experiment. the hole diameter (ho) is about 2-3 mm (figure 2). mechanical properties of the substrate alloy are [4]: • 0.2 % yield strength 469–655 mpa • minimum elongation 12 %; expected about 25 % • contractile strain ratio between 1.5 and 2.25 1 http://dx.doi.org/10.14311/app.2019.24.0001 http://ojs.cvut.cz/ojs/index.php/app g. bourdon, m. ševeček, j. krejčí, l. cvrček acta polytechnica ctu proceedings • texture in radial, hoop and axial directions ≈ 0.6, 0.35 and 0.05 alloy micro. nb sn fe optimized zirlo™ prxa 1 0.67 0.1 table 1. chemical composition of optimized zirlo™ [%] other properties of optimized zirlo™ used in design of experiments can be found in [5]. the transition of heat capacity of optimized zirlo™ shown in figure 1 shows expected transformation region from hcp alpha to mixed alpha+beta region and finally to bcc beta phase at high temperature. some of the optimized zirlo™ properties are similar to zircaloy4 such as thermal conductivity, coefficient of thermal expansion, elastic and shear modulus. the properties of the substrate are used for fuel performance modelling that is ongoing and will be published in the near future and for optimization of the deposition technique that slightly differs for different substrate alloys. figure 1. heat capacity for optimized zirlo™. these values are taken from frapcon/fraptran correlation, thus they are approximate. figure 2. schematics of two geometries of samples used. uncoated samples have cylindrical geometry. coated samples are almost half cylinder. r’ is measured to determine the α parameter in order to calculate the accurate surface of coated samples. 2.2. coated cladding the unbalanced magnetron sputtering method was chosen as the deposition technique of protective coating for the current study. it is considered as one of the physical vapor deposition (pvd) methods that is able to produce high-quality relatively thick coatings at low temperature. however, it produces line-of-sight coatings which means that it cannot coat convex geometries such as internal diameter (id) of the tubes used as cladding. the tubular cladding segments can be coated only on the outer surface but then the inner uncoated surface will strongly affect the results when testing oxidation resistance. two approaches were chosen to neglect the effect of inner oxidation of uncoated material: (1.) welding of end plugs on both ends of the tubular segments and coating the whole sample including end caps and later use tig welding for pressurization and sealing (2.) cutting the tubular samples into two halves and coating the whole sample surface including the id the first approach was presented in previous works [6, 7], the second approach was chosen in this study due to lower costs and time requirements. the sample geometry of both coated and uncoated samples is shown in figure 2. it can be seen that tubular segments were machined first and cut into two halves before coating deposition. 2.2.1. magnetron sputtering the coating was deposited using magnetron sputtering technology. this technique is based on depositing a thin layer of coating using gaseous plasma. unbalanced magnetron sputtering has been chosen for the coating since it allows an improved control of the coating deposition process. in particular, this avoids an overly intense bombardment of the substrate by the ions of the placement, which could modify its physical properties. more information about similar techniques can be found in [8, 9]. the coating of the samples is carried inside the hauzer flexicoat 850 machine which is a semi-industrial commercial machine that has been modified in order to produce required coating properties suitable for nuclear cladding applications. the samples were first ultrasonically cleaned in acetone and dried with a blower. then the surface of the samples was cleaned by ion etching in argon plasma for 20 minutes. this procedure removes the thin oxide layer and impurities from the samples. dc mode has been used with two cathodes of cr (99.6%) powered to 6kw. the deposition has been done at 250°c and 0.2 pa with a gas flow argon at 90 sccm. final coating consisted of ten chromium interlayers because every 70 minutes the deposition of the coating was interrupted for 60 minutes. during these nine interruptions, the temperature dropped to 100 °c. the total thickness of the cr coatings was measured with a calotest instrument (csm, switzerland) and was found to be 18.6 ± 0.1 µm. 2 vol. 24/2019 high-temperature oxidation of cr-coated optimized zirlo™ 3. experimental there are several differences between air and steam oxidation of cladding materials in general. the oxidation in air leads to the early cracking of the protective zro2 oxide layer (breakaway) which leads to the formation of zrn with a very porous structure that can be further reoxidized [10]. moreover, the oxidation of zr-based alloys in air is usually faster than in steam. in steam at high temperature, the breakaway may be also observed but usually in later stages. additionally, in steam there is a problem of hydrogen absorption in the cladding material, which degrades the cladding properties [11, 12]. oxidation in steam occurs during dbas such as loca [13], oxidation in air or steam/air mixture occurs during more severe accidents with air ingress such as accident in spent pool with a failed heat removal system [3]. 3.1. oxidation in air each sample is held by a platinum wire at a known distance from the thermocouple. as it is impossible to place the thermocouple on the surface of the sample, it is placed as close as possible to the sample. inside the furnace there is a 24 mm diameter quartz tube (item 21 in figure 9), where the sample will be placed. the temperature profile was measured before the start of the experiment in order to determine the temperature of the sample during the experiment. it is assumed that the temperature profile will be the same once the specimen is placed inside the furnace. the specimen is then placed outside the furnace (position 2 in figure 9). the furnace and the glassware are subjected to a flow of argon for 5 minutes to remove atmospheric gases, and then to a flow of argon (50 l.h−1) and synthetic air (100 l.h−1) for conditioning the furnace. the gases are mixed in the controlled evaporating system (cem) from bronkhorst used also at different institutes [14]. the specimen is later inserted into the preheated resistance furnace (position 1 in figure 9) within predefined temperature zone (± 6 °c) which is about 4–5 cm long. the flow of argon and air remains constant. the temperature is recorded throughout the experiment. once the specimen has reached the desired temperature ± 3°c, the exposure time is measured. once the desired exposure time has elapsed, the sample is placed outside the furnace and cooled with argon at room temperature. cooling the sample makes it possible to preserve the microstructure. the experiments were done at several temperature levels between 1000 and 1300°c. 3.2. oxidation in steam same as for oxidation with air, the enclosure is first conditioned with argon and then with a mixture of argon (50l.h−1) and steam. the steam is obtained thanks to the cem which is heated to 180 °c. water is incorporated in the cem with a flow rate of 85 g.h−1. a pump with a tank supplies a smaller tank in height (see elements 7,8,9 in figure 9). the water level in the small tank must remain within a specific region to allow the flow-meter to operate properly. the pump is adjusted accordingly. the procedure is then the same as for oxidation with air, except for cooling. here, water at 0 °c cooled with ice cubes is used to quench the specimen. it is more effective than cooling with argon and also more prototypical for loca accident in which rapid quench is expected at the end of the transient. because it is possible that cooling water may evaporate during the experiment and change the oxidation environment, this technique is not used for oxidation in air. at the end of the experiment, the specimen is placed inside an oven in order to dry it and to characterize later. 3.3. material characterization the experiments focus on high-temperature oxidation both in steam and air. therefore, before and after each experiment the sample is weighed, measured and visually evaluated. the weighing is done with a scale accurate to the nearest 0.01 mg and the measurement with a caliper rule with an accuracy of 0.01 mm. as the deformation is not isotropic, the length is measured at 3 locations on the sample and the final length is the average of these three measurements. however, it may happen that the sample is considerably deformed so that dimensions cannot be measured. in this case, no measurement is made. the visual evaluation consists of noting any relevant remarks on the appearance of the sample (color of the oxide, location of the oxide layer etc.). rings are then cut from the samples and further analyzed. the characterization includes also mettalography. microhardness, mechanical testing or hydrogen concentration measurements will be carried out and published later. 4. results this section presents results of oxidation results with coated cladding first and then with reference uncoated material both in steam and air. visual evaluation and results of mettalography are presented afterward. 4.1. weight gain – cr-coated cladding the initial evaluation of the effectiveness of the protective coating with respect to oxidation, can be done by studying the mass gain. to evaluate and compare the results it is practical to deduce the oxidation kinetic parameters described by eq. 1 [15]. for the purpose of the presented experiment, the weight gain is normalized by the surface of the specimen. the oxidation kinetics can be first described by a parabolic law (eq. 2) where the value of n (oxidation exponent) is 2. k1 is the rate of oxidation in mg.dm−2.min−1/2 . it is important to note that the parameter k1 is described by an arrhenius law (eq. 4) [16]. this parameter depends on the temperature and activation energy, thus also on the type of material tested as well as the gas composition in the 3 g. bourdon, m. ševeček, j. krejčí, l. cvrček acta polytechnica ctu proceedings oxidizing environment. k (mg.dm−2.min−1/2) and b (k) are experimental parameters. from arrhenius law a linear relation between log(k) and 1/t is expected. the results are then fitted using this model to deduce these parameters. the values of k1 as a function of temperature are shown in figure 5. table 2 summarizes experimental values of k and b. alloy k b cr-coated optimized zirlo™ 11617231 18652 uncoated optimized zirlo™ 87137683 16259 table 2. k (mg.dm−2.min−1/2) and b (k) values for coated and uncoated cladding materials from eq. 4 for air oxidation. however, after a certain time, there is the oxidation kinetic change from the parabolic law described in previous paragraph. this moment is called breakaway and it is due to the fact that the oxide layer formed (protective oxide layer) at the beginning of the oxidation disrupts due to excessive stresses and reoxidation of the nitrides that formed. thus, the oxidation strongly accelerates. after the transition, the n value is equal to 1 leading to linear kinetics of oxidation eq. 3. combining both regions and equations (preand post-breakaway), the correlations can be derived and plotted (see figure 3). dwgn dt = k(t ) (1) wg(t) ∼ k1(t ).t1/2 (2) wg(t) = k2(t ).t+wg0 − k2.to ≡ k2(t ).t+ c (3) k1(t ) = ke −ea rt ≡ ke −b t (4) figure 3. the results for oxidation with cr-coated samples in steam at 1200 °c and air at 1200 and 1300 °c. the fits show parabolic pre-breakaway part of the kinetics. delayed breakaway point for oxidation in steam is visible. 4.2. weight gain – uncoated optimized zirlo™ the same procedure was followed and measured weight gains of optimized zirlo™ were fitted. the graph below shows the results for air oxidation with optimized zirlo™ together with derived fits. it can be seen that there are two types of oxidation kinetics at 1000 °c with a transition between 20 and 30 minutes. on the other hand, there is no transition at 1200 °c when the accelerated oxidation is obvious from the beginning. it should be noted, that the values around 6000 mg/dm2 correspond to basically complete oxidation of the sample. figure 4. weight gains for uncoated optimized zirlo™ tested in air at 1000 and 1200 °c. preand post-breakaway regions are visible for 1000 °c but at there’s not breakaway at 1200 °c since the kinetics is accelerated from the beginning. to directly compare the oxidation of two materials (coated and uncoated), it is common to represent k as a function of 10000/t to compare the oxidation kinetics of different materials (see figure 5). since most of the experiments were performed in air, k values are only shown for optimized zirlo™ and cr-coated optimized zirlo™ in air. it can be seen that k value of the coated samples is more than 30 times lower than for uncoated reference alloy. it should be noted, that more data points are needed in order to confirm this trend also for different temperatures. figure 5. k as a function of 10000/t for cr pvd coated and uncoated optimized zirlo™. 4 vol. 24/2019 high-temperature oxidation of cr-coated optimized zirlo™ 4.3. visual evaluation visual analysis makes it possible to qualitatively report on the condition of the sample after the experiment. in particular, it is obvious that coated samples are much more resistant to oxidation (figure 6). it can be seen that the oxygen-rich white oxide layer forms for air oxidation and and sub-stoichiometric black oxide for steam oxidation. in addition, it is interesting to note that for coated samples, the chromium oxide layer only cracks around the drilled hole. the elongation trend of the samples can also be qualitatively observed from the pictures. the uncoated samples grow due to the oxidation creep when strongly oxidized. the changes in length are not considerable for coated samples. it is expected to see larger lenght changes at lower temperature due to phase transformations. optimized zirlo™ in air at 1200°c cr-coated optimized zirlo™ in air at 1200°c cr-coated optimized zirlo™ in steam at 1200°c figure 6. photos of some samples after oxidation it can be also noted that for the uncoated sample at 1000 °c after 60 minutes, the sample has fully oxidized and lost its mechanical integrity. however, during the experiment, the samples are kept in the furnace thanks to their hole. this sample must have fallen before it reached 60 minutes of exposure time, therefore the real oxidation time is lower. 4.4. microstructure the detailed microstructural analysis of all the samples is ongoing. it is, however, important to look at it to understand the oxidation phenomenon of these samples and the differences between coated and uncoated material. thus, figure 7 shows the microstructure of the cr-coated optimized zirlo™ after oxidation in air. to allow the comparison between coated and uncoated cladding material, microstructure of zr1nb alloy is also shown for reference comparison. cr pvd coated optimized zirlotm optimized zirlotm 1 1 2 2 3 3 3 4 4 5 5 figure 7. microstructure after oxidation in air for cr pvd coated sample oxidized for 4.5 and 30 minutes in comparison with uncoated sample oxidized for 1.5 and 9 minutes. (1. outer cr oxide 2. remaining cr metal layer 3. α zr(o) 4. zro2 5. porous zrn) during the oxidation in air for uncoated samples, a layer of zro2 (5) oxide is first formed. this layer is a protective one. but during oxidation this layer will eventually break which results in initiation of breakaway oxidation. from then on, the reaction accelerates. in addition, as oxygen is consumed by the oxidation reaction, the partial pressure of nitrogen increases, which leads to interaction of zirconium also with nitrogen to form nitrides (3). a porous zrn and an oxide layer consisting of zro2 and zrn are formed (6). it was shown that the formation of nitrides might lead to their re-oxidation and further oxidation acceleration. oxidation in air with coated samples is a little different. first a protective chromium oxide layer is formed (1). due to diffusion of oxygen into the substrate, α-zirconium stabilized by oxygen is formed (4). as for the uncoated claddings, the protective layer can break, which leads to the acceleration of oxidation. as oxygen is consumed by the oxidation reaction, partial pressure of nitrogen increases, which causes reaction between nitrogen and the cladding and formation of thin zrn layer (3). microstructure differences between coated and uncoated samples after oxidation in steam are shown in figure 8. with cr-coated sample, there is a chromium oxide layer and with uncoated one a zirconium oxide layer on the outermost surface. however, it may happen that after extremely long time of oxidation, steam penetrates the cr coating and forms zr oxide underneath which was observed for extremely long expositions. 5 g. bourdon, m. ševeček, j. krejčí, l. cvrček acta polytechnica ctu proceedings cr pvd coated optimized zirlotm optimized zirlotm 1 2 3 4 5 figure 8. microstructure after oxidation in steam for cr pvd coated sample oxidized for 15 minutes in comparison with uncoated sample oxidized for 5 minutes. (1. cr oxide 2. cr metal layer 3. zro2 4. α zr(o) 5. prior β zr 5. discussion several points can be discussed based on the data measured. first, a breakaway was observed also for cr-coated samples which has not been reported to date. however, for uncoated zr alloys the transition can be seen only at lower temperatures which confirms strong protective effect of cr pvd coating also in air containing environment. the visual analysis suggests that the oxide layer cracks only around the hole for hanging which causes the initiation of the transition of the coated samples. it is therefore possible that the chromium coating at this location does have different properties and lower adhesion due to complex geometry. so maybe by studying longer samples or different hanging designs, there might be no breakaway observed or it can be further delayed. it was also found the the id coating is slightly thinner in comparison with od and it can be seen that the breakaway initiates here. it is related to the characteristics of the pvd method used and chosen approach for elimination of uncoated areas of the samples. experimental kinetic parameters such as k and b can be deduced by studying weight gain. but for the moment there are not enough data to produce reliable correlation (especially for oxidation in the air of optimized zirlo™ where only two temperature levels have been tested) but these parameters will be published in the near future. similarly, the value of n can be discussed because it may be slightly different from the theoretical value. to derive reliable correlation, it is necessary to fit two parameters so more data is needed to get a relevant result. 6. conclusion the study presented clearly shows that chromium pvd coated cladding samples have a better resistance to high temperature oxidation than uncoated ones in both steam and air for dba as well as dec conditions. it was confirmed by visual evaluation, microstructural analysis as well as derived oxidation parameters for both coated and uncoated materials. moreover, the results show that oxidation in air is much faster than in steam for the considered temperature range including earlier occurrence of the breakaway point. these results further confirm that chromium pvd coating is a promising atf cladding candidate from the perspective of chosen test conditions. it is necessary to continue the experiments to have more data to be able to answer the problems raised previously. it is therefore planned to continue the oxidation experiments in air but also in steam to test the samples in these two different high-temperature environments. the microstructure analysis of all the samples is ongoing as well as detailed characterization of the coating and substrate microstructure. other samples with different coatings are also studied to compare them with the coating presented in this article. however, although the study of these new materials is motivated by accidents, it is important to note that this study focuses only on oxidation and that other parameters must be taken into account during such accidents, such as creep, hydrogen pickup, adhesion or cladding ballooning. list of symbols wg weight gain [mg dm−2] n oxidation exponent, value is 1 or 2 k kinetic pre exponential factor [mg dm−2 min−1/2] ea activation energy [j mol−1] r perfect gas constant [j mol−1 k−1] t temperature [k] wg0 weight gainbefore the break away [mg dm−2] k1 oxidation rate break away [mg dm−2 min−1/2] k2 oxidation rate after break away [mg dm−2 min−1] t0 time where the break away happen [min] b = ea r [k] od outer diameter id inner diameter hd hole diameter l length acknowledgements the authors thank the westinghouse electric company for providing optimized zirlo™ substrate, ujp praha zirconium group (j. šustr, j. kabátová, d. rada, f. manoch, a. přibyl, and v. vrtílková) for sample preparation, and post-experimental evaluation. financial support of this research through čez a.s. company is gratefully acknowledged. also, this work was supported by the ministry of education, youth and sport of the czech republic, programme npu1, project no lo1207, national programme of sustainability ii project lq1603 (research for susen), iaea crp 21065 and technology agency of the czech republic grant no. th02020477, th02020691, cz.2.16/3.1.00/21563. references [1] n. oecd. state-of-the-art report on light water reactor accident-tolerant fuels, vol. 2018. [2] iaea. accident tolerant fuel concepts for light water reactors 2016. [cit. 2018-01-25]. 6 vol. 24/2019 high-temperature oxidation of cr-coated optimized zirlo™ figure 9. schematics of the experiment procedure 7 g. bourdon, m. ševeček, j. krejčí, l. cvrček acta polytechnica ctu proceedings [3] a. benjamin, d. mccloskey, d. powers, s. dupree. spent fuel heatup following loss of water during storage. nuclear technology pp. 274–294, 1980. [4] g. pan, d. b. mitchell, a. m. garde1, et al. advantages gained by optimized zirlo™ and axiom™ pwr cladding materials. top fuel p. 1278, 2016. [5] w. luscher, k. geelhood, i. porter. comparison between frapcon-4.0,fraptran-2.0, and matpro. u.s department of energy, 2015. [6] j. krejčí, m. ševeček, j. kabátová, et al. experimental behavior of chromium-based coatings 2018:13. [7] j. krejčí, m. ševeček, l. cvrček, et al. chromium and chromium nitride coated cladding for nuclear reactor fuel. in proceedings of the 20th international corrosion congress, eurocorr 2017. [8] s. swann. magnetron sputtering. physics in technology 19(2):67–75, 1988. doi:10.1088/0305-4624/19/2/304. [9] p. kelly, r. arnell. magnetron sputtering: a review of recent developments and applications. vacuum 56(3):159 – 172, 2000. doi:10.1016/s0042-207x(99)00189-x. [10] m. steinbrück, m. böttcher. air oxidation of zircaloy-4, m5® and zirlo™ cladding alloys at high temperatures. journal of nuclear materials pp. 276 – 285, 2011. [11] y. yan, t. burtseva, m. billone. high-temperature steam-oxidation behavior of zr-1nb cladding alloy e110. journal of nuclear materials 393(3):433 – 448, 2009. doi:10.1016/j.jnucmat.2009.06.029. [12] m. steinbrück, n. vér, m. große. oxidation of advanced zirconium cladding alloys in steam at temperatures in the range of 600–1200 °c. oxidation of metals 76(3):215–232, 2011. doi:10.1007/s11085-011-9249-3. [13] s. zinkle, k. terrani, j. gehin, et al. accident tolerant fuels for lwrs: a perspective. journal of nuclear materials 448(1):374 – 379, 2014. doi:10.1016/j.jnucmat.2013.12.005. [14] m. steinbrück, u. stegmaier, t. ziegler. prototypical experiments on air oxidation of zircaloy-4 at high temperatures, vol. 7257. fzka, 2007. [15] m. stempniewicz. air oxidation of zircaloy,part 1 review of correlations. nuclear engineering and design 2016. [16] g.schanz, b.adroguer, a.volchek. advanced treatment of zircaloy cladding hight temperature oxidation in sever accident code calculations. nuclear engineering and design pp. 75–84, 2004. 8 http://dx.doi.org/10.1088/0305-4624/19/2/304 http://dx.doi.org/10.1016/s0042-207x(99)00189-x http://dx.doi.org/10.1016/j.jnucmat.2009.06.029 http://dx.doi.org/10.1007/s11085-011-9249-3 http://dx.doi.org/10.1016/j.jnucmat.2013.12.005 acta polytechnica ctu proceedings 24:1–8, 2019 1 introduction 2 materials 2.1 substrate 2.2 coated cladding 2.2.1 magnetron sputtering 3 experimental 3.1 oxidation in air 3.2 oxidation in steam 3.3 material characterization 4 results 4.1 weight gain – cr-coated cladding 4.2 weight gain – uncoated optimized zirlo™ 4.3 visual evaluation 4.4 microstructure 5 discussion 6 conclusion list of symbols acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2020.26.0064 acta polytechnica ctu proceedings 26:64–70, 2020 © czech technical university in prague, 2020 available online at https://ojs.cvut.cz/ojs/index.php/app influence of material damping on the response of underground structures subjected to earthquake veronika pavelcová, tereza poklopová, michal šejnoha∗, tomáš janda czech technical university in prague, faculty of civil engineering, department of mechanics, thákurova 7, 166 29 prague 6, czech republic ∗ corresponding author: sejnom@fsv.cvut.cz abstract. the paper describes a finite element simulation of the response of a real underground structure subjected to earthquake using geo5 fem program. it concentrates on the influence of material damping with respect to a specific type of boundary condition prescribed at the bottom of the analyzed domain. it is seen that considering material damping is inevitable particularly in case of so called fixed boundary conditions to arrive at meaningful results. this is demonstrated on an artificial earthquake generated according to a design spectrum defined in eurocode 8. a viscous damping ratio combined with the results of eigenvalue analysis is used to derive parameters of rayleigh damping for three specific scenarios promoting the approach based on the lowest natural frequency as sufficiently accurate for the present task. keywords: earthquake, tunnel, material damping, accelerogram, finite element method, absorbing boundary, fixed boundary. 1. introduction numerical modeling of geotechnical structures subjected to earthquake typically distinguishes between damping associated with the application of radiation boundary condition [1] and intrinsic material damping associated with the energy dissipation within the material caused by various mechanisms including interparticle sliding and friction and pore fluid viscosity [2]. the former type of damping has been addressed in detailed in [3, 4]. the present contribution is therefore an extension of our previous work with emphasis on the effect of material damping. although valid for a single degree of freedom the complex structural analysis usually grounds on the knowledge of a single value damping ratio. such a parameter is typically estimated experimentally from a hysteric stress-strain loop as the energy loss in a cycle [5]. while some new measurement technique arise [6] to eliminate uncertainty in the determination of geometric damping thus increasing reliability in the damping ratio measurement, the determination of damping ratio for all mode shapes is still far from trivial. this is why most practical applications rely on a single value damping ratio even in case of multi degree of freedom tasks combined with the concept of rayleigh proportional damping [7]. this computational strategy is adopted also in the present study focused on the effect of material damping in combination with various types of boundary conditions introduced at the bottom of the computational model. the remainder of the paper is organized as follows. starting with a theoretical background in section 2 we continue in section 3 with the definition of material damping. the theoretical part of the paper is finalized in section 4 addressing the construction of artificial accelerograms. details of the computational models including the finite element mesh refinement are provided in section 5. the overview of computational results starts in section 6 with modal analysis to provide data for the derivation of parameters of rayleigh damping. the results of free field (ff) analysis are presented in section 7 followed by the summary of the essential outcomes delivered by two-dimensional (2d) analyses reflecting the presence of a tunnel. 2. governing equations a detailed derivation of the set of governing equations driving the earthquake analysis in geotechnical engineering can be found, e.g., in [1, 3, 8]. therein, the formulation was limited to damping associated with absorbing boundary conditions. when taking into account the material damping the discretized form of the equation motion with quiet boundary assumed at the bottom of the model becomes, see also fig. 1, mür + cu̇r +kur + cbbu̇r|y=0 + clbu̇r|x=0,l = −müi0 − cu̇i0 + cbbu̇i0|y=0 + clbu̇ffr |x=0,l (1) −rτ |x=0 +rτ |x=l, assuming the total displacement u(x, t) be split into a relative displacement ur(x, t) and the displacement ui0(t) applied to the whole domain such that u(x, t) = ur(x, t) + ui0(t). (2) in eq. (1), m,k are the mass and stiffness matrices of the domain, respectively. the damping matrix c 64 https://doi.org/10.14311/app.2020.26.0064 https://ojs.cvut.cz/ojs/index.php/app vol. 26/2020 influence of material damping in earthquake analysis figure 1. boundary and loading conditions for both ff and 2d analysis. corresponds to material damping only, while matrices cbb and clb reflect the effect of artificial viscous dampers applied along the absorbing boundaries (the superscripts bb and lb identify the bottom and lateral boundaries; henceforth the absorbing bb boundary will be called the quiet boundary and denoted as qbb whereas the notation fbb will be adopted to identify the so called fixed boundary [3, 8]). finally, the vector rτ stores the nodal forces associated with the prescribed boundary shear stress. recall that in case of qbb the vector ui0(t) stands for the incoming wave only. should the top soil be supported by an infinitely stiff bedrock we may introduce at the bottom boundary the fbb conditions suggesting a total reflection of the incoming wave back into the domain. the applied load than corresponds to the total motion u0(t) so that u(x, t) = ur(x, t) + u0(t), (3) and eq. (1) simplifies as mür + cu̇r +kur + clbu̇r|x=0,l = −mü0 − cu̇0 + clbu̇ffr |x=0,l (4) −rτ |x=0 +rτ |x=l, for simplicity, the present study assumes the incoming wave be equal to one half of the total motion. 3. rayleigh damping material damping may have a strong impact on the response of an underground structure when subjected to earthquake. however, a physically supported formulation of damping matrix c is not an easy task mainly because of the lack of experimental evidence. the proportional rayleigh damping is then a typical choice in most practical applications. in such a case, the damping matrix is expressed as a linear combination of the mass and stiffness matrices in the form, see e.g. [9, 10], c = αm + βk. (5) accepting eq. (5) allows us to write the damped natural frequency ωdi associated with the i-th orthogonal mode shape φi of undamped system in terms of the viscous damping ratio ξi and the i-th natural frequency of undamped system ωi 2ωdi = 2ξiωi = φi tcφi = α+ βω2 i , (6) because φi tmφi = 1, φitkφi = ω2 i . (7) thus if two viscous damping ratios ξi are known for the pair of natural frequencies ωi, the values of parameters α, β readily follow from eq. (6). however, most often only a single value of parameter ξ is provided. then, the hypothetical assumption of the first natural frequency being least damped is usually accepted. therefore, dξ dω = 1 2 ( − α ω2 + β ) = 0. (8) substituting ω = ω1 into eqs. (6) and (8) then gives α = ξ1ω1, β = ξ1 ω1 . (9) 4. synthetic accelerograms a finite element analysis of geotechnical structures subjected to earthquake-induced loading requires specifying the time history of free field accelerations. eurocode 8 [11] allows for using either recorded or artificial accelerograms. the artificial accelerograms have to be generated to match the elastic response spectrum defined within eurocode 8. an example of such a spectrum used in the present study is plotted in fig. 2. the algorithm for generating artificial accelerograms utilized in this paper is mostly inspired by the algorithm proposed in [12] and consists of the following steps: 65 v. pavelcová, t. poklopová, m. šejnoha, t. janda acta polytechnica ctu proceedings 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 time period t [s] 0.5 1.0 1.5 2.0 2.5 3.0 3.5 a cc el er at io n a [m / s2 ] ω1ω3 ωrs design response spectrum computed response spectrum figure 2. design response spectrum, computed response spectrum, location of frequencies to calculate parameters of rayleigh damping. (1.) a fourier spectrum with constant spectral amplitudes and random phases is generated. (2.) time history of acceleration is obtained from the spectrum by means of fourier transform. (3.) elastic responses of a single degree-of-freedom systems with natural periods corresponding to frequencies used in the fourier spectrum are computed for the time history. (4.) factors of the computed elastic response acceleration and the design response acceleration are evaluated. (5.) the spectral amplitudes of the original fourier spectrum are divided by the factors while the phases are kept. (6.) steps 2-5 are repeated for the updated fourier spectrum until the computed elastic response spectrum matches the design elastic response spectrum within 10% relative error, see fig. 2. although the generated signal matches the target elastic response spectrum, it is stationary and lacks the characteristic stages of a typical earthquake signal. to take the rise, the strong motion and the decay periods into account, the amplitude of the stationary signal is modulated by an envelope function [13] e(t) = atb exp(−ct), (10) with parameters b = −ε ln η 1 + ε(ln ε− 1) , (11) c = b εtw , (12) a = ( exp(1) εtw )b , (13) where tw denotes the duration of earthquake. peak of the envelope function occurs at fraction ε of the earthquake duration and the original amplitude is reduced to η fraction at time tw . an example of 0.0 2.5 5.0 7.5 10.0 12.5 15.0 17.5 20.0 time t [s] −1.5 −1.0 −0.5 0.0 0.5 1.0 a cc el er at io n a [m / s2 ] stationary modulated figure 3. an example of stationary (original) and modulated (with rise, strong motion and decline stages) accelerograms. stationary and modulated synthetic accelerogram is shown in figure 3. 5. model description two computational models were created to demonstrate the effect of material damping in relation to the selected boundary conditions applied at the bottom of the domain. the first model in fig. 4(a) consists of a soft layer supported by an infinite layer of a relatively stiff bedrock. truncating the model within the rock layer thus requires application of absorbing boundary conditions ensuring that the incoming wave is fully damped and not reflected back into the domain. the second model in fig. 4(b) assumes in turn the bedrock be infinitely stiff thus replacing this layer with the fixed boundary. both layers are assumed linearly elastic with material properties listed in table 1. soil density ρ 1960 [kg/m3] poisson ratio ν 0.40 [-] elastic modulus e 80.00 [mpa] rock density ρ 2170 [kg/m3] poisson ratio ν 0.30 [-] elastic modulus e 630.00 [mpa] table 1. material parameters. as in [3] the tunnel being 6 m in diameter is located in the soil layer at a depth of 37.35 m. the tunnel lining is modeled by linearly elastic beam elements. 6. rayleigh damping parameters as demonstrated in section 3, defining parameters α, β in eq. (5) requires the knowledge of a pair of natural frequencies, eq. (6), or at least of the first natural frequency ω1 should eq. (9) be used. 66 vol. 26/2020 influence of material damping in earthquake analysis point b point a2point a1 0,00 -6,00 -12,00 -18,00 -24,00 -30,00 -36,00 -42,00 -48,00 -54,00 -59,00 0, 00 -1 2, 00 -6 ,0 0 -1 8, 00 -2 4, 00 -3 0, 00 -4 0, 00 36 ,0 0 24 ,0 0 12 ,0 0 30 ,0 0 40 ,0 0 6, 00 (a) point b point a2point a1 0,00 -6,00 -12,00 -18,00 -24,00 -30,00 -36,00 -42,00 -49,00 0, 00 -6 ,0 0 -1 2, 00 -1 8, 00 -2 4, 00 -3 0, 00 -4 0, 00 -6 ,0 0 -1 2, 00 -1 8, 00 -2 4, 00 -3 0, 00 -4 0, 00 36 ,0 0 24 ,0 0 12 ,0 0 30 ,0 0 40 ,0 0 6, 00 (b) figure 4. computational model a) quiet boundary conditions (qbb), b) fixed boundary conditions (fbb). eurocode 8 suggests proportional damping with a single value of ξ = 0.05 (proportional damping of 5%). thus the way of calculating parameters α, β determines, how much will individual natural frequencies be damped. the following three options are examined in this study • type 1: the first natural frequency is assumed to be the least damped frequency. • type 2: the least damped frequency domain is between the first and the third natural frequency. this option results from the recommendation provided in the plaxis user manual [14]. • type 3: the least damped frequency domain is between the first natural frequency and the most dominant frequency ωrs = 12.5664 rad/s of the design response spectrum displayed in fig. 2. the required natural frequencies were found from a standard eigenvalue analysis. the first three natural frequencies corresponding to the shear dominated mode shapes were extracted from the list of calculated frequencies based on the value of a significant modal participation factor [15] for the x direction, recall the coordinate system displayed in fig. 1. three analyses in particular were carried out. the first one assumed the free field column (1dff), the second one adopted the 2d model in fig. 4(a) (2dt) and the third one considered the same model but for the state prior to excavation (2di). in all calculations, the bottom boundary was fixed. in 2d models the lateral boundaries were additionally constrained in the y-direction to recover the mode shapes compatible with the free field column analysis. the results are stored in table 2. the values in parentheses represent the actual order when considering all modes. ω1 ω2 ω3 1dff 3.7547 (1) 11.5307 (3) 20.1191 (4) 2dt 3.7481 (1) 11.4440 (5) 19.5180 (10) 2di 3.7505 (1) 11.3844 (5) 19.4204 (10) table 2. calculated natural frequencies. as can be seen, all analyses provided almost identical results. thus to determine parameters α, β in table 3 the results from 2di analysis were employed. damping ω ξ α β type 1 ω1 0.05 0.1875 0.0133 type 1b ω1 0.10 0.3750 0.0266 type 2 ω1 + ω3 0.05 0.3143 0.0043 type 3 ω1 + ωrs 0.05 0.2888 0.0061 table 3. rayleigh damping parameters. figure 5 shows how the natural frequencies are damped for the three types of damping with ξ = 0.05. figure 5. viscous damping ratio as a function of the natural period (frequency). 7. 1d free field analysis the influence of material damping was first addressed in the light of one-dimensional free-field column analysis. this is in fact also the first step in a twodimensional analysis needed in generating the time variation of free-field velocities and boundary shear stresses. it also allows us to test the influence of finite element mesh, which is in this case quite non-uniform as seen in fig. 4. the 1dff mesh corresponds to the refinement along the edges of the 2d domains and certainly is not optimal neither from the material, nor from the integration time step point view, which was 67 v. pavelcová, t. poklopová, m. šejnoha, t. janda acta polytechnica ctu proceedings set to 0.01 s in accordance with the adopted acceleration record. only the soil layer was assumed to be damped. all the results presented in figs. 6 9 show variation of a relative displacement along the x-direction urx observed on the terrain (point a1). 0 5 10 15 20 time [s] -0,12 -0,08 -0,04 0,00 0,04 0,08 0,12 u r x [ m ] no damp. damp.t 1 acc fixed bc point a1 figure 6. time variation of horizontal displacement urx monitored at point a1. blue line is the scaled variation of the prescribed acceleration. figure 6 compares variation of damped and undamped displacement urx considering the fixed boundary conditions, fig. 4(b). the scaled acceleration is added just to illustrate the time interval of strong motion. the results suggest that appropriate damping is absolutely essential in case of fixed boundary, because in undamped model the vertically propagating shear waves reflect not only from the terrain free boundary but also from the fixed bottom boundary. therefore, in the undamped model the motion does not cease even after the input signal diminishes. the influence of the selected type of damping, recall section 6 and table 3, is examined in figs. 7 and 8. while some differences in the model response can be observed in fig. 8 in the case of qbb, the results plotted in fig. 7 considering fbb suggest almost negligible effect of the type of damping, at least in the present example. it is also seen in fig. 8 that unlike the fixed boundary, the influence of damping in the case of qbb is far less pronounced. this is attributed to the fact that the downward traveling wave is intentionally damped at the bottom boundary having the same effect as allowing the downward traveling waves to leave the examined model at the truncated boundary, so the motion eventually stops even without material damping. to directly compare the results derived for fbb and qbb we first plot in fig. 9 the time variation of the horizontal displacement urx monitored at various points to see that the displacement at the interface of the two materials practically coincides with the motion monitored at the bottom boundary. this indicates that the vertically propagating wave travels very fast through the rock layer so the difference between the two models dimensions in fig. 4 can safely be neglected. this comparison of the response of damped model with fbb and undamped model with qbb is plotted in fig. 10. 0 5 10 15 20 time [s] -0,10 -0,05 0,00 0,05 0,10 0,15 u r x [ m ] no damping damp. type 1 damp. type 2 damp. type 3 point a1 fixed bc figure 7. influence of the type damping fbb. 0 2 4 6 8 10 time [s] -0,04 -0,02 0,00 0,02 0,04 0,06 u r x [ m ] no damping damping type 1 damping type 2 damping type 3 point a1 quiet bc figure 8. influence of the type damping qbb. it can be observed that even with 5% damping, the model with fixed bottom boundary yields nonnegligible displacements even beyond 20 s of monitoring where the prescribed motion comes to zero. the figure also shows the importance of correctly prescribing only the incoming wave for the model with qbb. adopting the total motion would yield the displacement at the terrain exceeding the motion found in the model with fbb, which is unrealistic. 8. 2d analysis of tunnel taking into account the results discussed in the previous section the 2d analyses were limited to the qbb model in fig. 4(a) assuming no damping, while only the damping type 1 was considered when examining the fbb model in fig. 4(b). for both models, the horizontal displacements plotted in fig. 11 were monitored at points a2 and b. similar conclusions to what has been observed in section 7 can be drawn here suggesting again the need for material damping with fixed boundary. because choosing the particular value of damping ratio has mostly no rigorous support, we compared the results pertinent to 5% of damping to those derived for 10% of damping. again the 5% of damping still overestimates the displacements at the end of earthquake, while the results found for 10% of damping resemble the response of qbb model at least at the terrain boundary. in the vicinity of tunnel lining, the motion is, however, overdamped. 68 vol. 26/2020 influence of material damping in earthquake analysis 0 5 10 15 20 time [s] -0,04 -0,02 0,00 0,02 0,04 0,06 u r x [ m ] terrain soil layer mid point interface bottom no damping point a1 figure 9. time variation of horizontal displacement urx monitored at various points qbb. 0 5 10 15 20 time [s] -0,10 -0,05 0,00 0,05 0,10 0,15 u r x [ m ] fixed bc (damping type 1) quiet bc (no damp. total motion) quiet bc (no damp. incom. wave) point a1 figure 10. comparison of damped model with fbb and undamped model with qbb. a smooth variation of damped displacements also suggests that particularly high frequencies of the motion are damped, which agrees well with fig. 5. another observation follows from a close inspection of figs. 10 and 11(a) showing only a minor difference between the response provided by 1dff and 2dt analyses, mainly because of a relatively stiff lining. this thus supports the results obtained already from modal analysis in section 6. 9. conclusions the present paper examined the influence of material damping in earthquake analysis particularly with reference to the boundary conditions prescribed at the bottom boundary of the model. the following conclusions can be proposed: • considering material damping in case of qbb has only a minor effect on the predicted response. • on the other hand, the fbb model should not be considered without material damping as rather erroneous results can be obtained. • the way of determining the rayleigh damping parameters has, at least in the present study, a relatively minor effect. acknowledgements the support provided by the sgs project no. sgs19/032/ohk1/1t/11 and the tačr project no. 0 5 10 15 20 time [s] -0,10 -0,05 0,00 0,05 0,10 u r x [ m ] quiet bc ξ=0% fixed bc ξ=5% fixed bc ξ=10% point a2 (a) 0 5 10 15 20 time [s] -0,03 -0,02 -0,01 0,00 0,01 0,02 u r x [ m ] quiet bc ξ=0% fixed bc ξ=5% fixed bc ξ=10% point b (b) figure 11. time variation of relative displacement at selected points during the period of strong motion: a) point a, b) point b. te01020168 is gratefully acknowledged. references [1] o. zinkiewicz, n. bicanic, f. shen. generalized smith boundary a transmitting boundary for dynamic computations, vol. 207. innme swansea, 1986. doi:10.1007/978-3-7091-2828-2_3. [2] a. ashmawy, r. salgado, s. guha, v. drnevich. soil damping and its use in dynamic analyses. in third international conferences on recent advances in geotechnical earthquake engineering and soil dynamics. missouri university of science and technology, 1995. paper no. 1.13. [3] v. pavelcová, t. poklopová, t. janda, m. šejnoha. the influence of boundary conditions on the response of underground structures subjected to earthquake. acta polytechnica, ctu proceedings 15:74–80, 2018. doi:10.14311/app.2018.15.0074. [4] j. pruška, m. šejnoha, t. janda, et al. analysis of underground structures subjected to earthquake using finite element method. in 14th international conference entitled "underground construction prague 2019". ita-aites czech tunnelling association, 2019. june 3-5, prague. [5] i. towhata. geotechnical earthquake engineering. springer-verlag berlin heidelberg, 2008. [6] s. badsar, m. schevenels, w. haegeman, et al. determination of the material damping ratio in the soil from sasw tests using the half-power bandwidth method. geophysical journal international 182:1493– 1508, 2010. doi:10.1111/j.1365-246x.2010.04690.x. 69 https://doi.org/10.1007/978-3-7091-2828-2_3 https://doi.org/10.14311/app.2018.15.0074 https://doi.org/10.1111/j.1365-246x.2010.04690.x v. pavelcová, t. poklopová, m. šejnoha, t. janda acta polytechnica ctu proceedings [7] a. zidan. modeling of soil damping for seismic ground response by nonlinear finite element analysis. in third international conference on advances in civil and structural engineering cse. institute of research engineers and doctors, usa, 2015. [8] v. pavelcová. evaluation of real underground structure subjected to earthquake fully dynamic analysis. bachalor’s thesis, czech technical university in prague, faculty of civil engineering, 2018. in czech. [9] o. řeřicha, z. bittnar. metoda konečných prvků v dynamice konstrukcí. nakladatelství technické literatury, 1981. [10] z. bittnar, j. šejnoha. numerical methods in structural engineering. asce press, 1996. [11] comité européen de normalisation. eurocode 8: design of structures for earthquake resistance, 2004. [12] a. kumar. software for generation of spectrum compatible time history. in proceedings of 13th world conference on earthquake engineering. canadian association for earthquake engineering, 2004. [13] d. boore. stochastic simulation of high frequency ground motion based on seismological models of radiated spectra. bulletin of seismological society of america 73(6):1865–1894, 1983. [14] plaxis manual, http://www.plaxis.nl/. [15] t. irvine. effective modal mass and modal participation factors revision h, 2013. 70 acta polytechnica ctu proceedings 26:64–70, 2020 1 introduction 2 governing equations 3 rayleigh damping 4 synthetic accelerograms 5 model description 6 rayleigh damping parameters 7 1d free field analysis 8 2d analysis of tunnel 9 conclusions acknowledgements references doi:10.14311/app.2021.30.0126 acta polytechnica ctu proceedings 30:126–130, 2021 © czech technical university in prague, 2021 available online at http://ojs.cvut.cz/ojs/index.php/app automated bim entity reconstruction from unstructured 3d pointclouds jan voříšek∗, bořek patzák, edita dvořáková, daniel rypl czech technical university in prague, faculty of civil engineering, department of mechanics, thákurova 7, 166 29 prague 6, czech republic ∗ corresponding author: jan.vorisek@fsv.cvut.cz abstract. laser scanning is used widely in architecture and construction to document existing buildings by providing accurate data for creating a 3d model. the output is a set of data points in space, so-called point cloud. while point clouds can be directly rendered and inspected, they do not hold any semantics. typically, engineers manually obtain floor plans, structural models, or the whole bim model, which is a very time-consuming task for large building projects. in this contribution, we present the design and concept of a pointcloud2bim library [1]. it provides a set of algorithms for automated or user assisted detection of fundamental entities from scanned point cloud data sets, such as floors, rooms, walls, and openings, and identification of the mutual relationships between them. the entity detection is based on a reasonable degree of human interaction (i.e., expected wall thickness). the results reside in a platform-agnostic json database allowing future integration into any existing bim software. keywords: 3d model, bim, laser scanning, point cloud. 1. introduction the building information modelling (bim) brings excellent opportunities for the efficient and more automated design process of building structures and maintenance and reconstruction planning of existing structures. obtaining a digital bim representation of the latter is often a challenging task, namely when no up-to-date (or none at all) floor plans are available. laser scanning is a convenient and accessible technology to document the existing infrastructure. the output is a dense set of data points in space, socalled point cloud. see figure 1 for an example of a laser-scanned building. each point carries its spatial coordinates, and some scanners also capture the rgb color value and intensity of the scanned point. while point clouds can be directly rendered and inspected, they carry no additional information about the scanned objects. there are no available tools to convert the raw point data into a bim representation with a reasonable degree of automation, although research in this field is highly emerging [2, 3]. currently, the civil engineers have to manually identify the bim entities from graphical visualization of point cloud and create their digital representation in bim model. the long-term goal of the project is to develop an efficient and user friendly tool for semiautomated conversion of point clouds to bim entities while maintaining user interaction. the idea is based on automated identification of entities, where each entity is labeled with estimated degree of identification reliability. the individual identified entities are presented to the user who can accept all suggested entities recognized with reliability higher than defined threshold and manually accepting/modifying remainfigure 1. laser scan of an unfurnished office building, consisting of 85m points, top view. ing suggestions. the presented paper describes the concept of pointcloud2bim library [1] for point cloud manipulation and bim entity identification. 2. pre-processing of the point clouds laser scanners produce very dense point clouds containing tens or hundreds of millions of points. the distance between the scanned points often ranges in millimeters. therefore the point cloud data processing must be handled carefully in order to achieve competitive computation times and memory efficiency. 126 http://dx.doi.org/10.14311/app.2021.30.0126 http://ojs.cvut.cz/ojs/index.php/app vol. 30/2021 automated bim entity reconstruction figure 2. wide figure. 2.1. storage point information such as the xyz coordinates, rgb color, or intensity value is stored in an uncompressed binary file. the main advantage is that binary data can be directly read into c++ structures instead of the costly parsing of text files line-by-line. all the other project data reside in the project’s json file, which serves as a database of point cloud metadata and other entities. the json format is widely supported across platforms and programming languages and was selected to allow smooth integration into existing software. an empty project file is created automatically before the first point cloud import. 2.2. downsampling while storing the points as binary data offers high reading speeds, with hundreds of millions of points, reading all the values into memory might not be possible on a standard computer with limited memory. for example, storing 100 million xyz coordinates (3x64 bit float) occupies 2,4 gb of memory, which is more than half of a commonly available 4gb. reducing the number of points to work with is inevitable. in our case, the points were downsampled in a 3d voxel grid using the efficient implementation of a c++ hash map [4]. reasonable selection of a grid step of say 1 × 1 × 1 cm reduces all the points within the 1 cm3 cube to a single point with weighted coordinates. result of downsampling to a 1 × 1 × 1 grid is summarized in the table 1. # before after 1 85 316 410 12 358 178 2 62 947 576 20 213 992 3 108 200 686 34 672 667 table 1. the effect of downsampling on three typical point clouds — the number of original points and the number of points after downsampling. one can see that just by downsampling to a 1 cm3 grid, the size of points goes down to at least 30%. 2.3. the normal vector of downsampled points further processing can take advantage of determining the normal vector at each downsampled point (voxel). this is done in the three following steps. a single point represents each voxel by averaging the coordinates of the inner points. there are up to 26 (3 × 3 × 3 − 1) voxels surrounding each voxel if all the neighbors are activated. therefore, the total number of points n for the normal evaluation is at most 27. the centroid of the n points is calculated as a weighted average of the xyz coordinates. µx = !n i=1 xi n , µy = !n i=1 yi n , µz = !n i=1 zi n , (1) the 3 × 3 covariance matrix of the point xyz coordinates relative to the centroid is calculated using the formula for the covariance of a and b below. cov(a, b) = !n i=1(ai − µa)(bi − µb) n − 1 (2) the smallest eigenvector of the covariance matrix is the plane normal [5]. the direct solver of eigenvalues was implemented for the 3 × 3 matrix using the arccos function, as described in [6]. 3. bim entity extraction the overall strategy of point cloud transformation into the digital bim representation of the building is based on a gradual segmentation of floors, rooms, walls, and openings. the overview of this complex process is demonstrated in figure 2. 3.1. floor segmentation the floor segmentation is based on the histogram of vertical coordinates. from the histogram, the ceiling and floor levels are extracted, corresponding to locations with significant location frequency. more accurate results can be obtained by only considering points with vertical normal in the calculation. the ceiling level can also be input manually by inspecting the point cloud in a suitable tool like autodesk recap. 127 j. vorisek, b. patzák, e. dvořáková, d. rypl acta polytechnica ctu proceedings figure 3. results of the room segmentation of the point cloud shown in figure 1. 3.2. room segmentation the ceiling level of the floor is known either from the histogram calculation or from user input. all the ceiling plane points and points within 30 cm below (as suggested in [3]) are projected onto a 2d grid (floor plan) resulting in a pixel-like image. the step of the grid should be at least twice the step used for the initial downsampling, or the grid will contain missing points as some points may not be appropriately captured. region growing [7] is applied to join adjacent pixels into separate regions. backfilling of each of the found regions is performed to include columns and skylights within the room. knowing the pixel count and pixel area (step x step), area of the subject rooms or whole floor can be estimated. finally, the original point cloud is iterated, and points inside of the found regions (in terms of x, y coordinates) and floor (given z coordinate range) are selected. separate point clouds of each room are stored in the project file. an example of room segmentation results can be seen in figure 3. 3.3. vertical plane segmentation the wall planes are identified in each room using the modified random sample consensus (ransac) [8]. only points with horizontal normal are considered for plane finding, a slight deviation of 5 degrees is allowed to account for deviations in construction. each wall finding starts with hypothesis generation. three non-collinear points a, b, c are randomly selected from the point cloud for which the four plane coefficients a, b, c, and d are calculated using the two vectors !ab and !ac !ab = (bx − ax, by − ay, bz − az) (3) !ac = (cx − ax, cy − ay, cz − az) (4) figure 4. vertical plane segmentation (top view). door planes are segmented as they cover sufficient area, also notice the noise around outer windows and in the center room. to satisfy the plane equation ax + by + cz + d = 0, plane coefficients are calculated as follows a = (by − ay)(cz − az) − (cy − ay)(bz − az) (5) b = (bz − az)(cx − ax) − (cz − az)(bx − ax) (6) c = (bx − ax)(cy − ay) − (cx − ax)(by − ay) (7) d = −(aax + bay + caz) (8) after establishing the hypothesis, more points are selected randomly, and the distance from an arbitrary point to the plane is calculated. points within sufficient distance are considered in the support set. for the first 100 accepted points, the plane coefficients are refined using the algorithm described in section 2.3. 3.4. wall segmentation wall segmentation is performed on each of the vertical planes found in the previous step. only walls of constant thickness are supported. therefore, comparing the angle between every two planes and the distance between the two subject planes is sufficient to mark wall segment candidates. for each wall candidate, its points are projected to a single 2d plane. such a transformation can be done simply without the transformation matrix. the normal vector is taken from plane segmentation by calculating the average value of all wall planes. local x-axis is selected by picking two plane points a, b, and clipping their vector ’s z coordinate. by knowing the local x-axis and the normal vector, the y-axis vector can be calculated using the cross product. this is sumarized below. xloc = (bx − ax, by − ay, 0) (9) yloc = n × xloc (10) 128 vol. 30/2021 automated bim entity reconstruction figure 5. wall segmentation (top view). the wall local axes xloc and yloc are then normalized, and for each wall point p, the local coordinates are calculated using the dot product. px,loc = (p − a) · xloc (11) py,loc = (p − a) · yloc (12) plane points lie in two intervals [a, b] and [c, d] in the local x-axis direction. the distance o in the wall direction in which the planes are overlapping is calculated o = [max(a, c), min(b, d)] wall candidates are filtered based on the user’s overlap requirements, as partially overlapping planes are likely to belong to a common wall. 3.5. opening segmentation a naive way of finding openings in walls (on two parallel planes) is to find overlap of missing points on both planes. however, such treatment produces a lot of false results. for example, two cabinets on each side of the wall in the same place do not form an opening. points present between the walls can be taken into consideration to improve the accuracy of opening detection. by a combination of inverse region growing and boundary detection, openings with a sufficient density of intermediate points can be identified. by calculating the bounding box of an opening in the local plane coordinates, dimensions can be calculated for rectangular openings (i.e., doors and some windows). doors are recognized by comparing their bottom coordinates to the corresponding floor level. opening visualized by their bounding box can be observed in figure 6. figure 6. opening detection — doors (red), windows (blue). 4. demonstration on a sample point cloud the example of a simple office building with just a single floor with eight rooms is selected to demonstrate the capabilities of the presented approach. as the whole floor was unfurnished, the overall floor envelope has little to none missing points except for the areas behind doors and heating elements. the original data set consists of 85 316 410 points is shown in fig. 1. at first, the original point cloud was downsampled into 12 358 178 using a 1×1×1 cm grid. by inspecting the point cloud, we identify the floor and ceiling levels, and we use all the points in between ceiling level and 30 cm below for room finding. the room finding performed on a 2d floor plan grid with a 2×2 cm step yields all the eight identified rooms, visualized in figure 3. in the same step, we also identify all the 93 vertical planes, as can be seen in fig. 4. the planes assigned to their source rooms are then passed to the next step for wall matching even if some of the identified planes might be out of our interest (i.e., doors or noise). in figure 5, we can observe all the matched walls on the subject floor. while all the inner walls were identified, the exterior walls are missing as the exterior scan of the office building was not available. however, some of the external walls are obtained thanks to the scan capturing the window frames’ internal and external planes. these reveal the estimated window positions as well as exterior wall location, excluding its wall thickness. 5. conclusions the library was implemented in c++ based on the previously formulated methodology. it can be inte129 j. vorisek, b. patzák, e. dvořáková, d. rypl acta polytechnica ctu proceedings grated into any bim software to assist the conversion of point clouds into digital bim representation. the library api is documented in the reference manual [9]. the library also works as a standalone toolkit without any dependency on bim software. a set of command-line executables is available to perform the individual steps introduced above. each of the tools operates on top of a common database-like file (json) with point data stored separately in binary files. a single floor of an unfurnished office building was successfully segmented into separate rooms (compare figures 1 and 3). plane matching results can be seen in figure 4 with the 25 distinct wall segments considering the exterior windows as walls. the success of opening detection can be examined in figure 6 where six doors were identified and eight exterior windows (two of which are found twice). 6. future work our main goal is to create a robust and easy-tointegrate solution of converting the raw point cloud files into the corresponding bim entities. while our library provides enough functionality to process, analyze, and visualize the point clouds and calculated results, more research is desired, especially in the area of opening detection and overall confidence rate evaluation of the reconstructed bim entities. further research is needed to integrate the library into some of the industry-leading bim applications (i.e. autodesk revit). such integration is required to validate and refine our methodology using a variety of point clouds from different environments. acknowledgements the development of this software was supported by the technology agency of the czech republic under the program of the national competence center 1 as a project “center for advanced materials and efficient buildings” (cameb) project registration no. tn01000056 and by the grant agency of the czech technical university in prague (sgs project no. sgs20/038/ohk1/1t/11), both gratefully acknowledged. references [1] pointcloud2bim library for automated identification of bim entities from point clouds. http://mech2018.fsv.cvut.cz/pc2bim. accessed: 2020-08-06. [2] c. wang, y. cho, c. kim. automatic bim component extraction from point clouds of existing buildings for sustainability applications. automation in construction 56, 2015. doi:10.1016/j.autcon.2015.04.001. [3] h. macher, t. landes, p. grussenmeyer. from point clouds to building information models: 3d semi-automatic reconstruction of indoors of existing buildings. applied sciences 7:1030, 2017. doi:10.3390/app7101030. [4] g. popovitch. the parallel hashmap or abseiling from the shoulders of giants. https://greg7mdp.github.io/parallel-hashmap/. accessed: 2020-04-14. [5] e. ernerfeldt. fitting a plane to noisy points in 3d. https://www.ilikebigbits.com/2017_09_25_plane_ from_points_2.html, 2017. accessed: 2020-04-14. [6] s. hartmann. computational aspects of the symmetric eigenvalue problem of second order tensors. technische mechanik 23:283–294, 2003. [7] m. montoya, c. gil, i. garcía fernandez. implementation of a region growing algorithm on multicomputers: analysis of the work load balance., 2000. [8] m. a. fischler, r. c. bolles. random sample consensus: a paradigm for model fitting with applications to image analysis and automated cartography. communications of the acm 24(6):381–395, 1981. doi:10.1145/358669.358692. [9] j. voříšek, d. rypl, b. patzák, e. dvořáková. pointcloud2bim library: pointcloud2bim library reference manual. http://mech2018.fsv.cvut.cz/ pc2bim/www/refman/html/index.html. 130 http://mech2018.fsv.cvut.cz/pc2bim https://doi.org/10.1016/j.autcon.2015.04.001 https://doi.org/10.3390/app7101030 https://greg7mdp.github.io/parallel-hashmap/ https://www.ilikebigbits.com/2017_09_25_plane_from_points_2.html https://doi.org/10.1145/358669.358692 http://mech2018.fsv.cvut.cz/pc2bim/www/refman/html/index.html acta polytechnica doi:10.14311/app.2020.27.0057 acta polytechnica 27(0):57–61, 2020 © czech technical university in prague, 2020 available online at http://ojs.cvut.cz/ojs/index.php/app laser-induced surface acoustic waves for thin film characterization radim kudělkaa, lukáš václaveka, jan tomáštíkb, sabina malecováa, radim čtvrtlíkc,∗ a joint laboratory of optics, palacky university olomouc, 17. listopadu 50a, 772 07, olomouc, czech republic b institute of physics of the czech academy of sciences, joint laboratory of optics of palacky university and institute of physics as cr, 17. listopadu 12, cz-771 46, olomouc, czech republic c regional centre of advanced technologies and materials, joint laboratory of optics of palacky university and institute of physics of the czech academy of sciences, faculty of science, palacky university, 17. listopadu 12, 77146 olomouc, czech republic ∗ corresponding author: radim.ctvrtlik@upol.cz abstract. knowledge of mechanical properties of thin films is essential for most of their applications. however, their determination can be problematic for very thin films. law (laser-induced acoustic waves) is a combined acousto-optic method capable of measuring films with thickness from few nanometers. it utilizes ultrasound surface waves which are excited via short laser pulses and detected by a pvdf foil. properties such as young’s modulus, poisson’s ratio and density of both the film and the substrate as well as film thickness can be explored.results from the law method are successfully compared with nanoindentation for young’s modulus evaluation and with optical method for film thickness evaluation and also with literature data. application of law for anisotropy mapping of materials with cubic crystallographic lattice is also demonstrated. keywords: anisotropy, laser-induced surface acoustic waves, nanoindentation, thin films, young’s modulus. 1. introduction thin films and coatings have become an essential part of our everyday lives and knowledge of their mechanical properties and durability directly affects their lifetime regardless of their primary role. thin films are often exposed to external stresses during their fabrication or service life. therefore, their reliable testing is essential for both their research and development as well as industrial production. nanoindentation has become the standard method for determination of local mechanical properties, but it has its inherent limitations [1]. mostly the thickness of the film is the main limiting factor, because if the indenter penetrates to a greater depth than 10 % of the film (commonly used rule of thumb), then substrate may affect the hardness values considerably [2–4]. determining the elastic modulus of very thin films is even more problematic because the substrate and the film behave like two springs in series hence the elastic modulus of the film is always affected by the substrate [5]. this means that the indentation of films with thickness under 50 nm is very challenging [6]. however, mechanical properties of very thin films can still be explored using the methods based on analysis of the dispersion curves of rayleigh surface acoustic waves. one of such methods is the law (laser-induced acoustic wave) technique [7, 8]. it is a combined acousto-optic method developed for the assessment of material characteristics of surfaces (young’s modulus, poisson’s ratio, density, film thickness) based on the generation of surface acoustic waves and their detection. the propagation of these waves in the material depends on the elastic properties of both the thin film and the substrate. in order to demonstrate the possibilities and strengths of the law, the representative results from thin films and single crystals are presented. in case of thin films the results are compared with the results from other methods such as nanoindentation for young’s modulus evaluation or optical methods for film thickness characterization. besides, the limit of the film thickness is explored, as the ability to characterize very thin films is one of the main advantages of law over other methods. 1.1. principle of law short laser pulses from nitrogen laser are absorbed by the material and cause generation of a wide frequency spectrum (typically in the range from units to hundreds of mhz) of thermally activated surface acoustic waves (saw). those waves are propagating from the point of incidence and mostly in the surface layers of the sample as their amplitude decays exponentially with the increase of the distance from the surface [7] as shown in the figure 1. the penetration depth hpen of the saw can be estimated by their wavelength (hpen ≈ λ). this 57 http://dx.doi.org/10.14311/app.2020.27.0057 http://ojs.cvut.cz/ojs/index.php/app r. kudělka, l. václavek, j. tomáštík et al. acta polytechnica amplitude de pt h film substrate low frequency, high wavelength (a) low frequency, high wavelength figure 1. propagation and amplitude-decay of saw with a) low frequency and b) high frequency means that the penetration depth depends on the frequency f(λ = v/f ). the greater the frequency the lower the penetration depth. because most of the wave energy is concentrated on the surface, saw are very sensitive to very thin films with thicknesses from few nanometers (even when they are much thinner than the penetration depth of saw) [8]. the phase velocity v of saw in a non-coated homogeneous and isotropic material is independent on frequency and can be described as: ν = 0.87 + 1.12ν 1 + ν √ e 2ρ(1 + ν) , (1) where ν is poisson’s ratio, ρ is material density and e is young’s modulus. this means that phase velocity is directly linked to the elastic modulus, poisson’s ratio and density of the material. however, situation is different in coated materials, where the penetration depth depends on frequency so the phase velocity of the saw must as well. this phenomenon is called the dispersion. in case of a substrate with low phase velocity coated with a thin film with a high phase velocity, then the phase velocity for the film/substrate system increases with frequency. for explanation let’s assume infinite substrate and saw propagating across the surface. theoretically for the wave with frequency approaching zero, the penetration depth would be approaching infinity. that in turn means that the contribution from the thin film would be negligible. on the contrary, if the saw would have infinitively high frequency, then the wave would be restricted only to the very top of the sample, in other words the wave would propagate only in the film according to the eq. (1). the precise relation between the frequency f and phase velocity v of the saw on coated materials depends on elastic properties and densities of both the substrate and the thin film as well as on the thickness of the film. this means that if the dispersion curve and some of the variables are known, the rest of them can be calculated. in order to get the dispersion curve the saw must be detected in various known distances from the point of incidence using a pvdf foil which transforms the mechanical vibrations into electric signal, see principal scheme of law method (instrument) in figure 2. the dispersion curve is then calculated using the following equation: ν(f) = 2πf (x2 − x1) φ2(f) − φ1(f) , (2) where x1 and x2 are two different distances between the sensor and the point of incidence, φ1(f) and φ2(f) are the phase values of the surface wave frequency f . the whole experimental set-up can be seen in figure 2. substrate laser oscilloscope saw v pvdf foil substrate figure 2. schematic representation of the law set-up. 1.2. experimental details the experiments were performed on a laser acoustic test system lawave (fraunhofer institute, dresden, germany). acoustic waves with frequency ranging from ∼ 1 mhz up to ∼ 250 mhz were generated using a 337 nm n2 laser with a 0.5 ns pulse duration and 58 vol. 27/2020 laser-induced surface acoustic waves for thin film characterization 35 93 142 163 34 90 136 156 0 50 100 150 200 1 min 60 min 120 min 180 min fi lm t h ic kn es s [n m ] law optical figure 3. film thickness of samples from dry thermal growth measured by law compared to results from optical method. 400 µj pulse energy. the signal from the pvdf foil was averaged from 64 measurements for each of the 5-7 measuring points along the 15 mm and 20 mm measuring path for glass and silicon substrates respectively. all measurements were done at a room temperature. 2. results and discussion 2.1. thin film characterization – film thickness evaluation silicon dioxide thin films on si(100) were used for the film thickness characterization. it is well known that high quality sio2 films are prepared by dry thermal growth which is considerably slower than for example wet thermal growth but yields high quality films – the oxide film is more uniform and its density is higher. this allows for more precise and reliable comparison of law and other methods as these sio2 films represent an ideal model system. the si substrates were oxidized at 1000 ◦c in quartz tube for 1, 60, 120 and 180 minutes, the heating rate was 20 ◦c/min. the resulting film thickness can be seen in figure 3. the optical reference measurement of the film thickness was performed on spectral reflectometer calibrated on polished silicon wafer with known profile of spectral reflectance. results from both methods are similar with the results from law being slightly higher in all cases. the highest percentage difference (4.6%) is for the thinnest sample. the standard deviation for law is very small. thus, it may be concluded that law gives reasonable results for film thickness if young’s modulus and density are uniform and known. the native sio2 oxide layer (grown at room temperature) was measured as well in order to find out how accurate can law be for very thin films. its thickness, when kept in dry pure oxygen, is around ∼1 nm [9], but thickness around 1.5-2.5 nm can be expected when exposed to air depending on the temperature and humidity with 2.1 nm given in [9]. since law is very sensitive to the top surface conditions as well as presence of (sub)surface flaws [10] and any structural imperfections, different si(100) wafers of the same type and doping 65 67 65 66 101 85 67 68 0 20 40 60 80 100 120 34 nm 90 nm 136 nm 156 nm yo u n g’ s m o d u lu s [g p a] law nanoindentation figure 4. young’s modulus of sio2 thin films with different thickness measured by law and nanoindentation. were explored (university wafer, usa). this was confirmed by a strong variance in measured thickness of native sio2 film between different wafers. the native sio2 thickness of (3.2 ± 0.5) nm was obtained after the exclusion of the most extreme values that might be among others related to the sub-surface damage. these values are slightly higher in comparison with the literature data. this overestimation might be caused by the silicon suboxide interface layer formed between the si and sio2, with thickness estimated to be ∼0.35 nm [11]. the interface behaves like a second thin film and might increase the resulting film thickness since in our fit only single film system with characteristics of sio2 is assumed. this means that even few nm thick films may be explored via law method provided the dispersion caused by the film can be easily seen and substrate of superb quality, with no surface and sub-surface damage, is used and effects of interface layers are included in the calculation. 2.2. thin film characterization – young’s modulus evaluation in order to explore the inherent limits of depth sensing indentation and law the sio2 films with different thicknesses were measured by both methods. the high quality sio2 films with thickness ranging from 34 to 156 nm grown on si(100) mentioned in previous chapter were used. nanoindentation experiments were performed using the fully calibrated nanotest instrument equipped with a brand new berkovich indenter. at least 15 measurements for each film were analyzed by standard oliver-pharr method. the indentation modulus e values were calculated using sio2 poisson’s ratio of 0.17 and considering the correction for diamond elastic deformation (poisson’s ratio of 0.07 and young’s modulus of 1141 gpa). it should be noted that indentation modulus can be considered as young’s modulus as long as pile-up and sink-in effects are absent [12]. comparison of the results can be seen in figure 4. figure 4 shows that the values obtained from law and nanoindentation are similar for the 156 nm and 136 nm thick films because nanoindentation was not strongly affected by the substrate. however, for the 90 nm film the indenter penetrated up to 25 % of 59 r. kudělka, l. václavek, j. tomáštík et al. acta polytechnica 120 80 40 160 120 200 80 40 0 200 160 y o u n g ’s m o d u lu s [g p a ] (a) p o is so n ’s r a tio 0.00 0.30 0.25 0.20 0.15 0.10 0.05 0.10 0.15 0.20 0.25 0.30 0.05 (b) figure 5. si(100) anisotropy (a) young’s modulus (b) poisson’s ratio. the film and the results are strongly affected by the substrate which has higher young’s modulus than the film. the 34 nm thick film shows even stronger substrate effect as 59 % of the film was penetrated and the resulting value of young’s modulus is somewhere in the middle between the value of the film and the value of the substrate. this clearly shows the unambiguous advantage of the law as the results obtained from law are constant despite the changing film thickness as opposed to nanoindentation. this is crucial for the characterization of very thin films. law can be affected by the film thickness as well, but only in the case of much thinner films as shown in the previous chapter reporting on characterization of native sio2 layer of ∼2 nm thickness. comparison in figure 4 shows that the elastic modulus values obtained by nanoindentation are almost identical to those measured by law as long as the film thickness is high enough. in this case for the films thinner than 100 nm the unavoidable substrate effect leads to overestimation of the elastic modulus. for such thin films law can be viable alternative. in order to present the robustness and universal use of law other films were also explored. young’s modulus was evaluated for aluminum film on glass, and titanium and chromium on silicon. the thickness of all three films was around 100 nm. values obtained by law are compared with the values given by other authors [13–17] and in case of al and ti also with the results from nanoindentation. table 1 shows that, the values of young’s modulus obtained by law correspond to those obtained from nanoindentation and are within the range reported in literature. 2.3. silicon anisotropy mapping single-crystal silicon is a well-known material with interesting properties and wide range of applications [18] that exhibits a strong anisotropy. therefore, it is an ideal object for testing the law ability to explore material anisotropy. material young’s modulus (gpa) law reference indentation al 63.5 55-81 69.6 ti 116.1 102-123 111.5 cr 204.4 185-215 – table 1. comparison of young’s modulus values of al, ti, cr thin film measured by law, nanoindentation and literature data in general, elastic behavior of cubic crystals may be fully described by three elastic stiffness constants c11, c12 and c44 (for silicon at room temperature c11 = 165.6–165.7, c12 = 63.9, c44 = 79.5–79.6 gpa [18, 19]). young’s modulus and poisson’s ratio can be calculated in any direction from those constants. for example, si(100) has young’s modulus ranging from 130.2 to 168.9 gpa [20], the minimal value is for the [100] direction and can be calculated as: e[100] = c11 − 2 c12 c11 + c12 c12. (3) in this work, the elastic stiffness constants were measured in various directions on several si (100) wafers. the resulting average values were c11 = 165.4, c12 = 63.5 and c44 = 79.6 gpa that is in excellent agreement with literature values listed above. using these constant it is possible to plot a polar dependence of young’s modulus and poisson’s ratio. figure 5 clearly shows the strong anisotropy of silicon elastic constants. using law, anisotropy of any cubic crystals can be explored and elastic constants can be calculated for any other plane orientations. 3. conclusions law is a combined acousto-optic method developed for assessment of material characteristics of surfaces and can be used to obtain values of young’s modulus, poisson’s ratio and density for both the film as well as 60 vol. 27/2020 laser-induced surface acoustic waves for thin film characterization the substrate. it can also be used to obtain the values of elastic stiffness constants for anisotropic materials with cubic symmetry. films’ thicknesses measured via law are very close to those from optical thickness measurements if input parameters are precisely known. young’s modulus measurements by law and by nanoindentation yielded same results for films with thickness over 100 nm, however as the indentation depth / film thickness ratio increased the data from nanoindentation started to be affected by substrate as the predicted by the commonly used 10 % rule of thumb (indenter should not penetrate deeper than to the 10 % of the film). acknowledgements the authors gratefully acknowledge the support from the project iga_prf_2019_008 of palacky university and the project th03020245 of the technology agency of the czech republic and the operational program research, development and education, project no. cz.02.1.01/0.0/0.0/17_049/0008422 of the ministry of education, youth and sports of the czech republic. references [1] w. oliver, g. pharr. measurement of hardness and elastic modulus by instrumented indentation: advances in understanding and refinements to methodology. journal of materials research 19(1):3–20, 2004. doi:10.1557/jmr.2004.19.1.3. 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[20] m. a. hopcroft, w. d. nix, t. w. kenny. what is the young’s modulus of silicon? journal of microelectromechanical systems 19(2):229–238, 2010. doi:10.1109/jmems.2009.2039697. 61 https://doi.org/10.1557/jmr.2004.19.1.3 https://doi.org/10.1016/s0257-8972(97)00522-7 https://doi.org/10.1016/0020-7683(88)90091-1 https://doi.org/10.1557/jmr.2009.0102 https://doi.org/10.1007/978-1-4757-5943-3 https://doi.org/10.1557/jmr.1999.0299 https://doi.org/10.1016/s0257-8972(01)01664-4 https://doi.org/10.1016/s0257-8972(96)03147-7 https://doi.org/10.1007/s12633-015-9366-8 https://doi.org/10.1016/j.mee.2006.11.001 https://doi.org/10.1116/1.2218865 https://doi.org/10.3403/bseniso14577 https://doi.org/10.3390/coatings7090143 https://doi.org/10.1016/0040-6090(95)06990-9 https://doi.org/https://doi.org/10.1016/j.tsf.2005.02.024 https://doi.org/10.1016/j.surfcoat.2012.02.039 https://doi.org/10.1016/s0040-6090(96)09029-3 https://doi.org/10.1107/s1600577514004962 https://doi.org/10.1007/978-3-642-59497-7_157 https://doi.org/10.1109/jmems.2009.2039697 acta polytechnica 27(0):57–61, 2020 1 introduction 1.1 principle of law 1.2 experimental details 2 results and discussion 2.1 thin film characterization – film thickness evaluation 2.2 thin film characterization – young's modulus evaluation 2.3 silicon anisotropy mapping 3 conclusions acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2018.15.0142 acta polytechnica ctu proceedings 15:142–147, 2018 © czech technical university in prague, 2018 available online at http://ojs.cvut.cz/ojs/index.php/app image synthesis of metal foam micro-structure with the use of wang tiles lukáš zrůbeka,∗, martin doškářa, anna kučerováa, marcela meneses-guzmánb, francisco rodríguez-méndezc, bruno chinéc a department of mechanics, faculty of civil engineering, czech technical university in prague, thákurova 7, 166 29 prague 6, czech republic b school of industrial production engineering, costa rica institute of technology, cartago, costa rica c school of materials science and engineering, costa rica institute of technology, cartago, costa rica ∗ corresponding author: lukas.zrubek@fsv.cvut.cz abstract. in this paper we present our recent work focused on the analysis of the abilities of wang tiles method and automatic tile design method to synthesize the micro-structure of cellular materials, especially particular type of metal foam. wang tiles method stores and compress the micro-structure in a set of wang tiles and by the means of stochastic tiling algorithms the planar domain is reconstructed. the used tiles are created by the automatic tile design method from respective number of small specimens extracted from the original micro-structure image. as an additional step the central areas of automatically designed tiles are patched to suppress the influence of repeating tile edges (and relevant tile quarters) on inducing artifacts. in the presented analysis the performance of raw and patched tiles of different sizes in conjunction of various tile sets is investigated. keywords: heterogeneous micro-structure, wang tiles, synthesis, cellular materials, metal foam. 1. introduction detailed understanding and insight in the characteristic behaviour of real world processes are integral part to withstand the continuously increasing pressure to the ultimate materials performance. usually the key information are hidden in details and therefore the research attention is typically focused on the micro-structural level of materials. as the majority of materials is random and heterogeneous the appropriate modelling techniques are required. (a) (b) (c) figure 1. periodic unit cell concept, a) heterogeneous micro-structure with regular lattice, b) unit cell, c) reconstructed micro-structure. for simply heterogeneous materials with regular lattice (fig. 1a) the common concept in multi-scale modelling called periodic unit cell (puc) can be used. this method compress the regular micro-structure into the single cell (fig. 1b) corresponding to the representative volume element (rve). by duplicating the puc in cardinal directions, the original microstructure is reconstructed (fig. 1c). on the other hand, the statistically equivalent periodic unit cell (sepuc) method can be used for heterogeneous materials with random micro-structures. in this case the unit cell holds the same properties described by statistical descriptors as the original micro-structure [1]. despite the utility of these methods (puc or sepuc) in cases like numerical homogenization [2] both apply the concept of single cell which leads to periodic patterns in reconstructed domains. to preserve stochastic layout, other method is needed utilizing set of rves, such as wang tiles method. contrary to unit cell frameworks, the wang tiles are capable to reproduce non-periodic micro-structural patterns. 2. wang tiles method the wang tiles method was first presented by logician and mathematician hao wang in 1961 [3, 4]. the method has a conformable concept as the classic game domino (fig. 2a) or the jigsaw puzzle (fig. 2b). it is modelled visually by square tiles with particular information (usually colour, but for purposes of this paper, colours are visualized as patterns) stored on each of the four edges (fig. 2c). a group of these tiles is called set and copies of the tiles from set are placed side by side in cardinal directions, such as the edge information of two adjacent tiles are coincident. rotating or mirroring the tiles is forbidden. 142 http://dx.doi.org/10.14311/app.2018.15.0142 http://ojs.cvut.cz/ojs/index.php/app vol. 15/2018 wang tiles and metal foam micro-structure synthesis (a) (c) (b) figure 2. wang tiles concept, a) domino game, b) jigsaw puzzle, c) wang tiles. one of the significant research topics based on the wang tiles is aimed on discovery of a set with the smallest number of tiles, such that domain tiled with these tiles is aperiodic. during the years the number of tiles in smallest set was reduced from first set of 20 426 tiles discovered by r. berger [5] to the latest findings by e. jeandel [6] to 11 tiles. in our work we utilize tiles sets that are not aperiodic by itself but enable creation of domain with stochastic layout of tiles. such kind of sets are presented by m. f. cohen and his colleagues in [7] for generating stochastic graphic patterns. 2.1. tiling algorithms the planar domain covered with copies of tiles from set is called tiling and when there are no missing pieces (i.e. holes) and all the touching edges of tiles are corresponding to each other, the tiling is valid. the process of laying down tile after tile to create the tiling is in row–column order. in other words the desired number of tiles is placed side by side horizontally from left to right followed by next row of tiles below the previous one. this inevitably leads to situation that tile will be placed in corner where its top edge (n as north) will have to correspond to the bottom edge (s as south) of tile above and its left edge (w as west) will have to correspond to the right edge (e as east) of the tile on the left. this is so-called nw (north–western) corner position (see fig. 3). ? n w (a) (b) figure 3. planar domain tiling, a) edge labels, b) north–western corner position with two candidate tiles. to ensure the final tiling is not only valid but also stochastic the tiling algorithm presented in [7] and called by us cshd (cohen-shade-hiller-deussen) has to be used. this algorithm defines simple rule, that for each nw position have to be at least two valid candidate tiles (fig. 3b) from which single tile is randomly selected and placed in the corner position. this give quite strict requirements on the used tile sets see section 2.2. the random selection from two candidates can sometimes lead to occurrence of groups of same tiles in tiling. the above described cshd algorithm can be improved to prevent this grouping, by allowing to repeat the random selection, if the current selection would lead to identical neighbouring tiles. the repetition is limited to k many attempts, so grouping can still occur in the tiling. to distinguish the two algorithms in later text, we denote the one with improvement as modified cshd [8]. 2.2. tile sets as described in section 2 groups of tiles are called sets. to differentiate between various sets the following labelling wnt/nc i−nc i is used. where w means wang, nt indicates number of tiles in set and nc i denotes number of unique colours on horizontal edges (i = 1) and on vertical edges (i = 2) [9]. tile set with nc i = 1 corresponds to w1/1-1 and the puc (section 1) which is the primary concept we are trying to substitute. the sequent choice is set for nc i = 2. all admissible unique combinations of tiles with two different horizontal and two different vertical edges are showed on fig. 4a. set of tiles that contain all combinations is called complete set and in this case it is labelled w16/2-2. w e n s 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 (a) (b) figure 4. wang tile set with nc i = 2, a) all possible combinations, i.e. complete set w16/2-2, b) minimal set w8/2-2 with particularly selected tiles for nnw = 2. the number of tiles in complete set is equal to ncs = (nc i · nc i )2 for(i) ∈ {1, 2}. (1) to comply with the requirements of cshd algorithm (section 2.1) the minimal number of tiles in set is equal to nt = nnw · √ ncs, (2) where nnw is the number of required candidate tiles in the nw position (2 ≤ nnw ≤ √ ncs). 143 l. zrůbek, m. doškář, a. kučerová et al. acta polytechnica ctu proceedings 2.3. design of tiles the tiles used in practical examples stick to the underlying schema of edge codes and sets depicted in previous section but the tiles must contain the real micro-structure. one of the methods to design tiles is the the multicriteria optimization of tile morphology according to statistical descriptors [9]. nevertheless this approach has extreme requirements on time and computational performance. method that is used for purposes of this work is called automatic tile design and its application can be found in [7]. the process is extensively described in [8] and therefore only brief explanation follows. the process starts by extracting randomly √ ncs many square samples with the edge size l from the original micro-structure (fig. 5a). for the each tile from set the respective samples are arranged into rhombus such that they are overlapping by ω (fig. 5b). then the samples are stitched together by means of image quilting algorithm (iqa) [10] which searches for path in the overlap ω with minimal square difference between pixel values. finally the square tile of size h is cut-out (fig. 5c). 1 2 4 (a) (b) (c) 2 1 4 3 3 l ω h figure 5. automatic tile design, a) √ ncs many square samples with the edge size l, b) arranged samples overlapping by ω, c) final cut-out tile created by iqa. as an enrichment for the automatic tile design process, the central area of created tiles can be patched to suppress the influence of repeating tile edges (and relevant tile quarters) on inducing artifacts [8]. for each tile a unique patch is extracted from original micro-structure. this patch is placed over the centre of tile and by means of the same iqa glued in. for the purpose of following text the tiles created by the standard way are called raw and tiles with patch enrichment are denoted as patched. 3. metal foam the investigated cellular material is particular type of metal foam manufactured in laboratorio macchine utensili e sistemi di produzione (musp), at politecnico di milano, italy and examined in the laboratories of the institute of technology in costa rica (itcr). the manufacturing process is described in detail in [11] therefore only a brief description follows. 1 3 4 5 6 7 8 2 figure 6. photo image of the produced aluminum foam [11] cut into 8 samples designated for scanning (fig. 7) and other experiments. 3.1. manufacturing process the foam specimens were produced from the precursor composed of alsi10 alloy mixed with a 0.80wt% of titanium hydride (tih2) which was cut into cuboids of dimensions 2.0 × 4.0 × 16.5 cm. these were put in the steel mould with approximate internal dimensions 4.0× 16.5× 16.5 cm such as the largest faces are perpendicular to the direction of gravity. 0 10 20 30 [mm] 1 2 3 4 5 6 7 8 figure 7. binary scans of existent samples (fig. 6) of aluminum foam [11]. the order of images corresponds to the order of subsequent cuts of larger piece. the mould was then situated in the laboratory convection oven and heated to start the expansion process. during that the temperature and heating rate was controlled until the value of 680° c was reached. the process ended when the titanium hydroxide (tih2) released the h2 gas into the molten alloy of alsi10 + ti and by regulated solidification process the aluminum foam was created. the outcome of the procedure is irregularly porous cellular metal, with a variable density throughout its volume. 4. image synthesis the reconstruction of material micro-structure starts with the original micro-structure scan. as eight different scans (image size 7000× 14 000 px) were available 144 vol. 15/2018 wang tiles and metal foam micro-structure synthesis a simple comparison of them was performed. each scan was subjected to image analysis when all the pores and their area were measured. from the obtained data we created a histogram showing the counts of pores according to their area in all eight samples (fig. 8). from this perspective all the samples can be considered as equally suitable for further use as the distributions are almost identical. therefore for further use only the sample 1 (fig. 6 and fig. 7) was chosen. figure 8. histogram showing the counts of different size pores in the original samples (fig. 7). 4.1. data as outlined in previous reading, for creating the synthesized micro-structure many different configurations can be used. to cover most of the options the microstructures with further described settings were created (table 1). raw and patched tiles (section. 2.3) with two different sizes h and five different overlaps ω were created for each one of three used set types (section. 2.2). the overlap sizes were set to h 10 , 2h 10 , 4h 10 , 8h 10 and h. therefore for each set type was obtained 10 sets of raw tiles and 10 sets of patched tiles, in total 60 different tile sets. set tile size h [px] overlap ω [px] w8/2-2 1000 100 200 400 800 1000 2000 200 400 800 1600 2000 w16/2-2 1000 100 200 400 800 1000 2000 200 400 800 1600 2000 w18/3-3 1000 100 200 400 800 1000 2000 200 400 800 1600 2000 table 1. preview of used combinations of settings. every single one of created tile set was used to synthesize 5 domains using the standard cshd algorithm and 5 domains with use of the modified cshd algorithm (number of repetitions k = 5) (section. 2.1). as a result, we obtained 600 synthesized micro-structures (image size 7000× 14 000 px) in groups of five, where each group has different settings. 4.2. results because of the large amount of obtained outputs the micro-structures were analyzed at first only visually to eliminate clearly faulty results and by exclusion method get to the best results. 7 4 8 7 3 2 5 4 6 6 7 4 8 5 2 6 7 2 8 8 8 6 6 6 5 1 3 3 3 1 2 8 8 8 8 5 1 2 7 2 8 8 3 2 8 8 7 3 4 7 1 4 5 3 4 7 7 4 8 7 1 3 1 1 1 2 7 4 5 2 2 6 7 1 2 8 5 2 5 4 6 5 4 8 8 6 5 1 3 4 8 8 7 2 5 3 1 4 table 2. tiling map generated by the original cshd algorithm with highlighted groups of same tiles. as first we compared the original cshd and modified cshd algorithms. the table 2 shows the underlying tiling map with highlighted unwanted groups of same tiles. tiling map generated by means of the modified cshd algorithm is presented for comparison in the table 3. the further described phenomena can be observed in almost all reconstructed domains, hence all further presented results are those that used the modified cshd algorithm. 1 2 8 7 4 8 6 5 4 8 7 3 4 8 6 8 7 1 2 7 2 7 2 7 2 7 1 3 1 3 1 2 7 2 8 5 4 5 3 1 2 8 2 8 5 3 2 7 3 4 6 7 4 6 8 7 8 7 3 4 7 1 4 5 2 6 5 1 3 1 7 2 7 2 6 5 1 3 4 5 3 2 7 2 5 4 5 4 5 4 5 4 5 3 4 7 1 3 table 3. tiling map generated by the modified cshd algorithm. another finding that can be stated from the visual observation is that the tile size of 1000px seems not sufficient for raw tiles. the tiles contain opened and very damaged pores (fig. 9) from the quilting process and the synthesized micro-structure is very unlike the original (fig. 10). figure 9. wang tile set w8/2-2, tile size h = 1000 px, overlap ω = 100 px, raw tiles. the increasing value of overlap ω improves the results as the quilting algorithm is able to find superior paths. however, if compared with tiles of size 2000 px the better visual appearance of bigger tiles is clear. in subsequent results the tiles of size 2000px and with higher values of overlaps are used. 145 l. zrůbek, m. doškář, a. kučerová et al. acta polytechnica ctu proceedings figure 10. synthesized domain utilizing the modified cshd algorithm and tiles from fig. 9) with underlying tiling map from table 3. next, we compared the raw and patched tiles. as previously stated in [8] the repeating tile edges and especially the relevant tile quarters induces repeating artifacts in the reconstructed domain. the repeated tile quarters can be seen in fig. 11a. this can be partially avoided by usage of the patched tiles (fig. 11b). a) b) figure 11. wang tile set w8/2-2, tile size h = 2000px, overlap ω = 200px, a) raw tiles, b) patched tiles. because each tile contains unique patch the only repeated artifacts in the domain are exactly these patches. frequency of these artifacts can be even more decreased by means of larger sets. therefore, the patched tiles should be the preferred choice. figure 12. histogram showing the counts of different size of pores in the original scan and in the synthesized samples for three different sets w8/2-2, w16/2-2 and w18/3-3. although the visual observation can be quite subjective, we managed to withdraw the presumably best setting (from the analyzed range of options) to obtain the best results. in the following we compare the original scan 1 with synthesized domains using the tile size 2000px, overlap 2000px, patched tiles, modified cshd algorithm and three different tile sets w8/2-2, w16/2-2 and w18/3-3 (fig. 12). as shown in the figure the distribution of pores in synthesized domains and the original scan are almost equal. three selected examples of synthesized domains for different tile sets are presented in fig. 13. a) b) c) figure 13. examples of synthesized domains by means of tile sets, a) w8/2-2, b) w16/2-2 and c) w18/3-3. 5. conclusion the above presented approach and results show a few limitations for reconstructing the micro-structure of aluminum foam by the means of wang tiles (section 2) and automatic tile design (section 2.3) . firstly, because of the manufacturing process and the influence of gravity force the character of the aluminum micro-structure is progressively changing along the direction of the gravity. as the h2 gas is released in the molten mixture it tends to rise up and thus larger pores are created at the surface while at the bottom the foam density is much higher (fig. 7). this fluctuating layout cannot be achieved by the wang 146 vol. 15/2018 wang tiles and metal foam micro-structure synthesis tiles as the tiles are selected randomly (section 2.1) and the density of synthesized micro-structure is approximately equal in all directions. secondly, the automatic tile design utilize the image quilting algorithm for creating the tiles samples of the original micro-structure. this algorithm works quite well for mono-disperse media [8] but for media with high porosity like foams the quilting path search sometimes hit the borders of the overlap and visible errors like unclosed pores or completely unconnected pieces may occur. this could be improved by utilizing the max-flow or in other words min-cut [12] graph cutting method and enhanced by additional enrichments [13]. however, quite satisfying results can be obtained (fig. 13) when the modified cshd algorithm and patched, sufficiently large tiles with a relevant overlap are used. furthermore, the visually noticeable repetitions can be decreased by means of larger sets with more edge variety. acknowledgements the authors gratefully acknowledge the financial support from the grant agency of the czech technical university in prague, the grant no. sgs18/036/ohk1/1t/11 (modelling heterogeneous materials and identification of parameters of macroscopic material models) (l. zrůbek, m. doškář and a. kučerová). authors would also like to thank for the support from vicerrectoría de investigación y extensión, project no. 1351022 (application of nondestructive techniques for control of properties of porous and cellular materials) (l. zrůbek, m. meneses-guzmán, f. rodríguez-méndez and b. chiné-polito). references [1] h. lee, m. brandyberry, a. tudor, k. matouš. three-dimensional reconstruction of statistically optimal unit cells of polydisperse particulate composites from microtomography. phys rev e 80:061301, 2009. doi:10.1103/physreve.80.061301. [2] f. fritzen. microstructural modeling and computational homogenization of the physically linear and nonlinear constitutive behavior of micro-heterogeneous materials, vol. 1. kit scientific publishing, 2011. [3] h. wang. proving theorems by pattern recognition–ii. bell system technical journal 40(1):1–41, 1961. doi:10.1002/j.1538-7305.1961.tb03975.x. [4] h. wang. games, logic and computers. scientific american 213(5):98–106, 1965. [5] r. berger. the undecidability of the domino problem. 66. american mathematical soc., 1966. doi:10.1090/memo/0066. [6] e. jeandel, m. rao. an aperiodic set of 11 wang tiles. arxiv preprint arxiv:150606492 2015. [7] m. f. cohen, j. shade, s. hiller, o. deussen. wang tiles for image and texture generation. acm trans graph 22(3):287–294, 2003. doi:10.1145/882262.882265. [8] m. doškář, j. novák, j. zeman. aperiodic compression and reconstruction of real-world material systems based on wang tiles. phys rev e 90:062118, 2014. doi:10.1103/physreve.90.062118. [9] j. novák, a. kučerová, j. zeman. compressing random microstructures via stochastic wang tilings. phys rev e 86:040–104, 2012. doi:10.1103/physreve.86.040104. [10] a. a. efros, w. t. freeman. image quilting for texture synthesis and transfer. in proceedings of the 28th annual conference on computer graphics and interactive techniques, siggraph ’01, pp. 341–346. acm, new york, ny, usa, 2001. doi:10.1145/383259.383296. [11] b. chiné, m. meneses-guzmán, f. rodríguezméndez. characterization of aluminium foams produced via a powder metallurgy route. in proceedings of the 1st iberic conference on theoretical and experimental mechanics and materials / 11th national congress on experimental mechanics. inegi/feup (2017), porto, portugal, 2018. accepted for publishing. [12] y. boykov, o. veksler, r. zabih. fast approximate energy minimization via graph cuts. ieee transactions on pattern analysis and machine intelligence 23(11):1222–1239, 2001. doi:10.1109/34.969114. [13] v. kwatra, a. schödl, i. essa, et al. graphcut textures: image and video synthesis using graph cuts. acm transactions on graphics (tog) 22(3):277–286, 2003. doi:10.1145/1201775.882264. 147 http://dx.doi.org/10.1103/physreve.80.061301 http://dx.doi.org/10.1002/j.1538-7305.1961.tb03975.x http://dx.doi.org/10.1090/memo/0066 http://dx.doi.org/10.1145/882262.882265 http://dx.doi.org/10.1103/physreve.90.062118 http://dx.doi.org/10.1103/physreve.86.040104 http://dx.doi.org/10.1145/383259.383296 http://dx.doi.org/10.1109/34.969114 http://dx.doi.org/10.1145/1201775.882264 acta polytechnica ctu proceedings 15:142–147, 2018 1 introduction 2 wang tiles method 2.1 tiling algorithms 2.2 tile sets 2.3 design of tiles 3 metal foam 3.1 manufacturing process 4 image synthesis 4.1 data 4.2 results 5 conclusion acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2018.17.0024 acta polytechnica ctu proceedings 17:24–29, 2018 © czech technical university in prague, 2018 available online at http://ojs.cvut.cz/ojs/index.php/app thin nitride layers as permeation barriers jakub veverkaa,b,∗, jiri matejicekb, frantisek lukacb, david pospisilc, ladislav cvrcekd, vincenc nemanice a faculty of nuclear sciences and physical engineering, brehova 7, praha 1, czech republic b institute of plasma physics of the czech academy of sciences, za slovankou 3, praha 8, czech republic c technical university of liberec, studentska 1402/2, liberec i, czech republic d department of materials engineering, faculty of mechanical engineering, czech technical university in prague, karlovo namesti 13, praha 2, czech republic e jozef stefan institute, jamova 39, ljubljana, slovenia ∗ corresponding author: veverka@ipp.cas.cz abstract. permeation barriers represent one of the crucial fields in materials development for thermonuclear fusion. primary objective of the barriers is to suppress the permeation of hydrogen isotopes (mainly tritium) from future thermonuclear fusion facilities. secondary objective is to reduce hydrogen retention in structural materials, potentially also improving their corrosion resistance. expected reactor conditions put high demands on the material, as well as on the final barrier quality. key properties are tritium permeation reduction, absence of defects (especially cracks), high-temperature stability and corrosion resistance, and compatibility with structural materials (mostly ferritic-martensitic steels). regarding industrial scale production, ability of the deposition method to coat large complex surfaces is desirable. thin nitride layers, identified as promising permeation barriers, were prepared by diffusion-based nitridation and physical vapour deposition (pvd) and characterized. keywords: permeation barriers, nitride layers, pvd coating, permeation measurement. 1. introduction nuclear fusion is an appealing means of energy production that can resolve growing energy demands of the mankind. facing inevitable exhaustion of fossil fuels and growing awareness of climate changes caused by greenhouse gases emissions, it is obvious that current means of energy production need to be replaced. renewable sources (e.g. solar, hydro, wind or geothermal power) are not expected to be capable of supplying global energy needs. therefore, entirely new source of energy is required, in an ideal case fulfilling the following criteria • sufficient fuel supplies • no harmful emissions • no major operational hazards thermonuclear fusion expected to be harnessed in the first generation of power plants exhibits promising results regarding all the criteria. energy production will be based on the following equations 2 1d +3 1 t −→4 2 he +1 0 n + 17.6mev 6 3li +1 0 n −→3 1 t +4 2 he. (1) abundance of deuterium and lithium is sufficient for thousands of years, tritium will be produced in-situ. emissions consist only of harmless helium, operational hazards do not involve risk of uncontrolled chain reaction or major radiation leaks. however, enormous complexity and demands (technological, as well as economical) on the fusion facilities have delayed the harnessing of thermonuclear fusion. joint international effort resulted in the construction of international thermonuclear experimental reactor (iter) that should demonstrate the feasibility of fusion and test the neccessary systems for future fusion power plants. permeation barriers are one of the most important parts of fusion facility concerning radiological safety and economical issues. these are typically thin coatings that lower the permeation and retention of hydrogen isotopes (mainly tritium) in the reactor structures. selected materials have to withstand harsh reactor conditions, including • strong thermal gradients and elevated temperatures • presence of significant neutron and secondary charged particle irradiation • potential contact with high temperature pbli (corrosion issues) depending on the reactor design (further information to be found in [1]) excessive tritium permeation could result in • degradation of the reactor material properties • radiological hazard during operation and decommisioning • lowering of the reaction probability and economical losses therefore, the presence of a permeation barrier is envisaged by numerous studies [2–4] to meet the safety 24 http://dx.doi.org/10.14311/app.2018.17.0024 http://ojs.cvut.cz/ojs/index.php/app vol. 17/2018 thin nitride layers as permeation barriers requirements of maximum allowable tritium losses (1g per year) and maximum tritium inventory (200g) [1]. permeation barriers are also essential for early and safe decommissioning of fusion facilities after their shutdown. besides reducing the permeation through metallic structures, typically easily permeable by hydrogen isotopes, other material and deposition properties of the permeation barriers have to be taken into account: • stability and reliability of the coating (regarding the abovementioned conditions) • compatibility with the substrate material • capability to form dense and defect-free layers on complex surfaces regarding the coating stability in corrosive environment, some studies show that, depending on the coolant and tritium breeder choice, fusion facility components can be subjected to corrosion attack up to 700 µm per year, equal to 4 kg of dissolved material per year and square meter [5–7]. that would pose a significant risk to the structural integrity and operational reliability of the facility. parts subjected to the corrosion attack are in most cases also threatened by tritium permeation, therefore the prospective coating ought to suppress both phenomena to a high extent. as candidate materials for the permeation barriers, commonly oxidic ceramics [2, 8] or nitrides [9, 10] are being studied. this study is focused on diffusion-based nitride layers, whose advantage lies in the applicability on large complex surfaces, including for example the inside of coolant tubes [11, 12]. such nitride layers are qualitatively compared to thin coatings prepared by physical vapour deposition (pvd), which is an established technology for the production of thin and dense coatings. on an industrial scale, large surface areas can be coated, including curved surfaces, but still it is primarily a line-of-sight coating technology. nitriding is a thermochemical treatment that consists of the introduction of nitrogen into metallic material. according to the method of nitrogen introduction, several nitriding techniques are being distinguished: • gas nitriding • plasma/ion nitriding • solid/liquid nitriding gas nitriding [13] is conducted at elevated temperature, submitting the samples to nitrogen-containing atmosphere, commonly ammonia or various nitrogen mixtures, e.g. h2 and n2; n2, nh3 and co2 etc. ion/plasma [14] nitriding uses a glow discharge to introduce nitrogen into the material; nitrogen ions are accelerated and impinge the surface. during solid nitriding [15], granulate containing nitrogen compound is used e.g. fe4kcn. the samples are surrounded by granulate and placed into heating oven for enhancing the diffusion. gas and plasma nitriding are well established and widely used and researched techniques, however the required facility is complicated. during gas nitriding, the process temperature may affect the hardness of the nitrided material. plasma nitriding is faster than conventional gas nitriding and provides for a better control of the treatment parameters. this allows the control of the nitrided layer microstructure and also enables higher reproducibility of the results. solid nitriding is a low-cost process in comparison to gas and plasma nitriding, when the only required facility is the furnace providing the heating. the heating generates atomic nitrogen that is able to diffuse into the material. nitriding promotes higher hardness of the specimen, increases wear resistance, fatigue resistance, corrosion resistance and, as it was observed, some nitrided materials have favourable permeation properties. regardless of the used technique, surface cleaning of the samples is vital, because the contaminants can form a barrier preventing nitrogen from diffusing into the material. during the deposition, different layers are produced diffusion and compound layer. diffusion zone is a region characterized by solid solution in the original core microstructure with hardening precipitations. compound layer typically consists of mixture of iron nitrides, in case of steel nitriding. with plasma nitriding, by controlling the process parameters, it is possible to determine the composition or even avoid the formation of the compound layer [16]. in this study, three types of diffusion-nitrided layers were prepared and characterized for their morphology, phase and elemental composition and thermal stability. a brief comparison with several nitride coatings prepared by the pvd technique is made. 2. experiment as a substrate, ferritic-martensitic steel p92 was used, whose characteristic composition is provided in [17]. it is very similar to eurofer97 steel, developed as a reference structural material for the demo tokamak [18]. such material is also commonly used in permeation measurements [19], so a direct comparison with untreated substrate could be made. the substrates were prepared in a disk shape with 0.5 mm thickness and 25 mm diameter, by electrical discharge machining [20], grinding with sand paper and metallographic polishing. the following types of diffusionbased nitride layers were prepared: • plasma nitridation (430 °c, 24h), at technical university of liberec • carbonitridation (560 °c, 10h) at bodycote ht, s.r.o. • salt nitridation (530 °c) at katring plus s.r.o. presence of the nitride layer and its elemental composition was investigated by energy-dispersive x-ray spectroscopy (eds) in an evo ma15 scanning electron 25 j. veverka, j. matejicek, f. lukac et al. acta polytechnica ctu proceedings microscope (carl zeiss sma, germany) equipped with a quantax system and x-flash 5010 detector (bruker, germany). phase composition was analyzed by x-ray diffraction (xrd) using a d8 discover diffractometer (bruker, germany). for the xrd measurements, rietveld method was used to determine the composition. to assess thermal stability of the coatings, the samples were annealed in a vacuum furnace for 1 hour at 550 °c, which is the foreseen temperature in the future reactor application, also used in the permeation measurements. after the annealing, the layer composition was verified again. additionally, six types of nitrides were deposited by pvd: wn, crn and cr2n by magnetron sputtering at the department of materials engineering (faculty of mechanical engineering, czech technical university in prague) and crwn, craln and zrn by cathodic arc plasma deposition at ionbond czechia s.r.o. similar, but more comprehensive characterization was performed, including the permeation measurements at the facility of jozef stefan institute in ljubljana, slovenia [21, 22]. the details of the pvd coating preparation and characterization will be provided in a dedicated paper [23]. 3. results 3.1. diffusion-nitrided layers surface morphology of the three layers is shown in figure 1. a semi-granular structure consisting of very fine grains can be observed; in case of ion nitridation, these seem to be more concentrated along substrate grain boundaries. this appearance is typical and has been observed in previous studies [13, 24]. no cracks were observed, which would be detrimental for the permeation barrier performance. thickness of the diffusion nitrided coatings, provided by the manufacturer, reached approximately 100nm. eds and xrd measurements confirmed presence of the nitride layer. elemental composition, as determined by eds, is shown in table 1. phase composition, as determined by xrd, is shown in table 2. due to the low thickness of the nitride layer, the substrate is detected by both methods. before additional characterizations, the samples were annealed to assess their thermal stability. however, xrd measurements after the annealing showed that the nitride layer was significantly decomposed. this can be seen in figures 2 and 3 for the plasma-nitrided samples. similar situation was observed for the carbonitrided and salt-nitrided samples. therefore, permeation measurements of these coatings were not performed. in the plasma nitrided sample, a change in surface morphology was observed as well, including the formation of occassional cracks. this can be seen in figure 4. for the carbonitrided and salt-nirided samples, the morphology remained very similar. figure 1. surface morphologies of nitrided layers: plasma nitridation (top), carbonitridation (middle), salt nitridation (bottom). 3.2. pvd coatings successful deposition of the pvd coatings was confirmed by eds and xrd measurements. smooth and crack-free coatings were obtained, with thicknesses between 1.5 and 4.5 µm. representative surface morphologies are shown in figures 5 and 6. scratch test showed good adhesion of the coatings. xrd measurements indicated all the pvd coatings to contain compressive stress, which is favorable as it suppresses 26 vol. 17/2018 thin nitride layers as permeation barriers fabrication method n o na mg al cr fe w c si plasma nitridation 6.3 0.9 9.6 81.9 1.3 carbonitridation 0.9 3.6 0.6 8.6 83.4 0.1 2.5 0.3 salt nitridation 0.1 21.2 1.0 0.2 0.8 6.2 70.1 0.4 table 1. composition of the nitride layers measured by eds, wt. %. fabrication method fe2n fe3n fe4n magnetite plasma nitridation 14.96 18.27 4.88 0.00 carbonitridation 16.95 0.00 0.00 0.56 salt nitridation 27.20 4.09 0.00 17.79 table 2. phase composition of the nitride layers measured by xrd, wt. % balance α-fe. figure 2. xrd spectrum of the plasma nitrided sample. figure 3. xrd spectrum of the plasma nitrided sample after thermal annealing. crack propagation. all the pvd coatings showed potential for permeation barriers with permeation reduction factor (prf) > 100, in case of zrn even > 10000. phase stability after the permeation test was verified again by xrd. 4. conclusions thin nitride layers on p92 ferritic-martensitic steel were prepared by plasma nitridation, carbonitridation and salt nitridation. formation of the nitride layer and its composition was determined by quantitative rietveld analysis of diffraction patterns. sem observations indicated the absence of surface cracking. however, after annealing at 550 °c, the layers were significantly decomposed. therefore, hydrogen permeation was not measured. pvd coatings of selected nitrides were also deposited on eurofer97 ferriticmartensitic steel, a reference material for demo construction. the composition, thickness and adhesion of the coatings were measured and permeation measurefigure 4. surface morphology of plasma nitrided layer after thermal annealing. figure 5. surface morphology of magnetron sputtered crn coating. ments were performed. thickness of the coatings was several µm, all coatings showed the desired composition and good adhesion. permeation measurements confirmed suitability of selected nitrides as permeation barriers. best result was achieved by zrn permeation reduction by three orders of magnitude. these coatings also demonstrated thermal stability at the test temperature. from these results, it can 27 j. veverka, j. matejicek, f. lukac et al. acta polytechnica ctu proceedings figure 6. surface morphology of cathodic arc plasma deposited craln coating. be concluded that the investigated diffusion-based nitride layers are not suitable for the application as permeation barriers at elevated temperature, due to the lack of thermal stability. from this point of view, as well as promising prf values, pvd coatings seem more suitable. however, it should be noted that only a limited number of materials and techniques were examined and various other modifications exist, including technological parameter variations as well as different layer compositions. acknowledgements this work has been carried out within the framework of the eurofusion consortium and has received funding from the euratom research and training programme 20142018 under grant agreement no 633053. the views and opinions expressed herein do not necessarily reflect those of the european commission. financial support from czech science foundation (grant no. 14-12837), czech academy of sciences (project strategy av21), grant agency of the czech technical university in prague (grant no. sgs17/138/ohk4/2t/14) and ministry of education, youth and sport of the czech republic (program npu1, project no. lo1207) is acknowledged. references [1] l. giancarli et al. overview of the iter tbm program. fusion engineering and design 87:395–402, 2012. [2] r. causey et al. tritium barriers and tritium diffusion in fusion reactors. sandia national laboratories 2009. [3] m. abdou et al. blanket/first wall challenges and required rd on the pathway to demo. fusion engineering and design 100:2–43, 2015. [4] d. demange et al. tritium management and anti-permeation strategies for three different breeding blanket options foreseen for the european power plant physics and technology demonstration reactor study. fusion engineering and design 89:1219–1222, 2014. [5] w. krauss et al. al-based anti-corrosion and t-permeation barrier development for future demo blankets. journal of nuclear materials 417:1233–1236, 2011. [6] j. konys et al. corrosion behaviour of eurofer steel in flowing eutectic pb-17li alloy. journal of nuclear materials 329-333:1379–1383, 2004. [7] j. konys et al. compatibility behaviour of eurofer steel in flowing pb-17li. journal of nuclear materials 386-388:678–681, 2009. [8] h. glasbrenner et al. hydrogen permeation behaviour of hot-dip aluminized manet steel. fusion technology 28:1159–1164, 1995. [9] k. forcey et al. the formation of tritium permeation barriers by cvd. journal of nuclear materials 200:417–420, 1993. [10] i. tazhibaeva et al. hydrogen permeation through steels and alloys with different protective coatings. fusion engineering and design 51-52:199–205, 2000. [11] x. xiang et al. preparation technique and alloying effect of aluminide coatings as tritium permeation barriers: a review. international journal of hydrogen energy 40:3697–3707, 2015. [12] t. sample et al. the hydrogen permeation behaviour of aluminised coated martensitic steels under gaseous hydrogen, liquid pb-17li/hydrogen and cyclic thermal loads. journal of nuclear materials 283-287:1272–1276, 2000. [13] m. kondo et al. nitriding treatment of reduced activation ferritic steel as functional layer for liquid breeder blanket. plasma and fusion research 6:2405117, 2011. [14] active screen plasma nitriding. [online], http://web.ald-vt.de/cms/vakuumtechnologie/technologien/vacuum-heattreatment/aspn-active-screen-plasma-nitriding/, 242017. [15] m. lee et al. a basic study on the solid-state nitriding of aluminum by mechanical alloying using a planetary ball mill. advanced powder technology 8:291–299, 1997. [16] c. costa et al. study of the nitrided layer obtained by different nitriding methods. revista matãľria 20:460–465, 2015. [17] p. giroux et al. mechanical and microtructural stability of p92 steel under unaxial tension at high temperature. materials science and engineering a 527:3984–3993, 2010. [18] eurofer-97 technical specification. [online], www.uni-lj.si/mma/1-f4e-2008-grt010.%20technical%20specification/2013070115554947/, 1122017. [19] j. engels et al. hydrogen saturation and permeation barrier performance of yttrium oxide coatings. fusion engineering and design 124:1140–1143, 2017. [20] sodick wire edm technology. [online], http://www.sodick.com/technology/wireedmtech, 1632017. [21] v. nemanic et al. sensitivity enhancement in hydrogen permeation measurements. journal of vacuum science and technology a 28 (4):578–582, 2010. 28 vol. 17/2018 thin nitride layers as permeation barriers [22] v. nemanic et al. hydrogen permeation through silicon nitride films. journal of alloys and compounds 539:184–189, 2012. [23] j. matejicek et al. characterization of some less-common nitrides as permeation barriers, paper in preparation . [24] b. miao et al. the effect of sand blasting pretreatment on plasma nitriding. vacuum 136:46–50, 2017. 29 acta polytechnica ctu proceedings 17:24–29, 2018 1 introduction 2 experiment 3 results 3.1 diffusion-nitrided layers 3.2 pvd coatings 4 conclusions acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2020.26.0100 acta polytechnica ctu proceedings 26:100–106, 2020 © czech technical university in prague, 2020 available online at https://ojs.cvut.cz/ojs/index.php/app design of tuned mass dampers for large structures using modal analysis jan štěpánek∗, jiří máca czech technical university in prague, faculty of civil engineering, department of mechanics, thákurova 7, 166 29 prague 6, czech republic ∗ corresponding author: jan.stepanek@fsv.cvut.cz abstract. tuned mass damper is a device, which can be highly useful when dealing with excessive vibration and is widely used in many engineering fields. however, its proper design and optimization is a complicated task. this study uses mode superposition method to speed up the evaluation of dynamic response. the speed of response calculation allows for a quick calculation of frequency response function and numerical optimization of tuned mass dampers. this optimization method is demonstrated on a numerical example of a cable stayed footbridge. the example compares a simplified and widely used design method of tuned mass damper with numerical optimization. keywords: dynamic response, modal coordinates, tuned mass damper, optimization. 1. introduction the tuned mass damper (tmd) is a device composed of a spring, mass and a viscous damper, which is widely used in order to reduce mechanical vibration. however, its high efficiency is conditioned by appropriate design. unfortunately, the closed form solution for its optimal parameters has been found only for special, simplified cases. numerous studies dealing with optimal design of tmd have been published since the beginning of the 20th cetury, but most of them only consider a single dergee of freedom (sdof) main structure. the first design method for stifness of tmd spring was proposed by den hartog [1]. it comes from observation of two invariant points on frequency response function (frf), which are independent of tmd damping. the optimal damping was derived by brock [2] for both motion and force excitation. these settings of parameters are close to the optimal ones for undamped main structure. thanks to simple application, it is still a recomended design method in several modern guidelines [3–5]. the closed form solution of optimal tuning and damping for undamped main sdof system was found by o. nishihara and t. asami [6]. abubakar and farid presented a numericaly derived formula for design of tmd on clasically damped sdof structure [7]. simplification of main structure by sdof model is sometimes insufficient and may lead to wrong tuning or unnecessary big mass of tmd. especially, large structures such as bridges or high buildings, which may be sensitive to dynamic load, cannot be modeled as sdof structures. rana and soong showed the importance of numerical tuning in case of multiple degrees of freedom (mdof) structures and shows the example of multiple tmds, each tuned for particular mode [8]. ozer and royston extended hartog’s idea to mdof systems [9], but the proposed method requires inversion of the dynamic stiffness matrix and numerical solution of system of equations, which may be impractical. this paper presents numerical optimization of single and multiple tmds, on a mdof damped stucture. the goal is to minimize the maximal amplitude (also called h∞ optimization) of the structure caused by simple harmonic load. in the case of h∞ numerical optimization, the problem is that large amount of solutions needs to be evaluated for various tmd parameter values and forcing frequencies. to reduce the camputational complexity of the problem, method proposed in [10] was applied. this method operates with transformation to modal coordinates and allows us to find steady-state response of structures with tmd without solving a system of equations. this method is also suitable for design of tmds for existing structures where excessive vibrations were measured and modal characteristics are known. 2. response calculation the main structure is represented by classically damped mdof system with n degrees of freedom and it is loaded by general dynamic load f (t). the system is described by equation of motion kx+ cẋ+mẍ = f (t) , (1) where k, c and m represent stiffness matrix, damping matrix, and mass matrix, respectively, f (t) is the loading vector and x is vector of displacement. the vectors of velocity and acceleration are represented by symbols ẋ and ẍ. 2.1. single tmd tmd is attached to the m-th degree of freedom. the total system can be represented by system of n + 1 equations, but the equation which corresponds to the 100 https://doi.org/10.14311/app.2020.26.0100 https://ojs.cvut.cz/ojs/index.php/app vol. 26/2020 example of an article with a long title mass of tmd can be separated [9], as can be seen in eqs. (2) and (3). [k +ka]x+ [c + ca]ẋ+mẍ −ka mxa − ca mẋa = f (t) , (2) kaxa + caẋa +maẍa − [ca m]t ẋ− [ka m]tx = 0 , (3) where: ka defined by eq. (4), ca defined by eq. (5), ka m m-th column of matrix ka, ca m m-th column of matrix ca, ka stiffness of tmd, ca damping of tmd, ma mass of tmd, xa displacement of tmd, ẋa velocity of tmd, ẍa acceleration of tmd. matrices ka and ca contain only a single non-zero element on the m×m position: ka =  0 0 · · · 0 0 . . . ... ... ... · · · ka(m×m) ... 0 · · · · · · 0  , (4) ca =  0 0 · · · 0 0 . . . ... ... ... · · · ca(m×m) ... 0 · · · · · · 0  . (5) assuming the loading force is harmonic with amplitude fa and circular frequency ω, we only need to find a solution in the form of steady-state response. the equations (2) and (3) can be transformed to modal coordinates according to eq. (7). substituting xa expressed from eq. (3) to eq. (2), the following result is obtained: [ ω + iωc ′ − ω2i+ + (iωca + ka)(−ω2ma) −ω2ma + iωca + ka φt m,∗φm,∗ ] q = φtfa , (6) x = φq , (7) where ω, φ, and c ′ are spectral matrix, modal matrix, and modal damping matrix, respectively. φm,∗ denotes the m-th row of modal matrix and q is the vector of modal coordinates. the modal matrix must be mass orthonormal with mode shapes φ arranged as its columns. the spectral matrix contains squares of natural circular frequencies ωj on its diagonal. for a classically damped structure, c ′ is also diagonal containing members 2ξjωj where ξj is the damping ratio of j-th natural frequency. eq. (6) is a system of n linear equations. thanks to its structure, sherman–morrison formula can be used to find the inversion of the matrix on the left-hand side. the solution of separate modal coordinate qj with used substitutions is given by following equations: [ ω2ma(iωca + ka)φm,js t 2 (−ω2ma + iωca + ka)− ω2ma(iωca + ka)s1 + + φt j ] fa ω2 j − ω2 + 2iξjωωj = qj , (8) s1 = n∑ j=1 φ2 m,j ω2 j − ω2 + 2iξjωωj , (9) st 2 = n∑ j=1 φm,jφ t j ω2 j − ω2 + 2iξjωωj . (10) as can be seen in eq. (8-10), all mode shapes must be known to find an exact contribution of one particular mode shape to the response. however, mode shapes with frequencies, which are far from forcing frequency have negligible influence on the response. therefore using only several selected mode shapes in eqs. (7-10) can lead to very precise results. the reduction of number of mode shapes is necessary for fast response evaluation. 2.2. multiple tmds in the case of multiple tmds, the evaluation of separate modal coordinates using eq. (8) is no longer possible, but sherman–morrison formula a−1 i+1 = [ai + uvt ]−1 = a−1 i − a−1 i uvta−1 i 1 + vta−1 i u (11) can be repeatedly applied to get a vector of modal coordinates q. each addition of tmd requires one evaluation of eq. (11) with substitutions given by eqs. (1214). a0 = ω + iωc ′ − ω2i , (12) u = (iωca + ka)(−ω2ma) −ω2ma + iωca + ka φt m,∗ . (13) vt = φm,∗ (14) for j tmds, the vector of modal coordinates is q = a−1 j φtfa . (15) this method may be inappropriate for large number of tmds because of its high numerical complexity caused by multiplication of full matrices. however, its usage still avoids the solution of the system of 101 jan štěpánek, jiří máca acta polytechnica ctu proceedings equations. in section 4.5, this method is successfully used for the evaluation of structural response with three tmds. as can be seen in eqs. (11-14), the system receptance matrix a−1 j is found without necessity of matrix inversion. the only matrix which must be inverted is a0, but it is a simple procedure, because a0 is a diagonal matrix. the receptance matrix can be used for fast evaluation of response to various load vectors. 3. simplified tmd design well known simplified design of tmd for minimization of maximal amplitude, first introduced in [1, 2] by hartog and brock, is used in this study as reference method and is compared to numerical optimization of tmds. this method was chosen for its easy application and the fact it is widely used and recommended. using following notation: meff effective mass of the main structure, ωj n-th natural frequency of the main structure, ωa = √ ka/ma natural frequency of tmd, µ = ma/meff mass ratio, β = ωa/ωj frequency ratio, ξa = ca/ca,cr damping ratio of tmd, ca,cr = 2 √ kama critical damping of tmd. the optimal values of parameters β and ξa for force excitation are βopt = 1 1 + µ , (16) ξa,opt = √ 3µ 8(1 + µ) . (17) this method was firstly proposed for sdof main structure, but it can be used for design of tmd which is supposed to reduce vibration of j-th mode shape. the effective mass of the main structure meff can be found according to the following equation: meff = 1 φ2 m,j = φ′tj mφ′j , (18) where φ′j denotes the j-th mode shape normalized with respect to the m-th ordinate where tmd is attached hence the m-th ordinate is equal to one. 4. numerical example this chapter analysis a cable stayed footbridge. the structure is suspected of being sensitive to dynamic pedestrian load. a numerical model of the structure was created in matlab. design of tmds was performed both using simplified method indicated in section 3 and numerical optimization. mode frequency [hz] damping ratio [-] 1 0.733 0.0190 2 0.923 0.0148 3 1.300 0.0106 4 1.595 0.0087 5 1.818 0.0078 6 1.924 0.0075 7 2.002 0.0071 8 2.218 0.0063 table 1. natural frequencies and damping ratios. 4.1. model of the structure the structure is 243 m long, symmetric, cablestayed bridge with prestressed concrete bridge deck. a scheme of the structure and numbering of important nodes can be seen in figure 1. two-dimensional finite element (fem) model was created in order to examine vertical vibrations. the model contains nodes which define geometry of the structure and also uses nodes added by fem mesh. the final model contains 810 degrees of freedom. eight lowest natural frequencies, mode shapes and associated damping ratios were measured on the real structure. only vertical bending mode shapes are taken into account in this paper. the numerical model was identified to be in accordance with measurement. natural frequencies and damping ratios are summarized in table 1. 4.2. load as can be seen in table 1, several frequencies are close to 2 hz, which is typical frequency of human walk and thus resonance effect may occure. according to guidebook [5], a stationary load model which represents a group of 8-15 walkers can be used to evaluate the response and to decide whether the vibration is excessive. the group of walkers is simplified to one harmonic force in resonance with mode, which has the frequency closest to 2 hz. the force should be applied to the most adverse position, which may be understood as a place with the highest vertical ordinate of the forced mode shape. in the case given, it was decided to find maximal displacement over the bridge span for three positions, which corresponds to the maximal ordinate of mode shapes. they are node 19 for the 5th mode shape, node 5 for the 6th, and node 27 for the 7th one. the results can be seen in figure 1. the highest peak in range between 1.5-2.5 hz is caused by force situated in node 5 in resonance with 6th natural frequency. therefore the harmonic force with amplitude fv is placed in node 5. fv = kv × 180 = 3× 180 = 540 n . (19) however, the amplitude fv is important only for taking the absolute value of the response, but it has no impact on the design of tmd. 102 vol. 26/2020 example of an article with a long title figure 1. static scheme of the bridge. figure 2. frfs of force in nodes 5,19, and 27. figure 3. mode shape no. 5. figure 4. mode shape no. 6. figure 5. mode shape no. 7. node 5 is appropriate place for simple harmonic load, because mode shapes 5,6 and 8 also have high ordinate in this node, therefore multiple modes are forced. for simplified tmd design from section 3, the load does not play a role, but it is important for numerical optimization and calculation of frf. 4.3. objective function an objective function must be defined for the optimization of tmd. in this paper, it was decided to use h∞ optimization maximal value of displacement amplitude along the bridge deck between 1.7 and 2.2 hz, where the load causes highest response. that means 103 jan štěpánek, jiří máca acta polytechnica ctu proceedings maximal values of vector of displacement x create frf, which creates a continuous frf with discontinuous derivative. highest peak of frf is then the value of the objective function which is minimized. the step of forcing frequency for evaluating the frf was set to 0.001 hz to catch the sharp peaks but with respect to the speed of calculation. the transformation to modal coordinates allows us to use reduced number of mode shapes to speed up the evaluation of response. observation of frf showed that using more than first 12 mode shapes and natural frequencies does not affect the values of peak response in chosen frequency range. therefore it was decided to use only first 12 mode shapes. the reduction of number of mode shapes reduced the size of the problem dramatically (from 810 equations to 12 equations) without noticeable decrease of accuracy. 4.4. single tmd 4.4.1. simplified design the simplified design method presented in section 3 was performed in order to reduce vibration of the sixth mode shape, because the pacing force causes the highest peak on frf in resonance with the sixth natural frequency, as can be seen in figure 2. tmd with one vertical degree of freedom and mass ma = 600 kg is attached to node 5, and its parameters were designed according to section 3, which resulted into values ka = 86300 nm−1, and ca = 772 nsm −1. the mass of 600 kg is approximately only 0.15 % of the total mass of the structure, but it can reduce the vibration significantly. 4.4.2. numerical optimization to optimize tmd numerically, function fmincon implemented in matlab was used. the function uses interior-point algorithm and is designed to solve constrained multivariable optimization problems. the only implemented constraints were ka > 0 and ca > 0. the function was used to find optimal values of ka and ca which minimize the objective function. generally, in the case of single tmd, the objective function can have at most one local minimum for each natural frequency. in our case, the function has only one local minimum for the sixth frequency, because the peak of frf reaches the highest level in resonance with the sixth natural frequency. the initial values of ka and ca define the local minimum to which interior-point algorithm converges, therefore it is advantageous to begin with tmd tuned close to the natural frequency which response is minimized. for parameters of tmd, which are "far from optimal", a gradient of objective function may be very low, therefore convergence problems may occur, or it can take a large number of iterations to find the local minimum. parameters designed according to section 3 are quality starting point for quick convergence. the parameters, which optimize the objective function ka = 84100 nm−1 and ca = 1259 nsm −1 were mode shape node ka [nm−1] ca [nsm−1] ma [kg] 5 19 26000 98 200 6 5 29100 150 200 7 27 31500 129 200 table 2. tmds designed to reduce vibration of the 5th, 6th, and 7th mode shape. found numerically. corresponding frfs for both methods of design can be seen in figure 6. 4.5. multiple tmds as we can see in figure 2, there are three dominant natural frequencies between range 1.7-2.2 hz. therefore three tmds are designed in this section to show that a higher number of tmds can reduce the objective function more effectively than one. it was decided to keep the sum of tmds mass constantly at 600 kg. the constant sum of weight allows for a relevant comparison of one and multiple tmds. 4.5.1. simplified design in order to reduce frf, three tuned mass dampers were designed according to section 3 to reduce vibration in resonance with the 5th, 6th, and 7th natural frequencies. all of them were positioned to the node where the highest ordinate of damped mode is located. their properties are summarized in tab. 2. the mass was equally distributed among tmds because its optimal distribution remains unknown. 4.5.2. numerical optimization the numerical optimization was performed with two basic settings. the first one uses position of tmds used by previous section (nodes 19, 5, and 27) and was expected to provide the best results. the second one places all tmds in node 5. this calculation was proceeded in order to show that dividing the mass among more tmds can perform better than one tmd, if more than one mode participate on the response. theoretically, one tmd is only a subset of multiple tmds with the same sum of mass. therefore, dividing the mass provided additional room for a performance improvement. the objective function was minimized finding ca,i, ka,i, and ma,i for i =1. . . 3. the following constraints were implemented: • ca,i ≥ 0, • ka,i ≥ 0, • ma,i ≥ 0, • ∑3 i=1 ma,i = 600 kg. from the numerical point of view, the problem is much more complicated than the one with single tmd because 9 variables were optimized. moreover, the function contains multiple local minima. therefore, 104 vol. 26/2020 example of an article with a long title settings no. design method node ka [nm−1] ca [nsm−1] ma [kg] maximal displacement [mm] no tmd 3.447 1 simplified 5 86300 772 600 1.349 2 numerical optimization 5 84100 1259 600 1.103 3 simplified 19 26000 98 200 1.5435 29100 150 200 27 31500 129 200 4 numerical optimization 19 26800 213 208 1.1005 24500 198 157 27 33500 312 236 5 numerical optimization 5 29800 191 189 0.9205 21600 147 176 5 34000 391 235 table 3. comparison of design methods and peak response between 1.7-2.2 hz. it was decided to run the optimization multiple times from one hundred randomly generated initial points, which fulfilled the constraints. this procedure allowed us to find good results, but it is not possible to say that the global minimum was found. 4.6. results the results are summarized in table 3. a total of 5 different tmd settings were found using simplified method and numerical optimization. as expected, tmds designed using den hartog’s criteria were able to reduce the vibration though they were originally proposed for structure with one degree of freedom. it also can be noticed in figure 6 that single tmd can positively affect vibration of adjacent mode shapes if their frequencies are close enough, and if the adjacent mode shapes have sufficiently high ordinate in the position of tmd. figure 6. frf settings no. 1-2. as is shown in figures 6 and 7, the numerical optimization was able to improve the performance of both single and multiple tmds. in the case when starting parameters of tmds were randomly chosen, we expected that each tuned mass damper would converge to a point where it is able to damp one of the figure 7. frf settings no. 3-5. dominant frequencies. this assumption was confirmed and can be seen in figure 8, which shows the frfs of tuned mass dampers. figure 8. displacement of tmds settings no. 5. unexpectedly, the best results were provided by settings where all three tmds were placed in node 5, despite the fact that this is not the place where the highest ordinates of some mode shapes are located. this phenomenon can be explained by closer observation of mode shapes in figures 3, 4, and 5. the force is placed in node 5, therefore all the active mode shapes oscillate with the same phase in this 105 jan štěpánek, jiří máca acta polytechnica ctu proceedings node. however, in nodes 19 and 27, the sixth mode shape oscillates in opposite phase than mode shapes 5 and 7. therefore neither node 19 nor node 27 are the best place for tmd. however, this situation is closely connected to the fact that the optimization was performed to reduce vibration caused by only one specific force. for design of this footbridge, symmetric placement of smaller tmds to both sides of the bridge would solve the problem. 5. conclusions modified mode superposition method was used to evaluate response of structure with tuned mass dampers to speed up the response calculation. utilization of this method provided a powerful tool for numerical optimization, which requires numerous evaluations of steady state response. this response evaluation method provides several important advantages in comparison to standard response calculation: • reduction of numerical complexity without noticeable error thanks to reduced number of mode shapes used. • allows for a fast evaluation of various load vectors, because the system receptance matrix is known. • provides possibility to use experimentally measured mode shapes and natural frequencies for more precise response estimate. • can be used for both single and multiple tmds. simplified tmd design method proposed by den hartog was compared to numerical optimization. the results demonstrate that numerical design is more appropriate for mdof structures and can improve the overall performance of tmds. in all cases of comparison, the analysis showed that optimal damping is higher than the one designed by simplified criterion. further it was shown that multiple tmds with the same sum of mass as a single tmd can provide better results. the best settings of tmds was able to reduce the peak response between 1.7-2.2 hz by 73 %, with tmds total mass of only 0.15 % of the structural mass. this result shows that the common recommendations for tmds to have 1-3 % of mass of the structure may lead to uneconomical design. acknowledgements the authors gratefully acknowledge support from the czech technical university in prague, project sgs19/032/ohk1/1t/11 development and application of numerical algorithms for analysis and modeling in mechanics of structures and materials. references [1] j. p. den hartog. mechanical vibrations. mcgraw hill, new york, 1934. [2] j. brock. a note on the damped vibration absorber. journal of applied mechanics 68(a):284, 1946. [3] setra. footbridges: assessment of vibrational behaviour of footbridges under pedestrian loading, 2006. [4] e. caetano, a. cunha, w. hoorpah, j. raoul. footbridge vibration design. crc press, 2009. [5] fib bulletin 32: guidelines for the design of footbridges, 2005. [6] o. nishihara, t. asami. closed-form exact solution to h infinity optimization of dynamic vibration absorber: ii. development of an algebraic approach and its application to a standard problem. proceedings of the spie, 3989, 2000. doi:10.1117/12.384589. [7] i. m. abubakar, b. j. m. farid. generalized den hartog tuned mass damper system for control of vibrations in structures. earthquake resistant engineering structures 104:185–193, 2009. [8] r. rana, t. soong. parametric study and simplified design of tuned mass dampers. engineering structures 20(3):193–204, 1998. [9] m. b. ozer, t. j. royston. extending den hartog’s vibration absorber technique to multi-degree-of-freedom systems. journal of vibration and acoustics 127(4):341–350, 2005. doi:10.1115/1.1924642. [10] j. štěpánek, j. máca. dynamic response of structures with tuned mass dampers in modal coordinates. vibroengineering procedia 23:13–17, 2019. doi:10.21595/vp.2019.20672. 106 https://doi.org/10.1117/12.384589 https://doi.org/10.1115/1.1924642 https://doi.org/10.21595/vp.2019.20672 acta polytechnica ctu proceedings 26:100–106, 2020 1 introduction 2 response calculation 2.1 single tmd 2.2 multiple tmds 3 simplified tmd design 4 numerical example 4.1 model of the structure 4.2 load 4.3 objective function 4.4 single tmd 4.4.1 simplified design 4.4.2 numerical optimization 4.5 multiple tmds 4.5.1 simplified design 4.5.2 numerical optimization 4.6 results 5 conclusions acknowledgements references acta polytechnica doi:10.14311/app.2020.27.0116 acta polytechnica 27(0):116–120, 2020 © czech technical university in prague, 2020 available online at http://ojs.cvut.cz/ojs/index.php/app electron radiation effect on indentation creep of construction polymers martin ovsík∗, michal staněk, adam dočkal, petr fluxa tomas bata university in zlin, department of production engineering, nám. t. g. masaryka 5555, 760 01 zlín, czech republic ∗ corresponding author: ovsik@utb.cz abstract. cross-linking is a process in which polymer chains are associated through chemical bonds. radiation, which penetrated through specimens and reacted with the cross-linking agent, gradually formed cross-linking (3d net), first in the surface layer and then in the total volume, which resulted in considerable changes in specimen behaviour. creep value is an important parameter that describes the behaviour of a material for the duration of its exposure to long term stress. most of the technical parts used in industry are subjected to the long term stress for their whole life cycle. this can lead to material creep, which directly results in the transformation of dimensions. to eliminate this problem, a number of construction materials was chosen and subsequently irradiated by a source of electrons. this created a 3d network within the polymer structure, which led to an increase of the micro-mechanical and micro-creep properties. evaluation of these modifications was done by state of the art device micro-combi tester manufactured by anton paar. this device lowers the time required to measure the creep by standard technology and it fluently records the changes of indentation depth in relation to applied force. this dependence is then used to calculate the creep values. due to the electron irradiation, a 40 % increase was reached in creep resistance; therefore the useful properties of selected construction materials were improved. keywords: construction polymers, creep, crosslinking, electron radiation, micro-indentation. 1. introduction in the last years, crosslinking of polymers using electron beam radiation has become increasingly used modification method to obtain better mechanical properties. it is an environmentally safe and managed method. during crosslinking, the polymer structure changes, a 3d network is formed from crosslinked bonds and the polymers become more mechanically resistant [1–4]. in the year of 2005, a team of researchers situated in iran investigated the crosslinking of pa6, enriched by 1 to 3 % of crosslinking accelerator triallyl isocyanurate (tac), exposed to accelerated electrons. in this experiment, an electron accelerator with 5 mev was used to expose the samples to radiation doses in range of 40 to 150 kgy. the results showed, that the molecular weight of the polymer samples rose with increasing radiation doses, which was confirmed by the viscosity measurements. furthermore, the gel test indicated, that the pa6 enriched by tac started the crosslinking process even after being exposed to relatively low amounts of radiation. the contents of absorbed water declined with increasing amounts of tac and the absorbed levels of radiation doses [5–9]. maria porubska investigated the radiation effect on pa6 filled with 30% glass fibres and its virgin version. the samples were injection moulded and later irradiated by accelerated electrons with 10 mev energy in doses of 50, 100, 200, 300 and 500 kgy, where 50% of the dosage was applied to each side. the results showed, that regarding the material properties the radiation is more advantageous for unfilled pa6 [10, 11]. the goal of the paper was the study of pa6 irradiated by the electron beams in doses of 0, 66, 99 and 132 of kgy and the radiation effect on micro-creep. 2. methods 2.1. material two types of polyamides were chosen for this experiment, polyamide 6 pa6 v-ptscreamid-11-amn 0 tld and polyamide 66 v-pts-creamid-12-amn 0 tld. these polyamides are used in technical practice. polymer was delivered in granular form, from company pts plastics technology service, germany. to ensure correct meshing, the supplier added 6% of meshing agent (special crosslinking agent taic triallyl isocyanurate) that secures the meshing reaction and creation of 3d network. 2.2. sample preparation process conditions were set according to the manufacturer. these samples were moulded by the injection moulding machine arburg allrounder 470h according to the same process conditions. process conditions (table 1.) all injected materials were dried by dry and transport unit arburg thermolift 100-2. test samples were manufactured with norm čsn en iso 527 in mind. 116 http://dx.doi.org/10.14311/app.2020.27.0116 http://ojs.cvut.cz/ojs/index.php/app vol. 27/2020 electron radiation effect on indentation creep of construction polymers figure 1. dependence of indentation depth vs. indentation time of tested pa 6. figure 2. dependence of indentation depth vs. indentation time of tested pa 66. parameters unit pa6 pa66 injection pressure mpa 65 80 cooling time s 17 17 mould temperature °c 70 70 zone 1 °c 220 220 zone 2 °c 250 250 zone 3 °c 270 270 zone 4 °c 310 310 table 1. injection conditions. all these samples were sent to the company b. g. s. to germany, where the samples were irradiated. the range of the dosages was set in compliance with experience gained from industrial practice to 33, 66, 99, 132, 165 and 198 kgy. each cycle in the accelerator exposed the test sample to the radiation dose of 33 kgy (3 cycle of 33 kgy = 99 kgy). beta rays are characterized by their ability to irradiate individual packages within a few seconds. all samples were irradiated with electron (beta) rays (electron energy 10 mev). 2.3. micro-indentation creep indentation creep was measured by means of microhardness tester (mht3) by anton paar, switzerland, using vickers indenter tip, according to the csn en iso 14577. on each material at least ten indents were made and the results were statistically treated. standard simple loading-unloading mode was used. the indentation parameters were set according to the standard; see table 2. 117 m. ovsík, m. staněk, a. dočkal, p. fluxa acta polytechnica figure 3. indentation creep. figure 4. gel content. parameters unit value maximum load n 1 load/unload speed n/min 2 holding time s 21600 table 2. indentation parameters. 3. conclusions cit = h2 − h1 h1 · 100, (1) the identification creep was calculated according to the equation 1,where h1 is the indentation depth at time t1 of reaching the test force (which is kept constant), h2 is the indentation depth at time t2 of holding the constant test force [12, 13]. 3.1. gel content a gel content (eq. 2) test is performed in order to determine the non-dissolved gel content of the given material-according to the astm d 2765 standardtest method c. a portion of 0.5 g (of electron-beam irradiated pa 6 and pa 6.6 material) weighed with a precision of five decimal places on a "swiss made ep 125 sm" weighing apparatus (dietikon, switzerland) was mixed with 100 ml of solvent. xylene was used on the pa 6.6 because it dissolves the amorphous part of this material, and the crosslinking part does not dissolve. the mixture was extracted for 24 118 vol. 27/2020 electron radiation effect on indentation creep of construction polymers h. then, the solutes were separated by distillation. after removing the residual xylene, the crosslinking extract was dried for 8 h, in a vacuum, at 100 °c. the dried and cooled residue was weighed again with a precision of five decimal places and compared to the original weight of the portion. the result is stated in percentage as the degree of crosslinking [14]: gi = m3 − m1 m2 − m1 · 100 (2) where gi is the degree of crosslinking of each specimen expressed in percentage, m1 is the weight of the cage and lid in milligrams, m2 is the total of the weights of the original specimen, cage and lid in milligrams, and m3 is the total of the weights of the residue specimen, cage and lid in milligrams. 4. results and discussion the contemporary indentation method allows the measurements of creep behaviour of different types of materials, including the polymers. this method is based on the principle of immediate detection of indentation depth in dependence on time. the differences in the depth, reached by the indentor during pre-set loading force (1n) with holding time 21 600s, can then be used to calculate the indentation creep. figures 1 and 2 display the indentation curves for individual radiation doses for two tested materials (pa6 and pa66). the variance in curves indicates the different behaviour of the tested materials as well as the change of creep behaviour. the results of the indentation creep measurements show (figure 3), that the radiation crosslinking improves the properties of the tested material. the virgin pa6 displayed higher value of indentation creep, while the material exposed to lower amounts of radiation showed a deterioration in properties (66 kgy). on the other hand, the samples exposed to higher amounts of radiation indicated an improvement in said properties, with the test sample irradiated by 165 kgy. the difference between the virgin pa6 and the irradiated pa6 was 16%. finally, the test sample exposed to the highest doses of radiation demonstrated a slight decrease in the creep behavior. for pa66, similar tendencies were found. the worst values of indentation creep were found in virgin pa66. with increasing radiation dosage, the indentation creep was measured to incrementally improve up to the maximum, which was found in test samples irradiated by 165 kgy. the difference between this sample and the virgin material was 39%. samples irradiated by 198 kgy displayed a minor decline in the indentation creep. the results of the indentation test were confirmed by the gel content test (figure 4). the value of gel represents the percentage volume of 3d network created in the structure of tested polymers. the virgin versions of both tested materials contained 0% of gel. on the other hand, for pa6, the maximum value of the gel (79%) was measured in samples irradiated by 165 kgy, which directly corresponds with the results of the indentation test. radiation higher than 165 kgy proved to decrease the content of gel, which was probably caused by the degradation of the material due to the high intensity of the radiation. for pa66, the highest content of gel was measured in test samples irradiated by 99 kgy, which also confirms the results of the indentation creep measurements. the goal of this study is to examine the effect of beta radiation on the indentation creep of the tested polymers (pa6 and pa66). the research that was done proved, that the irradiation of the samples has a positive effect on the creep behaviour of construction polymers which were chosen. the best improvement was measured in test samples irradiated by 165 kgy dosage for both materials. in comparison to virgin material, the indentation creep rose by 16% for the pa6 and by 39% for the pa66. the results of the indentation creep were confirmed by the gel content test, where the highest increase in crosslinking parts was found in test samples irradiated by higher amounts of radiation. the biggest radiation dosage that was used (165 kgy), proved to worsen the indentation creep as well as the gel content, which was probably caused by the degradation of the material due to the high intensity of the radiation. acknowledgements this work was supported by the european regional development fund under the project cebia-tech instrumentation no. cz.1.05/2.1.00/19.0376 and by the ministry of education, youth and sports of the czech republic within the national sustainability program project no. lo1303 (msmt-7778/2014). moreover, it was supported by the internal grant agency of tbu in zlin: no. iga/ft/2020/003. references [1] y. kamran, p.-l. larsson. second-order effects at microindentation of elastic polymers using sharp indenters. materials and design 32(6):3645–3653, 2011. qc 20110520, doi:10.1016/j.matdes.2011.01.043. [2] k. rajeesh, r. gnanamoorthy, r. velmurugan. effect of humidity on the indentation hardness and flexural fatigue behavior of polyamide 6 nanocomposite. materials science and engineering: a 527(12):2826 – 2830, 2010. doi:10.1016/j.msea.2010.01.070. [3] j. schone, d. tondl, r. lach, et al. analysis of pa 6 nanocomposites indentation and creep behavior as a function of temperature and load level using different indentation techniques. polimery 59:722–728, 2014. doi:10.14314/polimery.2014.722. [4] j. g. drobny. radiation technology for polymers. crc press, 2003. [5] o. n. tretinnikov, s. ogata, y. ikada. surface crosslinking of polyethylene by electron beam irradiation in air. polymer 39(24):6115 – 6120, 1998. doi:10.1016/s0032-3861(98)00075-5. 119 https://doi.org/10.1016/j.matdes.2011.01.043 https://doi.org/10.1016/j.msea.2010.01.070 https://doi.org/10.14314/polimery.2014.722 https://doi.org/10.1016/s0032-3861(98)00075-5 m. ovsík, m. staněk, a. dočkal, p. fluxa acta polytechnica [6] g. zamfirova, v. gaydarov, t. zaharescu. microindentation study of electron beam irradiated polyamide samples. chemicke listy 104:283–286, 2010. [7] j. dobransky, p. baron, m. kocisko, et al. solving depressions formed during production of plastic molding. metalurgija -sisak then zagreb54:496–498, 2015. [8] a. j. uddin, y. gotoh, y. ohkoshi, et al. crystal modulus of a new semiaromatic polyamide 9-t. polymer engineering & science 52, 2012. doi:10.1002/pen.22086. [9] k. makuuchi, s. cheng. radiation processing of polymer materials and its industrial applications. wiley, 2011. [10] m. porubska, d. babic, i. janigova, et al. the effect of gamma irradiation in air and inert atmosphere on structure and properties of unfilled or glass fibre-reinforced polyamide 6. polymer bulletin 2015. doi:10.1007/s00289-015-1576-0. [11] m. porubska, i. janigova, k. jomova, i. chodak. the effect of electron beam irradiation on properties of virgin and glass fiber-reinforced polyamide 6. radiation physics and chemistry 102:159 – 166, 2014. doi:10.1016/j.radphyschem.2014.04.037. [12] m. ovsik, d. manas, m. manas, et al. the effect of cross-linking on nano-mechanical properties of polyamide. in polymers and composites in engineering: processing, properties and applications, vol. 699 of key engineering materials, pp. 37–42. trans tech publications ltd, 2016. doi:10.4028/www.scientific.net/kem.699.37. [13] w. oliver, g. pharr. measurement of hardness and elastic modulus by instrumented indentation: advances in understanding and refinements to methodology. journal of materials research 19(1):3–20, 2004. doi:10.1557/jmr.2004.19.1.3. [14] m. ovsik, m. stanek, m. reznicek, l. hylova. study of nano-creep of unfilled and filled cross-linking polypropylene. materials science forum 919:103–110, 2018. doi:10.4028/www.scientific.net/msf.919.103. 120 https://doi.org/10.1002/pen.22086 https://doi.org/10.1007/s00289-015-1576-0 https://doi.org/10.1016/j.radphyschem.2014.04.037 https://doi.org/10.4028/www.scientific.net/kem.699.37 https://doi.org/10.1557/jmr.2004.19.1.3 https://doi.org/10.4028/www.scientific.net/msf.919.103 acta polytechnica 27(0):116–120, 2020 1 introduction 2 methods 2.1 material 2.2 sample preparation 2.3 micro-indentation creep 3 conclusions 3.1 gel content 4 results and discussion acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2020.26.0086 acta polytechnica ctu proceedings 26:86–93, 2020 © czech technical university in prague, 2020 available online at https://ojs.cvut.cz/ojs/index.php/app euler and exponential algorithm in viscoelastic analyses of laminated glass jaroslav schmidt∗, alena zemanová czech technical university in prague, faculty of civil engineering, department of mechanics, thákurova 7, 166 29 prague 6, czech republic ∗ corresponding author: jaroslav.schmidt@fsv.cvut.cz abstract. laminated glass combines two remarkable materials: glass and a polymer ply. while glass is stiff and brittle, the polymer ply is a rate-dependent compliant material. together, they form a material which keeps the aesthetic value of glass, and due to the polymer, no fragile collapse appears. the polymer ply exhibits timeand temperature-dependency, whereas glass suffers from brittle fracture, which makes the analysis difficult. in this article, a 2d sectional plane-stress model for the viscoelastic analysis of laminated glass is presented. this study presents one step in the development of a phase-field-based damage solver for laminated glass to select the optimal time-integration scheme for a quasistatic-analysis and later also for dynamics. the validation against experimental data is provided, and the model reduction is also discussed. keywords: laminated glass, viscoelasticity, generalised maxwell model, exponential algorithm, euler method. 1. introduction nowadays laminated glass is a popular composite multi-layer material which consists of several glass plates and polymer interlayers. the laminated process improves post-breakable behaviour compared to conventional glass but retains its aesthetic value [1]. this makes laminated glass a suitable material for transparent structures in architecture and other fields. unfortunately, despite its advantages, laminated glass is still a very brittle material which makes the simulations of the initiation and propagation of cracks difficult. this paper partially follows the project objectives specified in our research project focused on the design and advanced modelling of forced-entry and bullet resistant glass structures. one of the goals of the project is to simulate crack development in laminated glass using phase-field fracture method [2] to better understand and explain the preand post-fracture behaviour of laminated glass under impact loading. several dynamic and quasi-static experiments were performed, and the next step is the validation of the collected experimental data against numerical models for laminated glass. at this moment, the numerical solver is implemented for laminated beams and plates made of several elastic layers under quasi-static loading. this study presents another step in the development of the phase-field-based damage solver with the focus on the timeand temperature-dependent behaviour of the polymer interlayer. at this stage, the analysis deals with a quasi-static regime only. in particular, two questions concerning interlayer modelling are discussed: • what is the optimal time-integration scheme with respect to the computational time and implementation demands? • how is the global response of the model influenced by involving only certain cells of the generalised maxwell chain into the numerical analysis? moreover, this study provides also validation of material parameters of two interlayer foils obtained in the previous step of the project [3]. the paper is organised as follows: section 2 introduces the 2d plane-stress model and presents the constitutive relationships. next, in section 3, two time-integrators are recalled, and finally, section 4 provides numerical simulations, validation, and parametric study, concluded with the discussion of results in section 5. 2. laminated glass model in this section, the mechanical model for the numerical analysis is presented. the constitutive assumptions for glass and interlayer are as follows: • glass has an almost purely elastic response. therefore, it is modelled as an elastic isotropic material described by two constant: young’s modulus e and poisson’s ratio ν. • the interlayer polymer is strongly timeand temperature-dependent material effectively modelled by the generalised maxwell model [4, appendix a], see figure 1, where the material constants are the stiffnesses of springs gp and the viscosities ηp and the long-term shear modulus g∞ corresponding to the single stiffness. the constitutive relationship is expressed in the shear variables 86 https://doi.org/10.14311/app.2020.26.0086 https://ojs.cvut.cz/ojs/index.php/app vol. 26/2020 viscoelastic analysis of laminated glass instead of in the normal components as the interlayer material is almost incompressible, and shear in the interlayer is dominant in bending regime. then, τ and γ represent shear stress and strain respectively. γ(t) γe,1(t) γv,1(t) τ(t) g1 g2 g3 gp g∞ η1 η2 η3 ηp figure 1. generalised maxwell constitutive model. a practical choice for the mechanical model of laminated glass is a layer-wise model consisting of several independent beams or plates connected together at the interfaces. despite its advantages, we validate experiments against the plane-stress 2d model, where the longitudinal cross-section is discretizted. one of the reasons for this approach is to check if the throughthe-thickness compression of the interlayer is really negligible and can be neglected. 3. time integrators the brief overview of the exponential and backwardeuler integrators is presented in this section. 3.1. governing equations the following model assumptions are considered: • a special two-dimensional state of stress called plane stress is assumed to reduce the originally three-dimensional task and subsequently the computational cost. • because all of the laminated glass samples were simply-supported, the analysis assumes only small deformations and deflections. the comparison with experiments shows that this assumption is sufficient as no stabilisation due to membrane forces occurred. • we assume that the poisson ratio of the polymer interlayer is constant. for further comparison with a constant bulk modulus assumption see also [5]. as mentioned before, the polymer material point is described by the generalised maxwell chain which consists of one single linear spring with stiffness g∞ and several maxwell cells. each cell is characterised by a spring with stiffness gp and a damper with viscosity ηp. the cells are indexed by p ∈ p, where p = 〈0, p 〉 ⊂ n. the time integrators are used only for the viscoelastic interlayer and glass is considered as rate-independent. under these assumptions, the behaviour of the interlayer can be described by a weak form of the equilibrium equation ∫ ωfoil δε : g∞dν : ε+ ∑ p∈p σp  dω− δfext = 0, (1) complemented by ordinary differential equations (ode) σ̇p gp + σp ηp = dν : ε̇, p ∈ p. (2) in the former equations δfext stands for the virtual work done by the external loads on the virtual displacement δu, and the small strain tensor ε is obtained as the symmetric displacement gradient ε = ∇su. the dimensionless tensor dν corresponds to the stiffness tensor of an isotropic material expressed by a unit shear modulus and the poisson ratio of ν. the stress carried by p-th cell is denoted by σp. on the other hand, the governing equation for the deformation of glass does not contain viscous stress tensors σp, and the corresponding weak form becomes∫ ωglass δε : gglassdν : ε dω− δfext = 0. (3) the implementation for laminated glass, which combines the elastic glass domain and the viscoelastic interlayer, is described in section 4. equations (1)–(2) contain both space and time gradients. the space discretization is captured by using conventional finite element methods (fem), whereas the type of the time discretization is a part of this analysis. two discretization options are recalled in the following subsection. 3.2. numerical integrators the common feature for all presented time integrators is the discretization into a set of time instants t = {0, . . . , tmax}, where tmax is the largest time of interest. backward euler method for the backward euler method, we approximate the time gradient by the firstorder backward finite difference •̇(ti+1) ≈ •(ti+1)− •(ti) ∆ti . (4) to shorten the notation, the time increment ∆ti is defined as ti+1 − ti, and the explicit time dependency is omitted and replaced by a lower index, therefore •(ti) ≡ •i. if we use the above-mentioned gradient approximation of the ode evaluated in time instant ti+1, we get σp,i+1 − σp,i gp∆ti + 1 ηp σp,i+1 = 1 ∆tidν : (εi+1 − εi) . (5) then, we can express explicitly σi+1 as σp,i+1 = 1 ζp,i (dν : (εi+1 − εi)gp + σp,i) , (6) 87 jaroslav schmidt, alena zemanová acta polytechnica ctu proceedings where ζp,i = 1 + ∆ti/τp is an auxiliary dimensionless parameter. the stress tensor can be directly substituted into the equilibrium equation to get the weak form for εi+1 and the resulting equation ∫ ωfoil δε : g∞ + ∑ p∈p gp ζp,i dν : εi+1 dω − ∫ ωfoil δε : ∑ p∈p gp ζp,i dν : εi dω (7) + ∫ ωfoil δε : ∑ p∈p σp,i ζp,i dω− δfext = 0 governs the fe analysis for εi+1 determination. then, the stress tensors are updated using (6). this procedure is repeated for all time instants. exponential algorithm the main difference for the exponential algorithm compared with the euler method is the discretization of the stress tensor. as we suppose that the strain is distributed linearly within the time interval 〈ti, ti+1〉, the viscous stress tensor is solved exactly using the corresponding ode. the linearly distributed strain induces a constant strain rate, and the governing equation for the maxwell unit becomes σ̇p(ti + si) gp + σp(ti + si) ηp = 1 ∆ti dν : (εi+1 − εi) , (8) where si is an auxiliary time variable starting from ti and going forward in time. the relation between the original time t and the variable si is si = t − ti. considering the boundary condition σp(si = 0) = σp,i, the stress evaluated in t = ti+1 reads as σp,i+1 = σp,ie −∆ti τp + ηp ∆ti ( 1− e− ∆ti τp ) dν : (εi+1 − εi). (9) the backward substitution into the equilibrium condition (1) in ti+1 gives ∫ ωfoil δε : g∞ + ∑ p∈p ηp ∆ti ξp,i dν : εi+1 dω − ∫ ωfoil δε : ∑ p∈p ηp ∆ti ξp,idν : εi dω (10) + ∫ ωfoil δε : ∑ p∈p e− ∆ti τp σp,i dω− δfext = 0, where another auxiliary variable ξp,i = 1− e− ∆ti τp is introduced. 4. numerical results 4.1. example all calculations were performed on a longitudinal crosssection of a laminated glass beam assuming a planestress state, see figure 2. the dimensions of the beam are: the thicknesses h1 = h3 = 10 mm, h2 = 0.76 mm, the overhanging edges l1 = l3 = 30.0 mm and their span l2 = 1040.0 mm. h1 h2 h3 l1 l2 l3 figure 2. scheme of laminated glass sample. the laminated glass samples were simply-supported and loaded by prescribed uniformed pressure in a vacuum chamber. the time evolution of pressure was almost the same for the two tested samples with eva and pvb interlayer, see figure 3. at the beginning of loading, the pressure increase is almost quadratic, whereas later the slope becomes linear. 0 5 10 15 20 25 30 35 40 time [s] 5 0 5 10 15 20 25 30 35 40 pr es su re lo ad [k pa ] pressure load figure 3. time evolution of pressure recorded during the experimental testing. 4.2. material parameters glass is treated as a perfectly elastic material described by the young modulus eglass = 76.6 gpa obtained from indentation tests and by the poisson ratio νglass = 0.22. whereas the interlayer polymer is effectively described by the generalised maxwell model to capture its viscoelastic time/temperature dependent nature. in the tested samples, an ethylene-vinyl acetate (eva) and a polyvinyl butyral (pvb) foils were used as the interlayer material, with the material constants taken from [3] and given in table 1. the poisson ratio was assumed to be constant, νfoil = 0.49. 88 vol. 26/2020 viscoelastic analysis of laminated glass eva pvb τp [s] gp [kpa] gp [kpa] 10−9 6933.9 – 10−8 3898.6 – 10−7 2289.2 – 10−6 1672.7 – 10−5 761.6 1,782,124.2 10−4 2401.0 519,208.7 10−3 65.2 546,176.8 10−2 248.0 216,893.2 10−1 575.6 13,618.3 100 56.3 4988.3 101 188.6 1663.8 102 445.1 587.2 103 300.1 258.0 104 401.6 63.8 105 348.1 168.4 106 111.6 – 107 127.2 – 108 137.8 – 109 50.5 – 1010 322.9 – 1011 100.0 – 1012 199.9 – table 1. prony series for generalised maxwell model complemented with the long-term moduli g∞ = 682.18 kpa for evalam 80-120 (eva-based foil) and g∞ = 232.26 kpa for trosifol bg r20 (pvbbased foil) for t0 = 20 ◦c from [3]. the room temperature during the experimental testing was 24 ◦c. for the polymer ply, the temperature dependency is incorporated into the material model using the time-temperature superposition principle [6, chapter 11]. time and temperature are tied together via a multiplicative parameter at as τp(t ) = at(t ) ηp gp . (11) the parameter at can be evaluated through the william–landel–ferry equation (wlf), [7], log10 at = − c1(t − t0) c2 + t − t0 , (12) where the constants c1 and c2 are additional material parameters, and t0 is a reference temperature. for the reference temperature t0 = 20 ◦c, the calibration process described in [3] provides the following values: c1 = 339.1 and c2 = 1185.8◦c, or c1 = 8.635 and c2 = 339.102, for eva or pvb foil respectively. 4.3. solver for a given time instant ti+1, the unknown displacement field u(ti+1) is calculated numerically by the conventional finite element method. the part of the discretized domain is illustrated in figure 4, where the red colour represents glass layers and the blue colour the interlayer. due to the disproportionality between the glass and interlayer thicknesses, the mesh is refined near the centerline with at least two elements through the ply thickness. moreover, the quadratic basis functions were employed. the computational fem library fenics was used to quickly implement the presented models for all numerical experiments. this solver with a high-level python interface provides an efficient tool for solving partial differential equations [8]. figure 4. finite element mesh: glass domain in red and the interlayer in blue. for modelling of the glass/polymer coupling, an indicator function i(x) was introduced as follows i(x) = { 0, if x ∈ ωglass, 1, if x ∈ ωfoil. (13) all viscous members of the governing equations, section 3.1, are multiplied by this function. therefore, the rate-dependent expressions appear only in the interlayer domain ωfoil. let us highlight that no delamination, glass fracture, or damage of polymer is considered in this study. 4.4. validation and comparison the main advantage of the exponential algorithm is that the time interval can be covered by a series of logarithmically increasing steps without significant loss of accuracy. therefore, the computational cost and the number of time increments can be significantly reduced, for example for long-term stress relaxation or creep quasi-static tests. the results for 20 logarithmically or uniformly distributed time steps are presented in figure 5. the reference solution correspondes to the backward euler method with the time step ∆t = 0.05 s. the detailed comparison of responses on the right side shows the advantage of the exponential algorithm for logarithmically increasing time steps, as was supposed. on the other hand, the performance of both methods is the same for uniformly distributed time steps. for uniformly distributed time steps, three different time increments were tested, i.e., 0.05 s, 0.5 s and 2.0 s. the numerical responses are presented together with the experimental data in figure 6. this validation shows that even the numerical response with 89 jaroslav schmidt, alena zemanová acta polytechnica ctu proceedings 0.0 0.5 1.0 1.5 2.0 2.5 3.0 time [s] 0.00000 0.00005 0.00010 0.00015 0.00020 0.00025 0.00030 0.00035 0.00040 0.00045 m ax im um d ef le ct io n [m m ] reference exp. log be log 0.700 0.705 0.710 0.715 0.720 time [s] 0 2 4 6 8 10 m ax im um d ef le ct io n [m m ] 1e 7+3.4e 5 detail (a) logarithmically distributed time steps 0.0 0.5 1.0 1.5 2.0 2.5 time [s] 0.00000 0.00005 0.00010 0.00015 0.00020 0.00025 0.00030 0.00035 0.00040 0.00045 m ax im um d ef le ct io n [m m ] reference exp. be 0.7880 0.7885 0.7890 0.7895 0.7900 time [s] 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 m ax im um d ef le ct io n [m m ] 1e 7+4.2e 5 detail (b) uniformly distributed time steps with ∆t = 0.125 s figure 5. time evolution of deflection of laminated glass sample under uniformly distributed pressure with quadratically increasing intensity according figure 3 obtained by: reference – backward euler method with ∆t = 0.05 s, exp – exponential algorithm, and be – backward euler method. 15 20 25 30 time [s] 0 5 10 15 m ax im um d ef le ct io n [m m ] exponential backward euler experiment ∆t= 0.05 s 15 20 25 30 time [s] 0 5 10 15 exponential backward euler experiment ∆t= 0.5 s 15 20 25 30 time [s] 0 5 10 15 exponential backward euler experiment ∆t= 2.0 s (a) response for samples with eva foil 14 16 18 20 22 24 26 28 time [s] 0 5 10 15 m ax im um d ef le ct io n [m m ] exponential backward euler experiment ∆t= 0.05 s 14 16 18 20 22 24 26 28 time [s] 0 5 10 15 exponential backward euler experiment ∆t= 0.5 s 14 16 18 20 22 24 26 28 time [s] 0 5 10 15 exponential backward euler experiment ∆t= 2.0 s (b) response for samples with pvb foil figure 6. validation of numerical responses provided by the exponential and backward euler method against the experimental measurement for three different time steps: ∆t = 0.05 s, ∆t = 0.5 s and ∆t = 2.0 s. 90 vol. 26/2020 viscoelastic analysis of laminated glass 15 20 25 30 time [s] 0 5 10 15 m ax im um d ef le ct io n [m m ] dt=0.05s dt=0.5s dt=2.0s dt=5.0s dt=10.0s b-euler figure 7. numerical response of backward euler method for different time steps. the coarser time steps is in good agreement with the measured deflections. the dashed line representing the exponential algorithm almost coincides with the solid one for the backward euler method in all cases. for larger time steps, the differences of numerical predictions against the measured experimental response would increase, see figure 7. on the other hand, we aim to describe the crack initiation and propagation in glass layers, which requires small time steps. it results from this comparison that both methods are able to capture the time/temperature-dependent response of polymer interlayers well. further, the validation against experimental data proved that the parameters of the generalised maxwell model for both foils, obtained from material tests and summarised in [3], provides very good predictions for quasi-static behaviour of laminated glass samples with eva-based and pvb-based foils of the same type. the response is just slightly stiffer at the beginning of loading for the sample with eva foil and a slightly softer before the fracture of glass for the sample with pvb. however, the largest error is under 5%. 4.5. sufficient number of parameters the comparison of the viscoelastic response of laminated glass samples with three elastic simulations in figure 8 demonstrates the influence of shear coupling of glass plies due to the interlayer. the behaviour of the polymer was approximated as an elastic one with three different values of shear moduli. the response of the laminated glass sample is often bounded by the behaviour of a monolithic glass plate with the same overall thickness and by the response of two independent plies representing the two glass plates without any coupling. we narrowed this range, assuming a constant long-term or initial shear modulus, g∞ or g0. for the sample with eva foil, a reasonable approximation of the viscoelastic response was found using the shear modulus of the interlayer corresponding to a half of the loading time g(t/2). for pvb, this assumption gives still a softer prediction of deflections compared with the measured data. in the next stage, we tried to reduce the number of spring-dashpot cells as each of these maxwell cells is represented by a second-order viscous stress tensor. therefore, it is desirable to balance the number of cells and save the computational time. the exponential and backward euler methods provided the same response for the fine time step. thus, only the backward euler method was used for the analysis. the influence of the individual maxwell cell can be assessed using the multiplicator ζp,i. it can be seen from equation (7) that the significant viscous stresses are those where the expression 1/ζp,i is not close to zero. recalling the definition of ζp,i, 1 ζp,i = 1 1 + ∆ti τp = τp τp + ∆ti , (14) the whole fraction is close to one if the relaxation time is much more greater than the given time step τp � ∆ti, and the viscous stress is activated. on the other hand if τp � ∆ti, the denominator is large and the whole fraction is close to zero. to verify this observation we perform two numerical tests. figure 9 shows that if we remove the low-relaxation-time cells, the response is not affected (left), and conversely, removing of cells with large relaxation time have significant influence on the response (right). the reference solution plotted by the red line corresponds to the original generalised maxwell model with all cells, table 1. the number of cells was gradually decreased. figure 9 (left) illustrates the case, for which the cells with the smallest relaxation times were neglected. it can be seen that neglecting even first seven cells for eva or three for pvb with the smallest relaxation times did not affect the solution as the time evolution of deflection is indistinguishable from the response of the full generalised maxwell model. on the other hand, removing a few maxwell cells with the largest relaxation times significantly affected the deflection of the sample, figure 9 (right). this is caused by the fact that we start in the proposed formulation with the same long-term modulus. 5. conclusions the numerical analysis of laminated glass samples under quasi-static loading is presented in this paper and validated against experimental measurements. the laminated glass plate is discretized as a 2d longitudinal cross-section with two elastic glass layers and the viscoelastic interlayer. the polymer ply was characterised by the generalised maxwell model with parameters derived from material tests in [3]. the conclusions from this study are: • both tested methods for time integration, i.e., the exponential algorithm and the backward euler method, can be used for the subsequent finite element analysis of glass fracture. the difference in results is negligible for values of time steps needed in proposed phase-field formulation. 91 jaroslav schmidt, alena zemanová acta polytechnica ctu proceedings 15 20 25 30 time [s] 0 5 10 15 m ax im um d ef le ct io n [m m ] linear g_inf linear g_0 linear g(t/2) exponential ∆t= 0.05 s 15 20 25 30 time [s] 0 5 10 15 m ax im um d ef le ct io n [m m ] linear g_inf linear g_0 linear g(t/2) exponential ∆t= 0.05 s (a) sample with eva foil (b) sample with pvb foil figure 8. comparison of the viscoelastic response of laminated glass samples obtained by the exponential algorithm with the elastics ones with three different shear moduli of interlayer assumed. 15 20 25 30 time [s] 0 5 10 15 m ax im um d ef le ct io n [m m ] n=0 n=5 n=10 n=15 n=22 (ref.) 15 20 25 30 time [s] 0 5 10 15 m ax im um d ef le ct io n [m m ] n=0 n=5 n=10 n=15 n=22 (ref.) (a) samples with eva foil 15 20 25 30 time [s] 0 5 10 15 m ax im um d ef le ct io n [m m ] n=0 n=4 n=8 n=11 (ref.) 15 20 25 30 time [s] 0 5 10 15 m ax im um d ef le ct io n [m m ] n=0 n=4 n=8 n=11 (ref.) (b) samples with pvb foil figure 9. time evolution of deflections predicted by the backward euler method with time step ∆t = 0.5 s: reference – numerical simulation with all maxwell (spring-dashpot) cells or reduced – with n maxwell cells if the cells with the smallest relaxation times (left) or the largest relaxation times (right) are removed. 92 vol. 26/2020 viscoelastic analysis of laminated glass • for laminated glass samples with eva-based or pvb-based foils, the formerly fitted material parameters provides reliable numerical predictions of deflections. the errors in deflections were under 5%. • the computational cost can be reduced without the loss of accuracy using a constant shear modulus for the interlayer g(t/2) for eva-based samples. for pvb-based samples, three maxwell cells with the smallest relaxation times can be neglected without an observable change in response of the sample. finally, let us conclude that this study was focused on the comparison of the response of laminated glass before fracture with emphasis on the time/temperature-dependent behaviour of polymer interlayers. no phase-field description of glass fracture is presented and discussed in this paper, and also no damage of polymer foils is assumed. the reason for this is the fact that based on the phase-field model formulation and setting, the linear elastic phase can also be affected by the damage evolution if the growth of damage starts immediately when the strain becomes nonzero. then, the model does not reproduce the linear elastic behaviour of laminated glass in the first stage, and the stress-strain diagram becomes curved. therefore, the phase-field model for brittle fracture of glass will be discussed separately to avoid the possible deviation from linearity and misinterpretation of results. acknowledgements this publication was supported by the czech science foundation, the grant no. 19-15326s and by the grant agency of the czech technical university in prague, grant no. sgs19/033/ohk1/1t/11. references [1] m. haldimann, a. luible, m. overend, et al. structural use of glass. structural engineering documents. international association for bridge and structural engineering, 2008. [2] b. bourdin, g. a. francfort, j. j. marigo. the variational approach to fracture. journal of elasticity 91:5—-148, 2008. doi:10.1007/s10659-007-9107-3. [3] t. hána, t. janda, j. schmidt, et al. experimental and numerical study of viscoelastic properties of polymeric interlayers used for laminated glass : determination of material parameters. materials 12(14), 2019. doi:10.3390/ma12142241. [4] z. p. bažant, m. jirásek. creep and hygrothermal effects in concrete structures. springer, 2018. doi:10.1007/978-94-024-1138-6. [5] a. zemanová, j. zeman, m. šejnoha. comparison of viscoelastic finite element models for laminated glass beams. international journal of mechanical sciences 131-132:380–395, 2017. doi:10.1016/j.ijmecsci.2017.05.035. [6] j. d. ferry. viscoelastic properties of polymers. john wiley and sons, 1980. doi:10.1149/1.2428174. [7] m. l. williams, r. f. landel, j. d. ferry. the temperature dependence of relaxation mechanisms in amorphous polymers and other glass-forming liquids. journal of the american chemical society 77(14):3701–3707, 1955. doi:10.1021/ja01619a008. [8] m. s. alnæs, j. blechta, j. hake, et al. the fenics project version 1.5. archive of numerical software 3(100), 2015. doi:10.11588/ans.2015.100.20553. 93 https://doi.org/10.1007/s10659-007-9107-3 https://doi.org/10.3390/ma12142241 https://doi.org/10.1007/978-94-024-1138-6 https://doi.org/10.1016/j.ijmecsci.2017.05.035 https://doi.org/10.1149/1.2428174 https://doi.org/10.1021/ja01619a008 https://doi.org/10.11588/ans.2015.100.20553 acta polytechnica ctu proceedings 26:86–93, 2020 1 introduction 2 laminated glass model 3 time integrators 3.1 governing equations 3.2 numerical integrators 4 numerical results 4.1 example 4.2 material parameters 4.3 solver 4.4 validation and comparison 4.5 sufficient number of parameters 5 conclusions acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2019.21.0005 acta polytechnica ctu proceedings 21:5–9, 2019 © czech technical university in prague, 2019 available online at http://ojs.cvut.cz/ojs/index.php/app use of finely ground recycled concrete for improvement of interfacial adhesion in fiber-reinforced cementitious composites radim hlůžek∗, jan trejbal czech technical university in prague, faculty of civil engineering, thákurova 7, 166 29 prague, czech republic ∗ corresponding author: radim.hluzek@fsv.cvut.cz abstract. this paper deals with an improvement and an assessment of a polymeric macro-fibers adhesion to the cement matrix. for this purpose, two approaches were employed – (i) roughening of fibers using a plasma treatment and (ii) an addition of finely ground recycled concrete (amount 30 wt.%) to the matrix ensuring the roughness of interfacial zones. polyethylene terephthalate (pet) and polypropylene (pp) fibers, both ca. 0.3mm in a diameter, were used. these were surface roughened using a cold oxygen plasma treatment and then observed by means of the scanning electron microscopy. consequently, pull-out tests of an individual fiber embedded 25mm in the matrix were performed, while the force needed for fiber pullout was recorded. results have shown that plasma treated fibers reached on a better adhesion with the matrix by up to ca. 5% (pet) and 20% (pp), if compared to reference fibers. when recycled concrete was used, the adhesion increased further by about 5–10% for both fiber types. keywords: recycled concrete, waste, adhesion, fiber reinforcement, fibrous composite. 1. introduction it is widely known that waste production shows an unsustainable trend in global scale. this also hold true for the construction industry. it is reported that global production of construction demolition waste ranges from 3 to 10 billion tonnes per year of which 3.5million tonnes in the czech republic. these basic data provide the sufficient proof of why it is necessary to deal with the reduction of waste and its re-use. therefore, a law on a total ban on landfilling of inert waste with effect from 2024 was ready within the european union [1]. it is therefore necessary to find new efficient solutions for the reprocessing of construction waste including concrete from demolished buildings [2]. in civil engineering praxis, waste concrete is crushed and aggregate is then separated and re-used within production of new concrete mixtures. but the question of the use of dust emerging during crushing and grinding processes arises [3]. such dust may contain up to 10% of un-hydrated cement grains [4]. these can be exposed and isolated from dust using a highspeed milling [5]. it is possible to use such obtained material as replacement of cement at the form of socalled active micro aggregate. it is appropriate for production of lightweight concrete blocks, mortars, etc. [4–6]. however, the potential of this material is still limited due to particles different thickness and size and presence of various impurities [4, 7, 8]. some researches tried to use finely ground recycled concrete for filling of pores in various cement-based composites. individual particles can fill also spaces in interfacial zones in fibrous composite materials and thus contribute to the better adhesion between reinforcing fibers and the matrix [9, 10]. unfortunately, such phenomenon has not been so far clearly explained, tested and described. therefore, we tested the adhesion between smooth and roughened fibers and the cement matrix containing finely ground recycled concrete by means of pull out tests of individual fibers from specimens made from both matrices. 2. materials 2.1. fibers two types of polymeric macro-fibers were used: (i) polyethylene terephthalate (pet) and (ii) polypropylene (pp). these were made by the czech manufacturer spokar (plc). both types were surface treated using cold oxygen plasma to roughen and to activate their surface. this process has been described in more details in our previous study [11]. fibers are further marked as follows: pp and pet, reference; pp-t and pet-t, treated. the basic geometric and mechanical properties of fibers are summarized in table 1. 2.2. cement and finely ground recycled concrete two different types of matrices were made. the first one was composed only from portland cement (cem i 42.5 r) with water to cement ratio (w/c) equal to 0.4 (marked cem), while the second one contained finely ground recycled concrete in amount of 30% of weight of cement (marked rec), water to binder 5 http://dx.doi.org/10.14311/app.2019.21.0005 http://ojs.cvut.cz/ojs/index.php/app radim hlůžek, jan trejbal acta polytechnica ctu proceedings fiber type diameter density young’s modulus tensile strength [µm] [kg/m3] of elasticity [gpa] [mpa] pet / pet-t 335 950 5.8 238 pp / pp-t 305 900 6.1 440 table 1. basic geometric and mechanical properties of fibers [12]. (w/b = w/(cem+rec)) was increased to 0.41 (increased due to the high specific surface area of recycled concrete). waste recycled concrete used in this study, originated from railway sleepers, was ground using high-speed milling technology with maximum grain size of 0.25mm. the hardening and curing of all test produced specimens took place in standard laboratory conditions for 28 days. 3. experimental methods 3.1. scanning electron microscopy morphology of fibers was observed using the merlin zeiss scanning electron microscopy device. in order to perform this analysis, it was necessary to provide sufficient electrical conductivity of the fiber surface. therefore, all tested fibers were coated by a thin layer of gold using the boc edward scancoats six machine. process parameters were: 1.3 kv voltage, 35ma electric current, and 40 s plating time. it was assumed that such a treatment had not any effect on physical properties of fiber surfaces. 3.2. pull-out tests prismatic samples having dimensions of 25× 20× 25mm were made from both types of matrices. the single fiber was always placed in the center of the specimen with a embedded length equal to the prism height, i.e. 25mm. an illustrative specimen is shown in figure 1. pull-out tests were performed 28 days after hardening and curing of mixtures. in the first place, each tested sample was clamped into the static jaw of the hydraulic loading frame (web tiv ravestein fp100). secondly, the free end of the fiber was clamped into the opposite sliding jaw of the press. finally, the fiber was gradually pulled out from the sample at a constant rate of 2mm/min. during the process, the free-end displacement and resisting force were measured. the loading process was terminated when the displacement reached 5mm. such obtained data were evaluated using the dipro software based on the matlab software. the adhesion between the two materials was then expressed by interfacial shear stress (1) and (2), which is commonly used in the field of fiber reinforced composites because it provides values normalized to the fiber surface area. the stress was calculated in two stages: after reaching a fiber free-end pull out of 3.5mm (hereinafter referred to as “3.5”) and when the measured force achieved on its maximum (denoted by the “max” index). figure 1. prepared specimen for the pull-out test. τmax = fmax cf le (1) τ3.5 = f3.5 cf le (2) f3.5 and fmax represent the force necessary to reach the 3.5mm fibers free-end displacement and the maximal reached force, respectively. cf is a fiber circumference and le is the embedded fiber length. 4. results 4.1. scanning electron microscopy a scanning electron microscopy image of the reference pet fiber (figure 2) revealed its smooth surface, which is very inappropriate from the mechanical point of view. compared to that, the same fiber after 120 seconds of the plasma treatment was already gently roughened by irregular holes, as shown in figure 2. this desirable morphology change was caused by surface etching and the ion bombardment. it should be noted that the roughening of the treated fibers surface was observed only after 120 seconds. if the fibers were exposed to plasma for shorten time, there were no visible changes on their surface. similarly to pet fibers, the surface of reference pp fibers was too smooth, except for longitudinal profiling apparently originates from production technology (stretching process), as shown in figure 3. after 120 seconds lasting modification, minor changes – longitudinal recesses along the length of the fiber – in morphology were observed, see figure 3. it can be 6 vol. 21/2019 recycled concrete in fiber-reinforced cementitious composites assumed that these tiny morphology changes will not affect the adhesion of fibers to the matrix. extending the exposure time would certainly increase the effect of roughening, but cross-section of fibers significantly decreases. 4.2. pull-out tests based on pull-out test results, it can be observed that the highest value of the interfacial shear stress was recorded in the case of plasma modified pp fibers in combination with the matrix containing finely ground recycled concrete (rec). as shown in figure 5, the interfacial shear stress at maximum force was about 0.45mpa and 0.38mpa at the 3.5mm fibers free-end displacement, respectively. in the case of cementitious matrix with same fibers, results were as follows: ca. 0.4 resp. 0.34mpa. furthermore, it should be noted that for untreated fibers, values of the interfacial shear stress with respect to the matrix used did not differ substantially. in the case of pet fibers, the same trend as for pp fibers mentioned above can be observed, but only with significantly smaller differences, see figure 4. unlike the assumption, it has been found that the adhesion between reference pet fibers in combination with the matrix containing recyclate (rec) was a little bit worse than between same fibers and the matrix containing only cement (cem). the maximum interfacial shear stress dropped from ca. 0.31mpa (cem) to 0.28mpa (rec) and at the 3.5mm displacement from 0.31 to 0.3mpa. nevertheless, in the case of the use of plasma treated fibers, the adhesion was increased, especially in combination with the matrix containing recycled concrete. the maximum shear stress was increased by ca. 5–10% up to ca. 0.33mpa. to sum up, it can be stated that the use of finely ground recycled concrete as a partial cement additive can improve the adhesion between polymeric fibers and the cement-based matrix, but only in the case of the use of plasma treated fibers. in the case of reference fibers with a smooth surface, the use of recycled material appears to be ineffective. thus, physical interaction between fine particles of the recycled material and the plasma-roughened fiber surface plays a significant role in achieving the desired increase in the adhesion between the fiber and the matrix. 5. conclusion this study researched the adhesion between polymeric macro fibers (polyethylene terephthalate and polypropylene), having ca. 0.3mm in the diameter, and two types of cement matrices. the reference matrix was composed of cement (cem i 42.5 r, w/c 0.4), while in the second case, 30 wt.% of cement was replaced with recycled concrete at the form of finely ground powder. in order to roughen fiber surfaces and thus to ensure the adhesion between them and the matrix, fibers were treated by cold oxygen plasma. changes on fiber surfaces were observed using the sem microscopy. interaction between the two materials was assessed directly by means of the single fiber pull-out test from matrix specimens (25× 20× 25mm). it was shown that the surface roughening of pp fibers was much more intensive than in case of pet fibers. pull-out tests revealed that the adhesion between pp and pet fibers and the cement matrix was constant regardless to the fiber treatment. similarly, when these fibers were pulled out from the matrix containing finely ground concrete powder, the adhesion was not increased, if compared to reference materials. on the other hand, treated fibers (both pp and pet) showed significantly increased adhesion with the matrix containing recycled concrete. this phenomenon was attributed to fiber roughening and to working of recycled concrete grains in interfacial zones. acknowledgements this work was financially supported by the czech technical university in prague – sgs project sgs18/037/ohk1/1t/11 and the czech science foundation [grant number ga čr 17-06771s]. the authors would also like to thank to tereza horová for her help with this research. references [1] m. škopán. analysis of the current state of c&dw recycling in the czech republic and assessment of development options. recycling 129(9):19–27, 2005. [2] m. d. schepper, r. snellings, k. d. buysser, et al. the hydration of cement regenerated from completely recyclable concrete. construction and building materials 60:33–41, 2014. doi:10.1016/j.conbuildmat.2014.02.067. [3] t. pavlů, m. šefflová. environmental assessement of the structural elements with recycled aggregate. recycling 129(5):108–112, 2005. [4] j. xiao, j. li, c. zhang. mechanical properties of recycled aggregate concrete under uniaxial loading. cement and concrete research 35(6):1187–1194, 2005. doi:10.1016/j.cemconres.2004.09.020. [5] l. evangelista, j. de brito. mechanical behaviour of concrete made with fine recycled concrete aggregates. cement and concrete composites 29(5):397–401, 2007. doi:10.1016/j.cemconcomp.2006.12.004. [6] j. topič, j. trejbal, t. plachý, z. prošek. relationship between compressive strength and young’s modulus of cement paste with recycled concrete powder. key eng mater 722 kem:254–259, 2017. doi:10.4028/www.scientific.net/kem.722.254. [7] v. růžička. lightweight blocks based on recycled concrete. ph.d. thesis, czech technical university in prague, faculty of civil engineering, 2017. [8] m. etxeberria, e. vázquez, a. marí, m. barra. influence of amount of recycled coarse aggregates and production process on properties of recycled aggregate concrete. cement and concrete research 37(5):735–742, 2007. doi:10.1016/j.cemconres.2007.02.002. 7 http://dx.doi.org/10.1016/j.conbuildmat.2014.02.067 http://dx.doi.org/10.1016/j.cemconres.2004.09.020 http://dx.doi.org/10.1016/j.cemconcomp.2006.12.004 http://dx.doi.org/10.4028/www.scientific.net/kem.722.254 http://dx.doi.org/10.1016/j.cemconres.2007.02.002 radim hlůžek, jan trejbal acta polytechnica ctu proceedings figure 2. sem images of reference (left) and modified (right) pet fiber. figure 3. sem images of reference (left) and modified (right) pp fiber. figure 4. interfacial shear stress values for different combinations of matrices and pet fibers calculated from the pull-out test. 8 vol. 21/2019 recycled concrete in fiber-reinforced cementitious composites figure 5. interfacial shear stress values for different combinations of matrices and pp fibers calculated from the pull-out test. [9] n. sebaibi, m. benzerzour, n. e. abriak, c. binetruy. mechanical properties of concrete-reinforced fibres and powders with crushed thermoset composites: the influence of fibre/matrix interaction. construction and building materials 29:332–338, 2012. doi:10.1016/j.conbuildmat.2011.10.026. [10] k. r. akça, ö. çakır, m. i̇pek. properties of polypropylene fiber reinforced concrete using recycled aggregates. construction and building materials 98:620– 630, 2015. doi:10.1016/j.conbuildmat.2015.08.133. [11] r. hlůžek, z. prošek, j. trejbal, et al. plasma treatment impact on physical and chemical properties of polymeric fibers. acta polytechnica ctu proceedings 13:49–54, 2017. doi:10.14311/app.2017.13.0049. [12] r. hlůžek. analysis and modification of interfacial zones between cementitious matrix and fiber reinforcement. ph.d. thesis, czech technical university in prague, faculty of civil engineering, 2018. 9 http://dx.doi.org/10.1016/j.conbuildmat.2011.10.026 http://dx.doi.org/10.1016/j.conbuildmat.2015.08.133 http://dx.doi.org/10.14311/app.2017.13.0049 acta polytechnica ctu proceedings 21:5–9, 2019 1 introduction 2 materials 2.1 fibers 2.2 cement and finely ground recycled concrete 3 experimental methods 3.1 scanning electron microscopy 3.2 pull-out tests 4 results 4.1 scanning electron microscopy 4.2 pull-out tests 5 conclusion acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2017.13.0089 acta polytechnica ctu proceedings 13:89–92, 2017 © czech technical university in prague, 2017 available online at http://ojs.cvut.cz/ojs/index.php/app drying characteristic of rammed earth with illitic-kaolinitic clay content tereza otcovská∗, barbora mužíková, pavel padevět czech technical university in prague, faculty of civil engineering, thákurova 7, 166 29 prague 6, czech republic ∗ corresponding author: tereza.otcovska@fsv.cvut.cz abstract. clay is a traditional construction material which has got to background with introduction of modern materials to building practice. there is no proper material available for clay constructions design due to lack of proper examination of its mechanical properties. this paper focuses on drying rate of rammed earth. drying is a primary way in which clay gets its strength. it is thus essential to know a duration needed to a construction to get dry. after pass of this time the maximum strength is attained and it is possible to load a construction. properties of unburned clay are dependent on clay mix composition. in this contribution two sets of testing bodies with different composition are presented. as a bonding agent an illitic-kaolinite clay was used. it was presupposed that amount of used clay in clay mixture has major influence on the speed of drying and a final equilibrium moisture content. this presumption was disproved. keywords: clay, rammed earth, drying rate, water coefficient, load-bearing structure. 1. introduction an unburned clay was commonly used construction material of the past. however, with the boom of new modern construction materials, unburned clay was pushed out from the business, and therefore the necessary scientific research of unburned clay mechanical properties was never performed [1–5]. these days unburned clay returns to focus again generally for three reasons. first reason is reconstruction of already built clay buildings [6–8]. second, the unburned clay constructions are compatible with the principles of sustainable development [9, 10]. third, unburned clay has a blessed impact on the internal micro-climate of the building and consequent improvement in the health of inhabitants [1, 3, 11]. one of the major methods of unburned clay manufacture is a ramming technology which this subscription concerns of. a principle of the method is ramming of earth layers into a framework. rammed earth is mostly used for load bearing constructions and for that reason is important a knowledge of the earth drying speed [1, 3]. the drying speed of unburned clay is important because drying is the only process with which unburned clay gain its strength. it is thus necessary to know drying speed to be able to estimate the time that must pass from a manufacture to full construction load [1, 3, 9, 12]. the clay mixture consists of three basic components: sand, clay and water. the sand acts as a filling agent. the clay has a function of a bonding agent similarly as a cement in concrete. and water serves for activation of bonding properties of the clay and for proper processibility of clay mixture [1, 13–15]. the clay mixtures that were experimentally examined and described in this contribution contain just these three basic components. no any additive nor additional bonding agent such as cement or lime were added to examined clay mixtures. the reason to exclude the additives was our effort to properly describe plain clay mixture. it was proved that even plain unburned clay used as a construction material features long lifespan. it is true particularly for properly designed construction details. in some cases it was discovered that by adding of another organic bonding agents the result material strength decreased. that is crucial for load-bearing construction elements. another reason why we concentrated on unburned clay without additives is a fact that addition of a cement or lime reduces positive influence of unburned clay to inner microclimate and raises environment load [3, 16, 17]. 2. test mixture and test bodies manufacture two types of clay mixtures were tested. the mixtures differed in sand to clay ratio and in amount of used water (water coefficient). the testing bodies were manufactured by ramming technology. the published results were measured minimally on six testing bodies of each mixture. the used clay mixtures are composed from a sand (see fig. 1), illitic-kaolinitic clay (see fig. 2) and water (common tap water). a sand to clay ratio and water amount were prescribed for the both clay mixtures. the behaviour of the clay mixture during drying and the speed of drying were studied in connection 89 http://dx.doi.org/10.14311/app.2017.13.0089 http://ojs.cvut.cz/ojs/index.php/app t. otcovská, b. mužíková, p. padevět acta polytechnica ctu proceedings figure 1. granularity curve. figure 2. illitic-kaolinitic clay. with clay content and water content in the mixture. the clay mixture krii contained 1/4 more clay (bonding agent) than in the mixture kriii. the mixture krii was manufactured with water coefficient 0.37. the mixture kriii was manufactured using water coefficient 0.4. the water coefficient is a ratio between weight of water and weight of clay. the actual clay mixture composition is declared in the table 1. the test bodies were rammed using electric power hammer into steel moulds of size 40×40×160 mm. only the last layer was rammed manually (see fig. 3). immediately after manufacture the testing bodies were put out of mold and weighted. after that the testing bodies were inserted into air-conditioned chamber, where constant temperature (20 °c) and relative humidity (60 %) are maintained (see fig. 4). the testing bodies were left in the air-conditioned chamber until their humidity has settled (approximately 1 month). 3. measured results the test bodies were held in the air-conditioned chamber and regularly weighted. thus the decrease of their mass was measured (see table 2, fig. 6). a frequency of weighting was the highest in the first day after manufacture (approximately once per one to three hours). during the first day after manufacture the highest drop in weight was measured (approximately 90 % from total drop of weight) (see fig. 5, table 2). figure 3. production of the test bodies. figure 4. test bodies in the air-conditioning chamber. the equation 1 was used for calculation of weight moisture content from the measured values. initial weight moisture content of the both test body sets was 9.3 %. time course of average value of weight moisture content is introduced in the table 3 and in figures 7 and 8. w = mw − md md (1) it can be seen in the figures 7 and 8 that equilibrium moisture of kriii set is 1.6 % and thus higher than moisture of krii which is 0.8 % even though that kriii set contains less clay that krii set (see table 1). this finding is surprising because it is generally believed that clay minerals bond water [1, 3, 13]. it was expected that equilibrium humidity of krii set will be higher. the reason why the equilibrium humidity is higher for clay mixture with lower clay ratio is probably that kriii set was manufactured with higher water coefficient (see fig. 9). it can be further conclude from carried on experiment that the speed of drying is independent on the amount of used clay. for the both mixtures the 90 % of moisture content (calculated from the total moisture content drop) evaporated during the first 24 hours after manufacture. equilibrium moisture content was 90 vol. 13/2017 drying characteristic of rammed earth ... set clay sand/clay ratio water/clay ratio number of test bodies krii illite-kaolinitic 75/25 0.37 6 kriii illite-kaolinitic 80/20 0.4 6 table 1. composition of the clay mixture batches and number of test bodies. figure 5. time course of the test bodies weight after the first 30 hours from manufacture. figure 6. time course of the test bodies average weight. reached after approximately 25 days. the form coefficient of test bodies defined by equation 2 was 0.8889. cs = vtb atb (2) 4. conclusion the carried out experiment showed that the amount of used water in mixture can have more substantial influence to resulting equilibrium humidity than amount of used clay. however, it is necessary to verify this finding and state the actual influence in detail. it should be acomplished by testing another sets of clay mixtures that would be manufactured in wider range of sand/clay ratio and water/clay ratio. it is also necessary to focus on microstructure of tested material to reliably describe influence of water and clay ratios on unburned clay drying courses. figure 7. time course of weight moisture content. figure 8. time course of weight moisture content – logarithmic scale of time figure 9. ratio of the clay and water in mixture and resultant equilibrium humidity. 91 t. otcovská, b. mužíková, p. padevět acta polytechnica ctu proceedings set time elapsed after manufacture [hour] 0 0.7 1.7 3 4 7 22 24 28 31 48 144 157 192 528 709 krii 592 582 564 551 550 547 547 kriii 588 587 583 575 551 550 549 548 548 547 table 2. time course of the test bodies average weight [g]. set time elapsed from test bodies manufacture [hour] 0 0.7 1.7 3 4 7 22 24 28 31 48 144 157 192 528 709 krii 9.3 7.3 4.1 1.6 1.3 0.8 0.8 kriii 9.3 9.1 8.3 6.9 2.4 2.2 1.9 1.7 1.7 1.6 table 3. time course of the test bodies average weight moisture content [%]. the obtained results also indicate that the drying speed is not dependent on the amount of the clay in a clay mixture. for test bodies with form coefficient of 0.8889 the drying was almost complete (reaching of equilibrium humidity) after the first 24 hours. list of symbols w weight moisture content [–] mm weight of damp sample [g] md weight of dry sample [g] cs form coefficient [m] vtb volume of test body [m3] atb surface of test body [m2] acknowledgements the financial support of this work by the faculty of civil engineering, czech technical university in prague (sgs project no. sgs16/201/ohk1/3t/11) is gratefully acknowledged. we would like to express our thanks to lb minerals, s.r.o. company for free supply of material necessary for experimental measurement and we would also like to express thanks to heluz cihláhřský průmysl v.o.s. for giving statistical data and information. references [1] g. minke. building with earth. ökobuch verlag, staufen, 2006. [2] čsn 1168-1939 podmínky pro zednické a přidružené práce pozemních staveb, 1951. [3] i. žabičková. hliněné stavby. era 21, brno, 2002. [4] p. stejskal. heluz cihlářský průmysl v. o. s. individual consultation, 2015. [5] t. morton. earth masonry: design and construction guidelines. ihs bre press, bracknell, berks., 2008. [6] v. kovářů. hliněný dům, problematika jeho památkové obnovy a využití. in zděná architektura v čechách, na moravě a ve slezsku, pp. 31 – 33. 2004. [7] v. hájek. lidová stavení: opravy a úpravy. stavitel. grada, praha, 1st edn., 2001. [8] i. žabičková. přestavby hliněných budov. sdružení hliněného stavitelství, brno, 2009. [9] j. růžička. building structures in environmental context, mechanical physical properties of unburned bricks by impact of humidity. mezinárodní vědecký seminář zdravé domy 2005. [10] c. sjostorm. agenda 21 pro udržitelnou výstavbu. cib report publication 237. cib, rotterdam, 2001. [11] m. jokl. teorie vnitřního prostředí budov. ediční středisko českého vysokého učení technického, 2011. [12] q.-b. bui, j.-c. morel, s. hans, p. walker. effect of moisture content on the mechanical characteristics of rammed earth. construction and building materials 54:163–169, 2014. doi:10.1016/j.conbuildmat.2013.12.067. [13] p. walker. rammed earth: design and construction guidelines. ihs bre press, watford, 2010. [14] j. konta. jílové minerály československa. nakladatelství čsl. akad. věd, 1st edn., 1957. [15] z. weiss. jílové minerály: jejich nanostruktura a využití. karolinum, vyd. 1. edn., 2005. [16] q. b. bui, j. c. morel, b. v. venkatarama reddy, w. ghayad. durability of rammed earth walls exposed for 20 years to natural weathering. building and environment 44(5):912–919, 2009. doi:10.1016/j.buildenv.2008.07.001. [17] i. zabalza bribián, a. valero capilla, a. aranda usón. life cycle assessment of building materials: comparative analysis of energy and environmental impacts and evaluation of the eco-efficiency improvement potential. building and environment 46(5):1133–1140, 2011. doi:10.1016/j.buildenv.2010.12.002. 92 http://dx.doi.org/10.1016/j.conbuildmat.2013.12.067 http://dx.doi.org/10.1016/j.buildenv.2008.07.001 http://dx.doi.org/10.1016/j.buildenv.2010.12.002 acta polytechnica ctu proceedings 13:90–93, 2017 1 introduction 2 test mixture and test bodies manufacture 3 measured results 4 conclusion list of symbols acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2020.26.0081 acta polytechnica ctu proceedings 26:81–85, 2020 © czech technical university in prague, 2020 available online at https://ojs.cvut.cz/ojs/index.php/app the influence of the use of recycled gypsum on the properties of gypsum products in labor conditions hana sekavováa,b,∗, jakub herrmanna, zdeněk prošeka, miroslav nyčb, george karra’ac a czech technical university in prague, faculty of civil engineering, department of mechanics, thákurova 7, 166 29 prague 6, czech republic b knauf praha spol. s r.o., mladoboleslavská 949, 197 00 prague 9, czech republic c lavaris s.r.o., areál šroubáren 43, 252 66 libčice nad vltavou, czech republic ∗ corresponding author: hana.sekavova@fsv.cvut.cz abstract. the article presents the first results from research, which is deal with recycling of plaster boards and use of the resulting material. the research is carried out within the project mpo trio č. fv30359 “recyklace sádrokartonových desek a nová materiálová využití s přidanou hodnotou – gipsrec”. the plasterboards are produced of stucco, which is calcinated of flue gas desulfurization gypsum (fgd gypsum). there is possibility to replace a part of this stucco with recycled material. there is opportunity for saving costs for production. the question how this utilization can change the properties of the products is very important and it is necessary to find the possible improvement or deterioration. this is the objective of this research. the effect on properties was monitored on the samples which was prepared of only pure stucco and of stucco with part of recycled material. there was observed water ratio (splash test), setting times, volume changes, flexural strength, compressive strength and elasticity module. keywords: fgd gypsum, stucco, recyclation. 1. introduction the gypsum was used as a building material for thousands of years. there are references, that the gypsum occurs on buildings dated 7000 bc. the gypsum was used in ancient egypt and rome. great fame has gypsum in 16th century and it is very popular material till today [1]. there are two main sources of gypsum. at first we can find several sources of natural gypsum in the world. there is only one in czech republic and it is situated near the opava. this is part of polish gypsum pan. the purity of the natural gypsum is variable and it depends on the location of mine. the purity of gypsum in the mine kobeřice u opavy is 60–85 % [1]. higher level of purity has gypsum from industrial operations. the syntetic gypsum is still the waste product of the other production. there are for example fgd gypsum, and gypsum generated during the chemical production [1]. the amount of the natural gypsum is limited and the amount of fgd gypsum depends on working power plants which used coal for producing of electric energy (fgd gypsum is product of wet limestone scrubbing of flue gas contains sulfur oxides chemical reaction between limestone suspension and sulfur oxides). the plan for the future is that these power plants will end their life about year 2050, because of their service life and because of the end of coal mining [2]. it is necessary to search for the new possibilities, how to reduce the amount of gypsum, which is used for production of building material. the first way is to find the fillers and additives, which reduce the amount of stucco in production. and the second way is recycled the waste of the gypsum based products and put them back to the production. there is also the pressure to develop the no-waste-production. the recycling and way to the no-waste-production is very popular and the research how to recycle and reuse old building materials or waste from production is running. very popular is recycling of concrete, or using of recycled materials for concrete as a filler. the greater interest in this material is due to the large use of concrete in construction. the observation of the influence on mechanical properties in this area can be found in many researches by j. topič, p. tesárek et al. [3], [4], [5]. there is not only concrete in construction and for example gypsum based building materials are very popular for their properties and it is necessary to deal with this issue. gypsum products have a relatively low bulk density, which ranges from 600–1200 kgm−3, which is about half the bulk density of conventional concrete. this feature allows easy handling of products during construction and the another advantage is less load on the building structure. gypsum is used for its properties in fire protection construction. the gypsum looses during effect of high temperature crystal bound water. this water makes the fire protection of the construction, because 81 https://doi.org/10.14311/app.2020.26.0081 https://ojs.cvut.cz/ojs/index.php/app h. sekavová, j. herrmann, z. prošek et al. acta polytechnica ctu proceedings it decreases the temperature of the surface of the construction [6]. the another positive property of gypsum is that it is hygroscopic. it means, the rehydrated gypsum can absorb the air humidity. it is the regulator of humidity in living space, what brings comfort feeling in rooms. the volume changes are relative low during this process [7]. 2. material the materials, which were used for these experiments, were the white stucco and the waste material from two sources. the white stucco is calcinated of fgd gyspsum. the fgd gypsum is produced during the desulphurisation process flue gases in power plant, where is burned the brown coal. this fgd gypsum has very high level of purity (96 % on average). the white stucco calcinated of fgd gypsum is used as main material in production of plaster boards and gypsum plasters. these materials are showed on figure 1. the first option of waste material were the gypsum based blocks. the second option of waste material were the plaster boards (pb). both of these waste materials were crushed and milled. in the case of boards there was separated the paper above that and the grains were less fine than the grains of milled gypsum blocks. the crushing, milling and separating was done on lavaris equipment. figure 3 shows the material after primer crusher. this material included both of main parts of plaster boards – gypsum core and paper. figure 4 shows material without paper. the paper was separated and the gypsum material is ready for sorting. the fine milled material and paper are showed on figure 2. figure 1. natural and fgd gypsum, stucco. figure 2. fine milled gypsum and separated paper. figure 3. crushed plaster boards with paper. figure 4. milled gypsum before sorting and without paper. 82 vol. 26/2020 reuse of recycled gypsum amount of stucco amount of recycled material initial time final time splash [% wt.] [% wt.] [min] [min] [mm] 90 10 1 2 50 75 25 2 2 56 50 50 12 > 2 weeks 58 10 90 > 2 weeks > 2 weeks 61 table 1. basic properties. figure 5. volume changes over time – recycled blocks [8]. 2.1. the effect of recycled material on basic properties in the first step was observed the behaviour the gypsum slurry depending on the amount of recycled material in recipe. the slurry was mixed by hand without any equipment. the water ratio was in each of recipe 1.2. there were observed the setting times (initial time is and final time fs) and the diameter of splash. the results are showed in table 1. these results showed, that the 50 % is the maximal amount of recycled material, which is possible to use for preparing of samples for following tests. the bigger amount leads to loose of solidifying ability. 2.2. the effect of recycled material on volume changes the volume changes was observed on prisms 160 × 40 × 40mm. there were prepared 3 different recipes for each recycled material, which were selected in first part of experiment. the recipe 1 is reference sample of pure white stucco. the recipe 2 is 90 % of white stucco and 10 % of milled blocks. the recipe 3 is 75 % of white stucco and 25 % of milled blocks. the recipe 4 is 90 % of white stucco and 10 % of milled plaster boards (pb). the recipe 5 is 75 % of white stucco and 25 % of milled plaster boards and the recipe 6 is 50 % of white stucco and 50 % of milled plaster boards. all presented % are % wt. there were prepared 3 samples of each recipe. all of this samples are tested and the average value for each recipe was signed. the prism were measured and weighed over the next 28 days. the samples lost the free moisture (weight loss) and the volume shrinkage could be monitored too. the results are showed in figures 5 and 6. figure 6. volume changes over time – recycled plaster boards [8]. figure 7. volume changes over time – recycled plaster boards [8]. 3. experimental methods and results the shrinkage increased depending on the amount of recycled material. this is because in the samples with more recycled material is more free water, which is not included in crystal bonded water. 3.1. changes of elasticity module over time the prisms prepared for the observed volume changes (recipes 1–3) was used also for observed changes of elasticity module. this property was tested by nondestructive method – modal analysis. the elasticity module was counted on the base of resonant frequency of samples. the measurement was carried out with a impulse hammer with sensor which was linked to software that processed the values of frequencies and after that was possible to count the elasticity module 8. the course of results is showed on figure 7. the elasticity module is increases over time. it depends on 83 h. sekavová, j. herrmann, z. prošek et al. acta polytechnica ctu proceedings recipe content water ratio flexural compressive [nr.] stucco milled blocks milled pb [-] strength strength [% wt.] [% wt.] [% wt.] [mpa] [mpa] 1 100 0 0 1.4 16.85 2.06 2 90 10 0 1.4 11.47 0.63 3 75 25 0 1.4 6.79 0.13 4 90 0 10 1.2 17.55 0.97 5 75 0 25 1.2 21.53 1.48 6 50 0 50 1.2 25.04 1.92 table 2. flexural and compressive strength. lost of free and bounded moisture from the samples. the gypsum core is more compact and stronger. the bigger shrinkage leads to higher values of elasticity module. figure 8. impulse hammer [9]. 3.2. strength destructive methods the prisms were used in the end of this particular research for destructive strength tests. the both tests were executed according to standard čsn 72 2301. the flexural strength is the first part of the test. the test method is a three point flexural test technique and the flexural strength represents the highest stress experienced within the material at its moment of yield 9. the compressive strength test is performed on parts of samples after execution a three point flexural test technique. the force of the machine act at the whole surface of the sample 10. the results are showed in table 2. the decrease of values of compressive strength depending on the increasing amount of milled blocks are completely different from increase of these values depending on the increasing amount of milled plaster boards. the low breaking loads are caused by high content of water in gypsum slurry. the high water ratio was used because of bad consistency of slurry, which was with lower water ratio unprocessable. figure 9. three point flexural test technique [8]. figure 10. compressive strength test [8]. 84 vol. 26/2020 reuse of recycled gypsum 4. conclusions this research found in its very early begins several dependencies in behaviour the gypsum samples which were prepared with a gypsum based recycled materials. that showed that it is very important to find optimal water ratio. optimal consistency of gypsum slurry is necessary to get the gypsum product with the best properties. in practice is usually use the liquifiers, accelerators or retarders to optimize gypsum slurry properties. the optimal water ratio for the gypsum slurry is from 0.6 to 0.8. in these test was necessary to use the water ratio higher, because we used fine milled material which accelerates the solidifying reaction. the fine grain worked as a crystallization nucleus and the reaction was very fast and in this case was reaction slowed down by adding more water. it optimized the process of rehydration, but it brought the decrease the other properties which were tested. they depend on the structure of gypsum core and when the gypsum slurry contents too much water it generates more pores in volume of gypsum based products and this leads to a reduction in strength and bigger volume changes over time. on the other hand it brings also better thermal properties. acknowledgements this paper was based on research supported by grant mpo trio č. fv30359, where cooperate ctu in prague, faculty of civil engineering, lavaris s.r.o. and knauf praha spol. s r.o. references [1] p. rovnaníková, z. keršner. vlastnosti modifikované sádry. first printing. vysoké učení technické v brně, akademické nakladatelství cerm, s.r.o., brno, 2009. [2] v. pačes. budoucnost energetiky očima odborníků. [2019-07-15], https://www.siemens.cz/energo/ budoucnost-energetiky-ocima-odborniku. [3] j. topič, z. prošek. properties and microsture of cement paste including recycled concrete powder. acta polytechnica 57(1):49–57, 2017. [4] p. tesárek, k. šeps, j. topič, et al. influence of recycled materials on resulting mechanical properties of cement composites. modern methods of experimental and computational investigations in area of construction 825:53–56, 2016. doi:10.4028/www.scientific.net/amm.825.53. [5] j. topič, z. prošek, j. hrůza. mechanical properties of cement paste with fine ground recycled concerete depending on the microstructure. modern methods of experimental and computational investigations in area of construction ii 1144:22–27, 2017. doi:10.4028/www.scientific.net/amr.1144.22. [6] a. vimmrová, l.svoboda. stavební hmoty vápenosíranová pojiva. first printing. jaga group, s.r.o., bratislava, 2004. [7] w. schulze. necementové malty a betony. first printing. sntl, praha, 1990. [8] j. herrmann. možnosti recyklace výrobků na bázi sádry, 2019. https://dspace.cvut.cz/bitstream/handle/10467/ 84255/f1-dp-2019-herrmann-jakub-dp_herrmann_ bez%20podpisu.pdf?sequence=-1&isallowed=y. [9] omega-engineering, 2019. https://in.omega.com/pptst/ih101.html. 85 https://www.siemens.cz/energo/budoucnost-energetiky-ocima-odborniku https://www.siemens.cz/energo/budoucnost-energetiky-ocima-odborniku https://doi.org/10.4028/www.scientific.net/amm.825.53 https://doi.org/10.4028/www.scientific.net/amr.1144.22 https://dspace.cvut.cz/bitstream/handle/10467/84255/f1-dp-2019-herrmann-jakub-dp_herrmann_bez%20podpisu.pdf?sequence=-1&isallowed=y https://dspace.cvut.cz/bitstream/handle/10467/84255/f1-dp-2019-herrmann-jakub-dp_herrmann_bez%20podpisu.pdf?sequence=-1&isallowed=y https://dspace.cvut.cz/bitstream/handle/10467/84255/f1-dp-2019-herrmann-jakub-dp_herrmann_bez%20podpisu.pdf?sequence=-1&isallowed=y https://in.omega.com/pptst/ih101.html acta polytechnica ctu proceedings 26:81–85, 2020 1 introduction 2 material 2.1 the effect of recycled material on basic properties 2.2 the effect of recycled material on volume changes 3 experimental methods and results 3.1 changes of elasticity module over time 3.2 strength destructive methods 4 conclusions acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2019.22.0007 acta polytechnica ctu proceedings 22:7–11, 2019 © czech technical university in prague, 2019 available online at http://ojs.cvut.cz/ojs/index.php/app fresh state properties of lime mortars with flax oil admixture pavla bauerováa,b, pavel reitermana,c, milena pavlíkováb, magdalena kracík štorkánovád, martin kepperta,b,∗ a czech technical university in prague, university centre for energy efficient buildings, třinecká 1024, 273 43 buštěhrad, czech republic b czech technical university in prague, faculty of civil engineering, department of materials engineering and chemistry, thákurova 7, 166 29 praha 6, czech republic c czech technical university in prague, faculty of civil engineering, experimental center, thákurova 7, 166 29 praha 6, czech republic d art & craft mozaika, z.s., kapitulní 103/19, 252 62 únětice, czech republic ∗ corresponding author: martin.keppert@fsv.cvut.cz abstract. flax oil has been used as mortar improving, hydrophobic, admixture already in ancient times. the paper describes the identification of flax oil in mosaic mortar from ca. 1900 by help of ftir spectroscopy. this historic mortar was reproduced by nowadays raw materials and the influence of flax oil on consistency and air entraining of fresh mortar was tested. it was found that already small amount of oil caused significant air entrainment, which, simultaneously with water repellency action, may contribute to higher durability of mortar with oil admixture. the flux oil influenced also consistency of mortar; its presence caused higher flow value of mortar. moreover, the introduction of the paper summarizes principal knowledge about action of natural admixtures in lime mortars and plasters on basis of current literature. keywords: modern mosaics, flax oil admixture, consistency of fresh mortar, hydrophobic admixture. 1. introduction mortar, plaster or even concrete on basis of lime have been used for several thousand years and most likely will be used also in future [1]. the lime based binders offer important advantages – high accessibility of raw materials, simple production technology and consequently low production costs. on the other hand, lime has obviously numerous disadvantages. one has to distinguish air lime (containing predominantly cao/mgo) or hydraulic lime. air lime is obviously non-hydraulic binder, which has to be in contact with air to set and harden. hydraulic lime is able to reach higher strength and better durability [2]; it can be prepared by calcination of limestone with proper content of hydraulic oxides (nhl) or by mixing of lime with a pozzolanic additive (hl). all of these materials have been modified by numerous natural admixtures since ancient times in order to improve the binder performance – repellency to water, plasticity, rate of setting etc. [3]. moreover, organic fibrous materials have been used as dispersed reinforcement reducing volume changes and increasing strength and integrity of material [4]. nowadays, the application of synthetic organic admixtures (i.e. better defined, more effective, with higher stability) is dominant in engineering practice, nevertheless the studying of natural admixtures has an importance with respect to restoration works or possibly may contribute to higher sustainability of building structures. the natural species used historically as lime admixtures can be classified, according to their chemical nature, to three groups: proteins, polysaccharides and triglycerides (glycerin esters of fatty acids, i.e. oils and fats,). specifically, protein admixtures were caseins (curd, buttermilk, whey), collagens (animal glue) and egg proteins [5]. animal glue was found to increase the mortar’s strength significantly; the mechanism probably lies in refinement of pore system and reduction of total porosity [3]. egg was used as plasticizer and setting retarder [6]; egg white (optimum content 6% by lime) increased plasticity, strength and reduce water absorption [7]. casein containing admixtures improve workability and strength; casein forms in lime environment calcium caseinate, which is insoluble and acts as binder [8, 9]. animal blood can be also classified as protein admixture, even though its complex composition; it is significantly improving adherence of lime mortar and its durability due to reduction of water absorption and air-entraining effect [10]. the ancient polysaccharide admixture is opuntia powder (or mucilage). it increases the rate of carbonation of lime, what obviously results to faster hardening of mortar [3]. nowadays, derivatives of natural polysaccharides cellulose ethers, welan gum, diutan gum, guar gum, xanthan gum are widely used as viscosity-enhancing admixtures, increasing viscosity, cohesion and stability of mortars [11]. influence of starch on lime mortar was found to be dependent on its 7 http://dx.doi.org/10.14311/app.2019.22.0007 http://ojs.cvut.cz/ojs/index.php/app p. bauerová, p. reiterman, m. pavlíková et al. acta polytechnica ctu proceedings dosage; it acted as a thickener when the incorporated dosage was up to 0.30% of lime weight; conversely, above that dosage, it behaved as a plasticizer [12]. triglycerides admixtures are represented e.g. by olive oil; it reduced the porosity as well as pore size resulting to lower permeability of mortar [3]. thanks to their structure, triglycerides are also acting as hydrophobic admixture. linseed oil acted as absorption reducer (and durability enhancer), but without a large influence on pore system [13]. nowadays, the fatty acids are used as water repellant admixtures in form of soaps (e.g. calcium stearate) [14]. the present paper deals with influence of flax (linseed) oil on fresh state properties of lime based mortar. the direct motivation for this research lies in fact, that flax oil was identified in a sample of historical mosaic mortar; the flax oil identification is described as well. 2. experimental the initial material under study, inspiring the present experimental work, was a sample of mosaic mortar collected at peluněk sepulcher, malvazinky cemetery, prague. it was built round 1900. the sample of mortar was characterized by xrd (panalytical empyrean, cukα source); the content of filler insoluble in acid (i.e. quartz and silicates) was determined by help of 1:3 hcl dissolution and filtration. the historic mortar was also examined by sem (zeiss merlin). the oil admixture was identified by help of ftir spectroscopy (nicolet 6700) in extract of mortar (extraction 1:10 by chloroform). the reproduced mosaic mortar was prepared on basis of analysis of the historical mortar from sepulcher. lime hydrate was used as the principal binder (40%), as the filler were used three fractions: quartz 0-0.5mm (30%) and 0.5-1mm (10%) and milled limestone d8 (20%). the water/lime ratio was kept constant (0.87). the natural flax oil was dosed as admixture (0-10% by lime hydrate weight). the first step of mixing procedure [15] was homogenization of dry powders by help of electric mixer; than small amount of powder was mixed separately with oil and this paste was dispersed in the rest of dry powders. finally the water was added. the consistency and entrained air content of mortar was measured by standard procedures (en 1015-3 and 7) by help of flow table test and pressure type air content meter. 3. results and discussion characterization of historic mortar: the sample of historic mosaic mortar was examined by xrd (fig. 1). it revealed that the main phases are quartz and calcite; the used filler also introduced muscovite and albite to the mortar. surprisingly, in sample of age of 120 years, also portlandite was found. its presence was confirmed by thermal analysis as well. the results of thermal analysis also indicated, that the binder was closer to the air lime than to the hydraulic one, although edx analysis indicated some si and al atoms dispersed in binder phase. the residuum after acid dissolution of the sample was 15%; this amount thus may be attributed to siliceous filler. unfortunately, the amount of historic mortar sample was limited (ca 10 g) due to conservation of historic mosaic and thus this result cannot be assumed as fully representative; neither the granulometry of filler cannot be determined reasonably. nevertheless, visually one can sate that the mortar does not contain particles larger than 1mm (these would, obviously, make difficult the application of mosaic mortar). the sem (and edx) analysis shown, that besides siliceous filler, the mortar contains also calcite (marble) flour with size of particles in the range of a few of tenths microns (fig. 2). the carbonate dissolution liberated not only the siliceous aggregates, but also an unknown organic substance forming a brownish greasy layer on the filter paper. in order to identify this substance, 5 g of mortar powder was extracted by chloroform (50ml). the obtained extract was analyzed by ftir spectroscopy (fig. 3). the spectrum was compared with nowadays flax stand oil. the group of three signals around 2900 cm−1 belongs to c=c groups; in this case in fatty acids. flax oil triglycerides contain three unsaturated fatty acids triply unsaturated α-linolenic acid, doubly unsaturated linoleic acid and monounsaturated oleic acid. the peak at 1758 cm−1 in reference stand oil corresponds to c=o in esters (i.e. triglycerides); the shift 1709 cm−1 in extract is due to decomposition of triglycerides to ca-soaps of carboxylic acids and glycerol in basic environment of mortar. the peak at 1465 cm−1 belongs to –ch2– groups in acids, while the peaks in range 1000-1400 cm−1 are due to c-o bonds in glycerol. the ftir results can be understood as proof that flux oil was used as admixture in historic mosaic mortar. the quantification of organic substance in mortar was attempted on base of mass balance of the extraction experiment; the result was ca. 6% what seems to be a lot and reasonability of such high dosage of flax oil must be checked in future experiments with reproduced mortars. influence of flax oil on fresh state properties of reproduced mortar was tested in wide range of oil dosing (0-10% by lime weight) (fig. 4). the consistency of reference mortar was 130mm, what is less than 140-200mm, what is range of “plastic mortar”. in fact, the mortar is usually mixed for 160mm consistency. nevertheless, this initial (low) choice of mixing water amount was due to possibility to study the oil influence on consistency. the oil dosage generally increased the flow value, but even the very high dosage (10%) did not caused any unacceptable behavior of fresh mortar. the observed trend is opposite compared to publications [8, 15] where the reduced flow is explained by hydrophobicity of the oil and consequent low absorption of water to particles. in our opinion, this effect should on contrary increase the flow value 8 vol. 22/2019 fresh state properties of lime mortars with flax oil admixture figure 1. x-ray diffractogram of historic mortar. figure 2. sem of historic mosaic mortar. (at constant water content), since the mixing water is less adsorbed by particles of lime and thus reduces more effectively the mortar stiffness. the air content of reference mortar was ca. 2%, what is expectable value for lime mortar. already 0.5% of flax oil doubled the air content, while the further increasing doses did not have such significant effect and the value remained around 4-5%. as the air entraining admixtures are conventionally used various surfactants [16], nevertheless also oils are able to increase the air content and slump of concrete [17]. the content of entrained air in a binder is closely related to the viscosity – higher air content means lower viscosity [18], i.e. higher result of flow table test, as it was indicated by the present results. 4. conclusions flax oil admixture was identified by ftir spectroscopy in the sample of historic mortar from peluněk sepulcher, built around 1900 in prague malvazinky cemetery. the composition of this historic mortar was reproduced; the effect of wide range of dosing of flax oil on fresh state properties of mortar was tested experimentally. the oil admixture caused increase of flow value as well as entrained air content. these findings are in agreement with results published on behavior of oil in concrete. moreover the oil is inducing hydrophobic behavior of the mortar. the higher air content and hydrophobicity of mortar are supposed to be responsible for the improved durability of flax oil mortars reported in literature. 9 p. bauerová, p. reiterman, m. pavlíková et al. acta polytechnica ctu proceedings figure 3. ftir spectra of extract, reference oil and extraction medium. figure 4. influence of flax oil admixture on consistency (flow table test) and air content of reproduced mortars. acknowledgements the work has been supported by czech science foundation project nr. 18-13525s “modern mosaic mortars in a microscope – methods for their materials characterization and degradation studies”. references [1] m. stefanidou, v. pachta, s. konopissi, et al. analysis and characterization of hydraulic mortars from ancient cisterns and baths in greece. materials and structures 47:571–580, 2014. doi:10.1617/s11527-013-0080-y. [2] k. dvořák, i. hájková. the effect of high-speed grinding technology on the properties of fly ash. materiali in tehnologije 50:683–687, 2016. doi:10.17222/mit.2015.127. [3] l. ventolà, m. vendrell, p. giraldez, l. merino. traditional organic additives improve lime mortars: new old materials for restoration and building natural stone fabrics. construction and building materials 25:3313– 3318, 2011. doi:10.1016/j.conbuildmat.2011.03.020. [4] c. kennedy, w. a revie, l. troalen, et al. studies of hair for use in lime plaster: implications for conservation and new work. polymer degradation and stability 98:894– 898, 2013. doi:10.1016/j.polymdegradstab.2013.01.004. [5] i. krizova, j. schultz, i. nemec, et al. comparison of analytical tools appropriate for identification of proteinaceous additives in historical mortars. analytical and bioanalytical chemistry 410:189–200, 2018. doi:10.1007/s00216-017-0709-8. [6] r. nogueira, a. p. ferreira pinto, a. gomes. design and behavior of traditional lime-based plasters and renders. review and critical appraisal of strengths and weaknesses. cement and concrete composites 89:192– 204, 2018. doi:10.1016/j.cemconcomp.2018.03.005. [7] m. a. othuman mydin. preliminary studies on the development of lime-based mortar with added egg white. 10 http://dx.doi.org/10.1617/s11527-013-0080-y http://dx.doi.org/10.17222/mit.2015.127 http://dx.doi.org/10.1016/j.conbuildmat.2011.03.020 http://dx.doi.org/10.1016/j.polymdegradstab.2013.01.004 http://dx.doi.org/10.1007/s00216-017-0709-8 http://dx.doi.org/10.1016/j.cemconcomp.2018.03.005 vol. 22/2019 fresh state properties of lime mortars with flax oil admixture international journal of technology 8(5):800–810, 2017. doi:10.14716/ijtech.v8i5.442. [8] p. rovnaníková. omítky. stop praha, 2002. 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[18] f. cardoso, v. john, r. pileggi. rheological behavior of mortars under different squeezing rates. cement and concrete research 39:748–753, 2009. doi:10.1016/j.cemconres.2009.05.014. 11 http://dx.doi.org/10.14716/ijtech.v8i5.442 http://dx.doi.org/10.1016/j.ijms.2009.01.011 http://dx.doi.org/10.1016/j.cemconres.2015.06.006 http://dx.doi.org/10.1088/1757-899x/251/1/012039 http://dx.doi.org/10.1016/j.carbpol.2009.11.010 http://dx.doi.org/10.1016/j.conbuildmat.2016.04.029 http://dx.doi.org/10.1016/j.cemconres.2009.07.026 http://dx.doi.org/10.1016/j.cemconres.2014.03.011 http://dx.doi.org/10.1155/2016/6981091 http://dx.doi.org/10.1016/s0950-0618(03)00002-3 http://dx.doi.org/10.1016/j.cemconres.2009.05.014 acta polytechnica ctu proceedings 22:7–11, 2018 1 introduction 2 experimental 3 results and discussion 4 conclusions acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2018.18.0001 acta polytechnica ctu proceedings 18:1–5, 2018 © czech technical university in prague, 2018 available online at http://ojs.cvut.cz/ojs/index.php/app characterization of the fracture mechanical behavior of c-smc materials martin tiefenthalera,∗, philipp s. stelzera, chi n. chunga, volker reiseckerb, zoltan majora a institute of polymer product engineering, johannes kepler universität, altenbergerstraße 69, linz, austria b transfercenter für kunststofftechnik gmbh, wels, austria ∗ corresponding author: martin.tiefenthaler@jku.at abstract. the fracture mechanics of random discontinuous carbon fiber sheet molding compound (c-smc) materials compared to traditional carbon fiber composites are not well understood. an experimental study was carried out to characterize the fracture behavior of such c-smc materials. mode i tests, using double cantilever beam specimens, and mode ii tests, adopting the four-point bend, end-notched flexure configuration, were performed. results show high variations in the forcedeflection responses and scatter in the fracture toughness properties gic and giic, due to the complex mesostructure defined by random oriented carbon fiber chips. to investigate the influence of the mesostructure, tensile tests with varying specimen width and thickness were assessed by stochastic measures to find the representative specimen size. keywords: c-smc, carbon fiber composites, fracture mechanics, anisotropy. 1. introduction the exceptional strength to weight ratios presented by many advanced composite materials are desirable for industries (automotive, aerospace, . . . ) that constantly fight to improve efficiency [1]. random discontinuous c-smc is a special type of smc material, where a unidirectional carbon fiber prepreg is slit and chopped into chips, randomly distributed in a mold and finished in a single-stage compression and curing process. this material and process technology can be used to produce geometric complex, lightweight applications in an industrial (automated) and cost efficient way. the composition of the c-smc material leads to a complex mesostructure, due to the random distribution and discontinuous nature of the chips. research employing microscopical analysis of tensile, compressive and flexural tested specimens conducted by feraboli et al. [2] showed that failure in random discontinuous c-smc materials is a matrixdominated event with two prevalent failure modes of intralaminar chip cracking and interlaminar chip delamination, with little or no fiber breakage. further resin burn-off and de-plying of the tested specimens suggest that chip delamination is the main cause of structural failure. also nicoletto et al. [3] identified that final failure rather occurs due to chip debonding, than due to fiber fracture. the authors point out that delamination failure initiates in chips perpendicular to the load direction and propagates until delamination of other chips with higher alignment is involved. in another recently published work by kravchenko et al. [4] the tensile failure behavior of discontinuous c-smc with unidirectional aligned chips was investigated by numerical analysis. a critical length-to-thickness ratio of the chips governing the failure mode was identified, above which chip tensile failure is observed due to the efficient load transfer between the chips. for smaller length-to-thickness ratios chip delamination is prevalent. however, in random discontinuous csmc materials, the effective load transfer between the chips is greatly reduced because of a wide distribution of overlapping lengths caused by the stochastic distribution of the chips. consequently, the main weakness of random discontinuous c-smc materials was considered to be the interface between the matrix and the fiber chips. therefore, fracture mechanical tests of mode i and ii were carried out to characterize the fracture toughness of this material system and to study the failure behavior in the interface between the chips. in figure 1, the inhomogeneous strain field with high scatter is shown on the example of a tensile tested c-smc specimen analyzed by digital image correlation (dic). to get statistical significant values for the macroscopic mechanical behavior and to assess the influence of the mesostructure a experimental representative volume element (rve) study was conducted. rve can be defined by: • the rve must include a large number of microheterogeneities (inclusions, grains, voids, fibers, etc.) [5]; • the rve is a model of the material to be used to determine the corresponding effective properties for the homogenised macroscopic model. the rve should be large enough to contain sufficient information about the microstructure in order to be representative, however it should be much smaller 1 http://dx.doi.org/10.14311/app.2018.18.0001 http://ojs.cvut.cz/ojs/index.php/app martin tiefenthaler, philipp s. stelzer, chi n.chung et al. acta polytechnica ctu proceedings than the macroscopic body. (this is known as the micro-meso-macro principle) [5]. • the rve is defined as the minimum volume of laboratory scale specimen, such that the results obtained from this specimen can still be regarded as representative for a continuum [5]. an experimental study of tensile tests with specimen dimensions varying in width and thickness was performed. results were analyzed via oneway-anova. figure 1. dic analysis of a tensile test 2. c-smc material the hexcel prepreg tape (hexmc-i/c/2000/m77 + hexply m77cs/38%) consists of high strength carbon fibers with 62% fiber weight content and the hexply® m77 resin system [6]. hexmc® m77 is produced as shown in figure 2: the prepreg is slit and chopped into chips (50 × 8 mm2), randomly distributed and pressed into mats with a width of 460 mm and a thickness of 2 mm (uncured) [6]. the material is delivered in semifinished (uncured) rolls which is cut into shape and finished in a (single compression molding) curing step. plates with dimensions of 350 mm × 350 mm × varying thickness (2,4 or 6 layers) for tensile testing (see figure 4) were produced by hexcel holding gmbh, pasching. additional plates were manufactured for fracture mechanical tests with dimensions of 245 mm × 245 mm × 2 layers thickness containing an adhesive tape, with a width of 50 mm, in the midplane (see figure 3). 3. methodology all mechanical tests were performed with the servohydraulic testing machine mts 852 combined using the digital image correlation (dic) system aramis, except a few tensile tests (see table 1) which were figure 2. c-smc production process (reprinted from [6]) figure 3. composition of plate for fracture mechanical tests carried out with a zwick/roell z150 at tckt in wels due to loading limits of the mts load cell. 3.1. experimental rve study in order to assess the rve size of the used hexmc® m77, an experimental study was carried out: performed tensile tests with specimen dimensions varying in width and thickness (see table 1) were statistically analyzed using oneway anova of the results. anova consists of a comparison of two or more sample groups by testing if their expected values are different and if the variance between the groups is bigger than the variance within a group. assumptions of anova: • variances of population, treatment and errors are normally distributed • error variances are equal between the groups • values are independent the tensile test specimens were milled out of csmc plates (350×350 mm2) with a diamond coated cutter according to the dimensions (see figure 4). specimens were modified through aluminum tabs (25 mm ×width×2 mm) glued to specimen in grip area in order to prevent failure. tensile tests were carried out at 1 mm/min, room temperature and a humidity of 71 %. 2 vol. 18/2018 fracture mechanical behavior of c-smc materials tensile test specimen dimensions width thickness mm mm 200×15×2l 200×30×2l* 200×20×2l 200×30×4l* 200×25×2l 200×30×6l* 200×30×2l 200×40×4l* 200×50×2l* table 1. tensile test study: varying specimen dimensions; l=layers; *tests performed on zwick/roell z150 figure 4. c-smc plate: milled specimen pattern 3.2. mode i mode i tests were performed using five double cantilever beam (dcb) specimens with end blocks (180×40×6.5 and an initial crack length of 50 mm). test setup is shown in figure 5. specimens were tested with an unloading/loading procedure according to [7] starting with loading until reaching an axial displacement of 6 mm, where the specimen was first unloaded by 2 mm and then loaded by 4 mm. these steps where repeated until reaching 20 mm. mode i tests were carried out at room temperature and at 1 mm/min loading rate and at 5 mm/min unloading rate [7]. figure 5. mode i testing setup mode i fracture toughness (gic) was calculated via corrected beam theory (cbt) [8]: gic = 3pδ 2b(a+ |∆|) f n , (1) where p is the applied load, δ the axial displacement,b the specimen width, f the correction factor for large displacement (see equation 2) and n the correction for loading blocks (see equation 3). ∆ may be determined experimentally by plotting the cube root of compliance, c 1 3 , as a function of the delamination length a [8]. f = 1− 3 10 ( δ a )2 − 3 2 ( δl1 a2 ) , (2) n = 1− ( l2 a )3 − 9 8 δl1 a2 { 1− ( l1 a )2 } − 39 25 ( δ a )2 . (3) for the definition of l1 and l2 see figure 6. figure 6. mode i double cantilever beam (dcb) specimen (reprinted from [8]) 3.3. mode ii mode ii tests were performed using five four point bending end notched flexure (4enf) specimens (180×40×6.5 and an initial crack of 50 mm). monotonic four point bending tests were carried out at room temperature and a loading rate of 1 mm/min until failure. testing setup can be seen in figure 7. mode ii fracture toughness was calculated via experimental compliance method (ecm) [8]: giic = 3p 2ma2 2b , (4) where p is the applied load, b is the specimen width and m is the slope from the compliance plotted versus the cube delamination length a3. figure 7. mode ii testing setup 4. results and discussion 4.1. experimental rve study in the figures 8 and 9, results from the onewayanova of the tensile tests are presented, where the black points represent the measured values and the 3 martin tiefenthaler, philipp s. stelzer, chi n.chung et al. acta polytechnica ctu proceedings green colored diamond shapes display the average mean (horizontal center line), the population size (width of the diamond) and the standard deviation values (height of the diamond). results from specimens varying in both, width and thickness show high scattering with high standard deviation and error values. no clear trend could be observed by using anova over the experimental window. the scattering of the values was too high and did not show a normal distribution (see right hand side of the figures 8 and 9), thus it was not possible to determine the rve by using anova. one reason for the high scattering is the heterogeneous mesostructure due to the in-plane random orientation of the discontinuous chips. the experimental study performed in this work ended at a specimen width of 50 mm, which is also the length of the reinforcing fiber chips. this could be a reason for the high scattering of the values. the results of this study show, that the rve might be found in larger specimen dimensions. figure 8. oneway anova: a) e-modulus over width, b) tensile strength over width, c) e-modulus distribution, d) tensile strength distribution figure 9. oneway anova: a) e-modulus over thickness, b) tensile strength over thickness, c) e-modulus distribution, d) tensile strength distribution 4.2. mode i in figure 10, load-displacement curves and mode i fracture toughness are presented. high scattering of gic values (from 1.53 to 2.69 mj/mm2; avg = 1.83; std = 0.542) compared to classical ud laminates was observed. the load-displacement curves show several small force drops. these drops may occur when fiber chips, carrying the load, fully delaminate. however, load may increase further because the crack stops at the next chips. in c-smc (discontinuous fiber composite) the initial crack does not correlate to total failure. c-smc material stop or at least greatly slow crack propagation. however, in continuous fiber composites cracks propagate more easily. initial failure is visible in load-displacement curves as a sudden load drop and often correlates to ultimate failure [9]. this crack stopping behavior is a result of the random distribution of the discontinuous fiber chips. a similar behavior was also found by boursier et al.[9] by performing tensile tests with acoustic emission measurements. they found out that first cracks do not correlate with ultimate failure of the tensile specimens which supports further the crack stopping behavior of the c-smc materials. 4.3. mode ii in figure 11 load-displacement curves and mode ii fracture toughness are presented. high scattering of giic values (from 2.21 to 3.69 mj/mm2; avg = 3.13; std = 0.551) compared to classical ud laminates was observed. the load-displacement curves of mode ii tested specimens show a crack stopping behavior similar to mode i. values of mode ii are less scattering compared 4 vol. 18/2018 fracture mechanical behavior of c-smc materials figure 10. results: load over displacement (top) and mode i fracture toughnesses comparison (bottom) figure 11. results: load over displacement (top) and mode ii fracture toughnesses comparison (bottom) to mode i in the linear elastic region (see figure 11) because specimens are loaded in shear mode. 5. conclusion random discontinuous c-smc is a special type of smc material, where a unidirectional carbon fiber prepreg is slit and chopped into chips, randomly distributed in a mold and finished in a curing step. the production process leads to a heterogeneous mesostructure with high scattering. a representative specimen size could not be found in the investigated experimental window via anova. however, the conducted experimental rve study strongly suggests the use of statistical methods to account for the variability of the mechanical properties of c-smc materials. nominal values are not significant in the design process and modelling of such materials. the representative size is expected to be at bigger specimen dimensions. fracture mechanical test results showed higher scatter of gic, giic values of c-smc material compared to traditional ud laminates. the crack propagation was found not continuous like in traditional ud laminates, but a crack stopping behavior was observed due to the random orientation of the discontinuous chips. a combination of micromechanical simulations and experimental studies is proposed for a better understanding of the c-smc fracture behavior, which will be part of future research. acknowledgements this research was performed within the framework of the “0-waste” project with contributions by engel gmbh (schwertberg, aut) and alpex technologies gmbh(mils, aut), hexcel composites gmbh (neumarkt, aut). the “0-waste” project is funded by the austrian funding agency ffg. the authors would further like to thank the transfercenter für kunststofftechnik gmbh (wels, aut) for their contributions. references [1] david a. hawkins jr., anwarul haque. strain energy release rate and mode-i delamination growth in carbon-graphene/epoxy hybrid nanocomposites 2015. [2] paolo feraboli, elof peitso, francesco deleo, tyler cleveland. characterization of prepreg-based discontinuous carbon fiber/epoxy systems 2009. [3] gianni nicoletto, enrica riva, anellino stocchi. mechanical characterization of advanced random discontinuous carbon/epoxy composites 2016. [4] sergii g. kravchenko, drew e. sommer, r. byron pipes. uniaxial strength of a composite array of overlaid and aligned prepreg platelets 2018. [5] inna m. gitman, harm askes, lambertus j. sluys, oriol lloberas valls. the concept of representative volume for elastic, hardening and softening materials 2004. [6] hexcel. hexmc user guide. https://www.hexcel.com/resources/datasheets/ hexmc-molding-compound, 2014. [online; accessed 10-may-2018]. [7] standard test method for mode i interlaminar fracture toughness of unidirectional fiber-reinforced polymer matrix composites, 2001. [8] john m. hodgkinson. mechanical testing of advanced fibre composites. 2000. [9] bruno boursier, alfonso lopez. failure initiation and effect of defects in structural discontinuous fiber composites 2010. 5 https://www.hexcel.com/resources/datasheets/hexmc-molding-compound https://www.hexcel.com/resources/datasheets/hexmc-molding-compound acta polytechnica ctu proceedings 18:1–5, 2018 1 introduction 2 c-smc material 3 methodology 3.1 experimental rve study 3.2 mode i 3.3 mode ii 4 results and discussion 4.1 experimental rve study 4.2 mode i 4.3 mode ii 5 conclusion acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2018.20.0016 acta polytechnica ctu proceedings 20:16–28, 2018 © czech technical university in prague, 2018 available online at http://ojs.cvut.cz/ojs/index.php/app experimental and numerical assessment of a last stage st blade damping at low load operation andrea bessonea,∗, luigi carassaleb, roberto guidab, zdeněk kubínc, antonio alfio lo balboa, michela marrè brunenghia, lorenzo pinellid a ansaldo energia, via n.lorenzi 8, 16152 genova, italy b university of genoa, via balbi 5, 16126 genova, italy c doosan škoda power, tylova 1/57, 301 28 pilsen, czech republic d university of florence, piazza san marco 4, 50121 florence, italy ∗ corresponding author: andrea.bessone@ansaldoenergia.com abstract. as lp steam turbines are requested to work at strong part-loads, lsb stalled and unstalled flutter may occur. testing on a downscaled steam turbine in t10mw test plant have been carried out to measure lsb aggregated damping at low load. also numerical analysis to predict aerodynamic damping have been performed and results have been compared to experimental data, allowing software tool validation at low load. keywords: low pressure steam turbine blade, flutter, low load operation. 1. introduction due to the increasing share of unpredictable and floating renewable production on the energy markets, gas and steam turbines are requested to operate at strong off-design conditions, in order to fulfill the residual demand. within this framework, it is mandatory that the last stage steam turbine blades operate at strong part loads, where flow-structure interaction may result in aero-elastic instabilities such as unstalled and stalled flutter, leading to asynchronous blade vibrations and possible blade failures for high cycle fatigue. this happens in case the aggregated blade damping (which consists of a mechanical and aerodynamic contribution) is not sufficient to suppress blade vibrations as theseoccur. to this end, flutter is a quite well known phenomenon at design conditions. many documents in literature are available [1–5] showing that the aerodynamic damping prediction is possible and reliable enough when the steady flow field on the blade is as designed. on the other hand, there is a much lower confidence on the aerodynamic behavior as the blade is operated at strong part load conditions. in this case, the aerodynamic load on the airfoil is characterized by off-design incidence angles leading to flow separations, affecting the flow-structure interaction and, consequently, the aerodynamic damping. numerical methods for blade damping estimation require the computation of the unsteady pressure response caused by the row vibration. they are usually classified in time-linearized, harmonic balance and non-linear approach both in frequency and time domain [4, 6, 7]. the validation of all these methods firstly started in the branch of low pressure turbines for aviation [3, 8] and has been gradually extended to compressor blades and steam turbine rows [9]. for the analysis of steam turbine last rotor operating at strong part load conditions, non-linear approaches turns out to be recommended as this method is able to deal with non-linear phenomena as shock waves and wide flow separations. within the present work, a downscaled 3-stages low pressure steam turbine has been designed by aen and tested at the dspw t10mw test facility. the turbine operates in wet steam environment and runs in analogy with a real scale steam turbine. the main aim of the tests has been to measure both the steady flow field within the last stage turbine blade, as it is operated at strong part load and the overall damping of the blade itself. aggregated damping measurements have been done exciting the blade resonances by means of an electromagnetic excitation, induced by an ac magnet mounted on the vane carrier. furthermore, the mechanical and the aerodynamic contributions to the overall damping have been estimated. the data will be used to validate unsteady cfd analysis to predict aerodynamic damping in terms of logarithmic decrement: numerical results have been achieved through an upgraded version of cfd software tool, optimizing numerical settings for simulation of very low load conditions, such as increased condenser pressure. 16 http://dx.doi.org/10.14311/app.2018.20.0016 http://ojs.cvut.cz/ojs/index.php/app vol. 20/2018 last stage st blade damping at low load operation figure 1. t10mw steam turbine test rig in plzen, cz. 2. experimental setup and damping measurement technique 2.1. downscaled low pressure steam turbine tested on t10mw test rig tests have been carried out in the t10mw test facility located in the dspw’s experimental laboratory in plzeň (cz). a view of the facility is provided in fig. 1. the experimental apparatus consists of a threestages lp steam turbine designed and manufactured by aen whithin the ft project. the lsb is downscaled from a 37 inches height blade mounted on a real turbine, with a part-span snubber and without any tip shroud. similarity criteria have been adopted to correctly reproduce the same aerodynamic, mechanical and aero-elastic behavior of the full-size machine, in order to experimentally characterize the flutter behavior of the last stage blade. the geometrical similarity of the test turbine with respect to the full-scale machine is respected. a scale factor of about 0.4 has been adopted, resulting the best compromise for testing the widest range of operating conditions within the dspw t10mw test rig capabilities (in terms of vacuum pressure and mass flow rate). the aerodynamic similarity is achieved by keeping constant the reduced speed defined as in eq. 1 and the same lpt expansion line. nrid = nd √ p0v0 (1) where p0 and v0 are respectively pressure and specific volume of the steam at the turbine inlet. consequently, flow velocities and mach numbers are kept equal to the full scale turbine. hence, the aerodynamic loading on the airfoils is the same. the reynolds number scales by the geometrical scale factor, but its effect on this study can be considered as negligible. lsb have been manufactured with the same material as the full scale ones. hence, since the geometrical similarity is respected, the stresses and mode shapes remain unchanged, while the modal frequencies are up-scaled by the geometrical scale factor. as a consequence, the aero-elastic behavior is reproduced through the similarity of the strouhal number, which represents the ratio between the time taken by the flow to cross the blade channel and the time characterizing a vibration period, as in equation 2. st = f ′ · ch′ v′out = ( d d′ ) f · ( d′ d ) ch vout (2) where the quantities with prime refer to the downscaled blade, the others to the fullscaled one. rotor blade tip gaps, sealing flows and manufacturing tolerances are not scaled. anyway, these aspects are properly considered within the numerical analysis and their effects quantified. therefore, experimental data are consistent with the full size machine. the test facility adopts steam as working fluid, allowing to reproduce condensation in the last lpt stages. this enables to follow the influence of the liquid phase on the thermodynamic efficiency of the stages. hence, the experimental turbine gives the possibility to study the effects connected to the wet steam flow. the performance on the stage or on the group of stages is measured by using the water brake froude (maximum revolutions 8000 rpm, the brake performance is 10 mw). a dynamometer system may be used for lower turbine power instead of the brake. the maximum flow rate of the steam through the turbine is 80 t/hour. the inlet pressure may be up to 15 bar. two different test conditions, named op3 and op8, are studied in the following. the main operating parameters of the two ops are summarized in table 1. they are characterized by different mass flow rates and vacuum pressures, but same volumetric flow rate at the turbine exit. according to the similarity criteria previously described, this means that the aerodynamic loading on the lsb is the same for both the ops. the last stage blade specific power is approximately nihil because, at these levels of volumetric flow rate, ventilation occurs and the flow does not expand through the blade channel anymore. moreover, at this volumetric flow rate, the last stage blade operates in subsonic conditions with off-design incidence. this condition is evident looking at fig. 2 where the blade aerodynamic loading, calculated through traf 3d fns cfd solver, are shown in terms of iso-mach distribution on the airfoil at three different span sections. 2.2. steam turbine instrumentation and sg positioning lpt has been instrumented with more than 70 measurement points aiming to characterize the lsb flutter behavior in a wide range of working conditions. 17 a. bessone, l. carassale, r. guida et al. acta polytechnica ctu proceedings steam mass flow condenser pressure volume flow lsb te (kg/s) (bar) (m3/s) op3 13.7 0.213 ≈ 90 op8 17.6 0.280 ≈ 90 table 1. the main operating parameters of the op3 and op8. figure 2. op3 and op8 operating parameters and last stage blade loading at 10%, 50% and 90% span for the analyzed volume flow. flutter response of the blade is strongly dependent on the flow conditions resulting from the upstream rows and on the pressure distribution at the turbine outlet. for this reason, beside the dynamic measurements to characterize blade vibration such as btt and sg, time averaged flow quantities have been recorded too: the expansion line has been measured, as well as radial distributions of flow variables at the turbine inlet and downstream the last rotor blade trailing edge. in particular, a rake of 5-hole probes has been installed downstream of the last stage blade trailing edge in order to measure the radial distribution of the total and static pressure and flow angles. this also allows the definition of the aerodynamic load of the blade during ventilation and the height of the flow separation at the hub. fig. 3 shows the st meridian view with the qualitative positioning of the sensors is reported. different symbols correspond to different types of sensors (see the table on the right). the sensors are also distributed in the circumferential direction to evaluate the uniformity of the flow, since flow axial-symmetry is the underlying assumption for numerical analyses described in the next chapter. strain gauges have been positioned in order to measure the best possible response of the last blade in terms of amplitude of vibration. the choice was based on the results of the fe modal analysis of the component, taking into account the most reliable zone for the strain gauges wiring: the area above the fillet between the airfoil and the platform. two strain gauge optimal locations have been identified and four blades have been instrumented, as reported in fig. 4. furthermore, the last blade has been instrumented by directional strain gauges, in particular positioned in the radial direction. the strain gauges positioning is based on fe analyses at design operating condition, at a nominal speed of 7600 rpm. in fig. 5, the radial deformations along the suction side of the airfoil, at a radius just above the fillet between the airfoil and the platform, are reported. normalized deformations are shown for the first five vibration modes. the final position of the strain gauges was identified as the best compromise allowing to measure dynamic response for the most relevant mode shapes. 2.3. damping measurement technique: magnet setup and sweep method because of the turbine control system limitation, it is not possible to accurately hold the turbine rotor at constant speed (rpm). this limitation is unpleasant due to the excitation of blades natural frequency on specific nodal diameters. the excitation of rotating blades is usually carried out by an electromagnet and an oscillator, assuming a constant rpm. therefore, it is necessary to use another excitation approach, some type of a dynamic excitation that accepts turbine speed fluctuation. the magnet excitation voltage signal is given by the formula: ym(t) = a · sin(2 · π · fext · t) +dc (3) where dc is direct voltage component used to shift the excitation signal to be symmetrical (± amplitude [v]) and fext is the excitation frequency of the magnet needed to induce blades vibration at the calculated natural frequency. the magnet excitation frequency fext is given by the formula: fext(t) = fb ±nd · frot(t) (4) where fb is the calculated natural frequency of the blades, nd is the number of nodal diameters of the selected mode and frot(t) is the turbine speed. since the 18 vol. 20/2018 last stage st blade damping at low load operation figure 3. st instrumentation. figure 4. strain gauges positioning on the last blade row. figure 5. normalized radial deformations on lsmb along the suction side of the airfoil. 19 a. bessone, l. carassale, r. guida et al. acta polytechnica ctu proceedings figure 6. measurement and magnet excitation diagram. turbine speed is fluctuating, the excitation frequency of the magnet will also need to change. measurement and processing chain has been assembled. it consists of the turbine speed measurement and magnet excitation, which is evident from the diagram in fig. 6. the analog signal from the phase marker (placed on the turbine rotor) is processed in an amplifier and in a trigger, which converts it into ttl pulses. ttl pulses continue to the multifunction card with counters (pxi6281 with 2 counters at 80 mhz is used). ttl pulses are then processed in the laptop using labview code, where the instantaneous turbine rotor speed is calculated. from the actual speed of the turbine rotor, the magnet excitation frequency required to excite the blade eigenfrequency at the specific nd is calculated. the calculated frequency with given amplitude is sent via the analog output card to the magnet. the same multifunction card with two fast analog output channels is used. this is repeated until the excitation time expires (usually 100 seconds). at the excitation time, the calculated frequency is swept in a range, because it is slightly different from the real one. sweep is usually ± 3 hz. the measurement and magnet excitation algorithm (written in labview) is divided into two parallel branches. in the first branch, the magnet parameter setting is performed and it provides timing and sweeping of the excitation frequency. the second branch evaluates the turbine speed continuously and receives information about magnet parameters, timing and sweeping from the first branch. the magnet excitation frequency is then calculated and the excitation itself is performed. this is repeated until the excitation time expires. the diagram in fig. 7 is illustrative of the abovementioned process flow. 2.4. damping measurement technique: damping estimation method the blade total damping is estimated following the procedure proposed in carassale et. al (2015) [10] in order to avoid the systematic overestimation due to the finite sweep frequency excitation. the method consists on extracting the wavelet-based frequency response function (w-frf) of the system using the continuous wavelet transform (wt) of the blade response during the passage through resonance. see [11] and [12] for more details on the procedure and validation of the method. the wt of a generic signal g(t) with finite energy is given, according to eq. 5, by correlating the function itself with a set of function ψ(u, s) called wavelet atoms [13]: wu,s[g] = 1√ s ∫ ∞ −∞ g(t)ψ∗ ( t− u s ) dt (5) where the superscript * denotes the complex conjugate. the wavelet atoms are obtained by translating and scaling a prototype function with compact support, called mother wavelet ψ(t), by a time shift u and a scale factor s as in eq. 5. hence, by construction, this set of functions is well defined in the time-frequency domain by the time u and the central frequency η = ωc/s, where ωc takes the name of central (round) frequency and is a property of the mother wavelet. the w-frf is derived assuming that each blade acts as an independent ltv s-dof system. in this way, the natural frequency variation due to modification in the rotor speed has been taken into account. hence the blade response q(t) due to the magnet excitation f(t) is governed by the following differential equation: m(t)q′′(t) + c(t)q′(t) + k(t)q(t) = f(t) (6) wherem, c, k are the system properties and assumed to vary slowly with time. applying the wavelet transform to the right and left sides of the equation, the following relationship is obtained: wu,s[q] = h(u, s)wu,s[f ] (7) where h(u, s) is referred to as the wavelet-based frequency response function (wfrf). fig. 9 shows the continuous wavelet transform of the blade response 20 vol. 20/2018 last stage st blade damping at low load operation figure 7. measurement and excitation algorithm. figure 8. measurement and excitation labview user interface. 21 a. bessone, l. carassale, r. guida et al. acta polytechnica ctu proceedings figure 9. continuous wavelet transform for first mode shape, nd = 4, backward traveling wave. figure 10. ltv-frf best fitting of w-frf. wu,s[q] in the case when the magnet excites the nodal diameter -4 of the first mode. the dashed line highlights the blade eigenfrequency while the continuous black line fits the magnet excitation. since the rotational speed is kept almost constant throughout the tests, the natural frequency shows a constant value either. when the magnet excitation crosses the natural frequency, around t = 50 s and η = 463.7hz, a peak value of wu,s[q] is observed which denotes the blade resonance. furthermore, the system parameters can be obtained by fitting the w-frf, calculated according to eq. 7 along the excitation line, with the frf of the ltv s-dof system which can be expressed, in the neighborhood of the resonance, as follows: h(u, η) = s −η2 + ω(ωr(u))2 + 2iξηω(ωr(u)) (8) where s is a constant, ω is the natural frequency which is expressed as function of the rotational speed variation with time ωr(u) in fig. 10 the blue line represents the wfrf for the case presented in the previous figure. the red line is the best fitting frf which allows evaluating the damping factor ξ if the system which in this case results ξ = 0.027% in the present study, the damping factors resulted well below 10%, hence the damping logarithmic decrement δ is related to the damping factor approximately by δ ≈ ξ · 2π. thanks to this approach, it is possible to estimate the aggregated damping of the system, meaning the sum of mechanical and aerodynamic contributions to last stage blade damping under steam flow, for different nodal diameters, both for forward and backward travelling wave on the blade row. in fig. 11, measured aggregated damping in terms of logarithmic decrement δ for the two analyzed operating conditions op3 and op8 is reported. points on the chart represent the averaged value of the damping measurements for each strain gauge installed on the blade. the damping estimation method described above is accurate, but there is some uncertainty in the strain gauges measurement chain: the subsequent analyses to separate mechanical and aerodynamic damping contributions will be carried out considering two 4th order polynomial fitting curves of the log. dec. vs nd function for op3 and op8 respectively. 3. numerical analysis 3.1. computatuional method all the numerical analyses have been performed with the rans/urans traf code [14]. the solver has been extended to simulate unsteady flows around vibrating blades, thus implementing an aero-elastic nonlinear method. the non-linear uncoupled method for 22 vol. 20/2018 last stage st blade damping at low load operation figure 11. aggregated damping of lsb for the first mode shape, in op3 and op8. flutter analysis is here briefly described and a comprehensive method description and validation can be found in [8]. time-sinusoidal blade surface vibrations coming from modal analysis are assigned to the aero-elastic calculation as input data to actually deform the fluid domain at each time step. the non-linear unsteady flow equations are integrated on the deforming mesh by applying the same dual time-stepping algorithm used for aerodynamic unsteady simulations. traveling waves are represented on a single-angular-pitch domain by applying phase-lagged conditions to periodicity boundaries. since in uncoupled methods blade vibrations are input data, rather than a result of the computation, flutter stability is estimated by checking the sign of the energetic damping coefficient (or of the logarithmic decrement) applied by the fluid onto the blade during one vibration cycle. a positive logarithmic decrement means that energy is transferred from the blade to the fluid flow and the cascade oscillation is damped, hence a stable configuration. on the contrary, if the log-dec is negative, the unsteady aerodynamic pressure adds energy to the blade motion and the cascade is unstable. the different aero-elastic behavior mainly depends on the local phase shifting between unsteady pressure and blade vibration. 3.2. steady and unsteady cfd analysis off-design operation of low pressure steam turbines is achieved if the inlet mass flow is reduced and/or if condenser pressure is increased at constant rotating speed. this implies a decrease in volume flow at the last stage blade outlet, because, through the mass flow reduction or the outlet pressure increase, large decrease in last stage pressure ratio and enthalpy drop occurs. the losses grow up because the flow incidence angles are increased. both the isentropic enthalpy drop reduction and the increased losses lead to a decrease of stage specific work produced by the rotor. when such off-design conditions are enhanced, the last stage starts to ventilate and its specific work may become negative. this means that the last stage blade absorbs part of the work produced by the front stages instead of generating work itself, resulting in a pressure and the enthalpy increase instead of a pressure drop on the last stage. this is the case of the two analysed partload operating conditions of the present work, op3 and op8. the outlet axial velocity at the last stage blade trailing edge decreases, while the circumferential component of absolute velocity and, consequently, the associated centrifugal force increases. as this phenomenon takes place, flow streamlines cannot satisfy equilibrium between centrifugal and pressure forces, thus causing a flow separation from the hub endwall profiling. in the case of a strong reduction in volume flow, the centrifugal force increases and the region with reverse flow at the last stage blade outlet section expands in radial direction [15]. steady cfd simulation may suffer from convergence issues when such reverse flow conditions happen on the outlet boundary of the simulation domain. thanks to a rake of five 5-hole probes assembled on the vane carrier downstream the last stage blade trailing edge, radial height of flow separation at the hub has been estimated thanks to the yaw angle measurement (fig. 12, left): at 10% span higher than 90°yaw angle was detected, indicating a reverse flow in the corresponding region. this estimation has been used to define a region to be removed from the cfd domain as shown in fig. 12 (right), in order to avoid any reverse flow on 23 a. bessone, l. carassale, r. guida et al. acta polytechnica ctu proceedings figure 12. matching of yaw angle measurement at lsb te in op3 (left) and meridional view of lsb cfd domain (right). the outlet boundary and improve simulations convergence. traf rans simulation has been performed using this model and last stage blade loads measured at op3 and op8 have been matched. this is important for the subsequent urans analysis especially on the top of the blade, from mid-span to the tip, where the strongest flow structure interaction occurs. traf rans solution has been used to initialize urans simulations with moving grid for different number of nodal diameters of the first mode-shape family. for each nd, a different mode shape and frequency were assigned, according to results of modal analysis with cyclic symmetry conditions. each vibration period has been divided in 100 time steps and 20 sub-iterations for each time step have been run. a variable number of periods, generally lower than 15-20, was needed to achieve full convergence and solution periodicity. in the two operating conditions op3 and op8, aerodynamic damping (as logarithmic decrement) as a function of number of nd has been achieved as from the plot in fig. 13. as described before, the two operating points are characterized by same volume flow rate (i.e. same aerodynamic load on the airfoil), but different mass flow rate: the volume flow rate through the blade channel is the same, but the flow density in op8 is about 28% more compared to the one in op3. this means that, unless strong nonlinearities occur, also aerodynamic damping scales by the same factor. the same kind of flow-structure interaction takes place in the two ops (which are characterized by same flow mach number and same blade deformation and frequency), but, as the density of the flow rises, the aero-elastic work exchanged between the flow and blade structure proportionally increases. reynolds effects related to different values are reasonably considered as second-order for the purpose of this work. looking at the energetic damping surface density on the airfoil for the nd having minimum aerodynamic damping and comparing op8 to op3, it can be noted that the regions characterized by aerodynamic excitation (red) or aerodynamic damping (blue) on the airfoil surface are the same. in op8, the energetic damping intensity is always stronger if compared to op3 in terms of absolute value, keeping the same sign (fig. 14). furthermore, it is worth to note that the highest damping/exciting interaction contributing to integral value of logarithmic decrement are located in the upper region of the airfoil, where the highest modal displacements occur. 3.3. comparison of numerical and experimental results in 2.3 consistent measurements of aggregated damping in the two different mass flow rate conditions were shown. in 3.2, numerical assessment of the aerodynamic damping for the same conditions was presented. experimental data obtained in 3.2 were then used to separate the components of mechanical and aerodynamic damping, using the approach described in [5]. for each nd, results of aggregated damping measured at same volume flow, but different flow density, can be extrapolated to zero mass flow: in fact, without steam flow, the aggregated damping represents actually the mechanical damping of the blade. assuming constant mechanical damping as the mass flow rate increases, for each nd the difference between the measured aggregated damping and the projected mechanical damping gives the aerodynamic damping for that particular flow condition. according to the abovementioned approach, it is possible to define if a volume flow condition is inherently stable or not for the last stage blade: as the mass flow increases, if the aggregated damping increases too, the aerodynamic damping is positive and the blade is being operated safely. on the other side, if the aerodynamic damping is negative, as the mass flow grows the aggregated damping will decrease and there will be a limit value of mass flow, corresponding to zero aggregated damping (where negative aerodynamic damping fully overcomes mechanical damping), 24 vol. 20/2018 last stage st blade damping at low load operation figure 13. aerodynamic damping (logarithmic decrement) vs nd for the first mode shape in op3 and op8, assessed through traf urans simulations. figure 14. energetic damping surface density for first mode shape, nd = 20 backward traveling wave, in op3 and op8. 25 a. bessone, l. carassale, r. guida et al. acta polytechnica ctu proceedings figure 15. separation of mechanical and aerodynamic contributions for op3 and op8, from aggregated damping measurements. which can be considered as the blade stability margin. this simple linear model relies on the physical behaviour of aerodynamic damping described in 3.2. in fig. 15 results of projected mechanical damping and estimated aerodynamic damping achieved through this method are reported. results are obtained from polynomial fitting curves of the aggregated damping measurements. mechanical damping is smaller at low nd (about 0.1%), while it slightly gets larger at higher nd (almost 0.3%). a possible reason of this behaviour is the following: the snubber is designed to have stiff contacts between its suction and pressure sides when the centrifugal force at nominal speed untwists the blade airfoil. as the row vibration has increasing number of nd, also the phase shift between oscillations of adjacent blades increases. this could lead to snubber contact unstiffening, generating friction between the pressure and suction side surfaces, which is the reason of additional mechanical damping. subtracting mechanical damping from aggregated damping, also aerodynamic damping component as logarithmic decrement was estimated for op3 and op8. in fig. 16 a comparison between aerodynamic damping estimated from measurements and cfd results is reported. it is worth to note following considerations: • experimental measurements and numerical methods predict positive aerodynamic damping for both the analysed operating conditions, meaning that the blade is stable with regard to flutter at the corresponding volume flow, in ventilation, for the first mode shape, • the measured and calculated aerodynamic damping average level is very similar, varying between 0.0 and 0.6%, • also the minimum aerodynamic damping assessed through traf is similar to minimum aerodynamic damping measured on the test plant, and close to 0%, • the nd corresponding to the minimum aerodynamic damping is different: experimental data have a minimum at 7nd, while numerical results show a minimum at 20nd, both for backward travelling wave. as mentioned in [16], reasons of this discrepancy can be found in some cfd model simplifications, such as missing of snubber modelling. another reason of this can be the snubber contact modelling in the modal analysis: if the snubber contacts unstiffen, the modal displacements can be affected, with an impact on the blade deformation imposed in the urans analysis. 4. conclusion an extensive experimental campaign on a 3-stages downscaled low pressure steam turbine has been carried out at the t10mw test plant, in plzen, aiming to characterize a snubbered last stage blade flutter behavior at low load conditions. the rig has been heavily instrumented in order to provide an in-depth characterization of the flow behavior throughout the turbine as well as to measure the structural dynamic response of the lsb. the overall damping has been evaluated by exciting the blade resonance by means of an ac magnet 26 vol. 20/2018 last stage st blade damping at low load operation figure 16. aerodynamic damping in op3 and op8: comparison of experimental and numerical results. mounted on the vane carrier. a dedicated system has been developed to control the magnet excitation frequency as the turbine rotational speed oscillates during tests in order to make the damping estimation procedure more precise. furthermore, the mechanical and the aerodynamic contribution to the aggregated damping have been separated. a cfd numerical method has been adopted to predict the aerodynamic damping in analogous test conditions. numerical and experimental results have been discussed and compared. a satisfying agreement has been found from an industrial point of view, showing that the software tool is capable to properly predict the last stage blade flutter stability at low load condition, even if some discrepancies in results need still to be fixed. experimental data at very low lsb loads with stronger ventilation are available from the experimental tests and a wider campaign of cfd simulations is currently ongoing to extend the purpose of the present work to stronger part-load regimes. list of symbols aen ansaldo energia dsp w doosan škoda power t 10mw experimental test turbine in plzen ac/dc alternating/direct current lsb last stage blade lp t low pressure turbine n rotation speed d characteristic diameter p pressure v specific volume f blade eigen-frequency ch airfoil real chord f t flexturbine project op operating point f ns full navier-stokes sg strain gauges bt t blade tip timing nd number of nodal diameters acknowledgements this research work was supported by the european project flexturbine, h2020 research and innovation programme under grant agreement no. 653941 references [1] c. fuhrer, d. m. vogt. on the impact of simulation approaches on the predicted aerodynamic damping of a low pressure steam turbine rotor. in asme turbo expo 2017: turbomachinery technical conference and exposition, vol. 8, p. v008t29a007. 2017. doi:10.1115/gt2017-63401. [2] t. sun, p. petrie-repar, d. qi. investigation of tip clearance flow effects on an open 3d steam turbine flutter test case. in asme turbo expo 2017: turbomachinery technical conference and exposition, vol. 8, p. v008t29a024. 2017. doi:10.1115/gt2017-64021. [3] f. vanti, l. pinelli, f. poli, a. arnone. aeroelastic investigation of turbine blade assemblies: cluster systems and mistuned rows. 2017. doi:10.29008/etc2017-144. [4] m. may, y. mauffrey, f. sicot. numerical flutter analysis of turbomachinery bladings based on time-linearized, time-spectral and time-accurate simulations. 2011. 27 http://dx.doi.org/10.1115/gt2017-63401 http://dx.doi.org/10.1115/gt2017-64021 http://dx.doi.org/10.29008/etc2017-144 a. bessone, l. carassale, r. guida et al. acta polytechnica ctu proceedings [5] t. rice, d. bell, g. singh. identification of the stability margin between safe operation and the onset of blade flutter. vol. 131. 2007. doi:10.1115/gt2007-27462. [6] f. poli, l. pinelli, a. arnone. aeroelastic stability analysis of a non-rotating annular turbine test rig: a comparison between a linearized and a nonlinear computational method. in 22nd international congress on sound and vibration, (icsv22). 2015. [7] o. pütz, h.-p. kersken, g. ashcroft, et al. comparison of linear and nonlinear frequency domain methods for flutter analysis. in asme turbo expo 2018: turbomachinery technical conference and exposition, vol. 2c. 2018. doi:10.1115/gt2018-75626. [8] l. pinelli, f. poli, a. arnone, c. schipani. a timeaccurate 3d method for turbomachinery blade flutter analysis. in 12th international symposium on unsteady aerodynamics, aeroacoustics and aeroelasticity of turbomachines (isuaaat), pp. 1–4. 2009. [9] f. vanti, l. pinelli, a. arnone, et al. aeroelastic optimization of an industrial compressor rotor blade geometry. in asme turbo expo 2018: turbomachinery technical conference and exposition, vol. 2d, p. v02dt46a016. 2018. doi:10.1115/gt2018-76474. [10] l. carassale, m. marre-brunenghi, s. patrone. estimation of damping for turbine blades in non-stationary working conditions. in asme turbo expo 2015: turbomachinery technical conference and exposition, vol. 7b, p. v07bt32a018. 2015. doi:10.1115/gt2015-42945. [11] l. carassale, m. marre-brunenghi, s. patrone. modal identification of dynamically coupled bladed disks in runup tests. in asme turbo expo 2016: turbomachinery technical conference and exposition, vol. 7a, p. v07at32a018. 2016. doi:10.1115/gt2016-57251. [12] l. carassale, m. marre-brunenghi, s. patrone. wavelet-based identification of rotor blades in passage-through-resonance tests. mechanical systems and signal processing 98:124 – 138, 2018. doi:10.1016/j.ymssp.2017.04.023. [13] s. mallat. a wavelet tour of signal processing: the sparse way. 2008. [14] a. arnone, m. marconcini, a. scotti del greco, e. spano. numerical investigation of three-dimensional clocking effects in a low pressure turbine. journal of turbomachinery 126:375–384, 2004. doi:10.1115/1.1740780. [15] m. petrovic, a. wiedermann, s. m. nedeljkovic, m. banjac. part load behavior of the lp part of an industrial gas turbine. p. v02at40a033. 2017. doi:10.1115/gt2017-64778. 28 http://dx.doi.org/10.1115/gt2007-27462 http://dx.doi.org/10.1115/gt2018-75626 http://dx.doi.org/10.1115/gt2018-76474 http://dx.doi.org/10.1115/gt2015-42945 http://dx.doi.org/10.1115/gt2016-57251 http://dx.doi.org/10.1016/j.ymssp.2017.04.023 http://dx.doi.org/10.1115/1.1740780 http://dx.doi.org/10.1115/gt2017-64778 acta polytechnica ctu proceedings 20:16–28, 2018 1 introduction 2 experimental setup and damping measurement technique 2.1 downscaled low pressure steam turbine tested on t10mw test rig 2.2 steam turbine instrumentation and sg positioning 2.3 damping measurement technique: magnet setup and sweep method 2.4 damping measurement technique: damping estimation method 3 numerical analysis 3.1 computatuional method 3.2 steady and unsteady cfd analysis 3.3 comparison of numerical and experimental results 4 conclusion list of symbols acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2017.12.0010 acta polytechnica ctu proceedings 12:10–12, 2017 © czech technical university in prague, 2017 available online at http://ojs.cvut.cz/ojs/index.php/app qfd example in interaction with hmi inga chudjakova∗, jaromír tobiška škoda auto a.s., tř. václava klementa 869, mladá boleslav, 293 60, czech republic ∗ corresponding author: inga.kadlecova@skoda-auto.cz abstract. this article focuses on the application of the quality function deployment (qfd) method and shows how we can use this method in the automotive industry. the cockpit of the modern car is still developing and changing according to technical progress and customers’ requirements. the qfd method enables the setting of the customers’ requirements and then puts them into technical expression. the matrix of qfd sets those technical expressions that are the most important properties customers expect. these chosen properties can be tested during the technical development. the testing of new hmi concepts goes through a driving simulator and can suggest which one is suitable for being used in a real car. keywords: the qfd method, customers’ requirements, automotive industry, doe, hmi, driving simulator, house of quality. 1. introduction nowadays in the automotive industry, the aim of all car producers is customer satisfaction, but also technical progress, that can differentiate them in the current competitive environment. a set of quality methods serves/is used to achieve customer satisfaction in the automotive industry. they lead to production of high-quality production and are also related to higher profit. 2. quality assurance methods for the reasons above it is necessary to use the quality assurance methods in the automotive industry and use them at the beginning of the product development process. the qfd method, i.e. quality function deployment, is one of these methods. this method is used mostly at the first stage of the product development process because the qfd method allows transformation of customer requirements into technical expression[1]. its main task is the determination of customer requirements and their implementation in the process of the product preparation. thanks to qfd methods, the designers can understand what customers want and it designs the product. the methods used for optimizing the costs and the methods to prevent errors and faults in the development of a product also belong among the assurance methods. for more detailed classification of the quality assurance methods overview see the fig. 1. by using qfd method(s) we can assess the fulfilment of the customers’ requirement for products and processes. at the start of the qfd method application, there are the customer’s requirements (wishes). to find out what the customers want we need to focus on the following: a)technical parameters, a physical element, a specified part of system figure 1. the quality assurance methods overview. b)defined procecesses, assembly operations, handling operations, specific actions figure 2. the qfd application. the use of the qfd method is equal in the both cases. the difference is in the outcomes. we are able to find the key element of the system and the specific technical parameter, which we monitor to ensure the fulfilment of customers’ requirements or key activities; the process which implementation may impact the consequences effecting the customers’ satisfaction. 10 http://dx.doi.org/10.14311/app.2017.12.0010 http://ojs.cvut.cz/ojs/index.php/app vol. 12/2017 qfd example in interaction with hmi 3. the application of the qfd method the basic outcomes of the qfd (quality function deployment) method application are the answers to the two following questions: • what are the customer’s expectations? • how can we meet these expectations? [1] the qfd method is characterized by four application stages, which are as follows: product proposal (a proposal of characteristic product properties), proposal of components, processes planning and production planning. each of these stages is noted in a diagram and matrixes. 3.1. matrixes of the qfd method the qfd method is based on the qfd matrix, which is often marked as the "house of quality" and consists of a team of experts. the overall structure of the qfd matrix has 8 specified fields, but during the use the qfd matrix not all fields need to be completed. it is possible to use only a part of the qfd matrix, as required by the project. the qfd matrix that describes the most important field, can be seen at the graph bellow (see fig. 3). figure 3. qfd matrix "the house of quality". for the completing of the qfd matrix it is not required to use a special programme or software, one can just fill an excel table. the most important thing for the using the qfd method and filling in the qfd matrix is to know the succession of inputs and outputs – for detail see fig. 4. the qfd method can interpret the customers’ requirements, which are often defined only verbally, into the technical values that the designers can work with. to use the qfd method correctly, it is essential to get the inputs. this information can be obtained from: • surveys/questionnaires • thinking loud (lautes denken) figure 4. qfd method relations. • clinical studies • group discussions/think tanks ??? • j. d. power these procedures are established in quantitative and qualitative research techniques. in order to receive the information needed, it is necessary to choose the right method, group of respondents, questionnaire, and also the form of responses for each project. the application of the entire qfd method is based on obtaining these input data. then the matrix is completed with technical solutions that could have an influence on the customers’ requirements. the technical parameters may be defined by using accurate parameters or units. therefore, the best technical solutions are those that influence several customer wishes at once. the correlation between the customers’ expectations and the technical parameters is placed in the main field of the qfd matrix. this mutual correlation shows how much the technical solution will influence the customers’ expectations. the mutual influence may have the following values: 0 = no influence, 1 = low influence, 3 = average influence, or 9 = huge influence. 4. a practical example of application of the qfd method in the automotive industry at the present time, the load of the driver while driving is increasing. this is caused by the improvement of technology and development of the hmi systems in cars. also, the proportion of displaying and controlling components in the cockpit is increasing. an interaction between these car systems, displaying components and a driver may affect driving safety in a remarkable way[2]. for the practical example of using qfd method, we need to optimize the display to ensure safe operation while driving. for setting of the car display properties of we used qfd method. in order to use the qfd method, firstly we needed to find out which display properties customers found the most important. these were the inputs into the qfd matrix and we had used the qualitative and quantitative techniques and methods. the input data were received due to an evaluation of the questionnaires, 11 inga chudjakova, jaromír tobiška acta polytechnica ctu proceedings where respondents assessed the general properties of a display independent of the tested concepts. for these customers’ expectations, the technical experts set the technical parameters, which is are in the field p4. then the teams set the correlations between the customers’ expectations and technical parameters at the field p5. after the calculations of qfd matrix, in the field p6 we can see which technical parameters had effected the customers’ expectations the most. figure 5. example of qfd matrix. the highest rated technical expressions from the qfd matrix were tested in the technical clinic through a driving simulator. in these clinics, the highest rated expressions were: a font size, font legibility, text understanding and contrast. these parameters were tested separately on a separate screen. several chosen screens from the display menu were tested. they display various combinations of the script sizes and contrast. the respondents were expected to assess the individual items on each screen, such as a script legibility, text comprehension and contrast. the above-mentioned testing revealed that the best combination of good display legibility for customers is the second biggest letter size of the tested script with combination of the best contrast (contrast 1), because the biggest font size can be distracting[3]. figure 6. combination of fontsize and contrast. 5. conclusion the qfd method is an integral part of a product development in the automobile industry. its contribution is provable in all the stages of the car development process as well as in the area of the hmi concept testing. the main objective of the hmi systems testing is the minimization of the time that these systems are controlled and thus the time that the driver is not concentrating on driving. together, these methods contribute to customers’ satisfaction, which has been nowadays emphasized more and more in the automotive industry. references [1] j. machan, j. tobiška, et al. metody kvality užívané ve fázi vývoje výrobku – aplikace v automobilovém průmyslu. ii.a revised and expanded edition, prague 2012, 117 p., isbn 978-80-87042-50-2. [2] m. novák, p. bouchner, j. faber, et al. senioři za volantem. prague: faculty of transportation sciences, czech technical university in prague, 2008, isbn 978-80-87136-20-7. [3] zvláštní projekty elektrostrategie a výzkumu, technický vývoj, škoda auto a.s.: projekt hmi ve škoda auto. (prezentace), mladá boleslav, an internal document, 2013. 12 acta polytechnica ctu proceedings 12:10–12, 2017 1 introduction 2 quality assurance methods 3 the application of the qfd method 3.1 matrixes of the qfd method 4 a practical example of application of the qfd method in the automotive industry 5 conclusion references acta polytechnica doi:10.14311/app.2020.27.0006 acta polytechnica 27(0):6–12, 2020 © czech technical university in prague, 2020 available online at http://ojs.cvut.cz/ojs/index.php/app fracture behavior of fealsi intermetallics jaroslav čecha,∗, petr haušilda, miroslav karlíka, kateřina nováb, filip průšab, pavel novákb, jaromír kopečekc a czech technical university in prague, faculty of nuclear sciences and physical engineering, department of materials, trojanova 13, 120 00 prague, czech republic b university of chemistry and technology prague, faculty of chemical technology, department of metals and corrosion engineering, technická 5, 166 28 prague, czech republic c institute of physics, ascr, v.v.i., department of functional materials, na slovance 2, 182 21 prague, czech republic ∗ corresponding author: jaroslav.cech@fjfi.cvut.cz abstract. the study is devoted to the intermetallic alloy feal20si20 (wt.%) with the potential applications in high temperature aggressive environments. the samples of the same chemical composition were prepared by spark plasma sintering from the different mechanically alloyed powders (pure elements and pre-alloyed powders). differences in mechanical properties were characterized. whereas no significant differences were found in hardness and young´s modulus, fracture resistance was higher for the samples from pre-alloyed powders in which palmqvist and lateral cracks were observed (contrary to the sample made of pure elements where only palmqvist cracks were identified). keywords: fealsi, fracture toughness, nanoindentation, sps. 1. introduction iron aluminides are one of the most studied intermetallics [1, 2], but not much attention was paid on their ternary alloys with silicon because of their brittleness. despite this drawback, they exhibit promising mechanical properties and excellent high-temperature oxidation resistance [3]. potential replacement of stainless steels or nickel-based superalloys would be possible only if the production process will be more effective and the brittleness significantly improved. the standard metallurgical processes (e.g. casting and rolling) for the production of fealsi alloys are not convenient. for this reason, powder metallurgy, especially mechanical alloying (ma) with consequent spark plasma sintering (sps) seems to be potentially applicable processing way. severe deformation, repeated cold welding, fracturing and rewelding of the initial powders during mechanical alloying result in the production of homogeneous powders with fine microstructure [4, 5]. sps used for the consolidation of the powders is very fast sintering method which reduces grain coarsening and phase changes during the sample compaction. resulting very fine microstructure can improve fracture toughness of these alloys which is usually very low (comparable with glasses or ceramics). as this material is brittle, the cracks are usually observed around the residual imprints after indentation measurements. measuring their geometry can be used for the evaluation of the fracture toughness. various methods and formulas were proposed for the determination of fracture toughness from indentation measurements (e.g. [6]). they are usually based on the size of palmqvist (radial) or median cracks. [7–9]. lateral cracks [10] or energetic methods [11, 12] are used less often and proper formula has to be employed to meet the assumptions of the specific cracking system. in this study, feal20si20 intermetallic alloy prepared by mechanical alloying and spark plasma sintering from various initial powders was studied. the focus was on the fracture properties investigated by indentation and scratch tests. 2. experimental methods 2.1. material three powder mixtures were prepared by mechanical alloying (ma). the final chemical composition was 60 wt.% of fe, 20 wt.% of al and 20 wt.% of si (feal20si20 wt.%). differences were in the initial powders used for milling. the mixture denoted fe_al_si was prepared from pure elements fe, al and si. the mixture feal_si was mixed from feal25 (wt.%) pre-alloyed powder and pure si. the third mechanically alloyed powder (denoted fesi_al) was milled from fesi25 (wt.%) pre-alloyed powder and pure al. the powders were milled in planetary ball mill pm 100 cm (retsch, haan, germany) for 8 h. powder to ball mass ratio was 1:60, rotational speed 400 rpm and no lubricant was used to fulfill the conditions of so-called ultra-high energy mechanical alloying [13] which can reduce the time necessary for the homogenization of the powders. the shortest time for the homogenization of the powders (evaluated from x-ray diffraction, microstructural observations and nanoindentation measurements) was needed for the mixture fe_al_si (4 h), for the mixture fesi_al 6 http://dx.doi.org/10.14311/app.2020.27.0006 http://ojs.cvut.cz/ojs/index.php/app vol. 27/2020 fracture behavior of fealsi intermetallics it was about 6 h, and the longest time (8 h) was necessary for the mixture feal_si. the details about the powder preparation and homogenization can be found in [14]. the powders were consequently sintered by spark plasma sintering (sps) in hp d10 device (fct systeme gmbh, rauenstein, germany). the heating rate (300 °c/min up to 900 °c and then 100 °c/min to 1000 °c), hold on the maximum temperature 1000 °c for 10minutes, and the cooling rate (50 °c/min) were carefully adjusted to avoid temperature overshoot or cracking caused by thermal shocks. 2.2. characterization techniques the compacted samples for microstructure characterization, microindentation and scratch measurements were prepared by standard metallographic procedures with the final polishing by colloidal silica suspension (0.04 µm). the microstructure was observed in light optical (neophot 32) and scanning electron microscope (sem) jeol jsm 5510lv in the signal of backscattered electrons (bse). microindentation and scratch tests were carried out on mct tester (anton paar, graz, austria). maximum load for indentation with vickers indenter was 5n, loading and unloading rate 10n/min and hold period at the maximum load 10 s. at least nine indentations per sample were performed. the acquired data (force f penetration depth h dependency) were evaluated to obtain hardness h and young´s modulus e according to the iso 14577 standard [15] by the oliver-pharr method [16]. the poisson ratio of fealsi samples was assumed 0.3. fracture toughness was evaluated from the system of palmqvist cracks observed and measured after the indentation tests. no cracks under the residual imprint were observed and only the palmqvist cracks were confirmed by optical, electron and confocal microsopy. the equation introduced by niihara [7] was used to calculate the values of kic : kic = 0.035 ( l a ) ha1/2 1 φ ( eφ h )2/5 , (1) where l is the crack length, a is the half-diagonal of the indent (see figure 1) and φ stands for the constraint factor (assumed to equal to 3). this formula is valid for the palmqvist cracks and the crack lengths 0.25 ≤ l/a ≤ 2.5. three scratches were performed for every sample to obtain statistically relevant data about scratch resistance. the scratches of the length 1mm with the progressively increasing load from 0.03n up to 30n were carried out by rockwell sphero-conical diamond indenter with radius 100µm. no lubricant was used during scratch tests. the data of the depth, normal and friction load were recorded by the instrument during every test. pre-scan and post-scan procedures [18] (i.e. scanning of the surface topography at the location of the scratch by load 0.03n before and after the figure 1. palmqvist and lateral crack systems (adapted from [17]). test) enabled measuring of the correct values of penetration and residual depth not affected by the surface profile. the panorama images of the scratch, which correlate the measured signals with optical image and enable the subsequent evaluation of the data, were taken. critical load lc denoting the cracking load of the samples was consequently determined from the optical analysis. 3. results and discussion 3.1. indentation spark plasma sintering was chosen as a compaction method as it is fast and it prevents the coarsening of the fine microstructure obtained by mechanical alloying. the microstructure observations revealed very fine structure with the equiaxed grains of mean size approximately 1µm for all the samples. fine-grained microstructure resulted in very good mechanical properties (table 1). no significant differences in hardness and young´s modulus were found between the sintered compacts. young´s modulus was approximately 220gpa for the samples from pre-alloyed powders, slightly higher (234gpa) for the sample from pure elements. the values of the young´s modulus are close to young´s modulus of the steel, which is suitable for the potential replacement of the steel products by this intermetallic alloy. moreover, the measured hardness of the fealsi samples was about 13gpa (only slightly lower for the sample fesi_al) which is higher than for common steels and it can be beneficial in some applications. on the other hand, intermetallic alloys are usually very brittle and they have low fracture toughness which is very limiting for practical use of these alloys. this was confirmed for the studied alloy for 7 j. čech, p. haušild, m. karlík et al. acta polytechnica h [gpa] e [gpa] l/a [−] kic [ mpa.m1/2] lc [n] fe_al_si 13.48 ± 0.44 233.55 ± 6.47 1.28 ± 0.15 2.57 ± 0.18 11.8 ± 0.5 feal_si 13.46 ± 0.25 220.45 ± 1.71 0.62 ± 0.09 3.61 ± 0.32 16.1 ± 0.8 fesi_al 12.61 ± 0.15 220.66 ± 2.96 0.74 ± 0.18 3.21 ± 0.40 22.7 ± 0.8 table 1. mechanical properties of feal20si20 samples. 0 1 2 3 4 5 6 0 1 2 3 4 5 f [n ] h [µm] figure 2. indentation curve and the residual indent with palmqvist cracks in fe_al_si sample. no lateral cracks were observed on micrographs from light optical microscope or sem. 0 1 2 3 4 5 6 0 1 2 3 4 5 f [n ] h [µm] figure 3. indentation curve and the residual indent with palmqvist and lateral cracks in feal_si sample. lateral cracks are visible on light optical micrographs. 8 vol. 27/2020 fracture behavior of fealsi intermetallics 0 1 2 3 4 5 6 0 1 2 3 4 5 f [n ] h [µm] figure 4. indentation curve and the residual indent with palmqvist and lateral cracks in fesi_al sample. lateral cracks are visible on light optical micrographs. 0 5 10 15 20 25 30 -5 -4 -3 -2 -1 0 1 2 0 0.2 0.4 0.6 0.8 1 fo rc e [ n ] d e p th [µ m ] distance [mm] penetration depth residual depth normal force friction force lc figure 5. scratch panorama and measured signals on feal_si sample. 9 j. čech, p. haušild, m. karlík et al. acta polytechnica 0 1 2 3 4 5 6 -5 -4 -3 -2 -1 0 1 2 0 0.2 0.4 0.6 0.8 1 fr ic ti o n f o rc e [ n ] re si d u al d e p th [ μ m ] distance [mm] fe_al_si residual depth feal_si residual depth fesi_al residual depth fe_al_si friction force feal_si friction force fesi_al friction force figure 6. comparison of residual depth and friction force. figure 7. scratches on (a) fe_al_si, (b) feal_si and (c) fesi_al samples. 10 vol. 27/2020 fracture behavior of fealsi intermetallics which the measured values of fracture toughness were around 3mpa.m1/2. even if this value is about ten times higher than fracture toughness of casted fealsi alloys [19], it is still very low value. some differences in the values of fracture toughness and developed crack systems were found between the samples from pure and pre-alloyed powders (table 1, figures 2, 3 and 4). even if the indentation force-penetration depth curves are very similar and no pop-ins associated with the cracking can be observed, the various cracks were identified after the optical analysis. for fe_al_si sample, only palmqvist cracks initiated in all four corners of the residual imprint were observed and the calculated fracture toughness reached the lowest value (2.6mpa.m1/2). the samples from pre-alloyed powders had higher fracture toughness (3.2mpa.m1/2 for fesi_al sample and 3.6mpa.m1/2 for feal_si sample) and for some indents, the palmqvist cracks propagated only form 3 corners of the imprint. in these cases, lateral cracks were often observed. differences in the fracture behavior can be probably attributed to the amount of the stored plastic deformation during mechanical alloying of the powders [14]. the homogenization of the fe_al_si powder was the fastest and after its end, no microstructural changes were observed and only plastic deformation was accumulated in the powder. the homogenization of the samples feal_si and fesi_al was more progressive [14], it took more time and less plastic deformation causing the brittleness of the samples was stored in the powders. 3.2. scratch resistance typical example of the scratch correlated with measured signals is shown in figure 5. no significant differences between samples were found in the depth or friction force signals (figure 6). the coefficient of friction progressively increased from approximately 0.05 at the beginning of the test to the value of nearly 0.2 at maximum load for all tested samples. the small cracks inside the scratch path were observed from the lowest applied loads for all samples. critical load lc was optically determined as the normal load measured at the position where the large long crack gets outside the scratch path (figure 5). at this position, the fast increase in residual depth was also found. the values of critical loads are summarized in table 1 and the typical panorama images are presented in figure 7. same as for the indentation fracture toughness, lowest critical load was found for fe_al_si sample from pure elements. at higher loads, total destruction of the sample (figure 7a) was observed for some scratches. this failure has the character of chipping and crumbling of the sample. samples from pre-alloyed powders showed higher scratch resistance (higher critical load) and no evidence of chipping. 4. conclusions the feal20si20 samples were prepared by spark plasma sintering form various initial powders (pure elements fe, al, si and pre-alloyed feal25 and fesi25). mechanical properties including hardness, young´s modulus, fracture toughness and scratch resistance were measured. no significant differences between the samples were found in hardness and young´s modulus. the fracture toughness and scratch resistance were higher for the samples made of pre-alloyed powders than for the sample milled from pure elements proving the importance of processing route on mechanical properties. acknowledgements financial support by the european regional development fund in the frame of the project centre of advanced applied sciences (no. cz.02.1.01/0.0/0.0/16-019/0000778), czech science foundation (project no. 17-07559s) and grant agency of the czech technical university in prague (project no. sgs18/190/ohk4/3t/14) is gratefully acknowledged. references [1] s. deevi, v. sikka. nickel and iron aluminides: an overview on properties, processing, and applications. intermetallics 4:357–375, 1996. doi:10.1016/0966-9795(95)00056-9. [2] x. zhu, z. yao, x. gu, et al. microstructure and corrosion resistance of fe-al intermetallic coating on 45 steel synthesized by double glow plasma surface alloying technology. trans of nonferrous met soc china 19:143–148, 2009. doi:10.1016/s1003-6326(08)60242-3. [3] p. novák, m. zelinková, j. šerák, et al. oxidation resistance of shs fe-al-si alloys at 800 °c in air. intermetallics 19:1306–1312, 2011. doi:10.1016/j.intermet.2011.04.011. [4] c. suryanarayana. mechanical alloying and milling. prog mater sci 46:1–184, 2001. doi:10.1016/s0079-6425(99)00010-9. [5] l. bhadeshia. mechanically alloyed metals. mater sci technol 16:1404–1411, 2000. doi:10.1179/026708300101507361. [6] s. zhang, x. zhang. toughness evaluation of hard coatings and thin films. thin solid films 520:2375–2389, 2012. doi:10.1016/j.tsf.2011.09.036. [7] k. niihara. a fracture mechanics analysis of indentation-induced palmqvist crack in ceramics. j mater sci lett 2:221–223, 1983. doi:10.1007/bf00725625. [8] m. laugier. new formula for indentation toughness in ceramics. j mater sci lett 6:355–356, 1987. doi:10.1007/bf01729352. [9] b. lawn, a. evans, d. marshall. elastic/plastic indentation damage in ceramics: the median/radial crack system. j am ceram soc 63:574–581, 1980. doi:10.1111/j.1151-2916.1980.tb10768.x. 11 https://doi.org/10.1016/0966-9795(95)00056-9 https://doi.org/10.1016/s1003-6326(08)60242-3 https://doi.org/10.1016/j.intermet.2011.04.011 https://doi.org/10.1016/s0079-6425(99)00010-9 https://doi.org/10.1179/026708300101507361 https://doi.org/10.1016/j.tsf.2011.09.036 https://doi.org/10.1007/bf00725625 https://doi.org/10.1007/bf01729352 https://doi.org/10.1111/j.1151-2916.1980.tb10768.x j. čech, p. haušild, m. karlík et al. acta polytechnica [10] d. marshall, b. lawn, a. evans. elastic/plastic indentation damage in ceramics: the lateral crack system. j am ceram soc 65:561–566, 1982. doi:10.1111/j.1151-2916.1982.tb10782.x. [11] j. malzbender, g. de with. energy dissipation, fracture toughness and the indentation load-displacement curve of coated materials. surf coat technol 135:60–68, 2000. doi:10.1016/s0257-8972(00)00906-3. [12] j. chen, s. bull. assessment of the toughness of thin coatings using nanoindentation under displacement control. thin solid films 494:1–7, 2006. doi:10.1016/j.tsf.2005.08.176. [13] p. novák, f. průša, k. nová, et al. application of mechanical alloying in synthesis of intermetallics. acta phys pol a 134:720–723, 2018. doi:10.12693/aphyspola.134.720. [14] j. čech, p. haušild, m. karlík, et al. effect of initial powders on properties of fealsi intermetallics. materials 12:16, 2019. doi:10.3390/ma12182846. [15] iso 14557, metallic materials instrumented indentation test for hardness and material parameters, 2002. [16] w. c. oliver, g. m. pharr. an improved technique for determining hardness and elastic modulus using load and displacement sensing indentation experiments. j mater res 7:1564–1583, 1992. doi:10.1557/jmr.1992.1564. [17] k. jin, m. minggat, r. singh. sintered properties of stainless steel-doped y-tzp ceramics. matec web of conferences 152:13, 2018. doi:10.1051/matecconf/201815202012. [18] anton paar. scratch software manual v8, 2019. [19] k. nová, p. novák, t. vanka, f. průša. the effect of production process on properties of feal20si20. manuf technol 18:295–298, 2018. doi:10.21062/ujep/94.2018/a/12132489/mt/18/2/295. 12 https://doi.org/10.1111/j.1151-2916.1982.tb10782.x https://doi.org/10.1016/s0257-8972(00)00906-3 https://doi.org/10.1016/j.tsf.2005.08.176 https://doi.org/10.12693/aphyspola.134.720 https://doi.org/10.3390/ma12182846 https://doi.org/10.1557/jmr.1992.1564 https://doi.org/10.1051/matecconf/201815202012 https://doi.org/10.21062/ujep/94.2018/a/1213-2489/mt/18/2/295 https://doi.org/10.21062/ujep/94.2018/a/1213-2489/mt/18/2/295 acta polytechnica 27(0):6–12, 2020 1 introduction 2 experimental methods 2.1 material 2.2 characterization techniques 3 results and discussion 3.1 indentation 3.2 scratch resistance 4 conclusions acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2018.16.0037 acta polytechnica ctu proceedings 16:37–39, 2018 © czech technical university in prague, 2018 available online at http://ojs.cvut.cz/ojs/index.php/app soil reinforcement and ec7 (202x) martin vaníček geosyntetika ltd., nikoly tesly 3, 160 00 prague, czech republic correspondence: mvanicek@geosyntetika.cz abstract. soil reinforcement is not really covered in current version of eurocode 7 and therefore during the proposals for 2nd generation of ec7 the soil reinforcement was highly promoted for inclusion in new ec7 based on the comments from practising engineers. in new ec7 which is now being written in newly created part 3 there will be next to other clauses also complete clause on reinforced ground. the paper will inform on the proposed contents of this clause. keywords: soil reinforcement, eurocode 7. 1. introduction existing eurocode 7 covers soil reinforcement topic very vaguely. together with ground improvement topic it is written just on half page [1, 2]. once current eurocode 7 was introduced to praxis, this state was highly criticised by geotechnical engineers around the whole europe. this criticism was formally visible as outcome from periodic 5-year review. as a result of whole eurocode suite success, european commission decided already in 2010 to update and further evolve the eurocode suite towards its second generation. 2. development in between 2010 and 2015 in soil reinforcement area within the cen technical committee tc250/sc7, which is responsible for eurocode 7, fourteen evolution groups (eg) were founded. each eg concentrated on different specific subject of current ec7. eg 5 under the convenorship of the author focused on the introduction of the subject of reinforced soils into the eurocode 7. group members were from all around europe. within this period comprehensive background document was developed, which covers most of typical applications of reinforced soil. we concentrated on specification of relevant limit states that should be checked during the design of such structures. calculation methods for different limit states were discussed and compatibility of these methods was checked. the main differences were identified in the design approaches that were selected in specific countries. surprising recognition was combination of different design approaches used during checking of identified limit states even within one country. in some countries the former codes of praxis were combined with eurocode 7, even when it meant combination of limit states approach with overall stability approach and hence very high (pointless) safety margins were required. during this phase group was meeting face to face once a year within ec7 workgroup meetings, which were held around europe (e.g. in november 2014 in prague). in the meantime, the group held online meetings in about 7 times a year, where further advancement and tasks for periods in between meetings were discussed. it was agreed to focus in turn to the following types of reinforced soils: • reinforced embankment slopes • soil nailed slopes • walls from reinforced soil • subsoil reinforcement under embankments on soft soils • embankment on load transfer platforms over inclusions • embankments over possible voids • veneer stability the structures that were agreed not to cover at the moments are reinforcement under shallow foundations and working platforms reinforcement. within ultimate limit states following locations of limit states origin were identified: • in soil • in reinforcing element • on the interface between soil and reinforcing element • at facing element • the structure as a whole it is possible to illustrate the identified limit states for given structure in the following figure 1. for these limit states it will be necessary to define partial safety factors on both materials and resistances according to the design cases selected in particular country. 37 http://dx.doi.org/10.14311/app.2018.16.0037 http://ojs.cvut.cz/ojs/index.php/app martin vaníček acta polytechnica ctu proceedings retaining structures embankment base piled embankment voids overbridging veneer stability uls in soil stability along slip surface (external, internal and compound) x x x x sliding x x x bearing capacity (including the excentricity) x squeezing x x in reinforcement tensile rupture geosynthetic reinforcement x x x x x steel reinforcement x x1 x1 x1 x soil nails x anchors x2 shear rupture soil nails x rigid vertical bearing elements “rvbe” (piles, micropiles, …) x2 x2 tensile continuity (overlaps, connections, …) x x x x x on the interface between soil and reinforcement pull-out x x x x x stripping x x x sliding x x x x squeezing soil nails x rigid vertical bearing elements “rvbe” (piles, micropiles, …) x2 x2 on facing elements connection to reinforcing element x bulging (blocks, steel mesh, wrap around, …) x sls in reinforced structure itself total vertical / horizontal deformation x x x x x relative vertical / horizontal deformation post construction creep x x x x x relative deformation of the facing (bulging) x in reinforcing element total strain (elongation) x x x x x creep strain (post construction) x x x x x in facing element differential settlement along the facing due to subsoil deformation x notes: x1 rarely used x2 anchors + rvbes are not considered as soil reinforcements in current standards but are shown as an aide memoire for hybrid structures figure 1. matrix of identified limit states and reinforced soil structures. with respect to the need for definition of design strength of reinforcing element, there will be in the framework of second generation of eurocode 7 defined a procedure for determination of strength for geosynthetic reinforcing elements, steel reinforcing elements and soil nails. the evolution group suggested following procedures to be incorporated into the ec7. an agreement between sc3 and sc7 was made that tensile strength of steel soil reinforcing elements will be covered in 2nd generation of ec7 rather than in ec3 [3]. for steel reinforcing elements uk and french long-term experience and suggestions will be used. for geosynthetic elements the design strength will be adopted from iso/tr 20432:2007 [4] that will be further extended to incorporate additional reduction factors from german specification ebgeo [5]. for soil nails design strength current appendix to en 14490 (execution of soil nailed structures) will be adopted [6]. 3. eurocode 7 development since 2015 the actual development of eurocode 7 2nd generation started in 2015 when project teams 1 and 2 were established. team 1 was contracted to reorganize eurocode 7 into 3 parts with common structure in all main clauses which will mimic the structure of other structural eurocodes. part 1 will be general and will be common to all geotechnical structures, part 2 will cover ground investigation and part 3 will cover all different geotechnical structures. additional target for team 1 was to propose simplifications to current text. all team 1 tasks are finished as of september 2017. task of team 2 was to rewrite part 1 of eurocode 7, so it includes the general subjects common to most of geotechnical structures and to prepare an alternative to existing design approaches, which generate incompatibility in between individual countries with respect to design. at the same time the requirement was to make the eurocode 7 fully compatible with eurocodes 0 and 1, which specify the basis for limit states design and loadings on structures. at the end 38 vol. 16/2018 soil reinforcement and ec7 (202x) of october 2017 team 2 prepared the final draft of ec7 part 1 for final comments by cen national standards bodies. these comments have to be taken on board for the final part 1 deliverable at the end of april 2018. second and third part of eurocode 7 started to be updated in october 2017 as during the 2017 the project teams 3, 4 and 5 were selected and appointed. team 3 is responsible for updating of ec7 part 2 for ground investigations. this part will be redrafted in that way that the main focus will be on determination of soil properties needed for geotechnical design from different investigation techniques, rather than the current situation when the focus is on tests. as part of team 3 work will be specification of ground investigation requirements and outputs including reporting. team 4 is responsible for only selected clauses of part 3, which cover the subjects of slopes, shallow and deep foundations and ground improvement. these clauses white spaces will be filled in, current text will be amended, refined and simplified to achieve the ease of use target. the clause on ground improvement will be completely new. team 5, of which the author is member, is responsible for remaining clauses on geotechnical structures in ec7 part 2. more specifically these clauses are retaining structures, anchors and reinforced ground. the task of this team is also to fill in white spaces, amend, refine and simplify existing text, so it will be in accordance with basic structure of all eurocodes. this common structure of all eurocodes should lead to simpler navigation through all eurocodes parts. the plan is that teams 3 to 5 shall have ready in april 2019 first proper draft of their respective clauses. this draft will be than reviewed by tc250/sc7 committee and official comments submitted by them to project teams. after that project teams will prepare final draft by the end of october 2019. the october 2019 draft will be circulated to national standards bodies for official comments from cen member states. these comments will lead to final text of respected parts by project teams by the end of april 2020, when their tasks end. there is also planned 3rd phase of ec7 updating. this phase will be undertaken by project team 6, whose task is to amend the text of all 3 parts for rock mechanics and dynamic design. the schedule for project team 6 is 1 year behind project teams 3, 4 and 5, which means that they should conclude their work by april 2021. once all project teams conclude their work the ownership of eurocode 7 text will be transferred to tc250/sc7 committee, which will undergo the official process of acceptance voting by cen processes. subsequently the accepted text not only of eurocode 7 but of all eurocodes will be published by cen, which is not expected before the end of 2022. the clause on reinforced geotechnical structures will be based on output from evolution group as described above. the main task will be to define the main limit states for different types of reinforced soil structures as indicated in figure 1 and within informative annexes there will be suggested calculation methods for evaluation of selected limit states. 4. conclusions with respect to the situation that current version of eurocode 7 practically does not cover the design of reinforced soil structures, it is a very positive signal to include these structures into the second generation of eurocode 7. this inclusion means that these structures will finally be designed more or less the same way (by the same philosophy) all around europe and even possibly around the world, where the system of eurocodes is being used more and more. the design of reinforced soil structures will be more easily checkable even across borders. for the czech users of eurocode 7 can be my presence in drafting of the new generation a good signal for keeping most design principles of existing design rules in the czech republic (valid since 1997) especially due to the fact that they are used for 20 years without any problems and are fully in accordance with existing eurocode 7 and surely will be also with second generation. acknowledgements here i would like to thank to the czech geotechnical society for the support in my presence on the preparation of second generation of eurocode 7. references [1] čsn en 1997-1:2006. eurocode 7: design of geotechnical structures – part 1: general rules. [2] čsn p env 1997-1:1996. design of geotechnical structures – part 1: general rules. [3] čsn en 1993-5:2008. eurocode 3: design of steel structures – part 5: piling. [4] pd iso/tr 20432:2007. guidelines for the determination of the long-term strength of geosynthetics for soil reinforcement. [5] w. ernst, s. sohn. deutsche gesellschaft für geotechnik e. v., dggt (2011). recommendations for design and analysis of earth structures using geosynthetic reinforcements – ebgeo. 340 p. berlin. [6] čsn en 14490:2010. execution of special geotechnical works – soil nailing. 39 acta polytechnica ctu proceedings 16:37–39, 2018 1 introduction 2 development in between 2010 and 2015 in soil reinforcement area 3 eurocode 7 development since 2015 4 conclusions acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2020.26.0076 acta polytechnica ctu proceedings 26:76–80, 2020 © czech technical university in prague, 2020 available online at https://ojs.cvut.cz/ojs/index.php/app densitometry-based fem simulations of novel porous implants and corresponding stress distribution at the peri-implant area luboš řehounek∗, aleš jíra czech technical university in prague, faculty of civil engineering, department of mechanics, thákurova 7, 166 29 prague 6, czech republic ∗ corresponding author: lubos.rehounek@fsv.cvut.cz abstract. the presented work focuses on determining the stress distribution at the peri-implant area around dental implants. a numerical analysis simulating the conditions of chewing food has been performed on a fem model. this model has been created using anonymized real patient ct data and a dental implant model developed at ctu. the ct data served as a 3d geometry and also as a way to construct the global matrix of stiffness of the bone material. bone density was used as the defining parameter in determining the values of young’s moduli of individual finite elements by the computational program (mechanical finder). the implant was introduced as a user-created stl file, which was imported to the computational software and situated inside the geometry of the human mandible. the results show that, as predicted, porous implants achieve higher values of minimum principal stress in the bone as opposed to homogeneous implants (13.4 mpa vs. 7.0 mpa), thus reducing stress shielding. keywords: dental implant, fem, mechanical finder, porous, stress shielding. 1. introduction the dental implants market seems to be one of the most rapidly developing markets in the field of biomaterials [1] and recently, inhomogeneous structures started to emerge as a viable alternative to conventional implants. the goal of this research is to compare different geometrical solutions of a novel porous dental implant with the aid of fem. the main quantitative measure of this computation is the stress distribution in the surrounding bone and in the body of the implant. the research also aims to evaluate the viability of fem in regard to simulation of small-scale biomechanical models, such as the human mandible, dentition and dental implants. our main goal in designing new implants is to reduce stress concentrations, as they are known to be the reason for early implant failure [2], and also introduce a level of porosity into the implant. this approach is possible with the advent of additive manufacturing. direct metal laser sintering (dmls) enables for manufacturing of morphologically complex structures, which would otherwise not be possible to create with conventional means of production [3]. porous structures, such as the gyroid structure, demand use of additive manufacturing technologies. the 3d printing of metals and metal alloys serves this purpose well. the main goal of this study was to try to evaluate the feasibility of the fem software mechanical finder for analysing the human bone system, specifically human dentition. the authors were convinced that based on wolff’s law [4] developed by the german anatomist and surgeon julius wolff, bones which are not subjected to mechanical loading tend to decrease in density and quality. the idea of porous implants is based on this fundamental law so as to provide greater loads to the bone and prevent the effect of stress-shielding. porous implants, which have a much lower value of global young’s modulus e, present a viable alternative for increasing the stresses in bone at the peri-implant area. as the homogeneous, stiffer implants have much higher e, authors expect the stresses transfered to the bone to be much lower. 1.1. porous implants porosity is welcome as it increases the surface area of the implant, thus creating a better environment for osseointegration, and also enables osteons to grow inside the body of the implant. bone ingrowth then has the potential to create a better, interconnecting interface of bone and implant material, thus increasing the stability of the implant inside human bone. the structures and whole implants, such as the ones illustrated in fig. 2 and fig. 3, are usually designed in a cad software and subsequently 3d-printed using a metal powder. the authors of this study already know the possibility of bone ingrowth from their own in-vivo experiments [5] on laboratory pigs, where the osseointegration and bic (bone-implant contact) were investigated. the magnitude of the pores (or trabeculae) is, therefore, a subject of discussion. authors list some general clues [6] and their own recommendations for mean pore size, but opinions vary and authors suggest that more in-vivo experiments are needed to ascertain the optimal range of pore size. to give the reader 76 https://doi.org/10.14311/app.2020.26.0076 https://ojs.cvut.cz/ojs/index.php/app vol. 26/2020 densitometry-based fem simulations of novel porous implants figure 1. a model of a sample unit cell of the gyroid structure with end blocks (left) and a microscope image of a real, 3d-printed gyroid structure specimen (right) dedicated to macromechanical tests. as the cell is repetitive, the model can be made by multiplication of the unit cell. material of the print is ti-6al-4v. figure 2. the first, homogeneous geometrical solution of the dental implant used for numerical simulations. implant was previously patented in the czech republic [7]. image shows the intraosseous part. a good estimate, the values usually fall in the range of 300–900 µm. authors would also like to address benefits which are special to the gyroid structure. the gyroid structure (fig. 1) is a special 3d triply periodic structure which is governed by the following equation: sin x cos y + sin y cos z + sin z cos x = 0 (1) authors want to denote that in their experiments, the gyroid structure seems to be much less prone to crack formation during the process of 3d printing, as opposed to trabecular structures. one of the main shortcomings of trabecular structures is their figure 3. the second, porous geometrical solution of the dental implant used for numerical simulations. implant was previously patented in the czech republic [7]. image shows the intraosseous part. beam-like character that contributes to an uneven heat dissipation process. this process has proven itself to be somewhat limiting in the design of trabecular structures. with the gyroid structure, authors have yet to experience a problem of this nature during the production process. thanks to its wall-like character, the gyroid is also less prone to local damage, crack propagation, formation of debris and manufacturing errors. especially, debris formation needs to be ruled out in case of dental implants as eroded particles must not enter the body of the patient. considering osseointegration, one of its benefits is also the absence of sharp edges and corners, which inhibit the circulation of body fluids and the overall healing process. 77 luboš řehounek, aleš jíra acta polytechnica ctu proceedings 2. materials and methods to examine the stress distributions at the peri-implant area, two simulations were performed in the mechanical finder software with the aid of a densitometry model of human mandible. the first simulation was done with a homogeneous implant (fig. 2) and the second with a porous implant with ribs made of a gyroid structure and a homogeneous stem (fig. 3). the 3d-model of the mandible was created from a ct scan. material model of the bone is fully inhomogeneous – densitometry assigns values of young’s moduli to every element in the fem mesh. the software is able to simulate patient-specific conditions this way. a porous dental implant with a system of four stabilizing ribs [7] was situated inside a real ct model of human mandible, constrained and its upper structure loaded with a masticatory force of 150 n. 2.1. creating the 3d model the creation of the 3d model begins by taking a calibrated ct scan of the area the user wants to analyze. the model is usually calibrated using the phantom instrument with calibrating rods. after taking the sliced ct data, the user creates a roi – range of interest – they want to specifically include in the model. the process of creating a specific roi is shown in fig. 4. this process includes several tools like transparent imaging method, editing multiple slices at once, interpolating between slices and automated editing of roi. figure 4. user-specified roi from a patient-specific ct scan. gray lines indicate the regions recognized by the software from a pre-set threshold value. yellow lines indicate user-specified roi (a canine was extracted as a dental implant was placed in its stead. when the process of roi extraction is finished, the user can generate a complete 3d model of the roi and procede with additional operations, such as subtracting bone parts, cutting the model and adding individual bone parts into separate categories for the analysis. the wireframe model is shown in fig. 5. the implants can be introduced either by importing an .stl file extension or the .imp file extension. the figure 5. the wireframe model of the human mandible for the creation of the fem mesh. the left canine was extracted in the step of roi extraction as dental implants will be introduced. implant is then placed within the desired position by translation and rotation and/or by using the snap tool or the bone axis tool. in our case, two separate models were created, one for the homogeneous and one for the porous implant variant. 2.2. load and boundary conditions in mechanical finder, the user can specify usual boundary conditions by applying restrictions on an individual node and/or neighboring nodes. the force was specified as a total force with a specific vector bound to the local system of coordinates (fig. 6). the total load on the upper lid of the implant was set to a value of 150n. this value of force falls within the common range of masticatory forces and does not need to be specific, since the goal of the simulation is to compare the behavior of 2 different implants and the value of force only serves as a constant. figure 6. loads and boundary conditions are implemented by selecting individual and neighboring nodes and providing a direction in the form of a vector. 78 vol. 26/2020 densitometry-based fem simulations of novel porous implants figure 7. the minimum principal stress of the bone material at the peri-implant area in mpa for the homogeneous variant of the implant. the maximum value of compressive stress at this section is 7.0 mpa for an axial load of 150 n. the value is smaller as compared to fig. 8 because the implant is much stiffer and transfers much less load into the surrounding bone. figure 8. the minimum principal stress of the bone material at the peri-implant area in mpa for the porous variant of the implant. the maximum value of compressive stress at this section is 13.4 mpa for an axial load of 150 n. the value of stress is higher as compared to fig. 7 because lower stifness of the porous implant provides the peri-implant area with more load, thus increasing the stress. 79 luboš řehounek, aleš jíra acta polytechnica ctu proceedings 2.3. numerical simulation two simulations were performed on the same human mandible model – one for the homogeneous (fig. 2) and one for the porous (fig. 3) implant. the simulations were performed with the same boundary conditions and exactly the same load. the goal of the simulations was to verify the hypothesis that a porous implant would transfer more load into the bone at the peri-implant area, thus reducing the effect of stressshielding and subsequent bone loss. the material model of the bone which was used for this simulation was the keyak model [8]. the material used for the imported .stl implant model file was ti-6al-4v, which is a common implant material in the industry. the comparison was carried out by comparing the values of minimum principal stress of the bone at the peri-implant area. note that greater values of stress are beneficial in this case, as bone has high compressive strength, but often lacks mechanical stimulation when an implant is introduced, leading to bone loss and aseptic loss of stability and failure of the implant [2]. 3. results the results of the numerical analyses suggest that the second implant variant with a porous outer layer and ribs provides greater values of stress inside the bone under the same load conditions (13.4 mpa peak minimum principal stress for the porous implant and 7.0 mpa for the homogeneous implant). this finding supports the hypothesis laid out by the authors. the distribution of minimum principal stress in the bone of human mandible are shown in fig. 7 and fig. 8 for the homogeneous and porous geometrical variant, respectively. the difference in the values of minimum principal stress arises from different values of global moduli of both implants. it is very important to distinguish between the modulus on the microscale (the modulus of the print material ti-6al-4v) and the modulus of a homogeneous structure and a porous structure, such as the one shown in fig. 1. the modulus of the print material is eti-6al-4v = 120 gpa in both cases, but the global modulus of the gyroid structure itself is expected to be approximately 1 order of magnitude lower (authors have not yet done mechanical tests on this exact presented structure but have experience testing similar structures [9]). 4. conclusions numerical analyses show that by incorporating some degree of porosity into an implant, we can reduce the effect of stress shielding on the human bone. as presupposed, the homogeneous implant transfers less load into the bone when compared to the same implant with a porous outer layer. this fact can be attributed to lower overal stiffness at the peri-implant area. greater values of compressive stress (13.4 mpa for the porous implant vs. 7.0 mpa for the homogeneous one) are beneficial as they provide the bone with more load to maintain its density and prevent bone loss, as described by the wolff’s law [4]. another benefit of the structure is its ability to let osteons grow inside the body of the implant and provide an interlocking mechanism which can potentially better the primary stability of the implant. with the advancement of additive manufacturing technologies, such complex structures are becoming easier to manufacture and more financially accessible to the general population. when evaluating porous structures, authors also see the potential of the gyroid structure as compared to other porous structures, such as the trabecular structure – ease of manufacture during the 3d printing process and the lack of sharp edges and discontinuities between individual beams are beneficial to the overall quality of the final product. acknowledgements the financial support by the faculty of civil engineering, czech technical university in prague (sgs project no. sgs17/168/ohk1/3t/11) is gratefully acknowledged. the authors also gratefully acknowledge the support granted by the czech ministry of industry and trade (project op pik no. cz.01.1.02/0.0/0.0/17_102/0011518). references [1] c. m. abraham. suppl 1: a brief historical perspective on dental implants, their surface coatings and treatments. the open dentistry journal 8:50, 2014. [2] m. ridzwan, s. shuib, a. hassan, et al. problem of stress shielding and improvement to the hip implant designs: a review. j med sci 7(3):460–467, 2007. [3] a. el-hajje, e. c. kolos, j. k. wang, et al. physical and mechanical characterisation of 3d-printed porous titanium for biomedical applications. journal of materials science: materials in medicine 25(11):2471– 2480, 2014. doi:10.1007/s10856-014-5277-2. [4] s. cowin, d. hegedus. bone remodeling i: theory of adaptive elasticity. journal of elasticity 6(3):313–326, 1976. [5] l. řehounek, a. jíra. numerical and mechanical analyses of a 3d-printed titanium trabecular dental implant. acta polytechnica 57(3):218–228, 2017. [6] s. a. yavari, j. van der stok, y. c. chai, et al. bone regeneration performance of surface-treated porous titanium. biomaterials 35(24):6172–6181, 2014. [7] a. jíra, f. denk, f. denk. a dental implant shaft, 2017. czech patent n. 306457. [8] j. h. keyak, s. a. rossi, k. a. jones, h. b. skinner. prediction of femoral fracture load using automated finite element modeling. journal of biomechanics 31(2):125–133, 1997. [9] l. řehounek, p. hájková, p. vakrčka, a. jíra. geometry and mechanical properties of a 3d-printed titanium microstructure. acta polytechnica ctu proceedings 15:104–108, 2018. 80 https://doi.org/10.1007/s10856-014-5277-2 acta polytechnica ctu proceedings 26:76–80, 2020 1 introduction 1.1 porous implants 2 materials and methods 2.1 creating the 3d model 2.2 load and boundary conditions 2.3 numerical simulation 3 results 4 conclusions acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2021.31.0058 acta polytechnica ctu proceedings 31:58–60, 2021 © 2021 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague maintaining the competitiveness of the czech railway industry jan vogl∗, mária jánešová czech technical university in prague, faculty of transportation sciences, konviktská 20, 110 00 prague 1, czech republic ∗ corresponding author: vogljan@fd.cvut.cz abstract. the development of foreign trade plays an important role in the czech economy. the important foreign trade commodity item is machinery and transport equipment (sitc 7). railway vehicles is a part of this category, other means of transport (sitc 79). in order to maintain competitiveness and economic growth, it is necessary for the railway industry to intensify the innovation and the introducing of new technologies into production. one of the available options is a wider implementation of life cycle cost analysis, which enables to activate innovative efforts, as well as to improve technical and performance data. keywords: export, railway vehicles, lcc analysis, innovation. 1. introduction the czech republic is a traditional export industrial country. therefore, the development of foreign trade plays an important role in our economy. last year, exports from the czech republic grew by 3.6% to czk 4 563.3 billion. the largest volume of czech exports has been realized in the long term within the european union countries, such as germany, france, great britain and poland. in 2019, the share of exports to eu countries reached 83.6%. in terms of commodity structure (sitc), machinery and transport equipment (sitc 7) are traditionally dominant. their share in total exports in 2019 was about 59%. compared to 2018, it grew by 5.1% [1]. figure 1. cross-border movements of goods by commodities, year 2019, source: czso [1]. 2. description of product group railway vehicles is part of lager category, other transport equipment (sitc 79), which consists of: • 791 railway vehicles (including hovertrains) and associated equipment • 792 aircraft and associated equipment • 793 ships, boats and floating structures if we focus on the export development in the commodity item railway vehicles and related equipment (railway vehicles (including hovertrains) and associated equipment) we find out that in 2019 this item reached the value of czk 20.5 billion. the largest volume of czech exports of the mentioned commodity goes to europe (approx. 88%). these are, for example, deliveries of locomotives, metro trains, trams and components [1]. the published analysis of the association of the czech railway industry (acri) regarding the development of the czech railway industry in 2012-2019 shows that 2015 was a turning point for the companies, which are the members of this association. in 2015, they have reached the highest turnover of 91 billion czk. for example the total turnover in 2019 is about 16.5% lower in comparison with 2015, as well as the total number of employees is lower by 6.5%. due to the global competition in foreign markets, czech railway industry is not able to reach the successful year of 2015 up to now [2]. 3. innovation and lcc analysis in order to remain competitive on foreign markets, we need to increase the innovative potential of our products. the czech republic will not keep its significant competitive advantages of cheap labour for long time. according to a study by the european commission entitled "country-specific recommendations 2020 research and innovation analysis", the czech republic lags behind in the field of research and innovation within the eu. productivity growth is mostly driven by large foreign companies, while domestic companies are lagging behind. the same applies to the growth of expenditure on research and development [3]. 58 https://doi.org/10.14311/app.2021.31.0058 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 31/2021 maintaining the competitiveness of the czech railway industry figure 2. exports of railway vehicles (including hovertrains) and associated equipment, source: acri [2]. the annual analysis of the european commission "european innovation scoreboard 2020" showed that the czech republic was, according to the results of 27 indicators included in the moderate innovators [4]. figure 3. performance of eu member states’ innovation systems, source: eu [4]. innovation of the offered products is a key task for maintaining the competitiveness of manufacturing companies in the market. this can be achieved through innovation and close cooperation between companies and research institutions in research and development. the introduction of new technologies and innovations is important for the further successful economic growth of czech industry. efficient use of life cycle cost (lcc) analysis contributes to this growth. the aim of lcc analysis is to determine the total cost of a product throughout its life cycle [5]. the results of my research among leading manufacturer of railway vehicles have shown that lcc analysis is one of those methods that has the ability to help increase innovation effects, the quality of the offered product and reduces overall costs. furthermore, lcc analysis plays a significant contribution in tenders to select the railway vehicles. 4. conclusion the increasing use of the lcc analysis should be an important incentive for manufacturers to improve the reliability and quality of their products. the implementation of the lcc analysis will lead to innovative efforts and thus increase the competitiveness of the company. in the rail sector, where railway vehicles has been in operation for more than 30 years [6], operators can, together with manufacturers, optimize the total cost of ownership, which accounts for the largest share of the total lcc cost by railway vehicles [5]. based on university research conducted in 2019-2020 with leading manufacturer of railway vehicles, it turned out that the use of lcc analysis not only reduces overall costs, but also activates innovation. as the czech industry is dependent on the export of its products to foreign markets, it is necessary not only in the railway industry, which has a two-hundred-year tradition, to capture new trends in the autonomous control, alternative drives or intelligent transport systems [7]. this requires intensive and consistent cooperation with the government, research institutions, high-tech companies, but also other sectors such as energy and information and communication technologies. one of the approved government strategic framework plans is the innovation strategy of the czech republic 2019-2030, which aims to help move the czech republic among the most innovative countries in europe within a few years. if we manage to capture new world trends in this sector, the czech republic will continue the economical grow and the improving of the quality of offered products and services to our citizens. references [1] český statistický úřad [čsu]. pohyb zboží přes hranice. in: český statistický úřad. databáze zahraničního obchodu v přeshraničním pojetí [online]. ©2020. available in: https: //apl.czso.cz/pll/stazo/stazo.stazo?jazyk=en. [2] analýza asociace podniků českého železničního průmyslu. in: acri [online]. praha: asociace podniků českého železničního průmyslu, 13. 5. 2020. available in: https://acri.cz/2020/05/24/firmy-zeleznicnihoprumyslu-jsou-stabilnimi-tahouny-ceskeekonomiky-dokazuji-to-jejich-hospodarskevysledky-za-rok-2019/. [3] country-specific recommendations 2020 – research and innovation analysis. [online]. european commission. brussel, 2020. available in: https://rio.jrc.ec.europa.eu/library/2020country-specific-recommendations-research-andinnovation-analysis. [4] european innovation scoreboard 2020. [online]. european commission. brussel, 2020. available in: https://ec.europa.eu/commission/presscorner/ detail/en/qanda_20_115. [5] čsn en 60300-3-3. management spolehlivosti – část 3-3: pokyny použití – analýza nákladů životního cyklu. praha: český normalizační institut, 2004. 60 s. [6] kontrolní závěr z kontrolní akce 17/35: pořízení a obnova železničních kolejových vozidel. in: nejvyšší kontrolní úřad [online]. praha. available in: https://www.nku.cz/assets/kon-zavery/k17035.pdf. 59 https://apl.czso.cz/pll/stazo/stazo.stazo?jazyk=en https://apl.czso.cz/pll/stazo/stazo.stazo?jazyk=en https://acri.cz/2020/05/24/firmy-zeleznicniho-prumyslu-jsou-stabilnimi-tahouny-ceske-ekonomiky-dokazuji-to-jejich-hospodarske-vysledky-za-rok-2019/ https://acri.cz/2020/05/24/firmy-zeleznicniho-prumyslu-jsou-stabilnimi-tahouny-ceske-ekonomiky-dokazuji-to-jejich-hospodarske-vysledky-za-rok-2019/ https://acri.cz/2020/05/24/firmy-zeleznicniho-prumyslu-jsou-stabilnimi-tahouny-ceske-ekonomiky-dokazuji-to-jejich-hospodarske-vysledky-za-rok-2019/ https://acri.cz/2020/05/24/firmy-zeleznicniho-prumyslu-jsou-stabilnimi-tahouny-ceske-ekonomiky-dokazuji-to-jejich-hospodarske-vysledky-za-rok-2019/ https://rio.jrc.ec.europa.eu/library/2020-country-specific-recommendations-research-and-innovation-analysis https://rio.jrc.ec.europa.eu/library/2020-country-specific-recommendations-research-and-innovation-analysis https://rio.jrc.ec.europa.eu/library/2020-country-specific-recommendations-research-and-innovation-analysis https://ec.europa.eu/commission/presscorner/detail/en/qanda_20_115 https://ec.europa.eu/commission/presscorner/detail/en/qanda_20_115 https://www.nku.cz/assets/kon-zavery/k17035.pdf jan vogl, mária jánešová acta polytechnica ctu proceedings [7] memorandum o budoucnosti českého železničního průmyslu. in: acri [online]. praha, 2019. available in: https://acri.cz/2019/04/12/memorandum-obudoucnosti-ceskeho-zeleznicniho-prumyslu/. 60 https://acri.cz/2019/04/12/memorandum-o-budoucnosti-ceskeho-zeleznicniho-prumyslu/ https://acri.cz/2019/04/12/memorandum-o-budoucnosti-ceskeho-zeleznicniho-prumyslu/ acta polytechnica ctu proceedings 31:58–60, 2021 1 introduction 2 description of product group 3 innovation and lcc analysis 4 conclusion references acta polytechnica ctu proceedings doi:10.14311/app.2018.17.0015 acta polytechnica ctu proceedings 17:15–19, 2018 © czech technical university in prague, 2018 available online at http://ojs.cvut.cz/ojs/index.php/app effect of annealing on the real structure and microstructure of advanced laser processed aisi h13 tool steel karel trojana,∗, václav ocelíkb, nikolaj ganeva, stanislav němečekc, jaroslav čechd, jiří čapeka, jakub němečeka a ctu in prague, department of solid state engineering, faculty of nuclear sciences and physical engineering, trojanova 13, 120 00 prague 2, czech republic b department of applied physics, zernike institute for advanced materials, faculty of science and engineering, university of groningen, nijenborgh 4, 9747 ag, groningen, the netherlands c raptech s. r. o., u vodárny 473, 330 08 zruč-senec, czech republic d ctu in prague, department of materials, faculty of nuclear sciences and physical engineering, trojanova 13, 120 00 prague 2, czech republic ∗ corresponding author: karel.trojan@fjfi.cvut.cz abstract. the aim of this paper is to describe the effects of annealing on the microstructure of laser cladded aisi h13 tool steel using various methods. advanced laser technology has the potential to replace conventional methods to make and repair dies. however, it has to be determined whether the newly created surface still needs to be heat-treated, which would cause additional repair costs. no significant effect of heat treatment on the microstructure and real structure of the clads was detected, but further confirmation, in particular by measuring wear resistance, is needed. keywords: laser processing, cladding, annealing, microstructure, aisi h13 tool steel. 1. introduction hot working tool steel aisi h13 is one of the most common die material used in metal and casting industries. dies suffer damage due to wear and thermodynamic stresses during their lifetime [1]. therefore, various methods have been developed for its repair. a great benefit of laser cladding in this field is a high productivity with minimal influence on surrounding material by thermal stresses due to low heat input [2]. therefore, the aim of this contribution is to describe the effects of annealing on the real structure and microstructure of laser cladded aisi h13 tool steel using x-ray diffraction (xrd) to determine real structure and residual stresses (rs), instrumented indentation (nanoindentation) technique and orientation imaging microscopy (oim) based on electron backscatter diffraction (ebsd), and energy-dispersive x-ray spectroscopy (eds). during the laser cladding, rapid cooling and thus the formation of brittle martensitic microstructure could occur [3]. hence, it is important to observe the effect of annealing on the real structure and microstructure which subsequently influences the properties of the newly created surface of dies. 2. experimental laser cladding was carried using a diode laser laserline 5.5 kw with optical head precitec y52. laser power density 140 j/mm2 was applied to form a single clad with the length of 137 mm, width of 6 mm and height of approx. 1 mm above substrate made from construction s355 steel. the powder of the aisi h13 tool steel was used with an average particles diameter of 53.1 ± 15.9 µm. three samples were made from the clad with average dimensions 15×12×5 mm3. the first sample from the clad was analysed without heat treatment. the second sample was annealed at 550 ◦c for 2 hours and the third sample was heat treated twice at the same temperature for 2 hours. sem image of the cross-section of the single clad with used directions can be seen in figure 1. lack of the fusion on the sides of the clad could be observed, it is probably the result of improper setting of laser power density. x-ray diffraction analysis was carried out using an x’pert pro mpd diffractometer with chromium tube anode and pinholes determining irradiated area of primary beam, with size of 1×1 mm2 in longitudinal clad direction l and 2×0.5 mm2 in transversal direction t, respectively. diffraction lines crkα1 of the planes {211} of the α-fe were fitted by pearson vii function and rachinger’s method was used for separation of the diffraction lines kα1 and kα2. for residual stress evaluation, sin2ψ method and x-ray elastic constants 1/2s2 = 5.75 tpa−1, s1 = −1.25 tpa−1 were used [4]. rs were measured at the top of the clad from the upper side in two directions. full widths of the measured diffraction lines crkα1kα2 at half of the maximum were also evaluated for both directions. full width at half of the maximum (fwhm) parameter depends on microstrains (fluctuation of inter-planar distances), crystallite size (size regions of coherent scattering) 15 http://dx.doi.org/10.14311/app.2018.17.0015 http://ojs.cvut.cz/ojs/index.php/app k. trojan, v. ocelík, n. ganev et al. acta polytechnica ctu proceedings figure 1. sem image of the cross-section of the single clad aisi h13 tool steel with marked directions. and density of dislocations. fwhm increases with the increasing microstrains and density of dislocations and decreasing crystallite size. hardness was measured by the instrumented indentation (nanoindentation) technique. tests were carried out on nht2 nanoindentation instrument (anton paar, graz) with diamond berkovich indenter. indentation cycle consisted of loading to maximum force 500 mn for 30 s, holding at maximum load for 30 s and unloading again for 30 s. data were evaluated by oliver-pharr method [5] and the average values were computed from 3 indents for each depth under the surface of a clad. the error bar represents the standard deviation. the oim data for ebsd was collected using a lyra tescan scanning electron microscope equipped with a tsl oim system based on hikari camera and eds edax octane detector. the accelerating voltage of 15 kv and 50 nm step size of scanning were used. a grain boundary is defined in the microstructure as a boundary between two neighbouring scanning points having crystallographic misorientation larger than 5°. all ebsd data were analysed with tsl oim analysis 7.3.0 software and only data points with confident index higher than 0.05 were used. confident index is based on voting scheme during automated indexing of the diffraction pattern, where it is counted as a ratio of votes for the best solution minus votes for the second best solution divided by total possible number of votes from the detected kikuchi bands. certain crystallographic orientation receives a vote when observed angles between the three bands are the same as table values of given orientation. confident index 0.05 corresponds on a face-centred cubic material to approx. 70% probability of correct indexing. eds data was collected using a philips xl 30 feg scanning electron microscope equipped with a eds edax sutw+ detector with accelerating voltage of 25 kv. 3. results and discussion table 1 gives the range of the chemical composition of aisi h13 steel determined by iso 4957 [6], the next row shows average chemical composition and standard deviation obtained by eds on a cross-section of four randomly selected particles of the used powder; the bottom row shows the average chemical composition and standard deviation in four different clad crosssection areas of dimensions approx. 500×250 µm2. table 1 shows that in the clad only chromium is on average about one weight percentage less than prescribed standard. silicon is also slightly below the interval and vanadium is in the interval within the error. on the other hand, manganese has been observed on the contrary of 0.1 % more. results of surface rs and fwhm obtained by xrd are shown in table 2 and the comparison is plotted in figure 2. the fwhm is the average value obtained from both directions with the standard deviation as the error. after first annealing a noticeable decrease in rs and fwhm was observed. even more, in the l direction (along the clad) mild compressive rs were detected. after the second annealing there was a slight increase in rs and, above all, a significant reduction in the error. the fwhm has dropped slightly. this effect is due to a lower density of dislocation and a decrease in the value of microstrain caused by annealing. depth gradient of hardness is shown in figure 3. parameter x represents the distance from the surface of the clad. it can be seen from the figure that the as cladded sample exhibits a higher hardness of approximately 50 hv0.05. in addition, a slower drop in hardness to the bulk values compared to annealed samples may be observed. however, there is no hardness difference between the annealed samples. figure 5 graphically illustrates the effect of annealing on the microstructure of the clad. an area 20×20 µm2 in the middle of the cross-section of the clad was observed using oim. three differently annea16 vol. 17/2018 effect of annealing on advanced laser processed aisi h13 tool steel cr [wt. %] mo [wt. %] si [wt. %] v [wt. %] mn [wt. %] c [wt. %] en iso 4957 4.80–5.50 1.20–1.50 0.80–1.20 0.85–1.15 0.25–0.50 0.35–0.42 powder h13 5.10 ± 0.07 1.79 ± 0.08 0.92 ± 0.11 1.08 ± 0.02 0.45 ± 0.07 clad 3.77 ± 0.05 1.21 ± 0.05 0.72 ± 0.01 0.84 ± 0.03 0.62 ± 0.03 table 1. chemical composition of the steel aisi h13 according to iso 4957 [6], the powder and the clad. σl [mpa] σt [mpa] fwhm [°2θ] as cladded 215 ± 36 271 ± 48 5.22 ± 0.13 annealed 2 h, 550 ◦c −63 ± 49 54 ± 30 4.37 ± 0.03 annealed 2+2 h, 550 ◦c −21 ± 16 144 ± 9 4.35 ± 0.02 table 2. results of surface rs and fwhm of the clad. figure 2. surface rs and fwhm of the clad. figure 3. depth gradient of hardness, where x is distance from the surface of the clad. led samples were used, so the measured area was not the same. as shown also in figure 4, it can be stated that the annealing time increases the proportion of carbides and decreases the austenite fraction. this effect is well known and not surprising when diffusion is facilitated by higher temperature. however, the weight percentage of the carbides that were indexed during the measurement (see figure 4) is inconsistent figure 4. effect of annealing on the phase content of the clad. with table 1. carbides have been indexed three times more than the steel composition obtained by eds analysis showed. inverse pole figures of ferrite in the selected area before and after annealing are shown in figure 6, wherein individual colours correspond to the normal vectors of crystallographic planes that are parallel to the normal vector of the cross-section of the sample. after cladding, the martensitic structure could be observed and vice versa after annealing. however, a slight rounding of corners of martensitic laths can be found after annealing for 2+2 hours. 4. conclusions laser deposition of the aisi h13 tool steel showed a great application potential. by annealing, residual stresses have dropped, which could have a beneficial effect on the component life time, as the compressive stresses delay crack initialization and also slow its propagation [7]. the difference between phase composition detected by eds and ebsd has not yet been accurately explained and understood, so further measurements and adjustments of the indexing parameters will be needed. nevertheless, based on these results, 17 k. trojan, v. ocelík, n. ganev et al. acta polytechnica ctu proceedings figure 5. effect of annealing on the phase composition of the clad where ferrite is red, cr23c6 green, vc yellow and austenite blue (upper as cladded, middle after 550 ◦c annealing for 2 h, bottom after 550 ◦c annealing for 2+2 h). annealing is not needed after cladding. however, verification on real dies and also further measurements are required, in particular wear resistance testing. figure 6. effect of annealing on the microstructure of the ferrite of the clad (upper as cladded, middle after 550 ◦c annealing for 2 h, bottom after 550 ◦c annealing for 2+2 h). acknowledgements measurements were supported by the project th02010664 of the technology agency of the czech republic and by university of groningen. this work was supported by the student grant competition ctu in prague grant no. sgs16/245/ohk4/3t/14. 18 vol. 17/2018 effect of annealing on advanced laser processed aisi h13 tool steel references [1] g. telasang, j. d. majumdar, n. wasekar, et al. microstructure and mechanical properties of laser clad and post-cladding tempered aisi h13 tool steel. metallurgical and materials transactions a 46:2309–2321, 2015. doi:10.1007/s11661-015-2757-z. [2] m. vedani, b. previtali, g. m. vimercati, et al. problems in laser repair-welding a surface-treated tool steel. surface & coatings technology 201(8):4518–4525, 2007. doi:10.1016/j.surfcoat.2006.09.051. [3] j. chen, s.-h. wang, l. xue. on the development of microstructures and residual stresses during laser cladding and post-heat treatments. journal of materials science 47(2):779–792, 2012. doi:10.1007/s10853-011-5854-4. [4] v. hauk, e. macherauch. a useful guide for x-ray stress evaluation (xse). advanced in x-ray analysis 27:81–99, 1989. [5] w. oliver, g. m. pharr. an improved technique for determining hardness and elastic modulus using load and displacement sensing indentation experiments. journal of materials research 7(6):1564–1583, 1992. doi:10.1557/jmr.1992.1564. [6] en iso 4957:1999 tool steels. standard, british standards institution, 1999. [7] g. e. totten, m. howes, t. inoue, i. books24x7. handbook of residual stress and deformation of steel. asm international, materials park, 2002. doi:10.1361/hrsd2002p027. 19 http://dx.doi.org/10.1007/s11661-015-2757-z http://dx.doi.org/10.1016/j.surfcoat.2006.09.051 http://dx.doi.org/10.1007/s10853-011-5854-4 http://dx.doi.org/10.1557/jmr.1992.1564 http://dx.doi.org/10.1361/hrsd2002p027 acta polytechnica ctu proceedings 17:15–19, 2018 1 introduction 2 experimental 3 results and discussion 4 conclusions acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2018.20.0086 acta polytechnica ctu proceedings 20:86–97, 2018 © czech technical university in prague, 2018 available online at http://ojs.cvut.cz/ojs/index.php/app market and development trends in cogeneration and combined heat and power plants jan slad∗, andreas pickard, frank strobelt siemens ag, freyeslebenstr.1, 91058 erlangen, germany ∗ corresponding author: jan.slad@siemens.com abstract. the transition of energy mix in europe is placing greater focus on energy efficiency. lawmakers in some of eu countries have already recognized that combined heat and power generation (cogeneration, chp) can help increase energy efficiency. targeted promotion and subsidization have raised the cost-effective profitability of cogeneration plants significantly. but how can the economic value of this investment be maximized? keywords: ccpp, chp, power plants, cogeneration, repowering, gas turbines, operational flexibility, efficiency, power market. 1. introduction as concerns grow about environmental protection, global warming, and steadily rising energy prices, public interest is focusing more on energy efficiency. germany is promoting and subsidizing efforts to expand the use of renewable energy, better building insulation, and cogeneration to make power and heat supply systems more energy-efficient. current subsidy legislation, driven by economics and defined energyefficiency and climate protection goals, is aimed at expanding combined heat and power generation to the extent that 25 percent of the nation’s electric power will be produced by cogeneration plants by the year 2020. yet even as lawmakers recognize the significant contribution made by cogeneration to increasing energy efficiency, there are ongoing discussions about whether these subsidies make good sense. the doubters, in most cases, are contrasting figures and data that make no sense in comparative analyses, pitting, for example, the energy utilization factors of cogeneration plants against the electrical efficiency of plants generating purely electric power. ultimately, it is the efficiency of the systems in our energy supply network that are the decisive variable. when electricity has to be generated from fossil fuels because currently available renewable energy sources are unable to completely meet the demand, waste heat is generated that is discharged to the environment without being utilized. according to publications, cogeneration enables fuel savings of anywhere from 10 to 20 percent compared with the separate generation of heat and power. it should also be noted that the theoretical savings potential is actually much higher. in theory, design and operation would always strive for optimized pairing of heat and power generation, entirely ruling out the possibility for peak-load heat generation from direct-fired boilers. in industrial plants, savings potentials of up to 40 percent are readily achievable, because in most cases the consumption of heat and electric power remains relatively constantand enables a narrow optimization window. 2. what does cogeneration actually mean? cogeneration is understood to mean the simultaneous production of electricity and heat, with the latter generated for building heating or production processes. there is a very broad range of designs for cogeneration power plants, depending on the required generating capacity and the fuel being fired and transformed into useful energy. in the case of liquid or gaseous fuels, combined cycle processes have become the state of the art, in which a downstream water-steam cycle utilizes the waste heat from a topping cycle with internal combustion, e.g. a gas turbine or a gas engine (dieselor gas-fired). as illustrated in figure 1, the exhaust gases are then routed to a heat-recovery steam generator to produce steam or hot water. steam produced from this recovered waste heat can be used to drive a downstream steam turbine to generate more electric power before the steam is extracted for heating purposes. particularly in combined cycle power plant designs where a gas turbine serves as the primary cycle, the temperature for steam production in the heat-recovery steam generator is at a level that lends itself well to first allowing the steam to expand through a steam turbine to generate electric power before it is subsequently extracted for process steam or heating purposes. this combined cycle design concept allows the production unit to achieve electrical efficiency ratings of over 60 percent when used solely to generate electric power without steam extraction. when firing solid fuels like lignite and bituminous coal, it is more common to implement a conventional water-steam rankine cycle in which steam is generated in a direct-fired boiler. when operated solely in power generation mode, these types of plants can de86 http://dx.doi.org/10.14311/app.2018.20.0086 http://ojs.cvut.cz/ojs/index.php/app vol. 20/2018 trends in cogeneration and combined heat and power plants figure 1. triple pressure reheat cogeneration cycle (simplified). figure 2. simmering power station (repowered), vienna, austria. 87 j. slad, a. pickard, f. strobelt acta polytechnica ctu proceedings figure 3. steam capabilities for siemens gas turbines. liver electrical efficiencies well below 50 percent. their maximum efficiency is thus significantly lower than that of a state-of-the-art combined cycle power plant. what’s more, the high carbon content of the coal fuel generates much higher co2 emissions. these plants therefore fail to fulfill the subsidization criteria stipulated by lawmakers in germany’s combined heat and power act. instead, some plants have modernized by backfitting an upstream gas turbine to create a combined cycle configuration. simmering power station in vienna, austria (see figure 2) has been repowered to a combined cycle configuration to improve overall cost-effectiveness and reduce emissions. 3. performance classes for every application in the small power classes below 5mw, standard packaged solutions predominate, for which only peripherals need be adjusted to project-specific conditions, but nothing within the package is altered. in most cases, a gasor diesel-fired engine of this power class is combined with a simple heat-recovery system to provide hot water in a combined heat and power configuration. above the 5mw power range, plant design concepts are increasingly being optimized and customized as needed for specific applications. where a gas turbine is used, a combined cycle configuration is usually selected that incorporates a steam turbine running the water-steam cycle. siemens’ gas turbine portfolio ranges from the sgt-100 series – which, via the steam turbine, delivers heating steam at up to 25 t/h – to the sgt5-8000h series that has steam capabilities up to 1,100 t/h in a 1 × 1 arrangement. due to their basic design, industrial-class units are generally configured with simple water-steam cycles. up to gas turbine outputs of about 50mw, conventional solutions implement a heat-recovery steam generator (hrsg) with only one evaporator stage and no reheat section (single-pressure hrsg) as illustrated in figure 4. while this design may keep investment costs down, it is accompanied by a simple waste-heat recovery and lower efficiency ratings. this performance class is normally found in small-town settings or small industrial facilities. where higher thermal or electrical generating capacities are needed, it is recommendable to opt for higher-performance unit designs. larger plants not only deliver greater efficiency benefits due to their turbine’s design (the jump from d-class gas turbines to e-class), but the specific investment costs are also lower (based on economies of scale). in general, the higher the performance class, the greater the complexity of the water-steam cycles for increasing fuel efficiency. unit outputs of up to 170mw are typically paired with double-pressure hrsgs, most without reheating shown in figure 5. above 170mw, triple-pressure hrsgs equipped with reheat section are usually used. as the number of 88 vol. 20/2018 trends in cogeneration and combined heat and power plants figure 4. single pressure chp cycle (simplified). figure 5. double pressure non reheat chp cycle (simplified). evaporator stages grows, and with the inclusion of a reheat section, the utilization of a gas turbine’s exhaust gas improves in the rankine cycle of the water-steam cycle and so does overall plant efficiency. however, it is the customer’s own performance assessment that ultimately decides how complex the design of the water-steam cycle should be, and what is economically justifiable in designing the heat-recovery steam generator. to supply heat to larger industrial complexes or within major cities with a high population density, solutions on the scale of large power plants are viable. in these environments, electrical generating capacities in the range of 600mw or more and thermal outputs of 370mw per gas turbine can be achieved. these plant designs can achieve energy efficiency ratings of 85 percent while simultaneously attaining electrical efficiency of over 61 percent when operating solely in power-generation mode. 4. plant designs optimized for specific applications the design concept of combined heat and power plants depends primarily on the requirement profile of the heat consumers. those requirement profiles show quite a variation driven by ambient conditions and seasonal heat demands. however, once the design of a combined heat and power plant is fixed and optimized for a specific requirement profile electric power production and heat provision cannot be controlled and operated independently of one another. alternatively, it is possible to provide variable steam extraction points within a conventional fossil-fuel combined cycle 89 j. slad, a. pickard, f. strobelt acta polytechnica ctu proceedings configuration. this allows the plant to be used solely for power generation, if so desired. the advantages of both options are defined in greater detail below. 5. plant designs for heat-driven operation electric power as a by-product in industrial plants where energy production is primarily focused on supplying heat, and the electric power generated is consumed within the facility as a by-product, a heat-driven design is often implemented with a back-pressure steam turbine. in order for combined heat and power plants to be optimally designed in thermodynamic terms, it is ideal if the heat to be generated varies as little as possible in thermal output and temperature. in this case, a process would be selected in which the steam is allowed to expand precisely to those parameters needed by the heat consumer. design concepts using back-pressure steam turbines (see figure 6) benefit on the one hand from the fact that effectively no heat is discarded in the steam turbine condenser or transferred to the cooling water, and all thermal energy released from condensation in the heating condensers is routed away as useful heat – while at the same time the achievable electrical output of the steam turbine declines because the enthalpy gradient of the turbine is significantly less due to the higher back pressure. the relationship between power generation and heat provision depends directly on the steam turbine’s back pressure (driven by the supply temperature demand), and lies between the values for extraction condensing solutions and those without a steam turbine. if process steam lines of different steam parameters are required, steam turbine extraction points are ideally implemented in a cascading design. these steam turbine extractions can be designed as controlled extraction points or as uncontrolled extraction ports, depending on the range of fluctuation of extractions and pressures. in addition, the process steam lines can be supported by direct extractions bled from an appropriate location in the heat-recovery steam generator. the fact that the amount of electric power that it’s possible to generate depends on the amount of thermal energy to be supplied translates into a loss of operating flexibility, and can constitute an economic disadvantage for power plants participating in the electricity market. in heat-driven operation, electric power is merely a marketable “by-product.” to counter this disadvantage, when requirements for power and heat fluctuate widely independent of one another, design concepts with extraction condensing turbines are needed, as described in the next section. 6. plant designs for power-driven operation – supplying electric power and heat on demand while cogeneration plants have traditionally been designed for heat-driven operation, today plants are making greater use of condensing turbines in order to maximize energy efficiency at the nominal point. this trend is being driven by the increasing use of renewable energy sources and the associated high requirements placed on power plant flexibility for generating power. the weather-dependent volume of electric power fed in from renewable energy sources determines the feedin price and, via the merit order, the dispatch and therefore electricity production (hours of full-load operation) of power plants. it is therefore becoming increasingly important to decouple power supply from heat supply. achieving this separation generally involves designing the steam turbine conventionally solely for powergeneration operation. to supply heating steam, appropriate extraction points are provided that can be located e.g. on the low-pressure (lp) turbine section, on the crossover line between the intermediate-pressure (ip) turbine section and lp turbine section, and on the ip turbine section. steam extractions from the high-pressure (hp) turbine section are quite rare. the plant design concept selected also determines the ratio between power generation and heat supply. in plants equipped with extraction condensing turbines, this ratio is about 1.4 (in the corresponding load case). one factor that limits the maximum steam extraction is the required cooling steam flow in the low-pressure sections of the steam turbines, which is not available for heat or steam provision. in the decoupled low-pressure section, the power-to-heat ratio is approximately 1.3. this means that the unit’s thermal output and efficiency increase by about 10 percent when the lp sections are decoupled. when the steam turbine is fully decoupled, the power-to-heat ratio is less than 0.8. in the latter case, the gas turbine alone is generating electricity. the exhaust gas from the gas turbine is transformed in the hrsg to useful heat at an efficiency rate greater than 85 percent. the efficiency of the hrsg can be increased to over 90 percent in the cold boiler section by implementing hot-water district heat exchangers. 7. forecasting market trends to optimize the economic value of cogeneration plants the transition of our energy mix to integrate power generation from regenerative energy sources is changing the business model of fossil-fired power generation and heat supply systems. determining what future cogeneration plants will look like requires in-depth tracking and analysis of developments on the power exchanges, including the control reserve markets, and the derivation of anticipated trends. 90 vol. 20/2018 trends in cogeneration and combined heat and power plants figure 6. backpressure double pressure reheat chp cycle (simplified). as a result of the ongoing transition in our energy mix, wind and photovoltaic power are pushing fossilfuel power generation out of the base-load market. most cogeneration plants are still being operated to meet base-load demand in phases when heat is needed. due to the steadily growing power feed-in from renewable energy sources, in the future cogeneration plants will also be pushed step by step from continuous fullload operation into low partial-load segments or even to standby operation. just how important it is to be able to decouple power supply from heat supply is demonstrated by the fact that existing plants are backfitting thermal energy storage systems, usually in the form of atmospheric hot-water storage tanks. an oil tank on the grounds of timelkam combined cycle power plant in austria, for example, has been converted into an atmospheric hot-water storage tank. these energy storage systems are heated using whatever form of energy happens to be economically available, and then used to supply heat to consumers on demand. this enables a significant increase in plant flexibility with respect to decoupling electric power from heat. when demand for heat is high, the heat load can be controlled via the storage system nearly independent of power production. power generation, in turn, can orient itself directly to market demand and attractive electricity prices, because it is no longer an uncontrolled by-product. 8. intermediate-load operation – periods of shutdown are getting longer the t transition of our energy mix is increasingly pushing fossil-fuel power plants out of the base-load market. soon or later this same trend will also impact cogeneration plants. there will be a growing focus on enhancing operational flexibility when optimizing these plants. current forecasts project that phases at full-load operation will steadily shorten while standby phases grow longer. because the plant systems must operate in order to be able to supply heat, the operating regime becomes a critical factor with respect to periods of plant shutdown: the plant must recommence rotating operation at the very latest when its thermal storage systems have been bled empty. where needed, units will need to be operated at partial-load in order to be able to ensure heat supply. for the plant’s business model, this means that sales from intermediate-load operation will come under intense pressure. all in all, the number of operating hours will drop below the 5,000-hour marker that used to be considered normal in this operating mode. a precise forecast of the hours of full-load operation must be determined individually for every new unit installed and per fiscal year. siemens has developed a forecasting tool for this purpose that is capable of projecting probable operating regimes over the coming years for plants located in europe. when regular startup and shutdown procedures are planned, fuel consumption can be significantly reduced by optimizing these phases of transient operation. the "co-start" startup feature, for example, can shorten the startup process under hot operating conditions to well under 30 minutes, and thereby increase the average efficiency of startup operations by 14 percentage points. our co-start technology enables plant operators to start their units quickly and efficiently under hotstart conditions: in other words, to re-start units after periods of shutdown that typically last eight hours or less. we can therefore reduce fuel costs during the startup phase and increase plant responsiveness to rapidly changing market conditions, enabling plant operators to start units rapidly and precisely at that point in time when favorable market conditions are in effect. 91 j. slad, a. pickard, f. strobelt acta polytechnica ctu proceedings we’ve successfully reduced startup time under hotstart conditions by starting the gas and steam turbines simultaneously – contrary to conventional startup procedures for combined cycle units. the gas turbine starts and runs up, increasing its output at the steepest possible gradient right up to gas turbine full load, without waiting for a staggered startup of the steam turbine, as is customarily practiced. this means that inefficient waiting periods and hold points are made a thing of the past. intermediate-pressure steam is admitted to the steam turbine from the heat-recovery steam generator as early as possible, which supports the startup process. steam is not admitted to the high-pressure section of the steam turbine until later in the startup process. in addition, the bypass stations are closed early in the process to support pressure buildup and steam production in the hrsg. this new hot-start procedure using the co-start solution is managed by improved instrumentation and controls. warm starts – unit re-start after a weekend shutdown that typically lasts up to 48 hours – have also been accelerated using our co-start technology. the improved startup procedure is basically achieved by implementing the same technical measures and modifications to the startup process as for hot starts, the only difference being the different temperature and pressure conditions in the heat-recovery steam generator taken into account by the instrumentation and control system after the longer period of shutdown. despite extensive acceleration of these startup processes, component service life is not adversely impacted to a prohibitive degree. flexible power plants should enable operators not only to start up units quickly, but also to shut them down again just as rapidly so that the operator can respond immediately to electricity market prices that are changing unfavorably or to reduced production demand across the market. to accelerate the shutdown process, we use our quick-stop technology, which fundamentally changes the unit shutdown sequence. just like the fast starts enabled by our co-start feature, the gas turbine is shut down as quickly possible independent of the steam turbine and without the customary waiting periods and hold points observed in conventional combined cycle power plant operations. at the same time, improved instrumentation and control logic operations enable the steam turbine to commence shutdown very early. another innovative feature comes to bear for single-shaft power plant configurations where the gas turbine, steam turbine, and generator are arranged along a single common shaft by means of couplings. once the steam turbine has been braked to under 25 revolutions per minute, the coupling opened at the beginning of the shutdown procedure between the gas and steam turbine is rejoined, so that now the additional heavy mass of the gas turbine and the entire shaft train contributes to bringing the steam turbine to a standstill more quickly. 9. peak-load operation – growing demand as windand photovoltaic-generated power continues to increase to achieve their full profitability over the lifetime of the plant, power plants need to be optimized for future market potentials and needs. attractive potentials can still be expected in this market of short-term demand coverage, despite the fact that since about 2010 photovoltaic installations having increasingly been cutting into demand peaks over the midday hours and therefore the peak-load margin potential. the vde40 study (figure 7) forecasts demand peaks of up to 35gw at operating windows of under four hours. pitted against this demand stands a fleet of peak-load power plants with a combined generating capacity of about 10gw. shortages in demand coverage must therefore be anticipated. in contrast to the peaks in demand that regularly occur over the midday hours, today’s demand arises from weather-dependent supply shortages caused, for example, by temporary shadowing from clouds or temporary lulls in the wind. because these weatherrelated events can impact the entire fluctuating fleet of renewable energy resources, we will be confronted in germany with a peak-load demand in the double-digit gw range. especially for cogeneration plants that remain online to supply heat, there are opportunities to serve these market windows by quickly ramping up power production from partial-load operation, because no startup costs are incurred in these cases. this makes the marginal costs of covering demand significantly lower than for units that are intended to serve this market from a standby condition. what’s more, the gas turbine can be used to cover peak load even in summer if a bypass stack is provided between the gas turbine outlet and hrsg to allow only gas turbine operation with no water-steam cycle. for heating operations, unit output can be increased by providing a modest level of supplementary firing. this slight supplementary firing barely influences unit efficiency in normal operation because operating pressure stays close to the optimized values in the design case. if higher supplementary firing of the hrsg is implemented, however, swallowing capacity of the steam turbine to accommodate the change must be increased. this entails a drop of life steam pressure as well as in efficiency when the plant is operated without supplementary firing. 10. balancing power market forecasting the coming trends in balancing power markets is a significantly more complex affair than in the energy-only market, which can be projected with relative precision based on the extensive studies of future residual-load demand. despite this accuracy, however, a number of scenarios and studies are currently the 92 vol. 20/2018 trends in cogeneration and combined heat and power plants figure 7. residual-load demand 2020 – expected growth in the peak-load operating range. figure 8. load ramp test at lausward chp station. subject of discussion. it is plainly apparent that the business potentials of individual plants depend to a great extent on location and grid expansion. it can generally be assumed that grid services in the future will become increasingly more important to power generation contributing to demand coverage. while most scenarios assume that a major share of the primary frequency control will be provided by controllable consumers, in the case of the control reserves the picture is somewhat different. when renewable energy sources are unable to deliver adequate power feed-in to the grid, the higher marginal costs of fossil-fuel power plants taking up the slack will lead to higher electricity prices. it’s therefore likely that a portion of the controllable consumption will be reduced to a minimum. these capacities will therefore be removed from the market as positive control reserve, and will no longer be able to take part in covering grid loads. if power supply shortages occur that need to be compensated by the control reserve, fossil-fired units will have to cover this demand. this means that, if the fossil-fired units are currently in operation, the control reserve market offers opportunities to maximize the margin. we achieve an increase in the load-change gradients that allow marketing of a higher control reserve using our flexramp technology. the load-change gradient of a combined cycle power plant is determined by the combined ramping speeds of the gas turbine and steam turbine. in order to achieve the highest possible overall gradient, the gas turbine load is changed at the highest possible and technically allowable gradient. if a ramp-up in output is desired, in parallel and simultaneous with running up the gas turbine as quickly as possible, the steam turbine’s output is significantly higher than is conventionally possible in traditional combined cycle power plants. by temporarily opening bypass valves, energy stored in the hrsg in the form 93 j. slad, a. pickard, f. strobelt acta polytechnica ctu proceedings of additional steam from the high-pressure section of the hrsg is suddenly admitted to the intermediatepressure section of the steam turbine, which results in a considerable change in steam turbine output. this boost is only available for a short time, because after a few minutes the extra energy stored in the hrsg is consumed by the additional steam extraction. this brief time span of just a few minutes is enough, however, to sustainably bring the hrsg up to the new load level using the increased exhaust heat of the gas turbine. when rapid drops in power must be achieved, the load reduction in the steam turbine is greatly accelerated by routing the intermediate-pressure steam to the condenser via a bypass station. these measures allow operators to implement upward and downward load ramps of up to 55mw per minute. in test runs, so-called multiple double-hump curves were used to demonstrate that this fast ramping is not only possible on a one-time or limited basis; it can also be executed stably and reliably in the rapid sequencing of upward and downward load changes during continuous operation. if supplementary firing is considered in the plant design to cover peak loads, the nominal output can be maintained by supplementary firing in the frequency control mode, while gas turbine output is maintained at a level that demonstrates adequate control reserve capacity. while this leads to a slight drop in efficiency, this only occurs during peak-load demand. in the united states, where supplementary firing designs are customarily used, these control concepts are already being implemented. 11. forecasting future control regimes a number of flexibilization measures are available to optimize cogeneration plants and thereby maximize the plants’ economic value for the future. the question is how to determine what package is optimally suited for the given location and the specific application. until recently, design engineers had attempted to derive a deployment plan from forecasts of the extrapolated residual loads. however, this prognostic approach is unable to meaningfully consider location-specific parameters. that’s why siemens has developed a forecasting model that can represent the entire power plant fleet and all supply lines. this tool enables the computation of an initial prognosis of a plant’s future deployment plan. it even allows meaningful advance analyses of the profitability of grid services based on the supply grid model, including grid deficiencies. the forecast itself is based on a purely market-economy approach. retrospective analyses of these forecasts reveal that they accurately reflected actual operation 90 percent of the time. work is currently underway to improve the tools’ prognostic accuracy. once the operating regime has been determined, a feature configurator can be used to compile the optimal package of individual flexibilization measures. this enables solution packages to be offered that are optimally customized to individual business cases. 12. fortuna – the most efficient cogeneration plant in the world while these approaches look very promising in theory, are there any reference projects for the new features? the new fortuna power plant unit installed at lausward power station in düsseldorf’s inland harbor is a cutting-edge combined cycle power plant with a performance capacity far exceeding those customarily achieved by power generating units of this type. by using innovative technologies, state-of-the-art engineering, and our pooled expertise for optimal power plant solutions, we are ensuring that environmentfriendly electric power and district heat can be generated economically for a low-carbon future. the highest efficiency: fortuna’s 61.5 percent electrical efficiency exceeds even our previous world record of 60.75 percent set by the ulrich hartmann combined cycle unit at irsching power station in southern germany. a triple-pressure benson® heat-recovery steam generator that achieves steam temperatures of 600 °celsius at 170 bar is one element that makes this high electric efficiency rating possible. the best possible use of waste heat: to supply the city of düsseldorf with district heat, steam is extracted from the low-pressure steam turbine section at volumes of 300mw of thermal energy in combined cycle operation. the plant’s high electrical efficiency combined with its efficient use of heat generated in the power production process (in combined heat and power generation, otherwise known as cogeneration) increases the overall fuel efficiency of the natural gas to 85 percent. energy efficiency was further improved by optimizing startup and shutdown procedures. the newest generation of co-start technology has been successfully implemented. as a result, startup time for hot starts has been shortened from 50 minutes to less than 25 minutes. this translates to significant fuel savings by increasing efficiency during startup, which also reduces the associated co2 emissions. assuming that 200 hot starts will be run per year, this amounts to savings of €1.1 million in fuel costs alone. the co-start technology has also been implemented for warm starts. this has enabled startup time to be halved, reduced from the conventional 90 minutes to 45 minutes. and just as for hot starts, this results in fuel savings. unit cost-effectiveness and therefore profitability are improved thanks to the lower fuel costs. with 50 warm starts per year, this will produce savings of some €400,000. however, the potential for improvement is not just for startups: because power plant units also consume fuel during shutdown procedures, using quick-stop 94 vol. 20/2018 trends in cogeneration and combined heat and power plants figure 9. fortuna power station, düsseldorf, germany. figure 10. hot co-start test at lausward chp station. figure 11. warm co-start test at lausward chp station. 95 j. slad, a. pickard, f. strobelt acta polytechnica ctu proceedings figure 12. quick-stop test at lausward chp station. technology and the reduction in shutdown time it achieves results in more fuel savings. shutdown optimization has reduced shutdown time from 45 to 25 minutes. the fortuna power plant project implemented by the municipal utility stadtwerke düsseldorf in collaboration with siemens makes clear the success factors that are vital to building and operating a new power plant in germany’s challenging market today. 13. conclusions the transition of our energy mix is placing greater focus on energy efficiency. lawmakers have already recognized that combined heat and power generation (cogeneration) can help increase energy efficiency. targeted promotion and subsidization have raised the cost-effective profitability of cogeneration plants to a range that is already conducive to investment. but how can the economic value of this investment be maximized? to find the answer, siemens developed a number of tools that enable the forecasting of future operating regimes and the assessment of each individual flexibilization measure in terms of return on investment. above all, it can be assumed that to provide useful heat, it will be absolutely necessary to decouple power generation from the supplying of heat. design engineers are banking on power-driven plant designs that, solely in power-generation mode, can use all the waste heat to generate electric power in a downstream condensing turbine. efficiencies of over 61 percent in power-generation operating mode have already been achieved by the fortuna project. in the changed operating regime, energy efficiency doesn’t just focus on the nominal load case alone. even startup and shutdown procedures can be optimized for greater efficiency. plant designs with no peak-load reserves still dominate the market. given the forecasted market developments, designs incorporating supplementary firing with a control concept optimized in line with grid service are becoming more attractive. these designs enable plant operators to exploit business potential in the peak-load market while at the same time generating revenue from the balancing power markets. last but not least, the summer season can be used to generate sales and profit margin. by relying on a power-driven configuration, the plant can be positioned in the intermediate-load market. what’s more, implementing a bypass stack enables plants to run up from the shutdown condition to position themselves on the peak-load market. 14. disclaimer this document contains statements related to our future business and financial performance and future events or developments involving siemens that may constitute forward-looking statements. these statements may be identified by words such as “expect,” “look forward to,” “anticipate” “intend,” “plan,” “believe,” “seek,” “estimate,” “will,” “project” or words of similar meaning. we may also make forward-looking statements in other reports, in presentations, in material delivered to shareholders and in press releases. in addition, our representatives may from time to time make oral forward-looking statements. such statements are based on the current expectations and certain assumptions of siemens’ management, of which many are beyond siemens’ control. these are subject to a number of risks, uncertainties and factors, including, but not limited to those described in disclosures, in particular in the chapter risks in the annual report. should one or more of these risks or uncertainties materialize, or should underlying expectations not occur or assumptions prove incorrect, actual results, performance or achievements of siemens may (negatively or positively) vary materially from those described explicitly or implicitly in the relevant forward-looking statement. siemens neither intends, nor assumes any obligation, to update or revise these forward-looking statements in light of developments which differ from those anticipated. 96 vol. 20/2018 trends in cogeneration and combined heat and power plants trademarks mentioned in these documents are the property of siemens ag, its affiliates or their respective owners. references [1] a. pickard, f. strobelt. development trends in cogeneration and combined heat and power plants. in power-gen europe, milano, italy. 2016. 97 acta polytechnica ctu proceedings 20:86–97, 2019 1 introduction 2 what does cogeneration actually mean? 3 performance classes for every application 4 plant designs optimized for specific applications 5 plant designs for heat-driven operation electric power as a by-product 6 plant designs for power-driven operation – supplying electric power and heat on demand 7 forecasting market trends to optimize the economic value of cogeneration plants 8 intermediate-load operation – periods of shutdown are getting longer 9 peak-load operation – growing demand as windand photovoltaic-generated power continues to increase 10 balancing power market 11 forecasting future control regimes 12 fortuna – the most efficient cogeneration plant in the world 13 conclusions 14 disclaimer references acta polytechnica ctu proceedings doi:10.14311/app.2019.21.0016 acta polytechnica ctu proceedings 21:16–20, 2019 © czech technical university in prague, 2019 available online at http://ojs.cvut.cz/ojs/index.php/app resistance of mortars with gypsum, lime and composite binders against molds jitka krejsová∗, magdaléna doleželová czech technical university in prague, faculty of civil engineering, department of materials engeneering and chemistry, thákurova 7, 166 29 prague, czech republic ∗ corresponding author: jitka.krejsova@fsv.cvut.cz abstract. resistance of three mortars (the lime mortar, the gypsum mortar and the mortar with the composite binder) against molds was evaluated. mortars were also compared according to their ph, bulk density, strength and porosity. experiments showed that the lime mortar has the best resistance against molds and the lowest bending and compressive strength. the mortar with the composite binder has better mold resistance and higher bending and compressive strength than the gypsum mortar. keywords: calcium sulfate dihydrate, gypsum, mold resistance, mortars, plasters. 1. introduction gypsum is widely used material in building industry especially for production of gypsum plaster, plasterboard (drywall), gypsum blocks [1] and some composite materials. gypsum plaster often contains filler, which can improve thermal insulation properties, reduce binder consumption, reduce volume changes during exposure to high temperatures [2] etc. in terms of composition, gypsum plaster with filler is actually gypsum mortar (it contains calcium sulfate hemihydrate based binder, fine aggregate and water). production of gypsum based products is more environmentally friendly [3] and faster than production of cement based products. however, use of gypsum products is significantly limited by their low moisture resistance. according to recent research [4], moisture resistance can be improved by adding lime and silica fume to gypsum binder where lime acts as an alkaline activator of the pozzolan reaction. mortar with such binder is being developed and researched [5], but some of its properties are still unknown, for example mold resistance which is important but often neglected property. molds belong to microorganisms, that commonly appear on inner and outer surfaces of building structures and can cause sick building syndrome (people in a building feel ill from no apparent reason) and threaten human health [6]. molds also damage building materials by biodegradation (synergic process of chemical and physical biodegradation) [7]. in general, gypsum based materials have worse mold resistance than cement based materials [8], [9] which corresponds with different ph of these materials. however, e.g. water content in materials and additives also play an important role [8]. that’s why this paper is focused on the possibility of improving the mold resistance of gypsum mortat by adding lime and silica fume to the gypsum binder. also ph, bulk density, strength and porosity of the lime mortar, the gypsum mortar and the mortar with the composite binder (gypsum, lime and silica fume) is tested and compared. 2. materials the mortars were made of grey gypsum binder (gypstrend s.r.o.), white hydrated lime (vápenka čertovy schody a.s.), silica fume (stachesil s, stachema), standardized sand (according to the čsn en 196-1 [10] standard) and water. silica fume was used as a pozzolan additive and hydrated lime was added as an activator of the pozzolanic reaction. the compositions of the mortars are in 1. 3. experimental methods sample preparation and testing. squared samples of 30 × 30 × 3 mm size were prepared for mold growth testing. prisms of 40 × 40 × 160 mm size were prepared for physical properties testing according to the čsn en 13454-2+a1 [11]. dry components of binders were mixed, poured into water and placed in standardized automatic mixer. sand was added after 30 s of mixing. mixtures were put in moulds on 2 24 hours and then they were put out of moulds and stored in laboratory conditions for at least 28 days. after that, experiment was divided into four modifications (as shown in table 2) according to the type of sterilization and location of samples. mediums used in modification a, b, and c were following (d was without medium): • (cz) czapek-dox (commercial medium produced by oxoid) • (min) mineral medium (follow composition per l: 2 g nano3; 0.7g kh2po4; 0.3g k2hpo4; 0.5g mgso4 · 7h2o; 0.01g feso4 · 7h2o) according to the čsn en iso 846 [12]. each sample was placed in the center of petri dish with medium. all squared samples and mediums were inoculated by spreading of physiological solution with four commonly occurring molds cladosporium cladosporioides, asperigillus versicolor, stachybotrys 16 http://dx.doi.org/10.14311/app.2019.21.0016 http://ojs.cvut.cz/ojs/index.php/app vol. 21/2019 resistance of mortars with gypsum, lime and composite binders against molds sample calcined gypsum [wt. %] lime [wt. %] silica fume [wt. %] sand [wt. %] water/binder ratio [-] gm 33 67 1 lm 3.3 67 1 glsm 27 3.2 2.8 67 0.82 table 1. composition of mortars sample sterilization storage of inoculated samples a on mediums in petri dishes in biological thermostat b alcohol and uv radiation on mediums in petri dishes in biological thermostat c autoclaving on mediums in petri dishes in biological thermostat d 100% rh in desiccator table 2. modifications of experiment chartum and penicilium purpurogenum which are obtained from czech collection of microorganism (ccm). the microscope pictures of the molds are below (1a , 1b all pictures are in the same scale). the incubation was at 26 ◦c for three months. rate of mold growth was monitored for 4 12 weeks every week as a cover of sample and halo effect around the sample. after the finishing of experiment the samples were evaluated by microscopy. (a) left cladosporium cladosporioides, right asperigillus versicolor. (b) left stachybotrys chartum, right penicilium purpurogenum. figure 1. microscope pictures samples were stored for 28 days in laboratory conditions. their ph was determined by ph paper from dispersion of crushed samples in distilled water. bulk density ρv [kg/m3] was determined by gravimetric method on dried prismatic samples. bending strength ff [mpa] and compressive strength fc [mpa] were determined using press fp 100 (veb industriewerk ravenstein) in accordance with the čsn en 13279-2 [13] on dried prismatic samples. porosity and pore size distribution were determined using mercury intrusion porosimetry device pascal 140 + 440 (thermo scientific) on dried pieces of prismatic samples. 4. results physical and chemical properties. values of ph are summarized in table 3. as expected, the gypsum mortar has the lowest ph. bulk densities of all mortars are similar, as can be seen in table 3. bending and compressive strength of mortars are in 2. the mortar with the composite binder (glsm) has almost three times higher compressive strength than the lime mortar (lm) and more than one and half times higher compressive strength than the gypsum mortar (gm). sample ph bulk density [kg/m3] porosity [-] gm 6 1 458 42.5 lm 10 1 642 32.6 glsm 11 1 581 38.5 table 3. physical and chemical properties of mortars. figure 2. bending and compressive strength. total porosity of all mortars is similar (see table 3). the gypsum mortar has primarily pores of size about 1 µm 3. the mortar with the composite binder has these pores partially filled by csh-phases formed during the pozzolanic reaction [5]. size of pores in the lime mortar is between 10−8 m and 10−4 m. therefore, all three mortars have mainly capillary pores 17 jitka krejsová, magdaléna doleželová acta polytechnica ctu proceedings figure 3. pore size distribution. (10−9 10−3 m). in these capillary pores, water movement is induced by surface tension (capillary forces) and that is why rising dump may occur in these materials. moisture significantly support mold growth and therefore, moisture transport properties of materials are important. it can be expected, that moisture transport properties of the three mortars differs due to the different pore size distribution. resistance against molds. mortars resistance against molds was evaluated using the five point scale, where: 0 no molds, 1 mold growth is visible under microscope, 2 molds cover less than 25 % of sample surface, 3 molds cover 25 50 % of sample surface, 4 molds cover more than 50 % of sample surface, 5 molds completely cover sample surface. mean results are in table 4. mold growth on different samples made of same material and same modification was similar. results show that molds grow more and faster on samples with lower ph (about 6), respectively on samples without lime, which corresponds to literature [9]. results also show higher resistance to mold growth of unsterilized samples a. it could be caused by the fact that some microorganisms that decelerate mold growth were destroyed by sterilization. on the lime mortar (lm), mold growth did not appear during the experiment and so called halo effect (see 5) occurred around samples a and b. it means that medium near sample was influenced by the sample. leaches (probably ca(oh)2 leaches) formed around the sample and molds did not grow in this inhibition zone (see 5). the lime mortar (lm) can be classified as biocidal (for about four weeks) due to this phenomenon. halo effect did not occur around autoclaved samples c. it was probably caused by the release of free ca + 2 ions during autoclaving. halo effect also appeared around the mortar with the composite binder (glsm) probably due to its ca(oh)2 contain. this mortar sustains the biocidal properties for four days. microorganisms probably created the appropriate growth environment for molds during this time. mold growth occurred after one week on samples on the czapek-dox medium. molds did not grow on samples on the mineral medium. fast mold growth and no halo effect were observed on the gypsum mortar (gm) in accordance with literature [14]. molds were observed few weeks after inoculation even on surfaces of the samples on mineral medium. this mortar has therefore worse resistance against molds than the mortar with the composite binder. however, mold growth on all tested mortars is caused by the presence of medium in the pores of the samples. it is proved by the modification d. all samples in modification d were monitored for 12 weeks and mold growth was not observed. after 12 weeks, some molds were observed under microscope. these molds were probably spread on sample surface during inoculation at the beginning of experiment and they have not grown since that. it means that tested mortars do not have enough essential nutrients for mold growth. results show that relatively low content of lime and silica fume in gypsum binder significantly improved resistance of mortar against molds. 5. conclusion resistance against molds, ph, bulk density, strength and porosity of the lime mortar, the gypsum mortar and the mortar with the composite binder (gypsum, lime and silica fume) were investigated. experimental results showed that: • the mortar with the composite binder has almost three times higher bending and compressive strength than the lime mortar and about one and half times higher bending and compressive strength than the gypsum mortar (gm); • the gypsum mortar has the lowest ph value; • pore size distribution of the mortars differs while their total porosity and bulk density are similar, • the tested mortars do not have enough essential nutrients for mold growth (tested time was 12 weeks); • molds grow on the gypsum mortar and on the mortar with the composite binder if nutrients are added, • molds do not grow up on the lime mortar (during 6 weeks); the lime mortar has the best resistance against molds and the gypsum mortar has the worst resistance against molds, • inhibition zone occurred around the lime mortar and the mortar with the composite binder and these mortars can be classified as biocidal (for 4 weeks the lime mortar, resp. 4 days the mortar with the composite binder). the mortar with the composite binder has better mold resistance and higher bending and compressive strength than the gypsum mortar. the mortar with the composite binder is therefore more suitable for use in practice, e.g. as a plaster in building structures. future research might be focused on mold resistance improving for example by adding nanoparticles [8]. acknowledgements this research has been financially supported by the project sgs 16/199/ohk1/3t/11. the experiments were performed in cooperation with microbiological and chemical 18 vol. 21/2019 resistance of mortars with gypsum, lime and composite binders against molds sample modification time [week] (medium cz) time [week] (medium min) 1 2 3 4 5 6 1 2 3 4 5 6 gm a 2 2 2 2 0 0 0 0 b 3 4 4 4 4 4 0 0 0 2 2 2 c 4 4 4 4 4 0 0 0 0 0.5 d lm a 0 0 0 0 0 0 0 0 b 0 0 0 0 0 0 0 0 0 0 0 0 c 0 0 0 0 0 0 0 0 0 0 d glsm a 2 2 2 2 0 0 0 0 b 0 2 3 3 3 3 0 0 0 0 0 0 c 2 2 2 2 2 0 0 0 0 1 d table 4. mold growth gm lm glsm modification a (4 days) modification b (4 weeks) modification c (4 weeks) table 5. inoculated samples, czapek-dox medium. laboratory, department of building structures, faculty of civil engineering, czech technical university, prague, namely with mgr. pavla ryparová, ph.d. and ivana loušová. references [1] casado, rodrigues. construction technology of internal gypsum block partitioning, 2012. brick and block masonry conference. [2] doleželová, scheinherrová, krejsová, vimmrov. effect of high temperatures on gypsum-based composites. construction and building materials 168:82–90, 2018. [3] j. bakker, h. brouwers. mortar and concrete based on calcium sulphate binders. in proceedings 16th ibausil, international conference on building materials (internationale baustofftagung), weimar, 20-22 september 2006. fa finger institut für baustoffkunde, 2006. [4] m. doleželová, a. vimmrová. moisture influence on compressive strength of ternary gypsum-based binders. 19 jitka krejsová, magdaléna doleželová acta polytechnica ctu proceedings in aip conference proceedings, vol. 1863, p. 290003. aip publishing, 2017. [5] m. doleželová, j. pokorný, a. vimmrová. design of the ternary gypsum-based building composite with the help of simplex optimization, 2018. 2nd international conference on materials design and applications. [6] p. ryparová, z. rácová. the occurrence of mold in construction materials before inbuilt into new building and protection against this type of biodegradation, 2016. [7] p. ryparová, r. wasserbauer, z. rácová. the cause of occurrence of microorganisms in civil engineering and the dangers associated with their growth, 2016. procedia engineering. [8] z. rácová, p. ryparová. silver inhibition effect of nanoparticles on specific mold group growth, 2015. nancon. [9] méndez, pérez, aguilar, et al. red pigment production by penicillium purpurogenum gh2 is influenced by ph and temperature, 2011. [10] čsn en 196-1. methods of testing cement. determination of strength., 2005. prague. [11] čsn en 13454-2+a1. binders, composite binders and factory made mixtures for floor screeds based on calcium sulfate. test methods, 2008. [12] čsn en iso 846. plastics – evaluation of the action of microorganisms, 1998. [13] čsn en 13279-2. gypsum binders and gypsum plasters. test methods, 2014. [14] s. m. nieminen, r. kärki, s. auriola, et al. isolation and identification of aspergillus fumigatus mycotoxins on growth medium and some building materials. applied and environmental microbiology 68(10):4871–4875, 2002. 20 acta polytechnica ctu proceedings 21:16–20, 2019 1 introduction 2 materials 3 experimental methods 4 results 5 conclusion acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2017.13.0093 acta polytechnica ctu proceedings 13:93–96, 2017 © czech technical university in prague, 2017 available online at http://ojs.cvut.cz/ojs/index.php/app long-time investigation of cement composite material with micronized waste marble powder: dynamic modules zdeněk prošeka,b,∗, jaroslav topiča a czech technical university in prague, faculty of civil engineering, thákurova 7, 166 29 prague 6, czech republic b university centre for energy efficient buildings, czech technical university in prague, třinecká 1024, 273 43 buštěhrad, czech republic ∗ corresponding author: zdenek.prosek@fsv.cvut.cz abstract. this article focus on “blended cement”. the blended cement was created by using waste marble powder (wmp) as a partial replacement for cement. we investigated the influence of wmp on the developing of the dynamic modulus of elasticity and the dynamic shear modulus in time. four different cement composites with wmp as a partial replacement for cement were studied (5, 10, 15 and 50 wt. %) together with reference samples. dynamic modulus of elasticity was monitored during the first 377 days since manufacture by use of non-destructive testing (resonance method). the results showed that wmp in a small amount had a no effect on the dynamic modulus of elasticity and the dynamic shear modulus. keywords: micronized marble powder, cement composites, mechanical properties, long-time investigation. 1. introduction the recycled materials are usually used as an option to non-renewable resources. the problem is mainly variability of these materials, for this cause, is mainly applied to transport infrastructure [1]. examples of the use of waste or recycled materials were spelled out in the study, e.g. [2–4]. this article differs from another primarily utilizing waste from marble industry, directly waste marble sludge. the waste marble sludge generated during the cutting block, or during the finishing process making an environmental problem. the generation of waste is making environmental problems as the valuable national wealth is getting wasted mainly due to lack of management and lack of technology, air contamination, water contamination and loss of vegetation and fauna [5]. the amount of this waste was calculated for the section of the shaq el thoaban, egypt to one million tons per year [6] and for the section of estremoz anticline, alentejo, portugal to 600 thousand tons per year [7]. both areas have approximately the same production of marble stone, but the technical, operational and management practices in the industry are less developed in area shaq el thoaban and they have contributed significantly to greater waste generation. from all the waste produced from marble enterprise, only 10 to 20 % is used in construction. the remainder is refused and accumulated in mounds marble waste close to the extraction quarries [8, 9]. there are some articles that deal with utilized the marble sludge as the replacement for cement in producing the blended cement. the first study found that the addition of the marble sludge to the cement composite is changed crystals portlandite [10]. the second study showed the morphology of the internal structures of cement pastes with and without marble sludge is identical [11]. in all case, marble sludge in a small amount had a good influence on mechanical properties [12, 13]. grains of marble sludge clump together into lumps and thus grains of marble sludge are not homogeneously distributed in the mixture. new option to reduce or restrict this effect is the use of high-speed milling from lavaris ltd. the resulting output is waste marble powder (wmp) and it has better mechanical properties than marble sludge [14]. high-speed milling method separates all the grains of marble sludge. the high-speed milling method was successfully applied for micronized of recycled concrete [15]. 2. materials and samples the cement used was a commercial cem i 42.5r (radotín, czech republic) ordinary portland cement. the wmp used was obtained from the processing of limestone marble in the czech republic as unsorted waste (beroun, jež ltd.), which was micronized using a high-speed mill from lavaris ltd. (libčice, czech republic). the micronized marble powder with fraction 0–40 mm had an average grain and mean grain size of 9.74 and 4.59 microns, respectively. the testing mixtures were designed to keep the identical workability as the fresh reference mixture. the consistency of the mixtures was determined by workability slump test 20 (d20), (table 1). the water ratio is determined as water to cement and marble pow93 http://dx.doi.org/10.14311/app.2017.13.0093 http://ojs.cvut.cz/ojs/index.php/app zdeněk prošek, jaroslav topič acta polytechnica ctu proceedings set/ material cem i wmp water flow expansion bulk density shrinkage [wt. %] [wt. %] ratio [-] test d20 [mm] [kg.m−3] [%] ref. wmp_0 100 0 0.35 190 1919 ± 5 0.19 ± 0.06 wmp_5 95 5 0.32 190 1915 ± 6 0.27 ± 0.11 wmp_10 90 10 0.315 190 1920 ± 3 0.14 ± 0.05 wmp_15 85 15 0.31 190 1879 ± 6 0.21 ± 0.07 wmp_50 50 50 0.305 190 1760 ± 8 0.45 ± 0.16 table 1. composition of the individual set/mixtures. figure 1. experimental set-up for the resonance method. der weight ratio. cement was replaced by the waste micronized marble powder by the amount of 5, 10, 15 and 50 wt. percent (table 1). mixtures made according to table 1 were cast to the rectangular molds with dimensions 40×40×160 mm. all the rectangular molds were measured their dimensions before the mixture was implemented to the mold. these values were applied for determine shrinkage. after casting, these specimens were kept in the molds for 24 h at the laboratory with the temperature 22 ◦c. each set consisted of 6 specimens. after demolding, these specimens were deposited in a laboratory environment at 22 ± 1 ◦c and relative humidity 50 ± 2 %. 3. experimental methods the dynamic modulus of elasticity and the dynamic shear modulus were observed using the resonance method, based on determinations of the natural frequency measured on specimens. resonance method is a non-destructive test. the last measurement was made after 377 days. the resonance method was carried out using the measurement line from brüel & kjær, consisting of a measurement station type 3560-b-120, type 4519003 acceleration transducers, an 8206 impact hammer type and a control computer with software pulse labshop (figure 1.). the resonance method requires information about the dimensions and the weight of each sample earlier than testing was applied. the response signals were transformed from the time to the frequency using the fast fourier transform. the frequency response function (frf) was determined as the ratio of the response and the excitation force in the frequency. the proper basic natural frequencies were evaluated from the frfs. then the dynamic modulus of elasticity and the dynamic shear modulus were evaluated based on the basic longitudinal natural frequency of the samples and the basic torsional natural frequency of the sample, respectively [16, 17]. 4. result and discussion figure 2 shows the evolution of the bulk density, where the free water evaporates over time, thereby changing the bulk density of the samples. figure 3 and 4 shows the evolution of the dynamic shear modulus and the dynamic modulus of elasticity, respectively over time. all of the samples exhibit the steepest increase of the modulus of elasticity in the first day. after the first day, the cement composite rapidly lost the value of dynamic modulus of elasticity. this was due to loss of weight of the sample by natural drying.this phenomenon can be explained by gauging water evaporates from the sample and changes the inner environment in which the wave from the pulse method spreads. the values were stabilized between 7 and 377 days. the curves got smaller decline modulus between 7 and 377 days. there is a noticeable relation between the value of modulus of elasticity and the amount of wmp used in the mixture. the highest modulus had the reference mixture wmp_0 , namely 16.8 gpa for 28 days old samples and 16.0 gpa fot 377 days old samples in the case of the dynamic modulus of elasticity and the dynamic shear modulus was 7.3 gpa for 28 days old samples and 7.1 for 377 days old samples. the lowest value of modulus had the mixture with 50 wt. % of wmp, namely 14.6 gpa for 28 days old samples and 13.9 gpa for 377 days old samples in the case of the dynamic modulus of elasticity and the dynamic shear modulus was 5.9 gpa for 28 and 377 days old samples. 5. conclusions this work is focused on the mechanical properties of the cement composites composed of portland cement and different amounts of waste marble powder. based on the results it can be concluded that: • all of the samples exhibit the steepest increase of the modulus in the first day. 94 vol. 13/2017 long-time investigation of cement composite ... figure 2. time dependence of bulk density (with standard deviation). figure 3. time dependence of dynamic shear modulus (with standard deviation). figure 4. time dependence of dynamic modulus of elasticity (with standard deviation). 95 zdeněk prošek, jaroslav topič acta polytechnica ctu proceedings • after the first day, the cement composite rapidly lost the value of modulus, due to loss of weight by gauging water. • the values of the dynamic modulus of elasticity and the dynamic shear modulus were stabilized between 7 and 377 days. the curves got smaller decline modulus between 7 and 377 days. acknowledgements this outcome was supported by the czech technical university in prague under no. sgs16/201/ohk1/3t/11 and no. sgs17/060/ohk1/1t/11 and by the ministry of education, youth and sports within national sustainability programme i, project no. lo1605. references [1] p. tesárek, k. šeps, j. topič, et al. influence of recycled materials on resulting mechanical properties of cement composites. in modern methods of experimental and computational investigations in area of construction, vol. 825 of applied mechanics and materials, pp. 53–56. trans tech publications, 2016. doi:10.4028/www.scientific.net/amm.825.53. [2] v. corinaldesi, g. moriconi. behaviour of cementitious mortars containing different kinds of recycled aggregate. construction and building materials 23(1):289 – 294, 2009. doi:10.1016/j.conbuildmat.2007.12.006. [3] j. topič, z. prošek, j. hrůza. mechanical properties of the cement paste with fine ground recycled concrete depending on the microstructure. in modern methods of experimental and computational investigations in area of construction ii, vol. 1144 of advanced materials research, pp. 22–27. trans tech publications, 2017. doi:10.4028/www.scientific.net/amr.1144.22. [4] z. prošek, a. osvald, j. topič, et al. influence of recycled concrete composition on its elastic stiffness. in key engineering materials, vol. 677, pp. 288–291. trans tech publ, 2016. [5] m. el-gammal, m. ibrahim, e. badr, et al. health risk assessment of marble dust at marble workshops. nature and science 9(11):144–154, 2011. [6] k. k. alzboon, k. n. mahasneh. effect of using stone cutting waste on the compression strength and slump characteristics of concrete. international journal of environmental science and engineering 1(4):167–172, 2009. [7] l. lopes, r. martins. marbles from portugal. naturstein, online publication, naturalstone-online com 2012. [8] s. shahidan, i. isham, n. jamaluddin. a review on waste minimization by adopting in self compacting concrete. in matec web of conferences, vol. 47. edp sciences, 2016. [9] z. prošek, j. trejbal, j. topič, et al. utilization of the waste from the marble industry for application in transport infrastructure: mechanical properties of cement pastes. in iop conference series: materials science and engineering, vol. 236, p. 012092. iop publishing, 2017. [10] a. a. aliabdo, a. e. m. a. elmoaty, e. m. auda. re-use of waste marble dust in the production of cement and concrete. construction and building materials 50:28–41, 2014. [11] a. ergün. effects of the usage of diatomite and waste marble powder as partial replacement of cement on the mechanical properties of concrete. construction and building materials 25(2):806–812, 2011. [12] b. demirel. the effect of the using waste marble dust as fine sand on the mechanical properties of the concrete. international journal of physical sciences 5(9):1372–1380, 2010. [13] g. menéndez, v. bonavetti, e. irassar. strength development of ternary blended cement with limestone filler and blast-furnace slag. cement and concrete composites 25(1):61–67, 2003. [14] k. šeps, j. topič, z. prošek. the effect of micronized waste marble powder as partial replacement for cement on resulting mechanical properties of cement pastes. in modern methods of experimental and computational investigations in area of construction ii, vol. 1144 of advanced materials research, pp. 54–58. trans tech publications, 2017. doi:10.4028/www.scientific.net/amr.1144.54. [15] z. prošek, l. hlubocký. mechanical properties of cement composite with material based on waste marble powder and crushed limestone. in modern methods of experimental and computational investigations in area of construction ii, vol. 1144 of advanced materials research, pp. 9–13. trans tech publications, 2017. doi:10.4028/www.scientific.net/amr.1144.9. [16] j. topič, z. prošek, k. indrová, et al. effect of pva modification on properties of cement composites. acta polytechnica 55(1):64–75, 2015. [17] z. prošek, k. šeps, j. topič. the effect of micronized waste marble powder as partial replacement for cement on resulting mechanical properties of cement pastes. in advanced materials research, vol. 1144, pp. 54–58. trans tech publ, 2017. 96 http://dx.doi.org/10.4028/www.scientific.net/amm.825.53 http://dx.doi.org/10.1016/j.conbuildmat.2007.12.006 http://dx.doi.org/10.4028/www.scientific.net/amr.1144.22 http://dx.doi.org/10.4028/www.scientific.net/amr.1144.54 http://dx.doi.org/10.4028/www.scientific.net/amr.1144.9 acta polytechnica ctu proceedings 13:94–97, 2017 1 introduction 2 materials and samples 3 experimental methods 4 result and discussion 5 conclusions acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2020.26.0094 acta polytechnica ctu proceedings 26:94–99, 2020 © czech technical university in prague, 2020 available online at https://ojs.cvut.cz/ojs/index.php/app response and damage evolution of single edge notched timber beams under three-point bending eliška šmídová∗, petr kabele, michal šejnoha czech technical university in prague, faculty of civil engineering, department of mechanics, thákurova 7, 166 29 prague 6, czech republic ∗ corresponding author: eliska.smidova@fsv.cvut.cz abstract. two groups of small-size single edge notched beams (senb) made of european spruce (picea abies) were tested in three-point bending (3pb) until failure under displacement control. the first group comprised of eight solid and two glued laminated (gl) timber beams manufactured with (a) the single edge notch at the bottom of the mid-span and (b) the reduced ligament depth. the second group consisted of four gl timber beams with the single edge notch only. we employed digital image correlation (dic) to quantify strains and displacements, capture the damage evolution, and track the sequence of failure patterns. in this work, we present response of the beams in terms of load vs. crosshead displacement of the moving crosshead and load vs. crack tip opening displacement (ctod). keywords: three-point bending (3pb), single edge notched beams (senb), reduced ligament depth, timber, crack tip opening displacement (ctod). 1. introduction an experimental campaign has been carried out at czech technical university in prague to determine both the response and the damage evolution of single edge notched beams (senb) with fibers along the beam length. we loaded two groups of beams made of european spruce in three-point bending (3pb). the first group beams were sawn with the notch and an additionally reduced ligament depth to induce the fibers rupture (splinter) under tension stress parallel with the grain at or close to the ligament. compared to it, the second group was cut with the notch only to initiate cracking along the grains at the notch tip exposed to tension perpendicular to the grain. rilem and cib-w18 ([1]) recommend performing the 3pb test of senb (figure 1) to obtain a stable crack extension in the direction parallel with the grain from an initial notch to the complete separation of the crack faces. representing the fracture mode i, i.e. cracking under tension stress perpendicular to the crack plane, the test is used for calculation of fracture energy. many studies involving the senb bending test have been conducted ([2], [3], [4], [5], [6], [7]). out of timber structure failure cases where a crack could be detected, the most frequent failure type is cracking in the grain direction corresponding to 75% while shear, tension and other failures represent 8%, 6% and 11% of the cases, respectively ([8]). general failure modes of timber beams are depicted in figure 2 figure 4 ([9]). in the midspan zone (figure 2), the tension stress leads to fibers rupture that we can categorize as a brittle failure. longitudinal compression causes ductile failure in the form of kink bands. thus, load-bearing capacity of a beam in bending is usually limited by the brittle behavior of timber in the tension zone and can be more reduced with the presence of natural defects. nevertheless, shear stress can be governing for the design of some timber members, e.g. short beams. its maximum is reached around the supports causing brittle failure parallel with the grain where the fibers bundles or layers slide (figure 3). the area of a support or a loading point is typical for compression stress perpendicular to the fibers that leads to a ductile failure (figure 4). figure 1. single edge notched beam [1]. figure 2. sketch for bending failure [9]. 94 https://doi.org/10.14311/app.2020.26.0094 https://ojs.cvut.cz/ojs/index.php/app vol. 26/2020 single edge notched timber beams under three-point bending group l × h × w ln wli material count failure specimens[cm] [cm] [cm] [-] 1 40× 10× 5 1 2 solid 5 f1 #01 -#05 solid 3 f2 #06 -#08 gl 2 f2 #09 -#10 2 40× 10× 4 1 − gl 4 f3 #11 -#14 table 1. test program characteristics and first failures types (f1 splinter around the ligament, f2 splinter around the ligament + shear, f3 parallel with the grain). figure 3. sketch for shear failure [9]. figure 4. sketch for compression failure [9]. 2. test procedure and material we conducted three-point bending test (3pbt) of two groups of beams, both with the length l of 40 cm, cross-section height h of 10 cm and single edge notch with the length ln of 1 cm (figure 5) and the width wn of 0.1 cm. the first group was composed of eight solid and two glued laminated timber beams with crosssection width w of 5 cm and reduced ligament width wli of 2 cm. the second group consisted of four glued laminated timber beams with cross-section width w of 4 cm. we ran the experiment under displacement control at the rate of 2mm/min. in this regime, we were able to record the descending branch of the load-displacement curve. furthermore, digital image correlation (dic) was applied, with mostly 10 sec time interval, to capture strains, displacements (e.g., crack tip opening displacement ctod, see figure 5), damage evolution and failure type sequence. figure 5. three-point bending test (3pbt) configuration common for all the beams: length of 40 cm, cross-section height of 10 cm and single edge notch length ln of 1 cm. 3. results and discussion the following section describes damage evolution and response of the beams. overview of this experimental campaign is given in table 1 for observed first failure types. let us note that the failure of fibers crushing due to compression stress perpendicular to the grain around the loading plate and supports is present in every beam influencing the response. 3.1. group one: notched beams with reduced ligament depth the load-crosshead displacement responses of the group one beams with the single edge notch and reduced ligament depth are plotted in figure 6 and figure 7 for the first failures denoted f1 splinter around or at the ligament and f2 splinter around or at the ligament accompanied by shear failure, respectively. furthermore, for the beams with f1, the load-ctod curves are shown in figure 8. the following subsections present the damage evolution and failure type sequence of individual beams. 3.1.1. beam #01 the beam #01 represents the failure type f1 for the beams #01 #05. its damage evolution is shown in figure 9. observing the photos of the load stages, we can see failure f1 splinter at or in the vicinity of the reduced ligament being the only failure type that occurred. 95 e. šmídová, p. kabele, m. šejnoha acta polytechnica ctu proceedings figure 6. load vs. crosshead displacement of the group one beams with the first failure f1 splinter around the ligament. figure 7. load vs. crosshead displacement of the group one beams with the first failures f2 splinter around the ligament + shear. 3.1.2. beam #08 the beam #08 represents a specific type of the failure f2 occurring in two beams (#08, #10) out of the group one. the damage evolution is shown in figure 10. the photos of the load stages show that failure f2 is composed of (a) splinter at or in the vicinity of the reduced ligament and (b) shear failure of a grain bundle reaching at the beam length and occurring both inside and at the cross-section boundaries, namely at the painted front face of the beam, see figure 11 (left). the latter failure causes leaning of the sheared grains against the other ligament face affecting the overall response, especially ctod of the notch. 3.1.3. beam #09 the beam #09 represents the failure type f2 occurring in three beams (#06, #07, #09) out of the group one. the damage evolution is shown in figure 12. observing the photos of the load stages, we can see that failure f2 covers (a) splinter at or in the vicinity of the reduced ligament and (b) shear failure of a grain bundle inside the cross-section, see figure 11 (right). 3.2. group two: notched beams the load-crosshead displacement responses of the group two beams with the single edge notch only are plotted in figure 13 for the failure denoted here f3 parallel with the grain being initiated at the figure 8. load vs. crack tip opening displacement (ctod) with the first failure f1. figure 9. damage evolution of the beam #01 with the first failure f1 splinter around the ligament. notch tip. let us mention that we do not report on the fifth beam from group two due to the presence of cross grain although the first failure type was also f3 along the grain. we present the response of the beam #13 only up to the first crack initiation (f3) that was consequently accompanied by unequal vertical displacement along the cross-section width above one of its supports resulting in torsion of the beam. compared to it, the beam #12 response captures not only the first crack initiation (f3) but also its opening. the following subsections present damage evolution and failure type sequence of individual beams #11 96 vol. 26/2020 single edge notched timber beams under three-point bending figure 10. damage evolution of the beam #08 with the failure f2 splinter around the ligament and shear failure. figure 11. the end of the beam #08 (left) and #09 (right) demonstrating the shear failure. and #14. 3.2.1. beam #11 the beam #11 failed in tension. we can see in figure 14 that (a) the first crack was initiated at the figure 12. damage evolution of the beam #09 with the failure f2 splinter around the ligament and shear failure. figure 13. load vs. crosshead displacement of the group two beams with the first failure f3 parallel with the grain. notch tip and spread along the grain (f3) and then (b) splinter above the notch occurred. the similar crack pattern can be visible on the back face of the beam, see figure 15. 3.2.2. beam #14 the failure pattern of the beam #14 is the same as that of the beam #11 except that the splinter above the notch is accompanied by shear failure of a timber layer in the middle of the beam height reaching the end of the beam figure 16. the overall crack pattern 97 e. šmídová, p. kabele, m. šejnoha acta polytechnica ctu proceedings figure 14. damage evolution of the beam #11 with the failure f3 parallel with the grain followed by splinter above the notch. figure 15. back face of the beam #11. on the back face is shown in figure 17. 4. conclusion we conducted experiments of single edge notched beams made of european spruce loaded in threepoint bending under displacement control employing a high-speed digital camera for digital image correlation (dic). ten beams (group one), with dimensions l × h × w of (40× 10× 5) cm, were manufactured with both the single edge notch of the height of 1 cm and the reduced ligament depth. the other four beams (group two), with dimensions l × h × w of (40× 10× 4) cm were cut only with the notch. crushing of fibers under compression stress perpendicular to the grain occurred in all beams around the supports and the loading point. we observed the first failure of fibers rupture (splinter) under tension stress parallel with the grain in five beams of the group one. in the other five specimens of this group, the splinter was accompanied by shear failure in the form of sliding figure 16. damage evolution of the beam #14 with failure f3 parallel with the grain followed by splinter above the notch and shear failure. figure 17. back face of the beam #14. of fibers reaching the end of the beam. regarding the failure of the group two, the first crack parallel with the grain was initiated at the crack tip on one or both sides followed by the second failure of the splinter above the notch that was in some cases accompanied by sliding of the fibers up to the beam end. we captured the load-crosshead displacement response for all beams, mostly with the descending 98 vol. 26/2020 single edge notched timber beams under three-point bending branch. furthermore, we obtained the load-crack tip opening displacement (ctod) for the beams with the reduced ligament depth (group one) where the first failure of splinter occurred. list of symbols h cross-section height [cm] l beam length [cm] ln vertical notch length [cm] w cross-section width [cm] wli ligament depth [cm] wn vertical notch width [cm] acknowledgements the support by the gačr grant no. 18-05791s is gratefully acknowledged. references [1] design of end-notched beams, 1989. [2] m. ashby, k. eastering, r. harrysson. the fracture and toughness of woods. in proc r soc lond a, vol. 398, pp. 261–280. 1985. [3] l. le-ngoc, h. mccallion. on the fracture toughness of orthotropic materials. eng fract mech 58:355–362, 1997. [4] k. ando, m. ohta. variability of fracture toughness by the crack tip position of coniferous wood 45:275–283, 1999. [5] k. watanabe, s. shida, m. ohta. evaluation of endcheck propagation based on mode i fracture toughness of sugi (cryptomeria japonica) 57:371–376, 2011. [6] t. nakao, c. m. e. susanti, h. yoshihara. examination of the failure behavior of wood with a short crack in the radial–longitudinal system by singleedge-notched bending test. journal of wood science 58(5):453–458, 2012. doi:10.1007/s10086-012-1266-6. [7] b. franke, p. quenneville. analysis of the fracture behavior of radiata pine timber and laminated veneer lumber. engineering fracture mechanics 116:1–12, 2014. doi:10.1016/j.engfracmech.2013.12.004. [8] m. frese, h. j. blaß. failure analysis on timber structures in germany p. 9, 2011. [9] s. franke, b. franke, a. m. harte. failure modes and reinforcement techniques for timber beams – state of the art. construction and building materials 97:2–13, 2015. doi:10.1016/j.conbuildmat.2015.06.021. 99 https://doi.org/10.1007/s10086-012-1266-6 https://doi.org/10.1016/j.engfracmech.2013.12.004 https://doi.org/10.1016/j.conbuildmat.2015.06.021 acta polytechnica ctu proceedings 26:94–99, 2020 1 introduction 2 test procedure and material 3 results and discussion 3.1 group one: notched beams with reduced ligament depth 3.1.1 beam #01 3.1.2 beam #08 3.1.3 beam #09 3.2 group two: notched beams 3.2.1 beam #11 3.2.2 beam #14 4 conclusion list of symbols acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2019.21.0001 acta polytechnica ctu proceedings 21:1–4, 2019 © czech technical university in prague, 2019 available online at http://ojs.cvut.cz/ojs/index.php/app plasma modification of polyvinyl alcohol microfibers to improve cohesion with cement matrix jakub ďurejea,∗, zdeněk prošek a,b, jan trejbala, pavel tesáreka a czech technical university in prague, faculty of civil engineering, thákurova 7, 166 29 prague, czech republic b czech technical university in prague, uceeb, třinecká 1024, 273 43 buštěhrad, czech republic ∗ corresponding author: jakub.dureje@fsv.cvut.cz abstract. the article describes plasma modifications of the surface of polyvinyl alcohol (pva) microfibers using oxygen and hydrogen plasma in order to improve the properties of the composite material containing modified microfibers, cement and recyclate. five different modification times 30, 60, 120, 240 and 480 seconds were applied. changes on fiber surface were detected by sem analysis, packed cell wettability measurement, and weight loss during modification. the selected durations of plasma treatment were chosen to produce test samples on which the modulus of elasticity was continuously measured and then bending and compression tests were performed. the measured values were compared with the reference samples. oxygen modified fibers behavior is more hydrophilic compare with reference fibers, but hydrogen modified fibres behave more hydrophobic than reference fibers. keywords: plasma fiber modification, fiber surface modification, hydrogen plasma, oxygen plasma, polyvinylalcohol fibers, pva fibers. 1. introduction cement composite materials are widespread in the construction industry due to their advantageous properties and relatively low cost. unfortunately, unreinforced cement composites are typical for their low tensile and bending strength, and brittleness [1]. in the field of cement composites, this disadvantage is reduced by using reinforcement. one of the possibilities of reinforcing cement composites is the use of scattered reinforcing fibers. as early as 3500 years ago, dispersed reinforcements in the form of straw to reinforce unburnt bricks were used. the article deals with synthetic fibers that have been used since the early 1990s. currently, synthetic fibers are among the most commonly used fibers for reinforcing cement composites in practice [2]. the cohesion of fibers with cement matrix in fiber-reinforced cement composites has a significant effect on its properties. fiber-matrix cohesiveness depends on the interfacial transition zone (itz) that transfers the tension to fibers [3]. to increase (or reduce) the coherence of the fibers with the matrix, plasma surface modification can be employed. plasma is referred to as the fourth state of matter. it consists of charged particles positive ions and negative electrons and neutral particles [4]. plasma fiber modifications process standardly use low-temperature plasma because of low material temperature resistance. plasma modifies fiber surfaces mechanically as well as chemically. with the increasing time of plasma modification process, changes on the surfaces are more significant [5],[6]. as reported in many studies, carbon fibers have been successfully modified for application into epoxy binders and polyetherimide (pei) composites [7],[8]. polypropylene fibers have been modified by plasma to apply colors and adhesives to their surface [9]. plasma modification has successfully increased the resistance of sisal fibers in alkaline environments [10]. in this work, polyvinyl alcohol (pva) microfibers were modified to achieve a slip-hardening behavior of the cement composite. therefore, it is necessary to reduce the chemical bond and increase the physical bond between the fiber surface and the matrix. considering that the process of oxygen plasma termination has a time-unstable character, ie., the modified wettability gradually returns to the reference values, but on the other hand the time-stable character of the physical modification, it is actually the ideal type of modification. the fiber has also been modified in hydrogen plasma, which on surfaces of some materials (diamond) induces hydrophobic behavior, which, along with mechanical modifications, could be ideal for these fibers. 2. materials polyvinylalcohol (pva) microfibers made by kuraray were chosen for modification using oxygen and hydrogen plasma. pva fibers, unlike most other types of polymeric fibers, have good wettability, but the physical bond with the cement matrix is insufficient. the mechanical properties of the fibers used are shown in table 1. surface modifications by oxygen plasma were performed using tesla vt 214 device (inductively stimulated plasma), those by hydrogen plasma were per1 http://dx.doi.org/10.14311/app.2019.21.0001 http://ojs.cvut.cz/ojs/index.php/app j. ďureje, z. prošek, j. trejbal, p. tesárek acta polytechnica ctu proceedings fibers company type length [mm] diameter [µm] tensile strength [mpa] modulus of elasticity [gpa] density [g/cm3] pva kuraray rec 15 × 8 8 40 1560 40 1,3 table 1. fiber properties. formed in the roth & rau ak 400 (capacitance stimulated plasma). a radio frequency source of 100 w was used for the both instruments. the samples were placed in the chamber of the device. that was subsequently evacuated to pressure less than 22 pa. subsequently, oxygen or hydrogen gas was applied into the chamber and the pressure increased to about 60 pa. then the radiofrequency plasma generator was triggered for 30 s, 60 s, 120 s, 240 s or 480 s. the temperature of the fibers in the chamber gradually increased during the plasma modification, reaching the temperature in the longest time around 50 ◦c. upon completion of the plasma modification, working gas was re-pumped from the chamber and subsequently aerated to atmospheric pressure. based on the results of these measurements, fibers were selected for the test specimens with a modification time of 480 s in oxygen plasma and 480 s in hydrogen plasma. after the modification, the fibers were immediately placed into the sealed bags to prevent air exchange and unpacked just before production of the samples. another fibers with a 480 s plasma oxygen treatment were stored in a laboratory environmental at 22 ± 1 ◦c and relative humidity 50 ± 2 % (table 2). designation fibers modification time [s] plasma gas storage pva 480 s pva 480 oxygen sealed bags pva h2 480 s pva 480 hydrogen sealed bags pva 480 s old pva 480 oxygen open bags table 2. fibers modification. a total of 4 sets which each contains 6 samples were made. the dimensions of the test samples were 40×40×160 mm. portland cement cem 42,5r (radotin) and finely ground concrete recyclate were used to produce the test specimens. the composition of all test samples was identical, differing only in the modifications of the fiber surfaces (table 3). designation cement [g] recyclate [g] water [g] pva ref 400 100 205 pva 480 s 400 100 205 pva h2 480 s 400 100 205 pva 480 s old 400 100 205 designation w/(c + r) fibers [%] volume fibers [g] pva ref 0,41 2 7,32 pva 480 s 0,41 2 7,32 pva h2 480 s 0,41 2 7,32 pva 480 s old 0,41 2 7,32 table 3. composition of the samples. 3. experimental methods and results all fibers were weighed by kern with measuring accuracy 0.1 mg before and after their plasma modification. the highest weight loss occurred in the case of fibers modified by oxygen plasma for 480 s. loss was equal to 0.58 % of their original weight. this was caused by the bombardment of working gas. based on the measured data, it can be concluded that the loss of mechanical properties of the fibers (reduction of the fiber cross-section) is almost negligible. the fiber surface was examined using the merlin zeiss electron microscope, which allows the observed object to be magnified up to 2,000,000 times. to observe objects under an electron microscope, the surface of the fiber needs to be conductive. to obtain the conductivity of the surface of the fibers, a thin layer of gold was sputtered on them. sprinkling was carried out using a quorum q150r es with an au thickness of 5 nm. mechanical changes on fiber surfaces relative to reference fibers were observed by electron microscopy at plasma modification time 240 s and 480 s (figure 1). fiber wettability was measured by an indirect method, similar to that called the packed-cell. because of the small fiber diameter, it was not possible to use direct fiber wettability measurement like for macrofibers. the amount of water on fibers after the fibers were dipped was measured. the fibers were placed in a small container with perforated bottom and lid. subsequently, the fibers were compressed using lead weights. the whole of this system was weighed by kern with accuracy 0.1 mg and then immersed up to the water for 30 seconds. then, the container was taken out of the water and weighed after 120 seconds. the container with lead weights was weighed as well. from the measured data, the percentage weight of the water in the container compared to the weight of the fibers was calculated in figure 2. based on the measurement results, fibers modified by oxygen plasma for 480 seconds and those modified by hydrogen plasma for the same time were selected for the production of test specimens. after modification, the fibers were stored in sealed bags or in open bags for 5 days in a laboratory environment at 22 ± 1◦c and a relative humidity of 50 ± 2%. the samples were tested by bending test with a three-point configuration and under pressure at 28 days of age. samples were loaded with a controlled displacement in a heckert model fp100 hydraulic press at a constant rate of 0.4 mm per minute. the remaining parts of the test 2 vol. 21/2019 plasma modification of polyvinyl alcohol microfibers figure 1. comparison of reference (up) and 480 s modified (down) fibre surface by oxygen plasma. figure 2. the weight of water between the fibers. specimens were tested at pressure with loading speed of 0.8 mm per minute. bending strength for samples with oxygen-modified fibers increased by 15 and 36 %, bending strength for samples which contains hydrogen plasma modified fibers decreased by approximately 20% in compare with reference samples (figure 3). compressive strength was reduced by approximately 5% for hydrogen-modified samples. 4. conclusion pva fibers modified by hydrogen plasma for 480 seconds have been shown to have their more hydrophobic behavior due to the chemical effect of this modification. the mechanical effect of this modification, as with the oxygen plasma modification of these fibers, almost does not occur and the loss of bending strength after reaching the maximum is almost the same as for the reference fibers (figure 4). for pva fibers, it may be appropriate to use hydrogen plasma due to its hydrophobic chemical effect, but it is necessary to significantly increase the mechanical effect of the hydrogen plasma modification. increase of the radiofrequency power or extension of the plasma modification time could increase the mechanical effect of modification. figure 3. comparison of flexural strength. figure 4. force-displacement diagram of bending test. acknowledgements this outcome was supported by the ctu in prague under no. sgs16/201/ohk1/3t/11 and by the ministry of education, youth and sports within national sustainability 3 j. ďureje, z. prošek, j. trejbal, p. tesárek acta polytechnica ctu proceedings programme i, project no. lo1605. references [1] z. keršner. křehkost a lomová mechanika cementových kompozitů., 2005. brno. habilitační práce. vut v brně. [2] d. bharathi. the history of fibre reinforcement. [online], 2015. [cit. 2017-10-31], https://www.linkedin.com/ pulse/history-fibre-reinforcement-bharathi-d. [3] a. bentur, s. mindess. fibre reinforced cementitus composites, 2007. 2nd ed., new york: taylor, isbn 978-020-3088-722. [4] rose, margret. definition plasma. [online], 2017. [cit. 2017-10-31], http://whatis.techtarget.com/definition/plasma. [5] s. r. sabren. cold gas plasma surface modification – optimize plastics bonding adhesion. [online], 2012. [cit. 2017-10-31], http://www.adhesionbonding.com/2012/ 04/30/cold-gas-plasma-surface-modification optimize-plastics-bonding-adhesion/. [6] d. chrysostomou. plasma surface modification for the biomedical industry., 2009. presentation. [7] j. moosburger-will, m. bauer, f. schubert, et al. methyltrimethoxysilane plasma polymerization coating of carbon fiber surfaces. surface and coatings technology pp. 223–230, 2017. [cit. 2018-01-06], doi:10.1016/j.surfcoat.2017.01.017. [8] l. eung-seok, l. choong-hyun, c. yoon-soo, et al. effect of hydrogen plasma-mediated surface modification of carbon fibers on the mechanical properties of carbon-fiber-reinforced polyetherimide composites. composites part b: engineering 2016. [cit. 2018-01-06], doi:10.1016/j.compositesb.2016.10.088. [9] r. pulíček. optimalizace procesu hydrofilizace polypropylenového textilu pomocí dbd atmosférické plazmy, 2016. liberec. diplomová práce, technická univerzita v liberci. [10] b. barra, s. santos, p. bergo, et al. residual sisal fibers treated by methane cold plasma discharge for potential application in cement based material. [online], 2015. [cit. 2018-01-06]. 4 https://www.linkedin.com/pulse/history-fibre-reinforcement-bharathi-d https://www.linkedin.com/pulse/history-fibre-reinforcement-bharathi-d http://whatis.techtarget.com/definition/plasma http://www.adhesionbonding.com/2012/04/30/cold-gas-plasma-surface-modification http://www.adhesionbonding.com/2012/04/30/cold-gas-plasma-surface-modification http://dx.doi.org/10.1016/j.surfcoat.2017.01.017 http://dx.doi.org/10.1016/j.compositesb.2016.10.088 acta polytechnica ctu proceedings 21:1–4, 2019 1 introduction 2 materials 3 experimental methods and results 4 conclusion acknowledgements references acta polytechnica doi:10.14311/app.2020.27.0013 acta polytechnica 27(0):13–17, 2020 © czech technical university in prague, 2020 available online at http://ojs.cvut.cz/ojs/index.php/app microstructure and microhardness evaluation for nicraly materials manufactured by spark plasma sintering and plasma spraying iosif hulka∗, radek mušálek, františek lukáč, jakub klečka, tomáš chrástka czech academy of sciences, institute of plasma physics, za slovankou 1782/3, 182 00 prague 8, czech republic ∗ corresponding author: hulka@ipp.cas.cz abstract. nicraly deposited by different thermal spraying methods is commonly used as the bond coat material in thermal barrier coatings (tbcs). in the present study, two experimental coatings were deposited by hybrid water stabilized plasma (wsp-h) and radio frequency inductively coupled plasma (rf-icp) using the same feedstock powder. spark plasma sintering (sps) was used to manufacture a compact nicraly from the same feedstock powder as a reference material. microstructure, internal oxidation, phase characterization and quantification of the mechanical behaviour in terms of microhardness were studied. the investigations clearly showed microstructural and mechanical differences between the nicraly samples manufactured by different plasma technologies. the results confirmed that sps and rf-icp provide dense structures with no oxides due to the fabrication under protective atmosphere and similar mechanical properties. thus, rf-icp may be used for deposition of very dense coatings with microstructure and hardness comparable to compacted materials prepared by sps. keywords: microhardness, microstructure, nicraly, plasma spraying, spark plasma sintering. 1. introduction the degradation of materials during service of turbine components in the field of energy production and aircraft industry is one of the major concerns in materials science research [1]. applying thermal barrier coatings (tbcs) on the surface of the turbine components is a common method to protect the substrate material, which is usually manufactured from a ni-based superalloy, against high temperature oxidation, thermal shocks and hot corrosion. the tbcs usually consist of i) bond-coat, which is used to reduce thermal stress at the coating-substrate interface while it improves the coatings adhesion to the substrate and provides oxidation protection, and ii) ceramic top-coat which ensures a low thermal conductivity acting like an insulating layer [2]. the performance of tbcs is strongly influenced by the properties and chemical composition of the bond-coat. nicraly-based alloy is one of the most common bond-coats used due to good oxidation resistance and good adhesion to the substrate [3]. there are several thermal spray methods to prepare nicraly bond-coats. among these methods, plasma spraying (ps) is one of the most common deposition technique. hybrid water stabilized plasma spraying (wsp-h) is a new deposition method providing high plasma enthalpy allowing very high feed rates (tens of kg/h). the wsp-h plasma torch can operate at power up to 180 kw consuming about 15 slpm of argon making it ideal for cost-effective spraying of large scale components [4]. wsp-h spraying is typically carried out in open-air atmosphere. access of oxygen to hot metal may cause its intensive oxidation. another plasma method which might be suitable for the deposition of novel nicraly bond-coats, is radio frequency inductively coupled plasma (rf-icp) which is a coating deposition method that operates in a controlled atmosphere eliminating internal oxidation of the coating during spraying. for both technologies, development of lamellar microstructure, resulting from consecutive impacts of molten droplets, is typical. besides chemical composition, properties of the coating are dependent mainly on the quality of internal interfaces between these lamellae (so called splats), presence of voids (pores and cracks) and impurities (such as oxides). on the other hand, spark plasma sintering (sps) enables fabrication of bulk materials in a controlled atmosphere. this process uses pulsing electrical discharge and pressure to compact powders. the process allows sintering of the powder particles providing materials with homogenous microstructure and free of pores. the drawback of the lab-scale sps method is the manufacturing of rather small-sized samples typically measuring 20-50 mm in diameter and several millimetres high. nevertheless, such coupons are more than enough for materials testing. the oxidation of nicraly is an important issue because the oxides formed within the bond-coat during deposition may for example hinder the formation of protective oxide film and thereby decrease the hightemperature oxidation resistance as reported by s.f. chen at al. [5]. in the present study, the microstruc13 http://dx.doi.org/10.14311/app.2020.27.0013 http://ojs.cvut.cz/ojs/index.php/app i. hulka, r. mušálek, f. lukáč et al. acta polytechnica figure 1. microstructure of samples in cross-section: low magnification of sps sample (a), low magnification of wsp-h coating (b), low magnification of rf-icp coating (c), high magnification of sps sample (d), high magnification of wsp-h coating (e) and high magnification of rf-icp coating (f). ture, phases and the mechanical behaviour in terms of micro-hardness of sps bulk material, wsp-h and rf-icp coatings obtained from the same feedstock nicraly powder were studied and compared in order to evaluate influence of microstructural features, in particular oxides and pores on their mechanical properties. 2. experimental 2.1. samples preparation for the preparation of sps sample and plasma sprayed coatings, a commercial nicraly powder (amperit 413.006, h.c. starck gmbh, germany) was used with a grain size in the range of 45-125 µm. according to the manufacturer, the powder contains 21-23% cr, 9-11% al, 0.8-1.2% y and ni in balance. spark plasma sintering (sps) process was carried out using a sps system type sps 10-4 (thermal technology llc, usa). the sintering was done at 1000 °c at 80 mpa pressure for 4 minutes. the cooling rate was 100 °c/min. wsp-h 500 plasma torch (projectsoft hk, a.s., czech republic) operated at 400 a (∼120 kw) and with a feeding distance (fd) of 75 mm and 200 mm stand-off distance (sd) was used to deposit the wsp-h coating in the open-air atmosphere. the torch was mounted on a 6-axis robot arm. the grit-blasted steel substrates (20 x 30 x 2.5 mm) were mounted on a rotating carousel. cooling of the samples was carried out by air knives in order to prevent overheating of the substrate during deposition. the rf-icp coating was prepared using tekna 15 plasma system (tekna, canada). the torch was operated at 12 kw power using argon as protective atmosphere in the deposition chamber. the plasma was obtained from the mixture of ar+h2 and ar was used also as a carrier gas for the powder injection into the torch. all the coatings were deposited on grit-blasted hastelloyx substrate (∅25 mm, thickness 4.9 mm). 2.2. samples characterization cross-sections of all samples were prepared following a standard metallographic procedure using a metallographic saw secotom-50 (struers, denmark) with diamond blade, vacuum epofix epoxy impregnation (struers, denmark) and semi-automated polishing system tegramin-25 (struers, denmark). vickers hardness was measured by a qness q10a+ microhardness tester (qness gmbh, austria) on polished crosssections with 1 kgf load. microstructural investigation and observation of indents prepared during hardness measurements were carried out on a evo ma 15 (carl zeiss smt, germany) scanning electron microscope (sem) using back scattered detector (bsd) in low vacuum mode. the local chemical composition of the samples was determined by energy dispersive x-ray microanalysis (eds, xflash 5010, bruker) installed in the sem. phase composition was evaluated by x-ray diffraction (xrd) using d8 discover diffractometer (bruker, germany). 3. results and discussions the cross sections of the samples are presented in figure 1 at low and high magnifications. in the sps sample (fig. 1a,d), neither pores or cracks are observed. the microstructure consists mainly of the βnial intermetallic (dark grey) phase and the γ-ni(cr) 14 vol. 27/2020 microstructure and microhardness of nicraly figure 2. top-surface morphology of as-sprayed wsp-h (a) and rf-icp (b) coatings. solid solution (light grey), as measured by eds. wsph coating shows structure typical for atmospheric plasma sprayed coatings consisting of splats, microcracks, un-bonded interfaces between splats and pores. the wsp-h coating (fig. 1b,e) presents a rough surface due to the formation of asperities during spraying. these may be in beneficial in spraying of the whole tbc as they may improve adhesion of the top-coat. internal oxidation took place during spraying which can be identified as dark areas within the coating. the dark areas are typical al rich oxides and were found to be formed preferentially at the interfaces between metallic splats. some cracking of these oxides was observed. the metallic matrix is enriched in ni, identified as light grey areas by eds. the porosity results mainly from the splashing effect of the splats overlapping during spraying or due to gas inclusions between splats. with rf-icp (fig. 1c,f) method, a dense coating with smooth surface, good bonding to the substrate, negligible degree of porosity and dendritic structure was obtained. the dendritic structure consists of β-nial phase dendrites (dark grey) rich in al and γ-ni(cr) phase as interdendritic solid solution (light grey). the coating shows no oxidation, just residues of the grit-blasting alumina may be observed on the substrate-coating interface. figure 2 shows the surface morphology of the wsph and rf-icp sprayed coatings. the rough coatings deposited by wsp-h presents deformed and spread splats, re-solidified droplets and a visible interface between the metallic phase and oxides. during spraying, the molten powder particles hit the surface and flattened. within the oxide splats, some cracking was observed. the rf-icp coating has a smooth surface consisting of highly flattened metallic splats without any considerable oxidation or cracking. figure 3 shows the xrd diffractograms for the original feedstock powder and deposited coatings. the powder consists of two main phases: a γ-ni (fcc) phase rich in cr which contains the dispersed β-nial (bcc) phase which is typical for the nicraly powder, as reported as well e.g. by m. m. khorramirad et al. [6]. the sps and rf-icp samples have similar patterns and consist of two major phases: γ-ni phase rich in figure 3. xrd patterns for nicraly powder, sps compact and coatings obtained by plasma technologies. cr and a β-nial inter-metallic phase, same as the feedstock powder. the γ’-ni3al phase was identified in sps and rf-icp samples as well. the wsp-h coating did not show presence of this phase. al2o3 and cr2o3 based oxides were identified instead. for each prepared sample, cross-sectional microhardness measurements were measured by vickers intender with 1 kgf load. the mean value of microhardness was calculated as an average of 16 indentations made in the middle of the coating thickness and the results are presented in figure 4. the results showed values of 546.8 ± 8.5 hv1 for the sps bulk material, 280.0 ± 37.1 hv1 for the wsp-h coating and 556.6 ± 27.9 hv1 for the rf-icp coating. the difference between the sps and rf-icp coatings is subtle, however it can be noticed that the sps sample presents lower fluctuations in the measurements which might be attributed to a more homogenous composition of the material. the wsp-h coating has almost 50% lower hardness value, compared to the previous mentioned samples and also the highest fluctuation among the results. this fact is associated with a higher degree of porosity, presence of cracks and brittle oxides. 15 i. hulka, r. mušálek, f. lukáč et al. acta polytechnica figure 4. measured values of cross-sectional hardness in half thickness of coatings/sps sample. figure 5. sem micrographs of typical indents for sps sample (a), wsp-h coating (b) and rf-icp coating (c). figure 5 shows sem images of typical indents of analysed samples. the uniformity of microstructure and lack of cracks network formation in the vicinity of the indents were noticed in the sps and rf-icp samples indicating that both materials have a good ductility. the microstructure was fine enough to include a representative number of microstructural features present in these two materials, which may explain a low scatter in the hardness values. the wsp-h coating developed cracks in the oxide rich areas (dark areas) due to their inherently brittle behaviour. mutual sliding of the splats was also observed. 4. conclusions the aim of the paper was to compare at microscopic level mechanical behaviour of nicraly coatings and bulk material prepared by different plasma methods from the same commercially available feedstock powder. the morphology, phases and mechanical performances were studied and the main conclusion were as follows: • using different plasma techniques, nicraly bulk material and coatings with potentially interesting microstructures can be obtained; • due to high achievable feed rate, wsp-h technology can be used for efficient deposition of coatings on large areas. formation of splats with distinguishable splat boundaries was observed. spraying in the open air led to considerable internal oxidation of the coatings and formation of brittle oxides. as a result, lower ductility and reduction of hardness value by 50% compared to the other samples was observed. on the other hand, the surface presents a rough surface which is favourable for deposition of top coat; • the results confirmed that sps and rf-icp samples present dense structures consisting mainly by β-nial intermetallic phase and the γ-ni(cr) solid solution with no oxides due to the protective atmosphere in both processes. nevertheless, size of the components which may be processed by sps and rf-icp techniques is limited by the size of the sintering tool and the deposition chamber, respectively; • from mechanical behaviour point of view, the sps and rf-icp samples showed similar behaviour, thus rf-icp may be used for deposition of very dense coatings with microstructure and hardness comparable to those prepared by sps. acknowledgements the authors gratefully acknowledge czech science foundation for financial support of the present research under project no ga19-14339s entitled "high temperature preparation of advanced refractory materials by inductively 16 vol. 27/2020 microstructure and microhardness of nicraly coupled plasma in controlled atmosphere". references [1] m. mayoral, j. andrés, m. bona, et al. aluminium depletion in nicraly bond coatings by hot corrosion as a function of projection system. surface and coatings technology 202(9):1816 – 1824, 2008. doi:10.1016/j.surfcoat.2007.07.068. [2] r. musalek, o. kovarik, j. medricky, et al. fatigue testing of tbc on structural steel by cyclic bending. journal of thermal spray technology 24(1):168–174, 2015. doi:10.1007/s11666-014-0180-4. [3] j. song, k. ma, y. li, et al. high temperature microstructure and microhardness evolution in dense nicraly bulk material fabricated by spark plasma sintering. materials science and engineering: a 528(7):3210 – 3217, 2011. doi:10.1016/j.msea.2010.12.094. [4] j. medricky, r. musalek, m. janata, et al. cost-effective plasma spraying for large-scale applications. in itsc 2018 proceeding of international thermal spray conference. 2018. [5] s. chen, s. liu, y. wang, et al. microstructure and properties of hvof-sprayed nicraly coatings modified by rare earth. journal of thermal spray technology 23:809–817, 2014. doi:10.1007/s11666-014-0097-y. [6] m. khorramirad, m. rahimipour, s. hadavi, k. shirvani. preoxidation of bond coat in in-738lc/nicraly/lamgal11o19 thermal barrier coating system. ceramics international 44(18):22080 – 22091, 2018. doi:10.1016/j.ceramint.2018.08.318. 17 https://doi.org/10.1016/j.surfcoat.2007.07.068 https://doi.org/10.1007/s11666-014-0180-4 https://doi.org/10.1016/j.msea.2010.12.094 https://doi.org/10.1007/s11666-014-0097-y https://doi.org/10.1016/j.ceramint.2018.08.318 acta polytechnica 27(0):13–17, 2020 1 introduction 2 experimental 2.1 samples preparation 2.2 samples characterization 3 results and discussions 4 conclusions acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2020.26.0071 acta polytechnica ctu proceedings 26:71–75, 2020 © czech technical university in prague, 2020 available online at https://ojs.cvut.cz/ojs/index.php/app long-time investigation of cement pastes with micronized old concrete, influence of different type of mixing approach on mechanical properties zdeněk prošeka,b,∗, pavel tesáreka a czech technical university in prague, faculty of civil engineering, department of mechanics, thákurova 7, 166 29 prague 6, czech republic b czech technical university in prague, uceeb, třinecká 1024, 273 43 buštěhrad, czech republic ∗ corresponding author: zdenek.prosek@fsv.cvut.cz abstract. the article focuses on the impact of different type of mixing approach on resulting mechanical properties of cement composites. the monitored parameters of the mixing approach were the speed of mixing and the method of mixing the individual components of the fresh mixture. the mechanical properties of the resulting composites were the main comparative parameter. the determined mechanical properties were dynamic modulus of elasticity, dynamic shear modulus, flexural strength and compressive strength. dynamics moduli were detected using non-destructive resonance method during the whole experiment and compressive and flexural strength were determined by destructive method for 28 and 129 days old samples. testing samples had dimensions equal to 40× 40× 160 mm. keywords: cement composites, recycled old concrete, mixing approach, mechanical properties, long-time development. 1. introduction at present, the cement based composite materials (in the form of monolithic reinforced concrete, prefabricated elements, fittings etc.) present the most widespread and widely used building material. however, from the perspective of sustainable development, three basic components are needed for production – cement, sand and water. their extraction, treatment and processing represent a burden on nature and the environment [1–3]. in particular, cement production represents 7 % of global co2 production and large amount of nox. water consumption is also not desirable, it would be appropriate to reduce it. in many locations (e. g. the middle east), there is lack of construction sand or cement, the problem is solved by transporting it from distant locations, for example, saudi arabia imports suitable sand from australia and israel imports cement from turkey [4–6]. on the other hand, there are alternative options in the form of green composite or green concrete using alternative sources of raw materials, even secondary ones, eg. old waste concrete. the amount of waste old concrete is growing year after year and the need for its efficient use with it [7, 8]. nowadays it is relatively common to use recycled aggregate from concrete into new concrete, mostly fraction 1/16 mm [9, 10]. there are several types of treatments to suitably adjust the surface of recycled aggregate to function properly in new concrete, but even these solutions have some limitations. on the other hand, fraction 0/1 mm of recycled concrete remains out of focus whether the professional or corporate (construction industry) and this fraction is estimated to be up to 20 wt. % of recycled concrete. after suitable treatment, this material can be advantageously used in cementitious composite materials, for example by mechanical-chemical activation [11, 12]. the properties of the resulting composite are not only directly dependent on the grain shape which is not suitable in the case of recycled concrete but also on the size of the interfacial transition zone (itz). these effects have a negative influence on the resulting properties [13]. recently, there have been studies in which the itz can be partially eliminated using the correct mixing approach, i.e., the correct procedure of mixing of the components of the cement composite and the mixing speed [14, 15]. unlike mentioned researches which they focus on recycled coarse aggregate, we focused on mechanical activated old concrete. this is fine aggregate with modified grain shape by using high-speed milling process [16]. the micronizing of the old cement paste into fine particle results in uncovered unhydrated clinker minerals, which can further hydrate upon contact with water [17]. 2. materials and samples cement pastes samples were created to tests the mechanical properties. cement pastes composed of 50 wt. % portland cement cem i 42.5r from radotin and 50 wt. % fine recycled old concrete (roc) has been prepared for investigating the impact of different mixing approach. the cem i 42.5r is ordinary portland cement with high content of c3s (approximately 75 %) and low content of c2s and c3a (< 8 %). roc was obtained from an old construction concrete (sleep71 https://doi.org/10.14311/app.2020.26.0071 https://ojs.cvut.cz/ojs/index.php/app zdeněk prošek, pavel tesárek acta polytechnica ctu proceedings figure 1. approach of mixing fresh mixtures. ers and columns with fraction 0/1 mm), which was micronized using a high-speed mill from lavaris ltd. (libčice, czech republic). the micronized roc with fraction 0 − 0.5 mm had mean grain size of 61.8 microns and more than 90 % of all grains were less than 281 microns water ratio was equal to 0.35. the difference between individual samples was in the approach of mixing fresh mixtures. two different components were mixed at 132 rpm for 240 seconds in the first step of the mixture preparation. in the second step, the remaining third component was added and the mixtures were mixed for 240 seconds at two different speeds, namely 132 rpm and 421 rpm. the mixing approaches are shown in figure 1. mixtures made according to figure 1 were cast to the rectangular molds with dimensions 40× 40× 160 mm. after casting, these samples were kept in the molds for 24 h at the laboratory with the temperature 22 ◦c. each set consisted of 6 samples. after demolding, these samples were deposited in in a water bath at 22 ± 1 ◦c. 3. experimental methods investigated mechanical parameters were developments of dynamic modulus of elasticity and dynamic shear modulus and values of flexural strength and compressive strength. the development of the dynamic modulus of elasticity and dynamic shear modulus was measured non-destructively in the first 129 days after the start of cement hydration, namely at 10th, 14th, 28th, 48th, 69th, 129th day. the flexural and compressive strength values were measured destructively on 28 and 129 day old samples. measurement of the dynamic modulus of elasticity was carried out by using resonance method, namely by the measurement apparatus from brüel & kjær. the measurement line consisted of a measuring unit type 3560-b-x12, impact hammer type 8206, an acceleration transducer type 4519-003 53422 and software pulse labshop. the dynamic modulus of elasticity calculated based on the measured basic longitudinal natural frequency and basic flexural natural frequency of the samples. the dynamic shear modulus calculated based on the measured basic torsional natural frequency of the samples. the flexural and compressive strength were determined by using destructive methods on the device fhf strassentest. the flexural strength was determined by a three-point bending test. the testing of flexural strength was displacement controlled at a constant rate of 1 mm/min. the distance between supports for three-point bending test was equal to 100 mm. the compressive strength was determined by using uniaxial compressive test. the uniaxial compressive test was performed on the broken halves of the samples after bending test with effective dimensions of 40× 40× 40 mm. the testing of compressive strength was displacement controlled at a constant rate of 5 mm/min. 72 vol. 26/2020 long-time investigation of cement pastes with micronized old concrete ... 6 6.5 7 7.5 8 9 16 23 30 37 44 51 58 65 72 79 86 93 100 107 114 121 128 d yn am ic s h ea r m o d u lu s [g p a] time [days] rw+c_s rw+c_f cr+w_s cr+w_f cw+r_s cw+r_f figure 2. time dependence of dynamic shear modulus (with standard deviation). 15 16 17 18 19 20 21 9 16 23 30 37 44 51 58 65 72 79 86 93 100 107 114 121 128 d yn am ic m o d u lu s o f el as ti ci ty [ g p a] time [days] rw+c_s rw+c_f cr+w_s cr+w_f cw+r_s cw+r_f figure 3. time dependence of dynamic modulus of elasticity (with standard deviation). 0 1 2 3 4 5 6 7 8 rw+c_s rw+c_f cr+w_s cr+w_f cw+r_s cw+r_f fl ex u ra l s tr en gt h [ m pa ] 28 days old samples 129 days old samples figure 4. flexural strength of the samples (with standard deviation). 73 zdeněk prošek, pavel tesárek acta polytechnica ctu proceedings 0 10 20 30 40 50 60 70 rw+c_s rw+c_f cr+w_s cr+w_f cw+r_s cw+r_f c o m p re ss iv e st re n gt h [ m pa ] 28 days old samples 129 days old samples figure 5. compressive strength of the samples (with standard deviation). 4. results and discussion results of non-destructive testing are shown in figure 2 and figure 3. all samples must be measured and weighed before evaluating dynamic moduli. from the measured values it was possible to determine the bulk density. all tested mixtures had approximately the same bulk density, namely 1926 ± 24 kg/m3. the results of dynamic moduli show the positive effect of the mixing speed of the fresh mixture on the dynamic modulus of elasticity and the dynamic shear modulus. differences between the highest and lowest values of moduli for 28 days old samples were between samples cr+w_s and samples cr+w_f, namely 0.5 gpa for dynamic modulus of elasticity and 0.3 gpa for dynamic shear modulus. these differences are within the accuracy of the method. the highest increase of dynamic modulus of elasticity and dynamic shear modulus is in the first 14 days of the sample age. furthermore, the increase in values is gradual in the next 34 days. after 48 days, there are minimal changes of dynamic moduli. the flexural strength of 28 days old samples was approximately same value on all testing samples (figure 4). differences were up to the size of the value standard deviation. value of flexural strength was 4.5 mpa for 28 days old samples. for the 129 day flexural strengths, there are greater differences between the individual mixtures than for the 28 day flexural strengths. highest value of flexural strength had sample rw+c_f, namely 6.5 ± 0.8 mpa. lowest value of flexural strength had sample cw+r_f, namely 3.1 ± 0.7 mpa, ie more than double the difference. in contrast of it, the compressive strength values (figure 5) were different and they have the same trend as the values of dynamic modulus of elasticity. the greatest value of compressive strength for 28 days old samples had rw+c_f, namely 40.8 ± 2.8 mpa and lowest value of compressive strength had cw+r_f, namely 28.1 ± 1.1 mpa. the difference between the compressive strength of 28 days old samples and 129 days old samples was approximately the same for all mixtures, namely 18 mpa. the increase in compressive strength between 28 and 192 days is due to a combination of two effects. the first effect is the hydration of c2s, which is contained in cem i 42.5r (content approximately 8 %) and the second effect is the content of unhydrated clinkers in the roc, where this content can be up to 10 % [16]. value of standard deviation was approximately 5 % of compressive strength value in all tested cases. the results show that the mixing speed has a positive effect on the resulting compressive strength, with a difference of approximately 5 mpa for slow and fast mixing of rw + c and 4 mpa for slow and fast mixing of cr + w. this is the effect of better homogenization of the fresh mixture by fast mixing. the mixture rw+c (the old concrete was first mixed with water and then with the cement) had the highest compressive strength values, namely 5 mpa higher than the mix where the old concrete was mixed with the cement at first. the mixture where the cement was mixed with water at first had the lowest compressive strength by about 5 mpa lower than in the case of cement mixing with the old concrete at first and 10 mpa lower than the mixture where the recycle was mixed with water at first. it is generally agreed that as the pasteaggregate bond strength increases (reducing of itz), the strength of cement composites also increases [18]. so the process of mixing the individual components of the cement composite affects the itz zone between the cement matrix and the recycle grains. results of the work are in agreement with the statement of several authors, because the size of itz zone depends on the approach of mixing not only in recycled coarse aggregate but also in micronized recycled old concrete. this effect can be attributed to the porous nature of the recycled aggregate, and therefore the premix process may fill some pores and cracks, resulting in an improved inter transition zone around the recycled aggregate and hence higher strength compared to the traditional mixing approach [14]. 74 vol. 26/2020 long-time investigation of cement pastes with micronized old concrete ... 5. conclusion this work was focused on the influence of different type of mixing approach on mechanical properties of cement composites. the researched samples were composed of portland cement (50 wt. %) and micronized recycled old concrete (50 wt. %). the difference between individual samples was in the approach of mixing fresh mixtures. based on the results, it can be concluded that: • approach of mixing fresh mixtures had influence on mechanical properties of resulting cement composites by reducing of itz, • the greatest effect of type of mixing approach can be seen in the sample where water and micronized old recycled concrete were mixed in the first step and in the second step, the cement was added, • the mixing speed has a positive effect on the resulting compressive strength, the change of mixing speed from 132 rpm to 421 rpm resulted in an increase of compressive strength of 5 mpa, • there is an increase of compressive strength due to hydration of c2s contained in portland clinker of reference cement and in non-hydrated clinker of micronized recycled old concrete. in the future, the research will focus on confirming influence on approach of mixing on itz by using scanning electron microscope with x-ray microanalysis, namely an energy spectrometer. acknowledgements this paper was financially supported by the ga cr research a project under the number 17-06771s and by czech technical university in prague under no. sgs project sgs19/148/ohk1/3t/11. the authors also thank the company lavaris ltd, where a high-speed mill was used for micronizing of the recycled old concrete. references [1] j. schoon, k. de buysser, i. van driessche, n. de belie. fines extracted from recycled concrete as alternative raw material for portland cement clinker production. cement and concrete composites 58:70–80, 2015. doi:10.1016/j.cemconcomp.2015.01.003. 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[18] s. mindess, j. young, d. darwin. concrete. prentice hall, 2003. 75 https://doi.org/10.1016/j.cemconcomp.2015.01.003 https://doi.org/10.1016/j.conbuildmat.2014.04.117 https://doi.org/10.1016/j.wasman.2018.04.024 https://doi.org/10.1080/014461997373105 https://doi.org/10.1016/j.conbuildmat.2007.12.006 https://doi.org/10.1016/j.conbuildmat.2011.12.074 https://doi.org/10.4028/www.scientific.net/amr.1054.262 https://doi.org/10.1016/j.conbuildmat.2018.12.227 https://doi.org/10.1016/j.conbuildmat.2014.08.030 https://doi.org/10.1016/s0008-8846(99)00105-2 https://doi.org/10.1016/j.cemconres.2004.10.025 https://doi.org/10.4028/www.scientific.net/amr.1144.22 acta polytechnica ctu proceedings 26:71–75, 2020 1 introduction 2 materials and samples 3 experimental methods 4 results and discussion 5 conclusion acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2020.29.0025 acta polytechnica ctu proceedings 29:24–28, 2020 © czech technical university in prague, 2020 available online at http://ojs.cvut.cz/ojs/index.php/app bim – an application in geotechnical engineering ivan vaníček∗, jan pruška, daniel jirásko czech technical university in prague, faculty of civil engineering, department of geotechnics, thákurova 7, 166 29 prague 6, czech republic ∗ corresponding author: ivan.vanicek@cvut.cz abstract. bim model represents a significant step forward within the frame of the overall process of construction. this new approach goes from the assumption that up to date praxis, which is represented by different sets of 2d or 3d drawings of drawings, technical reports and calculations does not cover all problems of the above mentioned overall process of construction. basic principles of bim will be discussed, not only from the view of 3d models of the structure design. the utilization of the bim model during life time structure expectancy will be stressed from the initial phase (investigation, design) via the phase of structure execution, maintenance and ending with structure demolition at the end of life time expectancy. specificity of the geotechnical engineering is in this entire process fundamental, as each construction is in the interaction with ground. 3d model of the ground (geotechnical model) is therefore one of the basic individual parametric elements from which bim model consist. ground model is time dependant as geological profile and geotechnical properties are refinement during each phase of ground investigation as well during geotechnical structure construction. final 3d ground model together with 3d model of geotechnical structure represents a first significant step of the overall bim model. in the case of underground or earth structures such output can be primordial element of the bim model with parametric elements around it. finally some other possibilities or practical applications are mentioned. keywords: bim, geotechnical engineering, ground model, model of the realized geotechnical structure, sustainable construction. 1. introduction bim – building information modelling (management) as a tool for the process of optimizing construction activities is connected over the most recent period with a more general digitalization evident throughout all industries. the term průmysl 4.0 (industry 4.0) or stavebnictví 4.0 (building 4.0) is now standard for most elements of building engineering. as from now more attention is being devoted also in civil engineering and some significant czech investors and construction companies are demanding that they receive data in the form of the bim model in order to be able to digitalize the process of construction. however the main condition for the application of the bim model in the wider profession is a clear understanding of bim principles. there are many different aspects to a definitive bim. the first one, that bim is simply a 3d model of the engineering structure, is not correct as it does not take into account up to date instruments of information modelling concerning engineering structures and modern communication methods. the most widely used definition was specified by the american national committee for the bim standard: “building information modelling (bim) is a digital presentation of physical and functional characteristics of a facility. a bim is a shared knowledge resource for information about a facility forming a reliable basis for decisions during its life-cycle, so defined as existing from earliest conception to demolition” [1]. in other words bim can be understood as a process for optimization of the engineering structure right through conception, design, execution and operation. by this approach the 3d computer model is a fundamental part of bim but however is enlarged by additional information on such matters as time, financial expenses, structure maintenance etc – with new so called 4d, 5d, and generally nd models. 2. the situation in the czech republic the decree of the government no. 82: “a concept for the implementation of the bim model in the czech republic” instigated september 25 2017 can be taken as the basis of bim acceptance. the main driver is the ministry of industry and trade which accredited the czech agency for standardization (cas) responsible for the realization of this governmental decree. some results of cas activities are described in [2]. from this analysis some basic reflections, facts and expected developments can be mentioned. from the reflections there can be cited “bim is about change – respectively processes, ideas, working habits, and “ the improvement of cooperation, information sharing along with an interrelationship cultivation that helps the higher effectiveness of engineering structures projects”. from the actual facts, six 25 http://dx.doi.org/10.14311/app.2020.29.0025 http://ojs.cvut.cz/ojs/index.php/app i. vaníček, j. pruška, d. jirásko acta polytechnica ctu proceedings working groups can be mentioned and some results are presented at www.koncepcebim.cz. activities of cas are coordinated in a european framework by positive participation in cen tc 442 – building information modelling (bim). the advantages of the bim are now primarily emphasized with respect to the practical application for engineering facilities of buildings, e.g. in articles [3] and [4]. the authors of the second paper dealing with building of a hospital in náchod stress the ratio of the construction to the operating charges for the expected life time expectancy of the building which they estimate to be 20 : 80. at the same time of course they are stressing the main idea: “the more quality we buy at the beginning, the less we spend later on”. naturally, this should be valid also for civil engineering structures such as transport infrastructure. when the transport engineering structure is made to be more robust, then less should be needed in the future for maintenance, reconstruction or on the restoration of services after natural hazards such as e.g. floods, rock falls; as any restraint of services can be included into additional expenses. the third note refers to the activities of sfdi – governmental fund of transport infrastructure which is responsible for bim implementation for transport engineering. procedures of sfdi are very important with the respect to the above-mentioned governmental decree according to which all significant funded projects disbursed from the government budget should be contracted within the bim regime from 2022. sdfi, currently in september 2019, approved the provisional version of the document “directive for bim pro structures of transport infrastructure – data standard – for pdps – project documentation of structure realization (execution)”. in this provisional version attention is devoted to the project documentation for structure execution, and therefore up to some extent it is outwith the main aim of the bim – to interconnect previous steps of the project documentation, even when some parts of the dur (documentation for planning inquiry), dsp (documentation for building permit) are mentioned in annexes 1 and 2 dedicated to road and railway structures. a “directive” can specify rules for the collection of data for bim in such a way that it can be used by owners, designers, contractors in all phases of design, execution and transport infrastructures maintenance. the data standard is based on the open data format ifc, see čsn en iso 16739 (730100) from april 2017. the ifc data format is playing the role of the coordination model of construction, which is composed of the individual construction sets, operational sets or engineering objects. this co-ordination model is used for the mutual coordination of the fragmental models, for the detection of collisions, in all construction display, for display of individual phases of the construction process and also for creation of cross sections. the co-ordination model is in principle an individual set, which contains the fragmental models. 3. bim and geotechnical engineering geotechnical engineering has an indispensable role in bim , as all structures are in some interaction with ground and this interaction is provided by foundation structures – fig. 1. the term structure is composed not only of the upper structure but also of foundation structures, including ground which is influenced by the total construction and includes a view of stresses, deformation or permeability. fragmental models for which the geotechnical engineering is responsible should therefore consist of the ground model, the geotechnical design model, the calculation model and also by a model of the real execution of the geotechnical structure with its connection to the ground model. these fragmental models from which all others models proceed can bring a significant recognition of the role of geotechnical engineering in the whole construction process. at the same time these fragmental models can lead to a more significant utilization of the basic principles of, and demands on, geotechnical structures, which are in europe specified in sets of eurocodes, initially in eurocode (ec 7) geotechnical design. figure 1. interaction of structure and subsoil all the above mentioned is valid for foundation structures, see fig. 2.a., but similarly, if not more significantly, for other geotechnical structures such as earth structures (for transport, water or environmental engineering) or underground structures (tunnels, underground repositories, metro. . . ) as these geotechnical structures can even be the main element of the solved construction, fig. 2.b. the significance of fragmental geotechnical models stands front rank as the main project idea in the phase of alternative solutions of the construction situation (placements) that should arise from the first phases of the ground (geological) model. this first phase of the geotechnical investigation is denoted as desk study and uses all previous information, firstly different geological maps, then engineering geological maps, hydrogeological maps , more generally from the 26 vol. 29/2020 bim – an application in geotechnical engineering figure 2. the position of geotechnical bim model for various geotechnical structures. ad a) geotechnical structure creates a basic bim model. ad b) gs creates an individual parametric elementaround basic bim model. set of so called geo-environmental maps, as this set includes also information about raw materials, about quality of underground water which should be protected etc. their utilization is also for the eia process – environmental impact assessment – for the phase of the evaluation of different variants of the transport infrastructure lines. the ground model is step by step improved so as to be able to fulfil demands on the individual stages of the design and will be terminated by the model which was verified during the phase of structure execution. the final version can be used for whole life time expectancy should any new interaction start. 3.1. foundation structures the 3d model of the ground together with the 3d model of the foundation structure can be used for verification of the basic rules which are specified in the eurocode 7 geotechnical design and involve: • to what degree the investigation methods respect the significance of the geotechnical structure and the risk with which any failure is connected – which is in the ec 7 expressed by 3 different geotechnical categories; • whether the depths of the investigation works are catching the maximum depths influenced by the proposed construction; • whether the range of the proposed and completed field and laboratory tests make possible their credible evaluation; • what differences there are between the measured values of geotechnical parameters, and which were finally used during the design as characteristic values, and with the values obtained during structure execution. similarly this is valid for step by step improvement of boundaries between individual lithological layers. as the bim model is time dependant there is the possibility to intervene at any time to correct potential shortcomings. similarly this is valid for geotechnical structures for which limit states are very sensitive to time, as from the uls with respect to short-term bearing capacity (stability) so on for long-term bearing capacity (stability) and then so for the sls verification. therefore, geotechnical structures get benefit from the monitoring plan not only during the phase of construction but also for the phase of construction of the upper structure, as well as during the period of warranty, to confirm that all demands are satisfied. the monitoring plan or plan of maintenance can create an independent fragmental model. the bim model is also able to control the schedule of the construction process, e.g. piles with additional information about pile execution, testing and a quality of material confirmation. 3.2. earth structures as the set-up of the bim model is very significant for transport infrastructure our attention is focused on these earth structures in transport engineering. timing specification is shown in fig. 3. for individual parts of the geotechnical design report (gdr). there are two geological models, the first one for subsoil and the second one for borrow pit, and from which material will be used for fill. similarly tests will be carried out for these two basic materials. for embankment, the quality of subsoil is not controlled during execution, and attention is devoted to the control of deposited and compacted material from the borrow pit. in principle there should be two levels of control, for compaction and for material properties (strength, deformation, filtration parameters) as these were used during the design. however, the possibilities in the second direction are very limited. the 3d model of subsoil together with the constructed earth structure (embankment) is shown in fig. 4. there are also supplementary cross-references on the description of investigation points (boreholes) on measure properties, and on the final selection of characteristic values of geotechnical parameters. similarly, as for foundation structures, the bim model can be used for approval on whether the model of subsoil is sufficient specifically with respect to the geotechnical category into which the designed geotechnical structure is falling. 3.3. underground structures (tunnels) as the bim model is proposed for total life time expectancy a significant role is present also for the phase of structure preparation (study). this is so especially with the respect to traffic line selection, the selection of technology for tunnel performance, a statement about material demands, and financial cost estimation. for embankment or cuts the estimation is relatively easy compared to the estimation for tunnels. the main reason is that during a tunnel execution unexpected geological and geotechnical conditions can be discovered because during the actual geotechnical investigation only a limited volume of the rock massif can be in fact be subject to this investigation. in this 27 i. vaníček, j. pruška, d. jirásko acta polytechnica ctu proceedings figure 3. geotechnical design report figure 4. vision of 3d model of earth structure within bim case the proposed bim model is only approximated and any final modification according to the real geological and geotechnical conditions should be the result of the last phase of performance. however, this last bim version is very important for the phase of tunnel operation and maintenance – for facility management (fm). as in bim , so also fm is a multi-criteria process. sometimes fm is denoted in bim as a sixth nongeometrical dimension – 6d bim. the ita – ites (international tunnel association) devotes great attention to bim at present. this is namely with respect to tunnel information modelling (tim) with the main aim to approve and support closer cooperation in the context of information management [6]. 4. summary bim is a long-term process which requires the preparedness of the people involved in, and familiar with, 28 vol. 29/2020 bim – an application in geotechnical engineering the new methods and processes which bim is offering. with wider application of bim in building and civil engineering new and more user-friendly software is appearing in this market. it is obvious that in the future transport infrastructures projects will almost certainly be realized involving bim and information management. the rules for multi-disciplinary cooperation of individual subjects are already partly included in the standard of ifc. however, these standards are not applicable for some civil engineering structures such as tunnels. that is the reason why under the umbrella of ita-ites a new working group was established, namely for above mentioned tim. nevertheless, in the field of geotechnical engineering bim can bring many new positive features, predominantly from the viewpoint of limitation of the risk with which geotechnical structures are connected. creation of the 3d model of subsoil together with the 3d model of geotechnical structures (either earth, foundation of underground) will help bring a better recognition of the geotechnical profession, especially its role in the whole process of construction. a great possibility exists with connection to the basic code for geotechnical structures – eurocode 7 geotechnical design. the creation of individual models is the common endeavour of ec 7. as for the standpoint of interconnection of individual phases and also for their control, that basic recommendation of the ec 7 appears satisfied. a full exploration of bim is duly justified given the basic aim defined for the bim which is that it should be a reliable basis for the decisionmaking process during a full life time structure expectancy – from the early phase of conception up to the structure demolition. after that it would be easier to implement simply from principles of sustainable construction. however, it is very important to define the principles of sustainable construction from the earliest phase. for example, the effort directed towards lower land consumption (greenfield saving) should be defined at the beginning as it is connected also with transport line selection. also, the possibility of land savings due to application of steeper slopes which are reinforced should start as soon as possible. however, the principle of sustainability is connected also with savings of energy and natural materials as for example in the case of reinforced retaining structures – during the phase of final decision about their type – as this principle can play significant role, additionally with respect to the c02 path. similarly it is valid for application of large volume waste materials into embankment such as flying ash, construction and demolition waste etc. instead of using natural soil. the transfer of the currently used 2d subsoil model into 3d is not such a great model and the first signs of this are becoming recognisable. similarly this is valid for design verification, with continuous transfer from 2d models into 3d models, even if the transport infrastructure where one dimensionality prevails, is more typical for 2d. therefore, more positively discussed is the transfer of “our” now typical geotechnical models into the coordination model of bim for which the main coordinator is responsible. this transfer demands a good orientation to code čsn en iso 16739 industry foundation classes (ifc) for data sharing in the construction and facility management industry (iso 16739:2013), especially, with respect to the fact that it is an open neutral standard data model, the transfer should not be a great problem – certainly after acquaintance develops confidence. the main coordinator will guarantee that this data model will be free of charge. a final note merits focus on the utilization of bim during life time expectancy. a basic model of subsoil together with a 3d model of geotechnical structure will be utilizable in the case of any new interaction. interaction with a new construction affecting the old one or due to any natural hazards or indeed any manmade hazards such as different types of accidents, e.g. a lorry with chemical substance, can allow a quick reaction to protect subsoil contamination. acknowledgements the paper was prepared with financial support from the programme of the centre of competence of the technology agency of the czech republic (ta čr), no. of project te0120168. references [1] us national bim standard project committee. [online]. [28. 8. 2019]., available at: https://www.nationalbimstandard.org/faqs#faq1. [2] j. nechyba (2018). vládní koncepce bim – rok po jejím schválení. zprávy a informace čkait, 5/2018. [3] p. kuba, d. wallensfels (2019). nová vodní linka účov: cesta od projektování 2d k projektování 3d a bim. stavitelství 04/19. [4] j. slánský, p. bečička. (2019). využití bim na příkladu projektu nemocnice náchod. stavitelství 09/19. [5] m. žibert. bim in tunnelling – implementation of bim methodology to trans alpine tunnel karavanke. brisbim gathering, august 2017, [online]. [28. 8. 2019]. available at: http://brisbim.com/wp-content/uploads/2017/08/ 170822_ic_bim-brisbim-kartu_mzr.pdf. [6] working group 22: information modelling in tunnelling. international tunnelling and underground space association [online]. [28. 8. 2019]. available at: https://about.ita-aites.org/wg-committees/ working-groups/276-ita-active-working-groups/ working-group-22-new-2017-informationmodelling-in-tunnelling. 29 https://www.nationalbimstandard.org/faqs#faq1 http://brisbim.com/wp-content/uploads/2017/08/170822_ic_bim-brisbim-kartu_mzr.pdf http://brisbim.com/wp-content/uploads/2017/08/170822_ic_bim-brisbim-kartu_mzr.pdf https://about.ita-aites.org/wg-committees/working-groups/276-ita-active-working-groups/working-group-22-new-2017-information-modelling-in-tunnelling https://about.ita-aites.org/wg-committees/working-groups/276-ita-active-working-groups/working-group-22-new-2017-information-modelling-in-tunnelling https://about.ita-aites.org/wg-committees/working-groups/276-ita-active-working-groups/working-group-22-new-2017-information-modelling-in-tunnelling https://about.ita-aites.org/wg-committees/working-groups/276-ita-active-working-groups/working-group-22-new-2017-information-modelling-in-tunnelling acta polytechnica ctu proceedings 29:24–28, 2020 1 introduction 2 the situation in the czech republic 3 bim and geotechnical engineering 3.1 foundation structures 3.2 earth structures 3.3 underground structures (tunnels) 4 summary acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2019.25.0089 acta polytechnica ctu proceedings 25:89–92, 2019 © czech technical university in prague, 2019 available online at http://ojs.cvut.cz/ojs/index.php/app simulation and optimization of porous bone-like microstructures with specific mechanical properties adrian wit∗, sebastian wronski, jacek tarasiuk agh university of science and technology, faculty of physics and applied computer science, al. mickiewicza 30, 30-059 kraków, poland ∗ corresponding author: adrian.wit@fis.agh.edu.pl abstract. bone trabecular structure can be characterized as a connected network of mineral bars and plates with unique mechanical properties. standard methods of producing bone-like structures based on periodic structures or foams have same limitations. the organization of the trabecular bone (meso scale) is adapted to the values of stresses and strains affecting the skeletal system. to simulate bone-like structure, the methodology of generating stochastic structure based on hyperuniform spatial points distribution is proposed. statistical analysis of generated structure shows the possibility to generate clouds of points in wide range of random close packing density, up to 59.52 %. points connected by voronoi tessellation produce to unique porous topology with no closed-cells and with wide range of connectivity. manufacturing of a generated structure is only limited by used technique. the proposed algorithm was developed regardless of the manufacturing technique, however, same examples of the structure were printed using 3d addictive technology. the mechanical properties of developed structure are strongly dependent on the material from which they are made, but the modification of the structure allows to change the strength in specific and controlled way. keywords: bone modeling, voronoi tessellation, porous structure. 1. introduction simulating internal bones structure with proper mechanical behaviour is an extremely difficult task. numerous study shows that the bioactive implants need specific requirements: high porosity and optimal pore size, directional strength and stiffness in range of natural bone, interconnected porous structure [1]. the most often attempts to create trabecular structures based on repetitive nets composed of metallic or polymeric bars arranged on top of each other [2]. this periodic structures are characterized by isotropic mechanical properties and simple manufacturing process but they gave insufficient results in similarity to natural spongy bones structure. another solutions use the methods of generating foam-like structures. however, the topology of standard foams leads to the formation of large areas of closed pores that do not allow free migration of living cells or the growth of blood vessels. the most realistic algorithm based on covering the space with 3-dimensional repetitive polyhedrons like kelvin cells or quasicrystal polyhedrons as a results of a special decoration of penrose tiling [3, 4]. they can form natural-looking net with roundly-shaped and connected porous. the scaffolds created on the edges of the polyhedrons and can be formed into various shapes and connections. the motivation to start working with designed structures was a significant progress in applications of 3d printed materials. many studies present the possibility to print simple 3d porous structure, that can be successfully applied in the living organism [5, 6], however, they are far from obtaining a structure similar to natural bone. researches on bone structure simulation are becoming more common but still they often have many limitations and simplifications [7]. the requirements of the good algorithm for generating base for bone-like structure are as follows: random spatial distribution, high uniformity points distribution which means that the points are not too close or too far to one another, no local concentration or grouping tendency and no preferred direction of distribution. the proposed methodology of generating porous structure is based on stochastic algorithm with well-spaced points distributions and real-time generation. very important role in the generator plays the real-time processing which allow to build adaptive algorithm targeted to reproduce the bone-like implants based on medical images. 2. materials and methods the basis of the model is blue-noise spatial points generator. the well-spaced blue-noise combine two desirable properties: random distribution and uniform space coverage. the modification of bridson fast sampling method [8] allow to easily generate large amount of points in any limited space. it also ensure the great homogeneous points distribution in local volume as well as good repeatability of generations with the satisfactory minimal margin of error. the set of points are connected together by using voronoi tessellation. postprocessing of the structure allow us 89 http://dx.doi.org/10.14311/app.2019.25.0089 http://ojs.cvut.cz/ojs/index.php/app a. wit, s. wronski, j. tarasiuk acta polytechnica ctu proceedings to modify the shape and size of structure scaffolds. 2.1. spatial points cloud generator let’s initialize an limited 3-dimensional background space with size sx, sy, sz and starting seed with coordination’s: x0, y0, z0. then create the list of active points that contains points in which domain it is possibility to generate next points, and conflict list for checking if the newly generated point is not located too closely the others. next step is to randomly generate a new seed that meets conditions: xn, yn, zn > rf and xn, yn, zn < rc. the space between rc−rf is called a point domain, where new points will be generated. while the list of active points is not empty, continue generating additional points and check if there are not too close to other points. if the conditions about location are correct, save this points in list of active points and continue the generation. in respect of each points, algorithm try to generate k-points in the domain. while k number is reached, remove the seed from the active list and randomly choose another seed from the list. in table 1, they have been collected all the initial parameters for the generator. the generator continue while the list of active points is not empty. the generator cannot allow to set new points out of limited space, so the number of points is numerally finite. the size of the background, β-ratio, and k-hits affects the number of points and density of packing. the graphical interpretation of points generation process and final results of spatial points cloud generator are presented in figure 1. the value of β-ratio was tested in wide range between 1 and 100, depending on the planned density of the structure and the connectivity of scaffolds. the value of k-number was set between 1 and 2000, but optimal value was defined at about 50. higher values heavily extends the time of calculations. both parameters should be tested individually and settings depending on the type or size of structure generated and assumption of implant porosity and the 3d-printer available parameters. 2.2. voronoi-based structure voronoi diagrams are related to one of the most fundamental constructs defined by a discrete set of points. the aim of the voronoi is to automatically build a trabecular structure based on a set of points in the limited volume. the result of this operation is a group of adherent tetrahedrons connected into 3d network with the desired hyperuniform properties. cells obtained in this way are mostly regular and similar with sizes (expect connected with borders). the results of voronoi diagram based on 3d well-placed sets of points is present in figure 2. the output voronoi diagram was processed into series of rod using grasshopper, the graphical editor integrated into rhinoceros 5. trabeculae were built figure 1. illustration of initial parameters during domain filling. no more points can exist in white circle (upper image). results of filling the 3-dimensional space with uniform points distributions (lower image). figure 2. an example of using 3d voronoi diagram based on hyperuniform points cloud. on the tetrahedrons edges achieved from voronoi. in grasshopper, rods can be designed with the various shapes, sizes and connection. all these parameters affect the final parameters of the model. changing the thickness of the trabeculae causes a change in the porosity of the structure and further the mechanical properties. the final result of the grasshopper is a meshed model that has been saved in standard 90 vol. 25/2019 simulation and optimization of bone-like microstructures symbol name description sx, sy, sz background size sizes of the background x0, y0, z0 initial seed coordination’s coordination’s of initial seed inside the background. this point could be generated randomly or arbitraly rf conflict radius the minimum distance between any pairs of points rc coverage radius the maximum distance between any pairs of points k hits limit the maximum number of attempts to fill a points domain β β-ratio the ratio between the radius of the conflict and the range of sample calculated as β = rc/rf table 1. list of initial parameters for spatial points cloud generator triangle language (stl) format which is ready for printing. 2.3. addictive manufacturing the final stage of the work is to produce bone-like porous structures. for manufacturing the artificially porous models in macro-scale the modified b9creator dlp printer was used. digital light processing (dlp) uses a liquid photopolymer resin which is able to be solidify under a light source with specific wavelength (typically in the range of uv light). in dlp printer, the light source project a single slice at the bottom of the vat, which causes the liquid resin cure in specific pattern. the solidified resin stick to the table and are removed from the resin so the next layer can be lightening. this printer is characterised by printing detail at 30µm microns resolution and maximal model size 104 × 75 × 203 mm. however, our modification of the printer allow to print trabecular structure with minimums size of element less than 20µm. this is a sufficient resolution to reproduce the size of the natural bone trabeculae existing in the human bone. the mechanical properties of the printed sample strongly depend on the strength properties of the resin. the proposed methodology for creating structures is independent from the printing material, so the final structure strength was not measured. depending on the manufacturer, the properties of the resins can be significantly different. there are many materials with very high stiff, chemical resistance and biocompatibility. 3. results the distribution of different sets of points have been analysed and measured. the most important properties defined for isotropic distribution were global and local density. global density was measured as a sum of all spheres limited by conflict radius dividend by the volume of the entire space. total density for different β-ratio and k parameter were measured and presented at figure 3. increasing the number of k-hits strongly influence the global density of the points distribution. the local density was measured by constantly dividing the background for small (equal in volume) sectors and measure the average number of the points in every figure 3. the density of points in function of k-hits show the increasing packing density with increasing k number (upper image). the average numbers of points in different number of squares (lower image). sector. the standard deviation between the number of points in each volume was calculated. for the sample that contains 176,531 points, when divided into 65536 regions, the every sector contains proximity 2.81 of points with std = 0.056. for the best parameters with high k (more than 2000) parameter and low β-ratio (less than 2) we are able to achieve the close packing density of about 59.52 %. the largest theoretical packing density has a crystalline facecentered cubic structure (fcc) which is 74.048 %. the best packing methods using random distributions allow obtaining results between 60 − 65 %. the appropriate 91 a. wit, s. wronski, j. tarasiuk acta polytechnica ctu proceedings figure 4. the numerical model of porous structure prepared for the addictive manufacturing (left). the printed model using dlp technology. the size of the printed cube is 10 × 10 × 10 mm (right). model of our structure has been prepared and printed using dlp technique. the result was shown in figure 4. however, the printing was developed only for visual inspection. as the main goal of this work was to create methodology for trabecular structure designing, the mechanical properties of the structure and biocompatibility were not analysed. the simplest geometrical parameters like a porosity, trabecular thickness, number of scaffolds were not measured and will be continued in the upcoming works. the biocompatibility of the resin was not defined. the final mechanical properties of the structure was not yet measured. 4. conclusion the method of generating bone-like structure based on blue noise algorithm give promises results in the matter of equal distribution of points and wide range of density obtained. the minimal and maximal value of archived density by the method is very important for designing bones with a significant variation of density, from cortex bones to porous bone fragments. additionally the generator is characterized by a good repeatability in the satisfactory minimal size of structures. the methods of generating points cloud disturbing can be improved. several studies show the numerical methods to improve spatial distraining with postprocessing algorithms [9]. using voronoi diagram to create open and connected cell gives satisfactory results. the purpose of further work will be improving the shape of the cell to be more roundish and increase their space diversity. on the other hand, before continue of this work a correct biocompatible resin has to be chosen, when work on printing and testing the mechanical properties of the samples will be continued. the method requires developing a set of optimal parameters like exposure time and time of disappearance for a sample of different sizes and porosities. also the compression test to analyse the mechanical properties are planned. acknowledgements this work was financed by the polish national centre for science (ncn) under decision number: 2017/26/e/st5/00043. adrian wit has been partly supported by the eu project powr.03.02.00-00-i004/16. references [1] j. teo, k. si-hoe, s. teoh. relationship between ct intensity, micro-architecture and mechanical properties of porcine vertebral cancellous bone. clinical biomechanics 21:235–244, 2006. doi:10.1016/j.clinbiomech.2005.11.001. [2] b. thavornyutikarn, n. chantarapanich, k. sitthiseripratip, et al. bone tissue engineering scaffolding: computer-aided scaffolding techniques. progress in biomaterials 3:61–102, 2014. doi:10.1007/s40204-014-0026-7. [3] w. jang, a. kraynik, s. kyriakides. on the microstructure of open-cell foams and its effect on elastic properties. international journal of solids and structures 45:1845–1875, 2008. doi:10.1016/j.ijsolstr.2007.10.008. [4] m. sniechowski, j. kaminski, s. wronski, et al. heterogeneous materials based on aperiodic structures for bone tissue substitutes. in vi international conference on computational bioengineering. 2015. https://www.researchgate.net/publication/ 313040001_heterogeneous_materials_based_on_ aperiodic_structures_for_bone_tissue_ substitutes. [5] a. palmquist, f. shah, l. emanuelsson, et al. a technique for evaluating bone ingrowth into 3d printed, porous ti6al4v implants accurately using x-ray micro-computed tomography and histomorphometry. pp. 1–8, 2017. doi:10.1016/j.micron.2016.11.009. [6] j. megias-barrera, l. garcia-consuegra, a. novoa, et al. histological and radiological evaluation of subcutaneous implants in mouse of a 3d-printable material (fulcure 720) and experimental application in mandibular reconstruction. pp. 88–92, 2018. doi:10.1016/j.jormas.2017.11.004. [7] m. fantini, m. curto, f. de crescenzio. a method to design biomimetic scaffolds for bone tissue engineering based on voronoi lattices. virtual and physical prototyping 11(2):77–90, 2016. doi:10.1080/17452759.2016.1172301. [8] r. bridson. fast poisson disk sampling in arbitrary dimensions. acm siggraph 2007. doi:10.1145/1278780.1278807. [9] m. ebeida, m. awad, x. ge, et al. improving spatial coverage while preserving the blue noise of point sets. computer-aided design 46:25–36, 2014. doi:10.1016/j.cad.2013.08.015. 92 https://doi.org/10.1016/j.clinbiomech.2005.11.001 https://doi.org/10.1007/s40204-014-0026-7 https://doi.org/10.1016/j.ijsolstr.2007.10.008 https://www.researchgate.net/publication/313040001_heterogeneous_materials_based_on_aperiodic_structures_for_bone_tissue_substitutes https://www.researchgate.net/publication/313040001_heterogeneous_materials_based_on_aperiodic_structures_for_bone_tissue_substitutes https://www.researchgate.net/publication/313040001_heterogeneous_materials_based_on_aperiodic_structures_for_bone_tissue_substitutes https://www.researchgate.net/publication/313040001_heterogeneous_materials_based_on_aperiodic_structures_for_bone_tissue_substitutes https://doi.org/10.1016/j.micron.2016.11.009 https://doi.org/10.1016/j.jormas.2017.11.004 https://doi.org/10.1080/17452759.2016.1172301 https://doi.org/10.1145/1278780.1278807 https://doi.org/10.1016/j.cad.2013.08.015 acta polytechnica ctu proceedings 25:89–92, 2019 1 introduction 2 materials and methods 2.1 spatial points cloud generator 2.2 voronoi-based structure 2.3 addictive manufacturing 3 results 4 conclusion acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2019.22.0012 acta polytechnica ctu proceedings 22:12–16, 2019 © czech technical university in prague, 2019 available online at http://ojs.cvut.cz/ojs/index.php/app short-term corrosion response of metals in lime mortar pavla bauerováa,b, zdeněk prošeka,c, jitka krejsováb, martin kepperta,b,∗ a czech technical university in prague, university centre for energy efficient buildings, třinecká 1024, 273 43 buštěhrad, czech republic b czech technical university in prague, faculty of civil engineering, department of materials engineering and chemistry, thákurova 7, 166 29 praha 6, czech republic c czech technical university in prague, faculty of civil engineering, department of structural mechanics, thákurova 7, 166 29 praha 6, czech republic ∗ corresponding author: martin.keppert@fsv.cvut.cz abstract. even though the corrosion of steel in concrete is very well described, the corrosion processes in lime environment studied very rarely. this environment is obviously also basic and its ph decreases in time, but the obtained results indicate important differences in behaviour of these two systems. corrosion response of carbon steel, zinc platted steel, copper, brass and lead placed in lime mortar was studied by help of sem microscopy. keywords: metal corrosion, modern mosaics, lime mortar. 1. introduction the problems related to the steel reinforcement corrosion in concrete are of crucial importance with respect to lifetime of majority of contemporary concrete structures [1]. there the most important corrosion-related feature is high ph, which, together with low potential, ensures formation of a passive layer of fe3o4 or fe2o3 on steel. the ph of concrete pore solution is controlled by soluble species present in concrete – portlandite (ca(oh)2) and eventually free alkalis (naoh, koh) [2]. in case that portlandite is the controlling species, the ph of pore solution is 12.5, corresponding to solubility constant of portlandite; the presence of alkalis can increase ph to higher level (ca. 13). obviously, also other components of pore solution are of importance towards the steel corrosion – especially chloride ions are widely studied [3]. the present paper is inspired by demand on knowledge of corrosion behavior of metallic materials in lime mortar; the demand came from restores dealing with mosaics. modern mosaics (from 19th and 20th century) have been frequently created in atelier by help of a metal frame, mortar and glass or stone tesseraes, and after the hardening were installed [4]. the metal frames, as well as other mosaics’ components, are frequently subjected to exterior conditions inducing complex degradation processes. the goal of the research was to evaluate corrosion behavior of carbon steel, copper, brass and lead in lime mortar environment. it is similar to the concrete environment – the ph is obviously also controlled by portlandite, but higher permeability of lime systems, responsible for their faster carbonation, makes the conditions somewhat different. corrosion tests are commonly performed by help of an accelerated test [5]; here just a short term response of metals to the lime environment was studied. 2. experimental the short-term corrosion behavior of five metals (carbon steel, copper, brass, lead) was studied. the strips (10 × 50mm) of metal plates were degreased, rinsed and immersed in fresh lime mortar (lime hydrate + standard quartz sand 1:3) and left in laboratory conditions. the used lime hydrate was compliant with cl90s type; the xrf analysis revealed that cao content was 99.3%, mgo 0.5% and so3 0.1%. after three months, the metal strips were removed from the mortar, cross-cutted, fixed by epoxy resin, polished and the interface between metal and adhering mortar was examined by electron microscopy (sem phenom xl) in bse mode. the mortar was not yet fully carbonated; the xrd rietveld analysis (fig. 1) revealed that binder (aggregates excluded) contained 38% portlandite and 62% of calcite. it means that ph was still controlled by ca(oh)2 dissolution. 3. results and discussion carbon steel. the surface of carbon steel (fig. 2) has been covered by continuous layer of fe corrosion products fexoy; generally fe2o3 and fe3o4 may be formed at basic conditions, depending on the potential. the layer thickness ranged from 5 to 10µm. the gap between metal and oxidized layer appeared during the sample preparation. mortar adhered well fe-oxides layer. the profile “1 fe” (line scan 1) shows that fe atoms has diffused also to the mortar layer. in areas, where mortar did not cover the steel well, a porous 12 http://dx.doi.org/10.14311/app.2019.22.0012 http://ojs.cvut.cz/ojs/index.php/app vol. 22/2019 short-term corrosion response of metals in lime mortar figure 1. phase composition of lime mortar under study. figure 2. interface between carbon steel and lime mortar. the arrows indicate line scans 1 and 2. dendritic fexoy structure appeared. the line scan 2 illustrates that the dendritic corrosion structure has grown also on the mortar surface. taking into account the short time (3 months) of experiment and laboratory conditions, one can state that steel corroded more that would be expected for ca(oh)2 rich environment. this behavior indicates, that in lime mortar is likely higher potential than in concrete; moreover in lime are not any alkalis like in cement which are responsible for higher ph than that corresponding to portlandite [6]. the zinc coated carbon steel was also subject of investigation, but one did not observed any corrosion effects. copper. the surface of copper (fig. 3) has been covered by well adhering layer of mortar; one observes that mortar is more compact in the vicinity of copper sheet. a closer view (fig. 4) indicates that the more compact surface layer is not homogenous; there are two phases. both of them are dominated by caco3, as results from ca and c profiles, but the phase in vicinity of copper plate is enriched by sulfur; its concentration is equal across the layer. the second element enriched in this layer is copper; its content is decreasing gradually in the layer and drops to zero on the interface between the two phases. the only source of sulfur in the system is lime, containing 0.1% of so3. due to low content, xrd diffraction was not able to detect, whether the is a sulfide or a sulfate salt. generally copper is immune in basic environment, or eventually, at higher potential, passivated by cu2o or cuo. in the studied system, copper surface is not seemingly corroded. on the other hand, certain amount of copper dissolved and formed a cu-s compound accumulating at the interface between copper and lime mortar. there has to be a driving force for diffusion of sulfur from bulk of lime to the interface; this driving force is a difference between sulfur activity 13 p. bauerová, z. prošek, j. krejsová, m. keppert acta polytechnica ctu proceedings figure 3. interface of copper (left) and lead (right) with lime mortar. figure 4. detail of copper – lime interface. the arrow indicate line scan. figure 5. elementary mapping performed on the copper – lime interface. 14 vol. 22/2019 short-term corrosion response of metals in lime mortar figure 6. sem micrograph of brass – lime interface. the arrow indicate line scan. figure 7. sem micrograph of lime mortar on lead substrate. the arrow indicate pb line scan. in bulk and at the interface. it indicates that a cu-s compound precipitated in the adjacent layer, what lowered the interface activity of a dissolved s-species, consequently inducing the s transport to the copper vicinity. this process can be observed also by elementary mapping of the adjacent cu-s enriched layer (fig. 5). brass. the interface between brass and lime mortar did not evince any indication of ongoing corrosion process (fig. 6). neither the element profiles did not indicate transfer of cu or zn from metal to the mortar phase. the mortar adhered well to the substrate. the results indicate that in the lime environment, zinc provides a passive layer to the brass and prevent its oxidation [7], even though in brass alloys, zn is usually corroding preferentially to copper due to its lower standard electrochemical potential [8]. lead. the adherence of lime mortar to lead was very poor (fig. 3). the distinct crystals in lead are sic particles, introduced, due to low hardness of lead, during the sample preparation. according to the lead pourbaix diagram, it dissolves at ph above 11.5 to pb(oh)2− 4 anion [9]; correspondingly the lead gradient was detected in the mortar (fig. 7) where the formation of capb(oh)4 may be expected. 4. conclusions there is just a little attention paid, compared to concrete, to corrosion behavior of metals in contact with lime mortar. this environment is obviously also basic and its ph decreases in time, but the obtained results indicate important differences in behavior of these systems. carbon steel, which is passive in concrete, corroded in lime mortar already after 3 months; the corroded layer developed between metal and adhering mortar. the zinc coated steel did not corroded in lime environment within the studied time range. copper formed a cu-s corrosion product, dispersed in adjacent layer of mortar, despite it is relatively noble metal. on the other hand, zinc in brass provided 15 p. bauerová, z. prošek, j. krejsová, m. keppert acta polytechnica ctu proceedings passivation to the material and any corrosion has not been observed. the lead corroded to pb(oh)2− 4 anion dispersed in lime mortar. it can be concluded that the best corrosion compatibility with the lime mortar environment proved brass and zinc coated steel. acknowledgements the work has been supported by czech science foundation project nr. 18-13525s “modern mosaic mortars in a microscope – methods for their materials characterization and degradation studies”. references [1] a. poursaee (ed.). corrosion of steel in concrete structures. elsevier ltd., 2016. doi:10.1016/c2014-0-01384-6. [2] a. behnood, k. van tittelboom, n. de belie. methods for measuring ph in concrete: a review. construction and building materials 105:176–188, 2016. doi:10.1016/j.conbuildmat.2015.12.032. [3] j. němeček, t. koudelka, t. krejci. numerical modeling of accelerated chloride transport in concrete. vol. 1978. 2018. doi:10.1063/1.5043737. [4] i. perná, t. hanzlíček, m. kracík štorkánová. characterization of historic mosaic at pfeiffer-kral sepulcher, jablonec nad nisou: a study of the mortar and tesserae origin. ceramics silikaty 58:308–313, 2014. [5] y. ma, z. guo, l. wang, j. zhang. experimental investigation of corrosion effect on bond behavior between reinforcing bar and concrete. construction and building materials 152:240–249, 2017. doi:10.1016/j.conbuildmat.2017.06.169. [6] a. poursaee. corrosion of steel bars in saturated ca(oh)2 and concrete pore solution. concrete research letters 1(3):90–97, 2010. [7] s. thomas, n. birbilis, m. venkatraman, i. cole. corrosion of zinc as a function of ph. corrosion 68:1–9, 2012. doi:10.5006/1.3676630. [8] s. selvaraj, s. ponmariappan, m. natesan, n. palaniswamy. dezincification of brass and its control an overview. corrosion reviews 21:41–74, 2003. doi:10.1515/corrrev.2003.21.1.41. [9] y. xie. dissolution, formation, and transformation of the lead corrosion product pbo2: rates and mechanisms of reactions that control lead release in drinking water distribution systems. ph.d. thesis, washingthon university in st. louis, 2010. 16 http://dx.doi.org/10.1016/c2014-0-01384-6 http://dx.doi.org/10.1016/j.conbuildmat.2015.12.032 http://dx.doi.org/10.1063/1.5043737 http://dx.doi.org/10.1016/j.conbuildmat.2017.06.169 http://dx.doi.org/10.5006/1.3676630 http://dx.doi.org/10.1515/corrrev.2003.21.1.41 acta polytechnica ctu proceedings 22:12–16, 2019 1 introduction 2 experimental 3 results and discussion 4 conclusions acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2019.21.0010 acta polytechnica ctu proceedings 21:10–15, 2019 © czech technical university in prague, 2019 available online at http://ojs.cvut.cz/ojs/index.php/app microscopical and micromechanical features of high performance concrete containing low levels of fly ash vladimír hrbeka,b,∗, zdeněk prošeka, roman chylíka, lukáš vráblíka a czech technical university in prague, faculty of civil engineering, thákurova 7, 166 29 prague, czech republic b institute of theoretical and applied mechanics as cr, v.v.i., prosecká 76, 190 00 prague 9, czech republic ∗ corresponding author: vladimir.hrbek@fsv.cvut.cz abstract. alkali activated fly ash is a widely used admixture to the concrete. due to the pozzolanic activity, the fly ash can replace up to 60 mass percentage. the impact of fly ash on the concrete depend on the level of it content, i.e. in small levels it improves the macro-mechanical properties and durability as well as chemical resistance. on the other side, fly ash admixture negatively influence the set time and cause slow development of strength. therefore, the effect of small fly ash admixture levels on the micro-structure of high performance concrete (hpc) is discussed in this study. keywords: high performance concrete, ccp, fly ash, grid indentation, spectral deconvolution sem, edx, image analysis, structure, effective modulus. 1. introduction the high performance concrete (hpc) became a commonly used building material in civil over last years due to increased demands on construction costs reduction, quality of building materials, prolonged service life of constructions, etc. compare to ordinary concrete, additional admixtures are added to the mixture, such as fly ash, providing specific features of hardened concrete. fly ash (fa) falls into the category of coal combustion products (ccps) that are produced from either hard or brown coal burning in coal-fired power stations. it is obtained from the gases of furnaces fired with coal or lignite at 1100 to 1400◦c by electrostatic or mechanical precipitation of dust-like particles. fa is defined as fine powder, mainly composed of spherical glassy particles. its chemical composition and pozzolanic and/or latent hydraulic properties depend upon the type of boiler and the type of burned coal. in general, fly ash contains significant quantities of crystalline and amorphous silicon dioxide (sio2), aluminum oxide (al2o3), calcium oxide (cao) and traceable amounts of other elemnets, such as magnesium, chrome, mercury etc. [1, 2] according to statistics of european coal combustion products association (ecoba) from 2016 [3], production of coal combustion products (ccps) in eu 15 countries reached 40.33 million tons and was estimated over 105 million tons in eu 28 countries. wast majority of ccps (63.8%, approx. 25.73 million tons) consists of fa and its utilization in the construction industry is currently around 43% (11.4 million tons). individual ccps production in eu 15 group is depicted in fig. 1a. detailed statistics from eu 28 group are not reliable due to vague characterization of ccps and insufficient data collection. statistics from american coal ash association (acaa) obtained in 2016 [4], show overall ccps production of 107.43 million tons, where only 35.2% (37.82 million tons) is represented by fly ash and 59.9% of it (22.63 million tons) is further used in other industries. it is worth mentioning, that overwhelming majority (75.3%, 17.04 million tons) of fly ash in usa is used in blend cement, concrete and concrete products. fly ash specification and comparison to portland cement, according to american standard (astm c618), are summarized in tab. 2. in czech republic, according to association for the district heating of the czech republic (adh cr) and asvep from 2014, approximately 13 million tons per year of ccps are produced, from which 9.24 million tons (71.1% of estimated production) are classified as fly ash (fig. 1b) . compare to eu 15 countries, czech republic utilize most of ccps, 59%, are used as surface mine backfill and just 11% for cement, concrete and masonry production. even though data of average chemical composition with respect to national standard are not available, specific oxide mass percentage of fly ash obtained from different power plants are presented in tab. 2. national standard csn en 197-1 recognizes fly ash, based on chemical composition, as silica (v type) and calcium fly ash (w type). their average chemical composition, according to [1], is presented in tab. 2. silica fly ash particularly contains silicon dioxide (sio2) and aluminum oxide (al2o3), where content of active sio2 should be over 25% of weight. it also includes more than 10 weight% of active calcium oxide and free form cao should not exceed 1.0 weight%. its use, when free cao (lime) is in range of 1.0 to 2.5 weight %, is limited according to the standard. 10 http://dx.doi.org/10.14311/app.2019.21.0010 http://ojs.cvut.cz/ojs/index.php/app vol. 21/2019 re-cycled concrete (a) ccps production in eu15, 2016. (b) ccps production in czech republic, 2014. figure 1. statistics of ccps production. (note: sda semi dry absorption, fgd flue gas desulphurization, fbc fluidized bed combustion) v type fly ash reacts with calcium hydroxide (ca(oh)2), which is present due to cement hydration, while generating silica hydrates and calcium aluminates. in stage of cement hydration initiation can its particles serve as nucleation centers for hydration products crystallization. calcium fly ash consists of 10 – 15 weight % of active cao and more than 25 weight% of active sio2. compare to silica fly ash, the concentration of alkali and sulfates is higher in w type fly ash and, when calcium hydrate and water is present, it displays pozzolanic and/or hydraulic behavior [5, 6]. fly ash replacement at various levels of the cementitious binder and its impact on hardened composite has been observed on macroscopical level in many studies. higher levels of fly ash addition (30 to 50 weight% replacement of cement) have been in past used mostly in massive structures, like foundations and dams, to control heat generation of the mixture hydration. in recent years, high replacement levels (40 to 60 weight%) used in structural applications produced concrete with increased mechanical properties and durability [7]. dependency of fly ash containment (both low and high levels of replacement of cement) and evolution of concrete strength in time are described in [8, 9]. the improvements of fresh concrete mixture containing fly ash addition, specifically the effect on workability and possible decrease of water-cement ratio, as well as impact on segregation of cement paste and aggregate are discussed in [10–12]. positive impact of the fly ash on shrinkage mitigation depends on two major facotrs, size and size distribution of fly ash particles and water requirements on the mixture. as pointed out in [13, 14], application of 50% replacement of cement by fine fly ash led to the shrinkage reduction by 30%. comparably, when coarse fly ash replacement is used, less reduction of drying shrinkage is observed. other aspect of fly ash addition is its positive effect on permeability and chemical resistance concrete composite. as shown in [15–17], low levels of fly ash replacement decrease the permeability up to 10-15% compare to conventional concrete. however, as in case of more than 50 cement weight% replacement, higher permeability (compare to control specimens) can be observed. on the other hand, significant reduction of chloride permeability (i.e. improvement of chemical resistance), as well as impact on pores size and distribution, with each increment of fly ash replacement volume [18]. 2. materials, sample preparation for purpose of this study, 4 mixtures were selected for testing, referential mixture (ref) and 3 modified mixtures containing 10, 20 and 30 weight% fly ash replacement of cement (fa10, fa20, fa30). ingredients of referential mixture consisted of cementitious binder (cem i 42.5 r mokrá), filler (crushed unwashed basalt aggregate of particle sizes 0 4, 4 8 and 8 16 mm), water and poly-carbonate based plasticizer (stachement, stachema). mixture compositions can be found in tab. 1 component ref fa10 fa20 fa30 cem 42.5 r 800.0 720.0 640.0 560.0 aggreg. 0 4 730.0 730.0 730.0 730.0 aggreg. 4 8 390.0 390.0 390.0 390.0 aggreg. 8 16 320.0 320.0 320.0 320.0 fly ash 0.00 80.0 160.0 240.0 water 210.0 197.4 184.8 172.2 plasticizer 32.0 33.8 29.6 29.5 table 1. compositions of tested mixtures in [kgm−3] specimens (cubes of 100 mm dimension) were stored in water in laboratory conditions (20 22◦c) to ensure adequate cement matrix hydration and to reduce rheologic changes and degradation of tested materials. furthermore, samples were prepared, using grinding and polishing media, for purpose of electron microscopy and indentation. due to mechanical and structural heterogeneity of the composite, preparation method reducing selective abrasivity was adopted. by using this method, both adequate roughness of the specimens surface and compact interface transition zone between aggregate and cement matrix were secured. 11 v. hrbek, z. prošek, r. chylík, l. vráblík acta polytechnica ctu proceedings oxide class f class c v type w type detmarovice hodonin portland cement sio2 50.0 40.0 51.4 39.4 50.4 31.6 23.0 al2o3 26.0 17.0 30.0 30.0 25.5 17.0 4.0 fe2o3 7.0 6.0 4.2 2.2 7.9 6.6 2.0 cao 9.0 24.0 1.3 11.6 4.8 29.4 64.0 mgo 2.0 5.0 1.4 1.4 2.8 3.7 2.0 so3 1.0 3.0 0.6 3.8 0.7 7.8 2.0 table 2. oxide analysis in weight percentage [%] of fly ashes an portland cement 3. methods and data processing the testing of described high performance concrete specimens was performed with two independent methods, scanning electron microscopy (sem) and grid nano-indentation. in order to overcome heterogeneous complexity, both methods were implemented on two main levels of the compositematrix without basaltic aggregate and transition zone between aggregate and matrix with limited occurrence of clinker. this prevented duplicity of obtained data from different phases in the material with close mechanical and/or structural features (such as aggregate and clinker etc.). 3.1. scanning electron microscopy and image analysis scanning electron microscope mira ii lmu (tescan corp., brno) equipped with epma was used for purpose of microstructure investigation and phase recognition of hpc samples. chemical composition (i.e. elemental analysis) of each phase was established by energy-dispersive x-ray spectroscopy (edx). this method is precise for determination of weight percentage representation of each chemical element in measured area, nevertheless it is greatly time consuming and covers limited percentage of sample. in general, energy of back-scattered electron is dependent on atomic number z of the element which is relevant to brightness of the phase in bse diagram. this led to direct use of image analyses of back-scattered electron micro-graphs which provided effective description of composite matrix. based on rough estimate of number of phases from low number of edx measurement, it possible to determine proper number of phases and their gray-scale range from histogram of several bse sem graphs. furthermore it is possible numerically quantify percentage representation of each phase and/or convert the diagram onto rgb spectrum image for better imagination of material structure. in case of aggregate-matrix transition zone, twostage image analyses was applied on bse diagrams. the sodium-calcium feldspar forming a structure of basalt aggregate is on gray-scale close to phases present in matrix. differential image analysis than consists of main binding phases (c-s-h gel) separation and identification of other phases in next step of the process based on their internal heterogeneity. 3.2. quasi-static indentation and spectral deconvolution elastic micro-mechanical parameters of individual phases were established usnig grid indentation (ti 700 series, hysitron inc.). principle of indendentation technique is based on dependency of probe propagation with respect to the recorded material response. the force-displacement record is further processed and the mechanical properties are calculated from the unloading part of the record. incorporated errors of the measurement, such as creep and visco-elasticity of measured phases, are avoided by appropriate test setup [19–25]. the implemented indents were displacement driven with maximum depth of 150 nm. for both levels of measurement, load function consisting of loading and unloading over 5 seconds with inserted holding time segment lasting 60 seconds in which maximum depth was held to ensure limited impact of material creep. the indents, again for both levels of investigation, were placed in 21 by 21 grind with mutual separation of 10µm. due to material level separation and indentation setting, grid indentation criteria of heterogeneous material are met [26–30]. effective modulus of each individual phase present in the composite were derived based on spectral deconvolution of indentation data. the process of spectral deconvolution, unlike statistical deconvolution, takes into account mutual interaction of phases (such as interaction of soft c-s-h gel and tough clinker) and data interference. the deconvolution procedure was implemented individually on both levels of investigated material and and overall effective features were established from merged normalized histograms of indentation data. maximum allowed divergence between structural and mechanical percentage representation of measured data did not exceed 2.5%. 4. results the results of previously described methods and data analysis are stated in this section. mechanical features of individual phases, derived by spectral deconvolution of indentation measurements (fig. 2), are summarized in tab. 3. the structural description of the composite, in means of individual phase percentage representation, was acquired using image analysis of sem bse micro-diagrams (tab. 4). 12 vol. 21/2019 re-cycled concrete sample itz ld csh hd csh portlandite clinker aggregate fa10 16.74 ± 2.42 33.45 ± 5.89 54.54 ± 7.63 84.92 ± 14.07 142.13 ± 13.34 154.86 ± 4.89 fa20 16.48 ± 2.12 34.95 ± 5.36 54.99 ± 6.38 88.63 ± 13.16 140.22 ± 17.42 163.32 ± 7.81 fa30 18.80 ± 4.40 35.62 ± 5.87 57.72 ± 6.40 86.17 ± 7.59 127.57 ± 24.85 161.54 ± 8.28 ref 17.34 ± 2.52 31.08 ± 4.59 45.07 ± 2.61 81.60 ± 18.55 128.36 ± 6.23 162.78 ± 5.87 table 3. indentation modulus er [gpa]) of concrete composites containing fly ash replacement figure 2. histograms of indentation modulus er. sample porosity ld csh hd csh portlandite clinker aggregate fa10 1.19 9.61 15.34 11.77 4.20 57.89 fa20 1.56 12.25 13.96 8.50 2.93 60.78 fa30 2.34 13.48 13.79 8.40 2.81 59.18 ref 1.50 15.82 15.43 6.55 4.18 56.52 table 4. percentage representation of individual phases in composite [%] 5. conclusion from the indentation results of composites on microscopical level and phase representation (based on image analysis of sem bse diagrams), following conclusions can be stated: • lower rates of fly ash replacements decreased interfacial transition zone modulus by 3.6 % for fa10 sample and by 5.2 % in case of fa20 sample while 8.4 % increase can be observed for fa30 specimen. • the indentation modulus of both csh phases (low and high density) are increased in all fly ash replacements. the most influential is 30 % fly ash content, where ld csh modulus is up 14.6 % and hd csh up 28.1 %. • the increase of calcium hydrate (portlandite) micromechanical performance by fly ash additions is up to 8.6 % in case of fa20 sample. • except the lowest fly ash replacement (fa10), fly 13 v. hrbek, z. prošek, r. chylík, l. vráblík acta polytechnica ctu proceedings ash addition increase the microand nanoporosity. • the representation of both csh phases is significantly decreased, while calcium hydrate (portlandite) percentage increased (especially in 10 % fly ash replacement sample). the early stage fly ash containing concrete composite project lower macro-mechanical properties, even though the micro-mechanical features of the matrix are elevated with the fly ash addition. the effect can thus be linked to the structural changes of each composite (mainly to increased porosity and lower csh phases representation). the impact of fly ash addition on the interfacial transition zone is not conclusive, due to inconsistent aggregate sizes in the measurements. further investigation of fly ash replacement on the itz is proposed. list of symbols f measured response/force [µn] hi indentation depth [nm] er indentation modulus [gpa] h hardness [gpa] hc contact depth [nm] p d probability density [–] acknowledgements the research is financially supported by czech technical university in prague – project sgs17/168/ohk1/3t/11 and by the czech science foundation research project 17-19463s. the support is gratefully appreciated. references [1] b. pacewska, g. blonkowski, i. wilinska. investigations of the influence of different fly ashes on cement hydration. journal of thermal analysis and calorimetry 86 (1):179–186, 2006. doi:10.1007/s10673-005-7136-7. 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[26] g. constantinides, k. r. chandran, f.-j. ulm, k. v. vliet. grid indentation analysis of composite microstructure and mechanics: principles and validation. journal of material science and engineering a 430 (1-2):189–202, 2006. doi:10.1016/j.msea.2006.05.125. [27] f.-j. ulm, m. vandamme, c. bobko, et al. statistical indentation techniques for hydrated nanocomposites: concrete, bone and shale. journal of the american ceramic society 90 (9):2677–2692, 2007. doi:10.1111/j.1551-2916.2007.02012.x. [28] o. bernard, f.-j. ulm, e. lemarchand. a multiscale micromechanics -hydration model for the early-age elastic properties of cement-based materials. cement and concrete research 33 (9):1293–1309, 2003. doi:10.1016/s0008-8846(03)00039-5. [29] w. zhu, j. j. hughes, n. bicanic, c. j. pearce. nanoindentation mapping of mechanical properties of cement paste and natural rocks. materials characterization 58 (11-12):1189–1198, 2007. doi:10.1016/j.matchar.2007.05.018. [30] l. sorelli, g. constantinides, f.-j. ulm, f. toutlemonde. the nano-mechanical signature of ultra high performance concrete by statistical nanoindentation techniques. cement and concrete research 38 (12):1447–1456, 2008. doi:10.1016/j.cemconres.2008.09.002. 15 http://dx.doi.org/10.1557/jmr.1992.1564 http://dx.doi.org/10.1557/jmr.1997.0324 http://dx.doi.org/10.1557/jmr.2002.0386 http://dx.doi.org/10.1557/jmr.2004.19.1.3 http://dx.doi.org/10.1016/j.msea.2004.04.070 http://dx.doi.org/10.1016/j.msea.2006.05.125 http://dx.doi.org/10.1111/j.1551-2916.2007.02012.x http://dx.doi.org/10.1016/s0008-8846(03)00039-5 http://dx.doi.org/10.1016/j.matchar.2007.05.018 http://dx.doi.org/10.1016/j.cemconres.2008.09.002 acta polytechnica ctu proceedings 21:10–15, 2019 1 introduction 2 materials, sample preparation 3 methods and data processing 3.1 scanning electron microscopy and image analysis 3.2 quasi-static indentation and spectral deconvolution 4 results 5 conclusion list of symbols acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2019.21.0033 acta polytechnica ctu proceedings 21:33–37, 2019 © czech technical university in prague, 2019 available online at http://ojs.cvut.cz/ojs/index.php/app reconstruction of the monastery next to the prague castle jakub řeháka,∗, eva burgetováa, josef řehákb, stanislav řehákb a czech technical university in prague, faculty of civil engineering, thákurova 7, 166 29 prague 6, czech republic b speleo-řehák, úvoz 6, 118 00 prague 1, czech republic ∗ corresponding author: jakub.rehak.1@fsv.cvut.cz abstract. the paper deals with a rehabilitation of the historic building of carmel monastery next to the prague castle. a structural survey of building was carried out and the historical drainage and ventilation system was identified. based on structural survey in-situ the reasons of failures were analyzed and remedial works of historic monastery were proposed. keywords: static failures, rising damp, structural survey, historic drainage systems, natural ventilation, sewer, rehabilitation. 1. introduction protection of buildings from water and moisture relates to the most important measures in ensuring the service life and serviceability of buildings. of great importance, as well, is structural solution of the designed rehabilitation procedure. unqualified interventions and measures often lead to failures and in the final phase, the deterioration of the whole complex. for this reason, the problems of long-term efficiency and reliability of rehabilitation methods are becoming a subject of great interest, both in terms of research and practical applications. the focus is mainly on listed buildings where technological designs are the most complicated. figure 1. carmel monastery in the prague castle square, view to the southern terrace of monastery. 2. monastic water systems complex water systems in the high middle ages was a phenomenon that occurred throughout europe. construction of new complex hydraulic systems seems to have begun in the 11th century, and it became increasingly widespread during following centuries. the concentration of water systems looks higher in germany than elsewhere. the rapid and widespread diffusion of the orders was followed by a rapid and widespread diffusion of hydraulic technology [1]. this technical knowledge of water and drainage systems was brought to bohemia primarily by cistercians and benedictines as a part of the spread the creed in the 13th century. the monastery of discalced carmelite nuns (the carmel of saint joseph) is located on the prague castle square. a part of the monastery grounds is the st benedict’s church, first mentioned in 1353, later the church underwent a number of reconstructions. in 1626 the monastery complex was added to it. its present form dates back to the middle of the 18th century. the carmel monastery was built on a steep south side of the castle promontory (fig. 1 and 4). the subsoil consists of weathered clay schist, which are unstable and slushy. in the middle ages a number of drainage and ventilation systems had served to prevent penetration of moisture into the structures, they drained water and dampness out of the structures and maintained them reasonably dry [2]. the baroque monastery buildings were forcibly abandoned in 1950 and the entire complex was costly adapted for use as a luxury hotel. between 1985 and 1991, the monastery underwent extensive reconstruction and was returned to its original religious purpose in 1992. figure 2. carmel monastery view of prague by f. van ouden allen, 1685. however, serious static problems manifesting by cracks have appeared and the building was moving slightly downhill to the street radniční schody (townhall stairs). according to the report based on visual 33 http://dx.doi.org/10.14311/app.2019.21.0033 http://ojs.cvut.cz/ojs/index.php/app j. řehák, e. burgetová, j. řehák, s. řehák acta polytechnica ctu proceedings inspection it was proposed to stabilize the building by grouting of the foundations and by fixing with reinforced ring beam and ties (fig. 3). the costs of remedial measures were estimated at almost czk 100 million, which was above the possibility of the nuns, as the owner of the building. figure 3. carmel monastery: proposed remedial measures. 3. experimental methods therefore carmelite nuns have addressed speleořehák company, specialized in rehabilitation of drainage systems and associated failures in historic buildings. during the subsequent structural surveys serious problems, that were causing the stability disruption, were revealed. figure 4. location of the monastery on a steep south side of the castle promontory between prague castle square and radniční schody (town hall stairs). historic drainage systems, however by ignorance of their importance and function, were clogged or filled up during reconstruction, possibly walled up as unnecessarily due to amateurish or inadequately carried out surveys or due to ignorance or negligence of geological and hydrogeological conditions. lessons from the functioning of these systems and their importance are unfortunately often acquired negatively during the laborious repairing of damage caused by undue interference in their functions. stability disruption of masonry of the carmel monastery causing its degradation was the result of interaction between multiple negative effects listed below: • leaking pipes the first defect was diagnosed in the main courtyard of the monastery. after realization of the borehole into the damaged masonry and the following extraction of the wet fill, the cause of the problem was discovered a leaking waste pipe. the water from the pipeline flowed through the backfill and saturated the adjacent structures with water. • closure of the main gallery at the same time, by the drill hole the rest of a lateral gallery was discovered. the main gallery, according to later found historical documentation, led originally from fountain in the courtyard, but later was closed and abolished. the whole area, as it turned out, belonged to the historic drainage system of the monastery and was covered with earth and debris. from this part of underground, about 70 tons of building waste probably from the time of reconstruction in the 1990s before the return of the monastery to the nuns, was removed. • collapse of the gallery walls the side walls of gallery have collapsed due to lack of maintenance of the underground system. this resulted in the vault falling in and the cavern originated here. if this situation is not solved, it could threaten imminently the surrounding buildings. • break of the toilet pit in the main gallery, fully filled up with rubbish, under monastery corridor the toilet (latrine) pit was discovered. the pit originally led up to the first floor, where it gradually moved into the size of a larger chimney and was led up to the roof. thanks to the chimney effect, this shaft provided sufficient air dynamics to help drying of the entire building through the drainage system. the chimney effect produced a negative pressure in the system, ensuring the odor from the pit will remove above the roof and at the same time preventing the odor from entering the building. during the reconstruction of the monastery, after the expulsion of the nuns in the 1950s, this toilet pit was interrupted on the ground floor by the corridor, thus preventing natural air flow and effective drying of the building. figure 5. carmel monastery: cross section of the main gallery with marks of the caverns. • exposed footing bottom 34 vol. 21/2019 reconstruction of the monastery next to the prague castle behind the toilet pit below the perimeter wall a large cavern with a diameter of 2.5 3 meters was discovered. looking from the gallery to the cavern, the footing bottom of perimeter wall in length of about 2m is suspended ”in the air” (without contact with subsoil). exposed footing bottom could not fulfill its function of transferring the load to the subsoil. the reason for the cavern was the total neglect and subsequent destruction of the underground system in combination with insufficient rainfall down pipe during the aforementioned rebuilding. water leaking from rainfall pipe saturated soil resulted in form of a cavern under the footing bottom. • the gallery clogged with concrete in 2013, due to removing of waste fill and rubbish, new part of historic underground drainage system was found. newly discovered gallery led up to the edge of the retaining wall of the monastery terrace. however, the gallery was filled up with concrete at the level of the retaining wall foundation. it is clear, that the drainage system continues further down below the radniční stairs and ensures the drainage of moisture and water also for the houses laid down below, where the survey subsequently confirmed this assumption. by blocking the gallery, the system could not perform its function. in addition, before the concrete ”plug”, leaking sewage pipe is connected to the gallery, which explains the critical state of the plaster on the supporting wall of radniční stairs. spalling of the plaster was caused by a leaking pipeline that supplied backfill by water, so it functioned as a water reservoir and constantly subsidize the support wall with moisture. the survey carried out in the carmel monastery showed, that moisture imminently threatened construction and represented a heavy burden for the entire building. 4. the proposal for redevelopment there are in principle two possible approaches for proposal of moisture redevelopment: • to fully respect and rehabilitate the original historical moisture proofing methods based on the ventilation of moist air from the structures, while assessing their options in relation to the existing use of the building. the results of applying these methods will take effect in the long term and require careful maintenance [3]. • to use of the latest technologies and materials, which are a perfect for sealing surfaces (e.g. grouting, waterproofing sealants etc.). this method is fast, the results will come immediately after application, but this is an irreversible action on the historic building [3]. restoration of traditional systems of protection of buildings from water and moisture should be prifigure 6. carmel monastery: radniční schody (town hall stairs) spalling of the plaster due to water-saturated backfill, the gallery clogged with concrete. oritized over unsuitable contemporary methods or costly technological measures. based on the results of the survey and analysis of the failures of the carmel monastery the rehabilitation procedures that pose minimal intervention into the structure, were designed in the context of the entire historic area. with an emphasis on the heritage value of the building it was used the original historical technologies for removal of moisture based on the principle of natural venting dampness from the structures. water-saturated backfill was manually removed, sewer pipes were repaired, opened up and unwanted ground water discharged from the building. the connection to a ventilation shaft was completed. moisture from the gallery is now diverted into the cleaned ventilation shaft and due to the chimney effect permeates up through the building above the roof. at present, the galleries of the old system have been mended and they serve as drains and part of the measures securing the building from moisture. 5. evaluation of remediation currently, it can be stated that the drainage and ventilation system is functional and moisture regime in building is stabilized. realized remedial variant has proved to be functional and effective within three years it has completely succeeded in drying the walls. the process of plastic deformation with cracking was stopped, cracks were bonded and sliding stabilized. it is clear, that these measures are sufficiently effective to prevent any further development of former structural problems, no other structural intervention were needed. 35 j. řehák, e. burgetová, j. řehák, s. řehák acta polytechnica ctu proceedings figure 7. carmel monastery: reconstructed main gallery. investigation involving the testing of rehabilitation methods and procedures in existing buildings is apparently the most reliable approach for obtaining conclusive results on the effectiveness of damp-removal measures. the experimental testing is useful not only for the assessment of the efficiency of remedial technologies, but also for a deeper investigation of moisture transport phenomena [4]. this appears an attractive way for better approaching the real features of building materials and, at the same time actual climatic conditions experienced on site. the phenomenon of moisture movement is extremely variable from case to case, depending on the specific features of the masonry: direction, quantity and continuity of water supply from ground, depending on seasonal and accidental factors, evaporation condition, thickness and arrangement of the bricks in the masonry, nature of mortar joints etc. [4]. in 2019 the long term monitoring supported by the european structural and investment funds, op prague: analysis of feasibility, concept dk-07, ”measurement system for tests and verification of the algorithm for description of properties and functions of the historic conduits” will verify the remedial measures carried out in the carmel monastery. testing of the moisture transport in historic drainage and ventilation system in the carmel monastery will use climatic and microclimatic data which allowed estimating quite accurately the indoor and outdoor evaporation. the efficiency of the air system will be monitored through the velocity and direction of airflow in the system. the collected data could allow the visual presentation of the behaviour of treated structures, its prediction and could help in the design of remedial measures as well. 6. conclusions the survey of the monastery gave an overview of the hydrogeological situation of this locality and identified the historic drainage and ventilation system. the solution of historic waterproofing was found and respected and based on its effectiveness was rehabilitated. crucial places, where water accumulated, were eliminated by opening up the gallery and cleaning the shaft. the sewer pipes were repaired completely. thus at the same time air circulation was restored and the high air humidity removed from the building. underground drainage systems are important and inseparable part of the historic monument, to complement its building history and have for its existence and preservation of its irreplaceable function. in historical buildings, it is desirable within the investigation of moisture conditions to identify, properly understand and restore the frequently faulty or damaged original insulation, ventilation or drainage systems, which was based on many years of experience of traditional building [5]. the understanding of the original or older solution, its technical parameters and performance is the first step towards finding out the causes of potential failures of the building and their removal allowing, at the same time, restoration of the function of the waterproofing system [6]. the reconstruction of historical drainage systems belongs to the basic tasks of conservation of buildings. experience of numerous cloister complexes show that the principal cause of increased ground moisture content and changes in subsoil manifested through structural failures of buildings is the breakdown of older drainage and ventilation systems [5]. the case of the carmel monastery confirmed that certain static failures and defects can be solved by additional treatment of moisture regime in building subsoil only and the application of invasive static remedial interventions is not necessary. at present, the galleries of the historic system have been mended and served as drains and sliding accompanied by cracking was prevented. the long term measurements of moisture movement verifying the remedial measures are planned in 2019. references [1] r. magnusson. water technology in the middle ages: cities, monasteries, and waterworks after the roman empire. baltimore: johns hopkins university press, 2001. [2] j. řehák. speleological survey of schwarzenberg palace praha 1, hradčanské nám. čp. 185. 2000. speleo-josef řehák, archiv speleo. [3] e. burgetová, k. michalová. a survey of historic galleries in the area of the prague castle. in advanced materials research, vol. 1122, pp. 61–64. trans tech publ, 2015. [4] e. franzoni. rising damp removal from historical masonries: a still open challenge. construction and building materials 54:123–136, 2014. 36 vol. 21/2019 reconstruction of the monastery next to the prague castle [5] e. burgetová. analysis of rehabilitation methods for rising damp in masonry 2005. habilitation thesis. [6] j. řehák, e. burgetová, j. řehák. moisture survey of former convent in the vicinity of prague castle 2017. engineering material. 37 acta polytechnica ctu proceedings 21:33–37, 2019 1 introduction 2 monastic water systems 3 experimental methods 4 the proposal for redevelopment 5 evaluation of remediation 6 conclusions references acta polytechnica doi:10.14311/app.2020.27.0121 acta polytechnica 27(0):121–125, 2020 © czech technical university in prague, 2020 available online at http://ojs.cvut.cz/ojs/index.php/app benefits of use of acoustic emission in scratch testing lukáš václaveka,∗, jan tomáštíkb, hana chmelíčkováa, radim čtvrtlíkc a joint laboratory of optics of palacky university and institute of physics of the czech academy of sciences, faculty of science, palacky university, 17. listopadu 12, 77146 olomouc, czech republic b institute of physics of the czech academy of sciences, joint laboratory of optics of palacky university and institute of physics as cr, olomouc, czech republic, 17. listopadu 50a, 77207 olomouc, czech republic c regional centre of advanced technologies and materials, joint laboratory of optics of palacky university and institute of physics of the czech academy of sciences, faculty of science, palacky university, 17. listopadu 12, 77146 olomouc, czech republic ∗ corresponding author: lukas.vaclavek@upol.cz abstract. scratch test is regularly used for assessment of cohesive and adhesive strength of thin films and coatings. by default, its evaluation is based on analysis of depth-load-time record and microscopic observation of residual scratch groove. the visual analysis of the residual groove provides the most detailed description of the final damage of the surface (crack patterns, extent of plastic deformation, delamination, etc.), but it may be a time demanding approach. although the continuous recording of indenter penetration depth and applied load offers instantaneous information about the performance of the tested material, it may not provide sufficient description of the sample’s deformation behaviour. therefore, other complementary techniques for description of the deformation response to scratch loading are desirable. continuous recording of acoustic emissions (ae) generated during the test could be a possible solution. it is especially the ability of ae method to detect the very first and even subsurface failures of the material that is of the most importance and otherwise inaccessible by standardly used techniques. what is more, it is a non-destructive and real-time method. in principle, ae method can be beneficially employed for a wide range of materials explored via scratch test. the strength of the ae analysis of the nano/micro scratch test will be demonstrated on various types of materials including optical thin films, durable metal and hard ceramic films as well as bulk laser cladding. selected phenomena and features of the use of ae during scratch testing will be presented, including effect of scratch load on character of ae records (burst vs. continuous) for tio2 on glass, subsurface damage of sic films on silicon and selective failure of hard carbide phase embedded in a metal matrix in case of laser re-melted layers. keywords: acoustic emission, laser cladding, scratch test, thin films. 1. introduction 1.1. scratch test the scratch test is a standard tribo-mechanical test for determination of cohesive-adhesive properties of coated materials. the test is based on lateral movement of the loaded tip across the sample surface, while different types of damage and/or wear can be simulated depending on the experimental setup (tip geometry, load, loading rate, environment, etc.). in most cases, a progressive loading with a chosen maximum is used. nevertheless, abrupt loading can also be employed for example in wear test, where constant sub-critical load is applied several times over the same track [1]. in fact, various scenarios could take place under the action of the generated pressure field based on the nature of the film/substrate properties. depending on the actual loading conditions, the material deforms beneath the moving indenter and various types of failures could occur including brittle film cracking, tensile cracking, ploughing, and film delamination according to mechanical properties and adhesion of the studied film-substrate system [2]. the scratch test applicability for evaluation of layered materials durability is given by the reliable detection of the onset of their failures, either in films, substrate or film/substrate interface. standard methods of scratch test evaluation are based on accurate identification of the critical loads representing onsets of those failures by visual observation of residual scratch groove using a microscope and/or analysis of load-displacement-time data. specific failure modes including different crack types along with plastic deformation are usually observable using modern microscopic techniques with sufficient resolution. however, there are film-substrate systems, where initial cracking is not detectable by microscopic techniques or depth-change data. the examples may be cracking at the film-substrate interface or substrate cracking 121 http://dx.doi.org/10.14311/app.2020.27.0121 http://ojs.cvut.cz/ojs/index.php/app l. václavek, j. tomáštík, h. chmelíčková, r. čtvrtlík acta polytechnica figure 1. acoustic emission: a) ae holder implemented in the nanotest system, b) burst ae envelope, c) continuous ae envelope d) ae hit waveform; b) and c) represents data from thin optical film on glass substrate. sample film deposition techniques substrate spherical tip radius [µm] max. load [mn] thickness [nm] sic film sic magnetron sputtering si wafer 5 300 and 500 2500 cr film cr magnetron sputtering steel (csn 16 720) 5 and 10 500 250 tio2 film tio2 magnetron sputtering glass 5 100 and 300 285 laser cladding remelted iron based powder laser cladding steel (csn 12 050) 10 500 ∼6x106 table 1. overview of tested samples and experimental conditions used in this study. under the opaque film [3]. therefore, new reliable methods are desirable. acoustic emission can be an appropriate solution for such cases [4, 5]. 1.2. acoustic emission at macro scale, acoustic emission (ae) is established as a standard non-destructive method in defectoscopy for the interfacial and cohesion failure detection, localization and evaluation of material defects and cracks of large-scaled objects [6, 7]. the energy levels released during macromechanical tests are many times higher than those released from the highly localized process volumes in nanomechanical and nanotribological tests [8]. therefore, ae sensors have to feature an optimized design that is vital to achieve sufficient sensitivity and reliability for applications at nano/micro scale. besides, the standard ae approaches and protocols have to be also adopted for specifics of nano/micro mechanical tests. the ae method principle is detection of the elastic waves generated by a sudden release of energy from a localized source in the material [9, 10]. energy release occurs when the material undergoes changes in its internal structure, for example due to cracking or plastic deformation arising from temperature gradients or external mechanical forces. typically, the piezoelectric material-based sensors attached to the sample are used for detection of ae signals (fig. 1a). these sensors are capable of detecting very small events with low ae signal amplitude. traditionally, the acoustic 122 vol. 27/2020 benefits of use of acoustic emission in scratch testing emission record is represented by the so-called ae envelope. it is a time-compressed signal that provides a quick overview about ae activity during the test and can be categorized into two types: i) burst emissions and ii) continuous emissions. a burst emission is a discrete packet of waves associated with a single event, whereas continuous emission is considered as a superposition of many small events, see fig. 1b and 1c. the ae envelope mainly gives information about the onset of the first cracks or an apparent change in failure mode. however, more throughout analysis of individual ae events can also be performed based on evaluation of the high-frequency sampled ae signal with sub-microsecond resolution. the discrete ae events are then represented by so called “ae hits” and can be further analysed in terms of their energy, and time and frequency parameters, see fig. 1d. in this paper, the high frequency ae monitoring was used during scratch testing of different thin films, coatings and bulk in order to demonstrate the benefits and features of ae technique. in the following sections the effect of scratch load on ae character for optical tio2 films on glass, subsurface damage of sic films on silicon and selective failure of hard carbide phase embedded in a metal matrix in case of laser re-melted layers are shown. 2. experimental settings diverse types of samples with different mechanical properties were explored using scratch test. specifically, a laser cladding on steel, magnetron sputtered cr film on steel, sic film on si and tio2 on glass substrate, see table 1. in all cases, samples were fixed using a low temperature wax on the special ae holder installed in the nanotest instrument (micromaterials, wales), where all the nano/micro mechanical tests were performed, see fig. 1a. scratch tests were performed with a fully calibrated nanotest instrument at room temperature with diamond spherical indenters. tips with end radius of 5 and 10 µm were used in order to generate different stress fields. during the tests, the normal force was linearly increased up to pre-set maximum (see table 1) over the scratch track of 450 µm. acoustic emission was recorded during the test using the zedo system (dakel, czech republic). this ae system consists of a dedicated sample holder with a piezoelectric sensor and an in-built preamplifier, a recording unit and a controlling software. ae signals were obtained with 16 bits sampling resolution, sampling frequency of 10 mhz and frequency bandwidth from 30 khz to 2 mhz. the scratch test evaluation was performed using a combination of three independent approaches: post process evaluation of residual scratch groove using microscopic techniques, the depth-change record analysis, and the acoustic emission envelope signal analysis. measured samples along with used experimental details are listed in the table 1. figure 2. scratch test performed up to 300 mn on sic thin film: a) ae envelope b) residual scratch groove. 3. results and discussion 3.1. tio2 thin film – burst vs. continuous ae ae envelope recorded during scratch testing on transparent tio2 film on transparent substrate is shown in fig. 1. the tio2 film with thickness of 285 nm on glass was tested up to 100 and 300 mn. it can be clearly seen that with the increase of normal load the character of ae signal also changes. a scratch test of the sample with a maximum load of 100 mn leads to the burst ae, induced by film cracking and breakage, see fig. 1b. increase of maximum load to 300 mn changes the character of the ae envelope to a continuous one, as shown in fig. 1c. this change is caused by scratching through the film directly to glass substrate. cracking of the glass is then responsible for strong continuous ae signal. 3.2. sub-surface cracking of sic coating on si sub-surface damage is a typical effect that might lead to a substantial overestimation of the tribological/mechanical durability of the film/substrate system if not considered. despite the possibilities of up-to-date microscopic approaches, they are limited to detection of surface damage. in other words, very limited options for detection of the sub-surface damage are available. the effect of sub-surface damage on scratch test evaluation and beneficial use of ae method can be presented for amorphous sic film magnetron sputtered on si substrate. a series of scratch tests with a maximum load of 300 mn and 500 mn were performed, similarly to [5]. high resolution visual observation with the laser scanning confocal microscope combined with the analysis of depth-load-time record does not reveal any damage in the film tested up to 300 mn (fig. 2) and only faint cracking beyond 460 mn (fig. 3) suggesting high durability of the films nevertheless, ae signal clearly reveals extensive subsurface cracking taking place already from 270 mn. subsurface crack123 l. václavek, j. tomáštík, h. chmelíčková, r. čtvrtlík acta polytechnica figure 3. scratch test performed up to 500 mn on sic thin film: a) ae envelope b) residual scratch groove. ing was directly confirmed on similar samples using the sem-fib cross-sectioning [5]. when we compare these values to the visually identified critical load for the first surface distinguishable cracks, then we can conclude that microscopic analysis overestimates the critical load by 70 %. 3.3. effect of indenter radius on ae signal for cr thin films on steel in general, acoustic emission has been known to be a very effective method for brittle materials. nevertheless, it can be used also for systems of metal films deposited on steel substrates. figure 4 shows an ae envelope recorded during a scratch test on cr film sputtered on steel substrate up to 500 mn. since scratch tests on non-coated steel surface did not produce any detectable ae signal it can be concluded, that ae is produced solely by the cr film cracking. microscopic analysis of the residual scratch grooves also supports that i) cr film damage perfectly corresponds to the ae envelope as can be seen from fig. 4 and ii) no signs of cracking were observed on the uncoated steel substrate (steel csn 16 720). scratch test is a very sensitive method to the used experimental conditions [11, 12] and direct comparison between various studies have to be made with care. it is apparent that use of indenter with a different radius substantially changes the stress field beneath the moving indenter [13, 14]. figure 4 also compares ae envelopes for the cr film on steel substrate tested at the same experimental conditions with 5 µm and 10 µm sphere. it can be clearly seen that distinguishable ae signal undoubtedly above the noise level was detected only for the test made with the indenter with smaller radius of 5 µm. 3.4. laser cladding although, scratch test has been used primarily for adhesion/cohesion testing of thin films, it may provide useful information on mechanical behaviour of thick coatings deposited by laser cladding technology. in fact, these coatings with thickness up to several figure 4. cr thin film: a) ae envelope b) corresponding residual scratch groove made during the scratch test up to 500 mn with a sphere of radius of 5 µm, and c) ae envelope for the test performed up to 500 mn with a sphere of radius of 10 µm. figure 5. laser cladding sample: a) ae envelope graph and b) surface image with marked areas of cracks (bright spots represent carbide inclusions) millimeters, prepared by laser melting of micrometer size metal powder, may be considered as bulk. here we report on a scratch test performed on a coating deposited from an iron-based mixture with 4.1 % of c, 32.0 % of cr, 0.5 % of si and 0.8 % of mn on carbon steel substrate (steel csn 12 050) using a fiber-coupled diode laser type ldm 3000-100 (wavelength 900-1070 nm). although the depth changes records did not exhibit any dramatic fluctuations originating from structural changes, a distinct ae signal was detected during the test. subsequent microscopic observation of the residual groove revealed well defined microstructure with carbide inclusions. comparison of the ae signal and microscopic images clearly points 124 vol. 27/2020 benefits of use of acoustic emission in scratch testing out extensive cracking inside the carbide inclusions (see fig. 5). this means that ae method is able to reflect microstructural features of the investigated material. 4. conclusions detection of acoustic emissions generated during the nano/micro-mechanical/tribological tests provides valuable information about deformation and failure mechanisms in the tested materials. it was demonstrated that application of the acoustic emission detection system during nano/micro scratch test proved to be very beneficial as it brings a capability to detect events previously undetectable by the standard scratch test evaluation approaches. acoustic emission can provide information about deformation and crack initiation in subsurface areas or film-substrate interfaces, which is undetectable neither by microscopic evaluation of residual scratch groove or from depth-change record data. the acoustic emission signal, recorded simultaneously during the scratch test, can be clearly assigned to a certain point of residual scratch groove, hence to the initiation of specific failure mode. such analysis might reveal valuable information about deformation processes and their dynamics in both bulk as well as layered materials. described ae method can be easily adapted to other local deformation or destructive methods like nanoindentation, pillar compression or cantilever bending tests. acknowledgements the authors gratefully acknowledge the support by the project th03020245 of the technology agency of the czech republic and the operational programme research, development and education, projects nos. cz.02.1.01/0.0/0.0/17_049/0008422 and cz.02.1.01/0.0/0.0/16_019/0000754 of the ministry of education, youth and sports of the czech republic. this work was also supported by the project iga_prf_2019_008 of palacky university. references [1] l. šimurka, r. čtvrtlík, j. tomaštík, et al. mechanical and optical properties of sio2 thin films deposited on glass. chemical papers 72(9):2143–2151, 2018. doi:10.1007/s11696-018-0420-z. [2] s. bull. failure modes in scratch adhesion testing. surface and coatings technology 50(1):25–32, 1991. doi:10.1016/0257-8972(91)90188-3. [3] j. tomaštík, r. čtvrtlík, m. dráb, j. maňák. on the importance of combined scratch/acoustic emission test evaluation: sic and sicn thin films case study. coatings 8(5):196, 2018. doi:10.3390/coatings8050196. [4] r. čtvrtlík, j. tomaštík, l. václavek, et al. high-resolution acoustic emission monitoring in nanomechanics. jom 71(10):3358–3367, 2019. doi:10.1007/s11837-019-03700-8. [5] j. tomaštík, r. čtvrtlík, t. ingr, et al. effect of nitrogen doping and temperature on mechanical durability of silicon carbide thin films. scientific reports 8(1), 2018. doi:10.1038/s41598-018-28704-3. [6] c. grosse, m. ohtsu (eds.). acoustic emission testing. springer science & business media, 2008. doi:10.1007/978-3-540-69972-9. [7] w. sikorski (ed.). acoustic emission: research and applications. intech, 2013. doi:10.5772/50270. [8] s. gholizadeh, z. leman, b. baharudin. a review of the application of acoustic emission technique in engineering. structural engineering and mechanics 54(6):1075–1095, 2015. doi:10.12989/sem.2015.54.6.1075. [9] d. f. bahr, w. w. gerberich. relationships between acoustic emission signals and physical phenomena during indentation. journal of materials research 13(4):1065–1074, 1998. doi:10.1557/jmr.1998.0148. [10] n. i. tymiak, a. daugela, t. j. wyrobek, o. l. warren. highly localized acoustic emission monitoring of nanoscale indentation contacts. journal of materials research 18(4):784–796, 2003. doi:10.1557/jmr.2003.0109. [11] p. steinmann, h. hintermann. adhesion of tic and ti (c, n) coatings on steel. journal of vacuum science & technology a: vacuum, surfaces, and films 3(6):2394–2400, 1985. doi:10.1116/1.572845. [12] b. beake, a. ogwu, t. wagner. influence of experimental factors and film thickness on the measured critical load in the nanoscratch test. materials science and engineering: a 423(1-2):70–73, 2006. doi:10.1016/j.msea.2005.09.121. [13] n. randall, g. favaro, c. frankel. the effect of intrinsic parameters on the critical load as measured with the scratch test method. surface and coatings technology 137(2-3):146–151, 2001. doi:10.1016/s0257-8972(00)01097-5. [14] j. tomaštík, r. čtvrtlík. effect of intrinsic parameters on evaluation of critical loads in nanoscratch test. proceedings of 30th danubia adria symposium on advances in experimental mechanics 2013. 125 https://doi.org/10.1007/s11696-018-0420-z https://doi.org/10.1016/0257-8972(91)90188-3 https://doi.org/10.3390/coatings8050196 https://doi.org/10.1007/s11837-019-03700-8 https://doi.org/10.1038/s41598-018-28704-3 https://doi.org/10.1007/978-3-540-69972-9 https://doi.org/10.5772/50270 https://doi.org/10.12989/sem.2015.54.6.1075 https://doi.org/10.1557/jmr.1998.0148 https://doi.org/10.1557/jmr.2003.0109 https://doi.org/10.1116/1.572845 https://doi.org/10.1016/j.msea.2005.09.121 https://doi.org/10.1016/s0257-8972(00)01097-5 acta polytechnica 27(0):122–126, 2020 1 introduction 1.1 scratch test 1.2 acoustic emission 2 experimental settings 3 results and discussion 3.1 tio2 thin film – burst vs. continuous ae 3.2 sub-surface cracking of sic coating on si 3.3 effect of indenter radius on ae signal for cr thin films on steel 3.4 laser cladding 4 conclusions acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2018.20.0029 acta polytechnica ctu proceedings 20:29–33, 2018 © czech technical university in prague, 2018 available online at http://ojs.cvut.cz/ojs/index.php/app calibration of btt measurement with respect to sensor position over shrouded lsb tereza dadáková∗, zdeněk kubín doosan škoda power s.r.o., tylova 1/57, pilsen, czech republic ∗ corresponding author: tereza.dadakova@doosan.com abstract. in order to increase efficiency and reliability of steam turbines, last stage blades are equipped with mid-span tie-boss and shroud. we can use blade tip-timing method (btt) to measure vibration of shrouded blades, but the measured level of vibration is dependent on position of sensor over blade shroud. hence, the results of btt measurement have to be compared with other measurement techniques, like strain gauges, and with fem analysis, to interpret these results right. this article provides findings we made on measurement of shrouded blades in test rig with eddy-current sensors, optical sensors and strain gauges at different positions of sensors over blade shroud. measurement is also compared to fem analysis. keywords: blade tip-timing, calibration, sensor. 1. introduction blade tip timing (btt) measurement is in doosan skoda power used mainly for two reasons: to prevent blade failure on some operational states, where can be increased level of vibration because of ventilation unpredictable asynchronous vibration can be excited by small aerodynamic forces. for these cases, longterm monitoring with calculation of blade lifetime is used on power plants to plan shutdowns of turbine and measuring blade behaviour on wide range of operation points to optimalize operation of the turbine. second use of btt in doosan skoda power is blade testing in campbell test rig to compare measurements with fem analysis and prove that the blades are designed out of synchronous resonances. for shrouded blades we use campbell test rig to do the calibration of btt determination of suitable position for sensor location, comparison of results of btt with strain gages and verification of calculation. the need to do the calibration of btt occurred with the new design of blades with interconnecting elements. to last stage blade (lsb) design tie-bos in the middle of the blade and shroud at the tip of the blade were added. the centrifugal force causes untwisting of blades, resulting in force between these contact elements and hence in reduction of blade vibration. however these features brought many questions about measuring those blades with btt. the blade shroud has a great impact on signal shape from the sensor, in some positions (contact plane) it is complicated to receive a good data. another question is what point of shroud is exactly measured and it is necessary to interpret the amplitude of vibration for btt right. also the identification of cracks on shrouded blades became more demanding task. cracks usually cause a change in blade vibration behaviour like frequency shift, higher level of vibration, shift of a blade towards other blades change in inner blade distance, etc. however on free blades is this behavior manifested more significantly. an alternative measurement to btt are contact methods use of strain gages. strain gages give us direct information about the blade stress, but usually are installed only on few blades and in corrosive steam enviroment with high temperature usually have a short service life. also the installation is quite expensive and these reasons make this method unsuitable for wider use. btt as contactless method, or nsms (nonintrusive stress measurement system), is cheaper, sensors are placed circumferentially around stator and hence each sensor sees each blade of the rotating disk [1]. the btt system measures time arrival of each blade, the difference between time arrivals is converted to vibration in mm. this data can be analyzed in many ways, using various methods [2, 3]. most used method is fast fourier transformation (fft) converting the signal from a sensor to single-blade spectrums or allblade spectrum. single-blade spectrum gives us information about vibration of every blade. however, it is sampled only once per revolution what causes limited frequency range and problems with identification of natural frequency and nodal diameter. on the other side, all-blade spectrum shows us the whole disk vibration behaviour with possibility of determination nodal diameters and natural frequencies (it is sampled by number of blades in one revolution), but changes in vibration of one blade with defect may not occur in this spectrum. this usually leads to using a combination of different methods to receive a real amplitude of all blades. various types of sensors can be used, new sensors with different properties are developed [4]. sensors can be based on different physical principals, not in every case it is clear what exactly sensors measure. simulation of measurement can help here [5, 6]. 29 http://dx.doi.org/10.14311/app.2018.20.0029 http://ojs.cvut.cz/ojs/index.php/app tereza dadáková, zdeněk kubín acta polytechnica ctu proceedings this article contains observations based on measurement of shrouded blade in campbell test rig, using optical sensor, eddy-current sensors and strain gages. this measurement was special, because of using a system that gradually moved sensors over rotating blades. this way we were able to “scan” blade shroud and find best sensor position for measuring this type of blade. it also gave us a different point of view on measuring with eddy-current sensors. we also describe a method we use to calibrate btt with calculation. 2. measurement description the measurement was done in campbell test rig on lsb for 60 hz with diameter 2904 mm, the disk consisted of 50 blades. the campbell test rig used for measurement operates without steam, blades rotate in vacuum, because high-speed motion through air would result in aerodynamic heating and rise of temperature. the level of vacuum is usually about 3 mbar, maximal rotational speed is 4000 rpm and maximal disk diameter is 5m. to excite blades an electro-magnet, oil jets or torsional shaking can be used. the electo-magnet excites blades on frequency fe, fe = fn +nd · fr, (1) where fn is natural blade frequency, nd is nodal diameter which is excited and fr is rotational frequency. electro-motor torsional shaking is used to excite 0nd, because from our experience the amplitude is ten-times bigger, than by using the electromagnet. unfortunately, only this nodal diameter can be excited this way. 2.1. measuring system to measure the blade vibration contact and contactless methods were used, as mentioned in section 1. in the test rig it is possible to use strain gages with telemetry system, btt and high speed camera monitoring system. for this measurement only strain gages and btt were used. strain gages were placed at positions, where we expected maximal response to verify shapes of various nodal diameters. amplitudes are obtained from fast fourier transformation directly on natural frequency. btt consists of sensor, amplifier, trigger and measuring system with high sampling frequency clock (figure 1). in the test rig we successfully use optical sensors, but in steam enviroment the use of optical sensors is problematic. for use in steam enviroment eddycurrent sensors are more suitable, because they are not affected by steam and are designed to operate in temperatures up to 250 ◦c. this leads to the need of comparison of those two types of sensors, since we know, what exactly optical sensors measure, but what eddy-current measure exactly on shrouded blade is a question. eddy-current design is shown in the figure 2. the signal from eddy-current is sensitive on position of magnet and coil inside the sensor and on size of shroud compared to the size of sensor. the shape of signal is very dependent on position of sensor over blade shroud and on rotational frequency. the signal from sensor is amplified (a in figure 1) and transferred to ttl digital puls (trigger t in figure 1). the measuring system with clock (s in figure 1) measures time arrivals of edges of ttl pulses. the system accuracy is dependent on sampling frequency of measuring clock. for example, if the rotational frequency fr is 60hz and rotor diameter d = 2904mm, to see vibration of order 10−1 mm we would need sampling frequency about fs = 5.5mhz, to see vibration of order 10−2 mm frequency fs = 55mhz and to see vibration of order 10−3 mm even fs = 550mhz. longer blades need even higher sampling frequency, disk with diameter 5 m would have blade tip speed about 940m/s, so to measure with 10−3 mm accuracy, frequency fs = 940mhz would be needed. for this measurement the sampling frequency was 200mhz, which provided accuracy about 3µm. 2.2. measurement setup we used optical sensor and two similar eddy-current sensors with slightly different properties (size, number of loops in coil). those sensors were attached to a threaded rod with stepper motor, that allowed us to move those sensors along the blade shroud step by step to measure on different positions without requirement of running-down and moving the sensors manually (figure 3). we were able to chose the speed of sweep and position arbitrarily and we saved values of the positions so we could compare it with the measurement. by the optical sensor we measured both, leading and trailing edge of the blade (figure 4), so we were able to capture the whole blade shroud shape. results are summarized in next sections. 3. results we have done two types of measurement: first, we investigated the influence of magnet and coil position to blade and secondly we excited blades on its natural frequencies to compare amplitudes from different sensors. we didn’t move the sensors continuously, but we stopped every one mm for a few seconds to capture the vibration on every position. 3.1. sensor orientation we performed five measurements, where we measured the blade shroud with sensor moving there and back starting on trailing edge, so we were sure, that the captured shape is not affected by level of vibration or inaccurate movement of sensors. we chose sensor orientation as shown in figure 5, where circle marks the coil in the sensor. starting by positioning the coil towards the blade, next positions were determined by turning the sensor by 90◦, 180◦, 210◦ and 300◦. 30 vol. 20/2018 calibration of btt measurement a t s 1/rev so se figure 1. diagram of btt measuring system. coil magnet figure 2. eddy-current sensor. electro-magnet optical sensor eddy-current sensor stepper motors figure 3. measurement setup. figure 4. edges captured by optical sensor. to ilustrate the result we chose optical sensor and one eddy-current. those sensors centers were distant from each other 190 mm (1.04 blade distance) and the data form each sensor are related to the 1/rev sensor. if we choose one specific blade, for example the first one that arrives to optical sensor after 1/rev mark, it is actually the second one, that arrives to eddy-current. the position of k-th blade of sensor s in relative angle pk in revolution n can be expressed as pk,n = toask,n − toarn toarn+1 − toarn , (2) where toa stands for time of arrival, r for sensor 1/rev and s for optical sensor, resp. eddy-current. we take k-th blade from optical sensor and k+ 1-th blade from eddy-current, recalculate relative angle to mm, subtract the distance of sensors and the result is shown in figure 6. data from eddy-currents are displayed as colored dots, colors correspond to orientation of sensor from figure 5. the shape of blade shroud is taken from the optical sensor data. te in the figure 6 stands for trailing edge and le for leading edge. the sensor size is for this blade comparable with the size of the blade shroud, what is in our opinion the cause of some uncertainty of what eddy-current measures, especially on leading edge. from experience it is expected, that the sensor measures by the center of the coil, which unfortunately wasn’t confirmed by this measurement. this measurement was very important, because it confirmed, that on trailing edge is the 31 tereza dadáková, zdeněk kubín acta polytechnica ctu proceedings figure 5. eddy-current sensor orientation. le te figure 6. eddy-current sensor data (relative angle position of one blade in disk.) measurement almost independent on orientation and that here it captures the blade shroud shape appropriately (even if it may not be exactly on the edge). trailing edge is just the position, that is mainly used for btt measurement. more in section 4. 3.2. amplitude on natural frequency another measurement we performed, was exciting the bladed disk on its natural frequency and comparison of amplitude of optical and eddy-current sensors. we excited the blades on frequency fe, see equation 1, where the exact natural frequency fn was determined from strain gages measurement. we again measured the blade shroud there and back with few seconds stops, so we were able to use fft on data in one position with sufficient accuracy. to compare amplitude form both sensors we used amplitude from all-blade spectrum. from parts of the data, where the sensors were steady, the all-blade fft was computed. the amplitude on excitation frequency fe from this spectrum was taken and that is represented by color in the figure 7. in the figure 7 are results for two natural frequencies, 0nd and 6nd. as mentioned in section 2, we use torsional shaking for excitation of 0nd and electro-magnet for higher nodal diameters. from the figure 7 0nd results we can see, that on trailing edge both sensors measure so so so so se se so so so so se se figure 7. amplitude for optical and eddy-current sensors on 0nd (top) and 6nd (bottom). so optical sensor, se eddy-current. the same amplitude (color of eddy-current and optical sensor matches). that shows us the optimal position for measuring this blade. unfortunately, as we can see in results for 6nd, eddy-current sensor measurement was affected by the electro-magnet excitation increasingly, as the sensor was getting closer to the electro-magnet, that was placed on the leading edge side of the blade, see figure 3, where its position is shown. for higher nodal diameters only data from optical sensor from this measurement were reliable. 32 vol. 20/2018 calibration of btt measurement yc figure 8. deflection measured by btt. yc y1 y2 y1 αβ figure 9. recalculation of deflection measured by btt to the actual deflection. 4. calibration method let us conclude with a description of how we use the results we get from this measurement and how we compare it with calculation. the btt sensor measures deflection in the direction of rotation, in the figures 8 and 9 marked as yc, however the actual vibration of blade in this direction is y1. using angles α and β we have yc = ( 1 tanα tan β + 1 ) y1. (3) from here comes the importance of angles α and β. the angle α is given by blade design and angle β by mode shape is different for every nodal diameter and is computed by fem. if we take measurement from strain gages in µm/m and divide it by actual amplitude measured by btt, we get a ratio that expresses strain on one mm of blade tip deflection for specific strain gage position. this can be also calculated using fem and those two values we compare to verify computed blade properties. 5. conclusion the measurement brought a new look on measuring with eddy-current sensor and we found an optimal position of sensor over blade shroud for this type of blade. the results may be extended to graphical comparison of measured data to fem analysis depending on nodal diameter, type of sensor etc. to get better results with excitation higher nodal diameters using eddy-current sensors it is necessary to find a way of exciting the blades without affecting the sensor signal. we expect a repetition of this measurement in the test rig with another type of blade with larger diameter. this could demonstrate the difference of sensor signal behaviour, especially on leading edge, where the measuring point of eddy-current for this blade is unclear. list of symbols fs sampeling frequency of measuring clock [hz] fe excitation frequency [hz] fn natural frequency [hz] fr rotational frequency [hz] nd nodal diameter pk,n relative angle of k-th blade in n-th revolution t oask,n time of arrival of k-th blade in n-th revolution to sensor s t oarn time of arrival of 1/rev marker in n-th revolution yc displacement seen by btt [mm] y1 actual displacement of blade in tangential direction [mm] acknowledgements these research’s results are part of the turbo-reflex project, which has received funding from the european union’s horizon 2020 research and innovation programme under grant agreement no 764545. references [1] z. kubin, j. liska, t. misek, p. prochazka. application of non-contact vibration measurements on a 48” blade under low load condition. dymamesi 2017 prague: institute of thermomechanics as cr pp. 45–54, 2017. [2] g. battiato, c. firrone, t. berruti. forced response of rotating bladed disks: blade tip-timing measurements. mechanical systems and signal processing 85(1):912–926, 2017. doi:10.1016/j.ymssp.2016.09.019. [3] o. danek, f. vanek, j. kozanek, et al. the development of identification of dynamic parametres of rotating machinery parts. proceedings of the conf structural system identification 2001. [4] p. prochazka, f. vanek. new methods of non-contact sensing of blade vibrations and deflections in turbomachinery. ieee trans instrum meas 63:1583–1592, 2014. [5] n. jamia, m. friswell, s. el-borgi, r. fernandes. simulating eddy current sensor outputs for blade tip timing. advances in mechanical engineering 10(1), 2018. [6] z. kubin, t. misek, j. hlous, et al. calibration of blade tip-timing sensor for shrouded 40” last stage blade. mechanical systems and signal processing 108:88–98, 2018. doi:10.1016/j.ymssp.2018.02.001. 33 http://dx.doi.org/10.1016/j.ymssp.2016.09.019 http://dx.doi.org/10.1016/j.ymssp.2018.02.001 acta polytechnica ctu proceedings 20:29–33, 2018 1 introduction 2 measurement description 2.1 measuring system 2.2 measurement setup 3 results 3.1 sensor orientation 3.2 amplitude on natural frequency 4 calibration method 5 conclusion list of symbols acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2019.21.0024 acta polytechnica ctu proceedings 21:24–27, 2019 © czech technical university in prague, 2019 available online at http://ojs.cvut.cz/ojs/index.php/app glass waste powder utilization in high performance concrete diana mariakováa,b,∗, tomáš vlacha,b, tereza pavlůa,b a czech technical university in prague, faculty of civil engineering, thákurova 2077/7, 166 29 prague 6, czech republic b czech technical university in prague, university centre for energy efficient buildings, třinecká 1024, 273 43 buštěhrad, czech republic ∗ corresponding author: diana.mariakova@fsv.cvut.cz abstract. this paper deals with investigation of high-performance concrete (hpc) with full replacement of the silica powder by the waste glass powder. the silica powder was replaced by two types of the waste glass powder, originated from different sources (waste glass powder from grinding jewelry and milling of municipal waste glass). the properties of the waste glass powder were examined and compared with the silica powder. the mechanical and durability properties of three hpc mixtures were experimentally verified. the bulk density, flexural strength, and compressive strength were tested on beams 40 × 40 × 160mm at age 28 and 60 days and after 0, 25, 50, 75 and 100 freeze-thaw cycles. there were observed slightly worse properties of mixtures with the waste glass powder in comparison with reference hpc. keywords: waste glass powder, high performance concrete, durability, mechanical properties. 1. introduction high-performance concrete (hpc) becomes very popular. hpc has been mostly used in the precast industry, for pedestrian footbridges. nowadays, hpc is used in buildings structures (skyscrapers) or in transportation structures (bridges) [1]. due to its excellent mechanical properties the hpc structures could be more subtle. this could lead to the raw materials saving. the properties of hpc are suitable in many ways such as mechanical and durability properties. hpc is also possible to use in aggressive environment with aggressive substances such as sulphates or chlorides. permeability of hpc is also very low [2]. there is also the risk of carbonation of the concrete and further its corrosion due to presence of carbon dioxide in standard environment. for this reason, the utilization of hpc is appropriate as prevention against corrosion and concrete damage. due to its excellent durability properties, the long lifespan of constructions is provided, despite exposure to aggressive conditions. the long lifespan and also lower material consumption of hpc structures is the main and most important advantages of this material. however, the primary raw materials which are used for hpc has high environmental impact such as co2 emissions, energy and raw material consumption. the impact on the environment is an important question. nevertheless, new tendencies are heading towards sustainable building and ecological materials. this is the reason why it is appropriate time to think about improving hpc in an environmentally friendly way. silica powder, using as inner filler in hpc, is possible to replace with glass powder, because of similar properties. on one hand, the glass is a nondecomposable material which is necessary to landfilled. however, on the other hand, the material is almost 100% recyclable [3]. recycling is the necessary attitude for the next years. along with recycling it is desirable to minimize the amount of waste [4]. there are several studies [5–8], that examine the possibility of using glass waste as a partial replacement of aggregate in concrete. it is possible to replace coarse aggregate or fine aggregate. the replacement ratio depends on particles size due to possibility of alkalisilica reaction (asr). it is important to notice that on basis of alkali-silica reaction are significant only aggregates with amorphous silica [9]. the utilization of waste glass as replacement of different constituents in concrete mixture has been investigated (aggregate replacement, inert filler, partial cement replacement etc.). the use of waste glass as partial replacement of coarse aggregate for ordinary concrete is possible way in some cases. there are several studies to examine a partial cement and quartz powder (fine fraction) replacement by waste glass powder in ordinary concrete or ultra-high performance concrete (uhpc) [8, 10]. the utilization of waste glass powder as partial replacement in hpc has not been completely explored yet. for this reason, the aim of this investigation was to prove the use of waste glass powder as full replacement of silica powder in hpc. the experiments were focused on a verification of mechanical properties and durability of hpc with waste glass powder. the results were compared with the reference sample containing silica powder. 24 http://dx.doi.org/10.14311/app.2019.21.0024 http://ojs.cvut.cz/ojs/index.php/app vol. 21/2019 glass waste powder utilization in hpc 2. materials and experimental program samples of waste glass powder was collected and examined. there were two types of waste glass – milled waste glass from municipal waste and grinding glass. analysis were made – granulometry and chemical analysis. after that, the grain curves were investigated and compared with the reference sample – silica powder. the same thing was made with chemical composition. after completing analysis of waste glass powder, high performance concrete was made. the basic recipe was optimized in previous research project, cf. table 1. the optimized recipe is used as the reference sample (a). after that, the optimized basic recipe was changed – replacing silica powder with grinding glass (sample b), then with the milled waste glass from municipal waste (sample c). the weight of fine fraction stayed the same. there were two samples (b, c) which were examined and compared with reference sample (a). the differences were evident in the structure of concrete. table 1. basic recipe of hpc – reference sample a. size of prepared samples for tests of mechanical properties was 40 × 40 × 160mm. there were 54 samples made, 18 from each type. samples 1–15 of each type was ready for tests after 28 days, samples 16–18 was ready for tests after 60 days. it was also meant to compare differences of the mechanical properties between 28-days-old concrete and 60-days-old concrete. 2.1. tests of mechanical properties tests of mechanical properties were made using destructive methods. the tests were made on samples after 28 and 60 days. prepared samples were examined to determine: compressive strength. the tested samples can be cubes, cylinders. in this paper, compressive strength was tested on fragments of samples 40 × 40×160mm. the tested surface ac was 40×40mm. there are certain conditions to keep – about samples (en 12350-1, en 12390-1, en 12390-2 or en 125041), machine (en 12390-4) and procedure. tensile bending strength. tensile bending strength is described in čsn en 12390-5 [11]. the test is realized on block specimens with dimension 40 × 40 × 160mm. tensile bending strength is defined by three-point press according to čsn en 1015-11 [12]. the test is over after the sample breaks. the highest force registered during the test is used for calculation tensile bending strength. it is always necessary to test at least three samples. freeze-thaw resistance. the tested samples are 40 × 40 × 160mm. this test was made following čsn 73 1322. cooling is done by air – 4 hours to −20◦c and then 2 hours of warming up to 20◦c. the samples were tested in 25, 50, 75 and 100 cycles. 3. results and discussion the following chapters describe the results and discussion of all tests. 3.1. chemical analysis chemical analysis of samples a, b and c were examined during the spring of 2017. table 2 summarize the results. there are differences such as similarities in chemical composition. after checking amount of sio2 and nao2, cao, it is possible to see difference in number – dozens of percent. chemical composition has an impact to resulting properties. it is necessary to examine potential alkali-silica reaction. the topic of detailed examination of chemical composition could be used in the upcoming research. table 2. results of chemical analysis. 3.2. particle size distribution the size of particle has the main impact to the results. with the growing particle size increases the potential risk of asr. it would be convenient to consider another milling or regrinding of the particles to achieve appropriate size in next research. the comparison of particle size is attached in graph 1. graph 1. comparison of particle size distribution. 25 d. mariaková, t. vlach, t. pavlů acta polytechnica ctu proceedings 3.3. bulk density bulk density of the samples could have the main impact to the results. the porosity in the sample can change the compressive strength or the tensile bending strength values. this research used the same weight amount of waste glass powder to replace silica powder. it would be appropriate to try another attitude in next research – based on bulk density, calculate the appropriate amount of the replacement. graph 2. bulk density of the samples a, b and c – average values from 10 samples. it is possible to see difference between samples b and c compared to sample a, cf. graph 2. however, the difference is negligible – bulk density of sample b is 94, 9%, bulk density of sample c is 94, 3% compared to sample a – considered as 100%. bulk density of the concrete is calculated based on weight and volume. in graph 3, note that the bulk density of the reference sample a is comparable with sample b and sample c. graph 3. bulk density of the concrete after 28 days and 60 days – average values from 3 samples. difference between samples b and c is less than 10% compared to reference sample a in all cases. based on this knowledge, it is possible to compare the samples in following tests. 3.4. tensile bending strength longer hardening time has a positive impact to the results – tensile bending strength increased after 60 days, cf. graph 4. nevertheless, the values between cycles are slightly different (with the increasing number of cycles the values are decreasing), but the impact of freezing cycles is not as big as the impact, for example, particle size or chemical composition. sample b achieved better results than sample c, it is probably caused due to the chemical composition. the values of sample b and c were 68%–88% compared to sample a (considered as 100%). however, it would be appropriate to work on utilization of both samples in hpc and try to improve the results. graph 4. comparison of tensile bending strength – average values from 3 samples. 3.5. compression strength compressive strength values showed positive impact of longer hardening time – the results are better after 60 days (expect sample a – although it is reference sample, in this case it is a negligible value). it would be appropriate to make the other tests based on longer hardening time in next years. the compressive strength values after freeze-thaw resistance tests are satisfying, cf. graph 5, the values of sample b and c range from 70% to 79% compared to sample a (considered as 100%). however, the values are decreasing (sample a, b, c) – it is unusual for hpc. graph 5. comparison of compressive strength – average values from 3 samples. 4. conclusions utilization of waste glass powder in high performance concrete was examined. waste glass powder was used as replacement of silica powder in hpc. mechanical 26 vol. 21/2019 glass waste powder utilization in hpc and physical properties of concrete made with waste glass powder from different sources were investigated. two samples of high performance concrete contain waste glass powder were made, one sample of high performance concrete contains silica powder was made as well – the reference sample. two waste glass powder samples were examined and compared to the reference sample. first, the chemical composition was examined to find out the differences and similarities. there is a possibility to modify chemical composition to improve the results. the particle size distribution was investigated, because the particle size has the main impact to the structure of the concrete and therefore the impact to the results as well. it would be appropriate to consider another milling to reach the right particle size – comparable to silica powder. the bulk density of silica flour, glass powder and concrete was examined as well. the values were slightly different – it should not affect the results of the physical properties. the tests to find out the compressive strength and the tensile bending strength were made. the results were compared to reference sample (contains silica powder). there is a risk of an alkali-silica reaction, because of glass used in the hpc mixture. the freeze-thaw resistance tests confirmed that due to the dense hpc structure, the probability of the alkali-silica reaction was eliminated. this study can be used as a basic for further work. the waste glass powder can be improved in different ways in next years and the utilization in hpc as inner filler is possible. acknowledgements this work was supported by the grant agency of the czech technical university in prague, grant no. sgs17/010/ohk1/1t/11. references [1] l. myšíčková. vlastnosti, zkoušení a využití uhpc v konstrukcích. brno: vysoké učení technické v brně, fakulta stavební, ústav stavebního zkušebnictví 2011. [2] high performance concrete. http://civil-resources.blogspot.be/2010/06/ high-performance-concrete.html. [3] j. vacková. problematika třídění odpadu u různých věkových kategorií obyvatel. české budějovice 2013. [4] d. vávrová. recyklace skla 2011. [5] a. mohajerani, j. vajna, t. h. h. cheung, et al. practical recycling applications of crushed waste glass in construction materials: a review. construction and building materials 156:443–467, 2017. [6] j. khatib, e. negim, h. sohl, et al. glass powder utilisation in concrete production. european journal of applied sciences 4(4):173–176, 2012. [7] i. b. topcu, m. canbaz. properties of concrete containing waste glass. cement and concrete research 34(2):267–274, 2004. [8] z. z. ismail, e. a. al-hashmi. recycling of waste glass as a partial replacement for fine aggregate in concrete. waste management 29(2):655–659, 2009. [9] r. breitenbücher. alkalicko-křemičitá reakce–důsledky pro cementobetonové kryty. strasse+ autobahn 4, 2006. [10] n. soliman, a. tagnit-hamou. development of ultra-high-performance concrete using glass powder–towards ecofriendly concrete. construction and building materials 125:600–612, 2016. [11] čsn en 12390-5. testing of hardened concrete–part 5: bending strength. czech standardization institute, prague 2007. [12] čsn en 1015-11. zkušební metody malt pro zdivo. 27 http://civil-resources.blogspot.be/2010/06/high-performance-concrete.html. http://civil-resources.blogspot.be/2010/06/high-performance-concrete.html. acta polytechnica ctu proceedings 21:24–27, 2019 1 introduction 2 materials and experimental program 2.1 tests of mechanical properties 3 results and discussion 3.1 chemical analysis 3.2 particle size distribution 3.3 bulk density 3.4 tensile bending strength 3.5 compression strength 4 conclusions acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2019.21.0021 acta polytechnica ctu proceedings 21:21–23, 2019 © czech technical university in prague, 2019 available online at http://ojs.cvut.cz/ojs/index.php/app technology of preparation of barium and magnesium hydroxide nanodispersion and possibilities of their use in monument care klára kroftováa,∗, david škodab, ivo kuřitkab, jan kubátc a czech technical university in prague, faculty of civil engineering, dept. of architecture, thákurova 7, 166 29 prague 6, czech republic b centre of polymer systems, tomas bata university in zlin, tř. tomáše baťi 5678, 760 01 zlín, czech republic c czech technical university in prague, faculty of civil engineering, dept. of building structures, thákurova 7, 166 29 prahue 6, czech republic ∗ corresponding author: klara.kroftova@fsv.cvut.cz abstract. an application of hydroxide nanoparticles in preservation of cultural heritage attracted great interest. the nanodispersions of calcium, magnesium or barium hydroxides exhibit high performance in consolidation abilities. in this work we focused on preparation of magnesium and barium hydroxide nanoparticles. for the synthetic procedure we used acetate precursors and sodium hydroxide. obtained particles were dispersed in 2-propanol. hydroxide nanoparticles were characterized by powder x-ray diffraction, infrared spectroscopy and scanning electron microscopy. it was observed that smaller particles were achieved in case of magnesium hydroxide. presented work demonstrates that this reaction procedure can be applied for preparation of materials used in cultural heritage preservation. keywords: magnesium, barium, hydroxide, nanodispersion, cultural heritage, preservation, consolidation. 1. introduction present cultural heritage preservation is based on the requirements of reversible and noninvasive interventions in historical structures and buildings in general, so as not to disturb the intact parts of the structure and to lose historical values. for these reasons, at the present time, the reconstruction of protected monuments places high demands on the preservation of historical matter itself and the preservation of the authentic expression of the object as a representative of the value of old age. within the ”minimal invasion” perspective, new, virtually invisible methods are highly desirable that can not only stop but also ”reverse” the degradation processes of artworks, can restore the original appearance of artifacts. in recent decades, the meaning of nanotechnology has raised in the area of cultural heritage protection, which brings new ways of protecting building structures by using specific, functionalized properties of materials due to increasing the surface area ratio of particle surface to particle volume of nanomaterials. compared with macro-materials, nanomaterials represent extremely high ”performance” in contrast to very low material consumption. nanomaterials can, for example, due to a great penetration into the pore system and yet preservation of the original material, facilitate the cleaning of varnished surfaces (graffiti), provide temporary stiffening of the plastered surface, enhance uv protection, provide self-cleaning technology, and more. it can be reasonably assumed that the use of nanotechnologies could provide new protection options, especially the surfaces of building materials, which aim at speeding up, facilitating or slowing chemical reactions and providing various protective functions of surface treatments etc. [1], [2], [3], [4], [5]. practical examples include the consolidation and consolidation of plaster surfaces and natural stone porous elements using nanomaterials based on calcium or barium hydroxide, in some cases sulphates [6],[7]. the most common nanomaterials used in restoration are water-soluble inorganic nanomaterials, especially calcium hydroxide and magnesium hydroxide [8], [9]. calcium hydroxide nanoparticles can be used to deacidify, consolidate and solidify the material on the basis of ”remineralization”. combined with the barium hydroxide solution, it also serves to remove sulphates and the so-called ferroni-dini method to in situ revitalize the original binder in paintings [10]. magnesium hydroxide can be successfully applied when restoring and preserving wood and artifacts from paper. just like ca(oh)2 and mg(oh)2, carbonates react with airborne co2 to strengthen and fill the damaged parts of the restored monuments. in addition,mg(oh)2, for example, slows the cellulose photodegradation [11]. herein we present a synthesis of barium and magnesium hydroxides from barium and magnesium acetate precursors, respectively. these materials were prepared by precipitation technique with sodium hydroxide in 2-propanol. obtained materials were dispersed in 2-propanol to form suspensions. 21 http://dx.doi.org/10.14311/app.2019.21.0021 http://ojs.cvut.cz/ojs/index.php/app k. kroftová, d. škoda, i. kuřitka, j. kubát acta polytechnica ctu proceedings 2. materials and methods the naki dg16p02m055 research project ”development and research of materials, methods and technologies for the restoration, preservation and strengthening of historic masonry constructions and surfaces and systems of preventive conservation of cultural heritage buildings threatened by anthropogenic and natural hazards (2016 2022, mk0/dg)” addresses the issues of strengthening historic surface plaster layers with lime nanosuspensions. the institutions currently participating in the research project are the centre of polymeric materials, tbu in zlín and the faculty of civil engineering, ctu in prague. 2.1. chemicals barium acetate (ba(ch3coo)2, mw: 255.42 g/mol, 99 %, sigma aldrich), magnesium acetate tetrahydrate (mg(ch3coo)2 · 4h2o, mw: 214.45 g/mol, sigma aldrich), sodium hydroxide (p.a., sigma aldrich) and 2-propanol (p.a. vwr). 2.2. syntheses typical synthesis of mg(oh)2 was performed as follows: magnesium acetate tetrahydrate (2.00 g) was added into 200 ml of 2-propanol under vigorous stirring. afterwards, 50 ml of aqueous sodium hydroxide (1.16 g) solution was dropwise added into stirred magnesium acetate suspension. after addition of sodium hydroxide, the reaction mixture was stirred at room temperature for 24 hours and white cloudy suspension was observed. solid product was separated by centrifugation, dried at 90 ◦c and characterized by powder x-ray diffraction, scanning electron microscopy and ftir spectroscopy. typical synthesis of ba(oh)2 was performed by the same manner as in the case of previous magnesium hydroxide. barium acetate (3,00 g) was added into 200 ml of 2-propanol under vigorous stirring. afterwards, 50 ml of aqueous sodium hydroxide (0.94 g) solution was dropwise added into stirred barium acetate suspension. after addition of sodium hydroxide, the reaction mixture was stirred at room temperature for 24 hours and white precipitate was observed. solid product was separated by centrifugation, dried at 90 ◦c and characterized by powder x-ray diffraction, scanning electron microscopy and ftir spectroscopy. 2.3. characterization techniques the powder xrd patterns were recorded on a rigaku miniflex 600 equipped with a cokα (λ = 1.7903 å) xray tube (40 kv, 15 ma). the ir spectra were recorded on a thermo scientific nicolet 6700 spectrometer using an atr technique with the diamond crystal (4000-400 cm-1, resolution 2 cm-1, 64 scans). the sem images and edx analysis were recorded on a nova nanosem (fei) with schotky field emission electron source (0.02 30 kev) and tld detector at 5 kv. 2.4. results and discussion barium hydroxide and magnesium hydroxide were prepared by precipitation reaction of acetate precursors and sodium hydroxide aqueous solution in 2-propanol. crystalline phases of obtained products were determined by powder x-ray diffraction. as illustrated in fig. 1a), diffractions of prepared mg(oh)2 match with reference diffractogram of brucite (jcpds card 01-0715972) and thus successful formation of mg(oh)2 crystalline phase is confirmed. since the observed diffractions are broad, nanostructured character of this sample can be expected. in the case of prepared ba(oh)2 sample (fig. 1b)), diffractions of ba(oh)2 ·h2o (jcpds 00-26-0154) and ba(oh)2 · 3h2o (jcpds 01-077-2333) were assigned. this observation corresponds with reported data of barium hydroxide chemistry with the hydrate structure ba(oh)2 · (h2o)x. [12]. figure 1. a) powder xrd diffractogram of prepared mg(oh)2 sample; b) powder xrd diffractogram of prepared ba(oh)2 sample. ftir spectroscopy was used for characterization of functional groups and chemical bonds in prepared samples. fig. 2 displays the ftir spectrum of mg(oh)2 sample. sharp vibrational band at 3696 cm1 corresponds to stretching modes of oh groups in mg(oh)2. the mg o bonds are represented with the vibrational band below 500 cm-1. broad band at 3350 cm-1 is assigned to oh groups of adsorbed water or 2-propanol. vibrational bands between 1700 and 1250 cm-1 are attributed to residual acetoxy groups and groups of 2-propanol solvent. in the case of barium hydroxide sample, ftir spectrum is little bit complicated in compared to magnesium hydroxide. sharp vibrational band of oh stretching modes in ba(oh)2 is located at 3574 cm-1 (fig. 2). this observation confirms the presence of ba(oh)2. other vibrational bands recorded in ftir spectrum can be assigned to 2-propanol, residual acetoxy species and undefined reaction byproducts. scanning electron microscopy was employed in order to investigate a morphology of prepared samples. as depicted in fig. 3, mg(oh)2 formed needle shape nanoparticles with the lengths about 100 nm. in case of ba(oh)2 sample the size of particles is significantly higher (about 500 nm). this behavior is caused mostly by a different chemistry between magnesium and barium. for example, barium hydroxide is stronger base 22 vol. 21/2019 technology of preparation of barium and magnesium hydroxide nanodispersion figure 2. a) ftir spectrum of mg(oh)2 sample; b) ftir spectrum of ba(oh)2 sample. figure 3. sem images of product powders. left column: mg(oh)2 sample, right column ba(oh)2 sample. than magnesium hydroxide and this can affect the final form of prepared sample. both samples exhibit the character of aggregated particles. these aggregates can be for example separated in ultrasonic bath. 3. conclusions his work reports the preparation of magnesium hydroxide and barium hydroxide nanoparticles via precipitation reaction of acetate precursors with sodium hydroxide. analyses of both products confirmed the formation magnesium hydroxide and barium hydroxide crystalline phases and thus the use of acetate precursors in these reactions is suitable. in the case of magnesium hydroxide sample small needle-like crystals with the length about 100 nm were observed. barium hydroxide sample revealed bigger particles. from this point of view we can conclude that presented synthesis is more suitable for preparation of magnesium hydroxide nanoparticles. obtained nanoparticles can be dispersed in 2-propanol and this dispersion can be supported by use of ultrasonic bath. anyway, both products dispersed in 2-propanol have the potential to be used in cultural heritage restoration. acknowledgements this article was written as part of the naki dg16p02m055 research project ”development and research of materials, methods and technologies for the restoration, preservation and strengthening of historic masonry constructions and surfaces and systems of preventive conservation of cultural heritage buildings threatened by anthropogenic and natural hazard (2016 2022, mk0/dg)”. references [1] j. rathouský. micellar solutions and microemulsions for the cleaning of historical materials. nanomaterials in heritage conservation. [2] l. machačko, et al. konsolidace historických omítkových vrstev v druhém np ambitu bývalého kláštera rosa coeli v dolních kounicích pomocí vápenné nanosuspenze calosil, 2012. [3] v. daniele, et al. the nanolimes in cultural heritage conservation: characterisation and analysis of the carbonatation process, journal of cultural heritage, 2008. [4] d. chelazzi, et al. hydroxide nanoparticles for cultural heritage: consolidation and protection of wall paintings and carbonate materials. journal of colloid interface science 2016. [5] l. ventola, et al. traditional organic additives improve lime mortars: new old materials for restoration and building natural stone fabrics, construction and building materials, 2011. [6] m. drdácky, z. slízkova. nanomateriály v péči o památky. akamedický bulletin (10):10–11, 2012. [7] r. giorgi, et al. nanoparticles for cultural heritage conservation: kalcium and barium hydroxide nanoparticles for wall painting consolidation. chemistry a european journal 16:9374–9382, 2010. [8] p. baglioni, r. giorgi. soft and hard nanomaterials for restoration and conservation of cultural heritage. soft matter 2:293–303, 2006. [9] a. blee, j. matisons. nanoparticles and the conservation of cultural heritage. materials forum 32:121–128, 2007. [10] p. baglioni, et al. nanotechnology in wall painting conservation conference: meeting on self-assembly the future pp. 32–41, 2003. [11] r. giorgi, et al. nanoparticles of mg(oh)2: synthesis and application to paper conservation, langmuir, 2005. [12] r. kresse, u. baudis, p. jäger, et al. barium and barium compounds. in ullmann’s encyclopedia of industrial chemistry 2007. 23 acta polytechnica ctu proceedings 21:21–23, 2019 1 introduction 2 materials and methods 2.1 chemicals 2.2 syntheses 2.3 characterization techniques 2.4 results and discussion 3 conclusions acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2019.23.0014 acta polytechnica ctu proceedings 23:14–19, 2019 © czech technical university in prague, 2019 available online at https://ojs.cvut.cz/ojs/index.php/app preparations for remediation of a former surface mine – a technical reclamation area petr černocha,∗, jiří košťálb,c a čez energetické produkty, s. r. o., komenského 534, 253 01 hostivice, czech republic b inset, s. r. o., division of energetics, lucemburská 7, 130 00 prague, czech republic c czech technical university in prague, faculty of civil engineering, department of geotechnics, thákurova 7, 166 29 prague, czech republic ∗ corresponding author: cernoch.petr@inset.com abstract. the reclamation area for secondary float energy by-products is located on a spoil heap, 45m thick and 30 years old, and consists mostly of clay stone with high plasticity. when depositing a first 18m high stack, it was necessary to verify stability and to analyse deformations during the reclaiming operation. stability and deformation depend on the shallow groundwater level. therefore, it was necessary to propose a technical solution for drainage of the structure concerned. this can be divided into two parts: drainage of surface water and drainage of the shallow underground water. it means a complex solution for the entire construction that would not be possible without long-term monitoring of the construction and field mapping. completion of the proposed facilities will increase the stability of the reclamation area and it will then be possible to continue with remediation of the former surface mine. keywords: remediation, reclamation, stability, monitoring, drainage. 1. introduction brown coal mining entails a number of negative aspects, risks and difficulties that geotechnical study has to manage. one of them is the issue of addressing the impact on the environment after mining activities have ceased, and then restoring the sites where the mining activities were pursued. these aims are pursued by a technical reclamation and an area restoration project that is primarily directed at refilling low parts of the terrain that are the result of mining work. subsequent revitalization of the landscape in a way that gives careful consideration to the surrounding environment is a comprehensive and long-term process. for this, as a matter of preference for restoration purposes, the method used involves depositing soil and materials that qualify as certified energy by-products [1] taken from adjacent coal-fired power plants. in any technical reclamation by depositing energy by-products, potential geotechnical risks arise. in particular, among these are a possible rise in groundwater levels as a result of the additional load on the subsoil caused by the weight of the structures and getting to appropriate design solutions for the drainage of surface water, rainwater and shallow underground water from the repository area. another phenomenon is the stability and deformation-related characteristics of the material that is deposited [2]. 2. the structure concerned and the working objectives 2.1. the area of interest and its history the structure presented as identifiable with the technical reclamation area is located in the south-eastern part of an extended zone of surface mines and constitutes the refilling phase of a large-scale reclamation scheme to restore areas where there were formerly mining operations. the first written mention of mining activities in the wider area of the locality concerned date back to the second half of the 18th century when a brown coal seam was discovered here. in the early 1860s, there were already 36 mines in operation, with the first surface mine working a seam up to 8m thick. the largest open pit mine began to be mined in 1917 and gradually went on to supply coal to nearby power plants. the bottom of the mine practically corresponded to the bottom of the seam; only the safety pillar of fixed reserves remained free of actual coal mining. the area of the depleted mine where the technical reclamation area structure is located was gradually filled with dump soil and overburden materials from adjacent smaller surface mines [3]. 2.2. brief description of the structure and natural conditions the technical reclamation area is used in order to deposit energy by-products, specifically power plant fly ash mixtures, energy gypsum and other deposit materials. the depositing activities started in 1996. the materials are deposited with a belt stacker (figure 1) 14 https://doi.org/10.14311/app.2019.23.0014 https://ojs.cvut.cz/ojs/index.php/app vol. 23/2019 remediation of a former surface mine – a pvz2500 crawler wagon with a boom length of up to 45m and a total specified weight of 350 tons. the tripper car (weight 63 tons) supplying the stacker with material from the conveyor belt and the cable drum car (69 tons) that supplies power are essential parts of the material stacking technology. subsequently, the material is shaped and compacted by a bulldozer. the deposit material is prepared in the mixing centre of a nearby coal power plant. from a geomechanical point of view, the deposit material is a finegrained mixture consisting of 50–70% dust fraction and 35–45% sand fraction. the remainder of the grain size curve consists of a minor share of clay particles (< 0.002mm). the area of interest is situated at the western edge of the north bohemian brown coal basin, specifically in the area of the střezov ridge. the whole formation is broken by a number of minor transverse faults with tertiary volcanic outcrops. the northern and eastern edge of the area is built of basin sediments, while volcanic rocks prevail at the western and southern edges. the north-western edge of the technical reclamation area is bordered by rocks of crystalline complex, which are exposed in the wider zone and form the subsoil of the basin sediments in the former mine area and its surroundings. around the technical reclamation area, only the overlying strata are developed in lowest thicknesses of up to 15m, and are only present in the northern and eastern parts of the zone. the coal seam formation is found in most parts of the area of interest almost in practice at the level of the original or present surface of the terrain and alongside coal seams, and it contains xylites, oxyhumolites, grey-brown clay stones and powdery clay stones with a fluctuating content of coal admixture and coal components. in the subsoil there are clays of green shades with a high content of tuffitic material, which in places shift to tuffites and volcanic breccias. their thickness is very variable (from 50m to the lower hundreds of metres), depending on the morphology of the crystalline bedrock. as in many similar cases, the technical reclamation area structure is here also located on the spoil heap. the immediate subsoil of the deposit body is the surface of an outer dump of overburden materials, consisting of clay soils with high plasticity, which is more than 30 years old and up to 45m thick. given the length of time the spoil heap has been exposed to climatic conditions, water infiltration and self-weight, its surface can be considered effectively consolidated. the northern part of the structure concerned already interferes with a part of the inner spoil heap. the hydrogeological conditions in the wider area concerned have changed several times in recent history. initially this was as a result of mining in the open pit mine and the consequent creation of the outer dump, and then later through the actual depositing of energy by-products. because of the impermeable nature of the dump soils, they cannot be expected to have any presence in terms of a significant groundwater horizon; the groundwater level more or less copies the outer surface of the dump, except in the southern part of the structure. in the early stages of the depositing, the groundwater level in the dump ranged, depending on the terrain morphology, from 282m above sea level (in the western part) to 315m (in the eastern part). under these circumstances, water flowed in a northerly direction towards the mine, and also southwards, to the local reservoir, which forms the drainage base of a wider area with a free water level at about 270m above sea level. deeper groundwater circulation is tied to coarse-grained sediments – miocene sedimentary sands mixed with clay elements. in the southern and north-eastern front zone of the structure, there are permanently waterlogged places which are relatively large in area. the region concerned is characterized by low rainfall and relatively high air temperatures. western winds prevail; the climate is warm, with a cooler spring and warm autumn. winter is mild to moderately warm. the original slope of the terrain on the site of the technical reclamation area was towards the south. after backfilling with material from the outer dump, the general slope of the surface changed to that of falling slightly towards the north. the present slope and terrain morphology are the result of how the depositing of energy by-products progressed [4]. the entire structure has been subject to long-term monitoring both by hydrogeological probes (observation boreholes) and through geotechnical inspection monitoring elements (inclinometric probes, pore pressure gauges installed in boreholes, and geodetic points). 2.3. objective of the work over the course of depositing the first layer of the deposit material, about 18m thick in the technical reclamation area, the need emerged to verify the stability of the emergent deposit body of energy by-products and to analyse the expected deformations in connection with the operation of stacking machinery at the minimum permissible distance of the stacker from the edge of the slope crest. the aim of the stability and deformation-related calculations was to determine the distance at which the earthwork carrying the weight of the stacking machinery has adequate long-term stability. furthermore, partial stability calculations were carried out – primarily to determine the preliminary stability of the northern slopes of the structure near the service road of the surface mine. another area of focus was the slopes delimiting the borrow pit in the east of the technical reclamation area (for short-term stability at the time of excavation). finally, there was also a focus on the southern slopes of the area of interest, which had the least favourable morphology and geological conditions – here the long-term and permanent stability of the deposit body and the southern front zone was assessed. 15 petr černoch, jiří košťál acta polytechnica ctu proceedings figure 1. stacking the deposit material in the technical reclamation area using a belt stacker. the stability and deformations of the body of energy by-products that was examined are significantly dependent on the shallow groundwater level. therefore, apart from the aforementioned calculations, it was also necessary to propose a technical solution for the drainage of the structure concerned. this also doubled in function as reference documents for updating the project for the technical reclamation of the area of interest. on the basis of the results of the field mapping and geodetic surveying of important points, a proposal for the drainage system of the structure was subsequently conceived. the drainage solution can be split into two logical parts: surface water and rainwater drainage and runoff, and then also drainage of shallow underground water. this was therefore a complex geotechnical solution, involving long-term monitoring of the area of interest, and including extensive archival research, all used as a basis for the stability and deformation-related calculations. subsequently, on the basis of their results and the field mapping, a comprehensive draft design of the structure and its shape, including drainage, was conceived. 3. implementation of the work and operational results 3.1. stability and deformation calculation the geo 5 and plaxis software were used to calculate stress-strain behaviour and slope stability. the first of these uses the limit force equilibrium method to perform the calculation. the bishop’s shearing surface optimization method was used to determine the critical least favourable shearing surface with the lowest local degree of stability. the optimisation was worked out by gradually changing the positions of individual points on the surface and determining which change in a given point’s position resulted in the lowest degree of stability. the numerical model specified in the plaxis software works on the basis of the finite element method (fem), and determines the degree of stability by automatic simultaneous reduction of the cohesion values and the tangent of the internal friction angle (ϕ and c reduction). the calculations were divided into steps corresponding to the expected work procedures. for each phase of the construction and load effect, a calculation focusing on the deformations caused by the static load was also performed. as an additional part of the calculation, a state was also modelled with an additional load from the stacker, taking into account the dynamic effects or simultaneous operation of pertinent machinery (the tripper car, cable drum car, middle parts, and material being transported) – thus procedural safety. the contact area of the caterpillar bands was modelled using two contact rectangles. the characteristic contact stress was calculated from the total weight of the stacker and the total area of the caterpillar bands, i.e. from the relation: f = mtotg shous = 500000 × 10 2.4 × 8.6 × 2 = 121 kn/m2, (1) where f is the characteristic contact stress, mtot the total weight of the stacker and shous the area of the caterpillar bands. the terrain morphology data that was used in the calculation model was based on the geodetically measured input. the boundaries between geological quasi-homogeneous units (geo-types) and the line of the groundwater level were processed on the basis of archival research and the results of long-term monitoring. in total, four computational profiles were created, three of which dealt with problematic areas of the structure (see chapter 2.3), while one section was used to determine the usability of the stacker for future levels of the deposit body (see above). 16 vol. 23/2019 remediation of a former surface mine the groundwater level was entered as a polygon, under which the mass unit weight of saturated soil and upward hydrostatic pressure was considered by the software in the calculation, and above it the entered mass unit weight value. the pore water pressure at a given point was calculated as the hydrostatic pressure, with the line of the groundwater level being taken into account. in dealing with the effect of the load on a level from the stacker, the groundwater level was assumed to be sufficiently deep below the bottom surface of the deposit body that in consequence its effect was negligible. in the remaining profiles, however, the groundwater level played a significant role. it was not possible to enter the course of infiltrated water, leachate water and drainage water (discontinuous and varying groundwater levels) into the calculation. however, thanks to the results of the monitoring of the structure that was implemented, the places where there was permanent surface waterlogging in the front zone of the technical reclamation area were sufficiently mapped, which made it possible to include them in the numerical modelling. the results of the limit force equilibrium method showed that approaching the edge of the slope to a distance of 5m with the stacker causes a deformation stress state in the earthwork with a degree of safety fs =1.69 and a maximum total deformation of the deposit body of 0.25m. in numerical verification using the finite element method, the total degree of stability was determined at fs =1.63. the shortest possible distance the stacker can travel to the edge of the earthwork, determined in this way, does not pose a technological restriction on operations, which is of key importance for further depositing processes. the additional load from the stacker caused deformations with their direction and size evident from the shift vectors (figure 2). the model showed a significant predominance of vertical over horizontal deformations, providing the stacking is within acceptable limits. the calculations performed did not indicate tensile cracking and only then to a negligible extent on the surface of the deposited material [5]. the stability calculation showed compliance with the requirement of short-term stability for profiles running through the northern slopes of the technical reclamation area and in the area near the borrow pit in the eastern part of the area of interest (fs =2.30 and fs =1.47 respectively). for the southern front zone of the structure, the resulting degree of stability was determined at fs =1.05. in conclusion, the resulting slope stability only met the requirements to a certain extent and did not meet the requirement of long-term stability. the results obtained were affected both by the shear parameters of the soils (which depend, inter alia, on the degree of consolidation, which in turn is influenced by the age and thickness of the deposit layers) and by the line of the groundwater level. it was therefore recommended to draft construction and technical measures that minimize unwanted soil saturation with rainwater in the southern front zone of the structure and that at the same time facilitate the runoff of shallow underground water from the technical reclamation area and from the affected area of permanent waterlogging in the southern front zone of the structure concerned. another output from the calculations that were performed was the conclusion that in the further stacking of the deposit material, any significant rise in the groundwater level in the subsoil should be avoided. it was therefore recommended to increase the frequency of measurements of groundwater levels in the monitoring wells and to install additional observation probes in the risk area for the shallow groundwater level horizon – observation piezometers and boreholes [6]. 3.2. earthworks from the above, the necessity arose of detailed mapping of the technical reclamation area with a focus on the current state of the surface waterlogging and drainage of the structure. the reconnaissance of the area of interest included a precise geodetic survey of significant hydrological elements and phenomena. during the fieldwork, concrete troughs were found to be clogged (with a layer of mud up to 40 cm thick) in many places, or entirely missing. in some places, undesirable aquifers thus formed (figure 3). surface water and rainwater from the mine service road should be channelled through an open ditch to the existing accumulation sump in the eastern part of the area of interest; however, the ditch is blinded and hence not functional. 4. draft technical solution and recommendations on the basis of the work carried out, a comprehensive solution of the technical reclamation area was proposed. this included in particular the draft design of the drainage method split into two main parts: a solution to surface water and rainwater runoff, and a solution to the drainage of shallow groundwater. 4.1. drainage of surface water and rainwater in designing the method for the drainage of surface water and rainwater, it was proposed to use the existing concrete trough combined with new piping. the existing troughs will need to be cleaned and those that are broken restored. subsequently, a dn 500 pipe of flexible material (pvc) that allows bending will be laid in the concrete trough. this piping will provide drainage of surface water even after the backfilling of the trough. it will be necessary to connect the piping at the southern portal through a connection under the service road (which includes lifting the blinding) and terminate it at the point of the drainage inflow to the sump area – the pipe outlet. the entire section concerned has a length of 523m, and from 401m to 523m the concrete troughs are actually missing. 17 petr černoch, jiří košťál acta polytechnica ctu proceedings figure 2. total deformation caused by additional load from the belt stacker. figure 3. waterlogging as a result of clogging or absence of surface drainage. there we recommend laying new concrete troughs while maintaining the proposed slope. 4.2. drainage of shallow underground water the drainage of shallow underground water involves the implementation of a ground trench (1.0m in depth with a minimum bottom width of 1.0m), lined with a separation geotextile and supplemented with a dn 800 perforated drainage pipe of flexible material (pvc), covered with quarry stone (grain size >32mm) and a separation geotextile. the bottom of the excavation pit, consisting of impermeable dump soil, will be compacted. should any places with coarse-grained fractions be found, these will be replaced with the surrounding fine-grained dump soil and subsequently compacted. the drainage of shallow groundwater will be terminated at the point of the drainage inflow from the area of the front dump of the surface mine. from there, the route will continue on a downward slope with troughs all the way to the sump. the drainage pipe will be 377m long, the entire section concerned being 499m in length. water surfaces cause waterlogging of the bottom of the deposit body; thus it was recommended to dispose of this by pumping and backfilling to a sufficient level that a renewed water retention is prevented. this measure, along with the implementation of the drainage, will significantly increase the stability of the slopes concerned. their stability should be verified before continuing with the depositing of another level [7]. 5. conclusions a comprehensive series of works was carried out for the project of the technical reclamation of the structure concerned, all aimed at restoring the site of former mining activities by refilling with a certified product of a coal-fired power plant; these included the archival research, stability and deformation numerical calculations using data from long-term monitoring of the structure, and field mapping. the result of the works 18 vol. 23/2019 remediation of a former surface mine that were carried out was presentation of a draft technical solution for the drainage of the area of interest, intended to increase the stability of the structure under monitoring and enable its continued existence. in further stacking, any significant rise in the groundwater level in the subsoil should be avoided, and so a higher frequency of groundwater level measurements in the monitoring probes, as well as some additional probes, was recommended. references [1] p. černoch, j. košťál. příprava sanace bývalého povrchového dolu/odkaliště. in proceedings of conference geotechnika 2018, vysoké tatry. orgware, 2018. [2] p. černoch, j. košťál. geotechnical risks of foundation of warehouses built on brownfields. repair and maintenance strategies of geotechnical structures 2(2-3):981–986, 2018. [3] p. černoch, j. košťál. gt práce na rozšíření úložného prostoru. inset s. r. o., 2013. [4] p. černoch. hydrogeologické a hydrochemické sledování. výsledky za rok 2017, inset s. r. o. [5] j. janků. stabilitní výpočet provozního svahu z deponátu úložiště. inset s. r. o., 2016. [6] p. černoch, j. janků. ptr. přepočet stability. čez ep, s. r. o., 2017. [7] p. černoch, p. straková. návrh odvodnění předvýsypky ptr. inset s. r. o., 2016. 19 acta polytechnica ctu proceedings 23:14–19, 2019 1 introduction 2 the structure concerned and the working objectives 2.1 the area of interest and its history 2.2 brief description of the structure and natural conditions 2.3 objective of the work 3 implementation of the work and operational results 3.1 stability and deformation calculation 3.2 earthworks 4 draft technical solution and recommendations 4.1 drainage of surface water and rainwater 4.2 drainage of shallow underground water 5 conclusions references acta polytechnica doi:10.14311/app.2020.27.0042 acta polytechnica 27(0):42–47, 2020 © czech technical university in prague, 2020 available online at http://ojs.cvut.cz/ojs/index.php/app micro-scale study of residual stresses in cr2o3 coatings sprayed by aps franck decroosa,∗, cécile langladea, eric bourillota, geoffrey daruta, manuel francoisb a université de bourgogne franche-comté, laboratoire interdisciplinaire carnot de bourgogne, site de sevenans, 90 010 belfort cedex, france b université de technologie de troyes, charles delaunay institute, life assement of structures, materials and integrated systems, 12 rue marie curie, 10 004 troyes cedex, france ∗ corresponding author: franck.decroos@utbm.fr abstract. whichever the application field, every material forming process generates residual stresses on the surface. while they are likely to enhance the aimed properties of the final mechanical part, these stresses may also drastically reduce them and result in early failures. therefore, understanding the residual stress state remains a major challenge when coating complex parts, especially as most characterization methods at the microscopic scale involve specific sample preparation procedures which may affect the residual stresses field. this work investigates the residual stress state that exists in chromium oxide coatings deposited via atmospheric plasma spray (aps), using two pioneering techniques featuring high spatial resolution: scanning microwave microscopy and raman microspectroscopy. the first technique combines the measurement of microwave electromagnetic capacities of a vector network analyzer with the subnanometric resolution of an atomic force microscope: it thus enables performing depth investigations at very accurately defined positions of the probe on the surface. the second technique relies on the principle of photons inelastic scattering and involves a laser beam aiming at the material sample: it allows a fine characterization of the microstructure as well as defects and stresses detection via molecular vibratory signatures. the investigation scale is limited here to a few cubic micrometers. due to the highly localized scales of our investigations, which also depend on the device, the objective of our procedure required that the comparison should be made not on individual points but on definite mapped areas, every spot being analyzed and post-treated one after another, with optimum measuring parameters. results have been correlated with xrd measurements to cross-check the average amount of stress observed over a wider area. keywords: coatings, chromium oxides, raman micro-spectroscopy, residual stress, scanning microwave microscopy. 1. introduction 1.1. residual stresses in thermal spray coatings it is today widely-known that residual stresses (rs) have significant implications in both the preparation and the performance of thermally sprayed coatings. on one hand, tensile stresses are generally recognized to reduce service lifetime, weaken interfacial adhesion, and sometimes cause early delamination when they exceed the elastic limit of the material. on the other hand, compressive stresses are most often associated with better performance and fewer micro-cracks, which are known to enhance corrosion resistance during service [1–3]. thus, assessing the rs field of a coating is a major concern to optimize spraying parameters and match technical specifications. by doing so, a more accurate prediction of the in-service behavior of the coating is possible. important properties of rs are magnitude, sign, and even more specifically their superficial distribution. 1.2. origins of residual stresses thermal spray processes involve various stress generating phenomena. when dealing with ceramic coatings deposited via atmospheric plasma spray (aps), the most significant are the following. the first one is the effect of quenching, when the shrinkage of individual splats leads to local stress generation as the coating is cooling from its melting temperature to the substrate temperature. simultaneously, stressrelaxation mechanisms such as interfacial sliding and extensive microcracking may occur, microcracking being very common in ceramic materials [4–7]. the second phenomenon is due to the mismatch between the coefficients of thermal expansion (cte) of the coating and the substrate [1], [8]. a further effect lies in the transient injection of heat fluxes into the substrate, hence generating important thermal gradients which cause differential thermal expansion at different depths as well as changes to the local misfit strains [1]. this point is important in our study as the depth of investigation varies for the two tech42 http://dx.doi.org/10.14311/app.2020.27.0042 http://ojs.cvut.cz/ojs/index.php/app vol. 27/2020 residual stresses in cr2o3 coatings niques employed. macro stresses (type 1 rs) and joint stresses (type 2 rs) are worth investigating in our case in consideration of the engineering objective [9]. 1.3. purpose of the work in practice, the superimposition of the various phenomena mentioned above generates a complex rs field and makes the in-service behavior of aps coatings difficult to predict. therefore, the objective of this work is to combine new investigation methods for local determination of the rs field, targeting the two kinds of stress mentioned above. the first method is the raman micro-spectroscopy, which is increasingly used in this field [9–14]. this technology enables fast micro-scale measurements on raman-active materials such as oxides [9, 10].here, lattice bondings will be submitted to vibration through the inelastic photon scattering phenomenon using a low power laser emission. hence, all molecules are characterized by a unique spectrum. the fingerprint spectrum and behavior law of raman peaks regarding stress are today known for common types of materials including cr2o3 [11, 12]. the second method is a custom scanning microwave microscope (smm) performing nano or micro-scale measurements, here used for the first time on ceramics [13]. hence, the study also aims to calibrate this new instrument using raman spectrometer as reference. smm features two advantages: a 20 nm diameter area of investigation at surface, and the data collection at different depths underneath this small area. moreover, this is a non-destructive investigation method. indeed, rs measurement is assessed while measuring the signal amplitude, which will vary with changes in lattice organization of the investigated material. a more complete theory is available in “experimental section” of aybeke, en and al. paper [14]. this in-depth technique represents a significant change in stress investigation, especially when dealing with ceramic coatings. finally, x-ray diffraction (xrd) measurements based on the sin2(ψ) method will be correlated to the previous results. xrd stands for the reference in our field [1, 9]. to do so, mappings have been performed with both more local devices in order to be compared to xrd wider area. raman spectrometer and top surface smm mappings results can be expected to exhibit similar tendencies as both investigate changes in lattice organization. these investigation technologies have been applied on cr2o3 coatings performed by aps: the spraying temperature was chosen as the varying parameter to elaborate different residual stress fields. 2. methods 2.1. materials and coating deposition chromium oxide coatings were deposited via aps upon a 30 mm × 30 mm × 5 mm steel substrate using commercially available chromium oxide powder from saint-gobain. before deposition, substrates were sandblasted so that the ra was superior to 4 µm, then they were cleaned in ethanol. in order to investigate differences between rs fields, this paper deals with two samples: (1.) ref1 which was performed using optimized parameters and procedure for thermal barriers coating applications at a depositing temperature of 200 ◦c. thickness is 200 µm. (2.) ht2 which only differs from the previous one by its depositing temperature that was about 450 ◦c. thickness is 400 µm. the main spraying parameters are listed in table 1. measurements were conducted on as-deposited coatings. spraying parameters value current intensity (a) 650 ar/h2 ratio 40/13 gas flow rate (l.min-1) 3 spraying distance (mm) 110 table 1. main spraying parameters 2.2. scanning microwave microscopy the used scanning microwave microscope is an original device from agilent which has been widely customized since it was bought. it is basically made out of two parts as showed in figure 1. figure 1. scanning microwave microscope the first one is an atomic force microscope (afm), equipped with a probe which will be used as an antenna to emit as well as to receive the microwave signal. to do so, being in the near field becomes a necessary condition in order to emit a directive signal. thus, the tip of the probe was put directly in contact with the sample surface. besides, the design of the tip also guarantees the high resolution of the measurement as its diameter is no more than 20 nanometers. the second part of the device is a vector network analyzer (vna). at first, this vna generates a wave train of a sinusoidal microwave signal at a very low power. then, it compares the generated signal to the one which is reflected. in short, vna outputs the s11 reflection coefficient. as a matter of fact, generating a wave train of various frequencies enables in-depth investigation of 43 f. decroos, c. langlade, e. bourillot et al. acta polytechnica figure 2. a typical microwave signal a material. hence, a finite number of depths can be investigated, all of them corresponding to specific resonant frequencies which are machine dependent. the lower the frequency, the deeper the investigation underneath the surface. a typical spectrum of scanning microwave microscopy is shown as an example on figure 2 with its resonant frequencies denoted by asterisks. last but not least, it is commonly acknowledged that stress variation inside a material will lead to a variation in amplitude of the microwave signal [12, 13]. in brief, this is exactly the ordinate variation of the asterisks that are studied in this paper. in practice, measurements were carried out from 0.5 ghz to 13 ghz, which means that the waves go from the surface coating (13 ghz) down to the substrate (0.8 ghz). the temperature was kept between 22.3◦c to 24◦c and the air humidity between 25 % to 27 %. 2.3. raman micro-spectroscopy the raman micro-spectroscopy examinations were performed with a xplora plus from horiba which was calibrated with a (111) si single crystal. measurements were performed using the parameters listed in table 2. the diameter of the irradiating laser spot is about 0.87 µm and depth penetration is about 0.380 µm. laser irradiance was kept under 1 gw as advised in previous works [14]. parameters value acquisition time (s) 15 accumulations 15 objective x50 gratings 1800 filter (%) 10 table 2. raman spectroscopy parameters this paper focuses on the shift of the most intense mode 555 cm−1 a1g vibration [15, 16] as it is recogfigure 3. edge effects in smm nized to produce a large change in the effective mode volume, thus undergoing the largest stress-induced frequency shifts [17]. the peak was fitted using either the pseudo-voigt or the lorentzian curve model based on the variation of the cumulative residuals. iterative curve fitting (icf) was performed under origin2016. 2.4. x-ray diffraction measurements were performed with a d8 discover from bruker equipped with a cu source, in ψ (or χ) type assembly configuration and θ-2θ set up. the post treatment was carried out with commercial software leptos using the sin2 ψ method. we focused on the shift of the peak at 125◦ with a peak evaluation chosen as standard and a stress model defined as normal in the software. 3. results and discussion 3.1. x-ray diffraction results at first, xrd measurements highlighted that there were very low amounts of stress at the surface of both samples, although the one coated at high temperature seems to be in a slightly more compressive state as shown in table 3: sample stress value (mpa) measurement uncertainty (mpa) ref1 5.0 ± 10.9 ht2 -52.4 ± 4.5 table 3. x-ray diffraction results 4. scanning microwave microscopy results 4.1. stress variation over centerlines an exploration along the centerline involving about thirty measurements was performed on both samples. 44 vol. 27/2020 residual stresses in cr2o3 coatings figure 4. averaged amplitude of smm signal against frequency results show that edge effects like the one visible on figure 3 at 9.1 ghz are likely to be found between 7.1 ghz and 10.9 ghz. change in solidification process may indeed occur near the edges of the sample as the number of degrees of freedom increases. this phenomenon is also to be considered when investigating closer to the very last sprayed layer where solidification was left to happen freely at ambient temperature. steel substrate is likely to exhibit edge effects too regarding cutting and drilling processes it underwent. in order to compare the state of stress at different depths between the two samples in a sound and relevant manner, only data between -5 mm to +5 mm were considered for the in-depth stress study. 5. in-depth stress variation we computed the average amplitude value obtained for each frequency within the delimited area. we therefore plotted the average stress value against the working frequency which reflects the investigation depth in the material. results are shown in figure 4: one can easily notice that both samples can be considered isotropic as levels of amplitude variate the same along the two directions. this result is consistent with what is expected from aps. moreover, the four curves exhibit the same steadily stress level as we investigate the [8-0.8] ghz range. such a tendency leads to think that this reflected signal characterizes the substrate, which is consistent with results from deep hole drilling investigation on thermally sprayed ceramics [2]. around the 9 ghz region would be the interface, whereas the 11.9 ghz to 10.9 ghz characterize the stress level within chromium oxides. collecting such few data in the coating can be explained by the evanescent behavior of microwave within ceramics. in concrete terms, the wave train easily penetrates the material following an exponential law of propagation as mentioned in literature [18]. this could also explain that, despite the thickness difference between ref1 and ht2, we collected the same amount of data in the two coatings. the authors want to set clear that smm resonant frequencies are very unlikely to match with the same depth of investigation between the two samples. 6. raman spectroscopy results 6.1. stress variations over centerlines centerlines stress measurements were performed with raman spectrometer to be compared to highest frequency (11.9 ghz) smm results. the resulting raw raman data are shown in dashed line on figure 5. 45 f. decroos, c. langlade, e. bourillot et al. acta polytechnica figure 5. correlation of stress measurements along y centerlines for the two samples to highlight clearer tendencies from them, a 5-points smoothing was applied, which does not impact curve extremities. from ht2 y, raman data reveal a strong wavy curve especially in the 6 millimeters area from either side of the center which is also exhibited by smm results. regarding ref1 y, raw raman data oscillate around 551.25 cm−1 from -12 mm to 0 mm, while smm data does around -12.75 db. then, as raman data decrease down to 548.5 cm−1, smm ones increase up to -12.3 db. at the last extremity of the curve, raman data drastically increase whereas smm ones start to decrease. although in practice stage position between the two instruments cannot perfectly match, resulting stress curves exhibit strong tendencies to face each other like mirror images. as 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[17] m. saka, y. ju, d. luo, h. abé. a method for sizing small fatigue cracks in stainless steel using microwaves. jsme international journal series a 45(4):573–578, 2002. doi:10.1299/jsmea.45.573. [18] r. zoughi. microwave non-destructive testing and evaluation. springer netherlands, 2000. doi:10.1007/978-94-015-1303-6. 47 https://doi.org/10.1002/jrs.734 https://doi.org/10.1039/c4nr04017a https://doi.org/10.1021/acs.langmuir.9b01979 https://doi.org/10.1039/j19700000980 https://doi.org/10.1016/0010-938x(92)90014-t https://doi.org/10.1299/jsmea.45.573 https://doi.org/10.1007/978-94-015-1303-6 acta polytechnica 27(0):42–47, 2020 1 introduction 1.1 residual stresses in thermal spray coatings 1.2 origins of residual stresses 1.3 purpose of the work 2 methods 2.1 materials and coating deposition 2.2 scanning microwave microscopy 2.3 raman micro-spectroscopy 2.4 x-ray diffraction 3 results and discussion 3.1 x-ray diffraction results 4 scanning microwave microscopy results 4.1 stress variation over centerlines 5 in-depth stress variation 6 raman spectroscopy results 6.1 stress variations over centerlines 7 conclusions acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2018.18.0010 acta polytechnica ctu proceedings 18:10–14, 2018 © czech technical university in prague, 2018 available online at http://ojs.cvut.cz/ojs/index.php/app instrumentation of split hopkinson pressure bar for testing of cellular metallic materials jan falta∗, tomáš fíla, petr zlámal, marcel adorna czech technical university in prague, faculty of transportation sciences, department of mechanics and materials, konviktská 20, 120 00 prague 1, czech republic ∗ corresponding author: falta@fd.cvut.cz abstract. this paper presents an overview of the custom design instrumentation of a split hopkinson pressure bar modified for dynamic testing of materials with low mechanical impedance, particularly for cellular metallic materials (e. g. metal foams, laser sintered structures). design and implementation of the components related to the strain wave measurement based on strain gauges (i.e. strain-gauge measurement unit, power supply unit, filtration) and the components used for the control and synchronization of the experiment, such as module of laser trough-beam photoelectric sensor are summarized in the paper. aside from the design of the hardware components, the contribution deals also with development of a control software with graphical user interference using labview (national instruments, usa) programming environment, that allows selection of parameters of the dynamic tests and their storage for the evaluation of experiments. keywords: shpb, instrumentation, strain-gauge measurement, labview. 1. introduction 1.1. motivation the mechanical properties of specific metal structures may interest some researchers and engineers because their various behavior at different loading rates. in terms of mechanical behavior, it has been observed that the stress-strain response is closely linked to variation in loading rates. due to this, it is required to be described material response for loading rates that can be expected after initiation in service [1, 2]. the split hopkinson pressure bar (shpb) apparatus has rapidly become the widely used device for material testing at high strain rates [3]. one of the directions in current research using shpb is the testing of porous structures with regular or irregular internal structure [4–6]. the problem of testing porous materials and investigating the mechanisms of their deformation is dealt with by many authors. a summary of the basic results of these works can be found in the work of g.w. ma, z.q. ye et al. [7]. 1.2. aims and objectives the shpb apparatus located at department of mechanics and materials ctu in prague, is designed for the research in the field of fast deformation processes of materials and structures with low mechanical impedance. due to possible change of experimental setup in terms of used measurement bars (currently, both aluminum and pmma bars are used) and number of strain gauges and their type (foil or semiconductor strain gauges) it was necessary to implement our own design of the components. the main advantages of the proposed instrumentation include full control of the signal processing before it is brought to the digitizing card in the pc. there is no unintentional distortion of the measured signal compared to better, but more complex solved commercially available solutions. the solution is also easily expandable with additional measuring points, implementation of different strain gauge sensors or targeted noise reduction in the selected frequency range. it also implements an ability to synchronize external devices such as high-speed camera operating on the ttl trigger principle. the aim of these partial solutions is to improve quality and reliability of the test components while increasing overall comfort of operation of the shpb instrumentation. 2. shpb setup the instrumentation was performed on a modified kolsky shpb setup. the incident, transmission, and striker bars had the same nominal diameter 20mm. material of the bars (usually aluminum alloy enaw-7075 or pmma) varied according to the material tested, but this does not affect the instrumentation. a gas-gun system with 16 bar maximum pressure was used to accelerate the striker bar. the incident bar and the transmission bar had the same length 1600mm and were supported by eight low-friction polymer-liner slide bearings with aluminum housing. both the incident and the transmission bars were equipped with the strain gauges for the measurement of deformation waves. the assembly also included a high-speed camera for optical inspection of the experiment. schematically, the entire assembly is depicted in figure 1, where the main parts of the instrumentation are three in-house designed hardware units (strain-gauge measurement unit, power supply unit and laser sensor and camera trigger unit) 10 http://dx.doi.org/10.14311/app.2018.18.0010 http://ojs.cvut.cz/ojs/index.php/app vol. 18/2018 example of an article with a long title figure 1. shpb assembly instrumentation scheme. marked 1-3 and gui in labview programing environment marked 4. 3. instrumentation 3.1. strain-gauge measurement unit the basic idea of the solution was the development of a modular assembly of separate measuring cards equipped with resistors matching the type of strain gauge. the simplified circuit diagram for one measuring point is shown in figure 2. the red framed parts are on the card, the other elements are fixed in the unit. balancing wheatstone bridge is implemented using a potentiometer to one of the bridge branches. the measuring cards are located in the sockets with the possibility of replacing by simply pulling out and inserting a different card. an example of a technical solution for placing six cards into the slots within the measuring unit is shown in figure 3. the module also includes a switch to set bridge configuration to half-bridge or quarter-bridge. figure 2. electrical circuit scheme of the realized wheatstone bridge. 3.2. power supply unit as power supply for strain gauge measurement the custom battery power supply unit was designed. motivation for the implementation of this unit consisted in the unsatisfactory properties of the laboratory power supplies used so far. the output of the laboratory figure 3. internal distribution of strain-gauge measurement unit. power supply exhibited considerable ripple and signal noise and had only a limited number of output terminals. after a series of tests "incident bar apart" void test and "bars together" void test the proper use of the battery sources has been proven to have a less ripple and noise of the output signal. the design of the power unit was also aimed to place all six independent voltage sources, for each measuring point, into one unit and ensure charging of all cells with full charge indication. lithium cells with a capacity 2600mah and a maximum voltage 4v are used as a voltage source. for these cells, a voltage drop is not expected during a series of measurements. the charging circuit of all cells is also integrated in the unit, with the possibility of disconnecting it as a potential source of electric noise as it is shown in the simplified circuit scheme in figure 4. noise reduction due to the use of battery power unit was up to 80%. the picture of the assembly is shown in figure 5. 3.3. strain gauges signal correction after the power supply was implemented, the signal noise was significantly reduced. but the noise level was still significant and distorted the measured data. another significant source of noise can be induced by wiring (lead wires, measuring cables, etc.). for this reason, shielded twisted pairs were used for cabling. further reduction of the signal noise required 11 jan falta, tomáš fíla, petr zlámal, marcel adorna acta polytechnica ctu proceedings figure 4. battery power supply solution scheme. figure 5. power supply unit front side. the passive element working as a frequency filter to be included. relevant frequencies for the signal reconstruction are in the frequency range up to 200 khz. higher frequency filtering can be performed using a low pass filter realized by a capacitor of a suitable capacitance in the so-called double-wiring when the capacitor is connected in parallel to the series-connected resistor. the threshold frequency f0 of the low pass filter realized by this connection is expressed by the relation: f0 = 1 2πrc , (1) where r is the value of the series-connected resistance in our case the resistivity of the strain gauge used and c is the capacity of the capacitor. in case of using foil strain gauges with lowest resistivity 120ω compared to other types of strain gauges, a capacitor of 6.6 nf was used realizing a low pass filter f0=201 khz. for semiconductor strain gauges with resistivity 1000ω, the value f0 is 241 khz. the capacitor c is connected to the internal section of the bnc output terminal of the strain gauge unit. comparison of the noise of the measured signal before and after the implementation of the frequency filter is in figure 6. 3.4. laser sensor and camera trigger unit the data obtained from strain gauges provides information about the course of the deformation wave. however, it is not possible to estimate in detail the deformation behavior of the sample with a complex internal structure. therefore, during the experiments, the deforming samples were observed using a digital figure 6. noise reduction after low-pass filter implementation (green) and before (blue). camera (fastcam sa5, photron, japan). captured images were evaluated using a dic technique to obtain in-plane displacement and strain fields [8]. the data were also used to estimate the expected measured values during the shpb experiment. synchronization is achieved using a signal from the optical gates as ttl (transistor-transistor logic) pulse signal at the camera’s up-link. the module further adjusts the magnitude of the output voltage of the optical gates before being brought to the high-speed digitizer where is it used as an experiment trigger and for the evaluation of impact velocity. the scheme of the module is shown in figure 7. figure 7. block scheme of laser sensor and camera trigger unit. 3.5. labview gui the main reason for choosing the labview environment is wide support of measurement devices from many manufacturers and their easy implementation [9]. in the case of the created solution for shpb, labview is used to control two four-channel digitization cards evaluating both optical gate signals and strain gauge sensors. the responsive user interface shown in figure 8, is divided into functional partitions for better orientation. when saving an experiment record, a data header is added to the measured data, which contains the parameters of the measurement performed for subsequent evaluation in the matlab environment (mathworks inc.). the resulting file is saved in the *.tdms format (technical data management sharing). 12 vol. 18/2018 example of an article with a long title figure 8. graphical user interface in labview. 4. experimental verification verification of correct function of all parts of the instrumentation as a whole was carried out by testing the deformation behavior of auxetic structures at different strain-rates. auxetic structures are cellural materials with negative poisson’s ratio [10]. three types of auxetic structures were tested: 2d cut missing-rib, 2d inverted honeycomb and 3d inverted honeycomb. these structures were produced by selective laser sintering method from a 315l-0407 austenitic stainless steel alloy. the visualization of individual structures is shown in figure 9. figure 9. tested auxetic structures, (a) 2d cut missing-rib, (b) 2d inverted honeycomb, (3) 3d inverted honeycomb. a prepared samples were placed between the incident bar and the transmission bar and carefully aligned with the faces of the bars. adjacent ends of the bars to the sample were marked by black and white texture for subsequent dic analysis. incident wave was generate by the striker bar with length 500mm accelerated by gas-gun pressurized on 5 bars with resulting impact velocity of 33 ms−1. high speed camera recording deformation process were set at 100 000 fps with 320x192 px image resolution. two high intensity led illuminators was used for sufficient illumination of the scene [11]. 5. experimental results laser sensor and camera trigger unit reliably recorded the transit of the striker through the first and second optical gate, synchronously triggering the high-speed camera record and signal from the strain gauges. the strain gauge signal has achieved sufficient quality for subsequent evaluation and the waveform at the realized measuring points on the incident and transmission bar was recorded. the measured signal from the strain gauges is shown in figure 10. figure 10. measured signal from the incident and transmission bars. the measured data were exported, along with the header containing the record parameters, to the *.tdms file for subsequent evaluation. example of evaluated data showing stress-strain and strain rate-strain diagram for 2d cut missing-rib structure is shown in figure 11. figure 11. 2d cut missing-rib structure a) stressstrain diagram b) strain rate-strain diagram by synchronizing the camera with optical gates a video recording of the entire deformation process was prepared. the high intensity led illuminators ensure sufficient scene brightness for subsequent dic evaluation. an example of selected images of a recorded deformation process of a 3d inverted honeycomb sample is shown in figure 12. 13 jan falta, tomáš fíla, petr zlámal, marcel adorna acta polytechnica ctu proceedings figure 12. deformation of the sample 3d inverted honeycomb. 6. conclusions in this paper, technical and software solutions for the shpb assembly were designed and implemented for testing of soft material structure at high strain-rates. all partial solutions were continually tested to verify their proper functioning before they were used in the verification experiment. as part of a validation experiment, where different auxetic structures were tested, all instrument parts showed the required properties and the measured data were subsequently successfully evaluated. the experiment has also shown that the instrumentation has improved the quality and reliability of the acquired data, as well as the convenience of the operator of the shpb instrumentation. list of symbols c capacitance [f] r resistance [ω] f0 threshold frequency [hz] acknowledgements the research was supported by the czech science foundation (project no. 15-15480s) and the internal grant of the czech technical university in prague (project no. sgs17/148/ohk2/2t/16 and sgs18/154/ohk2/2t/16). all the financial support is gratefully acknowledged. references [1] a. t. owens. development of a split hopkinson tension bar for testing stress-strain response of particulate composites under high rates of loading. m.sc. thesis, faculty of auburn university, 2007. [2] k. evans, a. alderson. auxetic materials: functional materials and structures from lateral thinking! advanced materials 12(9):617–628, 2000. cited by 406, doi:10.1002/(sici)1521-4095(200005)12:9<617::aidadma617>3.0.co;2-3. [3] p. wang, s. xu, z. li, et al. experimental investigation on the strain-rate effect and inertia effect of closed-cell aluminum foam subjected to dynamic loading. materials science and engineering a 620:253– 261, 2014. cited by 36, doi:10.1016/j.msea.2014.10.026. [4] o. duncan, l. foster, t. senior, et al. quasi-static characterisation and impact testing of auxetic foam for sports safety applications. smart materials and structures 25(5), 2016. cited by 9, doi:10.1088/0964-1726/25/5/054014. [5] b. yang, l. tang, y. liu, et al. localized deformation in aluminium foam during middle speed hopkinson bar impact tests. materials science and engineering a 560:734–743, 2013. cited by 18, doi:10.1016/j.msea.2012.10.027. [6] b. hou, h. zhao, s. pattofatto, et al. inertia effects on the progressive crushing of aluminium honeycombs under impact loading. international journal of solids and structures 49(19-20):2754–2762, 2012. cited by 44, doi:10.1016/j.ijsolstr.2012.05.005. [7] g. ma, z. ye, z. shao. modeling loading rate effect on crushing stress of metallic cellular materials. international journal of impact engineering 36(6):775–782, 2009. cited by 60, doi:10.1016/j.ijimpeng.2008.11.013. [8] s.-w. khoo, s. karuppanan, c.-s. tan. a review of surface deformation and strain measurement using two-dimensional digital image correlation. metrology and measurement systems 23(3):461–480, 2016. cited by 9, doi:10.1515/mms-2016-0028. [9] c. elliott, v. vijayakumar, w. zink, r. hansen. national instruments labview: a programming environment for laboratory automation and measurement. journal of laboratory automation 12(1):17–24, 2007. cited by 124, doi:10.1016/j.jala.2006.07.012. 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[11] t. fíla, p. zlámal, o. jiroušek, et al. impact testing of polymer-filled auxetics using split hopkinson pressure bar. advanced engineering materials 19(10), 2017. cited by 2, doi:10.1002/adem.201700076. 14 http://dx.doi.org/10.1002/(sici)1521-4095(200005)12:9<617::aid-adma617>3.0.co;2-3 http://dx.doi.org/10.1002/(sici)1521-4095(200005)12:9<617::aid-adma617>3.0.co;2-3 http://dx.doi.org/10.1016/j.msea.2014.10.026 http://dx.doi.org/10.1088/0964-1726/25/5/054014 http://dx.doi.org/10.1016/j.msea.2012.10.027 http://dx.doi.org/10.1016/j.ijsolstr.2012.05.005 http://dx.doi.org/10.1016/j.ijimpeng.2008.11.013 http://dx.doi.org/10.1515/mms-2016-0028 http://dx.doi.org/10.1016/j.jala.2006.07.012 http://dx.doi.org/10.1016/j.euromechsol.2003.10.006 http://dx.doi.org/10.1002/adem.201700076 acta polytechnica ctu proceedings 18:10–14, 2018 1 introduction 1.1 motivation 1.2 aims and objectives 2 shpb setup 3 instrumentation 3.1 strain-gauge measurement unit 3.2 power supply unit 3.3 strain gauges signal correction 3.4 laser sensor and camera trigger unit 3.5 labview gui 4 experimental verification 5 experimental results 6 conclusions list of symbols acknowledgements references acta polytechnica doi:10.14311/app.2020.27.0053 acta polytechnica 27(0):53–56, 2020 © czech technical university in prague, 2020 available online at http://ojs.cvut.cz/ojs/index.php/app hardness of nitrided layers treated by plasma nitriding zdeněk joska∗, zdeněk pokorný, jaromír kadlec, zbyněk studený, emil svoboda university of defence, faculty of military technology, department of mechanical engineering, kounicova 65, 662 10 brno, czech republic ∗ corresponding author: zdenek.joska@unob.cz abstract. stainless steels, particularly the austenitic stainless grades are widely used in many industries due to good corrosion resistance, but very poor mechanical properties as surface hardness and wear resistance limit its possible use. plasma nitriding is one of the few ways to increase the surface hardness of these steels, even though this will affect its corrosion resistance. this paper focuses on the description of the mechanical properties of nitrided layers in the two most widespread austenitic stainless steels aisi 304 and aisi 316l. the microstructure and properties of nitrided layers were evaluated by metallography and microhardness measurement. surface properties of nitrided steels were characterized by martens hardness. the results show that plasma nitriding created very hard nitrided layers with thickness about 40 µm and microhardness about 1300 hv0.05. surface hardness measurements have shown that the maximum values for both steels are about 8.5 gpa, but have different behaviour under higher loads, when the aisi 316l nitrided layer began to crack on the surface and sink. keywords: austenitic stainless steel, microhardness, plasma nitriding. 1. introduction plasma nitriding is one of the few technologies that allow us to increase the surface hardness and abrasion resistance of austenitic steels, and thanks to this surface treatment we can use austenitic stainless steels in other more demanding applications in industries such as chemical, petrochemical food or medical articles. [1–5] the issue of nitrided layers formed on austenitic steels has been intensively solved since the 1990s, when the properties of these layers such as their depth, hardness, abrasion resistance and their influence on the corrosion resistance of steels are investigated. [6–10]. another possible application of such treated surfaces is as a transition layer between a soft backing material such as austenitic steels and a very hard and thin pvd coating [11–14]. the aim of this paper is to describe the mechanical properties of nitrided layers formed on aisi 316l and 304 austenitic stainless steels. 2. experiment the aisi 304 steel samples were 50 x 30 mm and 2 mm thick in the unprocessed state, and the 316l steel had a 30 mm diameter and 7 mm thickness. the surface of the samples was ground and polished to a mirror finish and cleaned in an ultrasonic bath. the values of the chemical composition are given in tab. 1. subsequently, the samples were nitrided in plasma furnace rübig pn 60/60 at condition see tab. 2. sputter cleaning was conducted in an ar–h2 atmosphere prior to plasma nitriding with a pulsed dc glow discharge. all samples were cooled after plasma nitriding in vacuum. depth profiles of plasma nitrided layers were measured by gdoes/qdp method. gdoes measurements were performed in leco sa-2000 equipment. calibration of nitrogen: jk41-1n and nsc4a standards were used. confocal laser microscope lext ols 3000 was used for observation of the cross section morphology of nitrided layers. the microhardness profile was measured on the leco amh 47 automatic microhardness tester. the vickers indenter was selected, the load was 0.049 n, dwell time at load was 12 s. the profile of microhardness was measured at 5 locations on each sample. surface hardness of nitrided samples was measured on the instrument zwick zhu 2.5. the martens hardness of nitrided layers in macroscale was determined. vickers indenter was used, load was 196 n, dwell time at load of 12 s. average value was determined from five measurements. 3. results the nitriding process was carried out under specified conditions; methane was added to the nitriding atmosphere during the nitriding process. gdoes measurements (figs. 1, 2) showed that the plasma nitrided layer had a different chemical composition than the layer formed during standard nitriding. in both steels there were 1 zone in the nitrided layer where the amount of carbon increased. in aisi 304 steel, this zone occurred at a depth of about 10 µm below 53 http://dx.doi.org/10.14311/app.2020.27.0053 http://ojs.cvut.cz/ojs/index.php/app z. joska, z. pokorný, j. kadlec et al. acta polytechnica chemical composition (wt%) c mn cr ni si p s mo aisi 304 0.045 1.78 18.7 8.75 0.48 0.030 0.001 aisi 316 l 0.03 1.50 16.97 10.70 0.31 0.030 0.026 2.18 table 1. chemical composition of stainless steels cleaning nitriding temperature (◦c) 520 550 duration (hours) 0.5 8 h flow h2 (l/min−1) 20 8 flow n2 (l/min−1) 2 32 flow ch4 (l/h) 0 1.5 bias (v) 800 530 pulse length (µs) 100 100 pressure (pa) 80 280 table 2. parameters of plasma nitriding process figure 1. gdoes depth profile of nitrided aisi 304steel the surface, and this layer is very well visible in fig. 3. the second significant zone is below the nitrided layer; this significant zone was formed during the diffusion process as carbon atoms are displaced nitrogen atoms. in aisi 316l steel, a zone of higher carbon content was also found at a depth of 15 µm below the surface, but it is not apparent in the cross-section fig. 4. it can be clearly seen from fig. 4 that the diffusion layer formed has a different character at the bottom, not as sharply delimited as the aisi 304 steel. the total layer thickness of the aisi 304 steel was 45 µm. in aisi 316l steel the nitrided layer reached a thickness of 40 µm. figure 2. gdoes depth profile of nitrided aisi 316l steel figure 3. micrographs of the nitrided layer of the aisi 304 steel the thickness and profile of the nitrided layers were measured by microhardness see fig. 5., 6., thickness of both nitrided layers of aisi 304 and aisi 316l were in good agreement with cross section morphology. the highest values of microhardness were similar but for aisi 316l was higher, reached 1383 hv0.05 and for aisi 304 the highest values of microhardness reached 1200 hv0.05. the highest values of surface hardness were the 54 vol. 27/2020 hardness of nitrided layers treated by plasma nitriding figure 4. micrographs of the nitrided layer of the aisi 316l steel figure 5. microhardness profile of nitrided aisi 304 steel figure 6. microhardness profile of nitrided aisi 316l steel same for both of the steels reached 8.5 gpa. as can be seen from the graphs fig. 7. 8., each nitrided layer behaved differently during loading. with the aisi 304 material, the curve to an indentation depth of about 6 micrometres was stable and then gradually decreased with increasing indentation depth. thus, in the case of aisi 316l, the curve was diametrically different; the layer exhibited stability to an indentation depth of about 15 µm and then ruptured and continued to crack even at an increasing indentation depth. figure 7. force displacement curve of nitrided layers figure 8. hardness vs indentation depth of nitrided layers 4. conclusions the results of the experiment showed that plasma nitriding creates a sufficiently deep nitrided layer on both steels that can be used as a transition layer between a soft substrate and a pvd coating. in aisi 316l steel, the nitrided layer has more suitable properties because it achieves higher hardness and thickness of nitrided layer than aisi 304 steel nitrided layer which has lower microhardness and thickness and in addition, at higher loads, the surface of the nitrided layer is broken. acknowledgements the paper has been prepared thanks to the support of the project the development of technologies, design of firearms, ammunition, instrumentation, engineering of materials and military infrastructure “výzbroj (dzro k201)”. references [1] k. lo, c. shek, j. lai. recent developments in stainless steels. materials science and engineering: r: reports 65(4-6):39–104, 2009. doi:10.1016/j.mser.2009.03.001. 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[10] z. joska, j. kadlec, v. hruby, et al. characteristics of duplex coating on austenitic stainless steel. key engineering materials 465:255–258, 2011. doi:10.4028/www.scientific.net/kem.465.255. [11] z. joska, m. pospíchal, t. mrázková, j. sukáč. mechanical properties of duplex system: zrn coating on plasma nitrided stainless steel. chemické listy 104:322–325, 2010. [12] e. dalibon, d. heim, c. forsich, s. br uhl. mechanical behavior of nitrided 316l austenitic stainless steel coated with a:c-h-si. procedia materials science 9:163–170, 2015. doi:10.1016/j.mspro.2015.04.021. [13] metallic materials – instrumented indentation test for hardness and material parameters – part 1: test method. čsn en iso 14577-1:2015 . [14] j. stinville, c. tromas, p. villechaise, c. templier. anisotropy changes in hardness and indentation modulus induced by plasma nitriding of 316l polycrystalline stainless steel. scripta materialia 64(1):37–40, 2011. doi:10.1016/j.scriptamat.2010.08.058. 56 https://doi.org/10.1007/s11223-008-0031-y https://doi.org/10.1016/j.ijhydene.2016.09.220 https://doi.org/10.1016/j.wear.2003.07.006 https://doi.org/10.1016/j.surfcoat.2003.10.056 https://doi.org/10.1016/j.matchar.2008.08.011 https://doi.org/10.1016/j.cap.2009.01.030 https://doi.org/10.1016/j.surfcoat.2009.09.052 https://doi.org/10.4028/www.scientific.net/kem.465.255 https://doi.org/10.1016/j.mspro.2015.04.021 https://doi.org/10.1016/j.scriptamat.2010.08.058 acta polytechnica 27(0):53–56, 2020 1 introduction 2 experiment 3 results 4 conclusions acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2017.9.0001 acta polytechnica ctu proceedings 9:1–7, 2017 © czech technical university in prague, 2017 available online at http://ojs.cvut.cz/ojs/index.php/app comparison of parameters of surface integrity of machined duplex and austenite stainless steels in relation to tool geometry jiří čapeka,∗, zdeněk pitrmucb, kamil kolaříka, libor beránekb, nikolaj ganeva a department of solid state engineering, faculty of nuclear sciences and physical engineering, czech technical university in prague b department of machining, process planning and metrology, faculty of mechanical engineering, czech technical university in prague ∗ corresponding author: capekjir@fjfi.cvut.cz abstract. the goal of this contribution was to describe parameters of surface integrity of two machined materials; austenite and duplex stainless steel. residual stresses and presence of straininduced martensite were studied as a function of the side rake angle. residual stresses of surface and sub-surface layers were determined using x-ray diffraction techniques and hole-drilling method. by using x-ray diffraction, it is possible to determine residual stresses in each phase separately, in comparison with hole-drilling method. the presence of strain-induced martensite was investigated using barkhausen noise and optical microscope. keywords: stainless steels, machining, x-ray diffraction, residual stresses, strain-induced martensite. 1. introduction actual state of technology of sewage disposal plants (sdp) basically does not solve the problem of purification of fats and oils. using mobile tricanter device substantially changes the attitude of waste processing to sdp. it removes transporting of waste from small, problematic, or dangerous environmental sources and enables using device on the polluted place. the mobile version of tricanter device poses increased requirement on reducing of energy consumption and mobility given by size and weight reducing. the mobile separation tricanter may not be discernible from mobile separation decanter device. the differences are mainly in centrifuge and accessories of device. tricanter device enables three-phase separation with simultaneous separation of two immiscible liquids with different densities and one solid phase, provided that the solid phase is the heaviest phase using high revolution, i.e. centrifugal force. the main problems are removing of individual phases and purity observation. duplex and austenite steels are usually used for their properties, primarily for the high corrosion resistance. duplex steels combine properties of both phases and moreover, due to two-phase microstructure, some properties are better than high-alloyed austenite steel, e.g. abrasion resistance [1]. abrasion and corrosion resistance have to be one of the main required properties. not only properties of used material but it is necessary to investigate quality of machined surface — surface integrity, too [2, 3]. both, austenite and duplex steels have relatively low thermal conductivity (approx. 16 w/mk) [4] which leads to insufficient heat distribution into chip and workpiece and to excessive heat accumulation in cutting zone. this heat generation can result in microstructural changes, local changes of chemical composition, surface discoloration or inducing undesirable tensile residual stresses. the gradient of residual stresses (rs) is more important indicator of machined material than surface rs. a situation may arise that there are the favorable compressive rs on the surface but with a steep rs gradient which can result in very high unfavorable tensile rs in the sub-surface layers, and vice versa [5]. for this reason, it is very important to investigate the rs gradient. analysis of polycrystalline materials by x-ray diffraction methods is suitable for gaining information about state of rs of both the surface and subsurface layers. on the other hand, the other methods as hole-drilling determine the rs gradient from total relieved deformation of material after disruption of rs balance. the strain-induced martensite can be caused by sufficiently high rate of plastic deformation of surface layers and also by temperature instability of austenite [6, 7]. martensite is harder and more brittle in comparison with austenite steels and thus changes their properties. for strain-induced martensite determination, e.g. the optical microscopy or barkhausen noise analysis can be used. 2. theory duplex stainless steels have high corrosion resistance in many environments, where the standard austenite steel is consumed, and where its properties sig1 http://dx.doi.org/10.14311/app.2017.9.0001 http://ojs.cvut.cz/ojs/index.php/app j. čapek, z. pitrmuc, k. kolařík et al. acta polytechnica ctu proceedings nificantly exceed austenite steel. thereby, smaller amount of material from duplex steel is necessary to manufacture function components. austenite and duplex steels are susceptible to mechanical reinforcement, i.e. local changes in mechanical properties of surface layers. local changes, e.g. hardness, can lead to tools vibration during machining of the final component, which results in additional material inhomogeneity and blunting tool [8]. realizing that austenite steel has face centred cubic (fcc) lattice with close-packing structure of atoms, the primary slip system goes along {111}〈110 〉 the number of slip systems is 12, which is the sufficient amount to plastic deformation. moving dislocations form so called stair-rod dislocations which have small stacking fault energy, i.e. high energy is necessary to have for intersect or cross slip of these dislocations [9]. therefore, the austenite steels are prone to workhardening, which cause mechanical modification and inhomogeneity on the machined surface, and leads to e.g. unstable chip formation. the second type of deformation of high alloyed austenite steel takes place via twins. on the contrary, the ferrite crystallizes in a body centered cubic lattice (bcc). the direction slip in bcc materials is always 〈111 〉. since in the bcc lattice is not close-packing structure of atoms, more slip planes assert during the deformation, mostly planes {110} and {211}. in engineering practice, the residual stresses are often determined on the basis of total sample deformation (e.g. hole-drilling method). this procedure considers the solid state as a compact continuous body, and therefore could not take into account degradation processes which rise separately into each phase components. using x-ray diffraction (xrd) method, the residual stresses can be determined into both the phases separately [10]. for strain-induced martensite determination, several method can be used. x-ray diffraction and barkhausen noise belong to non-destructive techniques and e.g. microhardness measurements or optical microscopy to destructive analysis. 3. experiment the tested samples of tube shape of 100/86 mm in diameter were made of aisi 304 (austenite) and aisi 2205 (duplex) type of stainless steel. the samples were annealed in air laboratory furnace for 5 hours at 420 ◦c in order to reduce bulk macroscopic residual stresses. for machining of the surfaces, four types of side rake angle were used (−6◦; −2◦; +7◦ and +12◦). side rake angles are considered in combination with particular insert holder, which has negative rake angle (−6◦). dclnr/l r-clamp tool-holders with lead angle of 95◦ (side cutting edge angle −5◦) for four 80◦ negative rhombic inserts were used, namely f3m, sf, nf, and pp chip breakers of iscar cutting tools. all inserts had the same tip radius 0.4 mm. for elimination of blunting tool effect, the cutting tool was always new for machining of each tube segment. the surface of the austenite sample was machined with different tool’s feed, f = 0.1, 0.15; 0.2 and 0.31 mm/rev, using constant value of cutting depth 0.5 mm and cutting speed 155 m/min. the second series of machined samples were set up the cutting conditions of experiment using different tool geometry: feed rate 0.14 mm/rev, cutting speed 140 m/min, and depth of cut 2 mm. direction of feed rate was parallel to axis of the sample (tube) a and perpendicular to tangential direction t. according to the principles of design of experiments (doe) method, three 1cm tube segments were machined using the same cutting conditions. using mnkα and crkα radiation, x’pert pro mpd diffractometer was used to measure lattice deformations in austenite and ferrite, respectively. the average penetration depth of x-ray radiation is approx. 4 µm and 6 µm for ferrite and austenite phase, respectively. diffraction angles 2θhkl were determined from the peaks of the diffraction lines kα1 of planes {311} and {211} of austenite and ferrite, respectively. diffraction lines kα1 were fitted by pearson vii function and rachinger’s method was used for separation of the diffraction lines kα1 and kα2. for residual stress determination, winholtz & cohen method [11] and x-ray elastic constants 1/2s2 = 7.18 tpa−1, s1 = −1.20 tpa−1 and 1/2s2 = 5.75 tpa−1, s1 = −1.25 tpa−1 were used for austenite and ferrite phase, respectively. in order to analyze the stress gradients beneath the samples surface, layers of material were gradually removed by electro-chemical polishing in the center of the sample. hole-drilling method was performed using sintered carbide milling drill of 1.8 mm in diameter and the holes had depth of 2 mm. the detection of released deformations was done by 3 rectangular tensometric rosettes. the stresses were calculating using macroscopic elastic constants: young modulus 200 gpa and poisson ratio 0.3. the measurements of barkhausen noise were performed using a commercial unit stresstech rollscan 350 magneto-elastic analyser with a standard sensor. main parameters of the applied method were: sinusoidal shape of magnetic signal, magnetic voltage 3.5 v, and frequency 75 hz. in practice typical expectable penetration depth in this experimental arrangement is in the range of 20 µm, but generally depend on frequency and the analysed material. for obtaining optical microscopy images, the radial cut of each sample was made. the electrolytic etching with 10% oxalic acid of investigated surfaces was employed. the images were achieved by inclined illumination with 250× magnification. 2 vol. 9/2017 parameters of surface integrity of duplex and austenite steels figure 1. averaged values of residual stresses 〈σa〉, mpa (top) and 〈σt 〉, mpa (bottom) of both phases depending on tool’s feed f, mm/rev. 4. results and discussions 4.1. dependence on tool’s feed f in figs. 1, there are dependence of residual stresses values of ferritic and austenitic phases on the tool’s feed. the dependence of residual stresses 〈σ〉 on the tool’s feed f is increasing, see figs. 1. this trend is identical in the both analyzed directions; axial and tangential. it can be concluded that using a smaller tool’s feed leads to lower values of residual stresses. this effect is related to "an amount of chip". higher tool’s feed, cutting depth and cutting speed lead to higher cutting force and higher heat generation. if the heat effect is dominant, the tensile stresses are generated. generally, the temperature influence causes the tensile rs and contrarily, the plastic deformation leads to compressive rs. the type of the rs and their value deeply depend on the mechanical and thermal properties of the machined material [5, 12]. 4.2. dependence on the side rake angle 4.2.1. surface residual stresses in figs. 2, there are influences of surface macroscopic residual stresses 〈σa〉, 〈σt 〉, mpa on the side rake angle, ◦. these residual stresses were averaged from three values of rs of tube segments machined using same side rake angle. generally, the increasing of the side rake angle in the positive direction leads to a lowering of cutting force and temperature in the cutting zone [5]. for prediction of rs dependence on the side rake angle, the yield strength ratio rm/rp0.2 of the given material is necessary to take into account. for austenite steel, the yield strength ratio is approx. 2.5, which is typical value for plastic material. the tensile rs are created during machining using great load on the cutting tool, i.e. using the negative side rake angle, and smaller load causes the compressive rs, i.e. using the positive side rake angle [5]. for this reason, higher compressive (axial direction) and smaller tensile (tangential direction) rs were determined with increasing of the side rake angle, see fig. 2a. on the other hand, for ferrite steel, the yield strength ratio is less than 1.25, which is typical value for elastic materials. according to [5], the shear type chips should be created which should cause the interruption of the connection between chips and the material. the additional effect of strain filed of chips is not transferred in the machined surface. for this reason, the greater force causes that the plastic deformation influence is predominant and higher compressive or smaller tensile rs may be determined with increasing of the side rake angle. furthermore, for duplex steel, which consists of both phases, it is possible to presume that the dependence of rs on the side rake angle is generally not monotonic for both the phases because of their mutual influence during plastic deformation, see figs. 2b, 2c. 4.2.2. residual stresses gradient the rs gradients depending on the side rake angles were determined, see figs. 3. as can be seen in figs. 3a, 3b, the rs gradients are without any dependence on the side rake angle in the case of austenite steel. on the contrary, for duplex steel, there are clearly differences of the rs values depending on side rake angle in both austenite and ferrite phase. the reason resides in two phase material and mutual influence of both phases during plastic deformation. in the case of one phase steel, the difference of plastic deformation power is not so significant to change the state of rs in the sub-surface layers using the different side rake angle, see figs. 3a, 3b. on the contrary, for two phase steel, the differences of rs are evident, see figs. 3c, 3d. with the increasing side rake angles, the maximum stress position is moved deeper into the material which is in line with results in [13]. in figs. 4, there is a comparison of rs gradients determined by xrd and hole-drilling method. mostly, the macroscopic rs of bulk material can be estimated as approx. zero, see austenite steel in figs 3a, 3b. nevertheless, the compressive rs of ferrite and the tensile rs of austenite phase of bulk are seen in figs. 3c, 3d. by xrd, the determination of the rs is possible in each phase, separately. because of the similar mass ratio of ferrite and austenite phase in the duplex steel, the rs of bulk can be approximately predicted by value σbulk ≈ (σferrite + σaustenite)/2. however, 3 j. čapek, z. pitrmuc, k. kolařík et al. acta polytechnica ctu proceedings (a) . austenite steel. (b) . duplex steel — austenite phase. (c) . duplex steel — ferrite phase. figure 2. axial and tangential residual stresses 〈σa〉, 〈σt 〉, mpa on the side rake angle, ◦. the hole-drilling method is based on deformation of the measured material as the whole. it is evident from fig. 4 that σbulk ≈ σhole−drill ≈ 0 which is in line with expectation [10]. moreover, the continuity of both methods of rs determination was verified. 4.2.3. strain-induced martensite determination in fig. 5, there are the optical microscopy images of the radial cut for two types of chip breakers, for maximum negative (f3m chip breaker, i.e. side rake angle −6◦ ) and maximally positive (sf chip breaker, i.e. side rake angle +12◦ ). from theory follows that the primary shear zone is decreasing with increasing positive side rake angle. in other words, increasing side rake angle results in lower cutting force and temperature [14]. due to (a) . austenite steel — axial direction. (b) . austenite steel — tangential direction. (c) . duplex steel — austenite phase. (d) . duplex steel — ferrite phase. figure 3. residual stress gradients σa, σt using side rake angles −6◦, −2◦ and +12◦. 4 vol. 9/2017 parameters of surface integrity of duplex and austenite steels (a) . axial direction; side rake angle −6◦. (b) . tangential direction; side rake angle −6◦. (c) . axial direction; side rake angle +12◦. (d) . tangential direction; side rake angle +12◦. figure 4. residual stress gradients σa, σt determined by xrd and hole-drilling method. lower cutting force, the primary shear zone tends to get wider and the plastically affected surface layers is thinner. in figs. 5a and 5b, there is evident the possibility presence of strain-induced martensite. for confirmation of this statement, other analysis is necessary to do, e.g. barkhausen noise analysis, see fig. 6. after confirmation of presence of strain-induced martensite, it is evident that there are differences between thickness of surface layer with strain-induced martensite, see figs. 5a and 5b. increasing positive side rake angle, the thickness of mentioned layer is decreasing which is in line with presented theory. in the case of duplex steel, see figs. 5c and 5d, it is possible to observe differences in amount of straininduced martensite, i.e. dark blue grains. with increasing positive side rake angle, the amount of straininduced martensite decreases. using higher positive side rake angle, the cutting tool causes lower plastic deformation and therefore, the lower amount of straininduced martensite is created. the thickness of layer containing strain-induced martensite is approximately identical. in fig. 6, there is a comparison of barkhausen noise records of machined surface with and without straininduced martensite of austenite steel. the presence of ferromagnetic phase in paramagnetic austenite, i. e. strain-induced martensite, was absolutely confirmed by barkhausen noise. the barkhausen noise record of non-machined surface has constant value, i. e. equal to signal creating by measuring probe. the confirmation of the presence of strain-induced martensite in the duplex steel is more complicated due to presence ferromagnetic ferritic phase. the higher value of magneto-elastic parameters are not only due to higher ratio of ferromagnetic phase but also due to residual stresses, dislocation density, grain-boundaries etc. 5. conclusions the present study showed: • regarding dynamical loading of the final product, it is necessary to have a suitable state of residual stresses in the surface layers, i.e. high value of compressive stresses. for this reason, lower tool’s feed is more preferable when the final product is manufactured. with respect to our results, see figs. 1, and recommendation of tools producer, the value of tool’s feed 0.14 mm/rev was selected. • the surface rs distribution is dependent not only on the side rake angle but on the material, too. for austenite (one phase steel), the dependence of rs on the side rake angle is decreasing. on the contrary, for austenite and ferrite (in the two phase steel), the dependence is not monotonous. the reason is yield strength ratio which is different for austenite and ferrite. • the rs gradients are without any dependence on the side rake angle in the case of austenite steel. 5 j. čapek, z. pitrmuc, k. kolařík et al. acta polytechnica ctu proceedings (a) . austenite steel machined by f3m chip breaker, i.e. side rake angle −6◦ . (b) . austenite steel machined by sf chip breaker, i.e. side rake angle +12◦ . (c) . duplex steel machined by f3m chip breaker, i.e. side rake angle −6◦ . (d) . duplex steel machined by sf chip breaker, i.e. side rake angle +12◦ . figure 5. optical microscopy images of the radial cut of four samples; magnification 250×, electrolytic etching 10% oxalic acid. figure 6. barkhausen noise records of austenite steel of machined surface with and non-machined surface without strain-induced martensite. on the contrary, for duplex steel, there are clearly differences in rs values depending on side rake angle in both austenite and ferrite phase. the reason is mutual influence of the both phases during plastic deformation. • for the similar mass ratio of ferrite and austenite in the duplex steel, the rs determined by hole-drilling method are approx. the average between the rs determined by xrd of ferrite and austenite phase, separately. • moreover, the rs of austenite phase are generally more tensile in comparison with the rs of ferrite phase; see figs. 4, which is in correlation with [5]. • in figs. 5a and 5b, there is not evident difference between twins and strain-induced martensite. for confirmation of strain-induced martensite presence in the austenite steel, the barkhausen noise analysis was used, see fig. 6. • it follows from figs. 5 that with increasing positive side rake angle, the amount of strain-induced martensite and the layer containing strain-induced martensite is decreasing for duplex and austenite steel, respectively. • due to correlation between amount of straininduced martensite and microhardness, it is possible to presume the dependence of microhardness on side rake angle. acknowledgements j. čapek acknowledges support of the grant agency of the czech technical university in prague, grant no. sgs16/245/ohk4/3t/14. others were supported by ta04020658, the technology agency of the czech republic. references [1] r. dakhlaoui, et al. mechanical properties of phases in austeno-ferritic duplex stainless steel—surface stresses studied by x-ray diffraction. mater sci eng: a 444(1):6–17, 2007. doi:http://dx.doi.org/10.1016/j.msea.2006.06.074. 6 http://dx.doi.org/http://dx.doi.org/10.1016/j.msea.2006.06.074 vol. 9/2017 parameters of surface integrity of duplex and austenite steels [2] d. y. jang, et al. surface residual stresses in machined austenitic stainless steel. wear 194(1):168–173, 1996. doi:10.1016/0043-1648(95)06838-4. 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[14] h. saglam, f. unsacar, s. yaldiz. investigation of the effect of rake angle and approaching angle on main cutting force and tool tip temperature. int j mach tool manu 46(2):132–141, 2006. doi:http://dx.doi.org/10.1016/j.ijmachtools.2005.05.002. 7 http://dx.doi.org/10.1016/0043-1648(95)06838-4 http://dx.doi.org/http://dx.doi.org/10.1016/s0924-0136(99)00359-3 http://www.lucefin.com/wp-content/files_mf/stainlesssteels_low.pdf http://www.lucefin.com/wp-content/files_mf/stainlesssteels_low.pdf http://dx.doi.org/http://dx.doi.org/10.1016/j.matdes.2012.08.058 http://dx.doi.org/http://dx.doi.org/10.1016/j.vacuum.2014.01.032 http://dx.doi.org/http://dx.doi.org/10.1016/s0921-5093(02)00022-9 http://dx.doi.org/10.4028/www.scientific.net/amr.996.277 http://dx.doi.org/10.1107/s0021889804029516 http://dx.doi.org/10.1007/s11740-012-0389-3 http://dx.doi.org/http://dx.doi.org/10.1016/j.ijmachtools.2005.05.002 acta polytechnica ctu proceedings 9:1–7, 2017 1 introduction 2 theory 3 experiment 4 results and discussions 4.1 dependence on tool's feed f 4.2 dependence on the side rake angle 4.2.1 surface residual stresses 4.2.2 residual stresses gradient 4.2.3 strain-induced martensite determination 5 conclusions acknowledgements references doi:10.14311/app.2021.30.0109 acta polytechnica ctu proceedings 30:109–113, 2021 © czech technical university in prague, 2021 available online at http://ojs.cvut.cz/ojs/index.php/app micronized concrete powder as mircofiller in cement paste: influence on hydration and structure jaroslav topič∗, zdeněk prošek czech technical university in prague, faculty of civil engineering, department of mechanics, thákurova 7, 166 29 prague 6, czech republic ∗ corresponding author: jaroslav.topic@fsv.cvut.cz abstract. the reuse of old concrete as a coarse and fine recycled aggregate in cement composites is relatively well researched, and therefore a very fine fraction of recycled concrete was used in this paper. when using a very fine fraction of recycled concrete, it is appropriate to focus primarily on less technologically demanding treatment processes with minimal costs and environmental impact. therefore this paper focus on using the fine fraction of recycled concrete as a microfiller. for this purpose, the material is treated by high-speed milling process and micronized concrete powder (mcp) is produced. using several micronized concrete powder types that differ by origin, separation process and fineness of grinding should help understand how mcp influences the hydration process and cement paste structure. the samples with 0, 10, 20, 30, 40 and 50 wt. % of mcp were made to determine proper dosage in cement paste. specific surface, matrix density and dtg analysis of micronized concrete powder were carried out. the calorimetric measurement was used to confirm how microfiller affects the hydration process. finally, bulk density and porosity measurements were performed to determine the effect of mcp and its amount on the cement paste structure. keywords: cement paste, hydration process, micronized concrete powder, porosity, recycled concrete. 1. introduction the reuse of old concrete as a coarse and fine recycled aggregate in cement composites is relatively well researched. therefore, a very fine part of recycled concrete is examined in this paper. a very fine fraction of recycled concrete is produced as waste in the production of recycled aggregate. currently is landfilled as waste and has no further use. when we take into account that composed eligible from the cement matrix, we are talking about a production of 45 million tonnes per year within the eu [1]. finding the application of a very fine fraction of recycled concrete in cement composites meant not only complete recycling of old concrete but also approaching the ideal circular economy [2]. when using a very fine fraction of recycled concrete, it is appropriate to focus primarily on less technologically demanding treatment processes with minimal costs and environmental impact. due to the relatively small amount of non-hydrated clinker in the old cement matrix [1, 2], it seems appropriate to focus on the function of the fine fraction of recycled concrete as a microfiller. for this purpose, the material is treated by high-speed milling process and micronized concrete powder (mcp) is produced. the use of several types of micronized concrete powder, which differ in origin, separation process, and fineness of grinding, should help understand how mcp affects the hydration process and the cement paste structure. samples with different amounts of mcp were made to determine the correct dosage in the cement paste. 2. materials and samples in this paper, several types of micronized concrete powder (mcp) were used. mcp-s were made of old railway sleepers. sleepers were crushed to concrete rubble of grain size 0 – 16 mm. mcp-d were made of drainage channels which were crushed to concrete rubble of grain size 0 – 16 mm. mcp-ca and mcpcb came from old column. mcp-ca were fraction 0 – 1 mm separated after crushing. mcp-cb were made by crushing to concrete rubble of grain size 0 – 16 mm. all types of mentioned samples were milled in two steps by a high-speed electric mill. all types of micronized concrete powder were used to preparation of cement paste samples with 0, 10, 20, 30, 40 and 50 wt. % of mcp. all mixtures contained the portland cement cem i 42.5 (radotín, czech republic) and had the same water/mixture ratio 0.35. each set contained 6 prismatic samples having dimensions of 40 × 40 × 160 mm. the samples were removed from their casts after 2 days. after that, the samples were cured for 28 days in the water at the temperature of 21 ± 2 ◦c. 3. experimental methods specific surface area and matrix density were performed by matest e009 using the blain method. 109 http://dx.doi.org/10.14311/app.2021.30.0109 http://ojs.cvut.cz/ojs/index.php/app jaroslav topič, zdeněk prošek acta polytechnica ctu proceedings 0 70 140 210 280 350 420 490 0.0000 0.0010 0.0020 0.0030 0.0040 0.0050 0 1 2 3 4 5 cu m ul at iv e hy dr at io n he at [j /g o f ce m en t] he at fl ow [w /g o f c em en t] time [days] cem (ref.) cem+mcp-s cem+mcp-g cem+mcp-ca cem+mcp-cb figure 1. curves of heat flow (solid curves) and cumulative hydration heat (dashed curves) of cement pastes during 5 days normalized to 1 g of cement. properties cem mcp-s mcp-d mcp-ca mcp-cb specific surface area [m2/kg] 380 265 324 860 177 matrix density [kg/m3] 3105 2574 2579 2593 2600 table 1. size properties of used materials. phases compositions were investigated by using thermogravimetric and differential thermal analysis performed by tg-dta setsys evolution with spectrometer omnistartm. measurements were conducted in an oxide environment with temperature range 25 − 1100 ◦c. calorimetric measurements were performed on a tam air isothermal calorimeter to measure heat flux. mixtures were tested for five days at a constant temperature of 20 ◦c and stored in sealable plastic containers, each containing from 25 g to 40 g of the mixture with 50 wt. % of mcp. based on each sample’s weight (measured before testing), the results of heat flux and cumulative hydration were related to 1 g of cement for better detection of changes caused by different types of mcp. the bulk density of the samples was evaluated from the dimension and weight of the prismatic samples. total porosity was determined by using pascal 140 and pascal 440 device. those two devices can detect pores with a diameter ranging from 0.00425 to 10 µm. 4. results and discussion when comparing the specific surfaces, it can be seen that the mcp-ca is finely ground and is finer than cement. the mcp-d sample is slightly coarser than cement. samples mcp-s and mcp-cb are coarser than cement. the roughest ground is the mcp-cb sample. according to table 1, mcp-ca and mcpcb have the highest matrix density. this is probably due to the high content of cement matrix in mcp (table ref tab: 2) and partly due to the higher quality aggregate used in the original concrete. tga and dtg analysis investigated the phase compositions of micronized concrete powders. c-s-h gel (ca. 100 ◦c), portlandite (ca. 450 ◦c) and caco3 (ca. 750 ◦c) were detected [3]. the evaluation of the proportion of individual phases was performed manually from the dtg analysis curves and is evident from table 2. it can be seen from the calculated values that mcp-ca has the highest content of c-s-h gel, while mcp-s and mcp-d have the lowest proportion of portlandite. the evaluation shows that the origin of the material and the treatment method can have a significant effect on the ratio of individual phases in micronized concrete powder. the phase composition may be partially influenced by coincidences with other phases. when comparing the curves of the cumulative amount of hydration heat (figure 1), the effect of mcp on the increase in heat output is visible. all cement pastes containing mcp gave 27 to 40 % more 110 vol. 30/2021 micronized concrete powder as mircofiller in cement paste 1900 1950 2000 2050 2100 0 10 20 30 40 50 bu lk d en si ty [k g/ m 3 ] amount of mcp [%] cem+mcp-s cem+mcp-d cem+mcp-ca cem+mcp-cb figure 2. dependence of bulk density of cement paste on micronized concrete powder content. phase amount of phases by dtg [%] mcp-s mcp-d mcp-ca mcp-cb c-s-h gel 23 15 29 24 portlandit 3 2 12 11 caco3 16 17 7 11 table 2. phase composition by dtg analysis. heat than the reference cement when hydrated after 5 days. calorimetric measurements and differences between individual pastes can be explained in two ways. the influence on the heat flow during hydration can be caused by residual clinker presence in the mcp and at the same time by the action of mcp as a microfiller. given the amount of non-hydrated clinker in the mcp (below 5 %), the cumulative hydration heat emitted by pastes with mcp should be only a few percent higher. therefore it can be assumed that the microfiller effect will cause the majority effect on the development of heat flow. clinkers in mcp consist mainly of c2s, which has a slow onset of hydration and should not significantly affect the initial phase of hydration. however, the heat flow of cement pastes with mcp is higher from the initiation phase, which supports the function of mcp as a microfiller. higher heat flow and cumulative hydration heat of cement composites containing micronized concrete powder, which does not correspond to the reaction of residual clinkers in concrete powder (also reported by other authors [3]). oksri-nelfia [4] further confirms this fact by testing samples where an inert filler replaced the concrete powder. a similar verification was performed in a previous paper [5] when an inert microfiller replaced the micronized concrete powder in the form of silica powder. explanation of the different kinetics when using a microfiller can be found in the work of berodier & scrivener [6]. based on a thorough analysis of calorimetric measurements and detailed examination by sem, they explain changes in the hydration process of samples by microfillers. the microfillers were related to the increase in nucleation area for c-sh growth and the relatively complicated interaction mechanism of cement grains. thanks to microfiller grains, the distance between the grains and the mutual friction of the cement grains and the microfiller during mixing is reduced. due to mixing fraction, the layers around the dissolving surface of the cement grains are likely to be disrupted and the ions are better dispersed, resulting in more nucleation centers for c-s-h formation. at the same time, due to the friction of the cement grains and the microfiller, these nucleation centers are released and distributed on the surface of the pore system of the microfiller. they also discovered that the reduction of the grain distance is influenced by their size so that with smaller grains, the mutual distance decreases and the friction during mixing increases. the cumulative hydration heat curves clearly show 111 jaroslav topič, zdeněk prošek acta polytechnica ctu proceedings 20 22 24 26 28 30 0 10 20 30 40 50 to ta l p or os ity [% ] amount of mcp [%] cem+mcp-s cem+mcp-d cem+mcp-ca cem+mcp-cb figure 3. dependence of total porosity of cement paste on micronized concrete powder content. that the increase of hydration heat depends on higher fineness resp. smaller grain sizes of mcp. from the point of view of the berodier & scrivener work [6], this is due to the decreasing distance between cement grains and microfillers. the clear increase in the heat flow of mcp-ca at the peak of the acceleration phase and the beginning of the deceleration phase by about 4% will probably be due to residual clinker presence. due to the high fineness of mcpca the clinkers contained were probably exposed on a higher scale compared with other types of powders and were thus better made available for further reaction. it can be seen in figure 2 that the bulk densities of the samples containing 10 wt. % of mcp for most types of mcp remained the same or increased slightly. only in the cem+mcp-cb samples compared with the reference cement paste, there was a slight decrease in bulk density already at 10% of the mcp content, but by only 1%. further increase in the amount of mcp leads to a decrease in bulk density in most pastes. a more significant decrease than other sets can be seen in the set cem+mcp-d at 20 wt. % of mcp. if we compare the individual sets of samples, it can be stated that up to 30 wt. % is a decrease very similar to about 2.5% compared to the reference cement paste. for the set of samples cem+mcp-cb and cem+mcp-s with the content of 40 wt. % mcp, there was a slight increase in bulk density compared to other samples. when comparing the bulk density of individual sets with 50% cement replacement, the smallest decrease is in the cem+mcp-cb by less than 4% and the highest decrease is in the case of cem+mcp-ca by less than 7% compared to the reference cement paste. the dependence of total porosity of cement paste on the amount of mcp can be seen in figure 3. in general, it can be said that for all sets, the addition of 10 wt. % mcp reduces the total porosity by 0.3 to 0.7% compared to the reference cement paste. with a 30% mcp cement replacement, this trend no longer applies to all kits. for cem+mcp-s and cem+mcp-cb samples with 30 wt. % of mcp, the porosity reduced by 0.4 and 1.7% compared with the reference cement paste, while the set cem+mcp-d and cem+mcp-ca increased porosity by 0.8 and 1.4%. all samples with 50 wt. % mcp shows an increase in porosity compared to the reference cement paste. the lowest total porosity was in the cem+mcp-d sample with 50 wt. % mcp and the highest sample cem+mcp-ca with 50 wt. % mcp. compared to the reference sample, the porosity increased by 3.4 resp. 4.7%. the reduction of cement pastes porosity with mcp under 30 wt. % is probably due to the mentioned function of mcp as a microfiller. the pores are filled with fine particles and a larger number of nucleation centers are formed and distributed, leading to better filling the pores with c-s-h gel. replacement cement over 30 wt. % cause that small cement ratio to the total surface area of the added mcp prevails the microfiller function of mcp and leads to an increase in the total porosity and bulk density of the cement paste. based on results published in other papers [4, 7], the influence of residual content clinker in cement paste can be neglected due to the small amount of it. by comparing the results of bulk density and total porosity, it can be concluded that the decrease in 112 vol. 30/2021 micronized concrete powder as mircofiller in cement paste bulk density as a function of the increase in mcp is not only due to the increase in porosity. in case of mcp-ca (50 wt. % of mcp), the porosity increased by 4.7% compared to the reference paste, but the decrease in bulk density is almost 7%. therefore, the density of the mbm matrix, which is about 16% lower than cement, also has a comparable effect on the bulk density. in the article šeps & broukalová [8], which deals with the effect of mcp on bulk density, the same trend of decreasing bulk density depending on the increasing amount of mcp is seen. bordy, et al. [3] discusses, among other things, the effect of the amount of concrete powder on the porosity of samples. their work shows that the porosity increases with an increasing amount of concrete powder (specific surface area similar to mcp-d). even with small replacements, there is no apparent reduction in porosity. the difference compared to the results in figure 3 with a small cement replacement is because they tested cement mortars and using a less accurate method. 5. conclusions this paper focuses on the influence of different types and amounts of micronized concrete powder on the hydration properties and structural properties of cement paste. in this experiment, the rcp was used to fulfills the role of micro filler. based on the results, the following findings are summarized: • origin of the material and the method of treatment can significantly affect the ratio of individual phases in micronized concrete powder, which further influenced the matrix density. • the majority effect of mcp on the development of heat flow was caused by microfiller effect. hydration of residual clinker in mcp may be neglected due to his low amount. • addition of 10 wt. % mcp reduces the total porosity by 0.3 to 0.7% compared to the reference cement paste. • the reduction in porosity is more pronounced in samples with coarser concrete powder. • addition of mcp under the 30 wt. % decrease the porosity. acknowledgements the authors wish to express their gratitude for financial support to the faculty of civil engineering, czech technical university (ctu) in prague under sgs project no. sgs19/148/ohk1/3t/11. the authors also thanks to the center for nanotechnology in civil engineering at the faculty of civil engineering, ctu in prague. references [1] e. commission. eurostat, 2019. [online] available at: https://ec.europa.eu/eurostat/data/database [access 18 prosinec 2019]. [2] e. commission. european commission communication com closing the loop an eu action plan for the circular economy, 2015. [online] available at: https://eur-lex.europa.eu/legalcontent/en/txt/?uri=celex:52015dc0614 [access 18 prosinec 2019]. [3] a. bordy, a. younsi, s. aggoun, b. fiorio. cement substitution by a recycled cement paste fine: role of the residual anhydrous clinker. construction and building materials 132:1–8, 2017. doi:10.1016/j.conbuildmat.2016.11.080. [4] l. oksri-nelfia, p. y. mahieux, o. amiri, et al. reuse of recycled crushed concrete fines as mineral addition in cementitious materials. materials and structures/materiaux et constructions 49(8):3239– 3251, 2016. doi:10.1617/s11527-015-0716-1. [5] j. topič, z. prošek. hydration proceess and mechanical properties of cement paste with recycled concrete powder and silica sand powder. acta polytechnica ctu proceedings 13:125, 2017. doi:10.14311/app.2017.13.0125. [6] e. berodier, k. scrivener. understanding the filler effect on the nucleation and growth of c-s-h. journal of the american ceramic society 97(12):3764–3773, 2014. doi:10.1111/jace.13177. [7] z. prošek, p. tesárek, j. trejbal, t. horová. recycling of construction waste using high-speed milling process: determination of waste concrete. acta polytechnica ctu proceedings 22:88–93, 2019. doi:10.14311/app.2019.22.0088. [8] k. šeps, i. broukalová. properties of cement based composite with fine ground recycled concrete. in advanced materials research, vol. 1000, pp. 110–113. trans tech publ, 2014. doi:10.4028/www.scientific.net/amr.1000.110. 113 https://doi.org/10.1016/j.conbuildmat.2016.11.080 https://doi.org/10.1617/s11527-015-0716-1 https://doi.org/10.14311/app.2017.13.0125 https://doi.org/10.1111/jace.13177 https://doi.org/10.14311/app.2019.22.0088 https://doi.org/10.4028/www.scientific.net/amr.1000.110 acta polytechnica ctu proceedings doi:10.14311/app.2019.23.0054 acta polytechnica ctu proceedings 23:54–57, 2019 © czech technical university in prague, 2019 available online at https://ojs.cvut.cz/ojs/index.php/app a comparison of testing methods for determination of sprayed concrete tensile strength martin závacký brno university of technology, faculty of civil engineering, institute of geotechnics, veveří 331/95, 602 00 brno, czech republic correspondence: zavacky.m@fce.vutbr.cz abstract. sprayed concrete is important construction material in tunnelling. primary lining is essential in natm where the sprayed concrete can be loaded by tension due to bending moments. the tension is common reason of failure because concrete has a relatively low tensile strength. the tensile strength is usually determined by splitting tensile test in laboratory. however, the results can be distorted because the specimen is not loaded by pure tension in this case. the paper compares results of concrete tensile strength determined by two methods: indirect by the splitting tensile test and direct by the modified tensile test. keywords: tensile strength, sprayed concrete, modified tensile test, splitting tensile test. 1. introduction sprayed concrete is an important construction material, especially in underground supporting structures. thus, knowledge of the mechanical properties of the material is necessary for project design. unconfined compressive strength is a very common and easily managed testing method. however, tensile strength is also a significant parameter. the tensile strength of concrete is relatively low compared to the compressive one, and therefore failure of the material often occurs through the action of tensile forces. direct tensile strength testing comes with several complications such as the requirement of special testing equipment, a high precision in specimen preparation and especially the gripping of a specimen during testing. these identified problems are often overcome by using various indirect tensile testing methods, e.g. a splitting tensile test (stt) or a flexural test. however, indirect methods may result in different values of tensile strength and they often overestimate parameter values [1]. comprehensive comparative studies have been carried out also in the field of rock mechanics [2, 3]. thus, direct tensile testing is still important. the modified tension test (mtt) was suggested in order to eliminate the previously stated technical problems [4] and the method has already been well established in practice e.g. at graz university of technology [5]. tension is induced by special geometry of a specimen (figure 1), therefore a minimal adaptation of the commonly used compression apparatus is needed. the main benefit of the method is the entire elimination of specimen gripping problems. the main objective of this presentation is to compare the tensile strength of sprayed concrete obtained by two different methods. a series of splitting tensile tests and modified tensile tests were carried out with figure 1. modified tension test setup [8]. the results showing variation between the methods. the study also extended previous experience of the authors with laboratory testing of sprayed concrete [6], which can be employed for example in the determination of input parameter values for the shotcrete model [7]. 2. materials and methods the concrete mixture used for preparation of specimens is described in table 1. one block of sprayed concrete was used for extraction of all specimens by core drilling (figure 2). a set of 8 specimens with diameter of 55mm and length to diameter ratio 1.0 were used for the splitting tensile test (stt) and a set of 3 specimens for the modified tension test (mtt). the mtt specimens were prepared with an outer diameter of 100mm and a length to diameter ratio of 1.5. the inner drilling diameter was 43mm and the middle one was 69mm. the extraction and further 54 https://doi.org/10.14311/app.2019.23.0054 https://ojs.cvut.cz/ojs/index.php/app vol. 23/2019 determination of sprayed concrete tensile strength figure 2. scheme of specimens extraction. loading of the specimens gave attention to orientation of the failure plane. thus, the failure plane was in all cases parallel to the layering of sprayed concrete. the tests were carried out by servo-hydraulic compression load frame under load control and a rate of loading 200n/s in both the used methods. the maximal force reached was recorded and tensile strength was calculated according to the following formula: σstt = (2fmax) / (πdl) (1) for the splitting tensile test, where fmax – maximal force; d – specimen diameter; l – specimen length σmtt = fmax/ ( πr2 1 − πr2 2 ) (2) for the modified tension test, where there is fmax – maximal force; r1 – radius of larger core hole (middle drilling); r2 – radius of smaller core hole (inner drilling). the dry unit weight of specimens for stt was determined as well, in order to evaluate the material homogeneity. the obtained values of dry unit weight and σstt for each specimen were assessed by the kolmogorov-smirnov test (k-s test) if the data has a normal distribution. the one-sample k-s test is a nonparametric test of the equivalence of a continuous probability distribution that can be used to compare a sample with a reference probability distribution [9]: dα ≥ dn = max{ ∣∣f ( x(i) ) − (i− 1) /n ∣∣ ,∣∣f ( x(i) ) − i/n ∣∣}, i = 1, 2, ..., n (3) where there is dα – critical value (significance level α=0.05 was used); f(x) – reference probability distribution (considered as normal distribution); n – number of samples. the splitting tensile tests were carried out with respect to testing standard requirements čsn en 123906. the modified tension test procedure was carried out according to suggestions and reported experience published in previous studies [3, 8]. the tension annulus in the specimens was prepared with a thickness of 13mm. the maximal aggregate diameter was 8mm, and hence the obtained tensile strength should not be influenced by oversized aggregates. 3. results results of the splitting tensile tests carried out according to the method described in the previous chapter are listed in table 2. the table contains also data of dry unit weight and statistical parameters as averages and standard deviations. results of modified tension tests are listed in table 3. the dry unit weight (table 2) showed low variation when the maximal value (2247 kg/m3) is only 1.3% above the average and the minimal value (2175 kg/m3) is only 1.9% below the average. a standard deviation of σstt (0.33 mpa → 8%) is more significant than in the case of dry unit weight (table 2). however, both parameters appeared as relatively consistent, and thus the tested material can be considered as homogeneous. vice-versa, the mtt results showed up as non-consistent, due to a significant drop of tensile strength in the case of specimen 2 (table 3). eccentricity of drilling was found by investigation of the specimen after the test (figure 4). hence, distribution of load in the specimen was not uniform during the test and concentration of stress probably caused failure under the lower level of loading [3]. therefore, only the results of tests 1 and 3 were considered as valid. in comparison with the stt average, mtt tensile strength was determined as 28% lower. plotting of the cumulative distribution function against data of dry unit weight and stt tensile strength are presented in figure 3. in both cases dn value of k-s test was lower than the critical value dα (see formula 3). therefore, the normal distribution of the data was judged as appropriate. 4. conclusions a series of stt and mtt tests were carried out with 28% higher value of tensile strength by stt. thus, overestimation of tensile strength by the indirect method was indicated, as was stated in previous studies [3, 8]. the main technical problems of direct tensile testing are eliminated by mtt. however, successful application of the method remains strongly dependent on the precision of specimen preparation. this fact was investigated in [3]. and was also observed during the conducted tests (figure 4). uncertainty about stress distribution within the geometry of mtt specimens was noted by molenda [3], and thus additional research from a mechanical point of view would be beneficial. the experience obtained during the presented experiments may be useful in further laboratory testing of either shotcrete or rocks regarding the purpose of determining input parameter values for various material models used in geotechnical analysis. acknowledgements this paper has been researched and written under the project no. lo1408 "admas up – advanced materials, structures and technologies", supported by the ministry of 55 martin závacký acta polytechnica ctu proceedings cement water/cement ratio aggregate 0/1 aggregate 0/4 aggregate 4/8 500 kg/m3 0.4 174 kg/m3 919 kg/m3 585 kg/m3 table 1. concrete mixture specification. specimen % [kg/m3] σstt [mpa] statistic 1 2214 3.97 average % 2 2234 4.28 2217 3 2230 4.13 std % 4 2231 3.63 24 5 2247 3.83 average σstt 6 2218 4.38 4.15 7 2175 4.66 std σstt 8 2189 4.33 0.33 table 2. splitting tensile test and dry unit weight results. specimen σmtt [mpa] 1 3.13 2 2.25 3 3.28 average (1 and 3) 3.21 table 3. modified tension test results. figure 3. normal distribution test: left – dry unit weight; right – stt tensile strength. vertical axis represents probability [-]. 56 vol. 23/2019 determination of sprayed concrete tensile strength figure 4. spec. 2 after mtt, detail of eccentricity. education, youth and sports under the "national sustainability programme i" and under the project no. fast-s18-5356 – "stanovení vstupních parametrů materiálových modelů pro potřeby podzemního stavitelství s možností využití optimalizačních metod" supported by brno university of technology – faculty of civil engineering. references [1] s. kang, j. park, g. ryu, et al. comparison of tensile strengths with different test methods in ultra high strength steel-fiber reinforced concrete. key engineering materials 417-418:649–652, 2010. [2] d. li, l. wong. the brazilian disc test for rock mechanics applications: review and new insights. rock mechanics and rock engineering 46:269–287, 2013. [3] m. molenda, f. stöckhert, s. brenne, m. alber. comparison of hydraulic and conventional tensile strength tests. effective and sustainable hydraulic fracturing pp. 981–992, 2013. [4] m. luong. a new test for tensile strength measurements. revue francaise de géotechnique 34:69–74, 1986. [5] m. blümel. improved procedures for laboratory rock testing. in proceedings of conference eurock 2000. eurock, aachen, germany, 2000. [6] m. závacký, chalmovský, l. miča, p. bílek. stanovení hodnot vstupních parametrů pokročilého materiálového modelu pro stříkaný beton. in proceedings of conference zakládání staveb. česká geotechnická společnost čssi, 2017. [7] b. schädlich, h. schweiger. a new constitutive model for shotcrete. numerical methods in geotechnical engineering pp. 103–108, 2014. [8] r. plinninger, b. thomée. experimental and model studies on the modified tension test (mtt) – a new and simple testing method for direct tension tests. in proceedings of conference on geotechnical measurements and modelling. a.a. balmeka publishers, 2003. [9] kolmogorov-smirnov test. https://en.wikipedia. org/wiki/kolmogorov-smirnov_test. 57 https://en.wikipedia.org/wiki/kolmogorov-smirnov_test https://en.wikipedia.org/wiki/kolmogorov-smirnov_test acta polytechnica ctu proceedings 23:49–52, 2019 1 introduction 2 materials and methods 3 results 4 conclusions acknowledgements references acta polytechnica doi:10.14311/app.2020.27.0131 acta polytechnica 27(0):131–135, 2020 © czech technical university in prague, 2020 available online at http://ojs.cvut.cz/ojs/index.php/app influencing the indentation curves by the bluntness of the berkovich indenter at the fem modelling jaroslav kovářa,∗, vladimír fuisa,b, jan tomáštíkc a brno university of technology, institute of solid mechanics, mechatronics and biomechanics, technická 2896/2, 616 69 brno, czech republic b czech academy of sciences, centre of mechatronics, institute of thermomechanics, technická 2896/2, 616 69 brno, czech republic c czech academy of sciences, institute of physics, joint laboratory of optics of palacky university olomouc and institute of physics of the as, na slovance 1999/2, 182 00 praha 8, czech republic ∗ corresponding author: jaroslav.kovar@vut.cz abstract. the influence of the berkovich indenter bluntness on the indentation curves at nanoindentation test of fused silica is determined by the fem (finite element method) in this paper. the berkovich indenter, which is mostly used for the nanoindentation test, was assumed for calculations. the indenter is always blunted in real tests. the bluntness of the indenter has an influence on the results of the nanoindentation test. the combination of the nanoindentation test and the fe modelling can be used for the calculation of the parameters of fused silica plasticity and estimation of the bluntness of the indenter. in this paper, the fem was used for calculating of the indentation curves. two cases of bluntness were assumed. the curves for ideally sharp or blunted indenter were determined. the results of these calculations showed that the indentation curves are dependent on the bluntness of the berkovich indenter. the greatest relative influence of the nanoindentation curves by the bluntness is in the area of small values of the indenter displacement. the impact of assuming blunted indenter to calculated parameters of plasticity was assessed. it is often difficult to measure the bluntness of the indenter but it has an influence on the results of the nanoindentation test and should be included into calculations to make it more precise. keywords: berkovich indenter, bluntness, finite element method, nanoindentation. 1. introduction the nanoindentation test is often used for the testing of materials which cannot be tested by conventional tests. it is often used for the determination of the material parameters of the thin layers or plasticity of the fused silica [1]. the results of the nanoindentation test are often influenced by many conditions. if the fem is used for modelling of the nanoindentation test, these conditions should be included to the calculation to get more precise results. it was proved that the bluntness of the indenter has a significant influence on the results and it should be assessed to get accurate results especially when the displacements of the indenter are similar like height of the blunted part of the indenter. because it is very difficult to measure the real shape of the blunted indenter and use it for fem calculations, it is often modelled like a sphere with known radius but it is unknown if other edges near the tip should be rounded too. in this paper two types of the roundness of the tip of the berkovich indenter were assessed with four radii of the roundness, the fem calculations were done for every case, the results were compared and the best case was chosen. 2. nanoindentation test 2.1. nanoindentation of the fused silica the nanoindentation test can be used for the testing of the material properties of the fused silica. unlike the tension test, in which there is not any significant plastic deformation and the tested specimen fails by brittle fracture after elastic deformation, if the nanoindentation test is used the plasticity occurs in the small volume near the tip of the indenter [1]. fused silica is used in the oliver-pharr analysis as a material for the determination of the shape function of the berkovich indenter [2]. there are few difficulties in the fem modelling, if the fused silica is assumed. firstly, the mechanism of the plasticity of the fused silica is not known yet. the most often used models of stress – strain behaviour are either bilinear [3] or drucker-prager [4]. the bilinear model of the material was chosen for calculations because it should be enough accurate for the main aims of this paper. another problem in the fem modelling of the fused silica nanoindentation was found by the oliver and pharr [5]. the unloading curve should be mostly influenced by the elastic parameters and can be used for the determination of the young’s modulus but if the fem is used the results show steeper unload131 http://dx.doi.org/10.14311/app.2020.27.0131 http://ojs.cvut.cz/ojs/index.php/app j. kovář, v. fuis, j. tomáštík acta polytechnica ing curve than results obtained by the experiment and it leads to the overestimation of the young’s modulus from fem. the reason of this error is still unknown. 2.2. indenter many indenters (like berkovich, spherical, conical etc.) can be used for the nanoindentation [1]. the most used indenter is the berkovich indenter, which was used in this paper. the indenters with the rounded tip like spherical, show steeper loading part of the indentation curve than indenters with the sharp tips (berkovich indenter) [5]. if the bluntness of the berkovich indenter is assumed, the indentation curve is steeper than indentation curve obtained from the calculation with the sharp indenter [6]. this difference is significant in some cases and then the bluntness of the indenter should be included into the calculations otherwise the error in the estimated parameters of the plasticity by the fem could be significant. the two types of the bluntness of the indenter and few radii of the bluntness of the berkovich indenter will be assessed in this paper and the indentation curves from the fem will be compared with the indentation curve obtained by the experiment. 3. methods the fem was used for the determination of indentation curves. 3.1. models of geometry the berkovich indenter pressing into the specimen from the fused silica was assumed for calculations. the size of the indenter and the specimen was chosen to be much bigger than the displacements of the indenter (fig. 1). then the influence of the edge of the specimen on the results is small. only one sixth of the specimen and the indenter was modelled because there are three planes of symmetry. then the bluntness of the indenter was modelled. two types of the bluntness were assumed. in the case a (fig. 2), the tip of the indenter and the edges of the indenter were blunted. in the case b (fig. 3) another roundness, with the same radius as in the case a, was applied on the point where intersect the lines at the border of the roundness. for each case of bluntness, four radii of the bluntness were assessed. the values of the roundness were 100 nm, 200 nm, 400 nm and 800 nm. to compare the results, the case without bluntness was calculated too. 3.2. boundary conditions the indenter was loaded by the displacement of the specimen in two loadsteps. in the first loadstep, the displacement 612 nm plus the gap between the specimen and the blunted indenter against the way of the z axis was applied at the upper area of the indenter. in the second loadstep, the unloading by the displacement of the indenter back to the 200 nm was done. the displacement of the lower area of the specimen in figure 1. geometry of the indenter and specimen [mm]. figure 2. mesh of the tip of the indenter with the bluntness 200 nm, (case a, red line shows the border of the blunted area, yellow the border between the blunted tip and the edge). figure 3. mesh of the tip of the indenter with the bluntness 200 nm (case b, red line shows the border of the blunted area, yellow the border between the blunted tip and the edge). the way of the z axis was fixed. on the areas of the symmetry, the displacements perpendicular to these areas were fixed. 132 vol. 27/2020 fem modelling of indentation curves 3.3. models of material the diamond indenter was modelled with the linear elastic model of material with the young’s modulus e = 1 141 000 mpa and the poisson’s ratio µ = 0.07. the fused silica was modelled with the bilinear model of material. the elastic parameters were taken from [1] (e = 72 000 mpa, µ = 0.17). the parameters of plasticity (yield strength and hardening) were estimated from the experimental data by this procedure. from the comparison of the experimental data and the results from the fem, the bluntness of the indentor in the case b was determined like 200 nm. the indentation fits well in the area of the low loads where the plasticity is not so important. then the yield stress was estimated like re = 4000 mpa. the hardening of the material was not assumed. 3.4. mesh and the fem calculation the mesh was made coarse at the edge of the specimen but very fine at the centre to obtain precise results (fig. 4). similar procedure was used on the indenter (fig. 2 and 3). the elements of the type solid 185 were used on the specimen and the quadratic solid 186 elements on the indenter to describe the edge of the indenter more precise. between the indenter and the specimen, the contact region was made and conta 174 / targe 170 elements were used. to obtain more accurate results, the penetration tolerance factor was reduced to 0.01. both loadsteps were divided into a few substeps and the large displacements algorithm was applied. because of the small values of the displacements, the force convergence criterion was lower to 0.00001, to obtain more precise results. the calculation with this algorithm was done for every value of the bluntness of the indenter. figure 4. the mesh in the area near contact. 4. results and discussion the indentation curves (fig. 5) were obtained as the results of the fem calculations. the curve obtained by the model without shape imperfection showed the least steep loading curve and unloading curve too. the steepness of the curves grew with the bluntness. this agreed with the oliver-pharr analysis, where the higher steepness of the unloading curve is taken into account by the indenter shape function. if the sharp indenter was assumed, the unloading curve was still steeper than the experimental. this error was firstly observed by [5] and it is not explained yet. it could be caused by not enough accurate model of material but this was not important for this paper, which was focused on the assessing of the influence of the bluntness of the indenter. the difference between the results with the bluntness a (blunted only the tip and the edges fig. 2 and the bluntness b (blunted the tip, the edges, and rounded points, fig. 3) was small, if the bluntness was low but it grew with the growing bluntness. the relative differences of the maximal values of the force related to the bluntness were: 1.54 % for bluntness 100 nm, 2.92 % for 200 nm, 5.78 % for 400 nm and 11.4 % for 800 nm. the bluntness b should be nearer to the reality because the edges should be blunted at least a little but for new indenters, which are not so much blunted, the difference between these models was smaller than 2 %. when the bluntness b with radius 200 nm was assumed, the loading curve obtained from the fem calculation fitted the curve from the experiment. the similar but a little less accurate results were obtained by the bluntness a with radius of 400 nm. the bluntness influenced every part of the curve because it shifted the curve except the lower part of the curve which was mostly influenced because there is the spherical part of the tip of the indenter (fig. 6). the steepness of the curve in this area dramatically grew with the growing bluntness of the indenter. this fact is in the agreement with the results of the indentation curves obtained by the sharp indenters like berkovich and spherical indenter. the indentation curves obtained by the spherical indenters show steeper indentation curves [6]. from the fig. 6 is clear that the bluntness b with radius of the 200 nm was very near to the experimental curve even in the area of the low displacements. the similar relative error showed bluntness a with radius 400 nm. other cases show bigger relative errors in the area of the small displacements than in the maximal values of the forces. because the influence of the plasticity is smaller in this area than in the end of loading and the influence of the bluntness is higher, the agreement of the curves from the experiment and the fem calculation in the area of the small displacements is great criterion for the determination of the bluntness of the indenter for the next calculations. 133 j. kovář, v. fuis, j. tomáštík acta polytechnica figure 5. indentation curves for all cases. figure 6. indentation curves in the area of small displacements (detail of the fig. 5). 5. conclusions the two types of the bluntness of the berkovich indenter with four radii of the roundness have been compared in this paper. the loading and the unloading curves were getting steeper while the radius of the roundness grew. the bluntness b, in which are the edges of the indenter, the tip of the indenter, and the points, where the edges intersect, blunted, gave steeper indentation curves than bluntness a, in which are blunted only the edges and the tip of the indenter. the indentation curve for the indenter with the bluntness b with radius 200 nm was the closest to the experimental curve. there still remained the problem with higher steepness of the unloading curve in the results. it could be caused by the inaccuracy of the used model of material. the bluntness had the biggest impact in the area with the small displacements, where the blunted spherical tip caused higher steepness of the indentation curve. elastic parameters are known and the fitted parameters of the plasticity have only small impact in the area of the small displacements. then this could be used as a great criterion for the determination of the bluntness of the indenter with the fem in the future work. acknowledgements the experimental data were measured by the institute of physics of the czech academy of sciences, joint laboratory of optics of palacky university olomouc and institute of physics as cr. 134 vol. 27/2020 fem modelling of indentation curves references [1] j. kovář, v. fuis. fem simulation of the nanoindentation test with rigid and non-rigid indenter. engineering mechanics 2019 pp. 189–192, 2019. doi:10.21495/71-0-189. [2] w. oliver, g. pharr. an improved technique for determining hardness and elastic modulus using load and displacement sensing indentation experiments. journal of materials research 7(6):1564–1583, 1992. doi:10.1557/jmr.1992.1564. [3] d. torres-torres, j. muñoz-saldaña, l. a. g.-l. de guevara, et al. geometry and bluntness tip effects on elastic-plastic behaviour during nanoindentation of fused silica: experimental and fe simulation. modelling and simulation in materials science and engineering 18(7):075006, 2010. doi:10.1088/0965-0393/18/7/075006. [4] k. gadelrab, f. bonilla, m. chiesa. densification modeling of fused silica under nanoindentation. journal of non-crystalline solids 358(2):392–398, 2012. doi:10.1016/j.jnoncrysol.2011.10.011. [5] s. shim, w. oliver, g. pharr. a comparison of 3d finite element simulations for berkovich and conical indentation of fused silica. international journal of surface science and engineering 1(2/3):259, 2007. doi:10.1504/ijsurfse.2007.015028. [6] n. yu, a. a. polycarpou, t. f. conry. tip-radius effect in finite element modeling of sub-50 nm shallow nanoindentation. thin solid films 450(2):295–303, 2004. doi:10.1016/j.tsf.2003.10.033. 135 https://doi.org/10.21495/71-0-189 https://doi.org/10.1557/jmr.1992.1564 https://doi.org/10.1088/0965-0393/18/7/075006 https://doi.org/10.1016/j.jnoncrysol.2011.10.011 https://doi.org/10.1504/ijsurfse.2007.015028 https://doi.org/10.1016/j.tsf.2003.10.033 acta polytechnica 27(0):131–135, 2020 1 introduction 2 nanoindentation test 2.1 nanoindentation of the fused silica 2.2 indenter 3 methods 3.1 models of geometry 3.2 boundary conditions 3.3 models of material 3.4 mesh and the fem calculation 4 results and discussion 5 conclusions acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2019.22.0031 acta polytechnica ctu proceedings 22:31–37, 2019 © czech technical university in prague, 2019 available online at http://ojs.cvut.cz/ojs/index.php/app effects of abrasives during accelerated simulation of mechanical corrosion of cement composites using abrasive water flow petr hlaváčeka,b,∗, libor siteka,b, dagmar klichováa, lenka bodnárovác a institute of geonics of the cas, studentska 1768, 708 00 ostrava-poruba, czech republic b institute of clean technologies for mining and utilization of raw materials for energy use, 17. listopadu 2172/15, 708 33 ostrava-poruba, czech republic c brno university of technology, faculty of civil engineering, antonínská 548/1, 601 90 brno, czech republic ∗ corresponding author: petr.hlavacek@ugn.cas.cz abstract. the paper deals with the use of the abrasive water flow technology in an accelerated simulation of the process of mechanical degradation of surface layers of cement-based composites by flowing liquids. effects of the abrasive particle size and the mass flow rate of abrasives in an abrasive water flow impacting on the surface of concrete samples have been studied. in addition, suitability of the application of the abrasive water flow for the simulation of real situation of the surface wear by flowing liquids has been evaluated. keywords: cement-based composite, erosion, abrasion, cavitation, high-speed water flow, abrasive material. 1. introduction gradual erosion of concrete surface layers exposed to the effects of flowing liquids is predominantly caused by abrasion and cavitation or, where appropriate, by chemical action. abrasive erosion of concrete in hydraulic structures is caused by water-transported silt, sand, gravel, ice, or debris while cavitation erosion is a result of the collapse of vapour bubbles formed by pressure changes within a high-speed water flow. the degradation of concrete can be further intensified by chemical attacks or possible action of bacteria [1]. generally, the physical (abrasion and cavitation) and chemical damage complement each other. by the physical damage (such as abrasion), concrete surfaces are often exposed to agents such as acid rain, and therefore the attack by chemical compounds is encouraged. in case of the chemical damage (such as leaching), the concrete becomes more porous, facilitating the process of abrasion, and so on. these facts make both processes of deterioration, physical or chemical, a cycle difficult to dissociate or stabilize [2]. resistance of a particular concrete structure against these types of degradation is influenced by many factors. the most significant are the environmental conditions, dosing of aggregates, concrete strength, the mix ratio, the use of special cement and the use of supplementary materials, such as the addition of fibres and fly ash. two additional factors have an important effect on the abrasion resistance, i.e. the surface finish and curing conditions [3]. due to numerous factors, computational modelling of the concrete composition and resistance of the concrete to flowing fluids is a relatively difficult task. thus, experimental investigation of real structures or test specimens is the only reliable method. the development of new concrete mixtures, or the determination of suitable surface treatments of concrete long-term-resistant to fast flowing fluids, is often complicated by long and slow degradation of real concrete structures (spillways, stilling basins, walls of the upstream reservoir, drain pipes and hydraulic tunnels, etc.). measurable degradation occurs within a few months, often after several years or decades. while it is undoubtedly good that the deterioration of real constructions is slow, generally the long-term degradation process unreasonably prolongs any laboratory tests examining the resistance of concretes against the degradation effects of flowing liquids. there are numerous laboratory methods and recommendations focused on processes of the concrete degradation by the influence of liquids (e.g. [1], [4], [5]). however, due to the nature of the deterioration process and the effort to maintain conditions that are the most similar to the real situation, the testing methods are often time-consuming [6]. an extreme case of the long-term laboratory testing of the hydro-abrasive erosion is reported by binici et al. [7]. the total testing lasted for 3 years. the loss of concrete was investigated once a year. the apparently impractical and inflexible character of these methods encourages the development of new accelerated testing methods. certain similarities to the flowing process of frequently contaminated liquids with higher or lower content of free solid particles of various sizes which may already be part of the flowing liquid or were gradually eroded from the original surface of the con31 http://dx.doi.org/10.14311/app.2019.22.0031 http://ojs.cvut.cz/ojs/index.php/app p. hlaváček, l. sitek, d. klichová, l. bodnárová acta polytechnica ctu proceedings crete and consequently carried by the liquid can be seen in the technology of high-speed water flows, i.e. flows moving after the exit from a high-pressure nozzle at speeds of hundreds of m·s−1. material is eroded quickly and this method can be applied to faithfully simulate the real erosion processes, if the flow parameters and the testing setup are suitably combined. the mentioned method is relatively commonly used during accelerated testing. momber and kovacevic [8] investigated the wear particles occurring during the erosion of concrete after applying the high-velocity water flow. hu et al. [9] focused on the hydro-abrasive erosion tests on plain concrete and on steel-fibre reinforced concrete. they particularly studied the method of disintegration of concrete layers at various speeds and angles of impact of the hydro-abrasive suspension. erosion resistance of several types of concrete and reinforced concretes with a low volume of steel-fibres is compared in another study written by the same authors [10]. experimental work by momber et al. [11] focuses on the use of acoustic emission sensing technique for the on-line monitoring of hydro-abrasive erosion of five types of pre-cracked concretes. due to the capability to quantify the amount of the material removed, the method can be recommended for material removal process monitoring. in another work [12], momber examined the short-time cavitation wear of concrete using the cavitation chamber. the quantification of effects of the kinetic energy and exposure time of the abrasive water flow on the erosion of concretes is described in [13]. behaviour of concretes eroded by water-driven particles and modelling of the process are the subjects of other works [14] and [15]. many years of experience with the disintegration of cement composites using the high-speed water flows encouraged our team to use this technology for an accelerated mechanical simulation of the degradation process of concrete surfaces affected by flowing fluids. finding the appropriate configuration and setup of high-speed water flow parameters to faithfully simulate the real erosive process is quite demanding and it requires a large experimental program and a great amount of experience with the water jet technology. in particular, the paper focuses on the effects of the abrasive particle size and the mass flow rate of abrasives during the hydro-dynamic disintegration of concrete using the abrasive water flow. 2. abrasives in the water flow abrasives added to the water flow are commonly used to cut materials. the speed of the abrasive material after the exit from the focusing tube is closely related to the speed of the primary water flow. while the abrasive is accelerated by the water flow, the speed of the water flow decreases. speed of the suspension of water and abrasives vs at the moment of impact on the cutting material can be expressed for a particular arrangement of the mixing-and-acceleration device. one of the relations determining this dependence was crushing load [kn·m−1] 57.9 concrete water absorption [%] 3.9 concrete compressive strength [mpa] 64.5 concrete density [kg·m−3] 2300 table 1. properties of concrete shaft unit. published, for example, by hu et al. [9]: vs = χt vw 1 +r , (1) where χt is a momentum transfer parameter and r is the ratio between water mass flow rate and particle mass flow rate: r = ṁp ṁw . (2) the speed of the water flow vw after the exit from the water nozzle can be expressed using the bernoulli’s law as vw = ϕ √ 2p ρ , (3) where ϕ is the discharge coefficient, p is the water pressure upstream the nozzle exit and ρ is water density. similar relations can be found in [16]. however, at usual speeds of the abrasive material (about 600m·s−1 at the water pressure of 415mpa, abrasive flow rate of 300 g·min−1 and abrasive grain size of 180 – 250µm in the arrangement of standard abrasive mixing head [17]), the abrasive cuts off all structural elements of the concrete, which is far from the erosive wear by abrasion caused by flowing liquids (for instance [13]). one way to “slow down” the process in order to achieve lower speeds of abrasives is to use lower water pressures which generate the water flow (e.g. the speed of abrasive grain is only about 118m·s−1 at the water pressure of 80mpa [18]). another ways to reduce the cutting capacity of abrasives in the water flow to erode the surface rather than to cut it off are (i) to decrease the abrasive mass flow rate in the flow or (ii) to use finer abrasive. all methods were incorporated into the experimental program. simultaneously, erosion tests with the water flow without the presence of abrasive were realized for the purpose of comparing the test results. 3. experimental setup and procedure behaviour of concrete under short-time erosion tests has been performed. concrete testing specimens were cut off the 800/500/80mm concrete shaft unit (nominal size dn 800, internal height 500mm, thickness of the wall 80mm) by the abrasive water jet to facilitate the manipulation and consequent measurements. concrete shaft unit was produced by the vibro-compacting and pressing process. properties of concrete shaft unit are presented in tab. 1. crushing load was measured according to en 1917 concrete manholes and inspection chambers, unreinforced, steel fibre and reinforced. 32 vol. 22/2019 effects of abrasives during simulation of mechanical corrosion. . . figure 1. combination of trajectories used for one erosion cycle during treatment of concrete specimens by the water flow. water absorption was measured according to en 1917 concrete manholes and inspection chambers, unreinforced, steel fibre and reinforced, part d. concrete strength was measured according to iso 4012 concrete – determination of compressive strength of test specimens. concrete density was measured on cubes according to en 12390-7 testing hardened concrete part 7: density of hardened concrete. the concrete specimens were gradually eroded by either the abrasive water flow or the pure water flow acting perpendicular to the impact surface. the nozzle generating the flow moved above the specimen in trajectories shown in fig.1 in order to affect a larger area of concrete since the abrasive water flow used had a diameter of only 1mm. one milling cycle was composed of two zig-zag trajectories with a crossing of 90°. the concrete specimens produced were exposed to the water flow at the standoff distance of 50mm (i.e. the distance between the exit of the focusing tube and the specimen). a standard cutting head slice i with the water nozzle diameter of 0.33mm, focusing tube diameter of 1.02mm and focusing tube length of 76.2mm was used in erosion tests. based on previous tests, the operation pressures of 40 and 80mpa were selected. impact angle of the water flow used in this study was 90°. traverse velocity of the flow above surface was set at 500mm·min−1. thus, one erosion cycle lasted about 110 seconds. the australian garnet gma of grain size of 80 and 200 mesh (i.e., average grain size of 177 µm and 74 µm according to [19]) was used for the erosion tests (see fig. 2). for all combinations of operation pressure and abrasive grain size, three abrasive mass flow rates were tested: 10, 20 and 30 g·min−1. 4. evaluation of erosion eroded volume ve was the basic parameter used for the evaluation of erosion processes. this parameter was determined by mobile digital microscope keyence vhx 5000. once the eroded concrete volume of individual pockets ve was measured, the volumetric erosion rate vr was specified as follows [20]: vr = ve te , (4) where te is the exposure time te = n · tc. (5) n is the number of erosion cycles and tc is the time for one erosion cycle. the volumetric erosion rate vr indicates the rate of erosion of surface layers of the testing samples and better determinates the erosion abilities of the flowing water or flowing water with abrasives particles. 5. results and discussion the results have clearly demonstrated sufficient erosive capabilities of the abrasive water flow applied on the cement-based composites. obviously, with the increasing water pressure upstream the water nozzle and thus higher suspension speed (according to (1) to (3)), also the material removal of surface layers of the test specimens is higher (figs. 3 and 4). for similar findings, see, for example [9]. at the speeds of the suspension generated at the pressure of 40mpa, especially the hardened cement paste is removed, except of the coarse aggregate that remains uncovered after the application of the flow. at the pressure of 80mpa, the erosion process is more aggressive and in addition to the removal of smaller structural elements of the concrete (hardened cement paste and small aggregates), softer aggregate grains in the mixture (here, for example greywacke) are cut-off or disintegrated, see fig. 5. however, harder aggregate (for example quartz) resists the flow even at higher speeds (figs. 5 and 6). the increase of the abrasive concentration causes similar phenomena as the increase of the abrasive speed [10]. in addition, at higher speeds, the abrasive material has higher energy to disintegrate and erode the concrete. this fact is documented in figs. 3 and 4 and is also obvious in the computer reconstruction of the real eroded area (figs. 5 and 6). the change in abrasive grain size has a similar effect on the erosion process as the change in abrasive concentration. with increasing grain size, the abrasive has greater erosion ability and thus the erosion rate increases (compare figs. 3 and 4). higher grain size (in our experiment 80 mesh) already causes cutting of hard and durable aggregate, especially at high abrasive concentrations in the flow and higher flow impact speeds. erosive capabilities of the pure water flow are considerably lower compared to the hydro-abrasive flow (figs. 3 and 4), as documented e.g. in [21] or [16]. particles of the surface layer are removed by the water flow at the point of impact to the material. the particles are consequently carried by the flow along the eroded area. this phenomena resembles the erosion of real concrete structures, whereby the released grains of the substrate flowing on the surface cause the abrasive surface erosion. when the erosion cycles are repeated, cement stone and smaller aggregate are 33 p. hlaváček, l. sitek, d. klichová, l. bodnárová acta polytechnica ctu proceedings figure 2. grains of australian garnet 80 mesh (left) and australian garnet 200 mesh (right) used in experiments as an abrasive. figure 3. results of the erosion tests performed in the concrete samples (water pressure of 40 mpa). figure 4. results of the erosion tests performed in the concrete samples (water pressure of 80 mpa). 34 vol. 22/2019 effects of abrasives during simulation of mechanical corrosion. . . figure 5. eroded layers of concrete sample (water pressure of 80mpa, abrasive mass flow rate of 20 g·min−1, australian garnet 200 mesh). the amputated part of the aggregate (a), which prevents deeper erosion. the erosion (hardened cement paste and fine aggregates) is much deeper in case of the absence of greater aggregates (b). tangential scratches caused by the abrasives particles are well visible on the surface of some aggregates (c). almost intact quartz aggregate (d). figure 6. eroded layers of concrete sample (water pressure of 80mpa, abrasive mass flow rate of 20 g·min−1, australian garnet 80 mesh). the removal of hardened cement paste and small aggregate occurs at a lower concentration of abrasives particles. large quartz grain (a) are uncovered but intact. due to the lack of erosive energy of the abrasives particles in the flow, also the softer aggregate remain intact, for instance greywacke (b). gradually washed out and large aggregates become uncovered. if the power of the water flowing along the surface exceeds the adhesive forces between the uncovered aggregate and the substrate, the aggregate is split-off from the matrix (fig. 7) and the abrasive erosion process is repeated. regarding the number of erosion cycles applied at the same area of the concrete specimen, it is evident that multiple cycles cause greater erosion of the material. however, the amount of the material removed decreases in every subsequent cycle (e.g., fig. 8 for water flow without abrasives). this is mainly due to the presence of large aggregate at greater depths from the original surface and also due to greater distances of the newly created surface from the nozzle orifice and thus smaller impact energy per unit area. at the same time, the ratio of the abrasive erosion rate between faster and slower water flows increases with every subsequent cycle (fig. 8). the comparison of surfaces of identical aggregate exposed to pure water (left) and abrasive water flow (right) is visible on the fig. 9. pure water stream removes the cement stone to the surface of the aggregate only, contrariwise the abrasive stream removes and disintegrates the aggregate. there is a visible change in the surface of the aggregate. 6. conclusions from the experimental research in the field of the pure and abrasive water flow moving and acting perpendicular to the concrete surface, it follows that: (1.) application of high-speed water flows is a suitable quick method for simulating the erosion wear of concrete exposed to the effects of flowing liquids. (2.) in order to achieve similar effect to real concrete construction erosion, the pure high-speed water flow or hydro-abrasive flow with low impact speeds of the abrasive material, low abrasive concentration in the water flow and small grain sizes should be used. (3.) the erosion rate increases with the increase in the flow speed, abrasive content and abrasive grain size. however, at high erosion rates, the concrete surface is no longer eroded and structural elements of the concrete are cut off, which does not correspond to the real erosion caused by flowing liquids. (4.) in order to increase the erosion wear, multiple erosion cycles can be applied on the same surface. at first, the hardened cement paste and small aggregates are removed by the high-speed water flow (pure or abrasive) from the disintegrated concrete surface. large aggregate thus becomes uncovered and it is removed in subsequent erosion cycles. acknowledgements this work was supported by grant of the czech science foundation (18-25035s) and by project of the institute of clean technologies for mining and the utilization of raw 35 p. hlaváček, l. sitek, d. klichová, l. bodnárová acta polytechnica ctu proceedings figure 7. split-off stone after erosion test by pure water flow (water pressure of 80 mpa). figure 8. influence of number of erosion cycles on volumetric removal rate. figure 9. left – surface of aggregate exposed to pure water flow; right surface of aggregate eroded by abrasive particles contained in hydro-abrasive suspension flow. changes in colour of the surface are well visible. 36 vol. 22/2019 effects of abrasives during simulation of mechanical corrosion. . . materials for energy use – sustainability program, reg. no. lo1406 financed by the ministry of education, youth and sports of the czech republic. the authors are very thankful for the support. references [1] aci 210r-93 erosion of concrete in hydraulic structures. standard, american concrete institute, 1998. [2] j. c. a. galvão, k. f. portella, a. c. m. kormann. abrasive effects observed in concrete hydraulic surfaces of dams and application of repair materials. in m. adamiak (ed.), abrasion resistance of materials, chap. 2. intechopen, rijeka, 2012. doi:10.5772/27964. [3] e. horszczaruk. abrasion resistance of high-strength concrete in hydraulic structures. wear 259:62–69, 2005. doi:10.1016/j.wear.2005.02.079. [4] astm c 1138-97 standard test method for abrasion resistance of concrete (underwater method). standard, american society for testing and materials, west conshohocken, 2002. [5] e. horszczaruk. hydro-abrasive erosion of high performance fiber-reinforced concrete. wear 267:110–115, 2009. doi:10.1016/j.wear.2008.11.010. [6] e. horszczaruk. abrasion resistance of high strength fibre-reinforced concrete. in 6th rilem symposium on fibre-reinforced concretes – befib 2004. varenna, italy, 2004. [7] h. binici, o. aksogan, e. bahsude gorur, et al. hydro-abrasive erosion of concrete incorporating ground blast-furnace slag and ground basaltic pumice. construction and building materials 23:804–811, 2009. doi:10.1016/j.conbuildmat.2008.03.003. [8] a. momber, r. kovacevic. accelerated high speed water erosion test for concrete wear debris analysis. tribology transactions 39:943–949, 1996. doi:10.1080/10402009608983616. [9] x. g. hu, a. w. momber, y. g. yin. hydro-abrasive erosion of steel-fibre reinforced hydraulic concrete. wear 253(7):848–854, 2002. doi:10.1016/s0043-1648(02)00215-6. [10] x. g. hu, a. w. momber, y. yin. erosive wear of hydraulic concrete with low steel fiber content. journal of hydraulic engineering 132(12):1331–1340, 2006. doi:10.1061/(asce)0733-9429(2006)132:12(1331). [11] a. momber, r. mohan, r. kovacevic. on-line analysis of hydro-abrasive erosion of pre-cracked materials by acoustic emission. theoretical and applied fracture mechanics 31:1–17, 1999. doi:10.1016/s0167-8442(98)00062-7. [12] a. momber. short-time cavitation erosion of concrete. wear 241:47–52, 2000. doi:10.1016/s0043-1648(00)00348-3. [13] a. momber. effects of erodent flow energy and local exposure time on the erosion of cement-based composites due to high-speed hydro-abrasive flow. wear 378-379:145–154, 2017. doi:10.1016/j.wear.2017.01.120. [14] a. w. momber. stress–strain relation for water-driven particle erosion of quasi-brittle materials. theoretical and applied fracture mechanics 35(1):19–37, 2001. doi:10.1016/s0167-8442(00)00046-x. [15] a. momber. a probabilistic model for the erosion of cement-based composites due to very high-speed hydro-abrasive flow. wear 368-369:39–44, 2016. doi:10.1016/j.wear.2016.08.011. [16] a. momber. principles of abrasive water jet machining. elsevier, london, 1998. [17] m. zeleňák, j. foldyna, p. martinec, et al. measurement and analysis of abrasive particles velocities in awj. in water jet 2015. proceedings of the int. conf., pp. 221–232. ostrava, 2015. [18] m. zelenak, j. scucka, k. soucek. velocity measurements on continuous micro water jet using fluorescent pmma seeding particles. in water jet 2017. proceedings of the int. conf. ostrava, 2017. [19] p. martinec, j. foldyna, l. sitek, et al. abrasives for awj cutting. in inco – copernicus no. ic 15-ct98-0821. institute of geonics, 2002. [20] p. hlaváček, l. sitek, r. hela, l. bodnárová. erosion test with high-speed water jet applied on surface of concrete treated with solution of modified lithium silicates, pp. 135–143. springer, basel, switzerland, 2019. doi:10.1007/978-3-319-99353-9_15. [21] d. a. summers. waterjetting technology. e & fn spon, london, 1995. 37 http://dx.doi.org/10.5772/27964 http://dx.doi.org/10.1016/j.wear.2005.02.079 http://dx.doi.org/10.1016/j.wear.2008.11.010 http://dx.doi.org/10.1016/j.conbuildmat.2008.03.003 http://dx.doi.org/10.1080/10402009608983616 http://dx.doi.org/10.1016/s0043-1648(02)00215-6 http://dx.doi.org/10.1061/(asce)0733-9429(2006)132:12(1331) http://dx.doi.org/10.1016/s0167-8442(98)00062-7 http://dx.doi.org/10.1016/s0043-1648(00)00348-3 http://dx.doi.org/10.1016/j.wear.2017.01.120 http://dx.doi.org/10.1016/s0167-8442(00)00046-x http://dx.doi.org/10.1016/j.wear.2016.08.011 http://dx.doi.org/10.1007/978-3-319-99353-9_15 acta polytechnica ctu proceedings 22:31–37, 2019 1 introduction 2 abrasives in the water flow 3 experimental setup and procedure 4 evaluation of erosion 5 results and discussion 6 conclusions acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2019.21.0028 acta polytechnica ctu proceedings 21:28–32, 2019 © czech technical university in prague, 2019 available online at http://ojs.cvut.cz/ojs/index.php/app recycling and its use in concrete waste processing by high-speed milling zdeněk prošeka,b,∗, pavel tesáreka, jan trejbala a czech technical university in prague, faculty of civil engineering, thákurova 7, 166 29 praha 6, czech republic b czech technical university in prague, uceeb, třinecká 1024, 273 43 buštěhrad, czech republic ∗ corresponding author: zdenek.prosek@fsv.cvut.cz abstract. this article discusses the possibility of recycling of concrete waste using the high-speed milling method. the resulting of milling is micronize old concrete. used old concrete was created by crushing of old concrete, which served as a structural concrete for the construction of a supporting column. two level of milling process was used to recycle old concrete. the main use of waste is the possibility of partial replacement of commonly used binder and microfillers in concrete. for this reason, properties as particle size distribution, dynamic modulus of elasticity, flexural strength and compressive strength were observed. the aim is to replace as much cement as possible while maintaining mechanical properties. keywords: micronized old concrete, high-speed mill, cement pastes, mechanical properties. 1. introduction amount of produced waste increases with new concrete structures per year. most of the concrete waste is recycled, approximately 10 20 wt. % of old concrete is stored in landfills [1]. this waste is largely contaminated and can not be used. the most common way of recycling is to reuse old concrete in the form of aggregate to new concrete. first, the reinforcement is removed and then concrete is crushed into fractions. new waste is generated during crushing. this waste is composed of particles sized from 0-1 mm. this waste is not used in new concrete because it increases the water content of the concrete recipe and thus also increases the shrinkage and reduces the mechanical properties [2–4]. waste generated during recycling contains most of the old cement matrix. it is already hydrated cement, which also contains a part of unhydrated clinker because during hydration the water does not reach the center of the clinker minerals and thus the hydration is stopped. the amount of unhydrated clinker is about 10 % in the old matrix [5]. amount of unhydrated clinker directly depends on the age of concrete, its use, and quality. using the high-speed milling method, these unhydrated clinker centers can be uncovered and mechanically activated. recycled concrete can act as a microfiller and binder in the future composite [6–8]. the process of highspeed milling uses the principle that has been known for thousands of years. it is the rotation of friction elements between which the milled material passes. this process has been enhanced by the use of patented friction elements (teeth, pins) at lavaris ltd. and the use of high milling speed [9]. however, this process becomes very energy-intensive and thus creates an economic burden for recycling. for this reason, it is necessary to optimize the milling process and thereby economically improve the recycling process using high-speed milling [10]. 2. materials and samples experimental work deals with cement pastes, where part of cement was replaced with micronized concrete powder. micronized old concrete was formed by micronization of old concrete. it was old structural concrete used for the load-bearing structure, directly for the columns of the old engine factory. two levels of milling were used to recycle old concrete. for the experiment, fractions from 0 to 1 mm were used and it contained a large amount of the old cement matrix (oc). at the first level of the milling process, patented teeth with a diameter of 400 mm were used as the milling element. the result was a micronized old concrete (moc a), which was taken from two places directly behind the mill (moc a _1) or behind the filter (moc a _2). at the second level of the milling process, patented pins with a diameter of 300 mm were used as the milling element. the result was finer micronized old concrete powder (moc b), which was, as in the previous case, taken from the recycling line in two places, behind the mill (moc b _1), or behind the filter (moc b _2). collection points and the entire recycling line can be seen in figure 1. individual grain curves are shown in figure 2. it is clear from results that in the case of recycled moc a _2, moc b _1 and moc b _2 we get a finer material than the used cement. this effect can also be seen in the table 1, where these three micronized old concretes had the highest specific surface area and therefore the possible future activity. cement pastes were made up of 70 wt. % of port28 http://dx.doi.org/10.14311/app.2019.21.0028 http://ojs.cvut.cz/ojs/index.php/app vol. 21/2019 recycling and its use in concrete waste ... figure 1. recycling line with individual components and filing points. sample density specific grain size of matrix surface dx (10) dx (50) dx (90) [kg/m3] area [µm] [µm] [µm] [m2/kg] cem 3104 380 1.07 12.8 44.1 oc 2418 58 38.3 355 752 moc a_1 2489 144 7.77 75.3 267 moc a_2 2495 736 1.19 10.6 39.2 moc b_1 2592 860 0.793 5.95 18.9 moc b_2 2612 1351 0.551 3.04 11.5 table 1. characterization of grains of used materials. figure 2. partical size distribution curve of used materials. land cement cem i 42.5 r radotin (cem) and the remaining 30 wt. % formed a differently micronized old concrete. the resulting micronized old concrete differed the particle size and the milling time with each other. four different micronized old concretes were tested, which were then compared with the two reference mixtures. the first reference mixture was made up of portland cement only and the second reference mixture was made up of portland cement and the non-milled old concrete (fraction of 0-1 mm). the water-binder ratio was set to w / b = 0.4 in all cases (table 2). six beams of 40 × 40 × 160 mm were produced from each mixture. they were stored in a water bath with 100 % humidity and air temperature 23 ± 3 ° c for 28 days. the size of grains affected the workability of mixtures (table 2). the workability was measured by the cone spill method after 15 impulses. the cone spill was not measured on the mixture with non-milled old concrete because the mixture was too fluid. 3. experimental methods the resulting activation of the old unhydrated clinker was measured by the indirect method using the mechanical properties of the resulting cement paste. set of parameters describing the mechanical properties of the resulting cement paste was chosen for this purpose, namely the dynamic shear modulus, the dynamic modulus of elasticity, the flexural strength and the compressive strength. the dynamic shear modulus and dynamic modulus of elasticity were measured by the non-destructive method the resonance method. the advantage of a non-destructive measurement was that the property was traced over time. in our case, 29 z. prošek, p. tesárek, j. trejbal acta polytechnica ctu proceedings mixtures composition workability bulk cement old concrete water [mm] density [wt. %] [wt. %] ratio [kg/m3] cem 100 0 0.40 280 2030 ± 2 cem + oc 70 30 0.40 2028 ± 15 cem + moc a_1 70 30 0.40 290 1989 ± 11 cem + moc a_2 70 30 0.40 227 1930 ± 3 cem +moc b_1 70 30 0.40 225 1944 ± 8 cem + moc b_2 70 30 0.40 139 1993 ± 36 table 2. composition of the individual set/mixtures. figure 3. development of dynamic modulus of elasticity. samples were measured at 7, 14, 21 and 28 days. dynamic modulus of elasticity was accomplished by measuring the basic longitudinal natural frequency of the samples. the acceleration transducer was located in the centre of the vertical side perpendicular to the length of the sample, and the impact hammer strikes were carried out in the middle of an opposite side. dynamic shear modulus was measured by measuring the basic torsional natural frequency of the sample. the acceleration transducer was located in the corner of the vertical side parallel to the length of the sample, and the impact hammer strikes were carried out diagonally the opposite corner on the same side. samples were destructively tested after 28 days of curing. first, a three-point bending test was used to determine the flexural strength of a 40 × 40 × 160 mm beams and then the compression strength was determined using a single-axis pressure test on broken halves from bending tests using the heckert device, model fp100. the tests were controlled by shifting in the case of a three-point bend of 0.1 mm/min and the case of a pressure test of 0.3 mm/min. the distance between supports for the three-point bending test was equal to 100 mm. all of the results have been discussed below. 4. discussion figure 3 and 4 shows the development of the dynamic shear modulus and the dynamic modulus of elasticity. results show a gradual increase in the dynamic modulus of elasticity between 7 and 28 days in all samples with micronized old concrete. the dynamic modulus of elasticity increased by about 2 gpa. highest average values of the dynamic modulus of elasticity after 28 days had a sample with the finest micronized old concrete (cem + moc b_2) of 23.6 ± 0.5 gpa. this is 0.7 gpa less than the reference portland cement (cem) and 0.2 gpa more than the reference sample of a paste composed of cement and non-milled old concrete (cem + oc). a similar trend as with the dynamic modulus of elasticity is seen in the dynamic shear modulus. highest average values of the dynamic modulus of elasticity had a sample with the finest micronized old concrete (cem + moc b_2) of 9.5 ± 0.1 gpa. this is the same value as a reference sample with portland cement (cem) and 0.3 gpa more than a reference sample of a paste composed of cement and non-milled old concrete (cem + oc). also, a reference sample composed of cement (cem) shows a faster increase in the dynamic modulus of elasticity between 7 and 28 days by 3.5 gpa. it is because in old 30 vol. 21/2019 recycling and its use in concrete waste ... figure 4. development of dynamic shear modulus. figure 5. flexural strength of tested samples. figure 6. compressive strength of tested samples. concrete the majority of unhydrated clinkers are composed of belite that hydrates in a long time. figure 5 shows the flexural strength of the tested materials. all recycled material mixtures have a higher flexural strength than the reference cement. this is because old concrete is mostly inert and therefore does not release large amounts of hydration heat and thus smaller volumetric changes. these aspects have resulted in a decrease in the amount of microcracks and thus an increase in flexural strength. figure 6 shows the results of the compressive strength of tested materials. the highest average compressive strength was achieved at the sample with the coarsest micronized old concrete (cem + moc a_1) at 74.0 ± 1.9 gpa and it is 30 mpa less than the reference cement sample (cem). results of the compressive strengths of the sample with recycled materials were approximately 30 % less than reference sample. because it was used 30 wt. % of recycled material, it can be stated that during milling there was minimal activation of unhydrated clinkers in old concrete. 5. conclusions this work focuses on the effect of micronized old concrete as a partial substitute for portland cement. the resulting cement pastes contained 70 wt. % portland cement and 30 wt. % different kinds of micronized old concrete. based on the results, it can be concluded that: • the results show a gradual increase in the dynamic modulus of elasticity between 7 and 28 days in all samples with micronized old concrete. • the highest average values of the dynamic modulus of elasticity were achieved for the sample with the finest micronized old concrete (cem + moc b_2) of 9.5 ± 0.1 gpa. • all recycled material mixtures have a higher bending strength than the reference cement. 31 z. prošek, p. tesárek, j. trejbal acta polytechnica ctu proceedings • the results of the compressive strengths of the sample with the recycled materials are approximately 30 % less than reference sample. in the future, the research will focus on direct detection of the amount of unhydrated clinker and confirmation of the effect of micronization on their uncover. acknowledgements this outcome was supported by the czech science foundation research projects gacr 17-06771s. references [1] s. ahmari, x. ren, v. toufigh, l. zhang. production of geopolymeric binder from blended waste concrete powder and fly ash. construction and building materials 35:718–729, 2012. [2] j. schoon, k. de buysser, i. van driessche, n. de belie. fines extracted from recycled concrete as alternative raw material for portland cement clinker production. cement and concrete composites 58:70–80, 2015. [3] a. pasandin, i. pérez. overview of bituminous mixtures made with recycled concrete aggregates. construction and building materials 74:151–161, 2015. [4] f. rodrigues, m. t. carvalho, l. evangelista, j. de brito. physical–chemical and mineralogical characterization of fine aggregates from construction and demolition waste recycling plants. journal of cleaner production 52:438–445, 2013. [5] z. shui, d. xuan, w. chen, et al. cementitious characteristics of hydrated cement paste subjected to various dehydration temperatures. construction and building materials 23(1):531–537, 2009. [6] z. shui, d. xuan, h. wan, b. cao. rehydration reactivity of recycled mortar from concrete waste experienced to thermal treatment. construction and building materials 22(8):1723–1729, 2008. [7] j. schoon, l. van der heyden, p. eloy, et al. waste fibrecement: an interesting alternative raw material for a sustainable portland clinker production. construction and building materials 36:391–403, 2012. [8] v. corinaldesi, g. moriconi. behaviour of cementitious mortars containing different kinds of recycled aggregate. construction and building materials 23(1):289–294, 2009. [9] z. prošek, j. trejbal, j. topič, et al. utilization of the waste from the marble industry for application in transport infrastructure: mechanical properties of cement pastes. in iop conference series: materials science and engineering, vol. 236, p. 012092. iop publishing, 2017. [10] information on http://www.lavaris.cz. 32 http://www.lavaris.cz acta polytechnica ctu proceedings 21:28–32, 2019 1 introduction 2 materials and samples 3 experimental methods 4 discussion 5 conclusions acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2018.16.0040 acta polytechnica ctu proceedings 16:40–45, 2018 © czech technical university in prague, 2018 available online at http://ojs.cvut.cz/ojs/index.php/app calibration of input parameter values for an advanced material model of sprayed concrete martin závackýa,∗, juraj chalmovskýa, lumír mičaa, petr bílekb a brno university of technology, faculty of civil engineering, institute of geotechnics, veveří 331/95, 602 00 brno, czech republic b geotest, a. s., šmahova 1244/112, 627 00 brno, czech republic ∗ corresponding author: zavacky.m@fce.vutbr.cz abstract. the paper briefly presents an advanced material model for shotcrete involving many aspects of the actual behavior of concrete. however, the contribution is focused on determination and calibration of selected input parameter values for practical use of the model in structural analysis. two cross sections of different geological conditions were chosen from a utility tunnel in brno in order to obtain sprayed concrete samples. the laboratory tests conducted are described and then compared with results from numerical approximations. keywords: shotcrete, triaxial tests, calibration, material model, numerical analysis. 1. introduction nowadays numerical analysis is often used for design or assessment of more and more complicated structures in civil engineering including geotechnics. thus, more advanced material models have been developed and used both for ground and structures [1]. the shotcrete (sc) model was developed at graz university of technology [2] originally for simulation of tunnel linings made from sprayed concrete. however, the model is suitable for description of concrete and cement-based materials in general. the model is used here to demonstrate a powerful and practical tool for modelling of geotechnical structures of help to professionals in geotechnics. a brief theoretical introduction to, and description of, the input parameters calibration process illustrated by data obtained from an existing tunnel structure is the main objective of this paper. 2. the shotcrete model the shotcrete model belongs to a group of elastoplastic constitutive models involving strain hardening and softening. the following main features are involved in the sc model [2]: • mohr-coulomb yield surface for a deviatoric loading and rankine yield surface for a tensile loading regime. • strain hardening and softening in tension and compression. • regularization during the softening regimes in order to avoid mesh dependency (figure 1). • increase of shotcrete stiffness and strength over time. • decrease of shotcrete ductility over time. • creep and shrinkage strains. 3. application of the shotcrete model theoretical studies on the calibration of input parameters for the sc model to verify its function have already been done [3]. alternatively, this paper presents a practical example based on a set of laboratory tests performed on shotcrete samples taken from an existing underground structure. a utility tunnel in the city of brno was chosen for this purpose. two varying cross-sections in different geological conditions were examined for any potential degradation of the tunnel lining using an installed geotechnical monitoring system [4]. 3.1. description of the site – the primary utility tunnel in brno the first cross-section (cs-1) in figure 2 is located near shaft š13a under leitnerova street in a soil environment represented by neogene clay with occasional thin interlayers of sandy sediments containing water. the thickness of the tunnel lining was measured as 30 cm and the lining was installed sequentially in three layers of sprayed concrete. the second cross-section investigated, (cs-2) in figure 2, is situated near shaft š12 under hybešova street. there, the rock mass is formed by devonian sandstones and conglomerates, and thus the ground environment is considered as having a hard rock condition. ground water inflow into the tunnel is also significant there. thickness of the lining was measured as 20 cm and concrete was sprayed at once in whole thickness. the input parameters for the sc model from each locality should be obtained from results of laboratory tests and numerical simulations described in the following sections. the process of the calibration is focused on only part of a wide spectrum of involved parameters, particularly on strain hardening and soft40 http://dx.doi.org/10.14311/app.2018.16.0040 http://ojs.cvut.cz/ojs/index.php/app vol. 16/2018 material model of sprayed concrete figure 1. normalized stress strain curve in compression (taken from [2]). ening in triaxial load conditions. the small amount of material for testing and requirements for the next intended numerical analysis of the whole structure were the reasons behind selection of the properties analysed. 3.2. laboratory tests shotcrete from the lining of the utility tunnels was obtained during installation of geotechnical monitoring sensors in large drill cores with a diameter of 23 cm. smaller cylindrical samples with diameters of 38 mm and 55 mm were prepared by core drilling for laboratory testing later on. a set of experiments was carried out on dry concrete samples from each cross-section: splitting tensile tests, uniaxial compression and triaxial compression tests. the results are listed in table 1 and table 2. dry unit weight was determined at first by measuring the dimensions and weight of samples. three samples with diameter 55 mm and l/d ratio 1.0 were used for the splitting tensile test to determine tensile strength of the material [5]. the load was applied with a rate of 190 n/s. dimensions of 38 mm in diameter and l/d ratio 2.0 were used in samples for the uniaxial and triaxial compression tests. axial load was controlled by deformation through lvdt sensors with rate 0.5 µm/s in both cases. the hoek-brown pressure cell was used to induce radial pressure during triaxial tests. the possibility of monitoring after failure behaviour of the material was achieved by the deformation control. a relatively slow rate of loading was applied to meet the ductile mode of failure to prevent the brittle one. results from one uniaxial and three triaxial compression tests with different radial pressures were considered as representative for one cross-section examined. 3.3. calibration of the material model determination of input parameters related to the strain hardening during loading is presented in this section. input parameters governing the strain hardening are listed in table 3. the uniaxial tensile strength ft,28 was derived from the performed splitting tensile tests. the parameter a governing the shotcrete ductility and the maximum friction angle ϕmax were derived from a series of triaxial tests. for determination of the remaining parameters, the uniaxial test was used. calibration was performed on a stress point level using soiltestlab in the plaxis 2d 2016 software. the final model parameters for both cross sections are listed in table 4 measured and predicted stress strain curves are displayed in figure 4 (cs-1) and figure 5 (cs-2). 3.4. strain softening softening – post-failure reduction of stress with increasing strain is an important part of the actual behaviour of a sprayed concrete. however, prior to reaching the peak strength and during softening, the assumption of a homogeneous stress distribution ceases to be effective because shear bands and transverse tensile cracks occur in a sample. the strain softening rate further depends on the size of sample [6]. thus, to obtain the softening parameters under the pressure load defined in table 5, it is necessary to assemble a 3d model of the laboratory test with real sample dimensions and boundary conditions. calibration is presented only for the case of the cs-2 profile and was performed for triaxial tests with confining pressures of 2, 4 and 8 mpa, respectively. for measured and predicted stress – strain curves are displayed in figure 6. output from the mathematical 3d model of the laboratory test is presented for the test with a confining pressure of 4 mpa and a vertical deformation of 3 mm (figure 3). concentration of shear strains in thin zone (“shear band”) inside the sample is clearly visible. the material above and below the shear zone is in unloading mode, and there are no plastic or failure points. the incidence of diagonal shear cracks was also observed in the laboratory tests presented in this paper (figure 7). 4. analysis of results 4.1. strain hardening certain variations between the properties of concrete from different cross-sections of the tunnel can be recognized from data in table 1. higher dry unit weight with smaller standard deviation was detected in cs-2 in comparison with cs-1. the lower variation of this parameter in cs-2 can be explained by construction of the lining in one layer of shotcrete. however, higher performance of other mechanical parameters is observed in cs-1. this fact can be explained by a higher attack of ground water on the structure in cs-2 and consequently higher degradation of the concrete. the following aspects of actual shotcrete behaviour are correctly predicted by the numerical simulations: • a nonlinear stiffness decrease during strain hardening (before reaching peak strength). 41 m. závacký, j. chalmovský, l. miča, p. bílek acta polytechnica ctu proceedings figure 2. cross section of the utility tunnel; left – in soft soil conditions; right – in hard rock conditions [4]. parameter units cs-1 cs-2 dry unit weight kg/m3 2161 2169 std kg/m3 ±37 ±19 tensile strength mpa 3.76 2.40 std mpa ±0.12 ±0.44 uniaxial compressive strength mpa 44.0 35.9 table 1. properties of the concrete obtained by laboratory tests. cell pressure cs-1 cs-2 2.0 56.0 45.9 4.0 63.5 53.4 8.0 84.2 66.2 table 2. peak axial stresses in triaxial tests [mpa]. parameter description e28 young’s modulus of cured shotcrete ν poisson’s ratio fc,28 uniaxial compressive strength of cured shotcrete ft,28 uniaxial tensile strength of cured shotcrete fc0n normalized initially mobilized strength εcp p uniaxial plastic failure strain a parameter governing increase of εcp with mean stress p’ ϕmax maximum friction angle table 3. input parameters related to strain hardening. 42 vol. 16/2018 material model of sprayed concrete figure 3. simulation of triaxial test with confining pressure 4 mpa, vertical deformation 3 mm – isosurfaces of total deformations (a); deformed elements mesh – 2× enlarged (b); isosurfaces of shear strain (c); failure points and points of plasticity (d). parameter units cs-1 cs-2 e28 mpa 13,500 15,500 ν 0.2 0.2 fc,28 mpa 45 35 ft,28 mpa 3.4 2.4 fc0n mpa 0 0 εcp p 0.0015 0.0016 a 18 24 ϕmax ◦ 40 39 table 4. sets of calibrated parameters. figure 4. measured and predicted stress – strain curves for profile cs-1. • a compressive strength and ductility increase with increasing confining pressure in triaxial loading conditions. figure 5. measured and predicted stress – strain curves for profile cs-2. • a decrease in the rate of strain softening with increasing confining pressure. however, it is possible 43 m. závacký, j. chalmovský, l. miča, p. bílek acta polytechnica ctu proceedings parameter description fcfn normalized failure strength fcun normalized residual strength gc,28 fracture energy of cured concrete in compression table 5. input parameters for strain softening in compression. figure 6. measured and predicted stress-strain curves with strain softening in after failure part from cs-2 profile. figure 7. failure of samples in triaxial loading conditions – confining pressure of 4 mpa (a); confining pressure of 8 mpa (b). to analyse this aspect only from a qualitative point of view; the quantitative prediction requires a complete 3d model. lower initial stiffness is observed for the tests with 4 and 8 mpa confining pressures (cs-1). this effect can be caused by incidence of initial micro-cracks in the samples which cannot be taken into account during simulation. predicted compressive strengths for both sections are slightly different in comparison with measured ones in the case of the test with the cell pressure of 8 mpa. this might be caused by curvature of the actual failure envelope when the sc model has carried through a linear mohr-coulomb failure criterion. regardless of the causes, variation of concrete properties was recognized. thus, the requirements of two different sets of input parameters for the sc model described in table 4 is considered to be necessary in order to achieve better accordance of the numerical simulations with results from laboratory tests. 4.2. strain softening the best fit with the measurement is achieved for the test with a confining pressure of 4 mpa. ductile behaviour is overestimated by simulation in the case of the 8 mpa confining pressure, so the initiation of softening occurs later than in the case of the laboratory test. the predicted rate of softening at a 2 mpa confining pressure is slightly lower than the measured one. 5. conclusions differences in the mechanical and physical properties of the concrete were observed between the two cross-sections of the tunnel that were analysed. the variation can be explained by the different process of construction and also by different conditions of degradation caused by the ground water regime. a reasonable match between the measured stressstrain curves and those obtained by numerical simulations was found in the part of strain hardening when the advanced sc model was used. by combining the 3d mathematical model of the triaxial test and the sc material model, it is possible to predict post-failure behavior with an accuracy which is sufficient from an engineering point of view. the predicted lower softening rates for some of the tests could be caused by incidence of micro-cracks before the start of the laboratory tests. the process of strain softening and its influence on, and correct prediction in, real geotechnical structures is a subject for further research. acknowledgements the research for this paper has been carried out under the project no. cz.01.1.02/0.0/0.0/15_013/0004874 “vytvoření partnerské kooperace mezi firmou geotest a regionálním výzkumným centrem admas” supported by the ministry of industry and trade under the “european regional development fund operational programme enterprise and innovations for competitiveness”. 44 vol. 16/2018 material model of sprayed concrete references [1] b. schädlich, t. marcher, h. schweiger, e. saurer. application of a novel constitutive shotcrete model to tunnelling. in isrm regional symposium – eurock. isrm, vigo, 2014. doi:10.1201/b16955-137. [2] b. schädlich, h. schweiger. numerical methods in geotechnical engineering. crc press, london, 2014. doi:10.1201/b17017-20. [3] r. schütz, d. potts, l. zdravkovic. advanced constitutive modelling of shotcrete: model formulation and calibration. computers and geotechnics 38:834–845, 2011. doi:10.1016/j.compgeo.2011.05.006. [4] p. bílek, o. pazdírek, d. rupp. design of control geotechnical monitoring system of primary collector in brno. in proceeding of international conference of doctoral students juniorstav 2016. brno university of technology, 2016. [5] en 12390-6 – testing hardened concrete – part 6: tensile splitting strength of test specimens. [6] d. jansen, s. shah. effect of length on compressive strain softening of concrete. journal of engineering mechanics 123.1:25–35, 1997. 45 http://dx.doi.org/10.1201/b16955-137 http://dx.doi.org/10.1201/b17017-20 http://dx.doi.org/10.1016/j.compgeo.2011.05.006 acta polytechnica ctu proceedings 16:40–45, 2018 1 introduction 2 the shotcrete model 3 application of the shotcrete model 3.1 description of the site – the primary utility tunnel in brno 3.2 laboratory tests 3.3 calibration of the material model 3.4 strain softening 4 analysis of results 4.1 strain hardening 4.2 strain softening 5 conclusions acknowledgements references acta polytechnica doi:10.14311/app.2020.27.0022 acta polytechnica 27(0):22–31, 2020 © czech technical university in prague, 2020 available online at http://ojs.cvut.cz/ojs/index.php/app role of fluid on the contact deformation response of biological tissue michael v. swaina,b a the university of sydney, faculty of dentistry, biomaterials unit, amme, sydney nsw 2004, australia b don state technical university, biomechanics and biomaterials, rostov-on don, russia correspondence: michael.swain@sydney.edu.au abstract. this paper will focus on the role of fluids on the indentation deformation response of tooth and eye tissues. all natural biological materials contain fluid and function in a fluidic environment, which plays a critical role in responding to loading events as well as tissue nutrition. the location of this fluid varies and is considered as both bound and mobile with much of it located in cell compartments that are also able to respond directly to loading. the extent of the fluid content varies from less than 10 % in the case of the highly mineralised enamel to more than 80 % in the case of soft eye tissues. the role of the fluid and its response during loading is also complicated by the hierarchical structure of biological tissues, be they mineralised or not. the mechanisms by which the presence of fluid in these materials influences the mechanical response is still poorly understood and has not been systematically investigated. the present paper presents data generated over many years on both the above biological tissues and attempts to present indications as to the mechanism(s) by which the presence of fluid contributes to the deformation. the situation associated with contact loading with the presence of mobile fluid in the tissues results in a more complex situation than the classic elastic-plastic contact situation, but the latter still forms the basis for much of the analysis of instrumented indentation force-displacement load-unloading curves using various shapes of indenters, especially for mineralised structures. in the case of soft tissues the absence of agreed protocols for interpretation of force-displacement-time responses is restricting clinical/biological applications. keywords: biological tissues, fluid, indentation. 1. introduction indentation tests provide a simple basis to interrogate the local mechanical behaviour of materials. the developments possible with the advent of computer based instruments over the past 50 years have enabled high precision force-displacement data to be generated with nm and nn precision [1–4]. while these instruments were primarily designed for classic elasticplastic materials, evermore widespread application to biological tissues has occurred especially over the past few decades [5]. in many instances minimal differences in test/instrument parameter requirements are necessary for conducting tests for instance in the case of tooth enamel. however, the situation for softer biological materials such as eye and soft organs generally is that the instrument requirements are very different from the original design applications of instrumented indentation machines. these instruments were initially developed to measure the properties of thin elastic/plastic/brittle films on classic engineering materials, which generally required high displacement resolution (<nm) with reasonable forces (10mn). the situation for soft biological systems however is for very low loads (<1mn) and substantial displacements (up to 100 µm). currently there are virtually no specific nano-indentation instruments developed for soft biological tissues and as such this community has had to adopt atomic force based systems for this purpose [5]. the limitations of both approaches often generates different outcomes when testing the same tissues, this arises because of the size of the required indenter, generally spherical tipped, for afm versus nano-indenter systems [5]. the situation regarding software packages to assist with the interpretation of the resultant indentation force-displacement curves has become well advanced in the case of elastic/plastic and thin coating systems but this is not the case for biological soft tissues and associated materials [6–8]. for the elastic/plastic community well defined iso standards have been developed that enables the role of tip geometry and non-linear response to be addressed [6]. this unfortunately is not the situation for biological tissues. however, as oyen has recently suggested, there is a consensus beginning to emerge as to viable approaches for interpreting the deformation response of soft tissues and structures [9]. in the case of interpretation of force-displacement behaviour, using a standard berkovich diamond pyramid for elasticplastic materials, the iso standard has been shown to be sufficiently robust as it utilises the basic oliver and pharr approach developed more than 25 years ago [6]. the outcomes from the incorporated software in the various instruments are the associated hardness and contact modulus. with specific assumptions this can be extended to the case of elastic/plastic thin 22 http://dx.doi.org/10.14311/app.2020.27.0022 http://ojs.cvut.cz/ojs/index.php/app vol. 27/2020 role of fluid on the contact deformation response of biological tissue films on various elastic/plastic substrates [10]. more detailed outcomes for elastic/plastic materials such as the determination of the stress-strain, yield stress and work hardening have been empirically known for more than 60 years based upon the work of tabor [11]. the use of spherical tipped indenters to quantify the nano-indentation response of elastic/plastic materials was pioneered by field more than 25 years ago [12, 13]. details such as the role of work hardening and surface property gradients and their role on pile-up and sinking-in of the impression have limited the uptake of this approach although a recent study by leitner et al [14] shows great progress in being able to match small pillar compression tests with nano-indentation test outcomes. as will be shown below in the case of teeth, the elastic/plastic analysis with pointed or spherical tipped indenters has provided considerable detail of the properties of enamel, dentine and as well as their diseased states (caries and molar incisor hyper-mineralisation) and in particular to their spatial variation [15–20]. this information has been able to show a raft of outcomes relevant to the effect of clinical procedures, basic hierarchical size dependence and a basis to interpret creep [21–23]. however as will be shown below more critical recent ultrastructural analysis of teeth, especially for enamel, has led to the questioning of some of these outcomes [24]. in the case of soft tissues, with a focus on eye tissue, there are serious issues regarding what is the basis for the deformation observed. eye tissue, like many soft gel systems, contain a very high percentage of fluid, up to 80 %, some of which can be considered bound and some mobile as elegantly illustrated by oyen [9, 25]. however even with the bound fluid some of this may exist in cellular structures, which limit its general movement but does not prevent local displacement as the outer cell membranes are deformed. while gel like materials may be considered isotropic, biological tissues display hierarchical structure, which will result in likely scaling effects and in addition they have structural anisotropy [26]. these latter features have evolved with organism development and are genuinely optimised for function with variations associated with species etc. the very high fluid content, the high degree of extensibility of such biological tissues plus their ability to fully recover limit the adequacy of current (elastic/plastic/viscoelastic) approaches to interpret the outcomes. there have been two major approaches for interpreting the outcomes (for soft tissues) of force-displacement response generated during testing of these structures; visco-elastic [27] and more recently poro-elastic [9]. the former considers the material as primarily a viscous body, which can be considered as equivalent to various spring and dashpot combinations or prony series that are fitted to match the experimental outcomes [28]. whereas the poroelastic approach considers the material to be bior multi-phasic with one phase being porous and continuous but behaving elastically while the other is fluid that must flow through the porous elastic structure as a consequence of a stress or pressure gradient [29]. the latter field was first introduced to soil mechanics by darcy [30] and developed further by biot [31]. application of poro-elastic behaviour of biological tissues was given a major boost by the work of mow and colleagues who investigated the deformation response of cartilage tissue, especially of joints, using contact deformation [32, 33]. other investigations by cowin et al explored the role of hierarchy on poro-elastic behaviour of bone under simple compression tests [34]. a relatively simple methodology to extract the various elastic components (4 for an isotropic material) and permeability of the body was developed by galli and oyen but involved an optimisation parameter fitting of a specific model that becomes slightly more complex when realistic loading rates are used to generate loading, then upon achieving max load, creep data [35, 36]. more recent strategies to determine the poroelastic response of gels have been developed by lai and hu [37] who have proposed the use of dma systems involving frequency sweeps along with testing at different contact dimensions. this approach enables determination of whether visco-elastic or poro-elastic behaviour is the controlling mechanism as visco-elastic systems should be specimen or indenter size invariant while the fluid flow associated with poro-elastic behaviour is contact size dependent as deformation with the latter is related to fluid transport. there have also been novel approaches using fractional visco-elastic theory, which is somewhat easier to analytical model and invert than standard visco-elastic and poro-elastic solutions [38]. 2. anatomical aspects 2.1. teeth figure 1a shows an overview of the anatomy of tooth tissue [39]. there are three major structural components, the outer external layer of enamel that is directly exposed to the oral cavity environment and is non-vital, the dentine which surrounds the pulp space which is vital and contains extensive innervation and fluid transport and in addition there is the cementum layer that connects the dentine in the root region of the tooth to the periodontal ligament that binds the tooth into the jaw. the focus here will be only on the enamel and dentine. the composition of the enamel layer shows hierarchical behaviour that is illustrated in fig 1b and is predominately mineralized 85vol%, 10 vol% fluid and 4 to 5vol% residual proteins [40]. the smallest feature are the hydroxyapatite nanowires, approximately 25 by 50 nm in dimension that are also continuous from dej to the surface and are bundled into rods (approx. 5 µm diameter) that form the second level of the hierarchy. these rods also are bundled into aligned groups called hunter-schreger bands that are some 50 to 100 µms in width that are readily observed on a polished surfaces of enamel that form the 3rd level of hierarchy [41, 42]. 23 michael v. swain acta polytechnica a) b) c) figure 1. a) over-view of the structure of teeth [39]. b) hierarchical structure of enamel [42], and c) atom probe tomography of enamel crystallites [43]. there are also gradients in microstructural uniformity and composition as well as minor amounts of incorporated remnant proteins that increase from the surface to the dentine-enamel junction (dej). a more in-depth compositional analysis of enamel using atomic probe tomography also shows that the chemical composition of crystallites varies and that there are localised organic components present at some triple points as well as mg rich amorphous lenses present in these locations [42]. a typical illustration of the chemical composition of these structures is shown in fig 1c. dentine is a no less complex structure than enamel as is illustrated in fig 2. the hierarchy is more complex but consists of near unidirectional tubules aligned perpendicular to the dentine enamel junction that extend to the pulp. the density and size of the tubules increase towards the pulp from 0.5 to 1 µm diameter. surrounding these tubules is a mineral rich peri-tubular cuff approximately of 0.5 to 1 µm thick, which also contains finer microtubules approx. 100 nm diameter normal to the axis of the main tubules and that extend through the mineralised cuff into the inter-tubular dentine. the latter region contains mineralised type iv collagen as well as fluid and remnant growth factors. the volume fraction of fluid is approximately 30vol%, while that of collagen iv is 20vol % and mineralized tissue at 50vol%. fluid is able to move within the dentine when loaded and this can trigger pain sensation from the enervation arrangement at the pulp and within the dentine [43]. caries is the most prevalent affliction of humans and is associated with the development of acidic conditions from the fermentation of carbohydrates by various pathogens in the mouth [44]. the formation of these acids in protected locations associated with difficulty of adequate oral hygiene results in acid induced demineralisation of tooth structure. two common areas where these conditions occur and demineralisation results are about the fissures of molar teeth and the proximal space between teeth [45]. both areas are difficult for clinical detection often resulting in substantial extent of demineralisation including into the underlying dentine. the form of the demineralisation is very subtle, with this occurring at both the boundaries between rods and within individual crystallites in the rods of enamel [46]. while in dentine dissolution of the minor tubules precedes that of the major tubules. loss of mineral also occurs within the inter24 vol. 27/2020 role of fluid on the contact deformation response of biological tissue figure 2. overview of the structure of dentine showing the complex tubule features as well as microtubules through the peri-tubular mineralised cuff. tubular space. in the case of enamel there is often greater mineral loss beneath the surface with a harder surface layer present while in the case of dentine there is usually an almost linear gradient in mineral density from the surface with a hyper-mineralized region just in front of the active area of dissolution that is associated with the pulp/odontobast defence system, which deposits mineral occluding the tubules to prevent the migration of pathogens into the pulp space [47]. typical examples of demineralisation for enamel and dentine are shown in fig 3. a plot of the mineral density of a proximal lesion in enamel is shown in fig 4, which indicates that up to 50% loss of mineral occurs, yet the overall enamel structure remains intact. note the small locations of mineral loss from within the cores of the tubules as well as at some triple point regions of crystallites. in the case of dentine (fig 3c) the loss of mineral results in a higher fluid content and partially degraded collagen that is much more permeable than sound dentine [48]. 2.2. eyes here we shall only focus on the external surface of the eye namely the cornea, limbus and sclera regions which are shown in figure 5 [51]. the composition of eye tissue is predominantly fluid with collagen fibres and proteoglycans holding the collagen in specific arrangements to provide the optical clarity of the cornea and with partially permeable membranes restraining the fluid. the sclera and limbus regions are less well organised than the cornea and with slightly different volume fractions of liquid present. a high resolution cross-sectional view of cornea reveals the nature of the collagen fibre bundles that are aligned parallel to the surface resulting in an orthotropic structure with radial variations in the density and organisation of the fibers and the associated mechanical properties [50]. there are a range of degenerate diseases of the eye that influence the organisation of the collagen arrangement, which have a consequential outcome on the ability of the lens to focus images on the retina. 2.3. observations 2.3.1. teeth nano-indentation testing of enamel and dentine is very mature and was a focus of research shortly after the availability of high resolution indentation and afm based systems [52]. the analysis of force-displacement date followed that of elastic/plastic materials to enable the hardness and modulus to be estimated. variations of h and e as a function of position identified gradients across enamel, dentine and higher resolution studies showed changes across the dej. a slightly different strategy was adopted by he et al, who as well as probing the variation of h and e spatially, used spherical tipped indenters to investigate the contact stress-strain response and observed that with such indenters non-linear behaviour began at very low contact stresses (h/5) and was associated with substantial energy loss and hysteretic behaviour [53, 54]. this also agreed with the odd report that showed the apparent disappearance of indentations when viewed sometime after formation. he et al also found for enamel that both the initial loading behaviour and creep at max load and upon subsequent near complete unloading was dependent upon the environment with far less creep occurring for desiccated samples, see fig 6 [55]. this behaviour was rationalised in terms of the role of the remnant proteins present in enamel because of their perceived presence surrounding all crystallites and ability to unfold and refold with the breaking and reformation of weak hydrogen bonds. an alternate approach to rationalise the creep of enamel was developed considering the behaviour as a series of spring and dashpots as well as plastic loss mechanisms [55] and also by considering enamel as a brick like structure held together by proteins which behave as a viscous component [58]. these two approaches are schematically shown in fig 7. a more recent study has also shown that the hardness and friction response of enamel is dependent upon the fluid content with hardness increasing with desiccation while the friction 25 michael v. swain acta polytechnica figure 3. backscattered sem images of caries in enamel showing loss of mineral [49] both within the rods a) as well as at the inter-rods b). c) optical image of cross-section of tooth with fissure caries showing the extensive subsurface demineralisation. note the limited influence on the enamel [46]. figure 4. a) microct image of mineral density of enamel proximal white spot lesion [47], and (b) tem observations of the manner in which mineral is lost at the nanoscale [50]. a) b) figure 5. a) schematic illustration of a cross-section of an eye showing the various regions on the surface of the eye that are mentioned in the text [56]. b) histology section of the cornea [57] decreases with the presence of fluid as shown in fig 8 [59]. the concept of crystallites of enamel being surrounded by protein and resulting in a hierarchical structure led to the development of an analysis rationalising the observed size dependent behaviour of the properties of enamel [61, 62]. this approach was built upon the work of ji and gao which worked well for bone [63]. with the advent of more recent atom probe tomography [43] and hrtem [24] work indicating the lack of a uniform protein/-peptide layer between crystallites has raised some questions as to the role of proteins on the resultant mechanical behaviour. an issue that has not been widely canvassed is the role of fluid transport, this is despite knowing that enamel typically contains up to 10 vol% fluid which is mobile. more than 3 decades ago fox in a pioneering study found, during the cyclic loading of teeth at relatively low orally relevant loads, that considerable hysteresis occurred, which was loading rate and cation presence dependent [64]. indentation tests by bertassoni et al [65] as well as compression 26 vol. 27/2020 role of fluid on the contact deformation response of biological tissue figure 6. indentation force-displacement a) plus creep response of enamel b) at max load and c) at very low load ?? figure 7. schematic illustrations of creep behaviour mechanisms for of enamel a) and b) viscous fluid squeezed between rigid apatitie crystals [58] and c) viscoelastic-plastic body "spring + plastic element + dashpot + 2 kelvin-voigt bodies" [55]. figure 8. observations of a) friction coefficient of enamel wet vs dry, and b) hardness of enamel with dehydration time [59]. tests [66] of dentine also displayed creep at maximum load. the former authors showed that the presence of proteoglycans also influenced the extent of creep and subsequent recovery. these authors, like the earlier studies of enamel [54], suggested that the creep response of dentine was influenced by the presence of proteins. it is also known that both enamel and dentine are permeable with minor amounts of fluid from the pulp exiting into the oral cavity. the permeability of dentine, with its high density of open tubules, is 27 michael v. swain acta polytechnica figure 9. force-displacement curves during the indentation of various regions of the external surface of the eye [60]. these force-displacement curves are also strongly loading rate dependent. greater than enamel and both tissues show increased permeability when caries is present. in fracture tests on dentine [43] it was found that the presence of fluid played a major role in the fracture toughness and was observed to be drawn into the tubules in the tensile stress region about the crack tip but was expelled from tubules after the passage of the crack tip, that is in the wake of the crack tip. in another study it was found that in flexure tests of dentine beams the flexural behaviour including the strength and nominal e modulus was strain rate dependent for fluid filled but not for dehydrated samples [67]. caries has a major influence on the deformation behaviour of enamel and dentine. the extent of mineral loss in teeth is best illustrated using micro computed tomography (mct) as shown in fig 4 for a proximal lesion in enamel. as shown in studies by huang et al this loss of mineral has a major influence on nano-indentation derived hardness and elastic modulus properties [68]. the interpretation of the force-displacement curves for the latter outcomes was based upon the assumption of elastic-plastic behaviour whereas the behaviour of highly porous materials, which are generally fluid filled. the latter is far more complex involving both fluid displacement and crushing or densifying the porous structure directly beneath the indenter tip as shown by studies of porous bioceramics [69] and fitted the failure criterion proposed by gurson et al [70]. to date the motion of fluid transport within such a porous structure has not been addressed. in the case of carious dentine it was observed that there was a significant difference in the behaviour of saturated versus dehydrated specimens [71]. these authors also noted that the force-displacement response of fluid filledhighly demineralised dentine showed a substantial recovery during unloading, presumably associated with fluid flow returning to the consolidated material beneath the area of contact. 2.3.2. eye results indentation tests on eye tissues have become popular over the past decade, especially with the ability to use afm systems with small spherical tipped indenters (r< 20 µm) [72]. nano-indentation tests have also been conducted but have required systems with low force but large displacement using relatively large indenter spherical tips (r ~500 µm) [60, 73]. the afm approach has typically involved rapid load/unload tests and relied upon hertzian contact analysis for the determination of the elastic modulus. whereas the nano-indentation tests usually involve a slower loading rate and incorporate a hold period at max load generating substantial creep response, see fig 9. interpretation of ni results for eyes based upon an inverse fea visco-elastic analysis has been conducted by abyaneh et al [73]. an alternate highly simplified poro-elastic approach developed by the author and colleagues [60] considers localised deformation of these highly fluid filled tissues as being determined by the collagen elastic response and movement of fluid caused by indentation induced transient pressure gradients. at longer hold times at max load the resultant displacement is determined by the elastic properties of the structure whereas the short term behaviour is dependent upon the loading rate and hydraulic conductivity. the aim of the latter approach was an attempt to develop an output, namely an effective e and effective hydraulic conductivity k, that was comparable to the hardness and contact modulus that has so successfully been used for elastic/plastic materials. these effective e and k values are recognised as approximations because as outlined above eye tissues are highly anisotropic with orthotropic mechanical properties because of the aligned collagen fibres parallel to the surface, they have gradients in collagen density and are likely to have orthotropic hydraulic conductivity parameters. 3. conclusions the presence of fluid plays a significant role in the deformation response of biological tissues. in this overview only two types of tissue are considered, namely highly mineralised tooth tissue and highly fluid containing eye tissue. in both instances there is 28 vol. 27/2020 role of fluid on the contact deformation response of biological tissue clear evidence of the influence of fluid but the mechanisms are not well understood. there is a clear necessity for further critical investigations to understand the mechanisms responsible in these and other biological tissues. of even greater need is for collective agreement as to an approach to interpret the force-displacement-time curves generated during the indentation of such complex tissues. 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grepl@dipont.cz abstract. after optimization of the railway geometry for higher speed, a continuous settlement of the track occurred. improvement of the embankment body was needed; however, a complete traffic interruption was not allowed. the reconstruction work has been done using the methods of special ground engineering such as vibro-stone columns and driven ductile piles. keywords: railway embankment settlement, vibro-stone columns, ductile piles. 1. introduction with optimization of the double track to a higher speed, an increase of the curve radius occurred. the embankment had to be extended. later, deformations of the extended earth body appeared. to achieve a safe service on the track, both tracks needed to be rectified very often. based on several geotechnical surveys and monitoring, it was ascertained that the deformations originated inside the embankment. the influence of subsoil deformation as well as sliding influence were excluded. the low value of the global deformation modulus of the embankment had probably been the cause of the deformation. the article describes the design and the in-service implementation of the 16m high railway embankment repair. work was done according to the requirements of the track operator. at least one track of the double track needed to be in service given the demand for a maximum 90 days service disruption. 2. description of location and the original condition the double corridor track is located at the left sided curve with a radius of 725m and elevation of 135mm. the parameters had been designed for a track velocity of 120 km/h. the part of the track which has been dealt with is 400m long and is at the embankment of a variable height of 5–16m with bridges on both sides. the curve radius needed to be extended. due to the constricted conditions, the embankment extension close to the abutment has been built from a reinforced earth structure. the slope of the embankment extension close to the abutment was very steep. the crown of the old embankment, which was unused after optimization work, was adjusted into a slope of 2% with a direction out of the curve. the surface of the top has been compacted and stabilised with cement (see figures 1 and 2). the track has been drained by an open ditch between the rails and the loosened top of the original embankment. 3. recorded defects on the track and its causes 3.1. defects on the track the deformation of the rails had been the biggest problem on the track. the first indications about problems with embankment settlement occurred just a few months after the track was back in service. not long after that, problems with the earth structure connected with high water content occurred (cracks and gaps in the facing wall, a watery base of the slope at the facing wall, local outflow from the earth structure). the tendency of embankment settlement was not decreasing, and there was a need to rectify the track frequently. with respect to these problems geotechnical surveys using geophysical measurement were performed as well as the cone penetration testing and bore holes. 3.2. probable causes of the embankment defects due to these constant problems with the embankment settlement the causes of these vertical deformation could be defined additionally thanks to a complementary survey. insufficient value of the global deformation modulus of the embankment material has been discovered as the origin of the deformation. this has been caused by a combination of the following: • the incorrect technological procedure of the embankment building process (insufficient compaction, free swelling etc.), • the inappropriately chosen material for the embankment extension (slate of different levels of weathering, time degradation). the earth structure material is very sensitive to the water content. other influences contributed to the continual deformation. those influences caused infiltration of the rainwater into the earth structure. mainly it was about the open top of the original embankment and a non-functional drainage of the track 20 https://doi.org/10.14311/app.2019.23.0020 https://ojs.cvut.cz/ojs/index.php/app vol. 23/2019 in-service remediation of a railway embankment to prevent settlement figure 1. existing condition – extension with geogrids. figure 2. existing condition – unreinforced embankment. figure 3. concept for improvement of the non-reinforced embankment. figure 4. concept for improvement of reinforced embankment. 21 jan grepl acta polytechnica ctu proceedings with an open ditch. the non-functional drainage close to the bridge abutment contributed to the deformation of the transition zone. this abutment drainage was interrupted. the gradual deformation of the earth structure and constant rectification of the track caused a penetration of the ballast to the subgrade and non-function of the drainage. this fact led to the saturation of the embankment with rainwater which caused the degradation of the coarse material into a fine clay. 4. main steps to improve the embankment when the causation of the embankment deformation was clarified, the main steps of the improvement were defined. these were intended to eliminate the deformation and stabilize the geometrical track position. the following steps were taken: • embankment drainage to lower the water content. • homogenization and compaction of the earth body, which had degraded. • prevention of significant rainwater inflow to the earth body. • recovery of the subgrade and its functional drainage. 5. boundary conditions for the improvement design the main steps in the improvement had been defined. boundary conditions had been taken into account for the choice of technology. at least one track in service (electric traction included) had been the essential condition restricting the improvement design. maximal disruption time had been limited to 90 days in accordance with other works on the track. these conditions excluded complete excavation of the inappropriate material of the earth structure and its rebuilding from a suitable material. the electric traction in service also influences very significantly the technological procedure. the access on the embankment body was allowed only from one side, at the place where the track is coming to its cut. the loosened top of the original embankment provided sufficient manipulation space. the condition of the track in service caused the one track in service to limit access to the other track. 6. method and technology of the improvement 6.1. the improvement concept considering the basic steps of the improvement and the given conditions, the concept for the improvement was chosen. the substructure would be improved with installation of vibro-stone columns [1]. the released crown of the old embankment would be arranged for a greater slope and sealed with watertight membrane. membrane would be also laid under the sub-ballast. vibro-stone columns installed to the earth structure would drain the body, solving the problem associated with the high water content. when installing the vibro-stone columns, the gravel is pushed to the sides and it would also compact the existing material around them [2]. the watertight membrane installed around all the area of the embankment top would provide a permanent barrier to the rainwater (see figure 3). at the part where the embankment extension was reinforced with geogrids the vibro-stone columns would not be possible to use. reinforced geogrids would obstruct the technology of the columns installation. if the columns crossed the geogrids, its anchor length would break and the stability of the steep slope could be also broken. these are the reasons why the improvement of the bridge transition zone and its close parts were designed with driven ductile piles with precast reinforced concrete heads. ductile piles have been installed in the regular triangle grid, around 2m under the subgrade. on the piles concrete spreading heads have been placed and the construction has been covered using gravel with geogrids (see figure 4). 6.2. procedural progress work procedure had to be adjusted to the requirements of the lockout of the track and also to limited access to the embankment top. it was designed firstly under the track no. 1, which was located closer to the earth structure slope. the improvement under the track no. 2 followed just after the work of the track no. 1 improvement had finished. to access the track no. 1 it was necessary to build an access platform as it was not allowed to cross the track no. 2 in service. before the track no. 1 lockout, the super-elevation in the track no. 2 needed to be removed, the traction included. when the track no. 1 was not in service, the superstructure was removed, and the sheeting of the ballast shoulder was done. in the transition zone a soldier pile wall with heb 140 profiles was chosen. after the removal of superstructure and ballast, a space with the width of 5.5m was formed for the installation of the stone columns. the workspace was bounded by the track in service from one side and the embankment slope from the other side. it was very difficult to plan the work procedure. the area close to the machine for the stone columns production was very narrow and it was not able to pass by with other forms of mechanization. due to the length of the columns (15m), it was necessary to do a pilot borehole with a cfa drilling machine (see figure 5). after all stone columns were fixed, the excavation at the transition zone could be done. the installation of the ductile piles followed. at the same time repair to the insulation of the bridge abutment was made. finally, the watertight geo-membrane, the sub-ballast and ballast were laid. the superstructure was laid to 22 vol. 23/2019 in-service remediation of a railway embankment to prevent settlement the original geometry with zero super-elevation (see figure 6). figure 5. stone columns procedure under the track no. 1. (top) and procedure of the ductile piles under the track no. 1 (bottom). figure 6. procedure of the spreading heads under the track no. 1. as the track no. 1 was brought into service, the superstructure of the track no. 2 was removed. the procedure was repeated. the vibro-stone columns were fixed. the bridge transition zone was excavated for the ductile pile installation. the bridge abutment insulation was repaired as well. work on the crown of the old embankment was finishing at the same time. after the stone columns and the ductile piles installation, the backfill of the transition zone was done. this was laid and connected to the membrane under track no. 1 and the membrane on the crown of the old embankment. finally, the track superstructure was laid and both tracks were adjusted to its final geometry. the service was restored in both tracks and the final work was performed away from the track in service (see figure 7). figure 7. vibro-stone columns and excavation for ductile piles under the track no. 2 (top) and ductile piles hammering under the track no. 2 (bottom). 7. essential construction parts 7.1. vibro-stone columns vibro-stone columns have been designed in 700mm diameter, placed in triangular raster with 1500mm distance. the drainage layer on the base of the embankment had to be reached by the columns in order to achieve the best drainage effect. the maximum column length was 15m. in order to achieve the maximum length of the columns and to speed up the works, it was decided to install the columns into the pilot bore diameter of 500mm. under each track, 402 pieces of vibro-stone columns were fixed, a total of 804 pieces. in the first stage under track no. 1 there were 5,400 meters of columns installed in 19 days. in the second stage under track no. 2, 5,900 meters of columns were completed in the same period (see figure 8). 23 jan grepl acta polytechnica ctu proceedings figure 8. vibro-stone columns installation under the track no. 1. 7.2. driven ductile piles the use of ductile piles was chosen with regard to the necessity for passing through the geogrid reinforced body. the industrially pre-fabricated piles from highstrength ductile cast iron are joined to a pile shaft of variable length without using special tools and then driven into the soil. ductile piles are driven while simultaneously grouting the concrete mortar or cement suspension into the annulus between the tube and the grout shoe (this gives an increase in load capacity and corrosion protection). the driving of additional tubes to the required depth is achieved. resistance is recorded during the installation to indicate the load capacity. this also paid off during construction, when the sufficient load-bearing capacity was not achieved at the planned depth of 12m. the piles were operatively extended. the pilot head was equipped with a prefabricated reinforced concrete distribution head. the head provided a connection with the geogrid reinforced backfill. in the first phase under track no. 1, more than 700 meters of the piles, including grouting, were done in less than two days. a similar installation speed was achieved in the second phase. 8. conclusion the reparation of the railway embankment to prevent its settlement was carried out in a very short time and in a place with a very difficult access. one track was always in service. the given task would not have been completed in time without the use of modern methods of deep foundation. in this case, the cooperation of the designer and the deep foundation works contractor proved successful. references [1] r. fiala, j. boštík, a. kotačková, et al. technologie štěrkových pilířů pro praxi: (navrhování, provádění a kontrola). akademické nakladatelství cerm, brno, 2014. [2] p. svoboda. hloubkové zlepšování zemin v české praxi. akademické nakladatelství cerm, brno, 2009. 24 acta polytechnica ctu proceedings 23:20–24, 2019 1 introduction 2 description of location and the original condition 3 recorded defects on the track and its causes 3.1 defects on the track 3.2 probable causes of the embankment defects 4 main steps to improve the embankment 5 boundary conditions for the improvement design 6 method and technology of the improvement 6.1 the improvement concept 6.2 procedural progress 7 essential construction parts 7.1 vibro-stone columns 7.2 driven ductile piles 8 conclusion references acta polytechnica ctu proceedings doi:10.14311/app.2019.23.0038 acta polytechnica ctu proceedings 23:38–43, 2019 © czech technical university in prague, 2019 available online at https://ojs.cvut.cz/ojs/index.php/app evaluation of underground structures subjected to seismic loads jan pruška czech technical university in prague, faculty of civil engineering, department of geotechnics, thákurova 7, 166 29 prague, czech republic correspondence: pruska@fsv.cvut.cz abstract. the paper is focused on the evaluation of the effect of earthquakes on underground structures. free-field analysis is one solution of this task common mainly in engineering tunnelling practice, but it has some rather simplified aspects (e.g. equivalent shear strain is constant). pseudostatic finite element calculation combines free-field analysis and the advantages of a fem model. dynamic effects are introduced in the form of displacements prescribed along the vertical boundaries of the fem model in a usually static manner. this approach also implies constant material parameters for the geological profile in the horizontal direction, an arbitrary geometry of excavation, soil structure interaction and description of share waves as a time-dependent 1d analysis of the so called free-field column. moreover, there is shown an example comparing pseudo-static fem analysis with an analytical method. finally, the advantages of the pseudo-static fem method are presented. keywords: finite element method, earthquake, 1d free field analysis, dynamics, pseudo static analyses. 1. introduction the response of underground structures subject to seismic load can be solved in several ways, which can be roughly classified into four categories: laboratory testing, analytical solutions, pseudo static finite element calculations and full dynamic analysis. (1.) centrifuge modelling techniques allow well for control testing conditions regarding investigations of soil structure interaction in simulated seismic events. however, there are several difficulties for a dynamic centrifuge model such as accurate modelling of the boundary conditions or the scaling law for dynamic velocity. (2.) most simplified methods were derived from closed form solutions but involving neglect of some factors. in general, the methodology of the analytical method is that the seismic loads for an underground structure are characterized in terms of strains imposed on the structure by the surrounding ground or their interaction. although this is the most conservative method having only a limited application, it does provide a first-order estimate of the liming stresses so to potentially serve as an accuracy check on more complex calculations. (3.) a pseudo-static finite element calculation provides a solution for any shape of underground structure in a generally non-homogeneous layered rock/soil mass with a potentially nonlinear material response. the basic idea of this approach is that the maximum shear deformation in a free-field condition of the layer represents the maximum dynamic earthquake stress in this layer and can be an input to the numerical model as a boundary condition. the shear strain equivalent to the actual dynamic loading can be calculated from the particle velocity estimated either in a simplified manner based on limited information or from the solution of a one dimensional simulation of free-field response to an actual earthquake. (4.) full dynamic analyses are based on a direct application of recorded accelerations to the actual computational model of the tunnel where the soil and tunnel responses are mechanically coupled and analysed via numerical modelling, such as finite element methods. 2. analytical method the methodology of the analytical method for a seismic loading design is based on the principle that the static condition has to include the additional loading imposed by seismicity. in the case of underground structures, the seismic loads are described in terms of strains imposed on the structure by the surrounding ground mass or their interaction. 2.1. seismic coefficient method this method was originally developed for dynamic analysis of over ground buildings. inertial forces due to earthquakes f1 of the structure and f2 of the upper soil mass are described by the following equations: f1 = a g ·q = kc ·q, (1) f2 = ηc ·kh ·m1 · g. (2) this method is not suitable for tunnels with large diameters because this method does not reflect the real dynamic property of tunnel structure. 38 https://doi.org/10.14311/app.2019.23.0038 https://ojs.cvut.cz/ojs/index.php/app vol. 23/2019 underground structures subjected to seismic loads 2.2. free – field deformation approach tunnel structures are assumed to move together with the surrounding soil/rock mass during earthquakes and therefore undergo the following deformations: • ovaling, • axial, • curvature. the ovaling deformation of a circular tunnel lining is primarily caused by seismic waves that propagate perpendicular to the longitudinal axis of the tunnel (figure 1). the vertically propagating shear waves produce usually the most critical ovaling distortion of the lining. the free field deformation approach assumes that the deformation of the underground structure should conform to the deformation of the soil in the free field under seismic shaking. in these simplified methods described in [1–4] the soil – structure interaction and the effects of the waves are neglected. moreover the maximum shear strain γmax (equation 6) induced by shear waves propogating in vertical planes is constant along the depth z. this may lead to an overestimate of tunnel lining deformations especially in loose rock and soils. figure 1. ovaling deformation of a circular cross section. in the case of a circular tunnel in the homogeneous medium the ovaling deformation is determined by the following steps: (1.) according to the seismic report we get the pick ground acceleration (3) and the site-specific pickground-acceleration (4): agr = 40% · ss, (3) amax,s = s · agr (4) (2.) and then we determine the pick acceleration at the depth of the tunnel: az,max = c · s · agr, (5) (3.) az,max max is used to determine maximum shear deformation in free-field condition as shown in equations (6) and (7): γmax = vz cs , (6) vz = k · az,max (7) (4.) and finally we obtained the ovaling deformation of the circular tunnel: ∆dff = γmean d 2 . (8) 2.3. soil-structure interaction approach the kinematic interaction between the tunnel and the surrounding soil/rock mass is used to improve the accuracy of the widely used free-field methods. generally, analytical solutions for estimating soil-structure interaction for tunnels are based on the following assumptions: • the tunnel is circular; • the soil is an infinite, elastic, homogeneous and isotropic medium; • the tunnel lining is generally an elastic tube under plane strain conditions or it is based on the theory of an elastic beam on an elastic foundation; • the interface between the soil and the tunnel lining has a full-slip or a no-slip condition; • the soil-structure interaction effects operate in a quasi-static manner, ignoring any inertial interaction effect. there exist many approaches in the technical literature describing seismic design and analysis of underground structures using a soil structure interaction approach. for example: billota et al. [5] discussed four different closed-form elastic solutions to evaluate the maximum shear strain γmax (all approaches are based on the equilibrium of the soil column from the surface to the given depth z), while expressions for displacements and internal lining forces in the circular tunnels excavated in a rock mass with a near fault are presented in [6], and closed-form expressions to calculate the circular tunnel liner forces due to the compressive seismic p-wave propagation are described by kouretzis et al. [7]. in addition an analytical solution for estimating internal forces in the lining of a circular tunnel in the homogeneous medium is published in [8, 9]. the ovaling deformation is determined by the assumed shear strain ∆d (figure 1). we get for parameters ∆dff ,∆dlin the follow-on formulas: ∆dlin = rn∆dff ,∆dff = γmean d 2 , rn = 4(1 − νm) αn + 1 , αn = 12el(5 − 6) d3gm(1 − ν2 l ). (9) it is evident that the material and geometric parameters rn and αn take into account the rigidity of the subsoil-lining system. expressing the curvature of an elliptical excavation using ∆dlin and parameters describing soil structure interaction then the moment in the lining (figure 2) can be written using the polar 39 jan pruška acta polytechnica ctu proceedings coordinate ϕ (positive clockwise from horizontal) as: m(ϕ) = 6el∆dlin d2(1 − ν2 l ) cos (2(ϕ+ φ 4 )). (10) with reference to figure 2 (c), the normal force on the curved element can be expressed as: n(ϕ) = − 1 r dm2 dϕ + pnr (11) and after rearranging using equation (10): n(ϕ) = 48eli∆dlin d3(1 − ν2 l ) cos (2(ϕ+ φ 4 )) +rpn (12) figure 2. internal forces and contact stress on the differential element. 3. pseudo static fem implementation of free-field analysis results into the static fem analyses can be made in several ways. generally, the shear strains in the 2d solution without lining have to be in conformity with 1d free-field analyses and the shear deformation in the free-field deformation is applied by prescribed displacements at the boundaries of the numerical model. in the first method, we consider a homogeneous soil/rock mass with an effective shear modulus g. we determine the maximum shear deformation in free-field condition γmax (6). the value of γmax corresponds to the maximum horizontal displacement applied in the boundaries of the numerical model (figure 3), calculated by equation: ∆xmax = γmax · hm. (13) the second method considers the layered soil/rock mass (figure 4) where some sort of homogenization is needed. adopting the voigt assumption of constant deformation at each point of the solid we obtained effective shear modulus ghomv : ghomv = h1 h g1 + h2 h g2 (14) and average shear deformation of the numerical model γmean: γmean = h1 h γ1 + h2 h γ2. (15) figure 3. homogenized mass and constant shear strain. figure 4. layered mass and constant shear strain . for the layered mass the better method is based on the setup constant corresponding movements by parts – each layer has a different shear deformation – figure 5. the prescribed shear deformation values should be in the ratio of the stiffness of the materials in layers to ensure the continuity of the stress at the layer interface: γ1 γ2 ≈ g2 g1 (16) figure 5. layered mass with different values γ at layers 1 and 2. 4. full dynamic calculations using fem the numerical solution of dynamic load effects using the finite element method is based on the principle of virtual displacements, which express the external volume load by the effects of inertial forces and leads to equations of differential motion in the form: mü(t) + cu̇(t) + ku(t) = f(t). (17) 40 vol. 23/2019 underground structures subjected to seismic loads the assembly of the stiffness matrix c is usually a very complicated task thus the rayleigh formulation is used i.e. as a linear combination of the mass tensor m and the stiffness tensor k: c = αm + βk. (18) coefficients α and β have been calculated according to the double frequency method that avoids an overestimate of damping throughout the considered frequency range [10]. assuming that the first natural frequency is the least damping, the equation (17) can be decomposed to the natural vibrations. using the damping ratio ξ we get a relationship for individual frequencies (for real systems with a large number of degrees of freedom): ξi = 1 2( α ωi + βωi). (19) in order for there to be a damping ratio ξ minimal for ω=ωi it must fulfill the next equation: dξ dω = 1 2(− α ωi + β). (20) after substitution ω=ωi into equation (20) we get α = ω2 1β (21) and then from equation (20) β = ξ1 ω1 . (22) in common practice, equation (17) is solved by direct integration using the newmark integration method. the discrete form of equation (17) already includes the influence of boundary conditions and the effect of loading in the form of a prescribed acceleration of displacements at the lower limit. 5. case exemplification the fem calculation results will be compared with the results of the analytical calculation published in [11]. the geometry of the model and the finite element mesh is shown in figure 6 and table 1 contains the parameters of the subsoil and the tunnel lining. the seismic loading has pga 2ms−2 and the wave period is 0.01 s. all fem calculations were made by geo5 tunnel software [12]. for the sake of clarity the representative extreme values obtained for ϕ = φ 4 have been set – see table 2. for illustration the resulting moment in the lining is also shown in figure 7. the two approaches in terms of the absolute percentage of the difference in the last column of table 2 can be mutually assessed. it is evident that in the case of a homogeneous mass and a circular excavation both methods give comparable results. the exception is the value of normal force in the case without a considering of the interaction between the lining and the soil (effect of the contact stress). if we take into account the value of contact stress in the equation (18), the resulting values ∆dff a ∆dlin of normal force for both methods are almost identical. type unit value unit loading γmean 0.0105 [-] soil gm = ghom 8.65 [mpa] νm 0.3 [pa] ρm 2000 [kg/m3] lining γmean 0.0105 [-] νl 0.3 [-] gl 12.5 [gpa] d 6 [m] h 0.3 [m] i 0.00225 [m4] table 1. input parameters of the numerical model figure 6. numerical model. 6. conclusions the paper is focused on the evaluation of the seismic effects on the tunnel lining by free-field analysis and pseudo static calculation using fem. generally, both calculation methods give comparable results in the case of a homogeneous soil environment and ignoring soil structure interaction. however, from a practical point of view, the fem combined with the 1d freefield analysis yields clear advantages: • it allows a possibility to derive the load for any time of the earthquake (measured or synthetically generated accelerograms). • it analyses any shape of excavation (lining). • it takes into account the potentially non-linear subsoil response. • it take into consideration the layered rock/soil mass depending on the "real" material parameters. another indisputable advantage is taking into account the soil structure interaction whose neglect in the case of an analytical method may lead to a significant overestimation of the normal force. such a non-realistic value of the normal force leads to a lower degree of reinforcement in the effects of the bending moment for the considered cross section. 41 jan pruška acta polytechnica ctu proceedings variable fem analytical difference [%] ∆ dff [mm] 31.3 31.5 0.6 ∆dlin [mm] 31.0 36.1 14.1 m [knm/m] –367.6 –422.8 13.1 v [kn/m] 244.5 281.0 13.0 n [kn/m] –380.8 –562.0 32.2 n∗ [kn/m] –380.8 –390.1∗ 2.4∗ * the influence of cont. nor. stresses was taken into the calc. table 2. output parameters. figure 7. moment in the lining. list of symbols a earthquake acceleration an ratio between the stiffness of lining and massive az,max pick acceleration at the depth of the tunnel d diameter of the excavation g gravitational acceleration h thickness of the lining k ratio of pgv to pga m1 weight of soil mass u actual displacement vector c ground motion at depth z c damping matrices cs apparent propagation velocity of s-wave d tunnel radius el young modulus of the lining f vector of nodal loading g shear modulus of soil/rock gm elasticity modulus of the massive i moment of inertia of the lining k stiffness matrices kv seismic coefficient in vertical direction kh seismic coefficient in horizontal direction m mass matrices q weight of structure pga peak ground acceleration r reduction factor of depth influence r radius of excavation rn lining – massive racking ratio under normal loading s coefficient based on the types of elastic response spectra suggested in eurocode 8 ss short period spectral acceleration vs peak ground velocity α parameter that determines the influence of mass in the damping of systems αm damping proportional to displacements β parameter that determines the influence of the stiffness in the damping of the system βk damping proportional strain rate γmean average shear strain of the model ηc integrate response coefficient related to the tunnel depth, soil property and project significance νm poisson number of the massive νl poisson number of the lining σv poisson number of the lining ω angular frequency ω1 angular frequency of the first natural mode ωi angular frequency of i-th mode acknowledgements the article was produced with financial support from the competence centres program of the technology agency of the czech republic (ta cr), project no. te01020168 centre for effective and sustainable transport infrastructure (cesti). references [1] j.-n. wang. seismic design of tunnel – a simple state of the art design approach. parsons, brinckerhoff, quade & douglas, inc. 1993. monograph 7. [2] m. s. power, d. rosidi, j. kaneshiro. strawman: screening, evaluation, and retrofit design of tunnels. national center for earthquake engineering research, buffalo, new york. report draft. vol. 3. [3] j. penzien, c. wu. stresses in linings of bored tunnels. international journal of earth-quake engineering and structural dynamics 3(27):283–300, 1998. [4] y. m. a. hashash, j. j. hook, b. schmidt, j. i.-c. yao. seismic design and analysis of under-ground structures. tunnelling and underground space technology (16):247–293, 2001. [5] e. bilotta, g. lanzano, g. russo, et al. pseudo-static and dynamic analyses of tunnels in transversal and longitudinal directions. in proceedings of 4th international conference on earthquake geotechnical engineering. icege, thessaloniki, 2007. 42 vol. 23/2019 underground structures subjected to seismic loads [6] m. corigliano, l. scandella, c. lai, r. paolucci. seismic analysis of deep tunnels in near fault conditions: a case study in southern italy. bulletin of earthquake engineering 9(4):975–995, 2011. [7] g. kouretzis, k. andrianopoulos, s. sloan, j. carter. analysis of circular tunnels due to seismic p-wave propagation, with emphasis on unreinforced concrete liners. computers and geotechnics 55:187–194, 2014. [8] aashto. technical manual for design and construction of road tunnels – civil elements. first edition. american association of state highway transportation officials, 2010. [9] c. hunk, j. monses, n. munfah, j. wisniewski. technical manual for design and construction of road tunnels – civil elements. first edition. federal highway administration, 2009. report fhwa-nhi-10-034. [10] g. lanzo, a. pagliaroli, b. d’elia. influenza della modellazione di rayleigh dello smorzamento viscoso nelle analisi di risposta sismica locale. in proceedings of 11th national conference l’ingegneria sismica in italia. 2004. [11] d. kučera. posouzení geotechnické konstrukce na seismické zatížení. diplomová práce. čvut v praze, fakulta stavební, 2017. in czech. [12] geo 5 fem. online, available from: https: //www.fine.cz/geotechnicky-software/mkp-tunel/. 43 https://www.fine.cz/geotechnicky-software/mkp-tunel/ https://www.fine.cz/geotechnicky-software/mkp-tunel/ acta polytechnica ctu proceedings 23:33–38, 2019 1 introduction 2 analytical method 2.1 seismic coefficient method 2.2 free – field deformation approach 2.3 soil-structure interaction approach 3 pseudo static fem 4 full dynamic calculations using fem 5 case exemplification 6 conclusions list of symbols acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2018.17.0006 acta polytechnica ctu proceedings 17:6–9, 2018 © czech technical university in prague, 2018 available online at http://ojs.cvut.cz/ojs/index.php/app investigation of surface macroscopic residual stresses of cutting ceramics after mechanical machining jakub němeček∗, jiří čapek, nikolaj ganev, kamil kolařík department of solid state engineering, faculty of nuclear science and physical engineering, czech technical university in prague ∗ corresponding author: jakub.nemecek@fjfi.cvut.cz abstract. currently, the extensive research in the field of cutting ceramics is conducted and there are efforts to replace cemented carbides by these materials. however, the problem is to improve the production of ideal compact samples. therefore, the influence of mechanical machining technologies on the values of macroscopic residual stresses was investigated by x-ray diffraction. the resulting values were discussed depending on machining parameters and surface structure of the studied samples. keywords: cutting ceramics; x-ray diffraction; residual stress; machining technology. 1. introduction a development of cutting ceramics has begun at the beginning of 20th century. nowadays, there is an attempt to optimize the production process and machining parameters depending on the real structure of ceramic tiles [1–4]. in cooperation with moscow state technological university stankin (mstu stankin), which provides preparation of samples, the macroscopic residual stresses were analysed by x-ray diffraction. analysed samples were prepared from different materials such as trigonal al2o3 (samples 2.x), hexagonal si3n4 (samples 3.x), hexagonal sic (samples 4.x), and tetragonal zro2 (samples 5.x). they were finished with different mechanical surface treatments, specifically grinding by diamond disc (samples x.1−x.3) and air abrasive treatment (aat, samples x.9−x.11), see table 1 and 2. samples of first series (1.x) were analysed in [5]. production of samples differed for individual materials. samples of al2o3, sic and zro2 were made by so called standard sintering, when they were pressed at low temperature and then put into a furnace, where the bonds in material were created. samples of nitride ceramics were made by cold sintering, but they were put into compressible ampoules which the hydrostatic pressure exerted during the sintering process. this has resulted in high material densities, see [6]. sample material 2.x al2o3 3.x si3n4 4.x sic 5.x zro2 table 1. materials of individual series of samples. sample technology x.1−x.3 grinding by diamond disc x.9−x.11 air abrasive treatment table 2. machining technologies of individual series of samples. 2. experiment the influence of machining parameters on the surface values of macroscopic residual stresses was studied by x-ray diffraction. for grinding by diamond disc, the impact of transverse displacement on the lift f d was studied, and for the aat, where the sample rotated, the influence of loading time t was analysed (see tables 3 and 4). values of residual stresses were determined in two orthogonal directions l (direction of grinding) and t (direction of transverse displacement). the tensometric analysis of macroscopic residual stresses had different parameters for individual series of samples. for al2o3 diffraction line {0210} was analysed by manganese anode on diffraction angle 2θ = 146o and x-ray diffraction constants s1 = −0.61 tpa−1, 1 2s2 = 3.18 tpa−1, for si3n4 diffraction line {201} by chromium anode on diffraction angle 2θ = 63o in ψ-geometry and x-ray diffraction constants s1 = −0.87 tpa−1, 1 2s2 = 3.87 tpa−1, for sic diffraction line {028} by manganese anode on diffraction angle 2θ = 149.5o and x-ray diffraction constants s1 = −0.41 tpa−1, 1 2s2 = 2.74 tpa−1 and for zro2 diffraction line {220} by chromium anode on diffraction angle 2θ = 128o and x-ray diffraction constants s1 = −1.45 tpa−1, 1 2s2 = 6.42 tpa−1. for more details see [6]. 3. result and discussion at first, all samples were machined by diamond disc and some of them subsequently machined by air abra6 http://dx.doi.org/10.14311/app.2018.17.0006 http://ojs.cvut.cz/ojs/index.php/app vol. 17/2018 investigation of surface macroscopic residual stresses of cutting ceramics sample f d , [mm/displ.] x.1 0.5 x.2 1.0 x.3 1.5 table 3. parameters of grinding by diamond disc. sample t, [s] x.9 15 x.10 30 x.11 60 table 4. parameters of aat. sive treatment. depending on the structure, various effects of machining technology on the surface structure were found. for trigonal al2o3, the values of residual stresses have expecting character, i.e. while after grinding by diamond disc in direction l, values of compressive residual stresses decrease with increasing f d, in direction t, the compressive residual stresses increase (see figure 1). this effect is caused by moving the cutting tool in the l direction, when the so-called stretching of the subsurface areas occurs due to the mechanical interaction of the cutter blades with the material. in the t direction, the higher feed rate leads to higher stresses. in the case of air abrasive machining, it is evident that with the increasing time of machining, the values of the compressive residual stresses increase while the isotropy of the values of the macroscopic residual stresses in the l and t directions are increased. this is caused by the rotational movement of the sample during machining. with longer time of machining the depth of the infuenced surface layer of material increases. to prove that is necessary to determine precisely the depth gradient of macroscopic residual stresses by forceless polishing. in the case of si3n4 (see figure 2), the residual stresses were analysed by different experimental geometry due to overlap of diffraction lines with diffraction angles above 110o 2θ. it means the resulting values of residual stresses are from thinner surface (ca. 4× thinner). an accuracy of determination of position 2θ is about 8, 5× smaller than for diffraction lines with higher angles 2θ. however, the relative dependence for determining the effect of mechanical machining on surface residual stresses is indicative. after diamond wheel grinding, the compressive residual stresses increase in the both directions with increasing f d. this is caused by hexagonal close packed structure of si3n4 and its resistance to machining. as it has been shown in [7], machining materials with the hexagonal close packed lattice can cause an increase of compressive residual stresses in the grinding direction. this effect figure 1. values of macroscopic residual stresses after grinding by diamond disc (samples x.1-x.3) and aat (samples x.9-x.11) of al2o3. figure 2. values of macroscopic residual stresses after grinding by diamond disc (samples x.1-x.3) and aat (samples x.9-x.11) of si3n4. is induced by the reaction of the structure to a force which resists plastic deformation. after air abrasive treatment of si3n4 samples, the effect of the previous machining technology can be seen. at a short machining time, the macroscopic residual stresses have a relatively large anisotropy and, with increasing machining time, this anisotropy disappears. at the same time, it can be seen that with the machining time, the values of the surface compressive residual stresses increase, which is again caused by the hexagonal structure. compressive stresses dropped for sample 3.11 probably by sliding along the grain boundaries (there was an increase in crystallite size, see [8]). for hexagonal sic, it can be seen that in case of grinding with a diamond disc in the direction of grinding (direction l), the values of the compressive residual stresses increase and in the direction perpendicular (direction t), the tensile residual stresses decrease. this dependence is inconsistent with expectations, since in general grinding causes tensile stresses in the direction of grinding and in the perpendicular direction the compressive residual stresses. it is caused 7 j. nemecek, j. capek, n. ganev, k. kolarik acta polytechnica ctu proceedings figure 3. values of macroscopic residual stresses after grinding by diamond disc (samples x.1-x.3) and aat (samples x.9-x.11) of sic. figure 4. values of macroscopic residual stresses after grinding by diamond disc (samples x.1-x.3) and aat (samples x.9-x.11) of zro2. again by hexagonal close packed structure of sic, see [7]. values of macroscopic residual stresses after air abrasive treatment of sic samples show very high experimental errors. these are due to the large dimensions of crystallites. however, it is evident that in the l direction, the values of the compressive residual stresses decrease, even tensile stresses increase, and in the t direction the compressive stresses on the contrary increase. this is caused again by the influence of the hexagonal lattice and its resistance to plastic deformation (see figure 3). after grinding by diamond disc of zro2, it is evident that the values of the compressive macroscopic residual stresses decrease with the increasing f d. this is due to the thermal overload of the surface. the thermal conductivity coefficient for zro2 is approximately one-tenth compared to other studied materials (λ = 2, 7 wm−1k−1 for zro2, other ceramics have a temperature coefficient λ = 30 wm−1k−1 and more). for air abrasive treatment, it is obvious that due to the rotational movement of the zro2 samples, an isotropic distribution of macroscopic residual stresses figure 5. surface defects of sic cutting ceramic after grinding by diamond disc (5a, 5b) and aat (5c, 5d). has occurred, the values in the l and t directions are same within the error (see figure 4). for samples from sic the analysis of surface with sem was made. the purpose was to investigate a presence of cracks, which may be caused by large tensile residual stresses. however, it is evident from 8 vol. 17/2018 investigation of surface macroscopic residual stresses of cutting ceramics figure 5 that no cracks were found. but the images show, alongside smooth cutting traces, surface defects caused by grinding, such as various holes or ejected grains. the morphology of surface is significantly changed since the cutting traces of diamond wheel disappeared and instead can be seen deep craters after the impact of individual grains of abrasive. 4. conclusions the analysis of macroscopic residual stresses of surfaces of four different ceramic materials was performed by x-ray diffraction and the effect of mechanical machining on the surface structure was determined. different reactions of hexagonal structures on mechanical machining were observed. it was found that zro2 due to poor thermal conductivity had to be machined at lower speeds or pressures, otherwise the heat load of the structure and the tensile residual stresses would increase. acknowledgements this work was supported by grant student grant competition ctu no. sgs16/246/ohk4/3t/14 and grant agency of the czech republic (no. 14-36566g). references [1] a. humár. materiály pro řezné nástroje. first printing. mm publishing, 2008. [2] v. v. kuzin, et al. correlation of diamond grinding regimes with al2o3-tic-ceramic surface condition. refractories and industrial ceramics 57(1):520–525, 2017. doi:https://doi.org/10.1007/s11148-017-0016-0. [3] v. v. kuzin, et al. correlation of diamond grinding regimes with sisic-ceramic surface condition. refractories and industrial ceramics 58(2):214–219, 2017. doi:https://doi.org/10.1007/s11148-017-0108-x. [4] v. v. kuzin, et al. correlation of diamond grinding regime with surface condition of ceramic based on zirconium dioxide. refractories and industrial ceramics 57(1):625–630, 2017. doi:https://doi.org/10.1007/s11148-017-0035-x. [5] j. němeček, k. kolařík, j. čapek, g. n. influence of machining technologies on values of residual stresses of oxide cutting ceramics. acta polytechnica ctu proceedings (9):13–18, 2017. doi:https://doi.org/10.14311/app.2017.9.0013. [6] j. němeček. difrakcní studium vybraných charakteristik řezné keramiky, prague 2017. master thesis (ing.). czech technical university in prague, faculty of nuclear science and physical engineering, department of solid state engineering, 2017-06-16. [7] k. t. ramesh. effects of high rates of loading on the deformation behavior and failure mechanisms of hexagonal close-packed metals and alloys. metallurgical and materials transastions a 33:927–935, 2002. doi:https://doi.org/10.1007/s11661-002-0162-x. [8] z. yin, et al. friction and wear behaviors of al2o3/tic micro-nano-composite ceramic sliding against metals and hard materials. ceramic international 42:1982–1989, 2016. doi:https://doi.org/10.1016/j.ceramint.2015.10.001. 9 http://dx.doi.org/https://doi.org/10.1007/s11148-017-0016-0 http://dx.doi.org/https://doi.org/10.1007/s11148-017-0108-x http://dx.doi.org/https://doi.org/10.1007/s11148-017-0035-x http://dx.doi.org/https://doi.org/10.14311/app.2017.9.0013 http://dx.doi.org/https://doi.org/10.1007/s11661-002-0162-x http://dx.doi.org/https://doi.org/10.1016/j.ceramint.2015.10.001 acta polytechnica ctu proceedings 17:6–9, 2018 1 introduction 2 experiment 3 result and discussion 4 conclusions acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2019.23.0025 acta polytechnica ctu proceedings 23:25–30, 2019 © czech technical university in prague, 2019 available online at https://ojs.cvut.cz/ojs/index.php/app a headrace tunnel reconstruction in turkey matouš hilara,b,∗, martin srbb, jakub nosekb a czech technical university in prague, faculty of civil engineering, department of geotechnics, thákurova 7, 166 29 prague, czech republic b 3g consulting engineers ltd., na usedlosti 16, 147 00 prague, czech republic ∗ corresponding author: hilar@3-g.cz abstract. the operation of a hydro power project in north turkey started in 2015, included the 12 km long headrace tunnel. an 8 km long lower tunnel section was excavated by double shield tbm technology with 8.7m id. due to problems with the mechanised excavation, a 4 km long upper tunnel section was excavated conventionally by the drill and blast method with single shell lining generated from fibre reinforced sprayed concrete and rock bolts. significant operational problems were observed in september 2016. the flooded tunnel was investigated by underwater remotely operated vehicle which indicated significant collapses. operation of the power station had to be stopped and water from a supply dam at the headrace tunnel inlet had to be discharged to enable dewatering and inspection of the tunnel. consequent investigation revealed three major collapses and a considerable variety of other damages in the d&b section of the tunnel. repair and strengthening of the tunnel was completed in october 2017. keywords: tunnel, reconstruction, conventional excavation, sprayed concrete, tbm. 1. introduction the water tunnel is located in north turkey in the area of the north anatolian fault between the eurasian plate and the african plate (figure 1). the lower part of the tunnel is geologically very variable including sandstones, mudstones, conglomerates, ophiolitic and continental units, metalavas, marbles, etc. the upper part of the tunnel is situated in volcanic rocks (basalts and andesites with dykes). the maximum tunnel overburden is about 600m, and the difference of water tables in upper dam and lower dam is about 80m. a surge shaft is situated above the tunnel close to the power house. 2. tunnel construction the water tunnel excavation by a double-shield tbm technology [1] started from the lower (outfall) portal in february 2012 [2]. traditional reinforced segmental lining (6+1) with 400mm thickness, 1.5m ring length and internal diameter 8.7m was installed during tunnel excavation. originally mechanical excavation was planned for the whole tunnel, and the segmental lining was proposed only for the initial 3 km of the tunnel, with the rest intended to be supported with shotcrete and rock bolts. the double-shield was blocked by ground squeezing soon after the start of excavation [3]. the shield had to be released using by-pass adit which was a time-consuming process. this scenario had to be repeated several times during shield excavation due to very unfavourable ground conditions. thus it was decided to start with a conventional drill and blast (d&b) excavation [4] from the upper (inlet) portal as a mitigation measure to save time. the conventional excavation was realised with full profile using steel fibre reinforced sprayed concrete and ct bolts. finally a 4 km tunnel section ending with a dismantling chamber for double-shield was excavated conventionally by drill and blast method from the inlet portal, and an 8 km tunnel section was excavated by shield from the outlet portal. the whole part excavated by shield was permanently supported by segmental lining with steel cages, the whole d&b part was permanently supported by single-shell steel fibre reinforced sprayed concrete, ct bolts and a cast in situ concrete floor at the tunnel bottom. excavations were completed in june 2014, and power generation started in august 2015. 3. investigation of damage significant problems were observed in the power station in september 2016 (lower water pressure, muddy colour of water, stones in water), and thus its operation had to be stopped. the flooded tunnel was inspected by underwater remotely operated vehicle (rov) allowing 3d scanning (figure 2). the inspection revealed three different and bigger failures in the tunnel at approximately 0+210, 0+590 and 3+280 (chainage starts at the intake portal). the collapse 1 at 3+280 was the largest, as it obstructed about three quarters of the tunnel profile and did not allow further inspection of the remaining section of d&b tunnel from 3+280 to 4+000 since there was a significant risk that the rov cable would be trapped in the collapse. the tunnel and upper dam had to be dewatered afterwards, and this procedure took about one month (figure 1). 25 https://doi.org/10.14311/app.2019.23.0025 https://ojs.cvut.cz/ojs/index.php/app m. hilar, m. srb, j. nosek acta polytechnica ctu proceedings figure 1. discharged dam with the inlet structures and mountain range crossed by the headrace tunnel. figure 2. record from scanning of collapsed area by rov. an access to the d&b tunnel was prepared by ramp construction and the opening trash rack at the inlet structure. in november 2016 the d&b tunnel was inspected by walk from the intake portal to 2+160 and on a boat (due to water blocked by the collapse) from 2+160 to 3+280. water depth at the collapse 1 blockage was 5.5m. also the tunnel with segmental lining was walked through and inspected via access from the outlet portal through a man-hole in penstock in the power house just before the turbines. the collapse 2 (3+885) and the collapse 3 (3+905) in the d&b tunnel were also inspected, and the area between collapse 1 and 2 was flooded. an additional visit was done from the inlet portal, and the collapse 1 was walked through and the area between collapse 1 and collapse 2 in the d&b tunnel was inspected on boat. several minor failures were observed in this section. initial geodetic measurements of the collapses 1, 2, and 3 were completed in january 2017. the geodetic measurements undertaken gave a better idea about the shape of caverns and the volume of fallen material (i.e. the total volume of caverns). the original survey of collapses and failures was improved later (after accumulated water was discharged and after removal of obstructing material, giving a better access for equipment and machines, with a better lighting, etc.). after the initial investigation of damage, the detailed investigation of damage was conducted and recorded. the detailed inspection was completed for the whole tunnel (i.e. both d&b tunnel and tbm tunnel). the inspection schedule was affected by accessibility of various sections and some sections were accessible only after the repair of collapses and after water pumping. 4. collapses 4.1. collapse 1 the collapse 1 area (about a 30m long tunnel section) starting at km 3+280 (figure 3) was additionally filled by sand and access to the collapse cavern sealed by foam. for ventilation and access behind the collapse, a 30m long pipe of 1.2m diameter was placed on the top of the blockage before closing the tunnel profile. also a pipe for water pumping through the collapse 1 was installed prior to its filling. then the lower part of the cavern generated by the collapse 1 was filled with concrete from the tunnel. filling of the rest of the cavern was done through boreholes drilled from the surface. the purpose of surface works above the collapse 1 was a filling of the cavern above the collapse by self26 vol. 23/2019 a headrace tunnel reconstruction in turkey figure 3. the collapse 1 after water pumping completion. levelling concrete. firstly, the point for drilling had to be determined by surveyors and a road to this area had to be constructed. then drilling of boreholes started. the first borehole encountered the cavern above the collapse 1 at a depth of 215m, and it was used for concrete pouring to the cavern. the borehole was equipped with a plastic casing. after some time of concrete pouring the first borehole was blocked by concrete, caused by higher speed of concrete pouring with limited possibilities for air release. the attempt to re-drill the first borehole was not successful. the new drilling did not reach the cavern, and the drill probably deviated from its original direction. drilling of the second and third boreholes was successful; their depth was also about 215m. completion of the third borehole allowed faster concrete pouring due to air release, with generally about 100m3 of concrete being poured into the borehole daily. the strong air flow with a smell of concrete was blowing from the third (air release) borehole during concreting of the second borehole, and it confirmed connection of the boreholes via cavern. concrete pouring to the cavern from the surface was completed in april 2017, with a total of about 3,500m2 of selfcompacting concrete being poured to the area of the collapse 1 and the cavern above it (approx. 500m3 from the tunnel and 3,000m3 from the surface). the situation in the cavern was checked daily by a camera with attached lights lowered to the boreholes from the surface on a cable. tunnel lining strengthening in front of the collapse 1 area was realised prior to start of re-excavation. this was needed to generate a safe working area to minimize the risk for the working crews. a systematic rockbolting using 6m long galvanised sn bolts was done from 3+240 to 3+270. additional support using steel meshes, lattice girders and shotcrete was installed as from km 3+265 to km 3+270. then conventional excavation of top heading with 1m advances through the collapse 1 was carried out. applied support included sprayed concrete, steel meshes, lattice girders, 6m long galvanised rockbolts sn or ibo, umbrellas from 8m long ibo 51 bolts, etc. probe drilling with a length up to 15m was conducted every 6m. all voids encountered by drilling were filled by cement grout. material removal from the bench area started afterwards. excavation was initially ongoing on the right side from the downstream view, then bench excavation on the left side started. the original tunnel lining in the bench area was not damaged, thus material removal and installation of support was straightforward. only very low deformations below 20mm were recorded during re-excavation due to successful filling by concrete and grout. 4.2. collapse 2 breakthrough to the collapse 1 allowed access to the collapse 2. however, the area between the collapses 1 and 2 was flooded, and had to be dewatered. after water pumping completion, access could be cleaned up and the collapse 2 could be investigated. investigation revealed about a 34m high and a 5m wide cavern with the shape of a square vertical chimney. the cavern mouth was situated on the left shoulder at km 3+885. strengthening of area ahead of the collapse 2 started afterwards, with shotcrete spraying and rockbolting being realised. removal of collapsed material followed. then initial support was partly installed, but working was slow due to safety concerns (risk of falling stones from the cavern). a decision was taken to cover the cavern by the high-tensile steel mesh minax 80/3 (figure 4). after initial support and permanent rockbolting up to 3+880, the first meshes and 13 lattice girders were installed over the cavity opening from 3+878 to 3+890. the area was sprayed by shotcrete and plastic pipes were installed 27 m. hilar, m. srb, j. nosek acta polytechnica ctu proceedings through the lining to allow concrete pouring into the cavern. pumping of the self-levelling concrete c35 into the cavern started after tunnel lining under the collapse 2 (including permanent rockbolting, second meshes covered by shotcrete, and also the left side propping). the pumping was slower in the beginning; speed was increased once the concrete level reached 2m above the tunnel crown. the pipes for concrete pouring and air release were extended 3-times during concreting using a telescopic piping arrangement, and finally pipes reached the cavern roof. about 50m3 of concrete was pumped per day. then, grouting of voids and open spaces behind the lining was carried out. the collapse 2 cavern filling was completed in july 2017, involving a pumping of about 800m3 of concrete in total. 4.3. collapse 3 installation of the high-tensile steel mesh minax 80/3 over the collapse 2 before its filling allowed safe access to the collapse 3. a narrow mouth of the collapse 3 was situated to the right side wall at km 3+905, and as the mouth was blocked by a bigger block of rock, it did not allow sight and survey of the cavern behind it. after strengthening of the area, probe drilling performed by a standard tunnel boomer started, with the length of drills varied from 6 to 9m. generally loose material was encountered by probe holes, but no big cavern was detected. drilling machine for longer drills was also utilised, and the purpose of longer drills was to locate a cavern in higher areas. probe hole 20 drilled from 3+912 reached a length 32m, and as it encountered a loose material, it was used for grouting (figure 5) with a packer 9m deep. during grouting leakage in the cavern mouth area was observed which confirmed the connection of loose areas. after the probe hole 20 blockage, it was re-drilled by the probe hole 21. grouting was finalized by probe hole 22 which was crossing the probe hole 21 in a diagonal way. when a leakage from the hole 21 was observed during grout pumping into the hole 22, it was confirmed that the whole possible cavern was reasonably filled. some additional systematic 6m long drilling and grouting was performed afterwards. in total over 300m3 of grout was pumped into the collapse 3 area. then material removal from the bench in area of collapses 2 and 3 started. excavation was ongoing from downstream side; upper 2m of bench were removed on the whole section. then excavation continued from upstream side and the lower part of bench was removed. original tunnel lining inspection was done after removal of material from bench area of the collapses 2 and 3. original lining in bench area was disturbed on two locations – under the collapse 2mouth and under the collapse 3mouth. both areas of exposed ground were covered by a 10 cm thick layer of sprayed concrete as temporary support. lining damage under the collapse 2 exposed about 5m2 of ground. the lining damage was not connected with the cavern mouth, and a reasonably hard original rock was exposed in the area. lining damage under the collapse 3 was different. the area of exposed ground was about 7m long and 4m high (i.e. about 28m2). a very soft ground was exposed, and it was probably collapsed material washed out by water from rock mass after the collapse. the support of bench corresponded with top heading support. permanent rockbolting by galvanized sn bolts was carried out. two layers of meshes with lattice girders were installed under the collapse 2. other areas were supported by 2 layers of steel meshes covered by shotcrete. bench support installation was completed together with completion of all strengthening works of the d&b tunnel at the beginning of november 2017. 5. tunnel section with segmental lining repair of the tunnel section excavated by doubleshield with segmental lining was significantly affected by a limited access. access for people to this section was originally allowed by a small opening in the steel tube in the area of the power station. the second access was allowed by a surge shaft situated close to the power station. preparatory work to get an access through the shaft started in april 2017 – installation of crane with basket, arrangement of safety measures, etc. access from the intake portal was affected by collapses and activities in the d&b tunnel, with a barrier separating the segmental tunnel and the dismantling chamber in d&b tunnel being removed in july 2017. however, a good access was ensured after completion of the bench excavation in the collapse 2 and 3 areas. work in the segmental tunnel section started with core drilling in may 2017 using access though the surge shaft (figure 6). only problematic sections were investigated by core drilling. 5m long cores were taken from 5 locations in each profile (9:00, 10:30, 12:00, 1:30, 3:00). pressure water tests were realised in drills after removal of cores. trial grouting in the segmental tunnel section started in june 2017. it was executed on 18 rings (27metres). grouting continued afterwards on other problematic sections, with a total of 6 sections being grouted. trial grouting was done through 5 drills in each profile (9:00, 10:30, 12:00, 1:30, 3:00), then the number of drills in each profile was reduced to 3 (11:00, 12:00, 1:00) and then only to 1 (12:00). repair of damaged segments was done in september 2017. the worst section of segmental lining (about 11 rings) included cracked segments with exposed reinforcement. problematic segments had to be repaired (removal of loose concrete, cleaning of area, covering by appropriate material). steel ribs (galvanized heb 260 profiles) were installed to the section. also permanent rockbolting and grouting was completed. 28 vol. 23/2019 a headrace tunnel reconstruction in turkey figure 4. the collapse 2 mouth protection by the high-tensile steel mesh. figure 5. grouting of the collapse 3, its mouth situated behind a barrel with grout. figure 6. core drilling in the tbm tunnel. 29 m. hilar, m. srb, j. nosek acta polytechnica ctu proceedings 6. conclusions very specific and rare problems had to be solved during the water tunnel repair (collapses of the operated tunnel with a volume in thousands of m3) and with a very significant time pressure. the tunnel repair was completed within approximately one year. preparation took about half a year (investigation of problems, draining the dam, preparation of contracts, design, site preparation, provision of machines and materials, etc.). reconstruction took about another half-year (repair of three big collapses, repair of many minor failures, strengthening of the d&b tunnel, repairs and grouting of segmental lining). it is difficult to estimate the lifetime of the finalised repairs, but it is possible to expect that further repairs may be required in the future based on observations and measurements. the tunnel will be regularly inspected by rov (in roughly half-year intervals). the whole d&b tunnel and problematic sections of the tunnel section with segments were scanned and geodetically measured. thus, future tunnel lining deformations can be checked after dewatering of the tunnel to allow access for surveyors. detailed design and construction preparation will have to be done prior to further repair in order to ensure that the final solution will be optimal from both structural (functionality, quality, life time, etc.) and financial viewpoints (repair cost including impact of interruption of energy generation). acknowledgements financial support from the tačr grant te01020168 is gratefully acknowledged. references [1] g. girmscheid, c. schexnayder. tunnel boring machines. practice periodical on structural design and construction 8(3), 2003. doi:10.1061/(asce)1084-0680(2003)8:3(150). [2] j. clark, s. chorley. the greatest challenges in tbm tunneling: experiences from the field. in proceedings of the conference north american tunneling, chap. 12. knovel, 2014. [3] g. anagnostou, k. kovári. tunnelling through geological fault zones. in proceedings of international symposium on design, construction and operation of long tunnels. eth library, zürich, 2005. [4] g. girmscheid, c. schexnayder. drill and blast tunneling practices. practice periodical on structural design and construction 7(3), 2002. doi:10.1061/(asce)1084-0680(2002)7:3(125). 30 https://doi.org/10.1061/(asce)1084-0680(2003)8:3(150) https://doi.org/10.1061/(asce)1084-0680(2002)7:3(125) acta polytechnica ctu proceedings 23:25–30, 2019 1 introduction 2 tunnel construction 3 investigation of damage 4 collapses 4.1 collapse 1 4.2 collapse 2 4.3 collapse 3 5 tunnel section with segmental lining 6 conclusions acknowledgements references acta polytechnica doi:10.14311/app.2020.27.0126 acta polytechnica 27(0):126–130, 2020 © czech technical university in prague, 2020 available online at http://ojs.cvut.cz/ojs/index.php/app why monitor acoustic emissions during nanomechanical tests? radim čtvrtlíka,∗, lukáš václavekb, jan tomáštíkc a institute of physics of the czech academy of sciences, joint laboratory of optics of palacky university and institute of physics as cr, olomouc, czech republic, 17. listopadu 50a, 77207 olomouc, czech republic b joint laboratory of optics of palacky university and institute of physics of the czech academy of sciences, faculty of science, palacky university, 17. listopadu 12, 77146 olomouc, czech republic c regional centre of advanced technologies and materials, joint laboratory of optics of palacky university and institute of physics of the czech academy of sciences, faculty of science, palacky university, 17. listopadu 12, 77146 olomouc, czech republic ∗ corresponding author: ctvrtlik@fzu.cz abstract. acoustic emissions (ae) monitoring has been proved as an effective non-destructive technique at the macro scale. nevertheless, it may also be employed at nano/micro scale during nanomechanical and nanotribological testing. local mechanical properties of surfaces or micro object are routinely explored using nanoindentation, scratch test or dynamic impact tests that are evaluated based on analysis of depth-load-time records or microscopic observation of residual indents, scratch grooves or impact craters, respectively. although these approaches have been proven to be sufficient in most cases for a variety of materials, there are many situations where they do not provide sufficient information for a complex understanding of the deformation response. on the other hand, analysis of ae signals generated during these tests may provide valuable complementary information and provide some insight into the dynamics of phenomena like cracking, phase transitions, plastic instabilities, etc. that means that the analysis of acoustic emissions provides a better understanding and more complex interpretation of the results obtained by the above-mentioned tests. the strength of the ae based method is demonstrated for optical thin films deposited on glass tested via scratch test. combination of ae and nanoindentation will be presented for various types of glasses. the complementary analysis of the time and frequency domain will draw the potential of the ae extended approach. especially dynamics of the failure and fracture processes will be distinguished and elucidated. in both cases the analysis of ae events will be correlated with standard approaches. keywords: acoustic emission, indentation, scratch test, thin films. 1. introduction surfaces play an essential part of our everyday lives as they are involved in most of the interactions. there have been developed a wide variety of methods for surface modification based on either their compositional/structural modification or deposition of a surface layer. a tremendous effort has been devoted especially to thin films and coatings technology that allows tailoring of surface properties via numerous pvd (physical vapor deposition), cvd (chemical vapor deposition) and pacvd (plasma assisted chemical vapor deposition) methods. in fact, the thin film technology has become an inherent part of almost any manufacturing process. since the fabrication processes often operate under the non-thermodynamic conditions the physical properties of the films may substantially differ from bulk analogs with the same chemical composition, if even exist. therefore, the exploration of their physical or chemical properties is of great importance in order to understand their nature and practical limits as well as ways of their modifications. regardless of the primary role of the surface property (optical, electrical, protective, lubricating, wetting, catalytic, etc.) predetermined by their application, the surface mechanical durability is vital for a long lifetime of the component/device. besides, surfaces are often exposed to external stresses during their fabrication. therefore, reliable testing of mechanical properties is essential for their research and development as well as industrial production. this is very true especially in optics and electronics where most of the surfaces are coated with thin films or multi-layered structures [1, 2]. probably the most important tests are nanoindentation and scratch test based on quasistatic mechanical contact between the diamond indenter and the tested surface [3]. dynamic tests are also available at nano/micro scale. although these tests have been improved and optimized for years for testing of most classes of materials (metals, ceramics, polymers, biomaterials) in various forms (thin films, coatings, composites, etc.), there has been always a pursuit of increasing the information content obtained during these tests. since the standardly used nano/micro126 http://dx.doi.org/10.14311/app.2020.27.0126 http://ojs.cvut.cz/ojs/index.php/app vol. 27/2020 monitoring ae during nanomechanical tests figure 1. scratch testing of tio2 film on glass substrate performed up to 100 and 300 mn a) depth-load-distance records b) ae envelopes c) residual scratch grooves and d) spectrogram for the 300 mn test. mechanical/tribological tests are based on the analysis of depth-load-time record, which potential is inevitably limited and probably has approached its limit, new techniques are sought. in this perspective, proper application of the acoustic emission method to the nano/micro scale testing seems to be very promising as it proved to be irreplaceable at macro scale due to its ability to reveal the deformation dynamic, subsurface damage, defective area localization, real time monitoring etc. acoustic emission might be defined as the phenomenon of transient elastic waves radiation in solids that occurs during a dynamic reconstruction of its internal structure. this may be represented by crack formation, plastic deformation, phase transition, delamination or debonding as typical examples of material acoustic emission. besides, the mechanical ae is also distinguished and is connected with the mechanical origin (leakage, friction, impact etc.). the mechanical loading of a material may lead to its structural changes and in turn to generation of elastic stress waves. consequently, small surface displacements of a material can be detected. this effect is especially strong when the accumulated elastic energy is released rapidly. in practice, the ae method is based on the mechanism described above and is used for example for structural health monitoring, quality control, system feedback, process monitoring, etc. 2. instrumentation nano/micro-mechanical tests were performed using a fully calibrated nanotest instrument (micromaterials, uk) at room temperature. berkovich indenter was used for indentation, while rockwell indenter with nominal radius of 10 µm was used for scratch testing. the samples were loaded on the custom built ae holder based on dedicated piezo-element and inbuilt pre-amplifier. the glass samples were fixed using the special low melting point wax; though for ae experiment also super glue can be used. ae signals were monitored and recorded during the whole test using the zedo system (dakel, czech republic) featuring maximum sampling frequency of 10 mhz. the high dynamic range and the broad frequency range of 2 mhz allow exploring strong as well as weak signals with sub-microsecond resolution. 3. results and discussion 3.1. optical thin films scratch test is considered as a standard method used for exploration of adhesion and cohesion of thin films and coating. during the test, the diamond tip is pulled over the surface while the normal force applied on the indenter is increasing. the goal of the test is to find the critical loads representing the onset of specific failures. there are various ways how to find these loads based on either i) visual observation of the residual groove or ii) analysis of depth-loaddistance record or evaluation of the iii) friction probe and iv) ae signal. it should be noted that especially microscopic analysis is mostly used in practice. it has been shown that for reliable nano/micro scratch test evaluation the combination of all the above mentioned techniques should be used [4]. as a benefit it is also possible to get insight into the dynamics of the failure process [5]. in practice, the ae signal envelope, that is in fact a compressed signal, has proved to be reliable source of information. nevertheless, the thorough evaluation of the sampled ae signal considerably enhances the ae analysis and allows distinguishing between different failure types (cracking, chipping or delamination etc.) and/or the initial and catastrophic film/substrate failures. in general the ae signal may be analysed in both the time and frequency domain. the visual representation 127 r. čtvrtlík, l. václavek, j. tomáštík acta polytechnica figure 2. overview of a, b) elastic properties and density, and c) residual crack patterns together with ae records for various glasses tested with pyramidal berkovich indenter. of the spectrum of frequencies as a function of time for the whole ae signal is called spectrogram. in some cases, the ae is the only method able to unambiguously determine the film and substrate damage, like in case of optical tio2 thin film with thickness of 280 nm deposited on soda-lime silicate glass substrate, see fig. 1. the discrete ae envelope reflects the film cracking, while the continuous ae reveals total film wipe off and glass damage. the continuous character of the ae envelope reflects consecutive damage of the glass substrate in the form of successive ae events. it should be noted that depth-load-distance as well as optical observation of the residual scratch groove themselves do not provide sufficient information for reliable evaluation of the test. based on the ae record analysis the critical load of 18 mn was evaluated. acoustic emission technique is not only useful addon for the scratch test. it rather represents an important and reliable approach how to detect the very first cracking in the film/substrate system. furthermore, it offers a real-time detection of the subsurface damage in the very convenient way. 3.2. optical glass nanoindentation (depth sensing indentation) is a contact based technique widely used for evaluation of mechanical properties at small scales. during a typical test a diamond tip of known geometry is pressed into the sample surface, while the indentation load and depth are continuously recorded. hardness and elastic modulus are measured in most cases, but other phenomena like pressure-induced phase transformations [6, 7], incipient plasticity or time dependent effects can be studied. furthermore, it is possible to perform such test at elevated temperatures and study temperature stability of the material or temperature related phenomena [8]. nanoindentation is very effective for hardness and modulus evaluation provided several assumptions are fulfilled. for example absence of cracking, fully developed plastic zone, no substrate effect, thermal stability etc. ignoring these facts might lead to underestimation of hardness and elastic modulus values. nevertheless, since normal load is applied in a very controlled way during the indenter penetration the indentation test is well suited for studying the glass surface mechanical durability as conditions for stable crack growth are formed. it is well known that sharp point contact is one of the most often failure mode in glass, especially in chemically tempered glass. the glass surface damage induced by a sharp point contact is a serious problem in practice and still not fully understood comprehensive issue encompassing fracture mechanics and glass science. therefore nanoindentation using sharp indenter is a very convenient way of testing this phenomenon as diamond tip can be used to mimic the damage from sharp contact. what is more, this approach allows using of different tip geometries (pyramidal and spherical with different contact angles and radii, respectively) to simulate different loading conditions (load, loading rate etc.). in this study we indented 9 samples of optical glasses with different elastic moduli, poissons ratios and densities with berkovich indenter, see fig. 2. glasses were selected in order to cover as broad as possible range of elastic modulus (55-108) gpa, poissons ratio (0.170-0.302) and density (2.20-5.18) g/cm3. infor128 vol. 27/2020 monitoring ae during nanomechanical tests figure 3. crack evolution during indentation with sharp indenter a) theoretical diagram (based on a drawing by lawn [9]) b) schott optical glass k10 – ae envelope and detail od residual indent made by berkovich indenter at 20n. figure 4. indentation testing of fused silica from 100 to 500 mn a) indentation curves b) ae records and c) residual indents for 100 and 200 mn. mation about the properties of glasses used in this study was obtained from the datasheets provided by the manufacturers (shott, ohara, sumita). different types of glasses can crack during both the loading and the unloading sequence with various crack patterns as shown in fig. 2. it can be clearly seen that the resistivity against crack formation and propagation differs extensively. different types of cracks and their combinations can be visually distinguished. analysis of the ae envelopes might then assigns the specific types of cracks to either loading (first 10 seconds) or unloading (last 10 seconds) stage of the indentation process, see fig. 3a. for example the lateral cracks are formed during the unloading when the elastically deformed material recovers. the ae peaks during the loading can be ascribed to the formation of the so called median cracks that originate beneath the indenter and are closed during unloading. it should be noted, that especially in glasses the median cracks interfere with radial cracks emanating from the residual imprint corners and form so called median/radial cracks. microscopic observation of the residual indents for the optical glass k10, a classic glass type with lead oxide, did not reveal any traces of cracking, instead pile up was observed as can be seen in inset in fig. 3b. on the other hand, the ae record clearly points out on structural changes beneath the indenter taking place during loading and dwell period (creep). this implies that median cracking is the main cracking mode. it should be noted that median cracks propagate perpendicular to the surface and have stronger effect on glass strength in comparison to lateral cracks spreading more or less parallel to the surface. when 129 r. čtvrtlík, l. václavek, j. tomáštík acta polytechnica lateral crack reaches the surface then large fragments might delaminate causing material loss. 3.3. fused silica use of ae detection system during indentation of fused silica revealed marked cracking already at 100 mn, see fig. 4. this finding might have serious consequences on reliability of calculated elastic modulus values as fused silica is routinely used as a calibration material for assessment of indenter shape function. cracking of the material is inconsistent with the assumption of the elastic-plastic loading and purely elastic unloading [10]. it should be noted that no traces of cracking are detectable on indentation curves. 4. conclusions information provided by the nano/micromechanical/tribological tests can be easily extended by the use of acoustic emission detection system. new and unique information on material deformation behavior can be obtained in a non-destructive way. combination of depth-load-time records (indentation curve / scratch record / impulse depth) and images of residual indents / scratch grooves / impact craters with ae record provides a better insight in to the deformation dynamics. acoustic emission broadens the possibilities in scratch test evaluation as it allows detection of the very first subsurface cohesion and/or adhesion failures otherwise inaccessible. in specific cases it is also possible to separate film and substrate failures. it was shown that use of ae and indentation is very effective in exploring mechanical durability and cracking resistivity of optical glass surfaces against sharp point contact. different cracking patterns together with different ae records were observed. however, no unambiguous relationship between crack type and glass elastic properties and density was observed under used experimental conditions. acknowledgements the authors gratefully acknowledge the support by the project th03020245 of the technology agency of the czech republic and the operational programme research, development and education, projects nos. cz.02.1.01/0.0/0.0/17_049/0008422 and cz.02.1.01/0.0/0.0/16_019/0000754 of the ministry of education, youth and sports of the czech republic. this work was also supported by the project iga_prf_2019_008 of palacky university. references [1] l. martinu, d. poitras. plasma deposition of optical films and coatings. journal of vacuum science & technology a vacuum surfaces and films 18:2619–2645, 2000. doi:10.1116/1.1314395. [2] m. mozetič, a. vesel, g. primc, et al. recent developments in surface science and engineering, thin films, nanoscience, biomaterials, plasma science, and vacuum technology. thin solid films 660:120 – 160, 2018. doi:10.1016/j.tsf.2018.05.046. [3] a. gouldstone, n. chollacoop, m. dao, et al. indentation across size scales and disciplines: recent developments in experimentation and modeling. acta materialia 55(12):4015 – 4039, 2007. doi:10.1016/j.actamat.2006.08.044. [4] j. tomastik, r. ctvrtlik, m. drab, j. manak. on the importance of combined scratch/acoustic emission test evaluation: sic and sicn thin films case study. coatings 8(5), 2018. doi:10.3390/coatings8050196. [5] j. tomastik, r. ctvrtlik, t. ingr, et al. effect of nitrogen doping and temperature on mechanical durability of silicon carbide thin films. scientific reports 8, 2018. doi:10.1038/s41598-018-28704-3. [6] v. kulikovsky, v. vorlicek, p. bohac, et al. mechanical properties of amorphous and microcrystalline silicon films. thin solid films 516(16):5368–5375, 2008. doi:10.1016/j.tsf.2007.07.047. [7] v. kulikovsky, r. ctvrtlik, v. vorlicek, et al. mechanical properties and structure of tio2 films deposited on quartz and silicon substrates. thin solid films 542:91 – 99, 2013. doi:10.1016/j.tsf.2013.06.070. [8] r. ctvrtlik, m. al-haik, v. kulikovsky. mechanical properties of amorphous silicon carbonitride thin films at elevated temperatures. journal of materials science 50, 2015. doi:10.1007/s10853-014-8715-0. [9] b. lawn, m. swain. microfracture beneath point indenters in brittle solids. journal of materials science 10:113–122, 1975. doi:10.1007/bf00541038. [10] a. c. fischer-cripps. nanoindentation. springer, new york, ny, 3rd edn., 2011. doi:10.1007/978-1-4419-9872-9. 130 https://doi.org/10.1116/1.1314395 https://doi.org/10.1016/j.tsf.2018.05.046 https://doi.org/10.1016/j.actamat.2006.08.044 https://doi.org/10.3390/coatings8050196 https://doi.org/10.1038/s41598-018-28704-3 https://doi.org/10.1016/j.tsf.2007.07.047 https://doi.org/10.1016/j.tsf.2013.06.070 https://doi.org/10.1007/s10853-014-8715-0 https://doi.org/10.1007/bf00541038 https://doi.org/10.1007/978-1-4419-9872-9 acta polytechnica 27(0):126–130, 2020 1 introduction 2 instrumentation 3 results and discussion 3.1 optical thin films 3.2 optical glass 3.3 fused silica 4 conclusions acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2018.20.0098 acta polytechnica ctu proceedings 20:98–107, 2018 © czech technical university in prague, 2018 available online at http://ojs.cvut.cz/ojs/index.php/app experimental and numerical study of controlled flutter testing in a linear turbine blade cascade václav slámaa,∗, bartoloměj rudasa, petr eretb, volodymyr tsymbalyukb, jiří irac, aleš macalkac, lorenzo pinellid, federico vantid, andrea arnoned, antonio alfio lo balboe a doosan skoda power s.r.o., tylova 1/57, pilsen 30128, czech republic b university of west bohemia, department of power system engineering, univerzitni 8, pilsen 30614, czech republic c num solution s.r.o., u pergamenky 1145/12, prague 17000, czech republic d university of florence, via s.marta 3, 50139 florence, italy e ansaldo energia s.p.a., via nicola lorenzi 8, 16152 genova, italy ∗ corresponding author: vaclav.slama@doosan.com abstract. in this paper, experimental testing of flutter and numerical simulations using a commercial code ansys cfx and an in-house code traf are performed on an oscillating linear cascade of turbine blades installed in a subsonic test rig. bending and torsional motions of the blades are investigated in a travelling wave mode approach. in each numerical approach, a rig geometry model with a different level of complexity is used. good agreement between the numerical simulations and experiments is achieved using both approaches and benefits and drawbacks of each technique are commented in this paper. it is demonstrated that both used computational techniques are adequate to predict turbine blade flutter. it is concluded that validated numerical tools can provide a better insight of flutter phenomena of operationally flexible steam turbine last stage blades. keywords: steam turbine, last stage blades, flutter, experiment, test rig, measurement, numerical analysis, cfd, validation, blade cascade. 1. introduction with the increase of steam turbine operational flexibility, the risk of asynchronous blade vibrations induced by flow becomes higher and higher and may lead to undesired failures of last stage blades (lsbs). for this reason, the aerodynamic and structural design of lsbs, which nowadays may be over 1 meter in length, is a critical aspect faced by designers. the need for robust numerical tools to predict flutter of blade cascades is a key aspect for the aeromechanical design of new modern lsbs [1]. a more flexible way to operate steam turbines highly increases the risk of flutter induced vibrations therefore it is essential to validate applied numerical tools. as the validation is not possible in industrial environments (real turbines) controlled flutter tests on simplified experimental models must be used. such an experimental rig has been built at the department of power system engineering at the university of west bohemia (uwb) where the unsteady aerodynamic forces and moments in the oscillating turbine blade cascade can be investigated. the flexturbine project (ec horizon 2020, no. 653941) gives the opportunity to design and test flutter-resistant blades for a wide range of steam turbine operating conditions. experimental and numerical studies in this project are fundamental for a deeper understanding of flutter occurrences, so that free flutter design rules for a next generation of flutter-resistant steam turbine blades can be drawn. as a part of this project, a subsonic test rig installed at the uwb is employed for a controlled flutter testing of last stage blade tip profiles in air flow. unsteady aerodynamic forces and moments induced by blade cascade oscillations are measured and a work exchange between the blade and the flow is estimated. two different numerical methods were applied for the flutter assessment of blade cascade: a commercial code ansys cfx and an in-house code traf. numerical methods for flutter assessment have been firstly validated in low pressure environment for the aeronautical industry [2, 3]. more recently, the code validation has been extended to compressor blades and steam turbine rows [4, 5] so that the actual design process now includes an extensive use of these methods to achieve a free flutter design [6]. such methodologies are usually classified in time-linearized, harmonic balance and non-linear approaches both in frequency and time domain and require high quality experimental data for the validation and tuning, especially in off-design conditions. the stability results are evaluated by aerodynamic work which describes the exchange of energy between blade and flow [7, 8]. 98 http://dx.doi.org/10.14311/app.2018.20.0098 http://ojs.cvut.cz/ojs/index.php/app vol. 20/2018 study of controlled flutter testing figure 1. uwb test-rig, wind tunnel. 2. test rig description and measured data acquisition the overall view of the subsonic wind tunnel employed in experiments is shown in fig. 1. air flows from the atmosphere through a filter system (the inlet filters) into a smooth contraction section (the calming section) and then into a test section duct (the test section). inside this test section, there is a linear blade cascade consisting of eight blades (fig. 2). inner four blades (3-6) are flexibly mounted with two degrees of freedom (the torsion and bending motion). the blade height is 80 mm, the blade chord is 50 mm, the maximum blade thickness is 2.25 mm, the blade pitch is 45 mm and the cascade stagger angle is 72◦. a variable angle of attack from −15◦ to +15◦ can be set and measured. downstream of the blade cascade, the air is expanded in the drum chamber and runs through the outlet duct towards the roots blower vacuum inlet. there are two important measurement places in the test section as can be seen in fig. 2. the channel inlet is instrumented by a pitot probe, a flush-mounted static port and a thermometer sensor to evaluate the inlet flow velocity and prepare boundary conditions for numerical analyses. downstream of the blade cascade, a traversing mechanism is fitted with a pitot-static probe to evaluate the pressure profiles (the total and static pressure profile). these profiles are important for the comparison with steady-state numerical results. bhv 5355 differential pressure transducers are used to measure all the pressures. traverser motion is provided by a linear stepper motor. blade motion is measured using a pair of non-contacting eddy current displacement sensors schenck in-085 as it is shown in fig. 4. the bending motion can be evaluated as a mean of two signals and the torsion is proportional to the difference of signals. a simultaneously updating analog output module ni 9264 was programmed in labview to generate reference signals for eight analog controllers. a ni pxi 1042 chassis with three pxi 4472b modules has been used for a simultaneous sampling of all signals. figure 2. schematic of the test section part, top view. figure 3. supporting frame with four electromagnetic shakers. fig. 3 shows four electromagnetic shakers which are alternately attached at the top or at the bottom of the supporting frame. motions of blades (torsion and bending) are realized by elastic suspensions of the electromagnetic shakers (fig. 4). main and auxiliary elastic elements are specifically designed in a way which provides bending and torsion motions. two moving coils fixed on a cross beam are partially inserted into larger solenoids (permanent magnets). an oscillation mode is defined by a phase delay between the electric currents running through two moving coils. these electric signals are proportional to forces and moments. each moving coil has its feedback control system in order to set a precise blade motion. further details about the experimental setup are described in [7]. a technique developed in [9] is applied in order to measure unsteady aerodynamic forces and moments. this technique assumes a linear model of aerodynamic loading and it is suitable especially for low-amplitude vibrations. to put it simply, identical blade cascade motion is set up and measured with and without flow so that the aerodynamic loading could be estimated by the difference between these two measurements. furthermore, there are three basic conditions which are necessary for an accurate measurement and evaluation: (i) the system operates in resonance (exciting frequency) and the natural frequency for the bending 99 v. sláma, b. rudas, p. eret et al. acta polytechnica ctu proceedings figure 4. elastic suspension. figure 5. experimentally measured blade flexible deformation in quiescent air at bending vibration. mode and the natural frequency for the torsion mode are the same (system is tuned to 82.2 hz), (ii) the centre of the mass of the elastic suspension coincides with the axis of rotation and (iii) mechanical damping is small. due to the fact that the blade tip is subjected to larger amplitudes than the blade root during bending vibration (fig. 5), the blade deflection must be accounted for the analysis of unsteady aerodynamic forces and moments [10, 11]. for the test cases described in this paper, the travelling wave mode in the bending motion was defined with the vibration amplitude of 0.7 mm at the blade root. similarly, torsion motion was defined with the vibration amplitude of 0.5◦. figure 6. original model of experimental test section. 3. ansys cfx numerical analysis as one of the cfd numerical tools, the commercial code ansys cfx 18.2, which is widely used in dspw, has been used to carry out numerical simulations. the original model of the wind tunnel test section is shown in fig. 6. this model has been simplified for numerical simulations based on detailed cfd investigations [12]. final computational domains for steady-state and unsteady simulations are shown in fig. 7. the inlet of the domains is represented by the red planes and the outlet of the domains is represented by the blue planes. unstructured hexahedral meshes (fig. 8) were created in whole computational domains using ansys icem cfd 18.2. the first element height in the boundary layer was set to 1 · 10−5 m which was found to be adequate for this study [13]. the total number of elements for steady-state simulations is about 16 million. because unsteady simulations were extremely time-consuming, the domain for unsteady simulations was further simplified and the computational mesh was refined to the final number of elements of 3.4 million. for example, the total cpu time on twenty-core 3.10 ghz system was about 1 hour for one steady-state simulation. the computational time for unsteady simulations using the same machine was considerably longer. the resulting cpu time was about 16 hours for 6 periods (a number of periods which were necessary in order to achieve solution periodicity). the flow field in the computational domain was modelled as viscous and fully turbulent. thermodynamic properties of the medium (air) were described by the ideal gas law. the two-equation eddy-viscosity sst k-ω turbulent model with an automatic wall function was used for the turbulence modelling and second 100 vol. 20/2018 study of controlled flutter testing figure 7. cfx computational domain for steadystate simulations (up) and unsteady simulations (down). figure 8. computational mesh for steady-state simulations (up) and unsteady simulations (down). order accurate approximations for all terms in the governing equations were applied in all cfx numerical simulations. the time period for the unsteady simulations was defined as 1/82.2 s, which the reciprocal value of the tuned frequency 82.2 hz, and the total number of time steps per period was defined as 100. the total time duration of unsteady simulations was specified using the total number of periods per run. it was investigated that 6 periods per run were sufficient enough to calculate the required aerodynamic work [7, 12]. to quantify convergence criteria of steadystate simulations, the residual values were used where the rms (root mean square) residual type was chosen and 1 · 10−5 residual target value was defined. the global imbalances were less than 0.1% for all cases. for unsteady simulations, the same target residual was used in order to control the termination of iterations. the maximum number of coefficient iterations per time step was defined as 5. since a total pressure and a static temperature are measured in the inlet, they were used as initial inlet boundary conditions. then their values were iteratively adjusted so that the total pressure in the traverse plane downstream of the blade cascade would be approximately equal to the measured values (fig. 9 down). the mass flow rate was defined at the domain outlet and its value was modified in a close range until the mean static pressure value in the traverse plain was approximately equal to the measured value (fig. 9 up). for unsteady simulations, a total pressure, a total temperature and velocity vectors taken from the steady-state simulation were used as the boundary conditions at the inlet and the mass flow rate value defined at the domain outlet remained the same. reflecting boundary conditions could be used since no issues with reflections were observed in the simulations. the blade bending motion was defined according to the measured relative blade deformation and defined amplitude. the same blade attachment was considered as in the experiment (fig. 3). this means that the first vibrating blade (blade #3) is attached at the top, the second is attached at the bottom etc. the blade bending was defined as the translational motion perpendicular to the oncoming flow direction with the motion amplitude plus the blade deformation specified by a function where the independent variable is a blade length coordinate (see fig. 5 for more details). the blade torsion motion definition is easier. it is just the motion around the rotation axis in the middle of blade chord. before starting the unsteady simulations, the static and total pressure distributions at 50% of the blade height across the channel downstream of the blade cascade were compared with the experimental data (fig. 9). as the numerical results are based on the adjustment of boundary conditions according to the experimental results, the agreement is good especially in the total pressure distribution. in the static pressure distribution, we can see some differences, especially in the cfx approach. in case of the traf code results, the domain is simplified (fig. 10) so that the outlet static pressure can be defined as a boundary condition exactly according to the experimental data very close to the traverse plane. in case of the cfx, the domain is more complex (fig. 7) and outlet with defined boundary condition is more distant. furthermore, the mass flow rate is defined as the outlet boundary 101 v. sláma, b. rudas, p. eret et al. acta polytechnica ctu proceedings figure 9. comparison of the experimental and cfd data of cfx and traf (static pressure (up) and total pressure (down) in the traverse plane downstream of the blade cascade shown in fig. 2). condition. after the sensitivity analysis of the outlet mass flow rate influence on the static pressure distribution in the traverse plane was carried out, it was concluded that all results have similar differences as in fig. 9 and that the differences between the mean value and the peaks are supposed to be small. therefore, no other boundary condition adjusting would be necessary. further details of the differences caused by experimental approach are commented in [7]. 4. traf code numerical analysis the traf code is other solver employed to predict steady-state and flutter response of the uwb testrig. this is an in-house cfd solver developed at the university of florence used for aerodynamic, aeromechanic and aeroacoustic analyses of turbomachinery environments [14]. the 3d aerodynamic solver traf was extended to analyze unsteady flows around vibrating rows by using a re-meshing strategy, thus implementing an aeroelastic non-linear method able to deal with tuned rows, cluster systems and mistuned disks and linear cascades. the validation of the flutter procedure in subsonic and transonic tuned turbine environments can be found in [2, 3]. to perform a flutfigure 10. simplified domain for traf computations. figure 11. details of the 3d grid. ter analysis, time-sinusoidal blade vibrations coming from modal analysis or from vibration measurements are assigned to the unsteady calculations as input data. a re-meshing technique is used to actually deform the grid at each time step during the unsteady run. the non-linear unsteady flow equations are integrated on the deforming domain by applying the same dual time-stepping algorithm used for aerodynamic unsteady simulations. traveling waves are solved on a single-angular-pitch annular domain by applying phase-lagged conditions to periodicity boundaries. fourier transforms are used to assign quantities on periodicity boundaries with appropriate time shifts, limiting the memory storage requirements. it is also possible to compute blade cluster system with different blade geometries applying an arbitrary phase shift between adjacent blades within annular and linear packet [3]. for all the steady-state and flutter (unsteady) analyses, it was decided to simplify the geometry of the actual test-rig to evaluate the possibility to obtain an accurate flutter assessment with the tailored computational model. basically the two solid walls located at both sides of the 8-blade linear cascade (clearly visible in the cfx model (see fig. 6), were removed and replaced with periodic boundary condition. also the inlet plenum was not included in the computational domain and all the analyses were focused on the domain shown in fig. 10. this simplification decreases a lot the computational cost and allows a better discretization of the flow channel. a h-type mesh was created and duplicated to reproduce the whole linear cascade. the h-type grid dimensions are 396x80x8 (streamwise x pitchwise x 102 vol. 20/2018 study of controlled flutter testing figure 12. torsion mode-shape and cfd domain deformation (up), bending mode-shape and cfd domain deformation (down). spanwise, see fig. 11 for direction definitions) and the total number of elements is about 2.5 million. as can be seen in fig. 11, the blade to blade grid is viscous, while along the span-wise direction the mesh is inviscid. this type of mesh allows a further reduction of the computational cost, especially for the following unsteady simulations with vibrating profiles. a fine mesh with a viscous stretching in the span-wise direction was also generated for comparison purposes, the results do not differ significantly. steady-state results were computed with traf code using a k-omega turbulence model. inlet and outlet boundary conditions were provided by uwb in terms of total and static pressure, total temperature and flow angles. steadystate results in terms of pitch-wise distributions at the outlet midspan section of total and static pressure are compared to experimental values. since numerical simulations do not include cascade sidewalls and all the vanes are periodic, just the distribution after the central blade is included in fig. 9 for comparisons. as it can be noticed, the agreement with the inner part of the blade cascade is very good in terms of both static and total pressure. for flutter assessment, the work exchange between vibrating profiles and the flow is evaluated by using an uncoupled non-linear flutter method implemented in the traf code [10, 11]. in the test rig, the 4 inner blade profiles are vibrated by mean of shakers and the flexible component of the figure 13. instantaneous pressure field due to blade torsion oscillation. suspension is alternately located under the root or above the tip of single blade, the blade modes were basically considered as "rigid" mode-shapes (bending and torsion). for this reason, the cascade oscillation imposed by the shakers can be reproduced just knowing an oscillation axis, an oscillation amplitude and the inter blade phase angle (ibpa). in order to deform the computational domain according to whole cascade oscillation, a mode shape transfer technique is used to interpolate the rigid real modes artificially built to the blade surfaces within the multi-block cfd mesh. when referring to classical flutter problems, the row oscillation is the only source of unsteadiness and, obviously, the whole fluid domain (where navierstokes equations are numerically integrated by the solvers) has to be deformed following the blade-row vibration. the non-linear method employs a grid deformation strategy to actually rebuild the computational domain at different equally-spaced instants over the oscillation period. to avoid cell intertwining, the grid deformation is built by using an algebraic method which distributes the largest deformations where the biggest mesh elements are located, thus maintaining the shape of the smallest elements located, for instance, within the boundary layer or near the blade clearance. then, the non-linear unsteady flow equations are integrated on this deforming domain by applying the same dual time-stepping algorithm used for aerodynamic unsteady simulations. the computations are iterated until the unsteady pressure on the profile surfaces due to blade oscillation becomes periodic (see instantaneous static pressure in fig. 13). finally, once the solution is periodic, the aerodynamic work (eq. 1) can be computed during one blade oscillation period. since in uncoupled methods blade vibrations are input data, rather than a result of the computation, flutter stability is estimated by checking the sign of the aerodynamic work done by the fluid onto the blade during one vibration cycle as explained in the following section. the first vibrating test-case, which has been analyzed, is characterized by 4 inner blades moving of rigid torsion with different ibpas. the torsion oscillation axis is imposed at the middle of the profile chord and the oscillation angle is α = 0.5◦. as it can be seen in fig. 12 up, the actual mode-shape of the blade and the suspension system shows a rigid torsion vibration of the blade surfaces. this aspect confirms the rigid mode approximation hypothesis used herein for all the unsteady analyses. 103 v. sláma, b. rudas, p. eret et al. acta polytechnica ctu proceedings figure 14. blade torsion: aerodynamic work for the 4 vibrating profiles vs. ibpas. these rigid mode-shapes were transferred to the blade surface within the cfd mesh and, for each ibpa a different mesh deformation was created. fig. 12 down shows on the bottom the 3d view of cfd domain with the moving blades in the two extreme positions during the oscillation cycle. once the grid deformation was generated, urans computations with the vibrating cascade were run. the steady-state solution, already computed, is used as flow initialization and the unsteady analyses are iterated until the periodicity is reached. usually 5/6 blade oscillation periods are required for periodicity and a further period is run to compute the aerodynamic work and to extract unsteady forces on the vibrating blades. the aerodynamic work is computed by the solver while running the last period over each vibrating profile for each ibpa. the aerodynamic work curves for torsion testcase are shown in fig. 14. as it can be seen, the aerodynamic work curves have a typical sinusoidal trend and, some ibpas show positive aerodynamic work and so flutter instability, as expected. there is an excellent agreement between numerical and experimental data as shown in the following section. this suggests that the simplified setup used for the flutter computations is suitable to evaluate the flutter assessment of the linear cascade. the second test-case is dedicated to blade bending motion. as for the torsion test-case, the blade oscillation is approximated with a rigid mode and no blade deformation is modeled. the oscillation axis is oriented in the direction of the blade chord and for the sake of simplicity, the pivot point is located below the blade root. the blade oscillation systems are installed in the rig at the hub and tip of the channel alternately. a 3d view of blade deformation in two extreme positions for ibpa = 60◦ is depicted in fig. 12 down. as for the torsion test-case, the unsteady simulations were performed starting from steady-state flow solutions. again, when the periodicity is reached, aerodynamic work and unsteady blade forces were extracted. these bending test-case points out an overall stability of the cascade (see figs. 18-18 where all the profiles for each ibpa have a negative value of aerodynamic work, meaning that the energy goes from the blade to the fluid and the system is damped). as it is shown in the following section, the agreement with experimental data and numerical result with cfx is good confirming the applicability of domain approximations already described. 5. aerodynamic work definition the aerodynamic work per cycle done by the fluid on the blade per vibration cycle was numerically computed as follows in eq. 1 [3]: awcycle = t0+t∫ t0 ∫ a −pvn dadt, (1) where t is the period of one vibration cycle, t0 is the time at the start of the vibration cycle, p is the instantaneous fluid pressure, v is the local velocity of the blade surface due to imposed vibrational displacement (a defined blade motion), a is the surface of the blade and n is the unit normal vector to the surface a. if the aerodynamic work per cycle done on the blade is positive, the work is being imparted to the blade, generating negative damping, which indicates a potentially unstable situation leading to a self-sustained vibration (flutter) likely to cause a material fatigue failure. on the other hand, if the aerodynamic work per cycle done on the blade is negative, the blade motion is doing work on the fluid and leads to a stable or damped vibration. in general cases, it is necessary to evaluate some dimensionless parameter such as a logarithmic decrement, a damping ratio or a critical damping ratio [4–6], because the value of vibration amplitude affects the results of the aerodynamic work. the vibration amplitude is in real cases (real turbine rotor blades) different than it is defined in numerical simulations. however, in this study, the vibration amplitude is exactly defined in the experimental measurement so that the same value can be defined in the numerical analyses. therefore, the aerodynamic work can be directly compared. further details can be found in [9–11]. 6. bending motion results comparison the aerodynamic work expressed in eq. 1 is the main quantity to assess flutter stability and was experimentally and numerically evaluated for all the ibpas of the two vibration test cases (i.e. bending and torsion) at ma = 0.42 and aoa = 0◦. figs. 15-18 show the comparisons of the aerodynamic work in the bending motion with the vibration amplitude of 0.7 mm for each flexibly mounted blade. the data scatter between three sets of data (experiment exp and two numerical simulations cfx and traf) is obvious, both numerical results may differ by more than 100% in comparison with the experimental measurements, however blade cascade stability has been found in all results. the main reason for the discrepancy in the 104 vol. 20/2018 study of controlled flutter testing figure 15. aerodynamic work of blade bending motion blade #3: experimental (exp), cfx and traf comparison. results can be found in the difference of blade deformation in bending motion with and without flowing air. while the blade bending deformation in quiescent air (see fig. 5) was easily measured, as the supporting frame with the shakers (see fig. 3) was placed outside the test chamber, and the data was employed in cfx numerical simulations, a true bending deformation in flowing air could not be obtained experimentally. it is expected that the flow will significantly affect the blade bending deformation and therefore experimental and numerical results must differ. in order to confirm the model performance sensitivity on a blade bending definition, a new bending motion without the blade deformation was carried out in cfx for ibpas 135◦ and 270◦. these results are also shown in figs. 15-18 (cfx different bending) and they disagree with the previous solutions. the fact that they are closer to the measured values might suggest that the flow has a tendency to reduce the blade bending deformation. however, such a conclusion is immature due to the lack of data and the issue can be investigated further. moreover, simplifications of the definition of the bending motion in the traf code (i.e. rigid blade with the pivot point, which is located below the blade root, in reality, considerably below the blade root) also contribute to the disagreement between the experiment and computational data. therefore, the experimental and the numerical results show different blade bending motions and the discrepancies in the results in figs. 15-18 are correct. further work is expected to improve the absolute accuracy of the predictions and the measurements. this will be necessary for situations, where aerodynamic instability in bending motion can occur (i.e. testing at different angles of attack, lower (reduced) frequencies or coupled bending-torsion motions). anyway, it is important to pinpoint that the aerodynamic work for the bending configuration in this particular test case is always negative for each blade and this aspect was correctly captured by both numerical methods despite the discrepancies in blade oscillation definition. this bending configuration at ma = 0.42 and aoa = 0◦ shows an overall stable response with no danger of flutter occurrence. figure 16. aerodynamic work of blade bending motion blade #4: experimental (exp), cfx and traf comparison. figure 17. aerodynamic work of blade bending motion blade #5: experimental (exp), cfx and traf comparison. 7. torsion motion results comparison figs. 19-22 show the comparisons of aerodynamic work curves as a function of the ibpas for the flexibly mounted blades 3-6 in the case of blade torsion with the vibration amplitude of 0.5◦ at ma = 0.42 and aoa = 0◦. contrary to the blade bending motion test case, the discrepancies in terms of aerodynamic work are reasonably small and the agreement between all the curves is more than satisfactory for each blade. this agreement can be achieved due to the fact that the torsional motion is better defined than the bending case. although no evident blade deformation was measured in quiescent air for the torsion motion, flowing air can change slightly the shape of oscillating blade, however this effect cannot be experimentally observed. such a torsional deformation of blade might also contribute to the small discrepancy between the experiments and the numerical simulations. it can be noticed that there is a certain range of positive ibpas where the aerodynamic work is positive, thus highlighting a flutter instability for all the vibrating profiles. both codes delivered similar values of energy exchange, but in the direct comparison to experimental data, the range of unstable ibpas is better predicted by cfx. in this test case for torsion motion at ma = 0.42 and aoa = 0◦, both computational models have been successfully 105 v. sláma, b. rudas, p. eret et al. acta polytechnica ctu proceedings figure 18. aerodynamic work of blade bending motion blade #6: experimental (exp), cfx and traf comparison. figure 19. aerodynamic work of blade torsion motion blade #3: experimental (exp), cfx and traf comparison. validated and proved suitability for flutter simulations of slender blade profiles. 8. conclusions experimental measurements of the flutter behaviour of model of lsb were carried out using an in-draft wind tunnel where a turbine cascade with slender blades in a linear configuration is installed. four inner blades of the cascade were flexibly mounted each with two degrees of freedom to generate bending and torsion motion. two different numerical methods were applied for the flutter assessment of this blade cascade: a commercial code (ansys cfx) and an in-house code (traf). in each method, a rig geometry model with different level of complexity was used. a travelling wave mode for bending motion with vibration amplitude of 0.7 mm and torsion motion with the vibration amplitude of 0.5◦ was investigated at ma = 0.42 and aoa = 0◦. to some extent, very good agreement between all compared cases has been achieved. in detail, the bending test case showed discrepancies between the numerical and experimental aerodynamic work values. this can be explained by different blade oscillation behaviour (due to the blade flexibility) during the experimental campaign with respect to the blade oscillation imposed for the numerical computations. however, the overall stability of blade cascade figure 20. aerodynamic work of blade torsion motion blade #4: experimental (exp), cfx and traf comparison. figure 21. aerodynamic work of blade torsion motion blade #5: experimental (exp), cfx and traf comparison. has been observed using both numerical approaches and also in the experiments. on the other hand, the torsion test case, where the blade shape is not so modified by the shakers (but to a lesser extent by the flow), shows very good agreement between the experimental data and the numerical results. the multi-purpose commercial code cfx confirms its flexibility in simulating aeromechanical investigations of complex geometries (including sidewalls and the actual domains up/downstream the blade cascade). on the other hand, the in-house code traf is basically conceived to internal flow simulations for turbomachinery environments and a few approximations in the geometry discretization are required. yet, the traf code suite takes advantage of low computational cost, the possibility of a better discretization of the flow channel, an easy and quick mesh deformation strategy and of dedicated tools for flutter post-processing which speed up typical flutter analyses. the results in this paper confirm that the used flutter tools are capable to predict aerodynamic work exchange between fluid and vibrating profiles in air flows. these tools can be now widely tested for multi-phase flows and subsequently used during the design phase, especially in off-design conditions required by flexible operations to avoid hcf failure due to flutter vibrations. 106 vol. 20/2018 study of controlled flutter testing figure 22. aerodynamic work of blade torsion motion blade #6: experimental (exp), cfx and traf comparison. acknowledgements this research effort was founded by the european project flexturbine, h2020 research and innovation programme under grant agreement no. 653941. references [1] t. tanuma. advances in steam turbines for modern power plants. woodhead publishing, 2017. https: //doi.org/10.1016/b978-0-08-100314-5.00001-4. [2] l. pinelli, f. poli, a. arnone, c. schipani. a time-accurate 3d method for turbomachinery blade flutter analysis. 12th international symposium on unsteady aerodynamics, aeroacoustics and aeroelasticity of turbomachines (isuaaat), london, uk, 2009. paper i12-s8-3, http://arnone.de.unifi.it/tgroup/node/891. [3] f. vanti, l. pinelli, f. poli, a. arnone. aeroelastic investigation of turbine blade assemblies: cluster system and mistuned rows. 12th european conference on turbomachinery fluid dynamics & thermodynamics etc12, 2017. https://doi.org/10.29008/etc2017-144. [4] c. fuhrer, d. m. vogt. on the impact of simulation approaches on the predicted aerodynamic damping of a low pressure steam turbine rotor. asme turbo expo, 2017. https://doi.org/10.1115/gt2017-63401. [5] t. sun, p. p. repar, q. di. investigation of tip clearance flow effects on an open 3d steam turbine flutter test case. asme turbo expo, 2017. https://doi.org/10.1115/gt2017-64021. [6] f. vanti, l. pinelli, a. arnone, et al. aeroelastic optimization of an industrial compressor rotor blade geometry. asme turbo expo, 2018. https://doi.org/10.1115/gt2018-76474. [7] v. slama, b. rudas, j. ira, et al. the validation of flutter prediction in a linear cascade of non-rigid turbine blades. asme turbo expo, 2018. https://doi.org/10.1115/gt2018-75502. [8] i. mcbean, k. hourigan, m. thompson, l. f. prediction of flutter of turbine blades in a transonic annular cascade. asme j. fluids eng., 2005. https://doi.org/10.1115/1.2060731. [9] v. tsimbalyuk. method of measuring transient aerodynamic forces and moments on a vibrating cascade. strength mater, 1996. 28: 150, https://doi.org/10.1007/bf02215842. [10] v. tsymbalyuk, j. linhart. corrections of aerodynamic loadings measurement on airfoil cascade at bending-torsion vibrations. 19th international conference on force, mass & torgue. cairo: national institute of standards egypt, 2005. isbn: 953-7124-00-2. [11] v. tsymbalyuk, j. linhart. corrections of aerodynamic loadings measurement on vibrating airfoils. xvii imeko world congress, dubrovnik, croatian metrology society, 2003. isbn: 953-7124-00-2. [12] v. slama, b. rudas, j. ira, et al. cfd prediction of flutter of turbine blades and comparison with an experimental test case. aeanmifmae-2018, matec web of conferences 168, 2018. https://doi.org/10.1051/matecconf/201816802005. [13] j. d. denton. some limitations of turbomachinery cfd. asme turbo expo, power for land, sea, and air, volume 7: turbomachinery, parts a, b, and c: 7, 2010. https://doi.org/10.1115/gt2010-22540. [14] m. giovannini, m. marconcini, a. arnone, f. bertini. evaluation of unsteady computational fluid dynamics models applied to the analysis of a transonic high-pressure turbine stage. proceedings of the institution of mechanical engineers, part a: journal of power and energy, 2014. https://doi.org/10.1177/0957650914536170. 107 https://doi.org/10.1016/b978-0-08-100314-5.00001-4 https://doi.org/10.1016/b978-0-08-100314-5.00001-4 http://arnone.de.unifi.it/tgroup/node/891 https://doi.org/10.29008/etc2017-144 https://doi.org/10.1115/gt2017-63401 https://doi.org/10.1115/gt2017-64021 https://doi.org/10.1115/gt2018-76474 https://doi.org/10.1115/gt2018-75502 https://doi.org/10.1115/1.2060731 https://doi.org/10.1007/bf02215842 https://doi.org/10.1051/matecconf/201816802005 https://doi.org/10.1115/gt2010-22540 https://doi.org/10.1177/0957650914536170 acta polytechnica ctu proceedings 20:98–107, 2018 1 introduction 2 test rig description and measured data acquisition 3 ansys cfx numerical analysis 4 traf code numerical analysis 5 aerodynamic work definition 6 bending motion results comparison 7 torsion motion results comparison 8 conclusions acknowledgements references acta polytechnica doi:10.14311/app.2020.27.0079 acta polytechnica 27(0):79–83, 2020 © czech technical university in prague, 2020 available online at http://ojs.cvut.cz/ojs/index.php/app relation between hardness of (ti, al)n based multilayered coatings and periods of their stacking mehdi zaouia,b, alexandre bourcereta, yves gaillarda, sylvain giljeanc,∗, christophe rousselota, marie-josé pacc, fabrice richarda a université bourgogne franche-comté, institut femto-st, cnrs/ufc/ensmm/utbm, 32 avenue de l’observatoire, 25000 besançon, france b institut de recherche technologique, matériaux, métallurgie, procédés, 4 rue augustin fresnel, 57070 metz, france c université de haute alsace, lpmt, 2 rue des frères lumière, 68093 mulhouse, france ∗ corresponding author: sylvain.giljean@uha.fr abstract. this study aims to model, by using a finite element method, the relationship between the hardness and the period λ of metal/nitride multilayer coatings (ti0.54al0.46/ti0.54al0.46n)n in order to understand the increase of the hardness at the low periods [1] and then optimise the multilayer coating architecture to obtain the best mechanical properties. a 2d axisymmetric finite element model of the berkovich nanoindentation test was developed. the coating was designed as a stacking of ti0.54al0.46 and ti0.54al0.46n nanolayers with, in the first hypothesis, equal thickness and perfect interface. the elastoplastic behaviours of the metal and nitride layers were identified by berkovich nanoindentation experiments and inverse analysis on thick monolayer samples. the indentation curves (p-h) obtained by this model depend on the period λ of the stacking. simulated (p-h) curves were compared with experimental data on 2 µm thick films with different periods λ ranging from 10 to 50 nm deposited by rf magnetron sputtering using reactive gas pulsing process (rgpp). the model forecasts are very consistent with the experience for the largest period but the model does not reproduce the hardness increase at the lowest periods. the λ = 10 nm coating was analysed by electron energy loss spectroscopy (eels) on a transmission electron microscope. results show intermixing of the layers with the presence of nitrogen atoms in the metal layer over a few nanometers [1]. it was concluded that the metal/ceramic interface plays an important role at low periods. the addition in the model of a transition layer in the metal/nitride stacking, with an elastoplastic metal/ceramic medium behaviour, allows to reproduce the nanoindentation experimental curves. the thickness of this transition layer deduced from model updating method is in very good agreement with eels observations. keywords: finite element method, hardness, indentation modulus, nanoindentation, tial/tialn multilayer films. 1. introduction titanium and aluminium nitrides (tial)n coatings are extensively used in the industry since many years for their hardness, toughness and wear and oxidation resistances [2, 3]. it has been shown that the indentation modulus and the hardness of ti1-xalxn coatings depend on the aluminium content and that the optimal properties are obtained when the maximum al content is substituted for ti in face centred cubic structure close to x = 0.46 [1, 4]. one way to improve the hardness of that kind of coatings consists in depositing multilayer films. in this study, multilayer coatings are studied and consist in metal/ceramic stacking with alternate deposition of ti0.54al0.46 and ti0.54al0.46n nanolayers with three different periods of the bilayer metal/ceramic λ = 50, 16, 10 nm. in order to facilitate the reading of the paper, ti0.54al0.46 and ti0.54al0.46n are respectively written as tial and tialn. the main objective of the paper is to study the relationship between hardness of as-design multilayer films and periods of their stacking to understand the elastoplastic phenomena that are involved in the hardness increase experimentally observed at the lowest periods [1]. a 2d axisymmetric model of the berkovich nanoindentation is developed. then a method to identify the elastoplastic mechanical properties of the monolayer tial and tialn is proposed. the results are introduced in a multilayer stacking model that allows to obtain the indentation curves (p-h) as a function of the period λ. as a first hypothesis the interface between tial and tialn layers is considered as perfect without thickness. then a transition layer is added and allows to reproduce the experimental indentation curves. 79 http://dx.doi.org/10.14311/app.2020.27.0079 http://ojs.cvut.cz/ojs/index.php/app m. zaoui, a. bourceret, y. gaillard et al. acta polytechnica 2. materials and methods tial and tialn monolayer thin films were deposited onto si (100) wafer by rf magnetron sputtering using one sintered titanium/aluminium 66/33 at.% alloy target as described elsewhere [4]. same experimental conditions were used to deposit (tial/tialn)n multilayer films with different periods using reactive gas pulsing process (rgpp) [5] with a controlled pulsing flow rate of the nitrogen reactive gas as described elsewhere [1]. finally, the samples considered in this paper are summarised in table 1. sample period thickness λ (nm) ep (µm) tial / 3.0 tialn / 3.0 50 2.0 (tial/tialn)n 16 2.0 10 1.8 table 1. sample characteristics. nano-indentation tests were performed using an ultra nano-indentation hardness tester provided by anton paar. experiments were conducted using a berkovich tip and the multicycle procedure in load control up to 8 mn [1, 4]. a 2d axisymmetric finite element model of the berkovich nanoindentation test was developed with the ansys finite element software as described on figure 1. an implicit method with non-linear geometric formulation has been used with higher order 2d elements. a deformable indenter, with isotropic linear elastic law, was modelled with a half contact angle of 70.3° and a tip radius to fit the experimental tip defects. a friction coefficient of 0.1 was imposed at the contact between the indenter and the film. the mesh density has been refined so that the smallest elements are localised in the contact region. the film can be either monolayer tial or tialn or multilayer (tial/tialn)n. for the monolayer tial and tialn, isotropic elastoplastic behaviour laws were considered with linear hardening. elastic properties were obtained from experimental nanoindentation and plastic properties were estimated using finite element model updating. the approach used to extract the constitutive parameters of plastic law of tial and tialn consists in an optimisation problem; the objective of which is to minimise a cost function ω by using an algorithm based on the levenberg-marquardt [6, 7] method using the mic2m software [8]: θ̂ = argmin θ ω (p (t; θ) , p exp (t)) (1) where θ̂ is the vector of the unknown parameters and ω the cost function i.e., the gap between p exp (t) the figure 1. description of the 2d axisymmetric finite element indentation model. experimental data and p (t; θ) the numerical results. then the cost function can be defined as: ω(θ) = 1 2t t∑ k=1 ( pk(θ)− p expk p expmax )2 (2) where t is the number of acquisition times of the load pk(θ) = p (tk; θ) and p expmax the maximum value of the experimental indentation load. 3. results and discussion 3.1. mechanical properties of tial and tialn monolayers all the experimental data of the indentation tests performed on the tial and tialn monolayer films have already been published elsewhere [1, 4]. hardness and indentation modulus values of the metal tial are significantly lower than the ones of nitride tialn. by postulating the poisson’s ratio as known, the elastic properties of the monolayer films can be deduced from the experimental data. then, plastic parameters were numerically deduced from the optimisation approach described in method section. results are presented in table 2. the comparison of the experimental and model indentation load curves are plotted in figure 2 and show a good agreement. 3.2. simulation of (tial/tialn)n multilayer films with perfect interfaces all the experimental data of the indentation tests performed on the multilayer (tial/tialn)n films have already been published elsewhere [1]. interesting observation is that both the hardness and the elastic modulus of the multilayer films follow a hall and petch law as the values linearly increase as a function of 1/ √ λ. in a first approach, the multilayer film was modelled as a stacking of nanolayers of tial and tialn with a perfect interface without thickness as showed in figure 3.a. the model was set for each 80 vol. 27/2020 young modulus poisson ratio yield stress linear hardening (gpa) (gpa) (gpa) tial 199 0.25 0.88 56.6 tialn 417 0.21 5.70 405.0 table 2. elastoplastic parameters of the metal tial and the nitride tialn. figure 2. loading indentation curves (p-h) of (a) tial and (b) tialn monolayers. figure 3. modelling of multilayer films, (a) with perfect interfaces (b) with a transition layer. figure 4. comparison of the (p-h) experimental and simulated curves for λ = 10, 16 and 50 nm with a perfect interface between tial and tialn nanolayers. stacking period λ = 10, 16 and 50 nm. a convergence study was carried out with 1, 2 or 3 elements in the thickness of the nanolayers. since the relative variation of the load is less than 0.5%, only one element, with a λ/2 size, was finally used in the thickness (figure 3.a), in order to minimise the calculation time. results of the as simulated (p-h) curves are plotted on figure 4. for the largest period λ = 50 nm, the model is in good agreement with the experience. furthermore, the gap between the experimental curves and the simulated ones increases when the period decreases to become important for the smallest period. therefore, the phenomenon that generates an increase in hardness when period decreases is not taken into 81 m. zaoui, a. bourceret, y. gaillard et al. acta polytechnica figure 5. for λ = 10 nm, (a) comparison of the (p-h) experimental and simulated curves with a transition layer, (b) eels results. account in the coating model without interfaces. 3.3. simulation of (tial/tialn)n multilayer films with a transition layer in the stacking, between each nanolayer of tial and tialn, a transition layer was added (figure 3.b.). for this transition layer, an elastoplastic metal/ceramic medium behaviour was considered. each elastoplastic parameter of the transition layer was assumed to be an average (mixture law 50/50) of the parameters of the tial and tialn nanolayers. the thickness of this transition layer was optimised by minimising the gap between experimental and simulated data as previously described. result of the simulated (p-h) curve with a transition layer is plotted on figure 5.a and show a good agreement between experimental and simulated curves for the lowest period. for λ = 10 nm, the simulated thickness of the transition layer, eptr = 2.12 nm, is close to the one of tial and tialn layers. this result is in very good agreement with the thickness of interfaces observed between tial and tialn layers evaluated by electron energy-loss spectroscopy (figure 5.b). 4. conclusion the elastoplastic properties of ti0.54al0.46 metal and ti0.54al0.46n ceramic monolayers were identified by comparison of berkovich nanoindentation experimental data and 2d axisymmetric finite element simulations. the mechanical properties of the monolayers were used in a multilayer model to forecast the mechanical properties of (ti0.54al0.46/ti0.54al0.46n)n multilayer coatings, with various periods, designed by rf magnetron sputtering using rgpp technique. the multilayer model was considered, in first hypothesis, as a stacking of tialn and tial nanolayers with equal thickness and perfect interface. that model allows to reproduce the indentation curves of the highest 50 nm period films. to fit the experimental indentation curve at the lowest periods, a transition layer was introduced in the model between each metal/ceramic interface. using an optimisation process of the model, this transition layer was found to be 2.12 nm thick for the lowest period λ = 10 nm, which is in good agreement with the results of electron energy-loss spectroscopy experiments. acknowledgements this work was performed in the frame of the research project resem 2019, managed by the institut de recherche technologique matériaux métallurgie procédés (irt m2p) and financially supported by the french program plan d’investissement d’avenir (pia). references [1] m.-j. pac, y. pinot, s. giljean, et al. investigation of ti0.54al0.46/ti0.54al0.46n multilayer films deposited by reactive gas pulsing process by nano-indentation and electron energy-loss spectroscopy. thin solid films 634:96 – 106, 2017. doi:10.1016/j.tsf.2017.05.015. [2] w. münz. titanium aluminium nitride films: a new alternative to tin coatings. journal of vacuum science & technology a 4:2717–2725, 1986. doi:10.1116/1.573713. [3] s. paldey, s. deevi. single layer and multilayer wear resistant coatings of (ti,al)n: a review. materials science and engineering: a 342(1):58 – 79, 2003. doi:10.1016/s0921-5093(02)00259-9. [4] m.-j. pac, s. giljean, c. rousselot, et al. microstructural and elasto-plastic material parameters identification by inverse finite elements method of ti(1-x)alxn (0<x<1) sputtered thin films from berkovich nano-indentation experiments. thin solid films 569:81 – 92, 2014. doi:10.1016/j.tsf.2014.07.037. [5] n. martin, j. lintymer, f. sthal, et al. procédé de pulvérisation réactive à signal de commande cyclique et dispositif correspondant 2008. [6] k. levenberg. a method for the solution of certain non-linear problems in least squares. quarterly of applied mathematics 2(2):164–168, 1944. 82 https://doi.org/10.1016/j.tsf.2017.05.015 https://doi.org/10.1116/1.573713 https://doi.org/10.1016/s0921-5093(02)00259-9 https://doi.org/10.1016/j.tsf.2014.07.037 vol. 27/2020 [7] d. w. marquardt. an algorithm for least-squares estimation of nonlinear parameters. journal of the society for industrial and applied mathematics 11(2):431–441, 1963. doi:10.1137/0111030. [8] f. richard. mic2m software: modeling and identification of the mechanical behavior of materials, 2000. 83 https://doi.org/10.1137/0111030 acta polytechnica 27(0):79–83, 2020 1 introduction 2 materials and methods 3 results and discussion 3.1 mechanical properties of tial and tialn monolayers 3.2 simulation of (tial/tialn)n multilayer films with perfect interfaces 3.3 simulation of (tial/tialn)n multilayer films with a transition layer 4 conclusion acknowledgements references acta polytechnica doi:10.14311/app.2020.27.0141 acta polytechnica 27(0):141–144, 2020 © czech technical university in prague, 2020 available online at http://ojs.cvut.cz/ojs/index.php/app indentation size effect in high pressure torsion processed high entropy alloy petr haušilda,∗, jakub čížekb, jaroslav čecha, jiří zýkac, hyoung seop kimd a czech technical university in prague, faculty of nuclear sciences and physical engineering, department of materials, trojanova 13, 120 00 praha, czech republic b charles university in prague, faculty of mathematics and physics, department of low temperature physics, v holešovičkách 2, praha 8, cz-18000, czech republic c ujp praha a.s., nad kamínkou 1345, 156 00, praha, czech republic d postech, department of materials science and engineering, pohang, 790-784, south korea ∗ corresponding author: petr.hausild@fjfi.cvut.cz abstract. high entropy alloy hfnbtatizr in as cast conditions and after high pressure torsion straining was characterized by nanoindentation. the length-scale dependent material response (indentation size effect) was characterized by indentation at various indentation depths. hardness dependence on the characteristic length (depth of penetration) indicated decomposition of disordered high entropy alloy in the as cast sample, which probably occurred during slow cooling after casting. subsequent severe plastic deformation by high pressure torsion led on the other hand to the short-range disorder of (originally partially decomposed as cast) structure. further hardening was generated during high pressure torsion by the mechanisms of grain refinement and increasing dislocation density. keywords: high entropy alloys, high pressure torsion, indentation size effect, nanoindentation. 1. introduction in high entropy alloys (heas), the configurational entropy of a multicomponent solid solution phase is maximized so that the gibbs energy of random solution may be lower than that of possible intermetallic phases. generally, to achieve high entropy of mixing, the alloys contain typically five or more major elements in equimolar concentrations [1]. the heas show promising mechanical properties in applications ranging from (high temperature) structural to biocompatible materials [2, 3]. mechanical properties of such alloys can further be improved by grain refinement especially by severe plastic deformation. however, studies of ultrafine grained heas are rather scarce in the literature. an increase of strength with decreasing grain size was achieved through processing by high pressure torsion (hpt) in the probably most investigated hea cantor alloy i.e., equiatomic cocrfemnni with face-centered cubic (fcc) structure [4]. fewer attempts were made to process in such a way heas with body-centered cubic (bcc) structure. recently hfnbtatizr bcc hea was successfully nanostructured by hpt straining [5, 6]. it was reported that grain refinement by hpt resulted in a significant enhancement of the strength of this bcc hea, keeping excellent ductility during room temperature straining. nevertheless, there is still a lack of information about the development of microstructure and physical properties of this refractory metal hea subjected to severe plastic deformation processing. recent investigations [5, 6] revealed that thermodynamically stable system of hfnbtatizr alloy at room temperature is a mechanical mixture of zr, hf rich hcp phase and ta, nb rich bcc phase. recrystallization annealing after cold-rolling at 800 °c led to the transformation of the single-phase bcc material into a two-phase structure composed of bcc1 and bcc2 phases with lattice parameters a1 ∼ 0.3427 nm and a2 ∼ 0.3338 nm, respectively [7]. chemical analysis revealed that the matrix was slightly enriched in hf and zr, while the precipitated second-phase particles were on the other hand slightly enriched in ta and nb. the decomposition of the solid solution (and formation of short range order) after long-term annealing obviously leads to the deterioration of mechanical properties (loss of ductility and decrease of strength). the difference in hardness of both phases is relatively small and both are softer than the random (disordered) solid solution [6]. on the other hand, considerable contribution to the solid solution strengthening can arise from atomic size misfit (phase separation on the nano-meter scale) which is provoked by the high density of vacancies introduced by hpt [5]. the aim of this work was clarification of the relationship between phase (de)composition, microstructure, lattice defects, and length-scale-dependent material response of hfnbtatizr hea in as cast conditions, after solution (disordering) thermal treatment and hpt straining. the length-scale-dependent material response was characterized by indentation at various 141 http://dx.doi.org/10.14311/app.2020.27.0141 http://ojs.cvut.cz/ojs/index.php/app p. haušild, j. čížek, j. čech et al. acta polytechnica as cast annealed 1/4 turn 1 turn 15 turns εpl [1] 0 0 5 20 300 h0 [mpa] 4049 4438 4947 5717 5953 h∗ [nm] 45.7 76.3 49 33.5 29.3 table 1. list of the samples equivalent plastic strain εpl, identified characteristic lengths h∗ and hardnesses in the limit of infinite depth h0. indentation depths. the contributions of different mechanisms were attributed to distance between dislocation pinning defects (length scale) and are reported here. 2. experimental details 2.1. material the alloy of equimolar composition hfnbtatizr was prepared by vacuum arc melting from pure hf, nb, ta, ti and zr metals and cast into water cooled cu crucible. in order to ensure proper mixing, the ingot was flipped and remelted eight times. disc of 20 mm diameter and 1 mm thickness were hpt strained at room temperature and pressure of 2.5 gpa. straining was performed to 1/4 turn, 1 turn and 15 turns of the piston (see fig. 1). corresponding equivalent plastic strains εpl (at the characterized areas) are 5, 20 and 300 (see table 1). more details can be found in ref. [5]. one non-deformed (as cast) sample was also annealed for 2 mins at 1600 °c (followed by rapid cooling) for reasons described in section results and discussion. sample figure 1. scheme of high pressure torsion straining. 2.2. nanoindentation surface of samples was polished by standard metallographical procedure with final step made using 0.04µm colloidal silica to avoid the surface layer affected by mechanical grinding and polishing. nanoindentation measurements were performed using an anton paar nht2 nanoindentation tester with berkovich diamond indenter using instrumented indentation technique [8, 9]. in order to obtain the mechanical response from different depth, so called continuous multi cycle (cmc) indentations with increasing load were performed varying the maximum load at each cycle from 1 mn up to 500 mn. this type of load cycle allows characterization at different load (depth) levels at the same position (see e.g. refs. [10, 11]). loading time was 10 s per each cycle for cmc, followed by 5 s hold at the maximum load and unloading time 10 s per each cycle. the results (indentation hardness) were evaluated according to the iso 14577 standard. the indentations were made on sample surface in the position corresponding to the half radius, i.e. in the location of most homogeneously distributed deformation after hpt straining. at least 20 cmc indentations were performed for each material state (deformation condition). 2000 3000 4000 5000 6000 7000 8000 9000 0 1000 2000 3000 h (m pa ) h (nm) 15 1 1/4 as cast figure 2. indentation size effect in as cast sample and samples hpt strained to 1/4 turn, 1 turn and 15 turns. 3. results and discussion results of hardness measurements as a function of penetration depth for as cast sample and samples hpt strained from 1/4 turn to 15 turns are shown in fig. 2. it can be seen in this figure that hardness increases with increasing strain and that all samples show strong indentation size effect in sub-micron penetration depths. using nix nad gao approach [12] based on the geometrically necessary dislocations, the 142 vol. 27/2020 indentation size effect in hpt processed high entropy alloy depth dependence of the hardness h can be related to the characteristic length h∗ through the relation: h h0 = √ 1 + h∗ h (1) where h is a given depth of penetration, and h0 is the hardness in the limit of infinite depth (∼ macroscopic hardness). depth dependence of the hardness is plotted according to eq. 1 for different levels of straining in fig. 3 and the parameters h0 and h∗ obtained by least square fit are listed in table 1. 0 10 20 30 40 50 60 0 0.001 0.002 0.003 0.004 0.005 0.006 0.007 h 2 (g pa 2 ) m ill io n s 1/h (nm-1) as cast 15 turns 1 turn 1/4 turn figure 3. depth dependence of the hardness plotted for as cast sample and samples hpt strained from 1/4 turn to 15 turns according to eq. 1. as can be expected, the hardness in the limit of infinite depth h0 is increasing with the increasing level of hpt straining (hpt leads to the strain hardening [13]). on the other hand, the characteristic length h∗ is decreasing with the increasing level of deformation. it is to note that the values of characteristic lengths h∗ are quite low, which indicates fine microstructural features involved in the dislocation pinning process. according to nix and gao [12], the characteristic length h∗ depends on the hardness in the limit of infinite depth h0 as follows: h∗ ∼ 1 h02 (2) fig. 4 shows that characteristic depth h∗ dependence on the hardness in the limit of infinite depth h0 fulfils relation (2) for hpt deformed samples, but the as-cast sample clearly does not follow this relation. as mentioned in the introduction, the anomalous behavior of the as cast sample is likely caused by the decomposition of disordered (high entropy) bcc solid solution into two bcc1 and bcc2 phases. such short-range atomic redistribution is hard to be characterized by conventional techniques such x-ray diffraction or transmission electron microscopy as the variation of lattice parameter is very small 0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0 20 40 60 80 100 1 /h 0 2 (g pa -2 ) h* (nm) as cast annealed1/4 turn 1 turn 15 turns figure 4. characteristic depth dependence of the hardness in the limit of infinite depth for as cast sample, annealed sample and samples hpt strained from 1/4 turn to 15 turns. 2000 3000 4000 5000 6000 7000 8000 0 500 1000 1500 2000 2500 h (m pa ) hc (nm) as cast annealed figure 5. comparison of depth dependence of the hardness in the as cast and annealed samples. and the size of domains is in the order of tens of nm (see parameter h∗). therefore, the as cast sample was annealed at (high) temperature of 1600 °c with subsequent rapid cooling in order to preserve the random solid solution and subsequently characterized by nanoindentation. the results of hardness measurements in as cast and annealed state are compared in fig. 5. as can be seen in table 1, the parameter h∗ significantly increased after annealing, which is consistent with the hypothesis of short-range order disappearance. the tendency given by the relation (eq. 2) is now in good agreement for both annealed and hpt strained samples (fig. 4). thus, similarly to high temperature annealing the hpt led to the short-range disorder of (originally partially decomposed) as-cast sample (severe plastic deformation leads to an extension of solid solubility [14]). subsequent hardening in 143 p. haušild, j. čížek, j. čech et al. acta polytechnica hpt strained samples is related rather to grain (cell) refinement and increasing dislocation density as observed in ref. [5] using transmission electron microscopy. 4. conclusions the results of the present study can be summarized as follows: • high pressure torsion led to a significant (and progressive) hardening of high entropy hfnbtatizr alloy. • both as cast and hpt strained samples showed length-scale dependent material response (indentation size effect). • as cast sample presented different dependence of characteristic length h∗ on the hardness in the limit of infinite depth h0 than hpt deformed and annealed samples, which indicates short-range decomposition of disordered high entropy alloy. • it can be concluded that severe plastic deformation by hpt produced the short-range disorder of originally partially decomposed as cast structure, which was followed by hardening due to the grain refinement and increasing dislocation density intorduced by hpt process. • annealing of as cast sample at 1600 °c with subsequent rapid cooling in order to stabilize the disordered structure led to the same dependence of characteristic length on the hardness in the limit of infinite depth as in the case of hpt strained samples. acknowledgements this research was carried out in the frame of the projects cz.02.1.01/0.0/0.0/16_019/0000778 (centre of advanced applied sciences) and 17-17016s (czech science foundation). references [1] m.-h. tsai, j.-w. yeh. high-entropy alloys: a critical review. materials research letters 2(3):107–123, 2014. doi:10.1080/21663831.2014.912690. [2] d. miracle, o. senkov. a critical review of high entropy alloys and related concepts. acta mater 122:448 – 511, 2017. doi:10.1016/j.actamat.2016.08.081. [3] j. chen, x. zhou, w. wang, et al. a review on fundamental of high entropy alloys with promising high-temperature properties. j alloy comp 760:15 – 30, 2018. doi:10.1016/j.jallcom.2018.05.067. [4] a. heczel, m. kawasaki, j. lábár, et al. defect structure and hardness in nanocrystalline cocrfemnni high-entropy alloy processed by high-pressure torsion. j alloy comp 711:143–154, 2017. doi:10.1016/j.jallcom.2017.03.352. [5] j. čížek, p. haušild, m. cieslar, et al. strength enhancement of high entropy alloy hfnbtatizr by severe plastic deformation. j alloy comp 768:924–937, 2018. doi:10.1016/j.jallcom.2018.07.319. [6] b. schuh, b. völker, j. todt, et al. thermodynamic instability of a nanocrystalline, single-phase tizrnbhfta alloy and its impact on the mechanical properties. acta mater 142:201–212, 2018. doi:10.1016/j.actamat.2017.09.035. [7] o. n. senkov, s. l. semiatin. microstructure and properties of a refractory high-entropy alloy after cold working. j alloy comp 649:1110–1123, 2015. doi:10.1016/j.jallcom.2015.07.209. [8] w. oliver, g. pharr. an improved technique for determining hardness and elastic-modulus using load and displacement sensing indentation experiments. j mater res 7(6):1564–1583, 1992. doi:10.1557/jmr.1992.1564. [9] w. oliver, g. pharr. measurement of hardness and elastic modulus by instrumented indentation: advances in understanding and refinements to methodology. j mater res 19(1):3–20, 2004. doi:10.1557/jmr.2004.0002. [10] p. haušild, a. materna, l. kocmanová, j. matějíček. determination of the individual phase properties from the measured grid indentation data. j mater res 31(22):3538–3548, 2016. doi:10.1557/jmr.2016.375. [11] p. haušild, j. čech, a. materna, j. matějíček. statistical treatment of grid indentation considering the effect of the interface and the microstructural length scale. mech mater 129:99 – 103, 2019. doi:10.1016/j.mechmat.2018.11.006. [12] w. nix, h. gao. indentation size effects in crystalline materials: a law for strain gradient plasticity. j mech phys solids 46(3):411–425, 1998. doi:10.1016/s0022-5096(97)00086-0. [13] a. p. zhilyaev, t. g. langdon. using high-pressure torsion for metal processing: fundamentals and applications. progress in mater sci 53(6):893 – 979, 2008. doi:10.1016/j.pmatsci.2008.03.002. [14] o. senkov, f. froes, v. stolyarov, et al. microstructure of aluminum-iron alloys subjected to severe plastic deformation. scripta mater 38(10):1511 – 1516, 1998. doi:.10.1016/s1359-6462(98)00073-6. 144 https://doi.org/10.1080/21663831.2014.912690 https://doi.org/10.1016/j.actamat.2016.08.081 https://doi.org/10.1016/j.jallcom.2018.05.067 https://doi.org/10.1016/j.jallcom.2017.03.352 https://doi.org/10.1016/j.jallcom.2018.07.319 https://doi.org/10.1016/j.actamat.2017.09.035 https://doi.org/{10.1016/j.jallcom.2015.07.209} https://doi.org/{10.1557/jmr.1992.1564} https://doi.org/{10.1557/jmr.2004.0002} https://doi.org/{10.1557/jmr.2016.375} https://doi.org/10.1016/j.mechmat.2018.11.006 https://doi.org/{10.1016/s0022-5096(97)00086-0} https://doi.org/10.1016/j.pmatsci.2008.03.002 https://doi.org/.10.1016/s1359-6462(98)00073-6 acta polytechnica 27(0):141–144, 2020 1 introduction 2 experimental details 2.1 material 2.2 nanoindentation 3 results and discussion 4 conclusions acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2017.13.0066 acta polytechnica ctu proceedings 13:66–70, 2017 © czech technical university in prague, 2017 available online at http://ojs.cvut.cz/ojs/index.php/app evaluation of micromechanical properties of carbon fiber fabric using nanoindetation pavel klapáleka,∗, zdeněk prošeka,b, aleš jíraa, lenka melzerováa a czech technical university in prague, faculty of civil engineering, thákurova 7, 166 29 prague 6, czech republic b university centre for energy efficient buildings, czech technical university in prague, třinecká 1024, 273 43 buštěhrad, czech republic ∗ corresponding author: pavel.klapalek@fsv.cvut.cz abstract. this paper is focused mainly on nanoindentation of carbon fibers. fibers are in form of carbon fiber fabric that is used in larger research that is focused on reinforcing beams made of glued laminated timber. knowledge of this material on macro and micro level will help to understand its behavior in this specific type of use. nanoindentation is method used in this paper to obtain material characteristics on micro level such as hardness and modulus of elasticity. samples of the carbon fiber fabric had to be prepared for this specific testing method by polishing samples of carbon fabric attached in epoxy resin. in particular, it was found that the indentation hardness of the fibers ranges around 3.65 gpa and modulus of elasticity ranges around 26 gpa. keywords: carbon fiber fabric, nanoindentation, micromechanical properties. 1. introduction this paper is part of extensive research focused on reinforcing glued laminated timber beams with use of carbon fiber fabric. because in nowadays civil engineering is the timber very popular material, as it is on rise again mainly because of the high demands on energy efficiency of buildings, it is appropriate to devote this material to try to find ways for its optimum utilization. when solid timber ceased to meet the demands of civil engineers, it was necessary to find a way to improve its performance. accordingly, there was also first thought to remove these drawbacks by means of gluing timber elements to create a composite material with better properties than solid timber. that is glued laminated timber [1]. it consists of layers of lamellas that are glued together under pressure, using a waterproof adhesive [2]. at present, this material is mainly used in modern timber structures and roof structures of halls. despite the frequent use of this material, the technology of production is still in development. for this reason, the main material, which is an essential part of this research is glued laminated timber reinforced with carbon fiber fabric. as we examined timber with nanoindentation before [3, 4], there was a need for values from carbon fibers [5]. carbon fibers history goes back to 1860, where joseph swan produced carbon fibers for the first time, for use in light bulbs [6]. in 1879, thomas edison baked cotton threads or bamboo slivers at high temperatures carbonizing them into an all-carbon fiber filament used in one of the first incandescent light bulbs to be heated by electricity [7]. but first real use in industry started in 1960 when richard millington of h.i. thompson fiberglas co. developed a process (us patent no. 3,294,489) for producing a high carbon content (99%) fiber using rayon as a precursor. these carbon fibers had sufficient strength (modulus of elasticity and tensile strength) to be used as a reinforcement for composites having high strength to weight properties and for high temperature resistant applications. in the coming years there were a lot of improvement in manufacturing processes and nowadays the carbon fibers are used in many sectors of industry (mostly in aviation, cars, composite materials etc.). carbon fiber fabric combined with glued laminated timber seems to be suitable for development in the civil engineering. 2. testing 2.1. tested material tested material. tested material was carbon fiber fabric that is used to reinforce glued laminated timber beams. this material was used for additional reinforcement of glt beams on the bottom surface. type of used fabric is carbon unidirectional ud cst 200 made of unidirectional carbon fiber with pes grid. it has thickness of 0.4 mm and grammage of 200 g/m2 (figure 1). samples of carbon fiber fabric were photographed with use of a scanning electron microscope (sem) to verify properties such as thickness and diameter of individual fibers. this carbon fiber fabric is made of layer of carbon fibers that are aligned in one direction. figure 2 is detail of carbon fabric magnified 250× and alignment of individual fibers is evident. at figure 3 is bunch of individual fibers magnified 5.7k×, diameter of each fiber is around 6 µm. 66 http://dx.doi.org/10.14311/app.2017.13.0066 http://ojs.cvut.cz/ojs/index.php/app vol. 13/2017 evaluation of micromechanical properties figure 1. second set of glued laminated timber beams. figure 2. carbon fiber fabric magnified 250×. to prepare samples of carbon fiber fabric for examination on nano/micro level with nanoindentation the polished slices had to be prepared. before grinding of samples, the fibers had to be attached with suitable substance to avoid fraying of fibers during sample preparation. for this purpose, a method of vacuum impregnation with epoxy resin (epofix kit) was used. grinded cuts were prepared with machine tegramin from company struers located at university center of energy efficient buildings. samples were grinded and polished in multiple steps to achieve the best quality of sample surface. in the first step, sandpaper with a grain size of 220 grains/cm2 was used to eliminate the greatest unevenness after cutting. in the following steps, finer sandpaper was used: 500 grains/cm2, 1200 grains/cm2, 2000 grains/cm2 and 4000 grains/cm2. each step lasted 5 minutes while water was used as a lubricant. 2.2. testing method – static nanoindentation nowadays is the static nanoindentation is widely used and it is a very popular experimental technique for measuring the elastic stiffness and hardness of various materials (glass, metal, wood, ceramics). advantage figure 3. carbon fiber fabric magnified 5.7k×. of this testing method is that it can performed on a very small volume, so it is suitable for investigating materials at microscale. the principle is based on imprinting a micrometer sized diamond tip into the investigated material and recording the loading force and the indentation depth [8]. in most of the measurements, the indentation depth is in the order of hundreds of nanometers. there are various tips available for nanoindentation, e.g. berkovich, wedge or spherical tips. also, several techniques can be used to ensure that the analysis also provides information about the plastic hardening, the viscosity or the yield stress of the material [9]. standardized data is based on the assumption of perfectly homogeneous isotropic material in offset volume, and elastic and non-elastic material parameters are usually derived from nanoindentation data using an analytical solution. the pioneering work of hertz (1882) [10] dealt with the imprint of an elastic tip in a homogeneous medium. sneddon (1965) [11] derived an analytical relationship between the loading force, the depth of an imprint and the contact area for individual indentation tips. typical output of measured nanoindentations consists of two elastic constants: hardness and stiffness parameters. the hardness parameter is defined as the contact pressure diameter at maximum load. during deformation, the deformed material deforms in both the elastic and the plastic range. the plastic response on the load-pull curve is excluded during unloading, allowing the user to determine the stiffness of the material in the local environment known as the reduced modulus of elasticity (figure 4 and 5). the value for the reduced modulus can therefore be obtained from the unloading part of the recorded force-displacement diagram [12, 13]. 2.3. measurement and results nanoindentation was performed by using a csm instruments device with wedge tip, located in at the faculty of civil engineering at ctu in prague. the results of the static loading are in set of nanoindentation curves. this curve describes the response of 67 p. klapálek, z. prošek, a. jíra, l. melzerová acta polytechnica ctu proceedings figure 4. typical indentation curve – load vs. depth. figure 5. scheme of the simulation under the indenter. the material to mechanical loading, in particular the relation between the loading force and the depth of penetration. loading diagram (figure 6) of standard controlled load test of an individual indent consisted of three segments: loading on maximum force fn, holding at the peak and unloading. applied load force was in three steps with. speed of loading and unloading force was 40/40 mn/min and 80/80 mn/min [14]. the reduced modulus can be related to the sample young‘s modulus e using the derived relations [9, 15]. for forces <12 mn, indents were standard, with no evidence of brittle fracture. the dispersion of values is about 10%, which is acceptable due to the not completely smooth sample and the nature of the analyzed material. results in form of hardness and modulus of elasticity (table 1) are for indentation at the cross-section of the fiber. evaluation of the mechanical properties in the longitudinal section was complicated by the high fiber hardness and their orientation. there has often been a shift of the indentor tip outside the surveyed area 3. conclusion the presented results obtained by the static indentation in form of modulus of elasticity (table 1) are 26.0± 2.3 gpa and hardness is 3.60± 0.35 gpa. the values are reasonable with respect to the properties of carbon fibers [6, 7]. it has been found that with a loading force greater than 12 mn, a brittle fracture of fibers occurs (figure 7) and it is not possible to determine the required values at these forces and the results are misleading and disorted. however, the results from the lower load stages are almost constant and are usable. in the next work, these nanoindentation results will be compared to the macromechanical properties of carbon fiber fabric and micromechanical properties od timber. acknowledgements the financial support of this outcome by the faculty of civil engineering, ctu in prague (sgs project no. sgs17/168/ohk1/3t/11 and no. sgs16/201/ohk1/3t/11) and by the ministry of education, youth and sports within national sustainability programme i, project no. lo1605 is gratefully acknowledged. the authors also thank the center for nanotechnology in civil engineering at the faculty of civil engineering, czech technical university in prague, and the joint laboratory of polymer nanofiber technologies of the institute of physics, academy of science of czech republic, and the faculty of civil engineering, czech technical university in prague. special thanks also go to company dektrade s.r.o. for providing us with glt beams and to company grm systems for providing carbon fiber fabric. 68 vol. 13/2017 evaluation of micromechanical properties loading force [mn] 5 10 15 loading/unloading speed [mn/min] 40/40 80/80 80/80 hardness [gpa] ∼3.6 ∼3.7 ∼1.0* standard error of hardness 0.32 0.39 0.18 modulus of elasticity hr [gpa] ∼26 ∼26 ∼13* standard error of modulus of elasticity 2.1 2.5 0.6 contact depth hc [nm] ∼570 ∼810 ∼1970* standard error of contact depth 28 37 90 note * for the vast majority of indentations the fiber has broken through the fracture the resulting value is misleading and distorted table 1. the results of indentations in the cross-section of the fibers – indented in the direction of the longitudinal axis of the fiber. figure 6. typical indentation curves for forces greater than 12 mn causing fracture failure in the sample. figure 7. image from the optical microscope of the broken fiber. figure 8. a sample of indent outside the fiber cut section. note: this is not a pure longitudinal cut, but a diagonal cut. 69 p. klapálek, z. prošek, a. jíra, l. melzerová acta polytechnica ctu proceedings references [1] r. bergman, z. cai, c. g. carll, et al. wood handbook, wood as an engineering material (all chapters). forest products laboratory. wood handbook–wood as an engineering material. general technical report fplgtr-190 madison, wi: us department of agriculture, forest service, forest products laboratory 2010. [2] v. angst. handbook 1: timber structures 2008. [3] p. klapálek, z. prošek, v. králík. comparison of modulus of elasticity of glued laminated timber. in key engineering materials, vol. 714, pp. 29–32. trans tech publ, 2016. [4] z. prošek, j. topič, p. tesárek, et al. micromechanical properties of spruce tissues using static nanoindentation and modulus mapping. in applied mechanics and materials, vol. 732, pp. 115–118. trans tech publ, 2015. [5] s. syed asif, k. wahl, r. colton, o. warren. quantitative imaging of nanoscale mechanical properties using hybrid nanoindentation and force modulation. journal of applied physics 90(3):1192–1200, 2001. [6] y. deng. carbon fiber electronic interconnects. ph.d. thesis, university of maryland, college park, 2007. [7] j. gorss. high performance carbon fibers national historic chemical landmarks. [8] s. s. asif, k. wahl, r. colton. nanoindentation and contact stiffness measurement using force modulation with a capacitive load-displacement transducer. review of scientific instruments 70(5):2408–2413, 1999. [9] a. fisher-cripps. nanoindentation, 2002. new york, springerverlag . [10] h. hertz. über die berührung fester elastischer körper und über die härte-verhandlungen des vereins zur beförderung des gewerbefleißes 1882. [11] i. n. sneddon. the relation between load and penetration in the axisymmetric boussinesq problem for a punch of arbitrary profile. international journal of engineering science 3(1):47–57, 1965. [12] i. w. bailey, t. kerr. the visible structure of the secondary wall and its significance in physical and chemical investigations of tracheary cells and fibers. journal of the arnold arboretum 16(3):273–300, 1935. [13] z. prošek, v. králík, j. topič, et al. microstructure description and micromechanical properties of spruce wood. acta polytechnica 55(1):39–49, 2015. [14] p. hájková, a. jíra. micromechanical analysis of complex structures by nanoindentation. in applied mechanics and materials, vol. 731, pp. 60–65. trans tech publ, 2017. [15] z. prošek, j. topič, p. tesárek, et al. micromechanical properties of spruce tissues using static nanoindentation and modulus mapping. in applied mechanics and materials, vol. 732, pp. 115–118. trans tech publ, 2015. 70 acta polytechnica ctu proceedings 13:67–71, 2017 1 introduction 2 testing 2.1 tested material 2.2 testing method – static nanoindentation 2.3 measurement and results 3 conclusion acknowledgements references acta polytechnica doi:10.14311/app.2020.27.0018 acta polytechnica 27(0):18–21, 2020 © czech technical university in prague, 2020 available online at http://ojs.cvut.cz/ojs/index.php/app mathematical modeling of experiment on berkovich nanoindentation of zrn coating on steel substrate evgeniy sadyrin∗, andrey vasiliev, sergei volkov don state technical university, research and educational center “materials ”, 1 gagarin square, rostov-on-don, 344000, russia ∗ corresponding author: ghostwoode@gmail.com abstract. in the present paper the experiment on berkovich nanoindentation of zrn coating on steel substrate is modelled using the proposed effective mathematical model. the model is intended for describing the experiments on indentation of samples with coatings (layered or functionally graded). the model is based on approximated analytical solution of the contact problem on indentation of an elastic half-space with a coating by a punch. it is shown that the results of the model and the experiment are in good agreement. keywords: coating, contact mechanics, layered composite, mathematical modeling, nanoindentation, scanning electron microscopy. 1. introduction nanoindentation is a widespread technique aimed at researching of the physical and mechanical properties of materials [1]. during experiment the dependence of the penetration depth of the special probe – indenter – into the sample on the applied load is acquired (loaddisplacement curve). for different purposes indenters with different tip shapes are required. a pyramid with a triangular cross-section and a spherical blunting of the tip (berkovich indenter) is one of the most popular shapes. since it is difficult to avoid plastic deformation in such an experiment even with a small load, oliver and pharr proposed to analyze the unloading part of the load-displacement curve to determine the elastic properties of the sample [2]. they described the method for evaluation of the young’s modulus of the sample based on the analysis of the indentation stiffness – the derivative of the force with respect to the indentation depth – at the maximum indentation depth. in such a case, the contact area must be determined. for homogeneous materials, contact area can be found from calibration experiments on samples with previously studied properties [3]. however, it was shown [4, 5] that the presence of an inhomogeneous or layered coating on the material significantly influences the size of the contact area. one can determine elastic moduli of an isotropic elastic material by establishing the real indentation contact area [6] but it is nearly impossible for submicron indentation depth. in the present study, an effective mathematical model, taking into account the layered or continuously inhomogeneous structure of the sample, is proposed for the description of nanoindentation experiment of the zrn coating on the steel substrate. zrn coatings are used in practice to improve the ability of various tools for drilling and milling aluminum and titanium alloys [7, 8], for increasing hardness and abrasive resistance of surgical tools [9], decorative purposes [10] etc. the results of the mathematical modeling are compared to the results of nanoindentation experiment. 2. materials prior to coating deposition, the preparation of steel 3cr3mo3v (analogue to h10 [11]) substrate was carried out by a standard metallographical method using linear precision saw isomet 4000 (buehler, usa) and grinding-polishing machine metaserv 250 (buehler, prc). the substrate was mirror polished in several steps with the abrasive particles of various diameters (down to 0.05 µm). the zrn coating was deposited on the ion-plasma sputtering unit bulat 6. the substrate was heated to 550-600 ◦c, ionic surface cleaning was conducted, then deposition was carried out at 60-65 v supply voltage of the substrate and 130-140 a current. the microgeometrical characteristics of the sample were studied for using contour elite 3d optical microscope (bruker, belgium). figure 1 shows surface profiles constructed in the software vision64 (bruker, belgium) in horizontal and vertical directions (filter type: gaussian regression, long cutoff 250 µm). the values for the average roughness ra and the maximum roughness height rt were calculated according iso 4287 [12] standard and are summarized in the table 1. these values were used for choosing optimal parameters for the nanoindentation experiment. the extremely high rt values can be explained by the presence of the artefacts of deposition on the surface of the coating: caverns and crystallized droplets (figure 2a). one of the crucial characteristics for the mathematical modelling, the coating thickness, was determined using ion beam etching on the scanning electron microscope (sem). its value reached 2.7 µm after ap18 http://dx.doi.org/10.14311/app.2020.27.0018 http://ojs.cvut.cz/ojs/index.php/app vol. 27/2020 mathematical modeling zrn coating figure 1. surface profile of zrn coating across directions: a) horizontal; b) vertical. characteristic horizontal direction, [m] vertical direction, [µm] ra 0.073 0.052 rt 0.984 0.73 table 1. zrn coating microgeometrical characteristics. a) b) figure 2. zrn coating research using sem: a) artefacts of deposition; b) coating thickness. propriate tilt correction in the software of the device (figure 2b). 3. methods nanoindentation research was conducted on the nanotest 600 platform 3 (micro materials, uk) device. all the experiments were made in the closed chamber at a constant temperature of 27.0 ± 0.1 ◦c, the berkovich indenter with a diamond tip was used. the following load profile was applied: the load linearly increased for 30 seconds, held constant for 30 seconds, then linearly decreased for 30 seconds. before the coating investigation the value for the young’s modulus e of the substrate was obtained for the needs of mathematical modelling, it reached 267.11 ± 10.65 gpa. the value of poisson’s ratio of the steel substrate was taken ν = 0.3 [13]. the series of nanoindentation experiments was conducted for different maximum loads pmax: load varied within 5 mn ≤ pmax ≤ 280 mn. the corresponding maximum indentation depth hmax was as follows: 65 nm ≤ hmax ≤ 1414 nm. for each pmax, 8 to 12 identical indentations were carried at different locations, the results were averaged. 4. results and discussion the experimentally collected characteristics allowed us to perform the mathematical modelling of the indentation process. the berkovich indenter was modeled by an elastic body of conical shape with the elastic moduli epunch = 1131 gpa and νpunch = 0.07. the zrn on the steel substrate system was modeled by a half-space with a coating since the contact area between indenter and the sample is much smaller than the sample size. the contact problem was reduced to the solution of the dual integral equation, the solution of which was constructed by the authors earlier [14]. using the bilateral asymptotic method, the approximated analytical expressions for the indentation stiffness, force and indentation depth were obtained (1)− (3). in the formulas above a− the radius of the contact area (m);α− the half of the angle of the conical punch opening; λ = h/a− the dimensionless thickness of the coating; h− the coating thickness (m); e (s) ef = e/ ( 1− v2)− the effective elastic modulus of the substrate (pa); ai, ci, di− the constants depending on the elastic properties of the coating and the substrate [14]. expressions (1)-(3) are asymptotically exact for λ → 0 and λ → ∞. the functions p0(λ), δ0(λ) describe the influence of the coating on the characteristics of the contact. it is clear that if the coating thickness tends to zero ( λ ⇒ 0), then p0(λ)→ 1, δ0(λ)→ 1, the formulas (1) coincide with the formulas for the homogeneous half-space without coating [15]. the conical punch opening angle was chosen similarly to [3] on the basis of the following considerations. let abeek c − the contact area during the berkovich indentation, obtained from the calibrations of the device: aperk c = 23, 33χ2 + 3213, 76χ, and acon c − the contact area during the conical punch indentation. from geometric considerations acon c = 19 e. sadyrin, a. vasiliev, s. volkov acta polytechnica s = 2ae(s) ef p0(λ) δ0(λ) , p = e (s) ef aχ 2 p0(λ), δ = πχ 2 δ0(λ), χ = a ctgα (1) p0(λ) = 1 + 2λ n∑ i=1 ( ci ai [ ch ( ai λ ) − λ ai sh ( ai λ )] + di ai [ λ ai − λ ai ch ( ai λ ) + sh ( ai λ )]) (2) δ0(λ) = 1 + n∑ i=1 λa−1 i ( ci ch ( aiλ −1)+di sh ( aiλ −1)) (3) πa2 = πχ2/ cot2 α. equating aberk c = acon c , we obtained: α = 180 π arccot ( 17, 725√χ √ 2333χ+ 321300 ) (4) figure 3 shows the graphs of the indentation stiffness as the function of the maximum indentation depth hmax . it is clearly seen, that the experiment and the theory agree well on the entire investigated range (even despite the steps associated with the destruction of the sample were observed on the loading curves at ∼500 nm and ∼800 nm indentation depths). figure 3. dependence of the indentation stiffness s on the maximum indentation depth for the zrn coating on the steel substrate according to mathematical modelling and nanoindentation tests 5. conclusions the paper presents the second step in testing of the mathematical modelling for describing the results of the nanoindentation experiments on zrn coatings [16]. the results obtained show that the chosen mathematical model gives possibility to effectively describe the nanoindentation experiments with zrn coatings deposited on steel substrates. as a consequence, the proposed mathematical model may be used to reconstruct young’s modulus of such coatings for improving the technology of zrn coating deposition. in this regard, the further research concerning simplification of the model proposed (so the kernel transform to be approximated by a ratio of two quadratic functions containing only one parameter) for the engineering applications [17] is of significant interest. acknowledgements this work was supported by the russian science foundation (rsf) through grant no. 19-19-00444. e.v. sadyrin was supported by the scholarship of the president of the russian federation no. sp-3672.2018.1. experiments on nanoindentation were made in nanocenter of research and education center "materials", don state technical university (http://nano.donstu.ru). references [1] a. c. fischer-cripps. nanoindentation testing. in nanoindentation, pp. 20–35. springer new york, 2002. doi:10.1007/978-0-387-22462-6_2. [2] w. oliver, g. pharr. an improved technique for determining hardness and elastic modulus using load and displacement sensing indentation experiments. journal of materials research 7(6):1564–1583, 1992. doi:10.1557/jmr.1992.1564. [3] y. i. golovin. nanoindentation and its capabilities. mashinostroyeniye 2009. [4] a. vasiliev, s. volkov, a. belov, et al. indentation of a hard transversely isotropic functionally graded coating by a conical indenter. international journal of engineering science 112:63–75, 2017. doi:10.1016/j.ijengsci.2016.12.002. [5] e. v. sadyrin, b. i. mitrin, l. i. krenev, et al. evaluation of mechanical properties of the two-layer coating using nanoindentation and mathematical modeling. in springer proceedings in physics, pp. 495–502. springer international publishing, 2018. doi:10.1007/978-3-319-78919-4_39. [6] v. b. zelentsov, e. v. sadyrin, a. g. sukiyazov, n. y. shubchinskaya. on a method for determination of poisson’s ratio and young modulus of a material. matec web of conferences 226:03027, 2018. doi:10.1051/matecconf/201822603027. [7] j. vetter. vacuum arc coatings for tools: potential and application. surface and coatings technology 7677:719–724, 1995. doi:10.1016/0257-8972(95)02499-9. 20 https://doi.org/10.1007/978-0-387-22462-6_2 https://doi.org/10.1557/jmr.1992.1564 https://doi.org/10.1016/j.ijengsci.2016.12.002 https://doi.org/10.1007/978-3-319-78919-4_39 https://doi.org/10.1051/matecconf/201822603027 https://doi.org/10.1016/0257-8972(95)02499-9 vol. 27/2020 mathematical modeling zrn coating [8] h. randhawa, p. johnson. technical note: a review of cathodic arc plasma deposition processes and their applications. surface and coatings technology 31(4):303–318, 1987. doi:10.1016/0257-8972(87)90157-5. [9] j. kadlec, z. joska, j. kadlec. study of biocompatible zrn and zrn/dlc coating deposited on medical tools. ecs transactions 48(1):315–318, 2014. doi:10.1149/04801.0315ecst. [10] m. nose, m. zhou, e. honbo, et al. colorimetric properties of zrn and tin coatings prepared by dc reactive sputtering. surface and coatings technology 142-144:211–217, 2001. doi:10.1016/s0257-8972(01)01196-3. [11] t. okuno. effect of microstructure on the toughness of hot work tool steels, aisi h13, h10, and h19. transactions of the iron and steel institute of japan 27(1):51–59, 1987. doi:10.2355/isijinternational1966.27.51. [12] geometrical product specifications (gps) – surface texture: profile method – terms, definitions and surface texture parameters. standard, international organization for standardization, geneva, ch, 1997. [13] v. bhandari. machine design data book. mcgraw-hill education, 2014. [14] s. aizikovich, a. vasil’ev, s. volkov. the axisymmetric contact problem of the indentation of a conical punch into a half-space with a coating inhomogeneous in depth. journal of applied mathematics and mechanics 79(5):500–505, 2015. doi:10.1016/j.jappmathmech.2016.03.011. [15] i. n. sneddon. fourier transforms mcgraw-hill. new york pp. 445–49, 1951. [16] a. s. vasiliev, e. v. sadyrin, b. i. mitrin, et al. nanoindentation of zrn coatings on silicon and copper substrates. russian engineering research 38(9):735–737, 2018. doi:10.3103/s1068798x18090289. [17] e. v. sadyrin, a. s. vasiliev, s. s. volkov, et al. simplified analytical solution of the contact problem on indentation of a coated half-space by a spherical punch. in boundary elements and other mesh reduction methods xli. wit press, 2018. doi:10.2495/be410191. 21 https://doi.org/10.1016/0257-8972(87)90157-5 https://doi.org/10.1149/04801.0315ecst https://doi.org/10.1016/s0257-8972(01)01196-3 https://doi.org/10.2355/isijinternational1966.27.51 https://doi.org/10.1016/j.jappmathmech.2016.03.011 https://doi.org/10.3103/s1068798x18090289 https://doi.org/10.2495/be410191 acta polytechnica 27(0):18–21, 2020 1 introduction 2 materials 3 methods 4 results and discussion 5 conclusions acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2019.24.0009 acta polytechnica ctu proceedings 24:9–14, 2019 © czech technical university in prague, 2019 available online at http://ojs.cvut.cz/ojs/index.php/app coated cladding behavior during high-temperature transients adéla chalupováa,b,∗, jakub krejčíb, ladislav cvrčekc, martin ševečeka, vojtěch rozkošnýb, antonín přibylb, patricie halodovád, petra gávelovád a czech technical university in prague, faculty of nuclear sciences and physical engineering, department of nuclear reactors, v holešovičkách 2, 180 00 prague 8, czech republic b ujp praha a.s., nad kamínkou 1345, 156 10 prague – zbraslav, czech republic c czech technical university in prague, faculty of mechanical engineering, department of materials engineering, karlovo náměstí 13, 121 35 prague 2, czech republic d research centre řež, hlavní 130, 250 68 husinec-řež, czech republic ∗ corresponding author: chaluade@fjfi.cvut.cz abstract. the concept of accident-tolerant fuels has been proposed and widely investigated over the past decade. the contribution is focused on one of the near-term approaches the modification of the surface of existing zr-alloy claddings by protective coatings. the studied specimens are based on zr-1%nb substrate with chromium coating, multicomponent chromium nitride and chromium coating and multi-layer chromium nitride/chromium coating deposited by a physical vapor deposition process. thermomechanical tests were designed to understand the cladding deformation and the burst conditions during the loca phenomena. presented results show both the positive and the negative effect of coating on cladding behavior. all coatings exhibited a reduction of ballooning size and prolongation of time to burst. on the contrary, coating can be connected with larger opening size after burst, higher hydrogen content and deterioration of the local mechanical properties. keywords: nuclear fuel, fuel cladding, loca, atf, coating, pvd. 1. introduction one of the main candidates for the near-term accident tolerant fuels (atf) are coated cladding materials. surface modifications by specific technologies and various types of coatings have been recently adopted to treat and improve the surface of zr-based alloys used as cladding materials. the ultimate objective of their development and implementation is to provide longer time before core degradation in case of severe accidents of light water reactors (lwrs) while maintaining or improving the performance during normal operation and transients [1, 2]. cr-based coatings are currently the most promising from the perspective of both steady-state and accidental performance of lwrs [3–5]. during loca transient in wwer reactors, nuclear fuel cladding faces two high-temperature peaks. the first peak can reach temperatures above 1000 ◦c for a short time with inner over-pressure ranging between 1 to 2 mpa. the temperature during the second peak is about 800 ◦c and the cladding is exposed to significantly higher inner over-pressure up to 8 mpa. the cladding performance during high-temperature transients defined by both peaks with their characteristic parameters were studied and the results are presented. coated as well as uncoated reference zr-based materials are included. the present paper follows the previous articles focused on mechanical and oxidation behavior of coated cladding material [6–8]. 2. methods 2.1. material and coatings all experiments were performed with the reference material zr-1%nb, which is used mainly in pressurized water reactors as the fuel cladding material. nonirradiated segments with the outer diameter of 9.1 mm and length of 90 mm were tested. the thickness of the cladding’s wall was approximately 0.6 mm. the tests were carried out with reference material as well as with the coated samples. as the coatings to be tested, chromium, multicomponent chromium nitride and chromium (crn+cr) and multi-layer chromium nitride/chromium coating (crn/cr) were selected. coatings were deposited by magnetron sputtering from (99.6 %) cr targets in the semi-industrial system hauzer flexicoat 850. before deposition, the samples were ultrasonically cleaned in acetone, ethanol, demineralized water and dried in a furnace. their surfaces were also cleaned by ion etching in the argon plasma. this process removes the thin zr-oxide and other impurities in order to improve adhesion of the coating. coatings were subsequently deposited on outer surfaces in different atmospheres depending on the presence and concentration of nitrogen. pure chromium coatings were prepared in 9 http://dx.doi.org/10.14311/app.2019.24.0009 http://ojs.cvut.cz/ojs/index.php/app a. chalupová, j. krejčí, l. cvrček et al. acta polytechnica ctu proceedings coating cr crn+cr crn/cr layer 1: cr [µm] 15.3 11.4 10.8 layer 2: crn [µm] 5.3 1.6 layer 3: cr [µm] 1.2 layer 4: crn [µm] 1.2 layer 5: cr [µm] 1.0 layer 6: cr [µm] 1.3 layer 7: cr [µm] 0.6 total 15.3 16.7 17.6 table 1. measured thicknesses of studied coatings figure 1. burst-test specimen before exposition in argon argon atmosphere, whereas crn coatings in the gas mixture of argon and nitrogen. the temperature of deposition was around 250 ◦c. a thin metallic layer was deposited on the substrate first in all cases to improve adhesion of the coating. thicknesses of the deposited layers are presented in table 1. 2.2. burst test both the reference and the coated specimens were first sealed using end-plugs and then connected to the argon containing reservoir at required gas pressure. each specimen was equipped with k-thermocouple as shown in figure 1. afterwards, the specimen was inserted into the preheated resistance furnace with predefined temperature zone (± 3 ◦c) which is about 11 cm long. the experimental apparatus is shown in figure 2. temperature and inner gas pressure propagation were measured. the experiments were done at several temperature levels between 600 and 1100 ◦c and with inner pressure from 1 to 10 mpa, in an argon environment. the experiments were terminated after cladding failure and consequent pressure decrease. 2.3. material characterization electron backscatter diffraction (ebsd) map was obtained for coatings in the as-received state using figure 2. burst-test experimental apparatus the tescan lyra gmu scanning electron microscope equipped with energy dispersive x-ray spectroscopy (eds) x-maxn detector and nordlys ii ebsd detector (oxford instruments). the obtained data were post-processed in the "hkl channel 5" software package. after burst-test, the cladding diameter was measured in the position of the maximal deformation as well as in the position of 20 mm from the burstopening (same as in work of forgeron et al. [9]), where uniform clad deformation occurs. the measurements were carried out both using paper stickers and micrometer. the diameter obtained by measuring with paper stick reflects the creep phenomena while the diameter measured by micrometer has its relevance in potential water flow blockage inside an assembly channel. the deformation in the other cross-sections was evaluated using five diameter measurements with a micrometer. the width and length of the failure was measured using caliper. then, specimens were cut into several rings for further investigation. the metallographic cross sections were prepared using the standard polishing procedure and metallographic evaluation followed. the thickness of the coating, zro2 layer and the α-zr(o) layer were evaluated. the optical microscope nikon elipse ma200 and lucia g image analyzer were used for 10 vol. 24/2019 coated cladding behavior during high-temperature transients figure 3. time to burst at temperature 800 ◦c in relation to the hoop stress for different types of coatings and uncoated specimens the metallographic analysis. 3. results to analyze data and to compare them with other data obtained, the actual hoop stress σθθ [mpa] was calculated according to the equation 1: σθθ = p (d − t) 2t (1) where p [mpa] is the difference between the internal and the external pressure, d is the external cladding diameter and t is its thickness. 3.1. time to burst first set of tests was simulating the first peak of loca transient. the specimens were inserted into the furnace pre-heated at 1100 ◦c (heating rate > 25 ◦c/sec up to 1000 ◦c). table 2 shows the results. the second set of experiments focuses on the second loca temperature peak. the specimens were inserted into the furnace pre-heated at 800 ◦c and were exposed at constant temperature. the main goal was to identify the time to burst. the results of the experiments are summarized in figure 3. all of the coated specimens were more resistant then uncoated specimens. both, time to burst and burst temperature increased when comparing to the reference samples. 3.2. cladding deformation figure 4 shows the maximal deformation at temperature 1100 ◦c in relation to the hoop stress. the comparison of maximal deformation for coated and uncoated specimens at 800 ◦c is shown in figure 5. the size of deformation reduced significantly for all coated specimens. the biggest reduction was for metallic cr-coated specimens. deformation along the length of the specimen has its relevance in water flow blockage in an assembly. comparison of coated and uncoated specimens deformation after exposure to temperature 800 ◦c and inner pressures of 4 mpa, 6 mpa and 10 mpa are shown figure 4. maximal deformation at temperature 1100 ◦c in relation to hoop stress for different types of coatings and uncoated specimens figure 5. maximal deformation at temperature 800 ◦c in relation to hoop stress for different types of coatings and uncoated specimens in figures 6, 7, 8. the biggest reduction of deformation was for metallic cr-coated specimens. multi-layer crn/cr coating also significantly reduced deformation of the cladding in all cases. multicomponent crn+cr coating indicates comparable deformation as the reference uncoated sample. the size of opening was measured for the coated specimens and reference uncoated specimens as well. the results are summarized in figure 9. it seems that two mechanisms of cladding failure occur. crcoated specimens show a low concentration of long cracks with significantly larger openings, whereas on crn+cr coated specimens is indicated a larger quantity of shorter cracks with smaller openings. this phenomenon is shown in figure 10. measured deformation in the position of 20 mm from the burst-opening, where uniform clad deformation occurs was used to determine creep rate. results are shown in figure 11. the rapid discontinuity in creep rate is caused by exposition time and α-zr to β-zr transformation. when hoop stress exceeds 40 mpa, time to burst is short and the alloy partially stays in α-zr phase with low creep rate. whereas for hoop stress lower than 40 mpa, time to burst is prolonged and alloy forms β-zr structure which causes 11 a. chalupová, j. krejčí, l. cvrček et al. acta polytechnica ctu proceedings specimen inner pressure [mpa] time to burst [s] burst temperature [◦c] uncoated 3 13.8 1047 uncoated 4 12.7 1061 cr 2 34.0 1079 cr 4 12.7 1029 crn+cr 2 22.6 1017 crn+cr 4 17.9 1051 crn/cr 2 22.6 1040 crn/cr 4 17.9 998 table 2. time to burst at temperature 1100 ◦c in relation to the hoop stress for different types of coatings and uncoated specimens figure 6. deformation at temperature 800 ◦c under inner pressure of 4 mpa in relation to position for different types of coatings and uncoated specimens higher creep rate. 4. discussion as expected, there are many positives related to cr coating usage such as reduction of ballooning size and prolongation of time to burst. in the case of using cr and crn combined layers the ballooning reduction and time to burst are slightly lower compared to metallic chromium but still beneficial. for all coated specimens it was observed that after reaching some level of cladding deformation, the coating cracks and the creep rate locally increases. therefore, for most of the tested specimens the opening size was bigger for coated than for uncoated specimens. coated cladding is locally thinned in the cracked area and the stress is localized. afterward, during the burst it is torn along the whole crack length. thus it is needed to consider the danger of fuel relocation from a cladding provided that burst openings are larger for coated specimens. in 2016, a set of isothermal internal pressure creep and preliminary temperature ramp tests were perfigure 7. deformation at temperature 800 ◦c under inner pressure of 6 mpa in relation to position for different types of coatings and uncoated specimens formed at cea on cr coated zircaloy-4 cladding segments, in a steam environment. [10] it was shown, that the burst resistance of the cr-coated cladding was improved over the non-coated cladding. the chromium coating was still fully adherent after having experienced ballooning and burst. the results of time to burst and maximal deformation provided by cea are in good agreement with experiments performed at ujp praha a.s. on chromium coated specimens. however, the author in [10] claims that chromium coating causes smaller burst openings when exposed to temperatures above 800 ◦c. experiments presented in this paper show the opposite effect. the burst opening of metallic chromium coated specimens appears to be larger compared to uncoated specimens. in the case of ceramic coatings, burst openings are smaller compared to uncoated specimens. the burst opening may be influenced by α-zr to β-zr transformation and thus on the heating rate as well as deposition method and thickness of the coating. 12 vol. 24/2019 coated cladding behavior during high-temperature transients figure 8. deformation at temperature 800 ◦c under inner pressure of 10 mpa in relation to position for different types of coatings and uncoated specimens figure 9. opening size at temperature 800 ◦c in relation to hoop stress for different types of coatings and uncoated specimens 5. conclusions presented results show the influence of studied coatings on high-temperature cladding creep and related phenomena such as ballooning size, time to burst and cracking of the coating. there are many benefits related to coating usage such as reduction of the ballooning and prolongation of time to burst. furthermore, oxidation from the outer side of the coating is reduced and hydrogen release is limited. nevertheless, some negative effects were identified as well. the coating may be connected with larger opening size, the hydrogen content in the substrate is higher due to oxidation through cracks and the local mechanical properties can be deteriorated due to cr diffusion into the substrate in the cracked areas. it is also necessary to mention that burst-tests examine only a part of cladding material performance and for further description of coated cladding behavior more complex loca experiments are needed. figure 10. coated specimen with cr (above) and crn+cr (below) after exposure to 800 ◦c and inner pressure 6 mpa figure 11. creep rate at temperature 800 ◦c in relation to hoop stress for different types of coatings and uncoated specimens acknowledgements the authors thank the whole ujp-zirconium department (namely: j. šustr, j. kabátová, d. rada, f. manoch, a. přibyl, and v. vrtílková) for sample preparation, and post-experimental evaluation. financial support of this research through čez a.s. company is gratefully acknowledged. also, this work was supported by the ministry of education, youth and sport of the czech republic, programme npu1, project no lo1207, national programme of sustainability ii project lq1603 (research for susen), and iaea crp 21065, technology agency of the czech republic grant no. th02020477, th02020691, cz.2.16/3.1.00/21563. references [1] s. j. zinkle, k. a. terrani, j. c. gehin, et al. accident tolerant fuels for lwrs: a perspective. j nucl mater 488(1-3):374–379, 2014. [2] n. oecd. state-of-the-art report on light water reactor accident-tolerant fuels 2018. [3] d. j. park, h.-g. kim, y. il jung, et al. behavior of an improved zr fuel cladding with oxidation resistant 13 a. chalupová, j. krejčí, l. cvrček et al. acta polytechnica ctu proceedings coating under loss-of-coolant accident conditions. journal of nuclear materials 482, 2016. doi:10.1016/j.jnucmat.2016.10.021. [4] c. tang, m. s. . h. j. seifert, m. steinbrück. protective coatings on zirconium-based alloys as accident-tolerant fuel (atf) claddings. corrosion reviews (35), 2017. [5] j.-c. brachet, c. lorrette, a. michaux, et al. cea studies on advanced nuclear fuel claddings for enhanced accident tolerant lwrs fuel (loca and beyond loca conditions). 2014. doi:10.13140/2.1.5105.6325. [6] j. krejčí, m. ševeček, l. cvrček, et al. chromium and chromium nitride coated cladding for nuclear reactor fuel. eurocorr 2017. [7] j. krejčí, m. ševeček, j. kabátová, et al. experimental behavior of chromium-based coatings. 2018. [8] j. krejčí, m. ševeček, l. cvrček. development of chromium and chromium nitride coated cladding for vver reactors. wrfpm 2017. [9] t. forgeron, j.-c. brachet, f. barcelo, et al. experiment and modeling of advanced fuel rod cladding behavior under loca conditions: alpha-beta phase transformation kinetics and edgar methodology. astm special technical publication pp. 256–278, 2000. doi:10.1520/stp14303s. [10] j. brachet, m. l. saux, m. dumerval, et al. behavior under loca conditions of enhanced accident tolerant chromium coated zircaloy-4 claddings 6:2–3, 2016. 14 http://dx.doi.org/10.1016/j.jnucmat.2016.10.021 http://dx.doi.org/10.13140/2.1.5105.6325 http://dx.doi.org/10.1520/stp14303s acta polytechnica ctu proceedings 24:1–6, 2019 1 introduction 2 methods 2.1 material and coatings 2.2 burst test 2.3 material characterization 3 results 3.1 time to burst 3.2 cladding deformation 4 discussion 5 conclusions acknowledgements references acta polytechnica doi:10.14311/app.2020.27.0136 acta polytechnica 27(0):136–140, 2020 © czech technical university in prague, 2020 available online at http://ojs.cvut.cz/ojs/index.php/app comparison of two techniques for evaluating spherical indentation data aleš materna∗, petr haušild, jan ondráček czech technical university in prague, faculty of nuclear sciences and physical engineering, department of materials, trojanova 13, praha 2, czech republic ∗ corresponding author: ales.materna@fjfi.cvut.cz abstract. flow curves of 15kh2mfa, sv 08kh19n10g2b and 08kh18n10t steels used for fabrication of wwer-440 nuclear reactor pressure vessel and core internals were obtained using the automated ball indentation (abi) test technique and compared with flow curves evaluated from the same measured load-displacement data and widely used oliver-pharr method. differences in results obtained by both studied methods do not exceed 12 % and are attributed to the amount of material pile-up. keywords: automatic ball indentation, oliver-pharr method, pile-up, true stress-strain curve. 1. introduction perhaps the most important test of a material’s mechanical response is the tensile test. for small volume of material, from which the standard tensile test specimens cannot be manufactured, one can use an instrumented indentation. for estimation of a stress-strain curve from indentation data, a spherical indenter has advantage among others with e.g. conical or pyramidal shapes. contact angle of spherical indenter changes with loading (contrary to conical or pyramidal indenters), which allows determination of more than one point of the stress-strain curve. on the other hand, the shape of the ball limits the achieved strains to 0.2. at this limit value, both indent and ball diameters equal. there are numerous models available to reduce the spherical indentation data to uniaxial stress-strain behavior of the test material (e.g. [1–3]). they differ in the level of physical simplification and in the computational complexity. the accuracy of such models relies substantially on proper determination of the contact area between the indenter and test material. the simple model based on the well-known hertz law is suitable for low indentation forces, when the deformations under the indenter are elastic. a more advanced method was introduced by oliver and pharr (op) [4], which calculates the projected contact area from elastic contact stiffness at peak load. this method, however, can lead to underestimation of the true contact area for materials exhibiting pile-up [5, 6]. to account for the material pile-up, the partial unloading technique and set of iterative equations was introduced by haggag [7] in automatic ball indentation (abi) testing. after determination of the contact radius, two tabor’s expressions [8] can be used to obtain representative stress and strain. these indentation values should be equivalent to true stress-strain uniaxial ones. in this work, the op and the abi techniques are compared on data sets obtained during the standard abi test procedure with progressive indentation and intermediate partial unloadings. all materials involved in this study are used for manufacturing of pressure vessel and core internals of nuclear reactor of wwer-440 type. 2. tested materials three various steels present at wwer 440 nuclear power plant reactor were involved in this study: (i) chromium-molybdenum-vanadium low alloy 15kh2mfa steel, which is used as pressure vessel base metal, (ii) 19 chromium/10 nickel niobium-stabilized austenitic stainless steel sv 08kh19n10g2b, which is used as pressure vessel outer layer cladding, and, (iii) chromium-nickel titanium-stabilized austenitic stainless steel 08kh18n10t, which is used as main structural material for the reactor internals (namely the core barrel, the core basket and the block of guide tubes). 08kh18n10t steel was studied in two deformation states: (a) as manufactured, and, (b) colddeformed with the thickness reduction 20 %. 3. methods and analysis mechanical properties of all test materials were evaluated from load p displacement h curve recorded during a progressive ball indentation with multiple intermediate partial-unloadings at one location on specimen’s surface. as a basis for evaluation, tabor’s relations between the representative strain εr and the representative stress σr on left sides and the contact diameter d and the applied force p during spherical indentation on the right side were used: εr = 0.2 d d , (1) σr = 4p δπd2 , (2) 136 http://dx.doi.org/10.14311/app.2020.27.0136 http://ojs.cvut.cz/ojs/index.php/app vol. 27/2020 two techniques of spherical indentation data evaluation where d is the indenter diameter, and δ is the constraint factor. it follows from equation (1) that strain up to a maximum value of 0.2 can be achieved. in that limit case the ball penetrates material surface to its full diameter (d = d). representative values εr and σr are regarded as material true plastic strain εp and true stress σt. during the instrumented indentation, contact diameter d is not directly measured and must be approximated from p h curve. there are numerous approaches to that and two of them were chosen in this work for comparison. 3.1. oliver-pharr method the most widely used method for determining elastic modulus from instrumented indentation data is oliver-pharr (op) method [4], which evaluates contact diameter d as intermediate result from the contact depth hc according to equations d = 2 √ hc (d − hc), (3) hc = hmax − ε pmax s . (4) here, pmax is the force at maximum indentation depth hmax, s=dp/dh|h = hmax the initial unloading slope, and, constant ε = 0.75 for spherical indenter. the constraint factor δ in equation (2) was set to 3 in this study. 3.2. haggag method haggag in his abi test procedure [7] uses tabor’s equations (1) and (2), iteratively computed plastic indentation diameter dp (after unloading) instead of d, and, the constraint factor δ in in equation (2) dependent on the stage of deformation beneath the indenter. the value of dp comes from the hertz’s classical theory and has the form dp = 3 √√√√√√0.5cd h2 p + ( dp 2 )2 h2 p + ( dp 2 )2 − hpd , (5) where the indentation depth after unloading hp is extrapolated from measured unloading data and c = 5.47 · p ( 1 e1 + 1 e2 ) , (6) where e1 and e2 are the young’s moduli for the indenter and test material, respectively. according to statistical analysis of many experimental data it was proposed by francis [8] that the constraint factor δ in equation 2 is a function of the indentation variable φ = εpe2 0.43σt . (7) contrary to francis’s work, the abi procedure uses slightly modified functions for the constraint in the form: δ =  1.12 φ ≤ 1 1.12 + τ ln (φ) 1 < φ ≤ 27 δmax φ > 27 (8) where δmax = 2.87αm, τ = (δmax 1.12)/ln(27) and αm is the constraint factor index whose value varies between 0.9 and 1.25 for various structural steels. since no experimental data were available for calibration, value αm = 1.1 in the middle of the interval was used. 4. experimental details 4.1. mechanical testing all abi tests were carried out at room temperature using inspekt 20 kn testing machine and custommade testing device equipped with the replaceable tungsten-carbide ball and with two-arm extensometer for the measuring of the ball displacements. two different ball diameters were used: 2.5 mm for testing of 15kh2mfa and sv 08kh19n10g2b steels, 1.5 mm for 08kh18n10t steel. indentation loading sequence for each test material is evident from figures 1-3. for comparison purposes, standard tensile tests at room temperature using inspekt 100 kn testing machine were performed for 15kh2mfa and sv 08kh19n10g2b steels. tensile test specimens of 6 mm diameter were oriented in circumferential direction of the pressure vessel that corresponds to the direction of the indentation. 4.2. confocal microscopy in order to determine the amount of pile-up, it was necessary to obtain the topography of the spherical indentation. the depth profile was measured with olympus ols5000-saf confocal microscope employing a 405 nm wavelength laser diode. figure 1. abi test load displacement curve for 15kh2mfa steel. 137 a. materna, p. haušild, j. ondráček acta polytechnica figure 2. abi test load displacement curve for sv 08kh19n10g2b steel. figure 3. abi test load displacement curve for undeformed a cold deformed 08kh18n10t steel. figure 4. stress/plastic strain curves for 15kh2mfa steel obtained by various techniques. 5. results and discussion measured load displacement curves are for 15kh2mfa steel plotted in figure 1, for sv 08kh19n10g2b steel in figure 2 and for two deformation states of 08kh18n10t steel in figure 3. evaluated points which form the true stress/plastic strain curves are for 15kh2mfa steel plotted in figure 4, for sv 08kh19n10g2b steel in figure 5 and for two deformation states of 08kh18n10t steel in figure 6. figure 5. true stress/plastic strain curves for sv 08kh19n10g2b steel obtained by various techniques. figure 6. true stress/plastic strain curves for undeformed a cold deformed 08kh18n10t steel. steels with higher hardness exhibit lower penetration depths at the same load level and the higher flow stresses at the same strain levels. it can be observed in figure 3 and figure 6 for 08kh18n10t steel hardened by the cold working. however, more important differences are between the curves in figures 4 6 evaluated using simple oliver-pharr method and more sophisticated iterative abi test method. except the cladding material, op leads to higher stresses and lower strains compared to the abi technique. as can be seen in figures 4 and 5, true stress-strain data points evaluated by abi iterative procedure are in good agreement with standard tensile curves and this agreement can even be improved by the correction of tensile data to specimens necking. one possible explanation for differences between the op and abi results lies in chosen standard values of constraint factors, which can be inaccurate for tested steels. alternatively, it could be attributed to the amount of material pile-up or sink-in around the indent. in case of material pile-up, op method underestimates the contact area which could lead to increase of estimated stresses. it was proved experimentally in figures 7 9, 138 vol. 27/2020 two techniques of spherical indentation data evaluation where cross-sectional profiles for all indents are plotted. whereas materials pile-up around the indents in 15kh2mfa (figure 7) and 08kh18n10t (figure 9) steels, surface around the indent in sv 08kh19n10g2b remains flat (figure 8). susceptibility to pile-up can also be observed in p h plots. for linear indenters (e.g. conical or pyramidal), high final-to-maximum depth ratio hf/hmax > 0.8 indicates material pile-up [5]. for non-linear spherical indenter, the hf/hmax criterion is not as straightforward, because the deformation state under the indenter depends on the depth of penetration. it starts in pure elastic regime with sink-in at small depths and ends in fully plastic regime with possible pile-up. from this point of view, 15kh2mfa and sv 08kh19n10g2b steels are tested at the similar h/d ratios with expected higher hf/hmax ratio for piling-up 15kh2mfa steel (compare figures 1 and 2). on the other hand, pile-up is typical for materials with higher elastic modulus-to-yield stress ratio e/σy, which is higher for sv 08kh19n10g2b comparing to 15kh2mfa. this discrepancy in observed effect of the yield stress on pile-up occurrence can be attributed to ability of sv 08kh19n10g2b cladding to workharden to much higher plastic strains comparing to 15kh2mfa steel (compare tensile curves in figures 4 and 5). figure 7. cross-sectional profile of the spherical indent in 15kh2mfa steel. 6. conclusions the instrumented spherical indentation was used for characterization of tensile properties of base metal (15kh2mfa steel) and cladding (sv 08kh19n10g2b steel) of wwer-440 reactor pressure vessel and of main structural material of reactor core internals in two deformation states (undeformed and colddeformed 08kh18n10t steel). the aim of the study was to evaluate true stress/strain curves of all studied materials and to compare two methods of their evaluation: more complex abi test iterative procedure proposed by haggag and the relatively simple and available method of oliver-pharr mainly used figure 8. cross-sectional profile of the spherical indent in sv 08kh19n10g2b steel. figure 9. cross-sectional profile of the spherical indent in undeformed a cold deformed 08kh18n10t steel. for determination of elastic modulus and hardness of materials. whereas tensile curves computed according to abi equations match standard uniaxial tensile curves for 15kh2mfa and sv 08kh19n10g2b steels, the op method predicts about 12 % higher stresses for 15kh2mfa steel and about 7 % and 11 % higher stresses for undeformed and cold-deformed 08kh18n10t steel, respectively. these higher stresses are probably connected to the material pile-up, which has increased the real contact area contrary to the hypothetical one evaluated using simplified elastic assumptions of the op model. in case of sv 08kh19n10g2b steel, no pile-up was observed and both methods led to similar results. acknowledgements this work was supported by the technology agency of the czech republic under the project no. th02020565 and by the european structural and investment funds under the project cz.02.1.01/0.0/0.0/16_019/0000778 (centre of advanced applied sciences). 139 a. materna, p. haušild, j. ondráček acta polytechnica references [1] h. francis. phenomenological analysis of plastic spherical indentation. journal of engineering materials and technology, transactions of the asme 98:272–281, 1976. doi:10.1115/1.3443378. [2] p. au, g. lucas, j. sheckherd, g. odette. flow property measurements from instrumented hardness tests. non-destructive evaluation in the nuclear industry pp. 597–610, 1980. [3] b. taljat, t. zacharia, f. kosel. new analytical procedure to determine stress-strain curve from spherical indentation data. international journal of solids and structures 35(33):4411 – 4426, 1998. doi:10.1016/s0020-7683(97)00249-7. [4] w. oliver, g. pharr. an improved technique for determining hardness and elastic modulus using load and displacement sensing indentation experiments. journal of materials research 7(6):1564–1583, 1992. doi:10.1557/jmr.1992.1564. [5] a. bolshakov, g. m. pharr. influences of pileup on the measurement of mechanical properties by load and depth sensing indentation techniques. journal of materials research 13(4):1049–1058, 1998. doi:10.1557/jmr.1998.0146. [6] b. taljat, g. pharr. development of pile-up during spherical indentation of elastic-plastic solids. international journal of solids and structures 41(14):3891 – 3904, 2004. doi:10.1016/j.ijsolstr.2004.02.033. [7] f. haggag. in situ measurement of mechanical properties using novel automated ball indentation system. astm special technical publication pp. 27–44, 1993. doi:10.1520/stp12719s. [8] d. tabor. the hardness of metals. clarendon press, uk, 1951. 140 https://doi.org/10.1115/1.3443378 https://doi.org/10.1016/s0020-7683(97)00249-7 https://doi.org/10.1557/jmr.1992.1564 https://doi.org/10.1557/jmr.1998.0146 https://doi.org/10.1016/j.ijsolstr.2004.02.033 https://doi.org/10.1520/stp12719s acta polytechnica 27(0):136–140, 2020 1 introduction 2 tested materials 3 methods and analysis 3.1 oliver-pharr method 3.2 haggag method 4 experimental details 4.1 mechanical testing 4.2 confocal microscopy 5 results and discussion 6 conclusions acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2019.21.0038 acta polytechnica ctu proceedings 21:38–41, 2019 © czech technical university in prague, 2019 available online at http://ojs.cvut.cz/ojs/index.php/app assessment of adhesion between bituminous binder and mineral aggregate using digital image analysis jan trejbal∗, tereza valentová, tereza horová czech technical university in prague, faculty of civil engineering, thákurova 7, 166 29 prague, czech republic ∗ corresponding author: jan.trejbal@fsv.cvut.cz abstract. asphalt mixtures often fail due to a low adhesion of bituminous binder to mineral aggregate, which leads to surface coarse damages like potholes and fatigue cracking. to avoid this phenomenon, different types of adhesion promoters may be admixed into bituminous binder but a new question about their effectiveness arises. this paper presents two semi-automatic methods, which reliably replace the subjective assessment. both of them use a digital image of asphalt mixtures as an input. the first is based on a gray level thresholding, while the second one on an entropy-based image segmentation. asphalt mixtures composed from zbraslav aggregate (fraction 8–16mm), paving grade bitumen 50/70 and several types of adhesion promoters were made and subjected to the adhesion assessment. it was shown that aggregate grains coated by binder was equal to ca. 83–88% in the case of reference binder, while that was increased by ca. 10–13% if whatever adhesion promoters were used. keywords: asphalt mixture, bituminous binder, adhesion promoters, adhesion assessment, interfacial interaction. 1. introduction in europe, more than 90% of roads are constructed from asphalt mixtures [1]. over the past few decades, such a material has been partially replaced by concrete based on the cement matrix due to its better abrasive resistance and longer lifetime. unfortunately, it has been shown recently that these road structures often fail due to issues connected with shrinkage cracking [2]. an attention of many engineers and researches is therefore drawn back to asphalt mixtures and efforts to improve this material are newly developed. the poor adhesion between bituminous binder and mineral aggregate is considered to be one of the most problematic issue, leading to a premature damage of the surface coarse. mineral aggregates, especially those containing quartz-based granites, are typical for its affinity for water, much more than for binder. once the asphalt mixture is exposed to water, this can easily penetrate between the two materials and thus acts as a separation, if binder to aggregate adhesion is too poor [3, 4]. consequently, individual aggregate particles are stripped, i.e., the thin bituminous film is removed from their surface. this phenomenon is called “stripping” [5], resulting in road structure degradation like potholes and fatigue cracking, see figure 1. to reduce or avoid stripping, bituminous binder can be modified with adhesion promoters. the function of these synthetic compounds or those from natural resources is based on disruption of intermolecular interactions between water and aggregate surface [6]. in civil engineering practice, the interaction between the two materials is in most cases assessed as adhesion of the bituminous thin film to aggregate figure 1. a pothole through the surface coarse of the asphalt structure road (road nr. 28110 between villages jinolice and javornice). particles. two observers evaluate the ratio between binder-coated and stripped areas visually on asphalt mixtures prepared in laboratory [7]. it is obvious that such approach cannot provide consistent and accurate data due to human factor. therefore, some researchers developed semi-automatic methods. these are usually based on digital picturing of asphalt mixtures, while the ratio mentioned above is then assessed automatically. most of such methods are based on classification of characteristic colors [8, 9]. unfortunately, such an assessment is inaccurate or impracticable due to aggregate dark colors or light reflection from binder-coated aggregate surface. a digital camera is not capable to record such a contrast picture; the captured photo does not contain all shad38 http://dx.doi.org/10.14311/app.2019.21.0038 http://ojs.cvut.cz/ojs/index.php/app vol. 21/2019 adhesion of bituminous binder to mineral aggregate ows or brightness – the color spectrum is limited. in such cases, the software is not able to recognize the threshold between the two materials due to low information in the photo. therefore, it is suitable rather for edges analysis based on brightness and shadows recognition [10]. to overcome this limit, the entropybased image segmentation method can be used. such a procedure provides much more accurate outcomes. areas with high entropy are considered to belong to aggregate while low entropy areas represent the bituminous surface [11, 12]. 2. materials and methods 2.1. mineral aggregate mineral aggregate zbraslav (czech republic) composed of proterozoic spiliness and metaphrash was used for a production of asphalt mixtures. this was crushed to fraction of 8–16mm. from the geological point of view, this rock is typical for its very fine grain and porous surface. 2.2. bituminous binder standard paving grade bitumen 50/70 [13] was used at the alternatives – reference and modified with several different types of adhesion promoters. these were applied at amount of 0.2 or 0.3% of the binder weight according to recommendation of their manufacturers. composition of each asphalt mixture, differing in adhesion promoter type, is stated in table 1, where the abbreviation mr indicates the reference mixture, while ma–md belong to modified mixtures. 2.3. asphalt mixture samples asphalt mixtures were prepared by procedures following the relevant technical standard [14]. those were photographed in order to obtain basic input files for the digital image based analysis. a sample image is depicted in figure 2. 2.4. adhesion assessment two digital image based approaches were employed to assess the adhesion between aggregate and both reference and modified bituminous binder: (1.) gray level thersholding (glt): this method is based on recognition of brightness and shadows on the digital image of asphalt mixtures. areas which are brighter than dark bituminous binder are considered to be parts of stripped aggregate particles. (2.) entropy-based image segmentation (eis): the principle of this method rests on a local entropy calculation in order to assess roughness of the texture. areas with high entropy are considered to belong to aggregate while low entropy areas represent the bituminous surface. figure 2. a sample image of the asphalt mixture to be used for the adhesion assessment. 3. results the adhesion analysis of asphalt mixtures revealed that the surface area of the reference asphalt mixture (mr) reached on 88.0% and almost 83.0%, when assessed employing glt and eis method, respectively. the mixture was classified as "satisfactory" according to the relevant technical standard [14]. if an adhesion promoter is used (ma–md), the coated area increased to 95.2–99.3% (depending on method used). verbal classification following the standard cited above was either “good” or even “excellent”. it is therefore clear that the effect of applied adhesion promoters is significantly positive. it can not be said which mixture reached on the best result due to slight difference between them. all results are summarized in figure 3, where the verbal classification is also stated. figure 4 shows an original image of the reference asphalt mixture (mr), where stripped edges of aggregate grains are illustrated. these were exactly found by both methods used (glt and eis). this held true also for the mixture mb, which is depicted in figure 5. it is also worth noting that the difference between the adhesion of mr and ma mixture is visible clearly. the slight difference between results obtained using the two methods can be attributed to issues connected with dark and shiny surfaces of the aggregate grains. if these are rather dark, the software of the glt method can define them as binder-coated. therefore, such obtained results reached on better values than in case of the eis method. the problematic image is shown in figure 2, where the high contrast of whole mixture is presented. 4. conclusions the presented work was focused on adhesion assessment of bituminous binder to mineral aggregate using 39 j. trejbal, t. valentová, t. horová acta polytechnica ctu proceedings mixture aggregate fraction [mm] promoters amount [wt % ] mr proterozoic spiliness, metaphrash 8-16 no promoters 0 ma tto-a 0.2 mb valojames 0.3 mc adhere 0.3 md ad-2 0.2 table 1. summarization and composition of asphalt mixtures used. 0 20 40 60 80 100 su rf ac e of a gg re ga te c oa te d by b in de r [% ] mixture classification unsatisfactory satisfactory excellentgood legend mr ma mb mc md 88 .0 82 .9 98 .6 95 .8 99 .3 95 .2 95 .8 98 .3 98 .2 97 .0 grey level thresholding entropy-based image segmentation figure 3. the surface of aggregate coated by binder. figure 4. coverage of zbraslav aggregate with reference bituminous binder (mr), from left to right: an original image, glt, and eis results. 40 vol. 21/2019 adhesion of bituminous binder to mineral aggregate figure 5. coverage of zbraslav aggregate with modified bituminous binder (ma), from left to right: an original image, glt, and eis results. two different digital image based semi-automatic analyzes – a gray level thresholding (based on identification of aggregate areas brighter than dark bituminous binder) and an entropy based image segmentation (aggregate areas rougher than binder). asphalt mixtures composed of paving grade bitumen 50/70, zbraslav aggregate crushed at the fraction of 8-16mm, and several types of adhesion promoters were made. promoters were employed in trace amount – from 0.2 to 0.3% of the binder weight, according to recommendation by their manufacturers. after finishing of the asphalt mixtures preparation, those were photographed in order to attain input files for the semi-automatic adhesion analysis. the findings were as follows: • the aggregate area coated by reference binder was equal to ca. 83–88%, as assessed using both semiautomatic methods. • if bitumen was modified by adhesion promoters, regardless to their type and amount, the adhesion was increased by ca. 10–13%. • the eis method was considered to be more accurate than the glt because of the issue connected with dark and shinny surfaces of asphalt mixtures. these provide misleading information for the image-based analysis. acknowledgements this work was supported by the czech science foundation (research project no. 18-13830s) and by the czech technical university (no. sgs16/201/ohk1/3t/11) and the support is gratefully acknowledged. special thanks to václav nežerka, phd. for the software development. references [1] s. liu, a. shukla, t. nandra. technological, environmental and economic aspects of asphalt recycling for road construction. renewable and sustainable energy reviews 75:879–893, 2017. doi:10.1016/j.rser.2016.10.080. [2] v. šmilauer, p. slováček. new highway d1 is already broken again. svět motorů 31:4–8, 2018. [3] n. ahmad. asphalt mixture moisture sensitivity evaluation using surface energy parameters – phd thesis. the university of nottingham, 2011. [4] j. grönninger, m. wistuba, p. renken. adhesion in bi tumen-aggregate-systems. road materials and pavement design 11(4):881–898, 2010. doi:10.1080/14680629.2010.9690311. [5] b. amit. development of methods to quantify bitumen-aggregate adhesion and loss of adhesion due to water – phd thesis. texas a&m university, 2006. [6] c. rossi, b. teltayev, r. angelico. adhesion promoters in bituminous road materials: a review. applied science 7(524), 2017. doi:10.3390/app7050524. [7] en 12697-11: bituminous mixtures – test methods for hot mix asphalt part 11: determination of the affinity between aggregate and bitumen. [8] r. lamperti, c. lantieri, c. sangiorgi, et al. semiautomatic evaluation of the degree of bitumen coverage on bitumen-coated aggregates. in conference proceeding of 8th rilem international symposium. rilem bookseries, 2016. doi:10.1007/978-94-017-7342-3_2. [9] h. källén, a. heyden, k. åström, p. lindh. measuring and evaluating bitumen coverage of stones using two different digital image analysis method. measurement 84:56–67, 2016. doi:10.1016/j.measurement.2016.02.007. [10] v. nežerka, m. somr, j. trejbal. contact angle measurement tool based on image analysis. experimental techniques 2018. doi:10.1007/s40799-017-0231-0. [11] v. nežerka. available online: http://mech.fsv.cvut.cz/ nezerka/software.html. [12] v. nežerka, j. trejbal. assessment of aggregatebitumen coverage using entropy-based image segmentation, submitted to journal of materials science and technology. [13] en 12591: bitumen and bituminous binders – specifications for paving grade bitumens. [14] čsn en 76 6161: determination for adhesion of asphaltic binders to aggregate. 41 http://dx.doi.org/10.1016/j.rser.2016.10.080 http://dx.doi.org/10.1080/14680629.2010.9690311 http://dx.doi.org/10.3390/app7050524 http://dx.doi.org/10.1007/978-94-017-7342-3_2 http://dx.doi.org/10.1016/j.measurement.2016.02.007 http://dx.doi.org/10.1007/s40799-017-0231-0 acta polytechnica ctu proceedings 21:38–41, 2019 1 introduction 2 materials and methods 2.1 mineral aggregate 2.2 bituminous binder 2.3 asphalt mixture samples 2.4 adhesion assessment 3 results 4 conclusions acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2020.26.0045 acta polytechnica ctu proceedings 26:45–49, 2020 © czech technical university in prague, 2020 available online at https://ojs.cvut.cz/ojs/index.php/app micro-mechanical properties of cement and slag composites measured by nanoindentation jiří němeček 1∗, jiří němeček 2 czech technical university in prague, faculty of civil engineering, department of mechanics, thákurova 7, 166 29 prague 6, czech republic ∗ corresponding author: jiri.nemecek.1@fsv.cvut.cz (ph.d. candidate) abstract. in this study, the micromechanical response of two cementitious composites was characterized by nanoindentation. pure portland cement paste and portland cement with 50 vol. % replaced with granulated blast furnace slag (gbfs) paste were investigated at the age of 28 days. grid nanoindentation, statistical deconvolution and scanning electron microscopy were used to characterize the main hydration products. several grids with approximately 500 indents on each sample were performed to obtain modulus of elasticity, hardness and creep indentation parameter. similar mechanical phases containing calcium silica hydrate, crystalline calcium hydroxide and un-hydrated clinker were found in both samples varying by volume fraction. blended cement, moreover, contains a phase of slag hydration products with a significantly lower modulus of elasticity. this phase with a high portion of unreacted gbfs is mostly responsible for the difference of mechanical properties of the whole composite. keywords: nanoindentation, microstructure, blast furnace slag, cement paste. 1. introduction concrete is one of the most used materials in the construction industry. one of its ingredients is portland cement which is one of the major producers of carbon dioxide. in order to reduce the amount of cement used in concrete, admixtures are used. one of the most common admixtures is blast furnace slag which is created as a by-product of iron making as a liquid at 1350–1550 °c. if this liquid is cooled rapidly, it forms a glass which is a latent hydraulic cement. thus, using blast furnace slag is advantageous for two reasons, economical and ecological. concrete which has uses slag has different properties such as decreased total heat evolution in massive structures, improved durability, slower compressive strength development [1] or decreased creep compliance [2]. when portland cement is mixed with water, the hydration process starts immediately. if the mixture is incorporated with slag, the hydration rate is much slower, and activators need to be present in the mixture. according to chen [3], the most common activators are alkaline based e.g. li, k, na [4]. in the later stages of hydration, slag reacts with portlandite (ch) in pozzolanic reaction. this results in different microstructure of slag blended cements which affects the macroscopic properties. since the information about micromechanical properties is limited, obtaining more data would be beneficial. some research has been done in terms of effect of thermal damage [5], 50 % cement replacement; cured in moist condition for 120 days [6] or alkali activated slag [7]. 1ph. d. candidate at ctu in prague 2associate professor at ctu in prague this study concentrates on the characterization of individual hydration products of pure portland cement and cement with slag admixtures. moreover, it compares the micromechanical response and creep parameters of individual hydration products. 2. material and sample preparation two hydrated cement pastes for microstructural analysis were prepared. the first paste was made from pure portland cement cem i 42.5r (from mokrá, czech republic), labeled pc. the second paste was mixed with 50 volume percent of cem i 42.5r and 50 volume percent of blast furnace slag from štramberk, labeled sl. both pastes were mixed with water to binder ratio 0.4 and cast into cylindrical molds with height of 30 mm and diameter of 20 mm. the samples were demolded after three days and put in the water for the next 25 days. the samples were cut into 5 mm thick slices and dried at 50 °c for 24 hours. then the surface of the samples was polished with sic paper with grit sizes from 2000 to 4000. lastly, the samples were put in ethanol in an ultrasonic cleaner to remove all free particles. 3. microstructural characterization individual chemical phases of cementitious composites were found and characterized by scanning electron microscopy (sem) and energy dispersion x-ray spectroscopy (edx). the sample pc (fig. 1 a)) is primarily composed of three phases: calcium-silicahydrate (c-s-h gel), crystalline calcium hydroxide 45 https://doi.org/10.14311/app.2020.26.0045 https://ojs.cvut.cz/ojs/index.php/app jiří němeček 1, jiří němeček 2 acta polytechnica ctu proceedings 50 μm a) b) 50 μm clinker clinker gbfs inner p. inner p. ch ch outer p. outer p. figure 1. typical sem-bse images of cement composites with highlighted phases a) pc b) sl. phase r. (%) main hydrates ch gbfs clinker pores, cracks cp 71.47 14.35 – 9.45 4.73 cs 51.53 – 34.02 7.74 6.71 table 1. phase representation calculated from sem-bse images. portlandite (ch) and anhydrous clinker. it is well documented that c-s-h gel exists with two packing densities called high density and low density [8]. mixed with other hydration products, high density gel creates rims around the grains of unhydrated clinker called inner product. the outer product is created mostly of low density c-s-h gel mixed with other hydrates. the sample with slag sl (fig. 1 b)) is composed of the same phases as the sample pc. moreover, it includes unreacted gbfs and reaction product of the slag which is mostly c-s-h gel where some silica is replaced by alumina to form c-a-s-h gel [7]. also, both samples contain minor phases such as ettringite, hydrogarnet and monosulfphate which were not detected on polished surfaces by images analysis. the image analysis can make a histogram for all the different phases in the images based on their grey levels from which it is possible to calculate the volume fraction of the identified major chemical phases. twelve sem images with a view field of 300 x 300 µm were used for the analysis. the results are summarized in table 1. on both samples, cracks and porosity were visible which corresponds to the color black. due to similar color intensity of inner and outer products, both phases were combined and labeled as main hydrates. also, low amount of ch in sample sl makes it impossible to separate them. thus, ch was grouped with gbfs. 0 100 200 3000.0 0.5 1.0 1.5 2.0 2.5 displacement [nm] lo ad [m n ] clinker ch inner p. outer p. figure 2. typical load-displacement curves. 4. nanoindentation micromechanical properties such as modulus of elasticity or hardness of individual chemical phases can be measured by nanoindentation. a principle of nanoindentation is based on pressing a tip with known properties into a material at small scale. hysitron tribolab ti-700 with berkovich diamond tip was used for test46 vol. 26/2020 micro-mechanical properties of cement and slag composites 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 1500.00 0.01 0.02 0.03 0.04 0.05 modulus of elasticity [gpa] fr eq ue nc y de ns ity [-] a) #1 #2 #3 #4 #5 exp. pdf 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 1500.00 0.01 0.02 0.03 0.04 0.05 modulus of elasticity [gpa] fr eq ue nc y de ns ity [-] b) #1 #2 #3 #4 #5 exp. pdf figure 3. frequency plots of the samples a) pc b) sl. the bin-size for deconvolution of modulus of elasticity e = 1.5 gpa. ing. five grids of 10 x 10 indents with regular spacing of 12.5 µm were prescribed on both samples in order to cover heterogeneity of the samples. trapezoidal loading diagram was prescribed with a linear loading up to 2 mn at a constant rate of 40 mn/min, followed by 20 seconds holding period and linear unloading same rate as loading. during the measurement, load displacement curves are measured. the typical curves are shown in fig. 2. indentation modulus was evaluated from load displacement diagram using the oliver and pharr method [9]. the method is based on sneddon solution [10] of a rigid indenter action on an elastic half space. the indentation modulus er, and hardness h, are given as er = s √ π 2β √ ac , (1) h = pmax ac , (2) where s represents elastic unloading stiffness, ac is the projected contact area, β is coefficient which considers different geometry of the indentation tip (β = 1.304 for berkovich tip) and pmax is the maximum load. indentation modulus represents a combination of elastic stiffness of the sample and the tip. modulus of elasticity e, of the sample can be obtained from equation 1 er = 1− ν2 e + 1− ν2 i ei , (3) where ei and νi are modulus of elasticity and poisson’s ratio of the indentation tip (for the diamond tip ei = 1141 gpa and νi = 0.07), ν is the poisson’s ratio of the sample, it was assumed as 0.2 for the cement paste. analysis of the large amount of data produced by grid indentation is possible due to a technique known as statistical deconvolution. using deconvolution technique, it is possible to evaluate and separate the mechanical response of the individual phases in cementitious composites and identify volume fraction. this method is frequently used for heterogenous systems as it was also successfully done on cementitious material, e.g. [8, 11] . 47 jiří němeček 1, jiří němeček 2 acta polytechnica ctu proceedings #1 #2 #3 #4 #5 pc e [gpa] 24.9 ± 3.7 33.8 ± 4.8 45.0 ± 2.2 66.6 ± 11.6 127.1 ± 18.9 h [gpa] 1.10 ± 0.21 1.71 ± 0.19 2.29 ± 0.16 3.68 ± 1.07 10.88 ± 2.39 cit [%] 11.3 ± 3.1 9.5 ± 3.3 6.4 ± 3.7 4.9 ± 3.1 2.1 ± 1.9 vol. f. [%] 23.0 42.0 15.5 18.1 1.4 cs e [gpa] 12.3 ± 3.7 27.4 ± 6.2 45.9 ± 5.0 69.9 ± 9.5 116.1 ± 14.4 h [gpa] 0.56 ± 0.53 1.55 ± 0.64 2.24 ± 0.95 3.50 ± 1.21 6.24 ± 1.98 cit [%] 12.1 ± 3.2 9.3 ± 3.1 8.7 ± 2.2 6.8 ± 4.1 5.2 ± 2.8 vol. f. [%] 30.2 28.5 15.0 12.8 13.5 table 2. mean values and standard deviation of individual deconvoluted phases of modulus of elasticity, hardness, cit and volume fraction. short-time creep of material can be evaluated from holding period as indentation creep parameter cit is given by equation cit(p,t1,t2) = h2 − h1 h1 × 100, (4) which is defined as the relative change of indentation depths h1 and h2 encountered at times t1 and t2, respectively. the parameter is dependent on contact force pmax and time which is kept during the holding period. 5. results and discussion statistical deconvolution was used on experimental data of modulus of elasticity to calculate the mechanical phases of the samples. since the measured mechanical response is related to the finite indentation volume, the individual mechanical phases were assigned to the most probable chemical composition. both samples were separated into five mechanically similar phases. in sample pc (fig. 3 a)), phase #1, contains mostly outer product. the main phase, labeled #2 is primarily formed by inner product, however, the gauss peak overlaps with neighboring phases thus, this phase also contains some portion of outer product and portlandite. #3 includes primarily portlandite and small amount of un-hydrated clinker particles. phases #4 and #5 were deconvoluted with modulus of elasticity higher than 50 gpa which belongs only to un-hydrated clinker, these stiff particles ordinarily have e = 110 160 gpa [12]. such a high modulus was obtain only for clinker of higher dimension in phase #5. residual clinker of smaller dimension, while loaded, interacts with surrounding matrix of c-s-h gel resulting in lower modulus of elasticity 50-100 gpa in phase #4. in sample sl (fig. 3 b)) phase #1, is similar to more porous outer product composed with c-a-s-h gel and slag hydration products. phase #2 is mostly formed by c-s-h gel, mostly inner product. #3 includes portlandite and small particles of gbfs. phases #4 and #5 are created with same composition as sample pc moreover both phases contain particles of gbfs. the results of the statistical deconvolution for both samples are shown in fig. 3 and values of the modulus of elasticity, hardness, cit and volume fraction are summarized in table 2. phase #2 is the most dominant for sample pc with (e = 33.8± 4.8 gpa) which is comparable to phase #2 of sample sl (e = 27.4 ± 6.2 gpa), the higher value for sample pc is due to the presence of portlandite in this phase. phase #1 deconvoluted in sample pc still has a higher modulus and compared to phase #1 in sample sl with volume fraction of 30.2 %. this shift is visible in fig. 4. phase #3 present in sample sl belongs to portlandite with e = 45.0 gpa (according to literature e = 43.9 gpa, [11]), also identified volume fraction differs with sem image analysis only about 1.15 % which is in good correlation. phase #3 of sample sl is created by different chemical phases, however, the mechanical response and volume fraction is almost identical to sample pc. the volume fraction of stiffer particles (e > 50 gpa) is 6.8 % higher in sample sl. the difference in volume fraction is even more visible for larger particles with e = 100 gpa. the results of hardness correlates with modulus of elasticity, as well as cit parameter which is the highest for outer product followed by inner product. ch, gbfs and residual clinker do not creep and values are dependent on their dimension floating in c-(a)-s-h matrix. figure 4. probability distribution graph of modulus of elasticity of the pc and sl samples 48 vol. 26/2020 micro-mechanical properties of cement and slag composites thus, there are two major significant differences between both samples. first one is the presence of phase #1 in sample sl with modulus of elasticity (e = 12.3 ± 3.7 gpa) and volume fraction of 30.2 %. this value corresponds well with literature results, the naoh activated slag with e = 12–20 gpa [7] and slag blended cement measured in 120 days, where e = 15.5 gpa [6]. second is the high portion of unreacted gbfs in sl sample which is spread to the phases #3 #5 with total volume fraction 41.3 % this corresponds to results from image analysis 41.76 % (gbfs + clinker). according to study of shaikh [13] which studied macroscopic properties of 28 days old slag blended cement pastes, the compressive strength up to 70 % slag replacement is increased for blended pastes compared to ordinary portland cement paste. the compressive strength of the whole sample is affected by the microstructure with porosity distribution and other defects. from our microstructure investigation it is highly probable that high volume fraction of unreacted slag particles contributes to higher compressive strength of sample sl. the strength was, however, not assessed in this work. 6. conclusions microstructure of pure portland cement (pc) and cement with slag admixture (sl) was characterized by sem images, edx and nanoindentation. according to edx, sample sl contains two additional chemical phases over the sample pc, c-(a)-s-h gel and unreacted gbfs. lower volume fraction of portlandite of sample sl was observed by sem image analysis and nanoindentation which confirms consumption of the slag in hydration reaction. the main hydrated material of sample sl has significantly lower modulus of elasticity and hardness than sample pc. both samples contain un-hydrated material stiffening the samples. the volume fraction of un-reacted particles identified by nanoindentation is almost two times higher for sample sl. cit parameter of all phases of sample sl, except the inner product, reaches higher values than sample pc, meaning sample sl exhibits more compliant response. the values of cit parameter of un-hydrated particles are dependent on their dimension floating in c-(a)-s-h matrix and must not be treated as intrinsic phase properties. acknowledgements financial support of the czech science foundation (project 17-05360s) and the grant agency of the czech technical university in prague (sgs18/114/ohk1/2t/11) are gratefully acknowledged. references [1] h. f. taylor. cement chemistry. thomas telford, 1997. doi:10.1680/cc.25929. [2] i. pane, w. hansen. early age creep and stress relaxation of concrete containing blended cements. materials and structures 35(2):92, 2002. doi:10.1007/bf02482107. [3] w. chen, h. brouwers. the hydration of slag, part 2: reaction models for blended cement. journal of materials science 42:444–464, 2007. doi:10.1007/s10853-006-0874-1. [4] b. kolani, l. buffo-lacarrière, a. sellier, et al. hydration of slag-blended cements. cement and concrete composites 34:1009–1018, 2012. doi:10.1016/j.cemconcomp.2012.05.007. [5] v. z. zadeh, c. p. bobko. nanoscale mechanical properties of concrete containing blast furnace slag and fly ash before and after thermal damage. cement and concrete composites 37:215 – 221, 2013. doi:10.1016/j.cemconcomp.2012.09.003. [6] c. hu, z. li, y. gao, et al. investigation on microstructures of cementitious composites incorporating slag. advances in cement research 26:222–232, 2014. doi:10.1680/adcr.13.00029. [7] f. puertas, m. palacios, h. manzano, et al. a model for the c-a-s-h gel formed in alkali-activated slag cements. journal of the european ceramic society 31(12):2043 – 2056, 2011. doi:10.1016/j.jeurceramsoc.2011.04.036. [8] g. constantinides, f.-j. ulm. the nanogranular nature of c–s–h. journal of the mechanics and physics of solids 55(1):64–90, 2007. doi:10.1016/j.jmps.2006.06.003. [9] w. c. oliver, g. m. pharr. an improved technique for determining hardness and elastic modulus using load and displacement sensing indentation experiments. journal of materials research 7(6):1564–1583, 1992. doi:10.1557/jmr.1992.1564. [10] i. n. sneddon. the relation between load and penetration in the axisymmetric boussinesq problem for a punch of arbitrary profile. international journal of engineering science 3(1):47–57, 1965. doi:10.1016/0020-7225(65)90019-4. [11] j. němeček, v. králík, j. vondřejc. micromechanical analysis of heterogeneous structural materials. cement and concrete composites 36(0):85 – 92, 2013. doi:10.1016/j.cemconcomp.2012.06.015. [12] k. velez, s. maximilien, d. damidot, et al. determination by nanoindentation of elastic modulus and hardness of pure constituents of portland cement clinker. cement and concrete research 31(4):555–561, 2001. doi:10.1016/s0008-8846(00)00505-6. [13] f. u. a. shaikh, a. hosan. effect of nano silica on compressive strength and microstructures of high volume blast furnace slag and high volume blast furnace slag-fly ash blended pastes. sustainable materials and technologies 20:e00111, 2019. doi:10.1016/j.susmat.2019.e00111. 49 https://doi.org/10.1680/cc.25929 https://doi.org/10.1007/bf02482107 https://doi.org/10.1007/s10853-006-0874-1 https://doi.org/10.1016/j.cemconcomp.2012.05.007 https://doi.org/10.1016/j.cemconcomp.2012.09.003 https://doi.org/10.1680/adcr.13.00029 https://doi.org/10.1016/j.jeurceramsoc.2011.04.036 https://doi.org/10.1016/j.jmps.2006.06.003 https://doi.org/10.1557/jmr.1992.1564 https://doi.org/10.1016/0020-7225(65)90019-4 https://doi.org/10.1016/j.cemconcomp.2012.06.015 https://doi.org/10.1016/s0008-8846(00)00505-6 https://doi.org/10.1016/j.susmat.2019.e00111 acta polytechnica ctu proceedings 26:45–49, 2020 1 introduction 2 material and sample preparation 3 microstructural characterization 4 nanoindentation 5 results and discussion 6 conclusions acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2020.28.0015 acta polytechnica ctu proceedings 28(0):15–22, 2020 © czech technical university in prague, 2020 available online at http://ojs.cvut.cz/ojs/index.php/app investigation of critical heat flux on abraded inconel 625 tube ladislav suka, kamil števankaa,∗, taron petrosyana, daniel vlčekb a brno university of technology, faculty of electrical engineering and communication, department of electrical power engineering, technická 3058/10, 616 00 brno, czech republic b czech technical university in prague, faculty of nuclear sciences and physical engineering, department of nuclear reactors, břehová 78/7, 115 19 prague, czech republic ∗ corresponding author: kamil.stevanka@vutbr.cz abstract. aim of this work was to study flow boiling in an annular channel at low pressure and low flow on a tube with modified surface roughness. the tube with the outer diameter of 9.14 mm and the heated length of 380 mm was made of inconel 625 and was manually modified using 150 grit sandpaper. the tube was placed in a glass tube with an inner diameter of 14.8 mm. outlet pressure was set to 120, 200 and 300 kpa with varying mass flow from 400 to 600 kg/(m2.s). a high speed camera was used to record several experiments to fully understand ongoing phenomena. surface roughness was analysed using a confocal laser microscope and the effects of different mass flux and pressure on the chf value were observed. above all, the optimization of the flow parameters was done from the collected data and from the observed behaviour of the experimental loop. keywords: annulus, critical heat flux, flow boiling, inconel 625. 1. introduction departure from nucleate boiling decreases the heat transfer from the fuel cladding to the coolant, this in turn leads to a rapid increase of temperature and can cause failure of the material. therefore, it is vital for nuclear reactor operation to avoid critical heat flux (chf). it is important to maintain a safety margin to prevent possible fuel damage. despite the fact that boiling crisis was researched extensively during the early development and deployment of pwr reactors, it is still being researched today, especially with the inception of accident tolerant fuels (atf). boiling phenomena can be studied either in pool boiling arrangement or flow boiling. equipment needed for pool boiling experiments is simpler and thus they are relatively accessible as they are usually performed at atmospheric pressure and on small steel plates or thin wires. examples of recent pool boiling experiments aimed at comparing surface characteristics are the works by o’hanley et al. [1], kam et al. [2], ali et al. [3] and liang et al. [4]. flow boiling experiments are more complex and harder to compare between each other, because the results depend on flow parameters such as mass flow, pressure, inlet temperature, as well as the geometry of a test section. critical heat flux was researched broadly at high pressures [5], however there are far less available data on chf at low pressures. this work and test facility was inspired by haas [6, 7], who carried out his experimental work at the cosmos test facility at karlsruhe institute of technology in germany. haas discovered that unstable flow conditions can occur without necessary precautions. these may lead to oscillations of the flow and premature chf with much lower heat flux values. haas [7] conducted a thorough research of the flow boiling experiments at low flow and pressure performed, focusing mainly on annular geometry. chun [8] experimented with a wide range of pressures from 570 kpa to 15010 kpa. another investigation was done by fiori and bergles [9] who researched chf in annular geometry at low pressures of 148–600 kpa. experiments at pressures slightly above atmospheric pressure were done by el-genk [10], schoesse [11] and rogers [12]. each experiment at different mass fluxes but constant pressures of 118, 128 and 156 kpa, respectively. rogers investigated chf using inconel 718. other work includes the investigation of a very low subcooling of inlet water at the pressure of 100 kpa by mishima and nushihara [13]. mayer et al. [14] focused on the boiling crisis in an annulus at different mass fluxes and pressures between 110 kpa and 225 kpa, the obtained data were afterwards compared with several correlations developed by the researchers mentioned above. more detailed description is provided in the paper by vlcek et al. [15]. in this work, a set of experiments were performed on inconel 625 with the roughness of the surface manually modified with 150 grit sandpaper. the experiments were conducted at different pressures, mass fluxes and inlet temperatures. surface roughness was analyzed using a confocal laser microscope. high speed camera was used during the experiments to better understand the phenomena and improve the experimental setup. the aim of the work was to collect data on boiling crisis on the tube with increased roughness and opti15 http://dx.doi.org/10.14311/app.2020.28.0015 http://ojs.cvut.cz/ojs/index.php/app l. suk, k. števanka, t. petrosyan, d. vlček acta polytechnica ctu proceedings figure 1. setup for slow motion recording mize the experimental procedure. the results will be compared to the chf on other surfaces, unfortunately at the time of the conference only the experiments on sandpapered tube were measured and processed. 2. experimental setup experiments were conducted on the mobile research critical heat flux apparatus (mrcha) at but. the loop can operate at pressures of up to 1.5 mpa. dc power source was used to supply energy to the test section. dc power supply was tested up to 97.5 kw at 1500 a and 65 v, furthermore for a short time it can provide up to 1700 a and 70 v. the chf was detected using thermocouple probe which was fitted with 9 thermocouples (tcs). the test section consisted of a test tube that was pressed together with nickel-treated copper electrodes. the tube was placed in a glass tube, creating an annular geometry, and the probe was then placed inside the test section. the detailed description of the loop and measuring apparatus is provided in an article by vlcek et al. [15]. two inconel 625 tubes with an outer diameter of 9.14 mm were used for the experiments. the tubes were placed in a glass tube with an inner diameter of 14.8 mm. all details of the annular geometry of the test section can be seen in tab. 1. parameter outer diameter dout [mm] 14.8 inner diameter din [mm] 9.14 gap width δ [mm] 2.83 heated length [mm] 380 wetted perimeter [mm] 75.20 heated perimeter [mm] 28.71 length to heated eq. diameter ratio [−] 25.63 length to hydraulic diameter ratio [−] 67.13 length to inner diameter ratio [−] 41.57 table 1. the details of test annulus geometry. some of the experimental runs were recorded by the high speed camera, the information obtained will figure 2. photron high speed camera fastcam sa1.1 be discussed later. the test section and recording apparatus can be seen in figs. 1 and 2. the roughness of the tubes was increased with 150 grit sandpaper, the resulting modification can be seen in figs. 3 and 5. sandpapering was done manually at a 45° angle to the vertical axis, see fig. 4. surface roughness measurements were performed by a confocal laser microscope olympus lext ols4100 according to csn en iso 4288 standard [16]. the samples were cleaned with demineralized water and isopropyl alcohol before the measurements. comparison of roughness of smooth and sandpapered tube is in the tab. 2. material rz rsm rt ra [µm] [µm] [µm] [µm] smooth tube 3.937 77.74 4.803 0.437 abraded tube 8.173 97.02 8.640 0.771 table 2. roughness 3. methodology thirty one experiments were performed at different pressures, mass fluxes and inlet temperatures. the start-up sequence has been optimized to avoid excessive oscillations in the test section during the experiments. in instances when start-up sequence was rushed, chf values could be as low as half of the average value obtained during standard operation, thus producing unreliable values. the oscillations were induced by insufficiently heated backflow valve, condenser and inlet hoses. the inlet hoses started to vibrate if the start-up sequence was rushed. if there was no steam in the condenser, the backflow valve had the tendency to periodically close. these issues were solved by heating up the test section and loop for at least 15 minutes every time before ramping up the power supply. 16 vol. 28/2020 inconel 625 tube: critical heat flux investigation figure 3. surface of the tube figure 4. microscopic image of abraded inconel 625 tube another important finding was that the degassing procedure had to be done each time the loop was shut off. this was most likely caused by the dissolving of nitrogen into water, which was used to pressurize the loop. the parameters of experiments with deviations from target values are shown in tab. 3. parameter target avg. std. value value dev. mass flux 400 399.2 5.1 g [ kg/(m2.s) ] 500 499.2 1.5 600 597.7 3.1 inlet temperature 64 64.07 0.43 tin [◦c] 78 78.30 1.43 91 90.93 1.76 outlet pressure 120 122.2 4.3 pout [kpa] 200 199.2 12.5 300 299.1 13.1 table 3. experimental parameters with deviations figure 5. detail of the tube surface the measured quantities and their values are given in tab. 4. the power is calculated by the multiplication of electric current and voltage. the heat flux is then estimated by dividing the power by the heated surface which is equal to π · din · l, where din is the inner diameter of the annulus and l is the heated length. the relative deviation of the critical heat flux is calculated as: ∆q q = ∆q q + ∆din din + ∆l l . (1) the mass flux is calculated by dividing the measured mass flow rate f by the flow area of the annulus and the relative deviation can be therefore determined as: ∆g g = ∆f f + 2 · dout∆dout d2 out − d2 in + 2 · din∆din d2 out − d2 in . (2) measured quantity range accuracy pressure [kpa] 100 300 2.78 kpa temp. inlet [◦c] 64 91 0.3 ◦c mass flow [kg/hod] 152 229 0.45 % mass flux [ kg/(m2/hod) ] 400 600 3.7 % power [kw] 30 0.6 % heat flux [ kw/m2] 1050 2650 1.7 % table 4. accuracy of measurement devices 3.1. operational experience temperature of the surface rose above 500 ℃ several times during the boiling crisis and the dried-out spot started glowing. this happened due to a relatively long distance between thermocouples. that resulted in a delay between temperature excursion and its detection, whenever boiling crises happened in between tcs and at different sides of the test tube than the tcs were pressed to. most of the time, glowing occurred at the top of the test section, but it could occur in the middle as well, see fig. 6. an example of observed glowing of the test section can be seen in the figs. 7 and 8, where the glowing spots are clearly visible. as can be seen in the pictures, there is also a noticeable layer of oxides. this layer most likely consists of nickel oxides from the inconel and deposits from the loop, since the experimental loop components are mostly made of carbon steel. however, the effect of the oxide layer is not straightforward and will be the subject of further investigation. using a high speed camera, it was detected that prior to boiling crisis, dryout of the surface can occur without developing into chf. fig. 9 shows the evaporation of the liquid film and growth of the dry area. the dry area continues to grow, but leidenfrost temperature was not exceeded and therefore the surface 17 l. suk, k. števanka, t. petrosyan, d. vlček acta polytechnica ctu proceedings figure 6. glowing during chf figure 7. detail of the test section with post glowing spots figure 8. test section after experiments was rewetted as can be seen in the right picture in fig. 10. this particular dryout incident happened between 7.3 and 7.2 seconds before chf was reached1. figs. 11 and 12 show the developed boiling crisis during which glowing occurred. the picture on the left in fig. 11 shows already developed boiling crisis. the darker shade of grey at the bottom of the highlighted area indicates the onset of glowing. despite the high temperature, tcs did not detect chf yet and the surface continued to heat up2. the picture on the right shows that the water came into contact with the surface, however the temperature was higher than the rewetting temperature and the water evaporated immediately after it interacted with the surface. the 1each picture shows the same frame with and without highlighted parts. 2this happened 0.3 s before the detection. figure 9. dryout of tube surface figure 10. rewetting of tube surface moment when tcs detected temperature excursion is depicted in fig. 12. on the right picture the power source is already turned off, nevertheless water does not rewet the surface right after the shut-down and water flows around the spot, this is visible at the bottom of the highlighted area. 4. results and discussion the critical heat flux values for different mass fluxes are shown in fig. 13. most of the experiments were conducted at mass fluxes of 400 and 600 kg/(m2.s), but a few were done at 500 kg/(m2.s). the values differ substantially, as was mentioned above, some of the lowest values are unreliable due to the occurrence of oscillations. one of those instances was the experimental run at 400 kg/(m2.s) and 120 kpa with chf value of 1090 kw/m2, captured on the camera (fig. 11). the experiment was rushed and it led to instabilities, there was a pause between runs and the condenser cooled down, it would be desirable to keep heating up the loop longer than was the case with other experiments. the same is the case for the lowest chf value, 18 vol. 28/2020 inconel 625 tube: critical heat flux investigation figure 11. glowing during chf while more than half of the results at 120 kpa and 400 kw/m2 varied between 1350 1450 kw/m2, the lowest recorded chf value was 825 kw/m2. thus it can be assumed that values lower than 1200 kw/m2 are not reliable. this was the first experiment of the day, the degassing procedure was done the day before, and the heating period was too short. similarly, the lowest value for 300 kpa and 400 kg/(m2.s) is inaccurate because it was the first experiment of the day and the loop was not properly heated up. in general, the first experiments on any given day did not yield trustworthy results. the trend of ramping up power is depicted in figs. 15, 17 and 19 which show tcs temperatures during runs. each experimental run started with a 15 min heating period, followed by a slow continuous increase in supplied power. the power increase was usually between 15 and 20 kw over 30 min depending on the mass flow and pressure, as can be seen from the linear growth of temperature. chf was reached during this period, the sudden increase of temperature is also visible in the graphs. preceding the temperature excursion and chf, there seems to be a higher variation of temperature in the test section. this might indicate surface dryout and re-wetting without reaching boiling crisis, this could be one of the reasons of the relatively high discrepancy between results. nevertheless, flow boiling is a highly dynamic process and most of the results for the same parameters are within 100 kw/m2 range from each other. pressure graphs in figs. 14, 16 and 18 show oscillations of inlet pressure, this does not affect the outlet pressure too much, however it still has a negative effect on the experiments. this is most likely caused by the hoses that connect the test section with the figure 12. post chf overheated surface figure 13. results of the experiments preheater, a throttling valve is placed directly behind the preheater, thus allowing the inlet pressure to be affected by dynamic changes of the flow within the test section. it is assumed that the situation would be improved by moving the throttling valve directly in front of the test section to minimize the volume of water between the valve and the inlet of the section. another issue is the rigidness of the hoses, which is much lower than steel tubes and allows for a higher level of vibration. 5. conclusion the flow boiling chf experiments were done using the annular geometry of the test section, heated by dc power source. the experiments were performed on inconel 625 tube abraded with 150 grit sandpaper at low pressure and low flow. 19 l. suk, k. števanka, t. petrosyan, d. vlček acta polytechnica ctu proceedings figure 14. pressures during 120 kpa and 400 kg/(m2.s) figure 15. temperatures during 120 kpa and 400 kg/(m2.s) figure 16. pressures during 300 kpa and 400 kg/(m2.s) 20 vol. 28/2020 inconel 625 tube: critical heat flux investigation figure 17. temperatures during 300 kpa and 400 kg/(m2.s) figure 18. pressures during 300 kpa and 600 kg/(m2.s) figure 19. temperatures during 300 kpa and 600 kg/(m2.s) 21 l. suk, k. števanka, t. petrosyan, d. vlček acta polytechnica ctu proceedings the chf was substantially affected by the startup sequence as it was important to properly heat up the loop to prevent excessive oscillations. after studying high-speed camera recordings, it was found out that the surface may dry out without overstepping leidenfrost temperature and initiating a boiling crisis. this could be one of the reasons of the relatively high variations between results. oxide layer formed on the tube surface during the investigation, however the exact nature of the layer and its effect on the chf could not be discerned during this work and will be studied further. these experiments will be followed by experiments on smooth and bead blasted surface and on smooth zirlo tube. after these experiments the loop will be upgraded based on the knowledge and experience gained during the operation and it will be used to further study chf at low pressure and flow, the effect of oxide layer on chf and boiling crisis on atf fuels. list of symbols di inner diameter [mm] do outer diameter [mm] dhe heated equivalent diameter [mm] δ gap width [mm] f flow rate [kg s−1] g mass flux [kg m−2 s−1] ∆hin inlet subcooling [kj kg−1] l length [mm] pin inlet pressure [kpa] q heat flux [kw m−2] tin inlet temperature [◦c] rsm mean width of the profile [µm] ra roughness average [µm] rt max. height of the profile [µm] rz average max. height of the profile [µm] atf accident tolerant fuel chf critical heat flux but brno university of technology acknowledgements the research was supported by the project ltausa18198 and is part of a cooperation between but and texas a&m university. another support was also possible via czech technical university in prague with its student grant contest project with the code sgs20/187/ohk4/3t/14. authors are thankful to their supervisor and pi of the project, karel katovsky, ph.d., for his support and advice during the project. references [1] h. o’hanley, c. coyle, j. buongiorno, et al. separate effects of surface roughness, wettability, and porosity on the boiling critical heat flux. applied physics letters 103 (2)(2):1–5, 2013-07-08. doi:10.1063/1.4813450. [2] d. h. kam, j. h. lee, t. lee, y. h. jeong. critical heat flux for sicand cr-coated plates under atmospheric condition. annals of nuclear energy 76:335–342, 2015. doi:10.1016/j.anucene.2014.09.046. [3] a. f. ali, j. p. gorton, n. r. brown, et al. surface wettability and pool boiling critical heat flux of accident tolerant fuel cladding-fecral alloys. nuclear engineering and design 338:218–231, 2018. doi:10.1016/j.nucengdes.2018.08.024. [4] g. liang, y. chen, h. yang, et al. nucleate boiling heat transfer and critical heat flux (chf) from micro-pit surfaces. international journal of heat and mass transfer 152:119970, 2020. doi:10.1016/j.ijheatmasstransfer.2020.119510. [5] thermohydraulic relationships for advanced water cooled reactors. iaea, vienna, 1st edn., 2001. [6] c. haas, l. meyer, t. schulenberg. flow instability and critical heat flux for flow boiling of water in a vertical annulus at low pressure. asme/jsme 2011 8th thermal engineering joint conference 2011:11, 2011-03-13. doi:10.1115/ajtec2011-44292. [7] c. haas, t. schulenberg, t. wetzel. critical heat flux for flow boiling of water at low pressure in vertical internally heated annuli. international journal of heat and mass transfer 60:380–391, 2013. doi:10.1016/j.ijheatmasstransfer.2012.12.038. [8] s.-y. chun, h.-j. chung, s.-k. moon, et al. effect of pressure on critical heat flux in uniformly heated vertical annulus under low flow conditions. nuclear engineering and design 203(2-3):159–174, 2001. doi:10.1016/s0029-5493(00)00307-1. [9] m. p. fiori, a. e. bergles. model of critical heat flux in subcooled flow boiling. cambridge, mass. : m.i.t. dept. of mechanical engineering, 1968. [10] m. s. el-genk, s. j. haynes, k. sung-ho. experimental studies of critical heat flux for low flow of water in vertical annuli at near atmospheric pressure. international journal of heat and mass transfer 31 (11)(11):2291–2304, 1988. doi:10.1016/0017-9310(88)90161-5. [11] t. schoesse, m. aritomi, y. kataoka, et al. critical heat flux in a vertical annulus under low upward flow and near atmospheric pressure. journal of nuclear science and technology 34 (6)(6):559–570, 1997. doi:10.1080/18811248.1997.9733709. [12] j. t. rogers, m. e. salcudean, a. e. e. tahir. flow boiling critical heat fluxes for water in a vertical annulus at low pressure and velocities. proceeding of international heat transfer conference 7 pp. 339–344, 1982. doi:10.1615/ihtc7.1180. [13] k. mishima, h. nishihara. effect of channel geometry on critical heat flux for low pressure water. international journal of heat and mass transfer 30 (6):1169–1182, 1987. doi:10.1016/0017-9310(87)90046-9. [14] g. mayer, r. nagy, i. nagy. an experimental study on critical heat flux in vertical annulus under low flow and low pressure conditions. nuclear engineering and design 310:461–469, 2016. doi:10.1016/j.nucengdes.2016.10.026. [15] d. vlcek, l. suk, k. stevanka, t. petrosyan. experimental investigation of critical heat flux in annulus at low pressure and low flow parameters [preprint]. acta polytechnica ctu proceedings 2020. [16] čsn en iso 4288. czech office for standards, metrology and testing, 1999. 22 https://doi.org/10.1063/1.4813450 https://doi.org/10.1016/j.anucene.2014.09.046 https://doi.org/10.1016/j.nucengdes.2018.08.024 https://doi.org/10.1016/j.ijheatmasstransfer.2020.119510 https://doi.org/10.1115/ajtec2011-44292 https://doi.org/10.1016/j.ijheatmasstransfer.2012.12.038 https://doi.org/10.1016/s0029-5493(00)00307-1 https://doi.org/10.1016/0017-9310(88)90161-5 https://doi.org/10.1080/18811248.1997.9733709 https://doi.org/10.1615/ihtc7.1180 https://doi.org/10.1016/0017-9310(87)90046-9 https://doi.org/10.1016/j.nucengdes.2016.10.026 acta polytechnica ctu proceedings 28(0):1–8, 2020 1 introduction 2 experimental setup 3 methodology 3.1 operational experience 4 results and discussion 5 conclusion list of symbols acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2018.20.0051 acta polytechnica ctu proceedings 20:51–55, 2018 © czech technical university in prague, 2018 available online at http://ojs.cvut.cz/ojs/index.php/app comparison of total pressure fluctuations in steam turbine stages with different degree of reaction tomáš jelíneka,∗, martin němeca, petr milčákb a czech aerospace research centre, beranových 130, prague 9, czech republic b doosan škoda power, tylova 1/57, pilsen, czech republic ∗ corresponding author: jelinek@vzlu.cz abstract. the contribution deals with analysis of unsteady pressure measurement in one and half axial turbine stages. the measurement was performed in vzlu air test turbine. two stages geometries with different degree of reaction were tested in this work. comparison of unsteady flow field behaviours is discussed. keywords: turbine stage, stator-rotor interaction, driving forces. 1. introduction a multistage arrangement design of turbines is more and more complex and it is necessary to combine aerodynamic and strength aspects. this evolution of design procedures is followed also in a long-term cooperation of vzlu with the leading producer of steam turbines doosan škoda power (dspw). blades excitation is strongly affected by axial gap, blade count ratio and overall design of the stage. combined effect of the axial gap and blade count ratio was deeply studied by chen et al. in [1]. their results show resonance effect of stator and rotor row for huge number of combinations of blade count ratios and axial gaps. the resonance effect is caused by interaction of the rotor potential field with the stator outlet flow field. moreover, in the case of a multistage turbine, the interstage interaction and clocking effect influences the flow field development what presented reinmöller at al. in [2] and porreca at al. in [3]. a sensitivity of stage geometry on the excitation is presented by gao et al. in [4], where decrease of stimulus was achieved by using a negative bowed stator geometry. many other works are focused on optimization of the stage geometry. such studies are performed with help of numerical simulations and usually it is necessary to use scaling a number of blades to reduce computational costs. this could generate a fairly large discrepancy as showed clark in [5]. this work is unique because of two degrees of reaction and various stator aspect ratios were experimentally investigated on one and half stage configuration. experimentally obtained unsteady flow field parameters were analyzed and comparison of the stimulus behaviours of novel steam turbine stage design was performed. driving forces were not determined, but only pressure fluctuations were computed from unsteady flow field parameters measurements. this approach is sufficient for the comparison of various cases. a reaction stage and a stage with increased reaction figure 1. the one and half stage cofiguration of the test rubine. (marked as reac and ir, respectively) were studied in nominal and off-design regimes. the experiments caught unsteady flow field behaviours at the outlet of the rotor and also at the outlet of the second stator. an influence of degree of reaction in combination with stator aspect ratio and rotor rotational speed is studied in this work. 2. test turbine configuration the experimental work was performed in vzlu in the axial turbine stage test rig. the rig was adapted to the one and half turbine stage to study the multistage flow structures [6]. the one and half stage facility is depicted in fig. 1. the configurations of tested stages are characterized in tab. 1. an axial gap ratio (gr) and a vane/blade count ratio (bcr) are important parameters in stimulus point of view, therefore these parameters are also included in this table. detailed flow field investigation was performed by a miniature five-hole pressure probes (tip diameter 1.2 mm) and with a fast response pressure probes (frpp) developed in vzlu [7]. 51 http://dx.doi.org/10.14311/app.2018.20.0051 http://ojs.cvut.cz/ojs/index.php/app t. jelínek, m. němec, p. milčák acta polytechnica ctu proceedings reac v2-1 & v2-2 ir v1-2 & v1-2 stator aspect ratio 1st / 2nd 2.0 / 1.1 1.0 / 1.0 rotor aspect ratio 1.71 1.94 stage isentropic mach number 0.30 0.40 mid-span reaction 0.43 0.12 optimum velocity ratio u/cis 0.65 0.55 midspan gap ratio plane 1 / 2 0.23 / 0.67 0.11 / 0.63 blade count ratio plane 1 / 2 0.61 / 1.09 0.53 / 1.88 table 1. the turbine stage geometry and operational regime parameters. (a) . reac stage. (b) . ir stage. figure 2. the first rotor excitation for two stator aspect ratios. 3. the first stator excitation the first stator is specified by uniform inlet flow in this test turbine configuration, so the excitation of vanes is caused only due to rotor potential field. it is shown in [1], that the worst case occurs for bcr approximately 0.7, where the resonance appears. the bcr is sufficiently far from the resonance in this case. the excitation by the rotor potential field is captured in the case of the ir stage also by the fast response pressure probe placed in the plane 1, see fig. 1. measured amplitude is very low what is consistent with bcr value 0.53. radial distribution of the absolute total pressure amplitude is shown in fig. 4b. the absolute value of the amplitude is approximately four times lower compared to the rotor excitation, that is in agreement with [1]. influence of the rotational speed on the excitation of the first stator is shown in fig. 6a. 4. the first rotor excitation the first rotor excitation is mainly caused by passing the first stator wakes. the excitation of four cases is shown in following graphs fig. 2a and fig. 2b. these graphs show comparison of two stator families and two aspect ratios for each family. high aspect ratio stator geometry is characterised by a stronger decrease of the amplitude with increasing degree of harmonic component. low aspect ratio stator has a very similar excitation for three harmonics in the midspan region. the highest excitation is captured in secondary flow regions. 5. the second stator excitation the excitation of the second stator is caused by rotor wakes at the inlet of the stator. the relative pressure fluctuations are the highest in turbine stage (shown in fig. 5a and fig. 5b). nevertheless, the second stator excitation in absolute values point of view is approximately three times lower compared to the rotor excitation radial distribution is shown in fig. 4a and fig. 4b amplitude scale is kept the same in both graphs. the excitation of the second stator at the outlet caused by the first rotor wakes is slightly lower compared to the inlet excitation. as the second rotor is not present in this configuration, there is no excitation by second rotor potential field, which could cause additional stator excitation. the second stator excitation is moreover dependent on the clocking with the first stator. clocking effects are discussed in [8] for the case of the reac stage. 6. the second rotor excitation although the second rotor was not present in the test turbine, the excitation of the second stage rotor can be obtained from the second stator time averaged outlet flow field. radial distribution of the rotor excitation is shown in fig. 3a (relative values) and in fig. 3b (absolute values). both second stator geometries had 52 vol. 20/2018 comparison of total pressure fluctuations in steam turbine stages (a) . relative total pressure amplitudes. (b) . absolute total pressure amplitudes. figure 3. the second rotor excitation reac and ir stage. (a) . reac stage. (b) . ir stage. figure 4. absolute total pressure amplitudes. (a) . reac stage. (b) . ir stage. figure 5. relative total pressure amplitudes. 53 t. jelínek, m. němec, p. milčák acta polytechnica ctu proceedings (a) . the first stator of ir stage. (b) . the second stator of ir and reac stage. figure 6. influence of rotational speed on stators excitation. (a) . reac stage. (b) . ir stage. figure 7. the second rotor excitation influence of rotational speed. low aspect ratio, so a decrease of the amplitude with increase of harmonic component was less significant. 7. the rotor rotational speed influence the rotational speed strongly influences the flow filed in the turbine stage. as a blade incidence angle is changed, separation regions may occur, radial distribution of velocity is affected and thus unsteady flow field behaviours are changed. the potential field of the rotor is also slightly affected by the rotational speed, so the excitation of the first stator is also changed, as it is shown in fig. 6a. as it was said, the first rotor is excited due to its interaction with first stator wakes. the most important for the rotor excitation is time averaged distribution of the pressure at the stator outlet. the time averaged first stator pressure distribution is not seriously affected by the rotational speed, thus the excitation of the rotor is also not changed. the second stator excitation by the rotor outlet flow field is shown in fig. 6b. the variation of the excitation in radial distribution is clearly visible in this figure, namely for the ir stage. a lower expansion in the case of the ir stage causes a higher influences of the rotational speed on the second stator excitation. the highest excitation of the second stator is captured for nominal regimes. the second rotor excitation radial distribution with the rotational speed influence is shown in fig. 7a and fig. 7b for both stages. the radial distribution of the excitation is influenced more in the case of reac stage. extremely overloaded stage has a region of high excitation in the midspan. this effect is not as strong in the case of the ir stage, which is specified by more intensive expansion in the stator. 8. conclusion the excitation of one and half stages was studied for two degrees of reaction in wide range of regimes. the comparison of excitation results of stages with two degrees of reaction shown excitation insensitivity in the relative point of view. absolute values of excitation are of course higher in the case of lower reaction stage, because of higher dynamic pressure, which is used for the normalization. the first and the second rotor relative excitation stays nearly the same. this statement is valid for average excitation, because distribution of the excitation along blade height is strongly affected by the second stator secondary flow development. the second stator is strongly affected by the interaction with 54 vol. 20/2018 comparison of total pressure fluctuations in steam turbine stages rotor tip leakage flow field. in the case of off-design regimes, the secondary flow development is affected by a change of the second stator incidence angle. the second rotor absolute amplitudes are slightly higher compared to the first rotor. this is caused by a higher outlet mach number of the second stator compared to the first one. stator aspect ratio can influence damping of the excitation of higher harmonic components. it was tested in the case of the first rotor. excitation of the stator by the rotor potential field was also discussed and tested in the case of lower reaction stage. the results show that the excitation is the weakest in the nominal regime and grows with deviation from the nominal regime. the excitation caused by the potential field is generally weak for the cases where resonance does not occur. list of symbols cis isentropic stage velocity [m/s] l nondimensional blade height [1] q dynamic pressure [pa] u rotor circumferential velocity [m/s] ar aspect ratio bcr blade count ratio dsp w doosan škoda power f rp p fast response pressure probe gr axial gap ratio h harmonic component ir increased reaction stage reac reaction stage rp m nominal rotor rotational speed acknowledgements the work was done with financial support of the technology agency of the czech republic on the project ta04020129 "investigation of flows in a multi-stage axial turbine stages arrangement". references [1] t. chen, k. patel, p. millington. combined effects of both axial gap and blade count ratio on the unsteady forces of a steam turbine stage. asme paper no. gt2012-68874. [2] u. reinmöller, b. stephan, s. schmidt, n. r. clocking effects in a 1.5 stage axial turbine steady and unsteady experimental investigations supported by numerical simulations. asme paper no. 2001-gt-0304. [3] l. porreca, a. i. kalfas, r. s. abhari. interstage flow interactions and loss generation in a two-stage shrouded axial turbine. journal of turbomachinery 131(january), 2009. [4] k. gao, t. lin, z. xie, d. zhang. investigation on unsteady flow of a turbine stage with negative bowed stator. international journal of energy and power engineering 11(4), 2017. [5] j. p. clark, g. m. stetson, s. s. magge, et al. the effect of airfoil scaling on the predicted unsteady loading on the blade of a 1 and 1/2 stage transonic turbine and a comparison with experimental results. asme paper no. 2000-gt-0446. [6] m. nemec, t. jelinek, p. milcak. an investigation of a flow field in a one and half axial turbine stage. aip conference proceedings 1889(020025), 2017. [7] m. nemec. development of fast response pressure probe for turbomachinery applications (in czech). pse 2009, pilsen, 2009. [8] m. nemec, t. jelinek, p. milcak. clocking of stators in one and half stage of axial steam turbine. epj web of conferences 180(02071), 2018. 55 acta polytechnica ctu proceedings 20:51–55, 2018 1 introduction 2 test turbine configuration 3 the first stator excitation 4 the first rotor excitation 5 the second stator excitation 6 the second rotor excitation 7 the rotor rotational speed influence 8 conclusion list of symbols acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2018.16.0032 acta polytechnica ctu proceedings 16:32–36, 2018 © czech technical university in prague, 2018 available online at http://ojs.cvut.cz/ojs/index.php/app the interaction of floods with transport infrastructure ivan vaníčeka,∗, jiří vaníčekb a czech technical university in prague, faculty of civil engineering, department of geotechnics, thákurova 7, 166 29 prague, czech republic b geosyntetika ltd., nikoly tesly 3, 160 00 prague, czech republic ∗ corresponding author: ivan.vanicek@cvut.cz abstract. current demands on transport infrastructure are very often associated with the terms of “sustainability, availability and affordability”. the first term directly refers to the basic principles of sustainable construction in transport infrastructure, primarily motorway and railways, and how they are related to a very significant consumption of land, energy and natural aggregates. this paper is focused on the other two terms, availability and affordability. these principles are intended to guarantee proper functioning of communication even in nonstandard situations such as natural hazards, most typically floods. to avoid a total collapse of transport infrastructure, the term robustness is often applied, recognizing that this term is not a substitute for another outcome – namely a more expensive structure. the paper shows the possibilities for elimination of the negative impact of floods on transport infrastructure as they relate to different types of interaction and different types of floods. keywords: flood protection, transport infrastructure, dams. 1. different types of floods and a basic case of interaction in principle three types of floods are differentiated: • flash floods, caused by extreme rainfall on a relatively small river basin. here, prognosis is very difficult. • spring floods – situated at the foot of mountains, where floods are caused by the synergy of snow melting with heavy rainfall. prognosis with respect to place and time is much better here and countermeasures can be given a long term character. • regional floods, affecting a large area involving more river basins, and having duration of a few days up to several weeks. prognosis is average here, based on climate and especially rainfall forecasting. with respect to the relatively small area affected by flash floods, the interaction with transport infrastructure is focused on two basic examples: • erosion of the communication route running parallel with a river stream. • the threat to bridges crossing this river stream. dam systems, which in the czech republic are more than a century old, are the main countermeasures against spring floods. the dam system in jizera mountains, or rather at the foot of these mountains, is such an example. when the capacity of these dam reservoirs is not sufficient, then the interaction has a similar character as for the previous case. floods in 1997 and 2002 can be seen as classical examples of large regional floods. during the floods in 2002, there were two peaks in intensity of rainfall very close to each other, separated by roughly 2 or 3 days only. after the first peak, the land was saturated and therefore nearly all precipitation was running directly to the water course. in this case historical small dams on the small rivers and new large dams and the main water courses (as in the vltava river cascade) acted as the main engineered structures in order to retain and flatten the flood wave. therefore to the previously mentioned interactions, we can add overflowing with the potential failure of small historical dams as their dam crest is often used as a transport corridor. overflowing with potential failure of dikes along a river is another important case of interaction, as it can cause flooding of a wider developed area also containing transport infrastructure. with respect to transport infrastructure, the focus here is on motorways and railways and not on other possibilities such as airports or parking space. flooding of the prague metro during the floods in 2002 would really deserve specific attention [1]. the basic case of interaction studied here is the flooding of transport infrastructure, where the water is above its level and a structure is thus rendered unusable. nevertheless, often the damage is relatively small and the transport infrastructure can be used shortly after flood wave cessation. the bridge over the river labe, flooded on august 15 and reopened on august 20, 2002, would be a good example, see figure 1. in other cases the interaction can lead to part or complete failure of the structure of transport lines. the different types of failure are discussed in the next chapter together with practical examples. classifica32 http://dx.doi.org/10.14311/app.2018.16.0032 http://ojs.cvut.cz/ojs/index.php/app vol. 16/2018 floods and transport infrastructure figure 1. re-opening of the bridge in ústí nad labem five days after flooding. tion of these failures is consistent with the eurocode 7 – en 1997 geotechnical design. 2. the character of transport infrastructure failures practical knowledge based on particular cases shows that failure of the earth structures in transport network systems is very often connected with a limit state of failure of the type hyd, that is with erosion. this erosion can be either surface or internal. surface erosion is typical for crest overflow or as the result of flowing water contact with an upstream slope. a typical case of slope instability was in practice not actually observed and such a failure would deserve a more demanding explanation. 2.1. floods in the blatná region in 2002 the specificity of floods in the blatná region was due to the combination of heavy rainfall with additional waves from failed dams. seven small dams were affected on the small river of lomnice, of which five failed completely. for this small river basin the authors specified a so called domino effect of failures. at the beginning of this basin, there is situated the most significant dam (the highest with a larger reservoir volume) and if it fails, then the others situated below have a limited chance to survive [2–4]. failure by surface erosion prevails, but at least two failures can be attributed to internal erosion where seeping water, due to a high hydraulic gradient, used the most disposed path along which internal erosion started. for the lomnice river basin, there was a chance to compare the floods which occurred in 1895 with those in 2002. although for the first case the crest overflow was smaller, all 7 dams failed. in 2002 two dams did not fail completely, namely the dams the crests of which were covered by an asphalt carpet. figure 2 shows one such example. another example of the interaction was observed on this river basin. the river lomnice crosses the railway track blatná – nepomuk. the relatively large culvert was not able to transfer the supplemental flood wave from the above situated failed dam. this was probably also due to a partially blocked flow profile caused by transported vegetation, by shrub and smaller trees. a subsequent overflowing onto the track started and the erosion left the rails on the track raised above the ground, as shown in figure 3. numerical modelling of the flood wave is very important for the basin where a domino effect can be expected. this brings an estimation of its peak when arriving below the situated dam. this modelling should include also the potential interaction with transport infrastructure. in some safer cases, this interaction can play a positive role. delay and decrease of the flood wave passing through the culvert can have this positive impact when reaching below the situated reservoir and dam crest [5]. 2.2. floods in the frýdlant region in 2010 significant rainfall in the northwest part of jizera mountains brings a very quick rise to flood wave on the small rivers smědá and řasnice which flow through the town of frýdlant. most of the town was flooded on saturday august 7th, 2010. the roads changed into rivers. regrettably some people died. damage to properties, on transport lines and on infrastructures was very high. figure 4 shows the example where a parallel communication line was partly eroded by water stream in interaction during floods. a brief note can be added on the process of reconstruction of a damaged transport infrastructure similar to the above mentioned case. in the frame of an intention to repair the transport line as soon as possible, the reconstruction was solved under extra-ordinary conditions without a deeper discussion and preparation about the most appropriate solution. therefore there is some possibility that with the next floods the transport infrastructure will fail in the same place. however, this type of lateral interaction – erosion – also can be connected with slope instability, which can 33 ivan vaníček, jiří vaníček acta polytechnica ctu proceedings figure 2. example of disturbance of the downstream slope and crest of a dam which is used as an important communication line – e 49. figure 3. the erosion of blatná – nepomuk railway embankment. the water table shown on photo is typical for an ordinary flow rate. 34 vol. 16/2018 floods and transport infrastructure figure 4. an example of lateral erosion of a transport embankment in the frýdlant region, 2010. figure 5. failure of the railway track bari – taranto, 2005. 35 ivan vaníček, jiří vaníček acta polytechnica ctu proceedings be simulated later on. failures in similar cases always represent a limit situation, a limit state of failure, on which present-day codes are fixed. regrettably the back analysis is not performed immediately as no team of experts is present or sent. eventually, in most cases it is difficult to specify the extreme loading, or maximal water level, during which failure occurred. 2.3. floods in the bari region in 2005, italy the problem of floods and their interaction with transport infrastructure was the main concern of the study by the post-graduate student m.burdo [6]. during heavy rainfall the surface runoff was greater than the capacity of the culvert below the railway track bari – taranto. the embankment was damaged, and only due to a rather positive coincidence the problem had no exceptional impact as the train stopped just in front of the damaged part of the embankment, see figure 5. therefore the major part of this post graduate study was connected with analytical and numerical methods of investigating slope stability, and later on with a combination of more individual aspects. the infiltration of retained water into the partially saturated embankment was simulated, and then the subsequent impact on slope instability was checked. after this the rainfall on the “downstream slope” was also simulated. air bubbles were enclosed between saturated zones, and with continuing infiltration, the pressure in the air bubbles went up and this can cause the uplift of the saturated downstream slope. therefore the subsequent collapse can be associated both with internal erosion and with slope instability. the credibility of such a scenario was subject to a simple modelling in the laboratory. dry sand was deposited in a vertical glass tube and covered on top by a saturated layer. a vertical tube was submerged into a vessel with water. this water, due to capillary force, went up and closed the air bubbles between the saturated zones. the air pressure went up to the limit state when the saturated zone at the top was separated, and lifted up. this simple model proved that this scenario was plausible and it can explain many spread shallow landslides which occur in partially saturated soils during heavy rainfall. 3. conclusions the interaction of floods with transport infrastructure has a very significant impact on functionality of the system. in most cases this impact is in a negative direction. the presented examples point to the limit states of failure of an earth structure during extreme loading and through this show how to proceed in the future. for a current state this means selecting the most critical places and subsequently supervising their reinforcement, or in cases of reparation of failed parts by selecting the most appropriate methods, preferably in advance. at the same time it is appropriate to distinguish the earth structures which are in contact with water in the long term as in historical small dams the crest of which are used as transport line, and which possibly are in contact with water only in an exceptional case. the last case represented by motorway or railway embankments and by dykes, need some special attention as soils are unsaturated during first contact with flood wave. references [1] k. soga, i. vaníček, a. gens. micro-measurement and monitoring system for ageing underground infrastructure. ctu press, prague, 2011. [2] i. vaníček. embankments of small water reservoirs and dykes, keynote lecture, czech icold dam days, české budějovice, 2004. [3] i. vaníček, j. vaníček. rehabilitation of old earth dams failed during heavy floods in 2002. in proceedings of the conference on new development in dam engineering. a. a. balkema publishers, london, 2004. [4] i. vaníček, m. vaníček. earth structures in transport, water and environmental engineering. springer, netherlands, 2008. doi:10.1007/978-1-4020-3964-5. [5] i. vaníček, j. vaníček, t. pecival. risk evaluation of a domino effect of dam failure on small river-basins. in proceedings of the european conference on flood risk management. e3s web of conferences, lyon, 2016. doi:10.1051/e3sconf/20160707009. [6] m. burdo. final report of the postgraduate traineeships, 2016. 36 http://dx.doi.org/10.1007/978-1-4020-3964-5 http://dx.doi.org/10.1051/e3sconf/20160707009 acta polytechnica ctu proceedings 16:32–36, 2018 1 different types of floods and a basic case of interaction 2 the character of transport infrastructure failures 2.1 floods in the blatná region in 2002 2.2 floods in the frýdlant region in 2010 2.3 floods in the bari region in 2005, italy 3 conclusions references acta polytechnica ctu proceedings doi:10.14311/app.2018.20.0108 acta polytechnica ctu proceedings 20:108–113, 2018 © czech technical university in prague, 2018 available online at http://ojs.cvut.cz/ojs/index.php/app cfd flow analysis of the cantilever micro-turbine tomáš sykaa,∗, andreas p. weißb a new technologies research centre (uwb), univerzitní 8, pilsen, czech republic b oth amberg-weiden, kaiser-wilhelm-ring 23, amberg, germany ∗ corresponding author: tsyka@ntc.zcu.cz abstract. the article deals with a description of results from research and development of the cantilever micro-turbine designed for the orc power plants. the experimental turbine stage and used numerical models are briefly described. in the first part the comparison of measured and cfd results is presented. the second part deals with an investigation of the outlet geometry influence on the turbine working parameters. the results are relevant for next studies in the research and development process. keywords: cfd simulation, numeca, orc, cantilever micro-turbine. 1. introduction organic rankine cycle (orc) power plants are a viable option for decentralized small scale stationary energy converters (< 100 kwel) heated by e.g. waste heat in industry, waste heat of internal combustion engines, geothermal heat or even solar radiation. an orc power plant works like a coal fired steam power plant: a steam power plant pumps water on high pressure, heats and evaporates it in the steam generator, expands the steam in a turbine which drives the generator and finally condenses it back to liquid state in a condenser. the difference of orc to conventional steam cycle is that instead of water, another fluid like e.g. an alcohol, a refrigerant or siloxanes is used. more information about micro-turbines problematic in general can be found in [1] and [2]. this article deals with possibilities of cfd simulations of small turbines which usually work with fluids mentioned above. gained results are compared to measured values from measurement on the test rig. concretely, results from cfd simulations and measurement of the cantilever turbine with radial inlet direction into stator channels and axi-radial outlet from rotor are shown. as a working fluid hexamethyldisiloxane was used. in order to investigate real characteristics the cantilever turbine was tested in the orc research plant at the center of energy technology at the university of bayreuth. main goal of this article is to show evaluated turbine working parameters and find out the differences between computed and measured results. it is possible to evaluate many turbine stage parameters but for a purpose of this article we were focused on a total-tostatic efficiency evaluation computed from the torque and a turbine stage pressure ratio evaluation which are the most important parameters. next goal of our work was to find out an influence of the outlet casing geometry on the turbine efficiency and try to design more suitable shape of the outlet figure 1. architecture of the radial inflow cantilever turbine. casing because we were expecting large flow separation areas in this part. 2. the tested turbine and the orc research plant design of the cantilever micro-turbine is compact and simple in comparison to a general water steam turbine. it is necessary to develop a very flexible "construction kit" due to a wide range of temperature and pressure operating conditions dependent on a specific heat source. architecture of the investigated turbine is shown in the figure 1. the turbine design including generator is really compact and dimensions of the whole unit are relatively small. maximum diameter of the rotor wheel is 120 mm. main parts of this unit are inlet casing, stator wheel with nozzles, rotor wheel, outlet casing and generator. at this point there are no sealings in the turbine to lower leakage losses. in order to investigate real characteristics the cantilever turbine was tested in the orc research plant at the center of energy technology at the university of bayreuth. the research plant was designed to investigate waste heat recovery from a 250 kw biogas internal combustion engine with an exhaust gas temperature of about 500 °c. process simulations and theoretical investigations showed that for this rather high exhaust temperature hexamethyldisiloxane is a suitable working fluid [3] 108 http://dx.doi.org/10.14311/app.2018.20.0108 http://ojs.cvut.cz/ojs/index.php/app vol. 20/2018 cfd flow analysis of the cantilever micro-turbine figure 2. orc plant layout. figure 3. photo of the orc plant. and [4]. the layout of the research plant is shown in figure 2. orc plant consists of a feed pump, an evaporator, an expander, a recuperator and a condenser. however, the evaporator is directly heated by the exhaust gas and does not use an additional thermal oil loop. the mass flow rate is measured by a coriolis device ("fi-204" in figure 2). pressures (pi) and temperatures (ti) are measured upstream and downstream of each component. thus, the efficiencies of all components can be calculated. for full load, i.e. high mass flow rates, the turbine efficiency can be calculated reliably on the basis of the measured turbine exit temperature. however, for small part load the heat losses are not negligible. therefore, to avoid using a measured exit temperature, the applied electrical conversion chain was investigated separately in advance. for this purpose, the generator was driven by an electric motor. a torque meter between motor and generator was used to measure torque and rotational speed to get knowledge about the mechanical power input. electric power fed into the grid was measured by the 25 kw feed-in unit. thus, the overall electrical efficiency of the entire electrical conversion chain could be determined as a function of power [5]. discussed total-to-static isentropic turbine efficiencies (eq. 1) use actual enthalpy drop determined by measured turbine power (pt ur/ṁ) divided by the ideal total-to-static isentropic enthalpy drop (ht0 − hs2,is). ηis,ts = pt ur ṁ(ht0 − hs2,is) = pel ṁ ηel(ht0 − hs2,is) (1) 109 tomáš syka, andreas p. weiß acta polytechnica ctu proceedings 3. cfd setup and mesh parameters 3.1. micro-turbines cfd simulation issues cfd simulations of micro-turbines have specifics given by turbines functional principle. working medium is entering into the turbine with a low velocity and high pressure values but the expansion to very low pressure occurs already in the stator nozzles. this is connected with a high flow velocity increase. flow velocity at the stator nozzles outlet is supersonic and mach number value is usually higher than 2.5. gained kinetic energy is afterwards transformed in the rotor wheel into work. flow velocity increase is achieved by a special stator nozzles geometry. in case of water steam turbine we could talk about stator blades but in case of microturbines the stator nozzles are relatively long and their shape is similar to the laval nozzle. the stator nozzles geometry of the investigated turbine is also characteristic by its non axi-symmetric cross-section. due to this reason non-standard demands are placed on a grid generator abilities. rotor blades geometry is not much interesting on the first sight but due to high rotor inlet velocity it is necessary to design a correct blade geometry at the leading edge. high mach numbers and the presence of shock waves bring complications in modeling of the flow transition between static and rotating domains of the microturbine. in rotating machinery simulation the interface type "stage" is usually used because it is not necessary to keep the same pitch on both sides of the interface. but unfortunately in our case this interface type fails because "stage" can’t handle shock waves deflections from interface boundary condition at such high flow velocities. so we had to use the interface type "frozen-rotor" whose disadvantage is a necessity to model the same domain pitches on both sides of the interface and a consequence of this is the computational grid with higher number of cells. complications connected to presence of shock waves wasn’t removed totally since they were reduced to a local error but the stable solution was achieved. we have tested this behavior in ansys fluent 18, ansys cfx 18 and numeca fine/turbo 11. the results were more or less the same with similar error. due to high changes of physical properties of working gas (hexamethyldisiloxane) in the micro-turbine domain it is suitable to use some variant of medium real gas model instead of usually used ideal gas model. main problematic points of the micro-turbine cfd simulations are: • higher demands on the mesh quality and grid generator abilities. • properties of working medium are defined by real gas model or by thermodynamic tables. • simulation of flow field with high mach numbers. figure 4. stator nozzles. figure 5. rotor blades. • it is not possible to avoid local physical errors at the rotor-stator interface due to the shock waves deflection. • presence of shock waves increases the computational time of the cfd simulation. taking into account these points we chose the numeca fine/turbo software package to prepare, compute and evaluate solved cases. numeca autogrid5t m is a full automatic hexahedral grid generator for all types of rotating machinery: complex axial, radial, and mixed-flow configurations. it is also able to mesh leakage domains and non axi-symmetric domains automatically. properties of the working fluid are defined by generated tables in numeca tabgen. 3.2. mesh parameters as was mentioned we used the numeca autogrid5t m to create the computational mesh. the main asset of the autogrid5 mesh generator is a possibility to create channel and leakage meshes almost automatically. general geometries such as the turbine inlet casing had to be meshed manually. the resulting mesh is hexahedral and multi-block structured. these properties allow to solver to take advantage of the multigrid computation approach. multigrid simulations with a structured mesh usually converge faster than a solution with a unstructured mesh. to achieve precise results we had to create a mesh with good cells quality. the height of the first cells at the wall was 0.0005 mm and this corresponds to maximum y+ value of 3.24. we used 77 cell layers in 110 vol. 20/2018 cfd flow analysis of the cantilever micro-turbine figure 6. isolines of mach number. the radial direction in the main channel domains. the maximum cell expansion ratio was 3.5 and orthogonal quality was over 20 (higher is better up to maximum of 90). 3.3. investigation of the inlet casing influence on the turbine performance at the beginning of the turbine model preparation we had to check a possible level of the geometry simplification due to a computational time saving. main question was if it is necessary to keep the turbine inlet casing in the model because the inlet casing domain is not rotationally periodic. to be able to make decision we had to create simplified and also complete micro-turbine model and compare simulation results. simplified model covers two stator nozzles, five rotor wheel channels, leakage domains and the outlet casing and it’s mesh consists of 32 mil. of cells. this model is shown in the figure 9. the complete model of the micro-turbine is shown in the figure 7. in this case the mesh consists of 220 mil. of cells. comparison of the results from the simulations of both variants showed us that the turbine efficiency evaluated from the complete model is lower approximately by 0.15 % than the efficiency of the simplified model. this difference is caused by friction losses in the inlet casing and more realistic flow field behavior in the outlet casing. in case of the complete model the outlet flow field is not affected by the periodic boundary condition. the flow velocity magnitude at the stator nozzles inlet was quite low (6 ms−1) so we wasn’t expected variable influence on the turbine stage parameters if there would be some diversion of the inlet flow distribution. to confirm that we did analysis of the velocity magnitude profiles at the stator nozzles outlet. the evaluation showed that the flow distribution is not almost affected by inlet casing and the fact that the distribution is mainly affected by actual rotor blades position. from this reason we decided to neglect inlet casing in the model and to use only simplified version for the further simulations. figure 7. complete model of the cantilever microturbine. figure 8. velocity profiles at the stator outlet. 3.4. boundary conditions in the figure 9 the boundary conditions applied on the simplified micro-turbine model are shown. at the stator inlet a total pressure and total temperature boundary conditions were used. at the exit from the outlet casing an average static pressure was used. also an angular velocity was defined on rotating domains and surfaces. for the purpose of our study we used sst k-ω as a default turbulence model but we were expecting separated flow structures in the outlet casing so we also used more complex earsm turbulence model which better describes a flow field separation or an adhesion on walls. 4. results and discussion 4.1. comparing cfd results to measured values as it was mentioned in the introduction the main goal of our work was to do cfd analysis of the cantilever micro-turbine and compare achieved results with measured data. we also used two different turbulence models sst k-ω and earsm in our simulations but we found that it won’t be simple to compare simulated and measured data because of the way how a pressure in the outlet casing was measured. there was only one pressure gauge on the wall determined for the static pressure measuring. in cfd there are of course many ways how to evaluate certain variables an area or a mass flow weighted average, simple average or just evaluating in the specified point. but problem is that user has no possibility to define the static pressure boundary condition as a point boundary condition. to achieve solution similar to measurement we had 111 tomáš syka, andreas p. weiß acta polytechnica ctu proceedings figure 9. boundary conditions. figure 10. outlet velocity streamlines. to literally try to setup the outlet pressure in loops until the needed static pressure in specified point on the outlet casing wall was obtained. the second thing is that there is high probability of the outlet static pressure measuring inaccuracy. the outlet casing is wide opened and the pressure gauge is probably located in an area with the massive flow separation. also it would be better to measure the pressure in more locations than one. the pressure gauge location is indicated in the figure 11. in following figures the flow field in the cantilever turbine with 28 000 rpm is showed. it is a meridional averaged velocity in a meridional plane. the red lines show a flow direction and flow separations from the wall. figure 12 shows simulation results with the sst k-ω turbulence model used and figure 13 shows flow field from the simulation with earsm. there is obvious difference of the flow field character in the outlet casing. in case of simulation results with the sst k-ω turbulence model, the separated flow attaches back to the wall but in case of earsm turbulence model the separated flow is separated in the whole outlet domain. similar behavior can be seen also in other working characteristic operation points. unfortunately we don’t have measurement yet which could help us to decide which solution is correct. meanwhile we are inclined to the solution with the earsm turbulence model which more precisely models complex flow structures. the nature of the outlet flow field influences total efficiency of the cantilever turbine. comparison of results from both simulated variants with measured data is shown in figure 14. figure 11. approximate location of the pressure gauge. figure 12. 28 000 rpm, sst k-ω. figure 13. 28 000 rpm, earsm. figure 14. comparison of achieved results. 112 vol. 20/2018 cfd flow analysis of the cantilever micro-turbine figure 15. cfd results with the new outlet casing geometry. figure shows quite big differences between the results from cfd simulations and measured values. the efficiency difference is in the range from 2 to 4 %. as we expected efficiencies with the earsm turbulence model are lower approximately by 0.4 % than with the sst k-ω. big difference between computed and measured values can be caused by the inaccuracy of the outlet pressure measuring and by local inaccuracy on the rotor-stator interface. but the shape of characteristics is similar in all cases so it is possible to use cfd simulations for the investigating of the influence of several geometry changes on the turbine parameters. 4.2. influence of the outlet casing geometry as it was indicated in previous chapter the current construction design of the outlet casing of the cantilever turbine is not optimal. there are large structures of the separated flow in outlet and this causes a decrease of the turbine efficiency. we tried to design better outlet casing geometry in which a smoother transition between rotor and outlet surfaces was applied. current geometry was replaced by the diffuser as it is shown in the figure 15. we successfully increased the cantilever turbine efficiency approximately by 1 % with this new geometry. 5. conclusions we succeeded in the cfd simulating of the cantilever micro-turbine but we encountered some limits in a form of the local inaccuracy at the interface between stator and rotor turbine domains which probably affects an overall simulation results. this physical inaccuracy is caused by the shock wave deflection on the interface. for a purpose of this work we tested several cfd software but an achieved results were similar. next factor affecting results of this work is probably the very simple pressure measuring at the micro-turbine outlet where some deviation in the measured values can occur. based on the achieved results the new outlet casing geometry was designed. this helped to increase the cantilever micro-turbine efficiency by 1 %. results are relevant for next studies in the research and development process. list of symbols v velocity [m s−1] p pressure [pa] t temperature [°c] h enthalpy [j kg−1 k−1] ṁ mass flow [kg s−1] p power [w] η efficiency [%] ma mach number y+ dimensionless wall distance acknowledgements the work has been supported by the grant project ziel etz interreg v project 53 grenzüberschreitendes f&i netzwerk für energieeffizienz und kraft-wärme(kälte)-kopplung / přeshraniční síť pro výzkum a inovace v oblasti energetické účinnosti a kombinované výroby tepla a elektřiny (2016 2020). references [1] a. p. weiß. micro turbine generators for waste heat recovery and compressed air energy storage. 15th conference on power system engineering, thermodynamics & fluid flows es2016 2016. pilsen, czech republic. [2] a. p. weiß. volumetric expander versus turbine which is the better choice for small rc plants? 3rd international seminar on orc systems (22):301–310, 2015. http://www.asme-orc2015.be/content/publication. [3] m. preißinger, d. brüggemann. thermal stability of hexamethyldisiloxane (mm) for high-temperature organic rankine cycle (orc). energies 9(183), 2016. doi:10.3390/en9030183. [4] m. preißinger, j. schwöbel. multi-criteria evaluation of several million working fluids for waste heat recovery by means of organic rankine cycle in passenger cars and heavy-duty trucks. applied energy 206:887–899, 2017. doi:10.1016/j.apenergy.2017.08.212. [5] t. turunen-saaresti. design and testing of high temperature micro-orc test stand using siloxane as working fluid. journal of physics: conference series (821), 2017. doi:10.1088/1742-6596/821/1/012024. 113 http://www.asme-orc2015.be/content/publication http://dx.doi.org/10.3390/en9030183 http://dx.doi.org/10.1016/j.apenergy.2017.08.212 http://dx.doi.org/10.1088/1742-6596/821/1/012024 acta polytechnica ctu proceedings 20:108–113, 2018 1 introduction 2 the tested turbine and the orc research plant 3 cfd setup and mesh parameters 3.1 micro-turbines cfd simulation issues 3.2 mesh parameters 3.3 investigation of the inlet casing influence on the turbine performance 3.4 boundary conditions 4 results and discussion 4.1 comparing cfd results to measured values 4.2 influence of the outlet casing geometry 5 conclusions list of symbols acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2017.13.0162 acta polytechnica ctu proceedings 13:162–165, 2017 © czech technical university in prague, 2017 available online at http://ojs.cvut.cz/ojs/index.php/app wang tiles local tilings created by merging edge-compatible finite element meshes lukáš zrůbek∗, anna kučerová, martin doškář department of mechanics, faculty of civil engineering, czech technical university in prague, thákurova 7, 166 29 prague 6, czech republic ∗ corresponding author: lukas.zrubek@fsv.cvut.cz abstract. in this contribution, we present the concept of wang tiles as a surrogate of the periodic unit cell method (puc) for modelling of materials with heterogeneous microstructures and for synthesis of micro-mechanical fields. the concept is based on a set of specifically designed cells that compresses the stochastic microstructure into a small set of statistical volume elements – tiles. tiles are placed side by side according to matching edges like in a game of domino. opposite to the repeating pattern of puc the wang tiles method with the stochastic tiling algorithm preserves the randomness for reconstructed microstructures. the same process is applied to obtain the micro-mechanical response of domains where the evaluation as one piece would be time consuming. therefore the micro-mechanical quantities are evaluated only on tiles (with surrounding layers of tiles of each addressed tile included into the evaluation) and then synthesized to the micro-mechanical field of whole domain. keywords: heterogeneous microstructures, wang tiles, micro-mechanical fields, local tiling, finite element method. 1. introduction the heavily increasing pressure to the materials utmost performance leads to necessity of excellent understanding of characteristic mechanical processes taking place at a microstructural level of materials. therefore the numerical material models needs to incorporate also the micro-scale level. however, even though the materials may look like homogeneous from macro-scale view, the majority of materials used in engineering applications are heterogeneous with a stochastic microstructure layout (fig.1). for modelling of heterogeneous materials many methods exists, but the most widely used one is the numerical homogenization method [1]. heterogeneous materials are modelled using the concept of periodic unit cell (puc) or statistically equivalent periodic unit cell (sepuc). the puc approach is used for material with periodic patterns, where the modelled microstructure is produced by duplication of a representative part of the original microstructure in cardinal directions (see fig.2). on the contrary the stochastic microstructures are modelled using the concept of sepuc. in this case the unit cell representing the original microstructure holds the same properties given by statistical descriptors as the source one [2]. nevertheless, this approach uses only a single cell to describe the whole material and the stochastic heterogeneous material is transformed into unwanted periodic pattern of the same cells. and to be able to study micro-mechanical quantities (e.g. stresses, strains or displacements) at micro-scale level we need to preserve not only its properties but also the stochastic layout. (a) (b) figure 1. the appearance of the structure at different scales, a) homogeneous at macro-scale, b) heterogeneous at micro-scale. (a) (b) (c) figure 2. modelling of periodic microstructure, a) periodic microstructure, b) puc, c) reconstructed microstructure. 162 http://dx.doi.org/10.14311/app.2017.13.0162 http://ojs.cvut.cz/ojs/index.php/app vol. 13/2017 local tilings from merged edge-compatible fe meshes (a) (b) 1 3 5 7 8 figure 3. principle of wang tiles method, a) domino game, b) wang tiles. preserving the randomness of the recreated material cannot be achieved with single cell. hence the method utilizing the set of representative cells must be used. to be able to place these cells side by side, they must be specifically designed so they are mutually compatible on the edges. if this criterion is met the infinitely large material microstructure can be created with the use of simple algorithm. the resulting microstructure preserves stochastic distribution and the statistical description of the source media. the above described method is called method of wang tiles. 2. wang tiles method the basic principle of wang tiles method was introduced by mr. hao wang [3, 4] in the early sixties of the 20th century. the concept is very similar to the domino game. the objective is to construct a straight or zigzag line series of game pieces with a matching number of dots on the adjacent sides of touching stones (fig. 3a). the wang tiles method works nearly the same with only few differences. instead of double coded rectangular domino pieces, the tiles are squares with four edge codes, colours in this case (fig. 2b). and contrary of creating a linear series of game pieces the tiles are placed into four cardinal directions (north – n, south – s, east – e, west – w) and the two-dimensional area covered with tiles is produced. the tiles cannot be rotated or mirrored through the process of laying tiles as any of these two actions would create a new type of tile. the tiles are placed sideby-side according to matching edge codes until the desired size of area is completely created. the result is called tiling and if there are no missing tiles or unequal edge codes, the tiling is valid. 2.1. cshd algorithm except the condition that the tiling must be valid we also need it to be stochastic. that can be achieved by utilization of cohen-shade-hiller-deussen (cshd) tiling algorithm [5]. if the creation of tiling is performed row by row, the most difficult position to place a new tile would be the corner position like in figure 4. in this picture is shown the corner position where the north (n) and western (w) edges of new tiles must n w 1 3 4 8 2 6 2or ? figure 4. tiling process and valid tiles for the corner position. 1 2 3 4 5 6 7 8 figure 5. smallest wang tiles set called w8/2-2. correspond with already placed tiles. among other rules of the cshd algorithm, the most important one is that it there must be at least two valid tiles (like tiles number 8 or 2 in the fig. 4) for this kind of corner position. then the new tile is randomly selected from these valid tiles and placed in the tiling. this creates another corner position and the process is repeated until the tiling of required size is created. the smallest set that complies the requirements of cshd algorithm and has at least two distinct vertical edge codes and two distinct horizontal edge codes is set consisting of 8 tiles. we call this set w8/2-2 (fig. 5) and we use it as a representative set in all further explanations in the following paper. 2.2. design of tiles when designing the set of tiles the process must ensure that the resulting tiles will satisfy the requirement of the cshd algorithm and of course the tiles must hold the properties of the original microstructure. couple of methods to design tiles can be used and one of them is based on multicriterial optimization of tile morphology according to statistical descriptors. the goal is to create tiles such as the recreated microstructure share the same statistical properties as the original microstructure. however this process is extremely time and computationally consuming (days to weeks). moreover as presented in the paper [6] with increasing pressure on optimizations all the tiles tend to become identical and the reconstructed microstructure became visibly periodical. another method called automatic tile design [5] creates each tile by stapling four small samples (fig. 5) cut out of the original microstructure in different combination. in addition is almost instantaneous if compared with first method. completely different approach based on particles dynamics, movement and collisions inside of the interconnected tiles bounding boxes are using methods described in [7, 8]. these method can be used only for microstructures with distinguishable particles that can be described by simple geometrical shapes. 163 l. zrůbek, a. kučerová, m. doškář acta polytechnica ctu proceedings 1 3 2 4 1 2 3 4 a) b) c) figure 6. automatic tile design [5], a) cut-outs for stapling, b) stapling, c) new tile. 3. micro-mechanical quantities the wang tiles method can be used in many different fields like modelling of quasi-crystals and generating naturally looking textures for computer graphic [5]. or as a substitution for the unit cell concepts (puc and sepuc) in material modelling. or to obtain the micro-mechanical response for enrichment functions in numerous generalized finite element methods, e.g. [9, 10]. in our work, the following (numerical homogenization like) decomposition of a sought macroscopic field u(x) is assumed u(x) = ue(x) + u∗(x) (1) where u∗(x) is the microscopic fluctuation field synthesized by means of wang tiles method and ue(x) is the macroscopic part solved by finite element method (fem) with a sparse grid. 3.1. simple synthesis the whole microscopic domain can of course be discretized by very fine fem mesh and evaluated to obtain demanded micro-mechanical fields like stresses, strains and displacements. but according to the fineness of the mesh and size of the domain, the evaluation time is rapidly increasing. the computational requirements can be diminished with the wang tiles method as the process of microstructure reconstruction can be applied to the micro-mechanical field. the micro-scale quantities are evaluated on individual tiles and then synthesized back according to the same tiling as the microstructure. however, as presented in the paper [11] the non-local character of mechanical quantities is causing discontinuities on tile edges and therefore the underlying grid of tiles is recognizable. that is because of the mechanical response of each tile is affected by a different combination of surrounding tiles (fig. 7). 3.2. finite element meshes furthermore, to be able to use the finite element method (fem) as a solver brings another difficulty. because the domain is represented by the tiles which are mutually compatible on the edges, the finite element meshes created on the individual tiles has to fulfil the exactly same compatibility. example of artificially created edge-compatible finite element meshes on tiles are shows in the figure 8. 2 1 6 62 6 3 4 8 3 6 4 5 8 7 1 8 5 4 511 1 1 1 1 12 2 2 2 2 2 2 2 2 2224 4 4 4 4 4 4 8 8 8 88 8 8 3 3 33 3 3 5 5 5 5 55 5 7 7 7 7 7 7 77 7 6 6 6 6 1 6 m=3 figure 7. single layer of surrounding tiles (dashed squares) for different tiles with no. 1. 1 2 3 4 5 6 7 8 figure 8. mutually edge-compatible finite element meshes on tiles. 3.3. local tiling our attempt to solve this problem is to include these surrounding tiles into the evaluation of the mechanical response for each tile [12]. therefore for every tile ti (grey square in the in figure 9) from the macro tiling we create so called local tiling. this local tiling is represented by the center tile ti and the arbitrary number of surrounding layers of tiles taken from the macro tiling. then the local local fe mesh is synthesized (see section 3.4) and the micro-mechanical quantity is evaluated. because we need only the results for the centre tile ti we crop the results of all surrounding tiles and save the results under label t285416148. this procedure is repeated for every tile ti in macrotiling and because these processes are not dependent, they can be solved using parallel computation. after results of all tiles are obtained, the micro-mechanical field is synthesized. 3.4. fe meshes merging tool as mentioned in section 3.2 the fe meshes on tiles must be mutually edge-compatible so they can be merged together into single fe mesh (e.g. local tiling fe mesh). to create the edge-compatible meshes 164 vol. 13/2017 local tilings from merged edge-compatible fe meshes evaluated mechanical response 2 8 5 4 81 4 6 1 2 8 4 4 8 5 6 1 7 1 8 5 4 51 1 1 1 2 2 2 22 4 4 4 4 8 88 3 3 5 5 7 7 7 7 77 6 6 6 1 812 2 2 57 2 2 4 5 7 6 11 2854 61481 figure 9. local tiling process: macro tiling ⇒ local tiling ⇒ local fe mesh ⇒ evaluation ⇒ cropping results ⇒ results for tile ti. figure 10. real adaptive mesh of tile. the software called t3d is used. next the output from t3d must be loaded, sorted, renumbered, etc. for these purposes we are working on robust tool implemented in c++ language. in addition to above mentioned operations, the tool can already merge arbitrary number of tile meshes (without duplicate nodes or gaps in node numbering). any created mesh can be saved to a file or loaded from file (once saved) and especially visualized using the vtk visualization tool-kit as an unstructured grid. the tool is capable to process any finite element mesh from simple (fig. 8) to quite complex like on fig. 10. the entire current work is devoted to implement the process described on figure 9 and on the effective storing and working with big volume of data results. 3.5. future work once the implementation of the local tiling process (see 3.3) is completed and we will be able to quickly generate results we are planning to implement some statistical descriptors, automatized call of t3d software or even tile design methods. our main goal is to run sensitive analysis and determine the influence of characteristic microstructural lengths, tile size and number of necessary included layers in local tilings on the overall error of synthesized micro-mechanical fields. acknowledgements the authors gratefully acknowledge the financial support from the grant agency of the czech technical university in prague, the grant no. sgs17/042/ohk1/1t/11 (numerical methods for modelling uncertainties in civil engineering) (l. zrůbek, a. kučerová and m. doškář). references [1] f. fritzen. microstructural modeling and computational homogenization of the physically linear and nonlinear constitutive behavior of micro-heterogeneous materials, vol. 1. kit scientific publishing, 2011. [2] h. lee, m. brandyberry, a. tudor, k. matouš. three-dimensional reconstruction of statistically optimal unit cells of polydisperse particulate composites from microtomography. phys rev e 80:061301, 2009. doi:10.1103/physreve.80.061301. [3] h. wang. proving theorems by pattern recognition–ii. bell system technical journal 40(1):1–41, 1961. doi:10.1002/j.1538-7305.1961.tb03975.x. [4] h. wang. games, logic and computers. scientific american 213(5):98–106, 1965. [5] m. f. cohen, j. shade, s. hiller, o. deussen. wang tiles for image and texture generation. acm trans graph 22(3):287–294, 2003. doi:10.1145/882262.882265. [6] j. novák, a. kučerová, j. zeman. compressing random microstructures via stochastic wang tilings. phys rev e 86:040–104, 2012. doi:10.1103/physreve.86.040104. [7] d. šedlbauer, m. lepš. dynamic packing of stochastic wang tiling for heterogeneous material modelling. in proceedings of the 20th international conference on computer methods in mechanics, pp. 447–448. 2013. [8] j. stránský, m. doškář. stochastic wang tiles generation using the discrete element method. in engineering mechanics 2015, vol. 821 of applied mechanics and materials, pp. 392–398. trans tech publications, 2016. doi:10.4028/www.scientific.net/amm.821.392. [9] j. m. melenk, i. babuška. the partition of unity finite element method: basic theory and applications. computer methods in applied mechanics and engineering 139(1):289–314, 1996. doi:10.1016/s0045-7825(96)01087-0. [10] t. de freitas j.a. formulation of elastostatic hybrid-trefftz stress elements. computer methods in applied mechanics and engineering 153(1):127–151, 1998. doi:10.1016/s0045-7825(97)00042-x. [11] j. novák, a. kučerová, j. zeman. microstructural enrichment functions based on stochastic wang tilings. modelling and simulation in materials science and engineering 21(2):025–014, 2013. doi:10.1088/0965-0393/21/2/025014. [12] l. zrůbek, a. kučerová, j. novák. synthesized fields fluctuations by means of wang tiles and local tilings. in modern methods of experimental and computational investigations in area of construction ii, vol. 1144 of advanced materials research, pp. 178–183. trans tech publications, 2017. doi:10.4028/www.scientific.net/amr.1144.178. 165 http://dx.doi.org/10.1103/physreve.80.061301 http://dx.doi.org/10.1002/j.1538-7305.1961.tb03975.x http://dx.doi.org/10.1145/882262.882265 http://dx.doi.org/10.1103/physreve.86.040104 http://dx.doi.org/10.4028/www.scientific.net/amm.821.392 http://dx.doi.org/10.1016/s0045-7825(96)01087-0 http://dx.doi.org/10.1016/s0045-7825(97)00042-x http://dx.doi.org/10.1088/0965-0393/21/2/025014 http://dx.doi.org/10.4028/www.scientific.net/amr.1144.178 acta polytechnica ctu proceedings 13:161–164, 2017 1 introduction 2 wang tiles method 2.1 cshd algorithm 2.2 design of tiles 3 micro-mechanical quantities 3.1 simple synthesis 3.2 finite element meshes 3.3 local tiling 3.4 fe meshes merging tool 3.5 future work acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2019.22.0038 acta polytechnica ctu proceedings 22:38–47, 2019 © czech technical university in prague, 2019 available online at http://ojs.cvut.cz/ojs/index.php/app the fatigue behaviour of gfrp bars experimental study ondřej januša,∗, františek girglea, iva rozsypalováa, vojtěch kostihaa, lenka bodnárováa, petr štěpáneka, jan prokešb a brno university of technology, faculty of civil engineering, veveří 95, 602 00 brno, czech republic b prefa kompozity, a.s., kulkova 10/4231, 615 00 brno, czech republic ∗ corresponding author: janus.o@fce.vutbr.cz abstract. the paper describes an experimental program for studying the fatigue performance of gfrp bars, which has been initiated by the authors. two different test configurations were used to assess the fatigue behaviour. the bare specimens were tested within the first series. a modified gripping system was used to reduce eccentricity when the bar was not directly fixed. however, the boundary conditions seem to affect the results. the second series consisted of a set of specimens of bars embedded in concrete. this configuration seems appropriate for determination of fatigue life of gfrp bars. two s-n curves for bare bars and bars embedded in concrete were created and compared. significant reduction of interlaminar shear strength at the beginning of fatigue loading proved matrix or fibre/matrix interface damage. keywords: composite, cyclic loading, fatigue, gfrp, reinforcement, stiffness, s-n curve. 1. introduction at present, many bridge constructions are in a very bad, almost emergency state, and require remediation due to the corrosion of steel reinforcement, which is greatly accelerated by chlorides, moisture and freezing cycles. composite reinforcement in these application areas appears to be a suitable alternative to steel reinforcement. the suitability of its application is confirmed by many successful applications. however, as the bridge decks are directly subjected to repeated transport loads, the fatigue design is an important and often decisive part of the design. however, the potential of composite reinforcement can only be exploited after a perfect description of their long-term properties, which are the key to a reliable design. fatigue tests of frp bars are usually performed on bars with steel anchors or with bars embedded in concrete. the common difficulty of samples with steel anchors is early failure in the vicinity of the steel anchor. this phenomenon is mainly due to the stress concentration near the anchors together with the low transverse strength of the frp bars. anchors that function flawlessly in monotonic tensile tests are often required to be modified for cyclic loading. el refai [1] and noël & soudki [2] used modified reusable anchorage system. its advantage is the possibility of re-use without the need to glue the bar to the anchor. however, during the cyclic loading, there is slipping of the bar in the anchor, which can be in the order of millimetres, [1]. concrete blocks for reinforcement anchoring were used in experiments by adimi [3], rahman [4] and benmokrane [5]. the length of the end (anchorage) blocks must secure anchorage of samples. the sample sizes thus may be significant. the advantage, however, is the elimination of early failure of the frp bars. in addition to the level and nature of the load, an important factor that potentially negatively affects the long-term characteristics of the composite material is the alkaline environment, the humidity and the temperature, which generally reduce the tensile strength of frp reinforcement [5, 6]. even with relatively small displacements in the contact of reinforcement and concrete during cyclic loading, the surface treatment and outer layer of the matrix are impaired due to abrasions from sharp edges of concrete aggregate. katz [7] observed a significant damage of the frp bar surface due to the repeated friction of the reinforcement against the concrete. rahman et al. [4] compare the fatigue life of bare cfrp bars with the fatigue life of encased cfrp bars. the bare bars show a better fatigue life across the s-n curve, which the authors attribute to the negative impact of the concrete environment. noël & soudki [2] determined the fatigue behaviour of bare gfrp bars and beam specimens with the same gfrp bars. the bare bars showed a fatigue life of approximately one level higher than for bars embedded in the beam specimens. 2. experimental program – tested samples gfrp bars with a 10mm diameter composed of ecr fibres (80% by weight) and an epoxy matrix were tested to create s-n curves for bare bars and bars embedded in concrete. bond with the concrete was provided by the silica sand-coating with a nylon winding. the short-term properties are shown in table 1. 38 http://dx.doi.org/10.14311/app.2019.22.0038 http://ojs.cvut.cz/ojs/index.php/app vol. 22/2019 the fatigue behaviour of gfrp bars experimental study bar diameter bar diameter with surface treatment average tensile strength ± sd average young modulus ± sd average interlaminar (longitudinal) strength ± sd [mm] [mm] [mpa] [gpa] [mpa] 10 11.03 1018.8 ± 5.2 52.2 ± 0.3 63.32 ± 2.32 table 1. properties of the reinforcement. for more details of ongoing experimental program of gfrp bars, see stepanek et al. [8]. 2.1. monotonic tensile test the tensile testing of frp bars, including the subsequent evaluation, was performed according to iso 10406-1 [9]. six test specimens, 1.0m in length, were fitted with steel anchors for mounting on a test device. the anchors eliminate the risk of crushing the composite bar by the transverse pressure exerted by the jaws of the device. the middle (free) part of the sample was then fitted with lvdts. simultaneously with the strain of the sample, the applied tensile force was read. the load was applied at a deformation rate of 3mm/min until the sample was completely broken. the configuration of the test and the typical way of failure is shown in fig. 1. the obtained results are shown in table 1, the stress-strain diagram is shown in fig. 2. all samples were broken in an expected manner. there was a significant delamination of the individual layers of the cross-section (broomlike failure mode). however, all samples were found to have an intact core diameter of approximately 6mm. the creation of this core indicates a different cure of the matrix at the centre of the cross-section. 2.2. interlaminar (longitudinal) shear test the purpose of interlaminar shear test was to determine the strength of matrix and/or interface between fibre/matrix. the tests were performed according to astm d 4475 [10]. the distance between the supports was chosen with respect to the desired mode of failure, i.e. a crack parallel to the direction of the fibres (perpendicular to the load direction). a set of samples of 60mm length (i.e. the distance of the theoretical supports corresponded to 5 sample diameters) was tested. the average strength in the longitudinal (interlaminar) shear is shown in table 1. 3. cyclic (fatigue) tensile tests the samples during fatigue tests were loaded by sinusoidal function. one of the main parameters influencing the results, i.e. the fatigue life of composite materials, is the loading frequency [11, 12]. material is heated due to energy dissipation when the frequency is high. the acceptable load frequency range is 1-10hz, the recommended value is 4hz, see aci 440.3r-12 [13]. the value of 4hz was chosen for this study. increasing the stress ratio r (the difference between the minimum and maximum stress) leads to a decrease of fatigue life, see [12]. in aci 440.3r-12 [13] is recommended to set the stress ration equal to 0.1. the suggested method (c), according to aci 440.3r12 [13], was used in this study to create the s-n curves; the stress ratio r is equal to 0.1 and load values are always adjusted. the samples were loaded until failure or up to maximum 2 million cycles. three stress levels have been tested for two axially loaded test configurations: bare bars with steel anchors and embedded bars in concrete. levels with a maximum stress value in the loading cycle equal to 40%, 50% and 60% of the monotonic strength were tested so far. 3.1. fatigue test of bare bars the first set of samples subjected to cyclic loading was tested with a similar solution to the anchors used in the monotonic tensile test. to ensure a better connection of the steel anchor with the grouting, the steel anchor body was perforated by openings (fig. 5). however, during cyclic loading, these anchors did not work sufficiently. all samples were prematurely broken. the sample behaviour analysis had shown that the failure was accelerated by the eccentricity arising from the imperfect/inaccurate bar mounting in the anchor. this imperfection can be limited to a certain extent by inserting joints that are mutually rotated by 90°(fig. 4). the adjusted test configuration has been verified on six samples at load level fmax = 60%fult. with this configuration, a failure in the anchor occurred only in one case. 3.2. fatigue test of bars embedded in concrete the configuration of fatigue tests of reinforcement embedded in concrete was set according to rahman et al. [4]. the length of the end (anchorage) blocks of 500mm was considered with regard to securing a sufficient bond length for the tested bars. a 100mm long central block was separated from the anchor blocks using cardboard. the total length of the whole sample was 1100mm (fig. 6). to prevent splitting of concrete, the anchor blocks were compressed by steel frames. to eliminate possible eccentricities caused by the production, the sample was provided with hinges. the average compressive strength of the concrete was 63mpa. all samples were broken in a desirable manner, i.e. always in the middle block area. no undesired end block splitting or bond failure was observed in any of 39 o. januš, f. girgle, i. rozsypalová et al. acta polytechnica ctu proceedings figure 1. failure of samples during monotonic test. figure 2. stress-strain diagram during monotonic tensile test. figure 3. configuration of interlaminar (longitudinal) shear test. 40 vol. 22/2019 the fatigue behaviour of gfrp bars experimental study figure 4. cyclic test configuration with lvdts (left), broom-like failure mode with an undamaged core (right). figure 5. anchor for a cyclic test of bare bars. the samples. in the first phase of loading, a transverse crack in the concrete was formed at approximately half the height of the middle block due to the tensile force applied (fig. 8). the further development of cracks in the middle block of the specimen was observed just before the reinforcement failure. a longitudinal crack was created, which in many cases led to the splitting of the middle block. this longitudinal crack was caused by the transverse pressure of the breaking bar on the concrete, where most of the samples tested broke in a broom-like failure mode. after the test, the area of the reinforcement in contact with the concrete in the middle block was examined in more detail. the surface of the concrete or reinforcement in most cases did not show signs of macroscopic failure. however, a white residue on the surface of the concrete was evident locally (most often near the transverse crack), apparently created by the crushing of the silica sand-coating of the tested bar (fig. 7c). the observed phenomenon occurred only in samples that were exposed to the lowest tested level of loading (40% fult) and, therefore, the highest number of cycles. from the observation, one can conclude that a higher number of cycles can damage the contact between the tested reinforcement and the surrounding concrete. for that reason, the next series of samples will be subjected to a deeper exploration of the surface. it is obvious that the surrounding concrete actively (until reaching the tensile strength of the concrete) prevents reinforcement from delamination. this finding is also confirmed by the observed frequent failure of reinforcement at the unbonded length of the bar (especially at lower load levels), where the reinforcement is allowed free transverse expansion. 41 o. januš, f. girgle, i. rozsypalová et al. acta polytechnica ctu proceedings figure 6. bars encased in concrete specimen geometry. (a). (b). (c). figure 7. a) cyclic test configuration, b) broom-like failure mode, c) concrete surface with a white residue. 42 vol. 22/2019 the fatigue behaviour of gfrp bars experimental study figure 8. transverse (left) and longitudinal (right) crack during cyclic loading. 3.3. stiffness degradation after cyclic loading to quantify the degree of potential damage of tested samples at the beginning of the loading, an experiment was designed to determine the residual mechanical characteristics of the sample (modulus of elasticity and strength in interlaminar shear) after the first 100 cycles. three levels of cyclic loading were verified (table 2). the samples were loaded at a rate of 0.5 kn/s until the mean load value in the cycle was reached. after executing the set number of cycles at the frequency of 4hz, the sample was unloaded to the starting position at the same speed and then loaded back to the same value. for each level, one sample was tested. the elasticity modulus was determined by lvdt measurement before exposure to cyclic loading and after the first 100 cycles. reduction of the modulus was not observed for any of the tested samples. this level of fatigue loading thus did not cause a failure of the fibres, which would affect stiffness of the samples. samples were then cut into 6 pieces of 60mm for the interlaminar (longitudinal) shear test. the test was carried out in the same configuration as the reference samples (see fig. 3). at all tested levels after 100 cycles a significant reduction of the matrix or fibre/matrix interface strength was observed (see table 2 results). 3.4. effect of stress range the fatigue life of samples tested so far is shown in tables 3 and 4. only samples that have been broken in a satisfactory manner (i.e. failure of the free part of the bare bars or failure of the bar in the central block area of the encased bars) are used to construct the s-n curve (fig. 9). the fatigue life of the samples increases exponentially with decreasing stress level. equation (1) for bare bars and equation (2) for bars embedded in concrete is expressing the s-n dependence. fmax fult = 1.123 − 0.182 log n (1) fmax fult = 1.026 − 0.135 log n (2) it can be seen from fig. 9 and the equation (1) and (2) that the fatigue life of the samples embedded in concrete is higher. in the macroscopic scale, in most cases, the embedded samples did not show indications of failure of the surface treatment of the bar caused by friction between bars and concrete. in addition, the surrounding concrete prevented the broomlike failure mode of the bars. on the other hand, the fatigue life of bare bars is significantly reduced by the steel anchors. this is especially noticeable at 40% fult, load level. at this level, at most samples occurred undesirable failure near the anchors. however, fatigue life is likely to be affected at all tested levels. fig. 10 shows the results of the bare bars and bars embedded in concrete tests performed in this study presented along with other experiments with the same configuration and gfrp reinforcement [2, 3]. demers [12] stated that the shape of the sample influences the resulting fatigue life of the composite materials. therefore, only the results obtained from the experiments on the samples of the gfrp bars are included here. the tests published in [2] are performed with a variable stress ratio, which affects the fatigue life, see [12]. the lower fatigue life of the bare bars tested in this study indicates the negative impact of anchoring (especially at the lowest tested level). the results obtained from the tests of bars encased in concrete are comparable with other presented results (see fig. 10). 4. conclusions the fatigue behaviour was determined for two different configurations. for both axially loaded test configurations, s-n curves were created. these s-n curves were also compared with s-n curves of gfrp bars available in literature. from the experiments carried out so far, the following findings have been made: (1.) the configuration of samples with encased bars in concrete seems to be appropriate for determining the fatigue life of composite reinforcement. the 500mm length of the anchor block is sufficient to safely anchor the tested reinforcement. at 40%, 50% and 60% fult load levels, all the samples showed a failure of the bar in the middle block or in the unbonded area. the degradation of the outer layer 43 o. januš, f. girgle, i. rozsypalová et al. acta polytechnica ctu proceedings maximum stress in the cycle modulus of elasticity before the cyclic loading elasticity modulus after 100 cycles average strength in interlaminar (longitudinal) shear at monotonic test average strength in interlaminar (longitudinal) shear after 100 cycles [gpa] [gpa] [mpa] [mpa] 40% fult 53.43 53.65 (+0.41%) 63.32 60.26 (-4.83%) 50% fult 52.71 53.48 (+1.46%) 59.80 (-5.89%) 60% fult 52.51 53.19 (+1.30%) 57.5 (-10.03%) table 2. comparison of mechanical properties. figure 9. s-n diagram for tested bare gfrp bars and bars encased in concrete. figure 10. s-n diagram for gfrp bars. 44 vol. 22/2019 the fatigue behaviour of gfrp bars experimental study test type specimen stress [mpa] (% fult) fatigue life failure modeminimum maximum range n bare bars b.gfrp.e.e p.s.10-a.1 61 (6%) 611 (60%) 550 (54%) 695 broomlike b.gfrp.e.e p.s.10-a.2 61 (6%) 611 (60%) 550 (54%) 307 specimen for load adjustment. (broomlike) b.gfrp.e.e p.s.10-a.3 61 (6%) 611 (60%) 550 (54%) 634 broomlike b.gfrp.e.e p.s.10-a.4 61 (6%) 611 (60%) 550 (54%) 889 longitudinal cracks b.gfrp.e.e p.s.10-a.5 61 (6%) 611 (60%) 550 (54%) 870 broomlike b.gfrp.e.e p.s.10-a.6 61 (6%) 611 (60%) 550 (54%) 861 anchor b.gfrp.e.e p.s.10-b.1 51 (5%) 509 (50%) 458 (45%) 1571 specimen for load adjustment. (broomlike) b.gfrp.e.e p.s.10-b.2 51 (5%) 509 (50%) 458 (45%) 2483 longitudinal cracks b.gfrp.e.e p.s.10-b.3 51 (5%) 509 (50%) 458 (45%) 2426 longitudinal cracks b.gfrp.e.e p.s.10-b.4 51 (5%) 509 (50%) 458 (45%) 2104 longitudinal cracks b.gfrp.e.e p.s.10-b.5 51 (5%) 509 (50%) 458 (45%) 2398 broomlike b.gfrp.e.e p.s.10-b.6 51 (5%) 509 (50%) 458 (45%) 2422 broomlike b.gfrp.e.e p.s.10-c.1 41 (4%) 408 (40%) 367 (36%) 9925 anchor + longitudinal crack b.gfrp.e.e p.s.10-c.2 41 (4%) 408 (40%) 367 (36%) 8611 longitudinal cracks b.gfrp.e.e p.s.10-c.3 41 (4%) 408 (40%) 367 (36%) 10899 longitudinal cracks b.gfrp.e.e p.s.10-c.4 41 (4%) 408 (40%) 367 (36%) 9758 longitudinal crack b.gfrp.e.e p.s.10-c.5 41 (4%) 408 (40%) 367 (36%) 8614 longitudinal cracks b.gfrp.e.e p.s.10-c.6 41 (4%) 408 (40%) 367 (36%) 8752 anchor + longitudinal crack table 3. tested specimens fatigue life. 45 o. januš, f. girgle, i. rozsypalová et al. acta polytechnica ctu proceedings test type specimen stress [mpa] (% fult) fatigue life failure modeminimum maximum range n bars embedded in concrete c.gfrp.e.e p.s.10-a.1 61 (6%) 611 (60%) 550 (54%) 1249 broomlike c.gfrp.e.e p.s.10-a.2 61 (6%) 611 (60%) 550 (54%) 975 broomlike (unbonded length) c.gfrp.e.e p.s.10-a.3 61 (6%) 611 (60%) 550 (54%) 2226 longitudinal cracks c.gfrp.e.e p.s.10-a.4 61 (6%) 611 (60%) 550 (54%) 2489 broomlike c.gfrp.e.e p.s.10-b.1 51 (5%) 509 (50%) 458 (45%) 15871 broomlike (unbonded length) c.gfrp.e.e p.s.10-b.2 51 (5%) 509 (50%) 458 (45%) 4243 broomlike (unbonded length) c.gfrp.e.e p.s.10-b.3 51 (5%) 509 (50%) 458 (45%) 11785 broomlike (unbonded length) c.gfrp.e.e p.s.10-b.4 51 (5%) 509 (50%) 458 (45%) 11900 broomlike c.gfrp.e.e p.s.10-c.1 41 (4%) 408 (40%) 367 (36%) 23301 broomlike (unbonded length) c.gfrp.e.e p.s.10-c.2 41 (4%) 408 (40%) 367 (36%) 31508 longitudinal cracks c.gfrp.e.e p.s.10-c.3 41 (4%) 408 (40%) 367 (36%) 44876 broomlike (unbonded length) c.gfrp.e.e p.s.10-c.4 41 (4%) 408 (40%) 367 (36%) 35610 broomlike (unbonded length) table 4. tested specimens fatigue life. of the bar caused by friction with the surrounding concrete was observed only to a small extent and only in the 40% fult samples. the cover layer of the concrete in the middle part of the sample prevented the early breakage of the reinforcement by broomlike failure mode. (2.) the fatigue life of bars embedded in concrete is higher than fatigue life of the bare bars. however, it is likely that the chosen anchorage method has a negative influence on the fatigue life of bare samples in this study. in case of imperfect direct placement of the bar in the anchor, a combined load axial load and flexure will occur. (3.) all tested levels show the evident failure of matrix or fibre/matrix interface (if the load level is 60% fult by more than 10%) after the first 100 cycles. however, the modulus of elasticity was not adversely affected. it can, therefore, be assumed that in the case of test load levels, there is no failure of the fibres. acknowledgements the presented results were obtained with the financial support of project th03020067 “frp reinforcement with increased reliability and durability against fatigue loads” and fast-j-18-5224 “interlaminar shear test of frp bars for quick and effective evaluation of product quality and determination of degradation rate”. references [1] a. el refai. durability and fatigue of basalt fiber-reinforced polymer bars gripped with steel wedge anchors. journal of composites for construction 17:1– 11, 2013. doi:10.1061/(asce)cc.1943-5614.0000417. [2] m. noël, k. soudki. fatigue behavior of gfrp reinforcing bars in air and in concrete. journal of composites for construction 18(5):1–8, 2014. doi:10.1061/(asce)cc.1943-5614.0000468. [3] m. r. adimi, b. benmokrane, h. a. rahman. fatigue behaviour of gfrp bars embedded in concrete. in annual conference of the canadian society for civil engineering, vol. 6, pp. 121–130. 1997. [4] a. h. rahman, m. r. adimi, b. benmokrane. fatigue behaviour of frp reinforcements encased in concrete. advanced composite materials in bridges and structures 2:691–698, 1996. [5] b. benmokrane, f. elgabbas, e. ahmed, p. cousin. characterization and comparative durability study of glass/vinylester, basalt/vinylester, and basalt/epoxy frp bars. journal of composites for construction 19:1– 12, 2015. doi:10.1061/(asce)cc.1943-5614.0000564. 46 http://dx.doi.org/10.1061/(asce)cc.1943-5614.0000417 http://dx.doi.org/10.1061/(asce)cc.1943-5614.0000468 http://dx.doi.org/10.1061/(asce)cc.1943-5614.0000564 vol. 22/2019 the fatigue behaviour of gfrp bars experimental study [6] f. girgle, l. bodnárová, o. januš, v. kostiha. influence of alkalinity and ambient temperature on long-term properties of gfrp reinforcement. in special concrete and composites 2017, vol. 760 of key engineering materials, pp. 213–218. trans tech publications, 2018. doi:10.4028/www.scientific.net/kem.760.213. [7] a. katz. bond to concrete of frp rebars after cyclic loading. journal of composites for construction 4(3):137–144, 2000. doi:10.1061/(asce)1090-0268(2000)4:3(137). [8] p. štěpánek, o. januš, f. girgle, et al. long term strength of internal gfrp reinforcement by alkaline, temperature and cyclic loading. in xiv international conference on building pathology and constructions repair (cinpar 2018), vol. 11, pp. 12–19. 2018. doi:10.1016/j.prostr.2018.11.003. [9] iso 10406-1:2015 fibre-reinforced polymer (frp) reinforcement of concrete test methods part 1: frp bars and grids. standard, american concrete institute, 2008. [10] astm d4475-02 standard test method for apparent horizontal shear strength of pultruded reinforced plastic rods by the short-beam method. standard, american concrete institute, 2016. [11] m. r. adimi, a. h. rahman, b. benmokrane. new method for testing fiber-reinforced polymer rods under fatigue. journal of composites for construction 4(4):206– 213, 2000. doi:10.1061/(asce)1090-0268(2000)4:4(206). [12] c. demers. tension-tension axial fatigue of e-glass fiber-reinforced polymeric composites: tensile fatigue modulus. construction and building materials 12:51–58, 1998. doi:10.1016/s0950-0618(97)00059-7. [13] aci 440.3r-12 guide test methods for fibre-reinforced polymers (frps) for reinforcing or rtrengthening concrete structures. standard, american concrete institute, 2012. 47 http://dx.doi.org/10.4028/www.scientific.net/kem.760.213 http://dx.doi.org/10.1061/(asce)1090-0268(2000)4:3(137) http://dx.doi.org/10.1016/j.prostr.2018.11.003 http://dx.doi.org/10.1061/(asce)1090-0268(2000)4:4(206) http://dx.doi.org/10.1016/s0950-0618(97)00059-7 acta polytechnica ctu proceedings 22:38–47, 2019 1 introduction 2 experimental program – tested samples 2.1 monotonic tensile test 2.2 interlaminar (longitudinal) shear test 3 cyclic (fatigue) tensile tests 3.1 fatigue test of bare bars 3.2 fatigue test of bars embedded in concrete 3.3 stiffness degradation after cyclic loading 3.4 effect of stress range 4 conclusions acknowledgements references doi:10.14311/app.2021.30.0104 acta polytechnica ctu proceedings 30:104–108, 2021 © czech technical university in prague, 2021 available online at http://ojs.cvut.cz/ojs/index.php/app fire protection of steel elements using lightweight hybrid cement mortar stanislav šulca,∗, vít šmilauera, jakub šejnab, františek waldb a czech technical university in prague, faculty of civil engineering, department of mechanics, thákurova 7, 166 29 prague 6, czech republic b czech technical university in prague, faculty of civil engineering, department of steel and timber structures, thákurova 7, 166 29 prague 6, czech republic ∗ corresponding author: stanislav.sulc@fsv.cvut.cz abstract. material properties of steel structures are significantly reduced at high temperatures, so a fire protection has strong positive impact on the fire resistance of the structure. fire resistance of steel elements can be increased using a layer of cement-based materials as a fire protection. most of commonly used cement-based materials do not withstand high temperatures without noticeable reduction of mechanical properties. hybrid cement showed some interesting properties in the way of resistance to high temperatures and adhesion to steel surfaces, thus its behavior during fire exposure should be investigated. one experimental analysis with numerical simulation is presented in this article. it examines thermal material properties of lightweight hybrid cement mortar with expanded perlite from a simple experiment with a lab gas burner. keywords: fire protection, fire resistance, hybrid cement. 1. introduction mechanical behaviour of steel structures is very sensitive to high temperatures [1, 2] and it is essential to protect steel structures from fire exposure. there are various ways of such protection based on creating physical barrier with low thermal conductivity, such as fire-protective paintings, gypsum plasterboard lining or protection using cement-based materials. this article aims at a relatively new material called "hybrid cement" (h-cement) [3], produced by company "považská cementáreň, a. s." in slovakia. it has ambitions to be used as a fire-protective material, because the preliminary testing of this material by daxner [4] showed good characteristics at high temperatures compared to commonly used cementbased materials which do not withstand high temperatures without cracking and spalling [5, 6]. another one of its advantages is its suitability for steel structures, because it chemically binds to the steel surface, which makes noticeably stronger connection compared to other cement-based materials, but the adhesion itself has not been measured yet. there were several initial tests made to inspect the material properties. one of these tests is presented in this article. the overall target is to determine advantages of this material, explore and test its possible usages in the structures. 2. lab burner test the thermal characteristics of the material during fire exposure can be calculated by analysis of experimental data. as classic experiments in big furnaces are very expensive, the need of more experimental data lead to a small experiment with a lab gas burner. 2.1. experiment description the subject of interest is a h-cement mortar with expanded perlite. the target of this experiment was to determine its thermal conductivity. the composition of this light-weight h-cement mortar is in table 1. the amount of expanded perlite was composed of 43 % of fraction 0-1 mm and 57 % of fraction 12 mm. bulk density of the mixture in fresh state was 437 kg.m−3, while in the state of the experiment it was 335 kg.m−3. the bulk density is highly dependent on the level of intensity and duration of mixing, which mills the granulas of expanded perlite, see figure 1. it may not be the most suitable material for this purpose due to very small mechanical resistance. h-cement 72.1 g expanded perlite 9.17 g plastifier "stachement 508" 1,11 g water 55,6 g table 1. composition of h-cement mortar with expanded perlite. a steel element of 12 mm thickness was covered with h-cement mortar with expanded perlite from the bottom side. the other sides were covered with mineral wool. see the geometry scheme in figure 6 and the photos in figures 2-5. the hot air coming from the bottom of the element was being dissipated using thin steel plate aligned with the bottom side of the element. 104 http://dx.doi.org/10.14311/app.2021.30.0104 http://ojs.cvut.cz/ojs/index.php/app vol. 30/2021 fire protection of steel elements using hybrid cement figure 1. comparison of regular (left) and careful (right) mixing of the mortar /the weight of both elements is the same/. figure 2. experiment setup. figure 3. experiment setup. the properties of used materials are summarized in tab. 2. before the experiment was realized, the mortar was being dried in 40 ◦c for 9 days, during which the mortar lost 30 % of its weight. in the 9th day the weight losses were about 0.8 % per day. the lab gas burner was placed under the center of the exposed surface. the temperatures were measured with three thermocouples mtc 10 type k. the first one measured the temperature of the 12 mm thick steel element. the second one was placed on the center of the exposed surface. the third one was figure 4. experiment setup. figure 5. experiment setup. h-cement mortar bulk density 335 kg.m−3 capacity 1150 j.kg−1.k−1 conductivity unknown steel bulk density 7850 kg.m−3 capacity 500 j.kg−1.k−1 conductivity 46 w.m−1.k−1 mineral wool bulk density 100 kg.m−3 capacity 800 j.kg−1.k−1 conductivity 0.041 w.m−1.k−1 table 2. simulation material parameters. also on the exposed surface, but positioned close to the corner of the surface, see the photo in figure 3 and a position scheme in figure 7. all the three temperature curves are depicted in figure 9. we hoped in reaching higher temperatures, but the thicknesses of the mortar and the steel element were too big and the experiment was stopped after 45 minutes, thus the temperature of the steel element reached only 167 ◦c. but the layer of h-cement was in a state of high temperature, see the simulation results in figure 11. immediately after the 45 minute fire load the mor105 s. šulc, v. šmilauer, j. šejna, f. wald acta polytechnica ctu proceedings figure 6. geometry of the experiment. figure 7. position of the thermocouples on the exposed bottom surface. 3×3 fem elements are indicated. tar was put into water to test its mechanical resistance for this kind of immediate change of temperature, while the steel element remained just above water. the state of the mortar is shown in figure 8 and shows no visible damage. 2.2. simulation the simulation was realized with oofem software [7]. the domain was composed of quadratic brick elements. the domain of steel member was composed of 3 × 3 × 1 fem elements. the domain of h-cement was composed of 3 × 3 × 4 elements. the thickness of mineral wool was divided into 3 elements. material properties follow values from table 2. the temperatures ast center and ast side are considered to be the "adiabatic surface temperatures" figure 8. h-cement mortar after the experiment and after fast cooling in water. and thus come into both convective and radiative boundary conditions on the exposed surface. the temperature ast center is applied on the surface of the brick element in the center of the domain, while the rest of the exposed surfaces are loaded with the ast side temperature. the element division is noticeable in figure 7. the initial temperature of the whole domain was 28.6 ◦c. the outer temperature in boundary conditions of mineral wool surfaces was considered to be 30 ◦c. the simulation was calculated with all timesteps being 1 second. the emissivity of exposed surfaces was 0.8 and the heat transfer coefficient was 20.0 w.m−2.k−1. the output of this simulation is a curve representing temperature of the steel element for given conductivity value of h-cement. three curves for different values of conductivity are depicted in figure 10 to show the impact of this parameter on the results. 106 vol. 30/2021 fire protection of steel elements using hybrid cement figure 9. adiabatic surface temperatures from the thermocouples and temperature of the steel element. figure 10. temperature comparison for three values of h-cement conductivity. figure 11. temperature distribution at time 2700 s. according to the simulations, the thermal conductivity of this h-cement mortar is 0.09715 w.m−1.k−1. these results slightly do not correspond with experimental data until 100 ◦c temperature is reached, which is caused by water content and its evaporation. after that moment the results match the experimental data very well and also seem to have the same tendency for higher temperatures, which were not reached in this experiment. the temperature field in a section through the center of the domain at time 2700 s is presented in figh-cement mortar bulk density 335 kg.m−3 capacity 1150 j.kg−1.k−1 conductivity 0.09715 w.m−1.k−1 table 3. h-cement material parameters with conductivity yielding from the simulations. ure 11. it shows that despite the low resulting temperature of the steel element, the temperature of the 107 s. šulc, v. šmilauer, j. šejna, f. wald acta polytechnica ctu proceedings mortar has reached quite high values. however, it would need another experiment to show its behavior on its limit at much higher temperatures, but it should be completely different experiment setup with another fuel. 3. conclusions h-cement is a material with promising characteristics and resistance to high temperatures during fire. this article determined its thermal properties based on a simple experiment with a lab gas burner and it also showed its behavior during fire load with temperatures comparable with the standard temperature curve iso 834. in principle, once we know the thermal material properties, simulations can replace the experiments in the process of design of fire protection. but more experiments with higher temperatures must be done to have data about the material behavior at full possible temperature range. the material will be further tested and analysed to find its strongest sides and its most suitable applications. acknowledgements this research was funded by czech science foundation, grant 19-22435s "performance of structures with timber fire protection – multi-physics modelling" and also by czech technical university in prague, grant number sgs20/038/ohk1/1t/11. references [1] g. li, p. wang. properties of steel at elevated temperatures. advanced analysis and design for fire safety of steel structures 2013. doi:https://doi.org/10.1007/978-3-642-34393-3_3. [2] z. nie, y. li, y. wang. mechanical properties of steels for cold-formed steel structures at elevated temperatures. advances in civil engineering 2020. [3] p. martauz, i. janotka, j. strigáč, m. bačuvčík. fundamental properties of industrial hybrid cement: utilization in ready-mixed concretes and shrinkagereducing applications. materiales de construccion, 2016. [4] j. daxner. use of hybrid cement in fireresistant plasters. 88 khi/2016, 2016. [5] i. hager. behaviour of cement concrete at high temperature. bulletin of the polish academy of sciences, technical sciences 61, 2013. [6] j.-c. mindeguia, p. pimienta, h. carré, c. l. borderie. experimental analysis of concrete spalling due to fire exposure. european journal of environmental and civil engineering 2013. doi:10.1080/19648189.2013.786245. [7] oofem https://oofem.org, 2020. 108 https://doi.org/https://doi.org/10.1007/978-3-642-34393-3_3 https://doi.org/10.1080/19648189.2013.786245 acta polytechnica ctu proceedings doi:10.14311/app.2019.22.0048 acta polytechnica ctu proceedings 22:48–51, 2019 © czech technical university in prague, 2019 available online at http://ojs.cvut.cz/ojs/index.php/app analysis of the influence of transducer frequency on the ultrasonic measurement of concrete homogeneity dalibor kocáb∗, petr misák, barbora jindrová, martin alexa, tomáš vymazal brno university of technology, faculty of civil engineering, veveri 331/95, 602 00 brno, czech republic ∗ corresponding author: dalibor.kocab@vutbr.cz abstract. the paper analyses the influence of transducer frequency on the determination of concrete homogeneity using the ultrasonic pulse velocity test. transit time measurements were made on a 590 × 590mm concrete slab, with 110mm in thickness, in a raster of 5 × 5 points, which means the slab was tested in 25 places. the tests were made using a pundit pl-200 ultrasonic tester using transducers set at 54, 82, and 150 khz. two types of measurements were performed – spot measurements of the ultrasonic pulse transit time at each point and full area scanning. the paper is concluded by an evaluation of the concrete slab’s homogeneity measured by different transducers and techniques in addition to a statistical analysis of how the results are affected by the transducer frequency. keywords: ultrasonic pulse velocity test, concrete, concrete homogeneity, transducer frequency. 1. introduction the ultrasonic pulse velocity test, alongside the rebound hammer test discussed e.g. in [1], is one of the most widespread testing methods in civil engineering [2]. the principle of the ultrasonic pulse velocity test is sending ultrasonic pulses into the material and measuring their transit time. the pulses travel through different materials at a different velocity, depending on their properties [3]. in concrete it is generally true that the pulse velocity is lower in poor quality concrete or in damaged areas (including micro-defects) [4]. ultrasonic pulse transit time is regulated by čsn en 12504-4 [5] and čsn 73 1371 [6], and can be used for measuring: • concrete homogeneity, • the presence of cracks or air voids, • material properties – modulus of elasticity, compressive or tensile strength, • changes in the above-listed properties over time (e.g. by degradation). some of the main advantages of the ultrasonic pulse velocity test is that it is completely non-destructive, which makes it suited for repeated measurements in the same place and convenient for testing in a laboratory or on site. civil engineering usually uses frequencies of 20 – 150 khz; sometimes even up to 500 khz. higher frequencies improve the instrument’s resolution, making the measurement more accurate, but also substantially weakening the pulse, which asks for a compromise depending on the measured element’s thickness as well as other parameters [7]. the motivation for this experiment was the fact that a vikasonic ultrasonic tester [8] was procured by the co-authors [9]. this device is primarily designed for the non-destructive determination of the setting time in cement pastes or for monitoring the progress of their setting and hardening at an early age, which is something that has been addressed in a number of papers [10, 11]. the vikasonic uses transducers that operate at 54 khz. a specimen has the shape of the vicat ring and can be made of a paste, mortar, or concrete, which makes this device rather suitable. the vicat ring, however, is only 40 mm tall and thus the transducers are positioned at this distance as well. with a measuring base of this size, however, it would be better to use higher transducer frequencies [5, 6]. the goal of the paper was to determine how the transducer frequency affects the ultrasonic pulse transit time in concrete during homogeneity measurement. 2. experiment because it offers a wider range of settings during transit time measurement, the experiment was performed with a pundit pl-200 [12] instead of a vikasonic, since they both operate on the same principle. the actual measurement involves repeated sending of ultrasonic pulses and detecting them as they travel through the material. when applied to concrete, the ultrasonic pulse velocity test usually uses two transducers (a transmitter and a receiver) placed on opposite sides of the specimen or structure, measuring the time it takes for the ultrasonic pulse to travel from one transducer to the other, which is then used to calculate the velocity vl [7]. measurements were performed on a concrete slab with 110mm in thickness and 590 × 590mm in width and length using three types of transducers – 54, 82 and 150 khz. a 5 × 5 raster of 25 measurement places was laid out on the slab, see fig. 1. the slab was 48 http://dx.doi.org/10.14311/app.2019.22.0048 http://ojs.cvut.cz/ojs/index.php/app vol. 22/2019 analysis of the influence of transducer frequency. . . figure 1. spot measurement layout. scanned three times using each pair of transducers in an area scanning mode. this is a mode that the pundit pl-200 offers and is able to draw the quality distribution over an area of the concrete being tested. the results take the form of contour lines that show different areas in different colours based on the ultrasonic pulse velocity vl. even during the measurement, one can see which areas are of better or worse quality (in other words, where the ultrasonic pulses travel more quickly or slowly). the area scanning mode was used to determine the velocity vl 75 times – three measurements in the raster of 25 points, using all three transducer types. then the pundit pl-200 was switched to a mode measuring the ultrasonic pulse transit time t0. given the fact that the device does not support 82 khz transducers, the points of the raster on the test slab were measured using only transducers of 54 and 150 khz. otherwise the measurement procedure was the same as during area scanning (the sequence of measuring points) and both transducer frequencies were tested three times. thus, 75 values of transit time t0 were measured, from which 75 values of vl were calculated (three per measuring point). aside from homogeneity testing, a statistical analysis of measurements by each transducer was performed. the homogeneity testing followed čsn 73 2011 [13], where the key criterion is the value of the coefficient of variance vl, which, in homogeneous concrete, must not exceed 4.0%. 3. results and discussion the statistical comparison of test results was made following procedures of the analysis of variance (anova) and a two-sample t-test on a level of significance of 0.05. these statistical tests were set up so as to enable the assessment of the mean values of the test results; in case this hypothesis is rejected, the sets of results can be considered significantly different. first, an assessment was made whether the results are comparable at repeated measurements and unchanged instrument settings. no statistically significant deviation was found there. fig. 2 shows the results in the form of contour lines created from the average value of vl for the corresponding point of measurement. for the purposes of further processing, the results were compiled into five files of 75 values of vl. additionally, data normality was tested on a level of significance of 0.01. histograms of the test results are shown in fig.3. fig. 4 shows a box plot of the values of vl. in the case of spot measurements using 54 and 150 khz transducers, there was a statistically significant deviation in the values of vl, which was also confirmed by one-way hypothesis testing. it can thus be said that using 150 khz transducers produces much higher values of vl than using 54 khz ones. the same conclusion was also reached by an analysis of the results of area scanning, which says that measurements with 150 khz transducers return values that are statistically more significant than measurements made with 82 khz transducers. the values of vl measured by 54 or 82 khz transducers, however, do not show any difference. in addition, data from spot and area scanning at the same frequency were compared. no statistically significant deviation was found there. it can thus be stated that the resulting values of ultrasonic wave velocity is influenced primarily by transducer frequency. however, it must be noted that the experiment outcomes may have been affected by the small measurement base or the non-homogeneity of the slab being tested. this would explain why the coefficient of variance exceeded 4.0% in all measurements. 4. conclusion the experiment invites the following conclusions: • transducer frequency has a statistically significant role in determining the final value of ultrasonic pulse velocity vl in concrete. • the precise value of vl thus cannot be precisely ascertained, nor can it be used to calculate any property of concrete (e.g. dynamic modulus of elasticity), which corresponds with the recommendations in astm c597-16 [14]. • on the other hand, the ultrasonic pulse velocity test appears well-suited for determining concrete homogeneity, since it only observes variability in the values of vl. this method can just as well be used for determining the progress of degradation in concrete, e.g. due to exposure to freeze-thaw cycles, as the absolute value of vl is not deciding; only its decrease over time. • the vikasonic can be used for measuring the setting time of cement composites. however, given the small measurement base and the low frequency of 49 d. kocáb, p. misák, b. jindrová et al. acta polytechnica ctu proceedings figure 2. a contour diagram of the test results of area and spot scanning. figure 3. test result histograms. 50 vol. 22/2019 analysis of the influence of transducer frequency. . . figure 4. box plots of test results. transducers (40mm and 54 khz), this instrument is not entirely suitable for determining material properties such as the modulus of elasticity. • any ultrasonic device thus appears ill-suited for measuring the elastic modulus. there is a possible option of correcting the values based on some other simultaneous measurement (e.g. loading under compression). acknowledgements this paper has been written as part of the project no. ga17-14302s “experimental analysis of the early-age volume changes in cement-based composites”, supported by the ga0 czech science foundation and project no. fast/fch-j-18-5595 “reduction of chemical and autogenous shrinkage of alkaline activated slag”. references [1] p. cikrle, d. kocáb, b. kucharczyková, o. anton. in situ determination of the elastic modulus of concrete by means of ultrasonic pulse method. in 24th concrete days 2017, vol. 272 of solid state phenomena, pp. 70–75. trans tech publications, 2018. doi:10.4028/www.scientific.net/ssp.272.70. [2] v. m. malhotra, n. carino (eds.). handbook on nondestructive testing of concrete. crc press inc., 2nd edn., 2004. [3] v. özkan, i. h. sarpün, s. tuncel. nondestructive testing of materials and structures, vol. 6 of rilem bookseries, chap. relative effects of porosity and grain size on ultrasonic wave propagation in marbles, pp. 373– 379. springer, 2012. doi:10.1007/978-94-007-0723-8_54. [4] a. m. neville. properties of concrete. pearson, new york, 5th edn., 2011. [5] čsn en 12504-4 testing concrete part 4: determination of ultrasonic pulse velocity. standard, čni, prague, 2005. [6] čsn 73 1371 non-destructive testing of concrete – method of ultrasonic pulse testing of concrete. standard, únmz, prague, 2011. [7] b. kopec, et al. nedestruktivní zkoušení materiálů a konstrukcí: (nauka o materiálu iv). akademické nakladatelství cerm, brno, 2008. [8] s. t. systems. vikasonic measuring the early setting and hardening with ultrasonic waves. http: //www.schleibinger.com/cmsimple/en/?setting_ and_maturity:ultrasonic_setting_measurement. [9] i. rozsypalová, o. karel, b. kucharczyková, et al. monitoring of the setting and early hardening with ultrasonic waves. in rehabilitation and reconstruction of buildings, vol. 776 of key engineering materials, pp. 51–54. trans tech publications, 2018. doi:10.4028/www.scientific.net/kem.776.51. [10] a. gibson, d. ciancio. nondestructive testing of materials and structure, vol. 6, chap. early-age ultrasonic testing of concrete and shotcrete using embedded sensors, pp. 485–490. springer, dordrecht, 2013. doi:10.1007/978-94-007-0723-8_69. [11] n. de belie, c. grosse, j. kurz, h.-w. reinhardt. reinhardt hw ultrasound monitoring of the influence of different accelerating admixtures and cement types for shotcrete on setting and hardening behavior. cement and concrete research 35:2087–2094, 2005. doi:10.1016/j.cemconres.2005.03.011. [12] proceq. pundit ultrasonic pulse velocity and pulse echo tomography. https://www.proceq.com/compare/ pundit-ultrasonic-pulse-velocity-and-pulseecho-testing/. [13] čsn 73 2011 non-destructive testing of concrete structures. standard, únmz, prague, 2012. [14] astm c597-16 standard test method for pulse velocity through concrete. standard, astm international, west conshohocken, 2016. 51 http://dx.doi.org/10.4028/www.scientific.net/ssp.272.70 http://dx.doi.org/10.1007/978-94-007-0723-8_54 http://www.schleibinger.com/cmsimple/en/?setting_and_maturity:ultrasonic_setting_measurement http://www.schleibinger.com/cmsimple/en/?setting_and_maturity:ultrasonic_setting_measurement http://www.schleibinger.com/cmsimple/en/?setting_and_maturity:ultrasonic_setting_measurement http://dx.doi.org/10.4028/www.scientific.net/kem.776.51 http://dx.doi.org/10.1007/978-94-007-0723-8_69 http://dx.doi.org/10.1016/j.cemconres.2005.03.011 https://www.proceq.com/compare/pundit-ultrasonic-pulse-velocity-and-pulse-echo-testing/ https://www.proceq.com/compare/pundit-ultrasonic-pulse-velocity-and-pulse-echo-testing/ https://www.proceq.com/compare/pundit-ultrasonic-pulse-velocity-and-pulse-echo-testing/ acta polytechnica ctu proceedings 22:48–51, 2019 1 introduction 2 experiment 3 results and discussion 4 conclusion acknowledgements references doi:10.14311/app.2020.28.0032 acta polytechnica ctu proceedings 28:32–41, 2020 © czech technical university in prague, 2020 available online at http://ojs.cvut.cz/ojs/index.php/app thermal analysis of dry storage and transportation casks castor using cobra-sfs martin ševečeka,b,∗, mojmír valacha, chih-hao leeb a czech technical university in prague, faculty of nuclear sciences and physical engineering, department of nuclear reactors, v holešovickách 2, 180 00 prague 8, czech republic b national tsing hua university, institute of nuclear engineering and science, no. 101, section 2, guangfu road, east district, hsinchu city, taiwan 300 ∗ corresponding author: martin.sevecek@fjfi.cvut.cz abstract. state office for nuclear safety is a regulator body in the czech republic handling also licensing of storage and transportation casks (scs) of the spent nuclear fuel (snf). one of the main concerns for the dry storage of snf is the safe removal of the residual heat resulting from the decay of fission products, actinides, and activated construction materials. there are many storage systems in use around the world with different storage configuration, fuel designs and boundary conditions that need to be evaluated. two thermal models of snf storage casks castor 440/84 and castor 440/84m were developed using the cobra-sfs code, which is an internationally accepted and validated code used for licensing of dry storage casks and safety evaluation. the casks are in operation at the nuclear power plant dukovany for dry storage of the wwer-440 snf. both models were validated and used for evaluation of several problems in different configuration and particularly for licensing purposes at the sujb. keywords: cobra-sfs, heat removal system, licensing, spent nuclear fuel, storage and transportation cask, thermal analysis, wwer. 1. introduction the accurate prediction of heat transfer and temperature fields in a dry storage cask castor has been undertaken. due to the nature of the dry storage casks, the temperature cannot be directly controlled and calculations are performed to ensure safe storage and transport of snf in all postulated conditions during decades of dry snf storage. recently, many applicants for a license of storage and transportation cask (also in the czech republic) have used computational fluid dynamics (cfd) codes to simulate the thermal behavior of dry storage systems. cfd analysis was mentioned also in the iaea report [1] or recently performed by [2–8]. however, the cfd codes have their limitations and specific problems (errors in spatial discretization, meshing, numerical instability, and high computational power). for that reason, the state office for nuclear safety in the czech republic (sujb) decided to perform confirmatory independent calculations of the thermal behavior of sc castor using cobra-sfs code [9] in cooperation with department of nuclear reactors, ctu in prague and ujv rez as recommended by international standards [10, 11]. the subchannel code cobra-sfs based on the well-known cobra thermohydraulical code has been used for thermal analysis of different systems with pwr, bwr but also wwer fuel geometries [12–16]. the applicant for the license of storage casks for wwer-440 snf storage [17] supported the application with cfd thermal analysis using the fluent code [18]. the sujb staff decided to perform an independent evaluation of the heat removal design of the storage casks castor using cobra-sfs code and to use the models for new potential applications in case of new design changes as well. 1.1. cobra-sfs and cfd codes several studies concerning safe heat removal from dry storage casks using cobra-sfs were presented [6, 19–21]. most of the research and work was conducted more than 20 years ago at the time of validation of the first version of the code. recently, some institutions (kaist, epri, npp paks) have been using this code for comparison or validation of new codes [22, 23]. other authors published their results and recommendations on the usage of cfd computer codes. horizontal dry storage systems using phoenics code were investigated by [24], other studies showed thermal analysis methods for a vertical concrete sc with phoenics and fit-3d cfd codes [25], there were also practical guidelines for modeling a concrete sc using fleunt that were confirmed by experimental data from the vsc-17 sc [26]. the standard review plan nureg-1536 [27] defined by the us nrc allows applicants to use cfd codes together with the simplified porous media approach and its effective thermal conductivity. however, it recognizes the benefits of using more complex approaches modelling detailed geometry similar to cobra-sfs. 32 http://dx.doi.org/10.14311/app.2020.28.0032 http://ojs.cvut.cz/ojs/index.php/app vol. 28/2020 thermal analysis of dry storage casks castor the models presented in this paper are based on the detailed description of the inner complex geometries of the dry scs castor 440/84 and castor 440/84m. models in the cobra-sfs code are based on the detailed design and equations defining the heat transfer rather than using a general approach of cfd codes (effective thermal conductivity, porous media, etc.). the porous media concept adopted in most of the cfd scs flow calculations usually requires experimental data for determining of effective conductivity. the experimental validation of porous media and fuel effective conductivity models is needed for application of the code, on a case-by-case basis[6]. in contrast, because of the extensive validation of the cobra-sfs code by comparison with experimental data in single-assembly experiments and test casks loaded with spent fuel (see references [14, 23, 28, 29]), and the fundamental thermal-hydraulic modeling approach used in the code to represent spent fuel assemblies within a cask [30], extensive case-by-case validation is not necessary for each application of the code. the code requires only detailed knowledge of the geometry of the storage system. from this perspective, the model created in cobra-sfs is suitable for licensing purposes in the czech republic. on the other hand, the creation of the model is a much more difficult and time-consuming task than cfd modeling. 2. licensing process and requirements in the czech republic there were several safety functions of dry spent fuel storage facilities defined. safety functions must be maintained during normal operation, anticipated operational occurrences, and design basis accident conditions by either the storage cask, the storage facility itself, or both [31]. the detailed requirements are specified in the atomic act [32] or other regulatory legal documents. the basic safety functions are illustratively shown in figure 1 and can be simply summarized as [31]: • maintain the subcriticality of stored snf • ensure removal of heat generated by the stored snf • prevent the release of radioactive material • maintain retrievability of the snf throughout the whole lifetime of the storage facility • ensure that radiation dose rates and doses do not exceed valid limits the applicant shall verify by analysis, surveillance, testing, and inspection that the physical status of the installation and its operation continue in accordance with technical specifications, safety requirements, and safety assessment. the independent reassessment shall be periodically updated if there are modifications or new regulatory requirements and standards. each type of snf sc has to be licensed. however, there are no facility-specific international requirements, but only guidelines exist [33]. this paper focuses on the safe removal of heat from the spent nuclear fuel, however, the design of a sc has to ensure all safety functions. figure 1. five fundamental safety functions of the dry storage casks as defined by the state office for nuclear safety. the other characteristics of both scs are: dry transportation and 60 years storage of the snf; minimum of 5 years cooling of the snf before backfilling the sc; sc is filled with helium with low pressure around 700 kpa; the basket consists of 84 casings; the central position is empty. 2.1. decay heat removal at the storage facility for spent nuclear fuel dukovany (sfsnf), the basic (regulatory) heat-related requirements originating from the licensing process of the scs castor are as follows: • the peak cladding temperature (pct) during transport and storage must not exceed 350◦c [37] • the total thermal output of all snf assemblies in one cask must not exceed the limiting value (e.g., 25 kw for castor440/84) or must not exceed the limits of heat generation for a particular snf single assembly • the licensed cask has to be capable of withstanding an external fire for 30 minutes with the temperature of 800◦c • pct during the drying process of the sc can exceed 350◦c only for a limited time. the thermal evaluation of a sc must also include: definition of the snf inventory, cask material properties, snf thermal output, cask loading pattern, operational and accidental conditions, boundary conditions, stationary and transient calculation of temperatures at selected points, compliance with regulatory and design criteria, inputs to emergency procedures, 33 m. ševeček, m. valach, c.-h. lee acta polytechnica ctu proceedings figure 2. residual heat generated in the wwer snf by the most significant actinides (left) and fission products (right) for the first 500 years after discharge from a reactor. it was calculated using origen-arp decay code, with cross-section libraries generated using 2-d depletion module triton from the scale package for a reference condition of wwer-440 fuel as recommended by [34–36]. parameter castor 440/84 castor 440/84m diameter [mm] 2660 2660 height [mm] 4080 4170 thickness of the wall [mm] 370 410 storage weight [t] 116.11 118.34 max. activity [bq] 2.7 × 1017 2.6 × 1017 max. ḋ on surface < 2 msv/h < 2 msv/h max. ḋ in 2 m from sc < 0.1 msv/h < 0.1 msv/h max. number of fas 84 84 max. residual power of sc heterogeneous pattern 24.66 kw max. residual power of sc homogeneous pattern 21 kw 24.50 kw max. residual power of 1 fa heterogeneous pattern 340 w(6x), 290 w(78x) max. residual power of 1 fa homogeneous pattern 250 w 291.7 w table 1. fundamental parameters of the storage and transportation casks castor 440/84 a 440/84m derivation of technical specifications and thermal assessment of the cask drying procedure [31, 32]. for heat removal calculations, the snf is defined by the evolution of its residual heat (source term). it was calculated for different cases using origenarp [38] code from the scale 6.1 package. the wwer-440 cross section libraries were generated using the 2d depletion module triton from the scale package, and a similar approach was validated against isotopic assay experimental data for spent fuel in [35]. an example of residual heat generated by actinides and fission products in one of the wwer-440 snf assemblies is presented in figure 2. the evolution of the total generated residual heat for each of the 84 snf assemblies has been introduced into the cobra-sfs model as boundary conditions. 3. construction of the scs castor there have been two types of storage casks in the dukovany storage facilities used: castor 440/84 and castor 440/84m. the older castor 440/84 was licensed by the sujb in 1995, and the new modified type castor 440/84m obtained its original license in 2005. both licenses were reviewed, and the designs were reassessed several times due to the design changes of wwer fuel and the sc itself. the casks are dual-purpose casks for transportation and storage of 84 fas of undamaged spent nuclear fuel from reactor wwer-440. both types of scs have been developed and produced by german company gns gesellschaft fãijr nuclear-service mbh (gns), headquartered in essen, germany. the fundamental characteristics of both scs are summarized in table 1. 3.1. heat removal system of the scs the cross-section diagram of both types of scs are illustrated in figure 3. the residual heat originates mainly from the radioactive decay of fission products and actinides that are presented in the spent fuel pellets. the construction materials of the snf are also 34 vol. 28/2020 thermal analysis of dry storage casks castor figure 3. plain cut in the dual purpose storage and transportation casks castor 440/84 (left) and castor 440/84m (right) activated and generate considerable heat energy in comparison with fission products and actinides. the heat is transferred from the snf pellets mainly by conductive heat transfer to the fuel cladding. radiative heat transfer plays also a role mainly in the heat transfer from the fuel rods to the internal structures of scs. a sc is filled with helium flowing around snf, more precisely around the fuel rods. helium carries the heat away from the snf using natural convection heat transfer. helium is circulating inside the cask and is cooled down on the sc’s shell inner surface. the heat passes through the cask shell to the outer surface of the sc. the outer surface includes ribs to improve convection heat transfer with the environment. the ambient air convects the residual heat from the sc, and the storage facility itself ventilates the hot air outside. 3.2. design differences the scs castor 440/84 and castor 440/84m are very similar in terms of design. there are only a few differences and modifications to accommodate higher burnup fuel. the biggest change is the design of the basket. the improved design for castor 440/84m is using almg steel with atabor dividers. atabor is a special type of steel with a high content of boron for neutron absorption. the different construction of both baskets is shown in figure 3. the dimensions of the snf are the same for both types of scs. therefore, the inner dimensions of the basket tubes are the same as well. however, the basket of the castor 440/84m is more compact, and the walls are thicker. the next difference is a new construction of a cask shell. there was an extra row of polyethylene rods added to improve the shielding capability of the cask and the construction of the outer ribs was improved. there are two moderator plates added to the bottom and top of the sc to improve the neutron shielding in the top and bottom part. figure 4. relation of a subchannel control volume to a general snf storage or transportation system [30] 4. cobra-sfs thermal models 4.1. cobra-sfs code cobra-sfs code [30] is a computer code for the prediction of material temperatures and fluid conditions in various systems for spent fuel storage. it performs thermal-hydraulic analysis and has been successfully validated and verified [19]. cobra-sfs solves three basic equations in finite volume form: 1) conservation of mass, 2) momentum, and 3) energy. the equations assume an incompressible single-phase heat transfer fluid (typically inert gases). finite volume method is used also for the calculation of the conduction inside and between the solid structures of the dry storage system which is then coupled to the fluid solution [39]. in cobra-sfs, the general eulerian control volume is defined as the fluid subchannel in the rod array. the geometry is illustrated in figure 4. as can be seen, the fluid flow is constrained by the surface of fuel cladding. on the smaller scale, the fuel rods section the channel into many subchannels that communicate laterally by crossflow. applying the integral balance relations to the control volume of each subchannel yields a set of subchannel equations which can be approximated in finite difference form and numerically solved. 4.2. numerical models the configuration modeled is a sc filled with snf standing alone (vertically) inside a dry storage facility dukovany. the model was simplified using a symmetry of the sc. only one-sixth was modeled. this is a valid approach because 84 fas with generally similar characteristics were stored in the sc. if the number of stored fas is lower and the differences between particular assemblies are bigger, full-scale model needs to be developed (e.g., dry storage of wwer-1000 snf). there are three main areas of interest when modeling using cobra-sfs: upper plenum, channel region, and lower plenum [19]. the main de35 m. ševeček, m. valach, c.-h. lee acta polytechnica ctu proceedings tailed modeled section of the sc is a channel region (basket in this particular case). plenums provide a one-dimensional mixing node for helium entering or leaving the flow channels and subchannels in the basket region. this model enables the code to directly calculate the “closed loop” natural recirculation of helium gas within a cask or canister. the plenum model also includes the capability of modeling conduction heat transfer paths above and below the basket region. the feature is typically used to represent the solid structures of the cask lid (in the upper plenum), and the cask base (in the lower plenum). there are solid structures around the basket. they are defined by several boundary conditions. outer shell covering the sc’s basket is considered as a plenum as well. first, models of the wwer-440 fuel assemblies were created. since only one-sixth of scs was modeled, models of the half-assemblies were also required. there are two types of nodes used in these models: fuel rods and subchannels around these rods. a simplified sketch of the fuel model with fuel rod numbering is presented in figure 5. there are 127 fuel rods in 67 groups. in these groups, several fuel rods were merged into one bigger fuel rod. this technique simplified the models and saved computational time. similar numbering process was used in numbering subchannels around the fuel rods. this approach was developed in the validation work for cobra-sfs in the 1980âăźs and has been shown to yield accurate predictions of peak temperature and temperature distributions in spent fuel assemblies and casks. however, it requires careful selection of rods with similar decay heat and geometric symmetry within the rod array, to avoid distortion of the temperature distribution within the fuel assembly. if sufficient computational resources are available, a rod-by-rod representation of the fuel assemblies is the preferred approach, as it avoids the flattening of local temperature gradients, that is, the inevitable result of merging adjacent rods. figure 5. illustration of wwer-440 fuel assembly models with fuel rod numbering. outer fuel rods are merged to simplify the model, fuel rods with potentially higher pct are modeled individually. after the definition of fuel assemblies, the design and connections of the sc basket and particular casings were modeled. there are 16 fuel assemblies (12 full assemblies and four halves) incorporated into onesixth of the basket model. additionally, there are 24 channels without fuel filled only with helium. they substitute the periphery construction of the basket, one represents the central empty position, and the rest are channels around the actual fuel assemblies. the illustration of the computational model and its numbering is shown in figure 6. all defined assemblies and channels with or without fuel are connected by 326 solid slabs. 317 slabs connect assemblies inside the basket, and the others represent sc’s body construction, shielding, and connection to an environment. the heat transfer models were individually incorporated into the model for all channels and solid slabs. figure 6. one sixth of the modeled sc with subchannel numbering. there were 12 full-assemblies (no. 1-12), 4 half-assemblies (no. 13-16) and 24 empty channels filled with he (no. 17-40) implemented into the model. 5. results 5.1. reference calculations and models validation both storage casks are licensed for storage of the wwer-440 snf. however, the newer type castor 440/84m enables to store the snf with different parameters (e.g., residual heat generation, burnup, uranium enrichment). in the safety documentation provided by the applicant, one reference "boundary" configuration of the snf (pattern) was approved for castor 440/84 and two patterns for the castor 440/84m. the reference loading patterns are illustrated in figure 7 together with the actual configuration of the sc castor 440/84 no. 10 from the sfsnf dukovany. 36 vol. 28/2020 thermal analysis of dry storage casks castor temperature sensor no. measured temperature calculated temperature abs. difference [−] [◦c] [◦c] [%] 1 47.50 45.34 4.764 2 46.91 45.34 3.463 3 58.75 63.95 8.131 4 57.12 63.90 10.610 5 50.88 46.64 9.091 6 24.85 24.85 0.000 table 2. comparison of measured temperatures on the sc no. 10 and temperatures calculated by the cobra-sfs heat-removal model figure 7. the reference homogeneous pattern for castor 440/84 (left), reference heterogeneous pattern for castor 440/84m (center) and the initial pattern of the real sc castor 440/84 no.10 from the sfsnf (right) the reference configurations were calculated using cobra-sfs code and compared with the results provided by the applicant. the applicant used cfd code fluent [18] to prove the safe design of the scs. the results of the reference calculations calculating the pct are presented in figure 9. two reference calculations are presented together with results from the actual sc no.10. the highest peak cladding temperatures are: 255.0 ◦c and 256.8 ◦c for the two maximal patterns for castor440/84 and 298 ◦c for the reference pattern of the sc castor 440/84m. the fluent results provided by the applicant are not presented here, but they are similar lower by 35 ◦c. this is in agreement with other studies where cobra-sfs is presented as a code providing conservative results [40]. the wwer-440 fuel assembly consists of 127 fuel pins that do not have uniform power peaking. heat source term was calculated for the whole assembly and radial peaking factors were defined based on the expected radial power profile. the difference when using uniform radial peaking (homogeneous) in an assembly and non-uniform peaking (heterogeneous) with a peaking factor 1.2 is shown in figure 8. higher peaking factor results in slightly higher temperatures, but the effect are almost negligible. therefore, other figure 8. radial temperature distributions for homogeneous and heterogeneous radial peaking factors in the sc castor 440/84 with the reference pattern. calculations were performed with a homogeneous pattern of power (peaking equals to 1.0). as mentioned above, the resulting pcts are very similar to the applicant’s results. moreover, six thermocouples were installed on the outer surface of the sc castor 440/84 no. 10, and the actual temperatures on the outer surface of this cask were measured. the thermocouples were on the shielding lid and on the outer surface in various axial positions. the data obtained were compared with the cobrasfs simulation and applicant’s calculations to validate cobra-sfs model. the comparison of the measurements and calculations is summarized in table 2. based on the comparison of the applicant’s calculations and to the data experimentally measured, both thermal models of scs were validated and verified. this, in conjunction with the other validation data available, suggests that cobra-sfs predicts the physics appropriately for calculating surface temperature and pct and can be used for calculation of other problems and in other applications as approved by the sujb staff. 37 m. ševeček, m. valach, c.-h. lee acta polytechnica ctu proceedings figure 9. radial (left) and axial (right) temperature distribution in sc castor 440/84 for two reference cases and for the real sc no.10. in the case of filling the older castor 440/84 with the licensed reference filling of the modified type castor 440/84m, the pct will reach almost 300 ◦c which is more than 30 ◦c difference in the pct in comparison with the castor 440/84m. figure 10. axial pct distributions for the hottest rod no.66 in fuel assembly no. 3 at different times of snf dry storage in sc castor 440/84m. 5.2. quasi-steady state calculations cobra-sfs can perform transient calculations but only for short transients such as drying of a sc and accidental conditions. the thermal models that were developed perform only steady-state calculations and they are not suitable for long-term transient calculations. however, there is a possibility of calculating the boundary conditions for different time frames and using them for the steady-state model. this approach was chosen for the calculation of the pct evolution during 100 years of dry storage. other time-dependent effects such as degradation, geometry changes, etc. were neglected. as the source term is decreasing, the pct is decreasing as well. both types of scs hold a valid license for 60 years of dry storage, but there were some suggestions by the applicant that the license period shall be lengthened in the future. the results of the pct evolution are summarized in figure 10. the temperature profile is similar to the original heat generation profile, but it is influenced by the natural circulation of helium that is coolest on the bottom of the sc and is reheated as it is moving upwards. 5.3. disposal configuration the czech government approved a plan for opening a final disposal site for the snf in 2065. research and development of the project have been running for several years and brought several new ideas, including the using of the dual-purpose storage cask castor for disposal purposes. this approach would significantly reduce the cost and number of manipulations of the snf. however, scs castor were designed, fabricated, and licensed as storage and transportation casks, and there are some doubts about using them for disposal purposes. one of the concerns is a removal of residual heat. for that purpose, several modifications were made to the original cobra-sfs models of sc castor to accommodate the conditions inside the disposal facility. the design of the disposal facility originates from sweden [41] with horizontal placement of the storage/disposal cask castor. new materials with their fundamental physical properties were implemented. the granite is assumed as a homogeneous material with no faults. temperature boundary condition was defined on the outer surface of the granite 10 meters from sc’s surface. the model of disposal configuration inside the disposal facility is illustrated in figure 11. the goal was to calculate the limiting cooling time necessary for sufficient cooling of the snf. in the disposal configuration, there is a fundamental limit of 100 ◦c for the bentonite/sc interface. bentonite is a special material surrounding the disposal cask that has optimal isolation and confining properties for the disposal of the snf. however, the material must remain wet to keep the optimal characteristics and, therefore, limits the temperature on the interface: disposal cask/bentonite to 100 ◦c. the method of semitransient calculation described in section 5.2 was used to evaluate the maximal residual heat generation in the disposal configuration, 38 vol. 28/2020 thermal analysis of dry storage casks castor figure 11. model of disposal configuration based on the swedish disposal concept with sc castor horizontally disposed. sc is confined by bentonite 0.3 m thick in horizontal position and sealed with concrete. it is fully surrounded by granite. which might be permitted by the sujb in the future. the figure 12 shows the relation between the peak temperature on the interface bentonite/sc castor and the corresponding residual power of the snf. the limiting value of the residual heat is 23 watts per fuel assembly as shown in the graph to keep the temperature under the limit. however, it should be pointed out that the model is based on the storage configuration of the sc and only the boundary conditions were changed based on the disposal design. 6. summary and conclusions two types of dry storage casks have been used at the dukovany storage facility. every design of storage, transportation, or disposal cask must be licensed in the czech republic by the sujb based on the application from the responsible utility. cfd codes have been recently widely used by applicants, but there are several disadvantages connected to the usage of such codes. for that reason, the sujb decided to perform the independent heat removal system evaluation of scs castor using the cobra-sfs code. two heat removal models: castor 440/84 and castor 440/84m were created and validated using experimental data and independent code-to-code validation. the presented calculations suggest that cobra-sfs is able to accurately predict the pct temperature which is the limiting factor for dry storage. it also confirms that the code accurately predicts cases with hexagonal geometry such as wwer fuel assemblies. the models were used to reassess the safe removal of residual heat for reference snf patterns and the evolution of pct during 100 years storage. in the worst case scenario, the pct is not higher than 256.8 ◦c for castor 440/84 and 298 ◦c for castor 440/84m, which is well below the limiting value defined by the sujb and international guides. addifigure 12. linear relation between the residual heat generated by the snf and peak sc/bentonite interface temperature in the disposal configuration. the limiting temperature corresponds to the 23 watts per one wwer-440 snf assembly in homogeneous configuration of the sc castor 440/84m. tionally, these conservative limits are well above the real best-estimate pcts. the calculation of the real pattern from the sc number 10 shows pct below 210 ◦c. one of the possible solutions for final disposal is to use scs castor also as a disposal cask in the czech republic. the swedish concept of disposal configuration was adopted, and the original disposal cask was replaced by the sc castor. the calculation using cobra-sfs 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[41] j. andersson, k. skagius, a. winberg, et al. site-descriptive modelling for a final repository for spent nuclear fuel in sweden. environmental earth sciences 69(3):1045–1060, 2013. 41 https://doi.org/10.2172/986788 https://doi.org/10.1080/00295450.2017.1305190 acta polytechnica ctu proceedings doi:10.14311/app.2018.17.0001 acta polytechnica ctu proceedings 17:1–5, 2018 © czech technical university in prague, 2018 available online at http://ojs.cvut.cz/ojs/index.php/app deformation of fe3si single-crystals under nanoindentation jaroslav čech∗, petr haušild, aleš materna czech technical university in prague, faculty of nuclear sciences and physical engineering, trojanova 13, 120 00 praha, czech republic ∗ corresponding author: jaroslav.cech@fjfi.cvut.cz abstract. knowledge of the complex deformation behavior in the anisotropic materials is one of essential issues in materials science and it is crucial for the applications of a given material. in this study, mechanical response of fe3(wt.%)si single crystal to nanoindentation with spherical indenter was investigated. hardness and indentation young´s modulus were determined experimentally and by finite element modelling. observed pop-in phenomenon, shape of the residual imprints and origin of the slip lines were explained on the basis of resolved shear stress computed by finite element model. keywords: nanoindentation, bcc, slip activation, mechanical properties, finite element model. 1. introduction nanoindentation is a very useful tool for characterization of mechanical properties of various materials in micro(nano)-volume. nowadays, it is still more and more widespread method as the preparation of the specimens is not difficult and the amount of the material needed for testing is low. it gives the information about basic material properties, e.g. hardness, young´s modulus or fracture toughness. nanoindentation is a well-established and normalized method [1] and basic procedures of data analysis are already implemented in commercial devices. more profound analysis of the entire indentation test including the investigation of force-depth curves and residual imprints together with finite element modelling can result in complex description of deformation behavior of the materials under the examination. these detailed studies are usually necessary when investigating the anisotropic materials with crystallographic structure. many models based on the finite element (fe) method [2–5] or molecular dynamics [6, 7] were recently developed to describe the distribution of stresses and strains under the indenter. they provide not only the information about material properties, but they can also describe the evolution and propagation of defects (e.g. dislocations, twinning) in the studied material. in this paper, deformation in different orientations of body centered cubic (bcc) fe3(wt.%)si single crystal was investigated. nanoindentation was used to characterize the changes in mechanical properties with crystallographic orientation. finite element model was developed to describe the stress field under the indenter and to explain the differences in the shape of residual imprints. 2. experimental details the single crystals of bcc fe3(wt.%)si were produced by the floating-zone melting technique. the material was chosen for this study as its properties depend on the crystallographic orientation [8, 9] and it is often taken as a model material of ferritic steels and iron based alloys. tests were performed on the samples oriented with the directions [100] and [110] perpendicular to the specimen surface (figures 1 and 2). surface was prepared by standard metallographic procedures (mechanical gridding and polishing up to 1µm diamond paste) and finished by electrolytic polishing to remove residual stresses in the surface layers. nanoindentation tests were carried out on nht2 nanoindentation instrument (anton paar, graz) with diamond spherical indenter with nominal radius 20 µm. maximum loading force 75mn was reached after 30 s. loading sequence was followed by 10 s of the holding at maximum load. unloading time was the same as for loading, i.e. 30 s. to obtain values of hardness and indentation young´s modulus, data were treated by oliver-pharr method [10, 11]. poisson´s ratio was supposed 0.3. in order to enable the observation of activated slip systems and residual imprints by means of optical and scanning electron microscopy, indentations to maximum force 500mn were performed. fem nonlinear code msc.marc was used for numerical evaluation of the stress field under the indenter. the contact between the rigid indenter and deformable material was considered frictionless due to very low tangential angle between sample and indenter tip. the finite element mesh of indented material consisted of 343 500 eight-node linear brick elements and 35 sixnode linear wedge elements placed directly under the indenter center (figure 3). the smallest elements with a size approximately 0.1 µm were in the area of the supposed contact and its close surroundings. the entire model has the diameter of about 1000 µm 1 http://dx.doi.org/10.14311/app.2018.17.0001 http://ojs.cvut.cz/ojs/index.php/app jaroslav čech, petr haušild, aleš materna acta polytechnica ctu proceedings figure 1. orientation of atoms in the plane parallel with the surface and residual imprint with shown slip systems for [100] crystal orientation. figure 2. orientation of atoms in the plane parallel with the surface and residual imprint with shown slip systems for [110] crystal orientation. and height of about 500 µm. the specimen was fixed only on its bottom, as the relatively large size of the specimen minimized the effect of boundary conditions on its exterior on the achieved results. elastic constants used in the model were taken from ref. [12] as: c11 = 222.3gpa, c12 = 135.5gpa, and c44 = 123.0gpa. resolved shear stress τrss corresponding to all possible slip system for bcc structure (i.e. 48 slip systems for planes {110}, {112}, {123} and directions <111>) was computed using schmid´s law in the form for multiaxial stress state: τrss = lnxldxτxx + lnyldyτyy + lnzldzτzz+ + (lnyldx + lnxldy)τxy + (lnzldy + lnyldz)τyz+ + (lnxldz + lnzldx)τzx, (1) where l are direction cosines between the axes and slip direction d or normal to the slip plane n and τij is stress tensor. 3. results experimental indentation curves showing the dependence of applied force on penetration depth are in figure 4. for both orientations, the same behavior was observed. sudden increases in penetration depth at constant load called pop-ins were spread through whole range of applied forces. for some curves the popin did not occur even for the maximum applied load of 75mn and these curves were completely elastic, which was confirmed by zero residual depth and no residual imprint after the unloading. on the other hand, the curves with nearly immediate development of plastic deformation were also observed. in figure 4c, elastic and completely plastic˝curves are compared. it can be seen that both type of curves for orientation [110] are stiffer than for orientation [100]. hardness and indentation young´s modulus, measured experimentally and computed by finite element model are summarized in table 1. hardness evaluated from the indentation curves with developed plastic deformation was approximately 1.7gpa and it was slightly higher for orientation [110] than [100]. the differences in indentation young´s modulus were more pronounced. both approaches, experiments and finite element modeling, showed that orientation [110] is stiffer than orientation [100]. shape of residual imprints made by indentation to maximum force 500mn is shown in figures 1 and 2. for orientation [100] significant deviation from expected spherical shape was observed. the deformation spreads preferably in the directions of diagonals of the cubic lattice in the dense directions <111> creating characteristic four-fold symmetry of the imprint. the 2 vol. 17/2018 deformation of fe3si single-crystals under nanoindentation figure 3. finite element mesh. shape of the residual indent for orientation [110] is more regular. the spherical imprint is only slightly elongated into the ellipsoidal shape with two-fold symmetry. for both orientations, intersections of slip lines with the surface are clearly visible. resolved shear stress in the available slip systems for bcc structure were computed and the critical slip systems with their maxima were identified by finite element analysis (table 1). example of the distribution of the resolved shear stress is shown in figure 5. for both orientations, highest values of resolved shear stress were in {112} planes. only slightly lower values were computed for the planes {123} and the lowest resolved shear stress acts in planes {110}. in all critical systems, maximum shear stress was identified for <111> out-of-plane directions (i.e. <111> directions not located in the surface layer). differences between two studied orientations were found in the values of the maximum resolved shear stress and in the number of available slip systems. while the maximum of the resolved shear stress is approximately 1.5times higher for orientation [110], the number of the slip systems with maximum resolved shear stress is two times lower for this orientation than for the orientation [100]. 4. discussion generally, pop-in behavior can be related to several phenomena in the material (e.g. film rupture, beginning of plasticity). as, in the case of the studied material, plastic deformation did not occur before the pop-in event, it is unambiguous that the observed popins are associated with the activation of slip and/or nucleation of dislocations with consequent develop0 10 20 30 40 50 60 70 80 0 100 200 300 400 500 f [m n ] h [nm] [100] elastic curve curves with developped plastic deformation pop-in (a) 0 10 20 30 40 50 60 70 80 0 100 200 300 400 500 f [m n ] h [nm] [110] elastic curve curves with developped plastic deformation pop-in (b) 0 10 20 30 40 50 60 70 80 0 100 200 300 400 500 f [m n ] h [nm] [100] elastic [110] elastic [100] plastic [110] plastic (c) figure 4. experimental indentation curves for (a) [100] orientation, (b) [110] orientations and (c) comparison of elastic curves and plastic curves with nearly immediate development of plasticity after the contact for both orientations. ment of plastic deformation. the spread in the force for which the pop-in event occurs is caused by the distribution of pre-existing dislocations in the material and the position of the maximum of resolved shear stress under the indenter. the shear stress is not symmetric (figure 5) and its maximum is located in depth under the surface in a different positions depending on the slip system (figure 6). if the maximum is near the pre-existing dislocation, only low value of stress is needed for the activation of the slip and the plastic deformation starts nearly immediately after the contact of the indenter with the surface. this value of shear stress should correspond to the values of critical resolved shear 3 jaroslav čech, petr haušild, aleš materna acta polytechnica ctu proceedings table 1: mechanical properties and resolved shear stress in available slip systems for orientations [100] and [110] of fe3si single crystal. orientation h [mpa] eexp [gpa] efe [gpa] slip plane τrss [mpa] (fmax = 75 mn) no. of equivalent slip systems with maximum τrrs [100] 1698 ± 55 183.0 ± 4.5 202.6 {110} 2827 8 {112} 2905 4 {123} 2893 8 [110] 1754 ± 10 221.2 ± 5.7 229.3 {110} 4269 4 {112} 4634 2 {123} 4587 4 figure 5: resolved shear stress in (2-11) [11-1] slip system for the crystal in orientation [100] (cut in (011) plane). figure 6: location of the maximum of resolved shear stress under the indenter for critical slip systems (only one quarter of slip systems is displayed because of the symmetry). table 1. mechanical properties and resolved shear stress in available slip systems for orientations [100] and [110] of fe3si single crystal. figure 5. resolved shear stress in (211) [111] slip system for the crystal in orientation [100] (cut in (011) plane). stress for uniaxial tensile tests which is in the range of hundreds of mpa [13]. on the other hand, if there is no dislocation near the maximum of the resolved shear stress, significantly higher stress is needed for homogeneous dislocation nucleation. this stress is for fe3(wt.%)si defect-free single crystal in the range of g/20 [14], where g is the shear modulus. this value is comparable (for the orientation [110]) or even higher (for the orientation [100]) than the values of resolved shear stress computed for maximum load 75mn. it explains the existence of the indentation curves which are completely elastic. thus, the majority of popins spread through the whole range of applied loads corresponds to the activation of the slip on pre-existing dislocations located near the maximum of the resolved shear stress. the values of indentation young´s modulus determined experimentally are slightly lower than the values from fe model. besides the values of elastic constants used in the model which can be slightly different than the values taken from the literature, it is probably caused by the shape of the residual imprint. in elastic fe model, imprint is only slightly deformed as can be seen in figure 6. on the other hand, during the experiment, indentation young´s modulus is determined at the beginning of the unloading [10] when the shape of the imprint can significantly deviate from the ideal elastic shape (figures 1 and 2). consequently, the contact area can be different which leads to mismatch in measured and modeled indentation young´s modulus. another reason causing the difference in the shape of the experimental and modeled imprint (and consequently in the indentation young´s modulus) is the shape of the indenter itself [15]. imperfect shape of the spherical indenter leads to inaccurate determination of actual contact area and it can have effect on the resulting values of mechanical properties. the shape of the residual imprints can be explained based on activated slip systems whose intersections with the surface can be seen near the residual imprint on the surface. the intersections of the active planes determined by fe model correspond well to these slip lines for both material orientations (figures 1 and 2). for orientation [100], the number of active slip system is two times higher than for orientation [110]. this amount is sufficient for the development of deformation in the preferred <111> out-of-plane directions and the shape of the residual imprint consequently deviates from the spherical shape. for orientation [110] there is not enough of slip systems which enables the deformation only in <111> out-of-plane directions. nevertheless, stresses in other slip systems are high enough to facilitate the deformation in other systems and directions, which leads to more spherical shape of the residual imprint. 5. conclusions mechanical properties and deformation behavior of fe3(wt.%)si single crystals under spherical indenter were investigated in this study. it was found that orientation [110] is stiffer and slightly harder than orientation [100]. pop-in behavior was observed for both orientations. based on the resolved shear stress computed by finite element method, it was found that most of the pop-ins is caused by activation of slip on pre-existing dislocations. four-fold symmetry of the 4 vol. 17/2018 deformation of fe3si single-crystals under nanoindentation figure 6. location of the maximum of resolved shear stress under the indenter for critical slip systems (only one quarter of slip systems is displayed because of the symmetry). residual imprint for the crystal orientation [100] and two-fold symmetry for orientation [110] was explained by the different activation of slip systems. description of deformation behavior of this anisotropic material with bcc structure can be helpful in the future research and data interpretation of more complex systems. acknowledgements this research was carried out in the frame of the projects 14-36566g and 17-07559s (czech science foundation). references [1] iso 14557, metallic materials instrumented indentation test for hardness and material parameters, 2002. 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author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague a revival in façades: textile reinforced concrete panels are light, safe and aesthetically pleasing matthias roika,∗, matthias tietzeb, alexander kahntc a halfen gmbh (part of leviat), liebigstr. 14, 40764 langenfeld, germany b technische universität dresden, dresden, germany c hochschule für technik, wirtschaft und kultur leipzig, leipzig, germany ∗ corresponding author: matthias.roik@halfen.de abstract. textile reinforced concrete was developed in recent years into a construction technique that has its benefits and advantages in applications where conventional types of reinforcement have their limits. the current, minimum slab thickness for steel-reinforced concrete façade slabs is 7.0 cm; this is due to the minimum, required concrete cover to ensure adequate corrosion protection. façade slab anchors for these slab thicknesses are building authority approved. as corrosion protection is not an issue for textile reinforced concrete, the minimum thickness for the concrete cover can be significantly reduced. the requirement for component thickness is now determined by the load-bearing capacity and by production-related boundary conditions. for practical building reasons, panel thicknesses of 3.0 cm have proven to be the best choice. compared to steel-reinforced façade panels, this is a weight and thickness reduction of almost 60%. thin concrete elements are of great interest in cases when the thickness or the weight of the panels is largely limited e.g. because of adjoining concrete elements in renovation or upgrade projects, retrofitting or improvements. compared to other building materials, concrete has characteristic advantages in building physics and fire protection properties, irrespective of the thickness. obviously, minimal thicknesses place extra demands on planning and construction. especially effects on concrete, punching, splitting and concrete breakout must be examined in experiments. this is an overview of calculation and test methods. results are provided to show the bearing behaviour of fixings in thin, textile reinforced concrete slabs. the design rules are explained and the results are illustrated. keywords: building innovation, carbon reinforced concrete façades, fixings, new materials, textile reinforced concrete. 1. introduction currently, more than half of the world’s population lives in cities. in 2050, this figure is expected to rise to two thirds. this will lead to a strong increase in the density of urban spaces, which in turn will lead to a high demand for façade constructions that are efficient in terms of raw materials and space. the general availability of raw materials, sufficient loadbearing capacity and durability, as well as the ease of production have made reinforced concrete the building material of the 20th and the beginning 21st century. in 2017, approximately 4.3 billion tonnes of cement, 28.6 billion tonnes of aggregates and 2.85 billion tonnes of water were used to produce concrete [1]. and this is the other side of the coin of the universal building material concrete: no other material is responsible for greater raw material extraction and higher co2 emissions. the huge quantities of concrete used worldwide are particularly problematic. if it were possible to achieve a leaner design through a new type of construction, considerable savings could be achieved carbon concrete offers this potential. reinforcing structures (grids) made of carbon fibers can already be used today to replace some steel reinforced concrete applications in new buildings, i.e. to achieve greater resource and energy efficiency [2]. in addition, it is possible to repair solid structures in need of renovation by means of reinforcing layers of carbon concrete, thereby increasing their service life. for architects and designers, carbon concrete and other types of textile-reinforced concrete offer great design freedom, as they can be used to produce any shape, format, surface structure and colour in high quality. furthermore, additional functions such as heating, lighting or building automation can be integrated into the components. due to the arrangement of the reinforcement according to the force distribution, components made of textile concrete also require less reinforcement than conventional, fibrereinforced concrete, in which cut short fibres made of glass, plastic or carbon fibre are introduced into the concrete mix in a non-directional manner. in germany, experiments with reinforcement made of technical textiles were already being carried out in the 1980s. the first joint research projects in dresden, chemnitz and aachen followed in the mid-1990s. they formed the basis for two dfg-funded collab497 https://doi.org/10.14311/app.2022.33.0497 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en m. roik, m. tietze, a. kahnt acta polytechnica ctu proceedings figure 1. principle cross-section curtain wall façade. orative research centres in dresden [3] and aachen [4], in which basic research in the field of non-metallic reinforcements was advanced between 1999 and 2011. in what is currently the largest german construction research project c3 carbon concrete composite more than 150 partners from science and industry are working on the development and market launch of carbon concrete. carbon concrete will change the construction world, the technology is already there and is gradually being used in more and more areas of construction due to its advantages. the first applications have been realized in the field of façades, in addition to the repair of reinforced concrete structures. through the further development of the components concrete matrix and carbon reinforcement of the composite material, further fields of application can now be opened up. carbon concrete offers the possibility to rethink old approaches in the building industry. 2. carbon reinforced concrete in reinforced concrete, the concrete primarily absorbs the compressive forces occurring in the component and the reinforcing steel compensates for tensile stresses. in addition, the concrete cover protects the steel against corrosion, corrosion-promoting media (e.g. de-icing salts) and fire. depending on the paving situation, the concrete cover course must be up to 5 cm thick. in the case of carbon concrete, car� figure 2. carbon reinf. concrete façade white concrete curtain wall. bon, as the equivalent of steel reinforcement, ensures the tensile strength of the composite material. the principles and design bases are very similar to those of reinforced concrete. since carbon is about six times more load-bearing (3000 instead of 500 n/mm2 tensile strength) and four times lighter (1.8 instead of 7.8 g/cm3) than steel and also does not corrode, this reinforcement allows a much freer design. since the corrosion protection for the reinforcement is no longer required, concrete slabs for façade cladding, for example, can be produced with a thickness of 2 cm instead of the previous minimum of 7 to 10 cm [5]. even thinner layers are sufficient for reinforcing existing reinforced concrete components. currently, filigree mat-like structures made of one or more layers of multifilament yarns (rovings) are used for reinforcement. rovings in turn consist of several thousand individual filaments (fibres). since the fabrics are relatively soft, they can be used to produce almost any shape. however, for entire structures, rod-shaped reinforcements will be required in the future. various research institutes and industrial companies in germany are currently working on the development of suitable carbon rods. in addition to carbon, fibers made of ar glass are becoming increasingly important for multifunctional applications. these are somewhat less load-bearing (1500 n/mm2) and durable than carbon fibers, but are significantly less expensive. furthermore, unlike carbon, they do not conduct electricity, but are good light guides. up to now, normalstrength or high-strength fine concrete with a grain size of 1 to 8 mm was used as concrete depending on the component and application. this is also a difference to steel reinforced concrete, where grains up to 32 mm in diameter are common. this concrete matrix is particularly suitable for thin building components such as façade panels [6]. 3. carbon concrete façades for façade construction, carbon-reinforced concrete elements offer the advantage that they have very uniform, fine-pored and sharp-edged surfaces and can be produced much slimmer and lighter than steel re498 vol. 33/2022 textile reinforced concrete panels figure 3. storey-high façade element with new fixing system for carbon reinforced concrete. inforced concrete elements. the reduction in weight leads to greater economic efficiency in production and transport and reduces environmental pollution. the elements are mainly used for curtain wall rear ventilated façades (figure 1). furthermore, the panels are suitable for the production of particularly slim sandwich façades. the first office and commercial building with a curtain-wall ventilated façade made of carbon concrete was completed in neumarkt in 2016. the 2.88 * 4.48 m large façade elements here are only 30 mm thick. a reinforced concrete facing layer would have been around three times as thick at this point. the white concrete façade elements on a modular single-family house in leipzig are also only 30 mm thick and reinforced with a carbon fiber fabric (fig. 2). the panels are up to 0.75 × 3.00 m in size and are held in place by a clasp system. an office building in essen shows the possibilities of three-dimensional surface design. here, too, a curtain-wall, rear-ventilated façade was realized, whereby the concrete slabs are anthracitecolored throughout and provided with a binary code with the help of a formwork matrix. a fair-faced concrete façade element only 11 cm thick with passive house insulation standard (0.15 w/m2k) was developed within the framework of the research project "vakutex vacuum-insulated façade elements made of textile concrete" in leipzig [7]. the non-load-bearing sandwich element has similar physical properties as a four times thicker reinforced concrete element. this results in floor space gains of up to 15 % for the same building cubature. for the implementation of the first projects in the façade area, approvals were required in individual cases. in order to make this easier, general building authority approvals were obtained, which allow the approved component to be used throughout germany. façade elements have also been used in non-european countries. the pylons of the new borfigure 4. construction of the façade in dormettingen (ger). porus bridge yavuz-sultan-selim were clad with textile and carbon concrete façade elements at a height of 326m. for the further practical suitability of the technology, a façade anchor system was developed which is particularly suitable for large-format thin (min. d = 3 cm) concrete elements. with this system, concrete façade panels and ventilated façades made of textile and carbon reinforced concrete can be easily installed and adjusted. this technology has already been implemented in a practical project, the schiefererlebnis in dormettingen (germany). (figure 3 and 4) this shows that carbon-reinforced concrete technology has arrived in practice. 4. fixtures in concrete after the bearing construction is erected, the façade panels will be fixed. inserts are necessary for handling and support to provide a safe connection between the bearing construction and the panels. the general approach to calculate anchors in concrete is the cc (concrete capacity) method [8, 9]. the mechanisms of load-transfer from a steel anchor into the concrete are either mechanical interlock, friction or bond. anchors can be cast-in-place or post installed. usually, cast-in anchors are fixed by mechanical interlock whereas post installed anchors, such as dowels placed in a drilled hole introduce a load by friction or bond. raising the load and assuming the anchor material is strong enough to carry the tensile load, the fixing fails because the tensile capacity of the concrete is exceeded. this failure mode has a shape of a cone with a slope of approximately 35◦ from the horizontal. the concrete cone failure load is proportional to h1,5 ef , where hef is the effective anchorage depth, the depth where the anchor introduces the load into the concrete. the concrete capacity method is based on calculation of this failure cone. the cone capacity can be influenced by geometry of the concrete member, by reinforcement or by additional cracks. 499 m. roik, m. tietze, a. kahnt acta polytechnica ctu proceedings figure 5. stress distribution in uncracked concrete. figure 6. stress distribution in cracked concrete due to splitting. 5. experimental approach tests have been carried out to investigate the support behaviour of fixings in thin concrete slabs: calculations according to the cc-method set the minimum depth of anchorage at hef ≥ 25 mm. this minimum anchor depth is not feasible for applications with carbon reinforced concrete and the calculation method should be reviewed for the use case of thin slabs. tests were carried out to extend the effective range of the anchors for short depths. these tests were carried out on thin concrete panels, with and without reinforcement. 5.0.1. punctual fixings additionally to [10], tests with headed bolts installed in 30 mm thick concrete plates were carried out to investigate the bearing behaviour of anchors installed in thin plates. bolts were installed at the maximum feasible anchorage depth and the shaft diameters and different head sizes varied. all bolts were tested in tension. first results were published in [11], additional test series followed. unexpectedly large scatterings of the test results were observed in all test series. these results were not sufficiently reliable due to a coefficient of variation well in excess of 20 %. despite the observed tendency for the resistance of concrete fracture bodies to depend directly on the lateral area, it was not possible to derive a practically applicable mathematical rule for this dependence. the cause of these large scatterings can be attributed to the fracture mechanism itself. the order of crack initiation was assessed, and it was found that the initial surface cracks occurred during the test. the final failure of figure 7. supporting mechanism of a head bolt anchorage due to shear load (eligehausen, mallée & silva 2006) figure 8. load capacity depending on the relation between axial / lateral load, (dafstb 2008). the anchor is almost always due to splitting rather than the expected failure of the concrete. the failure mechanism of splitting is shown in figure 6. the compressive load introduced into the concrete by the anchor head results in tensile stresses near the surface of the concrete member, as shown in figure 5. if initial cracking occurs near the anchor due to bending stress in the concrete, further splitting stresses will open the cracks and reduce their resistance. therefore, fixtures which are sensitive to splitting failures will have a lower resistance value and a higher variability of test results. if a fixture is sensitive to splitting, a corresponding coefficient should be introduced [12]: ψh, sp = ! h 2 · hef "2/3 ≤ 1.0 (1) this factor reduces the basic resistance nrk,c in case when h < 2 hef . when additional shear forces are applied to the concrete, the tensile load component of the headed anchor increases (see figure 7). this additional load component can lead to so-called prying failure, which also leads to a reduction in the overall tensile resistance. this effect is investigated in figure 8 [10]. loads were applied to the anchors at large distances and at different angles towards the edge of the panel. the resulting crack diagrams in the report show additional 500 vol. 33/2022 textile reinforced concrete panels figure 9. tests with additional reinforcement. the resistance of the unreinforced concrete forms the reference value. it can be seen from the pictures that the load-bearing capacity is significantly influenced by the crack development. the inserted carbon meshes prevented cracks and a concrete breakout cone could form. figure 10. fixing system for façade panels. splitting and fracture of the concrete cone, in addition to the overlapping of the cracks due to prying failure. this may lead to a further increase in scattering (see figure 8). finally, it can be said that timed fixings are not very suitable for applying large loads to thin fibrereinforced panels, especially when the loads contain a large shear component or when h < 2 − hef . depending on the size of the panel, the resistance of the fixings may not be sufficient to support the façade panel. 6. methods to reduce the influence of splitting to increase the load-bearing capacity of an anchorage and to reduce the scattering of the test results, various measures can be taken. two measures are described in more detail below, on the one hand the use of reinforcement and on the other hand the distribution of the load to be introduced over several figure 11. test setup for shear/tension. load-bearing components. 6.1. use of additional reinforcement additional reinforcement can be arranged in several ways. in addition, different types of reinforcement were considered in tests, namely bar-shaped reinforcement and reinforcement mats. in the case of barshaped reinforcement, it must have its own corrosion protection due to the low concrete cover, i.e. it should consist of e.g. stainless steel or carbon. the same reinforcement was used as mat-shaped additional reinforcement, which is also used as slab reinforcement. the results of the tests with additional reinforcement are shown in figure 9 6.2. multi directional fixings special anchoring systems with suitable insert elements are required to fix concrete facades. to prevent eccentricity of the supporting structure, it is advantageous that the insert can be loaded in different directions. it is convenient to equip this type of insert element with several components to introduce loads independently into the concrete. an example of this is shown in figure 10. headed anchors carry the loads 501 m. roik, m. tietze, a. kahnt acta polytechnica ctu proceedings figure 12. test results vs. calculation results of multidirectional fixing. perpendicular to the surface of the facade, i.e. tensile loads, while reinforcement bars, for example, support the shear direction. these inserts were subjected to tensile tests at different angles. the test set-up is shown in figure 11. the concrete panels were fixed to a special test rig and loaded at different angles. the insert system was designed to accommodate loading angles of between 25◦ and 45◦. this range was used for the tests. it was found that even when relatively large loads were introduced into the concrete, the loads did not depend significantly on the loading angle. the final fractures were of sufficient size to achieve a noticeably higher resistance than the time-fixed ones; for values below 10%, the scatter was well within acceptable limits. these findings demonstrate that multidirectional fixings have significantly better loadbearing properties than the fixings. these inserts were subjected to tensile tests at different angles. the test set-up is shown in figure 11. the concrete panels were fixed to a special test rig and loaded at different angles. the insert system was designed to accommodate loading angles of between 25◦ to 45◦. this range was used for the tests. it was found that even when relatively large loads were introduced into the concrete, the loads did not depend significantly on the loading angle. the final fractures were of sufficient size to achieve a noticeably higher resistance than the time-fixed ones; for values below 10%, the scatter was well within acceptable limits. these findings demonstrate that multidirectional fixings have significantly better loadbearing properties than the fixings. the results of the test are shown in figure 12. to describe this interaction, a computational model has been developed according to [8, 9]. the test results are presented together with the relevant safety levels according to [13, 14]; the difference between the safety levels in [13, 14] is based on the difference in confidence levels (the confidence factor in [13] is 75% and in [14] it is 90%). in the case of reinforced concrete structures, [13] is the restraining force and in the case of fixed structures, [14] is the restraining force. tests have shown sufficient safety and a good correlation with the calculated values. the inserts have been specially developed for thin carbon-reinforced concrete panels and have been approved by the general building authority. for the horizontal load component due to wind loads, a spacer such as a compression element is usually installed. this compression element can take the horizontal load without affecting the load-bearing equipment. the thinner plates are more sensitive to wind loads, as they carry less dead load than thicker concrete façade panels. depending on the shape of the slab, the weight of the slab and the expected wind loads, horizontal anchors (e.g. compression bolts or restraint anchors) may need to be additionally fixed for wind suction. in stacked, suspended façade slabs, the lower spacer bolts are replaced by dowels for aligning the elements. these elements are also included in the general building approval. 7. numerical investigations numerical investigations of the effects observed in the experiment are in progress. based on the undisturbed concrete failure cone, i.e. concrete breakout without edge and thickness influence, the resulting resistances are currently to be determined, which occur if the influencing factors are guided to the limits of applicability. 502 vol. 33/2022 textile reinforced concrete panels as can already be seen from the test results, the influence of the splitting coefficient ψh,sp, for example, is considerably greater than stated in current publications. the influence of the reinforcement is currently being investigated. 8. summary / outlook the construction of carbon reinforced concrete can be described as a mega-trend, as this technology will be used in many areas of the construction industry. façade construction in particular has been one of the first areas of application to show the advantages of carbon reinforced concrete. above all, the savings of resources is an essential aspect which, in addition to the many technically advantageous properties, has benefits for society as a whole. of course, not all design methods known from reinforced concrete can be transferred to carbon concrete. therefore, new design methods are being developed at the universities in dresden and aachen (germany). in addition, the current technical specifications for fastening techniques developed for conventional steel-reinforced structures must be adapted to the specific boundary conditions of carbon concrete construction. for this purpose it is necessary to adapt the relevant parameters to the specific properties of the fabrics (reinforcement), which is usually done by tests. although the effective anchorage depth and the edge distance of an anchor according to the formula should have the greatest influence on the load bearing behaviour, the influence of cracking of the concrete dominates. in further investigations by means of tests and numerical analysis further special features of carbon reinforced concrete are to be analysed, such as the influence of the special bond behaviour or the reduced shear resistance compared to steel reinforcement, in order to derive efficient systems for practical use. acknowledgements we would like to thank the german federal ministry of education and research for its support of carbon concrete research within the framework of the twenty20 program, and our corporate partners for their successful cooperation. references [1] mineral commodity summaries 2019, 2019. https://doi.org/10.3133/70202434. [2] a. kahnt, s. kirmse, k. holschemacher. global challenges for the construction industry. investicii, stroitel’stvo i nedvizhimost kak materialnyi bazis modernizacyi i innovacionnovo razvitya ekonomiki, p. 55-59, 2014. [3] sfb 528, http://www.sfb528.tu-dresden.de. [4] sfb 532, http://www.sfb532.rwth-aachen.de. [5] scheerer, s.; schladitz, f.; curbach, m.: textile reinforced concrete from the idea to a high performance material. in: brameshuber, w. (hg.): proceedings of the ferro-11 and 3rd iccarbon reinforced concrete (pro 98), aachen, bagneux, 15-33 2015,. [6] f. schladitz, m. tietze, m. lieboldt, et al. carbon and concrete the future of construction? iabse symposium report, iabse conference guangzhou 2016: bridges and structures sustainability-seeking intelligent solutions, pp. 733-743, 2016. https://doi.org/10.2749/222137816819259167. [7] s. kirmse, a. kahnt, r. kraft, et al. energy façades made of textile reinforced concrete. cpi concrete plant international. issue (3):158-171, 2017. [8] cen tc250/sc2/wg2. design of fastenings for use in concrete, prcen/ts 1992-4-1:2009, 2009. [9] fib. fédération international du béton. design of fastenings in concrete guide to good practice parts 1 to 5, lausanne, 2011. [10] dafstb. concentrated load application into thin-walled components made of textile reinforced concrete, dresden, 2008. [11] r. matthias. new equipment for new technologie: fixing systems for thin, textile reinforced concrete facades, icsa 2016, guimaraes, portugal, 2016. [12] r. eligehausen, r. mallée, j. f. silva. anchorage in concrete construction, ernst und sohn, 2006. [13] din en 1990:2010-12 eurocode: basis of structural design; german version en 1990:2002 + a1:2005 + a1:2005/ac:2010, 2010. [14] etag 001. eota: metal anchors for use in concrete, 2013. https: //www.eota.eu/sites/default/files/uploads/etag s/etag-001-annex-a-amended-2013-04-08.pdf. 503 https://doi.org/10.3133/70202434 http://www.sfb528.tu-dresden.de http://www.sfb532.rwth-aachen.de https://doi.org/10.2749/222137816819259167 https://www.eota.eu/sites/default/files/uploads/etags/etag-001-annex-a-amended-2013-04-08.pdf acta polytechnica doi:10.14311/app.2020.27.0149 acta polytechnica 27(0):149–154, 2020 © czech technical university in prague, 2020 available online at http://ojs.cvut.cz/ojs/index.php/app mechanical properties of forged tungsten heavy alloys ondřej kováříka,∗, jaroslav čecha, jiří čapeka, michal hajíčekb, jakub klečkaa,c, jan siegla a czech technical university in prague, faculty of nuclear sciences and physical engineering, department of materials, trojanova 13, praha 2, czech republic b ujp praha a.s., nad kamínkou 1345, praha-zbraslav, czech republic c czech academy of sciences, institute of plasma physics, za slovankou 1782/3, praha-libeň, czech republic ∗ corresponding author: ondrej.kovarik@fjfi.cvut.cz abstract. tungsten heavy alloys are composite materials containing spherical tungsten particles embedded in binder matrix. their excellent mechanical properties can be further improved by rotary forging. this paper aims to gain deeper understanding of the forging process by investigating the local elastic modulus, hardness, and residual stress of individual phases in w6ni3co pseudo-alloy. the resulting global properties of the composite material such as stress-strain behavior, fracture toughness and fatigue crack growth rate behavior are also studied. the results show that sintered and quenched material consists of highly textured matrix containing nearly perfect single crystal spheres of pure w. the rotary forging leads to significant lattice deformations destroying the texture and significantly increasing the hardness of both wnico matrix and w particles and making residual stresses in w particles anisotropic with increased compression along the longitudinal axis of the forged part. keywords: instrumented indentation, residual strain, stress-strain, tungsten heavy alloy, w6ni3co. 1. introduction tungsten heavy alloys (wha) are two phase materials consisting of tungsten particles embedded in binder matrix typically consisting of w-ni-cu, w-ni-fe, wni-co or w-ni-fe-co. these materials feature good wear resistance and high strength, density, thermal conductivity, corrosion resistance and radiation absorption. these properties determine the use of whas for counterweights, kinetic penetrators, electrical contacts, radiation shielding etc. the high temperature application of wha’s is limited by the melting point of ni rich binder which lies above 1400 °c. despite this limitation, wha’s are considered as candidate materials for first wall of future fusion devices [1, 2]. the mechanical properties of wha’s can be compared to pure tungsten. however the fracture toughness of wha’s at room temperature can be approximately one order of magnitude higher than pure w [3]. moreover the mechanical properties of wha’s can be further improved by thermomechanical treatment such as rotary forging [4, 5]. this paper summarizes global or macroscopic engineering properties of the w6ni3co tungsten heavy alloy in as sintered and quenched (sq) and forged and annealed (fa) states. the observed changes are related to the local changes of the residual stress distribution, microhardness and elastic modulus of individual phases of the wha, i.e. of the ni rich matrix and w particles. 2. experimental the w6ni3co alloy was prepared from powder mixture by cold isostatic pressing, sintering at 1530 °c, annealing and quenching. the sintered and quenched specimens were designated sq and had cylindrical shape with diameter of 11.5 mm and length of 120 mm. additional rotary forging and subsequent annealing was applied to specimens designated fa (final diameter of 9 mm and length ≈ 160 mm). small rectangular bar specimens of dimension 4×3×32mm were cut using electric discharge machining and fine ground to final shape. this type of bar specimen was used for all performed measurements. stress-strain behavior was measured by the method of herbert [6, 7]. the used four point bending fixture with outer span of 27 mm and inner span of 13.5 mm is in figure 1. the measurement evaluates the stress strain curve from the records of loading force and strains on compression and tensile surfaces of the specimen (see strain gages sc and st in figure 1) and is limited by the 5% strain. in order to avoid tedious strain gaging work, the strains were measured using digital image correlation (dic) with help of the ncorr software [8]. the stress-strain curves in uniaxial tension were also measured for the investigated materials and evaluated according to standard iso 6892-1 [9]. fracture toughness was measured using the jintegral approach according to astm standard e1820 [10]. the j-integral characterizes the energy release by rate of the growing crack, i.e. the amount of elastic energy released by crack grown by unit area. 149 http://dx.doi.org/10.14311/app.2020.27.0149 http://ojs.cvut.cz/ojs/index.php/app o. kovářík, j. čech, j. čapek et al. acta polytechnica figure 1. the schematic drawing of the three point bending setup used for fracture toughness measurement and four point bending setup used for stress-strain curve measurement. the loadline displacement y and compressive strain sc at the top face and tensile strain st at the bottom face of the specimens are marked. temper sample et ec rp0.2,t rp0.2,c rm,t (gpa) (gpa) (mpa) (mpa) (mpa) sq ad1 307 285 685 690 ≥ 960 sq ad13 304 258 683 701 ≥ 973 fa ae1 313 393 1617 1546 ≥ 1701 fa ae13 329 357 1574 1505 ≥ 1673 table 1. mechanical properties of individual specimens obtained by four point bending. subscripts c and t denote tension and compression respectively. figure 2. r-curves of sintered and quenched (sq) and forged and annealed specimens (fa). the construction lines intersecting the mesured curve at jic are marked. the crack length is measured by dic or from unloading compliance. temper rp0.2,u rm,u a et rp0.2,t rp0.2,c kic ∆kth (mpa) (mpa) (%) (gpa) (mpa) (mpa) (mpa √ m) (mpa √ m) sq 630 1005 26.7 306 683 695 172 28 fa 1630 1627 7.3 321 1569 1525 65 50 table 2. average values of global properties. mechanical properties obtained by the uniaxial test are denoted by subscript u, subscript t and c denote properties in tension and compression obtained from four point bending test. 150 vol. 27/2020 mechanical properties of forged tungsten heavy alloys temper phase hit,l hit,g eit,l eit,g εl εrot (gpa) (gpa) (gpa) (gpa) (10−3) (10−3) sq w 7.5± 0.5 4.3± 0.15 412± 19 299± 20 −1.0± 0.3 −1.0± 0.1 ni 6.8± 0.3 296± 14 −2.1± 0.3∗ fa w 8.9± 0.5 6.8± 0.25 404± 17 275± 24 −1.3± 0.2 −0.9± 0.2 ni 7.8± 0.4 311± 37 2.1± 0.9 0.6± 0.2 table 3. local properties of w particles and ni matrix of two investigated material tempers: instrumented nanohardness hit , indentation modulus eit , longitudinal and isotropic residual strains εl, εrot in w particles and ni rich matrix. * indicates possible measurement artifact. the specimen with fatigue precrack of nominal length 2 mm was loaded in three point bending (see figure 1). the loadline displacement y was measured using an lvdt sensor, crack opening displacement (cod) was evaluated from dic. the crack length was measured using dic by the method described in [11] and also using unloading compliance. the fatigue crack growth rate was measured in resonance bending using the method described in [12]. the nominal stress ratio was r = −1.the crack initiated from a straight notch machined using a slow speed diamond saw. the nominal notch length was 0.5 mm, its width was about 0.15 mm. the test was performed in so called rate controlled mode. in this mode, the crack growth rate is controlled by a closed loop technique and the setpoint is prescribed by an exponential function da dn = da dn ∣∣ 0 e c(a−a0). this approach eliminates possible crack arrests that can experienced using standard load shedding procedures, at the same time, the duration of the experiment is known in advance. the fatigue crack growth rate experiment consisted of two parts. in the load shedding part, the crack growth rate was decreased from da dn = 10−9 m/cycle to da dn = 10−12 m/cycle at a crack length of a = 1.1 mm. in the following load increase part, the crack growth rate was increased up to da dn = 10−6 m/cycle at a = 2.4 mm. nanoindentation was carried out on nht tester (anton paar, graz, austria) using berkovich indenter. indentations with partial unloadings to maximum load of 0.5 n were performed in order to study the evolution of mechanical properties with increasing penetration depth hm. the acquired data (force-penetration depth dependency) were evaluated to obtain hardness hit and young´s modulus eit according to the iso 14577 [13] standard by the oliver-pharr method [14]. poisson ratio ν = 0.3 was assumed during elastic modulus computation. the x’pert pro mpd diffractometer (panalytical, malvern, united kingdom) was used to measure lattice deformations of {220} and {311} planes of the w and ni phase, respectively, using the manganese radiation. to determine residual strain, the sin2 ψ method was used. the irradiated area was specified by pinhole of the size of 2x1 mm2. the residual strains were measured in longitudinal direction, however, such measurement was unsuccessful in the matrix due to its heavy texture. therefore the measurements were also performed with sample rotating around the axis perpendicular to the measured surface in order to characterize the ’hydrostatic’ strain. such measurement is, however, unsuitable for the measurement of other than ’hydrostatic’ types of strain. microstructure of metalographically polished samples was observed in electron channeling contrast mode using high resolution scanning electron microscope jeol jsm-it500 hr (jeol, tokyo, japan). the final polishing was performed in colloidal silica using the vibromet vibratory polisher (buehler, lake bluff, il, usa). 3. results 3.1. global properties the results of stress-strain measurements are presented in figure 3. the mechanical properties obtained from these curves are summarized in table 1 and average values are in table 2. the rotary forging increased the yield strength rp0.2 nearly 2.6 times. this increase of yield strength was accompanied by nearly fourfold reduction of ductility a. figure 3. stress-strain curves in tension and compression obtained from four point bending. the r-curves describing the j-integral j as a function of crack extension ∆a are shown in figure 2. two 151 o. kovářík, j. čech, j. čapek et al. acta polytechnica figure 4. crack growth rates da dn vs. the maximum stress intensity factor kmax for sintered and quenched (sq) and forged and annealed (fa) specimens. specimens of each type were investigated. one specimen was tested without unloading with crack length measured only by dic. the second specimen was tested with periodical unloading and crack length was estimated from both unloading slope and also by dic. the r-curves show rising behavior up to the jic value marked by the construction line in figure 2. for higher crack extensions the r curve is relatively flat indicating the presence of brittle fracture mechanism. the resulting values of fracture toughness are summarized in table 2. the increased yield strength of rotary forged specimens was connected with nearly reciprocal decrease of fracture toughness kic . it should be noted, that the measured fracture toughness is significantly higher than that for pure tungsten which is ≈ 5 mpa √ m at room temperature [15]. the fatigue crack growth rate da dn as a function of the maximum stress intensity factor kmax is plotted in figure 4. the threshold values ∆kth defined at crack growth rate da dn = 10−10 m/cycle are presented in table 2. 3.2. local properties in order to explain the effect of rotary forging on global properties of the material, the local role of the w particles and ni-rich matrix was studied by instrumented indentation and x-ray diffraction measurement. the results of instrumented indentation are included in figures 5 and 6. the indents were placed in centers of w particles and in the centers of ni matrix regions. despite the relatively low applied loads and high stiffness of the material, the volume characterized by the indentation may reach out of the phase of interest at higher load. at low loads, however the measured hardness was influenced by the indentation size figure 5. hardness of w particles and ni matrix determined by instrumented indentation by berkovich indentor for different penetration depths hm. figure 6. elastic modulus of w particles and ni matrix determined by instrumented indentation by berkovich indentor for different penetration depths hm. effect [16]. the optimal load for characterization of local properties of each phase should be high enough to eliminate the indentation size effect on one hand. on the other hand, the load must be low enough to prevent the probed volume to reach out of the phase of interest. analyzing figures 5 and 6, the indentation depth of about 250 nm appeared as suitable compromise and thus local individual phase properties hit,l, and eit,l were estimated at this indentation depth. at maximum indentation depth, the measured values characterize the global macroscopic properties of the material hit,g and eit,g. the representative values of both local and global indentation properties are summarized in table 3. the local hardness hit,l of w particles is about 10% higher than the hardness of the ni matrix, both values increase by ≈ 20% after rotary forging. even higher increase of about ≈ 60% was observed for the global hardness. this indicates the 152 vol. 27/2020 mechanical properties of forged tungsten heavy alloys figure 7. the influence of rotary forging on microstructure of w particles and ni matrix observed using sem in backscattered electron channeling contrast mode. synergistic effect of w particle and ni matrix hardness increase probably originating from the w-w and w-ni interfaces. elastic modulus, on the other hand, shows no significant relation to the forging process neither at local or global scale. it was shown, that rotary forging influences w particles, ni matrix and probably also well as the w-w or w-matrix interfaces. the results of residual strain measurement by x-ray diffraction are summarized in table 3. longitudinal strains εl as well as isotropic strains εrot obtained by rotating the specimen are included. compressive strain is always indicated in w particles. for sq specimens, the values of isotropic and longitudinal strains of w particles coincide, indicating ’hydrostatic’ strain state. at the same time, the longitudinal strain in sq ni matrix could not be measured due to heavy texture. under heavy texture, the performed measurement of isotropic strain results in a strain component of unknown orientation and thus should not be evaluated. after forging and annealing the compressive strains in w appear to get oriented as the isotropic strain εrot is different from the longitudinal strain εl. similar situation, i.e. oriented tension strain can be observed in ni matrix of forged specimens. the above mentioned orientation of residual strains invokes higher longitudinal strain mismatch with increased compression of w particles by ≈ 0.6× 10−3 and corresponding increase in matrix tension by ≈ 1.5 × 10−3 when comparing longitudinal strains εl and isotropic averaged strains εrot. this increased stress mismatch can protect the inherently brittle w particles by imposing more compression on them up to the point of matrix yielding. this protection will be active at low loads, and together with the elimination of texture may cause the observed increase of fatigue performance of fa specimens (see figure 4). on the other hand, at high load, the matrix yields and the protection is decreased which can reduce fracture toughness (see figure 2 and table 2). the texture changes observed by x-ray diffraction indicated that individual w grains had near perfect single crystal structures (however each particle was oriented differently) is sq state. this structure was then heavily deformed during rotary forging. similar effect was indicated in the ni matrix, where the whole volume of the matrix had coherent crystal structure, that was later destroyed by forging. the described changes can clearly be observed on micrograph in figure 7. 4. conclusion 1. stress-strain behavior, fracture toughness and fatigue crack growth rates of w6ni3co wha in sintered and quenched (sq) and forged and annealed (fa) state were measured. the following effects of rotary forging were observed: . significant increase of hardness, yield strength, tensile strength and fatigue performance. . significant decrease of ductility and fracture toughness 2. hardness, elastic modulus and residual stress of w particles and ni rich matrix were measured. the following effects of rotary forging were observed: . the increase of hardness of w particles and ni matrix by ≈ 20% results in macroscopic hardness increase of ≈ 60%. . increasing compressive strain in w particles in the longitudinal direction vas observed the increased compressive strain in w may explain the increase of fatigue behavior at relatively low loads, whereas the hardening of individual phases results in decreased ductility and resulting decrease of fracture toughness. 153 o. kovářík, j. čech, j. čapek et al. acta polytechnica list of symbols cod crack opening displacement dic digital image correlation rp0.2 yield strength [mpa] rm ultimate tensile strength [mpa] kic fracture toughness [ mpa √ m ] jic critical value of j-integral [kj/m2] jcod j-integral from cod [kj/m2] hit indentation hardness [gpa] eit indentation modulus [gpa] ε strain [10−3] acknowledgements financial support by czech science foundation (project no. 19-14339s), ministry of industry and trade (project no. fv10165) as well as the grant agency of the czech technical university in prague (project no. sgs18/190/ohk4/3t/14) is gratefully acknowledged. references [1] r. neu, h. maier, m. balden, et al. investigations on tungsten heavy alloys for use as plasma facing material. fusion engineering and design 2017. doi:10.1016/j.fusengdes.2017.01.043. [2] r. neu, h. maier, m. balden, et al. results on the use of tungsten heavy alloys in the divertor of asdex upgrade. journal of nuclear materials 2018. doi:10.1016/j.jnucmat.2018.05.066. [3] d. rittel, g. weisbrod. dynamic fracture of tungsten base heavy alloys. international journal of fracture 112(1):87–98, 2001. doi:10.1023/a:1013581609836. [4] r. kocich, l. kunčická, d. dohnalík, et al. cold rotary swaging of a tungsten heavy alloy: numerical and experimental investigations. international journal of refractory metals and hard materials 61:264–272, 2016. doi:10.1016/j.ijrmhm.2016.10.005. [5] l. kunčická, r. kocich, c. hervoches, a. macháčková. study of structure and residual stresses in cold rotary swaged tungsten heavy alloy. materials science and engineering: a 704:25–31, 2017. doi:10.1016/j.msea.2017.07.096. [6] h. herbert. ueber den zusammenhang der biegungselastizität des gußeisens mit seiner zugund druckelastizität. forschungsarbeiten auf dem gebiete des ingenieurwesens pp. 39–81, 1910. doi:10.1007/978-3-662-02218-4_2. [7] r. a. mayville, i. finnie. uniaxial stress-strain curves from a bending test. experimental mechanics 1982. doi:10.1007/bf02326357. [8] j. blaber, b. adair, a. antoniou. ncorr: open-source 2d digital image correlation matlab software. experimental mechanics 55(6):1105–1122, 2015. doi:10.1007/s11340-015-0009-1. [9] iso. iso6892-1: metallic materials. tensile testing. method of test at room temperature. iso 2016. [10] astm. e1820: standard test method for measurement of fracture toughness. astm 2015. doi:10.1520/e1820-15a. [11] o. kovářík, p. haušild, j. medřický, et al. fatigue crack growth in bodies with thermally sprayed coating. journal of thermal spray technology 25(1-2):311–320, 2016. doi:10.1007/s11666-015-0329-9. [12] o. kovarik, a. materna, j. siegl, et al. fatigue crack growth in plasma-sprayed refractory materials. journal of thermal spray technology 28(1-2):87–97, 2019. doi:10.1007/s11666-018-0790-3. [13] iso. iso14577: metallic materials instrumented indentation test for hardness and material parameters. iso 2015. [14] w. oliver, g. pharr. an improved technique for determining hardness and elastic modulus using load and displacement sensing indentation experiments. journal of materials research 7(6):1564–1583, 1992. doi:10.1557/jmr.1992.1564. [15] b. gludovatz, s. wurster, a. hoffmann, r. pippan. fracture toughness of polycrystalline tungsten alloys. international journal of refractory metals and hard materials 28(6):674–678, 2010. doi:10.1016/j.ijrmhm.2010.04.007. [16] w. d. nix, h. gao. indentation size effects in crystalline materials: a law for strain gradient plasticity. journal of the mechanics and physics of solids 46(3):411– 425, 1998. doi:10.1016/s0022-5096(97)00086-0. 154 https://doi.org/10.1016/j.fusengdes.2017.01.043 https://doi.org/10.1016/j.jnucmat.2018.05.066 https://doi.org/10.1023/a:1013581609836 https://doi.org/10.1016/j.ijrmhm.2016.10.005 https://doi.org/10.1016/j.msea.2017.07.096 https://doi.org/10.1007/978-3-662-02218-4_2 https://doi.org/10.1007/bf02326357 https://doi.org/10.1007/s11340-015-0009-1 https://doi.org/10.1520/e1820-15a https://doi.org/10.1007/s11666-015-0329-9 https://doi.org/10.1007/s11666-018-0790-3 https://doi.org/10.1557/jmr.1992.1564 https://doi.org/10.1016/j.ijrmhm.2010.04.007 https://doi.org/10.1016/s0022-5096(97)00086-0 acta polytechnica 27(0):149–154, 2020 1 introduction 2 experimental 3 results 3.1 global properties 3.2 local properties 4 conclusion list of symbols acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2020.26.0050 acta polytechnica ctu proceedings 26:50–55, 2020 © czech technical university in prague, 2020 available online at https://ojs.cvut.cz/ojs/index.php/app diagnostics of unfired earth with methylene blue test tereza plaček otcovská∗, barbora mužíková, pavel padevět czech technical university in prague, faculty of civil engineering, department of mechanics, thákurova 7, 166 29 prague 6, czech republic ∗ corresponding author: tereza.otcovska@fsv.cvut.cz abstract. unfired earth is important historic building material. final properties of unfired earth constructions are influenced by amount and kind of clay in earth mixtures. methylene blue test is a method that helps determining the type and amount of clay in earth mixtures. this paper describes research focused on using methylene blue test for analysis of methylene blue dye absorption by montmorillonite clay. the first part of the paper describes principle of methylene blue test and course of experimental measurement. the second part deals with analysis of obtained results. on the basis of experimental measurement it can be stated that methylene blue test is influenced by the stirring time of earth samples with methylene blue solution and dependence between amount of methylene blue dye absorption and amount of montmorillonite clay in earth samples is linear. keywords: clay, montmorillonite, methylene blue test, methylene blue dye, absorption, methylene blue solution. 1. introduction unfired earth is important historic building material and nowadays unfired earth obtained its importance too. there are a lot of historic and modern buildings from unfired earth, e.g. the great wall of china or berlin’s rammed-earth reconciliation chapel. positive properties of unfired earth are especially that fit into principles of a sustainable building and improving of micro-climate in interiors. there is increasing interest in this building material. therefore companies include unfired earth in their product range [1–5]. unfired earth is created from earth mixtures and earth mixtures are created from three basic components: sand, clay, mixing water. final properties of unfired earth are greatly influenced by amount and kind of basic components. this composition of earth is very various in nature. this is the main reason why using of unfired earth in construction is complicated. especially kind and amount of clay is significant because clay fulfils a function of binder, similarly like cement in concrete [4, 6–8]. ability to determine amount and type of clay in earth mixtures could help solve problem with various composition of earth. methylene blue test is a method that could help determine the type and amount of clay in earth mixtures [9, 10]. the main idea of our research is that every kind of clay has different ability to absorb the methylene blue dye. if the absorbtion ability of different types of clay could be determined, it would be possible, for example in combination with geological maps, to determine the amount or type of clay in unknown earth. the basic step in this research is to determine methodology of methylene blue test for earth and then determine dye absorption capacity of individual kinds of clay. montmorillonite clay is a basic kind of clay and it is a common component of earth. experimental measurement described in this paper is focused on ability of methylene blue dye absorption by montmorillonite clay. 2. principle of methylene blue test principle of methylene blue test is absorption of methylene blue dye (methylthioninium chloride c16h18cln3s) by clay. this experimental method is described for testing of aggregate’s geometry in standard čsn en 933-9 [9] but procedure of this experiment for unfired earth testing was necessary to modify because there are much more clay types in common earth. methylene blue solution (mb solution) of concentration 20 g/l is created from methylene blue dye (fig. 1) and distilled water. examined earth sample is created form earth mixture (sand and clay) and distilled water. methylene blue dye and earth sample are dried in a dryer at 95 ◦c for reached of equilibrium moisture. mb solution is inserted into earth sample by burette (fig. 2). mb dye absorption by earth samples is investigated. amount of one dose of methylene blue solution into earth sample is different. the added dye solution in earth sample is stirred for 1 minute for every added 1 ml of mb solution. time for stirring of earth samples with mb solution is very important. you can see in chapter 3 that shorter stirring time distorts obtained results. after mixing of earth sample with mb solution dose, a drop of earth sample is placed on a filter pa50 https://doi.org/10.14311/app.2020.26.0050 https://ojs.cvut.cz/ojs/index.php/app vol. 26/2020 diagnostics of unfired earth with methylene blue test figure 1. methylene blue dye. figure 2. methylene blue dye is dosed into earth sample. per. weight 87 g/m2 of filter paper is used. this process is repeated and each sample drop with increasing amount of methylene blue solution is marked (fig. 3). methylene blue test is finished when a blue ring spreads around a drop of sample (fig. 4) because clay has already absorbed the maximum amount of methylene blue dye. result of methylene blue test depends on the kind of clay in earth samples and amount of clay in earth samples. dye solution consumption increases with increasing amount of clay in clay samples. amount of mb dye absorbed is what we investigate. amount of mb dye absorbed is determined by the equation 1. amount of dye absorbed is defined as the amount of weight of dye in gram that is absorbed by 1 kilogram of test material. mmb = ms · mmb,1ml · 1000 mes (1) mmb amount of mb dye absorbed [g/kg], ms amount of methylene blue solution [g], mmb,1ml amount of methylene blue in 1 g of methylene blue solution [g/g], figure 3. drops of earth samples on a filter paper. figure 4. a blue ring around a drop of sample. mes amount of tested material [g]. 3. description of methylene blue test with montmorillonite clay this paper is focused on investigation of earth with montmorillonite clay by methylene blue test. earth samples are created from montmorillonite clay (fig. 5), sand and distilled water. a grain curve of sand is shown in figure 6. sand and montmorillonite clay were dried in dryer before production of earth samples. there are two similar experimental measurements with the same earth samples but different stirring time of earth samples with mb solution. different results from methylene blue test are due to different stirring time. composition of tested earth samples for the measurement where stirring time 1 minute is used for each 1 ml of mb solution in earth samples is shown in 51 t. plaček otcovská, b. mužíková, p. padevět acta polytechnica ctu proceedings figure 5. montmorillonite clay. figure 6. grain curve of sand. table 1 (earth samples gem11, gem12, gem13). composition of tested earth samples for the measurement where shorter and undefined stirring time is used than 1 minute for each 1 ml of mb solution in earth samples is shown in table 2 (earth samples gem22, gem23). different sand and clay ratios are used for this experimental measurements. amount of distilled water is same for all measurements. dosing of mb solution to earth samples is different for individual earth samples. mb solution is dosed to earth samples in several steps. concrete amount of dye dose for each earth sample is shown in table 3 for experimental measurement where 1 minute stirring time is kept. one earth sample is used for each sand and clay ratio. concrete amount of dye dose for each earth sample is shown in table 4 for experimental measurement where is stirring time shorter then 1 minute for each millilitre of mb solution in an earth sample. in addition to this, no stirring time rules are followed for this experiment. three earth samples are used for each sand and clay ratio. based on the values obtained and shown in table 4, it is evident that the non-observance of stirring time has substantially affects at amount of mb dye absorption. figure 7. resulting values of amount of mb solution absorbed. 4. measured results earth samples gem11, gem12 and gem13 are used for experimental measurement with defined stirring time. earth samples gem22 and gem23 are used for experimental measurement without defined stirring time. amount of mb dye absorbed are calculated by equation 1 from obtained values showed in tables 3 and 4. amount of mb dye absorbed is shown in tables 5 and 6. average values of results from calculated values is shown in table 7. all resulting values of amount of mb dye absorbed are shown in fig. 7. obviously, significantly greater amounts of mb dye absorbed are in earth samples where stirring time was maintained. difference between amount of mb dye absorbed with and without defined stirring time is approximately 50 % (see table 8). therefore, we can assume that stirring time has significant impact on amount of bm dye absorbed and it is necessary to define it. methylene blue test without defined stirring time seems to have negligible informative value. therefore, in following part attention is devoted to results from measurement with defined stirring time. 4.1. methylene blue test with defined stirring time there are hypothesis that dependency between amount of clay in earth samples and amount of mb dye absorbed is linear. three types of earth samples are tested by methylene blue test with defined stirring time (gem11, gem12, gem13). only one test sample of each type of earth samples is created. amount of mb dye absorbed in gem11 earth sample is 85.81 g/kg, in gem12 earth sample is 39.60 g/kg and in gem13 earth sample is 16.50 g/kg (see table 5). dependence between amount of clay and amount of mb dye absorbed is demonstrated in fig. 8. 52 vol. 26/2020 diagnostics of unfired earth with methylene blue test mark of sample kind of clay sand/clay ratio [%/%] amount of clay [g] amount of sand [g] amount of distilled water [g] gem11 montmorillonite 0/100 15 0 75 gem12 montmorillonite 50/50 7.5 7.5 75 gem13 montmorillonite 75/25 3.75 11.25 75 table 1. composition of earth samples for the measurement where stirring time 1 minute is used for each 1 ml of mb solution in earth samples. mark of sample kind of clay sand/clay ratio [%/%] amount of clay [g] amount of sand [g] amount of distilled water [g] gem22 montmorillonite 50/50 7.5 7.5 75 gem23 montmorillonite 75/25 3.75 11.25 75 table 2. composition of earth samples for the measurement where shorter stirring time then 1 minute is used for each 1 ml of mb solution in earth samples. number of dose mark of sample 1 2 3 4 5 ... 9 10 11 ... 14 15 16 17 18 19 gem11 visibility of blue ring no no no no no ... no no no ... no no no no no yes sand/clay ratio 0/100 amount of mb solution [ml] 5 5 5 5 2 ... 2 15 5 ... 5 10 15 15 15 10 total amount of mb solution [ml] 130 gem12 visibility of blue ring no no yes sand/clay ratio 50/50 amount of mb solution [ml] 40 10 10 total amount of mb solution [ml] 60 gem13 visibility of blue ring no yes sand/clay ratio 75/25 amount of mb solution [ml] 20 5 total amount of mb solution [ml] 25 table 3. doses of methylene blue solution to gem earth sample where stirring time 1 minute is used for each 1 ml of mb solution in earth samples. 53 t. plaček otcovská, b. mužíková, p. padevět acta polytechnica ctu proceedings number of dose mark of sample measurement no. 1 2 3 4 5 6 gem22 first visibility of blue ring no no no no no yes sand/clay measurement amount of mb solution [ml] 10 4 4 4 4 2 ratio 50/50 total amount of mb solution [ml] 28 gem22 second visibility of blue ring no no no no yes sand/clay measurement amount of mb solution [ml] 15 5 5 3 2 ratio 50/50 total amount of mb solution [ml] 30 gem22 third visibility of blue ring no no no yes sand/clay measurement amount of mb solution [ml] 15 5 5 3 ratio 50/50 total amount of mb solution [ml] 28 gem23 first visibility of blue ring yes sand/clay measurement amount of mb solution [ml] 15 ratio 75/25 total amount of mb solution [ml] 15 gem23 second visibility of blue ring no no no yes sand/clay measurement amount of mb solution [ml] 5 2 3 3 ratio 75/25 total amount of mb solution [ml] 13 gem23 third visibility of blue ring no yes sand/clay measurement amount of mb solution [ml] 10 2 ratio 75/25 total amount of mb solution [ml] 12 table 4. doses of methylene blue solution to gem earth sample where stirring time shorter than 1 minute is used for each 1 ml of mb solution in earth samples . mark of sample sand/clay ratio [%/%] amount of mb dye absorbed [g/kg] gem11 0/100 85.81 gem12 50/50 39.60 gem13 75/25 16.50 table 5. amount of mb dye absorbed by earth samples during methylene blue test with defined stirring time. mark of sample measurement no. amount of mb dye absorbed [g/kg] gem22 1 18.48 sand/clay 2 19.80 ratio 50/50 3 18.48 gem23 1 9.90 sand/clay 2 8.58 ratio 75/25 3 8.58 table 6. amount of mb dye absorbed by earth samples during methylene blue test without defined stirring time. it can be seen from fig. 8 that the hypothesis could be correct. mark of sample sand/clay ratio [%/%] average amount of mb dye absorbed [g/kg] standard deviation [g/kg] gem22 50/50 18.92 0.62 gem23 75/25 9.02 0.62 table 7. average values of amount of mb dye from methylene blue test without defined stirring time. 5. conclusion methylene blue test could be useful tool for identification of composition of unknown earth. our experiments show that it is important to maintain observed and uniform approach of methylene blue test, especially the stirring time (see fig. 7). next important point is that dependence between amount of mb solution absorbed and amount of clay in earth samples seems to be linear (see fig. 8). however, these conclusions of our work need to be verified by further experimental measurements because we tested only one type of clay and we used only one earth sample for every amount of clay in the earth sample. list of symbols mb methylene blue mmb amount of mb dye absorbed [g/kg] ms amount of methylene blue solution [g] 54 vol. 26/2020 diagnostics of unfired earth with methylene blue test sand clay ratio 50/50 [%/%] sand clay ratio 75/25 [%/%] mark of sample gem12 gem22 gem13 gem23 amount of mb dye absorbed [g/kg] 39.60 18.92 16.50 9.02 difference between amount of mb dye absorbed [%] 52.22 45.33 (100 %=39.60 g/kg) (100 %=16.50 g/kg) table 8. difference between amount of mb dye absorbed with and without defined stirring time (gem12, gem13 defined stirring time; gem22, gem23 undefined stirring time). figure 8. dependence between amount of clay in earth samples and amount of mb dye absorbed. mmb,1ml amount of methylene blue in 1 g of methylene blue solution [g/g] mes amount of tested material [g] acknowledgements the financial support of this experiment by the czech science foundation (gačr project no. 18-10884s) and faculty of civil engineering, czech technical university in prague (sgs project no. sgs16/201/ohk1/3t/11) is gratefully acknowledged. we would like to express our thanks to lb minerals, s.r.o. company for free supply of material necessary for experimental measurement. references [1] p. jaquin, c. augarde. earth building : history, science and conservation. report, bracknell : ihs bre press, 2012. [2] agenda 21 united nations environment programme (unep). http://www.unep.org/documents. multilingual/default.asp?documentid=52. [3] a. jing. a history of the great wall of china. world scientific publishing company, 2015. [4] g. minke. building with earth. ökobuch verlag, staufen. [5] p. stejskal. e-mail consultation with heluz company, 2017. [6] i. žabičková. hliněné stavby. era 21, brno, 2002. [7] p. walker. rammed earth: design and construction guidelines. ihs bre press, watford, 2010. [8] z. weiss. jílové minerály : jejich nanostruktura a využití. karolinum,, vyd. 1. edn., 2005. [9] evropský výbor pro normalizaci. čsn en 933-9, zkoušení geometrických vlastností kameniva část 9: posouzení jemných částic zkouška methylenovou modří, 2013. [10] y. yeliz, k. abidin. suitability of the methylene blue test for surface area, cation exchange capacity and swell potential determination of clay soils. engineering geology 2008(102):38–45, 2008. 55 http://www.unep.org/documents.multilingual/default.asp?documentid=52 http://www.unep.org/documents.multilingual/default.asp?documentid=52 acta polytechnica ctu proceedings 26:1–6, 2020 1 introduction 2 principle of methylene blue test 3 description of methylene blue test with montmorillonite clay 4 measured results 4.1 methylene blue test with defined stirring time 5 conclusion list of symbols acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2021.31.0030 acta polytechnica ctu proceedings 31:30–35, 2021 © 2021 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague calculation of the minimal length of the high-speed line petr nachtigall∗, erik tischer university of pardubice, jan perner transport faculty, department of transport technology and control, studentská 95, 532 10 pardubice, czech republic ∗ corresponding author: petr.nachtigall@upce.cz abstract. this paper explores the minimum length of continuous sections of the high-speed line. the minimum length is examined in terms of the maximum speed of high-speed vehicles. both traditional trainsets consisting of traction units and coaches, and train units were selected for examination. the graphs present the difference in the ability of the different vehicles to reach and use the maximum speed. keywords: high-speed line, opentrack, simulation, track speed. 1. introduction the construction of a high-speed network is going to be a reality in the czech republic. after a long time of uncertainty, there is a real plan of activities that will make us part of the map of a high-speed railway. as the construction of the entire network will take a long time, this paper is trying to calculate the minimum length of a continuous line to make the high-speed traffic attractive for passengers. the opentrack simulation tool was used for simulation [1]. subsequent calculations were performed in ms excel. to find out which track length is effective for the defined maximum track speed, we analyzed rides of predefined trainsets on a hypothetical infrastructure. the history of high-speed transport dates to the 20th century and has experienced its greatest upswing since the beginning of the 21st century. the total length of high-speed lines has already exceeded 50,000 km and it is planned to reach the target of 100,000 km in the next 15 years. as for individual countries, the most progressive country is china with its more than 35,000 km, with a view to reaching 70,000 km by 2035. europe lags behind in this respect with about 9,000 km. the czech republic is still without a high-speed railway; however, pilot projects are already being implemented and some parts of the conventional infrastructure are being built to allow a speed of 200 km·h−1. with the construction of the high-speed network, there comes the question of how long a section of the high-speed line must be to allow for the maximum track speed, making efficient use of the funds invested in this infrastructure [2, 3]. in their previous research, the authors investigated the optimum distance between two stops ensuring energy reliability. at the same time, they explored the effectiveness of introducing a speed of 200 km·h−1 on short sections of the conventional network. this paper examines the minimum length of a section of the high-speed line, which will enable the trains to reach the planned track speed. this problem is covered by the question of the efficiency of investment into the construction and operation of this specific infrastructure. there is a clear linear correlation between the track speed and construction, noise and safety measures. at the same time, a higher track speed puts stronger demands on vehicle design, power, and traction power consumption. a littler aside is the question of the minimum length of the high-speed line. this research can be used in the preparation of hsl network [4] constructions or overall preparation of the systematic timetables [5]. the czech republic is not as big as for instance china, but during the expansion of the hsl network, we can use an optimization mathematical apparatus to ensure the maximum transportability and utility of the highspeed system [6, 7]. the issue of energy efficiency is dealt with in literature [8], describing the perspective of the use of the coasting and economic modes of high-speed trains. the question of the interlocking system is answered by the etcs because the czech republic is going to install this system on the main corridors, including high-speed lines [9–14]. the costsâăof infrastructure are figuratively paid by passengers in the price of the ticket, and this price should be in accordance with the travel time and the quality of service. another important question is energy consumption. this issue is the focus of other papers [15–17]. 2. preparation of the simulation model the opentrack simulation software was used for the research. the authors have been using this tool for a long time with good results, and this software is a globally established simulation tool [18]. the model was designed with the emphasis on allowing all the trainset movements modelled to be performed in a very simple way. the simulation model was therefore 30 https://doi.org/10.14311/app.2021.31.0030 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 31/2021 calculation of the minimal length of the high-speed line prepared as simply as possible. the simulation model was created in two phases. in the first phase, an infrastructure model was created. the infrastructure model developed for this high-speed vehicle research consists of four tracks of 50 km each with no curves or slopes. in the second phase, the parameters of each trainset were entered. most important are the tractive characteristics of trainsets, maximum speed, weight, adhesive weight, and vehicle resistance. as for the research trainsets, the 151.0 engine with 7 bmz coaches, škoda 109e with 7 bmz coaches, siemens viaggio comfort non-traction unit with taurus engine and multiple units of the 680 čd "pendolino" series, ave, and ice 3 were chosen. for a more detailed description of the model, see previous article [19]. what is important for the actual simulation are the input characteristics of the individual trainsets. these characteristics are provided for in the following table 1. to calculate the rolling resistance of the individual trainsets, the davis equation was used, as expressed in equation 1. the individual parameters are included in the vehicle database within the opentrack system used for the simulation. had there been tunnels on the line, we would have had to add resistance caused by using the tunnel. however, no tunnel was included in the model. rlz = a+b ∗ ν + c ∗ ν2 [n ] (1) where: rlz= train air resistance [n], ν = train speed [km·h−1], a, b, c= parameters. the simulation was running continuously. depending on the required accuracy [20], it was possible to establish the situation of the train in every moment in time. this is important in examining the impact of variables that can be easily included in our generic model, as appropriate. the actual train speed and distance covered can be calculated using integration according to equation 2 and 3. the resulting values can be used to create tachograph curves or the monitored parts thereof. ν = νp + ∫ t2 t1 a · dt [m · s−1] (2) s = sp + ∫ t2 t1 ν · dt [m] (3) where: ν = speed [m·s−1], νp= initial speed [m·s−1], t = time [s], t1 = initial time [s], t2 = target time [s], a = acceleration [m·s−2], s = distance covered [m], sp= initial distance covered [m]. all simulations were carried out under good adhesion conditions; the value of adhesion was set at 100% of the adhesion coefficient. by setting the value of adhesion utilization, it is possible to simulate potential degraded adhesion conditions. the adhesion coefficient can be calculated according to equation 4. µ = 2.1[m · s−1] ν + 12.2[m · s−1] [−] (4) where: µ = adhesion coefficient, ν = speed [m·s−1]. through the simulation, the authors intended to show the distance needed for the acceleration of each trainset. each trainset was simulated with its maximum speed. 3. results the output of the simulation is quite simple: it is a distance-speed graph for each trainset, where we can observe the basic dynamic characteristics. table 2 shows the distance necessary for a trainset to reach the speed limit. the distance required for acceleration varies greatly. it certainly depends on the tractive power of each trainset and its weight. based on this, a graph of the speed-distance dependence was created. figure 1 shows this graph illustrating the curve for each trainset. we can see that the minimum length of the highspeed line for a train to reach the limit of 200 km·h−1 lies between 4 and 10 km. if we consider a speed limit of 230 km·h−1, this distance will increase to 8 km for ice 3 and to 18 km for pendolino and railjet.âăon the other hand, those values are just minimum ones. the actual length for practical useâăshould be higher. another very important variable is acceleration. for the presentation of results, the trainsets were divided into two groups with a distance-speed graph created for each of them. shown in figure 1, the one group involves typical trainsets with a locomotive. the other group in figure 3 contains train units. it is clear that the modern railjet vehicles and locomotive 380 achieve much better results than a locomotive representing the older generation 151.0. particularly at lower speeds, both these modern locomotives have dynamic properties close to those of train units. clearly the weakest out of the train units is pendolino, which is mainly due to its older design and, therefore, its higher weight. even on a straight section, pendolino’s maximum acceleration is 0.45 m·s−2 and declines relatively quickly. starting at a distance of approximately 1 km (at approx. 100 km·h−1), its acceleration is already lower than 0.3 m·s−2. 31 petr nachtigall, erik tischer acta polytechnica ctu proceedings trainset maximum speed [km·h−1] weight [t] maximum tractive effort [kn] maximum power [kw] maximum acceleration [m·s−2] average deceleration [m·s−2] 151.0 + 7 coaches 160 443 210 4000 0.353 -0.6 škoda 109 e + 7 coaches 200 445 274 6400 0.560 -0.6 siemens viaggio comfort 230 437 300 6400 0.627 -0.6 ave s-102 300 355 200 8800 0.526 -0.6 pendolino 230 384 200 3920 0.461 -0.6 ice 3 300 463 300 8000 0.581 -0.6 table 1. basic characteristics of the vehicles simulated, source: authors. speed distance [m] [km·h−1] engine 380 engine 151.0 railjet ave pendolino ice 3 100 890 1573 737 860 970 730 160 2993 5284 2967 2610 4096 2280 200 6588 6914 4900 9400 4450 230 20804 8950 18247 7100 300 20612 330 36372 table 2. distance to be covered to reach the respective speed, source: authors. figure 1. the distance needed for acceleration [km], source: authors. 32 vol. 31/2021 calculation of the minimal length of the high-speed line figure 2. acceleration of typical trainsets [m·s−2], source: authors. figure 3. acceleration of units [m·s−2], source: authors. 33 petr nachtigall, erik tischer acta polytechnica ctu proceedings 4. conclusions this paper explores the minimum length of a section of the high-speed line for vehicles to be able to reach the maximum speed. the simulation showed that for a maximum speed of 230 km·h−1, this distance is approximately 10 to 15 kilometers. this is the distance to be covered by high-speed units to achieve this speed. an important question to be answered before the line construction is whether the line would only be used by high-speed units, or also by conventional trainsets with a maximum speed of 200 or 230 km·h−1. the simulation shows that even these trainsets can achieve their respective maximum speeds in a relatively short period of time. by contrast, older models of traction units are not suitable for operation on these newly constructed lines due to their low maximum speed and traction characteristics. encouraging is the fact that even short sections of the high-speed line are a convenient and efficient tool for the development of the railway network: not only are they parts of the high-speed network, but they can also take the strain off current conventional lines. mainly close to major urban areas, the capacity of these lines is often saturated, and they are not able to cover the ever-increasing demand for suburban transport. also freight railway undertakings are waiting for the clearing of these sections. acknowledgements this article is published within realization of the project "cooperation in applied research between the university of pardubice and companies in the field of positioning, 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[19] p. nachtigall, e. tischer. efficiency of increasing of the track speed using simulation in opentrack. proceedings of the 32nd european modeling & simulation symposium (emss 2020), https: //www.cal-tek.eu/proceedings/i3m/2020/emss/017/ issn 2724-0029. [20] j. bulíček. cancellation of delayed trains: passengers’ and capacity points of view. žilina: matec web. conf., 2018. vol. 235. 35 https://www.cal-tek.eu/proceedings/i3m/2020/emss/017/ https://www.cal-tek.eu/proceedings/i3m/2020/emss/017/ acta polytechnica ctu proceedings 31:30–35, 2021 1 introduction 2 preparation of the simulation model 3 results 4 conclusions acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2020.26.0030 acta polytechnica ctu proceedings 26:30–33, 2020 © czech technical university in prague, 2020 available online at https://ojs.cvut.cz/ojs/index.php/app mechanical properties of cement composite containing recycled concrete and alkali activated blast furnace slag jan horych∗, pavel tesárek, zdeněk prošek czech technical university in prague, faculty of civil engineering, department of mechanics, thákurova 7, 166 29 prague 6, czech republic ∗ corresponding author: jan.horych@fsv.cvut.cz abstract. recycling of materials is very popular and very important in these days. finding the new ways to process and use these materials is a key to get rid of a lot of construction waste. the amount of landfilling needs to be reduced. this study observes mechanical properties of the cement composites containing recycled concrete powder and alkali-activated blast furnace slag processed on a high-speed mill as a potential binder replacement up to the 60 wt. %. these materials have a positive effect on hydration process, increase flexural strength. it can reduce compressive strength loss when an amount of cement in the mixture is reduced. keywords: composites, waste, blast furnace slag, recycled concrete, cement matrix. 1. introduction according to the current recycling trend, we want to find a way to use waste material as a material to create something new. in this research we use recycled concrete and blast furnace slag to replace cement and reduce impact of its production. cement is the most important but the most expensive ingredient in concrete. the production of cement is based on natural resources and its processing needs a lot of energy. unfortunately, these used materials have their own issues and problems that have to be solved. many studies are focused on similar topic [1–6]. some of these studies are using blast furnace slag or recycled concrete to improve mechanical properties. the canadian study [7] is similar to our research. it uses the blast furnace slag as a binder and examines all physical properties of concrete including workability, air content, elastic modulus, strength, shrinkage, durability and carbonation. the results of all test confirmed the opinion, that the blast furnace slag can be used as a binder. it has a positive effect on mechanical properties and durability of concrete. another study [8] solves the issue of recycled concrete powder used as a binder. they also used the high-speed mill to activate the material. they created samples with varying cement and recycled concrete percentages and observed mechanical properties of the samples. results of this study showed the possibility to use recycled concrete in practise. concrete with cement and recycled concrete produce higher hydration heat. it is caused by unhydrated cement grains contained in recycled concrete powder. research [9] was based on fly ash, blast furnace slag and recycled concrete aggregate. the study investigated compressive and tensile strengths, elastic modulus, coefficient of permeability and durability in chloride and sulphate solutions. blast furnace slag had positive effect for a concrete with recycled aggregate and could reduce strength losses. fly ash had worse results and was recommended for further testing before being used in practice. the study [10] was focused on creation of alkali-activated foamed concrete with part of ground granulated blast furnace slag. all samples were produced using a pre-formed foam with no filler or aggregate. the results revealed that foamed concrete with blast furnace slag has a potential to be used in practise. it had even better mechanical properties than foamed concrete only with portland cement. 2. materials and samples testing samples were composed of portland cement cem i 42.5 r from radotín. another component was the recycled concrete micronized by high-speed mill in collaboration with lavaris corporation. the source of concrete is 100 years old construction concrete from the demolished building on waltrovka in prague. another component was the blast furnace slag kept from kladno. it is 100 years old air-cooled and carbonated slag stored on a heap and it’s the waste from slag aggregate production. this kind of slag is not suitable as a concrete binder. it needs to be activated. we use the same high-speed mill to activate it, because the blast furnace slag had a slow reaction for a long period of time. all of the samples were 40 × 40 × 160 mm and they were stored in water for 28 days. during this time we made tests to find dynamic modulus of elasticity using a non-destructive resonance method and at the end, we made destructive bending test and compresion test to figure out final strength. samples contain 50 or 40 wt. % of portland cement, 50 wt. % of recycled concrete and changing share of blast furnace slag and slaked lime from 0 to 10 wt. % (ta30 https://doi.org/10.14311/app.2020.26.0030 https://ojs.cvut.cz/ojs/index.php/app vol. 26/2020 mechanical properties of cement composite ... mixtures portland recycled slaked blast furnace flow cement [wt. %] concrete [wt. %] lime [wt. %] slag [wt. %] expansion test [cm] a 50.0 50.0 0.0 0.0 22.0 b 40.0 50.0 0.0 10.0 21.2 c 40.0 50.0 2.5 7.5 19.0 d 40.0 50.0 5.0 5.0 19.0 e 40.0 50.0 7.5 2.5 18.0 f 40.0 50.0 10.0 0.0 17.3 table 1. composition of individual samples. ble 1). it also shows the results of flow expansiom test. the part of slaked lime changes the result even when the water coefficient w/b is the same. the slaked lime is very finely ground and because of this fact, the mixture has a different consistency. the water coefficient w/b is set to 0.4. figure 1. final bulk density of the samples. figure 2. development of the dynamic young’s modulus for the samples. figure 3. development of the dynamic shear modulus for the samples. figure 4. flexural strength of the samples. figure 5. compressive strength of the samples. 3. measurement methods during our research we measured dynamic young’s modulus, dynamic shear modulus, flexural and compressive strength. dynamic young’s modulus and dynamic shear modulus were monitored using nondestructive resonance method during 28 days. the method is based on measuring the natural frequencies of longitudinal, transverse and torsional vibration of the samples. samples were supported with a soft elastic pads at nodal points of basic natural mode shapes. through the test we used brüel & kjær measurement station type 3560-b-x12 to record the excitation and response signals. as an acceleration transducer was used brüel & kjær type 4519-003 and another component is brüel & kjær impact hammer type 8206. everything was connected to the computer with software pulse labshop. the excitation and response signals were transformed from the time domain to the 31 j. horych, p. tesárek, z. prošek acta polytechnica ctu proceedings frequency domain by using the fast fourier transform. frequency response function was calculated as a ratio of the response and the excitation force. the basic natural frequencies were evaluated with frequency response function and then were evaluated dynamic young’s modulus and dynamic shear modulus. in this method we also need to know the dimensions and the weight of the samples. [11, 12]. flexural strength was measured on 28 days old samples using three-point bending test. the testing was displacement controlled at a constant rate of 1 mm/min and the distance between supports of the sample was 100 mm. during the test, samples were broken approximately in half. these halves (40 × 40× ∼ 80 mm) were used on the uniaxial compression test. the testing was displacement controlled at a rate of 5 mm/min [13]. 4. results and discussion figure 1 shows the final bulk density of all samples. the highest bulk density had the sample a as a reference. it had 50 wt. % of portland cement and 50 wt. % of recycled concrete powder. the second highest bulk density had the sample c with 40 wt. % of portland cement, 50 wt. % of recycled concrete powder, 7.5 wt. % of blast furnace slag and 2.5 wt. % of slaked lime as the activator. as we can see in figure 2 and figure 3, sample c had also the highest dynamic young’s modulus and the dynamic shear modulus from the samples with 40 wt. % of portland cement. however the reference sample had the higher dynamic young’s modulus approximately by 3 gpa and dynamic shear modulus by 1 gpa. each sample was measured during 28 days every 7 days including its weight and dimensions. in both cases, dynamic young’s modulus and dynamic shear modulus had the highest progress in the first 7 days. then the progress was much slower from day 7 to day 21 and in the last 7 days, the values were almost unchanged. figure 4 presents a flexural strength results of all samples. it is obvious that the blast furnace slag had a positive effect on flexural strength. the worst strength had a reference sample a without the slag. samples b, c and d had a higher strength almost by 100 %. however, the highest strength had the sample c and confirmed the previous measurement. figure 5 shows the results of the compression test. it proved the effect of recycled concrete and slag. the highest compressive strength had a sample a because of the highest amount of cement in the mixture. as we can see, the samples c, d and e had approximately the same strength. the combination of blast furnace slag and slaked lime seemed to be effective. through all stages of testing, the results seemed very promisinig. it seemed that the most promising sample was sample c because of the highest flexural strength. compressive strength was also one of the highest, so the ratio 3:1 of the slag and slaked lime seemed to be ideal. recycled concrete could be used as a microfiller but, the study [6] proved that very fine ground concrete has approximately 10 % of unhydrated cement grains and these grains helps with hydration process. there is a need for more research, but used materials definitely have the potential to be used in practical applications. 5. conclusion this paper examined the recycled concrete powder and blast furnace slag as a cement replacement at the level of 60 wt. % with various ratios of blast furnace slag and slaked lime and its effect on mechanical properties of samples. based on the results, it can be concluded that: • amount of cement in concrete can be reduced without a massive loss of mechanical properties. • highly carbonated blast furnace slag can be used as a filler and also a binder with proper processing. • blast furnace slag had a very positive effect on flexural strength. • the ratio 3:1 of blast furnace slag and slaked lime seems to be ideal. acknowledgements this outcome was supported by the czech technical university in prague under no. sgs19/148/ohk1/3t/11 and by technology agency of the czech republic (ta cr) research project tj01000435, which is solved in cooperation with the company pozemní komunikace bohemia, a.s.. the authors also thank the center for nanotechnology in civil engineering at the faculty of civil engineering, czech technical university in prague. references [1] s. demirdag, l. gunduz. strength properties of volcanic slag aggregate lightweight concrete for high performance masonry units. construction and building materials 22(3):135–142, 2008. doi:10.1016/j.conbuildmat.2006.10.002. 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[7] r. d. hooton. canadian use of ground granulated blast-furnace slag as a supplementary cementing material for enhanced performance of concrete. canadian journal of civil engineering 27(4):754–760, 2011. doi:10.1139/l00-014. [8] j. topič, z. prošek. properties and microstructure of cement paste including recycled concrete powder. acta polytechnica 57(1):49–57, 2017. doi:10.14311/ap.2017.57.0049. [9] m. l. berndt. properties of sustainable concrete containing fly ash, slag and recycled concrete aggregate. construction and building materials 23(7):2606–2613, 2009. doi:10.1016/j.conbuildmat.2009.02.011. [10] k.-h. yang, k.-h. lee, j.-k. song, m.-h. gong. properties and sustainability of alkali-activated slag foamed concrete. journal of cleaner production 68:226– 233, 2014. doi:10.1016/j.jclepro.2013.12.068. [11] astm 1876-01. standard test method for dynamic young’s modulus, shear modulus, and poisson’s ratio by impulse excitation of vibration, annual book of astm standards, american society for testing and materials 2006. [12] čsn 73 1372. in nedestruktivní zkoušení betonu rezonanční metoda zkoušení betonu, úřad pro technickou normalizaci, metrologii a státní zkuševnictví,. 2012. [13] čsn en 12390 (731302). in zkoužení ztvrdlého betonu. 2013. 33 https://doi.org/10.1088/1757-899x/236/1/012092 https://doi.org/10.1139/l00-014 https://doi.org/10.14311/ap.2017.57.0049 https://doi.org/10.1016/j.conbuildmat.2009.02.011 https://doi.org/10.1016/j.jclepro.2013.12.068 acta polytechnica ctu proceedings 26:1–4, 2020 1 introduction 2 materials and samples 3 measurement methods 4 results and discussion 5 conclusion acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2019.24.0044 acta polytechnica ctu proceedings 24:44–49, 2019 © czech technical university in prague, 2019 available online at http://ojs.cvut.cz/ojs/index.php/app analysis of neutron fields generated in spallation targets of b-uran experimental assembly using monte carlo method ondřej šťastnýa,b,∗, miroslav zemana,d, dušan krála, karel katovskýa, elmira melyana,c, jindřich adama,d, alexander alexandrovich solnyshkind a brno university of technology, faculty of electrical engineering and communication, department of electrical power engineering, technická 3082/12, 616 00 brno, czech republic b state office for nuclear safety, department of non-proliferation, senovážné nám. 9, 110 00 prague 1, czech republic c yerevan state university, faculty of physics, 1 alex manoogian street, 0025 yerevan, armenia d joint institute for nuclear research, laboratory of nuclear problems, joliot-curie 6, 141980 dubna, russia ∗ corresponding author: xstast30@stud.feec.vutbr.cz abstract. the aim of this paper is to introduce experimental assembly b-uran and the results of monte carlo simulations of neutron fields, which will be generated by using various spallation targets. this experimental assembly was constructed in joint institute of nuclear research in dubna, russian federation, in order to study accelerator driven systems fundamental characteristics. beam of 660 mev protons should be used for that purpose. the mcnp model of such set-up has been developed at brno university of technology, czech republic. the goal is to get data needed for prediction of reaction rates in detectors placed in b-uran experimental channels. such data will be experimentally validated later. furthermore, simulations of radiation exposure around this experimental assembly were performed. keywords: accelerator driven systems, b-uran, monte carlo simulations, reaction rates, spallation reactions. 1. introduction there is an area of nuclear power production, which is still considered as its weakness – spent nuclear fuel, which contains a lot of highly radioactive fission products and long-lived actinides; therefore, it has to stay separated from the environment forever. deep geological repositories can serve as the most conservative solution for this purpose. nevertheless, there are other options, such as reprocessing, which provides possibility to use plutonium again as a part of mox fuel. however, fission products and long-lived actinides remain as a heritage of nuclear power production and they are treated as radioactive waste. several decades ago, one promising solution was proposed – accelerator driven systems (ads) [1], which have the potential to "incinerate" such kind of problematic isotopes and therefore minimize the amount of radioactive waste after reprocessing the spent nuclear fuel. ads usually consist of accelerator, spallation target and blanket. accelerator produces accelerated particles (protons, deuterons) to high energies (hundreds of mev or ones of gev) which than trigger spallation reactions in target material. spallation reactions produce high-energy neutron fields around the target that can be used to transmute ("incinerate") radionuclides located in surrounding blanket (see the example at figure 1, neutron yield by using 800 mev protons and tungsten target). there has been a lot of research activities worldwide focused on this area so far, but still the compact models and nuclear data libraries describing the parameters of generated neutron fields are missing (for more information see [3–6]). one of the research facilities with long experience in this research is joint institute of nuclear research (jinr) in dubna, russian federation. group e+t raw (energy & transmutation of radioactive waste) with experimental assembly quinta [3] can be mentioned in this regard. local accelerator phasotron produces particles with energies up to 660 mev and various targets are available, as well. this creates a great opportunity for many researches for studying spallation reactions and parameters of generated neutron fields. 2. description of proposed experiment with b-uran experimental assembly 2.1. b-uran experimental assembly latest experimental assembly b-uran was designed at jinr for studies of spallation neutrons behavior in quasi-infinite geometry. this experimental assembly is a cylinder made from depleted uranium with diameter 44 http://dx.doi.org/10.14311/app.2019.24.0044 http://ojs.cvut.cz/ojs/index.php/app vol. 24/2019 analysis of neutron fields generated in spallation targets figure 1. neutron yield from fission and spallation inside tungsten target [2] figure 2. schematic drawings of b-uran, front and rear view [7] of 120 cm and length of 100 cm. it is coated with 10 cm thick steel layer and weights app. 20 tons. central zone allows positioning replaceable cylindrical targets with 20 cm in diameter. for experimental purposes, b-uran offers large amount of axial experimental channels [3]. more details are shown in figure 2. 2.2. spallation targets several types of spallation target were considered – lead, graphite and bismuth. lead target was 100 cm in length, but due to the high leakage of neutrons from the front part of b-uran, it was decided to make a hole 8 cm in diameter and 40 cm in length so the particle beam could penetrate deeper into the target geometry in order to have more convenient distribution of the generated neutron field inside the inner figure 3. b-uran sectional view, lead target [8] part of b-uran. for further details, see figure 3. 2.3. mcnp model of the proposed experiment first, the mcnp 6.2 [9] model of b-uran experimental assembly was developed. it was decided to simplify it in order to decrease the cpu time needed, therefore, only one plane of experimental channels (9 in total) was considered. each experimental channel is 3 cm in diameter, and they are located at positions 14, 18, 22, 26, 30, 34, 38, 44 and 52 cm from the b-uran axis. the source of particles is a proton beam of en45 o. šťastný, m. zeman, d. král et al. acta polytechnica ctu proceedings figure 4. b-uran sectional view, graphite target, red arrow is a symbol of proton beam (created by mcnp) ergy 660 mev impacting on target in b-uran central zone. for neutron spectra measurement a set of 10 threshold activation detectors is placed inside each considered experimental channel along the b-uran’s length. preliminary, it was decided that small "coin" detectors (diameter 2 cm, height 0.5 cm) made from bismuth will be used. the aim is to detect (n,xn) reaction rates up to several hundreds of mev. according to the results of calculation, different materials can be used for this purpose in further research. see the figure 4 for detailed image of b-uran experimental assembly and location of threshold activation detectors inside experimental channels. it should be stated, that the b-uran experimental assembly is located at story with height of 250 cm, so there is 45 cm of space between the ceiling and the top of concrete shielding. both ceiling and the floor are made from 1 cm thick steel plate. all these details correspond with the real conditions at jinr. surrounding environment of b-uran was represented by air in shape of cylinder with radius 200 cm. there were also additional "control volumes" filled with air around b-uran which were used for monitoring of neutron leakage from the experimental assembly. graphite target was 100 cm in length, despite bismuth target was 50 cm only and was placed in the rear position inside b-uran target channel – see figure 4. for neutron and gamma dose calculation, a simplified model of b-uran has been created. it also involved its surroundings (concrete shielding) at the story, where this experimental assembly is located. all calculations were performed using default physics settings of mcnp (model cem03.03 [10]) and photofission settings was turned on. concerning used nuclear data libraries, see table 1. for thermal treating a free-gas model was involved, no s(a,b) lifigure 5. b-uran sectional view, bismuth target, red arrow is a symbol of proton beam (created by mcnp) braries were used. for all other reactions, internal default mcnp models were used. 2.4. expected model outcomes the main aim of the model was to determine neutron flux values inside the threshold activation detectors (tally f4 – flux) in order to get data for further reaction rates calculation. subsequently, a visual information about several types of particles spatial distribution (mesh tally, cmesh type 1 – flux) was also seek. finally, rough estimation of radiation dose around b-uran during irradiation was investigated (mesh tally, rmesh type 1 – dose), as well. 3. results and discussion results of neutron flux calculation for threshold activation detectors can be found in figure 6. average relative error of values for detectors using spallation target made from graphite, lead and bismuth are 0.17% – 0.6% – 1.08%. it clearly shows that graphite target provides several orders higher values than other target materials (lead, bismuth) – this needs to be evaluated by comparison with experimental results. from the neutron yield point of view, graphite target seems to be the best option. however, other features of target materials should be taken into consideration, such as thermal stability, which could be in favor for heavy metals such as lead. original idea was to perform these calculations for targets made from depleted uranium and thorium as well. however, it turned out, that these heavy metals with fast fission factor need longer computation times at ordinary workstation. furthermore, neutron flux mesh was calculated for several targets – graphite, lead and bismuth and results of the neutron flux from various materials is 46 vol. 24/2019 analysis of neutron fields generated in spallation targets library materials endf/b-7.1 nat. c, 14n, 16o, 55mn, 209bi, 234u, 235u, 238u endl-85 nat. ar endf/b-5 nat. cr, nat. ni, nat. pb endf/b-5 t2 group upgrade [11] nat. fe mcplib-v.04 from endf/b-5 (photon data) nat. c, nat. n, nat. o, nat. ar, nat. cr, 55mn, nat. fe, nat. ni, nat. pb, 209bi, nat. u endf/b-7.0 (photonuclear data) 14n, 55mn, 209bi, 235u, 238u la150 (photonuclear data) 16o endf/b-7.0 (proton data) 14n, 16o, 209bi el03 (electron data) [12] nat. c, nat. n, nat. o, nat. ar, nat. cr, 55mn, nat. fe, nat. ni, nat. pb, 209bi, nat. u table 1. nuclear data libraries for mcnp calculations figure 6. comparison of several targets, neutron flux (neutrons/cm2), 9 sets of detectors in experimental channels (calculated by mcnp) possible to see the in figure 7, where is their comparison. these results fully corresponds with graph in figure 6. since the bismuth target was placed in the rear position of the b-uran target channel, the maximum values of neutron flux were located at the end of each experimental channel. in case of lead target, which fills the whole target channel (with hole in the center), the shape of neutron flux graph almost equals to normal distribution. for graphite target, the maximum of neutron flux in each channel can be found at detectors in the middle of b-uran length. moreover, radiation dose around b-uran during irradiation was computed, as well. drawing of the b-uran surroundings including concrete shielding is shown in figure 8. results can be found in figure 9 – neutron and gamma dose. influence of concrete shielding located in front of b-uran is clearly visible as well as a small tunnel which leads the particle beam to target. the different behavior of neutron and gamma ra47 o. šťastný, m. zeman, d. král et al. acta polytechnica ctu proceedings figure 7. comparison of neutron flux mesh of several targets (created by mcnp) diation is clearly visible. this particular concrete shielding provides very efficient protection against gamma radiation, unlike neutron radiation still creates quite compact neutron field behind the shielding, which decreases by three to five orders of magnitude only, probably because of neutron transport through figure 8. b-uran surroundings including concrete shielding [8] figure 9. neutron and gamma dose around b-uran experimental assembly with lead target, at level of proton beam, irradiated by 4e15 protons (created by mcnp) the space between the top of the shielding and the ceiling. 4. conclusions in this paper, the neutron flux for bismuth threshold activation detectors in experimental channels of b-uran assembly was calculated. these values can be then used for reaction rates determination. furthermore, several targets (graphite, lead and bismuth) were taken into consideration and neutron flux mesh was calculated for them, as well. based on values for neutron flux in threshold acti48 vol. 24/2019 analysis of neutron fields generated in spallation targets vation detectors, it can be stated, that graphite target provides several orders higher values than other target materials – this needs to be validated by comparison with experimental results. other question is graphite thermal stability, which could be in favor for heavy metals such as lead. since the targets made from depleted uranium and thorium were proposed as well, the same calculations should be also performed for them in further research. moreover, geometrical properties of targets should be also considered. the aim should be to minimize neutron leakage from the front and rear part of buran assembly and to maximize neutron field usage inside the threshold activation detectors located in experimental channels. as a result, the aim for further research in this area should be to propose a target design combining light and heavy materials together. based on the gained knowledge, most efficient material composition of detectors should be investigated. for purposes of this article, only single material (bismuth) has been considered. considering neutron and gamma dose calculations, the radiation dose around b-uran has been estimated. according to obtained values, which have been calculated for typical irradiation session at jinr phasotron (4x1015 protons), the doses from gamma and neutrons could be very high (see figure 9). following to that, radiation protection measures have to be clearly defined and regularly verified. moreover, the influence of neutron radiation to the concrete shielding should be investigated (especially with regard to possible concrete activation by neutrons). acknowledgements part of this research work has been carried out in the centre for research and utilization of renewable energy (cvvoze: http://www.cvvoze.cz) and part within energy and transmutation of radioactive waste collaboration (e&t raw: http://et.jinr.ru). authors gratefully acknowledge financial support from the ministry of education, youth and sports of the czech republic under brno university of technology specific research programme (project no. fekt-s-17-4784 "new technologies for sustainable power engineering") and under international research cooperative programme of jinr dubna and the czech republic (projects of joint 3+3 committee no. 11/159/2018 "neutron spectral characterization using spectral shield analysis" and 12/159/2018 "study of neutron flux in spallation targets"). references [1] c. bowman, e. arthur, p. lisowski, et al. nuclear energy generation and waste transmutation using an accelerator-driven intense thermal neutron source. nuclear inst and methods in physics research pp. 336–367, 1992. doi:10.1016/0168-9002(92)90795-6. [2] d. král. investigation of thorium utilization in accelerator driven nuclear reactors. master’s thesis, brno university of technology, brno 2017. [3] w. furman, j. adam, l. závorka, et al. recent results of the study of ads with 500 kg natural uranium target assembly quinta irradiated by deuterons with energies from 1 to 8 gev at jinr nuclotron. proceedings of science 2012. http: //www.sujv.cz/cz/index.php?ns=202&id=1000085. [4] a. koning, d. rochman. modern nuclear data evaluation with the talys code system. nuclear data sheets 113 2012. http://www.sciencedirect. com/science/article/pii/s0090375212000889. [5] d. rochman, a. koning, j. sublet, et al. the tendl library: hope, reality and future. proceedings of the international conference on nuclear data for science and technology 2016. bruges, belgium, https://tendl. web.psi.ch/bib_rochman/tendl.nd2016.pdf. [6] j. thomason. the isis spallation neutron and muon source – the first thirty-three years. nuclear instruments and methods in physics research section a: accelerators, spectrometers, detectors and associated equipment 917:61–67, 2019. issn 0168-9002, doi:10.1016/j.nima.2018.11.129. [7] jinr. energy and transmutation of radioactive waste [online], dubna, moscow region, russian federation, 2019 [quote: 2019-05-26], http://et.jinr.ru/pict/pic_buran.jpg. [8] p. tichý, j. adam, a. baldin, et al. experimental investigation and monte carlo simulations of radionuclide production inside the uranium spallation target quinta irradiated with a 660-mev proton beam. epj web conf 204 04003 2019. doi:10.1051/epjconf/201920404003. [9] c. werner. mcnp users manual – code version 6.2. los alamos national laboratory, report la-ur-17-29981 2017. [10] cem03.03. monte-carlo code system to calculate nuclear reactions in the framework of the improved cascade-exciton model. stepan mashnik and arnold sierk: cem0303 user manual, los alamos national laboratory, report la-ur-12-01364 2012. http://www. oecd-nea.org/tools/abstract/detail/psr-0532/. [11] e. arthur, p. young. evaluated neutron-induced cross sections for 54,56fe to 40 mev. los alamos national laboratory report no la-8626-ms 1980. [12] k. adams. electron upgrade for mcnp4b. los alamos national laboratory internal memorandum, x-5-rn(u)-00-14 2000. 49 http://www.cvvoze.cz http://et.jinr.ru http://dx.doi.org/10.1016/0168-9002(92)90795-6 http://www.sujv.cz/cz/index.php?ns=202&id=1000085 http://www.sujv.cz/cz/index.php?ns=202&id=1000085 http://www.sciencedirect.com/science/article/pii/s0090375212000889 http://www.sciencedirect.com/science/article/pii/s0090375212000889 https://tendl.web.psi.ch/bib_rochman/tendl.nd2016.pdf https://tendl.web.psi.ch/bib_rochman/tendl.nd2016.pdf http://dx.doi.org/10.1016/j.nima.2018.11.129 http://et.jinr.ru/pict/pic_buran.jpg http://dx.doi.org/10.1051/epjconf/201920404003 http://www.oecd-nea.org/tools/abstract/detail/psr-0532/ http://www.oecd-nea.org/tools/abstract/detail/psr-0532/ acta polytechnica ctu proceedings 24:1–6, 2019 1 introduction 2 description of proposed experiment with b-uran experimental assembly 2.1 b-uran experimental assembly 2.2 spallation targets 2.3 mcnp model of the proposed experiment 2.4 expected model outcomes 3 results and discussion 4 conclusions acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2020.26.0013 acta polytechnica ctu proceedings 26:13–18, 2020 © czech technical university in prague, 2020 available online at https://ojs.cvut.cz/ojs/index.php/app size effect on the ultimate drying shrinkage of concrete experimental evidence and engineering practice lenka dohnalová∗, petr havlásek czech technical university in prague, faculty of civil engineering, department of mechanics, thákurova 7, 166 29 prague 6, czech republic ∗ corresponding author: lenka.dohnalova@fsv.cvut.cz abstract. the design and behavior of creep-sensitive structures can be to a large extent dependent on the evolution of drying shrinkage. drying shrinkage of concrete measured on standard laboratory specimens is very time demanding, furthermore, the extrapolation based on the short-term measurements is an ill-posed problem and can lead to large errors. additionally, the final magnitude of shrinkage is size-dependent which makes the transition from the laboratory data to real structure even more challenging. the present paper evaluates this size effect based on the data gathered in the creep and shrinkage database developed at the northwestern university and the data from the literature. finally, this size effect is compared against the current codes of practice and recommendations. keywords: drying shrinkage, size effect, concrete, design code, prediction model. 1. introduction concrete drying is followed by its gradual contractive volume changes referred to as drying shrinkage. similarly to autogenous shrinkage, which happens even under sealed conditions and is characteristic for modern high-performance concretes with small water-to-cement ratios, also the drying shrinkage is considered to be bounded. however, on contrary to autogenous shrinkage which is believed to be uniform within the volume of the member, drying shrinkage which is driven by the changing field of relative humidity gives rise to non-uniform stresses and can result into surface cracking. the drying shrinkage of concrete is a complex phenomenon which is not fully understood even nowadays. the evolution and magnitude of drying shrinkage is influenced by many factors, and for this reason the codes of practice and the design codes comprise many parameters. the aim of this paper is to assess the influence of the size of concrete member on the final magnitude of the drying shrinkage. in the case of concrete structures sensitive to creep and shrinkage, it is recommended to perform shortterm measurements on laboratory specimens and consequently, if necessary, to adjust the parameters of the computational models based on the results. the purpose of these short-term measurements is to capture the influence of parameters such as the water-tocement ratio, cement type, type of aggregate, additives, admixtures, etc., which significantly affect the behavior of concrete. the updating of concrete compliance function works reasonably well. in the case of drying shrinkage the final value cannot be reached in a reasonable time; not even in standard tests on laboratory specimens, 75 × 75 × 285 mm according to astm c157 [1] and 100 × 100×400 mm as given by the iso 1920-8 [2] standard. in both cases the values stabilize after approximately 1,000 days of drying [3] which is unacceptable in civil engineering practice. typically, the already measured drying shrinkage evolution can be accurately captured with a very simple formula with two parameters only: the characteristic time and the ultimate shrinkage. despite that the drying shrinkage updating remains an ill-posed problem [4]. the estimated ultimate shrinkage strongly depends on the choice of the characteristic time which is unknown. both drying shrinkage magnitude and the characteristic time are functions of concrete composition, dimensions and ambient humidity which influence concrete diffusivity and thus the drying rate. if the size effect on drying shrinkage magnitude is fully understood and described, then the problem might be overcome by decreasing the specimen size, the characteristic time is approximately a square function of the characteristic size. the results obtained on smaller specimens might be polluted by the loss of representativeness if the ratio of the specimen size to the aggregate is too small. however, this influence can be incorporated in the size effect law if the governing mechanisms are fully understood. the size of typical structural members exceeds the characteristic dimension of the laboratory specimens many times. for this reason, the duration of drying and shrinkage can easily reach decades until these processes cease. the currently available size effect laws on the ultimate drying shrinkage are based only on relatively limited laboratory measurements, beyond that range their applicability is questionable. two recent studies have already approached this topic. the first one [5] compares the size effect on 13 https://doi.org/10.14311/app.2020.26.0013 https://ojs.cvut.cz/ojs/index.php/app lenka dohnalová, petr havlásek acta polytechnica ctu proceedings drying shrinkage according to ec2 with two experimental series only and concludes that even the current coupled fem simulations are unable to capture the phenomenon. the second, more recent paper [6] is oriented more on the extrapolation of the drying shrinkage rather than on the size effect law, however, the original data from this paper are adapted in the present study. the first part of this paper summarizes the size effect on the ultimate value of drying shrinkage as it is described in the codes of practice and recommendations. the following section examines the relevant experimental data some of which served, very probably, as a reference used for the development and calibration of the codes and prediction models. the final part compares the models with the experimental data and summarizes the conclusions. 2. design codes and prediction models this section summarizes the definitions of the size effect on drying shrinkage according to the most common codes of practice—the american standard aci 209.2r–08 [7] and eurocode 2 [8]—and selected recommendations for the long–term behavior of concrete—b3 model [9], b4 model [10] and fib model code 2010 [11]. in the models the total shrinkage is either taken equal to the drying shrinkage (ec2, aci 209, b3) or is split into drying and autogenous shrinkage (fib mc 2010, b4), both being bounded. the influence of specimen size on time evolution of drying shrinkage is not of interest in the present study and for this reason the related formulae are not presented here. 2.1. models b3 and b4 even though the newer b4 model introduces the split of total shrinkage into drying and autogenous shrinkage, the general structure of the formula for drying shrinkage is adapted from its predecessor, model b3 whose capability is limited only to concretes with higher water-to-cement ration and which recognizes only drying shrinkage. the b3/4 models reflect not only the influence of the specimen size, but also its shape. these parameters play role both in the evolution of shrinkage and in the formula for the ultimate shrinkage. different shapes of the specimens can be treated by means of the cross-section shape factor ks (1.0–1.55) which enters the formula for the shrinkage halftime and which influences the magnitude of the ultimate shrinkage. the specimen size is expressed using the effective cross-section thickness d defined as d = 2v/s (1) where v and s are the volume and drying surface of the member, respectively. 0.4 0.6 0.8 1 1.2 1.4 0 100 200 300 400 500 600 s iz e ef fe ct effective thickness, d [mm] b3, t0 = 7 b3, t0 = 28 ec2 aci factor aci coefficient figure 1. size effect on ultimate drying shrinkage evaluated according to selected design codes and prediction models. the ultimate shrinkage is defined by equation ε∞sh = ε∞s e(607) e(t0 + τsh) (2) where ε∞s is a size independent constant depending on concrete composition, curing method and 28-day compressive strength, t0 is the age at the onset of drying, e is the modulus of elasticity and τsh is a shrinkage half-time expressed as τsh = kt (ks ·d)2 (3) where kt is parameter depending on 28-day compressive strength. red color in fig. 1 corresponds to the size effect on drying shrinkage magnitude given by the b3 model. from the figure it is evident that the size effect is observable only in the case of very small specimens while for structural members with realistic dimensions it completely vanishes. the size effect on drying shrinkage indirectly changes with the parameters which are in a certain way related to the evolution of stiffness during the initial phase of drying, especially the onset of drying t0 and the compressive strength f c. however, these parameters only slightly influence the shape of the curve for very small specimens, d < 50 mm, and for d > 100 mm the size effect remains negligible. 2.2. eurocode 2 similarly to the b3 model also the eurocode 2 recognizes the influence of the specimen size on the drying shrinkage rate and on the ultimate value. the specimen shape does not come into the calculation and the specimen size is represented by the notional size defined as h0 = 2ac/u (4) where ac is the area of cross-section and u is the perimeter of the member in contact with ambient environment. this expression coincides with (1) in the case of very (infinitely) long prismatic members. 14 vol. 26/2020 example of an article with a long title the ultimate shrinkage strain is adjusted by factor kh which is a decreasing piece-wise linear function of the notional size. the reference value kh = 1 is specified for h0 = 100 mm, kh = 0.85 for h0 = 200 mm, kh = 0.75 for h0 = 300 mm and kh = 0.7 if h0 reaches or exceeds 500 mm. for smaller notional sizes than 100 mm the code does not provide any guidance. the size effect is shown in fig. 1 by blue dashed line. 2.3. aci 209.2r–08 the size effect on drying shrinkage is in the american standard introduced by means of multiplicative correction terms which affect the ultimate value of shrinkage strain. the evolution of drying shrinkage strain in time can be according to this standard treated by two distinctly different ways. similarly to other standards, the first method prescribes a time function which depends on the v/s ratio; on contrary to this, the second method enables to use constant parameters independently of the size and shape of the concrete member and thus this method gives size-independent results. the correction term reflecting the size effect on the ultimate drying shrinkage is introduced by two different ways, either it is called a coefficient or a factor. in order to be consistent with the preceding sections and the figures, here we slightly modify the original formulae presented in the code, because the aci uses a different definition of the characteristic thickness. the size-dependent correction coefficient is prescribed by a single expression γsh,vs = 1.2 exp (−0.00236d) (5) the correction factor uses a piece-wise linear decreasing function for members with 25 mm < d < 75 mm, for which the value is in the range 1.35–1.00, for larger members 75 mm < d < 190 mm it offers a simple linear expression γsh,d = 1.17 − 0.00228d (6) the correction coefficient and factor are shown in fig. 1 by solid black and dash-dotted black lines, respectively. 2.4. fib model code 2010 identically to ec2, the specimen size is in fib mc 2010 expressed by means of the notional size given by (4). yet, the notional size affects only the shrinkage evolution which becomes faster with decreasing size. the ultimate shrinkage is entirely independent of the specimen size, for this reason the size effect does not need to be illustrated in fig. 1. 3. experimental data the experimental studies in which the evolution of drying shrinkage was measured on specimens with different sizes allow for identification of the size effect 0 100 200 300 400 10 100 1000 s hr in ka ge , ε sh [ 10 -6 ] drying duration, t-t0 [days] d = 31 mm d = 62 mm d = 93 mm d = 311 mm figure 2. the experiment of l’hermite (data adapted from [16]) shows incomplete drying shrinkage of the largest specimen with d = 311 mm. such measurements were not discarded but are denoted by empty symbols in fig. 4. law on drying shrinkage. naturally, the specimens had to be manufactured from the same concrete mixture and stored at the same conditions for sufficiently long period of time to reach the ultimate shrinkage strain. except for the findings reported by samouh [6], keeton [14], and bryant [12], the experimental data investigated in this study are adapted from the creep and shrinkage database developed at the northwestern university [16]. keeton’s original data on total shrinkage was preferred to the database which contains somehow modified values. the database contains 61,512 values measured on 3,308 specimens (1,439 for creep and 1,869 for shrinkage) investigated within 340 experimental surveys (143 creep, 197 shrinkage). for the purpose of the present analysis it was necessary to narrow the shrinkage database only to those studies which examined shrinkage of concrete specimens of (at least two) different sizes made of the same concrete mixture. furthermore, the curing type and duration had to be the same and so had to be the ambient relative humidity. in total 21 experimental surveys met these criteria and from these 14 had to be discarded for some reason, most often because the duration of the experiment did not suffice to obtain stable value of the shrinkage strain of at least two specimens with different size. the setup and basic material properties of the suitable experiments are summarized in table 1. 4. discussion a comparison of the size effect on drying shrinkage is presented in fig. 1. it can be immediately noticed that the european and american standards use different member size as a reference. if both curves are normalized with respect to d = 60 mm, the differences (at least for 100 mm < d < 200 mm) considerably diminish as documented in fig. 5. the eurocode 2 15 lenka dohnalová, petr havlásek acta polytechnica ctu proceedings experimental year shape, d t0 a/c c w/c f c rh tmax study [mm] [day] [-] [kgm−3] [-] [mpa] [%] [day] bryant p [12] 1987 p 50–200 8 5.09 390 0.47 50.0 60 1,950 bryant s [12] 1987 s 100–400 8 5.09 390 0.47 50.0 60 1,880 hansen † [13] 1966 c 46–252 8 6.00 303 0.71 41.3 50 1,750 keeton 20 [14] 1965 c 33/44/65 8 3.74 452 0.46 46.5 20 897 keeton 50 [14] 1965 c 33/44/65 8 3.74 452 0.46 47.5 50 897 keeton 75 [14] 1965 c 33/44/65 8 3.74 452 0.46 46.8 75 897 l’hermite † [15] 1970 p 31–311 1 5.06 350 0.51 na 55 699 mazloom † 2004 c 35/60 7 3.7–4.4 425–500 0.35–0.41 58–70 50 586 samouh i [6] 2017 c 39/57/82 1 4.89 ∗) 320 ∗) 0.59 ∗) 49.6 50 300 samouh ii [6] 2017 c 39/57/82 1 6.03 ∗) 258 ∗) 0.74 ∗) 32.6 50 300 shritharan † 1989 p 44–178 8 5.09 393 0.47 50.1 60 3,492 wittmann † 1991 c 33/64/120 7 5.43 350 0.48 33.2 65 1,105 table 1. details of the experimental setup of the selected shrinkage experiments. the specimen shape is either a prism (p), cylinder (c), or a slab (s), d is the effective thickness, t0 is the onset of drying, c, w, a is the cement, water and aggregates content, respectively, in the concrete mixture. the average compressive strength on cylinders at 28 days is denoted fc, rh is the ambient relative humidity, and tmax is the duration of the experiment (i.e. drying duration). in samouh i and ii, asterisks ∗) indicate that c corresponds to the content of opc, not to the total amount of cementitious material, the data with symbol † are adapted from the nu database [16]. 0 200 400 600 800 0.1 1 10 100 1000 s hr in ka ge , ε sh [ 10 -6 ] drying duration, t-t0 [days] d = 44 mm d = 67 mm d = 89 mm d = 133 mm d = 178 mm sealed figure 3. evolution of drying shrinkage measured by shritharan, the data adapted from [16] serve as an example of experiment in which the shrinkage of all specimens can be considered to have terminated. does not define size effect on drying shrinkage magnitude for d < 100 mm; the value for 60 mm was linearly extrapolated from the interval 100–200 mm. in contrast to the other models, the fib mc 2010 and for d > 100 mm also the b3 model completely neglect the size effect on the ultimate shrinkage. compared to other models, the b3 model is the only model which incorporates the onset of drying into the size effect on the ultimate drying shrinkage. however, this effect is limited only to specimens with d < 30 mm which makes the purpose of the entire proposed and rather complex formula (2) rather questionable. a comparison of the processed experimental data is displayed in fig. 4. the value of shrinkage strain at the end of the experiment was normalized with respect 0.4 0.6 0.8 1 1.2 1.4 60 0 100 200 300 400 n or m al iz ed u lt im at e sh ri nk ag e, ε~ s h, ∞ effective thickness, d [mm] wittmann shritharan l′hermite bryant s bryant p mazloom keeton 20 keeton 50 keeton 75 hansen samouh i samouh ii figure 4. influence of the effective thickness on the normalized experimentally measured ultimate shrinkage strain. the strain is normalized with respect to the value corresponding to d = 60 mm, empty symbols mark the last data points in prematurely terminated experiments. to the value corresponding to d = 60 mm (if missing, the value was obtained by the linear interpolation or linear extrapolation such as in the case of “slabs” specimens from bryant’s experiment). the filled points denote the experiments which were treated as terminal while the empty marks correspond to the experiments which did not last sufficiently long and in which further increase of shrinkage can be expected. the symbol type was selected to resemble the shape of the specimen (circle for a cylinder, square for a prism). as documented by the fig. 4, the experiments show a uniform and almost linearly decreasing trend in the range 30 mm < d < 150 mm. however, there are almost no data for d > 150 mm. this does not infer 16 vol. 26/2020 example of an article with a long title that the experiments above 150 mm do not exist at all but rather the experimental setup did not explore different sizes. a very small size effect on drying shrinkage can be observed in the case of the “slab” specimens tested by bryant. however, the results presented in fig. 4 might be misleading: the smallest specimen size was d = 100 mm while the values are normalized with respect to d = 60 mm. this was done by extrapolation from the experiment with d = 100 mm and d = 150 mm which had almost the same value of shrinkage. similar result offers also l’hermite in whose experiment the specimens started drying already at the age of 1 day but very probably this is not the only reason. on the other hand, the steepest size effect on drying shrinkage was documented by keeton. his experiment also demonstrates that the size effect is humidity independent. yet, there is no experimental evidence which could provide a clue to identification of the size effect in more massive members. it must be noted that the ultimate drying shrinkage of thick members might not be of interest because the design life-time might actually come earlier than the member dries. figure 5 compares the experimental data with the prediction models. the models which do not consider size effect on drying shrinkage — fib mc 2010 and model b3 (above 100 mm) — are evidently incorrect. the best agreement is reached with both alternative formulations in aci 209 which somehow create a lower and upper-bound to almost all experimental data. since the eurocode 2 provides a recommendation only for d > 100 mm where very few experimental data are present, it is difficult to assess its performance. however, the slope of the ec2 curve in fig. 5 is similar to the aci 209 which provides a good agreement. unfortunately, the newly prepared version of eurocode 2 [17] which origins from the model code 2010 omits the size-dependent coefficient kh from the formulation and thereby completely neglects the size effect on the ultimate drying shrinkage. this will lead to more conservative but less economic design. 5. conclusions the presented study analyzed the size effect on the ultimate drying shrinkage of concrete and compared the processed experimental data to the recommendations and codes of practice. the following conclusions can be drawn: • according to the currently valid codes of practice, namely the eurocode 2 and aci 209.2r-08, the ultimate drying shrinkage is decreasing with increasing size (effective thickness). it can be stated that for members with effective thickness d between 100 mm and 200 mm this size effect is captured similarly. 0.4 0.6 0.8 1 1.2 1.4 60 0 100 200 300 400 s iz e ef fe ct effective thickness, d [mm] b3 ec2 aci factor aci coefficient figure 5. comparison of the size effect according to the design codes and recommendations (curves) with the experimental data (filled black symbols = finished shrinkage, empty grey symbols = prematurely terminated experiments). the functions and the data are normalized with respect to the size d = 60 mm. • as described by the b3/4 models the size effect is a function of the age when concrete starts drying and is very pronounced only for specimens with d < 50 mm, otherwise the size effect almost vanishes, similarly to the fib model code 2010 which ignores this size effect completely (and very probably so will do the new ec2). • the experimental database [16] contains 6 suitable experimental studies which together with 2 additional studies from the literature provide consistent results confirming the presence of size effect on the ultimate drying shrinkage. none of the experimental studies exhibits opposite or no size effect on the drying shrinkage magnitude. however, it must be noted that sound experimental data for d > 150 mm are missing and thus cannot provide any evidence above this limit. • for d in the range from 20 mm to 150 mm both approaches of the aci 209 code exhibit by far the best agreement with the experimental data. • the experimental data on the ultimate shrinkage of specimens with d > 200 mm are not available and very likely will not be available in the near future. in opinion of the present authors, the code-like expressions should not be constructed by a simple extrapolation beyond the experimentally explored range. more preferably, these expressions should origin from the fem simulations exploiting properly calibrated and physically based material model which will perform well in the well-documented range 20 mm < d < 150 mm. acknowledgements the authors gratefully acknowledge financial support from the grant agency of the czech technical university in prague, project number sgs19/032/ohk1/1t/11, and from the czech science foundation, project number 19-20666s. 17 lenka dohnalová, petr havlásek acta polytechnica ctu proceedings references [1] american society of testing and materials. astm c157, standard test method for length change of hardened hydraulic-cement mortar and concrete. 2008. [2] international organization for standardization. iso 1920-8 testing of concrete part 8: determination of drying shrinkage of concrete for samples prepared in the field or in the laboratory. 2009. [3] c. aguilar. estudio del comportamiento y desarrollo de una metodologia de prediccion de la retraccion hidraulica de hormigones, phd. thesis (in spanish). pontificia universidad catolica de chile. escuela de ingenieria, 2005. [4] z. bažant, m. jirásek. creep and hygrothermal effects in concrete structures. springer netherlands, 2018. doi:10.1007/978-94-024-1138-6. [5] j. m. torrenti, f. benboudjema. desiccation shrinkage of large structures: is there a size effect? pp. 404–411. 2013. doi:10.1061/9780784413111.048. [6] h. samouh, e. rozière, a. loukili. influence of specimen size and measurements duration on the long-term extrapolation of drying shrinkage. construction and building materials 150:276 – 286, 2017. doi:10.1016/j.conbuildmat.2017.06.009. [7] aci committee 209. guide for modeling and calculating shrinkage and creep in hardened concrete. aci, 2008. [8] european committee for standardization. en 1992-1-1 eurocode 2: design of concrete structures–part 1-1: general rules and rules for buildings. cen, brussels, 2005. [9] z. bažant, s. baweja. creep and shrinkage prediction model for analysis and design of concrete structures: model b3. adam neville symposium: creep and shrinkage structural design effects 2000. [10] rilem technical committee tc-242-mdc (z.p. bažant, chair). model b4 for creep, drying shrinkage and autogenous shrinkage of normal and high-strength concretes with multi-decade applicability (rilem technical committee tc-242-mdc multi-decade creep and shrinkage of concrete: material model and structural analysis). materials and structures 48:753–770, 2015. [11] fédération internationale du béton. model code 2010. no. vol. 65 in fib bulletin. international federation for structural concrete (fib), 2012. [12] a. h. bryant, c. vadhanavikkit. creep, shrinkage-size, and age at loading effects. aci materials journal 84:117–123, 1987. [13] t. hansen, a. mattock. influence of size and shape of member on the shrinkage and creep of concrete. journal of the american concrete institute 63-10:267–290, 1966. [14] keeton. study of creep in concrete, phase 1 (i-beam). u. s. naval civil engineering laboratory, port hueneme, california, 1965. [15] r. l’hermite, m. mamillan, c. lefevre. nouveaux résultats de recherches sur la déformation et la rupture du béton. annales de l’institut technique du bâtiment et des travaux publics 18(207-208):323–360, 1965. [16] m. hubler, r. wendner, z. bažant. comprehensive database for concrete creep and shrinkage: analysis and recommendations for testing and recording. aci 112:547–558, 2015. [17] european committee for standardization. eurocode 2: design of concrete structures–part 1-1: general rules, rules for buildings, bridges and civil engineering structures, final version of pt1-draft pren 1992-1-1 2018 d3. 2018. 18 https://doi.org/10.1007/978-94-024-1138-6 https://doi.org/10.1061/9780784413111.048 https://doi.org/10.1016/j.conbuildmat.2017.06.009 acta polytechnica ctu proceedings 26:1–6, 2020 1 introduction 2 design codes and prediction models 2.1 models b3 and b4 2.2 eurocode 2 2.3 aci 209.2r–08 2.4 fib model code 2010 3 experimental data 4 discussion 5 conclusions acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2020.28.0023 acta polytechnica ctu proceedings 28(0):23–31, 2020 © czech technical university in prague, 2020 available online at http://ojs.cvut.cz/ojs/index.php/app methods of xs data preparation for geometry with fuel dummy pavel suk czech technical university in prague, faculty of nuclear science and physical engineering, department of nuclear reactors, v holešovičkách 2, 180 00 prague 8, czech republic correspondence: sukpave2@fjfi.cvut.cz abstract. 3d deterministic core calculation represents important category of the nuclear fuel cycle and safe nuclear power plant operation. the appropriate solution was not published yet. data preparation process for non-fuel elements of the core represents the challenge for scientists. this report briefly introduce the problem of the data preparation process and gives the information about new input format for macrocode parcs (pmaxs). the best homogenization process approach is to prepare data in infinite lattice cell for fuel assemblies, which are placed next to the another fuel assembly. data for fuel assembly located next to the non-fuel region are better with preparation in the real geometry with the real boundary conditions. results of the neutron spectra study show that the pmaxs file format is well prepared for the 2 group calculation, but it is not well prepared for the multigroup calculations, however the xsec file format still gave reasonable results. keywords: eigenvalue, flux, fuel dummy, homogenization, scale, serpent, parcs. 1. data preparation process and reflector calculation nuclear power plant (npp) cores are in general too large to calculate them with exact approach of the transport equation in acceptable calculation time. based on this fact, the data preparation process and full core calculation via deterministic macrocodes are still remaining comprehensive parts of the fuel loading patterns preparation. these two processes are, in general, connected together, but in the reality, the complexity of this problem is replaced by the easier and faster approach. this approach is based on the separation of the data preparation process for each part of the core and 3d full core deterministic calculation. the research reactor cores are sometimes also calculated with the simplified methods. especially the fuel loading pattern in the high power research reactors can be designed via simplified homogenization approach. the total neutron-termohydraulic solvers are also based on this methodology. therefore the homogenization process is also important during the analysis of research reactors. the data preparation process for the fuel elements is historically based on the exact transport simulation of the fuel assembly with reflective boundary conditions [1]. the main complication of this approach is fact that the fuel assemblies are not in the infinite medium environment during the 3d calculation. the reactor core does not consist of the same fuel assemblies, some flux tilt inside the fuel assemblies can be presented. some advantages of this approach can be found in the rehomogenization process, which is described in papers [2] or [3]. the research reactors and npps are almost always operated in the critical state, but during the data preparation process, the infinite medium of the the fuel assemblies can be sub-critical or super-critical in case of their enrichment and geometry. some solution of these issues can be found in the b1 approximation, described bellow or in the article [1]. the comparison of b1, p1 and casmo methods can be found in the paper [1]. the data preparation process for non-fuel elements represents more challenge problem. the optimal solution of that problem has not been found yet, however some approaches are developed for data preparation. the main complication of the data preparation process for the non-fuel elements is based on the non-leakage medium and impossibility to calculate exact neutron spectrum during the data preparation process. the most codes are based on the diffusion solver, but the diffusion theory is not met near the boundaries [4]. data for non-fuel regions, or reflector regions can be prepared by various approaches. for instance 1d approach or more dimension approach [5]. for better criticality and power distribution prediction, the macroscopic data are sometimes optimized. the main complication of this approach is in the high calculation time. next complication of the fuel loading pattern optimization is in the uniqueness of the pattern. the optimization should to be made for each pattern separately. the optimization method can be found, for instance, in the paper [6]. numerous details of data preparation for nodal calculation codes can be found also in the paper [7]. the author explain complications of the b1 method, diffusion coefficient calculation, and spatial homogenization for data preparation process there. 23 http://dx.doi.org/10.14311/app.2020.28.0023 http://ojs.cvut.cz/ojs/index.php/app pavel suk acta polytechnica ctu proceedings 1.1. macroscopic cross section preparation macroscopic cross sections (xs) are, in general, quantities which provide information about the influence of that part of the core to the calculation. xs are defined in simple approach: σi,g,r = ni · σi,g,r, (1) where ni is atomic density of the region i and σi,g,r is microscopic cross section of region i, energy group g, and reaction r. the more rigorous approach is based on the type of the calculation code. the lattice codes can be divided into the discrete energy codes and continuous energy codes. in these two cathegories, the macroscopic cross sections are generated: σi,g,r = ∑eg,max h=eg,min ni · σi,h,r · φi,h∑eg,max h=eg,min φi,h , (2) for lattice codes which calculated with the discrete energies. φi,h is the neutron flux inside region i and energy group h. for the continuous energy calculation codes, the macroscopic cross sections are calculated via: σi,g,r = ∫ eg,max eg,min ni · σi,r(e) · φi(e)de∫ eg,max eg,min φi(e)de , (3) where σi,r(e) is the microscopic cross section of the region i, reaction r, and energy e and φi(e) is the neutron flux inside region i, energy e. 1.2. b1 correction b1 correction, also called critical spectra correction, is a method used for treating the leakage of neutrons in the fuel assemblies places in the real core pattern. as was mentioned above, the 3d deterministic calculations are based on the two level approach. the macroscopic data for fuel assemblies are prepared for the fuel assemblies in the infinite lattice of the same assemblies, thus, the neutron leakage from the fuel assembly is zero. the b1 method prepares data based on the calculation of critical spectra in the fuel assembly and due to it treats the non zero neutron leakage. the main idea of the b1 approximation is to find appropriate buckling factor and thus appropriate neutron spectra for xs preparation. the b1 corrected homogenised xs are then prepared with the new neutron spectra. the buckling factor adds or removes additional neutron leakage in the fuel assemblies based on the equation: σt,gφg ± ibjg = χg ∑ h σf,hφh + ∑ h σ0 s,h→gφh, (4) where σt,g is the total macroscopic cross section in energy group g, φg is the neutron flux in energy group g, b is the buckling factor, jg is the neutron current in energy group g, χg is the neutron spectra in the energy group g, σf,h is the fission macroscopic cross section in energy group h and σ0 s,h→g is the zero moment of scattering macroscopic cross section in energy group g. the critical spectra is to find iteratively by increasing buckling factor. the first step is based on the calculation with b2 = 0, the second step is based on the calculation with b2 = 10−6 and the next steps are calculated with extrapolated buckling factor from the previous calculation. the whole method is well described in the paper [1] and [7]. when the fuel assemblies in the infinite lattice have the eigenvalue less than 1, the neutron leakage is decreasing, when the eigenvalue is larger than 1, the leakage is increasing, in practical sense. 2. calculation codes calculation codes are essential for nuclear reactor operation. the codes can be divided into two main groups, macrocodes and microcodes (lattice codes). macrocodes are used for the full core calculations with diffusion or simplified transport solution [8]. microcodes are used for exact full core transport calculation or data preparation for macrocodes. microcodes can be than divided into the deterministic codes [9] and stochastic codes [10]. parcs parcs calculation code is developed at purdue university [11] for 3d npp core calculations. parcs is purdue advanced neutron core simulator code which offers various calculation methods based on the diffusion solver or simplified transport solution (sp3). some of the solvers are based on the finite difference methods, others are based on the nodal methods. [8] in the study presented below, the finite difference method based on the diffusion solver (fdm, nemmg) or simplified transport solver (sp3) are analysed. all cases were calculated with parcs v3.3.1 code version. scale scale is a comprehensive calculation package which contain deterministic codes (newt, triton) and a stochastic code (keno) [9]. the deterministic codes newt and triton are commonly used for the data preparation. both these codes are based on the solution of the transport equation in multigroup approach. xs can be prepared with actual flux spectrum or with b1 approximation. triton code is able to carried out burnup calculations in contrast with newt. all cases were calculated with scale 6.2.3 code version [12]. 3. test case the model consisted of the simplified fuel assemblies and simplified fuel dummy filled with water was developed as a test case. this model is not usual, however 24 vol. 28/2020 xs data it includes region for that the data preparation is complicated. the visualisation of the test case is shown in figure 1. this geometry can be found in the research reactors, where the water can be replaced by the research equipment like irradiation channels, detectors or other devices. figure 1. visualisation of the test case model test case consists of the 3 elements types. the whole structure is placed in the infinite lattice of the same structures. fuel assembly 1 (fa1) seems to be like a fuel assembly in the infinite lattice of the same assemblies, because each boundary is connected to the fuel assembly. fuel assembly 2 (fa2) has three sides connected with the fuel assembly and one side is connected to the water fuel dummy. water fuel dummy (dum) is simulated with the same temperature and density conditions as the moderator inside the fuel assemblies (ρ = 0.71667 g/cm3, t = 578 k). fuel assemblies consist of 17×17 fuel pins. description of the material and geometry of the test case is given in table 1. two different enrichments of the fuel pins were simulated: 0.7% sub-critical system and 4.3% super-critical system. test case parameters fuel diameter 0.82 cm cladding diameter 0.95 cm pin pitch 1.26 cm fuel density 10.219 g/cm3 moderator density 0.71667 g/cm3 fuel temperature 1100 k cladding temperature 600 k moderator temperature 578 k table 1. description of the material and geometry of the test case 4. calculation many different approaches were realised during the analysis of behaviour in the test case. in the first stage, the influence of b1 approximation and noncritical flux were analysed. the next part of the study was focused on the downscatter treatment. last part of the study deals with the multigroup approach and spectra comparison. some of the diffusion codes are designed to use only downscattering, therefore the scattering cross sections are modified via: σ̂g,h = σg,h − φh φg σh,g, (5) where the σg,h, respective σh,g is scattering cross section from group g to group h, respective from h to g. φh, respective φg is the neutron flux of group h, respective g. the compliance of the simplified solution calculated via parcs was rated by the eigenvalue compliance, see equation (6) and the neutron flux values inside homogenised areas, see equation (7). the reference solution was obtained by the whole geometry simulation in the scale 6.2.3 calculation code. ∆keff = (keff,scale − keff,parcs) · 105 (6) ∆φg = √√√√ 1 n n∑ i=0 ( φi g,scale − φi g,parcs φi g,scale 100 )2 (7) 4.1. two group approach the two group xs were prepared with the scale triton calculation code in this section. the data for macrocode parcs can be prepared by 2 ways, with pmaxs files and xsec files. the data from scale triton calculation code was prepared by the genpmaxs [13] program, which prepares pmaxs files for each region. the obtained results with pmaxs files showed good agreement, therefore the xsec file format was not used during two group analyses. many calculation modes and homogenization approaches were carried during this analysis: • case1 data for each different part of model was prepared by simulation in the real geometry with the real boundary conditions with b1 correction. • case1_cor case1 with downscatter correction via equation (5). • case2 data for the fuel assemblies was prepared by simulation with the reflective boundary condition, data for dum was prepared by simulation in the real geometry with the real boundary conditions with b1 correction. • case2_cor case2 with downscatter correction via equation (5). • case3 data for fa1 was prepared by simulation with the reflective boundary condition, other data was prepared by simulation in the real geometry with the real boundary conditions with b1 correction. 25 pavel suk acta polytechnica ctu proceedings • case3_cor case3 with downscatter correction via equation (5). • case4 case1 without b1 correction. • case5 case2 without b1 correction. • case6 case3 without b1 correction. • case7 data for fuel assemblies was prepared by the simulation with the reflective boundary condition with b1, data for dum was prepared by the simulation in the real geometry with the real boundary conditions without b1 correction. • case8 data for fa1 was prepared by the simulation with the reflective boundary condition with b1, data for other regions were prepared by the simulation in the real geometry with the real boundary conditions without b1 correction. 4.1.1. 0.7 % enrichment fuel enriched only with 0.7% was simulated to investigate agreement for the sub-critical core regions. the results of eigenvalue calculation and relative flux differences are shown in table 2, visualised in figures 2 and 3. modes ∆keff (pcm) ∆φ1 (%) ∆φ2 (%) case1 51.8 4.52 1.85 case1_cor 71.7 4.56 1.92 case2 91.4 3.47 1.40 case2_cor -46.0 8.60 1.92 case3 8.6 4.23 1.67 case3_cor 26.0 4.26 1.73 case4_cor 189.5 7.92 2.61 case5_cor 183.7 6.15 2.00 case6_cor 127.6 7.34 2.30 case7_cor 66.3 4.11 1.15 case8_cor 59.1 6.15 2.00 table 2. two group results of the the test case with fuel enrichment 0.7% (reference keff = 0.82853) -50 -25 0 25 50 75 100 125 150 175 200 case1 case1_cor case2 case2_cor case3 case3_cor case4_cor case5_cor case6_cor case7_cor case8_cor k e ff d iff er en ce ( pc m ) keff relative difference in each case for 0.7 % enrichment fuel type figure 2. relative eigenvalue differences in the calculation modes with 0.7% fuel enrichment test case in case of eigenvalue, the case1, case2_cor, case3 and case3_cor were in the best agreement with the lattice code. on the opposite, the cases without b1 correction (case4, case5 and case6) were 0 1 2 3 4 5 6 7 8 9 case1 case1_cor case2 case2_cor case3 case3_cor case4_cor case5_cor case6_cor case7_cor case8_cor p di ff er en ce ( % ) relative power difference in each case for 0.7 % enrichment fuel type g1 g2 figure 3. relative power differences in the calculation modes with 0.7% fuel enrichment test case 1 4 -2.19 0.49 4 5 -1.88 0.14 7 6 -2.19 0.49 2 3 -1.88 0.14 5 10 -11.30 -4.89 8 7 -1.88 0.14 3 2 -2.19 0.49 6 1 -1.84 0.15 9 8 -2.19 0.49 1 2 3 1 2 3 legend n type group 1 group 2 figure 4. 2d flux differences for each part of the test case in case3 and enrichment 0.7% in the worse agreement. the neutron flux comparison showed that the thermal neutron group deviates around 2% in all cases, but the fast neutron group flux difference reached up to 9% in case2_cor, case4_cor and case6_cor. the best results in case of eigenvalue and φg are in case3. 2d flux differences of case3 are shown in figure 4. figure 4 shows the best agreement in the thermal group (only 0.49%) for fuel assemblies and only 2% deviation in the fast energy group. the neutron fluxes in the fuel dummy region inside the test case were determined with worse agreement. up to 5% for thermal energy group and up to 11.5% for the fast energy group. 4.1.2. 4.3 % enrichment the nuclear fuel in the npp cores is in general enriched up to 5%. the fuel pellets with enrichment 4.3% are usually in the active length of the fuel pins and, moreover, they make the simulated system supercritical. the results of the eigenvalue and the relative flux differences are shown in the table 3, visualised in figures 5 and 6. the best agreement in case of eigenvalue were reached in the cases without b1 correction (case4_cor, case5_cor, and case6_cor). on the other hand, the fast neutron flux differences 26 vol. 28/2020 xs data modes ∆keff (pcm) ∆φ1 (%) ∆φ2 (%) case1 688.6 5.17 4.89 case1_cor 690.6 5.14 4.97 case2 767.6 5.43 4.74 case2_cor 730.6 5.49 4.87 case3 625.6 5.16 4.81 case3_cor 626.6 5.14 4.89 case4_cor 173.6 7.82 4.61 case5_cor 121.6 9.61 4.52 case6_cor 63.6 8.23 4.41 case7_cor 792.6 5.15 5.84 case8_cor 418.6 5.93 6.36 table 3. two group results of the test case with fuel enrichment 4.3% (reference keff = 1.32588) 0 100 200 300 400 500 600 700 800 case1 case1_cor case2 case2_cor case3 case3_cor case4_cor case5_cor case6_cor case7_cor case8_cor k e ff d iff er en ce ( pc m ) keff relative difference in each case for 4.3 % enrichment fuel type figure 5. relative eigenvalue differences in the calculation modes with 4.3% enrichment fuel test case 0 1 2 3 4 5 6 7 8 9 10 11 case1 case1_cor case2 case2_cor case3 case3_cor case4_cor case5_cor case6_cor case7_cor case8_cor p di ff er en ce ( % ) relative power difference in each case for 4.3 % enrichment fuel type g1 g2 figure 6. relative power differences in the calculation modes with 4.3% enrichment fuel test case were the highest in that three cases. the 2d flux differences of case6_cor is shown in figure 7. case6_cor model showed the best result in case of eigenvalue, with difference only 63 pcm. figure 7 shows relatively good agreement in thermal neutron flux in fuel assemblies and fast neutron flux in dum, only up to 3%. fast neutron flux in fuel assemblies and thermal neutron flux in the dum were determined with worse agreement, up to 11%. 1 4 8.63 -2.28 4 5 8.76 -3.21 7 6 8.63 -2.28 2 3 8.76 -3.21 5 10 -2.08 -10.62 8 7 8.76 -3.21 3 2 8.63 -2.28 6 1 8.78 -3.23 9 8 8.63 -2.28 1 2 3 1 2 3 legend n type group 1 group 2 figure 7. 2d flux differences for each part of the test case in case6_cor and enrichment 4.3% 4.1.3. discussion the above results showed that the data preparation based on the actual boundary condition cannot bring good results, because the parcs calculation code does not know the actual neutron leakage from the system and due to it there is lack of the informations to properly simulate the system. the best agreement in the lower enriched case was obtained with the b1 correction. this situation corresponds with the physical solution. if the solved problem is sub-critical, the b1 approximation simulates less neutron leakage to reach criticality during the data preparation process. because the each part of the test case is "locally" sub-critical, it means that the neutron spectra is changed in each part with the same direction and due to that the results are better with b1 approximation. the situation in the higher enriched case is different. the whole test case is super-critical, but the water fuel dummy is "locally" sub-critical. the b1 correction increases neutron leakage from the whole system, but the water fuel dummy is "locally" sub-critical. the b1 correction changes the neutron spectra in both regions with the same direction and due to that the neutron spectra in the water fuel dummy is very different during the xs preparation. the better results were obtained without b1 correction and it also corresponds with the above theory. 4.2. multigroup approach to better understand the behaviour of the water-fuel neutron transport in the macrocodes, there was pressure to analyse and compare multigroup neutron flux in the parcs and scale code systems. the fuel assembly with the mirror boundary conditions was analysed in the multi group approach. the neutron flux is constant in the assembly with the mirror boundary condition and due to it only eigenvalue was analysed. as it was told before, parcs calculation code offers to use two types of libraries structures (pmaxs and xsec). the eigenvalues were calculated with many different ways: • xsec xsec without downscatter correction, • xsec cor xsec with downscatter correction, 27 pavel suk acta polytechnica ctu proceedings -400 -350 -300 -250 -200 -150 -100 -50 0 50 100 150 200 250 2 4 8 16 23 40 56 r el at iv e di ff er en ce ( pc m ) group number keff relative difference between scale and parcs xsec corr b1 xsec b1 xsec corr without b1 xsec without b1 figure 8. keff relative deviation with xsec input files • pmaxs pmaxs without downscatter correction, • pmaxs cor pmaxs with downscatter correction. results of the simulations are in table 4. the pmaxs input format brings better results in two group approach, but the large deviations from the reference solution was found in many energy group analysis. the reference solution of the test case geometry was calculated with scale triton sequence keff = 0.86041. it seems that the pmaxs input format is well prepared for two group calculations, which are most common in the full core calculations, but it is absolutely inappropriate for multi groups calculation. the xsec input format seems to be more appropriate for multigroup analysis. there are only small deviations between diffusion solution and appropriate transport solution. the highest deviation with the b1 correction is in case of xsec for two group calculation. this deviation is caused by the fact, that parcs was used in the diffusion mode and the inputs scatter matrix is limited into the downscatter from fast to thermal group. due to that fact, the two group solution needs to be corrected by downscattering via the equation (5). when the downscatter was corrected, the results were similar to those obtained with pmaxs cor. this situation can be seen in both simulations (with b1 and without b1 approach). deviations between the xsec and xsec cor for more group approach are smaller, because for more group calculations, there are total scatter matrix input data. due to it there is no necessary to use correction via equation (5). based on this fact, the xsec without downscatter correction input format is used for other more group calculations. the relative difference of keff between scale and parcs for xsec input format is in figure 8. the convergence problem with the calculation were found in case of higher group number. the calculation did not converge automatically, but after it reached maximum number of iteration, it finished and gave a result without any warning message. the influence of b1 correction decreases with higher number of energy groups. this behaviour begins during the data preparation process by microcode scale triton, because the group structure for the data preparation process was 56 group endf/b-vii.0 library and microcode scale prepares critical spectra in 56 groups. when user wants to calculate xs with b1 leakage correction in 56 groups, the collapsing to the energy groups is independent on the neutron spectra. 4.3. neutron flux spectra comparison the neutron spectra was created to better understand the behaviour in the fuel regions near to the water fuel dummy region. the spectra was calculated with 56 energy groups. there was also the same convergence complications as were described in the previous sections. because of the results of two group calculation, there was analysed only three cases without b1 leakage correction (case4, case5 and case6). the data for scattering were not corrected for downscattering, because the results from section 4.2 showed that the downscattering correction is not necessary. the neutron flux spectra comparison gives the information about the spectra and slowing down in the scale triton and parcs codes. the spectra were analysed separately in fuel regions and in the dum region. 4.3.1. 0.7 % enrichment the test case with 0.7% fuel enrichment represents the sub-critical problem. the neutron spectra in the fuel assembly fa1, respective in the dum region are in figures 9, respective 10. the neutron flux spectra 28 vol. 28/2020 xs data groups with b1 without b1 pmax cor pmax xsec cor xsec pmax cor pmax xsec cor xsec 2 -38 -47 -38 -389 25 25 25 -323 3 -2296 -2307 -93 -106 -1950 -1950 229 228 4 -2168 -2174 40 33 -1952 -1952 231 230 8 -2192 -2195 -7 -10 -1954 -1953 206 207 16 -2110 -2112 77 74 -1955 -1955 205 205 23 -31510 -31512 112 117 -31479 -31479 141 147 40 -31495 -31496 63 68 -31479 -31479 76 83 56 -31985 -31987 -21 -20 -31987 -31987 -21 -20 table 4. keff relative difference between scale and parcs with multigroup approach for 0.7 % enriched fuel assembly with mirror boundary condition 1x10-12 1x10-10 1x10-8 1x10-6 0.0001 0.01 0.01 0.1 1 10 100 1000 10000 100000 1x10 6 1x10 7 fl ux ( cm -2 s1 ) energy (ev) neutron spectra in fa1 scale case4 case5 case6 figure 9. neutron flux spectra in the fuel assembly fa1 with 0.7% fuel enrichment 1x10-12 1x10-10 1x10-8 1x10-6 0.0001 0.01 0.01 0.1 1 10 100 1000 10000 100000 1x10 6 1x10 7 fl ux ( cm -2 s1 ) energy (ev) neutron spectra in dum scale case4 case5 case6 figure 10. neutron flux spectra in the dum with 0.7% fuel enrichment in fa1 and fa2 are similar and the differences are not visible in the spectra comparison. the eigenvalues and their differences from the reference calculation are in table 5. main result from figures 9 and 10 is that the fuel spectrum in the fuel assemblies is well calculated with the parcs calculation code. differences between the parcs and scale calculation were analysed via equation: ∆φg = φscale g − φparcs g φscale g (8) the neutron spectra differences in figures 11, 12 -14 -12 -10 -8 -6 -4 -2 0 2 4 0.01 0.1 1 10 100 1000 10000 100000 1x10 6 1x10 7 flu x di ff er en ce ( % ) energy (ev) neutron spectra diferences between scale and parcs in fa1 parcs case4 parcs case5 parcs case6 figure 11. relative difference in spectra between scale and parcs calculation for fa1 for 0.7% fuel enrichment -2 0 2 4 6 8 10 12 0.01 0.1 1 10 100 1000 10000 100000 1x10 6 1x10 7 flu x di ff er en ce ( % ) energy (ev) neutron spectra diferences between scale and parcs in fa2 parcs case4 parcs case5 parcs case6 figure 12. relative difference in spectra between scale and parcs calculation for fa2 for 0.7% fuel enrichment and 13 show that the spectrum calculated in the fa1 has the large deviations in the slowing down region and particularly in the high energy neutron region. the fa2 has great deviations only in the high energy neutron region. in the fuel assemblies, the relative deviations are only up to 12% in the absolute values, but the deviations in the dum region are up to 225%. that means that the data prepared for the dum region are not prepared well and the behaviour of neutrons near the dum region is not described correctly. 29 pavel suk acta polytechnica ctu proceedings -250 -200 -150 -100 -50 0 50 100 0.01 0.1 1 10 100 1000 10000 100000 1x10 6 1x10 7 flu x di ff er en ce ( % ) energy (ev) neutron spectra diferences between scale and parcs in ref parcs case4 parcs case5 parcs case6 figure 13. relative difference in spectra between scale and parcs calculation for dum for 0.7% fuel enrichment 4.3.2. 4.3 % enrichment fuel pins with 4.3% enrichment represent the supercritical problem, which, based on the information obtained during the two group study, can brings more deviated results. the eigenvalue differences are in table 5. the neutron spectra were similar to the neutron spectra in the lower enrichment case with little bit higher discrepancies. mode ∆keff 0.7% (pcm) ∆keff 4.3% (pcm) case4 120 923 case5 24 783 case6 61 837 table 5. keff relative difference between scale and parcs with 56 groups calculation 4.4. sp3 multigroup solution the parcs calculation code is able to calculate simplified transport approach (sp3) instead of the diffusion approach. the nemmg solver with the nspn 3 was used to calculate the test geometry. the only appropriate approach of case4, case5 and case6 were calculated. the results of the eigenvalue for both enrichment cases are in table 6. mode ∆keff 0.7% (pcm) ∆keff 4.3% (pcm) case4 -52 732 case5 -148 590 case6 -111 646 table 6. keff relative difference between scale and parcs with 56 groups calculation and sp3 the neutron spectra for sp3 calculations are very similar to the neutron spectra calculated with diffusion approach. the main difference is in the higher energies in the fa2 fuel assembly with 0.7% enrichment. the neutron spectra are more accurate in the higher energies in comparison with the diffusion approach for higher enriched fuel. the eigenvalue deviations increased in comparison to the diffusion approach in lower enrichment case, but deviation decreased with higher enrichment case. 5. conclusion various different approaches were analysed in this paper. two different enrichments were studied. results of the two group calculation showed that in case of lower enrichment, the data prepared by the simulation of the fa1 fuel assembly in the infinite lattice and fa2 and dum in the real geometry with b1 approximation (case3) well reproduce the eigenvalue. the relative flux differences were also calculated with the smallest difference in the case3. on the other hand, the cases, which were calculated without b1 approximations gave more-times worse results in case of lower enrichment. the results were opposite in the case of higher enrichment than in the lower enrichment calculation. the best agreement were obtained during the calculation without b1 approximation and the same data preparation scheme (case6). the eigenvalue was bellow 100 pcm from reference calculation, but the fast group neutron fluxes were calculated with higher difference (up to 10%) in each calculation without b1 approximation. the upper results are consistent with b1 correction. in the case of lower enrichment, the situation corresponds with the physical solution. the whole geometry is sub-critical and b1 approximation decreases the neutron leakage to reach criticality during the data preparation process. in case of higher enrichment, the whole geometry is super-critical, but there is part which is "locally" sub-critical dum. the b1 approximation increases neutron leakage from the system to reach the criticality, but this does not correspond with the physical solution in the dum part. the neutron spectra is changed there and due to that the b1 correction cannot brings better results in the case of higher enrichment. the main objective of the spectra study is that pmaxs files, which parcs use newly, are not able to calculate core with multigroup approach. the eigenvalue differences showed that there is any problem with data preparation for that calculation. the results of calculations with xsec file format bring reasonably results of eigenvalue despite the convergence problems. based on the results of multigroup study, the neutron spectra in multigroup approach were calculated for test cases without b1 approximation. calculations withoutb1 approximationwere chosenbecause the final geometry was not critical and there are still unsolved problemswithb1approximation. thepaper [7] informs that the b1 calculation is not rigorous and due to the reaching critical spectra by the changing absorption in each energy group, the result are not representative for lwr calculations, where the criticality is reached by the absorption of only thermal neutrons. the neutron spectra in the fuel assemblies were determined with a good agreement with the reference 30 vol. 28/2020 xs data calculation, but the spectra in the dum was determined with larger discrepancies. it is well known, that the diffusion theory is not fulfilled in the water fuel dummy region and the discrepancies are mostly based on this fact. there are also methods, which can bring better results, for instance the influence of neutron scatter anisotropy in the dum. the different approach of the dum preparation data will be study in the future. 6. acknowledgement this work was supported by the project no. cz.02.1.01/0.0/0.0/16_013/0001790 supported byoperational programme research, development and education co-financed from european structural and investment funds and from state budget of the czech republic and it was supported by the grant agency of the czech technical university in prague, grant no. sgs19/114/ohk4/2t/14. list of symbols φi(e) neutron flux of region i and energy e [cm−2s−1] φi,g neutron flux of region i and energy group g [cm−2s−1] σi,g microscopic cross section of region i and group g [cm2] σi(e) microscopic cross section of region i and energy e [cm2] σi,g macroscopic cross section of region i and group g [cm−1] ni atomic density of region i [cm−3] eg, min minimal energy of boundary related to the energy group g [ev] eg, max maximal energy of boundary related to the energy group g [ev] n number of regions in the test case [–] σt,g total macroscopic cross section for energy group g [cm−1] b buckling factor [cm−1] jg neutron current in group g [cm−2s−1] χg neutron spectra in group g [–] σf,h fission macroscopic cross section [cm−1] σ0 s,h→g scattering macroscopic cross section [cm−1] keff,x eigenvalue calculated with mode x [–] ∆keff difference of the eigenvalue [pcm] ∆φg relative difference of φg for the whole test case [%] references [1] s. choi, k. s. smith, h. kim, et al. on the diffusion coefficient calculation in two-step light water reactor core analysis. journal of nuclear science and technology 54(6):705–715, 2017. doi:10.1080/00223131.2017.1299648. [2] a. dall’osso. a spatial rehomogenization method in nodal calculations. annals of nuclear energy 33:869–877, 2006. doi:10.1016/j.anucene.2006.05.008. [3] p. suk. advanced homogenization methods for pressurized water reactors. acta polytechnica ctu proceedings 19:14, 2018. doi:10.14311/app.2018.19.0014. [4] p. reus. neutron physics. 2008. isbn: 2759800415. [5] k. i usheva, s. kutsen, a. a khruschinsky, l. babicheu. generation of xs library for the reflector of vver reactor core using monte carlo code serpent. journal of physics: conference series 781:012029, 2017. doi:10.1088/1742-6596/781/1/012029. [6] t. clerc, a. hébert, h. leroyer, et al. an advanced computational scheme for the optimization of 2d radial reflector calculations in pressurized water reactors. nuclear engineering and design 273:560 – 575, 2014. doi:10.1016/j.nucengdes.2014.03.044. [7] k. s. smith. nodal diffusion methods and lattice physics data in lwr analyses: understanding numerous subtle details. progress in nuclear energy 101:360 – 369, 2017. special issue on the physics of reactors international conference physor 2016: unifying theory and experiments in the 21st century, doi:10.1016/j.pnucene.2017.06.013. [8] t. downar, y. xu, v. seker. parcs v3.0 u.s. nrc core neutronics simulator theory manual. res/u.s. nrc, rockville, md, 2012. [9] b. t. rearden, m. jessee. scale code system. ornl/tm-2005/39, 2016. [10] j. leppänen. serpent a continous-energy monte carlo reactor physics burnup calculation code. vtt technical research centre of finland, 2015. [11] purdue advanced reactor core simulator. purdue university accessed: 2019-05-14. https://engineering.purdue.edu/parcs. [12] b. t. rearden, m. jessee. scale code system. ornl/tm-2005/39, 2018. [13] a. ward, t. downar, y. xu. genpmaxs v6.2 code for generation the parcs cross section interface file pmaxs. res/u.s. nrc, rockville, md, 2016. http: //nuram.engin.umich.edu/software/genpmaxs/. 31 https://doi.org/10.1080/00223131.2017.1299648 https://doi.org/10.1016/j.anucene.2006.05.008 https://doi.org/10.14311/app.2018.19.0014 https://doi.org/10.1088/1742-6596/781/1/012029 https://doi.org/10.1016/j.nucengdes.2014.03.044 https://doi.org/10.1016/j.pnucene.2017.06.013 https://engineering.purdue.edu/parcs http://nuram.engin.umich.edu/software/genpmaxs/ http://nuram.engin.umich.edu/software/genpmaxs/ acta polytechnica ctu proceedings 28(0):1–9, 2020 1 data preparation process and reflector calculation 1.1 macroscopic cross section preparation 1.2 b1 correction 2 calculation codes 3 test case 4 calculation 4.1 two group approach 4.1.1 0.7 % enrichment 4.1.2 4.3 % enrichment 4.1.3 discussion 4.2 multigroup approach 4.3 neutron flux spectra comparison 4.3.1 0.7 % enrichment 4.3.2 4.3 % enrichment 4.4 sp3 multigroup solution 5 conclusion 6 acknowledgement list of symbols references acta polytechnica ctu proceedings doi:10.14311/app.2020.29.0017 acta polytechnica ctu proceedings 29:17–24, 2020 © czech technical university in prague, 2020 available online at http://ojs.cvut.cz/ojs/index.php/app geotechnical analysis of transforming a water management structure to a transportation structure ivan slávik∗, ľuboš hruštinec slovak university of technology in bratislava, faculty of civil engineering, department of geotechnics, radlinského 11, 810 05 bratislava, slovakia ∗ corresponding author: ivan.slavik@stuba.sk abstract. this paper analyzes the transformation of the culvert channel under the supply channel of the gabčíkovo waterworks into a road tunnel structure and the impact this had on the surrounding rock environment. it presents the engineering-geological conditions in the area, the current structuralmaterial characteristics of the culvert channel and its proposed transformation into a road tunnel. the results of the geotechnical analysis, realized through numeric modelling, are presented as a comparison of deformation changes in the rock environment with respect to changes in loading states corresponding with functional changes in the analyzed culvert. keywords: culvert channel, deformation analysis, loading state, quaternary gravels, settlement, supply channel. 1. introduction during construction work on the supply channel of the gabčíkovo waterworks, a culvert channel was built under the supply channel, situated at 4.0 km. the culvert channel is used to drain seepage waters of the hrušov reservoir captured by the left-side seepage channel under the supply channel. the captured seepage waters, from both left-side and right-side seepage channels, are drained together into the danube river branches. the position of the culvert channel under the supply channel is shown in fig. 1. figure 1. position of the culvert channel under the supply channel the culvert channel has been designed for a maximum flow rate of 60 m3.s−1, while the passage of such a volume of water is provided by five rectangular openings with dimensions 4.5 m × 2.2 m, as is shown in fig. 2. the sixth opening, 2.0 m × 2.2 m, serves as a communication passageway that contains infrastructure. as early as during the design stage of the projected figure 2. cross-section of the culvert under the supply channel at km 4.0 [1] waterworks, it was presumed that seepage into the left-side channel would decrease over time due to colmatation of the subsoils of the hrušov reservoir and that three openings of the culvert channel would be sufficient to pass the captured seepage waters. for this reason, an alternative use of two openings for other than passage of seepage waters has been considered. the flow rates measured during operation of the seepage channels and the culvert channel situated at 4.0 km under the supply channel of the gabčíkovo waterworks in 1994 – 2016 confirmed the possible use of two openings for another purpose. based on the above, an alternative use of part of the culvert channel has been considered as a road transportation structure for permanent connection of communities divided by construction of the supply channel of the gabčíkovo waterworks. the analysis of reliability risks for use of this water management structure as a road tunnel structure included geotechnical assessment of the culvert channel. 17 http://dx.doi.org/10.14311/app.2020.29.0017 http://ojs.cvut.cz/ojs/index.php/app ivan slávik, ľuboš hruštinec acta polytechnica ctu proceedings 2. a structural engineering solution of the culvert channel the culvert channel consists of inlet and outlet sections, which include sluice mechanisms and regulatory gates, and the middle section consists of the culvert tunnel. the overall length of the culvert channel is 715 m, with the middle section representing 499.74 m and the remaining length being represented by the inlet and outlet sections. the middle section of the culvert channel consists of a total of twenty separate dilatation units (d1 to d20), each 19.6 m long, with average width of 30.9 m, and seven dilatation units under the safety dams of the supply channel (3 under the left dam – a1, b1, c1 and 4 under the right dam – c2, b2, e, f), 12.0 to 19.6 m long, with an average width of 31.2 m. a longitudinal section of the culvert channel is shown in fig. 3. figure 3. longitudinal-section of the culvert channel under supply channel in km 4.0 [1] the structure of the culvert channel is a framed reinforced concrete structure comprising 27 blocks, which is made of impermeable concrete with classification at least c 35/45, strengthened with 10 425 (v) reinforcing steel. at the points of contact between the blocks, dilatation joints were made, 20 mm wide with inner sealing around each opening of the culvert channel. the structure of the culvert channel is isolated and the surface of backfill over the ceiling is made of sealing clay. 3. geological conditions at the area of interest the geological environment in the study area is composed of fluvial deposits of quaternary age reaching a thickness circa. 250 m [2]. this was successively accumulated in the gabčíkovo-győr depression of the danube basin as a result of a neotectonic basin inversion [3]. the strata consist mostly of gravelly and sandy-gravelly channel-fill facies of the danube and western carpathian rivers and were accumulated in braiding depositional settings. the deposition in high sediment supply and low accommodation rate conditions caused a low content of sandy, silty and clayey layers [2]. an exception is given by the uppermost few meters of the basin fill, which were deposited during the holocene as floodplain, mostly silty and clayey facies of the meandering danube [4]. to characterize the geological conditions of the subsoil at the culvert channel location, we have assessed the geological survey [5] made in 1978 for construction of the culvert channel. survey drills dkp 8 – dkp 12 were carried out for the culvert channel. besides this, information on the geological condition was supplemented by older survey works vk 390, vk 391 and hvk 23, carried out in the past. depths of all survey work were within 15.0 m with the sole exception of the hvk 23 survey, which was 30.0 m deep. the survey works were located at the axis of the culvert channel situated at 4.0 km. within the examined depth of 15 – 30 m, the subsoil of the culvert channel is formed exclusively by quaternary sediments, which can be classified into three groups: • surface layer of loamy sands or sandy clay soils 0.5 – 1.3 m thick; • transition layer between surface fine-grained soils and gravelly soils of sands with loamy and clay ingredients; • underlying gravel layer with 10 – 60% of sandy ingredient where sandy or less sandy gravel positions are alternating and mutually interlocked. the full scope of the culvert channel’s footing bottom is situated within the quaternary gravel position. from the geotechnical point of view, the quaternary gravel layer, located under the footing bottom of the culvert channel is critical for analysis of the reliability risks of the culvert channel situated at 4.0 km under the supply channel of the gabčíkovo waterworks and its use as road tunnel. according to the survey results [5], the quaternary gravels can be characterized as well-graded (gw – g1) or poorly graded (gp – g2) gravels. the range of granular composition of the gravelly soil samples taken from the culvert channel subsoil is shown in fig. 4. figure 4. range of grain size distribution of quaternary gravel subsoil further properties of the quaternary gravels of the culvert channel subsoil, described in the survey [5], are as follows: • relative density: medium dense; • deformation parameters: . β = 0.83; pisson’s ratio ν = 0.25; . edef = 50 mpa – for depths 0.0 – 5.0 m; . edef = 100 mpa – for depths 5.0 – 30.0 m; . edef = 114.5 mpa – for depths 30.0 – 90.0 m; 18 vol. 29/2020 transformation of water management structure figure 5. engineering geological profile of subsoil [1] • effective parameters of shear strength: ϕef = 38◦; cef = 0.0 kpa; • filtration coefficient: kf = 3.37 × 10−3 m.s−1. location of the survey works with an indication of the axes of the right-side and the left-side dam of the seepage channels, as well as the axis of the supply channel and the culvert channel axis, are presented alongside the geological profile of the culvert channel axis and with an indication of the culvert channel’s footing bottom position in fig. 5. at the time of the survey work [5], the groundwater level was encountered at 3.0 – 7.5 m underground. subsoil in the area of interest is made of permeable quaternary gravelly sediments, which implies that before the culvert channel was constructed, the groundwater level was interconnected with the danube. the culvert channel body was built is a sealed excavation pit. this fact can also affect the dependence of the groundwater level, and especially its fluctuations, on the danube’s water level. 4. deformation analysis of the construction with current and new proposed operation of the culvert channel deformation analysis of the complex interaction problem of construction, along with both current and new proposed operation of the culvert channel was carried out based on assumptions of the theory of flexible half-space. interaction calculations used the numerical finite element method (fem). the task was solved as planar using the computational software geo5 – fem module. in the computational models, soil characteristics of the culvert channel’s subsoil were considered based on evaluation of geological conditions in the area of interest verified by all survey works [5]. definition of deformation properties of the gravelly subsoil respected the discontinuous anisotropy, i.e. changed (increased) deformation modulus 5.0 m underground. the computations considered the original groundwater level 3.0 m below the original ground level. characteristics of reinforced concrete culvert channel structures were considered based on results of the “diagnostics of concrete structures” carried out within a separate part of this task. characteristics of the subsoils and structural elements of the culvert channel used in the geotechnical calculations are listed in tab. 1. deformation calculations were carried out in a cross profile under the supply channel, a cross profile under the crown of a safety dam (within the axis of the dam crown), and in a longitudinal profile under the dam and the supply channel for two stages. stage no. 1 represented excavation of the pit for the culvert channel structure and stage no. 2 represented construction and operation of the culvert channel. considering the extensive amount of achieved computational outputs, this paper will only present the results of deformation calculations in the longitudinal profile under the supply channel for stage no. 2 representing construction and operation of the culvert channel. loading states, representative for assessment of the reinforced concrete structure of the culvert channel and geotechnical calculations, were defined for the stage no. 2, representing construction and operation of the culvert channel, as follows: • loading state 0 (ls0): represents the original state (effective stress in the subsoil after the excavation and construction of the sealed excavation pit) used as reference for comparison of the loading states that provide a model of stepwise construction 19 ivan slávik, ľuboš hruštinec acta polytechnica ctu proceedings table 1. properties of subsoil and structural elements of the culvert [6] of structural elements of the culvert channel and its current and new proposed operation. • loading state 1 (ls1): ls0 + realization of reinforced concrete culvert channel under the supply channel and the safety dam. • loading state 2 (ls2): ls1 + realization of backfills and rock cover over the culvert channel and in the safety dam area. • loading state 3 (ls3): ls2 + filling of all five culvert channels by water + water pressure from the water level in the supply channel at the minimum (operational) level altitude (128.2 m) + uplift of water from the water level in the left and right seepage channels at the operational level altitude (120.63 m). • loading state 4 (ls4): ls2 + filling of all five culvert channels by water + water pressure from the water level in the supply channel at the maximum (flood) level altitude (131.1 m) + uplift of water from the water level in the left and right seepage channels at the flood level altitude (123.32 m). • loading state 5 (ls5): ls2 + filling of three culvert channels by water + water pressure from the water level in the supply channel at the minimum (operational) level altitude (128.2 m) + uplift of water from the water level in the left and right seepage channels at the operational level altitude (120.63 m). • loading state 6 (ls 6): ls2 + filling of three culvert channels by water + water pressure from the water level in the supply channel at the maximum (flood) level altitude (131.1 m) + uplift of water from the water level in the left and right seepage channels at the flood level altitude (123.32 m). the aforementioned overview of the loading states implies that the loading states 3 and 4 provide a model of parallel operation of the culvert channel with all its five channels filled at minimum (operational) water level and maximum (flood) water level. loading states 5 and 6 provide a model of the new proposed operation of the culvert channel with three channels filled and the emptying of the other two channels (pumping off the water) of the culvert channel for road tunnel use at the minimum (operational) water level and at the maximum (flood) water level. the results of the calculation modelling the construction and operation of the culvert channel in its longitudinal profile under the supply channel have implied the following: • the effects of construction of the reinforced concrete culvert channel (ls1) resulted in maximum settlement of footing bottom smax = 17.6 mm. • the effects of culvert channel construction and realization of backfills and rock cover of the culvert channel (ls2) (floor of the supply channel and body of the earth embankment) resulted in maximum settlement of footing bottom smax = 50.1 mm. • the effects of culvert channel construction, realization of backfills and rock cover of the culvert channel and filling of 5 channels of the culvert channel by water at minimum water level in the supply channel (ls3) resulted in maximum settlement of footing bottom smax = 47.9 mm. • the effects of culvert channel construction, realization of backfills and rock cover of the culvert channel and filling of 5 channels of the culvert channel by water at maximum water level in the supply channel (ls4) resulted in maximum settlement of footing bottom smax = 46.1 mm. • the effects of culvert channel construction, realization of backfills and rock cover of the culvert channel and filling of 3 channels of the culvert channel by water at minimum water level in the supply 20 vol. 29/2020 transformation of water management structure figure 6. isosurfaces of deformations for loading state ls3 – longitudinal profile of the culvert [6] figure 7. isosurfaces of deformations for loading state ls4 – longitudinal profile of the culvert [6] figure 8. isosurfaces of deformations for loading state ls5 – longitudinal profile of the culvert [6] figure 9. isosurfaces of deformations for loading state ls6 longitudinal profile of the culvert [6] channel (ls5) resulted in maximum settlement of footing bottom smax = 41.2 mm. relative change due to pumping off the water out of two channels of the culvert for the planned use (transportation) resulted in a maximum rise of the footing bottom smax = − 6.9 mm. • the effects of culvert channel construction, realization of backfills and rock cover of the culvert channel and filling of 3 channels of the culvert channel by water at maximum water level in the supply channel (ls6) resulted in maximum settlement of footing bottom smax = 42.0 mm. relative change due to pumping off the water out of two channels 21 ivan slávik, ľuboš hruštinec acta polytechnica ctu proceedings figure 10. deformations resulting for loading state ls2–ls6 – longitudinal profile of the culvert [6] figure 11. comparison of measured and calculated deformations for loading state ls2 – ls6 – longitudinal profile of the culvert [6] of the culvert for the planned use (transportation) resulted in maximum rise of the footing bottom smax = − 9.6 mm. the resulting deformation values from the numerical calculations for stage no. 2 are presented as deformation isoplanes for the loading state 3 in fig. 6, loading state 4 in fig. 7, loading state 5 in fig. 8 and loading state 6 in fig. 9. for comparison of deformation magnitudes in the respective loading states, deformation values are graphically evaluated for representative points of the culvert channel’s footing bottom in the longitudinal profile in fig. 10. besides the resulting deformations for the loading states 3 to 6, fig. 10 also presents deformations resulting from the loading state 2, which represents construction of the culvert channel before putting the supply channel and the actual culvert channel into operation, that is without water in the culvert channel and in the supply channel. results of deformation calculations are presented in fig. 6 to fig. 10 for a half length of the culvert channel in longitudinal direction because the computational model also made use of the symmetry in the critical section of the culvert channel. 22 vol. 29/2020 transformation of water management structure 5. comparison of measured and calculated deformations during technical safety supervision, the structure of the culvert channel was subject to measurements of deformation in 11/1996 and in 11/2006 [7]. these measurements were used as a reference for comparison of the calculated deformation values for respective loading states. comparison of measured and calculated deformations in the longitudinal direction of the culvert channel for half of the longitudinal profile is shown in fig. 11. 6. conclusions the results of measured deformation values presented in fig. 11 provide general information on the magnitude and distribution of subsoil deformations under the culvert channel in the longitudinal direction triggered by operation of the culvert channel as a water management structure. the results of calculated deformation values presented in fig. 11 provide general information on the magnitude of deformation triggered by change in the culvert channel operation – part of the water management structure to be transformed into a transportation structure. additionally, fig. 11 also indicates the considered uplift effect of water on the floor of the culvert channel. measured deformation values are within the range of calculated deformations considering the uplift effect of water on the culvert channel’s floor depending on the maximum or operational water level in the supply channel and deformations not considering the uplift effect of water on the culvert channel’s floor. comparison of the results of geotechnical calculations (ls1 to ls4) and in situ measurements of vertical deformations of the assessed structures implies that the measured and calculated values are in significant agreement. based on observable experience, it can be assumed that the forecast values of vertical displacement for the considered change in culvert channel use (ls5 and ls6) will be in good alignment with reality after the proposed changes have been implemented. evaluation of maximum values of the final settling as well as values of irregular settling for the assessed loading states of the culvert channel (ls1 to ls6) indicate the possible transformation of the culvert channel structure under the supply channel of the gabčíkovo waterworks into a road tunnel structure while still meeting the conditions for a reliable and desired state pursuant to the european (eurocodes) and slovak (stn) standards in force. in solving the relatively complex engineering problem of the interaction of a building structure with the subsoil, the numerical finite element method was used the suitability of which has already been verified in solving similar interaction problems [8], [9], [10]. acknowledgements this paper was supported by grant projects vega 1/0452/17 and vega 1/0412/18. references [1] m. šúň, m. kováčik. feasibility study a. technical report assessment of the possibility of rebuilding part of the culvert channel at km 4.0 under the supply channal – the gabčíkovo waterworks for road transport, basler&hofmann slovakia s.r.o., 2017, 72 p. (in slovak). [2] m. šujan, r. braucher, s. rybár, et al. revealing the late pliocene to middle pleistocene alluvial archive in the confluence of the western carpathian and eastern alpine rivers: 26al/10be burial dating from the danube basin (slovakia). sedimentary geology 377, 2018. doi:10.1016/j.sedgeo.2018.10.001. [3] o. sztanó, m. kovac, i. magyar, et al. late miocene sedimentary record of the danube/kisalföld basin: interregional correlation of depositional systems, stratigraphy and structural evolution. geologica carpathica 67, 2016. doi:10.1515/geoca-2016-0033. [4] m. moravcová, k. fordinál, j. maglay. late quaternary history and palaeoclimatic implications of danubian flat based on dating, geochemistry, lithology, isotope analyses and land snail assemblages 18:31–70, 2020. [5] o. dolhá, š. tichý. svd gabčíkovo-nagymaros, supplement to the detailed geological survey for buildings a-d, buildings bb, bc3, culvert channel in km 4,0, bridge in km 4,0 additional survey, 01/1978, ighp n. p. žilina, 34 p. (in slovak). [6] ľ. hruštinec, i. slávik. analysis of reliability risks (resistance, usability, service life) of the culvert channel in km 4.0 under the supply channel to vd gabčíkovo in its application as a road tunnel , part: geotechnics, 09/2017, slovak university of technology – faculty of civil engineering – detartment of geotechnics bratislava, 85 p., 120 a4 app. (in slovak). [7] hruštinec, ľ., sumec, j.: numerical analysis of vertical displacements of the interim spent fuel storage building for different loading states. in sgem 2016. 16th international multidisciplinary scientific geoconference. book 1. science and technologies in geology, exploration and mining: conference proceedings. albena, bulgaria, 2016, sofia: stef 92 technology, 2016, p. 202-209, issn 1314-2704, isbn 978-619-7105-57-5, wos: 000395499800026. [8] geodetic annex to ses no. 14 technical safety supervision “the gabčíkovo waterworks“, geodetic measurements from construction to 2016, vodohospodárska výstavba š.p. bratislava – part tbd, 2017 (in slovak). [9] hruštinec, ľ., sumec, j., pollák, p.: numerical analysis of the storage boxes interaction with subsoil at different loading states. in sgem 2017. 17th international multidisciplinary scientific geoconference. volume 17. science and technologies in geology, exploration and mining: conference proceedings. albena, bulgaria, 2017. sofia: stef 92 technology, 2017, p. 587-594, issn 1314-2704, isbn 978-619-7105-99-5, doi: 10.5593/sgem2017/12/s02.075. [10] l. hrustinec, j. kuzma, j. sumec. the use of the finite element method in the optimization of the foundation of a bridge structure over the danube river. in 19th international multidisciplinary scientific geoconference sgem 2019, vol. 19 of international multidisciplinary 23 https://doi.org/10.1016/j.sedgeo.2018.10.001 https://doi.org/10.1515/geoca-2016-0033 ivan slávik, ľuboš hruštinec acta polytechnica ctu proceedings scientific geoconference-sgem, pp. 557–564. bulgarian acad sci; acad sci czech republ; latvian acad sci; polish acad sci; russian acad sci; serbian acad sci & arts; slovak acad sci; natl acad sci ukraine; natl acad sci armenia; sci council japan; world acad sci; european acad sci, arts & letters; ac, stef92 technology, 51 alexander malinov blvd, sofia, 1712, bulgaria, 2019. 19th international multidisciplinary scientific geoconference sgem 2019, 30 june 6 july, 2019, doi:10.5593/sgem2019/1.2/s02.071. 24 https://doi.org/10.5593/sgem2019/1.2/s02.071 acta polytechnica ctu proceedings 29:17–24, 2020 1 introduction 2 a structural engineering solution of the culvert channel 3 geological conditions at the area of interest 4 deformation analysis of the construction with current and new proposed operation of the culvert channel 5 comparison of measured and calculated deformations 6 conclusions acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2020.26.0034 acta polytechnica ctu proceedings 26:34–38, 2020 © czech technical university in prague, 2020 available online at https://ojs.cvut.cz/ojs/index.php/app experimental and numerical investigation of cement paste microstructure and its hydration under short-term exposure to acetone yuliia khmurovska∗, petr štemberk, jiří němeček, magdaléna doleželová czech technical university in prague, faculty of civil engineering, department of concrete structures, thákurova 7, 166 29 prague 6, czech republic ∗ corresponding author: yuliia.khmurovska@fsv.cvut.cz abstract. the solvent exchange methods are commonly used in order to arrest cement hydration reaction. this paper presents preliminary results of experimental investigation of cement paste microstructure under short-term (24 hours) exposure to acetone as a solvent in order to estimate the influence of solvent soaking time. the methodology to determine the effect of soaking time based on numerical prediction is also presented and described. the immersion of cement samples with the cross-section of 10× 10 mm in acetone for 24 hours at the sample age of 15 hours does not enable to fully stop the hydration reaction, however, it can slow down the hydration reaction significantly. according to the comparison of several measured data (nanoindentation, scanning electron microscopy, observation made by optical microscope, mercury intrusion porosimetry) at the sample age of 1 month with the numerical simulation, the equivalent sample age is determined as approximately equal to 69 hours. keywords: acetone, cement paste, experiment, hydration reaction, numerical simulation. 1. introduction the hydration process of cement paste often has to be stopped in order to allow precise investigation and measurements of the alteration of cement paste properties over time. there are a numerous amount of methods to stop the hydration reaction [1–4], which can be divided into the following two groups: (1.) direct drying. the essence of the direct drying methods is to transform water into the vapor. the water vapor may escape freely and hydration reaction stops due to the lack of water, however, the direct drying methods are accompanied by the rapid change in temperature or pressure, which may damage microstructure of the sample or alter the cement paste properties and distort the results of the measurements [5–7]. the following types of direct drying are used in the available literature: • oven drying [5, 8, 9], • microwave drying [10–12], • vacuum drying [13], • freeze drying [14], • d-drying [15], • p-drying [15], etc. (2.) solvent exchange. the mechanism of solvent exchange methods is following: the solvent penetrates into the sample, mixes with free water and evaporates rapidly due to its low boiling point. it is believed that solvent exchange methods have lower impact on cement paste microstructure, however, it was reported [16, 17] that solvents may react with cement paste hydrates and alter its properties. also, it is difficult to entirely remove solvents, therefore, the chemical composition of the cement paste may be affected by the solvent. the following types of solvents are used in the available literature: • acetone [8, 18], • methanol [19, 20], • ethanol [21, 22], • isopropanol [14, 19], • tetrahydrofuran [1], • dimethyl sulfoxid [20], • benzene [16], • pentane [23], etc. from the above, it is obvious that the method to stop the hydration reaction should be chosen according to the subject of interest. this study is focused on cement paste microstructure investigation, therefore, the solvent exchange method was chosen and the acetone was used as the solvent. the necessary solvent soaking time to stop the hydration reaction depends on many factors, such as porosity, diffusivity, sample cross-section. porosity and diffusivity of early age cement paste change rapidly over time, therefore, the necessary solvent soaking time has to be a function of cement paste age. the majority of the research does not provide a sufficient explanation of the chosen soaking time. 34 https://doi.org/10.14311/app.2020.26.0034 https://ojs.cvut.cz/ojs/index.php/app vol. 26/2020 another complication is that it is impossible to determine whether the reaction was stopped during the solvent soaking and the additional experiments are needed for this purpose. therefore, the experimental investigation of the influence of solvents on cement paste has to be performed in order to determine the necessary solvent soaking time to stop the hydration reaction of early-age cement paste. this paper presents preliminary results of the experimental investigation of early-age cement paste microstructure change under short-term (24 hours) exposure to acetone in order to estimate the influence of acetone soaking time. the methodology to determine the effect of soaking time based on the numerical prediction is also presented and described in detail below. 2. experiment the cement paste which consists of cem i 42.5r extracem and water with water-to-cement ratio of 0.38 was used to produce small-scale 10x10x80 mm samples. the cement clinker composition is shown in table 1. the samples together with the plexiglass moulds were insulated by polyethylene foil till the age of 15 hours. after demoulding, all the samples were immersed in acetone for 24 hours at the sample age of 15 hours in order to stop the hydration process. after that all the samples were dried in oven at 50 °c till equilibrium in order to remove acetone. the samples were packed in polyethylene foil and were stored in box with silica gel until the examination in order to avoid possible moisture ingress [24]. environmental temperature and relative humidity were recorded starting from the mixing till packing with the interval of 1 minute. the average environmental temperature was 20.7 °c and relative humidity was 72.8% (excluding oven drying). the amount of anhydrous clinker was obtained using three different techniques, namely, nanoindentation (nano), scanning electron microscopy (sem) and observation made by optical microscope (opt) at the sample age of one month. the surface of the sample sections, which were used for nanoindentation and sem and opt investigation, was polished with silica carbide paper to achieve a smooth surface. the nanoindentation of the polished sample sections was carried out with a hysitron tribolab ti-700 with the berkovich diamond tip. five grids of 10 x 10 indents with the spacing of 10µm were predetermined on the center part of the sample surface. the nanoindentation was performed using the force control test with the trapezoidal loading diagram. the sample surface was loaded by the diamond tip linearly during three seconds till the maximum force of 2mn was reached with a following 20 seconds holding period and 3 seconds of unloading. the indentation elastic modulus was determined from the obtained load-displacement curves using the oliver and pharr theory [25]. the values of the elastic modulus higher than 50gpa and hardness higher than 1.5gpa correspond to the anhydrous clinker. after the nanoindentation, the polished sample sections were dried in a vacuum for three hours with a vacuum pump. the microstructure investigation of these sample sections was carried out by a phenom xl desktop sem and optical microscope. the amount of anhydrous clinker was obtained using the image processing technique which was performed by the fiji code. the measured volume fractions of anhydrous clinker and the average value are shown in table 2. the sample porosity and its distribution was measured using mercury intrusion porosimetry (mip) at the sample age of one month. the sample sections were broken into pieces and the center pieces were selected for further porosity investigation by the mercury porosimeter pascal 140 a 440. the measured porosity was in the range from 32.6% to 36.7% for different samples. more detailed description of the experiment is given in [24]. 3. numerical simulation in order to understated whether the hydration reaction was fully stopped after the samples immersion in acetone, the matlab code of hydration and microstructure models, which is based on the models described in [26–28], was developed. the data presented in table 1 and the measured environmental conditions were used in the analysis. the preliminary results are described below. the degree of hydration was calculated using the multi-component model, where the rate of cement hydration heat generation is obtained from the sum of the hydration heat rates of cement components according to their proportions in cement. the degree of hydration was calculated according the following formula αhyd = ∑ pi · (qi/qi,∞), (1) where pi is the mass ratio of the chemical component i of cement (c2s, c3s, c3a, c4af, gypsum, etc.), qi is the accumulated heat generation of the chemical component i, qi,∞ is the maximum theoretical specific heat of the component i and αhyd is the averaged degree of hydration. the calculated degree of hydration of the individual clinker components and their average and time when the samples were immersed in acetone are shown in figure 1 [24]. the cement paste microstructure was modeled using the expanding cluster model, volumetric balance and simplified calculation of the total pore surface area described in detail in [26–28]. after that the pore size distribution was calculated using a simplistic raleighritz distribution function. 35 y. khmurovska, p. štemberk, j. němeček, m. doleželová acta polytechnica ctu proceedings c3s, [%] c2s, [%] c3a, [%] c4af, [%] 63.99 14.30 9.11 8.26 c3s: alite; c2s: belite; c3a: celite; c4af: felite. table 1. clinker composition of cem i 42.5r extracem according to the manufacturer [24]. nano, [%] sem, [%] opt, [%] average, [%] 18.40 18.30 18.89 18.53 table 2. volume fractions of anhydrous clinker. figure 1. calculated degree of hydration [24]. figure 2. amount of anhydrous clinker. the volume fraction of anhydrous clinker was calculated as follows vcl = (1 − αhyd) ·wp/ρp, (2) where wp is the weight of powder materials per unit paste volume, ρp is the density of powder materials and vcl is the volume fraction of anhydrous clinker. the decrease of the amount of anhydrous clinker due to the hydration reaction over time with the depiction of the averaged measured value is shown in figure 2 [24]. the capillary porosity was calculated using the following equation φc = 1 − vs − vcl, (3) figure 3. capillary porosity. figure 4. pore size distribution. where vs is the volume of hydrated products and vcl is the volume fraction of anhydrous clinker. the pore size distribution was calculated as follows φ(r) = φl + φg · vg + φc · vc, (4) where φl, φg and φc is the interlayer porosity, the gel porosity and the capillary porosity, respectively, vi = 1 − exp(−bir) is the normalized pore volume, bi is the porosity distribution parameter and r is the pore radius. the calculated capillary porosity over time with the depiction of measured porosity (the total porosity 36 vol. 26/2020 obtained using mip is considered to be predominantly capillary porosity) is shown in figure 3, while the measured and calculated pore distribution are shown in figure 4 [24]. as can be seen in figure 2, figure 3 and figure 4, the hydration reaction was not fully stopped at the sample age of 15 hours after sample immersion in acetone for 24 hours, but was slowed down significantly and the equivalent sample age is approximately equal to 69 hours according to the preliminary results of the numerical simulation at the real sample age of 1 month [24]. however, more detailed examination of the sample microstructure is still needed. 4. conclusions the immersion of cement samples with the crosssection of 10 x 10 mm in acetone for 24 hours at the sample age of 15 hours could not fully stop the hydration reaction, however, it can slow down the hydration reaction significantly. according to the comparison of the measured data at the sample age of 1 month with the numerical simulation, the equivalent sample age is approximately equal to 69 hours. the calculated volume fraction of anhydrous clinker, capillary porosity and pore size distribution have a good correlation with the measured values. however, the additional experiments with the variable acetone soaking time and different sample cross-section are necessary in order to draw the final conclusions. list of symbols αhyd averaged degree of hydration [–] ρp density of powder materials [kg m−3] φc capillary porosity [–] φg gel porosity [–] φl interlayer 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https://doi.org/10.1201/9781482288599 acta polytechnica ctu proceedings 26:34–38, 2020 1 introduction 2 experiment 3 numerical simulation 4 conclusions list of symbols acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2020.29.0011 acta polytechnica ctu proceedings 29:11–16, 2020 © czech technical university in prague, 2020 available online at http://ojs.cvut.cz/ojs/index.php/app change of a highway route caused by a rockslide miloslav kopeckýa,∗, martin brčeka, jana frankovskáa, martin ondrášika, marek fraštiab, pavel liščákc, jan novotnýd a slovak university of technology in bratislava, faculty of civil engineering, department of geotechnics, radlinského 11, 810 05 bratislava, slovakia b slovak university of technology in bratislava, faculty of civil engineering, department of surveying, radlinského 11, 810 05 bratislava, slovakia c state geological institute of dionýz štúr, mlynská dolina 1, 817 04 bratislava, slovakia d czech geological survey, klárov 3, 118 21 praha 1, czech republic ∗ corresponding author: miloslav.kopecky@stuba.sk abstract. during the spring of 2013, a relatively large rockslide with a total area of 96,952 m2 occurred in the eastern part of the malá fatra mountains. the anticipated depth of the slip surfaces was from 30 to 40 metres and the volume of moving mass was estimated to be 2,000,000 m3. the maximum movement of the rockslide on the first day in march 2013 was estimated to be 43 m. the front of the rockslide interfered with the projected d1 highway route. the paper presented here offers an analysis of the causes of the rockslide and its activity in association with the potential threat to the proposed highway. keywords: highway route, rockslide, slope stability. 1. introduction in march 2013, an extensive rockslide occurred in the kraľovany-rieka quarry, also known as the quarry šútovo. in addition to the remarkable visual and acoustic effect, the initial sudden and significant movement was also recorded on the three fixed geodetic points installed by the nds. the dimensions of the rockslide are 580 m x 280 m. the expected depth of the sliding surfaces was estimated from 30 to 40 m without investigatory works and the volume of the moving masses was 2,000,000 m3 [1]. the maximum movement of the rockslide at its instigation in march 2013 was estimated to be 43m. the rockslide posed a significant threat not only to the rock quarry and the nearby lake used for recreational purposes, but mostly as a threat to the construction of the d1 highway, which was planned before the rockslide occurrence in the rockslide front (fig.1 blue line). after the rock slide, one of the variants of the new route was planned on the south (fig.1black line variant v-1) which was also investigated [2] and its stability assessed [3]. 2. conditions and causes of rockslide development the geological-tectonic conditions we consider to be a decisive cause in the formation of the large-scale rockslide. there is a tectonic thrust fault in the given area, where mesozoic sedimentary rocks of mainly dolostones and broken limestone were pushed over granites. the carbonate complex as well as the rockslide is significantly disrupted tectonically, karstificated and divided into blocks of differing extent [4]. the thrust fault between granites and dolostones has a steep dipping toward the south to southwest and represents the main scarp area of the entire rockslide. granites in the area of the thrust fault have the character of mylonites (clays with medium to high plasticity). the presence of the mylonites enabled the movement of dolostones along the layer with significantly low share strength. as a second cause of the rockslide we consider the more than 50 years duration of mining activities in the quarry, which has significantly weakened the heel of the slope. these facts created permissive conditions for formation of a large rockslide. the cause of the slope movement was the extreme climatic conditions over winter and the spring of 2013. repeated warming caused multiple melting of the snow cover and saturation of the dolomite rocks with water. the trigger of the rockslide was the melting of the thick snow cover in march and infiltration of the melted water into the open cracks in the crown of the rockslide ("rozsadlina" cave fig. 1). 3. characteristics of individual parts of the rockslide and their activity in the period 2013-2014 the rockslide is characterized on the basis of engineering geological mapping, drilling and geodetic and inclinometric monitoring performed in the time period 2013-2014. with regard to the kinematics of the motion, the rockslide can be divided into 3 parts: a)western part of the rockslide 11 http://dx.doi.org/10.14311/app.2020.29.0011 http://ojs.cvut.cz/ojs/index.php/app m. kopecký, m. brček, j. frankovská et al. acta polytechnica ctu proceedings figure 1. rock and rockslide with two alternative routes of the highway, before (blue line) and after the rockslide (black line variant v1) figure 2. vectors and absolute values of motions (in mm) of geodetic points. red color – geodetic terrestrial method (14.5. till 14.8.2013), green color gnss measurements (14.5. till 19.9.2013) this part of the rockslide has not been surveyed by drilling and does not touch the highway route. it consists mostly of displaced blocks of limestone with rare dolostone blocks. the relatively compact rock block is also evidenced by the quite well-preserved forest on the surface of the central part of the block, where no significant cracks were open. the depth of the planar sliding surface is assumed to be up to 35 m at the contact of mesozoic rocks with paleozoic granodiorites which, due to tectonic failure, have the character of mylonites. above the main scarp in the rockslide crown, there is a progressive expansion of the slide, which resulted in creation of crown cracks into the granite massif. the active character of this part of the rockslide was indicated by the measurement at point b9 (fig 2). for the period may 2013 may 2014, the movement of the geodetic point in the direction of the slope was 1.2 m and its decrease was 0.76 m. also, at the foot of the rockslide, the pressing of the toe of the rockslide was confirmed by laser scanning (fig. 3). a simplified mathematical model of this part of the rockslide is shown in fig. 4. b)central part of the rockslide it is characterized by the most pronounced motion activity and is divided into a series of smaller 12 vol. 29/2020 change of a highway route caused by a rockslide figure 3. sliding motion activity based on changes in the surface of the rockslide body detected by laser scanning (29.5. and 22.8.2013) figure 4. computational model of the slope in profile s-1 (fig. 1) out of contact with the highway blocks of tectonically disturbed dolostones (fig. 5). the blocks show, in the upper part, motion vectors towards sw and ssw and in the lower accumulation part, the direction of motion turns in a n – s direction, which is documented by the motion vector of point nds 640 (fig. 2). the central part of the rockslide also has a significantly developed main scarp with height about 20 m (fig. 6). there is a movement of the dolostone blocks along the mylonite zone. this was verified by an inclinometric borehole ink-6 at a depth of about 25.0 m [5]. rockslide activity was confirmed mainly by geodetic measurements for the period 14.5.2013 to 14.8.2013. a shift 804 mm was recorded at point nds 640 (fig. 2) [6]. the average displacement speed of the point 640 in the observed time period reached 9 mm per day. the activity of the front part of the rockslide was documented by laser scanning (fig. 3). cracks above the rockslide front were also visually well observable (fig. 7). c)eastern part of the rockslide this part of the rockslide is closest to the newly proposed d1 variant (variant v1 black line in fig. 1). therefore, this part was examined by drilling works [2] and the slope stability calculated [3]. according to the mentioned works, it was found, as in other parts of the rockslide, that the rockslide body consisted of recrystallized tectonically disturbed dolostones. the sliding surface of the active rockslide was found at the contact of dolostones with the mylonite zone of a clay character (fig. 8). the depth of the sliding surface is shallower than in the western and central parts of the rockslide and was determined on the basis of boreholes and geophysical measurements at a depth of 9.6 (ink-2) to 12.4 m below the surface. the rockslide activity in this part was confirmed by geodetic measurements (fig. 2) significant shifts were shown by points b-1 (263 mm), b-2 (173 mm), with a shift vector to the sw and point b-4 (148 mm) with vector shift to the south. 13 m. kopecký, m. brček, j. frankovská et al. acta polytechnica ctu proceedings figure 5. significant radial cracks between two blocks of the central part of the rockslide figure 6. crown of the rockslide with the sliding surface in the central part of the rockslide (detail of sliding surface in mylonites) 4. slope stability calculations and a proposal for rockslide remediation the highway route was designed as a viaduct near the front of the active rockslide. the conceptual design of the slope stabilization consisted of 2 basic methods [3]: • remediation by force elements in the form of anchored reinforced concrete blocks and pile walls, • massive loading benches in the heel of the slope, reinforced or not reinforced according to the inclination of the forehead. stability calculations have led to the conclusion that a solution using force remediation elements is almost impossible due to the extent of the rockslide. therefore, this remedial action was not recommended. on the other hand, a solution using massive loading benches seemed to be potentially a possible method of remediation. the loading benches themselves were designed from a stony material, permeable enough figure 7. cracks above the front of the rockslide in the central part near the point 640 even after compaction to ensure trouble-free drainage of water from the rockslide and to prevent a rise of the groundwater level at the contact between the rockslide and the loading bench. the stony material used for the construction of the load benches must meet the condition of long-term stability of the fragments. they should not soften after their incorporation into the structure, as this would cause undesired deformations of the bench and closure of macro pores, and as a result of which creep and undesired reduction of permeability could occur. the method of remediation using massive loading benches must be supplemented by remediation measures ensuring both surface and deep drainage of the rockslide. the heel of the loading benches will extend into the current lakes. here, it is necessary to pay attention to the unverified properties of rocks at the bottom of the lakes when setting up a high loading bench. the nature of the underlay rocks will need to be verified by investigation work. one of the several considered alternatives of the slope stability remediation with a loading bench is shown in fig. 9. 5. conclusions in march 2013, an extensive rockslide occurred in the quarry kraľovany-rieka. in addition to the quarry, the quarry workers and people using the lake for recreational purposes, the rockslide endangers the route of the projected d1 highway. therefore, a new variant of the d1 highway route (taking it further from the rockslide front) was proposed, which was then examined and its slope stability calculated. monitoring observations (geodetic and inclinometric measurements) confirmed in 2013 that the rockslide is still active, e.g. the geodetic point nds showed a movement of about 9 mm per day. stability calculations have shown that stabilization of the rockslide above the highway route cannot be performed by use of force elements only, but needs more measure objects, 14 vol. 29/2020 change of a highway route caused by a rockslide figure 8. computational model of the slope in the profile c = s-3 (fig.1) in contact with the highway. the active character of the rockslide in the vicinity of v-2 / ink is also confirmed by the results of inclinometric measurements, which for the period from 18 march 2014 to 21 may 2014 showed movement of the casing by 10 mm figure 9. computational model of the slope in the profile c = s-3 (fig. 1) with a loading bench such as a loading bench in front of the rockslide. at present, however, it has already been decided that the so-called “valley variant” of the highway route with the rockslide will not be implemented and the d1 route will be led through the korbeľka tunnel. based on the latest data from geodetic monitoring, the activity of the rockslide movement has a diminishing tendency. acknowledgements this contribution was made possible thanks to the support of projects vega 1/0530/19 and 1/0842/18 by the research agency of the ministry of education, research and sport of the slovak republic. references [1] j. šimeková, p. liščák, p. ondrejka, m. fraštia, m. kopecký, a. žilka, m. kováčik, p. pauditš, p., d. balík, 2014: monitoring of physical activity of an emergency rockslide in the village of kraľovany. geological works, reports 122, ed. dionýz štúr state geological institute, bratislava, p. 7 27. issn 0433-4795. 2014. [2] a. grenčíková, a. et al. highway d1 turany hubová. orientational engineering geological investigation for modified section variant v1 and subvariant v1a. final report. geofos s.r.o. žilina. 2014. [3] j. novotný, i. novotná i. report on the geotechnical evaluation of variant 1 of the construction of the d1 motorway in the section turany hubová at the landslide site within the project documentation in the stage of the comparative study, arcadis praha, 46 p. 2014. [4] j. kondela, m. prekopová, v. budinský, et al. the importance of seismic methods application for geological reconstruction of rockslide threatened open pit. journal of applied geophysics 159, 2018. doi:10.1016/j.jappgeo.2018.09.005. [5] p. tupý p. a. ilkanič, z. jasovský z. emergency measures in the locality of kraľovany (šútovo quarry). orientative investigation. envigeo a.s. banska bystrica. 17 pages, 8 appendices. 2014. [6] m. fraštia, m. marčiš, m. kopecký, et al. complex geodetic and photogrammetric monitoring of the kral’ovany rock slide. journal of sustainable mining 13:12–16, 2014. doi:10.7424/jsm140403. 15 https://doi.org/10.1016/j.jappgeo.2018.09.005 https://doi.org/10.7424/jsm140403 acta polytechnica ctu proceedings 29:11–15, 2020 1 introduction 2 conditions and causes of rockslide development 3 characteristics of individual parts of the rockslide and their activity in the period 2013-2014 4 slope stability calculations and a proposal for rockslide remediation 5 conclusions acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2019.22.0001 acta polytechnica ctu proceedings 22:1–6, 2019 © czech technical university in prague, 2019 available online at http://ojs.cvut.cz/ojs/index.php/app the influence of a shrinkage reducing admixture on the long-term development of dynamic properties in alkali-activated slag martin alexaa, dalibor kocába,∗, barbara kucharczykováa, jan kotrlab a brno university of technology, faculty of civil engineering, veveri 331/95, 602 00 brno, czech republic b brno university of technology, faculty of chemistry, purkynova 464/118, 612 00 brno, czech republic ∗ corresponding author: dalibor.kocab@vutbr.cz abstract. the paper investigates the way a shrinkage-reducing admixture affects the long-term development of dynamic properties in mortars made with alkali-activated slag (aas). two aas mortars were tested one contained a shrinkage-reducing admixture, the other did not. the specimens (prisms of 40 × 40 × 160mm) were observed for changes in the dynamic modulus of elasticity at ages of 3 days through 2 years using the ultrasonic pulse velocity test and the resonance method. even though the statistical analysis of the results gave no proof that the shrinkage-reducing admixture affected the final values, the development of the elastic modulus appeared to follow a very interesting trend which is completely different from trends commonly observed in cement-based composites. keywords: alkali-activated slag, dynamic modulus of elasticity, ultrasonic pulse velocity test, resonance method. 1. introduction shrinkage, its reduction, or the means of mitigating its consequences are critical points in the development of all types of composite materials. the magnitude of shrinkage depends primarily on the composition of the material and its curing. the type of binder and the progress of hydration play a crucial part as well [1, 2]. alkali-activated binders have been investigated as alternatives to other concrete binders since the 1960s [3]. research shows that that the way alkaliactivated materials perform and behave depends on the raw aluminosilicate material, the activator, and the ratio of the two [4]. the alkali-activated slag composite discussed herein sometimes performs better than composites made with portland cement (pc) – specifically in such properties as durability, resistance to high temperatures, lower hydration heat, or a better interfacial transition zone [5]. the biggest downsides of aas composites are severe shrinkage and rapid setting (especially when the activator is water glass) [6–8]. despite the effort of many investigations to describe the hydration process and mechanical properties of these composites [9–11], some questions pertaining to their behaviour still remain unanswered. shrinkage by desiccation, which is far more severe in aas than in pc [12, 13], results in internal tensile stresses which locally exceed the material’s tensile strength and cause microcracks. this phenomenon can reduce both strength and elasticity. using static tests to observe changes in the structure of the composite and how they may occur over a long period of time is rather time-consuming and requires a large number of specimens. even choosing the right static test is far from straightforward. this is why the structure of these materials is far better observed using dynamic methods based on mechanical wave propagation or forced vibration. these methods can determine the values of the dynamic modulus of elasticity (tensile/compressive, flexural, or shear), which are directly affected by structural changes inside the material. the authors of this paper have already published some of the measurement results (taken between the ages of 3 and 28 days) [14]. however, experience showed that it is better to study the effect of shrinkage-reducing admixtures (sra) over a longer time period. a minimum recommended time is 90 days. 2. experiment 2.1. material and specimens the experiment’s main goal was to determine how a shrinkage-reducing admixture affects the long-term behaviour of aas materials as regards the development of their dynamic modulus. two mortars were tested; both made with granulated blast-furnace slag activated by water glass (sodium silicate). the slag was produced by kotouč štramberk, spol. s r. o. and the water glass (na2o + sio2 + h2o) by vodní sklo, a.s. the water glass was added so that na2o would correspond to 10% of slag mass, resulting in a water-binder ratio of 0.42 (the calculation takes into account water contained in the water glass as well). the aggregate was normalized silica sand cen of a 0–2mm fraction. the weight of the sand corresponded to three times the weight of slag. the basic components and their ratios were identical in both mortars 1 http://dx.doi.org/10.14311/app.2019.22.0001 http://ojs.cvut.cz/ojs/index.php/app m. alexa, d. kocáb, b. kucharczyková, j. kotrla acta polytechnica ctu proceedings they only differed in the shrinkage-reducing admixture (the commercially available stachement ac600) the first aas contained no admixture and the other contained the sra at 2% of slag weight. all aas components were added to a mixer in the following order: sand, slag, and lastly water with water glass and shrinkage-reducing admixture (if required). the mortars were mixed for 5 minutes at 30rpm. each of the mortars was made into 9 specimens with the nominal dimensions of 40 × 40 × 160mm. they were cast in high-strength plastic moulds and stored in a climate chamber with stable ambient conditions – temperature of (21 ± 2) °c and relative humidity of (60 ± 10)% – where they were left to harden without cover or curing for 72 hours. this method was intentional so as to underline the influence of the sra. after the specimens were removed from their moulds, they were left in the same conditions, still without curing, for the next 2 years. 2.2. test methods the experiment focused on observing the young’s modulus e and the shear modulus g. the measurements were made using two non-destructive methods – the ultrasonic pulse velocity test and the resonance method. the basic principle of the former lies in the repeated sending of ultrasonic pulses through the material at very short intervals. a pundit pl-200 with two 150khz transducers was used to determine the ultrasonic wave transit time, measured longitudinally in three different positions (fig. 1a). the measured data were then used to calculate the ultrasonic wave velocity vl, which is then used for determining the young’s modulus as follows: ecu = ρ · v2 l · (1 + µ) · (1 − 2 · µ) 1 − µ , (1) where ecu is the dynamic young’s modulus in mpa, ρ is bulk density in kg/m3, vl is the ultrasonic pulse velocity in km, and µ is the dynamic value of the poisson’s ratio. the dynamic modulus of elasticity was also determined by the resonance method, which operates by introducing vibration to the specimen and detecting the natural frequencies. the specimens received a mechanical pulse delivered by an impact hammer (ih). the natural frequencies were then scanned using a handyscope hs4 oscilloscope with acoustic emission sensors and software that uses a fast fourier transform, see fig. 1b. the natural frequencies were then used to calculate the young’s modulus according to the following formula: ecrl = 4 · l2 · f2 l · ρ, (2) where ecrl is the dynamic young’s modulus in mpa, l is the specimen length in m, fl is the measured natural frequency of longitudinal vibration in khz, and ρ is bulk density in kg/m3, or using the formula: ecrl = 0.0789 · c1 · l4 · f2 f · ρ · i−2, (3) where ecrf is the dynamic young’s modulus in mpa, c1 is the correction coefficient, l is specimen length in m, ff is the natural frequency of transverse vibration in khz, ρ is bulk density in kg/m3 and i is the radius of gyration of the specimen’s cross-section in m. data obtained by the resonance method can also be used to calculate the shear modulus of elasticity using the formula: gcr = 4 · k · l2 · f2 t · ρ, (4) wheregcr is the dynamic shear modulus of elasticity in mpa, k is a coefficient determined by the specimen’s cross section, l is specimen length in m, ft is the measured natural frequency of torsional vibration in khz and ρ is bulk density in kg/m3. all nine specimens of each aas mortar were tested for dynamic properties and bulk density at ages of 3, 7, 14, and 28 days. at the age of 28 days, three specimens were tested for the static young’s modulus by compressive loading. the remaining six were later tested for dynamic properties at 90, 180, 365, and 730 days. the pundit pl-200 used the same settings for every measurement – gain: 2× and voltage: 100v. the resonance method settings were also kept the same throughout the experiment. the dynamic poisson’s ratio used in formula (1) was obtained from the resonance method results. 3. results and discussion fig. 2 through fig. 5 show box plots of the measured dynamic parameters. in order to assess the influence of the shrinkage-reducing admixture, all the data sets, i.e. nine or six values of each property from every measurement date, were verified for normality on a significance level of 0.05. in addition, the test results were statistically compared using a two-sample t-test on a significance level of 0.05. the statistical tests were set up so as to assess the equality of the mean values of the test results. the comparison involved all results for all the ages contrasting aas mortars without shrinkage-reducing admixture (aas-1) with aas mortars that did contain it (aas-2). if the hypothesis is rejected, differences between the sets of results can be considered statistically significant. tab 1 shows the results of the statistical analysis. the data show that none of the mortars reaches values that would be statistically more significant in any of the property being tested at any time during the experiment. an influence of sra on the dynamic moduli of elasticity in aas mortars cannot be identified nor quantified in this case. over the first 14 days of age, the two materials show no statistically significant difference in the values of the young’s modulus, but in other parameters the mortar with no sra has slightly 2 vol. 22/2019 the influence of a shrinkage reducing admixture. . . figure 1. measurement by the ultrasonic pulse velocity test (t transmitter, r receiver); measurement by the resonance method (s acoustic emission sensor, ih impact hammer). age [days] ecu ecrl ecrf gcr 3 aas-1 ≈ aas-2 aas-1 > aas-2 aas-1 > aas-2 aas-1 > aas-2 7 aas-1 ≈ aas-2 aas-1 > aas-2 aas-1 > aas-2 aas-1 > aas-2 14 aas-1 ≈ aas-2 aas-1 > aas-2 aas-1 > aas-2 aas-1 > aas-2 28 aas-1 < aas-2 aas-1 > aas-2 aas-1 > aas-2 aas-1 > aas-2 90 aas-1 > aas-2 aas-1 > aas-2 aas-1 > aas-2 aas-1 ≈ aas-2 180 aas-1 < aas-2 aas-1 < aas-2 aas-1 < aas-2 aas-1 < aas-2 365 aas-1 < aas-2 aas-1 ≈ aas-2 aas-1 < aas-2 aas-1 < aas-2 730 aas-1 > aas-2 aas-1 > aas-2 aas-1 < aas-2 aas-1 < aas-2 table 1. a statistical comparison of test results with and without sra: where the results show no statistically significant difference, the table says “aas-1 ≈ aas-2”; where they do, the table uses a “greater-than” or “less-than” sign accordingly. figure 2. box plot of the development of the young’s modulus ecu. figure 3. box plot of the development of the young’s modulus ecrl. 3 m. alexa, d. kocáb, b. kucharczyková, j. kotrla acta polytechnica ctu proceedings figure 4. box plot of the development of the young’s modulus ecrf . figure 5. box plot of the development of the young’s modulus gcr. higher values. no clear trend can be seen at the age of 28 and 180 days, however, at the age of 1 year the situation becomes opposite to that of 14 days. the mortar with sra performs better at this age; with the exception of the young’s modulus measured by the resonance method (compression wave) where the two mortars do not differ with any statistical significance. at the age of 2 years it again depends which property is being observed – two properties reach significantly higher values in the aas mortar with sra, whereas the remaining two are higher in the mortar without sra. the progress of the aas properties is very interesting, see fig. 6 and fig. 7. they show the progress as a percentage, with 100% being the average 28-day value for all properties. over the first 28 days there is a clear increase in all the properties of both mortars. their young’s modulus reaches over 80% of its 28-day value in as few as 3 days and the shear modulus rises to more than 70%. after 28 days, however, there is a fairly rapid decrease in all the properties. this is where the aas mortar with sra begins to differ from the aas mortar without it. the mortar containing sra saw a drop in the values of the young’s modulus below the 3-day value down to 70%, however, after that they began to slowly rise again – ecu and ecrl return to nearly 90% of their 28-day values in two years, and ecrf and gcr even exceed them. the decrease in all the properties of the mortar without sra continues until the age of 180 days, with their minimum value being lower than in all the properties of the mortar containing the shrinkage-reducing admixture. after that, the mortar also begins to see an improvement; in ecu and ecrl the trend is even more pronounced than in mortar aas-1. 4. conclusion in conclusion, an influence of the shrinkage-reducing admixture on the dynamic moduli of elasticity in aas mortars was not statistically proved. however, the development of the young’s moduli and the shear modulus follows a very interesting trend. over the first 28 days these values increase, followed by a marked decrease, which applies to the mortar without the shrinkage-reducing admixture as well. while the aas mortar with the admixture begins to slowly recover 4 vol. 22/2019 the influence of a shrinkage reducing admixture. . . figure 6. development of the young’s modulus ecu and ecrl, the 28-day value represents 100%. figure 7. development of the young’s modulus ecrf and the shear modulus gcr, the 28-day value represents 100%. at the age of 90 days, the mortar without it continues to suffer a decrease until the age of 180 days when the values begin to rise again. acknowledgements this paper has been written as part of the project no. ga17-14302s “experimental analysis of the early-age volume changes in cement-based composites”, supported by the ga0 – czech science foundation and project no. fast/fch-j-18-5595 “reduction of chemical and autogenous shrinkage of alkaline activated slag”. references [1] h. ye, a. radlińska. shrinkage mitigation strategies in alkali-activated slag. cement and concrete research 101:131–143, 2017. doi:10.1016/j.cemconres.2017.08.025. [2] p. acker. swelling, shrinkage and creep: a mechanical approach to cement hydration. materials and structures 37(4):237–243, 2004. doi:10.1007/bf02480632. [3] j. provis, j. s. j. van deventer (eds.). alkali activated materials: state-of-the-art report, rilem tc 224-aam, vol. 13. 2014. doi:10.1007/978-94-007-7672-2. [4] y. ding, j.-g. dai, c.-j. shi. mechanical properties of alkali-activated concrete: a state-of-the-art review. construction and building materials 127:68–79, 2016. doi:10.1016/j.conbuildmat.2016.09.121. [5] v. bílek, l. kalina, r. novotný, et al. some issues of shrinkage-reducing admixtures application in alkali-activated slag systems. materials 9(6):462, 2016. doi:10.3390/ma9060462. [6] m. palacios, f. puertas. effectiveness of mixing time on hardened properties of waterglass-activated slag pastes and mortars. aci materials journal 108:73–78, 2011. [7] b. kucharczyková, v. bílek jr., d. kocáb, o. karel. shrinkage of fine-grained composites based on alkali-activated slag. in non-traditional cement and concrete, vol. 761 of key engineering materials, pp. 7–10. trans tech publications, 2018. doi:10.4028/www.scientific.net/kem.761.7. [8] v. bílek jr., l. kalina, j. koplík, et al. effect of a combination of fly ash and shrinkage-reducing additives on the properties of alkali-activated slag-based mortars. materiali in tehnologije 50(5):813–817, 2016. doi:10.17222/mit.2015.133. [9] f. collins, j. sanjayan. workability and mechanical properties of alkali activated slag concrete. cement and concrete research 29:455–458, 1999. doi:10.1016/s0008-8846(98)00236-1. [10] c. shi, r. l. day. a calorimetric study of early hydration of alkali-slag cements. cement and concrete research 25(6):1333–1346, 1995. doi:10.1016/0008-8846(95)00126-w. [11] f. collins, j. sanjayan. cracking tendency of alkali-activated slag concrete subjected to restrained shrinkage. cement and concrete research 30:791–798, 2000. doi:10.1016/s0008-8846(00)00243-x. 5 http://dx.doi.org/10.1016/j.cemconres.2017.08.025 http://dx.doi.org/10.1007/bf02480632 http://dx.doi.org/10.1007/978-94-007-7672-2 http://dx.doi.org/10.1016/j.conbuildmat.2016.09.121 http://dx.doi.org/10.3390/ma9060462 http://dx.doi.org/10.4028/www.scientific.net/kem.761.7 http://dx.doi.org/10.17222/mit.2015.133 http://dx.doi.org/10.1016/s0008-8846(98)00236-1 http://dx.doi.org/10.1016/0008-8846(95)00126-w http://dx.doi.org/10.1016/s0008-8846(00)00243-x m. alexa, d. kocáb, b. kucharczyková, j. kotrla acta polytechnica ctu proceedings [12] a. neto, m. cincotto, w. repette. drying and autogenous shrinkage of pastes and mortars with activated slag cement. cement and concrete research 38(04):565–574, 2008. doi:10.1016/j.cemconres.2007.11.002. [13] f. collins, j. sanjayan. effect of pore size distribution on drying shrinking of alkali-activated slag concrete. cement and concrete research 30(09):1401– 1406, 2000. doi:10.1016/s0008-8846(00)00327-6. [14] d. kocáb, l. topolář, v. bílek jr., et al. experimental analysis of the development of compressive strength, modulus of elasticity and acoustic emission parameters of alkali-activated composites. in special concrete and composites 2017, vol. 760 of key engineering materials, pp. 266–271. trans tech publications, 2018. doi:10.4028/www.scientific.net/kem.760.266. 6 http://dx.doi.org/10.1016/j.cemconres.2007.11.002 http://dx.doi.org/10.1016/s0008-8846(00)00327-6 http://dx.doi.org/10.4028/www.scientific.net/kem.760.266 acta polytechnica ctu proceedings 22:1–6, 2018 1 introduction 2 experiment 2.1 material and specimens 2.2 test methods 3 results and discussion 4 conclusion acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2018.15.0025 acta polytechnica ctu proceedings 15:25–30, 2018 © czech technical university in prague, 2018 available online at http://ojs.cvut.cz/ojs/index.php/app isogeometric beam element extended for gridshell analysis edita dvořáková∗, bořek patzák department of mechanics, faculty of civil engineering, czech technical university in prague, thákurova 7, 166 29 prague 6, czech republic ∗ corresponding author: edita.dvorakova@fsv.cvut.cz abstract. the design of gridshell structures is a complicated process, as the resulting shape of structure depends on initial undeformed grid geometry as well as on the history of boundary conditions used to form the final structure. in practice, the physical model is often used to determine the shape of the structure and the initial grid at the same time. introducing isogeometric analysis into a design of gridshells simplifies the design process as the problem can be easily recalculated when initial grid or boundary conditions change and the resulting shape can be immediately illustrated. the presented paper discusses possibilities in isogeometric gridshell modeling and proposes possible solutions of identified problems. keywords: curved beam element, gridshell structure, isogeometric beam element, nurbs. figure 1. realization of a multipurpose gridshell pavilion in třebešov [1]. 1. introduction gridshell is a light-weight structure gaining its strength from a doubly-curved geometry. such a structure is typically made of wood and it is constructed from initially straight grid of laths. the laths are mutually connected by joints allowing only for the in-plane rotation. the final shape of the structure is reached by subsequent introduction of a load or a prescribed displacement at selected nodes. once the desired shape is reached, the joints are tightened in order to fix the in-plane rotation and stiffen the structure. see fig. 1 for an example of a gridshell structure. it is important to design the structural shape properly to reach high low-bearing capacity. the design process of a gridshell can be divided into three main parts: form-finding, analysis, and construction. formfinding is typically performed by an architect who is seeking for the optimal shape of the gridshell taking into the account the aesthetics and structural behavior of the structure. either physical modeling or computer simulation is used for form-finding, nevertheless these methods (and available tools) do not provide knowledge about structural behavior. when the desired shape is defined the structure is assessed in a finite element method (fem) solver, however the analysis considers only the final shape of the structure. there are no available tools which would take into account the history of the construction steps and would allow to assess all the intermediate construction stages. currently the construction itself relies on trial-error methods supported by a human intuition and experience. our goal is to develop a complex tool for analysis assisted form-finding which would allow to find the optimal shape of gridshell and which would help to propose sequence of construction steps. the idea is to use the isogeometric approach which eases incorporation of the analysis within available design tools and which may bring higher accuracy into the analysis of doubly-curved geometry thanks to the more suitable geometry description. the presented paper discusses the advantages of isogeometric gridshell modeling and also points out some issues connected to its application. 2. isogeometric analysis of beams isogeometric analysis has been introduced by thomas hughes in 2005 [2] with the aim to bridge the gap between the computer-aided design (cad) and the finite element analysis (fea). this goal has been achieved by using the same cad basis functions for geometry description and unknowns approximation in the analysis. thus one model can be shared between the design and the analysis. 25 http://dx.doi.org/10.14311/app.2018.15.0025 http://ojs.cvut.cz/ojs/index.php/app edita dvořáková, bořek patzák acta polytechnica ctu proceedings figure 2. description of b-spline (nurbs) finite element geometry. the geometry is modeled by one patch consisting of two knotspans with quadratic nurbs approximation. 2.1. nurbs-based analysis the most wide-spread technology in cad industry are nurbs (non-uniform rational b-splines). the nurbs curve is obtained by linear combination of cartesian coordinates of the control points p using nurbs functions rpi c(ξ) = n∑ i=1 rpi (ξ)pi. (1) each nurbs function is generated by weighting bspline functions np i with a given weight wi associated with control point pi rpi (ξ) = ni,p(ξ)wi∑n j=1 nj,p(ξ)wj . (2) b-spline functions form a special subset of nurbs functions (corresponding to constant wi) and are derived recursively starting with a piecewise constant functions ni,0(ξ) = { 1 if ξi ≤ ξ < ξi+1 0 otherwise , (3) ni,p(ξ) = ξ − ξi ξi+p − ξi ni,p−1(ξ) + ξi+p+1 − ξ ξi+p+1 − ξi+1 ni+1,p−1(ξ), (4) where p is the degree of the b-spline function, ξi is the coordinate of the ith-knot and ξ ∈ 〈0, 1〉 is a parametric coordinate. the computational domain in isogeometric analysis (iga) is firstly divided into patches, which are further divided into knotspans (often referred to as elements in iga). understanding of knotspans is similar to elements in standard fem, nevertheless higher continuity up to cp−1 between knotspans can be achieved naturally using nurbs, unlike c0 in standard fem. moreover, the nurbs basis functions are generally non-iterpolatory. in the knots (points which are dividing patch into knotspans), the continuity can be locally decreased up to c0 by knot multiplication, which is a standard technique in nurbs technology. this procedure imposes the interpolatory behavior of the functions at the particular point. for better understanding of nurbs see fig. 2, where mapping between real and parametric geometry is illustrated. for more information the reader could refer to [3]. 2.2. beam element formulation the formulation of the three-dimensional beam element [4] is derived in local coordinate system (t, n, b), where t, n, b are tangent, normal and binormal vectors and s is the curvilinear coordinate measured along the centerline of the beam. presented formulation is based on timoshenko beam theory and therefore, in three-dimensional space, there are six independent unknowns: tangential displacement ut, normal displacement un, binormal displacement ub and rotations θt, θn, θb (see fig. 3). a strain-displacement matrix b is defined as ε = br, (5) where ε = {εm, γn, γb, χt, χn, χb}t and r = {ut, un, ub, θt, θn, θb}t . b is derived from the following relations for membrane strain εm, shear strains γn and γb, torsional strain χt, and bending strains χn 26 vol. 15/2018 isogeometric beam element extended for gridshell analysis b(s) r(s) t(s)n(s) ut θt θb ub θn un figure 3. local coordinate system and degrees of freedom of a three-dimensional beam element. and χb εm = u′t − κun, χt = θ′t − κθn, γn = κut + u′n − τub − θb, χn = κθt + θ′n − τθb, (6) γb = τun + u′b + θn, χb = τθn + θ′b. curvature κ is given as κ = ∣∣∣∣∣∣∣∣d2r(s) ds2 ∣∣∣∣∣∣∣∣ (7) and torsion τ τ = dn(s) ds b(s), (8) where r(s) is the position vector. the material stiffness matrix d is given by constitutive relations n = eaεm, mt = gikχt, qn = ganγn, mn = einχn, (9) qb = gabγb, mb = eibχb, where e is the young’s modulus, g is the shear modulus, ik is the torsional moment, in, ib are the moments of inertia, a is the cross-sectional area, ab, an are the reduced shear areas. finally, the element stiffness matrix k is defined as k = ∫ l 0 btdb ds, (10) where the integral is evaluated using gaussian quadrature, which is not exact for nurbs functions, but it has a sufficient accuracy. 2.3. numerical locking well-known problem of numerical locking of timoshenko beam elements in standard fem occurs also 0 0.5 1 x -0.5 0 0.5 q n shear force q 0 0.5 1 x 0 0.1 0.2 0.3 m b bending moment m figure 4. simple gridshell structure modeled by four patches with cubic approximation. joint is modeled by connection of patches. resulting internal forces overlap exact solution. for the isogeometric formulation. the same approximation order for displacements and for rotations leads to the shear strains of higher order than bending moments, while it should be vice-versa. moreover the constant strains cannot be approximated along the entire patch due to the field inconsistency in the shear terms. the reduced integration, popular locking-treatment in standard fem for its computational efficiency, is not convenient for iga, as the optimal reduced integration rule depends on the choice of approximation order and continuity and such a rule is hard to be determined. on the contrary, discrete shear gap (dsg) method [5, 6] or b̄-method [5] can be used independently of the choice of approximation, the latter being used in the presented implementation of the element. the main idea of the b̄-method is to project membrane strain εm and shear strains γn, γb onto a basis of lower order. for example, the contribution of membrane strain εm to the b̄ matrix is b̄m = [ b̄ 1 m b̄ 2 m 0 0 0 0 ] , (11) where for example b̄ 1 mij = ∫ l 0 ñin ′ j ds (12) and where nj is the jth basis function of the original basis and ñi is the ith basis function of the lower order basis. the membrane contribution to stiffness matrix km is km = ea b̄ t mm̃ −1 b̄m (13) where m̃ is the element “mass matrix” calculated in 27 edita dvořáková, bořek patzák acta polytechnica ctu proceedings 0 0.5 1 x -0.5 0 0.5 q n shear force q 0 0.5 1 x -0.2 0 0.2 0.4 m b bending moment m 0 0.5 1 x -1 0 1 q n shear force q 0 0.5 1 x -0.2 0 0.2 0.4 m b bending moment m a) c3-continuity b) c2-continuity 0 0.5 1 x -1 0 1 q n shear force q 0 0.5 1 x -0.2 0 0.2 0.4 m b bending moment m 0 0.5 1 x -0.5 0 0.5 q n shear force q 0 0.5 1 x 0 0.1 0.2 0.3 m b bending moment m c) c1-continuity d) c0-continuity figure 5. resulting internal forces (blue continuous line) and exact solution (red dash-dot line) for different continuity at the position of concentrated force load. the lower order basis m̃ij = ∫ l 0 ñiñj ds. (14) the analogical procedure can be followed also for the shear strains γn and γb. 3. application to gridshells isogeometric formulation of beam element allows to perform the analysis directly within the cad system, however the modeling of gridshells with presented isogeometric beam element formulation is not straightforward. there are two possible approaches to modeling gridshells: a) multiple patches per one lath, b) single continuous patch per lath. 3.1. lath modeled by multiple patches this approach is similar to the use of standard fem, where the joint is usually modeled by inserting a node. in iga, it means to assign at least one patch to each lath segment between two joints leading to the interpolatory degrees of freedom at particular points (joints). this approach is simple and does not require any additional implementation, nevertheless it leads to loss of some advantages of isogeometric formulation, namely loss of higher inter-element continuity along the entire lath and loss of possibility to handle one lath as a single entity, which might be highly convenient for the development of a complex design tool. a simple test has been performed to verify the "multiple-patches" approach. two laths mutually connected in the middle, each modeled by two patches with one knotspan and cubic b-spline approximation, have been subjected to the transverse force placed at the joint. the model and resulting shear force γn and bending moment χb are shown in fig. 4. the results 0 0.5 1 -0.1 0 0.1 n * 0 0.5 1 0 0.05 0.1 n v * figure 6. additional basis functions to support case of concentrated force load on 2d straight beam with nurbs approximation. overlap the exact solution and prove the possibility to use this modeling technique for gridshell analysis. 3.2. lath modeled by a single patch isogeometric approach offers a possibility to model each grid lath by a single patch. such a model requires to add additional constraints to enforce the compatibility of displacements and rotations in joints as the compatibility cannot be achieved by sharing particular degrees of freedom due to the non-interpolatory nature of isogeometric basis functions. a special boundary condition based on the lagrange multipliers method has been implemented to support this approach. the potential energy is enhanced by an additional term yielding the required continuity wint + ∑ k λk(r̄ki − r̄kj ) = wext, (15) where λk is vector of lagrange multipliers for kth joint, r̄ki and r̄kj are the vectors of constrained unknowns at the kth joints on the ith and jth beams, respectively. note, that due to the non-interpolatory nature of nurbs basis functions, the vectors r̄ki and r̄kj have to be expressed as a linear combination of the control points values using nurbs basis functions evaluated 28 vol. 15/2018 isogeometric beam element extended for gridshell analysis 0 0.5 1 x -1 -0.5 0 0.5 1 q n shear force q 0 0.5 1 x 0 0.1 0.2 0.3 m b bending moment m 0 0.5 1 x -2 -1 0 1 2 q n shear force q 0 0.5 1 x 0 0.1 0.2 0.3 0.4 m b bending moment m figure 7. resulting internal forces for simply supported beam subjected to a) a concentrated force, b) a concentrated force and continuous force loading. nurbs approximation enriched by the functions tailored to support case of concentrated force has been used. the results overlap the exact solution. at the joint position. the local coordinate ξi (ξj) of the joint on the beam i (j) has to be calculated according to the parametrization. the physical meaning of lagrange multipliers λ corresponds to the forces and moments required to enforce the compatibility and thus such an approach results in concentrated force and moment loadings at the joints positions. the problem of such a formulation is the inability of nurbs basis functions to represent exact solution with discontinuities in strains and internal forces corresponding to concentrated loadings. this inability is a consequence of the higher continuity of nurbs basis along the entire patch. this problem is documented on the similar test as the one used in section 3.1 (see fig. 4). unlike in previous section, each lath is now modeled by a single patch using just a single span with cubic bspline approximation, which results in c3-continuity at the joint position causing the oscillations in internal forces, see fig. 5a for the results. the continuity can be lowered to c2 by placing a knot at the particular position, the continuity can be further reduced up to c0 by increasing the knot multiplicity. the resulting internal forces for all choices of continuity are shown in fig. 5. the oscillations in the numerical solutions are sufficiently removed only in the case of c0-continuity. note, that the procedure of decreasing the continuity to c0 results in the same model as in sec. 3.1 and thus the advantages of isogeometric analysis are eliminated. the main idea, how to preserve the higher continuity while avoiding oscillations in numerical solutions, is to enrich the nurbs basis by the special basis functions tailored to match the exact solution. in this paper, the derivation of the additional basis functions is illustrated on the case of two-dimensional straight beam subjected to concentrated force. from the knowledge of the exact solutions of bending moment mex and shear force qex under the unit force mex = eiβ′(x), (16) qex = kga (v′(x) + β(x)) , (17) where β is a rotation and v is a transverse displacement, the additional basis functions n∗β , n∗v can be derived n∗β = β = ∫ 1 ei mex dx, (18) n∗v = v = ∫ 1 kga qex − β dx. (19) see fig. 6 for the resulting basis functions. the final approximation for the enhanced solution is v(x) = ∑ nivi +n∗v v ∗, (20) β(x) = ∑ niβi +n∗ββ ∗. (21) the integration constants were determined with respect to the boundary conditions of a simply supported beam. the performance of the enhanced approximation has been tested by means of two simple test. firstly, simply supported beam subjected to the concentrated force load was considered, in the second test the concentrated force has been combined with continuous loading. the resulting internal forces for both load cases overlap exact solutions, see fig. 7. 29 edita dvořáková, bořek patzák acta polytechnica ctu proceedings 4. conclusions two possible approaches to gridshell modeling have been presented. the simple fem-like approach, where the joints are modeled by connection of two patches, provides good results, however the advantages of isogeometric approach are lost. in order to keep the higher continuity along the entire lath, it has to be modeled by a single patch. in such a case, the enforcement of continuity of displacements and rotations in joints by lagrange multipliers inevitably leads to application of concentrated forces and moments. it has been shown, that enhancement of nurbs basis functions by a set of the special functions is sufficient to support concentrated loadings. the paper documents the first necessary steps of the development of a complex design tool for gridshells. the presented idea of enhancement of the nurbs basis has to be combined with locking treatment to provide robust element formulation, moreover due to the large deformations, the formulation has to be extended for geometrically non-linear analysis. acknowledgements the financial support of this research by the grant agency of the czech technical university in prague (sgs project no. sgs18/037/ohk1/1t/11) is gratefully acknowledged. references [1] a. vaňek. multipurpose gridshell pavilion in třebešov, czech republic, physical model as a groundwork for realization, harvest super, 2016. [2] t. j. r. hughes, j. a. cottrell, y. bazilevs. isogeometric analysis: cad, finite elements, nurbs, exact geometry and mesh refinement. comput methods appl mech engrg 194:4135–4195, 2005. [3] l. piegl, w. tiller. the nurbs book. springer-verlag berlin heidelberg, new york, 1997. [4] g. zhang, r. alberdi, k. khandelwal. analysis of three-dimensional curved beams using isogeometric approach. engineering structures 117:560–574, 2016. [5] r. bouclier, t. elguedj. locking free isogeometric formulations of curved thick beams. comput methods appl mech engrg 245-246:144–162, 2012. [6] r. echter, m. bischoff. numerical efficiency, locking and unlocking of nurbs finite elements. comput methods appl mech engrg 199:374–382, 2010. 30 acta polytechnica ctu proceedings 15:25–30, 2018 1 introduction 2 isogeometric analysis of beams 2.1 nurbs-based analysis 2.2 beam element formulation 2.3 numerical locking 3 application to gridshells 3.1 lath modeled by multiple patches 3.2 lath modeled by a single patch 4 conclusions acknowledgements references doi:10.14311/app.2021.30.0093 acta polytechnica ctu proceedings 30:93–97, 2021 © czech technical university in prague, 2021 available online at http://ojs.cvut.cz/ojs/index.php/app detection of the presence of microorganisms during production of plasterboard hana sekavováa,b,∗, zdeněk prošeka, pavla ryparováa a czech technical university in prague, faculty of civil engineering, department of mechanics, thákurova 7, 166 29 prague 6, czech republic b knauf praha spol. s r.o., mladoboleslavská 949, 197 00 prague 9, czech republic ∗ corresponding author: hana.sekavova@fsv.cvut.cz abstract. this article is focused on detection microorganisms appearing during production of gypsum plasterboards. the research is carried out within the project tačr zéta tj02000235 sádrokarton se zvýšenou odolností proti bilogické degradaci (gypsum plasterboard with increased biodegradation resistance). this article shows first part of this issue what was detection of the most critical place of production, where is the highest risk of bacterial contamination and it open the area to find suitable modification of production process or recipes of gypsum plasterboards. keywords: gypsum plasterboard, microorganism, mold, stucco. 1. introduction gypsum plasterboard is basically a composite material that consists primarily of a gypsum core and paper. it is the most widely used element of dry construction, which spread in europe in the 1970s. there was a massive expansion of this type of construction in the early 1990s in the czech republic, when imported gypsum boards from neighboring countries first began to appear on the market, and in the mid-1990s, gypsum boards produced domestically [1]. the term dry construction, of course, includes not only plasterboard, but also other types of building boards and they of course include accessories for their assembly profiles, screws, sealants, etc. the research of detection of the presence of microorganisms during production of plasterboard has two main targets. the first one is find solution for presence molds during storage, construction and use and the second one is prevention of mold during storage of waste before recycling and its subsequent introduction back into production process. recycling in general has been a much-discussed and supported topic with regard to sustainable development in recent years. research in the field of concrete recycling is very popular, but even gypsumbased building elements are not left behind. the recycling of gypsum itself is, by its nature, simple and repeatable. it is only a conversion of calcium sulphate dihydrate to its hemihydrate and back again. recyclability in the case of gypsum plasterboard is partially limited by the carton and other components added to the gypsum slurry, which reduce the purity of the calcium sulphate and some fundamentally affect the rehydration behavior. the information about the effect of the use of recycled material based on gypsum on the properties are mentioned in tasks from years 2018 and 2019 [2, 3]. influence of living organism to buildings, therefor building materials has many aspects. what can we observe at first sight is esthetic damage, but there is also mechanical damage and health effect [4]. that is the reason why there is lot of researches focus on this topic in many researches. the influence of the microoraganisms what occupied the building parts is very time solved topic. from many of them is popular detection and solution for etics [5, 6] and last but not least problematic with organism in wood construction [7]. with plasterboards have these topics similar base – stucco and gypsum like binder and paper like a product of wood. the target of this research is find the most critical place of production, where is the highest risk of bacterial contamination. 2. material at the beginning of this research was detected places and materials across the plant počerady. the schema of production of plasterboards is showed on figure 1. place no.1 is unwinding of bottom paper (face side paper). no.3 is unwinding of upper paper (backside paper). position no.2 shows main mixer preceded by preparatory station of dry and wet component. all components are mixed in main mixer and gypsum slurry is put between bottom and upper paper and this slurry together with paper continues through forming table to the setting (no.4). the gypsum slurry hardens on the setting belt. the important feature of this wet belt of product is possibility to cut. the cut is made on knife (no.5), where is cut the endless belt of plasterboard product on the individually boards. the boards are completed and transport by cross conveyor and turner (no.6) to tipple (no.7), which distributes the boards to dryer (no.8). the dryer has 8 floors and standard boards are dried in 93 http://dx.doi.org/10.14311/app.2021.30.0093 http://ojs.cvut.cz/ojs/index.php/app h. sekavová, z. prošek, p. ryparová acta polytechnica ctu proceedings figure 1. schema of production of plasterboards. sample identified microorganisms paper pulp sphingomonas paucimobilis, pseudomonas aeruginosa process water pseudomonas aeruginosa, penicillium sp. gypsum plasterboard stenotrophomonas maltophilia, penicillium sp., mucor stucco penicillium sp. table 1. identification of the samples used to determine the resistance of the materials. two lines. počerady plant has dryer which is consist of prezone, 1st zone, 2nd zone and the cooling section. during way through the dryer is evaporated all of not necessary water. the temperature of gypsum core has to be under 100 °c all the time. boards are transported after dryer by tipple (no. 9) to turner and they are completed on turner and they continue to bundler (no. 10), where are boards cut (possibly mill) to final dimension (beveling). the last process is assembly of plasterboards on transport pallets (no.11).the boards then travel to the finished goods warehouse the samples of input raw materials and final products were collected in the period from march to june 2019. the sampled plasterboards were two main products knauf white and knauf green. knauf white is the gypsum plasterboard type a. it is plasterboard with a face to which suitable gypsum plasters or decoration may be applied. knauf green is the gypsum plasterboard type h. it is plasterboard with reduced water absorption rate. these boards are produced with additeves to reduce the water absorption rate and they may be suitable for special applications in which reduce water absorption properties are required to improve the performance of the board [8]. knauf green has total water absorption under 10 % and it fulfill the category h2 according to eurepean standard en 520. the main products for production of plasterboards are stucco, water and paper. stucco is calcinated of fgd gypsum, which is produced during desulphurisation process in power plant. process water is filtrated surface water. input raw materials, what were sampled, were stucco, fgd gypsum, process water, paper pulp and paper. intermediate were gypsum slurry (after the mixer) and wet board (cut at knife, before drying process). 3. experimental methods and results 3.1. determination of a consortium of microorganisms identification of microorganisms was performed microscopically according to significant features on suitable media and according to their enzyme properties using commercial kits. enriched medium total agar (oxoid) was used to identify bacteria. cultivation was performed at 30 °c for 48 hours. molds were identifiable after culturing on czapek dox agar (oxoid) at 25 °c for three to seven days. similar microorganisms occur throughout the production process and therefore a specific source of contamination cannot be clearly identified. the most likely source of mold is the free flow of air and the movement of people and vehicles between the outdoor area and the production hall. the largest source of bacteria is pulp and process water. similar microorganisms were found in the air 94 vol. 30/2021 detection of the presence of microorganisms in plasterboard figure 2. identification of naturally occurring molds. sample kind of sample 1 gypsum plasterboard type h with paper 2 gypsum plasterboard type h without paper 3 gypsum plasterboard type a with paper 4 gypsum plasterboard type a without paper 5 gypsum slurry (hardened) type h 6 gypsum slurry (hardened) type a 7 gypsum labor slurry (stucco + water; hardened) 8 paper green 9 paper white table 2. identification of the samples used to determine the resistance of the materials. and on the surface of the production line, but their amount is negligible. results are showed in table 1. the designation of the samples in the microbiological study is given in the following table 2. individual materials were contaminated with molds of the genus penicillium, aspergillus and mucor. these molds were also found on the samples taken in production process. the highest contamination was detected in materials with paper (sample 1 and sample 3). the figure 2 shows identification of naturally occurring molds. 3.2. monitoring material degradation microbial degradation was performed according to the standard iso en 846 [9] that determines the degree of resistance of the material to the occurrence of molds (determines their resistance to biodegradation). the samples were 1 x 1 cm in size, their height depended on type of material. sterilization was performed using uv radiation, when microorganisms were deactivated from the surface while maintaining material and chemical parameters of individual samples. sterilization methods degrading the original properties of the material was not used. samples were inoculated with a consortium of molds and then placed at 25 ± 3 °c and monitored for 41 days. the consortium of fungi contained microorganisms from the czech collection of microorganisms: paecilomyces variotti, chaetomyum globosum, penicillium funiculosum, aspergillus terreus, penicillium brevicompactum. the evaluation of the resistance of the material took place visually on the 14th, 22nd, 29th and 41st day. sample 1 has isolated colonies of molds after 14 days of experiment. on day 22, coverage was already 80% and more (figure 3). sample 2 is very resistant to mold. the samples showed isolated mold hyphae, but they grew more out of the medium. at the end of the experiment, fungi were visible at the bottom of the flanks and isolated colonies of mold at the top (figure 4). mold colonies were already visible on day 14 on sample 3 and on day 22 the samples were completely covered (figure 5). sample 4 was the most resistant to the occurrence of mold, after 41 days only isolated colonies of mold were visible on the surface, the sides of the samples were only partially covered (approximately 30%). the mixture of sample 5 had a similar result, only the coverage rate after 41 days was several percent higher (figure 6). sample 6 had almost the entire sides covered and mold colonies were already present on the surface on day 22. the coverage rate remained almost unchanged by the end of the experiment. sample 7 prevented the growth of mold until day 41, where iso95 h. sekavová, z. prošek, p. ryparová acta polytechnica ctu proceedings figure 3. sample 1 after 14 (left) and 22 (right) days. figure 4. sample 2 after 25 (left) and 41 (right) days. figure 5. sample 3 after 11 (left) and 25 (right) days. figure 6. sample 4 (left) and sample 5 (right) after 41 days. 96 vol. 30/2021 detection of the presence of microorganisms in plasterboard figure 7. sample 6 (left) and sample 7 (right) after 41 days. figure 8. sample 8 (left) and sample 9 (right) after 11 days. lated colonies were visible, both on the surface and on the sides (figure 7). paper samples were most prone to mold growth and growth began as early as the first week (figure 8). 4. conclusions from the above results it is clear that the most risky part of gypsum plasterboards is the paper part. the best results achieve the samples of hardened gypsum slurry type h (impregnate) and hardened gypsum slurry prepared in laboratory from stucco and water. the experiment was designed to simulate a very demanding environment and in the case of the use of these materials in real constructions, the time of microbial contamination depends on the conditions, which are humidity, temperature and possible pollution as a source of nutrients for microorganisms. these results are the basis for subsequent modifications to prevent the development of microorganisms in gypsum plasterboard systems. acknowledgements this paper was financially supported by the ta cr research a project under the number tj02000235 and by czech technical university in prague under no. sgs project sgs19/148/ohk1/3t/11. the authors also thank knauf, praha ltd. for the supplied samples and materials and the center for nanotechnology in civil engineering at the faculty of civil engineering, czech technical university in prague. references [1] m. nyč. sádrokarton. grada publishing as, 2005. [2] j. herrmann. možnosti recyklace výrobkŭ na bázi sádry 2019. [3] h. sekavová, j. herrmann, z. prošek, et al. the influence of the use of recycled gypsum on the properties of gypsum products in labor conditions. acta polytechnica ctu proceedings 26:81–85, 2020. [4] r. wasserbauer. biologické znehodnocení staveb [biological damage of structures]. abf arch: praha, prague 2000. [5] n. antošová. impact of biocorrosion on the durability of etics and empirical findings about the periodocity of maintenance. slovak journal of civil engineering 21(2):21–28, 2013. [6] v. kučeriková, m. kraus, d. kubečková. analysis of the degradation of external plasters in the buildings with etics. in advanced materials research, vol. 1041, pp. 15–18. trans tech publ, 2014. [7] m. havrlik, p. ryparová. protection of wooden materials against biological attack by using nanotechnology. acta polytechnica 55(2):101–108, 2015. [8] čsn en 520+a1. sádrokartonové desky definice, požadavky a zkušební metody. praha: úřad pro technickou normalizaci, metrologii a státní zkušebnictví 2010. [9] čsn en iso 846. plasty hodnocení působení mikroorganismů. praha: úřad pro technickou normalizaci, metrologii a státní zkušebnictví 2019. 97 doi:10.14311/app.2021.30.0007 acta polytechnica ctu proceedings 30:7–11, 2021 © czech technical university in prague, 2021 available online at http://ojs.cvut.cz/ojs/index.php/app plasma modification of microfibers application to lightweight cement composite containing recycled concrete jakub ďurejea,∗, zdeněk prošeka, jan trejbala, pavel tesáreka, štěpán potockýb a czech technical university in prague, faculty of civil engineering, department of mechanics, thákurova 7, 166 29 prague 6, czech republic b academy of sciences of the czech republic, institute of physics, cukrovarnická 10, 162 53 prague 6, czech republic ∗ corresponding author: jakub.dureje@fsv.cvut.cz abstract. the article deals with the optimalization of composition for reinforced lightweight cement composite containing micronized recycled concrete, which will be used to produce masonry blocks. the composite material is reinforced with polypropylene microfibers. to increase the cohesion between the fibers and the cementitious matrix, the optimal modification using oxygen plasma was chosen. furthermore, a suitable foaming agent was chosen to lighten the cement matrix. a suitable ratio of cement and micronized recycled concrete was determined. finally, a cement composite was made from the optimized components. the mechanical properties of this composite were tested. the resulting mechanical properties of the lightweight samples were compared with the non-light samples. keywords: lightweight cement composite, micronized recycled concrete, oxygen plasma, plasma treatment. 1. introduction one of the most important properties of cement composites is the cohesion of the fiber with the matrix. cohesion depends on a matrix in close proximity to the fiber, on the interfacial transition zone [1, 2]. chemical and mechanical modifications of fiber surfaces are used to increase the cohesion between the fibers and the matrix. manufacturers of fibers most often apply mixtures of various chemicals to their fibers in order to increase the cohesion of the fibers with the cement matrix. however, chemical treatments are non-ecological [3]. using plasma, it is possible to modify the surface both mechanically and chemically. the process of plasma modification depends on many parameters, such as the time of modification, the working gas, the input power, etc. plasma modifies only the surface of fibers, therefore occurs to almost negligible loss of mechanical properties of the fiber itself. in addition, the process of plasma modification is more environmentally friendly than conventional methods of chemical modification [4, 5]. the most common use of recycled concrete (more than 90%) is in its unbound form as a backfill under the road. another possibility is its use in bonded form as a substitute for aggregates in cementitious composite materials. with a suitable production process and composition, cementitious composites containing recycled concrete can achieve similar mechanical properties as same composites without recycled concrete. for the production of composite material, micromilled recycled concrete was chosen, which was made from the finest fractions of recycled concrete, which is currently considered waste [6–8]. to lighten the cement composite, it is necessary to choose a foaming agent that has sufficient stability and frothiness after mixing with water and foaming. high foaming causes the formation of many air cavities in the material. the stability of the foam must ensure that the foam transfers load from the cement mixture until the material has sufficient load-bearing capacity to transfer its own weight. stability affects the resulting mechanical strength [9]. 2. materials and samples portland cement cem i 42.5 r from českomoravský cement, a.s. (radotín) was used. micronized recycled concrete was made from drainage gutters. to produce micronized recyclate was used waste recycled concrete with a fraction of 0/16 mm, which was subsequently ground by high-speed mills. the grain size of the resulting micronized recyclate was less than 0.25 mm. a suitable foaming agent was selected not only from the foaming agents intended for concrete, but also the foaming agents intended for industrial cleaning were tested. polypropylene microfibers with a diameter between 18–32 µm and a length about 12 mm were chosen to reinforce the samples. plasma modification by oxygen plasma under reduced pressure was performed using a tesla vt 214 device. microfibers were inserted into the chamber of the device 7 http://dx.doi.org/10.14311/app.2021.30.0007 http://ojs.cvut.cz/ojs/index.php/app j. ďureje, z. prošek, j. trejbal et al. acta polytechnica ctu proceedings figure 1. surface of the polypropylene microfiber, left – reference fiber, right – fiber after treatment by oxygen plasma for 480 seconds. and then air was sucked out of the chamber, after that the pressure in the chamber was only about 22 pa. subsequently, oxygen was filled into the chamber, the pressure in the chamber increased to 60 pa. subsequently, the plasma modification process was performed for 60 or 480 seconds. after completion of the plasma modification process, the chamber was filled with air to atmospheric pressure and then the fibers were removed. images of the microfiber surface before and after plasma modification process are shown in figure 1. to determine the stability of the modifications, part of the fibers were left in an environment at a temperature of about 20 ◦c and a relative humidity of 50% for 5 days (pp 480 air) before application into the cement matrix, the other fibers were applied into the cement matrix immediately after the modification. selected surface modifications of the fibers that were subsequently used to produce reinforced samples are listed in table 1. sample modification storage time [s] time [days] pp ref 0 0 pp 60 60 0 pp 480 480 0 pp 480 air 480 5 table 1. selected surface modification of polypropylene microfibers by oxygen plasma. the composition of the lightweight samples without microfibers reinforcement is in table 2. the com8 vol. 30/2021 plasma modification of microfibers set cement recyclate water w/ (c + r) foaming agent [g] [g] [ml] [−] [ml] c 1500 0 480 0.32 0 cr 750 750 480 0.32 0 cs2 1500 0 480 0.32 1.5 crs2 750 750 480 0.32 1.5 cs4 1500 0 480 0.32 3.0 crs4 750 750 480 0.32 3.0 table 2. composition of the lightweight samples without microfibers reinforcement. set cement recyclate water w/ (c + r) foaming agent fibers fibers plasma [g] [g] [ml] [−] [ml] [g] [kg.m−3] modification crf 750 750 480 0.32 0 0.75 ≈ 1 no crfp 750 750 480 0.32 0 0.75 ≈ 1 yes crs2f 750 750 480 0.32 1.5 1.43 ≈ 1 no crs2fp 750 750 480 0.32 1.5 1.43 ≈ 1 yes crs4f 750 750 480 0.32 3.0 1.875 ≈ 1 no crs4fp 750 750 480 0.32 3.0 1.875 ≈ 1 yes table 3. composition of lightweight samples reinforced by plasma treated microfibers. position of the final lightweight samples (with microfibers reinforcement) is shown in table 3. 3. experimental methods the dynamic modulus of elasticity was measured for all non-lightened samples. the resonance method was used to measure the dynamic modulus of elasticity. because it is a non-destructive method, measurement was repeat in different times (1, 7, 14, 21 and 28 days). the method is based on the measurement of fundamental resonant frequencies of transverse, longitudinal and torsional oscillations. the dynamic modulus of elasticity was measured by brüel & kjær measuring set. the measuring set contains a brüel & kjær impact hammer type 8206, a brüel & kjær acceleration sensor type 4519-003, a brüel & kjær fron-end 3560b-120 measuring control panel and a control unit. the test specimens were placed on one support in the middle when measuring longitudinal or torsional oscillations; when measuring the transverse oscillation, the samples were placed on two supports at the prescribed distances (figure 2). the response sensor (s) was glued to the prescribed position, subsequently the oscillation was excited by the impact of the impact hammer (b – exciter). the measurement was recorded by measuring control unit and then the resonant frequency for each oscillation was determined using pulse labshop software version 14.0.1. finally, the fundamental resonant frequencies were determined and dynamic modulus of elasticity and shear modulus were calculated. destructive measurement was performed on laboratory samples 40×40×160 mm. each set of samples figure 2. diagram of the sensor location (s) and hammer impact point (b) when measuring longitudinal (left), transverse (middle), and torsional (right) oscillations using the resonance method [10]. contained 6 test specimens. samples were stored for 28 days into water basin in a laboratory environment at 22 ± 1 ◦c. first, three-point bending tests were performed using a hydraulic press, during which the test specimens were halved. then compressive tests were performed on the halves of the test specimens (approximately 40 × 40 × 80 mm) using a hydraulic press. 4. results and discussion to select the optimal composition of the cement composite material, the individual components were first selected during testing on non-light samples, then the components were applied to a lightweight matrix. the polypropylene fibers were modified by oxygen plasma. the plasma modification of the fiber surface has a positive effect on the mechanical properties of the tested cementitious composite containing micronized recyclate. during plasma modification process the fibers lose almost no weight, which proves 9 j. ďureje, z. prošek, j. trejbal et al. acta polytechnica ctu proceedings figure 3. flexural strength of samples containing fibers with different plasma modification times. figure 4. compressive strength and density of samples with different amount of foaming agent. figure 5. compressive strength and density of lightweight cementitious composites containing recycled concrete reinforced by plasma modified fibers. 10 vol. 30/2021 plasma modification of microfibers that the diameter of the fibers is almost not reduced and mechanical properties of fibers are approximately the same as before the modification. when examining the surface of the fibers by sem, the mechanical effect of the plasma modification on the surface was noticeable only for the fibers that were modified for 240 seconds or more. the mechanical properties of the test specimens were best for samples containing fibers modified with oxygen plasma for 60 seconds, with longer modification times the mechanical properties were no longer improving (figure 3). oxygen plasma modification for 60 seconds was chosen as the optimal modification for polypropylene microfibers. the optimal amount of recycled concrete was chosen based on previous experiments [11, 12]. the ratio between cement and micronized concrete recycled was determined to 1:1. a total of six different foaming agents were tested to lighten the samples. the best results for lightening the cement matrix containing micronized recycled concrete were achieved by the foaming agent sika lightcrete 400. the most lightened samples had a bulk density about 800 kg.m−3 and a compressive strength about 4 mpa (figure 4). finally, lightweight samples reinforced with plasma-modified fibrous reinforcement were tested. the amount of fibers was chosen based on the recommended dosage from the manufacturer, the samples contained one kg of fibers per m3 of cement composite. at the recommended amount of fibers, samples containing plasma-modified microfibers achieved almost the same flexural strengths as samples containing fibers without plasma modification. samples containing plasma modified fibers achieved slightly higher compressive strengths compared to samples without plasma modification. however, the increase in compressive strength was not very significant. the results are shown in figure 5. 5. conclusion plasma modification by oxygen plasma increases the cohesion of polypropylene microfibers with the cement matrix. the mechanical effect of plasma modification by oxygen plasma on the fiber surface is insufficient. when dosing micronized concrete recycled and cement into the matrix in a ratio of 1:1, good mechanical properties of the cement composite material are achieved. when the microfiber reinforcement was applied to the lightweight cementitious matrix, there was an increase in bulk density compared to the same lightweight material without fibrous reinforcement. the use of oxygen plasma to modify fibers surface and their application into a lightweight cement matrix does not have a significant effect on the mechanical properties of this lightweight material. in order to achieve better mechanical properties of this lightweight material, it is necessary to increase the amount of fibers and to choose another type of plasma modification. to achieve better mechanical properties of the reinforced lightweight cement matrix, it may be appropriate to modify fibers surface by argon plasma. the modification by argon plasma is expected to have a significantly higher mechanical effect on polypropylene fibers surface compared to the oxygen plasma modification. 6. acknowledgements this work was financially supported by the czech technical university in prague – the project sgs19/148/ohk1/3t/11. the authors also thank to the center for nanotechnology in civil engineering at the faculty of civil engineering, czech technical university in prague. references [1] a. bentur, s. mindess. fibre reinforced cementitious composites. new york, 2007. [2] j. p. ollivier, j. c. maso, b. bourdette. transition zone in concrete. advanced cement based materials 2(1):30–38, 1995. doi:10.1016/1065-7355(95)90037-3. [3] m. m. kabir, h. wang, k. t. lau, f. cardona. effects of chemical treatments on hemp fibre structure. applied surface science 276:13–23, 2013. doi:10.1016/j.apsusc.2013.02.086. [4] s. r. sabreen. cold gas plasma surface modification ű optimize plastics bonding adhesion, 2012. [5] j. tinneke, m. rino, n. geyter, et al. plasma surface modification of biomedical polymers: influence on cell-material interaction. plasma chemistry and plasma processing 32:1039–1079, 2012. doi:10.1007/s11090-012-9394-8. [6] v. w. y. tam, c. m. tam. a review on the viable technology for construction waste recycling. resources, conservation and recycling 47(3):209–221, 2006. doi:10.1016/j.resconrec.2005.12.002. [7] i. vegas, j. a. ibanez, a. lisabona, m. cortazar, a. s. abd frias. pre-normative research on the use of mixed recycled aggregates in unbound road sections. construction and building materials 25(5):2674–2682, 2011. doi:10.1016/j.conbuildmat.2010.12.018. [8] z. prošek, p. tesárek, j. trejbal. recycling and its use in concrete waste processing by high-speed milling. acta polytechnica ctu proceedings 21:28–32, 2019. doi:10.14311/app.2019.21.0028. [9] k. ramamurthy, e. k. nambiar, g. i. s. ranjani. a classification of studies on properties of foam concrete. cement and concrete composites 31(6):388–396, 2009. doi:10.1016/j.cemconcomp.2009.04.006. [10] čsn 73 1372 nedestruktivní zkoušení betonu–rezonačnní metoda zkoušení betonu, 2012. [11] z. prošek, j. trejbal, v. nežerka, et al. recovery of residual anhydrous clinker in finely ground recycled concrete. resources, conservation and recycling 155, 2020. [12] z. prošek, j. topič, j. ďureje, j. trejbal. srovná́ní vlivu mikromletého betonu a mramorové moučky na mechanické vlastnosti cementových past. in waste forum 2:262–267, 2018. 11 https://doi.org/10.1016/1065-7355(95)90037-3 https://doi.org/10.1016/j.apsusc.2013.02.086 https://doi.org/10.1007/s11090-012-9394-8 https://doi.org/10.1016/j.resconrec.2005.12.002 https://doi.org/10.1016/j.conbuildmat.2010.12.018 https://doi.org/10.14311/app.2019.21.0028 https://doi.org/10.1016/j.cemconcomp.2009.04.006 acta polytechnica ctu proceedings doi:10.14311/app.2020.26.0007 acta polytechnica ctu proceedings 26:7–12, 2020 © czech technical university in prague, 2020 available online at http://ojs.cvut.cz/ojs/index.php/app on tuning the dynamic load balancing fem framework michal bošanský∗, bořek patzák czech technical university in prague, faculty of civil engineering, department of mechanics, thákurova 7, 166 29 prague 6, czech republic ∗ corresponding author: michal.bosansky@fsv.cvut.cz abstract. developments in computer hardware are currently bringing new opportunities for numerical modelling. the current trend in technology is parallel processing making use of multiple processing units simultaneously to solve a given problem. this paper deals with exploring the parallel dynamic load balancing framework implemented in the finite element software. this framework is based on a domain decomposition paradigm for distributed memory model. the paper describes the improved technique to determine the actual processor weights related to performance of individual processing units. the load recovery consisting in mesh (re)partitioning is based on actual processor weights. the (re)partitioning process has to be performed during the simulation and whenever the load imbalance is significant. the performance of the proposed technique is tested on the benchmark problem and discussed. keywords: distributed memory, domain decomposition, load balancing, message passing, processor. 1. introduction the main goal of parallel algorithms is partitioning the problem into a set of smaller tasks that can be solved simultaneously. the problem can be partitioned only once before the solution process, which represents static load balancing. when the problem is partitioned during the solution, it represents dynamic load balancing, see [1]. ideal scalability is difficult to obtain because of the overhead cost of the parallel algorithm (synchronization and communication) and because some parts of the problem are essentially sequential. an effective load balancing process that (re)distributes the work to individual computing units is needed in many cases to obtain reasonable scalability. one of the important characteristics of the parallel algorithm is its computational scalability, which is the most important goal in parallel computing. the scalable parallel algorithm achieves a reduction in execution time by using more processing units, ideally in a linear trend. ideal scalability is difficult to obtain because of the overhead costs of the parallel algorithm (synchronization and communication). almost every parallel algorithm has an overhead cost compared to a sequential version. individual tasks cannot be executed concurrently without synchronizing and communicating with other tasks. some parts of the algorithm are also essentially serial and can only be executed by a single thread. in addition to speedup, parallel computing allows large and complex problems to be solved that could not be solved on a single, well-equipped machine. the finite element method (fem) has become a widely used tool for solving problems described by partial differential equations and been widely adopted by engineering and scientific communities as a reliable numerical tool. in mathematics, the fem is a numerical method for finding a solution to boundary value problems for an ordinary differential equation (ode) or a partial differential equation (pde). in the fem, differential equations are converted into an algebraic system of equations by using variational methods aided by decomposition of the problem domain into sub-domains called elements with an appropriate choice of interpolation functions. in many cases the resulting system of equations is nonlinear. in structural mechanics, the non-linearity can originate from nonlinear geometrical relationships (large deformations), from constitutive relationships, and from non-linear boundary condition (e.g. follower type of loading). nonlinear problems are solved incrementally, typically using the newton − raphson algorithm. this makes the nonlinear problem solution more demanding than linear problems. the problem is solved in a series of load or displacement increments in which the equilibrium state is iteratively searched. the paper examines the solutions of large scale engineering problems by using parallel computation based on a parallel load-balancing framework. this framework can significantly reduce computational time by using available hardware more efficiently. this paper presents improved algorithm for determining processor weights in load-balancing fem framework. when solving the real problem on the parallel computer, the workload of individual processors can change during the solution. the first source of imbalance originates from the character of the problem. for example, in nonlinear problems, the transition from initial elastic material response to nonlinear regime is often associated with increased computational cost and this 7 http://dx.doi.org/10.14311/app.2020.26.0007 http://ojs.cvut.cz/ojs/index.php/app michal bošanský, bořek patzák acta polytechnica ctu proceedings transition is often associated only to certain regions of overall domain. the second source of load imbalance includes external factors, which can change performance of individual processing nodes or communication network. this typically happens in non-dedicated cluster environments, where processing nodes and communication infrastructure is shared between users. in both cases, the gradual grow of imbalance can have significant effect on performance and on scalability. the load-balancing process is driven not only by emerging applications but also by emerging parallel architectures. these parallel architectures span many scales. for example, clusters have become viable alternatives to tightly coupled parallel computers in small scale systems. on the medium scale, supercomputers are constructed as networks of shared memory multiprocessors (smps) and have complex and nonhomogeneous interconnection topologies [2]. finally, on the largest scale, grid technologies have enabled computations on widely distributed systems, combining distributed clusters and supercomputers into a single computational resource. these grids are a source of extreme computational power and high network heterogeneity. in order to effectively distribute data from any program on such systems, partitioning must account for heterogeneity in the solution environment. load balancing requires evaluation of computing environment parameters (e.g. computing speed, memory and network availability) and determining how to apply this information in the load-balancing process (e.g. adjusting computational domain sizes, selecting partitioning algorithms). the capabilities and performance of the loadbalancing framework depends on many aspects, including problem at hand and hardware configuration and are illustrated on the solution of selected engineering problem. the advantages of implementing this approach are discussed in this paper. 2. introduction and overall design of load balancing framework the models are often solved on complex domains leading to many degrees of freedom and often nonlinear effects have to be taken into account. parallel algorithms should account for load imbalance between particular sub-domains. the imbalance can occur due to several reasons: (i) first load imbalance factor is part of the character of the solution process, as if switching from a linear to nonlinear response in some regions, (ii) second load imbalance factor is an external factor concerning resource relocation, typical in cases when a solution process is run in cluster environments where the available individual computing units are shared by other processes owned by the system or users and lead to changes in allocating processing power. the imbalance is detected in the solution process by monitoring processes during the run time of the process. the decision depends on the amount of load imbalance and the cost of load redistribution. redistributing the work leads to serializing the problem data (elements, nodes, boundary conditions) into messages sent over the network and a receiving process based on unpacking, followed by a topology update reflecting the new work distribution. the idea of parallelization strategy is based on the domain decomposition paradigm. in general, two dual partitioning techniques for the parallel distribution of finite element codes exist, node-cut and element-cut strategy [3]. in this paper, the node-cut strategy, dividing cut dividing the problem mesh into partitions is given in the node-cut technique. the node-cut strategy is assumed when cut dividing the problem domain runs through the nodes. nodes on mutual partition boundaries are called shared nodes, and nodes inside individual partitions are called local nodes. the node-cut partitioning scheme can be interpreted as mesh decomposition using cuts passing through shared nodes of the mesh without crossing any element. the cut strategy ensures duplication of the shared nodes. each processing node is responsible for assembling its contribution to the global system matrix by summing the contributions from individual elements. in order to minimize communication, it is natural that each processing node will assemble and maintain in its local memory the block of global stiffness matrix rows corresponding to unknowns on its partition. this is uniquely defined for local nodes, which are exclusively shared by local elements on individual partitions. for shared nodes, which are shared by elements from multiple partitions, ownership must be defined. in this work, the partition with the lowest rank sharing the shared node is the owner of the shared node and thus responsible for maintaining the corresponding row entry of the global stiffness matrix. it is clear that for shared nodes, the contributions to the corresponding row have to be received from partitions sharing a particular node. the assembly process requires global, unique numbering of equations to be established across the computing nodes in addition to local numbering on individual partitions [4]. partitioning based on the load balancing process can be affected by many factors. ideally, no processor during the parallel solving process should be waiting for another processor to complete the solution. in the typical fem code, the most of the work is proportional to individual elements. the total computational work is determined as the sum of the contribution of individual elements. the computational work of individual element is expressed relatively to computational work of reference element. the load balancing process is based on distributing the overall computational work to a group of available processors with respect to their relative performance. another significant factor concerns minimizing and reducing communication between computational partitions. the assembly of equilibrium equations for shared nodes requires to 8 vol. 26/2020 on tuning the dynamic load balancing fem framework sum up contributions from local as well as remote partitions. the cost of accessing remote memory is much higher than the cost of accessing the local memory. it is therefore important to reduce communication between computational partitions. the present contribution examines the design of an object-oriented framework for dynamic load balancing implemented in oofem code [5]. 3. design of micro benchmarks tests overall processor performance depends on many factors, notably on its frequency, the performance of memory subsystem, and type of code executed. the performance of two cpus can be different for integer and floating point operations, on some smps, some resources are shared between processing cores, etc. the adopted approach to evaluate the individual processor performances is based on a set of so-called micro benchmark tests, that evaluate processor performance for different typical tasks and computing overall performance as the weighted average of performances of individual micro-benchmarks. the processor weights are updated each time, when rebalancing should occur. it is therefore important, that the individual benchmarks have very low time demands and on the other hand provide reliable estimation of individual processors. the first micro benchmark test proposed is a function to compute a numerical integral. in the test, the integral of cosine function is evaluated using numerical integration using newton-cortes formula. the parameter ni defines the number of integration points used to divide the area. therefore, with increasing parameter ni (increasing number of integration points), we have a solution process closely approximating the integral value. this test measures performance in floating point operations. the next type of micro benchmark problem was based on solving a linear system of equations. a fem model of the cantilever beam, divided into neq elements is used to define a linear system. the right hand side of the force vector in our case is a simple vector with its first member set to 1, while all the other elements of the vector are set to zero. the number of equations of the linear system directly depends on parameter neq , which also represents the dimension of the stiffness matrix and right hand side of the force vector. another micro benchmark problem called whetstones is a test that attempts to measure the speed and efficiency of a computer performing floating-point operations. the whetstone benchmark was the first designed for benchmarking [6]. this benchmark is very simple, comprising several sub-tests with active loops executed via procedure calls. this module represents a mix of operations typically performed in scientific applications. the test involves integer arithmetic, floating point arithmetic, "if" statements, calls and so on. at the end of this benchmark, a statement with the results is printed. weights were attached to the different modules (realized as loop bounds for loops around the individual modules statements). the weight distribution of whetstone instructions for the benchmark matched the distribution seen in the program sample. the execution frequency of each module was proportional to the input value of such that the scaling factors for nl = 10 gave the modules a total weight corresponding to one million whetstone instructions for more details see [7]. the final load balancing processor weighs are obtained as a weighted average of individual weights from micro bench-mark tests. the final weights are assembled in the ratio of 15% micro benchmark problem based on a function to compute numerical integral, 80% benchmark problem based on the solution of a linear system of equations and 5% benchmark problem based on measuring the speed and efficiency of a computer performing floating-point operations. the time requirements of individual micro benchmark problems were controlled using parameter n as follows: benchmark to compute numerical integral with parameter ni = 106, solution of a linear system of equations with parameter neq = 103 and computer performing floating-point operations with parameter nl = 6 ∗ 104. 4. dynamic load balancing framework the software design process is an important part of dynamic load-balancing research. the design of a dynamic load balancing framework based on a problem partitioned during the solution process is discussed in this section. dynamic load balancing is typically used for problems involving multiple solution or time steps, where work redistribution is performed to reflect potential work imbalance. partitioning the problem during solution is based on monitoring computation time of individual processors during the solution step. once the solution step is finished, the load balance process is activated by evaluating the measured imbalance. if the imbalance is larger than the user defined tolerance, the (re)partitioning is performed, taking into account the actual performance of processing nodes and processing cost of individual elements. as already noted, when solving a real problem on a parallel computer, the load balance can change during the solution. the only way of reflecting the growing imbalance is to adaptively redistribute work between processing units in order to restore load balance and thereby ensure optimal use of resources. the monitoring and evaluation of imbalance is the task of load balance monitor. this monitor can detect load imbalance by monitoring the time required to perform allocated work on individual processing units. the differences in processing time indicate an imbalance. after imbalance is detected, the decision of whether to restore the load balance or continue is made. this can be 9 michal bošanský, bořek patzák acta polytechnica ctu proceedings a complex task, as load redistribution may in fact be a very complex problem with non-negligible time requirements. the cost of load (re)balancing may be higher than the cost of continuing with a slight loadimbalance. all these aspects have to be considered and are, unfortunately, specific to the problem and implementation. when the imbalance is detected, the work (expressed in terms of individual elements) has to be redistributed to reflect the performance of individual processing nodes. individual elements have assigned weights that reflect the relative computational costs. these depend on element type, material state (elastic, plastic, etc.). these weights are typically obtained in advance from benchmark measurements. the processing power of individual processing units also has to be determined. the load (re)balancing process in this chapter is based on (re)distributing the computational work proportionally to the performance of individual processing units. in principle, additional factors, including available communication bandwidth between individual processing units or available memory, can be taken into account. any load balancing should not only distribute the work according to processing powers but also attempt to minimize the communication cost between individual processing units. in the fem, this means the cuts between partitions (number of shared nodes) should be minimized. failing to meet the secondary criteria can significantly impact overall performance, as the cost of communication (in terms of the time required) is much higher than the cost of computation. load (re)balancing should also attempt to minimize the reallocation of elements as much as possible in order to minimize communication costs. the parmetis library [8] was used in this work, however, the parmetis implementation is not restricted to one specific partitioning library. a parallel partitioner can take advantage of the increased memory capacity of parallel machines (distributed memory model) and improve overall performance. a general load balancing algorithm is responsible for dynamic repartitioning using the processor weights provided by the load balance monitor during the micro benchmark problem and should provide the new partitions with numbers for all local elements on each partition. after the updated element partition assignment is determined, the distribution and classification of nodes also have to be determined. each node is classified as either a local node that remains local on an existing partition or a shared node that is assigned to a remote partition. node classification can be determined from element partitioning. the dynamic load balancing framework implemented in oofem is using the weights based on measuring of individual time involved in the computation. these times represent how long each cpu work on the predefined number of solution steps. the repartitioning is based on default weights set as the ratio of total solution time to individual computational thread time. 5. example using a dynamic load balancing framework the above-mentioned strategy is illustrated on nonlinear 3d finite element analysis of an anchor pullout test. the fe model involved nonlinear nonlocal anisotropic damage model to describe fracture process and consists of 1456 nodes and 16772 tetrahedral elements, which was subsequently refined in 20 steps into a final mesh with 125400 elements and 22441 nodes. the number of solution steps was set to 80 steps. due to the character of the solution, the number of required equilibrium iterations was increasing with progressing solution steps. the number of required equlibrium iterations gradually increased form 9 (first solution step) to 263 (solution step number 80). the existing and proposed processor weight evaluation methods were tested on a workstation (running ubuntu 16.04 os) with an intel ® core ™ i7-4790 cpu @ 3.60 ghz with four cores consisting of two logical processors per core connected to a workstation with an intel® core™ i3-2370m cpu @ 2.40 ghz with two cores consisting of two logical cores. each workstation could simultaneously run a maximum of eight threads and four threads on their cpus and had 15 gb and 8 gb of system memory, respectively. the iterative linear equation solver from the petsc library was used with a block jacobi preconditioner. the dynamic load balancing framework parameters were set up. load (re)balancing was activated after each fifth solution step. the relative wall clock imbalance parameter represents the relative imbalance between wall clock solution time of individual computational threads. when greater than the provided threshold the rebalancing procedure is activated. the additional parameter called absolute wall clock imbalance parameter allows to triggered rebalancing procedure when an achieved absolute imbalance between solution times of individual processing threads is greater than the threshold. the absolute wall clock imbalance parameter was set to 10.0 [s]. the minimum absolute wall clock imbalance parameter allows setting minimum absolute wall clock imbalance threshold for performing rebalancing and was set to 0.5 [s]. the obtained results are presented in tables 1, 2 and 3. in the presented tables, the number of processing units used in the parallel computations is marked as "np". the results obtained using the existing dynamic load balancing framework without the micro benchmark tests to set up processor weights were compared to the results of the computation process without load balancing. these load balancing strategies were finally compared to the presented method based on evaluating the microbenchmark tests. tables 1 and 2 show examples of computation done by processors with hyper-threading [9] technology disabled (intel i3 max. 2 threads, intel i7 max. 4 threads). the computational process with hyper-threading technology enabled on the i3 processor is shown in table 3. 10 vol. 26/2020 on tuning the dynamic load balancing fem framework np no dlb [s] dlb [s] dlb w. [s] 2 1659 1473 1411 3 1408 1399 1230 4 1372 1215 1181 5 1277 1179 1131 table 1. execution times comparing solutions with estimated processing weights (dlb w.), uniform processing weights (dlb) and without a load balancing (no dlb) framework (intel i3 1 computing unit + intel i7 4 computing units). np no dlb [s] dlb [s] dlb w. [s] 4 1524 1479 1373 5 1365 1314 1154 6 1330 1302 1140 table 2. execution times comparing solutions with estimated processing weights (dlb w.), uniform processing weights (dlb) and without a load balancing (no dlb) framework (intel i3 2 computing units + intel i7 4 computing units). np no dlb [s] dlb [s] dlb w. [s] 2 1847 1662 1373 3 1606 1339 1303 4 1610 1340 1435 5 1853 1642 1578 table 3. execution times comparing solutions with estimated processing weights (dlb w.), uniform processing weights (dlb) and without a load balancing (no dlb) framework (intel i3 4 computing units (ht) + intel i7 4 computing units). the results confirm that better performance is obtained when appropriate weights are used. in the case with hyper-threading technology enabled, the scalability trend is not ideal in the solution based on six and eight computational threads and is worse, for example, than the solution based on two and four computational threads. the significant decrease in performance can be attributed to the hyper-threading technology of intel processors, which assembles and shares some cpu resources between hyper-threaded cores and only takes place with 4 threads (note that the workstation with intel® core™ i3 had two physical hyper-threaded cores). however, by using dynamic load balancing and appropriate weights (processor performance parameter), we can achieve better performance than in the solving process without load balancing or with the existing dynamic load balancing framework not using micro benchmark tests. 6. conclusions the contribution based on an improved methodology to determine processing weights parameters as a part of the load balancing framework was presented in this paper. the parallelization strategy was based on the dynamic load balancing process using weights that represented the performance of computational units. the performance of the upgraded dynamic load balancing process (processor weights parameters) was compared to the previously implemented dynamic load balancing process. in this work a nonlinear benchmark problem was considered to evaluate the performance of dynamic load balancing framework. the results showed differences in the performance of the upgraded and previously implemented dynamic load balancing process for the considered benchmark. the upgraded dynamic load balancing process had better performance than the previously implemented dynamic load balancing process. future studies will investigate other testing examples (simple linear equation system solutions) to more precisely assess variables as a representation of computational unit performance. it is clearly a benefit to using the appropriate measures of performance of individual computational units performance of computational units in the load balancing process as a necessary part of any finite element parallel code. the performance of the described load balancing framework could potentially be significantly improved by considering communication speeds between processing nodes by using parameters that represent communication speeds through different, heterogeneous networks. the load balancing framework is based on parallel computation using a distributed memory model. this model uses a message passing interface. for example, many distributed systems are now being constructed using a variety of different communication networks, such as ethernet and asynchronous transfer mode (atm). in addition to this hardware heterogeneity, there is a heterogeneity in the types of messages produced by parallel programs. short synchronization messages require low latency. conversely, large data messages require high-bandwidth, though they can tolerate high start-up latency. the different types of networks have different performance characteristics, while the different types of communication messages may have different communication requirements. the performance of parallel computations based on a distributed memory model is typically limited by communication overhead. high-performance networks and the use of multiple heterogeneous networks can help reduce this overhead. further research could focus on designing the load balancing framework’s input parameters, which represent different types of networks providing a data path between the same pair of network nodes. these load balancing parameters (network weight parameters) may allow a solution process to maximize efficient use of different types of networks rather than passively accept the given features of a single network. 11 michal bošanský, bořek patzák acta polytechnica ctu proceedings acknowledgements this work was supported by the grant agency of the czech technical university in prague, grant no. sgs16/038/ohk1/1t/11 advanced algorithms for numerical modelling in mechanics of structures and materials. the authors acknowledge the support of the op vvv meys funded project cz.02.1.01/0.0/0.0/16_019/0000765 "research center for informatics". references [1] k. schloegel, g. karypis, v. kumar. parallel static and dynamic multi-constraint graph partitioning. concurrency and computation: practice and experience 14(3):219–240, 2002. doi:10.1002/cpe.605. [2] k. d. devine, e. g. boman, r. t. heaphy, et al. new challenges in dynamic load balancing. applied numerical mathematics 52(2-3):133–152, 2005. doi:10.1016/j.apnum.2004.08.028. [3] p. krysl, z. bittnar. parallel explicit finite element solid dynamics with domain decomposition and message passing; deal programming scalability. comput struct 79(3):45–60, 2001. [4] m. bošanský, b. patzák. parallel approach to solve of the direct solution of large sparse systems of linear equations. acta polytechnica ctu proceedings 13:16, 2017. doi:10.14311/app.2017.13.0016. [5] b. patzak. oofem. http://www.oofem.org, 2000. [6] h. j. curnow. a synthetic benchmark, vol. 19. oxford university press (oup), 1976. doi:10.1093/comjnl/19.1.43. [7] s. harbaugh, j. a. forakis. timing studies using a synthetic whetstone benchmark. acm sigada ada letters iv(2):23–34, 1984. doi:10.1145/998395.998396. [8] v. k. g. karypis. parmetis: parallel graph partitioning and sparse matrix ordering library. department of computer science, university of minnesota„ 1997. [9] d. h. e. a. d. marr, f. binns. hyper-threading technology architecture and microarchitecture. intel technology journal 6(1), 2002. 12 https://doi.org/10.1002/cpe.605 https://doi.org/10.1016/j.apnum.2004.08.028 https://doi.org/10.14311/app.2017.13.0016 https://doi.org/10.1093/comjnl/19.1.43 https://doi.org/10.1145/998395.998396 acta polytechnica ctu proceedings 26:7–12, 2020 1 introduction 2 introduction and overall design of load balancing framework 3 design of micro benchmarks tests 4 dynamic load balancing framework 5 example using a dynamic load balancing framework 6 conclusions acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2018.20.0073 acta polytechnica ctu proceedings 20:73–77, 2018 © czech technical university in prague, 2018 available online at http://ojs.cvut.cz/ojs/index.php/app regarding certain aerodynamic sources of non-stationary force loads on turbine profiles tomáš radnic∗, martin luxa, david šimurda institute of thermomechanics of the czech academy of sciences, dolejškova 1402/5, prague, czech republic ∗ corresponding author: radnic@it.cas.cz abstract. the paper deals with selected phenomena present in a blade cascade flow field. the presented research was performed on prismatic blade cascades composed mostly of the tip and root sections of the last stage rotor blade of a large output steam turbine. the unstarted supersonic flow on the cascade inlet, the separation of the flow and the swirl line behind the trailing edge have been identified as the possible sources of the unsteady force effects. keywords: turbine blade cascade, unstarted supersonic flow, swirl line, off design conditions. 1. introduction the modern steam turbines of large power output utilize very long blades in the last stage of the rotor. the tip of the blade operates at supersonic inlet velocities, while the root of the blade operates at subsonic inlet velocities. in this setup, it is certain that the transonic inlet flow conditions will occur on the blade at its nominal speeds, hence unstarted supersonic flow will always be present. during the start up or shut down of the turbine, off design conditions appear in the turbine, which can generate swirl lines and flow separation. although these phenomena can only be present during off design conditions, they can prove fatal for the turbine. last but not least, in modern times, renewable sources are widely incorporated into the energetic grids and their non-reliable power output requires turbines to be able to operate in off design conditions on standard basis. it is important to note the majority of phenomena leading to unsteady force effects occurs in the region of tip section which is not rigid. 2. experiments the investigated phenomena were measured on different sections of a long turbine rotor blades made by doosan skoda power, formerly škoda pilsen. the investigation of the flow field is usually performed on several sections of the blade. tip sections and root sections were used for creation of prismatic models that could be fitted into test section of the wind tunnel. the prismatic blades cascades with finite number of blades were assembled for maintaining best possible periodicity of the flow field that would models as best as possible the flow field in the real turbine. perforated tailboard was utilized during measurements to prevent reflection of the shock waves and help maintaining periodicity. blades with tailored trailing edge with sharp edges, see figure 1, were used for investigation of the swirl line behind the trailing edge. figure 1. one of the shapes of the trailing edge used for the swirl line investigation. figure 2. scheme of the supersonic indraft wind tunnel. 1) silica gel drier 2) pebble filter 3) inlet nozzle 4) adjustable supersonic nozzle 5) test section 6) settling chamber 7) control nozzle 8) quick acting valve 9,10) duct to vacuum chamber. measurements were conducted in the supersonic indraft wind tunnel of the institute of thermomechanics of the czech academy of sciences in nový knín. the tunnel is equipped with deformable inlet nozzle so that supersonic inlet speeds could be achieved. 3. unstarted supersonic flow the unstarted supersonic flow can be characterized as a regime, where the inner branch of inlet shock wave has a shape of a normal shock, see figure 3. when the inner branch of the shock wave turns into chevron shape, the flow is considered to be started supersonic flow, see figure 4. the unstarted supersonic flow is also a boundary between transonic and supersonic flow past the cascade. in case of the unstarted supersonic flow, the inner branch of the inlet shock wave 73 http://dx.doi.org/10.14311/app.2018.20.0073 http://ojs.cvut.cz/ojs/index.php/app t. radnic, m. luxa, d. šimurda acta polytechnica ctu proceedings figure 3. unstarted supersonic flow field. figure 4. started supersonic flow field. has always unstable and unsteady behaviour and can induce instability of the whole flow field [1]. the transition of unstarted flow to started flow can be achieved by increase of inlet speed while maintaining low back pressure or by reduction of backpressure if the inlet speed is high enough. it is important to note that the unstarted supersonic flow cannot be avoided in the turbine where the tip of the blade operates at supersonic inlet velocities. the phenomena will be present on the blade and will have force effect on the blade. the force effects need to be assessed to reckon with them during the design phase of the turbine. the transition of unstarted supersonic flow to started supersonic flow can be seen in figure 5. the outlet isentropic mach number was roughly (a) . m1 = 1.141 m1 = 1.180. (b) . m1 = 1.209 m1 = 1.209. figure 5. development of the started supersonic flow. figure 6. scheme of torque load of the tip of the blade with non started supersonic flow field. m2 is ≈ 1.8. in the top left image, the inlet mach number is m1 = 1.141. the inlet shock wave has a shape of a normal shock wave that interacts with the boundary layer on the pressure side of the adjacent blade. the interaction causes local separation of the flow. on the top right image there is m1 = 1.180. the inlet shock wave is now much closer to the leading edge and the local separation which it causes is now much more limited. on the bottom there are two images with identical inlet speed m1 = 1.209. the images show unstable nature of transition between unstarted and started supersonic flow. on the left image the inner branch of the inlet shock wave hits the trailing edge of the adjacent blade, while on the right image the inlet shock wave has chevron shape. the inner branch of the inlet shock wave on the right image does not collide with the adjacent blade at all, however it can still affect the area of near wake. the shift of the shock wave shape does not have any measurable influence on the time averaged flow field parameters [2]. the effect of this phenomenon must be investigated quantitatively to assess the induced stress in the blade. 74 vol. 20/2018 aerodynamic sources of non-stationary force loads on turbine profiles figure 7. distribution of dimensionless tangential force on the blade, points in the circle correspond to figure 5. in the figure 6, there is a scheme which describes force effects due to the unstarted supersonic flow. it is apparent, that the flow will induce torque stress in the blade. figure 7 shows distribution of the dimension-less tangential forces that affect the blade during the transition from unstarted to started supersonic flow. the torque stressing is also cyclic during the whole start up process. because the phenomenon is unstable, even small variations in the operating conditions will result in quite large changes in the stress of the blade. 4. flow separation the main cause of flow separation is adverse pressure gradient which induces back flow in the boundary layer. the boundary of the separation and position of the reattachment point are unsteady and are sources of pressure pulsation in the flow field. the boundary layer gains thickness and the streamlines detach from the surface at the point of zero velocity gradient. it is possible that further downstream the pressure gradient becomes more favourable and the flow reattaches to the surface. in this case the separation area is closed. negative incidence angle can have very similar effect, the separation forms on the opposite side of the blade. 4.1. separation at the root the root of the blade is much more resistant to the flow separation. the cascade has a very low pitch to chord ratio hence the density of the profiles in the cascade is high. relatively long interblade channel and typical profile shaping provides the root section with a wide range of operational conditions in which there is no flow separation. the experimental investigation showed, that root section cascade will maintain unseparated flow field in the range of inlet angles ι ∈ (−20◦, 30◦). the separation at the root does decrease the mass flow in the interblade channel by limiting the flow through area. the shift of effective shape of the profile can change the position and shape of the sonic line in the cascade. figure 8. flow separation on the pressure side of the blade with negative angle of incidence. (a) . ι = 0◦. (b) . ι = 20◦. (c) . ι = 30◦. figure 9. flow field in the root section cascade at different incidence angles, m2is ∼ 1.5. 4.2. separation in the tip region the experimental investigation of the tip cascades does not allow to assess effects of varying incidence angles. the models have very high aspect ratio and the blades are thin. excessive force effects induced by positive incidence angles will endanger the model and the measuring equipment. negative incidence angles will lead to very low loads and increase of flow velocity in the cascade, possibly resulting in unstarted supersonic flow. the investigation can be conducted in the region of ι ∈ (−3◦, 3◦). therefore, investigation of the tip section is limited, and the separation phenomena are not very well mapped. measurements conducted by šafařík [3] on an older last stage blade with more robust design in the tip region had the minimal inlet angle ι = −6.5◦. 75 t. radnic, m. luxa, d. šimurda acta polytechnica ctu proceedings figure 10. section near the tip, ι = −6.5◦, m2is = 1.51. figure 11. section near the tip, ι = −6.5◦, m2is = 0.6. the measurement showed that in the area behind blade’s maximal thickness there is closed area of separation, see figure 10. this separation is caused by relatively high inlet speeds and by interaction of the inlet shock wave with the boundary layer on the adjacent blade. this separation was not present during measurements in the subsonic regions, see figure 11. 5. swirl line at the trailing edge one of important aerodynamic sources of unsteady flow fields, which usually are the dominant reason of unsteady loading of blades, is a swirl line arising at the trailing edge in the near wake [4–6]. the swirl line downstream one of the blades affects the pressure distribution along a part of a suction side of the adjacent blade. this phenomenon is more noticeable in section near the tip of long blade. there, the unsteady pressure field affects (due to the geometrical arrangement of the profile cascade) a very large portion of pressure distribution on the suction side of the adjacent profile. the formation of the swirl line downstream the figure 12. tip section consisting of profiles with the tailored trailing edge, nominal condition of flow m2is = 1.68 [4]. trailing edge is characterized by the strouhal number: sh = f · d u (1) and by reynolds number, for swirl line: re = u · d ν . (2) the characteristic length d in case of swirl line developing at the trailing edge is the diameter of the trailing edge added to the height of the boundary layer. the criteria for evaluation of the swirl line are dependent on wide scale of flow field parameters. in a transonic flow field, the supersonic parts of the flow field in the profile cascade are subordinate to the hyperbolical description of the flow. the supersonic expansion both in the interblade channel and at the trailing edge can very favourably affect the effective dimension of the near wake, which is responsible for the swirl line origin, see figure 12. the area of confluence at the end of the near wake, has very small characteristic length. it is mainly caused by the large supersonic expansion on the sharp edges of the tailored trailing edge. the swirl line downstream the profile is insignificant and the pressure distribution along the suction side of the adjacent profile is stable. the situation in the same cascade operating at off design conditions, when the flow field is significantly more subsonic, is completely different. the supersonic expansion at the trailing edge vanishes, and the decisive size for wake and swirl line formation is the real geometric dimension of the trailing edge. the very significant swirl line takes place downstream in the wake. the individual vortices float downstream the trailing edge and periodically affect the pressure distribution along the suction side of the adjacent profile, see figure 13. 76 vol. 20/2018 aerodynamic sources of non-stationary force loads on turbine profiles figure 13. tip section consisting of profiles with the tailored trailing edge, off-design condition of flow m2is = 0.912 [4]. application of this kind of profiles in the tip region of a long rotor blade looks promising, while the aerodynamic parameters of the profile in the region of design flow conditions are good. unfortunately, it is a problem to reach the design conditions. during the start-up process the steam turbine cannot surmount the dangerous off design regimes without seriously damaging the machine. 6. conclusion the paper deals with selected undesirable phenomena present in a blade cascade. analysis of unstarted supersonic flow, flow separation and swirl line at the trailing edge of the blade had been conducted based on long term experimental research. the unstarted supersonic flow that is present in the design conditions proved to be unstable and transient. force effects, especially torque, has been identified to affect the blade when unstarted supersonic flow occurs. the flow separation does occur in non-design operating conditions, during start up, stopping or limited power output operation of the turbine. the tip region proved much more prone to detachment of the flow than the root. incidence angle deviation ∆ι = 3◦ causes separation at the tip of the blade, possibly destroying the blade cascade. the root of the blade is quite resistant to the separation which occurred at ∆ι = 20◦ and ∆ι = 30◦. the separation causes higher losses, sonic plane shift, pressure pulsations and other effects. the swirl line at the trailing edge can be caused by its round shape, which cannot be avoided for technological and structural reasons. it can induce significant pressure pulsations in the flow field. investigation of the off design transonic flow fields in blade cascades is vital for development of turbines able to standardly operate at partial power output and that can survive frequent shutdowns and start-ups in the modern intelligent power grids. list of symbols a area [m2] d characteristic length [m] f frequency of vortex shedding [hz] f force [n] m mach number [1] p pressure [pa] re reynolds number [1] st strouhal number [1] u velocity [ms−1] ι incidence angle [◦] ν kinematic viscosity [m2s−1] indexes: 1 in front of the cascade 2 behind the cascade is isentropic max maximal acknowledgements acknowledgements of the authors belong to the technology agency of the czech republic, which supported this research under grant no. th02020057. an institutional support rvo61388998 is also gratefully acknowledged. special thanks are due to the doosan škoda power co. ltd. for making this research possible. references [1] r. dvořák, p. šafařík, m. luxa, d. simurda. optimizing the tip section profiles of a steam turbine blading. in asme 2013 turbine blade tip technical symposium and course week, hamburg. 2013. doi:10.1115/tbts2013-2027. [2] m. luxa, j. příhoda, d. šimurda, et al. investigation of the compressible flow through the tip-section turbine blade cascade with supersonic inlet. journal of thermal science 25(2):138–144, 2016. doi:10.1007/s11630-016-0844-0. [3] m. luxa, et al. images of flow fields in turbine profile cascades i, ii, research report it cas no.: z-1382/06 (in czech), 2006. [4] p. šafařík. measurements on cascade tr-n-1, internal report, it csas (in czech), 1981. [5] p. šidlof, m. štěpán, v. vlček, et al. flow past a self-oscillating airfoil with two degrees of freedom: measurements and simulations. in european physical journal web of conferences, vol. 67 of european physical journal web of conferences, p. 02108. 2014. doi:10.1051/epjconf/20146702108. [6] v. vlček, j. horáček, m. luxa, j. veselý. visualization of unsteady flow around a vibrating profile. in 9th international conference on flow-induced vibration, prague. 2008. 77 http://dx.doi.org/10.1115/tbts2013-2027 http://dx.doi.org/10.1007/s11630-016-0844-0 http://dx.doi.org/10.1051/epjconf/20146702108 acta polytechnica ctu proceedings 20:73–77, 2018 1 introduction 2 experiments 3 unstarted supersonic flow 4 flow separation 4.1 separation at the root 4.2 separation in the tip region 5 swirl line at the trailing edge 6 conclusion list of symbols acknowledgements references doi:10.14311/app.2021.30.0098 acta polytechnica ctu proceedings 30:98–103, 2021 © czech technical university in prague, 2021 available online at http://ojs.cvut.cz/ojs/index.php/app optimization of tuned mass dampers attached to damped structures minimization of maximum displacement and acceleration jan štěpánek∗, jiří máca czech technical university in prague, faculty of civil engineering, department of mechanics, thákurova 7, 166 29 prague 6, czech republic ∗ corresponding author: jan.stepanek@fsv.cvut.cz abstract. a tuned mass damper is a device, which can be highly helpful while dealing with dynamic behaviour of structures. its proper design is conditioned by knowledge of both loading and the structure properties. in many cases, the structure can be represented by single degree of freedom model, which simplifies the design and optimization of tuned mass dampers. most of studies focus only on minimization of displacement of the main structure under harmonic force load, however, in many cases, different frequency response function would be more appropriate. this paper presents an extension of design formulas for the h∞ optimization of tuned mass dampers for damped structures and various frequency response functions. keywords: acceleration, damped structure, displacement, dynamic response, optimization, tuned mass damper. 1. introduction optimal parameters of a tuned mass damper (tmd) have been a subject of many researches since the first half of the 20th century. since then, several different optimization criteria have been proposed. the most common ones are called h2, which minimizes the area under a frequency response function (frf) and h∞, which minimizes the maximum amplitude magnification factor. a first set of nearly optimal tmd parameters for undamped structure came from the existence of invariant points, where the amplitude is independent of tmd damping [1, 2]. a closed form solution of h∞ optimum parameters of tmd attached to a damped structure have not been found. in 1978, ioi and ikeda presented empiric design formulas [3], however, they were derived only for an insufficiently large mass ratio µ > 0.03, which makes them inappropriate for certain civil engineering structures. it is interesting that despite the formulas being derived for a large mass ratio, they are recommended in footbridge design guidebook setra [4]. design charts allowing use of lower mass ratio µ > 0.01 were published in 1981 [5]. asami et al. used a perturbation method to modify the fixed-point solution regarding a main system damping ratio as a perturbation [6]. they obtained a series solution, which is, however, too complicated for engineering application. therefore, they have proposed a simplified version. abubakar presented a simple modification of the fixed-point tmd optimum for damped structure. this modification provides very accurate solution for minimization of displacement [7]. despite many useful formulas and design methods being proposed, some modern guidebooks for civil engineering, for example [4, 8], keeps using the original approximate solutions thanks to their easy application. most of studies focus on a reduction in a maximum amplitude of displacement. this is a suitable criterion for the reduction of structural stress due to a dynamic load. however, in many cases, the main purpose of tmd is to reduce the maximum amplitude of acceleration of the main structure. these cases contain machines and sensors which are sensitive to vibration, because the vibration tolerance is usually defined in the units of acceleration. furthermore, human comfort criteria inside buildings or on footbridges are usually limited by acceleration. this paper points out the importance of distinction between displacement and acceleration criteria of the h∞ optimization. the approximate optimal stiffness and damping ratio for both criteria are proposed in this paper covering both ground and force excitation. considering the importance of easy practical application of the result, the formulas for optimal parameters are composed of standard solutions given by the fixed-point method and a correction, which takes nonzero damping into account. therefore, in the case of zero structural damping, we obtain standard result given by the fixed-point method. in the case of nonzero damping, we obtain results given by the modified formulas. 2. equations of motion the main structure is represented by a single degree of freedom (sdof) system and it consists of a mass m1, stiffness k1, and damping c1. tmd is composed of a mass m2, an elastic member with stiffness k2 and a viscous member with damping c2. the struc98 http://dx.doi.org/10.14311/app.2021.30.0098 http://ojs.cvut.cz/ojs/index.php/app vol. 30/2021 optimization of tuned mass dampers in damped structures figure 1. a) force excitation, b) support excitation. ! k1 + k2 −k2 −k2 k2 " # x1 x2 $ + ! c1 + c2 −c2 −c2 c2 " # ẋ1 ẋ2 $ + ! m1 0 0 m2 " # ẍ1 ẍ2 $ = f (t) (1) ture can be loaded by a harmonic force (figure 1a) or excited by support motion (figure 1b). this paper covers both loading cases, because the optimal parameters of tmd are not identical for force and support excitation. the equations of motion are described by eq. 1, where f (t) = # f(t) 0 $ in the case of force excitation, and f (t) = # −ẍ0(t)m1 −ẍ0(t)m2 $ in the case of support excitation. x, ẋ and ẍ denote the displacement, velocity and acceleration, respectively. it is important to note that x induced by support excitation is a relative displacement between the support and the mass. the absolute displacement is xa = x + x0. 3. optimal parameters in order to solve equation 1, it is advantageous to use following dimensionless substitutions and notation for further operations [9]: ω1 = % k1/m1 natural frequency of the main structure, ω2 = % k2/m2 natural frequency of tmd, µ = m2/m1 mass ratio, β = ω2/ω1 frequency ratio, ω = ω/ω1 forcing frequency ratio, c1,cr = 2 √ k1m1 critical damping of the main structure, c2,cr = 2 √ k2m2 critical damping of tmd, ξ1 = c1/c1,cr damping ratio of the main structure, ξ2 = c2/c2,cr damping ratio of tmd, nf (ω)= (2ξ2βω)2 + (β2 − ω2)2, na(ω)= [1 + (2ξ1ω)2][(β2 − ω2)2 + (2ξ2βω)2], nr(ω)= [(1 + µ)β2 − ω2]2ω4 + (1 + µ)2(2ξ2βω)2ω4, d(ω) = [(1 − ω2)(β2 − ω2) − µβ2ω2 − 4ξ1ξ2βω2]2 + +4ω2[(β2 − ω2)ξ1 + (1 − ω2 − µω2)βξ2]2. assuming a harmonic load according to figure 1, a steady state part of response can be expressed from eq. 1. the frequency response functions which describe the normalized response of the structure are summarized in table 1. 3.1. fixed-point approach the approximate h∞ optimum for main structure with no or negligible damping can be reached by the fixed-point method. this method was firstly described by den hartog [1]. he postulated that the approximate optimum frequency ratio βu opt is reached if the value of frf is equal in invariant points p and q, where the value of frf is not affected by tmd damping c2. brock lately stated that the optimum damping ratio ξu 2,opt can be taken as the average of two optima, each calculated separately for one of two points p and q and he derived the well known formula of optimum damping for frf no. 1 in table 1 [2]. using this approach, we can derive the approximate optimum solutions for various frfs. the results are summarized in table1. it should be noted that the optimum parameters are identical for transfer functions 1 and 3, as well as for 2 and 4. this is can be seen in na(ω, ξ1 = 0) = nf (ω). however, this is true only if ξ1 = 0. 3.2. non-zero damping the fixed-point approach is not available if we introduce damping of the main structure. an analytical solution of optimum tmd parameters attached to a damped main structure have not been found and according to asami et al., the solution is probably impossible [6]. therefore, we need to rely on empiric and approximate solutions. in order to define an empiric formulas for optimum frequency ratio βopt and optimum damping ratio ξopt,2, the values of the parameters were found numerically on a surface of 44x41 points for 0.001 ≤ µ ≤ 0.2 and 0 ≤ ξ1 ≤ 0.2. the density of mesh was increased in the area of low mass ratio 0.001 ≤ µ ≤ 0.005, because in this range, the optimum values of βopt and 99 jan štěpánek, jiří máca acta polytechnica ctu proceedings no. excitation frequency response function type frequency response function βu opt ξu 2,opt 1 force displacement &&&& x1 f0/k1 &&&& = ' nf (ω) d(ω) 1 1 + µ ' 3µ 8(1 + µ) 2 force acceleration &&&& ẍ1 ω2 1f0/k1 &&&& = ω2 ' nf (ω) d(ω) ( 1 1 + µ ' 3µ 4(2 + µ) 3 support abs. displacement &&&& xa 1 x0 &&&& = ' na(ω) d(ω) 1 1 + µ ' 3µ 8(1 + µ) 4 support abs. acceleration &&&& ẍ1 a ω2 1x0 &&&& = ω2 ' na(ω) d(ω) ( 1 1 + µ ' 3µ 4(2 + µ) 5 support rel. displacement &&&& ω2 1x1 ẍ0 &&&& = 1 ω2 ' nr(ω) d(ω) ' 2 − µ 2(1 + µ)2 ' 3µ 4(1 + µ)(2 − µ) 6 support rel. acceleration &&&& x1 x0 &&&& = ' nr(ω) d(ω) ' 2 + µ 2(1 + µ)2 ' 3µ 8(1 + µ) table 1. optimum parameters of tmd based on the fixed-point method. ξopt,2 change more rapidly then in the rest of the function. a convergence problem may occur while finding ξopt,2 numerically, because a structural response is much more sensitive to tmd tuning than to its damping. the empiric formulas are combination of the fixedpoint solution and a correction function, which takes the structural damping into account. the original approach [3] uses a correction function composed of 2nd order polynomial of µ and ξ1, but we found it neither simplest nor best-fitting. numerous types of functions, including polynomial, exponential, logarithmic and hyperbolic ones, were tested using a non-linear regression to find the best approximation of the optimum tmd setting regarding a simple application. thanks to similar characteristics of the optimum parameters for all frequency response functions, two common function were defined: βopt = βu opt + ξ1 ) a1ξ1 + a2 * µ 1 + µ + 1 3 , (2) ξ2,opt = ξu 2,opt + ξ1[b1 + b2 ln(µ) + b3µ + b4µξ2 1 ] (3) as we can see in the shape of eqs. (2) and (3), the optimum values of tmd parameters βopt = βu opt and ξ2,opt = ξu 2,opt if ξ1 = 0. in other words, the formulas preserve the fixed-point approach if the main structure remains undamped. the coefficient of determination was chosen as the comparative criterion for function fitting and it reached higher than 0.997 in all cases of frfs for our selected function types. the maximum error of βopt was lower than 1%. the maximum error of ξ2,opt reached up 5%, but it occurred only for extremely small mass ratio, where the optimum damping is close to zero and such a high error may be produced rather by a precisions of the numerical solution than by an inaccurate approximation. the average relative error of ξ2,opt was lower than 1% for all transfer functions. the coefficients ai and bi were found using linear regression and they are summarized in table 2. figure 2 presents the improvement of frf using desing formulas (2) and (3) for relative displacement of structure under support excitation. figure 2. improvement of tf 5 (µ = 0.05, ξ1 = 0.05) using desing formulas (2) and (3). figures 3-8 compare the numerical results with the approximations given by design formulas. we can see that in most cases, the empiric formulas give very precise results. it is also visible that ξ2,opt rises as the structural damping increases. however, the effect of ξ1 to the optimum frequency ratio is slightly more 100 vol. 30/2021 optimization of tuned mass dampers in damped structures frf no. a1 a2 b1 b2 b3 b4 1 -0.7636 -0.8748 0.1801 0.0140 0 -4.6350 2 1.2470 0.2644 0.2101 0.0178 1.264 0 3 -0.5561 -0.8466 0.1782 0.01367 0.01865 -4.3310 4 1.5600 0.2778 0.1985 0.01552 1.1420 29.0900 5 -0.7639 -1.5300 0.2005 0.01650 0.4629 0 6 0.9731 -0.2176 0.1783 0.0145 0 0 table 2. coefficients ai and bi for design equations (2) and (3). figure 3. optimum tmd parameters: tf no. 1. figure 4. optimum tmd parameters: tf no. 2. figure 5. optimum tmd parameters: tf no. 3. 101 jan štěpánek, jiří máca acta polytechnica ctu proceedings figure 6. optimum tmd parameters: tf no. 4. figure 7. optimum tmd parameters: tf no. 5. figure 8. optimum tmd parameters: tf no. 6. complicated. as we can see in figures 3-8, increase of ξ1 leads to decrase of βopt in the case of displacement frfs (no. 1,3,5). in the case acceleration frfs (no. 2,4,6), the increase of ξ1 has an opposite effect. sensitivity of optimum parameters to the damping ratio ξ1 varies among frfs. it is interesting to note that in the case of acceleration frfs, the optimum tuning βopt may slightly rise above one, which is an unusual situation in tmd design. 4. conclusions the design formulas for h∞ optimization of tmd attached to a damped structure are presented in this paper. the formulas are appropriate for tmds with the mass ratio µ between 0.001 and 0.2, and for the damping ratio of the main structure between 0 and 102 vol. 30/2021 optimization of tuned mass dampers in damped structures 0.2. this is a sufficient range for most of tmds localized in civil engineering structures and we consider the error of design parameters negligible in this range. the formulas were optimized for six frequency response functions, which describe relative and absolute displacement or acceleration for both force and support excitation. it is necessary to chose a suitable frf for tmd optimization regarding the purpose of tmd. for the purpose of a stress reduction of the main structure, frfs which describe displacement should be used. on the other hand, in order to increase a comfort of people in high-rise buildings and pedestrian on footbridges, or to reduce the vibration of equipment localized in the main structure, the acceleration frfs are an appropriate choice. we also find out that reduction of acceleration may lead to tmd tuned to a resonant or even slightly higher frequency. an easy application is preserved. only two simple formulas are presented, however, they can be used for all considered transfer functions with different parameters ai and bi. acknowledgements the authors gratefully acknowledge support from the czech technical university in prague, project sgs20/038/ohk1/1t/11 development and application of numerical algorithms for analysis and modelling in mechanics of structures and materials. references [1] j. p. den hartog. mechanical vibrations. mcgraw hill, new york, 1934. [2] j. brock. a note on the damped vibration absorber. journal of applied mechanics 68(a):284, 1946. [3] t. ioi, k. ikeda. on the dynamic vibration damped absorber of the vibration systems. bulletion of the jsme 21(151):64–71, 1978. [4] setra. footbridges: assessment of vibrational behaviour of footbridges under pedestrian loading, 2006. [5] s. e. randall, d. m. halsted, d. l. taylor. optimum vibration absorbers for linear damped systems. journal of mechanical design 103:908–913, 1981. [6] t. asami, o. nishihara, a. m. baz. analytical solutions to h∞ and h2 optimization of dynamic vibration absorbers attached to damped linear systems. journal of vibration and acoustics 124:284–295, 2002. [7] i. m. abubakar, b. j. m. farid. generalized den hartog tuned mass damper system for control of vibrations in structures. earthquake resistant engineering structures 104:185–193, 2009. [8] fib bulletin 32: guidelines for the design of footbridges, 2005. [9] o. nishihara, t. asami. closed-form exact solution to h infinity optimization of dynamic vibration absorber: ii. development of an algebraic approach and its application to a standard problem. proceedings of the spie, 3989, 2000. doi:10.1117/12.384589. 103 https://doi.org/10.1117/12.384589 acta polytechnica doi:10.14311/app.2020.27.0112 acta polytechnica 27(0):112–115, 2020 © czech technical university in prague, 2020 available online at http://ojs.cvut.cz/ojs/index.php/app local nano-mechanical properties of cross-linked polybutylene martin ovsík∗, michal staněk, adam dočkal, petr fluxa tomas bata university in zlin, department of production engineering, nám. t. g. masaryka 5555, 760 01 zlín, czech republic ∗ corresponding author: ovsik@utb.cz abstract. cross-linking is a process in which polymer chains are associated through chemical bonds. the cross-linking level can be adjusted by the irradiation dosage and often by means of a cross-linking booster. the polymer additional cross-linking influences the surface nano and micro layers in the way comparable to metals during the thermal and chemical-thermal treatments. polybutylene terephthalate (pbt) can be found in a group of structural polymers, which are often used in industry, especially in automotive. applying the technology of electron radiation induces a creation of 3d network structure, which improves the local mechanical properties. these were later measured by a depth sensing indentation (dsi) test. this state of the art method is based on immediate detection of indentation depth in relation to applied force. the creation of 3d network caused an increase in nano-mechanical properties values, such as indentation hardness and indentation modulus, in comparison to the virgin material. the indentation hardness rose by 80%, while the indentation modulus elevated by 62%. the selected structural materials, e.g. pbt, were modified by the electron irradiation in a positive way and as such could be moved to a group of high performance materials. keywords: cross-linked, electron radiation, hardness, nano-indentation, polybutylene terephthalate. 1. introduction polybutylene terephthalate (pbt) is a semicrystalline polymer. some of the remarkable properties of pbt are as follows [1–3]: • good chemical resistance • high dielectric strength, excellent electrical properties • high heat resistance and temperature performance • good strength and modulus at elevated temperatures • very good processability • flame resistance and ease of automated soldering. the major application of pbt is in the automotive sector. it is used for the manufacturing of electronic components, parts of instrument panels, and housings of automotives. exterior components, namely bumper fascias, mudguards, door handles, mirror housings, and wiper arms are also produced from pbt. its electrical applications include connectors, smart network interface devices, power plugs and electrical components, switches and controls, circuit breaker enclosures, and outdoor telecommunication enclosures. it has also found application in fiber-optic tubing and electrical appliances where the components require a high surface gloss [4–7]. e-beam crosslinking is a powerful tool used to improve the properties of a wide range of polymers in the creation of value-added specialty products. the crosslinking of polymers through electron-beam processing changes a thermoplastic material into a thermoset. when polymers are crosslinked, the molecular movement is severely impeded, making the polymer stable against heat. crosslinking is the interconnection of adjacent long molecules with networks of bonds induced by chemical treatment or electron-beam treatment [8–11]. this study is concerned with the effect of ionizing radiation upon the local nano-mechanical properties of filled pbt. the main goal was to evaluate the influence of cross-linking (creation of the 3d network) on the local nano-mechanical properties of structural materials, which are commonly used in technical practice. 2. methods 2.1. material a structural material pbt filled with 35% of glass fibres (pbt + 35%gf) was chosen as a test subject for this experiment. it is used in numerous technical applications within the automotive industry. the material is labelled pbt v-ptscreatec-b3hzc * m800/25 and was obtained from company pts. it was shipped in the shape of granules that were later enriched by 4 % of cross-linking agent labeled taic (triallyl isocyanurate). exposure of the cross-linking agent to radiation causes a creation of 3d network within the polyamide structure, which is faster than the degradation of the polymer caused by the ionizing radiation. 112 http://dx.doi.org/10.14311/app.2020.27.0112 http://ojs.cvut.cz/ojs/index.php/app vol. 27/2020 cross-linked polybutylene properties parameters unit pbt + 35%gf injection pressure mpa 80 cooling time s 35 mould temperature °c 85 zone 1 °c 210 zone 2 °c 220 zone 3 °c 235 zone 4 °c 250 table 1. injection molding parameters. 2.2. sample preparation test samples were manufactured by injection molding technology on injection machine arburg allrounder 470h (loßburg, germany). the sample were manufactured according to the standard čsn en iso 179, the dimensions were (80 x 10 x 4) mm. the injection molding process parameters were set in agreement with the material sheet of the tested polyamide. table 1 displays these parameters. the crosslinking causes the connection of polymeric chains to each other, most often using covalent bonds to form the spatial network. test bodies were irradiated under industrial conditions on a commercially available irradiation device in a broader range of radiation doses (0, 66, 132 and 198 kgy) compared to the doses corresponding to the experience in the practice. the irradiation process of the specimens was performed under general conditions (air atmosphere, ambient temperature of 23 °c) just as it is done in engineering practice. one pass in the accelerator exposed the material to 33 kgy of radiation. all samples were irradiated with electron (beta) rays (accelerated electrons a rhodotron r e-beam accelerator, electron energy 10 mev) in the firm bgs beta gamma service gmbh & co, saal am danau germany. 2.3. nano-indentation test nano indentation test was done on most modern indentation device; nano indentation tester (nht3) made by company anton paar (graz, austria). measurements were done by method of instrumented test (dsi depth sensing indentation) according to standard čsn en iso 14577. method used for calculation of mechanical characteristics was oliver pharr. berkovich pyramid was used as measuring tip. the poisson ratio was set to 0.47. it was subtracted from the material sheet for the test material pbt including the 35% glass fibers. device parameters are shown in table 2. the indentation hardness (hit) was calculated as maximum load (fmax) to the projected area of the hardness impression (ap) and the indentation modulus (eit) is calculated from the plane strain modulus (e*) using an estimated sample poisson’s ratio according parameters unit value maximum load mn 50 load/unload speed mn/min 100 holding time s 90 table 2. instrumented test parameters. to [12–15]: hit = fmax ap (1) eit = e ∗ (1 − ν2 s ) (2) the indentation creep (cit) was calculated: cit = h2 − h1 h1 · 100, (3) where h1 is the indentation depth at time t1 of reaching the test force (which is kept constant), h2 is the indentation depth at time t2 of holding the constant test force [12–15]. measurement of all above mentioned properties was performed 10 times to ensure statistical correctness. 3. results and discussion measurement done by the instrumented hardness test is based on the principle of immediate detection of indentation in dependence on the applied load (figure 1). the dependence provides the basic information about the behaviour of the tested material, which is subsequently used to calculate the mechanical properties, e. g. indentation hardness and modulus. figure 1. indentation characteristic of tested pbt 35%gf. figure 2 describes the dependence of the indentation depth on time that is used for the calculation of the indentation creep. table 3 shows the average values of the measured parameters. indentation hardness is a basic parameter, which is used to describe the local mechanical properties of the filled pbt surface layer. it is calculated by the oliver and pharr method. the unaltered material displayed 113 m. ovsík, m. staněk, a. dočkal, p. fluxa acta polytechnica parameters unit 0 kgy 66kgy 132kgy 198kgy indentation hardness mpa 65,04 79,35 110,57 116,83 indentation modulus gpa 1,26 1,35 1,83 2,04 indentation creep % 9,80 11,58 9,20 9,83 table 3. indentation parameters. figure 2. indentation characteristic (creep) of tested pbt 35%gf. figure 3. indentation hardness of tested pbt 35%gf. 65 mpa indentation hardness. due to the exposure to the electron radiation, a 3d network was created within the tested pbt, thus significantly increasing the indentation hardness. the indentation hardness measured in test samples irradiated by 66 kgy was 79 mpa, while the test samples irradiated by 198 kgy showed that the indentation hardness rose to 117 mpa. the difference in hardness between the unaltered material and the test sample irradiated by the highest amounts of radiation was 80%. from these measurements, it can be said that due to the filled pbt irradiation, its properties were nearing the properties of the more expensive polymers. furthermore, similar tendencies were measured in the case of indentation modulus. it was 1.3 gpa for the unaltered material and with the exposure of increasing dosages of radiation, the indentation modulus rose up to 2.1 gpa in the material irradiated by 198 kgy. so the improvement was a solid 62%. figure 4. indentation modulus of tested pbt 35%gf. figure 5. indentation creep of tested pbt 35%gf. indentation creep belongs among important local mechanical properties, which are commonly used to describe the behaviour of a specific material exposed to long term stress. as can be seen in figure 5, the irradiation of the samples did not cause any significant changes in the creep behaviour of the filled pbt. the difference between the virgin material and the irradiated pbt was approximately 6%. 4. conclusions this article describes the local nano-mechanical properties of the filled pbt. due to the irradiation by the electron beam the macromolecules were joined within the structure, thus creating a 3d network. this led to a significant improvement of the material properties. the local mechanical properties were increased even after the exposure to the lowest radiation dosage. furthermore, with higher intensity the samples’ inden114 vol. 27/2020 cross-linked polybutylene properties tation hardness and modulus rose by 80% and 62%. the results indicate, that the irradiation of the pbt has a positive effect on its local mechanical 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[15] m. ovsik, m. stanek, m. reznicek, l. hylova. study of nano-creep of unfilled and filled cross-linking polypropylene. materials science forum 919:103–110, 2018. doi:10.4028/www.scientific.net/msf.919.103. 115 https://doi.org/10.1016/j.matdes.2011.01.043 https://doi.org/10.1002/app.38945 https://doi.org/10.1016/j.polymdegradstab.2009.03.013 https://doi.org/10.1016/j.compscitech.2017.11.007 https://doi.org/10.3390/ma8125451 https://doi.org/10.1007/s00289-015-1576-0 https://doi.org/10.1016/j.msea.2010.01.070 https://doi.org/10.14314/polimery.2014.722 https://doi.org/10.1016/s0032-3861(98)00075-5 https://doi.org/10.4028/www.scientific.net/kem.699.37 https://doi.org/10.1557/jmr.2004.19.1.3 https://doi.org/10.4028/www.scientific.net/msf.919.103 acta polytechnica 27(0):112–115, 2020 1 introduction 2 methods 2.1 material 2.2 sample preparation 2.3 nano-indentation test 3 results and discussion 4 conclusions acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2019.22.0026 acta polytechnica ctu proceedings 22:26–30, 2019 © czech technical university in prague, 2019 available online at http://ojs.cvut.cz/ojs/index.php/app optimization of mix-design of chemically resistant sprayed concrete radek hermanna,∗, rostislav drochytkab, jiří kolískoc, pavel reitermand a brno university of technology, admas centre, purkynova 139, 612 00 brno, czech republic b brno university of technology, faculty of civil engineering, veveri 95, 602 00 brno, czech republic c czech technical university in prague, klokner institute, solinova 7, 166 08 prague 6, czech republic d czech technical university in prague, faculty of civil engineering, thakurova 7, 166 29 prague 6, czech republic ∗ corresponding author: xherma23@vutbr.cz abstract. this paper addresses optimization of mix-design and granulometric curves of aggregate of chemically resistant cementitious sprayed mix. the target of the paper is to improve properties of currently developed mix-design, which has no chemical resistance, and to utilize the secondary raw materials. the mix was optimized by additions improving chemical resistance and binder substituted by secondary raw materials. the binder was substituted by finely ground waste glass and high-temperature fly ash. filler was substituted with slag from deposit yard and waste sand with soluble glass from steel manufacture. main observed properties for design of optimized mix were workability time, consistence and physical-mechanical properties. optimized mix-design successfully replaced all fine parts of filler and up to 30% of binder while the physical-mechanical properties were preserved or even enhanced. optimized mix-designs show the same consistencies and workability times. keywords: sprayed mix, optimization, secondary raw materials, water absorbing capacity, granulometric curve. 1. introduction the problem of sprayed mixes has been solved since the beginning of the 20th century, equipment for application of sprayed concrete was patented in 1911 under the name cement gun together with sprayed material, gunite. sprayed mixes are usually connected mainly with tunnel works, but use is not limited only to those applications. use of sprayed concrete has advantages mainly because of its high density, low porosity and overall high quality of sprayed material as such. this material is suitable mainly for applications, where visual properties of final surface are not required. design of sprayed mixes requires mainly optimization of granulometric curves of aggregate to ensure high density and at the same time very good pumpability of the mix. the important factor is sufficient amount of fines supported by higher proportion of binder and admixtures. sprayed mixes have their own test procedures, which are described in a set of standards čsn en 14488 [1]. current time focuses on reduction of ecological impacts caused by manufacture of building materials, for this reason, waste or secondary raw materials are used. slag. this solid, inorganic and partly melted residuum is produced during combustion of coal or combustible shales on fire grates and in furnaces. this material has high contents of mineral residua of original combustible substance. it is suitable for concrete and mixes; however, properties of slag are not homogeneous, which may considerably change its quality. slag very often contains unburned residua of coal, humins, free oxides of calcium and magnesium and other substances, which influences considerably its final behavior in synergy with cement composite. time of deposit is a very important factor; generally – the longer time of deposit, the higher quality of the slag as such. during deposit time, rain flushes out salts, which may have adverse effect on final mix and create for example efflorescence. another negative aspect of slag is the fact, many deposit yards are covered with naturally seeded plants, which grow through the deposited slag and final product contains high amount of organic combustible products and hence certain volume instability. in accordance with legislative requirements, slag shall contain the maximum of 10% of combustibles and for use in concrete, high content of amorphous sio2 is assumed [2]. sand with soluble glass. this material is a waste product from steel manufacture. it is a residual material from forms after form removing of cast iron products. this material consists mainly of very pure siliceous sand with content of 80–85% sio2, and lower amount of soluble glass. annual production of waste sand with soluble glass in the czech republic is over 30 000 tons. there is a generally prevailing opinion, that sand with soluble glass is not suitable for concrete and mortars because of the soluble glass, as this increases amount of alkalis in matrix and may 26 http://dx.doi.org/10.14311/app.2019.22.0026 http://ojs.cvut.cz/ojs/index.php/app vol. 22/2019 optimization of mix-design of chemically resistant sprayed concrete contribute to alkali-silica reaction (asr). however, in lower amounts it has no impact on matrix and is almost required for sprayed concrete because soluble glass as addition accelerates setting and hardening of concrete. specific weight is similar as that of pure silica in the interval of 2500−2650 kg·m−3. sand with soluble glass is suitable because of its granulometry, when used as screened by screens 0.5mm. however, because of soluble glass, it forms agglomerates of size up to 16mm without any external influence [3, 4]. glass. glass is a transparent solid material with high content of amorphous sio2, produced by manufacture from glass batch. the most common glass products are packing material and sheet glass, which is nowadays manufactured with flow method, when the glass is poured on a layer of melted tin and an endless sheet of glass is produced. recycled glass used as a secondary raw material for manufacture of concrete comes mainly from packing glass. however, glass from automotive industry is used too, in particular windscreens of automobiles, where it is necessary to separate flexible membranes between layers of glass forming the windscreen composite. waste glass produced during manufacture and glass from sorted waste containers is used for manufacture of glass to reduce cost as temperature of melting broken glass is lower than temperature of melting glass batch. unfortunately, not all waste glass can be reused for glass manufacture and glass waste is a serious ecological problem. most frequent glass is soda-lime glass with 50% of silica sand, 10% na2co3 (soda), 12% caco3 (lime stone), 18% crushed splinters of waste glass and 4% of other substances. this glass represents up to 80% of overall glass waste. because of its composition and content of sio2 it shows high pozzolanic activity, which, together with cement, supports formation of csh gels in cement matrix. studies of keren zheng [5] state that if size of glass grain is lower than 75µm, the material shows sufficient pozzolanic activity. use of ground glass and generally glass in concrete presents problem of alkali silica reaction (asr), which is a reaction, when reaction of alkalis (cement) and amorphous silica material (glass) creates in cement matrix new forms of larger volume and these can gradually lead to static failure of the whole structure. gerry lee proved in his paper, if the glass is ground to higher specific surface and medium size of particles is lower than 300 µm, asr does not occur [5–7]. high temperature fly ash. high temperature fly ash is incombustible residuum formed in thermal power plants during combustion of pulverized coal at temperatures between 1200 and 1700 °c. to separate fly ash from waste gases, power plants use mostly highly effective electro-static separators; fly ash can be easily used for other purposes. as a reaction to growing demand for fly ash in building industry, standard csn en 450 [8] was elaborated, which prescribes properties and criteria of suitability of fly ash for concrete and mortars. suitability of fly ash for concrete and mortars requires minimal content of 45% amorphous sio2 and maximal content of free cao, mgo and alkalis. high temperature fly ash presents nowadays up to 70% of all energetic by-products. thanks to its composition, high temperature fly ash is similar to natural pozzolans. these pozzolanic properties combined with cement help the formation of csh gels. specific weight of fly ash is between 2100 and 2600 kg·m−3. fly ash influences mainly long-term physical-mechanical properties, which brings considerable enhancement and help enclose the structure cement matrix. this phenomenon also enhances chemical resistance in time [3]. 2. materials and methods for optimization, one of currently developed sprayed fine-grained mixes was selected, which did not show increased chemical resistance before adjustments. manufacturer supplies partial components; binder was cement cem i 42.5 r, additions, fillers, polypropylene fibers and admixtures for adjustments of specific properties of the material. composition of reference is in following table 1. mix was adjusted with secondary raw materials. part of the binder was substituted with finely ground recycled glass from soda-lime glass (glass) and high temperature fly ash (hfa). filler was substituted with sand containing soluble glass (scsg) and slag from former thermal power plant and nearby deposit yard (ssl). substitution filler, glass with soluble glass and slag were pre-treated with jawed crusher and subsequently with sieve analyzed so that final granulometric curve corresponded with that of original fillers as much as possible. substitution binders were pretreated with laboratory ball grinder so that specific surface was comparable with specific surface of cement, in particular 400m2·kg−1(± 50m2·kg−1). grain size of individual substitution binder is stated in following tab. 2. all tests and experiments were carried out in accordance with regulations and standards. mix-designs. in total, five reference materials were made with all content of binder and filler, the only modified part was content of additions influencing specific properties. out of these basic mix-designs, one representative was selected based on physicalmechanical properties after 28 days, in particular tensile strength, compressive strength and water absorbing capacity. this mix-design was then modified with secondary raw materials. the influence of these secondary raw materials on the physical-mechanical properties of the high strength cementitious composites was studied in cooperation of brno university of technology and klokner institute czech technical university in prague. as regards filler, 100% substitution of given size fraction was tested; as for binder, three levels of substitution were tested for each secondary raw material. after evaluation of physical-mechanical properties after 28 days, above mentioned binders and 27 r. hermann, r. drochytka, j. kolísko, p. reiterman acta polytechnica ctu proceedings component proportion in the mix [wt.%] portland cement cem i 42,5 r 17-25 silica sand 0-0.5mm 13-18 silica sand 0-5.4mm 38-45 waste from ground feldspar 4-9 ground lime stone 10-17 mix of additions 1-1.9 table 1. composition of reference mix. secondary raw materials < 0.01mm 0.01 − 0.063mm > 0.063mm high temperature fly ash 28.84 63.35 7.87 recycled glass 30.71 53.12 16.18 table 2. grain size of substitution binders [wt.%]. fillers were combined in various ways to ensure maximal utilization of secondary raw materials. materials were mixed separately for each mix from individual raw materials and then mixed with mixing water in the proportion 0.14 kg of water per 1 kg of dry mix. amount of mixing water was slightly adjusted depending on given secondary raw materials to keep the same consistency in fresh state (tested on flow table in accordance with csn en 1015-3 [9]). physicalmechanical properties were tested on test specimens with dimensions 160 × 40 × 40mm, which were placed in water after form removal; tests were carried out after 2, 7 and 28 days of curing. laboratory tests. fresh mixes were subjected to tests to determine density, consistency and time of setting and hardening. hardened specimens were subjected to tests of physical-mechanical properties – density, tensile strength, compressive strength and after 28 days also water absorbing capacity. 3. results and discussion physical-mechanical properties. fig 1 shows compressive strength of reference mix-design and other mix-designs with adjusted composition of additions. these adjusted mix-designs are labeled bet 0-3 and have completely identical composition and watercement ratio, the only change is amount of additions influencing specific properties of material. based on the results, one mix-design with the highest compressive strength and lowest water absorbing capacity at the same time will be selected, as the best chemical resistance of such mix-design can be presumed. selection is assumed after 28 days of maturation and selected mix-design will be subsequently optimized with secondary raw materials. the results imply that because of using additions affecting specific properties of the material, considerably higher compressive strengths were achieved only after two days while water-cement ratio was the same. to verify properties and determine exact results, suitable mix-design was selected after 28 days of maturation. above mentioned mix-designs were subjected to determination of water absorbing capacity. with few exceptions it was confirmed that the higher was the compressive strength value, the lower was water absorbing capacity after 28 days. mix-design bet 1 was selected for further optimization, as it showed the best results of physical-mechanical properties. this mix-design was then optimized with secondary raw material to achieve better results than material bet ref. following fig 2 shows compressive strengths of optimized mix-designs based on bet-1, from which, 5 out of 21 mix-designs were selected for further testing. compressive strengths of optimized mix-designs are pictured in contrast with reference mix-design bet ref. in optimized mix-designs, 100% of fine aggregate is replaced with waste sand with soluble glass from manufacture of steel (scsg) and subsequently, combinations were tested with waste glass (glass), high temperature fly ash (hfa) in the amount of 2030% of binder substitution (cement) and slag from deposit yard (ssl) as 100% replacement of coarse aggregate. test results show that when secondary raw materials are combined with additions, it is possible to achieve strengths which are comparable or even higher than those of reference mix after 28 days – combination of waste glass, high temperature fly ash and 30% substitution of cement. in this mix-design, pozzolanic properties of given materials proved. the results show that 20% substitution of cement with waste glass increased compressive strength by 4%, 20% substitution of cement with high temperature fly ash increased compressive strength by 7% and 30% substitution of cement with high temperature fly ash increased compressive strength by approx. 1% after 28 days. when slag from deposit yard was used, it was necessary to add more than 10% mixing water to keep the value of workability, which caused considerable reduction of compressive strength and volume weight, results are stated only for information. 28 vol. 22/2019 optimization of mix-design of chemically resistant sprayed concrete figure 1. compressive strengths of reference mix-design. figure 2. compressive strengths of optimized mix-designs. labeling of mix-design water absorbing capacity after 28 days [%] bet ref 10.44 bet x 10.29 bet 0 10.12 bet 1 9.37 bet 2 10.39 scsg 100 + glass 20 10.83 scsg 100 + glass 30 10.87 scsg 100 + hfa 20 10.85 scsg 100 + hfa 30 10.92 ssl 100 + scsg 100 + hfa 40 12.65 table 3. water absorption of individual mix-designs after 28 days. 29 r. hermann, r. drochytka, j. kolísko, p. reiterman acta polytechnica ctu proceedings following tab 3 show values of water absorbing capacity of samples made per individual mix-designs. water absorbing capacity was determined on fractions of test specimens after the compressive strength test. tab 3 shows water absorbing capacities of specimens made per individual mix-designs, compared to specimens made per reference mix-design. it is obvious at first sight that water absorbing capacity of optimized mix-designs is higher by 0.4 – 0.5 than reference sample. water absorbing capacity was determined on samples only 28 days old, when it is not expected that properties of used additions and pozzolanic properties of secondary raw materials show their effect. however, because of character of used raw materials it can be assumed, that the value of water absorbing capacity will decrease in time. 4. conclusion optimization of additions used in manufacture of above-mentioned sprayed mix could preserve or enhance physical-mechanical properties of reference material. reference material reached relatively high compressive strengths even after 2 days of curing; these strengths remained sufficient for sprayed application after optimization with secondary raw materials. use of secondary raw materials verified the hypothesis, that water absorbing capacity of matrix will be increased to certain extent. the usage of the selected secondary raw materials did not cause delay of setting or hardening. it was proved, that when 100% of finer aggregate was replaced with waste sand with soluble glass, combined with 30% replacement of binder (cement), the resulting mix shows after 28 days comparable or even better physical-mechanical properties and thanks to selection of secondary raw materials with pozzolanic properties it can be assumed, properties of the material will improve in the course of time. applicability of the material in more demanding exposition conditions is assumed after at least 60 days, when properties of additions and pozzolanic properties of used materials will manifest. because of pozzolanic additions, long-term properties are determined too, after 90 days. however, these are not yet available at the time of publishing this paper. selected mix-designs will be used for a test spraying and samples made in this way will be subjected to testing of physical-mechanical properties after 2, 7, 28 and 90 days including testing water permeability, resistance to chemical de-icing substances and resistance to chemical substances. the target is to create fine-grained sprayed mix with increased resistance to chemical substances and increased life time in demanding exposition conditions. acknowledgements this paper has been worked out under the project no. lo1408 “admas up – advanced materials, structures and technologies”, and under the project fast-s-18-5406 “study of structure of advanced silicate composites focusing on characteristics of action of secondary raw materials”. references [1] čsn en 14488-6: testing sprayed concrete part 6: thickness of concrete on a substrate. standard, czech office for standards, metrology and testing, 2007. in czech. [2] shotcrete design guidelines: design guidelines to avoid, minimise and improve the appearance of shotcrete. tech. rep., roads and traffic authority of nsw, 2005. [3] p. pytlík. technologie betonu. vutium, brno, 2nd edn., 2000. [4] l. svoboda, et al. technologie betonu. luboš svoboda, praha, 3rd edn., 2013. [5] c. shi, k. zheng. a review on the use of waste glasses in the production of cement and concrete. resources conservation and recycling 52:234–247, 2007. doi:10.1016/j.resconrec.2007.01.013. [6] v. bílek. možnosti využití odpadních písků z výroby vodního skla ve stavebnictví. master’s thesis, brno university of technology, faculty of chemistry, 2013. [7] h. lee, a. hanif, m. usman, et al. performance evaluation of concrete incorporating glass powder and glass sludge wastes as supplementary cementing material. journal of cleaner production 170:683–693, 2018. doi:10.1016/j.jclepro.2017.09.133. [8] čsn en 450-1: fly ash for concrete part 1: definition, specifications and conformity criteria. standard, czech office for standards, metrology and testing, 2005. in czech. [9] čsn en 1015-3: methods of test for mortar for masonry part 3: determination of consistence of fresh mortar (by flow table). standard, czech office for standards, metrology and testing, 2000. in czech. 30 http://dx.doi.org/10.1016/j.resconrec.2007.01.013 http://dx.doi.org/10.1016/j.jclepro.2017.09.133 acta polytechnica ctu proceedings 22:26–30, 2019 1 introduction 2 materials and methods 3 results and discussion 4 conclusion acknowledgements references doi:10.14311/app.2021.30.0069 acta polytechnica ctu proceedings 30:69–75, 2021 © czech technical university in prague, 2021 available online at http://ojs.cvut.cz/ojs/index.php/app comparing the hoek-brown and mohr-coulomb failure criteria in fem analysis tereza poklopová∗, veronika pavelcová, michal šejnoha czech technical university in prague, faculty of civil engineering, department of mechanics, thákurova 7, 166 29 prague 6, czech republic ∗ corresponding author: tereza.poklopova@fsv.cvut.cz abstract. this paper revisits the issue of a potential substitutions of the hoek-brown failure model by the standard mohr-coulomb model in the stability analysis of rock masses. the derivation of equivalent shear strength parameters of the mohr-coulomb proposed by hoek et al. [1] is addressed with emphases on the suitable range of stresses for which the equivalence of the two failure criteria applies. to that end, a simple numerical analysis of the oedometric test is carried out. it is seen that a correct choice of the upper limit of the minimum compressive principal stress is crucial for the mohr-coulomb model to provide predictions comparable to the hoek-brown model. this issue is addressed next in the light of the solution of slope stability problem. all the presented results were derived with the help of the geo5 fem finite element software [2]. keywords: equivalent shear strength, finite element method, hoek-brown criterion, nonlinear yield function, rock mechanics. 1. introduction application of the mohr-coulomb (mc) model is a common approach when addressing the stability of soil slopes. although for a rock mass the hoek-brown (hb) failure criterion [1, 3–5] proved more appropriate, the use of the mohr-coulomb model in the stability assessment of rock slopes is still quite popular in practice. this can be attributed to the fact that the factor of safety can easily be defined in terms of the basic shear strength parameters such as the cohesion c and the angle of internal friction ϕ. this is supported by the possibility of estimating these parameters through simple empirical formulas on the basis of hb model parameters [1]. this, however, requires the knowledge of the range of stresses for which the linear mc model provides results comparable to those predicted by the nonlinear hb model. so it is perhaps more straightforward to proceed in the footsteps of benz et al. [6] who introduced a simple modification to the original hb model, which brings the two models to the same footing and allows for a standard strength reduction approach to estimate the factor of safety in the light of the hoek-brown model. both issues are examined in this paper. 2. hoek-brown model the hoek-brown model was first introduced by hoek [3] in application to the analysis of an underground excavation of intact rock masses. later in [4] this criterion was extended to weak rock masses by adjusting the material constants according to a geological quality. the geological strength index (gsi) is used as a qualitative classification number. by implementing the influence of prior excavation process, the current form of the hb model covers the whole range of rock masses. the generalized hoek-brown failure criterion is defined in terms of principal stresses as σ3 = σ1 − σci ! mb −σ1 σci + s "a , (1) where σ1 and σ3 are the maximum and minimum effective principal stresses, respectively1. the material constants mb, s and a follow from empirical equations as mb = mi exp gsi − 100 28 − 14d , (2) s = exp gsi − 100 9 − 3d , (3) a = 1 2 + 1 6 ! exp −gsi 15 − exp −20 3 " , (4) where the disturbance factor d account for prior excavations and ranges from 0 (undisturbed rock) to 1 (disturbed by production blasting). the uniaxial compressive strength σci and the model parameter mi are considered for an intact rock. the rock mass stiffness is assumed constant and independent of the current stresses. following [1], the modulus of elasticity is estimated by em(gpa) = ! 1 − d 2 " # σci 10010((gsi−10)/40), (5) em(gpa) = ! 1 − d 2 " 10((gsi−10)/40). (6) 1unlike typical geotechnical notation, the tensile stresses are assumed positive. 69 http://dx.doi.org/10.14311/app.2021.30.0069 http://ojs.cvut.cz/ojs/index.php/app t. poklopová, v. pavelcová, m. šejnoha acta polytechnica ctu proceedings equation (5) is called when σci ≤ 100 mpa and suggests a rapid decrease of the modulus with a decreasing uniaxial compressive strength. equation (6) is then used when σci > 100 mpa. the above equations were exploited in this paper regardless of their updated version presented in [5]. 3. analogy with the mohr-coulomb criterion in the finite element analysis the failure criterion has a meaning of a yield function bounding the elastic response. the geo5 fem program assumes the stressstrain relation to be elastic perfectly plastic. the hoek-brown yield function then follows from eq. (1) in the form f(σ1, σ3) = σ1 − σ3 − σci ! mb −σ1 σci + s "a . (7) when presented in the principal stress space, the hoek-brown yield function is similar to the mc model independent of the intermediate principal stress σ2. in case of plastic yielding, the stress return mapping is driven by a non-associated flow rule with the plastic potential function given by g(σ1, σ3) = 1 + sin ψ 1 − sin ψ σ1 − σ3, (8) where ψ is the dilation angle and for the assumed perfect plasticity it remains constant. similar to mc model, the hb model plots as irregular hexagon, but unlike the mc model it experiences a nonlinear variation in the deviatoric plane, see also the projection into σ1-σ3 plane displayed in fig. 1. figure 1. hoek-brown yield function plotted in plane of maximum and minimum principal stresses. 3.1. equivalent shear strength parameters the simplicity of the mc model and its prevailing popularity among design engineers promote its use in applications concerning the stability issues of a rock mass. the similarity of the mc and hb models was examined by hoek et al. in [1, 4] in order to derive the shear strength parameters ϕ, c controlling the mc failure criterion as, compare with eq. (1), σ3 = −σcm + kσ1, (9) where σcm is the uniaxial compressive strength of the rock mass and k is the slope of the mc plot in the σ3 and σ1 principle stress space, see fig. 2. effective values of the shear strength parameters c, ϕ are then given by c = σcm 2 √ k , sin ϕ = k − 1 k + 1 . (10) comparing eqs. (1) and (9) it is seen that no mathematical relationship between the two models can be determined. to that end, an iterative approach balancing the area above and below the mc model, see fig. 2, was developed in [1] to get c = σci[(1 + 2a)s + (1 − a) mbσ1n](s + mbσ1n)a−1 a $ 1 + (6amb(s + mbσ1n)a−1)/a , (11) a = (1 + a)(2 + a), sin ϕ = % 6amb(s + mbσ1n)a−1 2(1 + a)(2 + a) + 6amb(s + mbσ1n)a−1 & , (12) for the range of stresses of σt > σ1 > σ1,max, where the bi-axial tensile stress σt = σcis/mb is found by setting σ1 = σ3 = σt in eq. (1), and σ1n = |σ1,max|/σci depends on the upper limit of the confining stress σ1,max seen in fig. 2. it is this parameter we accord our attention to in the next section. figure 2. nonlinear hoek-brown criterion and equivalent mohr-coulomb model in σ1-σ3 plane. 4. simple oedometer test to investigate the influence of the choice of the upper stress limit σ1,max we consider a simple oedometric test. assuming elasticity we get σ1 = σ2 = ν 1 − ν σz, σ3 = σz, (13) 70 vol. 30/2021 hoek-brown and mohr-coulomb failure criteria parameter symbol value unit poisson’s ratio ν 0.3 [−] uniaxial compressive strength σci 20.0 [mpa] hoek-brown constant mi 8.0 [−] geological strength index gsi 30.0 [−] disturbance factor d 0.0 [−] reduced hoek-brown constant mb 0.656680 [−] parameter s 0.000419 [−] parameter a 0.522340 [−] modulus of elasticity em 1414.20 [mpa] table 1. input and calculated parameters of rock mass. figure 3. geometry, boundary and loading conditions of fem model of oedometer. where σ3 < 0 is the applied vertical compressive stress. as mentioned in the previous section, arriving at a comparable response predicted by the mc and hb models using the equivalent shear strength parameters strongly depends on the adopted range of stresses bounded by σt and σ1,max. while σt depends on the hb model parameters, the choice of σ1,max is governed by specific loading conditions. thus an incorrect choice of this parameter may lead to erroneous predictions of the mc model. to see this, we begin with a value σ1n = 0.25 recommended in [4] for triaxial simulations. assuming the values of the hb model provided in table 1 we get from eqs. (11) and (12) c = 649.0 kpa, ϕ = 22.8◦. substituting eq. (13) into the hoek-brown yield function (7) ν 1 − ν σz − σz − σci ! mb ν 1 − ν −σz σci + s "a = 0, (14) gives the limit value of the vertical stress for the first plastic yielding as σz = −16.165 mpa. moving beyond elasticity calls for numerical simulations. to that end, we adopted the geo5 fem software and set up the most simple geometrical model of an oedometer under plane strain conditions consisting of only two constant strain 3-node triangular elements. boundary and loading conditions of the model are evident in fig. 3. the model was loaded by vertical compression such as to pass the elasticity limit point. (a) elastic stress path (b) figure 4. case 1 σ1n = 0.25: a) evolution of stresses based on hb and mc models, b) intersections of elastic stress path with yield surfaces. this simulation clearly demonstrates the influence of the chosen σ1n ratio on the degree of equivalence. unlike the triaxial test2, where the stress state is known, the evolution of principal stresses in oedometer is controlled by the yield function, i.e. by the strength parameters of the rock mass. therefore, the analytical determination of the limit stress value σ1max is not trivial. the consequence of an incorrect choice of σ1n = 0.25 appears in fig. 4(a) plotting the evolution of stresses in terms of invariant stress measures, where σm is the mean stress and j is the square root of the second invariant of the deviatoric stress. clearly, the hb model predicts the deviation from the elastic 2see for example [7] for more details on the hb model verification using triaxial simulations. 71 t. poklopová, v. pavelcová, m. šejnoha acta polytechnica ctu proceedings (a) elastic stress path (b) figure 5. case 2 σ1n = 0.79: a) evolution of stresses based on hb and mc models, b) intersections of elastic stress path with yield surfaces. response precisely at the elastic limit point for the applied load σz = −16.165 mpa, while the mc model still suggests an elastic behavior. the inadequacy of the mc model setting is better seen in fig. 4(b) plotting the elastic stress-path with the onset of yielding identified by the plus sign for both yield surfaces. evidently, the assumed range of stresses for which the equivalence holds, recall fig. 2, is outside the generated stress state controlled by the mc model. the new estimate of σ1n ratio was obtained numerically adopting the hb model to get the maximum lateral principle stress σ1,max = −15.79 mpa for the maximum applied load σz = σ3 = 30 mpa. this gives σ1n = 0.79 and consequently new estimates of strength parameters c, ϕ c = 1345.5 kpa, ϕ = 15.6◦. the corresponding evolution of stresses is shown in fig. 5(a). a reasonable choice of σ1n ratio is further supported in figs. 5(b) and 6(a) comparing the yield surfaces for individual settings. the actual stress path corresponding to both models appears in 6(b) suggesting a close match for the expected stress range. it is worth mentioning that the stress state generated either in oedometer or triaxial apparatus is constant within the sample. however, in real applications a non-homogeneous stress state arises. one thus has to be extremely careful when specifying the expected range of stresses if adopting just one pair elastic stress path (a) (numerical solution) (b) figure 6. a) influence of estimated stress scale, b) evolution of stresses based on hb and mc models. of equivalent shear strength parameters, essentially independent of the actual stress state. nevertheless, for some specific applications simple relations for the determination σ1,max have been proposed in [1]. one such a relationship for the case of slope stability analysis is examined in the next section. 5. slope stability analysis the slope stability analysis typically aims at providing the factor of safety. for the family of elastic perfectly plastic mc models the factor of safety is defined as the ratio of the current shear strength parameter, e.g. cohesion, to the minimum one for which the convergence of the nonlinear problem still exists. this approach is thus not directly applicable to the hb model. however, the authors in [6] introduced an elegant way to overcome this obstacle. so the analysis presented next proceeds in their footsteps. 5.1. hoek-brown model in stability analysis the approach outlined in [6] builds upon a gradual reduction of the strength of a rock mass by modifying the yield function as f(σ1, σ3) = σ1 − σ3 − σci η ! mb −σ1 σci + s "a , (15) 72 vol. 30/2021 hoek-brown and mohr-coulomb failure criteria parameter symbol value unit unit weight γ 25.0 ' kn/m3( poisson’s ratio ν 0.3 [−] uniaxial compressive strength σci 30.0 [mpa] hoek-brown constant mi 2.0 [−] geological strength index gsi 5.0 [−] disturbance factor d 0.0 [−] reduced hoek-brown constant mb 0.067225 [−] parameter s 0.000026 [−] parameter a 0.019210 [−] modulus of elasticity em 410.73 [mpa] table 2. input and calculated parameters of rock mass. where η is the reduction factor. a different reduction factor is introduced to reduce the shear strength parameters ϕ, c as tan ϕd = tan ϕc γϕ , cd = cc γc , (16) where ϕc and cc are the actual strength parameters of the subsoil material and γϕ and γc are their corresponding reduction coefficients. a typical setting in the stability analysis assumes γϕ = γc = γ. the objective now is to determine the relationship between the two reduction factors η and γ. this is achieved by constructing a tangent to the hoek-brown yield function at the current state of stress sitting on the yield function and comparing this slope to the slope of the mc yield surface to get η = 1 2 ) **+γ , 2 − f̃ ′ .///01 + , 1 γ2 − 1 1 −f̃ ′22 1 2 − f̃ ′22 + f̃ ′ 3 445 , (17) where f̃ ′ = ∂f̃(σ1) ∂σ1 = −mba ! mb −σ1 σci + s "a−1 . (18) notice that η is a function of the current stress state and must be evaluated for every element of the mesh in every loading step separately. for complete derivation of eq. (17) we refer the interested reader to [6]. 5.2. estimating σ1,max in stability analysis the application of the mc model requires setting the value of σ1,max to estimate the shear strength parameters. by performing a large parametric study on a variety of slope geometries and rock mass properties using bishop’s limit state analysis, hoek et al. [1] proposed the following relationship for σ1,max |σ1max| = 0.72σcm ! σcm γh "−0.91 , (19) where h is the height of the slope, γ is the unit weight of the rock mass and σcm is the rock mass strength defined by eq. (10) σcm = 2c cos ϕ 1 − sin ϕ , (20) the rock mass strength expressed using parameters of the hoek-brown model is then given by substituting eqs. (11) and (12) into eq. (20). it holds σcm = σci (mb + 4s − a(mb − 8s))(mb/4 + s)a−1 2(1 + a)(2 + a) . (21) 5.3. practical use of the methods to test reliability of eq. (19), we performed the slope stability analysis of a homogeneous rock slope employing both constitutive models. the geometry of the computational model of a slope with the height of h = 10 m and inclination of 35.5◦ is shown in fig. 7 together with the finite element mesh. the domain was discretized using either linear 3-node (t3) or quadratic 6-node (t6) triangular elements. an average element edge size was about 0.5 m. the material data listed in the upper part of table 2 were taken from [6]. the model parameters presented in the lower part of this table were determined from eqs. (2) (6). figure 7. geometry of fem model, boundary conditions and finite element mesh. the analysis was carried out using standard shear strength parameters reduction approach implemented in geo5 fem. adopting 73 t. poklopová, v. pavelcová, m. šejnoha acta polytechnica ctu proceedings fem tool geo5 fem plaxis 2019 material model hb hb mc mc hb element type t3 t6 t3 t6 t15 safety factor 1.44 1.37 1.42 1.33 1.35 table 3. results of slope stability analysis. (a) (b) (c) (d) figure 8. potential slip surfaces represented by the localized equivalent deviatoric plastic strain: a) hb criterion, t3 elements, b) hb criterion, t6 elements, c) equivalent mc criterion, t3 elements, d) equivalent mc criterion, t6 elements. eqs. (19), (11) and (12) yields |σ1max| = 189 kpa, c = 20 kpa, ϕ = 21◦. in the case of hb model the parameter η was calculated for the prescribed reduction factor γ using eq. (17). the results are summarized in table 3 for both types of approximations. the result acquired by employing the plaxis 2d [8] software is also presented for the sake of comparison. note that it was derived for the 15-node triangular element and a slightly different finite element mesh. as expected, the t3 element delivers with the same mesh refinement the most optimistic predictions. the results provided by the t6 element, the default setting in geo5 fem, are comparable to that of plaxis 2d. also, comparing the hb and mc predictions in terms of both the factor of safety and graphical representation of the slip surfaces in fig. 8 supports eq. (19) in providing a suitable estimate of σ1,max. it should, however, be pointed out that we considered a particularly simple case of a homogeneous slope. thus if possible, the use of the hb model for the slope stability analysis as proposed in [6] is preferable. 6. conclusions the principal objective of this contribution was to highlight the importance of a correct choice of the stress ranges in the definition of equivalent shear strength parameters when replacing the hoek-brown failure model with the mohr-coulomb model. this was demonstrated by running a numerical analysis of a simple oedometer test for two different estimates of σ1n ratio. while the first choice assuming simply the value recommended for triaxial simulations resulted in vastly different predictions when compared to a direct analysis with the hb model, the second choice based on a numerical estimate of the value of σ1,max led to comparable predictions. next, the slope stability analysis was performed taking advantage of the procedure proposed in [6] so that standard approach based on a gradual reduction of strength parameters was possible to apply to both models. although for the simple case of a homogeneous slope the estimate of σ1,max proposed by hoek et al. [1] delivers satisfactory results with the mc model, the approach based on a direct application of the hb model is preferable, particularly in the case of more complex subsoil conditions. acknowledgements the support provided by the sgs project no. sgs20/038/ohk1/1t/11 is gratefully acknowledged. references [1] e. hoek, c. carranza-torres, b. corkum. hoekbrown failure criterion 2002 edition. proceedings of the 5th north american symposium narms-tac 2002. [2] fine-ltd. geo5 fem, user manual. http://www.fine.cz/, 2020. 74 vol. 30/2021 hoek-brown and mohr-coulomb failure criteria [3] e. hoek, e. t. brown. underground excavations in rock. london: institution of mining and metallurgy, 1980. [4] e. hoek, e. t. brown. practical estimates of rock mass strength. international journal of rock mechanics and mining sciences 34.8:1165–1186, 1997. [5] e. hoek, e. t. brown. the hoek-brown failure criterion and gsi 2018 edition. journal of rock mechanics and geotechnical engineering 2018. [6] t. benz, r. schwab, r. kauther, p. a. vermeer. a hoek-brown criterion with instrinsic material strength factorization. international journal of rock mechanics and mining sciences 45:210–222, 2008. [7] t. poklopová. prediction of the response of geotechnical structure using hoek-brown model implementation, verification. master’s thesis, czech technical university in prague, faculty of civil engineering, 2020. in czech. [8] p. ltd. plaxis manual. http://www.plaxis.nl/. 75 doi:10.14311/app.2021.30.0087 acta polytechnica ctu proceedings 30:87–92, 2021 © czech technical university in prague, 2021 available online at http://ojs.cvut.cz/ojs/index.php/app beam element under finite rotations emma la malfa ribolla∗, milan jirásek, martin horák czech technical university in prague, faculty of civil engineering, department of mechanics, thákurova 7, 166 29 prague 6, czech republic ∗ corresponding author: emma.la.malfa.ribolla@fsv.cvut.cz abstract. the present work focuses on the 2-d formulation of a nonlinear beam model for slender structures that can exhibit large rotations of the cross sections while remaining in the small-strain regime. bernoulli-euler hypothesis that plane sections remain plane and perpendicular to the deformed beam centerline is combined with a linear elastic stress-strain law. the formulation is based on the integrated form of equilibrium equations and leads to a set of three first-order differential equations for the displacements and rotation, which are numerically integrated using a special version of the shooting method. the element has been implemented into an open-source finite element code to ease computations involving more complex structures. numerical examples show a favorable comparison with standard beam elements formulated in the finite-strain framework and with analytical solutions. keywords: finite rotations, nonlinear beam, shooting method. 1. introduction highly slender fiberor rod-like components represent essential constituents of mechanical systems in many fields of application such as civil, mechanical and biomedical engineering. it is widely recognized that slender bodies can be efficiently modeled applying a beam theory instead of a 3d continuum mechanics theory. kirchoff proposed the first beam formulation which includes large three-dimensional deformations [1, 2], and reissner completed the theory for both two-dimensional [3] and three-dimensional cases [4] with two additional deformation measures representing the shear distortion of beam segments. reissner’s finite-strain beam theory is one of the most important geometrically nonlinear models, subsequently extended and used by many other authors for 2-d and 3-d analysis of static as well as dynamic problems. simo developed a dynamic formulation for reissner’s beam [5] and together with vu-quoc [6] initiated the finite element implementation. he also introduced the useful concept of a geometrically exact beam, based on recasting reissner’s theory in a form which is valid for any magnitude of displacements and rotations. in this paper, a geometrically nonlinear beam model is formulated for the 2-d case. this model is applicable when the strains remain in a limited range while the rotations of beam sections can become arbitrarily large, and it properly accounts for the effect of curvature on the change of distance between end sections measured along the chord. therefore the model considers large rotations and displacements while strains are treated as small and simple hooke’s law is used (an extension to a nonlinear material law would be relatively straightforward). cross sections are still assumed to remain planar and perpendicular to the deformed beam axis. the formulation exploits the equilibrium equations in their strong form. they are combined with the kinematic equations and generalized material equations (linking internal forces to the curvature and centerline extension) and then numerically integrated. 2. model description 2.1. basic kinematic assumptions in this section, the kinematics of the element is presented, leading to the strain-displacement equations. each beam is analyzed in its local coordinate system, with the x-axis passing through the end joints in the undeformed configuration and the z-axis rotated by 90◦ clockwise, while the rotations are positive counterclockwise (see figure 1). we use u and w for horizontal and vertical displacements at an arbitrary point, while the displacements at the centerline are denoted by us and ws. let us consider an infinitesimal segment of the centerline of initial length dx, which is after deformation transformed into a segment of length dx̄ =! (dx + dus)2 + dw2 s . the corresponding centerline stretch can be expressed as λs = dx̄ dx = ! (1 + u′ s)2 + w′2 s (1) where the prime denotes differentiation with respect to x. the assumption of planarity of the deformed section and of its perpendicularity to the deformed centerline implies that the rotation angle is linked to the centerline displacements by the relation tan ϕ = − dws dx + dus = − w′ s 1 + u′ s (2) 87 http://dx.doi.org/10.14311/app.2021.30.0087 http://ojs.cvut.cz/ojs/index.php/app e. la malfa ribolla, m. jirásek, m. horák acta polytechnica ctu proceedings figure 1. kinematics of deformation of the presented nonlinear beam model. and that the displacements of a general point with coordinate z can be expressed as u = us + z sin ϕ (3) w = ws − z(1 − cos ϕ) (4) in contrast to the geometrically linear beam model with u = us + zϕ and w = ws, here the vertical displacement varies along the height of the section. however, the change of distance from the centerline due to deformation perpendicular to the beam axis is still neglected. differentiating (3)–(4) with respect to x, we obtain u′ = u′ s + z cos ϕ · ϕ′ (5) w′ = w′ s − z sin ϕ · ϕ′ (6) and we can express the longitudinal stretch at an arbitrary location λ = ! (1 + u′)2 + w′2 = = ! λ2 s + 2zλsϕ′ + z2ϕ′2 = λs + zϕ′ (7) it is worth noting that the stretch is a linear function of the spatial coordinate z. consequently, if we use a linear stress-strain law formulated in terms of the biot strain defined as ε = λ − 1, the resulting work-conjugate stress will be a linear function of z, too. the choice of working with linear stress-strain relation between the biot strain and stress can be justified in a regime where the strains are assumed to remain small. making use of (7), we can express the biot strain as ε = λ − 1 = λs − 1 + zϕ′ = εs + zκ (8) where εs = λs − 1 = ! (1 + u′ s)2 + w′2 s − 1 (9) is the strain at the centerline and κ = ϕ′ (10) is the curvature. 2.2. stress resultants and equilibrium based on (8) combined with hooke’s law σ = e ε, quantities εs and κ that characterize the deformation of an infinitesimal beam segment can be linked to the normal force n and bending moment m by the standard linear relations n = " a σ da = " a e(εs + zκ) da = ea εs (11) m = " a zσ da = " a ez(εs + zκ) da = ei κ (12) in which ea is the sectional axial stiffness and ei is the sectional bending stiffness. as usual, e denotes young’s modulus, a is the sectional area and i is the sectional moment of inertia. the structure is assumed to be loaded only at its joints, i.e., no external loads are applied on individual beams, and so the internal forces in each beam can be expressed in terms of the end forces and moments, based on equilibrium conditions written for the deformed configuration. from equilibrium of the finite beam segment between the left end section and a generic section x, one can, according to fig. 2, derive expressions n(x) = −xab cos ϕ(x) + zab sin ϕ(x) (13) m(x) = −mab + xabws(x) − zab(x + us(x)) (14) in which xab, and zab are the left-end forces (positive to the right and downwards) and mab is the left-end moment (positive counterclockwise). for the sake of brevity, we will consider the moment as one of the (generalized) forces, and, in a similar spirit, the rotation as one of the (generalized) displacements. since the internal forces are involved only algebraically, they can be easily eliminated and equations (2) and (9)–(14) can be converted into the following set of first-order differential equations for unknown functions us, ws and ϕ: ϕ′(x) = −mab + xabws(x) − zab(x + us(x)) ei (15) u′ s(x) = −1 + cos ϕ(x)+ + −xab cos ϕ(x) + zab sin ϕ(x) ea cos ϕ(x) (16) 88 vol. 30/2021 beam element under finite rotations m(x) n(x) x z xab zab mab x us ws ! x̃ z̃ ã a figure 2. schematic representation of beam internal forces at a general cross section x and left-end forces and moment. w′ s(x) = − sin ϕ(x)+ + xab cos ϕ(x) − zab sin ϕ(x) ea sin ϕ(x) (17) these equations are accurate even for very large rotations. the only limitation is that the strain remains small, which is true if εs ≪ 1 and hκ ≪ 1 where h is the depth of the cross section. in the context of a finite element analysis, the joint displacements are global degrees of freedom that are determined by equilibrium iterations on the structural level. in each iteration, one needs to determine, for each beam, the end forces that correspond to the given joint displacements and rotations. this means that constants xab, zab and mab in (15)–(17) are not known in advance. on the other hand, the displacements at both end sections are prescribed. at the left end, conditions ϕ(0) = ϕa (18) us(0) = ua (19) ws(0) = wa (20) can be considered as initial conditions for first-order differential equations (15)–(17). if the values of xab, zab and mab are somehow estimated, these equations can be integrated, at least numerically, and the values of ϕ(l), us(l) and ws(l) can be determined. the values of forces xab, zab and mab then need to be adjusted such that the resulting kinematic quantities at the right end satisfy the remaining boundary conditions ϕ(l) = ϕb (21) us(l) = ub (22) ws(l) = wb (23) for a given set of end displacements, the initial estimate of the left-end forces can be constructed based on linear beam theory, or on the values at the end of the previous step if the calculation is done in the context of an incremental iterative structural analysis. in fact, the suggested approach is a special version of the shooting method. normally, the shooting method iterates on some of the initial values. actually, the left-end forces would appear in static boundary conditions at the left end, which make it possible to integrate the differential equations of equilibrium and proceed from external forces to internal forces. here, the equilibrium equations were directly presented in their integrated form (13)–(14), and the unknown variables play the role of integration constants instead of boundary values. 2.3. numerical solution the suggested numerical approach is based on the full form of equations (15)–(17). the right-end displacements ub, wb and ϕb can be evaluated if the left-end displacements ua, wa and ϕa and the left-end forces xab, zab and mab are given. to this end, the derivatives in (15)–(17) can be replaced by finite differences and the interval [0, l] is divided into n equally sized numerical segments. mapping of the right-end displacements, rotation, forces and moment based on the left-end displacements and rotation can formally be described by ub = g(fab, ua) (24) where ua = # $ ua wa ϕa % & , ub = # $ ub wb ϕb % & , (25) fab = # $ xab zab mab % & and g is a column matrix of three functions of six variables. for given values of ua and ub, equation (24) represents a set of three nonlinear algebraic equations for unknowns fab. the solution is found by the newton-raphson method, using the recursive formula δf (k) ab = g−1 ' f (k) ab , ua ( ' ub − g ' f (k) ab , ua (( (26) f (k+1) ab = f (k) ab + δf (k) ab , k = 0, 1, 2, . . . (27) where g = ∂g ∂fab (28) is the jacobi matrix of mapping g with respect to variables fab. the entries of the jacobi matrix are evaluated numerically using the linearized version of the computational scheme. once the left-end forces have converged, the right-end forces are easily evaluated from equilibrium conditions written for the whole beam. the foregoing equations were formulated in a local coordinate system aligned with the undeformed 89 e. la malfa ribolla, m. jirásek, m. horák acta polytechnica ctu proceedings m figure 3. pure bending of a cantilever beam subjected to end moment. beam. since the deformed shape of the beam and the distribution of internal forces must be invariant with respect to rigid-body translations and rotations, one can also use a rotated coordinate system x̃ − z̃ with the origin located at the left end of the beam in the deformed configuration (point ã in fig. 2) and with the x̃ axis in the direction of the tangent to the deformed centerline at the left end. with respect to this coordinate system, the left-end displacements are always zero, and so ua can be omitted from the list of arguments of function g. of course, the rightend displacements ub must be computed from the global components of the joint displacements using an appropriate transformation rule, and the end forces obtained by the iterative process (26)–(27) must be transformed back into the global coordinate system. consistent linearization of the relation between the end forces and end displacements leads to the element tangent stiffness matrix. details will be presented in a full journal paper. 3. numerical examples a nonlinear beam element based on the proposed approach has been implemented into oofem [7, 8], an object-oriented finite element code. to verify the implementation and demonstrate the potential of the suggested approach, two examples involving simple structures are shown next. 3.1. pure bending of a cantilever beam the first test is a cantilever of length l = 8 and bending stiffness ei = 1 loaded by a concentrated end moment m on its right-end. the exact solution to this problem is a circular arc with radius r = ei/m . an applied end moment, m = π/4, will force the rod to deform into a full closed circle. in this example, the moment is applied in six load steps, making the rod wind around itself at the end of the sixth step. the deformed shape of the beam at the end of each step is depicted in fig. 3. the solution of the presented figure 4. close-up view of the solution at the end of the sixth step using 6,10,20, and 50 segments. nonlinear model is compared with the one obtained by employing the geometrically exact finite beam element by simo and vu-quoc [6] with a mesh of eight elements. the exact solution is reported as well. the overall agreement is good, and the simulation based on the present model, which uses only one two-noded element, is closer to the analytical solution. the example demonstrates that the present model allows for a dramatic reduction of the number of global degrees of freedom, but of course the number of segments n used for numerical integration of the governing equations (15)–(17) must be chosen high enough to provide a good approximation. the presented results have been obtained using 100 segments. a close-up view of the sixth step circle is showed in fig. 5 for calculations in which 6, 10, 20 and 50 numerical segments were employed. to ease the interpretation of the results we plot straight segments in between the integration points even though the curvature is constant along the beam. it turns out that 20 segments provide an acceptable solution, which is further improved and gets close to the analytical one if 50 segments are used. it is also noted that even with just 10 segments the method provides an accurate estimation of displacements at the integration points. to better quantify the previous comparisons, we calculated the sum of the distances between the the analytical and the computed solutions, normalized with respect to the number of sample points. the results for the sixth load step are reported in table 1. already for 8 integration segments, the error is smaller that 5%. the results indicate that comparable accuracy can be obtained if the present method replaces traditional finite elements by the same number of integration segments located within one single finite element. 3.2. clamped-hinged circular arch subject to point load the second example deals with an arch instability after large asymmetrical deflections. this specific 90 vol. 30/2021 beam element under finite rotations model error [%] simo and vu-quoc [6] 2.98 present approach, 101 segments 0.23 present approach, 50 segments 0.29 present approach, 20 segments 0.70 present approach, 10 segments 2.13 present approach, 8 segments 3.17 present approach, 6 segments 5.39 table 1. sum of the absolute values of the differences between the exact and the computed displacement solutions calculated for the sixth load step. the model of simo and vu-quoc [6] uses an eight-element mesh while the presented beam model uses one element with a variable number of segments of the integration scheme. figure 5. circular arch geometry. problem was investigated by many authors [6, 9] and the exact solution based on the kirchhoff-love theory was given by dadeppo and schmidt [10]. the arch is circular with one boundary clamped and the other boundary hinged, and is loaded by a point load, as shown in fig. 5. the cross section of the beam is a rectangle of depth h = 2.289 and width t = 1.0. the elastic modulus is set to e = 1.0 × 106 and the poisson ratio is zero. geometrical and constitutive parameters are dimensionless meaning that any length or force scale can be used for these values. simo and vu-quoc [6] performed a fe analysis with a forty-element mesh, leading to a buckling load of 9.0528 ei/r2, while wood and zienkiewicz [9] calculated a buckling load of 9.24 ei/r2. the exact value of the buckling load reported by dadeppo and schmidt [10] is 8.97 ei/r2. in our simulations, we used forty elements as well, each with four integration segments, and we obtained a buckling load of 8.9874 ei/r2, which is very close to the analytical solution and exhibits a relative error less than 0.02%. the full comparison with the analytical solution is reported in fig. 6 where we plot the load-displacement and the load-rotation curves relative to the top of the arch (loaded point). again, the analytical solution is satisfactorily reproduced. the deformed configurations for the four load levels numbered in the curves are also shown together with the bending moments indicated by colors. the calculation was completed in 612 load steps using the arc-length method with a 1 2 3 4 1 2 3 4 1 2 3 1 2 3 4 figure 6. load-displacements and load-rotation of the top of the arch (top). grey-dashed line represents the analytical solution reported by dadeppo and schmidt [10]. deformed shapes and moment trend along the arch relative to the four load levels numbered in the top curves. maximum number of 3 iterations per step. it is noted that simo and vu-quoc [6] and wood and zienkiewicz [9] show a longer snap-through behavior together with the associated deformed shapes even though the response is not reasonable because of non-physical penetration of the left support into the structure. this aspect was investigated by simo et al. [11], who showed that a contact constraint condition on the left support needs to be introduced to obtain a more realistic solution. for the aim of 91 e. la malfa ribolla, m. jirásek, m. horák acta polytechnica ctu proceedings this paper we did not consider contact activation and the subsequent stiffening effect in the structure even though this extension could be added. 4. conclusions we present an efficient formulation for a geometrically nonlinear beam model that accounts for arbitrarily large rotations of the beam sections and the effect of curvature on the change of distance between end sections measured along the chord. in the first stage, the model is restricted to the small-strain framework and cross sections are still assumed to remain planar and perpendicular to the deformed beam axis. equilibrium equations in their strong form are combined with the kinematic equations and generalized material equations and then numerically integrated using a special version of the shooting method. two examples show the capabilities of the element to replicate analytical results in a more accurate way with respect to existing beam finite elements that account for geometric nonlinearities. moreover, the h-refinement of finite elements, which increases the number of global unknowns and is thus computationally demanding, can be substituted by employing a proper number of segments in the integration scheme, while keeping the number of elements limited. the element will be extended into a formulation that takes into account an initial curvature and also accounts for a possible contact mechanism activation. acknowledgements the authors acknowledge the support of the czech science foundation (project no. 19-26143x). references [1] g. kirchhoff. ueber das gleichgewicht und die bewegung eines unendlich dünnen elastischen stabes. journal für die reine und angewandte mathematik 1859(56):285–313, 1859. [2] e. h. dill. kirchhoff’s theory of rods. archive for history of exact sciences 44(1):1–23, 1992. [3] e. reissner. on one-dimensional finite-strain beam theory: the plane problem. zeitschrift für angewandte mathematik und physik zamp 23(5):795–804, 1972. [4] e. reissner. on finite deformations of space-curved beams. zeitschrift für angewandte mathematik und physik zamp 32(6):734–744, 1981. [5] j. c. simo. a finite strain beam formulation. the three-dimensional dynamic problem. i. computer methods in applied mechanics and engineering 49(1):55–70, 1985. [6] j. c. simo, l. vu-quoc. a three-dimensional finite-strain rod model. part ii: computational aspects. computer methods in applied mechanics and engineering 58(1):79–116, 1986. [7] b. patzák, z. bittnar. design of object oriented finite element code. advances in engineering software 32(10-11):759–767, 2001. [8] b. patzák. oofem—an object-oriented simulation tool for advanced modeling of materials and structures. acta polytechnica 52(6), 2012. [9] r. d. wood, o. zienkiewicz. geometrically nonlinear finite element analysis of beams, frames, arches and axisymmetric shells. computers & structures 7(6):725–735, 1977. [10] d. dadeppo, r. schmidt. instability of clamped-hinged circular arches subjected to a point load. journal of applied mechanics, transactions asme 42(4):894–896, 1975. [11] j. simo, p. wriggers, k. schweizerhof, r. taylor. finite deformation post-buckling analysis involving inelasticity and contact constraints. international journal for numerical methods in engineering 23(5):779–800, 1986. 92 acta polytechnica doi:10.14311/app.2020.27.0032 acta polytechnica 27(0):32–36, 2020 © czech technical university in prague, 2020 available online at http://ojs.cvut.cz/ojs/index.php/app local mechanical properties of atmospheric sprayed molybdenum coatings deposited with cascaded plasma torch jakub antoša,∗, petra šulcováa, kateřina lencováa, šárka houdkováa, josef duliškoviča, jan hajšmanb, karolína burdováb, antonín račickýb a research and testing institute, tylova 1581/46 , 301 00 plzeň, czech republic b university of west bohemia, univerzitní 8, 306 14 plzeň, czech republic ∗ corresponding author: antos@vzuplzen.cz abstract. increasing interest for industrial use of thermally sprayed coatings leads to development in most thermal spraying technologies, including atmospheric plasma spraying (aps). the latest cascaded torch sinplexpro from oerlikon metco incorporates the efficiency advantages of cascaded arc technology into a single-cathode spray gun. it leads to more stable plasma arc across a wide range of gas flows, mixtures and pressures and also highly increases powder throughput. thermal sprayed molybdenum coatings are widely used for improving wear resistance and sliding properties in many mechanical applications. results of pure molybdenum coatings sprayed with cascaded plasma torch are not yet fully investigated. in this paper, mechanical properties of atmospheric plasma sprayed molybdenum coatings on steel (s235) substrate are evaluated. optimization of spraying parameters for spherical mo powder sprayed by cascaded plasma torch sinplexpro is carried out and the influence on final microstructure, mechanical and tribological properties of molybdenum coatings is analysed. keywords: atmospheric plasma spraying, cascaded plasma torch, mechanical and tribological properties, molybdenum coatings. 1. introduction atmospheric plasma spraying (aps) technology is widely used for spraying of function layers. plasma arc technology offers high temperature and fast and stable flow of plasma jet. high temperature is required for melting and consequential efficient deposition of refractory metals, ceramics and cermets. capability to deposit such materials makes plasma spraying particularly suitable for the production of wear resistant coatings, corrosion protective layers and thermal barriers [1]. molybdenum layers are used in many industry applications for its wear and scuff resistance and to reduce friction between sliding components [2]. whereas the spraying of pure molybdenum and molybdenum alloys by conventional plasma torches has been closely investigated in the past (e.g. ref. [2–4]), molybdenum spraying with cascaded plasma torch is not yet fully examined. this paper concerns spraying of pure spherical industrial grade molybdenum powder amdry 313x by oerlikon metco by cascaded plasma arc torch sinplexpro. correlation between spraying parameters and mechanical and structural properties of molybdenum coating on mild steel substrates are investegated and the optimization of spraying parameters of molybdenum powder for plasma torch sinplexpro is carried out. industrial aps with conventional torches is often struggling with reproducibility and reliability [5]. this is caused due to the instability of arc and subsequent oscilation of the output power level of the conventional torch during spraying process. sinplex pro cascaded torch deals with this issue by stacking copper rings insulated one from each other into a cascade. those rings are called neutrodes. the whole cascade of neutrodes is positioned between cathode and the nozzle, which serves as an anode. electric arc developing in such configuration is more stable and reduces almost all problems of conventional torches [5–7]. 2. setup of experiment in this experiment, argon is used as the primary gas and hydrogen as the secondary gas. argon and hydrogen are mixed to form plasma gas. this mixture runs through the electric arc, dissociating and ionizing into high temperature plasma current. rapid increse of temperature up to 14 000 k [1] is accompanied by substantional volume expansion. after plasma current leaves the torch, spraying material in form of a fine powder is injected into the plasma jet. argon is commonly used as the carrier gas for the powder. plasma jet transfer both thermal and kinetic energy into the particels. melted and accelerated particles are deposited on the substrate material [1]. in the first stage of design of this experiment, it was required to choose variable parametrs among all process parameters of spraying. main process parameters of sinplex pro system are as follows: diameter 32 http://dx.doi.org/10.14311/app.2020.27.0032 http://ojs.cvut.cz/ojs/index.php/app vol. 27/2020 atmospheric sprayed molybdenum coatings mechanical properties of torch nozzle, plasma current, primary gas flow and secondary gas flow. these four parameters take critical role in forming of plasma jet they determine both temperature and plasma current velocity, thus directly affecting melting of the particles and their velocity. secondary process parametrs are spraying distance, relative movement speed of the torch over the sprayed specimen, mass flow of the powder and carrier gas flow. according to ref. [2, 3] concerning the influence of process variables, three critical parameters were determined: velocity of particles (vp), temperature of particles (tp) and temperature of substrate (ts). those three parameters have the most significant effect on the forming of the layer as well as its final properties. the state of particles described by velocity and temperature express its kinetic an thermal energy state. among the setup parameters, the electric current has the most determinating effect on the output power level and temperature of the plasma jet and thus directly affecting the particle temperature and melting. plasma gas flow rate as another main setup parameter has crucial effect on the velocity of the particles, and secondary effect on the plasma jet temperature. substrate temperature as the last critical parameter is a result of combined effects of plasma jet conditions, spraying pattern, torch movement speed, spraying distance, eventual cooling cycles and the dimensions and thermal properties of the substrate istelf. it should be noted that the previous statements about influence of main setup parameters (current, gas flow, spraying distance etc.) on the three critical parameters (vp, tp, ts) are simplified in order to briefely describe the main effect of each setup parametr. in reality all setup parameters take effect in a most complex manner as one dynamic system, simultaneously influencing each others as well as all ciritcal parameters. for this reason it is essential, in order to simplify the experiment, to reduce the number of variable parametrs. in the design stage of experiment it was decided to use 9 mm diameter torch nozzle and constant spraying distance of 125 mm. main reason for choosing the 9 mm nozzle (the largest diametr of commercially availible nozzles) is economical aspect. according to ref. [8], larger nozzle diametr increases spraying power output, improving the spraying process efficiency and leading to faster and thus more economical spraying times. the choosing of constant spraying distance may be considered questionable, because the impact of different spraying distances on the final properties of the sprayed layers could be substantial. as mentioned in ref. [4], increasing the spraying distance generally leads to higher hardness, which possitively correlates with wear ressistance but may not have possitive influence on the fritcion and sliding properties. this hardness growth happens due to the increased level of oxidized particles that is because of longer exposure time of melted particles to the ambient atmosphere. the amount of oxidized particles is again not only funciton of the spraying distance alone, but comes in a complex system with other input parametrs especially powder feed rate, grain size, plasma jet velocity and temperature etc. overall, in order to clarify the basic relations in setup parameters properties, it was decided to maintain constant spraying distance during the experiment [3–5]. another constant parameters was powder feed rate (63 grams per minute) and carrier gas flow rate (6 nlpm). also secondary plasma gas (hydrogen) flow rate was set on constant level for the whole experiment (5 nlpm). after setting most setup parameters to constant levels, electric current and primary gas flow rate was the only two parameters left to vary. table 1 displays the chosen combinations of those two parameters for the experiment. current varried between 350 a and 600 a in 50 a intervals, while primary gas flow rate between three levels 40, 50 and 60 nlpm. not all sets of those parameter were included. to decide which sets should be sprayed, a cpsp (critical plasma spraying parametr [9–11]) parameter was used to determine the thermal state of plasma jet for particular set of conditions. cpsp is defined as the ratio between the torch input power and primary plasma gas flow rate [10]. from all the possible combination of parameters, ten final parameter sets were chosen in order to fill the interval of cpsp between 0.5 and 1.4. all specimens were degreased and grid blasted shortly before the spraying process. spraying took place in a plasma spraying chamber of research and testing institute pilsen. conditions for all spraying were the same, using same batch of the powder, same spraying program and also all equipment remained the same for all spraying sets. temperature monitoring was conducted during spraying process in order to prevent overheating of the substrate and eventual cooling cycles were included. ten passes of the torch were used on all sets. 3. testing methods layer thickness, surface roughness and macro hardness hr15n were measured on all sprayed specimens. layer thickness was measured as a difference between specimen thickness before and after coating deposition process and cooling to the ambient temperature using micrometer screw gauge. the measured thickness was double checked using optical metallography measurement on the scratch pattern. scratch patterns of samples from all sets were inspected and photographed using optical microscopy. all samples also underwent scanning electron microscopy and eds analysis. adhesive strength of coating to the substrate was evaluated according to iso en 14916 on specimens from 3 chosen set of parameters. a major effort was devoted to the detailed measurement of hardness and modulus of molybdenum coatings. rockwell hardness test hr15n was employed 33 j. antoš, p. šulcová, k. lencová et al. acta polytechnica figure 1. microstructure of samples from selected sets (100x). input current primary gas fow rate ar 40 nlpm ar 50 nlpm ar 60 nlpm 350 a cpsp 0.5 n. 1 400 a cpsp 0.6 n. 2 450 a cpsp 1.1 n. 9 cpsp 0.9 n. 7 cpsp 0.7 n. 3 500 a cpsp 0.8 n. 4 550 a cpsp 0.9 n. 5 600 a cpsp 1.4 n. 10 cpsp 1.2 n. 8 cpsp 1 n. 6 table 1. matrix of chosen variable parameters for experiment. cells displays a serial number and an estimated value of cpsp for particular set of spraying parameters on the polished surface of each specimen. tests concerning vickers hardness included hv0.3 for random measurement on scratch pattern and indentation map of hv0.1, which was used to measure hardness as a function of distance from the surface of the coating. nanoindentation with berkovich tip for hardness measurement and for evaluation of modulus was carried out in two layers 150 microns underneath the surface and 150 microns above substrate. rather large indentation force of 5 n was required in order to obtain indents large enough to cover a representative area of coating including all typical coating formations. smaller indents would have measured only properties of splats, oxide layers or unmleted particles alone. hardness tests hr15n and hv0.3 were performed on specimens from all sets of parameters, whereas hv0.1, nanoindentation, indentation fracture toughness test (ift) and ball-on-flat tests were performed on three chosen sets. the three sets that underwent more detailed test were chosen according to cpsp values and microstructure in order to represent three typically obtained structures. ball-on-flat tribological test according to astm g-133 standard was performed on three chosen sets (n.4, n.5 and n.10). dry contact conditions, 25 n normal force, length of track 10 mm, 5 hz cycling frequency and 1000 s test duration was used. stainless steel ball of 62 hrc and 6.3 mm diameter was used against the polished surface of tested samples. three tests were performed on each specimen. the tracks left after this test were measured with a profilometer device to obtain volume loss values and underwent detailed sem screening and eds testing to clarify the tribological processes. 4. results and discussion 4.1. microstructure based on comparison of microstructure images, three sets were chosen for further testing: n.4, n.5 and n.10 (see 3 testing methods). images from optical microscopy of those three sets are shown above (fig. 1). 4.2. hardness and modulus results of hr15n surface measurement showed only small differences between individual sets, mostly in magnitude of standard deviation. hv0.3, hv0.1 as well as nanoindentation were performed on specimens of three chosen sets, with 10 random indents each. fig. 2. shows progression of hv0.1 as a function of distance from the coating surface. n.5 displayed a distinctive compact structure of bottom half of the coating (no pores were visible and splat boundaries were more difficult to recognize 34 vol. 27/2020 atmospheric sprayed molybdenum coatings mechanical properties figure 2. results of hv0.1 indentation as a function of distance from surface. using the optical microscopy). this compact layer is accompanied with an increase in hardness. specimens representing n.4 and n.10 lacks this compact layer. n.10 represents the most porous and coarse microstructre of the three chosen sets, while n.4 is finer, with smaller and more evenly located pores. tab. 2 displays the results of nanoindentation, where values are highlighted by colored columns. it showed that n.5 containing visible bottom compact layer has the highest modulus and hardness in both measured layers (below surface and above substrate). especially significant increase in modulus implies that n.5 has the largest residual compressive stress, especialy in the compact layer [11]. this may happen due to the rather large input power level combined with high flow rate of plasma gas. table 2. resulst of nanoindentation correlated to cpsp parametr 4.3. tribological properties results of ball-on-flat test performed on the three representative types showed two stages of dry friction contact (see fig. 3). in the first stage there was an increasing progress of coefficient of friction (cof). this initial stage lasted about 350 s for n.4 and n.5 and around 280 s for n.10. after this period of time, sudden drop in cof appeared and the progress remained constant and stable for the rest of the test. sem images with eds analyses implied, that the main mechanism for this cof decrease was creation of oxidized films. with the shortest initial period, n.10 also displayed lowest cof value of all tested specimens for both stages 0.64 in the first stage following a drop to 0.62. n.4 and n.5 cof values were 0.68 (dropping to 0.66) and 0.7 (dropping to 0.67) figure 3. comparison of cof progress of ball-onflat tested specimens. note: for illustration only one track for every specimen is displayed respectively. those results correspond to wear coefficient k, which was similar for n.4 and n.5 1.67e-4 and 1.91e-4 mm3/nm respectively. n.10 showing the lowest cof values also displayed significantly lower k, valuing 1.1e-4 mm3/nm it is 34% less to n.4 and 42% less to n.5. this difference in k value may be partly contributed to the difference in the duration of the first cof progression period, where presumably most material removal occurred. 5. conclusions results of experiment showed, that with increasing power input also grows mechanical properties hardness and modulus. higher plasma gas throughput has the similar effect. the increase in modulus can be a result of the increase in residual stress, with higher modulus indicating higher compressive stress [11]. as implied in ref. 12 that higher deposition temperature leads to increase in splat cohesion and thus higher modulus. higher deposition temperature also shifts the stress towards compression. all those aspects are already known and have been described in detail in the past. therefore these knowledges can be successfully applied to design mechanical properties of plasma sprayed molybdenum with cascaded torch as well. however, as the main industrial application for molybdenum coatings is wear resistance and sliding properties, the principal property for designing the coating should be also its final wear resistance and sliding properties. therefor sets of parameters represented by specimen n.10, although not having the highest hardness nor modulus, can be proclaimed as the best tested coating for the required industrial applications. structure of this coating with the best cof and wear resistance was achieved with high power level of the plasma jet of 600 a, and rather low primary argon flow rate of 40 nlpm. as a result of those parameters, an increase in oxidation level can be expected in this coating compared to others, but eds analyse did not indicate any relevant difference. 35 j. antoš, p. šulcová, k. lencová et al. acta polytechnica acknowledgements the paper has originated in the framework of project no. tj02000135. references [1] m. i. boulos, p. fauchais, j. heberlein, et al. thermal plasma technology. vol. 2. international union of pure and applied chemistry, 1999. [2] an integrated study of thermal spray process-structure-property correlations: a case study for plasma sprayed molybdenum coatings. materials science and engineering a, structural materials: properties, microstructure and processing 403, 2005. doi:10.1016/j.msea.2005.04.056. [3] s. sampath, x. jiang, a. kulkarni, et al. development of process maps for plasma spray: case study for molybdenum. materials science and engineering: a 348(1):54 – 66, 2003. doi:10.1016/s0921-5093(02)00642-1. [4] t. stolarski, s. tobe. the effect of spraying distance on wear resistance of molybdenum coatings. wear 249(12):1096 – 1102, 2001. doi:10.1016/s0043-1648(01)00842-0. [5] r. chidambaram seshadri, s. sampath. characteristics of conventional and cascaded arc plasma spray-deposited ceramic under standard and high-throughput conditions. journal of thermal spray technology 28(4):690–705, 2019. doi:10.1007/s11666-019-00841-9. [6] oerlikon metco. plasma spray gun sinplexpro: user manual. 2018. [7] an introduction to thermal spray. bro-0005.6 . oerlikon metco, july 2016. issue 6. [8] s. t. aruna, n. balaji, j. shedthi, v. k. w. grips. effect of critical plasma spray parameters on the microstructure, microhardness and wear and corrosion resistance of plasma sprayed alumina coatings. surface & coatings technology 2012. doi:10.1016/j.surfcoat.2012.08.016. [9] d. zhao, f. luo, w. zhou, d. zhu. effect of critical plasma spray parameter on complex permittivity and microstructure by plasma spraying cr/al2o3 coatings. applied surface science 2013. doi:10.1016/j.apsusc.2012.10.061. [10] s. yugeswaran, v. selvarajan, d. seo, k. ogawa. effect of critical plasma spray parameter on properties of hollow cathode plasma sprayed alumina coatings. surface and coatings technology 203(1):129 – 136, 2008. doi:10.1016/j.surfcoat.2008.08.030. [11] j. matejicek, s. sampath, d. gilmore, r. neiser. in situ measurement of residual stresses and elastic moduli in thermal sprayed coatings: part 2: processing effects on properties of mo coatings. acta materialia 51(3):873 – 885, 2003. doi:10.1016/s1359-6454(02)00477-9. 36 https://doi.org/10.1016/j.msea.2005.04.056 https://doi.org/10.1016/s0921-5093(02)00642-1 https://doi.org/10.1016/s0043-1648(01)00842-0 https://doi.org/10.1007/s11666-019-00841-9 https://doi.org/10.1016/j.surfcoat.2012.08.016 https://doi.org/10.1016/j.apsusc.2012.10.061 https://doi.org/10.1016/j.surfcoat.2008.08.030 https://doi.org/10.1016/s1359-6454(02)00477-9 acta polytechnica 27(0):32–36, 2020 1 introduction 2 setup of experiment 3 testing methods 4 results and discussion 4.1 microstructure 4.2 hardness and modulus 4.3 tribological properties 5 conclusions acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2017.13.0152 acta polytechnica ctu proceedings 13:152–156, 2017 © czech technical university in prague, 2017 available online at http://ojs.cvut.cz/ojs/index.php/app the occurrence of synchronized jumping crowd at sports stadiums martin verner czech technical university in prague, faculty of civil engineering, thákurova 7, 166 29 prague 6, czech republic correspondence: martin.verner@fsv.cvut.cz abstract. since the age of the rome empire, people have been going for fun at the stadiums. today, important reasons for visiting the stadium are sport games or cultural performances. the text is about occurrence of specific types of behaviour during sport games. the experiment was realized at four different sports stadiums and during three kinds of sports. the worst case of loads for the grandstand is the synchronized jumping of a crowd. this is type of behaviour, which can cause over limit vibration, visitors discomfort on the tribune, in the extreme case collapse of the grandstand. in any country today, the standards or references or math models are not for application of dynamic load to grandstand. only the british standards are making recommendation how the designer should work with dynamic loads. keywords: grandstands, synchronized jumping, football, ice-hockey, biathlon. 1. introduction the jumping (especially the synchronized jumping of all rows of fans synchronized) is the main element of fans supporting their lovely team at stadiums. in 2010, the grandstands were shaking at football stadium in the nuremberg, germany. behaviour of spectators caused “huge” vibration of grandstands during match of bundesliga between 1. fc nuremberg and fc eintracht frankfurt. the tribune survived the rising level of vibrations [1]. the main question of the text is: what is the frequency of the occurrence of synchronized jumping crowd at sports stadiums in the czech republic. the research took place in the years 2013–2017. there was monitored the fans behaviour during the different sports matches and the acceleration of the grandstand structure was measured. the experiments were realized at the football stadium – prague letná, ice-hockey stadium – pilsen, football stadium – prague eden and biathlon stadium – new town in moravia. example types of construction figure 3, 4. 2. types of fans behaviour during the experiment, nine types of fans behaviour was recorded: walking and running, jumping, bouncing/bobbing/jouncing, swaying, handclapping, static state, goal scoring and mexican wave. the response to the grandstand vibrations are the higest during jumping. the type of behaviour -jumpingis divided to four subtypes [2]. jumping of the 1st type: the fans are jumping alone independently. the synchronization is realized only by drum. the drum is usually placed between fans named "ultras". figure 1. photo of fans: football stadium – prague letna(upper) [2], football stadium – prague, eden (lower) [3]. 152 http://dx.doi.org/10.14311/app.2017.13.0152 http://ojs.cvut.cz/ojs/index.php/app vol. 13/2017 the occurrence of synchronized jumping crowd figure 2. photo of fans: ice-hockey stadium – pilsen (upper) [4], biathlon stadium – new town in moravia (lower) [5]. jumping of the 2nd and the 3rd types: the fans are grabbing around shoulders and they are jumping all rows (vertically (type 2) or horizontally (type 3)). the synchronization is the fastest of all types of behaviours and vibrations on the grandstand are maximal. jumping of the 4th type: the type is called: sit and jump. spectators are sitting then they jump up when the ultras leader order to do it. the text focuses on the 2. and the 3. types of jumping. these types of fans behaviours caused the maximal vibration of the grandstand. 3. realized experiments 3.1. experiment new town in moravia the fans of behaviour (figure 2) influenced by the instability of the assembled grandstands in the new town in moravia. the jumping was prohibited because the organizers were afraid of high vibrations of the grandstands during this type of behaviour. interdiction of the jumping was respected during the race. 3.2. experiment prague letna the monitoring of fans was realized at football stadium prague letna during the spring part of the czech football league (2014/2015). the stadium figure 3. construction of monitored grandstands: football stadium – prague letna(upper) [2], football stadium – prague eden (lower) [3]. is the home stadium of the team ac sparta praha. the team has one of the faithful and dangerous fans (figure 1) in the czech republic. ac sparta praha reached around second place of football table. the attendance of fans was falling down during the end of the football season. sparta finished at the second place of the football season. in the table 1, there are the described football matches and the occurrence of the synchronized jumping of the 2nd and the 3rd type (figuge 7). 3.3. experiment pilsen the experiment was realized at the ice-hockey stadium pilsen during all the czech ice-hockey league (2015/2016). long-term monitoring was cocerned to spectators´ behavivour during ice-hockey matches as for the football matches. specifically, the monitoring 153 martin verner acta polytechnica ctu proceedings figure 4. construction of monitored grandstands: ice-hockey stadium – pilsen (upper) [5], biathlon stadium – new town in moravia (lower) [4]. was focused on fans (figure 2) supporting team hc skoda plzen. hc skoda plzen reached upper part of the icehockey table. the team finished at the fourth place of the ice-hockey regular season. after regular season, pilsen team played ice-hockey play-off and it finished at the semi-final. the experiment recorded twenty games of the regular season of possible twenty-six and six games of play-off. the synchronized jumping of all rows did not occur when the camera was recording during the experiment. this behaviour is not a part of the ice-hockey fans style to support their team. this is a marginal issue of cheering. in 2016, the winter classic game (outdoor game) was played in brno. the match is a part of ice-hockey season between hc kometa brno and hc škoda pilsen. assembled grandstands (figure 5), similar to the one in new town in moraqvia, where build for this match. as the grandstands had similar construction, jumping was also prohibbited, but this ban was broken. the vibrations of grandstands could be felt, tribunes were very uncomfortable. figure 5. construction of grandstands in brno [6]. 3.4. experiment prague eden the monitoring of fans (figure 1) was realized at the football stadium prague eden again during the spring part of the czech football league (2015/2016). the stadium is the home stadium of the team sk slavie praha. the team is like ac sparta praha, it has one of the faithful and dangerous fans in the czech republic. sk slavie praha reached around the fifth place of the football table. attendance of fans was not falling down with the end football season. in the table 2, there are the described football matches and the occurrence of the synchronized jumping of the 2nd (figure 6) and the 3rd type. 4. conclusion the results of the experiments show the big differences between fans of behaviours at football stadium and at the other stadiums. behaviours of crowds were affected by the instructions from the organizers during the biathlon races. the synchronized jumping was not observed at any ice-hockey game during the experiment in pilsen. only the author of text was a direct witness of the synchronized jumping crowd during the winter classic game in brno. fans realized the synchronized jumping of all rows at least once at every football match during the experiment. difference of behaviour between stadium prague letna and stadium prague eden was the different used types of jumping. in the stadium prague eden, fans were preferring jumping all rows – horizontally, on the other hand the spectators were jumping all rows vertically in the stadium prague letna. the results of the experiment are the fundamentals for the next evolution. it describes the behaviour of the fans, the gained data will be determined further in the other research: how many people do it and how the attendance influences the level of the vibration of grandstands. acknowledgements this paper has been supported by the czech science foundation project no. ga15-15728s and faculty of civil engineering, czech technical university in prague (sgs project no. sgs16/201/ohk1/3t/11). 154 vol. 13/2017 the occurrence of synchronized jumping crowd figure 6. the sample of the grandstand vertical vibration (praha eden) – jumping rows – vertically [3]. figure 7. the sample of the grandstand horizontal vibration (praha letna) – jumping rows – horizontally [3]. date matchweek rival types of jumping jumping of the 2nd jumping of the 3rd march 8, 2015 19. fk dukla praha 2× 2× march 21, 2015 21. fk teplice 1× 1× april 26, 2015 25. fc zbrojovka brno 1× may 15, 2015 28. 1. fc slovacko 2× 1× may 30, 2015 30. fc bohemians praha 1× table 1. table of occurrence of synchronized jumping crowd during the match – ac sparta praha. date matchweek rival types of jumping jumping of the 2nd jumping of the 3rd february 28, 2016 19. fc banik ostrava 1× 1× march 11, 2016 21. fc fastav zlin 1× april 23, 2016 26. fk teplice 1× may 8, 2016 28. 1. fc slovacko 1× table 2. table of occurrence of synchronized jumping crowd during the match – sk slavie praha. 155 martin verner acta polytechnica ctu proceedings references [1] m. verner. real crowd behavior on the grandstands. 4th conference nano & macro mechanics 4(6):215–220, 2013. [2] m. verner, t. plachy, m. polák. an experiment focused on fans behaviour and induced grandstand vibrations during a football match. trans tech publications offices (4):75–78, 2016. [3] m. polák, t. plachy, m. verner. a comparison of spectators induced grandstand vibrations and fans behavior during two ice hockey games between the same teams in the regular season and in the playoffs. 3rd international conference on structural and physical aspects of construction engineering 3(4), 2016. [4] j. hubka. dynamic analysis of grandstand. master’s thesis. 2017. [5] m. verner. behaviour of fans during the football match. trans tech publications 102(4):77–80, 2016. [6] peri. winter classic lužánky, brno. https://www. peri.cz/projekty/kulturni-a-sportovni-akce/ winter-classic-luzanky.htmll. 156 https://www.peri.cz/projekty/kulturni-a-sportovni-akce/winter-classic-luzanky.htmll https://www.peri.cz/projekty/kulturni-a-sportovni-akce/winter-classic-luzanky.htmll https://www.peri.cz/projekty/kulturni-a-sportovni-akce/winter-classic-luzanky.htmll acta polytechnica ctu proceedings 13:153–157, 2017 1 introduction 2 types of fans behaviour 3 realized experiments 3.1 experiment new town in moravia 3.2 experiment prague letna 3.3 experiment pilsen 3.4 experiment prague eden 4 conclusion acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2020.29.0030 acta polytechnica ctu proceedings 29:30–34, 2020 © czech technical university in prague, 2020 available online at http://ojs.cvut.cz/ojs/index.php/app recent advances in simulating landslide and debris flow wei wu universität für bodenkultur wien, institut für geotechnik, department für bautechnik und naturgefahren, feistmantelstraße 4 , 1180 wien, austria correspondence: wei.wu@boku.ac.at abstract. this paper summarizes the outcome of a research project funded by the european commission on the simulation of landslide and debris flow. the numerical techniques adopted include the mesh-based method (fem), the point-based method (sph) as well as a hybrid method (lattice-boltzmann method and dem). centrifuge model tests were carried out for the purpose of validation. keywords: debris flow, dem, fem, landslide. 1. introduction landslides are caused by slope failures. the stability of slopes is a classical topic in soil mechanics. debris flows are often triggered by landslides either partially or completely mobilizing into debris flows. globally, landslides cause billions of dollars in damage and thousands of deaths and injuries each year. methods based on limit equilibriums have been most popular for slope stability analysis, as in the slice method of bishop. however, the limit equilibrium approach cannot capture the initiation of hydrologically driven slope failure, which is characterized mainly by transient processes. the modelling of infiltration, evaporation, propagation of the wetting front and reduction of matric suction involves different time scales governing both short-term and long-term behaviour. moreover, the initiation and propagation of the failure surface cannot be explained by the limit equilibrium approach [1]. a consistent analysis of slope stability requires a fully coupled analysis of the hydrological and mechanical processes in multiphase media, and which considers spatial and temporal multiscale properties. whereas slope stability has been modelled mainly in the realm of soil mechanics, the modelling of debris flows has been developed mainly within the terms of fluid mechanics, in particular the shallow water model [2]. there are two main distinct approaches to computational modelling of debris flows: the continuum and discrete methods. in most continuum models, the flow is considered as a one-phase medium with constant density. much effort has been devoted to the debris viscosity. several debris flow runout models have been developed. contrasting the one-phase approach with constant viscosity are field observations demonstrating that such an approach is unable to describe unsteady (spatial and temporal) flow behaviour properly. however, some important phenomena, such as segregation of coarse particles and the transition between solid-like to fluid-like behaviour are still elusive to such models. consistent continuum modelling of debris flows requires an advanced constitutive model capable of describing the local inhomogeneity and solid-like/fluid-like behaviour and a robust numerical scheme, e.g. sph (smooth particle hydrodynamics), mpm (material particle method) and pfem (particle finite element method). the project mumolade (multiscale modelling of landslides and debris flows) was funded within the framework of the marie curie itn (initial training networks) by the european commission in order to address needs in research and training in the numerical simulation of landslides and debris flows. the funding duration was three years between 2012 and 2015 and involved a budget of about 4.3 xmillion euros. this paper gives a brief account of the results achieved in mumolade. 2. strain localization and triggering of slope failure by fem soil often fails through localized deformation in the form of shear bands, which give rise to progressive failure in slopes with over-consolidated soil. the precursor of failure is the localisation of deformation within thin shear bands. the bifurcation theory and the related numerical schemes provide a rational approach to this multiscale phenomenon. the multiphase fem analysis provides a realistic description of the failure process from initiation and aftermath both under drained and partially undrained conditions. once a shear band is formed, the underlying partial differential equation (pde) becomes rather poorly expressed and requires regularization. the elliptical pdf for quasi-static loading will change to parabolic. this change brings about some difficulties for the numerical calculations using the finite element method (fem). the numerical solutions show a spurious dependence on the mesh alignment and mesh size. more30 http://dx.doi.org/10.14311/app.2020.29.0030 http://ojs.cvut.cz/ojs/index.php/app vol. 29/2020 recent advances in simulating landslide and debris flow figure 1. equivalent viscoplastic strain profiles (a) and contours for (b) coarse mesh (5 × 17), (c) intermediate mesh (10 × 34) and (d) fine mesh (20 × 68) in the case viscosity η = 30 s. [3] over, the thickness of the shear bands and the spacing between the shear bands require a theory with a characteristic length. it should be noted that conventional continuum mechanics does not possess a length scale. there are several techniques to regularize the pdf, e.g. nonlocal theory, micropolar theory, strain gradient theory and viscosity. in most of these theories a characteristic length is introduced. the team led by prof. sanavia at padova, italy studied the regularization by nonlocal theory. the nonlocal theory assumes that the response of the material at some specific point is determined not only by the state at that point but also by the state of its neighbouring points. the variables are then substituted with their non-local counterparts obtained by weighted averaging over a spatial neighbourhood of each point under consideration. obviously, the size of the neighbourhood introduces a characteristic length scale. the following examples shows a plane strain compression test on water saturated sand. an elasticviscoplastic constitutive model is used. the finite element calculations are carried out using three meshes with an 85, 340 and 1340 element respectively. “the finite element mesh consists of eight-node quadrilateral isoparametric elements with a reduced gaussian (2 × 2) integration scheme to eliminate shear locking. the bottom of the sample was assumed to be fixed and rough whereas the boundaries are impervious and adiabatic. quasi-static loading conditions are assumed, and gravity acceleration is taken into account. axial compression is applied to the specimen by imposing vertical velocity (1.2mm/s) on the top [3]. as can be seen from fig. 1, the thickness of the shear bands remains fairly independent of the mesh size. this gives us some confidence when using the fe code that the problem of slope stability, where shear band propagation and progressive failure are concerned, can indeed be solved. 3. stability of vegetated slope by fem most shallow landslides are triggered by rainfall. vegetation is known to play an important role in reducing landslide hazard. we carried out fe simulations to assess the impact of rainfall and vegetation on slope stability. the vegetated slope is considered as a combined mechanical–hydrological system. a sophisticated elastoplastic constitutive model for partially saturated soil was used to describe the mechanical behaviour of the soil. the reinforcement by plant roots is regarded as a hardening mechanism in the constitutive model. the hydrological effect of plant by root uptake is considered in the underlying richards equation. the architecture of plant roots is also taken into consideration. fig. 2 shows the displacements of a slope with and without vegetation after a rainfall of up to 120 hours with an intensity of 18mm/h. the slope without vegetation loses stability before the vegetated slope by about 10 hours. the slope has an inclination of 45◦ and a height of 10metres. the root architecture is assumed to be half spherical with a diameter of 1 m. the numerical model has some advantages over the traditional slice method based on limit state analysis: “the model is capable of capturing and coupling the main features that should be considered when considering the stability of vegetated slopes. the mechanical root reinforcement is considered based on a macroscopic approach, which is a significant advantage when applying it to large-scale problems. the root architecture can be approximated with geometrical representations. the soil parameters within the root zones are enhanced. however, the ‘constant additional cohesion’ assumption is avoided, one which is widely used in many existing models. the soil strength is enhanced by the plant roots, whose strength is not constant, but hardens and softens depending on strain. importantly, the numerical model can simulate the 31 wei wu acta polytechnica ctu proceedings figure 2. contours of displacements, obtained at the end of calculations, for the two extreme cases: (a) non-vegetated slope; (b) vegetated slope with both mechanical root reinforcement and evapotranspiration considered. different shapes of failure surfaces can be observed. note that the circles are a projection of root zones (values of displacements are given in metres) [4] root water uptake of plant roots, which leads to the desaturation of the soil mass and gives rise to further enhancement of soil strength [4]. 4. discrete element simulation following the work by cundall, the discrete element method (dem) has become quite commonly used for granular materials. debris flows involve a large number of granules. therefore, a working programme led by prof. nicot at inrae, grenoble, france is devoted to the numerical simulation of flexible barriers against debris flow by dem. check dams are often used as protection structures against debris flow. recently, flexible barriers (steel meshes) are widely used as protection structures against debris flow in mountainous regions. such flexible structures absorb the impact energy better than the rigid check dams. the complex interaction between debris flow and flexible structures presents some challenge to the continuum simulation. the dem offers an attractive alternative to the continuum approach. moreover, the dem offers possible links between the microscopic properties of granular materials and their macroscopic behaviour. the grains of debris flow are usually modelled by spheres. the normal and tangential forces of the particles are obtained from interactions with the help of the normal and tangential aspects of stiffness. the tangential force can be limited by some failure criteria, e.g. mohr-coulomb. usually the model parameters are calibrated by considering some simple and welldefined problems. fig. 3 shows a snapshot of the interaction between the debris flow and the flexible barrier. the latter is modelled by net elements. the calculations provide also the forces in the flexible barrier and in the anchor elements. the numerical calculations are carried out with the dem code yade developed by colleagues in grenoble. figure 3. snapshots of the granular flow impact on the flexible barrier [5] figure 4. set-up of numerical simulation (a) geometry of the configuration before the simulation starts. (b) undeformed shape of the barrier. the snapshots on the right show the geometry of the flow before (c), during (d), and after (e) the impact on the barrier [6] 5. a hybrid method combining lbm with dem although dem may provide some insight into the complex interaction between debris flow and the flexible barrier, large scale problems require continuum modelling. the group led by prof. herrmann at eth, switzerland works on the numerical simulation with the hybrid method by combining lbm (lattice boltzmann method) and dem. the lbm is well established in computational fluid dynamics (cfm). the fluid (water together with fines) is simulated by lbm while the large particles are modelled by dem. the fluid is 32 vol. 29/2020 recent advances in simulating landslide and debris flow considered as a viscous fluid with yield limit by the bingham model. the interaction between the large particles and the surrounding fluid is considered by an interaction term. the flexible net is modelled by finite elements. this hybrid method benefits from the strength of both methods and offers an optimal way to model the complex interaction between debris flow and a flexible barrier. fig. 4 shows the set-up of the numerical simulation. a fluid saturated granular mass flows down a chute with an inclination of 15◦. the seepage of fluid and the holding back of large particles upon impact with the flexible net can be clearly observed in fig. 3 (e). figure 5. development of failure surface depicted by total equivalent strain with reduction factor f = 1 : 9 [7] 6. smooth particle hydrodynamics for problems with large deformation the fem often suffers from accuracy and convergence problems as a result of skewed mesh. in recent years, various meshless methods have been proposed to remedy this flaw. among these, the first and most widely used meshless method is the sph. originally proposed to handle hydrodynamic problems in astrophysics, sph has become very common in fluid dynamics and more recently in solid mechanics. sph is a pure lagrangian method, which makes use of fictive particles with assigned properties like mass, density, velocity, stress and strain. the approximation of the physical properties of the particles is based on an integration interpolation over a support domain. the size of this domain defines the interaction among the particles within its neighbourhood. through an updated lagrangian scheme, sph can handle large deformation and complex material interface with some ease. like fem, a constitutive model for the granular material is still required. until now, slope failure and debris flow are usually treated in different domains with different methods. sph offers a promising approach to handle both problems within one single consistent framework and with one numerical method. fig. 5. shows the development of the failure surface in a slope over the time. the slope is 5m high and 10m wide. a hypoplastic constitutive model was used for the soil. some 9000 particles were used for the simulation. the progressive failure process is well captured by the sph. in the sph calculations, the thickness of shear band is mainly dictated by the particle spacing and size of the support domain. the thickness of the shear band is known to depend on the mean grain diameter of soil. this needs to be improved. 6.1. centrifuge tests on a scaled slope model there are only a few cases of landslides with well documented data and instrumentation in the real world. sophisticated numerical models require more data for calibration and validation. geotechnical centrifuge offers a cost-effective alternative to study slope stability under well-defined initial and boundary conditions on a scaled model. in the scaled model, the size of the prototype slope is reduced by a factor n to maintain the geometrical similarity. the mechanical similarity is maintained by using n-fold of the earth acceleration. centrifuge tests on a scaled slope model are carried out to study the stability of soil slopes under rainfall. the setup and the technical specifications of the geotechnical centrifuge at boku can be seen from fig. 6 and table 1. a fine sand with d50 = 0.232 mm with fines was used. the grading was as follows: 83% sand, 13.4% silt and 2.8% clay. we carried out triaxial tests on unsaturated soil. the friction angle is about 35◦ but depends on the initial density and the confining pressure. the cohesion depends mainly on the suction i.e. water content. the slope height is 20 cm and the slope inclination is 45◦. the slope was prepared by moist tamping. the displacement was measured through a glass window and evaluated by piv. rainfall was simulated by purpose designed nozzles. fig. 7 shows the shear strain from piv measurements. the acceleration is 30g, which corresponds to a prototype slope of about 6 m. the test was carried out with initial dry density 1.4 g/cm3, the initial water content 11.5 %, and the rainfall intensity 11.8 mm/h. some 20 secs after rainfall is initiated, a shear band can be observed. this shear band propagates and becomes highly localized at t = 30 s. by varying the parameters, we can establish a relationship between rainfall intensity and duration. this compares well with the available data in the literature. our dataset can be used for validation of the numerical simulations. 7. conclusion landslide and debris flow remain an active research area. the research is highly rewarding for actual practice but also highly challenging. better understanding of soil behaviour and better numerical techniques are needed. in order to model slope failure and debris flow within one consistent framework, we need a constitutive model for solid-like and fluid-like behaviour. 33 wei wu acta polytechnica ctu proceedings figure 6. geotechnical centrifuge at boku diameter of centrifuge [mm] 3, 000 radius to base of swinging platform, rmax [mm] 1, 308.5 radial acceleration [g] 0 to 200 angular velocity [rpm] 0 to 400 deviation in angular velocity [%] ± 0.1 max. load capacity [g− kg] 9, 070 max. payload [kg] 90.7 max. model dimensions w × d × h [mm] 538 × 556 × 540 table 1. technical specifications of the centrifuge at boku 6 500 1000 1500 2000 200 400 600 800 1000 1200 1400 1600 y´ [px] z´ [p x] 0 5 10 15 20 25 30 35 40 45 (a) t= 10 s 500 1000 1500 2000 200 400 600 800 1000 1200 1400 1600 y´ [px] z´ [p x] 0 5 10 15 20 25 30 35 40 45 (b) t= 20 s 500 1000 1500 2000 200 400 600 800 1000 1200 1400 1600 y´ [px] z´ [p x] 0 5 10 15 20 25 30 35 40 45 (c) t= 30 s 500 1000 1500 2000 200 400 600 800 1000 1200 1400 1600 y´ [px] z´ [p x] 0 5 10 15 20 25 30 35 40 45 (d) t= 40 s figure 6.15: development of shear strain of test crf-03. the solid black lines represent the shape of the slope prior rainfall initiation. drop as the wetting front descends. in addition, the cohesive and frictional components of the soil strength increase with depth. in comparison to the slope failure observed upon increasing self-weight (chapter 5), the volume of the initial slide of the type a failure mode is smaller due to the reduced depth of the slip surface. the evolution of the shear zone is visualised in figure 6.15. the highest shear strain in evident at the mid-height of the slope. similar observations are made on self-weight induced slope failures. in the figures on cumulative soil displacements (fig. 6.14) and shear strain (fig. 6.15) the mobilised soil mass seems to follow a straight plane inclined at about 33 degree, arising in the figures of 20 s rainfall duration. yet, under continuous rainfall the shear zone develops under a steeper angle of about 55 degree in the upper half of the slope towards the crest region. based on the presented pre-failure deformation with maximum values in the top region, the rain-induced slope failure 167 figure 7. snap shots of shear strain for the rainfall duration of t = 10, 20, 30, 40 s [8] soil mechanics handles the former, fluid dynamics the latter. a model unifying both regimes remains to be developed. acknowledgements i am grateful for funding from the european commission within its people programme (marie curie actions) of the seventh framework programme fp7/2007-2013/ under research executive agency grant agreement no. 289911 with the title: multiscale modelling of landslides and debris flows (mumolade). references [1] h. b. mühlhaus, i. vardoulakis. the thickness of shear bands in granular materials. géotechnique 37(3):271–283, 1987. doi:10.1680/geot.1987.37.3.271. [2] s. b. savage, k. hutter. the motion of a finite mass of granular material down a rough incline. journal of fluid mechanics 199:177–215, 1989. doi:10.1017/s0022112089000340. [3] lazari, m. (2015) strain localization analysis of multiphase geomaterials. phd thesis, university of padova, italy. [4] switała, b. m. (2016). analysis of slope stabilisation with soil bioengineering methods. phd thesis, university of natural resources and life sciences, vienna, austria. [5] albaba, a. (2015) discrete element modeling of the impact of granular debris flows on rigid and flexible structures. phd thesis, universite grenoble alpes, france. [6] leonardi, a. (2015) debris flow simulations with discrete elements and lattice-boltzmann, phd thesis, eth, switzerland. [7] c. peng, w. wu, h.-s. yu, c. wang. a sph approach for large deformation analysis with hypoplastic constitutive model. acta geotechnica 10, 2015. doi:10.1007/s11440-015-0399-3. [8] idinger, g. (2016) experimental study on failure initiation in partially saturated slopes. phd thesis, university of natural resources and life sciences, vienna, austria. 34 https://doi.org/10.1680/geot.1987.37.3.271 https://doi.org/10.1017/s0022112089000340 https://doi.org/10.1007/s11440-015-0399-3 acta polytechnica ctu proceedings 29:30–34, 2020 1 introduction 2 strain localization and triggering of slope failure by fem 3 stability of vegetated slope by fem 4 discrete element simulation 5 a hybrid method combining lbm with dem 6 smooth particle hydrodynamics 6.1 centrifuge tests on a scaled slope model 7 conclusion acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2017.13.0157 acta polytechnica ctu proceedings 13:157–161, 2017 © czech technical university in prague, 2017 available online at http://ojs.cvut.cz/ojs/index.php/app methodology for measuring the shear stress of plasterboards ondřej zobala,b,∗, robert járaa,b, martin hataja,b a czech technical university in prague, university centre for energy efficient buildings, třinecká 1024, 273 43 buštěhrad, czech republic b czech technical university in prague, faculty of civil engineering, thákurova 7, 166 29 prague 6, czech republic ∗ corresponding author: ondrej.zobal@fsv.cvut.cz abstract. plasterboards have to meet many requirements for certification. the mechanical properties of itself plasterboard are one of them. the important mechanical property of the plasterboard is the shear strength perpendicular to the plane of the board. the european standard refers to performing the relevant test for the procedure set out in the wood standard en 789. but is this methodology for plasterboard the most suitable? this paper compares this recommended method with the alternative method according to the american standard astm d5379 with indicating the results of a specific measurement. keywords: plasterboard, shear strength perpendicular to the plane of the board, en 789, astm d5379. 1. introduction the plasterboard is pressed gypsum mass between two paper cartons for dry construction. this board can be used for reconstructions and new buildings for the construction of partitions, floors and ceilings. the plasterboard can protect against noise, moisture and fire [1]. marking, definition of properties and test methods for gypsum boards are prescribed standard en 520 + a1 from the 2010 which, moreover, unifies requirements and designations throughout europe. essential requirements include the declared tensile strength in bending and sag, shear strength, compressive strength, hardness and minimum required values of modulus of elasticity for individual stresses [1–5]. all required mechanical properties for certification are given in tab. 1. standard en 520 + a1 defines 8 types of plasterboard: (1.) type a – gypsum plasterboard, (2.) type h – plasterboard with reduced water absorption rate, (3.) type e – gypsum sheathing board, (4.) type f – gypsum plasterboard with improved core adhesion at high temperature, (5.) type p – plasterboard, (6.) type d – gypsum plasterboard with controlled density, (7.) type r – gypsum plasterboard with enhanced strength, (8.) type i – gypsum plasterboard with enhanced surface hardness. this paper addresses the determination of shear strength perpendicular to the plane of the board. european standards din 18180 from 2014 and ead 07001-01-0504 from 2016 refer to the shear strength test perpendicular to the plane of the board to en 789, which is, however, focused to wood-based boards [6– 8]. it has been found that the shear strength test perpendicular to the plane of the board according to en 789 is performed very rare in test institutes in the czech republic (two places) and only one test facility has been discovered on the university campus. for these two reasons, we decided to try a different and more accessible way of testing. the us standard astm d5379 for testing the shear properties of composite materials was finally chosen as an alternative shear strength test perpendicular to the plane of the board [9]. the paper compares results and measurement procedures these two standards. 2. methodology of measurement 2.1. standard en 789 the test specimen had the dimensions of 450×900×thickness of board [mm] and each board was cut one in the longitudinal direction and one in the transverse direction. we needed wooden or steel guide parts for fixing the test specimen. the guide parts had to meet the criteria (minimum dimensions, same humidity as the test specimen, minimum strength and rigidity). the connection of the guide parts and test specimen had to be done with a suitable adhesive. the static scheme is shown in fig. 1 and fig. 2 shows a load arrangement for a shear strength test perpendicular to the plane of the board. 157 http://dx.doi.org/10.14311/app.2017.13.0157 http://ojs.cvut.cz/ojs/index.php/app o. zobal, r. jára, m. hataj acta polytechnica ctu proceedings mechanical properties designation dir. [°] nr. of tests spec. width [mm] spec. length [mm] bending strength perpendicular to the plane of the board – in the longitudinal direction fm,0,k 0 3 400 300 bending strength perpendicular to the plane of the board – in the transverse direction fm,90,k 90 3 400 300 modulus of elasticity in bending perpendicular to the plane of the board – longitudinally em,0,mean 0 modulus of elasticity in bending perpendicular to the plane of the board – transversely em,90,mean 90 shear strength in the plane of the board – longitudinally fv,0,k 0 3 500 600 shear strength in the plane of the board – transversely fv,90,k 90 3 500 600 modulus of elasticity in shear in the plane of the board – longitudinally gv,0,mean 0 modulus of elasticity in shear in the plane of the board – transversely gv,90,mean 90 compressive strength perpendicular to the plane of the board – longitudinally fc,0,k 0 3 400 400 compressive strength perpendicular to the plane of the board – transversely fc,90,k 90 3 400 400 bending strength in the plane of the board – in the longitudinal direction fm,0,k 0 3 400 300 bending strength in the plane of the board – in the transverse direction fm,90,k 90 3 400 300 modulus of elasticity in bending in the plane of the board – longitudinally em,0,mean 0 6 6 modulus of elasticity in bending in the plane of the board – transversely em,90,mean 90 compressive strength in the plane of the board – longitudinally fc,0,k 0 3 400 400 compressive strength in the plane of the board – transversely fc,90,k 90 3 400 400 modulus of elasticity in compressive in the plane of the board – longitudinally ec,0,mean 0 modulus of elasticity in compressive in the plane of the board – transversely ec,90,mean 90 tensile strength in the plane of the board – longitudinally ft,0,k 0 3 50 350 tensile strength in the plane of the board – transversely ft,90,k 90 3 50 350 modulus of elasticity in tensile in the plane of the board – longitudinally et,0,mean 0 modulus of elasticity in tensile in the plane of the board – transversely et,90,mean 90 shear strength core + carton fshear,max 6 600 170 determination of surface hardness of the board 6 300 400 table 1. comparison of test of the shear strength perpendicular to the plane of the board according to the standards en 789 and astm d5379. 158 vol. 13/2017 methodology for measuring the shear stress of plasterboards figure 1. static scheme of the test of shear strength perpendicular to the plane of the board [6]. figure 2. the load arrangement for a shear strength test perpendicular to the plane of the board according standard en 789 (dimensions are in mm) [8]. all test specimens were loaded with incremental strain at a constant rate of 2 mm/min until failure. the shear strength perpendicular to the plane of the board was given by the following equation 1: fv = fmax l.t , (1) where fv is shear strength perpendicular to the plane of the plate [mpa], fmax is the maximum load at failure [n], l is the length of the test specimen measured along the centerline of the shear area [mm], t is the average thickness of the test specimen measured at two points along centerline the shear area [mm]. 2.2. standard astm d5379 the test specimen had the dimensions of 19.1 × 76.2× thickness of board [mm] and it was necessary to cut the specimens in the longitudinal and transverse directions from each board. no guide parts were needed to fasten the test specimen. loading scheme was the same as for the standard en 789 (fig. 1). the loading arrangement with the shape and size of the test specimen for the shear strength test perpendicular to the plane of the board according to astm d5379 is shown in fig. 3. the strength evaluation was performed according to eq. 1. figure 3. the load arrangement for a shear strength test perpendicular to the plane of the board with test specimen according standard astm d5379 (dimensions are in mm) [10]. 3. results of measurement the number of test specimens according en 789 was 6 pieces (3 different boards, 1× longitudinal and 1× transverse direction from each board). the number of test specimens according astm d5379 was 30 pieces (3 different boards, 5× longitudinal and 5× transverse direction from each board). fig. 4 shows a shear strength test perpendicular to the plane of the board according to astm d5379. the characteristic value of shear strength according to en 789 was in the longitudinal and transverse direction same 2.8 mpa. the shear strength according to astm d5379 had characteristic value in the longitudinal direction of 3.1 mpa and in the transverse direction of 2.7 mpa. the differences of values of shear strength with the standard deviation results under 10% were minimal. 159 o. zobal, r. jára, m. hataj acta polytechnica ctu proceedings standard size of the test specimen [mm] difference of characteristic values [% ] note en 789 600×900×thickness fv,0,k = 10.7 problem with fastening test specimen astm d5379 19.1×76.2×thickness fv,90,k = 3.5 table 2. comparison of test of the shear strength perpendicular to the plane of the board according to the standards en 789 and astm d5379. figure 4. test of shear strength perpendicular to the plate plane according to astm d5379. figure 5. characteristic values of shear strength perpendicular to the plane of the plate according to the standards en 789 and astm d5379. 4. conclusions as is shown in tab. 2, values of shear strength measurements according to en 789 and astm d5379 are very similar. the en 789 standard is determined primarily for wood-based materials. the test specimens according to en 789 are very large and there is a problem with their attachment and ensuring stability during the test. a numerical model will be created to comparing the results according to en 789 and astm d5379 on the basis of the obtained test parameters in the next phase of our work. acknowledgements this work has been supported by the faculty of civil engineering, czech technical university in prague (sgs project no. sgs16/201/ohk1/3t/11) and by the ministry of education, youth and sports within national sustainability programme i, project no. lo1605. references [1] gypsum unique properties in buildings. http: //www.eurogypsum.org/about-gypsum/properties/. [2] plasterboards according to european standards. http://imaterialy.dumabyt.cz/rubriky/ drevene-a-montovane-konstrukce/ sadrokartonove-desky-podle-evropskych-norem_ 101335.html. [3] cni. gypsum plasterboards – definitions, requirements and test methods. first printing. prague, 2010. [4] d. supik. analysis of selected mechanical properties of construction materials for wood building. thesis statement. mendel university in brno, 2009. [5] c. petrone, g. magliulo, g. manfredi. mechanical properties of plasterboards: experimental tests and statistical analysis. journal of materials in civil engineering 28(11):04016129, 2016. [6] din. gypsum plasterboards – definitions, requirements and test methods. first printing. berlin, 2014. [7] ead. gypsum plasterboard for load-bearing applications. first printing. eota, 2016. [8] cni. timber structures – test methods – determination of mechanical properties of wood based panels. first printing. prague, 2005. [9] d. astm. 5379:“standard test method for shear properties of composite materials by the v-notched beam method”. annual book of astm standards 100:235–47, 2012. 160 http://www.eurogypsum.org/about-gypsum/properties/ http://www.eurogypsum.org/about-gypsum/properties/ http://imaterialy.dumabyt.cz/rubriky/drevene-a-montovane-konstrukce/sadrokartonove-desky-podle-evropskych-norem_101335.html http://imaterialy.dumabyt.cz/rubriky/drevene-a-montovane-konstrukce/sadrokartonove-desky-podle-evropskych-norem_101335.html http://imaterialy.dumabyt.cz/rubriky/drevene-a-montovane-konstrukce/sadrokartonove-desky-podle-evropskych-norem_101335.html http://imaterialy.dumabyt.cz/rubriky/drevene-a-montovane-konstrukce/sadrokartonove-desky-podle-evropskych-norem_101335.html vol. 13/2017 methodology for measuring the shear stress of plasterboards [10] shear test fixture astm d5379. http://www.grip.de/pdf/ths600_datasheet.pdf. 161 http://www.grip.de/pdf/ths600_datasheet.pdf acta polytechnica ctu proceedings 13:157–160, 2017 1 introduction 2 methodology of measurement 2.1 standard en 789 2.2 standard astm d5379 3 results of measurement 4 conclusions acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2020.26.0024 acta polytechnica ctu proceedings 26:24–29, 2020 © czech technical university in prague, 2020 available online at https://ojs.cvut.cz/ojs/index.php/app on evaluation of the three-dimensional isogeometric beam element edita dvořáková∗, bořek patzák czech technical university in prague, faculty of civil engineering, department of mechanics, thákurova 7, 166 29 prague 6, czech republic ∗ corresponding author: edita.dvorakova@fsv.cvut.cz abstract. the exact description of the arbitrarily curved geometries, including conic sections, is an undeniable advantage of isogeometric analysis (iga) over standard finite element method (fem). with b-spline/nurbs approximation functions used for both geometry and unknown approximations, iga is able to exactly describe beams of various shapes and thus eliminate the geometry approximation errors. moreover, naturally higher continuity than standard c0 can be provided along the entire computational domain. this paper evaluates the performance of the nonlinear spatial bernoulli beam adapted from formulation of bauer et al. [1]. the element formulation is presented and the comparison with standard fem straight beam element and fully three-dimensional analysis is provided. although the element is capable of geometrically nonlinear analysis, only geometrically linear cases are evaluated for the purposes of this study. keywords: bernoulli theory, curved beam element, isogeometric analysis, nurbs. 1. introduction the desire for the automatic connection between computer-aided design (cad) and finite element analysis (fea) has been the crucial impulse for the development of isogeometric analysis [2]. the idea of iga is to use the basis functions used for the geometry description in cad also as the approximation functions for the analysis. this results in the possibility of only one model shared between the design and analysis. isogeometric approach introduces into the analysis some other very important features. one of them is a possibility to exactly model arbitrarily curved geometries, including a conic sections which can be only approximated by standard polynomial basis functions. this makes it very convenient for the use in the analysis of curved beams. the focus of this paper is placed on the geometrically nonlinear three-dimensional bernoulli beam [1, 3]. the beam formulation is briefly introduced and the performance over standard fem approaches is evaluated. 2. nurbs-based analysis the most wide-spread technology in cad industry are nurbs (non-uniform rational b-splines). the nurbs curve is obtained by linear combination of cartesian coordinates of the control points p and nurbs functions rpi c(ξ) = n∑ i=1 rpi (ξ)pi. (1) each nurbs function is generated by weighting bspline functions np i with a given weight wi associated with control point pi rpi (ξ) = ni,p(ξ)wi∑n j=1nj,p(ξ)wj . (2) b-spline functions form a special subset of nurbs functions (corresponding to constant wi) and are derived recursively starting with a piecewise constant functions ni,0(ξ) = { 1 if ξi ≤ ξ < ξi+1 0 otherwise , (3) ni,p(ξ) = ξ − ξi ξi+p − ξi ni,p−1(ξ) + ξi+p+1 − ξ ξi+p+1 − ξi+1 ni+1,p−1(ξ), (4) where p is the degree of the b-spline function, ξi is the coordinate of the ith-knot and ξ ∈ 〈0, 1〉 is a parametric coordinate. the computational domain in isogeometric analysis (iga) is firstly divided into patches, which are further divided into knotspans (often referred to as elements in iga). understanding of knotspans is similar to elements in standard fem, nevertheless higher continuity up to cp−1 between knotspans can be achieved naturally using nurbs, unlike c0 in standard fem. moreover, the nurbs basis functions are generally non-interpolatory. in the knots (points which are dividing patch into knotspans), the continuity can be locally decreased up to c0 by knot multiplication, 24 https://doi.org/10.14311/app.2020.26.0024 https://ojs.cvut.cz/ojs/index.php/app vol. 26/2020 on evaluation of the three-dimensional isogeometric beam element n4,2 ξ knotspan patch continuity: c0 c0c1c∞ c∞ knotspan p = 2, ξ = {0,0,0,0.5,1,1,1} n1,2 n2,2 n3,2 knots control points p1 p2 p4p3 parametric space physical space x y figure 1. description of b-spline (nurbs) finite element geometry. the geometry is modeled by one patch consisting of two knotspans with quadratic nurbs approximation. which is a standard technique in nurbs technology. this procedure imposes the interpolatory behavior of the functions at the particular point. for better understanding of nurbs see fig. 1, where mapping between real and parametric geometry is illustrated. for more information the reader could refer to [4]. 3. beam element formulation the formulation of the presented three-dimensional beam element [1] is based on bernoulli beam theory and accounts for the geometrically nonlinear behaviour. bernoulli kinematics assume that the orthogonality of the cross-section to the center line is preserved after deformation and that the cross-sectional dimensions remain unchanged. the element has four degrees of freedom in each control point: three global displacements u, v, and w and additional degree of freedom corresponding to rotation around the center line ψ. the rotational degree of freedom enables the element to develop a torsion (warping effects are neglected here) and also to model initially twisted beams. 3.1. geometric description in the following, the standard notation using uppercase and lower-case letters for the undeformed and deformed configuration, respectively, is adapted. the beam formulation (see figure 2) starts from threedimensional approximation reduced using bernoulli kinematic assumptions to the displacements of the center line, which is given by the position vector xc (xc). the position vector of the center line is described as a linear combination of the control points coordinates x̂i and the corresponding basis functions rpi xc = ∑ i rpi x̂ i, (5) xc = ∑ i rpi x̂ i. (6) deformed position vector is given as x̂i = x̂i + ûi, (7) where ûi is a vector of global degrees of freedom (u, v, w). additionally to the center line, the cross-section orientation is described by a moving trihedral given by the base vectors ai (ai) with i ∈ {1, 2, 3}. a position vector of an arbitrary point of a beam continuum given by coordinates x (x) can be expressed as x(θ1, θ2, θ3) = xc(θ1) + θ2a2(θ1) + θ3a3(θ1), (8) x(θ1, θ2, θ3) = xc(θ1) + θ2a2(θ1) + θ3a3(θ1), (9) where θi are the convective contravariant coordinates. the first components of a moving trihedral a1 and a1 are aligned with a normalized tangents t and t, respectively, and are computed as a1 = ∑ i rpi,1x̂ i, (10) a1 = ∑ i rpi,1x̂ i, (11) where (·),i denotes the derivative with respect to the coordinate θi. the remaining components aα with α ∈ {2, 3} are orthogonal to the tangent of the center line (resp. a1) and are described by the reference trihedral, given by a0 α,t0, as aα = r̄t(ψ)λ(t0,t)a0 α, (12) where ψ is a initial rotation about a tangent of a beam, and λ and r̄t are the two key operations used 25 edita dvořáková, bořek patzák acta polytechnica ctu proceedings a1(θ1) a2(θ1) a3(θ1) a1(θ1) a3(θ1) a2(θ1) u(θ1, θ2, θ3) x(θ1, θ2, θ3) xc(θ1) xc(θ1) x y z figure 2. illustration of the beam in its undeformed and deformed configurations. t0 t0 a0 2 a0 3 t λ(t0, t ) λ(t0, t )λ(t0, t )a0 3 a0 3 λ(t0, t )a0 2 t0 a0 2 a0 3 t λ(t0, t )a0 3 λ(t0, t )a0 2 r̄t (ψ) r̄t (ψ) a3 a2 figure 3. the λ(t0,t) and r̄t(ψ) operations used for the alignment of the cross-section at the current position. for the alignment of a moving trihedral illustrated in figure 3. while the mapping matrix λ(t0,t) aligns the reference trihedral given by the tangent t0 to the tangent t at the current position, the rotation matrix r̄t(ψ) rotates the aligned reference trihedral about t with given rotation ψ. the rotation ψ = ψ(θ1) is calculated using basis functions and values ψ̂i assigned to the control points. the euler-rodriguez formula [5] is used to define both the mapping matrix λ and the rotation matrix rt. analogical procedures denoted as λ(t, t) and r̄t(ψ) are used to align the moving trihedral of the undeformed to the deformed configuration. the base vectors aα of the deformed configuration are given as aα = r̄t(ψ)λ(t, t)aα, (13) where ψ = ψ(θi) is a rotational degree of freedom evaluated at the current position θi. the base vectors of the undeformed continuum gi are defined as gi = x,i leading to g1(θ1, θ2, θ3) = a1(θ1) + θ2a2,1(θ1) + θ3a3,1(θ1), g2(θ1) = a2(θ1), (14) g3(θ1) = a3(θ1) with analogical equations for the base vectors gi = x,i of the undeformed configuration. in the sequel, dot products gi ·gj ( gi · gj) and ai ·aj ( ai · aj) are denoted as gij (gij) and aij (aij), respectively. 3.2. green-lagrange strain tensor the green-lagrange strain tensor e calculated for the curvilinear coordinate system is defined as eij = 1 2 (gij −gij) . (15) and for the orthogonal base vectors, the transformation to the cartesian coordinate system denoted with (̃·) is given by ẽij = eij ‖gi‖2‖gj‖2 . (16) by substituting geometric relations into the strain tensor, the individual components of the tensor can be derived. during the derivation additional simplifications are made assuming that only a slender beam is considered (b, h << lwhere b and h are cross-sectional dimensions and l is a length of the beam) resulting in the following equations for the diagonal term e11 e11 = 1 2(a11 −a11)︸ ︷︷ ︸ ε +θ2 a2,1 · a1 −a2,1 ·a1︸ ︷︷ ︸ κ21 +θ3 a3,1 · a1 −a3,1 ·a1︸ ︷︷ ︸ κ31 . (17) the diagonal terms eαα as well as the shear term e23 are equal to zero. this yields from the bernoulli assumptions (undeformable cross–section). for the off-diagonal terms e12, e13 e1α = 1 2θ β (aβ,1 · aα − (aβ,1 ·aα)︸ ︷︷ ︸ κβα , (18) where (α, β) ∈ {(2, 3), (3, 2)}. 26 vol. 26/2020 on evaluation of the three-dimensional isogeometric beam element 3.3. constitutive equations the energetically conjugated stress tensor to the green-lagrange strain tensor is the second piolakirchhoff tensor s. elastic isotropic material is considered within this work and st. venant–kirchhoff material is applied. as the beam formulation is reduced to the center line and bernoulli theory is assumed, the shear forces s̃23 and s̃32 and normal forces s̃22 s̃33 vanish and the full constitutive relation can be reduced to s̃11 s̃12 s̃13  =  e 0 0 0 g 0 0 0 g  ︸ ︷︷ ︸ d  ẽ11 ẽ12 ẽ13  , (19) where e is young’s modulus, g is the shear modulus and d is a reduced elasticity matrix. 3.4. principle of virtual work the system is in equilibrium when the overall virtual work of the system is equal to zero δw = −δwint + δwext = 0. (20) the internal end external virtual work of the system is given by δwint = ∫ ω s : δedx, (21) δwext = ∫ γn t : δudx+ ∫ ω ρ0b : δudx, (22) where ω and γn are the domain and the neumann boundary, respectively. the external virtual work depends on the boundary forces t, body forces b and material density ρ0. by substitution of expressions for the virtual greenlagrange strain tensor obtained from (15), constitutive relations (19) and performing linearization the following incremental relation can be obtained ∑ s krs∆us = −rr, (23) where ∆us are the displacement increments and rr = −(f intr + f extr ) (24) = ∂wint ∂ur + ∂wext ∂ur , (25) krs = ∂rr ∂us = ∂2wint ∂ur∂us . (26) 4. numerical example the described beam element has been implemented and tested on the example of helicoidal spring (see figure 4). this problem represents fully threedimensional structure where the all membrane, bending and torsion effects are present. the geometry is f = 1 t t e = 108 ν = 0.0 figure 4. helicoidal spring cantilever subjected to the unit force tip load and its cross-section. figure 5. helicoidal spring geometry for different thickness/length ratios: 0.1, 0.05, 0.025, 0.0125. given by xc(θ1) =  10 sin ( θ12π ) 10 cos ( θ12π ) 20 θ1  . (27) the structure is subjected to the unit tip force load. different dimensions of the square cross-section have been used in the analysis, in order to evaluate the performance and the applicability of the underlying assumptions. the studied geometries can be observed in figure 5, where helicoidal springs with different thickness/length (t/l) ratios (0.1, 0.05, 0.025 and 0.0125) are illustrated. the results have been compared with results obtained using standard fem straight beam element with cubic approximation [6]. this element allows for the analysis of both thick and thin beams and is naturally locking-free. the beam has been used in two different configurations: i) the element satisfying bernoulli assumptions; ii) the element accounting also for the shear effects. the results can be observed in the figure 6. as expected, for the thick beam configuration the results obtained using classical fem or iga elements with bernoulli assumptions differ from the standard fem beam element with account for the shear deforma27 edita dvořáková, bořek patzák acta polytechnica ctu proceedings -1.01e-06 -1.00e-06 -9.90e-07 -9.80e-07 -9.70e-07 -9.60e-07 -9.50e-07 -9.40e-07 -9.30e-07 -9.20e-07 -9.10e-07 10 15 20 25 30 35 40 45 t/l = 0.1 -1.60e-05 -1.58e-05 -1.56e-05 -1.54e-05 -1.52e-05 -1.50e-05 -1.48e-05 -1.46e-05 10 15 20 25 30 35 40 45 t/l = 0.05 -2.55e-04 -2.50e-04 -2.45e-04 -2.40e-04 -2.35e-04 -2.30e-04 10 15 20 25 30 35 40 45 t/l = 0.025 -4.05e-03 -4.00e-03 -3.95e-03 -3.90e-03 -3.85e-03 -3.80e-03 -3.75e-03 -3.70e-03 10 15 20 25 30 35 40 45 t/l = 0.0125 iga p=3 iga p=4 iga p=6 fem thin fem shear -1.01e-06 -1.00e-06 -9.90e-07 -9.80e-07 -9.70e-07 -9.60e-07 -9.50e-07 -9.40e-07 -9.30e-07 -9.20e-07 -9.10e-07 10 15 20 25 30 35 40 45 t/l = 0.1 -1.60e-05 -1.58e-05 -1.56e-05 -1.54e-05 -1.52e-05 -1.50e-05 -1.48e-05 -1.46e-05 10 15 20 25 30 35 40 45 t/l = 0.05 -2.55e-04 -2.50e-04 -2.45e-04 -2.40e-04 -2.35e-04 -2.30e-04 10 15 20 25 30 35 40 45 t/l = 0.025 -4.05e-03 -4.00e-03 -3.95e-03 -3.90e-03 -3.85e-03 -3.80e-03 -3.75e-03 -3.70e-03 10 15 20 25 30 35 40 45 t/l = 0.0125 iga p=3 iga p=4 iga p=6 fem thin fem shear -1.01e-06 -1.00e-06 -9.90e-07 -9.80e-07 -9.70e-07 -9.60e-07 -9.50e-07 -9.40e-07 -9.30e-07 -9.20e-07 -9.10e-07 10 15 20 25 30 35 40 45 t/l = 0.1 -1.60e-05 -1.58e-05 -1.56e-05 -1.54e-05 -1.52e-05 -1.50e-05 -1.48e-05 -1.46e-05 10 15 20 25 30 35 40 45 t/l = 0.05 -2.55e-04 -2.50e-04 -2.45e-04 -2.40e-04 -2.35e-04 -2.30e-04 10 15 20 25 30 35 40 45 t/l = 0.025 -4.05e-03 -4.00e-03 -3.95e-03 -3.90e-03 -3.85e-03 -3.80e-03 -3.75e-03 -3.70e-03 10 15 20 25 30 35 40 45 t/l = 0.0125 iga p=3 iga p=4 iga p=6 fem thin fem shear figure 6. comparison of isogeometric beam element with cubic (iga p = 3), quartic (iga p = 4) and hexic (iga p = 6) approximation with standard fem straight beam element with bernoulli assumptions (fem thin) and with account for shear effects (fem shear). the vertical axis corresponds to the vertical tip displacement w, the horizontal axis corresponds to the number of nodes (control points). tion. this difference is diminishing with the smaller thickness/length ratio. in addition to the beam elements, the fully threedimensional analysis has been assessed. the enormous number of nodes, in comparison with beam elements, have to be used in order to obtain sufficiently accurate results. finally, the mesh consisting of 102800 nodes (20× 20× 257) has been used. for the thin beam with t/l = 0.0125, the obtained deflection is w = −3.877e−3. this result is slightly higher than the results obtained using beam assumptions. the difference between the full 3d and beam models is getting bigger as the height to length ratio increases. this difference is indicating, that the beam theory assumptions are no longer adequate. overall, the results demonstrate the superior convergence properties of the iga element. this is due to its ability to capture curved geometry and natural high-order interpolation. 5. conclusions the spatial isogeometric element based on the work of a. m. bauer et al. [1] has been implemented and its convergence properties have been evaluated in this contribution. the element formulation is geometrically nonlinear, however only the geometrically linear analysis has been considered in this paper. the performance of the presented element has been compared with standard fem beam element using benchmark problem of helicoidal spring. for the higher thickness/length ratios t/l = 0.1, 0.5 the element does not provide good results due to the limitations of bernoulli assumptions. for the thin beams analysed, the excellent results are obtained. acknowledgements the financial support of this research by the grant agency of the czech technical university in prague (sgs project no. sgs19/032/ohk1/1t/11) is gratefully acknowledged. 28 vol. 26/2020 on evaluation of the three-dimensional isogeometric beam element references [1] a. m. bauer, m. breitenberger, b. philipp, et al. nonlinear isogeometric spatial bernoulli beam. computer methods in applied mechanics and engineering 303:101–127, 2016. [2] t. j. r. hughes, j. a. cottrell, y. bazilevs. isogeometric analysis: cad, finite elements, nurbs, exact geometry and mesh refinement. comput methods appl mech engrg 194:4135–4195, 2005. [3] l. greco, m. cuomo. b-spline interpolation of kirchhoff-love space rods. computer methods in applied mechanics and engineering 256:251 – 269, 2013. doi:10.1016/j.cma.2012.11.017. [4] l. piegl, w. tiller. the nurbs book. springer-verlag berlin heidelberg, new york, 1997. [5] j. s. dai. euler–rodrigues formula variations, quaternion conjugation and intrinsic connections. mechanism and machine theory 92:144–152, 2015. [6] z. bittnar, j. šejnoha. numerické metody mechaniky 1. české vysoké učení technické, praha, 1992. 29 https://doi.org/10.1016/j.cma.2012.11.017 acta polytechnica ctu proceedings 26:24–29, 2020 1 introduction 2 nurbs-based analysis 3 beam element formulation 3.1 geometric description 3.2 green-lagrange strain tensor 3.3 constitutive equations 3.4 principle of virtual work 4 numerical example 5 conclusions acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2018.20.0078 acta polytechnica ctu proceedings 20:78–85, 2018 © czech technical university in prague, 2018 available online at http://ojs.cvut.cz/ojs/index.php/app ambient temperature impact on micro gas turbines: experimental characterization iacopo rossi∗, alberto traverso thermochemical power group – university of genoa, 16145 genoa, italy ∗ corresponding author: iacopo.rossi@edu.unige.it abstract. in the panorama of gas turbines for energy production, a great relevance is given to performance impact of the ambient conditions. under the influence of ambient temperature, humidity and other factors, the engine performance is subject to consistent variations. this is true for large power plants as well as small engines. in combined cycle configuration, variation in performance are mitigated by the hrsg and the bottoming steam cycle. in a small scale system, such as a micro gas turbine, the influence on the electric and thermal power productions is strong as well, and is not mitigated by a bottoming cycle. this work focuses on the turbec t100 micro gas turbine and its performance through a series of operations with different ambient temperatures. the goal is to characterize the engine performance deriving simple correlations for the influence of ambient temperature on performance, at different electrical loads. the newly obtained experimental data are compared with previous performance curves on a modified machine, to capture the differences due to hardware degradation in time. an active management of the compressor inlet temperature may be developed in the future, basing on the analysis reported here. keywords: turbine flexibility, ambient temperature, micro turbine, cogeneration, experimental analysis. 1. introduction the penetration of renewables is pushing the need for flexibility even further in the energy generation environment [1, 2]. together with deregulation of energy markets, non-dispatchable sources increased the need for traditional power plant of providing ancillary services to the electrical grid. the consequence is that power plants undergo several rapid transients during operations as well as daily startups and shutdowns. in this environment, gas turbine power plants are very much considered for their capability in being flexible and capable of supporting the frequency modulation of the distribution grid. furthermore, they are highly efficient when coupled with steam-based bottoming cycle. the efficiency at part load is a key point to create a remunerative service system [3, 4]. all these factors are linked to penetration of renewables in the grid: the higher the penetration, the higher the capability in absorbing production fluctuations is required. new layouts are emerging including fast electrical storage [5], or heat pumps [4] to improve flexibility without penalizing performance. for similar reasons, the small scale microgas turbine systems are expected to improve their share in energy market as they match very well the load variations in smart grid environments, in order to assist renewable systems in energy production. the problem of small scale gas turbine is linked to their performance. usually these systems are always based on a recuperated cycle, involving a recuperator at the exit of the compressor. the problem of the recuperator is both in terms of performance [6, 7] and long-term life [8] due to thermal cycles. in perspective, the global performance of these systems can be enhanced by considering the coupling with a fuel cell in a hybrid configuration [9], which however has a drawback on the capital costs associated to such technology. furthermore, the emissions of ghg is in any case present and these systems can be considered environmental friendly only through a coupling with other devices or exploiting renewable fuels, such as biogas. this is why new technologies for carbon sequestration are under investigation also for these small systems [10]. nevertheless, the possibilities for fuel flexibility is higher in these small scale systems than in larger power plants. a dual fueled 100 kw turbine operating with natural gas and ethanol is investigated at its part load operations in [11, 12]. in [13] bauer et al. investigated operability limits for a 100 kw turbine burning hydrogen. these systems can be used to enhance existing technologies and to produce new layouts where renewable and fossil sources coexists: this is proposed in [14], where a hybrid photovoltaic system and a microturbine create a trigenerative power plant. integration of microturbine in real smart grid environment is however a reality: in [15] the case study of the savona campus, university of genoa (italy), is illustrated, while in [16] an integration of micro turbine with an external solar source interacting with a ceramic storage is analyzed. this layout represents a possibility for enhancing traditional system with innovative renewable-assisted layouts. the dispatching of such systems is however dependent on ambient conditions. considering a single smart grid, the day-ahead programming makes large uses of weather forecast78 http://dx.doi.org/10.14311/app.2018.20.0078 http://ojs.cvut.cz/ojs/index.php/app vol. 20/2018 ambient temperature impact on micro gas turbines ing to plan which movers should be used and when, according to market-based optimization algorithms. the forecast is fundamental to understand what is expected from renewable systems as well. the forecast is useful also to estimate the required availability of the traditional energy systems with consequences on system scheduling and operations. similar procedures are carried out also for the day-ahead programming of larger power plant. under the influence of ambient temperature, humidity and other factors, the engine performance is subject to consistent variations: ambient temperature is certainly the ambient parameter having the largest impact on a specific installation. in [17, 18] the decrease in power with the ambient temperature increase is considered to be greater as the size of the system is reduced. in small scale systems, the maximum power results to be capped as temperature increases and the share between electric and thermal production changes [14, 19, 20]. in [21] a study carried out with neural networks based on experimental data highlighted that the main parameter affecting the performance (i.e. electric power and efficiency) is the compressor inlet temperature, whilst ambient pressure and humidity have e negligible influence on the system performance of small scaled engine. in [19] experimental curves were introduced to outline the influence of the ambient temperature on turbine performance. in that case a modified version of a t100 was adopted and the inlet air temperature was controller through a heat exchanger that was installed at the compressor inlet. in [20] a similar study was carried out. this is based on the results of a detailed model, which has been previously trained against experimental data deriving from a real test plant. in [20] a traditional engine equipped for advanced testing in a laboratory was considered. the present work focuses on a turbec t100 and its performance through a series of operations with different ambient temperatures. the analyzed engine is a standard out-of-the-box t100 generating energy for the smart grid test rig and the savona campus. the goal is to compare the correction curves proposed in [19] and in [20], which were measured on a modified machine, against data obtained from several hundred hours of real operation of a standard machine. in the end a characterization can be outlined. 2. test rig description the work is based on field data from a t100 micro turbine (fig. 1). this engine is derived from experience obtained by volvo in automotive powertrains in the ’80s. the specific engine under investigation was produced in 2008 and installed in savona campus [15] in late 2011 and operates since 2012 until nowadays. the t100 is based on a recuperated cycle and generates 100 kw of net electrical power and 150 kw of thermal power, revolving at 70000 rpm. the system is installed outdoor in the innovative energy system figure 1. powertrain representation of the t100. electrical power 100 kw thermal power 150 kw efficiency 30% pressure ratio 4.5 fuel consumption 7.2 g/s turbine inlet temperature 950 °c rotational speed 70000 rpm table 1. nominal performance of the t100 @ 15 °c. laboratory (ies lab) of the thermochemical power group (tpg), university of genoa (unige) [22]. the system is provided with a centrifugal single stage compressor and a radial turbine operating with a nominal pressure ratio of 4.5 and a tit of 950 °c. the analyzed system is equipped with an embedded fuel compressor. since the engine was installed in 2011 and never modified nor special operations were carried out on the system since then, the electronics, control system and hardware are still the original as provided by turbec. according to the nominal data, the engine consumes at full load 7.2 g/s of natural gas for a net electrical efficiency of 0.3. in table 1, a summary of the nominal values is reported. the engine is installed in a smart grid environment [23, 24] and it is part of the campus smart polygeneration microgrid (spm). the thermal energy delivery system is based on two rings, which drive water to the customers (the hot side, kept at 75 °c) and return it back to the recovery system of the generators (the cold side, at 55 °c) [23, 24]. the system is equipped with local energy storage for each generator and a grid vessel of 5000l capable of storing up to 400 mj thermal energy. the whole system is governed through labview® and can interact with a real time matlab/simulink station via udp. further information can be found in [23, 25, 26]. since the main background is given by [19] and [20], a brief description of those work is here provided. in [20] an operating test rig was not presented. the results were obtained through a model trained against a real test rig. still, the obtained results are derived from a model of a commercial t100. no information of operating hours and startups were provided. the model was used to outline different operative charts, presenting the operating conditions from which the influence of the ambient was evaluated. in the end, percentage linear variations of electrical power and ef79 iacopo rossi, alberto traverso acta polytechnica ctu proceedings ficiency, thermal power and efficiency, thermal to load ratio are given [20]. these variations were outlined considering a 1 °c growth of ambient temperature from 15 °c condition. in [19] the approach was different: a real test rig based on a t100 was operated at controlled compressor inlet conditions. the procedure gave light to a series of correction curves that were obtained controlling the temperature at the inlet of the compressor through a heat exchanger. the engine was running in steady-state conditions. the atmosphere of the room was controlled itself and the humidity of the inlet air was approximately constant throughout the test. those correction curves were built focusing on electrical power output, thermal power output and fuel consumption. three different intake temperatures have been considered for that investigation: 17 °c, 22 °c and 27 °c. the present work compares the two results obtained in [19, 20] and validates the two approaches on the basis of the field data from an operating engine in standard layout. the papers [19, 20] investigate the influence of inlet conditions on the electrical and thermal power variations – and this work is focused on them. the target engine delivers cogenerative energy to the savona campus of unige and operates mainly during the cold season. during the summer season, when few students live at the campus, the engine is usually switched off. in the campus there is a set of generators, which always operates to feed the campus consumptions and another set which is used for backup operation or experimental tests. the investigated t100 belongs to this second group. as a consequence, since the turbine was installed in 2012, the engine has performed a significant amount of startups (almost 400) and severe transient operations. 3. experimental data analysis in the current analysis, information on air humidity and ambient pressure were not considered, as it is recognized that they have a lower order impact on performance, while the main environmental variable for this small system is considered to be the inlet temperature [21]. as stated previously, the investigated engine operated several years for experimental testing or generating energy for the campus grid. this means that the engine was very stressed (in test rig configuration, the system was mainly used to test smart control systems) with several start-ups. in particular, during test rig configuration, rapid transients were very frequent due to the controlling tests that have been carried out [23–26]. therefore, all of the data available have been filtered in order to consider only steady-state conditions at the desired temperatures. in particular: • the data were considered agreeable if far enough from cold conditions i.e. this analysis considers only working points obtained 20 minutes from the startup; correction 1 [19] correction 2 [20] pel -0.92% -1.22% pth +1.19% -0.10% table 2. variation obtained in [19] per each 1 °c from 17 °c; variation obtained in [20] per each 1 °c from 15 °c. • a working point was considered to be stable if the electrical power derivative was lower than 0.05 kw/s in absolute value; • ambient condition were centered around ±0.5 °c for each condition; in [19] a set of correlations was proposed to link energy production and fuel consumption to the compressor intake temperature (in general, intake temperature is equivalent to ambient temperature, unless air conditioning units are employed before the filters). these are reported in equations 1-3 and define the variation of each parameter, starting from 17 °c ambient temperature. ṁf ṁf,0 = 1 + 0.0047 · ∆t (1) pel pel,0 = 1 − 0.0092 · ∆t (2) pth pth,0 = 1 + 0.0119 · ∆t (3) in [20] no correction curves were defined, but a percentage variation linked to ambient conditions. in particular in [20] it stated that, for each 1 °c growth from 15 °c, the system undergoes: • a reduction of electrical power of 1.22% • a reduction of electrical efficiency of 0.51% • a reduction of thermal power of 0.10% • an increase in thermal efficiency of 0.70% • an increase of the thermal to electrical power ratio of 1.30% normalizing the results of the two cited works, the percentage variation of electrical and thermal power for each increase of 1 °c; from the standard ambient temperature, results in table2 are obtained for comparison. in this work, data analysis is focused around the first operating year, i.e. the 2012. during that year, the t100 experimented 115 startup for 450 operative hours. data were filtered on the basis of the aforementioned methodology. three different graphs gather the operating points as a function of three different intake temperatures: 12 °c 15 °c and 27 °c. with respect to previous work [19], different temperature values where considered, still the proposed range of temperature is useful to understand the behavior of the engine both 80 vol. 20/2018 ambient temperature impact on micro gas turbines figure 2. operatinal field data for three ambient temperatures. figure 3. fuel consumption for operating points at three different ambient temperatures. under and over the 15 °c. the investigated parameters are electric power, thermal power, fuel consumption and electric efficiency. all values are normalized on the basis of the nominal data included in table 1. electric efficiency is a parameter computed during the data analysis, considering equation 4, with lhv equal to 41mj/kg. η = pel ṁf · lhv (4) figure 2 shows collected raw data for three ambient temperatures: below 15 °c the performance remains similar, while a shift is evident for 27 °c. in particular, while increasing the ambient temperature, the reduction of maximum electric power is noticed, as well as a small increase in thermal power production. this is in agreement with [19], since the proposed correction curves give a reduction in electric power and augmentation in thermal power as ambient temperature increases (see table 2). figure 3 shows the same operating points but focuses on fuel consumption: in this case, the variation is small and the points are distributed around the same line. the influence on efficiency is given by figure 4, where the impact on peak efficiency (-4.5%) and efficiency at maximum load (-3.7%) is clearly visible. from these data, validation of the curves [19, 20] has been carried out. in particular, a series of points around the maximum load has been collected and averaged per each of the investigated temperature. as a consequence, the field data included in figure 5a and 5b are mean values of operating points at maximum load. the same figure shows two trend lines: correction curve 1 and 2. these are the trend lines obtained from [19] and [20], respectively. from figure 5a it is clear that in general the correction curve 2 overestimates the reduction in electric power linked to intake temperature variation, while 81 iacopo rossi, alberto traverso acta polytechnica ctu proceedings figure 4. variation of efficiency as a function of electric load and ambient temperature. figure 5. (a) operating points for electric power production and (b) for thermal power production as a function of ambient temperature. the correction curve 2 slightly underestimates the same variation. in 5b the thermal variation is consistent with correction curve 2 even though this overestimates the variation. the correction curve 1, as proposed by [20], shows a small negative trend, which is clearly not in agreement with the real behavior of the engine. 4. degradation due to aging a system degradation can be in general defined as a deterioration in quality, level or standard of performance of the system due to different reasons. in energy power plant, a degradation of performance is expected during lifetime. the effect of aging on system degradation is linked to all of those components prone to wear and tear. the large availability of field data allowed preliminary short analysis on ageing of the engine. as stated before, the investigated t100 was installed outdoor, exploited to perform numerous startups (related to the actual operating hours) and figure 6. maximum electrical power as a function of the number of startups. 82 vol. 20/2018 ambient temperature impact on micro gas turbines figure 7. performance trend of the engine at 15 °c ambient temperature for (a) electrical and thermal power and (b) efficiency, as a function of electrical power. underwent severe operating cycles, linked to series of rapid transients to test the control system performance. for these reasons, performance degradation of the system was, in the end, evident. the analysis here proposed is mainly based on the number of startups of the system: in general, each startup event can be associated to a number of equivalent operating hours, which affect significantly the system performance and cause degradation. the investigated t100 underwent 360 startups against only 1400 operating hours; therefore the impact of the startups can be considered preeminent on the aging of the system. in particular, here the variation of engine maximum power is analysed as a function of the number of startups. this has been conducted with compressor intake temperatures around 15 °c, which guarantees the nominal performance of the engine. figure 6 reports the means of maximum electrical power versus cumulative startups. the reduction in power with the startup number is evident: the second axis of the figure shows the number of operating hours at each startup. it is evident a quadratic shape in power degradation with respect to startup number. finally, two images summarizes how the performance of the engine changed over the years: figure 7a shows the trend line at 15 °c for the first year of operations of the engine (same data showed in figs. 2 4) compared to the current trend line obtained in the same operative conditions. these points were obtained analyzing the last year of operation of the engine with the same criteria adopted for the data analyzed previously. a decrease in maximum power is evident, which reflects on increase of thermal production side. figure 7b shows the same comparison for electrical efficiency. consistently with the trend of performance, the efficiency is reduced both at its maximum peak and at maximum load (-7.5% and -5% respectively, on relative basis). 5. conclusions the analysis of t100 microturbine performance is based on real operating data at various ambient temperatures obtained in different laboratories and test rigs. the study outlined the differences of behavior of the system against the two proposed model. three different temperatures have been considered by the analysis. the data at 12 °c were in agreement with those at 15 °c, i.e. without showing significant variation in performance. an influence on the performance is indeed evident as the intake temperature rises. considering the ambient effect, on the analyzed t100, it has been highlighted that: • the electrical power is reduced by 0.83 kw/ °c per each 1 °c growth from 15 °c • the thermal power is increased by 0.5 kw/ °c per each 1 °c growth from 15 °c • the efficiency is reduced by 0.31%/ °c per each 1 °c growth from 15 °c in addition, some information based on the aging of the system has been derived, showing a clear decrease in maximum output power based on the number of startups. nevertheless, this analysis cannot be used to define a parameter for the aging of the system due to severe working conditions and frequency of startups and shutdowns against the effective operating time. what it can be concluded is that the engine, which has been never subject to maintenance and/or cleaning over the years, shows a clear reduction in performance – and it stabilizes around a loss of 8% from design value. even though a quadratic shape is evident, it is useful to quantify an average of performance losses paid by each start-up. in this way it would be possible to determine the impact in terms of equivalent hour associated to each startup procedure. giving that, the average losses can be quantified in terms of: • a reduction in electrical power of 0.022% per startup in relative terms • a reduction in efficiency at full load of 0.014% per startup in relative terms in the end, the analysis can be used to give light to a simplified dynamic model able to describe the engine 83 iacopo rossi, alberto traverso acta polytechnica ctu proceedings performance and the influence the intake temperature has on its performance. the model can be constituted by two transfer functions which represent the electrical and thermal output to input variations. these are coupled with a temperature-dependent parameter as obtained from the analysis, for off-design correction. this model can be eventually used to estimate performance variation due to ambient temperature variation, for instance, when the compressor intake temperature can be artificially modified through inlet conditioning techniques. the study shows also that what was obtained in [19] and [20] is reasonably repeatable, in particular for the electrical power. however, the proposed correction curves for thermal power shows a significant spread with respect to field data. acknowledgements this research work has been carried out within the pump-heat project, which has received funding from the european union’s horizon 2020 research and innovation programme under grant agreement no 764706. references [1] m. de groot, w. crijns-graus, r. harmsen. the effects of variable renewable electricity on energy efficiency and full load hours of fossil-fired power plants in the european union. energy 138:575 – 589, 2017. doi:10.1016/j.energy.2017.07.085. 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[26] i. rossi, l. banta, a. cuneo, et al. real-time management solutions for a smart polygeneration microgrid. energy conversion and management 112:11 – 20, 2016. doi:10.1016/j.enconman.2015.12.026. 85 http://dx.doi.org/10.1016/j.applthermaleng.2015.12.108 http://dx.doi.org/10.1016/j.apenergy.2014.02.025 http://dx.doi.org/10.1016/j.egypro.2014.11.1101 http://dx.doi.org/10.1016/j.enconman.2013.10.044 http://dx.doi.org/10.1016/j.enconman.2015.12.026 acta polytechnica ctu proceedings 20:78–85, 2018 1 introduction 2 test rig description 3 experimental data analysis 4 degradation due to aging 5 conclusions acknowledgements references doi:10.14311/app.2021.30.0036 acta polytechnica ctu proceedings 30:36–40, 2021 © czech technical university in prague, 2021 available online at http://ojs.cvut.cz/ojs/index.php/app simulation of t-joints between rhs steel members with offset in abaqus cae svitlana kalmykova czech technical university in prague, faculty of civil engineering, department of steel and timber structures, czech technical university in prague, thákurova 7, 166 29 prague 6, czech republic correspondence: svitlana.kalmykova@fsv.cvut.cz abstract. the current paper focuses on numerical simulation peculiarities of offset welded rectangular hollow section joints. understanding the modelling techniques can result in easier and faster and above all correct outcomes from fea for future use. the steel joints under discussion are composed from cold-formed regular rectangular hollow sections where rhs brace members are laterally shifted from chord axis. joints work under monotonically increasing compression load applied to a brace top. numerical models were developed in fe programme abaqus. fe-models is composed of c3d8r 8-noded solid linear brick elements with an emphasis on mesh size effect and modelling of a weld seam. fe advanced model were compiled considering both material and geometric nonlinearities. for validation purposes, the full-scale laboratory tests were conducted. proposed fe models reliably predict the structural behaviour of welded offset t-joints thanks to good agreement achieved on deformation limit 3 % b0 with the maximum deviation 10.3 %. keywords: fe programme abaqus, hollow structural sections (hss), numerical modelling, offset t-joints, rectangular hollow sections (rhs), truss connections. 1. introduction steel structural hollow sections, including rectangular hollow sections (rhs), are nowadays one of the most efficient load-bearing elements of the lattice structures. despite its wide-spread application, there is still lack of design techniques for joints produced with offsets. since the mid-1960s, vast study on hss connections structural behaviour has been undertaken. the results were implemented in current regulations and standards such as en 1993-1-8 (2005), ansi/aisc 360-05 (2005), csa-s16-09 (2009), iso 14346:2013 or a design guide cidect dg3. it is essential to note that existing methods of hss connections design are for coaxial ones only, i.e. centreline of a branch member should have an intersection with centreline of a chord. the term "standard" is used for geometry of aforesaid connections. at the same time, the question concerning mechanical resistance of joints under discussion is practically uncovered by design recommendations and guidelines. as of today, there are few studies published on the topic, namely, the earliest experimental research of laterally offset tand n-joints done by dawe et al. [1], kalmykova and wald [2, 3] including laboratory and numerical research, and wei and packer [4] who performed a set of the offset shs tand xjoints laboratory testing and proposed certain design approaches simultaneously with an analytical equation to predict resistance of the offset joints. the strength, modes of failure and loaddisplacement curves can be gained from a proper numerical analysis. considering costliness of physical laboratory experiments, it is of high impotence to find ways of effective numerical simulation of joints (see figure 1) to trustworthy predict their structural behaviour. figure 1. welded offset t-joint geometry. the aim of the current study is to present a practical numerical simulation guideline for offset joints and connections using abaqus cae. 2. finite element modellings 2.1. general numerical modelling of the fully welded offset tjoints was done in abaqus. it is general-purpose finite element programme, release 6.14. fe models of joints were composed in compliance with pren 36 http://dx.doi.org/10.14311/app.2021.30.0036 http://ojs.cvut.cz/ojs/index.php/app vol. 30/2021 rhs steel members t-joints simulation figure 2. finite element mesh of t-joint. 1993-1-14: 2020. static, riks procedure available in the abaqus library was employed to run loaddisplacement nonlinear analysis. fe models of offset joints were built considering both geometric and material nonlinearities. carefully performed sensitivity study allowed to define the certain element type, fe mesh size of finite elements as well as welding material property. it brings accurate results with adequate computational costs. boundary conditions, loading, weld seam modelling and simulation of materials are fundamental inputs in performed fe analysis. 2.2. geometry of joints experimental programme includes two types of tjoints employed for the validation procedure. the first type presents a regular coaxial geometry of tjoint without any offset; the second one was produced with the brace lateral offset (see figure 1). both joints were loaded by the monotonically increased axial compression applied at the brace top. brace member is made of cold-formed rectangular hollow section rhs 50 × 30 × 4 mm en10219. coldformed rhs 150 × 100 × 4 mm en10219 is used for chord member. 2.3. element types and fe mesh size in a brick model the element type used for joint modelling is 3d 8node solid element with reduces integration (c3d8r) from abaqus library. it is fully isoperimetric element with the first order interpolation. finite element mesh of a model is presented in figure 2. it consists of two regions with two various fe element sizes. smaller mesh region comprises fe bricks with the sizes up to 4 × 4 × 4 mm and is distributed along the direct chord axis on the distance of 240 mm. the distance of smaller mesh region for the brace member along the direct brace axis equals 300 mm. the rest of the fe mesh consists of 8 × 8 × 8 mm elements for both members of the joint. additionally, at the corner areas, the finer 4-element mesh was employed because of great importance to transfer stresses from a flange to a web of a joint member. isoparametric angles were monitored as there was no distortions in the fe bricks. aspect ratio of the fe bricks were limited to be less than 3. the number of in-depth fe layers for chord and brace walls were monitored in order to analyse model sensitivity. the number of layers were varied from 1 to 6. based on the obtained data and subsequent analysis, 3-layer mesh was chosen for further simulations by virtue of ability to provide accurate results with reasonable computational costs. apart from c3d8r brick fe units, two more types of solid 3d elements were utilized. there are c3d8i and c3d20r. it is to check processing time and precision of obtained results for fe models made by it. 2.4. loading and weld simulation joints work under monotonically increasing compression load applied at a brace top. axial compression was imposed to the reference point on an analytical rigid plate (see figure 3). weld seam, in general, has considerable impact to the mechanical resistance of t-joints. this is particularly the case for the fillet weld corner areas. thus, weld was employed in fe models of offset joints. as for the brick model, weld seam is the only member that joining brace and chord. thereto, a yawns or gaps were modelled between external face of the chord top flange and bottom section of the brace with the help of advanced simulation on initial geometry creation step. specific procedure was applied to the meshing. as for the shell model, one should go to the section 2.7. for simplicity, the material properties of the weld seam and steel members were assumed the same. it is due to complexity of weld material property investigation when an electrode’s composition is mixed with the main material of a joint. fe solid elements types coinside with the chosen type for a model. 2.5. boundary conditions as for boundary conditions of the joint, the chord member was stacked on the rigid base, i.e. it is fully restrained from rotation and displacement. additionally, the displacement in the direction of the applied 37 svitlana kalmykova acta polytechnica ctu proceedings figure 3. displacement of the reference point and boundary conditions on the model. load is allowed for the top brace edge, while other displacements and rotations are prohibited. 2.6. material tensile coupon tests (see figure 4) were performed in order to provide real material properties to an advanced fe model. the coupons were cut in the rolling direction from additional pieces of rhs made from the same material. a total of 5 tensile tests were conducted: 3 tests on s355 j2h material taken from a chord element section, and 2 tests on s355 j2h taken from a brace element profile. the middle part of a profile face was chosen for cutting coupons to avoid zones affected by hardening. material properties including young’s modulus, the ultimate tensile stress and the corresponding strain were recorded. more detailed information including measured data and true stress-strain curves is provided in kalmykova and wald [2]. in general, for elastic-plastic bodies analysed by abaqus, material properties of a specimen are assigned in two dialog boxes, namely, mechanical elastic and plastic material behaviour dialog boxes. the elastic response of steel is modelled by linear elasticity with young modulus. elastic properties are specified as isotropic (see table 1). to model isotropic plasticity in abaqus, the mises yield surface is used. the plasticity data are determined by true stress versus logarithmic plastic strain (see table 2). fracture strain stress triaxiality strain rate 1 0.3 0 0.3 1 0 table 1. material behavior assigned in abaqus (elastic isotropic). for the specimen modelling in abaqus, the real material model resulting from the coupon tensile tests was used. the initial part illustrates proportional yield stress plastic strain 320 0 355.594 2e-005 365.568 0.01806 420 0.04679 459.563 0.07013 492.25 0.09297 517.5 0.11532 537.625 0.1372 567 0.17962 587.5 0.22035 table 2. material behavior assigned in abaqus (plastic isotropic). dependency of stress and strain, an evidence of elastic property, up to the tensile yield strength fy with recorded young’s modulus e and poisson’s ratio ν. beyond the elastic limit, ultimate tensile strength fu and elongation after fracture εf measured on coupon tests create the post-yield response of the bilinear material model. additionally, von-mises yield rule and kinematic hardening model were employed into the model. extensive plastic strain is involved into nonlinear fe analysis. for that reason, static stress-strain diagram measured on coupon tests was converted to true stress-logarithmic plastic strain diagram. abaqus user’s manual contains recommendations based on which the true stress σtrue and logarithmic plastic strain εpl were computed. during the simulation, an attempt to simulate the elimination from calculations of individual physically damaged finite elements was made since the strength criterion for such elements is no longer satisfied. damaged cells were excluded from the calculation by introducing the ductile damage option (see table 3). 38 vol. 30/2021 rhs steel members t-joints simulation figure 4. coupon tests specimens and scheme of loading. young’s modulus poisson’s ratio 201000 0.3 table 3. material behavior assigned in abaqus (ductile damage). 2.7. shell fe model in addition to the main procedure of numerical model validation, it was decided to conduct comparative analysis of similar fe-models composed by solid elements (c3d8r) and their counterparts constructed by shell finite elements. the element type used for joint modelling is a 4node doubly curved shell element with reduced integration (s4r) from abaqus library. a 5-point integration was utilized by the thickness of the shell with all degrees of freedom per node. in modelling, sections of the joints’ brace and chord were presented by their middle surfaces with assigned thicknesses which are correspondent to the real thicknesses of components. as it was stated in more earlier studies (korol and mirza [5], matos et al. [6]), shell finite elements are able to assure precise computation to overwhelming majority of applications owing to transverse shear deformation presence that crucial for thick shell elements simulation (see figure 5). a four-node doubly curved shell element with reduced integration (s4r) has been used to model brace and chord members. a five-point integration was applied through the shell thickness with a full complement of six degrees of freedom per node. in modelling, sections of the joints’ brace and chord were presented by their middle surfaces with thicknesses which are correspondent to the real thicknesses of components. as it was stated in more earlier studies (korol and mirza [5], matos et al. [6]), shell elements provide an accurate solution to most applications by allowing for transverse shear deformation, which is important for simulation of thick shell element figure 5. as a simplification, some of researchers omit the weld seam in shell fe models of welded hss joints. this usually creates undervaluing of the joint strength. nevertheless, for the specimens under discussion, the weld seam was considered. filled weld with throat thickness of 10 mm at the intersection of brace and chord was modelled according to van der vegte et al. [7]. inclined at an 45 angle circle of elements is displaced along the brace and chord intersection area. some of the researchers omit the welds for simplicity in the modelling of welded tubular joints using shell elements, which leads to underestimating the joint strength. however, for the specimens under discussion, the weld seam was considered. filled weld with throat thickness of 10 mm at the intersection of brace and chord was modelled according to van der vegte et al. [7] using a "ring" of shell element along the brace and chord intersection region. 3. experimental study full-scale laboratory experiments were carried out by kalmykova and wald [2] with the aim to study failure modes of the standard and offset t-joints specimens and to get data for the validation of fe models developed for numerical studies. 4. validation for the purposes of this paper, results from numerical fe advanced model in abaqus were calibrated against relevant full-scale experimental data for both 39 svitlana kalmykova acta polytechnica ctu proceedings figure 5. "shell" fe models weld seam. standard and laterally offset t-joints. abaqus fe models were validated against experimental results conducted by the authors mentioned hereinabove. validation procedure and its complete outcomes have been provided in detail in [3]. the comparison of the "load-displacement" diagrams coming from the fullscale experiments and fe analysis is shown on figure 6 and figure 7. figure 6. validation of fe models against laboratory tests for a standard joint. test and finite element analysis results show good agreement achieved for both standard and offset models on allowed deformation level of 0.03 b0 with the maximum difference of 10.3 % and the minimum difference of 2.6 %. 5. conclusion fe models are an essential tool for structural behaviour assessment of steel offset joints between hss members. the structural response predicted by these numerical models adequately reflects the actual behaviour of the joints. to guarantee it, the fe models were validated on the results of physical laboratory tests. the fe analyses results accurately aligned with the test outcomes. hence, it could be said that the proposed fe models reliably predict the structural behaviour of welded offset t-joints with rhs members. 3d 8-node solid elements with reduces integration (c3d8r) is recommended for further simulation of offset t-joints between rhs members as the ones figure 7. validation of fe models against laboratory tests for an offset joint. that provide accurate results in adequate computation time. 6. list of symbols e young’s modulus ν poisson’s ratio σu ultimate tensile stress εu ultimate strain fy tensile yield stress fu ultimate tensile strength σtrue true stress εpl logarithmic plastic strain 7. acknowledgements the research was carried out under the grant sgs19/150/ohk1/3t/11. references [1] j. l. dawe, y. liu, r. zhang, a. dukuze. experimental study of offset hss connections. journal of structural engineering, american society of civil engineers 132(5):717–725, 2006. [2] s. kalmykova, f. wald. full-scale experimental study of welded rhs t-joints with offset and its main outcomes. grant journal 8(1):76–81, 2019. [3] s. kalmykova, f. wald. welded t-joints between rectangular hollow section members with offset. in proceedings of xii conference on steel and composite construction 1:1157–1167. [4] f. wei, j. a. packer. experimental study of laterally offset rhs x-connections. [5] r. m. korol, f. a. mirza. finite element analysis of rhs t-joints. journal of the structural division 108(9):2081–2098, 1982. [6] r. m. m. p. matos, l. f. costa-neves, l. r. o. lima, et al. resistance and elastic stiffness of rhs "t" joints: part i axial brace loading. latin american journal of solids and structures 12:2159–2179, 2015. [7] g. j. van der vegte, c. h. m. de koning, puthli, et al. numerical simulation of experiments on multiplanar tubular steel x-joints. international journal of offshore and polar engineering 1(3), 1991. 40 acta polytechnica ctu proceedings doi:10.14311/app.2017.13.0080 acta polytechnica ctu proceedings 13:80–84, 2017 © czech technical university in prague, 2017 available online at http://ojs.cvut.cz/ojs/index.php/app adaptive quasicontinuum simulation of elastic-brittle disordered lattices karel mikeš∗, milan jirásek czech technical university in prague, faculty of civil engineering, thákurova 7, 166 29 prague 6, czech republic ∗ corresponding author: karel.mikes.1@fsv.cvut.cz abstract. the quasicontinuum (qc) method is a computational technique that can efficiently handle atomistic lattices by combining continuum and atomistic approaches. in this work, the qc method is combined with an adaptive algorithm, to obtain correct predictions of crack trajectories in failure simulations. numerical simulations of crack propagation in elastic-brittle disordered lattices are performed for a two-dimensional example. the obtained results are compared with the fully resolved particle model. it is shown that the adaptive qc simulation provides a significant reduction of the computational demand. at the same time, the macroscopic crack trajectories and the shape of the force-displacement diagram are very well captured. keywords: quasicontinuum method, adaptive crack propagation, elastic-brittle lattice model. 1. introduction discrete particle models can effectively capture complex material responses, especially localized phenomena such as damage or plastic softening. the main disadvantage of particle-based approaches is that a huge number of particles is needed to describe the response of large-scale physically relevant models. this results in an extensive system of equations, which is expensive to solve. furthermore, the process of assembling this system is also computationally expensive because all discrete connections must be individually taken into account. a quasicontinuum (qc) based method can remove both of these disadvantages by combining continuum and atomistic approaches. in order to simulate crack propagation, the qc method needs to be combined with an adaptive algorithm that allows crack growth in arbitrary directions and initialization of new cracks. in this work, the material is represented by particles interacting via elastic-brittle links forming a disordered two-dimensional lattice. only axial interaction between particles is considered and the behavior of links is assumed to be perfectly elastic-brittle, with link breakage occurring at a critical level of tensile strain. 2. qc method the qc method can efficiently handle high-resolution particle models by combining continuum and discrete approaches. this method was originally proposed in 1996 [1] for regular atomistic lattices with long-range interactions. since that time, the qc method has been widely used and extended to applications for a variety of materials represented by regular lattices [2]. an extension of the qc method to irregular lattices has recently been developed by the authors [3]. the main idea of the qc method is to reduce the number of degrees of freedom (dofs) and the associated computational cost without losing the exact atomistic description in regions of interest. therefore, two types of regions in the investigated domain are considered. in regions of high interest, the pure particle approach is used and all particles carry their own independent dofs. by contrast, in regions of low interest, continuum assumptions are applied and the computational model is significantly simplified. the procedure that results from the qc method can be briefly presented in the following three steps: (1.) interpolation (2.) summation (3.) adaptivity 2.1. interpolation interpolation of dofs is used in regions of low interest. to simplify the full particle model, only a small subset of particles is selected to represent the entire system. these so-called repnodes serve as the nodes of a background triangular mesh that is used to interpolate the dofs of other particles in regions of low interest. on the other hand, in regions of high interest, all particles are selected as repnodes to provide the exact particle representation. this interpolation leads to a significant reduction of the number of dofs without losing the exact particle description in regions where a high resolution is needed. 2.2. summation the interpolation provides a significant reduction of the number of dofs but all particles still need to be visited to construct the system of governing equations, which makes the process computationally expensive. 80 http://dx.doi.org/10.14311/app.2017.13.0080 http://ojs.cvut.cz/ojs/index.php/app vol. 13/2017 adaptive quasicontinuum simulation of disordered lattices the summation rule can be applied in order to eliminate the requirement of visiting all particles during the assembly of the system. if the summation rule is adopted, the contribution of all particles in each interpolation triangle is estimated based on sampling of the links that surround one single particle and properly scaling the contribution. this makes the computational process faster but some problems can occur on the interface between regions of high and low interest. because of the interpolation and the summation, the deformation is considered as constant within each interpolation element in the regions of low interest while the deformations of individual links in the regions of high interest are evaluated exactly. consequently, forces of nonphysical character, called the ghost forces, appear on the interface between the regions of low and high interest [4]. in this work, the summation procedure is realized by a homogenization of links contributing to the interpolation elements. to eliminate the ghost forces, some specific links are selected to be processed exactly, in order to capture the interface between the fully resolved and interpolated domains. 2.3. adaptivity adaptivity allows to adjust the regions of low and high interest during the simulation process. the type of region can be changed by adding repnodes before each step. an appropriate modification of the regions of high interest leads to a substantial increase of accuracy. moreover, in specific cases such as simulation of crack propagation or damage evolution, adaptivity is necessary in order to represent the correct physical behavior. after the insertion of new repnodes, a new triangulation of the interpolation mesh can be constructed, in order to improve the accuracy of interpolation and summation in areas of low interest. 3. qc-based approaches in this section, the exact approach and two approaches based on the idea of qc with different levels of simplification are introduced. 3.1. fully resolved approach this approach does not use any simplification. every single particle represents a node with independent dofs. all links are taken into account explicitly and contribute directly to the stiffness matrix. consequently, this approach provides the “exact” result, which is used as a reference solution for evaluation of accuracy and efficiency of the following, simplified approaches. 3.2. qc approach with interpolation in this approach, only the interpolation rule is used to simplify the full particle model. in the regions of high interest, all particles are selected as repnodes. by contrast, in the area of low interest, only the particles forming the vertices of interpolation elements are selected as repnodes. all remaining particles are kept in the model and their dofs are linearly interpolated by using the underlying interpolation mesh. such particles are called hanging nodes because their dofs are not independent but are “hanging” on appropriate repnodes. all link connections are considered explicitly in the whole domain. 3.3. qc approach with interpolation and homogenization in this approach, 2d finite elements are used not only to interpolate dofs but also to replace the stiffness that corresponds to the microstructure. consequently, a substantial number of links and hanging nodes can be removed from the particle model. material properties of 2d elements are identified by homogenization of the effective elastic stiffness tensor de representing the microstructure of the links. applying the equivalence of the overall virtual work expressed for the continuous material and for the microstructure represented by discrete links, the following formula for the effective elastic stiffness tensor can be derived: de = 1 v nt∑ i=1 lieaini ⊗ ni ⊗ ni ⊗ ni (1) here, ni is the unit vector specifying the direction of the given link, e is the young modulus, a is the cross-sectional area and l is the length of the link. the summation is taken over nt links occupying volume v . the effective stiffness tensor de can be evaluated globally for all elements, or locally for each element separately. afterwards, the material parameters of 2d elements can be identified as isotropic, orthotropic, or arbitrarily anisotropic. different homogenization procedures for disordered lattices with normal interactions are described in [3]. in this work, only the most accurate homogenization rule with a local anisotropic stiffness tensor and an arbitrarily anisotropic material is used. evaluation of the effective material stiffness tensor given by (1) is done for each element separately. the stiffness tensor of each element is obtained only from the contributions of the parts of the links that are located in that particular element. 4. adaptive algorithm adaptivity is one of the key ingredients of the qc method. the area of high interest and the geometry of interpolation mesh can be arbitrarily changed during the simulation to optimize the accuracy and computational costs. furthermore, for qc simulation of materials with softening or damage, an adaptive 81 karel mikeš, milan jirásek acta polytechnica ctu proceedings algorithm is required to be able to predict the exact failure mechanism. in this paper, it is assumed that cracked links tend to form a macroscopic crack. the qc adaptive algorithm is designed to be able to simulate the growth of existing macroscopic crack as well as the initialization of new cracks. in each simulation step, this algorithm is able to change both the area of high interest and the interpolation elements in the area of low interest. the overall algorithm of an adaptive cq simulation is organized as follows: algorithm 1 adaptive qc simulation 1: load input geometry 2: generate interpolation mesh 3: calculate local stiffness tensors and material parameters 4: assemble stiffness matrix 5: assemble load vector 6: repeat 7: solve one elastic step 8: break link with maximal strain 9: update area of high interest 10: update interpolation mesh 11: update local stiffness tensors and material parameters 12: update stiffness matrix and load vector 13: until <stopping criterion> 14: postprocessing 4.1. area of high interest update the change of the area of high interest is realized around newly cracked links to allow crack growth. when a new link is broken, all particles within a given updating radius are labeled as repnodes. this update is realized only if the breaking link is located not too far from existing macroscopic cracks. therefore, a number of previously cracked links within a checking radius is evaluated and the update is realized only if a given critical number of previously cracked links is reached. consequently, the area of high interest is updated frequently but not necessarily in all steps. this allows to account not only for crack branching but also for nucleation of new macroscopic cracks in different locations. this procedure frequently requires searching for particles within a given radius, which can be timeconsuming for large sets of particles. therefore, it is convenient to store particles in an octree structure that enables fast spatial searching. 4.2. interpolation mesh update if the area of high interest has changed, it seems natural to change the geometry of the interpolation mesh as well to provide a better shape of interpolation elements at the interface between areas of low and high interest. however, this change is extremely expensive, as explained in section 4.3, and does not bring any significant improvement of accuracy. therefore, it turns out to be more effective to use relatively small elements with fixed geometry instead of changing the size of elements during the simulation. only the elements that are completely covered by the extended area of high interest become useless and can be removed from the model. if the interpolation elements are not changed, the updating radius must be larger than the size of interpolation elements to ensure that the nodes of one interpolation element are not located on the opposite sides of the crack (otherwise, special finite element techniques such as xfem must be used). 4.3. stiffness tensors update generally, there are three reasons why stiffness tensors must be updated. (1.) breakage of a link (2.) change of a region of high interest (3.) change of the interpolation mesh the breakage of one link occurs in each simulation step. the resulting update of stiffness tensors can be simply realized by subtracting the stiffness contributions of broken links from all corresponding stiffness tensors. the change of the area of high interest is realized by adding new repnodes. consequently, the type of ending particles of some links is changed. all links changed to type repnode–repnode and some specific links changed to type repnode–hangingnode need to be solved explicitly from this moment. all such links must be identified and their stiffness influence is subtracted in the same way as for broken links. the change of the geometry of interpolation mesh requires a recalculation of local stiffness tensors. this recalculation is extremely demanding because all links must be visited. even if the mesh is changed only locally near a newly created area of high interest and only some elements are affected, it is not easy to identify which links contribute to these elements and this step significantly slows down the simulation time. the recalculation of stiffness can be sped up by storing the information of element contributions during the assemblage of stiffness tensors at the beginning of the simulation. but this can be done only at the price of additional memory demands, which may become critical for large numbers of links. 5. results to assess the efficiency and accuracy, the presented adaptive qc algorithm has been used to simulate failure in two dimensions. the l-shaped specimen with dimensions 100×100mm (which may correspond to a joint of a frame) is fixed at the bottom section and loaded by prescribed vertical displacements imposed at the right end section; see figure 1. as a result of this loading, the non-convex corner is opened 82 vol. 13/2017 adaptive quasicontinuum simulation of disordered lattices 0 20 40 60 80 100 0 10 20 30 40 50 60 70 80 90 100 ↑ ↑ ↑ ↑ ↑ ↑ 0 20 40 60 80 100 0 10 20 30 40 50 60 70 80 90 100 ↑ ↑ ↑ ↑ ↑ ↑ 0 20 40 60 80 100 0 10 20 30 40 50 60 70 80 90 100 ↑ ↑ ↑ ↑ ↑ ↑ figure 1. interpolation elements and area of high interest (black) and broken links (red) in the first (left), 100-th (middle) and the last step of simulation (right). 0 20 40 60 80 100 0 10 20 30 40 50 60 70 80 90 100 figure 2. broken links computed with exact particle approach a0 (black) and with qc approach with local anisotropic homogenization a3 (red). and cracking of links is expected to start in this region. therefore, a small initial area of high interest is prescribed around this corner; see figure 1 (left). the random microstructure is generated with a density that corresponds to 67 particles along the short edge, leading to a total of 19,000 particles connected by 64,064 links. basic characteristics of models used by full particle and qc simplified simulations are listed in table 1. material parameters are considered to be the same for all links. as expected, cracking starts in the non-convex corner. duringcrack growth, the area of high interest is increased around newly broken links. interpolation elements completely covered by the area of high interest are removed from the model, but the geometry of the remaining interpolation elements is not changed. the areas of high interest in different loading steps are depicted in figure 1. overall 240 simulation steps 0 1 2 3 4 0 5 10 15 20 25 30 35 40 45 displacement [mm] f o rc e [ n ] exact particle approach a0 qc simplified approach a3 figure 3. force-displacement diagrams computed with exact particle approach a0 (black) and with qc approach with local anisotropic homogenization a3 (red). a0 a1 a3 particles 19,000 19,000 2,199 links 64,064 64,064 2,929 elements 0 1,542 1,542 repnodes 19,000 1,665 1,665 hnaging nodes 0 17,335 534 dofs 37,799 3,129 3,129 table 1. numbers of particles, links, elements, repnodes, hanging nodes and unknown dofs for exact approach (a0), qc approaches with interpolation only (a1) and qc approach with local anisotropic homogenization (a3). are performed until the specimen is almost completely split; see figure 1 (right). three different levels of simplification are used and compared. • full (exact) particle approach (a0). • qc simplified approach with interpolation only (a1). 83 karel mikeš, milan jirásek acta polytechnica ctu proceedings • qc simplified approach with interpolation and local anisotropic homogenization (a3). the results obtained for both qc simplified approaches are almost equivalent. both approaches predict the same broken links in the same order. generally, approaches involving homogenization tend to a stiffer response in comparison with the interpolation approach. in this case, the influence of homogenization is negligible and the maximum difference in relative stiffness is just 0.24 %. in comparison with the exact approach, the qc approaches give a slightly stiffer response. the relative stiffness error of the initial elastic branch is 11.8 %. the maximum relative stiffness error is below 18 % during almost the whole simulation. only in the last six steps (when the specimen is almost completely split) the error increases up to 25 %. the prediction of cracked links is not perfectly exact, but more than 90 % of links are broken correctly and the difference in the final shape of the macroscopic crack is almost negligible; see figure 2. the simplified results are also very accurate in terms of the force-displacement diagram. despite a stiffer elastic response, the trend of the unloading branch is very well captured and the shape of the qc simplified force-displacement diagram resembles the exact solution; see figure 3. times consumed by individual parts of the simulation for various approaches are listed in table 2. the initialization procedures are realized just once at the beginning of the simulation. on the other hand, the repeated procedures are realized in each simulation step. for qc approach a1, with interpolation only, the realization of qc simplification is relatively fast, but the initial assemblage of the stiffness matrix and load vector is extremely slow because of a huge number of hanging nodes with interpolated dofs. by contrast, qc approach a3, involving local homogenization, needs a relatively long time to realize qc simplification including local effective stiffness tensors calculation. however, the initial assemblage is very fast because the number of links and hanging nodes is significantly reduced. for both qc approaches, a long extra time is needed only at the beginning of the simulation. this initial time becomes negligible in simulations with a large number of repeated steps. the most important fact is that each simulation step of the simplified approach is twice as fast for a1 and almost 19 times faster for a3 in comparison with the full particle model. 6. conclusions this paper described a qc adaptive algorithm with automatic changes of area of high interest designed for simulation of crack propagation in disordered lattices. the presented algorithm allows not only propagation of an existing crack but also initialization of new cracks. a0 a1 a3 times of initial procedures [s]: qc simplification 0.83 8.08 assemble stiffness matrix 1.09 5.81 0.15 assemble load vector 1.92 6.36 0.21 times of repeated procedures [s]: solve one step 3.99 0.25 0.15 update one step 1.08 2.31 0.12 table 2. time consumed by individual parts of simulation for various approaches. the presented example has shown that qc based approaches in combination with an adaptive algorithm are able to capture the exact macroscopic crack trajectory even if the area of crack propagation is not known at the beginning of simulation. the force-displacement diagram can be predicted with high accuracy in spite of the fact that the initial elastic response of simplified models is slightly stiffer. the adaptive extension of the area of high interest has turned out to be sufficient to obtain accurate results and a new triangulation of interpolation mesh is not necessary. the application of qc approaches provides a significant reduction of the number of unknown dofs and of links used in numerical models. this reduction leads to a significant speed-up of one simulation step, at the price of an increased cost of the initial preparation (model simplification) before the first step. if both interpolation and homogenization are applied, the speed-up factor of one step is 18.8 and the whole simulation of crack propagation (including 240 steps) gets more than 15 times faster. acknowledgements financial support received from the czech technical university (sgs project 17/043/ohk1/1t/11) and the czech science foundation (gačr projects 14-00420s and 1704150j) is gratefully acknowledged. references [1] e. b. tadmor, m. ortiz, r. phillips. quasicontinuum analysis of defects in solids. philosophical magazine a 73(6):1529–1563, 1996. [2] d. m. kochmann, j. s. amelang. the quasicontinuum method: theory and applications. in multiscale materials modeling for nanomechanics, chap. the quasicontinuum method: theory and applications, pp. 159–193. springer, 2016. [3] k. mikeš, m. jirásek. quasicontinuum method extended to irregular lattices. computers and structures 192:50–70, 2017. [4] r. e. miller, e. b. tadmor. a unified framework and performance benchmark of fourteen multiscale atomistic/continuum coupling methods. modelling and simulation in materials science and engineering 17(5):053001, 2009. 84 acta polytechnica ctu proceedings doi:10.14311/app.2019.23.0044 acta polytechnica ctu proceedings 23:44–48, 2019 © czech technical university in prague, 2019 available online at https://ojs.cvut.cz/ojs/index.php/app reconstuction of the historical building foksal 13 and 15 in warsaw tomáš ředina fg consult, s. r. o., k jezu 1937/1, 143 00 prague, czech republic correspondence: redina@fgc.cz abstract. we were asked to design geotechnical structures to enable reconstruction of historical buildings. buildings have to meet current standards so load bearing capacity had to be increased and underground garages can be built. a description of the securing construction pit is described below as well as the design of the deep foundation. the construction pit was secured by jet-grouting columns with struts and soil nails. parts of the walls were underpinned by micropile trestles. jet-grouting columns were also used for the deep foundation of existing walls. keywords: jet grouting, historical buildings, underground garages, shoring. 1. introduction and historical review the block of flats on foksal street was built in the late 19th century and, as one of the few buildings in warsaw, it survived the second world war. after the war it was initially used as rented housing, but nobody invested in their maintenance because it was intended that they be demolished due to the construction of a tram line. in the 1990s residents were evicted because of the threat to health and life. a major rebuilding began in 2016. the development of the whole project was characterized by the investor’s effort to find the easiest solution, while on the other hand the requirements of heritage conservation institutions was to protect historically valuable buildings in the centre of warsaw. the investor wanted to build for luxury living and a necessary condition for this standard was the obligation to add underground garages to the floor plans of both buildings. the compromise solution eventually became an approved version involving the demolition of the rear and middle wings of foksal 15. in place of the demolished wings, underground garages were built, while new wings of the same extent and the same facade construction were also built. the state of the building before and after reconstruction is shown in figure 1. 2. geological and hydrogeological provisions below a layer of artificial backfill with thicknesses of 1–2.3m, pleistocene formations are found. these formations reach a depth of 12–15m below the terrain and are formed with sandy clay, with a depth of consistency varying from solid to firm in the lower parts, and then very firm. locally, the layers of clay were lined with fluvioglacial sand lenses with an id=0.8–1.0. all sediments are heavily disturbed glaciotectonically. in a basic survey (april 2016), underground water was found at depths of more than 15m under the terfigure 1. historical building foksal 13 before (top) and after (bottom) reconstruction. rain. in a supplementary survey, water-filled sanded lenses were found at depths of about 4–5m below the terrain. 44 https://doi.org/10.14311/app.2019.23.0044 https://ojs.cvut.cz/ojs/index.php/app vol. 23/2019 historical building foksal 13 and 15 in warsaw 3. design solutions of underground constructions 3.1. shoring of the construction pit in order to build an underground garage, it was first necessary to demolish two original court wings. the investor’s endeavour was naturally to increase the scope for future garages. maximum excavation for newly built garages is 8.7– 11m below the existing terrain. the perimeter of the building pit is 140m, of which 110m is secured by two rows of jet grouting ø 1000mm with reinforcement pipes ø 108/12. the remaining 30m of the construction pit is secured by a microrider bracing made of profiles heb 160. all shoring walls were shotcreted with concrete of thickness 10–25 cm, secured with nail length of 4.5– 7.5m at 2–4 levels. shotcrete was performed from the maximum excavation level to 1m below existing terrain. at a level of –3.2m the pit is secured by a steel frame consisting of struts and horizontal beams. struts had lengths up to 19m. horizontal beams along the whole construction pit was made of steel profiles 2×ipe 330 to 2×ipe 400. it created a solid and stiff element that prevented the movement of the perimeter walls towards the building pit. struts were designed from heb 200 and heb 300 profiles. the longest struts were designed from a pair of sheetpiles l604. some of the struts passed through the micropile foundation. in the area of a 30m long microrider bracing, the steel frame emplaced at three levels, since no anchor elements could be used beyond the investor’s own land. the pit is captured in figure 2 while its technical scheme including all the designed solutions is shown in figure 3. 3.2. increasing the bearing capacity of existing foundations the reconstructed apartment buildings foksal 13 and 15 have been given a complete replacement of the original wooden ceilings and now all the ceilings are made of concrete. the number of floors is also increased. this resulted in an increased load. at the same time floors in the cellars were deepened. therefore, it was necessary to design appropriate special foundation elements to take account of these changed conditions [? ]. probably the only acceptable option was the use of jet grouting technology. for this purpose, columns with diameter 1000mm were designed at a spacing of 1.2–2m. the space between the columns was then filled with shotcrete. in existing cellars, floor-to-ceiling heights range from 1.45m to 2.1m and the depth of foundation walls are below the existing floor level of 5–15 cm. excavations in the cellars increased by an average of 1m and in some places where the engineering networks were designed, excavation was up to 1.8m. the greater extent of the excavations had an impact on the lengths of the jet grouting pillars as the pillars were partially exposed during excavations. finally, the lengths of the ti columns ranged from 3 to 8m. the columns of jet grouting were made from the ground floor (see figure 4), which was partly under the protection of a heritage conservation institution. in the original design of demolition, all non-load-bearing walls on the ground floor were to be demolished. based on this assumption, the starting points for drilling jet grouting were determined. however, after a renegotiation between the investor and the heritage conservation institution, changes were made and part of the non-bearing walls had to be preserved. in addition, there was a strict ban on doing any damage to the existing stairways, which were lined with marble. this resulted in a major redesign of starting points for the jet grouting columns. the cellars of neighbouring buildings have been recently refurbished and are now used as a luxury restaurant (on the west side – foksal 17) and warehouses (on the eastern side – foksal 11). due to the fact that the floors of adjoining cellars were at the same level and even deeper than the cellars of the reconstructed buildings, special constructions had to be designed along the boundaries of these buildings. at foksal 11, where the basement floor was below the existing cellar floor in foksal 13, the technology was changed. instead of the jet grouting, which would certainly damage the neighbouring basement areas, micropiles in combination with concrete beams were designed. this was done by sections and at one time only one meter of wall was under-excavated. this procedure was performed before excavations in the cellars. on the west side, the reconstructed foksal 15 building and the adjacent foksal 17 building had a common supporting wall. jet grouting columns reinforced with a 108 / 12mm tube were designed to take the vertical load and used as a shoring construction. however, before jet grouting columns were executed, a concrete beam of 1m height was made, which allowed the head of the jet grouting columns to be 90 cm below the floor of the adjacent building. 3.3. micropile underpinning of existing masonry walls part of the floor plan of the apartment buildings was located above the newly built underground garages, so it was necessary to underpin these parts. in the original technical solution, all walls were underpinned by massive jet grouting columns, and concrete walls were designed to be beside the jet grouting columns [? ]. however, this solution was space-intensive and greatly reduced the actual utility of the underground garage. therefore, another solution has been chosen. micropile towers were designed to support the structures in question. the system was made of vertical steel pile pins of 108/16mm in combination with a carrier steel grid of heb beams 200 to 300mm. micropiles 45 tomáš ředina acta polytechnica ctu proceedings figure 2. construction pit – blinding concrete in process. figure 3. layout – all the designed construction in one drawing. 46 vol. 23/2019 historical building foksal 13 and 15 in warsaw figure 4. cross section of the underground part of the building. were braced between each other, both vertically and horizontally. in the first stage, the micropiles were made from the existing yard terrain on one side of the cast wall and from the side of the basement of the building. complications were brought by drilling of micropiles just from the basement, where the spatial conditions were very tight. due to the low height of the ceiling micropiles had to consist of pieces with lengths of 1.0 and 2.0m. all micropiles were inserted into boreholes ø 160mm, filled with cement grouting. underneath the level of the future definitive excavation of the garages, injected micropile roots in lengths of 6.0– 8.0m according to load were made. in the second phase, the micropiles were fitted with steel heads and the distribution grid was proceeded with. the distribution grid was designed above the future ceiling of the 1st underground floor of the new building. the reason for this was the simple connection of the ceiling to the supporting wall. first, holes were made in the masonry to accommodate the steel cross members. then the whole steel grid was thoroughly welded. in the end, the wall perforations were filled with concrete, thereby bringing into contact the steel grid and the masonry. the third phase consisted of micropiles bracing in both directions. individual phases are indicated in figure 5. all phases in the implementation of micropile underpinning have been elaborated. all cross–sections were designed (each with bracing). the step of excavation, which could be done without bracing, was 1.5–2.0m. bracing had to be adapted for passing steel struts through the micropile towers. after the foundation slab was concreted, the process of building walls and removing bracing was begun. in total, 40 micropiles with a total length of 760m were made. the layout of micropile underpinning is shown in figure 6. polish engineers attested to the fact that this was the first time micropile underpinning was used in warsaw. 3.4. monitoring with regard to the proposed design for deep excavations creating an underground garage, a monitoring project was prepared prior to the construction work. a network of control points for geodetic measurement was set up for foksal 13 and foksal 15. some points were placed on adjacent buildings, and in total 52 measuring points were created. the frequency of measurement was adapted to the working tempo and rate of execution of the substructure. the measurement results are analyzed continuously by the building management, the research authors and include investor presence. the following tolerances have been adopted for existing buildings: [sk]u =5–7mm, [sk]n =15–18mm, [sk]u – maximal vertical displacement for serviceability limit state (sls), [sk]n – maximal vertical displacement for ultimate limit state (uls). the results of the monitoring showed that the recorded measurements were within the tolerances throughout the construction, which indirectly confirms the correctness of the design of the technologies involved and the construction of the special foundation. 4. conclusion the underground garage project on foksal 13 and 15 was a very interesting task from a design perspective. unfortunately, it was difficult to meet the changing 47 tomáš ředina acta polytechnica ctu proceedings figure 5. cross section of micropile underpinning – selected phases. figure 6. layout of micropile underpinning. requirements of the investor and the chief designer. in addition, the language barrier did not do much to facilitate the project. the project was prepared in czech and english with minor changes being made in polish. however, all the difficult situations that emerged have been positively addressed by intensive communication with the investor and the management of the building. proof of this is that the underground garage construction is now successfully completed. references [1] k. terzaghi, r. peck. soil mechanics in engineering practice. second edition. wiley & sons inc, new york, 1967. [2] j. lehtonen, v.-v. hyyppä. classification of micropile underpinning methods exemplified by projects in turku. electronic journal of geotechnical engineering 15:295–310, 2010. 48 acta polytechnica ctu proceedings 23:39–43, 2019 1 introduction and historical review 2 geological and hydrogeological provisions 3 design solutions of underground constructions 3.1 shoring of the construction pit 3.2 increasing the bearing capacity of existing foundations 3.3 micropile underpinning of existing masonry walls 3.4 monitoring 4 conclusion references acta polytechnica ctu proceedings doi:10.14311/app.2020.26.0019 acta polytechnica ctu proceedings 26:19–23, 2020 © czech technical university in prague, 2020 available online at https://ojs.cvut.cz/ojs/index.php/app cement paste containing micronized recycled concrete influence of hardening accelerators on the modulus of elasticity jakub ďureje∗, zdeněk prošek czech technical university in prague, faculty of civil engineering, department of mechanics, thákurova 7, 166 29 prague 6, czech republic ∗ corresponding author: jakub.dureje@fsv.cvut.cz abstract. the article deals with the selection of a suitable hardening accelerator for cement composite material for mass production of masonry blocks. the blocks contain cement and finely ground recycled concrete in ratio 1:1. three different hardening accelerators in different quantity were tested for possibility of fast removing formwork. the dynamic modulus of elasticity was measured by non-destructive resonance method to determine the initial strengths. the modulus of elasticity was measured 7 and 12 hours after sample production to determine the increase in initial strength. subsequently, the modulus of elasticity was measured at 1, 7, 14, 21 and 28 days after production of the test specimens. the resulting moduli of elasticity were compared with reference samples. keywords: finely ground concrete, hardening accelerator, recycled concrete, recycled masonry blocks. 1. introduction old waste concrete from damaged or destroyed structures has the risky properties because of different attributes and inhomogeneity of this waste, which are caused by the presence of other materials from building demolitions (bricks, wood, glass, reinforcement, etc.). the quality of the recycled concrete thus depends on the degree and diligence of the recycling of the recycled concrete. unfortunately, waste concrete sorting is negatively reflected in the price of this raw material. recycled concrete is most often used as a base layer under the roadway. for the common application, a recycle fraction greater than 1 mm is used, the use of finer fractions brings many complications and additional financial costs [1, 2]. the paper deals with the measurement of the dynamic modulus of elasticity of samples from the early hardening phase up to the age of 28 days. the samples contain cement and recycled concrete in weight ratio 1:1, mixing water and various hardening accelerators. recycled concrete containing the finest fraction 0/1mm was used, this material was gently ground by high speed mill. the use of this fine fraction, which today has almost no use, is an advantage of this material, because fine recycled concrete fractions are often stored in landfills as waste material, it follows that using of this material is cost effective and environmental friendly. the finely ground recycled concrete is used in cement composite as a filler with certain binding properties [3]. this cementitious composite will be lightened and will be used to produce thermal insulation perimeter blocks. due to the possibility of mass production of blocks on the production line, a rapid increase in initial strength is needed because of the blocks formwork can be removed and used for the production of other blocks in the shortest possible time. for this reason, different hardening accelerators have been added to the mixtures, and the modulus of elasticity has been measured since the early hardening phase [4]. from previous experiments it has been shown that with increasing percentage of recycled concrete in the mixture there is a decrease in compressive strength. to maintain a mixture satisfactory compressive strength, it is planned to add suitable additives to the cement composite, or to reinforce the blocks by scattered reinforcement [5, 6]. 2. materials and specimens a total of 9 sample sets were made, each set containing 3 test specimens. all test samples contained cement and finely ground recycled concrete in ratio 1:1. the water ratio w/c + r was 0.32 for all samples. the sets differed in the solidification and hardening accelerator used and its amount. one set was a reference without the use of a solidification and hardening accelerator. to produce samples, portland cement cem i 42.5 r from radotín from the českomoravský cement company was used. recycled concrete comes from drainage gutters. a fraction 0/16 mm was used, this material was grinded by the high speed mill. the resulting grain size was under 0.25 mm. the fineness of the ground recycled concrete was similar to the cement, which has a surface area of 361 m2/kg. based on previous works, three types of solidification and hardening accelerators has been selected [7, 8]. betodur gold (b) is a chloride-free liquid additive that accelerates the hardening of mortars and concretes containing portland and mixed cements. ekosal l (e) is a liquid alkali-free solidification and hardening 19 https://doi.org/10.14311/app.2020.26.0019 https://ojs.cvut.cz/ojs/index.php/app jakub ďureje, zdeněk prošek acta polytechnica ctu proceedings designation product based on normal dosing density ph [wt. % of cement] [kg/m3] [-] b non-chloride liquid 1.0 – 3.0 1450 ± 30 5 – 7 e non-alkaline liquid 2.0 – 8.0 1320 ± 30 2 – 4 s polycarboxylates 0.2 – 2.5 1050 4.5 table 1. selected hardening accelerators and its properties. micronized hardening water ratio set / material cement concrete recyclate accelerator w/c + r [g] [g] [ml] [-] ref 750 750 0.00 0.32 b5 750 750 3.75 0.32 b10 750 750 7.50 0.32 b15 750 750 11.25 0.32 e5 750 750 11.25 0.32 e10 750 750 7.50 0.32 e15 750 750 11.25 0.32 s1 750 750 11.25 0.32 s2 750 750 7.50 0.32 s3 750 750 11.25 0.32 table 2. composition of the samples. accelerator that is used primarily for dry-sprayed concrete production. unlike commonly used hardening accelerators, it is not strongly alkaline and does not adversely affect the environment and workers’ health. viscocrete 20 gold (s) is a versatile, highly efficient polycarboxylate-based superplasticizer (pce), particularly suitable to produce concrete with the requirement of a rapid rise in initial strength (table 1). the water ratio was chosen based on previous measurements and hardening accelerator quantities as recommended by the manufacturer. an amount of 5, 10 and 15 l of setting and hardening accelerator per m3 of mixture was tested (table 2). for the production of specimens, 40× 40× 160 mm laboratory molds were used. the samples were removed from the mold after 6 hours and stored under water in a laboratory environment at a temperature of 21± 2 ◦c and a relative humidity of 50± 5% for 28 days. 3. experimental methods for all samples, the dynamic modulus of elasticity was measured at the age of 7 and 12 hours and subsequently at the age of 1, 7, 14, 21 and 28 days. a resonance method was used to measure the dynamic modulus of elasticity of samples. advantage of this method is a speed of measurement and repeatability because it is non-destructive method. the method is based on the calculation of dynamic modulus of elasticity and dynamic shear modulus based on measured fundamental resonance frequencies of longitudinal, transverse (bending) and torsional oscillation. the resonance frequencies were measured in the laboratory using the brüel & kjær measuring system, consisting of the brüel & kjær type 4519-003 response sensor, the brüel & kjær type 8206 impact hammer, the brüel & kjær front-end 3560b-120 and the control computer. the sample was placed on the supports of sufficiently low stiffness so that the supports were placed at the nodal points of the first waveform corresponding to the given oscillation – longitudinal, transverse or rotational (fig. 1). the response sensor (s) was attached on the sample to the appropriate position and the oscillation was excited by the impact hammer (b). both signals, response and excitation were recorded and converted to a frequency domain using a fast fourier transform. subsequently, the pulse labshop software version 14.0.1 has calculated the transmission function. based on the resonance peaks in the transmission function, the fundamental resonance frequency of the given oscillation was determined. from the measured resonance frequency, weight, and sample dimensions, the dynamic modulus of elasticity and shear modulus were determined [9]. 20 vol. 26/2020 cement paste containing micronized recycled concrete figure 1. measurement basic natural frequencies from longitudinal (left), transverse (center), and torsional (right) oscillations [10]. 4. results and discussion the dynamic modulus of elasticity value of each set was calculated by the arithmetic mean of all 3 samples in the set. samples were always measured and weighed. the results were calculated on the basis of measured frequencies of longitudinal oscillations (fig. 2). to verify the correctness of the results, the values of the dynamic modulus of elasticity from the transverse oscillation were calculated too. moduli of elasticity from the transverse oscillation are almost identical with the values of the moduli of elasticity from the longitudinal oscillation, which confirms the correctness of the measurement. from the growth of the dynamic modulus of elasticity edyn follows that the maximum increase in edyn is within the first 7 days. between 7 and 28 days edyn growing only slightly. the rapid increase in the modulus of elasticity in the initial hours is important for the mass production of blocks. the highest increase in the modulus of elasticity after 8 hours was achieved by samples containing accelerator of hardening b (fig. 3). with the increasing amount of hardening accelerator, the modulus of elasticity also increased, but the increase in the modulus of elasticity from 10 to 15 l/m3 (7 % modulus increase) was not as pronounced as from 5 to 10 l/m3 (22 % modulus increase). the increase in the modulus of elasticity of b15 samples versus reference samples was 93 %. for samples containing the e hardening accelerator, there was only a slight increase in the modulus of elasticity compared to the reference samples (13 %). we can conclude from the measured values that the modulus of elasticity increased immediately after sample production, which would correspond to the primary purpose of using this hardening accelerator for sprayed concrete. samples with s accelerator of hardening could not be removed from the mold at 8 hours after production due to low strength. samples were removed from the mold after 24 hours. measurement of dynamic modulus of elasticity for samples with hardening accelerator in amount of 5 l/m3 was successfully at the age of 24 hours, in amount of 10 and 15 l/m3 at the age of 7 days. for set edyn [gpa] ref 1.5 b5 2.2 b10 2.7 b15 2.9 e5 1.3 e10 1.7 e15 1.7 table 3. modulus of elasticity after 8 hours. samples s2 and s3 there were a significant decrease in the modulus of elasticity compared to other specimens due to many cracks. the modulus of elasticity after 28 days was approximately the same for all samples, ranging from 20.5 to 20.7 gpa, except for s2 and s3 specimens. for s2 samples, the modulus of elasticity decreased by 36 % compared to reference samples and s3 samples decreased by 20 %. 5. conclusion the paper deals with the selection of the most suitable solidification hardening accelerator to produce concrete blocks containing micronized recycled concrete. three different hardening accelerators (b, e and s) have been selected based on previous measurements and recommendations from manufacturers. to determine the efficiency of the hardening accelerators, the samples were removed from the molds after 6 hours and the dynamic modulus of elasticity was then measured by the resonance method. based on results, it can be concluded that: • the greatest increase in the dynamic modulus of elasticity of the mixtures occurs in the first 7 days of hardening. most then within 24 h. • as the volume of hardening accelerators increases, the modulus of elasticity also increases, but nonlinearly – a suitable amount of hardening accelerator needs to be determined. 21 jakub ďureje, zdeněk prošek acta polytechnica ctu proceedings figure 2. development of dynamic modulus of elasticity from 0 to 28 days. figure 3. development of dynamic modulus of elasticity from 7 to 12 hours. 22 vol. 26/2020 cement paste containing micronized recycled concrete • hardening accelerators b and e do not negatively affect the final modulus of elasticity even in the highest used amount. • the e accelerator of hardening is likely to increase the modulus of elasticity very soon, but it has not been able to achieve sufficient strength for earlier removal from the mold at the used amount. • hardening accelerator s proved to be unsuitable, samples were removed from the mold after 24 hours and the modulus of elasticity was lower than for reference samples. • hardening accelerator b could be used to increase initial strengths, after 8 hours the samples had almost double modulus of elasticity compared to reference samples. 6. acknowledgements this work was financially supported by the czech technical university in prague the project sgs16/201/ohk1/3t/11, by the ministry of industry and trade the research project fv20503. the authors also thank to the center for nanotechnology in civil engineering at the faculty of civil engineering, czech technical university in prague. references [1] a. barbudo, f. agrela, j. ayuso, et al. statistical analysis of recycled aggregates derived from different sources for sub-base applications. construction and building materials 28(1):129–138, 2012. doi:10.1016/j.conbuildmat.2011.07.035. [2] i. vegas, j. a. ibáñez, a. lisbona, et al. pre-normative research on the use of mixed recycled aggregates in unbound road sections. construction and building materials 25(5):2674–2682, 2011. doi:10.1016/j.conbuildmat.2010.12.018. [3] a. barbudo, f. agrela, j. ayuso, et al. mechanical properties of recycled binder/micro-filler cement-based material. construction and building materials 1054(1):234–237, 2014. doi:10.4028/www.scientific.net/amr.1054.234. [4] m. m. sabai, m. g. d. m. b. cox, r. r. mata, et al. concrete block production from construction and demolition waste in tanzania. construction and building materials 72(1):9–19, 2013. doi:10.1016/j.resconrec.2012.12.003. [5] m. lidmila, j. topič, t. plachý, et al. influence of increasing amount of recycled concrete powder on mechanical properties of cement paste. in iop conference series: materials science and engineering, vol. 236, p. 012094. iop publishing, 2017. doi:10.1088/1757-899x/236/1/012094. [6] j. hrůza. design of masonry elements with use of recycled concrete, 2016. [7] e. anastasiou, k. g. filikas, m. stefanidou. utilization of fine recycled aggregates in concrete with fly ash and steel slag. construction and building materials 50:154– 161, 2014. doi:10.1016/j.conbuildmat.2013.09.037. [8] f. cartuxo, j. de brito, l. evangelista, et al. rheological behaviour of concrete made with fine recycled concrete aggregates–influence of the superplasticizer. construction and building materials 89:36–47, 2015. doi:10.1016/j.conbuildmat.2015.03.119. [9] j. ďureje, z. prošek, j. hrůza, p. tesárek. cement matrix containing micronized recycled concrete and the influence of additives. acta polytechnica ctu proceedings 15:20–24, 2018. doi:10.14311/app.2018.15.0020. [10] čsn 73 1372 nedestruktivní zkoušení betonu–rezonačnní metoda zkoušení betonu, 2012. 23 https://doi.org/10.1016/j.conbuildmat.2011.07.035 https://doi.org/10.1016/j.conbuildmat.2010.12.018 https://doi.org/10.4028/www.scientific.net/amr.1054.234 https://doi.org/10.1016/j.resconrec.2012.12.003 https://doi.org/10.1088/1757-899x/236/1/012094 https://doi.org/10.1016/j.conbuildmat.2013.09.037 https://doi.org/10.1016/j.conbuildmat.2015.03.119 https://doi.org/10.14311/app.2018.15.0020 acta polytechnica ctu proceedings 26:19–23, 2020 1 introduction 2 materials and specimens 3 experimental methods 4 results and discussion 5 conclusion 6 acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2017.13.0085 acta polytechnica ctu proceedings 13:85–88, 2017 © czech technical university in prague, 2017 available online at http://ojs.cvut.cz/ojs/index.php/app water absorption capacity coefficient and mass moisture of rammed earth material barbora mužíková∗, tereza otcovská, pavel padevět czech technical university in prague, faculty of civil engineering, thákurova 7, 166 29 prague 6, czech republic ∗ corresponding author: barbora.muzikova@fsv.cvut.cz abstract. this article presents the development of mass moisture of rammed earth material and determines the water absorption capacity coefficient for rammed earth with illite-kaoline clay. specimens of two prescriptions were rammed in the moulds. they were settled in the box with soft foam that was moistened. the level of moistening was kept constant. the specimens were regularly measured and weighted. two measuerements were carried out – one of montmorillonite clay during 13 days and one of illite-kaoline clay during three hours. the development of moisture increase was captured and the water absorption capacity coefficient for illite-kaoline clay was determined and compared to coefficients of common building materials. keywords: rammed earth, illite-kaoline clay, mass wetness, water absorption capacity coefficient. 1. introduction this article presents earth material as a modern building material based on the long history of using, but in last centuries it was replaced by modern materials. the material characteristics were never fully researched due to the replacement. the demand for the earth material is increasing nowadays following the principles of sustainable buildings development [1, 2]. the main advantages of using earth as a building material are good air humidity balance, lower costs for material and construction, and also the fact that the earth based material has a positive effect on human health. rammed earth is accessible, environmentally friendly and suitable for low-cost building in person. according to the sustainable buildings development not only the mechanical properties are important in the structure design. the quality of building material is also given by economic and socio-cultural criteria. for example the rammed earth primary energy consumption connected with production of building material is about 44 kwh/m3. this is 18× less than for prefabricated concrete material [3–6]. the increasing interest in unburned earth material can be seen in the sales of unburned clay bricks in portfolio of heluz company shown in table 1 [7]. the mechanical properties of the unburned earth depend mainly on the composition and the method of processing. the basic methods of using earth are mudwalls, rammed earth, earth bricks, earth infill in a timber frame construction etc. the material which the research is focusing on is rammed earth. rammed earth walls are constructed by the compacting (ramming) of moistened subsoil into place between temporary framemwork panels. the process of building is shown in figure 1. first of all, the reinforced plywood framework is built and then the first layer moist earth (mixture of sand, gravel year sold unburned bricks [pcs] 2010 700 2011 8,600 2012 19,800 2013 21,500 table 1. number of sold unfired bricks of heluz company [7]. figure 1. the process of building rammed earth wall. and clay) is filled in. secondly, the layer of moist earth is compressed by a pneumatic backfill tamper. thirdly, next layer of moist earth is added and then succesive layers of moist earth are added and compressed. finally, the framework is removed leaving the rammed earth wall with visible layers of compacted earth. when dried, the result is a dense, hard monolithic wall [4–6]. 85 http://dx.doi.org/10.14311/app.2017.13.0085 http://ojs.cvut.cz/ojs/index.php/app b. mužíková, t. otcovská, p. padevět acta polytechnica ctu proceedings figure 2. the production of rammed earth specimens. 2. testing of unfired earth material 2.1. rammed earth and water the rammed earth material is considered for building bearing walls of construction so that the material is often exposed to rain and moisture of external conditions. all materials with open porous structures like loam are able to store and transport water within their capillaries. water always travels from regions of higher humidity to regions of lower humidity. the value of the water absorption coefficient w is known for common building materials, some of them are shown in table 2. in the research the value for rammed earth with illite-kaoline clay was searched for to be compared with values of other building materials. the premise was that the value of the water absorption coefficient w for rammed earth would be higher than for common building materials. 2.2. preparing material prescriptions for the specimens were designed of clay, sand and water. there were two prescriptions gem i and gem ii of montmorillonite clay and agl i and agl ii of illitic-kaolinite clay. prescriptions i and ii differed in the sand/clay ratio. the amount of the components was expressed by the weight percent. the amount of water was settled as a water/clay ratio, same at both prescriptions, all prescriptions are shown in table 3. figure 3. specimens in the climatic chamber. material w ρ [kg/m2 √ hour] [kg/m3] silty loam 3.7 1900 clayer loam 1.6 1940 cement concrete 1.8 2290 hollow brick 8.9 1165 solid brick 25.1 1750 table 2. water absorption capacity coefficient of common building materials [5]. first of all, the sand was well mixed with two thirds of water. secondly, the clay and the rest of water was added. it was mingled by a drilling machine with a special ending. specimens of size 40×40×160 mm were prefabricated for the testing in the laboratory. the production was made by ramming into moulds by hand and by drilling machine. the moulds were wiped by oil and the earth was rammed in four to five layers. layers were compresed by a mechanical tamper and the last one was always compresed by hand. produced specimens are shown in figure 2. specimens were tested after four weeks from production. by this time the humidity from the prefabrication is stable and is not changing any more. the hardening process of clay is based only on the drying out the mixed water. during these four weeks the specimens were settled in constant condition of temperature 20 °c and moisture 50 % in the climatic chamber (specimens in climatic chamber figure 3). 2.3. practical tests firstly, specimens were put in the covering of nylon to avoid the erosion of particles from the submerged surface. secondly, they were weighted and finally settled in the box with soft foam that was moistened. the level of moist was constant, the water was added during the test. the specimens were regularly measured and weighted. some of the specimens were broken during the test, they were removed from the testing and not evaluated. 86 vol. 13/2017 water absorption capacity coefficient of rammed earth material prescription type of used clay sand [%] clay [%] water/clay ratio [-] bulk density [kg/m3] agl i illite-kaoline 80 20 0.37 2191 aglii illite-kaoline 75 25 0.37 2159 gem i montmorillonite 80 20 0.37 2073 gem ii montmorillonite 75 25 0.37 2040 table 3. prescriptions of used specimens and bulk densities of final specimens. twenty specimens of gem and ten specimens of agl were tested. the test of gem was 14 days long and the agl was evaluated for three hours of the test beginning. 3. evaluation of the measurement 3.1. mass moisture development the mass moisture mw was calculated from measured weigh using relation: mw = m−md md . (1) the mass moisture mw was calculeted as a weight of the wet specimen m minus the weight of dry specimen md and then it was divided by the weight of the dry specimen md (the weight of the specimen in the beginning of the test) and it was multiplied by 100 to transform it to percentage of weight. the data of specimen with different starting weight can be compared with the mass moisture. the data represented the specimens that were not broken during the whole testing. they had the same conditions at the same time. the mass moisture increase is shown in the figure 4. the mass moisture in percent is shown on the vertical axis and the time (in hours or days) is shown on the horizontal axis. 3.2. water absorption capacity coefficient the water absorption cofficient w determines the ability of material absorp water by an area in defined time period. the formula for its calculation is: w = w√ t . (2) units of w are kg/m2√t. the quantity of water w that can be absorbed by an area a is defined by the formula: w = m−md a . (3) units of weight of the wet specimen m and weight of the dry specimen md are in kg and the time t is measured in hours. the weights were measured in a time period of one hour for agl prescriptions. the absorbing area of specimens is 40×40 mm. the coefficient w was calculated for prescription agl i and agl ii. figure 4. mass moisture development of prescriptions agl and gem. 4. discussion the mass moisture development of agl (blue column) and gem (red column) is shown in figure 4. the mass moisture development of agl i rose to 1.39 ± 0.28 % and of agl ii to 1.32 ± 0.16 % after four hours. the biggest increase was in the first hour of testing then the mass moisture rose quite slower. the specimen of gem i rose to 14.89 ± 0.13 % and of gem ii to 16.70 ± 0.92 % after 14 days. the trend of development was simillar to the first measurement, the biggest change was in the first days then it slowed down. the coefficient w was calculated to 2.80± 0.55 kg/m2√t for agl i and 2.70± 0.30 kg/m2√t agl ii. the calculated values were compared to known characteristics of common building materials as it is shown in figure 5. the coefficients w for two types of loam, cement concrete and two types of bricks were compered to 87 b. mužíková, t. otcovská, p. padevět acta polytechnica ctu proceedings figure 5. water absorption capacity coefficient of common building materials and tested rammed earth agl. illitic-kaolinite rammed earth material. the results of made tests agl are between the values of silty and clayey loam. the coefficient w for agl i is only 1.56× higher than for cement concrete. the value of agl i is 0.11× lower in comparation to the solid brick. the value above 10 kg/m2√t is consider to be not so good for building material. non of the prescriptions exceeds this limit, so in the point of view of the coefficient w, the rammed earth material with illite-kaoline clay is competitive to other common building material. 5. conclusion the rammed earth material properities in relation to mass moisture and water absorption capacity coefficient was observed. two kinds of specimens were prepared, one prescription of illite-kaoline clay and the other of montmorillonite clay. the increasing of mass moisture in rammed earth of illite-kaoline clay in three hours and montmorillonite clay in forteen days was observed. the progress of mass moisture increasing was fastest in the beginnig of moistenig for both cases. the other observed value was the water absorption capacity coefficient w for rammed earth with illite-kaoline clay (prescriptions agl i and agl ii). the calculated values are comperative to other buiding material as cement concrete and they are according to the values found in literaterature [5] for silty loam and clayey loam. from that point of view, rammed earth have better properities than for example the solid brick. generally the calculated values of water absorption capacity coefficient are lower than it was expecting in the begining of the test. acknowledgements this research was financial supported by the faculty of civil engineering at ctu in prague (sgs project no. 16/201/ohk1/3t/11). authors gratfully aknowledge free supply of material necessary for experimental measurement to lb minerals, s.r.o. company. references [1] p. suske. hlinené domy novej generacie. first printing. alfa bratislava, 1991. [2] p. suske. nepalena hlina v moderni architekture. dokument era21: ekologie, realizace, architektura (2):69–70, 2004. [3] b. little, m. morton. building with earth in scotland: innovative design and sustainability. first printing. scottish executive central research unit, 2001. [4] j. norton. building with earth, a handbook. first printing. intermediate technology development group limited, 1986. [5] g. minke. building with earth design and technology of sustainable architecturer. second printing. birkhauser publishe, 2006. [6] i. zabickova. hlinene stavby. first printing. era vydavatelstvi, 2002. [7] i. zabickova, t. otcovska. compressive strength of unburned clay masonry. applied mechanics and materials pp. 31–34, 2016. 88 acta polytechnica ctu proceedings 13:86–89, 2017 1 introduction 2 testing of unfired earth material 2.1 rammed earth and water 2.2 preparing material 2.3 practical tests 3 evaluation of the measurement 3.1 mass moisture development 3.2 water absorption capacity coefficient 4 discussion 5 conclusion acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2019.22.0133 acta polytechnica ctu proceedings 22:133–138, 2019 © czech technical university in prague, 2019 available online at http://ojs.cvut.cz/ojs/index.php/app characterization of lightweight lime mortars containing brick waste powder martin vyšvařil∗, tomáš žižlavský, patrik bayer, pavla rovnaníková brno university of technology, faculty of civil engineering, veveří 331/95, 602 00 brno, czech republic ∗ corresponding author: vysvaril.m@fce.vutbr.cz abstract. this work deals with the effects of an expanded perlite (ep) on the hardened properties of air lime mortars with a 50% brick waste powder content. the influence of different substitution of quartz sand by ep is also monitored here. a relationship between ep content and physical-mechanical properties of mortar (flexural and compressive strength, pore system) and mortar frost resistance has been found. very good contact between the binder matrix and ep surface was observed in microstructural images of mortars that conclusively exhibits the formation of products by pozzolanic reaction on the surface of ep. this work could be helpful for the establishment of the essential proportions of ep and quartz sand to be used in air lime mortars designed for restoration works. keywords: lightweight mortar, lime, brick powder, perlite, strength, porosity, frost resistance. 1. introduction lightweight aggregates are used in plasters to improve their acoustic and thermal insulation, or fire resistance [1–5]. lessening the density of mortars often causes a strength reduction and porosity enhancement, which limits the utilization of the mortar. expanded perlite, expanded clay, expanded glass, and hollow microspheres are lightweight materials that are pozzolan active and can therefore afford additional advantages in mortars by improving their mechanical properties. nowadays, expanded perlite (ep) is used as a component of lightweight cement or lime-cement pre-mixed coating mortars fulfilling today hygrothermal requirements on buildings performance [6]. these materials are usually hydrophobized, consequently they show worse properties in the comparison with traditional lime mortars, mainly a limited moisture transport to the plaster surface. application of these renovation mortars led to the shift of level of capillary rise above the renovated part of masonry [7, 8]. recently, the application of ep in air lime mortars proposed for restoration works was reported [9–11]. in this work, the influence of ep on the mechanical properties, microstructure and frost resistance of lime-brick powder mortars are investigated. waste brick powder was used to improve the strength characteristics of lime mortar as a cement replacement due to the suppose applications of mortars in restoration works. it has previously been found that waste brick powder from skiving of heat-insulating brick is a material with very good pozzolanic properties and it has a great potential in lime mortars production [12]. according the latest results, the combination of ep and waste brick powder in lime mortars seems to be auspicious [13, 14]. 2. materials and experimental procedures commercial lime cl 90-s supplied by carmeuse czech republic, ltd. was used to prepare mortar samples. the brick waste powder from the heluz brickworks factory, v.o.s. located in hevlín (czech republic) was added as a pozzolanic admixture to improve the strengths and frost resistance of hardened mortars. pozzolanic activity of the brick waste powder determined by the chapelle test was 348mg ca(oh)2/g after 1 d treatment and 556mg ca(oh)2/g after 5 d treatment. natural quartz sand (fraction 0/2mm from českomoravský štěrk, inc., hulín) and expanded perlite (experlit 180, fraction 0/2mm from perlit, ltd., šenov u nového jičína) were used as aggregates. some physico-mechanical properties of the ep, as imparted by the manufacturer, are summarized in figure 1 together with a granulometry of all raw materials. the chemical composition of initial materials is stated in table 1, the mineralogical composition in table 2, respectively. the mixtures composition is given in table 3. the water:binder coefficient was different for each mixture to achieve a normal fresh mortar consistency (160 ± 5mm). the composition of mortar mixtures considers the effort to prepare the lightweight mortars with sufficient strength, so the resulting binder/aggregate volume ratios are not constant. by reason of high water absorption of dry expanded perlite, this was immersed for 24 hours in water and subsequently mixed with dry mortar components. the prismatic moulds of size 40 × 40 × 160mm were used for casting of the mortar mixtures. hardened mortar samples were demoulded after 48 h and stored at ambient temperature (22 ± 2 °c) and rh of 50 ± 5%. the mechanical properties of mortars were determined 133 http://dx.doi.org/10.14311/app.2019.22.0133 http://ojs.cvut.cz/ojs/index.php/app m. vyšvařil, t. žižlavský, p. bayer, p. rovnaníková acta polytechnica ctu proceedings sio2 al2o3 fe2o3 cao mgo k2o na2o p2o5 tio2 so3 l.o.i. lime 0.9 0.7 0.4 68.1 1.3 0.5 0.2 <0.1 0.1 0.3 27.9 brick powder 57.4 14.6 5.5 9.8 4.0 2.8 1.4 0.1 0.3 1.2 0.7 quartz sand 86.8 6.5 1.2 0.5 0.3 2.3 0.9 <0.1 0.2 0.1 1.0 ep 68.0 16.0 1.9 4.5 0.4 2.5 4.6 0.1 0.1 <0.1 0.3 table 1. chemical composition of initial materials (mass %). mineral lime brick powder quartz sand ep albite 13.5 1.9 anatase 2.8 anorthite 3.2 biotite 3.0 2.8 brucite 0.5 calcite 1.8 epidote 3.6 hematite 3.1 0.7 microcline 12.4 3.9 muscovite 8.9 orthoclase 2.9 3.0 porthlandite 97.1 quartz 31.2 75.6 0.4 rutile 3.1 sandine 2.4 0.6 staurolite 6.1 amorphous phases 19.6 1.1 90.8 table 2. mineralogical composition of raw materials obtained by xrd analysis (mass %). figure 1. particle size distribution of raw materials and selected properties of expanded perlite. mixture hydrated lime brick powder quartz sand ep h2o [g] [g] [g] [g] [ml] ref-l 100 400 120 ref-lb 50 50 400 85 lbp-i 50 50 50 135 lbp-ii 50 50 12.5 37.5 125 lbp-iii 50 50 25 25 105 lbp-iv 50 50 37.5 12.5 100 table 3. mortar mixtures composition. 134 vol. 22/2019 characterization of lightweight lime mortars. . . after 7, 28 and 90 curing days. flexural and compressive strengths were measured according to en 1015-11. the water absorption of hardened mortars was determined according to en 13755 after 28 curing days. six mortar specimens at least were used to conduct each of the mentioned tests. the porosity of the specimens was assessed using a mercury intrusion porosimetry (mip). the microstructure images of mortars were obtained by a scanning electron microscope (sem). frost resistance tests were performed in accordance with czech standards čsn 722452 after 28 curing days. the test required 15 freeze-thaw cycles. one cycle was comprised of 6 h freezing at -20 °c and 12 h thawing in 20 °c water. the frost resistance coefficient df was evaluated as the ratio of flexural strength of samples subjected to 15 freeze-thaw cycles to the flexural strength of non-frozen samples. 3. results and discussion the bulk densities of reference mortars were considerably higher compared to the mortars with ep (figure 2). the reference samples decreased the bulk density due to gradual drying over time. on the other hand, lightweight mortars slightly increased their bulk density over time due to the higher porosity of the samples and their easier carbonation. the strengths of mortars increased over time because of mortar hardening. the partial replacement of lime by brick waste powder resulted in improvements in the strengths of the mortar, the compressive strength particularly (figure 3). it agrees with the previous observation [12]. hydrated products formed by pozzolanic reaction of the brick powder provided the higher strength of the modified lime mortar. the slightly lower strengths of the ref-l sample were also caused by higher amount of mixing water. the capability of the applied brick waste powder to utilization in lime plasters was thus confirmed. the lightweight samples with ep reached practically the same values of strengths as the reference samples but with the marked reduction in bulk density of the mortars. it can be stated that the strengths of lightweight mortars were almost independent on content of ep (just lbp-ii sample showed slightly lower strengths). the effect of ep content on the values of strengths is out of accord to the previous observations on lime-brick powder plasters with expanded clay aggregate, where the strengths decreased with increasing amount of lightweight aggregate [15]. pore size distribution in the mortar samples, evaluated by mip, is compared in figure 4. the reference mortars included mainly large capillary pores with radii of 0.3 − 1µm, which is typical of lime mortars. these mortars were evidently less porous than lightweight mortars (see table 4). the use of ep as an aggregate caused dramatically growth in porosity and also changed the pore size distribution in the mortars. the lightweight mortars contained a markedly higher proportion of larger pores with a diameter exceeding 1µm. in all mortars with ep, a second growth of cumulative pore volume is present in an area of pore diameter between 0.01 and 0.1µm. these smaller pores are commonly observed in cement mortars; they are formed by the network of csh [3]. the presence of ep in mortars results in the development of a csh gel network exhibiting an increased number of these small pores. this is therefore another proof of the pozzolanic reaction of ep with a lime binder. the volume of these pores is slightly higher in lbp-iii sample, which may advert to higher content of csh phases; this also correlates with the highest values of strengths of this mortar. the open porosity of mortars (table 4) expectantly enlarged with increasing amount of ep. the water absorption of mortars grew hand-in-hand with increasing open porosity. the frost resistance of mortars was examined by 15 freeze-thaw cycles; flexural strengths of the mortars were determined after this period and the frost resistance coefficient (df ) for each mortar was established, table 4. the reference mortar mixtures did not meet the standard criterion of df > 0.75 because of their disintegration after 5 freeze-thaw cycles. only the mortars with adequate ratio of quartz sand and ep (lbp-iii and lbp-iv) conformed to the frost resistance standard criterion. the lime-brick powder mortar with quartz sand/ep mass ratio of 1:1 (lbp-iii) was rated, based on its properties, as the best among the studied mortars. for this reason, only the microstructure images of lbp-iii mixture are shown in the paper (figure 5). the microstructure of mortar samples was determined at 28 curing days by sem. in the images, the typical microstructure of expanded perlite is observed – large porous amorphous objects. the crystals of calcite and csh amorphous phases can be identified in the mortar matrix. the close contact between the mortar matrix and surface of ep in interfacial transition zone (itz) is clearly conspicuous in a more detailed image in the center of figure 5. image in the right exhibits the formation of products of pozzolanic reaction on the surface of ep. 4. conclusions the effect of non-hydrophobized expanded perlite (ep) on the mechanical properties, microstructure, and frost resistance of blended lime-brick powder mortars is investigated in this paper. it has been observed that the partial replacement of lime by brick waste powder causes a slightly strength enhancement of mortars and reduces the amount of mixing water. the capability of the applied brick waste powder to utilization in lime plaster was thus confirmed. the application of ep in the mortar mixture is a positive factor: the lightweight samples with ep have practically the same strengths as the reference samples but with the appreciable reduction in bulk density. expanded perlite significantly changes the pore size distribution in the mortars by 135 m. vyšvařil, t. žižlavský, p. bayer, p. rovnaníková acta polytechnica ctu proceedings figure 2. bulk density of mortars over time. figure 3. flexural and compressive strength of mortars over time. mortar mixture open porosity water absorption df [%] [%] [-] ref-l 40.2 18.0 ref-lb 33.5 15.8 lbp-i 63.3 66.7 0.66 lbp-ii 62.8 62.3 0.70 lbp-iii 62.2 53.5 0.97 lbp-iv 60.1 46.4 1.02 table 4. open porosity, water absorption and frost resistance coefficient (df ) of mortars. 136 vol. 22/2019 characterization of lightweight lime mortars. . . figure 4. cumulative volume of pores in mortars after 28 curing days. figure 5. sem images of lightweight mortar lbp-iii: left – porous structure of ep in the mortar; in the center – view on interlocking microstructure in the mortar; right – detail of itz between mortar matrix and expanded perlite. the formation of large pores with a diameter between 1 and 10µm and also medium capillary pores formed by the network of csh. the usage of ep as an aggregate dramatically increases the open porosity of mortars resulted in improvement of their frost resistance. the lime-brick powder mortar with quartz sand/ep mass ratio of 1:1 (lbp-iii), which was rated as the best among the studied mortars, has a very good potential for practical applications because its properties are expressly improved, as compared with the common lime render with quartz sand. acknowledgements the work has been achieved with the financial support from the ga čr research project no. 18-07332s and from the ministry of education, youth and sports under the “national sustainability programme i” – the project no. lo1408 “admas up – advanced materials, structures and technologies”. references [1] j. l. clarke. structural lightweight aggregate concrete. taylor & francis, oxon, 2005. [2] a. m. rashad. a synopsis about perlite as building material – a best practice guide for civil engineer. construction and building materials 121:338–353, 2016. doi:10.1016/j.conbuildmat.2016.06.001. [3] l. m. silva, r. ribeiro, j. labrincha, v. ferreira. role of lightweight fillers on the properties of a mixed-binder mortar. cement & concrete composites 32:19–24, 2010. doi:10.1016/j.cemconcomp.2009.07.003. [4] m. lanzón torres, p. garcía-ruiz. lightweight pozzolanic materials used in mortars: evaluation of their 137 http://dx.doi.org/10.1016/j.conbuildmat.2016.06.001 http://dx.doi.org/10.1016/j.cemconcomp.2009.07.003 m. vyšvařil, t. žižlavský, p. bayer, p. rovnaníková acta polytechnica ctu proceedings influence on density, mechanical strength and water absorption. cement & concrete composites 31:114–119, 2009. doi:10.1016/j.cemconcomp.2008.11.003. [5] i. palomar, g. barluenga, j. puentes. lime–cement mortars for coating with improved thermal and acoustic performance. construction and building materials 75:306–314, 2015. doi:10.1016/j.conbuildmat.2014.11.012. [6] s. barbero, m. dutto, c. ferrua, a. pereno. analysis on existent thermal insulating plasters towards innovative applications: evaluation methodology for a real cost-performance comparison. energy and buildings 77:40–47, 2014. doi:10.1016/j.enbuild.2014.03.037. [7] w. brachaczek. the hydrophobicity of renovation plaster in manufacturing technology optimized by statistical methods. construction and building materials 49:575–582, 2013. doi:10.1016/j.conbuildmat.2013.08.051. [8] z. pavlík, m. pavlíková, l. balík, r. černý. in-situ analysis of hygric performance of piaristic monastery building. aip conference proceedings 1648:410006, 2015. doi:10.1063/1.4912635. [9] p. brzyski, m. widomski. the influence of partial replacement of hemp shives by expanded perlite on physical properties of hemp-lime composite. aip conference proceedings 1866:040006, 2017. doi:10.1063/1.4994486. [10] a. arizzi, g. cultrone. aerial lime-based mortars blended with a pozzolanic additive and different admixtures: a mineralogical, textural and physical-mechanical study. construction and building materials 31:135–143, 2012. doi:10.1016/j.conbuildmat.2011.12.069. [11] u. bulut. use of perlite as a pozzolanic addition in lime mortars. gazi university journal of science 23:305–313, 2010. [12] e. navrátilová, p. rovnaníková. pozzolanic properties of brick powders and their effect on the properties of modified lime mortars. construction and building materials 120:530–539, 2016. doi:10.1016/j.conbuildmat.2016.05.062. [13] m. čáchová, d. koňáková, e. vejmelková, m. vyšvařil. ternary binder based plasters with improved thermal insulating ability. iop conference series: materials science and engineering 251:012008, 2017. doi:10.1088/1757-899x/251/1/012008. [14] m. čáchová, d. koňáková, e. vejmelková, et al. hygric and mechanical parameters of ternary binder based plasters lightweighted by expanded perlite. iop conference series: materials science and engineering 379:012004, 2018. doi:10.1088/1757-899x/379/1/012004. [15] m. vyšvařil, t. žižlavský, p. rovnaníková. rheological and mechanical properties of lime-brick powder mortars with expanded clay aggregate. iop conference series: materials science and engineering 385:012063, 2018. doi:10.1088/1757-899x/385/1/012063. 138 http://dx.doi.org/10.1016/j.cemconcomp.2008.11.003 http://dx.doi.org/10.1016/j.conbuildmat.2014.11.012 http://dx.doi.org/10.1016/j.enbuild.2014.03.037 http://dx.doi.org/10.1016/j.conbuildmat.2013.08.051 http://dx.doi.org/10.1063/1.4912635 http://dx.doi.org/10.1063/1.4994486 http://dx.doi.org/10.1016/j.conbuildmat.2011.12.069 http://dx.doi.org/10.1016/j.conbuildmat.2016.05.062 http://dx.doi.org/10.1088/1757-899x/251/1/012008 http://dx.doi.org/10.1088/1757-899x/379/1/012004 http://dx.doi.org/10.1088/1757-899x/385/1/012063 acta polytechnica ctu proceedings 22:133–138, 2019 1 introduction 2 materials and experimental procedures 3 results and discussion 4 conclusions acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2019.24.0015 acta polytechnica ctu proceedings 24:15–20, 2019 © czech technical university in prague, 2019 available online at http://ojs.cvut.cz/ojs/index.php/app debris-fretting test of coated and uncoated zr-1%nb cladding marcin kopeća,∗, martina maláa, ladislav cvrčekb, jakub krejčíc a research centre řež, hlavní 130, 250 68 husinec-řež, czech republic b czech technical university in prague, faculty of mechanical engineering, deparment of materials engineering, karlovo náměstí 13, 121 35 prague 2, czech republic c ujp praha a.s., nad kamínkou 1345, 156 10 prague – zbraslav, czech republic ∗ corresponding author: marcin.kopec@cvrez.cz abstract. debris fretting is one of the most significant fuel rod failure causes. an approach to mitigate this is to apply special coatings on the surface of the cladding that are supposed to increase the wear resistance. this paper summarizes the experiments performed in research centre řež on fuel cladding tube specimens. the tube segments made of uncoated and coated zr-1%nb alloy were exposed to fretting tests simulating the wire-debris vibrations impact on cladding during the reactor operation. the goal of these tests is to obtain results about the differences in wear resistance of uncoated and coated specimens. this paper shows the testing procedure in the air at room temperature and obtained impact markings on the specimens’ surfaces. keywords: cladding, fretting, nuclear fuel. 1. introduction the leaking fuel rods are an issue having been solved for a long time and remaining still relevant. in most cases the root cause of the failure is found which is visible in figure 1. nevertheless, in some causes the root cause cannot be identified at the power plant and then it is necessary to cool down the assembly in a spent fuel pool for few years to decrease its activity, then transport it to a hot cell facility, and perform detailed post-irradiation examination. this is associated with higher costs. the leaking fuel rod phenomenon is associated also with possible deteriorative handling and storage. this can stand as a challenge for utilities, who either have to invest in the repair of fuel assembly or loose space in their spent fuel pools. indirectly this issue affects also the fuel vendors when their fuel product is not so reliable and needs improvement. generally, the improvement can be beneficiary for the whole nuclear fuel market but the costs, to reach it, are excessive and may have been pointless. the nei report from 2016 specifies the leaking fuel rod costs as $1.5b per reactor [1] globally. one of the most important degradation mechanism of a nuclear fuel rod is the fretting wear coming from the flow-induced vibrations (fiv). the weak wear resistance of the cladding material can lead to its rupture. according to the nuclear news (in cooperation with epri), for the year 2010, the majority of failed fuel rod causes in the u.s. was related to the wear problem [2]. the more renowned mechanism of wear damage is grid-to-rod fretting. some studies and models have addressed this phenomenon [3–5], including power plant experiences [6]. on the opposite side there is the debris fretting failure mechanisms. the studies concerning the debris impact onto the fuel assembly are limited to new designs of anti-debris filters mounted in the fuel assembly bottom nozzles, e.g. filtering wires of diameter more than 1 mm or plates of the thickness of more 0.3 mm and lengths reaching more than 10 mm [7]. an attempt to limit the problem is the introduction of the foreign material exclusion (fme) policy, which is more and more incorporated by the nuclear power plants into their internal regulations. this, although, can proof ineffective in the case of object of diameter less than 1 mm and less than 10 mm in length, e.g. wires coming from the maintenance of the power plant. the fiv in those cases gets into high-frequency vibrations. hence the force load of a single debris-cladding contact is relatively very small, the numerous impacts can lead to fretting wear when the affected range is limited. another approach lowering the debris-impact onto the fuel cladding integrity is the introduction of coatings on the cladding surface, which are usually formed as a very thin film (microns) of highly wear-resistant material. this issue is directly related to the accident tolerant fuels (atf) concepts, being the matter of a research world widely. the most known is the crn layer coating which dates back to the 70’ and has its own history in e.g. halden reactor project (hrp) activities [8]. those days, the coatings and the process of their application, were extremely costly and ineffective. with the development of technology and the research constantly pending in this field, the coatings’ performance has been increased. similarly the range of potentially useable material for coatings. no ultimate material has been specified and in the aim of mechanical tests is to select the group of the 15 http://dx.doi.org/10.14311/app.2019.24.0015 http://ojs.cvut.cz/ojs/index.php/app m. kopeć, m. malá, l. cvrček, j. krejčí acta polytechnica ctu proceedings figure 1. the percentages of nuclear fuel rod failure causes in u.s. for 2010 [2]. most promising in the view of their endurance also under the load of debris in high-frequency vibrations. such process constitutes a qualification process for new atf materials and new coatings on standard cladding materials. the r&d tasks to specify the best accessible material is world widely ongoing the fuel vendors are equally involved into this process as the research organizations [9, 10]. research centre řež (cvř) has developed its own methodology for debris fretting of nuclear fuel cladding testing in dry conditions with the possibility to widen the spectrum for wet and high-temperature dry tests. in this way, it is possible to test and compare different types of atf claddings, including even sic/sic composites, which are supposed to withstand the debris impact even in elevated temperatures. in the cooperation with the czech technical university (ctu) in prague and ujp praha a.s. several types of coatings on standard zr-1%nb cladding were tested for fretting wear resistance. all tests included the reference on the as-received specimen of zr-1%nb cladding. this work presents the testing method and preliminary outputs of the materials’ comparison. 2. testing method of debris fretting wear as no standards have been found in the open literature for debris fretting wear testing as well as no models have been found for the prediction of the debris particle inside the reactor core during reactor operation, the authors decided to foreseen the potentially worst scenario. hence it is on the verge of possibilities to model the behavior of the small wire particle inside the reactor core, the case where one of the ends of the wire is caught inside a fuel assembly spacer grid was selected. in this way, the second end exerts a repetitive load on the cladding surface, depending on the fiv of the particular fuel assembly. as the anti-debris filters capture foreign objects of ca. 1 mm in each dimension [7], a stainless steel wire of the diameter of 0.4 mm was specified for the fretting tests. the testing equipment can house as many as 8 specimens including the reference one. it is based on the rotation cycling of the foreign object imitator (i.e. wire) with the possibly constant radial speed of ca. 1000 rpm. the precise number of contacts between the foreign object imitator and the specimens are recorded with voltage circle closed by the wire and the specimen. the wire has one free end that cyclically gets into smooth slide-contact with testing specimens. it is ensured by the bending of the used wire as can be seen in figure 3a. to avoid sliding of the wire over the surface of the specimen, it is kept between two clamps which are leaving no more space than twice the diameter of the used wire. in this way, the slide-spot of the wire over the specimen is at the same level as the fastening of the wire itself, what can be seen in figure 3b. the specimens are mounted symmetrically against the rotating shaft inside the testing equipment. the length of the wire is ca. 2 mm longer than the distance 16 vol. 24/2019 debris-fretting test of coated and uncoated zr-1%nb cladding figure 2. testing equipment with 2 specimens in parallel testing: right – uncoated zr-1%nb; left – crn-coated zr-1%nb. (a) schematic upper view – specimen (red), wire (blue), the arrow shows wire’s movement direction, black – pivot. (b) clamps (black) and wire positions. figure 3. the wire scheme. between the shaft and the specimen. in this way, the equal angle distances between the specimens and the length of the wire ensures almost equal force that the wire exerts on the specimens’ surfaces. what is more, the contact force is extremely low to keep the wire-specimen in the conditions that will enable free sliding of the wire against the specimen. the impact of additional coating layer thickness is due to the above negligible. to keep the reference between each batch of testing specimens and to preserve the correlation between the various types of coatings, 4-time ranges were specified: 120 min, 200 min, 350 min, and 500 min. the cycling of the specimens and the recording of the impact appearance is controlled from the computer. the equipment allows testing the specimen in wet condition up to 100 °c. while mounted in a special chamber, it is able to run a couple-of-days long experiment in temperatures reaching up to 800 °c (dry environment). the hot cells are able to house a long-lasting experiment (50 days) in the furnace in temperatures reaching 1100 °c (dry environment). in these hot-cell conditions, irradiated samples can be tested. the testing in elevated temperatures is important for the candidates of gen-iv cladding materials as sic/sic, which are meant to work in temperatures exceeding 800 °c. those materials should be able to withstand the conditions of possible accident, when the temperature could exceed 1000 °c [11]. 3. testing specimens of coatings for the testing, there were selected five specimens of different cr-based coatings, including the composite coatings. the coatings were prepared and applied at ctu in prague in cooperation with ujp praha a.s. the cr-based coatings were deposited on outer surfaces of zr-1%nb tubes by magnetron sputtering from cr (99.6 %) targets in the hauzer flexicoat 850 industrial system. pure metallic chromium coatings were prepared in the argon atmosphere and ceramic crn and crn2 coatings in a gas mixture of argon and nitrogen at the deposition temperature of about 250 °c. as the coatings to be tested, simple layers of pure chromium (cr), chromium nitride (crn) and super stochiometric chromium nitride crn2. as the mul17 m. kopeć, m. malá, l. cvrček, j. krejčí acta polytechnica ctu proceedings ticomponent coatings were testerd chromium nitride and chromium (crn+cr) and super stochiometric chromium nitride and chromium (crn2+cr). before deposition, the tubes were ultrasonically cleaned in acetone, ethanol, and demineralized water and dried with in furnace at 90 °c. the tubes were placed then hanged into the vacuum chamber on rotating holders in the flexicoat system where the tube surfaces were cleaned by ion etching in the argon plasma. this process removes the thin zr-oxide and other impurities thus improving adhesion of the coating. the deposition parameters vary depending on the particular coating. a thin metallic cr-layer was deposited on the substrate first when depositing ceramic coatings to improve adhesion. the thickness of the as-deposited coatings was measured with a calotest (csm, switzerland). the resulting thicknesses are presented from the outermost coating towards the substrate in table 1. coating layer 1 (µm) layer 2 (µm) total (µm) cr 18.6 – 18.6 crn 16.4 – 16.4 crn2 10.7 – 10.7 crn + cr 16.4 18.6 35.0 crn2 + cr 10.7 18.6 29.3 table 1. measured thicknesses of applied coatings. 4. the preliminary outputs at first, the tested specimens and their fretting marks were visually examined. they have not been thoroughly measured yet. they are prepared for replicamethod measurements with the use of high-resolution scanning – 5 micron precision. nevertheless, using the vertex automated precision measurement system it was possible to prepare good quality pictures of the fretting grooves. those preliminary outputs already can show the increase in wear resistance when the coatings basing on cr are applied. the tables below compare each batch of the tested specimens. however, the surface treatments of "as received" – uncoated and coated specimens are quite different, no additional improvement like polishing of coated specimens was applied. the tests were deliberately conducted in such conditions because of the fact, that any potential lta assembly will house either both uncoated and coated fuel rods or will have only coated rods but will be placed next to uncoated assembly. no information was found about the planned additional treatment and the supposition is that in the case of debris contact, the coated specimen can be polished "freely" with the debris. this is to verify the behavior of the mixed-core, which can ease the change from regular fuel to atf without the necessity to change the whole fuel batch at once. what is more, testing (a) sediments after testing of crn2+cr-coated zr1%nb. (b) sediments after testing crn-coated zr-1%nb. figure 4. sediments. the specimens in "as received" state – both coated and uncoated – gives more information about the inherent strength of the coating. in this way, the potentially ultimate candidates’ range can be limited. a very important output that can be distinguished in those preliminary outputs are the rest-overs coming from the friction between the wire and the specimen. their highest accumulation was observed by the crn2+cr composite coating and the crn coating. as no such accumulation of particles was visible in the vicinity of reference specimen and as the length of the wire did not alter after all the tests, it is supposed that the sediment origins in the coated specimen. the most probable is the coating wear in the contact area but without breaking the coating structure. figures 4a and 4b show sediment accumulation. the precise answer about the sediment can be delivered by microstructure analysis which is planned for the second half of 2019 – after the replica measurements. 5. conclusions the cvř has developed its own methodology and equipment for debris fretting testing of cladding materials. the testing equipment can be extended for tests in elevated temperature and long-lasting tests in temperatures exceeding 1000 °c. in the cooperation with ctu in prague and ujp praha a.s. five different types of cladding materials were tested in the same conditions for their comparison. the photos obtained with the vertex equipment clearly indicates the positive effect of coating application on the wear resistance of standard zr-1%nb cladding. on the reference uncoated "as-received" samples the grooves from 18 vol. 24/2019 debris-fretting test of coated and uncoated zr-1%nb cladding 120 min 200 min 350 min 500 min crn coated uncoated crn coated uncoated crn coated uncoated crn coated uncoated 120 min 200 min crn+cr coated cr coated uncoated crn+cr coated cr coated uncoated 350 min 500 min crn+cr coated cr coated uncoated crn+cr coated cr coated uncoated 120 min 200 min crn2+cr coated crn2 coated uncoated crn2+cr coated crn2 coated uncoated 350 min 500 min crn2+cr coated crn2 coated uncoated crn2+cr coated crn2 coated uncoated 19 m. kopeć, m. malá, l. cvrček, j. krejčí acta polytechnica ctu proceedings fretting increases with the number of loads, whereas, in the case of coated material, the zr-1%nb matrix seems untouched. high-precision measurements, using replica-method, are planned to specify the real depth of the obtained debris fretting marks. this activity will allow to compare the wear resistance of the coating materials and serve for the qualification reference of the coating in the case of its utilization in the nuclear fuel industry. the powder particle sediment was observed in the vicinity of coated specimens only, with its highest accumulation close to coatings made of crn2+cr and crn. the origin of the powder particles is supposed in the surface abrasion of the coating. to investigate the loss of material and friction abilities of the tested coating, the microstructural analysis in the area of fretting marks is planned for later this year. the next steps considered for these specimens are testing at elevated temperatures and testing in water environment. this will bring another data to complete the material resistance against fretting damage. the dry conditions testing is a first step to specify the most promising candidates for future atf cladding materials. more representative data for direct testing will be obtained from the wet test that can include also more aggressive and corrosive environment. acknowledgements financial support of this research through čez a.s. company is gratefully acknowledged. also, this work was supported by the ministry of education, youth and sport of the czech republic, programme npu1, project no lo1207. the presented work was financially supported by the ministry of education, youth and sport czech republic project lq1603 (research for susen). references [1] nuclear energy institute, nuclear cost in context retrieved march 2016 from: https://www.nei.org/ corporatesite/media/filefolder/policy/papers/ nuclear-costs-in-context.pdf?ext=.pdf, 2016. [2] the nuclear news interview, k. edsinger. epri and the zero fuel failure program fuel special edition, december 2010. [3] m. pettigrew. the vibration behavior of nuclear fuel under reactor conditions. nuclear engineering and design 114 pp. 179–189, 1992. [4] s. kovács, j. stabel, m. ren, b. ladouceur. comparative study on rod fretting behavior of different spacer spring geometries. wear 266 pp. 194–199, 2009. [5] j. yan, k. yuan, e. tatli, z. karoutas. a new method to predict grid-to-rod fretting in a pwr fuel assembly inlet region. nuclear engineering and design 241 pp. 2974–2982, 2011. [6] k.-t. kim. a study on the grid-to-rod fretting wearinduced fuel failure observed inthe16x16 kofa fuel. nuclear engineering and design 240 pp. 756–762, 2010. [7] j.-k. park, s.-k. lee, j.-h. kim. development of an evaluation method for nuclear fuel debris-filtering performance. nuclear engineering and technology 50 pp. 738–744, 2018. [8] j. h. jackson, d. paraventi, m. wright. proceedings of the 18th international conference on environmental degradation of materials in nuclear power systems – water reactors. portland, usa 2017. [9] l. hallstadius, s. johnson, e. lahoda. cladding for high performance fuel. progress in nuclear energy 57:71–76, 2012. [10] e. lahoda, f. franceschini. advanced fuels for future light water reactors. westinghouse electric company llc, pittsburgh 2011. [11] k. barrett, s. m. bragg-sitton, d. galicki. advanced lwr nuclear fuel cladding system development trade-off study. idaho national laboratory 2012. 20 https://www.nei.org/corporatesite/media/filefolder/policy/papers/nuclear-costs-in-context.pdf?ext=.pdf https://www.nei.org/corporatesite/media/filefolder/policy/papers/nuclear-costs-in-context.pdf?ext=.pdf https://www.nei.org/corporatesite/media/filefolder/policy/papers/nuclear-costs-in-context.pdf?ext=.pdf acta polytechnica ctu proceedings 24:1–6, 2019 1 introduction 2 testing method of debris fretting wear 3 testing specimens of coatings 4 the preliminary outputs 5 conclusions acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2019.22.0017 acta polytechnica ctu proceedings 22:17–21, 2019 © czech technical university in prague, 2019 available online at http://ojs.cvut.cz/ojs/index.php/app the lightweight textile reinforced concrete for thin-walled structures tereza cibulka∗, luboš musil, jan vodička czech technical university in prague, faculty of civil engineering, department of concrete and masonry structures, thákurova 7, 166 29 prague, czech republic ∗ corresponding author: tereza.cibulka@fsv.cvut.cz abstract. the article deals with lightweight concrete with non-conventional reinforcement and its application in thin-walled structures. as part of experimental research, several sets of thin-walled slab and complementary specimen were made to determine the material characteristics of lightweight concrete. the porous aggregate liapor was used in the recipe. two-dimensional carbon and 3d glass textiles were used as reinforcement. the impact study of different casting technologies and recipe on the material characteristics of lightweight concrete was included in the research. fresh concrete for the slab production was placed in special wooden molds. the slabs were concreted in vertical and horizontal position. the casting method has a significant impact on the element material characteristics. reinforced specimens have shown high strength, even in thin-walled structures with low bulk density. keywords: lightweight concrete, lightweight aggregate, fibre reinforced concrete, textile reinforced concrete. 1. introduction at present, the demand for concrete with special characteristics is growing. not only physical-mechanical but also aesthetical characteristics are desired. an emphasis is placed on low overall costs together with high load bearing capacity demand. one of the criteria is the requirement to reduce the weight of the structure itself. concrete composite with classified low bulk density is called lightweight concrete (lc). the structure can be efficiently lightened by reducing the concrete matrix weight itself. this can be realized by partial or full replacement of the natural aggregate by a porous aggregate. [1] special reinforcement methods, as compared with standard concrete reinforcement, offer an interesting alternative in making complex and thinner structures with a high load bearing capacity. when the fibres are fixed in a textile mesh, this lightweight concrete is called the textile reinforced lightweight concrete (trlc). research on lc with special reinforcement, which takes place at the faculty of civil engineering of the ctu in prague, deals with the possibilities of applications, technologies and material characteristics of trlc. thin-walled structures are one of the suitable applications. the experimental verification of hypothesis is an important part of the research. the ongoing initial experiments are focused on production of interior furniture. the next step is an application of trlc in street furniture. 1.1. lightweight concrete (lc) lightweight concrete is classified as a concrete with a dry bulk density of more than 800 kg/m3 and less than 2,000 kg/m3. porous aggregates, such as ceramsite figure 1. the coffee table [3]. (liapor) or expanded perlite, are used as replacements for natural aggregates. porous stone, brick gravel, expanded vermiculite or other filler material can also replace natural aggregates. [1] because the concrete modulus of elasticity is directly related to the modulus of elasticity of the used aggregate, lc has lower static elastic modulus values. [2] the concrete strength is mainly determined by the strength of used cement binder, since the porous aggregate strength is usually lower than the strength of the cement stone. lower thermal expansion of lc has a positive effect on cracking. however, due to lower stiffness, lightweight concrete is prone to deformation, such as creep and shrinkage. 1.2. textile reinforced lightweight concrete (trlc) the reinforcing fibres of textile reinforced concrete (trc) are evenly distributed in a flat (2d) or spatial 17 http://dx.doi.org/10.14311/app.2019.22.0017 http://ojs.cvut.cz/ojs/index.php/app t. cibulka, l. musil, j. vodička acta polytechnica ctu proceedings figure 2. 2d carbon and 3d glass textiles. specimens reinforcement formwork pieces set dimensions [mm] trlc 1 700/425/25 unreinforced vertical 1 2d carbon textile vertical 2 2 700/425/20 vertical 2 3 700/400/18 3d glass textile vertical 1 horizontal 1 table 1. overview table of slab specimens. (3d) mesh and are placed in a suitable position relative to the stress considered. basalt, glass and carbon fabrics are used as reinforcement. these materials are not prone to corrosion. therefore, the cover layer is significantly lower compared to standard reinforced concrete and very thin trc elements can be produced. [4] the combination of grid structures with suitable mesh density and the appropriate size of grain aggregates is essential to ensure satisfactory cohesion between the fabric and the concrete. a fine-grained concrete matrix is usually used to ensure effortless casting and aesthetic quality of the surface of the reinforced element. [5] an additional surface treatment by grinding or painting is desirable for an architectural concrete. the absorbency of the final product can be greatly influenced by surface treatment. 2. results and discussion as part of the research on this issue, testing specimens were made. a slab of 700mm was selected as a reference test specimen for thin-walled structures. the thickness of the slab was reduced during the experiments 25mm, 20mm, 18mm for technology verification of maximized lightening of construction, while maintaining sufficient load bearing capacity. a coffee table (figure 1) was made in a vertical formwork as an example of the final product in a bachelor thesis [3]. to compare trlc specimen behaviour, several reinforced and non-reinforced slabs and accompanying test specimens were produced to determine the strength characteristics of composite according to the standard. 2d carbon and 3d glass meshes were used for slab reinforcing (figure 2). an overview of the slabs tested in bending is given in table 1. an expanded clay liapor was used to replace natural aggregate. the maximum grain size of the porous aggregate was chosen to be 2mm due to the intended application to the thin-walled elements. a bulk density of this fraction for liapor is declared to be 575 kg/m3. this reduced aggregate size also ensured sufficient co-operation with the reinforcement. the composition used for experimental research contains cement cem i 52.5, fine-grained sand, ash, microsilica and superplasticizer to assure a sufficiently fluid consistency of fresh concrete. for each set of specimens, the composition varied only in the watercement ratio and the amount of superplasticizer. the non-reinforced lc selected for experimental verification reaches bulk density ranging from 1,400 kg/m3 to 1,600 kg/m3. an average concrete compressive strength of 32,02mpa was proved on cube specimens with dimensions 100/100/100 mm. the experimental research has brought some insights. because of using the liapor aggregate for the production of fresh concrete, it is necessary to solve the method of compaction in thin-walled slabs. the compaction on the vibratory tables produces a non-homogeneous composite. the cement stone is placed at the bottom and the liapor aggregate at the upper surface. this phenomenon leads to a high tensile bending strength of concrete composite and consequently high load bearing capacity of the slabs during loading (table 2). the fixing of the reinforcing mesh position appeared to be difficult. 18 vol. 22/2019 the lightweight textile reinforced concrete for thin-walled structures formwork weight bulk density force f [kn] moment m [kg] ρ [kg/m3] first crack maximal force mmax [knm] vertical 8.02 1591.27 0.23 0.58 0.087 horizontal 6.91 1371.03 0.56 1.8 0.27 formwork flexural strength without cracks deflection w [mm] σb [mpa] first crack maximal force after unloading vertical 4.03 1.12 18.62 2.6 horizontal 12.5 1.19 41.97 4.3 table 2. flexural test results for trlc slabs with 3d glass textile. figure 3. formwork for trlc slabs with 2d carbon textile. figure 4. formwork for trlc slabs with 3d glass textile. 19 t. cibulka, l. musil, j. vodička acta polytechnica ctu proceedings figure 5. the static scheme of slab loading. figure 6. the flexural testing of trlc slabs with 2d carbon textile. figure 7. the flexural testing of trlc slabs with 3d glass textile. 20 vol. 22/2019 the lightweight textile reinforced concrete for thin-walled structures to eliminate both of these negative effects, the next production of the slabs was realized in a vertical position with double-sided formwork. the filling with fresh concrete was carried out by a metal funnel and the compaction was done only by tapping on the formwork walls. the fixing of the mesh was secured by clamping between the firmly bonded formwork walls (figure 3). 3. conclusion replacing natural aggregate by porous aggregate is one of the ways to reduce the weight of the concrete element. the lightweight concrete thus formed can be reinforced by special reinforcements, such as gridshaped fibres. by combining these approaches, it is possible to create lightweight and thin concrete elements with high load bearing capacity. the article deals with an initial research seeking new ways for trlc designing and casting. for the purpose of the research, several thin-walled slabs reinforced with 2d carbon and 3d glass fabrics and unreinforced specimens were produced by various casting processes to prove the material characteristics of lightweight concrete. the producing described in the article is original. no identical production was found in scientific literature. the method of casting has a significant effect on the material characteristics of the elements. bending tests confirmed a great increase in strength for all reinforced specimens. we decided to post the most interesting results, which are fundamental for further research compaction in horizontal position, fixation of lightweight concrete slabs, and homogeneity of the composite. the application of trlc to thin-walled elements is a real alternative to the present materials and technologies. acknowledgements this work was supported by the grant agency of the czech technical university in prague, grants number sgs15/036/ohk1/1t/1, sgs16/044/ohk1/1t/11, sgs17/049/ohk1/1t/11 and sgs18/115/ohk1/2t/11. references [1] l. svoboda. building materials. jaga, bratislava, 2007. [2] m. hubertova. static elasticity modulus of light-weight structural concrete. in beton tks, vol. 4, pp. 50–53. 2010. [3] f. vastl. desing of coffee table from lightweight concrete with mesh reinforcement. bachelor’s thesis, ctu in prague, 2017. [4] j. hegger, m. zell, m. horstmann. textile reinforced concrete realization in applications. in tailor made concrete structures. london, 2008. doi:10.1201/9781439828410. [5] s. gelbrich, h. l. funke, a. ehrlich, l. kroll. flexible fiber-reinforced plastic formworks for the production of curved textile-reinforced concrete. advances in structural engineering 21(4):580–588, 2018. doi:10.1177/1369433217732681. 21 http://dx.doi.org/10.1201/9781439828410 http://dx.doi.org/10.1177/1369433217732681 acta polytechnica ctu proceedings 22:17–21, 2019 1 introduction 1.1 lightweight concrete (lc) 1.2 textile reinforced lightweight concrete (trlc) 2 results and discussion 3 conclusion acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2019.22.0052 acta polytechnica ctu proceedings 22:52–56, 2019 © czech technical university in prague, 2019 available online at http://ojs.cvut.cz/ojs/index.php/app experimental analysis focused on the material characteristics of the cement-based polymer-modified mortars barbara kucharczyková∗, hana šimonová, romana halamová, dalibor kocáb, martin alexa brno university of technology, faculty of civil engineering, veveří 331/95, 602 00 brno, czech republic ∗ corresponding author: barbara.kucharczykova@vutbr.cz abstract. the article deals with the experimental analysis focused on the development of the physical and mechanical characteristics of the cement-based polymer-modified mortars (pcm) during ageing. two different commercial products commonly used for similar in-situ applications were used for the experiment. the results of the shrinkage, elastic, fracture and strength parameters determined within the time interval from 3 days to 2 years of ageing are summarized and discussed in the article. the performed experimental analysis showed different behaviour of tested pcms. the most significant differences were observed at the age of 90 days when one of the tested pcm showed a substantial decrease in most of investigated characteristics. keywords: polymer-modified mortar, cement, shrinkage, fracture, modulus of elasticity, strength. 1. introduction cementitious mortars modified by polymers, commonly known as polymer-cement mortars (pcm), find wide use in civil engineering especially in the rehabilitation and protection of both concrete and masonry structures [1, 2]. pcm have a monolithic matrix that uniformly combines the matrix of an organic polymer with the matrix of a cement gel [3]. thanks to the homogenization of the cement matrix with the polymer, these mortars receive an improvement in some properties – e.g. workability, tensile strength, adhesion, or chemical corrosion resistance [4, 5]. cementitious mortars can be modified by various polymers; e.g. latex, redispersible polymer powders, liquid resins, monomers, or acrylic and epoxy emulsions. polymers and polymer-modified materials can nowadays be considered an important component of modern, sustainable civil engineering [6]. applications where pcm are best used as well as how they are to be technologically implemented are regulated by documents and technical manuals supplied with each product; however, these instructions can often be rather vague. if pcm are wrongly applied, they may fail to perform and put the functionality of the entire rehabilitation system in danger [7]. differences in the behaviour of pcm can also be expected with varying quality and type of substrate. they may behave differently in combination with e.g. high-quality concrete compared to non-fired bricks. it is also important to properly treat the surface of the substrate (removing dust, wetting and saturating with water) as well as curing the final pcm surface once exposed to the environment. in order to understand the behaviour of pcm and their use it is necessary to know the value of compressive and tensile strength at the age of 28 days as well as the progress of their other material properties, such as volume changes or crack propagation resistance over a long period of time. 2. experimental part the main aim of the performed measurements was to determine the development of the selected material characteristics of the pcms during the ageing and show the specific behaviour which was observed during testing. to simulate inadequate curing conditions which can occur during in-situ applications, all tested specimens were not treated at all during the whole time of ageing and were stored in laboratory conditions with temperature of (21 ± 2) °c and relative humidity of (60 ± 10)%. for the first 72 hours they were stored in the moulds with uncovered upper surface. after demoulding all surfaces were left to dry freely. the results presented in this paper were partially presented in [7] and they are extended herein by the results of the tests performed at the specimens’ age of approx. 2 years. 2.1. materials two fine-grained polymer-modified mortars based on the portland cement were prepared for the experiments. because the commercial products were used, the trademarks and the composition of these products cannot be specified in the paper. only general information taken from the technical sheets were used for the specification of the materials. note, that both mortars are commonly used for similar in-situ applications. for the purpose of the results evaluation, the pcms were designated as “v” and “vii” sets. the specimens of the “v” set were made of a twocomponent mixture where a liquid component was an 52 http://dx.doi.org/10.14311/app.2019.22.0052 http://ojs.cvut.cz/ojs/index.php/app vol. 22/2019 experimental analysis focused on the material characteristics. . . aqueous copolymer dispersion and a powder component contained a mixture of portland cements and mineral fillers. the specimens of the “vii” set were made of a single-component powder mix, containing among others silica sand, portland cement, microfibers including plasticisers and polymers, which was mixed with the prescribed dosage of water to prepare a fresh mortar [7]. both pcms were prepared using a handheld mixer. the mixing process, including the mixing time, was proceeded in compliance with the technical sheet of each product. two types of the test specimens were manufactured for the purpose of experiment. the specimens with dimensions of 60 × 100 × 1000mm were used for the measurement of shrinkage process. remaining characteristics were determined using the prismatic specimens with dimension of 40 × 40 × 160mm. all experimental results were evaluated for the specimens’ ages of 3, 28, 90 and 730 days and were expressed by the mean value and sample standard deviation calculated from three and six independent measurements in the case of shrinkage and fracture tests respectively. 2.2. testing techniques 2.2.1. shrinkage the shrinkage process was determined using the shrinkage drains with a movable head [8] which enabled to start the measurement of the relative length changes very early after the fresh-state material was placed into the measuring moulds. the inner surfaces of the shrinkage drains with the internal dimensions of 60×100mm in cross-section and 1000mm in length were coated with a 2mm thick polyethylene foam mat to avoid a friction between the mould and test specimen during the measurement. in this way the relative length changes were measured along the central axis of the specimen using an inductive sensor leaning against the movable head of the mould during about 72 hours. special markers were also embedded into the upper surface of the test specimens during their manufacturing to facilitate long-term measurement of relative length changes after the specimens were removed from the drains. the arrangement of the measurement is shown in fig. 1. refer to [9] for the test procedure details. 2.2.2. dynamic modulus of elasticity each specimen was tested for the natural frequency of longitudinal vibration, see fig. 2. the vibration was produced by a mechanical impulse generated by an impact hammer and the natural frequency was measured using an oscilloscope handyscope hs4 with an acoustic emission sensor. the dynamic modulus of elasticity was then calculated from this measured natural frequency according to the standard [10] using the equation: edyn = 4 · l2 · f2 l · d, (1) where edyn is the compressive dynamic modulus of elasticity in [mpa], l is the specimen length in [m], fl is the natural frequency of longitudinal vibration measured in [khz] and d is the material’s bulk density in [kg/m3]. 2.2.3. fracture test the fracture characteristics were determined based on the results of the three-point bending test of prismatic specimens with dimensions of 40 × 40 × 160mm provided with an initial notch located in the middle of the specimens’ length. the depth of the notch corresponded in all cases approximately to the 1/3 of the specimen’s height. the span length was set to 120mm. a mechanical testing machine fp10/1 was used for testing which enabled the displacement increment loading of the test specimens with a rate of 0.02mm/min. the loading force (f ) and deflection (d) measured in the middle of the span length were continuously recorded into the data logger during the test. based on the f − d diagrams the elastic modulus was calculated from the initial part of the diagram. the effective fracture toughness was determined using the effective crack model and the work of fracture and the consequent specific fracture energy was calculated using work-of-fracture method [11, 12]. because of the loss of stability, which occurred during the loading, the value of the work of fracture w ∗ f was determined as an area under the proper part of f − d diagrams before stability loss occurred. refer to [13] for details about the determination of above mentioned characteristics. the informative value of compressive strength was determined on the specimens’ fragments after the fracture tests were finished. arrangement of the fracture and compressive test is shown in fig. 2. 3. results and discussion the results of performed measurements represented by the average values (markers on the lines) and sample standard deviations (introduced in the table) for the specific age of specimens are shown in fig. 3 – fig. 5. the results displayed in fig. 3 show development of the compressive strength (fc) and dynamic modulus of elasticity (edyn). different trend and also different absolute values of these characteristics were recorded for the set “v” and “vii”. in general, the “vii” set shows higher values of fc up to the age of 90 days in comparison with the set “v”. on the other hand, a visible decrease in this characteristic was recorded at the age of 730 days, when the magnitude of the fc is lower than the one at the age of 28 days and is very similar to the fc value determined for “v” set at the same age. the curve of edyn values determined for “v” set shows gradual increase during monitored time-interval, whereas the “vii” set values started to decrease at the age of 28 days. in general, it can be stated that increase of edyn values was insignificant during the monitored time-interval. the results of 53 b. kucharczyková, h. šimonová, r. halamová et al. acta polytechnica ctu proceedings figure 1. arrangement of the relative length changes measurement. figure 2. arrangement of the fracture test (left), compressive strength test (middle) and resonance test (right). figure 3. development of the compressive strength (left) and the dynamic modulus of elasticity. 54 vol. 22/2019 experimental analysis focused on the material characteristics. . . figure 4. development of the modulus of elasticity determined from f − d diagrams (left) and the shrinkage process. figure 5. development of the effective fracture toughness(left) and the specific fracture energy. both characteristics are accompanied by relatively high variability. in the case of the “vii” set, interesting trends are observed in the development of the characteristics obtained from the fracture tests (fig. 4 – left and fig. 5). a steep decrease in the values of elastic modulus (efr), effective fracture toughness (kice) and specific fracture energy (gf ) occur after the age of 28 days. the values of efr, kice and gf decrease by about 70% within the age of 28 and 90 days. one of the explanation of this steep decrease can be found in the shrinkage curve (fig. 4 – right) which shows a steep increase in the shrinkage values which reached almost the maximum within the 28 day of specimens’ ageing. the results are also accompanied by a high variability. the tests performed at the age of 730 days show an increase in the values of all fracture characteristics within the 90 and 730 days. the values increased approx. 2.5 times, 3 times and more than 4 times in the case of efr, kice and gf respectively. this increase is supposedly associated with a self-healing ability of the material. results of fracture characteristics for “v” set do not show any dramatic decline of the fracture characteristics. the efr values slowly increase during whole time, values of remaining characteristics (kice, gf ) increase up to the age of 28 days. after that both values decrease slightly. the gf value shows a gradual decrease up to the age of 730 days. the shrinkage curve (fig. 4 – right) of “v” set differs slightly from the curve of “vii” set. the shrinkage curve determined for “v” set is not so steep within the 3 and 90 days of ageing and the final shrinkage value is about 25% lower. the results show also much lower variability. 4. conclusions despite the fact that both tested pcms are used for similar in-situ applications, the performed experimental analysis, focused on the long-term development of the selected material characteristics, showed different behaviour of particular pcms. the greatest differences were observed at the age of 90 days when, in the case of “vii” set, most of investigated characteristics significantly decreased in their values. on the other hand, the tests performed at the age of 730 days for “vii” set showed some self-healing ability of this material which led to the re-increasing of the fracture characteristics especially. the shrinkage curves, determined on the test specimens intentionally exposed to the free desiccation during whole monitored time-interval, showed a steep increase in the shrinkage values for both pcms. this fact accentuates the necessity to use appropriate curing conditions during theirs in-situ applications. important is also the fact that most results for both pcms were accompanied with a high variability. 55 b. kucharczyková, h. šimonová, r. halamová et al. acta polytechnica ctu proceedings acknowledgements this paper has been written as part of the project no. ga17-14302s “experimental analysis of the early-age volume changes in cement-based composites”, supported by the gao czech science foundation and project no. fast-j-18-5516 “determination of modulus of elasticity in the early phase of maturing of cement composites and their thermal expansion”. references [1] w. malorny, m. plath. investigations on properties determining durability of novel pcc. in proceedings of the conference on the rehabilitation and reconstruction of buildings crrb 2012, vol. 688 of advanced materials research, pp. 130–138. trans tech publications, 2013. doi:10.4028/www.scientific.net/amr.688.130. [2] o. karel, j. kubanek, p. schmid, i. rozsypalová. sanace panelových bytových domů pomocí dodatečně vkládané helikální výztuže – vývoj, alternativy, zkoušení. beton tks 03:24–27, 2017. [3] y. ohama. handbook of polymer-modified concrete and mortars properties and process technology. park ridge, n.j: noyes publications, 1995. [4] l. aggarwal, p. thapliyal, s. karade. properties of polymer-modified mortars using epoxy and acrylic emulsions. construction and building materials 21(2):379–383, 2007. doi:10.1016/j.conbuildmat.2005.08.007. [5] p. łukowski. polymer-cement composites containing waste perlite powder. materials 9:839, 2016. doi:10.3390/ma9100839. [6] l. courard, j.-f. lenaers, f. michel, a. garbacz. saturation level of the superficial zone of concrete and adhesion of repair systems. construction and building materials 25:2488–2494, 2011. doi:10.1016/j.conbuildmat.2010.11.076. [7] b. kucharczyková, h. šimonová, z. keršner, d. kocáb. shrinkage, weight loss and fracture parameters of selected polymer-modified cement mortars during ageing. transactions of the všb: technical university of ostrava, civil engineering series 17(01):122–130, 2017. doi:10.1515/tvsb-2017-0014. [8] schleibinger gerate. schleibinger testing systems: ultrasonic setting measurement. http://www.schleibinger.com, 2018. [9] čsn 73 1372 non-destructive testing of concrete – testing of concrete by resonance method. standard, úřad pro technickou normalizaci, metrologii a státní zkušebnictví, 2012. [10] b. kucharczyková, l. topolář, p. daněk, et al. comprehensive testing techniques for the measurement of shrinkage and structural changes of fine-grained cement-based composites during ageing. advances in materials science and engineering 3:1–10, 2017. doi:10.1155/2017/3832072. [11] rilem tc-50 fmc recommendation, determination of the fracture energy of mortar and concrete by means of three-point bend tests on notched beams. materials and structures 18(4):287–290, 1985. doi:10.1007/bf02472918. [12] v. veselý. the role of process zone in quasi-brittle fracture. habilitation thesis, university of technology, faculty of civil engineering, institute of structural mechanics, brno, 2015. [13] p. rovnaník, h. šimonová, l. topolář, et al. carbon nanotube reinforced alkali-activated slag mortars. construction and building materials 119:223–229, 2016. doi:10.1016/j.conbuildmat.2016.05.051. 56 http://dx.doi.org/10.4028/www.scientific.net/amr.688.130 http://dx.doi.org/10.1016/j.conbuildmat.2005.08.007 http://dx.doi.org/10.3390/ma9100839 http://dx.doi.org/10.1016/j.conbuildmat.2010.11.076 http://dx.doi.org/10.1515/tvsb-2017-0014 http://www.schleibinger.com http://dx.doi.org/10.1155/2017/3832072 http://dx.doi.org/10.1007/bf02472918 http://dx.doi.org/10.1016/j.conbuildmat.2016.05.051 acta polytechnica ctu proceedings 22:52–56, 2019 1 introduction 2 experimental part 2.1 materials 2.2 testing techniques 2.2.1 shrinkage 2.2.2 dynamic modulus of elasticity 2.2.3 fracture test 3 results and discussion 4 conclusions acknowledgements references doi:10.14311/app.2021.30.0012 acta polytechnica ctu proceedings 30:12–17, 2021 © czech technical university in prague, 2021 available online at http://ojs.cvut.cz/ojs/index.php/app on comparison of 3d isogeometric timoshenko and bernoulli beam formulations edita dvořáková∗, bořek patzák czech technical university in prague, faculty of civil engineering, department of mechanics, thákurova 7, 166 29 prague 6, czech republic ∗ corresponding author: edita.dvorakova@fsv.cvut.cz abstract. application of isogeometric analysis (iga) for curved beams is very convenient for its ability of exact representation of curved geometries. several beam formulation has been presented since the introduction of iga. in this paper, two different beam formulations are presented: bernoulli beam formulation of a. m. bauer et al. [1], and timoshenko beam element introduced by g. zhang et al. [2]. both beam elements are implemented and their performance is documented on the fully threedimensional example of helicoidal spring. keywords: bernoulli theory, curved beam element, isogeometric analysis, nurbs, timoshenko theory. 1. introduction isogeometric analysis is an alternative of standard finite element analysis introduced in 2005 by t. hughes [3]. the crucial impulse for its development was given by the desire for the automatic connection between computer-aided design (cad) and finite element analysis. this goal has been achieved by the application of the same nurbs-based geometry description in cad also for the unknown approximation in the analysis. the use of cad basis functions enables the exact description of arbitrarily curved geometries including curved beams. the focus of this paper is placed on the isogeometric analysis of spatial curved beams. two different beam formulations are presented and their performance is compared on a benchmark problem. the first beam formulation is based on bernoulli beam theory, while the timoshenko kinematics is assumed in the second beam element presented. 2. nurbs-based analysis non-uniform rational b-splines (nurbs) [4] are the most widespread technology used in cad industry. their fitness for cad is rooted in the ability of the exact description of a vast range of geometries, including the conic sections widely used in architectural design. there are many effective algorithms developed for handling nurbs objects which can be also very useful for isogeometric analysis. the computational domain in isogeometric analysis (iga) is firstly divided into patches, which are further divided into knotspans (often referred to as elements in iga), see fig. 1. the nurbs curve is obtained by linear combination of cartesian coordinates of the control points p and nurbs functions rp i c(ξ) = n! i=1 rp i (ξ)pi. (1) nurbs functions are rational polynomials which are generated by weighting b-spline functions with a weights associated with a control points. b-spline functions are piecewise polynomials and can be derived recursively starting with a piecewise constant functions, see [4] for the detailed description. the use of nurbs basis introduces some special aspects to the analysis. the nurbs basis functions are generally non-iterpolatory, thus the degrees of freedom in the control points lack the direct physical meaning. additionally, the higher continuity between knotspans can be achieved naturally using nurbs, unlike c0 in standard fem. one of the main features of iga is the fact, that the basis can be enriched without altering geometry representation using knot insertion and degree elevation algorithms. 3. bernoulli beam element formulation the first presented beam formulation is adopted from the work of a. m. bauer et al. [1]. the element is based on bernoulli beam theory, thus the orthogonality of the cross-section to the center line after deformation is assumed to be preserved and the crosssectional dimensions remain unchanged. the formulation accounts for the geometrically nonlinear behavior, nevertheless only the linear behavior is studied in the presented examples. the element has four degrees of freedom in each control point: three global displacements u, v, and w and an additional degree of freedom corresponding to the rotation around the centerline ψ. 12 http://dx.doi.org/10.14311/app.2021.30.0012 http://ojs.cvut.cz/ojs/index.php/app vol. 30/2021 comparison of timoshenko and bernoulli beams n4,2 ⇠ knotspan patch continuity: c0 c0c1c1 c1 knotspan p = 2, ⌅ = {0,0,0,0.5,1,1,1} n1,2 n2,2 n3,2 knots control points p1 p2 p4p3 parametric space physical space x y figure 1. description of b-spline (nurbs) finite element geometry. the geometry is modeled by one patch consisting of two knotspans with quadratic nurbs approximation. in the following, the standard notation using upper-case and lower-case letters for the undeformed and deformed configuration, respectively, is adopted. the geometry description is illustrated in figure 2. the three-dimensional beam approximation can be reduced to the center line given by the position vector xc (xc) xc = ! i rp i x̂i, (2) xc = ! i rp i x̂i, (3) where x̂i are the control points coordinates and rp i are the corresponding basis functions. deformed position vector is given as x̂i = x̂i + ûi, (4) where ûi is a vector of global degrees of freedom (u, v, w). the cross-section orientation is described by a moving trihedral given by the base vectors ai (ai) with i ∈ {1, 2, 3}. with assumption of bernoulli kinematics, a position vector of an arbitrary point of a beam continuum given by coordinates x (x) can be expressed as x(θ1, θ2, θ3) = xc(θ1) + θ2a2(θ1) + θ3a3(θ1), (5) x(θ1, θ2, θ3) = xc(θ1) + θ2a2(θ1) + θ3a3(θ1), (6) where θi are the convective contravariant coordinates. the first components of a moving trihedral a1 and a1 are aligned with a normalized tangents t and t, respectively, and are computed as a linear combination of the control points coordinates and the derivatives of the basis functions. the remaining components aα with α ∈ {2, 3} are orthogonal to the tangent of the center line (resp. a1) and are derived from the reference trihedral, given by a0 α, t0, using two key operations used for the alignment at the desired position. these operations are illustrated in figure 3. analogical procedures denoted as λ(t, t) and r̄t(ψ) are used to align the moving trihedral of the undeformed to the deformed configuration. by substituting geometric relations into the definition of green-lagrange strain tensor, the individual components of the tensor can be derived as e11 = 1 2(a11 − a11) " #$ % ε +θ2 a2,1 · a1 − a2,1 · a1" #$ % κ21 +θ3 a3,1 · a1 − a3,1 · a1" #$ % κ31 . (7) the diagonal terms eαα as well as the shear term e23 are equal to zero. this yields from the bernoulli assumptions (undeformable cross-section). for the off-diagonal terms e12, e13 e1α = 1 2θβ (aβ,1 · aα − (aβ,1 · aα) " #$ % κβα , (8) where (α, β) ∈ {(2, 3), (3, 2)}. the relations between strains and displacements are not linear and to the purposes of the linear analysis considered in this paper need to be linearized. the energetically conjugated stress tensor to the greenlagrange strain tensor is the second piola-kirchhoff tensor. the stiffness matrix is obtained by a standard procedure [5]. the fully derived equations for the presented beam formulation can be found in [1]. 4. timoshenko beam element formulation the second beam element presented here has been proposed by g. zhang et al. [2]. the beam formu13 edita dvořáková, bořek patzák acta polytechnica ctu proceedings a1(✓1) a2(✓1) a3(✓1) a1(✓1) a3(✓1) a2(✓1) u(✓1, ✓2, ✓3) x(✓1, ✓2, ✓3) xc(✓1) xc(✓1) x y z figure 2. illustration of the bernoulli beam in its undeformed and deformed configurations. t0 t0 a0 2 a0 3 t ⇤(t0, t ) ⇤(t0, t )⇤(t0, t )a0 3 a0 3 ⇤(t0, t )a0 2 t0 a0 2 a0 3 t ⇤(t0, t )a0 3 ⇤(t0, t )a0 2 r̄t ( ) r̄t ( ) a3 a2 figure 3. the λ(t0, t) and r̄t(ψ) operations used for the alignment of the bernoulli beam cross-section at the current position. t(s) n(s) b(s) ut ✓t ub ✓b un ✓n r(s) figure 4. local coordination system and degrees of freedom of timoshenko beam element. lation is based o the timoshenko beam theory and is derived in local coordinate system (t, n, b), where t, n, b are tangent, normal and binormal vectors. there are six independent unknowns: tangential displacement ut, normal displacement un, binormal displacement ub and rotations θt, θn, θb (see fig. 4). a strain-displacement matrix b is defined as ε = br, (9) where ε = {εm, γn, γb, χt, χn, χb}t and r = {ut, un, ub, θt, θn, θb}t . b is derived from the following relations for membrane strain εm, shear strains γn and γb, torsional strain χt, and bending strains χn and χb εm = u′ t − κun, χt = θ′ t − κθn, γn = κut + u′ n − τub − θb, χn = κθt + θ′ n − τθb, (10) γb = τun + u′ b + θn, χb = τθn + θ′ b. curvature κ is given as κ = &&&& &&&& d2r(s) ds2 &&&& &&&& (11) and torsion τ τ = dn(s) ds b(s), (12) where r(s) is the position vector and s is the curvilinear coordinate measured along the centerline of the beam. conjugated generalized stress vector contains normal and shear forces, and torsional and bending moments. similarly to the previous formulation, by following standard procedure the stiffness matrix is obtained. 4.1. numerical locking the well-known drawback of the timoshenko beam element formulations is numerical locking. locking phenomena rises from the use of the same approximation order for displacements and for rotations, which leads to the shear strains of a higher order than bending moments. several methods for unlocking beam elements are available, b̄-method [6] is used for the purposes of 14 vol. 30/2021 comparison of timoshenko and bernoulli beams this study. this method is based on the projection of the membrane and shear strains onto a basis of a lower order, leading to the removal of the locking. see [6] for the detailed description of the method. 5. numerical examples both presented beam formulations have been implemented and their performance has been evaluated on the fully three-dimensional example of helicoidal spring cantilever subjected to the continuous constant force load [2] (see figure 5). three main aspects have been studied: numerical locking of timoshenko beam, comparison of convergence of the bernoulli and timoshenko beam formulations, and error of the bernoulli formulation caused by the neglect of the shear effects. 5.1. numerical locking firstly, the removal of the numerical locking of the timoshenko beam formulation has been verified. the presented timoshenko formulation without and with locking treatment have been compared using the example of the thin helix depicted in figure 5. the convergence of the tip vertical displacement has been observed. additionally, the results have been compared to the standard fem beam element with cubic approximation, which is naturally locking-free [5]. as expected, the numerical locking can be observed when no locking treatment is used. the convergence of the isogeometric beam is significantly deteriorated in comparison with the standard fem beam element. when b̄-locking treatment is used a superior convergence is obtained. the slow convergence of the traditional fem beam is due to the inability to exactly represent curved geometry. see figure 6 for the results. on the other hand, isogeometric formulation with the locking treatment provides superior convergency. 5.2. convergence comparison in order to compare the timoshenko and the bernoulli beam formulations the error of the vertical tip displacement has been calculated as e = wref − w wref . (13) the reference solution have been calculated using fine mesh consisting of 320 control points individually for each formulation. the results are shown in figure 7. the better convergence is provided by the presented bernoulli beam formulation, nevertheless, the timoshenko beam element performs also very well. 5.3. applicability of bernoulli formulation finally, the applicability of the bernoulli beam formulation has been assessed. the reference solution of the timoshenko beam formulation has been used r1 r2 helicoidal spring: r = 2 m h = 3 m load: f = 3 kn/m cross-section: r1 = 0.11 m r2 = 0.15 m material: e = 200 gpa figure 5. helicoidal spring cantilever subjected to the constant force load and its cross-section. in order to verify the error caused by the neglect of the shear effects. the analysis has been performed for five different cross-sections in order to assess the behavior of beams with various thickness/length ratios. the results can be observed in table 1. as expected, the error rises significantly with the increasing radius of the cross-section. r1 0.11 0.31 0.51 0.71 0.91 r2 0.15 0.35 0.55 0.75 0.95 r2/l 0.012 0.027 0.042 0.058 0.074 error 0.7% 4.5% 10.9% 18.7% 27.1% table 1. the error of bernoulli beam formulation to the timoshenko reference solution for different crosssections. 6. conclusions two isogeometric beam elements have been implemented: the bernoulli beam element based on the work of a. m. bauer et al. [1] and the timoshenko beam element presented by g. zhang et al. [2] with b̄method [6] used as a locking treatment. three main aspects have been observed in order to compare the performance of the presented formulations. firstly, the numerical locking of the timoshenko beam element has been documented and the suitability of the locking treatment used has been verified. secondly, the convergence properties of both 15 edita dvořáková, bořek patzák acta polytechnica ctu proceedings -0.023 -0.0229 -0.0228 -0.0227 -0.0226 -0.0225 -0.0224 -0.0223 20 40 60 80 100 120 140 160 ve rti ca l t ip d is pl ac em en t number of control points (nodes) ref timoshenko timoshenko bbar fem -0.02293 -0.02292 -0.02291 -0.02290 -0.02289 -0.02288 -0.02287 20 40 60 80 100 120 140 160 ve rti ca l t ip d is pl ac em en t number of control points (nodes) ref timoshenko bbar fem figure 6. comparison of timoshenko beam element with no locking treatment with timoshenko beam element using b̄-method and with naturally locking-free standard fem beam element. the vertical axis corresponds to the vertical tip displacement w, the horizontal axis corresponds to the number of nodes (control points). 16 vol. 30/2021 comparison of timoshenko and bernoulli beams 0 1x10-6 2x10-6 3x10-6 4x10-6 5x10-6 6x10-6 7x10-6 8x10-6 9x10-6 30 35 40 45 50 55 60 65 70 75 er ro r o f t he v er tic al ti p d is pl ac em en t number of control points (nodes) bernoulli timoshenko bbar figure 7. comparison of covergence of timoshenko beam element and bernoulli beam element. the vertical axis corresponds to the erro of vertical tip displacement w, the horizontal axis corresponds to the number of nodes (control points). the error is calculated with respect to the reference solution of particular beam theory. formulations have been analyzed, proving the better behavior of the bernoulli formulation. finally, the applicability of the bernoulli beam element only to the thin structures has been demonstrated. the superior convergence compared to the traditional fem formulation can be observed for both formulations. acknowledgements the financial support of this research by the grant agency of the czech technical university in prague (sgs project no. sgs20/038/ohk1/1t/11) is gratefully acknowledged. references [1] a. bauer, m. breitenberger, b. philipp, et al. nonlinear isogeometric spatial bernoulli beam. computer methods in applied mechanics and engineering 303:101–127, 2016. [2] g. zhang, r. alberdi, k. khandelwal. analysis of three-dimensional curved beams using isogeometric approach. engineering structures 117:560–574, 2016. [3] t. j. r. hughes, j. a. cottrell, y. bazilevs. isogeometric analysis: cad, finite elements, nurbs, exact geometry and mesh refinement. comput methods appl mech engrg 194:4135–4195, 2005. [4] l. piegl, w. tiller. the nurbs book. springer-verlag berlin heidelberg, new york, 1997. [5] z. bittnar, j. šejnoha. numerické metody mechaniky 1. české vysoké učení technické, praha, 1992. [6] r. bouclier, t. elguedj. locking free isogeometric formulations of curved thick beams. comput methods appl mech engrg 245-246:144–162, 2012. 17 acta polytechnica ctu proceedings doi:10.14311/app.2019.23.0049 acta polytechnica ctu proceedings 23:49–53, 2019 © czech technical university in prague, 2019 available online at https://ojs.cvut.cz/ojs/index.php/app a retaining wall reconstruction in kutná hora jiří svoboda∗, dana hadačová pragoprojekt, a. s. (plc.), k ryšánce 1668/16, 147 54 prague, czech republic ∗ corresponding author: jiri.svoboda@pragoprojekt.cz abstract. this paper addresses the reconstruction of a historical stone retaining wall at the busy kremnická – táborská road in the town of kutná hora. the structure was in very poor condition. there was the danger of an accident involving a wall slide, or even a total collapse of the road structure. the construction site in question is located near the st. barbara’s church, in the area of the formerly important but now historical mine osel. with respect to the possibility of undermining, various geodetic surveys and observations of the surrounding area and rock mass were performed. keywords: retaining wall, undermined area, st. barbara’s church, kutná hora. 1. introduction on the basis of expert static assessment and juridical assessment of the retaining wall in kremnická and táborská streets a preliminary permit was issued in 2015 by the special building authority for the reconstruction works on the road no. iii/3377 pertaining to the condition of the current retaining wall and the adjacent part of the road. the wall was in serious disrepair and it was necessary for it to be fixed immediately, possibly because of the hazard of accident such as a wall slide, or even a total collapse of the road embankment. in the first stage of construction the necessary securing works were carried out in order to avert an emergency or to eliminate its possible effects and thus preventing any danger to health or life regarding the public. the aim of the following second stage of the rehabilitation works was to finish the whole wall and road structure and ensure long-term durability, safety and functionality. the construction site is near st. barbara’s church in the location of the important historical mine osel. with regard to the possibility of undermining, various geodetic surveys and observations of the surrounding area and rock mass were performed. a detailed research of historical mining materials was also done with the cooperation of local citizens. on the basis of output evaluations a definitive technical solution was adopted, mainly in areas of cave-ins, large underground space rehabilitation and roadbed remediation. 2. sources and geology the sources and inputs for design work were: • design documentation for building permit by top con servis, s. r. o. (ltd.), 03/2015, • engineering-geological survey by forvia cz s. r. o. (ltd.), 03/2015, • static assessment of the retaining wall condition by libor pokorný (authorized engineer for bridges and engineering structures), 05/2015, • construction site inspection done by the designer on 01/07/2015, which declared that the condition of the wall endangered traffic and therefore safety measures had to be taken immediately – and the very same day they actually were. on the basis of the above mentioned sources it was concluded that the major part of the retaining wall was in serious disrepair and the work for its securing and subsequent reconstruction were to start immediately. from the geomorphological point of view the location is part of the kutná hora plateau, which is formed by crystalline rock. the plateau surface is subject to intrusion by the steeply carved valleys of local watercourses. watercourse erosion has had a predominant impact on the formation of local steep rocky banks with bare sandstone strata benches. because of gravity impact and erosion processes, horizontally deposited sandstones split off along the predisposed discontinuity planes and slid into the valley in the form of blocks and benches of various sizes. the phenomenon of open crack occurrence and the detached blocks slight inclination to the valley and the watercourse is important for stability assessment on the edge of the valley slope. this gives a very uneven shape to the rock base in the wall foundation joint. the formerly important silver mine osel is located in the area concerned as is the osel vein range. in the middle ages the mining works were done under the sandstone layers in a zone of clay rocks or geological faults. the foundations of these workings were inadequately carried out, and that is why after the drifts were exhausted and dismantled, the clay rock gradually reduced to mush causing the present cave-ins. we faced this reality several times during the execution of drilling work when the drilling rig slumped. 49 https://doi.org/10.14311/app.2019.23.0049 https://ojs.cvut.cz/ojs/index.php/app jiří svoboda, dana hadačová acta polytechnica ctu proceedings anchors bond lengths were also located in the mushy surrounding because of the ancient mining. that led to a considerable overconsumption of the injecting material during the anchor bond lengths execution. for illustration, the underground scheme is shown in figure 1. the view of the original and new wall near st. barbara’s dome is captured in figure 2. the detail of the wall is depicted in figure 3. 3. essential requirements for the new design in designing the retaining wall reconstruction, the following requirements had to be taken into account: • two-way traffic in at least one lane had to be maintained in kremnická street during the excavations. • proper drainage of the retaining wall back. underground water was not expected, but there was a risk of rain water accumulation (including water from the road surface), because of leaking at the wall back, which might cause degradation of the bedrock. • the underground services and the surrounding buildings including the retaining walls on the opposite side of the street should not be endangered by the excavations. • the aesthetic design had to be made according to the requirements of the national heritage institute. 4. conception of the new design after meetings on conceptualisation with the ordering party (central bohemia road agency – ksus and kutná hora municipality) a basic variant of the retaining wall design was set up. a monolithic reinforced concrete cantilever wall with stone lining was designed to be built in a temporarily supported excavation [1]. the temporary excavation was supported by anchored piling in such a way that any interference with the existing underground services was excluded. the continuous raised guiding wall for embedding micropiles prevented car slide to the excavation, and therefore ensured some traffic safety in kremnická street and also kept rainwater out of the excavation. the reinforced concrete cantilever wall itself has a variable height circa 1.5–5.0m. its total length is circa 315.0m. it is founded on an underlay concrete slab 0.15m thick. the breadth of the footing is within 1.9– 2.5m and the height 0.45–0.65m. the wall was mainly built in 5–6m long sections. in some sections, where the geology was unfavourable or where there were additional static reasons, the footing was founded on steel micropiles. the wall back drainage is provided by passages embedded to the stem and the longitudinal drainage system behind the wall. a small stone wall 0.4m thick and 0.9m high was built over the upper level of the retaining wall. with respect to the location (barrier curb, design velocity) there was no need to give this stone wall some dimensions as a safety barrier for traffic. the face of the new retaining wall is also walled in with stone. this lining is in the upper part gradually merged with the stone wall structure and capped with a flat stone roof 0.55 m wide. the aboveground stone part and the stone lining are integral parts of the retaining wall static system. local and overall stability of the retaining wall structure was assessed in its particular characteristic sections. 5. geotechnical monitoring and monitoring board geotechnical monitoring (gtm) was carried out for the contract owner directly. gtm is mandatory for structures where serious negative impacts are possible for the public or either built or neighbouring objects. the placing of some measuring points including the survey of them was undertaken in advance of the actual construction work. the rules for implementation of gtm and monitoring board leadership were given in the tender bid documentation. detailed passport of all affected buildings and underground services was made. for the second stage a new contractor was chosen. that meant, a re-passport of all objects had to be made in the zone of influence and at the same time all the observations and monitoring within the gtm had to be evaluated. the second re-passport was made at the end of construction, resulting in only two objects being damaged in the context of the construction. a geodetic survey on objects and walls, inclinometrical observation of the rock mass, crack monitoring on objects and also regular visual control of influenced objects and underground services were all done in the field. the outputs were regularly evaluated and construction procedure was continuously modified according to the monitoring board and its expert conclusions. simultaneously, the geological condition found on site was monitored with expert specialist geotechnical supervision. on the basis of output evaluations the definitive technical design was modified, mainly in the area of cave-ins and large underground space rehabilitation. subsequently static calculations had to be resolved with impacts on design detail. two major cave-ins happened on the site during the construction. right under the carriageway a free underground space of circa 65m3 was rehabilitated. some other surveying drills were made to find other free spaces especially under the carriageway. all these caves were filled with cement-ash mixture followed by a low-pressure cement injection. completion of the rehabilitation of surrounding buildings, started in advance, was also finished within the monitoring. 50 vol. 23/2019 a retaining wall reconstruction in kutná hora figure 1. underground space scheme. 6. on the history of mining in kutná hora on the basis of knowledge gathered from local citizens during re-passport and research of archival materials, especially the monograph "kutnohorské dolování" by j. bílek [2], it could be ascertained that the site is in the area of the important ancient mine osel (named by a burgomaster called asinus) in the osel vein range. at least 50 bigger mines are estimated to have been operated in the town area from the 13th to the 16th century. the majority faded out during the hussite wars. the osel mine manifested the wealth and heroic status of the miners in the medieval town of kutná hora. this mine became a status symbol of the miner and therefore a focus for mining authority. not just the beautiful and impressive churches of st. barbara and st. jacob (in kutná hora) were built from the mining profits, but also contributions had to be made to st. vitus cathedral in prague. mining in the osel range expanded in the 13th to 15th centuries and it penetrated to the depth of almost 500m. in the year 1541 an underground water outflow to osel took the life of eight miners and signalled the end of mining in 1545. the main shaft location has not been found until now. it is estimated to be located near the jesuit college. the other shafts were located close to each other nearby the limestone quarry behind st. barbara’s (mladé and staré rousy). the waste bank is documented in the area of žižkov and near všech svatých. the deepest point of this complex was located under the zelená hora street loop above the bylanka valley. the oldest shafts in the area concerned were opened probably before the end of the 13th century, when the upper parts of the ore veins were mined. these were settled within small horizontal distances of 20 to 50m with cross-sectional dimensions of 1.2×1.5m, or 1.5×2.0m. in loess and marl the shafts were timbered. the mines were drained by drainage galleries driven from the vrchlice valley. these galleries hooked up to the shafts at a level of 30 to 50m under the surface. shaft backfills were built almost entirely from the slag heaps. over centuries they were affected by underground water circulation. naturally, that would be the reason for the gradual relaxation and also often for the cave-ins. additionally, in the shafts the clay rock gradually reduced to mush, again causing cave-ins. the risk of cave-ins in these ancient shafts and galleries is nowadays small, although it cannot be completely excluded. therefore, any possibility must be taken into account for obvious safety reasons. the whole osel range consists of five rich ore veins stretching along the left side of the vrchlice river from holy trinity church to the western part of the town. the very old mining workings stretch from bylany over so-called cech and vyšata quarry to the west of st. barbara’s dome. indeed that is why the relics of mining were expected to be found on the construction site and that was proved by object movements in both táborská and kremnická streets. these movements were not caused by the security actions for construction but by the presence of these very old and defunct mine workings. 51 jiří svoboda, dana hadačová acta polytechnica ctu proceedings figure 2. view of the original (top) and new wall (bottom) near st. barbara’s dome. 7. georadar – a geophysical survey method with respect to the above mentioned facts the question of other disrupted places was raised during the construction process. these faults could in the future endanger not only the new retaining wall but also the adjacent carriageway that has itself been reconstructed during the construction including the new structural pavement layers. gravimetric data were processed and evaluated. the principal result of the survey led to the finding that there was several other places under the road where there existed relatively continuous ranges with a lack of material. from the geophysical point of view it was recommended to rehabilitate these places preferably with injections to the depth of 10m. the use of georadar in the final stage of the construction process in order to check the roadbed proved to be positive and valid [3]. it was decided to replace the original pavement design (reduced pavement structure in one half of the carriageway and only reconstruction of asphalt layers in the other half) for a complete reconstruction of the road structure in the whole rebuilt section. 8. conclusions design work started with the contract owner’s request to appraise the static assessment for the original retaining wall by libor pokorný. just after the first inspection of the wall and road in question the author of this appraisal pronounced the static situation to be very serious, and endangering not only traffic. the issue gradually developed from a small problem of carriageway slump and collapse of a rather small part of the wall to the bigger problem of global wall instability including the presence of undermining under the road. the amount of work needed gradually increased. the situation had to be solved quickly and as a complex whole. the designer’s thoroughness and often indeed intransigence paid off because a complex interconnected solution was carried right through to finality. the active cooperation of all affected authorities and organizations, contract owners – ksus and the municipality of kutná hora, with both contractors and the designer was a necessary condition for managing the situation. property relations responsibilities complicated the problem solving, because the road owner is the central bohemian region, while sidewalks and some of the walls belong to the town of kutná hora, and then leaving a major part of the walls belonging to private owners. for this reason a memorandum for a common course of action was initiated. besides the property municipality department and traffic municipality department, the officers from municipality department for national heritage, education and culture of kutná hora took part in the monitoring board. they consulted each other actively regarding the shape and appearance of the new wall and participated on the selection of proper stone, or possibly and more precisely the source quarry for the new wall lining. respected experts from the department of geotechnics of the fce of ctu prague (prof. jiří barták and assoc. prof. jan masopust) also made a considerable contribution. taking account of the danger of delay a very unusual instance of cooperation with the private owners of the surrounding estates was established, and this led to a prompt start to the process of implementing the geotechnical monitoring and then on to the construction work itself. the structure was deemed in the 3rd geotechnical category because of its complexity, and this made gtm necessary. local citizens themselves understood the importance of the situation and enabled their objects for passport and surveying point installation and facilitated incident-free surveys. the initial meeting was held in the municipal hall with the attendance of the town officials, representatives of the offering party, contractor and designer. the designer gave a detailed presentation on the design construction activities. an individual approach to the private owners in the affected zone proved to be crucial. personal exchange and discussion on any particular problems 52 vol. 23/2019 a retaining wall reconstruction in kutná hora proved much more advantageous than relying on written receipted and recorded correspondence. it brought benefits during the construction process, when hazardous works were performed near the objects and there was a need for safety monitoring and controlling the condition of objects. mutual reliance among all partners in the building process, involving the fulfilment of assurances and keeping to deadlines, proved to be an essential condition for successful construction management of such a complex structure. references [1] p. turček. foundation engineering (zakládání staveb). jaga, bratislava, 2005. [2] j. bílek. kutnohorské dolování. kuttna, 2000. [3] j.-l. briaud. geotechnical engineering: unsaturated and saturated soils. wiley, new york, 2013. figure 3. structural condition comparison: the old (top) and the new wall (bottom). 53 acta polytechnica ctu proceedings 23:44–48, 2019 1 introduction 2 sources and geology 3 essential requirements for the new design 4 conception of the new design 5 geotechnical monitoring and monitoring board 6 on the history of mining in kutná hora 7 georadar – a geophysical survey method 8 conclusions references acta polytechnica ctu proceedings doi:10.14311/app.2020.26.0001 acta polytechnica ctu proceedings 26:1–6, 2020 © czech technical university in prague, 2020 available online at http://ojs.cvut.cz/ojs/index.php/app homogenization of unreinforced old masonry wall comparison of scalar isotropic and orthotropic damage models vasco bernardo, tomáš krejčí, tomáš koudelka, michal šejnoha∗ czech technical university in prague, faculty of civil engineering, department of mechanics, thákurova 7, 166 29 prague 6, czech republic ∗ corresponding author: sejnom@fsv.cvut.cz abstract. masonry is a heterogeneous composite material made of bricks bonded by a mortar matrix. modeling such a material on macroscale typically calls for homogenization adopting a suitable constitutive model capable of capturing its quasi-brittle behavior. the present contribution concentrates on comparison and potential application of classical isotropic and orthotropic damage models in the framework of strain based first order numerical homogenization. as an illustrative example, a representative volume element in terms of a periodic unit cell is constructed to address the response of an unreinforced masonry (urm) structure typical of “placa” buildings (mixed masonry-reinforced concrete buildings) built in portugal. the performance of the two models is examined on the basis of macroscopic stress-strain curves constructed for both tensile and compressive loading. the selected geometrical constraints clearly identify the differences in the predictive capabilities of the two models. keywords: homogenization of unreinforced masonry wall, periodic unit cell, scalar isotropic damage model, orthotropic damage model, “placa” buildings. 1. introduction the portuguese housing stock results from more than eight centuries of history expansion. the building area consists mostly of residential buildings and it is estimated that half of the existing building stock in lisbon is represented by old masonry buildings [1]. four typologies of masonry buildings are typically recognized in the lisbon metropolitan area: buildings built before 1755, “pombalino” buildings built after the 1755 earthquake, “gaioleiro” buildings built between 1870 and 1930 and “placa” buildings 1930 to 1960 [2]. figure 1. scheme of "placa" building typology. the “placa” buildings, see fig. 1, now enjoy a particular interest as they represent 32 % of the portuguese housing stock [1]. as seen in fig. 1, the structure consists of a concrete slab simple supported by masonry walls. the exterior walls of these buildings are mainly composed of resistant stone masonry walls or by solid clay bricks with thickness greater than 0.50 m. the interior walls are usually made of solid or hollow clay bricks with thickness up to 0.30 m. the slabs are made of reinforced concrete typically with a total thickness of about 0.10 m and frequently with a single reinforcement layer and poor concrete. such building experience high vulnerability to damage when subject to seismic loads. this is attributed to insufficient strength or deformation capacity of masonry walls. it is also worth pointing out that no impact of earthquake has been considered in their design as the first portuguese seismic design regulation appeared not until 1958. predicting the bearing capacity of such buildings is thus of paramount importance, particularly in areas with a moderate to high seismic risk. a reliable prediction of the evolution of potential failure due to earthquake then strongly depends on the selected constitutive law for both concrete and masonry. herein, this issue is addressed in the light of a typically used isotropic scalar damage model [3] and more advanced orthotropic damage model allowing for an independent evolution of damage in three principal strain directions [4]. while concrete is usually assumed isotropic the masonry is only locally isotropic at the level of individual constituents whereas on macroscale it possesses an orthotropic material symmetry. to allow for treating masonry as a homogeneous anisotropic medium thus requires ho1 http://dx.doi.org/10.14311/app.2020.26.0001 http://ojs.cvut.cz/ojs/index.php/app v. bernardo, t. krejčí, t. koudelka, m. šejnoha acta polytechnica ctu proceedings mogenization. this issue is partially addressed here with reference to one specific topology of a masonry wall linked to the “placa" building. this choice is supported by available experimental data provided by the national laboratory for civil engineering in portugal (lnec) [5, 6]. the paper is organized as follows. following this introductory part, the two constitutive models are briefly reviewed in section 2. the basic grounds of the theory of homogenization are introduced next in section 3. the core of the present work appears in section 4 devoted to the numerical analysis of unreinforced masonry wall via homogenization. the principal findings are finally summarized in section 5. 2. damage models generally, damage models distinguish three states of the material virgin material, damaged material and pseudo-undamaged states. in a virgin state, the material contains no defects and behaves as linearly elastic. at points where the strength limit is exceeded the material experiences damage. in the most simple case of a scalar isotropic damage model, the damage evolution is governed by a dimensionless damage parameter ω written for one-dimensional case as ω = ad a , (1) where ad is the part of the cross section with evolved defects, i.e with the material in a damaged state and a is the total cross section area. the corresponding stress-strain relation reads σ = (1− ω)deε, (2) where σ is the stress vector, de is the elastic stiffness matrix and ε is the vector of strain components. the scalar damage parameter ω ∈ [0; 1] characterizes the material state, where ω = 0 represents an undamaged virgin state, while ω = 1 corresponds to fully evolved defects. the transition states from 0 to 1 are described by the damage evolution law. the present formulation builds on one-dimensional traction-separation law given by [7] σ = ft exp ( −wcr uf ) , (3) where ft is the tensile strength, wcr is the crack opening and uf is the parameter controlling the slope of the softening branch. to partially avoid mesh dependency of the results typical of materials with softening the crack opening is smeared over the element as [7] κ− κe = ωκ = wcr lch , (4) where the mazars equivalent strain κ is provided by κ = √√√√ 3∑ α=1 h(εα)2, (5) where h(·) is the heaviside function, κe is the elastic part of κ and lch is the element characteristic length. combining a 1d format of eq. (2) with eqs. (4) and (3) and replacing the strain ε with the maximum equivalent strain in the loading history κ̄ yields the resulting nonlinear equation to be solved for the damage parameter ω (1− ω)eκ̄ = ft exp ( −ωlchκ̄ uf ) , (6) for states where κ̄ exceeds the elastic threshold ε0. the scalar isotropic damage model can be used successfully for the description of 1d stress states, e.g., 1d tension or bending. a clear drawback is evident from a general multidimensional format of stress-strain relation (2) suggesting the reduction of the entire stiffness matrix even in cases when damage evolves in one direction only. the remedy can be provided by anisotropic [4] or orthotropic damage models. introducing two independent damage parameters dt α and dc α for tension and compression, respectively, gives an equivalent format of eq. (6) now written for individual principal strain directions as ( 1−dβ α ) e|εβα| = fβ exp ( −d β αlch|εβα| uβf ) , (7) where β ∈ [t, c] identifies either tensile or compressive failure. further details regarding both models can be found in [8] and [9]. 3. numerical homogenization in the present study, we limit our attention to a strain based first order homogenization only. to this end, consider a heterogeneous body loaded on its outer boundary by displacement field compatible with a macroscopically uniform strain e in an equivalent homogeneous medium. this allows us in the framework 1st order homogenization to split the local displacement and strain as [10] ∆u(x) = ∆e x + ∆u∗(x), (8) ∆ε(x) = ∆e + ∆ε∗(x), (9) where u∗(x) is the fluctuation part of the local displacement field u(x), ε∗(x) and ε(x) are the corresponding strains, and x is the spatial coordinate. next, suppose that the computational model is defined in terms of a periodic cell (puc) taking the actual brick layout into account. for “placa” building the assumed puc appears in fig. 2. the wall thickness is 250 mm, constituted by bricks with nominal dimensions of 250×120×70 mm and 10 mm for the mortar layer. because the present formulation assumes the macroscopic strain increment ∆e in eq. (8) be prescribed, the solution is searched in terms of the fluctuation rather than total displacements. the stepping stone in deriving ∆u∗ is the hill lemma written as〈 δεt(x)∆σ(x) 〉 = 0, (10) 2 vol. 26/2020 homogenization of unreinforced old masonry wall figure 2. periodic rve for the masonry wall. where the left hand side represents the volume average of virtual work done by local stress and strain fields. in the framework of finite element discretization, we approximate ∆u∗(x) in terms of their nodal values ∆r to write ∆u∗(x) = n(x)∆r, ∆ε∗(x) = b(x)∆r. (11) substituting from eq. (11)2 into (10) yields the final system of algebraic equations in the form k∆r = ∆f , (12) where k = 1 ω ∫ ω bt(x)l(x)b(x) dω, (13) ∆f = 1 ω ∫ ω bt(x)l(x)∆e dω, (14) where l is the material stiffness matrix. it is clear that in order to satisfy 〈∆ε(x)〉 = ∆e, (15) the volume average of the fluctuation part of the displacement field must disappear. this is achieved by adopting the periodic boundary conditions, which in this particular example of a rectangular puc amount to the same displacements u∗ on the opposite boundaries of puc. the rigid body motion is then constrained by simply fixing the corner displacements. note that eq. (12) can be sought as part of the solution of more complex computational framework based on multiscale analysis illustrated in fig. 3. in such a case, the actual masonry wall is replaced by an equivalent homogeneous one, so there is no need to consider all the geometrical details on macroscale. such a distinction between bricks and mortar is made on the mesoscale only being represented by a specific puc. in case of a fully uncoupled analysis, the theory of homogenization then serves to provide all the necessary data needed in the macroscopic constitutive model. the unit cell simulations then represent a virtual tester to substitute for complex or even unfeasible large scale laboratory experiments. this modeling concept is adopted henceforth providing both the effective elastic properties and various macroscopic stress-strain curves for the selected loading scenarios to compare performance of the two selected models. figure 3. framework for the homogenization of masonry wall. 4. numerical study the present section is devoted to the derivation of macroscopic response for various loading scenarios limiting attention to prescribed macroscopic uniform strains, as suggested in section 3. figure 4. masonry puc finite element model. the geometrical model displayed in fig. 2 is exploited. the corresponding, relatively fine, finite element mesh is plotted in fig. 4. this mesh consisting of four node quadrilateral elements is used first in the derivation of effective elastic properties and then in the prediction of a nonlinear response when loading the puc beyond the material elastic limits. 3 v. bernardo, t. krejčí, t. koudelka, m. šejnoha acta polytechnica ctu proceedings material e [gpa] ν [-] ft [mpa] gft [n/m] fc [mpa] ρ [kn/m3] brick 13.0 0.2 2.0 58.0 40.0 18.0 mortar 0.7 0.2 0.1 10.0 1.30 17.5 table 1. material parameters of brick and mortar. 4.1. effective elastic properties from homogenization the basic prerequisite in the application of homogenization is the knowledge of material data of individual phases as also intimated in fig. 3. in case of “placa” masonry, the computational model consists of clay bricks and mortar produced from the cement, lime and sand mixture in the proportions of 1:3. to collect the required properties, the specimens extracted during rehabilitation works from a building located in the center of lisbon were examined in the national laboratory for civil engineering in portugal (lnec), see fig. 5. the resulting material properties of both phases are listed in table 1. the same material properties for bed and head joint are assumed. further details regarding the actual experimental program are available in [5, 6, 11]. (a) (b) figure 5. axial compression tests: a) solid clay bricks, b) mortar [5]. the effective elastic plane-stress stiffness matrix is found first from four independent elasticity solutions when the unit cell is loaded in turn by each of the components of the macroscopic strain e equal to one while the remaining components vanish. the volume averages of the local stress then furnish individual columns of the homogenized effective stiffness matrix, see [12] for further details. the extracted young’s moduli and poisson’s ratios appear in table 2 clearly identifying the orthotropic material symmetry the “placa” masonry. e11 e22 g12 ν12 ν21 [gpa] [gpa] [gpa] [−] [−] 6.68 4.03 0.67 0.089 0.147 table 2. equivalent elastic properties for rve. 4.2. nonlinear response of “placa” masonry this section compares the macroscopic nonlinear response of “placa” masonry due to simple tension and compression applied independently in the x and y directions as plotted in fig. 6 also identifying the prescribed components of the macroscopic strain e. this is clearly a constrained problem as only one strain component in the principal material direction is assumed nonzero. however, because of heterogeneity all local strain and stress components become active to give rise damage evolution due to tensile failure even in case of compressive loading. (a) (b) figure 6. applied loading: a) et = {±exx, 0, 0}, b) et = {0,±eyy, 0}. the resulting macroscopic stress strain curves (σ– e) are plotted in fig. 7. the corresponding damage pattern associated with the last converged step for individual loading conditions appears in figs. 8 11. it is interesting to point out clear differences between the predictions provided by the two constitutive models observed in figs. 7(a-c). particularly the difference in the predicted strength is attributed to the fact that unlike in scalar isotropic damage model the orthotropic damage model reduces the stiffness independently in the principal strain direction according to eq. (7) providing the condition |εβα| > ε0 either for tension or compression is met. this is also supported by the evolution of stress component in the direction normal to the applied load. in this con4 vol. 26/2020 homogenization of unreinforced old masonry wall -0.002 -0.0015 -0.001 -0.0005 0 e xx [-] 0 -1.5 -1 -0.5 σ [ m p a ]σ xx orthotropic model σ yy othotropic model σ xx scalar isotropic model σ yy scalar isotropic model (a) -0.002 -0.0015 -0.001 -0.0005 0 e yy [-] 0 -1.5 -1 -0.5 σ [ m p a ] σ xx orthotropic model σ yy othotropic model σ xx scalar isotropic model σ yy scalar isotropic model (b) 0 0.0005 0.001 0.0015 0.002 e xx [-] -0.2 -0.1 0 0.1 0.2 0.3 0.4 σ [ m p a ] σ xx orthotropic model σ yy othotropic model σ xx scalar isotropic model σ yy scalar isotropic model (c) 0 0.0005 0.001 0.0015 0.002 e yy [-] 0 0.05 0.1 0.15 σ [ m p a ] σ xx orthotropic model σ yy othotropic model σ xx scalar isotropic model σ yy scalar isotropic model (d) figure 7. macroscopic stress-strain curves: a) compression (exx < 0) applied in x direction, b) compression (eyy < 0) applied in y direction, c) tension (exx > 0) applied in x direction, d) tension (eyy > 0) applied in y direction. (a) (b) (c) figure 8. damage pattern due to exx < 0 (compression): a) isotropic model, b) orthodam-11, c) orthodam-22. (a) (b) (c) figure 9. damage pattern due to eyy < 0 (compression): a) isotropic model, b) orthodam-11, c) orthodam-22. (a) (b) (c) figure 10. damage pattern due to exx > 0 (tension): a) isotropic model, b) orthodam-11, c) orthodam-22. (a) (b) (c) figure 11. damage pattern due to eyy > 0 (tension): a) isotropic model, b) orthodam-11, c) orthodam-22. 5 v. bernardo, t. krejčí, t. koudelka, m. šejnoha acta polytechnica ctu proceedings text, the results plotted in fig. 7(d) should not be surprising once realizing the geometrical arrangement of the puc. clearly, if tension is applied along the y axis, the weak mortar behaves as the weakest link. so, in this case, the stiffness along the x axis, which essentially keeps the original value in case of the orthotropic model, provides no support resulting in a similar behavior predicted by both models. although not exactly coincident with the axes of material orthotropy the damage parameter developed in the (11) principal direction (e.g., direction (11) is close to the x direction for the exx strain component prescribed) is significantly greater in comparison to the one in the second (22) principal direction, which appears negligible. it should also be mentioned that even though the two macroscopic normal stresses (σxx,σyy) figs. 7(a,b) are both compressive due to the applied constraints, e.g. exx 6= 0, while eyy = 0, the local failure is in tension because of positive strains developed as a result of material discontinuity and stress concentration at vertices of the brick and mortar interface. this causes the damage pattern in figs. 8 and 10 and figs. 9 and 11 be similar. however, because of the geometrical arrangement, the two loading conditions predict different strength limits. but as already mentioned, the maximum compressive stress reached should not be associated with the compressive strength of the masonry material. given the properties in table 1, it is not surprising that the damage localizes solely in the mortar phase for all loading scenarios. 5. conclusions the present paper was concerned with numerical modeling of masonry structure associated with the “placa” building in the framework of 1st order homogenization. apart from the derivation of effective properties of the selected masonry topology the research objectives focused on the prediction of macroscopic response employing two different constitutive models. limiting attention to tensile failure, the performance of a simple isotropic scalar damage model was compared to more advanced orthotropic damage model. the latter one allows for an independent evolution of damage in three principal directions of the local strain, thus leading in general to more stiff response as illustrated in this for both compression and tension loading condition. in this regard, the loading was introduced by prescribing the rate of macroscopic strain. only the loading in principal directions of material orthotropy was considered thus introducing some constraints to the macroscopic response. while such loading conditions appear useful in highlighting the essential differences in the behavior of the two damage models, they cannot be used in the estimation of effective parameters of the macroscopic model such as the tensile strength or the fracture energy in tension. prediction of such parameters would require combining the stress driven homogenization up to the onset of failure with a specific format of the strain-driven procedure to tract the descending part of the macroscopic stress-strain law consistent with a unidirectional stress state. this study will be the subject of our future research. acknowledgements the financial support supported by the fct project no.pd/bd/135325/2017 and by the gačr grant no. 18-24867s is gratefully acknowledged. references [1] f. tiago, j. milosevic, r. bento. seismic vulnerability assessment of a mixed masonry rc building aggregate by linear and nonlinear analyses. bulletin of earthquake engineering 14:1765–2327, 2016. 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[8] t. krejčí, t. koudelka, j. šejnoha, j. zeman. computer simulation of concrete structures subject to cyclic temperature loading. in b. h. v. topping, l. f. costa neves, r. c. barros (eds.), proceedings of the twelfth international conference on civil, structural and environmental engineering computing. civil-comp press, stirlingshire, united kingdom, 2009. paper 131. [9] t. koudelka, t. krejčí, j. šejnoha. analysis of a nuclear power plant containment. in b. h. v. topping, l. f. costa neves, r. c. barros (eds.), proceedings of the twelfth international conference on civil, structural and environmental engineering computing. civil-comp press, stirlingshire, united kingdom, 2009. paper 132. [10] j. c. michel, h. moulinec, p. suquet. effective properties of composite materials with periodic microstructure: a computational approach. computer methods in applied mechanics and engineering 172:109–143, 1999. [11] a. i. marques, p. x. candeias, j. g. ferreira, m. r. veiga. ensaios de compressao diagonal em paredes antigas de alvenaria de tijolo. in construcao 2018, porto. 2018. [12] m. šejnoha, j. zeman. micromechanics in practice. wit press, southampton, boston, 2013. 6 acta polytechnica ctu proceedings 26:1–6, 2020 1 introduction 2 damage models 3 numerical homogenization 4 numerical study 4.1 effective elastic properties from homogenization 4.2 nonlinear response of ``placa'' masonry 5 conclusions acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2018.20.0056 acta polytechnica ctu proceedings 20:56–64, 2018 © czech technical university in prague, 2018 available online at http://ojs.cvut.cz/ojs/index.php/app thermo-mechanical fatigue analysis of a steam turbine shaft martin nesládeka,∗, josef jurenkaa, michal bartošáka, milan růžičkaa, maxim lutovinova, jan papugaa, radek procházkab, jan džuganb, petr měšťánekc a czech technical university in prague, faculty of mechanical engineering, technická 4, 166 07 prague 6, czech republic b comtes fht a.s., průmyslová 995, 334 41 dobřany, czech republic c doosan škoda power s.r.o., tylova 1/57, 301 28 plzeň, czech republic ∗ corresponding author: martin.nesladek@fs.cvut.cz abstract. increasing demands on the flexibility of steam turbines due to the use of renewable energy sources substantially alters the fatigue strength requirements of components of these devices. rapid start-ups as well as the increased number of the load cycles applied to the turbines must be handled by design methodologies. the goal of the work presented in this paper was to provide a computational framework applicable to the thermo-mechanical fatigue (tmf) prediction of steam turbine shafts. the so-called damage operator approach by nagode et al. has been implemented to the software codes and applied to fatigue analysis of the thermo-mechanical material response computed numerically by the finite element analysis. experimental program conducted in order to identify the material thermo-mechanical behavior and to verify numerical simulations is introduced in the paper. some results of tmf prediction of a sample steam turbine shaft are shown. keywords: fatigue prediction, thermo-mechanical fatigue, finite element analysis, chaboche elasticplastic model. 1. introduction the energy industry has been experiencing significant changes recently. the overall share of renewable sources in the european union countries has reached 17 percent [1]. the leading country in this statistic is sweden having more than 50 percent of energy from renewable sources. in czech republic it is approximately 15 percent and the overall trend is that this share will continue to grow significantly in the future the eu strategy is to reach at least 27% by 2030. however, instability of the power output due to the nature of the renewable sources may cause problems in energy supplies if it is not compensated by some back-up sources. currently, there are no reliable ways to save energy for power shortages in the network in a sufficiently large scale. this role must be taken over by traditional thermal power sources such as gas-fired power plants providing "just-in-time" energy supplies. the problem is that these plants already have to or will have to cover shortages in the daily regime. increased number of start-ups and shut-downs as long as increasing demands on speed-up of the start-ups set new challenges in the gas and steam turbine design. components of steam turbines are often exposed to thermo-mechanical loading due to variable thermal and mechanical loading conditions induced by the turbine start-ups and shut-downs. the main damage mechanisms, which if they occur together constitute the therm "thermo-mechanical faigue" (tmf), are often referred to as oxidation, mechanical fatigue and creep [2, 3]. constitutive material model derived from the material testing, finite element analysis and suitable fatigue criterion is required for the reliable damage assessment of components subjected to cyclic thermo-mechanical loading conditions. this work is focused on the tmf assessment of a sample case of a steam turbine shaft. motivation for this work originates from the need for revising the current steam turbine design strategies due to the above-mentioned reasons. classical low-cycle fatigue (lcf) criteria as smithwatson-topper (swt) [4] and the nihei [5] methods in their original form can only cope with isothermal loading. for the case of thermo-mechanical loading, cycle reference temperature, which could be used to select fatigue curves for the isothermal models, is somewhat arbitrary and error-prone. the so-called damage operator approach (doa) by nagode et al. [6, 7] overcomes this issue by introducing a procedure for continuous damage parameter calculation. in the original form it is based on the swt method, however the damage induced by nonstationary thermal and mechanical loading is evaluated in a chronological manner, i.e. damage is computed and accumulated in any time point no matter if it is load reversal or not. such approach allows for handling arbitrary thermal and mechanical load 56 http://dx.doi.org/10.14311/app.2018.20.0056 http://ojs.cvut.cz/ojs/index.php/app vol. 20/2018 tmf analysis of a steam turbine shaft sequences without the need for evaluating the abovementioned cycle reference temperature. the work presented here was done within the flexturbine project, which aims to revise the design of key turbine components and propose new design methodologies so that the turbines can cope with the changing situation in the transmission grids. the paper describes the approach to prediction of thermomechanical fatigue of steam turbine shafts. especially the high-pressure sections of turbine shafts are exposed to complex thermo-mechanical loading due to variable service conditions. within the experimental part of the work, experiments for identifying parameters of the selected constitutive model as well as parameters of damage model were carried out. the assumed constitutive model of the material mechanical response is temperaturedependent elastic-plastic and for the purpose of the structural finite element analysis (fea) parameters of the chaboche non-linear kinematic hardening were identified. the doa allows for processing stress histories computed by fea in order to perform correction on the material viscous behavior, if needed. strain controlled fatigue tests with hold period may be used for finding the parameters of the maxwell model of viscoplasticity, which may then be applied for evaluating the viscous strain and subsequent stress relaxation. a set of tmf tests for verifying the prediction methodology have been scheduled. various phasing of temperature and mechanical load is tested. the experiments are conceived as strain-controlled with two channels of strain applied to tubular specimens axial and torsional. tmf test setup is briefly introduced in the paper and some preliminary results are presented. thermo-mechanical stress-strain response from the tests and numerical simulations is compared. the elastic-plastic material model is used in the structural fea of the steam turbine shaft, which is performed on the basis of the simulated temperature fields that are obtained from the previous transient thermal fea. finite element calculations for the steam turbine shaft are performed in the commercial finite element software. subsequently, tmf is predicted for the turbine shaft on the basis of the calculated stress, strain and temperature histories. 2. experimental work as mentioned within the introduction section, the experimental work may be divided into two main groups depending on the general purpose of the individual test on the one hand basic material tests to identify parameters of constitutive models and on the other hand tests intended for verifying the proposed modeling methodology. the material subjected to testing was a chrome-molybdenum creep resistant steel applicable to steam turbine shafts. figure 1. the temperature dependent young’s modulus. figure 2. the temperature dependent cyclically stable cyclic stress-strain curves. figure 3. the temperature dependent manson-coffin and basquin curves. 57 m. nesládek, j. jurenka, m. bartošák et al. acta polytechnica ctu proceedings (a) . detail of grips with the test specimen and instrumentation (b) . overall view of the tmf test setup (c) . tubular test specimen with surface speckle pattern and welded thermocouple figure 4. tmf test setup and specimen. 1 servohydraulic axial-torsional test machine, 2 resistance heating, 3 camera of the dic system, 4 infrared camera, 5 test specimen. 2.1. basic mechanical material tests static tensile tests as well as lcf tests under the isothermal conditions have been carried out. the assumed temperature range of the tests was 20 – 600 ◦c, which correspond to operating conditions of the sample steam turbine. circular specimens with 10 mm diameter in the gauge section have been tested. radiant heating of specimens was controlled by a split furnace. as a minimum, ten samples have been tested on each curve. fig. 1, 2 and 3 show the main results of the isothermal material tests, i.e. temperature dependent normalized young’s modulus, cyclic stress-strain curves (csscs) and the manson-coffin and basquin curves, respectively. the young’s modulus and the csscs have been used for identifying the elastic-plastic material model based on the chaboche’s procedure section 3. the temperature dependent manson-coffin and basquin curves are used as an input to the doa. numerical procedure based on this approach is described in section 4. lcf strain-controlled tests with hold period have been performed in order to identify temperature de0 500 1000 1500 0.2 0.4 0.6 0.8 1 1.2 t [s] n o rm a liz e d s tr e s s [ − ] 450 ° c 550 ° c 600 ° c figure 5. the temperature dependent stress relaxation curves. pendent stress relaxation curves fig. 5. this is an important input to the doa allowing for separating elastic-plastic and viscous strain and modeling stress relaxation section 4. note that the curves in fig. 5 are shown for illustrative purposes and will be further refined with continuation of the experimental program. 58 vol. 20/2018 tmf analysis of a steam turbine shaft figure 6. time dependence of axial strain and temperature during the in-phase tmf test. furthermore, creep tests should be conducted in order to identify constants of the larson-miller parameter that is applied to evaluating creep damage. 2.2. verification tests experimental setup for tmf tests is shown in fig. 4a and 4b. the test machine is equipped with a servohydraulic actuator allowing for combined axial and torsional loading. resistance heating is applied to specimens and the induced temperature is controlled by a thermocouple welded in the center of the specimen gauge section. online axial and torsional deformation control is done by an optical measurement system based on the digital image correlation (dic). tubular specimens with 8.5 mm in diameter and 0.75 mm wall thickness of the gauge section are sprayed with temperature-stable paint creating a random speckle pattern fig. 4c. design of the test specimens conforms to astm e2207 standard. triangular strain and temperature in-phase and out-of-phase loading is assumed within the tests fig. 6 and 7. until now, tests with axial strain loading at εa = 0.007 and r = −1 have been conducted. the maximum test temperature is 600 ◦c. more tests combining axial and torsional loading with various phases are scheduled in order to check the prediction quality of the selected model. 3. numerical simulations 3.1. constitutive material model a constitutive material model must be able to describe the cyclic material behavior for the service temperature range of the steam turbine shaft. the temperature-dependent chaboche non-linear kinematic hardening model [8, 9] is selected for the elasticplastic fea. the yield criterion may be expressed as f(σ −α) = k, (1) where k is the material temperature dependent yield stress, σ is the stress tensor and α denotes the backfigure 7. time dependence of axial strain and temperature during the out-of-phase tmf test. stress tensor. the function f() denotes the von mises stress norm, which may be in this case evaluated as f(σ −α) = √ 3 2(s −α′) : (s −α′), (2) where s and α′ are the deviatoric parts of the stress and the backstress tensors, respectively. the : operator denotes the tensor contraction. assuming the non-linear kinematic hardening model, the total backstress is composed of multiple backstress components as follows: α = ∑ i αi (3) the non-isothermal evolution of the backstress components is defined as: α̇i = 2 3ciε̇p − γiαiλ+ 1 ci ∂ci ∂t αiṫ . (4) ci and γi are temperature dependent material parameters, which may be derived from the material mechanical cyclic stress-strain response. εp is the plastic strain tensor and λ is accumulated plastic strain. in our case, the model with three backstress terms was selected, where γ3 is assumed constant and equal to zero. plots in fig. 8 and 9 show comparison of the measured temperature-dependent material mechanical response with the one simulated by finite elements using the above-mentioned constitutive model. the plots show good agreement of simulated and experimentally determined material response. 3.2. fea of a steam turbine shaft the non-linear transient elastic-plastic fea of a sample steam turbine shaft has been carried out in order to investigate local thermo-mechanical material response due to the turbine load regimes. in this paper, results of the cold-start load scenario are presented. at first, transient thermal analysis has to be conducted in order to analyze the time-dependent temperature spatial 59 m. nesládek, j. jurenka, m. bartošák et al. acta polytechnica ctu proceedings figure 8. comparison of measured and simulated mechanical response due to in-phase uniaxial mechanical and thermal loading. figure 9. comparison of measured and simulated mechanical response due to out-of-phase uniaxial mechanical and thermal loading. distribution. the cold-start regime is characterized by the turbine start-up from ambient temperature and after shut-down it cools down back to almost 20 ◦c. so the service temperature range corresponds to ∼ 20− 600 ◦c and especially during the start-up process the material exhibits severe temperature gradients in the subsurface layers since it is exposed to superheated steam. the heating rates are the matter of the individual turbine design that has to ensure that the material fatigue strength corresponding to the expected turbine life is not exceeded. three basic turbine load regimes are assumed in the design cold, warm and hot starts differing, from a material point of view, in the initial temperature and allowable heating rates. however, regarding the necessary computation strategy, these regimes are similar. temperature distribution in the turbine is a function of complex time evolution of heat flux governed by the material surface and steam flow difference and heat transfer coefficients. the induced thermal gradients result in the constrained material expansion with pronounced thermal stresses. furthermore, mechanical loads due to shaft rotation and steam flow pressure on blade airfoils have to be taken into account. fig. 10 shows time and subsurface distribution of thermo-mechanical material response within a single cold-start cycle in the domain marked in fig. 10a. the observable discontinuities in temperature are present due to the step changes in the boundary conditions, which partly reflect the nature of the service conditions. on the other hand, this also partly results from modeling imperfections caused by higher analysis incrementation than would be suitable to cover some steep transients. however, this is a reasonable compromise between accuracy and solution cost. the plots in fig. 10 illustrate how complex the material thermo-mechanical response could be in such structure. for instance, in the r3 notch root the cyclic plastic stress-strain state under variable temperature is present. this justifies selection of the doa as a method to assessing the tmf damage. 4. thermo-mechanical fatigue prediction fig. 11 graphically presents the main steps, inputs and outputs of the doa, which was adopted and implemented to software codes for the purpose of this work. as mentioned in the introduction section, the main advantage of this approach is that it treats thermo-mechanical material response in step-by-step manner (i.e. in increment-by-increment manner if translated into the finite element terminology). this allows for handling arbitrary stress-strain and temperature histories in tmf prediction, while assigning the proper parameters of the material damage model to the individual load time points. in general, it requires local elastic-plastic stress and temperature from fea as an input. the material inputs are temperature-dependent csscs, mansoncoffin and basquin curves, larson-miller parameter and stress relaxation curves for cases, where the material viscous effect is substantial. note that d = 1/n denotes mechanical fatigue damage per single cycle. prandtl hysteresis operator is employed in order to model the continuous fatigue mechanical damage influenced by temperature changes. the total damage is evaluated as summation of the mechanical part and the creep part computed separately. the oxidation effect is treated indirectly since the material fatigue and creep tests are performed under environmental conditions. palmgren-miner’s rule [10] and robnson’s life-fraction rule [11] are used for accumulating the mechanical fatigue and creep damage, respectively. this procedure has been coded to an in-house c++ procedure allowing for processing thermo-mechanical response from fea. refer to [12] for more details. 5. results and discussion tmf prediction results obtained by the doa are shown in fig. 12 and tab. 1. the contour plot in fig. 12b shows distribution of 6 √ d quantity, where d denotes the tmf damage due to the cold-start turbine load regime. it may be observed that the critical place is the r3 domain. note that these results were obtained by the doa without taking the 60 vol. 20/2018 tmf analysis of a steam turbine shaft (a) . the r3 domain and direction of the nodal path from which the results below have been extracted. (b) . time and spatial distribution of thermo-mechanical material response in the r3 domain. figure 10. fea results in the r3 domain of the sample steam turbine shaft. r2 r3 r4 r5 hp1 upper fillet hp1 lower fillet tmf damage 1.31 · 10−4 2.66 · 10−4 1.69 · 10−4 2.11 · 10−4 2.25 · 10−5 1.81 · 10−4 tmf life 7633 3759 5917 4739 44 444 5525 table 1. tmf fatigue prediction results by the doa in the selected domains of the steam turbine. 61 m. nesládek, j. jurenka, m. bartošák et al. acta polytechnica ctu proceedings strain controlled nonlinear maxwell model to compute the viscoplastic strain εvp thermo-mechanical elastic-plastic fea by ansys mechanical strain response modelling by using the prandtl hysteresis operators. assumes kinematic strain hardening. t1 t2 t3 cyclic stress-strain curves σa εa tinterp ˙εvp = ( σ − k(t ) k(t ) )n(t ) t1 t2 t3 stress relaxation curves σ t tinterp elastic-plastic strain calculation εep(t) = ε(t)− εvp(t) for t (t) exceeding the material creep temperature tc ε(t) relaxed stress calculation using the prandtl hysteresis operators. εep(t) t1 t2 t3 cyclic stress-strain curves σa εa tinterp t1 t2 t3 tinterp low-cycle fatigue curvespswt d = 1/n fatigue damage estimation (low-cycle fatigue damage operator approach) df creep damage estimation by using the larson-miller parameter dc σ(t), t (t) εep(t) σ(t) df dc d = dc +df total damage creep master curve σ lmp 1. 2. 3. 4. 5. 6. 7. material inputs doa processes inputs from fea figure 11. flowchart to illustrate the basics of the damage operator approach by nagode et al. 62 vol. 20/2018 tmf analysis of a steam turbine shaft (a) . steam turbine domain nomenclature adopted in this paper. (b) . sample contour plot of tmf damage distribution analyzed by the doa in the main sealing part of the steam turbine. figure 12. tmf prediction by the doa. material viscoplasticity into account. the difference between these values and those presented in [12] is due to different approach to handling the stress-strain histories. the results presented in this paper have been evaluated by using only the signed von mises stress from the fea as an input to the doa. the mechanical strain values have been obtained by using the prandtl hysteresis operators, which are part of the doa fig. 11. this approach gives a stress-strain response that more closely matches the physical behavior of the material than the approach that had been used in [12]. in that previous paper, the signing was applied to both the stress and the strain histories extracted from fea. this naturally may lead to distorted hysteresis loops and may have resulted to, for instance, different stress means and amplitudes. if these stress and strain histories are used as a direct input to the function evaluating the tmf damage, different results may be obtained as well. 6. conclusions this paper briefly presents the experimental program conducted in order to provide the necessary material and verification data for the adopted tmf approach. the tmf experimental setup was introduced and the experimentally determined stress-strain response was compared to the simulated one. the comparison shows good agreement of both experimental and numerical data. the doa in the form allowing for the viscous correction of strain histories and stress relaxation has been implemented to the software codes and the results of the tmf prediction on the sample steam turbine shaft are presented in the text. acknowledgements this project has received funding from the european union’s horizon 2020 research and innovation program under the grant agreement no. 653941. references [1] share of renewables in energy consumption in the eu reached 17% in 2016. http://ec.europa.eu/eurostat/ documents/2995521/8612324/8-25012018-ap-en.pdf/ 9d28caef-1961-4dd1-a901-af18f121fb2d. accessed: 2018-07-30. [2] r. w. neu, h. sehitoglu. thermomechanical fatigue, oxidation and creep: part i. damage mechanisms. metall trans a 20(9a):1755–1767, 1989. [3] r. w. neu, h. sehitoglu. thermomechanical fatigue, oxidation and creep: part ii. life prediction. metall trans a 20(9a):1769–1783, 1989. [4] k. n. smith, p. watson, t. h. topper. a stress-strain function for the fatigue of metals. j mater, jmlsa 5(4):767–778, 1970. [5] m. nihei, p. heuler, c. boller, t. seeger. evaluation of mean stress effect on fatigue life by use of damage parameters. int j fatigue 8(3):119–126, 1986. 63 http://ec.europa.eu/eurostat/documents/2995521/8612324/8-25012018-ap-en.pdf/9d28caef-1961-4dd1-a901-af18f121fb2d http://ec.europa.eu/eurostat/documents/2995521/8612324/8-25012018-ap-en.pdf/9d28caef-1961-4dd1-a901-af18f121fb2d http://ec.europa.eu/eurostat/documents/2995521/8612324/8-25012018-ap-en.pdf/9d28caef-1961-4dd1-a901-af18f121fb2d m. nesládek, j. jurenka, m. bartošák et al. acta polytechnica ctu proceedings [6] m. nagode, f. l angler, m. hack. a time-dependent damage operator approach to thermo-mechanical fatigue of ni-resist d-5s. int j fatigue 33(5):692–699, 2011. doi:https://doi.org/10.1016/j.ijfatigue.2010.11.009. [7] m. nagode. continuous damage parameter calculation under thermomechanical random loading. methodsx 1:81–89, 2014. https://doi.org/10.1016/j.mex.2014.07.004. [8] j. l. chaboche. time-independent constitutive theories for cyclic plasticity. int j plasticity 2(2):149–188, 1986. [9] j. l. chaboche. constitutive equations for cyclic plasticity and cyclic viscoplasticity. int j plasticity 5(3):247–302, 1989. [10] a. palmgren. die lebensdauer von kugellagern. zeitschrift des vereines deutscher ingenieure 68(14):339–341, 1924. [11] e. l. robinson. effect of temperature variation on the long-time rupture strength of steels. trans asme 74:777–780, 1952. [12] m. nesládek, m. bartošák, j. jurenka, et al. thermo-mechanical fatigue prediction of a steam turbine shaft. matec web conf 165, 2018. https://doi.org/10.1051/matecconf/201816522016. 64 http://dx.doi.org/https://doi.org/10.1016/j.ijfatigue.2010.11.009 https://doi.org/10.1016/j.mex.2014.07.004 https://doi.org/10.1051/matecconf/201816522016 acta polytechnica ctu proceedings 20:56–64, 2018 1 introduction 2 experimental work 2.1 basic mechanical material tests 2.2 verification tests 3 numerical simulations 3.1 constitutive material model 3.2 fea of a steam turbine shaft 4 thermo-mechanical fatigue prediction 5 results and discussion 6 conclusions acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2021.31.0045 acta polytechnica ctu proceedings 31:45–52, 2021 © 2021 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague vehicle/track interaction under the conditions of high speed railway operation jiří šlapák∗, tomáš michálek university of pardubice, faculty of transport engineering, department of transport means and dignostics, studentská 95, 532 10 pardubice, czech republic ∗ corresponding author: jiri.slapak@upce.cz abstract. this paper deals with the issue of dynamic effects of a high-speed railway vehicle on the track. some aspects of vehicle/track interaction at higher speeds are described. in this context, a multi-body model of high-speed railway vehicle was created and several simulation scenarios were performed with this model. furthermore, the simulations were evaluated with a focus on selected characteristics of the vehicle/track interaction. keywords: vehicle/track interaction, dynamic wheel force, damaging effect, multi-body simulation. 1. introduction high speed railway operation makes demands on all structural subsystems (infrastructure, energy, rolling stock, control command and signaling) of the railway system which have to meet relevant requirements of the technical specifications for interoperability (tsis). in the field of vehicle/track interaction, these requirements are connected especially with (vertical) wheel forces and vehicle running stability, i.e. with straining of the track. this paper is focused on the influence of vehicle speed on selected characteristics of the vehicle/track interaction representing damaging effects of the vehicle on the track. by means of multi-body simulations of vehicle running performance, a quantification of these damaging effects can be performed. 2. vehicle/track interaction at higher speeds during the run of a railway vehicle on a real track, track irregularities represent a kinematic excitation of the mechanical system of the vehicle. a direct consequence of this effect is an oscillation of the wheel forces (acting between individual wheels and rails in vertical direction) around their static values during the run. the static values of the wheel forces are given by the relevant axle load; amplitudes of the oscillations are influenced with the unsprung masses, vehicle speed and the track irregularities [1]. in order to minimize the dynamic interaction between the vehicle and the track in vertical direction, the axle load of the highspeed vehicles is limited. according to the relevant tsis, the maximum axle load of the vehicles intended for running at speed 250 km/h and more is 17 t [2]; locomotives with the axle load up to 22.5 t can be used up to 230 km/h. a minimization of dynamic components of the (vertical) wheel force interaction can be reached practically only by the reduction of unsprung masses in the vehicle running gear. another aspect of the vehicle/track interaction � the running stability � is related to the safety as well as running comfort. safeguarding of a stable run of the vehicle under all relevant conditions (equivalent conicity) is a necessary condition for the high speed operation. the technical solutions used to improve running stability (very rigid wheelset guiding, effective damping of bogie yaw motion etc.) usually lead to increased damaging effects of the vehicle run on rails in curves. however, the running stability is not the object of this paper. guiding forces and related frictional work in wheel/rail contact (creeps and tangential creep wheel forces) cause the damaging effects of rails in curve. in addition to the technical solution of the vehicle, the damaging effect also depends on the operating conditions of the vehicle (radii of curves, vehicle speed). 3. multi-body simulations for prediction of railway vehicle behaviour under the specific operation conditions and for the first consideration about vehicle/track interaction, a multi-body simulations of railway vehicles can be used. in this case, the high speed operation is the specific condition. the one of the advantage of simulations is that it can predict vehicles running behaviour if the running tests on a real railway vehicle can’t be performed. the suitable model of railway vehicle must be created for multi-body simulations. also the simulation scenarios (track properties) must be designed. the used model and scenarios of simulations correspond to the high speed railway conditions. the multi-body simulations can be performed in a various commercial software, for example adams/rail, vampire, simpack. of course, it is possible to create own simulation program. for purpose of this paper, the multi-body simulations of railway vehicle running were performed using the simulation software simpack 9.9.2. 45 https://doi.org/10.14311/app.2021.31.0045 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en jiří šlapák, tomáš michálek acta polytechnica ctu proceedings 3.1. multi-body model of railway vehicle for the purpose of this work, a multi-body model of a high-speed railway vehicle was created. this model represents one passenger coach. because the parameters of high-speed vehicles are different than parameters of conventional vehicles operating in the czech republic, the multi-body model represents chinese railway high-speed (crh) vehicle, which is described in [3]. selected fundamental parameters of the multi-body model are listed in table 1. parameter value unit mass of the carbody 35880 kg mass of the bogie frame 3300 kg mass of the wheelset 1751.8 kg bogie distance 17.5 m wheelset distance 2.5 m vertical stiffness of the primary spring (per wheel) 1.176 kn/mm vertical stiffness of the secondary spring (per side of bogie) 0.190 kn/mm table 1. fundamental parameters of the multi-body model of the railway vehicle. the wheel profile ore s1002 and the rail profile 60e1 were used. the rail inclination of 1:20 was considered to achieve better running stability of the vehicle model. for wheel/rail contact modeling, the method fastsim and the friction coefficient value f = 0.3 was selected. the reference track irregularities were used for all multi-body simulations. because of realistic modeling of the track, the model of elastic track foundation was used in order to describe the railway sleepers and elastic joint between a sleeper and solid ground. 3.2. simulation scenarios for the purpose of this paper, four scenarios of simulations were designed. these scenarios represent typical operating conditions on high speed railway lines. in all senarios, the 2200 meters long track sections were created. first 200 meters long part of each track section was considered without irregularities. then the reference track irregularities begin and continue until the end of track section. 3.2.1. influence of running on a straight track in the first scenario, each simulation of railway vehicle running was performed on the same straight track but at a different vehicle speed. specifically, the vehicle speeds are given in table 2 for this scenario. the minimum speed (160 km/h) was chosen in order to perform simulations at a vehicle speed higher than the czech conventional track lines allow. the maximum vehicle speed (350 km/h) was chosen with regard to the assumption of a maximal operation speed of 320 km/h. for vehicle speed higher than 300 km/h, the speed of the test run of railway vehicle must be increased by 30 km/h according to [5]. v [km/h] 160 200 225 250 275 300 320 350 table 2. vehicle speed parameters v of the first simulation scenario. 3.2.2. influence of cant deficiency the second scenario deals with railway vehicle running on curved tracks. the same constant radius of curve was set for all the tracks. for high speed railway vehicle operations, the large radii of curves are typical. the value of curve radius r = 4320 m was selected for these simulations. but the vehicle speed was changing for each simulation of running vehicle, it means that a cant deficiency i was changing too. the cant deficiency i was calculated according to the stated conditions and equation 1. for this scenario, the simulation parameters are listed in table 3, where d is the cant. i = 11, 8 · v 2 r −d (1) d [mm] i [mm] r [m] v [km/h] 180 -110.1 4230 160 180 -81.4 4230 190 180 -70.7 4230 200 180 -41.7 4230 225 180 -9.3 4230 250 180 26.6 4230 275 180 65.8 4230 300 180 99.7 4230 320 180 154.6 4230 350 table 3. the parameters of cant d, cant deficiency i, curve radius r and vehicle speed v of the second simulation scenario. 3.2.3. influence of curve radius another scenario is based on a constant value of cant deficiency i = 100 mm. here the radii of curves were changing. for each case of curve radius r, the vehicle speed v was calculated according to equation 2 and the values of cant d and cant deficiency i were constant. the simulation parameters are listed in table 4 for this scenario. v = √ (d + i) · r 11, 8 (2) 46 vol. 31/2021 vehicle/track interaction d [mm] i [mm] r [m] v [km/h] 180 100 3800 300.3 180 100 4100 311.9 180 100 4400 323.1 180 100 4700 334.0 180 100 5000 344.5 table 4. the parameters of cant d, cant deficiency i, curve radius r and vehicle speed v of the third simulation scenario. 3.2.4. influence of specific conditions around the speed limit of 300 km/h the last scenario of simulations was focused to show one of the specific condition of high speed railway operation according to tsi [2]. in the tsi, there is the table with cant deficiency conditions for high speed vehicle operation. the table says that the maximum value of the cant deficiency is i = 153 mm for vehicle speed v ≤ 300 km/h. however for vehicle speed v > 300 km/h, the maximum value of the cant deficiency i = 100 mm is defined. these conditions apply to railway vehicles and locomotives for passenger transport. so the value of cant deficiency is abruptly changed at this specified vehicle speed. the aim of this scenario is to investigate what happens around this vehicle speed. the radii of curves were defined in order to set the vehicle speed around the value v = 300 km/h. for this scenario, the equation 2 was used to calcuate the vehicle speed and the simulation parameters are listed in table 5. d [mm] i [mm] r [m] v [km/h] 180 153 2800 281.1 180 153 3000 291.0 180 153 3150 298.2 180 100 3800 300.3 180 100 4000 308.1 180 100 4200 315.7 table 5. the parameters of cant d, cant deficiency i, curve radius r and vehicle speed v of the fourth simulation scenario. 4. simulation results the multi-body simulations of running vehicle were performed according to the conditions for each scenario. time records of quantities characterizing the dynamic effects of a running vehicle on the track were exported for each multi-body simulation. then the exported data were processed in matlab software. figure 1 shows the marks of individual wheels of the vehicle model. the letters l and r indicate the left and right sides of the vehicle. figure 1. selected marking of individual wheels of the vehicle model. 4.1. results from straight track the vertical wheel forces q were monitored in all cases of straight track simulations. the mean value of vertical wheel force represents the static vertical wheel force qstat, its value does not depend on vehicle speed. in order to capture the dynamic effect of this force, it is necessary to focus on the oscillation around the mean value, i.e. the static value of the vertical wheel force qstat. the oscillating part of vertical wheel force is marked as ∆q. for the purpose of this paper, the force ∆q is called the partial dynamic wheel force. the dynamic wheel force qdyn is expressed in equation 3. qdyn = qstat + ∆q (3) this means that the recorded vertical wheel forces were the dynamic wheel forces qdyn. the 99.85th percentile of the time record of vertical wheel force was used to evaluate the dynamic wheel force qdyn. this evaluation was used for simulations performed at different vehicle speeds (table 2). the dependence between vehicle speed v and mean dynamic wheel forceqdyn_m is shown in figure 2. the mean dynamic wheel force qdyn_m was calculated as mean value of dynamic wheel forces qdyn on all wheels of the vehicle. this calculation was performed for each value of vehicle speed. figure 2. influence of vehicle speed v on dynamic wheel force qdyn under the conditions of straight track. figure 2 shows that the mean dynamic wheel force qdyn_m progressively increases with the vehicle speed v . however, according to the british standard gm/tt0088 [1], the dynamic wheel force is linearly dependent on the vehicle speed. the standard defines a force called p2, which represents the dynamic wheel force and it is described by the equation 4. p2 = qstat + a · v (4) 47 jiří šlapák, tomáš michálek acta polytechnica ctu proceedings figure 3. influence of vehicle speed v on partial dynamic wheel force ∆q under the conditions of the straight track. the parameter a in equation 4 depends on the unsprung mass in running gear of the vehicle [1]. the second part of equation 4 (a · v ) corresponds to the partial dynamic wheel force ∆q, it means that the partial dynamic wheel force is a function of the vehicle speed. the influence of vehicle speed v on partial dynamic wheel force ∆q is shown in figure 3. the linear approximation of the dependence of the partial dynamic wheel force on the vehicle speed shows the coefficient of determination r2 = 0.895. the linear function is shown in figure 3 by blue dashed line. the second degree polynomial (quadratic) approximation gives a better result. there is a coefficient of determination r2 = 0.997. this means that the second degree polynomial is a better function for approximating the dependence of the dynamic wheel force on the vehicle speed for high speed railway operation. ∆q = 0.0008 · v 2 + 0.01855 · v (5) equation 5 is the polynomial approximation, which is shown in figure 3 by red dashed line. the parameters of this equation are specific to the simulation conditions. for example, the parameters of equation 5 can be changed by various track irregularities. 4.2. results from curved tracks the situation of vehicle/track interaction in curved track is different. the track is significantly loaded in the lateral direction due to the effect of centrifugal (inertia) force. the track leads the railway vehicle through the curve. in addition to the force interaction, there is a sliding movement in the wheel/rail contact. a special characteristic called wear number wn is used to describe the effects of a railway vehicle on a curved track. in general, the wear number represents a specific friction work and it is used to evaluation of rail wear on a curved track, for example [4]. the wear number is defined by tangential contact forces t and creeps γ in the longitudinal (x) and lateral (y) directions. the formula for calculating the wear number is given by equation 6. wn = |tx · γx|+ |ty · γy| (6) the mean value of wear number was evaluated. to evaluate the wear number, the only part of the track where the whole vehicle was in the curve was selected. this part of the track starts at 700 m and ends at 1700 m, so only 1000 meters of the track were evaluated. 4.2.1. influence of cant deficiency in the first case of a multi-body simulation of railway vehicle in a curved track, the curve radius r and the value of cant d were constant. the vehicle speed v was increased from 160 km/h to 350 km/h in individual steps, as well as the cant deficiency i (according to equation 1). the simulation conditions are described in section 3.2.2. figure 4. dependence of the sum of the wear number w n values of all vehicle wheels on vehicle speed v at constant track parameters (cant, curve radius). firstly, for a very simplified introduction to this scenario, the dependence of the sum of the wear number wn values of all vehicle wheels on vehicle speed v is shown in figure 4. the figure shows that the wear number value increases progressively with the vehicle speed under conditions of constant curve radius. a very similar dependence is shown in figure 6. this figure shows the effect of the cant deficiency 48 vol. 31/2021 vehicle/track interaction figure 5. dependence of the wear number w n values on all outer wheels of the vehicle on the cant deficiency i at constant track parameters (cant, curve radius). i on the wear number wn . negative values of cant deficiency represent the cant excess. figure 6. dependence of the sum of the wear number w n values of all vehicle wheels on cant deficiency i at constant track parameters (cant, curve radius). according to figure 6, the sum of the wear number wn values increases with increasing cant deficiency i. this applies to the positive values of cant deficiency. but for the cant excess values (negative values of cant deficiency), the wear number values are in a narrow range. the situation where the values of the sum of the wear number were calculated for individual sides of railway vehicles is shown in figure 7. the figure shows the sum of wear number of the four outer wheels (left side in figure 1) and the four inner wheels (right side in figure 1) of the railway vehicle. the chart in the figure shows that the values of the sum of wear number on the inner wheels are almost symmetrical around the value of the cant deficiency i = 0 mm with a polynomial character. it is obvious that the wear number values on the outer wheels are bigger than on the inner wheels. the wear number values of the outer wheels for a positive values of the cant deficiency can be approximated using a quadratic function without constant parameter. because the parameters of a quadratic function may vary depending on the vehicle/track conditions (vehicle construction, track irregularities), the type and shape of the function is important. the quadratic function is shown in figure 7 by a gray dashed line. according to this figure, the sum of the wear number on the outer wheels of the railway vehicle for the cant deficiency i = 110 mm is about twice as large as the sum of the wear number for the cant deficiency i = 0 mm. figure 7. dependence of the sum of the wear number w n values on the inner and outer wheels of the vehicle on the cant deficiency i at constant track parameters (cant, curve radius). according to figure 7, it is obvious that the main part of the wear number (specific friction work) is performed on the outer four wheels. that is the reason why this paper will further deal with the situation on the outer wheels of the railway vehicle, which is shown in figure 5. the outer wheels of the front bogie (wheels 1l and 2l in figure 1) are shown in solid lines and the outer wheels of the back bogie (wheels 3l and 4l in figure 1) are shown in dashed line. as figure 5 shows, for cant deficiency values around i = 0 mm, the wear number values for the second (2l) and fourth (4l) wheels are close to zero. then, as the cant deficiency value increases, the value of the 49 jiří šlapák, tomáš michálek acta polytechnica ctu proceedings wear number also increases with a progressive trend. however, the wear number value is about 37 n on the first wheel and about 22 n on the third wheel if the cant deficiency value is i = 0 mm. for these wheels, the wear number values generally increase with increasing cant deficiency values. the wear number values on the first (1l) and third (3l) wheels increase slower then the values on the second (2l) and fourth (4l) wheels. finally, for the cant deficiency value of 150 mm (vehicle speed 344 km/h), the wear number values for the second and fourth wheels are higher than for the first wheel. this is due to high centrifugal force that pushes the vehicle out of the curve. for normal high speed railway operation (the cant deficiency up to 100 mm), the wear number value on the first outer wheel in the front bogie (1l) is the highest value. this means that the largest specific friction work is performed on this wheel. 4.2.2. influence of curve radius in another case of the simulation scenario, the cant deficiency value is constant i = 100 mm. this cant deficiency value is the maximum possible cant deficiency value for a vehicle speed higher than 300 km/h according to the tsi [2]. this means that the vehicle speed v changes with the curve radius r. the specific simulation conditions are descibed in section 3.2.3. for the first introduction of this scenario, the simple chart is shown in figure 8. dependence of the sum of the wear number wn values of all vehicle wheels on the curve radius r is shown under the constant cant d and cant deficiency i conditions. the radius range of the curve from 3800 m to 5000 m corresponds to the range of vehicle speed from 300.3 km/h to 344.5 km/h. it is obvious that the value of the sum of the wear number increases as the value of the curve radius decreases. this trend is also evident in methodology [4] which is focused on the conventional railway operation, so there is the range of the curve radius from 250 m to 1200 m and the values of wear number are much bigger depending on the smaller radius of the curve. figure 8. dependence of the sum of the wear number w n values of all vehicle wheels on the curve radius r at constant value of cant and cant deficiency. for a better view of this situation, the sum of the wear number values on all outer wheels (left side in figure 1) and inner wheels (left side in figure 1) of the vehicle is shown in figure 9. the sum of the wear number on the inner wheels is approximately equal to 10 n for the whole range of curve radii. the main part of the specific friction work is performed in the contacts of the outer wheels with rail as figure 9 shows. for the curve radius r = 5000 m, the value of the sum of the wear number is 75 n, then the value increases to 95 n for the curve radius r = 3800 m. this means that it is necessary to be interested in the wear number values on the outer wheels. figure 9. dependence of the sum of the wear number w n values on the inner and outer wheels of the vehicle on the curve radius r at constant value of cant and cant deficiency. further analysis of the wear number on each outer wheel (wheels 1l, 2l, 3l, 4l according to figure 1) is shown in figure 10. according to figure 10, the largest specific friction work is performed on the first outer wheel (1l) of the vehicle. the values of the wear number on first outer wheel increase from 25 n up to 39 n for the range of the curve radius from 5000 m to 3800 m. in context to these values, it can be assumed that the head checking rail defect will occur [4]. the wear number values on the third outer wheel (3l) have a similar trend as the first outer wheel, but the values are smaller. thewear number values on the second outer wheels of both bogies (2l, 4l) are very similar and the wear number value increases as the radius of the curve increases. in this scenario, the centrifugal forces have the same value for each simulation. thus, the trends of the wear number values are caused by the values of the angle of attack, which increase with decreasing values of the radius of the curve. the absolute values of the wear number depend on other vehicle parameters and track conditions, but it can be assumed that the trends of the graph curves will be the same. 50 vol. 31/2021 vehicle/track interaction figure 10. dependence of the wear number w n values of all outer wheels of the vehicle on the curve radius r at constant value of cant d and cant deficiency i. 4.2.3. influence of specific conditions around the speed limit of 300 km/h the specific conditions of this scenario are defined in section 3.2.4. this scenario of multi-body simulations shows what happens at the vehicle speed v = 300 km/h. the maximum permissible value of the cant deficiency changes abruptly from 153 mm to 100 mm at this vehicle speed. figure 11. dependence of the sum of the wear number w n values of all wheels of the vehicle on the vehicle speed v with the change of the cant deficiency value from 153 mm to 100 mm. the results of the multi-body simulations of this scenario are shown in figure 11. the figure shows the dependence of the sum of the wear number wn of all wheels on vehicle speed v . for vehicle speeds below 300 km/h, the cant deficiency is 153 mm. for vehicle speeds above 300 km/h, the cant deficiency is 100 mm. the radius of the curve varies according to the values of vehicle speed and cant deficiency. according to figure 11 it is obvious that the wear number values for vehicle speed below 300 km/h are greater than the values for vehicle speed above 300 km/h under the conditions of the maximum value of cant deficiency. for the high speed railway operation with a maximum value of cant deficiency, a vehicle speed greater than 300 km/h is more advantageous in terms of specific friction work and track damage. however, larger radii of curve are required. 5. conclusions after comparing some methodologies for the vehicle/track interaction evaluation for conventional railway operation with the vehicle/track interaction under the conditions of high speed railway operation, it’s obvious that the conditions and parameters of high speed railway operation are different from conventional operation. when a high-speed vehicle runs on a straight track, the damaging effect increases as a result of an increase of the dynamic wheel force, which progressively depends on the vehicle speed. a large radii of curve are required for high speed railway operation. this means that the wear number values are relatively small in compared to conventional operation and it can be assumed that the rail wear in the curve will be less intense than in conventional railway operation. but with increasing vehicle speed and cant deficiency, the values of wear number increase very quickly, as does the specific friction work in wheel/rail contact. this paper primary shows trends of the dynamic vertical wheel force q and the wear number wn values under the specific conditions of high speed railway operation and parameters of multi-body simulations, which are used for evaluation of vehicle/track interaction. the absolute values of these characteristics depend on the design of the railway vehicle and, of course, on the parameters and conditions of the track. acknowledgements this work was supported by the scientific research project of the university of pardubice no. sgs_2020_009. 51 jiří šlapák, tomáš michálek acta polytechnica ctu proceedings references [1] british railway board. permissible track forces for railway vehicles. gm/tt0088, issue 1, revision a, 1993. [2] commision regulation (eu) no 1299/2014 of 18 november 2014 on the technical specification for interoperability relating to the ’infrastructure’ subsystem of the rail system in the european union, 2014. [3] t. zhang, d. huanyun. on the nonlinear dynamics of a high-speed railway vehicle with nonsmooth elements. applied mathematical modelling 76:526-544, 2019. [4] base price wear in the train-path pricing system 2017 instruction for determining vehicle prices. bern: feredal office of transport, 2016. [5] en 14363:2016+a1:2018. railway applications – testing and simulation for the acceptance of running characteristics of railway vehicles – running behaviour and stationary tests. brusel: european committee for standardization, 2018. 52 acta polytechnica ctu proceedings 31:45–52, 2021 1 introduction 2 vehicle/track interaction at higher speeds 3 multi-body simulations 3.1 multi-body model of railway vehicle 3.2 simulation scenarios 3.2.1 influence of running on a straight track 3.2.2 influence of cant deficiency 3.2.3 influence of curve radius 3.2.4 influence of specific conditions around the speed limit of 300 km/h 4 simulation results 4.1 results from straight track 4.2 results from curved tracks 4.2.1 influence of cant deficiency 4.2.2 influence of curve radius 4.2.3 influence of specific conditions around the speed limit of 300 km/h 5 conclusions acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2021.31.0036 acta polytechnica ctu proceedings 31:36–40, 2021 © 2021 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague possible changes in transport conception of international and long-distance railway connections relating the construction of new high-speed railway lines in the czech republic pavel purkart∗, david vodák, tomáš javořík, martin jacura czech technical university in prague, faculty of transportation sciences, department of transportation systems, horská 3, 128 03 prague 2, czech republic ∗ corresponding author: purkapav@fd.cvut.cz abstract. the aim of this article is to deal with the issue of high-speed railway lines (hsl) in the czech republic in response to the fast connections conception ("rychlá spojení" in czech, abbreviation "rs"). currently the rs1, rs2 and rs4 projects are the most watched during the preparation of the hsl network construction. the authors would like to focus on this issue from the wider point of view and to assess possible impact on the current lines of long-distance trains, the location of graphical timetable nodes and the way of the transport service on the area of the state. keywords: high-speed railway lines, czech republic, graphical timetable, international transportation, long-distance transportation, travel time, transport junction. 1. introduction currently there is an ongoing development of czech hsl network in response to the fast connections conception. there are not any hsl in the czech republic now. this situation is not, with respect to the sustainable development and the situation in the neighbouring states of the former western europe, acceptable, but this topic is being revived and preparation of selected rs sections is accelerating. the construction of new high-speed railway lines (hsl) in the czech republic is currently an actual topic. the fast connections ("rychlá spojení" in czech, abbreviation "rs") system has defined following main directions of the fast railway network [1]: • rs1 praha (prague) – brno – ostrava – [poland], • rs2 brno – [slovakia/austria], • rs3 praha – plzeň (pilsen) – [germany] (western direction – münchen), • rs4 praha – [germany] (northern direction – dresden) with possible branch (most – chomutov, žatec, louny), • rs5 praha – hradec králové/liberec – [poland]. we can see the fast connections conception in figure 1. currently the rs1, rs2 and rs4 projects are the most watched during the preparation of the hsl network construction. the aim of this article is to focus on this issue from the wider point of view and to assess possible impact on the current lines of long-distance trains, the location of graphical timetable nodes and the way of the transport service on the area of the state. according to the správa železnic (the main czech railway infrastructure manager), first sections of the rs network should be put into operation by the year 2028, which makes this issue very actual. 2. goals of the article this article is mainly focused on the impacts of the rs network and changes in transport planning, especially: • evaluation of rs conception, • possible line routing in the international transportation, • discussing possible impacts in national long-distance transportation. 3. solution of the issue 3.1. evaluation of rs conception according to the research team, the basic design of the rs network is consistent. rs conception considers the most important national and international transport flows. decrease of the travel times to praha (prague) is shown in the figure 2. t is logical to solve rs1, rs2 and rs4 branches in the first place, but we should not omit rs3 branch, because it connects the area of plzeň and southern germany. this connection represents a strong transport relation, but currently it is not certain what parameters and routing should this route have between plzeň and germany. it is alarming, considering that more than thirty years have passed since the fall of the iron curtain. the defined rs5 direction is also suitable for reflection. rs5 route could be extended to the south bohemia (with connection to austria), because this area is currently not fully connected to the rs network. goals of rs network construction of railway transportation development are essential: 36 https://doi.org/10.14311/app.2021.31.0036 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 31/2021 possible changes in transport conception figure 1. rs network (continuous lines represent planned rs connections, dashed lines represent possible future connections, dotted lines represent alternatives), source: [2]. figure 2. estimated travel times decrease after the rs network construction, compared to the situation in 2018 (120/ 90 min = travel time in 2018/ prospective travel time after the hsl construction or conventional network modernization), source: [2]. 37 p. purkart, d. vodák, t. javořík, m. jacura acta polytechnica ctu proceedings • increasing the importance of the czech railway network for international transport, • cutting the travel times between praha and all regional capitals to 120 minutes or lower, to achieve this goal rs lines should be used both by high-speed and conventional trains, • lowering traffic density on the overloaded section of railway network, so suburban and freight trains could use it. another important issue is the organization of railway traffic on new rs lines. currently railway traffic is ordinarily organised according to the graphical timetable. this approach should be also applied on the new rs lines. principles of graphical timetable can be mathematically described by using two simple equations. the edge equation has the following formula (1): th = n · 1 2 tt (1) the formula (2) is the circumferential equation:∑ th = n ∗ tt (2) above-stated variables mean: • tt – tact time (service interval), • th – time weight of the edge between two tact nodes, • n – natural number (depends on individual situation) [3]. both these equations can be easily explained in order to understand their real meaning. the edge equation shows that vehicles from one line are passing each other after one-half of the tact time. in case of public transport line with tact time 60 minutes the vehicles of opposite directions pass each other every 30 minutes. vehicles pass on the route at the place where the symmetry axis is. on single-track railway line, we have to build a passing point. on the lines with two or more tracks, vehicles can pass each other on the route. we can assume that the perfect travel time between two important transport nodes equals one-half of the tact time. circumferential equation is related to the term circle (close sequence of edges and nodes) from the graph theory. it says that total count of system travel time on the all edges should ideally equal a multiple of natural number of the tact time. in such case, strong relations between the lines in the nodes are ensured. 3.2. possible line routing in long-distance international transportation construction of the rs1 and rs2 sections is crucial for national transport relationships, because it can significantly help to overloaded sections of first and third transit railway corridor. we can also assume, that lines on hsl will be integrated to the international connections – hsl are used both by national and international lines. there are already international lines on the czech railway network. those lines have high quality vehicles, but their travel speed does not exceed 100 km/h, which is not sustainable in the context of today’s "distance reduction". selected lines are even bypassing czech republic, because longer route outside of the czech republic offers better travel time. construction of the rs network should improve this situation and increase the importance of the czech railway network for international transportation. possible conception of international lines is shown on the network graphics in the figure 3. following lines of international trains can create the core of transport services in the rs network scenario: • wien – brno – praha – dresden – berlin. this line should ideally reach systemic travel time four hours along the entire route, which means one hour between each city. this should provide optimal travel time between individual nodes. current travel time is eight hours – construction of rs1 and rs2 can cut down the travel times by one half. it is alarming, that current fastest connection wien – berlin is not routed through the area of the czech republic, but only through germany and austria. travel time of connection routed through the area of the czech republic is slightly longer – this clearly proves unsatisfactory condition of our railway network for international transportation, • warszawa – ostrava – brno – praha – plzeň – münchen. this line also contains section of the rs3. current travel time of long distance train lines is eleven hours, but these lines are also not routed through the area of the czech republic. at the same time, there is not an intention to build the rs3 in the parameters of hsl. reaching the travel time under eight hours can be considered as a success. we should reach following travel times for the optimal locations of graphical timetable nodes: warszawa – ostrava three hours, ostrava – praha two hours and praha – münchen 3 hours. the above-stated network is basic and it consists of two lines with international meaning. for its addition and full use of the rs network, we can also assume following lines: • košice – ostrava – brno – praha – plzeň – nürnberg, similar to line warszawa – münchen in east-west direction. it connects east part of slovakia, czech republic and germany. travel time through czech republic should not exceed 4 hours. travel time on the area of other states should not be higher in order to keep the international line attractive for passengers, 38 vol. 31/2021 possible changes in transport conception figure 3. network graphics of possible line routing in the international transportation. 39 p. purkart, d. vodák, t. javořík, m. jacura acta polytechnica ctu proceedings • budapest – bratislava – brno – praha – most/chomutov – karlovy vary is a line, which connects the capitals of the czech and slovak republic. it does not have to end in the praha, but it can go to the area of most (on the designed rs line) and further to the karlovy vary (on the conventional line), • it is appropriate to mention the line linz – české budějovice – praha – warszawa in case of construction of the rs5. international lines can create following peak intervals on the area of the czech republic (assuming that they will be operated with the 60 minutes’ interval): • interval 15 minutes praha – brno, • interval 30 minutes praha – ostrava, • interval 30 minutes praha – plzeň. with these operational parameters and sufficient length of the trains, the international lines can serve the national relations without any additional national lines. for example, there is 15 minutes’ interval on the relation praha – brno, which is close to the parallel graphical timetable of subway. it is apparent that praha will become an important railway node at an international level. its significance could be very close to nodes such as wien and berlin. 3.3. possible impacts on the national long-distance transportation the above-mentioned international lines are going to bring significant travel times reduction also in the most important national lines. there will be also additional lines with lower importance. international lines are going to affect national transportation, because many passengers, who are using current national lines, are going to use new connections routed on rs/hsl. construction of rs/hsl is going to bring key changes in the current line routing. we can shortly mention following impacts on current lines: • line ex1 praha – ostrava is going to lose its importance; important cities (pardubice and olomouc) are going to remain outside of the rs, therefore it will be necessary to solve their connection. the rs network is probably not going to be important for pardubice, but we have to evaluate it for olomouc, • line ex3 praha – brno is going to lose its importance similar to line ex1 praha – ostrava, • ine ex5 praha – dresden – berlin also loses necessity for connection of those cities, in ústí nad labem case it depends whether it is on rs route or not (we can consider stopping of the line wien – berlin). we definitely have to change the conception of the line r15 praha – ústí nad labem – most – karlovy vary – cheb. if a new line with hsl parameters is built, including a branch to the most area, this line, in its current form, loses its importance for connection with praha. it is purposeful to divide this line into three lines, for example: . praha – most – chomutov – karlovy vary – cheb with systemic travel time praha – karlovy vary in phase one maximally two hours, then maximally one and half hour (for fast connection praha – karlovy vary), . praha – ústí nad labem – teplice – litvínov, . děčín – ústí nad labem – teplice – bílina – most – chomutov. • line ex6 praha – plzeň is primarily going to be routed on rs line, if there is not any segment of conventional line remained. we have to question the importance of the line r16 praha – plzeň – klatovy, which is partly regional, but, thanks to the strong transport connection between klatovy area and praha, it can be divided into two lines in plzeň – selected trains from klatovy area may be routed on rs line from plzeň. 4. conclusion we can expect smaller or bigger impacts from building the rs network on almost every line, however our main goal should be total passenger increase in public transportation, balanced use of railway network, reduction of travel times between the cities and reduction of environmental damages from the individual car transportation. it is necessary to point out, that high-speed railway transportation has to be attractive and affordable. otherwise our goals cannot be reached. acknowledgements all the facts presented in this article are based on the results of the research on the ctu faculty of transportation sciences, department of transport systems. this research was supported by the grant agency of the czech technical university in praha, grant no. sgs18/150/ohk2/2t/16 railway track parameters for transportation service optimization and no. sgs20/138/ohk2/2t/16 design and optimal use of railway infrastructure parameters. references [1] vysokorychlostní tratě v čr. [online], praha: správa železnic, 2020 [cit. 2020-02-22]. dostupné z: https://www.szdc.cz/vrt/. [2] m. švehlík, m. pinkava, p. provazník. postup přípravy vrt v čr. vědeckotechnický sborník čd. 2018, 25(46), 16. issn issn 1214-9047. [3] k. baudyš. časové prvky jř. systémové jízdní doby: technologie železnižní dopravy. praha, 2014. 40 https://www.szdc.cz/vrt/ acta polytechnica ctu proceedings 31:36–40, 2021 1 introduction 2 goals of the article 3 solution of the issue 3.1 evaluation of rs conception 3.2 possible line routing in long-distance international transportation 3.3 possible impacts on the national long-distance transportation 4 conclusion acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2018.20.0034 acta polytechnica ctu proceedings 20:34–42, 2018 © czech technical university in prague, 2018 available online at http://ojs.cvut.cz/ojs/index.php/app the effect of notches on the fatigue life of a nickel-base gas turbine disk material robert erikssona,∗, johan moverareb, zhe chenb, kjell simonssona a division of solid mechanics, linköpings universitet, 58183 linköping, sweden b division of engineering materials, linköpings universitet, 58183 linköping, sweden ∗ corresponding author: robert.eriksson@liu.se abstract. gas turbine disks carry significant load under high temperatures and may be subject to fatigue failure. disks contain several notches in the form of the fir tree blade attachments. low cycle fatigue tests were performed on blunt notch compact tension specimens made from alloy 718. the results indicated that notch support needed to be incorporated not to cause an overly conservative life prediction. the notch support diminished as the plastic strain range decreased, indicating that notch support is only present in the low cycle fatigue regime. a critical distance approach was applied to account for the notch support. an equation relating the critical distance to the notch root stress was derived. the chosen life model was formulated in terms of a variation on the smith–watson–topper (swt) parameter. the modified swt parameter taken at the critical distance was used in a life model calibrated for smooth specimens to successfully predict the fatigue life of notched specimens. keywords: fatigue, notch effect, gas turbines, disk alloy. 1. introduction gas turbine disks, in the turbine section, carry significant load under intermediate to high temperatures [1]. one, of many, critical aspects from a design point of view, is to ensure the disc does not fail through fatigue. in particular, the fir tree like blade attachments account for stress concentrations which may initiate cracks in the disk. although notches may promote crack initiation, there is also a tendency for conventional lifing methods to underestimate the fatigue life of the component if the lifing method is applied using the notch root stress. the phenomenon is sometimes referred to as notch support [2, 3] and arises from the fact that (typically) a very small material volume is exposed to the high notch root stress; ahead of the notch, stresses typically drop significantly. the current desire to reduce conservatism in design against fatigue (especially in the energy sector) [3] calls for notch support to be investigated and incorporated in lifing methods in an industrially applicable manner. in the current work, a method for incorporating notch support is applied to notched specimens made from the ni base alloy 718 subjected to low cycle fatigue (lcf). the theory of critical distances is taken as basis for the suggested lifing method. 1.1. theory of critical distances the theory of critical distances [4] accounts for the steep stress gradient at notches by suggesting that the failure of a notched specimen or component is more accurately based on the stress some distance from the notch root. that is, failure is assumed to occur when the stress, σ(r), a distance r from the notch root reaches a critical value, σ0, such that σ(r) = σ0 at failure (1) where r is the critical distance. for monotonic failure, the critical distance takes the form [4] r = 1 2π ( kc σ0 )2 (2) withkc being the fracture toughness and σ0 a strength parameter somewhat similar to the ultimate tensile strength. the theory of critical distances has also been extended to account for fatigue failure. for high cycle fatigue (hcf), with respect to the fatigue limit, the critical distance becomes [4] r = 1 2π ( kth σ0 )2 (3) with kth being the threshold stress intensity factor for crack growth and σ0 a strength parameter somewhat similar to the endurance limit. for finite fatigue lives, it has been suggested that the critical distance should take the form [4] nf = c1r c1 (4) where nf is the number of cycles to failure and c1 and c1 are fitting parameters. equation 4 has also been applied to low cycle fatigue of notched specimens made from a ni-based single-crystal alloy at 500 °c [5]. although eqn. 4 has been demonstrated to work for lcf, it is inconvenient to use for life predictions 34 http://dx.doi.org/10.14311/app.2018.20.0034 http://ojs.cvut.cz/ojs/index.php/app vol. 20/2018 effect of notches on fatigue as nf is not known a priori, requiring an iterative process for the fatigue life to be calculated. eriksson et al. [6] demonstrated that the critical distance could also be based on the notch root plastic strain range, ∆εrootp , via a simple power law r = p ( ∆εrootp )q (5) where p and q are fitting parameters. the rationale being that the notch root strain range correlates to the strain range further into the material. in the current work, the method suggested by eriksson et al. [6] will be further explored and a rationale for why the critical distance should correlate with notch root conditions will be provided. it will be shown that the critical distance can also be correlated to the notch root stress taken from a linear elastic finite element analysis (fea). 2. material behaviour 2.1. material model a simple material description, which sufficiently well captures strain ranges and mean stresses, was adopted. a two-step procedure was used to calculate needed load parameters: 1) a set of equations are used to determine the maximum stress and 2) another set of equations are used to determine the strain range. previously published data for alloy 718 were used to describe the studied alloy, see moverare et al. [7]. the data in moverare et al. [7] were established in lcf tests performed in strain control with rε = 0. firstly, the maximum stress is determined. for 450 °c, the maximum stress, σmax, is determined from σmax = { eεmax for εmax < σ′yl/e σ′yl for εmax ≥ σ′yl/e (6) i.e., by assuming ideal plasticity; e is the young’s modulus, σ′yl is the cyclic yield limit and εmax is the maximum total strain. as seen in fig. 1 a), the maximum stress is accurately described as ideal plastic at 450 °c. the parameters e and σyl are given in tab. 1. for 550 °c, on the other hand, the maximum stress is better described by a ramberg–osgood type equation εmax = σmax e + ( σmax k ′max )1/n′max (7) with the material parameters e,k ′max and n′max given in tab. 1; eqn. 7 is plotted in fig. 1 a). from fig. 1 a) it is evident that the material behaviour (with regard to maximum stress) changes between 450 °c and 550 °c; low strains at 450 °c will produce a significant mean stress (due to high maximum stress) whereas, at 550 °c, the mean stress is quite small at all strains. some microstructural change may be the cause for this shift in behaviour; however, it is out of the scope of the current work to study this phenomenon. here, it will max strain, % 0 0.5 1 1.5 2 2.5 m ax s tre ss , m pa 0 200 400 600 800 1000 1200 exper. data, 450 °c exper. data, 550 °c fit, 450 °c fit, 550 °c strain amplitude, % 0 0.2 0.4 0.6 0.8 1 1.2 st re ss a m pl itu de , m pa 0 200 400 600 800 1000 1200 exper. data, 450 °c exper. data, 550 °c fit, 450 °c fit, 550 °c a) b) figure 1. material behaviour at 450 °c and 550 °c based on data from ref. [7]. a) maximum stress versus maximum strain. b) stress amplitude versus strain amplitude. simply be accepted that a shift in material behaviour exists between 450 °c and 550 °c and that an accurate maximum stress can be predicted by assuming ideal plasticity at 450 °c but by using a ramberg–osgood type equation at 550 °c. secondly, the stress range, ∆σ is determined by assuming that the studied alloy follows masing behaviour described by a ramberg–osgood type equation ∆ε = ∆σ e + 2 ( ∆σ 2k ′ )1/n′ (8) where ∆ε is the total strain range and the material parameters e, k ′ and n′ are given in tab. 1. figure 1 b) shows eqn. 8 fitted at both 450 °c and 550 °c (the figure is plotted in terms of amplitudes, i.e., ∆σ/2 and ∆ε/2). the plastic strain range, ∆εp, can then be determined as ∆εp = ∆ε− ∆σ e (9) figure 2 shows that the outlined method accurately predicts maximum and minimum stresses (and, consequently, the mean stress) for smooth specimens. 35 r. eriksson, j. moverare, z. chen, k. simonsson acta polytechnica ctu proceedings t , °c e, gpa ν σ′yl, mpa k ′max, mpa n′max k ′, mpa n′ 450 178.97 0.32 1018.83 — — 1328.84 0.0560 550 172.25 0.33 — 3660.76 0.321 2406.08 0.184 table 1. material parameters fitted to data from ref. [7–9]. total strain, % 0 0.5 1 1.5 st re ss , m pa -1000 -500 0 500 1000 1500 calc. hysteresis calc. mean stress exper. max stress exper. min stress exper. mean stress total strain, % 0 0.5 1 1.5 2 2.5 3 st re ss , m pa -1000 -500 0 500 1000 1500 calc. hysteresis calc. mean stress exper. max stress exper. min stress exper. mean stress total strain, % 0 0.2 0.4 0.6 0.8 1 1.2 st re ss , m pa -800 -600 -400 -200 0 200 400 600 800 calc. hysteresis calc. mean stress exper. max stress exper. min stress exper. mean stress total strain, % 0 0.5 1 1.5 2 st re ss , m pa -1000 -500 0 500 1000 calc. hysteresis calc. mean stress exper. max stress exper. min stress exper. mean stress a) b) c) d) figure 2. comparison between experimentally determined stresses and calculated from eqn. 6, 7 and 8; for two different strain ranges at 450 °c, a) and b), and at two different strain ranges at 550 °c, c) and d). 2.2. fatigue life model for smooth specimens since mean stresses may not entirely relax (especially at 450 °c for low load levels), the smith-watsontopper (swt) parameter [10], σmax∆ε/2, is taken as the basis for the fatigue life model. the swt parameter can be used for handling mean stress effects [10]. here, since the majority of the studied specimens plasticise significantly, the variant of the swt parameter suggested by lorenzo and laird [11] will be used. the assumption involved is that the fatigue life under mean stress can be correlated to the fully alternating case such that the parameter σmax∆εp/2, for fatigue under mean stress, equals the parameter σa∆εp/2, for the fully alternating load, where σa is the stress amplitude. that is, σmax ∆εp 2 = σa ∆εp 2 (10) which, with the basquin equation, σa = σ′f (2nf )b (11) and the manson–coffin equation, ∆εp 2 = ε′f (2nf )c (12) yields σmax ∆εp 2 = σ′fε ′ f (2nf )b+c (13) where σ′f , ε′f , b and c are fitting parameters which after merging yields σmax ∆εp 2 = ana f (14) 36 vol. 20/2018 effect of notches on fatigue cycles to failure 10 2 10 3 10 4 10 5 0 1 2 3 4 5 6 7 exper. data, 450°c fit, 450°c exper. data, 550°c fit, 550°c σ m a x δ ε p /2 , m pa figure 3. fatigue life model calibration for smooth specimens at 450 °c and 550 °c. fatigue life is described in terms of the modified swt parameter, σmax∆εp/2. t , °c a, pa a 450 81.94× 106 -0.504 550 34.98× 106 -0.436 table 2. fitted parameters in the fatigue life model for smooth specimens, see eqn. 14. where a and a are fitting parameters. the parameter σmax∆εp/2 will here be referred to as the modified swt parameter. the fatigue life model given by eqn. 14 is calibrated with data from moverare et al. [7] for smooth specimens. the modified swt parameter is calculated from σmax and ∆εp/2 determined as described above in the section material model. figure 3 shows the fatigue life curves for smooth specimens at 450 °c and 550 °c and the fitted parameters are given in tab. 2. 3. experimental work a test programme has been carried out involving lcf tests on blunt notched compact tension (ct) specimens made from the ni base alloy 718. the specimen geometry is shown in fig. 4 and has outer dimensions of 32.4 mm × 31.2 mm × 12 mm, where the latter is the thickness (i.e. it is the size of a halfinch ct specimen). compared to a conventional ct specimen, the current specimen geometry differs in that it has a blunt notch (2 mm radius) to somewhat resemble a disk fir tree blade attachment. the specimens were subjected to lcf load at 450 °c and 550 °c for various load levels in the 4–10 kn interval. the lcf tests were performed in load control with a load ratio of r = 0.05. the tests were run until final rupture of the specimens. figure 4. the blunt notch ct specimen used in the current study; image from ref. [6]. 4. results and discussion 4.1. stress and strain ranges for notched specimens the stress ahead of the notch is obtained from a linear elastic finite element analysis of the blunt notch ct specimen loaded with 1 n load. stresses at all other load levels can be achieved by scaling the results from 1 n load. the stress’ y-component is extracted as function of distance, x, from the notch as shown in fig. 5. the y-component dominates the stress at the notch which enables the load to be treated as approximately uniaxial. quantities obtained from the linear elastic finite element analysis will be denoted by an asterisk superscript; the stress ahead of the notch as function of x is hence denoted as σ∗(x), the stress range as ∆σ∗(x) and the notch root stress as σ∗root = σ∗(x = 0). figure 6 a) shows an example of the ∆σ∗(x) profile for 9.9 kn applied load at 450 °c. a glinka shakedown is performed on the σ∗(x) profile to get σmax and ∆εp/2. a glinka shakedown has previously been shown to give accurate results for this material and specimen geometry, see ref. [6]. to get the maximum stress, the following equation systems are solved. for 450 °c, σmax is the outcome of solving  ∫ εmax 0 σdε = k2 f (σ∗)2 2e σ = { eε for ε < σyl/e σyl for ε ≥ σyl/e (15) for σ and for 550 °c, σmax is the outcome of solving ∫ εmax 0 σdε = k2 f (σ∗)2 2e ε = σ e + ( σ k′max )1/n′max (16) for σ. the stress range and the total strain range are 37 r. eriksson, j. moverare, z. chen, k. simonsson acta polytechnica ctu proceedings path figure 5. path at which the y-component stress profile is extracted from a linear elastic finite element analysis; image from ref. [6]. obtained by solving{∫∆ε 0 σdε = k2 f (∆σ∗)2 2e ∆ε = ∆σ e + 2 ( ∆σ 2k′ )1/n′ (17) which enables the plastic strain range to be calculated from eqn. 9. since σ∗(x) is the local stress, the fatigue notch factor, kf , is set to kf = 1 in eqn. 15, 16 and 17. figure 6 shows an example of the thus achieved stress and strain ranges. 4.2. comparison of the fatigue life of smooth and notched specimens figure 7 shows the modified swt parameter plotted versus the fatigue life for both smooth and blunt notch ct specimens; for the latter, the modified swt parameter has been calculated at the notch root. it is evident that, at both 450 °c and 550 °c, the predicted life of the notched specimens would be too conservative if based on fatigue life curves for smooth specimens and the notch root modified swt parameter. that is, notch support needs to be included. as seen in fig. 7, the notch support diminishes for low values of the modified swt parameter (roughly at 104 cycles to rupture). the low values of the modified swt parameter are caused by the plastic strain range approaching zero for low loads. the diminishing of the notch support could therefore be said to occur roughly at the transition into the high cycle fatigue region. to accurately account for notches in life prediction in the low cycle regime, the theory of critical distances will be applied. in the following sections, the distance from the notch at which the modified swt parameter coincides with that of a smooth specimen of the same fatigue life will be identified and used to calibrate an equation for establishing the critical distance. distance from notch, mm 0 0.5 1 1.5 2 st re ss ra ng e, m pa 500 1000 1500 2000 2500 3000 3500 from linear-elastic fea from glinka's rule distance from notch, mm 0 0.5 1 1.5 2 st ra in ra ng e, % 0 0.5 1 1.5 2 total, from linear-elastic fea total, from glinka's rule plastic, from glinka's rule a) b) figure 6. stress and strain ranges from a linearelastic finite element analysis (fea) at 450 °c and 9.9 kn load subjected to a glinka shakedown. a) stress range as function of the distance from the notch. b) total and plastic strain ranges as function of the distance from the notch. figure from ref. [6]. 4.3. a pragmatic equation for the critical distance since eqn. 4 has gained some use for the critical distance approach in fatigue (see the introduction section), it will be taken as basis also in the current work. although useful, eqn. 4 is somewhat inconvenient as it requires an iterative process to solve (since nf may not be known a priori). therefore, eqn. 4 will be adopted on a form which enables the critical distance to be calculated directly from notch root variables (from an elastic fea). a rationale for this is provided here. firstly, nf can be eliminated in eqn. 4 by using eqn. 14, giving σmax(r)∆εp(r) 2 = a(c1r c1)a = c2r c2 (18) in order to relate plastic strain with stress from an elastic finite element analysis, glinka’s rule is used∫ ∆εp(r)/2 0 σ(r)dε = k2 f (σ∗(r))2 2e (19) 38 vol. 20/2018 effect of notches on fatigue cycles to failure 10 2 10 3 10 4 10 5 0 2 4 6 8 10 smooth specimens notched specimens σ m a x δ ε p /2 , m pa cycles to failure 10 2 10 3 10 4 10 5 0 5 10 15 smooth specimens notched specimens σ m a x δ ε p /2 , m pa a) b) figure 7. comparison between the fatigue lives of smooth and notched specimens (data for smooth specimens are given as a fitted curve and data for notched specimens are experimental data). the fatigue life is described in terms of the modified swt parameter, σmax∆εp/2, which, for the notched specimens, has been calculated at the notch root: a) 450 °c and b) 550 °c. where σ∗(r) is the stress taken from an elastic finite element analysis the distance r from the notch root. since σ∗ represent the local stress, the fatigue notch factor is set to kf = 1. for simplicity, rigid–ideal plastic material behaviour is assumed (i.e. σ = σyl and ε = εp) which in eqn. 19 gives σyl ∆εp(r) 2 = (σ∗(r))2 2e (20) and, in eqn. 18, σyl ∆εp(r) 2 = c2r c2 (21) equating eqn. 20 and 21 yields c2r c2 = (σ∗(r))2 2e ⇒ r = [√ 1 2ec2 σ∗(r) ]2/c2 (22) or, after merging constant parameters, r = c3 (σ∗(r))c3 (23) assuming that the maximum stress occurs at the notch root and the critical distance of interest is close enough to the notch root (which it typically is), a sufficient approximation of the stress at the distance r ahead of the notch would be σ∗(r) = σ∗root(1 + χr) (24) which is a linear approximation of the stress in terms of the relative stress gradient, χ, taken as χ = 1 σ∗root ∂σ∗ ∂x ∣∣∣∣ x=0 (25) i.e. the stress gradient evaluated at the notch root normalised with the notch root stress. the approximation of the stress ahead of the notch can be improved by averaging χ over a distance roughly equal to the expected critical distance. therefore, χ will be replaced by an average relative stress gradient, χ̄, according to χ̄ = 1 σ∗root ∆σ∗ ∆x ∣∣∣∣ x=0 (26) where ∆x is taken as roughly the same as the expected critical distance. based on previous experiences [6], the critical distance should be in the order of a few tenths of a millimetre and is here (arbitrarily) set to ∆x ≈ 0.4 mm. inserting eqn. 24 in eqn. 23 (with χ̄ substituted for χ) gives r = c3 (σ∗root(1 + χ̄r))c3 (27) or, alternatively, σ∗root = drd 1 + χ̄r (28) with d = (1/c3)1/c3 and d = 1/c3. equation 28 is used in the current work to correlate the critical distance with the notch root stress from a linear elastic fea. although several simplifications and approximations were introduced in the derivation of eqn. 28, it is believed to be, at least, a good starting point for finding such a relation. equation 28 is sigmoid shaped and have some characteristics which seem intuitively reasonable: 1) for small values of σ∗root, r is approximately zero (in agreement with the notch support diminishing for long lives, see fig. 7). 2) the critical distance, r, asymptotically approaches 1/(−χ̄) for large values of σ∗root; it does seem reasonable that an upper limit in r should exist and be determined by the notch stress gradient. it should be noted that, due to the involved assumptions of material behaviour, the parameters d and d are material and temperature dependent. 4.4. calibration of the critical distance a few data points from 450 °c and 550 °c were used for calibrating the critical distance equation (eqn. 28). by examining eqn. 28, one realises that the upper limit of the critical distance is determined by the factor 1/(1 + χ̄r) which causes r to asymptotically approach r = 1/(−χ̄) for large σ∗root. therefore, the calibration 39 r. eriksson, j. moverare, z. chen, k. simonsson acta polytechnica ctu proceedings t , °c d, pa/md d 450 3.3045× 109 0.0493 550 4.9181× 109 0.1396 table 3. fitted parameters for eqn. 28. the fitting was done with σ∗root in pa and r in m. should be possible with just a few data points taken at low values of σ∗root (since the remaining r–σ∗root curve will be dominated by the 1/(1 + χ̄r) factor). the calibration is done as follows: (1.) for each calibration point, the experimentally observed fatigue life of the notched specimen, nnotch f , is given. (2.) a target modified swt parameter, σ̂max∆ε̂p/2, is determined from the σmax∆εp/2–nf curve for smooth specimens (i.e. from eqn. 14) as σ̂max∆ε̂p/2 = a ( nnotch f )a (29) (3.) the critical distance, r, is determined so that σmax(x = r)∆εp(x = r)/2 = σ̂max∆ε̂p/2 (30) which gives r; σmax(x) and ∆εp(x) profiles are obtained by glinka shakedown as shown in fig. 6. (4.) the hence found r values are plotted versus the corresponding σ∗root value and the parameters d and d in eqn. 28 are fitted. figure 8 shows the critical distance (obtained as described above) versus the notch root stress, σ∗root. at both 450 °c and b) 550 °c, the three points with lowest σ∗root were used to fit eqn. 28. as seen, eqn. 28 (when extrapolated) describes the critical distances for the remaining data points with reasonable accuracy. at 550 °c, there is a slight discrepancy between eqn. 28 and experimental data for high σ∗root values; most likely due to the critical distance exceeding the distance over which χ̄ was average, causing the approximation of the stress ahead of the notch to break down. however, the discrepancy is relatively minor. overall, eqn. 28 captures the critical distance with reasonable accuracy and will be used for predicting the fatigue life of notched specimens. the fitting parameters in eqn. 28 are given in tab. 3. 4.5. prediction of the fatigue life of notched specimens the calibrated equation for the critical distance (eqn. 28) can now be used together with the fatigue life model for smooth specimens (eqn. 14) to make a prediction of the fatigue life of notched specimens. it should be noted, though, that the notch support only is present in the lcf regime. the prediction is done as outlined below: σroot * , mpa 0 1000 2000 3000 4000 r, m m 0 0.1 0.2 0.3 0.4 0.5 used for fit used for validation fit * σroot * , mpa 0 1000 2000 3000 4000 5000 r, m m 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 used for fit used for validation fit * a) b) figure 8. the critical distance, r, as function of the notch root stress from an elastic finite element analysis, σ∗root, for: a) 450 °c and b) 550 °c. the fit was done using eqn. 28; a few points taken at low σ∗root were used for calibration. (1.) for each notched specimen, the σ∗(x) profile is determined from a linear elastic finite element analysis. the parameters σ∗root and χ̄ are established from the σ∗(x) profile. (2.) the critical distance, r, is determined from eqn. 28 with values from tab. 3. (3.) the stress σ∗(r) is retrieved from the σ∗(x) profile and a glinka shakedown is performed (see eqn. 15, 16 and 17), giving σmax and ∆εp. this allows the modified swt parameter, σmax∆εp/2, to be calculated at r. (4.) the hence obtained modified swt parameter is used in conjunction with fatigue life data for smooth specimens (see eqn. 14) to predict the life of notched specimens. figure 9 shows the results of the fatigue life prediction of notched specimens as outlines above. as in fig. 7, data from the notched specimens has been plotted in terms of the notch root modified swt parameter, but the prediction has been made based on σmax(x = r)∆εp(x = r)/2. the fatigue life prediction based on the critical 40 vol. 20/2018 effect of notches on fatigue cycles to failure 10 2 10 3 10 4 10 5 0 5 10 15 smooth specimens notched, used for calibration notched, used for validation prediction σ m a x δ ε p /2 , m pa cycles to failure 10 2 10 3 10 4 10 5 10 6 0 2 4 6 8 10 smooth specimens notched, used for calibration notched, used for validation prediction σ m a x δ ε p /2 , m pa a) b) figure 9. the modified swt parameter, σmax∆εp/2, versus cycles to failure for smooth and notched specimens at: a) 450 °c and b) 550 °c. for the notched specimens, the data is plotted versus the notch root modified swt parameter, σmax(x = 0)∆εp(x = 0)/2, but the prediction has been made based on the modified swt parameter at the critical distance, r, i.e., σmax(x = r)∆εp(x = r)/2. distance obtained from eqn. 28 agrees well with experimental results (see fig. 9) thus indicating that the suggested equation for the critical distance (eqn. 28) certainly is usable. in addition, it should be noted that only a limited number of data points were needed to calibrate eqn. 28, making it a fairly efficient approach to account for notch support. it should be noted, however, that the applicability of the life model is limited to loads which gives, at least some, plastic strain. the specimen with the longest life in fig. 9 agrees less well with the prediction due to the modified swt parameter being essentially zero. 5. conclusions low cycle fatigue tests were performed on blunt notch compact tension specimens made from alloy 718. the results indicated that notch support needed to be incorporated in the lifing method not to cause an overly conservative life prediction. the notch support, however, diminished as the plastic strain range decreased, indicating that notch support is only present in the low cycle fatigue regime for this alloy. to account for the notch support, a critical distance approach was attempted. an equation, relating the critical distance to the notch root stress from a linear elastic finite element analysis, was derived. the chosen life model was formulated in terms of a variation on the smith–watson–topper parameter, the modified swt parameter (based on the plastic strain range). the modified swt parameter taken at the critical distance was used in a life model calibrated for smooth specimens to predict the fatigue life of notched specimens. the fatigue life prediction for notched specimens agreed well with available experimental data. in summary, the current work has shown that: (1.) a critical distance approach can be applied to account for notch support in alloy 718. (2.) the critical distance can be correlated to the notch root stress from a linear elastic finite element analysis, considerably simplifying its use. (3.) the suggested equation for the critical distance requires relatively few data points for calibration. list of symbols a fatigue life model coefficient [pa] d coefficient in equation for the critical distance [pa/md] e young’s modulus [pa] k′ cyclic ramberg–osgood coefficient [pa] k′max cyclic ramberg–osgood coefficient for determining max stress/strain [pa] kc fracture toughness [ pa √ m ] kth threshold stress intensity factor for crack growth[ pa √ m ] kf fatigue notch factor nf cycles to failure nnotch f experimentally observed cycles to failure for notched specimens r load ratio, minimum load over maximum load rε strain ratio, minimum strain over maximum strain t temperature [°c] a fatigue life model exponent b fatigue strength exponent c fatigue ductility exponent d exponent in equation for the critical distance n′ cyclic ramberg–osgood exponent n′max cyclic ramberg–osgood exponent for determining max stress/strain r critical distance [m] ∆ε total strain range ∆εp plastic strain range ∆εroot p notch root plastic strain range ∆σ stress range [pa] ∆σ∗ stress range from a linear elastic analysis [pa] εp plastic strain εmax maximum total strain ν poisson’s ratio σ0 critical value of stress [pa] σ∗ stress from a linear elastic analysis [pa] σ∗root notch root stress from a linear elastic analysis [pa] 41 r. eriksson, j. moverare, z. chen, k. simonsson acta polytechnica ctu proceedings σyl yield limit [pa] σ′yl cyclic yield limit [pa] σmax maximum stress [pa] σa stress amplitude [pa] σ′f fatigue strength coefficient [pa] σmax∆εp/2 modified swt parameter [pa] σ̂max∆ε̂p/2 target modified swt parameter [pa] χ relative stress gradient [1/m] χ̄ averaged relative stress gradient [1/m] acknowledgements this project has received funding from the european union’s horizon 2020 research and innovation programme under grant agreement no. 653941. references [1] r. c. reed. the superalloys: fundamentals and applications. cambridge university press, 2006. doi:10.1017/cbo9780511541285. [2] l. mäde, h. gottschalk, s. schmitz, et al. probabilistic lcf risk evaluation of a turbine vane by combined size effect and notch support modeling. in asme turbo expo 2017: turbomachinery technical conference and exposition, vol. 7a: structures and dynamics. 2017. doi:10.1115/gt2017-64408. [3] c. kontermann, h. almstedt, a. scholz, m. oechsner. notch support for lcf-loading: a fracture mechanics approach. procedia structural integrity 2:3125 – 3134, 2016. 21st european conference on fracture, ecf21, 20-24 june 2016, catania, italy, doi:10.1016/j.prostr.2016.06.390. [4] d. taylor (ed.). the theory of critical distances. elsevier science ltd, oxford, 2007. doi:10.1016/b978-0-08-044478-9.x5000-5. [5] d. leidermark, j. moverare, k. simonsson, s. sjöström. a combined critical plane and critical distance approach for predicting fatigue crack initiation in notched single-crystal superalloy components. international journal of fatigue 33(10):1351 – 1359, 2011. doi:10.1016/j.ijfatigue.2011.05.009. [6] r. eriksson, k. simonsson, d. leidermark, j. moverare. evaluation of notch effects in low cycle fatigue of alloy 718 using critical distances. matec web of conferences 165:15001, 2018. doi:10.1051/matecconf/201816515001. [7] j. moverare, g. leijon, h. brodin, f. palmert. effect of so2 and water vapour on the low-cycle fatigue properties of nickel-base superalloys at elevated temperature. materials science and engineering a 564:107–115, 2013. doi:10.1016/j.msea.2012.11.079. [8] m. fukuhara, a. sanpei. elastic moduli and internal frictions of inconel 718 and ti-6al-4v as a function of temperature. journal of materials science letters 12(14):1122–1124, 1993. doi:10.1007/bf00420541. [9] h. ledbetter. temperature behaviour of young’s moduli of forty engineering alloys. cryogenics 22(12):653 – 656, 1982. doi:10.1016/0011-2275(82)90072-8. [10] k. smith, t. topper, p. watson. a stress-strain function for the fatigue of metals (stress-strain function for metal fatigue including mean stress effect). j materials 5:767–778, 1970. [11] f. lorenzo, c. laird. a new approach to predicting fatigue life behavior under the action of mean stresses. materials science and engineering 62(2):205 – 210, 1984. doi:10.1016/0025-5416(84)90223-4. 42 http://dx.doi.org/10.1017/cbo9780511541285 http://dx.doi.org/10.1115/gt2017-64408 http://dx.doi.org/10.1016/j.prostr.2016.06.390 http://dx.doi.org/10.1016/b978-0-08-044478-9.x5000-5 http://dx.doi.org/10.1016/j.ijfatigue.2011.05.009 http://dx.doi.org/10.1051/matecconf/201816515001 http://dx.doi.org/10.1016/j.msea.2012.11.079 http://dx.doi.org/10.1007/bf00420541 http://dx.doi.org/10.1016/0011-2275(82)90072-8 http://dx.doi.org/10.1016/0025-5416(84)90223-4 acta polytechnica ctu proceedings 20:34–42, 2018 1 introduction 1.1 theory of critical distances 2 material behaviour 2.1 material model 2.2 fatigue life model for smooth specimens 3 experimental work 4 results and discussion 4.1 stress and strain ranges for notched specimens 4.2 comparison of the fatigue life of smooth and notched specimens 4.3 a pragmatic equation for the critical distance 4.4 calibration of the critical distance 4.5 prediction of the fatigue life of notched specimens 5 conclusions list of symbols acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2018.17.0010 acta polytechnica ctu proceedings 17:10–14, 2018 © czech technical university in prague, 2018 available online at http://ojs.cvut.cz/ojs/index.php/app influence of radiation damage of reactor inner components on the results of indentation tests jan ondracek∗, ales materna department of materials, faculty of nuclear sciences and physical engineering, czech technical university in prague ∗ corresponding author: jan.ondracek@fjfi.cvut.cz abstract. irradiation of inner components of the wwer reactor type made from austenitic steel 08ch18n10t alters their material properties and may cause component degradation. numerical simulations of the indentation test on the nonand neutron-irradiated reactor's inner components were carried out. the aim of this study is to find out whether the indentation test is suitable for assessing the material radiation damage of studied components. keywords: neutron-irradiated material, hardness, indentation, wwer, fem. 1. introduction the extent of radiation damage of reactor pressure vessel internals (rvi) is a key factor in determining the remaining life span of nuclear power plants. in the wwer reactors, parts of the rvis are made from ti stabilized austenitic stainless steel 08ch18n10t. the overall radiation dose absorbed by the rvis after 40 years of operation can exceed 40 dpa [1]. the neutron irradiation produces a series of defects in material. with contribution of other factors, such as a higher temperature, dislocation loops, precipitates, cavities and chemical changes on grain boundaries are formed over time. this leads to changes in mechanical properties, namely an increase of yield stress and hardness and decrease of ductility. it is important to assess how the material properties change in dependence on the overall radiation dose. since the irradiated material is scarce, hardness testing, which needs only little testing material compared to other testing methods, might be used to estimate material properties of neutron-irradiated materials. numerical simulations of hardness testing (ht) of nonand neutron-irradiated materials were made in this study in order to decide whether the ht can reliably estimate the radiation damage of the studied material. the material used in this analysis is the 08ch18n10t austenitic stainless steel, analogous to astm a321 stainless steel. two experimental datasets of tensile tests taken from the literature were used in this study. the material for specimens in the first dataset [1, 2] was withdrawn from the decommissioned wwer 440 npp in greifswald. tensile tests of nonand neutron-irradiated samples with dose of 5.2 dpa conducted in 20°c air environment from [1, 2] were also accompanied by micro hardness tests. the specimens used in the second dataset [3] were cut out of a hot-rolled plate, then annealed at 1050°c for 2h and cooled in the air. the irradiation of these specimens was carried out in bor-60 reactor in ssc riar. the irradiation temperature was between 320 ° and 340 °c. the neutron-irradiation of the samples from the second dataset was 7 and 40-43 dpa. elastic-plastic behavior of the material was evaluated based on the tensile tests and then fed into numerical models of micro hardness testing. 2d and 3d fem models were used in order to obtain not only micro hardness results but also to evaluate surface of the material near the indent for a possible occurrence of sink-ins or pile-ups. 2. material data five tensile tests with four different neutron doses (0, 5.2, 7 and 40-43 dpa) taken from [1] and [3] were used to obtain yield strength and a plastic response of the material. the resulting experimental plastic responses as well as the yield stresses are plotted in figure 1. the non-irradiated samples show a hardening effect and their hardening curve can be fitted with a hollomon's equation. the sample irradiated to 7 dpa also shows some hardening effect but the hardening curve could not be fitted with a power law relation. therefore, no hardening was used in the range of plastic deformation between 0.2 and 1. in case of samples irradiated to 5.2 and 40-43 dpa no hardening was observed in the tensile tests, so an ideally plastic material response was used in the simulations. elastic constants of the 08ch18n10t stainless steel are following: young modulus equals 200 gpa and poisson ratio is 0.3. 3. numerical modeling in order to compute micro hardness of the nonirradiated and irradiated material using a fem model, a two-dimensional axisymmetric mesh as well as threedimensional mesh was used. the real shape of the indenter in case of the two-dimensional mesh is represented by an equivalent cone with cone apex semiangle of 70.3°. the cone indenter of this shape has the same projected contact area per penetration depth as 10 http://dx.doi.org/10.14311/app.2018.17.0010 http://ojs.cvut.cz/ojs/index.php/app vol. 17/2018 influence of radiation damage of reactor inner components figure 1. plastic response of studied neutron radiation damages the vickers indenter. this indenter is modeled as a perfectly rigid body with no friction with the sample and its movement is controlled by force with a maximum value 1 n. the indenter penetrates the mesh in the contact area. the size of the 4-node quadrilateral elements in the contact area is 0.5x0.2 µm and the size of the rectangular contact area is 40x40 µm. the rest of the two-dimensional mesh is auto-meshed with three node triangular elements. their size gradually grows with their distance from the contact area. the whole set-up of the 2d model can be seen in figure 2. boundary conditions are following: all nodes on the bottom edge of the model are fixed in the direction perpendicular to the direction of loading in order to achieve a rotational symmetry of the model and all nodes on the right-most edge are fixed in the direction of loading. the 3d mesh allows modeling a real shape of the vickers four-sided pyramid indenter. therefore, the true imprint as well as the pile-up profile can be studied. the three-dimensional mesh consists of 180 000 8-node hexahedral elements. only one fourth of the sample surface is modeled in order to save computing time. the indentation region with the smallest elements has size of 30x30x10 µm (10 µm in the direction of loading). the element size in this region is 0.5x0.5x1 µm (1 µm in the direction of loading). the three-dimensional mesh is shown in figure 3. all nodes at the symmetry boundary planes have fixed displacements in the direction perpendicular to the plane. the indenter is modeled by a rigid plane. the movement of the indenter plane is driven by displacement until the maximum loading force at the indenter tip 1 n is reached. this procedure provides a sufficient numerical stability. the plastic response of the material was modeled using isotropic hardening with von mises yield condition. plastic responses used in the simulations are shown in figure 1. all the simulations were run in the large-strain regime. figure 2. two-dimensional fem model of the indentation test. figure 3. three-dimensional mesh of a material surface with an indentation imprint 4. results and discussion the results compose of 2d and 3d indentation test simulations. five computations with different neutronirradiation damage were performed using msc.marc software for fem analyses. the mechanical properties data were taken from two different datasets. the corresponding dataset is labeled by its reference number in the results. indentation load – penetration depth curves and hardnesses can be evaluated using either twoor three-dimensional mesh. the differences are however within a numerical error range, so only the results for 2d mesh are shown. three-dimensional computations used in this study took approximately 10x times longer to compute than the two-dimensional ones, but they provide a better insight into the expected indent shape and surface profiles. indentation load – penetration depth curves are shown in figure 4. the maximum penetration depth reaches a little over 5µm for both non-irradiated specimens. the specimen from the dataset [1] has higher penetration depth than the one from [3] and their difference is 0.135 µm. the neutron-irradiated spec11 jan ondracek, ales materna acta polytechnica ctu proceedings figure 4. simulated indentation curves using a 2d model for nonand neutron-irradiated material, mechanical properties determined at room temperature figure 5. dependence of vickers hardness on neutron damage for different hardness evaluation methods: contact area (ca), oliver-pharr (op) [4] method imens are clearly harder and their maximum penetration depth is within a range of 3.32 and 3.53 µm. the curves are ordered by their radiation damage with the most irradiated specimen having the smallest maximum penetration depth. analogous results are shown in figure 5. vickers hardness is evaluated using a direct contact area method (ca) and the oliver-pharr method [4, 5]. in case of the ca method, the radius of the circular projected contact area at maximum load is read from the results. it is easy to recognize the furthest node in contact with the indenter at the maximum load. since the displacement in the direction of loading after a complete unloading of this node is minimal, the diameter of the circular contact area is evaluated based on this node. although the results of hardness are shown in the units of vickers hardness for the sakes of comparing to experimental data [1, 2], it is interesting to obtain hardnesses using the oliver-pharr method as well. the computed unloading force–penetration depth curves (fig. 4) were used to obtain hardness by the op method. the computed hardnesses from dataset [1] can be compared to experimental data. the computed hardfigure 6. surface profile cross-sections; face profile (red), diagonal profile (blue) ness evaluated by the ca method of non-irradiated and neutron-irradiated material with dose 5.2 dpa is approx. 25% smaller than experimentally assessed. it can be also seen than the computed hardness rises with rising neutron damage, although the increase in hardness between 7 and 40-43 dpa is insignificant. the resulting computed hardness also depends on a method used for evaluation. the op method gives higher hardnesses for all the specimens. the difference in hardness for nonirradiated specimens is approximately 6%. the methods differ by approx. 16% from each other for neutronirradiated material. such a large difference can be explained by the fact that the oliver-pharr method uses only penetration depth curves as an input for estimating the contact area using only an elastic contact solution (fig. 4). a little or no hardening material is used for simulations of neutron-irradiated specimens (see figure 1). therefore, pile-ups form in the vicinity of the indent and the estimated contact area differs greatly from the one coming from an elastic-plastic calculation. the non-irradiated strainhardening material exhibits smaller pile-ups compared to the neutron-irradiated one. created pile-ups are clearly visible in figure 7 showing the surface profiles for all the specimens. the surface profiles are plotted in the diagonal and face cross-section of the three-dimensional indent (as can be seen in 6) in colors, the dashed line shows a surface profile computed using a two-dimensional mesh. the 2d profile lies between the face and diagonal surface profiles as expected. the maximum pile-up height occurs at the three-dimensional indent face. the nonirradiated specimens (fig. 7a, c) have much smaller pile-ups than the irradiated ones because their material strain hardens. the biggest pile-up was calculated in fig. 7b because the yield strength of its material is smaller compared to more irradiated fig. 7e and no strain hardening was used for both computations. the overall visualizations of the indents in surrounding material can be seen in figure 8. 5. conclusions twoand three-dimensional fem models of indentation test were created in order to assess hardness of 12 vol. 17/2018 influence of radiation damage of reactor inner components (a) . 0 dpa [1] (b) . 5.2 dpa [1] (c) . 0 dpa [3] (d) . 7 dpa [3] (e) . 40-43 dpa [3] figure 7. computed surface displacement profiles; non-irradiated samples a and c, neutron irradiated samples b, d and e; , dataset [1] a and b, dataset [3] c, d and e; depicted surface area 40x40 µm 13 jan ondracek, ales materna acta polytechnica ctu proceedings (a) . 0 dpa [3] (b) . 7 dpa [3] (c) . 40-43 dpa [3] (d) . 0 dpa [1] (e) . 5.2 dpa [1] figure 8. computed indents; non-irradiated samples a and d, neutron-irradiated samples b, c and e; dataset [3] a, b and c, dataset [1] d and e; depicted surface area 30x30 µm neutron-irradiated stainless steel 08ch18n10t. the two-dimensional model used an equivalent cone indenter with a cone apex semi-angle of 70° to simulate vickers indenter shape and in the three-dimensional model the vickers 4-sided pyramid was directly modeled. necessary material input data was obtained using tensile tests taken from the literature. a part of the the tensile test of neutron-irradiated specimens was accompanied by experimentally measured hardnesses. the following conclusions can be made based on the work presented here: (1.) both twoand three-dimensional models predict the same hardness. the 3d model in addition is able to compute surface displacement profiles for face and diagonal cross-sections of the indent as well as the shape of the indent including possible pile-ups in its vicinity. (2.) the numerical models predict higher hardnesses for neutron-irradiated material which is in agreement with experiments. (3.) the oliver-pharr method overestimates hardness of neutron-irradiated material because pile-ups build up in the vicinity of the impression. acknowledgements this work was supported by the technology agency of the czech republic, project no. th02020565. references [1] a. hojna, m. ernestova, o. hietanen, et al. irradiation assisted stress corrosion cracking of austenitic stainless steel wwer reactor core internals. in g. i. j. t. busby, p. l. andresen (eds.), 15th international conference on environmental degradation of materials in nuclear power systems water reactors. john wiley and sons, inc., hoboken, new jersey, canada, 2011. doi:10.1002/9781118456835.ch131. [2] h. k. namburi, a. hojna, z. fencl. effect of tensile strain on microstructure of irradiated core internal material. in international conference nuclear energy for new europe, portoroz, slovenia. 2015. [3] a. sorokin, b. margolin, i. kursevich, et al. effect of neutron irradiation on tensile properties of materials for pressure vessel internals of wwer type reactors. j nucl mater 444:373–384, 2014. doi:10.1016/j.jnucmat.2013.10.016. [4] w. oliver, g. m. pharr. an improved technique for determining hardness and elastic modulus using load and displacement sensing indentation experiments. j mater res 7:1564–1583, 1992. doi:10.1557/jmr.1992.1564. [5] w. oliver, g. m. pharr. measurement of hardness and elastic modulus by instrumented indentation: advances in understanding and refinements to methodology. j mater res 19:3–24, 2004. doi:10.1557/jmr.2004.19.1.3. 14 http://dx.doi.org/10.1002/9781118456835.ch131 http://dx.doi.org/10.1016/j.jnucmat.2013.10.016 http://dx.doi.org/10.1557/jmr.1992.1564 http://dx.doi.org/10.1557/jmr.2004.19.1.3 acta polytechnica ctu proceedings 17:10–14, 2018 1 introduction 2 material data 3 numerical modeling 4 results and discussion 5 conclusions acknowledgements references doi:10.14311/app.2021.30.0081 acta polytechnica ctu proceedings 30:81–86, 2021 © czech technical university in prague, 2021 available online at http://ojs.cvut.cz/ojs/index.php/app numerical comparison of transgingival and subgingival dental implants in regard to their stress distributions luboš řehouneka,∗, aleš jíraa, gabriela javorskáa, daniel bodlákb a czech technical university in prague, faculty of civil engineering, department of mechanics, thákurova 7, 166 29 prague 6, czech republic b prospon spol. s.r.o., jiřího voskovce 3206, 272 01 kladno, czech republic ∗ corresponding author: lubos.rehounek@fsv.cvut.cz abstract. most modern dental implants differentiate in regard to the fixation of the abutment into two main categories the external or internal hexagon or octagon. we performed mechanical tests according to the čsn en iso 14801 standard on a dental implant variant using the external hex. we found that failure of all implant specimens occured below the screw head. to improve the current geometry, we performed numerical analysis of an alternative variant (internal hex) and compared it with analysis of the current geometry (external hex). it was found that the stress distribution of the variant with internal hex is preferable to the old variant. although extreme values of shear stress in the corresponding plane of loading are higher, they do not concentrate below the screw head, where the screw itself is thinner and more prone to breaking. therefore, it seems that the new variant of the dental implant is stronger, which is still to be proven by mechanical tests. keywords: external hexagon, fem, implant-abutment interface, internal hexagon, stress. 1. introduction the implant-abutment connection is one of the most important interfaces of a dental implant. it provides a means to apply torque required to screw the implant into bone and also serves as a connection between the intraosseous part and the abutment. there are many requirements and functions of this interface that may be less or more propagated in regard to the form of connection (internal or external). however, one of the most important functions is the prevention of rotation between individual parts [1]. this particular property is greatly influenced by the character of the interface. from a mechanical standpoint, there are several geometrical solutions among dental implants in regard to their placement and the fixation of the abutment. they include the internal morse taper, external morse taper, internal lobing, tube-to-tube systems and external and internal polygons. these last 2 main categories comprise the vast majority of clinically used implants as well as the majority of market share. the most common variants of the polygonal fixation are the hexagon or the octagon. our study includes 2 different variants of the polygonal variant – the external hexagon (transgingival variant) and the internal hexagon (subgingival variant). their names suggest their placement in the bone, as shown in fig. 1. historically, the first manufactured titanium dental implants by brånemark had the external hexagonal connection. it is a variant that is very simple a robust, however, it has some deficiencies, such as insufficient anti-rotational resistance or insufficient resisfigure 1. comparison of placement of the 2 analysed dental implant variants inside the bone. left – the internal hexagon variant (subgingival placement) and right – the external hexagon variant (transgingival placemenent). the lenght l indicates the portion of the intraosseous part that exceeds the bone after implantation. tance to intraoral forces with greater offset from the connection (e.g. top of the crown). to improve lateral and rotational stability, many other variants were developed (internal hex, spline connection, morse taper, tapered hexagon and other variants) [2]. 1.1. motivation for development the literature suggests that internal connections have mechanical advantages. it was found that the bending moments of zirconia implants are superior when paired with internal connection, compared to exter81 http://dx.doi.org/10.14311/app.2021.30.0081 http://ojs.cvut.cz/ojs/index.php/app l. řehounek, a. jíra, g. javorská, d. bodlák acta polytechnica ctu proceedings figure 2. documentation of the old external (eh) connection variant of the implant.the intraosseous part is the subject of a patent [7]. nally connected abutments [3, 4]. research suggests that external connections lead to greater stresses inside the components of the implant, mainly at the connection interface [5]. also, they pose as an alternative to external connections, which have proven to break in the abutment screw [6]. the abutment screw can be identified as a very weak component of the system as it is thin while being exposed to shear forces during mastication. the motivation to develop other variants of connection also stems from the fact that external connections have proven to decrease levels of crestal bone at the peri-implant area after 1 year of loading and also exhibit greater bone remodelling and bone loss when compared to internal connections within the same set of circumstances [8]. furthermore, internal abutment connections have better lateral stability as the lever arm of forces is shorter, thus reducing the value of maximum force that can be applied in the lateral direction. according to literature, this results in better stress distributions, less micromovement and higher survival probability [9]. other factors, such as surface treatment, anodization, implant length, abutment material etc. also influence bone loss. another aspect of potential improvement over the external connection types is improving the transfer of loads in the surrounding bone and minimizing the gaps at the implant-abutment interface which may figure 3. documentation of the new internal (ih) connection variant of the implant.the intraosseous part is the subject of a patent [7]. lead to bacterial colonization [10] and subsequent biofilm accumulation [11]. 2. materials and methods the means of comparison in this study are fem analyses of 2 selected dental implant variants in regard to their stress distribution. the selected variants are the external hexagon (eh) and the internal hexagon (ih), as shown in fig. 2 and fig. 3. the simulations were performed on both variants on the basis of conditions and results of mechanical tests of the eh variant so as to verify the viability of the newer ih variant in regard to stress distribution. 2.1. mechanical tests we performed mechanical tests on the old eh variant according to the iso 14801 dentistry–implants– dynamic fatigue tests for endosseous dental implants standard [12] on a total of 6 specimens of dental implants (fig. 4). the tests were performed on a double-column pneumatic press litem vertical double column vdc a-6. the machine complies with iso 7500-1 [13] and enables for constant crosshead displacement. a sample p-d diagram of the tested eh specimens is shown in fig. 6. all specimens were embedded in the dentacryl fixating medium (methylacrylate resin for technical use). the resin has good mechanical properties and thermal insulation properties. its bending strength is 50 mpa, tensile strength is 20 82 vol. 30/2021 comparison of transgingival and subgingival implants figure 4. a disassembled specimen of the eh variant used for the mechanical tests. although the body of the implant is different, the locking system including the abutment and screw are the same. specimen number force [n] 1 1278.7 2 1387.6 3 1301.2 4 1486.0 5 1275.8 6 1233.2 mean value 1327.1 table 1. maximum static load force for all eh specimens. mpa and compressive strength is 80 mpa. its elastic modulus is 3.2 gpa. with these properties, it complies with the mentioned standards for static testing. as the iso 14801 standard [12] requires a 30° slanted force, a fixation apparatus is needed to slant the specimens as the litem machine can only apply force in the vertical direction. we needed to make this preparation ourselves as the manufacturer does not provide any further preparations for the machine. it was modelled in netfabb and subsequently 3d printed on the sinterit lisa pro 3d printer by using the sls (selective laser sintering) technology. the used material was the pa12 polyamide, whose strength is comparable with aluminum. the fixating preparation is shown in fig. 7. after the fixation, the implant is connected with the abutment by the abutment screw with a m2.2 thread using a 250 nm momentum by a special ratchet supplied by the manufacturer of application instrumentation. the mechanical tests were controlled by a constant speed of displacement of 4 mm/min until specimen failure in the litem machine. the mean maximum force of the implant specimens at failure was 1327.1 n (table 1) and the failure occured below the head of the abutment screw for all eh specimens, as shown in fig. 5. this test confirmed the hypotheses that the abutment screw would be the weakest part of the assembly and also corrobofigure 5. two eh specimens after mechanical tests. the images show failure of the specimens right below the head of the abutment screw. note that all specimens failed exactly in this manner. figure 6. a sample p-d diagram of specimen n. 2 from the mechanical testing. the maximum force for this specimen was 1387.6 n. rated the research of [6]. from fig. 5, shear failure of all specimens is apparent. the failure of all implants occured by shear failure of the screw connecting the intraosseous part and the abutment. this connection proved to be a limiting factor for the tested eh specimens. a positive finding was the fact that the bodies of the implants themselves remained intact. 2.2. fem analyses as all of the eh specimens achieved failure below the head of the abutment screw, we set out to model these results using ansys workbench 2020 r1. the overall approach was to model the older eh variant, verify the model by comparing it with mechanical tests and subsequently analyse a newer version of the implant with an internal hexagonal connection to confirm whether distributions of stresses are better. the geometrical models were imported into ansys in the form of a parasolid (.x_t extension) and are shown in fig. 9. according to the previously conducted mechanical experiments, we chose a force with the magnitude of 900 n as it is approximately the highest force that is still in the linear region of the p-d diagram (fig. 6). the force is situated on top of the abutment rim and distributed equally among a few nodes of the mesh (fig. 9). to accurately represent the conditions of the experiment and the standard [12], we fixed the lower part of the implant that was embedded in the dentacryl resin and left 83 l. řehounek, a. jíra, g. javorská, d. bodlák acta polytechnica ctu proceedings figure 7. the mechanical testing assembly with the litem machine and a 3d-printed preparation for anchoring the specimens at a 30° angle. a 2 mm gap between the top of the fixed area and the theoretical level of bone (fig. 9). as the goal of the simulation was to compare stress distributions during the linear behavior, we did not consider nonlinear behavior in the model and chose the same force of 900 n for both simulations. simple tetrahedral linear elements and linear material behavior with using the ti-6al-4v material model were used to model the numerical problem. the mesh was designed so as to represent at least 5 elements across the width of the stabilizing ribs of the implant substructure and was further refined by size in the region of interest – the abutment screw. as the assembly consists of 3 solids – the abutment, the screw and the intraosseous part – the model contains basic contact conditions with specific distance break criteria. the contact conditions are necessary as some parts have conical shapes, e.g. the fit is assured by screwing the screw into the abutment conical cavity, the screw passing through the intraosseous part of the implant etc. the contact conditions are shown in fig. 8. due to the complicated geometry of the real screw, we decided to only model its threaded body as a cylinder as the goal of the simulation was not to observe stress distributions in individual threads or details, but rather stress distributions across the whole screw body, i.e. compare the stress concentrations or lack thereof. figure 8. contact regions of the new ih variant inside the ansys environment. figure 9. the two geometrical models inside the ansys environment. left– the old eh variant. right – the new, ih variant. blue shows the area of fixation (2 mm below the theoretical level of bone), red shows the location of the 30° slanted force. boundary conditions comply with previous mechanical tests and with iso 14801 [12]. 84 vol. 30/2021 comparison of transgingival and subgingival implants figure 10. the final results of both analyses (eh and ih variant) of the fem analyses displayed on the abutment screws. a – section through the abutment screw of the eh variant, typical stress concentration marked red. b – a detail of the area highlighted in "a". c – the new ih geometrical solution with significantly reduced shear stress concentrations. 3. results the initial hypothesis was that by introducing a modified geometry of the implant-abutment connection, we can reduce the shear stress concentrations and therefore either eliminate stress concentrations or increase the maximum force at which the screw will eventually break (for the eh variant, the mean force was 1327.1 n, see table 1). the results from the simulation of the old eh variant are shown in fig. 10-a and b. the image shows that the model behaves in the same way as the specimens in mechanical tests. although we did not encompass the region of failure as our analysis is only linear, it is apparent that the concentration of shear stresses occur exactly at the same place – the thin cross-section of the abutment screw below its head. with this information, we constructed the second model in the exact same way and compared the analyses. moreover, another region with significant stress concentration emerged at the bottom of the abutment screw. however, this occurrence was not accounted for as it did not have any direct impact during the mechanical tests. the results from the simulation of the new ih variant are shown in fig. 10-c. the shear stress concentrations were diminished and extremes moved from the thin cross-section of the screw into its head. although the overall values of shear stresses are higher across the whole body of the abutment screw, the concentration points were eliminated by incorporating the internal connection. 4. conclusions the initial hypothesis of this experiment was that by replacing the external hexagonal (eh) connection with internal (ih) connection, we can eliminate stress concentrations that emerge with the use of eh because of its geometry. the mechanical tests have shown that all of the dental implant specimens reached failure during the tests by shear failure of the screw below the its head. von-mises criteria and principal stresses were also considered, but their results were not included in this study as their evaluation has proven to be non-essential for confirming the hypothesis. the numerical analyses have shown that the eh variant induces concentrations of shear stress in the screw below its head. under the same conditions, we 85 l. řehounek, a. jíra, g. javorská, d. bodlák acta polytechnica ctu proceedings performed analysis of the ih variant and found a more viable distribution of shear stress with no apparent concentrations in the body of the abutment screw. therefore, this analysis confirms our initial hypothesis that the internal connection might be more favorable and corroborated the results provided by literature [6]. the trend to innovate in the area of implantabutment connections can be seen in the market share of dental implants, where more and more companies offer alternatives to the tried-and-true external connection, which proves to have its shortcomings, such as low anti-rotational resistance, bone loss at the peri-implant area, lower lateral stability, microgaps at the abutment-implant interface leading to bacteria colonisation and biofilm formation, etc. to further prove if this concept is true and finish the research, we need to manufacture specimens of this new geometrical solution with internal connection and subject them to mechanical tests to determine whether the character of failure changes or the maximum force required to break the abutment screw increases. acknowledgements the financial support provided by the by the faculty of civil engineering, ctu, prague, project n. sgs20/155/ohk1/3t/11 is gratefully acknowledged. references [1] l. gaviria, j. p. salcido, t. guda, j. l. ong. current trends in dental implants. journal of the korean association of oral and maxillofacial surgeons 40(2):50–60, 2014. [2] d. prithviraj, n. muley, v. gupta. the evolution of external and internal implant–abutment connections: a review. international dental research 2(2):37–42, 2012. [3] t. c. truninger, b. stawarczyk, c. r. leutert, et al. bending moments of zirconia and titanium abutments with internal and external implant–abutment connections after aging and chewing simulation. clinical oral implants research 23(1):12–18, 2012. [4] i. sailer, t. sailer, b. stawarczyk, et al. in vitro study of the influence of the type of connection on the fracture load of zirconia abutments with internal and external implant-abutment connections. int j oral maxillofac implants 24(5):850–858, 2009. [5] p. asvanund, s. m. morgano. photoelastic stress analysis of external versus internal implant-abutment connections. the journal of prosthetic dentistry 106(4):266–271, 2011. [6] w. att, s. kurun, t. gerds, j. strub. fracture resistance of single-tooth implant-supported all-ceramic restorations after exposure to the artificial mouth. journal of oral rehabilitation 33(5):380–386, 2006. [7] f. denk, f. denk, a. jíra. a dental implant shaft, 2017. [8] k.-t. koo, e.-j. lee, j.-y. kim, et al. the effect of internal versus external abutment connection modes on crestal bone changes around dental implants: a radiographic analysis. journal of periodontology 83(9):1104–1109, 2012. [9] c. a. a. lemos, f. r. verri, e. a. bonfante, et al. comparison of external and internal implant-abutment connections for implant supported prostheses. a systematic review and meta-analysis. journal of dentistry 70:14–22, 2018. [10] r. caricasulo, l. malchiodi, p. ghensi, et al. the influence of implant-abutment connection to peri-implant bone loss: a systematic review and meta-analysis. clinical implant dentistry and related research 20(4):653–664, 2018. [11] c. alves de sousa, j. j. conforte, k. s. caiaffa, et al. sealing agent reduces formation of single and dualspecies biofilms of candida albicans and enterococcus faecalis on screw joints at the abutment/implant interface. plos one 14(10):e0223148, 2019. [12] s. iof. international standard iso 14801–dentistry–implants-dynamic fatigue test for endosseous dental implants. geneva: international organization for standardization 2007. [13] e. iso. 7500-1. cen en iso 7500-1 metallic materialsverification of static uniaxial testing machines-part 1: tension/compression testing machines-verification and calibration of the force-measuring system. sl: european committee for standardization, august de 2004. document information . 86 doi:10.14311/app.2020.28.0042 acta polytechnica ctu proceedings 28:42–49, 2020 © czech technical university in prague, 2020 available online at http://ojs.cvut.cz/ojs/index.php/app analysis of fast neutron transport in chloride salts using monte carlo method ondřej šťastnýa,b,∗, karel katovskýa, dušan krála, antonín krásac, cable kurwitzd, kamil števankaa a brno university of technology, faculty of electrical engineering and communication, department of electrical power engineering, technická 10, 616 00 brno, czech republic b state office for nuclear safety department of non proliferation, senovážné náměstí 9, 110 00 prague, czech republic c belgian nuclear research centre, institute for advanced nuclear systems, sck cen, boeretang 200, 2400 mol, belgium d texas a&m university, department of nuclear engineering, ai engineering building 3133, 423 spence st, college station, tx 77843, united states ∗ corresponding author: xstast30@vutbr.cz abstract. the aim of this paper is to present results of fast neutron behavior analysis within the chloride salts environment using simulations based on monte carlo method (mcnp 6.2). three non-fueled salts (nacl, kcl, mgcl2) and two salts containing fissile material (ucl3, thcl4) were studied. results of this theoretical study will be used for design of an experimental assembly, which will serve to achieve goals of the international research project adar (accelerator driven advanced reactor). one of the project objectives is to investigate chloride salts as potential coolant and a dissolved fuel carrier of advanced nuclear reactor cooled by molten salts and driven by an accelerator. keywords: accelerator driven systems, fast neutrons, adar, chloride salts, mcnp, molten salt reactors, monte carlo simulations. 1. introduction molten salt reactors (msrs) have been identified as promising reactor technology with regard to thorium fuel cycle or for transmutation of spent nuclear fuel from light water reactors. there are several designs based on fuel dissolved in the molten salt coolant, such as fluorides or chlorides [1]. this article is focused on fast neutron behavior analysis within the chloride salts, since they have good nuclear, chemical and physical properties and are much easier to treat. they usually form solutions with lower melting point, and they have higher solubility for actinides. as a result, they can contain significant amounts of transuranic elements. on the other hand, the major isotope of chlorine, cl-35 gets activated to cl-36, long-lived energetic beta emitter, so cl-37 is much preferable in a reactor [1]. as stated above, the goal is to perform studies with fast neutron spectrum since it is used in fast spectrum msrs (msfrs) and or accelerator driven systems [2]. they can have conversion ratios within range burner – converter – breeder. furthermore, they can be optimized for burning minor actinides, for breeding plutonium from u-238, or they may be opencycle power plants since the fast neutron spectrum allows the possibility to omit onsite processing to remove transuranic elements. since the fission products have relatively large neutron capture cross sections in thermal energies, the capture cross sections at higher energies is much lower, resulting in much greater fission product build-up in an msfr than in a thermal-spectrum msr. higher absorption rate at fission products nuclei in thermal-spectrum msr leads to fission products transmutation and therefore they get closer to the valley of stability (after appropriate decay). moreover, the fuel salt heavily loaded with fission products can be sent occasionally for batch processing or allowed to solidify and be disposed of in a repository. for full breeder configuration the fissile material needs to be progressively removed [1]. recent research [3] shows significant discrepancies between evaluated nuclear data libraries in the fast neutron region. improving fast neutron data for chloride salts is essential requirement for the adar project (accelerator driven advanced reactor with molten chloride salt fuel coolant)1 . in order to perform such neutronic research, a theoretical study of an experimental assembly consisting of a fast neutron source in chloride salt environment was performed with the aim to optimize parameters for an experimental campaign using activation detectors. in the following sections, the model is of experimental assembly is described, the current status of 1detailed project information available at https://starfos.tacr.cz/en/project/ltausa18198. 42 http://dx.doi.org/10.14311/app.2020.28.0042 http://ojs.cvut.cz/ojs/index.php/app https://starfos.tacr.cz/en/project/ltausa18198 vol. 28/2020 monte carlo method for fast neutron transport figure 1. salt experimental assembly “solnicka” 3d model relevant nuclear data is summarized and then monte carlo calculations of neutron transport through chloride salts are presented. 2. design of salt experimental assembly 2.1. material in order to study neutronic properties of most commonly available chloride salts, a design of a salt experimental assembly “solnicka” (in english “salt cellar”) has been proposed (fig. 1). the assembly is cylindrical shape with diameter of 50 cm and height of 60 cm with coating from stainless steel (304 type, thickness of 0.6 mm on sides, 1 mm at the bottom and top cover). it contains the following channels (coated with 0.6 mm thick steel layer): • one central channel for neutron source (22 mm in diameter) which extends from top to the middle of the height (30 cm), • four axial experimental channels (28.1 mm in diameter) leading from to the top to the bottom located at radial distance of 5, 10, 15 and 20 cm from the vertical axis, • three radial experimental channels (28.1 mm in diameter) at heights of 35, 45 and 55 cm above the bottom. their length is 23 cm spanning from the outer surface of the assembly in direction to the vertical axis. the experimental channels have been filled with vacuum for simplicity and in order to avoid influence of other materials. as a source of neutrons, the ambe source (cylinder with diameter of 19 mm and height of 40 mm) located in the middle height (30 cm) with emission rate of 5.5615 × 106 neutrons/s was considered since it emits fast neutron field (0.1-11 mev, see table 1 for probability distribution function (pdf) e [mev] pdf e [mev] pdf 4,14e-07 0,00e+00 5,68e+00 2,33e-02 1,10e-01 1,44e-02 5,89e+00 2,06e-02 3,30e-01 3,34e-02 6,11e+00 1,82e-02 5,40e-01 3,13e-02 6,32e+00 1,77e-02 7,50e-01 2,81e-02 6,54e+00 2,04e-02 9,70e-01 2,50e-02 6,75e+00 1,83e-02 1,18e+00 2,14e-02 6,96e+00 1,63e-02 1,40e+00 1,98e-02 7,18e+00 1,68e-02 1,61e+00 1,75e-02 7,39e+00 1,68e-02 1,82e+00 1,93e-02 7,61e+00 1,88e-02 2,04e+00 2,23e-02 7,82e+00 1,84e-02 2,25e+00 2,15e-02 8,03e+00 1,69e-02 2,47e+00 2,25e-02 8,25e+00 1,44e-02 2,68e+00 2,28e-02 8,46e+00 9,68e-03 2,90e+00 2,95e-02 8,68e+00 6,52e-03 3,11e+00 3,56e-02 8,89e+00 4,26e-03 3,32e+00 3,69e-02 9,11e+00 3,67e-03 3,54e+00 3,46e-02 9,32e+00 3,81e-03 3,75e+00 3,07e-02 9,53e+00 5,06e-03 3,97e+00 3,00e-02 9,75e+00 6,25e-03 4,18e+00 2,69e-02 9,96e+00 5,52e-03 4,31e+00 2,86e-02 1,02e+01 4,68e-03 4,61e+00 3,18e-02 1,04e+01 3,70e-03 4,82e+00 3,07e-02 1,06e+01 2,78e-03 5,04e+00 3,33e-02 1,08e+01 1,51e-03 5,25e+00 3,04e-02 1,10e+01 3,63e-04 5,47e+00 2,74e-02 1,11e+01 0,00e+00 table 1. energy spectra for considered ambe neutron source according to iso-8529-1 [5] salt density ! g/cm3" keff pucl3 5.71 1.27283 +/0.00006 ucl3 5.50 0.12304 +/0.00001 thcl4 4.59 0.01936 +/0.00000 nacl 2.18 kcl 1.98 mgcl2 2.32 table 2. results of multiplication coefficient calculation (using 250 cycles with 106 particles from the source) overview) standardized iso-8529-1 [5] and can suitably serve for purposes of this study. to reach maximal homogeneity of the salt environment around the neutron source, a salt plug is considered to be placed above the source inside the source channel. for purposes of this study, a model of salt experimental assembly has been created using mcnp 6.2 (see fig. 2) [6]. three non-fueled salts (nacl, kcl, mgcl2) and two salts containing fissile material (ucl3, thcl4) were studied using calculation of reaction rates for set of selected reactions (see fig. 3). the non-fueled salts serve to investigate their potential influence on transport of neutrons (as a potential ternary salt coolant for nuclear reactors [7], [8], [9]). 43 o. šťastný, k. katovský, d. král et al. acta polytechnica ctu proceedings figure 2. salt experimental assembly "solnicka" vertical and horizontal cross section at height of 35 cm (from the mcnp model); nos. 1-4 are assigned to axial channels, 5-7 to radial channels figure 3. cross sections of activation reactions [4] ucl3 (with depleted uranium) and thcl4 represent a chloride salt carrying nuclear material – as a reference chemical compound which could be used in a molten salt reactor, where the fuel is dissolved in the coolant. pucl3 salt has also been considered, but salt assembly with abovementioned dimensions would be supercritical (see tab. 2). activation foils were selected as neutron detectors. the chosen foil materials are ni, in and au. they are mainly mono-isotopic or with one strongly dominating isotope, therefore, the identification of the reaction channel from the observed product is expected to be unambiguous. non-threshold and threshold reactions of these materials are shown in fig. 3. thanks to distribution of the threshold energies between 0.7 and 11.0 mev it will be possible to investigate sensitivity of activation to various parts of the fast component of the neutron field and their changes in the salt environment. 3. results and discussion source driven calculations were carried out with mcnp 6.2 and using the endf/b-vii.0 evaluated nuclear data library. radial and axial reaction rates from the fig. 3 have been calculated in all experimental channels (numbers 1-4 are vertically sorted by increasing distance from central source channel, 5-7 are horizontally sorted by increasing height) with 1010 neutrons from the source (resulting in relative error of results lower than 0.01), see examples of the gold foil activation in the following figures. the observed trends are similar for different activation foil materials (differences are mainly in the absolute values). the results are presented in a systematic comparison in three groups: • one salt, one foil material, one reaction – different radial and axial channels (fig. 4); • one salt, one foil material, one radial or axial channel – different reactions (fig. 5 and fig. 6); • one radial or axial channel, one foil material, one reaction – different salts (fig. 7 and fig. 8) the axial as well as radial distributions of nonthreshold (n,γ) reactions were found to be rather broad compared to the spatial distributions of threshold reactions, which are narrower, likely due to their higher sensitivity to the fast neutrons with energies greater than 1 mev that dominate in the vicinity of the ambe source. in the axial channel closest to the source, the peak factor of the threshold reactions is about an order of magnitude bigger than that of the non-threshold reactions. in the contrary, the peak 44 vol. 28/2020 monte carlo method for fast neutron transport figure 4. axial (top) and radial (bottom) reaction rates of non-threshold (left) and threshold (right) activation reactions in gold foils in a ucl3 salt assembly factors of both reaction types are very similar in the furthest axial channel. in absolute terms, the reaction rates of the nonthreshold reactions are about one and two orders of magnitude bigger than those of the threshold reactions at the positions in the middle part of the assembly and on its boundaries, respectively. comparison of the spatial distributions of reaction rates in different salt environments provide an interesting comparison, see fig. 7 and fig. 8. the spatial distributions of non-threshold reactions strongly differ in different salt environments in any of the axial and radial channels. the most intensive low energy neutron field causing the biggest (n,γ) rates was found to be, as expected, in the ucl3 salt, which contributes to the neutron field by fission. somewhat surprisingly, the second most intensive low-energy neutron field was found in nacl, while thcl4 is on the third place, which shows its rather small contribution of the fission neutrons (compare with keff in tab. 2). the weakest low-energy neutron field is in kcl. the reaction rates in different salts differ up to a factor of 4. the threshold reactions corresponding to the fast neutron field component show completely different trends. in the channel closest to the neutron source the reaction rates are almost the same for any of the salt environments. with increasing distance of axial channels from the source, the threshold reaction rates start differing in various salts. the strongest fast neutron field was found to be in kcl while the weakest one in ucl3 and thcl4. that suggests that the down-scattering and capture prevail over the fission neutron production in these fissile materials. this behaviour is valid for any of the reactions with threshold above 1 mev, while the inelastic scattering in 115in (eth ≈ 0.4 mev) represents a transition region between threshold and non-threshold reactions, see fig. 9. a strong influence of different salt composition can be also seen on specific behaviour in the radial channel furthest from the assembly centre, where the neutron leakage dominates causing a decrease in the threshold reaction rates for kcl, nacl and mgcl2, see fig. 8. however, in ucl3 and thcl4 fission locally prevails and causes the maximum to remain close to the assembly center axis. this will be especially interesting feature for comparison with the experiment. 4. conclusions monte carlo calculations of fast neutron transport through non-fueled salts (nacl, kcl, mgcl2) as well as neutron multiplication in salts containing fissile material (ucl3, thcl4, pucl3) were performed. the 45 o. šťastný, k. katovský, d. král et al. acta polytechnica ctu proceedings figure 5. axial reaction rates in gold foils in a ucl3 salt assembly figure 6. radial reaction rates in gold foils in a ucl3 salt assembly 46 vol. 28/2020 monte carlo method for fast neutron transport figure 7. axial reaction rates of non-threshold (left) and threshold (right) reactions in gold foils in five salt assemblies 47 o. šťastný, k. katovský, d. král et al. acta polytechnica ctu proceedings figure 8. radial reaction rates of threshold reactions in indium (left) and nickel (right) foils in five salt assemblies figure 9. axial reaction rates of threshold reactions in indium (left) and nickel (right) foils in five salt assemblies 48 vol. 28/2020 monte carlo method for fast neutron transport spatial distributions of threshold and non-threshold reactions in ni, in, au activation foils were investigated. all calculations have been so far performed using the endf/b-vii.0 library. based on presented calculation results for nonfueled salts, it can be stated, that mgcl2 or kcl should be more convenient if neutron fast spectrum is desired. on the other hand, nacl shows higher moderation capabilities. concerning salts fueled with u and th, using of thcl4 provides slightly higher yields of non-elastic reactions with higher threshold than the ucl3. acknowledgements authors gratefully acknowledge financial support from the ministry of education, youth and sports of the czech republic under international research cooperative program between texas a&m university, usa and the brno university of technology, czech republic (“adar – accelerator driven advanced reactor”, ltausa18198). references [1] molten salt reactors. world nuclear association [online]. london, united kingdom: world nuclear association, 2020 [cit. 2020-06-15] available at: https: //www.world-nuclear.org/information-library/ current-and-future-generation/ molten-salt-reactors.aspx. [2] p. mcintyre, s. assadi, k. badgley, et al. accelerator-driven subcritical fission in molten salt core: closing the nuclear fuel cycle for green nuclear energy 1525:636–642, 2013. doi:10.1063/1.4802405. [3] j. c. batchelder, s.-a. chong, j. morrell, et al. possible evidence of nonstatistical properties in the 35cl(n, p)35s cross section. phys rev c 99:044612, 2019. doi:10.1103/physrevc.99.044612. [4] m. chadwick, p. obložinský, m. herman, et al. endf/b-vii.0: next generation evaluated nuclear data library for nuclear science and technology. nuclear data sheets 107(12):2931 – 3060, 2006. evaluated nuclear data file endf/b-vii.0, doi:10.1016/j.nds.2006.11.001. [5] international organization for standardization. iso 8529-1:2001 reference neutron radiations — part 1: characteristics and methods of production. [6] c. j. werner. mcnp user’s manual – code version 6.2. los alamos national laboratory, report la-ur-17-29981 (2017). [7] e. shwageraus, p. hejzlar, m. driscoll. liquid salt cooled flexible conversion ratio fast reactor: neutronic design. nuclear engineering and design 239:2626– 2645, 2009. doi:10.1016/j.nucengdes.2009.07.011. [8] n. e. todreas, p. hejzlar, a. nikiforova, et al. flexible conversion ratio fast reactors: overview. nuclear engineering and design 239(12):2582 – 2595, 2009. flexible conversion fast reactors special section with regular papers, doi:https://doi.org/10.1016/j.nucengdes.2009.07.014. [9] y. li, x. xu, x. wang, et al. survey and evaluation of equations for thermophysical properties of binary/ternary eutectic salts from nacl, kcl, mgcl2, cacl2, zncl2 for heat transfer and thermal storage fluids in csp. solar energy 152:57 – 79, 2017. progress in solar energy special issue: concentrating solar power (csp), doi:10.1016/j.solener.2017.03.019. 49 https://www.world-nuclear.org/information-library/current-and-future-generation/molten-salt-reactors.aspx. https://doi.org/10.1063/1.4802405 https://doi.org/10.1103/physrevc.99.044612 https://doi.org/10.1016/j.nds.2006.11.001 https://doi.org/10.1016/j.nucengdes.2009.07.011 https://doi.org/https://doi.org/10.1016/j.nucengdes.2009.07.014 https://doi.org/10.1016/j.solener.2017.03.019 acta polytechnica ctu proceedings doi:10.14311/app.2019.24.0038 acta polytechnica ctu proceedings 24:38–43, 2019 © czech technical university in prague, 2019 available online at http://ojs.cvut.cz/ojs/index.php/app residual heat estimation of nuclear fuel in energy well reactor jana šoškováa,∗, antonio dambrosiob, guido mazzinib, marek ruščákb a czech technical university in prague, faculty of nuclear sciences and physical engineering, department of nuclear reactors, v holešovičkách 2, 180 00 prague 8, czech republic b research centre řež, hlavní 130, 250 68 husinec-řež, czech republic ∗ corresponding author: jana.soskova@email.cz abstract. energy well (ew) is a concept of a small modular reactor, designed as a source of energy for places with low infrastructure. its design combines triso fuel particles, graphite moderator and molten salt coolant. one of key features is that the active core is small and therefore easy to transport. residual heat estimation of ew fuel assembly is conducted using two different computation codes, scale and serpent. the obtained residual heat power is then used as an input in trace model of the ew to obtain the development of residual temperature in the coolant in the core inlet and outlet and also maximum temperature on the outer side of a fuel slab. the resulting temperatures are below the operational temperature. keywords: small modular reactor, energy well, triso fuel, liquid salt flibe, scale, serpent, decay heat. 1. introduction over the last few decades the nominal power of light water reactors was being increased. considering current technologies for fuel and materials as well as current safety requirements the limits are being reached and therefore other options for nuclear energy sources are being explored. small modular reactors were and are in development for decades. recently they have been gaining more attention due to the relatively small upfront capital investments as well as the ability to provide energy for remote areas off the main grid system. one of the key features of any new reactor designs is inherent safety. as a part of verification of small modular reactor energy well design, a code-to-code comparison of residual heat of its fuel is conducted. 2. energy well energy well is a concept of a small modular reactor, specifically fluoride-salt-cooled high-temperature reactor with triso fuel particles, graphite moderator and liquid salt flibe as a coolant. more details regarding the design of ew can be found in section 2.1. reactor energy well is to serve as a source of energy and heat for places with low infrastructure. the power is set to 20mwt with the expected time of nominal operation being 7 years. the whole power plant has modular construction and the reactor including primary circuit would be transportable to the place of need and afterwards taken back to the factory where it was made. the unique design of the reactor is based on an ahtr concept from oak ridge national laboratory (ornl), which is further described in studies [1] and [2]. the key feature of ew is transportability, therefore the core shroud will serve also as a transport container for used fuel. [3] 2.1. design of ew reactor as it was said above, the ew design was inspired by the ahtr concept and therefore a part of dimensions and material composition used in the ew design is identical to those in the aforementioned studies. however, in some cases adjustments were made so that the final concept was able to meet the requirements of the project, such as low power and smaller size that allows for easy transport. the active core consists of 19 hexagonal fuel assemblies with active height being 220 cm. between the assemblies there is a space 1.8 cm wide and filled with coolant in the form of liquid salt flibe. the equivalent diameter of the active core is 214.2 cm and the diameter including radial reflector is 320 cm. the material composition and horizontal dimensions of a fuel assembly are identical to figures described in [1]. the distance between two parallel outer faces of a fuel channel box which encloses the assembly is 45 cm. the box is made of 1 cm thick carbon-carbon composite with density 1.95 g/cm3. the assembly can be divided into three symmetrical segments, each including six fuel plates. these segments are separated by a graphite support structure. in each assembly there is a y-shaped control rod in the support structure, save for the middle assembly that has no rod due to the placement of the heat exchanger between primary and secondary circuit. this assembly also has lower fuel enrichment. the control rod is the single tool for controlling reactivity. the material of the absorber is b4c with 38 http://dx.doi.org/10.14311/app.2019.24.0038 http://ojs.cvut.cz/ojs/index.php/app vol. 24/2019 residual heat estimation of nuclear fuel in energy well reactor figure 1. triso fuel particle. picture taken from [6] . natural boron. the density is 1.79 g cm−3. the thickness of a blade of the control rod is 1 cm. different control rods are placed in the radial reflector of the active core and those help to keep sufficient subcriticality when the reactor is in shutdown. each fuel plate is 2.55 cm thick. between two fuel plates there is a cooling channel 0.7 cm thick. the first and the last cooling channel in each segment has only half thickness (0.35 cm). the fuel plate has a sandwich construction, where two fuel slabs are separated by a graphite plate. the fuel slabs consist of a graphite matrix with triso fuel particles embedded with packing fraction 25%. the fuel slabs are also covered by a layer of c-c composite on the outer sides to prevent erosion of triso particles into the coolant. attached at the end of the article there is figure 8 that displays the described geometry of the fuel assembly. in the pictures, brown stands for fuel plates, blue represents flibe coolant, the support structure and the assembly box are shown in grey and red stands for the control rod. 2.1.1. triso particle the idea of coated fuel particles dates back to 1957, when it was first suggested by roy huddle [4] as a part of the dragon project. since then, the particles were used in a variety of experimental reactors around the world. currently, the triso fuel is included in a basic reactor design for vhtr gen iv reactor. [5] the word triso comes from the term tristructural-isotropic. as the name suggests, there are three isotropic materials coating the fuel itself. those are graphite, silicon carbide and pyrolytic carbon. the materials are enveloping the fuel kernel in four layers as shown in figure 1. there were chosen triso particles with diameter 0.4235mm with uco fuel kernel (enrichment 15%) for ew design. the failiure temperature for triso fuel is 1873k, which is well above the highest temperature reached in ew (the fuel temperature 1200k). [3] 2.1.2. flibe coolant the binary system lif-bef2 has two eutectic invariant equilibria in its phase diagram. one at the temperature t =636k and the share of bef2 in the mix x(bef2)= 0.517. the other can be found at t =729k and x(bef2)= 0.328. for ew design the use of 2lifbef2 was chosen with the melting point t =732k and boiling point being t =1703k. with inlet and outlet coolant temperatures being 923k and 973k respectively, flibe allows the reactor to operate at atmospheric pressure. it was also chosen for its thermodynamic properties, such as the high volumetric heat capacity cv = 4540 kj · m−3 · k−1. [7] flibe properties give ew inherent safety because even in case of loss of flow accident the natural circulation would sufficiently cool the reactor. as for neutronics, the lithium has to be highly isotopically enriched in 7li as 6li has a high thermal neutron capture cross section. the beryllium also provides some moderation. fluoride salts are highly corrosive with cu and therefore the use of nickel alloys is necessary for construction materials. the alloys currently under consideration are h800, monicr and hastelloy n. [3, 8] 3. codes for residual heat estimation there were overall three codes used: scale, serpent and trace. scale was chosen as a deterministic code while serpent is a stochastic code. the decay heat power of an assembly was determined using both codes, giving the option of code-to-code comparison. the resulting values of decay heat power were then used as an input in an existing trace model of a power plant with ew reactor to gain maximum cladding temperature and an inlet and outlet coolant temperatures. scale 6.1 is a code package developed by the ornl. its computational modules include three deterministic radiation transport solvers. it can preform multiple tasks and fuel depletion and decay are among them. the package consists of different modules. the modules used in this case include: triton, origen-s, newt and opus. these are all included in a depletion sequence t-depl which was used in this case. origen-s starts the depletion of the material selected for depletion in the input. triton updates the data calculated with origen-s and newt is used for the flux post-processing in case of a 2d depletion. the opus module produces a condensed output file with data from output generated by the origens code. the opus tables contain information about total and individual nuclide or element concentrations in units set in the input. [9] [10] serpent 2 is a monte carlo reactor physics burnup calculation code developed by the vtt technical research centre of finland. it is a 3d code and it was designed specifically for lattice physics applications. the output is similar to scale. [11] [12] 4. model description 4.1. scale the depletion model of an ew fuel assembly was made to gain the assembly’s decay heat power in watts at 39 j. šošková, a. dambrosio, g. mazzini, m. ruščák acta polytechnica ctu proceedings different points in time from the opus module. the chosen nuclear data library is endf/b-vii.0. the composition of materials was specified for the fuel slab, flibe coolant, the graphites (in accordance with the densities) and for b4c. for reasons further explained in the following paragraph, the fuel was defined as a homogenous mixture, with the density calculated from overall volume and weight. as for celldata, the only deterministic calculation applicable to this geometry in scale 6.2.3 is latticecell. recently there was some development of the doublehet calculation, which keepsw the double heterogenity of the fuel slabs with triso particles, however, that is only applicable in keno module of scale which is monte carlo based. in this case, there has to be a deterministic calculation in order to make a comparison with serpent which is a monte carlo code. in opus the units were set to watts. the geometry itself contains only two units. a global unit with unit no.1 surrounded by flibe and the unit 1 itself, where the location of each coolant channel and each fuel element as well as all the other structures is given as a distance from the centre in centimeters. the unit is placed in a rhexagonal type array. this geometry setting requires a dense grid as almost the whole assembly is described in one unit and not in multiple units with their own grids. the boundaries are set to white, meaning an isotropic return at boundary. the resulting geometry is shown in figure 2, where graphite is in yellow, green and also dark blue (which is hardly visible due to the dense grid), flibe is in azure and peach, the control rod is in magenta and the fuel slabs are red. figure 2. geometry of ew fuel assembly in scale. 4.2. serpent while in scale the model was done for one assembly, in serpent there is a model for the whole active core. in order to gain results fitting for code-tocode comparison, the existing serpent model was simplified to include just one assembly as well. the parameters used were corresponding to the ones used in scale model. the geometry is shown in figure 3, where the graphite is bright red and light blue, flibe is dark red and dark blue and the fuel area is red with black grains. figure 3. geometry of ew fuel assembly in serpent. 5. results the decay heat of an ew assembly was obtained using deterministic and stochastic codes. the values were then normalized to the nominal power of the reactor. the comparison of the amount of 235u was made to show that both codes were in agreement. this is shown in tab. 1. the difference between the amount of uranium at time zero is 82.32 g. this difference presents less than 1.4%. a part of it is contributed to geometrical differences between the scale and serpent models. said difference is 76.68 g of 235u. then for the following years the difference varies, with maximum of 3.3%. results for the decay heat are shown in figures 4 and 5. figure 4 shows that a second after shutdown the heat is already below 7% of the nominal power and is declining exponentially. the drop to the approximately 7% of the nominal thermal power within the first second after shutdown is of typical occurrence in other reactors. figure 5 then follows the decline in years. there are small differences in the values obtained from scale and serpent, the difference being no more than 4.5%. the difference is declining with time, as can be seen in figure 5. the scale approach is more conservative. the values of decay power in watts were used as an input in the trace model to obtain information 40 vol. 24/2019 residual heat estimation of nuclear fuel in energy well reactor t [y] m [g] serpent m [g] scale 0 5796.73 5714.41 1 5319.98 5249.64 2 4878.28 4826.77 3 4465.29 4438.19 4 4077.16 4076.28 5 3712.07 3744.46 6 3368.34 3434.36 7 3044.81 3145.97 table 1. the amount of 235u in one assembly as calculated by scale and serpent during the seven year burnup. figure 4. the ratio of decay heat power to nominal power for both scale and serpent in seconds after shutdown. the time axis is logarithmic. about temperature. figure 6 shows the maximum temperature on the outer side of a fuel slab. figure 7 shows the coolant temperature in core inlet and core outlet. both results are below the operational temperature, which is almost 1000 k for the outer side of a fuel slab and almost 970 k for the coolant in core outlet. the difference between scale and serpent results doesn’t exceed 3k which is an acceptable result. 6. conclusions the models of ew fuel assembly in scale and serpent were used to obtain the decay heat power. in both cases the resulting decay heats fall well underneath the 7% ratio. the difference doesn’t exceed 4.5%. the results were used in the trace model of ew to obtain the values of the maximum temperature on the outer side of a fuel slab and the core inlet and outlet coolant temperatures. in both cases the results are below the operational temperature, which is almost 1000 k for the outer side of a fuel slab and almost 970 k for the coolant in core outlet. the difference in the resulting temperatures is negligible. figure 5. the ratio of decay heat power to nominal power for both scale and serpent in years. the time axis is logarithmic. figure 6. the chart shows the development of maximum temperature on the outer side of a fuel slab in kelvin. the time after shutdown is in seconds. the results are shown for scale and serpent. figure 7. the chart shows the development of coolant temperature in core inlet and core outlet in kelvin. the time after shutdown is in seconds. the results are shown for scale and serpent. acknowledgements ew project was supported by cv rez, czech technical university in prague and tacr’s theta programme. 41 j. šošková, a. dambrosio, g. mazzini, m. ruščák acta polytechnica ctu proceedings figure 8. assembly dimensions in cm. picture adopted from [1] 42 vol. 24/2019 residual heat estimation of nuclear fuel in energy well reactor references [1] b. petrovic, i. maldonado. fuel and core design options to overcome the heavy metal loading limit and improve performance and safety of liquid salt cooled reactors. tech. rep. doe/neup–12-3870, 1253940, 2016. doi:10.2172/1253940. [2] d. e. holcomb, d. ilas, v. k. varma, et al. core and refueling design studies for the advanced high temperature reactor. tech. rep. ornl/tm-2011/365, 1025857, 2011. doi:10.2172/1025857. [3] e. losa, m. ruščák, g. mazzini. návrh aktivní zóny a studie proveditelnosti malého modulárního reaktoru chlazeného roztavenou solí. [4] k. verfondern, h. nabielek, j. kendall. coated particle fuel for high temperature gas cooled reactors. nuclear engineering and technology 39:603 – 616, 2007. [5] g. d. del cul. triso-coated fuel processing to support high temperature gas-cooled reactors. tech. rep. ornl/tm-2002/156, 814326, 2002. doi:10.2172/814326. [6] j. hales, r. williamson, s. novascone, et al. multidimensional multiphysics simulation of triso particle fuel. journal of nuclear materials 443(1):531 – 543, 2013. doi:10.1016/j.jnucmat.2013.07.070. [7] j. richard, d. wang, g. yoder, et al. implementation of liquid salt working fluids into trace. vol. 3, pp. 1775–1785. 2014. [8] o. benes, r. konings. 4 thermodynamic calculations of molten-salt reactor fuel systems. in f. lantelme, h. groult (eds.), molten salts chemistry, pp. 49 – 78. elsevier, oxford, 2013. doi:10.1016/b978-0-12-398538-5.00004-4. [9] a. dambrosio, m. ruscak, m. guido, a. musa. neutronic analysis of the lvr-15 research reactor using the parcs code. annals of nuclear energy 117:145–154, 2018. doi:10.1016/j.anucene.2018.03.009. [10] b. t. rearden, m. a. jessee. scale code system 2018. doi:10.2172/1426571. [11] m. ruscak. rod ejection simulation on vver 1000/320 core using parcs/trace. p. 165, 2016. [12] j. leppänen. serpent–a continuous-energy monte carlo reactor physics burnup calculation code. vtt technical research centre of finland 4, 2013. 43 http://dx.doi.org/10.2172/1253940 http://dx.doi.org/10.2172/1025857 http://dx.doi.org/10.2172/814326 http://dx.doi.org/10.1016/j.jnucmat.2013.07.070 http://dx.doi.org/10.1016/b978-0-12-398538-5.00004-4 http://dx.doi.org/10.1016/j.anucene.2018.03.009 http://dx.doi.org/10.2172/1426571 acta polytechnica ctu proceedings 24:1–6, 2019 1 introduction 2 energy well 2.1 design of ew reactor 2.1.1 triso particle 2.1.2 flibe coolant 3 codes 4 model description 4.1 scale 4.2 serpent 5 results 6 conclusions acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2019.23.0031 acta polytechnica ctu proceedings 23:31–37, 2019 © czech technical university in prague, 2019 available online at https://ojs.cvut.cz/ojs/index.php/app 3d fem analysis of the construction pit for a tbm-driven flood discharge gallery jörg-martin hohberg iub engineering ag, eiger house, belpstrasse 48, 3007 bern, switzerland correspondence: martin.hohberg@iub-ag.ch abstract. as one speciality of the swiss hydraulic engineering tradition, several torrent diversion schemes were undertaken dating from the 18th century, with the aim of using the natural lake as a retention basin. the recent project of flood protection of the lower sihl valley and the city of zurich features a gallery of 2.1 km length with a 6.6m inner diameter designed for a 330m3/s free flow, discharging into lake zurich in an hq500 event. after a short presentation of the main features of the project, the paper concentrates on the target construction pit of the tbm drive close to a major railway line at the built-up lakeside. keywords: hydraulic engineering, torrent diversion, natural lake, retention basin, flood, flood protection. 1. historical background prolonged heavy rainfall, or the combination of rain pouring into melting snow fields during warm downhill winds (foehn conditions), can suddenly turn peaceful creeks into dangerous torrents, eroding hillsides upstream and depositing extensive gravel fans further downstream, which may block other rivers and permanently raise the valley floor. in medieval times this created malarial swamps and jeopardized the dwellings upstream. where the gravel bed load discharged into a lake, this risk did not occur as long as the lake was deep enough. the earliest man-made diversion scheme in switzerland was that of the kander torrent into lake thun. during 1711–1714 a gallery was dug in molasse conglomerate, which soon collapsed however, due to backward erosion of the invert [1]. 300 years later, the cut looks like a natural canyon, see figure 1. the problem with this pioneering project was that the additional influx led to a rise of the water table of the lake and increased the risk of flooding the city of thun. the problem was solved, when a 1.2 km long flood protection gallery of a clear 5.4m diameter was opened in 2009 [2]. after other river diversions – linth 1807–1816, glatt 1813–1830 (incl. a gallery), aare/jura 1868–1891 – plans were made in 1865–1897 also to divert the sihl into lake zurich. the most recent large flood in zurich occurred in 1910, figure 2 (left). although since 1937 a hydropower station (etzelwerk) is regulating the sihl lake, the current value at risk due to an inundation of zurich is estimated to be chf 20million per year. this is because the sihl flows through the outskirts of zurich and passes under zurich’s main railway station, where it joins the limmat river – a geographical situation similar to dresden (saxonia), where the flood of 2002 caused e 6,200million in damage. quite a similar loss is estimated for an extreme sihl flood. 2. the sihl flood protection gallery thalwil several remedies were studied, including a larger discharge of the etzelwerk during its current refurbishment ("kombilösung" in figure 2 (right)); but the preferred solution is the separate "stollen thalwil" to be built over a time period to 2023 with an investment of chf 130million. starting from an intake structure situated at the exterior bank in a river bend downstream of a recently built rake against driftwood, the gallery will have a length of 2.1 km, a clear diameter of 6.4m and a free-flow design capacity of 330m3/s. this sufficiently limits the damage in zurich in case of a hq500 (600m3/s). an important constraint is the future zimmerberg railway base tunnel to be overpassed, necessitating a kink in slope of 1.2% and 3.3% respectively, figure 3 and figure 4. the outlet structure will be placed at the shore of lake zurich in close proximity to a major railway line, an existing water purification plant and a public bathing beach. in order to avoid strong currents, the mouthpiece of the outlet will be hidden 3m under the water surface and extend 90m into the lake, as a concrete box-section founded on piles. 2.1. geology the gallery will traverse the upper freshwater molasse (osm) with an almost horizontal stratification of normaland fine-grained sandstone, siltstone and marl. the latter may exhibit very low strength in the presence of water. although the molasse rock itself has a low water permeability, decompression joints are expected to run parallel to the level course. the 31 https://doi.org/10.14311/app.2019.23.0031 https://ojs.cvut.cz/ojs/index.php/app jörg-martin hohberg acta polytechnica ctu proceedings figure 1. the kander canyon today (left) with the flood relief gallery (right) in the city of thun [2]. figure 2. the sihl at zurich main railway station in 1910 (left) and alternative flood relief projects (right) [3]. osm is overlain by moraine deposits. all the rock properties are summarized in table 1. the top 4–6m of the osm stratum are supposed to be weathered rock with low properties close to those of pure marl, figure 5. the moraine is shown in green. note the main railway line on the left-hand side. 2.2. geotechnical models for the analysis of the gallery proper, the osm stratification was homogenized to an equivalent stiffness according to the relative percentages of the strata, giving an elastic modulus of 1.5gpa. for the mohrcoulomb model a 40° friction angle and a 2.5mpa cohesion were adopted. the drainage of the rock during the tbm drive was simulated by lowering and re-raising the water table. for the analysis of the outlet construction pit, several scenarios were investigated with differing approximations of the ground properties listed in table 2. set a assumes characteristic values located about half-way between the mean and the inferior values ("cautious mean values"). set b1 and b2 distinguish between the rock matrix and the low values in horizontal sedimentation planes (multi-laminate model). finally, set c is a pessimistic combination of residual values ("ubiquitous joints"). the influence of ground water was neglected here, assuming full drainage due to excavation. a nonassociated flow rule was adopted throughout, with ψ = ϕ − 30°. 3. the three dimensional finite element model from the beginning it was clear that a 3d model with some reasonable resolution (say ≤50 cm element size) would just about allow a symmetrised representation of the construction pit. brick elements with 8 nodes in an enhanced assumed strain (eas) formulation of z-soil® were used [4]. the largest model of 70×30m length and width comprises 624’000 dof, a smaller one still 464’000 dof, figure 6. the latter is sufficient to study the behaviour of the gallery underneath the railway line, while the larger one is necessary to check the overall stability of the construction pit. the meshing of the geological strata together with the circular geometry of the gallery demanded some compromises, which might be avoided by meshing the gallery part and the construction pit part separately and by tying both meshes together in a global constraint approach. 32 vol. 23/2019 construction pit for a tbm-driven flood discharge gallery figure 3. longitudinal section of the gallery [3]. parameter fine sandstone siltstone marl moraine specific weight γ 26.3±0.3 kn/m3 26.4±0.2 kn/m3 23.8±1.0 kn/m3 22 kn/m3 friction angle ϕ’ 58.8±4.6° 55.3±5.4° 33° 34° cohesion c’ 7.0±2.2mpa 3.7±0.9mpa 21mpa 0.005mpa residual friction 50.8±3.7° 47.0±1.0° 23.0±12.4° rf direct shear 26.0±4.2° 19.8±5.9° residual cohesion (1.4±0.2mpa) (1.5±0.3mpa) (0.8±0.5mpa) rc direct shear 0.023±0.004mpa 0.038±0.015mpa e50 modulus 12.7±1.7gpa 9.6±3.0gpa 0.17±0.12gpa 0.050gpa v modulus∗ 14.3gpa 3.8gpa 0.6gpa 0.05gpa ∗from in situ dilatometer test. table 1. rock properties (triaxial and direct shear). set a osm weathered (marl) moraine man-made fill specific weight γ 26.4 kn/m3 23.8 kn/m3 22 kn/m3 18 kn/m3 friction angle ϕ’ 34° 16° 34° 29° cohesion c’ 480 kpa 120 kpa 10 kpa∗ 5 kpa∗ e modulus 3,800mpa 600mpa 50mpa 5mpa set b1 osm weathered (marl) moraine man-made fill specific weight γ 26.4 kn/m3 23.8 kn/m3 22 kn/m3 18 kn/m3 friction angle ϕ’ 46° / 23.9° 16.9° / 16.8° 34° 29° cohesion c’ 960 / 21 kpa 240 / 22.7 kpa 10 kpa∗ 5 kpa∗ e modulus 3,800mpa 600mpa 50mpa 5mpa set b2 osm weathered (marl) moraine man-made fill specific weight γ 26.4 kn/m3 23.8 kn/m3 22 kn/m3 18 kn/m3 friction angle ϕ’ 46° / 22° 11° / 14° 34° 29° cohesion c’ 1,200 / 19 kpa 300 / 23 kpa 10 kpa∗ 5 kpa∗ e modulus 1,500mpa 110mpa 50mpa 5mpa set c osm weathered (marl) moraine man-made fill specific weight γ 26.4 kn/m3 23.8 kn/m3 22 kn/m3 18 kn/m3 friction angle ϕ’ 30° 20° 34° 29° cohesion c’ 40 kpa 25 kpa 10 kpa∗ 5 kpa∗ e modulus 3,800mpa 600mpa 50mpa 5mpa ∗accounting stabilizing. table 2. geotechnical assumptions. 33 jörg-martin hohberg acta polytechnica ctu proceedings figure 4. situation at the lake site (outlet structure in blue) [3]. 3.1. serviceability limit state (sls) the concern of the railway line operator is the stability of the tracks during construction of the gallery. this is checked at serviceability level in the medium-size model. the gallery is assumed to remain unlined under the railway tracks while the tbm is slowly advancing to the disassembly point. but even with the very low stiffnesses and low strengths of parameter set b2 (table 2) the absolute displacements do not exceed 1.3mm (figure 7). the results do not look alarming at all, except for the fact that the displacements are increasing substantially during the last few meters of ground at the excavation face before the gallery daylights into the outlet construction pit. this indicates an unfavourable interaction of the gallery with the slope above the portal wall, which is due to the fact that the thickness of the loose overburden (man-made fill, moraine and weathered osm) is increasing towards the portal. 3.2. hazard scenario of deep-seated pit failure the construction pit was dimensioned with conventional limit equilibrium analysis software, featuring passive nailing and shotcrete sealing of the surface. in the medium-size model the nails are represented in a discrete fashion, while in the large-size model the cohesion of the overburden was simply increased by 5 kpa ("smeared nailing effect"). the large-size model is used to check the stability of the longitudinal construction pit walls. in predominantly elastic behaviour the inward pointing corner of the pit stabilizes the walls (figure 7). with the isotropic low parameters of set c, however, this stabilizing effect diminishes and the analysis indicates the risk of total collapse of the construction pit already during its excavation, long before the arrival of the gallery bore (figure 8). 3.3. hazard scenario of face slope failure with a slightly better friction angle and the high cohesion of parameter set a the construction pit does not fail, but a potentially critical situation arises during punching of the portal wall (figure 9). generally, a fos=3.0 would be more than sufficient, but it should be noted that the predicted strength parameters in the sedimentation planes are still lower than those of the matrix divided by three. it was thus concluded that a reliable analysis of the ultimate limit state (uls) demands an anisotropic model. 4. ultimate limit state (uls) of the gallery the following two analyses use a multi-laminate model, which combines a mohr-coulomb failure criterion for the matrix with a separate criterion for the horizontal stratification. the strength parameters are those of set b1 (table 2). 4.1. reduction of both frictional and cohesive strength the hazard scenario is that of sandstone plates detaching from the gallery crown ("coffin lid"), which would jeopardize the tbm advancement and might eventually progress upwards to the surface. because the safety margin of the loose overburden is inferior to that in the osm, such an analysis needs to exempt the overburden from the ϕ-c reduction. the failure nevertheless occurs in the slope rather than in the gallery crown (figure 10). further refinement of the model, i.e. by incorporating the shotcrete as plate elements, is not attractive (in terms of dof) and even illusive, as the plate elements could not rupture in shear. note also that a fos=1.46 on the characteristic strength is not sufficient to cover the wide spread of the lowest residual values, even though the partial safety factor γc =1.5 for cohesion, as stipulated by the swiss code sia 167, is almost attained (while exceeding γϕ =1.2 for friction by far). 4.2. reduction of cohesive strength, starting from design friction angles hence, an alternative method was used for uls assessment, starting from the design values of the lowest residual friction angle (γϕ =1.2): moraine ϕ’d =29.3°, marl ϕ’d =9.2°, osm ϕ’d =40.8° resp. ϕ’d =18.6° in sedimentation planes. all friction angles are kept constant, while the cohesion is reduced by ck/fosc → cd using the option of pure c-reduction in z-soil® (figure 11). if the analysis converges for fosc → γc =1.5, the safety factors for both friction and cohesion would be satisfied. as the convergence fails prematurely at fosc = 1.18, the unlined gallery is unsafe 34 vol. 23/2019 construction pit for a tbm-driven flood discharge gallery figure 5. geological profile through the planned outlet structure. figure 6. large (left) and medium (right) size fem model. figure 7. increase of sls displacements during boring of the gallery (from right to left); max. displacement (brown colour) is 5mm with contours 0.2mm apart. 35 jörg-martin hohberg acta polytechnica ctu proceedings figure 8. beginning failure of pit at the last-but one excavation step (left), loss of convergence during the last excavation step (right); colours of abs. displacements as before. figure 9. slip failure of the weathered osm in ϕ-c reduction for the final state (fos=3.0); colours of abs. displacements as before. figure 10. slope failure of the overburden at fos=1.30 (left) and fos=1.46 (right); colours of abs. displacements as before. 36 vol. 23/2019 construction pit for a tbm-driven flood discharge gallery figure 11. crown failure in the gallery at fosc=1.18; colours of abs. displacements as before. w.r.t. to the lowest values of residual cohesion and consequently the risk to the railway track is deemed too large. 5. conclusions regarding design the numerical analyses for such a wide spread of rock parameters proved quite tricky. although deformations in the serviceability limit state do not at all look critical, the sensitivity to residual values is too large – the more as seepage and visco-plastic effects are neglected – to take the risk of the gallery staying unlined until the tbm is drawn into the construction pit for disassembly. as a consequence a counter-drive is suggested which extends from the portal wall until right behind the rail-way tracks and will support the ground by a pipe canopy on horse-shoe shaped steel arches. moreover, additional post-tensioned anchors will be installed in the portal wall as a measure of precaution against the two hazard scenarios outlined in chapter 3. acknowledgements this contribution is dedicated to prof. marta doležalová, who had first invited me to prague to give a presentation on joint elements back in 1992. references [1] d. vischer. die geschichte des hochwasserschutzes in der schweiz. bundesamt für wasser und geologie, bwg, bern, 2003. berichte des bwg, serie wasser, nr. 5, online, available from: https://www.swv.ch/wp-content/uploads/2018/04/ geschichte-des-hochwasserschutzes_bwg.pdf. [2] iub engineering ag. references, 2019. online, available from: https://www.engineering-group.ch/ en/references/projekte/detail/ lake-thun-flood-protection-relief-tunnel.html. [3] awel. hochwasserschutz an sihl, zürichsee und limmat – synthesebericht. zurich, 2015, online, available from: https://awel.zh.ch/internet/ baudirektion/awel/de/wasser/hochwasserschutz/ hochwasserschutz_zuerich.html. [4] t. zimmermann, a. truty, k. poddles. numerics in geotechnics and structures, 1985 – 2015. online, available from: http://www.zsoil.com/publications/ zsoilday2015_book.pdf. 37 https://www.swv.ch/wp-content/uploads/2018/04/geschichte-des-hochwasserschutzes_bwg.pdf https://www.swv.ch/wp-content/uploads/2018/04/geschichte-des-hochwasserschutzes_bwg.pdf https://www.engineering-group.ch/en/references/projekte/detail/lake-thun-flood-protection-relief-tunnel.html https://www.engineering-group.ch/en/references/projekte/detail/lake-thun-flood-protection-relief-tunnel.html https://www.engineering-group.ch/en/references/projekte/detail/lake-thun-flood-protection-relief-tunnel.html https://awel.zh.ch/internet/baudirektion/awel/de/wasser/hochwasserschutz/hochwasserschutz_zuerich.html https://awel.zh.ch/internet/baudirektion/awel/de/wasser/hochwasserschutz/hochwasserschutz_zuerich.html https://awel.zh.ch/internet/baudirektion/awel/de/wasser/hochwasserschutz/hochwasserschutz_zuerich.html http://www.zsoil.com/publications/zsoilday2015_book.pdf http://www.zsoil.com/publications/zsoilday2015_book.pdf acta polytechnica ctu proceedings 23:26–32, 2019 1 historical background 2 the sihl flood protection gallery thalwil 2.1 geology 2.2 geotechnical models 3 the three dimensional finite element model 3.1 serviceability limit state (sls) 3.2 hazard scenario of deep-seated pit failure 3.3 hazard scenario of face slope failure 4 ultimate limit state (uls) of the gallery 4.1 reduction of both frictional and cohesive strength 4.2 reduction of cohesive strength, starting from design friction angles 5 conclusions regarding design acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2021.31.0041 acta polytechnica ctu proceedings 31:41–44, 2021 © 2021 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague the assessment of priorities in the construction of new high-speed railway lines in czech republic by using the stem method pavel purkart∗, jan kruntorád, lukáš týfa czech technical university in prague, faculty of transportation sciences, department of transportation systems, horská 3, 128 03 prague 2, czech republic ∗ corresponding author: purkapav@fd.cvut.cz abstract. the aim of the paper is to set priority of the fast connection sections ("rychlá spojení" in czech, abbreviation "rs") building mentioned above by using the stem method. the method is used to select projects with the maximum benefit if the budget is limited. there has not been set a limit for individual rs sections as a limitation of financial costs. the decisive factor is the order in which individual rs sections should be built on the basis of evaluation criteria. keywords: high-speed railway lines (hsl), the fast connections (rs), czech republic, mathematic model, the stem method, project choice, evaluation criteria. 1. introduction and goals today a great effort is being made in the czech republic to prepare the hsl network in relation to the previously developed rs concept. now, the czech railway network does not have any line in hsl parameters, which is not acceptable in the long term – with regard to sustainable development of the territory, taking into account the situation not only in the neighbouring western european states, but also global development [1]. the main directions of the fast railway network have been defined [2]: • rs1 praha (prague) – brno – ostrava – [poland], • rs2 brno – [slovakia/austria], • rs3 praha – plzeň (pilsen) – [germany] (western direction – münchen), • rs4 praha – [germany] (northern direction – dresden) with possible branch (most – chomutov, žatec, louny), • rs5 praha – hradec králové/liberec – [poland]. the implementation of the rs1, rs2 and rs4 sections is monitored as a priority. in these cases, the správa železnic (the main infrastructure manager of czech railway network) is already carrying out project preparation. the aim of this article is thus to open up the controversy as to whether the above-mentioned sections in terms of benefits should be actually built first, respectively whether there is a more appropriate order of implementation. the current design state is schematically shown in figure 1. 2. understanding and solving the issue the main aim of the paper is to set priorities in the construction of high-speed railway lines in the czech republic using the stem method (step method). the use of the stem method is defined in the following chapter. the method is used for project selection due to limited funding opportunities while maximizing the benefits of their implementation. in this case, the method was slightly modified (the method mandatory used for economic problems was modified for a purpose for maximization benefits within economic costs at first). 3. solution of the issue 3.1. method stem the stem method can solve linear mathematical problems with more purpose functions. the aim of this method is to find compromise solutions, whose realizations should bring most benefits. the main principle of the method is the calculation of purpose function ideal values for individual cases. this calculation is followed by minimizing compromise solution deviation from ideal purpose function values. the basic of the method is an interactive procedure of searching the compromise solution. benefit of the stem method is, that there is only minimal need of communication between a submitter and a solver (comparing to another methods). the scale method for individual criterions is set by calculation. the submitter must decide whether the result of the calculation is acceptable for him or not. so, the method consists of calculation and decision-making process. the calculation is stopped, if submitter finds the result acceptable, otherwise the solver must be informed by submitter in order to change the criteri41 https://doi.org/10.14311/app.2021.31.0041 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en p. purkart, j. kruntorád, l. týfa acta polytechnica ctu proceedings figure 1. suggestion of rs, source: správa železnic [3]. ons or their numbers, the whole calculation is made again [3]. the stem method consists of following steps: (1.) solver calculates optimal solution for individual criterions (purpose function) separately. the number of calculations fits the number of criterions. (2.) solver calculates the scales of individual criterions (χ;xj) according to the formula (1): wi = zii − min i=j...k zij zii α√ n∑ i=1 c2 ij (1) where: zij – element of optimalization criterions values matrix for optimalization in individual optimalization criterion (zij is the value of optimalization criterion j = 1, . . . , k in case of optimalization according to the criterion i = 1, . . . , k), cij – element of the price matrix – element of individual optimalization criterion coefficients matrix. value α comes from the equation (2): k∑ i=1 zii − min i=j...k zij zii α√ n∑ i=1 c2 ij = 1 (2) in reality we have to calculate coefficient α value first and then count the scales of individual criterions. if the scale fits the constraint wi > 0 for more criterions, the solver adds new variable and solves the model with new optimalization criterion (3). min f(x, d) = d (3) there is a form (4) for variable d: d = max i=j...k { wi ( zii − ∑ j∈j cijxj )} (4) we have to implement constraint (5) for correct calculation: wii ( zii − ∑ j∈j cijxj ) ≤ d (5) if constraint wi > 0 fits for only one value i = 1, . . . , k, solver cen simplify the constraint (5) to (6): min f(x) = k∑ i=1 wii ( zii − ∑ j∈j cijxj ) (6) (3.) solver presents the results to the submitter. the submitter can modify the criterions or add/remove some of them, if he does not find the results acceptable – but we haven’t to do that, if the first solution should be the best. solver goes back to step 2. (4.) solver has found compromise solution, if the submitter of satisfied with the result. the solution is optimal, if the value d = 0 is reached. 42 vol. 31/2021 the assessment of priorities in the construction 3.2. calculation according to the stem method and result for the purposes of the calculation, a total of six logical rs sections were identified: (1.) praha – brno, (2.) brno – ostrava – [poland], (3.) brno – [slovakia/austria], (4.) praha – plzeň – [germany], (5.) praha – ústí nad labem – [germany] (northern direction – dresden) with branch, (6.) praha – hradec králové/liberec – [poland]. the following evaluation criteria were used to determine priorities in the construction of rs sections: • average coefficient of reduced travel times in chosen connections: the research team (authors of the articles + colleagues from the department) determined rs sections, including international ones, for which the rs construction is crucial. for these sections there was calculated the number of shorter travel times compared to the current situation and in the case rs section is realized. then these numbers were distributed on individual rs sections and the average for these sections were calculated, • number of redirected long-distance lines expressed as an importance: three categories were set for each evaluated section based on the evaluation below: 1: up to 10 redirected long-distance routes in both directions per day, 2: up to 30 redirected long-distance routes in both directions per day, 3: over 30 redirected long-distance routes in both directions per day. in this criterion there was monitored the possible placement on the rs network of the current longdistance lines, especially close to agglomerations (prague, brno, ostrava), where the selected sections of the railway network are already overloaded, • routing difficulty: the average was calculated based on the questioning an expert team of four experts. the issue of routing was considered broadly (not only regarding natural barriers, but also routing through urban areas, etc.) and the logic of values is divided into these categories: 1: very difficult routing, 1,5:difficult routing, 2: medium difficult routing, 2,5:routing without major problems, 3: simple routing. • road traffic redirection potential: for all rs sections, a section on the road network was determined, from which the decisive load value in vehicles/24 hours was taken. the source of data is national traffic census 2016. in all cases, the section on a parallel highway network with logical minimum value of traffic volumes was taken into account, so this criterion represents the minimal potential for traffic redirection from roads. • agglomeration potential: the research team set agglomerations, including foreign ones, on which the built rs sections will have a direct influence. for all agglomerations the number of inhabitants was found and then their relation to individual rs sections was evaluated. for example, for pilsen agglomeration the rs3 section is crucial, but the rs2 section has a minimal importance – this logic was used to evaluate all agglomerations regarding a number of inhabitants. the importance of an agglomeration for a given section was multiplied with its number of inhabitants and the final values were cumulated into individual sections. all criteria have been compiled in such a way that as their value increases, their benefits are maximized. the specific values of evaluation criteria are summarized in table 1. the stem method is a very interactive method because it calculates the weight criteria for every mathematical model. there were two mathematical models considered in the calculation (the first one to select two projects out of six and the second one to select four projects out of six), therefore weights were determined twice. these weights are shown in table 2. in both models, the highest criteria weights were assigned to number of redirected long-distance lines expressed as an importance and routing difficulty, both above 30%. the average coefficient of reduced travel times in chosen connections has a value oscillating around a weight of 20%. on the other hand, in both calculations there were assigned very low values for road travel redirection potential and agglomeration potential, these weights are lower than 5%. the calculation was performed for both versions of the model, and in neither case d = 0 was achieved. it shows that compromise solutions, not optimal ones, have been achieved. the two top prioritized high-speed railway line sections have been set by the stem method: • praha – brno, • brno – [slovakia/austria]. these two sections have middle priority: • brno – ostrava – [poland], • praha – plzeň – [germany]. the rest of sections was described as a low building priority: • praha – ústí nad labem – [germany with branch], • praha – hradec králové/liberec – [poland]. 43 p. purkart, j. kruntorád, l. týfa acta polytechnica ctu proceedings section of railway line rs average coefficient of reduced travel times in chosen connections [-] number of redirected longdistance lines expressed as an importance [-] routing difficulty [-] road traffic redirection potential [vehicles per day in thousands in chosen point of a road network] agglomeration potential [-] praha – brno 2, 13 3 1, 6 39 22 brno – ostrava – pl 1, 85 2 2, 5 22 15 brno – sk/a 1, 85 2 3, 0 23 21 praha – plzeň – d 1, 73 3 1, 6 31 13 praha – ústí n. l. – d 2, 23 2 1, 5 24 20 praha – hk/lbc – pl 2, 39 1 1, 9 23 12 table 1. specific values of evaluation criteria, source: authors. criterion weight criterion for selecting 2 projects weight criterion for selecting 2 projects average coefficient of reduced travel times in chosen connections [-] 0.181 0.251 number of redirected longdistances lines expressed as an importance [-] 0.407 0.372 routing difficulty [-] 0.361 0.337 road traffic redirection potential [vehicles per day in thousands in chosen point of a road network] 0.024 0.013 agglomeration potential [-] 0.027 0.028 table 2. weight criteria set by stem method for cases of selecting 2 and 4 projects, source: authors. 4. conclusion it is obvious from the calculation, according to evaluation criteria, that the top priorities of construction have sections praha – brno and brno – [slovakia/austria]. while the plans of správa železnic (the main infrastructure manager of czech railway network) counts with the section praha – ústí nad labem – [germany], result of this paper prefers a construction of the section praha – plzeň – [germany]. it is also possible to assume that in the section prague ústí nad labem – [germany], the railway already uses its potential to a considerable extent, so its construction was not identified as a priority by the model. it is evident that the significant influence on the calculation according to the stem method had the number of long-distance transport lines transferred, which in the case of the route through plzeň is relatively significant (lines ex6, r16 and r26). this factor apparently helped significantly to increase the priority of construction of the rs through plzeň than through ústí nad labem. acknowledgements all the facts presented in the article are based on the results of research on ctu faculty of transportation sciences, department of transport systems. this work was supported by the grant agency of the czech technical university in prague, grant no. sgs18/150/ohk2/2t/16 railway track parameters for transportation service optimization and no. sgs20/138/ohk2/2t/16 design and optimal use of railway infrastructure parameters. references [1] p. purkart. optimal cooperation of public transport segments. praha: defense date 2019-12-06. doctoral minimum. ctu fts. department of transportation systems. supervised by týfa, l. [2] high-speed rails in the czech republic. [online], praha: správa železnic, 2020 [cit. 2020-02-22]. available from: https://www.szdc.cz/vrt/. [3] d. teichmann, m. dorda. comparison of two selected methods in evaluating of investments in transport infrastructure. in: finance and performance of firms in science, education and practice : proceedings of the 7th international scientific conference : april 23-24, 2015, zlín, czech republic. zlín: tomas bata university in zlín, 2015. pp. 1524-1536. isbn 978-80-7454-482-8. 44 https://www.szdc.cz/vrt/ acta polytechnica ctu proceedings 31:41–44, 2021 1 introduction and goals 2 understanding and solving the issue 3 solution of the issue 3.1 method stem 3.2 calculation according to the stem method and result 4 conclusion acknowledgements references doi:10.14311/app.2021.30.0063 acta polytechnica ctu proceedings 30:63–68, 2021 © czech technical university in prague, 2021 available online at http://ojs.cvut.cz/ojs/index.php/app influence of earth composition on adsorption capacity of methylene blue dye tereza plaček otcovská∗, barbora mužíková, pavel padevět czech technical university in prague, faculty of civil engineering, department of mechanics, thákurova 7, 166 29 prague 6, czech republic ∗ corresponding author: tereza.otcovska@fsv.cvut.cz abstract. final properties of unfired earth are influenced by composition of earth mixtures. methylene blue test could be useful method for analysis of earth composition. this would facilitate of designing unfired earth building structures because composition of natural earth is various. principle of methylene blue test is measure of amount of adsorbed methylene blue dye by clay. the essential component of earth mixtures is clay because clay fulfils a function of binder. experimental measure of adsorption capacity of individual kind of clay is described in this paper. adsorption capacity was investigated in montmorillonite, kaolinite, illite-kaolinite and illite clays. the obtained results show that the adsorption capacity of clays is significantly different. keywords: adsorption capacity, clay, methylene blue dye, methylene blue test. 1. introduction unfired earth is a building material which had become minor in 2nd half of 20th century because of modern building materials such as steel or concrete have have become readily available. nowadays, the unfired earth is getting back to forefront of interest for its properties that fit into principles of a sustainable building and improving microclimate in interiors [1– 4]. earth mixture is a basic material for production of unfired earth constructions. uncertain composition of natural earth is the main reason why using of unfired earth in construction is complicated. it is because of final properties of unfired earth constructions are greatly influenced by the earth composition [4, 5]. methylene blue test is described in standard čsn en 933-9. methylene blue test could be a good method for analysis of the earth composition. basic idea of this research is that every kind of clay (main component of earth mixture) has different adsorption capacity of methylene blue dye. if adsorption capacity of individual clay is known, this test could be a good method for analysing earth composition [5–8]. this paper is focused on experimental determination of adsorption capacity of clay by methylene blue test. four kinds of clay were investigated (montmorillonite, kaolinite, illite-kaolinite, illite). minimum number of measurements for each investigated material was 3. the result of this research is the determination of the adsorption capacity of mentioned clay and determination of dependence between the amount of adsorbed dye and amount of clay in earth. 2. description of experimental measurement a burette, a beaker, a filter paper, a magnetic stirrer and a glass rod are basic equipment for methylene blue test. solution of methylene blue dye (methylthioninium chloride c16h18cln3s) by concentration 10 g/l is used. earth samples are produced from earth and distilled water. methylene blue solution is inserted into earth sample by a burette (fig. 1). figure 1. an earth sample with methylene blue solution. earth sample with the solution is stirred on a magnetic stirrer 1 minute for each 1 millilitre of solution. after that a drop of the mixture is placed on a filter paper. this process is repeated and each sample drop with increasing amount of methylene blue solution is marked (fig. 2). methylene blue test is finished when a blue ring spreads around a drop sample (fig. 3) because clay has already adsorbed the maximum amount of methylene blue dye. amount of dye adsorbed depends on a kind and amount of clay. amount of dye adsorbed is determined by the eq. 1. amount of dye adsorbed is defined as the weight amount of dye in grams that is adsorbed by 1 kilogram of test material. 63 http://dx.doi.org/10.14311/app.2021.30.0063 http://ojs.cvut.cz/ojs/index.php/app t. plaček otcovská, b. mužíková, p. padevět acta polytechnica ctu proceedings figure 2. a drop of the mixture is placed on a filter paper. figure 3. a result of methylene blue test. mmb = ms.mmb,1ml.1000 mes (1) 3. investigated material earth samples are produced in laboratory from sand, clay powder (fig. 5) and distilled water. a grain curve of sand is shown in fig. 4. 4 kinds of clay are tested (montmorillonite, kaolinite, illite-kaolinite, illite). industrial marking which is used in this paper too and chemical composition are shown in table 1. earth samples are suspension of clay and sand in distilled water. clay and sand are mixed in prescribed ratio (table 2) and it is solid part of earth figure 4. grain curve of sand. figure 5. clay powder. samples. this solid part is always 15 g. distilled water is always 75 g. distilled water serves as a medium for adsorption. 4 clay/sand ratios are used for each kind of clay. 54 measurements are performed on 16 types of samples. the minimum number of measurements for each sample type was 3. table 2 shows composition of samples, amount of components and a number of measurements. mark of a sample contains mark of clay and percentage number of clay in a sample. table 2 shows mark of samples. 4. adsorption capacity of clay adsorption capacity is very different for each kind of clay. the highest adsorption capacity is found for montmorillonite clay (gem), lower for kaolinite clay (b4), even lower for illite-kaolinite clay (kr) and the lowest for illite clay (agl). table 3 and figure 6 shown results from methylene blue test. maximum average amount of adsorbed dye 88 g/kg (σ = 3.8 g/kg) is calculated for gem-100. the clay 64 vol. 30/2021 earth composition and mbd adsorption capacity kind of mark of amm sio2 al2o3 fe2o3 tio2 cao mgo na2o k2o clay clay [ml/g] [%] [%] [%] [%] [%] [%] [%] [%] montmorillonite gem 17 50.51 31.2 3.37 0.86 0.4 0.42 0.08 1.62 kaolinite b4 4.5 58.13 24.29 4.7 1.21 0.13 0.5 0.5 3.96 illite-kaolinite kr 8.4 59.31 24.71 3.37 1.09 0.19 0.4 0.3 2.82 illite agl 29.4 56.57 18.4 9.72 1.16 1.12 2.54 0.18 2.91 table 1. chemical composition of used clays. clay mark of sample clay/sand ratio amount of distilled water amount of clay amount of sand number of measurements [−] [g] [g] [g] [−] montmorillonite gem-100 100/0 75 15 0 3 gem-75 75/25 75 11.25 3.75 4 gem-50 50/50 75 7.5 7.5 3 gem-25 25/75 75 3.75 11.25 5 kaolinite b4-100 100/0 75 15 0 3 b4-75 75/25 75 11.25 3.75 3 b4-50 50/50 75 7.5 7.5 3 b4-25 25/75 75 3.75 11.25 3 illitekr-100 100/0 75 15 0 3 kaolinite kr-75 75/25 75 11.25 3.75 3 kr-50 50/50 75 7.5 7.5 4 kr-25 25/75 75 3.75 11.25 5 illite agl-100 100/0 75 15 0 3 agl-75 75/25 75 11.25 3.75 3 agl-50 50/50 75 7.5 7.5 3 agl-25 25/75 75 3.75 11.25 3 table 2. clays used for experimental measurements, their designation and composition. figure 6. amount of adsorbed methylene blue dye by clays. contained in a sample is montmorillonite and clay is 100 % of the solid part of a sample. on the contrary, the lower value of adsorbed dye 4.6 g/kg (σ = 0 g/kg) is determined for agl-25 sample. the clay contained in agl-25 sample is illite and clay is 25 % of the solid part of a sample. compared to montmorillonite clay, the adsorption capacity of kaolinitic clay is 34.3 % lower (e.i. 65.7 %), illite-kaolinite is 63.5 % lower(e.i. 36.5), illite is 78.2 % lower (e.i. 21.8 %). figure 6 shows that the same trend was also observed for the samples with a clay content of 75 %, 50 % and 25 %. the percentage difference in adsorption capacity for all clays is given in table 4. obtained values of adsorbed dye by individual samples are interpolated by a linear regression curve. it can be seen from figure 7 that dependence of adsorption on amount of clay is linear for all kinds of clays. 4.1. confidence intervals from the obtained adsorption values, limits of the confidence interval l1 and l2 were calculated by eq. 2 (α = 0.05). calculated confidence interval limits are given in table 5. l1,2 = x ± σ · tk√ n (2) confidence interval is calculated for all measurement values of adsorption dye. figure 8 shows dependence between amount of adsorbed dye and amount 65 t. plaček otcovská, b. mužíková, p. padevět acta polytechnica ctu proceedings clay mark of sample average amount of adsorbed dye standard deviation relative standard deviation [g/kg] [g/kg] [%] montmorillonite gem-100 88.0 3.8 4.3 gem-75 65.3 5.6 8.6 gem-50 41.8 6.9 16.4 gem-25 17.3 3.5 20.1 kaolinite b4-100 57.9 2.0 3.5 b4-75 44.5 1.7 3.7 b4-50 29.3 3.4 11.6 b4-25 14.5 0.7 4.5 illitekr-100 32.1 2.1 6.6 kaolinite kr-75 25.5 3.6 14.2 kr-50 18.0 1.4 7.6 kr-25 8.7 1.1 12.4 illite agl-100 19.2 0.6 3.3 agl-75 13.9 1.3 9.5 agl-50 9.5 1.0 10.7 agl-25 4.6 0.0 0.0 table 3. results obtained by methylene blue test. amount of adsorbed dye gem b4 kr agl amount of clay [g/kg] [g/kg] [g/kg] [g/kg] in a sample [%] [%] [%] [%] 15.00 g / 100 % 88.0 57.9 32.1 19.2 100 65.7 36.5 21.8 11.25 g / 75 % 65.3 44.5 25.5 13.9 100 68.1 39.1 21.2 7.50 g / 50 % 41.8 29.3 18.0 9.5 100 70.0 43.0 22.6 3.75 g / 25 % 17.3 14.5 8.7 4.6 100 84.0 50.4 26.7 table 4. comparison of the amount of adsorbed dye. of clay in sample with confidence interval area for each clay. the curves of adsorption are created by combining the calculated values of adsorption for amount of clay in samples 25 %, 50 %, 75 % and 100 %. extreme values of confidence interval area are join between the calculated values of the confidence interval limits l1, l2. it can be seen from figure 8, that illitic clay (agl) and illitic-kaolinitic clay (kr) can be well distinguished from each other in an amount of clay in solid part at least 25 %. confidence intervals of montmorillonite (gem) and kaolinite (b4) clay overlap when clay content is 25-50 %. based on figure 8, it can be assumed that the methylene blue test is applicable for amounts of 75 % and higher in the case of montmorillonite and kaolinitic clays. 5. conclusions methylene blue test is applicable to diagnose composition of earth. each of the investigated clays (montmorillonite, kaolinite, illite-kaolinite, illite) adsorbs a different amount of methylene blue dye. montmorillonite is characterized by the largest adsorption capacity. the adsorption capacity of methylene blue dye decreases for clays in the order: kaolinite, illitekaolinite and illite (fig. 6). furthermore, it was experimentally determined that the amount of adsorbed dye is directly dependent on the amount of clay in earth (fig. 7). using confidence intervals, it was determined that illitic and illitic-kaolinitic clays can be determined by methylene blue test in earth in quantities from 25 %. montmorillonite and kaolinite clay can be determined by methylene blue test in earth in amounts from 75 % (fig. 8). 66 vol. 30/2021 earth composition and mbd adsorption capacity figure 7. amount of adsorbed methylene blue dye by clay. figure 8. confidence intervals of clays. 67 t. plaček otcovská, b. mužíková, p. padevět acta polytechnica ctu proceedings clay mark of sample confidence interval [g/kg] l1 l2 g em gem-25 13.31 21.29 gem-50 29.18 54.43 gem-75 57.54 73.16 gem-100 81.01 95.01 b 4 b4-25 13.31 15.73 b4-50 23.04 35.49 b4-75 41.45 47.53 b4-100 54.16 61.57 k r kr-25 7.49 9.96 kr-50 16.10 19.88 kr-75 18.84 32.20 kr-100 28.23 36.02 a g l agl-25 4.62 4.62 agl-50 7.61 11.31 agl-75 11.44 16.29 agl-100 18.01 20.32 table 5. calculated confidence interval limits of adsorption for clays (α = 0.05). list of symbols mb methylene blue mmb amount of mb dye absorbed [g/kg] ms amount of methylene blue solution [g] mmb,1ml amount of methylene blue in 1 g of methylene blue solution [g/g] mes amount of tested material [g] l1,2 limits of the confidence interval [g/kg] x arithmetic mean [g/kg] σ standard deviation [g/kg] tk student’s t critical values n number of measurement acknowledgements the financial support of this experiment by the czech science foundation (gačr project no. 18-10884s) and faculty of civil engineering, czech technical university in prague (sgs project no. sgs19/148/ohk1/3t/11) is gratefully acknowledged. references [1] p. jaquin, c. augarde. earth building : history, science and conservation. report, bracknell : ihs bre press, 2012. [2] agenda 21 united nations environment programme (unep). http://www.unep.org. [3] čsn. čsn 1168-1939 podmínky pro zednické a přidružené práce pozemních staveb, 1951. [4] g. minke. building with earth. ökobuch verlag, staufen, 2006. [5] i. žabičková. hliněné stavby. era 21, brno, 2002. [6] p. walker. rammed earth: design and construction guidelines. ihs bre press, watford, 2010. [7] z. weiss. jílové minerály : jejich nanostruktura a využití. karolinum,, vyd. 1. edn., 2005. [8] evropský výbor pro normalizaci. čsn en 933-9, tests for geometrical properties of aggregates – part 9: assessment of fines – methylene blue test 2013. 68 acta polytechnica ctu proceedings doi:10.14311/app.2018.20.0065 acta polytechnica ctu proceedings 20:65–72, 2018 © czech technical university in prague, 2018 available online at http://ojs.cvut.cz/ojs/index.php/app isentropic efficiency of centrifugal compressor working with real gas jiří oldřich howden čkd compressors s.r.o., klečákova 347/5, 190 00, praha 9, czech republic correspondence: jiri.oldrich@howden.com, oldrich.jiri@seznam.cz abstract. the contribution deals with calculation of isentropic efficiency and also with calculation of isentropic process of real gas or gaseous mixtures. the method is based on numerical solution of basic definitional equation of isentropic process and equation of isentropic efficiency with direct implementation of real gas equation of state (eos). keywords: efficiency, isentropic, real gas, compressor. 1. introduction isentropic efficiency is an important parameter widely used in thermodynamic design of centrifugal compressor. reversible adiabatic process identical to isentropic process is used as reference process. the isentropic process is a process to which great attention is devoted in literature. in some articles certain inaccuracies are used repeatedly. for example dependence of heat capacities on temperature is ignored. very often poisson equations are used for calculation of real gases. isentropic exponents that are generally dependent on temperature and pressure are often substituted by mean values. adiabatic and isentropic processes are considered synonymous. these inaccuracies can generate noticeable errors in calculations. the aim of this contribution is to clarify the above mentioned inaccuracies and to suggest thermodynamically correct procedures that are appropriate for pc’s. in the first part of contribution equation for some simple special cases is described very shortly. the second more extensive part deals with techniques for various combinations of input and output variables. all these techniques are based on behavior of real gases and are independent on real gases eos used. 2. isentropic efficiency efficiency of compressor that does not exchange heat with the environment (compressor without intercooling or uncooled compressor section) is calculated by comparing the work that was actually spent on compression and the appropriate reference process. reference process is usually, depending on the compression process in the compressor, an isentropic process [1– 4], a polytropic process [2, 3, 5, 6], or a reversible isothermal process [2, 3, 7]. we will only deal with a reversible isentropic process and isentropic compression in the following text. in the actual process that occurs in the compressor, losses occur in the flowing gas and, as a result of it, some part of the kinetic energy is converted into heat. this process leads to an increase in temperature and figure 1. isentropic (area acdefa) and actual work (area gbcdefg) in t-s diagram when compressed from pressure p1 to pressure p2. entropy of compressed gas. a small part of the generated heat is discharged from the compressor casing to the surroundings. the process that is going on in the actual compressor is irreversible. the final temperature of the gas after its compression is always higher than temperature of gas after isentropic compression. in the diagram h-s in figure 1 the reversible isentropic compression work is represented by the area acdefa and the actual work spent on compression from pressure p1 to the pressure p2 = p2a = p2s is represented by the area gbcdefg. the work actually spent is irreversible and can be divided into isentropic work, compression losses (area abgfa) and increase of compression work due to losses (area abca) ∆ha = h2a −h1. (1) isentropic efficiency is defined as a relation between reversible isentropic work and actual work ηs = ∆hs ∆ha = h2s −h1 h2a −h1 . (2) short repetition of basic principles [2, 3, 8]. • adiabatic (isentropic) efficiency does not depend only on amount of losses. it depends on amount 65 http://dx.doi.org/10.14311/app.2018.20.0065 http://ojs.cvut.cz/ojs/index.php/app jiří oldřich acta polytechnica ctu proceedings figure 2. isentropic and actual processes in h − s diagram. of additional increase in compression work due to losses expressed by the area acba, too. • there are losses in gas flow in the actual process that occurs in compressor, and as a result, a part of the kinetic energy is converted into thermal energy and hence an increase in temperature and entropy of compressed gas. • at the same quality of compression mean value of additional increasing in compression work increases with increasing of total pressure. • at the same quality of compression the adiabatic efficiency will be at higher pressure ratio lower than at lower pressure ratio. • temperature of gas after actual compression is always higher than temperature after isentropic compression (see figure 2). 3. equation of isentrope adiabatic process is defined by the equation dq = 0, (3) where q is heat input into system ds = dq t = 0. (4) reversible adiabat is identical to isentrope. in irreversible process is ds > dq t = 0. (5) adiabat and isentrope are not identical. we rewrite equation (4) into form ds = ( ∂s ∂t ) v dt + ( ∂s ∂v ) t dv = = cv t dt + ( ∂p ∂t ) v dv = 0, (6) where p is the pressure of system and cv is heat capacity, which is derivative of internal energy with respect to temperature at constant volume cv = ( ∂u ∂t ) v . (7) that generally depends on temperature and on volume. for ideal gas the heat capacity cv = c0 v depends only on temperature. equation of isentrope is obtained by integrating the relationship (6). its specific form depends on the chosen equation of state from which we determine (∂p/∂t )v and the chosen equation for the temperature dependence of c0 v . 4. reversible process, ideal gas, constant c0 v (perfect gas) this simplest case leads to the known poisson equations. equation (6) we rewrite into form c0 v t dt = −r v dv, (8) where derivative (∂p/∂t )v is found from equation of state written for one mole pv = rt. (9) after integration of eq. (8) and after modification we receive well known poisson equation in variables t − v tv k−1 = const., (10) where k = (c0 v +r)/c0 v . poisson equations in variables t−p and p−v we obtain by inputting of volume v or temperature t from equation of state (9) tp 1−k k = const., (11) pv k = const. (12) 5. reversible process, general equation of isentrope from eq. (4) we obtain general equation of isentrope s(t, v ) = const. (13) then the problem is reduced to standard calculation of entropy for known (chosen) equation of state and equation of temperature dependence of c0 v (figure 3). in the following text we will use the molar density ρ = 1/v instead of the volume as an independent variable. we will write the equation of the isentrope in the form s(t2, ρ2)− s(t1, ρ1) = 0, (14) where index 1 denotes inlet state and index 2 denotes end state after isentropic compression (figure 2). 66 vol. 20/2018 isentropic efficiency of centrifugal compressor working with real gas figure 3. dependence of c0 p on temperature for several common used gases [9]. 6. methods of calculation of isentropic process and isentropic efficiency before we come to the description of the solution of each type of calculation, it will be useful to mention several methods of solving the isentropic process. • in the simplest case, we can assume that we are compressing the ideal gas. the calculation will be very easy and very inaccurate. • entropy was often read from diagrams or tables in the past. the accuracy was limited by the size of the diagram. however, this method is completely inappropriate for calculating gaseous mixtures and cannot be used on pc. • another of the approximate methods of calculating the isentropic process is the method based on the relations valid for the ideal gas, i.e. on the poisson equations (10), (11) and (12). isentropic exponent is calculated using of real gas eos and general relationships. kp,v = −v p cp cv ( ∂p ∂v ) t , (15) kp,t = 1 1− p cp ( ∂v ∂t ) p , (16) kt,v = 1 + v cv ( ∂p ∂t ) v . (17) usually we do not know all parameters of end point of isentropic change (figure 2). thus we have to use value of isentropic exponent calculated for mean pressure and temperature. we estimate parameters in the point of interest from the behavior of the ideal gas. this method is suitable for pc and is appropriate for calculation of gas mixtures. even in this case, we do not have enough accurate results. as an illustration of the inappropriate use of the relationships derived for the ideal gas in combination with the isentropic exponent calculated by means of the real gas equation of state from the relationship (15), the calculation of the isentropic change from the relation can serve calculation of isentropic enthalpy from equation ∆hs = p1v1 kp,v kp,v − 1 (p2 p1 ) kp,v −1 kp,v − 1  . (18) there are a number of other methods described in literature but further we focus only on methods which use the solution of a system of nonlinear equations, one of which is always a chosen real gas equation of state and one the requirement for zero change of entropy in transition from state 1 to 2 (figure 2). the following paragraphs provide algorithms of solution for individual types of calculations. in their design, particular emphasis was placed on working reliably and quickly. for every combination of input parameters the individual algorithm for solution of system of nonlinear equations was designed. important parts of every algorithm are relations for estimation of the initial approximations. in all cases, the calculation can be divided into a part independent of the real gas eos and the equation for the c0 p = c0 p(t ) of the ideal gas and a part dependent on these equations. the suitable method for solution of this system is newton’s method. functions fi that are used in the next detailed description of algorithms are shown in table 3. to construct the jacobi matrix of functions fi that we need for solution by the newton method, we need to know the partial derivatives of fi. these derivatives are shown in table 4. the abovementioned derivatives can also be obtained by numerical derivation of functions fi. in table 5 there are relations for calculation of entropy and enthalpy of real gas and relations for calculation of dimensionless quantities q [10]. any real gas eos can be chosen for the following calculations. dimensionless quantities q for chosen eos can be found in literature or can be derived from relations described in table 5. 7. calculations based on the real gas eos in practical calculations, we need to find unknown parameters when we know the initial state and two parameters of discharge state. the combination of input and output parameters is shown in table 1. in case that isentropic efficiency has value one i.e. ηs = 1 it is true p2s = p2a, t2s = t2a and ∆hs = ∆ha, then items 1, 2 and 3 from table 1 show combination of input and output parameters of isentropic change of thermodynamic state see table 2. 67 jiří oldřich acta polytechnica ctu proceedings input output 1 p1, t1, p2s t2s, ∆hs 2 p1, t1, t2s p2s, ∆hs 3 p1, t1, ∆hs p2s, t2s table 1. combination of output and input parameters for isentropic process. input output 1 p1, t1, p2a, ηs t2a, ∆ha 2 p1, t1, p2a, ∆ha t2a, ηs 3 p1, t1, t2a, ηs p2a, ∆ha 4 p1, t1, t2a, ∆ha p2a, ηs 5 p1, t1, p2a, t2a ∆ha, ηs table 2. combination of output and input parameters. 7.1. example of the solution by newton’s method all combinations of parameters are mentioned in [4]. for one of them we will show the solution. we will compute isentropic temperature t2 = t2s and isentropic change in enthalpy ∆h = ∆hs from known parameters p1, t1 and p2 (figure 2). it is variant 1 from table 1. in this case we assume that the isentropic efficiency is equal to one ηs = 1. since this is the isentropic process, the original system of four equations is now simplified to only two equations f1 = 0, f2 = 0. when we use general equation of isentropic process s1 − s2 = 0 (figure 2) and general expression of real gas eos we obtain the system of two nonlinear equations for two variables t2 and ρ2 f1 = s1(t1, ρ1)− s2s(t2s, ρ2s) = 0, (19) f2 = p2s ρ2srt2s − z2s(t2s, ρ2s) = 0. (20) this system of equations will be solved by newton’s method. because it is known and very often used method, we will show only an matrix notation of linear equations for increments in ∆t and ∆ρ[ ∂f1 ∂t2s ∂f1 ∂ρ2s ∂f2 ∂t2s ∂f2 ∂ρ2s ] · [ ∆t ∆ρ ] = [ −f1 −f2 ] (21) desired parameters t2 and ρ2 can be obtained from relationships t (k+1) 2s = t (k) 2s + ∆t, (22) ρ (k+1) 2s = ρ (k) 2s + ∆ρ. (23) the calculation is completed by fulfilling of the requiremen max(|δt |; |δρ|) ≤ ε where δt and δρ are relative deviations and ε is chosen value. detail description of solution for this example: for inlet condition p1, t1 we determine values s1 and h1 from real gas eos next we calculate c0 v c0 p = c0 p(t1, −→x ), c0 v = c0 p −r. (24) next step is solution of system of equations f1 = 0, f2 = 0 by using of newton’s method. initial approximations are relations valid for ideal gas t (0) 2s = t1 ( p2s p1 )c0 p−c0 v c0 p , (25) ρ (0) 2s = p2s rt (0) 2s . (26) as the results we obtain t2s and ρ2s. from general form of eos we obtain compressibility factor in point 2s (figure 2) z2s = p2s rt2sρ2s (27) and enthalpy h2s = h2s(t2s, ρ2s, −→x ). (28) finally we calculate change in enthalpy ∆h = h2 −h1, (29) where h2 = h2s. 7.2. calculation of t2s, ∆ha from known parameters p1, t1, p2a and ηs (figure 2, table 2 item 1) we solve the system of equations f1 = 0, f2 = 0, f4 = 0 and f5 = 0 for unknown parameters t2s, ρ2s, t2a, ρ2a. initial approximations are t (0) 2s = t1 ( p2a p1 )c0 p−c0 v c0 p , (30) ρ (0) 2s = p2a rt (0) 2s , (31) t (0) 2a = t (0) 2s − t1 ηs + t1, (32) ρ (0) 2a = p2a rt (0) 2a , (33) where c0 p is c0 p = c0 p(t1, −→x ), c0 v = c0 p −r. (34) change in enthalpy ∆ha we obtain from equation ∆ha = h2a −h1, (35) where h2a = f(t2a, ρ2a, −→x ). (36) 68 vol. 20/2018 isentropic efficiency of centrifugal compressor working with real gas 7.3. calculation of t2a, ηs from known parameters p1, t1, p2a and ∆ha (figure 2, table 2 item 2) we solve the system of four equations f1 = 0, f2 = 0, f5 = 0, f6 = 0 for unknown parameters t2s, ρ2s, t2a, ρ2a. initial approximation t (0) 2s we obtain from eq. (30), ρ (0) 2s from eq. (31) t (0) 2s = t1 + ∆ha c0 p (37) and ρ(0) 2a from eq. (33). isentropic efficiency ηs we calculate from eq. (2), where enthalpy h1 we know from calculation in initial point 1 and h2a = f(t2a, ρ2a, −→x ). 7.4. calculation of p2a, ∆ha from known parameters p1, t1, t2a and ηs (figure 2, table 2 item 3) calculation is based on solving of three equations f1 = 0, f4 = 0 and f8 = 0 for unknown parameters t2s, ρ2s and ρ2a. the first approximations are t (0) 2s = ηs(t2a − t1) + t1, (38) ρ (0) 2s = ρ1 ( t2s t1 ) c0 v c0 p−c0 v , (39) ρ (0) 2a = t (0) 2s ρ (0) 2s t2a , (40) where c0 p see eq.(34). after every step of the newton’s method the k-th approximation of the compressibility factor z(k) 2a is found from real gas equation of state and then the k-th approximation of the pressure p(k) 2a is calculated from equation p (k) 2a = z (k) 2a rt2aρ (k) 2a . (41) from equality p2s = p2a (see figure 2), we can calculate z (k) 2s = z (k) 2a t2aρ (k) 2a t (k) 2s ρ (k) 2s . (42) change in enthalpy ∆ha is determined from eq. (1), where h2a = f(t2a, ρ2a, −→x ). 7.5. calculation of p2a, ηs from known parameters p1, t1, t2a and ∆ha (figure 2, table 2 item 4) to calculate unknown parameters t2s, ρ2s and ρ2a system of three equations f1 = 0, f6 = 0 and f8 = 0 is solved. for the first approximation of temperature we choose t (0) 2s = t2a. density ρ(0) 2s we obtain from equation (40), ρ(0) 2a = ρ (0) 2s and z(0) 2a = z (0) 2s = 1. after every figure 4. ideal gas temperature t2s calculated from eq. (30) – blue curve and real gas temperature t2s – red curve. inlet conditions propane p1 = 550000pa, t1 = 293.15k. figure 5. ideal gas isentropic work ∆hs calculated from eq. (43) – blue curve and real gas isentropic work ∆hs – red curve. inlet conditions propane p1 = 550000pa, t1 = 293.15k. step of newton’s method compressibility factors z(k) 2a and z (k) 2s are calculated similar way like in previous case i.e. by solution of real gas eos and by using of equations (41) a (42). finally isentropic efficiency ηs we obtain from eq. (2), where h1 is known from calculation of thermodynamic properties in point 1 (figure 2) and h2s = f(t2s, ρ2s, −→x ). 7.6. calculation of ∆ha, ηs from known parameters p1, t1, p2a and t2a (figure 2, table 2 item 5) to calculate unknown parameters t2s, ρ2s and ρ2a system of three equations f1 = 0, f2 = 0 and f5 = 0 is solved. first approximation of temperature t (0) 2s we obtain from eq. (30), ρ(0) 2s from eq (31) and ρ(0) 2a from eq. (33). change in enthalpy ∆ha we calculate by using of eq. (1), where h2a = f(t2a, ρ2a, −→x ). isentropic efficiency ηs we finally obtain from eq. (2), where h2s = f(t2s, ρ2s, −→x ). 69 jiří oldřich acta polytechnica ctu proceedings figure 6. relative deviation of isentropic work calculated for real and ideal gas. 8. example as an example the calculation of temperature and isentropic work after isentropic change was chosen. in this example compression of propane from state 1 to state 2s (see figure 2) is calculate by two methods. in the first case above described method based on the real gas is used and in the second case method based on an ideal gas. parameters in point 1 are pressure p1 = 550000pa, temperature t1 = 293.15k. the third parameter is gas pressure p2s which varies in the interval 800000 to 2500000pa. for calculation of real gas isentropic change the method described in paragraph 7.1 was used. this method is equivalent to method of paragraph 7.2 when ηs = 1. for calculation of temperature t2s of ideal gas the equation (30) was for p2a = p2s = p2. isentropic change of enthalpy was calculated from eq. (18) in form ∆hs = z1rt1 k k − 1 [( p2 p1 ) k−1 k − 1 ] , (43) where z1 and k were calculated from bwr eos. results of all calculations are on figures 4, 5 and 6. 9. conclusion the methods described in this contribution exactly solved problem of calculation of the isentropic efficiency for various input parameters. in case that efficiency is equal zero above described procedures are suitable for calculation of isentropic process. the above described methods advantageously use the dimensionless variables defined in table 5 and real gas eos. it is evident from the work that some approximate procedures can lead to inaccurate results. methods described here together with method described in [6] and in [7] and suitable real gas eos form the basic tools for thermodynamic calculations of compressors or others flow machines. list of symbols t thermodynamic temperature [k] p pressure [pa] r molar gas constant [j/(mol k)] z compressibility factor [–] ρ molar density [mol/m3] v molar volume [m3/mol] k isentropic exponent [–] cv molar heat capacity at constant volume [j/(mol k)] cp molar heat capacity at constant pressure [j/(mol k)] h molar enthalpy [j/(mol)] s molar entropy [j/(mol k)] −→x vector of molar fractions of gas mixture [–] η efficiency [1] t0 standard temperature [k] p0 standard pressure [pa] eos equation of state indexes: s isentropic a actual 1 inlet, input 2 discharge, output id ideal gas 0 thermodynamic quantities of ideal gas (upper index) references [1] d. a. kouremenos, x. k. kakatsios. the three isentropic exponents of dry steam. forschung auf dem gebiete des ingenieurwesens 51:117–122, 1985. doi:10.1007/bf02558416. [2] d. misárek. turbokompresory (turbocompressors). sntl, 1963. in czech. [3] k. h. lüdtke. process centrifugal compressors. springer, berlin, heidelberg, 2004. doi:10.1007/978-3-662-09449-5_9. [4] j. oldřich, j. novák, a. malijevský. adiabatický děj v technické praxi, (adiabatic pro-cess in technical practice). strojírenství 40(2):69–76, 1990. in czech. [5] j. m. schultz. the polytropic analysis of centrifugal compressors. journal of engineering for power 84:69, 1962. doi:10.1115/1.3673381. [6] j. oldřich. advanced polytropic calculation method of centrifugal compressor. 2010. doi:10.1115/imece2010-40931. [7] j. oldřich. calculation of the isothermal efficiency of a cen-trifugal compressor compressing the real gas. in proceedings of conference “turbostroje 2016”. plzeň, czech republic, 2016. [8] b. eckert. axialkompressoren und radialkompressoren. springer-verlag, berlin, 1953. doi:10.1007/978-3-642-52720-3. [9] p. j. linstrom, w. g. mallard. nist chemistry webbook, nist standard refer-ence database number 69. doi:10.18434/t4d303. [10] j. p. novák, a. malijevský, j. pick. použití bezrozměrných veličin při výpočtu termo-dynamických funkcí, (use of dimensionless quan-tities for calculating of thermodynamic functions from equations of state). chemické listy 73:1178–82, 1979. in czech. 70 http://dx.doi.org/10.1007/bf02558416 http://dx.doi.org/10.1007/978-3-662-09449-5_9 http://dx.doi.org/10.1115/1.3673381 http://dx.doi.org/10.1115/imece2010-40931 http://dx.doi.org/10.1007/978-3-642-52720-3 http://dx.doi.org/10.18434/t4d303 vol. 20/2018 isentropic efficiency of centrifugal compressor working with real gas f1 = s1(t1, ρ1)− s2(t2s, ρ2s) f2 = p2s ρ2srt2s − z2(t2s, ρ2s) f3 = ∆hs − [h2(t2s, ρ2s)−h1(t1, ρ1)] f4 = ηs[h2(t2a, ρ2a)−h1(t1, ρ1)]− [h2(t2s, ρ2s)− −h1(t1, ρ1)] f5 = p2s ρ2art2a − z2(t2a, ρ2a) f6 = [h2(t2a, ρ2a)−h1(t1, ρ1)]−∆ha f7 = ηs∆ha − [h2(t2s, ρ2s)−h1(t1, ρ1)] f8 = z2(t2a, ρ2a)t2aρ2a − z2(t2s, ρ2s)t2sρ2s table 3. functions fi. ∂f1 ∂t2s = r t2s (1 + 2qu2s +qc2s)− ∑ xic 0 pi(t2s) t2s ∂f1 ∂ρ2s = rqt2s ρ2s ∂f2 ∂t2s = −qt2s t2s ∂f2 ∂ρ2s = −qρ2s ρ2s ∂f2 ∂t2a = ∂f2 ∂ρ2a = ∂f6 ∂t2s = ∂f6 ∂ρ2s = ∂f7 ∂ρ2a = 0 ∂f3 ∂t2s = ∂f4 ∂t2s = ∂f7 ∂t2s = −r(qt2s − 2qu2s −qc2s− −1)− ∑ xic 0 pi(t2s) ∂f3 ∂ρ2s = ∂f4 ∂ρ2s = ∂f7 ∂ρ2s = rt2s(qt2s −qρ2s) ρ2s ∂f4 ∂t2a = ηs[r(qt2a − 2qu2a −qc2a − 1)+ + ∑ xic 0 pi(t2s)] ∂f4 ∂ρ2a = ηs rt2a(qρ2a −qt2a) ρ2a ∂f1 ∂t2a = ∂f1 ∂ρ2a = ∂f5 ∂t2s = ∂f5 ∂ρ2s = 0 ∂f5 ∂t2a = −qt2a t2a ∂f5 ∂ρ2a = −qρ2a ρ2a ∂f6 ∂t2a = r(qt2a − 2qu2a −qc2a − 1)+ + ∑ xic 0 pi(t2s) ∂f6 ∂ρ2a = rt2a(qρ2a −qt2a) ρ2a ∂f8 ∂t2s = −ρ2sqt2s ∂f8 ∂ρ2s = −t2sqρ2s ∂f8 ∂t2a = ρ2aqt2a ∂f8 ∂ρ2a = t2aqρ2a table 4. derivative of functions fi. 71 jiří oldřich acta polytechnica ctu proceedings z = z(t, ρ) = p rtρ s(t, ρ,−→x ) = ∑ x1 [ s0 i (t0) + ∫ t t0 c0 pi(t ) t dt ] − −r ln zrtρ p0 −r ∑ xi ln xi +r(ln z −qf −qu ) h(t, ρ,−→x ) = ∑ x1 [ h0 i (t0) + ∫ t t0 c0 pi(t )dt ] + +rt (z − 1−qu ) qρ = z + ρ ( ∂z ∂ρ ) t qf = ∫ ρ 0 (z − 1)d(ln ρ) qt = z + t ( ∂z ∂t ) ρ qu = t ( ∂qf ∂t ) ρ qc = t 2 ( ∂2qf ∂t 2 ) ρ table 5. relations for calculation of entropy and enthalpy of real gas and relations for calculation of dimensionless quantities q [10]. 72 acta polytechnica ctu proceedings 20:65–72, 2018 1 introduction 2 isentropic efficiency 3 equation of isentrope 4 reversible process, ideal gas, constant cv0 (perfect gas) 5 reversible process, general equation of isentrope 6 methods of calculation of isentropic process and isentropic efficiency 7 calculations based on the real gas eos 7.1 example of the solution by newton's method 7.2 calculation of t2s, ha from known parameters p1, t1, p2a and s (figure 2, table 2 item 1) 7.3 calculation of t2a, s from known parameters p1, t1, p2a and ha (figure 2, table 2 item 2) 7.4 calculation of p2a, ha from known parameters p1, t1, t2a and s (figure 2, table 2 item 3) 7.5 calculation of p2a, s from known parameters p1, t1, t2a and ha (figure 2, table 2 item 4) 7.6 calculation of ha, s from known parameters p1, t1, p2a and t2a (figure 2, table 2 item 5) 8 example 9 conclusion list of symbols references acta polytechnica ctu proceedings doi:10.14311/app.2020.29.0035 acta polytechnica ctu proceedings 29:35–39, 2020 © czech technical university in prague, 2020 available online at http://ojs.cvut.cz/ojs/index.php/app foundation of the new railway bridge over the nosická dam on the river váh maroš zaťko zakládání staveb, a.s., organizačná zložka bratislava, košická 56, 821 08 bratislava, slovakia correspondence: zatko@zakladani.cz abstract. the new railway bridge over the nosická dam is a part of the complex modernization of the railway corridor from púchov to žilina. our company participated on the project using a wide variety of special foundation techniques on artificial islands for piers p6, p5, p4 and p3 and the pillar foundations. we applied the many years of experience gained by our company through working on projects involving water. bored piles, as well as vibrated sheet piles, welding and assemblage work have all been carried out from a floating pontoon platform and also there was jet grouting applied to stabilize embankments. at the same time we provided active assistance with temporary floating bridges and platforms for the kind of transfer on water required by our client. keywords: foundation in water, jet grouting, piles, sheet piles. 1. introduction as part of the first phase of the modernization of the púchov žilina railway line for speeds of up to 160 km/h on the interstation section púchov považská bystrica, our company has been carrying out special foundation work on structure so 44.33.21.1 on a new railway bridge over the nosická dam going back to autumn 2016 see fig. 1. the investor is the company železnice slovenskej republiky, a.s, bratislava, the main contractor is the association združenie nimnica, while its leading member doprastav a.s. is our client for the special foundation of the bridge. the general designer is the company reming consult, a.s. and the responsible designer of the bridge foundation is the company lkm consult s.r.o. figure 1. new railway bridge over the nosicka dam during construction 2. information concerning the bridge the bridge carries a railway line designed for a speed of 160 km/h from the diel tunnel across the nimnická road ii / 507 nimnica považská bystrica and further obliquely above the nosická dam water reservoir. at the end, it connects to so 44.33.21.2 the bridge under the double-track railway line, which transfers to the road iii / 5171 from milochov to považská bystrica. after its completion, the bridge will allow river navigation through two navigable corridors in fields 3 and 4. the proposed load-bearing structure of a 12−pole straight bridge of two-chamber crosssection made of prestressed concrete with a typical span of 51.5 m fits well to the nature of the bridged obstacle and the bridge length. the company zakládání staveb a.s. carried out pile drilling from the water surface on pillars p3, p4, p5 and from the terrain on p6, sheet piles vibrating and assembly work on the construction of sheet sumps for pillars p3, p4, p5 and a peninsula for pillars p6, p7, and finally jet grouting on all 4 foundations [1]. due to our many years of experience and involvement with the technological equipment of the company, the contract also included filling embankment in sumps, assembly work on the water surface and various other handling and auxiliary work that required the assistance of pontoon platforms and tugs. 3. geology as part of the detailed igp, no drilled probes were performed in the deeper riverbed of the river váh. the level of the mesozoic sandstone and claystone formations and the possible boulder zone was determined by simply using geophysical methods and sheet driving experiments, by which the possible driving depth was verified. typical geology at the site of drilled piles begins with a layer of alluvial muddy sediments with the character of clay sand and a thickness of 4m. under them was gravel with a mixture of fine-grained soil g3 / g-f to a depth of 7 m below the bottom. after 35 http://dx.doi.org/10.14311/app.2020.29.0035 http://ojs.cvut.cz/ojs/index.php/app maroš zaťko acta polytechnica ctu proceedings that gravels g3-g-f to g5gc with clay filling occurs to a depth of 12m. from 12m to 14.5m there begins the subsoil formed of weathered sandstones, which from 14.5 m deeper changed into healthy sandstones of medium to high strength r3-r2. 4. the working tasks 4.1. preparatory work as part of the preparatory work for operating on water, also embarkation of equipment and materials, there was realized a dock edge between pillars p11 and p12 made of vibrated sheet piles reinforced by a circumferential stiffening beam. at this landing place, the kobelco 550 crawler crane first replaced individual parts of the pontoon from the surface to water, where they were, one by one, joined by connecting rods into an assembly configuration according to the required scheme. at the turn of february and march 2017, a technological test of high-frequency sheet piles vibrating took place. the machinery set including wheel crane ltm40, hydraulic aggregate, vibrating hammer and auxiliary container, boarded a pontoon platform pushed by the tug boat veronikaii and set off to geodetically marked places for forthcoming works. the sheet vibrating was performed using one calibrated sheet profile with a marked scale in meter units. this test was applied on places of future pillars p3 – p8. experiments for pillars p9 to p12 were performed from the embankment. the objective of these attempts was to verify the actual levels of solid subsoil, and where sheet vibrating would be deemed ineffective. the result was that this geological layer was generally higher than had been assumed, which led to an acceptable reduction in the length of the sheet piles. 4.2. timeline of work the work started in may 2016 with pile drilling from the water surface on the p5 foundation and continued towards the other riverbank (direction nimnica) at p4 and p3. after 3 months of work on the water the drilling rig disembarked and work continued on the meanwhile completed peninsula p6, p7 where the drilling was realized from the terrain. for all islands there was designed double rectangular barrier sheeting walls around the bridge pillar base foot. sheet piles vibrating began on the p6, p7 peninsula. after completion pits for islands p5 and p4 was vibrated including welding and framing work. the realization of the island p3 walls could start only after partial withdrawal of p6p7 sheeting piles to maintain the water flow profile of the dam. all vibrating was realized from the pontoon on the water surface. the embankment body of the p6 p7 peninsula was built from the shore by gradually pouring the material into the water into the space bordered by the sheet pile walls. the p3 peninsula was similarly built from the other bank. temporary pontoon bridges were used to fill the p4 and p5 islands with material. embedding a heavy stabilizing rocky embankment on the outer sheeting wall perimeter was performed by a floating platform on which heavy duty trucks laden with rocks were shipped across the water. after the completion of the piles on p6, jet grouting was carried out, followed by p5 and p4 islands, always after the realization of an embankment of sheet. after that the complete construction of the p6 and p7 bridge pillars could begin sheet pile extraction and this material was subsequently used in realization of the island p3. after this completion and embankment filling, jet grouting also followed. one of the main boundary conditions for successful conduct of the construction work was the level regime at nosice dam. the minimum operating level of the dam is 274.60m above sea level. m. and maximum 279.60m above sea level whereas for the static design of temporary structures in the riverbed there was considered a minimum design level 276.00m above sea level. the normal regime of construction work was such that without restriction it was possible to perform in the interval above a min. design level 276.00m above sea level. and below a max. operating level 279.60m above sea level. mainly for assembly and welding work on the sheeting wall barriers the water level had to be kept to a value up to 277.6m n. m. 4.3. pile drilling pilot groups under the pillars p5, p4 and p3 were realized in a profile of 900mm. a drilling rig stood on a pontoon platform of suitable configuration which had been anchored by steel ropes to the sheet piles vibrated in advance in the river flow see fig. 2. for the needs of drilling works, the pontoon platform was modified by adding a welded steel frame used to guide the drill casing, to hang it and jam during the drilling process. on the pontoon there was placed two maintenance half-containers, a power generator, concrete pipes, along with funnel and drilling tools. the pontoon assembly was first guided geodetically from the shore and pushed by tugboat to the approximate place of drilling. during these maneuvers against the water current, the pontoon was mostly fixed by two ropes to the anchoring sheet piles to make it easier to handle in the water flow. for a more precise positioning of pontoons there were used four hand winches with steel ropes on each corner of the pontoon platform. when the position of the platform was approximately correct, the stabilizing legs were dropped down to the bottom of the riverbed to minimize horizontal movements due to drilling, current, torque and at the same time to prevent vertical movements that would be dangerous to the stability of the pontoon. the correct position of the drill casing was geodetically checked from the shore by aiming to the center of the casing. the piles on p3 were 11m long, on p4 12m on p6 16m and on p5, at the deepest point of the flow, it was 36 vol. 29/2020 new railway bridge over the nosická dam figure 2. realization of piles from pontoon figure 3. exposed piles in a sheet pile sump 16.5m. the requirement of the project was to adjust the length of the pile according to the specific geology and other matters, and by this the reinforcement cage was to be lengthened or shortened by a selected method. according to the control plan there was to be geological supervision present during the drilling of each pile, checking the match of the geological profile with the project and the quality of borehole bottom, which according to the project should be obliged to a min. 1m into the bedrock and thoroughly cleaned with a flat bottom drilling pot. for concreting, a floating pipe with a diameter of 150 mm was prepared composed of 6m long pieces. in the middle of each pipe was attached a polystyrene block so as to keep it above the water when the pipe is full of concrete. by connecting the parts there was created a pipeline of a length up to 150 m with which it was possible to transport the concrete mixture across the water to the drilled pile hole. for that purpose, the pump mecbo pulsar 6.80 was used. to ensure the full body of pile and the quality of its head in the muddy deposits, the piles were concreted much more than usual see fig. 3. pilot work on the p6 pillar was carried out from the terrain of the artificial peninsula p6 p7 which was built in the meantime. 4.4. sheet piles vibrating and assembly work along with the drilling rig pontoon platform, another one was designed for the ramming work, made of 5 fragments, so the dimensions of the platform were 15m× 9m. a liebherr ltm30 wheeled crane was placed on the platform, which was clamped and anchored to the deck with chains. behind it was placed a service semi-container and a hydraulic unit driving the ice1423 vibro-rammer. a ps50 military auxiliary pontoon was used to supply the main pontoon with sheet piles, rods, steel profiles and other necessary material. a smaller part of a pontoon of this type was used as a working platform for welders needed to carry out a large number of welding and locksmith work around the sheet piles structures. the workers were transported across the river by a motorboat, which also served as a means to inspect the quality of work. the vibrating began on the p6 p7 peninsula so that drilling and vibrating pontoon formations would not interfere with the water flow. figure 4. mounting the spacer frame in the sump this started by vibrating two parallel walls at a distance of 7.7m from each other from the tip of the embankment towards the water flow stream. gradually, beams of 2xipe270 were installed on the outer perimeter of the walls, and in the projected places, the walls were tied through gewi bars 50 mm in diameter. geodetically guided sheet piles were laid out, according to which the correct position of the rammed sheet piles was then maintained. during the double rectangular barrier sheeting walls, so called sumps, the inner sump was first vibrated and closed. on selected sheet piles, consoles were welded in an exact height position to fit the lower of the three spacer frames. on p6, these frames had dimensions of 12m× 11m and consisted around the perimeter of 2xipe400 profiles reinforced in the corners with two heb300 diagonals. first, the lower frame was welded and assembled above the water level, then lowered by a crane below the surface where it was to be landed on the mentioned consoles on the sheet piles. this operation was assisted by a diver, whose task was to check the position of the mounted frame and to install 37 maroš zaťko acta polytechnica ctu proceedings boundary wedges between the ipe profiles and the sheet piles to activate the frame. due to the higher level of mud sediments at the bottom, the lower frame could not be lowered to the projected depth but was fixed and wedged 120 cm higher. subsequently, the upper spacer frame was assembled above the surface in its projected position see fig. 4. the assembly of the middle spacer frame was carried out in the later phase of construction on the terrain of the already built island. on finishing the sheet pile walls and closing the outer walls, the assembly of stiffening beams and gewi tension rods continued. after the completion of part of the walls, the filling of the wall-bordered space began, which gradually created a new peninsula. at the same time, a heavy stone throw was made on the outside perimeter of the sheet pile walls by placing half meter large boulders next to the wall. poured material in front of the sump pushed the thin muddy sediments which sometimes had to be removed so as not to degrade the material of the embankment body. this effect was most visible when the filling reached the end of the peninsula. while carrying out the filling of the peninsula, visible deformations in the shape of the walls were detected and the sheet piles of the outer perimeter wall slightly bulged outwards. as a countermeasure, there was implemented an additional external embankment of the peninsula with a heavy stone dump in problem areas in order to ensure the stability of the foot of the sheet piles. during this phase of realization, the embankment pushed against the walls of the inner pit when it was not yet loaded from all 4 side evenly, which then began to tilt it and gradually deform it. this situation led to some reinforcing measures and project revision. 4.5. jet grouting based on monitoring of sheeting walls deformation after the filling of embankments, the designer was ordered to additionally reinforce the p6p7 peninsula by adding transverse rods and extending the jet grouting project see fig. 5. the use of jet grouting technology had 4 goals: • creation of a watertight sealing around the outer perimeter of the inner sheet pile walls in order to prevent water leakage through the wall • sealing and strengthening the bottom of the pit with pillars of j.g. between the foundation piles and thus creating a hydraulically resistant block with a thickness of 4m 5m • remediation of embankments in order to reduce uneven deformations of the embankment body, eliminate the risk of tilting of drilling rigs, and to minimize the overall settlement of the embankment around the foundations of the temporary bridges and around the crane platforms. at the same time, j.g. was used to increase the overall rigidity and resistance of the island body in the event of an extreme flood. • creation of a jet grouting block, which was designed in the vicinity of all micropile foundations of the temporary bridges on p3, p4, p5 and p6p7 the mobilization of jet grouting technology was planned after the completion of sumps filling and finishing of piles on the p6p7 peninsula. as the first stage there was improving the embankment body. in the previous period, during its construction inside the sheet pile sumps, there were significant deformations of sheet piles and local declines in the embankment terrain, and especially at the end of the peninsula, which thus required an improvement project. the resulting design included more ribs from j.g., a fullarea block of j.g. under the base of the temporary bridge and additional stiffening beams and rods in the transverse direction of the peninsula p6p7. grooves were dug in the projected places into which these reinforcing elements were inserted, and through the inserted strings they were connected to the sheet pile walls. after the implementation of these measures, jet grouting began. first, ribs from directed mono jet grouting to a depth of 6m were realized, then a block of pillars by the m2 method 900 and 1200mm in diameter to a depth of 6m below the temporary bridge foundations, also sealing of the perimeter sheet pile walls from directed mono jet grouting to a depth of 14m and finally a block of j.g. columns in the bottom of the inner sump between piles with a thickness of 5.2m was completed. on p5, p4 and p3, the inner sumps were not filled with backfill but with water, so special steel frames with massive beams were mounted above the sump. figure 5. implementation of jet grouting six-meter-long vl604 sheet piles were placed on them, which created a removable work platform for the drilling rig. at the point of drilling, single sheet pile was removed and the drill instrument was lowered through the water column to the bottom. the total length of the borehole was 15m. the designed pillars with a diameter of 1400mm and 900mm were grouted in ascending order by the m1 method or two-phase air m2 method. thus, blocks with a thickness of 5.2m 38 vol. 29/2020 new railway bridge over the nosická dam and a minimum strength of 2mpa were realized on the individual pillars. sealing walls on the outside of the inner sump were also made while drilling from steel platforms. here, a two-row mono directed jet ascending grouting with oscillation was used with borehole spacings of 1.5m (0.7m alternately and 0.4m between the rows), while the grouting length was 11 m − 15 m. the leached material remained at the bottom of the pit and was extracted during the excavation to the base foot level. the work process and especially the technological breaks were very strict in order to avoid deformations. as a foundation for a temporary bridge m60 for connection of islands p4, p5 with peninsula p6p7, there were realized two groups of three micropiles tr 114 / 12.5mm length 12m with injected root length 9m under each support. 4.6. working on water in addition to pile drilling, vibrating the sheet piles and assembly work, it was also necessary to deploy pontoon platforms of various configurations for less usual tasks during the transfer of material for external and internal embankments of sheet piles sumps. a pontoon platform 15 m × 18 m was used to place a heavy stone throw around the islands of p5 and p4 see fig. 6. it was handled by a pair of tugs veronika ii. for embarkation, a port edge with ramp was made on the edge of the p6p7 peninsula. figure 6. a heavy stone backfilling it was fixed by joint, and possible to lift and lower by crane. there could have been 4 trucks on board with a total weight of 160tons. as it was a relatively complex and risky activity, instruction was prepared by an authorized expert in the field, from whom the method of performing the entire operation was prescribed. the vehicles gradually reversed onto the pontoon and were secured in a precisely determined position in the order of 1, 3, 2 and 4 so that the pontoon was loaded evenly. before and after boarding each vehicle, the ramp was raised and lowered as the pontoon was moved along the port edge and the ramp was placed against the intended vehicle position. this was so that the cars did not need to maneuver on the platform but were only moving in a straight line. after securing them by inserting wedges under the wheels, the pontoon was pushed to a sheet pile wall, where it was hooked on by a rope and the cars gradually reversed to the edge of the pontoon and dumped the load in to the water just next to the sheet piles. after the implementation of these external embankments, the pontoon set was dismantled and rebuilt into a pontoon bridge between the artificial islands. over these floating bridges the cars transferred and poured the material into the pit p5 or p4. pontoon assemblies were needed for other work, such as moving material, laying cables to the bottom of the dam, or cutting sheet piles with the assistance of a diver. the bridge contractor used pontoons for excavation of the underwater embankment after demounting the artificial islands and returning the bottom of the dam to the original state. a pontoon was also used to dismantle some parts of the sliding concreting structure after finishing the bridge skeleton in the middle of the span above the water, a task which would otherwise be difficult to carry out. 5. conclusion work on the bridge should be completed in autumn 2020. the construction of bridges over the dams brings a number of unusual problems but offers opportunityfor interesting solutions especially in the field of special foundation, as the construction of the pillars no longer differs much from a similar bridge in terrestrial conditions. by the end 12, 200 m2 of sheet piles had been vibrated and 185 tons of steel had been seized to the structures and 2200m gewi rods had been installed. there were also completed 1500m of piles and 8400m of jet grouting. almost 36, 000 cubic meters of embankment had been incorporated into artificial islands p3 p5. references [1] m. zaťko, j. šperger, modernization of the railway line púchov – žilina, foundation of a railway bridge over the nosice dam on the river váh, journal fundation, záklání staveb a.s., vol. xxxi, n.4/2019, issn 1212 – 1711, prague 2019. 39 acta polytechnica ctu proceedings 29:35–39, 2020 1 introduction 2 information concerning the bridge 3 geology 4 the working tasks 4.1 preparatory work 4.2 timeline of work 4.3 pile drilling 4.4 sheet piles vibrating and assembly work 4.5 jet grouting 4.6 working on water 5 conclusion references acta polytechnica doi:10.14311/app.2020.27.0107 acta polytechnica 27(0):107–111, 2020 © czech technical university in prague, 2020 available online at http://ojs.cvut.cz/ojs/index.php/app estimation of anisotropic elastic properties of carbon fibers using nanoindentation thimmappa shetty guruprasad∗, vincent keryvin, alain bourmaud university of bretagne sud, umr cnrs 6027, irdl, 27 rue armand guillemot bp 92116, 56321 lorient, france ∗ corresponding author: thimmappa-shetty.guruprasad@univ-ubs.fr abstract. understanding the mechanical behavior of carbon fiber reinforced polymers requires knowledge on the deformation behavior of carbon fibers, they are highly anisotropic and heterogeneous. nanoindentation is an efficient method for determining the mechanical properties in small volumes of materials. for isotropic materials, a single nanoindentation test can evaluate an elastic properties of the material. but for anisotropic material, the difficulty increases since measured indentation modulus depends on five elastic parameters (el, et, glt, νlt,and νtt) of the material. nanoindentation experiments are performed on carbon fibers orientated between 0◦ to 90◦ at ten different orientations to the fiber axis. from theoretical models given by vlassak et al. and delafargue and ulm, the elastic constants are predicted numerically by comparing the results of indentation modulus versus orientation angle with the experiments. keywords: anisotropy, carbon fibers, nanoindentation. 1. introduction carbon fiber composites are an important class of materials, which have high tensile strength, high modulus, and lower weight. due to its advantages, the composites are used in many structural applications such as in the aerospace, automotive and naval industries. for the better design of composites having good structural strength, knowing the elastic properties of individual carbon fibers and also the knowledge of predicting its mechanical response is essential. the carbon fibers are generally categorized into four different types based on its strength, namely intermediate modulus (im), high modulus (hm), ultra-high modulus (uhm) and high resistance (hr). the mechanical strength of the composite material reinforced with unidirectional carbon fibers behave to be highly anisotropic in its fiber direction and the strength in the transverse direction is much weaker as compared to longitudinal direction [1]. the most notable experimental technique to determine the mechanical properties of small and delicate structures is through nanoindentation. the nanoindentation is generally used for measuring mechanical properties such as elastic modulus and hardness. since the carbon fibers are highly anisotropic, determining the modulus in any one direction will not determine the complete set of elastic properties of the material. assuming carbon fibers as transversely isotropic materials and due to symmetry we need to determine five elastic constants. so, the indentation modulus is a function of five elastic parameters of the material which are three modulus (el, et, glt) and two poisson’s ratios (νlt, νtt). due to less number of available theoretical models and also due to difficulty in implementing them numerically, determining elastic constants is a challenging task. similar to carbon fibers, there exists an anisotropy in natural fibers, for e.g. jäger et al. [2] determined the anisotropy in the elastic properties of wood cell walls using the nanoindentation technique. they predicted the elastic constants from the numerical solutions of vlassak et al. [3]. recently, csanádi et al. [4] studied the anisotropy in the carbon fibers from a ceramic based carbon fiber composites, here the elastic constants were predicted using the same numerical solutions of vlassak et al., but the experiments were done only on two independent directions i.e. at longitudinal and transverse directions. more recently, duan et al. [5] determined the transverse and the shear modulus of three different types of carbon fibers. the fiber surfaces are prepared by focused ion beam technique and both afm as well nanoidentation tests are performed to determine the indentation modulus. the results from the afm are considered to determine the elastic constants using swanson’s model [6]. in this work, we will determine the elastic constants of carbon fibers of type im2c using nanoindentation, they are pan-based carbon fibers. first, nanoindentation experiments are performed to determine the anisotropic response by cutting the carbon fibers at varying orientation angles concerning to the axis of the fiber. the fibers are cut between 0◦ (longitudinal direction) to 90◦ (transverse direction) in steps of 10◦. the conventional machining techniques are used to obtain the desired surface for the tests. the indentation modulus is calculated using a standard oliver-pharr method. then, we determined the elastic constants numerically using theoretical models; one of them was proposed by vlassak et al. [3]. the model relates the elastic constants with the indentation modulus for the 107 http://dx.doi.org/10.14311/app.2020.27.0107 http://ojs.cvut.cz/ojs/index.php/app t.s. guruprasad, v. keryvin, a. bourmaud acta polytechnica anisotropic materials. the other theoretical model is given by delafargue and ulm [7], which can predict the values only at 0◦ and 90◦. 2. experiments 2.1. materials the epoxy-based carbon fiber composite material is chosen to investigate the elastic properties. the composite consists of polyacrylonitrile (pan) type carbon fibers type im2c (hexcel corporation), and it belongs to the category of intermediate modulus. the average diameter of the carbon fibers are measured to be 5.2 µm. to perform nanoindentation, samples are prepared by cutting the composite at orientation angles between 0◦(longitudinal direction) to 90◦ (transverse direction) in steps of 10◦ with respect to the axis of the carbon fibers. then the composite samples are hot mounted in epoxy with orientation angle parallel to the cutting plane. the desired surface of the carbon fibers for the nanoindentation test is achieved by polishing the samples initially with sandpapers of 1000, 2000 and 4000 grit and then with diamond suspension having a particle size of 1 µm and finally with colloidal silica particles of size 30 nm. 2.2. microscopic analysis as carbon fibers are oriented between 0◦ and 90◦ in steps of 10◦ concerning to the carbon fiber axis, the actual orientation may be different than the cutting angle due to the use of conventional cutting and polishing methods. to measure the actual orientation angle (θ), samples are analysed using the scanning electron microscopy (sem, jeol ltd.), the angle is determined by measuring the size and shape of the carbon fibers from the sem images. for 0◦, the shape is a circle, for 90◦ it is a rectangle as it is perpendicular to the axis and for rest of the angles it is a ellipse. figure 1(a) and (b) shows the sem images of the carbon fibers with orientation angles of 0◦ and 70◦ respectively. to measure the actual orientation angle of the carbon fiber, imagej software is used, where the sem images of the carbon fibers are digitized and the dimensions of the cross section are measured. for 0◦ − 80◦, the major and minor axes of the ellipse is measured from their respective sem images, then the actual orientation angle is determined using the relation θ = cos−1( ba ), where a and b are the major and minor axes of the ellipse. figure 1(c) shows the schematic of the ellipse and the method for calculating the actual orientation angle from the carbon fibers cross section. 2.3. nanoindentation nanoindentation is an efficient method for determining the local mechanical response in small volume of materials. to determine the anisotopic elastic properties of the carbon fibers, indentations are performed perpendicular to its cross section which are having a 10 µm (a) 20 µm (b) a b actual angle of tilt, (c) figure 1. sem images of carbon fibers with cutting angle (a) 0◦ and (b) 70◦. a schematic with dimensions of ellipse and the formula to calculate the actual angle of tilt. orientation angle varying from 0◦ to 90◦ with respect to the axis of the carbon fibers. figure 2 shows the schematic of the indentation performed at an orientation angle (θ) with respect to the axis of the carbon fiber. the nanoindentation experiments are conducted in a continuous stiffness measurement mode using an mts nano xp nanoindenter having a diamond berkovich tip. all the indentations are performed for depths less than 150 nm, the larger depths the may affect the surrounding matrix along with the deformation of the carbon fibers. the hardness and elastic modulus are calculated using a standard method given by oliver and pharr (see 8 for detailed procedure for calculating the modulus and hardness). the effective 108 vol. 27/2020 anisotropy indenter carbon fiber figure 2. a schematic of the carbon fiber indentation at an orientation angle (θ) with respect to the carbon fiber axis. indentation modulus is given by 1 meff = 1− ν2 i ei + 1 m (1) wherem = es 1−ν2 s , which is indentation modulus of the material. the ei and es are the elastic modulus of the indenter and the sample respectively, similarly νi and νs are the respective poisson ratios. the elastic modulus is calculated using the above relation, and the value of ν has a minor influence on the value of the elastic modulus [9]. to determine the anisotropy in the carbon fibers, the samples prepared with varying orientation angle between 0◦ to 90◦ with respect to the axis of carbon fiber are indented. for each orientation angle, at least 20 are indents are performed to ensure the repeatability in the results. 3. theoretical models for anisotropic nanoindentation with the aim of determining the elastic properties of the material by nanoindentation, the indentation modulus for isotropic materials is given by m = e 1− ν2 , (2) where e is the elastic modulus of the material. for isotropic materials, m is independent of the indentation direction. in the case of anisotropic materials, the relation of indentation modulus with the elastic constants is very complicated as it depends on the indentation direction and the indenter shape. the closed form expressions between the indentation modulus and the elastic material constants (cij) for the transversely isotropic materials are proposed by delafargue and ulm [7] and the solution can be used when the indentation direction is aligned with direction of one of the principal material axes (see 7 for more details). the solution of the indentation modulus is available only at two independent directions i.e. at longitudinal direction 0◦ and transverse direction 90◦. the indentation modulus in the longitudinal direction (0◦) is given by m0◦ = 2 √ c2 31 − c2 13 c11 ( 1 c44 + 2 c31 + c13 )−1 (3) where, c31 = √ c11c33. similarly, the indentation modulus in the transverse direction (90◦) is given as m90◦ = √ m12m13 (4) where, m12 = √ c11 c33 m3 and m13 = c2 11 − c2 12 c11 . the other well defined solution schemes for general anisotropic elastic materials are proposed by vlassak et al. [3]. the solution relates the indentation modulus with the elastic properties of the material as well as indentation direction. the indentation modulus for the anisotropic material according to vlassak et al. are given by m = 1 α (e,φ) (1− e2) 1 4 (5) with the assumption of elliptical contact area formed with the indenter, the function α (e,φ) relates to eccentricity (e) of the contact ellipse and angle between the major axis of the ellipse and the reference direction of a function h (refer to 3 for further details). the solutions of vlassak et al. are used to determine the elastic constants by comparing the indentation modulus with the experimental results performed at varying orientation angles. the values of el, et, and glt are varied such that the solution matches the experimental indentation modulus closely. the values of νlt and νtt are kept constant as their effect on the indentation modulus is considered to be negligible [2]. similarly, the expressions of indentation modulus given by delafargue and ulm [7] are compared with the experiments to determine the elastic constants at two independent directions. 4. results and discussions from microscopic analysis of the carbon fibers i.e., according to section 2.2, we found that the actual angle of orientation is different than the cutting angle. for carbon fibers with 0◦ i.e., carbon fibers orientated in a longitudinal direction, there exists small deviation in the orientation angle, and the actual angle was found to be 12◦. but, for 90◦ practical orientation remains same as the cutting angle. also, it is found that there exists a small deviation in the orientation angle for the remaining cross-sectioned samples. the results of nanoindentation experiments of carbon fibers with varying orientation angle with its axis are presented. the load (p) versus the indentation 109 t.s. guruprasad, v. keryvin, a. bourmaud acta polytechnica 0 30 60 90 120 h [nm] 0.0 0.5 1.0 1.5 p [m n] 90 12 test_1 test_2 test_3 test_4 test_5 test_6 figure 3. load (p) versus indentation depth (h) behaviors from nanoidentation experiments for carbon fibers orientated at 12◦ (solid lines) and 90◦ (dotted lines). depth (h) from the indentation experiments are plotted for all the samples between 12◦ and 90◦. as the diameters of the carbon fibers indented are 5.2 µm and it is difficult to locate and perform indentation at the middle of the fibers. figure 3 shows the representative p − h curves of carbon fibers orientated at 12◦ and 90◦ with its axis. from the plot, we can see that there exists a repeatability in the p − h curves for two of those orientations and similar repeatability in the p − h are observed for other orientations as well. for the carbon fibers with orientation of 12◦, we observe from the p − h curves that there exists a hysteretic behavior and a small plastic deformation in the material but for the samples orientated at 90◦ shows negligible plasticity and are nearly elastic. the p − h also shows that the load required for the nearly longitudinal orientated fibers (12◦) requires approximately three times the load in comparison to carbon fibers oriented in transverse direction (90◦) to produce a same indentation depth. the indentation modulus (m) versus the actual orientation angle (θ) from the experiments are plotted in figure 4. due to anisotropy in the carbon fibers, the value of the indentation modulus decreases with increase in the orientation angle. apart from the indentation modulus, we also observed a similar decreasing behavior in the hardness with increase in the orientation angle. from the theoretical models presented in section 3, elastic constants are determined from the nanoindentation experiments. using vlassak et al. solutions, the elastic constants such as el, et, and glt are varied independently, such that a close match is obtained with the plot of experimental indentation modulus versus orientation angle. the final predicted values of three elastic constants are shown in table 1 and the poission’s ratio’s used are νlt = 0.3, νtt = 0.3. the corresponding fit with the experiments is shown in 10 10 30 50 70 90 [°] 0 20 40 60 80 100 m [g pa ] delafargue and ulm experiments vlassak et al. figure 4. the plot of indentation modulus (m) versus the orientation angle (θ) from nanoindentation experiments (circular markers), theoretical solutions of delafargue and ulm [7] (square markers) and vlassak et al. [3] (solid line). figure 4 and the obtained fit has a good agreement with the nanoindentation experiments. our predicted values of transverse modulus (et) and the shear modulus (glt) are comparable with other carbon fibers (type t-300), which was reported by csanádi et al. [4] and the values are et = 30.9 gpa, glt = 10.9 gpa. el (gpa) et (gpa) glt (gpa) 296 25.5 9 table 1. the predicted values of the elastic constants (el, et, and glt) of carbon fibers (im2c) via combined experimental-theoretical approach. similarly, the indentation modulus using the closedform solutions of delafargue and ulm are calculated for two orientations i.e., for 0◦ and 90◦ by varying the elastic constants. here, the results shows a good agreement with vlassak et al. as well as the indentation experiments. 5. conclusions the anisotropic elastic properties of carbon fibers (im2c) are determined using combined experimentaltheoretical approach from nanoindentation. the carbon fibers are cut at varying orientation angles between 12◦ and 90◦, and they are indented to determine the indentation modulus between longitudinal orientation and the transverse orientations. then, the plot of experimental indentation modulus versus the orientation angle is closely matched with the solutions of delafargue and ulm and also with the solutions of vlassak et al. to predict the elastic constants. 110 vol. 27/2020 anisotropy list of symbols el elastic modulus in logitudinal direction [gpa] et elastic modulus in transverse direction [gpa] glt shear modulus [gpa] νlt, νtt poisson’s ratios m indentation modulus [gpa] θ orientation angle [◦] cij elastic constants of the material p load [mn] h indentation depth [nm] references [1] r. gibson. principles of composite material mechanics. fourth edition. boca raton: crc press, 2016. doi:https://doi.org/10.1201/b19626. [2] a. jäger, t. bader, k. hofstetter, j. eberhardsteiner. the relation between indentation modulus, microfibril angle, and elastic properties of wood cell walls. composites part a: applied science and manufacturing 42(6):677 – 685, 2011. doi:https://doi.org/10.1016/j.compositesa.2011.02.007. [3] j. vlassak, m. ciavarella, j. barber, x. wang. the indentation modulus of elastically anisotropic materials for indenters of arbitrary shape. journal of the mechanics and physics of solids 51(9):1701 – 1721, 2003. doi:https://doi.org/10.1016/s0022-5096(03)00066-8. [4] t. csanádi, d. németh, c. zhang, j. dusza. nanoindentation derived elastic constants of carbon fibres and their nanostructural based predictions. carbon 119:314 – 325, 2017. doi:https://doi.org/10.1016/j.carbon.2017.04.048. [5] s. duan, f. liu, t. pettersson, et al. determination of transverse and shear moduli of single carbon fibres. carbon (158):772 – 782, 2020. doi:https://doi.org/10.1016/j.carbon.2019.11.054. [6] s. r. swanson. hertzian contact of orthotropic materials. international journal of solids and structures 41(7):1945 – 1959, 2004. doi:https://doi.org/10.1016/j.ijsolstr.2003.11.003. [7] a. delafargue, f.-j. ulm. explicit approximations of the indentation modulus of elastically orthotropic solids for conical indenters. international journal of solids and structures 41(26):7351 – 7360, 2004. doi:https://doi.org/10.1016/j.ijsolstr.2004.06.019. [8] w. oliver, g. pharr. an improved technique for determining hardness and elastic modulus using load and displacement sensing indentation experiments. journal of materials research 7(6):1564–1583, 1992. doi:10.1557/jmr.1992.1564. [9] w. oliver, g. pharr. measurement of hardness and elastic modulus by instrumented indentation: advances in understanding and refinements to methodology. journal of materials research 19(1):3–20, 2004. doi:10.1557/jmr.2004.19.1.3. 111 https://doi.org/https://doi.org/10.1201/b19626 https://doi.org/https://doi.org/10.1016/j.compositesa.2011.02.007 https://doi.org/https://doi.org/10.1016/s0022-5096(03)00066-8 https://doi.org/https://doi.org/10.1016/j.carbon.2017.04.048 https://doi.org/https://doi.org/10.1016/j.carbon.2019.11.054 https://doi.org/https://doi.org/10.1016/j.ijsolstr.2003.11.003 https://doi.org/https://doi.org/10.1016/j.ijsolstr.2004.06.019 https://doi.org/10.1557/jmr.1992.1564 https://doi.org/10.1557/jmr.2004.19.1.3 acta polytechnica 27(0):107–111, 2020 1 introduction 2 experiments 2.1 materials 2.2 microscopic analysis 2.3 nanoindentation 3 theoretical models for anisotropic nanoindentation 4 results and discussions 5 conclusions list of symbols references doi:10.14311/app.2021.30.0041 acta polytechnica ctu proceedings 30:41–47, 2021 © czech technical university in prague, 2021 available online at http://ojs.cvut.cz/ojs/index.php/app evaluation of impact of elevated temperatures on young’s modulus of glt beams lucie kucíkováa,∗, michal šejnohaa, tomáš jandaa, pavel padevěta, guido marsegliab,c a czech technical university in prague, faculty of civil engineering, department of mechanics, thákurova 7, 166 29 prague 6, czech republic b university of seville, high technical school of architecture, av. de la reina mercedes, 2, 41012 seville, spain c university of seville, instituto de matemáticas de la universidad de sevilla, (imus), avenida reina mercedes 41012 seville, spain ∗ corresponding author: lucie.kucikova@fsv.cvut.cz abstract. the influence of elevated temperatures on mechanical behavior of glued laminated timber beams is examined on the basis of tensile tests. dog bone samples prepared from beams exposed to fire of variable duration were categorized with respect to the type and position of the failure crack, type and number of discontinuities such as knots, and the level of browning. the acquired experimental results suggest that the wood variability and the effect of growth discontinuities are probably more significant than the effect of elevated temperatures. to support this conclusion, further study is currently under way, exploring samples from the second series of the fire tests. keywords: fire test, glt beam, tensile test, zero strength layer. 1. introduction glued laminated timber (glt) is a promising renewable material enabling wood to be used for more complex constructions. in this regard, the wood combustibility plays an important role which must be taken into account in every design. when fire occurs in the building, the key task becomes a residual load bearing capacity of wooden parts. reduced dimensions of the member cross-section can easily be measured in situ after removing the charcoal. nevertheless, a certain layer is affected by an elevated temperature and is assumed to have zero strength and stiffness. part 1–2 of eurocode 5 defines this layer as d0 = 7 mm for the fire duration longer than 20 min and unprotected surface [1]. some methods to evaluate this layer are presented in [2] depending on the type of load. the authors provide the zero-strength layer for bending in the range of 9.5 to 20.1 mm. in [3] the thickness of 15 mm is suggested to be more appropriate for bending. from that, at least 7 mm below the char-line, i.e. the base of the char layer, the material is expected to have lower load bearing capacity than in the remaining part of the cross-section, which in turn is considered unaffected. the elevated temperatures are assumed to influence the wood in general. the heat treatment affects various material properties depending on the temperature to which the member is exposed. several authors presented a rapid decrease of the modulus of elasticity [4, 5] and compression strength [6] with temperatures exceeding 220◦c and 150◦c, respectively. the present article should contribute to this research effort concentrating on young’s modulus of wood material forming the residual cross-section of glt beams exposed to fire. the article is divided into six sections. following the introductory part 1 we describe in section 2 large scale fire tests. these are supplemented by pilodyn measurements briefly outlined in section 3. the main part of this paper is concerned with small scale tensile tests discussed in section 4 categorized and subsequently evaluated on the grounds of the observed type of failure. the achieved results are further elaborated in section 5. section 6 finally reviews the essential findings. 2. fire test a possible way to examine the effect of elevated temperature on mechanical properties of glt are quick measurements of pilodyn indentation and small scale tensile tests. while the former tests can be carried out directly on the construction site on extinguished beams, the tensile tests performed in laboratory require production of samples identified with a specific position relative to the charred surface. these mechanical tests were preceded by large fire tests described in detail in [7] so only a brief summary is presented here for the sake of clarity. in particular, eight glt beams, with dimensions of 0.10×0.32×2.38 m and containing eight layers of lamellae, were exposed to fire for various duration of 20 min (samples 7 and 8), 30 min (samples 5 and 6), 40 min (samples 3 and 4) and 60 min (samples 1 and 2) and two types of loading curve. the first loading curve was set according to čsn en 1363-1 (samples 1 4) and the second one was set constant at 600◦c 41 http://dx.doi.org/10.14311/app.2021.30.0041 http://ojs.cvut.cz/ojs/index.php/app l. kucíková, m. šejnoha, t. janda et al. acta polytechnica ctu proceedings with the same initial stage as the previous one (samples 5 8). however, the actual furnace temperature exceeded the set temperatures in most cases. the horizontal experimental setup simulated ceiling layout and contained two beams in each test. the beams had to carry their self-weight only, no additional load was applied. a poorly laid stone wool insulation on the top of the beams enabled burning from all four sides. during the test, the temperature distribution within the whole cross-section was continuously measured via thermocouples placed in drilled holes in even-numbered beams. however, the fluctuations occurred on temperature curves of samples 1, 2, 3, and 4, resulting in unknown temperatures at the end of the measurement. consequently, these samples were excluded from the tensile tests, except for sample 4, which was accepted for further examination for the sake of comparison. to prevent the temperature fluctuations for the rest of the tested samples, the hole openings had to be covered by a ceramic blanket (sibral) resulting in three side fire in the middle part of even beams (the beams equipped with thermocouples). the difference between four side and three side fire is evident in figure 4, where in the case of sample 4 the holes were not covered. the pilodyn measurements were carried out along this experimental campaign both prior to and after completing the fire test. with reference to mechanical testing we may invite the interested reader to [7] for the results of three-point bending test performed on burned beams to address the reduction of stiffness and strength of wood exposed to elevated temperature. in this paper, we revisit this issue in the light of the measured young’s moduli only and compare the results provided by pilodyn and tensile tests. 3. pilodyn measurement the indentation by pilodyn 6j device was used to gain estimates of the longitudinal young’s modulus of glt beams both before and after the fire test. the measurement process is based on driving a spike of 2.5 mm in diameter into the wood in transverse direction with energy of 6 j. the only output is the penetration depth d [mm] with an accuracy of 0.5 mm. the value of young’s modulus e [gpa] then follows directly from an empirical equation with only one variable d [mm] as e = a − b d, (1) where parameters a, b are determined experimentally. two such expressions presented in [8] in the form e1 = 19.367 − 0.5641 d, (2) e2 = 20.15 − 0.766 d, (3) are examined in this study. the first equation (2) is generally adopted for wood, whereas eq. (3) was developed for a particular application of glt beams in bending. mind that both equations were proposed for wood not exposed to elevated temperatures. the indentation measurement was performed on all samples before the fire test in the defined grid of 5×8 points per beam located horizontally 0.2 m from the left side with 0.5 m gaps in between and vertically to the middle of each lamella (40 points/beam in total). after the fire test, beams 4, 6, and 8, cleaned from charcoal, were indented in cross pattern (70 points/beam in total). 3.1. results the mean values of measured depth acquired before and after the fire test together with the resulting young’s moduli computed using eqs. (2) and (3) are summarized in table 1. from that, eq. (2) appears more appropriate if no connection to the type of loading and structural application is given. apart from that we noticed that some of the depths measured after the test were too big and the resultant moduli provided by both equations became even negative. therefore, an application of these equations to wood exposed to fire is questionable, as these were originally derived for wood without thermal degradation. n. d [mm] e1 [gpa] e2 [gpa] b a b a b a 4 14.03 17.57 11.46 9.45 9.41 6.69 5 13.85 11.55 9.54 6 13.18 19.36 11.93 8.44 10.06 5.32 7 13.43 11.79 9.87 8 14.16 19.01 11.38 8.64 9.30 5.59 b before fire test; a after fire test table 1. pilodyn 6j indentation measurement. the mean indentation depth increased after the fire test suggesting reduction of young’s modulus. note that the resulting depth could be affected by unevenness of the burnt surface. nevertheless, this reduction was observed when testing different group of samples, where the surface at the location of indents was ground off to eliminate this effect. besides the identified decrease in values of the longitudinal young’s moduli, no correlation with the fire test duration was identified. 4. tensile test small dog bone samples were produced from about 25 cm long segments cut from five glt beams sustaining the fire load (samples 4, 5, 6, 7, 8) and two beams without thermal degradation (control group c). an example of dog bone sample having thickness of about 5 mm is depicted in figure 1(a). the samples were extracted across the width of the beam, with the maximum number of 8 samples depending on residual dimensions. disregarding the outer most lamellae the specimens were produced from two 42 vol. 30/2021 impact of elevated temperatures on glt beams a) dimensions b) failure crack location figure 1. dog bone sample. central lamellae (samples denoted as s) and four other lamellae (samples denoted as k) adjacent in pairs to the central ones. the specimens width was aligned with the lamella height. the wood fibers were oriented parallel to the specimen longitudinal axis. altogether, 406 samples were prepared and tested in the displacement control regime using the mts alliance 30kn electromechanical testing machine equipped with 30kn load cell. a standard extensometer with length of 100 mm was used to monitor the strain of the middle part. the modulus of elasticity is defined as the initial slope of the stress-strain curve. the key output of the tensile test is the load displacement curve. these curves were transformed into stress-strain curves by dividing the load by the initial area measured in the specimen mid section to get the stress and by dividing the extensometer expansion by its initial length (100 mm) to get the strain. all stress-strain curves were examined separately while eliminating those indicating measurement error, e.g. curves with large fluctuations. 4.1. data evaluation during careful visual inspection of all damaged samples, different types of failure modes were identified. comparing the characteristics of individual failures, the specimens were categorized into six groups according to the type of failure crack displayed in figure 2 in several illustrative examples. when the clear sample split into parts with irregular edges with splinters, the failure crack was categorized as ’a along fibers’. if the crack ran directly perpendicular to the fibers, it was denoted as ’b across fibers’. natural wood usually contains a certain amount of growth imperfections. the most pronounced are knots, which in many cases acted as a crack initiators depending on its size. the cracks propagated either across the knot (’e’) or along the knot (’f’), both with a similar frequency, see table 4. the group ’d discontinuity’ covers other discontinuities occurring in wood such as resin pockets, see figure 2, or change in the fiber direction due to vicinity of the knot. the last type (’c delamination’) figure 2. modes of failure. occurred only in a few cases, as the sample split into two plates or layers resembling delamination. further categorization was based on the location of the crack indicated in figure 1(b). if the crack extended to more than one location, the decisive factors were the majority of crack extension into a given location or the major impact. every sample was also rated considering the level of browning ranging from 0 for clear samples to 1 for black. the sample with failure crack running along (’a’) or across (’b’) the fibers and located in the middle (’d’) was considered as the ”ideal” one. 4.2. results the resulting mean and median values of young’s moduli determined from the loading part of the stress-strain curves are presented in tables 2 6 considering the described categorization. the specimens denoted as c were prepared from beams without thermal degradation, used as a control group. recall that the k and s letters refer to the edge and the middle part of the beam, respectively. the statistical data in table 5 were evaluated from both all samples (no elimination) and samples considering the type of failure ’a’ or ’b’ and position ’c’ or ’d’, thus excluding all samples identified with failure modes ’c’, ’d’, ’e’ and ’f’ and failures occurred in ’a’ or along ’b’ grip area (with elimination). the effect of discontinuities and other inaccuracies is, therefore, reduced and the resulting moduli are higher. in all cases, the mean value of (k) samples is lower than the one corresponding to (s) samples. despite eliminating all non ideal specimens no correlation with the position within the beam width has been found. considering the same elimination as mentioned previously, the mean value (9.01 gpa) and the median (9.30 gpa) of the browned samples (26) are only slightly lower than the not browned spec43 l. kucíková, m. šejnoha, t. janda et al. acta polytechnica ctu proceedings n. 4, 5, 6, 7, 8 c browning 0 >0 0 no elimination mean 9.13 8.15 7.18 median 9.06 8.43 7.02 num 282 43 40 with elimination mean 10.18 9.01 8.53 median 9.96 9.30 7.16 num 120 26 15 table 2. young’s moduli from tensile test in [gpa] depending on the level of browning. pos. num. e [gpa] mean median min max a 20 9.18 8.05 5.70 13.89 b 69 9.86 9.48 6.16 17.08 c 140 9.75 9.43 5.78 16.10 d 177 7.67 7.43 0.34 16.19 table 3. young’s moduli from tensile test according to failure position. imens (120) with the mean value (10.18 gpa) and the median (9.96 gpa). without elimination, the mean value (8.15 gpa) and the median (8.43 gpa) of browned samples (43) are still only slightly lower than the not browned specimens (282) with the mean value (9.13 gpa) and the median (9.06 gpa)1, see also table 2. in addition, the control samples which were not exposed to elevated temperatures (denoted as c) experienced the lowest values of young’s moduli compared to the fire test samples, see also table 5. mind that due to the process of sample preparation the most burnt parts of the beam were cut off, therefore, the parts most affected by thermal degradation were excluded. another examined parameter was the contribution of the position of the failure crack and the failure mode. the position of failure crack according to figure 1(b) seems not to affect the resulting young’s modulus. the values of young’s modulus listed in tables 3 and 6 do not differ significantly from each other, even when comparing different positions with the same failure mode. slightly smaller values of position ’d’ in table 3 are caused by a large number of discontinuities (knots and others). considering the most common failure mode ’a’, the mean moduli for all positions are 10.64 gpa (a), 9.74 gpa (b), 9.84 gpa (c), and 10.36 gpa (d), respectively. unfortunately, the number of data in position ’a’ is too small to draw any conclusion. more pronounced are differences among individual modes of failure, recall figure 2. again, comparing the values in tables 4 and 6, the most significant de1the number of samples is in parentheses. mode num. e [gpa] mean median min max a 146 10.07 9.90 1.45 16.19 b 96 9.55 9.16 4.41 17.08 c 10 9.69 8.02 6.72 14.30 d 21 8.21 7.94 4.30 13.86 e 65 5.43 5.42 0.34 15.13 f 68 7.79 7.89 1.53 13.88 table 4. young’s moduli from tensile test according to failure mode. a) a: along fibers b) b: across fibers c) c: delamination d) d: discontinuity e) e: across knot f) f: along knot figure 3. young’s moduli (e [gpa]) from tensile test for individual failure modes. crease is in the case of failure across the knot (mode ’e’) with the mean value of 5.43 gpa. this could be seen in figure 3, where the measured data of mode ’e’ 3(e) fall below those corresponding to mode ’a’ 3(a) or mode ’b’ 3(b). point out that this category (mode ’e’) still comprises all types and sizes of knots, which may lead to low as well as high values (see min and max values in table 4). differentiating between knots and adopting larger statistical data set would reveal more dependencies. because young’s moduli in the transverse direction are lower than in the longitudinal, this decrease is reasonable due to change of growth orientation. however, the values still occur in a similar interval (min, max) as in other failure modes. slightly lower values are observed for the failure along a knot (mode ’f’), where the mean young’s modulus is 7.79 gpa. this category comprises the effect of change in fiber orientation along the knot and an imperfect connection between the knot and the surrounding material. considering the failure along fibers (mode ’a’) and across fibers (mode ’b’) re44 vol. 30/2021 impact of elevated temperatures on glt beams no elimination with elimination time k s k s [min] num mean median num mean median num mean median num mean median c 0 21 5.19 5.89 19 9.38 9.08 10 6.91 6.98 5 11.76 12.86 4 42 28 7.18 8.14 24 9.41 9.70 10 7.83 8.69 14 9.34 9.70 5 31 26 7.57 7.65 29 10.55 10.58 10 9.58 9.34 15 11.91 12.50 6 31 29 8.43 8.04 26 8.46 8.52 17 8.91 8.25 4 9.77 10.21 7 23 37 9.87 10.20 37 10.77 10.48 17 10.59 11.00 15 11.96 12.76 8 23 21 8.80 9.17 25 9.31 10.32 6 9.87 10.21 12 10.73 11.07 table 5. young’s moduli from tensile tests in [gpa] with or without elimination. pos. mod. n. e [gpa] mean median min max a a 4 10.64 11.49 5.70 13.89 a b 3 10.02 8.20 8.09 13.79 a c 1 11.73 11.73 11.73 11.73 a d 2 a e 1 a f 9 7.82 7.75 7.00 9.31 b a 11 9.74 9.84 7.29 12.16 b b 50 9.87 9.42 6.16 17.08 b c 0 b d 0 b e 3 10.84 10.84 10.48 11.21 b f 5 9.48 9.60 9.08 9.76 c a 72 9.84 9.47 5.80 15.85 c b 26 10.25 10.07 6.54 16.10 c c 4 8.22 7.76 6.85 10.50 c d 6 8.82 9.48 6.03 11.27 c e 14 8.69 8.20 5.78 15.13 c f 18 9.81 8.86 7.34 13.88 d a 59 10.36 11.12 1.45 16.19 d b 17 7.52 7.92 4.41 9.73 d c 5 10.65 10.80 6.72 14.30 d d 13 7.93 7.52 4.30 13.86 d e 47 4.55 4.40 0.34 14.13 d f 36 6.77 7.08 1.53 10.53 table 6. young’s moduli from tensile test for combining positions of failure cracks and corresponding failure modes. sulted in similar mean values, namely 10.07 gpa and 9.55 gpa. the remaining modes (’c’ and ’d’) are represented by small data sets, so further conclusion could not be drawn. 5. discussion the results obtained from small-scale tensile tests suggest that the wood variability and the effect of growth discontinuities are probably more significant than the effect of elevated temperature. this is evident when comparing the values of young’s modulus obtained from all samples, recall table 2, and those pertinent to samples free of defects summarized in tables 4 and 6 also identifying the effect of various failure modes. however, we should still keep in mind that producing the specimens requires rubbing down the surface layer. this is because the outline of the beam’s cross-section is rather wavy, see figure 4, and the specimen surface needs to be flat. thus the parts of wood, most affected by elevated temperatures, were essentially removed. nevertheless, a certain amount of browning was observed on 43 specimens (table 2, which indicates temperatures of at least 221◦c (for a light brown) [5]. only four samples evinced dark brown color evaluated as 0.8 (level of browning). not a single black sample with browning level 1 was found. a) sample 4 b) sample 6 c) sample 8 (42.15 min) (31.40 min) (22.90 min) figure 4. temperature distribution [◦c] within cross-section at the end of the fire test (the last measured value before fluctuations occurred is presented in pictures in parentheses). figure 4 shows residual cross-sections in the midspan of the beams with temperatures measured by thermocouples at the end of the fire test. the numbers in parentheses denote the last measured values before fluctuations occurred on temperature curves. the temperatures close to the cross-section outline are relatively low, which also suggests a rather smaller thickness of the zero-strength layer, reminding a rapid decrease of the modulus of elasticity above 220◦c 45 l. kucíková, m. šejnoha, t. janda et al. acta polytechnica ctu proceedings [4, 5]. however, the values on the edges of the crosssection are below a widely accepted temperatures of 300◦c [1] or 288◦c (550◦f) [9, 10], which defines the base of the char layer (i.e. pyrolysis temperature). this could mean either position error within the picture or that the charring starts at lower temperatures, where e.g. lange et al. [3] defined the char front already at 270◦c. nonetheless, the results of the pilodyn indentation in table 1 shows a certain decrease of the longitudinal moduli. even if the presented equations (2) and (3) were not applicable to wood loaded by fire, the indentation depths after the fire test are still higher than that obtained before the test. therefore, an assumption of the softening of surface layers of charred beams is reasonable. 6. conclusions the tensile test was performed on small dog bone samples, produced from five beams sustaining the fire load and two beams without thermal degradation, to obtain the longitudinal young’s moduli. the samples were extracted across the beam width from two locations (k) and (s) samples. the specimens were also categorized into groups according to a defined failure mode, figure 2, and the failure position, figure 1(b), and they were also rated considering the level of browning (0–without browning, 1–black). the resulting young’s moduli are stated in table 5 considering all samples as well samples upon elimination of those with the failure crack located in or along the knot or other discontinuity and those occurring in or along the grip area. despite elimination, no correlation with position within the beam width has been found. the browned samples showed only slightly lower values than the not browned specimens, recall table 2. in addition, the control samples not exposed to elevated temperatures evinced the lowest values of young’s moduli. on the contrary, larger decrease of the moduli was observed by pilodyn 6j indentation. the differences between values measured before and after the fire test, see table 1, lead to an assumption that at least the properties of the surface layers are reduced. but keep in mind that eqs. (2) and (3) were originally derived for members not exposed to elevated temperature and their applicability could be considered questionable. nevertheless, the increased indentation depth still shows some level of softening of the surface. the largest reduction of the measured young’s moduli was observed for failure modes where the failure crack propagated across the knot (mode ’e’), see tables 4 and 6, and figure 3. therefore, the presence of knots affects the resulting material properties more than the elevated temperatures. less pronounced effect was also observed for modes ’f’ (along knot) and ’d’ (discontinuity). on the other hand, no correlation with the position within the dog bone sample was found as evident from tables 3 and 6. the results obtained from the small-scale tensile tests suggest that the wood variability and the effect of growth discontinuities are probably more significant than the effect of elevated temperature. to support this conclusion, further study is currently under way exploring samples from the second series of the fire tests. list of symbols e young’s modulus [gpa] d pilodyn 6j indentation depth [mm] k samples from edges (cross-section height point of view) s samples from middle (cross-section height point of view) c control samples a − d failure position (figure 1) a − f modes of failure (figure 2) acknowledgements the financial support provided by the czech technical university in prague within sgs project with the application registered under the no. sgs20/155/ohk1/3t/11 and by the gačr grant no. 18-05791s is gratefully acknowledged. references [1] european committee for standardization. eurocode 5: design of timber structures part 1-2: general structural fire design, 2004. [2] j. schmid, a. just, m. klippel, m. fragiacomo. the reduced cross-section method for evaluation of the fire resistance of timber members: discussion and determination of the zero-strength layer. fire technology 51(6):1285–1309, 2015. doi:10.1007/s10694-014-0421-6. [3] d. lange, l. boström, j. schmid, j. albrektsson. the influence of parametric fire scenarios on structural timber performance and reliability. tech. rep., sp technical research institute of sweden, 2014. [4] b. esteves, h. pereira. wood modification by heat treatment: a review. bioresources 4(1):370–404, 2008. [5] d. kačíková, f. kačík, i. čabalová, j. ďurkovič. effects of thermal treatment on chemical, mechanical and colour traits in norway spruce wood. bioresource technology 144:669–674, 2013. doi:10.1016/j.biortech.2013.06.110. [6] s. yildiz, e. d. gezer, u. c. yildiz. mechanical and chemical behavior of spruce wood modified by heat. building and environment 41(12):1762–1766, 2006. doi:10.1016/j.buildenv.2005.07.017. [7] l. kucíková, t. janda, m. šejnoha, j. sýkora. experimental investigation of fire resistance of glt beams. international journal of computational methods and experimental measurements 8(2):99–110, 2020. doi:10.2495/cmem-v8-n2-99-110. 46 https://doi.org/10.1007/s10694-014-0421-6 https://doi.org/10.1016/j.biortech.2013.06.110 https://doi.org/10.1016/j.buildenv.2005.07.017 https://doi.org/10.2495/cmem-v8-n2-99-110 vol. 30/2021 impact of elevated temperatures on glt beams [8] m. šejnoha, t. janda, l. melzerová, et al. modeling glulams in linear range with parameters updated using bayesian inference. engineering structures 138:293– 307, 2017. doi:10.1016/j.engstruct.2017.02.021. [9] american wood council. calculating the fire resistance of wood members and assemblies, technical report no. 10, 2020. [10] r. h. white, e. v. nordheim. charring rate of wood for astm e 119 exposure. fire technology 28(1):5–30, 1992. doi:10.1007/bf01858049. 47 https://doi.org/10.1016/j.engstruct.2017.02.021 https://doi.org/10.1007/bf01858049 acta polytechnica ctu proceedings doi:10.14311/app.2018.15.0020 acta polytechnica ctu proceedings 15:20–24, 2018 © czech technical university in prague, 2018 available online at http://ojs.cvut.cz/ojs/index.php/app cement matrix containing micronized recycled concrete and the influence of additives jakub ďurejea,∗, zdeněk prošeka,b, jaromír hrůzaa, pavel tesáreka a ctu in prague, faculty of civil engineering, thakurova 7, 166 29 prague 6, czech republic b ctu in prague, uceeb, trinecka 1024, 273 43 bustehrad, czech republic ∗ corresponding author: jakub.dureje@fsv.cvut.cz abstract. the article deals with the selection of a suitable additives in cement composite material for the production of a masonry block. the blocks contains recycled concrete which replacing 40% cement. the recyclate comes from concrete drainage gutters. based on previous measurements, two hardening accelerators and one superplasticizer were selected. to determine the effect of these additives on the recycled cement composite, pairs of samples sets were produced, of which one set contains concrete recyclate and the other one does not contain it. the dynamic modulus of elasticity and the dynamic shear modulus were measured by the resonance method at the age of samples 1, 7, 14, 21 and 28 days, and by an ultrasound method at 28 days of age. then was measured compressive strength. all values were compared with the values measured on the reference samples. keywords: recycled concrete, micro-milled concrete, influence of additives, resonance method, dynamic modulus of elasticity, dynamic shear modulus. 1. introduction concrete waste ,which comes from demolition of buildings, can be recycled but there can be problem with different properties and inhomogeneity of this waste due to the presence of other materials (bricks, wood, glass, reinforcement, etc.). the quality of the recycled concrete depends on the care of sorting this old waste. unfortunately, waste sorting is negatively reflected in the price of this raw material. recycled concrete is most commonly used as the underlayer under the road. usually is used fraction of recyclate greater than 1 mm, the use of finer fractions brings many complications and additional financial costs [1, 2]. the article deals with the measurement of the dynamic modulus of elasticity and compressive strength of samples containing cement, various additives and concrete recyclate. recycled concrete was used, including the finest 0-1mm fractions, which were then finely grinded using a highspeed milling process. the use of this fine fraction, which has almost no use today, is an advantage of the material. these fine fractions of concrete recyclates are often stored in landfills, and their use can reduce the volume of stored waste materials which is also environmental friendly. finely ground recycled concrete was used in cement composites as a filler with certain bonding properties. from previous experiments, it has been shown that with the increasing percentage of recyclate in the cementitious composite, compressive strength decreases [3, 4]. in order to reach a certain compressive strength, various additives to concrete has been used which cause an increase the initial and final compressive strengths. the examined material is planned to be used for the production of concrete blocks for the construction of single-storey buildings in the middle east. concrete blocks could then be lightened to achieve thermal insulation properties and then used as a perimeter masonry. 2. materials and specimens a total of 8 sets of samples were produced, each set containing 6 specimens. the first half of the samples were produced with no recycled concrete in the second half was replaced 40 % of cement by recycled concrete. the two sets were reference (with and without recyclate), the other sets contained additives. for the production of samples was used cem i 42.5 r portland cement radotin from českomoravsk y cement. recycled concrete comes from drainage gutter. a fraction 0/16 mm was used for high-speed milling process. the resulting grain size was <0.25 mm. the fineness of the ready-to-use recycled concrete approached the fineness of the cement with a surface area 361 m2. based on previous measurements, suitable additives were selected. superplasticizing additive (p3) based on modified polycarboxylates which is especially designed for concrete with requirements for high short and final compressive strengths. the accelerator (u1) which does not impair the workability of the fresh mixture, slightly reduces the amount of mixing water. it increases the initial and final strength of the concrete. the (u2) accelerator is composed of soluble mineral salts and organic compounds. combination of this substances has effect which accelerating hardening of concrete (table 1). the water ratio (weight of water divided by mass of cement and recyclate) w / (c + r) was determined on the basis of the flow expansion test after 10 and 20 impulses to achieve approximately equal expansion of the fresh concrete 20 http://dx.doi.org/10.14311/app.2018.15.0020 http://ojs.cvut.cz/ojs/index.php/app vol. 15/2018 cement matrix containing micronized recycled concrete... mixture for all mixtures. the water ratio ranged from 0.21 to 0.31, more mixed water was used for samples containing recycled concrete to achieve approximately the same workability (table 2). laboratory molds measuring approximately 40 × 40 × 160 mm were used to make samples. test specimens were removed from the molds after 24 hours and stored under water in a laboratory environment at a temperature of 21 ± 2 ◦c and a relative humidity of 50 ± 5% for 28 days. 3. experimental methods for all samples, the dynamic modulus of elasticity at the age of 1, 7, 14, 21 and 28 days was continuously measured. a resonance method was used to measure the dynamic modulus of elasticity of samples. advantage of this method is a speed of measurement and repeatability because it is non-destructive method. the method is based on the calculation of dynamic modulus of elasticity and dynamic shear modulus based on measured fundamental resonance frequencies of longitudinal, transverse (bending) and torsional oscillation. the resonance frequencies were measured in the laboratory using the brüel & kjær measuring system, consisting of the brüel & kjær type 4519-003 response sensor, the brüel & kjær type 8206 impact hammer, the brüel & kjær fron-end 3560b-120 measuring center and the control notebook. the sample was placed on supports of sufficiently low stiffness so that the supports were placed at the nodal points of the first waveform corresponding to the given oscillation longitudinal, transverse or rotational (fig. 1). the response sensor (s) was attached to the sample to the appropriate position and the oscillation was excited by the impact hammer (b). both signals, response and excitation were recorded and converted to a frequency domain using a fast fourier transform. subsequently, the pulse labshop software version 14.0.1 has calculated the transmission function. based on the resonance peaks in the transmission function, the fundamental resonance frequency of the given oscillation was determined. from the measured resonance frequency, weight, and sample dimensions, the dynamic modulus of elasticity and shear modulus were determined [5]. the dynamic modulus of elasticity was controlly measured by the ultrasonic pulse method at the age of 28 days. the method is based on the principle of measuring the velocity of ultrasound pulse in the material under investigation. ultrasound pulse spreads through each material at a different speed, it depends on its composition and its properties. implus is induced by an electroacoustic exciter attached on the surface of the sample to be tested. when the impulse passes through a path of known length (sample length), the vibration pulse is converted to an electrical signal by the sensor, and the electronic time circuit allows to measure the pulse passage time [6]. measurement was carried out by attaching sensors to the opposite sides of the sample, and then measuring the time of ultrasound wave spreading through the length of the sample. the sonic vita gel was used to achieve perfect contact between the specimen and the ultrasonic sensor. to calculate the dynamic modulus of elasticity by the ultrasonic method, the following equation was used: eult = ρ. l2 t2 . 1 k2 .10−9 (1) eult the modulus of elasticity measured by the ultrasonic method [gpa], ρ is the bulk density of the measured sample [kg·m−3], l is the length of the measured sample [m], t is the time of ultrasound wave spreading through length of the sample [s], k is the coefficient of the dimensionality [-]. subsequent destructive tests were performed on the samples. compressive strength was tested in an ev 40 hydraulic press with a maximum force of 400 kn according to en 12390-3 testing of hardened concrete part 3: compressive strength of specimens [7]. the hydraulic press spread pressure over the area of the 40 × 40 mm steel plates to the test sample until it was crushed. load speed was 0.3 mm/s. 4. results and discussions dynamic modulus of elasticity of each set are mean from all of 6 samples in the set. control measurements and weighing have been performed on samples before every measuring by resonance method. the evaluation was performed on the basis of measured frequencies from longitudinal oscillation (figure 2). to verify the accuracy of the results, the values of the dynamic modulus of elasticity from the transverse oscillations, which are almost identical to the values of the longitudinal oscillation modules, are also evaluated, which confirms the accuracy of the measurement. from the results of the dynamic modulus edyn, it is clear that the maximum increase in edyn is during the first 7 days. between 7 and 28 days there are no significant changes to edyn. the highest values of edyn reach samples with superplasticizer p3, in comparison with samples without recyclate (cp3 23.0 gpa) and in comparison with samples with recyclate (crp3 19.0 gpa). the difference between the cp3 and crp3 sets is approximately 4.0 gpa. the drop in values of about 4 gpa is noticeable for all pairs of ingredients with a given additive and without or with recycled concrete. we can declare that 40 wt. % of recycled material causes the dynamic elastic modulus to drop by about 4 gpa in 28 days due to less matrix coherence. unsatisfactory values reached samples with hardening accelerators (cu1, cu2, cru1, cru2) whose edyn values are only slightly higher or even lower than the edyn reference sets (c, cr). this additives cause initial increases in strength, therefore at the age of 7 days, the samples with recycled material reached higher values than the reference mixture. however, a subsequent slower increase in the modulus 21 j. ďureje, z. prošek, j. hrůza, p. tesárek acta polytechnica ctu proceedings designation product based on normal dosing density ph [wt. % of cement] [kg/m3] [-] p3 modified polycarboxylates 1.0 1060 ± 10 6.5 ± 2.0 u1 unchlorinated liquid 1.0 − 3.0 1450 ± 30 7.5 ± 2.0 u2 soluble mineral salts and 1.5 1300 ± 10 9.5 ± 1.0 organic substances table 1. selected additives and its properties. micronized set/ material cement concrete recyclate water ratio additive [g] [g] [-] [wt. % of cem. + recycl.] c 1000 0 0.29 0.0 cp3 1000 0 0.21 1.0 cu1 1000 0 0.29 1.5 cu2 1000 0 0.29 1.5 cr 600 400 0.31 0.0 crp3 600 400 0.25 1.0 cru1 600 400 0.30 1.5 cru2 600 400 0.30 1.5 table 2. composition of the samples. of elasticity of mixtures with hardening accelerators resulted in some cases (cu1, cru1) that modulus not reaching the value of reference (c, cr). the dynamic shear modulus is also affected by replacing cement with recycled concrete. in each of the two groups (with and without recyclate), samples with the superplasticizer p3 reached highest values of the gdyn (cp3 9.3 gpa, crp3 7.7 gpa). provable jump for all mixtures (figure 3) can be seen between the first day and 7 days of age when strength raises fastest in the all samples. with increasing time from 7 days onwards, the shear modulus increase is only gradual, practically negligible. negative effects on the dynamic shear module are mainly u1 and u2 solidification accelerators, especially for fine-grained recycled concrete mixtures (cru1, cru2), which achieve lower elastic moduli than reference samples. the compressive strength test was also carried out in the age of 28 days. for this test were used halfs of the beams from previous bending tests. total of 11 samples (half beams) were tested from each set. the resulting values are therefore an arithmetic mean of 11 values. the results of the test show higher values of compressive strength for mixtures without recycled concrete versus mixtures with recyclate. the greatest difference in strength between samples with and without recycled concrete is in the pair of mixtures with accelerators u1 and u2. significant reductions in the strength of cru1 and cru2 samples may be due to the adverse effects of accelerators on recycled concrete (figure 4). 5. conclusion the article deals with the selection of the most suitable additive for the production of concrete blocks containing micronized concrete recyclate. based on previous measurements, two solidification accelerators (u1 and u2) and one superplasticizer (p3) were selected [8, 9]. for comparison and identification of the influence of recycled concrete, pairs of sample sets were produced, which differed only by replacing part of the cement with micronized recycled concrete. based on results, it can be concluded that: • the highest increase in the dynamic modulus of elasticity of mixtures occurs in the first 7 days of hardening. most then within 24 hours. • 40 wt. % of recycled concrete will cause a dynamic elastic modulus drop in 28 days for all 24 samples with recycled concrete of about 4 gpa. • adding accelerators to the mixture cause slight increase of water ratio. • crp3 has proven to be the best in compression strength tests. it is the strongest tested composite with recyclate. • the u1 accelerator proved to be totally inappropriate, the value of the dynamic modulus edyn is even lower than reference. • the proposed modified crp3 mixture can be used to make lightweight concrete blocks, which can be used to masonry the perimeter walls of the singlestorey building. • the proposed modified resultant matrix (crp3 22 vol. 15/2018 cement matrix containing micronized recycled concrete... figure 1. measurement basic natural frequencies from longitudinal (left), transverse (center), and torsional (right) oscillations. figure 2. increase the dynamic modulus of elasticity over time. figure 3. increase the dynamic shear modulus over time. mixture) can be lightened because of sufficient compressive strength, eg. by foaming. list of symbols eult modulus of elasticity measured by the ultrasonic method [gpa] ρ bulk density of the measured sample [kg m−3] l length of the measured sample [m] t time of ultrasound wave spreading through length of the sample [s] re coefficient of the dimensionality 23 j. ďureje, z. prošek, j. hrůza, p. tesárek acta polytechnica ctu proceedings figure 4. compressive strength. acknowledgements this outcome was supported by the czech technical university in prague under no. sgs16/201/ohk1/3t/11, by the czech science foundation research projects gacr 17-06771s and by the ministry of education, youth and sports within national sustainability programme i, project no. lo1605. the authors also thank the center for nanotechnology in civil engineering at the faculty of civil engineering, the joint laboratory of polymer nanofiber technologies of the institute of physics, academy of science of czech republic, and the faculty of civil engineering, czech technical university in prague. references [1] a. barbudo, f. agrela, j. ayuso, et al. statistical analysis of recycled aggregates derived from different sources for sub-base applications. construction and building materials 28(1):129–138, 2012. [2] i. vegas, j. a. ibáñez, a. lisbona, et al. pre-normative research on the use of mixed recycled aggregates in unbound road sections. construction and building materials 25(5):2674–2682, 2011. [3] j. topič, z. prošek, t. plachý. influence of increasing amount of recycled concrete powder on mechanical properties of cement paste. in iop conference series: materials science and engineering, vol. 236, p. 012094. iop publishing, 2017. [4] j. hrůza. design of masonry elements with use of recycled concrete, 2016. [5] čsn 73 1372 nedestruktivní zkoušení betonu–rezonačnní metoda zkoušení betonu, 2012. [6] p. dvorský. diplomová práce využití ultrazvukové impulsové metody nástroj pro stanovení pevnosti cementů 2012. [7] čsn, en 12390-3–zkoušení ztvrdlého betonu–část 3: pevnost v tlaku zkušebních těles, 2009. [8] e. anastasiou, k. g. filikas, m. stefanidou. utilization of fine recycled aggregates in concrete with fly ash and steel slag. construction and building materials 50:154–161, 2014. [9] f. cartuxo, j. de brito, l. evangelista, et al. rheological behaviour of concrete made with fine recycled concrete aggregates–influence of the superplasticizer. construction and building materials 89:36–47, 2015. 24 acta polytechnica ctu proceedings 15:20–24, 2018 1 introduction 2 materials and specimens 3 experimental methods 4 results and discussions 5 conclusion list of symbols acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2018.17.0020 acta polytechnica ctu proceedings 17:20–23, 2018 © czech technical university in prague, 2018 available online at http://ojs.cvut.cz/ojs/index.php/app fitexc – diffraction profile fitting program run in ms excel petr vertata,b,∗, jan drahokoupila,c a department of solid state engineering, fnspe ctu in prague, trojanova 13, 120 00, prague 2, czech republic b department of magnetic measurements and materials, institute of physics as cr, na slovance 2, 182 21, prague 8, czech republic c department of material analysis, institute of physics as cr, na slovance 2, 182 21, prague 8, czech republic ∗ corresponding author: vertat@fzu.cz abstract. we present our custom made diffraction fitting program fitexc for the basic treatment of the 1d diffraction profiles, in particular, the intensity(2θ) of x-ray, synchrotron or neutron diffraction measurements. in the current state, the program allows for simultaneous fitting of up to three profiles in one data set and is intended to be used mainly for precise lattice parameters determination or fitting of overlapped peaks. the entire program is built in the common ms excel environment, it does not require any installation and is free to use. keywords: x–ray diffraction, diffraction profile fitting, ms excel. 1. introduction x-ray crystallography is a powerful tool with many applications: it is often used to determine the lattice parameters, the complete structure of solids, to examine the phase composition, residual stresses or texture. for multiple applications the precise examination of measured 1d intensity(2θ) diffraction profiles represents a crucial part of data processing. there are various programs for detailed diffraction profile fitting available. many of them offer complex solutions implementing the rietveld refinement process [1] (such as x’pert highscore plus [2], topas [3] or fullprof [4]). on the other hand, there are many applications, where quick and precise fitting of few diffraction peaks and their closest neighbourhood is sufficient. even in this field, a few programs are used, such as fityk [5]. unfortunately, the majority of the programs for x-ray diffraction (xrd) data analysis does not, to our knowledge and experience, allow the user to fully control or customize all the aspects of the fitting in an easily accessible way, nor are they userfriendly or quick enough for easier applications and batch processing. although the simple fitting of a few diffraction maxima might also be done manually (for example using the origin software [6]), the approach is often clumsy and require wider knowledge of such environment. we developed a simple user-friendly diffraction profile fitting program fitexc that does not suffer from mentioned disadvantages, yet provides precise results for basic applications. the program, first introduced in [7], is based on the microsoft excel environment, which is immediately available on the majority of computers today. excel’s interface together with visual basic for applications-based macros create a lightweight but powerful tool for the diffraction peak fitting. another advantage is the immediate possibility to implement user’s own scripts for various kinds of analyses such as macroscopic residual stress determination or the analysis of the crystallite size and strain using the williamson-hall plot. 2. program interface and fitting functions in the attempt to make fitexc as user-friendly as possible, the clickable interface with buttons, drop-down lists and check-boxes was created over the hidden worksheets used for the calculations. actions are macro-powered and the overall approach is straightforward. first, user is asked to select the file to be loaded. panalytical .xrdml files are supported as well as standard .xy, .xye or .txt files (multiple columns in the ordinary textfile). next, the fitting function and the radiation spectrum can be specified. standard cauchy, gauss, pearson vii and pseudo-voigt functions are implemented, as well as their split variants for dealing with possible asymmetry. as for the radiation, user may choose from different x-ray tube presets or define his own spectrum containing up to 7 components with desired wavelength, relative intensity and width (figure 1) [8]. therefore, the program might be used not only for xrd, but also for neutron or synchrotron diffraction data fitting. on the main fitting page, where the measured data and fitted function are plotted (figure 2), the entire data processing is controlled. in the current state, up to three profiles might be fitted for one loaded dataset, in the future, the ability to use more than three profiles is to be added. each of the profiles is defined by 20 http://dx.doi.org/10.14311/app.2018.17.0020 http://ojs.cvut.cz/ojs/index.php/app vol. 17/2018 fitexc – diffraction profile fitting program run in ms excel figure 1. program interface: initial settings worksheet shown after the data file is chosen. here the basic settings can be specified, such as the spectrum of the radiation, function for fitting or initialisation of the new peak parameters. function formula (f(2θ)) cauchy a1/(1 +a3(2θ −a2)2) gauss a1exp(−a3(2θ −a2)2) pearson vii a1/(1 +a3(2θ −a2)2)a4 pseudo-voigt a4 gauss + (1 −a4) cauchy table 1. implemented fitting functions with following parameters: intensity at maximum a1, angular position of the maximum a2 , the relative width a3 and the shape parameter a4 [9]. four parameters a1−4, where a1 is the intensity at maximum, a2 the 2θ position of the maximum, a3 is the relative width parameter and a4 is the shape parameter. the simple formulas for such defined functions are denoted in table 1 [9]. in the case of asymmetric profiles, instead of one a3 and a4 parameter, two sets of parameters (a31, a41 and a32, a42) are used – one pair for the left and one pair for the right side of the peak. for background fitting, the constant, linear and quadratic forms were implemented so far. the program also calculates the integral intensity of the fitted reflections using analytical formulas where possible and numerical approach for the pearson vii based functions. the loaded 2θ might be restricted if needed. as one of the advantages, the user is immediately able to fix some parameters that will not be changed during fitting. this makes the program very effective for fitting of complex patterns and multiple overlapped peaks. fitting algorithm is based on general reduced gradient non-linear least squares method. it is implemented using the solver (solver.xlam) add-in and the standard worksheet environment of ms excel run in the background. to evaluate the goodness of the fit the standard discrepancy factors rp , rw p , rexp and χ2 are calculated and their values might be compared with those before fit. 3. additional functionality for processing of multiple data, our program allows loading of multiple files. these are then fitted automatically one after another. in case that .xrdml file containing multiple scans is loaded, the batch feature is enabled automatically providing the tool to fit the datasets sequentially. this feature has already been used for determination of the temperature development of selected lattice parameters of magnetic shape memory (msm) alloys [10]. as our program enables determination of very small changes of the lattice parameters, their temperature evolution is clearly observable even in narrow temperature intervals (figure 3). fitting algorithm is stable enough even for fitting of multiple peaks with great intensity differences 21 petr vertat, jan drahokoupil acta polytechnica ctu proceedings figure 2. program interface: the main worksheet for data fitting. user is able to start the fitting, preset or check the fitted parameters, fix them if needed and to see the measured and fitted data including the difference curve. figure 3. application example: precisely determined lattice parameters a and b of ni–mn–ga msm alloy and their evolution while heating. the sudden change denotes the structural transformation [10]. (so far even with two orders difference the results were reliable). to review a situation, the user may immediately choose between the linear and logarithmic scale of the main graph by clicking a single button. once the good fit is obtained, all the results including the basic settings might be stored in the easily accessible results worksheet. the procedure for easy recovery of the parameters of the fit to the fitting worksheet is also implemented. 4. conclusions we developed a new simple diffraction profile fitting program fitexc. this lightweight and user-friendly program can be run on any pc withms excel (2003 or later) installed. it is intended mainly for the precise fitting of the single reflections (especially their 2θ positions) based on selected or user-defined spectra of the radiation source and for fitting of overlapped peaks. this allows the precise determination of the lattice parameters, observation of their evolution or study of the residual stresses. the program is free to use. to get your own copy, please visit http://people.fjfi.cvut.cz/vertapet/. acknowledgements this work was supported by the grant agency of the czech technical university in prague, grant no. sgs16/245/ohk4/3t/14, and czech science foundation, grants no. 16-00043s and 17-00062s. authors would like to thank o. heczko for useful remarks and motivation the tremendous amount of tasks requiring the development of our program :-) . references [1] h.m. rietveld: a profile refinement method for nuclear and magnetic structures. j. appl. cryst. 2 65–71, 1969. doi: 10.1107/s0021889869006558 [2] t. degen, m. sadki, e. bron, u. koenig, g. nenert: the highscore suite. powder diffraction 29 (supplement s2):s13–s18, 2014. doi: 10.1017/s0885715614000840 [3] diffrac.suite topas (bruker, karlsruhe, germany), https://www.bruker.com/products/x-raydiffraction-and-elemental-analysis/x-raydiffraction/xrd-software/topas.html 22 vol. 17/2018 fitexc – diffraction profile fitting program run in ms excel [4] j. rodriguez–carjaval: recent advances in magnetic structure determination by neutron powder diffraction. physica b 192 (1–2):55–69, 1993. doi: 10.1016/0921-4526(93)90108-i [5] m. wojdyr: fityk: a general-purpose peak fitting program. j. appl. cryst. 43 1126–1128, 2010. doi: 10.1107/s0021889810030499 [6] origin (originlab, northampton, ma). [7] p. vertat: diffraction profile fitting in ms excel. 5. studentska vedecka konference fyziky pevnych latek, nove hrady, ceske vysoke uceni technicke v praze, 2015. [8] v. k. pecharsky, p .y. zavalij: fundamentals of powder diffraction and structural characterization of materials, kluwer academic publishers: boston/dordrecht/london , 2nd ed., p. 115, 2009. [9] r. kuzel: informace v praskovem difraktogramu a jeho zpracovani. materials structure 10 (1a) 18–19, 2003. [10] l. straka, j. drahokoupil, p. vertat, j. kopecek, m. zeleny, h. seiner, o. heczko: orthorhombic intermediate phase originating from {110} nanotwinning in ni 50.0 mn 28.7 ga 21.3 modulated martensite. acta mater. 132 335–344, 2017. doi: 10.1016/j.actamat.2017.04.048 23 acta polytechnica ctu proceedings 17:20–23, 2018 1 introduction 2 program interface and fitting functions 3 additional functionality 4 conclusions acknowledgements references doi:10.14311/app.2021.30.0018 acta polytechnica ctu proceedings 30:18–23, 2021 © czech technical university in prague, 2021 available online at http://ojs.cvut.cz/ojs/index.php/app finite element implementation of geometrically nonlinear contact ondřej faltus∗, martin horák czech technical university in prague, faculty of civil engineering, department of mechanics, thákurova 7, 166 29 prague 6, czech republic ∗ corresponding author: ondrej.faltus@fsv.cvut.cz abstract. the oofem finite element software has been recently updated to include contact algorithms for small strain applications. in this work, we attempt to extend the contact algorithms to large strain problems. reviewing the current code and comparing it with approaches encountered in literature, we arrive at a specific algorithmic solution and integrate it into the current code base. the current code is explained, the necessary extensions are derived and documented, and the algorithmic changes are described. tests confirm the functionality and quadratic rate of convergence of the proposed implementation. keywords: contact, large strains, node-to-segment, oofem, penalty method. 1. introduction having originated with heinrich hertz in 1881 [1], contact mechanics has been a fringe discipline of structural mechanics for most of the 20th century, owing mainly to its limited options for analytical analysis. only with the advent of numerical methods, such as the finite element method (fem), has development in this field sprung forward [2]. this surge in new advances is only exacerbated by the ever more rapid evolution of computational hardware capabilities experienced today; ever more complicated contact problems necessitating ever more complicated methods previously unimaginable due to hardware constraints are coming into focus [2, 3]. the tool used for the purposes of this article is the finite element computational software oofem [4]. oofem is a free, open-source c++ code developed mainly at the czech technical university in prague. it is based on object-oriented design with emphasis on variability and extendability. currently, oofem’s finite element code supports a number of modules for solution of various types of numerical problems. of interest for our purposes is the sm module intended for structural mechanics. basic introduction of contact algorithms into the oofem software has been achieved in [5]. this implementation, however, has certain limitations, among them chiefly the lack of support for large strain, i.e., geometrically non-linear problems. in this work, we strive to remove this particular limitation and demonstrate a simple method of extending existing basic penalty-based formulation to handle large strain problems. 2. algorithm 2.1. basic equation without loss of generality, let us consider two deformable bodies bγ , γ = 1, 2. moreover, let ωγ 0 and ωγ represent the regions occupied by the body in the reference and deformed configuration, respectively. position of a point in the reference configuration is denoted by xγ , and a position xγ of a point in the deformed configuration is obtained be means of the one-to one deformation mapping ϕγ as xγ = ϕγ(xγ) (1) the basic equations for each body under consideration are the following: • kinematic equations f γ(xγ) = ∂ϕγ(xγ) ∂xγ ∀xγ ∈ ωγ (2) • equilibrium equations divp γ(xγ) + b̄γ(xγ) = 0 ∀xγ ∈ ωγ (3) • dirichlet boundary conditions uγ(xγ) = uγ(xγ) ∀xγ ∈ γγ u (4) • neumann boundary conditions t γ(xγ) = p γ(xγ) · n̄γ(xγ) ∀xγ ∈ γγ σ (5) • constitutive equation p γ(f ) = λγtr(eγ)f γ + 2µγf γ · eγ (6) here, we use the so-called saint-venant kirchhoff material model. note that the contact implementation is not affected by the choice of constitutive law and extension to inelastic law is straightforward. 18 http://dx.doi.org/10.14311/app.2021.30.0018 http://ojs.cvut.cz/ojs/index.php/app vol. 30/2021 fem implementation of geometrically nonlinear contact in the equations above, f is the deformation gradient, e is the green-lagrangian strain tensor, div is divergence operator in the reference configuration, p is the first piola-kirchhoff stress, b̄ are prescribed body forces in the reference configuration, u are prescribed displacements, n̄ is a unit normal vector in the reference configuration, γu and γσ are the dirichlet and neumann boundaries which are mutually disjoint, i.e., γγ u ∩ γγ σ = ∅, and γγ u ∪ γγ σ = γγ , λγ and µγ are lame’s parameters. moreover, contact conditions introduce essentially a new set of boundary conditions into the task. note that only normal contact is considered in what follows. the core issue of contact problems can be described by means of the contact constraints in the form of karush-kuhn-tucker optimality conditions [6]: tc ≤ 0 gc ≥ 0 gctc = 0 (7) the first condition represents the fact that contact forces tc are only compressive, the second condition is characterized by a gap function g(u) and express a non-penetration condition. the last equation is simply complementarity between two distinct and mutually exclusive states for the system; either contact occurs, and therefore the gap function gc is zero, while a non-zero compressive contact force is present, or the contact force is zero and the gap function is larger than zero. 2.2. penalty method due to the constraint conditions in form of ineqaulities, see (7), weak form of the above introduced formulation is represented by a variational inequality, see [2]. several methods how to handle the contact constraints are possible, e.g., the lagrangian multiplier method, the augmented lagrangian method, or the nitsche method [3, 6]. currently in oofem, the lagrangian multiplier method and the penalty method are implemented [5]. it is the penalty method which we have selected for the purpose of large strain extension, as its formulation is more straightforward and the necessary code complexity is notably lower. for the penalty method, the weak form can be written as ! γ " ωγ δf γ : p γ − δuγ · b̄γ dv − − " γγ σ δuγ · t̄γ dγ + w c = 0 (8) where δ represents variation and w c is the penaltybased contact contribution along contact interface γc defined as w c(u) = " γc 1 2h(−g(u))pg2 c (u) dγ (9) where h is the heaviside function and p is the penalty parameter. the main disadvantages of the penalty method is the that the achieved solution is imprecise (it would only be precise for p → ∞), but moreover, any improvement in precision acts directly against the stability of the computation, as larger penalty parameters introduce larger members into computational stiffness matrices, which disturbs most numerical solvers [2]. those disadvantages are balanced by the relatively simple formulation of the method and, in comparison with the lagrangian multiplier method, by simple resulting equation systems, as no new variables are introduced into the computation. 2.3. node-to-segment discretization an implementation of contact problems into the framework of the finite element method necessitates selecting a distinctive approach at contact discretization. as the physical world is described by fem in the form of finite elements and connecting nodes, contact can only be considered between those. typical contact discretizations include nodeto-node, node-to-segment, segment-to-segment and other approaches [3]. at current time, node-to-node and node-to-segment discretizations are implemented in oofem [5]. we are only extending the node-tosegment discretization for large strains, as the nodeto-node discretization is by definition inadequate for this purpose. in node-to-segment discretization, a single node comes into contact with a segment. various objects may be understood under the term "segment", most prominently a set of element boundaries (element edges in 2d domains or element surfaces in 3d domains) or an arbitrarily defined rigid analytical surface. contact detection in node-to-segment discretization necessitates determining three things [3, 6]: (1.) the gap function gc, essentially a projection of the node onto the contact segment, which determines whether there is contact or not; analogical to the penetration function in the analytical approach to contact problems. (2.) the normal vector in the deformed configuration n, which determines the orientation of the contact surface at the contact point. obtained for most segments in oofem as vector perpendicular to the contact segment’s surface at the contact point [5]. (3.) the extended n-matrix n∗, which ensures allocation of the external forces and stiffness to the proper degrees of freedom in the fem formulation. when the segment in question is an element boundary, the matrix takes the form [5] n∗ = # i(d×d) −ne(xc) $ (10) where i(d×d) is a square unit matrix of the appropriate dimension, which corresponds to degrees of freedom of the contacting node, and ne(xc) is the 19 ondřej faltus, martin horák acta polytechnica ctu proceedings matrix of element boundary interpolation functions evaluated at the contact point. 2.4. fem formulation to integrate contact constraints into the fem framework, considerations have to be made for the contact forces and energy discussed above within the principles of fem discretization. for the penalty method, contact energy is expressed as w c = 1 2pg2 c (11) note that the energy contact contribution above is calculated only in the contact point, which can be interpreted as lobatto integration of (9), see, e.g., [6] for more details. variation of this part of energy yields δw c = pgcδgc (12) it is worth noting that the term pg(u) represents the contact force. variation of the gap function can be further expressed as δgc = % n∗t n ||n|| & δdc (13) where δdc are the variational nodal displacements of relevant fem nodes and n is the normal vector in deformed configuration. the external forces of contact are therefore fc = −pgc % n∗t n ||n|| & (14) to obtain the associated tangential stiffness matrix. the internal forces have to be subjected to differentiation with respect to nodal displacements dc [5]. for small strains, it is generally assumed that the direction of contact does not change throughout the analysis, and therefore n remains independent of dc [5, 6]. kc = − ∂fc ∂dc = p % n∗t n ||n|| &t % n∗t n ||n|| & (15) 2.5. extension to large strain problems now we stand before a question: which of the previously described considerations change if we consider large strains? the mechanics of introducing contact constraints to fem remain the same, as well as the definition of contact energy. two critical differences appear. firstly, it can no longer be assumed that contact follows the same direction throughout the whole computation. the normal vector has to be reassessed after each computational step and also the stiffness matrix has to reflect this. secondly, large deformations result in change of contact point on the segment, which is reflected in the evaluation of matrix n∗. the normal vector is deformation dependent and can be expressed geometrically [2] n = t1 × t2 (16) where × is the vector cross product and ti are tangential vectors of the element boundary in the deformed configuration (in the case of an element edge, i.e., contact formulation in 2d, one is the tangent and the other is the e3, that is, a unit vector perpendicular to the plane). for the new expression of stiffness, we elect to utilize a formula presented in [2]. specifically for the case of node-to-2d-segment penalty method, it reads k = p # nvnt v − gc l # bvt t v + tvbt v + gc l bvbt v $$ (17) with nv = n∗t n ||n|| bv = b∗t n ||n|| tv = n∗t t (18) where b∗ is the extended b-matrix. note that the first member of the stiffness matrix corresponds with the previously derived term for the small strain case. for a linear edge in 2d, the extended matrices are defined as n∗ = ' −1 0 ξ − 1 0 −ξ 0 0 −1 0 ξ − 1 0 −ξ ( (19) b∗ = −∂n∗ ∂ξ = ' 0 0 −1 0 1 0 0 0 0 −1 0 1 ( (20) where ξ is the parametric coordinate on the element edge. 3. implementation 3.1. class structure the oofem code has a strictly object-oriented structure [4]. the integration of contact into this structure has been mainly accomplished in [5] and only slight additions were necessary for the large strain extension. figure 1 displays the relevant parts of oofem class structure together with the classes used for contact. the general idea of class structure centers around two main classes: elementedgecontactsegment and node2segmentpenaltycontact. elementedgecontactsegment is derived from the general contactsegment class and represents element edges in 2d. when initialized from an oofem input file, the class receives a set of element boundaries. internal processes ensure that contact is considered always with the edge which is closest to the contacting node [5]. node2segmentpenaltycontact is a subclass of activeboundarycondition, an oofem feature 20 vol. 30/2021 fem implementation of geometrically nonlinear contact figure 1. a diagram of oofem class structure, focused on contact-relevant parts, created with the help of [7]. which allows boundary conditions to contribute by themselves to the internal forces, external forces, and stiffness matrix. the utilization of this feature avoids creating any form of a "contact element", otherwise prevalent among fem contact implementations [3]. the contact boundary condition is likewise initialized from the oofem input file with information on the size of the penalty parameter, a set of nodes and a set of one or more contact segments. any node is always checked for contact with any segment [5]. 3.2. operations the assembly of external forces and stiffness matrix takes place in the node2segmentpenaltycontact class, specifically in its overridden methods assemblevector() and assemble(). for the determination of gap gc, the contact segment method computepenetration() is invoked, which computes it from a purely geometrical consideration, considering the node to have penetrated the segment when its deformed coordinates are on a different side of the segment than the undeformed ones. the contact segment is also responsible for the computation of the tangent and normal vectors. the tangent vector is determined as t = ∂x ∂ξ = −b∗ segxn = (21) = ' −1 0 1 0 0 −1 0 1 ( ) **+ **, x1 y1 x2 y2 **. **/ = 0 x2 − x1 y2 − y1 1 where b∗ seg is the segment part of the extended bmatrix and xi, yi coordinates of the segment nodes. meanwhile, the normal vector is determined as n = e3 × t (22) for large strains and from the element::edgeevalnormal() function for small strains (which returns the undeformed normal vector). neither of those vectors is normalized. the normalization of the normal vector happens in the contact condition, when the nv, bv and tv matrices are determined according to (18). in the assembly of stiffness, the contact conditions asks the contact segment whether the large strain part shall be assembled. this is accomplished by the contactsegment::hasnonlineargeometry() function and based on the setting of the element in contact. 4. demonstration to demonstrate the functionality of our approach, we select a simple computational example in a 2d plane strain idealization. the mesh used can be seen in figure 2a. a double-clamped beam is positioned above a cantilever. the length of the beam is 5 m and length of the cantilever is 3 m. both are 0.25 m high with a gap of 0.25 m between them. the top of the beam is loaded in every step by an uniform distributed load of 8000 kn/m, causing vertical deformation and thus 21 ondřej faltus, martin horák acta polytechnica ctu proceedings (a) . undeformed mesh. (b) . deformed state and vertical component of stress in step 4 (initiation of contact between beam and cantilever). (c) . deformed state and vertical component of stress in step 200 (end of analysis). figure 2. a computational example demonstrating the code and its convergence. iteration force err. x disp. err. x force err. y disp. err. y 1 5.074e-05 0.000e+00 5.450e-03 0.000e+00 2 7.311e-03 8.044e-02 1.583e-01 4.515e-02 3 1.879e-04 2.920e-03 4.120e-03 2.014e-03 4 1.465e-04 2.844e-03 3.202e-03 1.062e-03 5 2.041e-07 5.071e-04 1.488e-06 1.809e-04 6 1.418e-12 1.863e-06 3.298e-11 7.549e-07 table 1. evolution of global force and displacement errors in iterations of loading step 14. subsequently contact between the two bodies. loading is performed in 200 uniform loading steps. both beam and cantilever are modelled by rectangular plane strain elements (oofem element quad1planestrain). in total, the task mesh consists of 805 elements. refinement of the mesh has been accomplished with the help of a custom script in the matlab software [8]. all elements are made of the same saint-venant kirchhoff material with the parameters λ = 100 mpa and µ = 150 mpa. contact conditions are introduced between the lower edge of the beam and the upper edge of the cantilever below. on the latter interface, two boundary conditions are defined, one pairing the beam’s nodes with the cantilever’s element edges and the other vice versa. this ensures that no clipping caused by crude or otherwise imperfect meshing can take place. figure 2b shows the deformed state of the mesh and the vertical component of the stress tensor in step 4 of 200, when the beam and the cantilever first come into contact. likewise, figure 2c shows the same in step 200 at the end of the analysis. to render those 22 vol. 30/2021 fem implementation of geometrically nonlinear contact figure 3. evolution of error during the equilibrium iteration process. figures, the paraview open-source software was used [9]. values of stress in the visualizations are given in pa. note that between steps 4 and 200, the contact surface moves significantly to the left both on the beam and the cantilever. this was accomplished by alternating activation of the two opposite boundary conditions on the interface, which managed to subsequently introduce contact between neighboring nodesegment pairs, while the contact gap between the original pairs opened up again. the main objective of this test was to prove quadratic convergence of the new approach to large strain contact stiffness matrix. the vast majority of the 200 steps computed managed to converge in 6 iterations or less. the only exception is step 4, in which the contact occurs for the first time. table 1 shows error evolution in a typical computational step (step 14 was chosen). moreover, the dependence of error (norm of residual forces) on the number of iterations during the equilibrium newton-raphson process for steps 4, 14, and 164 is visualized in figure 3. thanks to the semilogarithmic scale used, it is apparent that approximate quadratic convergence has been achieved. 5. conclusions the paper describes an implementation of a two dimensional large strain node-to-segment penaltybased approach to contact mechanics into an opensource finite element code oofem. the emphasis is given on proper design of the hierarchy of classes with a focus on modularity and extensibility towards different methods for handling contact constraints, contact with friction, or three dimensional formulation. the functionality of the presented approach is successfully verified on a numerical example and a quadratic rate of convergence of the newton-raphson iteration procedure is demonstrated. acknowledgements the authors would like to acknowledge support received for work on this project from a ctu grant no. sgs20/038/ohk1/1t/11. references [1] h. hertz. on the contact of elastic solids. z reine angew mathematik 92:156–171, 1881. [2] p. wriggers. computational contact mechanics. springer, new york, 2nd edn., c2006. [3] v. a. yastrebov. numerical methods in contact mechanics. wiley, hoboken, nj, 2013. [4] b. patzák. oofem home page, 2000. http://www.oofem.org. [5] o. faltus. object-oriented design and implementation of contact mechanics into finite element code oofem. master’s thesis, czech technical university in prague, 2020. [6] a. konyukhov, r. izi. introduction to computational contact mechanics. wiley, chichester, west sussex, 2015. [7] staruml. version 3.2.2. mklabs co.,ltd., seoul, republic of korea, 2020. [8] matlab. version 9.3.0 (r2017b). the mathworks inc., natick, massachusetts, 2017. [9] u. ayachit. the paraview guide: a parallel visualization application. kitware, 1st edn., 2015. 23 acta polytechnica doi:10.14311/app.2020.27.0101 acta polytechnica 27(0):101–106, 2020 © czech technical university in prague, 2020 available online at http://ojs.cvut.cz/ojs/index.php/app micromechanical characterization of cement paste modified by nanoclays vojtěch zacharda∗, jiří němeček czech technical university in prague, faculty of civil engineering, department of mechanics, thákurova 7, 166 29 prague 6, czech republic ∗ corresponding author: vojtech.zacharda@fsv.cvut.cz abstract. this work deals with the comparison of micromechanical properties and microstructure of cement pastes with additives used in concrete to reduce the lateral pressure on the formwork. this work is a steppingstone for broader investigation of the lateral pressures on different cement mixtures. the work focused on two additives used for the purpose, namely calcined clay (metakaolin) and a type of nanoclay sepiolite. a scanning electron microscope was used to describe their microstructure. micromechanical properties of both cement composites were investigated by nanoindentation. large statistical grids of indents were performed on three sample types: plain cement paste and two mixtures containing the enhancing additive of metakaolin and nanoclay. from the evaluated results in the form of property histograms, the modulus of elasticity, hardness and creep parameter were derived. it was found that in the cement paste with metakaolin the amount of c-s-h gel increased compared to the control mixture. increased portlandite and the amount of unhydrous clinker was found in the cement paste with nanoclay. nanomechanical response of individual phases was derived from overall property histograms by statistical deconvolution. the results were confirmed by electron microscopy. the micromechanical research was supplemented with the measurement of the compressive strength on cubes at the macroscopic level. keywords: cement, hydration, metakaolin, nanoclay, nanoindentation. 1. introduction in the construction industry, concretes in their fresh state are cast into formworks. ordinary concretes are vibrated to support their flow and to overcome their high viscosity. the casting process of tall structures is done in steps to allow mixture compaction and stepwise vibration. lateral pressures introduced by the ordinary concretes on formwork diminish relatively soon after casting due to the very high mixture viscosity and prolonged casting times. in contrary, self-compacting concretes (scc) are characterized by low viscosity with no need of additional vibration to fill the formwork [1–4]. scc are usually poured into formwork in one step filling the whole height of the structure (e.g. a column or a wall). the long-lasting mixture flowability is usually maintained by additives (plasticizers) and hydration retarders and even after casting the mixture behaves like a fluid that introduces high lateral pressures on the formwork. however, the development of high lateral pressures in the scc mixtures can be controlled by special additives that control viscosity and workability of the mixtures in the fresh state and simultaneously control development of thixotropic behavior and early strength development of the mixture. such additives can be in the form of hydration modifiers or some nanoparticles [1–6]. so far, the effect of the additives on the microstructure and micromechanical properties of the resulting hardened concrete was not well studied. thus, this work deals with the comparison of micromechanical properties [7–10] and microstructure of cement pastes modified with additives that are used in sccs to reduce the lateral pressure on the formwork [1–6]. there are several ways how to reduce the lateral pressure in the mixture. one of the ways is to change thixotropy of the concrete by non-reactive additives. it was found that the thixotropy can be increased and the pressure can be reduced by nanoclay minerals or derived materials [1–6]. the other way of reducing pressures is to modify hydration kinetics and hydration products by additives such as metakaolin. metakaolin reacts with dissolved clinker minerals in cement paste and forms calcium-silica hydrates (c-sh) [6]. 2. tested materials cement pastes modified by two additives, metakaolin and nanoclay, were tested in hardened state in this work. firstly, metakaolin which is a calcined clay with the chemical composition: 53.1% sio2, 41.7% al2o3, 1.1% fe2o3 and other minor oxides, was used. it is prepared as a granulated powder with a similar grain distribution as cement. metakaolin particles have angular shape with mean size which can range from 1 to 10 µm (figure 1). metakaolin reacts with cement during hydration. it reduces the amount of portlandite in the hydrated cement and increases the main hydration product that is calcium-silica-hydrate gel (c-s-h) [6]. the nanoclay sepiolite was used for 101 http://dx.doi.org/10.14311/app.2020.27.0101 http://ojs.cvut.cz/ojs/index.php/app vojtěch zacharda, jiří němeček acta polytechnica sample cement [g] metakaolin [g] sepiolite [g] water [g] c 450 − − 202.5 cm 445.5 4.5 − 202.5 cs 445.5 − 4.5 202.5 table 1. composition of cement pastes. figure 1. sem image of metakaolin. the second part of the samples. its chemical formula is mg4si6o15(oh)2·6h2o. sepiolite particles have tiny elongated shape (figure 2). they have typical length of 1 to 2 µm and width of several tens of nanometers. sepiolite does not dissolve in water and it does not react with cement, but it works on reorganization of the crystallites in plastic stage of the mixture. it also binds more water upon mixing which lowers workability. the resulting microstructure contains larger amount of unhydrous clinker. the micromechanical properties of the three cement paste mixtures with the composition defined in table 1 were tested. all mixtures are based on portland cement cem i 42,5r and the additives described earlier. equal water to binder ratio 0,45 was used for all samples. the amount of additives was chosen as 1% of weight of cement. samples were cast into cylindrical plastic molds with height of 65 mm and diameter of 30 mm. they were demolded after three days from casting and stored in water for additional 25 days. after hardening, the samples were cut into discs and placed in an acetone bath to prevent further hydration. the surface of samples were grinded and polished by a metallographic procedure and prepared for testing by nanoindentation. along, six cubic specimens (40x40x40 mm) were made for compressive strength testing. figure 2. sem image of sepiolite. 3. methods 3.1. nanoindentation the main objective of the paper was to compare nanomechanical properties of the cement paste modified by metakaolin or sepiolite with pure cement paste. therefore, series of 400 indents were done on each sample. each consisted of a matrix of 20x20 indents. the separation of indents was 10 µm. a load-controlled test with the load function lasting for 26 seconds to the maximum force of 2000 µn was prescribed for each indent. the function had a trapezoidal shape (figure 3). the first part of the function was a linear loading with speed of 40 mn/min lasting for 3 seconds. the second part was a constant loading for 20 seconds and the last part of the function was a linear unloading with speed of 40 mn/min lasting for 3 seconds. a typical response (load-depth) for various phases in the material is shown in figure 5. material properties were evaluated for each indent by the oliver and pharr method [11]. some defective indentations caused by local porosity or roughness were eliminated from considerations. the oliver and pharr [11] method was used for obtaining reduced modulus, er and hardness, h for single indents and property histograms were constructed for each matrix. the evaluation was complemented by creep indentation parameter cit . the parameters are defined as 102 vol. 27/2020 cement paste modified by nanoclays figure 3. load function of indents. figure 4. topography of the surface. follows. er = s √ π 2β √ ac (1) where s is the unloading tangential stiffness, β is a tip shape correction coefficient and ac is the contact area, h = pmax ac (2) where pmax is the maximum indentation force, cit(p,t1,t2) = h2 − h1 h1 × 100 (3) which is defined as a relative change between indentation depths h1 encountered at the time t1 and h2 at the time t2, respectively. the cit is dependent on the contact force p and time of holding period. note, that the reduced modulus can easily be used to calculate young’s modulus of isotropic materials [11] knowing poisson’s ratio of the material (ν=0.2 was assumed for all phases in this work). figure 5. indentation curves. figure 6. bse images (phase composition). 3.2. microstructural analysis the basic microstructural analysis was performed by scanning electron microscope (sem). back scattered electrons (bse) images together with energy dispersive x-ray spectroscopy (eds) analysis are very effective way for exploring the phase of cement paste at the level of micrometers. topography of the surface (figure 4), bse images (phase composition) (figure 6) and eds maps of cement pastes were acquired. the distinction between the microstructural phases could be done from these images but it was not attempted in this work that concentrated on statistical evaluation of micromechanical results as shown later. 3.3. macroscopic level the paper was supplemented by the compressive strength measurements performed in a standard electro-mechanical press on 40x40x40 mm cubes (figure 7). 103 vojtěch zacharda, jiří němeček acta polytechnica figure 7. test of cement paste cube for compressive strength. 4. results and discussion 4.1. modulus of elasticity figure 8 shows histograms of modulus of elasticity for mixtures of cement pastes. values were calculated from merged results of nanoindentation. the histograms show a similar trend for all mixtures. the main peak in cm sample appears between 11 to 51 gpa, between 16 to 66 gpa on cs mixture and 13 to 61 gpa on c sample, respectively, with the mean of 25 gpa for cs and c and 22 gpa for cm (figure 8). the results show that the wrapping curve of elastic modulus for cm has shifted to the left compared to the control mixture. this confirms the assumption that the amount of portlandite in the hydrated cement is reduced and the calcium-silica-hydrate gel (c-s-h) content increases. the results of cs mixture show similar values compared to the control mixture c. 4.2. hardness figure 9 shows histograms of hardness of cement pastes. values were calculated from merged results of nanoindentation. similarly, to results of e, the histograms show similar trend for all mixtures. the main peak of hardness measured on cm and c is around 1.0 gpa. in cs mixture the hardness has the main peak around 0.9 gpa, thus similar. it can be concluded that the results are qualitatively as well as quantitatively equivalent. 4.3. creep indentation parameter cit figure 10 shows histograms of cit of cement pastes. values were calculated from merged results of nanoindentation. again, similar trend for all mixtures was encountered. the highest mean cit value is exhibited by the mixture with metakaolin suggesting the role of increased c-s-h content on increased creep. however, the differences between the samples are not large and the mixtures can be considered as equivalently creeping. 4.4. compressive strength the results of compressive strength were measured on the six samples cubes for each mixture. average value was calculated and was supplemented by standard deviation. the average compressive strength of control mixture (c) was 65,9 ± 5,7 mpa. for mixture with sepiolite (cs) the average compressive strength was 63,6 ± 1,1 mpa. the average compressive strength of mixture with metakaolin (cm) was 63,1 ± 3,4 mpa. it can be concluded that the average compressive strength of mixtures with additives were at about 95% of the control mixture. thus, the additives of metakaolin and sepiolite do not have considerable effect on macromechanical properties. 4.5. eds analysis on the samples eds analysis map were made (figure 11). the results are shown in table 2. comparison of results of mixture from eds analysis shows small change in volume concentration of chemical phases except carbon that is largely present in cs samples. this is a consequence of organic pollution of the raw sepiolite used for sample preparation. element c cm cs w. c. [%] w. c. [%] w. c.[%] o 72.63 68.74 59.17 ca 15.89 20.21 17.78 si 5.98 7.07 6.24 al 3.28 2.11 1.82 mg 1.4 0.89 0.53 s 0.82 0.97 0.48 c 12.95 table 2. weight concentrations of elements. 5. conclusions the presented paper provides nanomechanical and microstructural results of cement pastes with special additives in the form of nanoclays and calcined clay minerals used for the control of lateral pressures in fresh mixtures. nanoindentation was used to obtain microscale hardness, elastic and creep properties. values of young’s moduli, hardness and creep indentation parameter, cit , were calculated from large statistical sets of nanoindentation. the results of elastic moduli indicate a statistically significant shift towards lower values on cm samples. this is a consequence of the hydration process where the amount of portlandite in the hydrated cement is reduced and the main hydration product, the calcium-silica-hydrate gel increases. the sepiolite in cs samples does not 104 vol. 27/2020 cement paste modified by nanoclays figure 8. histogram of modulus of elasticity with trend line for cement pastes cs, cm and c. figure 9. histogram of hardness with trend line for cement pastes cs, cm and c. figure 10. histogram of cit with trend line for cement pastes cs, cm and c. 105 vojtěch zacharda, jiří němeček acta polytechnica figure 11. maps of chemical elements from eds analysis: a) c, b) cm and c) cs. appear to statistically influence elastic modulus on microscale. the hardness and creep were also found statistically equivalent on all samples. the average compressive strength of modified samples was found to be 95% of the control mixture meaning very little macroscopic influence of additives was observed. eds analysis shows small change in concentrations of chemical elements except the presence of carbon in cs samples which is a consequence of the raw material organic pollution. the work described in this paper confirmed mechanical compatibility of the additives with nil or very little influence on micro and macroscopic mechanical properties of the hardened samples. the work is a steppingstone for future lateral pressure investigations. acknowledgements this work was performed under the support of technological agency of the czech republic (trend fw01010521) and czech technical university in prague (project sgs20/107/ohk1/2t/11). references [1] j. h. kim, n. noemi, s. p. shah. effect of powder materials on the rheology and formwork pressure of self-consolidating concrete. cement and concrete composites 34(6):746 – 753, 2012. doi:10.1016/j.cemconcomp.2012.02.016. [2] n. roussel. a thixotropy model for fresh fluid concretes: theory, validation and applications. cement and concrete research 36(10):1797 – 1806, 2006. doi:10.1016/j.cemconres.2006.05.025. [3] s. kawashima, j. h. kim, d. j. corr, s. p. shah. study of the mechanisms underlying the fresh-state response of cementitious materials modified with nanoclays. construction and building materials 36:749 – 757, 2012. doi:10.1016/j.conbuildmat.2012.06.057. [4] j. h. kim, m. beacraft, s. p. shah. effect of mineral admixtures on formwork pressure of self-consolidating concrete. cement and concrete composites 32(9):665 – 671, 2010. doi:10.1016/j.cemconcomp.2010.07.018. [5] j. p. melo, a. s. aguilar, f. h. olivares. rheological properties of aerated cement pastes with fly ash, metakaolin and sepiolite additions. construction and building materials 65:566 – 573, 2014. doi:10.1016/j.conbuildmat.2014.05.034. [6] r. siddique, j. klaus. influence of metakaolin on the properties of mortar and concrete: a review. applied clay science 43(3):392 – 400, 2009. doi:10.1016/j.clay.2008.11.007. [7] v. nezerka, j. nemecek, z. slizkova, p. tesarek. investigation of crushed brick-matrix interface in limebased ancient mortar by microscopy and nanoindentation. cement and concrete composites 55:122 – 128, 2015. doi:10.1016/j.cemconcomp.2014.07.023. [8] j. nemecek, v. kralik, v. smilauer, et al. tensile strength of hydrated cement paste phases assessed by micro-bending tests and nanoindentation. cement and concrete composites 73:164 – 173, 2016. doi:10.1016/j.cemconcomp.2016.07.010. [9] s. diamond, j. huang. the itz in concrete a different view based on image analysis and sem observations. cement and concrete composites 23(2):179 – 188, 2001. special theme issue on image analysis, doi:10.1016/s0958-9465(00)00065-2. [10] j. nemecek, v. kralik, j. vondrejc. micromechanical analysis of heterogeneous structural materials. cement and concrete composites 36:85 – 92, 2013. special issue: nanotechnology in construction, doi:10.1016/j.cemconcomp.2012.06.015. [11] w. c. oliver, g. m. pharr. an improved technique for determining hardness and elastic modulus using load and displacement sensing indentation experiments. journal of materials research 7(6):1564–1583, 1992. doi:10.1557/jmr.1992.1564. 106 https://doi.org/10.1016/j.cemconcomp.2012.02.016 https://doi.org/10.1016/j.cemconres.2006.05.025 https://doi.org/10.1016/j.conbuildmat.2012.06.057 https://doi.org/10.1016/j.cemconcomp.2010.07.018 https://doi.org/10.1016/j.conbuildmat.2014.05.034 https://doi.org/10.1016/j.clay.2008.11.007 https://doi.org/10.1016/j.cemconcomp.2014.07.023 https://doi.org/10.1016/j.cemconcomp.2016.07.010 https://doi.org/10.1016/s0958-9465(00)00065-2 https://doi.org/10.1016/j.cemconcomp.2012.06.015 https://doi.org/10.1557/jmr.1992.1564 acta polytechnica 27(0):101–106, 2020 1 introduction 2 tested materials 3 methods 3.1 nanoindentation 3.2 microstructural analysis 3.3 macroscopic level 4 results and discussion 4.1 modulus of elasticity 4.2 hardness 4.3 creep indentation parameter cit 4.4 compressive strength 4.5 eds analysis 5 conclusions acknowledgements references doi:10.14311/app.2021.30.0058 acta polytechnica ctu proceedings 30:58–62, 2021 © czech technical university in prague, 2021 available online at http://ojs.cvut.cz/ojs/index.php/app numerical simulation of slump flow test of cement paste composites jiří němeček∗, jiří němeček czech technical university in prague, faculty of civil engineering, department of mechanics, thákurova 7, 166 29 prague 6, czech republic ∗ corresponding author: jiri.nemecek.1@fsv.cvut.cz (ph.d. candidate) abstract. this study presents a numerical simulation of the mini-slump flow test performed on the haegerman table, according to en 1015-3. the two-dimensional axisymmetrical volume of fluid (vof) model is used for the calculation. various scenarios with the different model set up and boundary conditions were calculated to show how the results are affected. simulations with different lifting velocities were carried out and compared to the case with instantaneous demolding, which is commonly used in numerical simulations. also, the effect on results is shown between noslip and specific shear boundary conditions on the mold. the results of simulations indicate that mold lifting should be considered in numerical modeling even if calculation time is highly prolonged. lifting velocities should not exceed the value of 0.2 m/s in order to maintain laminar flow and stable calculation. keywords: cement paste, cfd modeling, rheology, slump test, volume of fluid. 1. introduction concrete is a mixture of cement, fine, coarse aggregate, admixtures, and water. after mixing initial components with water, a fluid suitable for filling formwork is created. each mixture has different rheological properties depending on its composition, which could be characterized by various experimental tests such as slump test, l-box, v-funnel tests, viscosimeter, and vibration testing [1]. the most commonly used is the cone slump testing for its feasibility and reproducibility on in-situ and ex-situ conditions. the test is conducted with a metallic cone opened at both ends, which is placed on a metallic/wooden/glass table. the fresh mixture is filled inside the cone, and then the metallic cone is lifted in a vertical direction. when the initial shear stress of the mixture is overcome, the concrete flows by gravity [2]. the test is commonly simulated by numerical modeling using computational fluid dynamics (cfd) approach. most of the studies modeled the slump-test with instantaneous demolding [2–4]. however, such a boundary condition (b.c.) is leading to a higher slump spread. this behavior is then artificially correlated by changing constants of the material model. gao investigated the effect of a cylindrical mold lifting on the slump test of kaoline clay [5]. the results suggest the mold lifting velocity has a significant influence on the final slump spread. in this study, the setting up of a numerical model of the mini-cone slump test is shown. the effect of different boundary conditions on the final slump spread 1ph. d. candidate at ctu in prague 2associate professor at ctu in prague is investigated. 2. numerical simulation 2.1. volume of fluid model the eulerian multiphase model is the most complex model which can calculate dispersed-continuous phase interaction and continuous-continuous phase interaction. typically dispersed micrometer or millimeter size particles could be solid particles, liquids droplets, or bubbles of gas which are dissolved in a continuous phase. continuous-continuous phase interaction is represented by two immiscible phases, e.g., concrete and air. the eulerian model is solving continuity and momentum equations individually for each phase. the simplified version of the eulerian model used for continuous-continuous phase interaction is the volume of fluid (vof) model. the vof model is solving only one continuity equation and one momentum equation and tracks each phase’s volume fraction [6, 7]. the continuity equation could be simplified as ▽ · !v = 0, (1) where where !v is the velocity and time field. a single momentum equation is written as ∂ ∂t (ρ!v) + ▽ · (ρ!v!v) = − ▽ p + ▽ · ! η " ▽!v + ▽!vt #$ + ρ!g + !f , (2) where p is the static pressure, g is the gravitational acceleration, !f is the force resulting from surface tension at the interface, and ρ is the fluid density and η is the apparent viscosity. 58 http://dx.doi.org/10.14311/app.2021.30.0058 http://ojs.cvut.cz/ojs/index.php/app vol. 30/2021 slump flow test numerical simulation prog��zdoo����uljlg�erg\�prwlrq�� yhorflw\� ����������p�v d��qr�vols�e�f� e��vshfl¿f�vkhdu�e�f� d[lv�ri�v\pphwu\ fhphqw�s���� ghiruplqj�phvk dlu���ghiruplqj�phvk )j ��pp�jds e� d[lv�ri�v\pphwu\ fhphqw�sdvwh��� vwdwlrqdu\�phvk dlu���vwdwlrqdu\�phvk )j d� dlu�fhphqw�s��� lqwhuidfh dlu�fhphqw�s��� lqwhuidfh �vwdwlrqdu\�zdoo��qr�vols�e�f�� �vwdwlrqdu\�zdoo��qr�vols�e�f�� figure 1. schematic set up of mini-slump (symmetric part of the cone is shown) test with shown boundary conditions a) without mold b) with mold. in vof model volume fraction of phase in an individual cell is set to either 0 (none phase is present) or 1 (phase is fully present). in the sharp interface region, the value in the cell could range from 0 to 1. in order to preserve a sharp interface, one of few discretization schemes has to be used. the more details about discretization schemes could be found in ansys fluent theory guide [6]. in the current study, explicit scheme formulation with a geometric reconstruction scheme [8] was used to solve the volume fraction equation. 2.2. material model computationally, the concrete can be assumed as non-newtonian fluid. several models, such as bingham or herschel-bulkley, allow to model concrete flow. in both models it is required to overcome the yield stress threshold to initiate flow, the flow of fluid is further prescribed with plastic viscosity. for the bingham plastic model, after the stress is initiated, the shear stress is a linear function of the shear rate, and fluid behaves as newtonian. herchel-bulkley’s model exhibits shear-thinning behavior. in ansys fluent for shear rate γ̇ > critical shear rate γ̇c the viscosity is calculated as η = τ0 γ̇ + kγ̇n−1, (3) where τ0 is the yield stress threshold, k is the consistency index, and n is the power-law index. for bingham model n = 1, for herchel-bulkley: 0 < n < 1. critical shear rate γ̇c is the point beyond which material acts like very viscous fluid [6]. for γ̇ < γ̇c the viscosity is expressed by equation η = τ0(2− γ̇ γ̇c ) γ̇c + kγ̇n−1 c % (2 − n) + (n − 1) γ̇ γ̇c & . (4) 3. numerical modeling the presented numerical model was used to reproduce the slump flow test of mini-cone on the haegerman table described in en 1015-3. the dimensions of the mini-cone are defined as following: upper radius = 70 mm, bottom radius = 100 mm, height = 60 mm. the numerical model was solved as a 2d axisymmetric task. in general, two approaches could be followed to obtain slump spread and height of the mini-cone. in the first approach, the mold is not taken into account in the simulation. the mesh is stationary, and the overall calculation time is short. in the second approach, the mold and its lifting are added to the simulation, which requires dynamically changing mesh. the calculation time is then significantly prolonged. one set of parameters of the herschel-bulkley material model was used in the modeling to show differences in boundary conditions. the set of parameters was chosen for typical cement paste without admixtures as follow: τ0 = 25 pa, k = 10 pa·sn, n = 0.5, density ρ = 1800 kg/m3. critical shear stress was also fixed at a value γ̇c = 0.005 s−1. the air was modeled as newtonian fluid with density = 1.225 kg/m3 and viscosity = 1.7894e-05 kg/m·s. 3.1. slump test without mold the schematic situation of the model without mold is shown in fig. 1a. the domain of 300×100 mm was filled with linear quadrilateral cells. the frequency of cells near the base wall was increased to capture the velocity profile correctly, see fig. 2a. no-slip boundary condition was prescribed on the base wall. the initial trapezoidal shape of cement paste acts in the gravitational field, which is the only external force acting on the mini-slump. 3.2. slump test with mold lifting the scheme of the model with mold is shown in fig. 1b. the mold with thickness of 8 mm was added on the left side mini-cone, and its lifting was modeled 59 jiří němeček, jiří němeček acta polytechnica ctu proceedings ����pp ����pp ���pp ���pp ���pp ���pp ����pp ����pp ���pp d[lv�ri�v\pphwu\ �� �p p �� �� p p d[ lv �r i�v \p p hw u\ �� �p p �� �� p p &���vwdwlrqdu\�phvk�� %���vwdwlrqdu\�phvk�� $���g\qdplf�phvk�� d� e� � vwdwlrqdu\�phvk�� figure 2. mesh used for the slump test modeling a) without mold b) with mold. as rigid body motion in the vertical direction. several cases with various velocities: 0.01 m/s; 0.05 m/s; 0.2 m/s; 2.0 m/s were calculated to show differences in final slump spread. such a problem requires a dynamically changing mesh of the domain. triangular cells were chosen as most suitable for the analysis. in order to save calculation time, only one part of the domain was used for adaptive meshing. thus, the domain 300×200 mm was then divided into three regions. the first part (a) around the mold and cement domain was filled with the small size, 2.5 mm triangular cells. in this part, adaptive changes in the mesh: smoothing and remeshing were applied. the second part (b) is above the rest of the table with 2.5 mm cells . the cells were smoothened only above the table (wall b.c.). the third part (c) was filled with larger 15 mm cells. both the second and third parts of the mesh stayed stationary. the mesh is shown in fig. 2b. in the case of adaptive meshing in ansys fluent, the divergence problems occurred on the interface of mold and table (wall-wall boundary condition) where, after the initial lifting, the mesh is missing under the mold. to overcome such a difficulty 1 mm of mold was cut off in model, which should have minimal effect on the results. the gap was filled with triangular cells and air particles. the mold wallcement interface was modeled with two variants: a) no-slip b.c., b) specific shear b.c. with a value of 0.3 pa. 4. results and discussion fig. 3 shows three variants of the slump spreading results displayed in five different times. the first variant is the slump spread without mold (fig. 3a). after the calculation begins, all the volume of cement paste starts to flow. no obstacle prevents cement from movement, and due to the high inertia effect, the slump spread reaches the highest value (2×99.8 = 199.6 mm). the result with considering mold lifting with a velocity of 0.05 m/s and no-slip b.c. prescribed on the mold and table is shown in fig. 3b. obviously, the mold is partly holding the whole cement volume from motion, which is the factor decreasing the final spread (2×87.3 = 174.6 mm) which is 12.5% decrease in slump spread (199.6 mm) beside no-mold condition. due to no-slip b.c., part of the cement paste is sticked to the mold, which is another factor reducing the final spread. the third case (fig. 3c) shows mold lifting (v = 0.05 m/s) with shear b.c. prescribed on the mold, the chosen value 0.3 pa represent almost no shear between mold-cement interface. in this case, the cement is not sticking to the mold, which functions only as an obstacle. the overall result of slump spread (2×92.7 = 185.4 mm) is 60 vol. 30/2021 slump flow test numerical simulation ���pp w� ������v ���pp dlu fhphqw�s�� w� ��������vw� �����v ���pp ���pp w� ��������vw� �������v ���pp w� ��������vw� ������v w� ��������v ���pp w� ������v w� ������v dlu fhphqw�s�� ���pp d��qr�prog e��prog�oliwlqj� ������p�v��qr�vols�e�f� w� ������v dlu fhphqw�s�� ���pp f��prog�oliwlqj� ������p�v��vshfl¿f�vkhdu�e�f� w� ��������v ���pp ���pp w� ��������v �����pp ���pp �����pp ���pp ���pp ���pp w� ��������v ¿qdo�vsuhdg� �������pp ¿qdo�vsuhdg� �������pp w� ��������v ���pp �����pp ¿qdo�vsuhdg� �������pp figure 3. results of calculation of mini-slump test in five different times: a) without mold b) with mold lifting (v = 0.05 m/s) and no-slip b.c. c) with mold lifting (v = 0.05 m/s) and specific shear b.c. on the mold (0.3 pa). higher than in the previous case with no-slip b.c. but lower than in model without the mold. the decrease in the slump spread is only 7.1% besides the no-mold case. in terms of reproducing the real behavior of the mini-slump test, the case in fig. 3b is most promising. since during the experiment, cement paste is always sticking to the steel mold. applying the real value of friction would be even more suitable. however, establishing the coefficient’s value on a water wet steel-cement paste interface is not a simple task since the amount of water used, and admixtures used differs in each individual experiment. the results of the final spread and slump are summarized in tab. 1 and shown in fig. 4. it is apparent that lifting velocity affects overall results. basispread slump increase of [mm] [mm] spread v = 0.01 m/s 171.8 23.4 +0.0% v = 0.05 m/s 174.6 22.1 +1.6% v = 0.20 m/s 176.4 22.2 +2.7% v = 2.00 m/s – – – no-mold 199.6 17.2 +16.2% table 1. results of spread and slump calculated with different mold lifting velocities and no-slip b.c. cally, for lower lifting velocity (v = 0.01–0.20 m/s), the spread is increased when the lifting velocity is in61 jiří němeček, jiří němeček acta polytechnica ctu proceedings no mold v = 0.01 m/s v = 0.05 m/s v = 0.20 m/s 0255075100125 spread [mm] s lu m p [m m ] 10 20 30 40 50 0 figure 4. results of spread and slump calculated with different mold lifting velocities. creased. however, the difference between the smallest (v = 0.01 m/s) and maximum (v = 0.20 m/s) spread is only 2.7%. which is much lower than the difference 16.3% between mold and no-mold lifting. the high lifting velocities 2 m/s bring the cement paste to motion where other forces than gravitational are acting. the slumping process is not smooth, and the results are most probably incorrect due to the turbulent flow and calculation instability. similar findings were found in gao’s study, where different geometry and material were used [5]. the calculation time necessary for the model without a mold and stationary mesh is about 5 minutes. the time needed for calculation with dynamic mesh and mold lifting is 3-7 hours depending on the b.c. the total calculation time could be reduced using variable time step where the smaller time step is necessary at the beginning of the calculation and could be further increased during the calculation. 5. conclusions the present work provides a general study of setting up a numerical model of slump flow test and shows how different boundary conditions could affect the results. the calculation was done on typical cement paste using only one set of parameters of the herchelbulkley model to neglect different mixtures’ effect. several conclusions are made. • the consideration of the mold lifting during calculation has a significant effect on results. the final slump spread is increased by 12–16 % when the mold lifting is not used in the calculation. • mold lifting velocity has only a small effect on overall slump spread while slow lifting velocities are used. slow lifting velocities are considered in the range of 0.01–0.20 m/s, which should be typical lifting velocities used in laboratory conditions. the difference in slump spread between minimum and maximum velocity was calculated as 2.7 %. • lifting velocities with higher than 2 m/s bring fluid to turbulent motion, which is not recommended to calculate in ansys fluent with the herchelbulkley model suitable for laminar flow. moreover, these velocities are not commonly used in the experiments. • consideration of mold lifting and dynamic remeshing in the calculation results in increasing calculation time from minutes to hours. acknowledgements financial support of the technological agency of the czech republic (trend fw-01010521) and the grant agency of the czech technical university in prague (sgs20/107/ohk1/2t/11) are gratefully acknowledged. references [1] c. ferraris, f. de larrard, n. martys. fresh concrete rheology: recent developments. materials science of concrete vi, amer cer soc ed s mindess, j skalny pp. 215–241, 2001. [2] a. bouvet, e. ghorbel, r. bennacer. the mini-conical slump flow test: analysis and numerical study. cement and concrete research 40(10):1517 – 1523, 2010. doi:https://doi.org/10.1016/j.cemconres.2010.06.005. [3] f. dufour, g. pijaudier-cabot. numerical modelling of concrete flow: homogeneous approach. international journal for numerical and analytical methods in geomechanics 29(4):395–416, 2005. https:// onlinelibrary.wiley.com/doi/pdf/10.1002/nag.419 doi:10.1002/nag.419. [4] n. roussel, p. coussot. “fifty-cent rheometer” for yield stress measurements: from slump to spreading flow. journal of rheology 49, 2005. doi:10.1122/1.1879041. [5] j. gao, a. fourie. spread is better: an investigation of the mini-slump test. minerals engineering 71:120 – 132, 2015. doi:https://doi.org/10.1016/j.mineng.2014.11.001. [6] i. ansys. ansys fluent theory guide. vol 15317:514, 2011. [7] n. d. katopodes. chapter 12 volume of fluid method. in n. d. katopodes (ed.), free-surface flow, pp. 766 – 802. butterworth-heinemann, 2019. doi:https://doi.org/10.1016/b978-0-12-8154854.00018-8. [8] d. youngs. time-dependent multi-material flow with large fluid distortion, vol. 24, pp. 273–285. 1982. 62 https://doi.org/https://doi.org/10.1016/j.cemconres.2010.06.005 https://onlinelibrary.wiley.com/doi/pdf/10.1002/nag.419 https://doi.org/10.1002/nag.419 https://doi.org/10.1122/1.1879041 https://doi.org/https://doi.org/10.1016/j.mineng.2014.11.001 https://doi.org/https://doi.org/10.1016/b978-0-12-815485-4.00018-8 acta polytechnica ctu proceedings doi:10.14311/app.2017.13.0125 acta polytechnica ctu proceedings 13:125–129, 2017 © czech technical university in prague, 2017 available online at http://ojs.cvut.cz/ojs/index.php/app hydration process and mechanical properties of cement paste with recycled concrete powder and silica sand powder jaroslav topiča,∗, zdeněk prošeka,b a czech technical university in prague, faculty of civil engineering, thákurova 7, 166 29 prague 6, czech republic b university centre for energy efficient buildings, czech technical university in prague, třinecká 1024, 273 43 buštěhrad, czech republic ∗ corresponding author: jaroslav.topic@fsv.cvut.cz abstract. recycled concrete powder (rcp) mostly consisting of cement paste could be reused as partial cement replacement. the aim of this paper is to compare hydration and mechanical properties of rcp and two types of silica sand powder (ssp). comparison of those materials combined with cement can highlight the binder properties of recycled concrete powder. using of two types of ssp also show an influence of their fines on hydration process and mechanical properties. particle size analysis and calorimetric measurement were carried out and mechanical properties such as bulk density, dynamic young’s modulus and compression strength were examine. calorimetric measurement proves the presence of exposed non-hydrated particles in rcp that can react again. however lower density of old cement paste in rcp overweight the mentioned potential of rcp and mechanical properties are decreasing compared with reference cement paste and cement paste ssp. keywords: cement paste, recycled concrete powder, silica sand powder, hydration process, mechanical properties. 1. introduction the great popularity of cement composites such as concrete in the building industry, among other things, also bring difficulties of recycling old materials from demolitions and reconstructions. recycling process of old concrete reach a good effectivity of reusing of it. old concrete is usually used in earth works for backfill, sub-base layers of the roads [1, 2], unpaved road surfaces [3, 4] or concrete production [5–7]. using old concrete such way result to amount of not usable powder. this powder mostly consist of cement paste which could be reused for remaining binding properties of non-hydrated core of cement grains. in previous papers where the recycled concrete powder (rcp) were used as partial cement replacement and micro-filler [8, 9]. the aim of this paper is to compare hydration and mechanical properties of rcp with two inert fillers with similar range of granulometric curve or similar specific surface area. to eliminate influence of larger nucleation area (caused by presence of fine particles in rcp) on hydration process the inert silica sand powder was used to comparison. due to his crystalline character, the pozzolanic activity of sio2 should be minimal compared with amorphous sio2, which is used as reactive additive [10, 11]. comparison of those materials combined with cement can highlight the binder properties of rcp and influence of recycled concrete powder on hydration process can be defined [12, 13]. using of two types of silica sand powder also show an influence of their fines on hydration process and mechanical properties. results of those two types silica sand powder should help to further modification properties of recycled concrete powder. 2. materials and samples all mixtures for sample production composed of the portland cement cem i 42.5 (radotín, czech republic) and different amount of following admixture. samples detonate cem+rcp contained recycled concrete powder (rcp) produced by crushing and further micro-milling of old concrete railway sleepers. samples detonate cem+ssp contained silica sand powder st2 (ssp2) or st6 (ssp2) from company sklopísek střeleč, inc. four different mixtures were used for calorimetric measurement. first mixture (denote as cem) was reference cement past. second mixture (denote as cem+rcp 50) contained 50 % of cement and 50 % of rcp. third mixture (denote as cem+ssp2 50) contained 50 % of cement and 50 % of ssp st2 and fourth mixture (denote as cem+ssp6 50) contained 50 % of cement and 50 % of ssp st6. all samples had same water/mixture ratio equal to 0.45. for this measurement approximately 35 g of the mixture was used and put into plastic container. seven different mixtures were used for evaluation of mechanical properties. composition of the mixtures is shown in table 1. mixtures had different water/mixture ratio from 0.35 to 0.40 to compensate the 125 http://dx.doi.org/10.14311/app.2017.13.0125 http://ojs.cvut.cz/ojs/index.php/app jaroslav topič, zdeněk prošek acta polytechnica ctu proceedings mixture cement [g] recycled concrete silica sand water/mixture (cem i 42.5 r) powder (rcp) [g] powder (ssp) [g] ratio cem (ref) 1500 0.35 cem+rcp 10 1350 150 0.36 cem+rcp 50 750 750 0.40 cem+ssp2 10 1350 150 0.36 cem+ssp2 50 750 750 0.40 cem+ssp6 10 1350 150 0.36 cem+ssp6 50 750 750 0.40 table 1. composition of the mixture. figure 1. the particle size distribution of rcp, ssp st2 and ssp st6. deteriorated workability caused by increasing amount of rcp and ssp. each set contained 6 prismatic samples having dimensions of 40×40×160 mm. the samples were removed from their casts after 2 days. after that the samples were cured for 28 days in water at the temperature of 21 ± 2 ◦c. 3. experimental methods rcp particle size distribution and determination of middle grain size were performed using a fritsch analysette 22 microtec plus laser device. a matest e009 device was used to measure the specific surface area using the blain method. calorimetric measurement was conducted on an isothermal tam air calorimeter for measurement of heat flow. mixtures were tested for 7 days at a constant temperature of 20 ◦c and were stored in sealable plastic containers, each containing from 32 g to 36 g of mixture. based on the weight of each sample (measured before testing) the results of the heat flow were related to 1 g of cement for a better detection of changes caused by admixtures. bulk density of the samples was evaluated from dimension and weight of the samples before resonance testing. dimension and weight of the samples was used as input values for resonance testing. dynamic young’s modulus was monitored by using the non-destructive resonance method. this method is based on measuring the natural frequency of the prismatic 40×40×160 mm samples. for evaluation of dynamic young’s modulus the basic longitudinal and flexural natural frequency, dimension and weight of the samples were used. compressive strength was determined on 28 day old samples using a model fp100 heckert device. the testing was displacement controlled at a constant rate of 0.1 mm/s in the case of three-point bending and 0.3 mm/s during the compression test. a uniaxial compressive test was performed on the broken specimen halves with effective dimensions 40×40× ∼80 mm 4. result and discussion in figure 1 are presented results of granulometric measurement. as can be seen rcp has high amount of very fine particles but also has long range. ssp st2 was chose for comparison with rcp because of his high range of granulometric curve. ssp st6 was chose for comparison with rcp because of his similar fines. specific surface area of cem i 42.5r was equal to 10811 cm2/cm3 and very similar to rcp with value 10996 cm2/cm3. ssp st2 has specific surface area equal to 8676 cm2/cm3 and ssp st6 to 11604 cm2/cm3. results of calorimetric measurement are 126 vol. 13/2017 hydration process and mechanical properties ... figure 2. heat flow of cement paste and cement with 50 wt. % of recycled concrete powder (cem+rcp 50) and silica sand powder (cem+ssp2 50 and cem+ssp6 50). samples bulk density samples bulk density [kg/m3] [kg/m3] cem (reference) 1919 ± 9 cem+rcp 10 1851 ± 1 cem+rcp 50 1566 ± 3 cem+ssp2 10 2026 ± 11 cem+ssp2 50 1964 ± 3 cem+ssp6 10 1906 ± 6 cem+ssp6 50 1722 ± 5 table 2. bulk density of the samples. figure 3. dynamic young’s modulus of cement paste and cement paste with 10 and 50 wt. % of recycled concrete powder (cem+rcp) and silica sand powder st2 and st6 (cem+ssp2 and cem+ssp6) after 28 days. 127 jaroslav topič, zdeněk prošek acta polytechnica ctu proceedings figure 4. compression strength of cement paste and cement paste with 10 and 50 wt. % of recycled concrete powder (cem+rcp) and silica sand powder st2 and st6 (cem+ssp2 and cem+ssp6) after 28 days. shown in figure 2. by relating heat flow to 1 g of cement and not an entire rcp or ssp mixture it was possible to highlight differences in the hydration process caused by rcp. in case of mixtures cem+ssp2 50 and cem+ssp6 50 the increase of heat flow compared with reference cement was caused by presence of ssp which provided larger nucleation area to cement hydration. cem+ssp6 50 had little bit higher increase of heat flow compared with cem+ssp2 50 thanks to his higher specific surface. increase of heat flow of mixture cem+rcp 50 was partly (as in cem+ssp) caused by larger nucleation area, but mostly by presents of non-hydrated cement grains in rcp. increase of heat flow of mixture with rcp compared with mixture cem proves that exposed cement grains in rcp can react again and comparison with cem+mss 50 proves that increase of heat flow wasn’t caused only by larger nucleation area. results of bulk density of the samples are shown in table 2. there is obvious decrease of bulk density of the samples with rcp in both cases compared with reference samples or samples with. the high amount of fine particles and lower density of old cement paste compare with the new one caused decrease of bulk density in case of cem+rcp. samples cem+ssp6 10 exhibit comparable bulk density and cem+ssp6 50 had lower bulk density then reference samples. higher fines also cause the decrease of bulk density in case of samples with ssp st6 but decrease was not so steam because of higher density of ssp compared with rcp. addition of coarser ssp st2 resulted to increase of the bulk density in both concentrations. dynamic young’s modulus of the samples illustrated in figure 3 and compression strength illustrated in figure 4 closely related to bulk density. as can be seen the mechanical properties of samples with rcp are despite the hydration of cement grains in rcp decreased compared with other samples. this decrease was probably caused by lower density of old cement paste in rcp compared with ssp and new hydration products and simultaneously by high amount of very fine particles. dynamic young’s modulus and compressive strength of the samples cem+ssp2 10 increased compared with cement paste due to increased cohesiveness of matrix caused by grains that effectively work as micro filler. based on those two mechanical properties the ssp st6 in samples cem+ssp6 10 has similar micro filler effect but the specific surface area of was too high and contain higher amount of very fine particles that cause small decrease of compression strength. in all samples with 50 wt % of rcp or ssp there was huge decrease of compression strength cause by inappropriate ration of cement and filler. based on results, the potential improvements of mechanical properties could be achieved by using small amount of rcp in cement mortars or concrete. in this combination, the rcp would be fulfills the role of micro filler and partially the role of binder and lower density of rcp would not be obstacle as in case of cement paste. thanks to using two types of ssp with different fines can be assumed that better results would be achieved by using coarser rcp with similar fines as ssp st2. remaining question is if non-hydrated cement grains in old cement paste can exposed on this level and still fulfills the role of partial binder replacement. experiment that answers this question will follows in further research. 5. conclusions this paper is focused on influence of different type of additive and its properties on hydration and mechanical properties of cement paste. in this experiment the rcp was used to fulfills the role of micro filler and partial cement replacement. also two types ssp was used that thanks to its crystalline character works as inert micro filler. based on results the following findings are summarized: • based on heat flow during hydration process, the presence of exposed non-hydrated cement grains 128 vol. 13/2017 hydration process and mechanical properties ... in rcp has been proved and influence of larger nucleation area was also define as minimal thanks to comparison with influence of ssp with different fines. • despite the additional hydration of cement grains in rcp the mechanical properties of cement paste with rcp were negatively influenced by lower density of cement paste in rcp compared with ssp and new hydration products. • based on result two types of ssp the negative influence of high amount of very fine particles was recognized. • the rcp can be used as micro filler and binder replacement only in small amount into cement composites where aggregate is present. this way the influence of lower density of old cement paste can be eliminated. • based on results of samples with ssp can be stated that using rcp with lower amount of very fine particles and lower specific surface area might lead to improvement of mechanical properties of cement paste. but binder properties can be diminished by insufficient exposure of non-hydrated cement particles. acknowledgements the authors wish to express their gratitude for financial support to the faculty of civil engineering, czech technical university (ctu) in prague under sgs project no. sgs16/201/ohk1/3t/11 and no. sgs17/060/ohk1/1t/1 and czech science foundation gačr under project no. 17-06771s. the authors also thanks to the center for nanotechnology in civil engineering at the faculty of civil engineering, ctu in prague. references [1] a. barbudo, f. agrela, j. ayuso, et al. statistical analysis of recycled aggregates derived from different sources for sub-base applications. construction and building materials 28(1):129–138, 2012. [2] c. j. engelsen, g. wibetoe, h. a. van der sloot, et al. field site leaching from recycled concrete aggregates applied as sub-base material in road construction. science of the total environment 427:86–97, 2012. [3] i. vegas, j. ibañez, a. lisbona, et al. pre-normative research on the use of mixed recycled aggregates in unbound road sections. construction and building materials 25(5):2674–2682, 2011. [4] j. r. jimenez, j. ayuso, f. agrela, et al. utilisation of unbound recycled aggregates from selected cdw in unpaved rural roads. resources, conservation and recycling 58:88–97, 2012. [5] r. silva, j. de brito, r. dhir. properties and composition of recycled aggregates from construction and demolition waste suitable for concrete production. construction and building materials 65:201–217, 2014. [6] f. agrela, m. s. de juan, j. ayuso, et al. limiting properties in the characterisation of mixed recycled aggregates for use in the manufacture of concrete. construction and building materials 25(10):3950–3955, 2011. [7] l. evangelista, j. de brito. mechanical behaviour of concrete made with fine recycled concrete aggregates. cement and concrete composites 29(5):397–401, 2007. [8] j. topič, z. prošek. properties and microstructure of cement paste including recycled concrete powder. acta polytechnica 57(1):49–57, 2017. [9] j. topič, z. prošek, t. plachý. influence of increasing amount of recycled concrete powder on mechanical properties of cement paste . [10] p. barnes, j. bensted. structure and performance of cements. crc press, 2002. [11] s. shebl, h. seddeq, h. aglan. effect of micro-silica loading on the mechanical and acoustic properties of cement pastes. construction and building materials 25(10):3903–3908, 2011. [12] s. guettala, b. mezghiche. compressive strength and hydration with age of cement pastes containing dune sand powder. construction and building materials 25(3):1263–1269, 2011. [13] j. šeputytė-jucikė, m. kligys. the effects of modifying additives and chemical admixtures on the properties of porous fresh and hardened cement paste. construction and building materials 127:679–691, 2016. 129 acta polytechnica ctu proceedings 13:126–130, 2017 1 introduction 2 materials and samples 3 experimental methods 4 result and discussion 5 conclusions acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2019.22.0150 acta polytechnica ctu proceedings 22:150–154, 2019 © czech technical university in prague, 2019 available online at http://ojs.cvut.cz/ojs/index.php/app physical-mechanical properties and durability of hydraulic lime-based mortars with non-traditional biopolymers tomáš žižlavský∗, martin vyšvařil, pavla rovnaníková brno university of technology, faculty of civil engineering, veveří 331/95, 602 00 brno, czech republic ∗ corresponding author: zizlavsky.t@fce.vutbr.cz abstract. the study focuses on the influence of addition of four different biopolymers in various doses (0.1%, 0.5%, and 1%) on the physical-mechanical properties and durability of nhl 3.5 mortars prepared with constant water/binder ratio and the binder/aggregate ratio of 1 : 2 by weight. the flow table test was carried out and the specimens were tested for flexural and compressive strength at the age of 7 and 28 days. the durability coefficient for the flexural strength was calculated after 15 freezing-thawing cycles. all the admixtures have dosage-dependent slump reducing effect. the strengths were increased in comparison with mortar with no admixture, the most significantly in the case of highest dose of carrageenan and diutan gum. all biopolymers with the exception of diutan gum increased the durability coefficient with increasing dosage. sodium salt of alginic acid has been found the most effective in the case of durability improvement. keywords: natural hydraulic lime, sodium alginate, carrageenan, diutan gum, xanthan gum, compressive strength, flexural strength, durability. 1. introduction hydraulic lime has been, as the strongest and most versatile binder, used since the ancient times. natural hydraulic lime is prepared by burning limestone with clay impurities on similar temperatures as a pure limestone. during the burning process the clay decomposes creating silicate and aluminate phases in the binder, which reacts with calcium hydroxide creating minerals similar to clinker minerals in cement. the hydraulic lime hardens firstly by hydraulic reactions and afterwards by carbonation of the rest of the ca(oh)2. viscosity enhancing admixtures (veas) are lately widely studied for the use in concrete to improve properties especially of self-consolidating concrete. their use in lime mortars is far less studied, partially due to the amount of production of concrete in comparison with lime for the building purposes. veas are supposed to improve plasticity of the mortars, their fresh state adhesion, and to eliminate the sagging of renders, mostly by their water-retaining function. [1, 2] the previous results declare, that even though most of the veas are polysaccharides, due to the amount needed to improve concrete properties they do not slow the hydratation reactions of clinker minerals in cement, therefore, if the dosage is similar, they should not affect the hydratation reaction of nhl mortar neither. [1, 3] the current study focuses on the impact of addition of non-traditional biopolymerical veas on the hardened properties and durability of natural hydraulic lime-based mortar. the biopolymers used are microbial based (xanthan gum (xg), and diutan gum (dg)) and seaweed based (carrageenan (cg), and sodium salt of alginic acid (algna)). all of these biopolymers are currently used in food industry as gelling and thickening agents, and stabilisers [4–6]. the most well-known in the building industry are the xg and dg, which are partially in use in self consolidating concrete to reduce segregation and bleeding [4, 5, 7]. dg is more flexible in the use in comparison with xg for its properties are independent on monoand bi-valent (e.g. ca2+) cations concentration as well as the temperature [7–12]. the cationic sensitivity lead to significantly different behaviour of these two biopolymers in lime mortars, where dg improved the strength characteristics and xg notably reduce them, especially the compressive one [13]. the xg was chosen for the research on the hydraulic lime properties, even though it didn’t performed well, mainly because it is reported to have positive influence on concrete behaviour, and the nhl solution contains much less ca2+ cations than the lime putty. sodium and calcium salts of alginic acid have surpassed the commercial super absorbent polymers especially in the way of influence on the hydratation of cementbased mortars and their addition suppressed the same problems as dg and xg addition [8, 14]. in the aerial lime-based mortar [13] algna was one of the best performing admixtures, even though its initial strength reduction with growing dose caused by the highest humidity of the specimens. the use of cg has been reported mainly in geopolymers, for the fly-ash based one its addition increased strengths by creating more condensed structure. [15] this result was lately supported by z. abdollahnejad et al. [16] in the study on alkali activated cement based binder 150 http://dx.doi.org/10.14311/app.2019.22.0150 http://ojs.cvut.cz/ojs/index.php/app vol. 22/2019 physical-mechanical properties of hydraulic lime-based mortars mortars with fly-ash and waste glass addition, where cg addition lead to better workability and also to higher compressive strength, while adding the xg had no positive effect on the strength properties. in the lime mortar cg for the initial strength characteristics didn’t performed well, but showed interesting hint of improvement for the long-term properties. [13] 2. materials and methods 2.1. materials for the preparation of mortar samples same materials as for the previous work [13] were used: the nhl 3.5 lime according to en 459-1 has been mixed with the siliceous sand fraction 0–4mm (českomoravský štěrk a.s., hulín, czech republic) in a 1:1 volumetric ratio. then the biopolymer was added in the dose of 0.1%, 0.5% and 1% of the weight of binder. the admixtures used were products of sigma-aldrich co (algna) and cp kelco (kelco-crete dg-f – dg, genuvisco cg-131 – cg, and kelzan ap-as – xg). the dry mixture was introduced into water of constant water : binder ratio of 0.7. the flow value was determined according to en 1015-3. the samples were then cast into 40 × 40 × 160mm prismatic moulds, and demoulded after 48 hours. the beams were stored in the laboratory conditions (20 °c, 50% relative humidity) until the day of testing. 2.2. methods the methodology of the experiments is similar to the previously published results [13]: bulk density of the mortar was determined by weighting and measuring the samples before the strength tests, flexural and compressive strength tests were carried out according to en 1015-11 at 7 and 28 days of age of the mortars. after 28 days of curing time in the laboratory conditions samples underwent the freezing-thawing durability test for 15 cycles according to čsn 72 2452. the reference samples were stored under water to simulate similar conditions as the tested samples. the durability coefficient for flexural strength after 15 cycles has been determined. 3. results and discussion 3.1. flow values addition of any of the admixtures caused workability reduction varying on the dosage and type of the admixture. the results are presented in the figure 1. we can see, that the highest flow value reduction is, as well as for the lime mortar, in the case of dg [13]. the trends are similar to aerial lime-based mortar with exception of xg where the difference is more significant due to the earlier-stated concentration of ca2+ cations in the solution [7, 9–12]. 3.2. bulk density of mortar in the figure 1 there can be seen a decreasing trend in bulk density of the mortar. this is in this case caused by air entrained due to different workability of the mortars, for all of the specimens consisted of the same water : binder : aggregate ratio, the trend corresponds with the flow value reduction one and as has been stated earlier these biopolymers did not show air-entraining function [13]. 3.3. flexural and compressive strength the results obtained by strength tests are presented in figures 2 and 3. the 7 days strength was far less affected, than in the case of lime mortars with different biopolymers [13, 17, 18], but even less affected than the expectations based on the results obtained on cement or nhl mortars found in the literature [14, 19, 20]. all the biopolymers and doses with exception of lowest dosage (0.1%) of polymers for flexural strength surpassed the strength of mortar with none of the biopolymers. for the flexural strength the dg and cg in highest dosage (1%) exceeded significantly all other biopolymers confirming the earlier results on lime mortar [13] and the presumption based on literature research [15, 16]. the xg performed much better then while used in aerial lime-based mortar, but in comparison with other biopolymers, the 28 days results were one of the worst ones. the overall increase of strengths in this experimental set up may be as well interpreted as the impact of dewatering, which can be partially described by the flow value reduction, but the dewatering effect may not cause so enormous difference in strength as in the case of cg and dg [21]. 3.4. durability the results of durability tests carried out after 15 freezing-thawing cycles were summarized in table 1. the durability coefficient for flexural strength was chosen, for it is more representative if the material is used as a plaster. most of the specimens tested survived 15 freezing thawing cycles in the way, they could be tested for flexural and compressive strength. the most damaged were beams prepared from mortar with dg addition of 0.1% and 0.5% (fig. 4) out of the three beams for each mixture only one was compact enough for the three point bending test and even though the test results indicated several damage to the specimens, thus the durability coefficient of dg 1 and dg 5 mortars should be approached with a distance. overall the biopolymer addition lead to improvement of durability as expected based on results found in the literature [3, 17, 18, 22, 23]. the dg addition notably worsens the durability, so it’s even lower than the one of mortar with no admixture. with the growing dosage of biopolymeric admixture the mortar became more durable. the best performing were algna and xg, which even in the lowest dose (0.1%), improved the durability significantly. according to čsn 72 2452 151 t. žižlavský, m. vyšvařil, p. rovnaníková acta polytechnica ctu proceedings figure 1. flow values of fresh mortars and bulk density of 7 days old specimens. figure 2. flexural and compressive strength of mortar specimens 7 days old. figure 3. flexural and compressive strength of mortar specimens 28 days old. r ef a lg n a 1 a lg n a 5 a lg n a 10 d g 1 d g 5 d g 10 x g 1 x g 5 x g 10 c g 1 c g 5 c g 10 durability coefficient [–] 0.19 0.69 1.02 0.95 0.09 0.20 0.58 0.30 0.84 0.97 0.42 0.46 0.82 table 1. durability coefficient of tested mortars. 152 vol. 22/2019 physical-mechanical properties of hydraulic lime-based mortars figure 4. dg 1 and dg 5 (three of each from left to right) specimens after durability test. the specimens with the coefficient of 0.75 and higher are considered to be frost-resistant for the tested count of cycles, thus the algna 5 and 10, xg 5 and 10 and cg 10 specimens are frost resistant for 15 freezingthawing cycles. the values of durability coefficient of algna in the higher doses varied around 1, which indicates extreme durability in comparison with reference mortar, the coefficient higher than 1 can be explained as in the range of measurement errors. 4. conclusions the hardened physical-mechanical properties and durability of natural hydraulic lime mortars prepared from nhl 3.5 lime with addition of four different biopolymeric admixtures were studied with the following conclusions: • all the admixtures cause dosage-dependent flow value reduction; the most efficient ones are diutan gum and carrageenan while xanthan gum shows the highest dosage-dependency. • the addition of biopolymers increases the 7 and 28 days strength. diutan gum and carrageenan notably increase the 28 days flexural strength, while compressive one sustained similar to other two admixtures. • diutan gum addition worsens the durability of mortars, all other admixtures improve durability with dependency on growing dosage, the best performing is sodium salt of alginic acid, showing almost no strength reduction in higher doses. acknowledgements the 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[13] t. žižlavský, m. vyšvařil, p. rovnaníková. characterization of aerial lime-based mortars with addition of biopolymers. iop conference series: materials science and engineering 379:012006, 2018. doi:10.1088/1757-899x/379/1/012006. [14] a. mignon, d. snoeck, k. d’halluin, et al. alginate biopolymers: counteracting the impact of superabsorbent polymers on mortar strength. construction and building materials 110:169–174, 2016. doi:10.1016/j.conbuildmat.2016.02.033. [15] z. li, l. zhang. biopolymers and biotech admixtures for eco-efficient construction materials. woodhead publishing, duxford, 2016. [16] z. abdollahnejad, m. kheradmand, f. pacheco-torgal. short-term compressive strength of fly ash and waste glass alkali-activated cement based binder (aacb) mortars with two biopolymers. journal of materials in civil engineering 29(07):04017045, 2017. doi:10.1061/(asce)mt.1943-5533.0001920. [17] t. žižlavský, m. vyšvařil, p. bayer, p. rovnaníková. influence of guar gum derivatives on hardened properties of aerial lime-based mortars. in special concrete and composites 2017, vol. 760 of key engineering materials, pp. 22–29. trans tech publications, 2018. doi:10.4028/www.scientific.net/kem.760.22. [18] t. žižlavský, m. vyšvařil, p. bayer, p. rovnaníková. properties of aerial lime-based mortars with chitosan ethers. solid state phenomena 276:75–82, 2018. doi:10.4028/www.scientific.net/ssp.276.75. [19] y. zhang, q. zhao, c. liu, m. zhou. properties comparison of mortars with welan gum or cellulose ether. construction and building materials 102:648–653, 2016. doi:10.1016/j.conbuildmat.2015.10.116. [20] r. ravi, m. rajesh, s. thirumalini. mechanical and physical properties of natural additive dispersed lime. journal of building engineering 15:70–77, 2018. doi:10.1016/j.jobe.2017.10.009. [21] a. el-turki, r. ball, m. a. carter, et al. effect of dewatering on the strength of lime and cement mortars. journal of the american ceramic society 93(7):2074 – 2081, 2010. doi:10.1111/j.1551-2916.2010.03667.x. [22] k. khayat. frost durability of concrete containing viscosity-modifying admixtures. aci materials journal 92(6):625–633, 1995. [23] a. izaguirre, j. lanas, j. alvarez. characterization of aerial lime-based mortars modified by the addition of two different water-retaining agents. cement and concrete composites 33:309–318, 2011. doi:10.1016/j.cemconcomp.2010.09.008. 154 http://dx.doi.org/10.1088/1757-899x/379/1/012006 http://dx.doi.org/10.1016/j.conbuildmat.2016.02.033 http://dx.doi.org/10.1061/(asce)mt.1943-5533.0001920 http://dx.doi.org/10.4028/www.scientific.net/kem.760.22 http://dx.doi.org/10.4028/www.scientific.net/ssp.276.75 http://dx.doi.org/10.1016/j.conbuildmat.2015.10.116 http://dx.doi.org/10.1016/j.jobe.2017.10.009 http://dx.doi.org/10.1111/j.1551-2916.2010.03667.x http://dx.doi.org/10.1016/j.cemconcomp.2010.09.008 acta polytechnica ctu proceedings 22:150–154, 2019 1 introduction 2 materials and methods 2.1 materials 2.2 methods 3 results and discussion 3.1 flow values 3.2 bulk density of mortar 3.3 flexural and compressive strength 3.4 durability 4 conclusions acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2017.13.0148 acta polytechnica ctu proceedings 13:148–151, 2017 © czech technical university in prague, 2017 available online at http://ojs.cvut.cz/ojs/index.php/app modal analysis for revision of a fem model of a steel truss beam michal venglar∗, milan sokol, monika marfoldi slovak university of technology in bratislava, faculty of civil engineering, department of structural mechanics, radlinskeho 11, 810 05 bratislava, slovak republic ∗ corresponding author: michal.venglar@stuba.sk abstract. the paper deals with a preparation of a complex fem model for a local damage detection. the initial verified and validated three-dimensional fem model of a steel truss bridge in laboratory is revised step-by-step to achieve the accurate model according to the experimental model. the emphasis is on modelling of the joints with 4 rivets and modelling of correct boundary conditions, as well as mass parameters and cross-section dimensions.a modal analysis of the structure is performed in fem software. many experimental measurements were made to correctly revise the fem model. the calculated natural frequencies are compared with the measured ones. in addition, mode-shapes from the calculation are validated with the measured mode-shapes. the difference between the prepared fem model and the measured specimen is small enough after a few steps of tuning. the verified, validated and revised numerical model can be used in future for a local damage detection. keywords: modal analysis, revision of fem model, experimental measurements, rivet joint connection. 1. introduction civil infrastructure like bridge structures deteriorate with time due to various reasons. the reasons are, e.g., fatigue failure, extreme events, etc. in addition, many of these structures worldwide are currently nearing the end of their proposed design life. this situation can result into a necessity of major rehabilitation. according to the 2017 infrastructure report card [1] produced by the american society of civil engineers (asce) almost 4 in 10 bridges are over 50 years and older. this means that bridge structures in the usa are 43 years old on average. situation in slovakia is almost the same. because of mentioned state, structural health monitoring (shm) and system identification of bridges is currently reaching popularity among research teams around the world [2–7]. moreover, increasing safety demands of new-constructed bridges and buildings confirms the interest. the shm of bridge structures can help to prevent from the stated situations. therefore, this paper deals with one of the first parts of shm (preparation of a fem model of bridge specimen). 2. laboratory specimen a steel truss bridge was scaled to approximately 1:15. the whole length of the specimen was 3.4m. the cross-section of the truss is closed (through truss) with a width of 0.435m and a height of about 0.8m. the cross-section of upper and lower chords has been chosen as a square hollow section shs 15 × 1.5mm. diagonals of the truss beam form a 63 °angle with the bottom and/or with the upper chords. the diagonals at the joints have the same cross-section as the chords (shs 15 × 1.5mm) that changes to a full round bar figure 1. the boundary conditions. with a radius of 12mm. floor beams consists of two shs 15×1.5mm bars which have been welded together. rods with a diameter of 6mm are used for top and bottom lateral bracing. steel components included the following nominal properties: elastic modulus 200gpa and poisson ratio equals to 0.3. the experimental model has also been weighed for comparison with the numerical model. 3. experimental measurements the used boundary conditions of the experimental model were as a simply supported beam. the truss beam was supported at 4 places. you can see it in figure 1. accelerometers mmf ks901.100 were placed at 13 points. magnetic mounting was used because of pos148 http://dx.doi.org/10.14311/app.2017.13.0148 http://ojs.cvut.cz/ojs/index.php/app vol. 13/2017 modal analysis for revision of a fem model... figure 2. the natural frequencies from initial measurements. figure 3. the numerical model. sibility to change place of sensor [8] in accordance with investigated mode-shape. the air temperature reached approximately 20 °c during the measurements and it was measured because of possible influence on repeated dynamic measurements in the future. the first three natural frequencies from measurements are shown in figure 2. analysis of the measurement data was done in software national instruments labview and modalview r2. 4. model revision in accordance with [9], the comparison of experimental outcomes and simulation outcomes was done. the simulation outcomes were prepared in the fem code where the numerical model (figure 3) was made from shell, beam elements and mass elements was also used for lumped mass. firstly, the model was verified and validated. then, the experimental outcomes were acquired in accordance with section 3. as it was mentioned in section 2, the weight of the laboratory specimen with accessories was compared figure 4. revised boundary conditions. with the numerical model. the numerical model has also considered the weight of the used accelerometers mmf ks901.100. at the beginning, the whole mass did not correspond to the mass of numerical model. then, cross-sectional dimensions and mass of individual elements of the system were carefully measured. the occurred difficulty during the process of validation was different thickness of individual cross-sections. finally, the whole mass was adjusted to weight of the experimental specimen. another difficulty which occurred during the validation and following revision of the model represents the boundary conditions. after many simulations, the boundary conditions were achieved by modelling l-profiles (figure 4) with the same length placed in four corners of the specimen. modelling of the hinge joints by using rivets (figure 5) was done as the final step. the rivets were modelled using a single-axis element that allows to specify 149 m. venglar, m. sokol, m. marfoldi acta polytechnica ctu proceedings figure 5. rivets in the numerical model. figure 6. the first calculated mode-shape: view (top) and cross-section (bottom). the outer diameter and thickness of the cross-sectional area. the thickness of the used rivets influenced each of the global natural frequency as well as frequencies of local vibrations of diagonal members. the most suitable value of the thickness was equal to 0.45mm. 5. results of revision after verification, validation and revision of the numerical model, final modal analysis was done. the first natural frequency represents 9.37hz. when the structure is deformed in its the first mode-shape, the entire upper part of the structure with diagonals moved in the direction of the y axis, as you can see in figure 6. for a comparison with the first calculated modeshape, the modal analysis was done in modalview r2 software. the better results was achieved in the figure 7. the first measured mode-shape: view (top) and cross-section (bottom). fem code, because modalview r2 software used many times less elements. but generally, the measured mode-shape (figure 7) is comparable to the calculated one. the measured frequency was approximately 9.35hz. the frequency is also in good conformity. the comparison of another natural frequencies from the measurements and the numerical analysis is in figure 8. the first two mode-shapes and the fourth represent global mode-shapes. the third mode-shape is represented as vibration of diagonal members, so it is local mode-shape. the fifth and higher mode-shapes are also local modes. 150 vol. 13/2017 modal analysis for revision of a fem model... figure 8. comparison of measured and calculated frequencies. 6. summary and conclusions the laboratory investigation was utilized to test the numerical model. after mentioned revisions in the numerical model, the obtained results were satisfactory. the compared mass was equalled. the measured and calculated natural frequencies showed only small inaccuracies. the mode-shapes from the numerical model and the measurements gave us comparable outcomes. the tuned numerical model can be used for another calculations and measurements to detect local damages in joints. acknowledgements this paper was supported by the slovak research and development agency (srda), i.e. a grant from research program no. apvv-0236-12; it was also created with the support of the ministry of education, science, research and sport of the slovak republic within the research and development operational program for the project university science park of stu bratislava, itms 26240220084. it was also supported by grant from research program young researcher 2017. references [1] american society of civil engineers. the 2017 infrastructure report card. https://www.infrastructurereportcard.org/ wp-content/uploads/2016/10/ 2017-infrastructure-report-card.pdf. [2] z. orban. increasing the reliability of the assessment iof masonry archbridges by non-destructive testing. pollack periodica 1(3):45–56, 2006. doi:10.1556/pollack.1.2006.3.4. [3] s. c. siriwardane. vibration measurement-based simple technique for damage detection of truss bridges: a case study. case stud eng fail anal 4:50–58, 2015. doi:10.1016/j.csefa.2015.08.001. [4] k.-c. chang, c.-w. kim. modal-parameter identification and vibration-based damage detection of a damaged steel truss bridge. eng struct 122:156–173, 2016. doi:10.1016/j.engstruct.2016.04.057. [5] j. li, h. hao. health monitoring of joint conditions in steel truss bridges with relative displacement sensors. measurement 88:360–371, 2016. doi:10.1016/j.measurement.2015.12.009. [6] a. bayrakattar, a. c. altunisik, t. turker. structural health assessment and restoration procedure of an old riveted steel arch bridge. soil dyn earthquake eng 83:148–161, 2016. doi:10.1016/j.measurement.2015.12.009. [7] m. sokol, m. venglar. increasing the reliability of the assessment iof masonry archbridges by non-destructive testing. pollack periodica 12(3), 2017. [in press]. [8] j. s. wilson. test and measurement. first printing. newnes/elsevier, burlington, mass, 2009. [9] b. h. thacker, s. w. doebling, f. m. hemez, et al. concepts of model verification and validation. technical report, 2004. [la-14167-ms], https: //www.osti.gov/scitech/servlets/purl/835920. 151 https://www.infrastructurereportcard.org/wp-content/uploads/2016/10/2017-infrastructure-report-card.pdf https://www.infrastructurereportcard.org/wp-content/uploads/2016/10/2017-infrastructure-report-card.pdf https://www.infrastructurereportcard.org/wp-content/uploads/2016/10/2017-infrastructure-report-card.pdf http://dx.doi.org/10.1556/pollack.1.2006.3.4 http://dx.doi.org/10.1016/j.csefa.2015.08.001 http://dx.doi.org/10.1016/j.engstruct.2016.04.057 http://dx.doi.org/10.1016/j.measurement.2015.12.009 http://dx.doi.org/10.1016/j.measurement.2015.12.009 https://www.osti.gov/scitech/servlets/purl/835920 https://www.osti.gov/scitech/servlets/purl/835920 acta polytechnica ctu proceedings 13:149–152, 2017 1 introduction 2 laboratory specimen 3 experimental measurements 4 model revision 5 results of revision 6 summary and conclusions acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2017.13.0142 acta polytechnica ctu proceedings 13:142–147, 2017 © czech technical university in prague, 2017 available online at http://ojs.cvut.cz/ojs/index.php/app evaluation of effective properties of basalt textile reinforced ceramic matrix composites soňa valentováa,∗, vladimír hrbeka,b, michal šejnohaa a czech technical university in prague, faculty of civil engineering, thákurova 7, 166 29 prague 6, czech republic b institute of theoretical and applied mechanics as cr, v.v.i., prosecká 76, 190 00 prague 9, czech republic ∗ corresponding author: sona.valentova@fsv.cvut.cz abstract. the present paper is concerned with the analysis of a ceramic matrix composite, more specifically the plain weave textile fabric composite made of basalt fibers embedded into the pyrolyzed polysiloxane matrix. attention is paid to the determination of effective elastic properties of the yarn via homogenization based on the mori-tanaka averaging scheme and the 1st order numerical homogenization method adopting a suitable representative computational model. the latter approach is then employed to simulate the response of the yarn when loaded beyond the elastic limits. the required mechanical properties of individual material phases are directly measured using nanoindentation with in-build scanning probe microscopy. applicability of the proposed computational methodology is supported by the analysis of a unidirectional fibrous composite, representing the yarn, subjected to a macroscopically uniform strain. keywords: basalt fibers, textile composites, homogenization, periodic unit cell, damage. 1. introduction the textile reinforced composites have been gaining considerable attention in many different branches of engineering. structural applications of these composites are now rather common owing to their specific strength, high stiffness, high temperature resistance, and relatively low cost in comparison to traditional materials. in addition, the possibility of influencing the resulting macroscopic properties during fabrication makes them very interesting. on the other hand, predicting the effective elastic and fracture properties and thus estimating the real behavior under variety of loads calls for a quite complicated computational analysis. such an analysis grounds on the knowledge of properties of individual components, complex inner geometry depending on a technology of composite fabrication, and on a type of textile reinforcement. mostly the carbon and glass fibers as a reinforcement are used, but attention is also accorded to the basalt fibers especially due to their good thermal stability, important in the manufactured process using high temperatures when ceramic matrices are needed in the final product. they can also substitute the carbon fibers in most applications at a fraction of price. this paper deals with the evaluation of effective elastic properties of the basalt textile reinforced ceramic matrix composite at the level of yarns (microscale) using the mori-tanaka scheme and the finite element method, predicting its strength and characterizing the failure behavior. herein, a two-dimensional plain weave textile is examined. first, experimental measurements are carried out to determine properties of individual components. next, a suitable geometrical model based on the statistically equivalent periodic unit cell is designed and finally the nonlinear computational analysis is realized. 2. experimental measurements of the material parameters a specimen of basalt fiber ceramic composite was embedded into an epoxy resin and left for 24 hours to harden in a cylindrical mold (25mm in diameter). approximately a 15mm thick part of the sample was separated using a diamond cut-off wheel. to ensure an adequate surface roughness, the investigated sample was ground on an md-piano plate using 1200, 2000 and 4000 grain cm−2 grid and water as a lubricant under 5n force. consequently, a water based dpsuspension containing 3 and 1µm poly-crystalline diamonds on md-dur and md-nap cloths, respectively, was used for fine polishing. an md-chem cloth combined with the water-based dp-suspension containing 0.25µm and 0.1µm diamond particles was selected for final polishing. in such a way, two surfaces for the measuring of longitudinal (fig. 1) and transverse (fig. 2) properties of the basalt fiber were prepared. this allowed to address a potential anisotropy of the ceramic matrix in the vicinity of the fibers. when cutting the specimen in the direction normal to the fibers resulted in the average root mean square roughness rq = 95.08 nm, whereas in case of cuts along the longitudinal direction an increase of the average root mean square roughness rq = 300.75 nm was observed owing to the cleavage of fibers during preparation. the measurements were carried out analogously for both longitudinal and transverse directions start142 http://dx.doi.org/10.14311/app.2017.13.0142 http://ojs.cvut.cz/ojs/index.php/app vol. 13/2017 basalt fiber reinforced ceramics (a). (b). (c). (d). figure 1. indentation steps in longitudinal direction. (a). (b). (c). figure 2. indentation steps in transverse direction. ing with scanning the composite surfaces with an inbuild scanning probe microscopy (spm) (figs. 1a and figs. 2a) to identify location of individual indents. these are seen for the fibers in figs. 1b and 2b. the indented surfaces are evident in figs. 1c and 2c. each phase was subjected to a different loading function. separation of indents ensured no interaction of individual indents. the indented surfaces were finally scanned again to check the measurements accuracy, see e.g. fig. 1d. the load-controlled quasi-static indentation was performed with the load function consisting of “loading” and “unloading” segments lasting 5 seconds each with an in-between 300 seconds segment of “holding” time. the maximum applied force of 2500 µn was used for fiber indentation and 300 µn in case of the matrix. this procedure was applied to several positions on the sample. the resulting reduced moduli, (a). (b). (c). (d). figure 3. (a) mesoscopic 2d image of basalt fiber/ceramic matrix plain weave textile composite, (b) binary image of the 8-layer laminate cross-section, (c) description of two-layer sepuc on the mesoscale, (d) example of a real structure of yarn. corresponding to longitudinal and transverse indentations into the fibers and the matrix, were derived from unloading part of the indentation curve. next, the required young moduli were calculated based on the contact mechanics of colliding solid bodies [1, 2] assuming the poisson ratio ν = 0.24 for all phases and a diamond tip with material parameters e = 1140gpa and ν = 0.07. taking into account all indents gave the average values of the young modulus equal to 69.68 gpa and 64.82 gpa for the longitudinal and transverse fiber directions, respectively, and equal to 58.83 gpa for the matrix. further details including basic statistics are available in fig. 1. it is not surprising that the fiber values are close confirming an essentially amorphous structure of the basalt fibers. a slight difference can be attributed to the quality of sample of the examined textile composite. an expected isotropy of the amorphous ceramic matrix has also been confirmed, since no principal difference has been observed when indenting the matrix in two perpendicular directions. application of the isotropic theory of nanoindentation to both phases is therefore adequate. 3. formulation of geometrical model during construction of the geometrical model for a computational analysis of textile composite we need to distinguish several levels corresponding to its structure. fig. 3 identifies the mesoscale structure of individual yarns and a random distribution of fibers within the yarn cross-section at the microscale level. both scales require formulation of a suitable computational model. for illustration, a cross-section of a two-ply computational model to be used on mesoscale is plotted in fig. 3c, see e.g. [3] for more details. 143 s. valentová, v. hrbek, m. šejnoha acta polytechnica ctu proceedings position red. mod. young’s mod. hardness contact depth er [gpa] e [gpa] h [gpa] hc [nm] basalt fiber – longitudinal direction p127_2 67.81 ± 8.09 67.72 ± 7.68 5.27 ± 0.56 191.92 ± 6.55 p128_1 75.95 ± 6.55 75.66 ± 6.21 5.95 ± 0.88 106.09 ± 8.91 p128_2 76.85 ± 7.75 76.63 ± 7.35 5.71 ± 0.76 108.33 ± 7.98 p130_1 66.17 ± 10.44 66.08 ± 9.93 5.49 ± 0.75 161.55 ± 12.22 p130_2 71.76 ± 7.30 71.15 ± 6.92 5.79 ± 0.38 155.99 ± 5.62 p130_3 61.12 ± 6.68 60.85 ± 6.33 5.28 ± 0.74 239.19 ± 18.71 average 69.94 ± 7.80 69.68 ± 7.40 5.58 ± 0.68 160.51 ± 10.00 basalt fiber – transverse direction p120_1 74.09 ± 6.22 73.65 ± 6.22 5.88 ± 1.17 107.60 ± 11.80 p121_2 75.98 ± 3.79 75.69 ± 3.58 5.94 ± 0.54 105.58 ± 5.57 p121_3 57.31 ± 3.58 56.85 ± 3.38 5.00 ± 0.55 116.33 ± 7.25 p122_1 55.34 ± 10.55 54.80 ± 10.03 4.45 ± 1.16 126.64 ± 17.08 p122_2 68.77 ± 8.09 68.65 ± 7.68 5.15 ± 0.54 114.39 ± 6.80 p124_1 59.61 ± 11.70 59.26 ± 9.44 5.29 ± 1.22 117.95 ± 15.13 average 65.18 ± 7.32 64.82 ± 7.03 5.29 ± 1.19 114.52 ± 14.15 pyrolyzed polysiloxane matrix p127_2 67.57 ± 3.82 67.47 ± 3.61 7.29 ± 0.96 56.64 ± 4.25 p128_1 56.45 ± 3.77 55.95 ± 3.56 5.44 ± 0.44 110.65 ± 4.90 p128_2 60.20 ± 5.09 59.88 ± 4.82 6.01 ± 0.54 104.88 ± 5.21 p130_1 56.70 ± 8.53 56.22 ± 8.10 5.63 ± 1.02 109.81 ± 10.39 p130_2 58.39 ± 7.29 57.98 ± 6.91 5.68 ± 0.99 109.45 ± 11.72 p130_3 55.82 ± 5.00 55.30 ± 4.73 6.23 ± 1.08 103.85 ± 10.21 average 59.19 ± 5.58 58.83 ± 5.29 6.05 ± 0.84 99.21 ± 7.78 table 1. indentation results of individual material phases. (a). (b). figure 4. microscale level of yarn, (a) two-fiber sepuc-1, (b) four-fiber sepuc-2. in this paper the main attention is paid to the microscale, meaning the level of yarns. fig. 4 plots two types of the idealized unit cells being representative to the actual yarn. these are derived by comparing material statistics of real, fig. 3d, and artificial microstructures. such representative volume elements (rve) are call the statistically equivalent periodic unit cell (sepuc), see e.g. [4] for more details. apart from correctly capturing the real microstructure attention must also be accorded to the application of macroscopically uniform state of stress or strain and proper boundary conditions. to this end, the interested reader is referred to [3]. 4. evaluation of effective elastic properties when limiting our attention to linear elasticity the classical micromechanical models such as the moritanaka (mt) method are often preferred to more complex finite element based simulations employing sepuc. since the latter approach is adopted in the next section addressing the loading conditions beyond the elastic limits of material phases we compare both methods to support adequacy of the selected rves. the mori-tanaka averaging scheme builds upon the knowledge of the shape and orientation of the reinforcements, their material properties and volume fractions. the microstructural details are therefore not fully reflected. the material properties adopted in the presented study for individual phases are listed in tab. 2. these are derived from experimental measurements, recall tab. 1, and partly taken from literature [5]. the homogenized properties of the basalt yarn are then summarized in tab. 3. it is clear that in case of elasticity both analytical and numerical homogenization provide the results which are reasonably close. this also partly supports application of the two computational models for the nonlinear analysis, not quite possible with the mt method. 144 vol. 13/2017 basalt fiber reinforced ceramics material e g ν gf st [gpa] [gpa] [–] [n/mm] [gpa] basalt fiber – longitudinal direction 69.68 28.10 0.24 2 basalt fiber – transverse direction 64.82 26.14 0.24 0.5 pyrolyzed polysiloxane matrix 58.83 23.72 0.24 0.001 0.08 table 2. considered parameters of individual materials. model type e11 e22 e33 g23 g31 g12 cf [gpa] [gpa] [gpa] [gpa] [gpa] [gpa] [–] sepuc-1 fem 62.96 62.97 65.86 26.46 26.48 23.5 0.62 sepuc-2 fem 62.98 62.96 65.86 26.48 26.46 23.5 0.62 mori-tanaka method 62.97 62.97 65.88 26.47 26.47 23.5 0.62 table 3. effective elastic moduli. 5. damage modeling at the level of yarns henceforth, we concentrate on the description of damage evolution within a single yarn of the present textile composite. similar to the previous section, the yarn is being considered as a unidirectional composite (ud) subjected to macroscopically uniform strain. attention is accorded to transverse direction only. an improved damage evolution model proposed by fang et al. [6, 7] is applied to receive a more reliable response after the onset of cracking in comparison to, e.g. the direct stiffness reduction method. given the orthotropic character of ud composites we associate with each principal direction an independent damage parameter ωi, i=1,2,3. additionally, six damage parameters d1t , d1c , d2t , d2c , d3t and d3c are specified to further distinguish the failure in tension (t) and compression (c). their evolution is assumed in terms of an equivalent displacement χeq. the damage parameter d reflecting a material degradation with the evolution of cracks is expressed in the form dj = χjf eq (χj eq − χji eq) χj eq(χjf eq − χji eq) , j = 1t, 2t, 3t. (1) in eq. (1), χji eq represents the equivalent displacement at the onset of failure whereas χjf eq is the full damage equivalent displacement, see [6, 7] for more details. for both phases, fibers and matrix, the crack initialization is controlled by the simple maximum stress criterion. the corresponding tensile strengths are stored in tab. 2. note that compressive failure was not addressed in this study. 5.1. numerical experiment the two types of sepuc in fig. 4 representing the yarn were analyzed to show the influence of the damage parameter evolution on the mesoscopic stressstrain curve. the tensile load was controlled by the increments of the macrostrain in the two main directions perpendicular to the direction of fibers. the first figure 5. stress-strain curves under the macroscopic tensile strain in the first transverse direction. main direction corresponds to the x-axis, the second to y-axis, recall fig. 4. the resulting stress-strain curves are plotted in figs. 5 and 7. the blown-up parts in fig. 5 and 7 identify 2 points for both the sepuc-1 (points 1,2) and sepuc-2 (points 3,4) for which the evolution of damage parameter, ranging in general from 0 to 1, is plotted in fig. 6 and fig. 8 for both loaded directions. 145 s. valentová, v. hrbek, m. šejnoha acta polytechnica ctu proceedings (a) . sepuc-1 point 1 and 2 (b) . sepuc-2 point 3 and 4 figure 6. damage evolution in the matrix corresponding to the marked numbered points in fig. 5. figure 7. stress-strain curves under the macroscopic tensile strain in the second transverse direction. as you can see in fig. 6 and 8 the damage initiation in the matrix loaded in tension occurs in both directions relatively soon. this corresponds to the macroscopic stress value of about 40mpa. the damage in the matrix phase then progresses extensively before the fiber failure takes place. the mesoscopic strength parameter st can be derived as a value of stress in the stress-strain curve after which the damage parameter starts to rapidly grow. these values are indicated in fig. 5 and 7 for individual loading directions, respectively. point out that the hexagonal arrangement of fibers in sepuc-1 should give an isotropic response. a slight difference between the two directions can therefore be attributed the unstructured (nonuniform) finite element mesh. 5.2. summary the presented paper is focused on the evaluation of the effective properties of the basalt textile ceramic matrix reinforced composites at the level of yarns with a special attention paid to the determination of the macroscopic tensile strength using computational homogenization. first the homogenized elastic moduli were obtained with the help of the mori-tanaka averaging scheme and fem simulations to support the adequacy of the two selected computational models (sepuc-1 and sepuc-2). the elastic material properties of individual phases obtained from experimental measurement and partly also from literature. then the virtual experiment on these cells was carried out, the tensile macroscopic strain was applied to these cells aiming at the determination of the macroscopic tensile strength. apart from that, the mesoscopic fracture energy can also be 146 vol. 13/2017 basalt fiber reinforced ceramics (a) . sepuc-1 point 1 and 2 (b) . sepuc-2 point 3 and 4 figure 8. damage evolution in the matrix corresponding to the marked numbered points in fig. 7. determined from the homogenized stress-strain curves, recall blue lines in figs. 5 and 7. these parameters will then enter the numerical homogenization step on mesoscale adopting more complex unit cells as the one schematically plotted in fig. 4. point out, however, that the fracture properties of both the matrix and fiber were selected in the present study ad hoc and they still need to be properly measured. to that end, nanoindentation is again expected as a suitable method of attack. list of symbols rq root mean square roughness [nm] f applied force [µn] ν poisson ratio [–] er reduced modulus [gpa] e young’s modulus [gpa] h hardness [gpa] hc contact depth [nm] g shear modulus [gpa] gf fracture energy [n/mm] st critical strength in tension [gpa] eij homogenized elastic modulus [gpa] gij homogenized shear modulus [gpa] dj damage parameter [–] χeq equivalent displacement [m] σij applied stress [gpa] εij resulting strain [–] acknowledgements the financial support provided by the sgs projects with the application registered under the no. ohk1-014/17 and ohk1-031/17 is gratefully acknowledged. references [1] h. hertz. on the contact of rigid elastic solids. chap. 6. hertz’s miscellaneous papers, london, 1896. [2] w. oliver, g. pharr. measurement of hardness and elastic modulus by instrumented indentation: advances in understanding and refinements to methodology. journal of material research 19(1):3–20, 2004. doi:10.1557/jmr.2004.0002. [3] m. sejnoha, j. zeman. micromechanics in practice. wit press, boston, 2013. [4] j. zeman, m. šejnoha. from random microstructures to representative volume elements. modelling and simulation in materials science and engineering 15(4):325–335, 2007. doi:10.1088/0965-0393/15/4/s01. [5] m. černý, p. glogar, z. sucharda. mechanical properties of basalt fiber reinforced composites prepared by partial pyrolysis of a polymer precursor. journal of composite materials 43(9):1109–1120, 2009. doi:10.1177/0021998308097732. [6] g. fang, j. liang, q. lu. investigation on the compressive properties of the three dimensional four-directional braided composites. composite structures 93(2):392–405, 2011. doi:10.1016/j.compstruct.2010.09.002. [7] g. fang, j. liang, b. wang. progressive damage and nonlinear analysis of 3d four-directional braided composites under unidirectional tension. composite structures 89(1):126–133, 2009. doi:10.1016/j.compstruct.2008.07.016. 147 http://dx.doi.org/10.1557/jmr.2004.0002 http://dx.doi.org/10.1088/0965-0393/15/4/s01 http://dx.doi.org/10.1177/0021998308097732 http://dx.doi.org/10.1016/j.compstruct.2010.09.002 http://dx.doi.org/10.1016/j.compstruct.2008.07.016 acta polytechnica ctu proceedings 13:143–148, 2017 1 introduction 2 experimental measurements of the material parameters 3 formulation of geometrical model 4 evaluation of effective elastic properties 5 damage modeling at the level of yarns 5.1 numerical experiment 5.2 summary list of symbols acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2017.13.0121 acta polytechnica ctu proceedings 13:121–124, 2017 © czech technical university in prague, 2017 available online at http://ojs.cvut.cz/ojs/index.php/app coupled simulation for fire-exposed structures using cfd and thermo-mechanical models stanislav šulca,∗, vít šmilauera, františek waldb a czech technical university in prague, faculty of civil engineering, department of mechanics, thákurova 7, 166 29 prague 6, czech republic b czech technical university in prague, faculty of civil engineering, department of steel and timber structures, thákurova 7, 166 29 prague 6, czech republic ∗ corresponding author: stanislav.sulc@fsv.cvut.cz abstract. fire resistance of buildings is based on fire tests in furnaces with gas burners. however, the tests are very expensive and time consuming. this article presents a coupled simulation of an element loaded by a force and a fire loading. the simulation solves a weakly-coupled problem, consisting of fluid dynamics, heat transfer and mechanical model. the temperature field from the computational fluid dynamics simulation (cfd) creates cauchy and radiative boundary conditions for the thermal model. then, the temperature field from element is passed to the mechanical model, which induces thermal strain and modifies material parameters. the fluid dynamics is computed with fire dynamics simulator and the thermo-mechanical task is solved in oofem. both softwares are interconnected with mupif python library, which allows smooth data transfer across the different meshes, orchestrating simulations in particular codes, exporting results to the vtk formats and distributed computing. keywords: thermo-mechanical model, virtual furnace, computational fluid dynamics, multiphysics, fire resistance. 1. introduction several single physical models exist for description of particular physical phenomena. multi-physical coupled problems extend applications to more complicated tasks. in order to alleviate coupling among models, we created a python library which integrates solutions for computing simulations of structures exposed to fire. this paper presents approach how to interconnect single physical models. 1.1. fire dynamics analysis the issue of predicting fire process in a defined space domain is solved with the fire dynamics simulator software (fds) [1], developed at nist. we modified the structure of the code to be able to call each computational time-step individually, we added several interface functions to get the dimensions of meshes, the temperatures in nodes, etc. 1.2. thermo-mechanical analysis the thermo-mechanical analysis of the element takes place in oofem [2, 3] software. the thermal analysis uses a temperature field from fds, which defines cauchy and radiative boundary conditions on the element. then, the solution of thermal analysis passes temperature to the mechanical analysis, which induces the thermal strain and modifies the material parameters as yield strength and elastic modulus. temperature will also be used for timber structures where it controls carbonization process and changes the thermal conductivity. 1.3. adiabatic surface temperature the heat transfer to the construction is realized by convection and radiation. fds can compute a quantity called "adiabatic surface temperature" (ast), which includes both convection and radiation influence. we compute the total heat flux qtot into the element with a method by wickstrom [4] from the following equation: qtot = εσ ( t 4 ast − t 4) + h (tast − t ) (1) where ε is the emissivity, σ is the stefan-boltzmann constant and h is the heat transfer coefficient. it allows us to realize the complicated task of radiation transfer very easily. 2. interfacing fds and oofem using mupif interfacing fds and oofem using mupif [5, 6] library, which provides several useful tools such as exporting data to vtu format, parallel computations and advanced handling with data fields and meshes. both the fds fortran and oofem c++ codes provide application interface for communication and they are compiled as shared libraries. top-level steering script controls execution of both codes, data synchronization etc. the flowchart of the codes is given in fig. 1. the fds task consists of one or several uniform hexahedral meshes. in the beginning of the computation the geometries of all meshes are mapped from 121 http://dx.doi.org/10.14311/app.2017.13.0121 http://ojs.cvut.cz/ojs/index.php/app s. šulc, v. šmilauer, f. wald acta polytechnica ctu proceedings figure 1. flowchart of the codes. figure 2. data workflow. fds to mupif. the meshes contain discretized temperature field. we also load the coordinates of the points in which ast will be computed and create a mesh analogically. the only difference here is that the mesh consists only of points and has no elements. in this case, oofem uses no interpolation and uses a value of the closest point. we note that the ast points should not be exactly on the element surface, they must be in the air close to the surface. here we describe the loop over one oofem timestep, see the data workflow in fig. 2 and the flowchart in fig. 1. first, fds is called to process several computational steps until we reach the length of our defined time-step of the thermo-mechanical task. the fds application determines its time-step itself and it is usually much shorter than the oofem time-step. then, we load the temperature fields from fds and copy them into the mupif fields. the field of ast values is copied into the oofem boundary-condition field. then, oofem is asked to solve computational step of the thermal task and passes the computed temperatures of the construction to the mechanical task. the material parameters are updated due to the change of the temperature and the temperature strain is added. the mechanical response of the construction is obtained, which is the end of the computational loop. as the fds analysis is very time-consuming process and the computations can last couple of days, we use another mupif functionality to store the meshes with all the temperatures in all defined time-steps in files. in this regard, it is possible to precompute fds simulation and compute multiple thermo-mechanical analyses separately and much faster. coarse fds mesh can be refined in areas where structure elements occur. this improves precision in temperature predictions and gives more realistic results of the thermo-mechanical task. the mupif library has a tool to export the loaded fds meshes to vtu, so that the fds temperature fields can be exported into this useful format. oofem exports its results to vtu format as well. 3. example we demonstrate functionality of coupled cfd-thermomechanical model on a prototypical task, consisting of a steel beam placed into a furnace, see the geometry in fig. 3. the furnace consists of two meshes. the bigger one covers the whole furnace and has 1000 cells, while the second one around the beam has 4800 cells. the âůźmesh structure is displayed in the results in fig. 10. the inner space of the furnace is a cube with edge length 0.8 m. the furnace is heated by 1 natural gas burners, placed in the middle of its floor. the output of the burner is 400 kw during the whole simulation. in the middle of the ceiling is a chimney. both the holes are square-shaped with edge 122 vol. 13/2017 figure 3. furnace geometry. figure 4. cross-section of the beam. size 0.2 m. the simply-supported beam is made from steel i profile, see the geometry of its cross-section in fig. 4. it is placed 0.52 m from the floor, horizontally in the centre. it is loaded only by its weight and the temperature. this example aims to the thermal task, because the used mechanical material model has constant parameters, non-dependent on temperature. the material parameters are e = 200 gpa, ν = 0.3, ρ = 7850kg/m3 and α = 12×10−6. the length of the beam is 0.5m. steel conductivity is displayed in fig. 6 and capacity is displayed in in fig. 7. emissivity of the steel surface is 0.8 and the heat-transfer coefficient is 20 w.m−2.k−1. the oofemmodel of the beam consists of 540 cells, which is enough for our purpose, see fig 5. the results were computed end exported to vtu files. fig. 10 shows a cross-section through the middle of the furnace. we can see the two fds meshes, one coarse on the whole furnace and one finer around the beam. detailed temperatures of the beam are displayed in fig. 11, the stress magnitude σmax is in fig. 13. figure 5. oofem mesh of both thermal and mechanical task. figure 6. steel conductivity. figure 7. steel capacity. figure 8. thermal conductivity at t = 480 s. figure 9. thermal capacity at t = 480 s. 123 s. šulc, v. šmilauer, f. wald acta polytechnica ctu proceedings figure 10. temperatures at t = 480 s, section in the middle of the furnace. figure 11. temperature at t = 480 s. figure 12. stress at t = 480 s. figure 13. displacement z at t = 480 s. 4. conclusions this paper presents an approach for computing weakly-coupled problems, focusing on computational fluid dynamics coupled with thermo-mechanical analysis. temperature field from fds was validated previously [7], performance of structure elements presents a consecutive step. validation of this multi-physical approach happens during 2017 on steel and timber and concrete structural elements. acknowledgements we gratefully acknowledge the financial support from the czech technical university in prague, the grant sgs17/042/ohk1/1t/11. we also gratefully acknowledge the financial support from the czech science foundation, the grant gacr 16-18448s. references [1] nist. fds, fire dynamics simulator, project home page, 2017. http://www.pages.nist.gov/fds-smv/. [2] b. patzák. oofem project home page, 2000. http://www.oofem.org/. [3] b. patzák. oofem – an object-oriented simulation tool for advanced modeling of materials and structures. acta polytechnica 52(6):59–66, 2012. [4] u. wickstrom, d. duthinh, k. mcgrattan. adiabatic surface temperature for calculating heat transfer to fire exposed structures. in proceedings of the eleventh international interflam conferrence, interscience communications. london, 2007. [5] b. patzák. mupif project home page, 2010. http://www.mech.fsv.cvut.cz/mupif/. [6] b. patzák, v. smilauer, g. pacquaunt. design of a multiscale modelling platform. in 15th international conference on civil, structural, and environmental engineering computing. prague, czech republic, 2015. [7] k. cábová, v. asgf, g. sdfggf. modelling of standard fire test. in engineering mechanics. svratka, czech republic, 2017. 124 http://www.pages.nist.gov/fds-smv/ http://www.oofem.org/ http://www.mech.fsv.cvut.cz/mupif/ acta polytechnica ctu proceedings 13:122–125, 2017 1 introduction 1.1 fire dynamics analysis 1.2 thermo-mechanical analysis 1.3 adiabatic surface temperature 2 interfacing fds and oofem using mupif 3 example 4 conclusions acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2018.19.0030 acta polytechnica ctu proceedings 19:30–35, 2018 © czech technical university in prague, 2018 available online at http://ojs.cvut.cz/ojs/index.php/app uncertainty evaluation in criticality calculations using the tsunami methodology matěj šikla,b,∗, františek havlůjb a department of nuclear reactors, faculty of nuclear sciences and physical engineering, czech technical university in prague, v holesovickach 2, prague 8, cz b department of reactor physics and fuel cycle support, nuclear research institute rez near prague, cz ∗ corresponding author: siklmate@fjfi.cvut.cz abstract. procedure of criticality calculations and uncertainty evaluations currently has several insufficiencies which could lead to potential non–conservativeness. paper discuss and describes possibilities of elimination of these insufficiencies and thereby ensuring safely sub–critical results of calculations. selection of experiments for validation based on similarity coefficients between systems acquired from tsunami-ip is used for this purpose. tsunami-ip operates with sensitivity of multiplication factor to cross sections of individual isotopes and reactions between compared systems. moreover, tsunami-ip determines uncertainty of calculations brought in using insufficiently similar experiments. within research, effects of variable parameters used in calculations inputs are investigated. in conclusion optimized suggested approach for validations and uncertainty evaluation using tsunami module from scale based on knowledge acquired within research is described. keywords: validation, subcriticality, tsunami, similarity between systems, penalty, pool storage. 1. introduction sources of uncertainties are contained in current approach for criticality calculations . most of these uncertainties are considered in uncertainty evaluation, even so some of them remain and are not evaluated. it could possibly lead to non–conservative evaluation and even to violation of safety limits. computational code for criticality calculations is validated based on specific types of critical experiments. thereafter is said that code gives correct believable results. this paper focuses to selection of experiments used for validation, because main omitted source of uncertainties is contained there. one purpose for validating codes at department of reactor physics (nri rez) are calculations of spent fuel storages. code is usually validated using critical experiments with fresh fuel. however when code is applicated to spent fuel storage there is not considered effect of disparity between used experiments and newly suggested spent fuel storage. this brings potential risk and non–conservativeness described above. goal to be achieved was to evaluate effect of disparity and prepare instructions for validating code application on spent fuel systems. whole research was done by means of scale [1] with advantage of it’s module tsunami (uses keno monte carlo code, will be described later). 2. current process of validation and uncertainty evaluation process of validation and uncertainty evaluation in criticality calculations is currently carried out this way: (1.) firstly set of critical experiments for validation is chosen – selection usually depends on which experiments are available • for each experiment (i) are known both keff – measured (ke,im ) and calculated (ke,ic ) (2.) then biases bi for each experiment (i) are calculated (a) bi = ke,im − ke,ic , if ke,ic < ke,im (b) bi = 0, if ke,ic >= ke,im (3.) average bias is calculated from bi (4.) model of new system, which is newly designed (called application), is calculated → kac ± σka c • where σka c is statistic error of keno-vi monte carlo calculation (5.) finally sum of kac + 2σka c + bias + 2σbias (1) must be smaller than required safety limit (usually set by legislation) as was already pointed out above, in this procedure disparity between experiments and applications is not considered. it affect mainly two parts of the procedure: (1.) experiments for evaluation of bias are chosen without consideration of similarity between the application and the experiment (2.) in the final equation there is no term expressing and quantifying potential deviation caused by disparity between experiment and application 30 http://dx.doi.org/10.14311/app.2018.19.0030 http://ojs.cvut.cz/ojs/index.php/app vol. 19/2018 uncertainty evaluation in criticality calculations both of these aspects can be resolved using tsunami. 3. tsunami tsunami is part of complex computational code scale and its name stands for tools for sensitivity and uncertainty analysis methodology implementation. complete documentation can be found in [1]. when calculating systems parameters using tsunami-3d-k6 (monte carlo code keno is used for these calculations), one of the possible outputs is the sensitivity data file (.sdf). these contains sensitivity informations (how much keff changes because of change of cross sections – δk δσ ) for each nuclide, energy group and reaction. from these files tsunami-ip (indices and parameters) calculates amount of similarity between chosen systems. several similarity coefficient exist, which are calculated different ways based on sensitivities and uncertainties of cross section determination. most important coefficient, based on both sensitivity and uncertainty is ck. it expresses overall similarity of two systems (detailed description can be found in [1]). coefficient ck is normalized such way, that similar systems have ck = 1 and totally dissimilar systems have ck = 0. currently similarity with ck > 0.95 is considered sufficient. second important value is penalty. it is calculated based on both sensitivity and uncertainty. penalty expresses in % ∆k k uncertainty of calculating application system based on dissimilarity between experiments and applications. when calculating penalty it is possible to choose which experiments should or should not be used for penalty assessment. parameter cvalue can be set to ensure that only experiments with ck > cvalue will be used for penalty evaluation. using cvalue is necessary – it is not required to deduct informations from totally dissimilar systems. each system has maximum value for penalty, which responds with relative standard deviation of keff due to cross-section covariance data. usually in results of various systems calculations observed maximal penalty was around 0.6% ∆k k , which corresponds with [2]. value of maximum penalty is included as “relative standard deviation of keff due to cross-section covariance data” in output, when system is calculated with tsunami-3d-k6 module from scale. diagram of entire process of calculations and analysis using tsunami is shown in figure 1. 4. comparing system similarity and validation quality for various types of experiments for further usage determination of behaviour of similarity coefficient was needed. firstly several pincell systems with varying parameters were tsunami-3d-k6 tsunami-ip penalty c k .sdf files max. penalty cvalue applications inputs experiments inputs figure 1. diagram of evaluation process using tsunami. compared for this purpose. later, spent fuel wet storages as applications and different types of experiments were examined. 4.1. simplified systems infinite systems of fuel rods were modelled. for comparison some parameters within systems were changed: (1.) grid type (2.) enrichment (3.) pitch (4.) amount of absorber (h3bo3) (5.) isotopic composition of fuel due to burnup. in each case only one parameter was changed, others remained nominal, which means: • enrichment 4% • pitch between fuel rods 0.61 cm • 4 g/kg of h3bo3 in moderator. when comparing triangular and square grid the moderator-to-fuel ratio for compared systems remained unchanged. multiples from 0.8 to 2.5 of nominal pitch between fuel rods were used in infinite grids. for each multiple of nominal pitch ck between triangular and square grid was bigger than 0.99, thus the experiments with square grid can be used for validation for calculating triangular grids and same the other way. after comparison of square and triangular pitch and proving their similarity, all further comparisons were done for triangular grid. comparison of similarity between systems with different enrichments is displayed in figure 2. system with enrichment 4% was used as the experiment and 31 matěj šikl, františek havlůj acta polytechnica ctu proceedings 0 5 10 15 20 0.2 0.4 0.6 0.8 1 enrichment [%], original value 4.0 c k 0 0.2 0.4 0.6 0.8 1 p en al ty [% d k / k ] c k c k=0.95 penalty figure 2. evaluation of similarity between systems with different enrichments. 0.4 0.6 0.8 1 1.2 0.7 0.8 0.9 1 pitch [cm], original value 0.61 c k 0 0.2 0.4 0.6 0.8 1 p en al ty [% d k / k ] c k c k=0.95 penalty figure 3. evaluation of similarity between systems with different pitches. applications had from 0 to 20% enrichment. in figure 2 it can be seen that variation in enrichment from 1.5 to 8% has still ck > 0.95. that seems to be a sufficient range for a typical application. figure 3 shows comparison between systems with different pitches. grid with nominal pitch 0.61 cm was taken as the experiment. as can be seen in figure 3, systems with changes in pitch smaller than 0.1 cm are still sufficiently similar. value of 0.1 cm seems to be small variability, but in fact is enough, because experiments will be typically available for identical or similar types of fuel assemblies as in the applications. boron acid amount in moderator was next parameter changed. system with 4 g/kg boron acid in moderator was taken as the experiment. figure 4 shows that comparison and it can be seen that ck remains bigger than 0.99 for all applications. this fact is important, because it means it is possible to use 0 2 4 6 8 10 0.95 0.96 0.97 0.98 0.99 1 1.01 1.02 amount of h3bo3 [g/kg], original value 4.0 c k 0 0.2 0.4 0.6 0.8 1 p en al ty [% d k / k ] c k penalty figure 4. evaluation of similarity between systems with different amount of h3bo3 in moderator. 0 10 20 30 40 50 60 0.5 0.6 0.7 0.8 0.9 1 burn-up mwd/kgu c k 0 0.1 0.2 0.3 0.4 0.5 p en al ty [% d k /k ] c k penalty figure 5. comparison of irradiated fuel with fresh uo2 fuel. experiment without boron for application including boron with no big impact. later, systems with fuel rods in different degrees of irradiation were calculated and compared. to get closer to reality systems with fresh fuel only were used as experiments, because no experiments with irradiated fuel are included in evaluated databases of critical experiments. applications are systems with isotopic composition obtained with origen, input for origen was fresh uo2 fuel. firstly irradiated fuel applications were compared with fresh uo2 fuel as the experiment and is shown in figure 5. in figure 5 can be seen that if ck will be required bigger than 0.95, only spent fuel with irradiation smaller than approximately 10 mwd/kgu are sufficiently similar. 32 vol. 19/2018 uncertainty evaluation in criticality calculations element atom density pu-238 7.39e-06 pu-239 4.10e-03 pu-240 5.61e-04 pu-241 4.38e-05 pu-242 1.70e-05 u-235 1.20e-04 u-238 1.69e-02 o 4.37e-02 am-241 3.43e-05 table 1. fresh mox material composition. 0 10 20 30 40 50 60 0.4 0.5 0.6 0.7 burn-up mwd/kgu c k 0.34 0.36 0.38 0.4 0.42 0.44 0.46 0.48 0.5 p en al ty [% d k /k ] c k penalty figure 6. comparison of irradiated fuel with fresh mox fuel. secondly fresh mox fuel was used as the experiment. mox material composition used in calculations follows in table 1. mox was chosen because during burnup actinides are created (most important is pu) and in mox fuel pu and am isotopes are contained. output of comparison is shown in figure 6 and interesting progress in ck can be observed. applications with small burnup are not similar with experiment (ck < 0.5), but later with burnup raises to ck = 0.7. this progress led to comparing applications (irradiated fuel) with both experiments (uo2 and mox) in one time. this comparison is shown in figure 7. figure 7 brings one essential fact: ck > 0.95 is currently unfulfillable requirement. 4.2. realistic systems later more realistic systems were calculated, after first simplified comparison was completed. experiments were taken from international handbook of evaluated criticality safety benchmark experiments (icsbep, [3]) database. this database collects nearly 5000 critical experiment and benchmarks from nuclear facilities around the world. 0 10 20 30 40 50 60 0.75 0.8 0.85 0.9 0.95 1 burn-up mwd/kgu c k 0 0.1 0.2 0.3 0.4 p en al ty [% d k /k ] c k penalty figure 7. comparison of irradiated fuel with fresh uo2 and mox fuel experiments together. systems with low, intermediateand highlyenriched uranium, mox and other types of fuel are included. for some experiments are available input files (scale or mcnp) and for some are available .sdf files. for following comparison .sdf files from there were used, even though they were produced with another version of nuclear data libraries. this was done to save computational time and for possibility for using hundreds of experiments. applications were systems representing an infinite lattice of dukovany nuclear power plant wet spent fuel storage cells with varying fuel burnup. in this wet spent fuel storage two racks are located – the upper and the lower, where the lower one has boron metal sheets placed between fuel assembly positions. for the application a fuel assembly gd2m with mean enrichment 4.38% with burnup from 0 to 60 mwd/kgu was chosen. maximum possible penalty for the applications in question ranges from 0.4 to 0.6% ∆k k . as it was said, variable types of experiments were available and comparison was performed with various combinations of experiments. results of comparison were poor for application with small burnup when experiments were mox and for applications with high burnup when experiments were fresh low enriched uranium fuel, which corresponds with results from section 4.1. therefore result for comparison with mix of available experiments with low enriched uranium and mox will be discussed. first, comparison with variable cvalue (different for each application) was performed. in the first run ck were calculated and from these the cvalue for each application was set as 0.75 multiple of the highest ck achieved for individual application. value 0.75 was chosen because of two reasons. firstly experiment filtration is necessary, so only adequate system remains and are used in analysis. secondly acceptable amount of experiments is needed, so not to filter majority 33 matěj šikl, františek havlůj acta polytechnica ctu proceedings grid, mwd kgu cvalue penalty [%∆k k ] cvalue passed bottom, 0 0.68 3.24e-2 402 / 726 bottom, 15 0.62 3.88e-2 489 / 726 bottom, 30 0.58 6.48e-2 489 / 726 bottom, 45 0.59 1.52e-1 219 / 726 bottom, 60 0.57 2.09e-1 228 / 726 upper, 0 0.70 2.24e-2 359 / 726 upper, 15 0.64 5.13e-2 395 / 726 upper, 30 0.59 1.05e-1 291 / 726 upper, 45 0.58 2.02e-1 164 / 726 upper, 60 0.55 2.79e-1 166 / 726 table 2. comparison of wet spent fuel storage applications with low enriched uranium and mox fuel experiments, variable cvalue. grid, mwd kgu cmax k penalty [%∆k k ] cvalue passed bottom, 0 0.90 3.28e-2 401 / 726 bottom, 15 0.82 1.70e-1 367 / 726 bottom, 30 0.77 1.56e-1 139 / 726 bottom, 45 0.79 1.57e-1 139 / 726 bottom, 60 0.76 2.17e-1 82 / 726 upper, 0 0.93 2.24e-2 358 / 726 upper, 15 0.85 5.37e-2 273 / 726 upper, 30 0.79 1.11e-1 67 / 726 upper, 45 0.78 2.18e-1 64 / 726 upper, 60 0.73 3.09e-1 7 / 726 table 3. comparison of wet spent fuel storage applications with low enriched uranium and mox fuel experiments, cvalue=0.7 fixed. of all experiments is required. multiplication by 0.75 seemed to fulfil both requirements. the results of comparison of both grids and various levels of irradiation as applications and both together low enriched uranium and mox fuels as experiments are recorded in table 2. in comparison in table 2 for each application penalty value is smaller than maximum penalty. standard deviations of the penalties were approximately 2 orders smaller than the penalty values. as expected (because of choice of multiplicative factor) for each application some experiments passed cvalue test. on the other side cvalue for some applications was approaching 0.5, which is unacceptably low value, hence another system for setting cvalue had to be used. that system has to be independent on application’s maximal ck. paper [4] deals with requirements cvalue should fulfil. in accordance with that paper cvalue=0.7 was fixed in further investigations. results can be seen in table 3. in table 3 standard deviations of the penalties were approximately 2 orders smaller than the penalty values for each application. last comparison (results in table 3) still shows good results for penalty, its value is multiply smaller than value of maximum penalty. number of experiments passed through cvlaue test is on one side small, on the other side not all of the experiments were filtered. this process of calculating and validating seems to be best of all tried. for spent fuel applications mox and low enriched uranium experiments will be needed. in next section optimized recommended approach for validating will be described. 5. recommended optimized process based on research realized and knowledge from papers [4] and [5] an optimized validation and uncertainty evaluation in criticality calculations process will be recommended. the required value 0.7 for cvalue test is based on [4], requirement for at least 40 critical experiments passing cvalue test is based on [5]. as it was said in introduction, main issues in the current approach for validation are the way for choosing experiments for determining bias and the neglection if dissimilarity between experiments used in validation and applications. both of these can be resolved using tsunami. firstly, tsunami-ip analysis should be done for experiments considered for use in bias evaluation. the choice of experiments should be: • if at least 40 experiments with ck > 0.7 to application will be found → these experiments should be used for bias determination • if less than 40 experiments with such similarity will be found → 40 experiments most similar (with highest ck) should be used for determinating bias. for determining the value of penalty the following approach should be used: • if at least 40 experiments with ck > 0.7 to application will be found → value of penalty obtained from tsunami can be used • if less than 40 experiments with such similarity will be found → value of maximum penalty should be used. with this approach, the equation 1 is changed to kac +2σka c +bias +2σbias+penalty+2σpenalty < usl, (2) where usl stands for upper safety limit. 6. conclusion the issues in the current approach for validating and uncertainty evaluation in criticality calculation were pointed out and goal to resolve these was set. when evaluating criticality calculations, many sources of uncertainties are included, but influence of dissimilarity between system is currently neglected. this influence could be evaluated using tsunami methodology. for these purpose tsunami was described and calculations were performed. from these calculations possible ranges of individual parameters for sufficient similarity were determined. 34 vol. 19/2018 uncertainty evaluation in criticality calculations moreover recommendation for selection of critical experiment used for validation was found. all of these experiences were used to produce a recommended approach described in section 5, where easy to follow guide is placed. during the whole research and preparing recommended approach an effort to optimization was carried out, hence recommended approach has not too strict requirements while requesting all important inputs and being conservatively safe. correctly performed procedure can minimize deviation due to disparity of systems to level of calculation statistic error. the goal of preparing a complex approach for validating and uncertainty evaluation in criticality calculation was fulfilled and it was done in such way that it is possible to prepare automatic script for processing. references [1] b. t. rearden, m. a. jessee. scale code system, ornl/tm-2005/39, version 6.2.2. oak ridge national laboratory, oak ridge, tennessee, 2017. [2] m. klein, l. gallner, b. krzykacz-hausmann, et al. influence of nuclear data uncertainties on reactor core calculations. kerntechnik 76:174–178, 2011. [3] international handbook of evaluated criticality safety benchmark experiments, 2016. nea/nsc/doc(95)03, oecd, nea no. 7328. [4] b. rearden, m. williams, m. anderson jessee, et al. sensitivity and uncertainty analysis capabilities and data in scale. nuclear technology 174:236–288, 2011. [5] b. l. broadhead, b. rearden, c. hopper, et al. sensitivityand uncertainty-based criticality safety validation techniques. nuclear science and engineering 146:340–366, 2004. 35 acta polytechnica ctu proceedings 19:30–35, 2018 1 introduction 2 current process of validation and uncertainty evaluation 3 tsunami 4 comparing system similarity and validation quality for various types of experiments 4.1 simplified systems 4.2 realistic systems 5 recommended optimized process 6 conclusion references doi:10.14311/app.2021.30.0076 acta polytechnica ctu proceedings 30:76–80, 2021 © czech technical university in prague, 2021 available online at http://ojs.cvut.cz/ojs/index.php/app increasing the biological resistance of gypsum by means of silver, calcium and copper ions zdeněk prošeka,b,∗, hana sekavováa, pavla ryparováa a czech technical university in prague, faculty of civil engineering, department of mechanics, thákurova 7, 166 29 prague 6, czech republic b czech technical university in prague, university centre for energy efficient buildings, třinecká 1024, 273 43 buštěhrad, czech republic ∗ corresponding author: zdenek.prosek@fsv.cvut.cz abstract. the article focuses on the effects of additives increasing biological resistance of gypsum against degradation of the material. in this study we use silver nitrate, calcium oxide and copper sulphate as an additive that improves biological resistance of gypsum composites. the amount of addition was calculated so that the amount of ions was 0.1, 0.3 and 0.5 wt. %. microbial degradation was determined as the degree of resistance of materials to the occurrence of bacteria and molds by optical and electron microscope. the results obtained from these samples were compared with references materials created in the laboratory and available on today’s market. the results show a positive effect of silver and copper ions on the bioresistance of gypsum. keywords: biological resistance, calcium oxide, copper sulphate, gypsum, silver nitrate. 1. introduction gypsum is a widely used material in construction, especially for the production of gypsum plaster, wallboard, gypsum blocks [1]. gypsum is one of the longest used binders, it was already known in egypt or assyria. it has been used in europe since the early middle ages [2]. gypsum composites are more environmentally friendly and production is faster than cement composites. mainly due to lower greenhouse gas emissions and sustainability [3]. gypsum is produced by thermal decomposition of caso4 · 2h2o in a so-called boiling process at 130−150 ◦c [4]. the material for the production of gypsum can be natural or also waste after flue gas desulphurisation from thermal power plants (industry gypsum). the use of gypsum composites in construction is significantly limited because they have low resistance to damage of moisture. the moisture content of the material adversely affects the formation of bacteria, molds and the multiplication of other microorganisms [5]. molds are microorganisms that commonly occur on the inner and outer surfaces of building materials and can cause sick building syndrome, which limits or completely restrict the use of the building [6]. for this reason, it is very important to create biological resistance materials that resist the formation of mold on their surface and inside the material. one of the ways to increase the biological resistance of gypsum is to prevent the access of water or moisture. without the presence of water, mold and bacteria colonies cannot form. reduced water absorption is achieved by the use of hydrophobic additives such as waxes [7], saponifiers [8] and emulsifiers [9]. this method of protecting gypsum is very effective, but prevents possible future recycling of gypsum. during recycling, the hydrophobicizing additive cannot be separated from the gypsum and thus the calcined gypsum can no longer be mixed properly with the water and thus prevents it from solidifying properly [10]. another option is to create an environment that does not allow the growth of molds and bacteria. for example, it is a very alkaline environment that restricts the growth of bacteria and molds [11]. the last possibility to improve the biological resistance of gypsum composites is by adding antibacterial components (such as silver [12], titanium dioxide [13], copper [14]). this article deals with the direct use of silver and copper ions as additives that increase biological resistance. when penicillin was discovered, silver and other biocidal materials were almost abandoned. but today, with the emergence of antibiotic-resistant strains, it has gained new interest [15]. silver has been reported in several studies [16, 17] as an effective bactericidal antibacterial agent. gypsum composite with antibacterial components is also being developed and investigated [5], but some of its properties are still unknown. in an experimental study, the effect of silver ions on the mechanical properties of the gypsum composite was investigated and it was found that: the introduction of silver ions results in a less compact structure of hardened gypsum samples, leading to a 20% deterioration in flexural strength [18]. work dealing with other additives is completely missing. 2. materials and samples in our study of gypsum biodegradation, three reference materials were used: ref 1 was labora76 http://dx.doi.org/10.14311/app.2021.30.0076 http://ojs.cvut.cz/ojs/index.php/app vol. 30/2021 increasing the biological resistance of gypsum set gypsum [g] admixtures water [g] ions and their content ref 1 500 320 ag 0.1 500 agno3 – 1.33 g 320 ag – 0.1 wt. % ag 0.3 500 agno3 – 4.02 g 320 ag – 0.3 wt. % ag 0.5 500 agno3 – 6.69 g 320 ag – 0.5 wt. % ca 0.1 500 cao – 1.19 g 320 ca – 0.1 wt. % ca 0.3 500 cao – 3.57 g 320 ca – 0.3 wt. % ca 0.5 500 cao – 5.94 g 320 ca – 0.5 wt. % cu 0.1 500 cuso4 – 2.14 g 320 cu – 0.1 wt. % cu 0.3 500 cuso4 – 6.41 g 320 cu – 0.3 wt. % cu 0.5 500 cuso4 – 10.68 g 320 cu – 0.5 wt. % table 1. composition of tested sets. 100 µm figure 1. image from electron microscope, bse detector, magnified 400×. tory produced according to table 1, ref 2 is made of gypsum slurry for plasterboards (marked h2) designation with reduced water absorption and thus higher bioresistance, according to the standard čsn en 520 + a1 [19] and ref 3 is made of gypsum slurry for plasterboards marked a (common use), according to standard čsn en 520 + a1 [19]. ref. 2 and 3 were used to compare with current materials on today’s market. the resistance to biodegradation of the laboratory mixture was increased by ions of silver, calcium and copper. silver ions were obtained from the addition of silver nitrate (agno3), calcium ions from calcium oxide (cao) and copper ions from copper sulphate (cuso4). the amount of addition was calculated so that the amount of ions was 0.1, 0.3 and 0.5 wt. % (table 1). silver and copper ions were chosen because of their biocidal properties and calcium due to increased alkalinity of the environment which prevents the growth of molds and bacteria. five samples were made from each set according to table 1. the samples were stored for 9 days in a laboratory environment at temperature 22 ± 3 ◦c and relative humidity 50 ± 5 %. subsequently, the samples were tested. figure 2. sample placed on agar in petri dish after 41 days. in all cases, gypsum from the knauf praha company was used, originating from the industry gypsum of the počerady power plant. figure 1 show the typical character of gypsum grains, which are composed of only one phase and have an elongated, onesided oriented shape. the results of the eds analysis show that the gypsum grain is composed of calcium 19 wt. %, sulfur 17 wt. % and oxygen 64 wt. %. when converted to atomic percent and using stoichiometry, it can be stated that it is pure gypsum. 3. experimental methods microbial degradation was performed according to the standard iso en 846 [20], which determines the degree of resistance of materials to the occurrence of molds or determines their resistance to biodegradation. the produced samples had a size of 1 × 1 × 1 cm. sterilization was performed using uv radiation, which damaged microorganisms from the surface while maintaining the material and chemical parameters of individual samples. other types of possible sterilization, apart from radioactivity, degrade the original properties of the materials and were therefore not used. samples were inoculated with a 77 z. prošek, h. sekavová, p. ryparová acta polytechnica ctu proceedings ref. 1 ref. 2 ref. 3 10 mmmold colonies figure 3. surface of reference samples after 41 days of exposure to molds and bacteria. ag 0.1 ca 0.1 cu 0.1 ag 0.3 ca 0.3 cu 0.3 ag 0.5 ca 0.5 cu 0.5 mold colonies copper inclusion 0.18 mm figure 4. optical microscope images of tested materials. magnified 120×. consortium of molds and then plated on agar (czapek dox) at 25 ± 3 ◦c and monitored for 41 days (fig. 2). the consortium of molds contained microorganisms from the czech collection of microorganisms: paecilomyces variotti, chaetomyum globosum, penicillium funiculosum, aspergillus terreus, penicillium brevicompactum. the resistance of the materials was evaluated visually and monitored by optical microscope. optical microscopy was performed using a zoom microscope with 3d imaging capabilities. this is a device from zeiss axio zoom.v16. clusters of additive ions were discovered during optical microscopy. for this reason, optical microscopy has been supplemented by electron microscopy. a scanning electron microscope with a schottky cathode feg sem merlin zeiss was used for electron microscopy, which is located in the laboratory of electron microscopy and micro-analysis at the university center for energy efficient buildings in buštěhrad. quantitative and qualitative analysis of the chemical composition of the samples was performed by x-ray microanalysis directly with an energy dispersive spectrometer (eds) from oxford instruments. during the anal78 vol. 30/2021 increasing the biological resistance of gypsum 500 µm ag 0.1 ca 0.1 cu 0.1 ag 0.5 ca 0.5 cu 0.5 figure 5. image from electron microscope, bse detector, magnified 200× with eds element maps (yellow – silver, white – calcium and blue – copper). ysis, the microscope settings were as follows: accelerating voltage 20 kv, current 2 na, resolution 1024 px, work distance 8.5 mm and acquisition time 60 s. due to the higher electric current required for elemental analysis, 3 nm of platinum were sputtering on surface of samples. 4. results and discussion figure 3 shows a comparison of reference materials. the ref. 3 sample had almost the entire sides covered, and mold colonies were already present on the surface on day 22. the coverage rate did not change much at the end of the experiment, but the molds began to fructify. samples ref. 1 and 2 had similar results and were generally resistant to the occurrence of molds, after 41 days only isolated colonies of molds were visible on the surface, the sides of the samples were only partially covered, approximately 40 %. the laboratory mixture ref. 1 achieved the same results as gypsum slurry with lower water absorption (ref. 2) due to the fact that it did not contain paper pulp and other materials contained in gypsum slurry for plasterboards which reduces bio-resistance. in the case of added ions, all variants were highly resistant to biodegradation. for this reason, optical microscopy was performed. figure 4 shows the surfaces of the tested materials by optical microscope. the results show a positive effect of copper and silver, where after 41 days no microorganisms formed on the surface of the materials. in the case of calcium ions, a mold colony can be found on the surface of all materials tested. silver and copper ions 100% increase the bioresistance of gypsum even in the lowest tested amount of ions. in addition, it can be seen that with a higher concentration of copper ions, clusters occur. the individual ion clusters can be seen in figure 5 from electron microscopy. the images were taken using a bse detector and show the phase contrast. in addition, the left part of the images is supplemented by an element map, where elements were selected for individual ions. silver is shown in yellow, calcium in white and copper in blue. the results show that higher concentrations (0.5 wt. %) of silver and copper ions form clusters. in the case of a concentration of 0.1 wt. %, the ions are homogeneously distributed throughout the structure. individual cao grains can be seen in the calcium sample (ca 0.1 and ca 0.5). the results of the influence of silver and copper ions are in agreement with other authors who have used applications for other building materials such as lime and lime-cement plasters [21]. the formation of clusters explains the decreasing mechanical properties of gypsum with a high content of silver ions, which are described by other authors [18]. 5. conclusions this work was focused on the influence of different type of admixtures on biological resistance of gypsum. the researched samples were composed of gypsum from knauf praha (počerady) and various types of biocidal material. silver, calcium and copper ions in three different concentrations (0.1, 0.3 and 0.5 wt. %) 79 z. prošek, h. sekavová, p. ryparová acta polytechnica ctu proceedings were used as biocidal material. based on the results, it can be concluded that: • the addition of silver, calcium and copper ions has increased the resistance to gypsum biodegradation and the materials are comparable to those on the market today. • higher ion concentration (0.5 wt. %) form clumps and inhomogeneity of materials. • silver and copper ions were most effective, where no colony of molds and bacteria formed after 41 days for all ion concentrations. future research will focus on finding a minimum concentration of silver ions in gypsum that will be resistant to biodegradation. next research will not deal with calcium ions, due to lower efficiency and copper ions, due to toxicity and current legislation [22]. acknowledgements this paper was financially supported by the ta cr research a project under the number tj02000235 and by czech technical university in prague under no. sgs project sgs19/148/ohk1/3t/11. the authors also thank knauf, praha ltd. for the supplied samples and materials and the center for nanotechnology in civil engineering at the faculty of civil engineering, czech technical university in prague. references [1] m. doleželová, a. vimmrová. moisture influence on compressive strength of ternary gypsum-based binders. in aip conference proceedings, vol. 1863, p. 290003. aip publishing llc, 2017. doi:10.1063/1.4992440. 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[7] g. mumford, r. w. phillips. dimensional change in wax patterns during setting of gypsum investments. journal of dental research 37(2):351–358, 1958. doi:10.1177/00220345580370022301. [8] m. razali, m. aziz, w. hamdan, et al. synthesis of grease from waste oils and red gypsum. australian journal of basic and applied sciences 11:154–159, 2017. [9] y. zhang, k. wang, w. tao, d. li. preparation of microencapsulated phase change materials used graphene oxide to improve thermal stability and its incorporation in gypsum materials. construction and building materials 224:48–56, 2019. doi:10.1016/j.conbuildmat.2019.06.227. [10] h. sekavová, j. herrmann, z. prošek, et al. the influence of the use of recycled gypsum on the properties of gypsum products in labor conditions. acta polytechnica ctu proceedings 26:81–85, 2020. doi:10.14311/app.2020.26.0081. 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[19] čsn en 520+a1. sádrokartonové desky definice, požadavky a zkušební metody. praha: úřad pro technickou normalizaci, metrologii a státní zkušebnictví . [20] čsn en iso 846. plasty hodnocení působení mikroorganismů. praha: úřad pro technickou normalizaci, metrologii a státní zkušebnictví 2019. [21] a. jerónimo, a. camões, b. aguiar, n. lima. hydraulic lime mortars with antifungal properties. applied surface science 483:1192–1198, 2019. doi:10.1016/j.apsusc.2019.03.156. [22] n. a. ashford, c. c. caldart. environmental law, policy, and economics: reclaiming the environmental agenda. mit press, 2008. 80 https://doi.org/10.1063/1.4992440 https://doi.org/10.1007/bf01063410 https://doi.org/10.1016/b-978-0-08-100370-1.00025-1 https://doi.org/10.14311/app.2019.21.0016 https://doi.org/10.1080/00038628.2018.1461060 https://doi.org/10.1177/00220345580370022301 https://doi.org/10.1016/j.conbuildmat.2019.06.227 https://doi.org/10.14311/app.2020.26.0081 https://doi.org/10.1016/j.biotechadv.2008.09.002 https://doi.org/10.2217/fmb.11.78 https://doi.org/10.1016/j.ijantimicag.2008.12.004 https://doi.org/10.1093/jac/dkm006 https://doi.org/10.1007/s13762-018-1773-0 https://doi.org/10.1016/j.apsusc.2019.03.156 doi:10.14311/app.2021.30.0030 acta polytechnica ctu proceedings 30:30–35, 2021 © czech technical university in prague, 2021 available online at http://ojs.cvut.cz/ojs/index.php/app mechanical and chemical activation of blast furnace slag and its influence on the mechanical properties of the resulting cement paste jan horych∗, pavel tesárek, zdeněk prošek czech technical university in prague, faculty of civil engineering, department of mechanics, thákurova 7, 166 29 prague 6, czech republic ∗ corresponding author: jan.horych@fsv.cvut.cz abstract. the usage of waste materials is a very important global topic. the large amount of waste everywhere in the world needs to be processed or disposed. landfilling is not an option anymore, because of european legislation and restrictions. a lot of studies are trying to develop new options or possibilities of using waste materials. this research is trying to find a way to process blast furnace slag. a high-speed mill was used for the mechanical activation. chemical activation was used as the next step of activation. there are many materials that could be used, but in this study we used slaked lime and water-glass. slaked lime had a positive effect on mechanical properties. samples had higher compressive strength but the effect was limited only for 5 wt. %. another used material was water-glass, but in this case, there was a significant negative effect. compressive strength and flexural strength were significantly reduced. keywords: activation, blast furnace slag, cement replacement, slaked lime, water-glass. 1. introduction the objective of this study is to find a way to replace cement from concrete with useless waste materials. according to the trend of recycling we tried to find an option to use old and highly carbonated blast furnace slag as a partial substitute for portland cement. concrete is a trendy material thanks to its variability and reliability. cement is the main and the most important ingredient of concrete, but it is also the most expensive ingredient from natural resources. the production itself is highly economically and ecologically demanding. an issue with cement is not a new topic. for example, bellmann f. and stark j. [1] focused on chemical activation of blast furnace slag by adding calcium hydroxide and soluble calcium salts such as calcium chloride, calcium bromide, calcium nitrate, calcium formate and calcium acetate. early compressive strength was measured on mortar samples. compressive strength was increased by adding calcium hydroxide, calcium carbonate and calcium acetate from 3 mpa to 25 mpa after 7 days. after the 28 days period the final compressive strength was increased from 36 mpa to 53 mpa. another research from yum w. et al. [2] investigated the influence of calcium formate on the strength and microstructure of the ground granulated blast furnace slag activated with calcium oxide. the next study shows the effect of different slag characteristics on compressive strength in slag system with calcium oxide activation written by jeong y. et al. in 2016 [3]. four kinds of ground granulated blast furnace slags from different sources were collected to investigate changes in final mechanical properties. each of the slags developed significantly different compressive strength regardless of seemingly similar characteristics of the slags. the values of the final compressive strengths were spread from 25 mpa up to 52 mpa in 28 days. the reaction products were different as well. the goal of the study by wang y. et al. [4] was to determine the effect of wet grinding on the mechanical properties of blast furnace slag and slag-cement mixtures. testing was focused on different grinding times from 10 min to 50 min in 10 min intervals. the performance of well ground samples of slag indicated the effectiveness of wet grinding. the 50 min grinding could decrease the average particle size to 2.95 µm. other effects were high reactivity, improvement of mechanical properties and reduction of porosity. research of escalante-garcía j. et al. [5] observed a mechanically activated granulated blast furnace slag. samples were composed in a range of 50 to 95 wt. % to replace clinker in portland slag cement. structural properties were determined using x-ray diffraction, electron microscopy, thermogravimetry and differential thermal analysis. up to the 70 wt. % replacement was found an improvement of compressive strength in the first and 28th day. microstructural changes of the slag caused by mechanical activation provided enhanced reactivity and the improvement of cement strength. the paper from sobolev k. [6] examined the effect of grinding on high-performance cement containing granulated blast furnace slag. samples were produced in high volumes of blast furnace slag. testing results of mortars based on modified cement improved compressive strength up to 91.7 mpa. final mechanical properties were increased by 62 % over the reference sample. 30 http://dx.doi.org/10.14311/app.2021.30.0030 http://ojs.cvut.cz/ojs/index.php/app vol. 30/2021 mechanical and chemical activation of blast furnace slag 0 10 20 30 40 50 60 70 80 90 100 0.25 1 4 16 64 256 cu m ul at iv e am ou nt [% ] particle size [µm] slag o not ground slag a cem i 42.5 r figure 1. particle size distribution of used materials. mixtures cement [wt. %] slag [wt. %] type of slag slaked lime [wt. %] w/b [-] bulk density [kg/m3] c100 100.0 0.40 2020 ± 10 o30/a0 70.0 30.0 o 0.40 1978 ± 29 o30/av5 67.5 27.5 o 5 0.40 1958 ± 17 o30/av10 65.0 25.0 o 10 0.40 1940 ± 9 a30/a0 70.0 30.0 a 0.40 1992 ± 28 a30/av5 67.5 27.5 a 5 0.40 1998 ± 22 a30/av10 65.0 25.0 a 10 0.40 1942 ± 25 table 1. composition of individual samples with slaked lime. 2. materials and samples during the research, portland cement was still a basic component. specifically, it was portland cement cem i 42.5 r from radotín. another component was blast furnace slag. the source of slag was a foundry industry complex in kladno. it was a waste from slag aggregate production by recycling. used slag was approximately 100 years old air-cooled and carbonated blast furnace slag stored on a heap. at first, waste materials were not immediately ready for another use. it needed to be activated. as mentioned, mechanical activation was used on the base of previous study of prošek z. [7]. collaboration with lavaris ltd. was used again. the main goal was to find a material useable as a chemical activator of waste materials. dimensions of all produced samples were 40 × 40 × 160 mm. they were stored in water for 28 days. during this time, we tested every sample with non-destructive methods to determine dynamic young’s modulus of elasticity and dynamic shear modulus. at the end of this period, samples were destroyed with a destructive bending test and compression test to confirm final mechanical properties. slaked lime was used for the first experiment. in this test we also used two samples of slag with the different grinding capacity to find an ideal option and investigate the synergy of mechanical and chemical activation. figure 1 shows the difference between individual samples of the slags. the slag with no grinding was called slag o. as we can see, slag a was ground and the particle size distribution changed significantly and is similar to the curve of the reference cement cem i 42.5 r. samples were composed of changing shares of portland cement 65 to 100 wt. %, 25 to 30 wt. % of blast furnace slag and 0 to 10 wt. % of slaked lime (table 1). water binder ratio (w/b) was always set on 0.4. in the second phase water-glass was used based on sodium and potassium. the composition of samples was similar to the previous tests. there was 65 to 100 wt. % of portland cement, 25 to 30 wt. % of blast furnace slag and 0 to 10 wt. % of the activator (table 2). based on the results of the first phase, testing samples were produced using only slag a. 3. measurement methods several methods were used through testing to investigate mechanical properties. first of them was a nondestructive resonance method. in period of 28 days dynamic young’s modulus of elasticity and dynamic shear modulus were determined every week. a set of instruments and devices was used to measure natural frequency of all samples in longitudinal, transverse 31 j. horych, p. tesárek, z. prošek acta polytechnica ctu proceedings 0 5 10 15 20 25 30 0 7 14 21 28 yo un g' s d yn am ic m od ul us [g pa ] time [days] c100 a 30/a0 a 30/av5 a 30/av10 o 30/a0 o 30/av5 o 30/av10 figure 2. development of dynamic young’s modulus of the samples with slaked lime. mixtures cement [wt. %] slag [wt. %] sodium silicate [wt. %] potassium silicate [wt. %] bulk density [kg/m3] c100 100.0 2020 ± 10 a30/a0 70.0 30.0 1969 ± 36 a30/an5 67.5 27.5 5 1963 ± 27 a30/an10 65.0 25.0 10 1899 ± 10 a30/ak5 67.5 27.5 5 1945 ± 8 a30/ak10 65.0 25.0 10 1899 ± 13 table 2. composition of individual samples with water-glass. and torsional oscillation. the supports of samples were soft and elastic pads and had to be at node lines of basic natural mode shapes. impact hammer type 8206 was used to excite vibration of the samples. acceleration transducer type brüel & kjær type 4519-003 measured the response of the samples. both of devices are connected to the measurement station 3560-b-x12. it’s purpose is to record the excitation and response signals. the station transforms received signals to the frequency domain using the fast fourier transform. computer with specific software called pulse labshop then creates a frequency response function. the dynamic modulus of elasticity and the dynamic shear modulus were calculated according to astm 1876-01 and čsn 73 1372 [8, 9]. the measurement of the dynamic modulus of elasticity and the dynamic shear modulus was taken from the work of j. topič [10]. the flexural strength was measured at the end of the 28 days period by using three-point bending test on samples measuring 40 × 40 × 160 mm. samples were broken approximately in half (40 × 40 × ∼80 mm). the loading through the testing was displacement controlled at a constant rate of 1 mm/min. the distance between the supports was 100 mm. then these halves were used to determine compressive strength by uniaxial compression test. uniaxial compression testing was displacement controlled at a rate of 5 mm/min [11, 12]. 4. results and discussion as mentioned through testing, the development of young’s dynamic modulus and dynamic shear modulus was measured for all samples. in the first phase of the experiment, slaked lime was used to activate blast furnace slag. figure 2 shows the development of dynamic young’s modulus. first we can see a difference approximately 4 gpa between a reference sample with 100 wt. % of portland cement and the rest of the samples. it is caused by the replacement of portland cement with blast furnace slag. another slight difference, we can see between samples with a lower amount of slaked lime. samples a 30/av0, a 30/av5 and o 30/av0 had a little bit higher values of dynamic young’s modulus and dynamic shear modulus (figure 3). the highest progress of both modules was registered in first 7 days. later testing from day 7 to day 21 showed lower improvement. in the last week of testing, values were almost the same. results of non-destructive testing suggest there was an influence of mechanical activation. slag a had a slightly higher both dynamic young’s modulus and dynamic shear modulus. also, the effect of slaked lime as an activator was limited. only 5 wt. % of 32 vol. 30/2021 mechanical and chemical activation of blast furnace slag 0 2 4 6 8 10 12 14 0 7 14 21 28 d yn am ic sh ea r m od ul us [g pa ] time [days] c100 a 30/a0 a 30/av5 a 30/av10 o 30/a0 o 30/av5 o 30/av10 figure 3. development of dynamic shear modulus of the samples with slaked lime. figure 4. flexural strength of the samples with slaked lime. figure 5. compressive strength of the samples with slaked lime. slaked lime had a positive effect on final properties. at the end of the first phase, all samples were tested with destructive methods. the final flexural strength is shown in figure 4. we can see a significant positive effect of blast furnace slag on flexural strength. in some cases, values were almost tripled. in general, slag has a much slower reaction and produces less heat. it needs time to react appropriately. for these reasons, the concrete based on slag is used for a long time. the influence of slaked lime was registered on the samples a 30/av5 and o 30/av5. lower values of flexural strength suggest a more complex chemical reaction. figure 5 presents compressive strength and results overall confirmed previous conclusions. mechanical activation had a positive effect on the final properties. limited value of 5 wt. % of slaked lime supported a chemical reaction and helped to increase compressive strength. slag a was more finally ground and grains were more exposed. with slaked lime, we achieved the value 56 mpa. the positive effect of slaked lime is perceptible. in the second phase, water-glass based on sodium and potassium was used to investigate its influence on final mechanical properties. similar to the previous tests through 28 days period, dynamic young‘s modulus and dynamic shear modulus were measured every week. as we can see in figure 6 and figure 7, the final values are more spread. unfortunately, we can see a big difference between each samples. the reference sample had significantly higher values. the difference was approximately 5 gpa. the second highest value of dynamic young’s modulus and dynamic shear modulus had a sample a 30/a0 without any activator. by adding a water-glass as an activator, the final values were even more reduced. with more activator, the value drop was more obvious. the lowest values had samples with the highest amount of waterglass, regardless of sodium or potassium. the highest progress was also measured in the first 7 days. later development was almost completely stopped and dynamic shear modulus was even decreased in the last week. water-glass seemed unusable and negative effect was significant. the destructive tests confirmed the results of the resonance tests. flexural strength of the samples 33 j. horych, p. tesárek, z. prošek acta polytechnica ctu proceedings 0 5 10 15 20 25 30 0 7 14 21 28 yo un g' s d yn am ic m od ul us [g pa ] time [days] c100 a30/a0 a30/an5 a30/an10 a30/ak5 a30/ak10 figure 6. development of dynamic young’s modulus of the samples with water-glass. 0 2 4 6 8 10 12 14 0 7 14 21 28 d yn am ic sh ea r m od ul us [g pa ] time [days] c100 a30/a0 a30/an5 a30/an10 a30/ak5 a30/ak10 figure 7. development of dynamic shear modulus of the samples with water-glass. figure 8. flexural strength of the samples with water-glass. with water-glass was reduced. with a higher amount of water-glass, the flexural strength drop was even more significant. figure 8 shows the final results and values of flexural strength. the sample a 30/ak10 with 10 wt. % of water-glass based on potassium had figure 9. compressive strength of the samples with water-glass. almost the same flexural strength as the reference sample c 100 with 100 wt. % of portland cement. the negative effect of water glass almost removed the influence of the blast furnace slag. 34 vol. 30/2021 mechanical and chemical activation of blast furnace slag compressive strength shown on figure 9 proved water-glass does not work as an activator. the samples with water-glass had significantly reduced compressive strength. it seemed water-glass partially prevents the chemical reaction. the cement and blast furnace slag grains could not create a proper connection. the sample a 30/ak10 had the compressive strength reduced almost on half compared to the sample a 30/a0 and less than third compared to the sample c 100 only with portland cement. unfortunately, the result of the whole second phase was a complete failure. water-glass can not be used as the activator. 5. conclusions this study investigated the influence of mechanical and chemical activation of the blast furnace slag to support a possibility of cement replacement up to the level of 35 wt. %. another goal was to find an ideal chemical activator and determine synergy between mechanical and chemical activation. based on the results, it can be concluded that: • blast furnace slag had a positive effect on flexural strength. • highly carbonated blast furnace slag can be used as a binder to replace portland cement. • mechanical and chemical activation created a synergy and supported the hydration process. • slaked lime worked as an activator but at a limited amount of 5 wt. %. • water-glass did not work as an activator and even significantly reduced the influence of blast furnace slag regardless of sodium or potassium. acknowledgements this paper was financially supported by czech technical university in prague under no. sgs project sgs19/148/ohk1/3t/11. the authors also thank lavaris ltd. for the supplied materials. references [1] f. bellmann, j. stark. activation of blast furnace slag by a new method. cement and concrete research 39(8):644–650, 2009. doi:10.1016/j.cemconres.2009.05.012. [2] w. s. yum, j. i. suh, d. jeon, j. e. oh. strength enhancement of cao-activated slag system through addition of calcium formate as a new auxiliary activator. cement and concrete composites 109:103572, 2020. doi:10.1016/j.cemconcomp.2020.103572. [3] y. jeong, h. park, y. jun, et al. influence of slag characteristics on strength development and reaction products in a cao-activated slag system. cement and concrete composites 72:155–167, 2016. doi:10.1016/j.cemconcomp.2016.06.005. [4] y. wang, x. he, y. su, et al. efficiency of wet-grinding on the mechano-chemical activation of granulated blast furnace slag (gbfs). construction and building materials 199:185–193, 2019. doi:10.1016/j.conbuildmat.2018.11.245. [5] j. i. escalante-garcía, a. f. fuentes, a. gorokhovsky, et al. hydration products and reactivity of blast-furnace slag activated by various alkalis. journal of the american ceramic society 86(12):2148–2153, 2003. doi:10.1111/j.1151-2916.2003.tb03623.x. [6] k. sobolev. mechano-chemical modification of cement with high volumes of blast furnace slag. cement and concrete composites 27(7-8):848–853, 2005. doi:10.1016/j.cemconcomp.2005.03.010. [7] j. trejbal, z. prošek, p. tesárek, j. valentin. activation of blast furnace slag and its usage in practise. týden výzkumu a inovací pro praxi 2019 2019. [8] astm 1876-01. standard test method for dynamic young’s modulus, shear modulus, and poisson’s ratio by impulse excitation of vibration, annual book of astm standards, american society for testing and materials 2006. [9] čsn 73 1372. in non-destructive testing of concrete – testing of concrete by resonance method, czech office for standarts, metrology and testing. 2012. [10] j. topič, z. prošek. properties and microstructure of cement paste including recycled concrete powder. acta polytechnica 57(1):49–57, 2017. [11] čsn en 12390 (731302). in testing hardened concrete, czech office for standarts, metrology and testing. 2013. [12] j. kot’átková, j. zatloukal, p. reiterman, k. kolář. concrete and cement composites used for radioactive waste deposition. journal of environmental radioactivity 178:147–155, 2017. 35 https://doi.org/10.1016/j.cemconres.2009.05.012 https://doi.org/10.1016/j.cemconcomp.2020.103572 https://doi.org/10.1016/j.cemconcomp.2016.06.005 https://doi.org/10.1016/j.conbuildmat.2018.11.245 https://doi.org/10.1111/j.1151-2916.2003.tb03623.x https://doi.org/10.1016/j.cemconcomp.2005.03.010 acta polytechnica ctu proceedings doi:10.14311/app.2019.22.0057 acta polytechnica ctu proceedings 22:57–61, 2019 © czech technical university in prague, 2019 available online at http://ojs.cvut.cz/ojs/index.php/app monitoring and protection of timber-concrete bridges petr kuklík∗, lukáš velebil, anna gregorová, petr svora czech technical university in prague, university centre for energy efficient buildings, třinecká 1024, 273 43 buštěhrad, czech republic ∗ corresponding author: kuklik@fsv.cvut.cz abstract. the majority of timber bridges for ordinary road traffic are often made as timber-concrete composite structures. concrete slab is something like as umbrella for timber. the reason for problems in timber-concrete bridges is nearly always poor detailing for durability and neglected maintenance. mainly all aspects of timber protection and maintenance should therefore be considered even during the planning phase of bridge. paper deals with evaluation of timber properties, system of opto-fiber sensors for monitoring and diagnostics of mechanical stress of timber-concrete bridges, and degradation of timber due to uv radiation, atmospheric conditions and biological agents after application of photocatalytic materials on surface. keywords: bridges, concrete, timber, timber-concrete composite, monitoring, durability. 1. introduction timber-concrete composite bridges were first constructed in the united states in the early 20th century. this development was motivated by a shortage of steel that urged builders to use other available structural materials or combinations of such materials [1]. the first reported timber-concrete bridge in the united states dates back to circa 1925. in the 1950s, timberconcrete composite structures started to be applied to bridge construction in other locations, such as australia and new zealand. since the beginning of the 1990s, timber-concrete composite bridges have been increasingly used mainly in some northern and western european countries, such as finland, switzerland, france, germany, and austria. from the beginning of applications different systems and configurations have been used for bridges. in general, these can be grouped in two categories, see fig. 1. in the design and performance of timber-concrete composite bridges, a key issue is the connection system, which has a direct influence on both the serviceability limit states and the ultimate limit states. bridge structures are normally designed for a long service life, 100 years. the reason for decay in timber bridges is nearly always poor detailing for durability, low quality of materials applied and neglected maintenance (a regular check of the whole bridge should be made at intervals not exceeding three to five years). structural timber as a natural product is part of a life cycle of growing and decomposition. due to that is also the best to use of either naturally durable or chemically treated timber. important is also to keep timber constantly dry as is possible. this paper deals with evaluation of properties of structural timber by semi-destructive method similar figure 1. timber-concrete bridge configuration (a) timber beam, (b) timber deck. to evaluation of concrete, new possibilities of monitoring of bridge structure and new chemical preservation on timber surface. 2. evaluation of density and mechanical properties of structural timber one of the semi-destructive methods of timber evaluation in timber structures is based on measurement of material resistance against penetration of an indenter. the damage of a timber structural member is small after the tests. one of the well-known devices is pilodyn 6j forest that penetrates the surface layers of timber element by a short pin with the help of an internal spring. it is possible to determine wood density and even the related mechanical properties with the input of the depth of the pin penetration [2]. wood density is of key importance value of timber. very important also for the use of non-destructive and semi-destructive methods. wood density is determined by several factors, e.g. cell diameter and cell wall thickness, proportions of springwood and summerwood, cellulose and lignin content etc. den57 http://dx.doi.org/10.14311/app.2019.22.0057 http://ojs.cvut.cz/ojs/index.php/app p. kuklík, l. velebil, a. gregorová, p. svora acta polytechnica ctu proceedings sity has a significant influence on wood mechanical properties and the effect of a non-destructive parameter is frequently associated with the density [3]. the depth to which the pin penetrates is indicated on the instrument and is inversely proportional to the wood density. the equations below can be used for estimation of wood density ρ12. the calculations were developed to be used for in situ measurement of timber quality in the existing structures [4] and [5]. they are based on moisture content (mc) measurement, which is an integral part of each testing of wood, and the depth of penetration tp. ρ12 = −0.027102 · tp,12 + 0.727987 (1) tp,12 = tp · (1 − 0.007 · ∆w) (2) ∆w = w − 12 (3) where • ρ12 is density of wood with mc of 12% [g·cm−3], • tp,12 the depth of penetration into wood with mc of 12% [mm], • tp the depth of penetration into wood with measured mc [mm], • w measured mc [%]. not only the strength fm, but also the modulus of elasticity e can be estimated with help of penetration depth tp or more precisely density ρ12. the relationships between the quantities with specific constants empirically determined resulted from the research [4]. fm = −36.13 + 0.156743 · ρ12 (4) estat,05 = −14 441.49 + 55.7834 · ρ12 (5) 3. mechanical stress monitoring of timber-concrete bridges by fbg sensors due to the increasing number of bridge construction faults, there is a need for monitoring their condition, which could reveal the emerging issues in time and warn of imminent violations. one of the innovative methods particularly suitable for long-term continuous monitoring of structural stress is the use of a fiber optic sensor. the technology of fiber bragg grating (fbg) sensor is an optical system used for the same purposes as strain gauge sensors. however, unlike strain gauges, deformation measurements and information transmission are realized by means of light. energy requirements of the system are therefore reduced and the system is significantly more resistant to most potential sources of interference, including electromagnetic interference. the fiber optic sensor is essentially an optical fiber equipped with a reflecting grid (the so-called bragg grid), which is produced for example by uv laser firing during fiber production. it is a very small, germanium doped portion of the optical fiber, which in the case of joining this fiber with the stretched body changes its optical properties in a defined manner, and thus it is possible to measure very precisely the elongation of the body. on one fiber can be up to a few tens of independent fbg sensors, each measuring a different place on construction and working at a different wavelength. the fiber is equipped with a cover polymeric sleeve which acts as a fiber protection against moisture and mechanical damage. the fbg sensors are based on a principle of reflection central wavelength of light by the bragg grid. a part of a supplied signal passes through the grid and a part of the light spectrum is reflected back. in the measuring unit, it is compared to the reference (unloaded) grid tuned to the same wavelength, the deformation is obtained from the difference of values. in the case of a newly made glued laminated timber element, the sensor can be inserted into the wooden structure when it is glued. if an existing or even historical structure is to be monitored, the sensor can be attached to its surface. the system was tested under laboratory conditions when the fiber with the sensor was embedded into a beam structure of glued laminated timber. the beam was mechanically evaluated by a 4-point loading test until failure, see fig. 2a). the correct function of the sensor was examined and a obtained data were compared with an analytical approach, see fig. 2b). more detailed information about experiments and achieved results can be found in [6]. absolute error of measurement compared to analytical calculation is to 10%. from the comparison of the measured and calculated data it can be stated that the fbg sensors are also suitable for application to timber elements, for example for monitoring the stress of timber-concrete bridge structures. the system can be configured to track the behaviour of the load bearing element during the lifetime of the structure and if necessary, alert to structural overload or technical problems. the technology of fiber optic sensors could be one of the ways to control the state of the bridges in the future. 4. protection of timber by photocatalytic materials generally timber protection materials should prolong lifetime of timber elements. the main role of the group protective materials based on photocatalysis is to protect timber element before color changing, moulds and fungus. the most spread photocatalytic materials for wood protection are tio2, zno, ceo2 [7] and wo3. they are used in nano-form and usually applied as a transparent coating. there are two ways, particle can be directly mixed with the solution or encapsulated into porous, resistant material (e.g. silicates) and then apply on the timber. encapsulation 58 vol. 22/2019 monitoring and protection of timber-concrete bridges (a). (b). figure 2. laboratory testing of fbg sensors a) experimental arrangement of the test; b) comparison of measured and calculated data. should avoid damage of timber and save ability to decompose organic compound. nowadays is tendency made more efficient photocatalytic materials by dopping. dopping should caused decreasing band gap between valence band and conductive band which means that absorption expand in the visible region of light. until now the results in this research area are not satisfactory. dopped photocatalytic material are not such efficient like un-dopped one. less known is using of iron oxides. they are used in cases when it is not necessary keep wood color. other inorganic compound which were studied as a photocatalyzer were cds, cuo, moo3, cr2o3, but they are not used in bid scale. tio2 belong to the most known materials with the photocatalytic properties. it is mainly designed for decomposition of organic compound. a lot of materials with the photocatalytic properties were used earlier as a pigments or part of compound created pigments. it was observed that some of them, react with surroundings. exterior paints based on titania had tend to chalking which means that in surface stayed non-adhesive powder. tio2 is present in lot of minerals which were known already in ancient times but tio2 itself as a compound is known from nineteenth century. on the beginning of the twentieth century start industrial production. timber protection with tio2. tio2 was tested in different type of wood species different climatic conditions. the use of tio2 usually increased resistance against the fungi and mould. on the other hand it was mentioned in some works the use of tio2 negatively influenced mechanical properties. the morphology of tio2 is varied from spheres particle to tubes. in our focus is planar particles-leaflets of tio2. the type what we use is possible prepare as a photoactive and also as a non-photoactive. the synthesis of tio2 is simple but time consumed. the starting compounds are titanium sulphate nhydrate which is dissolved in deionized water. next step is precipitation by concentrated aqueous nh3. white precipitate is filtered, properly washed and transferred into a beaker and re-suspended in deionized water. after that is added h2o2 until solution became yellow. last step is lyophilisation. more details about synthesis is refer in literature [8]. we are also looking for the ways how to make it shorter and keep the morphology and same physico-chemical properties. the goal of our work is study interaction between planar particles of tio2 and wood matter. testing is running on the pine wood and beech wood. the samples were coated with different coating and concentrations of tio2. experimental data from uv-vis spectroscopy clearly show that the planar particle of tio2 absorbed wide range of uv light (uva, uvb) (fig. 3). tio2 was annealed on 200 °c, 250 °c and 300 °c due to remove residual peroxide after the lyophilisation. also we want to know whether the annealing temperature has an impact on the absorption of uv-light. tio2 annealed on 200 °c and 300 °c have the maximum on the same wave length (310 nm), maximum tio2 annealed on 250 °c is slightly shifted (320 nm). shape and maximum of the absorbance’s curves is in all cases similar. on the base of experimental data we assumed that in temperature range between 200 °c and 300 °c absorbance is same. the planar morphology was proved by electron microscopy (fig. 4). the surface of the particle is not plain but wavy and also it is not completely continuous. the holes cover 20% of the surface and aggregates 10%. holes and aggregates in this amount have no negative impact on the uv absorption of the material as was proven by uv-vis spectroscopy. 5. summary • timber-concrete composite bridges are a technical solution with great potential for bridge construction. the wide range of examples also prove that this 59 p. kuklík, l. velebil, a. gregorová, p. svora acta polytechnica ctu proceedings figure 3. uv-vis of planar, amorphous tio2. figure 4. uhrsem planar particle of amorphous tio2. technology very useful in the past but also has enormous potential for use in the future. • several aspects for future application of nondestructive techniques in bridge survey and restoration shall be considered. this may include enhancement of non-destructive methods accounting for specific applications in the structural assessment of existing bridges and preparation of draft proposals for technical guidelines in the field of bridges inspection. • the idea protect external timber construction with the photocatalytic material is not new. what is missing are information’s about the interaction between photocatalytic material and timber. we chose as a model material planar tio2 because it is possible prepare it in two forms amorphous (non-photoactive) and crystalline (photoactive). we can study interaction of two phases morphologically identical material which is very important because research community admit theory where the morphology is one of the main factor with big impact on the photocatalytic properties. we are able also study role of the coating solvent (water, water glass, acryl). in case of water solution we assume that water evaporate and the particles are bonded by van der waals force so we will study directly interaction particle – wood, in case of water glass solution the inorganic porous structure is created and particles are surrounded silicate chains (system to study particle silicate chain – wood) and in case of acrylic solution the organic porous structure is created (system to study particle – acrylic acid compounds). obtain data will be used for new evaluation method of influence photocatalytic materials on surface of timber structures, mainly timber bridges. acknowledgements this work is supported by the project ga18-26297s “study of interactions in system: wooden surface amorphus layer of tio2 crystalline layer tio2”. references [1] j. p. wacker, a. dias, t. k. hosteng. investigation of early timber–concrete composite bridges in the united 60 vol. 22/2019 monitoring and protection of timber-concrete bridges states. in proceedings, ictb 2017, 3rd international conference on timber bridges, p. 11. skellefteå, sweden, 2017. [2] h. hasníková, p. kuklík. various non-destructive methods for investigation of timber members from a historical structure. wood research 59(03):411–420, 2014. [3] b. kasal, r. anthony. advances in in situ evaluation of timber structures. progress in structural engineering and materials 6:94 – 103, 2004. doi:10.1002/pse.170. [4] a. kuklíková. kompozitní dřevobetonové konstrukce. ph.d. thesis, czech technical university in prague, faculty of civil engineering, prague, 2004. in czech. [5] j. maddox, p. kuklík. non-destructive testing methods for determining material characteristis values of historic timber structures. in m. c. forde (ed.), structural faults & repair 2016. 16th international conference and exhibition. edinburgh, 17.05.2016 19.05.2016. engineering technics press edinburgh, 2016. [6] l. velebil, r. zelený, j. včelák, et al. optical fibre sensors as a potential solution for monitoring wooden structures. in wcte 2016 cd-rom proceedings. vienna, 2016. [7] m. nikolic, j. lawther, a. sanadi. use of nanofillers in wood coatings: a scientific review. journal of coatings technology and research 12:445–461, 2015. doi:10.1007/s11998-015-9659-2. [8] j. šubrt, p. pulišová, j. boháček, et al. highly photoactive 2d titanium dioxide nanostructures prepared from lyophilized aqueous colloids of peroxopolytitanic acid. materials research bulletin 49:405 – 412, 2014. doi:10.1016/j.materresbull.2013.09.028. 61 http://dx.doi.org/10.1002/pse.170 http://dx.doi.org/10.1007/s11998-015-9659-2 http://dx.doi.org/10.1016/j.materresbull.2013.09.028 acta polytechnica ctu proceedings 22:57–61, 2019 1 introduction 2 evaluation of density and mechanical properties of structural timber 3 mechanical stress monitoring of timber-concrete bridges by fbg sensors 4 protection of timber by photocatalytic materials 5 summary acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2019.25.0001 acta polytechnica ctu proceedings 25:1–5, 2019 © czech technical university in prague, 2019 available online at https://ojs.cvut.cz/ojs/index.php/app evaluation of hopkinson bar experiments using multiple digital image correlation software tools marcel adornaa,∗, stefan bronderb, jan faltaa, petr zlámal a, tomáš fílaa a czech technical university in prague, faculty of transportation sciences, department of mechanics and materials, konviktská 20, 120 00 prague, czech republic b universität des saarlandes, institute of applied mechanics, campus a4.2, 66123 saarbrücken, germany ∗ corresponding author: adorna@fd.cvut.cz abstract. three different tools for digital image correlation (dic) were used for evaluation of dynamic experiments performed using custom open hopkinson pressure bar (ohpb) apparatus. high strain-rate measurements were performed on specimens of advanced cellular materials with predefined structure and negative poisson’s ratio. low impedance polymethyl methacrylate (pmma) bars instrumented with foil strain-gauges were used for dynamic loading of the specimens. experiments were observed using a pair of high-speed cameras for imaging of loading process in sufficient quality. custom developed evaluation dic tool implemented in matlab, open-source matlab tool (ncorr) and commercial dic software (istra 4d) were all used for evaluation of image sequences recorded by high-speed cameras. comparison of results obtained using all three different dic tools and results of complementary strain-gauge measurement are shown in this paper. verification of reliability of custom made dic software tool is presented. keywords: digital image correlation, hopkinson bar, impact loading, high-speed imaging. 1. introduction digital image correlation (dic) is a well established method used for contactless assessment of full-field maps of displacement and strain on the surface of the deformed specimen. it shows a great promise for evaluation of high strain-rate experiments performed using hopkinson bar experimental device with specimens of geometrically complex structures [1]. in this study, three different dic software tools were used for evaluation of direct impact open hopkinson presure bar (ohpb) experiments performed on specimens of advanced materials with predefined structure and negative poisson’s ratio (so called auxetic structures). the compared dic softwares were: i) custom developed dic tool implemented in matlab [2], ii) open-source matlab tool ncorr [3] and iii) commercial dic software istra 4d. full-field dic analysis of the specimens surfaces was performed using all three software tools with similar parameters settings. obtained results were compared visually and quantitatively. results were also compared with complementary strain-gauge measurement. correlation of single nodal point of the specimen structure was also performed and influence of correlation coefficient was discussed. 2. materials and methods 2.1. specimens the specimens geometry was based on a periodical assembly of 3 × 3 × 3 re-entrant honeycomb auxetic unit cells. the specimens were printed using the pro jet hd3000 3d printer (3d systems, rock hill, usa) from the uv curable polymer visijet ex200 at the highest available printing resolution (656 × 656 × 800 dpi). this polymeric core samples were dip coated with a thin layer of carbon and an electrodepostion of nanocrystalline nickel (ni) was subsequently carried out, resulting in a homogeneous layer of nickle (coating thickness of approx. 60 µm and 120 µm) within the whole structure. finally the core polymer structure was melted out of the specimens using pyrosis at approx. 1000 °c. for more information regarding the used specimens manufacturing process see [4]. the specimens of the 3d printed nickle coated structures are shown in fig. 1. the overall sample dimensions (selected specifically to suit the dimensions of the ohpb apparatus) of the specimens were approximately 13 × 13 × 19 mm. 2.2. digital image correlation high-speed imaging enables to capture the deformation process of the ohpb tests in a suitable quality (sufficient number of images with small changes between them can be captured during experiment) for evaluation using optical methods. dic method enables for the evaluation of strain fields on the surface of geometrically complex structures by tracking of a virtual pattern selected in the sequence of images where the displacements are to be evaluated. each point of the virtual pattern is subsequently tracked through the whole captured image sequence. the tracking algorithm is based on subsets of pixels which are defined as an area with specific pixel dimensions formed 1 https://doi.org/10.14311/app.2019.25.0001 https://ojs.cvut.cz/ojs/index.php/app m. adorna, s. bronder, j. falta et al. acta polytechnica ctu proceedings figure 1. specimens of 3d printed nickle coated structure (left) and the uncoated structure (right) around the individual correlation point (fig. 2). as the specimen is deformed, the reference pattern is deformed as well in the image sequence. the new position of the deformed subset is determined based on the extreme value of a correlation coefficient calculated using the selected criterion. note, that not the whole area of the deformed picture is searched, but only an offset area, which reduce the computational complexity. figure 2. parameters of the digital image correlation, m subset dimension, k offset dimension, n grid spacing 2.2.1. custom dic matlab tool in our department, a custom developed dic matlab tool is commonly used for evaluation of various experiments. it is based on the lucas-kanade tracking technique, the correlation coefficients are estimated during a two-step procedure, first at the pixel level and second at sub-pixel level to determine the displacement with a higher precision. at the pixel level, the sum-squared difference (ssd) criterion is used whereas at the sub-pixel level the lucas-kanade algorithm based on the zero-normalised ssd (znssd) criterion is used [5]. 2.2.2. ncorr matlab tool ncorr is an open source 2d dic matlab program, which implements several different correlation procedures and criterions. however, for the evaluation of ohpb measurements, only one of the procedures provided results comparable with our customdeveloped matlab tool. in this procedure the zeronormalized cross correlation criterion (zncc) in combination with the gauss-newton nonlinear iterative least squares method is used. 2.2.3. istra4d istra4d is a very robust commercial multi-camera dic software. it implements many optimized correlation procedures, however the user is not able to select them manually and is therefore not in a full control of the correlation procedure. correlation criteria and their inner parameters are not accessible or modifiable. results obtained using istra4d are compared with other software tools, however, precision and reliability of istra4d software is not discussed in this paper. 2.3. open hopkinson pressure bar high strain-rate experiments with the auxetic specimens were performed using the ohpb direct impact apparatus [6]. in contrary to conventionally used split hopkinson pressure bar (shpb) technique, in the ohpb no striker bar is used and instead an instrumented incident bar is accelerated by a gas-gun system and directly hits a specimen mounted at the end of a transmission bar in the setup. in our ohpb setup, the incident bar is guided by a gas-gun barrel and by a low friction linear guidance system. this apparatus can be used to compress specimens to a considerably high strain. moreover, as both the incident and the transmission bar are instrumented with strain-gauges, it is possible to evaluate the data from both contact faces of the mounted specimen. due to a low mechanical impedance of the tested specimens, the polymethyl-methacrylate (pmma) bars with diameter of 20 mm were used. both incident and transmission bar had a length of 1750 mm. one measurement point was placed on the incident bar, two measuring points were placed on the transmission bar. all measuring points were equipped with two pairs of strain-gauges (one pair semiconductor and one pair foil strain-gauges) in wheatstone halfbridge arrangement. for this study, only data from foil strain-gauges 3/120 ly61 (hbm, germany) with 3 mm active length were used since semiconductor strain-gauges exhibited non-linear behaviour in larger deformations and had a significantly reduced lifetime. the experiments were observed using a pair of synchronised high-speed camera fastcam sa-z (photron, japan) with resolution of 256 × 168 px at approx. 252 kfps. sufficient illumination of the specimen surface was achieved using a pair of high intensity led lights (constellation 60, veritas, usa). note, that only one of the two cameras observed the surface of measured sample and its close proximity. record from the second camera was not used for evaluation in this study but only for general inspection of the experiment. overview of the used experimental setup and the basic principle of ohpb are shown in fig. 3. 2 vol. 25/2019 evaluation of hopkinson bar experiments using multiple dic tools figure 3. principle of ohpb method with schematic of high speed camera used for dic evaluation (left), hopkinson bar experimental setup with a pair o high-speed cameras (right) 3. dic tools comparison 3.1. full-field dic comparison a virtual grid of 22 × 14 (columns × rows) correlation points was created to cover the area of the specimens surface and the ends of the measuring bars on the high-speed camera images. the same parameters of dic evaluation were set for all three compared softwares: subset size m = 23 px, offset size k = 33 px, grid spacing n = 4 px. evolution of the full-field in-plane displacement of the representative auxetic specimen with 60 µm layer of coating deformed using the ohpb apparatus and evaluated using the dic by all three correlation softwares is shown in fig. 6. correlation of high-speed camera images was possible only up to the 100th image in the sequence which corresponds to approx. 0.15-0.2 overall strain (35 px of subset displacement). for higher displacement, ncorr provides distorted results due to low value of correlation coefficient and loss of correlation points. istra4d and custom dic tool results are reliable up to 130th image (approx. 0.25-0.3 overall strain, displacement of 45 px). dynamic equilibrium was achieved during dynamic experiment as the sample was deformed uniformly on the both ends. 3.2. dic and strain-gauge results comparison the stress-strain curves with strain calculated using the three dic tools paired with the strain-gauge stress compared with the curve from conventional straingauge evaluation are shown in fig. 4. dic strain was for all software tools derived by tracking grid of correlation points created on the both edges of the deformed specimen. stress-strain curves derived using all correlation tools are in a very good agreement with the strain-gauge curve on both samples up to 0.16 of overall strain (100th image). for istra4d software and our custom correlation tool it is possible to perform correlation up to 0.25 of overall strain. differences of the dic curves to strain-gauge curve after 0.25 is due to large deformations of the individual cells of the tested structure resulting in a loss of correlation. figure 4. comparison of stress-strain diagrams with strain evaluated from strain-gauges and dic tools 3.3. dic single nodal point analysis to further investigate the process of correlation of the virtual grid, the correlation of a single grid point located in the node of the specimen structure was performed. tracked point of the structure and its location during the deformation is shown in fig. 7. original location of the point is marked by the green cross, the red cross marks the location evaluated from our custom tool and the blue cross from ncorr. note, that for relatively small displacement, the location provided by both software tools is identical (frame 30). the displacement of the same nodal point through the image sequence is shown in fig. 5. figure 5. displacement of single node point through image sequence 3 m. adorna, s. bronder, j. falta et al. acta polytechnica ctu proceedings figure 6. full-field dic analysis, ohpb measurement of auxetic structure: displacement in pixel, custom dic tool (top), ncorr (middle), istra4d (bottom); note, that istra4d uses different color range figure 7. single correlation point tracking, original point position (green), custom dic tool (red), ncrorr (blue) it is shown, that both the custom developed dic tool and ncorr track the nodal point reliably up to the 80th frame in the sequence. after that, the reliability of custom made software is much better. this is also illustrated by the graph shown in fig. 8, which shows the correlation coefficient mean value across all the correlation points in virtual grid. it is shown, that although the correlation criterions are different, the drop of correlation coefficient in ncorr software is much larger through the image sequence than in custom developed dic tool. 4. conclusion ohpb measuring device with low impedance pmma bars was successfully used for dynamic testing of the nickle coated 3d printed specimens of auxetic structure. three different dic tools were used for evaluation high-speed camera record of representative experiment. the results derived from all three software tools were compared and were found to be in a good agreement with the complementary strain-gauge figure 8. correlation coefficient of compared dic tools measurement. the results from ncorr open-source tool were reliable up to 16 % of deformation, the results from custom dic tool and commercial istra4d software were reliable up to 25% of deformation. single nodal point analysis was carried out for further proving that our custom developed dic software tool 4 vol. 25/2019 evaluation of hopkinson bar experiments using multiple dic tools is not only comparable with other software tools but provides even more reliable results. it was shown that both, the ohpb and the dic are suitable tools for advanced characterization of nickle coated auxetic structures under high strain-rate loading. acknowledgements the research was supported by the czech science foundation (project no. 19-23675s) and the internal grants of the czech technical university in prague (projects no. sgs18/153/ohk2/2t/16 and sgs18/154/ohk2/2t/16). all the financial support is gratefully acknowledged. references [1] t. fíla, p. koudelka, p. zlámal, et al. strain dependency of poisson’s ratio of sls printed auxetic lattices subjected to quasi static and dynamic compressive loading, 2019. doi:10.1002/adem.201900204. [2] i. jandejsek, j. valach, d. vavřík. optimization and calibration of digital image correlation method, 2010. [3] j. blaber, b. adair, a. antoniou. ncorr: open-source 2d digital image correlation matlab software. experimental mechanics 55(6):1105–1122, 2015. doi:10.1007/s11340-015-0009-1. [4] a. jung, m. weinmann, h. natter. electroforming and electrodeposition on complex 3d geometries: special requirements and new methods, pp. 941–970. springer international publishing, 2016. doi:10.1007/978-3-319-15266-0_46. [5] b. d. lucas, t. kanade. an iterative image registration technique with an application to stereo vision. in proceedings of the 7th international joint conference on artificial intelligence volume 2, pp. 674–679. morgan kaufmann publishers inc., san francisco, ca, usa, 1981. [6] r. a. govender, r. j. curry. the “open” hopkinson pressure bar: towards addressing force equilibrium in specimens with non-uniform deformation. journal of dynamic behavior of materials 2(1):43–49, 2016. doi:10.1007/s40870-015-0042-2. 5 https://doi.org/10.1002/adem.201900204 https://doi.org/10.1007/s11340-015-0009-1 https://doi.org/10.1007/978-3-319-15266-0_46 https://doi.org/10.1007/s40870-015-0042-2 acta polytechnica ctu proceedings 25:1–5, 2019 1 introduction 2 materials and methods 2.1 specimens 2.2 digital image correlation 2.2.1 custom dic matlab tool 2.2.2 ncorr matlab tool 2.2.3 istra4d 2.3 open hopkinson pressure bar 3 dic tools comparison 3.1 full-field dic comparison 3.2 dic and strain-gauge results comparison 3.3 dic single nodal point analysis 4 conclusion acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2018.15.0031 acta polytechnica ctu proceedings 15:31–35, 2018 © czech technical university in prague, 2018 available online at http://ojs.cvut.cz/ojs/index.php/app the effect of micro-silica on the microscopic features of the uhpc composite and its inter-facial transition zone vladimír hrbeka,b,∗, zdeněk prošeka, roman chylíka, lukáš vráblíka a czech technical university in prague, faculty of civil engineering, thákurova 7, 166 29 prague 6, czech republic b institute of theoretical and applied mechanics as cr, v.v.i., prosecká 76, 190 00 prague 9, czech republic ∗ corresponding author: vladimir.hrbek@fsv.cvut.cz abstract. the uplift of concrete overall macroscopic performance by way of alternative additives is commonly used technique. in case of ultra high performance concrete (uhpc), micro-silica is added to the mixture as a micro-filler to the structure of cement binder. as a result, the cementitious matrix macro-mechanical performance is elevated. this paper is aimed on the uhpc micro-scale enhancement by different micro-silica additive content in the mixture. more closely, the study investigate the impact of the micro-silica on the inter-facial transition zone (itz) between the binder matrix and basaltic aggregate. keywords: high performance concrete, indentation, spectral deconvolution sem, image analysis, itz. 1. introduction the rise of high performance concrete (hpc) usage in civil engineering practice over last decades can be generally attributed to increasing demands on sustainable development and cost reduction of constructions and building materials. the hpc exceeds the properties and construct-ability of normal concrete. this can be obtained, besides special mixing, placing, and curing practices, by addition of special materials specially selected for the concrete to meet required performance or combination of few. one of such additives, used in high performance concrete design, is microsilica (or silica fume). microsilica, in terms of its chemical composition, is a very fine solid sphere-like mineral composed of silicon dioxide (sio2). most silica fume particles are less than 1 micron in diameter. for example, average cement particle of cem i 42.5r mokrá has 34 µm in diameter and fly ash particles diameter ranges from 0.5 to 300 µm [1–3]. the silica fume is a by-product of the industrial manufacture of ferro-silicon and metallic-silicon in high-temperature electric arc furnaces. the ferrosilicon or silicon product is drawn off as a liquid from the bottom of the furnace. vapor rising from the 2000 ◦c furnace bed is oxidized and, after cooling, the condensate of particles is collected. processing the condensed fume to remove impurities and control particle size yields to microsilica production [4]. the effectiveness of silica fume as a pozzolan and a micro-filler depends on its composition and particle size, which are given by the design of the furnace and the composition of the raw charging materials of the furnace. dosage of microsilica addition range used in concrete is typically in about 5 to 20 % of cement weight, but up to 40 weight percentage can be found. used as an admixture, microsilica can improve both fresh and hardened state properties of concrete [5, 6]. most of current economic and environmental aspects favor the use of silica fume as an admixture, as growing body of studies of concrete with microsilica content shows improvement of its mechanical features, impermeability, chemical and abrasion resistance. from both micro-structural and mechanical angle, cement paste with silica fume containment comprise higher percentages of the calcium-silicate hydrates (csh) and less of the soluble calcium hydroxides, compare to ordinary cement pastes. because the microsilica particles size, its grains disperse in csh gel, forming fine and uniform cementitious matrix with improved mechanical properties [7–10]. 2. materials, sample preparation four different mixtures of cementitious specimens were selected for purpose of this study referential mixture (ref) and 3 modified mixtures containing 10, 20 and 30 weight% cement by mircosilica replacement (sf10, sf20 and sf30). ingredients of referential mixture consisted of cem i 42.5 r mokrá binder, aggregate filler (crushed unwashed basalt of particle sizes 0–4, 4–8 and 8–16 mm), water and poly-carbonate based plasticizer (stachement 508, stachema). the compositions is described in tab. 1. macroscopic specimens, cubes of 100 mm in each dimension, were kept in laboratory water environment (20–25◦c) over 14 days for adequate composite matrix maturation and to reduction of rheological processes and specimen degradation. furthermore, grinding 31 http://dx.doi.org/10.14311/app.2018.15.0031 http://ojs.cvut.cz/ojs/index.php/app v. hrbek, z. prošek, r. chylík, l. vráblík acta polytechnica ctu proceedings and polishing methods avoiding selective abrasion of phases were implemented on epoxy-resin embedded cuts of samples for purpose of electron microscopy and indentation. usage of these methods ensured both adequate roughness of the specimens surface and compact interface transition zone between aggregate and cementitious matrix. component ref sf10 sf20 sf30 cem i 42.5 r 800.0 720.0 640.0 560.0 aggreg. 0–4 730.0 730.0 730.0 730.0 aggreg. 4–8 390.0 390.0 390.0 390.0 aggreg. 8–16 320.0 320.0 320.0 320.0 microsilica 0.00 80.0 160.0 240.0 water 210.0 231.0 252.0 273.0 plasticizer 32.0 36.0 36.0 36.0 table 1. compositions of tested mixtures in [kg·m−3]. 3. methods and data processing the scanning electron microscopy (sem) and grid nano-indentation testing were independently used to evaluate high performance concrete specimens. in order to overcome complex heterogeneity of the composite, both investigating methods were implemented on two main material levels, i.e. matrix without basaltic aggregate and transition zone between aggregate and matrix with limited clinker occurrence. due to this separation, duplicity of obtained data (mechanical and/or structural features of different phases, such as aggregate and clinker) is prevented. 3.1. scanning electron microscopy and image analysis scanning electron microscope mira ii lmu (tescan corp., brno) equipped with epma was used for microstructure investigation and phase recognition of hpc samples with silica fume content. chemical composition (i.e. elemental analysis) of each phase was established by energy-dispersive x-ray spectroscopy (edx). despite precision of the edx in determining weight percentage representation of each chemical element in measured area, time demands and coverage limitations of this method are not suitable for proper overall investigation of specimen structure. in general, energy of back-scattered electron (bse) and their brightness captured by bse diagram is relevant to chemical composition of investigated compound. therefore, combination of appropriate number (20 positions for each sample) of chemical composition and gray-scale range from edx investigation and gray-scale sem bse diagrams image analysis led to numerical quantification of percentage representation of each phase, i.e. description of material structure (see fig. 2). in case of aggregate-matrix transition zone, two-stage image analyses was applied on bse diagrams as the sodium-calcium feldspar forming a structure of basalt aggregate is, on gray-scale, close to phases present in matrix. differential image analysis thus consisted of separating main binding phases (cs-h gel) and the rest of the material with following identification of these phases based on their internal heterogeneity. 3.2. quasi-static indentation and spectral deconvolution individual phases elastic micro-mechanical features were derived from grid indentation of prepared samples (ti 700 series, hysitron inc.). principle of indendentation technique is based on dependency of probe propagation with respect to the recorded material response. the force-displacement record is further processed and the mechanical properties are calculated from the unloading part of the record. to avoid incorporated errors during the measurement (creep, visco-elasticity of measured phases), appropriate test setup is neccessary [11–16]. the applied load function prescribed for each indent was displacement driven with maximum indentation depth of 150 nm. for each material level, the displacement driven function consisted of loading and unloading period lasting 5 seconds with in-between holding time segment with 60 seconds duration, over which maximum indentation depth was held in order for material to creep. the indents, again for both levels of investigation, were formatted in 21 by 21 grind with mutual 10 µm separation. due to material level separation and selected loading function, grid indentation criteria of heterogeneous material are met [17–21]. indentation modulus of individual phases, forming the structure of the composite, were derived from normalized histograms of the measured data by spectral deconvolution. the histograms are previously cleared from incorrectly measured data, caused by human error or environmental conditions. the process of spectral deconvolution, unlike statistical deconvolution, incorporates mutual interaction of phases with different stiffness, such as interacting low-stiffness ld c-s-h and any other phase (like high-stiffness clinker). even though both investigated material levels of all samples were measured and processed separately, overall effective features were established from merged data. maximum allowed divergence between structural percentage representation and derived mechanical features did not exceed 5.0%. 4. results the results of previously described methods and data analysis are stated in this section. mechanical features of individual phases, derived from normalized histograms of indentation measurements (fig. 1) by spectral deconvolution, are summarized in tab. 2. the structural description of the composite, in means of individual phase percentage representa32 vol. 15/2018 uhpc microsilica sample itz ld csh hd csh portlandite clinker aggregate ref 13.68 ± 2.38 30.05 ± 4.65 38.93 ± 2.47 66.63 ± 17.36 120.54 ± 17.99 175.40 ± 2.11 sf10 16.82 ± 2.18 32.91 ± 4.16 48.43 ± 5.78 66.55 ± 8.50 122.28 ± 8.50 159.58 ± 8.22 sf20 18.47 ± 2.41 33.37 ± 2.92 48.94 ± 4.24 66.12 ± 11.74 135.49 ± 11.74 167.84 ± 7.43 sf30 18.87 ± 3.91 34.70 ± 3.17 49.98 ± 3.48 65.82 ± 6.51 132.65 ± 6.51 168.47 ± 14.02 table 2. indentation modulus er [gpa] of concrete containing silica fume derived by spectral deconvolution. figure 1. histograms of indentation modulus er. tion, acquired by image analysis of sem bse microdiagrams (fig. 2), is shown in tab. 3. 5. conclusion from presented microscopical indentation results and phase representation (based on image analysis of sem bse diagrams) of investigated composites, following conclusions can be observed: • indentation module of all main csh phases (itz, ld csh, hd csh) are improved with silica fume content increment in specimens, while minor influence can be observed on calcium hydrate (portlandite) features; • the highest impact can be observed on sf30 specimen, where indentation modulus of (compare to referential sample) inter-facial transition zone is increased by 37.9 %. the ld csh and dh phases are enhanced by 15.5 % and 28.4 % respectively. the decrease of portlandite indentation modulus reaches 1.2 %; • the microsilica content does not impact indentation modulus of non-hydrated clinker and aggregate. the deviation of their micro-mechanical properties are caused purely by internal heterogeneity of these phases; • the amount of silica fume addition and microporosity reduction are vice verse to impact on mechanical properties, i.e. sf10 sample (with the lowest percentage of microsilica additive) indicates minimum micro-porosity of the cement matrix (22.7 % of referential sample micro-porosity); • comparison of sf30 sample to ref specimen shows that representations of ld csh and non-hydrated clinker were increased by 22.2 % and 61.7 %, while both hd csh and calcium hydrate (portlandite) decreased by 12.3 % and 13.9 %. the impact of silica fume replacement for binder component of concrete composite on micro-structural aspect inter-facial transition zone was not conclusive. the gray-scale range (basic chemical composition) of the itz is identical low density csh. furthermore, detailed investigation of silica fume content impact on itz thickness, with respect to aggregate size, is 33 v. hrbek, z. prošek, r. chylík, l. vráblík acta polytechnica ctu proceedings (a) . ref bse diagram (left) image analyses (right). (b) . sf10 bse diagram (left) image analyses (right). (c) . sf20 bse diagram (left) image analyses (right). (d) . sf30 bse diagram (left) image analyses (right). figure 2. example of matrix analysis: white=clinker, red=ch, magenta=hd csh, green=ld csh, blue=pores. sample porosity ld csh hd csh portlandite clinker aggregate ref 1.50 ± 0.08 15.82 ± 1.25 15.43 ± 1.35 6.55 ± 0.55 4.18 ± 0.12 56.52 ± 2.39 sf10 0.34 ± 0.02 16.09 ± 1.42 16.61 ± 1.44 4.07 ± 0.26 5.43 ± 0.56 57.45 ± 3.44 sf20 0.60 ± 0.03 18.30 ± 1.98 14.57 ± 1.17 4.49 ± 0.32 6.48 ± 0.72 55.56 ± 4.31 sf30 1.09 ± 0.09 19.33 ± 1.01 13.74 ± 1.38 5.75 ± 1.00 6.76 ± 0.65 53.33 ± 3.79 table 3. average percentage representation of individual composite phases. proposed through line-indentation technique and sem line-scan combination. this research was done as a part of project concern with hpc additives (microsilica, fly ash and metakaolin) impact on the microscopic features of the composite. the data are be further used in numeric modeling of the material and predication of material elastic properties based on known mixture composition. list of symbols f measured response/force [µn] hi indentation depth [nm] er indentation modulus [gpa] h hardness [gpa] hc contact depth [nm] p d probability density [–] acknowledgements the research is financially supported by czech technical university in prague project sgs17/168/ohk1/3t/11 and by the czech science foundation research project 17-19463s. the support is gratefully appreciated. references [1] v. m. malhotra, g. g. carette. silica fume a pozzolan of new interest for use in some concrete. concrete construction 86(1):443–446, 1982. 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[14] w. oliver, g. m. pharrr. an improved technique for determining hardness and elastic modulus using load and displacement sensing indentation measurements. journal of material research 7(6):1564–1583, 1992. doi:10.1557/jmr.1992.1564. [15] g. m. pharrr, a. bolshakov. understanding nanoindentation unloading curves. journal of material research 17(10):2660–2671, 2002. doi:10.1557/jmr.2002.0386. [16] w. oliver, g. m. pharrr. measuring of hardness and elastic modulus by instrumented indentation: advances in understanding and refinement of methodology. journal of material research 19(1):3–20, 2004. doi:10.1557/jmr.2004.19.1.3. [17] l. sorelli, g. constantinides, f.-j. ulm, f. toutlemonde. the nano-mechanical signature of ultra high performance concrete by statistical nanoindentation techniques. cement and concrete research 38(12):1447– 1456, 2008. doi:10.1016/j.cemconres.2008.09.002. [18] f.-j. ulm, m. vandamme, c. bobko, et al. statistical indentation techniques for hydrated nanocomposites: concrete, bone and shale. journal of the american ceramic society 90(9):2677–2692, 2007. doi:10.1111/j.1551-2916.2007.02012.x. [19] g. constantinides, k. s. ravi chandran, f.-j. ulm, k. j. van vliet. grid indentation analysis of composite microstructure and mechanics: principles and validation. journal of material science and engineering a 430(12):189–202, 2006. doi:10.1016/j.msea.2006.05.125. [20] w. zhu, j. j. hughes, n. bicanic, c. j. pearce. nanoindentation mapping of mechanical properties of cement paste and natural rocks. materials characterization 58(11-12):1189–1198, 2007. doi:10.1016/j.matchar.2007.05.018. [21] o. bernard, f.-j. ulm, e. lemarchand. a multiscale micromechanics -hydration model for the early-age elastic properties of cement-based materials. cement and concrete research 33(9):1293–1309, 2003. doi:10.1016/s0008-8846(03)00039-5. 35 http://dx.doi.org/10.1016/j.conbuildmat.2008.08.021 http://dx.doi.org/10.1016/0958-9465(95)00005-w http://dx.doi.org/10.1016/s0958-9465(00)00037-8 http://dx.doi.org/10.1016/j.resconrec.2011.09.016 http://dx.doi.org/10.1016/j.msea.2004.04.070 http://dx.doi.org/10.1557/jmr.1997.0324 http://dx.doi.org/10.1557/jmr.1992.1564 http://dx.doi.org/10.1557/jmr.2002.0386 http://dx.doi.org/10.1557/jmr.2004.19.1.3 http://dx.doi.org/10.1016/j.cemconres.2008.09.002 http://dx.doi.org/10.1111/j.1551-2916.2007.02012.x http://dx.doi.org/10.1016/j.msea.2006.05.125 http://dx.doi.org/10.1016/j.matchar.2007.05.018 http://dx.doi.org/10.1016/s0008-8846(03)00039-5 acta polytechnica ctu proceedings 15:31–35, 2018 1 introduction 2 materials, sample preparation 3 methods and data processing 3.1 scanning electron microscopy and image analysis 3.2 quasi-static indentation and spectral deconvolution 4 results 5 conclusion list of symbols acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2019.21.0042 acta polytechnica ctu proceedings 21:42–45, 2019 © czech technical university in prague, 2019 available online at http://ojs.cvut.cz/ojs/index.php/app effect of bacteria bacillus pseudofirmus and fungus trichoderma reesei on self-healing ability of concrete hana žáková∗, jiří pazderka, zuzana rácová, pavla ryparová czech technical university in prague, faculty of civil engineering, department of building structures, thákurova 7, 166 29 prague, czech republic ∗ corresponding author: hana.zakova@fsv.cvut.cz abstract. there are usually used inorganic substances for the strengthening of self-healing ability of concrete but some specific types of bacteria or fungi may also be considered for use. the bacteria’s ability to fill pores and micro-cracks was investigated on cylindrical concrete specimens with the diameter 55 mm and height 5 mm. bacteria bacillus pseudofirmus and fungus trichoderma reesei was used for the experiment. the main objective of the experiment was to investigate, if it is possible to use bacillus pseudofirmus and trichoderma reesei for self-healing concrete. the results show, that it is more advantageous to use bacillus pseudofirmus than trichoderma reesei in alkaline environment. it is desirable to create the most ideal conditions for microorganism’s growth, as possible. bacteria should have positive effect on self-healing ability of concrete. any effect of fungus on self-healing ability of concrete wasn’t confirmed. keywords: self-healing concrete, bacillus pseudofirmus, trichoderma reesei, filling of pores and micro-cracks. 1. introduction one of the highest priorities in civil engineering is to ensure the maximum durability of concrete structures in accordance with the theory of sustainable building. from this perspective, the typical problem is more financial requirement caused by necessary remediation measures in the concrete structure, related to the load carrying capacity or waterproofing, which is a key parameter to ensure durability. one of the options that have emerged in recent years is to use self-healing concrete. more specifically, this concrete structures have this ability significantly strengthened, because almost every concrete structure has a certain ability of autogenous healing, but this ability is limited (depends on boundary conditions). for strengthening of the self-healing ability of concrete, there are usually used inorganic substances, but some specific types of bacteria or fungi can also be used. the main objective of these bacteria and fungi is to penetrate the structure of the concrete and fill micro-cracks and pores [1],[2]. the waterproofing of the structure should increase by using microorganism, increasing of the load bearing capacity in some cases is also assumed [1],[2]. resistance of the microorganisms to the highly alkaline environment, typical for cement, is the necessary condition for using. this problem is mainly related to the fresh and low carbonated concrete, which can be found mainly in the new buildings, but there also can be various disorders that need to be rehabilitated. it is estimated that, in the world, 7% of co2 emissions are due to the cement production and, therefore, it would be advantageous to reduce the quantity produced as figure 1. concrete specimen (diameter 55 mm, height 5 mm). much as possible [1]. 2. methods the ability of microorganisms to fix micro-cracks and pores was investigated during the experiment on cylindrical concrete specimens with the diameter 55 mm and height 5 mm (figure 1). bacteria bacillus pseudofirmus (figure 2) and fungus trichoderma reesei (figure 3) was used for the experiment. concrete specimens were created from mixture consisting of portland cement (cem i 42.5), aggregate (fraction 04) and the water. the microorganisms weren’t added to the mixture in this phase. for verification, if the microorganisms are able to grow up in early-aged concrete with high alkalinity the specimens weren’t artificially carbonated. the experi42 http://dx.doi.org/10.14311/app.2019.21.0042 http://ojs.cvut.cz/ojs/index.php/app vol. 21/2019 effect of bacteria bacillus pseudofirmus and fungus trichoderma reesei figure 2. bacillus pseudofirmus, olympus bx41, 1000x. figure 3. trichoderma reesei, olympus bx41, 100x. mental measurement began 28 days after embedment of the specimens. first, moisture content and ph were measured. for determination of the moisture content the gravimetric method was used. subsequently, the specimens were saved in environment of 100% relative humidity for 4 weeks. after that, the microorganisms were implemented. only one type of concrete mixture was used for the experiment, but the test specimens were placed in different conditions. the main objective was to identify differences in the growth of microorganisms on specimens kept in ideal conditions and specimens kept in conditions more close to the real situation on site. there was measured a spectrum (figure 5) of bacillus pseudofirmus in 253 media using spectrophotometer spectroquant pharo 300. for bacterial growth investigation, 5 specimens were placed in 253 media, 5 specimens were placed in 253 media with 2% cacl2 solution (figure 4) and 5 specimens were placed in sterile water. into two erlenmeyer flasks, there were 253 media and 253 media with 2% cacl2 solution, without specimens. all of these solutions have been inoculated with the bacteria bacillus pseudofirmus in a ratio of 1 ml solution with bacteria to 100 ml media. growth curve was measured by spectrophotometry for next 28 days. measured curves initially show an increase, but gradually they have a decreasing tendency. after 28 days of the bacterial deployment, the growth curve measurement was terminated. for fungus growth investigation, 8 specimens were placed in petri dishes with malt agar. 5 specimens were inoculated with fungus trichoderma reesei. 3 specimens weren’t inoculated and one petri dish just with malt agar was inoculated with trichoderma reesei figure 4. concrete specimens in media (253 media on the left, 253 media with 2% cacl2 solution on the right). figure 5. spectrum of bacillus pseudofirmus in 253 media. for the reference test. all petri dishes were placed in a thermostat where they were kept at a constant temperature 24◦c. 13 specimens were placed in desiccators with 100% humidity (figure 6). 10 of them were regularly inoculated by solution. 5 specimens were inoculated with fungus trichoderma reesei and 5 specimens were inoculated with bacteria bacillus pseudofirmus. 3 specimens were placed in one desiccator for reference test and weren’t inoculated neither with bacteria nor with fungus. one specimen was placed in petri dish without agar and without inoculation and was left in normal laboratory environment as a reference test. 3. results and discussion concentration of bacteria in 253 media and 253 media with 2% cacl2 solution increased rapidly. growth curve measured in solution with specimens was similar as in the solution in erlenmeyer flasks. in solution of bacteria in water with specimens wasn’t so rapid increasing of bacteria concentration as in both media (figure 8). 28 days after experiment started, surface of the specimens was controlled using a microscope olympus bx41. on the surface of specimens placed in 253 media with 2% cacl2 solution and specimens 43 h. žáková, j. pazderka, z. rácová, p. ryparová acta polytechnica ctu proceedings figure 6. concrete specimens in desiccators. figure 7. development of fungus trichoderma reesei. placed in water, there were no visible changes (figure 9). on surface of specimens placed in 253 media, there were white crystals observed (figure 11). on the surface of the specimens placed in desiccator and inoculated by bacteria bacillus pseudofirmus, there were no visible changes 33 days after first inoculation. fungus trichoderma reesei grow up on malt agar around the specimens (figure 7). 33 days after experiment started, surface of the specimens was controlled figure 8. growth curves of bacteria in 253 media (on top), in 253 media with 2% cacl2 solution (in the middle) and in the water (on bottom). using a microscope olympus bx41. there were no visible changes on the surface of the specimens. this result was probably due to high alkalinity of the specimens. on the surface of the specimens placed in desiccator and inoculated by fungus trichoderma reesei, there were no visible changes 33 days after first inoculation. the experiment shows, that using of bacteria bacillus pseudofirmus for making self-healing concrete is more advantageous than using of fungus trichoderma reesei, because bacteria is able to grow up in high alkalinity conditions. the experiment also shows that it is necessary to provide the most ideal conditions to the microorganism’s growth, as possible. 44 vol. 21/2019 effect of bacteria bacillus pseudofirmus and fungus trichoderma reesei figure 9. surface of the specimen placed in solution of bacteria in 253 media in 2% cacl2 solution. 4. conclusion effect of bacteria bacillus pseudofirmus on self-healing ability on early-aged concrete was confirmed just in one case when concrete specimens were placed in 253 media. in other cases the effect wasn’t confirmed. but it is possible, that bacteria bacillus pseudofirmus should have positive effect on the self-healing ability of concrete in optimal conditions. it is necessary to continue in the research and make other experiments. effect of fungi trichoderma reesei on self-healing ability on early-aged concrete wasn’t confirmed. it probably was due to high alkalinity of concrete specimens. the research show, that using of the trichoderma reesei in concrete with high alkalinity is useless. acknowledgements this work was supported by ctu research project no. sgs17/117/ohk1/2t/11. references [1] h. m. jonkers, a. thijssen, g. muyzer, et al. application of bacteria as self-healing agent for the development of sustainable concrete. ecological engineering 36(2):230–235, 2010. [2] j. luo, x. chen, j. crump, et al. interactions of fungi with concrete: significant importance for bio-based self-healing concrete. construction and building materials 164:275–285, 2018. figure 10. surface of the specimen placed in solution of bacteria in the water. figure 11. surface of the specimen placed in solution of bacteria in 253 media. 45 acta polytechnica ctu proceedings 21:42–45, 2019 1 introduction 2 methods 3 results and discussion 4 conclusion acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2017.13.0075 acta polytechnica ctu proceedings 13:75–79, 2017 © czech technical university in prague, 2017 available online at http://ojs.cvut.cz/ojs/index.php/app solid burnt bricks’ tensile strength aneta maroušková∗, jan kubát czech technical university in prague, faculty of civil engineering, thákurova 7, 166 29 prague 6, czech republic ∗ corresponding author: aneta.marouskova@fsv.cvut.cz abstract. this paper deals with experimental testing of solid burnt bricks and mortar in pure (axial) tension. the obtained working diagrams will be further use for a detailed numerical analysis of whole brick masonry column under concentric compressive load. failure mechanism of compressed brick masonry column is characterized by the appearance and development of vertical tensile cracks in masonry units (bricks) passing in the direction of principal stresses and is accompanied by progressive growth of horizontal deformations. these cracks are caused by contraction and interaction between two materials with different mechanical characteristics (brick and mortar). the aim of this paper is more precisely describe the response of quasi-brittle materials to uniaxial loading in tension (for now only the results from three point bending test are available). for these reasons, bricks and mortar tensile behavior is experimentally tested and the obtained results are discussed. keywords: tensile strength, pure tension, brick, mortar. 1. overview the aim of this paper is determine solid burnt bricks’ and mortar tensile strength in “pure” tension. obtained working diagrams will be further used for detailed numerical analysis of whole masonry brick column. under concentric compressive load of masonry column, bricks are gradually damaged by lateral tension accompanied by appearance and development of cracks. these cracks are caused by contraction and interaction between two materials with different mechanical characteristics as a result of this mutual interaction, mortar that has a lower young’s modulus and has tendency to greater lateral strain, is transversely “pressed” and masonry units, on the contrary are transversely “stretched” [1, 2]. 2. experimental setup for bricks six p20 solid burnt bricks with dimensions of 290×140×65 mm (the declared compressive strength is 20 mpa) were experimentally tested in pure tension test. all specimens were measured and the specific dimension were used for each specimen. steel plates were glued on brick’s face in longitudinal directions by means two components epoxy glue akepox 2040 according to technological procedure in laboratory conditions. akepox 2040 reaches the maximal strength in seven days, after that the specimen were tested. brick with steel plates were placed into testing equipment labtest 4.100sp1 by means two snap hooks (fig. 1). this equipment adds load in form of deformation in regular steps. figure 1. experimental setup. 75 http://dx.doi.org/10.14311/app.2017.13.0075 http://ojs.cvut.cz/ojs/index.php/app aneta maroušková, jan kubát acta polytechnica ctu proceedings figure 2. failure of specimens no. 4 and 5. 3. discussion of the obtained results failure occurred at the interface between steel plate and brick with the exception of two samples. these two samples had initial damage from manufacturing. during experimental testing was observable a slowly opening of the crack following by brittle failure. failure mode of both specimen (no. 4 and 5) is showed in fig. 2. after failure, different level of burning is evident on the cross-section of specimens. grey core in specimen 5 indicates incomplete burning process. specimen 5 showed lower tensile strength, however it is figure 3. cross-sections of specimens no. 4 and 5. figure 4. failure of specimens no. 1, 2, 3 and 6. necessary take into account the size of initial crack from manufacturing. other specimen showed failure between brick and one of steel plates. reason of this type of failure is given by low interface adhesion. usage of insufficient amount of epoxy resin is probably the explanation. another stone specimens were also tested in pure tension test and the expected failure was reached – all specimens failed in stone (interface strength was higher than specimen tensile strength). fig. 4 shows failure of specimens 1, 2, 3 and 6. 76 vol. 13/2017 solid burnt bricks’ tensile strength figure 5. working diagram force-elongation of bricks. figure 6. tensile strength of bricks. figure 5 shows working diagrams of each specimen. the highest value of force was reached for specimen 6. specimen 6 (together with specimen 3) was quite enough glued (see fig. 4) and the failure occurred predominantly in very first layer of masonry unit [3]. this layer has improved properties thanks to glue penetration into porous system. for this reason, there is no failure direct at the interface, but a small layer of brick was thrown away. specimen 1 and 2 show poor gluing of steel plate, especially specimen 2 has the lowest value of reached force. specimens 4 and 5 had an initial manufacturing crack and have failed in brick. the maximal value of reached force is different, it could be caused by distinct depth of initial crack. it was not possible to measure the size of initial cracks, because there were only hair cracks observed. the possible solution is using specimen with notches whose accurate size would enable a conversion force to area. the initial slip of the specimens is not displayed in working diagrams (fig. 5). the diagrams show brittle rupture in all cases. average tensile strength was determined to figure 7. young’s modulus of bricks. figure 8. failure of mortar specimens. 0.64 mpa with standard deviation 0.15 mpa. specimen no. 2 was not taken into account. young modulus was obtained as secant modulus between two loads: 90 % num, 40 % num, where num is maximal load. average modulus of elasticity was determined to 0.24 gpa with standard deviation of 0.03 gpa. specimen no. 2 was not taken into account. 4. experimental setup for mortar for mortar pure tension test, three mortar specimens with dimensions 40×40×160 mm were made. specimens were tested after 28 days to ensure mortar’s hardness. steel plates were glued on mortar specimens after 21 day from construction (meaning the maximal strength of epoxy resin in 7 days was respected). mortar with steel plates were placed into testing equipment labtest 4.100sp1 by means two snap hooks as well as bricks. 77 aneta maroušková, jan kubát acta polytechnica ctu proceedings figure 9. failure of specimen m2. figure 10. working diagram force-elongation of mortar. 5. discussion of the obtained results from mortar testing two mortar specimens show failure close to the glued steel plates. after failure of specimen no. m1 small place of incomplete gluing is evident. specimen no. m2 failed approx. in a half of its length. calculated values of tensile strength and young modulus are reported in bar charts (fig. 11 and 12). the initial slip of the specimens is not displayed in working diagrams (fig. 10). the diagrams show brittle rupture in all cases. average tensile strength was determined to 0.25 mpa with standard deviation 0.04 mpa. young modulus was obtained as secant modulus between two loads: 90% num, 40% num, where num is maximal load. average modulus of elasticity was determined to 0.047 gpa with standard deviation of 0.005 gpa. figure 11. tensile strength of mortar. figure 12. young’s modulus of mortar. 6. summary reached values of young’s modulus and tensile strength of bricks are lower than the expected. the expected tensile of bricks strength was approx. around 1.5 mpa and the reached value with the exception of specimen 6 is not even half of that. simultaneously stone specimens were tested, whose failure matches to expected, and therefore the amount of used epoxy resin was determined as the reason of lower obtained values. also the low tensile strength of the specimens with the manufacturing crack (no. 4 and 5) for which were not possible to determine the effective area resisting to the force and thus it was not possible to recalculate the obtained tensile strength for the effective area. after this recalculation, the value of tensile strength of specimens 4 and 5 is expected higher than the determined ones. the experimental testing of bricks will be repeated and the results mutually compared and discussed. tensile strength of mortar met the expectation; however due to small amount of specimens the testing will further continue and reached values will be evaluated. acknowledgements the results were obtained with support from research project df12p01ovv037 – “progressive non-invasive methods of the stabilization, conservation and strengthening of historic structures and their parts with fibreand nanofibre-based composite materials”. the grant researcher is prof. ing. jiří witzany, drsc. 78 vol. 13/2017 solid burnt bricks’ tensile strength references [1] j. witzany, t. čejka, t. zigler. failure mechanism of compressed short brick masonry columns confined with frp strips. construction and buildings materials 63:180– 188, 2014. doi:10.1016/j.conbuildmat.2014.04.041. [2] a. maroušková. masonry column reinforced by frp wrapping: behavior and numerical analysis. applied mechanics and materials 825:27–30, 2008. doi:10.4028/www.scientific.net/amm.825.27. [3] r. fedele, g. milani. a numerical insight into the response of masonry reinforced by frp strips. composite structures 92:2345–2357, 2010. doi:10.1016/j.compstruct.2010.03.014. 79 http://dx.doi.org/10.1016/j.conbuildmat.2014.04.041 http://dx.doi.org/10.4028/www.scientific.net/amm.825.27 http://dx.doi.org/10.1016/j.compstruct.2010.03.014 acta polytechnica ctu proceedings 13:76–80, 2017 1 overview 2 experimental setup for bricks 3 discussion of the obtained results 4 experimental setup for mortar 5 discussion of the obtained results from mortar testing 6 summary acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2017.13.0109 acta polytechnica ctu proceedings 13:109–114, 2017 © czech technical university in prague, 2017 available online at http://ojs.cvut.cz/ojs/index.php/app variationally-based effective dynamic thickness for laminated glass beams jaroslav schmidt∗, alena zemanová, tomáš janda, jan zeman, michal šejnoha czech technical university in prague, faculty of civil engineering, thákurova 7, 166 29 prague 6, czech republic ∗ corresponding author: jarasit@gmail.com abstract. laminated glass, consisting of glass layers connected with transparent foils, has found its applications in civil, automotive, or marine engineering. due to a high contrast in layer properties, mechanical response of laminated glass structures cannot be predicted using classical laminate theories. on the other hand, engineering applications demand easy-to-use formulas of acceptable accuracy. this contribution addresses such simplified models for free vibrations of laminated glass beams, with the goal to determine their natural frequencies and modal damping properties. our strategy is to approximate the complex behavior of a laminated structure with that of an equivalent monolithic beam. its effective thickness is determined by the variational method proposed by galuppi and royer-carfagni for static problems, which we extended for modal analysis. we show that this new approach overcomes inaccuracies of the currently used dynamic effective thickness model by lópez-aenlle and pelayo. keywords: laminated glass, dynamic effective thickness, modal analysis. 1. motivation in the past, glass was seen as an infill material for transparent areas with no load-bearing function in a structural system. the invention of laminated glass became one of the milestones that allowed glass to attain a genuine structural role [1]. starting from windscreens in the car industry, its use has expanded into other fields of industry, including civil engineering. today, layered glass is used, for example, for floor and roof systems, columns, staircases etc. laminated glass is a layered composite material consisting of several layers of glass and polymer. apart from its primary function in enhancing the post-peak load-bearing capacity, the polymer interlayer also provides damping of vibrations [2]. therefore, it can also reduce the possible noise and vibration problems, which can occur in structures under dynamic loading. on the other hand, the nature of the polymer response is strongly frequency-dependent and also temperature-sensitive, e.g., [3], [4], or [5]. its behavior is often described by a complex-valued dynamic shear modulus, see figure 1. this complex-valued formulation allows us to transmit the information about the resonance behavior and damping of the structural element in a single complex number. for these reasons, the eigenvalue problem which describes the free vibrations of laminated glass becomes complex and nonlinear; see section 2.1 for more details. to the best of our knowledge, such type of problems cannot be currently addressed with standard finite element systems used in civil engineering. as engineering practice demands easy-to-use approaches of acceptable accuracy, this contribution addresses simplified models for free vibrations of laminated glass beams, with the goal to determine their natural frequencies and modal damping properties. our focus is on the effective thickness approaches – one group of the simplified methods popular in the design of laminated glass structures, e.g., [6], [7], [8]. in particular, we demonstrate on selected examples the (in)accuracy of the state-of-the-art effective thickness approaches from literature, and we discuss our proposal how to increase their accuracy by employing principles of variationally-based effective thickness methods. the structure of the paper is as follows. in section 2, we briefly describe all approaches used in this study. the examples are introduced and the results of our comparison are discussed in section 3. finally, the main findings of our study are summarized in section 4. 2. methods three methods for vibration analysis of laminated glass beams are used in this section. the first method is based on solving the nonlinear and complex eigenvalue problem using a finite element discretization and the newton method. this method provides us with a reference solution to assess the accuracy of effective thickness approaches. the second method is the dynamic effective thickness method (det) proposed by lópez-aenlle and pelayo [6]. finally, the last method is derived from the enhanced effective thickness approach (eet) by galuppi and royer-carfagni for deflections under static loading [7], which we adjusted for modal analysis. 109 http://dx.doi.org/10.14311/app.2017.13.0109 http://ojs.cvut.cz/ojs/index.php/app j. schmidt, a. zemanová, t. janda et al. acta polytechnica ctu proceedings 10−1 100 101 102 103 frequency [hz] 10−3 10−2 10−1 100 101 102 103 s h e a r m o d u l u s [m p a ] 25◦c 50◦c storage modulus loss modulus figure 1. the frequency dependence of the real and imaginary part of the complex shear modulus for polyvinyl butyral (pvb) according to [3]. for the sake of simplicity, we focus on the most common case of laminated glass configuration – a threelayered beam. however, the generalization of all methods for multi-layered beams can be also found in the literature [9]. in our analysis, we consider a threelayered beam with the same material parameters for both glass layers and with constant properties along the length according to figure 2. then, the crosssection areas ai and the second moments of area ii are defined by the standard relations ii = 1 12bh 3 i , ai = bhi, i = {1, 2, 3}, (1) where b and hi stand for the width and the thicknesses of the layers; see figure 2. 2.1. reference method (rm) our reference method is based on the finite element method (fem) together with the newton method due to the nonlinearity of the solved eigenvalue problem. the system of equations of natural vibrations is written in the fem matrix form as follows( k(ω)− ω2m ) u = 0, (2) where ω is the natural angular frequency, u is the corresponding vector of the mode shape, k(ω) is the complex frequency-dependent stiffness matrix, and m is the real constant mass matrix. the eigenvalue problem described by eq. (2) specifies the mode shape up to an arbitrary constant. therefore, we use a regularization condition u0 t(u −u0) = 0, (3) in order to ensure the uniqueness of the solution. here, u0 is the initial mode shape solving the real eigenvalue problem with the constant stiffness matrix accounting only for the initial shear modulus of the polymer interlayer. for each initial eigen-pair ω0 and u0, we solve iteratively a linearized system of equations, derived by the newton method [10] from eq. (2) and eq. (3), until the convergence is achieved. then, the natural frequencies fhz and the loss factors η (representing damping) correspond to the real part and to the ratio of imaginary and real part of the complex eigen-frequencies fhz = √ re[ω2] 2π , (4a) η = im[ω2] re[ω2] . (4b) this numerical solution converges for a small stopping tolerance to the exact solution. that is why this method is taken as the reference one, when assessing the accuracy of the simplified methods considered next. 2.2. dynamic effective thickness (det) to our best knowledge, only one effective thickness approach for modal analysis of laminated glass beams can be found in literature. this approach by lópezaenlle and pelayo [6] is derived from a closed-form solution of free vibration for a three-layer beam with simply-supported ends. for the other boundary conditions, the authors adjust the formulation using the wavenumbers of an euler-bernoulli beam, see ahead eq. (7) and table 2. the analytical expression for this dynamic effective thickness has the following form hef(ω) = 3 √√√√(h3 1 + h3 3) ( 1 + y 1 + h1 q(ω)(h1+h3) ) , (5) where y is a geometric parameter, which depends only on the thicknesses of layers and is given by y = 12h1h3(0.5h1 + h2 + 0.5h3) (h3 1 + h3 3)(h1 + h3) , (6) and q(ω) is a material parameter given by the identity q(ω) = g2(ω) e1h3h2β2 n . (7) in this equation, g2(ω) is the frequency-dependent complex shear modulus of the interlayer, e1 is the young modulus of glass layers, and βn is the wavenumber corresponding to the given boundary conditions and to the n-th mode, see ahead table 2. because the shear modulus g2(ω) is complex-valued, the effective thickness from eq. (5) is also complex. then, the complex-valued natural angular frequencies can be expressed analytically, similarly as for the monolithic beam ω2 = β4 ne1h 3 ef(ω) 12m̄ , (8) 110 vol. 13/2017 variationally-based effective dynamic thickness h1 h2 h3 l b figure 2. the layout of a three-layered laminated glass beam. where m̄ = ρ1h1 + ρ2h2 + ρ3h3 (9) is the effective mass per unit of length and width. the natural frequencies fhz and loss factors η are obtained from this complex-valued angular frequency ω according to eq. (4). 2.3. enhanced effective thickness (eet) firstly, we introduce the formula for the deflectioneffective thickness according to galuppi and royercarfagni [7], and subsequently, we propose its extension to the free vibration analysis. to shorten the notation, h is defined as a distance of centerlines of glass layers, so that h = h2 + 0.5(h1 + h3). (10) further, itot is the second moment of area of glass layers for the monolithic case itot = i1 + i3 + a1a3 a1 +a3 h2, (11) and is is the last term from (11) divided by the width of the beam b, thus is = h1h3 h1 + h3 h2. (12) using these quantities, the enhanced effective thickness for the deflection under static loading from [7] has the following form hef = 3 √√√√√ 1 ζ h3 1 + h3 3 + 12is + 1− ζ h3 1 + h3 3 , (13) where ζ is the coefficient of shear cohesion ζ = 1 1 + i1 + i3 µitot a1a3 a1 +a3 ψ , (14) and µ is the non-dimensional ratio of the glass and interlayer stiffnesses µ = g2b e1h2 . (15) finally, ψ is a coefficient which depends on the shape of the deflection curve and is described by the following equation ψ = ∫ l 0 (g′′(x))2dx∫ l 0 (g′(x))2dx , (16) geometry length l = 1 m width b = 0.1 m thicknesses of glass h1 = h3 = 10 mm thickness of interlayer h2 = 1.52 mm glass density ρ1 = ρ3 = 2500 kg/m3 poisson’s ratio ν1 = ν3 = 0.22 young’s modulus e1 = e3 = 72 gpa interlayer density ρ2 = 1100 kg/m3 poisson’s ratio ν2 = 0.49 prony series for gmm taken from [3] table 1. the properties of laminated glass beams (gmm – generalized maxwell model). where the deflection curve is represented by the function g(x), which describes the shape of the curve regardless of the stiffness of the beam. this deflection-effective thickness was derived by minimization of the strain energy functional for a laminated glass beam in bending under static loading. for its use in modal analysis, we made just two intuitive adjustments of this method: (1.) we used the complex-valued interlayer shear modulus g2(ω) in eq. (15). (2.) we replaced the shape function of the deflection under static loading in eq. (16) with the one corresponding to the n-th mode shape of a monolithic beam under given boundary conditions. this adjustment leads to a complex effective thickness in eq. (13), similarly to the det approach. for the evaluation of the natural frequencies and the damping, we again use eq. (4). 3. comparison of methods 3.1. examples in this section, we discuss the applicability of effective thickness approaches in modal analysis and how our adjustment changes the errors of the method. the comparison is provided for three types of boundary conditions: simply-supported, fixed-fixed, and free-free. 111 j. schmidt, a. zemanová, t. janda et al. acta polytechnica ctu proceedings mode shape functions simply-supported beam g(x) = sin nπx l fixed-fixed beam g(x) = cosh βnx− cosβnx− sinh βnl + sin βnl cosh βnl − cosβnl (sinh βnx− sin βnx) free-free beam g(x) = sinh βnx+ sin βnx− sinh βnl − sin βnl cosh βnl − cosβnl (cosh βnx+ cosβnx) table 2. the overview of the n-th mode shape functions for the coordinate x ∈ 〈0, l〉 for a simply-supported, fixed-fixed, and free-free beam. (the products of the wavenumbers and the beam length βnl for the first three modes are β1l = 4.7300, β2l = 7.8532, and β3l = 10.996.) the geometry and the material properties of glass and interlayer appear in table 1. whereas glass is treated as an elastic material, the viscoelastic behavior of polymer, which is in our case polyvinyl butyral (pvb), is described using a generalized maxwell model, whose parameters are taken from [3]. due to the temperature-sensitiveness of pvb, two different ambient temperatures were assumed – the room temperature 25◦c and the temperature 50◦c corresponding to an external panel under summer sunlight. 3.2. evaluation of shape coefficients recall that our extension of the enhanced effective thickness approach [7] to the modal analysis of laminated glass beams requires the evaluation of the shape coefficients ψ according eq. (16). for these coefficients, we need mode shape functions corresponding to the three given boundary conditions. their overview taken from [11] is shown in table 2. for a simplysupported beam, one closed-form expression can be written for all mode shape functions; however, the formulas for the other boundary conditions are more involved and contain a product of the wavenumbers βn and the beam length l that needs to be evaluated numerically from a characteristic equation for free vibrations [11]. then, we used the integral formula eq. (16) and evaluated, for all boundary conditions, the shape coefficients ψ summarized in table 3 using the mode shape functions for a monolithic euler-bernoulli beam from table 2. with these results at hand, we can calculate the complex enhanced effective thickness and subsequently the natural frequencies and the loss factors for our comparison. 3.3. results and discussion in this section, the results obtained by the reference method (rm), the effective thickness approaches from [6] (det) or from [7] (eet), their errors, and their comparison are shown. the first three natural frequencies fhz and loss factors η are summarized in table 4. the errors of the effective thickness approaches are evaluated against the reference method based on the finite element complex-eigenvalue solver. shape coefficients ψ beam mode 1 2 3 simply-supported π2 l2 (2π)2 l2 (3π)2 l2 fixed-fixed 40.7 l2 82.6 l2 148 l2 free-free 10.1 l2 34.9 l2 78.2 l2 table 3. summary of the shape coefficients ψ for three different boundary conditions and the first three mode shapes. it is evident from the first two subtables in table 4 that the adjusted eet approach gives the same results as the det method for the simply-supported beam for both temperatures. both effective thickness methods give the errors in natural frequencies less than 1% and in loss factors less than 10%. for the room temperature, even the errors in loss factors are under 1%. for the two other boundary condition, the det approach provides quite good predictions for natural frequencies with the errors under 13%. however, the errors in loss factors can be very high (up to 80%). therefore, this method is mostly unable to provide a good estimation of damping. the adjusted eet approach helps to overcome these inaccuracies. for the fixed-fixed beam, this approach gives the largest error in natural frequencies 3% (against the 8% error by the det) and in loss factors 9% (against the 44% error by the det). for the free-free beam, the largest value of errors in natural frequencies is 3% (against the 13% error by the det) and in loss factors 19% (against the 78% error by the det). this demonstrates that our intuitive extension of the variationally-based enhanced effective thickness concept from [7] provides better (or for the simplysupported beam the same) estimates of both the natural frequencies and loss factors for laminated glass beams. 112 vol. 13/2017 variationally-based effective dynamic thickness simply-supported beam at 25◦c mode natural frequency [hz] error [%] mode loss factor [%] error [%] rm det eet det eet rm det eet det eet 1 52.03 52.07 52.07 0.09 0.09 1 1.93 1.93 1.93 -0.25 -0.25 2 197.9 198.5 198.5 0.30 0.30 2 4.24 4.23 4.23 -0.32 -0.32 3 419.1 421.4 421.4 0.56 0.56 3 6.15 6.14 6.14 -0.20 -0.20 simply-supported beam at 50◦c mode natural frequency [hz] error [%] mode loss factor [%] error [%] rm det eet det eet rm det eet det eet 1 30.81 30.97 30.97 0.53 0.53 1 24.41 22.17 22.17 -9.18 -9.18 2 108.1 108.5 108.5 0.42 0.42 2 16.89 16.25 16.25 -3.81 -3.81 3 232.0 233.1 233.1 0.47 0.47 3 14.46 13.90 13.90 -3.84 -3.84 fixed-fixed beam at 25◦c mode natural frequency [hz] error [%] mode loss factor [%] error [%] rm det eet det eet rm det eet det eet 1 110.2 115.4 111.3 4.78 1.09 1 5.42 3.09 5.08 -43.05 -6.27 2 285.3 301.4 292.7 5.65 2.59 2 7.15 5.29 6.50 -25.94 -9.02 3 527.2 556.3 541.1 5.53 2.65 3 7.86 6.74 7.56 -14.28 -3.82 fixed-fixed beam at 50◦c mode natural frequency [hz] error [%] mode loss factor [%] error [%] rm det eet det eet rm det eet det eet 1 60.06 64.63 60.21 7.62 0.25 1 12.99 18.74 12.72 44.19 -2.14 2 160.2 164.6 160.9 2.79 0.46 2 11.51 14.91 11.80 29.52 2.49 3 308.8 314.0 310.2 1.66 0.43 3 10.98 12.89 10.87 17.39 -1.03 free-free beam at 25◦c mode natural frequency [hz] error [%] mode loss factor [%] error [%] rm det eet det eet rm det eet det eet 1 117.8 115.4 118.4 -2.00 0.56 1 1.73 3.09 1.50 78.05 -13.64 2 307.9 301.4 314.1 -2.12 2.00 2 4.13 5.29 3.37 28.20 -18.41 3 568.4 556.3 584.5 -2.12 2.84 3 5.77 6.74 5.04 16.74 -12.64 free-free beam at 50◦c mode natural frequency [hz] error [%] mode loss factor [%] error [%] rm det eet det eet rm det eet det eet 1 73.95 64.63 74.62 -12.60 0.90 1 26.44 18.74 25.39 -29.13 -3.96 2 170.5 164.6 175.6 -3.45 2.97 2 20.85 14.91 22.20 -28.51 6.48 3 322.5 314.0 325.3 -2.66 0.86 3 17.57 12.89 18.20 -26.61 3.62 table 4. natural frequencies and loss factors for the three-layer laminated glass beam for the first three modes determined by the reference method (rm), the dynamic effective thickness method (det), and the adjusted enhanced effective thickness approach (eet) with their errors against the reference method (rm). 113 j. schmidt, a. zemanová, t. janda et al. acta polytechnica ctu proceedings 4. conclusions finally, we would like to summarize that in this paper • we verified the effective thickness approaches against the finite element complex-eigenvalue solver, and • we proposed a new dynamic effective thickness concept derived from [7]. it follows from our comparison that this extended enhanced effective thickness method • provides the same results for the simply-supported beam as the dynamic effective thickness from [6] with the errors less than 1% in natural frequencies and 10% in loss factors, and • improves the estimates of natural frequencies and loss factors for the other two boundary conditions with the errors less than 3% in natural frequencies and 19% in loss factors. in our future work, we would like to justify this intuitive approach by rigorously extending the variationally-based derivation of the effective thickness from [7] to the dynamic problems. acknowledgements this publication was supported by the czech science foundation, the grant no. 16-14770s and by the grant agency of the czech technical university in prague, grant no. sgs17/043/ohk1/1t/11. references [1] m. haldimann, a. luible, m. overend. structural use of glass, vol. 10 of structural engineering documents. iabse, zürich, switzerland, 2008. [2] z. huang, z. qin, f. chu. damping mechanism of elastic-viscoelastic-elastic sandwich structures. composite structures 153:96–107, 2016. doi:10.1016/j.compstruct.2016.05.105. [3] y. shitanoki, s. bennison, y. koike. a practical, nondestructive method to determine the shear relaxation modulus behavior of polymeric interlayers for laminated glass. polymer testing 37:59–67, 2014. doi:10.1016/j.polymertesting.2014.04.011. [4] i. mohagheghian, y. wang, l. jiang, et al. quasi-static bending and low velocity impact performance of monolithic and laminated glass windows employing chemically strengthened glass. european journal of mechanics – a/solids 63:165–186, 2017. doi:10.1016/j.euromechsol.2017.01.006. [5] l. andreozzi, s. b. bati, m. fagone, et al. dynamic torsion tests to characterize the thermo-viscoelastic properties of polymeric interlayers for laminated glass. construction and building materials 65:1–13, 2014. doi:10.1016/j.conbuildmat.2014.04.003. [6] m. lópez-aenlle, f. pelayo. dynamic effective thickness in laminated-glass beams and plates. composites: part b 67:332–347, 2014. [7] l. galuppi, g. f. royer-carfagni. effective thickness of laminated glass beams: new expression via a variational approach. engineering structures 38:53 – 67, 2012. doi:10.1016/j.engstruct.2011.12.039. [8] l. galuppi, g. royer-carfagni. the effective thickness of laminated glass: inconsistency of the formulation in a proposal of en-standards. composites part b: engineering 55:109–118, 2013. [9] f. pelayo, m. lópez-aenlle. natural frequencies and damping ratios of multi-layered laminated glass beams using a dynamic effective thickness. journal of sandwich structures and materials 0:1–25, 2017. doi:10.1177/1099636217695479. [10] k. schreiber. nonlinear eigenvalue problems: newton-type methods and nonlinear rayleigh functionals. ph.d. thesis, technischen universitat berlin, 2008. [11] r. w. clough, j. penzien. dynamics of structures. computers & structures, inc, 2003. 114 http://dx.doi.org/10.1016/j.compstruct.2016.05.105 http://dx.doi.org/10.1016/j.polymertesting.2014.04.011 http://dx.doi.org/10.1016/j.euromechsol.2017.01.006 http://dx.doi.org/10.1016/j.conbuildmat.2014.04.003 http://dx.doi.org/10.1016/j.engstruct.2011.12.039 http://dx.doi.org/10.1177/1099636217695479 acta polytechnica ctu proceedings 13:110–115, 2017 1 motivation 2 methods 2.1 reference method (rm) 2.2 dynamic effective thickness (det) 2.3 enhanced effective thickness (eet) 3 comparison of methods 3.1 examples 3.2 evaluation of shape coefficients 3.3 results and discussion 4 conclusions acknowledgements references acta polytechnica doi:10.14311/app.2020.27.0145 acta polytechnica 27(0):145–148, 2020 © czech technical university in prague, 2020 available online at http://ojs.cvut.cz/ojs/index.php/app characterization of titanium laser welds hana chmelíčkováa,∗, helena hiklováa, lukáš václaveka, jan tomáštíkb, radim čtvrtlíkc a joint laboratory of optics of palacký university and institute of physics as cr, faculty of science, palacký university, 17. listopadu 12, 771 46 olomouc, czech republic b regional centre of advanced technologies and materials, joint laboratory of optics of palacký university and institute of physics as cr, faculty of science, palacký university, 17. listopadu 12, 771 46 olomouc, czech republic c institute of physics of the czech academy of sciences, joint laboratory of optics of palacký university and institute of physics as cr, 17. listopadu 50a, 772 07 olomouc, czech republic ∗ corresponding author: hana.chmelickova@upol.cz abstract. butt welding of commercially pure titanium grade 1 and ti6al4v alloy sheets using a pulsed nd:yag laser kls 246 102 lasag were carried out to determine optimal values of pulse energy and pulse length to create completely penetrated weld. surface peak power density of about 3.105 w.cm−2 was found as an optimal value. weld dimensions, both face width and penetration depth, are found to be proportional to increasing energy and decreasing pulse length. gentle sagging and root penetration were revealed by means of contact surface profilometry. the nanohardness tests on transverse cross-sections detected approximately 50% higher hardness in the fusion zone than in the base material. keywords: elasticity, laser welding, nanohardness, surface profile, titanium. 1. introduction the commercially pure titanium (cp ti) and its alloys especially ti6al4v have good weldability. gas tungsten arc welding (gtaw) and electron beam welding (ebw) are already used in wide range of applications in chemical, medical, nuclear and food industry, automotive and aerospace. development of high-power laser sources has opened possibility to investigate methods of laser beam welding (lbw) that are more efficient, flexible and precise with localized heat affect to workpiece than conventional ones. influence of laser beam parameters (power, pulse energy and frequency, scanning speed, spot diameter, focus position and shielding gas influence) on weld bead properties were examined in research laboratories all over the world. badgar et al. investigated effect of heat input during welding of cp titanium by means of cw nd:yag laser on microstructure, microhardness, relative elongation, tensile and yield strength in ref. [1]. lisiecki presented results of the disk laser welding of titanium alloy ti6al4v likewise in ref. [2]. fomin et al. added filler to ti6al4v butt welds welded by means of 8 kw continuous-wave ytterbium fiber laser ipg and study influence of post welding heat treatment on microstructure, microhardness, residual stress distribution and fatigue performance (ref. [3]). a significant influence of the laser welding high cooling rates to result weld mechanical properties was pointed out. the objective of this paper was to characterize butt welds of cp titanium gr.1 and ti6al4v alloy sheets, welded using a pulsed nd:yag laser kls 246 102 lasag under argon shielding gas atmosphere. 2. experimental setup two sheets (40 mm x 12 mm) of 0.8 mm thickness were fixed tightly on welding jig after cleaning by ethanol. the laser beam was focused by a 100 mm focusing lens, focus position + 3 mm was controlled by triangular laser system riftek, spot diameter d in this position has the theoretic value of 0.84 mm. pulse frequency 13 hz and linear axis speed 3 mm.s−1 were set according to previous parameter optimization to ensure laser spot overlap 67%. high purity argon was guided in laser beam axis with flow rate of 12 l.min−1. the influence of increasing pulse energy e at constant pulse length 3 ms on welding width and penetration depth was investigated with cp titanium sheets. pulse energy value was gradually set to 2.63 j, 3.15 j, 3.72 j, 4.33 j, 5.03 j and 5.58 j, butt weld was created for each of these pulse energies. the influence of increasing pulse length tp at constant pulse energy 5.03 j on welding penetration depth was investigated with ti6al4v alloy. pulse length was gradually set to 2.0 ms, 2.5 ms, 3.0 ms, 3.5 ms, 4.0 ms, 4.5 ms. 5.0 ms and 5.5 ms, butt weld was created for each pulse length. change of the pulse energy e or pulse length tp leads to a change of the peak power ppeak as well as to its surface density qppeak that is an important processing parameter. surface area s is dependent on laser spot diameter d, see equation 1. qppeak = ppeak.s −1 = 4e.(πd2.tp)−1 (1) 145 http://dx.doi.org/10.14311/app.2020.27.0145 http://ojs.cvut.cz/ojs/index.php/app h. chmelíčková, h. hiklová, l. václavek et al. acta polytechnica 3. methods the weld width both on the top and the bottom specimens’ surfaces were imaged and measured by means of laser scanning confocal microscope lext 5000. the penetration depth, heat affected zone depth and weld macrostructure were evaluated on the samples’ transverse cross-sections that were etched by kroll’s agent after grinding and fine polishing procedure. surface profile in fifty points per millimeter in (4 x 4) mm2 area around weld bead was measured by contact profilometer talysurf. graphs of 200 surface profiles and 3d axonometric display were created in talymap (ref. [4]) to measure and visualize sagging or overlap of fusion zone, as well as the root width and the root penetration under the sample back side plane. mechanical properties of the fully penetrated welds were determined by means of nanohardness tests on the transverse cross-sections. nano indentation instrument nanotest ntx, (micro materials, uk) was used. hardness and elastic modulus were determined by nanoindentation with a pyramidal diamond berkovich tip at maximal load 50 mn (ref. [5]). set of 8 lines in horizontal direction each with 17 points with spacing 100 µm creates a matrix that covered base material (bm), heat affected zone (haz) and fusion zone (fz). the nanoindentation curves were evaluated by oliver-pharr method for hardness and reduced elastic modulus determination. the indenter tip calibration was performed using the fused silica sample. 4. results and discussion 4.1. dimensions dependence on energy, pulse length and peak power density average values of weld face fusion zone width (fzw) and root width (rw) together with fusion zone penetration depth (fzpd) of not fully penetrated welds are summarized in table 1 for cp titanium samples, welded with constant pulse length of 3 ms and increasing pulse energy. weld dimensions grew up with increasing pulse energy. full penetration without significant surface defects was achieved with energy 5.03 j and peak power surface density qppeak = 3.105 w. cm−2. magnified view of this weld face shows distinctly overlapped laser spots (figure 1) and unbroken complete root (figure 2). higher energies led to deep sagging of fusion zone or melt pool collapse. measurement of ti6al4v alloy samples welded with constant pulse energy of 5.03 j and increasing pulse length denoted the same trend. the best fully penetrated weld was achieved for a pulse length of 3 ms. shorter pulse length 2.5 ms and 2 ms with the same energy resulted in a higher peak power surface density that in turn led to an appreciable weld bead sagging. penetration depth decreased with pulse prolongation from 3.5 ms to 5.5 ms and the penetration welding mode transformed to the heat conduction one. figure 1. a view of cp titanium weld face (e = 5.03 j, tp = 3 ms). figure 2. a view of cp titanium weld root (e = 5.03 j, tp = 3 ms). e qppeak fzw fzpd rw (j) (w. cm−2) (mm) (mm) (mm) 2.63 1.56. 105 0.77 0.11 3.15 1.89. 105 0.70 0.17 3.72 2.23. 105 0.93 0.32 4.33 2.59. 105 0.99 0.51 5.03 3.01. 105 1.07 0.54 5.58 3.37. 105 1.09 0.82 table 1. weld dimensions dependence on increasing energy for cp titanium samples. 4.2. surface profile average sagging of 50 µm is seen in the 3d axonometric display of cp titanium weld face (figure 3), welded with qppeak of about 3.105 w. cm−2. periodic alternation of peaks and depressions on the longitudinal weld profile is caused by the overlap of laser pulse spots, average distance between these points is about 130 µm. a set of 200 transverse profiles measured in 4 mm section over the weld root (energy 5.03 j, pulse length 3 ms) for the ti6al4v alloy is displayed in fig. 4. the root penetration also alternated regularly with the laser spot overlap, maximal value didn’t exceed 90 µm, while minimal value is situated close to the sample surface plane. the occasional occurrence of spatters on the sample surface was also detected for welds made with higher surface power densities. 146 vol. 27/2020 characterization of titanium laser welds figure 3. 3d axonometric display of cp titanium weld face (e = 5.03 j, tp = 3 ms). figure 4. a set of 200 surface profiles of ti6al4v alloy weld root (e = 5.03 j, tp = 3 ms), with the highlighted maximal, average and minimal profiles. 4.3. nanohardness test matrix of 17 nanohardness values in horizontal and 8 values in vertical direction was taken from ti6al4v sample transverse cross-section with step 100 µm. the average hardness value of base material was found to be 2.9 gpa. the individual values varied from 2.3 gpa to 3.1 gpa. the coarse-grained fusion zone possessed an average hardness of 4.3 gpa, varying from minimum 3.4 gpa to maximum 5.8 gpa. this means that the fusion zone is approx. 48% harder than the base material. heat affected zone between fusion zone boundary and the base material exhibits average hardness of 3.6 gpa. the nanohardness distribution in three selected horizontal matrix rows, measured in distance 100 µm, 400 µm and 700 µm from the weld face (figure 5), is shown in fig. 6. figure 5. macrograph of the transverse cross-section of ti6al4v butt weld (e = 5.03 j, tp = 3 ms), with the visible boundaries between fusion zone (fz), heat affected zone (haz) and base material (bm). white lines mark nanohardness data used in graph (figure 6). figure 6. nanohardness profiles on the transverse cross-section of ti6al4v butt weld (e = 5.03 j, tp = 3 ms) in the distance 100 µm, 400 µm and 700 µm from the weld face. in a similar way, matrix of 17 nanohardness values in horizontal and 7 values in vertical direction was taken from cp titanium sample transverse crosssection with step 100 µm (figure 7). full penetration was achieved here with the same parameters as in case of ti6al4v alloy (e = 5.03 j, tp = 3 ms). the average nanohardness value of the base material was found to be 2.5 gpa, varying from 2.1 gpa to 2.9 gpa. the fusion zone with coarse elongated grains has average hardness of 4.2 gpa, varying from 3.4 gpa to 5.5 gpa, it means about 70% more than hardness of the base material. heat affected zone with visible boundaries with the base material and fusion zone exhibits average nanohardness of 3.1 gpa. figure 7. macrograph of the transverse cross-section of cp titanium butt weld (e = 5.03 j, tp = 3 ms) with the visible boundaries between fusion zone (fz), heat affected zone (haz) and base material (bm). the nanohardness distribution across the whole measured area of cp titanium sample transverse cross-section is displayed in figure 8. values around 5.5 gpa have occurred sporadically, mostly close to weld centerline. although the cp titanium base material has a lower hardness than the ti4al6v alloy, the average hardness of both fusion zones is almost the same and is around 4.2 gpa. due to fast heating over titanium melting temperature and high cooling 147 h. chmelíčková, h. hiklová, l. václavek et al. acta polytechnica rates, that is characteristic phenomenon of laser welding, metastable phases can be formed, such as alfa martensite (ref. [1]). figure 8. surface graph of nanohardness matrix covering area of 1.4 mm x 0.6 mm on the transverse cross-section of cp titanium butt weld (e = 5.03 j, tp = 3 ms). 4.4. reduced elastic modulus and elastic recovery parameter in contrast to hardness, there are no evident differences between reduced elastic modulus er values of base material and fusion zone. the average reduced modulus value for ti6al4v alloy is 136 gpa, thought values from 113 gpa to 157 gpa were occasionally observed across the measurement area. the cp titanium sample has an average value of er = 141 gpa, with few extremes varying from 110 gpa to 166 gpa, which were all situated in the fusion area. this may reflect the local inhomogeneity of the weld macrostructure (figure 9). figure 9. detail of cp titanium weld fusion zone. 5. conclusions optimal parameters for fully penetrated butt weld for cp titanium gr.1 and ti6al4v alloy sheets with thickness 0.8 mm were found during welding experiments on pulsed nd:yag laser, which are the pulse energy of about 5 j and the pulse length 3 ms for laser spot diameter of 0.84 mm. the average sagging of 50 µm and the root penetration of 90 µm were measured by means of contact profilometer that also revealed a wavy profile of remelted material in longitudinal weld direction. dimensions of the fusion and heat-affected zone were measured on the weld transverse cross-sections, where these values growth with increasing surface peak power density. the weld face of optimal welds has width about 1 mm, while root width is two times smaller, about 0.5 mm. the coarsegrained fusion zone of ti6al4v alloy shows an average hardness value of 4.3 gpa with maximum of 5.8 gpa, similarly the fusion zone with coarse elongated grains of cp titanium optimal sample has average hardness value of 4.2 gpa with maximum of 5.5 gpa. analysis of reduced elastic modulus values did not reveal evident differences between the base material and the fusion zone. acknowledgements the authors gratefully acknowledge the support by the operational program research, development and education, project no. cz.02.1.01/0.0/0.0/17_049/0008422 of the ministry of education, youth and sports of the czech republic, the project iga_prf_2019_008 of palacký university and the project th03020245 of the technology agency of the czech republic. references [1] d. badkar, k. pandey, g. buvanashekaran. effects of laser phase transformation hardening parameters on heat input and hardened-bead profile quality of unalloyed titanium. transactions of nonferrous metals society of china 20(6):1078 – 1091, 2010. doi:10.1016/s1003-6326(09)60261-2. [2] a. lisiecki. laser welding of titanium alloy ti6al4v using a disk laser. mtm j 7:53–56, 2012. [3] f. fomin, v. ventzke, f. dorn, et al. effect of microstructure transformations on fatigue properties of laser beam welded ti-6al-4v butt joints subjected to postweld heat treatment. in t. tanski, w. borek (eds.), study of grain boundary character, chap. 6. intechopen, rijeka, 2017. doi:10.5772/66178. [4] h. hiklová, h. chmelíčková, m. havelková, l. řiháková. using the contact profilometer to measure the dimensional changes of metal samples after laser treatment. in nde for safety 2013 / defektoskopie 2013, pp. 39–44. [5] l. václavek, j. tomaštík, l. nožka, r. čtvrtlík. physical characterization of hafnium oxide thin films annealed in vacuum. in local mechanical properties xiii, vol. 784 of key engineering materials, pp. 135–140. trans tech publications ltd, 2018. doi:10.4028/www.scientific.net/kem.784.135. 148 https://doi.org/10.1016/s1003-6326(09)60261-2 https://doi.org/10.5772/66178 https://doi.org/10.4028/www.scientific.net/kem.784.135 acta polytechnica 27(0):145–148, 2020 1 introduction 2 experimental setup 3 methods 4 results and discussion 4.1 dimensions dependence on energy, pulse length and peak power density 4.2 surface profile 4.3 nanohardness test 4.4 reduced elastic modulus and elastic recovery parameter 5 conclusions acknowledgements references doi:10.14311/app.2021.30.0024 acta polytechnica ctu proceedings 30:24–29, 2021 © czech technical university in prague, 2021 available online at http://ojs.cvut.cz/ojs/index.php/app modeling time-dependent deformations of concrete members subjected to symmetric and asymmetric drying pavel horák∗, petr havlásek czech technical university in prague, faculty of civil engineering, department of mechanics, thákurova 7, 166 29 prague 6, czech republic ∗ corresponding author: pavel.horak@fsv.cvut.cz abstract. the time-dependent behavior of concrete members with uniform cross-section subjected to symmetric drying can be predicted with one of many models anchored in the design codes. since these models use the cross-sectional approach, only the average quantities can be obtained. more complex phenomena can be investigated by means of a coupled hygro-mechanical finite element analysis. owing to insufficient or missing experimental data, the input parameters for the moisture transport model are often tuned to match the development of axial shrinkage or moisture loss measured on small-scale laboratory specimens. in this paper it is assessed whether this approach leads to a unique set of parameters of the bažant-najjar model for moisture diffusion. additionally, for the selected sets of parameters the analysis is repeated with modified size and/or boundary conditions leading to asymmetric drying and the results are discussed. keywords: concrete, drying shrinkage, finite elements, moisture transport, parameter identification. 1. introduction and motivation creep and shrinkage are complex time-dependent phenomena in concrete whose magnitude and evolution is influenced by many factors. the distinction between creep and shrinkage, which are both related to delayed strains, is that creep is caused by loading. at constant temperature the creep deformation can be additively decomposed into basic creep and drying creep. basic creep is the deformation of hygrally sealed concrete while drying creep is an additional deformation caused by drying. in a similar fashion, autogenous and drying shrinkage refer to the volume change (typically contraction) of sealed and drying concrete, respectively. in normal-strength structural concretes with w/c ≥ 0.4, the autogenous shrinkage is typically negligible (≈ 40 × 10−6) in comparison to the magnitude of drying shrinkage (≈ 780 × 10−6 at 40% relative humidity according to aci 209). this time-dependent behavior of typical concrete members can be estimated using the prediction models (e.g. b3, b4, fib model code) or the formulae from the design codes (e.g. aci 209, eurocode 2). however, since these models operate on the cross-sectional level, only the average quantities can be obtained. in order to describe more complex phenomena, e.g. tensile cracking produced by stress gradients caused by nonuniform drying or the structural response to non-symmetric drying, a more versatile approach is a necessity. one of the options is a coupled hygro-mechanical finite element analysis. the bottleneck of this approach resides in often insufficient number of experiments which are necessary for proper calibration of the complex constitutive models. to overcome this obstacle, certain parameters are set to their recommended values or the parameters are adjusted to reproduce the observed macroscopic behavior. to illustrate this, in the recent paper [1], this approach was used by several research groups to calibrate the transport model based on the axial shrinkage and moisture loss of a standard laboratory specimen without any information on the distribution of relative humidity. the primary objective of this paper is to assess whether such approach yields a unique set of material parameters and what macroscopic measurements can narrow the range of the identified values. 2. methodology all results presented further in this paper were obtained using a one-way coupled hygro-mechanical simulations in open-source finite element package oofem [2]. the constitutive models described in more detail in the following sections were calibrated on creep and shrinkage data reported by bryant [3]. the parameters of the structural material model are then fixed while the parameters in the model for moisture transport are subject to optimization under various assumptions. the identified parameters are consequently utilized for predicting the curvature of an infinite slab subjected to asymmetric drying. 2.1. material models concrete drying is in the present study described by a widely recognized model proposed by bažant and najjar [4]. the governing equation for the diffusion of water vapor was derived under the assumption of 24 http://dx.doi.org/10.14311/app.2021.30.0024 http://ojs.cvut.cz/ojs/index.php/app vol. 30/2021 modeling of concrete subjected to symmetric and asymmetric drying constant moisture capacity k (slope of the desorption isotherm). when the moisture sink is neglected the equation reads ∂h ∂t = ∇ · (c(h)∇h) (1) where ∇h is the gradient of relative humidity and c(h) is the humidity-dependent diffusivity. this dependence is for the cementitious materials highly nonlinear and can be approximated as c(h) = c1 ! "α0 + 1 − α0 1 + # 1−h 1−hc $n % & (2) where c1 is the maximum diffusivity at h = 1, α0 determines the ratio between minimum diffusivity at h = 0 and c1, and parameters hc and n describe the relative humidity and the rate of the transition. this model was embedded into the fib model code 2010 [5] which recommends the following values α0 = 0.05, hc = 0.8, n = 15, while c1 is to be determined from the compressive strength. in the simulations, the initial value of relative humidity is set to h = 0.98 which is the typical value for the sealed conditions and normal-strength concrete with negligible self-desiccation. the ambient relative humidity is idealized as constant and is prescribed using the mixed boundary condition which uses the surface factor f to relates the humidity flux jh with the difference between the relative humidity at the boundary and the environment jh = f · (henv − h) (3) in each time step the moisture transport is followed by the structural sub-problem which utilizes the computed field of relative humidity. the constitutive vicoelastic model is the modified [6] microprestresssolidification (mps) theory [7]. under constant temperature and hygrally sealed conditions the material model reduces to the b3 model [8]. changes in relative humidity give rise to additional compliance, referred to as drying creep or the pickett effect, and to volume changes–shrinkage or swelling. the model has been modified to minimize the size-effect on drying creep which is controlled by parameter k3. shrinkage strain and relative humidity are linearly linked via their rates, ε̇sh = kshḣ (4) where ksh is a material parameter usually treated as humidityand age-independent constant. the model can be enhanced by tensile softening but this feature is not activated in the present study. 2.2. modeling strategy despite the unidirectional nature of the transport problem, the analysis was performed in two dimensions to provide the subsequent structural analysis with the distribution of relative humidity. the mechanical problem was carried out under the assumption of plane stress which allowed to run the computations very efficiently and repeatedly as described later. the assumption of plane stress is very realistic in the case of the experimental data. the parametric study is done on an idealized plane section cut out from an “infinite slab” drying from one or two surfaces. yet, the model which is hereafter referred to as “slab”, neglects the stresses perpendicular to the section and its behavior corresponds to a beam with prismatic cross-section, sealed lateral sides and drying from top and/or bottom surfaces. the effect of self weight is omitted in the study. the finite element meshes of the two sub-problems were identical and were composed of 60 quadrilateral elements with linear approximation of humidity and displacement. the meshes were refined near the drying surfaces. all problems were solved in 284 time steps with increasing length. as mentioned earlier, the material parameters were calibrated on experimental data by bryant and vahanavikkit [3]. their study was focused on creep and shrinkage of structural concrete (fc = 50.1 mpa, w/c = 0.47) measured on prismatic members with cross-section ranging from 100×100 to 400×400 mm. the specimens were either entirely sealed to investigate basic creep or partially sealed to study drying creep and shrinkage. drying at 60% relative humidity began at the age of 8 days. the majority of the creep tests imposing 7 mpa compression started at the age of 14 days. additional tests were done to identify aging. all loaded specimens had a companion without external loading. the constant room temperature was maintained throughout the experiment. calibration began with the basic creep compliance function. the parameters q1 − q4 of the b3 model were adopted from [9] and their values were q1 = 9, q2 = 75, q3 = 28 and q4 = 6.5 all in 10−6/mpa. drying creep and shrinkage was calibrated on experimental data of prism 150 × 150 mm drying from two lateral sides (i.e. 150 mm thick infinite wall). this specimen is the largest one which has reached the ultimate values and for this reason was selected as reference. with crudely calibrated parameters of the transport model the shrinkage constant was identified as ksh = 1.9 × 10−3, and drying creep parameter k3 = 25. the parameters of the bažant-najjar model and the surface factor were identified using a brute-force method implemented in a computer program. the repeatedly run analysis used the values with the ranges and stepping summarized in table 1 which makes altogether 31 360 combinations. the accuracy of the fit was assessed by means of a weighted error which considered the drying period on the log scale. the reference set of parameters which gave the best fit of with experimental data is listed in table 1 (see solid 25 pavel horák, petr havlásek acta polytechnica ctu proceedings 0 100 200 300 400 500 600 700 800 0.1 1 10 100 1000 10000 t0 = 8 d, henv = 60% calibration: experimenta xi al sh rin ka ge [1 0-6 ] duration of drying [day] ref exp 2 3 4 5 6 7 8 9 10 figure 1. best fits of axial shrinkage measured by bryant and vadhanavikkit [3] on 150 mm thick slabs, t0 = 8 days, henv= 60%. red line in figs. 1 and 2). this set was later used to define the reference behavior under a modified setup (i.e. different size or boundary conditions). in the parametric study the analysis starts at the age of 28 days and the ambient relative humidity is set to henv= 50% with the intention to minimize the effect of aging and to comply with the standard laboratory conditions. furthermore, as demonstrated in the following section, it was attempted to narrow the identified set of parameters by incorporating additional reference data: moisture loss (assuming k = 100 kg/m3), response to drying at different levels of relative humidity (30% and 70%) and shrinkage of a symmetrically drying slab with different thickness (75 mm and 300 mm). moreover, a model of 75 mm thick slab drying from one surface only is used to assess the influence of the identified diffusivity functions on the structural behavior. this model has the same effective thickness (and thus drying kinetics) but turns out to be significantly more sensitive to the shape of the diffusivity function. c1 α0 hc n f [mm2/day] [-] [-] [-] [mm/day] min. 35 0.02 0.68 6 1.1 max. 80 0.23 0.80 20 2.3 step 5 0.03 0.02 2 0.2 ref. 60 0.05 0.74 0.06 1.5 table 1. parameters of the transport problem: ranges, stepping for identification and the reference set. 3. results and discussion the solution obtained with the reference set of material parameters (tab. 1) is in the following figures shown consistently in thick red lines. as demonstrated in fig. 1, the axial shrinkage measured by 0 10 20 30 40 50 60 70 80 0.4 0.5 0.6 0.7 0.8 0.9 1 t0 = 8 d, henv = 60% calibration: experimentd i� us iv ity [m m 2 /d ay ] relative humidity [-] ref, f = 1.5 2, f = 1.5 3, f = 1.7 4, f = 1.7 5, f = 1.5 6, f = 1.5 7, f = 1.5 8, f = 1.3 9, f = 1.9 10, f = 1.7 henv figure 2. dependence of diffusivity on relative humidity for the 10 best fits in fig. 1. in the legend f is the surface factor [mm/day]. -1000 0 1000 2000 3000 4000 5000 6000 0.01 0.1 1 10 100 1000 10000 t0 = 8 d, henv = 60% calibration: experiment cu rv at ur e [ 10 -6 /m ] duration of drying [day] ref 2 3 4 5 6 7 8 9 10 figure 3. prediction of curvature of 75 mm thick slab exposed to drying from one side only at henv= 50% which started at the age t0 = 28 days. the line types and the material parameters correspond to figs. 1 and 2. bryant on prism 150 × 150 mm drying from two opposite surfaces is with very similar accuracy captured by numerous combinations of material parameters of the diffusivity function. figure 1 presents only the 10 best fits, the respective functions are shown in fig. 2 and the legend lists the values of the surface factor f [mm/day]. from the fits, it can be concluded that the evolution of axial shrinkage is almost insensitive to the diffusivity at high relative humidity (h > 0.8) where the values span from 45 to 75 mm2/day, but becomes very sensitive to the diffusivity in the region just above the ambient relative humidity henvwhere the functions form a narrow band. the response of a slab with the same effective thickness exposed to drying from one side only at standard laboratory conditions (t0 = 28 days, henv= 50%) is for the best 10 identified combinations of diffusivity shown in fig. 3. in contrast to symmetric drying, the computed response for asymmetric drying be26 vol. 30/2021 modeling of concrete subjected to symmetric and asymmetric drying 0 10 20 30 40 50 60 70 80 0.4 0.5 0.6 0.7 0.8 0.9 1 t0 = 28 d, henv = 50% calibration: simulationd i� us iv ity [m m 2 /d ay ] relative humidity [-] ref, f = 1.5 2, f = 1.5 3, f = 1.5 4, f = 1.5 5, f = 1.5 6, f = 1.5 7, f = 1.5 8, f = 1.5 9, f = 1.7 10, f = 1.5 henv figure 4. diffusivity functions giving the best agreement with the axial shrinkage from the simulation of symmetrically drying concrete slab (d = 150 mm, henv= 50%, t0 = 28 days) computed with the reference set of material parameters. -1000 0 1000 2000 3000 4000 5000 6000 0.01 0.1 1 10 100 1000 10000 t0 = 28 d, henv = 50% calibration: simulation cu rv at ur e [ 10 -6 /m ] duration of drying [day] ref 2 3 4 5 6 7 8 9 10 figure 5. prediction of curvature of 75 mm thick slab exposed to drying from one side only at henv= 50% and at the age t0 = 28 days. the line types and the material parameters correspond to fig. 4. comes surprisingly different. in all presented cases, the peak curvature is reached after ≈ 20 days of drying, and the individual solutions differ by up to 20% of the reference solution. in several cases the curvature exhibits a second local maximum one order of magnitude later (≈ 200 days). the value at the first peak is tightly connected with the diffusivity at high relative humidity, the higher this diffusivity, the lower the curvature and vice versa (compare e.g. the dashed blue and black lines #8 and #9). the higher this diffusivity the lower is the gradient of relative humidity between the sealed and drying surface of the cross-section and thus the lower is the resulting curvature. the second peak appears when the diffusivity function has a steep transition between high and low relative humidity which is controlled by parameter n in (2). the preceding observations are confirmed in fig 0 200 400 600 800 1000 1200 1400 0.1 1 10 100 1000 10000 t0 = 28 d calibration: simulation henv = 50% henv = 30% henv = 70% henv = 30, 50, 70% a xi al sh rin ka ge [1 0-6 ] duration of drying [day] ref 2 3 4 5 6 7 8 9 10 figure 6. best fits of (reference) axial shrinkage caused by symmetric drying at three levels of ambient relative humidity henv= 30, 50, 70% (d = 150 mm, t0 = 28 days). 0 10 20 30 40 50 60 70 80 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 t0 = 28 d calibration: simulation henv = 30, 50, 70% d i� us iv ity [m m 2 /d ay ] relative humidity [-] ref, f = 1.5 2, f = 1.5 3, f = 1.7 4, f = 1.3 5, f = 1.3 6, f = 1.7 7, f = 1.5 8, f = 1.3 9, f = 1.5 10, f = 1.7 figure 7. diffusivity functions giving the best agreement with the axial shrinkage from the reference simulations (symmetric drying at three levels of ambient relative humidity henv= 30, 50, 70%, d = 150 mm, t0 = 28 days). ures 4 and 5 where the calibration was done on the data from simulation with reference parameters but at “laboratory conditions” (t0 = 28 days, henv= 50%). the identified diffusivity functions are very close to the reference case in the region from henv to henv+ 10%–15%, while beyond this limit the functions are distinctively different which has an impact on the evolution of curvature. incorporating weight loss does not narrow the scatter of the identified diffusivity functions (figures are not presented here). from the findings presented above it is natural to expect that if the separate measurements of axial shrinkage are performed at different levels of relative humidity, then the identified diffusivity functions will be closer to each other in the relative humidity slightly above the prescribed levels. to validate this statement, 2 additional values of relative humidity 27 pavel horák, petr havlásek acta polytechnica ctu proceedings -1000 0 1000 2000 3000 4000 5000 6000 7000 0.01 0.1 1 10 100 1000 10000 t0 = 28 d, henv = 30, 50, 70% calibration: simulation cu rv at ur e [ 10 -6 /m ] duration of drying [day] ref 2 3 4 5 6 7 8 9 10 figure 8. prediction of curvature of 75 mm thick slab exposed to drying from one side only at henv= 50% which started at t0 = 28 days. the line types and the material parameters correspond to identification at 3 different levels of henv(figs. 6 and 7). 0 200 400 600 800 1000 0.1 1 10 100 1000 10000 t0 = 28 d, henv = 50% calibration: simulation 150 mm 75 mm 300 mm a xi al sh rin ka ge [1 0-6 ] duration of drying [day] ref 2 3 4 5 6 7 8 9 10 figure 9. best fits of (reference) axial shrinkage of 75 mm, 150 mm, and 300 mm thick slabs exposed to symmetric drying at henv= 50% (t0 = 28 days). 0 10 20 30 40 50 60 70 80 0.4 0.5 0.6 0.7 0.8 0.9 1 t0 = 28 d, henv = 50% calibration: simulation d i� us iv ity [m m 2 /d ay ] relative humidity [-] ref, f = 1.5 5, f = 1.5 6, f = 1.5 8, f = 1.5 figure 10. diffusivity functions giving the best agreement with the reference simulations of axial shrinkage of symmetrically drying slabs with various thicknesses (henv= 50%, t0 = 28 days, fig. 9). 0 10 20 30 40 50 60 70 80 0.4 0.5 0.6 0.7 0.8 0.9 1 t0 = 28 d, henv = 50% calibration: reverse d i� us iv ity [m m 2 /d ay ] relative humidity [-] ref, f = 1.5 2, f = 1.5 3, f = 1.5 4, f = 1.5 5, f = 1.5 6, f = 1.5 7, f = 1.7 8, f = 1.5 9, f = 1.5 10, f = 1.5 figure 11. diffusivity functions calibrated inversely on the evolution of curvature of 75 mm thick slab drying from one side (henv= 50, t0 = 28 days) obtained from the simulation with the reference set of material parameters. were added and the analysis was done at 30%, 50% and 70%. the 10 best fits are almost indistinguishable from the reference solution of axial shrinkage (see fig. 6) and indeed, the identified diffusivity functions form a narrow band which widens with increasing relative humidity (fig. 7). this has a direct impact on the computed curvature shown in fig. 8 where the scatter in the first peak persists (caused by high scatter of the maximum diffusivity), but beyond 100 days of drying the agreement between the solutions becomes significantly better. next, it was attempted to identify the diffusivity function based on the shrinkage evolution of specimens (i.e. models) with different effective sizes. the reference thickness d = 150 mm was doubled to 300 mm and reduced to 75 mm. the 10 best fits match the solution with reference parameters perfectly (fig. 9), yet the diffusivity functions show considerable scatter, see fig. 10. also it needs to be emphasized that such approach can turn out to be very demanding on the experimental duration since the evolution scales with d2. to summarize, and as illustrated in figs. 12 and 13 it turned out that the shrinkage of symmetrically drying specimen is significantly less sensitive to the diffusivity function compared to the curvature produced by non-symmetric drying. for this reason the diffusivity identified from axial shrinkage should be used with caution and to members with similar size and boundary conditions. on the other hand the diffusivity identified from the evolution of curvature (see fig. 11) exhibits significantly less scatter and so does the back-calculated axial shrinkage which matches the reference case perfectly (not presented here). 28 vol. 30/2021 modeling of concrete subjected to symmetric and asymmetric drying 0 10 20 30 40 50 60 70 80 0.4 0.5 0.6 0.7 0.8 0.9 1 t0 = 28 d, henv = 50% c1 60, alpha0 0.05, hc 0.74, n 6, f 1.5 c1 75, alpha0 0.10, hc 0.80, n 16, f 1.1 d i� us iv ity [m m 2 /d ay ] relative humidity [-] reference, f = 1.5 modi�ed, f = 1.1 henv figure 12. diffusivity function with the “reference” and “modified” set of material parameters. 4. conclusions this paper investigated drying shrinkage of concrete slabs exposed to symmetric and asymmetric drying at constant ambient relative humidity. the problem was approached by means one-way coupled finite element simulations in program oofem. the moisture transport was described using the the bažant-najjar model while the time-dependent behavior of concrete by the modified mps model. the emphasis was on put on identification of material parameters of the model for moisture transport and their uniqueness. the following conclusions can be drawn from the analysis: • identification of parameters based the evolution of axial shrinkage of symmetrically drying concrete does not yield unique set of material parameters. the reference data could be reproduced with various sets of parameters with accuracy similar or better than the typical experimental scatter. • incorporating the shrinkage data measured at different levels of relative humidity helps to narrow the identified diffusivity functions. this is not the case of the data on moisture loss or shrinkage of specimens with different size. • the behavior of concrete drying non-symmetrically is considerably more sensitive to the values of material parameters. the parameters identified from the evolution of curvature give more consistent diffusivity functions and result into almost identical axial shrinkage. • the presence and the character of the second maximum in the evolution of curvature is most sensitive to the exponent n which defines the steepness of the transition between small and high diffusivity. • the numerical simulations of concrete drying from one side only exhibit very little sensitivity to concrete creep (fig. 13). -200 0 200 400 600 800 1000 1200 0.01 0.1 1 10 100 1000 10000 -1000 0 1000 2000 3000 4000 5000 6000 t0 = 28 d, henv = 50% a xi al sh rin ka ge [1 0-6 ] cu rv at ur e [ 10 -6 /m ] duration of drying [day] reference modi�ed figure 13. evolution of axial shrinkage and curvature obtained with the “reference” and “modified” set of material parameters of the diffusivity function (fig. 12) and d = 150 mm, t0 = 28 days, henv= 50%. thin lines correspond to the behavior without viscoelastic effects. acknowledgements the authors gratefully acknowledge financial support from the grant agency of the czech technical university in prague, project number sgs20/038/ohk1/1t/11, and from the czech science foundation (ga čr), project number 19-20666s. references [1] v. šmilauer, p. havlásek, t. gasch, et al. hygro-mechanical modeling of restrained ring test: cost tu1404 benchmark. construction and building materials 229:116543, 2019. doi:https://doi.org/10.1016/j.conbuildmat.2019.07.269. [2] b. patzák. oofem home page, 2000. http://www.oofem.org. [3] a. h. bryant, c. vadhanavikkit. creep, shrinkage-size, and age at loading effects. aci materials journal 84:117–123, 1987. [4] z. p. bažant, l. j. najjar. nonlinear water diffusion in nonsaturated concrete. materials and structures 5:3–20, 1972. doi:10.1007/bf02479073. [5] fédération internationale du béton. model code 2010. no. vol. 65 in fib bulletin. international federation for structural concrete (fib), 2012. [6] z. bažant, p. havlásek, m. jirásek. microprestress-solidification theory: modeling of size effect on drying creep. in n. bicanic, h. mang, g. meschke, r. de borst (eds.), computational modelling of concrete structures, pp. 749–758. crc press/balkema, eh leiden, the netherlands, 2014. [7] z. p. bažant, a. p. hauggaard, s. baweja, f. j. ulm. microprestress solidification theory for concrete creep. i: aging and drying effects. journal of engineering mechanics 123:1188–1194, 1997. 29 https://doi.org/https://doi.org/10.1016/j.conbuildmat.2019.07.269 http://www.oofem.org https://doi.org/10.1007/bf02479073 pavel horák, petr havlásek acta polytechnica ctu proceedings [8] z. bažant, s. baweja. creep and shrinkage prediction model for analysis and design of concrete structures: model b3. adam neville symposium: creep and shrinkage structural design effects 2000. [9] p. havlásek. creep and shrinkage of concrete subjected to variable environmental conditions, phd. thesis. czech technical university in prague, 2014. 30 acta polytechnica ctu proceedings doi:10.14311/app.2018.20.0043 acta polytechnica ctu proceedings 20:43–50, 2018 © czech technical university in prague, 2018 available online at http://ojs.cvut.cz/ojs/index.php/app experimental and numerical research on flow in the last stage of 1090 mw steam turbine michal hoznedl∗, kamil sedlák experimental research of flow, doosan škoda power s.r.o., plzeň, czech republic ∗ corresponding author: michal.hoznedl@doosan.com abstract. the paper deals with experimental and numerical research in the last stage of real 1090mw steam turbine with the last steel blade length 1220mm placed in nuclear power station. the last stage was equipped with twelve static pressure taps. it was also possible to probe in two planes before and behind the last stage using pneumatic or optical probes. a number of last stage flow parameters were determined at the root and tip wall for nominal turbine output. among those parameters are static pressures, mach and reynolds numbers, last stage reactions and steam wetness. all the directly measured and evaluated flow parameters are taken from locally measured points and that is why even 3d cfd calculation of the whole system last stage, diffuser and exhaust hood was implemented. measured and calculated parameters are compared. especially static pressures are in very good agreement; the only steam wetness has bigger difference due to different measurement position. locally measured values are enough to estimate the flow behavior of the last stage. on the other hand, the cfd simulations with well determined boundary conditions, meshes and geometry and is effective tool to simulate even very complicated flow structures in the last stage and exhaust hood. keywords: steam turbine, experimental measurement, last stage blade, pneumatic probe, optical probe. 1. introduction flow in the steam turbine last stage and exhaust hood still remains one of the serious problems in the area of steam turbine research. the complexity of the issues can be found mainly in the mutual flow coupling between the last stage, the subsequent diffuser and the exhaust hood. the importance of the research can be easily shown on an example when inappropriate configuration of the last lp stage and the subsequent exhaust hood causes loss up to 24mw on the turbine with 1090mw output in a nuclear power plant with six last stages. specifically for this case the pressure loss between the lsb outlet and condenser inlet is considered at the level of 2000pa. for precise modelling of the whole system of the last stage and the diffuser it is necessary, for numerical simulation, to know exact boundary conditions. obtaining or deriving exact boundary conditions is often accompanied with significant inaccuracy, sometimes it is even impossible. certain possibility here can be only the last stage calculation without the subsequent diffuser. however, in this case fundamental differences can be expected between the data obtained by numerical simulation and reality. e.g. in the tip area behind lsb, because of the diffuser effect, velocity will be definitely higher than in the case when the diffuser in the mode is neglected. for the reasons mentioned above it is recommended to obtain the boundary conditions from experimental measurements. they were in the past carried out on some turbines [1, 2], recently results have been published only on 300mw turbine with an air-cooling condenser [3, 4]. there are of course many numerical simulations of flow that suitably solve the behaviour of the lsb diffuser coupling [5, 6]. in these cases boundary conditions are based above all on measurements of experimental steam turbines, e.g. [7]. the aim of presented paper is to show the comparison of experimentally obtained data from the static pressure taps placed on the tip and the root of the 1090mw steam turbine last stage with the results of numerical cfd simulation. however, in order to obtain other stage parameters (mach and reynolds number, stage enthalpy drop) it was necessary to define thermodynamic parameters before and behind the last stage. for this reason traversing was carried out by both the pneumatic and optical probes to get the wetness profile along the blade length. from the measured profiles the points were separated near the root and tip limiting wall and then used for the calculation of the above mentioned quantities. a certain complexity of the problem lies in the fact that the whole last stage is in wet steam, which makes the evaluation of a number of quantities rather difficult. detailed comparison of pressures and wetness profiles behind l-1 and l-0 rotor blade rows obtained by the pneumatic and optical are out of the range of this paper and they are presented in [8] or in [9]. the second goal of the paper is to compare numerically and experimentally obtained data mainly in the area behind the last stage when firstly the local value of measured and calculated pressure and wetness is compared with the integral value for the entire cross-section. cfd and experimental results must be 43 http://dx.doi.org/10.14311/app.2018.20.0043 http://ojs.cvut.cz/ojs/index.php/app michal hoznedl, kamil sedlák acta polytechnica ctu proceedings figure 1. position of static pressure taps. mutually compared and, if other measured parameters (pressures, velocities etc.) of steam flow along the flow path are consistent, it is possible to consider the cfd simulation validated. thus the average value can be used to obtain flow parameters in the desired section. an example may be a cfd computation based on the measured boundary conditions before the last stage and the condenser neck outlet and validated using measured data behind the last stage. 2. experimental setup one of the six turbine last stages was set up with a number of static pressure taps. twelve of them were used to explore the flow in the last stage. static pressure taps were located circumferentially between 9° and 43° above the dividing plane on both sides of the turbine. meridional position of taps is shown in fig. 1. it was implemented using stainless steel impulse piping of 12mm diameter with declination to the turbine. the pressure tap hole had a diameter of 2mm. on the turbine shroud the taps were connected to pressure sensors netscanner 9401 with measurement uncertainty of b type on the level of 0.19% from the measured value. the uncertainty value is defined with confidence interval of probability 95.5%. uncertainties were assessed using gum methodology. static pressure taps were equipped with solenoid valves that automatically, at certain intervals connected the turbine space with the surrounding atmosphere. the overpressure forced any water droplets from the impulse piping back to the turbine. it was found after first data evaluation that the pressure p1t has a leakage and it was not used for next evaluation. in order to define the steam parameters before and behind the last stage the ports for probes with diameter of 50mm were installed. the probing plane 0 was in the position of about 70mm from the last but one stage trailing edges. its angle to the turbine axis was figure 2. position of traversing planes. 80°. plane 2 was located about 100 mm behind the trailing edges of the lsb stage with the axis declination 85°. probing was carried out on both sides of the turbine with angle about 45° from the dividing plane. positions of plane 0 and 2 are in fig. 2. besides both planes also the section was used upstream of lsb, i.e. plane 1, where it was not possible for technical reasons to carry out probe measurements. besides probing the steam wetness level using optical probe along the blade length behind the last and last but one stage, measurement was carried out of distribution of pressures, velocities and flow angles [9], as well as probing for defining turbulence parameters and for probing for evaluation of unsteadiness in the steam flow. optical probe for measuring steam wetness enables to obtain extinction data that characterize the polydisperse structure of the liquid phase contained in the wet steam. for wetness evaluation from extinction data more than 40 wavelengths were used in the range 200 1000 nm. during measurement the probe was oriented to the steam flow in accord with flow angle α2, obtained previously from the pneumatic probes measurements. wetness measurement uncertainty is expected at the level of ± 5 ÷ 10% from the measured wetness value. the whole system was calibrated before measurement using latex particles in the laboratory of czech technical university. picture of the optical probe head is in fig. 3. 3. cfd setup cfd flow research methods based on boundary conditions found in experimental steam turbines are a rather common form of flow research in last stages and exhaust hoods [10, 11]. in the given case complex 3d geometry of the system was solved with one last periodically repeated 44 vol. 20/2018 research on flow in the last stage of 1090mw steam turbine figure 3. optical probe head with open and closed measuring space. rotor blade and all 66 stator blades. the rotor blades outlet is followed by the diffuser, exhaust hood and condenser neck with all important elements, such as the tube reinforcement, low pressure heaters and steam inlet ducts. only one flow was simulated out of the double-flow exhaust hood. between the left and the right flow the symmetry boundary condition was entered. for detailed research in the suction slots area even hollow stator blades were modelled as well as the whole suction tract. the last stator blade was modelled including the rotor seal and psc. these are the elements that significantly influence the quality of transition lsb-diffuser from the viewpoint of flow and its losses. between the periodically repeated stator and rotor rows the mixing plane is set due to different spacing of stator and rotor blades. between the rotor blade and the diffuser, the frozen rotor interface is considered. this setting respects very good distribution regarding the circumferential non-uniformity of the flow entering the diffuser. on the other hand, the disadvantage of the frozen rotor method is the need to model all last blades of the l-0 stage. this makes it more demanding for computational mesh and time. simulation was carried out as stationary, fully turbulent (model turbulence sst k-ω) with steam as the working medium, with parameters computed using iapws if-97. for calculations hexahedral computational mesh was used in the blade part of the domain. hexa-core hybrid mesh was used for exhaust hood and condenser neck meshing. the reason was a large geometrical complexity and irregularity of these components. the whole mesh contained about 60 million cells. the two viewpoints on the solved geometry including the marked boundary conditions are in fig. 4. the computational mesh is in fig. 5. figure 4. calculated geometry. figure 5. computational mesh. 45 michal hoznedl, kamil sedlák acta polytechnica ctu proceedings as the inlet of the computed domain the profiles of total pressure, temperature, wetness and direction vectors along the l-1 rotor blade length were used. the inlet to the domain is in the position of plane 0. data about water steam quantities were obtained by repeated probing. outlet pressure boundary condition was set with given static pressure obtained by measurement at the same time when the probing was done. after calculation the value of y+ parameter was checked. recommended value of this parameter for the used turbulent model is y+ < 1. however, such value was not possible to keep everywhere due to a large computation area. maximum value of y+ was at the level of about 7. nevertheless, at places where flow phenomena such as flow separation or large pressure gradients can occur the above mentioned condition was met. 4. discussion of obtained results a rather significant unevenness of the lsb outlet flow field resulted from cfd calculations. distribution of absolute mach number in plane 2 can be found in fig. 6. there is an obvious circumferential asymmetry, caused by underloaded last stage and the angle of the outlet steam flow α2 > 90°. due to this asymmetry a larger amount of flow goes through the right half of the exhaust hood than through the left half. the area behind psc is also important as due to this obstacle losses are generated there. it influences the local velocity change [10, 12]. another significant phenomenon is the area of high outlet velocities. these high velocities are generated in places where the loss between lsb and condenser neck is the smallest. it is obviously in the bottom part of the lsb outlet. on the contrary the area of low velocities behind lsb occurs in the upper quadrant of the outlet cross section. the flow coming from these areas must overcome a pressure loss by flowing across the dividing plane and the inlet steam ducts to the lp part. the area of high velocities behind lsb corresponds with low losses areas and vice versa, with the exception of flow in radial slackness. from fig. 6 it is also obvious that probing (and also static pressure measuring) is in the last stage area in plane 2 carried out in the places with lower outlet velocity value. measured values of velocity, pressure and possible other parameters definitely does not correspond with the mean value of velocity in the given area cross section. for this reason it can be unsuitable to implement further data analyses only on the basis of experimental measurement. thus in this paper basic flow parameters are compared in plane 0 and plane 2 and in the stage obtained using cfd as well as measurement. based on the obtained data it can be judged whether the last stage flow parameters can be calculated only from cfd simulations or whether supported experimental data are needed to confirm cfd at all time. static pressure values serve as basic information for other parameters calculation. they are given in [%] exp cfd cfd_integral p0 100.0 100.3 101.7 p1 105.3 100.5 p2 98.2 91.6 table 1. comparison of static pressures in individual planes. [%] exp cfd p0r 100.0 102.8 p0t 98.1 p1r 102.1 p1t 107.8 p2r 97.6 p2t 98.9 table 2. comparison of static pressures in local points. fig. 7, fig. 8 and fig. 9. first two columns in graphs always indicate the mean value of static pressure from measurements and from cfd simulations. but these are average pressure values from local pressure taps. e.g. in plane 0 these are taps p0t and p0r, each on the left and the right side of the machine. the last column shows the mean value of the pressure obtained by averaging the flow in the given cross section. a better view of the differences in all three approaches to the pressure evaluation is provided by the data in tab. 1, when the value 100% always represents the pressure obtained by experimental measurement. it is evident that the biggest differences between individual approaches to evaluation are in plane 2. the cfd_integral method predicts the pressure value by 8.4% lower than the one obtained by measurement. the reason is circumferential distribution of static pressure that reaches the lowest values in the lower half of the lsb outlet, while the measurement took place in the upper half of the outlet. significant differences in static pressure are evident also from fig. 10, where the position of probe traversing is marked. a detailed view on the above mentioned data is provided in tab. 2, where local values of static pressures from measurements and from cfd are compared. in plane 0 and 2 there is a good agreement between experimental results and cfd simulation. a larger difference occurs due to pressure p1t. the cause of the difference is the fact that pressure taps are located just behind the heat extraction that sucks part of the steam outside the flow part of the turbine. this extraction was not considered in cfd. the second cause of the difference was mentioned above one of the paired pressures p1t was not tight and thus it was excluded from further evaluation. wetness value is also an important quantity for defining the thermodynamic state of steam. this value was measured in plane 0 and plane 2 using an optical probe. however, for further analyses only two points 46 vol. 20/2018 research on flow in the last stage of 1090mw steam turbine figure 6. absolute mach number in plane 2. figure 7. static pressures in plane 0. figure 8. static pressures behind stator row. figure 9. static pressures in plane 2. figure 10. static pressure flow field in plane 2. 47 michal hoznedl, kamil sedlák acta polytechnica ctu proceedings figure 11. absolute mach number in plane 2. will be used from the whole profile, one located near the root wall and other one near the tip limiting wall in plane 2. comparison of wetness values defined from cfd and from experimental measurement is shown in fig. 11. the value of steam wetness obtained from cfd is in the root by 7.7% and in the tip by 2.5% lower than the value obtained from experiment. this rather significant difference is caused by the fact that the probe measurement area is located minimally 30 mm above the root limiting wall and is 50 mm long, which is the length of the illuminated space. but in cfd the wetness data are read in points, at the place of static pressure tap. on the other hand, high wetness on limiting walls would mean high local efficiency, which is inconsistent with reality. the reason for this contradiction can be the usage of thermodynamic equilibrium cfd computation of the flow. the results from both cfd and measurement confirmed the fact that higher wetness and thus better efficiency is found near the tip limiting wall. for further analyses it is important to define individual relations for defining of basic stage parameters. calculation of thermodynamic parameters that were not measured is derived from water and water steam tables iapws_97. stage parameters of the flow are considered subsequently on the root and tip limiting wall. the stage reaction is defined based on the ratio of isentropic drop of the rotor to the isentropic drop over the stage in the given cross section, see relation (1). ρ = hisn/hisst [−] (1) then mach number of the stage is defined: mast = √√√√( 2 κ− 1 ) · [( p0 p2 )κ−1 κ − 1 ] (2) relation (3) defines mach number for stator blades: man = √√√√( 2 κ− 1 ) · [( p0 p1 )κ−1 κ − 1 ] (3) in the above mentioned relations it is necessary to put the kappa value κ = 1.11. this is a common setting for wet steam conditions and it is consistent with evaluation of data from cfd. reynolds number of the stage is defined as the last one. the chord of stator blade on the root or tip cross section is used as a characteristic dimension. for reynolds number calculation it is necessary to define kinetic viscosity ν2. it is obtained as a result of the function iapws_97 for two parameters (p2t, x2t) and (p2r, x2r). also the isentropic velocity cis from the enthalpy drop over the whole stage is defined: cis = √ 2000 · hisst (4) in this case his is defined by function iapws_97 from the pressure and dryness in the stator blades inlet and from the pressure and entropy in the lsb outlet. the entropy behind lsb is the same as the entropy in the stator blades inlet subsequently on the root and tip limiting wall. now it is possible to calculate the reynolds number of the stage: rest = cis · b ν2 (5) the values of all described parameters are given or as a local value for root and tip, or in the form of integral value defined only using cfd computations. the integral value is defined from the mass flow weighted parameters of the flow detected over the entire section 1, 2 or 3. only the integral value for reynolds number is missing. the reason is that it is impossible to define the characteristic dimension, i.e. the stator blade chord, valid for all its height. percentage comparison of values is shown in tab. 3. for evaluation of cfd and of experimental data the same relations are used. due to considering all the important components and cfd computation of the complex geometry the differences from the experiment results should be minimal. more significant differences can be expected in the stator blade root because of a bigger difference between the measured and calculated wetness value. further differences can occur on the tip limiting wall due to a higher pressure p1t, obtained from numerical simulation. the first compared value is the stage reaction. cfd simulations showed a higher stage reaction than the measured one, mainly because of the pressure p1t, that was by 7% higher than that from experiments. on the other hand, the integral value of the reaction is in fact identical with the average value of the reaction from measurement in the root and the tip of blading. there is a very good agreement for stage mach number in the root and the tip. the integral value mast that is not an arithmetic average of the same parameter shows certain difference. it indicates that the mast values along the stage length will be closer to the values near the tip limiting wall. there is a difference for mach number of the stator in the tip (again caused by inaccurate pressure 48 vol. 20/2018 research on flow in the last stage of 1090mw steam turbine [%] exp cfd [%] exp cfd ρr 100.0 102.9 rest r 100.0 93.6 ρt 109.0 rest t 96.2 ρintegral 98.7 mast r 102.2 manr 100.8 mast t 99.9 mant 90.5 mast integral 103.8 manintegral 99.7 table 3. comparison of last stage flow parameters. figure 12. stage reaction. figure 13. stage mach number. p1t). however, the overall agreement is again very satisfactory. there are also reasonable differences when defining reynolds number using both methods. reynolds number is used mainly for calculation of the influence of surface roughness on the stage efficiency. it is therefore not necessary to define it precisely but rather its range. static pressure taps in all three planes can be considered as the credible source of information about the entire last stage behaviour. if also the wetness value near the taps is known sufficiently (e.g. from the flow calculation of the stage) it is possible to define many parameters without a demanding probe traversing. figure 14. nozzle mach number. figure 15. stage reynolds number. 5. conclusions experimental measurement was carried out on the 1090mw output turbine in the last stage area using static pressure taps. root and tip steam wetness was defined by probing before and behind the stage. at the same time extensive flow simulations were executed with the same geometry of the last stage, diffuser and exhaust hood as the ones measured on the real work. due to only negligible differences in the measured and simulated geometries a very good agreement was reached between the results from experimental measurements and cfd computations. it can be said that properly placed static pressure taps can credibly describe the last stage behaviour as for pressures not only in specific points, but also in the entire 2d section. pressure drops, mach numbers of the stage and of the stator can be specified accurately. with the 49 michal hoznedl, kamil sedlák acta polytechnica ctu proceedings knowledge of steam wetness other stage parameters can be defined, such as its reaction, isentropic drop or reynolds number. unless detailed knowledge of the flow behaviour along the blade length is required, it is not necessary to execute technically and financially demanding measurement with pneumatic or optical probes, at least not behind lsb. on the other hand, correctness and accuracy of computational cfd methods was verified using experimental measurement. these methods can accurately predict the behaviour in such a very complicated component of the steam turbine as is the last stage and the subsequent diffuser. however, in order to obtain correct behaviour of computations, it is necessary to accurately specify the inlet and outlet boundary condition. in order to define boundary conditions it is suitable to experimentally determine the flow behaviour at least by using the mentioned static taps. only this will ensure the results accuracy as for steam flow behaviour also in the places where experimental measurement is very complicated e.g. between the rotor and stator. list of symbols b stator blade chord c velocity p pressure α circumferential angle of the steam ν kinematics viscosity ρ density abbreviations: gum the guide to the expression of uncertainty in measurement lsb last stage blade lp low pressure psc part span connector l− 1 last but one stage l− 0 last stage st stage subscripts: 0 before last nozzle 1 behind last nozzle 2 behind last rotor blades n nozzle r root t tip acknowledgements the work was supported from european regional development fund-project “lostr” (no. cz.02.1.01/0.0/0.0/16_026/0008.389) and from technology agency of the czech republic from the project tk01020029 efficiency increasing of turbine wet steam last stages references [1] a. kleitz, a. r. laali, j. j. courant. 26. fog droplet size measurement and calculation in wet steam turbines, pp. 201–206. british nuclear energy society. doi:10.1680/totpowws.13957.0029. [2] a. liberson, s. h. hesler, t. h. mccloskey. water droplet size measurements in operating steam turbines. in epri proceedings of 1998 heat rate improvement conference. 1998. [3] x. cai, t. ning, f. niu, et al. investigation of wet steam flow in a 300 mw direct air-cooling steam turbine. part 1: measurement principles, probe, and wetness. proceedings of the institution of mechanical engineers, part a: journal of power and energy 223(5):625–634, 2009. doi:10.1243/09576509jpe690. [4] x. cai, f. niu, t. ning, et al. an investigation of wet steam flow in a 300 mw direct air-cooling steam turbine. part 3: heterogeneous/homogeneous condensation. proceedings of the institution of mechanical engineers, part a: journal of power and energy 224(4):583–589, 2010. doi:10.1243/09576509jpe906. [5] z. burton, s. hogg, g. ingram. the influence of inlet asymmetry on steam turbine exhaust hood flows. journal of engineering for gas turbines and power 136:0426021–426029, 2014. doi:10.1115/1.4026003. [6] m. mihailowitsch, m. schatz, d. vogt. numerical investigations of an axial exhaust diffuser coupling the last stage of a generic gas turbine. journal of engineering for gas turbines and power 2b: turbomachinery:v02bt43a003, 2018. doi:10.1115/1.4040769. [7] m. häfele, c. traxinger, m. grübel, et al. experimental and numerical investigation of the flow in a low-pressure industrial steam turbine with part-span connectors. journal of engineering for gas turbines and power 138:072604, 2016. doi:10.1115/1.4032205. [8] a. živný, a. macalka, m. hoznedl, et al. numerical investigation and validation of the 1 090 mw steam turbine exhaust hood flow field. in turbomachinery technical conference and exposition, vol. 8, p. v008t29a015. 2017. doi:10.1115/gt2017-63576. [9] m. hoznedl, m. kolovratník, o. bartoš, et al. experimental research on the flow at the last stage of a 1090âăľmw steam turbine. proceedings of the institution of mechanical engineers, part a: journal of power and energy 232(5):515–524, 2018. doi:10.1177/0957650917749692. [10] m. häfele, j. starzmann, m. grübel, et al. numerical investigation of the impact of part-span connectors on aero-thermodynamics in a low pressure industrial steam turbine. vol. 1. 2014. doi:10.1115/gt2014-25177. [11] j. starzmann, m. schatz, m. casey, et al. modelling and validation of wet steam flow in a low pressure steam turbine. vol. 7. 2011. doi:10.1115/gt2011-45672. [12] t. radnic, j. hála, m. luxa, et al. aerodynamic effects of tie-boss in extremely long turbine blades. journal of engineering for gas turbines and power 140, 2018. doi:10.1115/1.4040093. 50 http://dx.doi.org/10.1680/totpowws.13957.0029 http://dx.doi.org/10.1243/09576509jpe690 http://dx.doi.org/10.1243/09576509jpe906 http://dx.doi.org/10.1115/1.4026003 http://dx.doi.org/10.1115/1.4040769 http://dx.doi.org/10.1115/1.4032205 http://dx.doi.org/10.1115/gt2017-63576 http://dx.doi.org/10.1177/0957650917749692 http://dx.doi.org/10.1115/gt2014-25177 http://dx.doi.org/10.1115/gt2011-45672 http://dx.doi.org/10.1115/1.4040093 acta polytechnica ctu proceedings 20:43–50, 2018 1 introduction 2 experimental setup 3 cfd setup 4 discussion of obtained results 5 conclusions list of symbols acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2020.28.0050 acta polytechnica ctu proceedings 28(0):50–58, 2020 © czech technical university in prague, 2020 available online at http://ojs.cvut.cz/ojs/index.php/app experimental investigation of critical heat flux in annulus at low pressure and low flow parameters daniel vlčeka,∗, ladislav sukc, kamil števankab, taron petrosyanb a czech technical university in prague, faculty of nuclear sciences and physical engineering, department of nuclear reactors, břehová 78/7, prague, czech republic b brno university of technology, faculty of electrical engineering and communication, technická 10, 616 00 brno, czech republic c brno university of technology, faculty of mechanical engineering, technická 2896/2, 61669 brno, czech republic ∗ corresponding author: daniel.vlcek1@gmail.com abstract. steady state flow boiling experiments were conducted on a technically smooth inconel 625 tube with outer diameter 9.1 mm at inlet pressures 131, 220 and 323 kpa, inlet temperatures 62, 78 and 94 °c and approximately 400, 600 and 1000 kg/(m2.s) mass flow. water of these parameters was entering into the vertically aligned annulus, where the uniformly heated tube was placed until the critical heat flux (chf) appeared. the experimental data were compared to estimations of chf by local pgt tube correlation and groeneveld’s look-up tables for tubes. the results imply that in the region of low pressure and low mass flux, the differences between calculations and experiments are substantial (more than 50 % of chf). the calculations further imply that look-up tables and tube correlations should be corrected to the annulus geometry. here, the doerffer’s approach was chosen and led to a substantial enhancement of chf estimation. yet, a new correlation for the region of low pressure and flow is needed. keywords: correlations, critical heat flux, flow boiling, heat transfer, nuclear fuel cladding. 1. introduction current trend in cladding materials of nuclear fuel is an accident tolerant fuel which is designated to replace the current zirconium-based alloy. there are several candidates for atf cladding material with different benefits and drawbacks. to choose the best material, an extensive experimental testing has been conducted, however, a heat transfer characteristics of these materials are still an active research topic of interest. towards making the atf cladding material heat transfer analysis, the experimental facility with a primary database of different materials is needed. heat transfer characteristics of cladding material focuses mainly on cooling the heated fuel tubes and since the fuel is cooled mostly by water, a thermohydraulic analysis needs to be done. in this analysis, the key factor is the critical heat flux (chf) phenomena which lead to a fast reduction of heat transfer coefficient with a steep rise of cladding temperature which can induce the rupture of the heated material. better understanding of the boiling process is important for new fuel design and crucial for nuclear power plant safety analyses. the boiling process can be separated in two groups by the flow of coolant. the first group is pool boiling, which is characterised by the natural circulation of the coolant. the experimental facilities can be usually easily maintained and therefore they have been carried out on lots of different materials. the results of pool boiling experiments show strong agreement on the impact of surface morphology on chf [1], [2], [3], [4]. the flow boiling experiment traditionally considers pressure, mass flow and inlet subcooling as major factors assuming that the impact of the material is limited [5]. most of the flow boiling experiments were carried out in tube or in rod bundles at normal operating conditions of nuclear reactors, i.e., high pressure and high flow rate and therefore there is a lack of flow boiling data in the area of low flow and low pressure. after the fukushima accident, the severe fuel damage conditions of reactors and fuel behaviour during them became one of the main research issues, therefore there is an interest in the low flow and low pressure parameters. for the safety analysis, the chf is usually predicted by a subchannel code. its deterministic approach is based on a large experimental database from which the correlations were created. however, in the region of low flow and low pressure, the precision of the correlations is worse if not completely off. most of the data were collected from tube experiments made from steel and zirconium. this has to be taken in mind because even the change of the geometry from tube to annulus can lead to significant changes of the predicted chf. one way how to take account of bundle-specific effects (e. g., element gap size and curvature) together with the effects of low flow and low pressure is using specific correlations designed for annular geometry. the other way can be using the 50 http://dx.doi.org/10.14311/app.2020.28.0050 http://ojs.cvut.cz/ojs/index.php/app vol. 28/2020 critical heat flux on annulus figure 1. process flow diagram of mrcha test facility. correcting factors on existing correlations. haas [6] summarised the experimental results from the available literature of flow boiling at low flow and low pressure in annular geometry. rogers [7] and elgenk [8] investigated chf for water in an annulus in the problematic region mentioned above. both of them developed separate empirical correlations which can unfortunately be used only in the small parameter range of their experimental data. rogers also compared four different empirical correlations (knoebel [9], mcadams [10], menegus [11] and katto [12] and all of them significantly overpredicted the experimental chf in the annulus. chun [13] compared their chf experiments with bowring [14], doerffer [15] and janssen and kervinen [16] correlation. the bowring correlation showed the best agreement with chun’s data, however all used correlations at lower pressures than 10 mpa tended to overestimate chf. several correlations were studied at kaist [17] and the equations from doerffer [15] were suggested with caution for very low flow and pressure even though it was designated for pressures over 1 mpa. it can also be used for chf look-up tables by groeneveld [18] to correct chf data from tube to annulus geometry. this approach led to the best result in haas’s research [6]. mayer et al. [19] also presented the chf results of annular flow at the pressure range from 110 to 225 kpa with changing mass flux and with comparison of their data with several correlations. except the literature comparison, haas [20], [6] also presented his experimental data from the cosmos-l experimental loop. this was the main inspiration for the author’s research team to develop a test facility of similar design [21]. haas pointed out that flow boiling experiments could be strongly affected by instability, which could lead to pulsating flow and therefore these oscillations could cause a premature chf at much lower heat fluxes. apart from the material structure, there are a lot of different effects that bias the chf. the effect of clad thickness was discussed in [5] and for specific materials (inconel 600 and ss316) it showed the decrease of chf with increasing clad thickness up to 0.711 mm. lee [22], [23] also demonstrated that roughness and wettability have no clear impact on flow boiling chf. these results were not in accordance with the pool boiling experiments. the main object of this work was to measure a set of experiments on a technically smooth inconel 625 tube. to achieve the least amount of instability, three different values of pressure, inlet subcooling and mass flux were chosen. experimental data were compared to local chf correlation pgt and groeneveld’s lookup tables. the doerffer’s approach was then used to improve the tube equations for the annulus model. 2. experimental setup the flow boiling experiments were performed on the mobile research critical heat flux apparatus (mrcha) which was constructed and operated by the university of technology in brno at faculty of electrical engineering and communication and its thorough description can be found in the article [21]. the flow boiling test loop is formed by two circuits. the primary circuit delivers the deionized water (with conductivity circa 30 µs/cm) to the test section under the desired conditions. the secondary circuit serves mainly as a heat exchanger between the primary circuit and the water mains, thus consisting of heat exchangers, a circulation pump and several bypasses. the primary circuit is more complex and connects the two circuits through a heat exchanger and condenser as can be seen in fig. 1. the pressurizer stores the working fluid and separates the heated liquid and vapour coming out from the test section under the desired pressure. heat exchanger cools down the heated water mainly from the condenser while the pressurizer is preheated to a 51 d. vlček, l. suk, k. števanka, t. petrosyan acta polytechnica ctu proceedings figure 2. the cross-sectional view of the mrcha test section with its actual view. certain temperature to compensate for temperature differences. the main circulation pump then delivers the cooled water from the heat exchanger to the preheater, where the inlet temperature is set. to adjust the flow rate, a regulation valve is used directly behind the preheater. the water of the desired parameters enters the vertically aligned test section where it is uniformly heated by the electric current. at the outlet of the test section, a backflow valve is used to ensure the safety of the facility. 2.1. test section the main part of the flow boiling test facility is a test section which is designed as a vertical annulus. a detailed cross-section of the test section next to its actual photo is shown in fig. 2. the first experimental data were obtained on a 380 mm long uniformly heated inconel 625 tube. the fluid enters the bottom part by two throats and goes through an unheated stability section, where the inlet temperature is measured. the power from the power supply to the heated tube is delivered through the copper power terminals, which are designed as a 133 mm long rod with an outer diameter of 9 mm at the inlet section and a similar copper rod with the length of 113 mm and outer diameter of 10 mm at the outlet. nickel is used to cover the copper conductors. inside of the heated tube, a measurement probe is placed. to prevent water leakage from the test section to the test probe, the test tube and copper terminals are pressed together and then soldered with lead-free tin solder. since the temperature of the heated tube can reach more than 400 °the bottom copper terminal includes a thermal expansion joint to compensate for possible thermal dilatation. the annular geometry of the test section is characterised in the tab. 1. to ensure the centred position of the test tube in the glass surroundings, a spacer is placed in the lower part of the test section and a centring pin is located in the upper part. with the glass tube it is possible to closely observe ongoing phenomena by capturing the flow by a high speed camera. the only drawback can be the heat loss of it, however it was estimated to 50 w at the input power of 20 kw. parameter value outer diameter do [mm] 14.5 inner diameter di [mm] 9.1 gap width δ [mm] 2.7 heated length [mm] 380 wetted perimeter [mm] 74.1 heated perimeter [mm] 28.6 length to heated eq. diameter [mm] 27.1 length to hydraulic diameter [mm] 70.4 length to inner diameter [mm] 41.8 table 1. the details of test annulus geometry. the test section is heated by the direct current power supply via copper power terminals. active power of the supply is 120 kw and it can steadily provide 1000 a and 100 v and it can burst up to 1700 a and 70 v for a desired length of the heated test tube. the temperature of the test tube is measured by nine k-type thermocouples with a sheet diameter of 0.5 mm installed on the inner surface of the heated length as a test probe. more precise measurement of the flow temperature is needed, so two 4-wire platinum pt100 sensors are installed at the inlet and outlet of the test section. the test probe was upgraded several times. the first version was a copy of haas’s design [6] and was created from steel components isolated with small plastic rings from peek material, see the first two parts of fig. 3. unfortunately, the first version was not working, so a new version was made. the second version was constructed completely from peek material with perfect isolation of thermocouples but weak temperature stability. it was not ideal for higher inlet temperatures, therefore it was not ideal for longterm use. finally, the third version of the test probe was made. the whole probe was created from ceramic rings. the last version of the probe can last high temperatures and it is completely nonconducting. the main purpose of the probe remained the same throughout the all versions to pin the thermocouples down to the test tube so that the temperature could be measured. the thermocouples were inserted in the 52 vol. 28/2020 critical heat flux on annulus figure 3. from left to the top is a design and its actual photography of the first, the second and the third version of measurement probe. probe at different axial positions from top to bottom at 10, 60, 110, 160 and 210 mm. the thermocouples were doubled at the four top positions to better register the onset of boiling crisis. most of the data in this paper were collected with the second version. 2.2. measurement procedure and uncertainties prior to the chf measurements, the degassing of the test loop had to be done. the process usually took about 20 minutes when the water had boiled until the oxygen content in the loop reached the value of circa 0.8 mg/l. after this, the loop was ready to use and the parameters were adjusted to the desired values and the power could be slowly increased until the chf was detected. before the beginning of the experimental work, the flow regimes had to be set. the selection of the inlet parameters was made on the basis of previous experience to achieve the least amount of instability and can be seen in tab. 2. the parameters were then kept constant while the power input was increased by 350 w/min. parameter average std. d. rel. d. mass flux g1 407.7 12.4 3.0 % g [kg/(m2.s)] g2 594.3 28.1 4.7 % g3 1024.1 5.6 0.5 % temperature t1 61.5 1.6 2.6 % tin [℃] t2 78.2 3.2 4.0 % t3 93.6 1.0 1.1 % pressure p1 130.5 5.1 3.9 % pin [kpa] p2 220.4 12.2 5.5 % p3 323.0 14.7 4.5 % table 2. experimental setup of parameters with deviations from 21 runs on smooth inconel 625 tube. during the power ramping, the temperatures of the k-type thermocouples in the test probe were closely watched and when the temperature limit was crossed, the input power was automatically shut off. ideally, during every temperature overshoot, the critical heat flux appeared and its location was registered. every experimental run took usually about 30 minutes. different measurement devices were installed in the test facility to control and adjust the flow parameters. mass flux was measured by coriolis flow meter promass 80f15, inlet fluid temperature was measured by pt100 sensors. pressure was captured with bd sensors dmp 331i. the output from the power source was measured by a shunt resistor. the devices were checked in calibration tests and the accuracy of every measured parameter is stated in the tab. 3. the accuracy of the test section thermocouples is also stated, but their precision was not crucial for chf detection. measured value range accuracy pressure [kpa] 100 300 1.07 % temp. inlet [℃] 64 93 0.3 °c temp. test section [℃] up to 350 5 °c mass flow [kg.h−1] 152 229 0.33 % mass flux [kg.m−2.s−1] 400 600 3.22 % power [kw] 30 0.67 % heat flux [kw.m−2] 1050 2650 3.5 % table 3. accuracy of measurement devices according to [21] the power is calculated by the multiplication of electric current and voltage. the heat flux is then estimated by dividing the power by the heated surface which is equal to π · di · l, where di is the inner diameter of the annulus and l is the heated length. the relative deviation is calculated as: ∆q q = ∆q q + ∆di di + ∆l l . (1) the mass flux was calculated by dividing the measured mass flow rate f by the flow area of the annulus and the relative deviation was determined as: ∆g g = ∆f f + 2 · do∆do d2 o − d2 i + 2 · di∆di d2 o − d2 i . (2) all calculated relative deviations together with the accuracy of the measured parameters can be seen in tab. 3. 3. results and discussion the steady state critical heat flux experiments were measured on several surfaces, but in this paper the first experiments on inconel 625 tube were chosen for comparison with calculations. the test parameters are summed up in the tab. 2. the values of mass flux, temperature and pressure were at first set to the desired values (to prevent oscillations) and then the results were averaged and the deviations were calculated. the summarized graphs 53 d. vlček, l. suk, k. števanka, t. petrosyan acta polytechnica ctu proceedings figure 4. chf against inlet mass flux with different inlet pressure and temperature according to tab. 2. and their comparison to literature experimental measurements are the first part of validation of the test facility. the overall results are comparable to haas’s [6], el-genks’s [8] and roger’s [7] data. the constant input parameters were kept as steady as possible, however, as the chf was close, the pressure started to oscillate. moreover, the first experiments were affected by a certain lack of experience and it led to a larger deviation from the input parameters than expected, see tab. 2. the deviations could be avoided by creating separate points of inlet pressure, temperature and mass flux, however this approach was chosen only in figs. 4, 5 and 6 where different effects on chf were captured. the figs. 7, 8, 9 and 10 show the comparison between experimental values and calculations and in these figures the average value of chf was used together with deviations from tab. 2. in essence, three sets of pressure, temperature and mass flux were conducted to achieve and measure the reproducibility of the experiments. the result can be seen that chf strongly depends on even small changes of these parameters and therefore it is crucial to stabilize them as much as possible. the trends in increasing the values of the parameters can be seen, however more measurements have to be done to lower the deviations and fill the blank spaces between regimes. 3.1. effect of flow parameters the data were at first analyzed according to the effects of increasing inlet pressure, mass flux and temperature. the chf behaved as expected and in agreement with el-genk et al. [8], mayer et al. [19] and haas et al. [6] who also captured the effects of flow parameters on chf. in the fig. 4 it can be seen that with rising mass flux the chf values also grow. the mass flux function was also chosen for calculation comparison and the effect can be also seen. the two values of midrange mass flux were unfortunately lower against the desired value, so the mass figure 5. chf against inlet pressure with different inlet mass flux and temperature according to tab. 2. figure 6. chf against inlet temperature with different inlet pressure and mass flux according to tab. 2. flux deviation in tab. 2 is considerably high. it was caused by the lack of experience in the early experiments and during these experimental runs the amount of instability was extensive. therefore, for the next analysis, the data were filtered with the number characterizing the amount of instability (the overall view on behavior of the loop). the experiments were then plotted against inlet pressure, see fig. 5. the pressure has an impact on the density, viscosity, surface tension, and finally the boiling temperature of the fluid. the rising of chf is also in agreement with theoretical assumptions and with the literature as, for example, chun et al. [13]. it can be seen that the pressure varies a lot around its desired value. better results of maintaining the pressure were at lower values, however the small amount of data at higher pressures and worse stabilization led to pressure deviations up to 5.5 %, as can be seen in tab. 2. the main outcome of the pressure effect was that the pressure has to be measured at the outlet of the test section because it is more stable and not violated by the instability. here the inlet 54 vol. 28/2020 critical heat flux on annulus figure 7. comparison of chf results against calculations with pgt correlation and groeneveld’s tables at pin = 131 kpa and tin = 62 ℃. figure 8. comparison of chf results against calculations with pgt correlation and groeneveld’s tables at pin = 220 kpa and tin = 62 ℃. values were chosen to use them in the calculations. for the next analysis, the computations should be set to the outlet pressure. the last total comparison illustrates the effect of inlet temperature to chf. again, the rising dependency of its increasing value can be seen in fig. 6. in the figure there can be seen a strong variation of the midrange inlet temperature, however, because of small values of temperature the deviation is still quite small. inlet temperature was set together with pressure and the two parameters created three sets except the experimental setup with lower inlet temperature and higher pressure. that means that the effect of these two parameters on chf was almost the same. 3.2. calculations since the tube or annulus geometry prevents the uprising of cross-flow, the use of complex subchannel codes is redundant. the local fnspe simple code tube 2.0 [24], which uses the method of isolated channel, was chosen. figure 9. comparison of chf results against calculations with pgt correlation and groeneveld’s tables at pin = 220 kpa and tin = 78 ℃. figure 10. comparison of chf results against calculations with pgt correlation and groeneveld’s tables at pin = 323 kpa and tin = 94 ℃. in this paper, the prediction of chf was made using the average values stated in the tab. 2. two different approaches were tested against the experimental data. the first method (clean) included tube geometry correlation pgt [25] and groeneveld’s look-up tables [18] for chf in tubes then doerffer’s [15] corrective correlation for annulus was used for either of them (annuli). although the pgt correlation was designed for pressures greater than 0.3 mpa, its functionality was tested beyond its validity range. the results of chf were rounded to 100 kw/m2 and because of quite large deviations from the average values of the inlet parameters of the pretests, the uncertainties of the calculations were not estimated. however, the overall view can be seen from the presented figs. 7–10. the axis range was chosen approximately for 100% deviation from the experimental data and therefore some of the calculated data with very large deviation were not shown (pgt clean). the deviations from the inlet parameters and chf (see tab. 2) were also plotted in the figures. 55 d. vlček, l. suk, k. števanka, t. petrosyan acta polytechnica ctu proceedings from the pictures, it can be seen that the tube results (pgt and gt clean) significantly vary (more than 50 %) from the experimental values and with doerffer’s correction the results were notably better (from 10 to 40 %). in some cases (see fig. 9 and 10), the pgt correlation underestimated the chf. the look-up tables from groeneveld with doerffer’s correction led to slightly better or the same results as pgt correlation with the correction because of the underpredicted chf at 323 kpa (see fig. 10) but the pgt was closer to the highest mass flux in the fig. 7. the correction to annular geometry substantially improves the predictions of chf, but at very low pressure there is still more than 35 % overestimation and with rising pressure the value decreases to almost 10 %. mayer et al. [19] also compared their data with lookup tables and they had better results with the older version of groeneveld’s tables from 1995, and they used different correlations from el-genk et al. [8] and it led to radical improvement of the chf prediction. haas et al. [6] used the correlation from rogers [7] because the geometry of their test section was similar. haas also used look-up tables from different years and he also corrected them with doerffer’s factors. he stated that all look-up tables tend to overestimate the chf and the best results were gained from rogers correlation. here, the older versions of the tables were not used because groeneveld stated that it is always updated from the older version. doerffer [15] presented also a separate correlation but it showed inconsistent behaviour with haas’s data. 4. conclusion the flow boiling critical heat flux was measured on a uniformly heated inconel 625 tube inserted in a vertically aligned annulus test section at various mass fluxes and pressures. the set of inlet parameters was initially selected to allow a direct and consistent comparison of the tested cladding. the measured results broaden the chf database for inconel 625 material at low pressure and low flow during which this particular material has never been tested. in addition, the comparison with the chf correlations has never been done with the selection of inconel 625 and inlet parameters stated above. the main conclusions from the experiments on a technically smooth inconel tube and the comparison with calculation methods are summarized here. (1.) the chf values showed an increasing tendency for pressures in the range from 131 kpa to 323 kpa for several measured mass flux values. (2.) the chf values also increased with rising mass flux, which was in accordance with the literature. (3.) chf was predicted using look-up tables and pgt correlation both in its original tube form and corrected to annular geometry. the geometry correction made a substantial improvement of the prediction, however the correlation together with the tables still tend to overestimate the chf. the results of the two corrected methods were similar. (4.) for very low pressure (131 kpa), the corrected methods are about 40 % higher and the deviation decreases with increasing pressure. for 220 and 323 kpa, it is only about 10–15 % higher than the experimental values. in the future, more flow boiling experiments should be performed to better support the observed findings. presented data should serve as a reference for comparison with other materials including accident tolerant fuel [26]. the facility should be improved for ionex filters preventing the corrosive particles from depositing on test tubes and therefore extend the operation time with a clean surface as it was discussed in [21]. to obtain better flow stability and less oscillations, the inlet section will be slightly modified together with the change of the measuring probe. these modifications should allow us to measure at higher mass fluxes. the effects of wettability, capillarity, porosity and roughness on chf should be evaluated. the chf analysis will be focused more on the surface characteristics of different materials and together with that a more thorough comparison of the measured data with the chf correlations will be done. as for the calculation, other correlations should be used to find the most suitable one for this test section. length to heated diameter ration should be put in mind when choosing the most suitable correlation. with more experimental data, a new correlation can be created or a new corrective factor can be developed. list of symbols di inner diameter [mm] do outer diameter [mm] dhe heated equivalent diameter [mm] δ gap width [mm] f flow rate [kg s−1] g mass flux [kg m−2 s−1] ∆hin inlet subcooling [kj kg−1] l length [mm] pin inlet pressure [k−1] q heat flux [kw m−2] tin inlet temperature [◦c] atf accident tolerant fuel chf critical heat flux cosmos-l critical-heat-flux on smooth and modified surfaces – low pressure loop ctu czech technical university in prague kaist korea advanced institut for science and technology mrcha mobile research critical heat flux apparatus acknowledgements the support from the student grant contest from ctu in prague with code sgs20/187/ohk4/3t/14 is gratefully acknowledged. many thanks belong to my supervisor 56 vol. 28/2020 critical heat flux on annulus ing. dušan kobylka, ph.d and my consultant ing. martin ševeček who provided insight and expertise that greatly assisted the research. special thanks belong to ing. ladislav suk and his team from brno who made all experiments possible. i would also like to thank ing. vojtěch caha for his constructive criticism and friendly advice during the project work. references [1] g. h. seo, g. jeun, s. j. kim. enhanced pool boiling critical heat flux with a fecral layer fabricated by dc sputtering. international journal of heat and mass transfer 2016(vol. 102):1293–1307, 2016. doi:10.1016/j.ijheatmasstransfer.2016.06.077. 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[25] r. pernica, j. cizek. general correlation for prediction of critical heat flux ratio in water cooled channels. in proceedings of the 7th international topical meeting on reactor thermal hydraulics, vol. vol. iv, pp. 2636–2653. saratoga springs, new york, 1995. 57 https://doi.org/10.1016/j.ijheatmasstransfer.2016.06.077 https://doi.org/10.1016/j.ijheatmasstransfer.2018.09.013 https://doi.org/10.1016/j.nucengdes.2018.08.024 https://doi.org/10.1016/j.anucene.2014.09.046 https://doi.org/10.1080/00295450.2019.1670010 https://doi.org/10.1016/j.ijheatmasstransfer.2012.12.038 https://doi.org/10.1615/ihtc7.1180 https://doi.org/10.1016/0017-9310(88)90161-5 https://doi.org/10.1021/ie50477a027 https://doi.org/10.1016/0017-9310(79)90061-9 https://doi.org/10.1016/s0029-5493(00)00307-1 https://doi.org/10.1016/0029-5493(94)90283-6 https://doi.org/10.1016/s0735-1933(96)00089-9 https://doi.org/10.1016/j.nucengdes.2007.02.014 https://doi.org/10.1016/j.nucengdes.2016.10.026 https://doi.org/10.1115/ajtec2011-44292 https://doi.org/https://doi.org/10.3390/en13195205 https://doi.org/10.1016/j.ijheatmasstransfer.2018.11.141 https://doi.org/10.1016/j.ijheatmasstransfer.2020.119970 d. vlček, l. suk, k. števanka, t. petrosyan acta polytechnica ctu proceedings [26] accident tolerant fuel concepts for light water reactors. no. 1797 in tecdoc series. iaea, vienna, 2016. 58 acta polytechnica ctu proceedings 28(0):1–9, 2020 1 introduction 2 experimental setup 2.1 test section 2.2 measurement procedure and uncertainties 3 results and discussion 3.1 effect of flow parameters 3.2 calculations 4 conclusion list of symbols acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2019.25.0017 acta polytechnica ctu proceedings 25:17–20, 2019 © czech technical university in prague, 2019 available online at http://ojs.cvut.cz/ojs/index.php/app compressive properties of auxetic structures with controlled stiffness of strut joints tomáš doktor∗, tomáš fíla, petr koudelka, daniel kytýř, ondřej jiroušek czech technical university in prague, faculty of transportation sciences, konviktská 20, 110 00 prague, czech republic ∗ corresponding author: doktor@fd.cvut.cz abstract. presented paper deals with experimental study on compressive properties of auxetics with controlled stiffness of strut joints. the variable strut joints properties were simulated by adding extra amount of material in the struts’ intersection regions. four groups of inverted honeycomb structures were prepared by multi-jet 3d printing and tested in quasi-static compression. the structure collapsed gradually, however after the first collapse, failure in entire cross-section occurred due to the brittle nature of the base material. the behavior up to the first collapse was consistent among the specimens within each group, while differed slightly subsequently. with higher reinforcement in the joints, results showed increasing stress at the first collapse (ultimate compressive stress) while the strain at the first collapse remained unchanged. the auxetic behaviour became less significant with increasing joints’ reinforcement. keywords: auxetics, cellular materials, quasi-static testing, optimisation, additive manufacturing. 1. introduction it has been already shown that cellular solids are able to absorb tremendous amounts of deformation energy [1, 2] during impacts with the possibility to introduce strain-rate dependent characteristics into their deformation response [3]. however, certain applications, such as blast and flying debris protection systems, may require material with relatively high strength in addition to excellent deformation absorption capabilities. such complex mechanical characteristics can be achieved without alteration of the existing lightweight alloys (such as aluminium based materials) used for the material’s production when the microstructure is constructed in such a way to exhibit negative poisson’s ratio [4]. mechanical characteristics of such auxetic lattices are given not only by overall geometrical arrangement of struts in the unit cell and their connectivity but also by deformation properties of the strut joints. there are two main mechanisms of the lattice behaviour during mechanical deformation. based on the struts’ connectivity the deformation is stretching dominated for lattices which comply with truss assumptions or bending dominated in lattices with incomplete connectivity [5]. hence the deformation and energy absorption properties are highly influenced by the bending in the struts’ connections. influence of all these factors on effective mechanical properties in both elastic and plastic regime has to be thoroughly evaluated and taken into account in analytical and numerical optimisation. in this paper a parametric study on influence of the strut joint stiffness on the effective deformation response of various auxetic lattices is presented. 2. materials and methods 2.1. specimen design and manufacturing auxetic lattices exhibiting negative poisson’s ratio were designed in parametric modeller solidworks (dassault systémes corp., france). three-dimensional inverted honeycomb was selected which exhibits auxetic behaviour in all three dimensions. design of the tested auxetic structure was based on our previous study [6], in which first preliminary results were presented. only entirely auxetic structures were used in contrary to the planar auxetics. the porosity and ratio between cell size and strut cross-section (square shaped) was accommodated for a better stiffness and strength. at the top and bottom plane plate was added in to ensure a better force transfer between the loading grips and the specimens. design parameters of the specimens are listed in table 1, geometry of the unit cell is depicted in fig. 1. specimen height [mm] 26 specimen cross-section [mm] 16.5 × 16.5 number of unit cells [−] 3 × 3 × 3 strut cross-section [mm] 0.75 × 0.75 table 1. design parameters common parameters. different joint stiffness was achieved by differently sized spheres placed in the intersections of the struts. three different radii were selected. upper limit value of the spheres’ radius (1.875 mm) was selected to avoid contact between neighboring spheres. the lower limit (0.938 mm) was selected with respect to a possible observation of the reinforcing effect, as smaller spheres 17 http://dx.doi.org/10.14311/app.2019.25.0017 http://ojs.cvut.cz/ojs/index.php/app t. doktor, t. fíla, p. koudelka et al. acta polytechnica ctu proceedings figure 1. geometry of the unit cell. would be entirely hidden in the intersection of the prismatic struts. parameters of the joints’ reinforcement are listed in table 2. group sphere radius [mm] porosity [%] 0 78.8 1 0.75 76.9 2 0.9375 72.4 3 1.125 64.9 table 2. design parameters joint reinforcement variations. stl models which were used for the 3d printing of tested samples are depicted in figure 2. the specimens were manufactured by direct 3d printing from uvcurable acrylic material visijet ex200 (3d systems, usa) using 3d printer pro jet hd3000 (3d systems, usa) in high definition mode. resolution in high definition mode was 387 × 387 × 300 dpi. figure 2. stl models for 3d printing. 2.2. compression tests for the compression experiments a custom loading device was used equipped with force transducer with loading capacity 2 kn (u9b, hottinger baldwin messtechnik gmbh, germany). the loading was provided by a stepper motor controlled by linuxcnc software solution used with real-time gnu/linux operating system [7]. the tests were displacement driven with loading rate 20µm · s−1. contactless displacement measurement was employed based on digital image correlation (dic) [8, 9]. the loading scene was captured by a ccd camera (manta g504b, alied vision technologies gmbh, germany) attached to a figure 3. loading scene with tracking features (left) and tracked displacement paths (right). black background was inverted due to a better visibility. bi-telecentric zoom lens (tzcr 072, optoengineering, italy). the image data was captured with a custom software tool based on opencv library [10]. zoom ratio 0.25 × was used for all compressive tests, which provided pixel-size 13.7µm. a set of tracking features was selected in the first (undeformed) loading scene which were arranged into 7 columns and 15 rows to ensure placement of the features in solid material of the struts. the array of loading features is depicted in figure 3 (left). the loading rate together with the frame rate 2 fps ensured a sufficient number of points in the obtained loading curve. figure 3 (right) shows displacement paths tracked by dic in selected specimen from group 2. 2.3. evaluation the measured displacements and forces were used to calculate engineering strain εeng and stress σeng which were converted into true strain εtrue and true stress σtrue using formulae (1) and (2), respectively. εtrue = ln(1 + εeng) (1) σtrue = σeng · (1 + εeng) (2) based on measured strain in transversal and longitudinal direction, poisson’s ratio was calculated according to formula (3). µ = −εx εy (3) to exclude values which correspond to both initial settlement and post-yield region after the first plastic collapse, the function of poisson’s ratio was limited only inside these limit values of strain. moreover, to assess the influence of joints’ stiffness on the mechanical behaviour, ultimate compressive stress of the tested structures was evaluated as well as strain corresponding to the first plastic collapse. 18 vol. 25/2019 compressive properties of auxetics with controlled joint stiffness group σm [mpa] εy1 [1] µmean [1] 0 0.876 ± 0.124 0.0383 ± 0.0032 −0.180 ± 0.040 1 0.887 ± 0.134 0.0348 ± 0.0029 −0.219 ± 0.058 2 0.942 ± 0.138 0.0346 ± 0.0030 −0.142 ± 0.030 3 1.168 ± 0.054 0.0352 ± 0.0027 −0.047 ± 0.065 table 3. average results in tested groups. stress and strain at the first collapse. average poisson’s number. 0 0.2 0.4 0.6 0.8 1 1.2 1.4 0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 t ru e s tr e s s [ m p a ] logarithmic strain [-] 0.1 0.2 0.3 0.4 0.5 figure 4. stress-strain curves of specimens in group 0. 0 0.2 0.4 0.6 0.8 1 1.2 1.4 0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 t ru e s tr e s s [ m p a ] logarithmic strain [-] 1.1 1.2 1.3 1.4 figure 5. stress-strain curves of specimens in group 1. 3. results 3.1. stress-strain curves a series of uniaxial compressive tests was performed on custom designed three-dimensional inverted honeycomb structures. from the captured loading scenes the evolution of the deformation of the tested structures is assessed. unfortunately, the base material exhibited a high brittleness which caused failure after the yield strain of the structure. after this failure the loading continued with similar stress-strain behaviour but with a slightly lower stresses at the consequent collapses. true stress vs. logarithmic strain diagrams are depicted in figures 4 to 7 for groups 0 to 3 respectively. to compare the main mechanical properties of the tested structures and to assess influence of the different reinforcement of the strut joints, mean values among the groups were evaluated with their standard 0 0.2 0.4 0.6 0.8 1 1.2 1.4 0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 t ru e s tr e s s [ m p a ] logarithmic strain [-] 2.1 2.2 2.3 2.4 2.5 figure 6. stress-strain curves of specimens in group 2. 0 0.2 0.4 0.6 0.8 1 1.2 1.4 0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 t ru e s tr e s s [ m p a ] logarithmic strain [-] 3.1 3.2 3.3 3.4 3.5 figure 7. stress-strain curves of specimens in group 3. deviations. in table 3 are listed mean ultimate compressive strength (stress at the first collapse), strains corresponding to the first collapse in each group of samples and mean poisson’s ratio in the region up to the first collapse. 4. conclusions an experimental study on deformation response of 3d printed auxetic structures was carried out to evaluate influence of joint properties which were physically simulated by adding an extra amount of material in the struts’ connections. the loading setup was equipped with a custom contactless strain measurement system which enabled a precise evaluation of the stress-strain curve. the obtained curves exhibited consistent behaviour among each group (i.e. sphere radius) in terms of stiffness in the linear part, ultimate compressive strength and strain at the first collapse. 19 t. doktor, t. fíla, p. koudelka et al. acta polytechnica ctu proceedings the stress level at the first collapse increases with increasing stiffness in the joints, while the strain at the first collapse remains unchanged. the auxetic nature exhibits slight increase when the spheres with the lowest radius are added in the joints’ interconnections. then, with increasing spheres’ radius the auxetic behaviour is less significant. these results show promising possibility of optimisation of the cellular structures based on controlled stiffness in the strut joints. in the further results this knowledge will be employed for optimisation of the auxetics using different materials in the interconnection regions. acknowledgements the financial support of the czech science foundation (project no. 19-23675s). references [1] q. chen, n. m. pungo. in-plane elastic buckling of hierarchical honeycomb materials. european journal of mechanics a/solids 34:120–129, 2012. doi:10.1016/j.euromechsol.2011.12.003. [2] r. harb, e. taciroglu, n. ghoniem. partitioning of elastic energy in open-cell foams under finite deformations. acta materialia 61(5):1454–1468, 2013. doi:10.1016/j.actamat.2012.11.022. [3] k. a. dannemann, j. lankford jr. high strain rate compression of closed-cell aluminium foams. materials science and engineering 293(1):157–164, 2000. doi:10.1016/s0921-5093(00)01219-3. [4] r. lakes. foam structures with a negative poisson’s ratio. science 235(4797):1038–1040, 1987. doi:10.1126/science.235.4792.1038. [5] v. s. deshpande, m. f. ashby, n. a. fleck. foam topology: bending versus stretching dominated architectures. acta materialia 49(6):1035–1040, 2001. doi:10.1016/s1359-6454(00)00379-7. [6] p. koudelka, o. jirousek, t. fila, t. doktor. compressive properties of auxetic structures produced by direct 3d printing. materiali in tehnologije 50(3):311–317, 2016. [7] v. rada, t. fila, p. zlamal, et al. multi-channel control system for in-situ laboratory loading devices. acta polytechnica proceedings 18:15–19, 2018. doi:10.14311/app.2018.18.0015. [8] b. d. lucas, t. kanade. an iterative image registration technique with an application to stereo vision. in: proceedings of imaging understanding workshop pp. 121–130, 1981. [9] i. jandejsek, j. valach, d. vavrik. optimization and calibration of digital image correlation method. in: proceedings of experimental stress analysis 2010 pp. 121–126, 2010. [10] g. bradski. the opencv library. dr dobb’s journal of software tools 2000. http://www.drdobbs.com/ open-source/the-opencv-library/184404319. 20 https://doi.org/10.1016/j.euromechsol.2011.12.003 https://doi.org/10.1016/j.actamat.2012.11.022 https://doi.org/10.1016/s0921-5093(00)01219-3 https://doi.org/10.1126/science.235.4792.1038 https://doi.org/10.1016/s1359-6454(00)00379-7 https://doi.org/10.14311/app.2018.18.0015 http://www.drdobbs.com/open-source/the-opencv-library/184404319 http://www.drdobbs.com/open-source/the-opencv-library/184404319 acta polytechnica ctu proceedings 25:17–20, 2019 1 introduction 2 materials and methods 2.1 specimen design and manufacturing 2.2 compression tests 2.3 evaluation 3 results 3.1 stress-strain curves 4 conclusions acknowledgements references doi:10.14311/app.2021.30.0001 acta polytechnica ctu proceedings 30:1–6, 2021 © czech technical university in prague, 2021 available online at http://ojs.cvut.cz/ojs/index.php/app size effect on the ultimate drying shrinkage of cement mortar: 1-year experiment and numerical modeling lenka dohnalová∗, petr havlásek, vít šmilauer, pavel reiterman, vendula davidová czech technical university in prague, faculty of civil engineering, department of mechanics, thákurova 7, 166 29 prague 6, czech republic ∗ corresponding author: lenka.dohnalova@fsv.cvut.cz abstract. the magnitude and time evolution of shrinkage are influenced by numerous factors which are implemented in the design codes often in a different way. the time-dependent behavior of concrete in structures sensitive to creep and shrinkage should be verified by means of short-term laboratory measurements. extrapolation of drying shrinkage from short-term measurements is an illposed problem. the process is extremely slow but can be accelerated by reducing the specimen size. the knowledge of the size-effect on drying shrinkage is a necessity to establish the transition from the laboratory to the structural size. in the literature, the experimental data on such size-effect are insufficient. for this reason a new experiment was developed to study this phenomenon on small-scale specimens made of cement mortar and the results from the first year are summarized in this paper. the measured data are validated by coupled fem hygro-mechanical simulations. keywords: concrete, drying, modeling, shrinkage, size effect. 1. introduction creep and shrinkage of concrete are complex timedependent processes whose physical origin remains still fully unresolved. the importance of both phenomena and the need for their correct prediction grew with the boom of pre-stressed concrete technology which allowed to design slender structures with larger spans and which allowed to utilize concrete potential economically. at the same time, such challenging structures are generally more sensitive to the timedependent deformations in concrete. the magnitude and time evolution of shrinkage is influenced by numerous factors (ranging from concrete composition to ambient conditions) which are not approached consistently among current prediction models and design codes for concrete. to name a few, the subtle interplay between the development of drying shrinkage and drying creep (investigated e.g. in [1]) can determine whether the durability of concrete structure will be jeopardized by tensile cracking or the shrinkage-induced tensile stresses are relieved sufficiently fast by increased compliance. a disagreement among the different models prevails also in the question of the size-effect on the ultimate value of drying shrinkage [2] which is addressed in this paper. the resulting discrepancies between the design codes give rise to many uncertainties which can drastically affect the requirements on the design of shrinkageand creep-sensitive structures. the prediction of the behavior of such structures can be improved by laboratory measurements on specimens prepared from the same concrete composition. this approach can be successfully utilized to specify the estimated concrete creep based on short-time measurements. however, the direct applicability to drying shrinkage is rather limited because the extrapolation presents an ill-posed problem. the characteristic time and the ultimate shrinkage cannot be determined uniquely until the rate plotted in log-scale significantly decreases [3]. in the case of standard laboratory specimens 70 × 70 × 285 mm or 100 × 100 × 500 mm, a subtle decrease in shrinkage rate (in log scale) occurs after approx. 100 days while the almost ultimate shrinkage strain is reached after approx. 1000 days of drying [4]. the size-effect on drying shrinkage is in a certain way introduced in majority of design codes or models for time-dependent behavior of concrete. the origin of the size-effect can be sought in nonuniform distribution of relative humidity over the cross-section which gives rise to non-uniform shrinkage strains. the shrinkage strains are internally restrained which produces highly nonlinear distribution of self-equilibrated stresses. owing to the presence of these stresses in the early stages of drying, concrete is extremely prone to surface cracking. the generated stresses become redistributed as the concrete dries and their magnitude is gradually relieved by concrete creep. this complicated interplay causes that the value of ultimate drying shrinkage decreases with increasing size of the concrete member. if the size-effect on the ultimate shrinkage is known, the time-demanding experiments can be significantly accelerated when the dimensions of a spec1 http://dx.doi.org/10.14311/app.2021.30.0001 http://ojs.cvut.cz/ojs/index.php/app l. dohnalová, p. havlásek, v. šmilauer et al. acta polytechnica ctu proceedings figure 1. images of the experimental setup: frames for automated (left) and manual (right) readings. imen become reduced. the rate of drying and thus (drying) shrinkage scales with the square of the effective thickness d defined in b3 model [5] as 2v/s where v is the volume of the specimen and s is the surface exposed to drying. for example in the case of an infinite prism with square cross-section, d coincides with half of its thickness. in this paper, parameter d is used to express the specimen size. the comprehensive creep and shrinkage database assembled at the northwestern university [6] comprises a remarkable number of experimental results gathered from the entire world. however, the promising content of this database becomes rather limited when only a specific phenomenon is to be identified. only a small number of experimental studies examined drying shrinkage of specimens with different sizes prepared from the same concrete mixture and subject to identical conditions. this selection needs to be reduced even further because in many cases the experiment was terminated prematurely before the ultimate shrinkage was reached. a recent paper by the present authors [2] attempted to evaluate the aforementioned size-effect based on the data available in the literature and the aforementioned database. the experiments unanimously indicate that the ultimate drying shrinkage decreases with specimen size, but this relationship cannot be evaluated more accurately owing to insufficient relevant experimental data. the lacking data stimulated the present authors to design and execute a new narrow-oriented experiment focused directly on the size-effect on drying shrinkage. this paper presents and summarizes the measured data collected within the first year of the experiment and compares the strain development in time with the results obtained from coupled hygromechanical finite element simulations. 2. materials and methods 2.1. experimental setup the proposed setup for the shrinkage and moisture loss measurement comprised a large variety of prismatic specimens with uniform length 400 mm but with cross-section ranging from 20 × 20 mm (d = 10 mm) to 100 × 100 mm (d = 50 mm). in order to guarantee material homogeneity and to obtain representative data even on the smallest specimens, cement mortar with 2 mm maximum aggregate size was preferred at the expense of concrete. the composition of the mortar mix had standard sand-to-cement ratio 3:1 by mass. standard cen siliceous sand and the blended binder cem ii 32.5 b-s with slag content 29% were used. the typical water-to-cement ratio was reduced from w/c = 0.5 to 0.45 to match the behavior of mortar in a typical structural concrete. compressive strength and young’s modulus measured at the age of 33 days on standard cylinders 100 × 200 mm were 33.9 mpa and 40.9 gpa, respectively. the slender prisms were cast into a custom-built steel formwork with plastic bottom. the goal was to resemble curing in sealed conditions. for this reason the top was covered with 1 mm thick pe sheet followed by a damp cloth. the following day the specimens were transported to a sealed container with henv = 96%. the specimens were demolded at the age of 31 days when the drying began. the ends of the prisms were equipped by metal sheets with a central dent to provide suitable surface for the measurement as well as moisture sealing. the reference measurement was done once the glue had hardened. altogether 11 mortar prisms with one companion steel specimen for temperature compensation were placed in a special measuring frame, see fig. 1 left. each specimen rested on a bolt with round head. axial shortening was measured at the top using linear transducer with internal spring return mmr1012 (effective stroke 12.7 mm, total resistance 10 kω). data logging occurred automatically, the setup did not allow to measure the moisture loss due to drying. for this reason, a second group consisting of 15 specimens was measured in a more conventional approach using stiff measuring frame equipped with digital indicator sylvac with 1 µm precision, see fig. 1 right. this setup allowed to determine also the evaporated water which was recorded using precision balance scales; kern 572-39 (range 4.2 kg, readout 0.01 g) and t-scale (range 15 kg, readout 0.1 g) for sample with cross-section 100 × 100 mm. minimum difference between the coefficient of thermal expansion of steel and mortar allowed relative reading, circumventing temperature compensation. the experiment was executed in a laboratory without humidity and temperature control. in order to take the inevitable fluctuations into consideration, both temperature and relative humidity were recorded. the frequency corresponded to the automatic and manual measurements. 2.2. numerical modeling the consistency of the measured data was verified using one-way coupled hygro-mechanical simulations 2 vol. 30/2021 size effect on the drying shrinkage 0 200 400 600 800 1000 1200 1400 400 1 10 100 0.3 0.4 0.5 0.6 0.7 0.8 0.9 humidity sh rin ka ge , ε sh [× 1 0-6 ] a m bi en t h um id ity , h en v drying duration, t t0 [day] d = 10 mm d = 13 mm* d = 15 mm d = 20 mm* d = 20 mm d = 25 mm d = 30 mm d = 50 mm 0 200 400 600 800 1000 1200 400 1 10 100 0.3 0.4 0.5 0.6 0.7 0.8 0.9 humidity sh rin ka ge , ε sh [× 1 0-6 ] a m bi en t h um id ity , h en v drying duration, t t0 [day] d = 10 mm d = 13 mm* d = 15 mm d = 20 mm* d = 20 mm d = 25 mm d = 30 mm d = 50 mm figure 2. evolution of shrinkage measured automatically (left) and manually (right). the shaded area corresponds to the experimental range in the case more identical specimens. asterisk denotes specimens with rectangular crosssection (square cross-section otherwise). 0 200 400 600 800 1000 1200 1400 400 1 10 100 0.3 0.4 0.5 0.6 0.7 0.8 0.9 humidity sh rin ka ge , ε sh [× 1 0-6 ] a m bi en t h um id ity , h en v drying duration, t t0 [day] d = 10 mm d = 15 mm d = 20 mm d = 25 mm d = 30 mm d = 50 mm figure 3. comparison of the average shrinkage strain measured automatically (solid lines) and manually (symbols). the two techniques gave good agreement during the first 6 months but afterwards started to diverge which was attributed to the deformation of the plywood back of the shrinkage frame for automated measurements. in open-source finite element package oofem [7]. concrete drying is, in the present study, described by a widely recognized model proposed by bažant and najjar [8]. under certain simplified assumptions, the governing equation for the diffusion of water vapor reads ∂h ∂t = ∇ · (c(h)∇h) (1) where ∇h is the gradient of relative humidity and c(h) is the humidity-dependent diffusivity. this dependence is for the cementitious materials highly nonlinear and can be approximated as c(h) = c1 ! "α0 + 1 − α0 1 + # 1−h 1−hc $n % & (2) where c1 is the maximum diffusivity at h = 1, α0 determines the ratio between minimum diffusivity at h = 0 and c1, and parameters hc and n describe the relative humidity threshold and the rate of the transition. the ambient relative humidity is prescribed using mixed boundary condition which relates the humidity flux jh with the humidity difference at the boundary via surface factor f . in each time step, the moisture transport is followed by the structural sub-problem which utilizes the computed field of relative humidity. the mechanical problem uses constitutive model based on the microprestress-solidification (mps) theory [9]. under constant temperature and sealed conditions, the mps model reduces to the b3 model [5]. however, changes in relative humidity give rise not only to volume changes (shrinkage or swelling), but also to additional creep (pickett effect). the model has been modified [10] to minimize the size-effect on drying creep which is controlled by parameter k3. shrinkage strain and relative humidity are linearly linked via their rates, ε̇sh = kshḣ (3) where ksh is a material parameter usually treated as a humidityand age-independent constant. the mps model is extended with the cohesive crack model and fixed orientation of cracks to consider material softening if the tensile strength is reached. however, the formed cracks do not influence the humidity transport, allowing one-way coupling scheme. the computational model was calibrated in several subsequent steps. the basic creep compliance, which is in the case of the b3 model described by 4 parameters q1 − q4, was determined from empirical prediction formulae using compressive strength and concrete composition as input parameters. afterwards, parameters controlling short-term creep q1 and q2 were adjusted to match the results of the 1-hour creep experiment. the value of the drying creep parameter, k3 = 26 was identified in the previous analysis of bryant and vadhanavikkit experiment 3 l. dohnalová, p. havlásek, v. šmilauer et al. acta polytechnica ctu proceedings b3 model mps model moisture diffusivity + mixed b.c. q1 q2 q3 q4 k3 ft gf ksh c1 α0 hc n f [×10−6/mpa] [ ] [mpa] [n/m] [ ] [mm2/day] [ ] [ ] [ ] [mm/day] 15.5 150 2.08 9.41 26 3.0 100 0.0016 30 0.15 0.7 12 0.3 table 1. summary of material parameters [11]. the response is only very slightly sensitive to the value of tensile strength ft and fracture energy gf , which were set to typical values for concrete (see tab. 1 which summarizes the values of all basic parameters used in simulations). the remaining parameter of mps model ksh, and the parameters of the transport model were calibrated to achieve reasonable agreement with the shrinkage results of the performed experiment. the aim was to achieve the best possible match for all prism sizes. the main emphasis was placed on the initial development of shrinkage (approx. first 6 months of drying) when both groups of the specimens show similar development of shrinkage. later, when there is no longer a match between the specimen groups with a different measurement method, the aim was to achieve the best possible match with the results of the manually measured prisms. for the purpose of identification of material parameters, the evolution of ambient relative humidity was defined by a piecewise-linear function according to the measured values. in addition, the analysis was repeated for a partially adjusted development of ambient humidity (beginning according to the experiment, after 91 days of drying assumed humidity fixed at henv = 60%) achieve final values of shrinkage and to demonstrate more clearly the size-effect on drying shrinkage predicted by the model. 3. results and discussion the development of shrinkage in time is for both automated and manual measurement techniques shown in fig. 2. in the case of more identical specimens, the mean response is shown in solid line while the shaded area corresponds to the measured experimental range. the obvious fluctuations in shrinkage strain are caused by oscillating value of ambient relative humidity which is shown in dashed black line. the average responses obtained with the two techniques are compared in fig. 3. except of the largest specimen, the measurements demonstrate very good agreement during the first 6 months of drying. sudden decrease in relative humidity from 60% to 40%, which occurred afterwards, caused rapid acceleration of drying accompanied by increase in shrinkage rate. this decrease also led to separation of the experimental trends determined by the two methods; in all cases the shrinkage strain measured automatically exceeded the manual readings. the reason for this was not clarified even by thorough analysis of the measured data. still, the most probable cause seems to 0 200 400 600 800 1000 400 1 10 100 sh rin ka ge , ε sh [× 1 0-6 ] drying duration, t t0 [day] d = 10 mm d = 15 mm d = 20 mm d = 25 mm d = 30 mm d = 50 mm figure 4. comparison of the experimental measurements (symbols) with the results obtained from fem simulations (solid lines). be connected to the frame for automated measurement. the measuring frame was attempted to be sufficiently stiff for the current measurement. since the frame serves only as a support for the specimens and is loaded merely by their self weight, the selected material was 26 mm thick plywood for concrete molds. however, this material turned out to be insufficiently stable under variable relative humidity of the ambient environment since non-negligible volume changes were observed. these changes could not be completely eliminated by incorporating the measurement of the steel standard primarily intended for temperature compensation. the difference between the shrinkage of geometrically identical specimens determined by the automated and manual measurement technique was not uniform and was found to be, in a certain way, influenced by their position, namely by the distance from the largest specimen placed at one end (see fig. 1, the weight of the prism labeled “1” reached almost 10 kg). the joint effect of these two factors seems to be the most probable explanation. therefore, the data from the manual measurements are preferred in the later stage of the experiment (solid steel structure of the measuring frame insensitive to level of relative humidity) despite the intervals between the individual measurements were gradually increasing. as was stated above, the manual measurements allowed to determine not only the axial shrinkage but also the evolution of weight loss due to drying. to the best knowledge of the authors, such data are not available in this extent in any of the preceding experiments related to the size effect on drying shrinkage. 4 vol. 30/2021 size effect on the drying shrinkage 0 10 20 30 40 50 60 70 80 400 1 10 100 0.3 0.4 0.5 0.6 0.7 0.8 0.9 humidity w ei gh t l os s [ kg /m 3 ] a m bi en t h um id ity , h en v duration of drying, t t0 [day] d = 10 mm d = 15 mm d = 20 mm d = 25 mm d = 30 mm d = 50 mm 0 10 20 30 40 50 60 70 80 400 1 10 100 w ei gh t l os s [ kg /m 3 ] drying duration, t t0 [day] d = 10 mm d = 15 mm d = 20 mm d = 25 mm d = 30 mm d = 50 mm figure 5. experimentally measured moisture loss (left) and its comparison to fem analysis (right). after 100 days of drying the model exhibits excessive sensitivity to changes in henv. 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 0 50 100 150 200 250 300 350 400 reference size, d = 10 mm si ze e ff ec t drying duration t t0 [day] d = 13 mm* d = 15 mm d = 20 mm* d = 20 mm d = 25 mm d = 30 mm 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 0 50 100 150 200 250 300 350 400 0.3 0.4 0.5 0.6 0.7 0.8 reference size, d = 10 mm si ze e ff ec t a m bi en t h um id ity , h en v drying duration t t0 [day] d = 15 mm d = 20 mm d = 25 mm d = 30 mm humidity figure 6. time development of the experimentally measured (left) and computed (right) size-effect on drying shrinkage. the data points correspond to the average values obtained from manual measurement. the ambient relative humidity recorded in the experiment resulted into the fem response shown in solid lines while the modified history of humidity (henv = 60% since 91 days of drying) led to the results shown in dashed lines. yet such data provide better understanding of the time development of the drying process. the measured evolution of moisture loss normalized per volume of the specimen is shown in the left part of fig. 5. the sensitivity to changes in ambient relative humidity which occurred in the later stage of drying is in the case of moisture loss not so pronounced as in the case of drying shrinkage. in the finite element simulations the driving force for the moisture transport is the relative humidity, therefore the moisture loss needs to be backcalculated. the value of moisture capacity (slope of the desorption isotherm, for simplicity assumed as linear) which gave the best agreement with the experimental data as shown in right part of fig. 5 was k = 120 kg/m3. in the simulations, the development of ambient relative humidity corresponded with the experimental measurements. the agreement with the experimental data is satisfactory only in the first 100 days of drying; afterwards, the model overestimates the rate of the weight loss or gain caused by changes in henv. the discrepancy in the initial value of weight loss stems from the linear desorption isotherm which is not realistic in the case of high relative humidity. the ongoing changes in henv did not allow the specimens to reach their ultimate value of drying shrinkage. for this reason the size effect on drying shrinkage is here evaluated within the entire duration of the experiment and is related to the thinnest specimen with cross-section 20 × 20 mm (d = 10 mm). naturally, the strongest size-effect can be observed shortly after the onset of drying when the difference between the moisture loss of specimens with different size is most remarkable. on the other hand with the increasing duration the size-effect is slowly diminishing until the equilibrium with the ambient conditions is reached. the expected evolution of the size-effect on drying shrinkage is confirmed by the experimental measure5 l. dohnalová, p. havlásek, v. šmilauer et al. acta polytechnica ctu proceedings ments as shown in the left part of fig. 6. even though the ultimate values of drying shrinkage have not been reach so far, the size-effect has become almost stable and there is no indication that is should vanish completely. the processed data are in correct order, the size-effect increases with size. with respect to the reference specimen (d = 10 mm) the size effect is within 20% of that value. as was shown in fig. 4, the experimentally measured evolution of shrinkage was captured by the numerical model very realistically up to ≈ 180 days of drying. however, the mismatch beyond this time causes that the numerically computed size-effect significantly deviates from the experiment as demonstrated by the smooth solid lines in the right part of fig. 6. in order to arrive at equilibrated values of drying shrinkage, the simulation was repeated with a modified history of henv, which was fixed at 60% after 91 days of drying. under such conditions the predicted size-effect becomes almost negligible within one year of drying. 4. conclusions this paper analyzed and presented the results from the first year of a pilot experiment developed to investigate the size-effect on drying shrinkage. the primary motivation of this research was to verify whether the shrinkage measurements could be accelerated using smaller specimens than is the current practice. the findings can be summarized as follows. • the difficulties encountered in the first run of the experiment provided the research team with a valuable feedback for its future improvement. the experimental technique itself is feasible, cheap and capable to deliver good-quality data. a stiff steel frame, automatic monitoring of ambient conditions are a necessity, while the currently used monitoring of moisture loss is sufficient. • the measured data were in expected order and exhibited very little scatter among specimens with the same dimensions. this indicates good homogeneity of the material in the specimens despite their small dimensions. moreover, the consistency of the data was verified by a computational model and the agreement was promising. • the size-effect on drying shrinkage cannot be properly evaluated unless the experiment is conducted at constant relative humidity, which was not the present case. despite this deficiency, in the currently examined size range a non-negligible sizeeffect on drying shrinkage was registered. • the constitutive model used in structural analysis needs to be improved as it strongly underestimates the size-effect on drying shrinkage. acknowledgements the authors gratefully acknowledge financial support from the czech science foundation (ga čr), project number 19-20666s. references [1] l. dohnalová, p. havlásek. comparison of drying shrinkage and drying creep kinetics in concrete. acta polytechnica ctu proceedings 15:12–19, 2018. [2] l. dohnalová, p. havlásek. size effect on the ultimate drying shrinkage of concrete experimental evidence and engineering practice. acta polytechnica ctu proceedings 26:13–18, 2020. [3] z. bažant, m. jirásek. creep and hygrothermal effects in concrete structures. springer netherlands, 2018. doi:10.1007/978-94-024-1138-6. [4] c. aguilar. estudio del comportamiento y desarrollo de una metodologia de prediccion de la retraccion hidraulica de hormigones, phd. thesis (in spanish). pontificia universidad catolica de chile. escuela de ingenieria, 2005. [5] z. bažant, s. baweja. creep and shrinkage prediction model for analysis and design of concrete structures: model b3. adam neville symposium: creep and shrinkage structural design effects 2000. [6] m. hubler, r. wendner, z. bažant. comprehensive database for concrete creep and shrinkage: analysis and recommendations for testing and recording. aci 112:547–558, 2015. [7] b. patzák. oofem home page, 2000. http://www.oofem.org. [8] z. p. bažant, l. j. najjar. nonlinear water diffusion in nonsaturated concrete. materials and structures 5:3–20, 1972. doi:10.1007/bf02479073. [9] z. p. bažant, a. p. hauggaard, s. baweja, f. j. ulm. microprestress solidification theory for concrete creep. i: aging and drying effects. journal of engineering mechanics 123:1188–1194, 1997. [10] z. bažant, p. havlásek, m. jirásek. microprestress-solidification theory: modeling of size effect on drying creep. in n. bicanic, h. mang, g. meschke, r. de borst (eds.), computational modelling of concrete structures, pp. 749–758. crc press/balkema, eh leiden, the netherlands, 2014. [11] a. h. bryant, c. vadhanavikkit. creep, shrinkage-size, and age at loading effects. aci materials journal 84:117–123, 1987. 6 https://doi.org/10.1007/978-94-024-1138-6 https://doi.org/10.1007/bf02479073 acta polytechnica ctu proceedings doi:10.14311/app.2019.22.0139 acta polytechnica ctu proceedings 22:139–144, 2019 © czech technical university in prague, 2019 available online at http://ojs.cvut.cz/ojs/index.php/app experimental studies of accelerated chloride transport in concrete vojtěch zacharda∗, jiří němeček czech technical university in prague, faculty of civil engineering, thákurova 7, 166 29 prague 6, czech republic ∗ corresponding author: vojtech.zacharda@fsv.cvut.cz abstract. this contribution deals with the efficiency of electromigration of chlorides used as a repair method for reinforced concrete structures. experimental studies of accelerated chloride transport tests were performed on samples of concrete without chlorides and with admixed sodium chloride during concreting. two concrete types from portland cement characterized with normal and low compressive strengths were studied. the electromigration was applied to penetrate chlorides into the chloride-free sample and for extraction of chlorides from the sample. the effectiveness of the chloride extraction process for rehabilitation of reinforced concrete in terms of lowering the chloride concentration in different concrete types and surface concentration was observed. electrical extraction was found to be effective for lowering of initial chloride concentration by 15-20% after 24 hours. the decrease in surface concentrations was found in the range of 40-50%. the extraction process was found to be feasible and effective for both concrete types. keywords: concrete, electromigration, chlorides, extraction. 1. introduction reinforced concrete structure is exposed to a variety of environmental processes during its lifetime that can cause degradation. the penetration of chlorides into the concrete from deicing salts or in sea splash areas are the most common deterioration processes. chloride ions diffuse through concrete towards steel reinforcement and cause its corrosion. after reaching a critical value of chloride concentration on the rebar surface the corrosion of the steel starts [1–3]. the corrosion is accompanied by an expansion of the oxidic products causing high pressures, concrete cracking and finally spalling of a cover layer. therefore, the resistance of concrete to penetration of chlorides is one of the very important properties for steel-reinforced concretes. since the natural diffusion of ions [4, 5] is a slow process taking years further complicated by concrete aging and chloride binding [6] testing of concrete chloride permeability is often done in an accelerated manner using an electric field [7–9]. both types of tests need to be properly interpreted and diffusion characteristics of concrete can be derived from the tests [9, 10]. electrochemical extraction [7, 8] is one of the solutions that can be used to repair reinforced concrete structure already attacked by chlorides. besides chlorides, other particles with the charge, such as corrosion inhibitors or nanoparticles, can also be transported via electric field [8, 11–13]. the convective flux of ions caused by the electromotive force is much larger that the diffusive flux and leads to radical shortening of the time needed for extraction of chlorides from the structure. in the reality, the electric field is realized between the steel reinforcement and the outer surface electrode (usually stainless steel mesh) placed on the concrete surface. in the lab scale, the tests are performed on concrete samples placed in between the mesh electrodes submerged in a suitable electrolyte. a standard test involves rapid chloride penetration [14] which categorizes samples into levels of vulnerability to the chloride attack. more precise measurements with chloride profile assessment will be presented in this paper. 2. materials two types of concrete mixtures were prepared for this study with low compressive strength (labeled as l) and with normal strength (labeled as n). each type was prepared without chlorides and with 1% of cement weight nacl addition to the mixture prepared from ordinary portland cement cem i-42,5r, sand and natural crushed aggregate. the mixtures composition is shown in the table 1. the concrete was mixed in a 50 l laboratory mixer for 10minutes. the samples were cylindrical with height of 200mm and diameter of 100mm. after casting they were vibrated for approximately 30 seconds. as prevention before water evaporation the specimens were covered with a foil. after 2 days after casting they were unmoulded and stored in water. the resulting 28-days compressive strengths and density of the l samples were 24.3mpa and 2216 kg/m3 and 50.5mpa and 2340 kg/m3 for n samples, respectively. for chloride penetration tests the specimens were cut to slices with the thickness of 50mm. in this study the lower strength concrete represents a weak or damaged material that is worth to be repaired. the different content of cement also play a role 139 http://dx.doi.org/10.14311/app.2019.22.0139 http://ojs.cvut.cz/ojs/index.php/app vojtěch zacharda, jiří němeček acta polytechnica ctu proceedings type of concrete cem i 42,5r sand 0/4 aggregate 4/8 aggregate 8/16 water w/c nacl [kg] [kg] [kg] [kg] [kg] [kg] n 436.4 872.7 290.9 581.8 186 0.43 0 n-1% 436.4 872.7 290.9 581.8 186 0.43 4.36 l 261.8 1150.0 290.9 581.8 210.2 0.8 0 l-1% 261.8 1150.0 290.9 581.8 210.2 0.8 2.62 table 1. concrete mixtures composition, mass per 1m3. figure 1. an electromigration setup used for accelerated chloride penetration/extraction tests. in chloride binding that is higher in higher strength concrete. 3. methods in order to test the effectiveness of the electromigration process two types of accelerated tests were performed in an migration chamber. first, the chloride penetration test was made on the concrete samples without chlorides (n and l series) and second, the accelerated chloride extraction test was performed on the samples with 1% nacl per cement addition (n-1% and l-1% series). the migration chamber composed of two containers with electrolyte solutions. in case of chloride penetration tests, a 3% nacl solution was used in the compartment with positive electrode and 0.3% naoh solution in the compartment with negative electrode. in case of chloride extraction, both compartments were filed with 0.3% naoh solution and chlorides driven into one of them. a dc power source with constant voltage of 20v was connected to stainless steel mesh electrodes submerged in the electrolytes. the analyzed samples were inserted between the electrodes and the sample sealed (fig. 1). the accelerated chloride penetration/extraction run for 24 hours. the scheme of tests is shown in fig. 2. before the tests of samples with 1% chloride addition, the original total chloride concentration was analyzed in 20mm depth steps by drilling of the powder. subsequently, the drilled holes were sealed in the samples and samples analyzed further. after the tests, the concentration was analyzed from powder collected in 5mm depth steps. always, the powder was mixed with extraction liquid based on potassium dichromate and acetic acid. after at least 24 hours, the solution was analyzed with cl ion selective electrode and the chloride profile constructed. 4. results and discussion 4.1. penetration of chlorides the results of individual accelerated chloride penetration tests were used for calculation of average chloride concentration profile from 3-4 samples as shown in fig. 3 for l samples and in fig. 4 for n samples. the profile exhibits a gradual reduction of chlorides from surface of samples, which was exposed to the 3% nacl solution. the shape of the profile is different in comparison with traditional diffusion tests. here, the chlorides are driven by an electromotive force which is dominant and the profile is a “convective” one with the “middle bulge” caused by the convective force. it can be seen in the figures that the surface concentration after 24 hours exposure is higher by 50% for n samples while the extent of the penetration, i.e. the penetration depth, is larger for l samples. substantial chloride concentration appears within 40mm of the l sample while only to 10mm in the case of n sample. the difference in the surface concentration between l and n samples is related to the amount of cement in the mixture and the ability of the concrete to bind chlorides. the binding is higher for n samples which has twice as cement compared to l samples. the density of the n samples is however higher compared to l samples. thus the permeability of the n samples is lower and the penetration depth is substantially lower compared to l samples. 4.2. extraction of chlorides original and after-the-extraction chloride concentration profiles are shown in figs. 5 and 6 for l and n samples, respectively. chlorides are extracted from the sample to the right compartment. the extraction causes decrease in concentration for both sample types. most significantly, the concentration drops down in the surface region (left part of the profile in figs. 5 and 6). the average decrease of chloride concentration was about 15% on l samples and 20% on n samples. the decrease in the surface region was more the 50% for l samples and about 40% for n samples. 140 vol. 22/2019 experimental studies of accelerated chloride transport in concrete (a). (b). figure 2. scheme of accelerated chloride a) penetration test and b) extraction test. figure 3. concentration profile in l samples. the profile is in agreement with the convective transport [10] which leads the chloride ions from left of the samples to the right compartment of the migration chamber. note again, that the transport lasted for 24 hours with voltage of 20v. further acceleration can be reached by using higher voltages or using larger times. increasing voltage is, however, impractical due to safety reasons and due to joule effect causing heating of the specimen. increasing of the exposure time is, on the other hand, possible and higher efficiency leading to further lowering of the concentration would take place. 5. conclusions the presented paper provides results of accelerated chloride penetration and extraction tests performed on two typical concrete mixtures with different compositions with normal and low compressive strengths. the principle of electromigration was applied to penetrate chlorides into the chloride-free sample and to extract chlorides from the sample with some initial cl concentration. the chloride profiles with concentrations that can be developed after long diffusion tests were gained here in 24 hours while applying safe voltage of 20v. the chloride concentrations were assessed from concrete core drills using an extraction liquid and ion selective electrode. typical convective profiles with steep concentration change and different surface concentrations were obtained. the different permeability of the samples caused differences in both surface concentrations as well as in penetration depths. the surface concentration was twice as high in n samples compared to l samples. the penetration depth was three to four times larges in l samples compared to n samples. the first phenomenon can be attributed to a larger binding capacity of n samples given by 141 vojtěch zacharda, jiří němeček acta polytechnica ctu proceedings figure 4. concentration profile in n samples. figure 5. concentration profile of l-1% samples before and after extraction. 142 vol. 22/2019 experimental studies of accelerated chloride transport in concrete figure 6. concentration profile of n-1% samples before and after extraction. the twice amount of cement in comparison to l samples. larger penetration depth in l samples is related to their increased permeability given by their lower density and higher porosity. electrical extraction was found to be an effective tool for lowering of initial chloride concentration. 1520% average concentration decrease was reached after 24 hours. the decrease in surface concentrations was even higher (40-50%) which corresponds to the convection of ions out of the samples. the extraction process was found to be feasible and effective for both concrete types. acknowledgements financial support of the czech science foundation (project 16-11879s) and the grant agency of the czech technical university in prague (sgs18/114/ohk1/2t/11) is gratefully acknowledged. references [1] u. angst, ø. vennesland. critical chloride content in reinforced concrete – state of the art. in alexander, et al. (eds.), concrete repair, rehabilitation and retrofitting ii. taylor and francis group, london, 2009. [2] j. cabrera. deterioration of concrete due to reinforcement steel corrosion. cement and concrete composites 18(1):47–59, 1996. doi:10.1016/0958-9465(95)00043-7. [3] k. tuutti. corrosion of steel in concrete. ph.d. thesis, swedish cement and concrete research institute, stockholm, 1982. [4] c. page, n. short, a. el tarras. diffusion of chloride ions in hardened cement pastes. cement and concrete research 11(3):395 – 406, 1981. doi:10.1016/0008-8846(81)90111-3. [5] y. xi, z. p. bažant. modeling chloride penetration in saturated concrete. journal of materials in civil engineering 11(1):58–65, 1999. doi:10.1061/(asce)0899-1561(1999)11:1(58). [6] q. yuan, c. shi, g. de schutter, et al. chloride binding of cement-based materials subjected to external chloride environment – a review. construction and building materials 23:1–13, 2009. doi:10.1016/j.conbuildmat.2008.02.004. [7] m. sánchez, c. alonso. electrochemical chloride removal in reinforced concrete structures: improvement of effectiveness by simultaneous migration of calcium nitrite. construction and building materials 25:873– 878, 2011. doi:10.1016/j.conbuildmat.2010.06.099. [8] h. e. cardenas, l. j. struble. electrokinetic nanoparticle treatment of hardened cement paste for reduction of permeability. journal of materials in civil engineering 18(4), 2006. doi:10.1061/(asce)0899-1561(2006)18:4(554). [9] p. hlaváček, j. němeček. accelerated chloride migration tests in concrete. in proceedings of 22nd international conference engineering mechanics, pp. 213–216. 2016. [10] j. němeček, j. kruis, t. koudelka, t. krejčí. simulation of chloride migration in reinforced concrete. applied mathematics and computation 319:575–585, 2018. doi:10.1016/j.amc.2017.07.029. [11] j. němeček, y. p. xi. nanoparticle injection into concrete using electromigration. in special concrete and composites 2014, vol. 1054 of advanced materials research, pp. 6–10. trans tech publications, 2014. doi:10.4028/www.scientific.net/amr.1054.6. [12] j. němeček, l. li, y. xi. electrokinetic nanoparticle injection for remediating leaks in oil well cement. construction and building materials 15:63–72, 2017. doi:10.1016/j.conbuildmat.2017.08.152. 143 http://dx.doi.org/10.1016/0958-9465(95)00043-7 http://dx.doi.org/10.1016/0008-8846(81)90111-3 http://dx.doi.org/10.1061/(asce)0899-1561(1999)11:1(58) http://dx.doi.org/10.1016/j.conbuildmat.2008.02.004 http://dx.doi.org/10.1016/j.conbuildmat.2010.06.099 http://dx.doi.org/10.1061/(asce)0899-1561(2006)18:4(554) http://dx.doi.org/10.1016/j.amc.2017.07.029 http://dx.doi.org/10.4028/www.scientific.net/amr.1054.6 http://dx.doi.org/10.1016/j.conbuildmat.2017.08.152 vojtěch zacharda, jiří němeček acta polytechnica ctu proceedings [13] j. němeček, r. šulc, j. němečková, j. kruis. nanoparticles in concrete: application in fresh and hardened state. in special concrete and composites 2017, vol. 760 of key engineering materials, pp. 3–9. trans tech publications, 2018. doi:10.4028/www.scientific.net/kem.760.3. [14] astm c1202-19 standard test method for electrical indication of concrete’s ability to resist chloride ion penetration. standard, american society for testing and materials, west conshohocken. doi:10.1520/c1202-19. 144 http://dx.doi.org/10.4028/www.scientific.net/kem.760.3 http://dx.doi.org/10.1520/c1202-19 acta polytechnica ctu proceedings 22:139–144, 2019 1 introduction 2 materials 3 methods 4 results and discussion 4.1 penetration of chlorides 4.2 extraction of chlorides 5 conclusions acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2017.13.0011 acta polytechnica ctu proceedings 13:11–15, 2017 © czech technical university in prague, 2017 available online at http://ojs.cvut.cz/ojs/index.php/app performance of cement composites reinforced with surface-modified polypropylene microand macro-fibers jakub antoša,∗, lukáš dejdara, jan trejbala, zdeněk prošeka,b a czech technical university in prague, faculty of civil engineering, thákurova 7, 166 29 prague 6, czech republic b czech academy of science, institute of physics, na slovance 1999/2, 182 21 prague 8, czech republic ∗ corresponding author: jakub.antos@fsv.cvut.cz abstract. this paper focuses on the mechanical properties investigation of cement pastes reinforced with surface treated polymer fibers. the cement matrix was composed of portland cement (cem i 42.5 r, w/c ratio equal to 0.4). two polypropylene fiber types (microand macro-fibers) were used as randomly distributed and oriented reinforcement in volume amount of 2 %. the fibers were modified in the low-pressure inductively coupled cold oxygen plasma in order to enhance their surface interaction with the cement matrix. the investigated composite mechanical properties (load bearing capacity and response during loading) were examined indirectly by means of four-point bending mechanical destructive tests. a response of loaded samples containing treated fibers were compared to samples with reference fibers. moreover, cracking behavior development was monitored using digital image correlation (dic). this method enabled to record the micro-cracks system evaluation of both fiber reinforced samples. keywords: cement composite, fiber reinforcement, polymeric fiber, surface modification, plasma treatment, four-point bending, dic, crack pattern. 1. introduction plain concrete can be described as a tension-weak material, where cracks often occur due to exceed of maximal tensile stress, rapid drying, shrinkage, and the like. the crack system development generally increases in the time and thus gradually propagates the concrete volume. as moisture (containing substances and ions), co2 and other degrading agents comes through formed cracks, service-life, mechanical performance and other properties of the concrete degrade [1]. to reduce all above mentioned phenomena, the utilization of fibers seems to be an effective possibility to control creation, development and width of cracks [2]. during the mixing process, fibers are randomly oriented and distributed throughout the concrete in all directions. consequently, the fresh concrete is thus protected against excessive formation of plastic shrinkage cracks (concrete tensile strength as well as modulus of elasticity thus increased). in the hardened concrete, the fibers prevent evolution of micro-crack system into macro-crack formation. this is ensured by redistribution of tension stress through the fibers “bridging” a crack (sometimes refereed to as “sewing” effect) and preventing disintegration of broken concrete [1]. in the civil engineering practice, there are several types of fibers used as scattered reinforcements in concrete, e.g. metal, polymer, glass or made of various other materials. the polymer fibers (polypropylene or polypropylene-based) are often preferred over steel fibers due to lower production cost, increased resistance to degradation and sufficient mechanical properties (such as tensile strength or toughness) [3]. moreover, polymeric fibers provide an increase of fire resistance for the structure due to low melting point of the polymers and inferential formation of water vapor capillaries in the structure of concrete [4, 5]. on the other hand, due to low chemical activity of most polymers (related to cement paste), the chemical interaction (bonding) between fiber surfaces and cement matrix is not ensured. the hydrophobic character of fiber surface does not create strong bond with polar liquid (water contained in cement paste). the inter-facial transition zone is thus quite porous, as captured in fig. 1. moreover, smooth surfaces of fiber provides insufficient mechanical interaction between the materials. in general, strong inter-phase interaction between matrix and reinforcement is crucial to ensure mechanical (and other) properties of composites [6]. in order to enhance the physico-chemical interaction between the two materials, several types of fiber surface modifications including chemical or physical treatment can be applied [7]. in most cases, the standard treatments have strong impact on fibers mechanical properties. moreover, the chemical treatment is not environmentally friendly and is too time and energy consuming (an exposition fiber to alkali solution lasts up to tens of hours and whole process requires high temperature) [8]. 11 http://dx.doi.org/10.14311/app.2017.13.0011 http://ojs.cvut.cz/ojs/index.php/app j. antoš, l. dejdar, j. trejbal, z. prošek acta polytechnica ctu proceedings figure 1. porous inter-facial transition zone between mono-filament fiber and cement matrix [7]. the plasma treatment seems to be an effective and thrifty method to modify both physical and chemical polymer surface. the physical treatment rests in a roughening via ion bombardment. unlike the conventional physical methods, this influences only the monomolecular layer, so fiber mechanical performance remains almost untouched. next to that, the chemical modifications rest in an activation of surfaces due to interaction between polymeric chains and free radicals. the active polar groups are implemented onto fiber surfaces and thus change hydrophobic characteristic to hydrophilic [9, 10]. the most suitable time to modify polymeric micro-fiber using plasma treatment is 30 seconds according to [11, 12]. in presented study, we focused on inter-face interaction between reference and plasma treated fibers with cement matrix. we modified two fiber types – polypropylene microand macro-fibers, as the common representatives of reinforcement in the field of concrete constructions. we modified the fibers by low-pressure cool oxygen plasma treatment. as an indirect indicator of interaction between the two materials, we have done four-point bending tests of fiber reinforced prismatic cement specimens when we focused on their mechanical response. moreover, we recorded the specimens behavior during loading using digital image correlation. 2. materials 2.1. polymeric fibers two types of polypropylene (pp) fibers differing in dimensions were used. micro-fibers having 18 µm in diameter and approx. 12mm of length (commercial name: stavon, manufacturer: trevos, s. r. o., czech republic) were used for reinforcement of the first set of cement composite samples. these fibers were surface treated by a fabric softener. according to product data sheet, material properties of the fibers are declared as: uts ≈ 273mpa (ultimate tensile strength calculated from dtex unit) and ductility = figure 2. top – polypropylene macro-fibers, bottom – polypropylene micro-fibers. min. 50 %. no other information about the fibers is provided by the manufacturer. for the second set of experiment samples, the dimensions of used fibers were 80µm in diameter and ≈ 19mm of length (commercial name: highgrade, manufacturer: brugg contec ag, switzerland). the basic material properties listed in data sheet of the product are as follows: % = 910 kgm−3 (volumetric mass density), uts = 400mpa, e = 4.9gpa (young’s modulus of elasticity), mp ≈ 150 °c (melting point). according to european standard code [5], cementitious composite reinforced by above described macro-fibers is classified as construction concrete, not plain. both types of fibers are captured in figure 2. 2.2. cement paste prismatic specimens composed of cement paste (cem i 42.5 r, water to cement ratio 0.4) and polymer fiber reinforcement were made using metal forms of 40×40×160mm dimensions. on the whole, four different mixtures were prepared (reference and treated, microand macro-fibers). the amount of fiber reinforcement was equal to 2 % of sample volume. after 1 day, the prepared samples were cast out from forms and stored in a water environment to prevent degradation for other 27 days (total age of samples equal to standard 28 days), when the samples were tested. 12 vol. 13/2017 polymeric fiber reinforced cement composite figure 3. crack development during four-point bending test of cement sample reinforced with reference (top) and 30 seconds plasma treated (bottom) microfibers. figure 4. crack development during four-point bending test of cement sample reinforced with reference (top) and 30 seconds plasma treated (bottom) macrofibers. 3. experimental methods 3.1. plasma treatment both macroand micro-fibers were modified in order to change their surface properties by means of plasma treatment. concretely, low-pressure cool oxygen coupled plasma treatment was done using tesla vt 214 (13.56 mhz) device. the plasma treatment duration was equal to 30 s. other treatment parameters were as follows: 100 w total power of rf source, 20 pa total gas pressure, 50 sccm o2 flow. after plasma treatment, the fibers were admixed into fresh cement paste immediately. 3.2. four-point bending test the interface interaction between fibers and cement matrix was assessed by the destructive four-point bending tests. the indirect testing was executed as reimbursement of standard single-fiber pull-out test due to complicated fiber dimensions and shapes. each type of mixture was represented by 6 specimens in order to obtain statistically relevant set of results. the prismatic cementitious composite specimens were tested on loading frame web tiv ravestein fp 100. base-span was equal to 120mm. the test was displacement controlled with 1mm/min rate of loading and it was interrupted when the mid-span displacement reached 4 mm and 3mm in the case of samples reinforced with microand macro-fibers, respectively. the main focus of the experiment was on the post cracking phase with expected increase of sample toughness. 3.3. digital image correlation the behavior of cracks was observed using digital image analysis (dic). dic analysis allowed to image the amount, shape and position of cracks graphically and thus answer the question how the specimens resisted to loading in the post-cracking phase (single vs. multiple cracking behavior) [14]. as the result of the analysis, cracking patterns were visualized with a map of maximum principal strain in which concentrations can be attributed the crack formation. to allow the dic observation, the samples were equipped with artificial stochastic contrast pattern onto their surfaces. images used for dic analysis were taken with dslr camera canon eos 70d fitted with canon zoom lens ef-s 17-85 is usm. the exposition parameters were as follows, shooting interval 10 seconds, iso speed 100, focal length 50 mm, aperture value 4.5, shutter speed 1/100 sec. 4. results 4.1. samples reinforced with micro-fibers it was shown, that maximum load-bearing capacity of cement samples reinforced with micro-fibers (d = 18µm) was lower than 7 kn when reference fibers were used and almost 8 kn in the case of plasma modified fibers. the residual toughness of specimens containing surface treated fibers in the post cracking phase (namely after 3mm mid-span displacement) was significantly higher, if compared to the referential specimens. this phenomenon can be attributed to increase of adhesion between pulling-out fibers from matrix when sample underwent deformation. nevertheless, slight increase of initial toughness in the linear-elastic phase of loaded samples was also recorded. it was probably caused by reduction of amount and wight of shrinkage cracks. the force-displacement diagram is shown in fig. 5. dic analysis showed that loaded cement sample was damaged by one single crack, which was opened when the displacement was increased in the case of reference fibers. on the other hand, sample reinforced with surface treated fibers was damaged by systems of cracks, known as multiple cracking behavior. the principal strain diagrams are shown in the fig. 3. 13 j. antoš, l. dejdar, j. trejbal, z. prošek acta polytechnica ctu proceedings figure 5. force-displacement diagram of cement samples containing micro-fibers. figure 6. force-displacement diagram of cement samples containing micro-fibers. 4.2. samples reinforced with macro-fibers loading response of cement samples reinforced with macro-fibers (d = 80 µm) exhibited similar results compared to micro-fiber reinforcement. the maximum reached force of reference specimens can be found close to 6 kn. the plasma treated fiber-reinforced samples reached load-bearing capacity close to 8 kn. however, the residual toughness of samples beyond mid-span deflection of 3mm did not meet the expectations, compared to the reference specimens. this phenomenon seemed to be accidental. the recording of the test (fig. 6) also captured slight increment modified composite elastic modulus. similarly to micro-fibers, the behavior monitored using dic technique during four-point bending tests can be described as single crack and multi cracking in the case of samples reinforced with reference and plasma treated fibers, respectively. the results of dic analysis are shown in the fig. 4. 4.3. conclusion the main goal of this work was to enhance inter-phase interaction between cement matrix and polymer fiber reinforcement. the matrix was composed of portland cement (cem i 42.5 r, w/c ratio equal to 0.4). as the randomly dispersed and oriented reinforcement, two types of polypropylene fibers having 18µm and 80µm in diameter were used. the interaction between the two materials was enhanced via fiber surfaces modification by low-pressure cool coupled oxygen plasma. a duration of fiber exposition to plasma was equal to 30 seconds. prismatic specimens having dimensions equal to 40×40×160 mm contained from the matrix and fibers (volume amount 2 %) were made and after 28 days of mixture curring and hardening subjected to four-point bending destructive tests. as an indirect indicator of both chemical and physical interaction between the two materials, the response (toughness and loadbearing capacity) of loaded specimens was monitored and evaluated. the results of samples reinforced with reference and treated fibers were mutually compared. the increase of load-bearing capacity of samples re14 vol. 13/2017 polymeric fiber reinforced cement composite inforced with modified fibers (both microand macrofibers) was shown. compare to reference mixtures, specimens containing modified fibers exhibited toughness increase in both linear and post-cracking phase. also, as for the plasma treated micro-fiber reinforced samples, significant increase of residual toughness (compared to reference specimens) was observed. similar findings were pointed out in the study examining the interaction between plasma modified pet (polyethylene terephthalate) and cement matrix [13]. dic analysis revealed a different behavior during four-point bending tests between samples containing reference and modified fibers. while the behavior can be described as single crack in the case of reference mixtures, the mixtures with modified fibers exhibited multi cracking behavior. this phenomenon clearly point to the required increase of the adhesion between treated fibers and the cement matrix. based on these findings, it can be pointed out, that polymer fiber modification via plasma treatment can be assumed as low-cost and very effective method to increase inter-phase interaction and mechanical properties of fiber reinforced cement composites. acknowledgements this work was financially supported by czech technical university in prague – sgs project sgs16/201/ohk1/3t/11 and by the czech science foundation research project 15-12420s. the authors would also like to thank to tereza horová for her help with this research. references [1] p.s. song, s. hwang, b.c. sheu, townsend t.g., strength properties of nylon and polypropylene fiber-reinforced concretes. cement and concrete research 35(8):1546–1550, 2005. doi: 10.1016/j.cemconres.2004.06.033 [2] n. banthia, r. gupta, influence of polypropylene fiber geometry on plastic shrinkage cracking in concrete, cement and concrete research 36(7): 1263–1267, 2006. doi: 10.1016/j.cemconres.2006.01.010 [3] s. kakooei, h. akil, m. jamshidi, j. rouhi, the effects of polypropylene fibers on the properties of reinforced concrete structures, construction and building material 27(1): 73–77, 2012. doi: 10.1016/j.conbuildmat.2011.08.015 [4] en 14889-2, fibers for concretepart 2: polymer fibers – definitions, specifications and conformity. [5] m. zeiml, d. leithner, r. lackner, h.a. mang, how do polypropylene fibers improve the spalling behavior of in-situ concrete?, cement and concrete research 36(5): 929–942, 2006. doi: 10.1016/j.cemconres.2005.12.018 [6] j. topič, j. bartoš, l. kopecký, j. fládr, z. prošek, j. trejbal, development of mechanical properties of a cement composite reinforced with synthetic fibers, trans tech publications ltd, 332–335, 2016. [7] a. peled, h. guttman, treatments of polypropylene fibres to optimize their reinforcing efficiency in cement composites, cement and concrete composites 14(4): 277–285, 1992. doi: 10.1016/0958-9465(92)90026-r [8] v. machovič, l. lapčák, l. borecká, m. lhotka, j. andertová, l. kopecký, l. mišková, microstructure of interfacial tranyition yone between pet fibres and cement paste, acta geodyn. geomater. 10(169): 121–127, 2013. doi: 10.13168/agg.2013.0012 [9] a.m. lópez-buendía, m.d. romero-sánchez, v. climent, c. guillem, surface treated polypropylene (pp) fibers for reinforced concrete, cement and concrete research 54: 29–35, 2013. doi: 10.1016/j.cemconres.2013.08.004 [10] j. trejbal, v. šmilauer, a. kromka, š. potocký, l. kopecký, wettability enhancement of polymeric and glass micro fiber reinforcement by plasma treatment, in: proceedings nanocon, brno, 2015. [11] j. trejbal, j. fládr, l. kopecký, utilization of plasma treated polymeric macro-fibers as reinforcement in concrete constructions, advanced materials research 1144: 70–75, 2016. doi: 10.4028/www.scientific.net/amr.1144.70 [12] b.h. yang et al., strength characteristics of modified polypropylene fiber and cement-reinforced loess, journal of central south university 24(3): 560–568, 2017. [13] j. trejbal et al., impact of surface plasma treatment on the performance of pet fiber reinforcement in cementitious composites, cement and concrete research 89: 276–287, 2016. doi: 10.1016/j.cemconres.2016.08.01 [14] b. pan et al., full-field strain measurement using a twodimensional savitzky-golay digital differentiator in digital image correlation, optical engineering 46: 1–10, 2007. doi: 10.1016/j.cemconres.2016.08.01 15 acta polytechnica ctu proceedings 13:11–15, 2017 1 introduction 2 materials 2.1 polymeric fibers 2.2 cement paste 3 experimental methods 3.1 plasma treatment 3.2 four-point bending test 3.3 digital image correlation 4 results 4.1 samples reinforced with micro-fibers 4.2 samples reinforced with macro-fibers 4.3 conclusion acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2019.24.0031 acta polytechnica ctu proceedings 24:31–37, 2019 © czech technical university in prague, 2019 available online at https://ojs.cvut.cz/ojs/index.php/app effect of nacl on neutron flux density in graphite block tomáš slančík, kamil števanka∗ brno university of technology, faculty of electrical engineering and communication, technická 10, 616 00 brno, czech republic ∗ corresponding author: kamil.stevanka@vutbr.cz abstract. aim of this work is to study the effect of nacl on neutron flux through graphite block. graphite block had 39 channels drilled through it and filled with nacl. au and in foils and in wire was used for neutron activation measurement of the neutron flux. ambe neutron source with nominal activity of 92.5 gbq was used. the results were compared with mcnp simulations with channels filled with salt and air. twelve different locations for au and in foils were used and in wires were place in three locations to measure vertical profile of neutron flux. keywords: hpge, nacl, chloride salts, neutron flux. 1. introduction molten salts were first considered for application in nuclear reactors in late 1940s as part of the aircraft reactor experiment. molten salts were chosen because of the high power density requirements for the reactor and multiple other reasons [1, 2]. the research continued with molten salt reactor experiment (msre), which was considered a success [3, 4]. however the research of msrs was discontinued after msre due to other priorities, lack of funding and other political reasons [5]. since fluorides were chosen as primary coolant, most of the research to date focused on these mixtures [1, 3]. nevertheless chlorides were considered as well and therefore some cl based mixtures were studied too [2]. main advantages of msrs compared to pwrs are higher fuel burn-up, higher operating temperature and low operating pressure usually around 0.3 mpa. there is also theoretical possibility of breeding fuel from both uranium and thorium. among the challenges are higher neutron flux, corrosive behavior of salts, more difficult maintenance and inspection [6]. the interest in msrs was renewed in 21st century. even though most concepts are based on fluorides, some concepts, especially fast reactors, rely on chloride mixtures. among the concepts that plan to utilize chlorides are msfr by terrapower, llc and ssr by moltex energy ltd [7, 8]. apart from usage in msrs, chlorides can also be used in accelerator-driven systems [9]. since the data for neutronics of chlorides are not as well studied as fluorides, the goal of the work was to construct the experimental setup and determine how the nacl affects neutron flux density and to determine whether the experiment provides any useful data. this experiment was conceived to be the first in the series of experiments aimed at gathering more data about chloride neutronics. 2. experimental setup the equipment used for the experiment consisted of graphite block, ambe neutron source, sodium chloride and golden and indium threshold activation detectors (tad). dimensions of the graphite block were 506 × 300 × 300 mm (length x width x height). altogether 39 channels were drilled all the way through the block, these were filled with nacl. one hole was drilled at the center of the shorter side of the block to accommodate neutron source. this hole was drilled to the depth of 175 mm. the layout of the channels can be seen in figure 1. the diameter of the nacl channels was 29.3 mm and the diameter of neutron source hole was 22 mm. the block was placed on the 5 mm thick steel plate, which was attached to 600 mm tall wooden base. ambe neutron source with neutron emissivity of 5.6 · 106 s-1 was used. it is made of 833 mg of amo2 and 7374 mg of 9be encased in stainless steel cylinder which has diameter of 19.1 mm and height of 48.6 mm. 3. methodology preliminary calculations were made to determine suitable diameter. eventually it was decided to drill channels with diameter of 29 mm based on the practical feasibility. the 29 mm drill was the largest readily available and smaller diameters would mean more channels which would be time consuming. one channel took about 20 minutes to drill. due to the concerns about graphite fragility it was decided that distance between edge of the holes and the block should be at least 15 mm and keeping sufficient distance between edges of holes meant that there would be actually less volume of salt if bigger diameter was used. the channels are 0.3 mm wider due to drill vibrations. the hole for neutron source was drilled to the 31 https://doi.org/10.14311/app.2019.24.0031 https://ojs.cvut.cz/ojs/index.php/app tomáš slančík, kamil števanka acta polytechnica ctu proceedings figure 1. layout of drilled channels. depth of 175 mm in order to vertically center the source. solidworks 2014 software was used to create model of the block and the base. each of the 29 mm channels was filled with 99.5 % purity nacl salt, the salt was put into polyethylene bag for easier manipulation. neutron activation measurement was used to measure the neutron flux density. 197au (figure 4) and 115in (figure 3) foils were used as activation detectors, which have high cross section for thermal neutron capture. foils were placed in paper wrap in the selected channels as seen in figure 2. foils were placed at the 3 o’clock side of the holes, approximate placement of the foils is designated by red dots in figure 2. all of the in foils had same diameter of 16 mm, thickness of 0.5 mm and 99.99 % purity. au foils had square shapes but their dimensions differed as can be seen in table 1. apart from the foils, in wires were used in channels iiα, iiγ and iiη, right in front of the neutron source. this was used to determine neutron flux over the entire height of the block. the diameter of in wire was 1 mm and length was 300 mm. the au detectors were placed lower than 150 mm from the base and in detectors were placed above 150 mm because the foils could not be placed side by side in the vertical center of the channels at height of 150 mm. in addition to the detectors in the channels, other detectors were placed on the external sides of the block. both au and in detectors were placed in the center of the respective sides of the block. detectors with cadmium shielding were added to the left and rear sides (from the source point of view) of the block to detect ratio between thermal and fast neutrons. in hole purity (%) thickness (mm) dimensions (mm) iiα 99.95 0.25 10x10 iiγ 99.95 0.25 10x10 iiη 99.90 0.02 13x13 3a 99.95 0.25 10x10 3c 99.90 0.02 13x13 3e 99.90 0.02 12x12 iiiα 99.95 0.25 10x10 iiiγ 99.90 0.02 12x12 iiiη 99.90 0.02 13x13 4a 99.90 0.02 13x13 4c 99.90 0.02 12x12 4e 99.90 0.02 12x12 table 1. au detectors characteristics. foils had same dimensions as the ones in the channels and au foils are described in table 2. hpge detector was used to measure activation of the foils. gamwin software was used to evaluate measurements. the reaction rate rrc (as seen in equation 1) was calculated after the area of measured peaks was manually determined using results from gamwin software. intensities and detector efficiency for designated gamma energies can be seen in table 3. 32 vol. 24/2019 effect of nacl on neutron flux density in graphite block figure 2. layout of detectors. figure 3. indium-115 radiation capture cross-section [10]. figure 4. au-197 radiation capture cross-section [10]. 33 tomáš slančík, kamil števanka acta polytechnica ctu proceedings side purity (%) thickness (mm) dimensions (mm) source 99.95 0.250 10x10 left cd 99.95 0.250 25x25 left 99.95 0.125 25x25 right 99.95 0.125 25x25 rear cd 99.95 0.500 25x25 rear 99.95 0.500 25x25 table 2. outside au detectors characteristics. material eγ (kev) iγ (%) εγ (-) in 416.860 27.7 0.03484 1097.326 56.2 0.00833 1293.558 84.4 0.00559 au 411.802 96 0.03525 table 3. measured energies and characteristics. rrc = s(eγ) · λ · treal tlive εγ(eγ) · iγ(eγ) · (1 − e−λtirr )· · (1 − e−λtreal) · e−λtdelay (s−1) (1) where s(eγ) is area of measured peak of selected energy level, λ is decay constant of particular isotope, treal is measuring time, tlive live time of measurement, εγ(eγ) is detector efficiency for given energy level, iγ(eγ) is intensity of gamma line, tirr is duration of irradiation and tdelay is time between the end of irradiation and beginning of measurement. afterward the weighted average of reaction rates for different energies for in was calculated. then reaction rates per gram of material was calculated using dimensions of detectors and densities of materials [11],[12],[13]. the irradiation lasted for 7 days and measuring intervals were 15 mins for in wire and 20 mins for in and au foils. the irradiation lasted for 7 days to ensure sufficient degree of activation because half-life of 198au is 2.694 days [14]. because samples could not be reinserted into the setup without disrupting the experiment, it was not possible to conduct multiple measurements of the samples. reaction rates measured and calculated on outside detectors with and without cadmium shielding were used to calculate cadmium ratio 2 rcd = rr rcdr (2) where rr is reaction rate of all of the neutrons, rcdr is reaction rate of fast neutrons figure 5. vised visualisation. 3.1. mcnp simulation mcnp 6 was used to simulate the experiment [15]. polyethylene was neglected in the model given that combined weight of the bags was approximately 100 g compared to the weight of the entire setup which was approximately 90 kg. the model contained additional 300 mm thick wall on the side of neutron source, this could have some impact on the results. activation detectors were not placed within the channels but simplified approach was taken which evaluated neutron flux density through designated sections of channels. channels were separated into four sections, the two evaluated sections were at heights between 140 to 150 mm and 150 160 mm. then the reaction rate was calculated using calculated neutron fluxes, atomic density and cross sections of au and in. simulations were done for holes filled with air and nacl. 4. results the results of experiment and simulations can be seen in graphs figure 6 to 13. some results have substantial error margins, this is due to low intensity of neutron flux of neutron source and relatively short time of measurements. the short measuring time was caused by high number of samples because there was only one hpge available. the measuring time could not be longer, because the activity of samples would decrease too much and the setup did not allow to reinsert foils. cadmium ratio in table 4 shows that neutrons were not very well moderated, for future research would be useful to have higher volume of graphite. as can be seen in the graphs figure: 6 to 9, the trend of in foils is similar to mcnp simulation results, however most of the reaction rates are higher in the experimental samples than in simulations. this could be due to aforementioned simplifications and also due to high degree of uncertainties. the result for in foil in position αiii (figure 8) is different, but the error margin for that sample is also substantial, therefore it cannot be concluded that the model is inconsistent with the experiment. the results for golden foils of 0.02 mm thickness differ substantially from simulations because reference reaction rate was calculated for thickness of 0.25 mm in mcnp simulations. the experimental results for 34 vol. 24/2019 effect of nacl on neutron flux density in graphite block α γ ε location 0 20 40 60 80 100 r r c ( − 1 g −1 ) reaction rate of indium in channel in po ition ii mcnp-salt mcnp-air experiment figure 6. results for in foils in positions ii. 0.25 mm thickness is much more similar to the simulation, whereas reaction rate for 0.02 mm is much higher, this is caused by self-shielding of the thicker foil material. the difference between simulations with salt and air is not very pronounced. this is due to relatively small amount of salt in the system, however all simulations show slightly higher reaction rates for air in the farthest positions, indicating that nacl captures small amount of the neutrons. wire measurements figure 14, figure 15, figure 16 showed high degree of uncertainty because of short half-life of in wire and short measuring time. each wire was divided into five segments, each 6 cm long. the wires could not be reinserted, therefore they were cut into 5 pieces due to short half-life [14] and these segments were measured. the first wire showed expected results with higher rate in the middle, other two wires showed different results with no clear trend. this was most likely caused by short measuring time, low activity of the samples farther from the neutron source and short half-life of indium. the results for in wire are not reliable and require further measurements. position indium gold rcd (-) rcd (-) left 1.4090 1.5579 rear 1.3243 1.6416 table 4. experimental results. 5. conclusions the paper studies neutron flux density within graphite block with drilled channels that were filled with nacl. the aim of the work was to design and construct experimental setup and get some experience with chloride salts. another goal was to see whether the mcnp α γ ε location 20 40 60 80 100 r r c ( s( 1 g −1 ) reaction ates of indium in channels in position 3 mcnp-salt mcnp-air experiment figure 7. results for in foils in positions 3. α γ ε location 10 20 30 40 50 60 r r c ( s( 1 g −1 ) reaction ates of indium in channels in position iii mcnp-salt mcnp-air experiment figure 8. results for in foils in positions iii. model is reasonably accurate compared to the experiment and to find out ways in which we could improve the experimental setup. the main purpose of the setup is to provide students with opportunity to measure and evaluate real life data and get hands on experience with scientific work. neutron activation measurement was used to determine neutron flux. twelve au and in foils were used and three 30 cm long in wire were used as well. the wire measurement had high error margin due to short measuring time and only the wire at the closest position to the neutron source showed expected trend with regard to vertical distribution of neutron flux. both in and au foils showed similar trends to the simulation, reaction rates were higher than those of the simulations in all cases except one and this could be attributed to measuring error. the self-shielding effect of gold affected the results significantly and measured values for thinner, 0.02 mm thick foils diverged from simulation considerably. this was caused by simulation input which used thicker 0.25 mm thick foils for normalisation. despite the fact that 0.02 mm foils showed significant divergence in values compared to model, it nevertheless showed similar trends. given the error 35 tomáš slančík, kamil števanka acta polytechnica ctu proceedings a c e location 10 20 30 40 50 r r c ( s− 1 g −1 ) reaction ates of indium in channels in position 4 mcnp-salt mcnp-air experiment figure 9. results for in foils in positions 4. α γ ε location 10 20 30 40 r r c ( − 1 g −1 ) reaction rate of gold in channel in po ition ii mcnp-salt mcnp-air experiment figure 10. results for au foils in positions ii. margins it is sufficient indication that the model is in good agreement with the experiment. the measuring errors were quite large, this was due to limitations of experimental setup which did not allow for reinsertion of the foils. this was particularly visible on in wires which were separated to 5 segments, but because of short half-life of indium could not be measured for a long time. in the future it is planned to upgrade the setup to allow for better manipulation of salt and also reinsertion of detectors. the samples could not be reinserted because they were inserted simultaneously with the polyethylene bags. once they were removed the polyethylene bag would not allow for the sample to be put back in. in the future it is planned to use vessels that could keep their shape, probably aluminium tubes. thus allowing for continuous measurement. after the upgrade of the experimental setup it is planned to repeat the experiment, perform another experiment with empty channels to compare the results and adjust the mcnp model. it is also planned to measure the effects of other chloride salts, such as licl, kcl, cacl2, mgcl2. it is also possible to add another layer, approximately 200 mm of graphite between neutron source and the block to get higher amount of thermal neutrons. a c e location 10 20 30 40 50 60 70 80 r r c ( s− 1 g −1 ) reactio rates of gold i cha els i positio 3 mcnp-salt mcnp-air experiment figure 11. results for au foils in positions 3. α γ ε location 10 20 30 40 50 60 r r c ( s( 1 g −1 ) reaction rates of gold in channels in osition iii mcnp-salt mcnp-air experiment figure 12. results for au foils in positions iii. acknowledgements the research was supported by project ltausa18198 and is part of cooperation between but and texas a&m university. authors are thankful to their supervisor and pi of the 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[9] p. mcintyre, s. assadi, k. badgley, et al. acceleratordriven subcritical fission in molten salt core: closing the nuclear fuel cycle for green nuclear energy. in 22. international conference on application of accelerators in research and industry. aip, 2013. doi:10.1063/1.4802405. 0-6 6-12 12-18 18-24 24-30 part of the wire (cm) 20 30 40 50 60 70 r r c ( s− 1 g −1 ) reaction rates of indium ires in position iiγ figure 15. results for in wire in position γ. 0-6 6-12 12-18 18-24 24-30 part of the wire (cm) 10 20 30 40 50 60 70 r r c ( s− 1 g −1 ) reaction rates of indium ires in position iiε figure 16. results for in wire in position η. [10] nuclear energy agency java-based nuclear data information system, 2013. https://www.oecd-nea.org/janis/. [11] the engineering toolbox densities of common solids, 2019. https://www.engineeringtoolbox.com/ density-solids-d_1265.html. 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[15] c. werner. mcnp users manual code version 6.2, 2017. https://mcnp.lanl.gov/mcnp_manual.shtml. 37 http://moltensalt.org/references/static/downloads/pdf/whymsrsabandonedornlweinbergsfiringv3.pdf http://moltensalt.org/references/static/downloads/pdf/whymsrsabandonedornlweinbergsfiringv3.pdf http://moltensalt.org/references/static/downloads/pdf/whymsrsabandonedornlweinbergsfiringv3.pdf https://public.ornl.gov/conferences/msr2015/pdf/05-current%20msr%20status%20and%20challenges_david%20holcomb.pdf https://public.ornl.gov/conferences/msr2015/pdf/05-current%20msr%20status%20and%20challenges_david%20holcomb.pdf https://public.ornl.gov/conferences/msr2015/pdf/05-current%20msr%20status%20and%20challenges_david%20holcomb.pdf http://www.world-nuclear.org/information-library/current-and-future-generation/molten-salt-reactors.aspx http://www.world-nuclear.org/information-library/current-and-future-generation/molten-salt-reactors.aspx http://www.world-nuclear.org/information-library/current-and-future-generation/molten-salt-reactors.aspx http://www.world-nuclear.org/information-library/current-and-future-generation/molten-salt-reactors.aspx https://doi.org/10.1063/1.4802405 https://www.oecd-nea.org/janis/ https://www.engineeringtoolbox.com/density-solids-d_1265.html https://www.engineeringtoolbox.com/density-solids-d_1265.html https://www.engineeringtoolbox.com/metal-alloys-densities-d_50.html https://www.engineeringtoolbox.com/metal-alloys-densities-d_50.html https://www-nds.iaea.org/relnsd/vcharthtml/vcharthtml.html https://www-nds.iaea.org/relnsd/vcharthtml/vcharthtml.html https://mcnp.lanl.gov/mcnp_manual.shtml acta polytechnica ctu proceedings 24:1–7, 2019 1 introduction 2 experimental setup 3 methodology 3.1 mcnp simulation 4 results 5 conclusions acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2018.15.0041 acta polytechnica ctu proceedings 15:41–45, 2018 © czech technical university in prague, 2018 available online at http://ojs.cvut.cz/ojs/index.php/app bayesian inference of heterogeneous viscoplastic material parameters eliška janouchová∗, anna kučerová faculty of civil engineering, czech technical university in prague, thákurova 7, prague, czech republic ∗ corresponding author: eliska.janouchova@fsv.cvut.cz abstract. modelling of heterogeneous materials based on randomness of model input parameters involves parameter identification which is focused on solving a stochastic inversion problem. it can be formulated as a search for probabilistic description of model parameters providing the distribution of the model response corresponding to the distribution of the observed data. in this contribution, a numerical model of kinematic and isotropic hardening for viscoplastic material is calibrated on a basis of experimental data from a cyclic loading test at a high temperature. five material model parameters are identified in probabilistic setting. the core of the identification method is the bayesian inference of uncertain statistical moments of a prescribed joint lognormal distribution of the parameters. at first, synthetic experimental data are used to verify the identification procedure, then the real experimental data are processed to calibrate the material model of copper alloy. keywords: parameter identification, bayesian inference, hierarchical modelling, aleatory uncertainty, kinematic and isotropic hardening. 1. introduction reliable predicting behaviour of an investigated system in a computational way requires proper calibration of the corresponding numerical model. this contribution concentrates on modelling heterogeneous materials, where the heterogeneity is captured by randomness of model input parameters. the spatial variability of mechanical properties in heterogeneous materials, e.g. elastic modulus or yield stress, affects behaviour of the investigated structural system under the loading. this can be demonstrated by repeating a laboratory test for an ensemble of specimens made of the same heterogeneous material. calibration of a heterogeneous material model can be perceived as identification of its parameters’ probability distribution when the corresponding model response has the distribution as much as possible similar to the distribution of the observed data. the probabilistic identification of random variables is called a stochastic inversion problem. stochastic inversion enables to infer a probability distribution of the unknown model parameters from indirect experimental measurements. in order to make the identification process successful, a proper distinction of occurring uncertainties is required. essentially, uncertainties can be separated into two basic groups. the first one contains uncertainties connected to a lack of knowledge, it is caused by e.g. measurement errors or a small number of measurements. this uncertainty is called epistemic and it is reducible by any new information. on the other side, there is irreducible stochasticity called aleatory uncertainty or simply variability. it represents an inherent randomness which originates from modelling a data collection e.g. from different locations or times as a random variable [1]. an illustrative examples of uncertainties related to material properties are presented in figure 1. identification procedure of heterogeneous material properties from indirect noisy measurements having a numerical model describing the nonlinear relationship between the unknown inputs and observed outputs is presented in section 2. the principle of the method is a choice of a certain form of probability density function (pdf) of the unknown stochastic model parameters whose statistical moments are considered as uncertain random variables. the probabilistic identification is then concentrated on these so-called hyperparameters of the parameters’ distribution which can be inferred in the bayesian way [2]. this approach has been already applied on a very simple numerical example of a cantilever in the authors’ previous work [3]. this contribution focuses on application of the identification method for a more complicated numerical model leading to the model calibration from real experimental data. in section 3, the material model of kinematic and isotropic hardening for viscoplastic material together with its unknown random input parameters is briefly introduced. section 4 deals with verification of the proposed identification procedure and following model calibration. the obtained results are then summarized in section 5. 2. stochastic identification method this section recapitulates the basics of the bayesian inference and its extension by hierarchical modelling 41 http://dx.doi.org/10.14311/app.2018.15.0041 http://ojs.cvut.cz/ojs/index.php/app eliška janouchová, anna kučerová acta polytechnica ctu proceedings figure 1. uncertainties in properties of a homogeneous and heterogeneous material. in order to enable an application of this method for identification of aleatory uncertainty presented in the authors’ former conference paper [3]. the principal idea of the bayesian identification is based on a common way of thought when the resulting belief about a random event is given by a combination of all available information [4]. this approach introduces the concept of uncertainty in our subjective knowledge of the identified parameters. the bayesian inference becomes increasingly popular and more widespread approach to parameter identification providing an elegant solution to this inverse problem by making it well-posed. it allows to estimate values of input parameters together with appropriate uncertainties by combining prior information and experimental measurements. in this case, observations are assumed to be performed for the specific yet unknown values of input parameters and epistemic uncertainty arising from experimental errors and lack of knowledge is reduced with an increasing number of experimental observations. the bayesian approach can be extended into multilevel setting which allows to take into account also aleatory uncertainties [5, 6]. while in the classical formulation the model parameters have unknown deterministic values, hierarchical modelling enables to consider the model parameters x as stochastic variables described by some statistical model with uncertain moments. specifically, the extension is done by introducing new random variables, known as hyperparameters θ defining the probabilistic specification of the model parameters x [2]. the parameters’ prior is then conditional on the hyperparameters θ with their own prior distribution called hyperprior p(θ), the joint prior distribution of the parameters and hyperparameters has a form p(x,θ) = p(x|θ)p(θ) (1) and according to bayes’ rule the corresponding joint posterior distribution up to a normalisation constant is p(x,θ|z) ∝ p(z|x,θ)p(x|θ)p(θ). (2) more specifically, now we have n observations zi, each of them is realized for n combinations of input values xi drawn from unknown distribution. each of these combinations is defined by its prior distribution f(xi|θ) and according to assumption of observation exchangeability [7] the joint prior distribution is given as p(x1, . . . ,xn,θ) = ( n∏ i=1 f(xi|θ) ) p(θ) (3) and the posterior p(x1, . . . ,xn,θ|z1, . . . ,zn) as p(x,θ|z) ∝ ( n∏ i=1 f(zi|xi) )( n∏ i=1 f(xi|θ) ) p(θ). (4) our knowledge about the hyperparameters grows by updating based on every new measurement while structure of the model parameters’ prior distribution remains unchanged. 3. numerical model the identification method is applied to calibration of a numerical model of kinematic and isotropic hardening for copper alloy defined by constitutive equations in figure 2. the available experimental data are depicted in figure 3 and correspond to 16 repetitions of a straincontrolled cyclic loading test at a high temperature. the material data are represented in relative units: stresses are given as ratios of rref and strains as ratios of εref . these reference values are constant throughout this document, so all presented figures are consistent. the experimental errors in each measured point of discretized stress-strain curves are assumed as i.i.d. gaussian random variables with zero mean values and identic standard deviations. at higher temperatures, the cyclic material behaviour is dominated by the kinematic hardening mechanism and the stress saturation is already reached after a few cycles [8]. the copper alloy does not exhibit a distinct yield limit and its kinematic hardening behaviour is non-linear. the model parameters to be identified are summarized in table 1. operating with the hardening moduli 42 vol. 15/2018 bayesian inference of heterogeneous viscoplastic material parameters figure 2. constitutive equations of the viscoplastic model with non-linear isotropic and kinematic hardening. figure 3. experimental data: stress-strain curves. q and c, given in equations in figure 2, is replaced by operating with the saturation values r∞ = q/β and x∞ = c/γ. the unknown model parameters are assumed as lognormally distributed random variables. parameter name σy [mpa] yield stress v∞ [mpa] kinematic hardening saturation γ [-] kinematic dynamic recovery constant r∞ [mpa] isotropic hardening saturation β [-] isotropic dynamic recovery constant table 1. model parameters to be identified. the corresponding mean values and standard deviations are the hyperparameters with uninformative uniform hyperprior. the information is combined according to bayes’ rule into joint posterior distribution of 90 random variables, specifically, 80 parameters (5 model parameters corresponding to each of 16 measurements) and 10 hyperparameters (2 statistical moments of every single parameter’s marginal pdf). the bayesian inference is realized by the markov chain monte carlo (mcmc) method providing posterior samples [9, 10]. the computational effort is reduced by substituting the full numerical model by its polynomial chaos-based approximation, specifically by the hermite polynomials of standard normal random variables [11, 12]. the polynomial coefficients are computed with a help of regression method [13] based on latin hypercube sampling [14] from prior parameter distribution. 4. model calibration in order to verify the identification method at first, a synthetic experimental data set of 16 model simulations, see figure 4, is employed. figure 5 shows histograms corresponding to the simulations’ inputs generated from the prescribed lognormal distribution figure 4. comparison of synthetic observations (black) with model responses (red) corresponding to the identified parameters’ distribution in a form of mean value ± standard deviation. 43 eliška janouchová, anna kučerová acta polytechnica ctu proceedings figure 5. verification: histograms of synthetic experimental inputs and identified parameters’ pdf corresponding to hyperparameters’ means accompanied by bounds corresponding to 90 % of hyperparameters’ values. figure 6. validation: identified parameters’ marginal pdf corresponding to hyperparameters’ means accompanied by bounds corresponding to 90 % of hyperparameters’ values. in a comparison with the identified parameter distributions estimated from mcmc posterior samples. the solid lines correspond to parameters’ marginal pdfs with identified mean values of hyperparameters representing the aleatory uncertainties in the parameters’ values while the dashed lines form an envelope of parameters’ pdfs corresponding to 90 % of hyperparameters’ values representing the epistemic uncertainties. as expected due to experimental setting, the cyclic material behaviour is dominated by the kinematic hardening mechanism at a high temperature, so while the isotropic hardening parameter β is not identified properly, the kinematic hardening parameter γ is inferred very precisely and epistemic uncertainties connected to its pdf are reduced significantly. the identified parameters’ pdf corresponding to hyperparameters’ mean values is used to quantify aleatory uncertainty in the model response, which is presented in figure 4. estimation of the aleatory uncertainty in the model response has a good match with the synthetic experimental data. from this point of view, the results of the proposed identification procedure are satisfactory. since the proposed method of model calibration passed the verification process successfully, the model calibration based on the real experimental data follows. the results of model calibration are presented in the same manner as the verification results. the figure 6 shows the identified most likely marginal parameters’ pdfs with depicted epistemic uncertainties. now, it is not possible to compare the results on a level of model parameters’ values, but the epistemic uncertainties are reduced the most in the case of the parameter γ again due to the experimental setup. the aleatory uncertainty caused by heterogeneous character of the modelled material is sufficiently captured, see figure 7 presenting comparison of experimental data and model responses corresponding to the identified parameters’ pdf. figure 7. comparison of experimental observations (black) with model responses (red) corresponding to the identified parameters’ distribution in a form of mean value ± standard deviation. 5. conclusions the multilevel setting of the bayesian inference provides an elegant solution for stochastic inversion problems. this efficient tool enables to separate epistemic uncertainties and inherent variability, which is fundamental for successful parameter identification of heterogeneous material model from indirect noisy measurements. the major disadvantage of this method is the requirement of assuming some specific type of parameters’ pdf which affects the result essentially and 44 vol. 15/2018 bayesian inference of heterogeneous viscoplastic material parameters it can be problematic for example in the case of phenomenological parameters. the formulated posterior probability distribution can be gained in a form of independent samples by the markov chain monte carlo method, however the computational demands increase significantly with a number of measurements, so this approach is suitable especially for small experimental data sets. in this contribution, the material model of kinematic and isotropic hardening for copper alloy is successfully calibrated on a basis of the experimental data set of 16 repetitions of a strain-controlled cyclic loading test at a high temperature. at first, identification of aleatory as well as epistemic uncertainties with a help of the bayesian inference is presented for the case of synthetically generated experimental data. after the successful verification of the identification procedure, the real experimental data are processed in the same way to calibrate the numerical model. due to the experimental setup, when kinematic hardening of the material plays the main role, the parameters of kinematic hardening are identified more precisely than the parameters of isotropic hardening. in order to identify all the parameters more accurately, some more data from another type of experiment have to be available to obtain the necessary additional information. acknowledgements this outcome was financially supported by the czech science foundation, project no. 16-11473y, and the grant agency of the czech technical university in prague, grant no. sgs18/036/ohk1/1t/11. references [1] w. l. oberkampf, s. m. deland, b. m. rutherford, et al. error and uncertainty in modeling and simulation. reliability engineering & system safety 75(3):333–357, 2002. [2] a. gelman, j. b. carlin, h. s. stern, d. b. rubin. bayesian data analysis. chapman & hall/crc, druhé edn., c2004. [3] e. janouchová, a. kučerová, j. sýkora. bayesian updating of aleatory uncertainties in heterogeneous materials. in modern methods of experimental and computational investigations in area of construction ii, vol. 1144 of advanced materials research, pp. 136–141. trans tech publications, 2017. [4] t. bayes, r. price. an essay towards solving a problem in the doctrine of chances. by the late rev. mr. bayes, f. r. s. communicated by mr. price, in a letter to john canton, a. m. f. r. s. philosophical transactions (1683-1775) 53:370–418, 1763. [5] j. b. nagel, b. sudret. a unified framework for multilevel uncertainty quantification in bayesian inverse problems. probabilistic engineering mechanics 43:68–84, 2016. [6] i. behmanesh, b. moaveni, g. lombaert, c. papadimitriou. hierarchical bayesian model updating for structural identification. mechanical systems and signal processing 64:360–376, 2015. [7] d. draper, j. s. hodges, c. l. mallows, d. pregibon. exchangeability and data analysis. journal of the royal statistical society series a (statistics in society) pp. 9–37, 1993. [8] w. schwarz, s. schwub, k. quering, et al. life prediction of thermally highly loaded components: modelling the damage process of a rocket combustion chamber hot wall. ceas space journal 1(1-4):83, 2011. [9] j. c. spall. estimation via markov chain monte carlo. ieee control systems magazine 23(2):34–45, 2003. [10] y. m. marzouk, h. n. najm, l. a. rahn. stochastic spectral methods for efficient bayesian solution of inverse problems. journal of computational physics 224(2):560–586, 2007. [11] n. wiener. the homogeneous chaos. american journal of mathematics 60(4):897–936, 1938. [12] d. xiu, g. e. karniadakis. the wiener–askey polynomial chaos for stochastic differential equations. siam journal on scientific computing 24(2):619–644, 2002. [13] g. blatman, b. sudret. an adaptive algorithm to build up sparse polynomial chaos expansions for stochastic finite element analysis. probabilistic engineering mechanics 25(2):183–197, 2010. [14] r. l. iman, w. conover. small sample sensitivity analysis techniques for computer models. with an application to risk assessment. communications in statistics-theory and methods 9(17):1749–1842, 1980. 45 acta polytechnica ctu proceedings 15:41–45, 2018 1 introduction 2 stochastic identification method 3 numerical model 4 model calibration 5 conclusions acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2017.13.0135 acta polytechnica ctu proceedings 13:135–141, 2017 © czech technical university in prague, 2017 available online at http://ojs.cvut.cz/ojs/index.php/app optimization-based approach to tiling of finite areas with arbitrary sets of wang tiles marek tyburec∗, jan zeman czech technical university in prague, faculty of civil engineering, department of mechanics, thákurova 7, 166 29 prague 6, czech republic ∗ corresponding author: marek.tyburec@fsv.cvut.cz abstract. wang tiles proved to be a convenient tool for the design of aperiodic tilings in computer graphics and in materials engineering. while there are several algorithms for generation of finite-sized tilings, they exploit the specific structure of individual tile sets, which prevents their general usage. in this contribution, we reformulate the np-complete tiling generation problem as a binary linear program, together with its linear and semidefinite relaxations suitable for the branch and bound method. finally, we assess the performance of the established formulations on generations of several aperiodic tilings reported in the literature, and conclude that the linear relaxation is better suited for the problem. keywords: wang tiles, binary and continuous linear programming, semidefinite programming, aperiodic tilings, relaxation. 1. introduction wang tiles, squares with colored edges, were invented by and named in honor of hao wang, originally serving as a tool for studying the ∀∃∀ decidability problem of the predicate calculus [1]. wang showed that the decidability problem is equivalent to the domino problem: assume a set of non-rotatable unit-sized (wang) tiles with edges colored according to the ∀∃∀ problem. if it is feasible to cover the infinite plane by translating copies of the tiles, such that all their vertices lie at the integer lattice points of the plane and the adjoining edges share the same color, the problem is said to be solvable; unsolvable otherwise. for an illustration of a sample 2× 3 wang tiling, see fig. 1. in [2], wang made a fundamental conjecture stating that the domino problem is solvable if and only if the tiling was periodic, i.e., if there existed a rectangular region of the tiling with identically colored horizontal, and vertical edges, respectively. a year later, kahr reduced the turing machine halting problem [3, 4] into the origin-constrained domino problem [5], which implies the domino problem is also undecidable. this can be illustrated by introducing a turing machine for each tile set, halting only if the domino problem is unsolvable. however, there is an infinite number of such tile sets, making the domino problem undecidable as well, and forbidding existence of a general finite algorithm for the generation of infinite tilings. 1.1. aperiodic tile sets undecidability of the domino problem was also proved by wang’s student berger, who used the principle of expanding squares for developing the sets of 20, 426 [6] and 104 tiles [7] that admit only aperiodic tilings of the infinite plane, contrary to the wang fundamental conjecture. ✓ ✗ figure 1. periodic tiling composed of 2 wang tiles over 2 colors. aiming to simplify the berger’s proof, smaller aperiodic sets of wang tiles have been constructed. in 1966, läuchli sent to professor wang an aperiodic set of 40 tiles over 16 colors, but it remained unpublished until 1975 [8]. meanwhile, unaware of läuchli’s result, knuth simplified berger’s set to 92 tiles over 26 colors [9]; and robinson developed sets of 104 and 52 tiles in 1967 [10], of 56 tiles over 12 colors in 1971 [11], and marked existence of a set of 35 tiles [11]. in 1973, penrose developed a new approach based on kites and darts tiling, leading to a set of 34 tiles. robinson, being in contact with penrose, modified the penrose’s approach to reach a reduced set of 32 tiles over 16 colors [12]. using the same technique together with penrose rhombs tiling, grünbaum obtained a set of 24 tiles over 9 colors in 1987 [12]. another two tile sets were discovered due to ammann. in 1978, he used ammann bars to reach 16 tiles over 6 colors [13]. building on the ammann’s a2 tiling, see, e.g., [12], robinson obtained a set of 24 tiles over 24 colors in 1977. subsequent size reduction of the smallest aperiodic set occurred in 1996, as kari developed a new method based on mealy machines multiplying beatty sequences, and presented a set of 14 tiles over 6 colors [14]. čulík, using the same approach, reduced the set even further to 13 tiles over 5 colors [15]. 135 http://dx.doi.org/10.14311/app.2017.13.0135 http://ojs.cvut.cz/ojs/index.php/app marek tyburec, jan zeman acta polytechnica ctu proceedings the search for the minimal aperiodic set is concluded by jeandel, who used a brute-force enumeration approach to find aperiodic sets of 11 tiles over 4 and 5 colors; and proved both that there does not exist an aperiodic set with 10 or fewer tiles and with less than 4 colors [16]. in addition to the classical wang tiles, in 2006, lagae introduced a subset of the wang tiles, the corner tiles, with the connectivity information stored in the colored corners instead of the edges [17]. in the same year, he constructed an aperiodic set of 44 corner tiles over 6 colors [18]. the set was further simplified by nurmi to 30 corner tiles over 6 colors [19]. note that because the corner tiles can be straightforwardly converted to the edge-based wang tiles [17], our approach naturally extends to the corner tiles, see section 4 for a specific example. 1.2. selected applications due to the ability of some tile sets to generate aperiodic patterns, wang tiles received a broad interest among disciplines. also, their original significance for automated theorem proving [2] was supplemented by proofs in cellular automata theory [20]. in 2003, cohen et al. introduced wang tiles to the computer graphics community [21]. since then, wang tiles have been successfully used, e.g., for efficient synthesis of stochastic textures or for generation of poisson disk distributions. molecular realization of wang tiles is due to winfree, who introduced a self-assembly of biological nanostructures into aperiodic crystals [22]. novák was probably the first one who used wang tiles in materials engineering for efficient compression of microstructures [23]; doškář for their reconstruction [24]. doškář further generated large stochastic samples of the alporas® foam and its finiteelement representations in [25]. in [26], tyburec used wang tiles to describe modular assembly of structures, whereas both the topology and arrangement of modules were subjects of optimization. 1.3. tiling generation algorithms existing tiling generation algorithms are designed specifically to the tile sets they handle. in computer graphics, for example, it is essential to generate visually appealing patterns quickly, which is best achieved with stochastic tile sets. tiling a finite area has then a o(n) complexity, adopting, e.g., the stochastic tiling [21], or the hash-based direct stochastic tiling [27] algorithms. the latter algorithm allows straightforward definition of boundary conditions. in the case of mathematical logic, it is crucial to prove that an investigated tile set can tile the whole infinite plane, and does so only aperiodically. as the problem is undecidable, no general algorithm exists, and thus all the algorithms need to exploit the specific structure of each individual (family of) tile sets. although the algorithms are fast, they remain tile set dependent. all tilings of finite-sized areas are restricted to be subsets of the infinite plane, which is not required in the finite domain, and it is almost impossible to introduce boundary conditions. on top of that, there provably exist aperiodic tile sets that admit only non-recursive tiling [28, 29], forbidding design of any problem-specific algorithm to tile the infinite plane. to the authors’ knowledge, the only method that has been used for tiling of finite-sized areas by arbitrary tile sets is the backtracking algorithm, see [30]. although the algorithm is general, it is generally inefficient, as it commonly creates impossible assemblies too early. moreover, distant boundary conditions make the problem more difficult to solve. 1.4. contributions this paper aims to overcome the shortcomings of the methods outlined in the previous section, and to develop an approach that handles tiling of finitesized areas using arbitrary tile sets, together with a straightforward approach to define edgeor tilebased boundary conditions. our exposition is structured as follows. in section 2, we develop a binary linear programming representation of the tiling generation problem. the formulation is relaxed, and its linear and semidefinite approximations are presented. in order to show generality of the formulations, we introduce their extensions to solve the tile-packing problem, to prove that a tile set can not tile the plane, and to prove that a tile set admits periodic tiling. the relaxations are finally employed in a branch and bound method in section 3 and their performance is assessed in section 4. 2. optimization problem formulation 2.1. valid tiling consider a finite rectangular area a of the size nt,h× nt,w, with nt,h and nt,w denoting its height and width, respectively. placing wang tiles (of a unit size) from the tile set t , such that their vertices lie at the integer lattice points of a, we obtain a tiling with nt,h rows and nt,w columns of tiles. each tile k ∈ {1, .. , nt} from the tile set t , with nt denoting the number of tiles, is described by the 4-tuple (nk, wk, sk, ek), assigning color codes c to the north, west, south, and east edge of the tile, respectively. in any valid tiling, for any two adjacent tiles k and `, with the tile ` being placed in the east of k, it holds that ek = w`. (1) similarly, for any two vertically adjacent tiles k and m, with the tile m being placed in the south of k, it stands that sk = nm. (2) both the cases are illustrated in fig. 2. 136 vol. 13/2017 optimization approach to tiling of finite areas with wang tiles sm nm wm em s` n` w` e` sk nk wk ek figure 2. horizontal, and vertical connectivity among tiles k-`, and k-m, respectively. 2.2. binary linear programming formulation let x ∈ {0, 1}nt,h·nt,w·nt denote a binary vector describing the placement of individual tiles within a. the vector consists of all the xi,j,k variables, where i ∈ {1, .. , nt,h} and j ∈ {1, .. , nt,w} are the row and column iterators, respectively. let us also define xi,j,k = { 0 if the tile k is not at (i, j), 1 if the tile k is at (i, j). (3) because each position is occupied by a single tile, we have ∑ k∈t xi,j,k = 1, ∀i ∈ h,∀j ∈ w. (4) the compatibility constraint (1) can be written in terms of x after realizing that the number of tiles at (i, j) that have east edge colored by c ∈ c has to be equal to the number of tiles at (i, j + 1) with west edge colored also by c. consequently, we have∑ k∈t xi,j,k[ek = c]− ∑ k∈t xi,j+1,k[wk = c] = 0. (5) as the sums in eq. (5) are either equal to 0 or 1, resulting from eq. (4), only two options are possible: if the edge is colored by c, the relation simplifies to 1− 1 = 0, (6) otherwise, it equals to 0− 0 = 0, (7) showing that (5) remains valid in both cases. after applying the same approach to eq. (2) and writing constraints through the entire area a, we obtain the following binary linear program: min x 0 (8a) s.t. ∑ k∈t xi,j,k[ek = c]− ∑ k∈t xi,j+1,k[wk = c] = 0, ∀c ∈ c,∀i ∈ h,∀j ∈ w \ {nt,w}, (8b) ∑ k∈t xi,j,k[sk = c]− ∑ k∈t xi+1,j,k[nk = c] = 0, ∀c ∈ c,∀j ∈ w,∀i ∈ h \ {nt,h}, (8c) ∑ k∈t xi,j,k = 1, ∀i ∈ h,∀j ∈ w, (8d) xi,j,k ∈ {0, 1}, ∀i ∈ h,∀j ∈ w,∀k ∈ t . (8e) despite the program (8) being formulated straightforwardly, it is hard to solve in this original form due to the (non-convex) integrality constraint (8e). note that the problem is np-complete because of its combinatorial nature [31]. 2.3. linear programming relaxation in order to make the problem (8) easier to solve, let us relax the integrality constraint (8e) into a linear form 0 ≤ xi,j,k ≤ 1, ∀i ∈ h,∀j ∈ w,∀k ∈ t . (9) clearly, eq. (9) allows for all configurations of (8e), but additionally also intermediate non-binary values. in addition, because the constraints in (8) are linear, the resulting approximation is convex and reads as min x 0 (10a) s.t. ∑ k∈t xi,j,k[ek = c]− ∑ k∈t xi,j+1,k[wk = c] = 0, ∀c ∈ c,∀i ∈ h,∀j ∈ w \ {nt,w}, (10b) ∑ k∈t xi,j,k[sk = c]− ∑ k∈t xi+1,j,k[nk = c] = 0, ∀c ∈ c,∀j ∈ w,∀i ∈ h \ {nt,h}, (10c) ∑ k∈t xi,j,k = 1, ∀i ∈ h,∀j ∈ w, (10d) 0 ≤ xi,j,k ≤ 1, ∀i ∈ h,∀j ∈ w,∀k ∈ t . (10e) 2.4. semidefinite programming relaxation the integrality constraint (8e) can be rewritten using the non-convex quadratic constraint x2 i,j,k − xi,j,k = 0, ∀i ∈ h,∀j ∈ w,∀k ∈ t , (11) which is satisfied only if xi,j,k = {0, 1}, being thus equivalent to (8e). this quadratic form does not simplify the solution, because the problem is still np-complete, but we will use it for the derivation of a semidefinite programming relaxation. let us now substitute the binary variables x ∈ {0, 1} with y ∈ {−1, 1}, which requires x = 1 2(y + 1). (12) inserting this into eq. (3) yields yi,j,k = { −1 if the tile k is not at (i, j), 1 if the tile k is at (i, j). (13) consequently, we can write a non-convex quadratically-constrained optimization program in terms of y 137 marek tyburec, jan zeman acta polytechnica ctu proceedings min y 0 (14a) s.t. ∑ k∈t (yi,j,k + 1)[ek = c] − ∑ k∈t (yi,j+1,k + 1)[wk = c] = 0, ∀c ∈ c,∀i ∈ h,∀j ∈ w \ {nt,w}, (14b) ∑ k∈t (yi,j,k + 1)[sk = c] − ∑ k∈t (yi+1,j,k + 1)[nk = c] = 0, ∀c ∈ c,∀j ∈ w,∀i ∈ h \ {nt,h}, (14c) ∑ k∈t yi,j,k = 2− nt, ∀i ∈ h,∀j ∈ w, (14d) y2 i,j,k = 1, ∀i ∈ h,∀j ∈ w,∀k ∈ t . (14e) let us further introduce a symmetric matrix y ∈ snt,y·nt,x·nt defined as y = yyt. (15) the definition directly implies that any solution to the quadratically-constrained optimization problem (14) renders the matrix y positive semidefinite, in our notation y � 0, and the rank of the matrix y equal to 1. moreover, the definition of y constrains all the elements in the main diagonal of y equal to 1. consequently, another equivalent formulation to the problem (8) reads as min y,y 0 (16a) s.t. ∑ k∈t (yi,j,k + 1)[ek = c] − ∑ k∈t (yi,j+1,k + 1)[wk = c] = 0, ∀c ∈ c,∀i ∈ h,∀j ∈ w \ {nt,w}, (16b) ∑ k∈t (yi,j,k + 1)[sk = c] − ∑ k∈t (yi+1,j,k + 1)[nk = c] = 0, ∀c ∈ c,∀j ∈ w,∀i ∈ h \ {nt,h}, (16c) ∑ k∈t yi,j,k = 2− nt, ∀i ∈ h,∀j ∈ w, (16d) diag(y) = 1, (16e) y− yyt = 0. (16f) the only non-convex constraint (16f) can be relaxed, see, e.g., [32], into a convex form y− yyt � 0, (17) and based on the schur complement lemma equivalently rewritten to ( 1 yt y y ) � 0. (18) finally, the semidefinite programming relaxation of the binary linear program (8) reads as min y,y 0 (19a) s.t. ∑ k∈t (yi,j,k + 1)[ek = c] − ∑ k∈t (yi,j+1,k + 1)[wk = c] = 0, ∀c ∈ c,∀i ∈ h,∀j ∈ w \ {nt,w}, (19b) ∑ k∈t (yi,j,k + 1)[sk = c] − ∑ k∈t (yi+1,j,k + 1)[nk = c] = 0, ∀c ∈ c,∀j ∈ w,∀i ∈ h \ {nt,h}, (19c) ∑ k∈t yi,j,k = 2− nt, ∀i ∈ h,∀j ∈ w, (19d) diag(y) = 1, (19e)( 1 yt y y ) � 0, (19f) − 1 ≤ y ≤ 1. (19g) 2.5. extensions introduced formulations can include additional requirements for generated tilings. we list some of them below, but only in terms of x, as substitution of eq. (12) into developed equations is obvious. 2.5.1. tile-based boundary conditions there are four types of tile-based boundary conditions. first, we can enforce placement of the tile k ∈ t at position (i, j): xi,j,k = 1, i ∈ h, j ∈ w, k ∈ t . (20) conversely, avoiding the tile k at (i, j) requires xi,j,k = 0, i ∈ h, j ∈ w, k ∈ t . (21) the requirement that the same tile is placed at both (i, j) and (p, q) can be written as xi,j,k − xp,q,k = 0, {i, p} ∈ h, {j, q} ∈ w,∀k ∈ t . (22) enforcing different tiles at (i, j) and (p, q) requires xi,j,k+xp,q,k ≤ 1, {i, p} ∈ h, {j, q} ∈ w,∀k ∈ t . (23) 2.5.2. edge-based boundary conditions starting from eq. (5), we can also easily formulate edge-based boundary conditions. to constrain the north edge at (i, j) to the color c ∈ c, we write∑ k∈t xi,j,k[nk = c] = 1, i ∈ h, j ∈ w, c ∈ c. (24) if the north edge at (i, j) has to differ from c, it holds that∑ k∈t xi,j,k[nk = c] = 0, i ∈ h, j ∈ w, c ∈ c. (25) 138 vol. 13/2017 optimization approach to tiling of finite areas with wang tiles equal color c of two edges, e.g., of the north edge at (i, j) and of the west edge at (p, q), is provided by∑ k∈t xi,j,k[nk = c]− ∑ k∈t xp,q,k[wk = c] = 0, {i, p} ∈ h, {j, q} ∈ w,∀c ∈ c. (26) finally, different coloring of the north edge at (i, j) and the west edge at (p, q) is guaranteed through∑ k∈t xi,j,k[nk = c] + ∑ k∈t xp,q,k[wk = c] ≤ 1, {i, p} ∈ h, {j, q} ∈ w,∀c ∈ c. (27) 2.5.3. periodic tiling periodic tiling is a tiling with equally colored both horizontal and also both vertical edges. thus, periodic tiling secures tilability of the infinite plane. building on the edge-based boundary conditions, a periodic tiling can be enforced by∑ k∈t x1,j,k[nk = c]− ∑ k∈t xnt,h,j,k[sk = c] = 0, ∀j ∈ w,∀c ∈ c, (28a) ∑ k∈t xi,1,k[wk = c]− ∑ k∈t xi,nt,w,k[ek = c] = 0, ∀i ∈ h,∀c ∈ c. (28b) 2.5.4. tile packing problem the tile packing problem, see, e.g., [30], consists in generation of a tiling with periodic boundary conditions, and each tile has to be placed exactly once. in order to describe the problem in our framework, we just need to use eq. (28) together with∑ i∈h ∑ j∈w xi,j,k = 1, ∀k ∈ t . (29) 2.5.5. objective function obviously, the developed formulations can contain arbitrary convex objective function. for instance, we can minimize the maximum occurrences of tiles, compose the tiling such that it fits some pattern, etc. 3. the branch and bound method the whole integer design space, i.e., {0, 1} for the linear (10) and {−1, 1} for the semidefinite (19) approximation, can be described using a tree data structure, with each node corresponding to variables x, or y. however, as there is exactly one tile per position, it is advantageous to use the nt-ary tree representation instead of the binary one, so that the nodes correspond to the positions and branch into nt children, avoiding solution to infeasible programs placing multiple tiles at the same position. solution to the developed approximations, (10) or (19), corresponds to exploring the root node of the tree. unfortunately, because the approximations widen the feasible design space, their solution does not assure to solve the original problem (8) due to the design variables being allowed to take non-integer values. in order to overcome that, we branch some nodes of the tree, i.e., gradually fix all variables that correspond to the specific tile position to all possible combinations of integer values, nt in our case. the relaxation is solved in each node, bounding the problem1, hence the name of the method branch and bound [33]. the branching and bounding continues until the optimal (feasible) solution to (8) is found. if no such solution exists, the solution space is proven to be empty and the tiling generation problem infeasible. without boundary conditions, infeasibility proves the tile set does not tile infinite plane. 3.1. branching rule in our implementation, we firstly branch the most promising nodes, in which the integer infeasibility of the convex relaxation is minimal. for the linear relaxation, we have iinf = ∑ i∈h ∑ j∈w ∑ k∈t (xi,j,k − x2 i,j,k), (30) or iinf = nt,hnt,wnt − ∑ i∈h ∑ j∈w ∑ k∈t y2 i,j,k (31) for the semidefinite relaxation. if iinf = 0, a feasible solution to the original problem was found. 3.2. variables ordering in each node to be branched, we have to define variables that will be fixed. it seems to be advantageous to fix the variables corresponding to the most difficult position, i.e., to the position with the highest integer infeasibility. in the case of the linear relaxation, the to-be-fixed position (i, j) requires max ∀i∈h,∀j∈w ∑ k∈t (xi,j,k − x2 i,j,k). (32) similarly, for the semidefinite relaxation we write max ∀i∈h,∀j∈w (nt − ∑ k∈t y2 i,j,k). (33) 4. examples building upon the previous sections, we implemented a custom branch and bound algorithm in c++. the linear relaxations are solved using gurobi [34], the semidefinite programming relaxations are optimized by mosek [35]. as gurobi contains a build-in state-ofthe-art branch and cut algorithm, usable only for the linear relaxations, we also measured its performance. five tile sets were tested altogether: jeandel’s 11 tiles over 4 colors [16], čulík’s 13 tiles [15], ammann’s 16 tiles [12], lauchli’s 40 tiles [12], and nurmi’s 1bounding occurs only if an objective fuction is employed. 139 marek tyburec, jan zeman acta polytechnica ctu proceedings 11(4) [16] 13(5) [15] 16(6) [12] 40(16) [8] 30(6) [19] tile set tiling sample lp 5× 5 0.13 s/12 rel. 0.10 s/14 rel. 0.10 s/17 rel. 0.01 s/1 rel. 0.01 s/1 rel. sdp 5× 5 12.65 s/78 rel. 11.07 s/27 rel. 21.66 s/33 rel. 360.40 s/121 rel. 126.17 s/31 rel. lp 6× 6 0.10 s/12 rel. 0.12 s/14 rel. 0.13 s/17 rel. 0.02 s/1 rel. 0.03 s/1 rel. sdp 6× 6 69.60 s/34 rel. 227.24 s/79 rel. 184.23 s/33 rel. 1517.59 s/121 rel. 1522.72 s/91 rel. lp 15× 15 151.82 s/2190 rel. over 6000 s 2.15 s/17 rel. 8.52 s/41 rel. 0.13 s/1 rel. lpg 15× 15 0.06 s/0 rel. 0.09 s/0 rel. 0.39 s/0 rel. 0.79 s/0 rel. 1.49 s/0 rel. lp 20× 20 1545.94 s/26137 rel. over 6000 s 8.52 s/17 rel. 32.09 s/41 rel. 4.23 s/31 rel. lpg 20× 20 0.08 s/0 rel. 0.12 s/0 rel. 0.79 s/0 rel. 1.57 s/0 rel. 8.01 s/0 rel. lpg 25× 25 396.5 s/7368 rel. 0.12 s/0 rel. 1.34 s/0 rel. 2.96 s/0 rel. 23.95 s/0 rel. lpg 30× 30 390.18 s/3821 rel. 1361.51 s/5950 rel. 2.31 s/0 rel. 5.23 s/0 rel. 48.97 s/0 rel. table 1. time demands and solved relaxations count in generation of aperiodic tilings. lp and sdp denote custom developed branch and bound method supplied with the linear and semidefinite relaxation, respectively; lpg stands for gurobi cut-and-branch method and the linear relaxation. set of 30 corner tiles [19]. the tile sets were selected to sample different construction methods, tile set sizes, and numbers of colors used. results of the benchmarks, ran on the intel® core™ i5-4210h processor, are summarized in table 1. they indicate that the linear relaxation surpassed the semidefinite one in terms of speed and in the number of explored nodes. the latter results from the property of the simplex algorithm, used for the solution of linear relaxations, to terminate in vertices of the feasible space polytope. these are more likely integer-feasible than an interior point found by the interior-point algorithm, used for the solution of semidefinite programs. comparison of our branch and bound algorithm and gurobi’s build-in branch and cut, both using the linear relaxation, is even clearer. our implementation lacks heuristics, parallelism, and generation of cuts, consequently being inefficient for tile sets with large number of degrees of freedom, such as the čulík one. 5. conclusions in this contribution, we demonstrated that the tiling generation problem of finite-sized areas can be formulated as an integer optimization problem; and we also introduced its linear and semidefinite programming relaxations. both the relaxations were employed in the branch and bound method and benchmarked on five tile sets. the results indicate that the linear relaxation is more suitable for practical applications. 6. nomenclature list of symbols a rectangular area c color set h set of rows t tile set w set of columns c color iterator ek east edge color of the tile k i row iterator j column iterator k, `, m tile iterators nc number of colors in c nk north edge color of the tile k sk south edge color of the tile k nt,h number of rows in h nt,w number of columns in w xi,j,k design variable, xi,j,k ∈ {0, 1} yi,j,k design variable, yi,j,k ∈ {−1, 1} wk west edge color of the tile k iinf integer infeasibility 140 vol. 13/2017 optimization approach to tiling of finite areas with wang tiles acknowledgements this work was supported partly by the grant agency of the ctu in prague, sgs17/042/ohk1/1t/11, by the grant agency of the czech republic, gačr 15-07299s, by the ministry of industry and trade, mpo fv10202, and by the technology agency of the czech republic, tačr th02020420. references [1] h. wang. dominoes and the aea case of the decision problem. symposium on the mathematical theory of automata pp. 23–55, 1962. 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[35] mosek aps. mosek fusion api for c++ 8.1.0.23, 2017. http://docs.mosek.com/8.1/cxxfusion. 141 http://dx.doi.org/10.1002/j.1538-7305.1961.tb03975.x http://dx.doi.org/10.1112/plms/s2-42.1.230 http://dx.doi.org/10.1090/memo/0066 http://dx.doi.org/10.1007/bf01418780 http://dx.doi.org/10.1007/bf01403163 http://dx.doi.org/10.1016/0012-365x(95)00120-l http://dx.doi.org/10.1016/s0012-365x(96)00118-5 http://arxiv.org/abs/1506.06492v1 http://dx.doi.org/10.1145/1183287.1183296 http://dx.doi.org/10.1016/0167-2789(90)90195-u http://dx.doi.org/10.1145/882262.882265 http://dx.doi.org/10.1038/28998 http://dx.doi.org/10.1103/physreve.86.040104 http://dx.doi.org/10.1103/physreve.90.062118 http://dx.doi.org/10.1016/j.compstruc.2016.01.003 http://dx.doi.org/10.13140/rg.2.2.23612.85126 http://dx.doi.org/10.1145/1095878.1095888 http://dx.doi.org/10.2307/2272641 http://dx.doi.org/10.1007/978-3-642-30870-3_34 http://dx.doi.org/10.1007/978-1-4684-2001-2_9 http://dx.doi.org/10.1007/978-1-4615-6281-8_15 http://dx.doi.org/10.1002/9781118627372 http://www.gurobi.com http://docs.mosek.com/8.1/cxxfusion acta polytechnica ctu proceedings 13:135–141, 2017 1 introduction 1.1 aperiodic tile sets 1.2 selected applications 1.3 tiling generation algorithms 1.4 contributions 2 optimization problem formulation 2.1 valid tiling 2.2 binary linear programming formulation 2.3 linear programming relaxation 2.4 semidefinite programming relaxation 2.5 extensions 2.5.1 tile-based boundary conditions 2.5.2 edge-based boundary conditions 2.5.3 periodic tiling 2.5.4 tile packing problem 2.5.5 objective function 3 the branch and bound method 3.1 branching rule 3.2 variables ordering 4 examples 5 conclusions 6 nomenclature list of symbols acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2019.22.0022 acta polytechnica ctu proceedings 22:22–25, 2019 © czech technical university in prague, 2019 available online at http://ojs.cvut.cz/ojs/index.php/app autogenous shrinkage of composites based on portland cement vendula davidová, pavel reiterman∗ czech technical university in prague, faculty of civil engineering, thakurova 7, 166 29 prague, czech republic ∗ corresponding author: pavel.reiterman@fsv.cvut.cz abstract. autogenous shrinkage of cement based composites is important property influencing number of their engineering application. its ultimate value is predominantly determined by mineralogical composition of cement and its particle size distribution. present paper introduces experimental study focused on the evaluation of various cements of grade cem i 42.5 produced in czech republic in terms of shrinkage under autogenous conditions. selected cement type is currently the most frequently used cement. conducted study confirmed essential differences in ultimate values of shrinkage, which is partially determined by its specific surface area. accompanying tests of mechanical properties indicate the influence of particle size distribution, which controls initial phases of cement hydration. keywords: volume changes, shrinkage, cement mineralogy, mechanical properties. 1. introduction portland cement based binder play an important role in structural engineering worldwide. it is caused by the efficiency of portland cement and accessible raw materials for its production. annual world production of portland cement is approximately 3.8gt and due to actual progress of economy worldwide is still rising. traditional cement based concrete exhibits suitable values of compressive strength, however its low tensile performance is the reason of the risk of crack occurrence. there are many reasons of crack propagation; besides impacts of mechanical loadings, action of environment, thermal contraction, the shrinkage ranks to the most frequent reason of cracks propagation [1, 2], figure 1. the presence of cracks significantly limits the service-life of concrete structures, because corrosion of steel reinforcement could reduce load capacity of the concrete element. in addition, expansive stresses, accompanying the steel corrosion, accelerate deterioration of the entire concrete. that is why, there is intensive attempt to mitigate cracks origin [1, 3]. shrinkage of the concrete is complicated process, which consists of several aspects. however, it is suitable to distinguish shrinkage in early-age and during hardening. especially, shrinkage at early-age went into general concern, because it presents period where the concrete is highly vulnerable to cracks propagation [4, 5]. commonly used technological provision is the partial replacement of portland cement by supplementary cementitious materials (scm) such as fly ash, granulated blast furnace slag, calcined clay, or silica fume [6, 7]. however, kinetics of the hydration of the binder is crucial aspect as well. it is well documented on the application of silica fume, which could cause considerable shrinkage due to massive consumption of portlandite; it is well illustrated on figure 2. figure 1. the example of cracks propagation due to shrinkage. besides factors of chemical shrinkage, there is number of elements affecting the shrinkage during hardening. the dominant one is the drying, thus loss of water content. the main problem of drying shrinkage is the non-linear distribution of originating stress, which culminates on the surface. possible crack opening consequently accelerates moisture evaporation. drying shrinkage has dominant importance during initial curing; poor discipline leads to massive rupture, well documented on figure 1. additionally, the shrinkage caused by carbonation could contribute, however its influence is minor in comparison with the drying shrinkage [1]. the measurement of concrete shrinkage is conducted by various experimental procedures. autogenous shrinkage of hardening concrete was successfully determined using waved form, this arrangement allow contact less measurement [3]. special silicate composites are often investigated by so called ‘absolute’ method, which is based on the volumetric 22 http://dx.doi.org/10.14311/app.2019.22.0022 http://ojs.cvut.cz/ojs/index.php/app vol. 22/2019 autogenous shrinkage of composites based on portland cement figure 2. illustration of concrete shrinkage [1]. measurement of sealed specimen immersed in a suitable liquid. this procedure is very complicated in its instrumentation, however offers the determination of entire chemical shrinkage. free shrinkage in drains is the most frequently used procedure in technical praxis [3, 5]. it is very simple and the test arrangement well corresponds with real conditions, because it allows to control the curing condition. close to that is the measurement using traditional specimens with glued metal target for the fitting of the dilatometer. all these tests allow to obtain clear data, however their different arrangement needs precious interpretation of results obtained, because different aspects dominate in individual procedures. the drying shrinkage dominates during the measurement in bed, on the other hand samples with glued targets could be easily cured under specific conditions. the main aim of this paper is comparison of portland cements of similar class cured under autogenous conditions to investigate their chemical shrinkage. intentionally, cements with various finesses were selected for the study. 2. experimental program performed experimental program was focused on the determination of volume changes of ordinary portland cement cem i 42.5 produced by various producers. chemical composition of the studied cements is introduced in table 1. selected cement type is currently the most frequently used cement type in czech republic. volume changes were measured using prismatic specimens 20 × 20 × 100mm made from mortar. used mortar performed mix of standard sands and cement in weight ratio 3:1; water to cement ratio was set to 0.45. the volume changes were monitored by using dial test indicator in specified time intervals. similar methodology was used by koťátková and reiterman figure 3. used prismatic specimens. [8]. used samples are shown in figure 3. concurrently was monitored weight of measured samples. mechanical properties were determined in terms of bs en 196-1 [9] on prismatic specimens 40 × 40 × 160mm after 3 and 28 days of curing. the aim of performed work was to assess the autogenous volume changes, hence all samples were cured in water. 3. results and discussion various cements of class 42.5 r produced in czech republic were studied in terms of shrinkage and basic mechanical properties in time. studied materials exhibited different chemical composition and specific surface. it was confirmed, as expected, that basic properties of cement have essential influence on the studied properties having a high engineering importance. results of the measurement of the shrinkage are shown in figure 4; it is necessary to note, that all samples were cured under wet condition to quantify the influence of chemical shrinkage. however, applied 23 vendula davidová, pavel reiterman acta polytechnica ctu proceedings cao sio2 al2o3 fe2o3 mgo so3 k2o na2o loi blaine [%] [%] [%] [%] [%] [%] [%] [%] [%] [m2/kg] a 63.70 19.60 4.80 3.20 1.40 3.10 0.76 0.19 3.50 414 b 63.80 20.60 4.80 3.40 1.40 3.20 0.74 0.20 1.40 311 c 61.20 21.70 5.72 3.22 2.58 2.94 0.80 0.30 1.29 351 table 1. properties of studied cements. figure 4. results of shrinkage measurement. figure 5. results of shrinkage measurement. 24 vol. 22/2019 autogenous shrinkage of composites based on portland cement condition caused slow increase of the volume for all studied materials. it is caused by the fixation of water and air in capillary system of hardened mortar. the lowest volume changes exhibited cement with the lowest value of specific surface area, it is well visible on figure 4, that this cement has also retarded initial kinetics. the measurement of weight changes well corresponds with the determination of volume changes. on the other hand, cements with various surface area reached inverse trend in terms of volume changes. it could be caused by the different particle size distribution, which was not, unfortunately, studied in the paper. however, number of practical experiences accent unsuitability the surface area determination for special problems such as volume changes and cement calorimetry. there are evident lacks on the standard level in czech republic. mechanical properties of studied cements are presented in figure 5. it can be concluded, that studied cements exhibited similar mechanical performance, what can be expected for cements of similar class. however, detailed view on the results can offer interesting findings; evolution of mechanical properties of cement “c” is rapidly faster in early ages in comparison with the others. it is well visible on the evolution of flexural strength. paradoxical, cement designated “c” showed the highest volume changes, but lowest mechanical performance in terms of compressive strength. 4. conclusion performed experimental program was aimed at the comparison of various cements of class 42.5 r produced in czech republic, which all meet the requirement of en 197-1 [10]. studied material exhibited similar mechanical performance, however differences in chemical composition and basic physical properties caused significant differences in terms of volume changes. the program showed, that standard requirements, especially surface area, are not reliable criterion for prediction of volume changes. present topic needs following research with focus on the hydration kinetics. acknowledgements this study was supported by the research project fv30062 “possibilities of utilization of coal-ash from power stations stored at stock piles”. references [1] l. wu, n. farzadnia, c. shi, et al. autogenous shrinkage of high performance concrete: a review. construction and building materials 149:62–75, 2017. doi:10.1016/j.conbuildmat.2017.05.064. [2] s. a. al-saleh. comparison of theoretical and experimental shrinkage in concrete. construction and building materials 72:326–332, 2014. doi:10.1016/j.conbuildmat.2014.06.050. [3] k. kolar, j. litos, z. bazantova, p. reiterman. comparison of two methods for determination of volume changes of cement based composites. in p.padevět, p. bittnar (eds.), ean 2015 – 53rd conference on experimental stress analysis, pp. 173–176. 2015. [4] a. zemanova, r. sovjak, j. litos. restrained shrinkage test of high performance concrete ring specimen. advanced materials research 982:38–43, 2014. doi:10.4028/www.scientific.net/amr.982.38. [5] b. kucharczyková, d. kocáb, i. rozsypalová, et al. measurement and evaluation proposal of early age shrinkage of cement composites using shrinkage-cone. iop conference series: materials science and engineering 379(012038), 2018. doi:10.1088/1757-899x/379/1/012038. [6] u. haider, z. bittnar, l. kopecky, et al. determining the role of individual fly ash particles in influencing the variation in the overall physical, morphological, and chemical properties of fly ash. acta polytechnica 56:265, 2016. doi:10.14311/ap.2016.56.0265. [7] i. nováková, l. bodnárová, t. stavař, r. hela. evaluation of disruption of concrete caused by exposure to high temperatures by initial surface absorption test. arpn journal of engineering and applied sciences 10:6299–6304, 2015. [8] j. koťátková, p. reiterman. preliminary study on design of biological shielding concrete selection of binder. in special concrete and composites 2016, vol. 722 of key engineering materials, pp. 173–177. trans tech publications, 2017. doi:10.4028/www.scientific.net/kem.722.173. [9] bs en 196-1:2016 methods of testing cement. determination of strength. standard, british standards document, 2016. doi:10.3403/30291447u. [10] bs en 197-1:2011 cement. composition, specifications and conformity criteria for common cements. standard, british standards document, 2011. 25 http://dx.doi.org/10.1016/j.conbuildmat.2017.05.064 http://dx.doi.org/10.1016/j.conbuildmat.2014.06.050 http://dx.doi.org/10.4028/www.scientific.net/amr.982.38 http://dx.doi.org/10.1088/1757-899x/379/1/012038 http://dx.doi.org/10.14311/ap.2016.56.0265 http://dx.doi.org/10.4028/www.scientific.net/kem.722.173 http://dx.doi.org/10.3403/30291447u acta polytechnica ctu proceedings 22:22–25, 2019 1 introduction 2 experimental program 3 results and discussion 4 conclusion acknowledgements references acta polytechnica doi:10.14311/app.2020.27.0096 acta polytechnica 27(0):96–100, 2020 © czech technical university in prague, 2020 available online at http://ojs.cvut.cz/ojs/index.php/app nanoindentation and accelerated property mapping of portland cement paste jiří němeček1a,∗, jiří němeček2a, jaroslav lukešb a czech technical university in prague, faculty of civil engineering, department of mechanics, thákurova 7, 166 29 prague 6, czech republic b czech technical university in prague, faculty of mechanical engineering, department of mechanics, biomechanics and mechatronics, czech republic ∗ corresponding author: jiri.nemecek.1@fsv.cvut.cz (ph.d. candidate) abstract. this paper presents a new nanoindentation mode: accelerated property mapping (xpm) applied to one year old portland cement paste. the principle of the innovative method is similar to classical quasi-static nanoindentation but the time necessary for performing one indent is significantly decreased from minutes to tenths of seconds. the xpm mode is capable of measuring maps of local mechanical properties (reduced modulus, hardness). the method was compared to classical nanoindentation mode. the main advantages of xpm measurement are higher spatial resolution, lower property fluctuation and lower acquisition time. however, xpm is not able to measure some parameters such as creep. also, high speed loading of xpm brings high strain effect which artificially shifts values of reduced modulus to higher values compared to quasi-static loading cases. keywords: cement paste, high speed nanoindentation, strain rate, xpm. 1. introduction nanoindentation is a widely used technique for micromechanical testing of materials. its principle lies in pressing a tip with known geometry to a material. during the indentation process the loading response is measured. classical nanoindentation performs one indent in the order of minutes which leads to long measurement time, especially if hundreds of indents needs to be performed [1]. in recent years, a new mode of testing called accelerated property mapping (xpm) has been developed by bruker-hysitron company. xpm presents a way to perform hundreds of indents in a few minutes and create maps of reduced modulus and hardness with very high spatial resolution. the testing itself is done by three capacitive plate transducer where the tip is attached to piezo ceramic and loading is then applied electrostatically. classical nanoindentation (ni) measurement consists of several steps: moving tip to the position (a few seconds), piezo settlement (80-200 s), drift measurement (40-80 s) and quasi-static loading (typically a few seconds). during the xpm measurement, the piezo settlement and drift measurement are done only before indentation. so, the process of xpm measurement consists only of performing the indent and changing the position. thus, such a process must be very quick in order to avoid changing the drift rate. elastic modulus is then derived from the unloading curve by oliver and pharr theory [2]. the theory is usually implemented as a standard in nanoindenter’s softwares for, both ni and xpm. however, if the ef1ph. d. candidate at ctu in prague 2associate professor at ctu in prague fect of strain rate is taken into account then it can result in an increase of mechanical properties for some materials tested by nanoindentation, i.e., metals [3, 4]. figure 1. typical sem-bse image of cement paste showing individual phases. our study is focused on cementitious materials where the strain-rate effect of macroscopic mechanical properties e.g. an increase of compressive strength is observable on macro-scale [5]. similar effect can be observed on micro-scale using xpm method which affects the results and has not yet been properly studied. however, using xpm method on cementitious materials would be very advantageous. generally, hundreds of indents are needed for the characterization of highly heterogeneous composites [6, 7] and saving time would be a huge benefit. on the micro-scale, it is possible to characterize individual chemical phases by scanning electron microscopy (sem) and energy dispersion x96 http://dx.doi.org/10.14311/app.2020.27.0096 http://ojs.cvut.cz/ojs/index.php/app vol. 27/2020 portland cement paste mapping ray spectroscopy (eds). hydrated cement paste is mainly composed of calcium-silica-hydrate (c-s-h gel) existing in two characteristic densities, low (ld c-s-h) and high (hd c-s-h), crystalline calcium hydroxide (ch), residual clinker and porosity [7]. the typical layout of the phases is shown in fig. 1. this study aims to compare both methods on a cementitious composite, highlight their advantages and quantify the strain-rate effect of xpm method with rate dependent visco-plastic constitutive model. 2. materials and methods 2.1. sample preparation a hydrated cement paste from portland cement cem i 42.5r (from mokrá, czech republic) was prepared for testing purposes. cement was mixed with water to cement ratio 0.4, cast into cylindrical molds with height of 32 mm and diameter of 20 mm. the samples were demoulded after three days and stored in the water. 28 days old samples were then cut into 5 mm thick slices on precise cut off machine and dried at 50 °c for one day. the surface of the samples was then grinded with sic papers up to grit size of 4000. finally, samples were put into ethanol into ultrasonic cleaner to remove all loose particles. 2.2. nanoindentation the standard quasi-static nanoindentation test was performed using hysitron ti-700 with berkovich diamond tip. seven grids of 10x10 indents with separation of 12.5 µm were prescribed to cover the heterogeneity of the sample. trapezoidal loading diagram with linear loading lasting 3 seconds up to 2 mn, then by 20 seconds holding period, followed by 3 seconds un-loading were prescribed. xpm measurement was performed on hysitron ti-950. maximum load and the separation between indents were carefully chosen to neglect the influence of individual indents. eight grids with 21x21 indents with separation of 2 µm were prescribed on the sample. the relatively small area of 42x42 µm was covered. thus, each map was prescribed next to the residual clinker to include this less present phase into the results. the testing was force controlled with linear loading lasting 0.1 s up to the maximum force of 1.2 mn, followed by holding and un-loading period both lasting also 0.1 s. 2.3. standard evaluation reduced modulus for both testing methods (grid ni and xpm) was evaluated from load displacement curves using the oliver and pharr method [2]. the method is based on sneddon solution [8] of a rigid indenter action on an elastic half space. the indentation modulus er is defined as er = s √ π 2β √ ac , (1) where s is the elastic unloading stiffness, pmax is the maximum load, β is coefficient which considers different geometry of the indentation tip (β = 1.034 for berkovich tip) and ac is the projected contact area. reduced modulus represents a combination of elastic stiffness of the sample and the tip. modulus of elasticity e, of the sample is given by 1 er = 1− ν2 e + 1− ν2 i ei , (2) where ei and νi are young’s modulus and poisson’s ratio of the indentation tip (for the diamond tip ei = 1141 gpa and νi = 0.07), ν is the poisson’s ratio of the sample (for the cement paste ν = 0.2 ). during holding period it is possible to evaluate short term creep of the material. creep indentation parameter cit is expressed by equation cit(p,t1,t2) = h2 − h1 h1 × 100, (3) which is defined as the relative change of indentation depths h1 and h2 determined between the start and the end holding period at times t1 and t2, respectively. the parameter is dependent on contact force pmax and the duration of holding period. 3. results and discussion 3.1. standard nanoindentation the results of ni gives young’s modulus and hardness maps (fig. 2a) with relatively low spatial resolution. but it still gives valuable information of mechanical properties of individual phases. the range of e values from 14 to 120 gpa and h from 0.4 to 14 gpa were obtained. the frequency density plot of e was constructed from 700 indents, see fig 3. the values of e corresponding to the main peak lie in the range of 23-38 gpa and are mostly composed with ld c-s-h and hd c-s-h gels, the peak of ch was found around 42 gpa and the rest of the indents with young’s modulus higher than 50 gpa belong to residual clinker. these results correspond well with literature [6, 7]. the total time needed for performing one grid of indents (100 indents) was around 500 minutes. 3.2. accelerated property mapping eight maps of mechanical properties were obtained by xpm method. totally 3548 indents were done with very high spatial resolution of 2 µm. the time needed for acquiring one map was about 5 minutes. an example of measured areas by xpm is shown in fig. 2b. young’s modulus and hardness maps corresponds well with sem-bse image. thus, it suits well for identification of individual phases and their boundaries. also, due to high spatial resolution, xpm is capable of identifying individual clinker minerals, red and orange color in fig. 2b with values of e in range of 115-181 gpa. the moduli of other phases were identified as: ld c-s-h (e=30-32 gpa), hd c-s-h (e=40-46 gpa), ch (e=50-56 gpa). these 97 j. němeček, j. němeček, j. lukeš acta polytechnica young’s modulus, e [gpa] 0 20 40 60 80 hardness, h [gpa] a) 90 120 1816030 sem-bse b) 42 μm 112.5 μm 0 1 3 5 7 7 10 1541 figure 2. sem-bse image, young’s modulus, hardness maps measured by a) ni b) xpm. values are higher than the literature results [6, 7]. however, the results correspond well with xpm study of sebastiani et al. [9]. again, the frequency plot of e was constructed, see fig 3. 0 20 40 60 80 100 120 140 160 1800.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 shift young’s modulus, e [gpa] fr eq ue nc y de ns ity [-] xpm ni figure 3. frequency plots of ni and xpm with highlighted shift of the main peak. 3.3. indentation creep parameter cit was evaluated from 20 second holding period of ni technique. the results of individual phases are shown in table 1 and a different parameter was found for each individual phase. the highest value was found for c-s-h gel around 9-12%. also, some deformation was found for ch and residual clinker, where it is well known that these phases do not creep. however, during indentation hard particles of clinker are pressed into surrounding matrix of hydrates, which affects the results. again, cit parameter was evaluated from the holding period of xpm. a very interesting fact was found though: the value of cit was 6.8 ± 0.3% xpm ni phase 1.2mn, 2.0mn, 2.0mn, 0.1s,0.2s 3.0s, 3.1s 3.0s, 23.0s ld c-s-h 6.8±0.3% 1.2±0.3% 11.3±3.1% hd c-s-h 6.8±0.3% 1.1±0.3% 9.5±3.3% ch 6.8±0.3% 0.8±0.2% 6.4±3.7% clinker 5.9±0.6% 0.7±0.2% 2.1±1.9% table 1. creep indentation parameter cit calculated in different phases for xpm and ni. regardless of the indentation phase. however, different indentation forces were used. thus, the values are not directly comparable for xpm and ni. the cit was evaluated for 0.1 s holding period from ni. smaller values with different values were found, even if higher force was used. such findings suggest that during xpm, creep of the material is not correctly measured and the results might be affected by creep of the piezo scanner. 3.4. mutual comparison grid ni brings very low spatial resolution with very good absolute values of mechanical properties of ld, hd c-s-h gel and ch. the cit parameter provides comparison of creep deformation between individual phases. xpm gives higher spatial resolution somewhere close to sem-eds maps. the area covered by individual maps is lower than that for ni. however, time needed for obtaining one map is significantly decreased from hours to minutes. the creep parameter 98 vol. 27/2020 portland cement paste mapping 1 μm 1 μm 0.15 0.13 0.11 0.09 0.07 0.05 0.03 0.02 0 unit: (μm/μm) 0.15422 0.15 0.13 0.11 0.09 0.07 0.05 0.03 0.02 0 unit: (μm/μm) 0.15422 b)a) figure 4. equivalent von mises strain under the tip at the end of loading a) at time = 3 s, speed = 0.67mn/s (ni) b) at time = 0.1 s, speed = 20mn/s (xpm). of material is not obtained from the method. quantitatively, xpm provides data comparable to ni but slightly shifted towards higher values. the shift of 14.8% was calculated as the ratio of young’s modulus of the points with the highest frequency density in the histogram measured by ni and xpm, respectively (fig. 3). such a deviation could be explained by strain-rate effect and was further investigated with numerical modeling. 4. numerical modeling the strain rate effect was studied by finite element method (fem) in ansys software as 2d asymmetrical task with transient dynamic solver. the real shape diamond indenter (elastic isotropic:ei = 1141 gpa and νi = 0.07) was brought to contact with an elastoplastic domain with size of 15x15 µm of the cement sample. loading rate effects were captured by viscoplastic perzyna model [10]. the model calculates equivalent plastic strain as ε̇pl = γ( σ σ0 − 1)1/m, (4) where γ is the material viscosity parameter, σ is the effective stress, m is the strain rate hardening coefficient, σ0 is the static yield point of the elasto-plastic model. two different nanoindentation machines were used and the tip geometry plays an important role during the fitting load-displacement data. thus, the parameters were identified only to best-fit the experimental ni curve (speed = 0.67 mn/s). the parameters were identify as: m = 0.8, γ = 0.025, density = 2000 kg/m3, e = 33.8 gpa, ν = 0.2, σ0 = 300 mpa, tangent modulus = 2000 mpa. after parameters were identified, the same set of parameters was used for xpm loading (speed = 20.0 mn/s). the results of fitting are shown in fig. 5. the vonmises equivalent strain under the tip was analyzed at the end of loading period, see fig 4. it was found the during fast loading, smaller penetration depths and smaller strain are reached compared to ni but shorter loading time is leading to higher 0 50 100 150 200 2500.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 displacement [nm] lo ad [m n ] avg. exp.; speed = 0.67 mn/s model ni; speed = 0.67 mn/s model xpm; speed = 20.0 mn/s experimental data spread figure 5. calculated responses of ni and xpm loading with rate dependent viscoplastic model. strain-rate for xpm method. the results are the following: vonmises strain = 0.1542 (xpm) and 0.2009 (ni), strain rate = 1.542 s−1 (xpm) and 0.067 s−1 (ni). from the above, it is obvious that xpm loading leads to 23x higher strain rate comprared to ni. to quantify the effect of strain rate the oliver and pharr theory [2] was applied on model responses. it was found that e increased by 10 % for xpm mode. both experiment and numerical simulation show the shift of e to higher values. the small difference between numerical modeling and experiment could be given by overlaps of individual phases within the main peak. 5. conclusions this study shows mechanical characterization of portland cement paste with two nanoindentation testing modes: classical quasi-static grid nanoindentation (ni) and accelerated property mapping (xpm). both techniques provide qualitatively comparable results of elastic modulus and hardness. standard grid nanoindentation is favorable for covering larger areas and measuring creep parameters. xpm provides very fast acquisition time of hundreds indents in minutes with very high spatial resolution. the method is very good 99 j. němeček, j. němeček, j. lukeš acta polytechnica in identifying individual phases in a single measured map. also, xpm artificially shifts results to higher values due to the strain rate effect. this shift was quantified as 14.8 % from the experiment and calculated as 10 % according to numerical simulation. in this context, the oliver and pharr theory which is implemented as standard in nanoindenter’s softwares should be used with caution when strain-rate sensitive materials are tested with xpm technique. acknowledgements financial support of the czech science foundation (project 17-05360s) and the grant agency of the czech technical university in prague (sgs20/107/ohk1/2t/11 ) are gratefully acknowledged. references [1] a. c. fischer-cripps. nanoindentation. springer, 2002. doi:10.1007/978-0-387-22462-6. 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[10] p. perzyna. fundamental problems in viscoplasticity. vol. 9 of advances in applied mechanics, pp. 243 – 377. elsevier, 1966. doi:10.1016/s0065-2156(08)70009-7. 100 https://doi.org/10.1007/978-0-387-22462-6 https://doi.org/10.1557/jmr.1992.1564 https://doi.org/10.1007/s11837-018-2752-0 https://doi.org/10.3390/ma10060663 https://doi.org/10.1007/bf02472016 https://doi.org/10.1016/j.cemconcomp.2012.06.015 https://doi.org/10.1016/j.jmps.2006.06.003 https://doi.org/10.1016/0020-7225(65)90019-4 https://doi.org/10.1016/j.matdes.2016.02.087 https://doi.org/10.1016/s0065-2156(08)70009-7 acta polytechnica 27(0):96–100, 2020 1 introduction 2 materials and methods 2.1 sample preparation 2.2 nanoindentation 2.3 standard evaluation 3 results and discussion 3.1 standard nanoindentation 3.2 accelerated property mapping 3.3 indentation creep parameter 3.4 mutual comparison 4 numerical modeling 5 conclusions acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2019.25.0032 acta polytechnica ctu proceedings 25:32–35, 2019 © czech technical university in prague, 2019 available online at http://ojs.cvut.cz/ojs/index.php/app direct measurement of reaction forces during fast dynamic loading jan falta∗, marcel adorna, tomáš fíla, petr zlámal czech technical university in prague, faculty of transportation sciences, department of mechanics and materials, konviktská 20, 120 00 prague 1, czech republic ∗ corresponding author: falta@fd.cvut.cz abstract. the presented paper is focused on embedding of the serially manufactured piezo-electric impact loadcell into open hopkinson pressure bar (ohpb) and it’s modifications for a direct force measurement during dynamic loading. conventionally, during the shpb test dynamic force equilibrium is investigated by a comparison of the transmitted signal wave and the difference between the incident, reflected and transmitted signals measured by strain gauges [1]. however, in the experiments with specimens with low mechanical impedance, a major portion of the incident wave is reflected back on the interface between the bar and the specimen. comparison between two-large amplitude incident and reflected pulse and a small-amplitude transmitted pulse can be influenced by large error and resulting force equilibrium can be inaccurate. therefore, a piezo-electric quartz impact force transducer was used to directly measure the axial forces in the vicinity of the specimen end surfaces. keywords: hopkinson bar, instrumentation, strain-gauge measurement, piezo-electric loadcell. 1. introduction over the past decades, dynamic mechanical testing become widely used method for determination of properties of modern materials (e. g. metal foams, laser sintered structures). reliable recording of the dynamic response of the material during impact testing is therefore essential for obtaining required material properties at high strain rates. the strain gauges are one of the most commonly used sensors in dynamic testing devices (e. g. split hopkinson pressure bar, charpy impact test, drop tower). another way to measure reaction forces can be use of piezo-electric loadcell. in order to obtain reliable data, these sensors must be properly calibrated in a expected range of measured forces starting at 2 kn up to 20 kn. according to fujii, et. al., 2003 [2], there are three methods for calibrating dynamic force transducers. calibrating against oscillation forces, impact force or against a step force. however, all of these methods require expensive equipment and a calibrated reference sensor. this paper presents embedding of the serial manufactured piezoelectric impact load cell into ohpb (open hopkinson pressure bar) and its calibration using two pairs of strain gauges as reference sensors. calibration of the strain gauges can be done using static loading procedure without much difficulty was demonstrated in the previous research [3]. 2. testing setup ohpb, modification of standard shpb apparatus was used for calibration of the force transducer. in this case incident bar was guided by a linear guidance system and was instrumented with strain gauges (see figure 1). the incident bar with length of 1600mm and diameter of 20mm made from high strength aluminum alloy (en-aw-7075) was used. the impact bar was fitted with foil strain gauges 3/120 ly61 (hbm, germany) with active length 3mm wired in the wheatstone half-bridge arrangement at a distance of 200mm from the face of the bar. the nominal resistance deviation of the strain gauge according to manufacturer is 1%. the transmission part consists of two bars of the same material with lengths of 230mm (first bar) and 1470mm (second bar) between which a load cell is mounted. these rods are supported by a low friction polymer housings (drylin series, igus, germany) and instrumented using same strain gauges as on the incident bar. calibration of the strain gauges was carried out by gradual increasing of static load by 100n step. both strain gauges are located at a distance of 60mm from the load cell. since the load cell is not designed to measure tensile forces, and in the standard configuration the tensile wave is reflected at the free end of the transmission bar, a third rod was added to avoid the tensile wave from returning to the load cell and preventing cause possible damage. instrumentation of the load cell and strain gauges can be seen in figure 2. 3. piezoelectric force transducer most of the dynamic force transducers are based on piezoelectric crystals. principle of piezoelectric crystals stands on the generation of electric charge due to compression by the external force [4]. thus, the generated electric charge is converted as a small change in voltage at the output and readout by a corresponding measuring device. the main reason for using of piezo32 http://dx.doi.org/10.14311/app.2019.25.0032 http://ojs.cvut.cz/ojs/index.php/app vol. 25/2019 direct measurement of reaction forces figure 1. ohpb setup scheme figure 2. experimental setup overview electric crystals for dynamic loading measurements is their high natural frequency limit (commonly in order of 100 khz) and quasi-linear behavior over a wide amplitude range (pcb piezotronics, 2012) [5]. however, these sensors are unable to measure static and quasistatic loads or tensile load without prestressing. in this paper, a quartz crystal force sensor type 200c20 (pcb piezotronics, inc.usa) was used in calibration experiments. this sensor is designed to measure compression forces in a range starting at units of n up to 88.96 kn with sensitivity of 56.2mv/kn and upper frequency limit of 40 khz [6]. this type of sensors in general have quasi-linear output above approx. 20% of full scale output (fso) (see figure 3). figure 3. output applied force [5] if the measured forces are lower than this threshold, it is necessary to preload the sensor (quartz) to shift into the linear region. in our case the dimension of thread in the load cell and both bars is 1/4"-28 with tensile strength approx. 10 kn for steel thread and bolt. the 20% load threshold for instrumented load cell is 17.8 kn. preload above this threshold would require the use of a strength screw of class 10.9. or 12.9 which significantly exceeds the strength capacity of the alluminium rod. therefore, it was not possible to applied enough preload force to reach linear output dependency. 3.1. load cell calibration force sensor was mounted between two bars with steel bolt using a threaded steel bolt (see figure 4). two strain gauges were instrumented on adjacent bars in the distance of 60mm (see figure 1). for the load cell figure 4. quartz force sensor arrangement [6] 33 j. falta, m. adorna, t. fíla, p. zlámal acta polytechnica ctu proceedings calibration, 38 void tests (direct impact of the incident bar without sample) were performed in load range of 2 kn 28 kn. the signal measured by the load cell was converted to the unit of force using sensitivity constant (17.241n/mv) given by the manufacturer. the force signal measured by the load cell was compared with signals from the strain gauges and a growing deviation was observed with decreasing applied force. selected waveforms of the measured signals for three different loads of 2 kn, 8 kn and 15 kn are shown in figures 5 7. the correction coefficient was determined as the ratio between the mean value of forces measured by strain gauges and average force measured by the load cell from the plateau area of the deformation pulse. it can be seen that the shape of the load cell signal corresponds with the strain gauges signal and the difference between the load cell and strain-gauge is decreasing with the applied force. the overall signal-tonoise ratio with used instrumentation is significantly lower for load cell signal. figure 5. calibration experiment at low impact force (approx. 2 kn) with deviation corresponding to force ratio coefficient of 1.315 figure 6. calibration experiment at low impact force (approx. 8 kn) with deviation corresponding to force ratio coefficient of 1.143 figure 7. calibration experiment at low impact force (approx. 15 kn) with deviation corresponding to force ratio coefficient of 1.048 in recommended installation, shown in figure 4, the load cell is preloaded with beryllium-copper (be-cu) thread stud which is supplied by the manufacturer. this configuration allows a portion of the force to pass through the stud. according to the manufacturer, the be-cu stud may cause deviation of 5% of the measured force. in our case, steel stud was used which could cause a further increase in deviation. the total deviation is also dependent on the applied preload force. if this force is less than 20% fso, calibration of the load cell is also necessary. 4. results all measured data are plotted in figure 8 as load cell transducer non-linear output dependency on applied force. because of low level of applied preload force, the strong non-linearity was observed throughout the measured range. as shown in figure 3 under 20% of fso the load cell has a strong nonlinear characteristics. in that case it is necessary to set the correction coefficient depending on the applied force to obtain the correct value. one possibility is to divide the nonlinear region into sufficiently small sections and linearize them to obtain the corresponding conversion factor for each of them based on the strain gauge signal. another possibility is to define the function approximating the non-linearity of the measured forces for the required measuring area to determine the corresponding value of the conversion coefficient according to this function. the load cell calibration is strongly dependent on the required range of measured forces with respect to the total measuring range of the load cell and the maximum technically achievable preload force. the situation can be divided into two basic cases: 1. preload force itself or with applied load is more than 20 % of fso output has quasi-linear dependency. one correction coefficient is used. the measurement can be strongly influenced by power transmission through the screw. 34 vol. 25/2019 direct measurement of reaction forces figure 8. measured nonlinearity of the force transducer 2. preload force with applied load is less than 20 % of fso output has non-linear dependency. it is necessary to perform calibration over the entire measured range and to determine the discrete or continuous correction coefficients. the effect of the preload bolt is considerably lower. 5. conclusion the performed experiments show that it is possible to use the ohpb for calibration and subsequent measurement with a piezoelectric force transducer. the load cell accuracy is limited by the type of strain gauges used to calibrate it.therefore, it is not possible to reach higher precision with load cell than with strain gauges. however, the reliability of strain gauges and their service life is strongly influenced by the process and quality of the instrumentation, causing a number of failures especially in high strain-rate tests. in contrast, the load cell exhibits high reliability and does not suffer significantly due to cyclic loading. the load cell signal quality is also substantially higher compared to the strain gauges, and in some measurements it can be critical for a reliable evaluation of the experiment. the main objective is to choose a suitable load cell with respect to the measured range of deformation forces. after selecting a suitable working range of the load cell, it is necessary to ensure that the maximum permissible pressure force will not be exceeded and the reflected tensile deformation waves are eliminated to protect the load cell from possible damage. these requirements can be very difficult to achieve with devices operating on the shpb principle. if a suitable load cell is selected, the quasi-linearity of the output values can be achieved even for measurements of small forces. in cases where it is not possible to use suitable load cell due to the price or installation (max. diameter of preload stud), the methods proposed in this work can be used. acknowledgements the research was supported by the czech science foundation (project no. 19-23675s) and the internal grants of the czech technical university in prague (project no. sgs18/153/ohk2/2t/16 and sgs18/154/ohk2/2t/16). all the financial support is gratefully acknowledged. references [1] t. fila, p. zlamal, j. falta, et al. testing of auxetic materials using hopkinson bar and digital image correlation. epj web of conferences 183:02045, 2018. doi:10.1051/epjconf/201818302045. [2] y. fujii. proposal for a step response evaluation method for force transducers. measurement science and technology 14:1741, 2003. doi:10.1088/0957-0233/14/10/301. [3] j. falta, t. fila, m. adorna, p. zlamal. optimization of hopkinson bar instrumentation for cellular and low impedance materials. engieneering mechanics, 25th international conference, svratka, czech republic pp. 97–101, 2019. [4] d. van nuffel, j. peirs, i. de baere, et al. calibration of dynamic piezoelectric force transducers using the hopkinson bar technique. 2012. [5] b. metz. optimizing 3-component force sensor installation for satellite force limited vibration testing. 28th space simulation conference extreme environments: pushing the boundaries 2014. [6] p. piezotronics. technical sheet of dynamic force transducer of type 200c20, 2019. http://www.pcb.com/products.aspx?m=200c20. 35 https://doi.org/10.1051/epjconf/201818302045 https://doi.org/10.1088/0957-0233/14/10/301 http://www.pcb.com/products.aspx?m=200c20 acta polytechnica ctu proceedings 25:21–24, 2019 1 introduction 2 testing setup 3 piezoelectric force transducer 3.1 load cell calibration 4 results 5 conclusion acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2019.22.0113 acta polytechnica ctu proceedings 22:113–122, 2019 © czech technical university in prague, 2019 available online at http://ojs.cvut.cz/ojs/index.php/app durability of concrete with binder based on sulfocalcic fly ash rostislav šulc∗, matouš krása, petr formáček czech technical university in prague, faculty of civil engineering, department of construction technology, thákurova 7/2077, 166 29 prague 6, czech republic ∗ corresponding author: rostislav.sulc@fsv.cvut.cz abstract. this paper deals with the durability of concrete with binder based on sulfocalcic fly ash. at the beginning, in the theoretical part, there is a research on the frost resistance of concrete, its resistance to chemical thaw substances and measures to extend the durability of the concrete. there is also a part about sulfocalcic fluid fly ash and concrete. practical part is devoted to individual input materials and experimental procedures. there is also described the methodology of the design of the individual mixtures and the results from the tests and their comparison. keywords: fbc fly ash, durability, chemical thaw substances, compressive strength. 1. introduction we often hear topics about the environment, waste management and recycling in the media. the power industry is one of the leading industries in the production of waste materials. it is 13.7 million tons (2014) of the secondary energy product produced in the czech republic in one year [1]. these are wastes from thermal power plants and heat plants, specifically fly ash, slag, ash and products generated during flue gas desulphurization. attempts are currently being made to utilise these products, especially in the construction industry. fluid ash, which as a by-product is produced by fluidized bed combustion, starts to harden with just mixing with water. the work is based on previous publications dedicated to concrete based on sulfocalcic fly ash [2]. the main point is the resistance of these concrete to chemical thaw substances, where concrete of this type did not meet the requirements of the standards. specifically, it is the preparation of a mixture designed for the construction of a concrete road panels in the categories cb i, cb ii and cb iii [3]. 2. concrete resistance against freezing cycles the topic of frost resistance and resistance of concrete against chemical thaw substances is a very actual. individual stages of environmental aggression are defined. it depends on the conditions in which concrete is located and we can simply define the requirements for concrete [5]. among other things, this standard specifies the following requirements for the properties and composition of the concrete mixture: maximal water coefficient, minimal amount of cement, minimal strength class, minimal air pores content and requirements for used aggregate [4]. 2.1. theory of degradation processes due to frost the mechanism of degradation processes is complicated and ambiguous. there are many theories that differ from each other. the cause is the tension which is caused by the increase in volume of changing water to ice. but this statement is very inaccurate. the pore system of the concrete is very complex and consists of a system of macropores (gel, capillary, air) that are differently saturated with water. in these pores water changes in ice at different temperatures. water does not convert to ice even at -30 to -40 °c in some pores. the gradual formation of ice creates a hydraulic pressure, that forces the water into the capillary system. the degradation of the concrete body occurs if the pressure is greater than the tensile strength of the cement matrix. we also have to take consideration of theories based on thermal expansion of aggregate, cement matrix and ice [5]. there are many theories for the degradation process of frost due to chemical thaw substances. it is referred as salt scaling. it must meet the following characteristics to consider any mechanism as the cause of this type of degradation: (a) degradation consists in the gradual removal of small fragments of sealant. (b) worst condition is usage of a solution of ∼ 3% (independent of the type of solution we use). (c) there is no peeling if there is no layer of solution on the surface. (d) degradation does not occur if the minimum temperature is above -10 °c. the amount of damage increases when the minimum temperature drops below -10 °c. (e) aeration improves durability. 113 http://dx.doi.org/10.14311/app.2019.22.0113 http://ojs.cvut.cz/ojs/index.php/app r. šulc, m. krása, p. formáček acta polytechnica ctu proceedings degree of environmental impact xf1 xf2 xf3 xf4 maximal water coefficient w/c [ ] 0.55 0.55 0.50 0.45 minimal strength class c 30/37 c25/30 c30/37 c30/37 minimal amount of cement [kg/m3] 300 300 320 340 minimal air pores content [%] 4.0 4.0 4.0 table 1. requirements for properties and composition of concrete according to en 206 [4]. (f) the salt concentration in the solution layer on the surface is more important than the salt concentration in the pores. (g) susceptibility to salt peeling is unrelated to the susceptibility to internal frost. (h) surface strength determines the ability to resist the peeling of concrete due to salt [6]. the main cause of salt scaling and the mechanism that meets all these characteristics is called glue spalling. this designation is used originally for the glass decoration method, which explains the mechanism of concrete peeling. this method consists of sandblasting and subsequent application of high temperature epoxy. the formed composite is then cooled. it cracks and creates a tensile stress around the perimeter of cracks because of a larger shrinkage of the epoxide. this results in the peeling off thin glass flaps. it is a composite made of concrete itself and a frozen salt solution on its surface in the case of basic concrete. frozen solution starts to shrink five times more than the concrete if the temperature drops below its melting point. the stress results in the rupture of a certain layer of ice. these cracks penetrate the cement sealant and continue parallel to the composite interface. then some of the concrete is peeled off [7]. 2.2. measures to increase the resistance of concrete against chemical thaw substances the simplest measure is not to expose the concrete to such an environment. but this is usually not possible. there are measures that help to increase and protect the concrete from degradation. aerating additives – we increase concrete resistance to chemical thaw substances by aeration. we must keep a spacing factor of less than 300µm to achieve adequate resistance. aeration reduces the bleeding of the concrete and increases surface strength of concrete. bleeding causes a different density of concrete in its cross section. the greatest value occurs at the bottom. these changes in density correspond to the strength changes and concrete has the smallest strength on the surface. the resulting air bubbles will stick to the cement particles and allow them to move upward. initial frost in the resulting air bubbles causes suction in the pore fluid. this suction results in compression of the skeleton of pores, which reduces the stress caused by thermal expansion to one-third. the mixture should have a pores content with a diameter of less than 300µm greater than 1.5% [6–8]. the water coefficient the water factor has a great effect on the durability of the concrete (strength, permeability), decreasing bleeding and increasing strength while decreasing w/c ratio and achieving greater resistance to salt scaling. concrete with low water coefficient (up to 0.3) do not need to be aerated and its bleeding is very small. it corresponds to concrete with w/c = 0.45 and 20% air [6, 8, 9]. a compressive strength of about 40-45mpa should be achieved for aerated concrete to be resistant against chemical thaw substances at a conventional water coefficient of 0.4 to 0.45 [6]. aging conditions the longer the concrete treatment in wet conditions, the higher the strength will be. exposing the concrete to high temperatures for a short time immediately after mixing results in a decrease in resistance to chemical thaw substances [6]. the high temperature during aging of concrete has a negative effect on the resistance of concrete against chemical thaw substances as it reduces its strength [6]. it has been proven in concrete that a longer wet treatment period has a positive effect on resistance [10, 11]. subsequent drying at room temperature has no significant effect. the resistance will deteriorate due to the weakening of the concrete surface if the temperature is higher. this weakening can also cause drying at relative humidity less than 30% [6]. sealing mixtures “sealers” only delays the onset of degradation. in some cases, subsequently resulting in sudden and significant drop off generation [6]. when testing three different types of sealants, the resistance test was only delayed by moisture absorption due to the created hydrophobic surface [12]. the moisture content of the treated sample was almost equal to the untreated after one day of immersion and the difference was zero after three days. even with the use of similar silicone treatment preparations [13]. this corresponds to avoiding the degradation during the first 5 to 10 days thaw cycles, but this does not mean that the treatment agent prevents moisture from entering. as mentioned above, the treatment preparations only postpone the onset of degradation but do not affect the resistance to chemical thaw substances [6]. 114 vol. 22/2019 durability of concrete with binder. . . 3. concrete with binder based on sulfocalcic fly ash ashes are made from the combustion of coal in thermal power plants. they are non-combustible inorganic admixtures. the amount of these materials increases with decreasing quality of coal. different types of ash are distinguished depending on the type of combustion chamber, such as high temperature or sulfocalcic fly ash [14, 15]. 3.1. fluid combustion this is a modern method of combustion based on the fluidization effect. it consists of heating inert material (ash or quartz sand, etc.) to the temperature of combustion of fuel and the flow of air to achieve a fluid state of bulk material. liquid properties are used in fluidized fireplaces, hence transferring heat between the liquid and the heating surface. it results in a better heat transfer coefficient than in other types. different types of fuels can be burned. a small amount of limestone is added to the fluidized bed to prevent formation of so3 and to reduce the exhaust of nitrogen oxides. fluid boilers are divided into atmospheric and pressure vessels. atmospheric is further divided into stationary fluid boilers and circulating boilers [15–17]. 3.2. sulfocalcic fly ash the difference is both chemical and mineralogical between sulfocalcic fly ash and classic fly ash. fluid fly ash contains up to 20 wt.% so3, up to 15 wt.% free high reactive cao, and occasionally exhibits greater annealing losses of up to 15 wt.%. when the sulfocalcic fly ash is mixed with water solidification and hardening occur due to the presence of anhydrite and free lime. sulfocalcic fly ash cannot be evaluated according to en 450-1 or en 450-2 due to non-fulfilment of the characteristics on fly ash used in concrete and most technical criteria [18]. 4. concrete with binder based on sulfocalcic fly ash experiments were carried out on concrete in which fly ash was used as a binder [2]. specifically, it was a sulfocalcic fly ash from the tisova power plant. part of the series had a binder composed of sulfocalcic bed and fly ash, referred to as eth10. as a filler was used aggregate composed of three fractions (0-4, 4-8, 8-16). mixtures were not aerated and plasticizing additive glenium ace 40 was used in a ratio of 1.575% of fly ash to achieve better consistency. testing was performed on 100mm cubes. strength compressive strength increases with longer grinding time. between the non-milled binder and the 45-minute milling was a significant difference of 27.38mpa, as shown in the table (tab. 2) [2]. resistance against chemical thaw substances – a comparison of two series was made. these series differed only in the water coefficient (b-fpc 57 w/c = 0.4575, b-fpc 48 w/c = 0.4025). with a decreasing water coefficient, the resistance to chemical thaw substances increases, despite they achieved almost the same strength [2]. the values of the resistance of the series using eth10 binders (b-fpc: 57, 58, 59, 64) are recorded in the following table (tab. 3). as can be seen, requirement of standard for waste generation 1000 g/m2 at 75 cycles for cb ii or 100 cycles for cb i does not meet any mixture. among other things, with a longer time of grinding the amount of waste decreases. 5. experimental procedures this part will describe the used materials and their properties and the design methodology and composition of individual samples. 5.1. materials aggregate consisting of three fractions is used is a filler: 0/4, 4/8, 8/16. aggregate consists from sand component originating from dobrin and coarse fractions of crushed aggregate, which are from the zbraslav quarry. the percentage of individual fractions was determined using the kennedy bolomey curve [20]. due to the appreciable absence of the sand fraction in the aggregate, the adjustment of the ratios was made even though the aggregate curve crosses the standard centre line (fig. 1). the resulting ratios are listed in the following table (tab. 4). sulfocalcic fly ash fly ash comes from ledvice coal-fired power station. this is a bed fluid fly ash. the sample of non-grinded fly ash was sent to the xrf laboratory (tab. 5). the order of representation of the individual elements is consistent with the eth10 ash used in the previous work. percentage is different up to 4%. the biggest difference is in the sio2 content, which is 3.81% less in eth 10. additionally, eth 10 has 2.67% more tio2, 2.4% more so3 and 2.26% less cao. the remaining elements do not differ by more than 2% [2]. the ash was ground for 45 minutes in the tumbling mill 0m20f consisting of a rotating drum and grinding charge. in this case, the grinding charge is a mixture of spheres of different diameters (�17mm 20 kg, �30mm 20 kg, �50mm 20 kg) with a total weight of 90 kg. the mill is controlled by a computer, where the rotating speed of the drum (rpm) and the time for grinding and emptying are set on the touch screen [21]. a sample was taken from the non-ground ash ele lo 0 and the ground mixture ele lo iv to determine granulometry (tab. 6, fig. 2, fig. 3). eth 10 fly ash, used in previous experiments, had d50 = 230.6µm in the non-ground state. a value is less than that of non-grinded ele-lo fly ash, which had d50 = 275.4µm. as well as for the ground fly ash. for eth 10 – iv value of d50 was 9.3µm and ele lo iv only with d50 = 14.1µm [2]. ingredients superplasticizing and aerating additives were used in the experiment. glenium ace 40 115 r. šulc, m. krása, p. formáček acta polytechnica ctu proceedings series binder compressive strength 28 days [mpa] grinding time [min] water coefficient [ ] b-fpc 57 tisova eth 10 62.9 45 0.458 b-fpc 59 tisova eth 10 35.5 0 0.460 b-fpc 64 tisova eth 10 54.9 15 0.458 table 2. features of the b-fpc series 57, 59, 64 [19]. series sample age [days] chemical thaw substances resistance 25 cycles | 50 cycles | 75 cycles | 100 cycles [g] [g/m2] [g] [g/m2] [g] [g/m2] [g] [g/m2] b-fpc 57 153 50.0 2873.4 102.9 5475.4 b-fpc 58 146 14.2 770.1 29.8 1616.2 42.4 2299.6 81.7 4431.1 b-fpc 59 139 189.5 9036.7 471.4 22479.7 b-fpc 60 139 70.2 2575.4 174.5 6401.8 b-fpc 61 118 88.7 2914.9 202.0 6638.2 b-fpc 62 111 21.6 945.3 41.7 1824.9 70.6 3089.6 81.8 3579.7 b-fpc 63 111 362.0 12387.1 694.4 23829.7 b-fpc 64 104 89.7 3860.6 212.4 9141.4 table 3. results of the chemical thaw substances resistance tests [19]. figure 1. design of aggregate according to kennedy bolomey curve. fraction of aggregate quantity [%] 0-4 42 4-8 18 8-16 40 table 4. aggregate composition. ele lo 0 sio2 al2o3 cao so3 fe2o3 tio2 ∑ 34.16% 27.97% 21.48% 7.89% 3.94% 2.56% 98.00% table 5. xrf chemical composition of fly ash ele lo. r = 50% ele-lo-0 ele-lo-iv dr [µm] 275.374 14.067 table 6. granulometry of the sample and ground sample. 116 vol. 22/2019 durability of concrete with binder. . . figure 2. fly ash granulometry – frequency of particle size. figure 3. fly ash granulometry – passing. superplastifying admixture from basf is composed of polycarboxyether polymers. it accelerates the hydration of cement due to its molecular alignment. the recommended effective dosage is from 0.6% to 1.0% of the weight of the cement. specific dosages are chosen depending on the requirements for processability, type of cement and production technology. glenium ace 40 is applied during mixing, ideally after all ingredients and at least 80% of the mixing water have been added [22]. the mischől lp 78 (now referred to as master air 178) is a synthetic aerating additive. it reduces the surface tension of the mixing water. it achieves good plasticisation and better cohesion of the mixture. the dosage depends on the desired workability, the type of cement, the production technology. the effective dose ranges from 0.05% to 1.5% [23]. 5.2. optimizing aeration additives the amount of aeration additives was optimized in series no. 10. a total of 6 mixtures with different ingredient contents were made. aeration were from 0 to 2.5% and the dosage was increased by 0.5%. the mischől lp 78 was used as an aerating ingredient. the individual mixtures were identical. the dose of aerating ingredient was only thing that changed. the partial composition of each mixture is given in the table below (tab. 7). the series no. 12 was selected as a second mixture for the subsequent chemical thaw substances resistance testing. the aeration was carried out in the same way as in the 10th series. variable is again a dose of aerating additives. there is a lower water factor compared to series no. 10, but it is used 0.5% more plasticizing additive. we can compare the effects of these parameters to the final test values. the composition of individual mixtures in series no. 12 is given in the next table (tab. 8). 6. results and discussion this part is devoted to the results of the different series and their mutual comparison, depending on the different composition and properties. these are the results of concrete slump test (consistency), air content in fresh mixture, compressive strength, and resistance of concrete based on sulfocalcic fly ash against chemical thaw substances. 6.1. consistency this test is based on en 12350-2. the results of slump tests are shown in the following graph (fig. 4). the consistency of series no. 10 is as expected. the slump is growing with the increasing amount of mischől lp 78 additive. the steep slope of the curve at the beginning from 0 to 1% is interesting, especially between 0.5% and 1%, where the 105mm gap is observed. series no. 12 also show this increasing tendency, but there is an abnormality in mixture with an aerosol content of 1.5%. the slump drops from 190mm to only 90mm and then increases again to 240mm at 2%. this special phenomenon has not been explained. it was originally thought to be due to a dry mixer. the mixture was therefore prepared again, but similar slump values were achieved. a greater difference in consistency is achieved with the gradual dosing of the aerating additive in a series with a greater water coefficient and a lower dose of plasticizer. 6.2. air content in fresh mixture to determine the air content in the fresh concrete mixture, a pressure gauge method was used, the procedure of which is described in the standard en 12350-7. this method is based on a known volume of air and its pressure, which is connected in a sealed container with the volume of air in the concrete mixture, which is unknown [24]. the results from the air content measurements in the fresh concrete mixture are shown in the following graph (fig. 5). 117 r. šulc, m. krása, p. formáček acta polytechnica ctu proceedings sample aggregate fly ash water glenium ace 40 mischől lp 78 air content slump 0-4 4-8 8-16 elelo-iv [kg] [kg] [kg] [kg] [l] [kg] [kg] [%] [mm] 10_0 13.749 5.892 13.094 8.4 3.864 0.126 0.000 1.8 30 10_0,5 13.749 5.892 13.094 8.4 3.864 0.126 0.042 2.8 75 10_1 13.749 5.892 13.094 8.4 3.864 0.126 0.084 3.7 180 10_1,5 13.749 5.892 13.094 8.4 3.864 0.126 0.126 3.9 200 10_2 13.749 5.892 13.094 8.4 3.864 0.126 0.168 3.3 210 10_2,5 13.749 5.892 13.094 8.4 3.864 0.126 0.210 3.7 220 table 7. composition of mixtures of the series no. 10. sample aggregate fly ash water glenium ace 40 mischől lp 78 air content slump 0-4 4-8 8-16 elelo-iv [kg] [kg] [kg] [kg] [l] [kg] [kg] [%] [mm] 12_0 14.272 6.117 13.593 8.4 3.528 0.168 0.000 2.1 190 12_0,5 14.272 6.117 13.593 8.4 3.528 0.168 0.042 2.9 210 12_1 14.272 6.117 13.593 8.4 3.528 0.168 0.084 4.5 190 12_1,5 14.272 6.117 13.593 8.4 3.528 0.168 0.126 5.6 90 12_2 14.272 6.117 13.593 8.4 3.528 0.168 0.168 5.4 240 12_2,5 14.272 6.117 13.593 8.4 3.528 0.168 0.210 7.6 235 table 8. composition of mixtures of the series no. 12. figure 4. series no. 10, 12 dependence of the amount of aerating additive on slump. figure 5. series no. 10, 12 dependence of the amount of aerating additive on air content. 118 vol. 22/2019 durability of concrete with binder. . . the air content rise with the increasing dose of aerating additive in series no. 10. it increases up to 1.5% and due to the too high flowability of the mixture afterwards the measured air content stop increasing. the air content of the mixture is gradually increasing in the series no. 12 as expected. there was a slight decrease in air content in fresh concrete mixture at the 2% of aerating additive content. the final 2.5% dose reached the highest measured value of all the mixtures produced 7.6% of the air content. 7. compressive strength testing the compressive strength of the cubes (150 × 150 × 150mm) was based on the test procedure in en 12390-3. weighing samples and measuring their height was prior to testing (in the case of samples that increased their volume, all three dimensions were measured). a slider scale was used to calculate the mass of the concrete samples and to determine the surface area [25]. the compression strength results are shown in the graph below (fig. 6). the compressive strength decreases as the amount of aerating additive increases as expected. the figure shows a decreasing trend of both curves. the strength decreases even though the measured air content with the increasing dose of aerating additives did not rise. the reason could be the inaccuracy of a pressure method that did not measure the real air content of the fresh mixture. this theory of inaccuracy of the pressure gauge method is confirmed in the following figure of the dependence of compressive strength on the air content (fig. 7). the samples 10_2 and 10_2,5 were too fluid in the series no. 10., which could also have an impact on low air content and possible measurement inaccuracy. the series no. 12 shows the expected decline in strength with increasing air content. 7.1. resistance to chemical thaw substances testing of samples for resistance to chemical thaw substances was performed by method a, which is described in čsn 73 1326 [26]. the requirement of standard čsn 73 6123-1 for the resistance of concrete against chemical thaw substances in method a is for group cb i 1000 g/m2 after 100 cycles and group cb ii 1000 g/m2 after 75 cycles. all tested samples met both requirements. the measured values were very low, it is necessary to consider measurement inaccuracies. there is seen the effect of the aeration in the case of series no. 10. the sample without aeration additive has the largest wastes. other mixtures are difficult to compare due to too little waste difference. the series no. 12 shows the positive impact of the lower water coefficient compared to the series no. 10. only two mixtures were measured with waste. there was no waste generation after 125 cycles even in the sample without aerating additive. excess doses of mischől lp 78 greater than recommended (1.5%) have a negative effect on resistance against chemical thaw substances. the compressive strengths of the samples were high enough to exceed the requirements of the c30/37 by standard. the dependence of the amount of waste on compressive strength is not observed (fig. 10). there is no apparent relationship between the air content and the measured waste. high air content of the series no. 12 resulted in greater waste generation. there have been cases where is always one mixture with less or zero waste generation and the other with a higher dose of aerating additives with larger waste generation. both mixtures had almost the same air content. we can speculate about the accuracy of the measurement of the pressure gauge method or about the possible negative effect of the high dose of aerating additive. the dependence of increasing doses and decreasing waste amount cannot be seen in the graph. either no waste or less was measured in samples with fewer aeration additives, than in more aerated mixtures. it is important to find an optimal and effective dose of the additive for the individual mixtures. 8. conclusions the following findings were observed during the optimization. the air content measurements using the pressure gauge method appeared to be inaccurate. in the optimized mix was a requirement for consistency of s4, which is slump from 160 to 240mm. this requirement was met with optimal doses of aerating additives in the resulting series no. 10 and no. 12. with increasing doses of aerating additives, the slump of the mixtures increased. the high consistency of s5 was reached with high doses. the requirement for air content of 4.0% or 4.5% was met only in the series no. 12. the desired value was reached in mixture with 1% of aerating additive. a maximum of air content was 3.7% in series no. 10. the compressive strength was greater than the requirements for concrete of the group cb iii (c25 / 30) and cb i and ii (c30 / 37). similar results were achieved despite the use of fly ash from another power plant. compared to the previous work, where a 63mpa strength of concrete based on sulfocalcic fly ash with a grinding time of 45 minutes was achieved [18]. the resistance against chemical thaw substances by the method a test showed that both series with different aeration levels meet the requirements for concrete. a maximum waste generation of 29.4 g/m2 was achieved after 100 cycles in the series no. 10. the positive effect of the lower water content and the negative effect due to the too high dose of the aerating agent on the resulting resistance were shown. the previous sulfocalcic fly ash eth10 has reached a higher levels of waste generation, although with the same granulometry and the chemical representation of the individual elements, a possible 119 r. šulc, m. krása, p. formáček acta polytechnica ctu proceedings figure 6. series no. 10, 12 dependence of the amount of aerating additive on compressive strength. figure 7. series no. 10, 12 dependence of air content on compressive strength. figure 8. series no. 10 results of the chemical thaw substances resistance tests. figure 9. series no. 12 results of the chemical thaw substances resistance tests. sample sample age [days] chemical thaw substances resistance 25 cycles | 50 cycles | 75 cycles | 100 cycles | 125 cycles [g] [g/m2] [g] [g/m2] [g] [g/m2] [g] [g/m2] [g] [g/m2] 10_0_5 34 0.0 0.0 0.3 9.8 0.75 29.4 0.75 29.4 0.91 35.7 10_0,5_5 34 0.0 0.0 0.0 0.0 0.00 0.0 0.00 0.0 0.00 0.0 10_1_5 34 0.0 0.0 0.04 1.6 0.13 5.1 0.13 5.1 0.13 5.1 10_1,5_5 34 0.0 0.0 0.04 1.6 0.12 4.7 0.12 4.7 0.27 10.6 10_2_5 34 0.0 0.0 0.03 1.2 0.11 4.3 0.11 4.3 0.11 4.3 10_2,5_5 34 0.0 0.0 0.01 0.4 0.04 1.6 0.04 1.6 0.31 12.2 table 9. series no. 10 results of the resistance to chemical thaw substances tests. 120 vol. 22/2019 durability of concrete with binder. . . sample sample age [days] chemical thaw substances resistance 25 cycles | 50 cycles | 75 cycles | 100 cycles | 125 cycles [g] [g/m2] [g] [g/m2] [g] [g/m2] [g] [g/m2] [g] [g/m2] 12_0_5 34 0.0 0.0 0.0 0.0 0.00 0.0 0.00 0.0 0.0 0.0 12_0,5_5 34 0.0 0.0 0.0 0.0 0.00 0.0 0.00 0.0 0.0 0.0 12_1_5 34 0.0 0.0 0.0 0.0 0.00 0.0 0.00 0.0 0.0 0.0 12_1,5_5 34 0.0 0.0 0.0 0.0 0.00 0.0 0.00 0.0 0.0 0.0 12_2_5 34 0.0 0.0 0.2 6.3 0.50 18.4 0.50 18.4 0.6 21.6 12_2,5_5 34 0.0 0.0 0.0 0.0 0.00 0.0 0.00 0.0 0.3 11.8 table 10. series no. 12 results of the resistance to chemical thaw substances tests. figure 10. dependence of the amount of waste after 125 cycles on the compressive strength. figure 11. dependence of the amount of waste after 125 cycles on the air content. figure 12. dependence of the amount of waste after 125 cycles on the amount of aerating additive. 121 r. šulc, m. krása, p. formáček acta polytechnica ctu proceedings reason could be the different ash morphology. for this reason, further testing would have to be done. acknowledgements this study was part of the research project th02020163 "development and industrial optimisation of manufacturing process of construction materials from coal ash for transport construction", fv30062 "possibilities of utilization of coal-ash from power stations stored at stock piles", sgs16/198/ohk1/3t/11 "ternary binders based on fly ash from circulating fluidized bed combustion", and we are grateful for cooperation with performed tests provided by the company basf stavební hmoty česká republika s.r.o., zkušební laboratoř betonu praha. references [1] a. bilodeau, v. m. malhotra. aci special publication sp-132, chap. concrete incorporating high volumes of astm class f fly ashes: mechanical properties and resistance to deicing salt scaling and to chloride-ion penetration, p. 319–349. american concrete institute, detroit, 1992. [2] z. možíš. fpc betony na bázi upravených sulfátovápenatých popílků. master’s thesis, fakulta stavební, čvut v praze, praha, 2017. [3] čsn 73 6123-1, stavba vozovek – cementobetonové kryty – část 1: provádění a kontrola shody. standard, úřad pro technickou normalizaci, metrologii a státní zkušebnictví, praha, 2014. [4] en 206, beton – specifikace, vlastnosti, výroba a shoda. standard, úřad pro technickou normalizaci, metrologii a státní zkušebnictví, 2014. [5] j. dohnálek. vliv mrazuvzdornosti betonu na jeho povrchové úpravy. beton technologie konstrukce, sanace 3:44–47, 2012. [6] j. j. valenza, g. w. scherer. a review of salt scaling: i. phenomenology. cement and concrete research 37(7):1007–1021, 2007. doi:10.1016/j.cemconres.2007.03.005. [7] j. j. valenza, g. w. scherer. a review of salt scaling: ii. mechanisms. cement and concrete research 37(7):1022– 1034, 2007. doi:10.1016/j.cemconres.2007.03.003. [8] d. n. richardson. review of variables that influence measured concrete compressive strength. journal of materials in civil engineering 3:95–112, 1991. doi:10.1061/(asce)0899-1561(1991)3:2(95). [9] t. c. powers. the properties of fresh concrete. john wiley and sons, inc., new york, 1968. [10] r. gagne, m. pigeon, p. c. aiticin. aci special publication sp-126, chap. deicer salt scaling resistance of high strength concretes made with different cements, p. 185–199. american concrete institute, detroit, 1992. [11] m. lanlois, d. beaupre, m. pigeon, c. foy. aci special publication sp-114, chap. the influence of curing on the salt scaling resistance of concrete with and without silica fume, p. 971–989. american concrete institute, detroit, 1989. [12] t. sedrah, t. pigeon, m. hazrati. durability of building materials and components, vol. 6, chap. the influence of penetrating sealers on the deicer salt scaling resistance of concrete, pp. 487–496. e & fn spon, london, 1993. [13] p. klieger, w. perenchio. silicone influence on concrete resistance to freeze-thaw and de-icer damage. highway research record 18:33–47, 1963. [14] p. fečko, m. kušnierová, h. raclavská, et al. fly ash. ediční středisko všb-tu ostrava, ostrava, 2005. [15] m. topinková. anorganická pojiva. fakulta metalurgie a metriálového inženýrství, všb-tu ostrava, ostrava, 2013. [16] j. andreovský. spalování paliv – kotle, odborné vzdělávání úředníků pro výkon státní správy ochrany ovzduší v české republice seminář č. 2. http://www.ekomonitor.cz/sites/default/files/ obrazky/seminare/ovzdusi/seminar2/10_dil_5b_ tisk_andreovsky.pdf, 2013. [17] v. polach. parní kotle. http://slideplayer.cz/slide/6816422/, 2011. [18] výzkumný ústav maltovin praha, spol. s.r.o., svaz výrobců cementu čr. popílek a jeho použití do betonu. http://www.vumo.cz/wp-content/uploads/2015/05/ popilek_a_jeho_pouziti_do_betonu.pdf, 2002. [19] en 12350-2, zkoušení čerstvého betonu – část 2: zkouška sednutím. standard, úřad pro technickou normalizaci, metrologii a státní zkušebnictví, praha, 2009. [20] en 206-1/z3, beton – část 1: specifikace, vlastnosti, výroba a shoda. standard, český normalizační institut, praha, 2008. [21] brio hranice s.r.o. omílací buben 0m20f. machine information system. [22] basf stavební hmoty česká republika s.r.o. glenium ace 40, technický list, č. 630, 2007. [23] basf stavební hmoty česká republika s.r.o. master air 178, technický list, 2015. [24] en 12350-7, zkoušení čerstvého betonu – část 7: obsah vzduchu – tlakové metody. standard, úřad pro technickou normalizaci, metrologii a státní zkušebnictví, praha, 2009. [25] en 12390-3, zkoušení ztvrdlého betonu – část 3: pevnost v tlaku zkušebních těles. standard, úřad pro technickou normalizaci, metrologii a státní zkušebnictví, praha, 2009. [26] čsn 73 1326, stanovení odolnosti povrchu cementového betonu proti působení vody a chemických rozmrazovacích látek. standard, vydavatelství úřadu pro normalizaci a měření, praha, 1984. 122 http://dx.doi.org/10.1016/j.cemconres.2007.03.005 http://dx.doi.org/10.1016/j.cemconres.2007.03.003 http://dx.doi.org/10.1061/(asce)0899-1561(1991)3:2(95) http://www.ekomonitor.cz/sites/default/files/obrazky/seminare/ovzdusi/seminar2/10_dil_5b_tisk_andreovsky.pdf http://www.ekomonitor.cz/sites/default/files/obrazky/seminare/ovzdusi/seminar2/10_dil_5b_tisk_andreovsky.pdf http://www.ekomonitor.cz/sites/default/files/obrazky/seminare/ovzdusi/seminar2/10_dil_5b_tisk_andreovsky.pdf http://slideplayer.cz/slide/6816422/ http://www.vumo.cz/wp-content/uploads/2015/05/popilek_a_jeho_pouziti_do_betonu.pdf http://www.vumo.cz/wp-content/uploads/2015/05/popilek_a_jeho_pouziti_do_betonu.pdf acta polytechnica ctu proceedings 22:113–122, 2019 1 introduction 2 concrete resistance against freezing cycles 2.1 theory of degradation processes due to frost 2.2 measures to increase the resistance of concrete against chemical thaw substances 3 concrete with binder based on sulfocalcic fly ash 3.1 fluid combustion 3.2 sulfocalcic fly ash 4 concrete with binder based on sulfocalcic fly ash 5 experimental procedures 5.1 materials 5.2 optimizing aeration additives 6 results and discussion 6.1 consistency 6.2 air content in fresh mixture 7 compressive strength 7.1 resistance to chemical thaw substances 8 conclusions acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2020.29.0001 acta polytechnica ctu proceedings 29:1–5, 2020 © czech technical university in prague, 2020 available online at http://ojs.cvut.cz/ojs/index.php/app the oldest underground railways in the world and their anniversaries jiří barták czech technical university in prague, faculty of civil engineering, department of geotechnics, thákurova 7, 166 29 praha 6, czech republic correspondence: bartakj@fsv.cvut.cz abstract. the five oldest underground railways started to be operational in the 19th century. these were the metros in london, athens, istanbul, budapest and glasgow. undoubtedly, world primacy of status in terms of age belongs to the london underground. however, the age ranking of the others in continental europe is more difficult to determine. regardless of this criteria, the undergrounds of the 19th century became the basis of the world’s most widespread and rational means of public transport. the further development of these named oldest undergrounds is here documented. keywords: metro anniversary, metro establishment, underground railway. 1. introduction the nineteenth century is often characterised, somewhat figuratively but justifiably, as a century of steam. in 1825, george stephenson, an english mechanical engineer, started the public operation of a train pulled by a steam locomotive along a track between stockton and darlington. his locomotive, rocket, and newly invented hot rolled rails passed successfully into technical operation. in 1830, an indication of the future use of the underground for transportation purposes appeared on the manchester – liverpool rail track which was the first railway tunnel on a track using steam trains. this was the 1,066m long edge hill tunnel. numerous fundamental things required for the commencement of operations in the underground (steam locomotives with the necessary power, railbound passenger cars, rolled rails and their fixing, design of trackwork and other technical equipment) were tried on railway tracks. thus, the conditions for the creation of an efficient urban mass transit system – the underground railway – had been prepared. world leadership in the development of the highestcapacity and most widely spread urban mass transport is undisputable – the first six kilometres long line of the london underground went into service on 10th january 1863. on continental europe however there is a certain problem with the determination of the first metro construction project – there are three candidates. arranged chronologically, they are the metros in athens (1869), istanbul (1874) and budapest (1896). the instigation and subsequent development of metro networks in those cities exhibits noteworthy particularities and differences, making it relatively difficult to award the second place to one of them unambiguously. nevertheless, it is a straightforward fact that celebrations of the original creation of the first metro in continental europe are held in all of the three countries. 2. london underground the first great exhibition was held at crystal palace in london in 1851. the possibility of developing an underground railway was first mentioned at the exhibition. the preparatory work continued, and a company commercially registered under the name of metropolitan commenced constructing the london underground by connecting the end stations of two railway lines – the great western railway and the great northern railway. the basis of the london underground was formed by developing a 6km long section between paddington station and the farringdon street railway station ([1], [2], [3]). with respect to an operation using steam engines and the limited capacity of ventilation through chimneys, the grade lines of tunnels were designed to be at small depths (4 to 8 metres). horseshoe-shaped profile double-track tunnel linings were constructed on wood centrings in pits dug in the space of streets and were backfilled after completion (see fig. 1). figure 1. historic subsurface tunnels of the london underground [3] the first line in the world using wooden cars pulled 1 http://dx.doi.org/10.14311/app.2020.29.0001 http://ojs.cvut.cz/ojs/index.php/app jiří barták acta polytechnica ctu proceedings by a locomotive was brought into service with great success in london at the beginning of 1863 (10th january 2018). the first day of operation was attended by 38,000 visitors, and later it was used on average by 25,000 passengers every day. given the extraordinary attendance rates and the interest of citizens in this new mode of transport, this subsurface variant of the london underground using steam engines was considered for further extension. however, it was only after the introduction of electrification into the underground in 1890 and the use of an innovated tunnelling shield with a circular profile that it became possible to successfully drive couples of single-track tunnel tubes with a diameter 3,6m through deeper parts of london. the connection of the 1st line of the london underground with the legendary thames tunnel is very interesting. the thames tunnel was constructed under the river in muddy and sandy alluvium by civil engineer marc isambard brunel, who, in 1918, took out a patent for a completely new tunnelling method of excavation and supporting the excavated space by means of the world’s first tunnelling shield, which became much later on the basis of tunnel excavation technologies using tunnel boring machines for all types of rock and soils. the thames tunnel, which was awarded the status of a technical monument, was meticulously reconstructed and, since 2010, has become part of the london overground railway network, which is an extensive system of commuter railways serving a significant part of london and its regional environs. the current london underground has 11 lines with 292 stations. with its length of circa 408km it was the world’s longest underground railway system until 2012. trains corresponding to common railway transport services are operated on the subsurface lines; specially adapted trains are used on deep subsurface lines, in single-tube tunnels with the diameter of only 3.6m (see fig. 2). outside of central london, the underground trains run along deep-seated lines at speeds up to 96km (60miles) per hour. figure 2. cars for subsurface and deep-seated london underground lines [3] 3. the metro in athens in 1969, the 100th anniversary of opening the second underground railway in europe and also the first one in the continental part of europe, (connecting the port of piraeus with the greek capital), was celebrated in athens. the circumstances of the instigation and development of this line have often raised doubts over whether it is really a metro; nevertheless, greeks (and many other people) consider it to be the fully-fledged germinal source of the later athens metro system. at about 10km long, at-grade single-track railway line was constructed during the course of two years on the basis of a contract with edward pickering, an english businessman ([2], [4]). it was inaugurated on 27th february 1869. it connected the railway station in piraeus with the thiseio railway station in athens. during the ceremonial beginning of this service supported by the state, and attended by queen olga and the prime minister, a steam locomotive with a train set consisting of ten cars covered the distance between the two cities in 15 minutes. the line was extended to the inner city, omonia square, by a subsurface section in 1893, and was electrified in 1904. the at-grade portion stretch of the line still predominated. the line waited for subsequent changes until 1930 (doubling of the track – see fig. 3), later for the extension of the track to attiki station in 1948; and the whole route up to the kifisia end station was brought into service in 1957 as the first metro line. the length of this single line 1 route amounts to 25.6km, with a 3km long section running underground. only after a subsequent 54 years (in 2011), was line 1 incorporated into the athens underground railway system (the 150th anniversary of the commencement of operation will be celebrated in 2019). figure 3. original athens metro line after doubling of the track [4] currently athens metro (aττικó mετρó ) has 3 lines (see fig. 4) with an aggregate length of 84.5km. the work on the underground lines 2 and 3 commenced as late as 1991. the initial sections were brought into service in 2000 and were further extended; the current length of the lines amounts to 58.9km. the construction of a line 4 is being planned. it should have a circular character. line 1 with its historic piraeus – 2 vol. 29/2020 the oldest underground railways in the world thiseio – omonia square section was operated until 2011 separately from the newly developed metro system; now it is a part of it. 4. the metro in istanbul the only underground line was inaugurated on 23rd september 1875. it is 573m long. the 140th anniversary of bringing the second metro in continental europe was held in turkey in 2015 ([5], [4]). however, it raises rather legitimate doubts whether it can be considered strictly to be a metro. in this case it is obvious that it is not really the original start point of the future metro because of the fact that it has not been incorporated into the modern istanbul metro system. nevertheless, one primacy of status cannot be denied to this “metro” – it is the shortest one in the world. this solitary underground line, which is currently known by the name of tünel, was designed by henri gavand to be provided with a rope drive. it allowed for a more more comfortable overcoming of the sixtymetre difference in elevation with a steep rise of the surface between karaköy and beyoğlu, two city districts lying at the european corner of a bay on the aegean sea. the line has only two stations, boarding (see fig. 4) and disembarking, both with side platforms. a double-car train set covers the distance between the stations in 1.5 minutes, running at a speed of circa 25km per hour. the single-rail track has a passing bay in the middle of the route. the car undercarriages are on rubber tyres and move along a concrete track. figure 4. karaköy station of istambul metro [3] in 1910, the line was electrified and between 1968 and 1971 it was significantly modernised. the line has been incorporated into the istanbul at-grade transportation system. it links to a common tramway track at the boarding station (itself linking to the karaköy port); the disembarking station links to the historic tramway track (transfer to line m2 of the classical metro being also possible). the line carries a daily average of 15,000 passengers. the current istanbul metro (istanbul metrosu), which started operating on 3rd september 1989, had eight lines with an aggregate length of 105.8km and 82 stations in 2018. there are 6 classical metro lines marked as m1 through to m6 and two lines of the light rail transit system marked as t1 and t2 also in operation. the system is undergoing substantial further extensive development – 5 additional lines with 62 stations are under construction. the metro is connected with the taksim-kabataş underground funicular (2006) and the trans-bosporus marmaray undersea railway tunnel (2013). 5. the metro in budapest while the budapest metro ([2], [6], [4]) was brought into service as late as 2nd may 1896, it must be considered that, in terms of its actual attributes, it unambiguously meets the qualification requirements for second place among the european metros, compared to the earlier operating structures in athens and istanbul. it was inaugurated 33 years after the london underground. in 2016, the 120th anniversary of the metro was celebrated in hungary. in 1896, the volume of traffic under andrassy avenue, the main link of central pest to the rapidly growing new residential development more remote from the left bank of the danube, reached an unacceptable level. the solution was found in constructing an underground rail line which was, with respect to its character, rightly named as kisföldalatti (ie small underground space), and with time this became abbreviated simply to földalatti). the commencement of construction work and its rapid continuation was probably significantly affected by a more obviously political and historic aspect – the construction of the metro and the bringing of it into service was simultaneously conceived as a celebration of the thousand-year anniversary of the arrival of hungarians to pannonia. the contract for the construction was awarded to the budapest railway company under the condition that it would be completed before the millenium celebrations. the railway company fulfilled the condition and the so-called jubilee line, 3.7km long with 11 stations, was brought into service in may 1896, only 20 months after the commencement of the construction work. virtually the whole line, with the exception of a short at-grade section at its end, was constructed in sloped or braced pits (see fig. 5), fast completion being therefore necessary even from the viewpoint of surface transportation. the framework structure of the reinforced concrete tunnel lining was minimised to the dimension of circa 3 × 4m, and the height of the overhead catenary was only 2.3m above the top of the rail. the interior design of the stations was subject to quite demanding conditions set by the budapest city hall. the art nouveau style was chosen. the walls of the stations were clad with white and black tiles, while cast-iron columns along the edge of the short platform (circa 40m long) were provided with decorative heads, 3 jiří barták acta polytechnica ctu proceedings figure 5. földalatti under andrassy avenue [6] figure 6. kossuth lajos square five-vault station on line m2 [2] rest benches and cashier kiosks designed in the same style (see fig. 6). the structure of entrance staircases leading to stations was noteworthy – the treads were unusually wide, and risers were low (even entering on horseback was reportedly possible for officers). metro cars were wooden; before their modification for the underground they were originally intended for tramway services. the first very necessary modification took place as late as the 1930s, when, in particular, the power of engines was increased, doors of the cars were reconstructed and the traction voltage was changed. however, the belgian-produced wagons were operated until the 1960s. the földalatti line is newly identified as m1. it remained a sole provision in budapest until 1970, when line m2 of the modern metro was brought into service. the current budapest metro (budapest metró) has experienced an interesting evolution. the construction of line m2 commenced in 1950; however, the work was suspended after three years and the building capacities were shifted in the direction of new residential development. until 1963, the development of the metro was discontinued, and the finished tunnels and stations were used as stores. after the resumption of the works, line m2 was inaugurated on 2nd figure 7. kossuth lajos square five-vault station on line m2 [2] april 1970. the stations on line m2 partially copied soviet-type 3-vault stations. but a new concept of budapest-type stations was also instigated (fiveto six-vault stations – see fig. 7). the budapest metro network is currently formed by 4 lines (m1 through to m4) with an aggregate length of 40km and 52 stations; the development of a line m5 is now under preparation. 6. the metro in glasgow the fourth metro, the last one brought into service in the 19th century, is the metro in glasgow, and the second on the british isles ([1], [4]). it went into service on 14th december 1896, in the same year as the metro in budapest, but seven months later. in terms of the actual system, the glasgow metro is a certain curiosity, but there are no doubts that it belongs to the category of underground railways. it is formed by only one 10.5km long orbital line, which was never expanded during following years. projects to expand it existed in several versions but none of them was realised, probably with respect to extensive mine galleries (in large part inundated) under the city surface. the glasgow orbital metro (officially named the glasgow subway, in guidebooks even referred to by a nickname „clockwork orange“, see fig. 9) is a double-track system, with each track running through a separate tunnel tube. the diameter of the tubes is probably unique in the world – only 3.35m (11 feet); the track gauge is also atypical – 1,219 mm (4 feet). figure 8. glasgow orbital metro system [4] in terms of technology, the metro was designed in a very original way. trains were hauled by endless cables powered from a steam power plant. on the inner circle, 4 vol. 29/2020 the oldest underground railways in the world the car sets moved in an anti-clockwise direction; on the outer circle, they moved in the opposite direction. all 15 stations were solely equipped with intermediate platforms. for that reason the metro cars had doors only on one side. there were no cross-over tunnels with switches between the circles; no pull cable for traversing the cars to a depot was installed. the cars had to be lifted from the metro route with a crane to be carried to the depot. the cable drive was removed as late as 1935, when the metro was electrified. current glasgow metro (glasgow subway) is, in terms of the extent, the same as it was 123 years ago – a length of 10.5km and 15 stations. in 1977, it was significantly modernised, equipped with new car sets, a connection with the new depot was installed and electrification of the system was modernised. the minimum diameter of the tunnel tubes (see fig. 10) and the reduced track gauge was preserved. a welldeveloped proposal for metro expansion, using the already mentioned old mine galleries under the city surface and a relic of historic coal mining, has existed since 2005. figure 9. current glasgow subway – kelvinhall station [4] in 2016, the glasgow transport agency issued a report that the metro operation system would be reconstructed in the coming years to an unmanned system with the cbtc (communication based train control) system of traffic control and signalling. 7. conclusion all five structures mentioned in this paper originated in the second half of the 19th century. by their mostly successful operation, they clearly showed the path that most rational solutions to urban mass transit in large metropolises would take – underground railways. despite the fact that the development of these oldest five metros was significantly variable, the metros have usually become bases of extensive networks of underground lines (london, athens, budapest) or have initiated further development of the metro (istanbul), or exceptionally the original arrangement of the metro has remained as a final solution until the present day (glasgow). it is necessary for the sake of fair and complete information to mention the urban underground railway in wien (vienna) [4], where operation actually commenced in 1898 even though it did not have the official status of a metro. a decision was made in the 1960s that the obsolete and unsatisfactory urban railway system would be reconstructed to metro (u-bahn) and a rapid transit system (s-bahn). the reconstruction of this already inadequate urban railway system into a metro system then commenced in 1969. references [1] j. streit. tunely všech dob a světadílů. publishing house karel synek, 1. edition, praha 1947. 130 pages. [2] j. křivánek, z. šmíd, j vítek. všechna metra světa. nakladatelství dopravy a spojů, 1. edition, praha 1986. 208 pages. [3] london. https://en.m.wikipedia.org/wiki/london. [4] metro. https://cs.m.wikipedia.org/wiki/metro. [5] j. straka. podzemní stavby. nakladatelství technické literatury, 1. edition, praha 1966. 442 pages. [6] u-bahn museum budapest. universitätsdruckerei, budapest 1977. 40 pages. [7] https://wikimediafoundation.org. 5 https://en.m.wikipedia.org/wiki/london https://cs.m.wikipedia.org/wiki/metro https://wikimediafoundation.org acta polytechnica ctu proceedings 29:1–5, 2020 1 introduction 2 london underground 3 the metro in athens 4 the metro in istanbul 5 the metro in budapest 6 the metro in glasgow 7 conclusion references acta polytechnica doi:10.14311/app.2020.27.0090 acta polytechnica 27(0):90–95, 2020 © czech technical university in prague, 2020 available online at http://ojs.cvut.cz/ojs/index.php/app application of steel fibres in alkali-activated mortars götz hüskena,∗, lars-christian wagnera,b, gregor j. g. glutha, stephan pirskawetza, hans-carsten kühnea a federal institute for materials research and testing (bam), department of safety of structures (dpt.7), unter den eichen 87, 12205 berlin, germany b technische universität berlin, group of building materials and construction chemistry, gustav-meyer-allee 25, 13355 berlin, germany ∗ corresponding author: goetz.huesken@bam.de abstract. alkali-activated materials are ideal for the repair of concrete structures in harsh environmental conditions due to their high durability in chemically aggressive environments. however, slag-based mortars, in particular, are prone to shrinkage and associated cracks. in this respect, the application of steel fibres is one solution to reduce the formation of shrinkage induced cracks and to improve post cracking behaviour of these mortars. this study investigated the influence of two different types of steel fibres on the tensile properties of two alkali-activated mortars. direct tensile tests and single fibre pull-outs were performed to analyse the determining failure modes both on macro and micro scale. mechanical testing was accompanied by non-destructive testing methods such as digital image correlation and acoustic emission for a detailed analysis of the fracture process. keywords: alkali-activated materials, fibre pull-out, steel fibres, tensile strength. 1. introduction alkali-activated materials (aams) have been an extensively investigated topic in material sciences as they are a suitable alternative to systems based on ordinary portland cement (opc), in certain applications. much of the research covers design aspects using different precursor materials (such as fly ash or slag) and suitable alkaline activators (for example sodium silicates or sodium hydroxide) and how these variables influence mechanical and durability properties. these investigations show that aam, depending on their varying composition, can meet or even surpass properties of opc concretes [1–3]. nevertheless, aam often tend to be prone to higher shrinkage and more brittle behaviour than opc concretes [4]. fibres, such as steel, glass, synthetic, and natural fibres, within the concrete matrix have often been used to increase ductility and to reduce the formation of shrinkage induced cracks. adding fibres to the brittle matrix improves the post-cracking behaviour of the composite material significantly as the fibres connect the crack edges after cracks are formed. the application of fibres is usually limited to plastic fibres to reduce effects resulting from the formation of shrinkage cracks, but steel fibres are also of special interest for the application in aam [5]. besides reducing the formation of shrinkage induced cracks, the bond behaviour between steel fibres and the surrounding aam matrix is in some cases stronger than in opc concretes [6]. however, this effect is not yet fully understood. the application of steel fibres up to 2 % to 3 % by volume significantly increases both flexural behaviour and fracture energy of the aam composite, which is mainly caused by the high tensile strength and ductility of the fibres [7]. single fibre pull-out tests are a suitable test method to characterize the bond strength between steel fibres and the surrounding matrix [8]. these tests are common for opc steel fibre-reinforced concretes but are seldom used for characterizing the fracture behaviour of aam. steel fibre-reinforced aams prove to be alternatives to steel fibre-reinforced opc in fields like repair tasks, underground engineering or pre-cast elements [9, 10]. it is thus important to investigate the failure mechanisms of steel fibre-reinforced aam regarding: (i)the influence of the fibre properties such as length, diameter and form on the mechanical properties of aams. (ii)the influence of the (chemical) composition of the aams on the bond between the matrix and the fibres’ surfaces. (iii)how different compositions may lead to different failure modes and post-cracking behaviour. 2. materials and experimental setup two types of aam based on ground-granulated blastfurnace slag (ggbs) and fly ash have been investigated in combination with two different steel fibres. the mortars are referred to as c7a (slag) and fa5 (fly ash), respectively. the composition of the investigated mortars is given in table 1. further details on 90 http://dx.doi.org/10.14311/app.2020.27.0090 http://ojs.cvut.cz/ojs/index.php/app vol. 27/2020 application of steel fibres in alkali-activated mortars the mortars can be found in [11, 12]. the properties of the applied steel fibres are summarized in table 2. material c7a (kg/m3) fa5 (kg/m3) slag/fly ash 574.3 579.7 activator 1 50.8 234.0 activator 2 89.0 71.4 water 176.1 13.3 sand 0.1-0.5 400.6 402.0 sand 0.5-1 328.1 239.4 sand 1-2 374.2 239.4 sand 2-4 220.3 452.1 activator 1 sodium silicate solution with a modulus of 2.1 (‘grade d’) activator 2 naoh solution. table 1. mortar composition. property fibre 1 fibre 2 length (mm) 13 20 diameter (mm) 0.2 0.3 l/d ratio 65 66.7 surface (mm2) 8.17 18.85 tensile strength (mpa) >2 600 >1 200 max. tensile force (n) 81.68 84.82 density (kg/m3) 7 850 7 850 fibres per kg 314 000 90 110 coating brass none table 2. properties of steel fibres used. mixes with a steel fibre content (fibre type 1) of 1 %, 2 % and 3 % by volume were tested for the mortar c7a and 2 % by volume for mortar fa5. the steel fibres of type 2 were added to mortar c7a in such a quantity that their number per volume is comparable to the mix with 1 % by volume (c7a_1%_f1). the direct tensile tests were performed according to the procedure described in [13]. the 56 days old specimens were tested in a displacement controlled uniaxial tension test with a loading rate of 0.0025 mm/s until a total displacement of 1.5 mm was reached. to ensure uniform crack opening, both ends of the specimens were glued in the sample holders fixed to the servo-hydraulic testing machine (see figure 1). two linear variable differential transformers (lvdts) were clamped laterally to the specimen to measure the deformation in the zone with constant cross section. the machine data (force and displacement) were recorded at a rate of 50 hz. the tensile tests were accompanied by digital image correlation (dic) and acoustic emission (ae) analysis. a high-contrast stochastic pattern was applied on the front side of the specimen for the dic measurements. the optical deformation measurement was performed using an aramis 5m system supplied by gom, gesellschaft für optische messtechnik mbh. ae-sensors clamp dic field lvdt figure 1. test setup. the dic system was either triggered by the testing machine and a signal was sent every 0.01 mm of piston displacement to the dic camera or a fast measurement with 20 frames per second was conducted. 4 acoustic emission sensors (2 vs150ms and 2 pac wd) were fixed to the specimen and acoustically coupled using viscoelastic adhesive pads bosticprestik®. the acoustic emission signals were processed and stored by an amsy6 acoustic emission system supplied by vallen systeme gmbh. the detection threshold was set to 37.1 dbae and the bandpass filter was set to 25 khz to 800 khz for all sensors. single fibre pull-out tests were conducted for all fibre-matrix combinations to characterize acoustic emission signals in terms of formation of micro cracks and fibre pull-out. the pull-out tests were performed displacement controlled with a loading rate of 0.0025 mm/s until a pull-out length of 0.5 mm was reached and was then increased to 0.0333 mm/s until total pull-out of the fibre. a detailed description of the sample preparation and further test details can be found in [14]. 3. results and discussion figure 2 shows the averaged stress-strain plots (3 specimens for each mix) for the tested steel fibre combinations. the loading response of the tested mortars can be divided into three characteristic segments. firstly, all samples show a linear-elastic behaviour until the maximum tensile stress (crack stress) is reached. this maximum tensile stress varies between 2.8 mpa to 91 g. hüsken, l.-c. wagner, g. j. g. gluth et al. acta polytechnica 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 t en si le s tr es s (m p a) strain (‰) c7a_1%_f1 c7a_2%_f1 c7a_3%_f1 c7a_3.5%_f2 fa5_2%_f1 figure 2. averaged stress-strain curves. 3.2 mpa for the mortar c7a. fa5 has a stronger matrix and shows with 6.3 mpa a maximum value that is about twice that observed for the tensile strength of c7a. further loading beyond peak results either in a major drop of the tensile stress that stabilizes at a constant low level for low fibre contents or the gradual decrease of the tensile stress for higher fibre contents. the performance of the tested mortars under uniaxial tensile loading can be described according to [15] as strain-softening behaviour (level 1). furthermore, the low deviation of the crack stresses of mortar c7a for all tested fibre contents demonstrates that incorporation of fibres does not increase the tensile strength of the material, but influences the post-critical behaviour beyond matrix cracking due to crack bridging [15]. here, not only the fibre content is an important factor, but also the bond behaviour between the fibre and the surrounding matrix is essential. this fact is expressed by the area under the stress-strain plots beyond peak for mortars with same fibre content (2 % by volume) but different matrix material (c7a and fa5). in this case, the material with higher tensile strength (fa5) seems to have also the better bond between fibre and matrix. further insights into the post-critical behaviour of this composite material can be derived from the results of ae analysis and dic measurements. figure 3 shows as example the results of the ae analysis of specimen fa5_2%_f1_3. in this diagram, the localized acoustic events and the applied tensile force are plotted versus time. furthermore, a picture of the dic measurement is added showing the strain distribution of the cracked specimen at the end of the tensile test. the localized acoustic events have been separated into two groups considering the weighted peak frequency (wpf) as suggested by [16]. the threshold of the wpf was set to 110 khz. this value is based on preliminary tests where a clustering of acoustic events at 70 khz and 150 khz were observed. 0 1 2 3 4 5 6 fo rc e (k n ) wpf > 110 khz wpf < 110 khz st ep 1 96 0 0 100 200 300 400 500 600 time (s) -8 -6 -4 -2 0 2 4 6 8 p os it io n (c m ) figure 3. results of the ae analysis for specimen fa5_2%_f1_3. only few acoustic events have been detected in the uncracked state before maximum tensile stress. this low ae activity is related to the formation of micro cracks and increases rapidly in the upper part of the specimen shortly before crack formation causes a sudden drop in force. after the first crack is formed, the upper part of the specimen shows a high ae activity (wpf ≤110 khz) that indicates the activation of steel fibres due to the formation of micro cracks. starting at 85 seconds, the ae activity increases in the lower part of the specimen, indicating the formation of a second crack. the ae activity near the upper first crack decreases significantly after the second crack is formed. ae events (wpf ≤110 khz), indicating the formation of micro cracks, were mainly detected in the lower part of the specimen until the end, but their number decreases and higher frequency events (wpf >110 khz) dominate the fracture process in the lower part. these higher frequency events indicate fibre pull-out near the second crack as it occurs during crack opening. the observations made by ae analysis regarding crack formation and crack opening are in good agreement with the results of the dic measurements. figure 4 shows the strain distribution of specimen fa5_2%_f1_3 for relevant loading levels (steps). additionally, the corresponding tensile stress (mpa) for each step is given in parenthesis in figure 4. the corresponding steps of the dic measurement are marked in figure 3 by two dashed black lines. the first dashed line near the peak covers the time interval for the formation of the first and second crack, which is represented in figure 4 by the steps 1070, 1071, 1087 and 1088. the second dashed line shows the time interval at step 1960 when mainly crack opening occurs. step 1070 shows the specimen shortly before the peak is reached. all other pictures taken before are similar and do not show any relevant strains. 92 vol. 27/2020 application of steel fibres in alkali-activated mortars step 1088 (5.0) µm/m 30000 24000 21000 18000 15000 12000 9000 6000 3000 0 27000 step 1070 (6.5) step 1071 (6.1) step 1960 (4.5)step 1087 (5.6) figure 4. strain distribution of sample fa5_2%_f1_3. step 1071 was taken 0.05 s later and shows the formation of the first crack indicated by the strain concentration in the upper part of the specimen. the crack is formed from the left side of the specimen to the middle and results in a drop of the tensile stress of about 0.5 mpa. step 1087 was taken 0.8 s later and shows just a minor increase of the strains in the upper part. however, a second crack was formed in the lower part of the specimen, growing from the right side to the middle. step 1088 was taken, again, 0.05 s later and shows the rapid opening of the second crack, whereas the upper crack shows no significant changes in its strain distribution. the formation of the second crack results in a further drop of the tensile stress of about 0.6 mpa. step 1960 shows the strain distribution of the specimen at higher loading level when the tensile stress decreased to 4.5 mpa. the strain distribution in the upper part increases slightly, indicating an opening of the upper crack, but no lateral expansion to the right. the crack opening of the second crack is more dominant as it becomes evident from the larger increase in the strain distribution. moreover, the lower crack runs through the entire front surface of the specimen. single fibre pull-out tests were conducted to get further insights into the fracture behaviour in the post-critical phase. figure 5 shows typical loaddisplacement plots for both steel fibre types embedded in mortar c7a. in the beginning, the pull-out force increases linearly up to the maximum pull-out force. after reaching the peak, the force drops rapidly until a stable plateau is reached, which is dominated by the friction between the steel fibre and the surrounding matrix. in this phase, the pull-out force usually 0 1 2 3 4 5 6 7 8 9 10 0 5 10 15 20 25 30 p ul lou t fo rc e (n ) displacement (mm) maximum pull-out force linear-elastic part friction during pull-out pull-out blocking before pull-out c7a_f2_4 c7a_f1_3 figure 5. load-displacement plots of steel fibres type 1 and 2 embedded in mortar c7a. reaches a stable value or decreases slightly as with increasing fibre pull-out the force transmitting contact area between steel fibre and surrounding matrix decreases [17]. the pull-out force increases short before the fibre is pulled-out. similar findings are reported by [15] and have been explained by the plastic deformation of the fibre end due to the cutting process. the deformation of the fibre ends provides a mechanical anchorage, which also may help increasing the pull-out resistance. furthermore, it is possible according to [15] that some matrix particles adhere to the fibre surface. the abrasion of these particles and their accumulation 93 g. hüsken, l.-c. wagner, g. j. g. gluth et al. acta polytechnica towards the fibre pull-out end might cause a wedge effect leading to higher pressure on the fibre and higher frictional bond. both effects could be confirmed for both steel fibre types by own sem images as depicted in figure 6 for the plastic deformation of the fibre end and figure 7 for adhering matrix particles. 100 µm figure 6. plastic deformation of the fibre end due to the cutting process (steel fibre type 1). 100 µm figure 7. adhering matrix particles (c7a) at the fibre end (steel fibre type 1). it was already assumed from the averaged stressstrain plots that the bond behaviour between fibre and surrounding matrix of mortar fa5 is better than for mortar c7a. this assumption was confirmed by the results of the single fibre pull-out tests depicted in figure 8 for steel fibres type 1. here, not only a higher maximum pull-out force was obtained, but also the friction dominated pull-out phase differs from the results obtained for mortar c7a. in this case, the pull-out force decreases clearly with increasing fibre pull-out as the force transmitting contact area max. pull-out force friction during pull-out pull-out blocking during pull-out 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 0 10 20 30 40 50 60 70 80 p ul lou t fo rc e (n ) displacement (mm) fa5_f1_4 fa5_f1_2 fibre rupture figure 8. load-displacement plots of steel fibres type 1 embedded in mortar fa5. between steel fibre and surrounding matrix decreases. a slight increase in the pull-out force short before complete fibre pull-out was also recorded for steel fibres embedded in mortar fa5. however, fibre rupture occurred for both types of steel fibres at different levels (see figure 9). therefore, it was not possible to obtain comparable load-displacement plots for steel fibres type 2 as they all ruptured before the fibre was pulled-out from the matrix. 100 µm figure 9. ruptured fibre (steel fibre type 1) with adhering matrix particles (fa5). 4. conclusions uniaxial tensile tests and single fibre pull-out tests have been conducted to characterize the determining failure modes of fibre-reinforced aam. two different aam mortars have been investigated in combination with two types of steel fibres. the uniaxial tensile tests revealed that the post-critical behaviour is affected by the matrix properties (e.g. fibre-matrix bond), fibre 94 vol. 27/2020 application of steel fibres in alkali-activated mortars content and type, whereas the cracking stress was only affected by the matrix strength. single fibre pull-out tests confirmed the findings that the bond behaviour between embedded fibre and surrounding matrix is the main factor to control the post-critical behaviour of fibre-reinforced aam. the uniaxial tensile tests have been performed along with ae analysis and dic measurements. both techniques delivered valuable information on the determining failure modes in the post-critical phase. the results of the ae analysis demonstrated that it is possible to distinguish between the formation of micro cracks with low frequency and fibre pull-out with high frequency. furthermore, increased ae activity near the formed cracks was measured which correlates well with the findings of the dic measurements. the combination of both non-destructive test methods proved to be a useful tool for analysing failure mechanism of this composite material and optimizing material properties. acknowledgements this study was undertaken as part of the bleib project within the federal institute for materials research and testing (bam) and funded by the federal ministry for economic affairs and energy (bmwi), germany. the authors gratefully thank ms berta mota gassó for her assistance with sem imaging. references [1] j. provis, j. van deventer (eds.). alkali activated materials. springer-verlag gmbh, 2013. 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[6] t. ng, t. htut, s. j. foster. mode i and ii fracture behaviour of steel fibre reinforced high strength geopolymer concrete: an experimental investigation. in proceedings of framcos-7. 2010. [7] s. s. kumar, k. c. pazhani, k. ravisankar. fracture behaviour of fibre reinforced geopolymer concrete. current science 113(01):116–122, 2017. doi:10.18520/cs/v113/i01/116-122. [8] k. wille, a. e. naaman. bond stress-slip behavior of steel fibers embedded in ultra high performance concrete. in proceedings of 18th european conference on fracture and damage of advanced fiber-reinforced cement-based materials, pp. 99–111. 2010. [9] f. pacheco-torgal, j. b. de aguiar, y. ding, et al. performance of alkali-activated mortars for the repair and strengthening of opc concrete. in handbook of alkali-activated cements, mortars and concretes, pp. 627–641. elsevier, 2015. doi:10.1533/9781782422884.4.627. 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[17] a. khabaz. analysis of sliding mechanism of straight steel fibers in concrete and determine the effect of friction. archives of civil and mechanical engineering 17(3):599–608, 2017. doi:10.1016/j.acme.2017.01.005. 95 http://dx.doi.org/10.21236/ada559113 http://dx.doi.org/10.1021/acs.iecr.6b00559 http://dx.doi.org/10.1617/s11527-012-9985-0 http://dx.doi.org/10.3989/mc.2009.41807 http://dx.doi.org/10.18520/cs/v113/i01/116-122 http://dx.doi.org/10.1533/9781782422884.4.627 http://dx.doi.org/10.4028/www.scientific.net/kem.761.61 http://dx.doi.org/10.1617/s11527-016-0857-x http://dx.doi.org/10.1007/978-94-024-1194-2_35 http://dx.doi.org/10.1016/j.cemconcomp.2013.12.015 http://dx.doi.org/10.1016/j.compscitech.2011.10.013 http://dx.doi.org/10.1016/j.acme.2017.01.005 acta polytechnica 27(0):90–95, 2020 1 introduction 2 materials and experimental setup 3 results and discussion 4 conclusions acknowledgements references acta polytechnica doi:10.14311/app.2020.27.0155 acta polytechnica 27(0):155–159, 2020 © czech technical university in prague, 2020 available online at http://ojs.cvut.cz/ojs/index.php/app nanoindentation of hydrogen enriched zr-1nb zirconium alloy nuclear fuel claddings ondřej liberaa,∗, patricie halodováa, petra gávelováa, jakub krejčíb a research centre řež, hlavní 130, husinec-řež, czech republic b ujp praha a.s, nad kamínkou 1345, praha-zbraslav, czech republic ∗ corresponding author: ondrej.libera@cvrez.cz abstract. zirconium alloys are being commonly used as a material of choice for nuclear fuel claddings in water cooled nuclear reactors for decades due to their good corrosion resistance and low neutron absorption. however, the increasing operation conditions of next generation nuclear reactors (gen-iv) in terms of higher temperatures, pressures and higher neutron flux requires evaluation of further zr cladding usability. the embrittlement of zr claddings due to hydrogen pickup from reactor coolant is one of the issues for its potential use in gen-iv reactors. nanoindentation is an effective tool for analysis of the change of mechanical properties of hydrogen enriched zr claddings from localised material volume. zirconium alloy zr-1nb (e110) with experimentally induced hydrides was analysed by the means of nanoindentation. zirconium hydrides were formed in the material after exposure in high temperature water autoclave. the optimized methodology of surface preparation suitable for nanoindentation is described and the resulting surface quality is discussed. the nanoindentation measurements were performed as an array of 10x10 indents across areas with hydrides. depth dependent hardness and reduced modulus values measured by nanoindentation were compared between the material with no hydrogen content, low hydrogen content (127 ppm h) and high hydrogen content (397 ppm h). complementary microhardness measurements at hv 0.1 were performed on all materials for bulk material hardness comparison. keywords: nanoindentation, nuclear fuel claddings, zr-1nb. 1. introduction the nuclear fuel cladding serves as the first containment barrier of the fuel pellets in water-cooled nuclear reactors and subsequently in spent fuel storage casks. due to their good mechanical properties, corrosion resistance and low neutron absorption, zirconium alloys are being commercially used as a nuclear fuel cladding material for decades. despite all the benefits, zirconium claddings chemically react with the cooling water in the reactor, which results in creation of hydrides in the material volume [1]. hydrides formed in zirconium claddings affect the mechanical properties of the material, making it prone to cracking due to the internal stresses caused by fuel swelling and the propagation of fission gasses during the nuclear fuel cycle. this process of cracking associated with hydrides in zr alloys is called delayed hydride cracking (dhc) [2]. this phenomenon is one of the key issues with use of zr alloy claddings in gen-iv nuclear reactors. nanoindentation is one of the analytical methods to evaluate the change of mechanical properties before and after hydridation in a very local volume of the material. the mechanical properties of various hydrogen enriched zr claddings [3, 4] and pure zr [5] have already been assessed by nanoindentation measurements in previous studies. however, most of the research in the field was performed on western type on zr claddings such as zircalloy and zr-2.5nb. the amount of available research articles published on nanoindentation of e110 alloy in english is very limited to non-existent, despite the fact that this type of cladding is still being actively used in a large part of the world. in this work, nanoindentation testing was performed on zr-1nb (e110) zirconium alloy nuclear fuel claddings. the tests were conducted on reference and artificially hydrided claddings for future comparison with material irradiated in real operating conditions in pressurised water nuclear reactor (pwr). measurements were performed across the areas containing hydrides. the comparison of mechanical properties between claddings with various amount of hydrogen content is assessed. 2. experimental 2.1. material the material subjected for testing was e110 zirconium alloy cladding tube (zr, 1 wt.% nb, 0.5 wt.% fe, 0.1 wt.% o) in reference state and two hydrogen enriched states. two cladding tubes were exposed to 425°c pressurised deionized water at 10.7 mpa in an autoclave for 63 and 243 days. hydrogen content in samples was evaluated by inert gas fusion (igf) technique and was estimated as 122 ppm and 397 ppm [6]. 155 http://dx.doi.org/10.14311/app.2020.27.0155 http://ojs.cvut.cz/ojs/index.php/app o. libera, p. halodová, p. gávelová, j. krejčí acta polytechnica figure 1. comparison of average hardness values of samples finished by chemo-mechanical polishing for 30 minutes and 1 hour. the final concentration of hydrogen in samples approximately represents the state of cladding during the active use in pwr (122 ppm h) and during dry storage at the end of nuclear fuel cycle (397 ppm h). one milimeter thick rings were cut from each cladding tube on buehler isomet gravitational saw with diamond cutting disc. cut rings were heat embedded in condufast (struers) conductive polymer resin and ground on automatic grinding machine struers tegramin-25 using sandpapers with 500-2000 grits. the samples were then polished on the same machine with 3 and 1 µm diamond suspensions and finished with 0.05 µm colloidal silica suspension ops (struers). the final polishing step was performed on a fine polishing cloth with a mixture of 0.05 µm colloidal silica suspension ops with h2o2 (30%vol.) and hf (40% weight) in 50:10:1 vol. proportion [4]. this chemo-mechanical polishing combination of polishing suspension and etching solution was essential for the preparation of damage-free surface with excellent visibility of the microstructure containing hydrides. commonly used chemical or electrolytic polishing methods for microstructure enhancement and reduction of mechanically hardened layer [7] proved inappropriate for nanoindentation measurements. the inhomogeneous etching rate of zr matrix and hydride phases during these processes introduces increased surface roughness and etch pits, which might compromise the validity of nanoindentation data. 2.2. indentation testing nanoindentation tests were performed on bruker/hysitron ti 950 triboindenter. diamond berkovich type indenter tips were used for all measurements. calibration of tip area function [8] for hardness and reduced modulus calculations were performed on fused silica calibration standard from hysitron (er = 69.6 gpa; h = 9.25 gpa). nanoindentation measurements on e110 samples were performed across the area with visible hydrides as a 10x10 indentation matrix with 20 µm separation between indents. partial unload load function [9] with gradual increase in maximum load up to 10 mn was selected to obtain various depth dependent results for each indentation measurement. hardness and reduced modulus values were calculated from the nanoindentation load displacement curves utilizing the oliver&pharr analytical model [8]. microhardness measurements on struers durascan were performed with diamond tip for hardness comparison at high indentation depths. an array of 10 indents with 100 µm spacing was performed around the hoop direction in the central part of the ring of each sample at hv 0.1 with vickers diamond tip. 3. results and discussion 3.1. sample preparation the duration of chemo-mechanical polishing surface finishing step was found to be crucial for correct nanoindentation measurement on e110 samples. despite the good microstructure visibility after 30 minutes of surface finish, the chemical-mechanical polishing did not completely remove the mechanically hardened layer formed by mechanical grinding and polishing. figure 1 shows the comparison of the measured nanoindentation hardness between chemomechanically polished samples prepared for 30 minutes and 1 hour. the effect of nano-crystallic hardened layer on mechanical properties is best visible on approx. 0.5 gpa increase in overall hardness between hydrogen enriched samples in both states. indentation results presented and discussed later in this work (see section 3.2 and further) are from samples with 1h of surface finish, where the mechanically hardened layer has been reduced by proper sample preparation. 3.2. indentation testing results the area for nanoindentation measurement on 122 ppm h sample was selected to be around the area 156 vol. 27/2020 nanoindentation of hydrogen enriched zr-1nb figure 2. a) light optical micrograph of hydride distribution in a sample with 122 ppm h. b) bse image of indentation matrix in area with hydride on 122 ppm h sample. c) light optical micrograph of hydride distribution in a sample with 397 ppm h. d) bse image of indentation matrix on 397 ppm h sample. figure 3. comparison of average hardness values of reference, 122 ppm and 397 ppm hydrogen enriched e110 samples. e110 reference e110 122 ppm h e110 397 ppm h h 400nm (gpa) 1.56 ± 0.18 1.56 ± 0.12 1.77 ± 0.16 er 400nm (gpa) 72.75 ± 7.63 72.92 ± 2.56 78.14 ± 3.24 hv 0.1 (gpa) 1.44 ± 0.06 1.44 ± 0.05 2.09 ± 0.18 table 1. nanoindentation hardness and reduced modulus at 400 nm indentation depth and microhardness (hv 0.1) results for all samples. with hydride from lom observation shown on figure 2a. at this level of hydrogen pick up the hydrides were formed as individual chains in the hoop direction of the ring. the spacing between individual hydride chains was large, approximately ¼ of the ring width. for this reason, there was only one major hydride chain reached in the nanoindentation matrix (figure 2b). the light and dark stains in bse images represent individual grains with various grain orientations. the hydrides on 397 ppm h sample were much shorter and distributed evenly across the whole area of the ring. the orientation of larger 397 ppm h e110 hydrides was mainly in the hoop direction with minor hydrides in radial direction shown on figure 2c. the 157 o. libera, p. halodová, p. gávelová, j. krejčí acta polytechnica figure 4. comparison of average reduced modulus values of reference, 122 ppm and 397 ppm hydrogen enriched e110 samples. indentation area on 397 ppm h sample was placed in the middle of the ring, covering the representative area with hydrides (figure 2d). the indentation matrix on reference sample was placed in the central part of the ring. the hardness vs. displacement results from nanoindentation measurements plotted in figure 3 show the overall hardness profile across all measured depths for all studied samples. results of the same measurements are shown in figure 4 for reduced modulus values. it is apparent that hardness measurements on all samples were affected by the indentation size effect (ise), causing the increase in hardness values in shallow indentation depths. this effect is well known and unavoidable at these indentation depths [10]. a model for estimation of correct hardness values at low indentation depths was proposed by nix & gao [11], however its application is not in the scope of this work. the increased reduced modulus at low indentation depths (<200 nm) might be caused by a thin oxide layer formed after sample preparation. this assumption has to be confirmed by further investigation of the surface after the preparation process. an increase of hardness and reduced modulus is visible at 397 ppm h sample. a dense mesh of hydrides in the material directly affects the mechanical properties of the whole material. measured mechanical properties of 122 ppm h sample are nearly identical to values from the reference sample. it can be assumed that the scarce presence of hydrides in 122 ppm h sample affects the mechanical properties of the whole material just slightly. however, a closer examination of the mechanical properties near the hydride has to be performed to confirm this assumption. comparison of hardness measured by nanoindentation and hv 0.1 microhardness is shown in table 1. the value of 400 nm was selected as a stable nanoindentation depth without the influence of the ise for comparison. a good agreement of measured values is shown for reference e110 and 122 ppm h sample. the microhardness results for 397 ppm h show approx. 0.3 gpa higher value than the nanoindentation measurement. the reason behind this difference is a matter of further research. 4. conclusions optimized sample preparation method was used for fine surface preparation of e110 samples for nanoindentation measurements. samples of zr-1nb e110 nuclear cladding tubes in reference, 122 ppm and 397 ppm hydrogen enriched states were measured by nanoindentation. it was shown that hardness and reduced modulus values of the 122 ppm h sample were very close to the reference sample. this result was confirmed by comparison to microhardness testing results. a further research needs to be performed closer to hydride chains on 122 ppm h sample to confirm the preservation of mechanical properties despite the hydrogen build-up. the measurements on 397 ppm h sample show an increase of hardness by approx. 13 % and reduced modulus increase by 8 %. the measured microhardness value was 18 % higher than nanoindentation hardness measured at 400 nm. this phenomenon is a subject of further research. acknowledgements the presented work was financially supported by the ministry of education, youth and sport czech republic project lq1603 (research for susen). this work has been realized within the susen project (established in the framework of the european regional development fund (erdf) in project cz.1.05/2.1.00/03.0108). references [1] c. coleman, d. hardie. the hydrogen embrittlement of α-zirconium-a review. journal of the less common metals 11(3):168 – 185, 1966. doi:10.1016/0022-5088(66)90003-8. [2] delayed hydride cracking in zirconium alloys in pressure tube nuclear reactors. final report of a coordinated research project 1998-2002. tech. rep. 41, international atomic energy agency, nuclear power technology development section, vienna (austria), 2004. 158 https://doi.org/10.1016/0022-5088(66)90003-8 vol. 27/2020 nanoindentation of hydrogen enriched zr-1nb [3] k. kese, p. olsson, a.-m. a. holston, e. broitman. high temperature nanoindentation hardness and young’s modulus measurement in a neutron-irradiated fuel cladding material. journal of nuclear materials 487:113 – 120, 2017. doi:10.1016/j.jnucmat.2017.02.014. [4] a. rico, m. martin-rengel, j. ruiz-hervias, et al. nanoindentation measurements of the mechanical properties of zirconium matrix and hydrides in unirradiated pre-hydrided nuclear fuel cladding. journal of nuclear materials 452(1):69 – 76, 2014. doi:10.1016/j.jnucmat.2014.04.045. [5] m. kuroda, d. setoyama, m. uno, s. yamanaka. nanoindentation studies of zirconium hydride. journal of alloys and compounds 368(1):211 – 214, 2004. doi:10.1016/j.jallcom.2003.08.094. [6] b. d. holt, h. t. goodspeed. determination of nitrogen, oxygen, and hydrogen in metals by inert gas fusion. a manometric method. analytical chemistry 35(10):1510–1513, 1963. doi:10.1021/ac60203a023. [7] g. finch, a. quarrell. the beilby layer. nature publishing group 137(3467):516–519, 1936. doi:10.1038/137516a0. [8] w. oliver, g. pharr. an improved technique for determining hardness and elastic modulus using load and displacement sensing indentation experiments. journal of materials research 7(6):1564–1583, 1992. doi:10.1557/jmr.1992.1564. [9] a. c. fischer-cripps. nanoindentation. springer, new york, ny, 3rd edn., 2011. doi:10.1007/978-1-4419-9872-9. [10] y. milman, a. golubenko, s. dub. indentation size effect in nanohardness. acta materialia 59(20):7480 – 7487, 2011. doi:10.1016/j.actamat.2011.08.027. [11] w. d. nix, h. gao. indentation size effects in crystalline materials: a law for strain gradient plasticity. journal of the mechanics and physics of solids 46(3):411 – 425, 1998. doi:10.1016/s0022-5096(97)00086-0. 159 https://doi.org/10.1016/j.jnucmat.2017.02.014 https://doi.org/10.1016/j.jnucmat.2014.04.045 https://doi.org/10.1016/j.jallcom.2003.08.094 https://doi.org/10.1021/ac60203a023 https://doi.org/10.1038/137516a0 https://doi.org/10.1557/jmr.1992.1564 https://doi.org/10.1007/978-1-4419-9872-9 https://doi.org/10.1016/j.actamat.2011.08.027 https://doi.org/10.1016/s0022-5096(97)00086-0 acta polytechnica 27(0):155–159, 2020 1 introduction 2 experimental 2.1 material 2.2 indentation testing 3 results and discussion 3.1 sample preparation 3.2 indentation testing results 4 conclusions acknowledgements references acta polytechnica doi:10.14311/app.2020.27.0160 acta polytechnica 27(0):160–163, 2020 © czech technical university in prague, 2020 available online at http://ojs.cvut.cz/ojs/index.php/app hot hardness measurements on materials up to 600 ◦c during the first hour of using bruno passilly∗, lara molenda dmas onera, université paris saclay, 29 avenue de la division leclerc f-92322 châtillon france ∗ corresponding author: bruno.passilly@onera.fr abstract. in the aeronautical field, materials are used in severe environmental conditions (temperature, atmosphere, exposure time ...), particularly for engine applications. in order to characterize the use of these materials in the evaluation of their properties, it is necessary to carry out tests in conditions close to their operating environment. hot hardness is a simple method which can be applied on many different materials such as oxidized layers, coatings, composite materials, brazing cords, additive manufacturing materials. onera is developing micromechanical characterization means to carry out vickers microhardness tests from room temperature up to 600 ◦c. in principle, a pyramidal punch is applied on the surface of a material and the applied load is continuously measured during indenter’s moving in the material. the material is tested locally under conditions close to the actual conditions of employment. the goal of this research is to improve microindentation in order to achieve temperature test campaigns up to 600 ◦c under a controlled atmosphere of argon and to validate a method to produce a series of results during the first hour of using up to 600 ◦c. stainless material is studied to compare the evolution of its hot hardness properties versus different parameters such as load, holding time at the maximum load, atmosphere, and thermal duration. a discussion about these measurements and the technical limits of hot hardness technology is presented. keywords: hot hardness, microindentation, stainless steel, temperature. 1. introduction in aeronautic applications, temperatures can reach over 1000 ◦c in work condition. the used materials have to withstand stresses, strains as well as temperature cycles which can lead to the damage of the part. therefore, material characterization plays an important role in the part design. microindentation and hardness tests present many advantages compared to classical characterization methods such as flexural or tensile test: a simple geometry, little material quantity needed, automated data collection and analysis. it can be performed on substrate, thin films or through several layers too [1, 2]. when temperature is increased, it has an impact on the structure and the properties of materials. measuring hardness at high temperature gives important insight of the thermophysical properties of the material. research on micro and nanoindentation at high temperature has increased for the past two decades and many issues remain [3–5]. techniques have been developed to reduce induces thermal dilatation, thermal drift and oxidation which affect the quality of the results. the goal is to validate a method to investigate the changes in hardness versus temperature under the first hour of heating and to avoid thermal drift and oxidation of the indenter or the sample [6–8]. 2. hot hardness set-up 2.1. presentation of the experimental set-up the microindentation machine is composed of the chamber which comprises the sample holder, an optical microscope, the indenter, the indentation station, 2 motorized (x,y) tables to move the sample from the chamber’s opening to the microscope or indentation station, 1 motorized table to move the indenter to the sample and 1 motorized table to move the microscope for focusing (figure 1 and figure 2). a primary pump and a turbo pump are necessary to empty the vacuum chamber. a gas tap is required to introduce argon + hydrogen in the chamber. a water cooling system regulates the temperature of the measurements. a computer and a developed software on @labview are needed to manage all the movements of the tables and to acquire all the datas versus time of the sensors like thermocouples, load sensor, displacement coordinates of the tables. the following table 1 sums up the characteristics of the machine. tmax 800∼◦c fmax 3n atmosphere ar+3%h2 indenter’s shape vickers indenter’s material diamond table 1. characteristics of high temperature microhardness set-up 160 http://dx.doi.org/10.14311/app.2020.27.0160 http://ojs.cvut.cz/ojs/index.php/app vol. 27/2020 materials hot hardness measurements figure 1. high temperature microhardness synoptic figure 2. high temperature microhardness set-up the two ovens are composed of the heating part, which is made of a platinum coil which is winded around an hollow cylindrical ceramic part. the sample is placed inside the ceramic part of the first oven, and the indenter is placed in the second oven. a current goes through the coil and heated up because of joule’s effect. a thermocouple sensor measures ovens temperature and the controller fixes the working temperature, the slope of the rise ramps temperature and power. as the experiments are carried out at high temperatures, the oxidation of the sample’s surface has to be carefully avoided. in order to work under argon and hydrogen gas, vacuum of the chamber first needs to be done to reach a residual pressure of 5.10−5 mbar minimum. the gas tap is opened and the chamber’s door closed at the same time and the working pressure near from the atmospheric pressure is reached in 3 minutes. the primary pump is turned off once the turbo pump no long turns. 2.2. method the experiments are performed under argon and hydrogen gas at atmospheric pressure. the imprint calibration and the force sensor calibration are made prior to any experiments. the duration of heating must be optimized and controlled for different temperatures. for each oven, the study is performed at 400 ◦c, 500 ◦c and 600 ◦c. the investigated slopes are 15 ◦c/min, 25 ◦c/min and 50 ◦c/min. from each temperature, the time to reach the temperature as well as the overshoot (maximum temperature reached by the oven) is determined. the delay between the oven and the sample’s surface is also determined. all of these temperature measurements are tested on the same material inox 316l-1 (table 2). sample ref. num. temp. test study remarks 316l_1 20, 200,400, 500,600 temperature parameters 316l_2 20 applied load and hardness 316l_3 600 time duration 316l_4 400 hardness 316l_5 500 hardness 316l_6 600 hardness table 2. samples reference 2.3. temperature profile figure 3a shows the temperature profile of the sample’s oven for different ramps and temperatures. it can be seen that the increase in temperature is steady and the set temperature is easily reached: the ovens are well regulated. however, an overshoot is observed. even if it cannot be avoided, it needs to be as small as possible in order not to overheat the sample. to highlight this phenomenon, the curve is zoomed in (figure 3b). the overshoot of the oven is lower than 2 ◦c. therefore, 50 ◦c/min ramp is chosen as settings for all the hardness tests as it allows a faster heating with a reasonable overshoot. 2.4. temperature delay and temperature deviation as the temperature of interest is the temperature of the sample, the following experiment focuses on the temperature delay and the temperature deviation of the sample’s surface to the set temperature. a thermocouple sensor is placed just at the surface of the sample. the sample is placed underneath the indenter and the surface temperature is measured. the investigated temperature is 400 ◦c at 50 ◦c/min. then, the distance between the ovens is decreased to see its influence on the surface temperature. there is 37s seconds delay between the oven’s temperature and the sample’s one (figure 4a). the sample reaches a constant temperature of 400 ◦c with an overshoot which is less than 2 ◦c. however, the temperature difference is 6 ◦c and the temperature does not seem to increase. the temperature deviation between the set temperature and the surface temperature is plotted against the distance between the two ovens (sample and indenter). for all temperatures, the deviation decrease may be observed when the ovens are brought close 161 bruno passilly, lara molenda acta polytechnica figure 3. temperature ramps for 400, 500, 600 ◦c set temperature and 15, 25, 50 ◦c/min ramp (b) overshoot example at 600 ◦c and 50 ◦c/min ramp together. at 2 millimeters the deviation reaches only 1 ◦c difference. for each temperature test, the results can be extrapolated and it can be considered that there is no temperature deviation at contact. 3. hot hardness results on stainless steel sample 3.1. samples preparation the material studied is a stainless steel 316l [9]. the material is cut in a parallelepiped shape 6 mm × 5 mm × 10 mm. sample surface (10 mm × 6 mm side) polishing is performed with polishing papers with decreasing grain sizes from p800 to p4000. diamond slurry is applied on velvet sheets with grain sizes of 3 µm, 1 µm and 1/4 µm. between each step, the sample is cleaned in an ultrasonic bath of water and dried. an optical microscope is used to check the quality of the surface of the sample and to avoid scratches. the last step consists in cleaning the sample in an ultrasonic bath in acetone for 2 minutes and dried. for this study, 5 samples are prepared, sample 1 is used only to develop temperature profile before the study. 3.2. measurement of hardness the following protocol is applied: an imprint is done for a recorded load. each imprint is separated by figure 4. (a) temperature difference between the thermocouple of regulation of the oven and the temperature of the sample (b) temperature deviation as function of the distance between the two ovens 100 µm of the following imprint. the imprints are measured using an electronic caliper square on the printed photo of the screen. for a vickers imprint, each diagonal of the pyramidal imprint is measured separately. hardness is then calculated by using equation (1): hvickers = p avickers = 2.p · sin ψ 2 d2 = 1.854. p d2 . (1) with p the load, d the mean of the measured diagonals of the imprint and ψ is equals to 136 ◦. 3.3. study of the effect of the applied load and duration time a series of 5 indents is performed for each following load 50, 100, 150 and 200 grams. equation (1) is used to calculate the hardness. if the mean values are considered, the hardness values are constant as the load is increased for a given temperature of 20 ◦c. the lowest standard deviation is obtained for a load of 150 grams, which will be the load to be applied for all the other tests. the effect of the duration of time at the maximum load is also investigated. a 150 g load and a temperature of 600 ◦c are chosen. the same protocol as the load effect is applied: 5 imprints are done for each load, separated by 100 µm. the results of hardness at 5, 180 and 360 seconds duration time at the 162 vol. 27/2020 materials hot hardness measurements maximum load are constant which show that no effect of duration time at maximum load is occurring. a 5 seconds duration time is chosen for all the following experiments. 3.4. thermal history and changes in hardness the ovens are heated to the set temperature, then the contact point and indentations are made for one hour. the imprints are observed at the optical microscope. the temperature is set to the temperature test, the load used is 150 g and the duration time at maximum load is 5 seconds. for each temperature a new sample of stainless steel 316l is taken to have the same thermal history for each series of points (see table 2). the figure 5 shows an hardness mean of 3.6 gpa at 20 ◦c which decreases at 200 ◦c to reach 2.5 gpa and is stable for a higher temperature up to 600 ◦c. in order to investigate the possible oxidation layer, a series at 20 ◦c was performed after each series at high temperature were done. the values of the second series at 20 ◦c matches the first one. thus, the sample’s oxidation is avoided. figure 5. variation of hardness on stainless steel up to 600 ◦c the final goal is to validate this method to measure the changes in hardness in less than one hour at 600 ◦c. figure 6 represents constant value of hardness over time once 600 ◦c is reached. the mean value of the hardness is 2.4 ± 0.08 gpa. at 20 ◦c there is no more evolution of hardness than at 600 ◦c. 4. conclusions to conclude, the study shows very stable and efficient thermal regulations that allow fast heating and little overshoot which are not reach on conventional microhardness apparatus. oxidation problems are avoided by working under argon atmosphere. changing samples for each temperature is a good method to compare the results with the same thermal history. the tested method seems promising and higher temperatures (up to 1000 ◦c) under ultra-high vacuum or argon atmosphere should be developed to characterize materials which are used under more severe conditions. figure 6. validation of the method at 600 ◦c and 150 g load on a new sample acknowledgements dr k.durst and j.petit for their fruitful conversations and p.lasserre for his technical help. references [1] w. oliver, g. pharr. an improved technique for determining hardness and elastic modulus using load and displacement sensing indentation experiments. journal of materials research 7(6):1564–1583, 1992. doi:10.1557/jmr.1992.1564. [2] a. r. f. jr., g. pintaúde, a. sinatora, et al. the use of a vickers indenter in depth sensing indentation for measuring elastic modulus and vickers hardness. materials research 7(3):483–491, 2004. doi:10.1590/s1516-14392004000300018. [3] j. wheeler, d. armstrong, w. heinz, r. schwaiger. high temperature nanoindentation: the state of the art and future challenges. current opinion in solid state and materials science 19(6):354–366, 2015. doi:10.1016/j.cossms.2015.02.002. [4] t. kutty, k. ravi, c. ganguly. studies on hot hardness of zr and its alloys for nuclear reactors. journal of nuclear materials 265(1-2):91–99, 1999. doi:10.1016/s0022-3115(98)00610-2. [5] r. sundar, t. kutty, d. sastry. hot hardness and creep of fe3al-based alloys. intermetallics 8(4):427–437, 2000. doi:10.1016/s0966-9795(99)00118-1. [6] j. m. wheeler, j. michler. invited article: indenter materials for high temperature nanoindentation. review of scientific instruments 84(10):101301, 2013. doi:10.1063/1.4824710. [7] j. wheeler, r. oliver, t. clyne. afm observation of diamond indenters after oxidation at elevated temperatures. diamond and related materials 19(11):1348–1353, 2010. doi:10.1016/j.diamond.2010.07.004. [8] x. liu. développement de la caractérisation du comportement local à haute température des alliages métalliques par micro indentation. ph.d. thesis, 2017. [9] r. k. desu, h. n. krishnamurthy, a. balu, et al. mechanical properties of austenitic stainless steel 304l and 316l at elevated temperatures. journal of materials research and technology 5(1):13–20, 2016. doi:10.1016/j.jmrt.2015.04.001. 163 https://doi.org/10.1557/jmr.1992.1564 https://doi.org/10.1590/s1516-14392004000300018 https://doi.org/10.1016/j.cossms.2015.02.002 https://doi.org/10.1016/s0022-3115(98)00610-2 https://doi.org/10.1016/s0966-9795(99)00118-1 https://doi.org/10.1063/1.4824710 https://doi.org/10.1016/j.diamond.2010.07.004 https://doi.org/10.1016/j.jmrt.2015.04.001 acta polytechnica 27(0):160–163, 2020 1 introduction 2 hot hardness set-up 2.1 presentation of the experimental set-up 2.2 method 2.3 temperature profile 2.4 temperature delay and temperature deviation 3 hot hardness results on stainless steel sample 3.1 samples preparation 3.2 measurement of hardness 3.3 study of the effect of the applied load and duration time 3.4 thermal history and changes in hardness 4 conclusions acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2018.15.0114 acta polytechnica ctu proceedings 15:114–119, 2018 © czech technical university in prague, 2018 available online at http://ojs.cvut.cz/ojs/index.php/app relation between stationary and moving pedestrian load models jan štěpánek∗, jiří máca czech technical university in prague, faculty of civil engineering, thákurova 7, 166 29 prague 6, czech republic ∗ corresponding author: jan.stepanek@fsv.cvut.cz abstract. the dynamic analysis of footbridges became common in recent years, but the approach to modelling pedestrian induced load is still inconsistent. this paper compares two different ways of modelling single pedestrian load. the response of the structure to stationary pulsating force is compared to the results given by the same force moving along simply supported beam. 300 randomly generated beams were used in this parametric study to describe the relation among these two approaches and parameters of the structure. the proposed formula was successfully applied to get more realistic estimation of response of the structure to a single walking pedestrian using stationary pulsating force. keywords: footbridges, pedestrian load models, forced vibration, response of structure. 1. introduction footbridges designed and built during recent years are very slender and flexible structures in many cases. such footbridges may be very susceptible to pedestrian induced load which may cause an uncomfortable level of vibration. natural frequencies of such structures are often very close to walking frequency of pedestrians, and therefore the resonance effect may occur. a number of studies have been made in past decades to define the procedure of effective and safe design of footbridges. a footbridge designed this way has to satisfy the criteria of serviceability limit state (sls). to be able to detect the risk of unacceptable vibration during design phase the dynamic analysis is often necessary. the european codes define the maximal level of acceleration of a bridge deck which can be reached to keep the sufficient comfort of pedestrians. numerical simulation can reveal the sensitivity of the structure due to pedestrians but unfortunately there is still a lack of pedestrian load models in current eurocodes. many single pedestrian load models have been developed [1, 2] but most of them assume the numerical analysis using moving forces or moving biodynamic models. nevertheless, most of the commercial programs used for static design of bridges do not provide the option of calculation with force moving along the structure. stationary harmonic pulsating force was presented in guidebook [3] to simulate single walking pedestrian. the advantage of this estimation is its simplicity when it comes to numeric analysis in static software. the main disadvantage of this load model is that it does not consider the time that one spends on the bridge. the stationary pulsating force without time limitation may cause overestimation of the response of the structure which leads to unnecessary design changes and potential increase of cost of the structure. this paper focuses on comparison of approaches of modelling single pedestrian load by stationary and moving pulsating force. the goal is to find a simply applicable formula which combines these two approaches. the formula can be used to find realistic estimation of response of a simply supported beam using stationary pulsating force. 2. pedestrian load models pedestrian load can be divided into several categories. normal traffic on the bridge can be defined as a single pedestrian load, group of walking people or stream of pedestrians. exceptional load situations (jumping, running, kneeling or vandalism) must not excite the intolerable vibrations either [2]. to be able to evaluate the response of the structure in normal operation, it is necessary to define a simple but accurate dynamic single pedestrian load model. 2.1. single pedestrian load dynamic force produced by a single person can be represented by three components in perpendicular directions: vertical, horizontal lateral and horizontal longitudinal components [1]. horizontal longitudinal force is usually considered negligible. the lateral component is caused by moving centre of gravity horizontally while walking. the frequency of such force is half than the stride frequency. a vertical component with the stride frequency is the cause of most of the documented problems with uncomfortable vibrations of footbridges. only the vertical component is further investigated in this paper. vertical pedestrian load, assuming to be perfectly periodical force, can be formulated by fourier series in equation (1). numerous different approximations of such formulation were developed in the past years [1, 4]. 114 http://dx.doi.org/10.14311/app.2018.15.0114 http://ojs.cvut.cz/ojs/index.php/app vol. 15/2018 relation between stationary and moving pedestrian load models fv(t) = g+g n∑ i=1 αi sin(2πifpt+ ϕi) (1) where: fv(t) vertical pedestrian load g load corresponding to pedestrian’s weight αi fourier coefficient of the respective member fp step frequency ϕi phase shift n total number of included members various coefficients αi and ϕi can be found in literature [1, 4, 5]. in general, it can be said that including more than three or four harmonic members has no effect on the final load due to low values of αi for higher components. the worst-case scenario, when the stride frequency is equal to one of the natural frequencies of the bridge, is usually adopted for simulating a single pedestrian walking along the structure [1, 4, 6]. pedestrian walking velocity vs also plays an important role in serviceability verification of the structure. the speed vs can be taken by eq. (2) vs = fsls (2) where fs is step frequency and ls is step length. the recommended mean value for ls also varies in different studies. guidebook [4] recommends ls = 0.9 m. according to guidebook [3] ls is dependent on step frequency and for 2 hz (defined as normal walk) the typical step length ls = 0.75 m is considered. 2.2. stationary pedestrian load model the stationary pulsating force can be a simple way to find out the response of structure to dynamic pedestrian load. the guidebook [6] recommends to apply stationary harmonic force with amplitude 280 n to the most adverse position of the structure. the value of the force is taken as 700×0.4=280 n which is in accordance with average pedestrian weight 70 kg and the first fourier coefficient 0.4. other approaches take the movement of a person in account. the amplitude of applied force is multiplied by the reduction constant r. allen and murray give r=0.7 while grundman takes r=0.6 [1]. the formula for reasonable reduction factor r for simply supported beam is set in further parametric study. 2.3. moving pedestrian load model the moving load models can be used to define the response of the structure to pedestrian load in a very realistic way [5, 7]. examples of more sophisticated load models, including biodynamic models and inverted pendulum can be found in [8]. many force models are summarized in [1, 4] and most of them contains first fourier coefficient α1 ≈ 0.4. for this reason, one of the simplest models proposed by charles and hooprah was used in this study to simulate a walking pedestrian. this model consists only of static force g and first harmonic member for α1 = 0.4. this simplified approach is also tolerated in guidebook [2] for simulation of a walking pedestrian. 3. parametric study in this chapter a numerical analysis of 300 simply supported beams with randomly generated parameters was performed to find the formula for the reduction factor r. both stationary a moving pulsating force were used to find the peak acceleration in vertical direction. value of r defined in eq. (3) was taken for every single structure. finally, the function of r was found using the least-squares approximation with parameters of the tested beams used as the basis functions. rcal = amov ast (3) where amov is peak acceleration all over the structure calculated using moving force model and ast is acceleration induced by stationary force model without time limitation. 3.1. set of simply supported beams all the tested structures share the scheme of simply supported beam. every beam was generated so that one of the three lowest natural frequencies equals to 2 hz. the i-th natural frequency of simply supported beam can be calculated from analytical equation (4). ωi = π2i2 √ eiy µl4 (4) where: ωi i-th natural circular frequency e modulus of elasticity iy cross sectional second moment of area µ mass of one meter of length l span length iy can be expressed: iy = µl4ω2 i π4i4e (5) and µ is defined in eq. (6). µ = ρa+ µad (6) if ωi = 2π × 2 hz, every other parameter excluding iy can be randomly generated and iy is found using eq. (5). this procedure allows to generate simply supported beams with i-th natural frequency equals to 2 hz. assuming all the theoretical structures have rectangular cross-section, its width b and height h can be found by solving system of two equations (7) and (8). a = bh (7) 115 jan štěpánek, jiří máca acta polytechnica ctu proceedings iy = bh3 12 (8) where: a cross sectional area ρ material density µad mass added on each meter of length without static effect a single material – reinforced concrete c 30/37 was chosen for all the theoretical beams. the ranges of randomly generated parameters: ρ = 2500 kg/m3 standard density of concrete c 30/37 e = 32 gpa modulus of elasticity of concrete c30/37 ωi = 12.5664 circular frequency for fi = 2 hz. l = 10-50 m the typical span length of footbridges with static scheme of simply supported beam. a = 0.03-3 m2 wide range of cross sectional area which allows sufficient diversity of structures in the set. µad = 0 300 kg/m i = 1-3 one of the first three natural frequencies equals to 2 hz ξ = 0.008-0.02 damping ratio of concrete footbridges [1] the goal of this chapter is to assemble a set of simply supported beams with wide range of parameters so that the statistical analysis can be done using the leas-squares method. it means that some of the structures could exist just as mathematical model, not a real structure, because of their slenderness or high susceptibility to vibration. some other conditions of the properties of generated beams are set to reduce the amount of such structures in the list: • h > 0.1 m, b > 0.1 m minimal height and width of the cross section • 0.1 < h/b < 10 minimal and maximal ratio of height and width • h/l < 0.1 maximal ratio of cross sectional height and span length • b/l < 0.5 maximal ratio of width and span length the final set of the structures is composed of three parts. each part contains 100 beams with i-th natural frequency 2 hz for i = 1-3. every structure is modelled using finite element method (fem) in matlab and consists of 50 beam elements. the shear deformation of elements is neglected according to bernoulli-navier hypothesis. this simplification can be applied thanks to condition h/l<0.1 . 3.2. response of the structure 3.2.1. response to stationary pulsating force the i-th mode shape with corresponding natural frequency 2 hz must be found at first. the vertical harmonic force fv(t) with amplitude of 280 n and frequency fv = 2 hz (9) is then applied in the most adverse position of the bridge. fv(t) = 280 sin(2πfvt) n (9) maximal displacement of the structure is found using newmark-β method for solving equation of motion: kr(t) + cṙ(t) +mr̈(t) = p(t) (10) where: k stiffness matrix c damping matrix m mass matrix r(t) vector of displacement ṙ(t) vector of velocity r̈(t) vector of acceleration p(t) load vector created from fv(t) maximal acceleration can also be found solving the system of 2n linear equations (11) for system of n degrees of freedom. this approach is technically much easier but it does not provide any information about time history. [ (k − ω2m) −ωc ωc (k − ω2m) ]{ rs rc } = { fs fc } (11) where ω is the circular frequency of pacing force and r(t) and p(t) are expressed by two components: r(t) = rs sin(ωt) + rc cos(ωt) (12) p(t) = ps sin(ωt) + pc cos(ωt) (13) the amplitude of acceleration r̈ of such harmonic motion is given by eq. (14). r̈ = ω2 √ r2 s + r2 c (14) the reference value ast is finally found as maximal value of acceleration vector r̈ in vertical direction. 3.2.2. response to moving pulsating force the pedestrian load model, used in this section, was adopted from [1]. it consists only of static weight of pedestrian g = 70 kg and the first member of fourier series α1 = 0.4 and is expressed by eq. (15). the model is moving along the structure. to define the velocity of its movement, the step length needs to be defined. the step length ls = 0.75 m which is appropriate for step frequency was adopted from [3]. the walking speed was set according to eq. (2). 116 vol. 15/2018 relation between stationary and moving pedestrian load models figure 1. separate contributions of basis functions. fv(t) = 700 + 280 sin(2πfvt) n (15) the peak acceleration amov in vertical direction reached on each structure was analysed using newmark-β method. 3.3. evaluation the peak accelerations ast induced by stationary force model and amov induced by the moving force model were evaluated for all the structures which resulted in 300 values of reduction coefficient rcal eq. (3) the least-squares method was used for determination of the relation between reduction coefficient r and the parameters of the structure. eleven basis functions bj were defined: (1.) l span length (2.) µ mass for 1 meter of the beam (3.) iy cross sectional second moment of area (4.) µl total mass of the structure (5.) 1 l multiplicative inverse of span length (6.) 1 µ multiplicative inverse of mass for 1 meter of the beam (7.) 1 iy multiplicative inverse of cross sectional second moment of area (8.) i the order of frequency which equals to 2 hz. (9.) constant (10.) ξ1 damping ratio of the first mode shape (11.) 1 ξ1 multiplicative inverse of damping ratio of the first frequency the coefficients βj of basis functions were found solving system of equations (16).  x11 x12 · · · x1n x21 x22 · · · x2n · · · · · · . . . ... xm1 xm2 · · · xmn   β1 β2 ... βn  =  y1 y2 ... ym  (16) where xij = 300∑ k=1 bi,kbj,k and yi = 300∑ k=1 bi,krcal,k and k stands for number of the structure in the list. the first calculation was primarily performed in order to define the parameters which have the greatest influence on the value of reduction factor. four functions (no. 5,8,9 and 10) were chosen for further analysis. the second calculation was performed using only four selected functions and the respective coefficients β were found. the function rcal can be approximated by rapp which is expressed as a linear combination of basis functions (17). the figure 1 shows the contributions of chosen basis functions. rcal ≈ rapp = ∑ bjβj (17) the formula for r is finally set by equation: r = −6.248 l + 0.0495i+ 14.42ξ1 + 0.726 (18) and is usable in reduction of the stationary pulsating force model of single walking pedestrian according to eq. (19). fv(t) = r280 sin(2πfvt) n (19) as can be seen in figure 2, all the values of rcal falls in range between 0.5 1 thus the range of meaningful values is defined as: 0.5 ≤ r ≤ 1 (20) 117 jan štěpánek, jiří máca acta polytechnica ctu proceedings figure 2. calculated and approximated values of r. the average relative error of the approximation ∆r calculated according to eq.(21) is equal to 2.3 %. taking all the uncertainties of pedestrian load models into account, the value of relative error is sufficiently small. ∆r = ∣∣∣∣rcal −rapp rcal ∣∣∣∣ (21) 4. verification of the proposed formula verification of the proposed formula has been performed on numerical model of experimental structure presented in [5]. the experimental footbridge is prestressed concrete beam with t cross section and short overhangs (figure 5). the tuned mass damper (tmd) was installed under the midspan of the beam to show its influence on the susceptibility to vibration. tmd allows the options to be locked, which is performed by stiff connection to the structure. the moving load model used in section 3 provided very accurate results of response of the structure in comparison to load tests [5]. calculations were performed with both locked and free tmd to verify the formula for reduction factor r (18). the first natural frequency of the structure f1 = 1.98 hz, and therefore the step length of 0.75 m can be applied according to guidebook [3]. several parameters of the structure are required to estimate the reduction factor r: • l = 19 m • i = 1 (the first natural frequency is close to 2 hz) • ξ1, damping ratio of the first mode shape = 0.0143 (locked tmd), 0.06 (free tmd) 4.1. locked tmd the peak accelerations at midspan amov using moving pedestrian load model according to eq. (19) was figure 3. response of the structure stationary pedestrian load, locked tmd. figure 4. response of the structure moving pedestrian load, locked tmd. evaluated and can be seen in figure 4. the value for reduction coefficient was calculated from eq. (18). calculation of r for locked tmd: r = −6.248 l + 0.0495i+ 14.42ξ1 + 0.726 r = −6.248 17.4 + 0.0495 × 1 + 14.42 × 0.0143 + 0.726 r = 0.623 the amplitude of the stationary pulsating force is then r× 280 = 174 n. applying this force according to eq. (19) in the midspan of the beam, the maximal acceleration ast was evaluated. 118 vol. 15/2018 relation between stationary and moving pedestrian load models figure 5. static scheme of experimental footbridge [5]. tmd r acceleration moving model [ms−2] acceleration stationary model [ms−2] locked 0.623 1.100 1.073 free 1 0.264 0.261 table 1. maximal acceleration at midspan. 4.2. free tmd the evaluation of acceleration with free tmd was performed using the same method as with locked tmd. due to high structural damping (≈6%), the reduction factor r exceeded limit value 1 which was presented in eq. (20). therefore, the value of r is set to 1. comparison of the results can be seen in table 1. 5. conclusions the formula for the reduction coefficient r, which can be used for simulation of single pedestrian load, was proposed. this formula simplifies the pedestrian load from moving force model to stationary harmonic force model but keeps the accuracy of moving model. the formula is applicable on footbridges with static scheme of simply supported beams and with one of the first three natural frequencies close to 2 hz. the proposed method could be a very effective way to evaluate response of the structure to a single pedestrian load, because it does not require a time analysis and can be simply performed in commercial static software. parametric study in section 3 proved it is mainly length of the bridge and structural damping that influences the reduction coefficient. the numerical analysis in section 4 proved the estimation of reduction factor r = 0.6 0.7 [1] for low damped structures. however, it also showed that these approximations can lead to underestimation of load, which was demonstrated on the structure with tmd in this case. verification of the proposed formula was performed with very accurate results. nevertheless, further experimental verification and investigation in this field shall be performed. acknowledgements the authors gratefully acknowledge support from the czech technical university in prague, project sgs18/037/ohk1/1t/11 development and application of advanced algorithms for numerical analysis and modeling in mechanics of structures and materials references [1] štimac grandić. serviceability verification of pedestrian bridges under pedestrian loading. tehnički vjesnik 22:527–537, 2015. doi:10.17559/tv-20131030105641. [2] setra. footbridges: assessment of vibrational behaviour of footbridges under pedestrian loading, 2006. [3] fib bulletin 32: guidelines for the design of footbridges, 2005. [4] s. živanović, a. pavić, p. reynolds. vibration serviceability of footbridges under human-induced excitation: a literature review. journal of sound and vibration 279(1-2):1–74, 2015. doi:10.1016/j.jsv.2004.01.019. [5] a. dazio. fundamentals of structural dynamics. an-najah national university, 2013. [6] guide to basis of bridge design related to eurocodes supplemented by practical design handbook 4. leonardo da vinci pilot project, 2004. [7] e. caetano, a. cunha, w. hoorpah, j. raoul. footbridge vibration design. crc press, 2009. [8] e. shahabpoor, a. pavic, v. racic. interaction between walking humans and structures in vertical direction: a literature review. shock and vibration p. 22, 2016. doi:10.1155/2016/3430285. 119 http://dx.doi.org/10.17559/tv-20131030105641 http://dx.doi.org/10.1016/j.jsv.2004.01.019 http://dx.doi.org/10.1155/2016/3430285 acta polytechnica ctu proceedings 15:114–119, 2018 1 introduction 2 pedestrian load models 2.1 single pedestrian load 2.2 stationary pedestrian load model 2.3 moving pedestrian load model 3 parametric study 3.1 set of simply supported beams 3.2 response of the structure 3.2.1 response to stationary pulsating force 3.2.2 response to moving pulsating force 3.3 evaluation 4 verification of the proposed formula 4.1 locked tmd 4.2 free tmd 5 conclusions acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2020.26.0133 acta polytechnica ctu proceedings 26:133–138, 2020 © czech technical university in prague, 2020 available online at https://ojs.cvut.cz/ojs/index.php/app comparing mori-tanaka method and first-order homogenization scheme in the viscoelastic modeling of unidirectional fibrous composites soňa valentová, michal šejnoha∗, jan vorel czech technical university in prague, faculty of civil engineering, department of mechanics, thákurova 7, 166 29 prague 6, czech republic ∗ corresponding author: sejnom@fsv.cvut.cz abstract. a comparative study of the viscous response of polymer matrix based fibrous composites predicted by the mori-tanaka method and finite element simulations based on the 1st order homogenization theory is presented. aligned basalt and carbon fibers embedded into a polymeric matrix are considered to represent a quasi isotropic and transversely isotropic two-phase systems. while differences in the prediction of the macroscopic elastic response are attributed merely to the properties of the fiber phase, the viscoelastic behavior is largely affected by the selected homogenization method. a stiffer response predicted by the mori-tanaka method for both creep and relaxation tests is observed for both material systems and supports similar finding found in the literature. thus suitable modifications of the original formulation of such two-point averaging schemes are needed for them to be applicable in the multi-scale modeling of generally anisotropic yarns in plane weave textile composites. keywords: viscoelasticity, homogenization, mori-tanaka method, periodic unit cell, transformation field analysis, fibrous composites. 1. introduction a plane weave textile composites made of woven yarns reinforced by unidirectional fibers bonded to a polymeric matrix are used in many engineering applications. since often exposed to rate dependent or long lasting variable loading the viscous behavior of the matrix phase should be properly represented to arrive at reliable predictions of generally time dependent macroscopic response of such composites. it has been shown in [1] that random nature of the yarn microstructure may play a significant role in the prediction of its macroscopic viscoelastic response particularly in case of high contrast in material properties of the fiber and matrix phase. therein, the concept of statistically equivalent periodic unit cell (sepuc) [2, 3] was exploited to address adequacy of the often assumed periodic hexagonal arrangement (pha) of fibers. this issue is also briefly addressed herein. the point is that if sufficiently accurate, the pha model may be used to validate the applicability of the mori-tanaka method because of their correspondence in the elastic regime. following the footsteps of [4] the virtual creep or relaxation tests carried out numerically at the level of yarn could be used to calibrate a suitable macroscopic viscoelastic model to represent the yarn behavior at the level of textile. however, the macroscopically isotropic response of asphalt mixtures assumed in [4] is hardly acceptable for macroscopically orthotropic fibrous composites. this opens the way to fully coupled multi-scale analysis. most often the fe2 computational scheme [5] is used, where the sepuc model at the level of yarns is loaded by increments of the local strain averages derived at the level of yarns for a given increment of the macroscopic load. the sepuc analysis provides the instantaneous homogenized yarn stiffness and corresponding stress increment. such a format of stress update may often be very computationally expensive. a potential route appears in replacing detailed finite element simulations by a suitable micromechanical model such as the mori-tanaka averaging scheme [6] assuming a piece-wise uniform distribution of stresses and strains in individual material phases thus utilizing the so called two-point averaging scheme in case of two-phase materials, see e.g. [7, 8]. however, using the mori-tanaka method may raise some doubts because of the method limitations in connection to time dependent or nonlinear analyses. in particular, much stiffer response delivered by the mori-tanaka method in comparison to unit cell finite element simulations has often been reported. the purpose of this study is to contribute to this subject in the light of purely viscoelastic behavior of the matrix phase and if needed to offer suitable modifications to the original formulation of such micromechanical models while still keeping their computational efficiency. the paper is organized as follows. section 2 introduces the two material systems examined herein. a formulation of the adopted viscoelastic model is presented next in section 3 followed in section 4 by short theoretical description of the two homogenization approaches. the principal achievements are summarized in sections 5 and 6. 133 https://doi.org/10.14311/app.2020.26.0133 https://ojs.cvut.cz/ojs/index.php/app s. valentová, m. šejnoha, j. vorel acta polytechnica ctu proceedings ea et ga gt νa a b n cf [gpa] [gpa] [gpa] [gpa] [-] [gpa]−1 [gpa]−1 [-] [-] carbon fiber 294 13 12 5 0.24 0.67 basalt fiber 69.68 64.82 28.10 26.14 0.40 0.69 epoxy matrix 2.03 2.03 0.725 0.725 0.40 0.0474 0.00214 0.3526 table 1. material properties of individual phases. 2. materials two material systems are examined in the present study to address both the influence of microstructural details and material properties of individual phases. the same epoxy resin was considered for both carbon and basalt fiber composite with their microstructure displayed in fig. 1(a) and (b), respectively. (a) (b) figure 1. example images of local microstructure: a) carbon fibers, b) basalt fibers. reprinted from [9] with permission from begell house publishers. the corresponding material data taken from [9– 11] are listed in table 1. while basalt fibers are quasi-isotropic the carbon fiber composite represents a highly anisotropic system. on the contrary, the contrast in material properties of the fiber and matrix in the transverse plane is relatively small in case of carbon fibers as oppose to basalt fibers. this particular mismatch in material properties will be reflected in section 4.2 when constructing the computational model. 3. local constitutive equations for the selected epoxy resin the last three parameters in table 1 are the model parameters of a simple power law like formula for the relaxation rate1 r(t, τ) = 1 a+ b(t− τ)n , (1) where r(t, τ) is the relaxation function representing the stress at time t due to a unit strain applied at time τ and held constant. to facilitate the numerical solution this function is typically approximated by the dirichlet series in the form r(t, τ) = m∑ µ=1 eµ(τ) exp [ τ θµ − t θµ ] , (2) 1note that these specific values of parameters a, b, n require the time t − τ be given in minutes to yield the viscoelastic modulus in [mpa]. where θµ are the a priory selected relaxation times. functions eµ are usually obtained by fitting the relaxation function, e.g. eq. (1), via eq. (2) using the method of least squares. (a) (b) figure 2. a) maxwell chain unit, b) relaxation function. assuming for example the generalized maxwell chain model in fig. 2(a) composed of ten maxwell elements, we get the spring moduli eµ listed in table 2. note that sum of all spring moduli provides the elastic modulus of the epoxy matrix in table 1. for illustration, fig. 2 plots the relaxation functions provided by eqns. (1) and (2). µ θµ [mpa·s] eµ [mpa] 1 2.949089 3.548983×101 2 9.506234 9.965608×101 3 8.905084×101 2.133588×102 4 7.939963×102 3.599087×102 5 6.874448×103 4.325570×102 6 5.709058×104 3.954614×102 7 5.221879×105 2.450281×102 8 4.561310×106 1.384739×102 9 4.654514×107 5.785885×101 10 4.749622×108 5.854287×101 table 2. parameters of maxwell chain model. for the generalized maxwell chain model it holds σ = m∑ µ=1 σµ, (3) where σµ are stresses in individual maxwell elements. integrating the differential form of constitutive equation for each maxwell element over the time increment 134 vol. 26/2020 viscoelastic modeling of unidirectional fibrous composites ∆t and using eq. (3) gives the increment of the total stress ∆σ as ∆σ = ê (∆ε−∆µ) , (4) where the instantaneous young’s modulus ê and the increment of viscoelastic strain ∆µ read [12] ê = n∑ µ=1 ∆teµ θµ ( 1− exp [ −∆t θµ ]) , (5) ∆µ = 1 ê n∑ µ=1 ( 1− exp [ −∆t θµ ]) σµ(ti−1), (6) where σµ(ti−1) represents the hidden stress at the beginning of the new time increment. the stress σµ(ti) at the end of the time increment is then provided by σµ(ti) = σµ(ti−1) exp [ −∆t θµ ] + (7) + eµ ( 1− exp [ −∆t θµ ]) (∆ε−∆µ). extension of the above set of one-dimensional equations to a general three-dimensional space is provided, e.g. in [1]. 4. homogenization schemes in this section we briefly review the mori-tanaka micromechanical model for a two phase material system in the framework of transformation field analysis [13] and the first-order homogenization scheme grounding on the existence of periodic fields. 4.1. mori-tanaka method consider a two-phase composite made of elastic transversely isotropic fibers of a circular cross-section embedded into a viscoelastic isotropic matrix. assuming piece-wise constant local fields in individual phases r = f,m2 allows us to write the local constitutive equations in the form ∆σf = lf∆εf , ∆σm = l̂m(∆εm −∆µm), (8) where l is the material stiffness matrix and l̂ represents the dependence on the viscoelastic modulus ê. suppose that the composite is loaded on its outer boundary either by the prescribed displacements (strain control) or tractions (stress control) compatible with macroscopic uniform strain ∆e or stress ∆σ increments. the local strains or stresses in terms of their applied macroscopic counterparts follow from dvorak’s transformation field analysis [13] as ∆εf = âf∆e + d̂fm∆µm, (9) ∆εm = âm∆e + d̂mm∆µm, (10) ∆σf = b̂f∆σ− f̂ fml̂m∆µm, (11) ∆σm = b̂m∆σ− f̂mml̂m∆µm, (12) 2subscripts f,m stand for the fiber and matrix phase, respectively. where âr, b̂r and d̂rm, f̂ rm, r = f,m, are the mechanical strain and stress localization factors and strain and stress transformation influence functions, respectively. the mori-tanaka method gives the mechanical localization factors in the form âm = [ cmi + cf t̂f ]−1 , âf = t̂f âm, (13) b̂m = [ cmi + cfŵf ]−1 , b̂f = ŵf b̂m,(14) where the so called partial strain and stress localization factors t̂f , ŵf can be expressed in terms of the eshelby tensor ŝ as t̂f = [ i− p̂ ( l̂m − l̂f )]−1 , (15) ŵf = [ i− q̂ ( m̂m − m̂f )]−1 , (16) p̂ = ŝm̂m, p̂ = −l̂m ( ŝ− i ) , (17) where m = l−1 is the material compliance matrix. note that in case of the mori-tanaka method the eshelby tensor ŝ depends on instantaneous properties of the matrix phase. for a cylindrical fiber3 its particular form is available, e.g. in [8]. for a two-phase composite the transformation influence functions are readily provided in terms of the localization factors as d̂fm = ( i− âf )( l̂m − lf )−1 l̂m, (18) d̂mm = ( i− âm )( l̂m − lf )−1 l̂m, (19) f̂fm = ( i− b̂f )( m̂m −mf )−1 m̂m, (20) f̂mm = ( i− b̂m )( m̂m −mf )−1 m̂m. (21) 4.2. 1st order homogenization assuming again the composite be loaded either by the macroscopic uniform strain ∆e or stress ∆σ admits the following decomposition ∆u(x) = ∆e x + ∆u∗(x), (22) ∆ε(x) = ∆e + ∆ε∗(x), (23) where ∆u∗(x) is the fluctuation part of the local displacement field ∆u(x). the former one is considered periodic to give the volume average of the fluctuation strain ∆ε∗(x) equal to zero. the stepping stone in the derivation of fluctuation displacements ∆u∗(x) is the hill lemma given by〈 δεt∆σ 〉 = δet∆σ. (24) 3note that for cylindrical fibers the eshelby tensor depends on the matrix phase poisson ratio only, which is assumed constant in our study, i.e. ŝ = s . also note that keeping the time increment ∆t constant yields the viscoelastic modulus êm also constant. thus the localization and transformation matrices need to be evaluated only once. 135 s. valentová, m. šejnoha, j. vorel acta polytechnica ctu proceedings where 〈·〉 stands for volume averaging. in the framework of finite element discretization we approximate ∆u∗(x) in terms of their nodal values ∆r to write ∆u∗(x) = n(x)∆r, ∆ε∗(x) = b(x)∆r. (25) substituting from eq. (25)2 into (24) yields the final system of algebraic equations in the form [ k11 k12 k21 k22 ]{ ∆e ∆r } = { ∆σ + ∆f 0 ∆f0 } . (26) individual matrices listed in eq. (26) are written as k11 = 1 ω ∫ ω l̂(x) dω, k12 = kt 21 = 1 ω ∫ ω l̂(x)b(x) dω, (27) k22 = 1 ω ∫ ω bt(x)l̂(x)b(x) dω, and components of the right-hand side vector are ∆f 0 = 1 ω ∫ ω l̂(x)µ(x) dω, (28) ∆f0 = 1 ω ∫ ω bt(x)l̂(x)∆µ(x) dω. recall that the system of equations 26 is adopted when loading the composite by the prescribed macroscopic stress such as in case of creep. simulating a relaxation test calls for the macroscopic strain ∆e be prescribed. the system of equations 26 then simplifies to (k22)∆r = −(k21)∆e + ∆f0. (29) the increments of macroscopic strains ∆e (σ prescribed) or stress ∆σ (e prescribed) follow from the volume averaging of their local counterparts. further details can be found in [1, 8]. 5. results the first study aimed at comparing the results provided by various unit cell models. figures 3(b-c) show 5-fiber and 10-fiber periodic unit cells (puc). note that these were constructed by simply changing the diameter and slightly also the position of sepucs derived originally in [8, chapter 3] for a carbon-carbon composite to meet the required volume fractions. such puc thus should not be termed sepuc for the present systems. inadequacy of such a simple approach is confirmed by the results plotted in fig. 5 for the basalt fiber composite subjected to both creep and relaxation loading conditions displayed in fig. 4. a highly anisotropic behavior predicted by 5-fiber and 10-fiber puc in comparison to pha model in fig. 3(a) is evident in figs. 5(a,c) showing a different (a) (b) (c) figure 3. periodic unit cells: a) pha model, b) basalt 5 fiber uc, c) basalt 10 fiber uc. time [s] σ (σ )xy = 100 [mpa]xx σ time [s] e xy xxe (e ) = 0.375 [%] (a) (b) figure 4. applied loading: a) creep test, b) relaxation test. response of the two systems when loading the composite along x and y axes, but also in figs. 5(b,d) showing an evolution of a non-negligible shear strain and stress for the creep and relaxation loading conditions, respectively, associated with the two loading cases. while, as expected, the response predicted by the pha model confirms a material isotropy in the transverse plane. further study comparing the predictions provided by the fem simulations and the mori-tanaka thus adopted the pha model only. the results appear in fig. 6. figure 6(a) shows the evolution of the macroscopic normal strain exx caused by the macroscopic normal stress σxx. for both material systems the initial elastic response predicted by the mori-tanaka method appears more compliant in comparison to fem simulations, while the subsequent time dependent evolution of strains confirms much stiffer behavior suggested by the mori-tanaka method. this is supported by the results derived for the shear loading, figure 6(b), as well as for the relaxation tests, figs. 6(c,d) the results clearly show that the predictions depend not only on the material system but also on the applied type of loading conditions. while the differences in the elastic behavior can be associated with the material properties of the fiber phase, the evolution of local creep strain is driven by the selected computational model. a non-uniform distribution of creep strain in the matrix phase caused by the microstructural details, 136 vol. 26/2020 viscoelastic modeling of unidirectional fibrous composites 0 60 120 180 240 300 360 time [s] 0,50 0,75 1,00 1,25 e [ % ] pha 5 fiber uc 10 fiber uc σ xx applied, e xx plotted σ yy applied, e yy plotted (a) 0 60 120 180 240 300 360 time [s] -0,10 -0,08 -0,06 -0,04 -0,02 0,00 e x y [ % ] pha 5 fiber uc 10 fiber uc σ xx applied σ yy applied (b) 0 60 120 180 240 300 360 time [s] 40 50 60 70 80 σ [ m p a ] pha 5 fiber uc 10 fiber uc e xx applied, σ xx plotted e yy applied, σ yy plotted (c) 0 60 120 180 240 300 360 time [s] 0,0 0,5 1,0 1,5 2,0 2,5 3,0 σ x y [ m p a ] pha 5 fiber uc 10 fiber uc e xx applied e yy applied (d) . figure 5. test of sepuc applicability: (a-b) creep test time variation of macroscopic strains due to prescribed macroscopic stresses σxx, σyy, (c-d) relaxation test time variation of macroscopic stresses due to prescribed macroscopic strains exx, eyy 0 60 120 180 240 300 360 time [s] 0,8 1,0 1,2 1,4 1,6 1,8 e x x [ % ] fem pha mori-tanaka basalt fiber carbon fiber (a) 0 60 120 180 240 300 360 time [s] 2,0 3,0 4,0 5,0 6,0 e x y [ % ] fem pha mori-tanaka carbon fiber basalt fiber (b) 0 60 120 180 240 300 360 time [s] 25 30 35 40 45 50 55 60 σ x x [ m p a ] fem pha mori-tanakabasalt fiber carbon fiber (c) 0 60 120 180 240 300 360 time [s] 6 8 10 12 14 σ x y [ m p a ] fem pha mori-tanaka basalt fiber carbon fiber (d) figure 6. comparison of macroscopic response predicted by finite element simulations and mori-tanaka method for both basalt carbon fiber composites: (a-b) creep test, (c-d) relaxation test a) σxx prescribed, b) σxy prescribed, c) exx prescribed, d) exy prescribed. 137 s. valentová, m. šejnoha, j. vorel acta polytechnica ctu proceedings recall fig. 5, and the mismatch in material properties of the fiber and the matrix phase then renders a significantly more compliant macroscopic response in comparison to the mori-tanaka method, which builds on the piecewise constant stress and strain fields in individual phases, recall eqns.(9) (12). 6. conclusions the present paper examined a potential application of the mori-tanaka method in prediction of effective viscoelastic response of polymer matrix based fibrous composite. the results presented in fig. 6 confirmed a relatively stiff response delivered by the mori-tanaka method when compared to finite element simulations already observed in some previous studies. thus for the mori-tanaka method presented in the framework of dvorak’s transformation field analysis to be applicable as a very efficient substitute of computationally demanding finite element simulations in multi-scale analysis requires some modifications to its original formulation. with regard to generally nonlinear material behavior the literature offers a number of potential approaches. reduction of stresses in the reinforcing phase, typically assumed linearly elastic, by introducing an artificial damage parameter is one particular example [7, 14]. slightly different approach has been proposed in [15] for the analysis of quasi-brittle materials where the effect of strain localization was addressed with the aid of interfacial coating to allow for gradual debonding and thus letting the matrix to take all the stresses. another approach offers more refined transformation field analysis where the concentration and transformation matrices are derived from finite element method using rather crude meshes where each element represents a single material [16]. all these approaches will be examined in our future studies depending on the results provided by an accompanied experimental program. acknowledgements the financial support provided by the gačr grant no. 1915666s and by the czech technical university in prague within sgs project with the application registered under the no. sgs19/032/ohk1/1t/11 is gratefully acknowledged. references [1] m. šejnoha, j. zeman. overall viscoelastic response of random fibrous composites with statistically quasi uniform distribution of reinforcements. computer methods in applied mechanics and engineering 191(44):5027–5044, 2002. doi:10.1016/s0045-7825(02)00433-4. [2] j. zeman, m. šejnoha. numerical evaluation of effective properties of graphite fiber tow impregnated by polymer matrix. journal of the mechanics and physics of solids 49(1):69–90, 2001. doi:10.1016/s0022-5096(00)00027-2. [3] j. zeman, m. šejnoha. from random microstructures to representative volume elements. modelling and simulation in materials science and engineering 15(4):s325–s335, 2007. doi:10.1088/0965-0393/15/4/s01. [4] r. valenta, m. šejnoha, j. zeman. macroscopic constitutive law for mastic asphalt mixtures from multiscale modeling. international journal for multiscale computational engineering 8(1):131–149, 2010. doi:10.1615/intjmultcompeng.v8.i1.100. [5] v. kouznetsova, m. g. d. geers, w. a. m. brekelmans. multi-scale constitutive modelling of heterogeneous materials with a gradient-enhanced computational homogenization scheme. international journal for numerical methods in engineering 54(8):1235–1260, 2002. doi:10.1002/nme.541. [6] y. benveniste. a new approach to the application of mori-tanaka theory in composite materials. mechanics of materials 6:147–157, 1987. doi:10.1016/0167-6636(87)90005-6. [7] r. valenta, m. šejnoha. hierarchical modeling of mastic asphalt in layered road structures based on the moritanaka method. acta polytechnica 52(6):48–58, 2012. [8] m. šejnoha, j. zeman. micromechanics in practice. wit press, southampton, boston, 2013. [9] j. vorel, e. grippon, m. šejnoha. effective thermoelastic properties of polysiloxane matrix based plain weave textile composites. international journal for multiscale computational engineering 13(3):181– 200, 2015. doi:10.1615/intjmultcompeng.2014011020. [10] f. fára. komplexní hodnocení objemových změn kompozitních soustav. ph.d. thesis, czech technical university in prague, faculty of civil engineerin, prague, czech republic, 1990. [11] s. valentová, v. hrbek, j. vorel, m. šejnoha. strength of composite yarn under biaxial loading. acta polytechnica ctu proceedings 15:131–136, 2018. doi:10.14311/app.2018.15.0131. [12] z. bittnar, j. šejnoha. numerical methods in structural engineering. asce press, 1996. [13] g. j. dvorak, y. benveniste. on transformation strains and uniform fields in multiphase elastic media. proceedings of the royal society of london series a mathematical, physical and engineering sciences 437(1900):291–310, 1992. doi:10.1098/rspa.1992.0062. [14] m. šejnoha, r. valenta, j. zeman. nonlinear viscoelastic analysis of statistically homogeneous random composites. international journal for multiscale computational engineering 2(4):645–673, 2004. doi:10.1615/intjmultcompeng.v2.i4.80. [15] j. vorel, v. šmilauer, z. bittnar. multiscale simulations of concrete mechanical tests. journal of computational and applied mathematics 236(18):4882–4892, 2012. [16] g. dvorak, y. bahei-el-din, a. wafa. the modeling of inelastic composite materials with the transformation field analysis. modelling and simulation in materials science and engineering 2(3a):571, 1999. doi:10.1088/2f0965-0393.2f2.2f3a.2f011. 138 https://doi.org/10.1016/s0045-7825(02)00433-4 https://doi.org/10.1016/s0022-5096(00)00027-2 https://doi.org/10.1088/0965-0393/15/4/s01 https://doi.org/10.1615/intjmultcompeng.v8.i1.100 https://doi.org/10.1002/nme.541 https://doi.org/10.1016/0167-6636(87)90005-6 https://doi.org/10.1615/intjmultcompeng.2014011020 https://doi.org/10.14311/app.2018.15.0131 https://doi.org/10.1098/rspa.1992.0062 https://doi.org/10.1615/intjmultcompeng.v2.i4.80 https://doi.org/10.1088/2f0965-0393.2f2.2f3a.2f011 acta polytechnica ctu proceedings 26:134–139, 2020 1 introduction 2 materials 3 local constitutive equations 4 homogenization schemes 4.1 mori-tanaka method 4.2 1st order homogenization 5 results 6 conclusions acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2020.26.0139 acta polytechnica ctu proceedings 26:139–143, 2020 © czech technical university in prague, 2020 available online at https://ojs.cvut.cz/ojs/index.php/app thecomparison of two different sporting events based on the typical behavior of the active spectators and grandstand vibrations induced by them martin verner czech technical university in prague, faculty of civil engineering, department of mechanics, thákurova 7, 166 29 prague 6, czech republic correspondence: martin.verner@fsv.cvut.cz abstract. the paper describes especially the typical behavior of two active spectators crowds involved in two substantially different sporting events, namely, ice hockey games and a biathlon world cup competition. in both cases, spectators behavior was observed together with grandstand structure dynamic response to their cheering activities. the experimental results obtained during both events were mutually compared and discussed whether the dynamic loading and its effects on the grandstand structures are similar for the two compared spectators crowds. keywords: behavior of fans, grandstands, vibrations of grandstands. 1. introduction hereinafter, the research of the dynamic response of the steel construction induced by audience of different sport events is emphasized. actual research, which follows the one focused on dynamic response of the tribune induced by football fans [1], is focused on two different winter sports – ice hockey and biathlon. selection of these sports is not accidental. ice hockey is one of the most popular sport in czech republic. biathlon was chosen because of the unique mounted construction which has been considered and the attendance of biathlon races which is still growing in numbers in czech republic. the goal of the observation was to find out, if the behavior of the audience is recurrent as well as their influence on the response of the construction. for both sports the following assumption is taken – audience does not induce construction by the jumping in contrast to football etc., audience induces construction by the applause, handclapping. 2. measurement method the piezoelectric sensors brüel&kjær type 4507 b005 were used for all the experiments. pilsen, the four sensors were placed on the bottom flange of ice arena construction. two sensors, no. 1 and 3, measured horizontal acceleration of the girders. another two sensors installed in the middle of the transverse girder/beam. sensor no. 2 and no. 4 were used for measuring horizontal and vertical acceleration respectively see fig 1. number of the measurement sensors was limited to four sensors by the character of the measurement link. nine sensors were installed, two measurement links were applied (4 and 5 channel) in nové mv̌sto na moravě. acceleration in all directions measured on three points of the construction, in accordance with experiment in pilsen. two construction points, no. figure 1. location of sensors at the grandstands [2]. 1 and 3, were located on the stands of the steel construction and one point, no. 2, was located in the middle of the plank span see fig. 2. the behavior of the audience was recorder using the camera. the installation placement of the camera was chosen in accordance to not to raise suspicion of the audience. the camera was installed on the opposite tribune nn pilsen,. the camera was installed on the television tower left from construction in nové město na moravě. the distance between camera and recorded place was approximately 100 m in both cases. 2.1. description of construction 2.2. ice arena pilsen the construction of the ice hockey stadium in pilsen began in 1950. the overall reconstruction of the stadium took place including roof construction and stand reconstruction in 1969. since 1969, among other reconstructions, the building envelope has been replaced and the concrete construction rehabilitated. construction of the stadium is made of reinforced concrete skeleton/frame see fig.3. 139 https://doi.org/10.14311/app.2020.26.0139 https://ojs.cvut.cz/ojs/index.php/app martin verner acta polytechnica ctu proceedings figure 2. location of sensors at the grandstands and measuring axis sensors [3]. figure 3. construction of the stands of the ice stadium. the main girders are fixed to the columns. the girders support transverse panels of a l-shape. there is no evidence of any project documentation of the ice arena. therefor the basic level of documentation – passport was drawn up where the basic construction elements are described including dimensions. based on the passport the whole construction is considered to become monolithic. the modal analysis was performed and the first measured eigenfrequency of the stand was 12.5 hz (evaluated by the software mescope). 2.3. biathlon stadium nové město na moravě in the biathlon stadium there is a permanent reinforced concrete grandstand. audience capacity of the stadium could be extended by the temporary steel stands. these steel stands are based on modular scaffold pieces, steel planks and poles and could be built up to 14 m above ground. in accordance with safety requirements the jumping on the steel stand is forbidden see fig 4. figure 4. construction of prefabricated stands. there is no reliable mathematical model describing dynamic behavior of the jumping crowd based on the information of the scaffold stand manufacturer. according to the only technical readout focused on dynamics of scaffold stands – british standard, the evaluation of the dynamic load has to be performed if and only if the first eigenfrequency is smaller than 8 hz. the modal analysis of the steel stand was performed and the first measured eigenfrequency of the stand was 3.0 hz (evaluated by the software mescope). 2.4. spectators behavior spectators behavior was assessed using the video recording see fig 5. different types of behavior were recognized on the record. the total time of each 140 vol. 26/2020 the comparison of two different sporting events figure 5. position of the camera relative to the monitored area [4, 5]. the type of spectator behaviour the time during for the game [%] static state 23.6 walking and running 5.5 jumping – all type bouncing 1.1 swaying 15.9 hand clapping – all types 51.3 goal scoring 2.6 mexican wave table 1. behavior of fans hc škoda plzeň vs. hc kometa brno (2.10.2015). the type of spectator behaviour the time during for the game [%] static state 23.6 walking and running 5.5 jumping – all type bouncing 1.1 swaying 15.9 hand clapping – all types 51.3 goal scoring 2.6 mexican wave table 2. behavior of fans ibu nmn 2019 – women mass start (18.12.2016). behavior type was added up through the whole recording. the seven behavior types were recognized during the hockey matches. static state – spectators are standing on the tribune without any other activity; walking and running – recorded at the beginning and at the end of the match when spectators walk in and out of the stadium; bouncing – bouncing of knees when heels do not loose contact with the floor; swaying – the spectators rock their bodies back and forth and to the sides; there were recognized 3 types of hand clapping – 1) normal applause, clapping hand in front of the chest, 2) clapping hands over head, 3) clapping hands move from the place in front of the chest above head, all of them are considered as one type behavior hand clapping; goal scoring and mexican wave (the sitting spectators are rising, one row after another and wave their hands above their heads) see at the tab 1. the four behavior types were recognized during biathlon races see at the tab 2. there are two types excluded. jumping is forbidden on the biathlon competitions and therefore was not recognized. and second type – goal scoring is excluded inherently. the experiment did not consider mexican wave because this type of behavior took place during competition breaks when the crowd was not recorded. 3. response of stand the main part of the experiment was focused on the dynamic response of the stand. fourier transformation on the measured acceleration of different part of the stand was applied leading to excitation frequen141 martin verner acta polytechnica ctu proceedings cies induced by spectators. each type of behavior is represented by different excitation frequency. one can see similarity in terms of excitation frequency for all the types of behavior. 4. summary two different sporting events, the ice hockey games and biathlon race, were compared mutually in the paper. the described experiments proved our expectations based on previous observations [1, 2]. the spectators behaved in similar way during both compared sport events. the two most used behavior types in the course of the ice hockey games were static state and hand clapping. the same two types were most often utilized by spectators also during the biathlon race. moreover, it is evident that fundamental excitation frequencies for individual behavior types are very similar for both compared events. it may be remarked that the first natural frequency of the temporary steel grandstand (3.0 hz) is comparable to the basic excitation frequency (2.7 hz) evaluated during the type of behavior – bouncing, see fig. 9. on the other hand, the level of induced grandstand vibration was quite different. the highest vibration level recorded on the temporary steel grandstand during the biathlon race was 1.2 m/s2 and it was substantially higher in comparison to 0.18 m/s2 that was registered on the reinforce concrete grandstand in the course of the ice hockey games. -0,20 -0,15 -0,10 -0,05 0,00 0,05 0,10 0,15 0,20 240 242 244 246 248 250 252 254 time [s] a cc el er at io n [m ·s -2 ] 0,0000 0,0005 0,0010 0,0015 0,0020 0,0025 0,0030 0,0035 0,0040 0 5 10 15 20 25 30 35 frequency [hz] a cc el er at io n [m ·s -2 ] figure 6. ice-hockey, dynamic response of the stand hand-clapping [2]. -0,30 -0,20 -0,10 0,00 0,10 0,20 0,30 255 265 275 285 295 305 315 time [s] a cc el er at io n[ m ·s -2 ] 0 0,0005 0,001 0,0015 0,002 0,0025 0,003 0,0035 0,004 0,0045 0,005 0 2,5 5 7,5 10 12,5 15 17,5 20 22,5 25 27,5 30 32,5 35 frequency [hz] a cc el er at io n[ m ·s -2 ] figure 7. ice-hockey, dynamic response of the stand scored goal [2]. -2,50 -2,00 -1,50 -1,00 -0,50 0,00 0,50 1,00 1,50 2,00 2,50 270 275 280 285 290 295 300 time [s] a cc el er at io n [m .s -2 ] 0,00 0,02 0,04 0,06 0,08 0,10 0,12 0,14 0,16 0,18 0,20 0 1 2 3 4 5 6 7 8 9 10 frequency [hz] a cc el er at io n [m .s -2 ] figure 8. ibb, dynamic response of the stand – hand-clapping after shoot [3]. 142 vol. 26/2020 the comparison of two different sporting events -1,00 -0,80 -0,60 -0,40 -0,20 0,00 0,20 0,40 0,60 0,80 1,00 115 120 125 130 135 140 145 150 155 160 165 time [s] a cc el er at io n [m .s -2 ] 0,000 0,005 0,010 0,015 0,020 0,025 0 1 2 3 4 5 6 7 8 9 10 frequency [hz] a cc el er at io n [m .s -2 ] figure 9. ibb, dynamic response of the stand – bouncing – axes y [3]. -0,15 -0,10 -0,05 0,00 0,05 0,10 115 125 135 145 155 165 time [s] a cc el er at io n [m .s -2 ] 0,0000 0,0005 0,0010 0,0015 0,0020 0 1 2 3 4 5 6 7 8 9 10 frequency [hz] a cc el er at io n [m .s -2 ] figure 10. ibb, dynamic response of the stand – bouncing – axes z [3]. acknowledgements this paper has been supported by the czech science foundation project no. ga15-15728s. and project of the faculty of civil engineering, czech technical university in prague (sgs project no.16/201ohk1/3t/11). references [1] t. plachy, m. polak, m. verner. an experimental study focused on spectators’ behavior and induced vibrations of a reinforced concrete grandstand on a football stadium. procedia engineering oxford: elsevier ltd) pp. 2871–2876, 2017. [2] m. polak, t. plachy, m. verner. an analysis of the fans’ behavior during two ice hockey games and its influence on the level of grandstand vibrations. 54th international conference on experimental stress analysis 2016 2016. [3] j. hubka. dynamic analysis of grandstand, 2017. master\t1\textquoterightsthesis. [4] bmw-ibu-world-cup-biathlon-2020. https://www.ticketlive.cz/cs/event/biatlon-2020. [5] z. stadion plzeň. https://www.zimnistadionplzen.cz/arena/plan-haly/. 143 master\t1\textquoteright s thesis acta polytechnica ctu proceedings 26:140–144, 2020 1 introduction 2 measurement method 2.1 description of construction 2.2 ice arena pilsen 2.3 biathlon stadium nové mesto na morave 2.4 spectators behavior 3 response of stand 4 summary acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2019.25.0079 acta polytechnica ctu proceedings 25:79–82, 2019 © czech technical university in prague, 2019 available online at http://ojs.cvut.cz/ojs/index.php/app deformation response of polydimethylsiloxane substrates subjected to uniaxial quasi-static loading vladimír vinarskýa,∗, fabiana martinoa, giancarlo fortea, jan šleichrtb, václav radab, daniel kytýřb a st. anne’s university hospital brno, international clinical research center, pekařká 53, 656 91 brno, czech republic b czech academy of sciences, institute of theoretical and applied mechanics, prosecká 809/76, 190 00 prague 9, czech republic ∗ corresponding author: vladimir.vinarsky@fnusa.cz abstract. to investigate cellular response of cardiomyocytes to substrate mechanics, biocompatible material with stiffness in physiological range is needed. pdms based material is used for construction of microfluidic organ on chip devices for cell culture due to ease of device preparation, bonding, and possibility of surface functionalization. however it has stiffness orders of magnitude out of physiological range. therefore, we adapted recently available protocol aiming to prepare substrates which offer stiffness in physiological range 5 − 100 kpa using various mixtures of sylgard. an in-house developed loading device with single micron position tracking accuracy and sub-micron position sensitivity was adapted for this experimental campaign. all batches of the samples were subjected to uniaxial loading. during quasi-static experiment the samples were compressed to minimally 40 % deformation. the results are represented in the form of stress-strain curves calculated from the acquired force and displacement data and elastic moduli are estimated. keywords: quasi-static loading, hyperelasticity, polydimethylsiloxane substrates, in-situ loading device. 1. introduction mechanical stimulus emerged as important factor for cell and organ development, homeostasis, and disease in vivo. to investigate effects of static and dynamic mechanical stimulation of in vitro grown cells two different approaches are being currently used. to determine the effect of material stiffness in static conditions polyacrylamide (pa) gels which can be prepared in the range of stiffness reflecting in vivo situation are used. on the other hand, effects of dynamic mechanical stimulation (stretching) are investigated using polydimethylsiloxane (pdms) based microfluidic devices, where cultivation membrane is orders of magnitude stiffer than what can be found in vivo. so far it has been shown that both static and dynamic mechanical stimuli influence cardiomyocyte specification and maturation in vitro. in order to integrate physiological stiffness and possibility to actuate (stretch) the cultivation surface in one device we aimed to prepare pdms based material with stiffness in physiological range, which can be used in pneumatically actuated microfluidic device and test its biocompatibility. as the actuation of the cultivation membrane in general changes material properties we decided to measure the stiffness with tensile strain up to 20 % of original length to mimic its application in microfluidic device. biomechanical analysis of pdms is challenging experimental procedure because of very low material stiffness. there are two basic approaches for this purpose. for the micromechanical tested an atomic force microscopy or dynamic nanoindentation [1] could be used. this experimental procedure is very precise but for bulk material testing macromechanical approach is more appropriate to obtain overall material properties [2]. moreover customised precise macroscopic loading experiments coupled with radiography imaging techniques could provide the information about internal microstructure changes during the loading [3]. for this purpose the material properties of pdms mixtures was obtained employing fully customised experimental system. 2. materials and methods 2.1. sample preparation based on previously published data (see [4]) different ratios of sylgard 184 and sylgard 527 were mixed to obtain stiffness ranging from 5 kpa up to 170 kpa. in order to further decrease stiffness we manipulated the ratio of components a and b of sylgard 527 from 1.2 to 1. the resulting mixtures were degassed, poured in desired vessels and polymerized at 65 ◦c overnight. the bulk material was polymerized in plastic test tubes with nominal inner diameter 12.00 mm. properly polymerized cylinders with diameter 12.00 ± 0.05 mm were carefully removed and carve up to the 79 http://dx.doi.org/10.14311/app.2019.25.0079 http://ojs.cvut.cz/ojs/index.php/app v. vinarský, f. martino, g. forte et al. acta polytechnica ctu proceedings figure 1. visualization of the disassembled loading device with the main components: 1 base plate (for rotary table), 2 loading device frame, 3 motor assembly with harmony gearbox, 4 linear guide assembly, 5 main body cover with clamps, 6 loadcell and loading plate, 7 loading chamber with loading plate samples with height 14 − 16 mm using the microtome blade. plan-parallel faces of the samples are required for relevant testing, therefore only the sampled with height precision ±0.10 mm were selected for the testing. precise height of the samples was derived from loading plates positions at the contact force 0.005 n and 0.010 n for load cells with nominal capacity 1 n and 10 n respectively. for the strain calculation the height was rounded to 50µm. 2.2. instrumentation estimated high compliance of the material require to use highly precise measuring procedure. therefore, an in-house developed loading device originally intended for 4d microct in detail description in [5] was employed. the device allows to perform analysis of biological samples and artificial product of tissue engineering in controlled ambient conditions. the design of compact table loading device allow to perform the mechanical testing up to 3 kn with single micron position tracking. using the magnetic linear encoder with nominal resolution 250 nm the device operates with sub-micron position sensitivity. using the load cells with appropriate capacity, in this case miniature s beam load cells (lsb200 series, futek inc., usa) with nominal capacity 10 n and 1 n respectively, allows to provide reliable deformation analysis of the materials with elastic modulus in range of single kpa. main components of the loading device are depicted in fig. 1. for this purpose the loading frame was made of a high strength aluminium alloy (en-aw-6082-t6) tube with 60 mm diameter with holes for sample manipulation and optional optical imaging. the loading plates made from the same material with diameter 16 mm ensure the proper support of the faces of the sample under high (≈ 50 %) deformation. the motion of the system and all other peripherals are controlled using a custom developed control software based on the open-source project linuxcnc running on the real-time kernel [6]. the control hardware consists of high-performance motion control components (mesa electronics, u.s.a.), sensor readout electronics (labjack, u.s.a.) and custom electronics. 2.3. experimental procedure to obtain general information about the deformation behaviour of the mixtures and confrontation with findings presented in [4] set of pilot test was performed using 1 to 2 and 1 to 9 (sylgard 184 to sylgard 527) mixtures. it was found that i) the behaviour of the material is fully hyperelastic to minimally 60 % strain ii) the sample is collapsing by sudden outsplashing at level of 70 − 80 % compressive strain iii) the stress-strain behaviour during both loading and unloading phase was strain rate independent on level 0.005 − 0.05 s−1. moreover, the samples due its very low stiffness exhibits high adhesion on the surfaces. it allows to measure tensile behaviour beyond the zero point at unloading of the compressed samples nearly to 10 % tensile strain without any special sample fixation. finally one sample of each type was stored in the fridge with constant temperature of 5 ◦c the other were stored in laboratory temperature of 20 − 25 ◦c. mechanical tests were performed on all samples at as delivered fresh state, after one week and after six weeks. no significant/measurable changes caused aging and/or storage condition were observed. based on the results of pilot test following experimental procedure was applied on all types of the samples. from each type of material three samples were measured. each sample was put to loading chamber and its height was measured by the indicated contact. from this initial point the sample was compressed to the level of 40 % strain on loading rate 20µm · s−1 (strain rate ≈ 0.015 s−1) and hold on for 5 s at this position. then with the same loading rate the sample was continually unloaded and stretched to 10 %. from this point an other two loading cycled was performed. for the stress-strain analysis and elastic modulus estimation the average unloading curve was calculated. 2.4. data evaluation fully automatic evaluation procedure was developed using matlab (mathworks inc., usa). primary 80 vol. 25/2019 deformation response of pdms substrates figure 2. detail of the compressed pure sylgard 184 sample under 60 % deformation in loading chamber data acquired by control software with sampling frequency 200 hz was imported and force log from load cell and position from encoder was post processed. from the record the unloading phases were extracted and averaged to one unloading force–displacement behaviour. for deformation behavior the stress σ in all experimental analysis was considered as engineering stress obtained using σeng = f ac , (1) where ac is cross-sectional area of the specimen calculated from minimal sample diameter measured before deformation. for the purpose of stress calculations, samples were considered ideally cylindrical, neglecting all geometrical irregularities. engineering strain εeng was calculated by formula εeng = u h0 , (2) where u represents displacement measured by encoder and h0 is height of the sample before the mechanical loading. magnitude of the young’s modulus at level of 0 %, 10 %, and 30 % strain labeled e0.0, e0.1 and e0.3 in stress-strain diagrams (see figs. 3–8) was estimated linear regression from 600 stress–strain values. the linear fit provide less than 0.5 % error. finally the gnuplot (command-line open source program for twoand three-dimensional data and function graphical visualization) code was generated and executed. 3. results material properties and deformation behaviour of twelve different pdms mixtures were derived from set of uniaxial loading tests. each type of material was tested with the batch of the three samples. stress– strain diagram of selected six clinically relevant mixtures together with young’s modulus are depicted and listed in figs. 3–8. light red stripe represents an envelope of measured data from whole batch. dark red curve represents mean values. −2 0 2 4 6 8 10 12 14 16 −0.1 0 0.1 0.2 0.3 0.4 s tr es s [k p a] strain [−] 1:2 (syl184:syl527) e0.0 = 17.0±0.2kpa e0.1 = 27.9±0.2kpa e0.3 = 44.0kpa±0.3 figure 3. deformation behaviour of 1 to 2 (sylgard 184 to sylgard 527) mixture −2 0 2 4 6 8 10 12 −0.1 0 0.1 0.2 0.3 0.4 s tr es s [k p a] strain [−] 1:3 (syl184:syl527) e0.0 = 13.5±0.1kpa e0.1 = 19.8±0.1kpa e0.3 = 31.6±0.2kpa figure 4. deformation behaviour of 1 to 3 (sylgard 184 to sylgard 527) mixture −0.5 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 −0.1 0 0.1 0.2 0.3 0.4 s tr es s [k p a] strain [−] 1:9 (syl184:syl527) e0.0 = 6.4±0.1kpa e0.1 = 8.3±0.2kpa e0.3 = 13.2±0.1kpa figure 5. deformation behaviour of 1 to 9 (sylgard 184 to sylgard 527) mixture −0.5 0 0.5 1 1.5 2 2.5 −0.1 0 0.1 0.2 0.3 0.4 s tr es s [k p a] strain [−] 1:1 (syl527a:syl527b) e0.0 = 3.8±0.1kpa e0.1 = 4.5±0.1kpa e0.3 = 7.5±0.1kpa figure 6. deformation behaviour of 1.0 to 1.0 (sylgard 527a to sylgard 527b) mixture 81 v. vinarský, f. martino, g. forte et al. acta polytechnica ctu proceedings −0.2 0 0.2 0.4 0.6 0.8 1 1.2 −0.1 0 0.1 0.2 0.3 0.4 s tr es s [k p a] strain [−] 1.2:1 (syl527a:syl527b) e0.0 = 1.0±0.1kpa e0.1 = 2.0±0.1kpa e0.3 = 3.6±0.1kpa figure 7. deformation behaviour of 1.2 to 1.0 (sylgard 527a to sylgard 527b) mixture −0.5 0 0.5 1 1.5 2 2.5 −0.1 0 0.1 0.2 0.3 0.4 s tr es s [k p a] strain [−] pure syl527 e0.0 = 3.7±0.1kpa e0.1 = 4.5±0.1kpa e0.3 = 7.0±0.1kpa figure 8. deformation behaviour of pure sylgard 527 4. conclusions to find optimised biocompatible pdms substrates for cardiomyocytes developed for further mechanobiological studies the bulk material with different ratio of sylgard 184, sylgard 527a, and sylgard 527b components. mechanical analysis of soft pdms require employment of custom designed precise loading device measuring the forces and displacement in millinewtons and micrometers respectively. from uniaxial mechanical test deformation behaviour and elastic properties was derived. reliability of the instrumentation and measurement procedure for hyperelastic materials with young’s modulus from single kilopascal corresponding to desired values for further organ-ona-chip systems application [7] was successfully tested . for further analysis focused on dynamic testing simulating physiological processes new voice coil based loading device will be developed for cyclic loading of proposed materials with frequency up to 5 hz. acknowledgements the research has been supported by the european regional development fund in frame of the projects kompetenzzentrum mechanobiologie (atcz133) in the interreg v-a austria czech republic programme. references [1] j. raczkowska, s. prauzner-bechcicki, j. lukes, et al. physico-chemical properties of pdms surfaces suitable as substrates for cell cultures. applied surface science 389:247–254, 2016. doi:10.1016/j.apsusc.2016.07.009. [2] j. rihova, t. suchy, l. vistejnova, et al. mechanical and structural properties of collagen nanofribrous layers under simulated body conditions. in 17th youth symposium on experimental solid mechanics, vol. 25 of acta polytechnica ctu proceedings. 2019. doi:10.14311/app.2019.25.0073. [3] d. kytyr, p. zlamal, p. koudelka, et al. deformation analysis of gellan-gum based bone scaffold using on-the-fly tomography. materials and design 134:400–417, 2017. doi:10.1016/j.matdes.2017.08.036. [4] r. palchesko, l. zhang, y. sun, a. feinberg. development of polydimethylsiloxane substrates with tunable elastic modulus to study cell mechanobiology in muscle and nerve. plos one 7(12), 2012. doi:10.1371/journal.pone.0051499. [5] t. fila, j. sleichrt, d. kytyr, et al. deformation analysis of the spongious sample in simulated physiological conditions based on in-situ compression, 4d computed tomography and fast readout detector. journal of instrumentation 13(11), 2018. doi:10.1088/1748-0221/13/11/c11021. [6] v. rada, t. fila, p. zlamal, et al. multi-channel control system for in-situ laboratory loading devices. in 16th youth symposium on experimental solid mechanics, vol. 18 of acta polytechnica ctu proceedings, pp. 15–19. 2018. doi:10.14311/app.2018.18.0015. [7] s. kratz, c. eilenberger, p. schuller, et al. characterization of four functional biocompatible pressure-sensitive adhesives for rapid prototyping of cell-based lab-on-a-chip and organ-on-a-chip systems. scientific reports 9(1), 2019. doi:10.1038/s41598-019-45633-x. 82 https://doi.org/10.1016/j.apsusc.2016.07.009 https://doi.org/10.14311/app.2019.25.0073 https://doi.org/10.1016/j.matdes.2017.08.036 https://doi.org/10.1371/journal.pone.0051499 https://doi.org/10.1088/1748-0221/13/11/c11021 https://doi.org/10.14311/app.2018.18.0015 https://doi.org/10.1038/s41598-019-45633-x acta polytechnica ctu proceedings 25:79–82, 2019 1 introduction 2 materials and methods 2.1 sample preparation 2.2 instrumentation 2.3 experimental procedure 2.4 data evaluation 3 results 4 conclusions acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2021.31.0027 acta polytechnica ctu proceedings 31:27–29, 2021 © 2021 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague high-speed rail – highway 2.0? pavel krupík czech technical university in prague, faculty of civil engineering, department of construction management and economics, thákurova 7/2077, 166 29 prague 6 dejvice, czech republic correspondence: pavel.krupik@fsv.cvut.cz abstract. high-speed rail lines should make it possible to fully replace air transport over the medium distances of europe (about 500 km). in the czech republic, they are beginning to design. estimates suggest that in five years, the first sections could begin to physically originate and within ten years, they will begin to ride on them. is it possible that the scenario and the continuous extension of deadlines accompanying the completion of the backbone network will be repeated? this article discusses the options from the project management perspective and the idea of construction 4.0 to prevent this. keywords: high-speed rail, project management, predictive project life cycle, stakeholders, construction 4.0. 1. introduction the supreme audit office made two basic conclusions after taking an inventory of the preparation and construction of motorways between 2013 and 2017. the better news is: motorway prices have fallen by 55 percent on average compared to the years 2008 to 2012. the bad one: the preparation of constructions is getting longer and the basic motorway network will not be completed even in 2050 [1]. in 2018, 833 kilometers of the planned 2,073 kilometers were to be built to complete the backbone network of motorways. to achieve this, the road and motorway directorate would have to open approximately 25 kilometers of motorway annually. but the preparation is not accelerating, on the contrary. compared to the previous inspection in 2012, the phase from obtaining a favorable opinion of the eia to the issuance of a building permit extended by four years to an average of 13 years [1]. is the state failing? is it possible that the situation will repeat itself in the preparation and construction of high-speed lines? the initial situation is as follows. regulation (eu) no 1315/2013 of the european parliament and of the council identifies the main principles for the development of road, rail, inland waterway, maritime and multimodal infrastructure. rail transport is further divided into two parts of ten-t [2]: • the comprehensive network aims to connect all european nuts ii regions to the ten-t network, with an indicative completion date of 2050. network continuity has become a basic requirement. great emphasis has also been placed on cross-border sections. for the territory of the czech republic, the ten-t regulation identifies the main directions of high-speed rail development for further examination. • the core network is a subset of the global network and has been established by the european commission on the basis of a uniform european methodology. according to this methodology, the main network includes all the links of the neighboring so-called primary nodes (simply the capitals of the states and agglomerations over 1 million inhabitants). the second condition is a guarantee for the completion of the project by the end of 2030, provided that its economic efficiency is demonstrated. ministry of transport of the czech republic released program for the development of fast railway connections in the czech republic, signed by resolution of the government of the czech republic no. 389 of 22 may 2017 [3, 4]. by 2030, three sections of high-speed lines are expected to be realized within the ten-t main network, namely the new prague – lovosice / litomerice construction including the exit from the prague junction, the major reconstruction of the brno prerov line and modernization and capacity increase of the brno – breclav line. the new construction of the prague – brno high-speed line in its entire length was not enforced into the ten-t main network, as it is not feasible until the end of 2030, given the current process delays in the construction preparation period. the ministry of transport reached this conclusion on the basis of an assessment of experience from abroad, where implementation of similarly large projects from the beginning of preparation to completion usually takes 15 – 20 years. however, the potential of this connection within the czech republic is considerable and this section should be significantly preferred in the preparation, see below. however, the construction time limit is neither the design preparation, the actual construction work, nor the provision of financing, but the processes associated with building permits. the inclusion in the comprehensive network contrasts with the social need for this part of the network, according to which the new construction of the high-speed line 27 https://doi.org/10.14311/app.2021.31.0027 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en pavel krupík acta polytechnica ctu proceedings figure 1. ten-t map for rail transport [1]. prague – brno should clearly functionally belong to the core network for passenger transport [4]. high-speed rail lines are also included in the national investment plan, which presents the investment potential of the czech republic until 2050 and which was discussed on 16 december 2019 by the government. it presents a total of seven new track constructions for speeds up to 320 km/h with the date of commencement of implementation from 2025 to 2040 [5]. 2. aims the aim of this article is to describe the possibilities from the perspective of advanced project management methods and the idea of construction 4.0 how not to repeat some significant historical errors in the planning and realization of motorways, which has delayed this process compared to the original plans. 3. methods the article mentions the possible use of advanced project management methods, which the author considers to be one of the important components of construction 4.0. among these advanced project management methods, they work with a predictive project life cycle and different approaches to project management depending on the life cycle. for selected approaches will be discussed the suitability of use for planning and preparation of high-speed rails. 4. results and discussion the pmbok guide [6] states that there is a so-called predictive life cycle, or plan-driven. in this cycle, the scope, time and cost of the project are defined as soon as possible, possible. it is advisable to use it in areas where the final product is known in advance, which is undoubtedly high-speed rails. it also emphasizes the influence and relationship between the project, the project team, and various stakeholders. predictive live cycle of high-speed rail projects in the czech republic shows figure 2. the importance of stakeholders is also indicated by different project management approaches such as iso 21500 standard guidance on project management and project management systems and lean construction philosophy [7]. the situation with the construction of highways only confirms their importance. and it is possible that this will be the case for high-speed rail projects too. the relation between stakeholders and the project in case high-speed rail projects in the czech republic shows figure 3. 28 vol. 31/2021 high-speed rail – highway 2.0? figure 2. predictive live cycle, source: custom. figure 3. the relation between stakeholders and the project, source: custom. 5. conclusion usually it is not possible to name the causes of delays in the construction of motorways constructions. one of culprits is the former minister of transport, who allegedly stopped the preparatory work on dozens of projects in 2010 on the pretext that they are at risk of corruption and because unlimited amounts of money were not available in crisis years. another popular culprit is environmental associations, which continuously delay construction by appealing during any administrative procedure that the construction has to go through before it can even begin. culprit may be also the state bureaucracy as such. for the preparation of buildings it creates such complex regulations that it is beyond its own power to meet their requirements. at the same time, it is unable to reach a civilized agreement with those affected by the construction, from landowners, villages and nature conservation to construction companies. whoever is to blame, the repetition of the highway situation in the case of high-speed rail must be prevented. one of the keys is the consistent identification of the whole predictive life cycle and their stakeholders (including roles). acknowledgements this work was supported by the grant agency of the czech technical university in prague, grant no. sgs20/100/ohk1/2t/11. references [1] supreme audit office. audit conclusion of audit action 17/05: construction, modernization and reconstruction of motorways. in: supreme audit office: by the autumn of 2017, none of the planned 15 motorway constructions had begun to be built. the blame is the preparation problems [online]. available online at: https://www.nku.cz/cz/pro-media/tiskove-zpravy/ do-podzimu-2017-se-nezacala-stavet-ani-jednaz-planovanych-15-staveb-dalnic-na-vine-jsouproblemy-s-pripravou-id9349/. [2] regulation (eu) no 1315/2013 of the european parliament and of the council of 11 december 2013 on union guidelines for the development of the transeuropean transport network and repealing decision no 661/2010/eu (text with eea relevance). available online at: https://op.europa.eu/cs/publicationdetail/-/publication/58c00e4e-b59c-4b64-ad833d1a02b33fe8/language-en/format-pdfa1a. [3] czech republic. resolution of the government of the czech republic of 22 may 2017, no. 389, on the program for the development of fast railway connections in the czech republic. available online at: https: //apps.odok.cz/attachment/-/down/ihoaamva7kom. [4] ministry of transport of the czech republic. program for the development of fast railway connections in the czech republic. 2017. available online at: https://www.mdcr.cz/dokumenty/strategie/ vysokorychlostni-trate. [5] government of the czech republic. national investment plan of the czech republic 2020-2050. 1. prague: government of the czech republic, 2019. isbn 978-80-7440-244-9. [6] a guide to the project management body of knowledge (pmbok guide). fifth edition. newtown square, pennsylvania: project management institute, [2013]. isbn 978-1935589679 available online at: https://dinus.ac.id/repository/docs/ajar/ pmbokguide_5th_ed.pdf. [7] x. brioso. integrating iso 21500 guidance on project management, lean construction and pmbok. procedia engineering. 2015, 123, 76-84. issn 18777058. available online at: https://linkinghub.elsevier.com/ retrieve/pii/s1877705815031616, https://doi.org/10.1016/j.proeng.2015.10.060. 29 https://www.nku.cz/cz/pro-media/tiskove-zpravy/do-podzimu-2017-se-nezacala-stavet-ani-jedna-z-planovanych-15-staveb-dalnic-na-vine-jsou-problemy-s-pripravou-id9349/ https://www.nku.cz/cz/pro-media/tiskove-zpravy/do-podzimu-2017-se-nezacala-stavet-ani-jedna-z-planovanych-15-staveb-dalnic-na-vine-jsou-problemy-s-pripravou-id9349/ https://www.nku.cz/cz/pro-media/tiskove-zpravy/do-podzimu-2017-se-nezacala-stavet-ani-jedna-z-planovanych-15-staveb-dalnic-na-vine-jsou-problemy-s-pripravou-id9349/ https://www.nku.cz/cz/pro-media/tiskove-zpravy/do-podzimu-2017-se-nezacala-stavet-ani-jedna-z-planovanych-15-staveb-dalnic-na-vine-jsou-problemy-s-pripravou-id9349/ https://op.europa.eu/cs/publication-detail/-/publication/58c00e4e-b59c-4b64-ad83-3d1a02b33fe8/language-en/format-pdfa1a https://op.europa.eu/cs/publication-detail/-/publication/58c00e4e-b59c-4b64-ad83-3d1a02b33fe8/language-en/format-pdfa1a https://op.europa.eu/cs/publication-detail/-/publication/58c00e4e-b59c-4b64-ad83-3d1a02b33fe8/language-en/format-pdfa1a https://apps.odok.cz/attachment/-/down/ihoaamva7kom https://apps.odok.cz/attachment/-/down/ihoaamva7kom https://www.mdcr.cz/dokumenty/strategie/vysokorychlostni-trate https://www.mdcr.cz/dokumenty/strategie/vysokorychlostni-trate https://dinus.ac.id/repository/docs/ajar/pmbokguide_5th_ed.pdf https://dinus.ac.id/repository/docs/ajar/pmbokguide_5th_ed.pdf https://linkinghub.elsevier.com/retrieve/pii/s1877705815031616 https://linkinghub.elsevier.com/retrieve/pii/s1877705815031616 https://doi.org/10.1016/j.proeng.2015.10.060 acta polytechnica ctu proceedings 31:27–29, 2021 1 introduction 2 aims 3 methods 4 results and discussion 5 conclusion acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2018.16.0006 acta polytechnica ctu proceedings 16:6–10, 2018 © czech technical university in prague, 2018 available online at http://ojs.cvut.cz/ojs/index.php/app prediction of the development of groundwater quality in a finished sludge lagoon petr černocha,∗, jiří košťálb a čez energetické produkty, s. r. o., komenského 534, 253 01 hostivice, czech republic b inset, s. r. o., divize energetika, lucemburská 7, 130 00 praha 3, czech republic ∗ corresponding author: cernoch.petr@inset.com abstract. this study deals with prediction of the development of groundwater quality in a finished sludge lagoon. for depositing and predicting secondary float energy by-products, it is appropriate to use mathematical modelling. the implementation of relevant input data from long-term monitoring is suitable as an application for verification as well. the main output was the determination of the necessary priorities and geotechnical risks in solving the final state of the area of interest, especially regarding solving of the drainage system in the subsequently finished sludge lagoon. keywords: transport of contaminants, sludge bed, geotechnical risks. 1. introduction when depositing secondary energy by-products from coal-fired power plants, several crucial questions arise in relation to the ambient environment and potential geotechnical risks. regarding the remediation and revitalisation of former sludge beds by way of depositing secondary energy by-product, it is necessary to point out fundamental phenomena related to the impact of the structure of interest on the ambient environment and potential geotechnical risks. this concerns mainly a rise in groundwater level due to superimposed load on the structure [1]. when the secondary energy by-products are deposited, this new layer can sag and consequently, open tensile fractures may appear in it. therefore, it is advisable to pay close attention to the analysis of changes that can arise when former sludge beds are covered with a remediation layer. in particular, there is a risk of restoring the problem as the groundwater outflow conditions may change, which can lead to the transport of potential contaminants from the former sludge beds to the ambient environment. at the same time, the stability of some parts of the structure may be affected locally and the overall stability in a wider area may also be affected, especially if the uplift pressure at the base of the structure increases [2]. the above-mentioned risks may be disclosed in time and subsequently eliminated thanks to the implementation of hydrogeological model calculations which focus on the simulation of changes in outflow conditions caused by depositing secondary energy by-products. typical tasks mainly include monitoring the impact of hydro-regime changes on the stability of a former sludge bed (or rather the finished sludge lagoon), verification of possible spreading of potential contaminants in the saturated ground environment, and groundwater level development in various alternative solutions for overall capacity extension and reclamation of the structure [3, 4]. the prediction of groundwater quality development (modelling of contaminant transport) was implemented in the area of two (former) sludge beds. the first sludge bed was created in a depressed terrain by enclosing it with a starter dam. it was hydraulically operated between 1967 and 1990. the neighbouring sludge bed was located in the area of a former quarry in which mining was discontinued in 1983. ash deposition by way of sluicing in the central part of the structure of interest, started in 1990 and has continued to this day while the sluicing is scheduled to stop at the end of 2017 or the beginning of 2018. a hydromixture (a mixture of slag and water) is sluiced to the sludge bed. simultaneously, the implementation since 1997 of coal-fired power plant desulphurisation using the wet lime scrubbing process, a pre-reclamation arrangement for landscape recovery has been modelled on the surface of a sludge bed ash beach using a stabilised mixture prepared from secondary energy by-products (a mixture of fly ash, lime, wastewater, etc.). 2. input data first, a geological model was created and hydrodynamic (hydraulic) properties assigned to partial geological units. after that, boundary conditions were defined and then a computing grid was established. for the partial tasks to be solved, it was essential to first solve the hydrogeological model with steady groundwater flow in a wider area of interest. thus, the precondition of setting actual initial and boundary conditions at the border of the model area is met. visual modflow flex 2015.1. was used for the modelling, including the implemented modules: modflow 2005 u.s. geological survey to find out the course of the groundwater flow in the saturated ground 6 http://dx.doi.org/10.14311/app.2018.16.0006 http://ojs.cvut.cz/ojs/index.php/app vol. 16/2018 ground water quality in sludge lagoon environment, mt3mds for the transport of contaminants and zonebudget to establish the groundwater amount in pre-defined 3d bodies (areas). afterwards, vmod 3d-explorer enabled us to display both inputs and outputs of individual models in 3d form. 2.1. model geometry and boundary conditions the locality of interest was modelled over a total surface area of approx. 3.2×2.8 km (i.e., an area of approx. 905 hectares was simulated). the lower limit of the model was selected in such a way that the total vertical profile adequately covered all the geological units concerned and their general course. great emphasis was laid on creating geological boundaries of tuffitic rocks (forming an impermeable barrier between the two sludge beds) and coal, including coalbearing clays (especially the course of the coal seam and its non-balanced residues). the geological model was identical for all partial numerical models, only the input of the terrain surface course differing in the variants. the current terrain, the filled body after slag sluicing was discontinued (12/2017) and after that depositing of stabilised mixture is finished, that is before the reclamation starts in 2025. when the geological model was implemented (figure 1), the conceptual model was solved, and later there followed the numerical model. the conceptualisation consisted in the schematisation of all aspects of the area of interest, i.e., the geometry of the analysed area and the groundwater flow conditions. moreover, the model material composition was simplified while respecting all the defined boundary conditions. a specific constant groundwater level was used as the first-type boundary condition for the modelled borders of the area of interest. the level was not entered as constant up to the border but as respecting the found data (groundwater level) from the hydrogeological model of the wider surroundings of the area of interest. in addition, a second-type boundary condition was introduced, namely recharge. to avoid separate entry of another boundary condition – evapotranspiration (difficult determination of amount and depth range), was already included when the recharge boundary condition was entered – and this value was reduced. superficial saturation and local watercourses were included in the model as a 3rd-type boundary condition – river. another 3rd-type boundary condition was the course of levels in the stream tubing (drain) and drainage system for the sludge bed seepage water. the boundary condition of the lake included the artificially maintained constant height of the free water level in the residual sludge bed lake and hydro-mixture sluicing. water recharge from the sluiced hydro-mixture can be derived from the known value of the hydro-mixture flow rate through the sluice pipeline minus the slag content. the entered 4th-type boundary condition, pumping wells, represented the effect of overflow towers. the initial condition (initial head) with the starting groundwater level calculated within the steady groundwater flow modelling in the wider area of interest, was used in the entire simulated model area. 2.2. material characteristics the following basic geological units were selected for the solved models: stabilised mixture; slag-ash sediment; stockpiled soil; holocene deposits including deluvial-fluvial soils; aeolian or aeolian-deluvial sediments of loess or of loess-soil nature; fluvial pleistocene gravel accumulations – terraces; overlying part of most formation – clays with sand fraction; coal series (seam and non-balanced residues) – coal; coal claystone; pyroclastic rock including the underlying part of most formation – tuffs and tuffits of volcanic series; varied claystones and tuffitic clays of underlying series; cretaceous rock of turonian age – marlites and lime claystones; vulcanites (neogene effusive magmatites) – olivine basalts, nephelinites and leucites. the selection of all 11 various geological environments made for a certain simplification consisting in the unification of soils and rocks into so-called quasi-homogeneous units (i.e., units with similar geological and hydrogeological properties), although they may be heterogeneous as to their lithology and genesis. hydrodynamic parameters for each geological unit were obtained from hydrodynamic tests, well logging, laboratory analyses, and also partially derived from archive searches in czech and foreign professional literature [5]. the determination of the ash hydraulic parameters is very difficult. from experience in other similar fields, it is known that filtration coefficients in a saturated ash body in analogous conditions, as verified by a pumping experiment, showed differences of one or two orders of magnitude. differences between filtration coefficients in the horizontal and vertical directions may even be a hundredfold. moreover, in the central part of the sludge bed (near the overflow towers) the finest-grain ash fraction tends to settle and, mainly thanks to the existence of a free water surface in this area, the difference between the vertical and horizontal permeability values is almost entirely blurred. for those reasons, flatly introduced filtration coefficient values were adequately adjusted within the modelling. at the same time, crevice permeability was not considered in the solved model calculations. it plays a part in underlying cretaceous rocks and the stabilised mixture body. considering the position of the cretaceous deposits (deeper groundwater circulation), only a negligible error results in the calculation. it is true for both the slag-ash sediment and stabilised mixture that in reality (on the macro-scale), their permeability will be greater than the laboratory value. however, this is insignificant in the presented calculations. within the solution, the reference groundwater levels obtained from long-term monitoring of the area of interest were entered. additionally, earlier chemical 7 petr černoch, jiří košťál acta polytechnica ctu proceedings figure 1. initial geological model (superelevated 10 fold). analyses were searched out, groundwater samples were taken, and their subsequent laboratory analyses undertaken. the input values entered in the calculation model were average values obtained from the reference period from 2009 to 2016. the groundwater level values measured in the monitoring hydro-wells served for the calibration and backward verification of the model. the latter compared whether the calculated course of the groundwater level in the model initial time corresponded to the actually found levels to the maximum extent, i.e., the long-term average in recent years. 3. calculation, calibration and backward verification when solving the transport of contaminants in the area of interest, it was necessary to implement partial groundwater flow models. in general, four basic models were solved: • model 1: groundwater amount and quality in the area of the sludge beds in simultaneous slag sluicing to the other sludge bed; • model 2: prediction model of groundwater quality after the sluicing is discontinued for selected chemical indicators (until 2025); • model 3: prediction model of the groundwater amount flowing out of the area of both sludge beds from the discontinuation of slag sluicing to the other sludge bed till the discontinuation of depositing secondary energy by-product – final shape before reclamation (until 2025); • model 4: prediction model of the groundwater level when the water regime has stabilised after the discontinuation of sluicing. except for the first and partly also the last model, with the steady state of groundwater, a so-called “steady-state simulation”, the variants were always time-varying. the groundwater course reacting to time changes was solved, as a response to the change in the entered boundary conditions. in the case of model 1, we used the standard computing module of pcg2 (preconditioned conjugate-gradient method) to numerically solve the simulated equations of the linear groundwater flow model using modified incomplete cholesky and polynomial predictions. for the other models, the computing method of gmg (geometric multigrid solver) served the purpose of solving the corresponding matrices. this method is based on the conjugate gradient algorithm like the previous pcg2, but unlike pcg2 it was developed especially for solving groundwater flow models with a numerical grid of finite differences. for models 1 and 2 (after timevariant groundwater flow iterations were successfully finished) the advective flow method of ufd (upstream finite difference) with the implicit computing module of gcg (generalized conjugate gradient solver) was used to calculate the transport of contaminants. once the obtained results had been reviewed, we proceeded to the calibration and backward verification of the numerical model. hydrogeological model calibration consisted in a gradual change in the input parameters until an acceptable agreement between the measured and calculated values was achieved. in the first step, the achieved groundwater levels in zero time were compared with the measured data in each model variant. then the concentration values for the monitored chemical indicators were calibrated as well. numerical model verification was made possible mainly thanks to a sufficient amount of data obtained 8 vol. 16/2018 ground water quality in sludge lagoon from the long-term monitoring of the area of interest performed for many years by inset s.r.o. in summary, we can conclude that, despite its extreme complexity (extensive territory, complicated geological structure, large number of boundary conditions, etc.), the implemented model of the solved area achieved, in all its partial variants, a very good agreement with the documented values from the hydrogeological and hydrochemical monitoring. 4. results of model calculations all model variants included the effect of the sludge bed drainage system. in the implemented mathematical modelling, the effect of the upper near-surface aquifer was connected with deeper groundwater circulation. thus, the resulting groundwater level courses always reflected the shallowest continuous aquifer in the locality of interest, which comprised the saturated zone of both sludge beds, the quaternary aquifer at the inflow (sandy gravels), the near-surface aquifer of the shallow groundwater circulation connected in the east to a broad alluvial plain of the original stream bed (connecting to surface waters), and also in the north-east part of the second sludge bed a partial show of tertiary water recharge (coal seam outbursts). thus, the determined groundwater level represented the highest possible level of the continuous course. considerable deep groundwater circulation (mostly a crevice aquifer in cretaceous sediments and tertiary vulcanites and deposits) was found under the modelled course of the shallow groundwater level (pore character of the aquifer) and had virtually no bearing on the transport of contaminants solved by us; therefore, it was not included in the implemented simulations. the modelled groundwater level course imitated to a large extent the terrain sloping and slope direction. furthermore, it was obvious that the closer to the residual lake the calculated groundwater level was, the more the shallow groundwater levels gradually evened out, approaching the level corresponding to the free water surface of the sludge lagoon. therefore, the simulated steady groundwater flow indicates communication between the groundwaters from both sludge beds. in the case of model 1, the groundwater quality was shown as constant, representing the current state. the comparison of calculated values of the monitored chemical indicators with average values documented by monitoring in the reference period from 2009 to 2016 showed very good agreement. the achieved values of individual potential contamination parameters served as input data for the further solution variant implemented in model 2. model 2 solved the groundwater quality prediction for the following chemical indicators: dissolved inorganic salts (ras), ammonium ions, sulphates and arsenic. as mentioned above, it was based on the current state calculation for groundwater quality (model 1) and simultaneously on model 3, which focused on the change in outflow conditions, or rather the state of the groundwater levels from sluicing discontinuation till the creation of the final secondary energy by-product body, i.e., till 2025. the simulation of groundwater quality development from the beginning of secondary energy by-product depositing is quite a complicated operation; therefore, it is partially burdened with error. from the initial phase of the numerical calculation, i.e., from the time when the first sludge bed operation started in 1967, a significant leap in groundwater quality can be observed from this “zero” point up to the current state. it follows that the gradual deterioration of contamination up to the current state (for approx. 50 years) has a long-term tendency and therefore, no rapid or considerable improvement can be expected in the upcoming years (i.e., from the time when sluicing / secondary energy by-product depositing is discontinued). the calculated values of the monitored parameters had a relatively good agreement with the measured values of these chemical indicators in the long term. inaccuracies in simulations compared with the measured data got worse towards the borders of both sludge beds. the mathematical calculation of the current state was followed by the calculation of groundwater quality prediction in time, i.e., its change from 12/2017 till 2025. it was obvious in all the monitored cases that the equilibrium of the hydrochemical environment did not hold steady at the time when secondary energy by-product depositing was discontinued. the contaminant cloud moved very slowly in time and the contaminants will most probably never be washed completely out of both sludge beds, because of the lack of a sufficient gradient and (ground) water recharge, as shown in the results of model 4. it should be noted that according to the calculations, the water regime will stabilise in approximately 2042, which means 25 years after sluicing to the sludge bed is discontinued and 17 years after depositing of the stabilised mixture is discontinued (before reclamation). in the case of all chemical indicators, the modelling showed that all their monitored values decreased in time after sluicing was discontinued. at the end of the solved period (from approximately 2022), a partial decrease in the monitored concentration parameters started to show also in wells located at the outflow from both sludge beds. a detectable decrease in values of the monitored concentration parameters inside the sludge bed bodies would require a longer time period. for most indicators, the concentration values documented by the model for the final year 2025 were lower in all monitored wells than the maximum value found in the selected monitoring period of 2009 – 2016. in half of the cases, the calculated values (for 2025) were lower than the average values in the above-mentioned reference period. 9 petr černoch, jiří košťál acta polytechnica ctu proceedings 5. conclusions we can conclude that the indicated development of groundwater quality showed a clear tendency. that is, after discontinuation of sluicing, the contaminant cloud was gradually washed out, diluted and moving outside the sludge bed. however, the development depended not only on depositing of the secondary energy by-product but primarily on the hydraulic transport operation. if, theoretically, the sludge bed was supplied only with clear water through the sluice pipeline, the contaminant cloud could be gradually pushed out of the sludge bed and almost eliminated in the end. it means the cloud movement also implies a certain dilution, i.e., a decrease in contaminant concentrations. it is also quite obvious that the washing-out of contaminants will be considerably affected by the volume of inflows from the gravel aquifer. to the contrary, if substantial recharge of the sluicing water is missing, both concentration decrease and contaminant cloud movement will slow down; according to numerical modelling there will be an increase in concentrations of the monitored parameters inside both sludge bed bodies. another issue connected with the potential adverse effect of finishing the sludge lagoon (a change in hydroregime and thus the stability of the area of interest) is maintaining the drainage system function until the water regime completely stabilises. the performed model calculations have implied that it is necessary to operate the drainage system at least until the groundwater flow stabilises, i.e., for approx. 15 years after depositing of the secondary energy by-product in the area of interest is discontinued. none of the model cases required any adjustment of limit values in the number of internal and mainly external iterations in order to increase the tolerance for achieving convergence above the size of one-tenth of a computing cell. a very good agreement with initial times was achieved for the values obtained from the model and the field measurement, thanks to calibration and backward verification. the main uncertainty in the results obtained was the issue of course continuity of the groundwater level. the numerical calculation did not allow modelling of isolated saturated places; it is intrinsically made to solve a continuous course of the groundwater level. this results in a certain simplification and distortion; in reality, there will be separated, preferential groundwater flows in some isolated localities which will also allow the transport of contaminants. references [1] p. černoch, j. košťál. studie vlivu změn hydrorežimu na stabilitu odkališť. in proceedings of conference geotechnics 2016. orgware, 2016. [2] p. černoch, j. košťál. implementation of geotechnical risks during realization of a slag dam based on the sediments in a former sludge lagoon. in proceedings of 16th conference ecsmge. ice publishing, 2015. [3] p. černoch, j. košťál. řešení ukládání vedlejších energeticých produktů do prostoru odkaliště. in proceedings of 14th congress of hydrogeology and 2nd congress of engineering geology. čah, 2014. [4] p. černoch, j. košťál. stanovení zatěsnění úložného prostoru vedlejších energetických produktu na základě experimentu in situ. geotechnika journal 2, 2016. [5] p. černoch, j. košťál. uplatnění hydrogeologického modelování při ukládání vep. in proceedings of 3rd international conference popílky ve stavebnictví and 27th international symposium sanace 2017. vut brno, 2017. 10 acta polytechnica ctu proceedings 16:6–10, 2018 1 introduction 2 input data 2.1 model geometry and boundary conditions 2.2 material characteristics 3 calculation, calibration and backward verification 4 results of model calculations 5 conclusions references acta polytechnica ctu proceedings doi:10.14311/app.2018.14.0042 acta polytechnica ctu proceedings 14:42–48, 2018 © czech technical university in prague, 2018 available online at http://ojs.cvut.cz/ojs/index.php/app imitating neutron irradiation damage using a combination of ion beams václav šísla,∗, martin ševečeka,b a department of nuclear reactors, faculty of nuclear sciences and physical engineering, czech technical university in prague, v holešovičkách 2, prague 8, czech republic b department of nuclear science and engineering, massachusetts institute of technology, 77 massachusetts ave, cambridge, ma, usa ∗ corresponding author: sislvacl@fjfi.cvut.cz abstract. there is a strong motivation for using ion beams to imitate neutron irradiation damage, mainly in order to reduce costs and time demands linked to neutron irradiation experiments. the long-term goal of the authors is to create an ion irradiation methodology, which could be employed in the development process of innovative nuclear fuel materials. this methodology will be based on combining of a set of ion beams in such a way that the final distribution of irradiation-induced defects in the material structure is similar to the one which would have been introduced by neutrons in a nuclear reactor. the first part of the methodology is represented by an optimization tool described here. the tool uses a third party monte carlo code srim to simulate ion transport in a target and to determine the distribution of radiation damage. subsequently, a custom genetic optimization algorithm is applied to a set of damage distribution profiles to find their optimal combination. keywords: accident tolerant fuel, fuel cladding, radiation damage, srim, defects distribution optimization, genetic algorithms. 1. introduction development of innovative materials for nuclear reactor applications (such as new cladding materials) is a complex multidisciplinary process. one of the criteria to be fulfilled is to prove resistance of the material against neutron irradiation and to quantify material characteristics that degrade with radiation. a traditional approach of proving irradiation tolerance is through irradiation in a research reactor and subsequent post-irradiation experimental testing (pie). however, using a research reactor for irradiation purposes has multiple disadvantages, of which the most significant ones are time demands and costs. the time demands are the result of two factors. first, the irradiation time itself is long, as the damage rate in a nuclear reactor is usually low, thus it takes a long time to accumulate the desired damage (often several months or even years). second, since the samples become radioactive, further time is needed for decaying of the samples before subsequent testing. the fact that samples become radioactive also results in requirements for special handling procedures and facilities certified for such purposes. another drawback of neutron irradiation is the need for designing of special capsules, in which the samples are placed during irradiation or need to handle the material in hot cells. this further increases the costs and complexity of the experiments. there are also advanced nuclear reactors under development around the world. the high dose reactor figure 1. comparison of approximated requirements for temperature and dpa for advanced reactor concepts and annual damage rates produced by current research and fast reactors. [1] concepts as molten salt reactors or traveling wave reactor are very challenging from the perspective of radiation damage of structural components that must survive radiation damage up to 250 dpa or higher. [1] this makes the development of new materials very complicated and leads to excessive prolongation of the development of advanced reactors. the requirements for temperature and level of radiation damage to be reached for different advanced reactors are summarized in figure 1. taking into account all the problems linked to neutron irradiation in research reactors, there is a strong motivation for using ion beams. the main advantages 42 http://dx.doi.org/10.14311/app.2018.14.0042 http://ojs.cvut.cz/ojs/index.php/app vol. 14/2018 simulation of irradiation processes of ion irradiation can be summarized as: • sources of ion beams (accelerators) are widelyspread. • dose rates are much higher compared to neutron irradiation, thus exposure times are as low as hours or tens of hours. • there is no need for special capsule design. • there is only a low or none residual activity of irradiated samples. • experiment conditions can be well controlled easily adjustable ion energy, dose rate, and sample temperature. • damage cascades produced mainly by primary knock-on atom (pka) in the case of irradiation by heavy ions of typical energies are similar to those created by a pka knocked by energetic neutrons. there are, however, differences between the effects of neutron and heavy ion irradiation. these will be discussed in the following section. the goal of current research is to achieve as comparable irradiation damage profile as possible resulting in imitation of irradiation damage by heavy ions. 2. differences in effects of ion and neutron irradiation in spite of the fact that ion irradiation offers significant advantages compared to neutron irradiation, there is a fundamental question to be answered: are the effects of ion irradiation comparable to those created by neutrons? according to was [2], even though there are significant differences between the direct effects of neutron and ion irradiation, it is possible, to a certain degree, imitate the effects of neutron irradiation using ion beams. an important fact is that it is the final state of the material that determines the equivalence, not the path taken. the main differences between effects of ion and neutron irradiation are [1–3]: • particle energy spectrum: neutrons in a reactor have a wide energy spectrum ranging between several orders of magnitude from thermal energies (0.025 ev) up to fast neutrons (mevs). on the other hand, heavy ions produced by an accelerator have very narrow range of energy (usually considered as monoenergetic beams). • penetration depth and damage distribution: neutron’s mean free path in structural materials of nuclear reactors is in order of centimeters, thus they can penetrate deep in a target. a large mean free path also means that the spatial damage distribution created by neutrons is homogeneous over many millimeters of the penetration depth. ions, on the other hand, due to their electric charge, lose energy much faster when traveling through a material figure 2. damage distributions caused by various energetic particles. [2] and their penetration depths are orders of magnitude shorter. they usually penetrate between 0.1 and 100 µm depending on the particle type and its energy, see figure 2. • damage rate: as the cross-section for ion-atom interaction is much higher compared to neutron-nuclear interaction, the damage rate relative to the particle fluence is orders of magnitude higher for ion irradiation than for neutron irradiation. • nature of induced defects: neutrons and heavy ions produce large damage clusters of comparable size and nature. light ions and protons produce smaller clusters or isolated point defects and electrons produce widely separated self-standing point defects. this behavior is more complex due to diffusion related effects that are closely linked to the temperature of the target. in the presented study, we decided to tackle the issue of different damage distributions resulting from neutron and ion irradiation. our goal was to combine a set of several ion beams of different energies and/or ion types. assuming that the damage profiles created by individual beams can be summed-up independently, it is then possible to find a particular optimized combination leading to a relatively flat damage profile similar to the one, which would have been created by neutrons. the proposed optimization method is described in the following section, an example of a typical damage profile created by a single ion beam (6.0 mev, ni) and an optimized damage profile, as we defined it, are shown in figure 3. in the past, there were multiple studies investigating a possibility to combine irradiation by multiple ion beams to mimic neutron irradiation [3–5]. however, the main aim was not to obtain a uniform damage 43 václav šísl, martin ševeček acta polytechnica ctu proceedings 0 1 2 3 4 5 6 7 8 9 0 5× 10−16 1× 10−15 1.5× 10−15 2× 10−15 dp a pe r flu en ce [d pa ·c m 2 / io n] target depth [µm] ti2alc + zircaloy-4, dpa per fluence ni, 6.0 mev cladcoat 0 1 2 3 4 5 6 7 8 9 0 5× 10−16 1× 10−15 1.5× 10−15 2× 10−15 dp a pe r flu en ce [d pa ·c m 2 / io n] target depth [µm] ti2alc + zircaloy-4, dpa per fluence ideal damage distribution cladcoat figure 3. damage profile created by 6 mev nickel ions irradiating 3 µm thick ti2alc coating layer deposited on zircaloy-4 slab (top) and an optimal damage profile as defined by the authors (bottom). distribution, but rather to address neutron-induced transmutation by injecting he while irradiating by another type of ions to obtain the desired level of damage. also, a procedure combining neutron irradiation for obtaining radiation damage and he irradiation for injecting bubbles is being used by the community. [6] the possibility of obtaining a layer1 of evenly irradiated material might be especially suitable for testing accident tolerant fuel (atf) multi-component cladding materials. during such tests, it is needed to evaluate properties under irradiation of a thin coating layer deposited on a material, which irradiation properties are already well known (zr-based alloys serving as substrates). another objective is to test the behavior of the interface substrate/coating. structural changes at the interface caused by radiation might lead to issues related to adhesion of the coatings, cracking or excessive stress in the atf cladding. neutron irradiation will be always required to qualify materials for new reactor applications, however ion irradiation, as a low-cost and fast damage method, can serve as a screening method for evaluating new material concepts. with the use of ion irradiation, the development process of new materials for current and future reactor applications can be much faster and cheaper. 1relatively thin, limited by a penetration depth of the beams. 3. methodology and example results the proposed optimization method will be demonstrated on an example of irradiation of a thin ti2alc (max phase) coating deposited on zircaloy-4 alloy. zircaloy-4 is a standard binary zr-sn alloy used by the nuclear industry for decades. the max phase coating is proposed as a potential candidate material enhancing the safety features of the standard zr-based cladding. the ultimate goal is to irradiate the coating layer with multiple ion beams in such proportion that the sum of their respective damage profiles will result in an optimal damage distribution. the optimal damage distribution is defined as a flat distribution. that means a situation, where at each point of the depth of the target, there is the same irradiation-induced damage expressed by the quantity dpa per fluence [ dpa · cm2/ion ] . only the damage distribution in the coating layer of the target is taken into account, the shape of the damage profile in the substrate (zircaloy-4) is not considered. however, it should be noted that for the purposes of irradiation experiments, it is crucial for the beam to be able to penetrate the full depth of the coating, since the behavior of the interface between the coating and the bulk cladding is of a significant concern in the development of multi-component cladding materials. for that reason, the thickness of the coating needs to be optimized to fit the requirements of the test. in general, an arbitrary number of beams can be used for the optimization. there are two basic conditions the beams shall fulfill: first, at least one of the beams should be able to penetrate the full width of the coating layer. second, the bragg peaks of damage distributions created by individual beams should not overlap significantly. for demonstration purposes, it is assumed here that four different ion beams are to be combined, namely: fe (6 mev) and ni (6, 3.4, and 1.7 mev). each of these beams creates a unique damage profile2 when irradiating the target. these profiles are denoted as a(z), b(z), c(z), and d(z), respectively, and are functions of z the penetration depth inside the target. these profiles can be also represented as vectors a = {a1, a2, . . . anb }, b = {b1, b2, . . . bnb }, c = {c1, c2, . . . cnb }, and d = {d1, d2, . . . dnb }, where nb is the number of bins present in the coating part of the target. the aim is to find such parameters a, b, c, d fulfilling the fundamental condition a+ b+ c+ d = 1, (1) so that the resulting damage profile fj = aaj + bbj + ccj + ddj , j ∈ {1, . . . nb} (2) will be as flat as possible. 2damage profiles can be obtained using monte carlo simulation tools, e.g. srim [7] as in the case of this study. 44 vol. 14/2018 simulation of irradiation processes the flatness of the distribution can be described by its standard deviation. a small standard deviation corresponds to an even distribution of the defects, optimal (flat) distribution having the standard deviation equal to zero. the standard deviation of a damage distribution f is calculated as sdev = √√√√ 1 nb − 1 [ s2 nb − ( s1 nb )2 ] , (3) where nb is the number of bins present in the coating part of the target and s1 = nb∑ j=1 fj = nb∑ j=1 aaj + bbj + ccj + ddj , (4) s2 = nb∑ j=1 f2 j = nb∑ j=1 (aaj + bbj + ccj + ddj)2 . (5) having defined the problem and the measure of quality of a solution of this problem, standard optimization tools can be employed. in this case, a proprietary implemented genetic optimization algorithm [8] was used to find an optimal set of parameters a, b, c, d. genetic algorithms work with five basic concepts: generation, population, individual, genotype, and gene. • each generation is represented by one population. • a population contains individuals, who represent possible solutions of the problem. • each individual is represented by its genotype a set of genes. • a gene can be a binary variable, an integer, a float or anything else. the algorithm iterates over generations with an aim to transfer the best individuals from the current generation to the following one. the quality of an individual is expressed by the value of its fitness function, the higher the fitness, the better the individual. a flowchart describing the genetic algorithm used for irradiation optimization is showed in figure 5. at the beginning, damage profiles are loaded from previously prepared files. (see number 1 in the figure 5.) an initial population is created using the method initialize_population as a set of random individuals. (nr. 2 in figure 5.) for each individual, the genotype (values a, b, c, d) is set as an array of random numbers from an interval 〈0, 1〉. eventually, the genotype is normalized so that the individual fulfills the condition (1). after creating the initial population, an iteration over the number of generations begins (nr. 3). following steps are executed in each iteration. • the quality of each individual in the population is calculated (nr. 4). individual’s quality is represented by the fitness function value. since the algorithm is searching for an individual with the highest fitness value, while our aim is to minimize the value sdev , the fitness of an individual is calculated as fitness = 1 sdev . (6) • a linear transformation is then applied on the fitness of each individual by subtracting the fitness of the worst individual in the population (nr. 5). the transformation improves the process of roulette selection, which will follow later. it ensures that high-quality individuals will be more likely to be selected for the process of crossing. • the index of the best individual in the population is saved. the best individual will be eventually artificially inserted into the new population (elitism). • a new population is created as an empty array (nr. 6). an iteration over a half of the number of individuals in each population (population_size/2) begins. following steps are executed in each iteration. • two individuals (parents) are randomly selected (nr. 7). • with the probability (1 crossing_rate) the parents are directly inserted into the new population (nr. 8), with the probability crossing_rate, they undergo the process of crossing (nr. 9). • in the crossing process, two new individuals (descendants) are created via random combination of genotypes of the parents. • after the crossing process, each gene of each descendant is, with a probability (1 mutation_rate) left as it is (nr. 10), or with a probability mutation_rate modified (nr. 11) by addition of a random value from the distribution g(x, y) = 0.01 √ −2 ln(x) sin(2πy), (7) where x and y are random values from an interval 〈0, 1〉. after adding a random value, it may happen that the value of a gene is negative. in that case, the negative value is replaced by zero. • the genotype of each descendant is normalized (nr. 12) so that he sum of all genes is equal to 1. • after the process of crossing (and mutation, in some cases), both descendants are inserted into the new population (nr. 13). • after population_size/2 iterations, the new population is completed (nr. 14). 45 václav šísl, martin ševeček acta polytechnica ctu proceedings 0 0.2 0.4 0.6 0.8 1 0 0.2 0.4 0.6 0.8 1 −0.03 −0.02 −0.01 0 0.01 0.02 0.03 mutation function g(x, y) = 0.01 √ −2 ln(x) sin(2πy) x y figure 4. two-dimensional mutation function used in the algorithm. • in the last step of the loop, the best individual from the previous population is artificially inserted into the new population and replaces the first individual created through natural selection. this method is called elitism (nr. 15) and is executed regardless of whether or not the best individual from the previous population is actually better than the one it replaces. the process of creating new populations based on crossing individuals from the previous one is repeated until a generation counter reaches a set value (nr. 16). after the loop terminates, the best individual is printed to the console (nr. 17) and the program is terminated. the described algorithm was applied on the previously mentioned case: irradiation of a thin layer of ti2alc coating deposited on zircaloy-4. this material is widely studied as a prospective candidate for the so-called accident tolerant fuel (atf) [9]. a visualization of the optimized damage profile obtained as a combination of four ion beams, fe (6 mev) and ni (6, 3.4, and 1.7 mev), is shown in figure 6. the ion beams (particles and their energy) were chosen based on available and widely used ion beams (e.g. [6]). the methodology and the algorithm are, however, very generally applicable. in fact, any combination of ion beams with any predefined energy can be used. the presented case serves only as an illustration of the presented method. the resulting parameters are: a = 0.05, b = 0.48, c = 0.22, and d = 0.25. the standard deviation was reduced by 98% compared to irradiation by a single beam (fe, 6 mev). the final values of parameters a, b, c, d can be then used to design an irradiation test of the particular fuel concept. the algorithm is very flexible and it allows users to optimize their own irradiation experiment. it can be easily modified based on available ions and their energies for a particular irradiation facility and particular material. there are certain simplifications compared to the real case. the recombination of defects and other temperature effects are neglected. furthermore, the beams act as independent meaning that the algorithm treats them separately without any interaction between them and neglects combined effects. it is, however, difficult to estimate the influence of these phenomena on the presented results. as for the issue of defect diffusion during irradiation, it would be convenient to perform the irradiation at decreased temperatures as it would make the process of diffusion slower. the diffusion related phenomena of radiation effects were studied by [10] who proposed a correlation that can be used to account for the different damage rates when using different types of beams. when using ions or protons imitating irradiation damage inside lwr reactors, the temperature of the target should be increased by 15–40 ◦c compared to the temperature in reactor. another approach might be to irradiate the sample with multiple beams at the same time not separately. in this case the defects are created uniformly, and the diffusion is slow due to a low defects concentration gradient in the material. this brings challenges to the operation of the accelerator and the experiment would be more complicated to perform. 4. discussion and conclusions it is believed that ion irradiation can serve as an inexpensive and fast verification method for application of new materials in the nuclear industry. the methodology of ion irradiation can be optimized to cope with the fundamental differences between the primary effects of ion and neutron irradiation as was shown in this paper. however, ion beam testing is unlikely to replace neutron irradiation in research reactors for the purposes of nuclear fuel licensing. the proposed method proved to deliver desired results a homogeneous distribution of irradiation damage in a layer, which is relatively thin, but thick enough to be used for testing atf multi-component cladding materials. on the other hand, a weakness of proposed solution is that only the issue of damage distribution was considered. all the other aspects, 46 vol. 14/2018 simulation of irradiation processes figure 5. flowchart describing the genetic algorithm used to optimize radiation damage distribution using multiple ion beams. 0 1 2 3 4 5 6 7 8 9 0 5× 10−16 1× 10−15 1.5× 10−15 2× 10−15 0 1 2 3 4 5 6 7 8 9 0 5× 10−16 1× 10−15 1.5× 10−15 2× 10−15 dp a pe r flu en ce [d pa ·c m 2 / io n] target depth [µm] ti2alc + zircaloy-4, dpa per fluence fe, 6.0 mev ni, 6.0 mev ni, 1.7 mev ni, 3.4 mev cladcoatdp a pe r flu en ce [d pa ·c m 2 / io n] target depth [µm] ti2alc + zircaloy-4, dpa per fluence optimized distribution cladcoat figure 6. damage profiles of four different ion beams irradiating 3 µm thick ti2alc coating layer deposited on zircaloy-4 slab (violet, green, blue and yellow) and their optimized combination (orange). such as damage morphology, temperature differences or defects diffusion have not been taken into account. it is, however, a perspective method enabling to imitate neutron irradiation using heavy ions. planned irradiation experiments will be optimized using the proposed method and the resulting damage distribution will be more representative for the real application inside nuclear reactor cores compared to current simple heavy ion irradiation. the optimization tool and its manual can be found on the github repository [8] and the application of this tool to real irradiation experiments with cr and crn coated cladding materials will be presented soon. the cr and crn coated zr-based alloys were irradiated also in the lvr-15 reactor in rez up to 1 dpa. one of the goals of the proposed research is to study the advanced cladding material irradiated at the research reactor and compare the results directly to the irradiation of the same cladding concept from an accelerator with the same irradiation damage. list of symbols atf accident tolerant fuel dpa displacements per atom pka primary knock-on atom sdev standard deviation 47 václav šísl, martin ševeček acta polytechnica ctu proceedings acknowledgements the support from student grant contest from czech technical university with code sgs16/252/ohk4/3t/14 is gratefully acknowledged. many thanks belongs to my supervisor ing. martin ševeček. references [1] g. s. was. challenges to the use of ion irradiation for emulating reactor irradiation. journal of materials research 30(09):1158–1182, 2015. doi:10.1557/jmr.2015.73. [2] g. was, r. averback. radiation damage using ion beams. comprehensive nuclear materials 1:195, 2012. [3] g. was, z. jiao, e. getto, et al. emulation of reactor irradiation damage using ion beams. scripta materialia 88:33 – 36, 2014. doi:http://dx.doi.org/10.1016/j.scriptamat.2014.06.003. [4] a. kohyama, y. katoh, k. jimbo. radiation damage study by advanced dual-ion irradiation methods. materials transactions 45(1):51–58, 2004. [5] y. serruys, m.-o. ruault, p. trocellier, et al. multiple ion beam irradiation and implantation: jannus project. nuclear instruments and methods in physics research section b: beam interactions with materials and atoms 240(1):124 – 127, 2005. accelerators in applied research and technology, doi:http://dx.doi.org/10.1016/j.nimb.2005.06.100. [6] s. taller, d. woodley, e. getto, et al. multiple ion beam irradiation for the study of radiation damage in materials. nuclear instruments and methods in physics research section b: beam interactions with materials and atoms 412:1–10, 2017. [7] r. stoller, m. toloczko, g. was, et al. on the use of srim for computing radiation damage exposure. nuclear instruments and methods in physics research section b: beam interactions with materials and atoms 310:75 – 80, 2013. doi:https://doi.org/10.1016/j.nimb.2013.05.008. [8] v. šísl. optimization tool for atf ion irradiation, github repository. https://github.com/vsisl/vyzkumny_ukol, 2017. [9] p. xu, e. lahoda, s. ray, et al. corrosion resistant coatings for zirconium-alloy cladding with improved accident tolerance. nuclear engineering and technology 48:16–25, 2016. [10] l. k. mansur. correlation of neutron and heavy-ion damage: ii. the predicted temperature shift if swelling with changes in radiation dose rate 78(1):156–160. doi:10.1016/0022-3115(78)90514-7. 48 http://dx.doi.org/10.1557/jmr.2015.73 http://dx.doi.org/http://dx.doi.org/10.1016/j.scriptamat.2014.06.003 http://dx.doi.org/http://dx.doi.org/10.1016/j.nimb.2005.06.100 http://dx.doi.org/https://doi.org/10.1016/j.nimb.2013.05.008 https://github.com/vsisl/vyzkumny_ukol http://dx.doi.org/10.1016/0022-3115(78)90514-7 acta polytechnica ctu proceedings 14:42–48, 2018 1 introduction 2 differences in effects of ion and neutron irradiation 3 methodology and example results 4 discussion and conclusions list of symbols acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2019.22.0123 acta polytechnica ctu proceedings 22:123–127, 2019 © czech technical university in prague, 2019 available online at http://ojs.cvut.cz/ojs/index.php/app mechanical properties improvement of fiber reinforced concrete jan trejbala,∗, václav nežerkaa, radim hlůžeka, zdeněk prošeka,b a czech technical university in prague, faculty of civil engineering, thákurova 7, 166 29 prague, czech republic b czech technical university in prague, university centre for energy efficient buildings, třinecká 1024, 273 43 buštěhrad, czech republic ∗ corresponding author: jan.trejbal@fsv.cvut.cz abstract. fiber reinforced concrete mechanical properties are limited due to low adhesion between polymer fibers and cement matrix. to ensure a strong interaction between the two materials, polypropylene fibers (d = 0.305mm) were modified by an oxygen plasma treatment. the interface interaction was moreover activated using finely ground concrete recyclate, whose individual grains (1–64µm) ensure an adhesion improvement in interfacial zones. the adhesion enhancement was verified by pull-out tests, when reference and modified fibers were pulled-out from cement matrix specimens. such obtained results were used as a crucial parameter to numerical simulations of bending tests of specimens (550 × 150 × 150mm) with properties following fiber reinforced concrete. it was shown that samples reinforced with modified fibers and contained activating recyclate reached on higher residual bending strength then those with reference fibers. keywords: fiber reinforced concrete, macro fibers, polymer fibers, interfacial shear stress. 1. introduction fiber reinforced concrete (frc) has became popular at production of prefabricated concrete materials, shotcretes, and industrial high-loaded floors. such material is composed from polymeric macro-fibers (amount ca. up to 1 % vol. of whole mixture), cement, and aggregate [1, 2]. technical standards en 14845-1 and en 14845-2 describe frc as structural concrete reinforced with fibers having static effect and fulfilling requirements of en 14889-2. during three-point bending test of notched specimens 550×150×150 mm, such frc has to exhibit, besides, residual strength at least 1.5 mpa at crack mouth opening displacement (cmod) of 0.5 mm (corresponding deflection 0.47 mm). it is clear that such behavior differentiates the frcs from strain hardening or engineered composites, where strain-hardening response is required after reaching the elastic limit [1]. d. j. kim et al. explained that strength limit of a fibrous composite material (including frc) is a function of fibers volume, fibers length to diameter ratio, and the interfacial interaction between fiber surfaces and matrix. in the field of frcs, it means that increasing fibers amount weakens the cement matrix mechanical properties in the stage of elastics response during loading. it is therefore clear that fibers amount should be as small as possible. on the other side, once matrix limit of proportionality is reached and the matrix is damaged by the crack, fibers transfer the acting stress across the crack (crack bridging) and thus ensure macroscopic integrity of whole material. amount of stress transfered via fibers depends especially on their number and on adhesion between fiber surfaces and the cement matrix [3]. however, such adhesion is mostly too poor, especially between polymeric fibers and the cement matrix due to fibers smooth and chemically inert surfaces (related to cement matrix) [4, 5]. mechanical potential of fibers – tensile strength – is therefore unused. to avoid issues connected with the poor adhesion between the two materials, some researches have applied additional treatment of fibers in order to decrease their surface free energy and to increase their morphology, both ensuring improvement of bond to the matrix. for these purposes, several types of treatment may be employed, e.g. chemical (use of high alkali solutions) and physical (mechanical roughening) [6–8]. plasma modification has shown to be a promising technology, combining both the chemical (etching) and physical (roughening caused by a ion bombardment) treatment, as proven by li et al. from the early 1990s [9, 10] and many other researches later, e.g. [11– 14]. it is also worth noting that such treatment has been extended through many industrial fields over the past few years, especially for the surface treatment (roughening, activating, cleaning) of polymeric materials [15]. therefore, there is no obstacle to apply such technology during surface treatment of the fibers. although a benefit of fiber surface treatment was proven from the perspective of “surface science” many times, this was not achieved from the practical point of view, including the field of frcs. to connect theoretical findings with praxis of civil engineering, we studied an influence of plasma modified fibers on 123 http://dx.doi.org/10.14311/app.2019.22.0123 http://ojs.cvut.cz/ojs/index.php/app j. trejbal, v. nežerka, r. hlůžek, z. prošek acta polytechnica ctu proceedings mechanical properties of frc samples using numerical simulations, following en 14845-1. 2. interaction between fiber and matrix post-cracking response of frc is influenced by behavior of one fiber that is pulled out from the matrix. this phenomenon was described by ch. li et al. and c. redon et al. [16, 17]. the behavior is divided into two stages, the first describes chemical interaction between the two materials (pdeb), while the second mechanical interaction activated by fiber movement out of the matrix (ppull), concretely: pdeb = √ π2efd3 f 2 (τ0u+gd) (1) and ppull = πdfτ0 1 + β ( u− ( 2τ0l 2 e efdf + √ 8gdl2 e efdf )) df × [ le − u+ ( 2τ0l 2 e efdf + √ 8gdl2 e efdf )] , (2) and τ0, as a frictional stress after sudden drop following the peak pull-out load, by: τ0 = ppull πdfle (3) where ef is young’s modulus of elasticity of fibers; df , fibers diameter; τ0, interfacial shears tress between fiber surface and matrix; u, fiber free-end displacement; gd, interfacial bond strength; β, a shear retention factor, parameter describing slip softening / hardening behavior; le, fiber embedded length. 3. fibers and their treatment 3.1. polymeric fibers polymeric macro-fibers were used for all experiments described below. their geometrical and mechanical properties were as follows: material, polypropylene (pp); morphology, smooth; length, 60 mm; diameter, 305 µm; density, 900 kg/m3; young’s modulus of elasticity, 6.1 gpa; tensile strength, 440 mpa; elongation, 8 %. mechanical properties have been determined experimentally, as reported in [18]. 3.2. fiber treatment two fiber types were used: reference (further marked as r) and plasma treated (p30 and p120 according to treatment duration). plasma treatment was executed using tesla vt214 device. treatment parameters were: plasma, cold; gas, oxygen; power of rf source, 100 w; gas pressure, 20 pa; treatment duration, 30 and 120 seconds. name cement recyclate w/c [wt. %] [wt. %] [-] ref 100 0 0.40 rec 70 30 0.41 table 1. weight proportions of cement matrices. 4. pull-out tests two matrices were used to carry out pull-out tests. the reference matrix (ref) was made from portland cement cem i 42.5r and the modified matrix (rec) contained 30 wt. % concrete recyclate as a substitution for cement at the form of finely ground powder. the recyclate was used to fill interfacial zones between fiber surfaces and surrounding matrix and thus to ensure the adhesion between the two materials with individual grains. grain size differed between 1–64µm, as measured by blain’s method. matrices compositions are summarized in table 1. specimens made from such matrices had dimensions equal to 25 × 20 × 30 mm, contained a single fiber in their centerline (fiber embedded length was equal to samples height – 30 mm). results of this experiment were used as basic input values for numerical modeling of frc bending tests. the whole experiment was carried out using loading frame veb tiw rauenstein fp100. the specimen was anchored by its matrix body to a static part of the frame, while the single fiber, protruded from the specimen body, was caught by a moving frame part. the experiment was displacement controlled at the constant rate of 3 mm/min, finished after reaching to 4.5 mm of fiber-free end displacement (only r and p30 were further used). results from pull-out tests – dependence between fiber free-end displacement and force resisting to that – are summarized in figure 1. it is clear from these results that the maximal force recorded during pullout reference fiber (r) from the reference matrix (ref) slightly overcame 10 n, while in the case of 30 seconds plasma modified fibers (p30) and matrix containing concrete recyclate (rec), the force reached on more than 14 n. despite of the assumptions, fibers exposed to plasma for 120 seconds (p120) exhibited adhesion to the matrix worse than these modified for 30 seconds. this could be caused by their diameter reduction due to too long treatment. therefore, such fibers were not used for numerical simulations described below. according to equation (4), shear stresses were calculated from thus obtained results. it was calculated that τ0,r = 3.24 · 105 pa andτ0,p30 = 4.56 · 105 pa in the case of reference and 30 seconds modified fibers, respectively. 5. numerical simulations numerical simulations followed procedure of threepoint bending test described in technical standard 124 vol. 22/2019 mechanical properties improvement of fiber reinforced concrete fo rc e [n ] 0 2 4 6 8 10 12 14 16 0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 fiber free-end displacement [mm] r p30 p120 figure 1. pull-out behavior of reference and modified fibers. figure 2. geometry of frc specimen (top) and mesh of computational model (bottom). en 14845-2. as already mentioned in introduction, notched frc specimen 550 × 150 × 150 mm, containing 0.5 vol.% of fibers, has to exhibit at least 1.5 mpa of bending strength in post-cracking phase during midspan deflection of 0.47 mm. to avoid lengthy experimental testing, numerical simulation was employed. shcc material model [19, 20], suitable also for frcs, was applied. mash of the 2d model, counting 3080 of predominantly triangle linear elements, was created in salome software [21]. non-linear numerical analysis was conducted using oofem software [22], proceeded in 800 steps. numerical experiment was controlled by displacement. solution was searched by newton-raphson’s method. the stiffness matrix was compiled in 2d plain-stress preposition. geometry of the specimen and mesh of finite elements are shown in figure 2. two calculations were done; the first contained reference, while the second one 30 seconds plasma modified fibers. mechanical properties of matrix was set to correspond common concrete. all parameters set to numerical model were as follows (reference / modified fibers): e young’s modulus of elasticity of matrix, 20 / 20 gpa; ν poisson’s ratio of matrix, 0.2 / 0.2; gf fracture energy of matrix, 5.0 / 5.0 n/m; ft tensile strength of matrix, 2.5 / 2.5 mpa; softtype a parameter describing post-peak behavior, 3 / 3: hordijk’s softening; sheartype a parameter describing shear stiffness of cracked material, 1 / 1: constant shear retention; shearstrengthtype a parameter limiting the magnitude of resulting shear stress acting on crack plane; 1 / 1: the threshold is set to the value of the tensile strength; vf fiber volume ratio, 0.005 / 0.005; df fiber diameter, 0.305 / 0.305 mm; df fiber length, 60.0 / 60.0 mm; ef young’s modulus of elasticity of fibers, 6.1 / 6.1 gpa; gf shear modulus of fibers, 1.0 / 1.0 gpa; τ0 frictional shear stress between the fiber and the matrix during debonding, 0.324 / 0.456 mpa; f snubbing coefficient; 0.5 / 0.5; kf fiber cross-section shape correction factor, 0.9 / 0.9; fsstype a class describing type of fiber bond shear strength, 0 / 0: constant shear strength; fibertype class of reinforcing fibers, 2 / 2: short randomly oriented fibers; ncracks maximal number of cracks, 2 / 2; m exponent related to fiber unloading, 1 / 1; fibreactivationopening, 10−6 / 10−6; dw0 lower bond allowing to smoothen the traction-separation law for fibers, 10−7 / 10−7; dw1 upper bond allowing to smoothen the traction-separation law for fibers, 10−7 / 10−7. 6. results it was found from numerical simulations that residual strength of frc specimen reinforced with plasma modified fibers (p30) at amount of 0.5 vol.% tightly exceeded 1.5 mpa at cmod of 0.47 mm. in the same stage, the samples containing reference fibers exhibited only ca. 1.3 mpa. results from both simulations are imagined in figure 3, where the green line highlights the minimal bending strength required by en 148452. it is obvious from these results that only frc containing modified fibers fulfilled these requirements, so this can be considered as structural. these simulations also revealed that the behavior of both specimens was practically identical in phases 125 j. trejbal, v. nežerka, r. hlůžek, z. prošek acta polytechnica ctu proceedings fiber free-end displacement [mm] 0 0 0.1 0.2 0.3 0.4 0.5 0.6 1.0 1.5 2.0 2.5 3.0 0.5 st re ss [m pa ] midspan deflection [mm] r p30 figure 3. normal tension as a function of midspan displacement. until the fibers have not been activated yet. after, fibers bridged the opening crack, transfered acting stress and thus ensured macroscopic integrity of specimens. based on the specimens post-cracking behavior, it is clear that the adhesion between modified fibers and the matrix was increased than in case of reference fibers. this finding proves that interfacial shear stress between the two materials plays an important role from the mechanical response point of view. 7. conclusions this work deals with searching of mechanical behavior of fiber reinforced concrete using numerical simulations of three-point bending tests. reference and plasma modified polypropylene fibers (d=0.305) were used as reinforcement. adhesion between these fibers and two types of the cement matrix was examined employing pull-out test. the purpose of the simulation was to find residual bending strength of specimens, following relevant technical standards. finding were as follows: • adhesion between 30 seconds oxygen plasma treated fibers and the cement matrix containing concrete recyclate was higher by approx. 15 % then in the case of reference fibers and reference cement matrix. • 120 seconds lasting plasma treatment did not bring any benefits in terms of adhesion improvement. conversely, such treated fibers showed worse adhesion to the matrix than those 30 seconds exposed to plasma. this was probably caused by their diameter reduction as a consequence of too intensive ion bombardment. • numerical simulations revealed that residual strength of reference frc at the midspsan deflection of 0.47 mm was less than 1.5 mpa, so this material did not meet requirements of technical standard en 14845-2. on the other side, if plasma treated fibers and the cement matrix containing 30 wt.% of concrete recyclate were used, residual strength overcame minimal 1.5 mpa. acknowledgements this work was financially supported by czech technical university in prague – sgs project sgs16/201/ohk1/3t/11 and sgs18/037/ohk1/1t/11, and by the czech science foundation research project 17-06771s. special thanks to tereza horová for research assistance. references [1] m. prisco, g. plizzari, l. vandewalle. fibre reinforced concrete: new design perspectives. materials and structures 42(9):1261–1281, 2009. 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[15] j. reece roth. industrial plasma engineering. volume 2: applications to nonthermal plasma processing. crc press, boca raton, 2001. [16] v. li, c. leung. steady-state and multiple cracking of short random fiber composites. journal of engineering mechanics 118(11):2246–2264, 1992. doi:10.1061/(asce)0733-9399(1992)118:11(2246). [17] c. redon, v. li, c. wu, et al. measuring and modifying interface properties of pva fibers in ecc matrix. journal of materials in civil engineering 13(6):399–406, 2001. doi:10.1061/(asce)0899-1561(2001)13:6(399). [18] r. hlůžek. analysis and modification of interfacial zones between cementitious matrix and fiber reinforcement. diploma thesis. ctu in prague, faculty of civil engineering, czech republic, 2018. [19] k. toshiyuki, p. kabele, h. fukuyama, et al. strain hardening cement composites: structural design and performance. in state-of-theart report of the rilem technical committee 208-hfc, sc3. springer, dordrecht, 2012. [20] p. havlásek, p. kabele. a detailed description of the computer implementation of shcc material model in oofem, ctu in prague. [21] salome platform, on-line, available from: http://www.salomeplatform.org/usersection/about/mesh. [22] b. patzák. oofem – an object-oriented simulation tool for advanced modeling of materials and structures. acta polytechnica 6(6):59–66, 2012. 127 http://dx.doi.org/10.1016/s1065-7355(97)90001-8 http://dx.doi.org/10.1016/j.jmatprotec.2009.02.015 http://dx.doi.org/10.1016/j.cemconcomp.2018.03.010 http://dx.doi.org/10.1016/j.surfcoat.2013.10.008 http://dx.doi.org/10.1016/j.polymdegradstab.2012.08.001 http://dx.doi.org/10.1061/(asce)0733-9399(1992)118:11(2246) http://dx.doi.org/10.1061/(asce)0899-1561(2001)13:6(399) acta polytechnica ctu proceedings 22:123–127, 2019 1 introduction 2 interaction between fiber and matrix 3 fibers and their treatment 3.1 polymeric fibers 3.2 fiber treatment 4 pull-out tests 5 numerical simulations 6 results 7 conclusions acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2019.25.0006 acta polytechnica ctu proceedings 25:6–11, 2019 © czech technical university in prague, 2019 available online at http://ojs.cvut.cz/ojs/index.php/app multiaxial investigation of pvc foams and analysis of the deformation mechanism by 3d-dic francesca concas∗, stefan diebels, anne jung saarland university, institute of applied mechanics, campus a4.2, 66123 saarbrücken, germany ∗ corresponding author: francesca.concas@uni-saarland.de abstract. closed-cell polyvinylchloride (pvc) foams are widely used as core for sandwich composites for applications, in which multiaxial loads are involved. in the present work a wide range of uniaxial (tension, compression and torsion) and multiaxial experiments (both simultaneous tension-torsion and compression-torsion) were conducted on a high performance pvc foam. failure data for each experiment were collected and depicted in the invariants plane. the whole cylindrical surface of the specimen was monitored by means of an 8-camera-system, strain fields were obtained by 3d-dic. hence, the occurrence and the evolution of deformation bands were inspected. the usage of an 8-camera system was essential for the observation of the deformation mechanism, especially for pure compression, pure torsion and combined axial load-torsion, in which the arising of deformation bands is affected by the occurrence of buckling and the orthotropy of the foam. keywords: polymeric foam, multiaxial testing, three-dimensional digital image correlation. 1. introduction closed-cell polyvinylchloride (pvc) foams are broadly used as core material for sandwich composites in the marine industry [1] and for the construction of wind turbine blades [2]. in such applications, the core withstands multiaxial loads. hence, a proper knowledge of the mechanical behaviour of the above-mentioned foams under multiaxial loading conditions is of great concern. this topic was addressed by several works, e.g. christensen et al. [3] and gdoutos et al. [4] conducted combined tension-torsion and compressiontorsion tests among other experiments on hollow cylindrical specimens made of pvc foams. in both previous works, adhesives were used to fix the specimen’s position. the multiaxial behaviour of the same foam grade was investigated more recently by shafiq et al. [5], who applied uniaxial, biaxial and triaxial loads by means of a specific testing device. hoo fatt et al. [6] carried out cyclic multiaxial tests on the same foam grade. in the present work uniaxial and multiaxial experiments were performed on a high performance pvc foam core grade. first, pure tension, pure compression and pure torsion were applied on the specimens. failure loads were calculated for each experiment. afterwards, simultaneous tensile-torsion experiments and simultaneous compression-torsion experiments were conducted by considering several percentages of the axial failure load. failure stresses were then collected in the invariants plane for outlining the failure surface of the high performance pvc foam. the deformation of the specimen was inspected for some specific loading cases by means of the threedimensional digital image correlation (3d-dic) and an 8-camera system. such procedure allowed the monitoring of 360◦ of the specimen. 2. materials and methods experiments were done on the high performance divinycell® hp100 pvc foams core grade (diab ab, laholm, sweden) using an electropuls™ e10000 universal testing machine (istron ltd., pfungstadt, germany, load cell capacity 10 kn for axial loading and 100 nm for torsional loading), which was equipped with two three-jaw chucks for clamping the cylindrical specimens. the 3d-dic was achieved through the 8-camera system and the software istra 4d v. 4.4.2 (dantec dynamics gmbh, skovlunde, denmark). the eight cameras manta g-235b (allied vision technologies gmbh, stadtroda, germany) were disposed in aluminium bars in order to compose an octagon around the specimen. the experimental set-up is shown in figure 1 (a). the maximal frame rate of the cameras was 50.8 fps. images had a resolution of 1936× 1216 pixels with a pixel size of 5.86µm× 5.86µm and the acquisition rate was 1 fps. three lamps were also needed for providing an illumination of the specimen with an adequate uniformity. the light sources consisted in two led bulbs and a halogen light source kl 2500 lcd (carl zeiss ag, oberkochen, germany) equipped with two flexible light guides. the specimen had a cylindrical gauge part with a diameter and a height of 25mm and 50mm, respectively. both ends of the specimen had a conical part and a shorter cylindrical part with a bigger diameter in order to allow the positioning of custom-made bushings as shown in figure 1 (b). this method prevented the usage of adhesives for fixing the specimen’s position. the same specimen was used for both uniaxial and multiaxial tests. such geometry was manufactured by using the lathe with a grinding wheel as cutting tool. it is widely known that pvc foams are orthotropic because of the elongation of the cells towards the through-thickness 6 http://dx.doi.org/10.14311/app.2019.25.0006 http://ojs.cvut.cz/ojs/index.php/app vol. 25/2019 multiaxial investigation of pvc foams and analysis of deformation (a) camera (8) upper bushing specimen lower bushing three-jaw chucks (b) half of the upper bushing half of the lower bushing specimen figure 1. set-up of the experiments (a) and cutaway of the bushing on the specimen (b). direction [7, 8]. based on the thickness of the available panels, i.e. 40mm, the axis of the cylindrical specimen was perpendicular to the direction of cell elongation and hence the in-plane properties were evaluated by the experiments described in this work. tensile, compression and torsion experiments were conducted and failure loads are determined. afterwards, combined tension-torsion and combined compression-torsion were performed by considering an increasing contribution of the axial load. the procedure for performing multiaxial tests is described as follows: the actuator of the universal testing machine applied the tensile or the compressive load to the specimen up to a percentage of the uniaxial failure load, i.e. 25%, 50% and 75% and a further percentage of 90% for combined tension-torsion tests. when the desired percentage was reached, the torsion was applied up to the rupture of the specimen or until reaching the limit switch, in case the specimen did not break. the reached tensile or compressive load was kept constant during the superposition of torsion, in order to make a simultaneous loading tension-torsion or compressiontorsion. failure limits were calculated for combined tension-torsion and combined compression-torsion as well. for each loading case, three experiments were conducted for the sake of statistics. finally, failure limits were collected in the invariants plane. tensile and compression experiments were performed under displacement-control with a machine speed of 0.1 mm/s. the preload of multiaxial experiments was applied by load-control with a speed of 5 n/s. the application of torsion was done by rotational-control with an angular velocity of 0.5 ◦/s for pure torsion experiments as well as for multiaxial experiments. for the pure tensile, compression and torsion tests and for the combined tension-torsion and compression-torsion experiments with 50% of the axial failure load, strain fields were obtained by 3d-dic and are displayed in the unwrapped cylindrical surface by means of the software h5pyviewer© (thierry zamofing, paul scherrer institute, switzerland). dic is a full-field optical method, which evaluates displacements and strains by tracking the motion of the specimen’speckle pattern during the experiment [9]. the input values for the facet size and the grid spacing were 29 pixels and 3 pixels, respectively. the speckle pattern was applied as aerosol varnish on the specimens. 3. results and discussion stress-strain diagrams are shown in figure 2 (a) for pure tensile tests and pure compression tests. stresses and strains are reported as absolute values for comparing directly the compressive diagrams with the tensile diagrams. the occurrence of the buckling in the specimen during compression loading is highlighted in one of the diagrams in fig. 2 (a). figures 2 (b-d) show the shear stress-shear strain diagrams for pure torsion tests, for combined tensile-torsion tests and combined compression-torsion tests, respectively. for pure tension, pure torsion and combined tension-torsion, the specimen fractured at the end of the tests, then the load leading to the rupture of the specimen was considered as failure load. for pure compression and combined compression-torsion the specimen did not fracture, whereas a peak yield stress is evident in the diagrams. for such loading cases, the load leading the specimen to yielding was considered as failure load. stresses and strains were calculated by the output file of the universal testing machine, hence the strains of figure 2 are global strains, which are distinguished by local strains obtained by the 3d-dic. afterwards, failure data were collected in the invariants plane, i.e. the first principal invariant and the squared root of the second deviatoric invariant (see figure 3). the yield function proposed by bier et al. [10–12] was adapted as failure criterion to the gathered failure data of the pvc foam. further details on the fitting are given by concas et al. [8] the arising and the evolution of deformation bands were inspected in the strain fields given by the 3d-dic. for sake of conciseness, strain fields are shown only for two loading cases, i.e. pure 7 f. concas, s. diebels, a. jung acta polytechnica ctu proceedings increasing percentage tension loading increasing percentage compression loading entering buckling figure 2. stress-strain diagrams for the tension tests and the compression tests (a) and the shear stress-shear strain diagrams for the torsion tests (b), the combined tension-torsion tests (c) and the combined compression-torsion tests (d). figure 3. failure data in the invariants plane and fitting of the failure criterion proposed by bier et al. [10–12]. 8 vol. 25/2019 multiaxial investigation of pvc foams and analysis of deformation lo ca ll ag ra ng ia n ve rt ic al st ra in figure 4. local lagrangian vertical strain fields with the corresponding absolute values of the global strain |ε| for a pure compression test. compression and combined tension-torsional loading, since they reflect the main typical characteristics of the strain maps of pvc foams for the other loading cases as well. concerning the combined tension-torsion experiment, the axial load was kept constant to the 50% of the tensile failure load during the superposition of torsion. for each loading case, three strain fields are shown, which refer to three global strain values in the respective stress-strain diagram. the local lagrangian vertical strain fields are shown in figure 4 for three absolute values of the global strain for a simple compression test. in this case, deformation bands were detected only when the plateau stage was reached. during the plateau, the specimen underwent a growing buckling. in the last strain field, areas with positive and negative local vertical strain values are distinguishable due to the severe buckling of the specimen, where local strains are positive in the convex part of the curved specimen and negative in the concave part. figure 5 shows the local lagrangian shear strain fields for three global shear strain values of the combined tension-torsion test. in the second global strain stage of figure 5, the positioning of deformation bands in two diametrically opposed areas of the specimen is observed. such deformation bands are kept also in the last global strain stage’s map of figure 5, but it is less visible because of the arising of a deformation band with the highest strain gradient in the lower part of the specimen, where the fracture begins. this feature was observed also in pure tensile tests, whereas the gathering of deformation bands in two diametrically opposed wedges of the specimen was visible also in pure torsion tests and combined compression-torsion tests. in all tests, the widening black areas in the strain maps for the increasing global strains are due to the high distortion of the specimen and the consequent loss of the tracked pattern. 4. conclusions the present work described the execution of uniaxial and multiaxial tests on cylindrical specimens of the divinycell®hp100 pvc foam. the usage of the same geometry for all tests allowed a direct comparison between the different loading cases, i.e. the diagrams of figures 2 (b-d). hence, the increasing percentage of the axial failure load, either tension or compression, combined with a torsional load caused a lower failure limit in comparison to pure torsion tests. failure data were calculated for all tests and collected in the invariants plane. the yield function of bier et al. [10–12] was adapted successfully to the failure data of the pvc foam. strain maps were obtained for the whole cylindrical surface of the specimen by using the 3d-dic and the 8-camera-system. in this work, the strain fields were reported for two different 9 f. concas, s. diebels, a. jung acta polytechnica ctu proceedings lo ca ll ag ra ng ia n sh ea r st ra in figure 5. local lagrangian shear strain fields with the corresponding global shear strain values γ of a simultaneous tensile-torsion test, in which the 50% of the tensile failure load is reached. loading cases, i.e. pure compression and combined tension-torsion. for the pure compression test, deformation bands were visible only when the specimen had entered the plateau stage. for higher global strains, deformation bands merged for composing areas with positive and negative local strain values, which were consistent with the curvature of the buckled specimen. for the combined tension-torsion test, deformation bands were mainly positioned in two diametrically opposed areas of the specimen due to the elongation of cells perpendicularly to the axis of the cylindrical specimen. the arising of a deformation band with the highest strain gradients allowed the prediction of the imminent fracture of the specimen. the usage of the 8-camera-system permitted a complete monitoring of deformation bands on the chosen specimen geometry. for the applied loading cases, the particular localisation of deformation bands could not be detected if a lower number of cameras were used. a 4-camerasystem is efficient for other specimen geometries, e.g. flat dog bone specimens [13], but it would redouble the extensions of black areas in the strain maps when the above-mentioned cylindrical geometry is observed. acknowledgements the authors acknowledge prof. f. aymerich and prof. a. baldi (university of cagliari, italy) for useful discussions and for supplying the pvc foams panels. the university of cagliari is also acknowledged for financial support. references [1] j. graham-jones, j. summerscales. marine applications of advanced fibre-reinforced composites. woodhead publishing, 2016. 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[13] c. röhrig, t. scheffer, s. diebels. mechanical characterization of a short fiber-reinforced polymer at room temperature: experimental setups evaluated by an optical measurement system. continuum mechanics and thermodynamics 29(5):1093–1111, 2017. 11 https://doi.org/10.1007/s11340-011-9519-7 https://doi.org/10.1016/j.jmps.2019.06.008 https://doi.org/10.1016/j.jmps.2019.06.009 https://doi.org/10.1016/j.euromechsol.2006.01.002 https://doi.org/10.1016/j.ijmecsci.2006.09.026 https://doi.org/10.1016/j.ijplas.2007.01.014 acta polytechnica ctu proceedings 25:6–11, 2019 1 introduction 2 materials and methods 3 results and discussion 4 conclusions acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2018.16.0018 acta polytechnica ctu proceedings 16:18–24, 2018 © czech technical university in prague, 2018 available online at http://ojs.cvut.cz/ojs/index.php/app the load-displacement behaviour of ground anchors in fine grained soils juraj chalmovský∗, lumír miča brno university of technology, faculty of civil engineering, institute of geotechnics, veveří 331/95, 602 00 brno, czech republic ∗ corresponding author: chalmovsky.j@fce.vutbr.cz abstract. ground anchors represent an important structural element in the area of geotechnical engineering. despite their extensive usage, a design process of these elements is usually performed using simple empirical and semi-empirical methods, neglecting several important influencing factors. this paper gives an analysis of the factor of non-uniform distribution of skin friction resulting in a progressive failure of ground anchors. first, the finite element method in combination with a material model involving regularized strain softening is utilized. next, an experimental program, including several investigation anchor load tests, was carried out. the goal of this program was to confirm preliminary conclusions drawn from numerical studies and to obtain relevant data for further back analysis. after, there is then described a newly developed application based on the load transfer method, in which all the findings from numerical computations and experimental measurements are incorporated. keywords: ground anchor, strain softening, investigation load test, load transfer function. 1. introduction mobilization of shear stresses along a fixed length of ground anchors is significantly non-uniform during anchor loading. in the first loading stages, shear stresses mobilized at the grout soil interface are concentrated at the top of the fixed anchor length. after reaching the peak value, shear stress is consequently reduced towards its critical and residual value. the location of peak shear stress moves along the fixed anchor length as schematically shown in figure 1. the non-constant (peak) shear stress distribution along the fixed anchor length has been experimentally confirmed by several authors: ostermayer [1], scheele [2], barley [3], woods, and barkhordari [4]. the described phenomenon has a severe influence on the ultimate bearing capacity, ground anchor efficiency and load displacement curves. the ultimate load carrying capacity is not directly proportional to the fixed anchor length. because the maximum bond stress is reached only in a small portion of the fixed anchor length, anchors with fixed lengths longer than 8 m are inefficient (barley [5]). in addition, the assumption of a constant bond stress may lead to an overestimation of the load carrying capacity. based on regression analysis of anchor loading tests performed mainly in london clay and glacial clay, barley [3] has proposed an analytical formula for the efficiency factor feff and for the load carrying capacity of low pressure grouted anchors tult (eq. 2), where lfixed is the fixed anchor length, d is the borehole diameter and cu is the undrained soil shear strength. the progressive failure mechanism is dominant in the case of stiff, to very stiff over-consolidated clays and dense, to very dense non-cohesive soil exhibiting peak shear strength followed by a strength reduction during strain softening towards a residual stress state. feff = 1, 6l−0,57 fixed (1) tult = πdlfeffcu (2) 2. finite element modelling in order to analyze the effect of progressive failure by numerical methods it is necessary to adopt a constitutive model involving post-peak strain softening. for this purpose the multi-laminate model for stiff overconsolidated clays (mlsm) proposed in schadlich [7] was used. mlsm requires 11 input parameters: the oedometric reference stiffness eoed,ref , the modulus of elasticity for unloading reloading eur,ref , the poisson ratio for unloading reloading νur, the shear hardening parameter amat, the critical state friction angle ϕcs, the hvorslev surface inclination ϕe, the initial preconsolidation pressure σnc, the reference pressure pref , the stress dependency index m, the softening scaling factor hsoft, and the internal length lcalc. ground conditions of modeled and tested anchors consist of highly over-consolidated stiff brno clay of the neogene age. svoboda, mašín [8] estimated the value over-consolidation ratio ocr=6.5 based on oedometric tests on non-disturbed samples. applying the formula proposed by mayne, kulhawy [9] leads to k0=1.25. input parameters for mlsm are calibrated based on three cu triaxial tests (svoboda et al. [10]) performed at different confining pressures (275 kpa, 500 kpa, 750 kpa). the calibration process is divided 18 http://dx.doi.org/10.14311/app.2018.16.0018 http://ojs.cvut.cz/ojs/index.php/app vol. 16/2018 the load-displacement behaviour of ground anchors figure 1. non-uniform shear stress distribution along the anchor fixed length [6]. figure 2. triaxial stress strain curves. into two parts described in the following sections. mlsm with determined input parameters is then applied to simulation of a low pressure (gravity) grouted anchor. finally, back analysis of the investigation anchor load test is performed. fem packages plaxis 2d brinkgreve et al. [11] and plaxis 3d brinkgreve et al. [12] were used for the analyses presented here. 2.1. strain hardening parameters first, parameters governing strain hardening plasticity were obtained (table 1). this can be done using a "single stress point approach". soiltestlab application was used for this purpose. comparisons between simulations on a single stress point level and measured data (stress strain curves and effective stress paths) are shown in figure 2 and figure 3. 2.2. strain softening parameters determination of parameters governing strain softening on a single stress point level is not possible, as the softening rate is size dependent and non-homogeneity (shear band) originates in a sample. a 3d model of a triaxial test with real sample dimensions is therefore figure 3. effective stress paths. necessary. the strain softening behaviour is governed by two parameters: the softening scaling factor hsoft which controls the rate of softening for a given damage strain increment and the internal length lcalc which determines the domain to be considered during the regularization. in order to transfer a calibrated softening rate into a boundary value analysis, softening scaling (brinkgreve [13], galavi [14]) can be utilized because the softening rate depends only on the ratio hsoft/lcalc. for the purpose of calibration of these parameters, a 2d plane strain numerical model of a biaxial test and a full 3d numerical model of a triaxial test were employed. measured and predicted stress strain curves for the cu test with confining pressure 275 kpa (closest to the stress range in the anchor vicinity) are shown in figure 4. three simulations of a biaxial test with different hsoft/lcalc ratios (8000, 24000, 32000) were performed. a ratio of hsoft/lcalc=32000 is used in the final prediction with a 3d triaxial test model and for further finite element models. localization of shear strains into the shear band at the end of the simulation is clearly visible in figure 5. in the case of 19 juraj chalmovský, lumír miča acta polytechnica ctu proceedings eoed,ref eur,ref νur amat φcs φe σnc pref m kpa kpa ° ° kpa kpa 1200 8000 0.2 15 20 16 1800 100 0.55 table 1. values of input parameters governing strain hardening. figure 4. triaxial stress strain curves – softening. (a) . deformed mesh (b) . shear band figure 5. post peak behaviour. solving a practical boundary value problem, the internal length lcalc is appropriately selected (according to the recommendations) and hsoft is scaled to get the required hsoft/lcalc ratio. 2.3. initial application in the first step after calibration, an fe – mlsm combination was used for simulating a load test of the gravity grouted anchor with a fixed length lfixed of 4 m. the axisymmetric finite element model using 6noded triangular elements was prepared. mobilization of skin friction along the fixed length on the groutsoil interface for three different load levels (65 kn, 125 kn, 146 kn) is shown in figure 6. after reaching the peak skin friction of 95 kpa in the second load step, shear stresses are decreasing towards the critical figure 6. shear stress distributions for different loading stages. value. with increasing load level, also the fixed length section in the softening regime is growing. 2.4. back analysis of the investigation anchor load test calculations adopting a simple model of a ground anchor confirmed that it is possible to model the progressive failure mechanism by combination of the finite element method and the constitutive model involving regularized strain softening. in the second step, back analysis of the investigation anchor load test was therefore performed. the investigation load test was carried out by misove [15]. the free and fixed length was 4 m and 8 m, respectively. the anchor was post-grouted by tube-a-manchette system (tam). both section were separated by a packer to prevent a grout inflow into the free length. the anchor was excavated after the test in order to determine its diameter. due to post-grouting, the diameter was enlarged from the original 140 mm to 190 mm. the process of grouting was modelled by applying a pre-calculated volumetric strain to the respective elements thus increasing radial stresses acting on the fixed length surface. three basic calculations were performed: 20 vol. 16/2018 the load-displacement behaviour of ground anchors figure 7. predicted and measured load – displacement curves. • standard calculation with mlsm involving the reaching of peak shear strength (hvorslev surface) followed by strain softening. • calculation with modified version of the mlsm with no peak strength, thus only critical shear strength can be reached. • calculation with the standard mohr – coulomb model with peak shear strength parameters cp=90 kpa and ϕp=14°. these values were determined from the shear box test assuming a normal stress acting on a fixed length surface between 200 and 400 kpa. predicted and measured load – displacement curves are shown in figure 7. the alternative using peak strength characteristics significantly overestimates the bearing capacity. on the other hand calculation with modified mlsm (only critical strength can be reached) underestimates the bearing capacity. a sufficient match in terms of displacements and the carrying capacity is only reached when a skin friction decrease from peak to critical is involved. predicted shear stress distributions for three different load levels (627 kn, 739 kn, and 800 kn) are shown in figure 8. skin friction mobilization is significantly non-uniform. progressive shear stresses decrease from the peak value of 150 kpa and gradually propagate to a distance of 3 m from the fixed length head. 3. experimental program analysis performed using the combination of fem and mlsm has revealed that neglecting a progressive skin friction decrease may lead to highly inaccurate results. it was therefore decided to perform a detailed experimental test program involving 6 investigation load tests (up to a failure) of anchors constructed in brno neogene clay. each anchor was monitored by two groups of systems: figure 8. computed shear stress distributions for three load levels. • standard monitoring: measuring of anchor head displacement (lvdt) and a pre-stressing force. • detailed monitoring: axial strain measuring along the tendon and in grout, using vibrating wire tensometers and electric resistance gauges. in sum 5 of the tested anchors were standard temporary anchors, the last one was sbma (single bore multiple anchor). the free length of standard anchors was 5 m, the fixed length was 6 m (2 tests), 8 m (1 test) and 10 m (2 tests). due to the limited extent of the paper only two representative results are presented. detailed test results and their interpretation can be found in chalmovský [16]. the time record of a measured pre-stressing force and the anchor head displacement is shown in figure 9. on reaching the ultimate bearing capacity (point no. 1) there followed a rapid decrease of pre-stressing force which was accompanied by clearly visible upward movement of the grout body. the anchor was fully unloaded after that (point no. 2) followed by re-increasing of prestressing force. it was found that the bearing capacity dropped to 55% of the original peak value (point no. 3). at this load level anchor head displacements stabilized (point no. 4). similar behaviour was observed on all tested anchors. taking into account that the tendon – grout interface was not broken, it might be concluded that the observed behaviour was a consequence of rapid progressive failure of the grout-soil interface. the second example (figure 10) presented in this paper are measured distributions of pre-stressing force along the fixed length of the anchor (lfixed=10 m). it is obvious from this figure that for three initial 21 juraj chalmovský, lumír miča acta polytechnica ctu proceedings figure 9. the investigation test record. figure 10. measured distributions of pre-stressing force. load levels (236 kn, 476 kn, 715 kn) only the first 5.5 m of the fixed length is utilized. considering that pre-stressing force of 715 kn is much higher than what would be considered as a design bearing capacity (applying standard design methods), the anchor economic efficiency is low. 4. development of application based on load-transfer functions analysis using the finite element method and experimental results confirmed that the progressive decrease of skin friction along anchor fixed lengths significantly influences an overall load – displacement behaviour and an ultimate bearing capacity of anchors. despite its complexity, numerical analysis with a constitutive model involving regularized strain softening, presents a time demanding task. in order to involve the aspect of a non-uniform shear stress distribution and other factors in a design process, an approach incorporating the load transfer method is therefore proposed. a load transfer function (t-z curve) is the dependence of shear stress mobilized on the surface of a fixed length segfigure 11. assembled load transfer function. ment and its vertical displacement. the load transfer method is frequently adopted for the determination of load settlement curves of vertically loaded piles (coyle, reese [17], coyle and sulaiman [18]). however its use for ground anchors is rare. the important feature (from a practical point of view) of the proposed approach is that the load transfer function shape is determined using standard laboratory tests. no prior loading tests are necessary. the newly developed algorithm includes the following main features: • derivation of load transfer functions from laboratory testing following the procedure stated in kraft et al. [19]. • axial stiffness change due to the occurrence of tensile cracks in the grout material ([20]). • radial stress increases due to the post-grouting. therefore, cylindrical cavity expansion theory (randolph et al. [21]) is adopted. • radial stress decreases due to the grout consolidation (bezuijen, talmon [22], talmon [23]). the investigation load test presented in the chapter 2.4 is re-analyzed using the developed algorithm. three other load tests in two different localities were further used for verification purposes (chalmovský [16]). the load – transfer function is shown on figure 11. the predicted and measured load displacement curves are shown in figure 12. slightly higher displacements are predicted, and a reasonable match is reached for the ultimate bearing capacity. predicted distributions of skin friction and axial displacements for five different loading stages are shown in figure 13 and figure 14. for two last loading stages a shear stress decrease in the front section of the fixed length is already initiated. the tested anchor was equipped by electric resistance gauges placed on the tendon in regular intervals of 1 m. determination of shear stress profiles was therefore possible. figure 14 presents the predicted and measured shear stress profile for the last loading stage. progressive failure propagation is obvious. predicted peak shear stress is in good match with observation. the fixed length section in the softening regime is longer than predicted. 22 vol. 16/2018 the load-displacement behaviour of ground anchors figure 12. computed and measured load displacement curves. figure 13. skin friction distributions. this might be due to slightly higher real bearing capacity (886 kn) which allowed further spreading of the progressive failure. 5. conclusions in order to analyse the non-uniform shear stress distribution along the anchor fixed lengths, three different approaches are presented in the paper: numerical, analytical and experimental. in the first approach the finite element method in combination with the constitutive model involving strain softening was used. the back analysis revealed that ignoring shear stress decrease after peak shear strength is reached may lead to significant bearing capacity overestimation. in-situ loading tests confirmed that the peak and residual bearing capacity should be distinguished. in the presented loading test, the residual peak capacity ratio was 0.55. the newly developed application is based on the load transfer method. load transfer functions can be assembled employing standard laboratory tests. detailed information about all three approaches can be found in chalmovský [16]. figure 14. pre-stressing force distributions. figure 15. comparison of shear stress distribution for last loading stage. list of symbols amat shear hardening parameter [–] cp peak cohesion [kpa] cu undrained cohesion [kpa] d fixed length diameter [m] eoed,ref reference primary oedometer stiffness [kpa] eur,ref reference unloading-reloading stiffness [kpa] feff coefficient of effectivity [–] hsoft hvorslev surface softening parameter [–] k0 coefficient of earth pressure at rest [–] lcalc internal length for regularization [m] lfixed fixed length [m] m power for stress dependency of stiffness [–] ocr over-consolidation ratio [–] pref reference stress [kpa] tult bearing capacity [kn] νur poisson ratio for unloading reloading [–] σnc,0 initial pre-consolidation pressure [kpa] φcs critical state friction angle φe inclination of hvorslev surface in τ -σ space φp peak friction angle acknowledgements this research was financially supported by the research project of the ministry of industry and trade no. frti4/329. 23 juraj chalmovský, lumír miča acta polytechnica ctu proceedings references [1] h. ostermayer. construction carrying behaviour and creep characteristics of ground anchors. proceedings diaphragm walls and anchorages, inst civ eng, london pp. 141–151, 1975. [2] f. scheele. tragfahigkeit von verpressankern in nichtbindigen boden neue erkentnisse durch dehnungsmessungen im verankerungsbereich. technische universität münchen, 1981. [3] a. d. barley. the single bore multiple anchor system. proc int conf: ground anchorages and anchored structures, london pp. 65–75, 1997. [4] r. i. woods, k. barkhordari. the influence of bond stress distribution on ground anchor design. proc int conf: ground anchorages and anchored structures, london pp. 55–64, 1997. [5] a. d. barley. theory and practice of the single bore multiple anchor system. anker in theorie und praxis, proc int symposium, salzburg pp. 297–301, 1995. [6] m. puller. deep excavations: a practical manual. 2003. [7] b. schadlich. a multilaminate constitutive model for stiff soils, ph.d. thesis. gruppe geotechnik graz, graz university of technology, austria, 2012. [8] t. svoboda, d. mašín. impact of a constitutive model on inverse analysis of an natm tunnel in stiff clays. proc ita-aites world tunnel congress 2008, agra, india 2:627–636, 208. [9] p. w. mayne, f. h. kulhawy. k0 ocr relationships in soil. proc asce j geotech engdiv 108(6):851–872, 1982. [10] t. svoboda, d. mašín, j. boháč. class a predictions of an natm tunnel in stiff clay. computers and geotechnics 37(6):817–825, 2010. [11] r. b. j. brinkgreve, w. m. swolfs, e. engin. plaxis 2d 2012 users manual. plaxis bv, delft, the netherlands 2012. [12] r. b. j. brinkgreve, w. m. swolfs, e. engin. plaxis 3d 2012 users manual. plaxis bv, delft, the netherlands 2012. [13] r. b. j. brinkgreve. geomaterial models and numerical analysis of softening, ph.d. thesis. delft university press, 1994. [14] v. galavi. a multilaminate model for structured clay incorporating inherent anisotropy and strain softening, ph.d. thesis. gruppe geotechnik graz, graz university of technology, austria, 2007. [15] p. mišove. construction of pres-tressed ground anchors and their bearing capacity (in slovak), ph.d. thesis. vuis, bratislava, 1984. [16] j. chalmovský. behaviour analysis of a ground anchor fixed length in fine grained soils (in czech), ph.d. thesis. brno university of technology, department of geotechnics, 2016. [17] h. coyle, l. c. reese. load transfer for axially loaded piles in clay. journal of the soil mechanics and foundations division 92:1–26, 1966. [18] h. coyle, i. h. sulaiman. skin friction for steel piles in sand. journal of the soil mechanics and foundations division 93(6):261–278, 1967. [19] l. m. kraft, t. kagawa, r. p. ray. theoretical t-z curves. journal of the geotechnical engineering division 107(11):1543–1561, 1981. [20] building code requirements for structural concrete (aci 318) and commentary. [21] m. f. randolph, j. s. steenfelt, c. p. wroth. the effect of pile type on design parameters for driven piles. proc seventh european conference on soil mechanics and foundation engineering 2 pp. 107–114, 1979. [22] a. bezuijen, a. talmon. grout pressures around a tunnel lining, influence of grout consolidation and loading on lining. proc underground space for sustainable urban development (ita singapore) 2004. [23] a. talmon, a. bezuijen. simulating the consolidation of tbm grout at noordplaspolder. tunnelling and underground space technology 24(5):493–499, 2009. 24 acta polytechnica ctu proceedings 16:18–24, 2018 1 introduction 2 finite element modelling 2.1 strain hardening parameters 2.2 strain softening parameters 2.3 initial application 2.4 back analysis of the investigation anchor load test 3 experimental program 4 development of application based on load-transfer functions 5 conclusions list of symbols acknowledgements references https://doi.org/10.14311/app.2022.33.0504 acta polytechnica ctu proceedings 33:504–510, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague how design concepts influence carbon footprints of load bearing structures anne rønninga,∗, kjersti prestruda, simon saxegårda, simen s. haaveb, magne lysbergb a norsus norwegian institute for sustainability research, stadion 4, 1671 kråkerøy, norway b arcon prosjekt as, verftsgata 11 pb. 224, 7801 namsos, norway ∗ corresponding author: anne@norsus.no abstract. in recent years, it has become essential to consider the total carbon footprint of a construction project. commonly, the question has been: ’what is the best material to be used in this context?’ in this paper we argue that this question is incomplete, not taking the complexity of design choices into consideration. this paper intends to share light on how to analyse some factors that influence the construction of buildings in order to contribute to climate change mitigation, taking this complexity into consideration. calculation of fossil greenhouse gas (ghg) emissions for two load-bearing structures for office buildings in 4, 8 and 16 storeys with equal functional requirements; e.g. load bearing capacity, acoustic performance, fire resistance and adaptability are addressed. the main materials for the load-bearing structures are cross laminated timber (clt) elements and precast concrete elements respectively. the result show that one cannot on a general basis conclude that either type of load-bearing structure cause less fossil ghg emissions. it is always important to consider the building design, functionality as well as external conditions such as location when considering different load-bearing structure materials. keywords: carbon footprint, epd, lca. 1. introduction over the last 30 years, life cycle assessments (lca) have been applied in the construction sector as the methodological foundation to evaluate environmental performance of construction works and materials over the life cycle [1–3]. standards for both product and building level assessment are developed. for the latter, the european standard en 15978:2011, specifies the calculation method for assessing the environmental performance of building [4]. for documentation of the environmental profile for construction products, en 15804 provides the requirements to develop environmental product declarations (epd) [5]. the system boundaries defined in en 15978 are a1-a3 (product stage), a4-a5 (construction process stage), b1-b7 (use stage), c1-c4 (end of life stage) and d (benefits and loads beyond the system boundary). the approach covers all stages of the building’s life cycle and is based on data obtained from epd, their information modules, and when appropriate other information necessary and relevant for carrying out the assessment of the environmental performance of the building. to meet the increasing demands for ghg calculations of buildings in the norwegian market, a standard, ns 3720:2018 that define the rules and requirements for calculation of so, is published by standards norway [6]. moncaster et al. reviewed and analysed the data from over 80 individual life cycle assessments of buildings [7]. they found that several authors have used such findings to identify routes to lower carbon buildings. the strategy considered to have the biggest impact is the substitution of high carbon materials with low carbon; this is considered particularly important for the main structural and cladding elements which are often shown to have the highest impacts [8–10]. others have found that adaptability and patterns of use are of more importance as design factors than the building materials and products themselves, as the latter are a consequence of those factors [11, 12]. additionally, high replacement rates of materials with high embodied carbon as consequence of low adaptability will have a great impact on life cycle performance. the average rental period for office buildings in norway is 7 years. every seventh year, buildings are extensively rebuilt due to new tenants’needs and requirements [1]. how extensive the rebuilding processes will be will depend on the degree of adaptability of the building. thus, design for adaptability for office buildings is of vital importance. the goal of this study is to increase the knowledge regarding how design for adaptability, choice of materials and locations affect greenhouse gas calculations for load-bearing structures in office buildings. 504 https://doi.org/10.14311/app.2022.33.0504 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 33/2022 carbon footprints of load bearing structures figure 1. illustration of the two different load-bearing structures for office buildings (wood-based structure to the left). 2. methods 2.1. the objects the objects under study are two load-bearing structures for office buildings in 4, 8 and 16 storeys respectively, with equal functional requirements; e.g. load bearing capacity, acoustic performance, fire resistance and adaptability for future changes.the design is similar for both the wood-based and the concrete based structure. a visualisation of the two alternative 4 storey load-bearing structures can be seen in figure 1. the floor area size per storey of 50.4 m × 15.9 m (801 m2) was chosen based upon recommendations given in sintef building research design guides for adaptable office buildings [13]. it includes guidelines for physical sizes such as building width and floor area as well as guidelines for flexibility in office buildings in connection with pillar locations, etc. a width of 15−17 m is optimal as it gives the possibility of different interior solutions and simultaneously an acceptable area efficiency [14]. ideally, columns and load-bearing walls should be avoided in areas that can accommodate different types of workplace solutions to ensure high degree of adaptability. based on building technical considerations, the optimal span for a building width of 16 m will be approx. 8 m. this means that a row of columns should be placed approximately in the middle of the building with a centre distance in the longitudinal direction of approx. 5 − 8 m. this recommendation is to limit the load and cross-sectional dimension of the beams, between the columns and the column itself. it is assumed that foundations for all alternatives consist of the same concrete quality b35, placed in a humid environment below ground level. finally, the soil type is assumed to be natural sandy for 4 storeys, and natural gravel for 8 and 16 storeys. the solution of the foundation in the study consists of a foundation slab at the two short sides. the constructions experience the same magnitude of horizontal forces from wind, but the dynamic effect will be different since the stiffness and self-weight of the two alternative 16 storey buildings is different. the gables in the concrete solution are assumed to be of 12 m width, consisting of vertical, rigid concrete slabs, whereas the two short sides in the woodbase structure are assumed to consist of trusses along the width of the building (15.9 m). volume of concrete walls in every gable is 126 m3 (stabilizing effect from self-weight is about 2800 kn), whereas volume of trusses is 46 m3 (stabilizing effect from self-weight is about 190 kn). this explains the increased need for ready-mixed concrete for 16 storey wood-based structure. the fire requirements in norwegian building regulation, tek17 is used [15]. the regulations consider the constructions to be in hazard class 2, independent of the number of storeys, given that all storeys are used as office space. fire design of the concrete solution would be unproblematic up until r120. this does not apply to wood-based constructions which have to meet requirements on design and measures. proposed measures are (≥ r 60) 12 mm gypsum and 2 layers of 14 mm fire rated gypsum [16]. the load-bearing structures are designed by use of fem-design by strusoft and os-prog and quantities are extracted from this and further organised in excel, see table 1 and table 2 for material specification and quantities. 2.2. system boundaries en 15978 [4] and ns 3720:2018 [5] was used as the underlying rules for carrying out the calculation of the ghg emissions for the load-bearing structures. several of the applied epds that are used as data source have not declared emissions connected to the c module (end-of-life). therefore, it was chosen to limit the study to include only a1-a5 modules, from the extraction of raw materials to the construction of the load bearing structures. building shell and façade system, non-bearing separating walls, or technical equipment such as ventilation, heating, sanitary 505 a. rønning, k. prestrudr, s. saxegård et al. acta polytechnica ctu proceedings construction part material 4 storeys 8 storeys 16 storeys unit epd number foundation ready-mixed concrete 423 556 2 713 tonne nepd-332-216-no reinforcement 17 22 106 tonne nepd-347-238-en columns − − 31 m3 nepd-1577-605 beams − − 25 m3 nepd-1577-605 clt-slabs − − 203 tonne nepd-345-236-no vertical structures columns 23 72 298 m3 nepd-1577-605 truss 11 23 109 m3 nepd-1577-605 fireproof plasterboard − 20 000 41 550 m2 nepd-1264-406-en plasterboard − 10 000 20 775 m2 nepd-1260-406-en steel plate 2.95 8.45 37.9 tonne nepd-402-281-en bolts (steel) 3.29 9.22 41.9 tonne nepd-402-281-en horizontal structures beams 99 198 397 m3 nepd-1577-605 clt-slabs 813 1 626 3 252 m3 nepd-345-236-no steel plate 3 6 11 tonne nepd-402-281-en floor plasterboard 3 100 6 250 13 500 m2 nepd-110-177-en insulation 222 517 1 182 m3 nepd-1696-683 elastic underlayment 3 100 6 250 13 500 m2 nepd-207-260-no wooden laths 18 43 99 m3 nepd-308-179 table 1. material amounts for the clt-based load-bearing structure. construction part material 4 storeys 8 storeys 16 storeys unit epd number foundation ready-mixed concrete 476 670 1 583 tonne nepd-332-216-no reinforcement 19 26 78 tonne nepd-347-238-en columns − − 79 tonne epd generator beams − − 62 tonne epd generator hollow core slabs − − 204 tonne epd generator vertical structures columns 47 124 565 tonne epd generator steel plate 0.646 1.26 2.54 tonne nepd-402-281-en bolts (steel) 2.07 4.15 8.29 tonne nepd-402-281-en wall panels 102 409 1292 tonne epd generator reinforcement reinforcement 13 42 98 tonne nepd-326-206-en prestressed reinforcement 14 29 57 tonne nepd-458-296-en horizontal structures beams 250 499 999 tonne epd generator insulation 3100 1634 13 500 m2 nepd-00131e_ rev1_rockwool hollow core slabs 817 6250 3267 tonne epd generator table 2. material amounts for the pre-cast concrete-based load-bearing structure. 506 vol. 33/2022 carbon footprints of load bearing structures figure 2. fossil ghg emissions for three alternative load-bearing structures located in kristiansand and trondheim given as kg co2e/m2 and electricity are not included, as it were considered independent from the load-bearing systems. the alternative structures are dimensioned for the same service life, being maintenance-free during the service life and that they will not affect the operational conditions of the buildings. thus, the use phase (b1-b7) is excluded. no assessment has been made of how large the contribution from the module c is, but erlandsson and malmqvist [17] demonstrated in their study that fossil greenhouse gas emissions related to demolition and waste management of concrete and wood-based buildings were in the order of 2 − 5 % of emissions from a1-a5. carbon dioxide that is absorbed over time as the tree grows, is stored in wood-based building products during the life of the building [18]. when the buildings are disposed of after the end of their life and the wood-based products are used for energy recovery, the carbon is emitted in the form of co2. the climate change of sequestering biogenic carbon, storing it in harvested wood products and substituting more emission-intensive materials are hard to quantify. although different methodological choices and assumptions can lead to different conclusions, there is no consensus on the assessment of biogenic carbon in life cycle assessment [19]. as we have not included end-of-life assessment, instant oxidation of biogenic carbon is used as the approach where biogenic carbon is considered "climate neutral". this is in accordance with the common practice with calculations that do not include all life cycle modules, and with the practice for ghg calculations for constructions [4, 18]. 2.3. environmental data epd for construction products published by epdnorway are used as data source for the ghg emissions for the building materials specified in table 1 and table 2. the amount of all materials used for the four-, eightand sixteen-storey load-bearing structures are given in table 1 and table 2 for cltand pre-cast concrete structure respectively. epd for construction products published by epd-norway are used as data source for the ghg emissions for the building materials specified in table 1 and table 2. where more than one epd for the same representative product was available, the product with lowest ghg emissions are used. emissions related to the a4 are usually calculated based on an average distance from the production site to a typical construction site in norway, or to a specific destination such as oslo. this means that the assumptions for a4 will vary and the values in the epd cannot be applied directly. in this study, the construction sites are kristiansand and trondheim. ghg emissions were calculated based on the same type of means of transport as stated in the epd. our transport calculations also include infrastructure related to transport, which is not necessarily the case for all a4 modules in the epd. the precast concrete manufacturers in norway use a pre-verified epd-generator that allows for producing project specific epds [20]. two different data set are used; average data from four different precast manufacturers’epd for columns, slabs, wall elements and hollow cores respectively named ’typical element recipes’and epd for the same product from the manufacturer epd with the lowest ghg emission for a1a3. for clt slabs the manufacturer with lowest documented ghg emissions for a1-a3 is chosen. 3. results in figure 2 ghg emissions per square metre for the wood-based and concrete load-bearing structures are given for all three storey heights located in kristiansand and trondheim respectively. the results show that the wood-based structure has lower fossil ghg emissions than the construction with pre-cast concrete elements when built on four storeys for both locations but can be reversed at 16 floors when using the best precast concrete elements. for all cases, materials used for both vertical 507 a. rønning, k. prestrudr, s. saxegård et al. acta polytechnica ctu proceedings kristiansand trondheim element 4 storeys 8 storeys 16 storeys 4 storeys 8 storeys 16 storeys clt-slabs 43 % 34 % 25 % 36 % 28 % 21 % ready-mixed concrete incl. reinforcement (foundation) 28 % 15 % 26 % 31 % 16 % 27 % fire measures (plasterboard) 6 % 25 % 19 % 8 % 27 % 20 % steel products 11 % 12 % 16 % 13 % 13 % 17 % table 3. share of fossil ghg emissions in percentage from different construction materials in wood-based loadbearing structures of 4-, 8and 16-storey located in kristiansand and trondheim. kristiansand trondheim element 4 storeys 8 storeys 16 storeys 4 storeys 8 storeys 16 storeys hollow core slab incl. reinforcement 37 % 37 % 32 % 37 % 37 % 31 % ready-mixed concrete incl. reinforcement (foundation) 28 % 20 % 20 % 28 % 19 % 45 % columns/beams 20 % 18 % 20 % 18 % 16 % 46 % reinforcement 7 % 9 % 9 % 10 % 12 % 12 % wall element 6 % 12 % 15 % 5 % 10 % 15 % table 4. share of fossil ghg emissions in percentage from different construction materials in concrete load-bearing structures of 4-, 8and 16-storey located in kristiansand and trondheim. and horizontal structures contribute most. table 3 and table 4 present the contribution analysis to the total fossil ghg emissions in more details. the contribution analysis shows that clt-slabs, and foundation (ready-mixed concrete incl. reinforcement) contribute most to ghg emissions for the wood-based structure. the share of emissions from clt decreases with the height of the building. due to need for fire protection at higher storey, the contribution from plasterboard is significant. for the 8and 16-storey structure, fire measures contribute with 25 % and 19 %, and 27 % and 20 %, of the total emissions for structures located in kristiansand and trondheim, respectively. furthermore, ghg emissions from transport of clt elements from the manufacturer in sweden to kristiansand accounts for 51% of the total ghg emissions from clt (49 % for a1a3). the distance to trondheim is shorter, and the transport contribution from clt is then 36%. this illustrates that transport to building site for heavy materials can be important for the results and is conformed by [21]. for the pre-cast concrete structure, hollow core slabs and columns/beams are the elements that contribute with the highest share of ghg emissions. also, for the heavy hollow core slabs, transport turned out to be significant. the manufacturer that produce the hollow core slabs with significant lower ghg emissions than the competitors, is chosen only for deliveries to kristiansand. the transport from production site to trondheim gives ghg emissions that makes manufacturers closer to trondheim favourable. 4. discussion the span of the construction in this study is set to 7.95 m. for solid wood constructions a span of 5 to 5.5 meters is an alternative to avoid dimensions that are too large. if the span length is reduced, the thickness of the slabs is reduced including column and beams dimensions, but there will be a need to add an extra load-bearing axis. thus, the total material amount may not be significantly changed. nevertheless, the structure will have a lower adaptability for changes and consequently lead to more materials e.g. when new tenants every seventh year require changes in the interior [2]. the gross area is representative for office buildings in norway [13]. when assessing the higher structures, the choice of shape implies constraints for how the foundation and fire protection is solved. the foundation design is affecting the choices of materials and the material amounts as the structures. as the higher structures are relatively tall and slender (16 storey) the foundation will be subjected to large tensile forces. this can be solved by two different approaches; by friction piles or as in this study, by a casted underground concrete plate to ensure enough deadload to reach equilibrium. as the tensile forces are greater for the wood-based structures, the amount of materials in foundation used to ensure equilibrium are greater, which explains the ghg emissions (26 and 26 %) for the 16-storey wood-based structures. it is worth mentioning that there are also different solutions as to how the foundations of this specific construction design can be structured. a construction that has a different shape, on the other hand, 508 vol. 33/2022 carbon footprints of load bearing structures will have different characteristics which will be best solved by utilizing a different approach due to the shape (tall and slender) of the higher structures, the building will have a propensity to oscillate due to wind forces. this is not taken into consideration in our study but may be a challenge for the highest wood-based structure. a possible remedy to satisfy the requirements on oscillation is to build weight and masses into the floors for the woodbased structure. however, this again will require more columns or increased dimensions for columns and beams that will affect the amount of material used. today’s available technology for fire safety in wood constructions is assumed. as the technology develops on this area, the future solutions might be better and less material consuming, and it can potentially lead to a lower ghg emissions from the fireproof plasterboard used in the wood construction or solved by other technologies. however, research in the field of fire safety of clt buildings concludes that there is a lack of knowledge for use of unprotected clt [22, 23]. the fossil ghg emissions are highly influenced by transport and the choice of supplier used, as the elements for both the woodand concrete-based structure are quite large. the shape of the elements leads to more transport than other materials. this shows a considerable potential for reduction of ghg emissions especially for large elements, and it is important to include transportation when requirements for ghg emissions are set. it is pointed out that the fossil ghg emissions from material production and transport alone is not sufficient to conclude what is the most climate and environmentally friendly. the results of this study show that how the load-bearing structures are designed will considerably affect the ghg emissions related to both the load-bearing structure itself and the ability for design for adaptability for future changes in the office building. we want to emphasize in the study that based on the context and different solutions that are applied in specific projects, it will be possible to reduce the ghg emissions regardless of the material choices. this can be further promoted by giving manufacturers and other actors more room to use their skills and practical knowledge to develop innovative solutions. giving the designers the opportunity to adjust the design of a construction in collaboration with manufacturers, will make it possible to develop innovative solutions and potentially a lower carbon footprint. the carbon footprint of a construction project will rely on the design and the materials chosen for the building, which in turn will affect the future need for materials for maintenance and lay premises for how extensive future renovation or refurbishment will be. can any general conclusions be drawn based on the results of this study? what is considered the best material or design choices with respect to climate change mitigation will be contextual for each unique construction. and even when assessing a load-bearing structure and not the whole building, a holistic approach is vital and one need to reflect on how the load-bearing structure will affect other activities’potential ghg emissions during the service life to ensure design for low ghg emissions. references [1] m. röck, m. r. m. saade, m. balouktsi, et al. embodied ghg emissions of buildings the hidden challenge for effective climate change mitigation. applied energy 258, 2020. https://doi.org/10.1016/j.apenergy.2019.114107. [2] a. rønning. travels with lca. the evolution of lca in the construction sector. phd dissertation. university of aalborg. 2017. [3] a. m. moncaster, k. e. symons. a method and tool for ’cradle to grave’ embodied carbon and energy impacts of uk buildings in compliance with the new tc350 standards. energy and buildings 66:514-23, 2013. https://doi.org/10.1016/j.enbuild.2013.07.046. [4] ns-en 15978:2011 sustainability of construction works assessment of environmental performance of buildings calculation method. standard norway, 2012. [5] en 15804:2012+a1:2013 sustainability of construction works environmental product declarations core rules for the product category of construction products. 2019, european committee for standardization: brussels, belgium. p. 55, https://doi.org/10.3403/30259256u. [6] ns3720:2018 method for greenhouse gas calculations for buildings. standard norway, 2018. [7] a. m. moncaster, f. n. rasmussen, t. malmqvist, et al. widening understanding of low embodied impact buildings: results and recommendations from 80 multinational quantitative and qualitative case studies. journal of cleaner production 235:378-93, 2019. https://doi.org/10.1016/j.jclepro.2019.06.233. [8] t. häkkinen, m. kuittinen, a. ruuska, et al. reducing embodied carbon during the design process of buildings. journal of building engineering 4:1-13, 2015. https://doi.org/10.1016/j.jobe.2015.06.005.. [9] t. malmqvist, m. nehasilova, a. moncaster, et al. design and construction strategies for reducing embodied impacts from buildings case study analysis. energy and buildings 166:35-47, 2018. https://doi.org/10.1016/j.enbuild.2018.01.033. [10] h. kreiner, a. passer, h. wallbaum. a new systemic approach to improve the sustainability performance of office buildings in the early design stage. energy and buildings 109:385-96, 2015. https://doi.org/10.1016/j.enbuild.2015.09.040. [11] a. manewa, c. pasquire, a. gibb, et al. towards economic sustainability through adaptable buildings, adaptable futures, loughborough university, uk, 2009. 509 https://doi.org/10.1016/j.apenergy.2019.114107 https://doi.org/10.1016/j.enbuild.2013.07.046 https://doi.org/10.3403/30259256u https://doi.org/10.1016/j.jclepro.2019.06.233 https://doi.org/10.1016/j.jobe.2015.06.005 https://doi.org/10.1016/j.enbuild.2018.01.033 https://doi.org/10.1016/j.enbuild.2015.09.040 a. rønning, k. prestrudr, s. saxegård et al. acta polytechnica ctu proceedings [12] a. gibb, s. austin, a. dainty, et al. towards adaptable buildings: pre-configuration and re-configuration two case studies, manubuild 1st international conference, the transformation of the industry, open building manufacturing, rotterdam, 2007. [13] sintef. sintef building research design guides for adaptable office buildings. 344.110. [14] the british council for offices. bco guide to specification, 2019. http://www.bco.org.uk/researc h/publications/bcoguidetospec2019.aspx-. [15] tek 17. ğ 11.3 & 11.4. fire classifications & carrying capacity and stability. in: norwegian building authority, 2017. [16] norsk treteknisk institutt. bygge med massivtreelementer hefte 4 brann, 2006. https://www.treteknisk.no/resources/filer/publ ikasjoner/teknisk-handbok/hefte-4-brann.pdf. [17] m. erlandsson, t. malmqvist. olika byggsystem av betong och trä där mix av material inklusive stal ger klimatfördelar, basert på ivl rapportnr: c355, bygg och teknik 7/18, 2018. https://byggteknikforlaget.se/olika-byggsyste m-av-betong-och-tra-dar-mix-av-material-inklu sive-stal-ger-klimatfordelar/. [18] l. tellnes, u. hundhausen. life cycle assessment of creosote treated wood and tall oil treated wood with focus on end-of-life, 2016. [19] c. breton, p. blanchet, b. amor, et al. assessing the climate change impacts of biogenic carbon in buildings: a critical review of two main dynamic approaches. sustainability 10(6), 2018. https://doi.org/10.3390/su10062020. [20] m. vold, t. edvardsen, t, a. rønning, a. epd-generator: a tool for generation of epds in norwegian concrete industry. concrete innovation conference, istanbul, 2015. [21] l. g. f. tellnes, k. nore. environmental performance of construction stage for glue and cross laminated timber. presentation at the 14th annual meeting of the northern network for wood science and engineering (ecwm9), tallin, estonia, 2018. [22] e. d. wormdahl, k. hox, et al. fire safety in cross laminated timber buildings. sp fire research, trondheim, 2017. [23] n. k. reitan, k. l. friquin, r. f. mikalsen. challenges concerning cross laminated timber in tall buildings. rise fire research, trondheim, 2019. 510 http://www.bco.org.uk/research/publications/bcoguidetospec2019.aspxhttps://www.treteknisk.no/resources/filer/publikasjoner/teknisk-handbok/hefte-4-brann.pdf https://byggteknikforlaget.se/olika-byggsystem-av-betong-och-tra-dar-mix-av-material-inklusive-stal-ger-klimatfordelar/ https://doi.org/10.3390/su10062020 acta polytechnica ctu proceedings doi:10.14311/app.2018.15.0081 acta polytechnica ctu proceedings 15:81–87, 2018 © czech technical university in prague, 2018 available online at http://ojs.cvut.cz/ojs/index.php/app determination of drying time of the rammed earth walls tereza plaček otcovská∗, barbora mužíková, pavel padevět czech technical university in prague, faculty of civil engineering, thákurova 7, 166 29 prague 6, czech republic ∗ corresponding author: tereza.otcovska@fsv.cvut.cz abstract. the unburned earth is a building material with long history of use. buildings made of unburned clay are all over the world. but at present, unburned earth is minority building material and its properties are not sufficiently investigated. the rammed earth is one of main kind of unburned earth. this paper is focused on drying rate of the rammed earth of known composition. the significant part of the paper is focused on principle of our own research and the main idea of our research is explained here. the second part of the paper is devoted to drying rate experiment and measured results from this experiment. determination of drying time of universal rammed earth walls is the main result of the experiment. keywords: rammed earth, unburned earth, clay, sand, water coefficient, drying rate, drying time. 1. introduction importance of the unburned earth in construction industry is minor, but there are a few reasons why the research of this building material is important. recently, there has been a growing interest in unburned earth which has resulted in a number of scientific articles on this topic. the growing interest in this historic building material is evident in construction industry too (see fig. 1). i can see in my practice of civil engineer that more and more people choose the unburned earth for build of their houses. the growing interest is the first reason why carry out research in the unburned earth (see fig. 2) [1, 2]. figure 1. sales of unfired bricks by heluz cihlářský průmysl v.o.s. the second reason is a need for reconstruction of historic buildings. there are many earth buildings in the world and in the czech republic that need reconstruction. it is very important to have the knowledges of unburned earth properties for correct reconstruction of this buildings [3–7]. third, an earth building is a good way for sustainfigure 2. building of family house from straw and earth in klášterecká jeseň. able development in the construction industry. the energy needed for production of the unburned clay is minimal and the energy for demolition of the earth construction is small, because earth waste can be completely recycled and earth waste does not need to be stored in a waste dump. a good indicator is primary energy investment (pei). pei is an energy used during live cycle of building materials [8–11]. the primary complication about the properties of the unburned earth is in composition of the earth. the properties depend on composition of the earth and the composition of earth can be very variable. currently, a design of load bearing structures from rammed earth needs a lot of civil engineer’s experiences because norms or methodology does not exist for the design of unburned earth constructions in the czech republic. the reliable methodology is very important for easier design of the earth buildings. that are reasons why is necessary to carry out the research focused on the composition of earth mixture and prop81 http://dx.doi.org/10.14311/app.2018.15.0081 http://ojs.cvut.cz/ojs/index.php/app t. plaček otcovská, b. mužíková, p. padevět acta polytechnica ctu proceedings figure 3. the rammed earth. erties of the final earth structures. the reliable dates from the research should be used for the creation of the methodology [3, 12, 13]. the earth constructions can be produced of the earth mixture without or with inorganic binder. the inorganic binder can improve water resistance of unburned earth. but the inorganic binders are not necessary for the earth constructions. there are old historic buildings and there are researches which proved that unburned earth can be sufficiently resistant against the moisture. the important reason for using unburned earth without the inorganic binders is environment and the sustainable development. the unburned earth with inorganic binder is not possible to recycle easily. this are the reasons why our research is focused on the properties of unburned earth without additives [3, 8, 14–16]. the earth construction can be produced from the earth mixture in several ways. the elementary type of the earth construction is the rammed earth. this paper is focused on the rammed earth. the principle of rammed earth technology is pressing down layers of the earth mixture to a formwork (see fig. 3). the rammed earth walls are usually used like load-bearing structures. that is the reason why the mechanical properties and information about drying rate are very important [3, 13, 17]. the main topic of this paper is the drying rate and drying time. information about the drying rate and drying time are important, because the drying is the main way how the unburned earth gains its final mechanical properties. the rammed earth construction are usually load-bearing types and it is necessary to know when the rammed earth construction is possible to load. 2. the idea of our research as stated in the introduction, the norm or relevant instructions about the earth building designing are not available in the czech republic. the quality research about the unburned earth have to be realized for creating of this instructions. it is necessary to analyse unburned earth properties in dependence on its composition. this is the reason why we investigate mark of clay kind of clay gem montmorillonite agl illite kr illite-kaolinite table 1. three basic kinds of clay. figure 4. the test bodies shape of block. the behaviour and properties of the rammed earth from earth mixtures of different composition. the test earth mixtures are produced in our laboratory from the three elementary components (clay, sand, water). clay fulfils a function of a binder, sand fulfils a function of a filling agent and water serves for an activation of clay bonding properties and for good earth mixture processing. the composition of the earth mixtures differ in kind of clay, amount of clay or amount of mix water. the amount of water is defined by the water coefficient (see equation 1). three elemental kinds of clay were used (see table 1). the summary of the earth mixture which have been made can be seen in table 2. w = mw/mc (1) w water coefficient [-] mw weight of water [g] mc weight of clay [g] determination of dependence between the rammed earth properties and the rammed earth composition is our goal. up to now, we have produced 19 types of the earth mixtures and analysed their properties. we focused particularly on the tensile bending strength, the compressive strength, young’s modulus, moisture’s properties, methylene blue test, shrinkage, creeping and drying rate. we used the test bodies of block shape of size 2/2/10 cm and 4/4/16 cm for majority experiments (see fig. 4). we used the test bodies of prism shape with octahedron base and height of 7 cm for measurement of creeping and shrinkage (see fig. 5). we used the test bodies of block shape of different heights with 82 vol. 15/2018 determination of drying time of the rammed earth walls set clay sand/clay weight ratio water coefficient gemi montmorillonite 80/20 0.370 gemii montmorillonite 75/25 0.370 gemiii montmorillonite 75/25 0.295 agli illite 80/20 0.370 agli illite 75/25 0.370 kri illite-kaolinite 80/20 0.370 krii illite-kaolinite 75/25 0.370 kriii illite-kaolinite 80/20 0.400 kriv illite-kaolinite 85/15 0.295 krv illite-kaolinite 85/15 0.370 krvi illite-kaolinite 85/15 0.400 krvii illite-kaolinite 80/20 0.250 krviii illite-kaolinite 80/20 0.290 krix illite-kaolinite 80/20 0.450 krx illite-kaolinite 75/25 0.295 krxi illite-kaolinite 75/25 0.400 krxii illite-kaolinite 70/30 0.295 krxiii illite-kaolinite 70/30 0.370 krxiv illite-kaolinite 70/30 0.400 table 2. summary of the earth mixtures. figure 5. the test bodies shape of prism with octahedron base. aluminium moulds for measurement of drying rate (see fig 6). 2.1. the production of the test bodies the earth mixture is produced from three elemental components (clay, sand, water). the clay is fine powder form (see fig. 7). size of sand grain is to 4 mm. the grain curve of the sand can be seen in the figure 8. the water was used from a water supply line. the three components were mixed and processed into the earth mixture. the earth mixture was pressed figure 6. the test bodies for drying rate experiment. down to the moulds (see fig. 9). the test bodies for mechanical properties experiments and moisture’s properties experiments were demoulded a few minutes after production and inserted to an air-conditioning chamber. the test bodies for drying rate experiments were left in the moulds and inserted to the air-conditioning chamber. the test bodies were left in the air-conditioning chamber until equilibrium moisture content was reached. relative atmospheric humidity in the air-conditioning chamber was 50 % and temperature was 20oc. 83 t. plaček otcovská, b. mužíková, p. padevět acta polytechnica ctu proceedings figure 7. montmorillonite clay. figure 8. the grain curve of the sand. 3. the measure of the drying rate the test bodies in the figure 6 were used for the measurement of the drying rate. the earth mixture was pressed down to to the moulds of four sizes. unidirectional moisture flow of moisture was provided in the test bodies because the test bodies could dry out only by two opposite surfaces. the size of surface for dry out of the test bodies was square shaped of 53 mm long side. the height of the test bodies (moulds) were 5 cm, 6 cm, 7 cm and 8 cm (see fig. 10). the test bodies in the moulds were inserted to the air-conditioning chamber and regularly weighed. weight decrease of the test bodies was monitored. the moisture content was calculated from the weight of the test bodies by the equation 2. w = (mws − mds)/mds (2) w moisture content [-] mws weight of a wet sample [g] mds weight of a dry sample [g] research was focused on the dependence of the drying rate on the thickness of the drying rammed earth (see fig. 10). drying rate was determined for the test bodies. the points of interest in this paper are amount of moisture 50 % and 15 % from original value. from the values figure 9. the pressing down to the mould. obtained the drying time from experiment start we tried to determine the drying time of a universal wall. 3.1. the measured data the set kriii is the set of the earth mixture for this drying rate experiment (see table 2). number of the test bodies of every height were 3. total of 12 test bodies were used for the experiment. the weighing interval was 15 to 60 minutes in the first 12 hours after production of the test bodies. after first 24 hours the weighing interval was extended. the moisture content was calculated by equation 2. the shape of the moisture content curve in time is shown in fig. 11. a total time for this experiment was approximately 7 day (exactly 167.25 hours). the moisture content after 7 days were 0.6 % to 0.9 % (approximate decrease by 85 % from original value of the moisture content). the main decrease of the moisture content was in the first 24 hours. the moisture content after first 24 hours were 2 % to 2.9 % (approximate decrease by 65 % from original value of the moisture content). the moisture content 50 % from original value was reached after 8 to 16 hours depending on the height of the test bodies (see fig. 12). dependence between the drying rate and the thickness of the rammed earth appears to be linear (see fig. 13). time to reach 50 % from original value of the moisture content of different thickness rammed earth is possible determine by drying coefficient fdry. the drying coefficient is defined by equation 3. fdry = ∑n i=1(ti/hi) n (3) fdry drying coefficient [hour/cm] ti time for drying out of the test bodies of a particular size [hour] hi hight of the test bodies of a particular size [cm] n number of the test bodies heights [-] the drying coefficient was calculated 1.8 hour/cm from the moisture content values obtained. base on 84 vol. 15/2018 determination of drying time of the rammed earth walls figure 10. the diagram of the test bodies in the moulds for drying rate experiments. figure 11. the time course of the moisture content. the values obtained is possible to predict that, for example, a 30 cm thick rammed earth wall will dry out from 50 % in approximately 2.25 days. this prediction was done using equation 4. t50 = ww.fdry (4) t50 time for drying out of a rammed earth wall [hour] ww thickness of a rammed earth wall [cm] fdry drying coefficient [hour/cm] the decrease of the moisture content from 85 % from original value was reached after 70 to 123 hours depending on the height of the test bodies (see fig. 14). dependence between the drying rate and the thickness of the rammed earth appears to be linear (see fig. 15). figure 12. the decrease by 50 % from original value of the moisture content. from the time to reach 85 % of the original value of the moisture content of different thickness rammed 85 t. plaček otcovská, b. mužíková, p. padevět acta polytechnica ctu proceedings figure 13. the decrease by 50 % from original value of the moisture content – linear dependence. figure 14. the decrease by 85 % from original value of the moisture content. earth is possible to determine the drying coefficient. the drying coefficient for 85 % decrease of the moisture content was calculated 14.9 hour/cm from the moisture content values obtained (see equation 3). base on the values obtained it is possible to predict that, for example, a 30 cm thick rammed earth wall will dry out from 85 % approximately 18.6 days. this prediction was done using equation 3. 4. conclusions the rammed earth structures are usually load-bearing types and that is the reason why it is necessary to know how long after their production the rammed earth has its final properties (in particular mechanical properties). determination of drying time of the rammed earth of the known composition (set kriii, see table 2) was the goal of the drying rate experiment described in this paper. the decrease of the moisture content was calculated from the measured values of weights of the test bodies. the drying coefficient was determined for the calculation of the drying time of the different thickness rammed earth walls. the points of interest were 50 % decrease of moisture content from original value and 85 % decrease of moisture content from original value. the prediction of drying time is determined by using the drying coefficient. from the obtained values was predicted that 30 cm figure 15. the decrease by 85 % from original value of the moisture content – linear dependence. thick walls will dry out from 50 % of the original moisture content approximately in 2.25 days. the drying coefficient for this value is 1.8 hour/cm. the same walls will dry out from 85 % of original moisture content approximately 18.6 days. the drying coefficient for this value is 14.9 hour/cm. list of symbols w water coefficient [–] mw weight of water [g] mc weight of clay [g] w moisture content [–] mws weight of a wet sample [g] mds weight of a dry sample [g] fdry drying coefficient [hour/cm] ti time for drying out of the test bodies of a particular size [hour] hi hight of the test bodies of a particular size [cm] n number of the test bodies heights [–] t50 time for drying out of a rammed earth wall [hour] ww thickness of a rammed earth wall [cm] acknowledgements the financial support of this experiment by the czech science foundation (gačr project no. 18-10884s) and faculty of civil engineering, czech technical university in prague (sgs project no. sgs16/201/ohk1/3t/11) is gratefully acknowledged. we would like to express our thanks to lb minerals, s.r.o. company for free supply of material necessary for experimental measurement and we would also like to express thanks to heluz cihlářský průmysl v.o.s. for giving statistical data and informations. references [1] scopus analyze search results. https://www.scopus.com/term/analyzer.uri?sid= 8ee866586b66d91ce67a3f34f5af5efe. 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[17] p. walker. rammed earth: design and construction guidelines, vol. 2010. ihs bre press, watford, 2010. 87 http://dx.doi.org/10.1016/j.buildenv.2008.07.001 acta polytechnica ctu proceedings 15:81–87, 2018 1 introduction 2 the idea of our research 2.1 the production of the test bodies 3 the measure of the drying rate 3.1 the measured data 4 conclusions list of symbols acknowledgements references https://doi.org/10.14311/app.2022.33.0431 acta polytechnica ctu proceedings 33:431–436, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague structural flexural strengthening through material bonded to the concrete substrate victor hugo dalosto de oliveiraa, rodolfo alves carvalhoa,∗, rodolfo rabelo nevesb, luana ferreira borgesa, paulo chaves de rezende martinsa, marcos honorato de oliveiraa, joão paulo de almeida siqueiraa, rafael de almeida sobrala, sidney luan teixeira de nazaréc, carlos henrique de moura cunhac a department of civil and environmental engineering, university of brasília, sg-12, campus darcy ribeiro, asa norte, brasília, df, 70910-990, brazil b department of materials engineering and construction, federal university of minas gerais, av. antônio carlos, 6627, pampulha, belo horizonte, mg, 31270-901, brazil c school of exact, architecture and environment, catholic university of brasília, qs 07 lote 01, taguatinga, brasília, df, 71966-700, brazil ∗ corresponding author: rodolfoalvescarvalho@yahoo.com.br abstract. this article proposes an alternative method for the structural design of reinforced concrete elements strengthened in bending by metallic plates or fiber-reinforced polymer (frp) bonded to the concrete substrate. it is proposed a new calculation procedure for the strengthening using thin adhered material bonded to the element surface that dispenses the iterative process generally used in the design. the proposed routine is validated by comparison with other methods. a practical example is also presented, applying the procedure to an element of a building where a load change was foreseen. as result, it was verified that the proposed procedure provides values similar to the trial-and-error method used in the frp strengthening design. results are also coherent with other methods available in the literature for metallic plates. therefore, since this routine obtains similar values without using an iterative method, its applicability in the design becomes advantageous. keywords: attached materials, calculation guide, structural strengthening. 1. introduction when constructions do not fit the required capacity for correct use, rehabilitation is necessary. the reasons for rehabilitation may be pathological manifestations, accidents, natural disasters or changes in the building use. to restore the necessary resistance of the building, actions such as recovery or structural strengthening may be necessary [1–3]. the primary function of a structural retrieval is to improve the resistant capacity of any structure. the strengthening can be performed by associating new materials to the original structure, forming a composite system that supports the applied loads. examples of these processes are the addition of steel bars to the original section, reinforced concrete jacketing, external prestressing tendons and elements adhered to the substrate, such as steel sheets and composite materials [4]. this study proposes a new calculation methodology for the flexural design of reinforced concrete elements associated with small thickness bonded materials to the surface, as external steel sheets or fiberreinforced polymer (frp). the main goal herein is to establish a calculation guide for structural strengthening design, which fits the requirements of the brazilian standard nbr 6118: 2014 [5]. it should be noted that only the flexural capacity is concerned in this paper. considerations related to anchor capacity or the possibility of debonding of the strengthening are not made. 2. literature review and numerical simulation this section presents some methods commonly used for the flexural strengthening design of reinforced concrete beams and slabs present in the literature. these methods consider the insertion of steel sheets and carbon fiber reinforced polymer sheets bonded to the surface. the calculation guide proposed in this paper follows the literature review. 2.1. calculation models for metal sheet strengthening there are several calculation methods in the literature for the flexural strengthening using bonded metal sheets and steel profiles. some of the most relevant are described below. 2.1.1. j. bresson model metal sheet dimensioning with bresson’s method is carried out by balancing the bending moments in relation to the most compressed fiber. materials are 431 https://doi.org/10.14311/app.2022.33.0431 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en v. h. d. d. oliveira, r. a. carvalho, r. r. neves et al. acta polytechnica ctu proceedings figure 1. stress diagram of a strengthened beam. source: adapted from silveira [6]. assumed to have a linear elastic behavior; concrete tensile strength is neglected and stresses are limited to their respective elastic admissible values [6]. thus, the strengthening plate area is obtained by equation 1: as = ! mdead + mlive " + (fc1 + fc2) b·y2 6 − (fs1 + fs2) as d fr · h (1) as shown in figure 1, design takes into account dead and live loads, referred, respectively, to the forces already acting in the section when the strengthening is carried out and after the rehabilitation. 2.1.2. campagnolo model in this model, strengthening design is performed using classical formula for bending, considering composite beams theory [1]. thus, the tensile stress calculation for the steel plate is obtained as follows in equation 2. fr = er ec · mu (h − xii) ix (2) 2.1.3. cánovas model the design is performed on the basis that materials behave under their maximum allowable elastic stresses. stress distribution before and after the strengthening execution are considered. balance and design equations are obtained by superimposing the stress diagrams [1], as illustrated in figure 1. thus, the necessary area of additional strengthening, ar, is obtained by balancing external and internal moments as in equation 3: mu = (as fs + ar fr) (d − x′) (3) 2.1.4. ziraba and hussein model the authors proposed a method in which the strengthening design is done at the ultimate limit state, following the specifications of aci 318 [1, 7]. stress resultants and the strengthening area are obtained from the balancing of moments in the section taken at the point of the compressive stress resultant in concrete, as reproduced in figure 2, resulting in an iterative process, as shown in equation 4. msd = rs # d − y 2 $ + rr # hr − y 2 $ (4) 2.2. calculation models for carbon fiber reinforced polymer design of strengthening systems composed of carbon fibers, according to machado [2] and the american code aci 440 [8], may be done through a trialand-error procedure, that gives the required area of strengthening at the ultimate limit state. according to these methods, a position for the neutral axis (x) is initially arbitrated. then, deformations (ε) and stresses (f) of materials are calculated using compatibility and equilibrium equations. it is then verified if the resulting forces are balanced (equation 5), and if the resistant bending moment gotten is greater than the external moment applied (equation 6). if any of the conditions is not satisfied, a new neutral axis depth is chosen, and the procedure is repeated. nsd = rs + r′ s + rr + rc (5) msd = rs # d − yx 2 $ + r′ s # yx 2 − d′ $ + rr # hr − yx 2 $ (6) 2.3. proposed calculation model this section contains the method proposed by the authors. it is a calculation guide used for structural strengthening design, addressing the association of thin-thickness materials adhered to the surface. 2.3.1. calculation premise the following hypotheses are adopted: • the design is carried out at the ultimate limit state, disregarding the concrete tensile strength. • plane sections remain plane after loading, and the deformations are proportional to their distance to the neutral axis (bernoulli criterion). 432 vol. 33/2022 structural strengthening through bonded material figure 2. stress and strain diagram for a section in the strengthened condition. • perfect adhesion exists between the concrete, the steel reinforcement, and the thin adhered material (navier criterion). • the cross-sectional failure is characterized when the specific deformation of any material reaches its limit (ultimate) value. figure 2 illustrates the behavior of a rectangular cross-section of a strengthened concrete element, submitted to simple normal flexure. the rectangular stress diagram presented in section 17.2.2 of the brazilian standard nbr 6118: 2014 [5] is adopted. the compression stresses in concrete are distributed in a rectangle of depth y, with peak stress equal to α · fcd, according to figure 2. the value of y is defined as (equation 7): y = λ · x (7) in which, λ is given by equation (8) for fck ≤ 50 mpa, and λ is given by equation 9 for fck > 50 mpa: y = 0.8 (8) y = 0.8 − (fck − 50) % 400 (9) the α parameter is defined according to the concrete class as follows: for concrete classes up to c50: α = 0.85 (10) for concrete classes from c50 up to c90: α = 0.85 & 1 − (fck − 50) % 200 ' (11) 2.3.2. longitudinal strains considering bernoulli’s hypothesis, the deformation of any point within the section is proportional to its distance to the neutral axis, given as a function only of its location (y) and the curvature radius (ρ). knowing that the strengthening material bonded to the element has a minimal thickness, about a few millimeters, it can be considered that its specific longitudinal deformation (εr) is equal to that of the concrete fiber substrate to which it is glued. thus, the deformation of the material, taken as constant along its thickness, can be obtained by equation 12: 1 ρ = εc x = εs (d − x) = εrf (h − x) = ε ′ s (d − x) (12) when the applied loads, including the dead load, are not eliminated before the strengthening application, the material added to the section is not stressed by all these loads. in this way, the original structure is subjected to pre-existing stresses and has already suffered deformations before strengthening. under these conditions, it is necessary to know these stresses to determine the actual stress acting on the strengthening. thus, strengthening deformations can be obtained using equation 13: εr = (εrf − εri) ≤ εu (13) in which εrf is the specific deformation of the concrete substrate when all loads are applied, and εri is the specific deformation in the section at the moment of strengthening application. 2.3.3. failure criteria analyzing the failure criteria, the ultimate concrete deformations are defined in section 8.2.10.1 of the brazilian standard [5], by εc2 and εcu, as shown in equations 14 to 17: for concretes classes up to c50: εc2 = 0.2 % (14) εcu = 0.35 % (15) for concretes classes from c50 up to c90: εc2 = 0.2 % + 0.0085 % (fck − 50)0.53 (16) εcu = 0.35 % + 3.5 % & (90 − fck) % 100 '4 (17) considering hooke’s law valid up to yield stress (fy) of any steel element, it is assumed that the 433 v. h. d. d. oliveira, r. a. carvalho, r. r. neves et al. acta polytechnica ctu proceedings figure 3. strengthened beam scheme. source: adapted from machado [2]. stresses are proportional to the deformations. in this way, the tensile resultant in the pre-existent steel reinforcement (rs) and in the strengthening added to the element (rr) can be done by equations equation 18 and 19. rs = as · es · εs ≤ as · fsy (18) rr = ar · es · εs ≤ ar · fry (19) it is important to note that, as the carbon fiber has high tensile strength, linear elastic behavior is expected for this type of material until failure of the critical section. finally, to prevent excessive plastic deformation in critical sections, the maximum allowable elongations are limited to = 1%. 2.3.4. ductility criteria in reinforced concrete structures, proper measures must be taken to ensure adequate ductility of all structural elements up to the ultimate limit state. according to aci 318-14 [7], if the tensile stress in the reinforcement is greater than εs = 0,5%, the section is characterized as ductile, presenting warnings about imminent rupture, such as the presence of large deformations and cracks. if the steel deformation is less than this value, a fragile rupture condition is expected, with few warnings of an imminent collapse. section 14.6.4.3 of the brazilian standard nbr 6118: 2014 [5], in order to guarantee a ductile behavior for the slabs and beams, establishes that the depth of the neutral axis at the ultimate limit state must be limited to the following values: equation 20 for ≤ 50 mpa, and equation 21 for 50 mpa < fck ≤ 90 mpa. x ≤ 0.45 · d (20) x ≤ 0.35 · d (21) 2.3.5. strengthening equation the equilibriums equation of strengthening supposes that dimensions of the cross-section, the reinforcement position, and the material properties are known. the procedure described below must consider the balance of the section, the compatibility of deformations, and its failure mode. it was considered that the internal resisting bending moment must be at least equal to the external moment applied to the section. in this way, the equilibrium equation of moments can be carried out at the contact surface between concrete and the newly added strengthening. one of the variables is eliminated with this procedure. therefore, the equilibrium of moments in a strengthened section is: ( m = msd msd = rc (h − x′) + rs (h − d) + r ′ s (h − d′) msd = α fcd bw y # h − y 2 $ + as fs (h − d) + a ′ s f ′ s (h − d′) (22) knowing that the compressed concrete block is given as a function of equation 7, the position of the neutral axis can be obtained by as the positive root of equation 23, derived from equation 22 by algebraic manipulation: ! −0.5 α fcd bw λ2" x2 + (α fcd bw λ h) x + [a ′ s f ′ s (h − d′) − as fs (h − d) − msd] = 0 (23) with the neutral axis position, equations 12 and 13 are used to identify the domain of ultimate limit state of the behavior of the section, and to determine the specific deformations along its height. the sectional stresses distribution from existing load action during the strengthening can be considered, taking 434 vol. 33/2022 structural strengthening through bonded material variable iterative process [2] suggested model (authors, 2021) εri 0.228 ! 0.272 ! εrf 7.000 ! 7.324 ! εr 6.772 ! 7.052 ! l · n 0.228 cm 22.31 cm fr 1544.0 mpa 1607.9 mpa strengthening area 0.588 cm2 0.563 cm2 table 1. calculated carbon fiber strengthened area. figure 4. representation of beam characteristics of the building. calculation model strengthening area difference suggested model 2.054 cm2 j. bresson 4.091 cm2 99.17 % campagnolo 3.684 cm2 79.36 % ziraba e hussein 2.905 cm2 41.43 % cánovas 2.286 cm2 11.30 % table 2. comparison of the calculated strengthening sheet area. into account the cracked section properties, if necessary. considering a cross-section submitted only to flexure forces, the resultant strengthening force is determined as presented in equation 24. ( f = nsd nsd = rc + rs + r ′ s + rr rr = nsd − α fcd bw λ x − as fs − a ′ s f ′ s (24) once the resulting force and specific deformation have been determined, the strengthening area necessary to resist to the applied moment can be calculated using equation 19. finally, it is still necessary to assess whether the section mode of failure satisfies the ductility criteria set out in section 2.3.4. it is worth to mention that the proposed equation 24 also allows the direct application of the environmental reduction factors, as exposed in aci 440 [8], to the strengthening resultant. 3. results and discussions a comparison between the design methods mentioned and the calculation model proposed in this paper was made. for this, some examples available in the literature and a study case were taken for. analyzing example 4.1 of the manual by machado [2], a concrete beam was supposed to be strengthened through the association of carbon fiber sheets. all necessary data about the beam, the applied loads, and the material properties are shown in figure 3. the proposed calculation routine was compared with the iterative procedure used in machado [2]. the design was carried out using the criteria of the brazilian standard nbr 6118: 2014 [5], adopting the same parameters for the materials. the comparative results are shown in table 1. the actual strengthening provided had an effective area of 0.594 cm2, composed of two carbon fiber layers, 0.165 mm thick and 18 cm width. it appears that the recommended procedure provides a value very close to the trial-and-error process for necessary carbon fiber strengthening area (less than 4.25%). this difference may be due to the tolerance criteria applied in the iterative process [2]. 435 v. h. d. d. oliveira, r. a. carvalho, r. r. neves et al. acta polytechnica ctu proceedings the second example is the case of a building, where the live load was foreseen to be increased. the additional strengthening was conceived through the association of metal sheets. figure 4 shows the characteristics of one of the beams that needed to be strengthened. table 2 shows the section sizing of the steel plate obtained from the different methods. it is essential to highlight that, although the proposed method presents similarities with that developed by ziraba and hussein both consider the strengthening design at the ultimate limit state, their results are quite different. this fact is due, not only, but mainly because of criteria that govern aci 318 [7] is different from those of nbr 6118 [5] and because of the consideration of pre-existing deformations considered in the proposed calculation. 4. conclusions this paper shows that it is possible to design fiberreinforced polymer strengthening using a different approach from the trial and error method, frequently used in technical design. the proposed method leads to encouraging results with reasonable accuracy if compared with the classical ones. no iterative calculations are needed. the method provides the depth of the neutral axis without evaluating the stress resultant of the internal reinforcement. consequently, it is possible to consider its application for other thin materials bonded to the substrate, such as metal sheets. there was a distinct difference in the results obtained with other literature models for sheet metal design. the authors attribute these divergences mainly to the difference between some calculation hypotheses adopted, such as the design based on ultimate limit state instead of the allowable stress design methods, where material resistances are limited to its limit of elasticity. the proposed method also allows the usage of the load and resistance factor design, the consideration of pre-existing stresses, and the application of environmental reduction factors directly to the strengthening, which are not possible in some other methods. thus, the numerical analysis shown that the proposed method presents good efficiency. more exhaustive laboratory tests, experimental and/or numerical, may be foreseen to improve and calibrate the parameters of the method proposed herein. acknowledgements this study was financed in part by the coordenação de aperfeiçoamento de pessoal de nível superior brasil (capes). references [1] v. c. m. d. souza, t. ripper. patologia, recuperação e reforço de estruturas de concreto, são paulo: pini, 1998. [2] a. p. machado. manual de reforço das estruturas de concreto armado com fibras de carbono, são paulo: viapol, 2015. [3] p. k. mehta, p. j. m. monteiro. concreto: microestrutura, propriedades e materiais 2, são paulo: ibracon, 2014. http://www.viapol.com.br/media/ 97576/manual-fibra-de-carbono.pdf. [4] a. p. a. reis. reforço de vigas de concreto armado por meio de barras de aço adicionais ou chapas de aço e argamassa de alto desempenho dissertação de mestrado universidade de são paulo, são carlos, 1998. [5] associação brasileira de normas técnicas. abnt nbr 6118: projeto de estruturas de concreto procedimento, rio de janeiro: abnt, 2004. [6] s. s. silveira. dimensionamento de vigas de concreto armado reforçadas com chapas coladas com resina epóxi dissertação de mestrado universidade federal fluminense, niterói, 1997. [7] american concrete institute 2014 aci 318-14: building code requirements for structural concrete, farmington hills: aci, 2014. [8] american concrete institute 2007 aci 440.2r-08: guide for the design and construction of externally bonded frp systems for strengthening concrete structures, farmington hills: aci, 2007. 436 http://www.viapol.com.br/media/97576/manual-fibra-de-carbono.pdf acta polytechnica ctu proceedings doi:10.14311/app.2018.15.0069 acta polytechnica ctu proceedings 15:69–73, 2018 © czech technical university in prague, 2018 available online at http://ojs.cvut.cz/ojs/index.php/app modeling of monocrystalline magnesium microbeam bending jiří němečeka,∗, jan maňákb, jiří němečeka a czech technical university in prague, faculty of civil engineering, department of mechanics, thákurova 7, 166 29 prague 6, czech republic b institute of physics of ascr, na slovance 1999/2, 182 21 prague 8, czech republic ∗ corresponding author: jiri.nemecek.1@fsv.cvut.cz (ph.d. candidate) abstract. this paper presents a numerical simulation of a micro-scale experiment on a magnesium alloy. micro cantilever beams were fabricated using focused ion beam technology in a single crystal of mg. the cantilever beams have dimensions in the order of a few micrometers and a pentagonal cross section. nanoindenter was used for cantilever beam bending and load-displacement curve was received. cantilevers with two different crystallographic orientations were chosen for the experiment. three dimensional numerical fe model with elastoplastic behavior respecting crystal anisotropy was used to fit experimental load displacement curves. strengths and deformation energy were evaluated from the models for each cantilever. keywords: magnesium, microbeams, crystal anisotropy, fe model. 1. introduction magnesium crystallizes in a hexagonal close packed structure (hpc) with various deformation mechanisms such as basal plane slipping {0001} <1120>, prismatic slip {1010} <1120> and pyramidal slip {1011} <1123> or twinning {1012} <1011> [1]. the deformations start from a defect or on boundaries between different crystalline grains. in the first scenario, slipping happens when the crystal planes slide along each other and in the second, twinning is a concentrated shift of atomic positions [2]. magnesium alloys fulfill the demand for a lightweight construction material with good recycling potential. nowadays magnesium is commonly used in automotive, train and aircraft industry for its high tensile strength, low strength to weight ratio and excellent corrosion resistance. however, magnesium also has negative properties such as tendency for cracking and brittle failure which is probably given due to intercrystalline failure and local transcrystalline fracture at twin zone [3, 4]. nanoindentation is a powerful tool which uses a small diamond tip pressed into the material to measure mechanical properties such as modulus of elasticity, hardness and creep from load-displacement curves [5]. nanoindentation usually operates at microscale and is a perfect tool for measuring properties of an individual crystal. besides classical nanoindentaion testing there are possibilities to use nanoindenter tip for bending microscale samples. micrometer samples can be fabricated using focused ion beam (fib) technology. fib instrument is very similar to a scanning electron microscope (sem) but instead of electrons it uses gallium ions to hit the material and cause its milling [6]. fib energy is typically between 10 and 50 kv, with beam currents varying between 1 pa and 10 na [7] and is capable of creating samples with varied geometry, mostly pillars or triangular and pentagonal micro beams. micro-beam bending is advantageous for evaluating tensile strength and fracture energy. in this work, micro cantilevers beams were tested in specific crystal directions where twinning occurs. during the testing, load-displacement curves were measured. also, a simple numerical model with good reproducibility of the curves gained on different crystalline orientations has been developed. figure 1. crystal orientation with respect to loading direction. 2. experimental part a sample from pure magnesium was prepared. the sample was cut by a precise diamond saw and finely polished to achieve average roughness in the range of a few nanometers. ebsd (electron backscatter diffraction) was performed on the polished surface to find its crystal orientation. micro beams were fabricated on the identified places by the fei quanta 3d feg dual beam instrument combining sem and fib. all samples were micro cantilever beams with length of about 22 µm and have a pentagonal cross section with a width of about 3 µm and variable height 69 http://dx.doi.org/10.14311/app.2018.15.0069 http://ojs.cvut.cz/ojs/index.php/app j. němeček, j. maňák, j. němeček acta polytechnica ctu proceedings figure 2. sem images of fabricated micro beam with c crystal orientation: side view (top) and front view (bottom). from 3.5 to 5 µm. two types of cantilevers with two prismatic orientations, labelled c and d, were created, see figure 1. sem images of the fabricated cantilevers are shown in figures 2 and 3. circle marks were created on each cantilever approximately 2 µm away from the free end. at this mark, nanoindentation was performed with a berkovich tip to bend the cantilever. standard displacement controlled tests were performed with a constant increment of displacement up to a maximal value of 3000 nm. in total, four cantilevers were bent for each orientation, however, the curves had the same trend. in this paper were picked only the typical curves. typical experimental load-displacement curves measured are shown in figure 5. in both cases firstly, elastic deformation characterized by reversible response of the lattice to the displacement. secondly, plastic deformation of the crystal, caused mostly by twinning for the cases figure 3. sem images of fabricated micro beam with d crystal orientation: side view (top) and front view (bottom). of magnesium crystal orientations, takes place. after reaching a critical strain, twinning causes a release of energy that is demonstrated as a radical drop of load in figure 5. 3. numerical modelling a numerical model was created in ansys workbench software 17.2. a three dimensional model was proposed due to the irregular cross section of the cantilever micro beam and due to previous experience of authors with micro beam modelling [10]. material constants for elastic anisotropy of pure mg were taken from the literature [8, 9] and are given in table 1. for the hexagonal crystal, five independent elastic constants are needed and a sixth is calculated from 70 vol. 15/2018 modeling of monocrystalline magnesium microbeam bending figure 4. directional deformation (z axis) at maximal prescribed displacement 3 µm of the cantilever labelled as c with shown mesh. c11 [gpa] c12 [gpa] c13 [gpa] c33 [gpa] c44 [gpa] c66 [gpa] basal 58.58 25.02 20.79 61.11 16.58 16.78 prismatic 61.11 20.79 25.02 58.58 16.58 20.16 table 1. elastic constant of pure mg for basal and prismatic crystal orientation [8, 9]. 0 1000 2000 30000 100 200 300 400 500 displacement [nm] lo ad [µ n ] c experimental d experimental figure 5. typical experimental load-displacement curves of two different crystal orientations measured during cantilever bending. plastic strain [µm µm−1] stress [mpa] c d 0 392 260 0.07 470 410 0.010 495 420 0.015 530 425 0.020 545 430 table 2. plasticity data for the two prismatic orientations of crystal c and d. the equation c66 = 1 2(c11 − c12). (1) the constants are given for basal crystal orientation only and need to be transformed for prismatic orientation. plasticity of the beams was modelled with multilinear isotropic hardening. the yield stress was directly calculated from the bending experiment, where the values for cantilevers c and d are 392 mpa and 260 mpa respectively. the data of plastic strain and stress are shown in table 2. the cantilever is filled with 3-d solid186 elements which are defined by 20 nodes and have three degrees of freedom per node, translation in the nodal x, y, and z directions. this element supports von mises plasticity and is suitable for large deflection [11]. the boundary conditions were set similar to the real cantilever with fixed support on one end and a displacement increment of up to 3000 nm. the deformed shape of the cantilever model is shown in figure 4. the deformation process of cantilever bending can be characterized with deformation energy. a part of the total deformation energy that occurs before the first twinning was calculated from the stress-strain curves as follows g = ∫ εcrit 0 σdε, (2) where σ is the axial stress and εcrit is the critical strain when the first twinning happens. 71 j. němeček, j. maňák, j. němeček acta polytechnica ctu proceedings 0 1000 2000 30000 100 200 300 400 500 twinning displacement [nm] lo ad [µ n ] c experimental c fit figure 6. experimental and calculated loaddisplacement curves of c prismatic crystal orientation with marked the first twinning. 4. results and discussion calculated load-displacement curves can be seen in figures 6 and 7. in both cases fitting curves exhibit the same behavior as in the experiment on the elastic part that matches perfectly. the curves are also with good agreement after yield point until the first twining which is observed on the experimental curves when a radical drop of load occurs. unfortunately this drop brings very difficult task for modelling and it was not considered in this work. according to research on austenitic high managanese steels showing twinning-included plasticity by c.y. sun et al. [12], it would be necessary to employ crystal plasticity incorporating the twinning mechanism, where eighteen material constants are needed to be obtained experimentally. the first twinning, labelled as wcrit, occurs at displacement of 1.2 µm for c and 2 µm for d orientation. the shape of the fitting curves after twinning has the same trend and would match well with the experimental curve if no twinning had happened with only one exception. the slope of the un-loading curve for d orientation is slightly different. the axial stress in the x axis direction is shown in figure 9. the strain was calculated as the equivalent total strain composed from elastic strain and equivalent plastic stain computed of von mises criterion. stress-strain curves calculated at the top fixed end of the cantilever are shown in figure 8. however, this diagram is again partly correct till the wcrit value; the data after this point are not relevant. thus, critical strain (εcrit) and stress were deducted from the model. the normal stress till the first drop of load is 619.9 mpa with a strain of 0.0323 for the c cantilever and 592.9 mpa with value of strain 0.0548. the stress-strain curves made it possible to compare the deformation behavior of micro-beams with different orientations. based on this comparison, the 0 1000 2000 30000 100 200 300 400 500 twinning displacement [nm] lo ad [µ n ] d experimental d fit figure 7. experimental and calculated loaddisplacement curves of d prismatic crystal orientation with marked the first twinning. 0.000 0.032 0.055 0.080 0.1200 100 200 300 400 500 592 620 700 800 900 strain [-] st re ss [m pa ] c d figure 8. calculated stress-strain curves of c and d prismatic crystal orientations with labelled critical stresses and strains. yield point is higher for c orientation while the stored energy before the first twinning is higher for d. the energies calculated according to equation 2 for c and d are 16.09 jm−3 and 28.08 jm−3 , respectively. that makes d orientation more ductile. 72 vol. 15/2018 modeling of monocrystalline magnesium microbeam bending figure 9. deformed shape and axial stress (x axis) at critical displacement of 2 µm with uninformed wireframe of d cantilever beam. 5. conclusions this paper shows numerical modelling of micro cantilever beams made from pure magnesium with two prismatic orientations of the crystals. the elastic part, modelled with five independent anisotropic constants, gave a perfect match to the experiments. inelastic deformations were captured with isotropic plasticity with multilinear hardening law until the first twinning. the twinning mechanism was not captured and left for future work. it was found that the fitting curves have the same trend as experimental curves if the twinning was removed, which suggests that the model is correct and only the twinning mechanism needs to be incorporated using e.g.crystal plasticity by sun et al. [12]. critical displacements wcrit, corresponding to the critical strains and stresses at the time of the first twinning were evaluated. it was found that the critical stress for c orientation is 30 mpa higher compared to d. however, the critical strain is higher for d. thus, the deformation energy stored in the material before the first twinning is higher for d. list of symbols wcrit displacement at first twinning [nm] εcrit strain at first twinning [–] σ axial stress (x axis) [mpa] g deformation energy [j m−3] c66 elastic constant fib focused ion beam sem scanning electron microscope ebsd electron backscatter diffraction acknowledgements financial support of the czech science foundation (projects 17-18652s) is gratefully acknowledged. references [1] k. fukuda, y. koyanagi, m. tsushida, et al. activation stress for slip systems of pure magnesium single crystals in pure shear test. materials transactions 58(4):587–591, 2017. [2] n. k. bourne. unexpected twins. physics 9:19, 2016. [3] f. kaiser, k. kainer. magnesium alloys and technology. john wiley & sons, 2003. [4] t. homma, n. kunito, s. kamado. fabrication of extraordinary high-strength magnesium alloy by hot extrusion. scripta materialia 61(6):644–647, 2009. [5] a. c. fischer-cripps. introduction to contact mechanics, vol. 87. springer, 2000. [6] f. a. stevie, l. giannuzzi, b. prenitzer. the focused ion beam instrument. in introduction to focused ion beams, pp. 1–12. springer, 2005. [7] s. reyntjens, r. puers. a review of focused ion beam applications in microsystem technology. journal of micromechanics and microengineering 11(4):287, 2001. [8] r. hearmon. the elastic constants of anisotropic materials. reviews of modern physics 18(3):409, 1946. [9] m. a. kumar, i. j. beyerlein, r. j. mccabe, c. n. tome. grain neighbour effects on twin transmission in hexagonal close-packed materials. nature communications 7:13826, 2016. [10] j. nemecek, j. nemecek, j. manak. fracture properties of cement studied by nanoindentation and fib in press 2018. [11] ansys, inc. ansys reference manual version 17.2, 2017. https://www.ansys.com/. [12] c. sun, n. guo, m. fu, s. wang. modeling of slip, twinning and transformation induced plastic deformation for twip steel based on crystal plasticity. international journal of plasticity 76:186–212, 2016. 73 https://www.ansys.com/ acta polytechnica ctu proceedings 15:69–73, 2018 1 introduction 2 experimental part 3 numerical modelling 4 results and discussion 5 conclusions list of symbols acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2019.22.0128 acta polytechnica ctu proceedings 22:128–132, 2019 © czech technical university in prague, 2019 available online at http://ojs.cvut.cz/ojs/index.php/app experimental verification of impregnated textile reinforcement splicing by overlapping tomáš vlacha,b,∗, lenka laiblováa,b, jakub řepkaa,b, zuzana jirkalováb, petr hájeka a czech technical university in prague, faculty of civil engineering, department of building structures, thákurova 7, 166 29 prague, czech republic b czech technical university in prague, university centre for energy efficient buildings, třinecká 1024, 273 43 buštěhrad, czech republic ∗ corresponding author: tomas.vlach@cvut.cz abstract. this paper presents an experimental verification of impregnated textile reinforcement splicing by overlapping using tensile test of small textile reinforced concrete slabs before its using in the product. the specimen dimensions were designed 80 × 360mm and thickness approximately 18mm. this specimen was reinforced using two pieces of impregnated flat technical fabric from carbon roving and epoxy resin. two overlap lengths were designed using data from previous cohesion tensile tests and necessary anchoring length. the purpose of this experiment was experimental verification before flat reinforcement splicing by overlapping on the final product – furniture with textile reinforcement. this paper shows possible problems and complications in the anchoring of the textile reinforcements and in splicing by overlapping, the importance of the accuracy reinforcement position in the thin concrete cross-sectional area. keywords: textile reinforcement, high performance concrete, textile reinforced concrete, reinforcement splicing, reinforcement overlapping. 1. introduction textile reinforced concrete (trc) [1, 2] is new modern composite material made from high performance concrete (hpc) [3] reinforced by technical fabrics. for new structures are often used impregnated composite fabrics by epoxy resin and other materials because of stabilization and also technology of concreting in comparison with non-impregnated. non-impregnated fabrics are often used for strengthening of reinforced concrete (rc) structures. carbon fabrics used in this experiment were impregnated and stabilized by epoxy resin. it was selected impregnated fabrics commonly available on the market and own developed recipe of hpc. these both trc materials are mechanically resistant, chemically resistant and corrosion free. since the component thickness for rc walls, facade panels and other elements is normally only determined in order to satisfy the concrete cover corrosion protection required in eurocode 2, exterior construction elements must be designed with a thickness 80mm and even higher. thanks to the noncorrosive new reinforcement elements can be made thinner. the concrete cover must be designed only for transferring of bonding stresses between the concrete and textile reinforcement and it is approximately 10mm. so the building trc elements can be approximately 30mm thick with two layers of textile reinforcement. concrete cover for trc specimens is designed only with regard to the transfer of forces. in particular, the basic strength properties of both materials and interaction conditions between technical fabric and hpc are studied using most often using tensile test [4]. tensile test was selected also for presented verification of splicing by overlapping. interaction conditions and bond behavior goes hand in hand with anchoring length and also with reinforcement splicing. many studies, models and experiments was already done [5–8] also at ctu in prague in previous research for example [9–11]. this paper presents experimental verification of impregnated textile reinforcement splicing by overlapping using small trc specimens before its using in the final products. these products can be for example footbridges constructions, concrete furniture and facade elements with larger dimensions and different shapes, where the joining of textile reinforcement is necessary. 2. materials used in experiment 2.1. concrete the hpc mixture used in this experiment was developed and optimized in last years at the ctu in prague for different applications. concrete recipe is presented in table 1. it is the self-consolidating fine grain concrete without any fibres. materials used for experiment are primarily from local sources and components are: cem i 42.5 r cement, silica sand with two particle sizes and maximum grain size 1.2mm, silica flour with one particle size, microsilica and one type of the pce superplasticizer. developed mixture reduces the amount of water on water/cement 128 http://dx.doi.org/10.14311/app.2019.22.0128 http://ojs.cvut.cz/ojs/index.php/app vol. 22/2019 experimental verification of impregnated textile reinforcement. . . mix content kg/m3 cement i 42.5 r 680 technical silica sand 960 silica flour (ground quartz) 325 silica fume (microsilica) 175 superplasticizers 29 water 171 total 2 340 table 1. hpc mix composition. ratio only 0.25 and therefore significantly improves the mechanical properties. direct tensile strength of used hpc is experimentally determined to 5.0mpa respected the csn 73 1318 standard. tensile strength in bending is 16.8mpa on prisms 40 × 40 × 160mm with distance between supports 100mm according to csn en 123905 standard. compressive strength is 106mpa on 100mm cubes according to csn en 12390-3 standard. static modulus of elasticity is 49.2gpa on prisms 100 × 100 × 400mm according to csn iso 1920-10 standard [12]. 2.2. reinforcement for presented experiment was used commonly available flat composite concrete reinforcement produced by ©solidian company. technical fabrics were made of carbon fibre roving and stabilized and impregnated by epoxy resin. flat textile had dimensions 5×1.2m and was cut to the required specimens dimensions. grid spacing was 21mm for both longitudinal and lateral directions. basic parameters of used carbon roving are: cross section area of the strands 1.81mm2, tensile strength of fibrils more than 4000mpa. basic parameters of impregnated reinforcement: cross section area of the reinforcement 85mm2/m for both directions, tensile strength 3300mpa (characteristic value 2500mpa) and youngs modulus more than 220gpa. other information and parameters of reinforcement are mentioned in the technical data sheet. 3. experimental investigation concreting of specimens with dimensions 80 × 360 × 18mm was done in two layers with the reinforcement applied in the middle of specimens. the casting process was done without any use of spacers. this procedure ensured concrete cover approximately 7mm for both surfaces. only one layer of technical fabric with two pieces were used as a reinforcement with 4 rovings in longitudinal direction. view on the reinforcement just before concreting is presented in fig. 1. two groups of three specimens were prepared with minimal length of fabrics overlapping approximately 35 and 55mm based on the previous calculations of anchoring length approximately 30mm [7, 9]. it means a distance of one or two meshes spacing plus free overlap of cut reinforcement. one both ends of specimens were fabrics doubled using small piece of composite reinforcement as a protection against specimen damage on the ends during the experiment and its possible negative effect on achieved results. uniaxial tensile test was chosen for the testing procedure of splicing by overlapping because it is the clearest experimental verification of reinforcement connection. in the middle of the specimens were created a small transverse notch approximately 1.0mm deep on both specimen sides so that the first tensile crack was initiated just in the middle of the monitored specimen part. both end of trc specimens were fixed between two steel plates with thickness 5mm using four screws in combination with chemical mortar on the entire contact area. the fitted length was 100mm for both ends, so free monitored length for testing was 160mm. using fifth screw on the end of steel plates simulated the joint and small next top steel plate were specimens fixed in the testing machine. tensile test was performed in mts 100 testing machine with controlled load speed 2.0mm per minute after 28 days of specimens hardening. in fig. 2 is presented the testing scheme with all dimensions. the loading process continued until the sample failure with constant load speed. measured values were force, time and displacement. 4. experimental results presentation and discussion results of experimental verification are presented in graph in fig. 3. the form of presented graph is force on y axis and displacement on x axis. one typical experimental curve for both group of three specimens is presented. it can be easily calculated the maximal theoretical tensile force 29.0 kn in the moment of reinforcement failure [13]. maximal characteristic tensile force 17.9 kn was calculated using characteristic value of tensile strength and composite reinforcement cross sectional area. the maximal force reached in experiment was 12.6 kn only for overlap 55mm and the failure was always in the concrete part of reinforcement contact. the beginning of the curve and loading process was irregular until a deformation of about 0.3mm when the sample position was stabilized in the testing machine. the linear part than followed when the concrete part is not cracked until the moment of the first crack initiation. the drop in force after the moment of the first crack initiation is minimal and it means that the carbon composite flat reinforcement was very quickly activated. next part of oscillated curve is called multiple cracking zone, which is nonlinear and represent the process of reinforcement full activation in its length. next zone without larger curve oscillation and it is called post-cracking behavior [14]. but in fig. 3 this part of curve is visible only for specimen with 2x spacing. it means a distance of two meshes spacing plus free overlap of cut reinforcement, so spacing approximately 55mm. the part of curve 129 t. vlach, l. laiblová, j. řepka et al. acta polytechnica ctu proceedings figure 1. view in the wooden mold just before casting process on the used flat carbon concrete reinforcement with overlapping in the middle part. figure 2. scheme of the testing set up with dimensions. figure 3. test results presented in the form of force – displacement graph. one typical experimental curve for both groups is presented. 130 vol. 22/2019 experimental verification of impregnated textile reinforcement. . . figure 4. typical detailed pictures of damaged specimens with shear collapse during testing procedure and just after the collapse of specimens and its uninstalling from the testing machine. was no longer as expected from the numerical nonlinear model, because the damage occurred in concrete contact part, not in reinforcement. in fig. 4 is presented the typical shape of crack in the moment during the loading process when both reinforcement layers are separated from each other. before this collapse point on all specimens were visible multiple cracking. on the right part of same fig. 4 is presented the view on the damaged both halves of specimen after the loading process and its uninstalling from the testing machine. one of the halves was logically turned upside down in comparison with the position in the testing machine so both broken areas are visible. also the bond behavior of composite reinforcement without surface treatment was lover as visible in fig. 4 on the right part. the interaction of reinforcement was caused more or less due to the small shapes of the reinforcement in its longitudinal direction due to the technological progress of the mesh production. the surface of the reinforcement was otherwise smooth [12]. it leads to the need for a larger concrete cover of the flat reinforcement for bonding stresses transferring between the concrete part and reinforcement or necessary reduction of cross sectional carbon roving area. after the samples examination it was found that the expected position of composite reinforcement in cross-sectional concrete area was not assured. overlapped reinforcement layers were sometimes more than 4mm apart from each other. this resulted in high eccentricity and additional stresses as well as significant reduction of the cover layer of the reinforcement. it was not included in the previous numerical model. the position of the reinforcement in the cross sectional concrete area must be precisely determined for the future applications. 5. conclusions this article presents experimental results of trc tensile test with splicing of technical textiles by overlapping. in the curve is visible the multiple cracking until the moment of maximal stress before the failure in the contact area of reinforced meshes. the real distance between textiles axis were approximately 4mm and higher. it was more than excepted values in previous numerical model because the yarn thickness was less than 1.0mm. it led to the higher eccentricity and higher additional negative stresses initiation in the cross-section of overlapped reinforcement part. during the loading test for all tested samples reinforcement layers were separated from each other in the concrete part without any damage of reinforcement in longitudinal direction. this experiment presents the importance of all details during designing and realization of trc applications. with extremely small thicknesses of constructions and small concrete cover also small millimetre details and deviations from the assumption can very negatively affected final results. it is very important to ensure the precise position of the composite reinforced fabrics in cross-sectional concrete area for example using small plastic spacers and splicing area should be fixed due to the unwanted eccentricity in the tensile load. also longer anchoring length should be designed for safety load transfer through the reinforcement contact. acknowledgements this particular outcome has been achieved with the financial support of the tacr 11124 311 3111702a124 – subtle concrete furniture and small structures for the railways stations, it has been supported by the ministry of education, youth and sports within national sustainability programme i, project no. lo1605 and also 131 t. vlach, l. laiblová, j. řepka et al. acta polytechnica ctu proceedings by sgs18/108/ohk1/2t/11 – environmental 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[14] r. contamine, a. si larbi, p. hamelin. contribution to direct tensile testing of textile reinforced concrete (trc) composites. materials science and engineering a-structural materials properties microstructure and processing 528:8589–8598, 2011. doi:10.1016/j.msea.2011.08.009. 132 http://dx.doi.org/10.4028/www.scientific.net/amr.982.130 http://dx.doi.org/10.4028/www.scientific.net/amr.1106.69 http://dx.doi.org/10.4028/www.scientific.net/amm.825.7 http://dx.doi.org/10.4028/www.scientific.net/kem.677.203 http://dx.doi.org/10.4028/www.scientific.net/amr.1054.104 http://dx.doi.org/10.1016/j.msea.2011.08.009 acta polytechnica ctu proceedings 22:128–132, 2019 1 introduction 2 materials used in experiment 2.1 concrete 2.2 reinforcement 3 experimental investigation 4 experimental results presentation and discussion 5 conclusions acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2019.23.0001 acta polytechnica ctu proceedings 23:1–8, 2019 © czech technical university in prague, 2019 available online at https://ojs.cvut.cz/ojs/index.php/app practical use of modern investigation and monitoring methods in polish landslide remediation projects zbigniew bednarczyk poltegor-institute – institute of opencast mining, parkowa 25, 51 616 wroclaw, poland correspondence: zbyszbed@gmail.com abstract. this paper discusses exemplary landslide investigations in natural carpathian and man-made opencast mine slopes. research methods included mapping, core drilling, laboratory testing, gpr scanning, on-line inclinometer, piezometer, and pore pressure monitoring and also numerical modelling. landslide triggers were complex and diverse. on flysch slopes movements were activated by heavy rainfalls and in opencast mines, by exploiting the parameters of clayey soils. precise prediction of landslide activation is impossible. identification of landslide triggers for mitigation measures requires a multiple analysis of integrated external and internal variables. keywords: lanslide, mine slopes, landslide triggers. 1. introduction ground movements cause considerable economic loss in many countries every year. most of the landslides in poland, approximately 95% occurred on natural slopes in the carpathian mountains [1]. other, large man-induced mass movements are also observed in polish lignite opencast mines. warning of landslide hazard in poland is mostly based on field inspection and mapping. these works could recognize landslide location and basic geometrical parameters as size and approximate depth. however, more advanced landslide monitoring and early warning studies requires identification of many external and internal activation factors and precise location of sliding surface depth using advanced investigation and monitoring methods. observed in the polish carpathians mass movement constitutes the debris slide type of landslide [2]. identifying activation of flysch soil-rock type colluviums is very complicated. it requires usage of expensive core impregnated drillings, geophysics, laboratory tests and complex monitoring methods. precise detection of sliding surfaces locations, movements ranges and directions is basic for recognition of remediation possibilities. it is possible to detect surface displacement using many types of modern monitoring systems. besides the standard geodesy comprehensive data could be delivered also by gps, implementation of satellite remote sensing or tls terrestrial laser scanning. insitu extensometers and inclinometers could precisely measure ground displacements. groundwater conditions could be studied using piezometers and pore pressure transducers. surface displacement warning systems cover large areas and allow precise monitoring. earlier and more precise detection of ground displacements is possible using in-depth monitoring. different parameters can be monitored to provide meteorological, ground displacement, groundwater and pore pressure data [3]. prediction of landslide activation time could be based the analysis of the movements and rainfall or groundwater conditions [4, 5]. measurements of these parameters and its interpretation could be used to define warning triggers for early warning. the paper presents selected examples of landslide investigations in which the author had the opportunity to participate. investigations in the polish carpathians and conventional monitoring measurements date from 2005. the first attempt using in-situ remote monitoring for landslide monitoring in poland was made in 2010. the second in-situ remote system was installed in belchatow opencast mine in 2016. however much new technologies can deliver significant data, very important is the proper interpretation of geotechnical engineering conditions. instrumentation methods always need careful design and calibration. 2. landslide investigations in the polish carpathians the author of this paper had the opportunity to investigate 25 landslides and participate in the design of 7 landslide road reconstructions in se poland [6, 7]. landslides occurred on mountain slopes in neogene flysch layers built from claystones and sandstones with a shallow groundwater level depth. creep type ground movements have a depth of 5–16m and a speed of a few millimetres to several centimetres a year. the proposed research methods were included in the design of stabilization projects or the preliminary counteraction concepts (figure 1). on two roads, due to the high cost of stabilization, only temporary repairs were performed. in one case, slope investigation was conducted for the design infrastructure at a landslide-prone slope for the polish oil & gas company [8]. one project was realized for protection of the cultural heritage of the saint john chapel in dukla [9]. the main purpose 1 https://doi.org/10.14311/app.2019.23.0001 https://ojs.cvut.cz/ojs/index.php/app zbigniew bednarczyk acta polytechnica ctu proceedings of the research was to define the possibilities and the methods of landslide remedial work. 2.1. site investigation the site investigation program was dependent on the risk posed by every landslide, the volume of unstable mass and observed displacements. it included preparation of geological engineering reports (figure 2) in risk areas (2005–2008). research methods included high-quality boreholes, undisturbed sampling, gpsrtk mapping (figure 3) and gpr near-surface geophysical scanning (figure 4). conventional instrumentation was applied in site investigations boreholes (2005–2014). on-line systems were installed in specially drilled boreholes (2010). an updated version of the actual landslide terrain morphology was very important due to the lack of current landslide maps and significant morphological changes caused by the ground movements of recent years. in some areas, besides standard geodesy surveying gps-rtk measurements were implemented given their efficiency and low cost in relation to conventional mapping. the gps post-processing measurements were found to be an effective way of landslide mapping. they had the horizontal and vertical accuracy of 1 cm. however, some difficulties occurred in the forests. drilling allowed recognition of layers and inclination inside the slopes. for this reason over 800m of drilling by the dual-core apparatus, involving a diameter of 132mm to a depth of 9–30m, was performed. during these works over 200 soil samples were taken for the laboratory tests. the relatively high yield of the core (80–90%) allowed descriptions of landslide profiles, cross-sections and in some cases recognition of the sliding surface depth. additional data connected with slope stratification and colluviums depths were detected using 2d gpr ramac scanning (figure 3). this allowed accurate measurements of changes in colluviums and the dielectric properties of bedrock layers between the boreholes. the gpr profiling was essential for construction of geotechnical cross-sections. it allowed recognition of landslide colluviums and the inclinations of layers and faults. interpretation by the gpr method and boreholes in sandy and clayey layers proved helpful in the identification of water infiltration-prone zones. however, it is to be noted that the identification of colluviums was made possible by gpr correlation with detailed site investigation and monitoring results. with the gpr method, layer depths are only the result of interpretation and should be calibrated very carefully. the gpr method had some limitations in places such as forests or locations close to power supply lines. interpretation of gpr landslide scanning results was not easy and should be made by experienced users of this equipment. 2.2. instrumentation and monitoring nearly 500m of inclinometer casing was installed in the carpathians by the author. boreholes drilled inside the site investigations were used also for inclinometer, piezometer, and pore pressure transducers. the conventional monitoring methods were used for standard inclinometer, piezometer and pore pressure monitoring before, during, and after the remediation. the network included 30 monitoring locations, which consisted of a 70mm abs inclinometer depth of 7– 21m and standpipe piezometers depths of 5m. for pore pressure monitoring, pneumatic and piezoelectric transducers were located at a depth of 5–10m. in 2010, the first on-line early warning system in poland, involving four field stations, was installed in a landslide in szymbark (figure 5). it included two on-line continuous inclinometer field stations, and an in-place inclinometer and weather station. continuous inclinometers consisted of 3d mems tilt sensors every 0.5m to a depth of 12–16m. they included a total of 66 mems tilt sensors. the maximum range of measurements, up to 500mm, is much greater compared to standard inclinometers. one segment included 3 tilt sensors, measuring a range of ±45 degrees, with an accuracy of 0.02mm/m, an admissible error of joints of ±0.250. every octet (8 segments) is equipped with a ground temperature sensor. the in-place inclinometer system with 3 uniaxial sensors has a length of 14m. the gprs data are registered every 6 hours and available online from 10 may 2010 (figure 6 and figure 7). the ground movement measuring devices were supplemented by three automatic pore pressure and groundwater level transducers and online weather station with measurements of rainfalls, air temperature, air pressure and air humidity. the conventional measurements were conducted every few months from 2006 until 2017, a period of over 11 years. in the first 5 years scheduled readings were taken more often, every 30–45 days, then two times a year. automatic pore pressure readings were performed every 6 hours from 2005. measurements allowed verification of representative values of displacements, their direction and depth. observed ground movements varied from 50mm to 500mm over 12 years in time. in some cases, displacements reached critical values that made it possible to measure only the shallowest sliding surface. it was observed that the greatest displacement occurred after the highest pore pressure variations. the pore pressure rose at such a period to 60–65 kpa and then decreased to 40–45 kpa. the on-line system delivered more comprehensive data due to small 10 minutes – 6 hours reading intervals. the on-line system registered the record high rainfalls just after the installation. the reading from the weather station on june 2 reached 100mm/m2 in 3 hours. total displacement reached a size of 30–60mm to the depth of 12–15m. the largest deformation occurred in may-june 2010 and in june-august 2011. initial threshold values were defined as a daily displacement higher than 1 cm and rainfall higher than 100mm a day. two of the on-line and three of the conventional inclinometer stations were already damaged by 2 vol. 23/2019 investigation and monitoring methods in landslide remediation figure 1. localization of the investigated landslides and slopes. figure 2. geological engineering cross-section of landslide in szymbark. figure 3. sekowa landslide gpr scanning results. 3 zbigniew bednarczyk acta polytechnica ctu proceedings figure 4. gps-postprocessing results, szymbark landslide. figure 5. online landslide monitoring station, szymbark landslide. large displacements. the conventional and on-line measurements were conducted until the end of 2018. 2.3. laboratory tests to define the geotechnical parameters of colluvium, index, il oedometer and direct shear, and ciu, cid triaxial tests were performed. the index tests included the grain size, the moisture content, the plastic and liquid limit, the bulk density, the grain-size density, and the content of organic/bituminous material. however, the preparation of samples into the oedometer and the strength test was complicated due to the occurrence of crushed rock particles in the soil samples. therefore, the strength tests were limited to the simple direct shear tests in the shear box apparatus and only a limited number of triaxial tests. the tests detected that colluviums represented silty clays, loams, gravelly loams, claystone, and sandstone (bedrock). the investigated soil samples, had the natural moisture content of 18–37%, varied in effective cohesion 6.5–10 kpa and an effective angle of internal friction of 11–15°. clayey colluviums were characterized by a very high compressibility. the highest values of the moisture content and plasticity were observed in samples taken near the slip surface to depths of approximately 2–10m, near the slip surface. 2.4. numerical modelling the slope stability was checked by lem numerical modelling using the janbu and bishop methods. the factor of safety fos was calculated before and after the remediation. the strength parameters for the analysis were implemented as the lowest values from the laboratory tests and comparable experience. limit equilibrium analyses showed that the analyzed slopes had low factors of safety fos=0.68–1.2. very low values of fos=0.7–0.8 were calculated for the lower slope parts at landslide tongues. the fem methods were implemented to predict the possible displacements and to compare it with the monitoring results (figure 8). the modelling included external factors such as static or dynamic loads, shallow groundwater level and preferred slip surfaces. the modelling detected that the expected total displacement could reach without any remediation up to 33 cm. 4 vol. 23/2019 investigation and monitoring methods in landslide remediation figure 6. ground displacement 3d model, szymbark landslide. figure 7. results of on-line displacement, rainfall and pore pressure measurements, szymabark. figure 8. fem numerical modelling, soilvision software, szymbark landslide. 5 zbigniew bednarczyk acta polytechnica ctu proceedings 3. landslide investigations in polish opencast mines presented research was conducted in belchatow mine. this opencast mine, the largest excavation in europe, is localized in central poland 50 km south-west from the city of lodz. the mine is situated in the kleszczow tectonic rift. the rift filled by neogene deposits thickness of 15-310m is formed in mesozoic limestone and marl blocks, and cut by many faults (figure 9). neogene deposits within the rift are 5 to 15 times thicker than outside it. the thickness of the main lignite seam is 20-60m. exploitation in the mine is conducted on of two operational fields separated by a salt dome at debina. the belchatow field, located at the east, will finish the exploitation in 2020, the szczercow field at the west in 2038. numerous landslides in this mine were mostly structural genesis and were activated by mining activity. at belchatow field landslides permanently occurred on the south slope of the pit and was formed on paleolandslide structural surfaces, over a deep ditch structure with the greatest thickness of lignite. other landslide prone zone that posed risks to transportation systems and power supply lines was the north slope of this mine. this slope is built from quaternary clays, characterized by low strength parameters. the main structural slip surfaces was formed on borders between quaternary and neogene deposits, and the neogene clays with the main lignite seam. faults cracks, glacitectonic surfaces, borders of low strength soils, varved clays were others surfaces that’s allowing movements activation. mining efficiency in this mine is affected by the complexity of the geological structure of the lignite deposit and overlay landslide-prone sediments. registered in the mine landslides had volumes up to 3,500,000 cubic meters, with displacements from a few centimeters to nearly one meters in one day [10]. eight landslide risk areas were identified at the west slope of belchatow field, investigated inside the presented research. 3.1. site investigation site investigations included description of core impregnated 132mm borehole to the depth of 100m, located at the northern part of the west slope at level of +42m. a. s. l. the borehole was located in ivw risk area in the contact zone of the salt dome. the core obtained from the borehole were described in details. it included soil type, moisture content, consistency and field tests of the soil strength parameters. thirty one nns undisturbed samples (a class) for the laboratory tests were collected into stainless steel cylinders using the osterberg sampler. 3.2. instrumentation and monitoring the instrumentation installed in december 2016 included an on-line continuous inclinometer system, the same type as installed in the carpathians. however the system in belchatow is deeper and consists of sensors, located every 0.5m, to a depth of 100m. the system consists from 200 ground displacement 3d measuring devices. it is built from rigid segments and includes 3 magnetometers for rotational control. additional vw pore pressure transducer is located at a depth of 30m. remote monitoring data are registered every 6 hours from 19th december 2016. till may 2018 the largest of displacements was observed in jan-feb 2017 during the first 40 days of measurements when they reached 70mm. the second activation of displacements occurred in aug-sept 2017 when these increased another 40mm. the largest displacements and shear strains in the direction of slope inclination (x) were recorded at the depths of 0-45m. in the y direction perpendicular (y) at 27m depth (figure 10). located at depth of 30m, pore pressure sensor, registered drop of pore pressures from 258 kpa in december 2016 to 50 kpa in may 2018. this significant change of pore pressure was probably caused by coal exploitation at lower levels and mine dewatering system. however, it was observed that usually acceleration of movements usually occurred after small secondary pore pressure drops. displacements were caused probably by the changes in stress state caused by mining and the salt dome. the initially alarm trigger was set at 30mm per day. to the current point in the time this value has not been detected. by the end of may 2018, observed movement magnitude was 130mm. 3.3. laboratory test laboratory tests included index, direct shear, il oedometer, triaxial ciu, cid tests. index tests covered grain-size, moisture content, unit weight, and dry unit weight analysis. tests included also content of organic material and attenberg limits. soils comprised sandy silts, sandy loams, loams and loamy sands. moisture content was the highest in loamy sands up to 32.8%, relatively high in loamy sand 17.2–28.8% and low in sandy loam 13.6–16.3%. the content of organic material was very high 3–19.3%. the unit weight varied from 1.64 g/cm3 for silty sand to 2.29 g/cm3 for loamy sand. the liquidity index varied from 0.17 to 0.37 for sandy loam with the highest at 0.41 for sandy silt. shear box tests performed detected lower values of apparent cohesion of 19.5 kpa and an apparent friction angle of 22.800 for sandy silts compared to 29 kpa and 27.900 for sandy loams. the il oedometer test for silty sand characterized by the highest plasticity detected a constrained modulus of primary consolidation m=1.74mpa and a constrained modulus of secondary compression m=8.4mpa. 3.4. numerical modelling numerical modelling was performed in two crosssections using flac 8.0 software (ssr method) and lem software (bishop and janbu method). the ssr method tends to reflect the actual condition on the slopes leading to the reduction of shear strength of soil up to the stage of losing stability. the implemented 6 vol. 23/2019 investigation and monitoring methods in landslide remediation figure 9. geological we cross-section of belchatow and szczercow fields. figure 10. magnitude of displac. (a), x shear stress (b), y shear stress (c), belchatow mine. mohr-coulomb elasto-plastic strength model required specification of bulk density, effective cohesion and effective angle of internal friction. the obtained values of factors of safety using the ssr method were low ranging 0.85–1.14 (figure 11). the movements were caused by multiple factors as mining influence, the slope height, strength parameters and salt dome influence. the salt structure influence and rheological properties of salt were not included in these preliminary local analyses due to the relatively high distance to the salt dome and a lack of representative data, but it should be implemented in future analysis. 4. summary and conclusions the practical use of different types of geotechnical engineering investigation and monitoring technologies in exemplary projects in poland was presented. investigations in the carpathians included complex geotechnical and geophysical methods that allowed recognition of landslides parameters to the depth of 15-20m. the investigations in belchatow opencast mine allowed recognition of layer parameters to the depth of 100m. presented the firsts in poland, two on-line monitoring systems, allowed early warning of landslide risk. the on-system in belchatow detected deep movements 235m below the natural terrain level. this results should help the mine owner lower the risk for exploitation. interpretation of displacements and pore pressure changes could be used as an early warning indicator. the research delivered comprehensive geotechnical data for the numerical modelling. however, investigation methods should always be cal7 zbigniew bednarczyk acta polytechnica ctu proceedings figure 11. results of ssr analysis 20we, fos=1.14, belchatow mine. ibrated for local conditions and possible threats. acknowledgements i would like to acknowledge the european investment bank (ebi) for financial support within the sopo landslide counteraction project. installation of the first on-line monitoring in polish carpathians would not be possible without the eu innovative economy project uda-poig.01.03.01-00-043/08 financed by the european agency for regional development. thanks are extended to the eu research fund for coal and steel and the polish ministry of science and higher education for the financing of the slopes rfcr-ct-2015-00001 project and the first installation of on-line landslide monitoring system in polish opencast mine. i would also like to thank to the pge gieksa company, the gorlice local road district administration, the polish academy of science research station in szymbark and the carpathian branch of polish geological survey (pgi). references [1] z. grabowski. instruction for landslide maps and risk areas in poland. polish geological institute, warsaw, 2008. [2] d. cruden, d. varnos. landslides types and processes. in landslides – investigation and mitigation. national academy press, washington d.c., 1996. [3] a. turner. landslides investigation and mitigation; national research council transportation research board special report 247. transportation research board, 1996. [4] e. gil, m. dlugosz. threshold values of rainfalls triggering selected deep-seated landslides in polish flysch carpathians. studia geomorphologica carpatho-balcanica 40:21–43, 2006. [5] l. starkel. geomorphology role of extreme rainfalls in the polish carpathians. studia geomorphologica carpatho-balcanica 30:21–38, 1996. [6] z. bednarczyk. geotechnical modelling and monitoring as a basis for stabilization works at two landslide areas in polish carpathians. in proceedings of 11th international & 2nd north american symposium on landslides. canadian geot. soc., taylor & francis, 2012. [7] z. bednarczyk. landslide monitoring and counteraction technologies in polish lignite opencast mines. advancing culture of living with landslides pp. 33–43, 2017. [8] z. bednarczyk, a. szynkiewicz. ground penetrating radar (gpr) scanning in geological and geotechnical recognition of mountain site for polish oil & gas company. in proceedings of 22 sageep symposium on the application of geophysics to engineering and environmental problems. fort worth, 2009. [9] z. bednarczyk. preservation of saint john chapel in dukla from landslide natural hazard. 8. springer international publishing, 2014. [10] s. rybicki. zjawiska osuwiskowe w krajowych kopalniach wegla brunatnego, ich skala, charakter i uwarunkowania. wydawnictvo politechniki krakowskiej, 1996. 8 acta polytechnica ctu proceedings 23:1–8, 2019 1 introduction 2 landslide investigations in the polish carpathians 2.1 site investigation 2.2 instrumentation and monitoring 2.3 laboratory tests 2.4 numerical modelling 3 landslide investigations in polish opencast mines 3.1 site investigation 3.2 instrumentation and monitoring 3.3 laboratory test 3.4 numerical modelling 4 summary and conclusions acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2018.15.0063 acta polytechnica ctu proceedings 15:63–68, 2018 © czech technical university in prague, 2018 available online at http://ojs.cvut.cz/ojs/index.php/app modulus of elascity of unfired rammed earth barbora mužíková∗, tereza plaček otcovská, pavel padevět czech technical university in prague, faculty of civil engineering, thakurova 7, 166 29 prague 6, czech republic ∗ corresponding author: barbora.muzikova@fsv.cvut.cz abstract. the article is focused on measurement and evaluation of elastic modulus of unfired rammed earth and determination of stress-strain curve in compressive test. sets of different containt of clay, sand and water were made and their properties were evalueted and compared. it was found out that for the hightest elascity modulus is ideal the ratio of sand and clay around 75/25 up to 80/20, the sets with montmorillonite clay had substantialy smaller value of elascity modulusthan the sets with illite-kaolinite clay and the ideal content of water for sand/clay ratio 75/25 is around 0.295. keywords: rammed earth, elastic modulus, mechanical properties, stress-strain curve. 1. introduction nowadays utilization of the unfired earth as a building material is rising. the reason of the trend is mainly the fact that unfired earth is according to the principles of sustainable building development. it is environmentally friendly thanks to using of the final product, there is no need to produce any intermediate products. more over unfired earth building is a hundred per-cent recyclable. on the other hand, the mechanical behavior of the material is not described enough to use the data for designing in practical engineering. this was caused by the leaving the rammed earth as a building material in the time when the characteristics of other materials started to be investigated. due to this fact, there are no standards for designing such constructions. that is why the only way how to build such buildings is to have some experience with the material and use gain knowledge in other constructions. nowaday few researches start to pay attention (for example [1–4]) on the topic of unfired earth. worldwide there is only a few companies that are capable to build unfired earth house with all requirements of modern living comfort. one of these constructions is a visitor centre in southwest wyoming (fig. 1). it is built of 2500 m3 rammed earth. the energy is gained from the renewable sources photovoltaic cells, five wind turbines and geothermal system unsuring the heating and cooling of the building. onother example can be a family house in new mexico (fig. 2). there are two big monolithic walls made of rammed earth. over 150 tuns of material were used to build these two walls. the color of layer take turns to look like a rock cliff. there are also very old examples of rammed earth construction (such as the great wall in chine, mayen pyramid in mexico) and even brand new construction are built. there is a map of unfired earth constructions worlwide in the fig. 3. the orange color are areas where the unfired earth is a traditional building material. the white marks are the unfired earth constructions that are stated in the unesco list. as you can see from the fig. 3 unfired rammed earth is used in different climatic zone in the wold [5–10]. figure 1. the tourist centre in wyoming is built of rammed earth [11]. figure 2. the family house in new mexico [12]. 1.1. properties of the material as it was written above rammed earth brings some advantages and of course even disadvantages compared to commonly used building materials such as concrete and steel. 63 http://dx.doi.org/10.14311/app.2018.15.0063 http://ojs.cvut.cz/ojs/index.php/app b. mužíková, t. plaček otcovská, p. padevět acta polytechnica ctu proceedings figure 3. the orange areas are the places where unfired earth is a typical building material and white ones are stated in the unesco list [10]. first of all, lets take a look at the advantages. the material is according to the principles of sustainable development, there are no intermediate products it is direct use of the final product, the building material (in case that there are no stabilisation such as lime or cement) is a hundred procenr recycable and it is a local material. these were the advanteges in the respect of ecology. another properity of unfired earth is that it is able to balances air humidity in rooms. the loam in unfired earth walls is able to absorbe and desorb humidity faster and greater extent than any other materials. from this results keeping very healty living condition with reduced humidity in summer and elevated humidity in winter. the wall of rammed earth (as other heavy materials) stores heat. it can balance indoor climate in areas with hight diurnal temperature differences [13]. another advantages is absorbing pollutants, cleaning the indoor air and shielding high frequency light. we can not forget to mention the fire resistant of unfired earth. the biggest development of massive unfired earth houses building was at the area of the czech republic in the beginning of 17th century. it was caused by big fires that destroyd wooden houses. even so called "fire patent" was published by marie terezie. it was ordering to have brick kitchen and chimney built from unfired bricks [9]. more over earth material preservs timber and other organic materials and it is ideal to do-it-yourself constuction in this case it is inexpensive. on the other hand there are also disadvanteges of the material. from the authors‘ point of view, the main one is a prejudice, skepticism and lack of trustworthy informations about unfired earth worldwive among laymen and even among civil engineers. this is conected to the fact that unfired earth is not a standardises building material. next the material is not water resistant. it must by shetltered against rain and frost, especialy in its wet state. the sheltering can be done by roof overhangs, dampproof courses or appropriate surface coatings [14]. another diadvantage is shringing of the material during its drying [7, 15, 16]. 2. composition of rammed earth the material that we need for creating rammed earth construction is a loam. loam is a product of erosion from rock. the composition and varying properties of loam depend on local conditions. loam is a mixture of clay, silt and sand and sometimes contains larger aggregates like gravel and stones. the particles can be sorted according to diameter: particles with diameters smaller than 0.002 mm are called clay, those between 0.002 and 0.06 mm are called silt, and those between 0.060 and 2.000 mm are called sand. particles of larger diameter are termed gravels and stones. like cement in concrete, clay function is to binde all larger particles in the loam. other larger parts silt, sand and aggregates constitute the fillers in the loam. depending on which of the three components is dominant, we can differentiate clayey, silty or sandy loam. figure 4. the process of building a rammed earth wall. 64 vol. 15/2018 modulus of elascity of unfired rammed earth set sand/clay ratio water-clay ratio pcs of specimens bulk density type of clay [-] [%/%] [-] [-] [kg/m3] [-] gem i 80/20 0.370 4 2073 montmorillionite gem ii 75/25 0.370 6 2040 montmorillionite gem iii 75/25 0.295 6 1929 montmorillionite kr ii 75/25 0.370 11 2134 illite-kaolinite kr iii 80/20 0.400 12 2138 illite-kaolinite s iii 75/25 0.295 4 2170 illite-kaolinite s iv 85/15 0.370 6 2036 illite-kaolinite s v 75/25 0.335 6 2179 illite-kaolinite table 1. composition of used mixtures. 2.1. clay the clay is a product of erosion of feldspar and other minerals. the basic types of clay are known based on the way of creation. these are kaolinite, illite and montmorillionite. they differ in the lamellar structure. the clay changes the properities of the final product mechaical properties and even the colour is different as can be seen in the fig. 5. 2.2. sand sand and gravel are used as fillers. they are described by the graph of grain size distribution. 2.3. water water activates the binding forces of the clay. the amount of mixing water has influence on the properities of the final product. figure 5. different type of used clay in specimens. 3. production of specimens the material for building specimens is loam and mixing water. that means it is a mixture of sand, clay and water. the loam is made in the laboratory, that means the chosen type of clay and sand (of known grain size distribution) are mixed together to create loam of exactly known composition to compare changes in composition with changes of mechanical properties of the final product. the amount of mixing water is determined depending on the amount of clay, and it is expressed by the water-clay ratio. the process of building rammed earth wall is as follows. first of all, the framework is built, then the first layer of rammed earth is filled in. secondly the layer of moist earth is compressed by a tamper. then next layers of moist earth are added and compressed up to the top of framework. finally, the framework is removed and the result is a hard monolithic wall. the principle is shown in fig. 4. eight different prescriptions were tested for determining modulus of elasticity e. sets gem with montmorillonite clay and sets s and kr with illite-kaolinite clay, sets differed in the ratio of sand and clay and in the water-clay ratio. the composition of the prescription is described in table 1. there are names of sets in the first column, then sand/clay ratio, water/clay ratio, the pieces of used specimen is in the forth column, average bulk density of specimens and type of used clay in the last one column. figure 6. testing of specimens: compressive test. the laboratory produce of specimens is similar to the building of real constructions. firstly the prescription is defined as a ratio between sand (the filler) and clay (the binder) and the water-clay ratio is determined. secondly the compounds were mechanically mixed together, then the moist earth en mixture was 65 b. mužíková, t. plaček otcovská, p. padevět acta polytechnica ctu proceedings set s/c w type of clay fc ft e [-] [-] [-] [mpa] [mpa] [mpa] gem i 80/20 0.370 montmorillionite 1.274± 0.157 0.072± 0.008 43.293± 6.865 gem ii 75/25 0.370 montmorillionite 1.211± 0.173 0.075± 0.007 33.548± 7.460 gem iii 75/25 0.295 montmorillionite 1.466± 0.433 0.045± 0.005 33.528± 7.442 kr ii 75/25 0.370 illite-kaolinite 1.150± 0.096 54.584± 16.856 kr iii 80/20 0.400 illite-kaolinite 1.077± 0.104 61.039± 22.202 s iii 75/25 0.295 illite-kaolinite 2.017± 0.029 0.143± 0.009 99.176± 12.200 s iv 85/15 0.370 illite-kaolinite 5.890± 0.067 0.073± 0.005 37.356± 17.075 s v 75/25 0.335 illite-kaolinite 2.165± 0.037 0.127± 0.008 74.339± 19.428 table 2. measured modulus of elascity e. layer by layer pressed in to the mould by the drill or manually by a steel block. the specimens were removed from the mould and set in to the climatic chamber where the stable conditions were settled. compressive tests (specimens of size 20×20×100 mm) were made after a month after the produce. the pictures from the testing are shown in the fig. 6 and 7 . figure 7. compresive strenght test with extesometer that records displacements δ on the specimens (range 25 mm). figure 8. principle of determinig of modulus of elascity from σ-ε curve. 4. method of determining e test for determining the static modulus of elasticity was the compresive strenght test. the loaddisplacement curve data were recorded during the test. from these data the stress-strain curve was calculated and then the modulus of elasticity was evalueted. the static modulus of elasticity (so called young‘s modulus) e is a material property, that describes stiffness. mechanical deformation puts energy into a material. the energy is stored elastically or dissipated plastically. the way a material stores the energy is summarized in stress-strain curve (σ-ε curve) that can be determined from load-displacement curve (f-δ curve). stress is defined as force per unit area and strain as elongation per unit length. graphically the modulus of elasticity can be defined as a slope of the linear portion of the stress-strain curve. the modulus of elasticity is defined by the hooke‘s law, this law says that for elastically deformation the stress σ is directly proportional to strain ε: σ = e · ε. (1) the stress-strain curve for gem i set is show in the fig. 8. it can be seen that the elasticity modulus can be determined as: e = ∆σ ÷ ∆ε. (2) the material behaviour is linear elastic, the stressstrain curve in the loading part has a shape of linear curve from the beginning to 0.8fc. due to the fact it was supposed that tangent modulus and secant modulus are of the same value. 5. results from the measurements the arithmetic mean and the standard deviation for each set were settled from the measured values, they are in table 2. there are the compositions of the sets, their compressive and tensile bending stregths (± standard deviation) [17–19] and the evalueted 66 vol. 15/2018 modulus of elascity of unfired rammed earth figure 9. prescription of used recepises in triangle diagram. figure 10. comparing values of elascity modulus for all sets. elascity modulus (± standard deviation). fig. 10 compares modulus of elascity of each set in a chart where the values are in mpa. the recepis are shown in the triangular diagram (fig. 9) where three components of the mixture (sand, clay and water) are expresed by percent and are on the axis of the diagrame. the composition and the difference between the prescription can bee senn from the diagram. 5.1. impact of sand and clay content the set s iv had the minimum content of clay and water and maximum content of sand (85/15 0.370). let us remember that the amount of water is settle by the water-clay ratio that means the less clay is used the less water is with same water-clay ratio. the clay that was used was the illite-kaollinite clay. this set reached the minimum value of illite-kaolinite clay sets 37.356± 17.075 mpa. sets gem ii and kr ii contained the maximum of water and minimum of sand, the ratio is the same at both sets (75/25 0.370). they differed in used clay. illite-kaolinite clay was used at kr ii and montmorillonite at gem ii. the set gem ii and gem iii reached the minimum value of all sets 33.548± 7.460 and 33.528± 7.442 mpa and the set kr ii reached the second minimum value 54.584± 16.856 mpa of sets with illite-kaolinite clay. as a result it seems that the ideal ratio of sand and clay is around 75/25 up to 80/20. 5.2. impact of type of used clay two types of clay were used illite-kaolinite and montmorillonite clay. take a look to already mentioned sets gem ii and kr ii. the ratio of clay, sand and water was the same (75/25 0.370), but the elascity modulus of kr ii with illite-kaolinite clay is 1.6 times higher. another sets gem iii and s iii had the same ratio of clay, sand and water (75/25 0.295) and again they differed in type of clay. illite-kaolinite clay was used at s iii and montmorillonite at gem iii. the elascity modulus of s iii with illite-kaolinite clay is nearly 3 times higher. the sets with montmorillonite clay had substantialy smaller value of elascity modulus so the type of used clay in loam is very important for the mechanical properies of the final product. 5.3. impact of water content the set s iii, s iv and kr ii had the same ratio of sand and clay 75/25. the differed in amount of mixture water 0.295, 0.335 and 0.375. the values of e were 99.176± 12.202 mpa, 37.356± 17.075 mpa and 54.584± 16.856 mpa. so evidently, the optimal value of water-clay ratio is around 0.295 for sand/clay ratio 75/25. 6. conclusions sufficient data of rammed earth properties in relation to its composition have to be find out for extension of these constructions. the article was focused on evalueting elascity modulus of different sets of rammed 67 b. mužíková, t. plaček otcovská, p. padevět acta polytechnica ctu proceedings earth. the main data that were found about dependence elascity modulus and composition of clay, sand and mixing water: • the ideal ratio of sand and clay is around 75/25 up to 80/20, • the sets with montmorillonite clay had substantialy smaller value of elascity modulusthan the sets with illite-kaolinite clay, • the ideal content of water for sand/clay ratio 75/25 is around 0.295. list of symbols e elascity modulus [mpa] f load force [kn] fc comressive strength [kpa] ft tensile bending strength [kpa] δ displacement [mm] ε strain [–] % bulk density [kg m−3] σ stress [kpa] gem sets with montmorillionite clay s sets with illite-kaolinite clay kr sets with illite-kaolinite clay acknowledgements the research was financial supported by the czech science foundation (gacr no.18-0884s) and by the faculty of civil engineering at ctu in prague (sgs project no.16/201ohk1/3t/11). references [1] h. araki, j. koseki, t. sato. tensile strength of compacted rammed earth materials. soils and foundation (56(2)):189–204, 2016. [2] l. miccoli, d. v. oliveira, r. a. silva, u. muller. static behaviour of rammed earth: experimental testing and finite element modelling. materials and structures (48(10)):43–56, 2015. [3] q. liu, t. l. tong. engineering properties of unstabilized rammed earth with different clay contents. journal of wuhan university of technology: materials science (32(4)):914–920, 2017. [4] r. el-nabouch, q. bui, o. ple, p. perrotin. characterizing the shear parameters of rammed earth material by using a full-scale direct shear box. construction and building materials (171):414–420, 2018. [5] b. little, m. morton. building with earth in scotland: innovative design and sustainability. first printing. scottish executive central research unit, 2001. [6] j. norton. building with earth, a handbook. first printing. intermediate technology development group limited, 1986. [7] g. minke. building with earth design and technology of sustainable architecturer. second printing. birkhauser publishe, 2006. [8] p. suske. nepalena hlina v moderni architekture. dokument era21: ekologie, realizace, architektura (2):69–70, 2004. [9] i. zabickova. hlinene stavby. first printing. era vydavatelstvi, 2002. [10] craterre cultures, constructives at developpement durable. 2017 architecture de terre dans le monde. http://craterre.org. [11] sirewall. sirewall inspiration ki,t 2017. http://sirewall.com/. [12] r reck. rammed/earth construction gets luxurz makeover, 2015. http://www.pagethink.com/v/blog-detail/ rammed-earth-construction-is-cool-again-and-hot-/ 47/. [13] x. dong, v. soebarto, m. griffith. strategies for reducing heating and cooling loads of uninsulated rammed earth wall houses. energy and buildings (77):323–331, 2014. [14] q. bui, j. morel, b. v. reddy, w. ghayad. durability of rammed earth walls exposed for 20 years to natural weathering. building and environment (44(5)):912–119, 2009. [15] m. rauch. refined earth construction and design with rammed earth. edition detail. munich, 2015. [16] kianfar, ehsan, v. toufigh. reliability analysis of rammed earth structures. construction and building materials (127):884–895, 2016. [17] t. otcovska, p. padevet. dependence of tensile bending strength of rammed earth on used clay composition and amount of mixture water. modern methods of experimental and computational investigations in area of construction ii pp. 48–53, 2017. [18] t. otcovska, p. padevet. dependence of compressive strength of rammed earth on used clay composition. experimental stresss analysis 2016. [19] i. zabickova, t. otcovska, p. padevet. compressive strength of unburned clay masonry. modern methods of experimental and computational investigations in area of constructions pp. 31–34, 2016. 68 http://craterre.org http://sirewall.com/ http://www.pagethink.com/v/blog-detail/rammed-earth-construction-is-cool-again-and-hot-/47/ http://www.pagethink.com/v/blog-detail/rammed-earth-construction-is-cool-again-and-hot-/47/ http://www.pagethink.com/v/blog-detail/rammed-earth-construction-is-cool-again-and-hot-/47/ acta polytechnica ctu proceedings 15:63–68, 2018 1 introduction 1.1 properties of the material 2 composition of rammed earth 2.1 clay 2.2 sand 2.3 water 3 production of specimens 4 method of determining e 5 results from the measurements 5.1 impact of sand and clay content 5.2 impact of type of used clay 5.3 impact of water content 6 conclusions list of symbols acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2018.14.0001 acta polytechnica ctu proceedings 14:1–7, 2018 © czech technical university in prague, 2018 available online at http://ojs.cvut.cz/ojs/index.php/app lateral coolant flow between fuel assemblies in mixed cores vojtěch cahaa,b,∗, jiří čížekb a department of nuclear reactors, fnspe, ctu in prague, v holešovičkách 2, prague, czech republic b chemcomex, a.s., elišky přemyslovny 379, prague, czech republic ∗ corresponding author: vojtech.caha@fjfi.cvut.cz abstract. this paper presents the results of an analysis of lateral coolant flow between adjacent fuel assemblies with non-identical spacing grids in a mixed core consisting of tvsa-t mod.1 and tvsa-t mod.2 fuel assemblies. the calculation was carried out using modified subchannel code subcal which allows to calculate 3d thermo-hydraulic characteristics of the coolant flow in the full three fuel assemblies model. this full three fuel assemblies model was created in two variants. the first variant consisted of three hydraulically identical fuel assemblies tvsa-t mod.1, whereas the second variant consisted of two fuel assemblies tvsa-t mod.1 and one fuel assembly tvsa-t mod.2 which mainly differ in types, number and axial coordinate of spacing grids and also in diameter of guide tubes. the influence of mixed core to lateral coolant flow and hence coolant temperature was obtained by comparing these two variants. the power distribution was taken from presumed mixed core fuel reload calculated by macro-code andrea. finally there were also provided a comparison of results achieved by subchannel analysis approach with calculation of similar problem using cfd code ansys cfx by tvel, the fuel supplier. keywords: subchannel analysis, subcal, mixed core, thermal-hydraulics. 1. introduction when a reactor core consists of more than one type of fuel assembly, the local flow redistributions due to differences in fuel assembly pressure drops may be introduced. the crossflow, as well as axial flow, results in flow-induced coolant temperature changes and thus may influence a heat generation in peripheral fuel rod rows. the mixed core can cause a departure from nucleate boiling ratio (dnbr) penalty with respect to the reference core consisting of only one fuel type. an important aspect of core performance evaluation is the ability to predict interchange transfer of mass and energy. two mechanisms are identified: crossflow and thermal mixing. crossflow involves net transfer of the coolant (both mass and energy) between assemblies or subchannels. thermal mixing is energy exchange due to a radial enthalpy gradient with no associated net mass transfer. modified subchannel thermal-hydraulic code such as subcal [1] can be used to account for the effects of crossflow in fuel assemblies and reactor cores due to hydraulic mismatch or power gradient, and perform the mixed core dnbr calculations. similar analysis (in simple geometry) can be also provided by cfd codes, e.g. ansys cfx [2]. the thermal-hydraulic subchannel code subcal is developed in chemcomex and used in the nuclear industry in czech republic for vver core design, licensing applications and safety assessments of fuel cycles [3]. subcal was developed on the basis of cobra code series. similar to other subchannel codes currently used for pwr and vver analyses, subcal predicts the three-dimensional velocity, pressure, and thermal energy fields and fuel rod temperatures for singleand two-phase flow in pwr, resp. vver. subcal solves the finite difference equations for mass, energy, axial and lateral momentum conservation for an interconnected array of channels, assuming incompressible and thermally expandable homogeneous coolant flow. although the formulation is homogeneous, empirical models are incorporated into the code to account for subcooled boiling and vapor/liquid slip in two-phase flow. the subcal code was validated and standardized by sújb (nuclear regulatory body of the czech republic) in 2010 for tvsa-t mod.0/1 fuel and is expected to be lincesed for tvsa-t mod.2 fuel in 2018. 2. three fuel assemblies model the three fuel assemblies model was created by connecting three separate tvsa-t fuel assemblies where the subchannels at the interface of these fuel assemblies were merged together and hence the new types of subchannels were created. the schematic drawing of three fuel assemblies model is shown in figure 1. the main characteristics of the three fuel assemblies model are listed in table 1. the axial length and nodalization were taken from previous neutron-physical calculation of power distribution by code andrea [4]. a nonuniform axial nodalization into 96 nodes was used at total length of 3730 mm, thus each node length was within range 20.7 to 44.5 mm. 1 http://dx.doi.org/10.14311/app.2018.14.0001 http://ojs.cvut.cz/ojs/index.php/app vojtěch caha, jiří čížek acta polytechnica ctu proceedings parameter number [-] subchannels 1986 types of subchannels 27 boundaries 3033 types of boundaries 27 rods 993 axial nodes 96 table 1. subchannel model of three fuel assemblies. type tvsa-t 1 mod.2 2 mod.2 3 mod.2 4 mod.1 5 mod.1 table 2. types of spacing grids. for the purposes of assessing the influence of the mixed core on the coolant temperature, two variants of the three fuel assemblies model were created. the first variant consisted of three hydraulically identical fuel assemblies tvsa-t mod.1 (figure 2), while the other variants were composed of two fuel assemblies tvsa-t mod.1 and one fuel assembly tvsa-t mod.2 (figure 2). the fuel assemblies tvsa-t mod.1 and mod.2 differ, for the purpose of subchannel code calculation, in particular in the number and types of spacing grids and the diameter of the guide tubes. all types of spacing grids are shown in table 2 and their coefficients of resistance can be found in [5]. the coefficients of resistance for type 4 mixing grid in tvsa-t mod.1 fuel assemblies are assigned to each type of subchannel individually [3], while the type 1 mixing grid in tvsa-t mod.2 fuel assembly requires the same coefficient of resistance for all types of subchannels. different types of spacing grids for tvsa-t mod.1 and mod.2 fuel assemblies are associated with different turbulent mixing coefficients values. for mod.2 fuel assemblies, the value of the trbulent mixing coefficient β1 is given along the entire axial length, while mod.2 fuel assembly is divided into two axial parts. a value of β2 is taken into account up to the first mixing grid (type 1 approx. 2500 mm), whereas the second part has a turbulent mixing coefficient equal to β3 > β2. all turbulent mixing coefficients were carefully set up on the basis of mixing experiments for stand-alone fuel assemblies [6]. at the interface between mod.1 and mod.2 fuel assemblies, the mean value is considered. the axial geometry of spacing grids for both types of fuel assemblies is shown in figure 3. 3. analyzed regime parameters all calculations were performed at the regime parameters corresponding to the diverted values for fuel cycles safety assessments at 4 working mcp (main circulation pump) and 100% thermal power [3]. the calculation regime parameters are shown in table 3. pin by pin power distribution was calculated by code andrea at 48 axial non-uniform nodes. two mixed core cycles (t1 and t2) were selected in which six different power profiles were chosen, see highlighted groups of fuel assemblies in figure 4. fuel assemblies whose name begins with the letter “a” are tvsa-t mod.1, while the names of fuel assemblies beginning with the letter “b” indicate the type tvsa-t mod.2. pressure [mpa] 15.34 inlet temperature [°c] 293.8 mass flow [m3 hod−1] 1425 (3 × 475) reactor thermal power [mw ] 3245 table 3. regime parameters. 4. results each of the six power profiles was set as an input and calculated for both three fuel assemblies model variants by subchannel code subcal, i.e. three tvsa-t mod.1 fuel assemblies and also two fuel assemblies tvsa-t mod.1 and one tvsa-t mod.2 (see figure 2), so that these variants can be compared. the evaluation was carried out in the boundary of mod.1-mod.1 fuel assemblies (var. 1), resp. mod.1mod.2 (var. 2), see highlighted rods and subchannels in figure 5. the evaluated boundaries, subchannels and rods are grouped in graphs with results according to colored rows. the following figures 6 and 7 show the axial distribution of cross-flow velocities. a positive value indicates that the flow proceeds from the first to the second subchannel listed in the legend of each figure. in all rows according to figure 5 the same direction of positive flow velocity is maintained. for example, the positive cross-flow velocity value at “b. 1072-981” expresses flow from subchannel no. 1072 to subchannel no. 981, in the case of negative value it is opposite direction. figures (8 and 9) plotting the axial distribution of the coolant temperature shows coolant temperature differences around the rod between variant 1 (three tvsa-t mod.1) and variant 2 (mod.2 and two mod.1). for example, a positive value indicates that the coolant temperature at a given axial position is lower in variant 2 than in variant 1. 2 vol. 14/2018 lateral coolant flow between fuel assemblies in mixed cores figure 1. three fuel assemblies model with rod and subchannel numbering. figure 2. three fuel assemblies calculation variants. figure 3. axial geometry of both types tvsa-t. [2] 3 vojtěch caha, jiří čížek acta polytechnica ctu proceedings figure 4. calculated power profiles t1 and t2. [4] figure 5. areas of evaluation of lateral coolant flow and coolant temperature around rods. 4 vol. 14/2018 lateral coolant flow between fuel assemblies in mixed cores figure 6. example of crossflow velocity variant 1. figure 7. example of crossflow velocity variant 2. 5 vojtěch caha, jiří čížek acta polytechnica ctu proceedings figure 8. example of rod coolant temperature differences fa1. figure 9. example of rod coolant temperature differences fa2. 6 vol. 14/2018 lateral coolant flow between fuel assemblies in mixed cores 5. conclusions a subchannel model of three fuel assemblies was developed to evaluate mixed cores with the tvsa-t mod.1 and mod.2 fuel assemblies. two variants were created the connection of three tvsa-t mod.1 fuel assemblies and the tvsa-t mod.2 fuel assembly with two tvsa-t mod.1. to evaluate the influence of mixed core on the coolant temperature and the crossflow, two mixed core fuel cycles were analyzed by code andrea. six groups of fuel assemblies forming these mixed cores were selected from different positions in the core and these power profiles were used for further evaluation. by comparing all the results it was found that differences between these different power profiles are not significant at the crossflows nor at the coolant temperatures [5]. the crossflow coolant velocities were evaluated for both variants. maximum values of the crossflow coolant velocities in the mixed core reached 1 m s−1 and the significant influence of the position of spacing grids and the value of their resistance was observed. the inaccuracies in the calculations were introduced by inputting the same value for each subchannel for mixing grids in tvsa-t mod.2 fuel assemblies, because the exact values of resistances specific to subchannel type was not known, while for mod.1 different resistances into subchannel types was used (more corresponding to reality). a comparison of the crossflow velocity calculated by subchannel code subcal and cfd code ansys cfx [2] was also performed. it should be noted that this comparison has several limiting factors. the cfd calculation, unlike the subchannel, was carried out only for the selected area of three spacing grids and with zero power on all fuel rods, i.e. without heating of the coolant. the quantities obtained by the subchannel analysis are on the other hand averaged over entire subchannel or boundary whose dimensions are several times larger than those used for cfd calculation. taking these differences into account, the results of both methods are in good agreement. evaluation of coolant temperatures around fuel rods was performed in several rows at the boundary of two fuel assemblies. from all calculation, it can be seen that the influence of the mixed core on the coolant temperature around the fuel rods may reach a maximum of 5 °c. it should be noted that the increase in coolant temperature also occurs in subchannels around guide tubes whose diameter has increased from 12.6 mm (mod.1). the prepared three fuel assemblies model can also be used to evaluate the combination of two tvsat mod.2 fuel assemblies and one tvsa-t mod.1 or mixed cores with other hydraulically non-identical fuel assemblies. another possibility is, for example, the sensitivity analysis of dnbr in mixed cores. references [1] j. čížek, j. krejčí. subcal-ete dokumentace programu, 108084.93857. chemcomex, 2010. [2] tvel. 03-tem-th-05.002 gidrodinamicheskiy raschet po cfd kodu dlya usloviy smeshannoy aktivnoy zony, rev.0, 2015. [3] j. čížek, v. caha, j. krejčí. 01-th metodika bezpečnostního hodnocení palivových vsázek, rev. 4, 108368.171437. chemcomex, 2017. [4] j. hejzlar. andrea mixed core calculation: cycles t1 and t2. újv řež, 2016. [5] j. čížek, v. caha. analýza vlivu příčného proudění mezi ps na th charakteristiky chladiva ve směsné zóně, rev. 1, 00.417.112. chemcomex, 2017. [6] v. caha, j. čížek. subcal-ete koeficient turbulentního míšení: tvsa-t mod.2, rev. 0, 00.432.179. chemcomex, 2016. 7 acta polytechnica ctu proceedings 14:1–7, 2018 1 introduction 2 three fuel assemblies model 3 analyzed regime parameters 4 results 5 conclusions references acta polytechnica ctu proceedings doi:10.14311/app.2019.25.0021 acta polytechnica ctu proceedings 25:21–24, 2019 © czech technical university in prague, 2019 available online at http://ojs.cvut.cz/ojs/index.php/app high strain-rate compressive testing of filling materials for inter-penetrating phase composites tomáš doktora,∗, tomáš fílaa, petr zlámala,b, daniel kytýřa,b, ondřej jiroušeka a czech technical university in prague, faculty of transportation sciences, konviktská 20, 110 00 prague, czech republic b academy of sciences of the czech republic, institute of theoretical and applied mechanics, prosecká 76, 190 00 prague, czech republic ∗ corresponding author: doktor@fd.cvut.cz abstract. in this study behavior of the selected types of filling material for the inter-penetrating phase composites was tested in compressive loading mode at low and high strain-rates. three types of the filling material were tested, (i) ordnance gelatin, (ii) low expansion polyurethane foam, and (iii) polyurethane putty. to evaluate their impact energy absorption bulk samples of the selected materials were tested in compression loading mode at strain-rates 1000 s−1 to 4000 s−1. the high strain-rate compressive loading was provided by split hopkinson pressure bar (shpb) which was equipped with pmma bars to enable testing of cellular materials with low mechanical impedance. based on the comparative measurement response to compression at both low and high strain-rates was analysed. the results show a significant strain-rate sensitivity of the ordnance gelatin and of the polyurethane putty, while strain-rate effect in the polyurethane foam was not observed. keywords: interpenetration phase composite, shpb, impact protection filling materials, strain-rate sensitivity. 1. introduction in recent decades cellular materials, both metallic and polymeric, are of interest in engineering design fields due to their lightweight nature and possibilities for structural and topological optimisation. among such cellular materials, metal foams [1], additively manufactured periodic structures and auxetics (materials with negative poisson’s ratio) may serve in impact energy absorption applications. for this purpose, materials exhibiting plateau region with increasing of strain keeping constant or only slightly increasing stress up to considerably high strain are required. a high potential for impact energy protection pose materials with strain-rate sensitive behaviour which increase their stress response with increasing strain-rate. such materials may be identified among closed cell metal foams, where strain-rate sensitive response is induced by air flow trough the cell walls during their failure [2], or coated open cell metal foams in which the strainrate sensitivity is caused by micro-inertia effects of the struts [3]. another way to enhance cellular metals’ impact energy absorption properties is to equip the porous structure with a suitable filling material to form inter-penetrating phase composite (ipc). the filling has to provide a good strain energy absorption with acceptable increase of the overall density of such ipc. in this study behavior of selected types of filling material were tested in compressive loading mode at quasi-static condition and using hopkinson pressure bar (shpb) at high strain-rates. 2. materials and methods 2.1. specimen preparation three groups of specimens were prepared from three different polymeric materials: (i) polyurethane foam, (ii) polyurethane putty and (iii) ordnance gelatin. cylindrical shape and dimensions (diameter 18 mm and length 5 mm) of the sample were selected with respect to bars’ diameter in shpb apparatus. both polyurethane foam and polyurethane putty were commercially available single component substances which cured in several hours under ambient conditions. due to its expandible nature, polyurethane foam was poured into a larger mold and subsequently cut by an in house metalic hollow drill to the desired shape. finally, length of the cylinder was finalized by sharp blade. the polyurethane putty was prepared in a confined mold, and after curing process, the cylinders shortened by sharp blade. the ordnance gelatin samples was prepared according to recipe published by jussila [4]. the liquid mixture was poured in a shallow mold to achieve uniform height and after curing cylindres were cut by a hollow drill. the same shape and dimensions of the specimens were used for both quasi-static and dynamic tests. 2.2. quasi-static tests to obtain reference stress-strain curves for evaluation of behaviour under higher strain-rates, a series of quasi-static tests of all three materials were performed. for the quasi-static tests, a custom uniaxial loading 21 http://dx.doi.org/10.14311/app.2019.25.0021 http://ojs.cvut.cz/ojs/index.php/app t. doktor, t. fíla, p. zlámal et al. acta polytechnica ctu proceedings figure 1. experimental setup used for quasi-static tests. frame [5] with loading capacity 3 kn was used. the loading setup was instrumented by a load-cell u9b (hottinger baldwin messtechnik, germany) with loading capacity 1 kn. loading was provided by a stepper motor attached to a harmonic drive and controlled by an in-house control software based on linuxcnc interface [6]. the loading setup for the quasi-static tests is depicted in figure 1. a custom control unit for both loading axis control and load-cell readout was used [7]. the displacement controlled test was performed up to overall strain of 50 % with loading rate of 4µm.s−1 corresponding to strain-rate appox. 5 · 10−3 s−1. 2.3. high strain-rate tests for the high strain-rate tests, an in-house shpb, developed at ctu in prague [8] was used. the apparatus is based on kosky setup, which was adapted for testing of materials with low mechanical impedance. the striker, incident and transmission bars were maufactured from poly methyl-metacrylate (pmma). diameter of the bars was 20 mm, lengths of striker, incident and transmission bars were 190 mm, 1600 mm and 1600 mm, respectively. the bars were supported by 8 low-friction polymer-liner slide bearings with aluminum housing (drylinfjum housing, igus, germany). the striker bar was accelerated by air gun with maximum pressure 16 bar with fast release solenoid valve (366531, parker, usa). different levels of pressure were used to accelerate the striker in range from 0.5 bar to 3.0 bar. based on the preliminary results, pressure levels of 0.5 bar, 1.0 bar and 1.5 bar were selected for testing of polyurethane foam and polyurethane putty and pressure levels of 1.0 bar, 2.0 bar and 3.0 bar were used for the samples of ordnance gelatin. the incident bar was instrumented with 3 measurement points using strain-gauges to take the wave propagation phenomena in visco-elastic medium into account. the transmission bar was instrumented with single measurement point. the foil strain-gauges (3/120 ly61, hottinger baldwin messtechnik, germany) were applied on each measurement point using a single component low-viscosity cyanoacrylate adhesive (cyberbond 2003,cyberbond europe gmbh, germany). the strain-gauges were connected in wheastone half-bridge arrangement to compensate the possible minor bending of the bars during the latter stage of the experiment. the signals were amplified by an active differential low-noise amplifier (el-lna-2, elsys ag, switzerland) and recorded by high speed 16bit digitizer (pci-9826h, adlink technology, inc., taiwan) with maximum sample rate of 20 mhz. due to a limited capacity of the digitizer, precise triggering of the measurement was required. the trigger was implemented by a pair of laser through-beam photoelectric sensor (fs/fe10-rl-ps-e4, sensopart, germany) which were installed on the barrel. moreover a high speed camera (fastcam sa5, photron, japan) was used to capture the loading scene at frame rate 124000 fps which enabled visual inspection of the deformation behavior of the tested specimens. the shpb setup is shown in figure 2. 3. results 3.1. quasi-static tests stress-strain curves of all tested specimens were calculated using the data recorded during the experiments. in the mechanical response of the polyurethane putty, progressive densification occured during the entire compressive loading. in the response of the polyurethane foam, the elastic region followed by the plateau and the densification was observed. response of the ordnance gelatin exhibits steady densification. the stress-strain curves are shown in figure 3. 3.2. high strain-rate tests in the high strain-rate tests, stress-strain curves were derived using the strain-gauges signals. from known wave propagation velocity in the bars’ material and measured strain, strain rate and strain was calculated based on the wave propagation theory in onedimension. then, from the strain and known crosssection area of both bars and sample and elastic modulus of the bars, stress was derived. to check validity of the performed tests, force equilibrium (convergence between input and output force at transmission bar) was evaluated. example of the equilibrium evaluation, where force calculated at incident and transmission bar is plotted over time, is presented in figure 4. in total, 5 tests were performed for every pressure level in each sample group. the stress-strain diagrams for the group of the polyurethane putty, the polyurethane foam and the ordnance gelatin, together with strain-rate-strain curves, are presented in figures 5-7. note, that strain-rate of the experiments varied from 1000 s−1 to 4000 s−1. 22 vol. 25/2019 high strain-rate compressive testing of filling materials for ipc figure 2. shpb setup. figure 3. stress-strain curves under quasi-static compression. figure 4. force equilibrium diagram for polyurethane putty specimen tested at 1.5bar. the characteristics of the plastic failure were evaluated using the high-speed camera images. comparison between the intact and the damaged state of the selected specimens in all three groups are shown in figures 8 to 10. the specimens of the polyurethane foam exhibit no significant radial expansion during the plateau region, which corresponds to its porous nature and crushing of the pores during the plateau. on the other hand, radial expansion of the polyurethane putty specimens is clearly detectable during the entire deformation evolution, which corresponds to the steady increase of stress without significant plateau. the specimens of the ordnance gelatin exhibit slight figure 5. stress-strain curves and strain-rate evolution for group of polyurethane foam specimens. figure 6. stress-strain curves and strain-rate evolution for group of polyurethane putty specimens. radial expansion during the initial phase of tests and subsequently fail may be identified at the surface, development of which occures in the plateau phase of the stress-strain curves. 4. conclusions based on the comparative measurement, the response to compressive loading of three types of filling material was evaluated. the obtained stress-strain curves were analyzed together with the sets of images of intact and damaged specimen during high strain-rate loading 23 t. doktor, t. fíla, p. zlámal et al. acta polytechnica ctu proceedings figure 7. stress-strain curves and strain-rate evolution for group of ordnance gelatin specimens. figure 8. comparison of intact and deformed state of polyurethane foam specimen. and the behaviour corresponds to the measured curves. the results show a significant strain-rate sensitivity of the ordnance gelatin and of the polyurethane putty. strain-rate effects in polyurethane foam occured starting at strain rates higher than 1500 s−1. however, this fact might be influenced by the limited dimensions of the specimens required for the shpb tests which did not allow to test the representative volume element in this case. acknowledgements the research has been supported by the czech science foundation (project no. 19-23675s) and by operational programme research, development and education in project engineering applications of microworld physics (cz.02.1.01/0.0/0.0/16_019/0000766). references [1] p. qiao, m. yang, f. bobaru. impact mechanics and high-energy absorbing materials: review. journal of aerospace engineering 21(4):235–248, 2008. doi:10.1061/(asce)0893-1321(2008)21:4(235). [2] k. dannemann, j. lankford jr. high strain rate compression of closed-cell aluminum foams. materials science and engineering a 293(1):157–164, 2000. doi:10.1016/s0921-5093(00)01219-3. [3] a. jung, s. diebels. synthesis and mechanical properties of novel ni/pu hybrid foams: a new economic composite material for energy absorbers. advanced engineering materials 18(4):532–541, 2016. doi:10.1002/adem.201500405. figure 9. comparison of intact and deformed state of polyurethane putty specimen. figure 10. comparison of intact and deformed state of ordnance gelatin specimen. middle: white bars highlight the radial expansion. right: overlap of the gelatin over the transmission bar highlighted by red lines. [4] j. jussila. preparing ballistic gelatine review and proposal for a standard method. forensic science international 141(2-3):91–98, 2004. doi:10.1016/j.forsciint.2003.11.036. [5] t. fila, j. sleichrt, d. kytyr, et al. deformation analysis of the spongious sample in simulated physiological conditions based on in-situ compression, 4d computed tomography and fast readout detector. journal of instrumentation 13(11), 2018. doi:10.1088/1748-0221/13/11/c11021. [6] v. rada, t. fila, p. zlamal, et al. multi-channel control system for in-situ laboratory loading devices. acta polytechnica proceedings 18:15–19, 2018. doi:10.14311/app.2018.18.0015. [7] d. kytyr, n. krcmarova, j. sleichrt, et al. deformation response of gellan gum based bone scaffold subjected to uniaxial quasi-static loading. acta polytechnica 57(1):14–21, 2017. doi:10.14311/ap.2017.57.0014. [8] t. fila, p. zlamal, o. jirousek, et al. impact testing of polymer-filled auxetics using splithopkinson pressure bar. advanced engineering materials 19(10):1–13, 2017. doi:10.1002/adem.201700076. 24 https://doi.org/10.1061/(asce)0893-1321(2008)21:4(235) https://doi.org/10.1016/s0921-5093(00)01219-3 https://doi.org/10.1002/adem.201500405 https://doi.org/10.1016/j.forsciint.2003.11.036 https://doi.org/10.1088/1748-0221/13/11/c11021 https://doi.org/10.14311/app.2018.18.0015 https://doi.org/10.14311/ap.2017.57.0014 https://doi.org/10.1002/adem.201700076 acta polytechnica ctu proceedings 25:12–15, 2019 1 introduction 2 materials and methods 2.1 specimen preparation 2.2 quasi-static tests 2.3 high strain-rate tests 3 results 3.1 quasi-static tests 3.2 high strain-rate tests 4 conclusions acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2017.13.0130 acta polytechnica ctu proceedings 13:130–134, 2017 © czech technical university in prague, 2017 available online at http://ojs.cvut.cz/ojs/index.php/app pullout behavior of oxygen plasma treated polymer fibers from cement matrix jan trejbala,∗, tereza horováa, zdeněk prošeka,b a czech technical university in prague, faculty of civil engineering, thákurova 7, 166 29 prague, czech republic b university centre for energy efficient buildings of technical university in prague, třinecká 1024, 273 43 buštěhrad, czech republic ∗ corresponding author: jan.trejbal@fsv.cvut.cz abstract. the aim of this work is to describe bonding properties between surface treated polymer fibers and a cement matrix. in order to increase an interaction between the matrix and fiber surfaces, two fiber types having approx. 0.5 mm in diameter were modified by mean of oxygen plasma treatment. surface physical changes of treated fibers were examined using sem morphology observation and interfacial adhesion mechanical tests. the principle of mechanical tests rested on a single fiber pulling out from the matrix (cement paste, cem i 42.5 r, w/c 0.4). the embedded length was equal to 50 % of original fiber length (50 mm), where the fiber free-end displacement and force resisting to the displacement were monitored. it was pointed out that interfacial shear stress needed to break the bond between the modified fibers and the matrix increased almost by 15–65 % if compared to reference fibers. when the fiber free-end displacement reached to 3.5 mm, the shear strength increased almost twice. keywords: cement composites, fiber reinforcement, plasma treatment, morphology, pullout. 1. introduction a fiber-reinforced concrete (frc) is a composite material containing fibers in an amount of 1 % of concrete volume standardly. the role of the fibers is to promote a crack distribution and to reduce crack widths. frc mechanical properties are dependent on (i) properties of each components (by which we mean mainly cement matrix as a continual phase and fibers as reinforcement) and (ii) a mutual interaction between the two basic phases. the strong interaction characterized by the bond and the adhesion between the two phases plays an important role in the mechanical performance of the frc [1–3]. the frc may be reinforced with polymer fibers. the use of polymer instead of more widespread steel fibers brings a number of benefits: a corrosion resistance, a lower propensity to a balling fibers creation during a mixing process, a frugality to hosses and nozzles of shotcrete devices, etc [4]. despite the above mentioned benefits, polymer fibers (including polypropylene, polyethylene, nylon, pva, etc.) have their significant limitations. this is primarily about poor adhesion and wettability to the cement matrix due to their chemical inertness, low surface energy and smooth surfaces [5, 6]. the smooth fiber surfaces can not guarantee required strong adhesion with the matrix. after the full debonding, when fibers are pulled out from the matrix, a weak abrasion effect results to frc integrity infraction. to ensure reinforcing effect, high amount of fibers can be applied. unfortunately, this access is inappropriate due to mixture workability reducing effect [3]. on the other hand, an interfacial bond strength enhancement seems to be the most effective way to improve frc mechanical performance without undesirable side effects, especially in the field of polymer fiber reinforcement [7–9]. in order to modify polymer fiber surfaces, several types of treatment may be applied, e.g. chemical (e.g. use of high alkali solutions) and physical (e.g. mechanical roughening). unfortunately, the first mentioned method we can not considered to be effective and environmentally friendly due to high time consumption (often crossing tens of hours), requirement of high temperature (almost 100 ◦c) and need to use aggressive chemical substances [9–11]. next, it is difficult to control the mechanical methods and moreover, its effect has a very strong impact on fiber mechanical properties [12, 13]. to modify polymer fiber surfaces effectively, a plasma treatment seems to be good alternative to conventional methods mentioned earlier. an effect of the plasma treatment is twofold: chemical as well as physical. the chemical treatment rests in a polymer surface activation, when active polar groups are implemented onto fiber surfaces. as a consequence, the surface is modified from hydrophobic to hydrophilic and thus its reactiveness with polar liquid (water contained in cement paste) is increased [8, 14]. next to that, the polymer surface is roughened via ion bombardment. in the case of the frc containing polymer fibers, the second mentioned phenomenon is very important to achieve strong adhesion between 130 http://dx.doi.org/10.14311/app.2017.13.0130 http://ojs.cvut.cz/ojs/index.php/app vol. 13/2017 polymer fiber pullout behavior figure 1. principle of physical surfaces treatment (from own resources). name material diameter young modulus length mass density [-] [-] [µm] [gpa] [mm] [kg/m3] concrix polyolefin 500 ≥ 10 50 910 benesteel pp, pe 480 5.17 ± 0.50 55 ∼913 table 1. summary of basic fiber parameters. the two materials when the fiber is pulled out from the matrix (called as bridging effect). in this study, we focus primarily just to the physical modifications and their effect. the theoretical principle of the plasma modification is imaged in the figure 1. as a proper indicator of the interaction rate between the modified fibers and the matrix, mechanical tests should be done. indirect mechanical methods have been applied – for example bending tests of the composite material [8]. nevertheless, pullout tests of the single fiber from the matrix provide clear information about the interaction between the two materials. therefore, we focused to the second mentioned method. moreover, the effect of ion bombardment may be examined using sem morphology investigation and afm surface scanning [8, 15]. 2. materials 2.1. polymer fibers two types of polymer macro-fibers were used. both of them are standardly used as randomly distributed and oriented reinforcement in the field of the frcs to reduce creation and development of shrinkage cracks and to preserve construction integrity. concrix fibers composing from two layers (shell and high-modulus core) were made from polyolefin (a specific type was not provided by the manufacturer) in switzerland. benesteel fibers composing from pe (polyethylene) and pp (polypropylene) mixture were made in the czech republic. the basic fiber parameters provided by their manufacturers are summarized in table 1. 2.2. cement samples cement prismatic samples composed of portland cement cem i 42.5 r (water to cement ratio w/c 0.4) having dimensions equal to 25×20×25mm were made and stored for 15 days (unmolded after 1 day of hardening) in standard laboratory conditions. a fiber embedded length was equal to 25 mm. the sample is captured in figure 2. figure 2. cement specimen with single fiber for pullout tests. 3. methods 3.1. plasma modification in order to modify fiber surfaces physically, the oxygen cold plasma treatment was done using tesla vt 214 device. the duration of plasma treatment was 30 and 480 seconds. the first mentioned duration (30 seconds) should guarantee required surface changes, but only to a minimal extend. on the other hand, the longer mentioned duration was established as the longest possible time (when the fiber were treated even longer, their melting temperature was reached). the power of rf source was 100 w, gas pressure 20 pa. 3.2. sem morphology analysis to asses physical changes onto fiber surfaces properly, the scanning electron microscope observation was done by zeiss merlin (carl zeiss microscopy gmbh). using the plasma sputtering system (boc edward scancoats six), the fibers were coated by thin gold layer to eliminate surface charging during sem analysis. 3.3. pullout tests ass a proper indicator of the interaction of both reference and plasma treated fibers and the cement matrix, pullout tests of the single fiber were realized. the displacement controlled testing took place on the loading frame web tiv ravestein fp100 at the constant rate of 0.8 mm/min until reaching 60 n and 0.6 mm/min further. as a result of the experiment, a dependence between the fiber free-end displacement and the force 131 j. trejbal, t. horová, z. prošek acta polytechnica ctu proceedings figure 3. sem images of reference (left), 30 s (middle) and 480 s (right) plasma treated concrix fibers. figure 4. sem images of reference (left), 30 s (middle) and 480 s (right) plasma treated benesteel fibers. resisting to the displacement was recorded. to provide values normalized by the fiber surface area, the interfacial shear stress was calculated. we focused to two stages: (i) when the pullout force reached its maximum and (ii) when the fiber free-end was displaced by 3.5 mm. the mentioned shear stress was calculated as τmax = fmax cf le (1) and τ3.5 = f3.5 cf le (2) where fmax and f3.5 represent the maximal force and the force needed to reach the fiber free-end displacement of 3.5 mm, cf is a fiber circumference and le is a fiber embedded length. 4. results 4.1. sem morphology analysis the morphology changes onto concrix and benesteel fiber surfaces are shown in the figure 3 and figure 4, respectively. the sem images with magnificence of 5k showed that both types of untreated fibers had smooth surfaces having tiny grooves that originated probably from a production technology. the same can be said of fibers modified 30 seconds by plasma treatment. this treatment time turned out to be too short to achieve fiber surfaces physical changes via the ion bombardment but despite of this, tiny morphology changes were revealed. on the other side, the surfaces were significantly changed after 480 seconds of modifications. in the case of concrix fibers, their surfaces were slightly damaged. a violation of the surfaces by scoops was observed. in the case of benesteel fibers, their surfaces after 480 seconds of plasma treatment can be described as significantly roughened by longitudinal grooves, if compared to reference fibers. the roughening of fiber surfaces has twofold importance, as mentioned below. • increase of the surface area. before full debonding, when is the frc exposed to tensile load, chemical bonds between the two materials are realized on a larger area and thus a stronger mutual interaction is ensured. • increase of the shear stress. after full debonding, when are fibers pulled out from the matrix, the shear strength is increased due to the fiber surface roughness. 4.2. pullout tests pullout tests of single fibers from cement matrix revealed the significant cohesion increase in the both stages – before and after the full debonding. before the full debonding, the maximal shear stress (τmax) was increased by ca. 10 and 60 % regardless to the duration of the plasma modification in the case of concrix and benesteel fibers, respectively. this phenomenon can be attributed to the increase of the contact area and to the activation of the surfaces by 132 vol. 13/2017 polymer fiber pullout behavior concrix es benesteel 2.0 0.4 1.2 0.8 1.6 duration of plasma treatment [s] 0 30 480 0 30 480 τ [m pa ] m ax figure 5. maximal interfacial shear stresses for various treatment duration. concrix es benesteel 2.0 0.4 1.2 0.8 1.6 duration of plasma treatment [s] duration of plasma treatment [s] 0 30 480 0 30 480 τ [m pa ] 3. 5 figure 6. interfacial shear stresses at 3.5 mm pullout for various treatment duration. functional polar groups. the results are graphically shown in the figure 5. in this study, we focused especially on physical fiber surface changes, so we put more emphasis to the shear stress (τ3.5) monitored when fibers pulled out from the matrix by 3.5 mm (after the full debonding). the results revealed that the shear stress increased after 30 seconds of the plasma treatment by ca. 70 and 80 % in the case of concrix and benesteel fibers, respectively. after 480 seconds of the treatment, an other shear stress increase was recorded. concretely by 15 % identical for both fiber types as is shown in figure 6. these phenomenons were caused by the increase of fiber surface roughness. 5. conclusions two types of polyolefin macro-fibers having ca. 0.5 mm in diameter were surface treated by mean of the cold low-pressure oxygen plasma treatment in order to achieve their surface physical changes via ion bombardment and thus to enhance an interaction between their surfaces and the cement matrix. the fibers were exposed to plasma by 30 and 480 seconds. to asses the plasma modification impact onto fiber surfaces, the scanning electron microscopy enabling an observation of morphology and single fiber pullout tests from the cement matrix (prismatic cement specimens composed of portland cement cem i 42.5 r, w/c ratio 0.4, dimensions 25×20×25mm, embedded length 50 % of the fiber original length) pointing out on an interaction between the two materials directly were done. interfacial shear stresses were calculated to mutual compare and normalize provided values. the stresses were assessed into two stages – when the value reached to its maximum (before the full debonding) and when the fiber free-end was pulled out from the matrix by 3.5 mm. because we concerned primarily to the physical fiber 133 j. trejbal, t. horová, z. prošek acta polytechnica ctu proceedings surface modification, we focused on the shear stresses when was the fiber completely debonded from the matrix in detail. based on the two experiments, we found out that: • fiber surfaces were roughened via ion bombardment especially in the case of modification lasting 480 seconds. compared to reference fibers characterized by the smooth surface, modified samples were etched. grooves and other surface damages were revealed (scales, depressions) using sem. • the cohesion between the two materials was significantly increased. the maximal interfacial shear stress increased by ca. 10 and 60 % for both treatment times (30 and 60 seconds) of plasma modification in the case of concrix and benesteel fibers, respectively. after the full debonding, the cohesion between both fiber types was increased by 70–80 %, if compared to reference fibers. all in all, it was demonstrated that plasma treatment is an effective method to modify polymer fibers surfaces in order to enhance their interaction with the cement matrix, as it was found also in [5, 8, 14]. this modifications may bring many benefits in the field of frc production. to extend the plasma modification into the industrial use, it will be necessary to accelerate the treatment process. acknowledgements this work was financially supported by czech technical university in prague – sgs project sgs16/201/ohk1/3t/11, by the czech science foundation research project 15-12420s and by the ministry of education, youth and sports within national sustainability programme i, project no. lo1605. references [1] v. 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[15] m. sharma, s. gao, e. mäder, et al. carbon fiber surfaces and composite interphases. composites science and technology 102(6):35–50, 2014. doi:10.1016/j.compscitech.2014.07.005. 134 http://dx.doi.org/10.3151/jact.1.215 http://dx.doi.org/10.1016/s1065-7355(97)90001-8 http://dx.doi.org/10.1007/bf02479507 http://dx.doi.org/10.1016/j.cemconres.2016.08.018 http://dx.doi.org/10.13168/agg.2013.0012 http://dx.doi.org/10.1016/j.matdes.2010.04.009 http://dx.doi.org/10.1016/j.jcis.2010.12.066 http://dx.doi.org/10.1016/j.cemconres.2004.05.046 http://dx.doi.org/10.1016/s1359-835x(97)00114-0 http://dx.doi.org/10.1016/j.compscitech.2014.07.005 acta polytechnica ctu proceedings 13:131–135, 2017 1 introduction 2 materials 2.1 polymer fibers 2.2 cement samples 3 methods 3.1 plasma modification 3.2 sem morphology analysis 3.3 pullout tests 4 results 4.1 sem morphology analysis 4.2 pullout tests 5 conclusions acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2019.25.0012 acta polytechnica ctu proceedings 25:12–16, 2019 © czech technical university in prague, 2019 available online at http://ojs.cvut.cz/ojs/index.php/app mechanical and optical investigation of laser welded structural steel poly(methyl-methacrylate) hybrid joint structures tamás csiszéra,b,∗, tamás temesic, lászló molnára a edutus university, engineering institute, stúdium tér 1., 2800 tatabánya, hungary b óbuda university, sándor rejtő faculty of light industry and environmental protection engineering, doberdó út 6, 1034 budapest, hungary c budapest university of technology and economics, faculty of mechanical engineering, department of polymer engineering, muegyetem rakpart 3, 1111 budapest, hungary ∗ corresponding author: csiszer.tamas@edutus.hu abstract. modern welding processes that can easily be automated (such as friction stir welding, laser welding and ultrasonic welding) are gaining popularity in joining metal-polymer hybrid structures. this field of science is intensively studied around the globe, as a dependable, productive joining method that directly produces structurally sound joints between a metal and a polymer structure could unleash unforeseen possibilities in the vehicle industry. in our experiments, we manufactured hybrid steel-poly(methyl-methacrylate) (pmma) joints with laser welding, using the 2p design of experiment method. we measured the effect of cellulose reinforcing fibres (in varying weight percentages) on the transparency and weldability of the pmma material and the effect of welding parameters on the mechanical properties of the joints. we also examined the vicinity of the welded seam with scanning electron microscopy. keywords: laser welding, metal-polymer hybrid structures, poly(methyl-methacrylate). 1. introduction the importance of integrated structures made from dissimilar materials, for example by joining steel and polymer structures together is increasing in the vehicle industry. an advantage of these integrated structures is that less dense materials can be used, and thus vehicle manufacturing and operating costs can be reduced. however, the mechanical properties of the integrated structure must be comparable to the mechanical properties of the structure it replaced. besides searching for compatible metal and polymer material pairs, a considerable amount of effort is made to convert and optimise existing joining technologies that can be automatized (for example friction stir welding [1–3], ultrasonic welding [4–6], laser welding [7–10], and also a combination of laser and ultrasonic welding [11]) to make them suitable for the mass production of integrated structures made from dissimilar materials. one factor that hinders this process is the fact that metals and polymers differ from each other both in physical structure and chemical composition and thus polymers and metals behave differently when joined together [12]. if the polymer material contains additives, reinforcing fibres or nanofibers, the polymer’s behaviour during joining (for example during laser irradiation) is even more complex [13]. laser beams can be used in multiple ways during the processing and manufacturing of hybrid or integrated metal-polymer structures: depending on the laser welding machine and other parameters like laser power and beam velocity, laser beams can be used to cut materials, to process and to prepare their surfaces and also to join them together. the latter is usually called laser assisted metal polymer, or lamp joining for short in publications [14]. the possibility of using laser welding to join metal and polymer structures together is already proven in publications [15]. rauschenberger et al. [8] proved that using the ideal process parameters during the joining of steel and polymer specimens (both reinforced and unreinforced) can result in even better joint strength than the strength of the polymer materials themselves. jung et al. [16] joined zinc-coated steel and carbon fibre reinforced polyamide 6: the ultimate shear force of the overlapped joint reached 3300 n . based on transmission electron microscopy (tem) and energy dispersive x-ray spectroscopy (eds), the authors claimed that the strength of the macroscopic joint was influenced by shape-connected mechanical joints: the molten polymer filled in the surface roughness grooves of the zinc coating. they also found that chemical bonding between zinc-oxide molecules and carbon atoms of the polymer chains further increased the joint strength. it is also stated by cheon et al. [17] that the crystalline structure, the primary chemical bonding structure and the conformation of the polymer chains have a significant effect on the strength of metal-polymer joints. bubbles can form in the polymer material during the joining process caused by the evaporation of remnant monomer molecules or 12 http://dx.doi.org/10.14311/app.2019.25.0012 http://ojs.cvut.cz/ojs/index.php/app vol. 25/2019 mech. and opt. investigation of laser-welded steel-pmma hybrid joints additives (this phenomenon is called humping), which can strengthen or weaken joints depending on their quantity and size [18]. adhesion between the metal and polymer joining partners can be increased by structuring the joining surface of the metal specimen using a laser beam either to remove material from the surface [19], to transform material on the surface [20, 21], or to add material to the surface [5, 22]. it is also possible to use technologies like corona-discharge plasma treatment to chemically modify the surface of either the metal and/or the polymer specimens. in our ongoing research, we joined structural steel to unreinforced and reinforced (with bio-based cellulose fibres in varying weight percentages) poly(methylmethacrylate) (pmma) specimens using two different laser welding machines. our aim was to identify and optimise the technological parameters that had a significant effect on the strength and appearance on the joint and to find and adjust measurement methods that are suitable for the measurement of the joints we made. 2. materials and methods 2.1. materials and specimens materials used in this work were structural steel (s235jr), 50×10×0.8 mm in dimension as metal specimen, and cellulose fibre-reinforced (in 0 . . . 10 wt% ratio, whole numbers only) pmma, 80 × 10 × 2 mm in dimension made by two manufacturers: • the first pmma type we used is sold under the trade name sitramac hw55, a pmma comonomerised with polystyrene (ps) made by sitraplas gmbh, • the second pmma type we used is sold under trade name altuglas vs-uvt, a pure and crystal-clear pmma made by the arkema group. firstly, we measured the effect of cellulose fibres on the transparency and weldability of the pmma material. for this, we manufactured cellulose fibre-reinforced specimens using the sitramac base material and cellulose fibres manufactured by kronospan-mofa hungary ltd., using cellulose from multiple tree species. both the pmma pellets and the cellulose fibres were dried in a drying oven for 8 hours on 80 °c (as specified by the manufacturers) before compounding on a twin-screw extruder (labtech lte 26-44, ascending temperature profile with 5 °c increments, temperature at the rod-type die: 200 °c). an endless filament was formed from the molten compounded material, which was pelletized for injection moulding on an arburg allrounder 270s 400-170 injection moulding machine. standard (dogbone-shaped) tensile test specimens were manufactured. later on, we manufactured further reinforced and unreinforced specimens for testing and joining using both the sitramac and the altuglas vs-uvt base materials. the reinforcement we used was 1 wt% cellulose fibre (manufactured by arbocel under the trade name b600) with the same process parameters on the same twin-screw extruder and injection moulding machine. flat, rectangular specimens 80 × 80 × 2 mm in dimension were injection moulded, from which, 80 × 10 × 2 mm specimens were cut using a disk cutting machine for joining. 2.2. experimental methods as stated in the introduction section, the chemical composition, material structure of the polymer and the reinforcing material can influence the behaviour of the specimen during laser welding, so we measured the transparency of the specimens to visible and nearinfrared (nir) light on a perkin-elmer lambda 1050 spectrophotometer. we found, that in the 950-1050 nanometre range (in which our laser welding machines operate), the direct transparency (the amount of light passing through the material without scattering) of the unreinforced specimens reached an average of 75 % and 82 % (for sitramac and altuglas base materials, respectively). when cellulose fibres were added to the base material in 1 wt% ratio, the direct transparency of the specimens fell to an average of 35 % and 22 %, respectively. adding more than 1 wt% cellulose fibres to the base material decreased the transparency so much that basically all the incident light scattered or were absorbed inside the material (see figure 1). we used two different laser welding machines to create two different kinds of overlapped joints between the steel and polymer specimens. all metal and polymer specimens were degreased and cleaned using methanol before joining. the first type of the overlapped joints was manufactured with the so-called transmission welding process, in which the polymer specimen is placed on top of the steel specimen. in this case, the laser beam passes through the polymer and then it is absorbed in the steel specimen, creating heat. the heat is then conducted to the polymer specimen, which in turn melts and fills in the surface roughness grooves of the steel, forming shape-connected (mechanical) joints. the second type of process we used was the so-called direct laser welding. in this process, the polymer specimen is placed below the steel specimen, while the latter is irradiated and heated up by the laser beam. with this method, higher laser power outputs may be used and thus, faster cycle times can be achieved without the degradation of the polymer material. we used a trumpf trudiode 151 diode laser with a maximum power output of 150 w for the transmission welding process, and a trumpf trudisk 4001 (yb:yag) solid state laser with a maximum power output of 4000 w for the direct laser welding process. the main process control parameter was the laser power. during the transmission welding with the diode laser, laser power was set to 30, 40 and 50 w , while during the direct laser welding with the solid 13 t. csiszér, t. temesi, l. molnár acta polytechnica ctu proceedings figure 1. direct transparency of unreinforced and reinforced pmma specimens rate meaning 0 no data 1 no flame 2 small flame 3 moderate flame 4 heavy flame table 1. the qualification of flaming rates state laser, laser power was set to 210, 230 and 250 w . these values were chosen based on preliminary measurements we conducted. we used the 2p design of experiment method to manufacture a total of 200 steelpmma specimens. all other parameters remained constant during the joining process: the specimens were joined in 3 cycles in a straight, 10 mm line, while the welding speed was set to 1 m/min. investigating the mechanical (shear) strength of the joined specimens is ongoing. in the next section, we will present a qualitative analysis of the joined specimens and some preliminary results. 3. discussion in most cases during the direct welding process, the polymer specimens burst into flames. the degree of flaming was qualified according to its intensity (see table 1) with visual inspection during the welding process. applying 250 w of laser power, the average rates of intensity of flaming were between 3 and 3.5 for each matrix-fibre combination, which proves the presumption that higher energy input can cause the intensive degradation of the polymer, which can lead to flaming. on lower laser power levels (at 210 and 230 w ), the flame intensity of the materials were similar, except for the sitramac pmma reinforced with 1 wt% cellufigure 2. average flaming rates during direct laser welding of steel-pmma specimens lose fibres at 230 w laser power (figure 2). a possible reason for this phenomenon can be that the chemical composition of the sitramac pmma differs from that of the altuglas pmma. based on the tensile shear testing of overlapped joints made by the solid-state laser with 250 w laser power between unreinforced (sitramac) pmma and steel, we can say that the joints are quite rigid and standard deviations of both ultimate shear force and displacement values are high (figure 3). this may be caused by inadequate contact (air gap) between specimens during joining, or the fact that the material surfaces were only degreased before the joining process. in later experiments, we made sure that there wasn’t an air gap between specimens during joining, and we are planning on trying different techniques to prepare the surfaces of the specimens in the hopes of reaching better shear strength values. we also noticed that after the tensile shear tests, some pmma remained on the steel specimen’s surface. using a scanning electron microscope (sem), we discovered that the connection between the steel and pmma was not evenly distributed on the join14 vol. 25/2019 mech. and opt. investigation of laser-welded steel-pmma hybrid joints figure 3. ultimate tensile-shear force values of unreinforced pmma-steel specimens figure 4. cohesive failure of the pmma near the steel-polymer interface (sem image of the steel surface) ing surface. in some areas, the adhesion between the materials was so good, that the failure was inside the pmma material (as shown in figure 4, where the plastic deformation of the pmma material can be seen), while in other areas, the joint failed with simple adhesive failure (debonding) on the interface. the pmma material also possibly degraded: the formation of gas bubbles on and near the joint interface can clearly be seen in figure 5, however, at lower laser power values, no joints could be manufactured. the degree of degradation of the pmma and the possibility of forming chemical bonds between the materials needs to be investigated further. 4. conclusions with our experiments, we proved that it is possible to manufacture joints between pmma and steel specimens with laser welding equipment. both the transmission welding (when the laser beam passes through the polymer material) and the direct laser welding (when the laser beam does not pass through figure 5. gas bubbles on and near the steel-polymer interface and adhesive failure of the pmma (sem image of the pmma surface) the polymer) techniques could be used to join the specimens. we also proved that cellulose fibres greatly decreased the direct transparency of the otherwise crystal clear pmma material even at low weight percent ratios, thus making the transmission welding joining of reinforced pmma and steel specimens difficult and time-consuming. the steel-polymer joints we manufactured were rigid and the standard deviation of measured tensile force values were high, which was possibly caused by the degradation of the pmma material during welding and the uneven connection between pmma and steel in the joint area. in most cases during the direct welding process, the polymer specimens burst into flames, which also decreased joint strength.the two prominent failure modes were adhesive failure (debonding) on the joint interface and cohesive failure of the pmma material. 15 t. csiszér, t. temesi, l. molnár acta polytechnica ctu proceedings 5. acknowledgements this research was supported by the grant no. efop3.6.1-16-2016-00009 and by the únkp-18-3 new national excellence program of the ministry of human capacities of hungary under grant no. únkp-18-3-ibme-183. references [1] f. khodabakhshi, m. haghshenas, s. sahraeinejadand, et al. microstructure-property characterization of a friction-stir welded joint between aa5059 aluminum alloy and high density polyethylene. materials characterization 98:73–82, 2014. doi:10.1016/j.matchar.2014.10.013. 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[22] d. p. graham, a. rezai, d. baker, et al. the development and scalability of a high strength, damage tolerant, hybrid joining scheme for composite-metal structures. composites part a: applied science and manufacturing 64:11–24, 2014. doi:10.1016/j.compositesa.2014.04.018. 16 https://doi.org/10.1016/j.matchar.2014.10.013 https://doi.org/10.1080/13621718.2016.1204171 https://doi.org/10.1007/s40194-017-0517-y https://doi.org/10.1002/adem.200800271 https://doi.org/10.1016/j.compstruct.2014.07.019 https://doi.org/10.1016/j.ijadhadh.2015.11.001 https://doi.org/10.1016/j.scriptamat.2008.08.026 https://doi.org/10.1117/12.2080226 https://doi.org/10.1016/j.optlastec.2016.09.028 https://doi.org/10.1016/j.proeng.2017.04.092 https://doi.org/10.1016/j.jmatprotec.2017.06.036 https://doi.org/10.1002/9781119429807 https://doi.org/10.1016/j.matdes.2016.03.044 https://doi.org/10.1016/j.matdes.2012.12.015 https://doi.org/10.11779/1362171814y.0000000236 https://doi.org/10.1016/j.phpro.2011.03.030 https://doi.org/10.1016/j.jmatprotec.2015.10.026 https://doi.org/10.1016/j.phpro.2011.03.065 https://doi.org/10.1007/s00170-016-9646-5 https://doi.org/10.1016/j.compositesa.2014.04.018 acta polytechnica ctu proceedings 25:12–16, 2019 1 introduction 2 materials and methods 2.1 materials and specimens 2.2 experimental methods 3 discussion 4 conclusions 5 acknowledgements references https://doi.org/10.14311/app.2022.33.0153 acta polytechnica ctu proceedings 33:153–159, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague critical assessment of co2 emission of different concretes: foamed, lightweight aggregate, recycled and ordinary concrete devid fallianoa,∗, salvatore quattrocchib, dario de domenicob, giuseppe ricciardib, ernesto gugliandoloc a department of structural, geotechnical and building engineering, politecnico di torino, corso duca degli abruzzi, 24, turin 10129, italy b department of engineering, university of messina, contrada di dio, messina 98166, italy c g. gugliandolo s.r.l., via galileo galilei, messina 98100, italy ∗ corresponding author: devid.falliano@polito.it abstract. construction materials contribute to about 75% of the co2 emission of all the construction processes. concrete is one of the most widely used construction materials and is thus primarily responsible for co2 emission. in particular, 8 − 9% of global greenhouse gas (ghg) emission are produced by concrete. co2 emissions can be considerably reduced in the construction phase through a careful selection of materials with low environmental impact or through specific admixtures. in this study, different concretes are taken into consideration, including foamed concrete, lightweight aggregate concrete, recycled concrete and ordinary concrete. a series of mix designs of these four classes of concrete, characterized by a comparable mechanical strength or a comparable density, are taken from the relevant literature and compared to one another in terms of co2 emission. some guidelines or possible research lines aimed at reducing co2 emission are finally outlined in this contribution. keywords: co2 emission, foamed concrete, lightweight concrete. 1. introduction in the last few decades, the attention to environmental issues is more and more increased. the european union issued several guidelines and recommendations in order to reduce global co2 emissions. the objective is to achieve a 40% reduction of the 1990 co2 emission values by 2030 [1]. environmental protection, saving of raw materials and effects of human activities on the climate changes are popular matters that are of interest in diverse social contexts. consequently, the construction industry is also committed to solve these challenges, as witnessed by the large range of research topics in the relevant literature. indeed, the building industry alone is responsible for about 40% of the total energy consumption [2]. more specifically, when coming to the concrete industry, the energy consumption is estimated to be around 3% of the total consumption value, which is associated with an overall co2 emission of about 8-9% of the total co2 emissions [3]. the environmental impact of concrete is due to the fundamental ingredient acting as binder, namely portland cement. indeed, the portland clinker is obtained by cooking a mix of natural and/or artificial soils, including clay, lime, pyrite ash etc. in the blast furnace at 1400◦c. during this preparation phase, the limestone included in such raw materials undergoes the calcination process with significant co2 emission. a twofold environmental impact can thus be recognized in this process: 1) the co2 emission due to the calcination process; 2) the co2 emission due to the energy consumption of the overall process [4]. as a result, the portland clinker is responsible for more than 90% of the overall co2 emission of the concrete industry [5]. based on the above remarks, it appears clear that the most straightforward strategy to mitigate the greenhouse gas (ghg) emission is to reduce the amount of clinker, by partly replacing it with supplementary cementitious materials, such as fly ash and ground granulated blast furnace. the latter technique allows the achievement of different objectives at a time: not only the reduction of the environmental impact in terms of ghg emission by the concrete industry, but also the re-use of by-products and recycled materials, which are converted from negative elements to a useful resource [6]. mineral additions in parallel to clinker portland are allowed by international standards. indeed, in the uni en 197-1 five different classes of cement are reported, depending on the clinker and mineral addition contents. evidently, cements having higher amounts of mineral additions as replacement of portland clinker are characterized by lower co2 emissions. therefore, the co2 emissions due to the concrete industry depend on the type of binder being adopted, which changes from country to country, as the specific culture and the available raw materials influence the preparation phases of concrete. this 153 https://doi.org/10.14311/app.2022.33.0153 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en d. falliano, s. quattrocchi, d. d. domenico et al. acta polytechnica ctu proceedings concept is clearly acknowledged in [7]. besides the introduction of mineral components as partial replacement of the portland cement, which is the conventional strategy adopted in the range of ordinary density concretes, the co2 consumption can be well reduced for special concretes, including foamed concretes (fcs) and aerated autoclaved concretes (aacs). these special concretes are more convenient for the preparation of low-to-medium density concrete mixes. considering the low density, these special mixes are also featured by good thermal insulation properties [8], which is associated with lower energy demands for heating and cooling processes, thus contributing to a lesser amount of co2 emissions not only in the preparation stage, but during the whole service life of the structure [2]. this is particularly convenient in hot climates during the summer season in terms of thermal inertia, when compared to classical insulation panels currently available in the market. recent research on foamed concrete has focused on the determination of the mechanical strength [9] and thermal properties [10], the impact of different ingredients such as foaming agent [11], [12], cement type and presence of superplasticizers [13], the influence of the curing conditions [13], the influence of the presence of by-products and slags, such as fly ash [14] and biochar [15], the influence of mixing intensity [16], the possibility of improving the mechanical characteristics via short fibers [17, 18], or bi-directional grid reinforcement [19, 20] and the fracture behaviour [21–23]. however, to the authors best knowledge, there is no extensive study in the literature concerning the co2 emissions of foamed concrete mixes compared to alternative concrete mixes such as ordinary concretes (ocs), lightweight aggregate concretes (lacs) and recycled concretes (rcs) either by recycled aggregates or by replacing part of the clinker with recycled components. to fill this gap, the present work aims to present a comparative study focused on the co2 emission of such different cementitious pastes. the comparison is made assuming mixes with similar values of mechanical strength or similar values of density. in this way, the evaluation of the co2 emission is made assuming homogeneous conditions among four classes of concrete (ocs, lacs, fcs, racs). a critical discussion is finally made regarding the possibility of adopting alternative concrete mixes (as substitute of ordinary concretes) characterized by lower co2 emissions. 2. materials and mix design in this section, a series of mix designs of different experimental campaigns taken from the relevant literature are presented. these mix designs concern several concrete mixes selected to assess the co2 emissions taking into account different parameters such as density, compressive strength, composition of the mix and main ingredients, presence of mineral additions or recycled elements. through this comparative study, it is possible to shed light on key aspects primarily responsible for the ghg emissions, so as to plan future experimental campaigns on special concretes wherein the reduction of the co2 emission is the main objective. moreover, the sources of the co2 emissions that are inherent in the conventional preparation processes of ordinary and special concretes will be discussed. in particular, the study comprises four different types of concretes, namely fcs, lacs, rcs, and ocs. a preliminary selection process is made in order to identify certain common properties of the different mix designs related to the different types of concrete. as an example, the co2 emission will be comparatively analysed for mix designs characterised by the same compressive strength (but different densities), or alternatively by the same density (but different compressive strengths). the motivation for including these different types of concrete is due to the significantly different compositions in terms of mix design. for instance, in fcs a part of volume is occupied by air voids generated by the presence of a preformed foam, or introduced in the matrix through proper chemical reactions during the initial phases of the preparation. besides these fc specimens, another widely used class of lightweight concretes is represented by lacs. in this case, the cementitious matrix is the same as in ordinary concretes, but the self-weight reduction is achieved by the partial replacement of normal aggregates with lightweight aggregates. in the cases discussed in this paper, expanded clay was used in the specimen preparation. finally, another interesting class of concretes is represented by rcs in which part of the aggregate is replaced by recycled materials. in this specific case, the recycled components are represented by lightweight recycled aggregates as partial replacement of normal aggregates. in this way, lightweight characteristics are achieved also in this class. with regard to fc, three reference studies are included. more specifically, the mix designs presented by some of the authors in [13] and labelled as #1, #2.1 and #4.1 in the referenced paper [13] are included, which are characterized by the presence of cement, water and preformed foam resulting in different densities spanning from about 400 kg/m3 to 800 kg/m3. moreover, another paper presenting mix designs of fcs addressed in this study is [24], where, unlike the previous experimental campaign, the specimens contained also fly ash. also in the latter study the densities span a wide range, from 750 kg/m3 to 1250 kg/m3, so as to supplement the previous experimental campaign [13] with higher-density specimens. finally, the third study is [25], where ultralightweight foamed concrete specimens (density ranging from 100 to 250 kg/m3) with the use of fly ash were documented. in this way, a very broad range of densities of fcs are included in this examination. with regard to lacs, only one benchmark study is considered [26], in particular the specimens labelled 154 vol. 33/2022 co2 emission of different concretes figure 1. distribution of co2 emissions related to 25% of world cement production (data from [7]). lt30, lt40 are included. in this case, the lightweight properties of the specimens were achieved by the addition of expanded clay. in particular, keeping a constant amount of 500 kg/m3 of cement, the authors investigated the effect of different contents of expanded clay ranging from about 200 to 330 kg/m3, obtaining densities from 1280 to 1600 kg/m3 approximately. with regard to rcs, only two specimens were taken from the extensity study in [27], labelled cm20rm and chd50rm. besides the classical aggregates and portland cement, these specimens contained lightweight recycled aggregates and expanded clay. finally, in order to compare the results pertinent to the above-described mixes with traditional concretes having densities of about 2400 kg/m3, two classical mix designs were also included in the study, which are characterized by compressive strengths of around 25 and 30 mpa (comparable to some of the aforementioned special concretes). these specimens have an aggregate-to-cement ratio in the range from 2.7 to 4.78, and a water-to-cement ratio of around 0.5. extensive details of the mix designs of all the considered concretes can be found in the quoted experimental studies, to which the reader is referred to. 3. co2 evaluation a basic procedure for the computation of the co2 is adopted in this study. based on the mix design proportions and the amount of the single components, an estimate of the contribution of each component to the overall co2 emission is performed through assumptions from the literature. in particular, the present study is limited to the assessment of the co2 emission related to the production phase. in other words, this evaluation does not include the emissions related to the transportation from the manufacturing place to the concrete plant, nor the emissions related to the casting in the construction site, nor the lifecycle data (e.g. recovering part of co2 emissions through carbonation of concrete [28]). as a first remark, it is worth noting that the main source of co2 emission in the mix design is represented by the cement. however, slightly different values of the kg co2 emissions per ton of clinker are reported in the literature from country to country [7], as illustrated in figure 1. the differences are due to the different technologies adopted in the various countries in the concrete industry, and due to the different fuels employed. moreover, besides cement, the other predominant ingredients being present in the mix designs analysed in this study are: i) sand; ii) expanded clay; iii) fly ash; iv) water; v) recycled aggregate; vi) foam. these secondary ingredients contribute to the overall co2 emission to a far less extent in comparison with cement. however, they are included for completeness. in particular, the specific co2 emissions of ash and cement are calculated according to the guidelines provided in [29]. whereas the specific co2 emissions of sand, recycled aggregates, water and components of foam are computed according to the instructions provided by the keiti (korea environmental industry & technology institute), environmental declaration office, available at website epd.or.kr/eng/lci/lcico200.do. finally, the specific co2 emissions of the expanded clay are evaluated according to the environmental product declaration provided by the italian company laterlite s.p.a. the calculation of the co2 emissions due to the foam in fcs is made taking into account the different concentration of the foaming agent and the actual density of the foams used in the mix designs documented in the referenced papers. based on the assumptions described above, figure 2 illustrates the co2 emission (expressed in kg of co2 per m3 of concrete) for fcs, lacs and ocs at a comparable compressive strength of 25 mpa and 30 mpa, 155 d. falliano, s. quattrocchi, d. d. domenico et al. acta polytechnica ctu proceedings figure 2. co2 emission comparison alongside related density values among three classes of concrete for equal compressive strength of 25 mpa (left) and 30 mpa (right). figure 3. co2 emission comparison alongside related compressive strength values among three classes of concrete for equal density of 1400 kg/m3 (left) and 1600 kg/m3 (right). figure 4. increase of co2 emission for foamed concrete with and without fly ash in the mix design as a function of the density. 156 vol. 33/2022 co2 emission of different concretes respectively. in the figure, the density of the various classes of concrete is also reported along a secondary vertical axis for completeness, wherein it is easily seen that oc is characterized by a much higher density than both fc and lac. by inspection of the plot, it is seen that for both compressive strength classes (i.e. 25 and 30 mpa), ocs are characterized by a lower quantity of co2 emission in comparison to fcs and lacs. this is justified by the fact that in the mix design of ocs a lower amount of cement is used. indeed, in fcs and in lacs the amount of cement is around 500 kg/m3 in order to attain reasonably good mechanical strengths with lower densities, whereas in the traditional concretes (ocs) typically this quantity does not exceed 350 kg/m3. moreover, lacs are characterized by a slightly higher co2 emission compared to fcs (for equal amount of cement) because the co2 emissions underlying the production process to realize the expanded clay are surely higher than the co2 emissions related to the production of the foam (one order of magnitude higher approximately, based on the previous guidelines adopted for the calculation). considering that the ocs produce lower amount of co2 emissions than lightweight concretes, attention is then paid to the comparison among three classes of lightweight concretes, namely fcs, lacs as well as rcs. figure 3 depicts the comparison of co2 emissions of the three lightweight concretes at equal density of 1400 kg/m3 and 1600 kg/m3, respectively. also in this case a secondary vertical axis is added on the right-hand side of each plot, showing the values of the compressive strengths. based on this comparison, it emerges that rcs are the most favourable ones in terms of both highest compressive strengths and lowest co2 emissions. varying the density, it is confirmed that lacs are characterized by the poorest performance, from both environmental and mechanical perspectives. comparing the two plots on the left and on the right, it is also noticed that the co2 emissions do not vary significantly in relationship to the two analysed values of density. in the case of lacs and rcs, this is due to the fact that the increase of density was obtained by reducing the lightweight aggregates while keeping the cement amount quite constant. analogously, in fcs the increase of density was achieved through a lower amount of foam while keeping the cement amount quite constant. the above statements concerning fcs are valid for medium-to-high densities. nevertheless, by significantly widening the range of the examined densities for fcs it is noted that the density plays a key role in the overall co2 emissions. this is shown in figure 4, which reports the co2 emissions for a range of densities from 150 (ultra-lightweight foamed concretes) up to 1600 kg/m3. in the low-density range, the amount of cement drastically decreases, while the quantity of the foam is simultaneously increased, thus generating a significant reduction of the resulting co2 emissions. values of co2 emissions even five times lower for ultra-lightweight foamed concretes compared to medium-density foamed concretes are indeed observed. however, for lower densities the compressive strength values decrease accordingly, thus making them unable to cope with structural applications [13, 25]. indeed, such ultra-lightweight elements are mostly used in non-structural applications for thermal insulation and acoustic absorption purposes. in figure 4 two sets of data are reported, including mix designs of fcs with and without fly ash. obviously, the fly ash has a considerable effect on the co2 emissions because it is used as partial replacement of the cement in the mix design, thus significantly reducing the co2 emission. as an example, for fcs having density of 750 kg/m3 the addition of fly ash allows the reduction of the co2 emissions of more than 300%. in figure 4 two regression curves for fcs with and without fly ash, respectively, are superimposed. these curves, whose parameters are determined through least-square regression, well agree with the reported or computed values of co2 emission, as confirmed by the high value of the coefficient of determination r2 being higher than 0.98. these curves can be used within the range examined in this paper to predict the co2 emission produced by fcs depending on whether fly ash is incorporated or not in the mix design. 4. discussion and conclusions this study has presented a numerical investigation on the co2 emission produced by four classes of concretes with different mix designs but comparable properties in terms of mechanical strength or density. since the main cause of co2 emission is due to the amount of portland cement, in order to keep the co2 emission to within an acceptably low level, the optimization of the mix design of special concretes plays a key role. in foamed concretes it is well known that abundant use of portland cement is made to obtain good mechanical strengths. therefore, the elaboration of novel mix designs wherein the clinker is replaced by alternative elements, such as fly ash and silica fume, is very useful. however, such replacement is rather limited in practice, because the quantity of cement (despite the introduction of fly ash or silica fume) remains still too high (≥ 500 kg/m3) in most of the studies from the literature. subsequent studies on foamed concretes should be directed toward a compromise between a further reduction of portland cement (e.g. ≤ 400 kg/m3) and good mechanical strengths that can be acceptable for structural purposes (e.g. rck ≥ 20 mpa). an attractive alternative solution is represented by geo-polymer foamed concretes, but relevant literature in this regard is rather scarce. instead, the co2 emissions are notably reduced for ultra-lightweight foamed concretes, in which part of the cement is replaced by fly ash or other mineral additions. similar remarks can be 157 d. falliano, s. quattrocchi, d. d. domenico et al. acta polytechnica ctu proceedings made 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[29] r. leese, d. casey. embodied co2e of uk cement, additions and cementitious material. report of mineral products association (mpa), fact sheet 18, 2012. 159 https://doi.org/10.1088/1757-899x/615/1/012018 https://doi.org/10.1016/j.proeng.2015.06.157 https://doi.org/10.1016/j.tafmec.2019.102297 https://doi.org/10.1016/j.prostr.2019.08.196 https://doi.org/10.1016/j.conbuildmat.2015.01.051 https://doi.org/10.1016/j.conbuildmat.2012.02.053 https://doi.org/10.1016/j.jclepro.2014.11.015 https://doi.org/10.1016/j.eiar.2014.01.004 acta polytechnica doi:10.14311/app.2020.27.0062 acta polytechnica 27(0):62–66, 2020 © czech technical university in prague, 2020 available online at http://ojs.cvut.cz/ojs/index.php/app hardness of hybrid pvd-pecvd w-c:h coatings vs. substrate type františek lofaj∗, margita kabátová slovak academy of sciences, institute of materials research, watsonova 47, 040 01 košice, slovakia ∗ corresponding author: flofaj@saske.sk abstract. the effects of substrate on the measurement of hardness (and indentation modulus) penetration depth profiles were investigated on hybrid pvd-pecvd w-c:h coatings made with the additions of c2h2 or ch4 using high power impulse magnetron sputtering (hipims) method. the substrates involved 100cr6 bearing steel, al2024 aluminum alloy and (111) si wafer. although no clear influence of the substrate type on hardness and indentation modulus of the coatings at different acetylene and hydrogen additions was observed, when both measured properties were combined in hiteit ratio, significant shift of hit/eit ratio dependencies obtained on al alloy vs. steel and si wafer substrates occurred. thus, the effect of substrate type on the measurementlevaluation was confirmed. keywords: hardness, indentation modulus, substrate effect, w-c:h coating. 1. introduction tribological properties of hard coatings, especially their wear resistance, coefficient of friction (cof) depend on large number of parameters including loading and environmental conditions but also material system used for the coating, its structure, surface roughness and such properties as hardness, elastic modulus, and others. it was reported that the wear properties of hard coatings more strongly correlate with h/e ratio than just with the hardness itself [1]. moreover, it was also found out that the h/e and/or h3/e2 ratios are related to the level of plasticity and can be used to estimate changes in the toughness of the coatings [2–4]. this is of significant interest because toughness is among the main parameters, besides hardness, young’s modulus, cof, wear, scratch, oxidation and corrosion resistance of the coatings for engineering applications. the measurement of mechanical properties, involving hardness, elastic modulus and toughness in the coatings with the thickness in the range of few micrometers is a challenge even by nanoindentation because of the inevitable influence of the substrate on the measurement of the coating properties. to reduce this influence, the indentations depths used for the determination of coating hardness should be substantially smaller than the thickness of the coating. physical explanation is that the stress field under the indenter, corresponding to the zone with the stresses above the yield stress (zone of plasticity), has to be fully confined in the coating. so called “10 % (of the coating thickness) rule” is often applied to determine such maximum depth and true hardness of the coating, respectively [5–10]. another approach to eliminate substrate influence and to determine true coatings properties is to use hardness (and elastic modulus) – indentation depth profiles obtained from continuous multi-cycle (cmc) or continuous stiffness measurements (csm) [11]. the coating properties correspond to the maximum or plateau on the corresponding depth profile at the depths smaller than 10 % of the coating thickness. however, the liability of 10 % rule is limited by the accuracy of the measurement at coating thicknesses below 1 µm related to tip surface area calibration function, by indenter tip radius [12], surface roughness, pile-up or sink-in behavior [13], residual stresses as well as by the ratio between the yield stresses and the elastic moduli of the coating and the substrate, respectively [9, 14]. the measurements of indentation modulus are much more sensitive to substrate properties than hardness because the elastic stress field under the indenter would be much larger than the zone of plasticity and therefore, 10 % rule may not be sufficient. thus, the problem of proper measurement of hardness simultaneously with the elastic modulus in thin hard coatings persists and it is even emphasized by the applicability of the h/e ratio for toughness and wear resistance estimations. in our earlier works, the influence of indenter tip radius on the hardness – depth profiles has been analyzed using a combination of the experimental and finite element modelling approaches [12]. however, the influence of the young’s modulus of the substrate on the nanoindentation measurements was not investigated. therefore, the aim of the present work is to investigate the influence of the substrate properties on the measurements of hardness, and indentation modulus in thin hard coatings used for engineering applications. the measurements were performed on hipims (high power impulse magnetron sputtering) w-c and w-c:h coatings deposited using hybrid pvd-pecd (physical vapor deposition – plasma enhanced chemical vapor deposition) process with different acetylene and hydrogen additions on three different substrates. 62 http://dx.doi.org/10.14311/app.2020.27.0062 http://ojs.cvut.cz/ojs/index.php/app vol. 27/2020 coatings vs. substrate type hardness figure 1. load – indentation depth curves from the nanoindentation tests on the hipims w-c:h coatings deposited with 2 sccm c2h2 on si wafer, 100cr6 bearing steel and al 2024 alloy substrates – a) the depth profiles of hardness – b) and indentation modulus – c). 2. experimental material and methods the studied w-c:h coatings were simultaneously deposited on three different substrates: polished tempered bearing steel 100cr6 discs (diameter – 25mm, thickness – 3mm ), polished al2024 aluminum alloy discs with similar size and on approximately 1cm2 polished (111) si wafer fragments with ∼ 200nm cr bond layer. the deposition was carried out at a minimum working pressure of 0.5 pa in a magnetron deposition system (cryofox discovery 500, polyteknik, denmark) using high power impulse magnetron sputtering (hipims) source at 350 w average power achieved at 2.62% duty cycle related to the frequency of 150hz and impulse length of 175 µs. a set of 14 coatings with variable additions of c2h2 precursor (0, 2, 4, 6, 8 sccm) and (0, 5, 10, 20 sccm) into ar atmosphere was prepared for the study. the thicknesses of the coatings were in the range from 0.7 µm up to almost 3 µm depending on the amount of precursor gas additions [15–18]. the nanoindentation tests were carried out on a nanoindenter (model g200, agilent, usa) using a diamond berkovich tip in csm mode with the preset maximum penetration depth of 1000 nm, strain rate of 0.05 s−1, frequency of 45 hz and amplitude of 2 nm on a set of 16 indents. the hardness and indentation modulus depth profiles were obtained by averaging the results of at least 10 measurements. the young’s modulus and poisson’s ratio of the diamond tip used for calculations were etip = 1141 gpa and νtip = 0.07 , respectively. the indenter tip was calibrated on a fused silica reference sample. the poisson’s ratios of the coatings of v = 0.25 was used for the evaluation of the corresponding properties. 3. results and discussion 3.1. substrate effects on hardness and indentation modulus in hipims w-c:h coatings fig. 1a 1c illustrate the differences between load indentation depth curves, hardness – indentation depth and indentation modulus – depth profiles in the studied w-c:h coating deposited with the addition of 2 sccm c2h2, respectively. the load – displacement curve in the coating on al alloy differs significantly from those on si and steel substrates, which were practically identical, at the displacements above 400 nm 63 františek lofaj, margita kabátová acta polytechnica figure 2. the comparison of the hardness, hit − a) and indentation modulus, eit − b) in hipims w-c:h coatings deposited at different acetylene (and hydrogen) additions on three types of substrates. the data-points corresponding to 5, 10 and 20 sccm h2 additions were intentionally shifted by 0.2, 0.4 and 0.4 sccm respectively, for better visibility. (fig. 1a). however, this difference is not important for the determination of the hardness and indentation modulus, because they have to be determined from the indentation depths within the range of 100 300 nm due to the earlier mentioned 10 % rule. fig. 1b shows the depth profile of hardness: on si and steel substrates, very wide plateau at their maxima of around 20 gpa extending up to almost 500 nm depth can be seen. in contrary, the influence of substrate is much stronger in the case of measurement on al substrate: the onset of hardness decrease occurred already at 250 nm. such difference is a natural consequence of lower young’s modulus of al alloy in comparison with steel and si wafer. however, the maximum of the hardness profile obtained on al alloy overlapped with those on the other two substrates and essentially no difference of the substrate on hardness can be seen. however, the situation was different in the case of indentation modulus profiles. a plateau at around 225 gpa extending up to 500 nm can be seen on steel substrates but the measurement on si wafer resulted in a maximum plateau at around 220 gpa only up to ∼ 300 nm. the effect of al alloy substrate was much more pronounced: the maximum was around 175 gpa and it extended only to 150 nm indentation depth. the values of hit and eit determined from the corresponding depth profiles according to the above mentioned way are summarized in fig. 2a and 2b, respectively. they reveal number of effects and differences. they involve the effects of substrate type but also of acetylene and hydrogen additions during hybrid pvd-pecvd. the pvd process without additional precursor gases resulted in (possibly over-stoichiometric [18]) wc coatings exhibiting much lower values of hardness (22 gpa) and modulus (< 220 gpa) in the coatings deposited on al alloy substrates than on si and steel substrates (26 – 29 gpa and 280 – 340 gpa, respectively). it is known from earlier works on dc magnetron sputtered and hipims w-c:h coatings [17, 18] that the structure of the pvd coatings without additions of acetylene may vary from nano-columnar to nanocrystalline, which resulted in the variations of hardness from 34 gpa to 28 gpa, respectively. the additions of acetylene caused gradual increase of the content of hydrogenated free carbon phase and a transition from nanocomposite to amorphous structure accompanied by a decrease of hardness [16–18]. the substrate itself may also play significant role in the formation of the coating structure and its morphology [15]. without detail structure observations, it is difficult to decide, which of the mentioned effects could be responsible for the changes of hardness and indentation modulus in the studied w-c coatings. as expected, the additions of acetylene resulted in gradual decrease of these properties. there were certain variations among the values obtained on various substrates, but they were within the scatter bars of the measurements and the differences decrease, especially in the case of elastic moduli (fig. 2b). apparently, the amorphization of the structure at high acetylene additions seems to result in the unification of the properties and has stronger effect than the possible influence of the substrate. thus, the differences in coating properties deposited on different substrates seem to be statistically significant only in the cases without or with very small acetylene additions. however, they may interfere with the effects of coating structure and therefore, cannot be fully attributed to the substrate effect. figs. 2 also show the influence of hydrogen additions; it should be noted that the data-points corresponding to 5, 10 and 20 sccm h2 additions were intentionally shifted by 0.2, 0.4 and 0.4 sccm from the position of the corresponding acetylene addition to show the tendency of the changes without overlap. it can be seen that the hydrogen additions have different effects on hit and eit in pvd w-c coatings and in hybrid pvd-pecvd w-c:h coatings made with acetylene additions. in the first case, hydrogen additions resulted in the increase of hardness and elastic modulus 64 vol. 27/2020 coatings vs. substrate type hardness whereas they were decreased in the coatings made with 4 sccm and 6 sccm acetylene additions regardless of the substrate. moreover, the corresponding values principally followed the dependencies described for the coatings without hydrogen additions and agree with the above conclusions concerning exclusivity of w-c coatings compared to w-c:h coatings. the above results indicate only a small influence of substrate type on hardness and indentation modulus. however, they are measured simultaneously and it is reasonable to check the simultaneous effect of both properties on different substrates. thus, the results from fig. 2a and 2b were replotted as a function of hit/eit ratio on acetylene (and hydrogen) additions in fig. 3. the obtained plots show that the acetylene and hydrogen additions usually result in gradual hit/eit increase and that the difference between substrates was strongly magnified. although the differences between hit/eit in the coatings on 100cr6 steel and si substrates were negligible in the studied additions range, the corresponding values on al alloy substrates followed the same tendency but they were by 0.02 – 0.025 higher. since both hardness and indentation modulus decrease with the acetylene addition increase, the increase of hit/eit has to result from faster decrease of indentation moduli. another consequence of the shift of the dependencies between steel, si and al alloy substrates was that all hit/eit values obtained on al alloy were above the plasticity limit of 0.1 while the identical coatings deposited on si and steel substrates at 0 and 2 sccm acetylene (and hydrogen) additions would be below that limit. obviously, such systematic differences in hit/eit on the same coatings do not have real physical reason. they have to be related to the influence of the substrate type on the measurement method and/or data evaluation rather than on true hardness and indentation modulus changes. it is a clear indication that the substrate type definitely affects the nanoindentation measurements. the question is what is the main parameter controlling the influence of the substrate. young’s moduli of steel and si substrates are rather similar (210 gpa vs. 193 gpa) while that of al alloy is considerably lower (∼ 73 gpa) and this difference is reflected in the difference obtained fig. 3. however, the yield strength and hardness of substrate materials should play a role because they control plasticity of the substrate 4. conclusions the investigations of the influence of the substrate type on the measurements of hardness and indentation modulus in hipims w-c and w-c:h coatings with different acetylene and hydrogen additions on si wafer, 100cr6 steel and al 2024 aluminum alloy substrates revealed: • significant influence of substrate type on hit and eit in pvd w-c coatings deposited without the 0 2 4 6 8 0.075 0.100 0.125 0.150 hipims w-c:h coatings 5,10,20 h 2 5,10,20 h 2 h i t /e it ra tio x c 2 h 2 (+ y h 2 ) flow, sccm al substrate si substrate fe substrate 5,10,20 h2 al 2024 substrate si substrate 100cr6 substrate figure 3. the dependence of hit /eit ratio in hipims w-c:h coatings on the substrate type at different acetylene (and hydrogen) additions. note that the data-points corresponding to 5, 10 and 20 sccm h2 additions to the corresponding acetylene flows were intentionally shifted to emphasize the role of hydrogen additions. additions of acetylene. however, these differences may overlap with the influence of substrates on the coating structure. • an enhanced scatter of the measured properties in hybrid pvd-pecvd w-c:h coatings with the additions of acetylene on the studied substrates. the differences between the substrates were within the scatter of measurement. • when the effects of substrate type on hardness and indentation modulus were combined in hit/eit ratio, significant shift between the dependencies obtained on al alloy vs. steel and si wafer substrates was obtained. thus, the effect of substrate type on the measurement/evaluation was confirmed. • the effect of substrate properties on nanoindentation measurements cannot be related only to the difference in young’s moduli of the substrates and further investigations are required to find the way to eliminate this influence. the above conclusions imply that the reliable evaluation of the wear resistance and fracture toughness of thin coatings based on hit/eit ratio requires the elimination of the influence of the substrate type on hit/eit ratio. acknowledgements the support provided by the projects apvv-17-0320, apvv 17-0059, apvv-17-0049 and vega 2/0017/19 is acknowledged. the equipment used in the work was acquired from the projects “research centre of advanced materials and technologies for recent and future applications” promatech, 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https://doi.org/10.1590/0366-6913201965s12577 https://doi.org/10.1016/j.surfcoat.2019.07.078 https://doi.org/10.1016/j.ijrmhm.2020.105211 acta polytechnica 27(0):62–66, 2020 1 introduction 2 experimental material and methods 3 results and discussion 3.1 substrate effects on hardness and indentation modulus in hipims w-c:h coatings 4 conclusions acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.38.0084 acta polytechnica ctu proceedings 38:84–89, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague development of scientific requirements for climate-neutral buildings and sustainable funding schemes in germany sylvi eckart∗, juliane jäger, nicolas kerz, andreas rietz federal institute for research on building, urban affairs and spatial development (bbsr), division sustainable building, straße des 17. juni 112, 10623 berlin, germany ∗ corresponding author: sylvi.eckart@bbr.bund.de abstract. the german climate protection plan 2050 describes interim targets and measures for the implementation of the paris climate agreement, including the building sector. with the “new european bauhaus”, the eu commission has currently initiated an ambitious process. while new construction has been in the foreground up to now, the existing building stock must make a significant contribution to achieving the climate protection goals in the future. robust building technology and a high degree of adaptability lead to a longer useful life and are thus more sustainable, both economically and in terms of ecological balance. resilience to the consequences of climate change is gaining in importance. the main goal of this contribution is to inform about the latest developments in governmental policies and subsidy schemes. as part of the national climate protection programme 2030, the new “federal funding for efficient buildings (beg)” was introduced in 2021. research projects investigated how, with the introduction of efficiency house classes, funding can take greater account of the life cycle approach and other aspects of sustainability. at the same time, general and special requirements for the ecological, socio-cultural and economic quality were developed as a supplement to the existing certification systems and introduced by the federal building ministry as “quality seal for sustainable buildings” (qng). keywords: sustainable building, new european bauhaus, sustainability assessment, climate protection plan, federal funding. 1. introduction climate change creates new challenges for building. in terms of sustainable building, environmentally sound and energy-efficient construction as well as climatefriendly adaptation of buildings must be seen as a holistic task. the overarching guiding principle of sustainable development – based on the three dimensions: ecology, economy and socioculture represents the basis for the development of the principles for sustainable building in germany. this approach goes back to the “our common future” report published in 1987, in which the un brundtland commission outlined the guiding principle of sustainable development for the first time. with the 2030 agenda adopted in 2015, the united nations defines sustainable development via 17 sustainable development goals (sdgs) into global goals, subdivided into approximately 169 sub-goals. the revision of the national sustainability strategy and its adoption in january 2017 form the essential framework for national implementation. under the auspices of the federal ministry of building, a start was made as early as the late 1990s on defining sustainability for the building sector and implementing it in a practical manner. building on the recommendations of the enquete commission “protecting people and the environment – goals and framework conditions for sustainable development” and the experiences of the federal energy commissioner in the context of the government relocation, frameworks for sustainable building were derived. as early as december 2001, the former federal ministry of building and urban affairs set up the “round table on sustainable building” as a body to support its implementation. in the same year, in the spirit of setting an example, the first guideline for sustainable building [2] was introduced as a mandatory planning aid for federal building. due to the systematic nature of the requirements for the technical, functional, ecological, economic and social performance of buildings, the relevant sub-goals of the sdgs are now addressed and covered in the current version of the guideline. in a two-year cooperation between the federal ministry of building and the german sustainable building council (dgnb), the development of a national assessment system was advanced in germany in 2007/08, based on the national framework conditions, bodies of law, standards, etc. and designed for building in germany. in january 2009, a first set of national sustainability criteria including a calculation and evaluation methodology was presented to the professional public. this methodology follows a holistic assessment approach taking into account the life cycle of a build84 https://doi.org/10.14311/app.2022.38.0084 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 38/2022 development of scientific requirements for climate-neutral buildings . . . figure 1. scheme of the united nations sustainable development goals [1]. ing as well as a comprehensive quantification. for the regulatory area of federal building, the assessment system for sustainable building (bnb) was derived from this in 2010. 2. the climate protection plan 2050 with the “new european building”, the eu commission has currently initiated an ambitious and farreaching process. with regard to climate neutrality, sdg 13 “climate action” plays a central role. it essentially refers to the goals of the paris agreement by listing measures for greenhouse gas reduction and adaptation to impacts as well as financing and capacity-building instruments. the climate protection plan 2050 adopted by the federal cabinet describes interim targets and measures for implementing the paris climate agreement. it defines “nearly climate-neutral buildings” in such a way that these buildings have a very low energy demand, the remaining energy demand is covered by renewable energies and further greenhouse gas emissions are avoided. in the brochure “energy efficiency strategy for buildings”, the federal ministry for economic affairs and energy (bmwi) calls for primary energy demand to be reduced by a combination of energy savings and the use of renewable energies in the order of 80 percent by 2050 compared to 2008. in addition to considering the energy demand of buildings for heating/cooling and electricity, the federal ministry of building focuses on the entire life cycle. in addition to the operation of buildings, emissions are primarily caused by the consumption of resources for materials, constructions, construction site equipment and processes, as well as during the deconstruction of buildings. only if we succeed in significantly reducing resource consumption will there also be a chance to achieve the energy transition away from fossil fuels and towards renewable energies and to sustainably and permanently reduce co2 emissions. only if we succeed in significantly reducing resource consumption will we have a chance to achieve the energy transition away from fossil fuels and towards renewable energies and to reduce co2 emissions sustainably and permanently. so far, energyand co2-reducing concepts and normative ideas have been developed primarily in relation to new construction. however, the focus of attention is on existing buildings. a socio-culturally and economically careful development of the building stock, taking into account the ecological framework conditions, will have to make a significant contribution to achieving the climate protection goals. robust building technology and a high degree of adaptability generally lead to a longer useful life and are thus more sustainable, both in economic terms, in terms of resource consumption and in terms of life cycle assessment. in this context, keywords such as “simplicity” and “low tech” play an important role. at the same time, resilience to the consequences of climate change (e.g. weather events, periods of heat) is important in climate-adapted construction. 3. federal sustainable building assessment system (bnb) this results in the obligation to develop and implement each planning task as an integrated overall concept in terms of sustainability. the guiding principle of the german assessment system based on the equal consideration of economic, ecological and socio-cultural qualities. in the federal assessment system for sustainable building (bnb), which based on european standardisation, technical quality and process quality been introduced as cross-sectional qualities in addition to the classic pillars of sustainability described above. on the one hand, this is intended to represent the high technical standard of buildings customary in germany on the basis of the existing set of rules, 85 s. eckart, j. jäger, n. kerz, a. rietz acta polytechnica ctu proceedings figure 2. german environment agency house 2019 in berlin [3]. and on the other hand, it is also intended to document the necessity of integral planning and quality assurance during construction as a basis for achieving the sustainability goals. the site characteristics are not included in the actual building rating, but been shown separately in the certificate. the bnb system is continuously revised and adapted to legal changes; at the same time, criteria that take into account the consequences of climate change have currently been included, e.g. the criteria profile “resistance to natural hazards”. with the guideline for sustainable building, the “silver level” is required nationwide as a minimum standard for new construction and refurbishment in the area of civilian federal properties, and the gold standard is targeted for selected buildings. for example, an office building (house 2019) of the federal environment agency in berlin-marienfelde was planned as a zero-energy building with the aim of actively conserving resources. with the two-storey building in timber construction, a design as compact as possible was chosen for reasons of energy efficiency, whereby the elongated canopy roof created a lounge zone in front of the building and enabled shading and thermal protection of the conference rooms on the first floor. in addition to the ecological balancing function, the green roof achieves a synergy effect with the photovoltaic system installed there. as a result of extensive monitoring, the zero energy level of the building was confirmed in the annual balance sheet and a clear surplus of electrical energy could be shown overall, which benefits the property as a whole. the extension of the office building of the federal ministry for the environment, nature conservation and nuclear safety (bmu) in berlin-mitte is a lighthouse project for implementing the goals of climate protection and sustainable building in germany in an exemplary manner. as early as the competition stage, the scope and depth of the preliminary assessment of the competition entries were oriented towards the implementation of the bnb standard in gold. the results of the preliminary assessment were graphically prepared for the jury and presented during the jury meeting, in order to illustrate both, the distance from the respective target corridor and, above all, the comparability of the competition entries with regard to key sustainability aspects. thus, based on the bnb system, among other things, the quantitative determination of the environmental footprint (greenhouse potential), supported by life cycle assessment modelling, was assessed. the preliminary assessment estimated the overall sustainability potential of the entries based on derived pre-design-relevant individual indicators from 17 sustainability criteria of the bnb system. in order to accentuate the specificity of the competition – which relies on a decidedly low-tech strategy in terms of sustainability – three topics were prioritised in addition to the overall assessment: the quantitative determination of the environmental footprint, expressed in greenhouse potential based on life cycle assessment modelling, indoor air hygiene and daylight availability. 4. introduction of the federal subsidy efficient buildings in the context of sustainable planning and construction, the federal government is focusing on the further reduction of greenhouse gas emissions in the life cycle of buildings. this is intended to make an active contribution to climate protection and the achievement of political goals. energy-efficient building has a long tradition in germany. on 16 november 2001, the “ordinance on energy-saving thermal insulation and energysaving systems technology in buildings (enev)” was enacted for the first time and came into force on 1 february 2002. it replaced the previously valid 1995 thermal insulation ordinance and 1998 heating installations ordinance. with the new building energy 86 vol. 38/2022 development of scientific requirements for climate-neutral buildings . . . figure 3. model 1st prize – competition federal ministry for the environment, nature conservation and nuclear safety – extension, c.f. møller architects [4]. act (geg) passed by the bundestag on 18 june 2020, the enev, the energy conservation act (eneg) and the renewable energies heat act (eewärmeg) were merged into one modern law. in a further step, it is planned to further develop and specify requirements for sustainable planning and building in the context of climate protection. against the background that the climate neutrality of the building stock is to be achieved by 2045, a minimum kfw 55 standard was assumed for the eligibility and/or sustainability assessment of newly constructed residential buildings. the report initially proposes benchmarks for buildings with the efficiency house 55 (eh 55) and efficiency house 40 (eh 40) standards. within the framework of the national climate protection programme 2030, new approaches for government funding programmes were developed and the new “federal funding for efficient buildings (beg)” was introduced in 2021. the existing programmes to promote energy efficiency and renewable energies in the building sector will be bundled in a modernised, simplified and further developed funding offer. in this context, the federal government will promote sustainability aspects for the first time within the framework of the beg from 1 july 2021 through a separate “nh class”. the required proof for the funding is provided by awarding the building-related qng. with the beg, the promotion of energy efficiency and renewable energies will be brought together under one roof for the first time. for new buildings and complete renovations, the use of renewable energies will be rewarded even more. at the same time, there are new, attractive funding offers for particularly ambitious renovations and new buildings. an efficiency house/building eligible for funding in beg additionally achieves the “efficiency house nh” class or “efficiency building nh” class if it has been awarded a “quality seal sustainable building plus (qng-plus)” or “quality seal sustainable building premium (qng-premium)” on the basis of certification of the building in accordance with the guarantee market statutes and the seal documents. proof of the successful award of the “sustainable building quality label” certificate after completion of the building project must be submitted with the proof of use. the energy efficiency experts confirm that the “efficiency house nh” class was achieved with the award of the quality seal. proof of the certificate must be provided upon request. 5. quality seal sustainable building the “quality seal sustainable building (qng)” is a german national quality seal for buildings. the prerequisite for the award of the quality seal is proof of fulfilment of general and special requirements for the ecological, socio-cultural and economic quality of buildings. the fulfilment of the requirements is to be proven by an independent audit after completion of construction on the basis of the completed planning and construction processes and on the basis of the review of selected realised qualities. at the start of the quality seal in july 2021, seal variants will initially be offered for the new construction of residential buildings. when applying, accredited energy efficiency experts confirm that the measure fulfils the minimum requirements 87 s. eckart, j. jäger, n. kerz, a. rietz acta polytechnica ctu proceedings of the efficiency house level and that a certification “quality seal sustainable building” is planned. an efficiency building achieves the “efficiency building nh” class if it has been awarded a quality seal by an accredited certification body on the basis of a certification of the building. whether the “plus label” or “premium label” has been achieved has no influence on eligibility for funding or the type and scope of funding. buildings awarded the quality seal make a special contribution to sustainable development. the requirement levels make the fulfilment of above-average requirements (qng-plus) and clearly above-average requirements (qng-premium) for sustainabilityrelevant features and characteristics recognisable at a glance. certification bodies are the testing and awarding bodies for the “sustainable building quality label” and are responsible for the technical-operational processing of the testing of the requirements for the award of the quality label and its awarding. they are commissioned by the applicants, for example the building owners. in principle, only certification bodies that demonstrate and maintain accreditation within the meaning of regulation (ec) 765/2008 in accordance with iso/iec 17065 may be active in the area of awarding the quality seal. the sustainable building office at the bbsr maintains a publicly accessible list of accredited certification bodies. all information on the “quality seal for sustainable buildings”, including a list of the certification bodies that may act as testing and awarding bodies for the qng, as well as the assessment systems that may be used as a basis for the qng, are published on the sustainable building information portal [5]. the quality seal is further developed on the basis of research projects of the building innovation programme for the future. representatives of industry, associations, administration, politics and science will be involved in the further development via the round table on sustainable building of the federal ministry of building. in various research projects commissioned by the bbsr, it was investigated how, with the introduction of efficiency house classes, funding can in future take greater account of the life cycle approach as well as other aspects of sustainable building, such as biodiversity or accessibility. the qng-plus and qng-premium quality seals of the federal government are a state seal of quality for buildings. a prerequisite for the award of the quality seals is the fulfilment of the special requirements for the contribution of buildings to sustainable development. the building requirements formulated by the seal issuer are described with evaluation criteria and assessment standards in annex 3 to the quality seal manual. the following chapters describe the currently applicable building requirements. 5.1. greenhouse gas and primary energy the building may only be awarded qng-plus if the greenhouse gas emissions determined in accordance with the methodology of the appendix “lca balancing rules of the qng for residential buildings” are a maximum of 28 kg co2eq/m2a and the calculated primary energy demand non-renewable is a maximum of 96 kwh/m2a in the building life cycle. for qngpremium, the greenhouse gas emissions in the building life cycle must not exceed 20 kg co2eq/m2a and the calculated non-renewable primary energy demand in the building life cycle must not exceed 64 kwh/m2a. the aforementioned minimum requirements for limiting the global warming potential gwp100 and the primary energy demand were determined on a suitable data basis in the research project “development of lca benchmarks for residential buildings” [6]. the calculations were carried out using the calculation tool “elca”, which was provided by the bbsr. on the basis of the calculations carried out, evaluations are made in order to be able to propose suitable values (benchmarks) for the questions raised. the calculation results were transferred in an editable data format (elca exchange format). 5.2. sustainable material extraction the building may only be awarded qng-plus if at least 50 % of the timber, timber products and/or timber materials used can be proven to originate from sustainable forestry. for qng-premium, a limit of 80 % applies here. the condition applies that the regulated, sustainable management of the forest of origin is proven by presentation of a certificate. the following certificates are recognised for certificates are recognised for verification purposes: • pefc (programme for the endorsement of forest certification schemes) • fsc (forest stewardship council) 5.3. pollutant avoidance in building materials the building may only be awarded the qng-plus if the client has contractually obligated all companies carrying out the construction to comply with the qng quality requirements for the avoidance of pollutants and the companies declare their compliance after completion of their services. for qng-premium, compliance with the qng quality requirements for pollution prevention must be demonstrated for all materials and products used. the specific requirements are described in detail in a separate document. 5.4. accessibility the building may only be awarded qng-plus if at least 80 % of the residential units and the common areas of the building are shown to meet 7 of the specified 8 requirements of the “ready for visitors” standard. for qng-premium, compliance with 7 88 vol. 38/2022 development of scientific requirements for climate-neutral buildings . . . of the specified 8 requirements of the “ready plus” standard must be demonstrated. these requirements are listed in detail in the profile. 6. conclusions with the introduction of the new “federal promotion for efficient buildings (beg)”, the previous programmes for the promotion of energy efficiency and renewable energies in the building sector will be merged into a unified funding offer. the aim is to achieve a significantly greater reach and thus a broadly effective implementation of sustainable building in germany. in this way, the federal government is implementing an important political goal within the framework of climate protection. the quality seal for sustainable buildings implements quality control in the promotion of sustainability. the regulations anchored in it are the result of various research projects within the framework of the federal ministry of building’s departmental research programme zukunft bau. list of symbols bbsr federal institute for research on building, urban affairs and spatial development beg federal funding for efficient buildings bmi federal ministry of the interior, building and community bmu federal ministry for the environment, nature conservation and nuclear safety bmwi federal ministry for economic affairs and energy bnb assessment system for sustainable building dgnb german sustainable building council eewärmeg renewable energies heat act eneg energy conservation act enev ordinance on energy-saving thermal insulation and energy-saving systems technology in buildings geg building energy act gwp global warming potential lca life cycle assessment sdg sustainable development goal qng sustainable building quality label un united nations references [1] undp (united nations development programme). the sustainable development goals, adopted on 25 september 2015 as a part of the 2030 agenda. [2022-06-10]. https://unric.org/en/united-nationssustainable-development-goals/ [2] federal ministry of the interior, building and community. guideline for sustainable building, 3rd updated edition, berlin, 2019. [3] qatsi.tv gmbh & co.kg, german environment agency house 2019 in berlin. [4] w. mateyka. model 1st prize, competition federal ministry for the environment, nature conservation and nuclear safety – extension, c.f. moller architects. [5] federal ministry of the interior, building and community. sustainable building information portal. [2022-06-10]. https://www.nachhaltigesbauen.de/ [6] h. könig, a. hafner. teilprojekt: entwicklung von lca-benchmarks bei wohngebäuden projekt: rv handlungsplan nachhaltiges bauen, 2021. aktenzeichen: 10.08.17.7-19.13. 89 https://unric.org/en/united-nations-sustainable-development-goals/ https://unric.org/en/united-nations-sustainable-development-goals/ https://www.nachhaltigesbauen.de/ acta polytechnica ctu proceedings 38:84–89, 2022 1 introduction 2 the climate protection plan 2050 3 federal sustainable building assessment system (bnb) 4 introduction of the federal subsidy efficient buildings 5 quality seal sustainable building 5.1 greenhouse gas and primary energy 5.2 sustainable material extraction 5.3 pollutant avoidance in building materials 5.4 accessibility 6 conclusions list of symbols references acta polytechnica ctu proceedings doi:10.14311/app.2019.22.0094 acta polytechnica ctu proceedings 22:94–98, 2019 © czech technical university in prague, 2019 available online at http://ojs.cvut.cz/ojs/index.php/app using secondary raw materials in lightweight open-structure concrete with good utility properties martin sedlmajer∗, jiří zach, jan bubeník brno university of technology, faculty of civil engineering, centre admas, purkyňova 139, 612 00 brno, czech republic ∗ corresponding author: sedlmajer.m@fce.vutbr.cz abstract. the paper presents the results of research in lightweight concrete with open structure made using a lightweight porous foam-glass aggregate produced from recycled glass powder. the goal was to develop lightweight concrete. in order to achieve the best possible properties while reducing binder content, the concrete was reinforced with by-product fibres, which helped reduce the weight of the concrete while delivering satisfactory mechanical properties. in the paper are proposed lightweight concrete with open structure made using foam-glass aggregate. mechanical, thermal-insulating and acoustic properties were determined on lightweight concrete. designed concrete is only made of crushed lightweight foam-glass aggregate with a combination of portland cement with the option of adding recycled pet fibres. the new concretes possess a very good ratio of thermal insulation to mechanical properties as well as good sound absorption. keywords: lightweight concrete, lightweight aggregate, recycled fibres, thermal insulation. 1. introduction concrete is nowadays one of the most widespread building materials and is indispensable for modern civil engineering. as the industry develops, hand in hand with the performance requirements for building materials, lightweight concrete has assumed a position where it cannot be replaced by any other kind of concrete. lightweight concrete combines thermal insulation and acoustic properties with good mechanical properties and thus can be used in the construction of new types of single-layer structures with multifunctional properties. the reduced bulk density of this concrete saves material and energy during construction. depending on the requirements, there are several methods of manufacturing lightweight concrete. the most common means of weight reduction is using a lightweight filler instead of a dense aggregate. this filler is usually a lightweight aggregate. it can be natural or artificially manufactured. artificial aggregates are used especially in cases where no sources of natural aggregate are available. one such material is foam glass, which is made from recycled glass powder. foam glass is manufactured artificially and is a very good source of lightweight aggregate. there are several varieties of lightweight foam glass aggregate, depending on manufacturing technology. they can be round pellets or sharp, angular particles produced by crushing larger sheets of foam glass. both methods yield different fractions, which are then separated into broad or narrow according to need. this is important for optimal formula design, specifically, the correct particle size distribution. foam-glass aggregate has properties that make it suitable for use in both reinforced and non-reinforced applications, specifically fills, foundation insulation, or concrete aggregate [1], where it provides a considerable weight reduction and alters both freshand hardenedstate properties. the aggregate reduces bulk density and thus improves thermal insulation properties far above traditional concrete that uses natural dense aggregate [2]. a disadvantage of foam-glass aggregate are typically poorer mechanical properties, resulting from the reduced bulk density because aggregate is a dominant component of every concrete and the main provider of mechanical properties. lightweight concretes with foam-glass aggregate can also be used in places where ordinary dense concrete is unsuitable, e.g. lightweight structures, thermal insulation and eliminating thermal bridges [3], improving fire resistance [4], etc. it still preserves all of the benefits of concrete, the most important being its mouldability. 2. test mixtures and methodology the research was performed with a lightweight crushed foam-glass aggregate produced by refaglass s.r.o. it is an angular aggregate made by crushing larger pieces of foam glass and separating them by desired fraction. the feedstock for foam glass manufacturing is recycled glass powder, most often types of bottle glass that are difficult to use elsewhere in the glass industry. in addition to glass powder, another important ingredient for foam glass manufacturing is the foaming agent, which causes expansion at high temperatures (around 900 °c) in electric furnaces. the refaglass aggregate 94 http://dx.doi.org/10.14311/app.2019.22.0094 http://ojs.cvut.cz/ojs/index.php/app vol. 22/2019 using secondary raw materials in lightweight open-structure concrete. . . component/mixture no. 1 2 3 4 5 6 7 portland cement [kg] 250 200 150 130 100 130 100 aggregates from foam glass 4–16mm [l] 1000 1000 1000 1000 1000 1000 1000 polycarboxylate ether superplasticizer [kg] 2 1.6 1.2 1 0.8 1 0.8 water 106 61 57 53 51 53 65 pet fibres [kg] 6.5 5.0 table 1. mixtures of lightweight concrete. used in this research was of a 4–16mm fraction (particle size of 4 to 16mm). this fraction was used in the manufacturing of the experimental lightweight concretes with open structure. drawing on previous research, the mixtures of the new concretes were designed. there were 7mixtures in total, out of which mixtures 1-5 contained no fibres and differed in binder content, which descended from 250 kg/m3 down to 100 kg/m3. mixtures 6 and 7 contained recycled pet fibres provided by ciur a.s. at the amount of 5% of cement weight. plasticiser content was uniform in all mixtures as well; 0.8% of cement weight. table 1 shows the composition of the mixtures. the lightweight concretes were observed for key properties so as to compare their mixtures in fresh and hardened state. the basic properties to be observed included freshand hardened-state bulk density, compressive strength, and thermal conductivity. mixtures were produced according to the formulae described above. the following tests were performed on the fresh mixtures: • determination of consistency in fresh concrete en 12350-2 [5], • determination of bulk density in fresh state en 12350-6 [6]. and in hardened state: • determination of bulk density in hardened state en 12390-7 [7], • determination of compressive strength en 12390-3 [8], • determination of thermal conductivity coefficient en 12667 [9] (according to iso 8301), • determination of sound absorption coefficient iso 10534-1 [10]. the recycled pet fibres had the thickness of ca. 13µm and length of 6.3mm. figure 1 shows a recycled pet fibre. 3. results and discussion the new lightweight concretes with open structure were first tested for consistency [5] and bulk density in fresh state [6]; the values are listed in table 2 and in figure 2. it was necessary to watch the consistency because of the danger of cement paste segregation due to the great difference between the bulk density of both figure 1. detail image of pet fibre. materials. this is why the cement paste consistency was set so as to ensure that all the foam-glass particles are completely coated in the paste without it flowing through and accumulating at the bottom. once correct consistency was determined, attention was paid to measuring hardened state properties, primarily compressive strength [8] and bulk density [7]. table 3 shows the measured values. figure 3 shows the values of compressive strength and bulk density in a hardened state. the reduction of binder content, which entailed reducing cement content from the original 250 kg/m3 (mixture 1) down to 100 kg/m3 (mixture 5), which is more than a half, resulted in a loss of mechanical properties, making the concrete poorly bonded and prone to crumbling. looking at mixture 7, the combination of 100 kg of portland cement with recycled pet fibres improved the concrete’s bond and compressive strength, which reached 1.2mpa. the benefit of the fibres is clear in this case. a similar effect can be seen in mixtures 4 and 6, both of which contained pet fibres and thus profited from an improvement in mechanical properties, specifically compressive strength, by more than 100%. next, the hardened concrete specimens were tested for steady-state thermal conductivity by the hot plate method (at a mean temperature of +10 °c and a temperature gradient of 10 °c) [9]. before that the test the specimens were dried at 105 °c down to stable mass. the results are listed in table 4 and plotted in figure 4. 95 m. sedlmajer, j. zach, j. bubeník acta polytechnica ctu proceedings properties/mixture no. 1 2 3 4 5 6 7 bulk density in fresh state [kg] 860 740 580 500 510 670 540 slump test [mm] 20 0 0 0 0 0 0 table 2. properties of the lightweight concrete in fresh state. figure 2. bulk density and consistency of fresh lightweight concrete. properties/mixture no. 1 2 3 4 5 6 7 bulk density in hardened state [kg] 780 670 550 460 500 600 490 compressive strength [mpa] 3.6 2.9 1.5 0.8 0 1.8 1.2 table 3. bulk density and compressive strength of lightweight concrete in hardened state. figure 3. compressive strength and bulk density of hardened lightweight concrete. properties/mixture no. 1 2 3 4 5 6 7 thermal conductivity coefficient [w/(m·k)] 0.1529 0.1419 0.1223 0.1112 0 0.1214 0.1013 table 4. thermal conductivity coefficient of lightweight concrete in hardened state. 96 vol. 22/2019 using secondary raw materials in lightweight open-structure concrete. . . figure 4. thermal conductivity coefficient and bulk density of hardened lightweight concrete. figure 5. sound absorption coefficient of hardened lightweight concrete 2. figure 6. overview of the calculated single-number values of sound absorption čsn en 1793. 97 m. sedlmajer, j. zach, j. bubeník acta polytechnica ctu proceedings the results of thermal conductivity tests do not show any major effect of the fibres on thermal conductivity. as could be expected, however, what matters more is cement content. the last parameter was the sound absorption coefficient, which expresses how well a material can absorb incoming acoustic energy. the measurements were performed using an acoustic interferometer according to iso 10534-1 [10]. the measurement was made within 100–6300hz in one-third octave bands. the measured values were used in the calculation of single-number values of sound absorption dlα [db] in accordance with results are listed in figure 6. the progress of the sound absorption coefficient is plotted in figure 5. the results show that mixtures 2 and 6 perform the best. 4. conclusions lightweight crushed foam-glass aggregate can be used for manufacturing open-structure concrete with a good ratio of thermal insulation and mechanical properties. the results show that a reduced amount of cement results in poorer mechanical properties, but better thermal insulation. reducing the cement content down to 100 kg/m3 resulted in a weak, insufficiently bonded concrete. the addition of recycled pet fibres to the mixture with 130 kg of cement raised its compressive strength by 125% from 0.8mpa to 1.8mpa. mixture 7 with 100 kg of cement and pet fibres had compressive strength of 1.2mpa. this corresponds to a mixture without fibres with a cement content of ca. 140 kg of cement. it can thus be said that the addition of pet fibres brought a dramatic improvement of the concrete’s properties and enabled a significant reduction of cement content without damaging the mechanical properties. in mixture 7 the reduction of cement content and addition of fibres further reduced the concrete’s thermal conductivity. its final value was 0.1013w/(m·k), which makes this concrete suitable for constructing structures with excellent thermal insulation properties. the newly developed lightweight concrete also shows very good acoustic properties (compared to ordinary dense concrete); mixtures 2 and 6 performed the best in this regard. acknowledgements this paper was written under the project no. lo1408 “admas up advanced materials, structures and technologies”, supported by the ministry of education, youth and sports under the “national sustainability programme i” and project fv20086 “development of light new-age building materials with utilization of light-weight aggregate based on waste glass powder” supported by the ministry of industry and trade. references [1] m. limbachiya, m. s. meddah, s. fotiadou. performance of granulated foam glass concrete. construction and building materials 28(1):759–768, 2012. doi:10.1016/j.conbuildmat.2011.10.052. [2] m. b. janetti, t. plaz, f. ochs, et al. thermal conductivity of foam glass gravels: a comparison between experimental data and numerical results. energy procedia 78:3258–3263, 2015. doi:10.1016/j.egypro.2015.11.713. [3] s. real, m. g. gomes, a. m. rodrigues, j. a. bogas. contribution of structural lightweight aggregate concrete to the reduction of thermal bridging effect in buildings. construction and building materials 121:460– 470, 2016. doi:10.1016/j.conbuildmat.2016.06.018. [4] c.-g. go, j.-r. tang, j.-h. chi, et al. fire-resistance property of reinforced lightweight aggregate concrete wall. construction and building materials 30:725–733, 2012. doi:10.1016/j.conbuildmat.2011.12.081. [5] european standard en 12350-2 testing fresh concrete part 2: slump-test. european standard, 2009. [6] en 12350-6 testing fresh concrete part 6: density. european standard, 2009. [7] european standard en 12390-7 testing hardened concrete part 7: density of hardened concrete. european standard, 2009. [8] european standard en 12390-3 testing hardened concrete part 3: compressive strength of test specimens. european standard, 2009. [9] european standard en 12667 thermal performance of building materials and products determination of thermal resistance by means of guarded hot plate and heat flow meter methods products of high and medium thermal resistance. european standard, 2001. [10] iso 10534-1 acoustics determination of sound absorption coefficient and impedance in impedance tubes part 1: method using standing wave ratio. european standard, 1999. 98 http://dx.doi.org/10.1016/j.conbuildmat.2011.10.052 http://dx.doi.org/10.1016/j.egypro.2015.11.713 http://dx.doi.org/10.1016/j.conbuildmat.2016.06.018 http://dx.doi.org/10.1016/j.conbuildmat.2011.12.081 acta polytechnica ctu proceedings 22:94–98, 2019 1 introduction 2 test mixtures and methodology 3 results and discussion 4 conclusions acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2020.26.0126 acta polytechnica ctu proceedings 26:127–133, 2020 © czech technical university in prague, 2020 available online at https://ojs.cvut.cz/ojs/index.php/app mechanical testing and numerical modelling of porous structures improving oseintegration of implants petr vakrčka∗, aleš jíra, petra hájková czech technical university in prague, faculty of civil engineering, department of mechanics, thákurova 7, 166 29 prague 6, czech republic ∗ corresponding author: petr.vakrcka@fsv.cvut.cz abstract. implants, such as dental, are ordinary devices in medical care nowadays, even though they are quite expensive. in the present study, the use of trabecular and gyroid structures as external layer of implants is examined. the advantage of porous structures compared to surface modification of compact implants is the possibility to be fabricated by additive manufacturing together with the whole implant. the additive manufacturing also allows us to produce various shapes with controlled porosity for bone ingrowth. the design of 6 types of trabecular and 4 types of gyroid structures is part of the study. the trabecular structures are strut-based, whereas the gyroid structures are based on a wall system. the study is focused on mechanical testing of samples which were 3d printed from the titanium alloy ti6al4v. the gyroid structures, which we evaluated as more reliable, were chosen for numerical modelling. other observed advantages and disadvantages of the structures are also discussed. keywords: implant, cancellous bone, trabecular structure, gyroid, titanium alloy, additive manufacturing, numerical model. 1. introduction thanks to modern medicine and advancing technological progress, people across the world live longer than ever before. the technology has become vital for giving patients the best care as possible. many patients in the past had to suffer because there was no technology which could support them. nowadays, people do not have to suffer anymore because technology is on their side. since engineers started improving medical care, new medical devices have been created, modern software has been adapted, and technology has increase hospitality for every patient. for example, laparoscopic surgeries have become the standard for many operations. they are minimally invasive, cause less pain, and shorten the recovery time. in recent years, the next great achievement in the field of medicine is targeted cancer therapy, which can interfere with the spread of cancer by blocking cells involved in tumor growth. worth mentioning is also czech plastic surgeon bohdan pomahač, who performed the first full face transplant, and the list of recent achievements in the medical field goes on. however, the presented study focuses on the particular field of medicine which is implantology. the evolution made the body as perfect as possible. that is why the attempt to adequate reconstruction has been close to pursuing the holy grail. for this reason, the majority of attempts have been unsuccessful. the human effort has made massive progress since the first primitive implants from ancient egypt until custom-made implants made from titanium alloys by 3d printers. at first, the implants meant to be for privileged ones. nowadays, implants have become a device which is available for the ordinary people. even now, there are many challenges for engineers. the crucial topic is the contact area between an implant and living tissue. the presented study sets a goal to knock down the problem by using a different approach than most of the current implants. in general, commonly used compact implants need various surface modifications for adequate oseintegration. thanks to additive manufacturing, we can print almost every shape using a 3d printer. it allows us to substitute the surface modification with a suitable structure which can minimize the risk of implant loss. trabecular and gyroid structures, which can be seen all around ourselves, can be the answer to this complicated question. figure 1 shows examples of presence of the structures in nature. 2. geometrical model the main aim of the study is to design an artificial model of 3d-printed microstructure with similar properties as cancellous bone. it could allow us to design an implant that does not need a surface modification. we substitute the surface modification with porous micro-structures. this method has a variety of reasons. first of all, there is no universal approach of surface modification suitable for all biomaterials, but the porous structures can be used for all additive manufactured products. the second huge advantage is the control of pore size. this gives us a chance to 126 https://doi.org/10.14311/app.2020.26.0126 https://ojs.cvut.cz/ojs/index.php/app vol. 26/2020 title figure 1. various application of trabecular structures in nature [1]. in the top we can see the structure of leaf. in the bottom, we can see trabecular bone morphology [2]. optimize the pore size to maximize the bone ingrowth. in the end, if we change the porosity of the microstructure, we can modify young’s modulus to minimize the stress shield effect which can lead to resorption of the human bone. at first, trabecular (strut-based) structures were selected. after designing and fabrication of the samples, the microstructure was observed due to problematic different cooling rate which is caused by placing one layer upon the other during fabrication process. thin bars seemed to be prone to fracture due to cooling process of the structures. these fractures can split in time and lead to necrosis or aseptic loos of implant. it was assumed that the change of morphology could give us more suitable structure and the problem could be eliminated. what is more, the strength of trabecular structures was not as high as it had been expected, probably due to the fractures. this was the main reason why structures based on a wall system were chosen for mechanical testing and numerical modeling in the next step. namely, the gyroid structures were selected for its constant pore size throughout the model. so the next goal of this study was to design 4 gyroid structures with different thickness of the walls and size of the pores and to test the structures mechanically for verification of the numerical models. gyroid is a continuous and triply periodic cubic morphology with a constant curvature surface across a range of volumetric fill fractions. it can be found in a variety of natural and synthetic systems [3]. the most convenient approximation of the gyroid structure for our purpose is through a level surface. it is defined by the function: f (x, y, z) = t , (1) where f determines the form of the surface via spacedividing function and t is the constant which determines the volume fractions of the divided space. if we set the parameter t to zero and substitute the f with an appropriate function, we receive the equation: sin(x̃) ∗ cos(ỹ) + sin(ỹ) ∗ cos(z̃) + sin(z̃) ∗ cos(x̃) = 0, (2) where x̃, ỹ and z̃ are scaled spatial ordinates, such that x̃ = 2π ∗ x/a, ỹ = 2π ∗ y/a, z = 2π ∗ z/a. the variable a is the cubic unit cell edge length [4]. we designed 4 types of gyroid structures in total. specimens of all the gyroid structures had same dimensions as the trabecular structures – 14×14×14 mm. the pore size was optimized based on the literature. for example, the optimal pore size for bone ingrowth is 600 µm according to taniguchi et al. [5]. hulbert et al. [6] mentioned that the pore size cannot be smaller than 100 µm for successful bone ingrowth. based on the available literature, the range of 350–800 µm was considered as applicable. gyroid structures number 1, 2 and 3 fulfilled the criterion for bone ingrowth according to the literature, gyroid structure number 4 was slightly out of range. the pore sizes of the designed trabecular and gyroid structures are summarized in table 1. 3. mechanical testing the trabecular and gyroid structures which were produced by additive manufacturing underwent mechanical testing. the processes of manufacturing and mechanical testing were similar for both types of the structures. the goal was to obtain material characteristics of the structures and compare them with the results of the numerical simulation, following numerical model verification. the essential material characteristic for this purpose was the global modulus of elasticity (or global elastic modulus) of each structure. mechanical testing was performed in the form of compression tests. for this purpose, at least three specimens of each structure were produced, 21 specimens of the trabecular structures and 12 specimens of the gyroid structures in total. tests were executed according to international standard iso 13314:2011 mechanical testing of metals – ductility testing – compression test for porous and cellular metals. the standard is intended for porous and foam metals with porosity higher than 50 %. the compression tests were performed on the machine mts alliance rt-30 and rt-50 (mts, usa), which the faculty of civil engineering ctu prague possesses. the maximal load of 30 kn for the trabecular structures and 50 kn for the gyroid structures was 127 p. vakrčka, a. jíra, p. hájková acta polytechnica ctu proceedings a) b) figure 2. the stress-strain curve which is part of the standard for compression testing of porous and foam metals (iso 13314:2011) with specific characteristics, such as 1 – quasi-static gradient, 2 – elastic gradient, 3 – the first maximum compressive strength (σfirst,max), 4 – the compressive offset stress (σ0.2). used. the tests were executed in the laboratory at room temperature. the load direction was the same as the direction of additive manufacturing and thus perpendicular to the printed layers. this was important because of orthotropic behavior, which is caused by the technological procedure. the specimens were loaded by remote displacement and the velocity of deformation was set to 1 mm/min which corresponds to the standard. material characteristics of the trabecular and gyroid structures were based on stress-strain curves which were obtained in the compression tests. the stress was calculated from the following equation: σ = f a , (3) where σ stands for stress, the a stands for the area of specimen cross-section and f stands for the force which is applied perpendicularly to the area of specimen cross-section. the strain (or relative deformation) of the specimen was calculated according to the following equation: ε = ∆h h , (4) where ε stands for strain, ∆h stands for measured deformation and h is the original height of the specimen. as we mentioned earlier, the most important mechanical characteristic for our purpose was the global modulus of elasticity (e). then we also determined the first maximum compressive strength (σfirst,max) and compressive offset stress (σ0.2). the first maximum compressive strength was set as the first local maximum of the stress-strain curve as can be seen in figure 2a. the compressive offset stress is defined as the stress in material when the material reaches the plastic compressive strain equal to 0.2 % (figure 2b). the quasi-static gradient specifies this moment. furthermore, the angle of inclination of quasistatic gradient was used to determinate the global elastic modulus. although the quasi-static gradient is not equal to the global elastic modulus according to the standard, the curve inclination in this point is the most suitable for the global elastic modulus. at the beginning, the stress-strain curve does not correspond with the real loading progress. behind the point of inflection, the inclination is significantly influenced by the plastic deformation. in more details, we can see the determination of quasi-elastic gradient in ffigure 2b. according to the figure, it can be stated that the quasi-static gradient is defined as the tangent line inclination of the stress-strain curve in the point of inflection. this point divides the curve into the convex and concave parts. the curve concavity is due to plastic deformation. numerically, the point of inflection is specified as the point where the curve inclination starts lowering. it can be calculated from the equation: ∆e = σi εi − σi−5 εi−5 , (5) where σi stands for the stress in the i-step of measurement, εi is the strain in the i-step of measurement, σi−5 stands for the i-5-step of measurement, and finally the εi−5 is the strain in the i-5-step of measurement. the inclination change was set after 5 steps to eliminate the error possibility caused by the measurement deviation of the applied force (f ) and deformation (∆h). the next step in determination of the compressive offset stress is detection of the point which is the intersection of the quasi-static gradient and the x-axis (ε = 0). numerically, the point can be computed: x0 = − b a , (6) where a,b stands for constants in linear equation of quasi-static gradient in the slope-intercept form. 128 vol. 26/2020 title when the plastic deformation 0.2 % is added (ε = 0.2), the intersection of the line with the stress-strain curve gives us the compressive offset stress. the process can be seen again in figure 2b. the slope-intercept equation of the new line is define as: y = a ∗ x+ c c = (x0 + 0.2) ∗ a , (7) where a still stands for constant of quasi-static gradient and also for the global elastic modulus of the relevant structure. numerically, the intersection of the mentioned line with the stress-strain curve is the point where following condition is fulfilled: |a ∗ εi + c| = |σi| , (8) where σi and εi stands for stress and strain in the istep of measurement. in addition, the last but not less important material characteristic is the porosity of the structures. porosity was determined as: n = 1− m− (vhom ∗ ρt i6al4v ) a ∗ h ∗ ρt i6al4v , (9) where m is the weight of the specimen, vhom stands for volume of the homogenous part of specimens, a and h are the dimensions of the porous structures and ρt i6al4v stands for a density of titanium powder which was used to manufacture the specimens. the porosity was used for comparison between the trabecular and gyroid structures. it is assumed that the higher porosity would correspond to the lower global modulus of elasticity. the results of mechanical testing (the compression tests) for each structure are listed in table 1. there are mean values for all sets of the trabecular and gyroid structures. based on the experiment, we can definitely conclude that the global elastic modulus of all the structures is significantly lower than the elastic modulus of the structure material ti6al4v, which is equal to 115 gpa according to the catalogue list. furthermore, the global elastic modulus of all the structures is close to the elastic modulus of human cancellous bone, which is approximately 2.71–9.10 gpa [7]. the crucial part of the design was to adapt global elastic modulus of the structures to the modulus of bone as much as possible to avoid negative consequences which are called stress shield effect. thus, the design of structures met the requirement. the global elastic modulus of trabecular structures is dependent on porosity as we expected. the greatest elastic modulus (3.822 gpa) was showed by the structure with the lowest porosity (0.446) which was rhombic dodecahedron (1.25 mm). on the other hand, the structure with the greatest porosity (0.653), which was rhombic dodecahedron (1.00 mm), had the lowest elastic modulus (2.631 gpa). however, we were surprised that a small difference in the porosity of structure diamond (1.00 mm) and diamond (0.75 mm) should have caused the difference of 18 % in the elastic modulus. the same effect can be seen between structures dode thick (1.00 mm) and diamond (0.75 mm). it is probably caused by the fractions of struts which are discussed at the end of the section. the comparison of trabecular and gyroid structures did not report the same phenomenon. gyroid structure nb 1 with the lowest porosity (0.413) showed significantly lower global elastic modulus (3.048 gpa) than trabecular structure rhombic dodecahedron (3.822 gpa) with similar porosity (0.446). more generally, the gyroid structure showed lower global elastic modulus than the gyroid structures with the same porosity. it is again important to mention that elastic modulus of both types of structures (trabecular and gyroid) was close to the elastic modulus of cancellous bone – 2.71–9.1 gpa. we can conclude that the differences in elastic modulus are not a suitable property for comparison of reliability of the structures. however, the first maximum compressive strength and the compressive offset stress showed huge differences across the structures. it is also important to mention that the compression tests of trabecular structures were executed by using a machine with a lower maximal force of 30 kn. this is the reason why the strength of dode thick (1.00 mm) and rhombic dodecahedron (1.25 mm) were not able to be obtained. although some of the values are unknown, we can state that, in general, the gyroid structures feature higher strength. this fact is crucial because strength is related to the durability of the implant. for our purposes, the global elastic modulus is mainly important in relation to the stress shield effect. on the contrary, the strength is related to the loading which implant can withstand before failure. in the figure 3, we can see the stress-strain curves of all the structures. for clarity, we chose just one characteristic strain-stress curve for each type of the structures. based on the figure, we can see differences in the strength of structures. the trabecular structures have much lower strength than the gyroid structures. what is more, there is a difference at the beginning of the curves. if we take a closer look, we can see that the gyroid structures are linear from the beginning. on the contrary, the trabecular structures have a non-linear beginning. it is assumed that this behavior was due to fractures which were initiated during the fabrication of trabecular structures. it took some time until the material “settled”. the fractures and its origin were described in details in our last paper [8]. in conclusion, we can state that the differences in the elastic modulus of trabecular and gyroid structures are negligible for our purpose. the main difference is that the gyroid structures have higher compressive strength than the trabecular structures. thus, the gyroid structures withstand more loading before failure. 129 p. vakrčka, a. jíra, p. hájková acta polytechnica ctu proceedings type of porous structure pore size e σfirst,max σ0.2 n [µm] [gpa] [mpa] [mpa] [-] gyroid structure nb 1 (1.40 mm) 400 3.048 228.501 166.990 0.413 gyroid structure nb 2 (1.80 mm) 450 2.868 214.805 161.554 0.473 gyroid structure nb 3 (2.40 mm) 750 2.837 190.670 157.764 0.496 gyroid structure nb 4 (3.00 mm) 850 2.772 191.434 154.119 0.515 diamond (0.75 mm) 350 2.884 88.649 86.647 0.540 diamond (1.00 mm) 450 3.508 141.924 126.449 0.549 dode thick (1.00 mm) 500 3.713 n/a 142.218 0.547 dode thick (1.25 mm) 630 2.838 98.211 84.694 0.577 rhombic dodecahedron (1.25 mm) 640 3.822 n/a n/a 0.446 rhombic dodecahedron (1.50 mm) 800 2.631 90.201 78.388 0.653 table 1. the pore size and mean values of the mechanical properties for each type of the trabecular or gyroid structures. figure 3. the overview of the stress-strain curves of trabecular and gyroid structures. we can see quite similar elastic modulus but significantly different compressive strength. 130 vol. 26/2020 title 4. numerical modelling during the compression tests and the microscopical analysis, we evaluated the gyroid structures as more reliable structures than the trabecular ones. it was mainly because of the fractures of struts in the trabecular structures, and because of greater strength of the gyroid structures. for these reasons, we focused only on processing of numerical model of the gyroid structures. the main problem was the process of geometrical modeling and meshing of the structures. it took lot of time to successfully model an appropriate structure without singularities in any modelling software. the most suitable way to achieve the structure without problems was directly in netfabb software by autodesk company. the huge constrain was the limit of ansys academic license. after taking into account the constrain, only element size of gyroid structures was modeled to represent the whole structure. according to gyroid structure number 1, the dimension of cutting box was set to 3 mm. all models were meshed successfully by the automatic meshing method. we had to lower the accuracy of gyroid plotting for structures number 3 and 4 because of the academic license constrain. loading program was set according to iso 13314:2011 mechanical testing of metals – ductility testing – compression test for porous and cellular metals which specify a test method for compressive properties of porous and cellular metals with a porosity of 50 % or more. therefore, all the specimens had to be loaded by the remote displacement 1 mm/min. in general, the rigid body has 6 degrees of freedom which we need to constrain. in consonance with the experiment, we fixed all the degrees on one side of the gyroid structure by fixed support. on the other side, we set the loading by remote displacement. the remote displacement loading had rigid behaviour and ramped rotation around all axes in order to represent the experiment as much as possible. we assumed just elastic part of the stress-strain curve because of implant safety reasons. we do not accept the state when there is plastic deformation, which can lead to the loss of implant. during observation of stress-strain curves of the gyroid structures, we noticed that 150 mpa is the point when all the gyroid structures start the process of hardening. naturally, this value of stress is achieved under different forces based on the gyroid morphology and different strain. now, we would like to discuss the computation of model input for one of the gyroid structures in details. first of all, we had to state the force reaction which is caused by remote displacement. we obtained the value of force which caused stress equal to 150 mpa figure 4. the deformation of the gyroid structure. the unit of deformation is meter. from the experiment data: fmodel gyroid = fexperimental gyroid ∗amodel gyroid aexperimental gyroid fmodel gyroid = fexperimental gyroid ∗ 3 ∗ 3 14 ∗ 14 , (10) where fmodel gyroid stands for force reaction caused by remote displacement, fexperimental gyroid stands for value of force obtained from the experiment which caused stress equal to 150 mpa, amodel gyroid stands for contact loading area of the gyroid which underwent the experimental testing, amodel gyroid stands for contact loading area of gyroid model. the calculated force had to correspond with the force probe on the top of the loaded model. the next step was to determine the final deformation of model. for our purposes, a simplification that strain is equal throughout the structure had to be made. afterward, we could compute the required deformation of our model: umodel gyroid = uexperimental gyroid ∗ hmodel gyroid hexperimental gyroid umodel gyroid = uexperimental gyroid ∗ 3 14 . (11) we can see the deformation of gyroid structure in figure 4. in the table 2 we can find the results of curvefitting approximations. we can see that the elastic modulus of material ti6al4v is significantly lower than value according to the material list which is 115 gpa. it is assumed that the reason for such a high difference is due to the problematic loading of samples. the producer of gyroid samples placed a selvage on the edge of platforms. it caused that the loading was distributed mainly through outer parts of gyroid structures, whereas the loading of the numerical model was placed on the complete platform. 131 p. vakrčka, a. jíra, p. hájková acta polytechnica ctu proceedings type of gyroid structure force probe strain of model elastic modulus of material [n] [%] [gpa] gyroid structure nb 1 1350 5.10 14.57 gyroid structure nb 2 1350 5.80 12.98 gyroid structure nb 3 1350 6.10 12.20 gyroid structure nb 4 1350 6.25 11.11 table 2. mean values of the mechanical properties for each of the designed gyroid structures. 5. conclusions the goal of the presented study was to design an effective porous structure which can be used for different types of implants on the interface between bone and artificial material of implant. it can substitute contemporary compact implants with necessary surface modification, and subsequently help to avoid the stress shielding effect and maximize the bone ingrowth. in order to achieve the goal of the study, the different types of trabecular and gyroid structures were designed. both types of the structures were tested macromechanically and their quality was checked microscopically. all the structures showed global elastic modulus close to a cancellous bone. on the other hand, the gyroid structures had greater strength. the gyroid structures, which also seemed to be more reliable based on microscopical analysis, were chosen for designing of the numerical model in the fem software ansys. the results of mechanical testing were also used for the verification of numerical model. all in all, the gyroid structures seem to be an appropriate structure for external layer of implants. as we mentioned earlier, it can speed up the production of implants, which are composed of compact body and porous external layer and do not need any surface modifications. because time and money are always tightly linked, it can also make the implants cheaper. hopefully in the future implants, such as dental, will become a standard and not a nightmare. acknowledgements the financial support provided by the technology agency of the czech republic (tačr), project n. tj01000328, is gratefully acknowledged. the financial support by the faculty of civil engineering, czech technical university in prague (sgs project no. sgs17/168/ohk1/3t/11) is gratefully acknowledged. references [1] t. vieru. mathematics links structure to function in leaves, [online] available at:. https://news.softpedia.com. [accessed: 5.4.2019]. [2] a. kitabjian. trabecular bone morphology changes may predict bone strength in girls [online] available at:. https://www.endocrinologyadvisor.com. [accessed: 5.4.2019]. [3] j. a. dolan, m. saba, r. dehmel, et al. gyroid optical metamaterials. acs photonics 3(10):1888–1896, 2016-09-22. doi:10.1021/acsphotonics.6b00400. [4] j. a. dolan, b. d. wilts, s. vignolini, et al. optical properties of gyroid structured materials. advanced optical materials 3(1):12–32, 2015. doi:10.1002/adom.201400333. [5] n. taniguchi, s. fujibayashi, m. takemoto, et al. effect of pore size on bone ingrowth into porous titanium implants fabricated by additive manufacturing. materials science and engineering: c 59:690–701, 2016. doi:10.1016/j.msec.2015.10.069. [6] s. hulbert, f. young, r. mathews, et al. potential of ceramic materials as permanently implantable skeletal prostheses. journal of biomedical materials research 4(3):433–456, 1970. [7] d. wu, p. isaksson, s. j. ferguson, c. persson. young’s modulus of trabecular bone at the tissue level. acta biomaterialia 78:1–12, 2018. doi:10.1016/j.actbio.2018.08.001. [8] a. jíra, p. hájková. trabecular structures as efficient surface of dental implants 2019. 132 https://doi.org/10.1021/acsphotonics.6b00400 https://doi.org/10.1002/adom.201400333 https://doi.org/10.1016/j.msec.2015.10.069 https://doi.org/10.1016/j.actbio.2018.08.001 acta polytechnica ctu proceedings 26:127–133, 2020 1 introduction 2 geometrical model 3 mechanical testing 4 numerical modelling 5 conclusions acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.34.0080 acta polytechnica ctu proceedings 34:80–84, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague mesh generation and mechanical tests of basic element cells of porous structures luboš řehounek∗, aleš jíra czech technical university in prague, faculty of civil engineering, thákurova 7, 166 29 prague 6, czech republic ∗ corresponding author: lubos.rehounek@fsv.cvut.cz abstract. advanced porous structures are novel, emerging materials with a broad range of applicability but a rather difficult means of design and manufacturing. the advent and availability of the additive manufacturing industry in the last decade has enabled for production of these structures via 3d printing of polymers and metals. this research deals with the design, mechanical testing and preparation of fem meshes of novel gyroid structures manufactured by pa12 sls 3d printing. as demonstrated, conventional means of cad generation of these structures in the *.stl format are sufficient for manufacture of specimens, but not precise enough for the purpose of mesh generation for fem due to errors in the geometry of tesselation. mechanical tests show that the sheet gyroid variant is the preferable geometry as it offers the greatest peak compressive stress among all variants at the same material density. keywords: 3d printing, gyroid, porous, mesh, mechanical. 1. introduction osseointegration of implants, which is closely related to the properties of their surfaces and materials, is a topic that has been widely covered in literature and the surface properties of conventional implants today have multiple ways of achieving a good connection at the bone-implant interface (bii). surface treatment usually aims to roughen the surface of the implant so as to increase its specific surface area. another approach, made possible by additive manufacturing technologies, is application of a porous surface as an integral part of the geometry. this approach also increases the specific surface of the implant, but not at the level of added material, but rather at level of the implant geometry itself. one such structure is the gyroid, which is also often found in nature [1]. it was first described by the american physicist alan schoen in 1970 [2]. the presented mechanical tests show that gyroid structures have very good strength while maintaining very low values of elastic moduli e (a very beneficial characteristic that greatly reduces the stress shielding effect [3–6]) and high porosity. the research shows that for the purpose of manufacturing specimens or real implants, it is acceptable to use a cad environment for the structure’s generation with satisfactory precision of geometry. however, if numerical simulations are considered (fem), this approach is no longer viable. the gyroid has many forms but it is always a tpms (triply-period minimal surface) structure with a system of interconnected pores constructed from the sine and cosine functions. an example of tpms structures is given on fig. 1. figure 1. a simple example of tpms structures. left – the gyroid structure, middle – the neovius structure, right – the fischer-koch s structure. the structures vary in porosity for demonstration purposes. generated with mslattice [7]. 2. materials and methods 2.1. the gyroid structure the gyroid surface can be approximated with the gyroid equation (eq. 1). notably, there are multiple variants of the gyroid structure. the following equation represents the "single gyroid" structure with one solid phase and one void domain: sin(x) cos(y) + sin(y) cos(z) + sin(z) cos(x) = t (1) where a is the length of the edge of a cube that the gyroid structure is circumscribed in, x, y and z are modified spatial coordinates so that: x = 2πx a , y = 2πy a , z = 2πz a and t is a parameter that influences the magnitude of the constant curvature of the gyroid structure. by manipulating the values of t, we can achieve variations 80 https://doi.org/10.14311/app.2022.34.0080 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 34/2022 mesh and mechanical tests of basic cells of porous structures figure 2. computer-generated (eq. 1) single gyroid tpms tubular surfaces with different values of parameter t. image shows the as-generated surface that needs to be enclosed to create a solid for fem simulations. boundary cube has a length of a = 2π. a) t = 1.2, b) t = 1.38, c) t = 1.41. parameter t has a domain of (−1.5; 1.5). as we approach the value of t = 1.5, the structure begins to lose its typical shape and discontinuities occur. between the two variants of the structure (trabecular beam gyroid system and the gyroid sheet system, fig. 3). however, it has its limitations and the overall shape of the gyroid is very sensitive to its value. for extreme values, discontinuities, deformities and overall loss of shape can be observed (fig. 2). this approach of generating a tpms surface is then followed by enclosing one domain (either the trabecular or the sheet domain) to create the desired partition. notably, a gyroid structure created as such always lends to the ability to create either partition as the process of creating the solid from the tpms surface always leads to decomposition of a circumsscribed cube into two (in case of the single gyroid eq. 1) separate domains. therefore, the other partition (domain) is always in a geometrically inverse relationship to the other in a circumscribed cube (fig. 4). figure 3. two variants of the single gyroid structure. left – the trabecular system (sometimes referred to as the tubular system), right – the sheet system. the properties of the gyroid can be manipulated by the parameter t, which determines the final appearance of the structure and the character of the system (trabecular, sheet or a fluid combination). structures have different porosities for purpose of clear demonstration. another variant of the gyroid structure is the double gyroid, represented by eq. 2. . figure 4. a section showing the inverse relationship between the two partitions of a circumscribed cube in which a gyroid structure is generated. if a structure is generated from a tpms function equation (eq. 1 or eq. 2), the other domain will always be an inverse gyroid structure with a different t value. on this picture, purple color represents the trabecular (tubular) gyroid and green represents the sheet gyroid. note that the differences in appearance of either partition are influenced by the parameter t (eq. 1) [ sin(x) cos(y) + sin(y) cos(z) + sin(z) cos(x) ]2 = t2 (2) where a, x, y, z and t have the same meaning as in eq. 1. another important property of the gyroid structure is its chirality. therefore, it is asymmetric and cannot be created or generated by mirror or translation operations. a gyroid basic element cell cannot be divided along any axis or plane and superimposed on itself [8]. 2.2. mechanical tests the specimens for mechanical tests were made in the autodesk netfabb software as single gyroid porous structures with dimensions of 25.12×25.12×25.12 mm. the specimens also had a top and bottom homogeneous plate fitted for a more uniform load distribution. the height of this plate was set to 2 mm. three variants of structures were manufactured, each with a porosity of n = 0.75. the variants were the dode-thick structure, sheet gyroid with a defined wall thickness of 0.5 mm and a trabecular gyroid with parameter value t = 0.78. the gyroid structures were made using eq. 1 with a period of 2π. the sheet 81 luboš řehounek, aleš jíra acta polytechnica ctu proceedings a b c figure 5. deformations of specimens during the uniaxial mechanical compression test. row a – dode-thick structure, row b – trabecular gyroid, row c – sheet gyroid. the time of taking the pictures is the beginning of the test (first column), ca. 0.5τ (middle column) and the final time at failure τ (last column). gyroid structure has t = 0 with a cad-defined wall thickness of 0.5 mm. the specimens were created using the 3d printer sinterit lisa pro and the pa12 material. subsequently, they were processed and tested on a pneumatic static press litem. the loading was controlled by displacement with a speed of 0.04 mm s . fig. 5 shows the specimens before and after loading. 2.3. mesh generation for the purpose of 3d printing, it is generally acceptable to use a well-established cad software for the purpose of generation of the desired geometry, export an *.stl file and prepare it for the printing process. the user usually chooses from a preset variant of the tpms (triply-periodic minimal surface) structure type (gyroid, linioid, etc.) and specifies the parameters given by the prescribed equation. however, the same is not true if we want to use the geometry for the purpose of fem analyses as the errors in geometry and tesselation of the output surface are not acceptable (fig. 6). therefore, the approach of generating a tpms surface according to eq. 1 or eq. 2 has to be adopted. subsequently, one domain of the circumscribed cube in which the tpms surfaces are generated, is enclosed and a solid gyroid structure is created. 3. results the results of mechanical tests are shown in table 1. the values of elastic moduli were calculated from linear parts of load curves obtained via uniaxial compression tests. the specimens before and after load are shown on fig. 5. from the given figures and tables, we can see that the sheet gyroid variant has much greater stiffness and peak compressive stress (more than double) at the same material density, making it a much more effective variant of material arrangement compared to the dode-thick and trabecular gyroid variants. the character of failure (specifically well 82 vol. 34/2022 mesh and mechanical tests of basic cells of porous structures figure 6. a detailed view of gyroid structure models generated via cad with bad tesselation (a, b) and via modelling with [7] (c, d) and good tesselation. a and c show cutouts of a gyroid structure, b and d show a detailed view. the geometry errors and intersecting small faces make it unable to succesfully generate a fem mesh, but are good enough for 3d printing. the cad approach of geometry generation is not suitable for numerical analyses, but sufficient for specimen manufacturing. variants (c, d) are good enough for fem. figure 7. geometry models of specimens for mechanical tests (upper row) and the element basic cells (bottom row). structures: yellow – dode-thick, red – trabecular gyroid, blue – sheet gyroid. 83 luboš řehounek, aleš jíra acta polytechnica ctu proceedings structure type σmax e [mpa] [mpa] dode-thick 1.69± 0.25 27.30± 3.81 trabecular gyroid 1.75± 0.11 28.74± 1.80 sheet gyroid 4.43± 0.47 72.43± 6.13 table 1. values of maximum peak stress σmax and elastic modulus e of tested trabecular and gyroid structures made from the pa12 material. values of e obtained from linear parts of load curves of specimens. captured for the trabecular gyroid structure) can be seen on fig. 5. 4. conclusions the results of this study are twofold – the first part investigated the mechanical properties of gyroid structures manufactured from the pa12 powdered plastic material via sls. the results are listed in table 1. gyroid structures are perspective in the field of implant materials as they represent a porous structure with a system of interconnected pores that is applicable on the surface of an implant. therefore, the gyroid serves as a medium at the bii, creating greater surface area for ingrowth of bone trabeculae [9], provides sufficient fluid flow for bone remodelling [10] and also provides an interface that diminishes the effects of stress shielding, as it (depending on the structural design) has an elastic modulus in the same order of magnitude as human cancellous bone (in case of ti6al-4v specimens, [11]). our experiments on pa12 specimens have shown that the sheet gyroid variant has the highest stiffness and peak compressive stress among all variants at the same material density. reduction of stress shielding, in particular, is a great benefit as many revision surgeries are needed each year [12] because of aseptic loosening and bone loss at the peri-implant area. the difference is apparent, as the values of moduli of human bone are ca. in the range of 1–25 gpa [13], depending on the location in the human body and whether the tissue is trabecular or cortical (trabecular bone occupies the lower range of moduli while cortical bone is stiffer). on the other hand, elastic modulus of common ti-6al-4v alloy is ca. 115 gpa and cp-ti modulus is ca. 105 gpa. the mechanical tests performed on pa12 specimens show that the sheet gyroid structure is much better suited for biomedical applications than trabecular structures with better values of peak stress (table 1). in the second part, the authors learned that in order to successfully generate a fem mesh and perform subsequent analyses, it is necessary to use the approach of tpms generation using the gyroid equations (eq. 1 and eq. 2) with subsequent closing of domains and creating solids. the commonly used process for 3d printing, where we create an .stl mesh from a cad environment, is not applicable for fem analyses as the intersecting bad quality tesselation impairs the overall geometry quality and prevents the enclosure of a solid body. performing of the fem analyses and comparing the moduli obtained from the mechanical tests and simulations is a subject of further research. acknowledgements the financial support provided by the by the faculty of civil engineering, ctu, prague, project n. sgs20/155/ohk1/3t/11 is gratefully acknowledged. references [1] l. wu, w. zhang, d. zhang. engineering gyroidstructured functional materials via templates discovered in nature and in the lab. small 11(38):5004–5022, 2015. [2] a. h. schoen. infinite periodic minimal surfaces without self-intersections. national aeronautics and space administration, 1970. [3] r. ummethala, p. s. karamched, s. rathinavelu, et al. selective laser melting of high-strength, low-modulus ti–35nb–7zr–5ta alloy. materialia 14:100941, 2020. [4] g. yamako, e. chosa, k. totoribe, et al. in-vitro biomechanical evaluation of stress shielding and initial stability of a low-modulus hip stem made of β type ti-33.6 nb-4sn alloy. medical engineering & physics 36(12):1665–1671, 2014. [5] j. bobyn, a. glassman, h. goto, et al. the effect of stem stiffness on femoral bone resorption after canine porous-coated total hip arthroplasty. clinical orthopaedics and related research (261):196–213, 1990. [6] h. u. cameron. the 3—2-6-year results of a modular noncemented low-bending stiffness hip implant: a preliminary study. the journal of arthroplasty 8(3):239–243, 1993. [7] o. al-ketan, r. k. abu al-rub. mslattice: a free software for generating uniform and graded lattices based on triply periodic minimal surfaces. material design & processing communications p. e205, 2020. [8] j. chin, p. v. coveney. chirality and domain growth in the gyroid mesophase. proceedings of the royal society a: mathematical, physical and engineering sciences 462(2076):3575–3600, 2006. [9] l. řehounek, a. jíra. experimental and numerical analyses of a 3d-printed titanium trabecular dental implant. acta polytechnica 57(3):218–228, 2017. [10] s. ma, q. tang, q. feng, et al. mechanical behaviours and mass transport properties of bone-mimicking scaffolds consisted of gyroid structures manufactured using selective laser melting. journal of the mechanical behavior of biomedical materials 93:158–169, 2019. [11] p. vakrčka, a. jíra, p. hájková. mechanical testing and numerical modelling of porous structures improving oseintegration of implants. acta polytechnica ctu proceedings 26:126–132, 2020. [12] c. chanlalit, d. r. shukla, j. s. fitzsimmons, et al. stress shielding around radial head prostheses. the journal of hand surgery 37(10):2118–2125, 2012. [13] h. h. bayraktar, e. f. morgan, g. l. niebur, et al. comparison of the elastic and yield properties of human femoral trabecular and cortical bone tissue. journal of biomechanics 37(1):27–35, 2004. 84 acta polytechnica ctu proceedings 34:80–84, 2022 1 introduction 2 materials and methods 2.1 the gyroid structure 2.2 mechanical tests 2.3 mesh generation 3 results 4 conclusions acknowledgements references https://doi.org/10.14311/app.2022.33.0525 acta polytechnica ctu proceedings 33:525–532, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague determination of in-situ concrete compressive strength under consideration of rearrangement effects rabea anna sefrin∗, christian glock department of civil engineering, university of kaiserslautern, gottlieb daimler straße, 67663 kaiserslautern, germany ∗ corresponding author: rabea.sefrin@bauing.uni-kl.de abstract. for the design of existing structures the concrete compressive strength and the derived mechanical parameters are of central importance. due to safety reasons the compressive strength of existing concrete is usually set comparatively low and thus underestimated. the reasons for this are the limited numbers and the large scatter of material properties of the drill cores, which are the basis for the experimental determination of the compressive strength. in contrast to experimental tests the load in structural components of buildings usually is transferred over the area with higher stiffness and consequently with higher compressive strength. therefore, existing strength variations within a component only play a subordinate role due to rearrangement effects. this paper deals with the experimental and numerical analysis of such rearrangement effects in order to determine the concrete compressive strength of existing structures more realistic and lay a basis for a economical design. by considering these rearrangement effects more realistic during the design of existing concrete structures, a higher number of existing buildings can be maintained without structural measures. the preservation of existing structures is not only decisive from an economic, sustainable and resource-saving point of view, but also represents an added value for cultural and social aspects. keywords: existing structures, in-situ concrete compressive strength, recalculation. 1. introduction the preservation of existing structures is of great interest, because of its contribution to sustainability and resource conservation as well as economic reasons. in the case of existing buildings, maintenance, reconstruction and revitalisation works often take place due to adjustments or changes of use. in terms of structural design the stability of the structure has to be verified in accordance with the currently valid regulations. the determination of the characteristic concrete compressive strength of existing structures, which forms the basis for the design according to the valid regulations, can currently be defined on european level according to en 13791. the standard en 13791:2007 [1] introduced in 2007 was replaced in 2018 by a new draft pren 13791:2018 [2]. to access the concrete compressive strength of structures or parts of structures, en 13791:2007 [1] contains the two approaches a (n ≥ 15) and b (3 ≤ n ≤ 15) depending on the number of the drill cores samples n. due to the weaknesses of approach b of en 13791:2007 [1], which (according to [3] and [4]) can significantly overestimate or underestimate the characteristic in-situ concrete compressive strength as a function of the coefficient of variation of the basic population, adjustments were made in this respect both in the annex a20 (dinăenă13791/a20:2017 [5]) introduced in germany in 2017 and in the development of prenă13791:2018. the regulations introduced with dinăen 13791/a20:2017 [5] and the associated background are presented in detail in [3] and [4]. analogous to en 13791:2007 [1], the evaluation of the characteristic compressive strength of structural concrete according to prenă13791:2018 [2] can also be carried out by means of drill core tests. instead of the approaches a (n ≥ 15) and b (3 ≤ n ≤ 14) of en 13791:2007, pren 13791:2018 [2] only provides an evaluation method that must be based on at least eight drill cores. for a limited test range, pren 13791:2018 [2] provides two options to determine the characteristic insitu concrete compressive strength based on at least three drill core results. an assessment of the procedures latest introduced in prenă13791:2018 [2] for restricted test areas and small sample sizes is carried out in [6] and is not part of this paper. 2. scattering of in-situ concrete compressive strength according to en 13791:2007 [1], din en 13791/a20:2017 [5] and pren 13791:2018 [2], the estimation of the scatter of the compressive strength based on the sample number n, regardless whether the scatter is expressed in the form of the 525 https://doi.org/10.14311/app.2022.33.0525 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en rabea anna sefrin, christian glock acta polytechnica ctu proceedings figure 1. relationship between standard deviation sx and mean in-situ concrete compressive strength fcm(n), is(n) (left) and relationship between coefficient of variation vx and mean in-situ concrete compressive strength fcm(n), is (right) [4] and [7]. figure 2. distribution of drill cores compressive strengths within a test area left: twl; right: anonymized building 1 [6]. figure 3. distribution of the concrete compressive strength within the test areas a to d of the anonymised structure 1. 526 vol. 33/2022 determination of in-situ concrete compressive strength standard deviation sx, the coefficient of variation vx or the spread r, has a decisive influence on the calculated value of the characteristic in-situ compressive strength. figure 1 shows the standard deviation sx (left) and the coefficient of variation vx (right) in dependency of the mean concrete compressive strength of the existing structure fcm(n), is(n) of the test data from [4] and [7]. according to en 1992-1-1 [8], a coefficient of variation of 0.15 is assumed for the determination of the partial safety factor for concrete γc, whereas the coefficients of variation of the existing structures examined in figure 1 partly exceed this value significantly. in order to illustrate the significant variation of the compressive strength of the single drill cores within a small area of a given structural element, two drill core removal areas were labelled with the corresponding core compressive strength (in [mpa]) in figure 2. two structures with a different volume v (left: v ≤ ă10 m3, right: v > 10 m3, the distinction was made in accordance with en 13791:2018) but with a similar coefficient of variation vx were compared. no in-situ compressive strength could be determined at the drill cores (drill core diameter d = 100 mm), which were drawn at the points marked with a red cross, e.g. due to insufficient length or existing defects. figure 3 shows the test areas a to d of the anonymised structure 1 (test area b is already shown in figure 2) with the drill core sampling points and the corresponding concrete compressive strength, in order to be able to consider the distribution of the concrete compressive strength not only within one test area, but also over several test areas. at the drill core locations where two concrete compressive strengths are entered in figure 3, longer drill cores were drawn which could be divided into several drill cores (here: two) to determine the concrete compressive strength. due to the manufacturing process, it could be assumed that a horizontal stratification structure is formed and that the concrete compressive strength therefore differs only over the height of the component. on the basis of figure 3, however, no distribution over the component height or the component width could be derived. such considerations were also made for other structures investigated within the framework of [4] and [7]. however, no clear distribution of the concrete compressive strength over the height or width of the building component could be derived here either. the variation of the concrete compressive strength within a building component must therefore be considered arbitrary. the large variation in the scattering of the concrete compressive strength in the form of the standard deviation sx or the coefficient of variation vx according to the current evaluation methods from [2] or [6] (annex d) together with the mean concrete compressive strength fcm(n), is and the sample number n are the decisive variables for the determination of the characteristic in-situ concrete compressive strength fck, is. when comparing slender with massive structures in terms of design aspects significant differences have to be considered. in slender structures the failure of the weakest part could cause total failure of the structure. in the case of massive structures or components, the load bearing behaviour will cause the redistribution of the stresses resulting from the load within a component towards the areas with a higher strength. subsequently the influence of areas with lower compressive strength is less than previously assumed via the standard deviation sx or the coefficient of variation vx valid also for very slender structures. based on this consideration, the handling of the scattering of the concrete compressive strength when calculating the characteristic in-situ concrete compressive strength could changed in order to increase the strength to be used for the assessment. by using outlier tests, which also work for small sample sizes, the scatter can be further reduced by excluding outliers upwards or downwards [9]. in addition, the concrete’s long-term effects must be taken into account in the assessment, [10] recommends the coefficient of long-term effects αcc of 0.85 for existing structures. the basis for the evaluation of the coefficient of long-term effects in [10] were longterm load tests on drill cores from various existing structures. furthermore, the partial safety factor can be reduced according to [11]. first tests concerning the determination of rearrangement effects on structural members with large scattering of concrete compressive strength were carried out by [12]. here slabs composed of concrete strips of different strength for shear force bearing capacity without shear reinforcement where examined and compared with the shear force bearing capacity of reference slabs homogeneously made of one concrete mixture. further information regarding these tests can be found in [12]. however, a generally valid statement regarding possible rearrangement effects within components cannot be derived from the six test specimen of [12], especially since a special failure case (shear failure without shear reinforcement) was examined and this information cannot be adapted to other failure cases without further investigations or considerations. for this reason, the following considerations were made regarding the investigation of rearrangement effects on small scaled tests. 3. experimental small scale tests to determine effects of rearrangement the following experimental investigations regarding rearrangement effects in concrete structures and components are carried out on small scale tests in the 527 rabea anna sefrin, christian glock acta polytechnica ctu proceedings figure 4. formwork construction and test specimen construction and dimensions. figure 5. illustration of the production process of the prism-shaped test specimens with different concrete mixtures. laboratory of technical university of kaiserslautern / germany (tuk). by producing small scale tests, a test database should be generated that is as extensive as possible in order to enable statistically reliable statements. the aim of these investigations is to proof and qualify the presence and impact of rearrangement effects within a component. also parameters should be determined which have an influence on the rearrangement effects. through numerous structural investigations carried out at the tuk, no clear distribution of the concrete compressive strength as a function of component height or component width within one or more test areas could be determined (cf. chapter 2). however, a distribution has to be assumed for the determination of rearrangement effects on small scale tests executed in the laboratory. for this reason, only two limiting cases of vertical layering and horizontal layering of different concrete strength were examined. in existing building structures aădistribution of the concrete compressive strengths will be found, which is irregular and lies in-between these two limiting cases. in this paper the tests results of the vertically layered test specimens are discussed. 3.1. test program and test setup the formwork for the prism-shaped test specimens, which are made of different concrete strength and are to be produced fresh in fresh, is shown in figure 4 and consists of a base plate, four side panels and two stainless steel inserts. to allow frequent use of the formwork and easy stripping, the base plate and the four side parts were made of polyoxymethylene (pom). the side parts are firmly connected to each other with screws, while the side parts a are additionally coupled to the base plate with a screw connection. the higher strength concrete c1 is always mixed first (c2 in table 1 and table 2). then the concrete is poured halfway either into the middle or the two outer chambers of the formwork and compacted with the a vibrating table. later the concrete is poured up to the top edge of the formwork and compacted again (figure 5 b)). the concrete with the lower strength is coloured with red pigments in order to be able to check whether mixing between the concretes has taken place. here too, the concrete is first poured into the respective chamber of the formwork, compacted to halfway, then filled to the top of the formwork and compacted again (figure 5ăc)). afterwards, the stainless steel sheets were removed (figure 5ăd)). to create a full-surface bond (fresh in fresh) between the concrete layers, the concrete was compacted again (figure 5ăe)). in order to check the bond, the test specimens that had been stripped off were reviewed from the outside. in addition randomly selected test specimens were cut into three equally sized pieces with a concrete saw to check whether a full-surface bond between the concrete layers was present even in the part not visible 528 vol. 33/2022 determination of in-situ concrete compressive strength test series test specimens geometry concreting target strength class combinationstest specimens of different strengths reference test specimens c1 c2 v1 prism cylinder b1 c20/25 c12/15 k1 k2 (c1-c2-c1) (c2-c1-c2) b2 c30/37 c12/15 k3 k4 (c1-c2-c1) (c2-c1-c2) b3 c30/37 c20/25 k5 k6 (c1-c2-c1) (c2-c1-c2) per concreting 5 × (c1-c2-c1) 5 × (c2-c1-c2) per concreting 3 × c2 3 × c2 b4 c40/55 c30/37 k7 k8 (c1-c2-c1) (c2-c1-c2) b5 c50/60 c30/37 k9 k10 (c1-c2-c1) (c2-c1-c2) b6 c50/60 c40/50 k11 k12 (c1-c2-c1) (c2-c1-c2) goal v1 checking whether rearrangement effects can be determined on uniaxial compression strength tests. table 1. overview of test series 1. test series test specimens geometry concreting target strength class combinationstest specimens of different strengths reference test specimens c1 c2 v2 prism cylinder prism b1 c20/25 c12/15 k1 k2 (c1-c2-c1) (c2-c1-c2) b2 c30/37 c12/15 k3 k4 (c1-c2-c1) (c2-c1-c2) b3 c30/37 c20/25 k5 k6 (c1-c2-c1) (c2-c1-c2) per concreting 5 × (c1-c2-c1) 5 × (c2-c1-c2) per concreting 3 cylinder × c1 3 cylinder × c2 3 prism × c1 3 prism × c2 b4 c40/55 c30/37 k7 k8 (c1-c2-c1) (c2-c1-c2) b5 c50/60 c30/37 k9 k10 (c1-c2-c1) (c2-c1-c2) b6 c50/60 c30/37 k11 k12 (c1-c2-c1) (c2-c1-c2) b7 c50/60 c40/50 k13 k14 (c1-c2-c1) (c2-c1-c2) goals v2 confirmation of the results from v1. examine influence of the sample geometry (reference sample). v2 hp prism cylinder prism b1-hp c12/15 / c1 = c2 b1-hp c20/25 per concreting 3 × per concreting 3 × prism 3 × cylinder b3-hp b4-hp c30/37 c50/60 goals v2 hp checking the influence of the manufacturing process. checking the influence of the test specimens geometry. table 2. overview of test series 2. 529 rabea anna sefrin, christian glock acta polytechnica ctu proceedings figure 6. rearrangement effects determined on small part tests in dependency of the coefficient of variation vx (left) and as a function of the mean value of concrete compressive strength fcm (right), yellow: results of k1, k3, k5, k7, k9, k11, k13, red: results of k2, k4, k6, k8, k10, k12, k14. figure 7. influence of the manufacturing process and the test specimen geometry on the concrete compressive strength. from the outside. all test specimen fulfilled these criteria. table 1 and table 2 show the test program of test series 1 and test series 2. in test series 1, six concreting operations were carried out to produce prisms from different concretes. two concretes (c1 and c2) were mixed per concreting and arranged as shown in figure 4. three reference cylinder are produced per concrete. the aim of test series 1 was to check whether rearrangement effects can be determined on uniaxial compression strength tests. in test series 2, prism-shaped test specimens were also produced from different concretes. prisms as well as cylinders were chosen as reference specimens to check whether the specimen geometry has an influence on the concrete compressive strength. this was also to check whether the choice of cylinders as reference specimens in test series 1 was appropriate. in addition, the production process was to be investigated in test series 2. for this purpose the prismshaped specimens were also produced as shown in figure 4, but the same concrete was poured into all three chambers. 4. results figure 6 shows the determined rearrangement effects from test series 1 and 2 in dependency of the coefficient of variation vx and the average concrete compressive strength fcm. the percentage rearrangement effect was determined by relating the concrete compressive strength of the prism-shaped test specimens made of different concretes to the mean value of concrete compressive strength of the reference specimens ((fcm, c1 + fcm, c2 + fcm, c1)/3) or 530 vol. 33/2022 determination of in-situ concrete compressive strength ((fcm, c2 + fcm, c1 + fcm, c2) /3). the coefficient of variation, which is present within the prism-shaped test specimens made of different concretes, was also formed using the results of the reference test specimens. for each concrete, at least three concrete compressive strength results were available for the determination of the coefficient of variation. for the prism-shaped test specimens, which are composed of different concretes, the coefficient of variation is determined on the basis of nine or more results (cf. table 1 and 2). in test series 2, due to the results shown in figure 7, both the results of the cylinders and those of the prism-shaped test specimens could be used to determine the coefficient of variation. the results in figure 6 show that rearrangement effects could also be determined on uniaxial compressive strength tests in which the test specimens exhibit large coefficients of variation (since these consist of different concretes). the percentage rearrangement effects vary between 2 and 18 %. no clear dependence on the average concrete compressive strength or the coefficient of variation could be established. so far, no other parameter could be identified on which the magnitude of the redistribution effects clearly depends. on the basis of the test series 2, it could be determined, that the production process has no influence on the experimentally determined concrete compressive strength (see figure 7). the prisms, which were manufactured from concrete as described in figure 4 (p), deviate only slightly from the results of the reference cylinders (rz) or the reference prisms (rp). figure 7 shows that the coefficient of variation between the compressive strength results of the different geometries or manufacturing processes is 0.02 to 0.04, which is too low, that an influence of the manufacturing process on the concrete compressive strength can be excluded. generally, the concrete compressive strength is determined according to the specifications of din en 12390-3 [13]. 5. conclusion and outlook the theoretical analysis and experimental tests show, that rearrangement effects even occur in comparatively small test specimens consisting of different concretes (with different concrete strengths), which are loaded uniaxially in compression. it could be observed that the prisms (as a function of vx and fcm) can bear up to 18% more load than the mean value calculated from the reference specimens due to rearrangement effects. however, it is currently still not quantified on which parameters these effects depend significantly. therefore in the next step, further small scale tests with horizontal stratification structure will to be carried out to consider the second limiting case for the arrangement of the concrete compressive strength within a component. in order to cover a larger parameter range, additional fem investigations will be carried out which include the variation of the concrete compressive strength, the coefficient of variation and the dimensions of the specimen. these further investigations are intended to provide a better understanding of the presence and impact of rearrangement effects within structural elements, so that these can ultimately be used in the assessment of existing structures. references [1] en 13791:2007. assessment of in-situ compressive strength in structures and precast concrete components, european committee for standardization, brussels, 2007. [2] e din en 13791:2018-06: bewertung der druckfestigkeit von beton in bauwerken oder in bauwerksteilen; deutsche und englische fassung pren 13791:2018, berlin, beuth verlag gmbh, 2018. [3] dafstb-heft 619. sachstandbericht bauen im bestand teil ii: bestimmung charakteristischer betondruckfestigkeiten, beuth verlag, berlin, germany, 2016. [4] m. weber. zur ermittlung mechanischer kennwerte von bestandsbetonen. dissertation, tu kaiserslautern, 2019. [5] dinăenă13791/a20. bewertung der druckfestigkeit von beton in bauwerken oder in bauwerksteilen; änderung a20, berlin, beuth verlag gmbh, 2017. [6] r. sefrin, c. glock, e. schwabach. ermittlung der charakteristischen betondruckfestigkeit von bestandstragwerken nach din en 13791:2020 für geringe stichprobenumfänge. betonund stahlbetonbau 116(1):35-44, 2020. https://doi.org/10.1002/best.202000041. [7] m. weber, e. schwabach, j. schnell. bestimmung der charakteristischen betondruckfestigkeit im bestand bei kleinem stichprobenumfang. betonund stahlbetonbau 111(4):188-99, 2016. https://doi.org/10.1002/best.201500073. [8] en 1992-1-1. eurocode 2: design of concrete structures part 1-1: general rules and rules for buildings, brussels, 2004. [9] m. weber, m. doktor, j.-p. stockis, et al. ausreißertests bei der bestimmung der in-situ-betondruckfestigkeit. beton fachzeitschrift für bau+technik 69. jahrgang (2019), erkrath: verlag bau+technik gmbh. p. 212-219, 2019. [10] m. weber, r. sefrin, j. schnell, et al. berücksichtigung von dauerstandseffekten im nachrechnungswert der betondruckfestigkeit. betonund stahlbetonbau 114(11):847-56, 2019. https://doi.org/10.1002/best.201900049. [11] m. weber, r. caspeele, j. schnell, et al. das neue fib bulletin 80 teilsicherheitsbeiwerte für die nachrechnung bestehender massivbauwerke. betonund stahlbetonbau 113(4):264-74, 2018. https://doi.org/10.1002/best.201700104,. 531 https://doi.org/10.1002/best.202000041 https://doi.org/10.1002/best.201500073 https://doi.org/10.1002/best.201900049 https://doi.org/10.1002/best.201700104 rabea anna sefrin, christian glock acta polytechnica ctu proceedings [12] y. yang, j. den uijl, j. walraven, et al. influence of spatial variability on the shear capacity of rc members without shear reinforcement. iabse conference, rotterdam 2013: assessment, upgrading and refurbishment of infrastructures2013. p. 532-3. https://doi.org/10.2749/222137813806521315. [13] din en 12390-3. prüfung von festbeton teil 3: druckfestigkeit von probekörpern; deutsche fassung en 12390-3:2009 mit din en 12390-3:2011, berlin, beuth verlag gmbh, 2009. https://doi.org/10.31030/1511392. 532 https://doi.org/10.2749/222137813806521315 https://doi.org/10.31030/1511392 student conference on nuclear engineering šimáně 2017 6th 7th november, 2017, prague, czech republic acta polytechnica ctu proceedings organised by: department of nuclear reactors, faculty of nuclear science and engineering, ctu in prague simane.fjfi.cvut.cz editors: martin ševeček martin cesnek scientific comittee: milan štefánik martin ševeček martin cesnek local organizing comittee: adam kecek vojtěch caha martin cesnek filip fejt martin ševeček milan štefánik guarantor of the peer review process: milan štefánik guarantor of language editing: martin ševeček iv vol. 14/2018 preface nuclear science and technology have a long history within the czechoslovakia. even though the czech republic and slovakia have been separated since 1993, wide cooperation on different levels including education and research still exists. in order to support cooperation and good relations between czech and slovak students, the organizing committee decided to hold the 2nd student conference on nuclear engineering šimáně 2017. the conference is named after professor šimáně, who graduated at the dr. edvard beneš technical university in brno. during his early career, he had the opportunity to work with fréderic joliot-curie at college de france. later, he significantly contributed to the foundation of the nuclear science, industry and education in the czechoslovakia. he became the first employee of the institute of nuclear physics at the czechoslovakian academy of science, later he became the first director of the nuclear research institute in řež. professor šimáně was well established even on the international level. he was a director at the joint institute of nuclear research in dubna or a division director at the international agency for atomic energy. his work was also significantly connected with our faculty where he acted as dean from 1967 to 1972. the main objective of the conference is to provide an opportunity for bsc., msc. and ph.d. students of nuclear-oriented study programmes to publish and present their scientific results achieved during their university studies. furthermore, it should give the participants experience in the field of presenting their own research, writing scientific papers and also broaden their knowledge throughout the related fields. the organizing committee would like to thank all participants for their presentations and also to the ctu in prague for funding this student conference. the committee also hopes that the work achieved and presented will fulfill the heritage of professor čestmír šimáně. v acta polytechnica ctu proceedings list of participants: dana barátová stu fei martin lovecký zču dana_baratova@stuba.sk lovecky@rice.zcu.cz pavel bartl čvut fjfi/kjch tomáš matlocha čvut fjfi pavel.bartl@fjfi.cvut.cz matlocha@ujf.cas.cz ivana bonková uk ba ondřej novák čvut fjfi ivana.bonkova@gmail.com ondrej.novak2@fjfi.cvut.cz viktor burian uk mff adela nováková čvut fjfi viktor.burian@protonmail.com novakad3@fjfi.cvut.cz vojtěch caha čvut fjfi sebastian nývlt čvut fjfi vojtacaha@gmail.com nyvltseb@fjfi.cvut.cz daniel campolina čvut fjfi filip osuský stu fei campolina@cdtn.br filip.osusky@stuba.sk lucie celbová čvut fjfi/kipl pavel suk čvut fjfi celboluc@fjfi.cvut.cz sukpave2@fjfi.cvut.cz martin cesnek čvut fjfi jakub svoboda čvut fjfi martin.cesnek@fjfi.cvut.cz svoboj68@fjfi.cvut.cz tomáš czakoj čvut fjfi martin ševeček čvut fjfi tomas.czakoj@gmail.com martin.sevecek@fjfi.cvut.cz michal farník čvut fjfi/kf václav šísl čvut fjfi farnimi1@fjfi.cvut.cz sislvacl@fjfi.cvut.cz filip fejt čvut fjfi/kjr jan škarohlíd čvut fs filip.fejt@fjfi.cvut.cz jan.skarohlid@fs.cvut.cz markéta floriánová čvut fjfi/kjch jan vimpel čvut fs jan.vimpel@seznam.cz dominika holková stu fei daniel vlček čvut fjfi dominika.holkova@stuba.sk daniel.vlcek1@gmail.com kateřina jiráková čvut fjfi lenka volfová uk přf jirakkat@fjfi.cvut.cz volfl@fzu.cz adam kecek čvut fjfi/kjr pavel vrbka čvut fjfi adam.kecek@fjfi.cvut.cz pavel.vr@seznam.cz jakub krejčí čvut fjfi jiří závorka čvut fs krejci@ujp.cz jiri.zavorka@fs.cvut.cz denisa kubániová uk mff kubaniova@mbox.troja.mff.cuni.cz vi https://doi.org/10.14311/app.2022.33.0133 acta polytechnica ctu proceedings 33:133–139, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague analyzing the impact of the aspect ratio of a building on concrete use in its structure manish k. dixita,∗, pranav pradeep kumara,b a texas a&m university, department of construction science, 574 ross st., college station, tx, 77843-3255, usa b texas a&m university, zachry department of civil and environmental engineering, 199 spence st., college station, tx, 77843-3255, usa ∗ corresponding author: mdixit@arch.tamu.edu abstract. buildings consume over half of annual energy supply as embodied and operating energy in their construction and operation releasing harmful emissions to the atmosphere. over 90% of the embodied energy is attributed to construction materials used in building structure, envelope, and interiors that must be reduced to minimize material use. concrete is one of the major materials that contributes significantly to the energy and carbon footprint of buildings, as it is responsible for 5-9% of global carbon emission. because most of the concrete use in the building sector occurs in building structures, assessing how building design parameters influence its environmental sustainability is important. one of the design parameters that impact the sustainability of buildings is the aspect ratio, which is defined as the ratio of horizontal to vertical surface area of a building. a building with the same floor area can be designed horizontally or vertically with different aspect ratios, which will influence its structural design and eventually the amount of concrete used in the building. in this paper, we examine how aspect ratio may affect the environmental sustainability of a buildings foundation, structural framing, and slab. we model the structure of a generic building with different aspect ratio to analyze if aspect ratio can help reduce the energy and carbon embodied in reinforced concrete structures. keywords: aspect ratio, concrete, embodied carbon, embodied energy, environmental sustainability. 1. introduction building sector consumes nearly half of global energy supply in building construction, operation, and maintenance as embodied and operating energy, releasing over 40% global carbon emission [1, 2]. for an effective reduction in a building’s energy and environmental footprints for long-term environmental sustainability, the use of both embodied and operating energy must be minimized [1]. with emerging advanced materials, energy-efficient technologies, and regulations, the operating energy use is decreasing gradually [3]. however, to reduce embodied energy, the use of energy intensive materials must be controlled [2]. over 90% of a building’s embodied energy can be attributed to construction materials [2]. this means that the greatest opportunity to minimize embodied energy impact lies within construction material domain. one such material is concrete, the use of which has always been extensive and would continue to grow realizing its mechanical strength as well as durability [4–6]. it is the most consumed solid material in the world by mass [4, 7] and is also a material of choice for emerging construction automation technologies such as 3d-printed or additive construction, which are garnering the attention of the construction sector due to their higher productivity, efficiency, and safety [6]. however, concrete is also responsible for 5-9% of global carbon emission, primarily due to the use of cement [8–10]. roughly 74%-81% of the total carbon dioxide emission from concrete production comes from portland cement use with just 13%-20% originating from coarse aggregates [11]. several studies have been targeting concrete to enhance its environmental sustainability [11]. however, effectively reducing its energy and carbon footprint is still challenging [4]. there are two approaches to advance the sustainability of concrete, particularly in the construction sector. the first approach involves material science to either modify or replace cement, the main ingredient responsible for a majority of carbon emission of concrete [4, 11]. the second approach is to minimize the use of concrete through design so that it is consumed only in components that actually need its mechanical properties and durability [6]. in this paper, we analyse one of the design parameters, the aspect ratio of buildings, to understand how the use of reinforced cement concrete is affected by the aspect ratio. 2. concrete sustainability among the widely applied approaches to enhance the environmental sustainability of concrete is replacing or reducing the amount of cement use in a concrete mix [11, 12]. reducing cement quantity by adding 133 https://doi.org/10.14311/app.2022.33.0133 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en manish k. dixit, pranav pradeep kumar acta polytechnica ctu proceedings pozzolanic materials such as fly ash, silica fumes, natural pozzolans, rice-husk/wood ash, or granulated blast furnace slag (gbfs) is one example of minimizing the carbon footprint (13%-15% reduction in co2 emission) [4, 7, 11, 12]. likewise, fiber-reinforced concrete (frc) is another example to enhance the overall sustainability of concrete through reduced construction time (37% decrease) and cost (12% decrease) as well [5]. studies also used inorganic polymers or geopolymers as well as calcined clays to supplement cement in concrete [4]. using recycled concrete aggregate (rca) has also been examined to lower the adverse environmental impacts of concrete [12]. the manufacturing process of cement (dry vs. wet process) as well as the type of fuel consumed is also being targeted to capitalize on any opportunities of saving energy [7, 12]. the other approach to improve the environmental performance of concrete involves design of concrete mixes for higher durability and strength and lower maintenance requirements so that the amount of concrete over a structure’s life cycle can be reduced [4, 12]. by modifying the packing factor, size, and shape of aggregates and controlling the amount of water, concrete mixes can be optimized for smaller carbon footprints. the use of concrete can also be optimized through innovative automated construction processes as well as designing a building or a structure in ways that boosts the material efficiency to decrease concrete use [6]. one of the building design parameters is the aspect ratio that has been studied by several studies for not just operating and embodied energy usage (heating and cooling loads) but structural optimization as well [13–16]. most studies defined aspect ratio as the ratio of the width (or depth) and the height of a building that influences structural behaviour under normal load conditions as well as wind loads [15]. some studies (e.g. [16]) revealed that aspect ratio can significantly influence the quantity of construction materials as well, affecting overall embodied energy and carbon emission. in this study, we conducted a preliminary analysis of how aspect ratio influences the amount of concrete and steel in a building’s structure. the goal is to examine whether tall or horizontal buildings help enhance the sustainability of concrete as a construction material. 3. research goal and methodology the primary objective of the present research is to assess the impact of aspect ratio on the use of concrete in buildings. the following are the key objectives of the research paper: 3.1. objective 1. define aspect ratio as surface aspect ratio which is the ratio of the horizontal (floor) surface area to the total vertical (peripheral) surface area. 2. calculate the quantity of steel and concrete required for buildings with different aspect ratios. 3. compare the results for different aspect ratio cases and discuss the impact of the variation in material quantities. 3.2. approach 3.2.1. defining surface aspect ratio (ars) in this paper, the surface aspect ratio (ars) of a building is defined as the ratio of the total area of the horizontal and the vertical surfaces of a building. the aim is to understand the relationship between surface aspect ratio and the quantity of materials (concrete and rebar steel) used in the structure of a building. this was achieved by conducting a structural analysis of a 12-storied commercial building. the surface aspect ratio of the building was incrementally changed to investigate its effect on the quantity of concrete and steel used in the building structure. this relationship is important to examine if a design parameter (ars) influences the amount of concrete and steel used in a building’s structure, and eventually affects the sustainability of concrete. the total floor area of the commercial building is approximately 10,600 m2. to obtain the average area per floor, for the given plan, the total area is divided by the number of floors, nf +1. the additional floor designates the reinforced concrete roof. the length and breadth are calculated from the average area per floor using the assumption that the length of the floor, l, is two times that of the breadth of the floor, b. the surface aspect ratio is varied by changing the number floors from twelve to nine, six, three, and one. the total usable floor area of the entire building is maintained constant to make the comparison of steel and concrete usage. the average area per floor is calculated based on the different nf values, which is eventually used to calculate length and breadth dimensions. evidently, when the total floor area is held constant, the horizontal dimensions i.e., length and breadth of the building, are observed to increase with a decrease in the number of floors. the interior floor to ceiling height of each floor, h, is also held constant to 13 feet. in this study, horizontal surface refers to the individual floor surface and the vertical surface represents the exterior peripheral surface as shown in figure 1. here, we defined aspect ratio (ars) differently as the ratio of horizontal to vertical surface area. the horizontal surface area, ha, is quantified using the length and the breadth of the floor plan. the vertical area, va, is calculated as the sum of the products of total building height and that of length and breadth. the surface aspect ratio of the building is computed as ha/va. assuming the thickness of the exterior wall assembly as 9 inches and slab thickness of 6 inches, the centre-to-centre dimensions of length (lclc), breadth (bclc), and height (hclc) are calculated. 134 vol. 33/2022 building aspect ratio analysis figure 1. horizontal and vertical surfaces of a building considered to define surface aspect ratio member assignment material assignment loading assignment parameters dimension parameters yield strength parameters load value [inches] [ksi] wall 9 concrete 4 dead load of wall 104 lb/ftdlwall slab 6 rebar steel 60 live floor / roof 100 psf load 20 psf table 1. structural modeling assumptions. number of floors, nf 12 9 6 3 1 surface aspect ratio 0.14 0.21 0.37 0.99 4.19 table 2. surface aspect ratio for different floors. 3.2.2. structural analysis and quantification of materials to perform the structural analysis of floor plans with different surface aspect ratios, etabs v18 structural software is used for the superstructure and rapid interactive structural analysis (risa) foundation is used for foundation/footing analysis. first, the structural model is prepared for each case of surface aspect ratio, by assigning horizontal and vertical structural members (reinforced concrete columns and beams) as well as 6-inch-thick slab. the dimensions of the beams and columns are fed to the autogenerated list in etabs using the "auto-beam" and "auto-column" options. the standard beam and column dimensions are input in the list which the software uses to randomly assign to the respective beams and columns for the initial model before the analysis. table 1 lists all the key assumptions used for structural modelling. the loads are assigned based on the standard loading reference of asce7 [17]. the exterior wall assembly of 9 inches thickness is assumed to be non-load bearing, and their dead load is assumed as uniformly distributed along the wall location. the dead load of the walls of 104 pounds per linear foot (lb/ft) is assigned based on the assumption that a normal partition stud wall with 0.5 in. thick gypsum board on each side for a wall/floor height of 13 ft, as per c4.3.2, asce7. the floor live load of 100 pounds per square foot (psf) and roof live load of 20 psf, respectively, are assigned as per asce7. asce7 auto lateral load is assigned to define the wind load case. all parameters (exposure, wind pressure and co-efficient) are kept the same as the default values from asce7 in etabs for this study. application of wind lateral load from different direction and of different wind parameters is not accounted for in this study. the analysis is run, and the optimal design is generated using the list of standard beam and column dimensions. the quantity take-off is estimated from the software and the results are plotted between the surface aspect ratio and the amount of concrete and steel used for the buildings with different number of floors. the foundation is designed using the risa foundation software. the dimensional range of the foundations are also provided in a similar way explained previously for the beam and column design in etabs. the dimensions ranged from a minimum of 2 ft to a maximum of 20 ft. isolated footings are assumed as the type of founda135 manish k. dixit, pranav pradeep kumar acta polytechnica ctu proceedings � � � (a). 12 floors (ars – 0.14) (b). 9 floors (ars – 0.21) (c). 6 floors (ars – 0.37) � � (d). 3 floors (ars – 0.99) (e). 1 floor (ars – 4.19) figure 2. different types of models prepared for surface aspect ratio analysis. � (a). 12 floors (ars – 0.14) (b). 9 floors (ars – 0.21) (c). 6 floors (ars – 0.37) � � (d). 3 floors (ars – 0.99) (e). 1 floor (ars – 4.19) figure 3. isolated footing foundation models. tion. the dead load and live load loads are assigned as point loads on the footings, using the base reactions of the superstructure analyzed and designed in etabs. 4. results table 2 lists different surface aspect ratios of the building considered in the structural modelling. figure 2 shows building models with different surface aspect ratios. the structural analysis of these five building models was carried out individually using etabs and risa foundation. as seen in figure 2, the horizontal floor size increases with the decreasing surface aspect ratio and increasing total height of the building, keeping the total usable floor space constant. figure 3 shows the foundation detailing for all five structural models. figure 4 illustrates the relationship between different surface aspect ratios and the quantity of steel and concrete used in building structures. the quantity take-off for each case is plotted against the respective surface aspect ratio. the surface aspect ratio is plotted on a log normal scale on the x-axis. the two materials whose quantities are plotted on the y-axis are steel and concrete. from the plot between the quantities of steel and concrete versus surface aspect ratios, the impact of surface aspect ratio on material quantities can be understood. the quantities of the construction materials decrease as the surface aspect ratio increases. the rate of decrease is higher for steel usage than the concrete. thus, the quantities of steel and concrete for a unit vary inversely with the surface aspect ratio raised to some exponent greater than unity. the quantities of steel and concrete are also compared with the number of floors in the unit. the number floors are also plotted on a log normal scale as the secondary x-axis as shown in figure 4. the quantities of steel and concrete increase with the increasing number of floors. 136 vol. 33/2022 building aspect ratio analysis figure 4. relation between quantities of concrete and rebar steel. figure 5. flow chart for determining the boundary limits to carry out further structural analysis. it may be structurally justified that as the height of the structure increases, the total gravity load onto the base columns increases, increasing the base reaction. this raises the quantity of steel and concrete required for individual footing. further, as the loads are higher on columns at the bottom level of tall buildings, the size of columns and beams goes up from top to bottom of the building, which tends to increase the total volume and quantity of materials required for each structural component. however, slab volume and material quantity remain approximately the same as the total floor area and slab thickness are kept constant. the drop in quantity of concrete and steel is, therefore, directly dependent on the total load transferred to each structural member. the results clearly show a decrease in the quantity of steel and concrete as the surface aspect ratio increases. this relates to the carbon footprint of the structure. it can be concluded that, the surface aspect ratio of the building may be judiciously used to ensure that the building construction is sustainable, while maintaining the utility of the unit. this can be achieved by choosing a reasonable combination that satisfies a minimum quantity of raw material consumption while maximizing the services desired from the building space and dimensions. this study shows that the surface aspect ratio can be a good tool in the planning of buildings with the objective of reducing the carbon footprint of the project, particularly in the construction stage because this analysis can lead to huge saving on the construction material by the selection of an optimum surface aspect ratio. 5. discussion from the results presented in the previous section, one may see that for the five cases of different surface aspect ratios, the total quantity of steel and concrete decreases with an increase in the surface aspect ratio. however, these results are based on just a preliminary analysis of the five cases. to improve the efficacy of the model to make robust conclusions on the impact of aspect ratio, a more rigorous structural analysis needs to be carried out that considers not just several aspect ratios but different building orientations as well. we are currently in the process of programming a code to run exhaustive structural simulations, which include a wide range of building orientation, floor size variations, and aspect ratios. two primary 137 manish k. dixit, pranav pradeep kumar acta polytechnica ctu proceedings inputs are considered: (a) the total usable volume of the building (v ), which is held constant throughout the analysis and is calculated using an existing 12 storied building plan, and (b) the total number of floors in the building (nf ), which is varied starting with one floor. as seen in figure 5, the simulation begins with the number of floor (nf ) as one and using the floor height (h) computes the total height of the building. the number of floor (nf ) is then increased incrementally, quantifying the corresponding total height of the building. for this study, only non-slender buildings are considered by performing a check of slenderness ratio (sr). the slenderness ratio (sr) is defined as the ratio of the smallest horizontal dimension and the height of a building (h). fu [18] suggests that most structural engineers consider buildings with sr < 0.1 to be slender. this parameter gives the maximum number of floors that may be considered for analysis. using v and nf , the total area per floor for each case is calculated, which is eventually used to estimate the length and breadth of the building. to ensure that either of the length and the breadth of the building is not too narrow to render the building non-functional, we will use single or double-loaded corridor/hallway configuration to derive a minimum dimension. this minimum dimension will control the least length (lmin) and breadth (bmin) of the building that could be considered in the simulation. these dimensions are then used to calculate the ratio of the length and breadth of the building. several studies (e.g. [19, 20]) suggest that buildings with the length and breadth ratio of 5.68 are the most economical in terms of energy costs as well as the quantity and quality of daylight. therefore, the cases with the length and breadth ratio 5.68 will be excluded from the analysis. the cases that pass all the initial checks will undergo structural modelling including lateral load analysis along with gravity loads. the main parameters considered here are variation in spatial dimensions (length, breadth, and height of the building) and change in the orientation of the building to face the predominant wind direction. geographic location parameters such as wind direction and speed and soil conditions are held constant. the goal is to evaluate the quantity of steel and concrete under different spatial and lateral load conditions to study the influence of aspect ratio on quantity of materials and the subsequent effect on the carbon footprint. even though the preliminary results suggest that the construction of taller buildings may need more quantities of steel and concrete than the horizontal ones, the sustainability of concrete should be viewed from a more holistic perspective that also includes other factors such as availability of land, land use and the area of impermeable surfaces on ground. for instance, a horizontal design of a building may help improve the sustainability of concrete through material savings, it may also intensify the urban heat island effect. in fact, horizontally designed buildings may also decrease ground permeability increasing the probability of flash floods, which is particularly concerning in the cases of radically changing climate. the amount of roof and wall surface also influences the amounts of heating and cooling loads of a building, which consequently affects the operating energy consumption. however, availability of land area for horizontal construction is a challenge and may call for vertical construction. taller buildings, on the other hand, offer opportunities of having more permeable and green space on ground that may not just help control the urban heat island effect to keep the urban air temperatures low but also offer vegetated ground to soak rainwater and reduce the chances of flash floods. increased green space may also mean enhanced quality of life as well as reinforced biodiversity through expanded natural habitat for different plant and animal species. note that this paper is not favouring a tall building over a horizontal one. it is rather arguing to apply a holistic perspective to analyse the sustainability of concrete as it relates to individual building design as well as urban environments. 6. conclusions this paper presented preliminary findings based on the structural modelling of five different aspect ratios to compute the amount of concrete and steel used in the buildings’ structure. the results showed that the quantities of concrete and steel increase with a decrease in the surface aspect ratio. in other words, designing a building vertically may require more material usage in its structure than a horizontal one. as these results are preliminary, we are currently developing a code to structurally analyze multiple combinations of the length, width, and height of a building model in different orientations to arrive at robust conclusions. our goal is to examine if a design parameter such as the surface aspect ratio impacts the quantities of concrete and steel, and eventually resulting environmental impacts. we also argue that analysing the sustainability of concrete based on just material quantities may lead to misleading conclusions, as other environmental phenomena such as urban heat island effect, expanding land use, and flash flood events may need to be included in the analysis of the sustainability of concrete. references [1] m. baum. green building research funding: an assessment of current activity in the united statesi (washington d. c.), 2007. https://www.usgbc.org/re sources/green-building-research-funding-asses sment-current-activity-united-states. 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https://doi.org/10.1016/c2015-0-06071-3 https://doi.org/10.32920/ryerson.14658090.v1 acta polytechnica ctu proceedings doi:10.14311/app.2019.25.0025 acta polytechnica ctu proceedings 25:25–31, 2019 © czech technical university in prague, 2019 available online at http://ojs.cvut.cz/ojs/index.php/app numerical modelling of wave shapes during shpb measurement radim dvořák∗, petr koudelka, tomáš fíla czech technical university in prague, faculty of transportation sciences, konviktská 20, 110 00 prague, czech republic ∗ corresponding author: dvorara9@fd.cvut.cz abstract. the paper aims at the numerical simulation of the wave propagation in compressive split hopkinson pressure bar (shpb) experiment. the paper deals with principles of shpb measurement, optimisation of a numerical model and techniques of pulse shaping. the parametric model of the typical shpb configuration developed for ls-dyna environment is introduced and optimised (in terms of element size and distribution) using the sensitivity study. then, a parametric analysis of a geometric properties of the pulse shaper is carried out to reveal their influence on a shape of the incident pulse. the analysis is algorithmized including the preand post-processing routines to enable automated processing of numerical results and comparison with the experimental data. results of the parametric analysis and the influence of geometric properties of the pulse shaper (diameter, length) on the incident wave are demonstrated. keywords: shpb, pulse shaping, finite element method, explicit dynamics. 1. introduction since material parameters are dependent also on strain rate, many experimental methodologies allowing to investigate material behavior during dynamic loading like fast impacts and explosions were developed. for example, yield strength of most of metals (e.g. copper) and alloys (including aluminium based alloys, steel, etc.) tends to increase with higher strain rates. the physics of this phenomena can be explained, for example, in a way that particles of material are affected by micro-inertia effects and do not have enough time to move and to cause yielding during high strain rates. however, quantification of this behavior is difficult. first experiments were performed by hopkinson [1], while an important improvement was proposed by kolsky [2] introducing the common split hopkinson pressure bar (shpb) apparatus [3]. shpb is an experimental device, which allows to measure the stress-strain diagram in dynamic compression, where the strain-rate and maximum compressive strain can be regulated within physical boundaries given by performance of the shpb apparatus and the investigated samples. in a typical configuration, the shpb consists of the impacting striker bar, the input incident bar, the output transmission bar, and the specimen placed between the incident and transmission bars (see figure 1). all the bars are manufactured from the same material and have the same diameter, while the incident and transmission bar also have the same length selected according to parameters of the experiment and properties of the investigated specimen. typical instrumentation of the experiment consists of strain-gauges mounted on the incident and transmission bars, through-beam sensors for measurement of the striker impact velocity, and a high-speed camera for optical inspection of the experiment. the accelerated striker impacts the incident bar forming a stress-wave propagating through the incident bar until the bar-specimen interface is reached. on the specimen boundary, the part of the incident wave is reflected and part is transmitted through the specimen causing its deformation. the transmitted part of the initial pulse is recorded by the transmission bar strain-gauge. during the experiment, incident, reflected and transmitted pulses are recorded with strain-gauges (see figure 2). the recorded strain-gauge signals and known properties of the experimental setup (e.g. bar dimensions, young’s modulus, density, wave propagation velocity etc.) are used for the evaluation of the stress-strain diagram of the specimen for the given strain-rate. for valid experiment, two crucial requirements have to be fulfilled: i. satisfactory dynamic stress equilibrium and ii. approximately constant strain-rate. in the specimen, the wave is also reflected on the interface of the specimen with the transmission bar. therefore, a defined number of reflections has to pass through the specimen before the dynamic stress equilibrium is reached (see figure 3). the dynamic equilibrium condition in the experiment is crucial as the standard mathematical methods for the evaluation of the specimen response are valid only in the equilibrium state. ramp-in effects (also known as ringing effects a transient state caused by sudden incidence of a sharp pulse) and spurious oscillations of the specimen, negatively affecting dynamic equilibrium, can be significantly reduced by a pulse-shaping technique. a pulse shaper is usually a small cylinder of a soft material 25 http://dx.doi.org/10.14311/app.2019.25.0025 http://ojs.cvut.cz/ojs/index.php/app r. dvořák, p. koudelka, t. fíla acta polytechnica ctu proceedings figure 1. shpb apparatus figure 2. three measured signals figure 3. space-time diagram of the wave (soft copper, soft aluminium, paper etc.) with length in order of tenths or units of mm placed between the striker and the input bar. the pulse shaper causes slower raising of the incident wave and also attenuation of high frequency oscillations (see figure 4). as a result, the pulse shaper improves conditions of the experiment leading to good quality dynamic equilibrium and constant strain-rate during loading of the specimen [4–6]. in this paper, we concentrate on numerical aspects of simulations of the shpb experiments. finite element (fe) model of shpb was developed in ansys ls-dyna. a sensitivity analysis of the fe model of the bars was performed as an important part of this study to assess the influence of the selected meshing algorithm and element size on numerical results. then, the numerical model was calibrated according to experimental data. the second part of this contribution summarizes a parametric study of a pulse-shaper. length, diameter and impacting velocity were varied parameters and evaluation of their influence on shape of stress pulse was analyzed. figure 4. original and shaped wave 2. models and methods a numerical model of shpb apparatus consisting of striker bar, incident bar and the pulse shaper was created in ansys. the bars had the same diameter of 20 mm. no global constraints were prescribed to the numerical model. two-way surface to surface algorithm was used for contact boundary conditions between the individual elements of virtual shpb apparatus. two algorithms of meshing, implemented in ansys toolkit were compared (see figure 5). the direct volume meshing algorithm creates elements from within of the bars and causes generating very small elements on the edges of the bar. these small elements have negative influence on the computing time, because the time step size is calculated (and is proportional to) from the smallest element size. the direct meshing algorithm was compared with the sweeping algorithm for mesh generation, which chooses two faces that are topologically on the opposite sides of the body. these faces are called the source and target faces. algorithm meshes the source face with quadrilateral and triangular elements and then copies that mesh onto the target face. then it generates either hexahedral or wedge elements connecting the two faces and following the exterior topology of the body [7]. using this algorithm, larger elements were produced in the middle of the bar (see figure 5). the variation of element sizes and internal angles is apparently higher, when compared to directly generated mesh, but provides control over the achievable time-step via setting the mesh size on the edge of the bar. of course it is possible to create a regular mesh using meshing software specialized for geometries such as cylinders (ls-dyna/lspp), however, ansys was used as a pre-processor in this case for the ease of 26 vol. 25/2019 modelling of wave shapes during shpb measurement figure 5. outcome of the direct (top) and the sweeping method (bottom). automation, hence the mesh is a result of its meshing algorithms with only limited controlability. however, in our experience with simulating the shpb problems, the irregularity of the sweeped mesh produces negligible differences to ls-dyna/lspp cylinder meshes for most cases. after the sensitivity studies are concluded and critical factors identified, we use best quality approach for mesh generation. however, it was necessary to compare both mesh generation algorithms in terms of influence on the virtual stress-wave propagation. for this reason, stress wave was recorded on the impacted front of the bar and in the distance of two and four multiples of bar radii measured from the front of the bar (see figure 6). the stress-wave is almost identical for both models as can be seen in figure 6, in the distance equal to 2 diameters from the impact face. it is evident that from the distance of 2 diameters from the impacted face of the bar both models have the same stress wave evolution. this results proves the sweeping algorithm as a more suitable method for our task due to ability of indirectly control the time step using the size of the element. in this case, the direct meshing algorithm produced elements with five times shorter time step than sweeping algorithm. mentionable is the difference in the evolution of the stress at the striker-bar interface (0 r). for both cases automatic surface to surface contact algorithm figure 6. an evolution of the stress wave measured at the front of the bar (top), one diameter distance (middle) and two multiples of the diameter distance (bottom) was used. boundary conditions are identically the same for both models. this anomaly is probably the consequence of the contact algorithm failure at the edge of the cylinder, which is supposed to happen very occasionally. the algorithm probably could not recognize if the bars are in or out of the contact and was alternately connecting and disconnecting the coupling between interfaces. this phenomena should be investigated in further studies, however now is not essential for our purpose, that is to analyze pulse shaper influence on the stress wave. although both model results are from determined border of 4 radii comparable with theoretical assump27 r. dvořák, p. koudelka, t. fíla acta polytechnica ctu proceedings figure 7. comparison of the finest, the middle and the coarse mesh tions of a wave propagating phenomena, it is well known that fem can produce very unreliable results particularly in the analysis of stress waves propagation using explicit numerical methods. basically, smaller elements cause that higher frequencies pass through the material in the model. the chosen sweeping algorithm model had to be calibrated according to the experimental data. a series of void tests (impacts of the striker bar on the incident bar without a pulse shaper) using shpb was performed and the experimentally measured signals were compared with the signals from the fe model. unphysical behavior of the stress signal in the model with the rough mesh (40 elements along the circumference of the bar) can be seen in figure 7. small sized elements in the model allowed for propagation of high-frequencies that were not observed during the experiments. however, choosing between the middle sized (160 elements along the circumference) element model and the finest one (280 elements along the circumference) is not possible without comparing with experimental data. results of the models with the middle-sized and coarse mesh are shown in figure 8. it can be seen that results of the coarse mesh model showed the best agreement with the experimental data. on the basis of the results of the sensitivity study, a set of automated parametric simulations comprising ansys environment, ls prepost tool, ls-dyna solver, and matlab was peformed to assess influence of pulse-shaping on stress-waves. for the parametric study, the thickness and diameter of a cylindrical pulse-shaper were selected as the control variables. in the simulations, the diameter of pulse-shaper in the range of 5 − 8 mm and its thickness in the range of 0.5 − 1.5 mm were considered and the simulations were performed for the striker impact velocity in the range of 5 − 50 mm · s−1. 3. results in total, 130 simulations were peformed and the results were derived from the stress-wave shapes captured at the distance of four radii of the bars from its impacted figure 8. comparison of experiment data with the middle (top) and the rough (bottom) meshed model face. changes of the wave shape caused by different pulse-shaper geometries during the impact with velocity of 25 mm · s−1 can be seen in figure 9. the fe modelling was identified as a very efficient tool to investigate the phenomena. observed attributes of the wave were: (1.) the wave-length of the pulse, defined as the time interval between the first nonzero value of stress and the first decrease of stress bellow the value of 5 % of the maximum value achieved during measuring, (2.) the rise time of the wave, defined as the time interval between the first nonzero value of stress and achieving the first peak, (3.) the rate of oscillations in the first half of the pulse, defined as the sum spread from smoothed data in the time interval between the first nonzero stress value and half of the above defined pulse length. the length of the pulse slightly incrreases with longer pulse shaper (see figure 10). the fluctuations are smoothed. the time rise also slightly rises, however the effect of changing length is not as remarkable as using pulse shaper by itself. changing the diameter has an apparent influence on the attributes of the wave (see figure 11). with smaller diameters the wave is longer (1.2 ∼ 1.4 times), the time rise is also longer and oscillations get smoothed. these attributes of the pulse are ap28 vol. 25/2019 modelling of wave shapes during shpb measurement figure 9. influence of the pulse-shaper geometry figure 10. influence of the length of the ps propriate for achieving the stress equilibrium in the specimen. 3.1. the influence of the strain rate on the time rise for different geometries of the pulse shaper the results have a comparable trend except two cases (see figure 12) that can be identified as numerical error (probably during automatic postprocessing, another possibility is an imperfect processing of these two simulations e.g., due to not defined friction at the contact faces resulting in slipping of the pulse shaper and that negatively influenced the data). for diameters of 5 and 6 mm no distortion of results occurred, which means this mesh is capable to describe high frequencies generated by high-velocity impact (40 − 50 mm · s−1). the other curves have a comparable manner and two mentioned cases should be an error. it is evident that decreasing the diameter causes remarkable prolonging (from 2 to 4.5 times) of the rise time and increasing the length causes insignificant prolonging of the rise time. both effects are stronger during lower strain rates. figure 11. influence of the diameter of the ps 3.2. the influence of the strain rate on the length of the pulse for different geometries of the pulse shaper data for 7 and 8 mm pulse shaper are again nonnegligibly distorted and it is caused by the identical numerical error mentioned in the section above. moderate influence of the strain-rate is evident (see figure 13). lower strain rates cause prolonging (from 1.06 to 1.22 times) of the wave and this effect is more remarkable with decreasing diameter. length of the pulse shaper has a minimal influence, the remarkable cases are is 5 and 6 mm. 3.3. the influence of the strain rate on the oscillations of the pulse for different geometries of the pulse shaper it is evident, that pulse shapers with smaller diameters behave as a better filter (see figure 14). also the filtering of high frequencies is more effective during lower strain rates. the length of the pulse shaper has a negligible influence during slow impacts (5−30 mm · s−1). during high velocity impacts (40 − 50 mm · s−1) also the length has a remarkable influence. 4. conclusions the numerical model of the shpb apparatus was calibrated including the mesh sensitivity study. two types of mesh generation algorithms were analyzed and boundary where the stress wave has the similar shape for both models was identified. the border where the stress curve has similar shape for both models was established, size of elements was set up according to experimental data. a phenomenon that the mesh actually behaves as a filter of high frequencies was observed. following conclusions can be drawn based on the main parametric study: (1.) pulse-shapers with smaller diameter are more effective as filters of oscillations, well prolong the 29 r. dvořák, p. koudelka, t. fíla acta polytechnica ctu proceedings figure 12. influence of impacting velocity on a time rise (l length of the pulse shaper, d diameter of the pulse shaper figure 13. influence of impacting velocity on a pulse length (l length of the pulse shaper, d diameter of the pulse shaper figure 14. influence of impacting velocity on oscillations of the pulse (l length of the pulse shaper, d diameter of the pulse shaper 30 vol. 25/2019 modelling of wave shapes during shpb measurement time rise and the pulse length which is crucial for achieving stress equilibrium. (2.) the length of the pulse shaper has an insignificant influence. each mentioned phenomena is more remarkable during low strain rates. these conclusions are fully compatible with other studies [4–6] and with the experiments. list of symbols σ stress [mpa] εi incident pulse [–] εt transmitted pulse [–] εr reflected pulse [–] t time [ms] acknowledgements the research was supported by the czech science foundation (project no. 19-23675s). all the financial support is gratefully acknowledged. references [1] b. hopkinson. the scientific papers of bertram hopkinson. cambridge university press, 1921. [2] h. kolsky. an investigation of the mechanical properties of materials at very high rates of loading. proceedings of the physical society section b 62(11):676– 700, 1949. doi:10.1088/0370-1301/62/11/302. [3] w. w. chen, b. song. split hopkinson (kolsky) bar: design, testing and applications (mechanical engineering series). springer, 2010. [4] r. naghdabadi, m. ashrafi, j. arghavani. experimental and numerical investigation of pulse-shaped split hopkinson pressure bar test. materials science and engineering: a 539:285–293, 2012. doi:10.1016/j.msea.2012.01.095. [5] r. gerlach, s. k. sathianathan, c. siviour, n. petrinic. a novel method for pulse shaping of split hopkinson tensile bar signals. international journal of impact engineering 38(12):976–980, 2011. doi:10.1016/j.ijimpeng.2011.08.007. [6] a. b. shemirani, r. naghdabadi, m. ashrafi. experimental and numerical study on choosing proper pulse shapers for testing concrete specimens by split hopkinson pressure bar apparatus. construction and building materials 125:326–336, 2016. doi:10.1016/j.conbuildmat.2016.08.045. [7] j. hallquist. ls-dyna keyword users manual. livermore software technology corp, 2006. 31 https://doi.org/10.1088/0370-1301/62/11/302 https://doi.org/10.1016/j.msea.2012.01.095 https://doi.org/10.1016/j.ijimpeng.2011.08.007 https://doi.org/10.1016/j.conbuildmat.2016.08.045 acta polytechnica ctu proceedings 25:25–31, 2019 1 introduction 2 models and methods 3 results 3.1 the influence of the strain rate on the time rise for different geometries of the pulse shaper 3.2 the influence of the strain rate on the length of the pulse for different geometries of the pulse shaper 3.3 the influence of the strain rate on the oscillations of the pulse for different geometries of the pulse shaper 4 conclusions list of symbols acknowledgements references https://doi.org/10.14311/app.2022.33.0125 acta polytechnica ctu proceedings 33:125–132, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague high thermal conductivity concrete for energy piles luca di girolamoa,∗, gigliola ausiellob, gianpiero russoa, gabriella maronea a university of naples federico ii, polytechnique and basic sciences school, department of civil, building and enviromental engineering, via claudio 21, 80125, napoli, italy b university of naples federico ii, polytechnique and basic sciences school, department of civil, building and enviromental engineering, p.le tecchio 80, 80125, napoli, italy ∗ corresponding author: luca.digirolamo@unina.it abstract. research is increasingly focusing on thermal properties of concrete with the aim of reducing the heat exchange between buildings and environment. on the other hand, concretes with high thermal conductivity could have interesting applications in the field of thermo-active ground structures as geothermal energy piles (geps). this kind of foundations represent an environmentally friendly technology that allows exploiting the heat of the shallow earth surface to supply renewable energy for the air conditioning of buildings. geps are needed for structural and geotechnical reasons and allow recovering the installation costs connected to vertical boreholes. concrete drilled or driven piles are equipped with a primary circuit (pc) of high-density polyethylene plastic pipes attached to the reinforcement cages. thermal energy is extracted from or injected into the ground thought a carrier fluid that flows into the pipes of the pc. to improve the heat exchange between the pile and soil the thermal properties of the concrete should be considered as design parameters. concrete thermal conductivity, contrary to what happens for the buildings, should be increased to optimise the thermal performance of the geps. different solutions that modify the mix design of concrete are proposed to the aim of increasing the thermal performance of geps. keywords: energy piles, high conductivity concrete, sustainability. 1. introduction energy demand increasing is linked to the rapid economic development that is taking place worldwide. however, the energy requirement cannot continue to be satisfied using fossil energy sources which are responsible for climate changes and environmental pollution [1]. the use of renewable energy source is promoted by european parliament directive 2010/31/ue on energy performances of buildings [2]. geothermal energy is one out of the possible and most easily available renewable energy sources. in fact, below the ground surface, at depths greater than 10 m b.g.l, temperature of ground is nearly constant throughout the year. using this relatively constant temperature, the air conditioning of buildings, both in the heating phase and in the cooling phase, it is more efficient than other heating or cooling systems [3]. in the early 80’s geothermal energy was initially extracted, in some parts of central europe like austria and switzerland, from deep foundation elements like piles. in this technology, differently from conventional ground heat exchangers made by one or more u-shaped plastic absorber pile inserted in aăborehole, geps use thermal conductivity and thermal storage capacity of concrete because high-density polyethylene plastic pipes are installed directly in piles structures before concrete casting. thermal energy is extracted from or injected into the ground via a heat carrier fluid that flows into the pipes of the pc [4]. since geps combine both the structural and the energetic functions in a single solution, they are known to be cost effective [5]. in gep’s technology, concrete can play a very important role. usually, the intrinsic characteristics of this material, in relation to its traditional applications for super-structures, do not allow to have high performances from the energetic point of view because its relatively high thermal conductivity is the main cause of heat dispersion inside the buildings. for geothermal applications, concrete can gain a redemption opportunity. its high thermal conductivity can be further increased, increasing at the same time the efficiency of the geothermal plant and without dramatic increases of the stress level within the structural sections of the piles. on this last aspect of course, a generalisation is not opportune and any case should be carefully designed and checked. in this paper, the role of concrete in geps technology is analysed. the target is to analyse the effects of thermal conductivity increasing of concrete on energy and mechanical performances of geps. after an introduction about thermal properties of concrete and a review of literature about ways to enhance its ther125 https://doi.org/10.14311/app.2022.33.0125 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en l. di girolamo, g. ausiello, g. russo, g. marone acta polytechnica ctu proceedings figure 1. description of mechanism of heat transfer from primary circuit of energy pile to ground. mal conductivity, the evaluation of stresses in a geps structural was carried out through a finite element (fe) code software. 2. heat transfer in geothermal concrete energy piles heat transfer mechanism from the primary circuit of geps to soil (and vice versa) during heating or cooling of built structures is a complex mechanism that involves all the underground components of geothermal system (geps). primary circuit consists of heat exchanging hdpe loops incorporated within piles foundation, heat exchangers fluid that flows inside pipes loops and concrete of the pile. between the fluid circulating into the pipes inside the pile and the surrounding soil there is a thermal gradient due to differences of temperature and thermal properties of the fluid and the soil [6]. heat transfer in a concrete energy pile occurs by heat convection between carrier fluid and wall of pipes and heat conduction between pipes’ wall and concrete of pile and between concrete and soil [7]. usually, for heat transfer in geps convection and radiation mechanisms are negligible [8]. according to loveridge et al. [9] the larger dimensions of the diameter of the geps compared to traditional geothermal wells ensure that it cannot be considered a thermal steady state. in fact, since the concrete around the pipes can take several days to reach the steady state, the difference in the average temperature between the heat exchanger fluid and the average soil temperature on the edge of the exchanger cannot be considered constant. for this reason, thermal resistance of concrete could become a fundamental parameter to improve thermal performances of geps. the heat transfer between the primary circuit fluid and the soil is given as [6]: q = t1 − t5 rt ot (1) where t1 and t5 are fluid and soil temperatures, respectively, and rt ot is the total thermal resistance transfer, given as: rt ot = rfluid + rpipe + rconcrete + rground (2) the resistance of the following fluid is given as: rfluid = 1 2nπrih (3) where ri is the internal pipe radius, n is the number of pipes, and h is the convective heat transfer coefficient. the pipe thermal resistance is given as: rpipe = ln (r0/ri) 2nπkp (4) where r0 is the pipe outer radius and kp is the thermal conductivity of the pipe material. regarding thermal resistance of the concrete annular section, its steady state value could be calculated with the equation of the thermal resistance of a cylinder, but an assumption must be made for the effective inner radius of that cylinder reff . the concrete thermal resistance using the equivalent diameter approach is given as: rconcrete = ln (rb/reff ) 2πkc (5) where rb is pile radius, kc is concrete thermal conductivity, and reff is the effective radius reff = r0 √ n (6) the equivalent cylinder approach does not consider actual positioning of pipes. the mechanism is explained in figure 1. 126 vol. 33/2022 high thermal conductivity concrete for energy piles figure 2. cooling (a) and heating (b) energy usage of a building in chicago compared to different concrete thermal conductivities (image reprinted from [6] and [10]). 3. heat transfer enhancement: thermal conductivity of concrete equation 2 and the followings show that thermal conductivity k and/or convective heat transfer coefficient h increasing correspond to thermal resistance decreasing. in this way, the heat transfer between the primary circuit fluid and the soil will increase. according to kwang et al. [10], increasing foundation thermal conductivity reduces the heating and cooling energy demands of a building (figure 2). therefore, to increase heat exchange between energy piles and soil, designers could work on thermal properties of thermal fluid, thermal properties of pipes, by optimizing pile dimensions and geometrical arrangement of pipes or by improving thermal properties of concrete modifying its mix design. this study focused on this later aspect. conduction is the most important mechanism of heat transfer for concrete. thermal and mechanical properties of concrete are influenced by its porosity [11]. according to asadi et al. [12], moisture, the specimen’s condition and aggregate volume fraction appeared to be the main factors influencing the thermal conductivity of concrete. however, the most effective factors on the thermal conductivity are the water/cement ratio and type of admixture. the analysis of the variables that influence thermal conductivity, although in many studies is aimed at its reduction, is oriented towards a useful definition of a relationship between the density of concrete and its conductivity. anyway, because the target is the realization of geps, any modification of mix design should not alter piles’ resistance. this kind of foundations in comparison to the ordinary piles are subjected to additional loading induced by thermal variations. additional strains and stresses induced by the thermo mechanical coupled loadings occur. from this point of view the improvement of both thermal and mechanical properties could be an interesting point. this aim could be achieved developing mix designs which can boost compactness and density at the same time. according to kim [13] thermal conductivities of concrete mixtures were revealed independent of curing age. in relation to the components of the mixture and the relationships between them, kim et al. (2003) show interesting results. first, the larger the amount of coarse aggregate in a concrete mix the higher the thermal conductivity of the mixture because of the well-known higher thermal conductivity value of aggregate than other constituents of the concrete mix. furthermore, the nature of the aggregate also appears to have an influence on conductivity [14]. the siliceous aggregate, compared to the carbonate one, having a more regular molecular structure (crystallinity), has greater thermal conductivity. even with an increase in the fine aggregate, an increase in thermal conductivity is obtained [13]. low values of w/c ratio lead to higher conductivity of the concrete mix. in fact, the thermal conductivity of the cement is higher than that of the water and above all the water is responsible for the formation of the voids that are created after its natural evaporation. so, a lower w/c ratio corresponds to a lower porosity of the concrete. thermal conductivity decreases linearly with temperature but this dependence is spread over large temperature ranges. as a matter of fact, in the temperature range in which the geps work the dependence of the conductivity from the temperature it is nearly negligible. finally, thermal conductivity increases when voids of concrete are filled with water instead than air. zhang et al. [15] shows that under saturated conditions thermal conductivity increases by at least 50%. other studies [16–19] provide reports to parameterize the increases in the k value as a function of the hu127 l. di girolamo, g. ausiello, g. russo, g. marone acta polytechnica ctu proceedings midity and of the weight of the concrete both due to the absorption of water. therefore, pile foundations made by concrete directly cast in place in soils below the groundwater table can boast greater performance as geothermal heat exchanger. starting from these assumptions, it is possible to identify some precautions that mix design should have to optimize both the structural and the energy performance. the compactness to be maximized is certainly the right solution, which can be obviously obtained following different procedures. high strength and significant compactness also make the cement matrix physically impenetrable to the aggressions of the environment, the microporosity being reduced too. both are a guarantee of durability, a fundamental aspect of the performance for any concrete structural object and this is especially true for a foundation pile and even more for an energy foundation pile. in the literature, several authors [20–24] have carried out lcc and lca evaluations of geothermal pump systems, equipped with energy piles also. they have also compared them with more conventional heating and cooling systems. these studies have shown a reduction of environmental impact of ghps than conventional plants. in economic terms, according to morrone et al, the npv (net present value) varies from about €12.000 to about €33.000 while the payback period varies from 4 to 11 years for geps installations depending on the climate zone considered in italy [24]. however, other considerations and insights on lcc and lca deserve a separate study. 4. guidelines for a mix design with high thermal conductivity for piles in new concrete mixes, fillers are increasingly used with the main function of enhancing the workability in the wet state. it is known, however, that the performance changes are mainly characterized by an increase in the resistance, since the presence of the filler reduces the microporosity due to acompactness increasing. calcareous filler, with its physical action, only increases the compactness, while the pozzolanic filler, with its chemical action, also improves the resistance, involving lime in the formation of a second family of hydrate compounds [25]. kim [13] and demirboğa [26] test the effects of some pozzolanic fillers in partial replacement of cement and detect reductions in thermal conductivity. in fact, compared to the large and fine aggregate, the main function of the filler is that of a micrometric aggregate that can reduce microporosity. therefore, inserted in the mix design as a new component, it can only increase the compactness at the micrometric scale and, consequently, increase density and mechanical resistance. this creates the conditions for greater thermal conductivity. another correction of the mixture is the use of the new superplasticizing additives, also called water reducers, which allow to keep with the same degree of workability a low value of the w/c ratio. this suggestion among many of the guidelines on mix composition is the most important because thermal conductivity is inversely proportional to the w/c ratio. as a result, the viscosity is reduced without causing the segregation of the concrete. in addition, with their deflocculating action, they limit the formation of clots of the cement granules, which would form when dispersed in the water. fillers and superplasticizers are two additional components that enter the new self-compacting concretes, in response to construction site management needs. with rather low w/c ratios and the consequence of reducing microporosity, self-compacting concrete (scc) [25] achieves high resistance values. a "technology in technology" of scc concrete is that of continuous flight auger (cfa) piles. it is a technique to install replacement piles combining and axial thrust and an action of a torque on a continuous flight auger inserted in soil. the scc could be pumped to the down hollow stem of the auger thanks to the exceptional workability characteristics. while scc is pumped, without rotation the auger is pulled out. in this way, temporary supported are not necessary. once the concrete has been cast in place pre-assembled reinforcement cage, is easily inserted into wet concrete. in the case of geps also geothermal pipes are fixed on the pre-assembled reinforcement cage. it could be added that the scc casting performed from bottom allows the mixture to incorporate less air and therefore to reduce even more the microporosity. the possibility of execution in soils of any nature, even in the presence of groundwater, widens the application horizon, combining greater conductivity and mechanical resistance with easy execution on site, without cost increases [25, 27]. asadi [12] relates the thermal conductivity of scc concrete with the increase in temperature, highlighting intervals with increases and decreases starting from the room temperature value and reports the results of some studies done by khaliq and kodur [28] suggesting the following relationship between thermal conductivity and temperature for a range of temperature of concrete between 20◦c and 400 ◦c (equation 7). k = 3.12 − 0.0045t (7) as can be seen from equation 7, the increase in conductivity is inversely proportional to the temperature of concrete. considering that the range of temperatures at which the geps work is very close to the lower limit of the range proposed by the authors, it follows that the conductivity values of the concrete remain quite high. the use of steel fibers seems another interesting guideline, considering that the thermal conductivity of steel is approximately 50 times greater than 128 vol. 33/2022 high thermal conductivity concrete for energy piles that of concrete and that the density of steel is approximately 3ătimes that of concrete. adeyanju and manoha [29] show that the fiber reinforced concrete (frc) with steel fibers reaches high levels of thermal conductivity, of about 2,0-2,5 w/m◦c compared to 1,01,3ăw/m◦c of a concrete without fibers, with increases well above 100%. this increase is also accompanied by a higher density, once again signing a direct proportionality of the conductivity with both compactness and density. nagy, nehme and szagri [30], comparing various investigations on frcs with steel fibers, they find out values of thermal conductivity ranging from 2,0 to w/m◦c 3,2 w/m◦c. on the other hand, khaliq, kodur, experiment with composite details. these are fiber-reinforced sccs with thermal conductivity values ranging from 3,0 w/m◦c to 3,5 w/m◦c. obviously, the size and quantity of the fibers also influence the formation of the voids. indeed, nagy, nehme and szagri [30] find out that by increasing the quantity of steel fibers the thermal conductivity starts to decrease. this phenomenon is due to the formation of voids for effect of the greater quantity of fibers. lie and kodur [14] also report an interesting relationship between thermal conductivity and temperature in a concrete in which the effects of steel fibers and the nature of the aggregate are added. equation 8 is related to siliceous aggregate for a range of concrete temperature between 0 ◦c and 200 ◦c, while equation 9 is related to carbonates aggregate for a range of concrete temperature between 0 ◦c and 500 ◦c. k = 3.22 − 0.007t (8) k = 2.00 − 0.001775t (9) the comparisons show the order of magnitude of the thermal conductivity k which, at temperatures close to those of the soil, presents high values when the compactness is high and there is the right amount of steel fibers. furthermore, in all these mixtures with steel fiber reinforcement, characterized by high mechanical properties, the action of the fibers does not induce any improvement in compressive strength, but only in tensile one. furthermore, metal fibers have little influence on the heat capacity at low temperatures [14] even for high quantities of fibers [30]. the integration with microcapsules in pcm is another possible guideline. asadi [12] found that the thermal conductivity of cementitious materials with pcm is lower than conventional cementitious materials and several authors have parameterized this decreasing [31, 32]. the encapsulation system is crucial because the use of small quantities of microcapsules could reduce the microporosity. pcm microcapsules could increase thermal capacity without having too strong effects on the reduction of thermal conductivity, improving the storage capacity of the concrete dampening indirectly the already modest temperature fluctuations affecting the soil supporting the steady-state hypothesis. as shown in the section there are a lot of potential paths to enhance the efficiency of concrete piles as heat exchangers optimizing the heat transmission mechanism. 5. effect of concrete thermal conductivityăon pile-soil thermo-mechanical interaction heat transfer enhancement in concrete improves the heat transfer process among the heat exchanger piles and the soil. several studies report the beneficial effects in terms of energy performance of geothermal systems, but the conductivity improvement should be considered also in terms of load carrying capacity of the pile foundation that should however not be compromised [5]. if the heat transfer phenomena inside the pile are modified, additional investigations about pile-soil thermo-mechanical interaction are needed. herein the effects of concrete thermal conductivity improvement on the short term thermo-mechanical behaviour of a single energy pile are briefly presented. four different cases are compared. case 0 where concrete thermal conductivity is assumed 2,4 w/mk, case 1, case 2 and case 3 where thermal conductivity is assumed 2,88 w/mk, 3,6 w/mk and 4,8 w/mk respectively. a cfa pile of 13 m length with 0,60 m diameter, embedded in a multi-layered pyroclastic deposit with a socket in yellow tuff 3,00 m deep is modelled. the pile is the prototype used recently in the foundation project of a new trading centre in city of napoli [33]. transient fully coupled analyses are carried out with the fe software plaxis 2d. one day of heating, characterized by a maximum temperature variation of 13 ◦c, and one day of cooling, characterized by a maximum temperature variation of 17 ◦c, are simulated [34]. to reproduce realistic operational conditions, thermal loadings are combined to the live mechanical top load. the mechanical load, assumed as the 40% of the pile bearing capacity, is thus applied and kept constant during thermal loadings fluctuations. the effects of the thermal conductivity on pile-soil thermo mechanical interaction are analysed in terms of axial loading along the pile shaft. all the thermo-mechanical cases presented (case 0, case 1, case 2 and case 3) are compared to the purely mechanical case (mechanical) where only the live mechanical top load, (i.e. 2400 kn), is applied to the pile. the axial load along pile shaft is reported during and at the end of the cooling phase (respectively labelled c for cooling and ec for end cooling) and during and at the end of the heating phase (h for heating and eh for end of heating) for each thermomechanical case (figure 3). from figure 3 it could be 129 l. di girolamo, g. ausiello, g. russo, g. marone acta polytechnica ctu proceedings figure 3. c axial loads along the pile shaft for different thermal conductivity of concrete. observed that an improvement of concrete thermal conductivity, for a daily thermal cycle, does not determine significant effect on the thermal axial loading both for cooling and heating modes. for case 1 and case 2 the maximum thermal axial load increasing during heating and decreasing during cooling are coincident and equal to 390 kn and 320 kn respectively. in case 3 the maximum thermal axial load increasing during heating and decreasing during cooling are almost 380 kn and 330 kn respectively. in terms of axial load, increments of 20%,50% and 100% of the thermal conductivity of concrete determine comparable effects. comparing case 0 to case 3, corresponding to minimum and maximum thermal conductivity respectively, a difference of almost 4% of the thermal axial load is observed. the enhance of concrete thermal conductivity, for a short time analysis, seems not to determine significant axial thermal load variations. for long term analyses should also be considered that an enhance of thermal conductivity could determine a beneficial effect in terms of temperature distribution on the cross section and therefore a more uniform axial stress distribution. as a matter of fact, normal stresses induced by mechanical and thermal loadings are not uniform in the cross section. the trend of the axial stress is characterised by sudden variations close to the primary circuit position. an improvement of thermal properties of concrete could mitigate this effect and could be beneficial even in terms of durability of concrete. 6. conclusions in this paper the thermal conductivity of concrete and the use of high conductivity concretes for special applications like geps was analysed. after an introduction about thermal properties of concrete and a review of literature about ways to enhance its thermal conductivity, guidelines for a mix design with high thermal conductivity was laid out and simulations through fe analyses about the potential effects of the thermal improvement on mechanical behaviour were reported. the axial stress induced by live loadings combined to heating or cooling was evaluated for a potential energy cfa pile characterised by different thermal conductivities. for a concrete with high thermal conductivity low w/c ratio is necessary to reduce the voids inside the cement matrix. for this reason, the use of fillers, superplasticizers or both at the same time is desirable to achieve the goal. the addition of steel fibers within the mix design of concrete is another possible expedient. in addition to improving the mechanical capabilities of concrete, they also increase its thermal conductivity with negligible influence on thermal capacity at low temperatures. the effect of microcapsules with pcm should not affect conductivity very much but an increase in the thermal capacity is typically expected. the simulations carried out with fe software for a prototype pile embedded in a layered soil profile similar to a real foundation project in the eastern area of the city of napoli has shown that the change in the thermal properties of the concreted is not producing large effects on the structural performance. computed axial compression loading changes for the thermal induced strains is in a range of ±15% compared to the axial loadings obtained by the head load corresponding to a typical 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[34] g. marone, l. di girolamo, g. russo. studio parametrico di un palo singolo sottoposto a carichi termomeccanici, iagig 2019, 2019. 132 https://doi.org/10.1016/j.cemconres.2011.06.012 https://doi.org/10.4236/jmmce.2011.1015111 https://doi.org/10.1016/j.egypro.2015.11.616 https://doi.org/10.1016/j.conbuildmat.2016.12.185 https://doi.org/10.1016/j.conbuildmat.2012.04.042 https://doi.org/10.1007/s11440-012-0177-4 acta polytechnica ctu proceedings doi:10.14311/app.2019.24.0021 acta polytechnica ctu proceedings 24:21–30, 2019 © czech technical university in prague, 2019 available online at http://ojs.cvut.cz/ojs/index.php/app diamond detectors for fast transient scram measurements – the detector development ondřej nováka,∗, lubomír sklenkaa, michal pomorskib, clement lyndec a czech technical university in prague, faculty of nuclear sciences and physical engineering, department of nuclear reactors, v holešovičkách 2, 180 00 prague 8, czech republic b cea (french atomic energy comission), list, diamond sensors laboratory, cea saclay, 91191 gif-sur-yvette cedex, france c cea (french atomic energy comission), list, sensors and electronic architectures laboratory, cea saclay, 91191 gif-sur-yvette cedex, france ∗ corresponding author: ondrej.novak2@fjfi.cvut.cz abstract. this paper presents a development of special diamond detector for time dependent neutron flux measurements in a nuclear reactor. neutron flux changes during scram are difficult to measure by existing detector technologies. therefore, a new detector is developed. this new type diamond based detector should allow to perform fast measurements of neutron flux during scram with high pulse count rate. the diamond detector design and signal processing are described in this paper including the pulse shape discrimination method. in addition to theoretical detector study, three detector was manufactured at cea saclay. experience gathered during manufacturing is described as well. keywords: diamond detector, neutron detection, nuclear reactor, pulse shape analysis, boron conversion, fast transient, scram, vr-1 reactor. 1. introduction reactor operation is connected with events that have direct impact on reactor operation and reactor nuclear safety. these events are commonly defined as postulated initiating events (pies) [1] and are used in deterministic safety theory. one area of pie contains fast changes of reactivity (like ria1 or scram2). those events initiate a fast transient process, where the neutron flux distribution and parameters are rapidly changed. deep and clear understanding of the scram fast transient processes are highly challenging due to several reasons. they are complex, space dependent and mostly, they happen very fast. in addition, they have significant impact on the nuclear safety. for better understanding, the currently available experimental instrumentation should be improved (new, faster detectors and detecting systems) which will result in better reactor core monitoring, faster, and more accurate prediction of core behavior. this improvement in the field of fast transient experiments will conclude in enhancement of codes assigned for fast transient studies. precise study of the ria or scram flux distribution changes that are happening during the event have not been carried out due to lack of high performance neutron detectors [2, 3]. in other words, a new detection system must be developed. the final goal of this study is to measure neutron flux behavior during 1reactivity initiated accident 2emergency shutdown of a nuclear reactor scram. this means to: (1.) develop fast performing detector (including electronic and data processing) that will be able to provide fast accurate measurement of neutron flux inside reactor core; (2.) provide calculation tools that will be able to simulate the experiment proposed; (3.) design and perform the scram measurement (at the vr-1 reactor) with new detector developed; (4.) analyze experimental and data calculated and conclude results. this article presents the progress in the first step – the detector development. 2. diamond detectors current detectors available on the market are not fast enough to perform high accuracy short measurements for the rod scram instrumentation. due to this, new detector needs to be developed. to demonstrate capabilities requested from the new detector, the following example is presented: one case of fast transient measurement application is a study of the neutron flux changes during the fast control rod insertion (scram). the control rods are usually inserted in few seconds. at the vr-1 reactor, scram duration is less than 0.5 s. during this very short time period, large amount of interaction needs to be recorded. if 500 time steps during scram are requested, each step should contain more than one thousand thermal 21 http://dx.doi.org/10.14311/app.2019.24.0021 http://ojs.cvut.cz/ojs/index.php/app o. novák, l. sklenka, m. pomorski, c. lynde acta polytechnica ctu proceedings neutron interactions, in conclusion, more than one million neutron pulses per second should be recorded. however, the total number of interaction will be several times higher because of the gamma, electrons and fast neutrons, which will interact in the detector active volume as well. based on the example presented, the experimental setup should be fast enough to collect and process more than ten million events per second. this puts demands on the detector, the amplifier and the analog digital (ad) converter. requirements stated are difficult to fulfill. based on results from the first tests [4]3, the detectors made of diamond crystals are highly promising (if this technology is developed and adjusted for this particular application). the diamond detectors contain a single crystal diamond that serves as active volume. the charge deposited by ionizing interactions is collected by an electrical field created by the applied voltage on two electrodes deposited on the surface. the diamond mono crystals have unique material properties and are very promising for the application in the detection of chemical compounds and radiation [5]. general overview of the diamond based detectors is summarized in [6]. the detectors offer variety of options in the crystal size, design and in the overall detector construction. main advantages of the diamond detector are: • pulse length from the diamond detector is around 10 ns. this very short duration results in almost no detector dead time and therefore theoretically, the detector yield can reach up to 108 pulses per second. this yield can be increased by decrease of the crystal thickness or increase of voltage applied. thanks to these modifications, the pulse duration can be shortened up to 5 ns. • detectors size is extremely small and therefore it can be used for point measurement. in addition, thanks to tiny size, there is almost no impact of the detector on studied system. • diamond is a solid form of carbon with a diamond cubic crystal structure. this special structure results in its beauty and in its special properties [7]. the diamond crystal is an unique material, stable in vast chemical environments, and especially it has high radiation hardness [5, 8–10]. crystal itself is the crucial part of the detector, compared to electrodes, which are not affected by radiation damage. the radiation hardness of the detector is necessary condition for the diamond detectors to be considered as appropriate for such application. • charge collection conserves total charge deposited and also the energy deposition location. the type of charge deposition (point or linear) is reflected in pulse shape. thanks to this detector property, 3the first tests [4] contains results of the first vr-1 reactor measurements with the diamond detectors. these experiments were done in 2014 and showed possible application of the diamond detectors for neutron measurements. source particle of interaction can be determined by pulse shape analysis [11] and later can be used for discrimination. moreover, this allows to count interactions with lower amplitude than gamma background. on the other hand, the diamond detectors are difficult to manufacture, requires advance technologies for data processing and have several problems that needs to be solved [5, 6]. main detector challenges are: • the performance of electronics – signal peak duration is less than 10 ns that results in high requirements on speed of signal and data processing. from our experience, the signal sampling should be higher than 2 gs/s4 and bandwidth above 250 mhz. for higher impulse rates, the ad converter needs to work in almost continuous mode without the dead time for data processing. • the crystal quality – the purity of crystals (low birefringence) is crucial for diamond application. our work is struggling with impurities in the crystals and with non ideal crystallographic structure. impurities and dislocation results in decrease of the breakdown voltage. this results in decrease of detector performance and limits the charge collection. manufacturing of pure diamond crystal underwent vast development during last decades and therefore the diamond crystals can be more used in scientific application. however, still the quality of the electronical grade single crystal diamonds has some fluctuations. • the quality of the conversion layer and electrode – the conversion layers and electrodes are deposited by the physical vapour deposition technology and sometimes the deposited layer laminates or cracks. thicknesses, materials and deposition technology therefore needs to be further enhanced. • the neutron flux intensity – active volume of the crystal is relatively small, and reaching high number of interactions per second can be difficult if the flux is not intense enough [12]. in conclusion, the diamond detectors are promising for precise measurements of neutron flux. however, the diamond detectors for scram measurement that detects thermal neutrons do not exist. therefore we are forced to develop our own detector. 2.1. current diamond development overview the diamond detector technology is currently under fast development [5, 6]. compared to silicon semiconductors, the diamond technology is delayed due to problems with crystal manufacturing. the process of making artificial diamond is difficult and preparation of clean diamond crystals is much more challenging. due to that, the diamond detectors are not often used 4giga samples per second 22 vol. 24/2019 diamond detectors for fast transient scram measurements and are currently being developed. diamond detectors are mostly used on particle accelerators. current research activities still include studies of diamond crystal itself (behavior, manufacturing, processing,...). in addition to this, the diamond detector applications are explored and enhanced. with the improvement of diamond manufacturing technology during last decades, more research is now carried out in field of the diamond detectors. general overview of chemical vapor deposited (cvd) diamond application in radiation detection is presented in [5]. the diamond detectors are developed for variety of application in medicine or accelerators. the diamond based detector with possible application for neutron measurement are currently developed only at ctu in collaboration with cea saclay (this work) and tu vienna under prof. erich griesmayer [13]. griesmayer’s research group is not focused at the fast transient measurements but more at the beam applications or spectrum measurements. they are developing diamond based beam monitors, diamond detectors for neutron spectroscopy for fast reactors [14], testing different convectors [13, 15], and more. their closest detector type to our is the li converter based detector for thermal neutron measurements [16]. the griesmayer’s research group use the pulse shape analysis for interpretation of measured interactions. they summarize this method here [11, 17, 18]. 3. proposed diamond detector design the diamond detectors are made of a single crystal electronic grade crystal (elsc), an electrode material and a conversion layer. two largest surfaces of the diamond crystal are covered with the super-thin electrode. the conversion layer is placed on the electrode surface. the electrodes and conversion layers are deposited by the physical vapour deposition (pvd). after the deposition, the wires are connected by a special conductive epoxy glue to the electrodes and the detector is ready for measurements. scheme of this "sandwich" detector design is shown in figure 1. for the neutron detection direct and indirect method can be used [19]. direct method uses direct interaction of neutrons with carbon nuclei [15], but this can be used for fast neutrons only. thermal neutrons works better with the indirect method, where the converting material is introduced. several materials [20–22] can be used for neutron conversion to charged particle, which is then detected. • b4c – the 10b isotope is frequently used conversion material for the neutron detection. the boron nuclei absorbs the neutron and emits alpha particle with energy of cca 2 mev. the 10b isotope has high cross sections for n alpha interaction. the alpha particle can be detected in the crystal. the energy is deposited point wisely so it creates signal with specific shape. based on the shape, the interaction can be identified. • li – the 6li isotope is frequently used as a conversion material as well. the cross section for the n alpha reaction is around one order smaller than for the 10b. on the other hand, the particle emitted has higher energy. • uranium – the neutron induced fission on 235u creates fission fragments that can be detected in the crystal. these fragments carry around 160 mev and has short range, so they deposit all energy in short distance from the fission interaction. this conversion material creates very high pulses in the detector, however the 235u isotope is a nuclear material which requires special attention (nuclear security, proliferation,...). • hydrogen – the neutron proton interaction in the hydrogen can be used to produce charged particles, that can interact in the detector volume [15]. for our application, it is difficult to place hydrogen on the electrode with sufficient density. in addition to direct deposition of the conversion material, the material can be also dispersed in the crystal itself (something like the gaseous detectors with b4f ). this option has problems with a decrease of breakdown voltage and low concentration of the conversion material and therefore it lowers detection efficiency. for our detector, we selected b4c as conversion layer due to several reasons: • high cross sections for neutron alpha interaction, • high reaction rate, • good mechanical properties, stability, • possibility to use pvd for deposition, • vast experience in nuclear field with this conversion material, • cheap and without restriction. another construction part of the detector is the epoxy glue for wire connection. conductivity of this glue is usually ensured by silver element. however, silver creates as any metal free electrons (from gamma interaction) and based on experience from first measurements [4], is advised to not use metal based glues for the reduction of electron generation. current market offers the carbon epoxy which can be used instead of silver based epoxies. 3.1. conversion layer thickness optimization the simple detector model was created in the mcnp code to calculate the optimal conversion layer thickness. the simple model contain – the crystal, the conversion layer and the neutron source (see figure 2). results of this calculation are plotted in figure 3. as shown, the increase of thickness provides more particles with lower energies. the limit thickness is around 23 o. novák, l. sklenka, m. pomorski, c. lynde acta polytechnica ctu proceedings figure 1. diamond detector: diamond crystal (white), electrodes (green), conversion layer (blue). figure 2. simple detector model in mcnp (detector – black rectangle, electrode – green, conversion layer – blue, neutron source – black dot) 3.5 µm. above this value, no more alpha particles are entering in the diamond crystal. the optimal thickness is strongly dependent on the data analysis (whether it is possible to discriminate gamma from low energy alphas or not). the pulse shape analysis can distinguish between alpha interactions and other although the alpha particles entering the diamond crystal have low energy. in other words, the thickness can be 3.5 µm. for the pulse height discrimination, low energy alphas cannot be detected, because the peak height is below the background level. in contrary, the thicker layer is more likely to suffer delamination or have other structural problems. therefore it is better to use thinner layer. in our detectors, the conversion layers were 1 – 1.5 µm thick. 3.2. interactions in the detector all charged particles interact in the diamond detector, however, they differ in the energy deposition method. the alpha particle deposits its energy point vise (bragg peak [21]), compared to gamma and elecparticle signal alpha square pulse electrons, gamma triangular pulse neutron mixed pulse table 1. signal shape for different particles trons which use the linear deposition. this difference is useful in separation of neutron signals from others. the energy deposited by alpha interaction is too low, so these peaks are mixed in other interactions, and therefore the pulse height discrimination is not applicable. on the other hand, the pulse shape separation can be used thanks to the different energy deposition method. for this application, the neutron detection is crucial. neutrons are commonly detected in this detector indirectly. at first, the neutron interacts with boron in the conversion layer. the neutron capture results in the alpha particle emission. then the alpha particle interacts with the diamond crystal, depositing its energy to holes and free electrons. due to electrical field introduced by high voltage applied on electrodes, holes and electrons drifts to electrodes and charge is collected. usually 100 v per 100 µm is used. collected charge creates typical rectangular pulse [11]. the peak height can be adjusted by high voltage change. if higher voltage is applied, the charge is collected faster and therefore the peak duration is shorter and the peak height is increased. neutron can also (rarely) interact directly in the crystal [15], with different signal shape. for this application proposed, direct interactions are not important. compared to previous cases, electron and gamma particles interact in the diamond crystal differently. the energy is deposited linearly on the whole path of the traversing particle. this results in different ionization and therefore the peak has different – triangular – shape [11]. for both interactions, pulse area corresponds to absolute value of deposited energy. pulse shape depends on type of energy deposition and can be used for the interaction separation [11]. particles and their interactions are noted in the table 1. the difference between shapes created by the alpha and the electron interaction in the crystal are shown in figure 4. the cases 1) and 2) show the alpha interaction in different crystal areas. on the right side of the figure, the theoretical shapes produced by the interaction are displayed. the case 3) represents the electron or gamma energy deposition in the crystal and its pulse shape. as it was mentioned previously, the direct neutron interaction creates something like stair shaped pulse. 3.3. pulse shape method implementation after the interaction, the charge is collected by electrical field. the detector produces pulses with duration 24 vol. 24/2019 diamond detectors for fast transient scram measurements figure 3. energy spectrum on border of the crystal for different conversion layer thicknesses figure 4. theoretical interaction of 1) and 2) alpha particles; 3) gamma or electrons [11] (a area, a amplitude, w width, bw base width) 25 o. novák, l. sklenka, m. pomorski, c. lynde acta polytechnica ctu proceedings 8 νs (deepens on applied voltage and diamond thickness) approximately. the signal from the detector is firstly amplified by the charge sensitive amplifier. then the whole pulse is recorded for the pulse shape analysis. the ad converter should have high bandwidth and high sampling rate so the the shape of signal is preserved. our recommendation is to have bandwidth above 250 mhz and sampling above 2 gs/s. for recording purposes, the pulse is identified by pulse-height separation (only for the basic noise signal separation) and values are recorded. 5 recorded pulses are later processed by the matlab script (developed in collaboration with cea, based on [11]) which characterize each pulse by several parameters: • peak height • mid height width – this is fwhm equivalent • drift time • qtot – total deposited energy • form factor – this special coefficient compares the calculated area vs the measured area. the calculated area is a rectangular shaped (peak height multiplied by peak length). if values of calculated and measured area are close, pulse shape should be rectangular as well (see figure 5). • qtail – area (energy) of the peak tail part (after peak maximum) • qtail vs qtot tail vs total area the processing matlab script produces two output files. one text file, with above mentioned parameters for each pulse, and one plot file with plotted results (see figure 6). for demonstration, data from an alpha source measurement (including gamma and background) were processed by the script and are presented in the figure 6. alpha interactions cannot be separated by the pulse height method only (commonly used discrimination method). as it is possible to see in the upper second chart, interactions from alpha particles includes all pulse heights (marked by green). some of them are higher than other pulses from different interactions, but most of them has similar peak height. this means, that pulse height separation will result in much lower yield than possible. however, as it was described previously, the alpha interactions can be separated by their shape using the form factor and the mid height width parameter. the orange circle in figure 6 marks all alpha interactions (low form factor and similar mid height width). form factor needs to be close to 1 and mid height width is around 6 νs in this case. as shown, by simple conditioning of the form factor value, the alpha interactions can be separated from other signals. combination of other parameters can be used to enhance the separation. 5in our case we use the dt5743 8 channel 12bit 3.2 gs/s switched capacitor digitizer (details at caen web page [23]). figure 5. the form factor schematic [11] 4. diamond detector manufacturing after large theoretical studies and simulations, three detectors were manufactured in cea saclay from one diamond crystal (el sc plate6 3.0x3.0mm, 0.50mm thick). during manufacturing and testing several problems occurred, some of them were successfully solved, some still needs more research effort. design parameters of manufactured detectors are noted in table 2. detailed description of the detector manufacturing, including problem faced is presented in the following sections. 4.1. the first detector parameter value diamond thickness 0.5 mm diamond size 3 mm x 3 mm electrode material aluminum conversion layer material b4c table 2. parameters of the diamond detector the first detector was manufactured after the crystal inspection and cleaning. two layers of material were deposited on each side of the diamond crystal by the technique of physical vapour deposition. the first, very thin layer of conductive metal (aluminum) serves as the electrode. the second, boron layer serves as the conversion layer that uses the n alpha interaction to convert non charged particles (neutrons) to charged particles (alpha). both layers are shown in figures 7 and 8. electrodes were round shaped and the conversion layers were square shaped. probably, due to higher thickens of the second conversion layer, several cracks appeared (see figure 8). however, these structural problems have no influence on the conversion itself. after the electrode deposition, the wires were connected to the electrodes by the conductive resin. in this case, the silver resin was used. for the other detectors, we used the conductive carbon epoxy (to decrease gamma – electron interaction). the first detector with wires connected is shown in figure 9. 6electronic grade single crystal, the diamond crystal was manufactured by the element six company. 26 vol. 24/2019 diamond detectors for fast transient scram measurements f ig ur e 6. r es ul ts fr om th e al ph a m ea su re m en t af te r th e sc rip t pr oc es sin g pl ot s sh ow di ffe re nt pa ra m et er s fo r m ea su re d in te ra ct io ns . o ra ng e ci rc le m ar ks th e al ph a in te ra ct io ns 27 o. novák, l. sklenka, m. pomorski, c. lynde acta polytechnica ctu proceedings figure 7. the electrode and the conversion layer at diamond crystal first side (first detector) figure 8. the electrode and the conversion layer at the diamond crystal second side (first detector) for the detector mechanical protection, the first detector was sealed in a small flask filled with a nonconductive epoxy. the detector was then installed in a plastic box and connected to the bnc connector as shown in the figure 10. after several measurements with the first detector, the bridge current occurred. this means, when the high voltage (up to 500 v) was applied, the current started to flow through the detector. the breakpoint was somewhere above 50 v. sometimes higher, sometimes lower. our first idea was that the current is caused by some structural problems in the crystal (probably some dislocations). therefore to repair the detector, we tried to heat the detector up to 70 ◦c (to remove the dislocations from crystal)7. this method did not help and thus the first detector was dismantled, the crystal was cleaned and used for a new detector. we expected, that the conductive connection is somewhere over the edge of the crystal (probably some impurities in the non conductive epoxy made conductive connection). this problem should be solved by cleaning. our other idea was that structural problems of the crystal resulted in bridger current and the previous treatment (70 ◦c) was not successful. however, during the cleaning process, the crystal is boiled in acid for long time period. this high temperature treatment can repair the crystal. 7the temperature 70 ◦c was the highest possible temperature that does not destroy the detector. figure 9. the diamond detector with wire glued to the electrode by silver resin (first detector) figure 10. the detector inside the boron plastic box (the first detector) 4.2. the second detector the second detector was produced in the same way as the first one, except the epoxy for wire connection. instead of the silver conductive epoxy, the carbon conductive epoxy was used. the deposition of two b4c layers was done as in previous case and did not show any structural problems right after the deposition. however, after weekend, one of the b4c layers was cracked and started to delaminate (see figure 11). the second layer was without structural problems. we were unable to find the reason for this delamination. under the polarized light, it was possible to observe, that the aluminum electrode delaminated with the b4c layer together, this means that the delamination occurred between the diamond surface and the aluminum layer. to solve this problem and finalize the detector, we removed the delaminated area, cleaned the diamond surface with the plasma cleaner and deposited only the aluminum electrode. at the end, we have received a crystal with the aluminum electrode on both surfaces and one b4c layer. the conversion layer measured around 1 µm in thickness. then the wires were connected to the aluminum electrodes by the carbon conductive epoxy. sadly, the current measurement showed that the detector is not functional, due to the current going through the detector. the root of this current was probably in the carbon epoxy bridge over the edge of the crystal. the process of placing small drops to the crystal surface was not properly done and resulted in the bridge conductive connection of both electrodes. the electrodes were 28 vol. 24/2019 diamond detectors for fast transient scram measurements figure 11. delaminating layer on the second detector probably connected over the edge of the diamond crystal. to solve this problem, we improved the consistency of the carbon epoxy and made several tests of different syringes and pipets to enhance the dropping technique. however, the detector was not functional and therefore it was dismantled again. 4.3. the third detector based on previous experience, the third version of the detector was created in the same way as the previous one. this detector has one b4c conversion layer, approximately 1.5 um thick. wires are connected to the electrode by the carbon conductive epoxy. the detector was tested and had no leakage current and no bridge current. in the end, the crystal with wires is closed in a plastic flask (see figure 12) compared to the first detector, the flask is not filled with the epoxy but contains air. the flask itself is glued to the box. this third detector was tested under large natural uranium source and showed expected responses to incoming radiation. 5. conclusion the time dependent measurement of neutron flux distribution changes during the scram is highly challenging and requires high performance from the detector system. however, the current detectors available at the market are not capable of such measurements. therefore, this paper presents the progress in the diamond detector development, aimed to the fast neutron flux change measurements in a nuclear reactor introduced by the scram. the diamond detector is based on the electrical grade single crystal with a boron conversion layer and aluminum electrodes. this configuration allows to detect thermal neutrons by the neutron alpha conversion in the boron layer. the alpha particle later interacts in the diamond crystal and deposits the energy. high voltage applied on the diamond detector figure 12. the detector inside the boron plastic box (the third detector) collects the charge. all pulses are recorded including gamma, electron and direct neutron interactions. neutron interactions are selected by the pulse shape analysis. to prepare the preliminary detector design, the current experience in the field of the diamond detector was studied. the first part of this paper presents overview of work in the diamond detector field with special attention to neutron flux measurements. theoretical study was combined with several mcnp calculations to prepare the design of the detector. the mcnp calculations were performed to understand the detector performance and to optimize the main detector parameters (e.g. conversion layer thickness). obtained results are presented for example in figure 3. detector design was used in three experimental detectors, that were prepared in cea saclay. as a main part of the study presented, especially the manufacturing process and detector design parameters were developed and enhanced. during this work, several problems arised (breakdown currents, conversion layer delamination,...). some problems were solved. these challenges are more described in section 4. for the purpose of the gamma background discrimination, the pulse shape analysis was selected and implemented in the data processing. the matlab script was prepared for data processing. description of the detector interactions and the pulse shape analysis is presented in section 3.2. the first results of a simple test mea29 o. novák, l. sklenka, m. pomorski, c. lynde acta polytechnica ctu proceedings surement with the alpha source are presented in figure 6. in the example presented, the location of alpha interaction is marked. in this case, the discrimination conditions were not stated. detail experimental work will examine and optimize the discrimination conditions (results will be published in the next paper). the progress achieved in the detector development is promising. this work will be followed by experimental testing with various radiation and neutron sources to study detector performance and to enhance the pulse shape analysis. the optimization of discrimination parameters should be performed before the detector will be tested in the vr-1 nuclear reactor, which will be another step of this scram study. acknowledgements support from iaea fellowship number c6/czr/17003 was used in this work. this work was supported by the grant agency of the czech technical university in prague, grant no. sgs18/146/ohk4/2t/14. references [1] international atomic energy agency. safety of research reactors. iaea safety standards iaea, vienna 2016. 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[10] y. sato, et al. radiation hardness of a single crystal cvd diamond detector for mev energy protons. nuclear instruments and methods in physics research section a: accelerators, spectrometers, detectors and associated equipment 784 pp. 147–150, 2015. doi:10.1016/j.nima.2014.12.036. [11] m. cerv. cvd diamond application for particle detection and identification in high radiation environments. phd thesis technische universitat wien supervised by univ doz dipl-ing dr erich griesmayer, wien 2016. [12] j. rataj, et al. benchmark experiments for validation of reaction rates determination in reactor dosimetry. radiation physics and chemistry 104 pp. 363–367, 2014. issn 0969-806x. [13] e. griesmayer. cividec instrumentation. 6th adamas collaboration meeting @ ect 2017. trento, available at: http://www-adamas.gsi.de/adamas06/ talks/griesmayer_2017.pdf. [14] m. osipenko. neutron spectrometer based on diamond detectors for fast reactors. 6th adamas collaboration meeting @ ect 2017. trento, available at: http://www-adamas.gsi.de/adamas06/talks/ osipenko_2017.pdf. [15] c. weiss. neutron application with diamond detectors. 6th adamas collaboration meeting ect trento, 2017, available at: http://www-adamas.gsi. de/adamas06/talks/weiss_2017.pdf. [16] m. hursin, et al. testing of a scvd diamond detection system in the crocus reactor. the european physical journal a 545 82, 2018. issn 1434-6001, doi:10.1140/epja/i2018-12519-1. [17] p. kavrigin, et al. pulse-shape analysis for gamma background rejection in thermal neutron radiation using cvd diamond detectors. nuclear instruments and methods in physics research section a: accelerators, spectrometers, detectors and associated equipment 795 pp. 88–91, 2015. isbn 0168-9002, doi:10.1016/j.nima.2015.05.040. 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[23] dt5743. 8 channel 12bit 3.2 gs/s switched capacitor digitizer caen, (accessed on july 16th, 2018) available at: http://www.caen.it/csite/caenprod. jsp?parent=14&idmod=823. 30 http://www-adamas.gsi.de/adamas03/talks/pomorski_cea.pdf http://www-adamas.gsi.de/adamas03/talks/pomorski_cea.pdf http://dx.doi.org/10.1016/s0168-9002(01)01668-0 http://dx.doi.org/10.1002/9780470740392 http://dx.doi.org/10.1016/j.diamond.2018.07.005 https://www.sciencedirect.com/science/article/pii/0168900295005455 https://www.sciencedirect.com/science/article/pii/0168900295005455 http://dx.doi.org/10.1088/1748-0221/12/01/c01066 http://dx.doi.org/10.1016/j.nima.2014.12.036 http://www-adamas.gsi.de/adamas06/talks/griesmayer_2017.pdf http://www-adamas.gsi.de/adamas06/talks/griesmayer_2017.pdf http://www-adamas.gsi.de/adamas06/talks/osipenko_2017.pdf http://www-adamas.gsi.de/adamas06/talks/osipenko_2017.pdf http://www-adamas.gsi.de/adamas06/talks/weiss_2017.pdf http://www-adamas.gsi.de/adamas06/talks/weiss_2017.pdf http://dx.doi.org/10.1140/epja/i2018-12519-1 http://dx.doi.org/10.1016/j.nima.2015.05.040 http://dx.doi.org/10.1051/epjconf/201714603004 http://www.lanl.gov/orgs/n/n1/panda/00326408.pdf http://www.lanl.gov/orgs/n/n1/panda/00326408.pdf http://dx.doi.org/10.1016/0955-2219(90)90048-k http://www.caen.it/csite/caenprod.jsp?parent=14&idmod=823 http://www.caen.it/csite/caenprod.jsp?parent=14&idmod=823 acta polytechnica ctu proceedings 24:1–10, 2019 1 introduction 2 diamond detectors 2.1 current diamond development overview 3 proposed diamond detector design 3.1 conversion layer thickness optimization 3.2 interactions in the detector 3.3 pulse shape method implementation 4 diamond detector manufacturing 4.1 the first detector 4.2 the second detector 4.3 the third detector 5 conclusion acknowledgements references https://doi.org/10.14311/app.2022.33.0417 acta polytechnica ctu proceedings 33:417–423, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague glued timber concrete composite walls using ultra-high-performance concrete thomas oberndorfera,∗, frank hungerb, oliver fischera a technical university of munich, department of civil, geo and environmental engineering, chair of concrete structures, theresienstraße 90, 80333 munich, germany b technical university of munich, department of civil, geo and environmental engineering, chair of timber structures and building construction, arcisstraße 21, 80333 munich, germany ∗ corresponding author: thomas.oberndorfer@tum.de abstract. this paper gives a short introduction of the development of cross laminated timber (clt) concrete composite walls with a glued connection. in the composite walls, prefabricated ultra-highperformance concrete (uhpc) lamellas replace timber lamellas at the core layer of the clt elements. to check the feasibility of gluing timber to uhpc small-scale shear, delamination and bonding tests were performed and showed promising results. the load bearing behaviour was analysed with centrically and eccentrically loaded tests on wall segments. analytic modelling of the wall experiments using an effective bending stiffness, based on shear analogy method, showed a good correlation to the experimental results. keywords: adhesive bond, cross laminated timber, timber concrete composite, ultra-highperformance concrete. 1. introduction cities are forced to grow, due to an increasing urbanization and population growth in general [1]. growth occurs vertically, horizontally or in both directions [2]. vertical growing cities need more high-rise buildings to accommodate the rising number of people. because of sustainability aspects and numerous technical innovations at the turn of the millennium, timber multi-storey buildings came into focus [3]. currently, there are visionary timber high rise projects with building height up to 300 m [4]. the increasing building height leads to high forces in vertically load bearing members in the lower floors. this causes a mass consumption of material and space in timber-only high-rise buildings, due to a relative low compression strength and youngs modulus of timber compared to ultra-high-performance concrete. compared to the sustainable construction material timber, ultra-high-performance concrete may be regarded as not-sustainable, because of its high content on non-renewable material and its energy consumption for processing. aim is the sustainable use of ultra-highperformance concrete (uhpc) through its targeted use for vertically load bearing members. therefore, timber and ultra-high-performance concrete are combined in walls. the wall cross section is comprised of a slender uhpc core, encapsulated within timber, as shown in figure 1. applied normal forces concentrate on the concrete core. the surrounding timber prevents the slender concrete core from buckling and bears compression and tensile forces, caused by bending moments due to eccentricities or figure 1. wall with hybrid cross section comprised of timber and uhpc [5] external loads. the walls are economically produced in the industrialized environment of the cross laminated timber (clt) production. three steps are necessary. first, a precast concrete plant prefabricates lamellas. in the regular production process of a clt plant, the manufacturer inserts the prefabricated concrete lamellas into the raw clt panels at previously defined places. after pressing the panels, wall elements are formed out with joinery machines, automatic cutting machines or machining centres. to ensure statically monolithic acting walls and to enable this production process, a glued connection between timber and concrete using standard glues in 417 https://doi.org/10.14311/app.2022.33.0417 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en t. oberndorfer, f. hunger, o. fischer acta polytechnica ctu proceedings � figure 2. typical stress-strain relationship of different types of concrete according to [7] clt production is inevitable. 2. fundamentals 2.1. ultra-high-performance concrete ultra-high-performance concrete is characterized by its, for concrete, high compressive strength, up to 200 n/mm2, and a young’s modulus between 45.000 and 55.000 n/mm2, which is shown in figure 2. the high compressive strength comes along with a brittle failure when no steel fibres are added to the concrete. a certain degree of ductility can be achieved by adding steel fibres [6]. the high compressive strength of uhpc is achieved through its optimised packing density of the aggregate. therefore, almost no pores exist in uhpc. this leads to a very dense surface of uhpc compared to normal strength concrete and the possibility to easily glue uhpc. [8] 2.2. cross laminated timber cross laminated timber is a quasi-rigid composite, plate-like engineered timber product, which is commonly composed of an uneven number of layers, each made of boards placed side-by-side, which are arranged crosswise to each other at an angle of 90◦c [9]. the layers are normally glued together using polyurethane (pu) or melamine-urea-formaldehyde resin (muf) adhesives [10]. the thickness of clt elements is commonly between 51 mm and 300 mm, whereas the single layer thickness generally varies between 20 mm and 40 mm. common lengths of clt elements is around 18 m with a width up to 3,0 m [9]. figure 3 gives an overview of the clt production process and figure 4 displays a simplified technical drawing of a clt element. clt bears in-plane and out-of-plane loads through its crosswise layering. the load bearing behaviour is influenced by the cross wise layering and for stability and deformation calculations the shear deformation must be considered [11]. � figure 3. production process of cross laminated timber according to [9]. figure 4. simplified technical drawing of a clt element according to [9]. 2.3. timber concrete composite a well-known timber concrete composite (tcc) member is the tcc slab. in this composite element the concrete is positioned in the compressive zone, whereas the timber is placed in the tensile zone. the two elements are connected with special connectors to form one statically acting cross section. the effectiveness of the tcc element is mainly characterized by its connection. the usage of systems with high composite action can reduce beam depth and allow for longer span length [12]. adhesives are a very effective connector for tcc elements and lead to a quasi-rigid connection which increases the member stiffness and strength. in addition, the shear forces are distributed uniformly over the entire surface and local concentrations are avoided. [13] in the past, lots of research on glued connections has been performed, e.g. [14–17], most of these research projects have the use of epoxy resin as adhesive in common. 3. small scale experiments 3.1. general in the beginning of the project numerous small-scale experiments were performed to determine the possibility to glue timber to uhpc using standard glues for clt production. this was an iterative process, as the information obtained served as basis for 418 vol. 33/2022 glued timber concrete composite walls figure 5. experimental test setup for shear experiments [5]. the following experiments. these small-scale experiments were shear, delamination and bond experiments, which will be discussed in this section. 3.2. shear first, small-scale shear experiments on bonded timber concrete composite specimen were performed to find combinations of concrete, concrete surface treatments and adhesives with a promising behaviour. the three-layered specimen consisted of a concrete layer, placed in the middle, and two layers of spruce, which were free of any defects. per specimen two shear joints existed and shearing off took place parallel to the grain. three different types of concrete (c1, c2 and c3) were tested with and without steelfibres. c1 and c2 were commercially available compounds and c3 was a laboratory mixture. the used concrete surfaces were a) smooth, directly out of the formwork, b) ground and c) sandblasted. five adhesives were used: a one-component polyurethane adhesive, a melamine-urea-formaldehyde resin, a phenol resorcinol resin, a two-component polyurethane resin and an epoxy resin. the adhesive application was 400 g/m2 and the pressure at bonding was about 0, 75 ± 0, 1 n/mm2 for all specimen. the test setup is displayed in figure 5. a combination of concrete, concrete surface treatment and adhesive was characterized as promising if it achieved high shear strength and timber fracture in the experiment. considering only standard clt production adhesives, suitable results could be achieved independently from the used concrete type, with a ground concrete surface in combination with the onefigure 6. technical drawing of experimental test setup of inclined bond tests [5]. component polyurethane adhesive. specimen glued using the melamine-urea-formaldehyde resin failed in the glue line, which was characterized through a low shear strength and a maximum timber fracture of 20%. 3.3. delamination although this research project concentrated on the short-term load bearing behaviour of clt walls with glued in lamellas from uhpc, durability keystroke tests with glued specimen were performed. the test procedure included storing the specimen in boiling water for six hours, the storage in 20 ◦c cold water for one hour and afterwards drying in an oven for approximately 16 hours. this induces shearand tensile stresses on the glue line due to the swelling and shrinkage behaviour of the timber. if the glued connection is insufficient there will be a gap opening of the glue line. according to din en 16351 [18] the maximum tolerated failure of a single glue line is 40% of its length and 10% of glue line length may be opened for the sum of all glue line lengths. based on the shear experiments, delamination tests with promising combinations were performed. here, both steel-fibre reinforced concrete types c1 and c2, a smooth and ground surface, the one-component polyurethane adhesive, the twocomponent polyurethane resin, and the epoxy resin were used to form the specimens. the manufacturing procedure of the specimen was identical as for the shear tests. for each series two specimen were produced. 3.4. bond to verify the assumption of a rigid connection inclined shear tests with three different bond lengths, 419 t. oberndorfer, f. hunger, o. fischer acta polytechnica ctu proceedings figure 7. schematic (left) and real (right) experimental test setup for full scale tests [5]. 14 cm, 28 cm and 45 cm, were performed. a technical drawing of the experimental test setup is displayed in figure 6. the specimens were comprised of concrete type c1 lamellas with smooth and ground surface, glued to spruce glulam gl24c using the onecomponent polyurethane adhesive. bonding parameters were identical to the shear and delamination tests. all experiments showed a brittle failure when reaching the maximum force. with increasing bond length, the average shear force leading to failure increased. the average failure shear force for the 14 cm bond length with the ground concrete surface was 77,1 kn and increased up to 217,1 kn for the 45 cm long bond length. no difference could be seen in the behaviour of the smooth or ground concrete surface. the maximum measured relative slip between concrete and timber, recorded with an optical measuring system, in the middle of the bond length was approx. 0,25 mm for all experiments. in most cases the failure occurred in the timber part. only in two experiments, with the 14 cm long bond length, the failure occurred in the glue line. 4. full scale tests 4.1. methodology to analyse the load bearing behaviour of the composite walls, centrically and eccentrically loaded, experiments at scale 1:1 have been performed. figure 7 displays the test setup. for every eccentricity, 0 mm, 10 mm and 20 mm, three wall segments were tested. the tested wall segments were 0,5 m in depth, 2,84 m high and had a thickness of 15 cm. two roller tilting bearings were used to test a pendulum rod with a buckling length of 2,97 m. the five layered wall segments with a complete concrete core, every layer was 30 mm thick, were produced in a clt production facility. the adhesive application of a onecomponent polyurethane was production based 170 g/m2 and the bonding pressure was 0,08 n/mm2, because the elements were pressed in a vacuum bed. the used timber was spruce with a strength class of c24, the concrete had a mean compressive strength of fck, cyl = 132, 5n/mm2 and a mean young’s modulus of 46.774 n/mm2. these values were determined according to din en 12390-13 [19] and din en 123903 [20]. the tests were conducted displacement controlled with a speed of 0,12 mm/min. laser distance sensors controlled the horizontal displacement of the wall segments during the experiments. strain gauges recorded the strain on the outermost timber fibres and in the middle of the cross section on the concrete. the strain gauges were placed in the centre, at the bottom and the top of the wall. 4.2. results figure 8 displays the force-horizontal deflection in the vertical wall centre and the force-piston stroke diagram for the centrically loaded experiments. all three experiments showed similar behaviour and the maximum force varied between 1331 kn for v_d_30_00_3 and 1693 kn for v_d_30_00_2. with increasing load, the horizontal deflection increases slowly until the critical point is reached and the walls buckle. this is indicated by a rapid increasement of the horizontal deflection, accompanied with a decreasing load. after the specimen were removed out of the testing machine material failure could visibly not be detected. 420 vol. 33/2022 glued timber concrete composite walls figure 8. force-horizontal deflection and forcepiston stroke diagram for centrically loaded experiments [5]. figure 9. force-horizontal deflection and forcepiston stroke diagram for 10ămm eccentrically loaded experiments [5]. the force-horizontal deflection diagram of the 10 mm eccentric loaded experiments is shown in figure 9. the three specimens showed a similar behaviour upon reaching the maximum force, which varied between 908 kn and 1025 kn. the horizontal deflection at the maximum load is close to 30 mm. experiments v_d_30_10_1 and v_d_30_10_2 showed a ductile behaviour, whereas v_d_30_10_3 failed brittle. material damage could clearly be seen on the compressive side of the wall segments in the outer most timber lamella. the recorded strains in the wall centre showed a similar behaviour for all experiments. figure 10 shows a typical force-strain diagram, explicitly for experiment v_d_30_10_1. the concrete middle layer suffers only compressive strain, whereas the outermost timber fibres are either under compressiveor tensile strain. 4.3. analysis to calculate the maximum buckling force for the centrically loaded experiments, euler’s formula using an effective bending stiffness, accounting for the additional shear deformation, according to [11], was used. the eccentric experiments were analytically modelled figure 10. development of strains at the vertical wall centre for timber and concrete for experiment v_d_30_10_1. [5]. figure 11. development of strains at the vertical wall centre for timber and concrete calculated (gray) and experiment (black). [5]. with the differential equation for second order problems, also using an effective bending stiffness. due to the scattering of the elastic timber properties the mean stiffness values and the characteristic material strength of nominal timber strength class c24 according to din en 338 [21], and for concrete the determined young’s modulus and compressive strength were used for the calculations. the calculated buckling forces were lower than the ones achieved in the experiments. figures 11 and 12 compare the calculated strain distribution and horizontal deflection for different timber strength grades in the vertical wall centre with ones obtained in an experiment. the curves fit well. because a linear elastic material model was used to model the experiments, the declining branch in the experiment cannot be considered. 421 t. oberndorfer, f. hunger, o. fischer acta polytechnica ctu proceedings figure 12. force-horizontal deflection calculated (gray) and experiment (black). [5]. 5. conclusions the completed test program showed that it is possible to bond timber to uhpc using standard glues for clt production regarding short-term behaviour. the tests on wall segments verified, that the cross section can be considered as monolithic and the elements can be calculated using an effective bending stiffness. the maximum load capacity depends on the slenderness of the walls and the eccentricities. compared to homogenous clt walls, the hybrid walls tested with a 10 mm eccentricity, show a 25% higher load bearing capacity, calculated with characteristic values for material strength. acknowledgements the authors want to thank the forschungsinitiative zukunftbau for funding this project. this paper is only a short introduction in the project, the described experiments, and results in section three, four and five are detailly described in [5]. references [1] united nations, department of economic and social affairs, population division. world urbanization prospects 2018: highlights (st/esa/ser.a/421), 2019. https://population.un.org/wup/publications /files/wup2018-highlights.pdf. 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[18] din en 16351:2015-12. holzbauwerke brettsperrholz anforderungen, 2015. 422 https://population.un.org/wup/publications/files/wup2018-highlights.pdf http://www.skyscrapercenter.com/quick-lists#q=&page=1&type=building&status=vis&structural_material=timber&min_year=0&max_year=9999&region=0&country=0&city=0 www.zukunftbau.de https://kobra.uni-kassel.de/handle/123456789/2009031226631# https://doi.org/10.1007/s00107-015-0999-5 https://doi.org/10.1061/(asce)st.1943-541x.0000353 https://www.cost.eu/uploads/2018/11/design-of-timber-concrete-composite-structures.pdf https://doi.org/10.1061/(asce)st.1943-541x.0000228 https://doi.org/10.1061/(asce)cc.1943-5614.0000063 https://doi.org/10.1016/j.conbuildmat.2010.12.060 vol. 33/2022 glued timber concrete composite walls [19] din en 12390-13:2014-06. prüfung von festbeton teil 13: bestimmung des elastizitätsmoduls unter druckbelastung (sekantenmodul), 2014. [20] din en 12390-3-2019:10. prüfung von festbeton teil 3: druckfestigkeit von probekörpern, 2019. [21] din en 338: 2016-07. bauholz für tragende zwecke festigkeitsklassen, 2016. 423 acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.37.0031 acta polytechnica ctu proceedings 37:31–37, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague proposal of point-wise power reconstruction for irt-4m fuel assemblies in coupled monte-carlo calculations pavel jíška∗, jan frýbort czech technical university in prague, faculty of nuclear sciences and physical engineering, department of nuclear reactors, v holešovičkách 2, 180 00 prague 8, czech republic ∗ corresponding author: jiskapav@fjfi.cvut.cz abstract. current efforts to develop high-fidelity multi-physics tools for research reactors with irt-4m fuel leads to the involvement of monte-carlo transport and subchannel thermo-hydraulic codes in the process of safety assessment. monte-carlo based neutronic codes are often used as a reference tool and are suitable for a prediction of a detailed power distribution. this paper proposes a method of the point-wise power reconstruction based on a radial basis function (rbf) interpolation for complicated irregular geometries and compares different detector functionalities of the serpent2 code from the time efficiency point of view. furthermore, a preliminary validation of the applied method for irt-4m in a subchannel code subsals mesh is presented. this code is currently under the development at the research centre řež and the czech technical university in prague and focuses on the thermohydraulic analysis of irt type fuel assemblies. keywords: power reconstruction, multi-physics coupling, irt-4m, serpent2, subsals. 1. introduction the main motivation behind the development and the application of advanced calculation tools and methods is the ensurement of ability to simulate the processes in the reactor core with a lower degree of uncertainity than nowadays computing resources allow, and even beyond operational safety analyses. most of the attention is predominantly given to power reactors. however, research reactors can also have difficulties to simulate specific phenomena. two czech research reactors lvr-15 [1] and vr-1 [2] are operated with irt-4m fuel assemblies. this type of fuel has an unusual irregular geometry as demostrated by figures 1a or 1b. its basic description is provided by table 1. because of its unique concentric square annular fuel elements with rounded corners, the coolant flow is subjected to a relatively significant pressure driven crossflow. a detailed calculation of such thermalhydraulic characteristics is beyond capabilities of standard dedicated one-dimensional system codes. a new subchannel code subsals (subchannel square annular layout solver) is under the development at the research centre řež and the czech technical university in prague. the code is currently in the phase of concept verification. nevertheless, its further utilisations are considered, for instance, its coupling with neutronic codes. monte-carlo based neutronic codes are often used as a reference tool and are suitable for prediction of a detailed power distribution. for multi-physical coupling, it is essential to ensure the appropriate data transfer between individual codes in a required data designation value fuel assembly type irt-4m standard number of fuel tubes in fa 8 fuel meat material uo2 + al enrichment 19.7 % cladding material sav-1 alloy mass of 235u in fa 300 g core pitch 71.5 mm cladding thickness 0.45 mm fuel meat thickness 0.70 mm water gap between fuel tubes 1.85 mm active height of fuel meat 600 mm outer dimensions of tubes and corner radius: 1st tube 2nd tube 3rd tube 4th tube 5th tube 6th tube 7th tube 8th tube (cylindrical) 69.6 mm; 9.3 mm 62.7 mm; 8.5 mm 55.8 mm; 7.7 mm 48.9 mm; 6.9 mm 42.0 mm; 6.1 mm 35.1 mm; 5.3 mm 28.2 mm; 4.5 mm 21.3 mm table 1. basic characteristics of standard irt-4m fuel assembly [3]. structure. different embedded funcionalities for power reconstruction (i.e. different detector types) are implemented in monte-carlo code serpent2 [4]. however, some approaches appear to be more computationally challenging or difficult to implement in such a complex irt-4m geometry. fuel assembly power mapping can be regarded as an interpolation problem of scattered data which is common in many fields. the radial basis function interpolation offers excellent tool for interpolating multidimensional scattered data whose value is dependant only on system coordinates. thus, it is an 31 https://doi.org/10.14311/app.2022.37.0031 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en pavel jíška, jan frýbort acta polytechnica ctu proceedings (a) . mesh 9 × 9 (b) . mesh 17 × 17 (c) . subsals mesh figure 1. cartesian mesh detectors 9 × 9 and 17 × 17, and subsals thermal-hydraulic model for 8-tube irt-4m fuel assembly. appropriate approach for a power and temperature mapping in situations where values of scattered variables varies gently. in addition, the round-off error of a piecewise constant distribution could be minimized by a continuous function. however, it should be noted that the application of rbf interpolation is prone to uncertainty of sampled data, which may apply especially for monte-carlo tallies. 2. theory within this work two different types of response detector meshes were considered for power reconstruction in the irt-4m fuel assembly. those meshes are denoted here as: (1.) mesh of cell detectors (mocd) is a general mesh made of individual cell detectors (see option dc in [5]) situated only in the fuelled regions in accordance with required subsals layout (see figure 1c), (2.) irregular cartesian mesh detector (icmd) is a standard serpent2 irregular cartesian mesh (see option dmesh with datamesh type 6 in [5]). mocd enables to determine values of a variable in a general spatial mesh superimposed on the base area of an fuel assembly, whereas irmd can be applicable only to an irregular cartesian grid problem. therefore mocd is here considered as a reference mesh for point-wise rbf-based power reconstruction testing. the icmd is here utilized as an auxiliary grid on which the mean values of an intensive variable (i.e. power density) are interpolated by rbf interpolation. here should be noted, that the term point-wise power reconstruction is here used to distinquish from a frequently used term pin-wise or plate-wise power reconstruction since the process of evaluation is different and the final value is not connected with any fuel pins or plates, but rather with a general volume and its center of mass. basic descriptions of mesh and cell detectors are mentioned in this section. further, basic information about rbf interpolation for the pin-wise power reconstruction in irt-4m fuel assemblies are provided. 2.1. mesh/cell detectors monte-carlo mesh detectors are able to determine various integral quantities in user-defined detector bins. the fundamental idea is based on a division of space into partial regions and a selection of events of particular interest. the resultant spatial description of integral quantities has the character of a piecewise constant function. for fission energy production p is evaluated integral of type p = ∫ v ∫ e κσf (r, e) ϕ(r, e) d3r de, (1) where κσf (r, e) is macroscopic total fission energy production cross section and ϕ(r, e) is neutron flux density [6]. in order to achieve the highest possible level of statistical accuracy, it is reasonable to increase the size of cells in mesh detectors or to increase the number of neutron tallies. on the contrary, in an effort to characterize the distribution function as accurately as possible, it is necessary to minimize the rounding error of the mesh detector approximation. however, the common limit condition is the computational performance. for any simulation, it is necessary to find the balance between the level of the detail and the simulation time [7]. 2.2. radial basis function theory radial basis functions interpolation is one of the commonly used methods to interpolate multi-dimensional data and have gained popularity over the last few decades in a variety of areas such as a surface reconstruction, multivariate interpolation, approximation theory, neural networks, machine learning, etc. rbfs are typically used to approximate an unknown func32 vol. 37/2022 tion f : rn → r by the continuous function s(x) = n∑ i=1 λi φ(||x − xi||), (2) where {xi, i = 1, 2, ..., n} ∈ rn are given scattered data coordinates, φ : ⟨0, ∞) → r denotes a radial basis function, and ||x − xi|| is the euclidean norm between coordinates x and xi [8]. the approximating function, to be constructed over n sampled coordinates, satisfies s(xi) = fi, i = 1, 2, ..., n, (3) where values fi of unknown function f are assumed to fulfill fi = f(xi). the equation (2) could be rewritten in terms of linear system of equations aλ = f , (4) or in matrix form φ(r1,1) . . . φ(rn,1) ... . . . ... φ(r1,n ) . . . φ(rn,n )   λ1 ... λn  =  f1 ... fn  , (5) where rj,i = ||xj − xi||. from the definition of the euclidean norm, it is clear that matrix a is symmetric with dimensions n × n . to ensure a unique solution, the matrix a has to be non-singular. specific form of the radial basis function φ(r) may have a different prescription (e.g. gaussian, multiquadric, polyharmonic, etc.). 3. power reconstruction currently, the serpent2 program focuses predominantly on reactor systems with regular grids, similar to standard power reactors. its coupling interface is not fully adapted to generally irregular geometries, as required in the subsals code for irt-4m representation. however, this minor deficiency can be easily circumvented using a specific type of mesh of cell detectors and an individual coupling wrapper. in the proposed subchannel model, for the irt-4m fuel assembly in subsals program (see figure 1c), three different types of heat sources occur: • one-eighth annular section, • large straight plate, • small straight plate. 3.1. models description serpent2 models includes simplified three-dimensional lvr-15 reactor core configuration with standard (8tube) fresh fuel assemblies irt-4m surrounded by beryllium blocks (see figure 2). depending on the level of detail of fuel meat division inside one preselected fuel assembly, two geometrically different models can be distinguished: (1.) simplified fa model where all fuel tube cells and water channels are considered as an unique material cell circumscribed by square prisms with rounded corners, (2.) detailed fa model which is almost identical to the simplified fa model with the only exception in the one core position (b6 or e6), where fuel meat materials are replaced by the identical material azimuthally divided into the subsals computational mesh (i.e. 8 fuel meat cells replaced by 168 cells). figure 2. serpent2 model with irt-4m fuel assemblies surrounded by beryllium blocks (framed positions b6 and e6 of particular interest). depending on the implementation of response detector meshes (see mocd and icmd), three different approaches were considered for determining the point-wise power distribution in the subsals layout. the nomenclature for each approach is stated below. these notations will be used throughout this work for a simplification: (1.) cell-based model where the detailed fa model is utilized and a mesh of cell detectors (mocd) is directly included in the main universe as individual fuel meat cells, (2.) universe-based model where the simplified fa model is utilized and a mesh of cell detectors (mocd) is included in a separate universe as mesh of individual detector cells overlapping the main universe, (3.) rbf-based model where the simplified fa model is utilized and an irregular cartesian mesh detector (icmd) is applied 33 pavel jíška, jan frýbort acta polytechnica ctu proceedings (a) . position e6 (b) . position b6 figure 3. reference power density imbalance factors k (i) q in the subsals mesh. for a subsequent two-dimensional rbf interpolation.1 3.2. testing settings all results were generated by running serpent2 (version 2.1.32) in a criticality source simulation mode. the run parameters were set with 40 inactive cycles to populate the fission source, 400 active cycles to collect results, and 500 000 neutrons per cycle. all calculations were performed on one computer using 32 openmp threads with processor amd epyc 7351. the host operating system was an ubuntu 20.04.4 lts. 3.3. implementation of rbf-based power reconstruction it is necessary to mention at the outset that tested rbf-based power reconstruction method utilizing rbf interpolation is established on rough simplifying assumptions. the method is based on the known distribution function in the auxiliary grid (e.g. icmd). the auxilliary mesh selection for irt-4m fuel assembly is considerably complicated task and can have a significant effect on reconstruction outputs. source data for the rbf-based power reconstruction are total values of power in auxiliary cartesian mesh (i.e. icmd), or respectively mean power density if related to the relevant fuel meat volume. however, the question how to assign function values (i.e. power densities) to the spatial coordinate remains. usually they are assigned to the center of mass of given fuel meat element (e.g. in pin-wise reconstruction). nevertheless, the center of mass of icmd cell may not coincide with the center of mass of the fuel meat cell. unfortunately, this may result in the incorrect assignation of function values (especially when multiple fuel cells are in cartesian mesh cell or when rounded corners are included) and thus resultant interpolant could be disorted. in this work, the mean values were 1please note, that the manner of response detector mesh implementation is external for universe-based and rbf-based models, whereas internal for a cell-based model. always assigned to the center of mass of cartesian mesh cell detector. the common aim was to create cartesian mesh so that center of masses matches (at least in planar segments). another assumption is that resulting power distribution is obtained by quantifying the interpolation function at predefined coordinates and by volumetric multiplication of given fuel meat element. predefined coordinates were chosen manually to correspond approximately to the center of mass of fuel meat element in subsals layout. however, the assumption is fulfilled only if the course of the function is not convex or concave in the given interval. within this work, the polyharmonic radial basis function φ(r) = r3 (6) was selected for testing. the choice of rbf was based on the good practice and meaningful reconstruction outputs. the utilisation of other radial basis function is possible, but their impact on results was not evaluated here. an auxiliary program for the rbf-based power reconstruction has been created. the program utilizes python3 language and is based on numpy and scipy libraries (version 1.23.1, respectively 1.9.0). the program has an automatic procedure, which can evaluate irt-4m fuel meat volumes almost in any icmd cell to ensure proper volume division and also includes plotter in subsals mesh (see figure 1c) for result visualisation such as figures 3. 3.4. testing rbf-based power reconstruction the point-wise rbf based power reconstruction was tested on two different auxiliary icmd, specifically 9 × 9 and 17 × 17. meshes were set as demonstrated on figures 1a and 1b. the division of the detector in the vertical direction was not considered. performed point-wise rbf based power reconstructions were compared against the reference data obtained directly in subsals layout (i.e. direct comparison with universe-based model results). it was 34 vol. 37/2022 (a) . position e6 (b) . position b6 figure 4. rbf interpolation of reference data. ensured, by the calculation settings, that the statistical uncertainity for individual fuel meat elements was below 0.5 % and the case of cartesian meshes for individual cells up to 1 % (especially for cells with lowest contain of fuel meat, i.e. cells (4,4) and (5,5) in mesh 17 × 17). in the figure 3, reference values of power density imbalance factors k (i) q for fuel tubes cells in core positions e6 and b6 are depicted. power density imbalance factors can be expressed by equation: k(i) q = q (i) v qv , (7) where q (i) v is a power density in cell i and qv denotes a mean fuel assembly power density in given axial layer. in the figure 4 are demonstrated rbf interpolations of reference data in positions e6 and b6. when comparing point-wise rbf-based power reconstructions for both cartesian meshes with reference data (see figures 5 and 6), it is possible to quantify relative deviations. those are summarized by the table 2. overall, presented and roughly simplified rbf based power reconstruction gives reasonable results, because the maximal fission power density difference between neigbouring fuel meat elements could reach roughly 13 % for fuel assembly in the b6 possition and 23 % in the e6. the round-off error of irregular mesh reconstruction is higher than the error of point-wise rbf based power reconstruction. position and icmd max. deviation rmse b6 mesh 9 × 9 3.29 % 1.15 % e6 mesh 9 × 9 4.29 % 1.28 % b6 mesh 17 × 17 2.33 % 0.91 % e6 mesh 17 × 17 3.51 % 1.03 % table 2. max. relative differences of rbf-based power reconstruction from reference data. figure 5. relative differences between rbf-based power reconstruction and reference data for cartesian meshes 9 × 9 (top) and 17 × 17 (bottom), both for fuel assembly in e6. 4. computational speed the computational performance of the monte-carlo simulation depends predominantly on the complexity of created model. the total number of cells and their boundary surfaces play an important role in this case. creating strongly heterogeneous discrete models (i.e. models whose properties are characterized by many 35 pavel jíška, jan frýbort acta polytechnica ctu proceedings parameter simplified fa w/o detectors detailed fa w/o detectors rbf-based model cell-based model universe-based model9 × 9 17 × 17 total cpu time [min] 2358.83 2694.85 2378.52 2522.38 5745.35 7721.40 total wall-clock running time [min] 92.74 105.12 92.73 95.19 197.83 260.95 wall-clock time for transport simulation [min] 92.00 104.50 92.00 94.52 197.18 260.33 cpu usage fraction in transport simulation [–] 25.70 26.10 25.90 26.85 29.23 29.78 allocated memory size [mb] 7586.46 7589.86 7586.46 7586.46 7590.06 7588.57 memory used for storing cross sections [mb] 2809.81 2809.81 2809.81 2809.81 2809.81 2809.81 memory used for storing material-wise data [mb] 1086.42 1086.42 1086.42 1086.42 1086.42 1086.42 total number of cells [–] 3305 3465 3305 3305 3465 3473 total number of cells using unions [–] 0 0 0 0 0 0 table 3. comparison of serpent2 simulation characteristics for different cases. figure 6. relative differences between rbf-based power reconstruction and reference data for cartesian meshes 9 × 9 (top) and 17 × 17 (bottom), both for fuel assembly in b6. materials and a common, usually mean, value of state variable) can be not only computationally inefficient, but also user-unfavorable. the comparison of main computational characteristics is provided in table 3. as can be seen, the utilisation of a finer model division of fuel meat in the position b6 leads to an extension of computing time by approximately 14 % (from the dirrect comparison of simplified fa and detailed fa model without any detectors). if other fuel meat elements were divided, for instance in other fuel assemblies, the effect of the time increase will be even more significant. consequently, performing full core calculations with cell detectors can be considerably time disadvantageous. in terms of the pin-wise power distribution reconstruction, a significant increase of computational time was observed in the case of cell-based and universebased models. from table 3 is evident that the utilization of cell detectors has considerable impact on the transport simulation time. in direct comparison with rbf-based models (i.e. standard icmd), this increase can be quantified for cell-based model by 144 % and for universe-based model even by 224 %. in both cases, an increase of fraction of time spent in openmp parallel loops was observed (by 15 % and 19 %, respectively). 5. discussion modification of the cell detectors implementation in monte-carlo code serpent2 may allow to detect local neutron-physical characteristics more efficiently, with lower computing time and a more user-friendly model definition (i.e. without any update of model, which also applies to models considering detailed burnup distributions). to focus on the cell of particular interest, which is not included in geometry, could be crucial in neutron-physical and thermal-hydraulic coupling. in that case, several iterations (or criticality source simulations) are performed until the set convergence criterion is met. observed time increase will be multiplied, and even more if simulating full core coupled calculations. however, the author is not fully acquainted with the implementation of cell detectors in the serpent2 code. this could be further discussed with code developers. if the modification is not feasible, a simplificated method of rbf-based power reconstruction for irt4m fuel assembly was presented to shorten the computational time. for application of this method, the reduction of statistical uncertainty is necessary and ensurement of proper values to coordinates pairing. further optimization of this method is possible as demostrated in the section 3.4. furthermore, this method could be applicable to the temperature characteristics of fuel and coolant materials in coupled 36 vol. 37/2022 calculations. the pleasant benefit is that coupled monte-carlo models could not have to be geometrically complicated (not necessary to use a discrete material segmentation) and potentially save computinal time spent in a neutron transport simulation. the local temperatures of each fuel meat element, or coolant channel, could be also described by rbf interpolation (i.e. one function prescription) for given material region. this may result in a potential reduction of computing time in coupled calculations, because of less number of cells in transport calculation. however, this temperature field interpolation has not been computationally verified so far. 6. conclusions in this work a new method for power reconstruction for irt-4m fuel assemblies was presented. the rbf-based power reconstruction method was tested to provide quantitative characterization of method error and the computational demands. testing was performed on simplified lvr-15 reactor core for two selected fuel assembly positions b6 and e6 with different assembly-wise power distribution. demonstrated application gives reasonable results with errors lower than a round-off error of neighbouring fuel element cells. although rough simplifications were used, the maximal observed deviation was approximately 4.3 %. maximal power density imbalance rmse for fuel assembly reaches less then 1.3 %. maximal deviations decrease with finer division of mesh. nevertheless, with finer mesh cells, larger local uncertainties may appear. attention need to be paid, when choosing input data from a cartesian data mesh. as a part of this work was found, that cell detectors implemented in monte-carlo code serpent2 (version 2.1.32) has a significant impact on computational time. when using cell-based model for direct power reconstruction (see section 3.1), an increase in computing time around 144 % was observed. if universe-based model (see section 3.1) was used, the time extension was approx. 227 %. this outcome is going to be further discussed with code developers. the new update of cell detectors of serpent2 may allow to detect local neutron-physical characteristics more efficiently, with lower computing time and a more userfriendly model definition (i.e. without any update of model). list of symbols a matrix of evaluated kernels cp u central processing unit e energy [j] fi function value in xi icmd irregular cartesian mesh detector κ recoverable energy per fission [j] k (i) q power density imbalance factor for node i [–] λi linear combination coefficient max maximum mocd mesh of cell detectors p total power [w] q (i) v power density in node i [w m−3] qv mean fuel assembly power density [w m−3] r position vector ||r|| euclidean norm rbf radial basis function rmse root mean square error σf fission macroscopic cross section [m−1] s(x) aproximating radial basis function interpolation ϕ neutron flux [m−2 s−1] φ radial basis function kernel v volume [m3] x coordinates x-axis [cm] y coordinates y-axis [cm] acknowledgements this work was supported by the grant agency of the czech technical university in prague, grant no. sgs22/189/ohk4/3t/14. references [1] research centre řež. research reactor lvr-15. http://reaktory.cvrez.cz/en/ research-reactor-lvr-15/, 2022. [2022-10-10]. [2] l. frýbortová, j. rataj, l. sklenka, et al. the training reactor vr-1 – 30 years of operation. 21st international scientific conference on electric power engineering (epe) pp. 1–7, 2020. https://doi.org/10.1109/epe51172.2020.9269254. [3] e. vilímová, p. dostál. provozní bezpečnostní zpráva reaktoru lvr-15. research centre řež, 3rd edn., 2021. [4] j. leppänen, m. pusa, t. viitanen, et al. the serpent monte carlo code: status, development and applications in 2013. annals of nuclear energy 82:142–150, 2015. https://doi.org/10.1016/j.anucene.2014.08.024. [5] vtt technical research centre of finland. input syntax manual – serpent wiki. https://serpent.vtt. fi/mediawiki/index.php/input_syntax_manual, 2022. [2022-10-10]. [6] j. leppänen. development of a new monte carlo reactor physics code. vtt technical research centre of finland, 2007. [7] d. p. griesheimer. functional expansion tallies for monte carlo simulations. ph.d. thesis, university of michigan, 2005. [8] g. song, j. riddle, g. e. fasshauer, f. j. hickernell. multivariate interpolation with increasingly flat radial basis functions of finite smoothness. advances in computational mathematics 36(3):485–501, 2012. https://doi.org/10.1007/s10444-011-9192-5. 37 http://reaktory.cvrez.cz/en/research-reactor-lvr-15/ http://reaktory.cvrez.cz/en/research-reactor-lvr-15/ https://doi.org/10.1109/epe51172.2020.9269254 https://doi.org/10.1016/j.anucene.2014.08.024 https://serpent.vtt.fi/mediawiki/index.php/input_syntax_manual https://serpent.vtt.fi/mediawiki/index.php/input_syntax_manual https://doi.org/10.1007/s10444-011-9192-5 acta polytechnica ctu proceedings 37:31–37, 2022 1 introduction 2 theory 2.1 mesh/cell detectors 2.2 radial basis function theory 3 power reconstruction 3.1 models description 3.2 testing settings 3.3 implementation of rbf-based power reconstruction 3.4 testing rbf-based power reconstruction 4 computational speed 5 discussion 6 conclusions list of symbols acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.32.0042 acta polytechnica ctu proceedings 32:42–48, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague assessment of shear strength parameters of soil from load micropile tests karel vojtasík všb – technical univerisity of ostrava, faculty of civil engineering, deparment of geotechnics and underground engineering, ludvíka podéště 1875/17, 708 33 ostrava-poruba, czech republic correspondence: karel.vojtasik@vsb.cz abstract. design and evaluation of the bearing capacity of micropiles are based on shear strength parameters of the soil within which a micropile is installed. these parameters are mostly received from laboratory tests or as an expert evaluation relying on soil description, classification and collected experience. this paper presents an approach that determines cohesion and angle of internal friction from diagrams of displacement on load, these being obtained by compression and traction load tests of real micropiles installed in the known soil. furthermore, the diagrams are interpreted by the mohr-coulomb strength criterion and the desired values of shear strength are thus derived by using it. keywords: displacement on load diagram, load test, micropile, mohr-coulomb, shear strength, soil. 1. introduction bearing capacity of the micropile is determined by the state and shear strength parameters of the ground within which a micropile is installed. the bearing capacity of the micropile is evaluated in terms of limit stability states regarding either peak or residual stability. the achievement of the respective state is conditioned by the displacements of micropile in the ground. calculations of the bearing capacity most often rely on data obtained either on samples tested in the laboratory or on an expert estimate by classification. both these input data do not take into account state conditions completely and indeed has never given technological procedures for micropile installation, yet both of which also contribute significantly to the bearing capacity. a more reliable picture of the micropile bearing capacity is offered by a load test on site, which will give a respective diagram of the displacement on load. unlike computational methods based on laboratory or expert data, this approach encompasses the state conditions and technological procedures of micropile installation. this diagram of the displacement on load comprises also the peak and residual limit stability states. the respective load forces extracted from the diagram may be utilised for further back analysis to evaluate the shear strength parameters of soil. the article here presented sets out a method employing a force calculation formula of micropile bearing capacity and an interpretation of the diagram following the mohr-coulomb criterion of shear strength. 2. introductory comment on method there is a fundamental assumption that the shear strength development in soil ground should be generally the same regardless of whether it is a laboratory test (figure 1) or a practical field situation, such as in the case of a micropile bearing capacity. figure 1. diagram of box shear tests. determination of strength parameters is based on results of two types of tests performed in a certain order on the same micropile. the compression test is performed first, followed by the traction test. this order of testing ensures that during the compression test both strength parameters are actively combined and at the end of this test procedure the cohesion of soil ground is derived completely. in the traction test that follows immediately there can be mobilised only the angle of internal friction because, after the previous pressure test, the cohesion on the shear surface became zero. the output of both kinds of test are the diagrams of displacement on load (figure 2) that have an analogous course with the shear strength on the displacement course of the soil sample obtained by standard 42 https://doi.org/10.14311/app.2022.32.0042 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 32/2022 assessment of shear strength parameters box shear tests. figure 2. graphs of the course of loading tests of a micropile. a second assumption is that the peak limit state contains both soil shear strength parameters and the residual limit state contains only one, namely the angle of internal friction. the peak limit state corresponds to the load of the peak force fp that represents the maximal recorded value during the test. the residual limit state corresponds to the load of the force fr that occurs at a displacement of 25 mm of micropile. a third assumption consists in analysing the peak force fp and the residual force fr by the shear strength morh-coulomb criterion τ = c + σ tanφ. the peak force fp extracted from diagram of displacement on load by compression test there is derived a peak shear resistance tp at the tube shape surface of the micropile and this equals the sum of a shear resistance due to cohesion tc and a shear resistance due to angle of internal friction tφ. tp = tc + tφ (1) the peak shear resistance is given by the peak force according to the formula: tp = fp πdl (2) where: d diameter of tube shape surface of mircopile l length of tube shape surface of micropile (length of micropile) the shear resistance due to the angle of friction is present in both kinds of test during the whole course of tests so the shear resistance due to angle of internal friction is gained by the residual force according to the formula: tφ = fr πdl (3) the resistance due to cohesion only is the difference between the peak shear resistance and the shear resistance due to angle of internal friction. the value of this difference is the cohesion. tc = tp − tφ = c (4) the residual force fr cannot be applied for evaluation of the angle of internal friction φ because the shear resistance due to the angle of friction involves also normal stress σ at the outer surface of the micropile body (tφ = σ tanφ). the normal stress is hard to verify experimentally on site by measurement. also, a common exact determination encounters a problematic coefficient of lateral pressure, for which there is no point in determining in the usual way, because the soil has been grouted with high grouting pressure and the grouting medium has undergone a phase transformation from a liquid to a solid one. thus, the possible way to determine the normal stress and also the unknown angle of internal friction in question offers a variation in approach with variation parameters σ and φ. the key parameter in the variation calculations is the peak force fp as it includes the both cohesion and angle of internal friction. the peak force is put into a modified formula 1. the remaining parameters are either derivable as tc or measured fp. fp πdl = σ tanφ (5) the variation approach provides many combinations of values σ and φ satisfying the formula 5. finally there are selected those ones fitting the relevant values expected at the respective ground site. 3. description of tests the displacement on load diagrams were processed with data obtained experimentally from two series of tests. a group of four micropiles was first subjected to a compression load force and after this the micropiles were loaded again, this time with a traction load force. the micropiles were installed on the site of the external testing frame at všb tuo, fast. 3.1. micropile the tests were conducted on a micropile from a steel hollow rod of a spindle shape casing, type r32. the length of each of the four micropiles was 3 meters. each micropile ended with a drill bit of diameter 45 mm. the micropile was installed by percussion drilling and after subsequently grouted from the micropile tip with a synthetic epoxy resin. the grouting pressure was about of 8-10 mpa and delivery of the resin was terminated when it ascended to surface (figure 3). this design is widely implemented in practice to enhance the bearing capacity of a foundation. 43 karel vojtasík acta polytechnica ctu proceedings figure 3. micropile from steel hollow rod of a spindle shape casing (type r32). atop of the micropile steel rod is incorporated a concrete prismatic cube. the concrete prismatic cube is necessary to facilitate the installation of a hydraulic press on the lender micropile body and to secure a coaxial transfer of compression load force in the micropile. the smallest deviation in coaxial transfer causes a bending of a slender micropile body and it consequently leads to serious distortion in the values of micropile axial displacement. the concrete prismatic cube has dimensions 40x40x50 cm (50 cm is the height of the cap). in the concrete prismatic there is embedded a short section micropile equipped with steel plate and a nut to fix the plate (figure 4). figure 4. concrete prismatic cube. in the traction load force test a traction load force is applied directly on the end of the micropile. the micropiles are installed inside an outdoor steel test frame. the micropiles are positioned in a square layout and the spacing of the micropiles axes is 1.2 m. the distance of the micropiles to the steel test frame foundations is 1.34 m. 3.2. ground all micropiles are installed in a ground profile that consists of an artificial layer of compacted clayey sand of thickness approx. 0.8 m, which replaces the original top layer consisting of anthropogenic sediments and soil with a high organic content (topsoil and sub-soil). the concrete prismatic cube is placed in it. then there follows a monotonous layer of loess to a depth of approx. 5.5 m and glacial sand and gravel are below this. the parameters of the loess are summarised in table 1. 3.3. compression load test the compression load test is carried out on the micropile together with the concrete prismatic cube on the micropile head. a compression load is generated by a hydraulic press, which is propped against the horizontal beam of the steel test frame through a steel support extension. the hydraulic press is placed on a steel plate with dimensions of 0.4 x 0.4 x 0.03 m. its role is to avert possible crushing of the top of a concrete prismatic cube and eliminate deformation when a higher load force sets in. it provides a base for four potentiometer transducers placed at plate corners to measure micropile settlement. the force is read out by a hydraulic press pressure sensor. the installation of the hydraulic press on the micropile is shown in figure 5. figure 5. installation of hydraulic press on the micropile at the micropile compression load test. all four micropiles were tested with the same regime of applying compression forces. the compression forces were increased in steps and after each step there followed a time break which was necessary to accomplish the full settlement of the micropile in each respective force step. the value of a force in the step was the same in all the steps and it was 10 kn. the break time was 5 minutes and it proved to be sufficient to complete the full micropile settlement in the respective force step. the compression test took place until the value of the 25 mm settlement of the micropile was exceeded. additionally, a concrete prismatic cube without a micropile, placed close to the micropiles in the same soil ground, was tested in a compression test. it was tested with the same instrumentation and load regime and during the test a respective diagram displacement on load was recorded. the diagram displacement on load of only the micropile without the concrete prismatic cube is derived as the difference of displacements of the micropile together with the concrete prismatic cube and a concrete prismatic cube. figure 6 shows the diagrams 44 vol. 32/2022 assessment of shear strength parameters h or iz on d ep th [m ] d en sit y ρ [g /c m 3 ] po ro sit y n [% ] w at er c on te nt w [% ] li qu id li m it w l [% ] pl as tic li m it w p [% ] sa tu ra tio n s r c oh es io n c e f [k pa ] a ng le of fri ct io n φ e f [◦ ] d ef or m at io n m od ul us e d e f [k pa ] 2.0 1.98 40 19.8 31.5 16.5 0.8 8 0 0 2.7 2.07 39 20.7 36.9 16.0 0.87 8 25 6900 table 1. the parameters of ground of displacement on load measured on the micropile together with the concrete prismatic cube (fm+h), on the sole concrete prismatic cube fh and the derived diagram for micropile only fm. figure 6. diagrams of the compressions tests. 3.4. traction load test the traction load test is carried out on the same micropiles shortly after the compressive load. the concrete prismatic cube is taken away and a traction rig installed above the micropile on ground around the micropile. the traction rig consists of a steel trestle specially constructed for this purpose. the trestle bears a hydraulic press and on its piston there is attached a traction mechanism assembled from two thick steel square plates and four steel rods that join both plates holding stiff together. one plate rests on a press piston the second is fixed with a screw to the micropile. the assembly of the traction rig and its installation on the micropile is shown in figure 7. figure 7. installation of the traction rig at micropile traction load test. the regime of applying traction forces was identical with the compression load test. the only difference between both kinds of tests is that traction displacement of the micropile is measured just with two potentiometer transducers. both transducers are placed on a lower steel plate close to the micropile top in a straight line going approximately through a micropile axe. after traction tests all four micropiles were pulled out from the ground at the end of the traction test. the diameters of the micropile column body along the whole length vary in the range between 70 to 80 mm. the steel rods are coated completely with grouted soil , from the bottom of the concrete prismatic cube to the drill bit. (figure 8). 45 karel vojtasík acta polytechnica ctu proceedings figure 8. the state of the tube shape surface of the micropile after being pulled out of the ground. 4. results the input data assigned straight from the diagram of tests (figure 2) are the load force of the peak bearing capacity fp taken from the diagram of the compression load test. it amounts to 59 kn. another is the load force at the residual bearing capacity fr. it is read as a displacement of micropile of 25 mm. in the case of a compression load test it amounts to 21 kn, while in the case of a traction load test it is greater and amounts to 32 kn. the last straightly assigned parameter is the diameter of a tube shape of micropile determined by measurement after the extraction micropiles out of the ground. this amounts to about 0.075 m (figure 8). the last parameter is the length of the micropile which is 3 m. before performing the calculations according to the relationships ??, it is necessary to explain the different course of the diagrams from both tests and further justify why there was no agreement between the courses and values of residual force in the residual strength stage, as would be expected. the diagram of the traction test does not display any peak in load force in contrast with the diagram for the compression test because during this test cohesion of soil ground should be zero, while it has been drawn out in the preceding compression test. the peak force at the compression test is due to cohesion at the tube shape surface of the micropile and resistance as the ground acts against the micropile base tip. the base is not large and therefore its contribution is disregarded in the analysis. the apparent difference in residual load forces at both tests is probably due to the different development of pore pressure on the tube shape surface of the micropile. in the pressure test the pore pressure will be positive, while in the tensile test it will be negative due to a suction. unfortunately, the pore pressure on the tube shape surface of the micropile is almost impossible to determine by measurement on site. the suction influences and raises the value of the measured residual force in a traction test. this is a reason why the measured value forces at the traction test must be corrected for the suction effect. the suction acts as cohesion on the tube shape surface of the micropile along with friction. the correction force due to suction fs is chosen intentionally as the difference between the residual forces of both tests for the 25 mm displacement and this is about 12 kn. the measured forces in the traction test are all reduced exactly by this value. the cohesion is calculated according to the modified formula 4 complemented with the effect of suction c = fp − (frt − fs) πdl (6) where frtis the residual force of traction test the cohesion is calculated from the values read from the graph of forces at a displacement of 8 mm at which the loading force in the pressure test reached a maximum. c = 59 − (33 − 12) π × 0.075 × 3.0 = 58kpa (7) the variation calculations formula is 59 π × 0.075 × 3.0 − 58 = σ tanφ (8) the result of variational calculations that satisfy the condition given by formula 8 is presented by the graph in figure 9. the relevant combinations of discovered parameters can be read from the graph. figure 9. the graph of combinations of values σ and φ satisfying formula 8. the determination of an angle of internal friction depends on the choice of a magnitude of the normal stress on the tube shape surface of the micropile. 46 vol. 32/2022 assessment of shear strength parameters in the case of a standard situation this magnitude changes with depth and should be same as the horizontal earth stress at rest. the micropile is grouted with a grouting pressure of 8-10 mpa, which is several times higher than a horizontal compound of earth stress value at geostatic stress state in the ground. further, the type of ground has 40% porosity and a degree of saturation between 80 and 90%. thus, there might be expected a normal stress definitely greater than horizontal earth stress at rest. keeping these facts in mind the normal stress might achieve a state where the horizontal compound of stress might be the same as the vertical compound of stress. an effect of grouting might be that the horizontal compound of stress does not depend on the depth and might have approximately constant magnitude along the micropile. the values on the axis σ range from 30 to 60 kpa, and these are consistent with the vertical compound of stress at a depth in the middle of the micropile and respectively at the micropile tip. a photograph on the figure 8, displaying the micropile after being pulled out of the ground and that was used to determine the diameter of it, can be also helpful. the tube shape surface of the micropile is visibly not smooth but rough. the roughness of the tube shape surface of the micropile can contribute to a decision about the magnitude of angle of internal friction indirectly. for the normal stress of 45 kpa, which is a middle point within the range 30 and 60 kpa, the respective angle of internal friction is 28°. this is slightly greater by about 3°than the value of 25°established at the laboratory which complies with a normal stress of 50 kpa. 5. discussion the derivation of the diagram of displacement on load for compression load of the micropile is based on an assumption that the total work performed by the micropile together with the concrete prismatic cube wt must be the sum of the work of the concrete prismatic headwh and the micropile without concrete prismatic cube wm. wt = wh +wm = fm+hs = fhs+ fms (9) where: s displacement fm+hcompression force at the micropile together with the concrete prismatic cube fhcompression force at concrete prismatic cube only fmderived compression force at micropile only without concrete prismatic cube fm = fm+h − fh (10) the laboratory data (table 1) do not correspond exactly to the results derived by the presented method. there is an obvious big difference in cohesion. the value of cohesion received from the laboratory test of 8 kpa corresponds with soft consistency and the value seems to be low with regard to a stiff consistency state of soil ground on the testing site. the laboratory value is likely devalued by sampling, transport, and handling of the ground sample before testing. another reason for the different values may be that the force boundary conditions in the laboratory test did not correspond to those on the site. the cohesion value of 58 kpa derived from the compression test is significantly greater than the laboratory value but not outside the range of the stiff soil ground consistency. in the value of the angle of internal friction there is a satisfactory agreement with the range of possible values of normal stresses and also the value determined in the laboratory. 6. conclusion the paper gives us knowledge that the search for, and finding a solution to, a specific geotechnical situation lies in the suitable application of general basic principles. in our case, the derivation of the shear strength parameters of soil ground was based on analysis of diagrams of displacement on load of micropiles. the parameters were derived from comparison with appropriate diagrams between shear strength and displacement of consolidated and soft soil that result from box shear tests and interpretation of the respective diagrams by the mohr-coulomb strength criterion. the results of shear strength determined according to the method were compared with laboratory results of soil samples taken at the testing site. the agreement between the two parameter determination approaches was only not found for cohesion where there occurred an apparent discordance. this discordance in the values of cohesion points to the low reliability of the determination of cohesion in the laboratory. this is due to boundary conditions under which the test is performed and the quality of soil sample. boundary conditions should be the same as on the actual construction site and the quality of the soil sample should be intact. both are difficult to achieve at the same time in laboratory tests, because the conditions on the construction site are difficult to reproduce in their entirety in the laboratory and the quality of the sample is also affected by the handling of samples. all this is a major obstacle to finding a reliable value of cohesion. if the cohesion assessment is based on site tests, both the boundary condition and the quality of the sample are implicitly included in the test. execution of compression and traction tests on the same micropile at the construction site and evaluation of their diagrams displacement on load open up a simple and fast way to a quite reliable determination of shear strength parameters of soil ground and surmount a number of important factors that affect 47 karel vojtasík acta polytechnica ctu proceedings these parameters without being excluded from the evaluation or else having been determined to be of low reliability. this method brings a different approach to determination of the parameters in question or also their verification if they were determined by other methods. acknowledgements the paper has been funded with the support of conceptual development of science, research and innovation in 2021, assigned to the všb-technical university of ostrava, the ministry of education, youth and sports in the czech republic. 48 acta polytechnica ctu proceedings 32:42–48, 2022 1 introduction 2 introductory comment on method 3 description of tests 3.1 micropile 3.2 ground 3.3 compression load test 3.4 traction load test 4 results 5 discussion 6 conclusion acknowledgements acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.37.0043 acta polytechnica ctu proceedings 37:43–47, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague determination of radionuclide impurities in pure beta radiation sources by semiconductor spectrometry pavel novotný czech technical university in prague, faculty of nuclear sciences and physical engineering, department of dosimetry and application of ionizing radiation, břehová 7, 115 19 prague, czech republic correspondence: pavel.novotny@fjfi.cvut.cz abstract. in many applications, the radionuclidic purity of a radioactive substance is a crucial property. it involves, for example, the production of radiopharmaceuticals or reference standards used in metrology. however, the identification of admixtures is not straightforward for certain nuclides. the paper presents a technique for the determination of radionuclide impurities in the mixtures of pure beta-emitting nuclides. the proposed method is based on the comparison of measured beta spectra with the detector responses to individual radionuclides. these are obtained using a monte carlo calculation performed in the geometry corresponding to the accurate description of a measuring assembly. keywords: beta spectrometry, monte carlo simulation, si(li) detector. 1. introduction beta-decaying radionuclides are used as sources of ionizing radiation in various industrial, medical, and scientific applications. in many of them, the key parameter of a radioactive substance is its radionuclidic purity, i.e. the absence of other radionuclides in the source. usually, most of the potential impurities can be determined by gamma spectrometry. however, this analytical tool is not applicable if both the nuclide of interest and an admixture are pure beta radiation sources, i.e. no gamma ray is emitted. in addition, the impurity may be an isotope of the same element as the main nuclide, which precludes the use of any chemical analytical method. the proposal of a method for the identification and quantification of radionuclide impurities in the mixtures of pure beta-emitting nuclides was one of the objectives of the recently completed metrobeta project, which was supported by the european metrology programme for innovation and research (empir) [1]. the research focused on the measurement of beta radionuclides was performed by several european national metrology institutes. one procedure for identifying radionuclides from a mixed beta spectrum has been proposed by [2]. the authors demonstrated the application of the spectrum stripping method for the deconvolution of spectra measured using a magnetic spectrometer. the spectra belonging to individual nuclides were described by a function defined by the formula as follows 8∑ i=1 ai sin(bie + ci), (1) where e stands for the energy of radiation. the equation (1) is empirical in nature, which means that it describes the spectra shape well from a mathematical point of view, but it has no deeper physical meaning. the parameters ai, bi, and ci were obtained by fitting the experimental data. the method was tested for the analysis of mixed spectra of three radionuclides 32p, 90y, and 106rh with different mixing ratios. the accuracy achieved in determining the radionuclide activity ratios was better than 2 %. a different approach to solving the discussed problem is offered by the following procedure [3]. the mixed beta spectrum measured is divided into several regions. considering a mixture of three radionuclides a, b, and c, the count rate ni in each spectrum region can be expressed as ni = ηa i aa + ηb i ab + ηc i ac, (2) where the symbol η denotes a detection efficiency. the desired activity values a are calculated by minimizing the expression representing the variance using the least square method. the procedures employed for the analysis of mixed beta spectra in this study partially utilize the principles mentioned above. however, values obtained from a precise monte carlo simulation were used to express the response to each single radionuclide instead of an empirical function. more details are given in the following section of the paper. 2. instrumentation and methods 2.1. detector and monte carlo model beta spectra were measured at the czech metrology institute spectrometry laboratory using a spectrometric system equipped with a lithium-drifted silicon detector. the detector was thoroughly characterized; therefore, its monte carlo model prepared for the mcnpx code [4] was available. a detailed description of the detector, its characterization, and the 43 https://doi.org/10.14311/app.2022.37.0043 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en pavel novotný acta polytechnica ctu proceedings model validation have been published in [5]. the source term for beta spectra calculations was defined as a histogram of electron energies derived from the fermi theory of beta decay. the simulated detector response to a given radionuclide was obtained using a pulse-height tally (type f8). the computation time of the simulations was chosen to achieve the relative type a uncertainty of the histogram bin values at the level of tenths of a per cent. the efficiency of calculations was improved by the variance reduction techniques (i.e. cell-by-cell energy cutoff and source biasing). all studied samples were prepared by drop deposition [6]. the droplet of a radioactive solution1 was deposited on a 0.1 mm thick polyethylene terephthalate foil. the solvent was subsequently evaporated, resulting in the formation of a thin radioactive layer. 2.2. two-window method suppose that the measured beta spectrum is a combination of contributions from two radionuclides a and b which differ in their beta decay endpoint energy. as the first step of an analysis using this method, two windows (regions of interest) are defined in the spectrum as indicated in figure 1. events originating from the radioactive decay of both components of a mixture are registered in region no. 1, whereas the detector response in region no. 2 is caused only by the radionuclide with a higher endpoint energy. figure 1. illustration of the two-window method idea. the following relations hold for the number of pulses n1 in region no. 1 and n2 in region no. 2 n1 = aaηa 1 t + abηb 1 t, n2 = abηb 2 t, (3) where ηi stands for the detection efficiency in the i-th region of the spectrum for the radionuclide indicated by the superscript value and t is the live time of the measurement. by dividing the first expression in equation (3) by the second one, a relation giving the ratio of radionuclide a and b activities is obtained aa ab = ηb 2 ηa 1 · ( n1 n2 − ηb 1 ηb 2 ) . (4) 1the aqueous solutions of srcl2 (20 mg of srcl2 per litre + 3 g of hcl per litre), ycl3 (50 mg of ycl3 per litre + 3 g of hcl per litre), and h3po4 (50 mg of h3po4 per litre) were used to prepare the samples analysed in this study. the spectrum in figure 1 corresponds to the situation where the endpoint energy value of an impurity is higher than the main nuclide endpoint. with a logarithmic scale on the vertical axis, its presence in a sample can be easily detected. on the other hand, if the impurity has an endpoint lower than the main nuclide, its identification may appear less clear, as the effect on the spectrum shape may not be visually apparent. nevertheless, the procedure for the quantitative evaluation is identical for both variants. 2.3. spectrum stripping method an alternative approach to the analysis of pure beta radionuclide mixtures is the spectrum stripping method. generally, this technique is based on the successive subtraction of the library spectra corresponding to individual nuclides present in a sample from the measured spectral response. the normalized library spectrum is subtracted channel by channel from the sample spectrum. this procedure proceeds from higher energies (i.e. for beta spectra from nuclides with a higher endpoint value) to lower ones. when analysing a mixed beta spectrum using this method, one region of interest is defined at first. its boundaries are chosen as follows. considering the difficulties in modelling the detector response for lowenergy electrons, it is advisable to choose the lower limit from approximately 500 kev upwards. the upper limit is restricted from above by the lowest value of an endpoint energy in the radionuclide mixture. in the selected region of interest, the integral of pulses is determined for both the measured and reference (i.e. admixture-free) spectra. for binary mixtures, the difference between these two values represents the number of pulses in the measured spectrum caused by the presence of an impurity in the sample ni. the number of pulses corresponding to the main nuclide nm is equal to the integral obtained from the reference spectrum. the activity ratio is then given by the following relation ai am = niη m nmηi (5) with ηi and ηm being the detection efficiencies for radionuclides i and m respectively. 3. results both methods of spectra deconvolution described above in sections 2.2 and 2.3 were tested on data from the measurement of samples with varying amounts of 89sr and 90sr. this mixture is commonly formed during the production of 89sr radionuclide due to parasitic reactions [7]. the samples also contained 90y, which is a radioactive decay product of 90sr. as the parent nuclide has a much longer half-life than the daughter nuclide2, it can be assumed that secular (long-term) radioactive equilibrium between these 2the half-lives of radionuclides 90sr and 90y are 28.80(7) a and 2.6684(13) d respectively [8]. 44 vol. 37/2022 radionuclide impurities in pure beta radiation sources figure 2. beta spectra of 89sr samples containg different amount of 90sr/90y impurity (top 0.3 %, bottom 3.0 %). nuclides was established in the sample. therefore, the activity of 90sr can be determined by the measurement of 90y. compared to 89sr, the endpoint energy of 90y is higher, which is favourable for the spectrum shape analysis because the high-energy part (> 500 kev) of beta spectra is not significantly distorted by self-absorption [9]. 3.1. two-window method the beta spectra of six samples were recorded using the si(li) detector. an example of two of these spectra is shown in figure 2. the ratio of the 90sr/90y impurity activity to the 89sr main nuclide activity was calculated using formula (4). the detection efficiencies were obtained from the monte carlo model of the detector. the regions of interest were defined in the energy range3 of (700–1400) kev and (1500–1700) kev. the results obtained are summarized in table 1. the standard uncertainty of the measured values was calculated as the uncertainty of an indirectly measured quantity (see [10]). 3the endpoint energy of 90sr equals 549.5(14) kev [8]. it implies that the radionuclide does not contribute to the detector response in the selected regions of interest. declared [%] measured [%] 0.100(1) 0.15(1) 0.300(3) 0.35(2) 0.500(5) 0.52(2) 1.00(1) 1.04(3) 2.00(2) 2.07(4) 3.00(3) 3.08(5) table 1. ratio of the 90sr/90y impurity activity to the main nuclide 89sr activity (two-window method). a second set of samples was analysed in the same way. these samples were also composed of strontium isotopes; however, 89sr was the impurity in 90sr/90y in this case. the calculated activity ratios are summarized in table 2. declared [%] measured [%] 1.00(1) 1.1(5) 2.00(2) 2.1(5) 5.00(5) 5.4(5) 8.00(8) 8.7(5) table 2. ratio of the 89sr impurity activity to the main nuclide 90sr/90y activity (two-window method). 3.2. spectrum stripping method the spectrum stripping method was tested on four spectra corresponding to different concentrations of the 89sr impurity in 90sr/90y. an example of one measured spectrum compared with the response to a 90sr/90y source of an older production date (i.e. virtually free of impurities4) is shown in figure 3. the ratio of 89sr to 90sr activity was calculated according to formula (5), with 90sr determined via 90y (assuming radioactive equilibrium between these two nuclides). the evaluation was performed in the energy interval of 1000 to 1490 kev. a comparison of the measured values with the declared ones is shown in table 3. the standard uncertainty of the results was calculated in accordance with the law of uncertainty propagation for an indirectly measured quantity [10]. declared [%] measured [%] 1.00(1) 1.0(4) 2.00(2) 2.1(4) 5.00(5) 5.2(4) 8.00(8) 8.3(4) table 3. ratio of the 89sr impurity activity to the main nuclide 90sr/90y activity (spectrum stripping method). 4the half-life of 89sr is 50.57(3) d [11], whereas it is equal to 28.80(7) a for 90sr [8]. 45 pavel novotný acta polytechnica ctu proceedings figure 3. beta spectra of two 90sr/90y samples (with and without 89sr impurity). 4. discussion specifically, the spectra of mixtures consisting of 89sr and 90sr/90y nuclides in various ratios were studied. it is easier to analyse a mixed beta spectrum if the admixture has a higher endpoint energy value than the main nuclide. the presence of the impurity is well evident in the spectrum, even for low concentrations of the order of tenths of a per cent. the measured and the declared activity ratios of the impurity to the main nuclide agree within the uncertainty for values greater than 0.5 %. the results are slightly overestimated for the lower concentrations. on the other hand, if the admixture endpoint energy is lower compared to the main nuclide, the analysis is more complicated. if only because the effect of the impurity on the spectrum shape is not obvious at first sight. moreover, the typical achievable uncertainties are larger due to the subtraction of close values in equation (4), which is used for the evaluation via the two-window method. the alternative evaluation of the same data set using the spectrum stripping technique provided slightly more accurate results (in terms of both measurement trueness and measurement precision). either way, both mentioned methods allow the activity ratio to be determined only at a level of a few per cent under these conditions. therefore, the sensitivity is about ten times lower in comparison with the situation mentioned in the previous paragraph. both proposed methods of beta spectra analysis are comparable in terms of instrumentation requirements or the complexity of calculations. however, the twowindow method seems to be slightly more universal. the spectrum stripping method is not suitable when the impurity has a higher endpoint energy than the main nuclide because the response from the admixture in the high-energy part of the spectrum is low and its statistics is poor. this fact complicates the normalization of reference spectra, which is crucial for the correct calculation of the impurity concentration. the advantage of the proposed method for the analysis of mixed beta spectra lies in its simplicity in terms of instrumentation. it requires only the equipment that is commonly available in radiometric laboratories. however, the results and the measurement repeatability are strongly dependent on the quality of measured samples. it is a limiting factor for the analysis of samples containing radionuclides with an endpoint energy value lower than circa 500 kev prepared by the universal drop deposition technique. figure 4. beta spectra of two 32p samples of the same design. the above-mentioned issue is demonstrated by the spectra in figure 4. both studied samples contained the same radionuclide 32p free of impurities and were of the same design. nevertheless, the measured spectra differ significantly from each other in shape in the low-energy part. as a result of the crystallization, the radioactive layer formed is not homogeneous and its thickness varies locally [9]. therefore, the level of selfabsorption in the samples also varies, which influences the low-energy part of the beta spectra. generally, there are two approaches how to overcome this limitation. the first relies on a change in the technology of samples production to improve their properties in terms of measurement repeatability. the second is based on the detailed characterization of each sample prepared by drop deposition, which is used to improve the accuracy of the monte carlo calculation of reference spectra. as part of the research on beta spectra metrology, the suitability of high-resolution x-ray fluorescence 2d scanning for this purpose was studied in detail and the results obtained have been presented in [12]. 5. conclusion the experiments performed showed that the analysis of beta spectra shape is a technique suitable for the quantification of radionuclide impurities. the method is promising, especially for the assay of mixtures consisting of pure beta minus emitters that are isotopes of one chemical element, i.e. other analytical tools, 46 vol. 37/2022 radionuclide impurities in pure beta radiation sources such as gamma spectrometry or chemical analysis, are not applicable. the findings are of direct practical relevance, and the proposed method can be easily implemented in radiometric laboratories where the radionuclide purity of substances is determined. references [1] metrobeta – radionuclide beta spectra metrology. [2022-04-28], http://metrobeta-empir.eu/. [2] u. lee, j. w. bae, h. r. kim. multiple beta spectrum analysis based on spectrum fitting. journal of radioanalytical and nuclear chemistry 314(2):617–622, 2017. https://doi.org/10.1007/s10967-017-5409-5. [3] y. kameo, a. katayama, a. fujiwara, et al. rapid determination of 89sr and 90sr in radioactive waste using sr extraction disk and beta-ray spectrometer. journal of radioanalytical and nuclear chemistry 274(1):71–78, 2007. https://doi.org/10.1007/s10967-006-6905-1. [4] d. pelowitz, j. durkee, j. elson, et al. mcnpx user’s manual version 2.7.0, 2011. [5] p. novotný, p. dryák, j. šolc, et al. characterization of the si(li) detector for monte carlo calculations of beta spectra. journal of instrumentation 13(01):p01021–p01021, 2018. https://doi.org/10.1088/1748-0221/13/01/p01021. [6] g. sibbens, t. altzitzoglou. preparation of radioactive sources for radionuclide metrology. metrologia 44(4):s71–s78, 2007. https://doi.org/10.1088/0026-1394/44/4/s09. [7] y. toporov, v. filimonov, r. kuznetsov, et al. production of 89sr using (n,γ)-and (n,p) reactions. in isotope production and applications in the 21st century, pp. 371–374. world scientific, 2000. https://doi.org/10.1142/9789812793867_0092. [8] m.-m. bé, v. chisté, c. dulieu, et al. table of radionuclides, vol. 3 of monographie bipm-5. bureau international des poids et mesures, sèvres, 2006. [9] k. siegbahn. alpha-, betaand gamma-ray spectroscopy. north-holland publishing co., amsterdam, 1965. p. 1367. [10] joint committee for guides in metrology. evaluation of measurement data – guide to the expression of uncertainty in measurement, 2008. [2022-04-28], https://www.bipm.org/utils/common/ documents/jcgm/jcgm_100_2008_e.pdf. [11] m.-m. bé, v. chisté, c. dulieu, et al. table of radionuclides, vol. 1 of monographie bipm-5. bureau international des poids et mesures, sèvres, 2004. [12] p. novotný. x-ray fluorescence analysis of samples for beta spectrometry and its application to precise monte carlo simulations. radiation physics and chemistry 192:109930, 2022. https: //doi.org/10.1016/j.radphyschem.2021.109930. 47 http://metrobeta-empir.eu/ https://doi.org/10.1007/s10967-017-5409-5 https://doi.org/10.1007/s10967-006-6905-1 https://doi.org/10.1088/1748-0221/13/01/p01021 https://doi.org/10.1088/0026-1394/44/4/s09 https://doi.org/10.1142/9789812793867_0092 https://www.bipm.org/utils/common/documents/jcgm/jcgm_100_2008_e.pdf https://www.bipm.org/utils/common/documents/jcgm/jcgm_100_2008_e.pdf https://doi.org/10.1016/j.radphyschem.2021.109930 https://doi.org/10.1016/j.radphyschem.2021.109930 acta polytechnica ctu proceedings 37:43–47, 2022 1 introduction 2 instrumentation and methods 2.1 detector and monte carlo model 2.2 two-window method 2.3 spectrum stripping method 3 results 3.1 two-window method 3.2 spectrum stripping method 4 discussion 5 conclusion references acta polytechnica ctu proceedings doi:10.14311/app.2017.13.0115 acta polytechnica ctu proceedings 13:115–120, 2017 © czech technical university in prague, 2017 available online at http://ojs.cvut.cz/ojs/index.php/app comparison of failure criteria for wood in tensile-shear stress state eliška šmídová∗, petr kabele czech technical university in prague, faculty of civil engineering, thákurova 7, 166 29 prague 6, czech republic ∗ corresponding author: eliska.smidova@fsv.cvut.cz abstract. an orthotropic failure criterion enhancing the lourenco's criterion by a shear strength multiplier and a maximum shear strength upper bound has been recently proposed and validated for timber under tensile and shear loading by the authors. the paper discusses its applicability for predicting strength in comparison with tsai-hill criterion, hankinson's and hyperbolic formula applying the two above mentioned enhancements of the lourenco's criterion. experimental data available in the literature for off-axis tensile and shear test of sitka spruce (picea sitchensis carr.), katsura (cercidiphyllurn japonicurn sieb. and zucc.), douglas fir (pseudotsuga menziesii), douglas fir laminated veneer and cupiúba (goupia glabra) are used for the purpose of this study. keywords: orthotropic failure criterion, timber, off-axis tensile test, shear strength upper bound. 1. introduction a large number of phenomenological strength criteria, that apply to the failure phenomenon in general instead of the failure mechanism itself, has been proposed for composite materials in recent decades. similarly to composites, wood is typical for twodimensional character and organized non-isotropic structure and as such can be categorised as orthotropic or transversely isotropic. it follows that strength theories developed for artificial composites can be used for prediction of wood strength [1, 2]. unfortunately, for some of these theories, it may be difficult to obtain specific coefficients experimentally, let us mention the issue of the interaction term [3–5]. all strength parameters may not be viewed in a deterministic way. stochastic analysis is an alternative approach that is on the rise. it takes into consideration, among others, correlation of strength properties that we can treat as independent random variables [2]. the off-axis test has become a widely used experiment for validation of failure criteria of non-isotropic materials. at a relatively low cost and operational requirements, it provides the strength of tensile or compressive specimens under uniaxial loading at various load-grain angles. specimens with the angle of 0°and 90°yield tensile strength parallel and perpendicular to the grain, respectively. the off-axis tensile test has been involved in numerous studies, especially in those related to the complex problem of determining shear strength of wood [6–10] or the applicability of failure criteria for a specific wood species [11, 12]. in the present study, we compared four existing failure criteria: (i) lourenco’s criterion modified for wood by the authors [13, 14], (ii) tsai-hill criterion [15], (iii) hankinson's formula [16] and (iv) hyperbolic formula [17]. furthermore, we applied some of the modifications of the lourenco's criterion to the other criteria. we made the comparison against the data available in the literature for sitka spruce, katsura [7], douglas fir [18], douglas fir laminated veneer [19] and cupiúba [20]. 2. failure criteria in this section, we give a brief overview of four failure criteria that are involved in this study. the formulas are expressed in terms of either a 2d stress state or the off-axis test using the following relations: σx = σ0m 2 (1) σy = σ0n 2 (2) τxy = −σ0nm (3) 2.1. lourenco's criterion modified for timber failure criterion that has been recently proposed by the authors [13, 14] is defined by means of rankine failure surface modified for orthotropy [21]. it is further enhanced by (i) parameter ps that multiplies shear strength fs in the failure surface formula, (ii) shear strength as an upper bound of the shear stress and (iii) crack type criterion. the first two enhancements enable a calibration of the failure surface through offaxis test results. crack type criterion defining whether a crack is initiated either along or across the grain is out of the scope of this work. failure surface and failure criterion are expressed in a 2d stress space by equation (4) and (5), respectively. 115 http://dx.doi.org/10.14311/app.2017.13.0115 http://ojs.cvut.cz/ojs/index.php/app eliška šmídová, petr kabele acta polytechnica ctu proceedings τxy,fs =  psfs ftxfty √ ftxfty(σx − ftx)(σy − fty) for τxy < fs fs for τxy ≥ fs (4) τxy = τxy,fs (5) 2.2. tsai-hill criterion the most frequently used failure theory for wood was proposed by norris [15]. it is also referred to as the tsai-hill criterion. it is an extension of the von miseshencky distortion energy hypothesis. in a 2d stress state, it has the form of equation (6). ( σx ftx )2 − σxσy ftxfty + ( σy fty )2 + ( τxy fs )2 = 1 (6) 2.3. hankinson’s formula this formula [16] has been used mostly for predicting the ultimate compressive strength of wood. some researchers have applied it also for predicting tensile strength [22]. the formula is as follows: σ0 = ftxfty ftxnh + ftymh . (7) 2.4. hyperbolic formula the hyperbolic formula has been developed to provide a better fit to douglas fir off-axis test results compared to other commonly used failure theories [17]. the definition is following: σ0 = 2ftxfty exp0.01θ(ftx + fty) + exp−0.01θ(fty − ftx) . (8) 3. results and discussion using the equations 1 3, the failure criteria are expressed in terms of the off-axis tensile test, i.e. σ0(θ), and compared with the experimental data from the literature for off-axis tensile and shear tests. for the purpose of calibration, two enhancements of the lourenco's failure criterion proposed by the authors (section 2.1) are applied to the other failure conditions, if possible. these are the use of: (1.) a shear strength multiplier ps in the failure surface formula (psfs instead of fs), (2.) shear strength as an upper bound of shear stress in a 2d stress space. 3.1. sitka spruce and katsura yoshihara and ohta conducted off-axis tensile and shear test for sitka spruce (picea sitchensis carr.) and katsura (cercidiphyllurn japonicurn sieb. and zucc.) [7]. specimens were conditioned at 65% relative humidity. for the first experiment type, dog-bone specimens with the outer dimensions of 140×10×8 mm were cut, five pieces for each grain angle out of 0°, 5°, 10°, 15°, 20°, 25°, 30°, 45°, 60°, 75° and 90°. shear strength was obtained from both off-axis tensile test (by transformation of the axial strength to the stress components of orthotropic symmetry) and torsion test. for the latter, ten dog-bone specimens with the outer dimensions of 180×20×5 mm were prepared. in this experimental campaign, shear strength from torsion test coincided well with that from the off-axis test for 15°– 30°. off-axis tensile test and shear test results for sitka spruce together with the uncalibrated failure curves (ps = 1.0, h = 1.0) are shown in the upper part of figure 1 in terms of off-axis tensile test variables. we can see that out of the uncalibrated curves, the tsai-hill curve represents a good estimate. calibrating the modified lourenco's, tsai-hill and hankinson's curves by the parameters of ps = 2.0, ps = 1.2 and h = 1.8, respectively, and activating the shear strength upper bound, we obtain the best fit of the average experimental data, see the lower part of figure 1. on the contrary, the hyperbolic curve does not reproduce the data well. experimental data for katsura are plotted together with uncalibrated and calibrated failure curves in the upper and lower part of figure 2, respectively. similarly to the results for sitka spruce, tsai-hill curve is the best estimate out the uncalibrated curves. nevertheless, by the calibration of the modified lourenco's, tsai-hill and hankinson's curves by the parameters of ps = 2.3, ps = 1.3 and h = 2.2 , respectively, and by application of the shear strength limit to the shear stress, we get a good prediction of the data. the hyperbolic curve strongly underestimates the data. 3.2. douglas fir woodward and minor measured tensile strength parallel and perpendicular to fibers and shear strength for douglas fir (pseudotsuga menziesii) [18] according to the specimen configuration and testing procedure specified by astm d-143 [23]. they used eight specimens for each type of experiment. for determination of the ultimate strength at grain angles of 15°, 30°, 45° and 60°, they used eight rectangular specimens for each grain angle. the specimens were 38 mm wide, 10 mm thick and they were of different length, excluding the gripping area: 480 mm, 330 mm, 250 mm and 200 mm for the grain angles of 15°, 30°, 45° and 60°, respectively. the specimens were dried to the moisture content of 12± 1%. uncalibrated and calibrated failure curves are shown in the upper and lower part of figure 3, respectively, together with the experimental results for douglas fir. both the modified lourenco's and hyperbolic criterion fit the average off-axis tensile test data well without any calibration. on the other hand, we can reproduce the data even better applying ps = 0.8 116 vol. 13/2017 comparison of failure criteria for wood 3 [°] 0 10 20 30 40 50 60 70 80 90 < 0 [m p a] 0 20 40 60 80 100 120 140 160 modified lourenco p s = 1.0 modified tsai-hill p s = 1.0 hankinson h = 1.0 hyperbolic off-axis avg off-axis avg'cov f s,avg f s,avg'cov 3 [°] 0 10 20 30 40 50 60 70 80 90 < 0 [m p a] 0 20 40 60 80 100 120 140 160 modified lourenco p s = 2.0 modified tsai-hill p s = 1.2 hankinson h = 1.8 hyperbolic off-axis avg off-axis avg'cov f s,avg f s,avg'cov figure 1. the experimental data of sitka spruce [7] with uncalibrated (top) and calibrated (bottom) failure conditions. and h = 1.5 to the tsai-hill and hankinson's formulas. let us note that for this wood species, the shear strength upper bound is not activated. in this study, we disregard size effect following conclusions of a related experimental work [24]. 3.3. douglas fir laminated veneer clouston et al. performed off-axis tensile test and shear test for douglas fir laminated veneer [19]. offaxis specimens of equal size (610×63×35 mm) were cut with grain angles of 0°, 15°, 30°, 45°, 60° and 90°, sixteen to eighteen pieces for each angle. shear strength was evaluated on nineteen standard astm shear block specimens using shear adjustment factor [25]. average moisture content of the specimens was 7.9%. looking at uncalibrated failure curves and experimental data in the upper part of figure 4, we can see that the modified lourenco's curve represents the best estimate. we can get similar results if we calibrate the tsai-hill's and hankinson's criteria by ps = 0.65 and h = 1.8, respectively, see the lower part of figure 4. we can notice that the shear strength upper bound is not activated. 3 [°] 0 10 20 30 40 50 60 70 80 90 < 0 [m p a] 0 20 40 60 80 100 modified lourenco p s = 1.0 modified tsai-hill p s = 1.0 hankinson h = 1.0 hyperbolic off-axis avg off-axis avg'cov f s,avg f s,avg'cov 3 [°] 0 10 20 30 40 50 60 70 80 90 < 0 [m p a] 0 20 40 60 80 100 modified lourenco p s = 2.3 modified tsai-hill p s = 1.3 hankinson h = 2.2 hyperbolic off-axis avg off-axis avg'cov f s,avg f s,avg'cov figure 2. the experimental data of katsura [7] with uncalibrated (top) and calibrated (bottom) failure conditions. 3.4. cupiúba todeschini conducted off-axis tensile and shear test for cupiúba (goupia glabra) [20] in accordance with the brasilian norm nbr 7190 [26]. for the purpose of the off-axis test, dog-bone specimens with the outer dimensions of 280×20×20 mm were cut, twelve pieces for each grain angle out of 0°, 15°, 30°, 45°, 60°, 75° and 90°. shear strength was measured using shear block specimens. the moisture content of the specimens varied from 12% to 14%. plotting the uncalibrated curves together with the experimental results of cupiúba (the upper part of figure 5), we can see that the hyperbolic curve yields the closest estimate of the average off-axis data. the lower part of figure 5 shows that we can reproduce well the data utilizing ps = 0.6, ps = 0.4 and h = 1.7 for the modified lourenco's, tsai-hill and the hankinson's criteria, respectively. similarly to the case of douglas fir and douglas fir laminated veneer, the shear strength upper bound is not activated. 4. conclusion the paper compares four orthotropic failure criteria against off-axis and shear experimental results avail117 eliška šmídová, petr kabele acta polytechnica ctu proceedings 3 [°] 0 20 40 60 80 100 < 0 [m p a] 0 20 40 60 80 100 120 140 modified lourenco p s = 1.0 modified tsai-hill p s = 1.0 hankinson h = 1.0 hyperbolic off-axis avg off-axis avg'cov f s,avg f s,avg'cov 3 [°] 0 20 40 60 80 100 < 0 [m p a] 0 20 40 60 80 100 120 140 modified lourenco p s = 1.0 modified tsai-hill p s = 0.8 hankinson h = 1.5 hyperbolic off-axis avg off-axis avg'cov f s,avg f s,avg'cov figure 3. the experimental data of douglas fir [18] with uncalibrated (top) and calibrated (bottom) failure conditions. able in the literature for the wood species of sitka spruce, katsura, douglas fir, douglas fir laminated veneer and cupiúba. the enhancements of the modified lourenco's criterion, which has been recently proposed and validated by the authors, are applied to the tsai-hill, hankinson's and hyperbolic formula, to a certain extent. specifically, (i) the shear strength multiplier and (ii) the shear strength upper bound are used. following conclusions can be drawn: (1.) the shear strength multiplier enables calibration of a failure formula that contains shear strength. in this way, both, the modified lourenco's and tsaihill criteria provide a better estimation of the average off-axis data for sitka spruce, katsura, douglas fir, douglas fir laminated veneer and cupiúba. (2.) the shear strength upper bound, that can be activated as a maximum limit of the shear stress component, provide a better fit for average off-axis data of sitka spruce and katsura. for these wood species, average shear strength measured by torsion test coincides well with the average off-axis strength. (3.) calibration of the hankinson's formula by the parameter h yields good estimates for all wood species involved in this study, similarly as the modified 3 [°] 0 10 20 30 40 50 60 70 80 90 < 0 [m p a] 0 10 20 30 40 50 60 70 modified lourenco p s = 1.0 modified tsai-hill p s = 0.8 hankinson h = 1.5 hyperbolic off-axis avg off-axis avg'cov f s,avg f s,avg'cov 3 [°] 0 10 20 30 40 50 60 70 80 90 < 0 [m p a] 0 10 20 30 40 50 60 70 modified lourenco p s = 1.0 modified tsai-hill p s = 0.65 hankinson h = 1.8 hyperbolic off-axis avg off-axis avg'cov f s,avg f s,avg'cov figure 4. the experimental data of douglas fir laminated veneer [19] with uncalibrated (top) and calibrated (bottom) failure conditions. lourenco's and tsai-hill criteria do. in contrast, the hyperbolic formula predicts well only douglas fir data as it does not contain any calibration or shear strength parameter. list of symbols θ load-grain angle [°] ftx tensile strength parallel with grain [mpa] fty tensile strength perpendicular to grain [mpa] fs shear strength [mpa] h parameter of the hankinson's criterion [–] m cos θ [–] n sin θ [–] ps shear strength multiplier [–] σ0 ultimate strength at a load-grain angle θ [mpa] σx normal stress parallel with grain [mpa] σy normal stress perpendicular to grain [mpa] τxy shear stress [mpa] acknowledgements this work was supported by the grant agency of the czech technical university in prague, grant no. sgs17/168/ohk1/3t/11. 118 vol. 13/2017 comparison of failure criteria for wood 3 [°] 0 10 20 30 40 50 60 70 80 90 < 0 [m p a] 0 20 40 60 80 100 modified lourenco p s = 1.0 modified tsai-hill p s = 1.0 hankinson h = 1.0 hyperbolic off-axis avg off-axis avg'cov f s,avg 3 [°] 0 10 20 30 40 50 60 70 80 90 < 0 [m p a] 0 20 40 60 80 100 modified lourenco p s = 0.6 modified tsai-hill p s = 0.4 hankinson h = 1.7 hyperbolic off-axis avg off-axis avg'cov f s,avg figure 5. the experimental data of cupiúba [20] with uncalibrated (top) and calibrated (bottom) failure conditions. references [1] r. s. sandhu. a survey of failure theories of isotropic and anisotropic materials. tech. rep., air force flight dynamics lab wright-patterson afb oh, 1972. [2] b. kasal, r. j. leichti. state of the art in multiaxial phenomenological failure criteria for wood members. progress in structural engineering and materials 7(1):3–13, 2005. doi:10.1002/pse.185. [3] s. w. tsai, e. m. wu. a general theory of strength for anisotropic materials. journal of composite materials 5(1):58–80, 1971. [4] a. a. zolochevskii. tensor relationship in the theories of elasticity and plasticity of anisotropic composite materials with different tensile and compressive strength. mechanics of composite materials 21(1):41–46, 1985. [5] r. e. rowlands, d. e. gunderson, j. c. suhling, m. w. johnson. biaxial strength of paperboard predicted by hill-type theories. the journal of strain analysis for engineering design 20(2):121–127, 1985. doi:10.1243/03093247v202121. [6] e. p. saliklis, r. h. falk. correlating off-axis tension tests to shear modulus of wood-based panels. journal of structural engineering 126(5):621–625, 2000. [7] h. yoshihara, m. ohta. estimation of the shear strength of wood by uniaxial-tension tests of off-axis specimens. journal of wood science 46(2):159–163, 2000. [8] j. y. liu. analysis of off-axis tension test of wood specimens. wood and fiber science 34(2):205–211, 2002. [9] h. yoshihara, t. satoh. examination of the off-axis tension test method for evaluating the shear properties of wood. forest product journal 53(5):75–79, 2003. [10] j. c. xavier, n. m. garrido, m. oliveira, et al. a comparison between the iosipescu and off-axis shear test methods for the characterization of pinus pinaster ait. composites part a: applied science and manufacturing 35:827–840, 2004. [11] n. t. mascia, r. todeschini, e. a. nicolas. uniaxial and biaxial test for evaluation of failure criterion of anisotropic material applied to wood. in world conference on timber engineering, riva del garda, italy. 2010. [12] n. t. mascia, r. a. simoni. analysis of failure criteria applied to wood. engineering failure analysis 35:703–712, 2013. doi:10.1016/j.engfailanal.2013.07.001. [13] e. bartunkova. constitutive model of timber. master’s thesis, czech technical university in prague, 2013. [14] e. smidova, p. kabele. constitutive model for timber fracture under tensile and shear loads. applied mechanics and materials 784:137–144, 2015. doi:10.4028/www.scientific.net/amm.784.137. [15] c. b. norris. strength of orthotropic materials subjected to combined stresses, 1962. [16] r. l. hankinson. investigation of crushing strength of spruce at varying angles of grain. air service information circular 259, 1921. [17] j. woodward, b. clinton. the elastic and strength properties of douglas fir in the radial-longitudinal plane. ph.d. thesis, new mexico state university, new mexico, 1986. [18] c. woodward, j. minor. failure theories for douglas-fir in tension. journal of structural engineering 114(12):2808–2813, 1988. [19] b. p. clouston, f. lam, j. d. barrett. interaction term of tsai-wu theory for laminated veneer. journal of materials in civil engineering pp. 112–116, 1998. [20] r. todeschini. ensaios uniaxiais e biaxiais para avalicao de criterio de resistencia de materiais anisotropicos aplicado a madeira. master’s thesis, faculdade de engenharia civil, arquitetura e urbanismo, campino, brasil, 2009. [21] p. b. lourenco, r. de borst, j. g. rots. a plane stress softening plasticity model for orthotropic materials. int j numer methods eng 40:4033–4057, 1997. [22] f. f. p. kollmann, w. a. cote. principles of wood science and technology: i solid wood. springer berlin heidelberg, berlin, heidelberg, 1968. oclc: 851389657. [23] a. s. d-143. standard methods for testing small clear specimens of timber, vol. 09. american society for testing and materials, philadelphia, pa., 1984. 119 http://dx.doi.org/10.1002/pse.185 http://dx.doi.org/10.1243/03093247v202121 http://dx.doi.org/10.1016/j.engfailanal.2013.07.001 http://dx.doi.org/10.4028/www.scientific.net/amm.784.137 eliška šmídová, petr kabele acta polytechnica ctu proceedings [24] t. tannert, t. vallee, s. franke, p. quenneville. comparison of test methods to determine weibull parameters for wood. world 15:19, 2012. [25] p. clouston. the tsai-wu strength theory for douglas-fir laminated veneer. master’s thesis, the university of british columbia, vancouver, 1995. [26] abnt. nbr 7190: projeto de estruturas de madeira. abnt, rio de janeiro, 1996. 120 acta polytechnica ctu proceedings 13:116–121, 2017 1 introduction 2 failure criteria 2.1 lourencos criterion modified for timber 2.2 tsai-hill criterion 2.3 hankinson's formula 2.4 hyperbolic formula 3 results and discussion 3.1 sitka spruce and katsura 3.2 douglas fir 3.3 douglas fir laminated veneer 3.4 cupiúba 4 conclusion list of symbols acknowledgements references https://doi.org/10.14311/app.2022.33.0452 acta polytechnica ctu proceedings 33:452–456, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague dowels effectiveness investigation between ultra high performance fiber reinforced concrete and reinforced concrete spyridon a. paschalisa,∗, andreas p. lampropoulosb a school of engineering and computing, university of west london, uk b school of architecture, technology and engineering, university of brighton, uk ∗ corresponding author: spyros.paschalis@uwl.ac.uk abstract. in the present study, the performance of reinforced concrete (rc) beams which were strengthened with ultra high performance fiber reinforced concrete (uhpfrc) and dowels at the interface was investigated. rc beams with a length of 2.2 m strengthened with uhpfrc layers at the tensile side. before the application of the layers, the interface between rc and uhpfrc was roughened. during the testing, the interface slips between uhpfrc and rc were recorded using linear variable differential transformers (lvdts). the beams were tested under four-point flexural test. the results of the present study indicated that the dowels at the interface reduce the slips at the interface, delay the formation of cracks and result in higher load carrying capacity. keywords: dowels, interface characteristics, reinforced concrete beams, steel connectors, strengthening, uhpfrc. 1. introduction safety of structures is of paramount importance especially in case of old substandard structures or structures damaged by earthquakes or other accidental actions. existing strengthening and retrofitting methods present crucial disadvantages which are related to the high cost, the difficulty in the application and the low performance of the technique. the present study investigates the effectiveness of ultra high performance fiber reinforced concrete (uhpfrc) as strengthening material using dowels at the interface between reinforced concrete (rc) members and uhpfrc layers. the are several studies in the literature investigating the properties of the uhpfrc [1–4]. the last years research has been focused on the application of uhpfrc as repair and strengthening material. an analytical model for the prediction of the response of composite uhpfrc and rc elements was developed by habel et al. [5]. uhpfrc was applied as repair material from bruhwiler and denarie [6], while lampropoulos et al [7] developed a numerical model that predicts the performance of rc beams which were strengthened with uhpfrc. paschalis et al. [8], conducted full scale tests to investigate the effectiveness of uhpfrc for the strengthening of rc beams. in this study, rc beams strengthened with uhpfrc layers, with and without the use of steel bars in the layers. the addition of steel bars in the layer resulted in high load carrying capacity of the strengthened beams. however, high values of slips at the interface between uhpfrc and concrete were recorded. in the present study the performance of rc beams which were strengthened with uhpfrc layers and dowels at the interface has been investigated. this is a topic which has not been investigated in detail and needs further investigation. 2. experimental program six beams have been tested in the present study. two rc beams prepared without any layer and used as control beams. in two beams, uhpfrc layers added at the tensile side and in two beam dowels added at the interface between uhpfrc and rc. the reinforcement and the dimensions of the beams without any reinforcement are presented in figure 1. in figure 2, the dimensions of the layers and the position of the dowels are presented. the dowels were designed based on greek retrofitting code [9]. 3. preparation of the specimens fine sand, ggbs, silica fume, superplasticizer, high strength cement and steel microfibers were used in the mix design. in addition, a high percentage of fibers (3 vol.%) was incorporated in the mix to achieve strain hardening. the interface was roughened using a needle scaler and the dowels placed in position using a drill. the preparation of the beams which were strengthened with dowels is presented in figure 3. 4. properties of the materials cubes with 10 cm sides, tested in compression according to bs en 12390-3:2009 [10]. the compressive 452 https://doi.org/10.14311/app.2022.33.0452 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 33/2022 dowels effectiveness investigation figure 1. control beam. figure 2. strengthened beams using dowels. figure 3. preparation of the beams. strength of concrete which was used for the preparation of the beams was 31 mpa and the compressive strength of uhpfrc layers was 136 mpa. the tensile characteristics of uhpfrc identified with a direct tensile test. the results are presented in figure 4. based on the average curve of figure 4, the tensile strength was 11.5 mpa and the modulus of elasticity 50 gpa. 5. testing of the beams in figure 5, a strengthened beam with layer before the testing is presented. as can be seen, lvdts were place at the interface between uhpfrc and rc to record the slip. in figure 7, the average experiments results are placed. the results of figure 7 indicate the effectiveness of dowels. in both techniques, the stiffness of the beams was increased. however, with the addition of dowels a higher load carrying capacity was achieved. in this case, the maximum load was increased by 22.5%. on the contrary, an increase of only 1.5% was achieved with the addition of layers only. in figure 8, the beams after the testing are presented. the control beams failed with a major crack at the tensile side (figure 8a). the failure mode of the two beams which were strengthened with uhpfrc 453 spyridon a. paschalis, andreas p. lampropoulos acta polytechnica ctu proceedings figure 4. stress-strain curve from the testing of uhpfrc in tension. figure 5. experimental setup for the testing of the beams. figure 6. a) lvdts on side 1 b) lvdts on side 2. 454 vol. 33/2022 dowels effectiveness investigation figure 7. results from the testing of beams. figure 8. failure mode a) control beam b) beam strengthened with layer c) beam strengthened with layer and dowels. layers was not the same. in one beam the bond at the interface was strong and a major crack at the layer progressively propagated to the rc beam. in the second beam, a de-bonding at the interface occurred (figure 8b). finally, identical was the failure mode of beams which were strengthened with layers and dowels. a major crack appeared in the layers and propagated to the rc beam (figure 8c). during the testing it was noticed that dowels resulted in the delay of the formation of cracks. this was also evident from the results of figure 7. the cracking on the control beams started at value of load 5 kn, on the strengthened beams with layers at 15 kn and on beams with layers and dowels at 24 kn. the results from the measurements of slips at the interface are presented in table 1. due to the fact that a local debonding commenced in one of the beam which was strengthened with uhpfrc layer, the reading of this beam were ignored. lvdt layers 1 dowels 1 dowels 2 2 0.06 0.04 0.01 3 0.11 0.01 0.18 4 0.36 0.05 0.06 7 0.03 0.03 0 8 0.18 0.14 0.01 table 1. reading of the lvdts. from the results of table 1, it can be noticed that almost in all positions, the values of slip for the beams with dowels at the interface, were significantly lower 455 spyridon a. paschalis, andreas p. lampropoulos acta polytechnica ctu proceedings compared to the slips of the beam without dowels. this indicates the better bonding at the interface when dowels are used. 6. conclusions the main aim of the present study was to investigate the effectiveness of dowels at the interface between uhpfrc and rc. full scale tests were conducted and rc beam with a length of 2.2 m strengthened with uhpfrc layers, with and without dowels. the slips at the interface were also recorded. based on the experimental results the following conclusions can be drawn: • dowels delay the formation of the cracking • dowels result in a better bonding • dowels result in a higher load carrying capacity based on the findings of the present research, the use of dowels should be considered in cases that better bonding and better performance of the strengthened elements is required. references [1] l. ferrara, n. ozyurt, m. di prisco. high mechanical performance of fibre reinforced cementitious composites: the role of casting-flow induced fibre orientation. materials and structures 44(1):109-28, 2010. https://doi.org/10.1617/s11527-010-9613-9. [2] s. paschalis, a. lampropoulos. fiber content and curing time effect on the tensile characteristics of ultra high performance fiber reinforced concrete. structural concrete 18(4):577-88, 2017. https://doi.org/10.1002/suco.201600075. [3] s. paschalis, a. lampropoulos. ultra high performance fiber reinforced concrete under cyclic loading. aci materials journal 113(4):419-427, 2016. [4] s.-t. kang, j.-k. kim. the relation between fiber orientation and tensile behavior in an ultra high performance fiber reinforced cementitious composites (uhpfrcc). cement and concrete research 41(10):1001-14, 2011. https: //doi.org/10.1016/j.cemconres.2011.05.009. [5] k. habel, e. denarié, e. brühwiler. structural response of elements combining ultrahigh-performance fiber-reinforced concretes and reinforced concrete. journal of structural engineering 132(11):1793-800, 2006. https://doi.or g/10.1061/(asce)0733-9445(2006)132:11(1793). [6] e. bruhwiler, e. denarie. rehabilitation of concrete structures using ultra-high performance fibre reinforced concrete the second international symposium on ultra high performance kassel, germany, 2008. https://infoscience.epfl.ch/reco rd/182195/files/010.pdf. [7] a. p. lampropoulos, s. a. paschalis, o. t. tsioulou, et al. strengthening of reinforced concrete beams using ultra high performance fibre reinforced concrete (uhpfrc). engineering structures 106:370-84, 2016. https: //doi.org/10.1016/j.engstruct.2015.10.042. [8] s. a. paschalis, a. p. lampropoulos, o. tsioulou. experimental and numerical study of the performance of ultra high performance fiber reinforced concrete for the flexural strengthening of full scale reinforced concrete members. construction and building materials 186:351-66, 2018. https: //doi.org/10.1016/j.conbuildmat.2018.07.123. [9] greek organization for seismic planning and protection, greek retrofitting code (greco), athens, 2013. [10] bs en 12390-3:2009. testing hardened concretepart 3: compressive strength of test specimens, 2009. 456 https://doi.org/10.1617/s11527-010-9613-9 https://doi.org/10.1002/suco.201600075 https://doi.org/10.1016/j.cemconres.2011.05.009 https://doi.org/10.1061/(asce)0733-9445(2006)132:11(1793) https://infoscience.epfl.ch/record/182195/files/010.pdf https://doi.org/10.1016/j.engstruct.2015.10.042 https://doi.org/10.1016/j.conbuildmat.2018.07.123 acta polytechnica ctu proceedings doi:10.14311/app.2018.15.0006 acta polytechnica ctu proceedings 15:6–11, 2018 © czech technical university in prague, 2018 available online at http://ojs.cvut.cz/ojs/index.php/app performance evaluation of different linear equation solvers for solving nonlinear fe problems on multicore architectures michal bošanský∗, bořek patzák czech technical university in prague, faculty of civil engineering, thákurova 7, 166 29 praha 6, czech republic ∗ corresponding author: michal.bosansky@fsv.cvut.cz abstract. the aim of this paper is to evaluate the performance of existing parallel linear equation solvers to solving large-scale, nonlinear finite element analysis problems on systems with distributed memory. the parallel approach allows us to take an advantage of the distributed memory enabling forming large system matrices and of multiple processing units to achieve significant speedups. our study is based on comparison of parallel direct solver and parallel iterative solver implemented in superlu dist library from portable, extensible toolkit for scientific computation (petsc). both considered solvers are designed for distributed system memory model and are based on a massage passing interface (mpi). the efficiency of individual solvers is evaluated on a selected benchmark problems, with different solution strategies by comparing computation times and obtained speedups. keywords: nonlinear system, distributed memory, direct solver. 1. introduction computational advancements have started a new trend in computational science and engineering that were powered by development and increases in the power and pervasiveness of computer and communication. capitalization on those advances by developing techniques for modern hardware enable the solution of large and complex problems. traditional finite element solvers run in serial mode and are limited by available resources represented by a single machine. the serial computers permit us to run simulation codes only sequentially and often have limited available resources represented by the single processing unit and available system memory. parallelization can significantly reduce the solution time by more efficient use of modern hardware and enable to solve large problems by utilizing distributed memory resources. parallel computers can be classified by type of system memory architecture. the shared, distributed and hybrid (combination of shared and distributed memory) memory systems exist [1]. this paper is focused on distributed memory systems. in distributed memory systems, the memory is physically distributed on individual processing units and there is no global shared memory as on shared memory systems, where all computing units can access the same physical global address space. the explicit communications between processing units are needed to establish data exchange. it is a task of the programmer to explicitly define how and when data is communicated. the cost of communication, compared to a shared memory systems, can be very high, on the other hand, the advantage is that overall memory is scalable with increasing number of processors an memory access is not limited by a single memory bus. in recent years a new trend is based on using graphics processing units together with traditional processors to accelerate the solution process is enabling. graphics processing unit computes intensive portions of the solution while the remainder of the code still runs on the processor. the idea of parallel algorithms is based on partitioning the problem into a set of smaller tasks, that can be solved simultaneously. scalability of computation is the most important goal in parallel computing [2]. the scalable parallel algorithm allows achieving decreasing execution time by using an increasing number of processing units, ideally in a linear trend. the ideal scalability is difficult to obtain due to the overhead cost of the parallel algorithm (synchronization and communication) and due to the fact that some parts of the problem are essentially sequential. moreover, despite obtained speed-up, the parallel computing allows solving large, complex problems that cannot be solved at a single, well-equipped machine. the finite element method (fem) has become a widely used tool for solving problems described by partial differential equations and it has been widely adopted by engineering and scientific communities as a reliable numerical tool. in fem the differential equations are converted to the algebraic system of equations by using variational methods with the help of decomposition of the problem domain into sub-domains called elements and smart choice of interpolation functions. numerical solutions of many engineering problems lead to the solution of nonlinear models consisting of very complex geometries and many degrees of free6 http://dx.doi.org/10.14311/app.2018.15.0006 http://ojs.cvut.cz/ojs/index.php/app vol. 15/2018 different linear equation solvers for solving nonlinear fe problems dom. nonlinear problems add additional complexity. in structural mechanics the nonlinearity can originate from nonlinear geometrical relations (large deformations), nonlinearity of constitutive relations on from boundary conditions (follower type of loading, for example) problem, typically using newton-raphson algorithm. this makes the nonlinear problem solution more demanding, compared to linear problems. the schema of common process for iteration processes is presented on next figure fig. 1. begin increment increase external load f ext d = 0∆ calculate balance force f f ext int (n) (n,1) predict change in displacements dδ d = d +∆ i+1 ∆ i dδ i+1 determine new internal force f int (n,i) problem is solved with required tolerance? end increment no yes figure 1. iteration process. nonlinear structural analysis is the prediction of the response of nonlinear structures by modelbased simulation. the concept of equilibrium path explaining the solution process of nonlinear structural analysis. this concept lends itself to graphical representation in the form of response diagrams. the form that is used in this paper is load-deflection response diagram. the mechanical behavior of structures can be characterized by load-deflection or force-displacement response. this response is represented in two dimensions as representative force quantity marked by f against a representative displacement quantity represented by d as illustrated in fig. 2. term path is a continuous curve in load deflection diagram and in typically the path is smooth. smooth path has a continuous tangent except at exceptional points. the state or configuration of the structure represents each point in the path. the equilibrium path represents configurations in static equilibrium. the inception of the response diagram (load is equal to zero, displacement is equal to zero) is characterized reference state and this state represents configuration from which loads and displacements are measured. for structural analysis there are four sources of nonlinear behavior. the corresponding nonlinear effects are identified by the term material, geometric, force boundary conditions and displacement boundary conditions. in this paper, material nonlinearity we consider that only being the source of nonlinear behavior. material behavior depends on current deformation state and possibly past history of deformation. other constitutive variables, prestress, temperature, time, etc. may be involved [3]. the problem representing discrete equilibrium equations at nodes is described by system of nonlinear algebraic equations fint(d) = fext, (1) where the fint represents the internal forces vector depending on unknown displacements d and fext representing external load. the problem can be linearized by means of taylor series expansion of fint(d). the internal force vector find represent nodal equivalent of internal stresses and defined as fσint(d) = btσ(ε(d))dv, (2) where b is strain-displacement matrix, σ is stress and ε is displacement. the taylor series expansion of fint(d) around the point (d) is given by fint(d) + ∂fint(d) ∂d ∆d = fext, (3) where ∂fint(d) ∂d represents the tangential stiffness matrix k of the structure. after substitution the equation 2 has a form k∆d = fext − fint(d), (4) newton‘s method is a commonly used to iteratively solve nonlinear systems. in the general form, load movement is applied and the incremental displacement is solved from linearized, discrete problem. the residual loading is evaluated from the difference of external and internal forces corresponding to achieved displacement, structure is loaded by residual and corresponding incremental change of displacement is evaluated. this iterative process is repeated, until required tolerance is achieved. in this contribution, we consider different in variants of the newton-raphson method, the modified newton-raphson method and the initial stiffness method [4]. the full newton-raphson method is based on using the tangent stiffness matrix in each iteration which is formed and factorized in each iteration. the advantage of the method is fast convergence, but it can be computationally expensive for some types of problems. 7 michal bošanský, bořek patzák acta polytechnica ctu proceedings f d prescribed load increment unknown displacement increment f ext (n) f ext (n-1) f int (n,1) f int (n,2) d (n-1) d (n,1) d (n,2) d (n,3) d (n) d (n) ∆ d (n,3) ∆ d (n,2) ∆ d (n,2) δd (n,1) δ d (n,i) d (n) as i ∞ reference state figure 2. newton raphson method. the modified newton-raphson method has the same algorithm, but stiffness matrix is loaded only after certain number of iterations or out the forming of each loading step. this approach can be computationally expensive as the stiffness matrix should be factorized only when changed however, the solution process has slower convergence rate compared to the full newton-raphson method. f d f ext (n) f ext (n-1) d (n-1) d (n,1) d (n) d (n) ∆ d (n,3) ∆ d (n,2) ∆ d (n,2) δd (n,1) δ d (n,i) d (n) as i ∞ d (n,2) d (n,3) reference state figure 3. modified newton raphson method. the initial stiffness method is using initial, elastic stiffness matrix constructed only once and which is held constant throughout the solution process. this method is robust but requires a large number of iterations to converge. the optimal choice is problem dependent. the iterative solver has basically the same solution cost of each problem, while direct solver can profit from existing factorization of system matrix in case there is no change (also iterative solver can profit, as no preconditioner setup is needed), but the effect is not in common with profit from existing factorization of system matrix, f d f ext (n) f ext (n-1) d (n-1) d (n,1) d (n) d (n) ∆ d (n,3) ∆ d (n,2) ∆ d (n,2) δd (n,1) δ d (n,i) d (n) as i ∞ d (n,2) d (n,3) reference state figure 4. initial stiffness method. 2. implementation superlu is a general purpose library for the direct solution of a large sparse symmetric or non-symmetric system of linear equations [5]. superlu library provides serial, multithreaded (shared-memory), and distributed memory versions. the performance of the distributed version of superlu solver based on message passing interface (mpi) [6] is evaluated nonlinear structural analysis and compared with implemented portable, extensible toolkit for scientific computation (petsc) as the iterative solver [7] in this paper. the comparison was made using the oofem opensource finite element solver [8], distributed under gnu public license, written in c++ programming language. this solver has been extended to support the superlu distributed version. the interface to superlu dist library consists of new classes to represent interface to superlu dist (direct solver based on distributed memory model) and classes implementing superlu dist compatible sparse matrix storage. the petsc (iterative solver based on distributed memory) support already exists. the superlu driver advantageously uses the symmetry of the matrix in the nonlinear system. the sparse matrix and the right-hand side vector are distributed among all the processes using the distribution based on block rows. that is, each process owns a block of consecutive rows of matrices. distributed sparse matrix is stored in a compressed row format. this choice comes from superlu library, which requires the sparse matrix in this format on input. in the compressed row format, only nonzero entries of the sparse matrix are stored in one dimensional array. additional integer array is needed to store column indices of the stored values. we assume, that the portioning of the discretized problem domain has been established and individual, non-overlapping sub-domains (partitions) are assigned to and stored locally on individual computing nodes. the so called node-cut strategy is assumed, where the cut dividing the problem domain runs through the nodes. nodes on mutual partition boundaries are called shared nodes, the nodes inside individual partitions are so-called local nodes. the process of parallel assembly is using group 8 vol. 15/2018 different linear equation solvers for solving nonlinear fe problems of processing nodes assembling the problem simultaneously. each processing node is responsible for assembling its contribution to the global system matrix by summing up the contributions from individual elements assembled to the node by portioning. to minimize the communication, it is natural that each processing node will assemble and maintain in its local memory corresponding block of global stiffness matrix. the ownership is uniquely defined for local nodes, which are exclusively shared by local elements on individual partitions. for shared nodes, which are shared by elements from multiple partitions, the ownership has to be defined by convention. the process of assembling the global right hand side vector is very similar to the process of assembling the stiffness matrix, but in many aspects is simpler, do to the assembly of local vector contributions. 3. results the mentioned nonlinear solution strategies have been evaluated on 3d finite element model of anchor pull out test using two different sparse linear solvers (superlu dist and petsc) using distributed memory programming model. the anchor is located close to the boundary, requiring 3d analysis with only one plane of symmetry. as the stell anchor is pulled out of concrete, the crack surface is initiated at the anchor head and starts to propagate toward the boundary as the loading increases. to model concrete fracture, an anisotropic, nonlocal damage based model has been used. the anchor pullout test mesh consists of 444 nodes and 799 triangular elements with nonlinear interpolation. the loading was controlled by prescribed increments of anchor pull out. in total 15 loading steps have been analyzed, corresponding to almost fully pulled anchor. the individual approaches were tested on linux workstation (running ubuntu 14.04 os) with the two cpu intel(r) xeon(r) cpu e52630 v3 @ 2.40ghz and 126gb ram. each of the two cpu units consists of eight physical and sixteen logical cores, allowing up to thirty-two threads to run simultaneously on one workstation. all the tests fit into a system memory. figure 5. anchor pullout test. the evaluated nonlinear solution strategies include newton-raphson method with stiffness matrix update after every iteration, modified newton-raphson method with stiffness matrix updated after every two and ten iterations, and initial stiffness method. the total number of iterations in our testing benchmark problem for all considered strategies (newtonraphson method, modified newton-raphson method with all cases, initial stiffness method) is presented in tab.1. it is clear that the number of iteration which is needed to successfully solve our benchmark problem is higher for initial stiffness matrix method than the newton-raphson method. the number of iterations in one loading step is in the range from 2 (newton-raphson method) to 30 (initial stiffness method) iterations to successfully solve this loading step as a part of the solution process. solution method num. of iteration matrix update n-r 204 241 mod. n-r s. 2 251 131 mod. n-r s. 10 297 37 initial stiffness 6049 1 table 1. number of iterations for different methods. to illustrate the performance of direct superlu dist solver and iterative solver petsc the execution times and speedups are presented. in case of iterative conjugated gradient solver from petsc, the convergence criteria based on relative solution error equal to 10−6 has been used. the performance has been evaluated for case without any preconditioning, as well as for the case when block jacobi preconditioning has been used. the obtained solution times (averaged over five consecutive runs) and corresponding speedups (relative to 2 cpu) are presented in fig. 6, fig. 7, fig. 8, fig. 9, fig. 10 and fig 11. the achieved results show that the effect of the parallelization using both solvers is quite substantial. in case of full newton-raphson method where the stiffness is updated after every iteration, the performance of direct solver is lower than performance of iterative solvers. this is because of dense character of stiffness matrix, from which iterative solver can profit. also direct solver could not profit from existing factorization, as the system matrix is updated after every iteration. the performance of preconditioned iterative solver is better compared to performance of non-preconditioned solver, see fig. 6 for reference. 9 michal bošanský, bořek patzák acta polytechnica ctu proceedings 100 1000 10000 100000 2 4 8 16 32 e xe cu ti o n t im e [ s] number of threads intel xeon (32 computing units) headedstud nsteps 15 superlu dist petsc petsc bjacobi figure 6. execution times using the newtonraphson method with secant stiffness updated after every iteration. the similar trend can be observed in case of modified newton-raphson method. again the performance of iterative solver is superior to performance of direct solver, see fig. 7, comparing solution times for both solver and stiffness update every 2 or 10 iterations. 100 1000 10000 100000 2 4 8 16 32 e xe cu ti o n t im e [ s] number of threads intel xeon (32 computing units) headedstud nsteps 15 superlu dist update every 2 steps superlu dist update every 10 steps petsc update every 2 steps petsc update every 10 steps petsc update every 2 steps bjacobi petsc update every 10 steps bjacobi figure 7. execution times using the modified newton-raphson method with secant stiffness updated after every 2nd and 10th iteration. finally, the achieved results of using initial stiffness matrix method with using direct solver and iterative solver are presented. in this case the best performance has been obtained using direct solver, which is in this case extremely profits from existing factorization. 100 1000 10000 100000 2 4 8 16 32 e xe cu ti o n t im e [ s] number of threads intel xeon (32 computing units) headedstud nsteps 15 superlu dist petsc petsc bjacobi figure 8. execution times using the initial stiffness matrix method. it is difficult to generalize obtained results, as the relative performance of direct and iterative solvers is highly depend on particular problem. however it is possible to evaluate the scalability of both solvers. the results for different settings are presented in figs. 9, 10 and 11. although far from ideal scalability, the results show reasonable linear scalability up to 16 cores. the significant decrease of performance for 32 threads, observed in all cases, can be attributed to hyperthreading technology of intel processors, assembling to sharing some cpu resources between hyper-threaded cores, which takes place only for 32 threads (note that workstation had 16 physical hyper-threaded cores) 1 1.5 2 2.5 3 3.5 4 4.5 5 2 4 8 16 32 sp e e d u p number of threads intel xeon (32 computing units) headedstud nsteps 15 superlu dist petsc petsc bjacobi figure 9. speedups using the newton-raphson method with secant stiffness updated every iteration. 10 vol. 15/2018 different linear equation solvers for solving nonlinear fe problems 1 1.5 2 2.5 3 3.5 4 4.5 5 2 4 8 16 32 sp e e d u p number of threads intel xeon (32 computing units) headedstud nsteps 15 superlu dist update every 2 steps superlu dist update every 10 steps petsc update every 2 steps petsc update every 10 steps petsc update every 2 steps bjacobi petsc update every 10 steps bjacobi figure 10. speedups using the modified newtonraphson method with secant stiffness updated after every 2nd and 10th iteration. 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 2 4 8 16 32 sp e e d u p number of threads intel xeon (32 computing units) headedstud nsteps 15 superlu dist petsc petsc bjacobi figure 11. speedups with using the initial stiffness matrix method. 4. conclusions the paper evaluates the performance of a direct superlu and petsc iterative solvers based on distributed system memory model in object-oriented, fem framework. the parallelization strategy is based on message passing programming model. the performance of direct and iterative solvers are compared on complex nonlinear benchmark problem of 3d simulation of an anchor pullout test. for the particular benchmark case considered, the performance of iterative solvers has been superior, except the case of initial stiffness method, where direct solver had better performance. despite particular problem considered, more general can depend or can be made regarding the scalability of both solvers. however, we can make general conclusion regarding scalability of both solvers. both solvers show relatively good scalability. as the relative performance of individual solvers anyway for different use case and problems, it is definitely an advantage of having both solver types implemented in any finite element code. acknowledgements this work was supported by the grant agency of the czech technical university in prague, grant no. sgs16/038/ohk1/1t/11 advanced algorithms for numerical modelling in mechanics of structures and materials. references [1] c. hufhes, t. hughes. parallel and distributed programming using c++. pearson education, 2003. [2] b. patzak. parallel computations in structural mechanics. ceske vysoke uceni technicke v praze, 2010. [3] c. fellipa. nonlinear finite element methods. https://www.colorado.edu/, 2017. [4] a. ibrahimbegovic. nonlinear solid mechanics. springer dordrecht heidelberg london new york, 2009. [5] x. s. li, j. w. demmel, j. r. gilbert, et al. superlu users‘ guide program interface. http://crd.lbl.gov/ xiaoye/superlu/, september 1999. [6] p. s. pacheco. a user‘s guide to mpi. univ. of san francisco, 1997. [7] s. balay, s. abhyankar, m. f. adams, et al. petsc web page. http://www.mcs.anl.gov/petsc, 2018. [8] b. patzak. oofem. http://www.oofem.org, 2000. 11 http://www.mcs.anl.gov/petsc acta polytechnica ctu proceedings 15:6–11, 2018 1 introduction 2 implementation 3 results 4 conclusions acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2018.15.0126 acta polytechnica ctu proceedings 15:126–130, 2018 © czech technical university in prague, 2018 available online at http://ojs.cvut.cz/ojs/index.php/app assessment of adhesion between mineral aggregate and bituminous binder using digital image analysis jan trejbala,∗, zdeněk prošeka,b, tereza valentováa a czech technical university in prague, faculty of civil engineering, thákurova 7, 166 29 prague 6, czech republic b university centre for energy efficient buildings of technical university in prague, třinecká 1024, 273 43 buštěhrad, czech republic ∗ corresponding author: jan.trejbal@fsv.cvut.cz abstract. this work presents how to assess a rate of adhesion between bituminous binder and mineral aggregate. asphalt mixtures composed from grade bitumen 50/70, reference or modified with adhesion promoters based on amines, and aggregate (brant, zbraslav, skuteč – 8-16mm) were made and then photographed. three adhesion assessment approaches were applied: (i) standardized adhesion visual assessment, (ii) gray level thresholding, and (iii) entropy-based image segmentation, both evaluated from digital images. it was shown that adhesion between both brant and skuteč and reference binder, expressed as a rate of binder-coated area onto aggregate particles, was equal to ca. 50-70%, while mixture composed from zbraslav exhibited ca. 70-80%. if adhesion promoters were used, these areas increased in all three cases up to 80-90%. it was shown that results obtained using visual and entropy segmentation analysis were very similar, while these differed in comparison with gray-level thresholding. keywords: asphalt mixture, bituminous binder, adhesion promoters, interphase interaction. 1. introduction asphalt mixtures are considered to be one of the most important material in many industries, especially in road structure engineering. after all, over 90% of roads in europe are constructed from asphalt mixtures, as summarized by s. liu et al. [1]. it is therefore obvious that whatever damages of such roads cause many economical, traffic-safety, and traffic-comfort negative consequences. one of the most common cause of pavement structure failures is so-called “stripping”, i.e., lost of adhesion between bituminous binder film and mineral aggregate particles in dry conditions or in presence of water [2]. this phenomenon is also known as “asphalt moisture damage”. once the adhesion between the two materials fails, water migrates between them and thus disallows their direct contact, as illustrated in figure 1. this is associated with reducing of pavement strength, resulting in damages known as potholes and fatigue cracking. therefore, water is considered to be one of the most enemy of asphalt mixtures. in asphalt mixtures, two types of water sources are known [3]: • internal: water is left in asphalt mixture with inadequately dried aggregate or with hot bitumen at the form of steam. • external: water entering pavements made from asphalt mixtures because of poor drainage or highly wet subgrade. in order to avoid the stripping, it is necessary to dry conditions wet conditions aggregate aggregate bitumen film bitumen film water adhesion lost figure 1. adhesion lost between binder and aggregate in presence of water. ensure a strong interphase interaction between the two materials and thus to prevent to water penetration between them. unfortunately, the adhesion often fails due to physical properties of aggregate (surface smooth texture), occurrence of impurities (clay, dust), or unfavorable bitumen and aggregate chemistry. the last mentioned item seems to be the most problematic issue; binder exhibits weak chemical affinity for aggregate, whereas aggregate is typical for its strong affinity for water, especially if contains high amount of acidic type mineral rocks (high silica contents – quartz-based granites). binder can be therefore replaced by water easily [3]. to avoid the phenomenon described above, the adhesion between the two materials can be enhanced using adhesion promoters – synthetic compounds or those from natural resources, mixed in trace amount to binder. their role is to alter interface properties of both binder and aggregate. their function is based on disruption of intermolecular interactions between 126 http://dx.doi.org/10.14311/app.2018.15.0126 http://ojs.cvut.cz/ojs/index.php/app vol. 15/2018 adhesion assessment between mineral aggregate and bituminous binder water and aggregate surface. consequently, contact energy between binder and aggregate is thus lower [4]. with using of adhesion promoters, an essential question arises: how to quantify their function. across both the technical and laboratory praxis, there are commonly used methods assessing adhesion between binder and aggregate that can be, in general, divided into following two groups: • mechanical testing: asphalt mixtures are made in dry conditions and then their mechanical properties are examined (modulus of elasticity, tensile and pressure strength, adhesion peeling tests, etc.). as the next step, those are exposed to water and then their mechanical properties are tested again. such obtained results are mutually compared [5, 6]. • laboratory interphase interaction testing: interaction between the two materials is tested in laboratory conditions, which cannot reflect many variable parameters like aggregate shape, surface, and so on (contact angle measurement, wilhelmy method, etc.) [7–11]. • stripping rate observation: asphalt mixtures are made similarly to procedure described in the previous item but a rate of stripping is then assessed visually, ie., a rate between binder-coated and stripped areas on aggregate particles is observed (rolling bottle test, detachment in boiling water, etc.) [12, 13]. stripping rate observation-based methods provide the most indicating results in terms of applicability and reliability. they are therefore most frequented across technical praxis, as summarized by grönninger et al. [9]. however, an evaluation of these methods is based on subjective assessment of at least two evaluators, who estimate a degree of bitumen-coated area on aggregate visually. to overcome their subjective and so unconscious conclusions, some researchers have developed semi-automatic techniques. källén et al. [14] and lamperti et al. [15] employed methods based on gray level thresholding (glt), assessing yuv or rgb color space. although these techniques seems to be promising, their usage is limited due to bitumen glossy surfaces. specularly reflected light from these surfaces causes over exposed areas in captured images. these areas merge in with light aggregates, so any gltbased software is not able to recognize them correctly as binder-coated. in order to overcome the issues connected with dark aggregate and over-exposed areas within images, software based on entropy image segmentation was employed. the software is able to assess the adhesion between the two materials even from under-exposed images, free from local over-expositions, but still containing detail drawings in dark parts, if captured properly. these images were used for digital image based analysis: gray level thresholding and entropybased image segmentation. moreover, to compare such obtained results, standardized adhesion visual assessment was also executed. 2. asphalt mixtures 2.1. aggregate three types of mineral aggregate, crushed to the fraction of 8-16mm, were used with respect to the availability of natural aggregate sources in the czech republic. these three representatives are considered to be the most typical aggregate being used for purposes of asphalt mixture production in the czech republic. moreover, their both volume and surface properties are mutually different; their grain is from very-fine to medium coarse and their surface is from sintered to porous. these differences are suitable for adhesion tests because they cover wide spectrum of all possible aggregate properties. their basic characteristics are summarized in table 1. 2.2. bitumen paving grade bitumen 50/70 was used, standardized by [16]. two alternatives of this bitumen were made – reference with no admixtures and modified containing 0.3 wt.% adhesion promoters based on chemical surfactants comprising amines. 2.3. mixture preparation and imaging following technical standard [13], 12.0±0.3 g of binder was heated to 170±5 °c and then mixed together with 300±3 g of aggregate preheated to 160±5 °c. after prescribed 24 hours lasting technological pause, such prepared asphalt mixtures were poured with 60±3 °c stripping water for 60minutes. subsequently, these mixtures were subjected to visual assessment according to above cited standard. then, they were photographed using dslr camera canon eos 70d equipped with large-area sensor (aps-c 22.2×14.8mm), canon zoom lens ef-s 17-85 is usm and polarizing circular filter hoya hd. these pictures were used for semi-automatic adhesion assessment. summarization of all mixtures is for clarity mentioned in table 2. 3. adhesion assessment 3.1. visual analysis the visual analysis, executed according to [13], is based on subjective assessment of at least two evaluators, who determine percentage of binder-coated area on aggregate within asphalt mixture (a reside is considered to be stripped areas). their findings are compared to model cases figured in the cited standard. there are estimated 7 such cases (from a to g and from 100% to less than 20% of binder-coated areas) of stripping. it has to be stressed out that their findings can be inaccurate. 3.2. grey level thresholding this assessment is based on recognition of brightness and shadows on digital image of asphalt mixtures. the areas being brighter than the dark bituminous binder are considered to be parts of stripped aggregate 127 j. trejbal, z. prošek, t. valentová acta polytechnica ctu proceedings quarry fraction rock grain surface brant 8-16 mm granodiorite, paleo-porphyrite fine porous zbraslav proterozoic spiliness, metaphrash very fine porous skuteč amphibolic-biotic granodiorite medium-coarse sintered table 1. basic properties of aggregate used. mixture aggregate adhesion promoters [wt. % to binder] br brant 0 bm 0.3 zr zbraslav 0 zm 0.3 sr skuteč 0 sm 0.3 table 2. asphalt mixtures summarization. 0 20 40 60 80 100 su rf ac e of a gg re ga te c oa te d by b in de r [% ] mixture a b c d e g f visual assessment grey level thresholding entropy-based image segmentation br srzr c la ss ifi ca tio n ac co rd in g to c sn e n 7 3 61 61 70 .0 57 .5 73 .9 80 .0 72 .0 82 .9 49 .7 73 .5 70 .0 figure 2. assessment of adhesion between reference bituminous binder and three types of aggregates. 0 20 40 60 80 100 su rf ac e of a gg re ga te c oa te d by b in de r [% ] mixture a b c d e g f visual assessment c la ss ifi ca tio n ac co rd in g to c sn e n 7 3 61 61 bm zm sm 90 .0 78 .0 93 .7 90 .0 78 .6 93 .8 90 .0 85 .4 97 .5 grey level thresholding entropy-based image segmentation figure 3. assessment of adhesion between modified bituminous binder and three types of aggregates. 128 vol. 15/2018 adhesion assessment between mineral aggregate and bituminous binder particles. as indicated in introduction (chapter 1), such an approach can be misleading; if aggregate particles are rather dark, the software can define them as binder-coated. on the other side, if bitumen surfaces reflect light specularly, their captured picture is relatively bright so the areas of aggregate assessed by the software are then considered to be stripped. 3.3. entropy-based image segmentation similarly to gray level thresholding, this method is also based on the digital image analysis. its main principle rests on local entropy calculation in order to assess roughness of the texture. although this method is computationally more demanding than that based on brightness/shadows recognition, it is still feasible, and primarily, it produces much more accurate outcomes independent on issues relating with light reflection from binder surfaces. consequently, an input image can be slightly under-exposed in order to eliminate over-exposed areas but only to such an extent to dark areas to be readable for the digital camera. the software is available in [17]. 4. results and discussion 4.1. brant it is clear from figure 2 and figure 3 that the adhesion between reference binder and brant aggregate, classified according to [13], was marked as “d” – coated area up to 70% (stripped edges, stripped small spots on other areas), while that was increased up to 90%, classification “b” (aggregate particles coated slightly, edges, corners, and small spots may be stripped) if adhesion promoters were used. when assessed by gray level thresholding based software, the examined coated area reached almost to 60% and 80% in the case of using reference and modified binder, respectively. while when the adhesion was assessed using entropy-based image segmentation, the area was over than 70%, respectively 90%. 4.2. zbraslav the adhesion between binder and zbraslav aggregate was, in accordance with [13], classified as “c” (80% – stripped edges and corners, residue well coated) and “b”, when reference and modified binder was used, respectively. in the case of glt analysis, the area was between 72-79%, regardless to binder type. on the other side, when it was assessed using entropybased image segmentation, the area slightly exceeded 70%, resp. 80%. the results are summarized in figure 2 and figure 3. 4.3. skuteč the total of binder-coated areas on skuteč aggregate, consistent with methodology referred to in [13], was equal to “d” and “b” in the case of reference and modified binder used, respectively. when glt analysis was employed, that was approximately 50%, resp. 85%, while results from entropy-based image segmentation were equal to almost 75%, resp. fewer 100%. these results are also summarized in figure 2 and figure 3. moreover, for clarity, captured images of asphalt mixtures made from skuteč aggregate and reference binder and their preview of consequent assessment using the both semi-automatic methods described earlier are shown in figure 4. 4.4. summarization skuteč and brant proved to be insufficient aggregate in terms of adhesion with reference binder. this was probably caused by their rock-origin; these aggregates, composed namely of acidic rocks, are typical for their fissile surface with high surface energy. on the other side, zbraslav exhibited high affinity for bituminous binder. if adhesion promoters were used, all three aggregates were coated by binder significantly better (in some cases by more than 20%). after results comparison, it was shown that conventional visual analysis provided very similar results to entropy-based image segmentation; their results differed on average by 2.1% only. on the other side, grey level thresholding analysis showed to be inaccurate, if compared to other two methods. such obtained results differed by up to 15%. this was attributed to issues connected with difficulty during asphalt mixture imaging. when the light turned out on the shiny surface of binder, it is then reflected back and causes bright areas in an image. these were next assessed as stripped areas by the software. high occurrence of such areas is captured in figure 4. 5. conclusions three types of aggregate at the fraction of 8-16 mm – brant, skuteč, and zbraslav – were covered by reference and by adhesion promoters modified paving grade bitumen 50/70 for production of asphalt mixture samples to be tested and assessed in terms of adhesion between the two materials. the rate of binder coated areas onto aggregate particles was assessed using three different approaches, concretely by: (i) conventional visual analysis executed according to standardized procedure, (ii) grey level thresholding semi-automatic analysis evaluating a ratio between shadows and brightness in digital image of the mixture, and (iii) entropy-based image segmentation evaluating roughness of image texture. following results were found: • skuteč and brant aggregate showed to be unsatisfactory in intarphase interaction with reference binder. binder-coated area was approximately 50-70% in both cases, depending on the method employed. if adhesion promoters were used, the adhesion increased by ca. 20%. • zbraslav exhibited high affinity for reference binder. the coated area was equal to ca. 80% and 90% in case od reference and modified binder, respectively. 129 j. trejbal, z. prošek, t. valentová acta polytechnica ctu proceedings figure 4. images of aggregate skuteč covered by reference binder, from left: original photo, gray level thresholding-, and entropy-based image segmentation. • grey level thresholding was intended as the least accurate, if compared to other two methods. this method was not able to recognize bright bitumen areas in captured image, caused by light reflection, as binder-coated. acknowledgements this work was supported by the czech science foundation (research project no. 18-13830s) and by the czech technical university (no. sgs16/201/ohk1/3t/11) and the support is gratefully acknowledged. special thanks to tereza horová for research assistance and to dr. václav nežerka for the software development. references [1] s. liu, a. shukla, t. nandra. technological, environmental and economic aspects of asphalt recycling for road construction. renewable and sustainable energy reviews 75:879–893, 2017. doi:10.1016/j.rser.2016.10.080. [2] b. amit. development of methods to quantify bitumen-aggregate adhesion and loss of adhesion due to water – phd thesis. texas a&m university, 2006. [3] n. ahmad. asphalt mixture moisture sensitivity evaluation using surface energy parameters – phd thesis. the university of nottingham, 2011. [4] c. rossi, b. teltayev, r. angelico. adhesion promoters in bituminous road materials: a review. applied science 7(524), 2017. doi:10.3390/app7050524. [5] s. cui, b. blackman, a. kinloch, a. taylor. durability of asphalt mixtures: effect of aggregate type and adhesion promoters. international journal of adhesion and adhesives 54:100–111, 2014. doi:10.1016/j.ijadhadh.2014.05.009. [6] t. valentová, j. altman, j. valentin. impact of asphalt ageing on the activity of adhesion promoters and the moisture susceptibility. transportation research procedia 14:768–777, 2016. doi:10.1016/j.trpro.2016.05.066. [7] c. rossi, p. caputo, n. baldino, et al. effects of adhesion promoters on the contact angle of bitumenaggregate interface. transportation research procedia 14:768–777, 2016. doi:10.1016/j.ijadhadh.2016.07.013. [8] c. rossi, p. caputo, n. baldino, et al. quantitative evaluation of organosilane-based adhesion promoter effect on bitumen-aggregate bond by contact angle test. international journal of adhesion and adhesives 72:117–122, 2017. doi:10.1016/j.ijadhadh.2016.10.015. [9] j. grönninger, m. wistuba, p. renken. adhesion in bi tumen-aggregate-systems. road materials and pavement design 11(4):881–898, 2010. doi:10.1080/14680629.2010.9690311. [10] a. khan, p. redelius, n. kringos. evaluation of adhesive properties of mineral-bitumen interfaces in cold asphalt mixtures. construction and building materials 125:1005–1021, 2016. doi:10.1016/j.conbuildmat.2016.08.155. [11] j. yi, x. pang, d. feng, et al. studies on surface energy of asphalt and aggregate at different scales and bonding property of asphalt-aggregate system. road materials and pavement design 19(5):1102–1125, 2016. doi:10.1080/14680629.2017.1300597. [12] čsn en 12697-11: bituminous mixtures – test methods for hot mix asphalt part 11: determination of the affinity between aggregate and bitumen, praha: czech office for standards, metrology and testing, 2006. [13] čsn en 76 6161: determination for adhesion of asphaltic binders to aggregate, praha: czech office for standards, metrology and testing, 2000. [14] h. källén, a. heyden, k. åström, p. lindh. measuring and evaluating bitumen coverage of stones using two different digital image analysis method. measurement 84:56–67, 2016. doi:10.1016/j.measurement.2016.02.007. [15] r. lamperti, c. lantieri, c. sangiorgi, et al. semiautomatic evaluation of the degree of bitumen coverage on bitumen-coated aggregates. in conference proceeding of 8th rilem international symposium. rilem bookseries, 2016. doi:10.1007/978-94-017-7342-3_2. [16] čsn en 12591: bitumen and bituminous binders – specifications for paving grade bitumens, praha: czech office for standards, metrology and testing, 2009. [17] v. nežerka. software python phase analyzer based on image segmentation [online], available online: http://mech.fsv.cvut.cz/nezerka/software.html. 130 http://dx.doi.org/10.1016/j.rser.2016.10.080 http://dx.doi.org/10.3390/app7050524 http://dx.doi.org/10.1016/j.ijadhadh.2014.05.009 http://dx.doi.org/10.1016/j.trpro.2016.05.066 http://dx.doi.org/10.1016/j.ijadhadh.2016.07.013 http://dx.doi.org/10.1016/j.ijadhadh.2016.10.015 http://dx.doi.org/10.1080/14680629.2010.9690311 http://dx.doi.org/10.1016/j.conbuildmat.2016.08.155 http://dx.doi.org/10.1080/14680629.2017.1300597 http://dx.doi.org/10.1016/j.measurement.2016.02.007 http://dx.doi.org/10.1007/978-94-017-7342-3_2 acta polytechnica ctu proceedings 15:126–130, 2018 1 introduction 2 asphalt mixtures 2.1 aggregate 2.2 bitumen 2.3 mixture preparation and imaging 3 adhesion assessment 3.1 visual analysis 3.2 grey level thresholding 3.3 entropy-based image segmentation 4 results and discussion 4.1 brant 4.2 zbraslav 4.3 skutec 4.4 summarization 5 conclusions acknowledgements references https://doi.org/10.14311/app.2022.33.0015 acta polytechnica ctu proceedings 33:15–19, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague effect of tio2 and zno powder mixtures on mechanical and photocatalytic performance of high performance concrete fouad amora,∗, lenka ingrišováa, zuzana rácováa, michal baudysb, petr hájeka a czech technical university in prague, faculty of civil engineering, department of building structures, thákurova 2077/7, 166 29 prague 6, czech republic b university of chemistry and technology prague, department of inorganic technology, technická 5, 166 28 prague 6, czech republic ∗ corresponding author: fouad.amor@fsv.cvut.cz abstract. the development of new modified cement-based materials is increasingly becoming a necessity for improving the durability and surface performance of building materials. titanium dioxide (tio2) photocatalyst has been widely used in building materials science due to its ability to break down pollutants. zinc oxide (zno) is often considered a substituent for tio2 because of its photocatalytic and photoluminescent properties. a new inorganic nanocomposite photocatalyst, based on titanium and zinc oxides, is introduced in this work in order to study its compatibility with high performance concrete (hpc). this research aims to study the mechanical and photocatalytic behavior of mixtures based on nanoparticles in hpc. the study of the efficiency in the nitrogen oxides (nox) degradation of modified hpc in tio2 and zno with different percentages is studied. the studies have shown that the introduction of titanium dioxide in hpc presents a significant efficiency for the nox degradation and a positive effect on the mechanical properties than zinc oxide, and thus represents potential contribution to sustainability of concrete structures. keywords: high performance concrete, nanoparticles, photocatalysis, titanium dioxide, zinc oxide. 1. introduction atmospheric pollution is an alteration in the quality of the air, which is due to one or more gaseous substances or particle materials present in concentrations and for a period of time sufficient to create a toxic or eco-toxic effect. the presence of these toxic gases in the atmosphere is due to industrial activity and road transport [1]. in this study, we are focused on emissions and reduction of nox gases. the nitrogen oxides family, grouped under the formula nox, mainly includes nitrogen monoxide (no) and nitrogen dioxide (no2). they can contribute to various environmental problems and have health consequences [2]. there are several methods to reduce nox, in particular photochemical oxidation by the use of photocatalysts [3]. among the many nanoparticles studied over the last ten years, titanium oxide (tio2) and zinc oxide (zno) have aroused particular enthusiasm because of their many properties. their photocatalytic properties have allowed them to be used for self-cleaning, self-disinfection and environmental clean-up applications [4–6]. tio2 is currently the most efficient catalyst in this field thanks to its strong photocatalytic activity under uv irradiation. zno has a bandgap similar to that of tio2, making it increasingly recognized as a suitable alternative to tio2. zno is the subject of intensive research because of its remarkable properties which promote the development of photoinduced phenomena [7, 8]. the interest of this study is to mix these two oxides to form a nanocomposite which can be used to improve the photocatalytic activity of concrete. the photocatalytic activity of the tio2-zno system is increasingly being studied with the aim of finding a more efficient material for photocatalytic applications [9, 10]. s. mayén-hernández et al. [11] studied the photocatalytic activity of mixed oxides of tio2 and zno with different ti/zn ratios, they concluded that the ti/zn ratio of 0.5 exhibited the best photocatalytic performance. furthermore, the incorporation of mixed oxides into the cement matrix stabilizes the distribution of hydration products (c-s-h) and improves mechanical surface properties [12–14]. the objective of this work aims to evaluate the effect of the inclusion of mixed oxides based on zn and ti on the hydration of high performance concrete by following the mechanical evolution and the photocatalytic performances as a function of time. 2. materials and methods 2.1. materials the hpc has been developed and optimized in recent years at ctu prague [15] and it is the self15 https://doi.org/10.14311/app.2022.33.0015 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en f. amor, l. ingrišová, z. rácová et al. acta polytechnica ctu proceedings oxides sio2 al2o3 fe2o3 cao mgo so3 k2o tio2 p2o5 % 18.7 4.5 3.4 65.9 1.3 4.3 0.8 0.3 0.1 table 1. chemical analysis of cem i 42.5 r (% by weight). binder samples technical sand % quartz flour % silica fume % superplasticizer % water % cement (c) % tio2 % zno (z) % r 41 13.92 7.47 1.25 7.31 29.05 0 0 ct1 41 13.92 7.47 1.25 7.31 28.05 1 0 ct2 41 13.92 7.47 1.25 7.31 27.05 2 0 ct3 41 13.92 7.47 1.25 7.31 26.05 3 0 cz0.3 41 13.92 7.47 1.25 7.31 28.71 0 0.34 cz0.6 41 13.92 7.47 1.25 7.31 28.38 0 0.67 cz1 41 13.92 7.47 1.25 7.31 28.05 0 1 ct1z0.6 41 13.92 7.47 1.25 7.31 27.38 1 0.67 ct2z0.3 41 13.92 7.47 1.25 7.31 26.71 2 0.34 table 2. formulation of hpc samples. consolidating fine grain concrete without fibres. the components of the raw material used are: cement i 42.5 r, silica fume, technical sands with maximum grain size of 1.2 mm, silica flour, water and superplasticizer. these materials come mainly from local sources. table 1 illustrates the chemical composition of cem i 42.5 r cement used for the hydration of hpc with the addition of the mixed oxides tio2 and zno in proportions varying from 1 to 3% by weight. titanium dioxide (tio2) and zinc oxide (zno) used in this research were produced by the czech manufacture precheza. tio2 powder consists of 99 wt% of pure anatase and also some traces of sulphates (0.7 wt%). the specific surface area is about 70-100 m2/g. 2.2. formulation and experimental methods for the development of this study, 9 series of hpc mixtures were produced, each group consisting of 4 prisms. the concretes were mixed with different percentages of the tio2 and zno nanoparticles according to table 2. after 28 days of hardening in water, the samples were dried for 24 hours at the room temperature. the flexural and compressive strength tests were carried out accordance with čsn en 12390-3 on prismatic specimens 40×40×160 mm in accordance with the instructions of standard čsn en 12390-1 [16]. after 28 days of curing under water, each specimen is first subjected to a 3-point flexion test. then, the two half-test pieces are tested in compression until they break. the compressive strength of a formulation is the arithmetic average of the four individual results obtained from four determinations made on a set of two prisms. if any of the four results vary by more than ±10 % from the average, that result is discarded and the arithmetic mean is calculated from the remaining three results. if any of the three results vary by more than ±10 % from their average, all of the results are rejected. the tests were carried out using a mechanical press of the brio haanice ® type with a capacity of 400 kn. nanoparticles of zno and tio2 were suspended in distilled water (2.5 g/dm3) and after 15 minutes of ultrasonic pretreatment were prepared drop casted layers on the soda lime glass (dimension 10×5 cm) containing 0.5 mg/cm2 tio2 or zno respectively. photocatalytic activity was determined using standard iso 22197-1 methodology (total flowrate 3dm3/min, initial concentration of no 1 ppm, relative humidity 50%, uv intensity 1mw/cm2). the photocatalytic performances of tio2 and zno nanoparticles were studied by nox degradation under uv irradiation for 2 hours (fig. 1). tio2 showed a significant nox degadation capacity which has reached 26%. on the other hand, zno was less reactive and had a value of 4%. photocatalytic activity of concrete samples was determined in stirred flow through reactor. due to the expected lower photocatalytic activity, total flowrate was decreased to 1dm3/min, initial concentration of no was 1 ppm, relative humidy 50%, intensity of uv light 1 mw/cm2. 3. results and discussion 3.1. compressive and flexural strength figures 2 and 3 showed the evolution of the addition of mineral powders on the compressive and flexural strength of hpc mixtures. the values of the compressive strength of concrete mixture ref, ct1, ct2, 16 vol. 33/2022 tio2 and zno in a high performance concrete figure 1. nox abatements for tio2 and zno nanoparticles under uv irradiation. figure 2. compressive strength of hpc samples. ct3, cz0.3, cz0.6, cz1, ct10.6 and ct2z0.3 are respectively 124, 129.2, 116.5, 102.1, 103.8, 88.4, 84, 87.6 and 120.2 mpa. the addition of a small amount of tio2 (1%) in the concrete increased its compressive strength compared to the reference concrete. on the other hand, from 2% of tio2 the strength slightly decreased. the flexural strength value of unmodified concrete (ref) was 14.5 mpa. as in the compressive strength measurements, the addition of 1 and 2 wt% tio2 increased the flexural strength and decreased slightly for the ct3 to reach a value nearly equivalent to the ref sample of 14.4 mpa. the highest values of the flexural strength were obtained for the samples with 1 and 2% by weight of tio2 of the value of 16.2 mpa. the lower value of flexural strength was observed in the sample with 1% by weight of zno. in general, the substitution of cement by nanoparticles led to a slight decrease in mechanical strengths. figure 3. flexural strength of hpc samples. unlike tio2, the mechanical properties of concrete strongly depend on the amount of this nanoparticle used. according to wang et al. [17], with increasing the dosage of tio2 nanoparticles in cement mortars, the strength initially increased rapidly compared to the reference mortar until the dosage reached 2% by weight of tio2, then there is a slowdown in the percentage of that increase. the incorporation of nanoparticles into the cement matrix led to increases and slight decreases in compressive and flexural strengths except for samples containing only zinc oxide which showed a significant decrease. the retarding effect of zinc on the hydration of cement, has a direct effect on the mechanical strength [18–20]. these results are not surprising since the added nanoparticles do not have the same performance as portland cem i cement. 17 f. amor, l. ingrišová, z. rácová et al. acta polytechnica ctu proceedings figure 4. nox removal under uv irradiation over ct1, ct3, cz1 and ct1z0.6. 3.2. nox decomposition the photocatalytic activity of hpc samples containing tio2 and zno was evaluated under uv irradiation by photodegradation of nox (figure 4). the maximum no, nox removal obtained was 8.8 and 6.7%, respectively, for ct3 sample. ct1z0.6 sample removed an average of 0.1% of nox and 1% of no, however, cz1 showed no photocatalytic reaction. on the other hand, the produced amount of toxic intermediate no2 is very low, which is crucial in environmental application [21]. it is clear that concretes incorporating tio2 have been shown to be superior to the photodegradation of nox compared to those containing zno. b. bica et al. [22] showed that with the same percentage of tio2 and zno introduced in the concrete, the concretes incorporating the tio2 presented a more degradation of the nox for various test conditions and even with low percentages. these results are not surprising since the zno powder used in this work was not very reactive to photodegradation. 4. conclusions different mixtures of hpc with the nanoparticles in substitution with cement have been prepared and their mechanical properties and capacities to degrade nitrogen oxides have been studied. the introduction of nanoparticles into the cement matrix resulted in increases and slight decreases in compressive and flexural strengths, except for samples containing only zinc oxide which showed a remarkable decrease. the addition of tio2 up to 2% by weight in the hpc did not decrease the mechanical properties but even increased the compressive and flexural strength. all samples showed photocatalytic degradation under uv irradiation, except for concrete containing 1% zno, achieving a higher nox removal rate with a higher tio2 dosage in concrete. the incorporation of zinc oxide alone into the cementitious matrix deteriorates the properties of the concrete. on the other hand, titanium oxide alone or mixed with zinc oxide can be used as additions to hpc by substitution of cement to produce a material with photocatalytic and durable properties. acknowledgements the authors would like to thank the program "mobility faculty of civil engineering", projects "mobility czech technical university in prague" for the financial support given in order to support this original research. references [1] h. t. t. nguyen. emissions polluantes des nox: mecanisme de formation et de reduction (université du littoral), 2013. [2] s. d. beevers, e. westmoreland, m. c. de jong, et al. trends in nox and no2 emissions from road traffic in great britain. atmospheric environment 54:107-16, 2012. https://doi.org/10.1016/j.atmosenv.2012.02.028. [3] m. perez-nicolas, i. navarro-blasco, j. fernández, et al. atmospheric nox removal: study of cement mortars with iron-and vanadium-doped tio2 as visible light-sensitive photocatalysts. construction and building materials 149:257-71, 2017. https: //doi.org/10.1016/j.conbuildmat.2017.05.132. [4] j. chen, c.-s. poon. photocatalytic construction and building materials: from fundamentals to applications. building and environment 44(9):1899-906, 2009. https://doi.org/10.1016/j.buildenv.2009.01.002. [5] f. pacheco-torgal, s. jalali. nanotechnology: advantages and drawbacks in the field of construction and building materials. construction and building materials 25(2):582-90, 2011. https: //doi.org/10.1016/j.conbuildmat.2010.07.009. [6] z. racova, m. baudys, j. krysa, et al. inactivation of aspergillus niger on paints containing zno proceedings 10th international conference on nanomaterials research & application, 2019. [7] m. nirmala, m. nair, k. rekha et al. photocatalytic activity of zno nanopowders synthesized by dc thermal plasma african j. basic appl. sci. 2 161-6, 2010. 18 https://doi.org/10.1016/j.atmosenv.2012.02.028 https://doi.org/10.1016/j.conbuildmat.2017.05.132 https://doi.org/10.1016/j.buildenv.2009.01.002 https://doi.org/10.1016/j.conbuildmat.2010.07.009 vol. 33/2022 tio2 and zno in a high performance concrete [8] t. welderfael, o. yadav, a. m. taddesse, et al. synthesis, characterization and photocatalytic activities of ag-n-codoped zno nanoparticles for degradation of methyl red. bulletin of the chemical society of ethiopia 27(2):221-32, 2013. https://doi.org/10.4314/bcse.v27i2.7. [9] cristina yeber m, rodríguez j, j. freer, et al. photocatalytic degradation of cellulose bleaching effluent by supported tio2 and zno chemosphere 41:1193-7, 2000. [10] deguchi t, imai k, matsui h, iwasaki m, et al. rapid electroplating of photocatalytically highly active tio2-zn nanocomposite films on steel. journal of material science 36:4723-9, 2001. [11] m. c. yeber, j. rodríguez, j. freer, et al. photocatalytic degradation of cellulose bleaching effluent by supported tio2 and zno. chemosphere 41(8):1193-7, 2000. https://doi.org/10.1016/s0045-6535(99)00551-2. [12] t. vulic, m. hadnadjev-kostic, o. rudic, et al. improvement of cement-based mortars by application of photocatalytic active ti-zn-al nanocomposites. cement and concrete composites 36:121-7, 2013. https: //doi.org/10.1016/j.cemconcomp.2012.07.005. [13] f. amor, h. ez-zaki, m. el alouani, et al. synthesis and photocatalytic performance of calcined zn-al-tilamellar double hydroxides for building material applications. journal of inorganic and organometallic polymers and materials 31(7):3137-53, 2021. https://doi.org/10.1007/s10904-021-01952-z. [14] f. amor, a. diouri, i. ellouzi, et al. development of zn-al-ti mixed oxides-modified cement phases for surface photocatalytic performance. case studies in construction materials 9, 2018. https://doi.org/10.1016/j.cscm.2018.e00209. [15] l. laiblová, j. pešta, a. kumar, et al. environmental impact of textile reinforced concrete facades compared to conventional solutions-lca case study. materials 12(19):3194, 2019. https://doi.org/10.3390/ma12193194. [16] anon méthodes d’essais des ciments partie 1: détermination des résistances mécaniques. nf en 196-1 avril, 2006. [17] l. wang, h. zhang, y. gao. effect of tio2 nanoparticles on physical and mechanical properties of cement at low temperatures. advances in materials science and engineering 2018:1-12, 2018. https://doi.org/10.1155/2018/8934689. [18] g. arliguie, j. grandet. influence de la composition d’un ciment portland sur son hydration en presence de zinc. cement and concrete research 20(4):517-24, 1990. https://doi.org/10.1016/0008-8846(90)90096-g. [19] m. yousuf, a. mollah, t. r. hess, et al. an ftir and xps investigations of the effects of carbonation on the solidification/stabilization of cement based systems-portland type v with zinc. cement and concrete research 23(4):773-84, 1993. https://doi.org/10.1016/0008-8846(93)90031-4. [20] f. amor, a. diouri, s. kamali-bernard. mortar’s design from shell powders associated with ldh nanocomposite ti-zn-type. journal of materials and environmental science 7:2739-47, 2016. [21] i. manisalidis, e. stavropoulou, a. stavropoulos, et al. environmental and health impacts of air pollution: a review. frontiers in public health 8, 2020. https://doi.org/10.3389/fpubh.2020.00014. [22] b. o. bica, j. v. s. de melo. concrete blocks nano-modified with zinc oxide (zno) for photocatalytic paving: performance comparison with titanium dioxide (tio2). construction and building materials 252, 2020. https: //doi.org/10.1016/j.conbuildmat.2020.119120. 19 https://doi.org/10.4314/bcse.v27i2.7 https://doi.org/10.1016/s0045-6535(99)00551-2 https://doi.org/10.1016/j.cemconcomp.2012.07.005 https://doi.org/10.1007/s10904-021-01952-z https://doi.org/10.1016/j.cscm.2018.e00209 https://doi.org/10.3390/ma12193194 https://doi.org/10.1155/2018/8934689 https://doi.org/10.1016/0008-8846(90)90096-g https://doi.org/10.1016/0008-8846(93)90031-4 https://doi.org/10.3389/fpubh.2020.00014 https://doi.org/10.1016/j.conbuildmat.2020.119120 acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.35.0014 acta polytechnica ctu proceedings 35:14–18, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague modern railways for the 21st century – a challenge not only for designers josef kolář czech technical university in prague, faculty of mechanical engineering, department of automotive, combustion engines and railway engineering, technická 4, 160 00 prague 6, czech republic correspondence: josef.kolar@fs.cvut.cz abstract. the paper outlines the basic goals for modern rail transport in the czech republic. this contribution points out some challenges and technical tasks that project engineers and operators of the rail transport in the czech republic will have to deal with if they want to succeed in the conditions of the liberalized european railway market after the year 2035. moreover, the article assesses the demands placed on safety of the railway transport and presents suggestions for reduction of the consequences of a rolling stock-car collisions in the regional transport. keywords: active safety, design vehicles, etcs, high speed railway, modern railway transport, passive safety, regional railways. 1. introduction in the 21th century, modern railways must not be just an interconnected and interoperable network of european high-speed and corridor lines, e.g. see figure 1. it must represent a highly ecological and safe transport system, which offers customers fast and reliable transport throughout europe by its services and quality. the transport system, which will be more timeand cost-effective and acceptable than competitive air or individual car transport (ict) for people and goods transportation to a distance of about 1000 km. modern rail transport in the czech republic has to meet the following basic goals: • to be a leader in the safe, attractive and barrier-free passenger transport, not only in the long-distance transport, but also in the regional and suburban transport, • with the quality services offered to passengers it has to offer a higher level of connectivity to the regional passenger transport to junctions (stations) and thus to contribute to a reduction of the part of the individual car transport in passenger transport for work or culture. the model of ecological, time-bound and affordable mobility needs to be strengthened, • it has to offer operationally safe, energy-efficient and low-emission (noise, co2, nox , dust particles) transport of goods and freight, which will be more competitive and more environmentally friendly than individual truck transport, • it must significantly increase its share on the international container transport and the european combined transport (road – railway), because this system is able to significantly decrease an accident rate of truck transport. trucks should be mainly used for the first and the last mile freight transport from the railway junction (modern transshipment point) to the customer and back, not for transport across the whole country or the central europe, • within the freight transport it has to ensure the ecological collection of municipal waste to selected large-capacity incinerators and sorting plants, because by this way it is possible to meet the objective of the ban on landfilling of municipal waste and its secondary use. by taking these steps with the high level of "traffic electrification" the modern railways, while maintaining a certain level of mobility, can make a significant contribution to meet the emission reduction targets defined in the eu program documents called as "white book of transport". 2. challenges not only for rolling stocks project engineers to meet these goals requires: • significant investments costs in the infrastructure (building hsr, modernization of the contemporary railways and a completion of electrification – transition to 25kv/50hz ac traction system, modernization of railway network junctions – transfer points from the regional railway to hsr, loading hubs for european combined transport. • extensive investments in vehicles (purchase of new rolling stocks, new high-speed electric multiple units for speed of 300 kph, cost-effective fleet modernization, etcs implementation, dual-powered vehicles for regional transport, new vehicles for combined transport, . . . ). • change of transport models and the passenger transport in order to achieve maximum and efficient 14 https://doi.org/10.14311/app.2022.35.0014 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 35/2022 modern railways for the 21st century figure 1. a trans-european rail lines ten-t [1]. utilization of new high-speed lines and vehicles. • new models of mobility in the area of the central europe and a thinking and behaviour change of the people and passengers in favour of safe and environmental-friendly transport. only the rapid fulfilment of these points and elaborated training of new railway experts and employees is able to prepare czech railway for a great competition in the conditions of the liberalized european railway market after 2035. achieving of low operating costs while meeting high level of operational safety and environmental requirements will be key factors, which will decide about the success or failure of our operators on this market. figure 2. high speed agv or ice 4 for passenger transport in czech republic? [1]. 2.1. high-speed railway transport versus corridor lines = new transport models the requirements for a high safety level of high-speed railway traffic on hsr are embodied in the technical specifications for interoperability (tsis) for the design of railway tracks and rolling stocks. tsi hs fulfilment and european train control system etcs implementation (level 2 and level 3) shows that the hsr system, with the appropriate operational load of high-speed units, significantly contributes to economic and environmental mobility and demonstrate competitiveness against air or road transport. the czech republic has to significantly accelerate building of hsr if it wants to offer an economic and fast connection between northern and southern europe to european operators after the year 2035. it would be inappropriate if the czech republic is bypassed and the international transit passes through the federal republic of germany and austria. the transfer of transport capacity in the international and the domestic long-distance passenger transport on high-speed railways with maximum axle load of 17 tonnes and speed limit up to 350 kph will require a change in transport models of regional services and transfer nodes. daily operation between approx. 5-23 hours can be realized by electric multiple units with distributed power, see figure 2. for night bed and lounger express trains with operation from about 9 pm to 7 am, it is more appropriate to use units with concentrated traction power in front and rear car with regard to passenger comfort. for hsr operation these multi-system drive head cars should not exceed the mass of 67 tonnes. at this point it is necessary to point to the fact that czech operators will have to buy these vehicles in near future. 15 josef kolář acta polytechnica ctu proceedings these new transport models should free up existing railway corridors for a high-speed regional transport and for international container and combined freight transport. for conventional freight-trains sž s. p. (railways administration s.e.) has to ensure suitable and efficient use of incline and directionally more demanding railway line kolín – kutná hora – havlíčkův brod – křižanov – brno, see figure 3. however, modern railways have to maintain a high level of operational safety when meeting the new requirements. figure 3. railway transit corridors in czech republic. 2.2. modern regional railway, safety versus railway ecology the safety of rolling stocks operation is divided into two groups: • active safety – represents elements and systems to prevent accidents of rolling stock among themselves or with road users. • passive safety – represents the components and systems designed to reduce the consequences of an accident. 2.2.1. modern speed regional railway for railway lines with a speed limit above 160 kph, level crossing with road infrastructure is required. this requirement of separation of the railway line from the road provides a sufficient level of active safety. in the field of passive safety of rolling stock, new requirements for the strength (en 12 663) and crash resistance of rolling stock (en 15 227) were defined at the end of the 20th century, see train avg – figure 4. the implementation of these new measures, together with the growing demands on multi-system (three-system, dual-powered or hybrid) vehicles, accessibility and better equipment of vehicle interiors (wifi, better information systems, noise reduction, etc.) is reflected in an increase of vehicle body weights. figure 4. visualization of energy absorption device – hsr train unit agv [2]. the manufacturers are trying to compensate for this increase in weight with a light carbody design and with a change in the concept of vehicle running gear. instead of bogies with outer frames, concepts with an inner frame are being developed. but even these solutions have their design limitations (eg. higher sensitivity to changes of wheel load forces, less space for installation of the wheelset drive, reduced angular stiffness of the primary and secondary suspension, which leads to utilization of torsional stabilizers, and others). 2.2.2. regional and suburban railway transport versus road transport the situation is somewhat more demanding in terms of safety and ecology of operation in the regional transport. the requirement for a regional rolling stock to meet the requirements of en 12 663 – category p2, en 15 227 – category ci represents an increase in its weight and the resulting higher rolling resistance, greater inertia effects and thus higher energy consumption. modern 21st-century regional railways must therefore use rolling stocks with electric power transmission, as only this type of drive makes possible to achieve a high percentage of vehicle kinetic 16 vol. 35/2022 modern railways for the 21st century figure 5. bogie siemens sf 7500 with inner frame and inside bearings – train unit mireo [3]. energy recovery with the consistent application of electrodynamic brakes on all vehicle wheelsets. figure 6. electric regional unit mireo [3]. regional railway lines quite often cross roads in the form of railway crossings. there are about 8,000 railway crossings in the czech republic with varying degrees of security, in [4]. any unprotected level crossing increases the risk of crash between regional train and road vehicle. compliance with the requirements of standards en 12 663 (category p2), en 15 227 (category ci) makes the railway vehicle a clear winner in the case of a collision with car. nowadays, most collisions of rolling stocks with the car leads to a significant car body destruction and serious injuries or death to car crew members. in this context, the question arises for a broader expert discussion: should we equip vehicles for regional transport with additional increased weight (resulting from the installation of the additional passive safety elements and the requirement for controlled deformation of vehicle fronts) and thus increase vehicle weight, extend starting and braking distances, increase consumption and reduce competitiveness with bus transport? meeting the requirements of standards en 12 663 (category p2), en 15 227 (category ci) de facto means that economically operated and ecologically advantageous, light two-axle motor vehicles, called as rail buses, will disappear from regional railways. would it not be more appropriate to require regional rolling stock operated on local railways to meet the requirements of en 12 663 category p3, en 15 227 – category cii and to equip both types of vehicles (rail and road) with active safety features that would significantly eliminate road vehicle collisions with a light rail vehicle (rail bus)? figure 7. design study of the rail – electric bus [5]. the potential to reduce the consequences of a collision between a rail vehicle and cars is possible: > in the field of active safety, it is appropriate to consider and use the possibility of communication of the mobile part of etcs placed in new rolling stocks with the navigation and control system of cars. when all rolling stocks are equipped with an etcs safety guard, which allows the vehicle ‘s position to be monitored on the track, it is possible to link the rolling stock communication with the level crossing or directly with the motor vehicle. thanks to the exchange of information about the railway vehicle drive, car control units could control the driver’s activity and possibly directly prevent the driver from driving over the railway crossing and thus prevent an accident. > in the field of passive safety, it is appropriate to assess the possibility and reality of the application of new safety features ("protective frames or panels") on the fronts of regional rolling stocks. these more flexible protective frames, which the driver would "shoot" in 17 josef kolář acta polytechnica ctu proceedings front of the rail vehicle in the event of an immediate avoidance of the accident. these forward-moved safety elements will absorb and "soften" the impact of the front of the light rail vehicle to the side of the car and reduce the penetration of the coupling into the passenger zone. figure 8. revolution very light rail [6]. 3. conclusions the requirement to implement etcs-level 2 will certainly make a significant contribution to a higher level of active safety in the regional rail traffic. however, the next, entirely logical step should be to start intensive cooperation between etcs creators and car navigation and control system manufacturers. the transfer of information about train’s position to the car navigation and its display to the driver and storage in a "black box" recording device can significantly increase the responsibility of car drivers for their behaviour at unprotected level crossings. this cooperation of the etcs mobile part with road vehicles will be necessary for autonomous vehicles. figure 9. very light rail for export to turke [7]. acknowledgements this research has been realized using the support of the technology agency of the czech republic, programme national competence centres, project #tn01000026 josef bozek national center of competence for surface transport vehicles this support is gratefully acknowledged. references [1] j. kolář. vysokorychlostní tratě a doprava v čr. [unpublished lecture], fakulta dopravní čvut v praze, 2020-11-10. [2] https://www.alstom.com,https://fdocuments.in/ document/alstom-english-agv.html. [3] https://www.mobility.siemens.com/global/en/ portfolio/rail/rolling-stock/ commuter-and-regional-trains.html. [4] j. kolář, j. seidl, v. zelený, o. vincik. statistika nehodovosti tramvají a vozidel regionální železnice. [research report], praha: ctu fme, department of automotive, combustion engine and railway engineering, 2020. report no. z20-03 u 12 120 fs čvut. [5] j. kolář. konstrukční studie částečně nízkopodlažního kolejového elektrobusu. in zborník prednášok i. – xxiv. medzinárodná konferencia – súčasné problémy v koľajových vozidlách. žilina: vts pri žu, 2019. p. 317–324, isbn 978-80-89276-58-5. [6] https://warwick.ac.uk/fac/sci/wmg/research/ hvmcatapult/research/rail/vlr. [7] https://www.transportxtra.com/publications/ local-transporttoday/news/47689/ uk-manufacturer-unveils-very-light-rail-vehicle, https://www.severn-lamb.com/images/products/ ulr-express/dsc_4741.jpg. 18 https://www.alstom.com, https://fdocuments.in/document/alstom-english-agv.html https://www.alstom.com, https://fdocuments.in/document/alstom-english-agv.html https://www.mobility.siemens.com/global/en/portfolio/rail/rolling-stock/commuter-and-regional-trains.html https://www.mobility.siemens.com/global/en/portfolio/rail/rolling-stock/commuter-and-regional-trains.html https://www.mobility.siemens.com/global/en/portfolio/rail/rolling-stock/commuter-and-regional-trains.html https://warwick.ac.uk/fac/sci/wmg/research/hvmcatapult/research/rail/vlr https://warwick.ac.uk/fac/sci/wmg/research/hvmcatapult/research/rail/vlr https://www.transportxtra.com/publications/local-transport today/news/47689/uk-manufacturer-unveils-very-light-rail-vehicle https://www.transportxtra.com/publications/local-transport today/news/47689/uk-manufacturer-unveils-very-light-rail-vehicle https://www.transportxtra.com/publications/local-transport today/news/47689/uk-manufacturer-unveils-very-light-rail-vehicle https://www.severn-lamb.com/images/products/ulr-express/dsc_4741.jpg https://www.severn-lamb.com/images/products/ulr-express/dsc_4741.jpg acta polytechnica ctu proceedings 35:14–18, 2022 1 introduction 2 challenges not only for rolling stocks project engineers 2.1 high-speed railway transport versus corridor lines = new transport models 2.2 modern regional railway, safety versus railway ecology 2.2.1 modern speed regional railway 2.2.2 regional and suburban railway transport versus road transport 3 conclusions acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.32.0007 acta polytechnica ctu proceedings 32:7–14, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague considerations for brittleness in tailings jiří herzaa,∗, ryan singhb a czech technical university in prague, faculty of civil engineering, depatment of geotechnics, thákurova 7, 166 29 prague, czech republic b hats consulting, 118 st georges terrace, pert, wa 6000, australia ∗ corresponding author: jiri.herza@fsv.cvut.cz abstract. brittleness is an important geotechnical parameter of soils as it describes the degree of reduction of the soil shear resistance after reaching the peak strength. [1] described soil brittleness by means of a brittleness index (ib), which is the ratio of the shear resistance loss to the peak shear strength. the ib has become a common soil parameter that is used as an indicator for the soil susceptibility to liquefaction. however, ib does not consider the rate at which the soil resistance reduces, the development of excess pore water pressure and it ignores the stress strain relationship. as a result the same ib can be calculated for a soil that collapses over a very small strain range and a soil that gradually reduces its shear resistance over extensive strain levels as long as both soils have similar peak and residual shear strengths. this paper discusses the root causes of the shear resistance loss and proposes a modification of the ib parameter to take account of the rate of the shear resistance loss, which will help to better depict the stress-strain behaviour of strain softening soils. this paper also distinguishes between structural brittleness and undrained brittleness and provides a clear definition of each to improve the understanding of tailings among practitioners. keywords: brittleness, critical state, shear resistance, tailings. 1. introduction following the failures of the tailings dams at samarco (fundão) and brumadinho (feijão) in brazil, brittle failure has become a topic of great interest within the geotechnical engineering community. brittle failure has been identified to be a contributing factor in the failures of many tailings storage facilities, as described by [2–4] and others. the post-failure review of the feijão dam [4] identified that the sudden failure of the dam was caused in part by tailings exhibiting brittle behaviour. of concern to the mining industry was the finding that there were no apparent signs of distress prior to failure, which is in fact a fundamental characteristic of brittle failure. it should be noted that the lack of indication of distress was not due to a lack of monitoring instrumentation for the dam which featured 113 piezometers, 14 surface survey markers, six inclinometers and 50 monitored seepage points. the industry’s concern regarding the presence of brittle materials within and supporting tailings dams, particularly in those featuring upstream raises is evident in the requirements of the newly published global industry standard on tailings management ("the global standard"), which includes requirement 4.6 that reads "identify and address brittle failure modes with conservative design criteria."(global tailings review, 2020). this requirement is also reflected in several other design guidelines, such as australian national committee on large dams [5] guideline on tailings dams [6], which provides recommendations for conservative design assumptions if materials are found to be susceptible to static liquefaction, which is noted to be a brittle subset of contractive materials. other forthcoming publications, guidelines and papers – including the forthcoming international commission on large dams (icold) bulletin on tailings dam safety (icold, in progress) and the international council on mining and metals (icmm) tailings management good practice guide (icmm, 2021) identify the identification of brittle failure modes as a critical evaluation factor for dam safety that should be a primary consideration in the design and review of tailings storage dams. despite the recent – and perhaps overdue – emphasis on brittleness and its importance to tailings dams, the meaning of brittleness and methods to characterize it currently lacks certain aspects of clarity and alignment with how it is not being used. further, the concept may not be fully understood by some professionals involved in evaluating and designing these facilities. generally, the use of the term "brittle" in the context of recent tailings dam failures and in the various standards and guidelines that refer to brittle behaviour alludes to a sudden loss of shear strength without signs of prior deformation. however, the currently available definition for soils as proposed by bishop (1967) does not fully align with this interpretation, namely the aspect of suddenness. without a formal definition of brittleness that reflects 7 https://doi.org/10.14311/app.2022.32.0007 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en jiří herza, ryan singh acta polytechnica ctu proceedings the intended use, the meaning or measure of terms like "brittle material or brittle failure mode" is left to the individual interpretation of the reader. 2. brittleness index bishop’s index (bishop, 1967) relates the peak shear strength (τp) to the residual shear strength (τr) as follows: ib = τp − τr τp (1) the index provides a sense of the magnitude between the peak and residual shear stresses, with a higher index indicating a greater difference between the peak and residual resistance of a material. the ib proposed by bishop does not consider the strain (deformation) levels at which a brittle failure is initiated or the strain at which the residual strength is achieved. the importance of strain has been well recognised in the field of rock mechanics, where it has been identified that the brittleness of a rock mass provides insight into how the rock mass will behave for various applications, e.g. hydraulic fracturing , drilling rate, dust generation, etc. [7]. over a dozen brittleness indices have been developed for use in rock mechanics to capture the various aspects of material strength loss, including consideration of the strain range within which the loss occurs [8]. [1] recognised that ib did not consider the suddenness of the shear strength loss and thus he also introduced the concept of brittleness in terms of the additional energy required to progress the shear failure from the peak (ep) to the residual state (ef ) as shown in figure 1. figure 1. an energy parameters for brittle soils (bishop, 1967). while the rupture index (introduced in figure 1) considered the suddenness of the shear strength loss, [1] recognised that the shape of the stains-stress line past the failure is strongly dependant on numerous parameters including test type, test procedure, sample preparation, geometry etc. as a result the rupture index has not been adopted by the geotechnical community and the strain considerations in the brittleness assessment for soil have not been addressed in the literature. in addition, bishop’s work was focused on clays, whereas tailings frequently (but not exclusively) occur as cohesionless silts and sands. while the magnitudebased brittleness index may be useful in both cohesive and cohesionless materials, the mechanism of strength loss is typically different between the two classes of materials, as discussed further below, and the strain levels at a which brittle failure occurs may vary significantly. [4] thus proposed renaming bishop’s brittleness index as the "strength loss index" because it considers solely the strength and not the strain aspects of brittleness. 3. different types of brittleness the research leading to the definition of bishop’s ib was primarily focused on the behaviour of plastic clays. the shear strength reduction depicted in bishop’s work was the result of the clay particles reorientating along the developing shear plane. [1, 2] drew attention to the decreasing shear resistance of plastic clays caused by the development of continuous bands of strongly orientated particles, which resulted in the strength of the clays lowering to a residual value. [2] observed the development of strains and displacement during simple shearing of the kaolin clay fabric. figure 2 (left) shows successive sections of kaolin clay, which were interrupted at various stages of shearing. each section has been labelled (e.g. v.2, v.3, etc.), to show the position of the section on the adjacent stress-strain curve. as can be seen, the development of structural strains and displacements of the clay structure coincides primarily with an increase in the peak strength as the original clay structure rotates (v.3). in the post-peak phase of shearing, discontinuities of the structure occur with progressive reorientation of the clay particles occurring at discontinuities (figure 2, right). once the clay minerals had reorientated along the direction of strain, the residual shear strength of the clay was reached. the originally proposed brittleness index therefore relates the loss of shear strength of the material to the structural change of the material caused by straining, which is an irreversible phenomenon. strain-induced shear strength loss also occurs in granular materials where the soil particle and intraparticle bonding can be compromised by shearing. in sandy soils, the structure of the sand and sand-like particles can be crushed and grinded as the particles move over and around each other during shearing. this change to the particles and intra-particle contacts can result in a reduction of the frictional resistance of the soil during shearing. this loss in shear strength is intensified for cemented granular materials in which straining can break the cementitious bonds resulting 8 vol. 32/2022 considerations for brittleness in tailings in a sudden (and potentially large) drop in the shear strength of the material. figure 3 shows an example of cemented aggregates of bauxite residue in which sandsite particles are composed of much smaller particles interconnected by iron oxide and aluminium hydroxide minerals. although it is acknowledged that the properties of soils change in time, the structural changes and the peak and the residual strength can be considered, for the purpose of this paper, as lasting properties of the soil induced by straining. in line with the original bishop’s brittleness concept, the shear strength in the context of this paper is considered to be a soil property that depends primarily on the soil composition and structure. in comparison, shear resistance is hereby defined as the ability of a soil to withstand shear stress under a given set of conditions. it must be emphasised that a generic function of the shear resistance has numerous parameters, some of which may still be unknown. the influencing parameters include the void ratio, soil composition, stress history, level of structuration, temperature, strain, strain rate and structure. the limitations of our testing and computing methods do not allow us to fully quantify the influence of these parameters; thus, the shearing relationships are simplified, such as in the mohr-coulomb equation which is the most common expression of the shear resistance of soils. the shear strength of a soil in the mohr-coulomb equation is expressed by the indices of an effective friction angle (φ′) and an effective cohesion (c′) and the shear resistance of the soil at failure (τf ) is expressed as: τf = c′ + σ′ n tanφ ′ (2) where σ′ n represents the normal effective stress acting on the shear surface, the sole variable in the relationship. to illustrate the concept of shear strength as a soil property and shear resistance as a function of stress conditions, the φ′ value has a definite minimum value that is greater than zero. however, the normal effective stress could reduce to zero due to significant increase in pore water pressures. using the example of the feijao tailings dam, the cause for the loss of shearing resistance was the rapid and significant increase of pore water pressures, which reduced the effective normal stress variable σ′ n to zero or near zero and resulted in a liquefaction of the tailings. the generation of excess pore water pressure was caused by the contractive nature of the tailings in which the collapse of the loose aggregate assembly transferred the stress from the interparticle interfaces to the water. it is noted that the breakdown of the iron oxide bonds (similar to those shown in figure 3 reduced the effective cohesion value of the feijao tailings. however, it is unlikely that the frictional resistance between particles, represented by c′ and φ′, changed in any substantial way and was the root cause of the failure. because of the major differences between the described processes leading to the reduction of the soil’s ability to resist shear stress, it is proposed that shear resistance loss caused the irreversible change of the soil’s structure as described by [1] and others be referred to as structural brittleness. the sudden loss in a soil’s shear resistance caused by excess pore water pressure should then be referred to as undrained brittleness. it is recognised that both types of brittleness can occur simultaneously, as structural brittleness can also lead to undrained brittleness and many soils can lose shear resistance due to a combination of shear strength reduction and excess pore water pressure generation. however, an understanding of the different underlying phenomena leading to a shear resistance loss is crucial because the minimum recommended factors of safety that are currently used to prevent embankment failures do not account for misconceptions of the underlying geotechnical processes [5]. 4. structural brittleness and the rate of strength loss as discussed earlier, the current brittleness index alone lacks a measure of the suddenness of a material’s resistance or strength loss, with bishop’s rupture index indirectly providing a method to assess the strain over which shear resistance is lost. a more direct consideration of the strain range over which the loss of shear strength occurs was assessed by estimating the rate of the greatest reduction in shear strength after the peak shear strength is reached normalised by the peak shear strength, as shown in figure 4 with the stress stain data adopted from [2] (v7 plot in figure 2, with the data translated to si units). this calculated value, called the brittleness slope (bs), provides for the aspect of "suddenness" of strength loss and expresses the percentage of the peak shear strength loss per 1 mm of shear strain. to illustrate the application of the bs, the index was calculated for sands and clays with available direct shear test data. of particular interest was the application of the index for sands of varying cementation, density in the form of relative density/void ratio and normal effective stress. direct shear test data for sands was obtained from [10, 11]. the tests were completed on uncemented samples and with an addition of 1% and 2% of cement and at different normal effective stresses. additional ring shear tests on interparticle bonded bauxite residue (figure 3) were also completed for the purpose of this study. for a comparison to the above non-plastic soils direct shear test data for clays was also included, based on direct shear tests for a clay material presented by [12] and the shear box test results from [2]. bishop’s brittleness index and the brittleness slope for the referenced tests are shown in figure 5. 9 jiří herza, ryan singh acta polytechnica ctu proceedings as can be seen, the development of structural strains and displacements of the clay structure coincides primarily with an increase in the peak strength as the original clay structure rotates (v.3). in the post-peak phase of shearing, discontinuities of the structure occur with progressive reorientation of the clay particles occurring at discontinuities (fig. 2, right). once the clay minerals had reorientated along the direction of strain, the residual shear strength of the clay was reached. the originally proposed brittleness index therefore relates the loss of shear strength of the material to the structural change of the material caused by straining, which is an irreversible phenomenon. strain-induced shear strength loss also occurs in granular materials where the soil particle and intra-particle bonding can be compromised by shearing. in sandy soils, the structure of the sand and sand-like particles can be crushed and grinded as the particles move over and around each other during shearing. this change to the particles and intra-particle contacts can result in a reduction of the frictional resistance of the soil during shearing. this loss in shear strength is intensified for cemented granular materials in which straining can break the cementitious bonds resulting in a sudden (and potentially large) drop in the shear strength of the material. fig. 3 shows an example of cemented aggregates of bauxite residue in which sand-site particles are composed of much smaller particles interconnected by iron oxide and aluminium hydroxide minerals. although it is acknowledged that the properties of soils change in time, the structural changes and the peak and the residual strength can be considered, for the purpose of this paper, as lasting properties of the soil induced by straining. in line with the original bishop’s brittleness concept, the shear strength in the context of this paper is considered to be a soil property that depends primarily on the soil composition and structure. in comparison, shear resistance is hereby defined as the ability of a soil to withstand shear stress under a given set of conditions. fig. 2: sequence of kaolin clay structures sheared normal to original clay fabric (left) and sequential development of discontinuity with arrow indicates direction of shearing (right) (morgenstern & tchalenko, 1967) figure 2. sequence of kaolin clay structures sheared normal to original clay fabric (left) and sequential development of discontinuity with arrow indicates direction of shearing (right) [2]. fig. 3: cemented aggregates of bauxite residue (herza, 2020) it must be emphasised that a generic function of the shear resistance has numerous parameters, some of which may still be unknown. mitchell and soga (2005) listed several influencing parameters including the void ratio, soil composition, stress history, level of structuration, temperature, strain, strain rate and structure. the limitations of our testing and computing methods do not allow us to fully quantify the influence of these parameters; thus, the shearing relationships are simplified, such as in the mohr-coulomb equation which is the most common expression of the shear resistance of soils. the shear strength of a soil in the mohr-coulomb equation is expressed by the indices of an effective friction angle (φ’) and an effective cohesion (c’) and the shear resistance of the soil at failure (tf) is expressed as: where σ’n represents the normal effective stress acting on the shear surface, the sole variable in the relationship. to illustrate the concept of shear strength as a soil property and shear resistance as a function of stress conditions, the φ’ value has a definite minimum value that is greater than zero. however, the normal effective stress could reduce to zero due to significant increase in pore water pressures. using the example of the feijao tailings dam, the cause for the loss of shearing resistance was the rapid and significant increase of pore water pressures, which reduced the effective normal stress variable σ’n to zero or near zero and resulted in a liquefaction of the tailings. the generation of excess pore water pressure was caused by the contractive nature of the tailings in which the collapse of the loose aggregate assembly transferred the stress from the interparticle interfaces to the water. it is noted that the breakdown of the iron oxide bonds (similar to those shown in chyba! nenalezen zdroj odkazů.) reduced the effective cohesion value of the feijao tailings. however, it is unlikely that the frictional resistance between particles, represented by c’ and φ’, changed in any substantial way and was the root cause of the failure. because of the major differences between the described processes leading to the reduction of the soil’s ability to resist shear stress, it is proposed that shear resistance loss caused the irreversible change of the soil’s structure as described by bishop (1967) and others be referred to as structural brittleness. the sudden loss in a soil’s shear resistance caused by excess pore water pressure should then be referred to as undrained brittleness. it is recognised that both types of brittleness can occur simultaneously, as structural brittleness can also lead to undrained brittleness and many soils can lose shear resistance due to a combination of shear strength reduction and excess pore water pressure generation. however, an understanding of the different underlying phenomena leading to a shear resistance loss is crucial because the minimum recommended factors of safety that are currently used to prevent embankment failures do not account for misconceptions of the underlying geotechnical processes (herza et al 2017). figure 3. cemented aggregates of bauxite residue [9]. figure 4. brittleness index for strength loss. 10 vol. 32/2022 considerations for brittleness in tailings a "zone of high brittleness" has been overlaid on the structural brittleness plane to identify areas where ib is greater than 0.4. this ib threshold has been adopted from [4]. it should be noted that this value of 0.4 was selected as a threshold by robertson as available data for flow liquefaction failures indicated a correlation of brittleness and liquefied undrained shear strength ratios. in reviewing the conclusions in [4], it is likely that structural brittleness and undrained brittleness has been combined for several reasons, including the testing being carried out in undrained conditions leading to the generation of excess pore water pressures. nonetheless, where ib values greater than 0.4 are observed it indicates that particular attention should be given to the potential for some form of brittle behaviour. figure 5. structural brittleness plane. the analysed test results indicated that: • brittleness of clean uncemented sand is smaller compared with the brittleness of the cemented materials and it increases with the increasing sand density as expected. • increasing cement content increases the peak shear strength, the dilatancy and the ib and bs values. it also results in the peak shear strength being achieved at lower shear displacements. • increasing normal stresses by 100% supresses the dilatancy and the ib and bs values of the samples with 2% cement content. however, the increase of normal stress had only a very little impact on the samples with 1% of cement added. • the spread of the points in figure 5 is most likely a result of the non-linear relationships between the sand density, cement content and the applied effective stress. in addition, several of the presented stress paths did not converge to the same residual shear strength at the same normal stress, indicating that the shearing may not have destroyed all cemented bonds within the sand samples. • the test results of the bauxite residue were showing brittleness caused by gradual destruction of the intra-particle bonds only on comparatively denser samples where the loss of shear strength was accompanied by the sample contraction. this behaviour contrasted with the behaviour of the cemented and uncemented compacted sands that exhibited dilation after reaching the peak shear strength. • the bauxite residue samples reached the peak shear strength values at higher displacement compared to the artificially cemented sands. for clays, it was noted that the range of brittleness varied significantly and the brittleness slope reduces with an increasing brittleness index. however, it is likely that due to the low permeability of the tested clays some of the measured loss of shear resistance was due to the development of shear-induced porewater pressures. in relation to the brittle behaviour identified in recent tailings dam failures, structural brittleness of the tailings was not the primary trigger mechanism, nor can it necessarily indicate that undrained brittleness is likely. as expected, the test results of loose uncemented clean sands did not show structural brittleness as the samples contract during drained shearing and the tighter packing of sand particles made shearing harder. conversely the same sands, but in a denser state, indicated moderate structural brittleness as they dilated under shearing. if saturated and tested in undrained (constant volume) conditions, the dilative sands would gain shear resistance due to reduced pore pressures generated during shearing. in contrast the loose sands tested in undrained conditions would show a loss of shear resistance during shearing due to shear induced excess pore water pressures. it can therefore be seen that for uncemented clean sands, the tendency or likelihood for structural and undrained brittleness are opposed. in cemented sands however, the structural brittleness may indicate the potential for undrained brittleness and liquefaction. in the case of drained conditions, structural brittleness can be observed for loose and cemented sand, as the cementitious bonds break, resulting in a collapse of the soil, or particle structure followed by further contraction. this behaviour was observed in the test results of the bauxite residue sample. under undrained conditions, this tendency for contractive behaviour would not be able to develop and would instead translate into generation of excess pore water pressures, a reduction in normal effective stress and loss of shear resistance potentially to the point of liquefaction. as such, structural brittleness can be used as a screening assessment for the potential of undrained brittleness and even liquefaction for loose and cemented sands. this relationship could be made even clearer, with a three-dimensional path of shear stress, shear strain and volumetric change (reduction). the volumetric change is perhaps a better indication of structural change than a void ratio for cemented materials as the change of the structure (breakage of particles) may not necessarily be reflected by the change of the void ratio. 11 jiří herza, ryan singh acta polytechnica ctu proceedings 5. undrained brittleness as discussed earlier, the undrained brittleness refers to the loss of shear resistance of a soil due to the straininduced pore water pressures. this type of brittleness is caused by a rapid increase of pore water pressures when the soil structure decays in so-called undrained conditions. such conditions occur in contractive, fully or partially saturated soils when the strain-induced porewater pressures do not concurrently dissipate. under such conditions, the loss of shear resistance is proportional to the reduction in effective confining stress in the soil. with the available testing apparatus, given excess porewater pressure generated in the shear zone cannot be directly measured using the current laboratory equipment, it is being inferred from observations made at the boundaries of the sample. together with the other influencing factors discussed earlier, this makes the conventional strain-stress diagrams unsuitable for description of the undrained brittleness. instead, the concept of critical state soil mechanics (cssm) provides an indication of the likelihood of a brittle failure, and this indeed has become common practice in the tailings industry. the use of the state parameter to assess a soil’s likelihood of brittle failure is based on the distance between the current state of the soil from its critical state, as expressed by a critical state line (csl). the csl is a locus of critical states, at which continuous shearing occurs without a change of mean effective stress (p′), deviatoric stress (q) and void ratio (e) [3]. due to its dependency on the void ratio as a measure of the soil structure, the cssm concept is not directly applicable to all materials, in which the orientation of aggregates and/or cementation plays an important role such is the case of the kaolinite clays and cemented sands discussed earlier in this paper. the csl of a soil is commonly depicted as a relationship between mean effective stress and void ratio, with any point on the plane representing a soil’s state prior to shearing. an example of a csl is shown in figure 6 [3]. per cssm, any soil being sheared will tend towards the csl as the soil structure decays from the initial state to the critical state. during undrained shearing of a soil, volumetric changes are prohibited and the shear stress path towards the csl is horizontal with respect to the csl plot as shown in figure 6. figure 6. csl [13]. the pre-sheared state of the soil is often described using a state parameter, ψo, which is the difference in the pre-sheared state and the critical state void ratios at the same mean effective stress (figure 6). a stress path following the vertical direction of the state parameter ψo can be obtained from a direct shear tests at a constant normal stress undertaken in perfectly drained conditions. however, as the undrained brittleness is caused by reduction of the mean effective stress in undrained shearing, the state parameter ψo alone is not sufficient to capture the potential for brittle failure. the susceptibility to brittle failure, rather, is proportional to the horizontal distance between the pre-sheared mean effective stress (p′ o) and the effective stress at the csl at constant p′ cs) which is equivalent to the ratio of the state parameter ψo and the csl slope (ψo/λe) as shown in figure 6. a greater ψo/λe indicates a greater potential for generation of excess pore water pressures during undrained shearing and thus a greater likelihood of brittle failure. [13] presented results of critical state lines for various tailings, which demonstrated that the csl cannot be reliably constructed from basic soil characterisation tests and csl specific testing is crucial to understand the potential for undrained brittleness. 6. discussion and conclusions this paper proposes to clarify terms for brittleness to align them with the use of the term brittle as it pertains to a soil exhibiting a sudden loss of shear strength without signs of prior deformation, particularly in the context of recent tailings dam failures. in this context the two following forms of brittle behaviour are identified: • structural brittleness – a loss of a soil’s shear strength caused by irreversible change to a soil’s structure during shearing, that can occur in clays and cemented soils • undrained brittleness – a loss in a soil’s shear resistance due to a change in the stress field applied to a material, often due to generation of excess pore water pressures the structural brittleness can be captured by bishop’s ib using conventional strain-stress plots although the rate of the shear strength loss can be 12 vol. 32/2022 considerations for brittleness in tailings treated with caution due to its dependence on the testing procedure. modern investigation techniques such as sem can be successfully applied to reveal the nature of the soil structure at the microscopic level and thus its susceptibility to structural brittleness. the undrained brittleness is the result of excess pore water pressure generated during undrained shearing and the use of ib estimated from stress-strain plots is considered insufficient. although the ib can used as a screening tool, the undrained brittleness should be considered using the cssm concept. a state parameter ψo greater than -0.05 (a dashed parallel line to the cs, in figure 7 is commonly taken as an indication of contractive soils that may be susceptible to brittle failure. rather than using ψo alone, the likelihood of brittle failure should be expressed by the ψo/λe ratio with increasing values indicating a higher likelihood of undrained brittleness. the potential susceptibility to undrained brittleness is indicated by shading of zones in figure 7. because not all contractive soils are susceptible to brittle failure, further delineation of the zones may be appropriate using the ψo/λe value. as figure 7 shows a log-linear relationship between the mean effective stress and the void ratio, the potential boundary of undrained brittleness may by a curved line as indicated in figure 7. the authors emphasise that the potential boundary of undrained brittleness is indicative only and it is not based on any testing. the structural brittleness can be captured by bishop’s ib using conventional strain-stress plots although the rate of the shear strength loss can be treated with caution due to its dependence on the testing procedure. modern investigation techniques such as sem can be successfully applied to reveal the nature of the soil structure at the microscopic level and thus its susceptibility to structural brittleness. the undrained brittleness is the result of excess pore water pressure generated during undrained shearing and the use of ib estimated from stress-strain plots is considered insufficient. although the ib can used as a screening tool, the undrained brittleness should be considered using the cssm concept. a state parameter ψo greater than -0.05 (a dashed parallel line to the cs, in fig. 7is commonly taken as an indication of contractive soils that may be susceptible to brittle failure. rather than using ψo alone, the likelihood of brittle failure should be expressed by the ψo/λe ratio with increasing values indicating a higher likelihood of undrained brittleness. the potential susceptibility to undrained brittleness is indicated by shading of zones in fig. 7. because not all contractive soils are susceptible to brittle failure, further delineation of the zones may be appropriate using the ψo/λe value. as fig. 7 shows a log-linear relationship between the mean effective stress and the void ratio, the potential boundary of undrained brittleness may by a curved line as indicated in fig. 7. the authors emphasise that the potential boundary of undrained brittleness is indicative only and it is not based on any testing. fig. 7: csl and critical zones beyond considering this state parameter delineation, further definition can be added to a csl plot in order to better define the area of concern. the authors propose that the assessment should focus on the range of mean effective stresses that the soil can be exposed to under the current and future loading conditions. this range includes the current conditions at relevant depths and the pressures imposed by potential development and closure of the tsf. this is important because  soils that would only exhibit undrained brittleness under mean effective stresses well outside the operation stress range can be defensibly deemed not to pose a concern for the structure.  soils that are dilative in the current state can become contractive and potentially susceptible to undrained brittleness if the mean effective stress increases due to the future tsf development or closure. operating stress range in structural zone of tsf ? ? ? ? ? ? ? ? ? potential boundary of undrained brittleness? csl ψ0 = 0.05 figure 7. csl and critical zones. beyond considering this state parameter delineation, further definition can be added to a csl plot in order to better define the area of concern. the authors propose that the assessment should focus on the range of mean effective stresses that the soil can be exposed to under the current and future loading conditions. this range includes the current conditions at relevant depths and the pressures imposed by potential development and closure of the tsf. this is important because: • soils that would only exhibit undrained brittleness under mean effective stresses well outside the operation stress range can be defensibly deemed not to pose a concern for the structure. • soils that are dilative in the current state can become contractive and potentially susceptible to undrained brittleness if the mean effective stress increases due to the future tsf development or closure. the authors propose that consideration of appropriate mean effective stresses be included in the assessment of materials susceptible to undrained brittleness in the design and assessment of all civil structures, including tsfs. to support this, the derivation and application of tailings-specific csls are critical, as recommended by [13]. it should also be noted that the use of the state parameter to identify a material’s susceptibility to brittle failure is dependent on a material’s void ratio, and any materials that do not exhibit a meaningful change in void ratio during brittle failure, such as cemented sands or structured clays, cannot be appropriately assessed within this framework. thus, further research into other appropriate measures of a material’s structure may allow a wider assessment of brittle behaviour in soils and tailings, in particular. future work extending from this paper include further definition and characterisation of the causes and mechanisms of undrained brittleness, as well as some proposed index to measure the susceptibility of a material to feature undrained brittleness and thus a formal potential boundary of undrained brittleness. this will allow a characterisation of materials that may be susceptible to undrained brittleness, which aligns with the principles of the performance based, risk informed approach to storage facility safety. acknowledgements this work was supported by the grant agency of the czech technical university in prague, grant no. sgs21/045/ohk1/1t/11. references [1] a. bishop. progressive failure with special reference to the mechanism causing it. 1967. proceeding of the geotechnical conference, oslo, sweden. 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[7] h. zhou, j. lu, y. jiang, f. meng. a new rock brittleness evaluation index based on the internal friction angle and class i stress-strain curve. 2018. rock mechanics and rock engineering vol. 51. 13 jiří herza, ryan singh acta polytechnica ctu proceedings [8] j. jiang, w. dong, h. xinping, d. shuai. relationship between a brittleness index and crack initiation stress ratio for different rock types. 2020. advances in civil engineering, vol. 2020, article id 8091895, 12 pages. [9] j. herza, d. jirasko, i. vanicek. the influence of aggregates on tailings behaviour. 2020. proceedings of the wmess conference, prague, september 2020. [10] y. amini, a. hamidi, e. asghari. shear strength characteristics of an artificially cemented sand-gravel mixture. 2013. challenges and recent advances in geotechnical and seismic research and practices, chengdu, china. october 25-27, 2013. [11] a. simoni, g. houlsby. the direct shear strength and dilatancy of sand-gravel mixtures. 2006. geotechnical and geological engineering. 24. 523-549., https://doi.org/10.1007/s10706-004-5832-6. [12] m. heidemann, l. bressani, j. flores. residual shear strength of a residual soil of granulite. 2020. soils and rocks, são paulo, 43(1): 31-41. [13] k. smith, r. fanni, p. chapman, d. reid. critical stae testeing of tailings – comparison between various tailings and implications for design. 2019. proceeding of tailings and mine waste 2019. november 17-20, vancouver, canada. 14 https://doi.org/10.1007/s10706-004-5832-6 acta polytechnica ctu proceedings 32:7–14, 2022 1 introduction 2 brittleness index 3 different types of brittleness 4 structural brittleness and the rate of strength loss 5 undrained brittleness 6 discussion and conclusions acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2018.15.0074 acta polytechnica ctu proceedings 15:74–80, 2018 © czech technical university in prague, 2018 available online at http://ojs.cvut.cz/ojs/index.php/app the influence of boundary conditions on the response of underground structures subjected to earthquake veronika pavelcová, tereza poklopová, tomáš janda, michal šejnoha∗ czech technical university in prague, faculty of civil engineering, thákurova 7, 166 29 prague 6, czech republic ∗ corresponding author: sejnom@fsv.cvut.cz abstract. the paper deals with the prediction of the response of a real underground structure subjected to earthquake. a fully dynamic analysis is carried out in the geo5 fem program using the finite element method. limiting our attention to a two-dimensional analysis we focus on the implementation of special boundary conditions along the vertical edges of the computational model. a simple study is carried out first to show that incorrectly applied boundary conditions may significantly influence the actual design of underground structures loaded by vertically propagating shear waves. this study promotes the combination of so called free-field and static boundary conditions as demonstrated on a simple example. keywords: earthquake, shear weave, accelerogram, finite element method, free-field boundary, absorbing boundary, fixed boundary. 1. introduction crowded cities, pushing the need for fast and safe transportation, are the main cause of ever faster growing number of underground structures. such structures, built in densely populated areas, often experience a high degree of complexity. to maintain reliable performance of tunnels during their life time calls for reliable designs. these are typically based on detailed finite element simulations. in this regard, the preliminary design typically adopts a certain simplification such as a dimensional reduction from 3d (three-dimensional) to 2d (two-dimensional). while simplified 2d formulations attempting to account for out-of-plane deformations do exit [1–3], the majority of practical engineers rely on a fully 2d plane-strain analysis employing the convergence confinement method (2dcc) either in deterministic [4] or stochastic [5] environment. owing to its popularity and availability in various geotechnical softwares, e.g. geo5 tunnel [6], the 2dcc analysis is often the choice even if addressing the tbm (tunnel boring machine) based excavation strategies. while in most applications the static loading conditions play the principal role in the design of tunnel lining or in assessing a potential failure during tunnel construction and when in service, the dynamic analysis in regions prone to the occurrence of earthquake may prove important. for a general and clear introduction to this subject the reader is referred to an excellent monograph by i. towhata [7]. when limiting our attention to the determination of lining forces caused by earthquake it is often sufficient to carried out a simplified pseudo-static analysis with the help of either analytical [8, 9] or fem based methods [10–12]. the principal concern in connection to a fully dynamic analysis, regardless whether 2d or 3d, is the representation of infinite boundary in the horizontal direction. assuring that horizontally propagating waves are not reflected back to the domain requires implementation of appropriate boundary conditions along vertical boundaries. these are typically in the form of viscous dampers relating the boundary tractions to velocities of the propagating waves. clearly, absorbing these waves at the boundary has the same effect as if transmitting them with no restrictions. the most widely used absorbing/transmitting boundary conditions are those introduced by lysmer and kuhlemeyer [13]. if represented by these boundary conditions the boundary absorbs the entire wave propagating across this boundary. when modeling the response of structure to earthquake it appears more appropriate to absorb only the portion of energy associated with the perturbation from so called free field conditions, the state preceding any construction. such perturbation can be attributed to, e.g. the built above-ground or underground structure such as embankment or tunnel. such boundary conditions, termed henceforth the free-field boundary conditions, were put forward by zinkiewicz, et al. [14]. to acquire the necessary free field response they build upon the solution of one-dimensional (1d) free field column problem [10, 12, 14]. point out that free-field boundary conditions should become active only if there are other than vertically propagating waves. thus to properly represent the results of 1d free field column analysis within a 2d environment calls for other types of boundary conditions that assure, in case of verti74 http://dx.doi.org/10.14311/app.2018.15.0074 http://ojs.cvut.cz/ojs/index.php/app vol. 15/2018 influence of boundary conditions in earthquake analysis cally propagating shear weaves, the stress state the same along every vertical section in the 2d model as derived from 1d analysis. starting with a brief introduction to a theoretical background in section 2 we continue in section 3 with a simple parametric study on the effect of various types of potential boundary conditions applied along the lateral boundaries of the analyzed domain. the most appropriate boundary conditions are then tested on an illustrative example of ground excavation in section 4. the essential findings are finally summarized in section 5. 2. theoretical background suppose that a simple homogeneous rectangular domain is loaded at its boundary by a vertically propagating shear weave as schematically shown in fig. 1(a). the terms uu, ud stand for the upward and downward traveling waves, respectively. we further assume an infinitely stiff base rock so the total motion in the form of outcrop motion can be applied at the bottom boundary. (a) (b) figure 1. (a) schematic representation of loading through outcrop motion, (b) loading and boundary conditions applied to 1d and 2d domains. the displacement field in space and time can then be decomposed as u(x, t) = u0(t) + ur(x, t), (1) where at the bottom boundary it holds u(x, t)|y=0 = u0(t), ur(x, t)|y=0 = 0. (2) the discretized form of the equation of motion then reads mür(t) + cu̇r(t) + kur(t) = −mü0(t), (3) where m,c,k are the mass, damping and stiffness matrices, respectively. note that no material damping is assumed so that the damping matrix c collects only the viscous dampers applied along the domain lateral boundaries as shown in fig. 1(b). therein, the viscosity cp = √ eoed/ρ corresponds to the pressure wave velocity whereas cs = √ g/ρ is associated with the shear wave velocity; ρ,eoed, g stand for the soil density and oedometric and shear moduli, respectively. further details can be found in [13–15]. the time integration of eq. (3) is typically performed with the help of newmark integration scheme. as already mentioned in the introductory part the analysis of 1d free field column problem seen in fig. 1(b) is needed to provide free-field velocities u̇f f r and shear tractions py = τxy. in the present study, a simple impulse plotted in fig. 2 was considered in both 1d and 2d analyses. (a) (b) figure 2. prescribed impulse: (a) acceleration, (b) displacement. to eliminate any potential errors caused by finite element discretization we assumed an optimal element size defined such that for the selected time step of 0.01s the shear wave passes through only just one element, i.e. lelem = cs∆t = √ g/ρ∆t =√ 28.6× 106/1960 · 0.01 = 1.2 m. the potential error arising from discretization if deviating from an optimal element length is clearly visible in fig. 3. thus only the optimal element length was adopted when performing all relevant calculations. one such analysis considers a 1d free-field column to provide results to be reproduced by a 2d analysis discussed in the next section. example of time variation of horizontal displacements of two selected points (on surface and at the center of the domain) is plotted in fig. 4 for illustration. 75 v. pavelcová, t. poklopová, t. janda, m. šejnoha acta polytechnica ctu proceedings (a) (b) figure 3. influence of finite element discretization. distribution of horizontal displacement u along vertical direction at time: (a) t = 0.4 s, (b) t = 4.0 s. (a) (b) figure 4. time variation of horizontal and vertical displacements u, v: (a) on surface, (b) at the center of the domain. as seen, the vertical displacements are clearly zero as only the horizontal displacements in terms of acceleration ü0(t) are prescribed to the entire domain. owing to the fixed boundary ur(0, t) = 0 and no material damping, the wave is free to bounce back to the domain when arriving at the bottom boundary. also note that the top boundary, free of shear stress, amplifies the propagating shear wave at the boundary two times in comparison to the wave within the domain, see e.g. [7]. 3. parametric study on the influence of lateral boundary conditions to see how well may various boundary conditions represent the free-field response we perform a simple parametric study. to that end, a rectangular homogeneous domain with the shear modulus g = 28.6 mpa, the poisson ratio ν = 0.4 and the density ρ = 1960 kg/m3 is considered. as in the previous section we reserve ourselves to fixed boundary at the bottom and loading conditions specified in fig. 2. in particular, four types of boundary conditions plotted in fig. 5 are examined in a sequel. (a) (b) (c) (d) figure 5. selected boundary conditions along lateral edges: (a) fixed vertical displacement v(t) = 0 (bc1), (b) free boundary-no restrictions (bc2), c) freefield boundary conditions (bc3), d) static boundary conditions py(t) prescribed (bc4). 3.1. fixed boundary conditions (bc1) with reference to the results provided by free field column analysis we begin by prescribing a kinematic constrain in the vertical direction, fig. 5(a). such boundary conditions assure zero vertical displacement along lateral edges while generating, through the reaction forces, the expected non-zero shear stress τxy. as evident from fig. 6, these are the correct boundary conditions producing the same response as the 1d free field column analysis. this is supported by the distribution of the shear stress found at the end of analysis in fig. 10(a). unfortunately, such boundary conditions are generally not acceptable as they do not allow for transmitting the horizontally propagating wave caused, e.g. by some structural perturbation. thus application of such boundary conditions could only be justify in cases when the lateral boundaries are sufficiently far 76 vol. 15/2018 influence of boundary conditions in earthquake analysis from the point of interest, e.g. tunnel, and the reflected horizontal wave would not interfere with the incoming wave. however, such a requirement may lead to computationally demanding geometrical models. creating unstructured meshes with coarsening in the vicinity of lateral boundaries as typical of static analysis may not be the solution bearing in mind the mutual connection between the integration time step and the element size. such a boundary condition is, therefore, generally not recommended. (a) (b) figure 6. time variation of horizontal and vertical displacements u, v: (a) on surface, (b) at the center of the domain. 3.2. free boundary (bc2) just for illustration we consider next a free boundary with no restrictions. similar to terrain surface, such a boundary requires the shear stresses along lateral edges be zero. this in turn generates unacceptable vertical displacements within the whole domain and yield a nonuniform distribution of the shear stress as evident in fig. 10(b). the evolution of ineligible vertical displacements is further seen in fig. 7(b), which generates a strong deviation of the horizontal displacement in every section of the 2d domain when compared to the 1d free field column analysis. for the sake of clarity, the influence of free boundary on the results pertinent to surface point on the left lateral edge are reported in fig. 7(a). the free boundary is therefore a totally incorrect boundary condition, even if the lateral boundaries are sufficiently far. (a) (b) figure 7. time variation of surface displacements: (a) horizontal displacement u, (b) vertical displacement v at the edge. 3.3. free-field boundary conditions (bc3) recall that free-field boundary conditions in fig. 5(c) should allow for transmitting the structural or material perturbation only causing, as will be seen in section 4, e.g. a horizontally propagating shear weave. however, no such perturbation is present in the present example. so both viscous dampers (the horizontal one transmitting the horizontally propagating pressure wave, the vertical one transmitting the horizontally propagating shear wave) should remain inactive during the analysis. however, the formulation of damper does not distinguish between the type and direction of the propagating wave. with reference to the previous section it is therefore expected the vertical damper to become active as attempting to damp the difference between the 1d (v = 0) and 2d (v 6= 0) analyses. this is evident from fig. 8(b) showing the variation of surface vertical displacement being 10 times smaller than in case of free boundary examined in section 3.2. the results, fully compatible with the 1d free field column analysis, could thus be achieved by setting the shear viscosity cs to infinity, see [15]. clearly, the infinite viscosity acts in this case as a penalty forcing the velocity of vertical displacement to zero thus in turn generating, similar to section 3.1, the nonzero shear stresses. nevertheless, even with damper of finite viscosity the horizontal displacements in fig 8(a) are repre77 v. pavelcová, t. poklopová, t. janda, m. šejnoha acta polytechnica ctu proceedings sented fairly well. similarly, the distribution of shear stresses displayed in fig. 10(c) is considerably improved. but keep in mind that the action of vertical damper is in this case purely artificial, as it is activated for purpose, for which it is not actually designed. (a) (b) figure 8. time variation of surface displacements: (a) horizontal displacement u, (b) vertical displacement v at the edge. 3.4. static boundary conditions (bc4) intuitively, and recalling the discussion in section 3.1, the most appropriate boundary conditions to reproduce the 1d free field column analysis are static boundary conditions represented by the prescribed shear tractions as shown in fig. 5(d). it has been demonstrated in [15] that such a boundary conditions yield identical displacements and stresses as produced with the bc1 type of boundary conditions, even if combined with the free-field boundary conditions (bc3), which in this case remain inactive. figure 9. periodic boundary conditions. it is interesting to point out that similar results would be derived with the application of periodic boundary conditions [14] schematically depicted in fig. 9. these boundary conditions, widely used in the field of homogenization [16], are essentially the tying constrains which force the homologous displacements on the opposite sides of the analyzed domain the same. [kpa] (a) [kpa] (b) [kpa] (c) figure 10. distribution of shear stress τxy at the end of analysis for different lateral boundary conditions: (a) bc1 and bc4, (b) bc2, (c) bc3. 4. example to illustrate the effect of combined free-field and static boundary conditions to demonstrate the applicability of the proposed boundary conditions we consider a geometrical model plotted in fig. 11(a) and loaded by the same impulse as used in the previous section, recall fig. 2. in particular, the effect of perturbation caused by the 90◦ wedge will be examined. in this particular example it is expected that the upward traveling shear wave, when hitting the wedge boundary, will be reflected in the 90◦ angle to become a horizontally propagating shear wave as illustrated in figs. 11(b,c). to appreciate the action of free-field boundary conditions we begin with the static boundary conditions only and follow the time evolution of the vertical and horizontal components of the displacement vector linked to one particular point marked in fig. 11(a) 78 vol. 15/2018 influence of boundary conditions in earthquake analysis (coordinates [40,30]). the results appear in fig. 12. it is clear from fig. 12(b) that in view of the horizontally propagating shear wave the horizontal boundary behaves as a free boundary and reflects the wave back into the domain. because the bottom boundary is assumed fixed, the wave is trapped in the domain, and in case of no material damping will continue to infinitely move between the two boundaries. the fact that the impulse is applied along the entire bottom boundary promotes interaction of individual waves which in turn is manifested by a continuous rise of the wave amplitude. (a) (b) (c) figure 11. testing boundary conditions: (a) geometrical model, (b) static boundary conditions assumed only, (c) combination of free-field and static boundary conditions. a considerably different response is observed when properly combining the static boundary condition with the free-field boundary condition, see the red curves in figs. 12. as evident from fig. 12(b), the latter boundary condition is able to transmit, damp, the horizontally propagating incoming wave across the lateral edges, so it is not reflected back. the subsequent vibration is again the result of interaction of the neighboring waves. the continuous vibration of the tracked point in the horizontal direction, see fig. 12(b), is caused by continuously traveling shear wave in the vertical direction caused again by the presence of fixed boundary assumed at the bottom of the model. to bring the results closer to reality one would have to either introduce the material damping or the absorbing boundary condition at the bottom boundary, see e.g. [11, 12, 15]. 5. conclusion application of various boundary conditions along vertical edges of a rectangular domain was critically exam(a) (b) figure 12. time variation of the displacement of the marked point: (a) horizontal displacement u, (b) vertical displacement v. ined in this paper. it was proved first in section 3 that static boundary conditions are the correct boundary conditions to represent the 1d free field column analysis in 2d simulations. these boundary conditions were then combined in section 4 with the free-field boundary conditions to allow for properly transmitting the horizontally propagating shear weave across the lateral boundaries perturbed in this particular case by an excavated wedge. the ability of free-field boundary conditions to absorb such waves has been confirmed. however, the performance of these absorbing conditions in cases of incident angles other than 90◦ is still an open issue and will be examined in our future studies. acknowledgements the support provided by the sgs project no. sgs18/037/ohk1/1t/11 and the tačr project no. te01020168 is gratefully acknowledged. references [1] t. janda, m. šejnoha, j. šejnoha. application of converegence measurements in 2d analysis of tunnel excavation in comparison with convergenece confinement method. tunel 19(4):52–57, 2010. [2] t. janda, m. šejnoha, j. šejnoha. modeling successive excavation within two dimensional finite element mesh. acta geodynamica et geomaterialia 8(1):69–78, 2011. [3] t. janda, m. šejnoha, j. šejnoha. modeling of soil structure interaction during tunnel excavation: an engineering approach. advances in engineering 79 v. pavelcová, t. poklopová, t. janda, m. šejnoha acta polytechnica ctu proceedings software 62-63:51–60, 2012. special issue dedicated to professor zdeněk bittnar on the occasion of his seventieth birthday: part i. [4] t. svoboda, d. mašín. comparison of displacement field predicted by 2d and 3d finite element modelling of shallow natm tunnels in clays. geotechnik 34(2):115–126, 2011. [5] t. janda, m. šejnoha, j. šejnoha. applying bayesian approach to predict deformations during tunnel construction. international journal for numerical and analytical methods in geomechanics 42:1765–1784, 2018. [6] fine ltd. geo 5 civil engineering sowftware. [online]: www.fine.cz. [7] i. towhata. geotechnical earthquake engineering. springer-verlag berlin heidelberg, 2008. [8] j. penzien. seismically induced racking of tunnel linings. earthquake engineering and structural dynamics 29:683–691, 2000. [9] y. hashash, j. hook, b. schmidt, j. yao. seismic design and analysis of underground structures. tunneling and undergrounding space technology 16:247–293, 2001. [10] d. kučera. posouzení geotechnické konstrukce na seismické zatížení metodou konečných prvku. master’s thesis, czech technical university in prague, faculty of civil engineering, 2017. [11] t. poklopová. evaluation of real underground structure subjected to earthquake pseudostatic analysis. bachalor’s thesis, czech technical university in prague, faculty of civil engineering, 2018. in czech. [12] t. poklopová, v. pavelcová, t. janda, m. šejnoha. evaluation of real underground structure subjected to earthquake pseudostatic approach. acta polytechnica 2018. accepted. [13] j. lysmer, r. kuhlemeyer. finite dynamic model for infinite media. journal of the engineering mechanics division, proceedings of the american society of civil engineers 95(em4):859–877, 1969. [14] o. zinkiewicz, n. bicanic, f. shen. generalized smith boundary a transmitting boundary for dynamic computations, vol. 207. innme swansea, 1986. [15] v. pavelcová. evaluation of real underground structure subjected to earthquake fully dynamic analysis. bachalor’s thesis, czech technical university in prague, faculty of civil engineering, 2018. in czech. [16] m. šejnoha, j. zeman. micromechanics in practice. wit press, southampton, boston, 2013. 80 acta polytechnica ctu proceedings 15:74–80, 2018 1 introduction 2 theoretical background 3 parametric study on the influence of lateral boundary conditions 3.1 fixed boundary conditions (bc1) 3.2 free boundary (bc2) 3.3 free-field boundary conditions (bc3) 3.4 static boundary conditions (bc4) 4 example to illustrate the effect of combined free-field and static boundary conditions 5 conclusion acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.38.0635 acta polytechnica ctu proceedings 38:635–641, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague how municipalities should approach the transformation of public spaces michaela maláa, martina sýkorováa,∗, licia felicionia,b a czech technical university in prague, university centre for energy efficient buildings, třinecká 1024, 273 43 buštěhrad, czech republic b czech technical university in prague, faculty of civil engineering, thákurova 2077/7, 166 29 prague, czech republic ∗ corresponding author: martina.sykorova@cvut.cz abstract. public spaces are the living rooms of cities, accessible to everyone without any restrictions. public spaces are the calling card of the city; they are where the community comes together. their design either enables or complicates community life. from a visitor’s point of view, public spaces are the first things to be noticed and encountered in a city. due to various circumstances, public spaces sometimes fail to meet the abovementioned features or do not fulfil them to the degree they should. thus, the presented methodology guidelines may help small cities representatives who do not have the professional or personnel capacity to deal with public spaces through planning, preparation and contracting the public spaces studies, which leads to a quality assessment of public spaces. the guidelines also include tips for improving public spaces, recommended practices for public participation in planning the transformation of public areas, and a site assessment form for a non-expert’s perspective. keywords: public space, public participation, applied research, interdisciplinary cooperation, guidelines. 1. introduction the city is a complex system that can be seen through a prism of many disciplines – from urban planning and geography, legal science and history, to social science [1]; it is a multifaceted system composed of the physical environment and people and their interactions. the city’s location and historical and demographic development also play a vital role. from the point of view of social sciences, a city is a territorial community [2]. even in this definition, it is noticeable that it is a multidisciplinary concept. it may seem paradoxical since it is not usual for this basic theoretical consideration of the city to be practically applied in planning human settlements and public spaces, which are the intrinsic essence of cities. the design of urban spaces is primarily based on architects’ perspectives and intuitions; designers usually perceive life in human settlements and provide for the end-users needs (e.g., different groups of inhabitants, visitors, workers) without inviting experts from the social sciences for reflecting on the creation or transformation of urban places and cities. this awareness led to linking urban planning and sociological expertise in the context of public space planning. thus, at the university centre for energy efficient buildings (uceeb) of the czech technical univesity in prague (ctu), czechia, a multidisciplinary team was formed, and it developed the socalled “methodology for planning public spaces in small cities” in order to merge the knowledge of different fields. this methodology guides the representatives of small municipalities through the planning process, from the preparation and commissioning to more specific studies and analysis of public spaces. this method aims to facilitate the initial orientation in the issue and negotiations with experts, state administration and users of public spaces – the methodology has been freely available in electronic form on the http://www.atraktivniobec.cz/ website since march 2021. the methodology guidelines are intended for the general public, including representatives of small communities without architectural or sociological backgrounds. these people often have no experience with architecture or participatory methods; however, they are in charge of deciding the future directions of the transformation of public spaces and setting the parameters for selecting a suitable contractor. this is also what makes the methodology guidelines different from many other methodologies that have been developed in the last decade for the needs of the public administration [3] or professionals [4, 5] in the czech environment. the already implemented methodologies are very beneficial for the construction culture in the czech republic, but those are written in a professional language that is not understandable to the general public. the originality of the “methodology for planning public spaces in small cities” is based on the establishment of sequential steps in the process of planning the transformation of public space, which are explained in a clear and comprehensible way for the general public. the methodology also con635 https://doi.org/10.14311/app.2022.38.0635 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en http://www.atraktivniobec.cz/ m. malá, m. sýkorová, l. felicioni acta polytechnica ctu proceedings tains several original educational tools that provide deeper insight into architectural issues (i.e. tips for improving public spaces and the on-site public space assessment form) or public participation (i.e. public participation in planning the transformation of public space). 1.1. scope of the paper the aim of this paper is to present the “methodology for planning public spaces in small cities” and, in particular, one of its unique tools (the on-site public space assessment form) for setting up better cooperation and discussion between relatively distant groups: (i) experts in urban planning and public participation, and possibly representatives of the state administration on the one side and (ii) representatives of small municipalities and their inhabitants on the other side. the methodology is the output of a three-year applied interdisciplinary research project that aims to guide potential stakeholders through creating and validating an urban planning process. 2. planning human settlements and public spaces – taking the user perspective into account the theoretical part of this manuscript introduces potential stakeholders to human settlement planning, its development and its context. it presents public spaces as a phenomenon in which two components are combined – the material or physical component, i.e. the form, and the social component, i.e. life and activities in urban areas. it also introduces several approaches to assessing the quality of both physical and social aspects of public spaces. 2.1. the material form of cities, their development and transformation against the background of social changes and tendencies the material form of cities, and consequently, public spaces, is based on the needs of the inhabitants. cities were founded in places with the best economic, transport and natural conditions available. man-made features also had a significant influence on choosing an optimal site. in this sense, important trade routes, their branching or crossing, played a primary role. these places also offered suitable outlets for artisans and farmers’ products and became centres of trade and regular market places [6]. the transformation of cities’ physical environment, which began in the 19th century, is usually a response to technological development or the evolution of society and its needs. gradually, on one side, along with the technological development, the importance of specific functions (e.g., fortification) got lost; on the other side, emphasis was placed on improving the quality of urban life (such as improvement of sanitary conditions or the needed space for everyday recreation) or on responding to socio-political changes (such as urbanisation due to industrialisation, suburbanisation, revolutionary movements, and wars). these tendencies were manifested by the transformation of the urban structure (greater spacing of buildings, their orientation and height) and, inevitably, also in the character and perception of public spaces (the need for new types of public spaces, such as park squares, waterfronts, public spaces of housing estates, boulevards, and cemeteries) [7]. gehl [8] pointed to a “clearly physically and materially oriented planning ideology” that neglected the psychological and social aspects of the design of buildings and public spaces. in cities or parts of cities planned within functionalist visions in the 1950s and 1960s (e.g., condominiums in toronto or municipal buildings in the former east berlin), there was a reduction of social activities in public spaces, even if this was not intentional. this result was mainly due to the great distances between people, events, functions, and needs [8]. the 1960s were thus marked by a move away from the discussion with local stakeholders and the public, and urban design, including the design of public spaces, was entirely the responsibility of urban planners [9]. the turn away from the modernist planning approach has long been evident worldwide [10, 11]. however, in the czech environment, we still encounter insufficient space for discussion in transforming public spaces. “the methodology for planning public spaces in small cities” responds to this situation and, beyond simply describing the process of preparing and commissioning the transformation of public spaces, offers unique tools to stimulate discussion among municipal representatives and experts (on-site public space assessment form), as well as with people among themselves (public participation in planning the transformation of public space). 2.2. public spaces as the intersection of the material form of place and life in it neither of these public realm components (physical and social) should be neglected, and they should be considered equally in the transformation of a place. public spaces should always be designed for people and their needs, not the opposite, i.e. people adapt themselves and their needs to the environment [7, 8]. thus, it can be argued that the connection between physical and social aspects is a fundamental prerequisite for designing attractive public spaces. “when we talk about public space, we must always keep in mind both the physical and the living aspects and the interrelationship between them.” [9] 636 vol. 38/2022 how municipalities should approach the transformation of public spaces 2.3. approaches to assessing the quality of public spaces specialists in the physical and social aspects of public spaces, respectively urban planners and public involvement planners seek answers to elementary questions by setting up different evaluation approaches – for example, why is one public space attractive to its inhabitants and another not? – however, there are several approaches to assessing the quality of the physical and social aspects of public spaces. a few of them are described in the following sections. 2.3.1. william h. whyte’s approach to the assessment of public spaces william h. whyte was a pioneer in thinking about public spaces in cities and their functioning. whyte’s research focused on new york city and explored why some public spaces are frequented while others are not. he sought to answer this question through participant observation and interviews, which he supplemented with a time-lapse film and photographic footage (his research also resulted in the 1980 documentary film “social life of small urban spaces”). his social-behavioural analysis of public spaces aimed to formulate recommendations that would help urban planners design and create attractive public areas [12–14]. “the significance of whyte’s research was not only that it turned planners’ attention to how people actually use public spaces, and initiated other similar research in other countries. his undisguised enthusiasm for the varied life of new york’s streets was part of his promotion of a return to inner cities [. . . ].” [12] whyte’s work was followed by several other scholars who further developed his theory. two of the more notable works were done respectively by the non-profit organisation project for public spaces (pps) [15–17], which has been working on transforming public spaces with the involvement of the users since 1975, and by the danish urbanist jan gehl. 2.3.2. approach to the evaluation of public spaces in the document “public spaces in pilsen – methodology for access to public spaces.” for the purpose of this work, it has been selected the document “public spaces in pilsen – methodology for access to public spaces” [18], one of the czech approaches that deal with the theoretical [4] and practical [19] assessment of the quality of public spaces. this approach, which is taken as best practice by the methodology for planning public spaces in small cities (see section 3), is the preferred one by the authors – they identified their method in this approach. figure 1 shows some outputs that emerged from assessing the quality criteria (such as the offer of available activities, microclimate, and social security) of public spaces following this approach. the case figure 1. ray diagram showing the assessment of the quality of the public space that surrounds smetana park in pilsen, czechia. figure readapted by authors from sedlak et al. [18]. study chosen is the public space surrounding smetana park in pilsen, czechia. the public space components are rated according to a point-based system: five points represent the highest quality solution, while zero points represent the lowest quality solution. the evaluation of the quality criteria is not based on an exact methodology or procedure (quantitative assessment) but instead on the knowledge and experience of the evaluator – it is a qualitative assessment made by experts. the dark green colour in the graph highlights the quality current state of the public space, and the light green colour displays the desired target. 3. methodology for planning public spaces in small cities 3.1. use of knowledge from urban and social science surveys the cause and result of the interdisciplinary urbanisticsociological approach to public spaces is the abovementioned methodology for planning public spaces in small cities designed by the uceeb team. during the qualitative research aimed at learning experiences of municipality leaderships and their agendas targets, it became clear that the original intention of targeting the methodology to cities with fewer than 20 000 people should have been abandoned. municipalities with 20 000 inhabitants are still large cities in the context of the czech environment. in these cities, several specialised authorities administer the wider area; they are “municipalities with extended jurisdiction” (in czechia, they are named by the abbreviation orp – obcí s rozšířenou působností). thus, the methodology proved to be much more helpful and desirable among small cities (up to 5 000 inhabitants), such as municipalities with no extended jurisdiction. the reason is that these small cities 637 m. malá, m. sýkorová, l. felicioni acta polytechnica ctu proceedings figure 2. bar chart showing the number of downloads of the methodology by population in the municipalities. figure 3. (a) bar chart showing the evaluation of the methodology in terms of clarity. (b) bar chart showing the evaluation of the methodology in terms of usefulness. do not have sufficient professional or personnel capacity to deal with municipal public spaces, as they have a broad agenda of very diverse topics under their responsibility. this observation is followed by the authors’ attempt to write the methodology as straightforward as possible, emphasising the clarity of the information presented. this is also reflected in the number of pages of the methodological text itself, around 50 pages of structured text. the assumption regarding the target group was confirmed not only during semi-structured interviews with representatives of the authorities and municipalities but also during the implementation of the methodology guidelines. the methodology is freely avaialble for download from the website http://www.atraktivniobec.cz/. considering the period from april to august 2021, it has been accessed by 420 municipalities, 307 of which were representatives of municipalities with a population that counts less than 5 000 inhabitants. figure 2 represents the number of downloads of the methodology by population in the municipalities in the period from april to august 2021. the authors’ goals of text clarity and perceived usefulness of the methodology guidelines were also met. feedback on the methodology (which was collected through questionnaires in october 2021 – number of respondents: 65) showed that 74 % of respondents rated the methodology guidelines as understandable – rating 6 and 7 (figure 3a), and 64 % rated it as useful – rating 6 and 7 (figure 3b). the practicality of the methodology guidelines was also highlighted in the questionnaires. here we have selected one of the quotes from a municipality representative: “from the perspective of a non-fulltime municipality representative, it offers me a professional, substantive, informative, legislative background, including methodological assistance, guiding steps, and essentials that we must not forget.” 3.1.1. first annex to the methodology: on-site public space assessment form the primary purpose of this methodological text is to invite mayors and other representatives of municipal authorities to become equal speakers in the dialogue with irreplaceable actors of this process in the ranks of architects, planners, sociologists, etc. the main idea is not to make them experts in spatial planning, communication or public participation but to induce them in an understandable and time-saving way to relevant topics or specific issues that they would need to discuss with experts when planning strategies to transform a given public space. therefore, this methodology includes unique materials for municipal representatives (tips for improving public spaces, the on-site public space assessment form and the public participation in planning the 638 http://www.atraktivniobec.cz/ vol. 38/2022 how municipalities should approach the transformation of public spaces transformation of public space), which will serve as tools for discussions with experts and people. this material was developed by combining expertise in urban planning and social sciences in the context of small towns. the main intersection of interdisciplinary collaboration is thus reflected in the on-site public space assessment form and in a document that introduces municipal representatives to public involvement in planning the transformation of public spaces and allows them to view space through the lens of an urban planner. it stimulates the thought process of evaluating a specific public space. another purpose of the form is to encourage those stakeholders to imagine and think about the space, so that they become more equal partners during discussions with urban planners when it is time to plan the transformation of a particular public space and formulate their ideas and requirements. this unique tool was created by an urban planning expert who provided information on how public spaces are evaluated in practice (i.e. what aspects are monitored and how they are evaluated). in doing so, the expert drew on different methods established to assess the quality of public spaces by urban planners and social scientists that are presented in section 2 of the paper. moreover, it was also based on the results gathered in the field and by questionnaire surveys in three small czech cities (drahelčice, mnichovice and bustěhrad – total number of respondents: 306) carried out in 2018. the results indicated that resident respondents did not assess the conceptual links in the public realm (i.e. sightlines, permeability, vibrancy and relationship between the building parapet and the public realm) and were unable to assess the appropriate character of the public areas (with reference to history, function and typology). on the contrary, they were competent to determine the safety and maintenance of the public space plus the traffic design (e.g., traffic volume, number and location of parking spaces, the safety of crossing, condition and width of pavements) and the presence of green spaces and urban furniture [20]. it can be summarised that inhabitants do not perceive what the urban planner forecasts and designs in public spaces, but they focus more on what influences their daily user’s experience of a specific place. however, this result is legit – citizens are not experts in the typology of public spaces, buildings or benches’ location, but they are in community life. the same outcome has been registered regards the municipal leaders. indeed, the mayor and his/her deputy are also “just” residents of the city, but simultaneously, people responsible for the initial reflections on the current state of public spaces in their city and the subsequent decisions that will lead to their transformation. this is also why the previous mentioned on-site public space assessment form has been created; it intends to guide municipal representatives in viewing public spaces from the perspective of an urban planner. it would help them understand what these experts research, assess and try to achieve. nevertheless, filling out the form is not a substitution for an expert’s site assessment, but it should show municipal representatives what needs to be considered and how complex the issue is in specific public spaces. based on a mutual discussion between the urban planner and the social scientist, the public participation expert then translated the questionnaire into a language that the general public could understand. each category contains several questions designed to get municipal representatives to think about the public space differently than they used before. in order to determine whether the form fulfils this objective, whether the topics defined in the form correspond to the reality of small cities and whether the questions are understandable for municipal representatives and correctly formulated with their purpose, the form was tested in practice (specifically in our czech small cities partners – drahelčice, hrusice). figure 4 shows the on-site public space assessment form, thematic area usability and functions. as mentioned in section 2.3.2, the creation of the form was inspired by the methodological text “public spaces in pilsen. a methodology for accessing public spaces” [18]. the form is divided into five thematic areas: • type of assessed public space – the evaluator should define what type of public space it is (e.g. square, street, park). • usability and functions – the evaluator has to select what function the public space has (e.g. representative, commercial, sports) as well as what groups of people may visit the public space and what activities they would do there. • pedestrian and cyclist safety – the evaluator should describe how safe the public space is for pedestrians or cyclists. • comfort – the evaluator should describe how the public space is comfortable, whether it has a suitable microclimate, and whether there are benches or other furnishings. • accessibility and availability – the evaluator should describe how access to the public space is addressed for pedestrians, cyclists and disabled people. 4. conclusion the paper describes various urban and social science approaches, methods and reflections on the quality of public spaces and the benefits of using methods to engage people in planning public spaces. after the qualitative research and discussion with municipality leadership, the authors have designed the methodology for planning public spaces in small cities merging urban and social science knowledge. this methodology is addressed to mayors and deputies for helping plan and design attractive public spaces 639 m. malá, m. sýkorová, l. felicioni acta polytechnica ctu proceedings figure 4. sample of the on-site public space assessment form. in a time-saving way with the involvement of the residents. it has been an open-access source since march 2021. however, the methodology is not limited to describing the process of preparing the transformation of public spaces but also contains some unique educational annexes that aim to develop a discussion between experts in the field and the general public. one of these tools, the on-site public space assessment form, has been presented in detail in this article. acknowledgements this work has been supported by tačr (technology agency of the czech republic) – contract number tl01000555. references [1] m. petrusek (ed.). malý sociologický slovník. svoboda, praha, 1970. 614 p. [2] m. petrusek (ed.). velký sociologický slovník. karolinum, praha, 1970. 1627 p. isbn 80-7184-311-3. [3] územní studie veřejného prostranství: metodický návod pro pořízení a zpracování. 2. vydání, revidované. mmr čr, odbor územního plánování, praha, 2018. 24 p. eisbn 978-80-7538-201-6. http://www.uur.cz/images/8-stanoviska-ametodiky/od-01-01-2018/21-usvp-06112018.pdf [4] z. ambrožová. metodika hodnocení hlavních náměstí malých měst pro potřeby krajinářské architektury a rozvoje městského prostředí. mendelova univerzita v brně, zahradnická fakulta, ústav zahradní a krajinářské architektury, brno, 2016, [2018-11-06]. http://web2.mendelu.cz/zf_563_krarch/1.1.3_ metodika-hodnoceni-hlavnich-namesti-malychmest/certifikovana_metodika-hodnoceni-hlavnichnamesti-malych-mest_komplet.pdf [5] p. návrat. manuál participace: jak zapojit veřejnost do plánování města. ipr praha, praha, 2015. [2016-01-06]. https://iprpraha.cz/stranka/2656/manualparticipace-v-planovani-mesta [6] j. hrůza, j. zajíc. vývoj urbanismu. i. díl. vyd. 2. vydavatelství čvut, praha, 2002. 186 p. isbn 80-01-02551-9. [7] j. hrůza, j. zajíc. vývoj urbanismu. ii. díl. vydavatelství čvut, praha, 1997. 160 p. isbn 80-01-01549-1. [8] j. gehl. život mezi budovami: užívání veřejných prostranství. nadace partnerství, brno, 2000. 202 p. isbn 80-85834-79-0. [9] j. jacobs. the death and life of great american cities. random house, 1961. isbn 978-0-679-60047-3. [10] k. schmeidler, a kol. sociologie v architektonické a urbanistické tvorbě. vyd. 2. zdeněk novotný, brno, 2001. 292 p. isbn 80-238-6582-x. [11] j. gehl. města pro lidi. partnerství, o. p. s., brno, 2012. 261 p. isbn 978-80-260-2080-6. [12] p. kratochvíl. městský veřejný prostor. zlatý řez, praha, 2015. 188 p. isbn 978-80-88033-00-4. [13] w. h. whyte. the social life of small urban spaces. the conservation foundation, washington dc, 1980. 125 p. isbn 0-89164-057-6. https://doi.org/10.1177/089124168201000411 640 http://www.uur.cz/images/8-stanoviska-a-metodiky/od-01-01-2018/21-usvp-06112018.pdf http://www.uur.cz/images/8-stanoviska-a-metodiky/od-01-01-2018/21-usvp-06112018.pdf http://web2.mendelu.cz/zf_563_krarch/1.1.3_metodika-hodnoceni-hlavnich-namesti-malych-mest/certifikovana_metodika-hodnoceni-hlavnich-namesti-malych-mest_komplet.pdf http://web2.mendelu.cz/zf_563_krarch/1.1.3_metodika-hodnoceni-hlavnich-namesti-malych-mest/certifikovana_metodika-hodnoceni-hlavnich-namesti-malych-mest_komplet.pdf http://web2.mendelu.cz/zf_563_krarch/1.1.3_metodika-hodnoceni-hlavnich-namesti-malych-mest/certifikovana_metodika-hodnoceni-hlavnich-namesti-malych-mest_komplet.pdf http://web2.mendelu.cz/zf_563_krarch/1.1.3_metodika-hodnoceni-hlavnich-namesti-malych-mest/certifikovana_metodika-hodnoceni-hlavnich-namesti-malych-mest_komplet.pdf https://iprpraha.cz/stranka/2656/manual-participace-v-planovani-mesta https://iprpraha.cz/stranka/2656/manual-participace-v-planovani-mesta https://doi.org/10.1177/089124168201000411 vol. 38/2022 how municipalities should approach the transformation of public spaces [14] p. pospěch. od veřejného prostoru k nákupním centrům: svět cizinců a jeho regulace. slon, praha, 2015. 194 p. isbn: 978-80-7419-186-2. [15] project for public spaces. new york. [2020-11-20]. https://www.pps.org/ [16] project for public spaces. william h. whyte, 2010. new york. [2020-11-20]. https://www.pps.org/article/wwhyte [17] project for public spaces. what makes a successful place? new york. [2020-11-20]. https://www.pps.org/article/grplacefeat [18] r. sedlák, m. římanová, m. nawrath, et al. veřejná prostranství v plzni. metodika pro přístup k veřejným prostranstvím. partnerství, o. p. s, brno, 2014. 105 p., [2015-06-25]. https://ukr.plzen.eu/uzemni-planovani/verejnaprostranstvi/metodika/metodika.aspx [19] k. beránek. územní studie – veřejná prostranství města tábora. praha, 2018. [2018-06-27]. https://taborcz.eu/html/soubory/tabor_us_ cistopis_20180625_web.pdf [20] j. hainc. podkladová studie projektu: atraktivní obec: metodika plánování veřejných prostranství v digitálním věku. české vysoké učení technické v praze, univerzitní centrum energaticky efektivních budov, praha, 2018. 641 https://www.pps.org/ https://www.pps.org/article/wwhyte https://www.pps.org/article/grplacefeat https://ukr.plzen.eu/uzemni-planovani/verejna-prostranstvi/metodika/metodika.aspx https://ukr.plzen.eu/uzemni-planovani/verejna-prostranstvi/metodika/metodika.aspx https://taborcz.eu/html/soubory/tabor_us_cistopis_20180625_web.pdf https://taborcz.eu/html/soubory/tabor_us_cistopis_20180625_web.pdf acta polytechnica ctu proceedings 38:635–641, 2022 1 introduction 1.1 scope of the paper 2 planning human settlements and public spaces – taking the user perspective into account 2.1 the material form of cities, their development and transformation against the background of social changes and tendencies 2.2 public spaces as the intersection of the material form of place and life in it 2.3 approaches to assessing the quality of public spaces 2.3.1 william h. whyte's approach to the assessment of public spaces 2.3.2 approach to the evaluation of public spaces in the document ``public spaces in pilsen – methodology for access to public spaces.'' 3 methodology for planning public spaces in small cities 3.1 use of knowledge from urban and social science surveys 3.1.1 first annex to the methodology: on-site public space assessment form 4 conclusion acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.35.0008 acta polytechnica ctu proceedings 35:8–13, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague digital track map for the vexa expert system adam hlubuček czech technical university in prague, faculty of transportation sciences, department of transport telematics, konviktská 20, 110 00 prague 1, czech republic, prague, czech republic correspondence: hlubuada@fd.cvut.cz abstract. this paper aims to summarize possible solutions of a digital track map proposal, designed to become a data source for the vexa decision-making system. the vexa project is carried out in cooperation of the department of applied mathematics at the czech technical university in prague – faculty of transportation sciences and the ažd praha s. r. o. company. the digital track map is an indispensable data source describing the railway line intended for the vexa supervised trains operation and its surroundings. being created in the arcgis pro software environment, the digital map allows individual objects to be visualized and handled as standard gis features and described by related data stored in a geodatabase. depending on the object class, the object location can be expressed in different ways. not only physical location (directly expressible using gis features) but also functional location (as the objects relate to a particular track) must be considered. keywords: digital track map, vexa, autonomous railway vehicle, expert system, gis. 1. introduction the effort to reduce the influence of the critical human factor in the railway sector leads to the development of numerous process information systems. such systems include, for example, automatic train control systems, automatic train operation systems, various forms of railway traffic management systems and decision-making systems. whereas many of these systems require accurate infrastructure data, the need for thorough description of the railway infrastructure is growing. this is not different in the case of the expert system that is being developed within vexa. the vexa research project is carried out in cooperation of the department of applied mathematics at the czech technical university in prague – faculty of transportation sciences and the ažd praha s.r.o. control and signalling systems supplier. the vexa expert system itself consists of several interconnected modules providing various functionalities leading to perform the tasks of an autonomous railway vehicle. based on the evaluation of defined inputs, it aims to substitute decision-making processes of a train driver. these inputs can be outputs from detectors (e. g. obstacle, fire) and status reporting systems (as regards the individual modules) and track condition monitoring. with the use of a knowledge base and decision rules (railway operation rules, decision algorithms taking into account the driver’s behaviour), vexa is supposed to evaluate the conditions for running a railway vehicle in order to grant a permit to start moving or to issue a request to stop the train. with the use of the vexa system, it is expected to reach the grade automation of the level goa3 or goa4 (i. e. driverless or unattended train operation) [1, 2]. in order to provide the vexa system with information concerning the environment in which the train is running, the digital track map is being developed. the map is supposed to describe the railway line intended for the vexa supervised trains operation as well as its surroundings. the digital track map data represent one of the important inputs for the vexa system decision making processes. among others, the data will be compared with detected scene in order to recognise unwanted objects. 2. logical architecture of the vexa system the onboard vexa unit is supposed to consist of four following modules: • perception input module • map matching module • incident analysis module • incident solver module [1] the perception input module is responsible for aggregating inputs from train detectors of different types. as the set of sensors may differ for individual implementations of an autonomous train, the module is being designed to be configurable. the map matching module is in charge of managing information about the known surroundings of the autonomous train. based on the unit location information, it provides a comparison of the data obtained from the perception input module with the digital track map data. the incident analysis module is designed to process the data related to the objects evaluated by the map matching module as potentially unwanted and additional information from detectors. the data are 8 https://doi.org/10.14311/app.2022.35.0008 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 35/2022 digital track map for the vexa expert system figure 1. logical architecture of the vexa system [1] analysed using a set of deterministic rules forming a nested multi-criteria expert system. on the basis of classification, it decides on the choice of reaction that is necessary to prevent or mitigate the impact of a potential incident. finally, the incident solver module is responsible for performing the correct reaction to possible incidents that the incident analysis module was unable to eliminate. the module includes relevant rules that define how to react to a given type of accident. as regards the vexa system surroundings, the perception subsystem includes various detectors providing data to the perception input module. among others, it is the object detector and the detectors of train control management system. the interface of other modules to various railway systems is provided by the vexa – train systems communication hub and vexa – trackside communication hub. the map matching module is also supposed to receive data from global navigation satellite system and to communicates with the digital track map. the logical architecture of the vexa system is shown in the figure 1. at the same time, the expected data flows are shown there [1]. 3. recognition of potentially dangerous objects as already outlined, the purpose of the digital track map is to provide the vexa system with the data describing the infrastructure used to operate the autonomous vehicle and its intermediate surroundings. even though the digital track map is not a part of vexa itself, it is being developed in close collaboration with the system in order to provide it with required inputs. whereas one of the vexa system goals is to detect unwanted objects around the railway line and especially on the running track, the map serves as a basis for comparison against the scene recognised on the basis of inputs captured by the means of onboard devices. while these inputs are supposed to be processed by the perception input module of the vexa system, the map matching module is responsible to compare the map with the recognised scene. for this reason, the map is supposed to include the location of expected objects around the railway line which are to be evaluated as harmless when being compared to the scene. the map matching module evaluates whether detected objects need to be classified as potentially dangerous. this case occurs if there is no corresponding data object included in the digital track map while a real object is detected by train sensors in a specific location. the information about the potentially dangerous object is to be provided the incident analysis module to decide on and carry out an appropriate reaction which affects the behaviour of the train. in the event of an irregularity being detected that cannot be solved by the incident analysis module and an accident threatens to occur, the incident solver module must also receive the appropriate data to take action [1]. 4. requirements arising from vehicle localization aspects considering the aspect of topological decomposition, the digital track map consists of two subsystems: digital map trackside and digital map onboard. the digital map onboard subsystem comprise the data tiles being loaded from the digital map trackside part depending on the specific train path. it works in close collaboration with the map matching module, responsible for further data processing. selection of individual data tiles to be loaded is based on the knowledge of the vehicle location [1]. the vehicle location information is also necessary to be known in order to determine the distance and direction in which the individual objects recorded in the digital track map are expected to be located 9 adam hlubuček acta polytechnica ctu proceedings relative to the vehicle. in order to calculate the values of these quantities, it is necessary to express the object location and the location of the vehicle in a common coordinate system. whereas linear coordinate systems are not suitable for this purpose (as they must always be assigned to a specific line or track) and there is a need to keep constant lengths, angles, and areas for measurement, it is necessary to express location through one of projected coordinate systems. it may require the recalculation of coordinates, in some cases. a projected coordinate system is defined on a flat, two-dimensional surface and is always based on a geographic coordinate system that is based on a sphere or spheroid. in a projected coordinate system, locations are identified by x and y coordinates. in addition, the z coordinate determining the elevation value can be expressed using a vertical coordinate system [3, 4]. as regard the railway vehicle, the location information is supposed to be provided by the gnss technology. it can be also determined using an odometry system of the vehicle, e. g. using collaboration with some of automatic train control or automatic train operation systems. the odometry information is especially important in such cases for which the location determined on the basis of gnss is not sufficiently accurate, in particular with regard to ensuring safetyrelevant functions. the location of a train cannot be determined by the means of the gnss technology in cases such as when the train is in a tunnel. gnss is also not able to reliably distinguish on which track in the station or on the multi-track line the vehicle is moving. this can only be determined in combination with route setting information provided by the railway traffic management system. for these reasons, a spatial description of the track guidance is also required to be included in the digital track map. this makes it possible to convert the odometric information related to the appropriate route into x, y and z coordinates. the same applies to descriptive data directly associated to individual points or segments defined within the network expressed at the level of individual tracks. 5. vexa and ato possible cooperation in the foreseeable future, vexa should be able to receive data from the avv system, which is the instance of ato used in the czech republic. the avv automatic train operation system was developed during the 1990s and put into revenue operation in 2000. it is a representative of the goa2 grade of automation system, which means the semi-automated train operation. the train is driven automatically, but the driver is still in the cab, e. g. to control the absence of obstacles in the track [2, 5, 6]. avv provides automatic driving for acceleration, cruise control and braking, comparing the actual speed with that requested by the driver and applying traction or braking to control the speed. in some cases, avv is able to work in close cooperation with the etcs automatic train control system. in such cases, these systems complement each other. avv can also use the same train parameters as etcs when calculating braking curves [6]. based on the principle of interconnection between the ato and the atc system, standardised european solution called ato over etcs (aoe) is being developed. this narrow harmonised cooperation brings also new concept of infrastructure data processing. instead of using fixed track map stored in avv, aoe uses more flexible concept. the description of the railway network corresponds to the decomposition into so-called segment profiles to which the relevant data relate. each running track is usually covered with several such segment profiles. the onboard ato part receives these data dynamically from the ato trackside subsystem. because the vexa system has the possibility to get some infrastructure data directly from ato, it is not needed to include them to the digital track map. the segment profiles, however, do not contain relevant spatial information which is necessary due to the need of recalculation of coordinates for the purposes of the digital track map and the map matching module functionalities. in order to localize the relevant data in geographic space, it is necessary to link individual segment profiles with appropriate geographically located elements. such a connection will make it possible to express the location of a point defined by the segment profile identifier and the depth of the segment profile penetration (absolute position from its beginning) also through geographical coordinates. this is essential both in terms of the ato infrastructure data localization and for the purpose of expressing the railway vehicle geographical location based on the odometry information [6]. 6. railtopomodel and gis principles application spatial expression of a railway transport route, which is the basis for localization of many infrastructure data description components, can take several different forms. the uic railtopomodel initiative aims to unify the various approaches to modeling railway infrastructure under one generic model allowing various user extensions managed in a common way. since the railtopomodel has become the irs 30100 solution in 2016, its application in various projects is gradually expanding. among others, the railml® 3 xml-based railway data format implements the concepts of the railtopomodel for data exchange purposes [7]. taking into account the railtopomodel principles, the multipurpose railway infrastructure model is being developed at the railway laboratory workplace at the czech technical university in prague 10 vol. 35/2022 digital track map for the vexa expert system figure 2. individual points of the track obtained through geodetic survey visualised in the arcgis pro software – faculty of transportation sciences. among several other projects, the digital track map represents a field for implementation and further development of this model. the multipurpose railway infrastructure model allows the user to view the recorded data in the context of individual objects or object classes, at the same time, however, it retains the form of a relational database, which allows it to be integrated into the geodatabase working under the gis software used [8]. as the platform for creating the digital track map, the arcgis pro software has been chosen. the software allows to open and edit various data formats from which the most convenient for storing data for the digital track map purposes is the file geodatabase format. it offers a comprehensive data repository integrating feature data sets, standalone tables and some other types of data sources. thanks to that, individual data records are not necessarily bound to specific gis features but they can simply be joined with them [4]. considering the principles of the railtopomodel recommendation, the topology of the digital track map network is designed using net elements and net relations. due to the nature of the task solved, the only possible approach is to use the micro level of detail with linear elements representing individual tracks interconnected by net relations at nodes representing switches. each linear element must be terminated in such a node and connected to the following linear elements with the use of a net relation [9, 10]. the digital track map, as it is created in the arcgis pro software, allows individual objects to be visualized and handled as standard gis features, i. e. points, lines and areas. therefore, the railtopomodel linear element class can be considered as a specialization of the line feature class, in the gis context. individual linear elements can therefore be represented and data-described as line features of the linear element feature class. these line features are fundamentally linestrings (sometimes also called polylines), i. e. sequences of points localized using geographical coordinates which can also have their altitude and stationing defined. in the case they represent linear elements, these points can be obtained through geodetic survey (as shown in the figure 2) or by sampling the analytical description of the track guidance (which is shown in the figure 3). when a geodatabase is taken into account, there is also the possibility to use the analytic description directly to define the line features (or parts thereof). arcgis pro offers the creation of arc and spiral segments in this way, even by entering their design parameters. it is necessary to consider whether such an option would be beneficial in the creation of the digital track map [4]. the net elements defined in this manner are supposed to become the basis for the location of various types of objects and properties of the railway infrastructure. among others, the aforementioned ato segments are assumed to be mapped to them in order to provide relevant data related to respective track sections. according to the railtopomodel terminology, railway infrastructure objects and properties which can be localized are collectively called net entities. after all, objects that are not directly related to the transport route can be considered as net entities, as well. this 11 adam hlubuček acta polytechnica ctu proceedings figure 3. horizontal curves rendered on the basis of the analytical description of the track geometry also make it possible to include among net entities those objects that are intended for comparison with the detected scene by the map matching module. net entities can be localized basically in two different ways. taking into account the class names defined in the multipurpose railway infrastructure model, it is possible to distinguish between physical and functional location. the physical location essentially corresponds to the localization with the use of the standard gis features and expresses the real location of physical objects in geographical space. such a location is therefore independent of the railway infrastructure and is not associated with any net elements. on the other hand, the functional location expresses the projection of the relevant object into associated point or segment defined within an (in our case linear) element or several (often consecutive) linear elements. this projection is typically perpendicular but in justified cases may be defined in another way to suit the functional nature of the localized object in relation to the railway transport route. depending on the method of functional location, we can further distinguish between spot, linear and area location. this division is based directly on the definition of the raitopomodel classes designed to describe the location aspects [9, 10]. as regards the digital track map, taking into account available data sources, the spot location is to be used for locating such entities as signals and balises, while the linear location is supposed to be used by horizontal curves, vertical curves, segment profiles, platform edges, level crossings and possibly other types of net entities. the area location could be used, for example, to locate turnouts, if it is decided that it is beneficial for the vexa project to record data about them. the main significance of the physical location from the point of view of the digital track map project is seen for the purposes of the railway surroundings objects which have no relation to a particular track. exclusively this type of location is supposed to be used, in such cases [1, 11]. 7. conclusions this article outlined possible approaches to creating the digital track map for the vexa project and introduced the basic issues which must be taken into account when defining the map feature classes. after the design of its structure is completed, the digital track map is supposed to be first used on the vexa system trial implementation on the čížkovice – obrnice railway line, also known as švestková dráha. the first testing is planned for the line section between the libčeves railway station and the židovice railway stop. the prospective intention is to extend the technology to a wider range of railway vehicles and parts of the railway network. acknowledgements the research presented in this paper is supported by the technology agency of the czech republic (ta čr) under project number ck01000111 "development of a train expert system for autonomous train behaviour". the author would like to thank ing. vítězslav lansfeld and ing. michal novák from ažd praha s. r. o. for valuable recommendations provided as part of the paper final review. references [1] m. matowicki et al. průběžná zpráva o činnostech, provedených v projektu vexa v roce 2020 [online]. tech. rep., čvut v praze – fakulta dopravní, 2021. [2] shift2rail. shift2rail multi-annual action plan, 2015. [3] arcmap. what are projected coordinate systems? [online], ©2016. https://desktop.arcgis.com/en/ arcmap/10.3/guide-books/map-projections/ about-projected-coordinate-systems.htm. [4] t. corbin. arcgis pro 2.x coockbook. packt publishing, 2018. [5] a. lieskovský, i. myslivec. etcs a ato – poprvé společně. reportér ažd praha (3): p. 46 – 48, 2011. [6] a. lieskovský, i. myslivec. ato and etcs can work together. railway gazette international p. 19 – 22, 2011. 12 https://desktop.arcgis.com/en/arcmap/10.3/guide-books/map-projections/about-projected-coordinate-systems.htm https://desktop.arcgis.com/en/arcmap/10.3/guide-books/map-projections/about-projected-coordinate-systems.htm https://desktop.arcgis.com/en/arcmap/10.3/guide-books/map-projections/about-projected-coordinate-systems.htm vol. 35/2022 digital track map for the vexa expert system [7] railtopomodel. railtopomodel [online], ©2021. http://www.railtopomodel.org. [8] m. leso, d. kamenický, p. koutecký, a. hlubuček. dopravní sál fd – železniční laboratoř pro výuku i výzkum. vts správy železnic 1, 2019. [9] uic. railtopomodel v1.0 [pdf], 2016. http://www.railtopomodel.org/en/download/ irs30100-apr16-7594bca1524e14224d0.html?file= files/download/railtopomodel/180416_uic_ irs30100.pdf. [10] uic. railtopomodel v1.1 [online], 2017. https://uic.org/rail-system/railtopomodel. [11] ažd. zdd [online], 2019. https://www.azd.cz/cs/ dalsi-informace/provoz-drahy/zdd. 13 http://www.railtopomodel.org http://www.railtopomodel.org/en/download/irs30100-apr16-7594bca1524e14224d0.html?file=files/download/railtopomodel/180416_uic_irs30100.pdf http://www.railtopomodel.org/en/download/irs30100-apr16-7594bca1524e14224d0.html?file=files/download/railtopomodel/180416_uic_irs30100.pdf http://www.railtopomodel.org/en/download/irs30100-apr16-7594bca1524e14224d0.html?file=files/download/railtopomodel/180416_uic_irs30100.pdf http://www.railtopomodel.org/en/download/irs30100-apr16-7594bca1524e14224d0.html?file=files/download/railtopomodel/180416_uic_irs30100.pdf https://uic.org/rail-system/railtopomodel https://www.azd.cz/cs/dalsi-informace/provoz-drahy/zdd https://www.azd.cz/cs/dalsi-informace/provoz-drahy/zdd acta polytechnica ctu proceedings 35:8–13, 2022 1 introduction 2 logical architecture of the vexa system 3 recognition of potentially dangerous objects 4 requirements arising from vehicle localization aspects 5 vexa and ato possible cooperation 6 railtopomodel and gis principles application 7 conclusions acknowledgements references my title https://doi.org/10.14311/app.2022.36.0237 acta polytechnica ctu proceedings 36:237–243, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague structural reliability of hollow core slabs considering compressive membrane action thomas thienpont∗, ruben van coile, wouter de corte, robby caspeele ghent university, department of structural engineering and building materials, technologiepark-zwijnaarde 60, 9052 ghent, belgium ∗ corresponding author: thomas.thienpont@ugent.be abstract. compressive membrane action can considerably improve the load bearing capacity of concrete slabs and beams in case of excessive loaded due to an accidental event. currently, only limited research has been focusing on compressive membrane action in prestressed concrete elements, or on concrete elements with large cavities, such as precast concrete hollow core slabs. therefore, a novel real-scale test setup has been developed in order to assess this effect in precast hollow core slabs, and how it can enhance the load-carrying capacity in accidental events. in parallel with these tests, a numerical finite element model has been developed in order to perform a more detailed structural analysis of this phenomenon, and to study the influence of various input parameters. the details of this test setup are briefly explained, and some relevant experimental test results are provided. considering the experimental findings and validated numerical model, this contribution aims to quantify the influence of compressive membrane action on the structural reliability of precast concrete hollow core slabs. to this end, probabilistic models for the most important material and geometric variables are gathered, and the structural reliability is assessed using latin hypercube sampling. overall, the results indicate that considering the formation of compressive membrane action strongly influences the variability of the ultimate load-carrying capacity of precast concrete hollow core slabs. keywords: cost optimization, structural reliability, target reliability. 1. introduction when an axially restrained concrete slab is excessively loaded (e.g. due to an accidental event), compressive membrane action (cma) can be activated, which can considerably enhance the load-carrying capacity. this can delay or even prevent a progressive collapse, increase the structural reliability and consequently increase the robustness of concrete structures. the general principle of cma in an axially restrained slab is illustrated in figure 1. although this effect was already investigated by other researches [1–4], most investigations focus on reinforced concrete members. so far, very little attention has been paid to the role of membrane action in prestressed concrete members. furthermore, only limited research has been focusing on cma in concrete members with large cavities, such as hollow core (hc) slabs. hc slabs are nowadays very commonly used, with the annual global production of these elements reaching 50 million m2 [5], and a better understanding of how cma could affect the behaviour of these slabs is needed. in a recent study, the beneficial effect of cma on the load-carrying capacity of precast hc slabs has been demonstrated experimentally [6]. in parallel, a numerical finite element (fe) model was established in order to model the structural behaviour (i.e. compressive membrane action) as per the investigated test program. in this contribution, the details of the experimental test setup and the numerical model are briefly discussed, and some relevant experimental test results are provided. next, the numerical model is employed to perform a stochastic fe analysis of the ultimate load-carrying capacity. herewith, this paper undertakes some first steps to quantify and incorporate the uncertainties which are associated to the development of compressive membrane action in precast hc slabs exposed to excessive loading. 2. novel experimental test setup the beneficial effect of cma on the load-carrying capacity of hc slabs was demonstrated in an experimental study by [6], as illustrated in figure 2. herein, three tests were performed: two axially restrained tests (labelled hc1 and hc2) and one unrestrained test (labelled hc3). the tests were carried out on hc slabs casted by a local belgian producer, with a length of 10.4 m and a 1200 mm × 320 mm cross section (see figure 3). in both the restrained and the unrestrained tests, the slabs were loaded in a fourpoint bending setup. the slabs were supported by two roller supports at a distance of 1 m from the slab end (resulting in a free span of 8.4m) and the loads were applied at 1/4th of the span length. in the restrained tests, the horizontal outward displacements of the hc slab were restrained by four concrete blocks (1300 mm 237 https://doi.org/10.14311/app.2022.36.0237 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en t. thienpont, r. v. coile, w. d. corte, r. caspeele acta polytechnica ctu proceedings figure 1. basic principle of compressive membrane action. effect in real-scale precast hc slabs. in parallel, a numerical finite element (fe) model has been established in order to model the structural behaviour (i.e. compressive membrane action) as per the investigated test program. in this contribution, the details of the experimental test setup and the numerical model are briefly discussed, and some relevant experimental test results are provided. next, the numerical model is employed to perform a stochastic fe analysis of the ultimate load-carrying capacity. herewith, this paper undertakes some first steps to quantify and incorporate the uncertainties which are associated to the development of compressive membrane action in precast hc slabs exposed to excessive loading. 2 novel experimental test setup to investigate the beneficial effect of compressive membrane action on the load-carrying capacity of prestressed hc slabs, an experimental test set-up has been developed as illustrated in figure 2. three tests were performed: two axially restrained tests (labelled hc1 and hc2) and one unrestrained test (labelled hc3). the tests were carried out on hc slabs casted by a local belgian producer, with a length of 10.4 m and a 1200 mm x 320 mm cross section (see figure 3). in both the restrained and the unrestrained tests, the slabs were loaded in a four-point bending setup. the slabs were supported by two roller supports at a distance of 1 m from the slab end (resulting in a free span of 8.4m) and the loads were applied at 1/4th of the span length. in the restrained tests, the horizontal outward displacements of the hc slab were restrained by four identical concrete blocks (1300 mm × 400 mm × 1600 mm) that were anchored in the laboratory strong floor. additionally, uplifting of the slab ends was prevented by two steel hem160 profiles, which were anchored to the lab floor with two high-grade steel rods. this particular setup allowed for the formation of significant compressive membrane forces inside a real-size hc slab. in the unrestrained test, these end restraints were removed, and the outward expansion and rotation of the slab was not hindered. a) b) figure 2: restrained four-point bending test: (a) schematic overview (dimensions in mm); (b) picture of the actual test set-up figure 2. restrained four-point bending test: (a) schematic overview (dimensions in mm); (b) picture of the actual test set-up [6]. × 400 mm × 1600 mm) that were anchored in the laboratory strong floor. additionally, uplifting of the slab ends was prevented by two steel profiles, which were anchored to the lab floor with high-grade steel rods. this particular setup allowed for the formation of significant compressive membrane forces inside a real-size hc slab. in the unrestrained test, these end restraints were removed, and the outward expansion and rotation of the slab was not hindered. 2.1. test results during the tests, the vertical and horizontal displacements were measured using 9 displacement transducers. the applied forces and horizontal forces on the restraint blocks were also continuously measured using 6 load cells. in the two restrained tests, the slabs reached a capacity of respectively 298 kn and 269 kn per load application point, which was significantly higher than the unrestrained setup (capacity = 180 kn). the maximum horizontal reaction force measured in the restrained tests was 1221 kn and 779 kn for hc1 and hc2 respectively. in both tests, the horizontal restraint stiffness of approximately 250 kn/mm was observed. this values was derived from the measurements of the restraint forces and horizontal displacement of the slab ends. the load displacement graphs from the three tests are depicted in figure 4a. both restrained slabs collapsed after a brittle web shear failure, whereas the unrestrained slab exhibited a more ductile behaviour. even though the test setup was identical in test hc1 and hc2 a significant difference in the ultimate capacity and deflection at failure was observed. for further details on the experimental campaign the reader is referred to [6]. 238 vol. 36/2022 hollow core slabs structural reliability figure 3. hollow core slab section (dimensions in mm). test age (days) fc,cube (mpa) fc,cyl (mpa) fct (mpa) hc1 68 75.7 64.3 3.85 hc2 71 74.9 65.6 3.63 hc3 85 78.1 68.1 3.63 table 1. sample characteristics of material properties for concrete. strand/wire fp0.2 (mpa) fm (mpa) εu (%) ep (gpa) hc1 1789 1975 6.25 221.5 hc2 1782 1938 5.21 218.1 hc3 1627 1848 5.94 193.9 table 2. sample characteristics of material properties for prestressing steel. 2.2. material tests the tested hc slabs were made of c50/60 concrete. to obtain the actual mechanical properties of the concrete, compressive tests on cube samples (150 mm × 150 mm × 150 mm), cylinder samples (diameter 150 mm, height 300 mm) and flexural bending tests on prism samples (400 mm × 100 mm × 100 mm) were performed, see table 1. the prestressing strands (bottom reinforcement) have a specified characteristic tensile strength of 1860 mpa. the prestressing wires (top reinforcement) have a specified characteristic tensile strength of 1770 mpa. to obtain the actual mechanical properties, tensile tests were performed on the three types of prestressing reinforcement. from the obtained stress-strain curves, the values for the 0.2 % strain limit fp0.2, tensile strength fm, ultimate strain εu and young’s modulus ep were derived, see table 2. these values were used as inputs for the material models in the numerical models. 3. finite element modelling 3.1. model validation in parallel with the experimental tests, a detailed 3d numerical model was developed using the finite element (fe) software abaqus [7]. herein, nonlinear material models were employed to model the behaviour of the concrete and prestressing steel under large deformations in an adequate manner. the concrete was modelled using the abaqus concrete damaged plasticity model. herein, the stress-strain diagrams for concrete in compression and tension were based on en 1992-1-2 [8] and fib model code 2010 [9], respectively. the prestressing steel was modelled using a bilinear stress-strain diagram. for each model evaluation, the material properties in accordance with table 1 and 2 were used. in the numerical evaluations, 8-noded hexahedral (brick) elements (c3d8) and 2-noded linear truss elements (t3d2) are used to model the concrete and steel prestressing strands, respectively. assuming perfect bond, the prestressing strands are embedded within the concrete element, using the embedded constraint option in abaqus. the supports and end restraints were modelled to precisely correspond to the test setup. overall, the model is able to predict the experimentally obtained load-displacement curves, the failure mode and crack patterns with high accuracy, as shown in figure 4a and figure 4b, respectively. only for the case of hc1, the model somewhat underestimates the deflection and load at failure. 3.2. influence of concrete tensile strength as observed in the experimental tests, the ultimate capacity fq,max and horizontal restraint force fh,max of the hc slabs in the restrained setup exhibit a large variability. even though the test setup was identical for hc1 and hc2, the difference in the measured ultimate capacity was close to 30 kn. furthermore, the difference in the maximum horizontal restraint force was nearly 450 kn. these large differences could be attributed to the brittle shear tension failure mechanism, which was observed in both tests. as shown in figure 4b, this failure mode is characterized by a diagonal crack, which forms at mid-depth and propagates towards the support and the compression zone, 239 t. thienpont, r. v. coile, w. d. corte, r. caspeele acta polytechnica ctu proceedings 3 finite element modelling 3.1 model validation in parallel with the experimental tests, a detailed 3d numerical model was developed using the finite element (fe) software abaqus (dassault systemes, 2014). herein, nonlinear material models were employed to model the behaviour of the concrete and prestressing steel under large deformations in an adequate manner. the concrete was modelled using the abaqus concrete damaged plasticity model. herein, the stress-strain diagrams for concrete in compression and tension were based on en 1992-1-2 (cen, 2004) and fib model code 2010 (fib, 2012), respectively. the prestressing steel was modelled using a bilinear stress-strain diagram. for each model evaluation, the material properties in accordance with table 1 and 2 were used. in the numerical evaluations, 8-noded hexahedral (brick) elements (c3d8) and 2-noded linear truss elements (t3d2) are used to model the concrete and steel prestressing strands, respectively. assuming perfect bond, the prestressing strands are embedded within the concrete element, using the embedded constraint option in abaqus. the supports and end restraints were modelled to precisely correspond to the test setup. overall, the model is able to predict the experimentally obtained load-displacement curves, the failure mode and crack patterns with high accuracy, as shown in figure 4a and figure 4b, respectively. only for the case of hc1, the model somewhat underestimates the deflection and load at failure. (a) (b) figure 4: comparison of experimental results and numerical model: (a) load-displacement graphs; (b) crack pattern at failure in restrained test hc1 3.2 influence of concrete tensile strength as observed in the experimental tests, the ultimate capacity fq,max and horizontal restraint force fh,max of the hc slabs in the restrained setup exhibit a large variability. even though the test setup was identical for hc1 and hc2, the difference in the measured ultimate capacity was close to 30 kn. furthermore, the difference in the maximum horizontal restraint force was nearly 450 kn. these large differences could be attributed to the brittle shear tension failure mechanism, which was observed in both tests. as shown in figure 4.b, this failure mode is characterized by a diagonal crack, which forms at mid-depth and propagates towards the support and the compression zone, thus causing a brittle failure (araujo, loriggio, & da figure 4. comparison of experimental results and numerical model: (a) load-displacement graphs; (b) crack pattern at failure in restrained test hc1 [6]. thus causing a brittle failure [10]. in the absence of transverse reinforcement in thin-walled webs of the hc slab, the shear capacity of slabs depends only on the tensile strength of the concrete [11]. hence the concrete tensile strength might considerably influence the capacity of these slabs. to test this hypothesis, the numerical model for hc1 was re-evaluated for a range of input value for the concrete tensile strength between 3.5 mpa and 5 mpa, as shown in figure 5. from the obtained load-displacement curves, it is clear that the tensile strength of concrete has a very large influence on the cma capacity of the hc in the studied test-setup, especially, in the range between 3.5 mpa and 4.5 mpa. the above result highlights the importance of considering the stochastic nature of the concrete tensile strength in a probabilistic assessment of concrete slabs prone to shear failure. 4. probabilistic model in the following section, a stochastic evaluation of the previously presented fe model is performed, in order to obtain the distributions of the ultimate capacity fq,max of prestressed hc slabs in restrained and unrestrained conditions. in the restrained configuration also the distribution of the maximum membrane force fr,max (i.e. the horizontal force in the slab) is evaluated. given the significant computational power required for a single fe model evaluation, it would be impractical to perform such an analysis by evaluating a set of traditional monte carlo simulations. hence, a latin hypercube sampling scheme was used to keep the number of model evaluations to a minimum. furthermore, four key variables were selected, which are considered as the most influencing with respect to the determination of the ultimate load capacity, especially in case of the occurrence of cma. the key figure 5. influence of the concrete tensile strength fct on the load-displacement behaviour of restrained hollow core slabs. variables are the concrete compressive strength fc, the concrete tensile strength fct, the initial prestress σpi and the spring constant k corresponding to the horizontal restraints. the probabilistic models for concrete material properties and the initial prestress of the steel reinforcement are based on [12, 13], as presented in table 3. herein, the mean values for the concrete compressive and tensile strength are based the values of hc1, in accordance with table 1. as a first approximation, the concrete compressive strength and tensile strength have been considered independent. to study the influence of the axial restraint stiffness, two values for the spring constant k are considered, i.e. 50 kn/mm and 250 kn/mm. the latter value corresponds to 240 vol. 36/2022 hollow core slabs structural reliability property distr. µ cov concrete compressive strength fc ln 64.3 mpa 0.06 concrete tensile strength fct ln 3.85 mpa 0.3 initial prestress σpi n 1116 mpa 0.06 spring constant k of horizontal restraints ln case dependent (kn/mm) 0.25 table 3. probabilistic models for most important basic variables involved in the uncertainty quantification of the compressive membrane action in the investigated hollow core slabs. figure 6. lhs samples of restrained and unrestrained hollow core slabs; (a) the load-displacement curves (b) restraint force versus displacement curves. figure 7. (a) pdf of the ultimate capacity fq,max; (b) pdf of the ultimate restraint force fr,max. the stiffness of the restraints in the experiments. the value of 50 kn/mm was chosen to study the influence of a structure with a lower restraint stiffness on the development of cma in precast hollow core slabs. the probabilistic model for the spring stiffness is based on [14]. 5. results in total, three stochastic fe analyses were performed: two restrained cases and one unrestrained case. for each case, a set of 12 latin hypercube samples of the input variables were generated according to the method described in [15]. figure 6a depicts the numerically obtained load-displacement curves from the fe evaluations, for the unrestrained case and two restraint stiffnesses. the curves are displayed up to the point of failure (i.e. bending or shear tension failure). from the graph, it is clear that a higher restraint stiffness corresponds to a higher ultimate capacity fq,max and a reduced deflection at failure. the curves in figure 6b show the horizontal restraint force fr as a function of the midspan deflec241 t. thienpont, r. v. coile, w. d. corte, r. caspeele acta polytechnica ctu proceedings calculation distribution µ cov restrained 250 kn/mm ln 264.1 kn 0.084 restrained 50 kn/mm ln 227.1 kn 0.062 unrestrained ln 169.8 kn 0.048 table 4. probabilistic models for the ultimate load-carrying capacity fq,max. calculation distribution µ cov restrained 250 kn/mm ln 986.7 kn 0.279 restrained 50 kn/mm ln 352.5 kn 0.286 table 5. probabilistic models for the maximum horizontal restraint force fr,max. tion. in these curves, the restraint force increases monotonically, up to the point at which the ultimate capacity fq,max is reached. thereafter the restraint forces reduce rapidly (not shown in the graph). the graph illustrates that a higher restraint stiffness corresponds to a higher maximum restraint force fr,max. figure 7a shows the distribution of the ultimate load-carrying capacity fq,max, for each of the three considered cases, assuming a lognormal distribution. from this graph, it is clear that the variability of fq,max increases significantly with an increased restraint stiffness. additionally, figure 7b depicts the distribution of the maximum restraint force fr,max. this graph clearly illustrates the presence of a very large variability in the ultimate restraint force. considering the lhs samples in combination with the fe analyses, probabilistic models for the loadcarrying capacity and maximum restraint force of both the restrained and unrestrained cases are presented in table 4 and table 5 respectively. a lognormal distribution is assumed for both variables. the results in table 4 show that the coefficient of variation (cov) of the ultimate capacity fq,max is higher for the restrained cases, and increases with increasing axial restraint stiffness. table 5 on the other hand, indicates no significant difference in the value of the cov of the maximum horizontal restraint force fr,max for both restraint cases. 6. conclusions in the present paper, a stochastic finite element analysis was performed to study the influence of stiff lateral restraints on the load-displacement behaviour of precast concrete hollow core slabs. the finite element model was validated against a set of real-scale experimental tests, and was applied in combination with probabilistic models for the most influencing variables to execute latin hypercube simulations of the loadcarrying capacity. based on the random simulations, probabilistic models for the load carrying capacity and horizontal restraint force are established. in general, it can be concluded that the presence of lateral restraints strongly influences the variability of the ultimate load-carrying capacity. the coefficient of variation (cov) of the ultimate capacity increases from 0.048 in the unrestrained case to 0.084 for the highly restrained case with an axial restraint stiffness of 250 kn/mm. furthermore, a high cov of the horizontal restraint force was observed. this value was observed not to change significantly with respect to the two stiffnesses of the surrounding structure considered. acknowledgements this research has been made possible through funding from the research foundation flanders (fwo), which is gratefully acknowledged (grant number 11f5520n). references [1] l. k. guice, e. j. rhomberg. membrane action in partially restrained slabs. aci structural journal 85(4), 1988. https://doi.org/10.14359/2517. [2] g. i. b. rankin, a. e. long. arching action strength enhancement in later ally-restrained slab strips. proceedings of the institution of civil engineers structures and buildings 122(4):461-7, 1997. https://doi.org/10.1680/istbu.1997.29834. [3] k. thoma, f. malisia. compressive membrane action in rc one-way slabs. engineering structures 171:395-404, 2018. https: //doi.org/10.1016/j.engstruct.2018.05.051. [4] f. j. vecchio, k. tang. membrane action in reinforced concrete slabs. canadian journal of civil engineering, 17:686-697, 1990. https://doi.org/10.1139/l90-082. [5] k. s. elliott, c. k. jolly. multi-storey precast concrete framed structures. john wiley & sons, oxford uk, 2013. [6] t. thienpont, w. de corte, r. van coile, et al. compressive membrane action in axially restrained hollow core slabs: experimental investigation. structural concrete, 2022. https://doi.org/10.1002/suco.202100684. [7] dassault systemes. abaqus 6.14: abaqus/cae user’s guide. providence, usa, 2014. [8] cen. en 1992-1-2. eurocode 2 design of concrete structures. part 1-2: general rules structural fire design. brussels. https://www.phd.eng.br/wp-conte nt/uploads/2015/12/en.1992.1.1.2004.pdf. 242 https://doi.org/10.14359/2517 https://doi.org/10.1680/istbu.1997.29834 https://doi.org/10.1016/j.engstruct.2018.05.051 https://doi.org/10.1016/j.engstruct.2018.05.051 https://doi.org/10.1139/l90-082 https://doi.org/10.1002/suco.202100684 https://www.phd.eng.br/wp-content/uploads/2015/12/en.1992.1.1.2004.pdf https://www.phd.eng.br/wp-content/uploads/2015/12/en.1992.1.1.2004.pdf vol. 36/2022 hollow core slabs structural reliability [9] fib. fib model code 2010, final draft volume 1. comité euro-international du béton, 2010. http://repositorio.unan.edu.ni/2986/1/5624.pdf. [10] c. a. m. araujo, d. d. loriggio, j. m. m. n. da camara. anchorage failure and shear design of hollow-core slabs. structural concrete 12(2):109-19, 2011. https://doi.org/10.1002/suco.201000024. [11] w. derkowski, m. surma. prestressed hollow core slabs for topped slim floors theory and research of the shear capacity. engineering structures 241, 2021. https: //doi.org/10.1016/j.engstruct.2021.112464. [12] jcss. probabilistic model code part 3.1: concrete properties. in the joint committee on structural safety, 2000. [13] jcss. probabilistic model code part 3.4: static properties of prestressing steel. in the joint committee on structural safety, 2000. [14] r. caspeele, d. gouverneur, r. van coile, et al. structural reliability of concrete slabs considering tensile membrane action. in safety, reliability and risk analysis, p. 2713-20, 2013. https://doi.org/10.1201/b15938-410. [15] a. olsson, g. sandberg, o. dahlblom. on latin hypercube sampling for structural reliability analysis. structural safety 25(1):47-68, 2003. https://doi.org/10.1016/s0167-4730(02)00039-5. 243 http://repositorio.unan.edu.ni/2986/1/5624.pdf https://doi.org/10.1002/suco.201000024 https://doi.org/10.1016/j.engstruct.2021.112464 https://doi.org/10.1016/j.engstruct.2021.112464 https://doi.org/10.1201/b15938-410 https://doi.org/10.1016/s0167-4730(02)00039-5 acta polytechnica ctu proceedings 36:237–243, 2022 1 introduction 2 novel experimental test setup 2.1 test results 2.2 material tests 3 finite element modelling 3.1 model validation 3.2 influence of concrete tensile strength 4 probabilistic model 5 results 6 conclusions acknowledgements references https://doi.org/10.14311/app.2022.33.0063 acta polytechnica ctu proceedings 33:63–69, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague long span post-tensioned box girder bridge in bratislava adrián chalupeca,∗, martin ondroša, petr novotnýb a dopravoprojekt a.s. kominárska 141/2,4 ; 832 03 bratislava nové mesto, divízia zvolen, pracovisko liptovský mikuláš, slovak republic b shp s.r.o. bohunická 50, 619 00 brno, czech republic ∗ corresponding author: chalupec@dopravoprojekt.sk abstract. the paper describes an analysis and detailed design of the 470 m long d4 motorway bridge in the slovak capital bratislava. this bridge is a part of 2 932.50 m long d4 motorway crossing of the danube river and its surrounding areas. the 210 m long main span of the post-tensioned three span box girder bridge is the longest span which has ever been built in slovakia. the ministry of transportation and construction of the slovak republic requirement was to cross all future rowing tracks by a single span. the depth of the cross section varies from 4.39 m at span to 13.09 m at piers. 35 m wide cross section carries 4-lane carriageway, median, sidewalk and cycling path. the central part of carriageway is built by the balanced cantilever method. the overhangs carrying outer lanes are built by wing traveller machine and supported by the precast struts. the bridge is currently under construction as a part of ppp project of bratislava ring road (the motorway d4 bratislava, jarovce bratislava, ivanka north section). keywords: balanced cantilever, box girder, main span. 1. introduction the bridge is a part of the southeast bratislava ring road d4 motorway, running between the jarovce interchange in the south and ivanka, north interchange in the east of bratislava. d4 motorway bratislava, jarovce ivanka, north connects the motorway d2 (south) and the motorway d1 (east). this 22.59 km long motorway section is a part of the running ppp project consisting of 58.13 km long d4 motorway and r7 expressway, 14 interchanges and 122 bridges. to avoid the european and national protected areas and to protect the danube river habitats as much as possible, the d4 motorway crossing the danube, required to build the 2 932.50 m long bridge structure which passes over the right bank seepage channel, the danube branch jarovce, the future rowing tracks, the danube, the danube branch biskupice and the left bank seepage channel. the bridge structure consists of 4 bridges divided by the expansion joints. the first structure above the right bank seepage channel and the danube branch jarovce is 782.5 m long, the second one which is subject of this paper, above the future rowing tracks is 470 m long, the third one crossing the danube itself is 430 m long and the forth bridge spanning above the danube branch biskupice, the left bank seepage channel and floodplains is 1250 m long (see fig.1). the design meets the minimal vertical and horizontal clearance requirements of 4 m (above a reference level of 132.20 m.n.m.) and 200 m respectively, at the future rowing tracks crossing. the horizontal clearance of 150 m and vertical clearance of 10 m above the above mentioned reference level were required at crossing of the danube. the bridge articulation, the type of foundation, the shape of substructure and superstructure were proposed by the concessionaire and accepted by the slovak ministry of transportation and construction in the public procurement. the bridge carries 4-lanes and paved central reserve of d4 motorway, which is a category d25.50/100 (width/design speed). the distance between the guardrails is 26.0 m on the bridge structure. the 3 m wide sidewalk is on the left side and 3 m wide cycling path on the right side of the bridge. pavement crossfall is positive on both sides from a motorway centre line. straight section of the d4 motorway runs on the bridge. 2. bridge description after taking into account the area nature and conditions and meeting the client’s requirements the bridge has been designed as a 470.0 m long continuous three-span box girder structure of post-tensioned concrete with the spans lengths 127.5 + 210.0 + 127.5 m. the bridge is resting on deep foundations. the bridge construction method, with the main span’s length of 210 m, is the balanced cantilever construction method. in cross section the superstructure is formed by a single-cell core of the section with top slab’s cantilevers (overhangs) on both sides which are supported in the transverse direction with precast steel63 https://doi.org/10.14311/app.2022.33.0063 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en a. chalupec, m. ondros, p. novotný acta polytechnica ctu proceedings figure 1. bridges crossing the danube and its branches. figure 2. b longitudinal section. concrete braces set in the section’s core. the superstructure is prestressed in longitudinal as well as transversal direction of class c50/60 post-tensioned concrete. the box girder core section’s depth varies from 4.30 m in midspan and at the abutments up to 13.0 m at piers. the superstructure top slab’s width is constant, 34.50 m, along the whole length. the superstructure is placed on steel bearings with the placement on fixed bearing on pier d2. the bridge substructure is formed by a common bridge’s pier supporting the structure above the danube branch jarovce, two intermediate piers and an end pier (abutment) common with the bridge over the danube river. the common piers are designed as pairs of column stems with the dimensions 3.15×2.10 m. each pair of piers’ stems supporting one of the adjacent bridges is interconnected in the bottom part with a dwarf wall, the parameters of which were optimised in order to enable, by its rigidity, the transmission of seismic effects. the joint piers are resting on deep foundations, on 20.0 m long large-diameter piles with the diameter of 1.200 m. the intermediate piers are designed as pairs of of skew piers’ stems embedded in 5.0 m high footing. the piers stems’ upper part is interconnected in transverse direction by means of 2.5m high stiff section with a frame effect. there is a pair of bearings on pier’s cap. the intermediate piers are resting on deep foundations, on 29.0 m long large-diameter piles with the diameter of 1.800 m. 3. construction sequence when preparing the bridge design the designer has proposed the piers foundations to be situated in veĺký zemník water body bank’s slope. the method and the sequence at the construction of piers foundations has been designed so as to eliminate in the maximum possible extent the action of groundwater and the action of water from the danube’s riverbed. the substructure foundations are designed to be constructed in pits protected with interlocking sheetpile walls strutted with steel shielding box. the piles were bored from the terrain level from artificially made jutting of fill material at the level of river’s reference water level. the length of pier foundation pit’s protection walls is 13.0 m, with the strutting in wall’s upper level. the pit’s bottom is grouted by means of jet grouting. the sealing with 3.50 m thick injection-grouted subgrade in combination with completed bored piles resists the water’s buoyancy forces. the jet grouting was carried out by drill rig from the pile-construction platform’s level. there were intermediate piles with temporary props constructed in pits which are providing for the superstructure’s stability during the construction. 64 vol. 33/2022 long span bridge in bratislava figure 3. cross-section in midspan and at piers. figure 4. deck cross-section construction stages. 65 a. chalupec, m. ondros, p. novotný acta polytechnica ctu proceedings figure 5. global analysis model. the superstructure construction is proposed in two stages. the section core formed by box-section girder with outer overhangs is constructed in stage one to which the deck slab’s outer overhangs supported with precast steel-concrete braces will be connected in stage two. the box-section core is constructed from two starting sections by segments symmetrically because of the piers axis. the temporary props are situated at the end spans. the props are leant to piers foundations with anchoring the joint gap by means of prestressing bars. the starting section is anchored through temporary prop to the foundations by means of unbonded tendons. the temporary prop’s side is connected with piers’ stems by means of steel braces. the number of segments of balanced cantilever’s overhang is 20. because of the used form traveller’s (mobile concreting carriage) loading capacity the length of 6 highest segments is 3.9 m, the length of remaining 14 segments is 5 m. upon completing the concreting of each segment the section is prestressed by using a group of prestressing tendons situated in the slab. the section core at bridge’s end spans is executed on temporary supporting scaffolding with temporary bracing by means of scaffolding prop in the vicinity of jointing segment. uniting the balanced cantilevers to form one unit by means of in-situ concreting will be performed first at the end spans and then at the middle span. upon completing the box girder’s core section the prestressing of bonded tendons situated in core’s bottom slab will be performed along the whole bridge’s length. after removing the casting form travellers the travellers for concreting the second stage of bridge superstructure will be assembled at the end span on both sides of the core. using the form traveller the precast steel-concrete braces will be positioned and the deck slab’s outer overhanging cantilevers will be concreted. following the top slab’s concreting and hardening the top slab will be prestressed in the transverse direction. in order to minimise the tensile stresses in section’s core and in deck slabs cantilevers the whole section in bridge’s longitudinal direction will be post-tensioned by means of tendons situated in top and bottom slab of section’s core, anchored in anchor blocks (blisters). the diameter of longitudinal prestressing bonded tendons is 15.7 mm, with the number of strands being 31 pieces. the diameter of transversal prestressing bonded tendons, with using the flat plastic tendon ducts, is 15.7 mm, with the number of strands being 4 pieces, alternatively 12 pcs above the end diaphragms. 4. bridge analysis and design the bridge, with its span of 210 m and the corresponding dimensions, with the used construction method, has required, when preparing the design and at the assessment, a special approach with the need to analyse the possibility of occurrence of risk construction details. there were various analysis models created for this purpose, in order to achieve as faithful behaviour of structure as possible. the bridge’s structure has been analysed by using the midas civil program system. the structure’s global model has been prepared of beam elements (figure 5), together with the substructure and piles. this model expresses the detailed construction sequence in two stages, progressive prestressing of longitudinal tendons in a number of stages and an impact of shrinkage and creep of concrete. for the very model it was necessary to enter as concisely as possible the subgrade for the bridge foundations construction. the subgrade’s reaction is modelled by means of springs’ rigidity along piles’ length, which express the composition of subsoil in foundation level and subgrade’s deformations when subjected to load. the different subgrade stiffness were verified by the company bgg by means of a special geotechnical software and the results of geotechnical monitoring. the structure has been assessed in the construction stages during balanced cantilever construction of bridge superstructure and in the final stage of the bridge use. for the analysis of transverse direction, in addition to the global model, a shell element (top slab, web, bottom slab) model of bridge’s part (figure 6) was used, which included also the prestressing of deck slab in transverse direction. to check the details subject to extreme stresses, where cracks can occur, a truss analogy together with the calculation on the model of volumetric elements (figure 7) has been used. 4.1. the structure design and assessment during the constructionstability supporting of balanced cantilever during the construction of balanced cantilevers their stability shall be provided for by means of a temporary pier situated situated 8.6 m from piers d2 and d3 axis towards the end span. temporary piers are connected with the balanced cantilever by means of prestressing tendons which are anchored in auxiliary diaphragm in balanced cantilever’s box. the analy66 vol. 33/2022 long span bridge in bratislava figure 6. shell element model. sis models for the assessment of temporary pier were created in the midas civil program by adjusting the global analysis model so that they would express the determining temporary state in order to assess the balanced cantilever’s stability at concreting the 20th segment to the left and to the right. the balanced cantilever’s stability before transformation of segments into one unit by in-situ concreting is checked for the load effects according to the stn en 1991-1-6 standard with assuming the load during section core’s construction. the concreting of segments is required symmetrically from the starting section’s centre line with the concreting of segment of the same number first at the end span and them at the main span. 4.2. the structure design and assessment during the construction and operation the structure’s global beam model described above has been used for the calculation of internal forces, stresses and deformations in each interim stage, as well as during the bridge use with the live load due to traffic, stn en 1991-2, till the end of bridge’s service life over the time t = 100 years. the concrete rheological effects were assumed according to the stn en 1992 standard. the relaxation of prestressing tendons is assumed according to the eurocode. the effects of seismic load (earthquake) according to the stn en 1998-1, the design seismic acceleration: ag = 0.63 m.s-2 are verified on the model as well. the analysis of construction sequence in stage two of construction works was difficult from the point of view of exact entering of cantilever slabs concreting sequence. it was necessary to analyse in detail a number of different concreting sequences, either using one or two deck slab form travellers. the analysis substance consisted in achieving the stress values as optimal as possible in section’s core as well as minimising of tensile stresses in cantilever slabs. figure 7. d model of cast-in situ pier elements. figure 8. model of temporary support at pier. the assessment of stresses during the construction has been performed according to the stn en 1992-2 standard, clause 113.3.2. the check of stresses (both compressive and tensile) at the top as well as the bottom surface of box girder was performed according to the stn en 19922 standard, according to the conditions for the characteristic, frequent as well as quasi-permanent combination both at the time of putting the bridge into service (t=0) and at the time of the end of bridge service life(t = 100y). 4.2.1. the structure assessment in transverse direction the criteria checked at the serviceability limit state were the criteria of stress limiting in the characteristic and the quasi-permanent combination. the cracks width was checked in the frequent combination. the reinforcement in transverse direction was designed for the combination of the effects of transverse bending, 67 a. chalupec, m. ondros, p. novotný acta polytechnica ctu proceedings figure 9. sample of stress distribution in the bottom of deck (mpa) at time t=100y frequent combination. figure 10. sample of stress distribution in the top of deck (mpa) at time t=100y quasi-permanent combination. figure 11. deflections for different creep and shrinkage functions. longitudinal shear, torsion as well as the local effects occurred due to prestressing tendons anchoring. 5. superstructure camber calculations the global beam element model was prepared to calculate the camber using midas civil. the construction stages and pile settlement (modelled by springs) were taken into the account. in general, the calculated camber depends on these two variables: creep and shrinkage functions and accuracy of the fem model (1d, 3d, type of elements, cracks and so on). the following creep and shrinkage functions were considered to get the camber prediction as accurate as possible: • stn en 1992, appendix b (eurocode) • ceb-fip model code 1978 • fib model code 2010 the calculated deflections using different creep and shrinkage functions gave significantly different results, e.g. the max deflection for ceb-fip model code 1978 was 400 mm and for fib model code (2010| was 288 mm, i.e. the difference was 112 mm using the same type of the model. to find out the proper combination of the fem model and creep and shrinkage functions, we used the calculated and measured deflections of the existing bridge of the similar type of superstructure and span. the camber was checked by using combined fem model created from beam elements (a top and bottom slab of the 16.3 m wide concrete core) and plate elements (the concrete core walls and overhangs). the fib model code 2010 was used for creep, shrinkage and prestressing steel relaxation (r1000 = 2.5%, mean development). the combined fem model en68 vol. 33/2022 long span bridge in bratislava figure 12. balanced cantilever under construction at pier d2. abled to take into the account the shear creep deformation in the concrete core walls and shear-lag effect of the overhangs. the simplified crack prediction in the concrete core walls and overhangs was taken into the account either. the unfavourable deviations of actions and prestressing from mean values in accordance with stn en 1990, stn en 1991-1-1 a stn en 1992-2 were considered for the final camber deflections. the main span camber deflections obtained using above described combined model with the crack prediction and creep and shrinkage from the fib model code 2010 were almost identical to the results obtained from the beam model and creep and shrinkage from the ceb-fip model code 1978. 6. conclusions the bridge construction commenced in november 2018. at the time of writing, 5 segments of balanced cantilever at the d2 pier and 3 segments of balanced cantilever at the d3 pier have been built. so far, the camber calculations are in accordance with the measured box girder core deflections. accurate camber calculations are key to the success of the construction of long span bridges built by the balanced cantilever method. the construction of superstructure should be finished in the fall of 2020. after the completion, the 210 m long span is going to be the longest span in slovakia built by the balanced cantilever method. 69 acta polytechnica ctu proceedings doi:10.14311/app.2018.15.0001 acta polytechnica ctu proceedings 15:1–5, 2018 © czech technical university in prague, 2018 available online at http://ojs.cvut.cz/ojs/index.php/app bond connections of high performance concrete (hpc) and steel: "experimental verification of selection glued for two different surfaces of steel" tomáš blažeka,b,∗, jakub řepkaa,b, tomáš vlacha,b a czech technical university in prague, faculty of civil engineering, thakurova 7, 166 29 prague 6, czech republic b czech technical university in prague, uceeb, trinecka 1024, 273 43 bustehrad, czech republic ∗ corresponding author: tomas.blazek@fsv.cvut.cz abstract. this article deals with the test of selected adhesives for glued connection of high performance concrete and steel plates with various surface treatments. results of this experiment can be used as a basis for the possible anchoring of concrete facade elements to the steel grate, the possible bonding of steel anchors on the surface of concrete facade elements. high performance concrete is a concrete that exceeds the compressive strength 100 mpa after 28 days. the second adherent is the uncoated steel and the steel with zinc coating as a second variant. samples were tested in two different ways. the first experiment verified the single tensile strength of bond connection. the second test verified the shear strength, which is very important for connection designing. keywords: hpc, concrete, steel, joint, connection, glue, epoxy resin. 1. introduction high performance concrete (hpc) has been increasingly used already also in the czech republic both in structural such as bridges and other structures such as indoor and outdoor furniture or facade elements. research at the department of building structures, faulty of civil engineering ctu in prague is much focused on the development of lightweight concrete facade elements [1, 2]. due to the corrosion of traditional steel reinforcement is more and more used alternative reinforcement in facade elements such as non-steel fibres, impregnated and non-impregnated technical textiles [3, 4]. it leads to the next lightening of elements hand in hand with development of hpc [5, 6]. their anchoring to the building structure is an essential part of the research. due to their lower weight less anchorage capacity is required and it is getting interested also the bonding in addition to the traditional mechanical anchors [7, 8]. the existing facade elements are usually anchored using mechanical anchors of various shapes with different mechanisms and systems for rectification. these mechanisms can be visible or hidden and sometimes affect the view on the building envelope. testing the mechanical anchors of the circumferential envelopes is addressed in more detail for example in etag 034 [9]. the aim of this work was selection of suitable adhesives commonly available on the market and their experimental verification on the connection of two basic materials. it was a high performance concrete (hpc) element representing for example facade element and steel plate representing steel grate. both materials hpc and steel have almost zero porosity and minimal water absorption. these are more demanding conditions for bonding. adhesives are usually tested for one-stage stress test blunt joint according to the csn en 15 870 [10] standard and they are also usually tested for shear stress according to the csn en 1994-1-1 [11] standard. the shear test is not exactly according to the standard. dimensions for presented experiment were approximately adapted to the dimensions of the facade elements and to the possibilities of the laboratory testing equipment. finally four adhesives were tested on two selected commonly used types of steel adhesion finishes, black steel without any surface treatment and heat-galvanized adherent [12]. 2. materials used for experiment hpc developed at the faulty of civil engineering ctu in prague (fce ctu) from local sources of raw materials has been already used for many applications and experiments. the same hpc has been also used for presented experiments in this paper. mix design of used fine-grain self-compacting hpc is presented in table 1. water cement ratio was 0.25 and water binder ratio was only 0.20. compressive strength of used hpc was 113 mpa on cubes with edge length 100 mm according to csn en 12 390-3 standard. flexural strength (three point bending test) is 13.2 mpa on prisms 40× 40× 160 mm according to csn en 101511 standard. in the past on the same mixture also direct uniaxial tensile strength has been tested and it is 6.7 mpa on dog-bones with cross sectional area 30× 30 mm according to csn 73 1318 standard and static modulus of elasticity in compression is 49.3 gpa 1 http://dx.doi.org/10.14311/app.2018.15.0001 http://ojs.cvut.cz/ojs/index.php/app t. blažek, j. řepka, t. vlach acta polytechnica ctu proceedings mix content kg/m3 cement i 42.5r (č.m. cement, mokrá) 680 technical silica sand 960 silica flour (ground quartz) 325 silica fume (microsilica) 175 superplasticizers 29 water 171 total 2,340 table 1. hpc mix design. on prisms 100× 100× 400 mm according to csn iso 6784 standard. two often used types of steel adhesion finishes were selected, black steel without any surface treatment and heat-galvanized adherent. the steel type was s235. in the experiment, four selected typical adhesives commonly available on the market were tested: mamut crystal glue, merbenit sf50, m3 and sika floor 156 – epoxy resin. the joint was experimentally subjected to tensile and shear stress and the mechanical behaviour of the adhesives was monitored. measured values were the crosshead displacement of testing machine, force and time of loading process. 3. experiments the basic hpc specimen dimensions for the tensile test were selected on the 100× 25× 25 mm. the hpc adherent part was glued to the steel part with a dimension of 50× 25× 25 mm. logically the fully glued cross sectional area was 25× 25 mm. all selected types of glues were applied on this whole area to achieve the ideal source just after 28 days of hpc hardening from the concreting. described tensile test design and view on the real specimen in testing machine is presented in fig. 1. specimens after one week maturing were tested for the uniaxial tensile test with the calculated time until the failure 60 s ± 20 s. the displacement load speed was calculated on the 0.5 mm/min and it was used for all tensile specimens. the hpc adherent for the shear test were designed on 100× 60× 40 mm. dimensions of both steel adherents were similar 100× 60 mm with different thickness 20 mm. the glued area in contact zone was 60× 50 mm, but the specimens for shear test had two glued area because of required testing symmetry. it eliminated other undesirable stresses in the glued area. described shear test design and view on specimen in testing machine after the testing procedure is presented in fig. 2. the displacement load speed was calculated on the 0.2 mm/min and also in the shear test was same for all groups of specimens. 4. results of the tensile test the goal of this tensile test was the determination of maximal tensile stress until failure of the adhesive complex consisting of hpc adherence, steel adherence and adhesive. the stress values were calculated using critical maximum breaking force and the bonded area approximately 25× 25 mm. the tensile stress dependence on the displacement of testing machine crosshead is presented in fig. 3 for all adhesives and also for both selected surface treatments of the metal adherence. presented curves are the average curves of three specimens for all selected types. the stress development was almost linear until the maximum value was reached. the description of stress development can be divided into three similar typical areas for all tested specimens. the first is the area before the linear trend of curves. this part was nonlinear due to the gradual activation and pre-stressing of all elements in testing machine. the second area is with linear trend of stress increasing on the cross-beam until the moment of maximal force was reached. different slope of curves suggests different stiffness of adhesives. it does not have to be the rule, but with higher adhesives stiffness also higher strengths have been achieved. epoxy resin sika floor 156 achieved the best results, the damage point was in the hpc part. commonly used construction adhesives in praxis on other chemical bases compared to the epoxy resin exhibited worse tensile properties. the third area is the part of curves behind the point of maximal force. for adhesives extruded from the tube (constructions or mounting adhesives), which has let’s say a rubbery soft character, this part shows considerable ductility. and it is a negative option of epoxy resin, high values of tensile stress, but fragile material behaviour. epoxy resin in general exhibits a sudden rupture, after the point of maximal force the epoxy resin is not able to transfer any load as visible in fig. 3. 5. results of the shear test in this part of the experiment the aim was to determine the maximum shear force and calculate the maximum shear stress before the failure of specimens. the shear stress was calculated based on the measured maximum force before the breaking point and the bonded two surfaces of the joint with cross sectional area approximately 60× 50 mm. the dependence of the stress on the displacement is presented in fig. 4 in the form of graph, taking into account the surface treatment of the metal adherence similarly like in the case of previous described tensile test. presented curves are also the average curves of three specimens for all selected types. the description of stress development can be divided into three similar typical areas for all tested specimens similarly with previous described tensile test. also the mechanical behavior of the joint is very similar. epoxy resin reached the highest shear strength, but its behavior is fragile. in the moment when the maximum force in the contact area is reached, the 2 vol. 15/2018 bond connections of hpc and steel figure 1. design of tensile test: 1. steel sleeve, 2. steel adherent, 3. selective glue, 4. hpc adherent, 5. steel sleeve, 6. construction glue. view on the real specimen in testing machine. figure 2. results of the tensile test. joint is unable to transfer any load. in contrast, other softer adhesives have a distinct plastic part of deformation and their behavior is more ductile. however, the shear strength is lower compared to the epoxy resin sika floor 156. metal adherent without any surface treatment achieves better results compared to the metal adherent with zinc surface treatment. only in the case of merb glue opposite result were achieved. may be the surface of non-treated steel parts was incorrectly cleaned by technical gasoline or the surface was dusty or wet also in comparison with results of the tensile test. the amount of resin in connected part and the method of resin application could have a significant effect on the results. epoxy resin sika floor 156 and epoxy resins in general are two-component adhesives, where curing occurs when the both components are mixed together in the correct ratio. but in the cases of other adhesives (variants in tubes merb, 3m and mamut resin) reaction occurs with the access of air to the adhesive. therefore, manufacturers recommend that the glue in tubes should be applied in the caterpillars side by side. this procedure provide access of air along the caterpillars and good process of hardening. this procedure was followed, but it was very difficult in praxis estimate how the caterpillars should be apart form others. on several samples, the caterpillars were joined together along the length of the glued surfaces and adequate access of air was not ensured. for a shear test with a 50× 60 mm of glued surfaces, the area was so large that the adhesive was not well hardened in 3 t. blažek, j. řepka, t. vlach acta polytechnica ctu proceedings figure 3. addiction stress – displacement, tensile stress. figure 4. addiction stress – displacement, shear stress. the middle of the sample in these cases. also use of hpc, which is waterproof, vapor-proof and airtight and does not allow access to air, is also of no effect. 6. conclusion the results of the tensile and shear tests correspond to each other. the best results with respect to strength values was achieved with epoxy two component resin sika floor 156. type of failure was for all samples was lost of adhesion on the side of hpc adherence. all other selected adhesives with soft rubber character (mamut, merbenit and m3) had a cohesive failure. it means that after the moment when the maximal force in the contact area was reached, the joint was able to transfer load with lower values of tensile stress. two selected commonly used types of steel adhesion finishes, black steel without any surface treatment and heat-galvanized, reached similar results. metal adherent without any surface treatment achieves may be little bit better average results compared to the metal adherent with zinc surface treatment. only in the case of merb glue opposite result were achieved because the surface of non-treated steel parts was incorrectly cleaned and prepared before the adhesive application. especially for soft rubber glues of lower strength (mamut, m3) the treatment of steel adherence is extremely important before adhesives are applied. the 4 vol. 15/2018 bond connections of hpc and steel manufacturer’s technical sheet must be always strictly observed. black steel is naturally characterized by a local occurrence of oxidized iron (scorch) which adversely affects the overall strength of the joint. the method of failure for presented adhesives was more or less cohesive with the visible small residues of oxidized iron. 7. summary the aim of this work was to verify the joining possibility of high performance concrete and steel adherence plates using adhesives. four commonly available selected adhesives were experimentally tested. the glued joint was verified in the tensile and shear strengths according to the relevant standards. the results show that the highest values were achieved using an epoxy resin based adhesive for both performed testing methods. the observation also showed a negative effect of the oxidized iron at the surface on the shear strength of the complex. tension strength is not fundamentally affected by this fact. this thesis does not solve the durability of used connection and its fire resistance, these propertieswill be verified in follow-up research. acknowledgements this outcome has been achieved with the financial support of the ta cr 11124 311 3111702a124 subtle concrete furniture and small structures for the railways stations and also has been supported by student grant competition, sgs18/108/ohk1/2t/11 – environmental aspects of high performance cement composites and concrete with recycled aggregate including their durability and service life. the authors would like to acknowledge with gratitude all the financial help that was provided in order to support this original research. references [1] t. bittner, p. bouška, m. kostelecká, et al. experimental investigation of mechanical properties of textile glass reinforcement. applied mechanics & materials 732, 2014. [2] a. chira, a. kumar, t. vlach, et al. textile-reinforced concrete facade panels with rigid foam core prisms. journal of sandwich structures & materials 2(18):200–214, 2016. [3] l. laiblova, t. vlach, a. chira, et al. technical textiles as an innovative material for reinforcing of elements from high performance concretes (hpc). advanced materials research 1054, 2014. [4] l. laiblová, t. vlach, ženíšek m. využití technických textilií jako alternativní výztuže do subtilních fasádních panelů z vysokohodnotného betonu. sympozium společnosti pro techniku prostředí pp. 155–160, 2015. [5] p. reiterman, m. jogl, v. baumelt, et al. development and mix design of hpc and uhpfrc. advanced materials research 982, 2014. [6] p. reiterman, o. holcapek, m. jogl, et al. long-term resistance of hpc to action of deicing salts. aip conference proceedings p. 090007, 2014. [7] etag 001. guideline for european technical approval of metal anchors for use in concrete, 2000. [8] p. bernardin. spojování kompozitních materiálů s kovem, 2016. disertační práce, zču v plzni. [9] etag 034. guideline for european technical approval of kits for external wall claddings, 2012. [10] čsn en 15 870. adhesives determination of tensile strength of butt joints, 2009. [11] čsn en 1994-1-1. eurocode 4: design of composite steel and concrete structures part 1-1: general rules and rules for buildings, 2006. [12] m. bajer, j. barnat. the glue–concrete interface of bonded anchors. construction and building materials 34:267–274, 2012. 5 acta polytechnica ctu proceedings 15:1–5, 2018 1 introduction 2 materials used for experiment 3 experiments 4 results of the tensile test 5 results of the shear test 6 conclusion 7 summary acknowledgements references https://doi.org/10.14311/app.2022.33.0644 acta polytechnica ctu proceedings 33:644–650, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague life cycle management of concrete structures based on sustainability indicators hiroshi yokotaa,∗, kohei nagaib, koji sakaic a hokkaido university, faculty of engineering, sapporo, hokkaido 060-8628, japan b the university of tokyo, institute of industrial science, tokyo 153-8505, japan c japan sustainability institute, sapporo, hokkaido 065-0021, japan ∗ corresponding author: yokota@eng.hokudai.ac.jp abstract. a concrete structure should be sufficiently planned, designed, executed and maintained to ensure its requirements during the life cycle. however, structures suffering from serious deterioration in structural members and sometimes subsequent loss in sustainability have been often found due to various reasons. one of the reasons is lack of total management for the structure. to meet these facts, it is extremely important to pursue coordination of engineering work in the design, execution and maintenance stages. the life cycle management is an organized system to support engineers decision to realize sufficient sustainability of the structure in the design, execution, maintenance, and all related work during its life cycle. the life cycle management is implemented based according to the life cycle management scenario in which balance of several sustainability indicators should be considered with ensuring overall sustainability. the sustainability indicators will be determined from the social, environmental and economic points of view. the scenario should be regularly reviewed based on the pdca cycle and be updated if necessary. this paper deals with the concept and framework of the life cycle management of concrete structures to ensure sustainability during the structural life. keywords: life-cycle management, life-cycle scenario, pdca cycle, performance, sustainability indicator. 1. introduction a concrete structure is constructed having its own purposes, such as protecting a society from disasters and ensuring a comfortable, safe life for people. the structure is required to ensure its functions and performance to achieve these purposes during its life cycle. however, serious damages are likely brought as the results of the application of physical and/or environmental actions, which provokes structural performance degradation and collapse. those damages may considerably cause a loss of sustainability not only of the structure but also of the society. the life cycle of a structure is made up of the planning, basic and detailed designs, execution including material selection, production and construction, maintenance including assessment and remedial action, and decommissioning stages. the life cycle management (lcm) is a coordinated system to ensure that the structure meets the associated performance requirements defined at the time of structural planning and may be subsequently updated during the service life of the structure. there are lots of standards and guidelines on how to make effective service life design and maintenance work. most of them cover durability design of concrete structures focusing on chloride-induced rebar corrosion, carbonation, chemical erosion, freeze/thaw deterioration, and fatigue. the maintenance standards deal with the basic principles of maintenance and repair for concrete structures and specify codes of practice underlying the basic principles covering inspection, prediction, evaluation, and remedial action. however, there are very few standards dealing with design and maintenance simultaneously with the consistent framework and concepts. it is of considerable importance to pursue collaboration between design and maintenance as well as execution. as the international standard on the lcm led by the authors, iso 22040 life cycle management of concrete structures, was published in january 2021 for specifying the concept of lcm. introduction of the lcm for a structure contributes to all aspects of sustainability while maintaining the functions and performances to fulfil its purposes. sustainability is defined from environmental, economic, and social aspects. it is likely to be considered in terms of resources and energy consumption, initial construction and/or life cycle costs, and environmental burden. sustainability is generally considered with one or a few sustainability indicators. the paper presents the principles and the framework of lcm of concrete structures by compiling the authors’ previous papers [1–3]. 644 https://doi.org/10.14311/app.2022.33.0644 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 33/2022 concrete structures life management figure 1. overall framework of the life cycle management. 2. concept of life cycle management 2.1. overall framework of lcm a concrete structure goes through different stages during its life cycle: from structural planning, design, execution, use, and to the life-end stages. this implies that it is essential to coordinate all the stages sufficiently and to transfer important information from one stage to another effectively. however, no system has been developed to date regarding the basic concept and specific methods to achieve this and manage the whole life of the structure in a consistent manner. the overall framework of the lcm is presented in figure 1. the lcm is undertaken according to the lcm scenario. the scenario is made up with basic strategies on how the whole life of the structure will be managed in consideration of the balance in sustainability indicators. the sustainability indicators will be determined from the social, environmental and economic aspects, which will be mentioned later. the scenario should be regularly reviewed and evaluated based on the pdca cycle and be updated if necessary. as shown in figure 1, it is easily understood that the lcm is a coordinated concept to support activities for managing the total life of structure. which consists of managements of each stage of life and overall management to cover whole life to ensure structural functions and performance and to achieve sustainability. 2.2. procedure of lcm figure 2 shows the standard procedure for lcm. for a new structure, the scenario is drawn up in the planning stage. the scenario describes the fundamental strategy on how the structure will be managed its structural performance and sustainability aspects. the structure is initially designed to ensure its requirements without major remedial measures; however, planned remedial measures should be declared in the scenario if they are necessary. the scenario links and coordinates the life cycle stages the structure. design is created to satisfy the scenario that was already drawn up in the planning stage. when the design does not meet the scenario, either the scenario is modified to be consistent with design considerations, or the design is created once again. construction work is engaged to follow the design outputs in the execution stage. then, in the use stage, initial inspection, diagnosis and evaluation is carried out to check the conditions of the structure. when defects or unexpected damages are found, remedial measures should be implemented as necessary. decision will be made whether the scenario is suitable for the subsequent maintenance work or not. when the scenario is found to be unsuitable at this point, it has to be updated. in the use stage, the above procedures will be repeated. when the scenario is updated, the updated scenario will be applied for the subsequent maintenance work. if remedial measures are found to be difficult mainly because they are rather costly or burdens, the structure will go to the life-end stage. in conclusion, the lcm can provide the overall strategy in order to ensure the structure to ensure the associated performanc in the life-end stage, where removal or renewal of the structure or diversion or change of use of the whole or part of the structure or members is investigated, consideration is given to reducing the amount of waste and promoting reuse/recycling of materials and members. 645 h. yokota, k. nagai, k. sakai acta polytechnica ctu proceedings figure 2. standard procedure of the life-cycle management. for an existing structure, the lcm starts with drawing up a scenario. then, assessment including inspection, diagnosis and evaluation is carried out for understanding the structural conditions. the scenario is drawn up based on the results of the assessment. when assessment results conclude that remedial measures are difficult to be taken to restore structural performance, the structure will go to the lifeend stage directly; otherwise, the lcm follows the same procedure as that for a new structure. being different from a new structure, an existing structure may have insufficient design, execution and previous maintenance records; accordingly, missing information should be assumed. every structure has specific functions that will be able to meet their purposes as mentioned earlier. to ensure those functions, performance requirements are ensured, such as safety, serviceability, restorability, and environmental preservation requirements. from the structural performance point of view, in the planning and design stages, the performance requirements of the structure are set, and the structural forms, materials, and geometric data will be specified so that the structure can satisfy the requirements during its service life. structural performance is verified during the design work. care in structural forms, materials and dimensions should be exercised so that maintenance can be appropriately carried out to ensure structural performance requirements in the use stage. in the execution stage, the structure should be constructed to fully ensure the expected performance set in the design stage. in the use stage, periodic maintenance is required to find deterioration of materials and even performance degradation. in addition, by predicting the future progress of performance degradation, the most suitable remedial measures can be chosen, which would optimize the decision-making indicators. maintenance work should follow the initially drawn up scenario, but the scenario is often necessary to be updated so that the scenario reflects the actual conditions of the structure relating to the structural performance. since the initial scenario is drawn up based on the results of future prediction on materials deterioration and performance degradation based on various assumptions, the lcm-related data acquired in the use stage can be effectively utilized to review the assumptions. 2.3. information transfer based on pdca cycle figure 3 shows the concept of information transfer particularly from the use stage. information obtained 646 vol. 33/2022 concrete structures life management figure 3. information transfer from the use stage. in the use stage is required to share not only among engineers in the use stage but also engineers in the planning, design and execution stages. the information would be utilized to improve future work in each stage based on the concept of pdca (plan-do-checkact) cycle. the pdca cycle for the target structure can also be extended to be utilized for other similar structures. it is considered that well planned and organized investigation or inspection and evaluation in the use stage are important to ensure performance of the structure. during the maintenance work, it is important to identify the mismatch between design assumption and real situation, which will be applicable for improvements on future maintenance, design and/or execution work. consequently, it is important to feedback and utilize the information collected in the use stage to the design and execution stages. when inspection data, technical knowledge and experience are accumulated, it is possible to draw up the more realistic lcm scenario. it is necessary to review the design outputs through the comparison with the inspection, diagnosis and evaluation results in the use stage because deterioration progress does not always follow the design outputs. the deterioration progress differs widely by its location and time because of inhomogeneous characteristics of concrete and diversity of local environmental conditions. it is absolutely necessary to update the scenario by using the real inspection data. 3. sustainability indicators 3.1. sustainability design sustainability is defined as a concept based on the environmental, economic, and social aspects, and is one of the key issues in a construction sector to be well considered now. sustainability is likely to be considered in construction work from the viewpoints of resources and energy consumption, initial construction and/or life cycle cost, durability, and environmental impact [4, 5]. in other words, sustainability is generally considered with one or a few of the above viewpoints, but it is not easy to find the best solution because no comprehensive sustainability indicator exists to represent all the aspects. it will be a typical example that the more the margin of safety is given, the more resources and energy may be needed for execution, which will require higher costs. this is the collision between the social aspect of sustainability that corresponds to the safety margin, and the economic and environmental aspects of sustainability that correspond to small costs and environmental loads. therefore, the decision on the safety margin should be essentially made after examining the influence of the margin on the costs and environmental burdens. it is not difficult to understand that structural collapse impairs the sustainability. huge energy should be devoted to treat the debris produced by the collapse, and a huge amount of resources is required to reconstruct structures. many people may be killed or injured, and employment and production bases will be at least temporally unavailable. these loss and restoration also require the huge amount of costs. engineers have to keep it in mind what might be caused by insufficient design, execution, and maintenance of the structures. as mentioned earlier, we realize that the safety margin principally links the sustainability. the safety margin will be determined at the general design process, which will be subsequently verified with some sustainability indicators. figure 4 shows the sustainability design flow for initial construction and repair. as indicated in figure 4, the sustainability indicators should be calculated and verified from the social, economic and environmental aspects. for the purpose of verification, requirements for these aspects should be 647 h. yokota, k. nagai, k. sakai acta polytechnica ctu proceedings figure 4. flow of sustainability evaluation and verification. made clear. resiliency and robustness that have recently been of great interest, are related to the safety margin social sustainability. as shown in figure 4, the sustainability design enables us to systematically consider the sustainability of the structure in the design stage. it may be also possible for designers to find a good balance between the social, economic and environmental indicators with the sustainability design system [6, 7]. the sensitivity analyses among the sustainability indicators for a simplified reinforced concrete beam [8] found that, when a 10 % increase in the cost because of an increase in the overall safety margin is assumed to be allowable, the overall safety margin can be set at approximately 1.2; and when a 10 % increase in co2 emissions is assumed to be allowable, the overall safety margin can be increased to approximately 1.3. in other words, the cost increase will be small with the increase in the overall safety margin of 10 % or with enhancing durability. the same trend can be found in estimating the environmental aspect. therefore, those sensitivity should be well considered for seeking the balance of sustainability indicators. 3.2. scenario evaluation in the design stage, durability is verified by the prediction of the performance degradation. based on the durability design and prediction, the fundamental concept on how to ensure the structural performance during its service life have to be thoroughly considered. structural conditions, design service life, structural characteristics, material properties, difficulties in assessment and remedial measures, and social and economic importance may have great influences on the concept. indicators that would be used for the scenario evaluation and selection should be determined among the sustainability indicators consisting of social, environmental, and economic aspects. these indicators will be quantified with respect to the period of the lcm. for the present scenario evaluation, costs, such as total cost or life cycle cost are widely used as the indicator. a scenario that provides the lowest cost will be chosen. from the viewpoint of sustainability, however, social and environmental indicators have to be considered as well for the scenario evaluation. as the environmental aspect, appropriate indicators are determined in consideration of environmental impacts in the execution and use stages of the structure, such as resource consumption, greenhouse gas emissions, and impacts on the ambient environment. as the economic aspect, all the direct and indirect costs during execution and the use and at the life-end of the structure, as well as the benefits and values provided by the structure, can be set as indicators. 648 vol. 33/2022 concrete structures life management figure 5. calculation example of sustainability indicators. indicators objectively expressing performance of the structure during the use period, such as safety of the structure itself and safety of users of the structure, can be determined for the social aspect. some indicators related to the social aspect are difficult to be quantified because of the non-representational features, such as adaptability, comport, cultural values, and social contribution. however, they may be considered by a social-scientific manner. the appropriate scenario should be selected among its alternatives in consideration of the balance among sustainability indicators. figure 5 shows an example of the scenario evaluation in terms of npv (net present value) and co2 emissions of a group of concrete structures during its life cycle [9]. the npv and the co2 emissions indicate the economic and environmental aspects, respectively. in figure 5, two scenarios were drawn up based on a preventive repair strategy (pm) and a corrective repair strategy (cm). also, two types of cements, such as opc (o) and bb (b) were considered. the deterioration progress was predicted with the markov process considering the diffusion coefficient of concrete. the scenario having higher npv would be preferable. in this calculation, the scenarios with pm-b, cm-b and pm-o produce almost the same npv, which is superior to that with cm-o. co2 will be emitted by repairing the deteriorated parts of a structure and the less emissions, the better scenario from the environmental aspect. the co2 emissions are the least for scenario pm-b and followed by cm-b and pm-o showing almost the same values. scenario cm-o would emit a great amount of co2. in conclusion, scenario pm-b is judged to satisfy economic and environmental requirements, and scenario cmo has the least possibility to be selected when the structures’ life cycle is 100 years. this example shows a simplified calculation, and more factors should be considered for drawing up a practical scenario. it is important to predict changes in sustainability indicators during a structure’s life cycle and draw up a suitable scenario according to the prediction. 4. conclusions it is extremely important to manage a concrete structure to ensure functions and performance that people and users expect during its life cycle. a concrete structure can be expected to have a long service life when it is well planned, designed, constructed, and maintained. coordinated management during the structural life cycle would provide the structure with longer life and would realize sufficient sustainability. for this purpose, all activities from the planning stage to the life-end stage of a concrete structure are well linked and coordinated as the lcm. 1. for doing infrastructure management, planning, design, execution, and use stages should be well coordinated with smooth information transfer and exchange. 2. the pdca cycle is a key point for realizing excellent lcm. for this purpose, necessary information should be shared in each stage of the structural life cycle and be transferred between the life cycle stages. 3. sustainability is one of the most important considerations during the lcm. sustainability should be well considered from the environmental, economic and social aspects when the management scenario is drawn up. 4. it is necessary to consider the balance in the sustainability indicators. the sustainability design and verification procedure are proposed for this purpose. 649 h. yokota, k. nagai, k. sakai acta polytechnica ctu proceedings acknowledgements the authors would extend their appreciation to the iso/tc71/wg1 members for fruitful discussion during developing the iso document on the life cycle management of concrete structures. references [1] h. yokota. practical application of life-cycle management system for shore protection facilities. structure and infrastructure engineering 13(1):34-43, 2016. https://doi.org/10.1080/15732479.2016.1198391. [2] h. yokota, k. nagai, k. matsumoto, et al. prospect for implementation of road infrastructure asset management. advanced engineering forum 21:366-71, 2017. https://doi.org/10.4028/www.scientific.net /aef.21.366. [3] h. yokota. considerations for life-cycle of concrete structures. proceeding of 8th internationa conference of asian concrete federation, fuzhou: 1-8, 2018. [4] k. sakai, t. shibata, a. kasuga, et al. sustainability design of concrete structures. structural concrete 17(6):1114-24, 2016. https://doi.org/10.1002/suco.201600069. [5] k. sakai. sustainability design for innovations of concrete technologies. proceedings of 2nd acf symposium for sustainable concrete infrastructures chiang, 2017. [6] h.-b. xie, w.-j. wu, y.-f. wang. life-time reliability based optimization of bridge maintenance strategy considering lca and lcc. journal of cleaner production 176:36-45, 2018. https://doi.org/10.1016/j.jclepro.2017.12.123. [7] j. j. wang, y. f. wang, y. w. sun, d. d. tingley, y. r. zhang. life cycle sustainability assessment of fly ash concrete structures. renewable and sustainable energy reviews 80: 1162-1174, 2017. https://doi.org/10.1016/j.rser.2017.05.232. [8] h. yokota, s. goto, k. sakai. parametric analyses on sustainability indicators for design, execution and maintenance of concrete structures. proceedings of the 2nd international conference on concrete sustainability, iccs 16, madrid, spain, p. 1046-1053, 2016. [9] s. d. puspitasari, y. kawabata, h. yokota. optimization of life-cycle management on port mooring facilities. proceedings of 74th annual conference of jsce takamatsu: cs2-13, 2019. https://www.eng.ho kudai.ac.jp/e3/alumni/files/abstract/m688.pdf. 650 https://doi.org/10.1080/15732479.2016.1198391 https://doi.org/10.4028/www.scientific.net/aef.21.366 https://doi.org/10.1002/suco.201600069 https://doi.org/10.1016/j.jclepro.2017.12.123 https://doi.org/10.1016/j.rser.2017.05.232 https://www.eng.hokudai.ac.jp/e3/alumni/files/abstract/m688.pdf acta polytechnica ctu proceedings doi:10.14311/app.2019.25.0036 acta polytechnica ctu proceedings 25:36–40, 2019 © czech technical university in prague, 2019 available online at http://ojs.cvut.cz/ojs/index.php/app custom-made rheometer for the experimental study of polyurethane resin pu9010 luciano fissore∗, stefan diebels saarland university, institute of applied mechanics, campus a4.2, 66123 saarbrücken, germany ∗ corresponding author: s8lufiss@stud.uni-saarland.de abstract. polyurethane resins are well-known examples in aerospace industry, electronics and automotive as an adhesive or coating because of their broad spectrum of properties. they are obtained by a process called curing, where in the initial uncured state the mixture of monomers exhibits a viscous behavior and at the end of the polymerization reaction, a solid with viscoelastic properties is formed. the shrinkage due to cross-linking and thermal expansion cause the generation of residual stresses which often result in broken adhesive layers. therefore, the reduction of such problems is of utmost importance. in this study, a rheometer capable to conduct oscillatory-strain-controlled experiments was built in order to apply large deformations not only to the sample at the beginning of the reaction but also during the curing process and to the fully cured adhesive. keywords: rheology, curing, dynamics. 1. introduction the studies on polymers have increased at a high rate in the past few years not only due to their broad spectrum of properties but also because of its high versatility of production. among them are cross-linked adhesives, which are a special kind of polymer because they undergo a chemical reaction that transforms the liquid sample into solid called curing. at the beginning, the uncured polymer solution behaves as a deformable viscous liquid which can only withstand hydrostatic loads. during curing, the polymer chains cross-link to each other forming a 3d network which means that there is an increase in its viscosity, in its stiffness and in relaxation or creep effects. the cured and solidified polymers are used in electronics and automotive parts, among others. there are researchers, like [1], that study the chemical reaction; others study the way the chemical reaction affects how the resin behaves mechanically, chemically or thermally [2–4]. moreover, during the curing process a shrinkage up to a volumetric strain of 10 % can be observed, leading to large residual stresses. depending on the mechanical properties this inhomogeneous stress distribution may damage the adhensive layer even without external loading [5]. there are also investigations that try to find suitable mathematical equations to model the curing reaction as well as material models to simulate the time-dependent mechanical behavior during and after curing [6, 7]. one way of studying how the mechanical properties change with time is by using a rheometer. the aim of this study is to build a rheometer capable of applying large deformations to the sample not only in liquid state but also in solid state. due to the fact that commercial rheometers are normally used to study the rheology of liquids, the torque needed in this case is 10 times lower than the maximum torque that this rheometer should apply for the fully cured solid. furthermore, once the rheometer is built, it will be used to study how the deformation, the frequency and temperature influence the properties of the fully cured resin. 2. experimental 2.1. materials the reactants to obtain polyurethane pu9010 are: desmodur vp.pu 1806 (isocyanate), baygal k55 (triol) withmn = 444 g/mol, desmophen 3600z (diol) with mn = 2000 g/mol. baygal k55 is the crosslinking agent because of its 3 functional hydroxils. the reaction is very sensitive to the presence of water. hence, the experimentall test must be perfomed in a humidity-controlled environment [8]. 2.2. rheological experimental plan dynamic measurements in oscillatory mode are planned with strain amplitudes from 2 % to 100 %. the tests will be performed during the reaction as well as once it is finished. the aim is to observe if there is a possible coupling between the curing reaction and the strain amplitude and how the mechanical properties are influenced by the curing and loading history. furthermore, each oscillatory test will be performed at different temperatures and frequencies, from t = 25 ◦c to t = 70 ◦c and from 1 hz to 15 hz, respectivetly. 3. rheometer the design of the rheometer and the mechanism to put it in motion is shown in figure 1. 36 http://dx.doi.org/10.14311/app.2019.25.0036 http://ojs.cvut.cz/ojs/index.php/app vol. 25/2019 custom-made rheometer for the experimental study of pu9010 figure 1. rheometer and motion mechanism drawn in catia v5®. 3.1. geometry the most important part of a rheometer is the implemented geometry because it determines not only how the sample is deformed during the process but also it determines the torque needed to apply large deformations to the sample. in this case, to examine the curing reaction of a resin without fillers or nanoparticles, a plate-plate or a cone-plate-geometry could be used. the main difference between them, and also the main reason to exclude the plate-plate geometry, is that with the former geometry the applied strain and strain rate do not depend on the radius of the sample. in table 1 and in figure 2 the dimensions are shown. given the fact that the tip of the cone could be in contact with the plate, it was decided to use a truncated cone forming a gap of 50µm in order to avoid possible wear on the lower plate and spurius tensions in the measurement. 1. radius 10 mm 2. angle 2◦ 3. gap 50 µm table 1. characteristic features of the cone-plate geometry. figure 2. implemented cone-plate geometry. because of the nature of the chemical reaction, once it finishes, both the cone and the upper part of the plate must be discarted. 3.2. motion based on the fact that a oscillatory motion is needed, a crank-slider mechanism was selected because it can transform the pure rotational motion of a normal motor into a planar translational oscillatory motion. the amplitude of oscillation is equal to the eccentric position of the main connecting rod related to the rotational axis of the motor. the problem is that this eccentricity, r, also induces dynamic forces or vibrations to the machine. in figure 3 a sketch of the mechanism is showed where r is the eccentricity, l is the length of the main connecting rod and v is the velocity of the slider. figure 3. crank-slider mechanism. 3.2.1. dynamic forces the reduction of dynamic forces is of best interest because they affect the absolute position of the cone in relation to the fixed plate and therefore affect the final measurements. as it can be seen in equation 1, fd ≈ mrw2 ( cos(wt) + λ cos(2wt) − cos(4wt)λ3 4 ) , (1) the dynamic force, fd, is aproximated to the 4th harmonic and depend on the mass m, the eccentricity r and λ = l r , which is the relation between the 37 luciano fissore, stefan diebels acta polytechnica ctu proceedings length of the main connecting rod and the eccentricity. therefore, one way of reducing the dynamic forces is by reducing the eccentricity as well. however, this means that the strain amplitude also decreases. where ω is the frequency of oscillation and t is time. 3.2.2. lever arm the planar translational oscillatory motion is transformed into a rotational oscillatory motion using a lever arm (figure 4) because, in principe, this would give the opportunity to apply high torque using small forces. furthermore, if it is possible to change the length of the lever arm, the mentioned reduction in strain amplitude could be balanced based on the similarity of triangles. this means that a constant deformation or, in other words, a constant angle of rotation can be obtained if the tangent of the angle of rotation is kept constant as well. figure 4. lever arm mechanism implementation. 3.3. sensors 3.3.1. temperature the temperature measurement range is accomplished by a ceramic heating plate from misumi®,beneath the plate in figure 1. this kind of device are small enough to fit in the rectangular hole that can be seen in figure 2 and its maximum operating temperature and power are high enough for this study. in order to be able to measure the temperature directly on the sample, the temperature sensor is placed near the bottom part of the truncated cone in figure 1). keeping in mind the dimensions of the cone and that it has to be changed after each test, the sensor must be small and cheap in order to be able to change it once each test is over. therefore, a ntc thermoresistor was selected since its high temperature reaction rate and its small size. 3.3.2. deformation as there are many parts in motion and many places where forces are transmitted from body to body, it is important to measure the deformation directly on the sample. the strain amplitude, γ, can be transform into degrees of rotation with the help of table 1 and equation 2, γ = θ tan(2◦) , (2) where θ is the angle of rotation. applied to the upper and lower limit of the measurement range, the strain amplitude limits are (1.) the corresponding angle of rotation for a γlower = 2 % is ≈ 0.046 ◦. (2.) the corresponding angle of rotation for a γupper = 100 % is ≈ 2 ◦. the aim is to measure the deformation in during the entire oscillatory motion, therefore, the sensor must have not only a fast reaction to changes but also very high resolution. a sketcht of the deformation measurement is shown in figure 5. the idea is to placed a laser on the oscillating surface of the axis of the cone and a photodiode sensor c10443-01 from hammamatsu® in a distance x mm. these sensors can measure how an incident laser beam moves along its surface with a resolution of 0.5µm giving the chance to measure the 0.046◦ of oscillation as the travel distance of the laser beam. figure 5. deformation measurement by a laser and a photodiode array. 3.3.3. torque in this rheometer the torque sensor that was chosen is a reaction torque sensor because it fits the design better than dynamic torque sensors. however, as the sample at early stages of the reaction is a liquid and later on it transforms into a solid, two sensors with different resolutions must be placed in the device. the idea is to build an adapter which allows both sensors to be placed at the same time and when a maximum value of torque is reached, the bigger sensor is activated avoiding possible damage on the smaller one. given the fact that the resolution of the sensor depends on the full scale value, the big sensor, on the one hand, must be selected according to the stiffness of the fully cured sample. on the other hand, 38 vol. 25/2019 custom-made rheometer for the experimental study of pu9010 figure 6. dynamic and applied torque components for: (a) lever length 25 mm and eccentricity 0.9 mm; (b) lever length 125 mm and eccentricity 0.1 mm the measurement range of the small sensor should include both the lowest value that the other sensor can measure and the minimum value for this kind of polymer during the liquid phase. therefore, a 20 nm reaction torque sensor from me-mesysteme gmbh® with a resolution of 0.1 % fs and also a 0.3 nm with a resolution of 0.1 % fs as well were selected. 4. results the needed torque to deform the fully cured sample up to 100 % is calculated using the radius of the cone r, showed in table 1 and the shear modulus g, of the fully cured pu9010 [8]. τ = gγ, (3) m = τ 2 3πr 3. (4) first the maximum shear stress τ , was calculated using equation 3 and then substituted into equation 4 to obtain ≈ 8.4 nm of torque. furthermore, the possibility of changing the length of the lever arm gives good results on keeping low the dynamic forces. the applied torque and the dynamic torque on the sample for minimum and maximum values of deformation were obtained. in 6 can be observed the change of both torques as a function of the motor degree of rotation. when the sample is deformed 100 %, an eccentricity of r = 0.9 mm and lever arm of 25 mm are needed(a), while in (b) when the sample is deformed 2 % the used eccentricity is r = 0.1 mm and the used lever arm is equal to 125 mm. moreover, deformations in between these limits will be obtained by finding the best combination of both variables. in consequence, keeping a small eccentricity constant, a certain deformation can be applied by changing only the position of the oscillating connecting rod in relation to the fixed point (figure 4). as stated earlier, one of the reasons for which a commercial rheometer can not be used in this kind of study was the lack of high-torque and high-frequency features. therefore, given the final design of the device, it can also be seen in figure 6 that the torque that the motor has to be able to apply is more than 20 times lower than the needed torque calculated previously. as a consequence, it is possible to find a suitable motor capable of suppling a torque of 0.3 nm at 15 hz. for instance, the mdrive 34 plus from schneider electric® is an appropiate motor because it has a torque/frequency ratio suitable for this application. 5. conclusions in the present work, a rheometer was succesfully design as far as mathematical calculations can predict. thereby, different rheological test can be performed changing the maximum deformation, the temperature and the frequency. furthermore, the torque that the motor has to apply could be reduced using crankslider mechanism and a lever arm making it possible to find a suitable motor. the final design can be seen in figure 7 in which also a glovebox is included for a controlled atmosphere. this leads to future work, in which the next step would be to design the system to control both the humidity and temperature inside the glovebox and also to generate the interface for the control of the device in labview® a software from national instruments®. 39 luciano fissore, stefan diebels acta polytechnica ctu proceedings figure 7. final design of a mda machine for rheometric analysis of pu9010. acknowledgements the research was supported by daad and directed by the chair of applied mechanics in saarland university. both finantial support and guidence during the reasearch is gratefully recognized. references [1] s. montserrat, j. malek. a kinetic analysis of the curing reaction of an epoxy resin. thermochimica acta 228:47–60, 1993. doi:10.1016/0040-6031(93)80273-d. [2] s. saseendran. effect of degree of cure on viscoelastic behavior of polymers. master’s thesis, luleå university of technology, 2016. [3] n. illie, a. simon. effect of curing mode on the micro-mechanical properties of dual-cured self-adhesive resin cements. clinical oral investigations 16:505 – 512, 2009. doi:10.1007/s00784-011-0527-x. [4] m. g.-m. e. papadopoulos, s. clarke. a thermal and rheological investigation during the complex cure of a two-component thermoset polyurethane. journal of applied polymerscience 114:3802–3810, 2009. doi:10.1002/app.30987. [5] m. h. a.j. hudson, s.c. martin, j. spelt. optical measurements of shrinkage in uv-cured adhesives. asme 124:352–354, 2002. [6] p. s. m. hossain. modelling and simulation of the curing process of polymers a modified formulation of the arruda-boyce model. archives of mechanics 63:621–633, 2011. doi:10.1002/pamm.201110186. [7] b. y. c. liebl, m. johlitz, a. lion. three-dimensional chemo-thermomechanically coupled simulation of curing of adhesives including viscoplasticity and chemical shrinkage. computational mechanics 49:603–615, 2011. doi:10.1007/s00466-011-0663-9. [8] m. b. w. possart. adhesive joints: ageing and durability of epoxies and polyurethanes. wiley-book, 2018. 40 https://doi.org/10.1016/0040-6031(93)80273-d https://doi.org/10.1007/s00784-011-0527-x https://doi.org/10.1002/app.30987 https://doi.org/10.1002/pamm.201110186 https://doi.org/10.1007/s00466-011-0663-9 acta polytechnica ctu proceedings 25:21–25, 2019 1 introduction 2 experimental 2.1 materials 2.2 rheological experimental plan 3 rheometer 3.1 geometry 3.2 motion 3.2.1 dynamic forces 3.2.2 lever arm 3.3 sensors 3.3.1 temperature 3.3.2 deformation 3.3.3 torque 4 results 5 conclusions acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2019.25.0041 acta polytechnica ctu proceedings 25:41–47, 2019 © czech technical university in prague, 2019 available online at http://ojs.cvut.cz/ojs/index.php/app additive manufacturing process parameter influence on mechanical strength of adhesive joints, preliminary activities mattia frascioa,∗, lorenzo bergonzib, michal jilicha, fabrizio moronib, massimiliano avallea, alessandro pirondib, margherita montia, matteo vettoric a universitá degli studi di genova, via opera pia 15a, 16145 genova, italy b universitá degli studi di parma, via universitá 12, 43121 parma, italy c mach3d srl, strada s. nicoló 19, 43126 parma, italy ∗ corresponding author: mattia.frascio@edu.unige.it abstract. the work illustrates how building parameters of the additive manufacturing process fused filament fabrication can affect not only the mechanical properties but also the surface wettability and morphology. wettability and morphology are relevant factors in bonded joints performance. advantages of polymeric additive manufacturing are to allow a re-design of components with locally controlled properties and integrated functions. major limitations are related to the lack of material testing standardization and constraints due to the build volume and to the object orientation for printability: the latter problems can be addressed with the use of bonded joints that allow to create bigger assemblies from smaller parts optimally designed to take advantage of material orthotropy and without the structural drawbacks. in this study, two materials are considered, acrylonitrile butadiene styrene and polylactideacid. wettability, surface morphology and mechanical strength have been determined at different combinations of nozzle temperature, print speed and layer thickness. keywords: additive manufacturing, fused filament fabrication, bonded joints, surface characterization. 1. introduction in this work it is presented the identification by experimental testing of the influence of the 3d printing process parameters on components characteristics, as tensile strength and surface finishing, relevant for the bonded joint process. it is implemented using the design of experiments doe to be able to investigate at the same time the effects of three factors (temperature of nozzle t, velocity of deposition v, thickness of the layer h) at three levels. the monitored output are the mechanical properties, the wettability and the morphology of 3d printed specimens. this allowed to obtain a more comprehensive reference to design a bonded joint that use the fused filament fabrication (fff) for the build of the adherends. the selected materials for the adherends are commercial polymers supplied for additive manufacturing purpose in order to produce data using polymer with chemical composition, molecular weight and dispersity developed for this technology. the material selection took in account many factors as the rate of utilization of the material in fff application, the mechanical properties, the durability and the processability. therefore acrylonitrile butadiene styrene (abs) and polylactideacid (pla) were chosen. the experimental data are analysed using statistical methods, the analysis of variance (anova), to achieve a better understating of the dependence on printing parameters of the mechanical properties, wettability and morphology measured. 2. motivation the advantages of additive manufacturing (am) to allow a re-design of components with locally controlled properties [1] and integrated functions [2] are yet insufficient to balance the technological drawbacks such as the lack of material standardization and constraints due to the build volume and to the object orientation for printability. the latter problems can be addressed with the use of bonded joints. bonded joints allow to create bigger assemblies from smaller parts optimally designed to take advantage of material orthotropy without structural drawbacks. while it is a wellknown fact that the process parameters of the am technology fff are relevant for the mechanical properties [3, 4] the influence of the build set up on the surface characteristics, as wettability and morphology, has not been investigated yet. however that are relevant factors in bonded joints performance [5]. when components made by 3d printing are assembled by bonding, the method of bonding and in particular the parameters of the bonding, are chosen on the base of the mechanical properties, wettability and morphology of the components to be assembled. it is common to refer to the specifications provided by the supplier of the filaments but it is proven [2] that these change significantly with the parameters of the printing process. so can be concluded that a 41 http://dx.doi.org/10.14311/app.2019.25.0041 http://ojs.cvut.cz/ojs/index.php/app m. frascio, l. bergonzi, m. jilich et al. acta polytechnica ctu proceedings figure 1. the used 3d printers, the delta wasp 4070 industrial by wasp and the portal witbox by bq figure 2. the mach3d specimen specimen compared to the bs en iso 527-2:2012 specimen and the prismatic specimen used for wettability and morphology characterizations bonding method for highest mechanical performance of the joint must be defined on actual properties to be measured on printed samples [6, 7]. 3. materials 3.1. 3d printers and process parameters two fused filament fabrication fff 3d printing machines are used, a witbox1fdm by bq and a wasp 4070 industrial by wasp (figure 1). the fused fff process consists in a liquefied filament of polymer pushed through a heated nozzle. the geometry is build by stacking layers of opportunely deposition patterns of the nozzle. the nozzle can be moved by different kinematic schemes of actuation: by portal scheme (ex. the one adopted by bq for the witbox1fdm) nominally higher geometrical accuracy is achievable, by delta (ex. technical solution adopted by the wasp) is possible to use higher print speed to cut the build time instead[8]. the choice to use both the 3d printers is done to test the kinematic schemes in order to verify if there is any actual influence on the measured properties. specimens of two materials are built using both 3d printers. acrylonitrile butadiene styrene (abs) is selected in relation to the compatibility characterization with different adhesives that is being performed [9, 10]; moreover it can be used when durability is a requirement for the application. polylacticacide (pla) is chosen as it is the most used in fff, but mostly because it is a biodegradable material [11–13]. the solid models are designed using creo parametric 2.0 software, different geometries of specimens are used for the different characterizations: the mach3d tensile test specimen for tensile test and prismatic specimens for morphology and wettability characterization (figure 2). the slicing and building parameters set up is done using the ultimaker cura software. the placement of the specimens is with the dominant length along the machine x-axis, resulting in a ±45 ◦ raster orientation, configuration compliant to the bonding application [1]. then multiples gcode are generated at the same time to obtain controlled variations of the investigated parameters, nozzle temperature, deposition speed and layer height, preventing the influence of undesired variations in the set-up. these parameters are chosen as relevant factors for the experiments because expected to affect the polymer crystallinity, the coalescence and the intra-layers 42 vol. 25/2019 additive manufacturing process parameter for adhesive joints material abs pla factor level 1 2 3 1 2 3 nozzle temperature t (◦ c) 230 240 250 200 215 230 deposition speed (mm/s) 30 75 120 30 75 120 layer height h (mm) 0.1 0.2 0.3 0.1 0.2 0.3 table 1. factors and levels adhesion [13]. three levels for each relevant factor are investigated, values are set on the basis of previous experiences [4, 14] and considering the ranges indicated in the manufacturers data sheets (table 1). worth nothing that the building time is highly affected by the specimen volume, the deposition speed and the layer height. by evaluating the needed accuracy of data analysis and by the building time esteem of each set-up, the design of experiments doe is done. it led to a taguchi orthogonal array experiment for tensile testing and to a full factorial design for wettability and morfology characterization. 3.2. characterization set-up the mechanical testing is performed using mach3d, a miniaturized universal testing machine (figure 3) developed for products and materials certification directly on the production site [14], peculiarity relevant for additive manufacturing. indeed additive manufacturing needs the development of cost effective and transportable tools. tensile test experiments are performed on 9 sets of three specimens (figure 2) built varying the printing set-up as described in the design of experiments section. figure 3. the mach3d miniaturized testing machine and mach3d specimen the wettability characterization is performed on the top layer of the prismatic specimens (figure 2) using a milli-q sessile drop tester with deionized water. static contact angle measurements, measured where the liquid-vapor interface meets the solid interface, are performed with a custom made set-up based on the dino-lite microscope (figure 4). the choice to measure the static contact angle is justified by the bonding application as the static contact angle reflects the relative strength of the molecular interaction at the interfaces. specimens surfaces were cleaned using ethanol alcohol diluted in 30 % (w/w) deionized water solution. images of the drop were taken by microscope from two different positions to consider the asymmetrical shape caused by the orthotropic surface morphology of the layer (figure 5). figure 4. the wettability measurement set-up the roughness parameters were measured on the same prismatic specimens (figure 2) used to characterize the wettability as it is a well known fact that wettability is influenced also by the morphology [15]. specimens morphology were investigated by scanning the surface of top layer according to the built direction because it is the one affected by the printing parameters; the bottom surface have a finishing related mostly to the build plate. measurements are performed compliant to en iso 25178 600:2019 using a cci taylor-hobson 3d optical profilometer. the areal roughness parameter arithmetic mean height roughness is used in order to characterize surface morphology. the arithmetic mean height roughness, sa, quantifies the deviations in height of the surface points in relation to the mean reference plane. 4. experimental methods 4.1. design of experiments doe design of experiment (doe) is a systematic approach to process optimization [16]. in this work it is used to 43 m. frascio, l. bergonzi, m. jilich et al. acta polytechnica ctu proceedings figure 5. wettability and morfology measurements determine how the factors (nozzle temperature, deposition speed and layer height), affect the response variables (the mechanical and surface properties). each factor is experimented on three discrete levels as described in table 1. full factorial design consists in testing all the possible combinations of all levels of the factors varying one factor at a time. it is feasible for morphology and wettability characterization due to the use of miniaturized specimens despite the high number of experiments required [17]. it is chosen considering the high precision of the estimation of the effects of the factors and interactions between factors on the response variables. the taguchi method is selected to study the effect of the print parameters on tensile testing output; it also introduces the use of orthogonal array (oa) to reduce the number of experiments despite the same number of factors and levels investigated. the orthogonal array (oa) is selected on the degrees of freedom of the problem. the degrees of freedom are the minimum number of independent experiments to be conducted [18]; as the degrees of freedom are one for the mean value and two each for the remaining factors the taguchi based l9 oa is chosen and nine experiment configurations for each material are carried out (table 2). repetitions and confounding, randomized testing sequence, are applied for the specimens testing in order to reduce experimental errors. the tensile test data are processed with the larger is best (equation 1) signal-to-noise-ratio (snr) to obtain from the several experiment factor 1 factor 2 factor 3 1 1 1 1 2 1 2 2 3 1 3 3 4 2 1 3 5 2 2 1 6 2 3 2 7 3 1 2 8 3 2 3 9 3 3 1 table 2. taguchi orthogonal array l9 repetitions the measure of the variation present due to the controlled factors. snr = −10 log (∑ 1 y2 i n ) (1) where yi is the observed response value for the i-th trial and n the number of observations for the trial. the advantage of the use of the balanced orthogonal array (oa) is that the data analysis is simplified, ex. the effects can be estimated independently. the main effect, the change in response produced by the change of the factors levels can be computed using et = fh − fl (2) 44 vol. 25/2019 additive manufacturing process parameter for adhesive joints where et is the main effect, fh is the average response at high level, fl is the average response at low level. the interaction effect, the change in response due to the combined effect of more than one factor, is computed as half of the difference between the effect of one factor f1 at high level of the other factor f2 and the effect of the same factor at low level of the other factor. the interaction effect is written in symbol as f1 × f2 . 4.2. analysis of variance anova the analysis of variance (anova) is performed to determine which of these effects are statistically relevant [16–18]. the analysis of variance (anova) tool allow to sub-divide the variation in the data in components that can be addressed to the main effects, the interaction effects and errors. in particular the f-ratio test allow to determine if a factor is significant. the effect of each factor or interaction relative to the error is significant if the f factor, the ratio between the effect variance and the error variance, is higher than a critical f. the value of f critical is given by the degrees of freedom of the experiment. the information from doe and anova are used to determine the process parameters set up for specimens with enhanced tensile and surface characteristics. 5. results and discussion experimental data are reported and elaborated to extrapolate main and interaction effects, f-statistic values for anova and signal to noise ratio (snr). these are relevant information to understand the effects of factors on the monitored output in order to define the build configurations to take advantage of these effects. the results are shown in the pictures, in which the main effects are abbreviated as t for the nozzle temperature, v for the deposition speed and h for the layer height. in the x axis is reported the level. 5.1. tensile testing results tensile testing are performed with three repetitions for each level. experimental ultimate tensile strength ranges from a minimum of 20 mpa to a maximum of 40 mpa. the main effect analysis and the interaction effect for tensile data are reported in figure 6. for the abs there is an evident dependence with the nozzle temperature and a correlation between velocity and both temperature and layer height. for pla there are significant effects of the temperature and velocity. relevant interaction between layer height and both temperature and velocity is noticed. anova analysis is performed. the f critical value for 95 % confidence interval is 2.51. the values in table 3, compared to the f critical, confirm the information drawn from the main effect analysis. the signal-to-noise-ratio snr analysis (figure 7) is performed (table 6) to identify a best building set up based on relevance of main effect and building time figure 6. main and interaction effect on ultimate tensile strength figure 7. signal-to-noise-ratio analysis on ultimate tensile strength source f abs f pla nozzle temperature 6.78 27.68 deposition speed 0.19 12.26 layer heigh mm 0.06 0.01 table 3. uts f factors. minimization, a critical factor for additive manufacturing [8]. 5.2. wettability results wettability testing is performed, nine repetitions for each specimen, experimental static contact angle ranges from a minimum of 50 ◦ to a maximum of 130 ◦. the values of the main effect and of the interaction effect are reported in figure 8. for both abs and pla, the deposition speed has the most relevant effect. the anova analysis is performed and the computed f values are shown in table 4. the f critical for 95 % confidence interval is 1.84. the comparison of the values to f critical confirm the outcome of the 45 m. frascio, l. bergonzi, m. jilich et al. acta polytechnica ctu proceedings figure 8. main and interaction effect on wettability main effect analysis. source f abs f pla nozzle temperature 0.74 0.21 deposition speed 5.24 1.3 layer heigh 0.57 0.23 table 4. wettability f factors 5.3. morphology results morphology testing is performed on three specimens and three repetitions for each 3d printing set-up. experimental sa ranges from a minimum of 7µm to a maximum of 26µm. the main effect analysis and interaction effect analysis are reported in figure 9. for abs the height h has a relevant effect, and, for pla, both t and v are relevant. the anova analysis is performed, results are reported in table 5. the value of the f critical for the 95 % confidence interval is 1.84. the computed values in table 5 compared to f critical confirm the information drawn from the main effect analysis. source f abs f pla nozzle temperature 0.38 2.05 deposition speed 1.15 17.15 layer heigh 2.6 1.05 table 5. morphology f factors 6. conclusions building parameters for maximum ultimate tensile strength and maximum and minimum values of wettability and roughness are identified (table 6). using the figure 9. main and interaction effect on morphology f-test the relevant effects have been identified. the different kinematic schemes do not affect the measured output. the showed set-up in the table 6 is drafted according to the relevant effects and in order to minimize the build time, another critical issue for additive manufacturing. worth nothing to notice that abs has the maximum ultimate tensile strength uts for the shorter build time set-up and that the two materials require sundry set-up to obtain the desired enhanced surface characteristics. other works investigated the adherends materials compatibility with many adhesives or experiencing adherends modification through additive manufacturing as surface tailoring with patterns or infill variations. the results presented in this work are the preliminary activities needed to investigate further the actual influence of the additive manufacturing (am) process parameters on mechanical strength of adhesive joints. 46 vol. 25/2019 additive manufacturing process parameter for adhesive joints factor level output absuts abs-theta abs-sa plauts pla-theta pla-sa min max min max min max min max nozzle temperature (celsius) 3 1 2 2 1 2 3 1 1 3 deposition speed (mm/s) 3 1 3 2 3 1 1 2 2 1 layer heigh (mm) 3 1 3 3 3 3 2 1 1 2 table 6. configurations for single lap experimental investigation acknowledgements this research was supported by universitá degli studi di genova, universitá degli studi di parma and mach3d srl. references [1] l. li. composite modeling and analysis for fabrication of fdm prototypes with locally controlled properties. journal of manufacturing processes 4:s129–s141, 2002. doi:10.1016/s1526-6125(02)70139-4. [2] m. jilich. development of a gripper for garment handling designed for additive manufacturing. proc imeche part c 1-12, 2019. doi:10.1177/0954406219857763. [3] d. popescu. fdm process parameters influence over the mechanical properties of polymer specimens : a review. polymer test 69:157–166, 2018. doi:10.1016/j.polymertesting.2018.05.020. [4] m.frascio. fatigue strength of plastics components made in additive manufacturing: first experimental results. 12, pp. 32–43. 2018. doi:10.1016/j.prostr.2018.11.109. [5] d. dillard. the mechanics of adhesion, vol. 2. 2002. [6] b. m. tymrak. mechanical properties of components fabricated with open-source 3-d printers under realistic environmental conditions. materials & design 58:242–246, 2014. doi:10.1016/j.matdes.2014.02.038. [7] j. kotlinski. mechanical properties of commercial rapid prototyping materials. rapid prototyping journal 20:499–510, 2014. doi:10.1108/rpj-06-2012-0052. [8] i. gibson. additive manufacturing technologies, 3d printing, rapid prototyping, and direct digital manufacturing. 2015. doi:10.1007/978-1-4939-2113-3. [9] j.m.arenas. multi-criteria selection of structural adhesives to bond abs parts obtained by rapid prototyping. int j adhes adhes 33:67–74, 2012. doi:10.1016/j.ijadhadh.2011.11.005. [10] d.espalin. analysis of bonding methods for fdm-manufactured parts. 21st annu int solid free fabr symp-an adit manuf conf 21, 2010. [11] g. wypych. handbook of polymers. 2016. [12] c. moliner. mechanical and thermal performance of pla and phbvbased biopolymers as potential alternatives to pet. chemical engineering transactions 57:1417–1422, 2017. doi:10.3303/cet1757237. [13] v. srinivas. correlating molecular and crystallization dynamics to macroscopic fusion and thermodynamic stability in fused deposi-tion modeling; a model study on polylactides. polymer 142:348–355, 2018. doi:10.1016/j.polymer.2018.03.063. [14] l. bergonzi. numerical and experimental validation of a non-standard specimen for uniaxial tensile test. procedia struct integr 12:392–403, 2018. doi:10.1016/j.prostr.2018.11.078. [15] k. kubiak. wettability versus roughness of engineering surfaces. wear 271:523–528, 2011. doi:10.1016/j.wear.2010.03.029. [16] j.antony. experimental quality, a strategic approach to achieve and improve quality. 2000. doi:10.1007/978-1-4615-5293-2. [17] v. srinivas. application of taguchi method for optimization of process parameters in improving the surface roughness of lathe facing operation. international refereed journal of engineering and science 1:13–19, 2012. [18] b.m.gopalsamy. taguchi method and anova, an approach for process parameters optimization of hard machining while machining hardened steel. journal of scientific and industrial research 68:686–695, 2009. 47 https://doi.org/10.1016/s1526-6125(02)70139-4 https://doi.org/10.1177/0954406219857763 https://doi.org/10.1016/j.polymertesting.2018.05.020 https://doi.org/10.1016/j.prostr.2018.11.109 https://doi.org/10.1016/j.matdes.2014.02.038 https://doi.org/10.1108/rpj-06-2012-0052 https://doi.org/10.1007/978-1-4939-2113-3 https://doi.org/10.1016/j.ijadhadh.2011.11.005 https://doi.org/10.3303/cet1757237 https://doi.org/10.1016/j.polymer.2018.03.063 https://doi.org/10.1016/j.prostr.2018.11.078 https://doi.org/10.1016/j.wear.2010.03.029 https://doi.org/10.1007/978-1-4615-5293-2 acta polytechnica ctu proceedings 25:41–47, 2019 1 introduction 2 motivation 3 materials 3.1 3d printers and process parameters 3.2 characterization set-up 4 experimental methods 4.1 design of experiments doe 4.2 analysis of variance anova 5 results and discussion 5.1 tensile testing results 5.2 wettability results 5.3 morphology results 6 conclusions acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.38.0057 acta polytechnica ctu proceedings 38:57–64, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague towards an automatized and objective assessment of data from visual inspections of building envelopes jan mandinec∗, pär johansson chalmers university of technology, department of architecture and civil engineering, sven hultins gata 6, 412 58 gothenburg, sweden ∗ corresponding author: jan.mandinec@chalmers.se abstract. the renovation planning process is filled with uncertainties and subjective decisions. these make the decisions upon what and when to renovate a complex and ambiguous problem. selection of renovation measures related to building envelope are often far from optimal as decisions are usually made based on visual inspections. these are manned and thus prone to subjective assessment and the knowhow of individual inspectors. furthermore, objective criteria which could indicate non-structural failures are often missing. the objective based planning process allowing the estimation of the current damage status of the building envelope by only using non-destructive measurements is still in its infancy. the first step requires establishing reliable and objective based data collection. these could be efficiently collected by unmanned aerial vehicles (uav) with subsequent image recognition algorithms allowing the identification of imperfections and store the position and extent of such deviations into the building’s digital assessment database. such tools do not exist. the aim of this study is to investigate the current objectivization possibilities in the domain of building inspections. the first part provides a literature review describing how an autonomous uav survey of a building envelope may be planned and what computer vision techniques may be used for automatic damage recognition and classification. subsequently, an objective detection model based on the yolo-tiny (you only look once) computer vision framework is employed in a case study investigating a building envelope of historical tjolöholm castle in sweden. this study contributes to developing a methodology for an objective based visual inspection process. keywords: uav, computer vision, object detection, yolo, case study, building, renovation, brick, masonry. 1. introduction in terms of environmental aspects, renovation of a building or its adaptive reuse has proven to outperform constructing a new building [1]. the environmental effectiveness of renovations may be further improved by transforming the maintenance paradigm from reactive (emergency renovation after a component breaks down) to proactive (planned renovation before a component breaks down). however, this paradigm shift requires a full digitalization of the maintenance planning allowing to perform data driven prediction of failures (predictive maintenance). inspiration could be taken from the world of wind turbines, or jet fighters where data can already predict whether any component will fail within a given period allowing not only to reduce down-times but also optimize the usage of resources and thus the environment. ideal maintenance system in buildings should be capable of doing the same type of assessment to recommend measures which can prevent a failure. however, reaching this level of predictive maintenance requires systematic changes in data collection processes. the performance of machine learning algorithms allowing a computer to find patterns in the data and thus automatically learn and decide over complex systems, are highly dependent on the type and the total amount of the data. building inspection provide invaluable information for future failure prediction as it shows how much damaged building is at the time of inspection. however, common manned inspections are insufficient in terms of large data collection. moreover, data are often not stored in a format suitable for applying machine learning algorithms. note that insufficient in this context means that building inspectors, however experienced, cannot fully quantify every aspect of how a building envelope is damaged (e.g., count the number of all cracks, measure, and sum up all damaged areas). contrastingly, unmanned aerial vehicles (uavs or drones) have such possibilities as drones can gather large sets of data by scanning whole building envelope with various sensors (i.e., rgb or infrared cameras, lidar, radar). given the premise from above, one need to objectively obtain the current status of the building envelope. the most readily available sensor technique is represented by rgb (red, green and blue) cameras. the overarching research questions this study answers are: 57 https://doi.org/10.14311/app.2022.38.0057 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en jan mandinec, pär johansson acta polytechnica ctu proceedings (1.) which data can objectively be used to assess what the current and future performance level of a building envelope are? (2.) how can a damage on a building envelope be quantified based on images from uavs? (3.) how can the whole process become as automatized as possible? this paper is divided into a literature review and the case study. the literature review is further divided into two distinct parts. based on the conclusions from the literature review, the case study is used to optimize the methodology into the practice. this is done by applying the in-house build sense model, developed based on the yolo-tiny (you only look once, tiny version) object detection framework [2], on a wall of tjolöholms castle in the west of sweden. finally, the study concludes with the discussion about how damaged areas may be quantified giving the outlook for future data-driven uav based building inspections. 2. literature review the first part of the literature review investigates how a flight pattern of uavs may be automatically generated for building envelope inspection. the second part presents automatic damage recognition from images to investigate how damages on a building envelope may be automatically recognized by the use of computer vision algorithms. 2.1. planning of uav based inspections of building envelope usage of unmanned aerial vehicles (uavs or drones) in a building inspection process yields many advantages compare to the classical manned inspections. the two of the most distinct advantages are the time efficiency and the reachability over the whole building envelope. the subsequent analysis of the images captured by drones may provide a comprehensive look on the status of the building envelope. the building inspection process based on a uav survey may be either conducted manually i.e., the drone is controlled remotely by an operator on the ground, or it can be conducted semi-automatically. in both cases local laws and restrictions regarding operations of the uavs need to be followed. duque [3] identified 4 necessary steps which need to be taken prior to the inspection of any construction: (1.) uav fly operation training, (2.) documentation review of inspected structure, (3.) observation of surroundings and (4.) uav pre-flight check (e.g., battery levels). in the latter semi-automatic case, drone is often preprogramed to follow specific gps coordination waypoints and flight path making the flight itself fully automatic. such an approach was adopted e.g. in a study by rakha [4] who used images captured by infrared camera to develop heat-leakage segmentation framework based on computer vision algorithms. however, the loss of the gps signal may lead to navigational hazards. to cope with this problem, a slam (simultaneous localization and mapping) technique, was developed, allowing drones to detect objects in the surroundings by employing lidar or similar sensor and thus to avoid collisions. current development in the semi-autonomous flight systems adopts 3d models (or bim models) of an inspected building as a source of prior information for uav inspection planning. freimuth [5] utilizes bim models to automatically generate safe flight paths for uavs. similar framework developed for automatized inspections of bridges, allowing to generate safe flight paths can be found in the work by morgenthal [6]. this framework was practically utilized for inspecting building envelopes by benz [7] who performed a case study to automatically generate flight paths for obtaining infrared images and subsequent thermal conductance value determination of the building envelope. note that flight path in this case needed to be manually adjusted to avoid collision with nearby obstacles (e.g. trees). regardless the type of flight control, uavs need to employ specific routing strategies to comply with the needs of different sensors and the intentions of the quest. for this purpose, a different flight pattern is needed for drone scanning to produce high resolution images suitable for subsequent defects analysis compared to a drone taking infrared images suitable for thermography analysis. 2.2. automatize damage recognition and classification from images recent development of computational power made the machine learning and computer vision algorithms accessible to the point where increasing numbers of research papers dealing with their application on building inspections are being published every year. the foundation for the majority of algorithm for image interpretations is a convolutional neural network (cnn) as it mainly specializes on extracting basic features from images like straight lines, round shapes etc. extracted features are then subsequently fed to a classification algorithm providing the predictions. the aforementioned method was successfully used for crack detection where support vector machine (svm) algorithm achieved 85.94 % of the validation accuracy [8]. note that substantial number of labelled images i.e., images where the labels are specified manually, are needed for reaching high levels of accuracy. the aforementioned study used 6 002 images of “cracks” and “non-cracks”. more examples of similar detection strategy using different classification algorithms can be found in a literature review conducted by sony [9]. an interesting option allowing to overcome the need of the high number of images is represented by the concept of transferred learning which is the 58 vol. 38/2022 towards an automatized and objective assessment . . . domain of computer vision frameworks e.g., r-cnn (region-based cnn), faster r-cnn, alexnet, oxfornet, yolo (you only look once), resnet50, vgg16. such frameworks are usually based on layer(s) of cnns subsequently connected to feed-forward neural networks or similar for classification purposes and in case of object detection problems also to a regression algorithm, which also allows to accurately draw a bounding box around the objects area. note that frameworks are obtainable as pre-trained i.e., the user doesn’t need to train the whole network from scratch but rather tune it for a specific problem. in some specific cases, this may lead to the reduction of the total number of images needed for achieving accurate predictions. özgenel [10] compared the performance of 7 different computer vision frameworks and concluded that transfer learning applied to crack detection problems is feasible. in terms of transfer learning, the study also advised to use limited number of training images especially when the variance among the images is low as the high number of images increases computational time as well as the risk that the model learn some specific crack patterns too well and fails to comprehend general appearance of cracks (overfitting). the majority of research papers investigating the usage of presented computer vision techniques comes mainly from three research areas: bridge health monitoring, pavement monitoring and the large-scale monitoring of buildings after earthquakes. the first two areas use the techniques described above for damage detection. the latter mainly uses 3d cloud of points (set of points in virtual 3d space usually forming a base for 3d models) to quickly find collapsed buildings and subsequently applies computer vision techniques to detect damages of smaller scale [11]. practical example for concrete bridges is provided by yang [12] who performed a field test with a drone and by the means of vgg-16 framework successfully detecting over 70 % of cracks and spalling areas on a ground level of a bridge. the means of transferred learning was used to detect cracks on pavements in the work of gopalakrishna [13] who used 1 056 images in total for training purposes. more computer vision based examples of bridge health and pavement monitoring alongside with examples of different applications (building health monitoring or inspection of underground structures) may be found in the work of sony [9]. practical applicability of pre-trained frameworks in detecting defects on building envelopes may be well illustrated on the work done by wang [14] who feed 500 annotated images to faster r-cnn framework to identify and draw bounding boxes around bricks subjected to spalling and efflorescence with resulting accuracy 95 %. another good example is provided by the research of cha [15] where modified faster r-cnn is used to differentiate concrete cracks, steel corrosion, bolt corrosion and steel delamination. in total 2 366 annotated training images were fed to the framework achieving mean average precision 89.7 % while testing on previously unseen images. image segmentation is another computer vision technique allowing to precisely highlight the area of interest i.e., to color out pixels which are associated with a crack or other type of damage. hoskere [16] used a cnn on a level of individual pixels rather than on whole images eventually adding a label to each pixel and thus highlighting six different types of damage. already mentioned work by gallareta [11] combined multiresolution segmentation allowing to differentiate building objects (i.e., façades, windows etc.) and spectral difference segmentation allowing to highlight damaged areas (i.e., cracks, holes etc.). outputting highlighted areas from image segmentation may be used for measuring distances within an image eventually allowing to measure the scope of a damage. such approach was illustrated in the work of lins [17] who used images to measure width and length of cracks. 3. case study in the following case study, an in-house computer vision algorithm (build-sense model) based on the yolo-tiny framework [2] capable of detecting damaged areas is applied to images of historical tjolöholm castle in the west of sweden. the main goal is to detect missing joints in a stone masonry wall. 3.1. object detection model the build-sense model was originally built for detecting damage on brick walls specializing on drawing bounding boxes around cracks and bricks subjected to spalling and efflorescence. the model is utilizing the concepts of transfer learning and was trained based on 225 images of damaged brick walls (approximately 75 images for each damage type) manually annotated with more than 2 000 bounding boxes providing the model with the interpretation of how a damage looks like. figure 1 shows bounding boxes around efflorescence as they were predicted by build-sense model. the current predictive power of the build-sense model may be characterized as good enough for initial stage of inspections as it in theory may notify the inspector about possible building envelope failure. however, given its rough estimation of the damaged area it is not yet suitable for accurate damage quantification i.e., measuring the area of damage, width of damage etc. 3.2. tjolöholms castle and data collection the construction of tjolöholms castle was finished in the beginning of 20th century. thenceforth, it has been subjected to many refurbishments given its continuous moisture related problems which occurs in the form of mold growth and ongoing disintegration of joints in the stone masonry walls forming the building envelope of the castle. problems are more obvious on 59 jan mandinec, pär johansson acta polytechnica ctu proceedings figure 1. example of output from the build-sense model – model draw bounding boxes around damaged area alongside with its level of confidence [18]. the façade facing the south-east with the open view towards the coast of north sea. as foundation for the status assessment a basic ground level visual inspection of the building envelope was performed and complemented by inspecting building insides, including the roof. following the many years of subsequent repair work in the castle, there is a substantial amount of data. this is stored in the castle management’s own databases containing technical drawings, data from hygrothermal wall sensors, 3d model of the building (based on cloud of points), panoramic images of each room from interior and panoramic images taken on 78 different locations around the main building. note that performing an uav survey for acquisition of close-up images of building envelope was not possible at the time of the visit. therefore, obtained outdoor panoramic images are used in subsequent computer vision analysis. 3.3. scope and the methodology of the case study given the fact that joints are continuously disintegrated, mainly on the east side of the south-west façade, the study is limited to the assessment of the build-sense model only to one panoramic image which closely depicts the area of interest. the positions of the panoramic image as well as the overall view over the wall of interest is on the figure 2. given the limitations of the position i.e., testing images will not be in direct opposition to the wall surface but rather tilted, and given the complexity of light exposure conditions (uneven surface is under direct sunlight casting a lot of shadows), one area of interest on a building envelope will be analyzed from different angles. the confidence threshold for the model was set to be 50 % i.e., the model draws bounding boxes around objects only where the probability of accurate detection is higher than 50 %. 3.4. retraining the build-sense model direct application of the current build-sense model is not feasible in this problem as it was not able to detect any missing joints in the castle’s stone masonry wall. this is expected as it was originally tuned for detecting cracks, efflorescence and spalling on brick walls i.e., each construction has different structure and different damage patterns which are different from missing joints in stone masonry walls. therefore, the model needed to be retrained specifically for the purpose of detecting missing joints in stone masonry walls. images for re-training purposes were collected in the streets of gothenburg city (in the west of sweden) by the in-built high-resolution camera in iphone se 2019. initial 115 images of stone masonry walls were annotated highlighting damaged joints. the total number of images was multiplied in the image augmentation process by performing 90°, 180° and 270° rotation. the images were then split to train/test samples by the ratio of 80/20. training itself was performed in the google collaboration environment allowing to borrow graphics processing units (gpu) for the purpose of calculation. the whole training session took 6.3 hours to complete on a cloud based dedicated gpu nvidia tesla k80 controlled by a local computer with 2,.3 ghz quad-core intel core i5 processor. note that the time of the calculation could been decreased by limiting the pixel size of images used for the training purposes. the model iterated over the images for 6 000 times internally using mean average precision (map) as a metric for network’s weights evaluation. final output of the training is a set of weights which are the best in predicting missing joints given the training images and the number of iterations. the best weights are then used for subsequent analysis of images from tjolöholms castle. 60 vol. 38/2022 towards an automatized and objective assessment . . . (a). (b). figure 2. (a) schematic plan on the building’s ground floor to the left shows the location where the panoramic photograph was taken (red dot). gray background indicates multi-storey parts while cyan colour indicates single storey part of the building. (b) photograph was taken on the stairs leading to the roof of single storey part [19]. figure 3. default view from the panoramic image directly facing the area of interest [19]. 3.5. results and discussion all images altogether presented in this subchapter were analyzed using retrained build-sense model. figure 3 shows the default (un-zoomed) view from the panoramic image directly facing the building envelope. note that there are missing joints on the left side of the window. the model there fails to detect any objects of interest. however, this was not a case for zoomed images. figure 4a shows missing joints in the vertical direction but the algorithm failed to comprehend damages in the horizontal direction. observe that the confidence of the model in predicting missing veneer is relatively high i.e., always above 70 %. complete opposite picture gives the figure 4b where the missing joints were detected mainly in horizontal direction while the model’s confidence is in general lower. the model failed to comprehend a majority of missing joints in figure 4c. however, vertical joints are in this point of view badly visible and difficult to correctly interpret even for human eyes. furthermore, the model again fails to comprehend joints in horizontal direction which makes the confidence of the model over individual bounding boxes rather low. observe that the model prefers detecting missing joints of relatively thin size but fails to detect thicker joints. this is probably caused by the limited number of training images and by the low variance of images i.e., training set does not include missing joints of this size. the presumption that the low confidence in the previous case is caused by the low number of relevant training images is further strengthened by figure 5 where most missing joints were correctly detected with high level of confidence. the model only fails to grasp missing veneers in the opposite wall in the distance and few parts of the joints on the sides. 61 jan mandinec, pär johansson acta polytechnica ctu proceedings (a). detailed view 1. (b). detailed view 2. (c). detailed view 3. figure 4. detailed views. credit: anders jansson, all rights reserved. figure 5. image facing stairs [19]. 62 vol. 38/2022 towards an automatized and objective assessment . . . 4. conclusion this study investigated how the inspection process of a building envelope may be automated by the means of uavs and subsequent computer vision based analysis of images. the first part of the paper is a literature review summarizing how drones may be automatically navigated to capture data from a building envelope. the second part of the literature review shows how computer vision algorithms are used in image analysis. the latter part of the review was exemplified using computer vision based model (build-sense model) for detection of missing joints on a wall of tjolöholms castle, a historical building in sweden. the model failed to comprehend majority missing joints in vertical direction and joints of larger thicknesses. it was concluded that the model may be improved by adding a variance to the training set (more different examples) and by expanding the total number of the training set. nevertheless, current predictive power of the model may be typified as good enough for notifying inspectors, who should be responsible for final damage evaluation, about potential building envelope failure. despite the performance limits of the presented model, the conjunction of fast and overreaching uavs with the computer vision promises to bring revolution to the building inspection process. when properly set, uavs may scan whole building envelope in terms of minutes. subsequent analysis may be used for accurate damage quantification of building envelope i.e., detecting the number of cracks per square meter, measuring the scope of a damage etc. by separating the envelope into smaller parts, one could differentiate the performance of the envelope over those parts and subsequently use this information for accurate data-based failure predictions, opening the gate of predictive maintenance. acknowledgements this work was supported by the swedish research council for environment, agricultural sciences and spatial planning (formas) [grant no. 2019-01402]. the authors also thank chalmers information and communication technology (ict) area of advance for additional financial support allowing the development of the build-sense model. references [1] v. hasik, e. escott, r. bates, et al. comparative whole-building life cycle assessment of renovation and new construction. building and environment 161:106218, 2019. https://doi.org/10.1016/j.buildenv.2019.106218 [2] z. jiang, l. zhao, s. li, y. jia. real-time object detection method based on improved yolov4-tiny, 2020. [2022-02-12]. https://doi.org/10.48550/arxiv.2011.04244 [3] l. duque, j. seo, j. wacker. bridge deterioration quantification protocol using uav. journal of bridge engineering 23(10):04018080, 2018. https: //doi.org/10.1061/(asce)be.1943-5592.0001289 [4] t. rakha, a. liberty, a. gorodetsky, et al. heat mapping drones: an autonomous computer-vision-based procedure for building envelope inspection using unmanned aerial systems (uas). technology|architecture + design 2(1):30–44, 2018. https://doi.org/10.1080/24751448.2018.1420963 [5] h. freimuth, m. könig. planning and executing construction inspections with unmanned aerial vehicles. automation in construction 96:540–553, 2018. https://doi.org/10.1016/j.autcon.2018.10.016 [6] g. morgenthal, n. hallermann, j. kersten, et al. framework for automated uas-based structural condition assessment of bridges. automation in construction 97:77–95, 2019. https://doi.org/10.1016/j.autcon.2018.10.006 [7] a. benz, j. taraben, p. debus, et al. framework for a uas-based assessment of energy performance of buildings. energy and buildings 250:111266, 2021. https://doi.org/10.1016/j.enbuild.2021.111266 [8] k. chaiyasarn, m. sharma, l. ali, et al. crack detection in historical structures based on convolutional neural network. international journal of geomate 15(51):240–251, 2018. https://doi.org/10.21660/2018.51.35376 [9] s. sony, k. dunphy, a. sadhu, m. capretz. a systematic review of convolutional neural network-based structural condition assessment techniques. engineering structures 226:111347, 2021. https://doi.org/10.1016/j.engstruct.2020.111347 [10] ç. f. özgenel, a. g. sorguç. performance comparison of pretrained convolutional neural networks on crack detection in buildings. in j. teizer (ed.), proceedings of the 35th international symposium on automation and robotics in construction (isarc), pp. 693–700. 2018. https://doi.org/10.22260/isarc2018/0094 [11] j. fernandez galarreta, n. kerle, m. gerke. uavbased urban structural damage assessment using objectbased image analysis and semantic reasoning. natural hazards and earth system sciences 15(6):1087–1101, 2015. https://doi.org/10.5194/nhess-15-1087-2015 [12] l. yang, b. li, w. li, et al. deep concrete inspection using unmanned aerial vehicle towards cssc database. in proceedings of the 2017 ieee/rsj international conference on intelligent robots and systems. 2017. 9 p. [13] k. gopalakrishnan, s. k. khaitan, a. choudhary, a. agrawal. deep convolutional neural networks with transfer learning for computer vision-based data-driven pavement distress detection. construction and building materials 157:322–330, 2017. https: //doi.org/10.1016/j.conbuildmat.2017.09.110 [14] n. wang, x. zhao, p. zhao, et al. automatic damage detection of historic masonry buildings based on mobile deep learning. automation in construction 103:53–66, 2019. https://doi.org/10.1016/j.autcon.2019.03.003 [15] y.-j. cha, w. choi, g. suh, et al. autonomous structural visual inspection using region-based deep learning for detecting multiple damage types. computeraided civil and infrastructure engineering 33(9):731– 747, 2018. https://doi.org/10.1111/mice.12334 63 https://doi.org/10.1016/j.buildenv.2019.106218 https://doi.org/10.48550/arxiv.2011.04244 https://doi.org/10.1061/(asce)be.1943-5592.0001289 https://doi.org/10.1061/(asce)be.1943-5592.0001289 https://doi.org/10.1080/24751448.2018.1420963 https://doi.org/10.1016/j.autcon.2018.10.016 https://doi.org/10.1016/j.autcon.2018.10.006 https://doi.org/10.1016/j.enbuild.2021.111266 https://doi.org/10.21660/2018.51.35376 https://doi.org/10.1016/j.engstruct.2020.111347 https://doi.org/10.22260/isarc2018/0094 https://doi.org/10.5194/nhess-15-1087-2015 https://doi.org/10.1016/j.conbuildmat.2017.09.110 https://doi.org/10.1016/j.conbuildmat.2017.09.110 https://doi.org/10.1016/j.autcon.2019.03.003 https://doi.org/10.1111/mice.12334 jan mandinec, pär johansson acta polytechnica ctu proceedings [16] v. hoskere, y. narazaki, t. a. hoang, b. spencer. vision-based structural inspection using multiscale deep convolutional neural networks, 2018. [2022-02-14]. https://doi.org/10.48550/arxiv.1805.01055 [17] r. g. lins, s. n. givigi. automatic crack detection and measurement based on image analysis. ieee transactions on instrumentation and measurement 65(3):583–590, 2016. https://doi.org/10.1109/tim.2015.2509278 [18] karagrubis. brick efflorescence. https://rainguardpro.com/ [19] a. jansson. tjolöholms castle. 64 https://doi.org/10.48550/arxiv.1805.01055 https://doi.org/10.1109/tim.2015.2509278 https://rainguardpro.com/ acta polytechnica ctu proceedings 38:57–64, 2022 1 introduction 2 literature review 2.1 planning of uav based inspections of building envelope 2.2 automatize damage recognition and classification from images 3 case study 3.1 object detection model 3.2 tjolöholms castle and data collection 3.3 scope and the methodology of the case study 3.4 retraining the build-sense model 3.5 results and discussion 4 conclusion acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.38.0196 acta polytechnica ctu proceedings 38:196–202, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague analysis of the geometrical properties of fine recycled aggregates using digital image processing technique jentel de vlieger, jiabin li∗ ku leuven, faculty of engineering technology, department of civil engineering, spoorwegstraat 12, 8200 brugge, belgium ∗ corresponding author: jiabin.li@kuleuven.be abstract. the acquisition of natural sand and dumping of fine recycled aggregates (fra) are two serious environmental problems that can be solved simultaneously by using fra as replacement of natural sand in high-end applications. in the past, fra has been considered an unwanted by-product of construction and demolition waste (cdw) processing, due to the high levels of contaminations and high content of fines. however, fra is currently considered as a new secondary raw material with large potential. due to the increasing interests in fra it is of utmost importance to extensively examine their characteristics. to date, research has been done on the water absorption, density, chemical and mineralogical composition, and size distribution of the fra. however, knowledge on the geometrical properties of fra is limited. in this paper the geometrical properties of several types of fra are examined using digital image processing (dip). the effect of processing methods, crusher openings and crushing cycles are considered to gain new insights into the characteristics of fra. keywords: fine recycled aggregates, geometrical properties, digital image processing. 1. introduction in the last decades, climate change has been the most persistent environmental concern. the accelerated urbanization and industrialization pose severe challenges to sustainable development around the world. depletion of the earth’s natural resources is one of these challenges. over the past five decades, depletion of mineral resources expanded fivefold [1]. therefore, it is essential to minimize the use of natural resources and replace them with alternative resources. one of the possible solutions is to utilize recycled aggregates, a secondary raw resource derived from processing construction and demolition waste, as replacement of natural aggregates in cement-based materials such as mortar and concrete. so far, numerous research has been done on the use of coarse recycled aggregates (cra) in cement-based materials. this has led to an increased use of the cra in high-end applications. however, the use of fra as replacement of natural sand in cement-based materials is a subject that is dramatically lagging behind. today, fra is mainly used in road construction as foundation and sub-foundation layers. however, this application causes a decrease in the intrinsic value of the secondary raw material and can thus be seen as downcycling [2]. figures from the mdo (2015) show that 99 % of the produced fra in flanders are used in low-end applications such as foundation layers and backfills [3]. moreover, the current dominant application area, namely foundation layers in road construction, will not remain large enough to meet the production of the ra [2]. fra has been mainly used in low-end applications due to the high contamination, high content of fines, high water absorption and large variability. therefore, there is need to extensively examine the fra to understand their characteristics in order to use it in more value-added applications such as cement-based materials. up to now, some research has been done on the water absorption, density, chemical and mineralogical composition and size distribution of fra [4]. however, there is rather limited knowledge on the geometrical properties of fra. it is often stated that fra has a more angular and irregular shape compared to natural sand but there is little research to prove this statement. in literature, nearly no research can be found on the geometrical parameters of fra and how these parameters are affected by crushing method, size of the crusher opening and amount of crushing cycles. the geometrical properties of fra can have a major influence on the rheological parameters of the cement-based materials affecting its workability, compactibility and pumpability. solyman [6] tested the geometrical properties of fra using scanning electron microscopy (sem). it was found that the fra particles are more angular and irregular in comparison to natural sand. the author also analysed the sphericity of the particles by use of photogrammetric techniques. it was found that the length/width ratio of the natural sand and fra was similar while the sphericity varies more, confirming the greater angularity of the fra particles. the increased angularity of the fra particles can cause a decrease in the workability of cement-based materials. 196 https://doi.org/10.14311/app.2022.38.0196 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 38/2022 analysis of the geometrical properties of fine recycled aggregates . . . figure 1. sub-quantities to describe the geometrical properties of a particle. figure 2. definition of roundness by wadel [5]. fumoto and yamada [7], examined the geometrical properties of fra using the bet technique and reported that the greater angularity of fra is caused by the brittleness of the recycled materials and their production process. in addition, it was found that fra has a large specific surface area due to the open pores on the surface. the increased specific surface area can cause a loss of efficiency of superplasticisers. in this study the geometrical properties of several types of fra is examined using the digital image processing technique. the effect of crushing procedures, crusher opening and amount of crushing cycles on the geometrical properties of the fra is examined. 2. materials and methods 2.1. materials a total of 25 aggregates are analysed which includes 1 natural aggregate (as reference) and 24 fra’s. the natural aggregates are washed natural riversand 0–2 mm provided by the company bouwpunt botha, belgium. in total 13 fine recycled concrete aggregates (frca’s) and 11 fine mixed recycled aggregates (fmra’s) are tested. the aggregates were produced with two types of crushers and different crusher openings and crushing cycles are used to determine the effect of the crushing procedure on the geometrical properties of the fra’s. 2.2. methodology to quantitatively classify the geometrical properties of a particle, a 3d or 2d analyses can be used. a 3d analysis requires sophisticated equipment to establish a 3d model of the particle. the 3d model can be made using 3d scanning or by combining orthogonal images to represent the 3 dimensions. 2d image analyses is easier to perform and can be done using a regular camera or microscope. in a 2d image analysis it is assumed that the particles lay random or over there dominant/stable axis [8]. due to the complexity of a 3d analysis and the fineness of the particles used in this work, a 2d image analysis is used. as shown in figure 1, three parameters can be used to describe the geometrical properties of the aggregates: sphericity, roundness, and roughness. each of the three sub-quantities describes the geometrical properties of a particle but at a different scale [8, 9]. at large scale the particle’s morphology is examined. the sphericity is often used to quantify this characteristic [8]. the particle sphericity can be defined in various ways. j. zheng at al. [10] compared five existing definitions of sphericity based on twodimensional particle projections and found that the ratio of width to length, often referred to as the aspect ratio, provides the best distribution of sphericity values between 0 and 1 and is also independent of the particle roundness. at intermediate scale, the corners and edges of the particles are analysed. the roundness is often used as a measure at this scale [8]. wadel [5] stated that the roundness can be described as the average radius of corners and edges divided by the radius of the maximum inserted circle as shown in figure 2. at the smallest scale, the term surface roughness is used to describe the texture of the surface. in this research each aggregate sample is analysed based on sphericity and roundness as mentioned above. a keyence 3d laser scanning microscope vk-x3000 is used to take images of the aggregate samples. for each 197 jentel de vlieger, jiabin li acta polytechnica ctu proceedings (a). (b). figure 3. analyses of sphericity and roundness with matlab 2020 – mathworks. aggregate sample 400 images of individual particles are taken and analysed. 2.3. digital image processing technique matlab 2020 – mathworks is used to process the images and analyse them based on the sphericity and roundness. the sphericity is determined using the minimum and maximum feret diameters, i.e. the distance between two parallel planes restricting the particle perpendicular to that direction as shown in figure 3a. the roundness of the particles is determined using the matlab code proposed by zheng j. and hryciw, r. [10]. the matlab code determines the average radius of corners and edges and the radius of the maximum inserted circle of the particle as shown in figure 3b. 3. results and discussion 3.1. fraction all samples are sieved in 4 different fractions to examine the change in geometrical properties of both the natural sand and fra’s with the fraction sizes. in figure 4, the sphericity and roundness of the natural sand and all tested fra’s combined are shown. the graphs represent the cumulative percentages of aggregates exceeding a certain sphericity or roundness. in the legenda of the graphs, the sum of the cumulative percentages of each curve is given to further clarify the results. it can be seen from the figures that the sphericity and roundness of the natural sand does not significantly change with the fraction size. however, for fra’s more spherical and rounded particles are found in the fraction 500–250 µm. fra with fraction 2–1.6 mm show the lowest roundness. it is believed that this phenomenon is caused by a shift in the chemical composition across the different fractions. however, this needs to be verified. 3.2. type of aggregates in this work, the geometrical properties of both mfra and frca are tested. 4 samples of frca and 3 samples of mfra are crushed with both an impact crusher and a jaw crusher. in figure 5a and 5c, the sphericity and roundness of the mfra and frca crushed with an impact crusher are shown (m.i and c.i respectively). in figure 5b and 5d, the sphericity and roundness of mfra and frca crushed with a jaw crusher are shown (m.j and c.j respectively). it can be seen that, for both the samples crushed with a jaw crusher and an impact crusher, the sphericity and roundness are not affected by the aggregate type. due to the high force used during crushing both mfra and frca are produced under similar failure patterns. although, it is found that the sphericity and roundness of the particles are not affected by the aggregate type, it still needs to be verified if this is also the case for the particles surface roughness. 3.3. crushing method earlier research has shown that the crushing method affects the geometrical properties of cra [11]. in this research, the effect of crushing method on the geometrical properties of both mfra and frca are tested. in figure 6a and 6c, the sphericity and roundness are shown for frca crushed with an impact crusher and a jaw crusher (c.i and c.j respectively). in figure 6b and 6d, the sphericity and roundness are shown for mfra crushed with an impact crusher and a jaw crusher (m.i and m.j respectively). from the figures, it can be seen that both the roundness and sphericity of the particles are affected by the crushing method. the impact crusher produces rounder and more spherical particles than the jaw crusher. this was also found for cra by j. hubert et al. [11]. it is believed that these differences in the geometrical properties are caused by the breakage patterns that occur within the crusher. 198 vol. 38/2022 analysis of the geometrical properties of fine recycled aggregates . . . (a). natural sand: sphericity. (b). fra: sphericity. (c). natural sand: roundness. (d). fra: roundness. figure 4. geometrical properties of natural sand and fra of different fractions. (a). frca and mfra: impact crusher. (b). frca and mfra: jaw crusher. (c). frca and mfra: impact crusher. (d). frca and mfra: jaw crusher. figure 5. effect of aggregate type on the geometrical properties of fra’s. 199 jentel de vlieger, jiabin li acta polytechnica ctu proceedings (a). frca: sphericity. (b). mfra: sphericity. (c). frca: roundness. (d). mfra: roundness. figure 6. effect of crushing method on geometrical properties of frca and mfra. 3.4. amount of crushing cycles during the production process, recycled aggregates are often subjected to several crushing cycles. therefore, it is examined if the amount of crushing cycles affects the geometrical properties of the fra’s. a sample of frca and mfra was crushed one, two and three times as shown in figure 7. the sphericity and roundness of the frca and mfra particles are shown in figure 8. it can clearly be seen from the figure that for both mfra and frca the geometrical properties are not affected by the amount of crushing cycles. these results are contradictory with the results of cra since the geometrical properties of cra are found to be affected by the crusher opening. this can be the result of the breaking processes that occur during the production of the fra’s. cra is mainly produced under cleavage failure as shown in figure 9. this means that cra will contain mainly elongated particles after an initial crushing cycle. in a second crushing cycle, these elongated particles will mainly break on their short axis, producing more circular particles. after the third breaking process, the sample will again contain more elongated particles due to cleavage failure. however, fra are not produced under cleavage failure but under compression failure. the geometrical properties of the particles produced under compression failure are not dependent on the number of crushing cycles since the breakage pattern will be similar in each cycle. 3.5. crusher opening sizes since different crusher openings are used in practice, this study examined the influence of the crusher opening on the geometrical properties of fra. a sample of frca and mfra are crushed with crusher openings of 5, 10 and 15 mm. the results are shown in figure 10. it can be concluded from the results that the sphericity and roundness of the fra are not affected by the crusher opening. this can be explained by the failure mechanisms of the particles in the crusher. in general, the fra are produced by compression failure and cra are produced through cleavage failure. a change in the crusher opening will affect the particles that are produced through cleavage failure i.e. cra but not the particles produced under compression failure i.e. fra. therefore, the crusher opening does not affect the sphericity and roundness of the fra. 4. conclusions in this research, an extensive analysis is done on the geometrical properties of fra’s. based on the test results, the main conclusions are: • the sphericity and roundness of natural sand do not change with fraction. for fra, fraction 500– 250 µm contain more spherical and rounder particles compared to fractions 500 µm–2 mm. fra particles in fraction 1.6–2 mm show lowest roundness (highest angularity). 200 vol. 38/2022 analysis of the geometrical properties of fine recycled aggregates . . . figure 7. procedure crushing cycles. (a). frca: sphericity. (b). mfra: sphericity. (c). frca: roundness. (d). mfra: roundness. figure 8. effect of crushing cycles on geometrical properties of mfra and frca. figure 9. failure patterns that occur in jaw crusher [12]. 201 jentel de vlieger, jiabin li acta polytechnica ctu proceedings (a). frca: sphericity. (b). mfra: sphericity. (c). frca: roundness. (d). mfra: roundness. figure 10. effect of crusher opening on geometrical properties of mfra and frca. • the sphericity and roundness of fra are not affected by aggregates type. both mfra and frca showed comparable geometrical properties. • the sphericity and roundness of the fra are significantly affected by the crushing method. an impact crusher produces rounder and more spherical particles compared to a jaw crusher. • the sphericity and roundness of the fra are not affected by the crusher opening. references [1] g. habert, y. bouzidi, c. chen, a. jullien. development of a depletion indicator for natural resources used in concrete. resources, conservation and recycling 54(6):364–376, 2010. https://doi.org/10.1016/j.resconrec.2009.09.002 [2] j. gillabel, n. d’haese, p. dierckx, et al. stimuleren van het gebruik van gerecycleerde (en secundaire) granulaten in hoogwaardige toepassingen. steunpunt duurzaam materialenbeheer, leuven, 2016. [3] c. de groot, s. renate, j. van roo, et al. inzet primaire delfstoffen en alternatieve grondstoffen in vlaanderen in 2015. monitoringssysteem duurzaam oppervlaktedelfstoffenbeleid, vlaanderen, 2015. [4] l. evangelista, j. de brito. concrete with fine recycled aggregates: a review. european journal of environmental and civil engineering 18(2):129–172, 2014. https://doi.org/10.1080/19648189.2013.851038 [5] h. wadell. volume, shape, and roundness of rock particles. the journal of geology 40(5):443–451, 1932. https://doi.org/10.1086/623964 [6] m. solyman. classification of recycled sands and their applications as fine aggregates for concrete and bituminous mixtures. ph.d. thesis, university kassel, kassel, 2005. [7] t. fumoto, m. yamada. influence of quality of recycled fine aggregate on properties of concrete. memoirs of the faculty of engineering 40:81–84, 2002. [8] j. m. rodriguez. importance of the particle shape on mechanical properties of soil materials. ph.d. thesis, lulea university of technology, lulea, 2013. [9] s. tafesse, j. m. robison fernlund, w. sun, f. bergholm. evaluation of image analysis methods used for quantification of particle angularity. sedimentology 60(4):1100–1110, 2013. https: //doi.org/10.1111/j.1365-3091.2012.01367.x [10] j. zheng, r. d. hryciw. traditional soil particle sphericity, roundness and surface roughness by computational geometry. géotechnique 65(6):494–506, 2015. https://doi.org/10.1680/geot.14.p.192 [11] j. hubert, z. zhao, f. michel, et al. recycled aggregates properties – influence of the washing process on aggregates quality, belgian concrete day, liege university, 2019. [12] e. köken, a. özarslan. new testing methodology for the quantification of rock crushability: compressive crushing value (ccv). international journal of minerals, metallurgy, and materials 25(11):1227–1236, 2018. https://doi.org/10.1007/s12613-018-1675-7 202 https://doi.org/10.1016/j.resconrec.2009.09.002 https://doi.org/10.1080/19648189.2013.851038 https://doi.org/10.1086/623964 https://doi.org/10.1111/j.1365-3091.2012.01367.x https://doi.org/10.1111/j.1365-3091.2012.01367.x https://doi.org/10.1680/geot.14.p.192 https://doi.org/10.1007/s12613-018-1675-7 acta polytechnica ctu proceedings 38:196–202, 2022 1 introduction 2 materials and methods 2.1 materials 2.2 methodology 2.3 digital image processing technique 3 results and discussion 3.1 fraction 3.2 type of aggregates 3.3 crushing method 3.4 amount of crushing cycles 3.5 crusher opening sizes 4 conclusions references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.37.0038 acta polytechnica ctu proceedings 37:38–42, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague spectrometry of neutron field based on p(20)+be source reaction ján kozica,b,∗, milan štefánika,b a czech technical university in prague, faculty of nuclear sciences and physical engineering, department of nuclear reactors, v holešovičkách 2, 180 00 prague 8, czech republic b nuclear physics institute of the czech academy of sciences p.r.i., řež no. 130, 250 68 řež, czech republic ∗ corresponding author: kozic@ujf.cas.cz abstract. the research deals with a spectrometry of neutron field based on the p(20)+be source reaction. the p(20)+be interaction stands for a nuclear reaction where protons accelerated to energy of 20 mev bombard be target. produced neutron field has broad energy distribution up to 18 mev. the research was carried out at the nuclear physics institute of the czech academy of sciences and was motivated by extension of experimental possibilities of the ng-2 target station. to determine the neutron spectrum, the activation foil technique was used. during the experiment, ten activation foils were irradiated. activated foils were measured using the hpge detector to determine the reaction rates of observed reactions. the neutron field was simulated in the mcnpx code, and the simulation served as a priori information for the unfolding of the neutron spectrum in the sand-ii code. a determined neutron spectrum is essential for various experiments, e.g., material research, measurements of nuclear data, etc. keywords: accelerator-driven neutron source, beryllium target station ng-2, activation foil technique, neutron energy spectrum, sand-ii. 1. introduction fusion reactors such as iter [1], demo [2], are expected to use a d+t reaction which produces neutrons with energy distribution up to 20 mev and with a peak at an energy of 14 mev. the p(20)+be source reaction presented in this paper produces a neutron field with a broad energy spectrum up to 18 mev which covers almost the whole energy range of the d+t reaction. this fact allows the usage of the neutron field produced by the p(20)+be source reaction for the material research for future fusion reactors. the new neutron field from the p(20)+be source reaction can also be used for experiments of measurements, benchmarks of nuclear data and fast neutron activation analysis which extends experimental possibilities of beryllium target station ng-2. 2. materials and methods 2.1. activation foil technique an activation foil technique is a method which is used to characterize an energy spectrum of neutron sources. to determine a neutron spectrum using this method a set of spectroscopically thin foils or wires are irradiated in an unknown neutron field. the foils or the wires mentioned are called activation detectors. reactions of neutrons with the activation detectors produce radioactive nuclei which emit gamma radiation. the gamma radiation is detected using, for example, a hpge detector. from the measurements of the gamma-ray spectra of the irradiated activation detectors, it is possible to determine the reaction rates per target nucleus of the observed reactions using an equation rr = λs(eγ) treal tlive i(eγ)ε(eγ)n0(1−e−λtirr )e−λtcool (1−e−λtreal ) , (1) where λ is a decay constant of the produced radioactive nucleus, s(eγ) is an area of a full energy gammaray peak, tlive is a duration of the measurement of gamma spectrum, treal is a duration of the measurement of gamma spectrum with consideration of a dead time, i(eγ) is an intensity of the gamma ray, ε(eγ) is a detection efficiency of the apparatus for a given gamma-ray energy, n0 is the number of target nuclei, tirr and tcool is an irradiation and a cooling time respectively. correction factors are also applied. a relative uncertainty of the reaction rate is determined by an equation [5] δrr = √( 0.03 ( treal tlive − 1 ))2 + (tcoolλδt1/2)2 + δ2 s(eγ) + δ2 i(eγ), (2) where δj is a relative uncertainty of j-th quantity. the reaction rate is defined by an equation rr = ∫ +∞ 0 φ(en)σ(en)den, (3) where φ(en) is a neutron spectral flux density, σ(en) is a microscopic cross-section of respective reaction and en is an energy of neutron. from the measurement of the irradiated activation detectors, a set of reaction rates for the observed reactions is obtained 38 https://doi.org/10.14311/app.2022.37.0038 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 37/2022 spectrometry of neutron field based on p(20)+be source reaction reaction energy of reaction [mev] threshold energy [mev] 9be(p,n)9b −1.85 2.06 9be(p,pn)8be −1.66 1.85 9be(p,nα)5li −3.54 3.93 9be(p,pα)5he∗ → α+n −2.31 2.57 9be(p,npα)4he −1.57 1.75 9be(p,α)6li∗ → p+α+n −2.19 2.31 9be(p,α)6li∗ → p+5he∗ → p+α+n −3.25 3.61 9be(p,α)6li∗ → 5li+n −3.53 3.92 9be(p,p’)9be∗ → α+5he∗ → α+n −2.46 2.74 table 1. reactions of protons with beryllium which produce neutrons [3, 4]. and a process of unfolding the unknown neutron energy spectrum is based on a solution of equations resulting from the definition of the reaction rate. 2.2. p+be as a neutron source reaction based on the suitable physical properties of beryllium, mainly a melting point of 1287 °c [6] and thermal conductivity of 190 wm−1k−1 [6], it is possible to use it as an accelerator-driven external neutron source target, despite its toxicity [7]. reactions of protons with beryllium which produce neutrons are summarized in table 1. the most important neutron producing reaction is 9be(p,n)9b which contributes to the fast component of the neutron energy spectrum. the energy of this reaction is −1.85 mev [3], and it has threshold energy equal to 2.06 mev [3]. the rest of the reactions listed in table 1 produce neutrons with energy of the order of mev units. depending on the beryllium target’s energy thickness, the external neutron source can produce a white or semi-monoenergetic spectrum. neutron field with white energy spectrum is a neutron field with broad energy distribution. although the beryllium target is mostly used as an energetically thick white neutron source, uwamino et al. [8] have studied an energetically thin beryllium target as a semi-monoenergetic neutron source. the energetically thick acceleratordriven beryllium neutron source has been intensively studied by lone et al. [9] and brede et al. [10]. experimental data measured by lone et al. and brede et al. are shown in figures 1 and 2 respectively. brede et al. derived an empirical equation for the determination of a fluence averaged neutron energy [10] en = 0.433ep − 1.39, (4) where en is the fluence averaged neutron energy and ep is an energy of incident protons. the equation is usable for neutrons with an energy higher than 2 mev. 2.3. experimental apparatus at the nuclear physics institute of the czech academy of sciences (npi cas) an isochronous cyclotron u-120m and neutron target stations ng-2 are figure 1. neutron energy spectra from the source reaction p+be measured by lone et al. for proton energies of 14.8 mev, 18 mev and 23 mev [9, 11]. figure 2. neutron energy spectra from the source reaction p+be measured by brede et al. for proton energies of 19.08 mev, 19.92 mev and 22.01 mev [10, 11]. operated. the cyclotron u-120m is a multi-purpose device which can accelerate the positive and negative ions. it was manufactured at the joint institute for nuclear research in dubna russia. at the beginning of the operation, the cyclotron was able to accelerate only positive ions. an option to accelerate negative ions was added during a modernization of the cyclotron between 1996 and 1998. the beam of accelerated ions could be extracted to a hall where the cyclotron is located or to an external experimental hall. parameters of the ion beams extracted from isochronous cyclotron u-120m are listed in table 2. the department of nuclear reactions of the npi cas operates two accelerator-driven neutron producing target stations ng-2. one of them has embedded 39 ján kozic, milan štefánik acta polytechnica ctu proceedings energy maximal current ion [mev] [µa] h+ 6 − 25 5 d+ 12 − 20 5 3he+2 18 − 52 2 4he+2 24 − 38 5 h− 6 − 35 13 d− 11 − 20 10 table 2. parameters of the extracted ions from the cyclotron u-120m [12]. figure 3. the beryllium target station ng-2 at the nuclear physics institute of the czech academy of sciences. an energetically thin lithium target and the other has an energetically thick beryllium target. the beryllium target has dimensions of 8 mm in thickness and 50 mm in diameter. because the beam of protons is fully stopped inside the beryllium target, cooling of the target is needed and it is cooled by liquid alcohol at a temperature of 5 °c. this target is used to produce a neutron field with broad energy distribution. the beryllium target station ng-2 is shown in figure 3. 2.4. data acquisition to determine an energy spectrum at a position p14 (see figure 4) of the neutron field produced by an accelerator-driven external neutron source based on a source reaction p(20)+be, an irradiation experiment has been carried out at the nuclear physics institute of the czech academy of sciences. the experimental arrangement is illustrated in figure 4. the activation foil technique has been used and a set of various high purity metallic spectroscopic foils, which has been delivered by goodfellow company, have been irradiated. the set of foils has contained materials au, in, fe, al, lu, y, co, ti, nb, and ni (see figure 5). during the experiment, the proton beam has been accelerated to the kinetic energy of 20.134 mev and a mean current on the surface of the beryllium target has been 12.3 µa. the irradiation lasted for 11 hours. after the irradiation has ended, the activation detectors have been transferred to a gamma spectrometric figure 4. a schematic of the experimental arrangement. figure 5. the set of activation detectors. laboratory where their gamma spectra have been measured on a hpge canberra semiconductor detector for four weeks. 3. results from the measured gamma-ray spectra of the irradiated set of activation detectors the necessary quantities for determination of reaction rates of the observed reactions, which have been identified based on characteristic gamma-ray energy, a half-life of their products and with respect to their activation cross-section, have been obtained. using equations (1) and (2), a set of reaction rates and their relative uncertainties for the observed reactions, which are summarized in table 3, have been determined. the reaction (n,x) denotes a combination of reactions for the natural atomic abundance of the element. a priori information about the neutron spectrum has been obtained from simulation in mcnpx v.2.7.0. code [13] using la-150h [13] and endf/b-vii.1 [14] nuclear data libraries. the geometry in the simulation has been the same as has been shown in the schematic in figure 4. the neutron energy spectrum has been successfully unfolded in sand ii code [15, 16]. during the unfolding process, 22 out of a total of 24 reaction rates have been successfully used with respective cross-sections from the eaf-2010 nuclear database [17], and the neutron energy spectrum obtained from the simulation in the mcnpx code has served as an initial guess spectrum. the unfolded neutron energy spectrum shows good agreement with the mcnpx predictions. both simulated and unfolded neutron energy spectra from the source reaction p(20)+be at the position p14 are illustrated in figure 6. during the process of neutron spectrum unfolding, calculated over experimental reaction rate ratios (c/e) of all used reactions have been monitored and they are listed in table 4. as can be seen from table 4 all of the c/e ratios are close to 40 vol. 37/2022 spectrometry of neutron field based on p(20)+be source reaction reaction rr [s-1] relative uncertainty [%] natlu(n,x)176mlu 4.10×10−16 0.52 natlu(n,x)174mlu 8.69×10−17 1.84 93nb(n,α)90my 7.13×10−19 1.04 93nb(n,2n)92mnb 5.46×10−17 0.72 natni(n,x)58co 1.29×10−16 0.33 natni(n,x)57ni 2.41×10−18 0.78 natni(n,x)57co 4.38×10−17 0.97 89y(n,γ)90y 3.34×10−19 2.85 89y(n,2n)88y 8.09×10−17 0.47 27al(n,α)24na 1.60×10−17 0.62 197au(n,γ)198au 3.97×10−16 0.11 197au(n,2n)196au 2.89×10−16 0.19 197au(n,2n)196mau 1.39×10−17 0.72 59co(n,γ)60co 7.04×10−17 1.23 59co(n,α)56mn 4.18×10−18 1.45 59co(n,p)59fe 1.02×10−17 0.83 59co(n,2n)58co 7.40×10−17 0.24 natfe(n,x)56mn 1.51×10−17 1.02 natin(n,x)115min 1.65×10−16 0.41 natin(n,x)116min 1.23×10−16 0.62 natin(n,x)114min 1.54×10−16 0.74 natti(n,x)46sc 6.12×10−18 1.12 natti(n,x)47sc 4.88×10−18 0.33 natti(n,x)48sc 6.42×10−18 0.36 table 3. the set of reaction rates and their uncertainties for observed reactions. unity which confirm the correctness of the unfolding process. the unfolded neutron energy spectrum has been compared to neutron spectra reported by other authors to check the correctness and to show good agreement in shape. moreover, the mean energy of the unfolded neutron spectrum is 7.3 mev which is in agreement with the computed mean energy from the empirical equation (4). the neutron flux density for a neutron energy higher than 1 mev reached a value of 5.6×109 cm-2·s-1 at the position p14. figure 6. the unfolded and determined neutron energy spectra from the source reaction p(20)+be at the position p14. 4. conclusions at the nuclear physics institute of the czech academy of sciences, an energy spectrum of the reaction c/e ratio 197au(n,2n)196au 0.98 197au(n,2n)196mau 1.12 197au(n,γ)198au 0.96 27al(n,α)24na 1.19 115in(n,n’)115min 0.96 59co(n,p)59fe 0.93 59co(n,α)56mn 1.05 59co(n,γ)60co 1.04 59co(n,2n)58co 0.90 89y(n,2n)88y 1.03 89y(n,γ)90my 0.8 natfe(n,x)56mn 1.08 natti(n,x)48sc 0.98 natti(n,x)47sc 1.07 natti(n,x)44sc 0.92 93nb(n,α)90my 1.08 93nb(n,2n)92mnb 1.04 natlu(n,x)176mlu 0.99 natlu(n,x)174mlu 1.03 natni(n,x)58co 1.01 natni(n,x)57ni 0.91 natni(n,x)57co 0.96 table 4. calculated over experimental reaction rate ratios. new neutron field based on the p(20)+be source reaction at the position p14 has been measured. 41 ján kozic, milan štefánik acta polytechnica ctu proceedings the p(20)+be source reaction produces a neutron field with a broad energy distribution up to 18 mev and mean energy of 7.3 mev. the neutron energy spectrum has been determined using the activation foil technique. for proton beam current 12.3 µa the neutron flux density for a neutron energy higher than 1 mev reached a value of 5.6×109 cm-2·s-1 at the position p14. furthermore, the determined neutron energy spectrum shows good agreement in shape with neutron spectra reported by other authors for similar energies of protons. the neutron field from the source reaction p(20)+be has an energy distribution which covers almost the neutron energy spectrum from the d+t reaction and it is suitable for material research for future fusion reactors. it also extends experimental possibilities of beryllium target station ng-2, mostly for measurements, benchmarks of nuclear data, and fast neutron activation analysis. acknowledgements this publication and irradiation time at the ng-2 neutron source was supported by op rde, meys (ministry of education, youth and sports), czech republic under the project canam, cz.02.1.01/0.0/0.0/16_003/0001812. the research work was carried out within the adar project (project no. ltausa18198) supported by meys, czech republic under inter-action research programme. references [1] international thermonuclear experimental reactor. iter – the way to new energy. https://www.iter.org/, 2022. 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[17] l. packer, j. sublet, j. kopecky, et al. recent progress in neutron-, protonand deuteron-induced reaction nuclear data for eaf-2010 and the european activation system. journal of the korean physical society 59:1100– 1103, 2011. https://doi.org/10.3938/jkps.59.1100. 42 https://www.iter.org/ https://www.euro-fusion.org/programme/demo/ https://www.nndc.bnl.gov/qcalc/ https://doi.org/10.1016/j.radphyschem.2018.06.046 https://doi.org/10.1023/a:1010937301028 https://www.webelements.com/ https://doi.org/10.1002/14356007.a04_011.pub2 https://doi.org/10.1016/0168-9002(88)90318-x https://doi.org/10.1016/0029-554x(77)90616-4 https://doi.org/10.1016/0168-9002(89)90399-9 https://www-nds.iaea.org/exfor/ http://www.ujf.cas.cz/en/ https://doi.org/10.1016/j.nds.2011.11.002 https://doi.org/10.1016/0029-554x(72)90482-x https://doi.org/10.3938/jkps.59.1100 acta polytechnica ctu proceedings 37:38–42, 2022 1 introduction 2 materials and methods 2.1 activation foil technique 2.2 p+be as a neutron source reaction 2.3 experimental apparatus 2.4 data acquisition 3 results 4 conclusions acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.38.0620 acta polytechnica ctu proceedings 38:620–629, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague investigating the change in preference of living and transport during covid-19 oskar fahlstedt∗, winnie ma norwegian university of science and technology, høgskoleringen 7a, 7034, trondheim, norway ∗ corresponding author: oskar.fahlstedt@ntnu.no abstract. the covid-19 pandemic meant that people had to perform most of their daily activities from home. this study intends to investigate if a period of working from home has changed living preferences concerning housing size and location. the data was collected through a web-based survey. the literature is divided regarding the potential benefits of reduced commuting due to increased home office utilization. there is a prevalent risk of rebound effects as saved travel time might lead to increased travel on leisure, which could counterweight the benefits of reduced daily commuting in cities. most of the respondents in this study want to work from home 2–3 days a week. the respondents that desire to move to a less central location state the lower square meter price as one important reason. analyses in prevailing research did not investigate the preference for larger homes combined with increased home office use. future studies should also investigate the desire for larger homes to achieve a holistic evaluation of the emissions related to telecommuting. keywords: covid-19, teleworking, telecommuting, emissions. 1. introduction the covid-19 pandemic has affected many people either directly or indirectly. in attempts to reduce the spread of the virus, a lot of people have worked from home aided by modern information and communication technology (ict). the long-term societal effects of the pandemic remains unclear, but it illuminate the importance of the the sustainable development goals (sdgs) for human and planetary prosperity [1, 2]. however, the results of impeded mobility and inactive industries have reduced greenhouse gas (ghg) emissions and improved the air quality in cities, which benefits sdg 13 (climate action) [1–3]. moreover, albeit good news, a sensible assumption is that this is a deviation whilst society will return to its old behavior patterns once the pandemic is over [3, 4]. nevertheless, it is imperative to investigate this endeavor as utilization of the home office for a long time might affect the perception people have of their work and housing situation [4, 5]. it might contribute to a situation where people demand larger homes due to a continuation of the home office. as housing prices generally decrease with greater distance to city centers, it might also increase urban sprawl. furthermore, as larger homes demand more energy and material whilst residents in the periphery, to a more considerable extent, depend on their cars, the positive environmental effects might be reversed instead. this paper intends to investigate this notion and ask the following research questions: (1.) how has a period of working from home changed people’s preference of distance to work and housing size? (2.) what potential challenges can derive from a changed preference in commuting and housing size? 1.1. literature review when investigating the potential positive and negative implications of increased utilization of the home office it is sensible to mention the aspects of “teleworking” and “telecommuting”. the literature defines a teleworker as a person that uses information and communication technology (ict) to work from another location than the physical workplace [6, 7]. furthermore, according to cerqueira et al. [8] a distinction is to be made between a teleworker and homeworker, with the former working part-time from home and part-time from work while the latter identifies the home as the physical workplace. o’keefe et al. [9] defines telecommuters as anyone that works from home at least one day of the workdays during the week. thus, a telecommuter is a person that partially works from home at least one day a week and commutes to work all other days not working from home [7]. in this paper, the term telecommuter is adopted and used in the results to describe a person that works from home at least one day of the workweek. thus, the term is overarching and includes the aspects of the term teleworker as well. moreover, the characteristics of teleworking and telecommuting are not limited to a change in environmental burdens such as emissions from commuting or energy use in buildings, there are also social aspects to consider [10]. however, the scope of this paper focuses on the potential environmental benefits or burdens of changed living and transport preferences in a situation when more people are choosing to work from home. 620 https://doi.org/10.14311/app.2022.38.0620 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 38/2022 investigating the change in preference of living and transport . . . 1.1.1. positive aspects of increased use of home office the advance in ict has enabled the home-space to become an environment for work as well as any office for many people. and some potential benefits of this type of work arrangement are the reduced congestion in traffic and the need for fewer square meters in office spaces [6, 11, 12]. furthermore, with less commuting, there are direct environmental benefits like the reduction in greenhouse gases (ghg) from vehicles with internal combustion engines (ice) [13]. the indirect benefits deriving from reduced demand for office space are reduced embodied (less material needed) and operational emissions (decreased energy demand for heating and cooling) [14, 15]. an analysis of travel patterns based on mobility reports provided by google during the covid-19 pandemic combined with multi regional-input output data (mrio) of the global economy demonstrates that there is a significant reduction potential of environmental stressors. this is the conclusion in the work of pomponi et al. [16] as the developed scenario for increased teleworking together with 50 % reduction in energy demand in offices would lead to a 5 % reduction of global ghg emission. the authors put into the context of the paris agreement and state that it is half of the yearly decrease in ghg needed to align with the goal for 2050 [16]. through stakeholder involvement and backcasting, quantitative scenarios were developed and calculated in the article by godínez-zamora et al. [17]. the authors conclude that telecommuting is essential in reducing transportrelated emissions in cities. in a framework developed by shabanpour et al. [12] the consequences of teleworking are investigated in three adoption levels and the results show that daily co2 and pm25 emissions can be reduced with 696 (-0,7 %) and 167 (-1,14 %) tonnes a day on average 0,7 % and 1,14 % in their study performed in chicago. in a study performed by giovanis [18] the change in air quality in a swiss city was analyzed in a scenario where increased teleworking decreases the traffic volume. the result after analysis of demographic household data indicates that it reduces toxic pollutants as fewer commute to work and should be included in sustainable urban planning strategies. o’keefe et al. [9] performed a study based on the results of census data from the greater dublin area and complemented it with a survey for deeper analysis. they determine that 27 % of the population work from home at least once a week and that a majority of the respondents (53–74 %) prefer to work from home at least once a week. thus, the authors conclude with the use of proxy values for kg co2 emissions that a scenario with 20 % of the population working from home one day a week for a year, the total saving would amount to 60,000 tons of co2 [9]. it amounts to about an 8.6 % reduction in total co2 emissions deriving from transport in dublin [9, 19]. finally, güereca et al. [13] investigate carbon abatement strategies for a research institute on a university campus in mexico city, and the results from a processbased life cycle assessment approach demonstrate that a scenario with reduced commuting will reduce the total co2 emissions. if half of the staff and students would work remotely two out of five days, then the total emissions would be reduced by 15 %, and combining this with increased car-sharing would lead to a 22 % reduction in total emissions. 1.1.2. potential negative aspects of increased use of home office zhu and mason [15] investigate the potential telecommuting has for reducing travel distance and indirectly ghg-emissions emissions in the usa by scrutinizing data gathered from the us national household travel surveys (nhts). the results reveal that co2 emissions have increased for the telecommuting population. there might be a potential rebound effect when telecommuting in combination with increased fuel efficiency induces people to accept longer distances to work and increase non-work-related trips instead. similarly, cerqueira et al. [8] discovered that co2 emissions deriving from teleworkers are higher than non-teleworkers due to rebound effects of increased distances for commuting and non-related work trips. there is a potential for an increase in this pattern when developments in ict make the home office more efficient. furthermore, if more people utilize their home as office space the home the electricity consumption will increase and time saved from not commuting leaves people with more time to travel longer distances in their spare time [20–22]. in the findings from cerqueira et al. [8] co2 emissions from teleworkers are higher than non-teleworkers due to increased kilometers needed during the days traveling to work. it also identifies men as being responsible for a higher share of direct emissions since they traveled long distances by car. in comparison to non-telecommuters, the telecommuter has a larger travel budget which entails that they are less dependent on having access to a physical workplace and other amenities [21]. zhu et al. [21, p. 25] describe them as “footloose” as working from home enables a lifestyle that is decoupled from geographical locations and consequences might be increased instead of decreased travel demand. as aforementioned, reduced commuting enables more time for non-work trips [20–22] and alonso et al. [22] demonstrate the potential rebound effects as vehicle kilometers (vk), energy use, and emissions increase in a scenario where increased teleworking is assumed. 1.1.3. holistic approach to increase telecommuting urban planners need to be cautious of how neighborhoods are designed and be aware of the consequences that demographic aspects have for the adoption of teleworking among inhabitants. the socio-economic, neighborhood characteristics and accessibility are factors that determine where inhabitants chose to live [7]. additionally, urban forms are also influential as high street connectivity, access 621 oskar fahlstedt, winnie ma acta polytechnica ctu proceedings to public transport, and density make fewer people work from home. for neighborhoods with higher land use per person, irregular street patterns, and less accessible public transport the utilization of the home office is higher [23]. if given the opportunity to work from home at least once a week workers are less inclined to let the distance to work be a deterministic factor as workers accept increased commuting time (5 % longer) between home and the work place [6]. allen et al. [7] provides a rich review of the literature when investigating the multifaceted outcomes of telecommuting and concludes that there is not a clear answer to whether telecommuting is beneficial or not. a multidisciplinary approach is sought as the topic of telecommuting must be addressed holistically. thus, not only consider the environmental but also acknowledge the softer social aspects that influence the adoption of working from home. the demand for local amenities such as affordable and diverse housing, ict, schools, daycare, and leisure influences the decision to work from home or not [7, 10]. for example, no accessibility to kindergarten might leave people with no other option than combining child care and working from home [7]. there is also an aspect of gender since women care for children to a higher extent than men and are more likely to have to combine work with childcare [7, 10]. the economical aspects are also prevalent. for example, tax reductions for the one working from home due to infrequent use of cars compared to a regular day commuter using a car to and from work [24]. 2. method and sample a self-assisted web-based survey was used to collect the opinions and experiences of people living conditions in trondheim (norway) during the pandemic. the data collection lasted two and a half months from mid-june to the end of august 2021. the samples were created through convenience sampling by distributing one thousand flyers in residential areas (mailboxes) and public spaces. the use of a web-based method was advantageous in mid-pandemic times regarding infection risk, being spatial-temporal flexible, and in collecting a number of respondents within the project’s framework. anonymity was an important aspect of the study, as for the layman transportation research often is associated with strong intentions towards sustainable transport which may contribute to social desirability bias; skewed results in favor of what is deemed “correct” behavior. from the information on the flyer, the survey could be accessed by scanning the qr-code or typing the url address. the residential areas were of mixed housing types and were centrally located in the city. there is a propensity of participation bias as the accessibility to mailboxes varied according to housing types (difficult to access mailboxes for some apartment complexes). aside from residents, the flyers were placed in public areas like at bus stops, outside groceries stores, public bulletin boards, and other busy areas. thus, the geographical scope of the study is not limited to the residences in central areas of the city but expands outwards to respondents that live further out but that happened to have errands in the central area of the city. responses from visitors or tourists were discarded from the results. the approach used when distributing the flyers means that it is not possible to provide any response rate as multiple people might have been informed through the same flyer. the survey consisted of four parts; the first part covering demographic information of the respondent, the second subjective questions on residence location and other facilities, the third investigating the prospect of work-from-homes, while the fourth and last part collected information regarding travel habits covering the primary mode and commute time, and preferred travel time for various activities. 3. results and discussion 3.1. housing location the respondents were asked if their preferences towards residence location had changed regarding centralization or decentralization. 57 % of the respondents stated unchanged preferences, about 19 % state they want or already have moved more centralized during the pandemic, and the rest 24 % state wanting to or already moved further away from the city center. furthermore, the respondents that stated changed preferences were asked to rate the degree of influence on these preferences on a list of different aspects of travel activity, living conditions, or residence characteristics. among the respondents stating an increased preference for centralization, the accessibility to public transport and mobility by active modes was deemed important aspects, see top rows in figure 1. on the opposite range, aspects having relatively low influence are “closeness to health services” and the aspect of “less residence area to maintain”. oppositely, the respondents stating an increased preference for more decentralized residence location, favor residencerelated aspects, see figure 2. living further away from heavy traffic areas as well as access to a garden are also deemed important aspects. on access to parking space for the private car, only 47 % deemed it as influential. the increased rate of telecommuting has not induced the strongest effect on preferences, having 60 % stating an effect, where only 25 % percentage being “substantial effect”. food delivery services and digital alternatives for socializing were not assessed as very influential aspects regarding location preferences. the aspect of residents wanting to move to larger homes was not covered in the literature as the consequences of telecommuting and teleworking were foremost discussed in terms of emission from reduced or increased travel [8, 9, 15, 16, 18, 21]. the potential increase in energy use in the homes was discussed [20–22] but the demand for larger homes due 622 vol. 38/2022 investigating the change in preference of living and transport . . . figure 1. the assessed effect of each aspect on preferences for centralization (n = 63). figure 2. the assessed effect of each aspect on preferences for decentralization (n = 80). to telecommuting was not covered is perplexing since it is an important aspect when discussing emissions in the built environment. however, it is imperative to investigate as holistic assessments are sought for when discussing telecommuting [7]. the impact of house size adds another dimension to the debate of the impacts of telecommuting. this is since residents living in larger homes felt more content with their home office situation. regarding telecommuting, the respondents preferring centralization have on average fewer commuting days and the least willingness or possibility to telecommute post-pandemic compared to the “unchanged” and “less central” respondents. these results may indicate a tendency of more positive attitudes towards telecommuting when having preferences towards decentralization rather than centralization or vice versa, an attitude on telecommuting that depends on location preferences. in general, 48 % of the respondents stated being just as content with their residence during the survey response moment (mid-pandemic) compared to prepandemic times. about 28 % stated being less content and 20 % being more content, while the rest stated not being sure. in combination with other variables, level of contentment seem to have no clear correlation with stated residence density, residence type, residence size, age, household type, gender, and income. 3.2. transport mode and travel times the respondents were asked to assess the maximum acceptable travel time for various trip purposes independent of their current residence location. not all purposes were applicable and the least relevant were kindergarten, primary school, and high school by only 30 % of respondents, see figure 3. the lowest average accepted travel time is for bus stops at 6 minutes, whereas the next lowest is grocery stores at 9 minutes. on the opposite end, trips to the workplace or cinema/theaters have on average an accepted travel time of 25 minutes. the fact that workers are willing to accept longer commute times makes an increased adoption of home office an interesting scenario in the post-pandemic future. the respondent’s general priorities in residence location may be based on their assessment of important aspects and their maximum acceptable travel time for different destinations. it 623 oskar fahlstedt, winnie ma acta polytechnica ctu proceedings figure 3. the average maximum preferred travel time by the various trip purposes. the dots’ sizes represent the share of applicability for the respondents. reveals that commute times may be one of the more flexible variables compared to trip purposes with much lower acceptable travel times. the lower the travel times, the more restrictive the vicinity to this destination becomes in residence location compared to other activities/destinations given the assumption that each destination is equally important for the individual. it is important to notice that the time assessment states a desire or a preferred estimate, but do not tell us the relative importance of each trip purpose when facing the dilemma of fulfilling the max criteria of several different destinations (e.g. more than 6 minutes to bus stop versus more than 14 minutes to the gym). to assess the importance of the destinations/activities, one could conduct a stated preference survey followed by a calculation of the travel time elasticizes per activity/destination. while not accounting for the individual destination importance, our sample reveals public transportation to be the dominant primary mode for both work and leisure purposes, making the proximity to bus stops at 6 minutes quite relevant as confirmed in the assessment of aspects that affect the preference for centralization. thus, the potential impact of increased car use due to teleworking [8, 15, 21, 22] is not revealed in this study as a geographical decoupling is potentially not viable when respondents are dependent on public transport services as much as they were in this study. the requirement for bus stops nearby reveals that a relocation further away is not preferable as the frequency of bus stops usually decreases with a lower density of residencies. the average commute is 23 minutes while the average accepted commute is 27 minutes, see figure 4 for the distribution of the time difference between the respondents’ acceptable commute times and their actual commute times. the density figure 4. the density distribution of the time difference between maximum acceptable commute times and actual commute times (n = 182). figure 5. transport mode distribution each for work and leisure purpose. distribution shows a peak at 0 minutes, indicating that a majority of respondents do not prefer any additional travel time by any means. the graph has more volume on the right side which reveals that most of the workers have commute times within their maximum travel time. though, it is also apparent that there are some incidents of negative values (the other side), indicating that a few individuals are currently commuting longer than they want to. the distribution of primary modes for commute trips (n = 183) show that public transport dominate with 45 %, see figure 5. mind the newly emerging mode e-scooters make up almost 2 % of the main modes in respect to commuting. given car as the main mode, the total pool of cars accessible (can be more than one car) consist of 40 % electrical powered cars, 36 % gasoline, 20 % diesel, and the rest powered by alternative energy sources. on a national level, 12 % of the private cars are electric-powered, but this share is higher for the average car commuter living in urban areas that cross a toll ring – as an electric car pays no fees passing the toll per this date 2022 [25]. the mode distributions are similar across genders, households, and education levels. the mode distributions are similar across genders, households, and 624 vol. 38/2022 investigating the change in preference of living and transport . . . education levels. tendencies for a higher share of car use are found among households with children, households with higher income, single-family residences, or row houses compared to apartments. the modal distribution seems to be strongly related to the travel times for work trips, with active modes as the dominant mode choice for shorter trips until public transport takes over. the car share does not vary the same as the other modes, where a possible explanation for this is its ability for trip chains (e.g., escort trips to school/activity), having space to bring more people and goods, reducing any walking and waiting time and requiring marginal physical effort. for leisure trips (n = 338) the mode distribution was 40 % for public transport, still a great share, but less than the modal distribution for work trips, see figure 5. given car as the main mode, 37 % of the accessible cars are powered by gasoline, 37 % by diesel, 20 % electric, and 7 % alternative energy sources. the percentage of car use is higher for leisure than commutes, which is a bit surprising as the fact that students’ mode choices are included in leisure (and not in the distribution for commute trips) was thought to have a reducing effect on the share of car usage. it was revealed that the cause of this was mostly due to workers having public transport as their main mode for commutes but switching to the car as their leisurely main mode. this is interesting as some findings in the literature indicated that car trip distances increase when working from home [8, 22]. when asked of walking time to the closest public transport stop/station, 76 % state less than 5 minutes and 20 % from 5 to 10 minutes. there was no clear correlation between mode choice and vicinity to public transport stops. 3.3. working from home and their willingness to move when asking full-time, part-time workers and students (total n = 302) about their preferences regarding telecommuting, 50 % were positive to telecommuting (n = 148), though of those 7 % and 6 % stated a wish for telecommuting but experienced challenges with inadequate workspace at home and not being sure of the employer would allow it, respectively. 30 % stated not wanting to telecommute post-pandemic, and 5 % was not sure. out of those wanting to work from home, 12 % stated a wish of working max 1 day a week, 65 % was willing to work up until 2–3 days a week and 18 % stated a willingness for 4–7 days a week. there is some inconsistency among the answers, as 4.3 % answered a willingness of 0 days a week after stating a willingness for telecommuting. zhu et al. [21, p. 25] described teleworkers as “footloose” since they are less dependent on the physical location of the workplace and amongst the respondents, half of them state a wish to, some extent, continue to work from home after the pandemic. the results show that among the full-time workers there is a greater share willing to telecomfigure 6. willingness to move for increased residence size given the possibility to telecommute in respect to number of days telecommuting mid-pandemic. mute compared to part-time workers. the part-time workers state more often that their work tasks cannot be conducted from home. of those n = 163 willing to telecommute post-pandemic (50 %), aside from asking about the willingness of the number of days telecommuting, they were asked about their willingness to move for increased residence size and if they’re willing to move further away from their current workplace. when asked about their willingness to move for a bigger residence given the possibility of telecommuting, 59 % were willing, 29 % were not, and 12 % were not sure. 41 % (n = 64) were willing to move further away from their workplace, 40 % were not, and 19 % were not sure. it is as expected that some would be willing to increase the residence size without the compromise of moving further away, though our results also show the opposite, where some individuals want to move further away without increasing the residence size (n = 8). the results show different relations between location preferences and days telecommuting compared to “willingness to increase residence size” and days telecommuting, as a viable increase in residence often involves relocation less central (not necessarily, but often a correlation). given the possibility of teleworking in the future, respondents answering “no” to a willingness of increased residence had an overall higher median number of days telecommuting during the pandemic compared to those positive to increase residence size, see figure 6. the results on the hypothetical post-pandemic telecommuters indicate that their willingness to move further from their workplace is not influenced by their work-from-home frequency mid-pandemic, as those willing to move further and those not wanting to move have the same wishes of working from home in the future. there are as many reasons to move residence as there are individuals, but the contentment level of their residence may be an overarching factor for a general indication of their willingness to move. this study asks respondents about their willingness to move residence relatively to their workplace, and further questioning on transport are only asked those who report a willingness to move further away from their workplace as opposed to those who do not want that. 625 oskar fahlstedt, winnie ma acta polytechnica ctu proceedings this is because moving further away would generally increase commute times, reduce the propensity for active modes of public transport and therefore increase traffic-related burdens (e.g. rush hour, congestion, local pollution). results indicate that the respondents being less content with their residence have a higher willingness to move given the possibility of telecommuting. though, the effect of contentment is not unidirectional as those being “more content” show a level of willingness lying just in between the “less content” and “unchanged”, when it would have been expected to show the lowest willingness (below the level of those answering “unchanged”). 3.3.1. transport and travel time given telecommuting the respondents that answered a willingness to move further away from their workplace given the possibility of telecommuting, were asked to assess the potential acceptable increase in commute times. for all transport modes except biking, there was stated a range of increased travel time, see figure 7. it is noticeable how some pedestrians would be willing to walk that much longer if not thinking of changing mode in such a context. mind that each sample is limited and a few extreme values may therefore have a relatively big influence on the aggregated results. a look at the current commute times and the willingness of increasing these show no clear tendency between the two variables. the number of stated maximum days of telecommuting post-pandemic do not seem to have a clear relation with the willingness for longer commute times as showing big variations among the answers. the group of workers (n = 64) stating a willingness to move further from their workplace (the 41 % positive to telecommuting), though a small group, have a transport distribution that reveals, compared to the distribution for all workers, that pt and e-bike users were slightly over-represented, pedal bike users underrepresented, pedestrians and car users were “balanced” represented in their willingness to move further away from their workplace. this is a bit surprising as one would expect the ones having the least distancesensitive modes (e.g. car) to show a greater willingness to move decentralized. given this study’s sample, among students and workers, 4–6 % self-predict increased car use given a reduction in pt/ biking (when willing to move further away from the workplace due to telecommuting). with a bigger sample size, it would be possible to look closer into which primary modes that predict the biggest modal shift. the potential benefits of telecommuting are foremost reported in the form of reducing emissions related commuting transport and traffic congestion. literature has shown that telecommuting reduces commute frequency, but may have an increasing effect on leisure trips. this since time saved on commuting from might increase car use instead as it is a popular figure 7. willingness for longer commute times given new residence location due to telecommuting, grouped by stated primary transport mode. mode of transport for leisure [8, 15, 21, 22]. although, the literature describes how the car is utilized to a further extent for spare-time trips than commuting there was no clear distinction of this rebound effect in the results from the questionnaire as a majority stated public transport and walking as the primary mode for leisure. a possible effect of fewer commute trips by pt users is the “lower value for money” when traveling on the same period ticket for fewer trips – which may result in omitting the purchases of period tickets altogether in favor of private modes like car or bike. other studies highlight that telecommuters tend to commute further when going to their workplace, indicating that their homes are located further from work compared to non-telecommuters which is problematic with the primary mode being car [8]. the respondents that used motorized transport stated that them working more from home would make them accept longer commutes. about half of the workers were willing to work from home 3–5 days a week post-pandemic and the accepted increase in commute times was 35– 45 minutes (depending on transport mode) which is significantly higher than suggested in literature [6]. however, the sample size is limited and has volatile results due to the marginal influence of a few extreme values. 3.4. limitations the study was conducted in a university city resulting in a data collection recruiting an overweight of students. the survey sample, although recruited as evenly as possible within limited time and financial resources, cannot be regarded as demographically representative of the entire population, but rather as indicative of the residential experience in a city during the pandemic. as a limitation of this study and discussion of the article, the presented analyses are mostly overarching, and rather point into general directions or tendencies of main mechanisms. as such, there is a demand for future research to go beyond the descriptive results presented in this article and focus on finding potential variables with the least or most influence on the observed tendencies. 626 vol. 38/2022 investigating the change in preference of living and transport . . . 4. conclusions the conclusion sets out to answer the two research questions asked in the introduction of this study. 4.1. how has a period of working from home changed people’s preference of distance to work and housing size? in total there are almost equal shares of respondents having changed preferences towards centralized and decentralized locations, where people favoring decentralized locations have had and wish to continue telecommuting more than those with other preferences. given the 50 % respondents being positive towards future telecommuting, the average willingness of maximum days telecommuting is 2–3 days a week. about three out of five future telecommuters state a willingness to increase their residence size due to telecommuting. there is no clear correlation between the frequency of home office mid-pandemic and the wish for a larger residence, though it is worth mentioning that resourceful individuals more often possess the types of jobs more suitable for home office; one of several potential mechanisms. but for the number of days wanting to telecommute post-pandemic, respondents willing to move had a higher median day per week of maximum days telecommuting than the individuals who did not want to move. 4.2. what potential challenges can derive from a changed preference? the literature is divided regarding the environmental benefits of telecommuting. an increase in telecommuters reduces the emissions related to omitted commute trips, but a possible rebound effect may occur in the form of longer leisure trips. in total, the changes in travel habits may cancel each other out or the rebound effect outweighs the benefits. the potential emissions from the changed preferences amongst the respondents were not addressed in this paper but scenario testing with a focus on embodied and operational emissions from transport and housing should be investigated further. the findings from the study demonstrated that respondents preferred to travel by public transport for commuting and leisure purposes. thus, infrastructure and transport services are important factors even if telecommuting increases. the rebound effect discovered in the literature was not prevalent in this study as public transport is used more than cars. the desire for larger homes when working from home adds another dimension to the potential rebound effects of the home office. the benefit of the reduction in required office space should be analyzed with the desire of people wanting to increase the size of their homes. finally, some suggestions for further research: (1.) a larger cross-sectional study similar to this study in cities with other work cultures; (2.) scenario testing with a focus on embodied and direct emissions from groups with various work arrangements; (3.) the potential consequences of the desire for larger homes when working from home should be investigated further. 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https://www.toi.no/getfile.php?mmfileid=53659 vol. 38/2022 investigating the change in preference of living and transport . . . a. appendices figure 8. share of respondents having telecommuting before, during and wanting to after the pandemic. 629 acta polytechnica ctu proceedings 38:620–629, 2022 1 introduction 1.1 literature review 1.1.1 positive aspects of increased use of home office 1.1.2 potential negative aspects of increased use of home office 1.1.3 holistic approach 2 method and sample 3 results and discussion 3.1 housing location 3.2 transport mode and travel times 3.3 working from home and their willingness to move 3.3.1 transport and travel time given telecommuting 3.4 limitations 4 conclusions 4.1 how has a period of working from home changed people's preference of distance to work and housing size? 4.2 what potential challenges can derive from a changed preference? references a appendices https://doi.org/10.14311/app.2022.33.0070 acta polytechnica ctu proceedings 33:70–77, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague natural fiber-reinforced light-weight cement blocks prepared from waste for sustainable development latda chandenga, thanawat meesaka, chakrapan tuaktab, tidarut wisuthseriwongb, tamon uedac, oratai jongprateepa,∗ a kasetsart university, faculty of engineering, department of materials engineering, 50 ngamwongwan road, lat yao, chatuchak, 10900 bangkok, thailand b kasetsart university, faculty of engineering, department of civil engineering, 50 ngamwongwan road, lat yao, chatuchak, 10900 bangkok, thailand c shenzhen university, guangdong provincial key laboratory of durability for marine civil engineering, college of civil and transportation engineering, shenzhen, guangdong, 518054, china ∗ corresponding author: fengotj@ku.ac.th abstract. low density of the light-weight cement blocks offers an advantage in terms of dead load reduction, which is advantageous in structural design-reduction of size and numbers of loadcarrying structural components is possible. production of cement blocks, however, generally requires ordinary portland cement (opc) which creates harmful environmental impacts. utilization of waste as alternative raw materials for cement production is a route to alleviate the problem. this research aims at synthesizing eco-friendly cement-like material for production of light-weight cement blocks. the cement-like material were prepared from eggshells, cockleshells, and rice husk ash (rha). with high content of calcium oxide and silica, eggshells and cockleshells are potential sources of calcium, whereas rha is a good source of silicon. additionally, a fuel-efficient solution combustion technique was employed in synthesis of the cement-like material. phase identification analysis of the synthesized powder indicated that tri-calcium silicate (c3s), di-calcium silicate (c2s), tri-calcium aluminate (c3a), and tetra-calcium alumino ferrite (c4af), which are main constituents of opc, were obtained. to fabricate eco-friendly light-weight cement blocks, the synthesized cement-like material were mixed with cement, water, and additional rha and cast into blocks. the optimal compressive strength and density of the cement blocks were in comparable range with the standard light-weight concrete defined by thai industrial standards institute (tisi) and american concrete institute (aci 213,2001). with jute fiber reinforcement, enhanced compressive strength of 20% was achieved, while elimination of spalling after compression test was clearly evident, implying a more ductile failure. keywords: fiber reinforcement, light-weight cement block, waste. 1. introduction as a predominantly agricultural country, agricultural sector in thailand plays a significant role in economic contribution. with increasing world population and growing demand for agricultural products, considerable amounts of agricultural residues are produced annually. it has been reported that approximately 5 million tons of rice husk are produced in thailand annually [1]. residues from poultry industry, one of thailand’s promising agri-food segments, is notable. production of eggshells in thailand is estimated to be around 650,000 tons per year [2]. in addition to rice husk and eggs, residue produced from aquaculture is abundant, with approximate amount of 10 thousand tons per year for cockleshells [3]. these remainders are often transformed into waste, disposed in landfill sites and possibly create pollution [4–7]. efficient utilization of the aforementioned waste is vital to both the economy and environment. it is well recognized that construction industry is the major consumer of natural resources. to preserve natural resources, emerging technology, such as innovative utilization of waste for construction, is commendable. since key constituents of cement comprise calcium silicate and calcium aluminate, wastes containing extensive amounts of calcium and silicon have been considered. according to numerous studies, rice husk ash (rha) contains significant silica content, with the amount exceeding 90 wt% [8, 9]. cockleshells and eggshells are reported to contain high level of calcium carbonate, in the range between 95 and 99% [10–13]. hence, rice husk ash, eggshells, and cockleshells can be potential sources of silicon and calcium used as cement-like material. along with waste utilization, an efficient route of waste management and reduction of natural resource consumption, production of eco-friendly cement-like material by fuel-efficient techniques can also be a route to save the environment. reduction of carbon dioxide emissions can provide a positive impact 70 https://doi.org/10.14311/app.2022.33.0070 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 33/2022 waste light-weight cement blocks components components ratio tri-calcium silicate (c3s) and di-calcium silicate (c2s) 0.75 tri-calcium aluminate (c3a) 0.12 tetra-calcium alumino ferrite (c4af) 0.10 gypsum 0.03 table 1. the components of cement-like material. to the environment. conventional production of cement requires heating process of raw materials, also called clinker, at the temperature ranging between 1400 and 1600◦c. with high temperature calcination, enormous fossil fuel consumption and massive carbon dioxide emission are intensified. it has been reported that 0.81 kg of cement production produces co2 1 kg on average [14]. carbon dioxide emission from cement production is a result of processing steps, including 7% from quarrying, 5% from clinker grinding, 3% transportation. calcination of the clinker is accounted for 85% of carbon dioxide emission [15]. the natural resources and environmental policy and planning of thailand reported that cement industry is responsible for carbon dioxide emission of more than 12 million tons per year [16]. solution combustion technique is a powder synthesis technique capable of producing fine ceramic powders with controlled chemical composition. the technique involves self-propagating exothermic reaction. in addition to high-quality powder production, this method offers advantages in terms of low fossil fuel consumption and minimal processing time [17]. since combustion reaction is initiated at the temperature lower than 400◦c, and low calcination temperature of 900◦c is required for production of cement-like material, considerably lower fuel consumption and diminished carbon dioxide emission is substantially evident. for practical applications, lightweight cement blocks are required to maintain compressive strength following industrial standards. thai industrial standards institute (tisi) type c12 demands minimum compressive strength of 2.5 mpa [18]. in addition to adequate strength, high durability and good integrity of the cement blocks are desired. poor installation of construction component, excessive stress, and environmental factors often cause disintegration of the materials. flaking and pitting are commonly observed. with addition of fiber reinforcement, alleviation of spalling is possible. fiber reinforcement can generally enhance mechanical properties of materials through crack abridgement. it has been reported that jute fiber is an effective reinforcing material to improve ductility, compressive strength, flexural strength, and tensile strengths in concrete [19, 20]. this research aims at utilizing waste in synthesis of eco-friendly cement-like materials. properties of fiber-reinforced light-weight cement blocks prepared from the cement-like materials were also examined. 2. experimental procedures 2.1. preparation of cement-like material an initial step to synthesis of cement-like material involves preparation of wastes. to obtain fine particle, with sizes smaller than 45 micrometers, rice husk ash (dhebkaset industrial co., ltd) was ground by ball milling for 2 hours and screened through a 325mesh sieve. eggshells and cockleshells were collected from local markets in bangkok, calcined at 900◦c for 3 hours and ball milled. solution combustion technique was employed in synthesis cement-like material, consisting of dicalcium silicate (c2s), tri-calcium silicate (c3s), tricalcium aluminate (c3a), and tetra-calcium alumino ferrite (c4af). di-calcium silicate (c2s) and tricalcium silicate (c3s) were produced by dissolving cockleshells or eggshells and rice husk ash (rha) in nitric acid 70% (hno3, rci labscan) with the weight ratio of cockleshell:rha equal to 0.84 : 0.41. for preparation of c2s and c3s using eggshells, the weight ratio of eggshell:rha of 0.85 : 0.41 was used. glycine (c2h5no2, daejung) and deionized water was added to the mixture to obtain aqueous solution with concentration close to 0.2 m, which was subsequently heated at 400◦c. combustion reaction was initiated and resulted in as-synthesized powders. the powder was collected and calcined at 900◦c for 3 hours. c3a and c4af powders were prepared in a similar approach. for c3a, cockleshells or eggshells were mixed with aluminium nitrate nonahydrate (al(no3)3, daejung) at weight ratio close to 0.73 : 2.77. c4af was attained by preparing mixture of cockleshells or eggshells, aluminium nitrate nonahydrate, and 95% ferric oxide red (fe2o3, lobachemie) at ratio of 0.81 : 2.36 : 0.51 by weight. to obtain chemical compositions similar to those of the opc, the synthesized powder of tricalcium silicate(c3s) and di-calcium silicate(c3s), tri-calcium aluminate (c3a), tetra-calcium alumino ferrite (c3af) and gypsum were mixed together with weight ratio of cs : c3a : c4af : gypsum equal to 0.75 : 0.12 : 0.1 : 0.03, as shown in table 1. 2.2. casting of eco-friendly lightweight cement blocks in order to utilize high content of waste and to minimize usage of opc, lightweight cement blocks con71 l. chandeng, t. meesak, ch. tuakta et al. acta polytechnica ctu proceedings components of light-weight cement blocks content of fiber cement-like opc rha water to binder ratio[%w/v] [wt%] [wt%] [wt%] 1 20 20 60 0.8, 0.9, 1, 1.1, 1.2 1 25 25 50 0.8, 0.9, 1, 1.1, 1.2 1.5 20 20 60 0.8, 0.9, 1, 1.1, 1.2 1.5 25 25 50 0.8, 0.9, 1, 1.1, 1.2 2 20 20 60 0.8, 0.9, 1, 1.1, 1.2 2 25 25 50 0.8, 0.9, 1, 1.1, 1.2 table 2. components of light-weight cement blocks consisting of cement-like material, opc, rice husk ash and natural jute fiber with water to binder ratio of 0.8 to 1.2 and autoclaving 6 hours after curing in water 7 days. figure 1. a) x-ray diffraction pattern of rice husk ash, b) x-ray diffraction pattern of cockleshells, c) x-ray diffraction pattern of eggshells taining 50 − 60 wt% of rha were fabricated. the mixtures containing cement-like material, opc, rha at weight ratio of cement-like material:opc:rha equal to 25 : 25 : 50 and 20 : 20 : 60, respectively, were prepared. to examine optimal water content, the mixtures were mixed with water at the water to binder ratio (w/b) of 0.8, 0.9, 1, 1.1 and 1.2. to examine effects of fiber reinforcement, 1 cm-long jute fibers were added to the mixtures at the content of 1, 1.5 and 2% w/v. the mixtures were casted in a 2.5 × 2.5 × 2.5 cm acrylic mold, cured in water for 7 days and autoclaved for 6 hours. components of the fiber-reinforced cement blocks are shown in table 2. 2.3. characterization the examination of chemical composition and phase identification of the raw waste materials, such as rice husk ash, cockleshells, and eggshells, as well as the cement-like material, were conducted by x-ray diffraction (philips, xpert). compressive strength of the lightweight cement blocks was determined by using a universal testing machine (hounsfield, h50ks), whereas optical micrographs were obtained from the optical microscope (lica, dm2700m). 3. results and discussion 3.1. phase identification of raw waste materials x-ray diffraction was employed in phase identification of rice husk ash, cockleshells, eggshells. according to x-ray diffraction patterns shown in figure 1, rice husk ash contained a single phase of silica (sio2, jcpds 01-074-9378). observations from this study was consistent with numerous researches [21–23]. the x-ray diffraction patterns of cockleshells and eggshells were similar in nature. the patterns, as shown in figure 1, contained prominent peaks corresponding to calcium hydroxide (ca(oh)2, jcpds 01-084-1263) and calcium carbonate (caco3, jcpds 01-085-1108). with high silica content and high calcium content, rice husk ash, cockleshells and eggshells had great potential for usage as raw materials for producing cement constituents such as tri-calcium silicate (c3s) and di-calcium silicate(c2s), tri-calcium aluminate (c3a) and tetra-calcium alumino ferrite (c4af). 3.2. phase identification of cement and cement-like material as mentioned in the previous section, high silica content and high calcium content facilitate the use of rice husk ash and cockleshells in the production of c3s, c2s, c3a, and c4af. in this section, xray diffraction was also employed in phase identification of the synthesized powders. x-ray diffraction pattern of the synthesized powders revealed prominent peaks corresponding to tri-calcium silicate (ca3sio5, jcpds 00-031-0301), di-calcium silicate (ca2sio4, jcpds 00-029-0369), tri-calcium aluminate (ca3al2o6, jcpds 00-008-0005) and tetracalcium alumino ferrite (ca4al2fe2o10, jcpds 00011-0124). the pattern, as showed in figure 2, was also similar to the diffraction pattern obtained from 72 vol. 33/2022 waste light-weight cement blocks figure 2. x-ray diffraction pattern of the ordinary portland cement (opc) and cement-like material containing tri-calcium silicate (ca3sio5), di-calcium silicate (ca2sio4), tri-calcium aluminate (ca3al2o6) and tetra-calcium alumino ferrite (ca4al2fe2o10) figure 3. a relationship between bulk density and w/b ratio of jute fiber-reinforced cement blocks containing rha50% and rha60%. opc. the results suggested that the cement-like material synthesized by solution combustion technique might be alternative materials used for substitution of ordinary cement. 3.3. effects of water content on density of lightweight cement blocks water-to-binder ratio (w/b) is one of the most critical parameters influencing properties of construction components containing cement, specifically workability, strength, and durability. determination of the w/b is therefore crucial for quality control during lightweight cement blocks production and general quality assurance purposes [24, 25]. in this study, the mixture containing synthesized cement-like material, opc, rice husk ash, and jute fibers were mixed with water at the w/b ratios ranging from 0.8 to 1.1, and cast into cement blocks. after 7 days of curing in water and autoclave, the cement blocks were tested for their density. the results, as shown in figure 3, revealed density values ranging from 0.873 to 1.050 g/cm3 and from 0.796 to 1.013 g/cm3 in the fiber-reinforced cement blocks containing 50 and 60 wt% rice husk ash, respectively. high water content diminished bulk density. this is attributed to formation of capillary water from excessive water content. the water subsequently evaporates and leads to formation of capillary pores. it was also found that the cement blocks containing high rha yielded lower density. this is attributed to low bulk density of rice husk ash (dhe73 l. chandeng, t. meesak, ch. tuakta et al. acta polytechnica ctu proceedings figure 4. a relationship between water to binder ratio and compressive strength of jute fiber-reinforced cement blocks containing rha50% and rha60%. figure 5. a relationship between compressive strength and jute fiber contents of cement blocks containing rha50% and rha60%. bkaset industrial co., ltd) of 0.33 g/cm3. according to type c12-thai industrial standards institute (tisi 2601-2556), it is required that the lightweight concrete must have density lower than 1.2 g/cm3. the overall density values of the cement blocks obtained in this study were in an acceptable range according to tisi 2601-2556. 3.4. effects of water content on compressive strength of lightweight cement blocks the results from compressive strength measurements of the fiber-reinforced cement blocks indicated that the strength values ranging from 2.6 to 5.1 mpa and 2.1 to 4.1 mpa was achieved in the cement blocks containing 50 and 60 wt% rice husk ash, respectively. in this study, the optimal w/b ratio for enhancement of compressive strength was 0.8. with high w/b ratio, compressive strength was diminished. minimal compressive strength was associated with low density, which was the result of excessively high water content. in addition, the results, as shown in figure 4, also revealed that higher content of rice husk ash could result in reduction of compressive strength. with high rha, content of cement and cement-like material, which play a role as binding materials and reagent for hydration reactions, was minimized. uneven and discontinuous distribution of the cementitious materials could occur, which possibly cause low compressive strength. 74 vol. 33/2022 waste light-weight cement blocks figure 6. lightweight cement blocks after compression: a) rha 50% w/b 0.8 autoclaved 6 hours (no jute fiber), b) rha 50% w/b 0.8 autoclaved 6 hours (with 1.5% jute fibers). figure 7. optical microscographs image (x5) of lightweight cement blocks after compression test. 3.5. effects of fiber contents on compressive strength of lightweight cement blocks in this study, compressive strength of cement blocks with w/b lower than 1.1 were higher than the minimum compressive strength values required by thai industrial standards institute (tisi 2601-2556)-type c12. with acceptable range of both density and compressive strength, jute fiber-reinforced cement blocks tended to have great potential for practical utilization. in the previous sections, it was found that the optimal water to binder ratio to achieve acceptable density while retaining high compressive strength was 0.8. in this section, optimal content of jute fibers for enhancement of compressive strength was examined. for the cement blocks containing 50 wt% rha, the minimal and optimal compressive strength was observed in the un-reinforced cement blocks and the reinforced cement blocks with 1.5% jute fiber, respectively. a similar trend was observed in the cement blocks containing 60 wt% rha, as shown in figure 5. the results indicated that with an optimal content of fiber reinforcement, compressive strength could increase from 4 to 5.12 mpa and from 3.5 to 4.13 mpa in cement blocks containing 50 and 60 wt% rha, respectively. an increment of 18 to 28% of compressive strength was consistent with numerous studies [26]. fibers with high tenacity, such as jute fibers, can be substantially beneficial for improvement of mechanical properties of cement blocks. in addition, high homogeneity mixing of short random fibers can increase the integral adhesion force within the cement blocks, resulting in reduction of spalling in cement blocks when subjected to compression, as showed in figure 6. observation from this study was concurrent with s. p. kundu et al, which revealed that the un-reinforced samples were disintegrated after the maximum load was applied. on the contrary, for the samples with jute fiber reinforcement-cracks appeared without spalling even when the samples were subjected to the maximum load [27–29]. it has been generally accepted that mechanical property enhancement of fiber-reinforced composites can be attributed to crack bridging mechanism. as shown in figure 7, jute fibers functioned as bridges, joining cement matrix, which consequently resulted in good integrity of the samples subjected to compressive stress. despite the aforementioned advantages, excessively high fiber content did not result in improved compressive strength. according to m. zakaria et al, addition of fiber of more than 2% could lead to fiber ball-up and intermeshing while mixing, and resulted in inferior mechanical strength [19, 30, 31]. 4. conclusion eco-friendly fiber-reinforced light-weight cement blocks with acceptable density and compressive strength were successfully fabricated. cement-like material, an alternative material for opc which was used as binder for cement block production, was synthesized by solution combustion technique. in addition to utilization of waste, specifically cockleshells, eggshells and rice husk ash, the synthesis technique required low energy consumption, which could be integral part toward sustainable development. additionally, effects of processing parameters, including water to binder ratio, rice husk ash content and jute fiber content, were examined. to achieve 75 l. chandeng, t. meesak, ch. tuakta et al. acta polytechnica ctu proceedings the optimal compressive strength, water to binder ratio of 0.8 and 50% of rice husk ash were required in this study. by adding jute fiber with the amount of 1.5%, compressive strength enhancement as high as 28% could be achieved. acknowledgements all authors would like to extend sincere gratitude to aun/seed-net for financial support. equipment support from the department of materials engineering, faculty of engineering, kasetsart university, and raw materials from dhebkaset industrial co., ltd are gratefully acknowledged. references [1] s. nattawut, t. rinnaphat, c. raungrut, 2019. http: //raungrut.tunjai.com/index.php?uid=1&pid=23&me nutype=rboard&qid=60&fbclid=iwar3mxglgyks09cqy 2hx5vkksiilgqv8vzexcvkedyszo5nucakf80ex7fbk. 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[31] l. yan, n. chouw. natural frp tube confined fibre reinforced concrete under pure axial compression: a comparison with glass/carbon frp. thin-walled structures 82:159-69, 2014. https://doi.org/10.1016/j.tws.2014.04.013. 77 https://doi.org/10.1016/j.cemconres.2009.06.013 https://doi.org/10.1016/j.conbuildmat.2019.04.036 https://doi.org/10.1016/j.conbuildmat.2018.10.045 https://doi.org/10.1016/j.jobe.2019.100786 https://doi.org/10.1016/j.conbuildmat.2018.09.048 https://doi.org/10.1016/j.tws.2014.04.013 doi:10.14311/app.2021.30.0047 acta polytechnica ctu proceedings 30:47–52, 2021 © czech technical university in prague, 2021 available online at http://ojs.cvut.cz/ojs/index.php/app localization analysis of damage for one-dimensional peridynamic model karel mikeša,∗, milan jiráseka, jan zemana, ondřej rokošb, ron h. j. peerlingsb a czech technical university in prague, faculty of civil engineering, department of mechanics, thákurova 7, 166 29 prague, czech republic b eindhoven university of technology, department of mechanical engineering, po box 513, 5600 mb eindhoven, the netherlands ∗ corresponding author: karel.mikes@cvut.cz abstract. peridynamics is a recently developed extension of continuum mechanics, which replaces the traditional concept of stress by force interactions between material points at a finite distance. the peridynamic continuum is thus intrinsically nonlocal. in this contribution, a bond-based peridynamic model with elastic-brittle interactions is considered and the critical strain is defined for each bond as a function of its length. various forms of length functions are employed to achieve a variety of macroscopic responses. a detailed study of three different localization mechanisms is performed for a one-dimensional periodic unit cell. furthermore, a convergence study of the adopted finite element discretization of the peridynamic model is provided and an effective event-driven numerical algorithm is described. keywords: damage, localization, nonlocal continuum, peridynamics. 1. bond-based peridynamics for natural as well as man-made materials, the resulting macroscopic response is dictated by the finescale structure of the material. individual building units (such as atoms on the nanoscale, inclusions and matrix on the microscale, or bricks on the mesoscale) have a certain characteristic size and their interaction takes place at finite distance. this can be taken into account by particle/lattice models, or by various enriched continuum theories. in this work, we consider the bond-based version of the peridynamic model and study the effect of the specific choice of the critical strain function on the evolution and localization of the macroscopic damage. peridynamics was introduced in 2000 by s. a. silling as a reformulation of the classical theory of elasticity [1]. in peridynamics, the partial differential equations of the classical continuum theory are replaced by integral equations. since differentiation of the displacement field is not needed, peridynamics is especially convenient for capturing discontinuities such as cracks without the complications of mathematical singularities. in a peridynamic model, the material is represented by infinitely many infinitely small particles that can interact at a finite distance. usually, interaction between two points is considered only if their distance does not exceed the so-called horizon h, see figure 1. the acceleration ü of a particle located at a spatial position x in the reference configuration at time t can h figure 1. two-dimensional representation of a discretized peridynamic model: regular grid of particles (black dots) and one selected particle (hollow circle) with interactions (grey lines) within horizon h (dashed line). be found from the peridynamic equation of motion ρü(x, t) = ! hx f(u(x′, t) − u(x, t), x′ − x)dvx′ + b(x, t) (1) ρü(x, t) = ! hx f(u(x′, t)−u(x, t), x′−x)dvx′+ b(x, t) where hx is the set of points that interact with the particle at x, u is the displacement vector field, b 47 http://dx.doi.org/10.14311/app.2021.30.0047 http://ojs.cvut.cz/ojs/index.php/app k. mikeš, m. jirásek, j. zeman et al. acta polytechnica ctu proceedings ⇠ h�h s0(⇠) ⇠ h�h s0(⇠) ⇠ h�h s0(⇠) figure 2. various functions describing the dependence of failure strain on the initial bond length: constant (left), decreasing (middle), and increasing (right). is the prescribed body force density field, ρ is the mass density in the reference configuration, and f is the so-called pairwise force function. the value of f is the force vector (per unit volume squared) that a particle located at the position x′ exerts on the particle located at x. the pairwise force function can be written in the form f(η, ξ) = ξ + η |ξ + η|f(|ξ + η|, ξ) (2) where ξ = x′ − x (3) is the relative position of the interacting particles in the reference configuration, η = u(x′, t) − u(x, t) (4) is their relative displacement, and f is the scalar bond force governed by a constitutive relation. in this work, bonds are considered as perfectly elastic-brittle, in line with the concept of microelastic brittle material, adopted from [2]. the corresponding bond force is defined as f(|ξ + η|, ξ) = c s(|ξ + η|, |ξ|) µ(t, ξ) (5) where c is a spring constant called the bond elastic micromodulus, s(|ξ + η|, |ξ|) = |ξ + η| − |ξ| |ξ| (6) is the bond stretch and µ(t, ξ) = " 1 if s(t′, ξ) < s0 for all 0 ≤ t′ ≤ t, 0 otherwise (7) is a history-dependent damage function that jumps from 1 to 0 when the bond breaks, which is assumed to happen when its stretch attains the critical level, s0. in this work, a fixed value of the elastic micromodulus c is considered for all bonds, whereas the critical stretch for bond failure, s0, is evaluated for each link as a function of its length. three different forms of length functions, namely constant, linear decreasing, and linear increasing, are employed, as illustrated in figure 2. l ux h = 0.3lx 0.1l ⇠ = 0.1l ⇠ = 0.2l ⇠ = 0.3l figure 3. geometry of an illustrative example of the one-dimensional finite element model of a periodic unit cell of length l with horizon h = 0.3l: primary nodes (filled circles), slave nodes (hollow circles), virtual node with prescribed displacement (gray circle), and an expanded illustration of the considered interactions with different lengths ξ (gray solid lines). dashed arrows denote connections of slave nodes with primary nodes. 2. numerical implementation for numerical implementation, we use an analogy with the finite element analysis, adopted from [3], to represent a bond-based peridynamic system by a finite element truss model. thus, individual bonds on the microscopic level are modelled by truss elements, so-called links, and the material parameters of the bonds, c and s0, are related to the macroscopic young modulus e and critical strain ε0, respectively. 2.1. one-dimensional model damage localization and evolution of failure mechanisms in a one-dimensional periodic lattice submitted to positive strain is investigated for different forms of the critical strain length function. the geometry of an illustrative example of the finite element model of a one-dimensional periodic unit cell is depicted in figure 3. all nodes in the periodic unit cell (except the last one) are modeled as primary nodes that carry independent degrees of freedom (horizontal displacements). to enforce periodicity, additional slave nodes are constructed as periodic images of primary nodes on one side of the periodic cell. the number of periodic slave nodes is given by the size of the horizon. loading in the form of prescribed displacement is applied to the cell through one virtual node which is linked to all slave nodes simultaneously. the displacement of each slave node is evaluated as the sum of the unknown displacement of its master (one of the primary nodes) and the prescribed displacement of the virtual node. bonds, in the form of one-dimensional truss elements, are placed between all pairs of primary nodes as well as between primary nodes and slave nodes within the given horizon h; see the solid grey lines in figure 3. the cross-sectional area of a truss element is the same for all links and it is scaled to provide a unit sum of overlapping link areas representing the macroscopic cross-sectional area. 48 vol. 30/2021 one-dimensional peridynamic model damage analysis during the loading process, the evolution of damage in a macroscopic cell is modelled as a sequence of failures (breakages) of individual links followed by stress redistribution. the macroscopic damage ω can be evaluated for each point x of the unit cell as ω(x) = nf(x) n(x) (8) where n is the number of all links passing through the given point x and nf is the number of broken links (i.e., those with µ = 0) passing through x. theoretically, it is possible that multiple links can reach the critical strain simultaneously. then the subsequent solution is not unique and additional choices must be made to distinguish one link or a group of links preferred for failure in the given step. for an ideal lattice geometry without imperfections, such a situation occurs quite often, not only at the beginning of the loading, but also almost every time when broken links form a symmetric pattern. to avoid this phenomenon and to more closely mimic the behavior of real materials, the initial position of the primary nodes is randomized by a small perturbation with a uniform distribution and a maximum value l × 10−6. the macroscopic response of the randomized model is evaluated statistically based on multiple random realizations. 2.2. solution algorithm to capture the exact sequence of breaking links, it is important to investigate not only the beginning of failure but also the full failure process of individual links and take into account that a new link can start failing before another (already failing) link becomes fully stress-free. for elastic-brittle links, failure means breaking, i.e., decreasing link stress under constant link strain. in this case, neglecting the possibility of partial unloading usually has only a minor effect on the macroscopic response. however, for elastic-damaging links with softening (which are not considered in this contribution), the possibility of partial unloading is essential. it is obvious that the size of the loading increment to reach link failure or unloading is different in each step. therefore, conventional solvers for an incremental nonlinear structural analysis are inconvenient for the present problem because they may miss the correct failure mechanism if the prescribed increment is too large. to solve the present problem exactly, an event-driven algorithm with step size control introduced in [4] has been employed. since the bond-level constitutive law is piecewise linear, the step size can be selected such that the tangential stiffness matrix does not change during the step. therefore, no equilibrium iteration is necessary. in one solution step, either a positive or a negative force increment is considered to achieve the onset of failure or complete failure of one link. furthermore, an effective method of inelastic forces can be used to avoid the necessity of decomposing the tangential stiffness matrix in each step; for more details see section 4.2 in [4]. 3. results in this section, results are presented for the aforementioned finite element representation of the peridynamic model of a one-dimensional periodic cell with elastic-brittle interactions subjected to positive (average) strain. the modelled cell is considered to have a length of l = 1 m, with the horizon h = 0.1l and cross-sectional area a = l2. the links have a constant young modulus e = 100 n m−2 and critical strain function with a maximum value ε0 = 1 × 10−2 and various forms, namely constant, linearly decreasing, and linearly increasing with link length. 3.1. convergence study first of all, a convergence study of the peridynamic model with decreasing computational grid spacing is performed. for that purpose, five simulations with the same macroscopic parameters but with 10, 20, 50, 100, and 200 primary nodes have been performed. the simplest case of constant critical strain for different link lengths has been considered. the results are shown in figure 4 in terms of normalized forcedisplacement diagrams, in which the elastic part of the displacement, f/kel, is subtracted from the total macroscopic displacement u and only the inelastic part is plotted. f is the macroscopic reaction force and kel denotes the initial elastic stiffness. for the purpose of clarity, only one representative random realization is shown for each discretization. figure 4. convergence of normalized forcedisplacement diagrams (elastic part of displacements has been subtracted) for one-dimensional periodic lattice with fixed size l, horizon h = 0.1l, for discretizations with different numbers of nodes. for each discretization, only one representative random realization is shown. the maximum force corresponding to the initial failure of the first link as well as the maximum displacement for the final damage mechanism are predicted exactly for all refinements. however, for a small number of nodes, significant oscillation can be 49 k. mikeš, m. jirásek, j. zeman et al. acta polytechnica ctu proceedings figure 5. results for constant critical strain: final damage profile (top left), positions and lengths of broken links (bottom left), and normalized force-displacement diagram where the elastic part of the displacement has been subtracted (right). figure 6. results for critical strain linearly decreasing with link length: final damage profile (top left), positions and lengths of broken links (bottom left), and normalized force-displacement diagram where the elastic part of the displacement has been subtracted (right). observed, while with an increasing number of nodes, the macroscopic response tends to a smooth curve. discretization with 100 nodes turned out to be sufficient to capture the important characteristics of the macroscopic response. note that for 10 primary nodes, there is only one link between each pair of neighbouring nodes and therefore damage localizes in one step by failure of the first link and the corresponding response is a straight line, see the gray line in figure 4. 3.2. damage mechanisms in this subsection, the influence of the form of the critical strain function is analyzed. the discretization with 100 nodes is used based on the previous study. the results are shown in figures 5, 6, and 7 for the constant, decreasing, and increasing functions of the critical strain, respectively. for each function, 10 independent random realizations have been evaluated to analyze the influence of randomness to the mechanisms of failure. however, for the purpose of clarity, only the results of one representative random realization are depicted. the resulting mechanisms of failure are illustrated by the final damage profiles (top left in each figure), evaluated according to eq. (8), and by the positions of all broken links (bottom left); here, each blue dot represent one broken link. the length of the broken link ξ/l is plotted against the position of its center. the values are normalized by the cell size l. moreover, the corresponding inelastic force-displacement diagram (right) is shown to illustrate the macroscopic 50 vol. 30/2021 one-dimensional peridynamic model damage analysis figure 7. results for critical strain linearly increasing with link length: final damage profile (top left), positions and lengths of broken links (bottom left), and normalized force-displacement diagram where the elastic part of the displacement has been subtracted (right). behavior during damage evolution. 3.2.1. constant critical strain in the initial (undamaged) configuration, the strain of the links is uniform. consequently, for the first case of constant critical strain, there is no preferred length of links for failure in the first step. in our simulations, one of the shortest links is made weaker to initialize damage localization. once the shortest link breaks, the stress is redistributed, two links in the second shortest layer spanning the broken link become the most loaded links and one of them fails. then, each subsequent broken link is situated in the next longer layer spanning all previously broken links until the first link in the longest layer is reached. once this sequence of propagating broken links from the shortest to the longest layer is finished, similar sequences follow starting from the second shortest to the longest, the third shortest to the longest, etc., until the final triangular pattern visible in figure 5 (bottom left) is formed. note that if the shortest layer of links is not artificially favored at the beginning, the initial failure can occur for a different link length due to the randomization. however, in such a case, the closest shortest links break in the next step and the same failure sequences can be observed with only the exception of the first link. for constant critical strain, the position of maximum damage (ω = 1), i.e., the position of macroscopic failure, is dictated by the first broken link. the final damage mechanism is simple. the resulting failure pattern and associated failure properties such as fracture energy can be evaluated from the geometry of the cell. also the force-displacement diagram does not exhibit any significant irregularities. 3.2.2. decreasing critical strain in the case of a critical strain decreasing with link length, the longest layer of links is the weakest one and the first broken link appears in this layer. subsequently, other links in the weakest layer are breaking one by one until the weakest layer is completely broken. this process is repeated for each next longest layer of links until a critical localization layer is reached. the gradual breaking of individual layers corresponds to horizontal parts of individual steps (jumps) in the force-displacement diagram. particularly, four steps can be observed at the levels of normalized force about 0.01×10−2, 0.08×10−2, 0.12×10−2, and 0.135×10−2, see fig. 6 (right). from the macroscopic point of view, this process can be understood as a uniform growth of damage. once the critical localization layer is reached, broken links localize in a triangular pattern similar to the previous case of constant critical strain; see fig. 6 (bottom left). in the force-displacement diagram, localization corresponds to the final unloading part, where both macroscopic force and macroscopic displacement are decreasing. the position of maximum damage is not affected by the first broken link but it is determined during the localization of the damage in the critical localization layer. 3.2.3. increasing critical strain for the last case of increasing critical strain, the shortest layer of links is the weakest one and the first broken link appears here. however, in this case, damage of the first link is followed neither by damage of the whole weakest layer nor by localization. on the contrary, links continue breaking in different layers without a specific pattern. consequently, the macroscopic damage is distributed randomly and no clear 51 k. mikeš, m. jirásek, j. zeman et al. acta polytechnica ctu proceedings localization is observed. in the end, the majority of broken links are situated in several weakest layers. however, these weak layers (except the last one) are not completely broken. on the other hand, in several regions, damage significantly propagates also into layers of stronger links, see fig. 7 (bottom left). in the central region, the broken links form an unusual mechanism of failure where no macroscopic cross-section exhibits complete damage (ω = 1), even when the macroscopic reaction force completely vanishes. such a mechanism is formed by overlapping sliding parts. as a result of this, the final macroscopic strain is localized not only into one but into three segments between neighbouring nodes where two of them exhibit positive strain increments and the remaining one a negative strain increment of equal magnitude. unlike the previous cases of constant and decreasing forms of critical strain function, in this case, the order of broken links is unstable and the position and shape of the final damage pattern as well as the evolution of the damage before localization are affected by the randomization of the initial lattice geometry. as a result, significant oscillations can be observed in the force-displacement diagram, see fig. 7 (right). 4. conclusions in this contribution, a finite element truss model representing a peridynamic system with elastic-brittle interactions has been introduced, together with an accurate and efficient event-driven numerical algorithm which is able to capture exactly not only the failure but also the full unloading of individual links. the convergence study of the presented model has shown that upon refinement the system tends to a smooth macroscopic response. the influence of various forms of the critical strain function on the type of macroscopic response has been investigated for a one-dimensional periodic unit cell. constant, decreasing, and increasing critical strain functions have been investigated and the resulting responses with different types of damage evolution have been analyzed, namely direct damage localization, uniform damage evolution preceding localization, and random damage distribution, respectively. the presented results demonstrate that the concept of critical strain values depending on link length can be used to reach various types of macroscopic responses even for such a simple geometry (1d) and one of the simplest nonlinear material models (elasticbrittle). the significant advantage of this concept is that the presented elastic-brittle model can be fitted to represent various macroscopic behaviors, but at the same time the micro-level constitutive law remains piecewise linear and therefore the exact and efficient event-driven algorithm can be used. in addition, the spectrum of different response types can be further extended by considering various micromodulus functions or by adopting elasticdamaging links with softening. list of symbols h material horizon [m] hx neighborhood of a particle vx volume of a particle neighborhood x particle coordinates [m] u displacement vector [m] ü acceleration vector [m s−2] ρ mass density [kg m−3] b body force density [n m−3] t time [s] ξ relative position of two particles [m] η relative displacement of two particles [m] f pairwise force function [n m−6] f scalar bond force [n m−6] s bond stretch [–] x horizontal coordinate [m] l cell size [m] a cross-sectional area of links [m2] ux prescribed horizontal displacement [m] u total lattice displacement [m] f reaction force [n] kel initial elastic stiffness [n m−1] c spring constant [n m−2] s0 critical stretch for bond failure [–] e young modulus [n m−2] ε0 critical strain [–] µ damage function [–] ω damage [–] n number of links [–] nf number of broken links [–] acknowledgements this research has been performed in the center of advanced applied sciences (caas), financially supported by the european regional development fund (project no. cz.02.1.01/0.0/0.0/16_019/0000778). financial support received from the czech technical university in prague under project sgs20/038/ohk1/1t/11 is gratefully acknowledged. references [1] s. a. silling. reformulation of elasticity theory for discontinuities and long-range forces. journal of the mechanics and physics of solids 48(1):175–209, 2000. [2] s. a. silling, e. askari. a meshfree method based on the peridynamic model of solid mechanics. computers & structures 83(17-18):1526–1535, 2005. [3] r. w. macek, s. a. silling. peridynamics via finite element analysis. finite elements in analysis and design 43(15):1169–1178, 2007. [4] m. jirásek, z. p. bažant. macroscopic fracture characteristics of random particle systems. international journal of fracture 69(3):201–228, 1994. 52 acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.38.0005 acta polytechnica ctu proceedings 38:5–10, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague toward a critical assessment of indoor-environmental quality standards ardeshir mahdavia,∗, christiane bergerb a department of building physics and building ecology, tu wien, karlsplatz 1040 vienna, austria b department of architecture, design and media technology, aalborg university, 9000 aalborg, denmark ∗ corresponding author: amahdavi@tuwien.ac.at abstract. building design and operation requirements regarding indoor-environmental quality (ieq) are of direct relevance to occupants’ needs. in most buildings, occupant-related requirements pertain to the provision of conditions that support optimal task performance and are subjectively perceived as comfortable or pleasant. standards and guidelines, which entail ieq requirements specifications are commonly viewed as the main sources of reference for practitioners, who are expected to follow the provisions in these documents and provide corresponding proof of compliance. however, actual code compliance processes are not always accompanied by critical reflections regarding the evidentiary basis of the included mandates and recommendations. it is thus necessary to critically scrutinise standards in view of explicit or implicit references to the scientific basis of the entailed mandates. the present contribution explores and details a path toward such a critical assessment. to this end, we considered several frequently referenced standards pertaining to thermal, visual, and air quality aspects of indoor environments. the results of this illustrative assessment effort highlight the scope and limitations of the standards’ default approaches to the definition of ieq-relevant requirements. moreover, they point to considerable gaps in the chain of evidence from standards’ immediate content to the underlying factual sources. keywords: indoor-environmental quality, standards, evidence. 1. introductory remarks for professionals and stakeholders involved in the building delivery process, codes, standards, and guidelines related to indoor-environmental quality (ieq) represent a critical source of information and guidance. these resources typically define and specify the spectrum of quality requirements (including those directly relevant to the needs of occupants) to be addressed in the course of buildings’ planning, erection, and operation. specifically, ieq-related requirements are directly relevant to occupants’ needs. broadly speaking, requirements pertaining to building occupants range from some rather basic principles (i.e., avoiding harm to human health, provision of the minimum thermal, visual, acoustic, and air quality conditions) to criteria that could be classified as less tangible (e.g., creating indoor environments that occupants perceive as being pleasing and pleasant). risks in the former category should be obviously avoided. those in the latter category could be presumably tolerated if the temporal extent of the exposure is limited. it is important to recognize that what constitutes a comfortable environment has a strong subjective background. but the same is not true of necessary conditions for occupants’ health. occupants do not always consciously perceive sources of adverse health effects. for instance, dangerous gases such as carbon monoxide and radon cannot be perceived by humans. standards represent a key source of information and support for professionals and stakeholders in the building delivery process. building designs and the actual buildings’ performance need to comply with the ieq-related mandates in these standards [1]. the question is, however, if by the virtue of their entailment in the standards, mandates and requirements can be automatically assumed to be entirely objective and based on solid scientific evidence. this, we argue, cannot be assumed a priori, but must be rather examined carefully. 2. in search of evidence one could assume that, in order to locate the scientific basis for standard-based ieq mandates, one would need to look no further than the evidence provided by disciplines such as physiology, psychology, and ergonomics. however, neither standards themselves, nor the related compliance processes appear to explicitly address arguments for the applicability of standards in general and the uncertainty of their recommendations in particular. this circumstance may have negative implications. professionals may concentrate solely on demonstrating compliance with minimum requirements instead of focusing on creating genuinely high-quality indoor environments. this underlines the importance of critical reflections regarding the availability and strength of sound scientific evidence behind the entailed prescriptions in the standards. in this context, a key question is as follows: do we 5 https://doi.org/10.14311/app.2022.38.0005 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en ardeshir mahdavi, christiane berger acta polytechnica ctu proceedings figure 1. illustrative exemplification of the chain of reference from certification systems, to different layers of standards, and technical literature. ([2–5], [6–10], [11], [12, 13]). sufficiently understand the influence mechanisms of indoor-environmental factors on the health, comfort, productivity, and well-being of the building occupants? despite the plethora of valuable research in the past, the identification and realization of occupants’ health and comfort requirements remains a formidable challenge. given the increasing popularity of various total building quality evaluation schemes and certificates, it would be useful, in principle, to examine not only commonly used ieq-related codes, standards, and guidelines (and their recommendations), but also building evaluation and certification systems in view of any cited theoretical background and empirical evidence. however, as illustrated in figure 1, common systems for building rating and certification (e.g., [14, 15]) appear not to entail explicit specifications of indoorenvironmental requirements. rather, they tend to refer to other thematically related national or international standards such as iso, en, din, or ansi [1]. some of these standards (e.g., din en 15251) refer, in turn, to other standards. there are very few cases, where standards include direct evidence and justification for the specified requirements. they do, however, include at times either general literature or specific studies (see “technical literature” box in figure 1). nonetheless, it is not a straight-forward matter to isolate, in the referenced technical literature, the evidence underlying the standards’ ieq-related criteria and the mandated values of the relevant occupantcentric performance indicators. 3. about formats for the specification of requirements for practitioners involved in the building delivery process, the primary utility of standards may be in the specification of ieq-relevant performance indicators and their respective mandated values. these values can be specified in terms of various formats. for instance, maximum allowable values may be mandated for parameters such as pollutants’ concentration, magnitude of glare, or sound level. likewise, minimum values can be specified, for instance, for illuminance levels or ventilation rates. the specification of performance indicator values can help organize and streamline the procedures aiming at demonstration of compliance with standards and general quality assurance. assuming that such values can be monitored after the commissioning phase, the examination of contractually specified performance can follow a rational path and liability issues can be resolved in an accountable man6 vol. 38/2022 toward a critical assessment of indoor-environmental quality standards figure 2. example of a standard-based ieq indicator (operative air temperature) and its value as a function of the outdoor air temperature (based on standards din en 15251 and din en iso 7730). figure 3. illustrative exemplification of standard-based specification of an ieq indicator (maximum permissible air flow speed) and its value as a function of indoor air temperature and turbulence intensity (based on standards din en 15251 and din en iso 7730). ner. requirements can be also formulated in terms of “zones”, involving the combined consideration of multiple variables and their values (e.g., thermal comfort zone that encompass air and radiant temperatures, air velocity, and humidity). to exemplify this, consider the following graphs (figures 2 and 3), which are based on standards din en 15251 and din en iso 7730. these graphs illustrate, in generic form, the dependency of the value of a performance indicator (i.e., recommended operative temperature, maximum allowable air flow speed) as a function of other variables (in this case, outdoor temperature, indoor air temperature, turbulence intensity). the logic behind these kinds of requirements in the ieq field has both similarities and dissimilarities with other code-based building regulations. basic building codes prescribe, for instance, minimum dimensions of certain architectural elements in building design and construction domains (e.g., width of doors, stairs, corridors). such requirements, for instance those pertaining to accessibility and universal design, are typically based on straightforward practical considerations such as human body and wheelchair dimension. they can be readily understood by designers and implemented in actuality. hence the compliance can be fairly easily examined in the course of building commissioning. but the translation of this approach, namely inference from scientific facts to design requirements, is in case of ieq-related requirements much more complex. it is thus important to explore if and to which extent ieq-related standards bolster their credibility by provision of explicit references to scientifically established evidence. 4. theory versus practice a cursory inspection of ieq-related standards leads to the conclusion that they entail much in terms of prescriptive requirements, but rather little in terms of explicit evidence behind those requirements. as mentioned before, rating systems typically refer to 7 ardeshir mahdavi, christiane berger acta polytechnica ctu proceedings figure 4. illustrative schema of indoor-environmental comfort equation components. thermal comfort visual discomfort presumed key indoor-environmental comfort parameters (independent parameters) ambient air temperature, radiant temperature, relative humidity, air velocity background luminance, glare source luminance indoor-environmental comfort constructs indicating building users’ subjective evaluation (dependent parameters) predicted mean vote, predicted percentage of dissatisfied unified glare rating (ugr), visual comfort probability (vcp) table 1. independent and dependent parameters with regard to thermal comfort and visual discomfort. standards, which sometimes refer to other standards. standards occasionally refer to technical papers and report, but a direct link from these resources to the stated requirements is rarely established. hence, such references appear to have the function of a general topical bibliography, rather than repositories of directly relevant evidentiary material. various reasons may be responsible for this circumstance. firstly, standards could be argued to represent resources for practitioners, rather than repositories of scientific knowledge. professionals involved in the building delivery process are more likely to consult standards in view of practical solutions and regulatory constraints, rather than contributors to their scientific understanding. secondly, ieq standards, in contrast to building construction and structure codes, cannot rely only on engineering sciences, but must also take into account findings from human and social sciences such as physiology, psychology, and sociology. this arguably adds an additional layer of complexity to the requirements specification process, given the significant role of subjective and qualitative parameters in the formation of occupants’ perception and evaluation phenomena. thirdly, the genesis of ieq standards does not follow in all cases an entirely systematic process. thereby, factors other than the occupants’ health or comfort requirements may play a role. these other factors could include financial issues, policy considerations, or special interests. as with many other areas involving regulatory aspects, development of standards may require, from the involved stakeholders (e.g., government, industry, academia), various degrees of consent and the willingness to comprise. consequently, not all aspect of the resulting standards could be expected to strictly adhere to scientific evidence. the above reflections, however, should not be misunderstood. many commonly applied ieq standards do not explicitly elaborate on their underlying evidentiary basis. but they do provide implicit pointers to the kinds of principles and methods that inform them. in other words, standards frequently entail features that point a kind of implicit methodological and theoretical underpinnings. it may be thus interesting and useful, to engage in a kind of reverse engineering of standards and their syntax, terminology, and logic, so as to uncover their implicit theoretical foundations. 5. sources of indoor-environmental comfort constructs conventionally, indoor-environmental comfort equations are used in standards focusing on thermal and visual comfort. figure 4 schematically illustrates the elements of such indoor-environmental comfort equations. thereby, a set of physical variables is intended to formulate dependent variables that capture building users’ level of comfort [1]. whereas physical variables can be typically derived by measurements, dependent variables result from subjective evaluations. table 1 shows such constructs that are assumed to capture occupants’ subjective evaluations concerning thermal and visual comfort/discomfort. thermal comfort standards typically refer to relevant (measureable) physical variables, such as air temperature, air speed, and water vapour concentration. moreover, thermal comfort constructs capture subjective building users’ evaluations by means of scales that are commonly used in studies in psychology. the underlying logic to map operations in the comfort equation often relies on two sources. on the one hand, the source is based on underlying physiological mechanisms. for instance, the humans’ thermo-regulatory system to keep the human body’s kernel temperature is of key significance when addressing thermal comfort [16]. on the other hand, occupants’ evaluations 8 vol. 38/2022 toward a critical assessment of indoor-environmental quality standards of experiments with different indoor-environmental conditions provide another source of information. in which way these two sources contribute in order to derive a comfort equation may be quite different. concerning thermal comfort, the understanding of thermo-regulatory processes is crucial in order to formulate thermal comfort models. moreover, studies involving subjective evaluations of participants provide relevant data of physiological-relevant variables. with regard to visual comfort, a physiological understanding of glare or light scattering is essential. discomfortable visual indoor-environmental conditions are primarily evaluated by occupants’ subjective evaluations. 6. reflections on limitations codes, guidelines, and standards seldom bring to light the relevant and precise explanation for the particular requirements they demand, some of which provide exact limitations for or allowed ranges of different variables. previously, we suggested some reasons for this situation. these include difficulties in accurately formulating occupant-oriented concepts for comfort, well-being, and health; recognising and estimating the proper ieq proxies; the multi-domain character of indoor-environmental circumstances; the variance of occupants’ requirements and the constantly changing nature of these requirements; the actual challenges of working procedures in standardization bodies. it is important to emphasize that we did not embark on the present treatment with an unreasonably high expectation. clearly, it is not likely that a single formula or scheme could provide definitive evidence for all certification systems and regulatory guidelines regarding indoor environment. however, this process revealed a repeated pattern, known from the fields of psychology, physiology, and medicine, which can be described in the following way. in standards, the values and ranges of the relevant variables meant to be representative of the indoor environment conditions are mapped onto the values of specific indicators of human comfort and health. this mapping process usually relies on the combination of two components, specifically (1.) a psychologically or physiologically based theory, and (2.) obtainable research data from experiments with human participants. in cases where explicit comfort calculations are needed (e.g., visual discomfort or thermal comfort), the guidelines provide specific models defining occupant comfort and health (including their required values and ranges). otherwise, these models may not be specifically provided. rather, the assumption appears to be that when specific indoor-environmental variables (e.g., indoor air carbon dioxide concentration) are kept in mandated ranges, the requirements related to occupants’ comfort and health are met. we can summarize the foregoing discourse regarding the current and future state of guidelines, codes, standards as follows: • to begin with, we need to acknowledge that it is at times difficult to follow the trace from guidelines back to their evidentiary basis. in practice, this may lead to a situation, where standards and their requirements do not serve as reliable sources of disciplinary guidance, but are reduced to opaque propositions that are thoughtlessly followed. it is not our intent to suggest that standards should reproduce the complete theoretical substructure and scientific evidence that they are built upon. their focus needs to be on operational matters and specific directives, as they mainly serve as regulative tools. however, it would be justified to look for an explicit link between the standard-based requirements and their scientific basis. this link appears to be frequently untraceable. • secondly, we face numerous challenges, while trying to identify the theories and supporting data that would document the credibility of the requirements formulated the standards. research in the fields of human health, well-being, and comfort continuously yields new and valuable results, but it also involves numerous challenges and uncertainties. while reputable scientists in the field commonly avoid expressing doctrinal standpoints and claiming the absolute truth, regulating authorities are obligated to derive from insufficiently uncertain knowledge mandatory rules and regulations. as such, it seems as if a certain level of uncertainty in the objectivity of mandated requirements is the price one is willing to pay in order to prevent chaos, confusion, and accountability issues in the building design and delivery process. • thirdly, the identified ieq-relevant standardization challenges (especially, the scantiness of explicit arguments for the entailed requirements) may also point to gaps in our scientific understanding in the related domains. it is not always clear which physical characteristics of the indoor environment represent the “correct” variables to be considered when evaluating comfort and health. further difficulties must be encountered when we try to define and validate the constructs for comfort and health that do not fully capture the relationship between psychological, physiological, and social aspects of occupants’ perception and assessment of indoor environment. moreover, the scope of available empirical data for the evaluation and validation of behavioural and perceptual theories is still rather limited. the indoor-environmental exposure circumstances are fundamentally multi-domain. hence, isolated treatment of these separate ieq domains is likely to fail in capturing their complexity. we have made some progress in our appreciation of occupants’ diversity and the dynamic nature of their preferences. 9 ardeshir mahdavi, christiane berger acta polytechnica ctu proceedings however, probably the level of progress has not been high enough to warrant a translation of the respective findings in the appliable ieq regulations. the investigation of the formulation and implementation of ieq-relevant standardization procedures structures points not only to their limitations, but also to certain shortcomings in our scientific understanding of occupants’ preferences and necessities in indoor environments. there is a need for clearer, better structured and more objective procedure, when it comes to implementing our current scientific knowledge, with all its undeniable limitations, into ieq standards, guidelines, and codes. at the same time, we urgently need to extend and refine the depth of our knowledge regarding the mechanisms by which indoor-environmental conditions influence human health, comfort, wellbeing, and productivity. references [1] a. mahdavi, c. berger, v. bochukova, et al. necessary conditions for multi-domain indoor environmental quality standards. sustainability 12(20):8439, 2020. https://doi.org/10.3390/su12208439. [2] r. de dear. thermal comfort in practice. indoor air 14(s7):32–39, 2004. https://doi.org/10.1111/j.1600-0668.2004.00270.x. [3] m. a. humphreys, j. f. nicol. understanding the adaptive approach to thermal comfort. ashrae transactions 104(1):991–1004, 1998. [4] f. nicol, k. mccartney. final report (public) smart controls and thermal comfort (scats), report to the european commission of the smart controls and thermal comfort project (contract joe3-ct97-0066). oxford brookes university, 2001. [5] k. j. mccartney, j. fergus nicol. developing an adaptive control algorithm for europe. energy and buildings 34(6):623–635, 2002. https://doi.org/10.1016/s0378-7788(02)00013-0. [6] r. dedear, g. s. brager. developing an adaptive model of thermal comfort and preference. ashrae transactions 104(1a):145–167, 1998. [7] m. fountain, e. arens, r. dedear, et al. locally controlled air movement preferred in warm isothermal environments. ashrae transactions 100(2):937–952, 1994. [8] b. w. jones, k. hsieh, m. hashinaga. the effect of air velocity on thermal comfort at moderate activity levels. ashrae transactions 92(2b):761–769, 1986. [9] s. tanabe, k. kimura. effects of air temperature, humidity, and air movement on thermal comfort under hot and humid conditions. ashrae transactions 100(2):16, 1994. [10] j. toftum, g. zhou, a. melikov. effect of airflow direction on human perception of draught. in proceedings of clima 2000. brussels, belgium, 1997. paper 366. [11] e. mundt, h. m. mathisen, p. v. nielsen, a. moser. ventilation effectiveness. guidebook n. 2. rehva (federation of european heating and ventilation association), 2004. [12] p. tregenza. mean daylight illuminance in rooms facing sunlit streets. building and environment 30(1):83–89, 1995. https://doi.org/10.1016/0360-1323(94)e0006-d. [13] n. baker, a. fanchiotti, k. steemers. daylighting in architecture. james & james, 1993. [14] u.s. green building council. leed rating system. [2021-10-14], https://www.usgbc.org/leed. [15] dgnb system – sustainable and green buildings. [2021-10-14], https://www.dgnb-system.de/en/. [16] a. mahdavi. the human factor in sustainable architecture. in k. al-sallal (ed.), low energy low carbon architecture: recent advances & future directions, pp. 177–198. 2017. https://doi.org/10.1201/b19882-12. 10 https://doi.org/10.3390/su12208439 https://doi.org/10.1111/j.1600-0668.2004.00270.x https://doi.org/10.1016/s0378-7788(02)00013-0 https://doi.org/10.1016/0360-1323(94)e0006-d https://www.usgbc.org/leed https://www.dgnb-system.de/en/ https://doi.org/10.1201/b19882-12 acta polytechnica ctu proceedings 38:5–10, 2022 1 introductory remarks 2 in search of evidence 3 about formats for the specification of requirements 4 theory versus practice 5 sources of indoor-environmental comfort constructs 6 reflections on limitations references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.38.0235 acta polytechnica ctu proceedings 38:235–240, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague the potential use of giant reed from portugal as a thermal insulation material raphaele malheiroa, aurora morillab, adriana ansolinc, jorge fernandesc, sandra silvac, ricardo mateusc,∗ a university of minho, department of civil engineering, ctac, campus de azurém, 4800-058, guimarães, portugal b universitat politècnica de catalunya, carrer de jordi girona, 31, 08034, barcelona, spanish c university of minho, department of civil engineering, isise, campus de azurém, 4800-058, guimarães, portugal ∗ corresponding author: ricardomateus@civil.uminho.pt abstract. the construction sector plays an important role in climate change. thus, there is a pressing need to construct buildings that reduce heat losses, use natural and local materials, exploit renewable sources and ensure high comfort levels with a minimum environmental impact. reed, considered carbon-neutral and a carbon dioxide sink material, has been used for centuries for diverse uses. its properties and high availability made it a popular building material, as seen in portuguese vernacular architecture. knowing the properties of the reed is a crucial step to ensure successful heritage conservation, optimising these materials, and developing innovative solutions. this paper studies the potential of using giant reed from different portuguese regions as a thermal insulation material. giant reed board prototypes (15 × 15 × 5 cm, about 235 kg/m3) were built. their thermal performance was tested in a hotbox, according to astm c1363 19. the results show that the giant reed harvested on the northern coast of portugal has better thermal performance than reeds from other regions. however, regardless of the region of the country where the giant reed was harvested, it has a satisfactory thermal resistance (re ≥ 0.30 (m2 · °c)/w), allowing its use as a thermal insulation material in the buildings. keywords: giant reed, thermal insulation material, natural material, low cost, sustainability. 1. introduction the construction sector is one of the largest energy consumers. it is under pressure to improve its energy efficiency and environmental performance and reduce non-renewable resource use. the largest part of buildings energy consumption is related to the operation phase. energy poverty is still high in the european union (eu), and portugal is one of the eu countries where the consequences of energy poverty are most evident. a large part of the portuguese building stock was built before the first thermal regulation came into force dl 40/90 [1], resulting in reduced thermal insulation and inadequate adaptation to the climatic context. the use of natural materials in construction can be a great advantage. using these materials in construction can reduce the environmental impact compared with non-natural materials, reaching similar or better characteristics in some cases. reed, considered carbon-neutral and a carbon dioxide sink material, has been used for centuries for diverse uses. its properties and high availability made it a popular building material, as seen in portuguese vernacular architecture. the arundo donax, giant reed, has been identified in portuguese vernacular architecture, mainly in walls [2] and roofs [3], to provide better thermal insulation to the buildings. several authors have studied the thermal insulation properties of natural materials (cork, reed, bagasse, cattail, corn cob) to assess the opportunity to use them in the buildings [4]. the potential of using reed as thermal insulation has also been investigated [5–7]. some studies have evaluated the thermal potential of boards where the reed is the main material [6], and others have evaluated boards made only with reed [7]. boards of giant reed harvested in portugal were used in a prototype of a building solution based on earth and reeds (stem and fibres) [8]. the prototype, built in lisbon, had its indoor and outdoor temperatures monitored during different seasons. the researchers concluded that the solution contributed to controlling the indoor air temperature, given the thermal amplitudes that were registered outside. regarding boards made only with reeds harvested in portugal, no results were found. the studies highlighted the thermal insulation potential of the reed around the world. since the properties of natural materials can be sitedependent [9], it is important to know the thermal insulation properties of reeds harvested in portugal. the plant growth and development are influenced by several environmental factors, such as temperature and humidity [10, 11]. thus, the origin of the plant can influence its characteristics. in this sense, this paper 235 https://doi.org/10.14311/app.2022.38.0235 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en r. malheiro, a. morilla, a. ansolin et al. acta polytechnica ctu proceedings figure 1. giant reed harvest regions: serpa (i), santa cruz do douro (ii) and apúlia (iii). studies the potential of using giant reed from different regions of portugal as a thermal insulation material. this investigation assessed the thermal behaviour of reed, aiming to produce knowledge that can be used to design new solutions for more sustainable construction and heritage conservation. 2. reed: specie identification and climate characterisation of harvesting regions reed species studied is arundo donax, also called as giant reed. this species is widespread throughout the portuguese territory, being an invasive plant. considering the country climatic differences, three different regions were chosen for harvesting the reed: serpa (i), santa cruz do douro (ii) and apúlia (iii) (figure 1). serpa is a city located at 200 m above sea level [12] and it is characterized for having a temperate climate – type c – according to köpp-geiger climate classification, sub type csa (temperate with hot and dry summer) [13]. serpa is the least rainy and hottest region studied. the average total annual precipitation is 400 mm, and the annual average mean temperature is 17.5 °c. regarding the extremes, the annual average minimum and maximum temperature are 12.5 and 25.0 °c, [13]. santa cruz do douro is a parish located at 400 m above sea level [12]. according to the köppen-geiger its climate is also type c, sub-type csa [13]. the average total annual precipitation is 800 mm and annual average mean temperature is 15.0 °c. regarding the extremes, the annual average minimum and maximum temperatures are 10.0 and 17.5 °c, respectively [13]. apúlia is a village on the northern coast of portugal located at 10 m above sea level [12]. according to the köppen-geiger its climate is also type c, but the sub-type is csb (temperate with dry or temperate summer) [13]. apúlia is the rainiest region studied. the average total annual precipitation is 1400 mm, and annual average mean temperature is 15.0 °c. regarding the extremes, the annual average minimum and maximum temperatures are 10.0 and 17.5 °c, respectively [13]. 3. experimental program 3.1. reed boards preparation three reed board prototypes were developed to evaluate the performance of giant reed harvested in different regions of portugal as an insulation material. one board with 150 × 150 × 50 mm was built for each studied region (figure 2). the giant reed board prototypes were prepared using a three stage process: cut the reed stems in parts with 150 mm length; steel wireframe preparation using the pre defined dimensions (figure 3); accommodation of reeds cut into steel wireframe (figure 4). the wireframe was developed using steel wire only in the board’s borders to minimise its influence on the heat flux during the tests. furthermore, the se236 vol. 38/2022 the potential use of giant reed from portugal . . . id quantity of reed in the board [unit] steel wire mass [g] board mass (initial): reed + wire [g] final thickness [mm] density of board [kg/m3] i 41 19.50 295.1 55 237.1 ii 33 19.50 260.5 50 231.6 iii 29 19.50 287.9 50 255.9 table 1. characteristics of the reed board prototypes. figure 2. reed board prototype. lection of reeds was made considering their regularity along the length. it was also used different diameters of reed to achieve a better accommodation between them. the final thickness of the reed board prototype is related to the accommodation of reeds into the frame. considering the reed as a hygroscopic material, the reed boards mass were verified before (initial mass) and after (final mass) the tests. the main characteristics of the reed board prototypes (including initial mass) are presented in table 1. 3.2. thermal performance the thermal performance of giant reed board prototypes was evaluated considering their thermal resistance (re) and thermal conductivity (λ). these parameters were determined using a calibrated hotbox designed and built at the department of civil engineering of the university of minho, based on astm specifications c1363 [14]. the hotbox is composed of two chambers, the cold and the hot one, and one mounting ring placed between the two chambers. the giant reed board prototype was placed in the centre of the mounting ring. it figure 3. steel wire frame. figure 4. accommodation of reeds into the frame. was enclosed between two medium density fibreboard (mdf) boards to provide a flat surface for installing the flux meter and controlling the air permeability between the two chambers [15]. since in a previous study, malheiro et al. [16] concluded that geometric configuration has no influence on the thermal performance of the giant reed board prototype, it was tested in a vertical position. the variation of the giant reed board prototypes mass during the test was evaluated. the tests were carried out considering the heat flow meter method, defined in iso 9869-1 standard [17]. the heat flux is measured through a heat flux sensor installed in the giant reed board prototype’s central 237 r. malheiro, a. morilla, a. ansolin et al. acta polytechnica ctu proceedings figure 5. region i reed board prototype: temperatures and heat flow. part, and thermocouples measured the temperatures. with the values of the heat flux (q) and the surface temperatures (t ), it was possible to determine the thermal resistance (re) of the set of materials (giant reed board prototype + mdf), using equation (1). ∆t is the difference between the surface temperature of the mdf in the hot and cold chambers. the thermal resistance of the giant reed board prototype was determined using equation (2). the reed board prototype’s thermal conductivity (λ) was assessed using equation (3), where e is the board’s thickness. reset [(m2 · °c)/w] = ∆t q (1) rereed [(m2 · °c)/w] = reset − (2 ∗ remdf ) (2) λreed [w/(m · °c)] = e rereed (3) 4. results 4.1. thermal performance of reed board prototype figures 5 to 7 represent temperatures and heat flow reached during 72 hours test [17] in the hotbox for each board studied. from figures 5 to 7, it is possible to see that temperature in the cold chamber and heat flow remained very stable during the test period, regardless of the giant reed board studied. concerning the temperature in the hot chamber, a slight perturbation is observed during the test, being more evident in the giant reed board from region iii. this variation in the hot chamber temperature does not influence the heat flow during all test periods. the heat flow remained very stable, maintaining an average value of 4.36 w/m2, 5.35 w/m2 and 4.39 w/m2 for regions i, ii and ii, respectively. table 2 summarises the final mass of prototypes and the average values obtained for their thermal figure 6. region ii reed board prototype: temperatures and heat flow. figure 7. region iii reed board prototype: temperatures and heat flow. properties. these values were calculated based on the results from figures 5 to 7, using equations (1) to (3). table 2 shows that the reed board prototype from region iii has the greatest thermal resistance and the least thermal conductivity between the board studied. the boards from regions i and ii show very similar values for the thermal properties studied. comparing final and initial (table 1) board mass, it is clear that the mass variation has no significance, being the loss always less than 0.70 %. 5. giant reed board prototype as insulation material the results presented in section 4 show a satisfactory thermal performance for the giant reed board prototypes, regardless of the region from where the reeds were harvested. the values achieved for thermal conductivity are very close to the values presented in the current literature, 0.045–0.056 w/m · °c [23]. furthermore, comparing the performance of boards made using only reeds, the thermal conductivity achieved in this study is lower than that achieved by asdrulali et al. [5] using the hotbox test method, 0.065 w/m.ºc. 238 vol. 38/2022 the potential use of giant reed from portugal . . . id board mass (final): reed + wire [g] reset [m2 · °c/w] remdf [m2 · °c/w] rereed [m2 · °c/w] λreed [w/m · °c] i 293.4 1.313 0.147 1.019 0.049 ii 258.7 1.247 0.147 0.953 0.052 iii 287.0 1.501 0.147 1.207 0.041 table 2. characteristics of the reed board prototypes. material thickness [mm] density [kg/m3] re [m2 · °c/w] reference rock-wool 60 25 1.60 [18] xps 50 32 1.40 [19] eps 50 20 1.30 [20] cork 50 110 1.25 [21] giant reed 50–55 237–266 0.95–1.21 this study table 3. characteristics of the thermal insulation materials (adapted from [22]). the board developed by asdrulali et al. [5] has a similar thickness, 56 mm, a lower density, around 90 kg/m3, and different reed species, phragmites australis (common reed). the difference in the density is probably related to the reed species used and, consequently, the number of reeds used and accommodation between them. common reed and giant reed have a marked difference in their average diameter: around 1.0–2.5 cm and 2.5–5.0 cm [24], respectively. in this sense, the comparison of results from different studies should be made with caution. considering the portuguese thermal regulation, the thermal resistance and thermal conductivity values achieved are in accordance with the requirements defined for thermal insulation material, that is re ≥ 0.30 (m2 ·°̧c)/w [25]. furthermore, when comparing the giant reed board prototypes studied with some insulation materials commercially used in portugal, a similar performance is observed (table 3). considering similar thickness, the thermal resistance of reed board represents at least 60 % of the thermal resistance of rock-wool and 76 % of the thermal resistance of cork, for instance. concerning the giant reeds origin, observing table 2 data, it is possible to say that the climate of the reed harvest region has no significant influence on the thermal performance of the giant reed board prototypes studied. despite that, the board made with reed harvested in region iii, the rainiest region studied, has the highest thermal resistance and the least thermal conductivity. this behaviour may be related to the density of the board since the reed board made with reed harvested in region iii has the highest density. it is important to know that the reed board prototype iii reached a highest density using fewer reeds than prototypes i and ii (table 1). these results confirm the potential of the giant reed harvested in portugal to be used as a thermal insulation material. in addition to the advantages in terms of insulation, reeds have other ecological benefits. they are considered a carbon-neutral and a carbon dioxide sink material and have the advantage of being biodegradable and low-cost. 6. conclusions an experimental investigation was carried out to evaluate the potential of using giant reed (arundo donax) harvested in portugal as a thermal insulation material. considering the influence of climate in the plant growth, reed board prototypes were made using giant reed from three different regions from portugal. the results show a satisfactory thermal performance for all the reed board prototypes, regardless the region where the reeds were harvested. the values achieved for thermal resistance, 0.9–1.2 m2 · °c/w, are in accordance with the requirements defined for thermal insulation materials in the portuguese thermal regulation. comparing these reed boards with some insulation materials commercially used, it is observed a similar thermal behaviour. concerning the giant reed origin, the reed board prototype made with reeds from region iii shows a slightly better thermal performance than the others. analysing the results, it is possible to conclude that, under the studied conditions, regardless the harvesting region studied, the reed boards showed satisfactory thermal performance. however, it is important to consider different possibilities to contain the boards (wood boards, plasterboards, mortars). additionally, as the reed is abundant throughout portugal, its use is an eco-friendly and low-cost option that gathers all conditions to be used in the construction market. acknowledgements the authors would like to acknowledge the support granted by the feder funds through the competitively and internationalization operational programme (poci) and by national funds through fct (the foundation for science 239 r. malheiro, a. morilla, a. ansolin et al. acta polytechnica ctu proceedings and technology) within the scope of the project with the reference poci-01-0145-feder-029328, which were fundamental for the development of this study. the authors would also like to acknowledge the support granted by danosa “derivados asfálticos normalizados, s.a.” industry for providing all the necessary insulation material to the hotbox construction. references [1] decree-law № 40/90 (1990). portugal: diário da república n.° 31/1990, série i de 1990-02-06. 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[2021-0930]. https://dre.pt/application/conteudo/672456 240 https://dre.pt/pesquisa/-/search/334611/details/maximized https://dre.pt/pesquisa/-/search/334611/details/maximized https://doi.org/10.3390/en13030624 https://doi.org/10.1016/j.susmat.2015.05.002 https://doi.org/10.1016/j.buildenv.2016.03.022 https://doi.org/10.1016/j.resconrec.2018.10.005 http://www.mires-and-peat.net/ https://doi.org/10.1016/j.wace.2015.08.001 https://doi.org/10.1626/pps.3.129 https://pt-pt.topographic-map.com/maps/gnn0/portugal/ https://pt-pt.topographic-map.com/maps/gnn0/portugal/ http://www.ipma.pt/resources.www/docs/publicacoes.site/atlas_clima_iberico.pdf http://www.ipma.pt/resources.www/docs/publicacoes.site/atlas_clima_iberico.pdf https://doi.org/10.1520/c1363-19 https://doi.org/10.3390/en14144276 https://termolan.pt/wp-content/uploads/2021/03/ft-roctermmn230-pt-compressed.pdf https://termolan.pt/wp-content/uploads/2021/03/ft-roctermmn230-pt-compressed.pdf https://portal.danosa.com/danosa/cmsservlet?node=484103&lng=4&site=3 https://portal.danosa.com/danosa/cmsservlet?node=484103&lng=4&site=3 https://secilpro.com/upload/documents/55acf7dba8e4b.pdf https://secilpro.com/upload/documents/55acf7dba8e4b.pdf https://www.amorimcorkinsulation.com/xms/files/fichas_tecnicas_2021/ft_corkboard_pt_2021.pdf https://www.amorimcorkinsulation.com/xms/files/fichas_tecnicas_2021/ft_corkboard_pt_2021.pdf https://doi.org/10.1016/j.rser.2016.05.045 http://desertfishes.org/cuatroc/organisms/arundo-vs-phragmites.html http://desertfishes.org/cuatroc/organisms/arundo-vs-phragmites.html https://dre.pt/application/conteudo/672456 acta polytechnica ctu proceedings 38:235–240, 2022 1 introduction 2 reed: specie identification and climate characterisation of harvesting regions 3 experimental program 3.1 reed boards preparation 3.2 thermal performance 4 results 4.1 thermal performance of reed board prototype 5 giant reed board prototype as insulation material 6 conclusions acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.34.0133 acta polytechnica ctu proceedings 34:133–137, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague automatic extraction of 3d building meshes from lidar data jan voříšek∗, bořek patzák, daniel rypl czech technical university in prague, faculty of civil engineering, thákurova 7, 166 29 prague 6, czech republic ∗ corresponding author: jan.vorisek@fsv.cvut.cz abstract. terrestrial laser scanning and drone-based scanning are often combined to gain a complete picture of the external envelopes of buildings. while the resulting data sets are easy to understand for humans, extracting any semantical information from the data is a challenging task. creating a bim model for large cities is desirable, especially for planning and inventory purposes. nowadays, engineers manually separate buildings and draw corresponding floor plans or any other required entities, making creating such a model nearly impossible for the whole city. in this contribution, we present the design and concept of a c++ library that provides fundamental algorithmic tools for automated detection of the ground points, separation of buildings into individual point clouds, and segmentation of walls and roofs. the buildings’ 3d model (both volumetric and surface mesh) can be constructed using the data. furthermore, the building area and volume calculation can be performed on such a model, which can then be compared with the existing city plan. keywords: laser scanning, bim, point cloud, city planning. 1. introduction building information modelling (bim) brings excellent opportunities for the efficient and more automated design process of building structures and maintenance and reconstruction planning of existing city structures. however, obtaining a digital bim representation of the latter is often challenging when no up-to-date city ground plans are available in digital form. the plans are usually just a set of polylines with no relation to individual buildings or other entities. laser scanning is a convenient and accessible technology to document the existing infrastructure. for cities, the combination of the terrestrial laser scanner and drone-based scanning is often used to capture the external envelope of the buildings. the output is a dense set of data points in space, the so-called point cloud. see figure 1 for an example of a laserscanned part of dublin. the dataset was obtained from [1]. while point clouds can be directly rendered and inspected, they carry no additional information about the scanned objects. therefore, it is desirable to convert the raw point data into a bim representation. civil engineers and architects must identify and separate buildings manually, draw the corresponding floor plans, and measure any desired quantity (i.e., building area and volume). our goal is to provide automated extraction of the individual buildings from the city point cloud and construction of the corresponding building meshes. the idea is based on the segmentation of the groundlevel points. then, those points are used as a filter to obtain all the points above a certain height. segmentation of the individual buildings is then performed on the filtered point cloud. furthermore, the outer figure 1. laser scan of dublin consisting of 226 million points before downsampling. the buildings and streets are clearly visible because the points were obtained from an airplane, so the point density is much higher than on the walls. unfortunately, no rgb color information is available. source: [1] surface planes are found, and a voxel-based 3d model is built for each building. the calculation of the build133 https://doi.org/10.14311/app.2022.34.0133 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en j. voříšek, b. patzák, d. rypl acta polytechnica ctu proceedings ing volume is straightforward. by extraction of the surface of the voxel mesh, the building area can be approximated. such a surface model may be suitable for texture mapping and city visualization in planning and architectural applications. we have used a 2×2 km area of dublin for the implementation and verification of our implemented algorithms. the raw dataset is described in [1]. 2. pre-processing laser scanners produce very dense point clouds containing hundreds of millions of points. the distance between the scanned points often ranges in millimeters. therefore the point cloud data processing must be handled carefully in order to achieve competitive computation times and memory efficiency. 2.1. downsampling while storing the points as binary data offers high reading speeds, with hundreds of millions of points, reading all the values into memory might still not be possible on a standard computer with limited memory. for example, storing 100 million xyz coordinates (3x64 bit float) occupies 2.4 gb of memory, more than half of a commonly available 4 gb. reducing the number of points to work with is inevitable. in our case, the points were downsampled in a 3d voxel grid using the efficient implementation of a c++ hash map [2]. reasonable selection of a grid step of say 1 × 1 × 1 cm reduces all the points within the 1 cm3 cube to a single point with weighted coordinates. as we have discussed in [3], one can see that just by downsampling to a 1 cm3 grid, the size of points goes down to at least 30%. in the case of city scanning, the grid step length may be much longer. for example, we have used a downsampling step of 5 cm for the dublin point cloud, and even a larger step of 10 cm still produced satisfying results. 2.2. the normal vector of downsampled points further processing can take advantage of determining the normal vector at each downsampled point. this is done in the following steps. for each point, its ten closest points are found. this is done by implementing and using a k-d tree data structure. the total number of points n for the normal evaluation is at most n = 10. the 3 × 3 covariance matrix of the point x, y, z coordinates relative to the point is calculated using the formula for the covariance of a and b below. cov(a,b) = ∑n i=1(ai − µa)(bi − µb) n− 1 , (1) where µx, µy and µz are the coordinates of the subject point. the smallest eigenvector of the covariance matrix is the plane normal [4]. the direct solver of eigenvalues figure 2. the results of a ground segmentation: the streets are dominating the picture. it can be seen that most of the inner areas of the building were captured as well. the large flat park area in the top right is also notable. was implemented for the 3×3 matrix using the arccos function, as described in [5]. 3. ground-level segmentation there are numerous algorithms for ground segmentation. for example, in [6], they use a ground plane fitting algorithm that is suitable for small areas of the point cloud. as we have wanted to obtain ground points for the whole dataset (2 km2 dublin area), we propose a different approach suitable for cities with mostly vertical building walls. we claim that this is a reasonable approach for most cities. however, the algorithm may not perform well for areas where no vertical building walls are present. the first step is to activate a two-dimensional grid in the horizontal plane. grid coordinates are calculated for each point, and the point is added to a list at a corresponding grid index. in the next step, we iterate over all the active grid cells. cells without a sufficient vertical coordinate difference ∆z will be removed from the grid. the minimum vertical coordinate difference equals the minimum expected building height. we have chosen ∆z = 3 m because we do not expect any building to be smaller than such a height. most of such selected grid cells will contain building wall points. however, lamps, some higher trees, or noise will be captured as well. a minimum number of points nmin in the cell is introduced to limit the number of false positives. the average point density ρ is calculated for the point cloud. for the dublin data set, the value is around 1 point per 3 cm3. the average point density ρcell is calculated for the grid cell and compared to the ρ. only cells with ρcell / ρ > 0.25 are accepted to accommodate walls with large openings (i.e., windows, balconies). finally, any high walls present on the buildings and not reaching the ground need to be removed. another horizontal grid with a much higher step length 134 vol. 34/2022 automatic extraction of 3d building meshes from lidar data figure 3. the results of building segmentation: each of the buildings are marked in a single color. it can be seen that noise is present in the results as small entities (primarily trees). multiple buildings may be identified as one when some joining element is present (i.e., bridge or a tree in between). is introduced (25 m in our case). points from the previous grid (z-coordinate is equal to the minimum vertical coordinate in the cell) are activated within the larger grid. we can then calculate the median of the z-coordinate per the larger grid’s steps and filter out the smaller grid by not taking into account the points above the median. finally, we can calculate the middle point (in terms of x, y coordinates) for each active cell, and the point’s z coordinate is considered the minimal z coordinate within the cell. those calculated points are considered to be the ground-level approximation. in order to separate the road surface and sidewalks, the z coordinate of the ground points should be increased by some small tolerance. we have used values around 0.5 m. it is then possible to iterate over all of the initial points and find its closest ground level point. if the point lies below the ground points, then it can be considered as a surface point. the results of a ground segmentation can be seen in figure 2. 4. building segmentation the ground level was estimated to perform building segmentation similarly as we have performed rooms segmentation in our previous work [3]. the closest ground point is found for each point. each point that is at least 3 meters higher than the ground point is projected onto a 2d grid resulting in a pixel-like image. the step of the grid should be at least twice the step used for the initial downsampling, or the grid will contain missing points as some points may not be appropriately captured. region growing [7] is applied to join adjacent pixels into separate regions. each region corresponds to a single building. backfilling of each of the found regions is performed to include missing areas (i.e., chimneys). knowing the pixel count and pixel area (step × step), the area of the regions can be estimated. finally, the original point cloud is iterated, and points inside of the found regions (in terms of x and y coordinates) and above the ground are selected. 5. plane segmentation the surface planes are identified for each building using the modified random sample consensus [8] with minimum area and continuity control. three separate sets of trial points for outer walls, horizontal roofs, and pitched roofs are constructed. running the ransac directly on the point cloud has a high chance of finding a generally oriented plane across the building. the three separated sets of points are defined as follows. outer walls only points with horizontal normal are considered for plane finding, a slight deviation of 15 degrees is allowed to account for deviations in construction. horizontal roofs only points with vertical normal are considered for plane finding, a slight deviation of 15 degrees is allowed to account for deviations in construction. pitched roofs only points not contained in the previous two datasets are considered for plane finding. the results of a plane segmentation for one of the building can be seen in figure 4. 6. building mesh each building point cloud is iterated, and its points are projected in a horizontal 2d grid. the single region is then backfilled to avoid losing volume in places with no points captured (i.e., shadows or reflective surfaces). finally, the grid is deactivated at places where any points exist below the ground level. the simplest mesh consists of blocks, each defined by the grid cells (in the xy plane), the ground level, and the highest point within the cell. such a model allows for a simple building volume calculation by summing up the volumes of the blocks. the area is the product of the sum of the active cells in the grid and the grid step squared. such a mesh can be seen in figure 5. calculating the building volume, including antennas or chimneys, may not be desirable. therefore, the previously found roof planes can be used to clip the blocks so that no z coordinate is above the subject plane. voxel-based mesh does not precisely represent the building near the surface if the planes are not parallel to the voxel grid. the previously found planes could be used to straighten up the voxel model by moving its vertices onto the planes within a small distance. 135 j. voříšek, b. patzák, d. rypl acta polytechnica ctu proceedings figure 4. the results planes of plane segmentation for one building: each of the planes is marked with a unique color. it can be seen that most of the surface planes were found with some smaller pieces missing. figure 5. the resulting voxel model of a single building. 7. conclusions this paper analyzed the methods of segmentation of the bim entities from the city point clouds. the primary motivation was to separate individual building point clouds from the large city point cloud and obtain water-tight mesh of such buildings. the meshes are suitable for approximation of the building volume and area. the proposed algorithms were tested on a 2015 aerial laser and photogrammetry survey of dublin city. the findings can be summarized as follows. • downsampling with a step of 0.05 m dramatically reduces the number of points to around 20% of the original dataset. such precision is still sufficient for the segmentation of the buildings and their features. • the ground segmentation procedure works well for cities with mostly vertical building walls. more complex buildings can still be identified if there are enough nearby ground points. • the building segmentation algorithm works well for buildings with a sufficiently large area. however, when using a small minimum area of around ab,min < 10 m2, it is not easily possible to distinguish between a small building (i.e., a garage) and a tree. in order to eliminate the trees, buildings with no found planes can be excluded. • modified ransac algorithm with minimum area and continuity checking is used to find the building surface planes. pre-filtering of the trial point clouds is done using the point normal vectors, which dramatically increases performance. the results are promising for vertical (outer walls) and horizontal (roof) planes. however, some skewed roof planes are often missing due to noise or additional structures (i.e., large chimneys). • 3d voxel mesh can be constructed for each building. the building area and volume can be easily estimated from the voxel mesh. the voxel mesh can be clipped by the found planes, removing small features (i.e., chimneys and antennas). the voxel points can also be projected onto the found planes to straighten out the surface. 136 vol. 34/2022 automatic extraction of 3d building meshes from lidar data acknowledgements the development of this software was supported by the technology agency of the czech republic under the program of the national competence center 1 as a project “center for advanced materials and efficient buildings” (cameb) project registration no. tn01000056 and by the grant agency of the czech technical university in prague (sgs project no. sgs21/037/ohk1/1t/11), both gratefully acknowledged. references [1] d. f. laefer, s. abuwarda, anh-vu vo, et al. 2015 aerial laser and photogrammetry datasets for dublin, ireland’s city center, 2017. https://doi.org/10.17609/n8mq0n. [2] g. popovitch. the parallel hashmap or abseiling from the shoulders of giants. https://greg7mdp.github.io/parallel-hashmap/. accessed: 2020-04-14. [3] j. voříšek, b. patzák, e. dvořáková, d. rypl. automated bim entity reconstruction from unstructured 3d pointclouds. acta polytechnica ctu proceedings 30:126–130, 2021. https://doi.org/10.14311/app.2021.30.0126. [4] e. ernerfeldt. fitting a plane to noisy points in 3d. https://www.ilikebigbits.com/2017_09_25_plane_ from_points_2.html, 2017. accessed: 2020-04-14. [5] s. hartmann. computational aspects of the symmetric eigenvalue problem of second order tensors. technische mechanik 23:283–294, 2003. [6] d. zermas, i. izzat, n. papanikolopoulos. fast segmentation of 3d point clouds: a paradigm on lidar data for autonomous vehicle applications. in 2017 ieee international conference on robotics and automation (icra), pp. 5067–5073. 2017. https://doi.org/10.1109/icra.2017.7989591. [7] m. montoya, c. gil, i. garcía fernandez. implementation of a region growing algorithm on multicomputers: analysis of the work load balance., 2000. [8] m. a. fischler, r. c. bolles. random sample consensus: a paradigm for model fitting with applications to image analysis and automated cartography. communications of the acm 24(6):381–395, 1981. https://doi.org/10.1145/358669.358692. 137 https://doi.org/10.17609/n8mq0n https://greg7mdp.github.io/parallel-hashmap/ https://doi.org/10.14311/app.2021.30.0126 https://www.ilikebigbits.com/2017_09_25_plane_from_points_2.html https://www.ilikebigbits.com/2017_09_25_plane_from_points_2.html https://doi.org/10.1109/icra.2017.7989591 https://doi.org/10.1145/358669.358692 acta polytechnica ctu proceedings 34:133–137, 2022 1 introduction 2 pre-processing 2.1 downsampling 2.2 the normal vector of downsampled points 3 ground-level segmentation 4 building segmentation 5 plane segmentation 6 building mesh 7 conclusions acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2020.26.0117 acta polytechnica ctu proceedings 26:117–125, 2020 © czech technical university in prague, 2020 available online at https://ojs.cvut.cz/ojs/index.php/app on optimum design of frame structures marek tybureca,∗, jan zemana,c, martin kružíkb,c, didier henriond,e a czech technical university in prague, faculty of civil engineering, department of mechanics, thákurova 7, 166 29 prague 6, czech republic b czech technical university in prague, faculty of civil engineering, departement of physics, thákurova 7, 166 29 prague 6, czech republic c czech academy of sciences, institute of information theory and automation, department of decision-making theory, pod vodárenskou věží 4, 182 08 prague 8, czech republic d university of toulouse, laas-cnrs, 7 avenue du colonel roche, 31400 toulouse, france e czech technical university in prague, faculty of electrical engineering, department of control engineering, karlovo náměstí 13, 121 35 prague 2, czech republic ∗ corresponding author: marek.tyburec@fsv.cvut.cz abstract. optimization of frame structures is formulated as a non-convex optimization problem, which is currently solved to local optimality. in this contribution, we investigate four optimization approaches: (i) general non-linear optimization, (ii) optimality criteria method, (iii) non-linear semidefinite programming, and (iv) polynomial optimization. we show that polynomial optimization solves the frame structure optimization to global optimality by building the (moment-sums-of-squares) hierarchy of convex linear semidefinite programming problems, and it also provides guaranteed lower and upper bounds on optimal design. finally, we solve three sample optimization problems and conclude that the local optimization approaches may indeed converge to local optima, without any solution quality measure, or even to infeasible points. these issues are readily overcome by using polynomial optimization, which exhibits a finite convergence, at the prize of higher computational demands. keywords: frame structures, global optimum, polynomial optimization, semidefinite programming, topology optimization. 1. introduction designing economical, efficient, and sustainable structures represents a major challenge of the contemporary society. while structural engineers literally explore designs that must satisfy the requirements of limit states analysis, there is usually an infinite number of such designs. regardless of the properties of this feasible design space, it is required to select only one, the best design. this design quality is measured by an objective function, which usually approximates the expenses of production. greatly simplified, one such (most common) criterion considers maximization of structural stiffness (while the amount of available material is limited), or, equivalently, minimization of structural volume (while requiring a certain structural stiffness) [1]. among the structural optimization problems the greatest progress has been achieved so-far in optimizing (the cross-sectional areas of) truss structures [2]. this development can be attributed to convexity of the feasible design space [3, sections 1.3.5, 3.4.3, and 4.8], as the (axial) structural stiffness is an affine function of the cross-sectional areas. on the other hand, when the bending stiffness comes into a consideration, convexity does not hold in general. quite surprisingly, these problems are also much less studied, and to our knowledge only local optimization algorithms have yet been developed [4, 5]. in this contribution, we investigate the problem of optimum design of frame structures. in particular, we develop four different methods in section 2: (i) general non-linear optimization solved by the interior-point method of fmincon, (ii) the first-order optimality criteria (oc) method [1, 5], (iii) reformulation of the problem into a non-linear semidefinite program (nsdp) solved by penlab [6], and (iv) a suitably modified polynomial optmization (po) problem (iii) solved globally using polynomial optimization methods [7] and the mosek [8] optimizer. we show that the latter po approach generates guaranteed lower and upper bounds on the solution, providing a means of assessing the design quality. finally, section 3 introduces a set of three sample optimization problems to compare the optimization approaches. 2. solution techniques to frame optimization 2.1. problem statement let us consider the problem of optimum design of frame structures composed a finite number of nodes, nn, and of admissible elements, ne, defining the socalled ground structure [2]. the frame elements are attributed with non-negative, and thus possibly zero, given-shaped cross-sectional areas a. for simplicity, 117 https://doi.org/10.14311/app.2020.26.0117 https://ojs.cvut.cz/ojs/index.php/app m. tyburec, j. zeman, m. kružík, d. henrion acta polytechnica ctu proceedings ki(ai) =  eiai `i 0 0 −eiai `i 0 0 12eiii `3 i 6eiii `2 i 0 − 12eiii `3 i 6eiii `2 i 4eiii `i 0 − 6eiii `2 i 2eiii `i sym. eiai `i 0 0 12eiii `3 i − 6eiii `2 i 4eiii `i  (3) we assume, in the following text, that the moments of inertia of individual cross-sections ii are polynomials of degree two1, i.e., ii = ci,ia 2 i for some ci,i > 0, which includes all cross-sections with given aspect ratios of all their components. note that in such a case, each cross-section is fully determined by its area. optimizing the cross-sectional areas a we search the maximum stiff structures within the available volume v of a linear-elastic material. the structural stiffness is measured (inversely) by the compliance c, work done by external forces, f(a), c(a) = f(a)tu = utk(a)u, (1) where u constitutes the generalized displacement vector, and k(a) is the symmetric positive semi-definite stiffness matrix— a polynomial function of a, k(a) = ne∑ i=1 ki(ai), (2) assembled from contributions of individual elements ki(ai). the lower the compliance, the stiffer is the structure with respect to the external forces. in this contribution, we use the element stiffness matrix of euler–bernoulli frame elements (3), with ei denoting the young modulus. in (1), f(a) is the external force column vector—a linear function of a—to allow for self-weight, assembled from the contributions of elements f(ai), f(a) = ne∑ i=1 fi(ai). (4) in the following text, we assume that ∀a > 0 : k(a) � 0, i.e., the structure is not a kinematic mechanism and the stiffness matrix is positive definite (which is denoted by “� 0”) for all positive cross-sectional areas. since k(a) has therefore the full rank, we also have f(a) ∈ im (k(a)), where im(•) is the image of •. 1the same procedure can be employed for higher-degree polynomials, so that all cross-sectional parameters can be optimized concurrently. we restrict ourselves, however, to the polynomials of degree two to maintain a simpler notation. this optimization problem is formalized as min a,u f(a)tu (5a) s.t. k(a)u = f(a), (5b) `ta ≤ v , (5c) a ≥ 0, (5d) with ` being the column vector of the frame elements lengths. notice that in general, (5) constitutes a nonconvex non-linear optimization problem because of the bilinear objective function (5a) and the polynomial equilibrium equality (5b) with possibly singular k(a). on the other hand, the volume constraint (5c) and the cross-sectional areas non-negativity constraint (5d) are affine functions of the design variables, u and a, and are in turn convex. the optimization problem (5) can be solved to local optimality using standard numerical optimization techniques. in particular, we will solve this problem using the interior-point method implemented in the fmincon function of matlab. 2.2. optimality criteria the inherent difficulty of singularity of k(a) in (5b) can be circumvented by assuming a small positive lower-bound on the cross-sectional areas [1], which is denoted by ε in this study. consequently, the former topology optimization problem (5) is effectively transformed into the sizing one and the displacement field u can be excluded from the design variables2. notice that in the limit when ε → 0, these problems are equivalent, but the smaller ε the higher the condition number of k(a), and thus it is more difficult to solve (5b). hence, we assume ε = 10−6 in this study. considering (5) with ε1 ≤ a, its lagrangian function reads as l(a,u,λ, µ,ν) = f(a)tu + λt [f(a)−k(a)u] +µ ( `ta − v ) + νt (ε1− a) , (6) with the lagrange multipliers λ, µ, and ν; and 1 denoting the vector of all ones. in addition to the primal feasibility (5b)–(5d), the karush–kuhn–tucker conditions require feasibility of the dual and complementary 2at the price of solving the equilibrium equation in each iteration. 118 vol. 26/2020 on optimum design of frame structures slackness, µ ≥ 0, (7a) µ ( `ta − v ) = 0, (7b) ν ≥ 0, (7c) νt (ε1− a) = 0, (7d) together with the stationarity of the lagrangian with respect to u, 0 = dl(a,u,λ, µ,ν) du = [f(a)−k(a)λ]t , (8) and a. however, because the equation k(a)λ = f(a) in (8) possesses a unique solution due to k(a) � 0, we have λ = u. using this observation, the necessary first-order optimality conditions read as 0 = ∂l ∂ai = 2ut ∂f(a) ∂ai − ut ∂k(a) ∂ai u + µ`i − νi. (9) conditions (9) and (7d) then imply that at the optimum, the frame elements with ai > ε have equal constant energy µ = 1 `i ut ∂k(a) ∂ai u− 2 `i ut ∂f(a) ∂ai . (10) aiming to satisfy (10), optimality criteria methods [1] build update schemes which increase stiffnesses of elements with energies higher than µ, and, conversely, decrease stiffnesses of elements with the energy lower than µ. the levels are balanced in an iterative process, based on the value of µ. in each iteration, the relative change of the design variables is bounded by the move limit ζ, assumed as ζ = 0.2 in this paper. consequently, the fix-point update scheme reads as a (k+1) i = max { max { (1− ζ)a(k) i , ε } , a (k) i [ b (k) i ]η} (11) with the tuning parameter η = 0.3 and with b (k) i = ut ∂k(a) ∂ai u− 2ut ∂f(a) ∂ai µ`i . (12) clearly, if ∀i ∈ {1, . . . , ne} : b(k) i = 1, we reach a local minimum as (10) is satisfied. combination of (11), (12) with (5c) allows us to write the (current) volume v = `ta(k+1) as a continuous function of the multiplier µ. it can be seen from (12) that v is a non-increasing function of µ. in fact, strict decreasing occurs when εi < a. consequently, the bisection algorithm is used to find µ such that the volume constraint is satisfied. 2.3. nonlinear semidefinite programming in this section, we describe another approach to eliminate the displacement field variables u from the optimization problem formulation. first, let us rewrite (5) as min a,c c (13a) s.t. c− f(a)tk(a)†f(a) = 0, (13b) `ta ≤ v , (13c) a ≥ 0, (13d) where k(a)† denotes the moore-penrose pseudoinverse of k(a). because we require that ∀a > 0 : k(a) � 0, the (possible) singularity of k(a) is caused exclusively by zero rows and columns belonging to the degrees of freedom without any attached finite element (or, equivalently, ai = 0 for all attached elements in that node). this assumption allows us to partition the stiffness matrix into a positive definite principal submatrix k̂(a) and zero blocks, so that the pseudo-inverse equals k(a)† = ( k̂(a)−1 0 0t 0 ) . (14) using the same partitioning, the force vector f(a) is split into ( f̂(a)t 0t )t , in which the term 0t appears due to the original assumption that f(a) ∈ im(k(a)). consequently, we see that (13b) can be rewritten to c− f̂(a)tk̂(a)−1f̂(a) = 0, (15) and (13) is therefore equivalent to (5). from (14) we have k†(a) � 0, so that c ≥ 0 based on (13b). moreover, iff f̂(a) 6= 0, we have both f(a)tk(a)†f(a) > 0 and c > 0. because we minimize c (13a) and f(a)tk(a)†f(a) is bounded from below, (13b) can be simplified to the one-sided inequality c− f(a)tk(a)†f(a) ≥ 0. (16) to use the generalized schur complement lemma, e.g., [9, theorem 16.1], we further need to show that[ i−k(a)k(a)† ] f(a) = 0 (17) holds, with i denoting the identity matrix. indeed, (17) is always satisfied because f(a) ∈ im(k(a)), so that we can substitute f(a) by k(a)v, where v is a vector of coefficients of the linear combination. then, (17) is equivalent to[ k(a)−k(a)k(a)†k(a) ] v = 0, (18) with the term in the square brackets always zero [9, lemma 14.1], as k(a) is symmetric. finally, application of the generalized schur complement lemma to (16) and (18) provides us with min a,c c (19a) s.t. ( c −f(a)t −f(a)t k(a) ) � 0, (19b) `ta ≤ v , (19c) a ≥ 0, (19d) 119 m. tyburec, j. zeman, m. kružík, d. henrion acta polytechnica ctu proceedings which is a non-linear semidefinite program equivalent to both (13) and (5). moreover, if we substitute c with 1/s, where 0 < s < ∞ is a measure of stiffness, (19) is, after the application of the (standard) schur complement lemma, e.g., [9, proposition 16.1], reducible to max a,s s (20a) s.t. k(a)− sf(a)f(a)t � 0, (20b) `ta ≤ v , (20c) a ≥ 0, (20d) which is a useful reformulation when f is constant, i.e., self-weight is not considered. it shall be noted that due to the polynomial matrix inequalities (19b) and (20b) both the optimization problems are non-convex in general. they can still be solved efficiently (to local optimality) using, e.g., augmented lagrangian methods [6, 10]. in this contribution, we solve these problems using the open-source penlab optimizer [6]. 2.4. polynomial optimization having introduced three different local approaches to frame structure optimization, it is natural and expected that one asks for a global optimization technique. in this section, we exploit the fact that although (19) is non-convex it is indeed a polynomial optimization (po) problem at the same time, which allows us to employ modern po techniques, namely the lasserre hierarchy [7, 11], successively building tighter and tighter convex outer semidefinite programming (sdp) approximations called relaxations [7, corollary 4.3]. to develop a more efficient formulation suitable for po, we first recognize that the design variables in (19) are all bounded both from below and above: 0≤ai≤ v `i , ∀i ∈ {1, . . . , ne}, (21a) 0≤ c ≤ ĉ. (21b) while the lower bound in (21a) is caused by the nonnegativity of the cross-sectional areas (19d), the upper bounds arise from the volume constraint (19c): none of the structural elements can occupy larger volume than v . in the compliance case (21b), the lower bound3 is due to k(a) � 0, recall the discussion in section 2.3, and the upper bound is provided by an arbitrary feasible design, e.g., the uniform cross-sectional areas a = v 1t` 1, (22) which are used for the upper-bound computation in this paper. then, the compliance bound is computed from ĉ = f(a)tk(a)†f(a), (23) 3in fact the strict inequality 0 < c is satisfied in all nontrivial optimization problems. where k(a)† reduces to k(a)−1 in the case of (22). to improve numerical stability of the solution process, we state the optimization problem in terms of the scaled cross-sectional areas asc and scaled compliance csc instead of the original a and c, where the scaled variables are defined in the [−1, 1] domain. therefore, we have ai = v (asc,i + 1) 2`i (24) and c = ĉ(csc + 1) 2 . (25) rewriting (21) in terms of asc,i and csc as unit ball constraints a2 sc,i ≤ 1, ∀i ∈ {1, . . . , ne}, (26a) c2 sc ≤ 1, (26b) we finally arrive at an equivalent formulation to (19), min asc,csc 0.5ĉ(csc + 1) (27a) s.t. ( 0.5ĉ(csc + 1) −f(asc)t −f(asc)t k(asc) ) � 0, (27b) 2− ne − 1tasc ≥ 0, (27c) 1− a2 sc,i ≥ 0, (27d) 1− c2 sc ≥ 0. (27e) in (27), the objective function (27a) and also the constraints (27b)–(27e) are all polynomial inequalities4 non-negative in the feasible region of (27), which is therefore a semi-algebraic set. 4the entries in the pmi (27b) are polynomials, so that the feasible region of the pmi is a semi-algebraic set. alternatively, we may require the roots of the characteristic polynomial of (27b) to be non-negative. we refer to [12] for more details. 120 vol. 26/2020 on optimum design of frame structures the optimization problem (27) can readily be solved by building the (lasserre) hierarchy of convex linear sdp relaxations, with a monotonously converging objective functions to the global optimum. the hierarchy is generated by the gloptipoly [13] software package interfaced with the yalmip [14] toolbox, and the underlying sdp relaxations are solved by the state-of-the-art mosek optimizer [8]. 2.4.1. solution process to simplify the notation, let us now denote the objective function polynomial (27a) by p0, and the constraining polynomials (27c) to (27e) by p1 to p3, respectively. further, let p4 denote the pmi (27b) and let x = ( csc asc,1 . . . asc,ne )t (28) be the vector of the design variables. moreover, let br(x) = ( 1 x1 x2 . . . xne+1 x2 1 x1x2 . . . x1xne+1 x2 2 x2x3 . . . x2 ne+1 . . . xr1 . . . xrne+1 )t (29) denote the polynomial space basis of the maximum degree r. then, we can express each of the polynomials pj , j ∈ {0, . . . , 3} as a linear combination pj(x) = |br(x)|∑ β=1 pα,j,β(xα)β (30) of monomials xα = ne+1∏ m=1 xαm m , ne+1∑ m=1 αm ≤ r, (31) indexed in the basis br(x). the vectors α associate a non-negative integer number with each element in x, and the vector pα,j contains coefficients of the linear combinations of the monomials. notice that∑ne+1 m=1 αm ≤ dj , where dj stands for the degree of the polynomial pj . a similar approach is also applied to the elements of p4 [12]. further, introducing y, with its components yβ corresponding to a monomial in the basis br(x), we build the lasserre hierarchy of convex linear semidefinite programming relaxations min y |br(x)|∑ β=1 pα,0,βyβ (32a) s.t. mr(y) � 0, (32b) mr−dj (y) � 0, ∀j ∈ {1, . . . , 4}, (32c) and solve it successively with an increasing relaxation order r until the globally optimal solution(s) are found. for all our test cases, this convergence was always finite. in (32), the matrix mr(y) is the moment matrix of the r-th order, and mr−dj is the (r − dj)-order localization matrix associated with pj or pj . for more details about these matrices, we refer the reader to [7, 12]. 2.4.2. recognizing global optimality there are (at least) two ways to recognize whether y∗r , a r-th order relaxation solution to (32), is globally optimal. the first (common) way is based a sufficient condition for global optimality of (32) [15, 16], rank mr(y∗r) = rank mr−d(y∗r), (33) in which y∗ constitutes the optimal solution in the basis br(csc,asc), and rank mr(y∗) is less or equal to the number of these solutions [7]. the values of the original design variables can be extracted using cholesky or singular value decompositions [16]. for the optimization problem (27), we introduce another sufficient condition for global optimality. let ãr be the (optimized) values of the cross-sectional areas and let c̃r denote the compliance found in the r-th order relaxation. indeed, c̃r ≤ c∗ [7]. moreover, because any feasible design a generates an upper bound ĉr, recall eq. (23), we have c̃r ≤ c∗ ≤ ĉr. (34) a globally optimal solution to (27) is found if there is no gap, ĉr − c̃r −→ 0. (35) note that (35) is substantially simpler to check than (33). if (35) is not satisfied in the (current) relaxation order r, we have at least the quality measure for the current solution in hand. 3. sample problems in this section, we describe and solve three rather small-scaled structural optimization problems, and compare the four optimization approaches. 3.1. cantilever beam consider a (simple) cantilever beam design problem, as shown in fig. 1a. the beam is discretized into ne finite elements of equal lengths, each of them assigned a prismatic square cross-section. the beam is loaded with a skew force at its tip. further, we assume (dimensionless) e = 1 and v = 0.1. it can be seen from table 1 that the problem is relatively “easy” to solve, as all of the tested algorithms converge to the unique global optima (shown in 1b–1e), proved by the po approach (figs. 1h–1j). in fact, we include this optimization problem mainly to show that po scales unambiguously worse with the problem dimension than the other methods, and that the optimality criteria (oc) method is the fastest one. using oc, we can optimize the discretizations of 150 or 300 elements in 0.4 and 0.8 seconds (figs. 1f and 1h). 121 m. tyburec, j. zeman, m. kružík, d. henrion acta polytechnica ctu proceedings t [s] ĉ c̃ a1 fmincon 0.7 107.50 0.100 oc 0.1 107.50 0.100 nsdp 0.2 107.50 0.100 po(1) 0.3 107.50 107.50 0.100 (a) 1 element t [s] ĉ c̃ a1 a2 a3 fmincon 0.6 80.30 0.142 0.102 0.056 oc 0.1 80.30 0.142 0.102 0.056 nsdp 0.3 80.30 0.142 0.102 0.056 po(1) 0.3 80.72 35.81 0.147 0.097 0.056 po(2) 0.3 80.30 80.30 0.142 0.102 0.056 (b) 3 elements t [s] ĉ c̃ a1 a2 a3 a4 a5 fmincon 0.4 77.19 0.151 0.128 0.103 0.075 0.043 oc 0.1 77.19 0.151 0.128 0.103 0.075 0.043 nsdp 0.5 77.19 0.151 0.128 0.103 0.075 0.043 po(1) 0.3 79.09 24.72 0.151 0.123 0.096 0.073 0.058 po(2) 0.7 77.37 76.34 0.154 0.131 0.103 0.072 0.040 po(3) 26.8 77.19 77.19 0.151 0.128 0.103 0.075 0.043 (c) 5 elements t [s] ĉ c̃ a1 a2 a3 a4 a5 a6 a7 fmincon 0.5 76.23 0.155 0.139 0.122 0.104 0.084 0.061 0.036 oc 0.1 76.23 0.155 0.139 0.122 0.104 0.084 0.061 0.036 nsdp 0.4 76.23 0.155 0.139 0.122 0.104 0.084 0.061 0.036 po(1) 0.3 79.81 20.00 0.149 0.130 0.112 0.096 0.081 0.069 0.062 po(2) 2.5 77.02 71.69 0.166 0.145 0.122 0.099 0.077 0.055 0.036 po(3) 550.1 76.23 76.23 0.155 0.139 0.122 0.104 0.084 0.061 0.036 (d) 7 elements table 1. comparison of the four optimization approaches on the design of the cantilever beam problem. bold text denotes the proven global optimum. 1 ne 1 ne 1− 2 ne 1 1 2 ne nn 1 ne 1 30◦x y (a) (b) c∗ = 107.50 (c) c∗ = 80.30 (d) c∗ = 77.19 (e) c∗ = 76.23 (f) c = 75.19 (g) c = 75.19 1 2 0 20 40 60 80 100 120 relaxation order r c om pl ia nc e c ĉr c̃r c∗ (h) 1 2 3 0 20 40 60 80 100 120 relaxation order r c om pl ia nc e c ĉr c̃r c∗ (i) 1 2 3 0 20 40 60 80 100 120 relaxation order r c om pl ia nc e c ĉr c̃r c∗ (j) figure 1. cantilever beam design problem. (a) shows the design domain, (b)–(e) are the optimal topologies for 1, 3, 5, and 7 elements; (f) and (g) display optimized topologies computed by oc with discretization by 150 and 300 elements. figures (h)–(j) show the convergance of po for 3, 5, and 7 elements. 122 vol. 26/2020 on optimum design of frame structures 1 2 x y 1 1 1 1 2 3 4 5 6 7 8 9 10 1 2 3 4 5 6 (a) (b) c = 1042.20 (c) c∗ = 959.32 1 2 0 500 1,000 1,500 2,000 relaxation order r c om pl ia nc e c ĉr c̃r c∗ (d) figure 2. 10-beam structure design problem. (a) shows the design domain. while all the tested local optimization algorithms converged to the topology in (b), the global optimum design (c) obtained by po possesses a clearly different topology. figure (d) shows the convergence of po. t [s] ĉ c̃ a1 a3 a4 a5 a6 a7 a8 a9 a10 fmincon 1.0 1042.14 0.144 0.040 0.018 0.000 0.003 0.000 0.210 0.039 0.038 oc 0.6 1042.33 0.146 0.039 0.017 0.000 0.002 0.000 0.212 0.039 0.037 nsdp 1.0 1042.20 0.144 0.040 0.018 0.000 0.003 0.000 0.210 0.039 0.038 po(1) 0.3 1429.31 443.20 0.103 0.082 0.000 0.029 0.000 0.000 0.121 0.095 0.069 po(2) 5.2 959.32 959.31 0.070 0.186 0.000 0.043 0.000 0.098 0.000 0.064 0.000 table 2. comparison of the four optimization approaches on the design of 10-beam structure. bold text denotes proven global optimum. the cross-sectional area a2 is zero in all test cases. 1 2 3 4 5 1 1 2 3 4 5 6 2 2 2 2 2 10 (a) tp 5tp 10 t p (b) 1 2 3 0 500 1,000 1,500 2,000 2,500 relaxation order r c om pl ia nc e c ĉr c̃r c∗ (c) figure 3. girder beam design problem. (a) shows the design domain, (b) the cross-sectional shape, and (c) the convergence of polynomial optimization. t [s] ĉ c̃ a1 a2 a3 a4 a5 fmincon 0.4 0.394 0.256 0.000 0.003 0.000 oc 0.1 1372.25 0.010 0.017 0.022 0.025 0.026 nsdp 2.0 0.000 0.000 0.000 0.000 0.000 po(1) 0.2 1456.75 297.34 0.007 0.015 0.022 0.027 0.029 po(2) 1.0 1426.05 1286.44 0.007 0.016 0.023 0.026 0.028 po(3) 37.7 1372.25 1372.25 0.010 0.017 0.022 0.025 0.026 table 3. comparison of the four optimization approaches on the design of the girder beam problem. bold text denotes the proven global optimum. 123 m. tyburec, j. zeman, m. kružík, d. henrion acta polytechnica ctu proceedings 3.2. 10-beam frame structure second, we consider the topology optimization problem of designing circular cross-sections of the 10-beam structure shown in fig. 2a. this structure is loaded by two moments of magnitudes 1 and 2, placed at the nodes 2 and 3 , respectively. note that there is no intersection between the elements 3 and 4 , and between 6 and 7 . we assume the young modulus e = 1 and the volume bound v = 0.5. evaluating the four optimization methods, table 2, we see that while the local optimization methods converged to nearby local minima of topologies shown in fig. 2b, po found the global optimum of a clearly different topology, fig. 2c. moreover, the global optimum possesses the compliance lower by 8%, which was reached in the second relaxation, fig. 2d. 3.3. i-shaped girder with self-weight last, we consider a simply-supported i-shaped girder beam of the span 20, loaded by a uniform load 1 and self-weight. due to the problem symmetry, we consider only one half of the problem, fig. 3a, and discretize it using 5 finite elements of equal length. this (half of the) girder beam is allowed to utilize at most v = 0.2 material of the young modulus e = 104 and of density ρ = 3. the beam cross-sections are parameterized by the parameter tp, which denotes the thicknesses of the flanges and web. the cross-sectional area equals a(tp) = 18t2p. we assume that the upper surfaces of the top flanges are aligned, so that we have i(tp) = 696t4p = 58/27a(tp)2. consequently, we optimize ai of individual elements, rather than tp. table 3 reveals that for this specific problem the oc converged to the global optimum. on the other hand, the fmincon and non-linear semidefinite programming approaches converged to an infeasible point. polynomial optimization exhibited convergence to the global optimum in three relaxations, fig. 3c. notice, moreover, that the designs found for r = {1, 2} were of very high qualities. 4. conclusions in this contribution, we investigated four topology optimization techniques for the design of frame structures with a given aspect-ratios all components of their cross-sections. three of the optimization techniques— general non-linear formulation solved by the fmincon solver, optimality criteria, and non-linear semidefinite programming—provide a local solution to the considered optimization problem, and in turn converge to a local optimum in general. unfortunately, the nsdp and fmincon techniques also converged to infeasible points. from the local optimization approaches, the optimality criteria method seems to be the most efficient one. on the other hand, the last technique — polynomial optimization — allows to solve the problem to proven global optimality, while generating both lower and upper bounds in each of the relaxation order. compared to the local approaches, the designs obtained in lower relaxation orders are of a known quality with respect to the optimum. however, finding a proven global optimum requires a considerably higher computational resources than solving the local optimization formulations. in all the test cases, the convergence of the po hierarchy was finite. list of symbols 0 column vector or matrix of all zeros 1 column vector or matrix of all ones a cross-sectional areas column vector asc scaled cross-sectional areas column vector ai cross-sectional area of the element i asc,i scaled cross-sectional area of the element i âr optimized cross-sectional areas at the relaxation r bi auxiliary variable associated with the element i br polynomial space basis of maximum degree r c compliance (external work) c̃ lower bound on compliance ĉ upper bound on compliance c∗ globally optimal compliance csc scaled compliance ci,i positive constant dj degree of the polynomial j e young modulus f generalized force column vector fi generalized force column vector of element i f̂ generalized force column vector associated with k̂ i element index j polynomial index ii moment of inertia of the element i i identity matrix k iteration number k stiffness matrix k̂ positive definite principal submatrix of k ki stiffness matrix of the element i l lagrangian function ` column vector of element lengths `i length of the element i m index of x mr moment matrix of the order r mr−dj localization matrix of the order r − dj ne number of elements nn number of nodes pj polynomial inequality pj polynomial matrix inequality pα,j coefficients of linear combinations of monomials r relaxation order, maximum polynomial degree s inverse of c tp parameter, web and flanges thickness t time u generalized displacement column vector v column vector of coefficients of linear combination v volume 124 vol. 26/2020 on optimum design of frame structures v volume upper bound x design variables column vector xα monomial indexed by α yβ moment associated with the basis br y column vector of moments y∗ optimal column vector of moments yr column vector of moments at r-th relaxation α vector of integers β br index η tuning parameter, 0.3 in this study ζ move limit, 0.2 in this study ε small positive number, 10−6 in this study λ lagrange multiplier column vector µ lagrange multiplier ν lagrange multiplier column vector acknowledgements the work of marek tyburec was supported by the grant agency of the ctu in prague, sgs19/033/ohk1/1t/11. martin kružík and jan zeman acknowledge the financial support by the european regional development fund through the project cz.02.1.01/0.0/0.0/16-019/0000778. references [1] m. p. bendsøe, o. sigmund. topology optimization: theory, methods, and applications. springer berlin heidelberg, berlin, heidelberg, 2003. doi:10.1007/978-3-662-05086-6. [2] w. s. dorn. automatic design of optimal structures. journal de mecanique 3:25–52, 1964. [3] a. ben-tal, a. nemirovski. lectures on modern convex optimization. society for industrial and applied mathematics, 2001. doi:10.1137/1.9780898718829. [4] g. steven, o. querin, m. xie. evolutionary structural optimisation (eso) for combined topology and size optimisation of discrete structures. computer methods in applied mechanics and engineering 188(4):743–754, 2000. doi:10.1016/s0045-7825(99)00359-x. [5] g. i. n. rozvany. structural design via optimality criteria, vol. 8 of mechanics of elastic and inelastic solids. springer netherlands, dordrecht, 1989. doi:10.1007/978-94-009-1161-1. [6] j. fiala, m. kočvara, m. stingl. penlab: a matlab solver for nonlinear semidefinite optimization. arxiv preprint arxiv:13115240 2013. 1311.5240. [7] j. b. lasserre. an introduction to polynomial and semialgebraic optimization. cambridge university press, cambridge, 2015. doi:10.1017/cbo9781107447226. [8] mosek aps. the mosek optimization toolbox for matlab manual. version 8.1., 2017. [9] j. gallier. geometric methods and applications. springer new york, 2011. doi:10.1007/978-1-4419-9961-0. [10] m. kočvara, m. stingl. pennon: a code for convex nonlinear and semidefinite programming. optimization methods and software 18(3):317–333, 2003. doi:10.1080/1055678031000098773. [11] j. b. lasserre. global optimization with polynomials and the problem of moments. siam journal on optimization 11(3):796–817, 2001. doi:10.1137/s1052623400366802. [12] d. henrion, j. b. lasserre. convergent relaxations of polynomial matrix inequalities and static output feedback. ieee transactions on automatic control 51(2):192–202, 2006. doi:10.1109/tac.2005.863494. [13] d. henrion, j. b. lasserre, j. löfberg. gloptipoly 3: moments, optimization and semidefinite programming. optimization methods and software 24(4-5):761–779, 2009. doi:10.1080/10556780802699201. [14] j. löfberg. yalmip : a toolbox for modeling and optimization in matlab. in in proceedings of the cacsd conference. taipei, taiwan, 2004. [15] h. e. curto, l. a. fialkow. the truncated complex k-moment problem. transactions of the american mathematical society 2000. doi:10.1090/s0002-9947-00-02472-7. [16] d. henrion, j. b. lasserre. detecting global optimality and extracting solutions in gloptipoly. lecture notes in control and information sciences 312:293–310, 2005. doi:10.1007/10997703_15. 125 https://doi.org/10.1007/978-3-662-05086-6 https://doi.org/10.1137/1.9780898718829 https://doi.org/10.1016/s0045-7825(99)00359-x https://doi.org/10.1007/978-94-009-1161-1 1311.5240 https://doi.org/10.1017/cbo9781107447226 https://doi.org/10.1007/978-1-4419-9961-0 https://doi.org/10.1080/1055678031000098773 https://doi.org/10.1137/s1052623400366802 https://doi.org/10.1109/tac.2005.863494 https://doi.org/10.1080/10556780802699201 https://doi.org/10.1090/s0002-9947-00-02472-7 https://doi.org/10.1007/10997703_15 acta polytechnica ctu proceedings 26:118–126, 2020 1 introduction 2 solution techniques to frame optimization 2.1 problem statement 2.2 optimality criteria 2.3 nonlinear semidefinite programming 2.4 polynomial optimization 2.4.1 solution process 2.4.2 recognizing global optimality 3 sample problems 3.1 cantilever beam 3.2 10-beam frame structure 3.3 i-shaped girder with self-weight 4 conclusions list of symbols acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.34.0006 acta polytechnica ctu proceedings 34:6–10, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague thermal properties of mineral wool insulation recovered from construction and demolition waste mária domonkosa,∗, ondřej zobala,b, zdeněk prošeka,c, jan trejbala a czech technical university in prague, faculty of civil engineering, thákurova 7, 166 29 prague, czech republic b knauf praha, s.r.o., mladoboleslavská 949, 197 00 prague 9, czech republic c university centre for energy efficient buildings of czech technical university in prague, třinecká 1024, 273 43 buštěhrad, czech republic ∗ corresponding author: maria.domonkos@fsv.cvut.cz abstract. mineral wool is one of the most commonly used types of thermal insulation in the european union at the present, however, it generates massive amounts of waste. from an environmental standpoint, recycling or reuse of insulation materials seems to be a wise solution. this paper aims to study the thermal and structural properties of recycled mineral wool insulation recovered from construction and demolition waste. the thermal conductivity, the basic parameter characterizing thermal insulation materials, was measured using a heat flow meter. test specimens with various bulk density (in the range 50–120 kg/m3) were made from micro-milled and chopped waste material (cladding of 30 years old building). the obtained results were compared with reference samples (non-contaminated blown mineral wool insulation supafil loft 045). keywords: construction and demolition waste, recycling, building insulation materials, mineral wool, thermal conductivity. 1. introduction 1.1. insulation material waste thermal insulation materials are essential in the design and construction of energy efficient buildings [1]. in the last decades, many efforts have been made to improve building comfort, however, along with that, the amount of waste from insulation materials has increased dramatically. moreover, most of the used building insulation materials simply end up in legal and illegal landfill sites [2]. the construction sector has a significant impact on the environment due to the amount of waste generated, among other issues (e.g., extraction of raw materials, high energy consumption and co2 emissions, etc.). unfortunately, the use of recycled materials in this area is not widespread yet. nevertheless, building materials produced from waste are considered green materials, thus recycling or reusing them is getting more and more attention. most insulation waste comes from construction and demolition (c&dw) [3]. it is estimated that c&dw accounts for 25 – 30 % (approx. 890 million tonnes annually) of all waste generated in the european union (eu) [4, 5]. the eu has set itself the goal of reusing, recycling or recovering 70 % of c&dw by 2020 [3]. it is also assumed that circular economy-inspired actions can improve the sustainability of building and construction sectors using the 3rs principle – reduce, reuse and recycle [6]. utilization of thermal insulation material waste could be an important step in improving the recycling percentage of c&dw [5, 7]. there are different types of thermal insulation materials (e.g., glass and stone wool, polystyrene, sheep’s wool, spray foam, polyurethane, fibreboard, cellulose) commercially available [1]. mineral wool is one of the most used materials in europe for thermal insulation, cold and fire protection, and noise insulation [5]. mineral wool refers to several different types of insulation, e.g., including various inorganic materials, e.g., rock, wool, glass, slag wool [2]. 1.2. mineral wool recycling mineral wool is difficult to recycle, and it’s recycling is not in practice in most countries [3, 8], despite the fact that the amount of mineral wool waste produced in the area of eu-28 in 2020 was estimated to be more than 2.5 million tonnes [5]. the czech republic generates more than 217,000 tonnes of mineral wool annually. furthermore, the transport and landfilling of insulation materials is expensive (approximately 1,750 czk/tonne in the czech republic) due to their unfavourable properties. poor compressibility and low weight-to-volume ratio (mineral wool insulation has a bulk density of 8–100 kg m−3) cause problems in landfills [8]. therefore, it is necessary to develop reliable methods for recycling this type of waste. it should be noted that insulation material waste may contain various contaminants (e.g., nails, screws, rubble, stones, wood, binders, lubricants, mortar, polyurethane foam, etc.) which must be considered when evaluating its 6 https://doi.org/10.14311/app.2022.34.0006 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 34/2022 thermal properties of mineral wool insulation recovered from c&dw reuse potential. from a technical point of view, a mineral wool recycling system should crush/grind the waste insulation material into short fibers and powder with the desired size and sufficiently separate the particles from the contaminants. an ideal machine is cost effective and movable, i.e., insulation materials can be collected or recycled on site. the short fibers can be then re-used as blownin/loose-fill insulation (in open attics, ceilings, etc.) or as filling insulation (in hollow structures, e.g. in concrete and ceramic masonry blocks, etc.). the effectiveness of thermal insulation materials is highly dependent on their thermal conductivity (the heat transfer coefficient). the lambda (λ) value represents the quantity of heat that passes through a unit area of a homogeneous material in a direction perpendicular to its isothermal planes induced by a unit temperature difference across the sample. the lower the lambda value, the better the thermal performance of the insulator material will be. a typical mineral wool has a thermal conductivity of 0.03–0.04 w/(mk), it varies with temperature, moisture content, type of pores and bulk density [1]. in this paper, the thermal conductivity of various samples recovered from construction and demolition waste is studied. 2. experimental part 2.1. materials and recycling three types of input materials were used for the experiments. set a includes non-contaminated virgin samples supafil loft 045 (knauf insulation) – blownin insulation made of pure natural fiber, which is made of recycled glass without binder and additives, used in new and existing open attics and ceilings above the highest heated floor. such material was used as a reference. these waste materials have the same origin: they served as thermal insulation of the peripheral cladding of a residential building in kralupy nad vltavou built in 1991. the original insulation system from basalt fibers (fos 125) was replaced with new materials. mark set 0 belongs to the "as-received" samples without any other processing, while set b and c (initial bulk density of 50 kg/m3) were subjected to different recycling technology and their properties thus differ from each other. while the first material is micro-milled using high speed mill (lav/k – 350 tex, lavaris, s.r.o., czech republic), the second is chopped by the similar technology where the mill was replaced with a gear crusher. then, the prepared material was uniformly sprinkled into a extruded polystyrene foam frame with internal dimensions 240×240×60 mm. after weighing the container, the bulk density was determined from the weight and the volume of the insulating material. test specimens with various bulk density (density in the range 50–120 kg/m3, 6 samples for each bulk density) were made from each material and then subjected to heat transfer coefficient measurement. the microstructural characterisation of the materials was achieved using optical (axio zoom, zeiss) and scanning electron microscopy (sem, thermo scientific phenom xl). figure 1. a) schematic drawing and b) photograph of the linseis heat flow meter instrument used for the measurement of the thermal conductivity coefficient. 2.2. thermal conductivity a thermal conductivity meter was used to determine the thermal conductivity properties of the insulation materials (figure 1). the linseis heat flow meter (hfm 300) is a robust and reliable instrument with a high level of accuracy and repeatability. it allows high precision temperature control due to the intelligent peltier heating and cooling system (in the temperature range from −20 up to +90 °c). the samples were put between a hot and cold plate in order to measure the heat flow. a temperature gradient (30–10 °c) was applied to the samples and the heat flow through the samples was measured by two sensors between the sample and each plate. the thermal conductivity was measured when stable conditions had been reached. the λ-value was determined from the thermal transmittance (calculated from the measured heat flow) and the thickness of the sample using equation (1) λ = q/(a · ∆t ) · d, (1) where: q – measured heat flow, a – area of the sample, ∆t – temperature gradient, d – thickness of the sample. the measurements were carried out in compliance with the standard čsn en 12667. 3. results and discussions figure 2 shows digital and sem photographs of the supafil loft 045 sample (set a). the sample is composed 7 m.domonkos, o. zobal, z.prošek, j.trejbal acta polytechnica ctu proceedings figure 2. representative optical and sem images of the non-contaminated supafil loft 045 sample (set a). figure 3. representative optical and sem images of demoliation mineral wool waste before processing in the recycling line (set 0). of long intertwined fibers. there is a high number of fibers in the unit volume. no impurities are present in the sample. figure 3 shows the waste mineral insulation from the demolition of an external thermal insulation composite system (etics) in kralupy nad vltavou before processing in the recycling line (set 0). there are also a high number of fibers in the unit volume and the fibers are long and intertwined. the presence of a large amount of impurities is evident. figure 4 shows the waste insulation after high-speed micro-milling (set b). the integrity of the original sample was disturbed, so the resulting material could serve as recycled blown insulation or as a filling insulation in the cavities of building materials and structures. the amount of impurities coincides with set 0. dust particles are visible in the sample. the fibers are short and damaged by the milling process. figure 5 shows waste insulation after the chopping process (set c). from a macroscopic point of view, the sample is very fine. sem images show large amounts of dirt and dust particles. the fibers are very short. the thermal conductivity coefficient of the samples was determined using a heat flow meter. figure 6 shows the dependence of the thermal conductivity on the bulk density of samples. the λ-value is an indication of how well a sample conducts heat. as is evident from figure 6, the set a (supafil loft 045) has very good thermal insulation performance – it has the lowest thermal conductivity coefficient values out of all sets. the sample with bulk density 64 kg/m3 had the lowest λ-value (0.0379 w/(mk)). it is worth noting that set a reaches 0.045 w/(mk) according to its manufacturer. however, such a thermal conductivity coefficient corresponds to a very low bulk density of 12 kg/m3 which is achievable only using a blowing machine. samples made of recycled insulation (set b and set c) exhibited higher thermal conductivity coefficients than reference set a, however, both still reached acceptable insulating properties, as their thermal conductivity coefficient oscillated between ca. 0.040–0.055 w/(mk). the results also show that reference material, set a, is characterised by very stable thermal conductivity (ca. 0.038 w/(mk) when its density is changed in the range of 50–120 kg/m3. compared to it, set b and c exhibited slightly different behaviour. their thermal conductivity decreases with increasing density (up to ca. 120 kg/m3), which is clear especially in case of set c. such a phenomenon is attributed to the principles of heat transfer; higher porosity of thermal insulation results in lower λ-values because the larger amount of 8 vol. 34/2022 thermal properties of mineral wool insulation recovered from c&dw figure 4. representative optical and sem images of micro-milled mineral wool waste (set b). figure 5. representative optical and sem images of chopped mineral wool waste (set c). trapped air within the material restricts heat transfer. however, when a certain value of bulk density is reached, which is individual for specific insulating materials, the coefficient increases. this is due to the fact that if the bulk density of the material is too low, its structure is very porous and heat is allowed to dissipate by convection. on the other hand, if the bulk density of the material is too high, the structure of the insulator is so dense that heat is dissipated to an increased extent by conduction. for our experiments it is evident that the value of the thermal conductivity can be regulated by the inner structure (particle size and porosity) of the samples, i.e. the characteristics of the output material of the recycler line. while the micro-milled set b tends to copy the behaviour of set a, set c showed massive decrease in thermal conductivity with increasing density. set b is thus suitable to be used as both recycled loose-fill/blown-in and filling insulation. set c is more appropriate as filling insulation. thermal insulation properties of the set c are poor in small densities (in contrast with blown-in insulation). some improvement occurs after the density is more than ca. 90 kg/m3. density lower than 60 kg/m3 can not be achieved in this case due to short and too compacted fibers. however, its compactness may cause stability in dismounting, which is strongly limited for filling insulation as stated in čsn en 14064-1. 3.1. recycling line the construction industry is a major source of waste in developed countries. the purpose of the international waste policy is to reduce the use of raw resources and convert waste material into usable recyclate and reuse it as much as possible. recycling minimizes the adverse effects of waste generation and reduces the environmental impact. insulation materials are an underutilized material for recycling. re-use options for mineral wool waste are limited yet, however after developing an efficient recycling process there is a possibility to turn mineral wool waste from unrecyclable waste into a raw material which can be re-used in the mineral wool manufacturing process or in manufacturing new construction materials such as brick, concrete, cement, ceramics, cement or fiber-based composites, tiles and soilless cultures [3, 7]. we have demonstrated that the utilization of a welldesigned recycling line for mineral wool processing could be a promising technique that would enable recycling mineral wool for a variety of purposes. the line was designed as both stationary and mobile. the utilization of a movable machine reduces the amount of waste sent to landfill, thus reducing the carbon footprint of the process. both assemblies have the 9 m.domonkos, o. zobal, z.prošek, j.trejbal acta polytechnica ctu proceedings 0.030 40 0.035 0.040 0.045 0.050 0.055 0.060 50 60 70 80 90 100 110 120 130 set a set b set c th er m al c on du ct iv ity c oe ff ic ie nt λ [w /(m k )] density ρ [kg/m3] figure 6. graph of the thermal conductivity coefficient as a function of the bulk density of samples. following basic parts: i) crusher or mill, ii) separating cyclone with bag, iiii) double filter. these parts can be combined and optimized, thus the process parameters can be adjusted on demand (e.g., various construction waste materials can be processed at different speeds, i.e., the size and structure of the output material is adjustable). 4. conclusions in this paper, samples recovered from mineral wool waste (made from basalt fibers, cladding of 30 years old building in kralupy nad vltavou) were prepared and their thermal conductivity was measured using a heat flow meter. the λ-value is crucial indicator for evaluating the performance of insulation materials. we tested the thermal conductivity of various samples (with bulk densities in the range 50–120 kg/m3): i) non-contaminated virgin sample supafil loft 045 (reference), ii) micro-milled and iii) chopped waste material. samples made of recycled insulation had higher thermal conductivity coefficients that reference samples, however they still exhibited acceptable insulating properties, since their thermal conductivity coefficient was in the range of 0.040–0.055 w/(mk). the micro-milled material seems to be promising to use as both recycled loose-fill/blown-in and filling insulation, on the other hand chopped waste material is more appropriate as filling insulation. acknowledgements this work was financially supported by technology agency of the czech republic the project number tj04000208. the authors also thanks to ing. vladimír vymětalík, ph.d., visco, s.r.o, ing. milan pokrivčák, mba, knauf insulation, s.r.o., and ing. george karra’a, ph.d., lavaris, s.r.o. for their technical and material support within the research. special thanks for joint laboratory of polymer nanofiber technologies of institute of physics of the cas and faculty of civil engineering, ctu in prague. references [1] l. d. hung anh, z. pásztory. an overview of factors influencing thermal conductivity of building insulation materials. journal of building engineering 44:102604, 2021. https://doi.org/10.1016/j.jobe.2021.102604. [2] o. väntsi, t. kärki. mineral wool waste in europe: a review of mineral wool waste quantity, quality, and current recycling methods. journal of material cycles and waste management 16(1):62–72, 2014. https://doi.org/10.1007/s10163-013-0170-5. [3] p. sormunen, t. kärki. recycled construction and demolition waste as a possible source of materials for composite manufacturing. journal of building engineering 24:100742, 2019. https://doi.org/10.1016/j.jobe.2019.100742. [4] construction and demolition waste. https://ec.europa.eu/environment/topics/waste-andrecycling/construction-and-demolition-waste_en, 2008. [5] c. piña ramírez, e. atanes sánchez, m. del río merino, et al. feasibility of the use of mineral wool fibres recovered from cdw for the reinforcement of conglomerates by study of their porosity. construction and building materials 191:460–468, 2018. https://doi.org/10.1016/j.conbuildmat.2018.10.026. [6] m. norouzi, m. chàfer, l. f. cabeza, et al. circular economy in the building and construction sector: a scientific evolution analysis. journal of building engineering 44:102704, 2021. https://doi.org/10.1016/j.jobe.2021.102704. [7] s. romaniega piñeiro, m. del río merino, c. pérez garcía. new plaster composite with mineral wool fibres from cdw recycling. advances in materials science and engineering 2015:1–9, 2015. https://doi.org/10.1155/2015/854192. [8] t. sattler, r. pomberger, j. schimek, d. vollprecht. mineral wool waste in austria, associated health aspects and recycling options. detritus (9):174–180, 2020. https://doi.org/10.31025/2611-4135/2020.13904. 10 https://doi.org/10.1016/j.jobe.2021.102604 https://doi.org/10.1007/s10163-013-0170-5 https://doi.org/10.1016/j.jobe.2019.100742 https://doi.org/10.1016/j.conbuildmat.2018.10.026 https://doi.org/10.1016/j.jobe.2021.102704 https://doi.org/10.1155/2015/854192 https://doi.org/10.31025/2611-4135/2020.13904 acta polytechnica ctu proceedings 34:6–10, 2022 1 introduction 1.1 insulation material waste 1.2 mineral wool recycling 2 experimental part 2.1 materials and recycling 2.2 thermal conductivity 3 results and discussions 3.1 recycling line 4 conclusions acknowledgements references https://doi.org/10.14311/app.2022.33.0119 acta polytechnica ctu proceedings 33:119–124, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague effect of superplasticizer on the strength of fly ash based geopolymer concrete rhem leoric cantos dela cruz∗, leah monica alaurin roriguez, emmanuel josh yambing tiongco, khim denize asilo yulas, jason maximino ongpeng de la salle university, gokongwei college of engineering, department of civil engineering, 8/f bro. andrew gonzalez fsc hall, 2401 taft avenue, 1004 manila, philippines ∗ corresponding author: rhem_delacruz@dlsu.edu.ph abstract. in this research, the effect of a polycarboxylate based superplasticizer on the strength of geopolymer concrete (gpc) was investigated. a fixed amount of superplasticizer (1.5 % of fly ash weight) was utilized along with alkali activators sodium hydroxide (naoh) and sodium silicate (na2sio3). the ultrasonic pulse velocity test determined the strength development of concrete. the quality of the gpc improved when a higher concentration of alkali activator was applied based on the concrete quality designation. in detecting the color development of gpc, samples were put through matlab and specimens became lighter as time passes due to dehydration (a process where water escapes from the sample). stress strain diagrams were generated which generally indicate that gpc specimens are ductile. the researchers were able to assess the workability of the mix designs using a rating from 1 to 5, with 1 being the least workable and 5 being the most workable. keywords: alkali activators, color detection, fly ash based geopolymer concrete, matlab, polycarboxylate based superplasticizer, ultrasonic pulse velocity test. 1. introduction throughout the years, the construction industry has developed and changed, paving the way to new improvements and innovation, especially to construction materials such as concrete. concrete consists of cement, fine aggregates, coarse aggregates, water, and additives to aid in the preparation of the mix. approximately 86.3 million metric tons of cement was manufactured in the united states and 4,100 million metric tons were produced worldwide in 2017 [1]. as the world continues to develop, more infrastructures are being made to meet the demands and necessities of people, which results in the increased usage of concrete. unfortunately, concrete usage greatly affects the environment since it is known to contribute around 5% of total world carbon emissions [2]. in the production of concrete, ordinary portland cement (opc) is commonly utilized as the binder. though opc may be a commonly used binder, it also has its downsides. a considerable amount of natural resources is depleted in order to produce ordinary portland cement [3]. the opc production emits large amounts of carbon dioxide. it contributes around 1.35 billion tons yearly or around 7% of the total greenhouse gas (ghg) emissions to the atmosphere of the earth [4]. cement production also requires a considerable amount of energy. in response to this, actions have been done to lessen the carbon emission. in the study regarding the reduction of the concrete industry’s impact on the environment, [5] suggested that less materials and energy should be used, and carbon dioxide emissions should be lessened. in order for the concrete to be more ecological, the opc used must be replaced with other binders. alternative binders such as rice-hull ash (rha) and fly ash (fa) are currently being used and tested to lessen carbon emission while maintaining the needed mechanical properties of a concrete mix as per astm. additionally, [6] presented in their review that it is feasible to reduce up to 80% to 90% of the current co2 emissions globally through the use of geopolymer technology. apart from reducing greenhouse gas emissions from the cement production, it also mitigates the issue of solid waste. geopolymer, an inorganic alumino silicate polymer made from secondary product materials which contain silicon and aluminum, can be utilized as an alternative to cement paste in the production of concrete [4]. geopolymer concrete undergoes the process of geopolymerization, similar to the hydration of cement. geopolymer concrete requires by-products with high aluminum and silicate content to react with an activator. different by-products may be utilized as a material for geopolymer binders, such as fly ash. fly ash, a by-product generated from coal fired power production, can be an alternative for cement. it is considered the fifth largest raw material resource in the world as 500 million tons of it, which is about 75% − 80% of the total ash, are produced annually [7]. fly ash is considered as an environmental pollutant when released into the atmosphere, however 119 https://doi.org/10.14311/app.2022.33.0119 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en r. l. c. dela cruz, l. m. a. roriguez, e. j. y. tiongco et al. acta polytechnica ctu proceedings composition (%) component point 1 point 2 point 3 average calcium (ca) 7.11 7.13 7.07 7.10 silicon (si) 22.37 22.37 22.24 22.33 aluminum (al) 9.34 9.5 9.58 9.48 table 1. xrf result of coal fly ash. there are numerous areas wherein fly ash can be applied, such as cement clinkers, road basement material, waste stabilization or solidification, concrete production, and geopolymer concrete [8]. as an additive to the cementitious material, fly ash decreases heat of hydration, reduces thermal cracking of concrete in early stages, improves mechanical and durability at late ages, and improves workability. a research conducted by [9] presented the different properties of a fly ash based geopolymer concrete and concluded that fly ash based geopolymer concrete has a compressive strength that is of high quality and can be used for construction purposes, and that its elasticity is similar to that of opc concrete, has excellent resistance against sulfate and acid, and undergoes minimal creep and drying shrinkage. despite the numerous research involving geopolymer concrete in the present time, studies on the utilization of admixtures in improving the strength of geopolymer concrete is still fairly limited. there is a need to further investigate the effect of the utilization of superplasticizer in gpc, as well as the impact of the curing methodology to be used in order to investigate it as the binder for concrete in construction. the objective of this study is to determine the optimum strength and workability of geopolymer concrete (gpc) with superplasticizer through the use of both mechanical and non-destructive testing. nondestructive testing in concrete has been advancing using nonlinear ultrasonic test [10, 11] and acoustic emission test [12, 13]. in this paper, ultrasonic pulse velocity test was used since it is practical and can estimate concrete strength [14]. 2. materials the study used an alumino-silicate coal fly ash obtained from a coal-fired power plant located in batangas, philippines. a hand-held x-ray fluorescence (xrf) spectrometer was used in order to acquire the elemental composition of the material, as per astm c188 shown in table 1. astm standards state that fly ash can be classified into three types: class f, c and n. commonly, class c and class f fly ash, which vary in terms of their calcium content, are used for the concrete binder. class c fly ash is high in calcium, while class f fly ash comprises less than 10% of calcium content [15]. observing table 1, it can be seen that the fly ash in this study contains low calcium and high silicon content. thus, the fly ash used in this study is a class f. a low calcium fly ash is favorable in making geopolymer concrete to obtain optimal binding properties [16]. sodium hydroxide also known as caustic soda flake; this alkali activator has a 98% minimum naoh purity. in this research, a molarity of 12m was used. in order to obtain such molarity, 480 grams of naoh pellets were gradually added to every 1 liter of distilled water. another alkali activator is the sodium silicate which is a colorless liquid. the said activator is known to be the most preferred activator for fly ash based geopolymer concrete [17]. the superplasticizer utilized in this research was a water-reducing admixture. its chemical base is a polycarboxylate ether with a white to yellowish color in appearance. the recommended consumption of the said admixture ranges from 0.8% 2.0% of the weight of the cementitious material. 15.51 20.62 12.99 19.25 0.00 5.00 10.00 15.00 20.00 25.00 1:1:1 1:1:2av er ag e c om pr es siv e s tre ng th , m pa design mixture without sp with sp figure 1. compressive strength of gpc without sp and gpc with sp. 3. methodology the study compared the gpc and gpc with superplasticizer with the aid of destructive and nondestructive tests. the destructive tests are astm c39 compressive strength test and strain gauge test. the nondestructive tests are ultra pulse velocity test (upv) and color detection test which were performed before and after the destructive testing. the following procedures were done upon mixing the raw materials: preparation of 12m naoh solution add 480 g of naoh pellets to every 1 liter of water; mix the dry materials such as fly ash, sand, and gravel; mix the wet materials such as sodium hydroxide (naoh) and sodium silicate (na2sio3); 120 vol. 33/2022 fly ash based geopolymer concrete materials (grams) ratio fly ash sand gravel na2sio3 naoh water-biner *sp (1.5% of fa) 1:1:1 1 1 1 982.953 887.953 880.814 351.055 140.422 245.738 14.7 1:1:2 1 1 2 737.215 665.964 1321.221 263.291 105.316 184.303 11 note: *sp is only for one set of specimens (those with superplasticizer) table 2. design mix of geopolymer concrete. design mix ratio rating description/remarks 1 : 1 w/o sp 3 difficulty in mixing was observed. the mixture can be mixed while exerting effort. 1 : 1 w/sp 5 the lightest and easiest to mix. the mixture easily spreads out. 1 : 2 w/o sp 2 notable lack of flow and relative dryness were observed. additional water was put so that it could be consolidated. 1 : 2 w/ sp 4 the mixture spreads out slowly with ease. the mix was dry before the superplasticizer was added. table 3. assessment of the workability of the geopolymer samples. figure 2. strength development of (a) geopolymer concrete without superplasticizer (b) geopolymer concrete with superplasticizer. figure 3. the concrete quality designation (cqd) derived from the upv. mixing of the dry and wet materials; add the water and add the superplasticizer. seen in table 2 is the design mix. 4. results and discussion 4.1. effect of superplasticizer on the workability workability, described as the ease and homogeneity of concrete during mixing, was one of the properties of concrete investigated in this study. the said fresh concrete property is reliant on several factors including water-binder ratio having its relationship as directly proportional. however, the strength of concrete is negatively affected as the water in the mixture increases. in gpc, alkali activators, superplasticizer, and water present in the mixture are the factors that affect the workability [18]. a high concentration of naoh and na2sio3 reduces the workable flow of geopolymer [19]. superplasticizer was used in this research to improve the workability of gpc. as expected, the mixture with superplasticizer resulted to be more workable than the mixture without super121 r. l. c. dela cruz, l. m. a. roriguez, e. j. y. tiongco et al. acta polytechnica ctu proceedings 0 5 10 15 20 25 30 0 1000 2000 3000 4000 5000 str es s, mp a strain, μԑ 1:1:2 # 2 1:1:2 # 3 1:1:1 # 2 1:1:2 s # 1 figure 4. stress strain diagram of geopolymer concrete. figure 5. average color detection of each design mix. plasticizer. additionally, the researchers were able to assess the workability of various mix designs since all the specimens were hand mixed. shown in table 3 below is the assessment of the workability of the geopolymer in this study. the assessment makes use of a rating from 1 to 5, with 1 being the least workable and 5 being the most workable. 4.2. effect of superplasticizer on the compressive strength shown in figure 1 below is the comparison of the average compressive strength of geopolymer concrete without superplasticizer and that of the geopolymer containing superplasticizer. the figure 1 shows that the superplasticizer did not positively impact the compressive strength of the specimens. percentage differences of 17.68% and 6.87% were calculated for the design mixtures 1 : 1 : 1 and 1 : 1 : 2, respectively. the superplasticizer content of the mixtures was seen as the vital factor that affected its effect on the geopolymer concrete’s strength. increasing the content of polycarboxylate-based superplasticizer to 2% could positively impact the strength [20]. 4.3. strength development ultrasonic pulse velocity test the strength development of geopolymer was monitored in this research through ultrasonic pulse velocity (upv) test which was done every seven days until the 28th day. the velocity obtained from the upv machine was converted into its equivalent strength in megapascal (mpa) using raouf’s equation [21]. observing figure 2, it can be seen that there is a constant decrease in strength development for most of the samples. possible factors such as varying amounts of water and the inaccuracy of the manual compaction may have influenced the outcome of the results. based on the cqd by [22] shown in fig122 vol. 33/2022 fly ash based geopolymer concrete ure 3, the quality of the geopolymer concrete cylinders without superplasticizer and geopolymer concrete cylinders with superplasticizer were good and poor to questionable, respectively. 4.4. compressive strength test compressive strength test was done on the 28th day of the concrete cylinder samples. strain gauges were also attached to the sample upon the application of the load to generate stress-strain diagrams. table 4 shows the compressive strength of the geopolymer concrete samples. it can be observed that 1 : 1 : 2 without superplasticizer had the optimum compressive strength equal to 24.752 mpa. design mixture (fa:s:g) 28th day strength (mpa) 1 2 3 1 : 1 : 1 18.538 14.897 13.089 1 : 1 : 2 18.373 24.752 18.729 1 : 1 : 1 w/ sp 15.954 12.032 10.988 1 : 1 : 2 w/ sp 17.711 19.646 20.384 table 4. assessment of the workability of the geopolymer samples. figure 4 displays the stress strain diagram of the geopolymer concrete. it can be observed that 1 : 1 : 2 # 2 has the optimum compressive among all the specimens. the diagram also implies that the stressstrain curve of gpc is similar to a typical stressstrain curve of an ordinary portland cement concrete. additionally, the curve dictates the geopolymer is a ductile material. 4.5. image processing: color detection images were taken every 7th, 14th, 21st, and 28th day, which were then manually cropped to maintain a fixed perspective of the surface of the gpc. images were then uploaded to matlab so that the software can automatically convert them to grayscale, giving values between 0 (black) and 255 (white). the main factor in the change in color is due to gradual dehydration of geopolymer and oxidation of iron present in the fly ash [23]. each sample in figure 5 follows a pattern from a darker shade to a lighter shade, indicating gradual dehydration. 5. conclusion in determining the optimum strength of fly ash-based geopolymer concrete, the compressive strength test was used. the optimum mix design ratio would result in a compressive strength greater than 3000 psi (21 mpa). for gpc specimens, a peak strength of 24.752 mpa was obtained in the mix design ratio of 1 : 1 : 2. moreover, based on the previously shown stress-strain diagram, it can be stated that the geopolymer concrete followed a curved orientation until its failure point. such orientation denotes that the material tested is ductile. higher alkali activator concentration and proper method of mixing greatly affect the performance of geopolymer concrete. with the use of superplasticizer, the specimen had a glossier appearance when compared to a specimen without it. it was found that the superplasticizer did not increase the strength of fly ash-based geopolymer concrete. however, it improved the workability of the concrete specimens. further investigation is recommended on the application of different types of superplasticizers and other materials in improving workability of geopolymer concrete. references [1] t. wang. cement production globally and in the u.s. from 2010 to 2018 (in million metric tons). https://www.statista.com/statistics/219343/ceme nt-production-worldwide/. 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[23] m. a. malik, m. sarkar, s. xu, et al. effect of pva/sio2 nps additive on the structural, durability, and fire resistance properties of geopolymers. applied sciences 9(9), 2019. https://doi.org/10.3390/app9091953. 124 https://doi.org/10.3390/app8030405 https://www.sid.ir/fileserver/je/103820160406.pdf https://doi.org/10.14359/51689238 https://doi.org/10.11113/aej.v7.15490 https://doi.org/10.1088/1757-899x/288/1/012019 https://doi.org/10.1109/natpc.2011.6136362 https://doi.org/10.1016/j.conbuildmat.2018.07.111 https://doi.org/10.1016/j.cemconcomp.2006.11.002 https://doi.org/10.1016/j.conbuildmat.2013.09.048 https://doi.org/10.1016/j.cemconcomp.2006.11.002 https://doi.org/10.1016/j.conbuildmat.2016.08.158 https://doi.org/10.3390/app9091953 https://doi.org/10.14311/app.2022.33.0146 acta polytechnica ctu proceedings 33:146–152, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague advanced hybrid nonmetallic composite reinforcement for concrete structures vyacheslav ruvimovich falikmana,∗, dmitry anatolievich ilyinb, valentina fedorovna stepanovaa a scientific research center "construction"/scientific research institute for concrete and reinforced concrete, department on structural concrete durability, 2nd institutskaya st., 6, 109428, moscow, russia b moscow state university of civil engineering (national research university)/ institute of construction and architecture, department of building materials, yaroslavskoye shosse, 26, 129337, moscow, russia ∗ corresponding author: vfalikman@yandex.ru abstract. during the last decades, fiber reinforced polymer (frp) reinforcing bars for concrete structure has been extensively investigated and a number of frp bars became commercially available. however, major shortcomings of the existing frp bars are its low elastic modulus and high initial cost compared to conventional steel bars. the possibility to obtain a hybrid composite reinforcement (hcr) with increased performance based on glass and carbon fibers (gcfrp) is considered. the optimal content of carbon fibers in the amount of 6.3 − 6.5 % of the mass of the hcr was established. further increase in the carbon fiber content gives a slight improvement in physical and technical characteristics, which is not comparable to the increase in the cost of the material. the manufacturing technology of hcr has been developed. the effect of hybridization on tensile properties of frp bars were obtained by comparing the results of tensile test with those of non-hybrid gfrp bars. operation regularities of hcr in the bent concrete beams are established. hcr can increase the stiffness of concrete beams by 15 % and crack resistance by 12 % in comparison with glass composite reinforcement. dependences for predicting the hcr elasticity modulus are established. physical and technical characteristics of hcr, including adhesion to concrete and resistance to the alkaline medium, were established. high durability of hcr for more than 50 years is experimentally shown. experimental-industrial concrete piles, reinforced with gcfrp bars were produced and tested. for further development, new types of hcr, as well as a study of prestressed concrete structures are recommended. keywords: durability, elasticity modulus, fiber reinforced polymer (frp), glass and carbon fibers, hybrid composite reinforcement (hcr). 1. introduction the russian industry for the production of composite polymer reinforcement (frpr) is rapidly developing. non-metallic reinforcements are becoming valuable alternatives for the classical concrete reinforcements [1]. this evolution started in the late eighties of last century. having high tensile strength, low density, dielectric properties and high chemical resistance, frpr is of great interest to builders and researchers. composite materials of different type were introduced to overcome the inherent disadvantages of concrete: strength, durability, corrosion of steel rods, creep and shrinkage, flexibility and retrofitting aspects. glass, aramid and carbon are the most popular fiber reinforcements, embedded in a pure polymer or in a polymer modified matrix. material properties, reinforcement design, analysis and layout became important research topics, but practical applications came immediately after research, and sometimes even preceded research. along with the positive properties, frpr has some disadvantages. among them, it’s necessary to mention a low modulus of elasticity and low heat resistance, increased creep under prolonged static load. all this hinders the widespread application of composite rebar in the construction practice. some of the above disadvantages could be eliminated when modifying the epoxy binder with nanoadditives. in the last decade, nanoparticles of various natures have been tested for modification of polymeric materials, wiz carbon nanotubes, fullerenes, astralenes, ultrafine diamond powders, nanodiamonds, montmorillonite, aerosil, schungite, metal-containing nanocomposites, etc. [2–4]. the positive effect of the modification by copper and nickel nanoparticles was confirmed by one of the russian frpr manufacturers. they achieved stable increased physical and mechanical properties of the reinforcement and its high corrosion resistance [5]. however, the modification of the polymer binder is not enough to significantly increase the elastic modulus of the frpr. this is due to the initial low modulus of elasticity of the glass fiber (70-80 gpa only). in turn, carbon fiber (cf) has a high modulus of elasticity (220 gpa), but the cfrp reinforcement is of 25-30 times more expensive. the optimal solution may be a hybrid com146 https://doi.org/10.14311/app.2022.33.0146 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 33/2022 hybrid nonmetallic composite reinforcement � �� �� figure 1. experimental samples of frpr; cf location. seq no. component kind quantity [%] 1 fibreglass jsc "fiberglass" 71.3 2 carbon fiber aksa a−49 3.7 3 multicomponent epoxy binder solidifier−imthfa1 25 promoter−tdmamph2 25 1 isomethyl tetrahydrophthalic anhydride 2 2, 4, 6−tris−(dimethylaminomethyl)−phenol table 1. composition of hcr experimental samples designation nominal cross density tensile ultimate elasticity unit bond diameter area strength modulus extension resistance dn ρ σb ef εb [mm] [mm2] [kg/m3] [mpa] [mpa] [%] [mpa] gfrp 11.89 111.03 2025 935 51500 1.72 11.92 acc−c 11.98 112.72 2009 1115 59500 1.87 13.90 acc−k 11.89 111.03 2046 1110 62000 1.60 12.76 table 2. test results posite reinforcement (hcr) based on carbon and glass fibers, impregnated with a nanomodified epoxy binder. the use of two fiber types allows increasing of the elasticity modulus. nanomodification of the epoxy binder provides high-quality joint work of the "fiber + binder" system. the result of this processing is a dense structure of the reinforcement, an inseparable relief from the bar, and a high proportion of reinforcing fiber. 2. experimental and discussion 2.1. making of hcr and its basic material properties previously, the hybrid effect was studied on the example of composite reinforcement made of glass and carbon fibers with their different locations. the authors noted that the final strain under tension of bars consisting of 37 % glass fibers and 23 % carbon fibers increased by 3 − 33 % compared to the carbon frpr [6]. in the course of experimental work, a hybrid composite rebar (hcr) was manufactured and tested, in which 3.73−6.3 % of the glass fibers were replaced by cf with a modulus of elasticity of 220 gpa. at the same time, experimental and conventional samples of gfrp bars with a diameter of 12 mm were studied. samples of hybrid composite (hcr) and glass composite rebar (gfrp bar) were produced using industrial line. in experimental samples, 3.7 % of the glass fibers were replaced with cf. the theoretical value of the elasticity modulus of hybrid reinforcement calculated in accordance with voigt’s formula is 63.9 gpa. at the same time, two schemes were adopted for the location of the cf (figure 1): 1−cf was concentrated in the center of the composite; the designation of this type of reinforcement is acc−c; 2−cf were evenly distributed along the profile outline; the designation in this case is acc−k. table 1 shows the ratio of hybrid composite reinforcement components. as the primary roving, optical fiber of jsc "fiberglass" located in gus khrustalny was selected. carbon fiber used was aksa a-49 brand. manufacture of frpr was carried out according to the needletrusion technology [7]. the main performance characteristics of experimental samples according to gost 31938-2012 are given in table 2. the rupture fracture pattern of the frpr sample is shown in figure 2a. the dependence of the strain on the stress (load) during the tensile test is shown in figure 2b; it shows repeated peak stresses, which 147 v. r. falikman, d. a. ilyin, v. f. stepanova acta polytechnica ctu proceedings are explained by the hybrid effect. the dependence of deformations on stress (load) during tests for axial expansion of acc−k−10 samples with a carbon fiber content of 6.3 % is also characterized by repeated peak stresses, which indicates the pseudo-plastic behavior of combined composite reinforcement samples at break. the maximum strain of 1311 mpa is reached at 1.93 % deformation, after which the strain drops to 1200 mpa with a further rise to 1250 mpa. then there is a breakage of the test sample acc−k. the deformation reduction is occurred normally in accordance with the "mixtures law". the substitution of 6.3 % of glass fibers with carbon fibers resulted in an strength increase of the reinforcement by 17.4 % and the modulus of elasticity by 31.5 % compared to the samples of gfpr bars. the quality of frpr depends, first of all, on its components used in the production, viz fiber, multicomponent polymer binder, and their ratio in the matrix of the composite material. in addition, a significant role is played by production technology of frpr, namely the tension force of the fiber, temperatures of polymerization of the binder, and cooling the finished product etc. selecting of the optimal frpr composition and the observance of technological processes allows obtaining a product with the specified parameters. 3. corrosion (deterioration) resistance of hcr to alkaline environment when achieving high physical and mechanical properties of frpr, the determining factor becomes its corrosion (deterioration) resistance to aggressive media. in russian [8–12] and foreign [13–15] studies, the aggressive medium for composite polymer materials is usually taken as an alkaline medium, which is being characteristic for wet concrete. the aggressiveness of this medium is estimated by the ph value in the range of 12-14. to simulate the liquid environment of concrete, wet concrete may used directly [11]; aqueous naoh solutions of various concentrations [10], saturated water solutions ca(oh)2 with ph=13 [15] or ph=12 [14] may be used also. in order to accelerate the aggressive action of the alkaline medium on the material under study, the medium is heated to 50◦c [14], 55◦c [11], 70◦c, 80◦c [13], 100◦c [8]. to date, however, there are no generally recognized theoretical dependencies that allow predicting the durability of the composite material on the basis of accelerated tests. thus, according to the authors [13], 50 years of operation may be imitated by holding of the bar for 28 days in a saturated ca(oh)2 solution with ph=13 at a temperature of 80◦c. in [14], samples of frpr were kept in a container with an alkaline medium at a temperature of 50◦c for 52 weeks. the authors believe that 1 day under such conditions corresponds to 101 days under natural operating conditions. test results show that the service life of the frpr in concrete is about 100 years. in [15], tests were performed on frpr samples placed in a saturated solution of ca(oh)2 with ph=12 at a temperature of 50◦c. calculations made by authors show that 1 day of such exposure is comparable in intensity to more than 100−day alkaline exposure in concrete. thus, the tested gfrp bars are able to serve in concrete for more than 47 years. in this work, the assessment of the hybrid composite reinforcement (hcr) stability to the alkaline environment was carried out according to gost 31938. preliminary tests of the reinforcement for axial tension showed that the samples of hcr with the edge arrangement of carbon fibers, akk-k series (figure 1), more efficient than with the central placement, taking in account of elasticity modulus increasing. when replacing 3.7 % of glass fibers with carbon fibers, the modulus of elasticity was increased to 18 %. in this regard, samples of gfrp bars and acc-k were studied. an alkaline solution with a ph of 12.6−13 was used as an aggressive medium in the study, since this medium is the closest to the composition of the liquid concrete phase. the alkaline solution had the following composition: 8.0 g naoh and 22.4 g koh per 1 liter of distilled water. the samples were immersed in an alkaline solution and kept in a closed container under a constant solution temperature of 60 ± 3 ◦c for 30 days, with only the middle section of the samples exposed to the alkaline environment; during the tests, the appearance of the samples was monitored before and after aging in an alkaline solution (figure 3 a, b). the ph value of the solution was maintained in the range of 12.6−13.0 during the entire experiment. after exposure, tests were performed to determine the axial tensile strength in accordance with gost 32492−2013. the results are presented in table 3. after aging in an alkaline environment, samples of frpr are observed: rupture of the profile-forming thread, fading, and the appearance of «whitishness» (figure 3). for gfrp samples, the rupture resistance decrease was by 5.2 %, for the akk-k samples it was 5.6 %. the reduction of the elastic modulus was obtained by 2.31 % for gfrp samples and 1.26 % for acc−c samples. interstate standard gost 31938-2012 did not specify an elastic modulus reduction of the frpr after exposure to an alkaline environment. so, it seems reasonable to make provision for this limitation during next revision of gost, as it was done in official edition for the strength degradation of the frpr under axial tension. figure 4 shows samples of acc−k tested for rupture after exposure to an alkaline medium. the samples were destroyed in the central part with a characteristic "fluff out" of the fibers. 148 vol. 33/2022 hybrid nonmetallic composite reinforcement ɚ�� e�� figure 2. frpr tests. a) the nature of frpr destruction at the break; b) tensile stress-strain diagram. ultimate tensile stress elasticity modulus, ef seq series reference value after exposure reference value after exposure no. frpr unit average unit average unit average unit average value value value value ∆1 value value value value ∆1 [mpa] [%] [gpa] [%] 1 gfrp 935.5 935.1 861.8 886 5.2 52.7 53.20 50.7 52.0 2.31 2 943.6 844.5 55.1 53.4 3 932.0 887.6 52.6 50.7 4 938.1 879.0 54.7 51.0 5 930.2 991.1 51.3 55.8 6 931.4 853.2 52.8 50.2 7 acc−k 1080.1 1085.1 1040.4 1024 5.6 63.9 62.82 59.7 62.0 1.26 8 1100.2 992.3 62.3 61.3 9 1050.4 1074.2 64.1 58.9 10 1090.6 986.7 61.9 64.6 11 1079.2 998.4 62.9 62.8 12 1110.1 1052.4 61.8 64.9 table 3. frpr strength under axial tension ɚ�� e�� figure 3. appearance of acc−k samples before (a) and after (b) exposure in an alkaline medium 149 v. r. falikman, d. a. ilyin, v. f. stepanova acta polytechnica ctu proceedings figure 4. acc−k samples after the rupture test figure 5. concrete beam reinforcement diagram 3.1. tests of concrete beams reinforced with hcr to study a behavior of the hcr in concrete structures, beams were made with a cross section of 120×220×1200 mm. prototype beams were made of ordinary concrete (strength class b 20; cement grade cem 1 42.5). as course aggregate, granite crushed stone was used (fraction of 5−25 mm); fine aggregate was quartz sand. tests of control samples (concrete cubes) were carried out in accordance with gost 10180. the beams were tested for short-term action of bending moments in order to determine the maximum breaking moment, as well as to establish the nature of the collapse, the development of cracks, patterns of development of deflections, and deformations of concrete depending on the type of frpr. the reference structures were reinforced with gfrp bars with a diameter of 10 mm (series b−1); experimental ones were reinforced with hcr having a diameter of 10 mm and a carbon fiber content of 6.3 % (series b−2−acc). the reinforcement scheme for testing of both beams series was adopted the same according to figure 5. bent elements made of smooth steel reinforcement of class a240 with a diameter of 6 mm were used as clamps. during the tests, the span between the supports was chosen of 106 cm; the distance from the supports to the point of the load application was 35.5 cm, and the distance between the clamps was accepted of 70 mm. deflections of concrete beams were measured with a digital deflection meter. according to the readings of the test machine sensors, the movements of the traverse were determined. data were recorded after the application of the load at each stage. concrete deformations were recorded using a multichannel measuring system by tokyo sokki kenkyujo co., ltd., japan (tds−530 model). as measuring instruments, conductive strain gauges with a polyester-based pl-60-11 type substrate with a measuring base length of 60 mm were used. sensors were applied by a special technique to a pre-prepared concrete surface. measurements of concrete deformation and deflection of beams were carried out until the load-bearing capacity of the beams was exhausted. the moment of crack formation and the kinetics of their development were controlled using a special template. the widths of normal cracks were fixed at the level of the stretched zone of the reinforcement, and inclined cracks in the middle of the beam section. the length of the cracks was measured after testing by the marks applied to the beams. the testing process was accompanied by photography. after the destruction of the beams, the destruction zone was fixed; the height of the cracks, the distance 150 vol. 33/2022 hybrid nonmetallic composite reinforcement between the cracks, and the thickness of the concrete protective layer were measured. the actual bearing capacity of the beams reinforced with gfrp bars and hcr was the same amounted to 89 kn, which is of 35 % higher than calculated for the ultimate limit state (uls). for both series of beams, the destruction occurred along inclined sections with a rupture of the frpr in the stretched zone. for beams reinforced with gfrp bars, the first cracks formation occurred under a load of 15 kn, which is of 12 % less than the estimated moment of crack formation. in turn, in the beams with hcr, the first cracks were formed at a load of 17 kn, corresponding to the calculated load. the normative crack opening width equal to 0.7 mm was formed in beams reinforced with hcr under a load of 37 % higher than in beams reinforced with gfrp bars. the longitudinal deformation of compression and extension at height of normal beams section reinforced with a gfrp bars and hcr are linearly with the maximum compression strain at the upper edge and the maximum deformations at the lower edge. at the same time, the deformation of the lower stretched zone of the beams with gfrp bars at maximum load is of 70 % higher than the deformation of the beams with hcr. the maximum deflections of beams reinforced with a hcr are of 15 % lower than the deflections of beams reinforced with gfrp reinforcement. tests of gfrp reinforcement for fire resistance according to russian standards have shown that it is highly inflammable, but non-burning. when testing concrete structures reinforced with gfrp bars, even a short-term combustion occurrence does not occur. the structures are fireproof. 4. conclusion the paper deals with important material properties of frps, on which application and durability as reinforcement based. thus, a hybrid composite reinforcement based on glass and carbon fibers was produced, which is resistant to the alkaline concrete environment. when replacing 3.7 % of glass fibers with carbon fibers, the elastic modulus of the frpr is increased to 18 %, and the elastic modulus remains high (62 gpa) even after exposure to an alkaline environment. analysis of the deterioration nature of samples and test results showed that the edge location of the cf is the most effective. a large convergence of the theoretical (calculated) modulus of elasticity and the test results was achieved; the discrepancy limit does not exceed of 3 %. theoretical calculations and tests have shown that the elastic modulus of the frpr increases with an increase in the content of cf and with a decrease in the amount of polymer binder. at the same time, the feasibility of nanomodification of an epoxy binder with varying amounts of cf was proved. the decrease in the strength of the reinforcement in the axial tension of hcr after exposure to an alkaline environment is not key question, since its strength is not fully used in structure performance, and the value of reduction is within the requirements of gost 31938. the analysis of research allows estimating the service life of hcr in concrete for about 50 years, which corresponds to known data. the regularities of operation of hcr based on glass and carbon fibers in bent concrete beams are established. it is shown that the hcr can increase the rigidity of concrete beams by 15 % and crack resistance by 12 % in comparison with gfrp reinforcement, and the elastic modulus remains high (62 gpa) even after exposure to an alkaline environment. references [1] v. falikman, v. stepanova , a. bouchkin. russian experience in non-metallic composite reinforcement production and use. the fourth international fib congress 2014 (mumbai, india). improving performance of concrete structures. proceedings. university press. vol. ii, p. 784-786, 2014. [2] v. g. khozin. i. a. starovoitova, n. v. maisuradze, et al. nanomodification of polymer binders for structural composites. building materials 2:4-11, 2013. [3] a. d. pomogailo, a. s. rozenberg, i. e. upland. metals nanoparticles in polymers. khimiya, p. 672, 2000. [4] a. l. treniova. preparation of composite materials based on an epoxy oligomer and nanofillers, p. 12-39, 2008. [5] komar. composite reinforcement and masonry mesh manufactured. http://www.komarmatura.ru/. [6] y.-j. you, y.-h. park, h.-y. kim, et al. hybrid effect on tensile properties of frp rods with various material compositions. composite structures 80(1):117-22, 2007. https: //doi.org/10.1016/j.compstruct.2006.04.065. [7] russian patent no. 2287431. method of composite reinforcement manufacturing, 2006. [8] n. p. frolov. production technology of fiberglass reinforcement and some of its properties. concrete and reinforced concrete 9:5-8, 1965. [9] l. g. kuznetsova, y. s. cherkinsky, n. p. frolov, et al. improving of fiberglass reinforcement resistance. concrete and reinforced concrete, 3:30-31, 1973. [10] n. a. moshansky. on the fiberglass reinforcement resistance in concrete. concrete and reinforced concrete, 9:33-34, 1965. [11] n. k. rosenthal, g. v. chekhniy, a. r. bel’nik, et al. corrosion resistance of polymer composites in the alkaline environment of concrete. concrete and reinforced concrete, 3:20-23, 2002. [12] a. a. dalinkevich, k. z. gumargalieva, a. v. sukhanov, et al. aging kinetics of basalt and some glass fibers in an alkaline environment. plastics 12:23-26, 2012. 151 http://www.komarmatura.ru/ https://doi.org/10.1016/j.compstruct.2006.04.065 v. r. falikman, d. a. ilyin, v. f. stepanova acta polytechnica ctu proceedings [13] hughes brothers. glass fiber reinforced polymer rebar. hughes brothers, inc. 15, 1997. [14] w. ramm. report concerning the tests regard to the alkaline consistency of an anchoring of plastic reinforced with glass fiber concerning three-layered facade panels according to the deha-tm system, p. 32, 1992. [15] w. ramm. report concerning tests regarding the alkaline durability of an anchoring system out of reinforced glass fiber plastic for three-layered facade panels according to the deha-tm system, p. 43, 1993. 152 acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.38.0672 acta polytechnica ctu proceedings 38:672–677, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague ecotoxicological assessment of recycled aggregate and concrete via aquatic biotests klára anna mocováa,∗, hedvika roztočilováa, diana mariakováb a university of chemistry and technology prague, faculty of environmental technology, department of environmental chemistry, technická 5, 166 28 prague 6, czech republic b czech technical university in prague, university centre for energy efficient buildings, třinecká 1024, 273 43 buštěhrad, czech republic ∗ corresponding author: klara.mocova@vscht.cz abstract. with the growing consumption of primary raw materials, the need for recycling of construction and demolition waste (cdw) arises. according to international and national regulations, any waste must be tested for ecotoxicity using a leaching test followed by chemical analysis and a set of aquatic toxicity biotests. standardized leaching procedures have also been developed for construction materials and products and are used in common practice. on the other hand, studies aimed at direct determination of ecotoxicity are still lacking. acute toxicity tests with unicellular algae, freshwater crustaceans, and marine bacteria are among the most popular for determining the ecotoxicological potential of recycled aggregates or concrete samples. the article deals with the perspective of semichronic and chronic tests with extended exposure, as well as testing of leachates obtained from leaching events for more than 24 hours. results of performed experiments were compared and evaluated. from the point of view of sustainability, it is necessary to develop an optimal experimental design for the ecotoxicological evaluation of recycled aggregate and concrete. the conclusion of the paper is the evaluation of possible methods and their combinations. keywords: recycled aggregate, recycled concrete, construction and demolition waste (cdw), leachate, ecotoxicity. 1. recycling of cdw – a step toward sustainability the construction sector is one of the largest consumers of mineral resources, which is also related to the subsequent production of waste. the production of the most widely used products in this sector, such as bricks and concrete, is currently dependent on the constant extraction of primary materials from nonrenewable resources [1]. reuse of recycled materials in the construction industry has considerable potential for saving primary resources [2–4]. at the same time, recycling is also beneficial due to the reduction of construction and demolition waste and its disposal in landfills. the life cycle of construction materials is shown in figure 1. in addition to the importance of the mechanical and chemical characteristics of the materials, mainly recycled aggregates, it is appropriate to assess the degree of impact on the environment. recently, many studies have relied on life cycle assessment (lca). during the recycling process, the cdw undergoes several steps of conversion to secondary raw material. efforts have been made to facilitate this process for several years, for example, by replacing classic demolitions with so-called controlled demolitions [5, 6]. during this process, the object (structure) is disassembled and sorted according to individual materials, to facilitate preparation for the next process, reuse, and recycle. this attitude is quite common, e.g. in belgium. however, due to different international and/or national regulations, different criteria may apply to waste and different to secondary raw materials (recycled aggregates). the problem is generally the testing of the physical and mechanical properties of these materials, because although they are mainly defined as waste, in the case of recycled aggregates or recycled concrete, tests are necessary according to the standards for aggregates and concrete mixtures [7]. in terms of further use, it is possible to apply modified cdw in construction. however, when dealing with cdw, the legislation is highly complicated. a problematic question arises in case that the cdw is classified as hazardous. 2. leaching tests generally, there are two ways to characterize construction materials or wastes from a chemical and/or environmental point of view. analysis of the samples in solid form using x-ray based methods brings insight into chemical composition and reveals potentially hazardous substances. however, information on chemical concentration does not fully refer to the bioavailability of the compounds and thus the level of (eco)toxicity. another option is to perform leaching tests and subsequent chemical analysis of both inorganic and selected organic compounds in the leachate. the leaching 672 https://doi.org/10.14311/app.2022.38.0672 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 38/2022 ecotoxicological assessment of recycled aggregate and concrete . . . figure 1. life cycle and recycling possibilities of primary raw materials. method experimental design duration leachant renewal reference en 12457-4 one-stage batch; homogenized sample 24 h no [8] en 16637-3 percolation; granular material 24 h no [9] cen/ts 15862 one-stage batch; monolith 24 h no [10] en 16637-2 multiple-stage 64 d in total; (stages 6 h – 28 d) yes [11] table 1. leaching standards used for characterization of construction materials, products, and wastes. methods mainly applied for the characterization of construction products, wastes and recycled aggregates are presented in table 1. one-stage batch leaching of homogenized material under continuous shaking [8], a method determined for wastes can also be chosen for the testing of recycled aggregates [12–15] or crushed concrete products [13, 16, 17]. using a one-stage batch or long-term multistage leaching procedure without the homogenization step [10, 11] represents a more suitable approach for testing construction products, such as concrete samples. in contrast with the en 12457-4 [8] where leachant volume is calculated according to sample weight, in case of both cen/ts 15862 [10] and en 16637-2 [11], the liquid amount addition is dependent on the solid sample surface area. the one-stage batch monolith leaching protocol was applied in a study dealing with slag cement-based concrete [18]. the multistage leaching method provides a more complex overview of the leaching potential of the material because both prolonged time and repeated leaching are taken into consideration. recently, an interlaboratory test was performed using various types of samples with both a percolation test and a dynamic surface leaching test [19]. a different leaching procedure was used in our previous study [14] to compare the leaching of glass waste before and after its use in recycled high performance concrete (hpc). for both raw materials, i.e., crushed glass waste and concrete products in the form of cubes, the same liquid/solid ratio and leaching time were applied to assess the immobilization of hazardous substances in the hpc. 3. ecotoxicological tests of leachates unlike the calculation of the ecotoxicological impact on both freshwater and marine aquatic ecosystems using the lca methodology, a direct ecotoxicological assessment can be conducted with leachates. the principle of ecotoxicological bioassays is to expose a model organism to a set of dilutions of a tested sample (leachate) together with a negative control (culture medium). culture medium also serves as a dilution water. the leachates can be alternatively treated such as ph adjustment in the case of extreme ph values unbearable for the survival of the model organism and/or nutrient addition. selected test organisms are exposed to the sample sets under given conditions (duration, temperature, and light regime). at the end of the exposure, the ecotoxicological endpoint (generally survival, growth, or reproduction of the organisms) is determined. the tested organisms are compared to the control organisms. in case of statistically significant decrease/increase of the endpoint tested compared to 673 k. a. mocová, h. roztočilová, d. mariaková acta polytechnica ctu proceedings figure 2. inhibition/stimulation effect in ecotoxicological biotests. the control, the inhibition/stimulation effect is considered, as shown in figure 2. the inhibition/stimulation effect is usually expressed in percentage and serves for further determination of ecotoxicological indexes and evaluation of ecotoxicity risk. for ecotoxicological assessment, it is common to select a battery of biotests which consists of organisms of all basic trophic levels, i.e. producers, consumers, and decomposers. freshwater and marine organisms can be used. the ecotoxicological evaluation of construction samples is usually based on acute toxicity tests, which are easier to perform and last from several minutes to several days. a list of ecotoxicity tests used in studies for the construction sector is shown in table 2. as table 2 shows, the most popular are acute toxicity tests with unicellular freshwater algae, freshwater crustacean, and marine luminescent bacteria. application of acute exposure is appropriate mainly in samples with high toxic potential, where acute toxic effects such as loss of locomotion, decreased growth, and mortality are observed. with a lower concentration of potentially toxic compounds, biotests with prolonged exposure, i.e. chronic, represent a more suitable choice. however, application of such tests may be problematic not only due to the time demands, but also to the increased sample volume and space. the semichronic toxicity test with aquatic plant duckweed (lemna minor) allows the determination of acute effects (necrosis, chlorosis) and chronic effects (inhibition of reproduction) with relatively short exposure time (7 days) and sample volume 100 ml per replicate [24]. at the same time, more parameters, such as growth and photosynthetic pigment content, can be determined at the morphological and biochemical levels, respectively. in contrast to unicellular algae, macroscopic plants can be exposed to unsterile samples. selected results from l. minor biotests with finerecycled aggregates [12] and concrete mixes containing these aggregates [20] are presented in figure 3. the photos of the duckweed plants grown for 7 days in 80 % leachates show clearly, that natural aggregate and masonry-derived recycled aggregate had no effect on plant growth, while concrete-derived recycled aggregate caused growth inhibition. however, no significant differences were observed when leachates of concrete mixes were tested. another option is to compare the effect of shortand long-term leaching using the same battery of bioassays. in the interlaboratory study, a series of biotests was performed with samples obtained from only 6 and 24 hours of leaching [19]. as the concentration of leached chemicals increases with time, the next step in the investigation may be to test samples obtained after several days of the leaching procedure. it is quite clear that the ecotoxicity of concrete is dependent on the porosity (i.e. surface area) of the solid samples and chemical composition of concrete eluates. up to date there is still limited number of studies on ecotoxicity of concrete eluates and the majority of them deals with crushed concrete. for instance, replacement of portland cement with fly ash and natural aggregates with recycled aggregates increased the ecotoxicity of concrete eluates using particle size below 10 mm [13]. another study on utilization of slag as fine aggregate showed, that with increasing slag amount in concrete, the leachate ph and ecotoxicity increased as well. however, the authors did not consider the level of toxicity as significant for aquatic environment [16] and similarly, only insignificant ecotoxic impact of slag cement-based concrete was determined when monolith leaching protocol was used [18]. when toxic particles are solidified in concrete mixes with small surface area, the leaching potential of the concrete is significantly reduced and thus reduced bioavailability of harmful 674 vol. 38/2022 ecotoxicological assessment of recycled aggregate and concrete . . . model organism (group) test type duration endpoint reference pr od uc er s freshwater unicellular algae •d. subspicatus acute 72 h growth inhibition/ [12, 14, 15, 17, 19, 20] •r. subcapitata1 stimulation [21] marine unicellular algae •p. tricornutum acute 72 h growth inhibition/ stimulation [21] terrestrial higher plants •sinapis alba acute 72 h root growth [12, 15, 16] freshwater higher plants •lemna minor semi-chronic 168 h growth inhibition; chlorophyll [12, 14, 17, 20] c on su m er s freshwater aquatic crustacean •daphnia magna acute 24 h; 48 h immobilization [12–15, 17, 19–23] marine aquatic crustacean •artemia franciscana acute 24 h immobilization [21] freshwater fish eggs •danio rerio acute 24 h; 48 h coagulated eggs; no heartbeat, no tail detachement [19] d ec om po se rs marine bacteria •aliivibrio fischeri2 acute 15 min; 30 min inhibition of bioluminescence [13, 19, 21, 23] •yeast (s. cerevisiae) acute 16 h growth inhibition [13, 23] intestinal bacteria •salmonella typhimurium genotoxicity 2 h umuc gene activation [19] 1 raphidocoelis subcapitata: former pseudokircheniella subcapitata. 2 aliivibrio fischeri: former vibrio fischeri. table 2. aquatic ecotoxicity tests performed with concrete and cdw leachate. figure 3. example of lemna minor semichronic toxicity test performed with 80 % leachates of natural and recycled aggregates fraction 0–4 mm (left) and 80 % leachates of concrete mixes (c25/30) based on the relevant aggregates (right). c-na – reference concrete, c-rac – concrete containing concrete-derived recycled aggregate, c-ram – concrete containing masonry-derived recycled aggregate. 675 k. a. mocová, h. roztočilová, d. mariaková acta polytechnica ctu proceedings substances can be expected [14, 20]. accordingly, for the selection or non-approval of certain types of concrete in aquatic environment, it is necessary to set stricter limits for concrete samples in the form of monoliths than for cdw or other homogenized materials. hence, more studies on monolith samples are needed to compare the leaching potential of homogenized and uncrushed samples. 4. conclusions with the growing concern about the impact of construction products and recycled materials on the environment, interdisciplinary methods are needed. the impact on the environment can also be understood in terms of ecotoxicological potential. as hazardous waste materials cannot be disposed of on the landfill surface due to their high leaching potential, a more suitable variant seems to be their solidification in concrete. the use of cdw in recycled concrete must meet safety requirements. there are several standardized leaching protocols for construction materials and products. the application of aquatic biotests can be combined with the chemical analysis of leachates. however, the number of ecotoxicological studies is still very low. further research is necessary to find effective methods that can be used in common practice. acknowledgements this work was supported by the ministry of education, youth and sports, cr – grant no. sgs21/096/ohk1/2t/11 – influence of chemical properties on the use of waste materials in concrete applications. references [1] j. dubiński. sustainable development of mining mineral resources. journal of sustainable mining 12(1):1–6, 2013. https://doi.org/10.7424/jsm130102 [2] p. o. awoyera, a. adesina, r. gobinath. role of recycling fine materials as filler for improving performance of concrete – a review. australian journal of civil engineering 17(2):85–95, 2019. https://doi.org/10.1080/14488353.2019.1626692 [3] m. t. rahman, a. mohajerani, f. giustozzi. recycling of waste materials for asphalt concrete and bitumen: a review. materials 13(7):1495, 2020. https://doi.org/10.3390/ma13071495 [4] h.-j. ho, a. iizuka, e. shibata. chemical recycling and use of various types of concrete waste: a review. journal of cleaner production 284:124785, 2021. https://doi.org/10.1016/j.jclepro.2020.124785 [5] a. h. menon, g. k. jayaraj. comparative study of demolition methods. international journal of advance scientific research and engineering trends 2(2):26–31, 2017. http://ijasret.com/volumearticles/ fulltextpdf/139_ijasret_comparative_study_of_ demolition_methods.pdf [6] j. de brito, f. agrela (eds.). new trends in eco-efficient and recycled concrete. woodhead publishing, 2018. [7] z. prošek, t. pavlů, m. rydval, et al. current possibilities of increasing the use of construction and demolition waste in construction. waste forum (3):176–186, 2021. [8] standardization, characterization of waste – leaching-compliance test for leaching of granular waste materials and sludges – part 4: one stage batch test at a liquid to solid ratio of 10 i/kg for materials with particle size below 10 mm (without or with size reduction) (no. 12457-4). cen, brussels, 2002. [9] standardization, construction products – assessment of release of dangerous substances – part 3: horizontal up-flow percolation test (no. 16637-3), cen, brussels, 2016. [10] standardization, characterisation of waste – compliance leaching test – one stage batch leaching test for monoliths at fixed liquid to surface area ratio (l/a) for test portions with fixed minimum dimensions (cen/ts 15862). cen, brussels, 2012. 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[20] d. mariaková, k. a. mocová, j. pešta, et al. ecotoxicity of concrete containing fine-recycled aggregate: effect on photosynthetic pigments, soil enzymatic activity and carbonation process. sustainability 14(3):1732, 2022. https://doi.org/10.3390/su14031732 [21] r. barbosa, n. lapa, d. dias, b. mendes. concretes containing biomass ashes: mechanical, chemical, and ecotoxic performances. construction and building materials 48:457–463, 2013. https: //doi.org/10.1016/j.conbuildmat.2013.07.031 [22] j. b. choi, s. m. bae, t. y. shin, et al. evaluation of daphniamagna for the ecotoxicity assessment of alkali leachate from concrete. international journal of industrial entomology 26(1):41–46, 2013. https://doi.org/10.7852/ijie.2013.26.1.041 [23] p. rodrigues, j. d. silvestre, i. flores-colen, et al. methodology for the assessment of the ecotoxicological potential of construction materials. materials 10(6):649, 2017. https://doi.org/10.3390/ma10060649 [24] standardization, water quality – determination of the toxic of water constituents and waste water on duckweed (lemna minor) – duckweed growth inhibition test (no. 20079), iso,geneva, 2005. 677 https://doi.org/10.3390/su14031732 https://doi.org/10.1016/j.conbuildmat.2013.07.031 https://doi.org/10.1016/j.conbuildmat.2013.07.031 https://doi.org/10.7852/ijie.2013.26.1.041 https://doi.org/10.3390/ma10060649 acta polytechnica ctu proceedings 38:672–677, 2022 1 recycling of cdw – a step toward sustainability 2 leaching tests 3 ecotoxicological tests of leachates 4 conclusions acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.38.0228 acta polytechnica ctu proceedings 38:228–234, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague hygric performance of new building components for vertical green gardens pavel kopecký czech technical university in prague, university centre for energy efficient buildings, třinecká 1024, 273 43 buštěhrad, czech republic correspondence: pavel.kopecky@fsv.cvut.cz abstract. the newly developed box-like panel lightweight element for vertical greenery was designed and built at university centre for energy efficient buildings of the czech technical university in 2020. a test site for testing green facade components is operated since spring 2021. test samples differ in shape, arrangement of functional layers, water distribution means and irrigation patterns. the article presents and discusses measured data at selected test samples during growing season in year 2021. physical quantities, such as surface temperature of plants, moisture content and matric potential of growing substrate, water inflow and outflow, and ambient boundary conditions, were recorded. keywords: vertical green gardens, real-scale experiment, hygric performance, watering patterns. 1. introduction the space next to buildings in urban areas is in particular used for pavements and roads. solid nonporous materials like concrete and asphalt are used. the built environment lacks green plants if compared with rural areas. building enclosures can be seen as potential carriers of green plants. whereas technical solutions for green roofs are established on the market the technical solutions for green vertical walls are still under development. the current state-of-the art of vertical greenery systems is summarized in [1]. vertical green gardens attached to building walls offer some benefits. greenery improves the visual quality of built environment, improves soundproofing properties of building enclosures, protects the building enclosure and the internal environment against external climatic loads and may create new ecological niches. massive usage of green roofs and vertical greeneries within urban areas could mitigate the urban heat island effect [2]. some disadvantages are however associated with vertical green gardens as well. first, it is known to only limited extent which plants are suitable for given type of climate, environmental conditions of site and the system for vertical greenery. a lack of long-term measurement data from real-scale installations exists. second, the raw building materials are needed. for instance, some containers to keep growing substrate at place and supporting structures carrying the greenery are needed. moreover, the irrigation system has to be used in order to compensate for unfavourable environmental conditions at vertical walls. the irrigation system and greenery itself need regular inspections. it can cause non-negligible maintenance costs during operation. it is the subject of current research if some of the disadvantages can be minimized by optimized and reliable technical solutions. this paper deals with newly developed planar boxlike elements for vertical green gardens. various test samples were built and equipped with measurement sensors in 2021. this article presents a side-by-side comparison of measured hygric performance of several test samples in real climatic conditions of the site located in semi-continental climatic conditions. 2. materials and methods 2.1. development of a new technical solution for vertical greenery the goal was to design a technical solution for vertical greenery which will be suited for vertical walls of industrial buildings (e.g. warehouses and production spaces). the walls of modern industrial buildings are currently being constructed as lightweight sandwich structures with low capacity to carry extra loads. therefore, it was required that the proposed solution should be lighter than 50 kg/m2 (i.e. less than half of the system for vertical greenery described in [3]). the requirement of low weight implies the limit on the thickness of soil-type growing substrate (< 5 ÷ 6 cm). the soil-type substrate was believed to be applicable for plants under consideration and for semi-continental climatic conditions. since only one worker should be able to carry the weight of the element, surface area of the element is practically limited to 0.5 m2. water availability is essential for long-term success of plants. the technical solution for vertical greenery must ensure reliable inflow and controlled flow of water through growing substrate leading to uniform and sufficient moisture content. the challenge is to control the vertical redistribution of water due to gravity. the goal is to maintain stable environment with good water availability. the development of a system for controlled distribution of irrigation water in the element was carried out iteratively by trial and error approach. several 228 https://doi.org/10.14311/app.2022.38.0228 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 38/2022 hygric performance of new building components . . . (a). 26/11/2020. (b). 14/04/2021. figure 1. the first prototypes placed on the test façade. laboratory models of the water distribution system in the characteristic sample were built (06–08/2020). the laboratory experiments tested two conceptual ideas: (1.) water distribution along the upper edge of the element can be provided by a suitable hydrophilic capillary active material, and (2.) water distribution in growing substrate can be enhanced by a drainage layer located at the rear side of the panel (behind soil substrate). testing of watering concepts finally led to an original solution using a block of hydrophilic mineral wool placed along the top edge of the element. two parallel plastic plates with teeth shaped bottom edge are pushed in the cut-outs made in the mineral wool. teeth release drips of water if hydrophilic mineral wool is oversaturated. water drips into the soil substrate which is enclosed in the honeycomb-like structure of interconnected cells made of recycled plastics (cell size 62.5 × 62.5 mm, thickness 4 cm). the growing substrate is firmly hold in cells by jute fabric and metal lattice placed on the external side. water also drips into drainage layer which is located behind growing substrate. the carpet made of recycled polyester was used (thickness 2 cm). water moves down through cells with growing substrate and through carpet by means of gravity. the carpet enables faster water transport than soil substrate. the overflow of water from carpet to growing substrate is created in the middle of the height of the element. the carpet is there discontinued by means of horizontal yet sloped cut. the plastic plate with teeth is placed in the cut and sealed to the rear side of the element. water flow through the carpet is thus diverted from rear side into growing substrate. the first three test samples (scale 1:2, full height, half width) were built during 09/2020. two test samples were seed by grass and hanged on the test façade. the test samples were fully green since the end of 10/2020. the test samples were irrigated weekly in the cold period. grass on both test samples survived cold season in a relatively good shape, albeit showing weaker coverage of the bottom side of both test samples and reduction of leaf area density than in initial state. the fade was mainly associated with one-week long cold period (daily average ambient temperature < 0 °c) at beginning of 03/2021. grass relatively quickly regrown during warmer weeks of spring 2021, even without new seeds (see figure 1b and figure 4a, b). the experience from the cold season showed that capillary redistribution of water in hydrophilic mineral wool does not work reliably. the uniformity of outflow from teeth was dependent on finding the good compromise between total water inflow, size and geometry of mineral wool, and geometry of teeth. moreover, mineral wool suffered from material degradation and changed its hydraulic properties during the cold season. the distribution of water along the top edge of test samples was therefore redesigned during spring 2021. a thicker plastic pipe with low diameter inlet holes (drilled in angle of 120° and regularly distanced to each other) was placed along the top edge of the sample instead the block of mineral wool. the tests have proven that combination of diameters 13 mm (main pipe) and 1.5 mm (individual outlet holes) leads to uniform outflow of water from holes if water pressure exceeds 2 bars. six real-scale test samples were gradually built in 2021. test samples differed in shape, arrangement of functional layers and water inflow distribution devices. five samples were hanged on the test façade of the research centre uceeb. test sample nr. 4 was built and sent to test site of industrial partner. for differences between samples see table 1. main parts of a test sample are depicted in figure 2. test samples nr. 3–6 were built in plastic euro containers (size 80×60×12 cm). containers for samples nr. 1 and nr. 2 were built from scratch using extruded polystyrene and plywood. 229 pavel kopecký acta polytechnica ctu proceedings figure 2. technical drawing of test sample nr. 3. sample dimensions substrate plants inflow overflow 1 12 × 7 cells (76 × 44 cm) s1, 43 mm grass (standard mixture of seeds) block of hydrophilic mineral wool with plastic teeth carpet with overflow (4th row) 2 6 × 15 cells (38 × 95 cm) s1, 43 mm grass + veronica spicata, rudbeckia hirta block of hydrophilic mineral wool with plastic teeth no carpet 3 12 × 8 cells (76 × 50 cm) s1, 43 mm grass + veronica spicata, rudbeckia hirta plastic pipe dn 16 mm with capillaries 2 mm carpet with overflow (5th row) 5 12 × 8 cells (76 × 50 cm) s1, 63 mm grass + arabis alpina, fragaria vesca plastic pipe dn 13 mm with holes 1.5 mm no carpet 6 12 × 8 cells (76 × 50 cm) s2, 63 mm thymus serpyllum, sedum reflexum plastic pipe dn 13 mm with holes 1.5 mm no carpet s1 – 3 vol. grass substrate / 1 vol. acre substrate. s2 – 3 vol. grass substrate / 1 vol. acre substrate / 1 vol. sand, 1 vol. expanded clay table 1. definition of test samples. 2.2. description of test site the test site for green facade components was built at university centre for energy efficient buildings in buštěhrad (uceeb) during spring 2021. the test site is located on south-east oriented window sill of the experimental curtain wall (at the ground level). the space behind test façade is unheated and ventilated by outdoor air. the following sensors were used to monitor conditions in test samples: (1.) temperature sensors (pt1000, accuracy ± 0.15 k), (2.) fdr volumetric moisture content sensors (analog truebner smt 50 and digital smt 100, accuracy ± 3 % vwc, no laboratory tests were performed for calibration prior measurement campaign), (3.) gypsum blocks for measurement of soil water potential (delmhorst gb2, measuring range −0.1 bar to −15 bar), (4.) non-contact temperature sensors (infrared radiometers, apogee si 421-ss). the boundary conditions were measured on the test facade by ems 33h, temperature and relative humidity sensor shielded against radiation, accuracy ± 0.15 k, ± 2 % rh. global solar radiation was measured by ems11, low cost global radiation sensor. global radiation data recorded at roof of the research centre were also available. outflow from test samples 1 and 2 is measured by rain gauge (pronamic pro). positions of sensors are shown in figure 3. 2.3. description of irrigation system the test site was equipped by an automatic irrigation system. the system was used for watering of test samples nr. 1 and nr. 2. the system consists of magnetic valves, balancing valves, water flow meters, compressor, fittings and plastic pipes for water supply to the test facade. the magnetic valves and compressor are both controlled by programmable logic controller (plc). the compressor enables emptying of the system during cold season in order to prevent damage of external pipes and fittings by water freezing. the logic of the controller can be programmed and tested in matlab/simulink environment [4]. the code is then built and run in plc. two standard control schemes were used at first: (a) timer with specification of time and water amount for watering pulse (used in test sample nr. 2) and (b) feedback control with irrigation controlled according to actual measured moisture content. 230 vol. 38/2022 hygric performance of new building components . . . figure 3. position of sensors in test samples (external view on test samples – since 08/2021). feedback control was used in test sample nr. 1. test sample nr. 2 used time controller scheme. morning and evening watering event were usually used in the very warm days. watering event was constituted by two consecutive pulses. the first watering pulse was used to saturate the upper part of the sample. the second watering pulse transported water from the oversaturated top to the bottom part of the sample. test samples nr. 3–6 were irrigated by standard commercially available garden timers. 2.4. a method used for estimation of real evapotranspiration the steady-state thermal balance of the plant surface was used to estimate the real daily evapotranspiration et from a test sample. the following equation can be derived: et = 1 lv ( αsolggt − hre(tae − tre) − ∆t ( hse + 1 r )) [kg/(m2day)], (1) where hse [w/(m2k)] is total heat transfer coefficient at plant surface, hre [w/(m2k)] is radiative heat transfer coefficient at plant surface, r [m2k/w] is thermal resistance between plant surface and ventilated air gap behind a test sample, αsol [-] is absorptivity of plants for short-wave radiation, ggt [w/m2] is daily mean global solar irradiance on the test facade, tre [°c] is daily mean radiant temperature of surrounding surfaces, lv [j/kg] is latent hat of water evaporation (2.5 × 106 j/kg), and ∆t [k] is the difference between daily mean plant surface temperature tpe and daily mean ambient air temperature tae (∆t = tpe − tae). for practical calculation, it is assumed that mean radiant temperature is approximately equal to apparent sky temperature (tre ≈ tsky). the apparent sky temperature is estimated according to [5]. real evapotranspiration can be alternatively estimated by method [6] as: et = kcet0 [kg/(m2day)], (2) where et0 is reference evapotranspiration, i.e. evapotranspiration from grass surface having fixed crop height, surface resistance and albedo, and kc is real crop coefficient (time variable). 3. results 3.1. visual investigation of test samples the test façade was visited almost every week during warm season 2021. a diary with impressions and comments was written. photographs were regularly taken by a mobile phone and by outdoor camera mounted on the stand in front of the façade. surface patterns recognizable on test samples shows the condition of plants and to some extent moisture content of soil substrate. figure 4 shows selected days in time period 06–09/2021. the visual appearance of test samples in june was promising with nice green colour and no brown coloured spots (see figure 4a). only the half-size test samples built in 2020 shows lower leaf area density at the bottom part but this is believed to be the consequence of spring freezing week. the figure photographed in july already shows some signs of fade at some locations of test samples (see figure 4b). there is a brown spot on the right side of test sample nr. 1 and brown area located at the bottom edge of test sample nr. 3. one month later almost all grass of test sample nr. 1 died (see figure 4c). in september, test sample nr. 1 shows improvement at the bottom and top edge whereas middle part remains brown. brown spots are evident at test samples nr. 2 and nr. 3 in august and september 2021 (see figure 4c and d). 231 pavel kopecký acta polytechnica ctu proceedings (a). 14/06/2021. (b). 19/07/2021. (c). 10/08/2021. (d). 07/09/2021. figure 4. photographs of test façade – selected days in growing season 2021. (a). 04/06/2021. (b). 12/08/2021. figure 5. surface temperature of plants tpe compared with ambient air temperature tae in two selected clear sky days. figure 6. difference of daily mean plant surface temperature and ambient air temperature (6–9/2021). 232 vol. 38/2022 hygric performance of new building components . . . figure 7. evapotranspiration of test samples compared with reference evapotranspiration. visual observation also showed signs that soil substrate contained excessive amount of water, i.e. brightness of the surface, extrusion of water when soil surface was pressed by fingers, muddy fingers after wiping, and frequent water leaks at the bottom edge of test samples. mould growth was observed on the jute fabric of test sample nr. 1. the difference of plant patterns between test samples nr. 3 and nr. 5 deserves comments (see figure 4c and d). seeds germinated more quickly on the upper (and moister) part of sample nr. 5, whereas grass grew better at the bottom (and not so moist) part of test sample nr. 5 (see figure 4, compare c and d). the upper part of test sample is moister since no overflow element was used in test sample nr. 5. test sample nr. 3 seemingly shows the opposite to test sample nr. 5. seeds germinated more quickly on the bottom part whereas grass grew well along the upper edge. in this case, the moister part is located at the bottom. the wetness of the bottom part of test sample was caused by the drainage carpet and overflow element used in test sample nr. 3. 3.2. the surface temperature of plants the measured surface temperature of plants was compared with ambient air temperature in two similar clear sky warm days of growing season (04/07, 12/08). the surface temperature of plants was lower than ambient air temperature by about 4 °c for both test samples in 04/07 (see figure 5a). the surface temperature of test sample nr. 1 exceeded ambient air temperature by 7 °c in 12/08 whereas test sample nr. 3 maintained undercooling of the plant surface (see figure 5b). the evolution of the difference between daily mean plant temperature and daily mean ambient air temperature is depicted in figure 6. 3.3. daily evapotranspiration daily evapotranspiration calculated by equation 1 is compared with reference evapotranspiration et0 in figure 7. 4. discussion the locations with poor plants were documented by photographs (rise of characteristic brown spots visible in photographs). dying of plants occurred during the second half of growing season and should not be attributed with the lack of available water in root area in this case. in fact, the reason for dying of plants was probably opposite – long term excessively high moisture content of growing substrate. the excess water in root area caused the lack of oxygen in growing substrate. when one would like to improve the whole concept, operation of irrigation, technical concept and selection of plants would have to be better balanced for given environmental conditions. the task is difficult due to many design variables coming into consideration, e.g. the geometry of cells, position of protruding holes amongst cells, hydraulic properties, thickness and composition of soil substrate, distance between the upper distribution pipe and the bottom edge of the greenery element, position and presence of the overflow elements, amount of water and watering patterns. 5. conclusions this study dealt with hygric performance of the newly developed building component for vertical greenery. investigations were based on measured data and regular visual observations of real-scale test samples in warm season of year 2021. the initial promising growth of plants was replaced by their gradual decline in the second half of growing season. overwatering is considered to be the main reason for decline of plants. the presented technical solution of the panel element should be therefore further optimized to obtain better environmental conditions for plants. the main design issue is to find trade-off setting between water storage and water transport properties of the element, i.e. to balance the composition of growing substrate, the shape of plastic cells, position of overflow elements and daily watering patterns. last but not least it is evident that better understanding of plants would be helpful. systematic overwatering showed that watering needs of plants should be better predicted. therefore, an algorithm for prediction of daily watering needs was programmed in [4] and embedded in plc. weather forecast apis [7, 8] provide access to data for environmental boundary conditions on site in the upcoming day or days. the first test runs were performed at the end of year 2021. the algorithm will be further tested in year 2022. the test period was limited to growing period of year 2021. the performance of test samples in nongrowing period must be studied carefully in conti233 pavel kopecký acta polytechnica ctu proceedings nental climatic conditions as well. from practical perspective, it is questionable how to protect pipes and fittings against freeze-thaw damage. acknowledgements this work has been supported by project tačr th 03030230. the author is very grateful to radim havlíček for his technical assistance. references [1] g. pérez, j. coma, i. martorell, l. f. cabeza. vertical greenery systems (vgs) for energy saving in buildings: a review. renewable and sustainable energy reviews 39:139–165, 2014. https://doi.org/10.1016/j.rser.2014.07.055 [2] m. santamouris. cooling the cities – a review of reflective and green roof mitigation technologies to fight heat island and improve comfort in urban environments. solar energy 103:682–703, 2014. https://doi.org/10.1016/j.solener.2012.07.003 [3] p. kopecký, j. tywoniak, k. staněk. experimental investigation of green façade components for industrial and storage buildings. iop conference series: earth and environmental science 588(5):052049, 2020. https://doi.org/10.1088/1755-1315/588/5/052049 [4] matlab, 2020. 9.9.0.1718557 (r2020b), natick, massachusetts: the mathworks inc. [5] m. aubinet. longwave sky radiation parametrizations. solar energy 53(2):147–154, 1994. https://doi.org/10.1016/0038-092x(94)90475-8 [6] r. g. allen, l. s. pereira, d. raes, m. smith. crop evapotranspiration – guidelines for computing crop water requirements – fao irrigation and drinage paper 56. [2022-01-20]. https: //www.fao.org/3/x0490e/x0490e00.htm#contents [7] weather forecast api. [2022-01-20]. https://wp2.pvforecast.cz/ [8] weather forecast api. [2022-01-20]. https://openweathermap.org/ 234 https://doi.org/10.1016/j.rser.2014.07.055 https://doi.org/10.1016/j.solener.2012.07.003 https://doi.org/10.1088/1755-1315/588/5/052049 https://doi.org/10.1016/0038-092x(94)90475-8 https://www.fao.org/3/x0490e/x0490e00.htm#contents https://www.fao.org/3/x0490e/x0490e00.htm#contents https://wp2.pvforecast.cz/ https://openweathermap.org/ acta polytechnica ctu proceedings 38:228–234, 2022 1 introduction 2 materials and methods 2.1 development of a new technical solution for vertical greenery 2.2 description of test site 2.3 description of irrigation system 2.4 a method used for estimation of real evapotranspiration 3 results 3.1 visual investigation of test samples 3.2 the surface temperature of plants 3.3 daily evapotranspiration 4 discussion 5 conclusions acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.32.0021 acta polytechnica ctu proceedings 32:21–24, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague the bílina tunnel – selected geotechnical aspects of a proposed underground structure viktor sotorníka,∗, petr černochb, jiří košťálc a inset s.r.o., divize energetika, lucemburská 1170/7, 130 00 praha 3 – vinohrady, czech republic b čez energetické produkty, s.r.o., komenského 534, 253 01 hostivice, czech republic c czech technical university in prague, faculty of civil engineering, depatment of geotechnics, thákurova 2077/7, 166 29 prague, czech republic ∗ corresponding author: sotornik.viktor@inset.com abstract. every workday the city of bílina, with the i/13 road running through it, suffers from traffic congestion. the road is designed as dual carriageway outside bílina but only as a single carriageway inside the city. three roundabouts further complicate the situation. as a solution a 1,3 km long tunnel going under the city was proposed. the preliminary geotechnical investigation describes the geological conditions of the area, suggests the most appropriate localisation details of the tunnel, points out several complications that can be expected and finally offers solutions to mitigate them. keywords: aquifer test, ciss, geotechnical investigation, pressure meter test, tunnel. 1. introduction most and teplice, two major cities in north bohemia are connected by the i/13 road, and on it halfway between them the city of bílina is located. the i/13 road is designed as a dual carriageway with only one exception – the bílina section. the city forms a metaphorical bottleneck that causes regular traffic congestions with queues of cars and trucks several meters long. this situation is further intensified by the existence of three roundabouts located close to each other and by a large number of passing trucks connected to industrial sectors of the city. since 1980’s a solution to mitigate this problem has been researched; several variants of city bypasses [1], a through road in the form of a long overpass or a combination of tunnels and overpasses [2] have all been proposed. however, in the first half of 2018 a so-called variant z that includes three alternatives of horizontal alignment in the form of a 1,3 km long tunnel was made public; the tunnel underpasses the bus station, the commercial area and the mostecké předměstí district. multiple geological units have appeared in the surroundings of bílina. the bedrock comprises several types of the orthogneiss, belonging to the ore mountains massif. units of tertiary age are present in the form of igneous rocks, genetically belonging to the central bohemian highlands. the investigation documented both thick basalt layers and layers of pyroclasts. quaternary sediments can reach a thickness up to 15 m. the naturally occurring fluvial deposits of the bílina river (of both a sandy and clayey type) are mostly covered with anthropogenic deposits that originated during earthworks that were linked to the coal mining industry located in the city and its surroundings. 2. purpose and goal of the investigation the main goal of the investigation was to complete the knowledge of the geological conditions in the alignment of the proposed tunnel. according to desk study no boreholes deeper than 15 m were created; the majority of these boreholes did not even reach the bedrock. a basic hydro-geological evaluation was also made. results of laboratory tests of soils and rocks were used, beside other things, for evaluation of the sustainability of excavated materials for embankment construction. based on these results, the most suitable alternative of the tunnel alignment was suggested, including classification of the natm excavation support classes. at the same time complications that are to be expected during following stages of both the investigation and design work were identified including suggestions to mitigate them. figure 1. site plan of the area of interest including variant z tunnel alignment alternatives. 21 https://doi.org/10.14311/app.2022.32.0021 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en v. sotorník, p. černoch, j. košťál acta polytechnica ctu proceedings 3. geotechnical investigation prior to initial field work an agreement with the client was made that split the investigation into two phases. the first phase, realised in the second half of 2018, was aimed at both portal sections. the second phase (first half of 2019) investigated geotechnical conditions of the mined part of the tunnel. the majority of works that were conducted within the investigation are described in the following subchapters. positioning of all boreholes were determined using a high-precision gnss receiver. nine boreholes were realized (maximum depth up to 45 m) with total length over 200 m; four of them were cased as groundwater monitoring wells. soil, rock and groundwater samples for subsequent laboratory tests were extracted from all boreholes, and aquifer and pressure meter tests were conducted in some selected ones. 3.1. preparatory works prior to a field work launch it is always necessary to secure entrance permission by the landowner on whose property the boreholes will be located. in the next step it is necessary to prevent potential conflict situations, for example to verify the nonexistence of public utility lines. as the boreholes were located within the city limits a high number of underground utility lines was indeed confirmed; because of this 1 m deep manually dug trenches were undertaken in almost all borehole localities. the trenches proved a key factor; thanks to them unexpected utility lines were discovered in places where they were not indicated by their respective companies. as a result, some of the boreholes were relocated up to several tens of meters away from their original positions. a local peculiarity of geotechnical investigation in the city of bílina is the existence of a mineral water source protection zone. boring into pre-tertiary rocks must be approved by the czech inspectorate of spas and springs (or ciss for short). this permission prescribed certain requirements that needed to be followed during the boring process. 3.2. core drilling in the first phase both portal sections were explored [3]. drilling up to the depth of 10 m was carried out without drilling fluid, using a tungsten carbide drill bit with a single tube core barrel. the second phase was aimed at the mined part of the tunnel [4]. the above-mentioned drilling technology was applied to reach the bedrock. after that, boreholes were cased with a water pressure resistant steel casing (the result of the ciss requirements); the boring continued with a diamond encrusted drill bit and a double tube core barrel up to the final depth. figure 2. temporary water pressure resistant steel casing of a borehole during realisation (located in an orange kg-system pvc sewage pipe backfilled in the manually dug trench). 3.3. sampling soil and rock samples, for determination of their physical and mechanical properties by means of laboratory tests, were extracted from all boreholes. in the course of boring groundwater samples for realtime determination of selected parameters, defined by the ciss, were taken. when the boring finished samples for assessment of any groundwater aggressiveness towards concrete and steel were collected. 3.4. field tests in the cased groundwater monitoring wells aquifer tests (short-term pumping and recovery tests) were performed. pressure meter tests were conducted in two boreholes; the tests were performed at three depth levels of each borehole. figure 3. groundwater monitoring well realisation, to the left in the background the historical building of bílina city hall. 4. results of the geotechnical investigation description of the geological environment of the planned tunnel was based on the work mentioned 22 vol. 32/2022 the bílina tunnel – selected geotechnical aspects above. for better clarity, the individual tunnel sections are discussed separately. 4.1. southern (most) portal the southern tunnel portal is located in the vicinity of a football field. the geological environment is formed of superficial anthropogenic deposits and underlying fluvial sediments of the bílina river. the upper clayey part acts as an aquitard that confines the riverbank infiltrated water into the lower sandy layer, thus forming a confined aquifer. in depths under 15 m orthogneiss bedrock is to be expected. 4.2. mined section of the tunnel the geological environment of the mined part is formed of three types of lower-paleozoic orthogneiss; these rocks differ in the amount of individual minerals present. the core drilling proved the existence of fault zones that were indicated in the geological map of the area. boreholes located close to the bílina river also proved a chemical weathering of orthogneiss in the form of kaolinization. classification of individual parts of the tunnel into natm excavation support classes is not trivial. in the central (deepest) tunnel part, where fresh or slightly weathered rock is expected, class no. 3 is presumed. in case of highly weathered or tectonically damaged rocks class no. 4 is necessary. in the portal parts where orthogneiss residual soil is expected class no. 5a is unavoidable. 4.3. northern (teplice) portal the northern tunnel portal is located near the train station. the geological environment is formed of superficial anthropogenic deposits and underlying tertiary igneous rocks. contrary to the results of the desk study very hard basalt layers were recorded; these layers originate from the nearby chlum hill. 5. selected noteworthy geotechnical aspects it is necessary to highlight certain aspects that, if neglected, could constitute complications for subsequent investigation and design. 5.1. southern (most) portal in the southern portal area a confined aquifer located in sandy layers of the bílina river sediments was registered. when the upper clayey layer is penetrated quicksand behavior occurs. during borehole realization wall instability and liquefied sand rising were observed. 5.2. mined section of the tunnel brittle tectonic rock deformation eases groundwater penetration into the upper parts of the orthogneiss bedrock. after a sufficient amount of time passes the water causes progressive chemical decomposition (kaolinization) of the rock, in worst case up to the residual soil state. another dangerous factor for the tunnel excavation is the alternation of more and less weathered rock layers; in selected boreholes this was proved even at depths greater than 20 m. 5.3. northern (teplice) portal laboratory tests proved to the high compressive strength of basalt layers located in the northern portal area. this means complications during construction of the portal, and even rock blasting is not to be excluded. at the same time the extracted rock presents a suitable aggregate source that can be utilised for embankment construction. 5.4. geotechnical and hydrogeological hazards the main hazards that complicate the tunnel construction are connected to water and geological environment characteristics. the whole structure is located on the floodplains of the bílina river and the syčivka brook. the structure is placed in the mineral water source protection zone. the mined section of the tunnel passes through the orthogneiss bedrock where the source of the biliner (or bílina acidulous water) is also located. all construction works have to be permitted by the ciss. in the southern portal area quicksand occurs, therefore application of larssen sheet piling during construction was proposed. in the mined section of the tunnel tectonic deformation of the bedrock was registered, locally associated to uneven chemical weathering of the orthogneiss. excavation in this environment can be confronted with tunnel face instability in the form of loosening and falling of individual rock blocks and also overbreak formation. 6. conclusion the preliminary geotechnical investigation for the bílina tunnel described the geological environment of the area. based on deeper boreholes that reached the bedrock and the field and laboratory tests carried out, the western tunnel alignment alternative was recommended for realisation. results indicate that with increasing distance from the river the quality of the orthogneiss also increases, or rather the degree of weathering of the tectonically deformed bedrock decreases. for subsequent investigation and design, certain measures were proposed that can facilitate these works, e.g. moving of the tunnel alignment further away from the river or relocation of the portal areas. this is however complicated by other nongeological factors (for example, existence of a railroad and district heating pipes in the area). currently only subsequent design work is taking place. for the time being however funding of the project is unclear; it will be addressed at state level. 23 v. sotorník, p. černoch, j. košťál acta polytechnica ctu proceedings figure 4. transverse geologic cross-section including individual tunnel alignment alternatives (vertical exaggeration x2). let us hope that the project will move to completion in the shortest time possible. references [1] v. köllner. podrobný inženýrskogeologický průzkum bílina – průtah. 1988. v.o. silnice i/27, i. stavba, pragoprojekt, praha. [2] k. dusbaba, et al. i/13 bílina, obchvat – varianta x: technicko-ekonomická studie. 2016. valbek, spol. s r.o., ústí nad labem. [3] p. černoch, v. sotorník. i/13 bílina, obchvat – realizační dokumentace předběžného geotechnického průzkumu. 2018. inset s.r.o., praha. [4] j. košťál, v. sotorník. i/13 bílina, obchvat – ii. etapa předběžného geotechnického průzkumu. 2019. inset s.r.o., praha. 24 acta polytechnica ctu proceedings 32:21–24, 2022 1 introduction 2 purpose and goal of the investigation 3 geotechnical investigation 3.1 preparatory works 3.2 core drilling 3.3 sampling 3.4 field tests 4 results of the geotechnical investigation 4.1 southern (most) portal 4.2 mined section of the tunnel 4.3 northern (teplice) portal 5 selected noteworthy geotechnical aspects 5.1 southern (most) portal 5.2 mined section of the tunnel 5.3 northern (teplice) portal 5.4 geotechnical and hydrogeological hazards 6 conclusion references https://doi.org/10.14311/app.2022.33.0052 acta polytechnica ctu proceedings 33:52–57, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague application of high performance concrete for reconstruction iva broukalová∗, alena kohoutková, anna horáková, hana schreiberová czech technical university in prague, department of concrete and masonry structures, faculty of civil engineering, thákurova 2077/7, 166 29 praha 6 dejvice, czech republic ∗ corresponding author: iva.broukalova@fsv.cvut.cz abstract. one of the sustainability aspects of the construction is a sufficient durability. in objectively justified cases, the durability is enhanced by reconstruction. to ensure economic design of the reconstruction, non-liner approaches are often applied. this paper deals with aănonlinear numerical simulation of disassembled reinforced concrete (rc) panels strengthened by an ultra-high performance concrete (uhpc) layer. based on the prior experimental programme, simulations of four-point bending tests of the original and strengthened panels are created using software atena science. calibration of the material parameters is based on the destructive and non-destructive investigations performed in the experimental programme. the comparison of the experimental and numerical model loading curves indicates that additional mechanical testing will be needed in order to achieve an accurate numerical simulation. although the bending test of the rc panel and the crack mouth opening displacement (cmod) test performed on the applied uhpc beam were calibrated, the final model of the uhpc strengthened panel does not correspond completely with the experimental measurements. in this paper the possible reasons for this result are discussed. keywords: analysis, high-performance concrete, reconstruction. 1. state of the art in recent decades, environmental aspects have been attracting more and more attention in the field of building structures. one of the possible approaches to an environmentally friendlier structure is prolongation of its service life. by improving the no-longer complying parts of structures or further utilization of removed building components, the negative impacts of construction can be reduced greatly. however, an increased demand for the load-bearing capacity or a different loading type of the recycled component very often results in necessary strengthening of the structure. in the case of concrete structures, an application of a new additional concrete layer, usually from a type of high-performance fibre concrete, has been proposed and investigated as a potential strengthening method [1–13]. in the previous studies, the layer from highperformance (hpc) or ultra-high performance (uhpc) concrete has been generally applied in four strengthening configurations in thickness from 20 to 100ămm (most frequently 30 mm). a strengthening layer located in the compression zone was most frequently used in the case of panels [2, 5, 8, 12]. the main advantage of this configuration lies in its easier performing as the strengthening layer is applied on the upper surface, thus no complex formwork is needed. concrete beams have been, in the conducted experiments, strengthened in the compressive zone [1, 4, 10, 11], the tension zone [3, 7, 11], two-sided (left and right surfaces) [7, 13], and three-sided (aăcombination of all of the above) [3, 7, 9, 11, 13]. in order to determine the most effective strengthening configuration, a number of studies compared the impact of the layer location on the load-bearing capacity of the element [7, 11]. the results indicated that the three-sided configuration reaches the highest values (an increase of up to 89% in the loadbearing capacity) following by the two-sided (around 47%) and the tensile configuration with as low as 16%. not surprisingly, the conducted experiments also showed that the magnitude of the tensile strength of the strengthening layer has a decisive influence especially in the three-sided configuration, whereas it has only a little effect in the case of compressive zone [7]. another important aspect of the hpc or uhpc strengthening method is the adhesion of the new layer with the original concrete component. in [7, 13] which have dealt with this topic, a transverse tensile strength test was performed on cylindrical or cubic specimens which were prepared from both the original basic concrete and uhpc. the results of these experiments were also further used in numerical finite element (fe) modelling in order to describe the behaviour of the contact layer. to further improve the adhesion of the layers, researchers have been also investigating different surface treatments, such as certain adhesion agents 52 https://doi.org/10.14311/app.2022.33.0052 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 33/2022 application of high performance concrete for reconstruction figure 1. a typical geometry of the non-strengthened panels based on destructive investigations. figure 2. typical geometry of the strengthened panels after the uhpc layer application [14]. (e.g. epoxy resin) or roughening (e.g. sandblasting, hydrodemolition, or wire-brushing). a study [7] showed, that specimens with an epoxy resin surface treatment reach higher values in the transverse tensile strength test but lower values in the shear strength test compared to specimens with a surface treated with sandblasting. importantly, the further bending test showed that the sandblasted surface leads to higher load-bearing capacity compared to the epoxy treatment. the finding indicates that the strength of the new-to-old concrete bonding depends primarily on its shear strength. this paper focuses on numerical modelling of the uhpc strengthening method. as high-performance concrete is rather economically and ecologically demanding material, careful optimization of its placement and characteristics is crucial for the sake of maintaining the positive effects of the application of the method. in order to achieve an effectively strengthened concrete structure by an uhpc layer, it is necessary to describe characteristics of the materials and their interaction as accurately as possible. for this reason, a large number of studies have been conducting numerical non-linear analysis alongside mechanical testing [7, 11–13]. by using an appropriate finite element software (e.g., atena, abaqus) and material models, the studies have been generally able to successfully describe the structure behaviour with only little (no more than 13.4%) deviations from the mechanical test results. however, several issues in the topic of fe modelling needs further clarification as they might influence the model accuracy. firstly, the compressive and tensile stress-strain relationship for the uhpc is a rather complex problem, as it is significantly different from the normal strength concrete. further, the characteristics of the bonding between the original substrate and new uhpc layer may distort the results, although the majority of previous studies considered perfect bonding. in our paper, we focus on fe numerical analysis of four-point bending tests performed on prefabricated reinforced concrete panels strengthened by a uhpc layer. in this study, several calibration procedures of material parameters are presented, and the outcomes are compared with experimental measurements. further, the specifics of designing recycled structures are discussed and further development is outlined. 2. experimental program 2.1. specimens bending tests were performed on several prefabricated reinforced concrete (rc) panels, which have been located in outside conditions for several years. the panels were around 120 mm high, 490 mm wide, 53 i. broukalová, a. kohoutková, a. horáková, h. schreiberová acta polytechnica ctu proceedings and 2750 mm long; however, the dimensions varied slightly in every specimen. destructive tests showed that the panels were reinforced with steel reinforcement two !12 mm bars at the bottom surface and two !12 mm bars at the upper surface. however, the performed destructive investigations revealed that the location of the bars was slightly different in every specimen. further, due to the inadequate reinforcement at the bottom surface, four !10mm b500b bars were glued into formed grooves to increase the loadbearing capacity of the panels. before casting of the uhpc layer, the upper surfaces of the panels were treated by hand hydro-jetting to ensure adequate bonding of the original substrate with the new layer. after the treatment, the uhpc layers were cast in the compressive zone in thickness of 30 mm. this study focuses on two experimental series reference panels with no strengthening layer (figure 1) and panels with 30 mm uhpc layer (figure 2). 2.2. material properties in order to determine the strength class of the original rc panels, a number of destructive and nondestructive tests were performed. according to the measurements, the material was categorized as c30/37. the material characteristics of the prepared ultrahigh strength fibre concrete (uhpc) were investigated by destructive mechanical testing. the compressive test conducted on cubic specimens determined its mean compressive strength as 133 mpa. further, a three-point bending test, with support span 500 mm, was conducted on pre-notched beam specimens (150 × 150 × 700 mm) to obtain curves of the dependence of the loading force on the crack mouth opening displacement (cmod). 2.3. testing methods to determine behaviour of the investigated strengthened and non-strengthened panels, a series of fourpoint bending tests were conducted. the span of the supporting pins was 2500 mm, the span of the loading pins was 800 mm. the loading force and deformation were monitored and recorded during the loading. 3. numerical modelling of the four-point bending tests all of the analysis was conducted using a finite element analysis software atena 5.6.1 science with gid interface. the actual geometry of the test setup was modelled in all cases. in the case of the fourpoint bending tests, the symmetry of the setup was considered, and only halves of the specimens were modelled. in all cases, the loading was introduced into the model as a pre-described deformation through steel distribution plates. 3.1. numerical modelling of the reference non-strengthened panels firstly, the four-point bending test of the nonstrengthened panels was modelled. as the information about its materials was limited, a careful calibration of the numerical model was crucial for further investigation of the uhpc strengthening method. in order to determine whether it would be possible to omit the spatial effect, the rc panel was modelled as both a plane stress (2d) and a three-dimensional stress (3d) structure model type. the geometry was identical in both cases. however, due to the significantly lower computational demands of the 2d model, the mesh size could be set to 15 × 15 mm in the case of the 2d model, whereas the 3d model used 25 × 25 mm. in order to obtain the loading curves, the loading force was monitored using a monitor placed under the force and the deflection was monitored on the lower surface in the middle of the span. 3.1.1. material models as mentioned above, the information about the rc original materials were limited. as a starting point, the material parameters of both concrete and steel bars were generated using the predefined materials in atena gid software. material concrete ec2 which uses the cc3dnonlincementitous2 material model was selected as a representation of concrete. as this material generates the parameters according to the eurocode2, the category c30/37 was set. the steel bars, both the original and the additional, were defined as reinforcement ec2 using ccreinforcement set to the category b500b. 3.1.2. parametric study and calibration of the fe models firstly, analysis of the 2d and 3d model with generated default material parameters, using the material models mentioned above, was run. subsequently, the loading-deflection curves obtained from the analysis were compared in order to evaluate the differences between the 2d and 3d modelling. figure 3. a comparison between the 2d and 3d loading curve a reference panel with default material parameters. as can be seen in figure 3, generally, the curves did not differ significantly. the crack formation and the 54 vol. 33/2022 application of high performance concrete for reconstruction parameter units default settin multiplication factor concrete material model young modulus mpa 32000 0.56 tension strength mpa 2.9 1.00 compressive strength mpa −38 0.79 fracture energy mn/m 0.000073 1.14 tension stiffening − no yes critical compressive displacement m −0.0005 1.10 original steel reinforcement model characteristic yield strength mpa 500 0.84 class − b c table 1. quality and operations assessment schedules [7]. figure 4. the experimental loading curves of the tested non-strengthened panels compared with the calibrated atena simulation. reinforcing bars yield occurred at almost the identical loading force (at approx. 15 kn and 63ăkn, respectively). however, before the yield point, the lower slope of the 2d curve indicated a slightly lower stiffness in the case of 2d analysis, especially after the crack formation. on the other hand, after the yielding, the slope of the 2d curve was higher compared to the 3d curve. furthermore, the failure of the 2d model took place at significantly higher values of the deflection, resulting in a higher maximum load force. on the base of a parametric study (the analysis was run several times, while changing the individual values of the default material models) the numerical models were calibrated (figure 4) by setting the material parameter values to required values, so that the loading curve obtained from the fe analysis would correspond as closely as possible with the experimental measurements. in table 1, the key default parameters together with the necessary multiplying coefficient for the changed values are listed. 3.2. numerical modelling of the panels with a uhpc layer after the calibration of the model of the nonstrengthened panel, analysis of the strengthened specimens could take place. conveniently, unlike the original materials, the newly introduced uhpc could have been subjected to several destructive tests. thus, the information about its compressive strength figure 5. the experimental force-cmod curve compared with the fe analysis results (with default material parameters and after the calibration procedure). and tensile behavior was much more certain. to correctly determine its tensile material parameters, a three-point bending test was modelled in the atena software and calibrated using the conducted experiment. subsequently, the strengthened panel was modelled using the parameters obtained from the numerical representation of the bending test. 3.2.1. the cmod analysis of the applied uhpc in order to define the uhpc material parameters, aăuhpc beam subjected to the three-point bending test was modelled using atena science in 2d. in order to obtain the force-cmod curves, the loading force and horizontal displacements in the notch mouth were monitored. as a representation of the uhpc, cc3dnonlincementitous2user was utilized as it allows the user to define its own fracture-plastic material model, and it is recommended for the fibre reinforced concrete (frc) modelling. the calibration procedure was performed according to the software documentation. the tensile function, which have a crucial importance for the frc materials, were carefully set so that the force-cmod curves corresponded and closely as possible (figure 5). calibrated values of the material parameters can be seen in table 2. 55 i. broukalová, a. kohoutková, a. horáková, h. schreiberová acta polytechnica ctu proceedings parameter units calibrated values parameter units calibrated values young modulus mpa 43000 tension strength mpa 6.5 compressive strength mpa −133 table 2. calibrated values of the key material parameters based on the cmod analysis. 3.2.2. modelling of the four-point bending test of the strengthened panels based on the four-point bending test of the strengthened panels, the strengthened panels were modelled using atena science as a plane stress structure model type. the geometry of the model remained identical, only the 30 mm uhpc layer was added. the connection between the old and new layer was considered perfectly solid, thus no mechanical characteristics were prescribed. the material parameters of the steel bars and original concrete were set according to the calibration of the non-strengthened panels. the uhpc parameters were taken over from the calibrated cmod analysis. results of the analysis run with the parameters based on the prior calibration procedures can be seen in figure 6. figure 6. the experimental loading curve of strengthened panels compared with the atena simulation with material parameters based on the prior calibration procedures. as can be seen, the load-deflection curve from obtained from the fe analysis did not match the experimental measurements completely. similarly, the formation of the tensile cracks occurred around the same loading force and deflection. however, the stiffness of the cross-section after the cracking differed quite significantly, as the curve slope of the fe model is notably steeper. the fe model also showed approx. 12 % higher maximum loading force than the experimental measurements. 4. discussion the finite element analysis of strengthened prefab concrete panels presented in this paper pointed out several crucial factors which influence the design of the uhpc strengthening method. as previously stated in the introduction, the environmental benefits of recycling or renewal of no-longer necessary or unsatisfactory concrete structures are unquestionable. however, as is apparent from the experimental part of the study, the knowledge of the original materials and its geometrical arrangement in the structure is frequently incomplete. for this reason, the numerical analysis inputs are then based on several destructive or non-destructive tests, which provide only limited information about selected specimens. as the parametric study of the non-strengthened panels conducted in our paper showed, the structure behaviour was fundamentally affected by the placement and characteristics of the original steel reinforcement. in our experimental program, the placement, and possibly the diameter, of the bars were slightly different in every specimen, and it was not possible to subject the bars to any mechanical measurements. thus, although the fe model of the fourpoint bending test corresponded well with the experiment after the calibration, it was adjusted only to the chosen specimens. therefore, the model could have been unsatisfactory for the further strengthened panels due to a different geometrical arrangement. more known inputs were available for the modelling of the strengthened panels. the cmod analysis of uhpc provided information about the material tensile properties and the compressive test determined its compressive strength. however, no loaddeflection curve was known from the pressure test, only the maximum loading force. thus, the compressive function (the stress-strain relationship) could not have been determined based on any measurements. the loading curve of the strengthened panel showed, that the calibrated model of the original panel and uhpc with defined tensile function based on the cmod analysis did not match with the experimental measurements. this unsatisfactory result could have been caused by several factors. firstly, as stated above, the model fitted on the selected non-strengthened panels could have been unsuitable for the selected strengthened panels due to the different geometry or materials. secondly, as mentioned in the introduction, the tensile properties of the strengthening layer in the compressive zone have only a limited impact on the load-bearing capacity. thus, the setting of the compressive function based on the load-deflection curve could have been crucial and more experimental measurement is therefore needed. further, no stresses due to shrinkage of the uhpc layer and mechanical properties of the connection between the original substrate and new material were considered. although no visible deterioration of the layers was apparent during the bending test, the effect may not be negligible in the fe analysis and determination of the interface parameters will be necessary in future investigations. 56 vol. 33/2022 application of high performance concrete for reconstruction 5. conclusions application of high-performance cementitious composites for reconstruction of concrete structures is aăstep towards sustainable construction. the reuse of damaged or no-longer satisfactory structural elements leads to saving both material and waste. however, an optimized design of the required strengthening is the key to ensure its advantageousness. when the non-linear behaviour of the materials is taken into account, it allows the design to be less conservative, thus more economic. in this paper, the performed non-linear numerical analysis pointed out several crucial factors which influence the numerical simulation suitability. for a sufficient numerical model, the geometry of the recycled structures must be known as precisely as possible. further, when modelling only a general geometry of the structures, the slight differences in the individual elements might affect the model accuracy greatly. however, most importantly, it is crucial to ensure inputs for the numerical analysis by conducting a sufficient number of mechanical tests of the materials present in the modelled structure. acknowledgements fv20472 application of high performance cementitious composites at rehabilitation of concrete structures, sgs19/149/ohk1/3t/11 durability of concrete structure and assessment of its life cycle. 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[14] a. čítek. experimentální ověření betonových prvků zesílených pomocí uhpc, čvut v praze, fakulta stavební, 2019. 57 https://doi.org/10.1002/stco.200910014 https://doi.org/10.1061/(asce)mt.1943-5533.0000008 https://digitalrepository.unm.edu/cgi/viewcontent.cgi?article=1094&context=ce_etds https://doi.org/10.1201/b17063-333 https://doi.org/10.1016/j.conbuildmat.2017.08.077 https://doi.org/10.1016/j.conbuildmat.2016.12.094 https://doi.org/10.1088/1757-899x/271/1/012064 https://doi.org/10.14359/51684405 https://doi.org/10.1016/j.engstruct.2015.10.042 https://doi.org/10.1155/2017/4387545 https://doi.org/10.1186/s40069-018-0330-z https://doi.org/10.14311/app.2022.33.0284 acta polytechnica ctu proceedings 33:284–289, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague lower bound analysis for shear assessment of full-scale rc girders subjected to axial tension takeru kanazawa hokkai gakuen university, 1-1, minami 26, nishi 11, tyuo-ku, sapppro, hokkaido, japan correspondence: t-kanazawa@hgu.jp abstract. axial tension force exerted as a result of a temperature change or shrinkage can cause the collapse of rc structural members. design code provisions and analytical models such as modified compression field theory (mcft) yield reasonable estimates of shear strength of rc beams subjected to axial tension. nevertheless, their semi-empirical nature is not necessarily appropriate for shear assessment of existing rc structural members. the extra conservativeness and empirically determined parameters might require unnecessary maintenance work. a generalised model with rigorous formulation must be developed. this paper presents a purely theoretical model to predict the shear strength of rc beams under axial tension based on limit analysis. without regressive functions and empirical functions, lower bound analysis enables shear strength derivation when the force equilibrium and strain compatibility are satisfied. accuracy of the analysis was verified by comparison of its predictions with three experimental shear strengths of full-scale rc girders. an equal level of accuracy was observed between the analytical solutions and mcft-based predictions. keywords: axial tension, full-scale rc girders, lower bound analysis, shear assessment. 1. introduction the sustainability of rc structures is an important issue worldwide because of their exposure to a variety of loading with environmental and mechanical actions. the partial collapse of the wilkins air force depot in 1955 [1] spurred the refinement of design practices for shear under axial tension caused by a temperature change or shrinkage. the brittle collapse was reproduced in laboratory investigation [1]. results demonstrated that tensile stress of approximately 1.4 n/mm2 can reduce shear strength by 50% [2]. design code provisions have difficulty in predicting such detrimental effects of axial tension because of empirically determined parameters with safety factors. in this respect, several well-established theories for shear [3–5] are based not only on the force equilibrium, but on the failure kinematics. the former and latter respectively correspond to lower and upper bound solutions. the lower bound analysis is the basis of the shear design expression in most international codes. this lower bound is the reason underlying their inherent conservativeness, which is appropriate for designing new structures. for structural assessment of existing rc members, however, such conservativeness is of less importance than determining the actual bearing capacity [6]. among the currently available theories, the modified compression field theory (mcft) has provided a consistent framework to consider the influence of axial force and to facilitate its incorporation into various international codes and specifications. nevertheless, the mcft predictions might elicit some discrepancy with several test results of rc beams under axial tension [2]. current understanding should be supplemented theoretically for rational shear assessment because the limitations of existing test results include laboratory-scale specimens without web reinforcement [7], despite recent progress [8, 9]. this paper presents a rational analysis based on the lower bound theorem. the analytical structure is based on the combined upper and lower bound analysis [10], which predicted the shear strengths of rc beams accurately with no axial force and no web reinforcement. the present work extends their original theory to consider the effectiveness of web reinforcement and the negative effects of axial tension. rigorous formulation for shear failure is presented, including shear resisting mechanisms such as aggregate interlock and dowel action of longitudinal reinforcement under both force equilibrium and strain compatibility. the developed analysis is validated by comparison of its solutions with mcft-based predictions and test results of three full-scale rc girders subjected to axial tension [7]. the notation used for this study is summarised in the appendix. 2. model formulation 2.1. analytical framework in the theoretical framework of limit analysis [11], the exact solution lies between lower and upper bound solutions, for which the former and latter respectively satisfy force equilibrium and compatible patterns of failure, in addition to yield criteria. one therefore finds the exact solution by maximising the lower bound solutions. to derive shear strength with 284 https://doi.org/10.14311/app.2022.33.0284 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 33/2022 analysis for shear assessment of full-scale rc girders figure 1. constitutive law of concrete. figure 2. constitutive law of steel reinforcement. reasonable simplification, the following assumptions are introduced. • concrete and steel reinforcement are in a plane stress state. • perfect bonding exists between steel reinforcement and concrete. • concrete strength equal to over a compression zone; concrete behaves as a rigid perfectly plastic material, as presented in figure 2. • steel reinforcement behave as an elastic perfectly plastic material, as shown in figure 2. it is noteworthy that the physical quantities such as forces, stresses and strains in compression are negative. those denoted with a prime have positive compression. 2.2. force equilibrium figure 3 shows the free-body diagram with the shear resisting components against the external shear and axial tension force, respectively denoted as v and n. this equilibrium condition yields the following expressions, respectively, for vertical force, horizontal force, and moment. v − vc − fd − fw − b ! t f sin θ dt = 0 (1) figure 3. free-body diagram. figure 4. critical crack shape. ! h αh bf ′ c dy − fs1 − f ′ s2 + n + b ! t f cos θ dt = 0 (2) [γa + cot β (h − d1)] fd + (h − d1) fs1 − (h − d2) f ′ s2 + a (1 + γ) 2 fw + avc − h 2 n − h (1 + γ) 2 ! h αh bf ′ c dy + b "! t f sin θ x dt − ! t f cos θ y dt # = 0 (3) therein, $ t dt denotes the path integral from point a to b in figure 3. one can replace these integrations with the following integrations with respect to x and y. ! t cos θ dt = ! a γa dx, ! t sin θ dt = ! αh 0 dy (4) mathematical treatment of the path integral requires an expression for the critical crack shape shown in figure 4 on the x − y coordinates. y2 = α2h2 (x − aγ) a (1 − γ) (5) dividing equations 1, 2 and 3 respectively by bd1f ′ c and abd1f ′ c, one obtains non-dimensionalised expressions as presented below. 285 takeru kanazawa acta polytechnica ctu proceedings figure 5. strain compatibility. v = (1 − α) τc + φs1τs + ωφswσw + α µ f (6) 1 − α µ − φs1σs1 + φs2σs2 + n + λ (1 − γ) f = 0 (7) % γ + cot β " ζ − 1 λ #& τsφs1 + " ζ − 1 λ # φs1σs1− " ζ − δ λ # φs2σs2 + 1 + γ 2 ωφwσw + (1 − α) τc − ζ 2n − ζ 2µ ' 1 − α2( − α 3µ (1 − 4γ) f = 0 (8) in those equations, the non-dimensional static variables are defined as shown below. ) **********+ **********, v = v bd1f ′ c , σs1 = fs1 as1fs1 , σs2 = f ′ s2 as2fs2 , σw = fw aswfsw , τc = vc bd1f ′ c (1 − α) , τs = fd as1fs1 , n = n bd1f ′ c , f = f f ′ c . (9) equation 6 is the objective function in this study. the non-dimensional shear strength of v is maximised with respect to the static variables. 2.3. compatibility conditions though lower bound solutions do not require compatibility conditions, an inequality constraint for aggregate interlock ' f ( can be derived from strain compatibility. figure 5 presents the strain compatibility on the cracked section. the assumption of bernoullieuler beam (plane sections remain planar) yields the linear distribution of longitudinal strain. those of bottom and top reinforcement are obtained respectively as shown below. εs1 = d1 − (1 − α) h (1 − α) h ε′ cu = 0.0035 µ 1 − α − 1 . (10) εs2 = (1 − α) h − d2 (1 − α) h ε′ cu = 0.0035 δµ 1 − α − 1 . (11) the neutral axis depth under the effect of axial tension is calculable by arranging equation 6 with respect to k = (1−α)h/d1 as presented in the equation below. k =ε′ cues f ′ c / (ρs1 + δρs2) µ 1 − α − (ρs1 + ρs2) 0 − f λ (1 − γ) − n (12) equation 12 gives an upper limit of the aggregate interlock in maximisation because k must be positive according to its definition. 2.4. yield criterion and solution technique the following von mises yield criterion is introduced between τs and σs1. 3τ2 s + σ2 s1 = 1 (13) although hweé et al. [10] used another yield criterion for concrete, it need not be addressed in this study because the rigid perfectly plastic behaviour is assumed. the complete set of equations enables the maximisation of the objective function of equation 6 with respect to the six static variables ' σs1, σs2, σw, τc, τs, and f ( . the possible ranges of those variables are presented as the following. σs1, σs2, σw ∈ 〈0, 1〉 τc, τs, f ∈ 〈0, ∞〉 (14) all of those expressions must be positive. the upper limits of σs1, σs2, and σw and correspond with yielding of reinforcement. although no upper limit is imposed on the others, equation 7, 8, 12, and 13 render them physically admissible values. maximisation is performed using the optimisation algorithm of matlab [12], under the equality and inequality constraints listed in table 1. all possible 286 vol. 33/2022 analysis for shear assessment of full-scale rc girders objective function equation 6 equality constraints (force equilibrium) equation 7 and 8 equality constraints (yield criterion) equation 13 inequality constraints (neutral axis depth) equation 12 ≥ 0 inequality constraints (yield criterion) 3τ2 s + σ2 s1 ≤ 1 table 1. list of constraints for optimisation. figure 6. cross-sectional details of inverted-t beams (unit: mm). figure 7. shear strength dependence on applied axial force. values of v are calculated within the following ranges of kinematic variables. 1 − µ ≤ α ≤ 1 αh a (1 − γ) ≤ β ≤ ∞ 0 ≤ γ ≤ 1 (15) the lower limits of and stem respectively from the assumption that the longitudinal strain of bottom reinforcement in equation 10 must be positive, and that the critical crack is directed upwards at y = 0 (figure 3). to obtain more optimal solution, the maximised solutions are then minimised with kinematic variables. the partial derivative of equation (6) with respect to determines whether the objective function is monotonically decreasing or increasing. the minimised shear strengths are calculable as presented below. ∂v ∂α = − τc + f µ (16) ) *******************+ *******************, vana = φs1τs + ωφswσw + f µ " if ∂v ∂α < 0, αmax = 1 gives vana # vana = µτc + φs1τs + ωφswσw + 1 − µ µ f " if ∂v ∂α > 0, αmax = 1 − µ gives vana # (17) 3. results and discussion 3.1. experiment results the analytical solutions have been compared with test results obtained for full-scale rc girders [7]. the studied girders had the inverted-t configuration (figure 6) to support the deck in flexural tension. figure 7 presents the detrimental effect of axial tension on shear strength. the axial force of 900 kn decreases the shear strength by 14%. the axial force magnitudes of 3-200-p and 4-300-p were found respectively to represent shrinkage-induced stress only, and both the shrinkage and temperature-induced stress. it is noteworthy that smith et al. [7] tested seven specimens in all. the three specimens shown in figure 7 were extracted for model validation because the others included the use of cut-off bars and epoxy injected specimen, of which the contributions were not formulated. 3.2. model validation to calculate the shear strength by the developed analysis, static and kinematic constants were determined as listed respectively in tables 2 and 3. these values were calculable using information referred from reports of the related literature [7]. the rectangular area of bd1 in figure 6 was used for calculation because the contributions of the flanges under flexural tension are negligible. in addition, the values of ε′ cu 287 takeru kanazawa acta polytechnica ctu proceedings n = n bd1f ′ c φs1 = ρs1fs1 f ′ c φs2 = ρs2fs2 f ′ c φw = ρswfsw f ′ c 1-0-p − 0.221 0.110 0.064 3-200-p 0.083 0.269 0.135 0.079 4-300-p 0.128 0.267 0.133 0.081 table 2. non-dimensional static constants. λ = a d1 ζ = h a µ = d1 h σ = d1 d2 ω = s d1 1-0-p 3-200-p 2.60 0.407 0.943 0.066 0.265 4-300-p table 3. non-dimensional kinematic constants. vexp vexp vana vaasht o vana vexp vaasht o vexp + vdl (kn) (−) (−) (kn) (−) (−) 1-0-p 902 0.0773 0.0809 810 1.05 0.88 3-200-p 780 0.0814 0.0879 743 1.08 0.93 4-300-p 783 0.0835 0.0847 712 1.01 0.89 table 4. accuracy of the developed analysis. and es in equation (12) are assumed respectively as 0.0035 and 200 kn/mm2 because the original data were not found in the literature [7]. table 4 presents comparisons among test results (vexp), present analysis (vana), and mcft-based prediction by aashto lrfd bridge design specification [13]. the analytical solutions show adequate accuracy (vana/vexp) with specimens under axial tension (3-200-p and 4-300-p) and without axial tension (1-0-p). in all cases, the static variable of σw was equal to one when the optimisation terminated. this value represents the yielding of web reinforcement, which was observed in the experiments as well. furthermore, results demonstrate better agreement than the mcft-based predictions, except for those for specimen 3-200-p. although the analytical predictions of 3-200-p and 3-400-p show a consistent decrease of shear strength with the increase of applied axial force, no such tendency was observed in the experiment as presented in figure 7. further investigation about this discrepancy will be reported in another paper with a broad range of experimental data. 4. conclusions shear strengths under the effect of axial tension loading were derived analytically based on the lower bound theorem. the theoretical basis treated the shear strength derivation as an optimisation problem under equality and inequality constraints obtained from force equilibrium, strain compatibility and yield criteria. model validation showed good correspondence between experimental and analytical shear strengths without any regression functions and empirically determined parameters. the accuracy was an equal level with mcft-based predictions. the generality of this formulation enables the consistent treatment of axial compression, and gives quantitative results of each shear component such as aggregate interlock of cracked concrete and dowel action of longitudinal reinforcement. although further verification by comparison with a number of test results is necessary, the developed analysis might contribute to the provision of comprehensive understanding of shear behaviour under axial force, together with the mcft [2] and other well-established theories [3–5]. research is currently underway to extend this theory to axially compressed and tensioned beams without web reinforcement. list of symbols f ′ c concrete compressive strength v external shear force n external force of axial tension vc shear component of uncracked concrete fw shear component of web reinforcement f aggregate interlocking force per unit area θ inclination angle of critical crack to x-axis fd dowel force of bottom reinforcement fs1, fs2 longitudinal forces of bottom and top reinforcement, respectively α critical crack height divided by total depth 288 vol. 33/2022 analysis for shear assessment of full-scale rc girders β slope of critical crack at point b γ longitudinal distance between support and point b divided by shear span a shear span b web width d1 effective depth h total depth d2 distance between top reinforcement and upper perimeter of beams s spacing between adjacent web reinforcement t coordinate axis along critical crack ω spacing between adjacent web reinforcement divided by effective depth µ effective depth divided by total depth λ slenderness (shear span-to-depth) ratio ζ total depth divided by shear span δ d1 divided by d2 k neutral axis depth divided by effective depth ε′ cu ultimate compressive strain of concrete (0.0035) es modulus of elasticity of longitudinal reinforcement (200 gpa) ρs1, ρs2, ρsw reinforcement ratios of bottom, top and web reinforcement, respectively as1, as2, asw areas of bottom, top and web reinforcement, respectively fs1, fs2, fsw yield strengths of bottom, top and web reinforcement, respectively εs1, εs2, εsw longitudinal strains of bottom and top reinforcement, respectively φs1, φs2, φsw reinforcement degrees of bottom, top and web reinforcement, respectively acknowledgements financial support from a grant-in-aid for scientific research from the japan society of promotion of science (19k15062) is greatly appreciated. references [1] r. c. elstner, e. hognestad, editors. laboratory investigation of rigid frame failure. journal proceedings, 53:637-668 1957. [2] s. b. bhide, m. p. collins. influence of axial tension on the shear capacity of reinforced concrete members. structural journal 86(5):570-81, 1989. [3] y. yang, j. den uijl, j. walraven. critical shear displacement theory: on the way to extending the scope of shear design and assessment for members without shear reinforcement. structural concrete 17(5):790-8, 2016. https://doi.org/10.1002/suco.201500135. [4] a. muttoni, m. fernández ruiz. shear strength of members without transverse reinforcement as function of critical shear crack width. aci structural journal 105(2):163-72, 2008. https://doi.org/10.14359/19731. [5] b. i. mihaylov, e. c. bentz, m. p. collins. two-parameter kinematic theory for shear behavior of deep beams. aci structural journal, 110(3):447-56, 2013. [6] fib. shear and punching shear in rc and frc elements (lausanne: international federation for structural concrete), fib bulletin 57:268, 2010. [7] sm. t. smith, d. a. howell, m. a. t. triska, et al. effects of axial tension on shear-moment capacity of full-scale reinforced concrete girders. aci structural journal 111(1):211, 2014. https://doi.org/10.14359/51686534. [8] d. fernández-montes, e. gonzález valle, e. díaz heredia. influence of axial tension on the shear strength of floor joists without transverse reinforcement. structural concrete 16(2):207-20, 2015. https://doi.org/10.1002/suco.201400063. [9] d. t. pham, b. fouré, n. pinoteau, et al. influence of axial tension on the shear strength of rc beams without stirrups. structural concrete 22(2):1037-53, 2020. https://doi.org/10.1002/suco.202000077. [10] k. h. tan, a. hasegawa, f. nishino. determination of ultimate shear strength of reinforced concrete beams without web reinforcement by a combined upper and lower bound analysis. doboku gakkai ronbunshu 1986(368):23-33, 1986. https://doi.org/10.2208/jscej.1986.368_23. [11] d. c. drucker. on structural concrete and the theorems of limit analysis. international association for bridge and structural engineering, division of engineering, brown university, p. 36, 1961. [12] s. j. chapman.matlab programming for engineers, cl engineering, toronto: nelson education, 2015. [13] aashto. aashto lrfd bridge design specifications, 7th ed. american association of state highway and transportation officials, washington, d.c., 2013. 289 https://doi.org/10.1002/suco.201500135 https://doi.org/10.14359/19731 https://doi.org/10.14359/51686534 https://doi.org/10.1002/suco.201400063 https://doi.org/10.1002/suco.202000077 https://doi.org/10.2208/jscej.1986.368_23 https://doi.org/10.14311/app.2022.33.0256 acta polytechnica ctu proceedings 33:256–262, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague chloride ion penetration behaviour in concrete containing an expansive additive and a calcium-aluminate-based additive shinya itoa,∗, takeshi iyodab a denka co.ltd., 2209, omi, itoigawa-city, niigata 949-0393, japan b shibaura institute of technology, 3-7-5, toyosu, kotoku, tokyo 135-8548, japan ∗ corresponding author: shinya-ito@denka.co.jp abstract. the effect of the material characteristics on the infiltration behaviour of chloride ions in concrete containing a combination of an expansive additive and a calcium-aluminate-based additive has been investigated. the same level of salt resistance as that of blast furnace slug cement was exhibited even for a small addition amount. in addition, the permeation behaviour of chloride ions was largely influenced by the immobilisation capacity and pore network. when these changes were dominant, the results suggested that the characteristics based on the fick diffusion equations may not necessarily reproduce the actual permeation behaviour of chloride ions. keywords: chloride ion, chloride ion immobilized material, diffusion coefficient, expansive additive. 1. introduction in japan, there is a large amount of airborne salt from the ocean, and degradation of reinforced concrete structures because of salt damage has been reported nationwide owing to spraying of antifreeze agents in winter. salt damage is generally indicative of steel corrosion in concrete owing to infiltration of chloride ions. for general-purpose materials, blast furnace cement and common additives, such as fly ash, are used. in recent years, an additive has been developed in which cao · 2al2o (ca2) of calcium aluminate is admixed with cement to react with ca(oh)2(ch), which is a cement hydrate, to form a hydrate with an immobilisation function, and free chloride ions, which cause corrosion of steel, are chemically immobilised as friedel’s salt. it has been reported that the additive exhibits high salt damage resistance at low dosage [1, 2]. in addition, if excessive cracks occur in the concrete structure, chloride ions directly reach the reinforcing rebar, which increases the possibility of premature deterioration. therefore, it is important to suppress crack occurrence as a salt damage countermeasure. in addition, by incorporating an expansive additive in concretes, it is possible to suppress excessive cracking by introducing a shrinkage reduction effect or a chemical pre-stressing effect [3]. therefore, it can be expected that the external permeation path of chloride ions can be minimised by using ca2 and an expansive additive in combination, and the permeation rate of chloride ions can be suppressed by the immobilisation function even in the cured body. although the basic characteristics and salt damage resistance of concretes containing ca2 and an expansive additive in combination have been reported in previous studies, it is essential to establish a method to reflect the results to check the durability of concrete for practical use. for commons additives, such as blast furnace slag and fly ash, the characteristic values for durability inspection are clearly indicated in the concrete standard statement of the society of civil engineers based on a vast amount of data, but they cannot be directly applied to concrete containing ca2 or an expansive additive. in this study, we investigated the material characteristics of concretes containing ca2 and an expansive additive, and we then investigated the effect of the material characteristics on the chloride ion infiltration behaviour for durability examination. 2. materials and methods 2.1. composition of materials and concrete ordinary portland cement (opc) and blast furnace cement b (bb) were used as the cements. ca2 was prepared by calcining at 1750 to 1850 ◦c. a cao/al2o3 molar ratio of 0.5 was used with calcium carbonate and aluminium oxide as the raw materials. the obtained clinker was pulverised by gradual cooling. a lime-ettringite composite was used as the expansive additive. the chemical composition and densities of ca2 and the intumescent materials are given in table 1. sand (density 2.62 g/cm3) produced by kimitsu city, chiba prefecture was used as the fine aggregate, and limestone crushed stone (density 2.70 g/cm3) produced by tsukumi city, oita prefecture was used as the coarse aggregate. the concrete mix proportions are given in table 2. the cement, ca2, and the expansive additive were regarded as a binder (b, table 2). 256 https://doi.org/10.14311/app.2022.33.0256 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 33/2022 chloride ion penetration behaviour in ca-al concrete chemical composition (%) density (g/cm3) cao sio2 al2o3 so3 fe2o3 opc 64.1 20.5 5.2 2.1 3.0 3.16 ca2 24.0 0.6 67.7 0.01 7.1 2.96 ex 70.6 1.0 7.2 18.5 0.8 3.10 table 1. chemical compositions and densities. unit weigth (kg/m3) no. w/b (%) s/a (%) w c ca2 ex s g n 55.0 48 170 309 − − 864 965 n + ca2 289 20 − 964 n + ca2 + ex 269 20 22 964 bb 310 − − 859 959 table 2. concrete mix proportion. 2.2. test materials and measurement methods 2.2.1. permeation characterisation of the chloride ions a salt water immersion test was performed to evaluate the permeation characteristics of chloride ions. in the salt water immersion test, a 100 mm × 100 mm × 400 mm prismatic specimen was cured in water at 20 ◦c for 28 days. the five surfaces other than the 100 mm × 400 mm surface were coated with epoxy resin and immersed in 10% aqueous nacl solution at 20 ◦c with one surface exposed to epoxy resin. at the specified age of the specimen, the depth from the exposed surface to the coloured part was measured by sequentially splitting and spraying silver nitrate solution (0.1 n) on the split surface. 2.2.2. chloride ion profile after 48 weeks aging of the immersion material, the test piece was cut at intervals of 1 cm in the depth direction from the permeation surface of the specimen. the total chloride ion content was analysed in accordance with jis a1154, and the chloride ion profile in the depth direction was constructed. the soluble chloride ions were extracted with warm water at 50 řc, and the amounts of free and fixed chloride ions were determined. 3. results and discussion 3.1. evaluation of the chloride ion permeability 3.1.1. salt immersion test the chloride penetration depths of the concrete specimens soaked in 10% strength nacl solution are shown in figure 1. the penetration depth of the reference sample n aged for 48 weeks was about 30 mm. the penetration depths of the n+ca2 and n+ca2+ex samples were about 20 and 16 mm, respectively, and figure 1. salt immersion test results. the penetration of chloride ions tended to be suppressed by the use of additives. in addition, the difference in the penetration depth compared with bb was several millimetres, and there was not a large difference in the penetration depth in the immersion period. it was presumed that this was because of the fact that hydrates with an immobilisation function, such as hydroculmite (hc), were produced by adding ca2 immobilised chloride ions as friedel’s salts, thereby suppressing penetration of chloride ions into the concrete. when the expansive additive was used in combination with ca2, the penetration depth of chloride ions was equal to or less than that of the ca2 sample. in addition, if the expansion ratio was within the range of the expansion ratio of shrinkagecompensating concrete, there was no effect on the immobilisation capability of chloride ions of the ca2 sample, and a synergistic effect on improving the salt shielding property was obtained by the combination of the expansion material and expansive additive. 3.1.2. chloride ion profile the chloride ion profiles of the different concrete samples are shown in figures reffig2 5. the apparent diffusion coefficients and amounts of superficial 257 shinya ito, takeshi iyoda acta polytechnica ctu proceedings figure 2. chloride ion profile for n. figure 3. chloride ion profile for n+ca2. chloride ions calculated by fick’s diffusion equations based on this chloride ion profiles are given in table 3, along with the maximum total chloride ion content in each concrete and the free/immobilised chloride content ratios. the amounts of surface chloride ions estimated from the salinity profiles by fick’s diffusion equations were higher for the n+ca2, n+ca2+ex, and bb samples compared with the n sample as a reference. for the distribution of the chloride ion amount in the depth direction from the penetration surface, the difference in the chloride ion amounts between the surface layer and inside the cured body was large, except for the n sample, and the chloride ion amount decreased with increasing depth. the tendency was remarkable for n+ca2+ex and bb in which ca2 and the expansive additive were used in combination. the apparent diffusivities of the n+ca2+ex, n+ca2, and bb samples were smaller than that of the n sample, by about 42% for the n+ca2 sample, about 54% for the n+ca2+ex sample, and about 69% for the bb sample. this confirmed that chloride ion diffusion was suppressed by using the combination of ca2 and the expansive additive compared with using ca2 alone. in this study, salt water immersion was performed for 48 weeks. therefore, it is considered that the first layer (0 to 10 mm) of the permeation surface, which exhibited figure 4. chloride ion profile for n + ca2 + ex. figure 5. chloride ion profile for bb. the maximum total salt content, was saturated with chloride ions present in the concrete. for the n sample, because the ratios of free/immobilised chloride ions in the depth direction from the penetration surface direction were not significantly different between the surface layer and inside the sample, it is considered that the permeated chloride ions moved and were immobilised at a constant ratio. in contrast, for the n+ca2 and n+ca2+ex samples, the immobilisation ratios of the surface layer and inside the sample were different, and it was inferred that immobilisation and adsorption preferentially occurred. in addition, for the bb sample, although the amount of immobilisation was small, penetration of chloride ions into the surface layer was greatly suppressed, suggesting that the pore structure was such that ion penetration was difficult. therefore, it was considered that the permeation behaviour of chloride ions can be expressed by fick’s diffusion equations for the n sample in which fixation/adsorption and ion permeation proceed at relatively constant rates. however, it is suggested that it is difficult to appropriately describe the permeation behaviour of ions by fick’s diffusion equations for a material that possesses the fixing function, such as ca2, or a material with pore structures that influence ion permeation, such as blast furnace cement. 258 vol. 33/2022 chloride ion penetration behaviour in ca-al concrete fick’s diffusion equation ratio (%) apparent diffusion coefficient (m2/s) chloride ion content at the concrete surface (kg/m3) total chloride ion content (kg/m3) free-chloride ion immobilized chloride ion n 1.02e-11 16.4 14.3 69% 31% n + ca2 5.88e-12 19.8 15.9 57% 43% n + ca2 + ex 4.72e-12 20.9 16.1 60% 40% bb 3.13e-12 22.1 15.0 79% 21% table 3. concrete mix proportion. w/b (%) n 55 opc : 100% n + ca2 opc : 93.5% + ca2 : 6.5% n + ca2 + ex opc : 86.4% + ca2 : 6.5% + ex : 7.1% bb bb : 100% table 4. mix proportions of the pastes. figure 6. hydrate before immersion. 3.2. material characteristics and penetration behaviour of chloride ions the results suggested that the penetration behaviour of chloride ions inside the hardened bodies and the distribution of free chloride ions differed in the concrete formulations, and that the values calculated with fick’s diffusion equations may not necessarily reproduce the actual infiltration behaviour of chloride ions, except for the n sample. therefore, we focused on two material properties: the hydrate and the pore structure. 3.2.1. hydrate the immobilisation performance of chloride ions in each sample was determined. the hydrate was verified by powder x-ray diffraction (xrd) using the paste test specimens with the mix proportions given in table 4. curing of the paste test specimens was performed for 7 days in water. the results are shown in figure 6. compared with the case of no mixing, the intensity the ch peak (10◦ −11.5◦) of the cement figure 7. hydrate before immersion. hydrate decreased when ca2 or ca2 and the expansive additive were mixed with ordinary cement. this means that ch was consumed by the hydration reactions of ca2 and afm(c) of hc and mono-carbonate or hemi-carbonate formed. the reason why hc and afm(c) were mixed in the reaction of ca2 can be attributed to the small amount of mixed components in the cement. in addition, the same hydrate formed when the combination of ca2 and the expansive additive was used. no peak was observed for ettringite, which is the hydrate of the expansive additive. it was presumed that this was because the so3/al2o3 molar ratio was reduced by the combined use with ca2. to confirm the hydrate change after salt immersion, the paste test specimens with the same mix proportions were immersed in 10% aqueous nacl solution for 2 months. the results of xrd measurements after immersion are shown in figure 7. the peaks of hc and afm, which were observed before salt immersion, disappeared after salt immersion, and they changed to friedel’s salts regardless of the mix proportion. therefore, hc and afm have the function of immo259 shinya ito, takeshi iyoda acta polytechnica ctu proceedings figure 8. friedel’s salt content. figure 9. pore volumes of the four samples. bilising hc and afm as friedel’s salts when chloride ions are present, and it is considered that afm also has immobilising ability even when it is afm(c), such as mono-carbonate and hemi-carbonate. thermal analysis was also performed using paste specimens with the same mix proportions to quantify the amount of friedel’s salt after salt immersion. the amount of friedel’s salt was determined by taking the mass reduction from 247 to 387 ◦c obtained by thermal analysis using the pure synthesised friedel’s salt as 100% and comparing it with the mass reduction rate of the paste in the same temperature region. the amount of friedel’s salt for each sample determined by thermal analysis is shown in figure 8. from the results, compared with the n sample, the amount of friedel’s salt was 1.5 times higher for the n+ca2 sample, 1.45 times higher for the n+ca2+ex sample, and 1.2 times higher for the bb sample, and the immobilisation capacity of chloride ions was remarkably increased in the ca2 blended system. in addition, the expansive additive did not significantly influence the immobilisation ability of chloride ions. 3.2.2. pore structure the influence of the pore structure was investigated, including the migration path of ions, total pore amount, continuous pore amount, and pore network. figure 10. steady-state electrophoresis test results. first, the ratio of the total pore amount to the continuous pore amount measured by the mercury intrusion method using the paste test specimens after salt water immersion was investigated (figure 9). the continuous pore amount was determined by press-fitting mercury twice up to a maximum pressure of 410 mpa, and the mercury press-fit amount at the time of re-pressurisation was the continuous pore amount [4, 5]. from the results, the continuous pore volume of n+ca2+ex was the smallest prior to salt immersion and that of n+ca2 was the smallest after salt immersion. from these results, it is considered that the pores tended to be slightly reduced by using ca2. the absolute value of the total pore amount was in the range 0.3 − 0.4 ml/ml before salt immersion and 0.2 − 0.3 ml/ml after salt immersion. in addition, there was a tendency for the continuous pore volume to slightly decrease after salt immersions for the composites containing ca2 and bb. it was presumed that the effect of densification caused by the change in the hydrate, such as formation of friedel’s salt, affected the continuous pore volume. however, the absolute amount ranged from about 0.09 to 0.15 ml/ml, and it was presumed that the difference was not sufficiently large difference for permeation of chloride ions to greatly change. next, the pore network was investigated. the steady-state electrophoresis test is a technique for evaluating pore networks. it is specified in the "method for testing the effective diffusion factor of chloride ions in concrete by electrophoresis (jsceg-571)" standard of the japan society of civil engineers. it determines the effective diffusion factor from the amount of ions eluted to the anode side by applying a voltage of 15 v to the hardened concrete to accelerate penetration of chloride ions. the results of the steady-state electrophoresis tests for the concrete samples given in table 2 are shown in figure 10. the amount of chloride ions eluted to the anode was in the order n > n + ca2 > n + ca2 + ex > bb, which is the same as the salt immersion test results described in the previous paragraph. however, when the respective behaviours were compared, the chloride ion permeation behaviours were clearly different between 260 vol. 33/2022 chloride ion penetration behaviour in ca-al concrete fixation ability total pore volume continuous pore volume pore network ratios chloride ion suppression effect n 100% 0.26 ml/ml 0.15 ml/ml − − n + ca2 150% 0.20 ∼ 0.26 ml/ml © 0.09 ∼ 0.15 ml/ml © −17% © n + ca2 + ex 145% −25% bb 120% © −52% table 5. summary of material characteristics and penetration behaviour of chloride ions. the sample containing ca2 and the bb sample. here, we will consider the pore structure inside the cured body on the basis of the results obtained from the steady-state electrophoresis test. in the steady-state electrophoresis test, chloride ions are forced to move to the anode side by a potential gradient, and the eluted amount of chloride ions is measured. that is, the ions eluted on the anode side pass through the continuous pores in the cured body, and the eluted amount in the same time depends on the pore network, which is the complexity of the continuous pores. in addition, because the permeation and migration behaviour of chloride ions clearly differed between the immersion method and electrophoresis method for the sample containing ca2, it is considered that the immobilisation function of chloride ions by ca2 is not reflected by the steady electrophoresis method. here, from the relationship between the energisation time and the chloride ion concentration on the anode side shown in figure 10, the cumulative amount of ions since the elution amount of ions became a steady state was approximated by the least squares method. then, assuming that the slope of the approximate straight line is an index representing the pore network, expressed in proportion to the n sample. the elution rate of chloride ions to the anode side were in the order n > n + ca2 > n + ca2 + ex >>> bb. in addition, based on the n sample, the pore network ratios were −52% for the bb sample, −17% for the n+ca2 sample, and −25% for the n+ca2+ex sample. that is, the pore structures tended to be more complicated using the expansive additive and ca2 than using ca2 alone. therefore, a synergistic effect can be obtained by using the expansive additive and ca2 from the viewpoint of suppressing permeation and migration of chloride ions. 3.2.3. summary of material characteristics and penetration behaviour of chloride ions the summary of study on the effect of material characteristics and penetration behaviour of chloride ions are shown in table 5. 4. summary the material characteristics of concrete containing a combination of ca2 and an expansive additive were investigated as a basic study for durability examination, and the effect of chloride ions on the permeation behaviour was investigated. the following results were obtained. 1. by dividing the amount of chloride ions into immobilised and free chloride ions based on the chloride ion profile obtained by the salt immersion test, the proportions greatly differed depending on the material characteristics. 2. the calculated and measured amounts of surface chloride ions calculated by fick’s diffusion equations differed, except for ordinary concrete. for a material that possesses the immobilisation function, such as ca2, or a material that influences the pore structures, such as blast furnace cement, it is difficult to appropriately describe the penetration behaviour of chloride ions by fick’s diffusion equations. 3. in the evaluation test using paste samples, the same hydrate was produced using ca2 alone and using the combination of the expansive additive and ca2, and friedel’s salt production when chloride ions were present was also equivalent. the combined use of ca2 and the expansive additive had no influence on the immobilisation ability of the chloride ions. 4. the total and continuous pore volumes of each paste specimen were determined by the mercury intrusion method, and there was no significant difference in the permeation of chloride ions. 5. for the pore network calculated on the basis of the steady-state electrophoresis test, the reduction rate of the pore network was larger when the combination of ca2 and the expansive additive was used compared with ca2 alone. it was inferred that the pore structure changed by introduction of expansion strain. 6. although the reduction rate of the pore network using ca2 was smaller than that of the bb sample, the higher immobilisation rate suggested that the same degree of resistance to salt damage was exhibited, albeit with a small total addition rate. references [1] k. tabara, k. yamamoto, m. ashida, et al. capacity of cemented cement hardened with cao and 2al2o3 261 shinya ito, takeshi iyoda acta polytechnica ctu proceedings to fix chloride ions, cement science and concrete technology 64(1):428-34, 2010. https://doi.org/10.14250/cement.64.428. [2] morioka, w. tawara, k. yamamamoto, et al. diffusion inhibiting effect of chloride ion of cao.2al2o3 and its mechanism, concrete technology series no.89 of civil engineering society, report no.2 of the study subcommittee on physical property change and performance evaluation of concrete using admixture materials (333 committee) report no.2, pp.443-448 (2010). [3] k. tsuji, k. suhara. performance evaluation of expanded concrete, technical journal publishing (2011). [4] ryoshi yoshida, kishiji. separation and extraction of continuous pores and ink bottle pores by stepwise injection of mercury, discoloration of samples by mercury injection, summary of lectures at the annual academic lecture of the japan society of civil engineers, 62:5043, 2007. [5] r. yoshida, r. kishi. study on differences in mercury injection processes in cement pastes with different water cement ratios and curing, report on annual articles of concrete engineering, 29:729-734, 2007. 262 https://doi.org/10.14250/cement.64.428 https://doi.org/10.14311/app.2022.33.0112 acta polytechnica ctu proceedings 33:112–118, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague limitations of life cycle assessment of green concretes a state-of-the-art review amina dacić∗, oliver fenyvesi department of construction materials and technologies, budapest university of technology and economics, müegyetem rkp. 3, 1111 budapest, hungary ∗ corresponding author: amina.dacic@epito.bme.hu abstract. there have been numerous researches focusing on the alternatives to conventional concrete regarding environmental benefits and trade-offs. for this purpose, nowadays, the life cycle assessment (lca) is widely used. until now, lca results of conventional and green concretes have shown some limitations, and this paper focuses on addressing them. firstly, the researches have been mostly based on a small number of concretes, which can limit the possibility to quantify the range of emissions for different set of assumptions. the second one is regarding the appropriate selection of functional unit where a divergence between different researches is visible. moreover, it has been recognised that the criteria involved in the investigation of transportation might lead to the variation of results. also, inventory allocation has to be assessed in detail to reach adequate distribution of the environmental impact of the concrete during its life cycle phases. finally, co2 uptake of the concretes throughout the life cycle should be considered and applied adequately to have representative results of overall co2 emissions. it is expected that the differences will depend on construction practices among the countries where the researches are executed. also, the results will surely vary between different system definitions when lca is applied. keywords: co2 uptake, environmental impact, functional unit, green concrete, inventory allocation, life cycle assessment. 1. introduction sustainable development has got numerous definitions, parkin recognised around 200. this reflects the complexity of the concept and its interpretation as well as possible ways of achieving it. however, it is for sure that it should include economic, environmental, and social dimensions. [1] on the other hand, it has been acknowledged that the structures become sustainable when at least one whole life cycle is assessed and both environmentally beneficial manufacturing processes and deconstruction concepts incorporate the development of circular economy solutions. these solutions are usually based on recycling and reuse of construction and demolition waste (cdw). [2] although the construction industry is one of the most significant for the economy, it carries the burden of one of the most harmful to the environment. the contribution of it to the gross domestic product (gdp) is about 9%, and it is providing 18 million direct jobs through around 3 million enterprises in the european union (eu). on the other hand, the negative effects of the industry are numerous, including dust and gas emissions, land depletion and deterioration, energy and non-renewable natural resources consumption, noise pollution and cdw generation. [3] the construction industry is accountable for the consumption of 50% of natural raw materials, 40% of the energy produced and the generation of 50% of global waste. [4] in countries of organisation for economic co-operation and development (oecd) and eu buildings contribute to about 30% of total primary energy consumption and more than 30% of greenhouse gas (ghg) emissions. [5] kofoworal and ghewaala investigated a typical concrete office building in thailand based on life cycle energy (lce) analysis and concluded that operation phase contributes 81.3% to the building’s lce profile. while the manufacturing of the materials was responsible for 16.8% where steel accounted for 42% and concrete for 35% of the initial embodied energy, respectively. the construction, demolition and maintenance phases values were equal to 0.6%, 0.4% and 0.8%, respectively. however, it was recognised that recycling of building materials can contribute about 9% to energy savings. [6] consequently, sustainable construction has been a focus of research for many decades. in that sense focusing on designing, building, and occupying more sustainable structures would benefit all three beforementioned dimensions of sustainable development. regarding the environmental aspect of sustainable development, it would lead to improvement in air and water quality, reduction in energy and water consumption, as well as in waste disposal. on the other hand, economic benefits would incorporate a decrease in operating and maintenance costs and an increase in revenue due to sale price or rental. finally, in a social sense, the enhancement 112 https://doi.org/10.14311/app.2022.33.0112 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 33/2022 green concretes life cycle assessment of the occupants’ comfort and health, reduction in absenteeism, turnover rate and liabilities would be included. [7] in the case of the construction industry, it has been recognised that sustainable development should be based on a decrease of raw materials, energy and amount of cdw being landfilled. [8] there are significant economic, environmental and social impacts of cdw landfilling. it can reduce the value of the land stock in the area, it can result in the contamination of soil and groundwater due to cdw’s composition and finally put public health at risk. [9] recycling has been recognised as a good practice that includes the reduction of landfilled cdw and the consumption of both energy and raw materials. as a result, recycled aggregate (ra) has been investigated and more widely used in the construction industry. [8] however, all possible alternatives to the conventional concrete should be investigated regarding environmental benefits and trade-offs. for this purpose, nowadays extensively used life cycle assessment (lca) is applied. [10] although until now many methods have been developed focusing on the evaluation of the environmental impact (ei), lca is the most widely used mainly because of its effectiveness to calculate the potential effects that a product, process or service has or may have on the environment during its whole life cycle. [3] 2. green concrete by definition, green concrete is concrete which uses waste material as at least one of its components, or its production process does not lead to environmental destruction, or it has high performance and life cycle sustainability. [11] in the eu one-third of total waste generated is cdw, making it the largest waste stream. [12] cdw may be resulting from construction activities, natural hazards, demolition or rehabilitation works. furthermore, the most substantial amount is corresponding to demolition and rehabilitation works. around 40 to 50% of cdw is made of concrete, asphalt and brick. [9] the most widely used building material in the construction industry is concrete. [13] moreover, it is the second most utilized material after water. in 2009 on a global level, it is estimated that between 13 to 21 billion tons of concrete are produced annually. [4] only in eu in 2018 total concrete produced was about 320 million cubic meters. [14] moreover, concrete plants globally consume 1 billion tonnes of water, 1.5 billion tonnes of cement and 10 billion tonnes of aggregate annually. [9] these numbers are expected to rise in the future as the production of concrete is expected to increase due to the rise in population and urbanisation. by the increase in the production of the concrete, consumption of natural aggregate (na) as concrete’s largest constituent raises accordingly as well. in the countries of eu annual production of aggregate is equal to three billion tons. [13] out of this value, about 45% is used in concrete. [3] hence, the challenge of the availability of na’s sources has been present, which resulted in the placing of the taxes on the use of na in the case of many european countries. [13] the recycling rate of cdw in eu is on average 83%. [15] there is a significant difference regarding this value among countries since some of the countries have established this practice for years and others have still a very low rate. [16] worldwide countries have adopted the strategy of management of cdw expressed by "3r" (reduction, recycling and reuse). moreover, the eu proposed that the best practice would be for preventing the production of waste. possible solutions to the reduction of cdw production are improving materials which are energy and environmentally efficient, usage of high-performance materials (resulting in the reduction of the amount of material used) and enhancing the durability of buildings (resulting in the increase of the life span of the raw material used). [2] on the other hand, the percentage corresponding to the cdw compared to the total utilisation of aggregate differs between countries, but there is no case where it can completely replace the use of nas. [16] cdw is recognised as one of the most significant solid waste streams and recycling of it into useful components of concrete is seen as an important part of sustainable development. however, there has been extensive research on the inclusion of different types of industrial waste into green concretes either as an aggregate or a binder. the precondition to the usage of such materials is their environmental acceptability and technical adequacy. [10] recycling of cdw can have benefits in reducing the amounts of it disposed to landfills and the preservation of natural resources. this potential can be enhanced if the ra is not just applied to lower quality products but also as a component of structural concrete. even though the research has been extensive in the area of ra, its usage for higher quality products has not been widely recognised. due to some national practices, ra is used for granular base or sub-base applications, embankment construction and earth construction works. [13] moreover, in the researches made, it was shown that na could be successfully substituted by ras in producing concrete that fulfils the requirements for structural use. on the other hand, the application for structural purposes has been recognised as feasible in the both commercial and technical sense. [17] waste recycling has been largely developed for about three decades now. as a result of this activity, many regulations have been established. as an example, the eu directive defines some conditions that waste must fulfil to be considered as a by-product. these conditions are: • further usage of the object or substance is definite; • the object or a substance is generated as an integral part of a production process; 113 amina dacić, oliver fenyvesi acta polytechnica ctu proceedings • the object or substance is usable excluding any further processing other than usual industrial practice; • further usage is legitimate meaning that it fulfils all necessary environmental, health and product requirements for that specific use and would not cause overall negative environmental or human health effects. [18] moreover, the design and process of recycling should take into account the saving of raw materials and energy and decreasing the potential of environmental pollution. consequently, recycling and reuse of cdw offer an important opportunity for sustainable development in the construction industry. [2] 3. life cycle assessment back in the late 1960s and early 1970s, environmental issues were addressed extensively, such as resource and energy efficiency, pollution, and solid waste. during that period, studies on ei of consumer products also started which were mainly based on simple principles to be able to compare two products. as a result of this soon enough, it was recognised that the biggest ei is not connected to the products’ use but rather to its production, transportation, or disposal. while these studies were evolving, there was a need for standardisation of the assessment tool, which would be widely applied. finally, the international organization for standardization (iso) published iso 14040 (2006) and iso 14044 (2006) standards. based on these standards, the methodology consists of four main steps which are the definition of a goal and scope, the inventory analysis, the impact assessment and the interpretation [3]. lca is a methodology used for evaluation of the environmental burden of the processes, products or services throughout their life cycle. [13] in the case of green concretes, lca can be a useful tool for generating indicators regarding ei in source separation, waste treatment and recycling technologies. however, it was recognised that aggregates’ recycling differs between developing and developed countries in terms of production and transportation methods. moreover, resources availability can vary among countries. for example, in the case of china, the shortage of resources has become a serious issue currently, and the possibility of alternatives to conventional concrete is of significant importance. furthermore, source locations are mainly on a longer distance from urban areas which then puts a greater environmental burden on conventional concrete [8]. on the other hand, many european countries have been employing regulations to answer to the decreasing availability of the na resources such as placing taxes on the use [13]. as well as focusing on recovery of cdw by implementing the waste framework directive 2008/98/ec aiming at 70% recycling rate by 2020 [12]. 3.1. quantity of concretes one of the limitations that have been acknowledged considering the lca of conventional and green concretes is that the researches are based on a small number of concretes. the range varies mainly between 3 to 18 mix designs. [4] moreover, the researchers mainly focused on the certain green solution of concretes, generally considering recycled concrete aggregate substitution to nas. it has been concluded that overall there are five types of green concrete on which the researches have been focused: 1. recycled aggregate concrete (rac) for which as aggregates, ras generated from cdw are used; 2. blended cement concrete in ordinary portland cement (opc) in the binder is replaced with a variety of supplementary cementitious materials; 3. alkali activated concrete where no opc is used but materials which are rich in alumina and/or reactive silica; 4. high performance concrete which is considered as a concrete of improved mechanical and durability properties compared to one generated using opc; 5. bio-concrete which is produced mainly using various bacteria. [19] these five types of concrete can have a beneficial behaviour in ei sense compared to traditional concrete. furthermore, most of the researchers have used the conventional mixture design method in case of rac, which is based on replacing a certain percentage of na with ra based on equivalent volume or mass [20]. in advance, it has been noticed that the amount of cement is increased in rac mixes to achieve those properties of natural aggregate concrete (nac) [21]. eventually, this could lead to more co2 emissions. this is why researchers worked on developing some alternative mix design methods that could lead to improved performance of racs and better and fairer comparison to nac. the alternative to the classical approach of mixture design in the case of the rac, equivalent volume mortar method (emv) has been proposed. this method is based on the fact that rac is two-phase material including na and mortar, so it seeks to use the same amount of total mortar volume in mixtures of both nac and rac [22]. other than mixture design methods, the development of the different mixing methods mainly based on the sequential mixing approach in order to improve properties of rac have been introduced [23]. furthermore, there have been just a few researches regarding the effect of mixing methods on lca of rac, which can lead to possible better performance of green concretes on both material and lca level. 3.2. functional unit another limitation, the functional unit (fu) is used as a key factor influencing lca. fu is a quantified 114 vol. 33/2022 green concretes life cycle assessment performance of a product system for use as a reference unit. [24] as the definition implies, it is an important factor for comparison of lca results for investigating alternative solutions for a certain product. that applies to the production of concrete also. so, many researchers based their lca of green concretes on the functional units (fus). however, the selection of fus differs, including volume of concrete, combining two or more variables such as volume and strength; volume, strength, durability; and strength reliability. the biggest disadvantage of having different fus in the conducted researchers is that these results can not be compared. [25] by many researchers volume of concrete as a fu was found as not reliable since it does not take into concern that for 1 m3 nac and rac compressive strength results are not equivalent. on the other hand, fu per strength, which combines both volume and strength, neglects the durability properties of the concrete. [26] however, it has been recognised that fu has to incorporate the workability, strength and durability/service life differences between investigated concretes. [3] 3.3. transportation non-renewable energy consumption and global warming potential (gwp) as ei in the case of concrete significantly depend on the transportation scenarios. [27] ding, xia and tam showed in their study based on data in china that aside from cement proportion, transportation is the top contributor to the co2 emissions and energy consumption for both conventional and rac. in the conducted sensitivity analysis, it was concluded that longer delivery distances for na lead to the possibility of decreasing ei of rac. so, by increasing the replacement ratio of na by ra, there is a possibility of reducing the negative ei of concrete. [8] on the other hand, marinković et al. based their lca in serbia, and the results showed that the total ei of conventional and rac depend on the na and ra transport distances and types. since the ei of the aggregate and cement production phase gave slightly disadvantageous results for rac compared to conventional concrete in their research, the transportation phase was recognised as a possible source for reducing the ei of rac. [13] knoeri, sanye-mengual and althaus examined rac mixtures according to laws, standards, and construction practices in switzerland. they concluded that the additional transportation distances more than 15 km result in higher ei for rac than conventional concrete. based on the present market mixtures in switzerland, the beneficial results can be expected for rac compared to conventional one if supplementary cement and transportation are kept limited. [28] 3.4. inventory allocation life cycle inventory (lci) corresponds to the collection of data, all inputs and outputs of the considered life cycle phases. [9] in green concretes, the research should adequately address inventory allocation for the recycled and/or reused products. for ra, this allocation is important to be suitably distributed between waste management and material production. [4] in general, for the inventory analysis for lca information can be collected from industries involved usually by questionnaires, publicly available annual environmental reports (ers), and environmental product declarations (epds), lca related journals and lca databases (e.g. ecoinvent, gabi). the most precise data can be obtained from questionaries. however, generally all the data can not be collected this way, so alternative beforementioned sources are also used. [3] when it comes to waste recycling, the main issue of the lca is the allocation process. in the present practice to the waste production, no environmental burdens are assigned, if we disregard waste disposal, because it is not generated on purpose. but when we consider waste as a by-product, it has been recognised that some allocation coefficient has to be applied. however, there is no consensus on the correct allocation procedure in this case in the previous researchers. so, it rather relies on the choice of the researcher, and since it can have an important effect on the results, it is one of the most debatable issues in lca of green concretes. according to the abovementioned iso standards when there is a possibility of several allocation procedures, it is recommended to conduct a sensitivity analysis demonstrating the impact of different choices. [18] 3.5. co2 uptake to evaluate the portion of ghg that concrete is responsible for, it has been acknowledged that it is needed to adequately cover the net balance of ghg in the use of concrete as a construction material. this implies that co2 uptake throughout the whole product life cycle shall also be considered. so, as the emissions are emerging by the concrete production, there is a parallel co2 uptake process going on in the already produced concrete. however, when the uptake is considered, there is one corresponding to primary concrete product (i.e. concrete used in the construction of the structures) and the secondary resulting from demolishing and crushing the primary one. this secondary product can be landfilled, used as a road base material or ra in green concretes. [29] if the co2 uptake is not considered in the lca of concrete, it can lead to the overestimation of the emissions. co2 is absorbed during the carbonation of the concrete during both the primary and secondary product phase. [4] collins proposed the formula for the calculation of co2 uptake from the atmosphere: co2uptake = x · copc · cao · r · a · m (1) where x corresponds to calculated depth of carbonation equation (2), copc is amount of opc in binder and cao refers to calcium oxide content of 115 amina dacić, oliver fenyvesi acta polytechnica ctu proceedings opc. while r is a fraction of cao in fully carbonated opc, a denotes surface area of the exposed concrete and chemical molar proportion of co2/cao is assigned by m . moreover, carbonation depth can be calculated using the following formula: x = k √ t (2) where k is carbonation rate coefficient which covers the chemical and physical characteristics of the concrete and environment (can be determined experimentally or measured on the structure) and t is exposure time. [30] from the above-presented equations, it can be concluded that the co2 uptake is dependent on the properties of the concrete such as cement content, co2 in the atmosphere which relies on the exposure condition and diffusion process between the atmosphere and the concrete effected by the exposed surface. [4] full carbonation of the concrete, hence, can lead to a substantial reduction in the gwp. however, this complete carbonation does not seem like a realistic outcome, since by researchers 75 % has been concluded as a maximum value. [31] moreover, just 11 % of theoretical maximum co2 uptake has been recognised as possible during the primary production phase. [29] however, when concrete is considered as a part of the cdw, demolishing and crushing results in the smaller particle size henceforth larger exposed surface area. [31] further investigation has to be conducted regarding the co2 uptake for green concretes, especially when ra is an ingredient of it. 4. conclusions lca is the standardised tool for determining ei of a product or process, and it has been the most widely used one for this purpose. some limitations have been recognised when it comes to its applicability to the evaluation of concrete’s performance. this is especially the case when alternative solutions to conventional concrete are assessed. although the research in this field has been present for some decades now, there are some parts of the lca of the concrete where consensus between researchers has not been made. consequently, the large-scale assessment of the ei of concretes, especially green concretes is scarce. in further researches, special attention should be paid to incorporating a larger quantity of investigated concretes. it is essential to investigate different mix design methods, especially since they may offer better performance in both material and lca sense for green concretes. on the other hand, one of the most critical questions when concretes’ lca is conducted is fu. the variety of fus used in research leads to the limited possibility of comparing the results of the different studies. this is one of the main issues on which consensus should be made to have a more precise picture about ei of concrete. as expected, transportation takes a significant portion of the gwp of concrete. moreover, after the cement production phase, it is the most influential factor. the results showed that by reducing the transportation distance for ra compared to na beneficial results can be conducted in the case of green concretes. in lca of any product or process, inventory allocation is a very important phase. it can lead to significantly overor underestimated results if not adequately conducted. moreover, in the case of green concretes, special attention has to be paid to the usage of materials as by-products. finally, to have representative results of ghg emissions of the concrete adequate net balance calculation has to be made. in that sense, co2 uptake takes a significant role. it is of great importance to consider both primary and secondary (one emerging after demolishing and crushing of the first one) product’s co2 uptake. co2 is absorbed during the carbonation of concrete as both primary and secondary product. if this net balance is not appropriately calculated, the results lead to overestimating ghg emissions of concrete. to sum up, lca of alternatives to conventional concretes require more detailed evaluation to have representative results. further investigations for sure have to be conducted, which will focus on the abovementioned limitations for improving the lca tool for the evaluation of concrete. references [1] s. parkin. sustainable development: the concept and the practical challenge. proceedings of the institution of civil engineers civil engineering 138(6):3-8, 2000. https://doi.org/10.1680/cien.2000.138.6.3. 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https://doi.org/10.1061/(asce)0899-1561(2009)21:10(601) https://doi.org/10.1016/j.conbuildmat.2020.121490 https://doi.org/10.1016/j.conbuildmat.2019.03.078 https://doi.org/10.1016/j.cemconcomp.2012.01.004 https://doi.org/10.1016/j.jobe.2020.101173 https://doi.org/10.1007/s11367-012-0544-2 https://doi.org/10.1021/es401775w https://doi.org/10.1007/s11367-010-0191-4 amina dacić, oliver fenyvesi acta polytechnica ctu proceedings [31] s. butera, t. h. christensen, t. f. astrup. life cycle assessment of construction and demolition waste management. waste management 44:196-205, 2015. https://doi.org/10.1016/j.wasman.2015.07.011. 118 https://doi.org/10.1016/j.wasman.2015.07.011 acta polytechnica doi:10.14311/app.2020.27.0037 acta polytechnica 27(0):37–41, 2020 © czech technical university in prague, 2020 available online at http://ojs.cvut.cz/ojs/index.php/app dynamic impact wear and impact resistance of w-b-c coatings josef daniela,∗, jan grossman a, vilma buršíková b, lukáš zábranský b, pavel součekb, saeed mirzaeib, petr vašinab a institute of scientific instruments of the czech academy of sciences, královopolská 147, brno 612 64, czech republic b masaryk university, faculty of science, department of physical electronics, kotlářská 2, brno 611 37, czech republic ∗ corresponding author: jdaniel@isibrno.cz abstract. coated components used in industry are often exposed to repetitive dynamic impact load. the dynamic impact test is a suitable method for the study of thin protective coatings under such conditions. aim of this paper is to describe the method of dynamic impact testing and the novel concepts of evaluation of the impact test results, such as the impact resistance and the impact deformation rate. all of the presented results were obtained by testing two w-b-c coatings with different c/w ratio. different impact test results are discussed with respect to the coatings microstructure, the chemical and phase composition, and the mechanical properties. it is shown that coating adhesion to the hss substrate played a crucial role in the coatings’ impact lifetime. keywords: boron carbide, fracture resistance, impact resistance, impact wear, w-b-c. 1. introduction industrially used components coated with protective coatings are often exposed to the external load. in some cases, this load may take the form of repeated dynamic impact. for the investigation of the lifetime of the coated components under this form of load, knotek et al. developed the method of dynamic impact test in the early 90s [1]. a tungsten carbide ball repeatedly impacted the surface of the specimen. the load force of the ball was well-defined, and the impact frequency was set to a constant value. coatings’ lifetime was evaluated as a maximum number of impacts before coating failure. knotek also showed that dynamic impact test makes a study of dynamic wear and fatigue behaviour of the whole coating/substrate system possible [1]. in the past decades, the dynamic impact testing was investigated by different researchers. bouzakis et. al. developed fem simulation of the elastoplastic deformations of material under dynamic impact load [2] or the inclined dynamic impact test testing of the coated specimens under various angles of inclination [3]. engel et. al. proposed a three-stage model of the dependence of the impact crater volume on the total number of impacts based on the coating deformation [4]. voevodin et. al. investigated the impact lifetime of various ti-based coatings and discussed it with respect to their chemical composition and hardness [5]. heinke et. al. used the dynamic impact test as a supplementary method to the evaluation of coatings adhesion [6]. sobota et. al. investigated the behaviour of dlc coatings under dynamic load in various air humidity [7]. in the past years, the dynamic impact testing also expanded to the field of the thick hvof sprayed coatings [8]. the aim of this paper is to present some crucial results of dynamic impact testing, important for the investigation of the behavior of the coating under dynamic impact load and for development of the new types of the protective coatings. novel parameters of impact resistance and impact deformation rate, suitable for better understanding of the behaviour of the coating under repeated impact load, were discussed. presented results were obtained from two w-b-c coatings with different chemical composition, structure, and mechanical properties. 2. experimental 2.1. dynamic impact tester the coatings were tested by a dynamic impact tester developed at the institute of scientific instruments of the czech academy of sciences in brno. the impact hammer with a lift set at a constant value of 2.4 mm was electromagnetically driven. attached to the impact hammer was a ball-shaped cemented tungsten carbide impact indenter with a diameter of 5 mm. the indenter ball was rotated to an unworn contact side before every test. the frequency of impact testing was set to 8 hz, and the duration of the contact of the ball indenter with the surface was 1.5 ms. the impact testing was carried out with impact loads of 200 n, 400 n and 600 n. due to its construction the impact tester is uniquely suitable also for the investigation of the behaviour of the samples under a low number of impacts (≤ 100). therefore, the number of impacts was in the range from 1 to 500 000. every test was repeated three times. the depth, 37 http://dx.doi.org/10.14311/app.2020.27.0037 http://ojs.cvut.cz/ojs/index.php/app j. daniel, j. grossman, v. buršíková et al. acta polytechnica c/w w (at. %) c (at. %) b (at. %) h (gpa) e (gpa) h/e h3/e2 (gpa) thick.(µm) 0.11 63 7 30 28.6 ± 1.5 500 ± 12 0.057 0.094 1.7 0.8 40 32 28 23.3 ± 1.0 400 ± 12 0.058 0.079 1.3 table 1. chemical composition and mechanical properties of the w-b-c coatings. figure 1. loading curves of the w-b-c coatings. dotted lines were added as a visual guide. the radius and the surface morphology of the impact craters were measured by talystep profilometer and lext ols 3100 confocal microscope. 2.2. w-b-c coatings preparation two w-b-c coatings with different c/w ratios were deposited on hss substrate by magnetron cosputtering of w, c and b4c targets in ar atmosphere. for the detailed description of the coatings’ deposition, see ref. [9]. 3. results 3.1. w-b-c coatings composition, structure and mechanical properties the chemical composition and the mechanical properties of the presented w-b-c coatings are summarized in table 1. both coatings exhibited an amorphous microstructure. the coating with a higher amount of carbon (c/w = 0.80) exhibited a higher amount of c-c bonds and b-c bonds and a lower amount of w-b bonds. the detailed description of the chemical composition and the microstructure analyses, the evaluation of the mechanical properties and the discussion of the relationship between the microstructure and the mechanical properties can be found in ref. [9]. 3.2. results of impact testing one of the first results of the dynamic impact test is a loading curve the dependence of the impact crater volume on the total number of impacts. the loading curves of the studied w-b-c coatings are depicted in figure 1. the amount of energy supplied from the tester to the tested specimen increased with the increasing number of impacts. this energy caused the deformation of the specimen and thus the formation of the impact crater. as the number of impacts increased further, the supplied energy was dissipated in the form of a continuously increasing internal stress. after reaching some critical number of impacts nc , the tested coating/substrate system was unable to receive more energy from the tester, and the coating failed. the depth and the radius and thus the volume of the impact crater then rapidly increased. these different behaviour modes of the substrate/coating system can be seen on the loading curve as different slopes and thus it is often possible to determine the nc and therefore the lifetime of the coating under dynamic impact load. the impact craters in the w-b-c coatings formed after 10 000 impacts are depicted in figure 2. the coating with the c/w ratio of 0.11 exhibited impact craters with a higher diameter than the coating with the c/w ratio of 0.80. moreover, in case of the load of 400 n, this coating exhibited an almost uncovered substrate indicating the first stage of the coating failure. the total coating failure and the uncovered substrate in case of the load of 600 n for the coating with the c/w ratio of 0.11 corresponded to a rapid increase of the slope of the loading curve of this coating depicted in figure 1. while the coating with the low c/w ratio exhibited pronounced chipping cracks in the case of the loads of 400 n and 600 n, the coating with the higher c/w ratio only exhibited small concentered cracks even in case of the highest load of 600 n. the detailed view on the edges of impact craters created with load of 400 n is depicted in figure 3. another parameter describing the behaviour of a coated specimen under the dynamic load is the impact resistance. the impact resistance is defined as follows. the volume of the impact crater created by n1 impacts is denoted as v1 and the volume of the impact crater created by n2, impacts is denoted as v2. an increment of the impact crater volume ∆v = v2 − v1. this 38 vol. 27/2020 dynamic impact wear and impact resistance of w-b-c coatings figure 2. comparison of impact craters created by 10 000 impacts. figure 3. detail view on the edges of impact craters created with load of 400 n and 10 000 impacts. increment depends on the number of impacts n1 and n2 [4]. assuming that the increment of ∆v is inversely related to the impact resistance of a coating, the impact resistance can be calculated from the volume of impact craters at a given number of impacts. this parameter with the unit of [ n/mm3] describes how many impacts (denoted as n ) are required to create a crater with the volume of 1mm3. the impact resistance of the w-b-c coatings is shown in figure 4. at first, the impact resistance increased with the increasing crater volume due to the elastoplastic deformation of coatings. as the number of impacts increased further, the energy from the tester was dissipated in the form of internal stress and not in the form of the crater deformation. the impact resistance thus reached the highest values. then, as the coating failed, the volume of the impact crater and also the increment of the volume ∆v rapidly increased, and the impact resistance correspondingly rapidly decreased. therefore, the critical number of impacts nc can also be evaluated using the impact resistance. the third way of evaluation of the critical number of impacts is from the determination of the deformation rate. assuming that contact time between the impactor ball and the specimen is the same for every impact, it is possible to calculate the deformation rate as the impact crater volume divided by the total contact time. the deformation rate of the w-b-c coatings is depicted in a log-log scale in figure 5. the deformation rate linearly decreased as the number of impacts increased. after reaching the critical number of impacts, the impact crater volume rapidly increased and thus deformation rate curve deviated from linearity. the final critical number of impacts was evaluated from the combination of the previously described methods from the loading curves, the optical investigation of impact craters (uncovered substrate as evidence of the coating failure), the impact resistance and the deformation rates. figure 6 shows the woehler-like dependence of the critical number of impacts on the load. the coating with the c/w ratio of 0.80 exhibited higher lifetime under dynamic impact at all of the applied loads. 4. discussion in general, the behaviour of coatings under impact load is closely related to the coating’s toughness [2, 5]. both w-b-c coatings exhibited an amorphous structure and the same h/e ratio that is related to the fracture resistance [9]. the coating with the c/w ratio of 0.11 exhibited higher hardness and elastic 39 j. daniel, j. grossman, v. buršíková et al. acta polytechnica figure 4. impact resistance of the w-b-c coatings. dotted lines were added as a visual guide. figure 5. deformation rate of the w-b-c coatings. figure 6. woehler-like curves of the w-b-c coatings. dotted lines were added as a visual guide. modulus and therefore, a higher h3/e2 ratio that is related to resistance to plastic deformation and the coating toughness [10]. however, better impact resistance was observed in the case of the coating with the c/w ratio of 0.80. the main reason was different adhesion of the coatings to the hss substrate. as was shown in figure 3, the impact crater on the coating with the c/w ratio of 0.11 created after 10 000 impacts with the load of 400 n exhibited cracking on its edge, while the impact crater on the coating with the c/w ratio of 0.80 created after the same number of impacts at the same load exhibited an uninterrupted coated edge. thus, the adhesion of the coating with the c/w ratio of 0.80 was higher than the adhesion of the coating with the c/w ratio of 0.11. as the h/e ratio was the same for both coatings, the adhesion of the studied coatings was the main parameters leading to better impact resistance. 5. conclusions two w-b-c coatings with 30 at. % of boron and different c/w ratios were deposited on hss substrates and analyzed using the dynamic impact test. the impact loads of 200 n, 400 n and 600 n were used. the critical number of impacts determining coating failure was estimated using the loading curves, optical investigations and two novel parameters – impact resistance and deformation rate. the higher adhesion and of the coating to the hss substrate led to the higher lifetime of the coating under repeated impact load. acknowledgements this work was performed under the support of the projects lo1212 and lo1411 funded by ministry of education, youth, and sports of the czech republic, and project ga19-03899s funded by the czech science foundation. references [1] o. knotek. a new technique for testing the impact load of thin films: the coating impact test. surface and coatings technology 54-55:102–107, 1992. doi:10.1016/0257-8972(92)90147-3. [2] k.-d. bouzakis, n. vidakis, t. leyendecker, et al. determination of the fatigue properties of multilayer pvd coatings on various substrates, based on the impact test and its fem simulation. thin solid films 308309:315–322, 1997. doi:10.1016/s0040-6090(97)00561-0. [3] k.-d. bouzakis, a. asimakopoulos, n. michailidis, et al. the inclined impact test, an efficient method to characterize coatings’ cohesion and adhesion properties. thin solid films 469-470:254 – 262, 2004. proceedings of the 31st international conference on metallurgical coatings and thin films, doi:10.1016/j.tsf.2004.08.093. [4] p. engel. impact wear of multiplated electrical contacts. wear 181-183:730–742, 1995. doi:10.1016/0043-1648(94)07105-5. [5] a. voevodin, r. bantle, a. matthews. dynamic impact wear of ticxny and ti-dlc composite coatings. wear 185(1-2):151–157, 1995. doi:10.1016/0043-1648(95)06603-9. 40 https://doi.org/10.1016/0257-8972(92)90147-3 https://doi.org/10.1016/s0040-6090(97)00561-0 https://doi.org/10.1016/j.tsf.2004.08.093 https://doi.org/10.1016/0043-1648(94)07105-5 https://doi.org/10.1016/0043-1648(95)06603-9 vol. 27/2020 dynamic impact wear and impact resistance of w-b-c coatings [6] w. heinke, a. leyland, a. matthews, et al. evaluation of pvd nitride coatings, using impact, scratch and rockwell-c adhesion tests. thin solid films 270(12):431–438, 1995. doi:10.1016/0040-6090(95)06934-8. [7] j. sobota, j. grossman, v. bursikova, et al. evaluation of hardness, tribological behaviour and impact load of carbon-based hard composite coatings exposed to the influence of humidity. diamond and related materials 20(4):596–599, 2011. doi:10.1016/j.diamond.2011.01.011. [8] k. bobzin, l. zhao, m. öte, et al. impact wear of an hvof-sprayed cr 3 c 2 -nicr coating. international journal of refractory metals and hard materials 70:191–196, 2018. doi:10.1016/j.ijrmhm.2017.10.011. [9] m. alishahi, s. mirzaei, p. souček, et al. evolution of structure and mechanical properties of hard yet fracture resistant w-b-c coatings with varying c/w ratio. surface and coatings technology 340:103–111, 2018. doi:10.1016/j.surfcoat.2018.02.054. [10] j. musil. hard and superhard nanocomposite coatings. surface and coatings technology 125(13):322–330, 2000. doi:10.1016/s0257-8972(99)00586-1. 41 https://doi.org/10.1016/0040-6090(95)06934-8 https://doi.org/10.1016/j.diamond.2011.01.011 https://doi.org/10.1016/j.ijrmhm.2017.10.011 https://doi.org/10.1016/j.surfcoat.2018.02.054 https://doi.org/10.1016/s0257-8972(99)00586-1 acta polytechnica 27(0):37–41, 2020 1 introduction 2 experimental 2.1 dynamic impact tester 2.2 w-b-c coatings preparation 3 results 3.1 w-b-c coatings composition, structure and mechanical properties 3.2 results of impact testing 4 discussion 5 conclusions acknowledgements references https://doi.org/10.14311/app.2022.33.0250 acta polytechnica ctu proceedings 33:250–255, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague axial behaviour of strengthened circular hollow reinforced concrete column with cfrp partial confinement ruqayyah ismaila, raizal saifulnaz muhammad rashidb, fariz aswan ahmad zakwana,∗, hazrina ahmada, farzad hejazib a school of civil engineering, college of engineering, universiti teknologi mara, cawangan pulau pinang, kampus permatang pauh, 13500 permatang pauh, pulau pinang, malaysia b department of civil engineering, faculty of engineering, universiti putra malaysia, 43400 serdang, selangor, malaysia ∗ corresponding author: fariz838@uitm.edu.my abstract. circular hollow reinforced concrete column will experience deterioration due to many causes such as natural disaster, corrosion, low quality and others. therefore, structural strengthening is required since replacing the deteriorated column is costly. carbon fibre reinforced polymer (cfrp) confinement is one of the common method used for column strengthening. however, being a nonbiodegradable material, the usage of cfrp will contribute to environmental issues. therefore, this study investigates the effect of using partial cfrp confinement in a strengthened circular hollow reinforced concrete column. six specimens of 2 m height with 250 mm and 110 mm for outer and inner diameter were tested. the effectiveness of partial cfrp confinement is analyzed by investigating the tested specimens’ axial and transverse displacement and strain behaviour. this study has shown that with cfrp partial confinement the strength is increased up to 44% from the unconfined circular hollow reinforced concrete column. the results of the study showed that, circular hollow reinforced concrete column with cfrp partial confinement able to enhanced the column load carrying capacity. keywords: cfrp, frp, hollow column, partial confinement. 1. introduction concrete is always known as one of the most reliable materials used in the construction industry. it was always acknowledged as a minimum maintenance material where extensive damage can always be repaired and strengthened. however, many other factors could cause the concrete structure to deteriorate during its service years. circular hollow reinforced concrete column (chrcc) is commonly used as a tower, concrete chimney, large bridge columns and piles, and offshore platforms structures. these structures’ cost could be minimized sufficiently, as the section area and the self-weight is reduced. it significantly increases the sectional moment-of-inertia with an enormous depth and concentrated flanges than regular solid sections with similar areas [1]. to date, cfrp is a well-known strengthening material that has elevated the structural strengthening method tremendously. many experimental works have been carried out resulted from more than 80 concrete frp stress-strain models developed. however, most works focused more on frp stress-strain full confinement but not partial confinement. even though partial cfrp confinement can provide too high strengthening enhancement to the concrete, the number of experimental works is still limited. regardless of all benefits of using cfrp fullconfinement, it does come with several limitations. full cfrp confinement experienced sudden failure explosion without early warning since no initial crack or spalling can be seen. furthermore, when improper full cfrp confinement is installed, it could cause air voids and de-bond between the cfrp and the concrete surface [2]. the use of full cfrp confinement will increase the cost unnecessarily and caused a hazard to the environment due to toxic gasses released during the cfrp manufacturing. it is harmful not only to the environment but also to any people directly in contact with this material either during the manufacturing or installation [3]. therefore, partial crp confinement is seen as a reasonable option compared to full cfrp confinement due to its reduced material used, faster and easy installation, and suitability for a structure requiring moderate strength enhancement [4]. however, there is no proper guideline in designing partial cfrp confinement since experimental and theoretical investigations are still inconclusive [4]. even though several researchers have highlighted the advantages of using partial cfrp confinement ([5–7]) its application is still limited compared to full cfrp confinement. it is due to a limited fundamental understanding concerning the development of stressstrain of cfrp partially confined concrete ([8, 9]). 250 https://doi.org/10.14311/app.2022.33.0250 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 33/2022 axial behaviour of hollow reinforced concrete column parameter nominal value concrete and steel average compressive strength of concrete, fc (mpa) 35.03 yield strength of the longitudinal bars, fy (mpa) 734.3 yield strength of the transverse bars, fys (mpa) 470.0 cfrp composite system type of frp used unidirectional cfrp sheets elastic modulus of cfrp, mpa 226.3 ultimate tensile strength of cfrp, mpa 3193.27 fracture strain 1.1% thickness of each layer, tf 1.0 mm table 1. material properties. not enough experimental work to validate numerical study in developing partial cfrp-confinement stressstrain concrete model is one of the significant factors that are not commonly used today in practice. 2. experimental program testing was carried out on six circular hollow reinforced concrete columns for this research work. all columns were constructed with two series. the first series is referred to the column specimen without cfrp confinement while the second series referred to the specimen with cfrp partial confinement. the column’s height is 2 meter with 250 mm and 114 mm outer and inner diameter using 30 mpa normal concrete strength. the base of each specimen was fixed with 800 × 800 mm square foundation. eight numbers of 12 mm diameter reinforcement bar with 500 mpa strength were used. 6 mm diameter was used for transverse reinforcement. in obtaining strain data to analyze the specimen’s behaviour, 10 mm strain gauges were installed at every 500 mm height of the specimen. 20 mm lvdts were also installed at each 500 mm height to validate the obtained data. details of the specimen are as in figure 1 below. 2.1. material materials used for this research work were all tested to validate with data provided by the supplier. cube testing was carried out for 7, 14 and 28 days after the curing time to verify the concrete strength. all testings were conducted at material laboratory universiti putra malaysia, serdang according to bs en 12390-3-2009. as for steel reinforcement, a tensile test was conducted for three 12 mm diameter steel reinforcement bar. testing was conducted using instron esh at lightweight structural laboratory, universiti teknologi mara, shah alam with all procedure as in bs en 1008:2005. the displacement rate of 0.001 mm/sec is used to control the machine during the testing until the ultimate tensile strength is reached. all materials properties are tabulated in table 2. figure 1. detail of specimen (dimensions in mm). 2.2. specimen preparation six chrcc specimens were prepared at structural laboratory, universiti putra malaysia (upm), malaysia. all specimens were cast using two different stages with the same batch for each stage. the first concrete stage was for the specimen foundation base and the second concrete stage was for the column. after casting, all columns specimens were cured with intermittent water spray every day to ensure hydration process did not affect the concrete strength. detail specimen cast is as figure 1 below. 251 r. ismail, r. s. m. rashid, f. a. a. zakwan et al. acta polytechnica ctu proceedings figure 2. partial cfrp installation; (a) adhesive application on cfrp strip (b) cfrp strip installatin on concrete surface. figure 3. failure behaviour of chrcc before and after concentric loading. 2.3. cfrp installation every column specimen surface was examined and grounded with sandpaper to ensure a clear surface before the cfrp installation is carried out. to ensure there is no dust or unwanted particles before applying primer to the column surface, acetone is used. a layer of primer was first applied to the column specimen. after at least 24 hours the specimen was left to dry, partial cfrp strip with 60 mm width was cut and adhesive mixture (4:1) was then applied on the cfrp strip before being applied on the column surface (figure 2). 2 layers of cfrp sheet was used in this study with the fibres oriented in the hoop direction. the spacing of the cfrp strips is based on study carried out by pham et al. [7]. 2.4. testing set-up all columns specimens were tested at structural laboratory, universiti putra malaysia with consideration of fixe end at the bottom and axial load is applied concentrically on the top part of the chrcc column. testing was carried out with a displacement control configuration with hydraulic jack capacity of 1500 kn. 20 mm lvdts and strain gauges 60 mm, 10 mm and 5 mm were used for concrete, cfrp and steel reinforcement. the location of lvdts and strain gauges, as shown in figure 1. 3. experimental result and discussion 3.1. behaviour of specimens under concentric loading all chrcc specimens were tested until failure. typical failures of all specimens, as shown in figure 3 below. most unconfined column specimens failed due to sudden loss after it was compressed. failure was observed happen at the top of the column. figure 3 shows that unconfined chrcc failed by flexure since the concrete was crushed at the top after the specimens’ internal steel reinforcements yielded. for partial cfrp confinement, the specimen fracture not as severe as the unconfined column specimen. however, specimens with partial cfrp strip is still experienced ruptured at ultimate load due to flexural tension with a delay compared to unconfined specimens. with partial cfrp confinement, initial cracks 252 vol. 33/2022 axial behaviour of hollow reinforced concrete column �e���d�� � � � ��� ��� ��� ��� ��� � � � � � �� �� �� �� �� �� �� /r dg ��n 1 � 9huwlfdo�'lvsodfhphqw��pp� /rdg�yv�9huwlfdo�'lvsodfhphqw� �8qfrqilqhg� &+&�����$ &+&�����$ &+&�����$ � ��� ��� ��� ��� ���� ���� � � �� �� �� �� �� �� �� /r dg ��n 1 � 9huwlfdo�'lvsodfhphqw��pp� /rdg�yv�9huwlfdo�'lvsodfhphqw��3duwldo� &)53� &)�����$ &)�����$ &)�����$ figure 4. load vs vertical displacement; (a) unconfined chrcc (b) partial cfrp chrcc. � � � � � � � � � � � � � � �d� �e�� � ��� ��� ��� ��� ��� ��� ��� ��� � � � � � /r dg ��n 1 � 7udqvyhuvh�'lvsodfhphqw��pp� /rdg�yv�7udqyhuvh�'lvsodfhphqw� �����pp��8qfrqilqhg &+&�����$ &+&�����$ &+&�����$ � ��� ��� ��� ��� ���� ���� � � � � � � � � � � �� /r dg ��n 1 � 7udqvyhuvh�'lvsodfhphqw��pp� /rdg�yv�7udqyhuvh�'lvsodfhphqw� �����pp��3duwldo�&)53 &)�����$ &)�����$ &)�����$ figure 5. load vs tranverse displacement; (a) unconfined chrcc (b) partial cfrp chrcc. �e�� � � � � � � � � � � � � � � �d�� ϭ ϱ ϭϭ ϭϱ ϯϭ ϭ͕ϭϭϭ ϭ͕ϭϭϯ ϭ͕ϭϭϰ ϭ͕ϭϭϲ ϭ͕ϭϭθ ϭ͕ϭϭϭ ϭ͕ϭϭϯ ϭ͕ϭϭϰ ^ƚ ƌğ ɛɛ �;d wă ϳ $[ldo�6wudlq��pp�pp� 6wuhvv�yv�$[ldo�6wudlq�������pp��8qfrqilqhg �,�ͳ;ϭϳͳ� �,�ͳ;ϯϳͳ� �,�ͳ;ϯϳͳ� � � �� �� �� �� �� ����� ����� ����� ����� ����� 6w uh vv ��0 3d � $[ldo�6wudlq��pp�pp� 6wuhvv�yv�$[ldo�6wudlq�������pp��3duwldo� &)53 &)�����$ &)�����$ &)�����$ figure 6. stress vs axial strain; (a) unconfined chrcc (b) partial cfrp chrcc. specimen ultimate load (kn) vertical displacement (mm) transverse displacement (mm) axial strain (mm/mm) transverse strain (mm/mm) cn-0-1 361.90 12.51 2.75 0.007 0.0017 cn-0-2 340.00 3.48 1.40 0.007 0.0122 cn-0-3 420.10 7.05 0.96 0.012 0.004 cf-0-1 1020.62 19.35 3.64 0.020 0.013 cf-0-2 960.04 21.75 5.74 0.016 0.010 cf-0-3 1029.02 17.69 1.25 0.010 0.007 table 2. summary of test result. 253 r. ismail, r. s. m. rashid, f. a. a. zakwan et al. acta polytechnica ctu proceedings can be monitored in between the cfrp strips spacing. 3.2. load-displacement behaviour vertical and transverse displacement of chrcc were measured along with the height at three critical position for every 500 mm. for each unconfined and cfrp partially confined, three specimens were tested to obtain more reliable data. for vertical displacement for unconfined specimens, maximum vertical displacement was 12.61 mm at ultimate load (361.9 kn) for chc-(1)-a. as shown in figure 4 (a), the result is not consistent among the three specimens. chc-(2)-a and chc-(3)-a behaved more linear compared to chc-(1)-a which leads to lower vertical displacement. for cfrp partial confinement specimens, all three specimens behaved similar which it increases non-linearly up to a maximum load before almost linearly reduced until failed. details value of the maximum load for each specimen with respected vertical displacement are presented in table ??. as for transverse displacement, only lvdt at the mid-height of the column specimen is considered for discussion, as shown in figure 5 below. for unconfined specimens, the behaviour of the three specimens was very different from each other. however, it can be observed, two specimens chc-(1)-a and chc(3)-a experienced a similar trend and recorded transverse displacement of 4.24 mm and 6.53 mm respectively. as for cfrp partially confined, all three specimens behaved differently with maximum transverse displacement experienced by cf-(2)-a with 8.95 mm before it is fractured and failed. however, the other two specimens were only experienced 3.64 mm and 1.21 mm, which is lower than the unconfined concrete transverse displacement. by taking the maximum transverse displacement of cf-(2)-a, it can be concluded that cfrp partial confinement sufficiently strengthened chrcc with an increment of 21% from transverse displacement for unconfined chrcc. 3.3. stress-strain behaviour figure 6 below shows the behaviour of stress-strain for unconfined and cfrp partially confined to chrcc specimens. it can be seen that the axial stress was kept increasing until it reached the yield strain of cfrp strips. for unconfined specimens, the maximum axial strain is recorded 0.012 with other two specimens experienced lower axial strain value. cfrp partially confined, cf-(1)-a shows an increment of axial strain up to 0.0202, which is higher than the other two specimens. the other two specimens experienced slightly lower strength and lower axial strain. it is sufficiently proven with cfrp partially confinement; it can increase the strength and the axial strength of chrcc specimens. as for stress vs transverse strain for chrcc unconfined and partially cfrp confined, the trend is increased non-linearly with hardening behaviour for chc-(2)-a. for unconfined chrcc, the transverse strain has reached 0.0122 at 17.74 mpa while for partially confined it reached 0.013 at 26.25 mpa. at 17.74 mpa for partial cfrp confinement, the transverse strain has only reached 0.0045. this has shown that at the strength applied; lower transverse strain occurred as the cfrp strip sufficiently confined chrcc from further expansion (figure ??). summary of the test results is tabulated in table ??. 4. conclusions this research concludes that with cfrp partial confinement, the strength of chrcc can increase up to 44.4%, which is sufficient for construction industry work. it can also be observed that with three specimens tested for every parameter, each parameter’s behaviour was found different. the maximum value was taken as references for every parameter. with data and result tabulated from table 2, the cfrp partial confinement strength enhancement of chrcc specimen can be achieved with lesser cfrp material used and contributing more to the sustainability of construction material. acknowledgements this work fully sponsored by the universiti putra malaysia (upm) research grant under the putra grant initiative (grant reference no: 9439300). the authors would like to express their most profound appreciation to upm for providing all facilities and human resources during the laboratory work phase. full gratitude dedicated to universiti teknologi mara, cawangan pulau pinang, kampus permatang pauh, pulau pinang. references [1] t.-h. kim, h.-t. kang. seismic performance assessment of hollow circular reinforced concrete bridge columns with confinement steel. journal of the earthquake engineering society of korea 16(1):13-25, 2012. https://doi.org/10.5000/eesk.2012.16.1.013. [2] t. w. park, u. j. na, l. chung, et al. compressive behavior of concrete cylinders confined by narrow strips of cfrp with spacing. composites part b: engineering 39(7-8):1093-103, 2008. https: //doi.org/10.1016/j.compositesb.2008.05.002. [3] t. sen, a. paul. confining concrete with sisal and jute frp as alternatives for cfrp and gfrp. international journal of sustainable built environment 4(2):248-64, 2015. https://doi.org/10.1016/j.ijsbe.2015.04.001. [4] j.-j. zeng, y.-c. guo, w.-y. gao, et al. stress-strain behavior of concrete in circular concrete columns partially wrapped with frp strips. composite structures 200:810-28, 2018. https: //doi.org/10.1016/j.compstruct.2018.05.001. [5] l. p. ye, k. zhang, s. h. zhao, et al. experimental study on seismic strengthening of rc columns with wrapped cfrp sheets. construction and building materials 17(6-7):499-506, 2003. https://doi.org/10.1016/s0950-0618(03)00047-3. 254 https://doi.org/10.5000/eesk.2012.16.1.013 https://doi.org/10.1016/j.compositesb.2008.05.002 https://doi.org/10.1016/j.ijsbe.2015.04.001 https://doi.org/10.1016/j.compstruct.2018.05.001 https://doi.org/10.1016/s0950-0618(03)00047-3 vol. 33/2022 axial behaviour of hollow reinforced concrete column [6] a. m. vasumathi, k. rajkumar, g. ganesh prabhu. compressive behaviour of rc column with fibre reinforced concrete confined by cfrp strips. advances in materials science and engineering 2014:110, 2014. https://doi.org/10.1155/2014/601915. [7] t. m. pham, m. n. s. hadi, j. youssef. optimized frp wrapping schemes for circular concrete columns under axial compression. journal of composites for construction 19(6), 2015. https: //doi.org/10.1061/(asce)cc.1943-5614.0000571 . [8] a. d. mai, m. n. sheikh, m. n. s. hadi. investigation on the behaviour of partial wrapping in comparison with full wrapping of square rc columns under different loading conditions. construction and building materials 168:153-68, 2018. https: //doi.org/10.1016/j.conbuildmat.2018.02.003. [9] h. al-nimry and m. jawarneh, effects of increased slenderness in short heat-damaged rc columns confined with frp composites, mater. struct. constr., vol. 50, no. 1, 2017, doi:10.1617/s11527-016-0965-7. 255 https://doi.org/10.1155/2014/601915 https://doi.org/10.1061/(asce)cc.1943-5614.0000571 https://doi.org/10.1016/j.conbuildmat.2018.02.003 10.1617/s11527-016-0965-7 acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.38.0310 acta polytechnica ctu proceedings 38:310–315, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague experimental monitoring of autonomous curtain walling facade module vojtěch zavřela,b,∗, tomáš matuškab, petr slaninac a czech technical university in prague, faculty of mechanical engineering, department of environmental engineering, technická 4, 166 07 prague 6, czech republic b czech technical university in prague, university centre for energy efficient buildings, department of energy systems in buildings, třinecká 1024, 273 43 buštěhrad, czech republic c wieden s.r.o., do čertous 2830/2, 193 00 prague, czech republic ∗ corresponding author: vojtech.zavrel@fs.cvut.cz abstract. this paper introduces an innovative concept for active wall-curtain façade modules aiming at high level of energy autonomy. the façade module consists of two opaque panels and one transparent panel integrating building integrated photovoltaics supported by façade integrated battery peltier air-conditioning unit, active shading, and led lighting. the developed demo-protype was deployed at testing facility and subjected to long-term experimental monitoring to assess the energy performance in real environment. the complex monitoring system captured key power and thermal fluxes, temperatures, mass flows and other operational states of the active wall-curtain façade modules. these measurements were assessed in form of monthly overview depicting total electricity consumption, on-site energy production, heating and cooling delivery as well as self-sufficiency and self-consumption indicators. keywords: active facades, energy autonomy, bipv, in-façade energy systems, peltier air-conditioning. 1. introduction the eu committed in the european green deal to decarbonize the building sector by 2050 [1]. however, according to bpie [2], over 97 % of buildings around eu have to undergo the retrofit to achieve this vision. they also stated that 75 % of the building stock are built before 1990 in outdated envelope standards leading to insufficient energy performance. this situation calls for high-performance façade solutions enabling to achieve nzeb standard for new or retrofitted buildings. the building envelope must cope with daily and seasonal variation in both interior and exterior environment due to the occupancy requirements and given weather conditions. however typical envelope has usually static properties, with lack of flexibility to reflect these variations or harvest the incident solar radiation. this paper introduces compact façade solution embedding multiple energy systems within the wallcurtain façade module aiming at high level of energy autonomy. the current research proposes an innovative active façade solution integrating bipv panels, flat battery, peltier air-conditioning (ac) unit, active shading system and artificial lighting embedded into one direct current (dc) microsystem developed by czech technical university in prague. this dc microsystem has been integrated into prefab curtainwalling module manufactured by wieden s.r.o., the industry partner within the project. the active façade solution may improve the local energy management, where the solar energy is locally utilized for the space-conditioning or other integrated energy systems. the current work represents experimental research, where the research institution and industry partner closely cooperated in the development of a demo-prototype of the active multi-functional façade curtain walling unit. the demo-prototype was deployed on testing facility located at the uceeb, ctu in prague (see in figure 1) and energy behaviour of the façade unit is being monitored and tested in real environment. so far, the current version of the demo-prototype does not represent a marketable product. the presented façade unit is developed mainly for demonstration purposes of the compact in-façade solutions and embedded energy system cooperation. the development is still in progress to reach the marketable level, that the active curtain walling module, complies all required standards. the demonstration façade unit is compounded from three main modules: an opaque module with bipv and integrated flat-plate batteries, an opaque module with bipv and façade integrated peltier ac unit and a transparent module with bipv and active shading device. the technical specification of these panels is provided in sections below. hereunder, the applied technologies are briefly discussed. the bipv technology was utilized as a main source of energy to supply the active façade unit. this technology represents the best-practice building component, that has been already established at the market as a promising solution for building stock decarbonization. for further information, this technology has been already comprehensively reviewed in several publica310 https://doi.org/10.14311/app.2022.38.0310 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 38/2022 experimental monitoring of autonomous curtain . . . figure 1. demonstration façade curtain walling unit deployed at testing facility. tion e.g. recently in [3, 4]. the intermittent operation of the bipv panels is in our concept supported by (i) prioritizing the direct use by the other energy systems within the façade unit and then (ii) storing the surplus energy in the flat-plate battery array integrated in the inner wall of the unit. the battery integration into the facade has not been as heavily researched as bipv. there are only limited literature sources available regarding such integration. e.g. kim et al. [5] reported integration of a solar power bank for user appliances. the reason is likely due to several practical issues related with questionable fire safety as well as high sensitivity of battery performance for operational conditions. these practical issues were neglected in this research in order to arrive to a compact solution demonstrating the potential of the energy autonomous concept. the next integrated system is façade integrated air-conditioning system. the design of façade unit assumes the integration of air-conditioning device into the curtain walling structure despite the fact, that the potential space for the unit is very limited. the conventional compressor systems cannot fit into the façade structure easily and vibro-acoustic properties together with limited possibilities for maintenance of mechanical components finally exclude such solution from design. peltier cells compiled from thermocouples can be regarded as promising alternative for such application. peltier cells with use of electricity absorb heat on cold side of the thermocouple and reject the heat on hot side [6]. such non-mechanical heat pump offers number of advantages such as minimal dimensions, no mechanical parts, no maintenance, no risk of leakage, suitability for direct current supply from pv system and simple control (heating and cooling mode based on electric current polarity). compared to compressor system, lower efficiency and a limited temperature drop is referenced as main disadvantages [7, 8]. peltier technology utilization in building technical systems has been researched in last decade and number of prototypes has been developed. the first peltier system for heating and cooling purpose integrated into building façade has been developed in 2016 [7]. the combination of a façade-integrated peltier airconditioning device with pv system has been reported a year later [9]. 2. autonomous façade curtain walling unit design 2.1. module 1: building-integrated pv with flat battery the section view of module 1 is depicted in figure 2a. the bipv panels are integrated into a standard design of curtain-walling unit structure. the thickness of the curtain walling unit together with ventilated cavity is about 400 mm. the allocated place for the flat battery is within the 25 mm thin cavity at the inner side. pv system consists of 8 polycrystalline panels considering 17 % efficiency with total peak power about 920 wp. the bipv operation is supported by flat battery array accounting 2 sections, each with 8 lifepo4 battery cells. the nominal voltage of each cell is at 3.2 v and capacity 60 ah. in total, the array is designed for maximum voltage 25.6 v and storage capacity of 3.1 kwh. in addition, the led strip is located at the inner side. 311 v. zavřel, t. matuška, p. slanina acta polytechnica ctu proceedings (a). module 1 with integrated flatplate battery. (b). module 2 with glazing and active shading. (c). module 3 with façade integrated peltier ac unit. figure 2. unit design – section view of modules. 2.2. module 2: glazing with external shading system the section view of module 2 is depicted in figure 2b. this module represents transparent structure for natural daylight combined with mechanical shading system in the front of the large-format triple glazing. servomotor for external louvers is powered by dc current. the shading system emplacement is covered by the pv panel. the shading system is controlled automatically by the embedded smart controller with respect to outdoor environment to prevent the high cooling load in summer season or harvest the solar gain in winter. 2.3. module 3: façade integrated peltier air-conditioning unit the section view of module 3 is depicted in figure 2c. this module is specific by the façade integrated air conditioning unit using peltier cells. the spaceconditioning is provided by 20 pcs of high-performance peltier cells arranged in two rows with design cooling and heating capacity 640 w, 1280 w, respectively. the rows can be operated separately in two stage control manner. the nominal electric power load is assumed at 750 w. the heat is absorbed/removed from cell surfaces by finned heat sinks located at both side of the cells. the heat is then transferred to the air circulating in exterior and interior duct channels. the circulation of the air is provided by two variable speed control fans. the highest airflow is considered at 800 m3/h. the entire ac unit fits into cavity with thickness of 165 mm. voltage polarity of peltier cells defines the operation mode of ac unit (heating, cooling). the air from indoor space enters to the interior channel and cooled down is delivered back to the space. the extracted heat is rejected to the ambient environment through the hot side of peltier cells via the exterior air channel. the operation in winter mode is reversed. 2.4. monitoring system the demo-prototype is equipped with complex monitoring system. this system measures main energy flows within the embedded energy systems of the façade unit, outdoor and indoor climate conditions (temperature and relative humidity of the air, solar irradiance). the thermal output of the peltier ac unit has been measured indirectly based on fan speed vs mass flow mapping and inlet-outlet temperature difference measured directly. the mapping was performed as one-at-time measurements using velocity probing. the outcome of this preparatory experiment was correlation curve between fan speed vs mass flow, that was further used in the energy performance assessment. in addition, the monitoring system as well as logged consumption of secondary heating and cooling systems in the testing facility. the secondary systems serve as back-up energy sources in case of limited heating or cooling capacity of the peltier ac unit. the measured variables are listed in table 1. 3. energy performance assessment the demo-prototype was deployed at testing facility in september 2020. the installation was followed by commissioning period to configure the control and monitoring system. during commissioning period, a faulty behaviour of the on-site power system was found. the battery system was being overloaded every time, when the second stage of the peltier ac unit was triggered. the electrical current required by peltier cells exceeded the nominal level of the battery system. 312 vol. 38/2022 experimental monitoring of autonomous curtain . . . metering definition unit metering definition unit peltier cells consumption kwh peltier ac unit inlet – interior °c fans consumption kwh peltier ac unit inlet – exterior °c back-up heating consumption kwh peltier ac unit outlet – interior °c back-up cooling – fan consumption kwh peltier ac unit inlet – exterior °c monitoring system consumption kwh back-up cooling – inlet water temperature °c led lighting consumption kwh back-up cooling – outlet water temperature °c total electricity consumption kwh back-up cooling – water mass flow °c pv production kwh indoor temperature °c battery supply kwh indoor relative humidity % pv surplus metering via electric heater kwh outdoor temperature °c outdoor relative humidity % solar irradiance on façade exterior w/m2 table 1. list of measured variables. year / month missing data total el. consumption pv system production heating supply and coverage factor cooling supply and coverage factor [%] [kwhel] [kwhel] [kwht] [%] [kwht] [%] 10/2020 3 268 nan 16 59 0.4 17 11/2020 0 322 24 24 71 0.0 n/a 12/2020 0 361 15 145 90 0.0 n/a 01/2021 17 352 21 192 95 0.0 n/a 02/2021 21 273 33 135 74 0.0 n/a 03/2021 2 278 43 136 96 0.8 100 04/2021 0 193 45 73 95 0.4 100 05/2021 0 102 49 13 75 0.0 n/a 06/2021 0 166 51 0 0 4.1 15 07/2021 0 48 42 0 55 0.1 100 08/2021 0 59 34 2 70 0.2 100 09/2021 0 44 25 3 80 0.0 n/a table 2. energy performance of the façade module: monthly overview. thus, the presented results were obtained without using second stage of the ac unit. regarding the operational regime, the heating setpoint of the peltier ac unit was set to 21 °c. the cooling setpoint 27 °c to keep the indoor environment of the testing facility within this range. flowrate through interior and exterior channel of peltier ac unit were different for winter and summer operation. the winter flowrate was approx. 600 m3/h (3800 rpm) for interior channel (fan) and approx. 300 m3/h (1900 rpm) for exterior channel (fan). the summer flowrate was approx. 450 m3/h (2800 rpm) for interior channel (fan) and approx. 650 m3/h (4200 rpm) for exterior channel (fan). in this experiment, secondary energy sources were also in operation. the settings aim to switch on the secondary heat sources only if the peltier ac unit reach its maximum capacity. therefore, the heating setpoint was set at 18.5 °c and cooling setpoint was 28.5 °c. the resulted energy performance is depicted as monthly overview in table 2. the table summarizes the total electrical consumption, measured pv system production and heating/cooling supply and associated coverage factor. remark on share of missing data due to lost internet connection in january and february 2021 is also added. the time-step of the monitoring data acquisition was 30 s and data were logged into cloud archive. detailed energy balance has been monitored for period from october 2020 to september 2021. the pv system electricity production was not measured in october 2020 due to fault of the battery system. various operation parameters have been adjusted within the monitoring and evaluation period. in november 2020, the winter flowrates for channels have been finally set. the summer flowrates for channels wre set in the beginning of may 2021. external mechanical shading was shut in june and the back-up system for cooling was switched off. the level of autonomy and energy efficiency was further processed using following energy performance metric. self-sufficiency ss indicator and selfconsumption indicator sc were used to indicate the level of energy autonomy. some literature denotes these indicators also as on-site energy fraction (oef) and on-site energy matching (oem). indicators ss and sc are defined between 0 to 1, where ss indicates 313 v. zavřel, t. matuška, p. slanina acta polytechnica ctu proceedings year / month missing data ss sc scop max. cop seer max. eer [%] [%] [%] [-] [-] [-] [-] 10/2020 3 n/a n/a 0.08 0.99 0.34 0.39 11/2020 0 8 98 0.10 0.46 0.05 0.17 12/2020 0 4 100 0.46 2.16 n/a n/a 01/2021 17 6 100 0.62 3.32 n/a n/a 02/2021 21 12 1.0 0.57 1.13 n/a n/a 03/2021 2 16 99 0.58 1.75 0.17 0.22 04/2021 0 24 99 0.46 1.91 0.20 0.24 05/2021 0 48 97 0.18 0.64 0.00 0.17 06/2021 0 31 99 0.16 0.38 0.05 0.33 07/2021 0 86 90 n/a n/a 0.01 0.11 08/2021 0 56 91 0.08 0.26 0.01 0.11 09/2021 0 57 92 0.09 0.25 n/a n/a table 3. energy autonomy and efficiency: monthly overview. (a). cop factor and heating load. (b). eer factor and cooling load. figure 3. peltier ac unit efficiency vs outdoor air temperature. the portion of power consumption, that is covered by the on-site source and sc indicates the portion of generated energy from the on-site source that is used within the system, rather than exported or dumped. the peltier ac unit provides the heating and cooling in the testing facility. this device also represents the major electricity consumer of the active facade unit. therefore, this study is focused on evaluation of the energy efficiency using cop for heating and eer for cooling. the monthly summary is shown in table 3. this table contains timestamp, percentage of missing data, self-sufficiency and self-consumption indicators and cop and eer factors. cop and eer factors were evaluated in two forms: (a) as ratio of “seasonal” (monthly) sum of thermal energy delivered/removed by the unit vs its electricity consumption accounted in kwh denoted as scop and seer in table 3 and (b) as ratio of nominal heating/cooling load vs its power load accounted in w. maximal values for given months are also depicted in table 3 denoted as max. cop and max. eer. both forms (a) and (b) include fans power. the efficiency of peltier ac unit was further investigated with respect to outdoor air temperature. figure 3 shows a scatter plot, where each dot represents a measured state for which cop or eer was evaluated related to outdoor temperature at the time of the measurements. in addition, each dot was coloured in scale representing the thermal load of the unit at the time of the measurements to give whole information regarding the energy performance of the peltier ac unit. the maximum cop = 3.2 was reached january 2021. the maximum eer = 0.39 was obtained in october 2020 during the full load. the unit in part-load performed at considerably lower efficiency. the seasonal cop is between 0.46 and 0.62 for winter season and the seasonal eer is between 0.11 and 0.33 for summer season. 4. lessons learned after successful laboratory testing of functional samples, the demo-prototype of the façade curtain walling unit was designed, constructed, and deployed at testing-facility located at uceeb (ctu) site, 314 vol. 38/2022 experimental monitoring of autonomous curtain . . . buštěhrad. the energy performance of the façade unit was experimentally evaluated within one year long monitoring campaign to assess the façade curtain walling unit operation in the real environment. hereunder, the presented results are summarized from perspective of: (i) energy autonomy expressed by power selfsufficiency and self-consumption of the testing facility including local pv, (ii) thermal demand coverage by the façade integrated peltier unit, (iii) efficiency in terms of cop and eer of peltier unit (as main power consumer). (a) in terms of energy autonomy, self-sufficiency during summer season and potential for solar assisted cooling has been evaluated. the highest self-sufficiency indicator was reached in july 2021, power load covered by the on-site energy generation was 87 % due to application of external shading. on the other side, high power self-sufficiency indicator in winter season was low, only around 10 % as indicated already in preliminary simulations. (b) the thermal demand coverage was limited due to second stage disallowance in operation of the peltier ac unit. the missing capacity was especially noticeable in cooling mode without shading. as an example, only about 15 % of cooling load in june 2021 was covered by the peltier system. on the other hand, operation within the transition season or with use of external shading resulted in possibility to cover the demand of testing facility space and it reduced the hours of space overheating. although the total capacity of the peltier ac unit was half of originally assumed, the coverage of heating load was high at approximately 80 % in average during the whole winter season (from october 2020 to march 2021). (c) peltier ac unit performed bellow the expectations. based on laboratory measurements, the cop was expected in range of 1.2 to 2 and the eer in range of 0.6 to 1. the unit in the real-life scenario could not reach the expected energy performance. the cop was found between 0.8 to 0.6 for nominal load and eer around between 0.1 to 0.2. the efficiency is worsened due to part-load operation as well as due to higher temperature difference between ambient and interior air temperature. the energy efficiency could be improved by several measures: (i) enabling the full-capacity, (ii) optimization of current-voltage settings of the peltier, (iii) optimization of fan speed. particularly the constant fan speed setting was found inappropriate. as first step, the variable fan speed control needs to be applied to reflect the regime of the unit and ambient temperature. the design of the optimized fan control is the ongoing work. to conclude, this paper introduced demo-prototype of active façade curtain walling unit, that integrates various energy systems. the demo prototype was successfully deployed at testing facility and long-term monitoring campaign was executed. the detailed monitoring system allowed to assess the level of energy autonomy and energy performance of the innovative façade components. acknowledgements experimental monitoring has been supported by the technology agency of czech republic within the project th03020341 autonomous curtain wall panel. references [1] european commission. establishing the framework for achieving climate neutrality and amending regulation (eu) 2018/1999 (european climate law), brussels, 2020. 25 p. [2] buildings performance institute europe. 97 % of buildings in the eu need to be upgraded, 2017. [2021-10-15]. https://www.bpie.eu/wp-content/uploads/2017/12/ state-of-the-building-stock-briefing_dic6.pdf [3] e. biyik, m. araz, a. hepbasli, et al. a key review of building integrated photovoltaic (bipv) systems. engineering science and technology, an international journal 20(3):833–858, 2017. https://doi.org/10.1016/j.jestch.2017.01.009 [4] a. k. shukla, k. sudhakar, p. baredar. recent advancement in bipv product technologies: a review. energy and buildings 140:188–195, 2017. https://doi.org/10.1016/j.enbuild.2017.02.015 [5] a. a. kim, d. a. reed, y. choe, et al. new building cladding system using independent tilted bipv panels with battery storage capability. sustainability 11(20):5546, 2019. https://doi.org/10.3390/su11205546 [6] s. lineykin, s. ben-yaakov. modeling and analysis of thermoelectric modules. in twentieth annual ieee applied power electronics conference and exposition, 2005. apec 2005, vol. 3, pp. 2019–2023. 2005. https://doi.org/10.1109/apec.2005.1453336 [7] m. ibañez-puy, j. bermejo-busto, c. martín-gómez, et al. thermoelectric cooling heating unit performance under real conditions. applied energy 200:303–314, 2017. https://doi.org/10.1016/j.apenergy.2017.05.020 [8] z. liu, l. zhang, g. gong, et al. review of solar thermoelectric cooling technologies for use in zero energy buildings. energy and buildings 102:207–216, 2015. https://doi.org/10.1016/j.enbuild.2015.05.029 [9] k. irshad, k. habib, f. basrawi, b. b. saha. study of a thermoelectric air duct system assisted by photovoltaic wall for space cooling in tropical climate. energy 119:504–522, 2017. https://doi.org/10.1016/j.energy.2016.10.110 315 https://www.bpie.eu/wp-content/uploads/2017/12/state-of-the-building-stock-briefing_dic6.pdf https://www.bpie.eu/wp-content/uploads/2017/12/state-of-the-building-stock-briefing_dic6.pdf https://doi.org/10.1016/j.jestch.2017.01.009 https://doi.org/10.1016/j.enbuild.2017.02.015 https://doi.org/10.3390/su11205546 https://doi.org/10.1109/apec.2005.1453336 https://doi.org/10.1016/j.apenergy.2017.05.020 https://doi.org/10.1016/j.enbuild.2015.05.029 https://doi.org/10.1016/j.energy.2016.10.110 acta polytechnica ctu proceedings 38:310–315, 2022 1 introduction 2 autonomous façade curtain walling unit design 2.1 module 1: building-integrated pv with flat battery 2.2 module 2: glazing with external shading system 2.3 module 3: façade integrated peltier air-conditioning unit 2.4 monitoring system 3 energy performance assessment 4 lessons learned acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.34.0138 acta polytechnica ctu proceedings 34:138–141, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague generation of mesostructure for lattice discrete particle model jan vozába,∗, jan vorela, marco marconb, jan podroužekc, roman wan-wendnerd a czech technical university in prague, faculty of civil engineering, 166 29 prague 6, czech republic b university of natural resources and life sciences (boku), department of civil engineering and natural hazards, christian doppler laboratory licrofast, 1190 vienna, austria c brno university of technology, faculty of civil engineering, 602 00 brno, czech republic d ghent university, laboratory for concrete research, 9052 ghent, belgium ∗ corresponding author: jan.vozab@fsv.cvut.cz abstract. a preliminary study of two approaches for the internal structure generation utilized in the lattice discrete particle models (ldpm) [1] is presented. the presented methods used for particle generation and placement are meant to capture the internal structure of materials realistically. the first approach governs the positioning of the generated spherical particles by a gradient field to mimic the casting process. the second approach considers the non-spherical shape of individual particles, i.e. ellipsoidal particles. therefore, the grain size, angularity, and flakiness can be controlled to match the real grain distribution closely. keywords: internal structure, ldpm, pggf-g, voronoi cell. 1. introduction concrete and particulate polymers are assumed to be homogeneous on larger scales, but they are considered heterogeneous at lower scales [2]. the material internal structure can be viewed as aggregates bonded together by the matrix (cement paste, epoxy, etc.). the aggregates vary in size according to the grain size curve. the failure of these materials is mainly around the grains in the mortar [3, 4]. breakage through the grains occurs mainly in high-strength concretes [1]. in this paper, we put our attention on the discrete particle model, more specifically lattice discrete particle model (ldpm) introduced by cusatis et al. [1]. the ldpm material model response is dependent on particle distribution, and thus multiple simulations are needed to provide credible results [5]. 2. ldpm model to take into account the underlying material structure, we chose the ldpm [1], utilizing the structure created from individual grains and allowing fracture only between these grains. as input for structure generation, we have the specimen’s geometry and the prescribed gradation curve, i.e. maximum and minimum aggregate sizes. the maximum aggregate size defines the higher bound of the sieve curve. in contrast, the minimum aggregate size defines its arbitrary lower cut-off, i.e. the diameter under which no particles are discretely generated and placed. thus, the minimum aggregate size affects the refinement of the discrete mesh and the computational cost. particles on the edges of the specimen are generated first, followed by the generation of particles on the faces. these points are uniformly distributed and have zero diameters. the result of this generation is shown in fig. 1a. next, the particles in the interior of the element are generated. the interior particles are generated before placement, following a grain size curve with a specific diameter until the sum of their volumes is greater than a predetermined value. this value will be high enough to be a good representation of the number of particles in the actual element, but at the same time not too high so that the generation of the structure is not too computationally demanding. in the next step, the particle centres are positioned throughout the specimen volume one by one (from the largest to the smallest). they are placed in a random position, and a possible overlapping is checked. if the overlapping occurs, a new position is generated. in fig. 1b you can see the result of the generation and placement of particles in the element. when all particles are generated, the next step is to calculate their interconnection using delaunay triangulation. this computational method finds the smallest distances between particles, and the result is the fundamental geometric objects in 3d, which are tetrahedrons. these are made up of the nearest four particles. in fig. 1c you can see the resulting connections of the delaunay triangulation. then the formation of individual ldpm cells from the tetrahedrons is employed. each tetrahedron, composed of four vertices, is decomposed into four parts, which belong to individual cells connected through the facets. these facets are the planes connecting centroid, centres of the tetrahedron edges and centres of the tetrahedron faces. the facets’ points on a tetrahedron take into account the diam138 https://doi.org/10.14311/app.2022.34.0138 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 34/2022 generation of mesostructure for lattice discrete particle model eters of connected particles since a more significant part of the tetrahedron belongs to the bigger particle. in fig. 1d the tetrahedron with four particles that represent its vertices is presented. fig. 1e shows a portion of the tetrahedron associated with a single cell. the individual parts of tetrahedrons around the particles are then used to assemble the resulting polyhedral cells (fig. 1g). when all the polyhedral particles are formed, their interaction with each other must be determined through the facets created by the splitting of the tetrahedrons. displacements and rotations of such adjacent particles form the discrete compatibility equations in terms of rigid body kinematics. at each cell facet, the mesoscale constitutive law is formulated such that it simulates cohesive fracture, compaction due to pore collapse, frictional slip and rate effect. for every single particle, equilibrium equations are finally formulated. 3. gradient field positioning the first change we made to the ldpm model was to investigate the effect of particle location on fracture location and development. when a concrete specimen is formed, the distribution of particles can change due to vibration and gravity in such a way that larger particles are more concentrated near one of the surfaces of the specimen [6, 7]. however, this distribution no longer corresponds to the random placement used in the original ldpm model. therefore, we tried to influence the distribution when creating the structure and modified the algorithm of particle placement according to a gradient-based field [8]. therefore, the particles are placed according to the intensity of the field, which leads to the positioning of larger particles along one side of the sample. the results of random and gradient-based particle placement procedures are shown in fig. 2a and fig. 2b, respectively. the blue particles have the largest diameter, and the red particles have the smallest diameter. comparing fig. 2a with fig. 2b, it can be seen that when the particles are placed according to the intensity field, the larger particles are placed mainly on the left side of the sample. the influence of different particle placement is mainly manifested by the moderate change of the crack paths (fig. 3). however, the resistance of the specimen during the loading is influenced minimally by using the current formulation of ldpm. 4. voronoid particle generation the second procedure is utilized to investigate the effect of grain shape on the crack development of the sample. the generation of particles is changed, and their shape is also controlled. more specifically, rotating ellipsoids are employed instead of spherical particles to match the actual shapes of grains. possible approximations of different particle shapes are shown in fig. 4. in the presented paper, the voro++ library [9] is used to generate particles with predefined (a) . points on the edges and faces (b) . interior particles (c) . delaunay triangulation results (d) . tetrahedron with particles as vertices (e) . part of tetrahedron associated with one particle (f) . two nearby particles (g) . specimen with resulting particles figure 1. generation of particles using ldpm. shapes. the library allows the whole tessellations of particles. however, the current voro++ library only allows the generation of voronoi diagrams for spherical particles. creating the voronoi cells for ellipsoidal particles is a complex task since the facets can be curved [10], and the cells are not necessarily convex [11]. this fact is illustrated in fig. 5. note that the effect of 139 j. vozáb, j. vorel, m. marcon et al. acta polytechnica ctu proceedings (a) . random distribution of particles (b) . gradient distribution of particles figure 2. ldpm model, distribution of particles: (a) random distribution of particles; (b) gradient distribution of particles. (a) . first subfigure (b) . second subfigure figure 3. results of the different particle distribution on the fracture propagation. nonspherical particles is out of the scope of this paper and will be published separately. 5. conclusion the initial study of material structure effects on the location and development of cracks in a concrete specimen and similar type of materials is presented in the paper. furthermore, a placement procedure taking into account the prescribed gradient field is described. according to the obtained results, changing the particle placement without affecting the material properties has a negligible effect on the response compared to the independent and random generation of particles. the description of the method, considering the nonspherical shape of particles, is also briefly described. acknowledgements the financial support provided by the gačr grant no. 21-28525s and by the czech technical university in prague within sgs project with the application registered (a) (b) figure 4. concrete grains: (a) grain with higher flakiness index (elliptical); (b) cell with lower flakiness index (spherical). (a) (b) figure 5. generating boundary between particles: (a) circular particles; (b) elliptical particles. 140 vol. 34/2022 generation of mesostructure for lattice discrete particle model under the no. sgs20/155/ohk1/3t/11 is gratefully acknowledged. references [1] g. cusatis, d. pelessone, a. mencarelli. lattice discrete particle model (ldpm) for failure behavior of concrete. i: theory. cement and concrete composites 33(9):881–890, 2011. [2] n. roussel. understanding the rheology of concrete. elsevier, 2011. [3] b. karihaloo, a. carpinteri, m. elices. fracture mechanics of cement mortar and plain concrete. advanced cement based materials 1(2):92–105, 1993. https://doi.org/10.1016/1065-7355(93)90014-f. [4] k. nagai, y. sato, t. ueda. mesoscopic simulation of failure of mortar and concrete by 3d rbsm. journal of advanced concrete technology 3(3):385–402, 2005. [5] m. marcon, j. vorel, k. ninčević, r. wan-wendner. modeling adhesive anchors in a discrete element framework. materials 10(8):917, 2017. [6] y. peng, r. lauten, k. reknes, s. jacobsen. bleeding and sedimentation of cement paste measured by hydrostatic pressure and turbiscan. cement and concrete composites 76:25–38, 2017. https://doi.org/10.1016/j.cemconcomp.2016.11.013. [7] x. gao, j. zhang, y. su. influence of vibration-induced segregation on mechanical property and chloride ion permeability of concrete with variable rheological performance. construction and building materials 194:32–41, 2019. https://doi.org/10.1016/j.conbuildmat.2018.11.019. [8] j. podrouzek, j. vorel, r. wan-wendner. discrete particle placement schemes for ldpm. in emi international conference. engineering mechanics institute (emi), 2017. [9] c. rycroft. voro++: a three-dimensional voronoi cell library in c++. tech. rep., lawrence berkeley national lab.(lbnl), berkeley, ca (united states), 2009. [10] i. z. emiris, e. p. tsigaridas, g. m. tzoumas. the predicates for the voronoi diagram of ellipses. in proceedings of the twenty-second annual symposium on computational geometry, scg ’06, p. 227–236. association for computing machinery, new york, ny, usa, 2006. https://doi.org/10.1145/1137856.1137891. [11] f. m. schaller. the structure of random ellipsoid packings. 2012. 141 https://doi.org/10.1016/1065-7355(93)90014-f https://doi.org/10.1016/j.cemconcomp.2016.11.013 https://doi.org/10.1016/j.conbuildmat.2018.11.019 https://doi.org/10.1145/1137856.1137891 acta polytechnica ctu proceedings 34:138–141, 2022 1 introduction 2 ldpm model 3 gradient field positioning 4 voronoid particle generation 5 conclusion acknowledgements references https://doi.org/10.14311/app.2022.33.0295 acta polytechnica ctu proceedings 33:295–299, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague optimizing the acid resistance of concrete with granulated blast-furnace slag luca-alexander kempfa,∗, rolf breitenbüchera, christian gertenb, andreas ehrenbergb a ruhr university bochum, department of civil and environmental engineering, institute for building materials, universitätsstrasse 150, 44801 bochum, germany b fehs building materials institute, bliersheimer straße 62, 47229 duisburg, germany ∗ corresponding author: luca-alexander.kempf@rub.de abstract. concrete structures exposed to high levels of chemical attacks are assigned to exposure class xa3, which recommends separate concrete protection or a special expert solution to ensure durability. due to the partial substitution of portland cement by blast-furnace slag, an increased resistance to acid attacks could be achieved within the framework of a research project. the technical and ecological advantages of cements containing granulated blast-furnace slag were exploited through chemical, granulometric and concrete technological optimizations. despite extensive parameters, a statistical test design (doe) was able to limit the experimental effort, thus defining principles for the conception of binder systems with increased chemical resistance.mortar prisms indicated that the use of (ultrafine) blast-furnace slags (up to 13,000 cm2/g according to blaine) with a broad particle size distribution can have a positive effect both on the capillary/gel pore ratio and on the calcium hydroxide content in the cement stone. furthermore, the chemical composition of the blast-furnace slag as well as the water-binder ratio are decisive influencing factors for the acid-resistance, which was confirmed in accelerated acid resistance tests on concretes (ph-stat method). after 13 weeks of storing concrete specimens in sulfuric acid (h2so4, ph 3.5), reduced damage depths and lower weight losses were observed compared to conventional binder compositions. the results serve as a basis for the development of highly acid-resistant concretes using blast-furnace slag-containing binder systems. currently, the acid resistance of those concretes is being investigated in a long-term study by outsourcing representative test specimens into the emscher sewer. keywords: blast-furnace slag, concrete, durability, sulfuric acid. 1. introduction concrete structures in agriculture as well as in industrial and wastewater engineering plants may be exposed to various types of chemical attacks. even though concrete is known for its high durability, the material is susceptible to acid attacks [1, 2]. acidic media can diffuse through the porous binder matrix and dissolve it. to increase the acid resistance of the binder, priority is given to reducing the porosity. this is done on the one hand by the choice of a low w/bratio and on the other hand using an optimized grain size distribution up to the finest aggregates. for a complete hydration of cement, a w/b-ratio of about 0.40 is required. the larger amount of water is chemically bound in the calcium silicate hydrates (csh), a minor part remains in the gel pores < 30 nm. with increasing w/b-ratio, more and more capillary pores are formed, causing a connected network of capillary pores at a w/b-ratio of approx. 0.60 [2, 3]. especially the usage of ordinary portland cement (opc) lowers the resistance of concrete towards acidic attack because it releases readily soluble calcium hydroxide (ca(oh)2) and large portlandite crystals while hydration [4]. the ca(oh)2 preferably develops where water is located as well as on the surface of the aggregates [5], which results in a three-dimensional interconnected structure of ca(oh)2 [6]. the substitution of opc by granulated blast-furnace slag (ggbfs) reduces those effects due to a reaction of the ggbfs with the surplus ca(oh)2 forming further csh. in addition, these csh are less soluble because of an increased silicate amount and the porosity of the binder matrix decreases which thus, obstruct harmful media from penetration into the material [5, 6]. the main objective of the research project is to optimize the acid resistance of commonly used concretes while at the same time maintaining good workability with the greatest possible avoidance of costly additives. the potential of ggbfs to strengthen the acid resistance is being evaluated by means of a comprehensive research programme involving a wide range of parameters. 2. research program 2.1. binder optimization due the research program was divided into two parts. in a first step, the binder was optimized on a mortar scale before a series of binder com295 https://doi.org/10.14311/app.2022.33.0295 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en l. kempf, r. breitenbücher, ch. gerten, a. ehrenberg acta polytechnica ctu proceedings figure 1. testing area of the binder compositions. figure 2. schematic sketch of the acid test rig. positions, based on the results of the mortar tests, got examined with coarse grain on a concrete scale. to evaluate a wide range of ggbfs, three granulated blast-furnace slags with different reactivities were considered. the reactivity is defined as the ratio of calcium and magnesium to silicon ((c+m)/s)). before the blast-furnace slag was used to produce concrete, it has been grounded into four degrees of fineness, resulting in two ggbfs (4,200 cm2/g and 7,000 cm2/g) and two fine ggbfs (10,000 cm2/g and 13,000 cm2/g). to investigate the characteristic values for the acid resistance, the w/b-ratio was variated between 0.30 and 0.44 and the percentages of opc and ggbfs got amended. due to the huge quantity of parameters, a statistical design of experiments (doe) with 45 different binder compositions was able to limit the experimental effort, thus defining principles for the conception of binder systems with increased chemical resistance. the testing area of the doe is shown in figure 1. 2.2. acid resistance test setup the acid resistance of the concretes was tested in an acid test rig, which is based on the ph-stat method according to the mpa-berlin-brandenburg testing figure 3. concrete specimen in a storage tank with sulfuric acid. procedure [7]. the test stand consists of a reservoir, which is connected to multiple storage tanks via acid-resistant hoses. the reservoir features a magnetic stirrer as well as a titrator and a ph-electrode, which continuously measures the oxonium ion concentration of the test medium. sulfuric acid (h2so4) has been used as the test medium, which was constantly maintained at a ph value of 3.5 µm 0.05 (automatic endpoint titration). when the ph-value of the sulfuric acid exceeded an equivalence point, concentrated sulfuric acid (h2so4, 2.0 molar) was automatically titrated and distributed by the magnetic stirrer. in order to prevent a high concentration of dissolved substances in the storage tanks, the sulfuric acid was also completely replaced every 14 days. the sulfuric acid was transported via pumps with a flow rate of approx. 15 dm3/h into the storage tanks, which always contained at least 34 litres of acid. via a built-in overflow, the test medium is returned to the reservoir. the overall structure of the acid test rig is shown schematically in figure 2. the test specimens were stored for 91 days in the storage tanks of the acid test rig starting at a preliminary age of 28 days as shown in figure 3. the mass of the specimens was determined every two weeks in the process of the acid exchange. figure 4 shows two mortar specimen after 91 days of storage, one in water and the other one in sulfuric acid with a ph-value of 3.5. 2.3. concrete scale after the binder optimization with mortar prisms, the acid resistance was tested on concrete specimens with optimized binder compositions in sulfuric acid. again, use was made of the doe with a statistical evaluation. for an optimal grain size distribu296 vol. 33/2022 concrete acid resistance optimization figure 4. mortar specimen after 91 days of storage in water (left) and sulfuric acid (right). figure 5. preparation outline of the concrete specimen for acid storage. tion, the fuller principle with a maximum aggregate size of 16 mm was applied to achieve a high packing density [8]. the concrete specimens were cut out of cubes with 150 mm edge length as shown in figure 5. figure 6 shows a cuboid-shaped concrete specimen (150 mm × 100 mm × 50 mm) with a high packing density of the coarse grain after preparation for the acid resistance test. the experimental programme for the acid tests consisted of 14 binder compositions were multiple parameters got changed to each other. the results were evaluated in comparison to reference samples with cem i 42.5 r and cem iii/b 42.5 n-lh/sr/na. to quantify the acid attack, the depth of damage to the specimens was determined three times (after 28, 56 and 91 days) under a stereo microscope during the 91-day acid storage period. to determine the depth of damage, two different specimens were split off. the fresh fracture surfaces were sprayed with an indicator (1% phenolphthalein solution) and evaluated microscopically. the area with a ph value of 0 to 8.2 and above a ph value of 13 remains colourless, while the area between 8.2 and 13 turns pink-violet. figure 7 shows an example of a fresh fracture surface after spraying on the phenolphthalein solution. the depth of damage was determined at each fragfigure 6. concrete specimen with high packing density of the coarse grain. figure 7. position of the microscopic pictures on the fracture surface of an acid-stored test specimen after spraying on the phenolphthalein solution. ment at four equally spaced measuring points, each with 4 individual measurements (see figure 7). the average damage depth was then calculated from two different fragments (thus 32 individual values). figure 8 shows one of those microscopic images of the acid-exposed concrete edge zone of a test specimen after 91 days of acid storage. to establish a reference line for the measurement, the acid-resistant aggregates lying on the concrete surface were used as reference point. 3. results 3.1. binder optimization a statistical evaluation of the test results was used to determine the influencing parameters that significantly affect the respective target values. the specimens were tested for capillary porosity, compressive strength after 7 and 28 days and for their ca(oh)2 content. as shown in figure 9, not only the w/b ratio but also the fineness of the binder (specific surface) decreases the capillary porosity of the specimens. regarding this and the results of the other target values, 15 binder compositions with a high potential 297 l. kempf, r. breitenbücher, ch. gerten, a. ehrenberg acta polytechnica ctu proceedings figure 8. microscopic image of a concrete specimen after 91 days of storage in sulfuric acid. figure 9. impact of w/b ratio and specific surface on the capillary porosity. of an increased acid resistance were chosen for acidtests in order to evaluate the correlation between the binder granulometry and the acid resistance of the mortars. figure 10 shows the location of the optimized binder compositions in the defined testing area. 3.2. concrete tests after the acid resistance tests, mathematical models have been created which allow the target values to be forecast within the selected system boundaries. the models were subsequently verified by separate experiments that were not part of the original experimental design. these models provided the basis to create further optimized binder compositions for the tests with concrete specimen. given the variation of multiple parameters of the test series, the evaluation is carried out by a statistical test assessment. figure 11 shows the output of the analysis by means of a contour chart. the chart consists of the calculated estimated damage depth after 91 days of storage in sulfuric acid, regarding the w/b ratio, the reactivity of the ggbfs and the parameter "n", which represents the rrsb slope according to din 66145 [9] to embody the particle size distribution (psd) of the finest particles. the contour chart shows that all three parameters figure 10. optimized binder compositions for acid resistance tests. have a massive impact on the acid resistance. in order to design an acid-resistant concrete, high reactivity should already be taken into account when choosing the blast-furnace slag. comparing the two diagrams, damage depth under 500 µm can only be achieved with a high (c+m)/s ratio of 1.45. the psd in the finest particle range is also of great importance. the modification from a narrow psd (n = 1.25) to a broad psd (n = 0.95) is about as effective as adjusting the w/b ratio from 0.4 to 0.3. 4. conclusion the main objective of the research project was to achieve the highest possible resistance of mortars or concretes against sulfuric acid attack in combination with sufficient strength development and suitable workability by using (ultra-fine) granulated blast furnace slag in the binder. it has been shown that a high (ultra-fine) granulated blast furnace slag content and a broad psd down to the finest particle size has a favourable effect both on the capillary/gel pore ratio and on the calcium hydroxide content in the hardened cement paste. 298 vol. 33/2022 concrete acid resistance optimization figure 11. influencing factors to the damage depth of the concrete specimen after 91 days of storage in sulfuric acid. the results presented serve as a basis for the design of highly acid-resistant concretes. the optimized concrete compositions developed in this project were able to undercut the damage depths of concretes currently regarded as extremely acid-resistant by more than 50%. it is shown that the choice of material and the preparation of the granulated blast furnace slag play a decisive role when it comes to the acid resistance of concrete. acknowledgements the igf-project 18949 n / 2 of the steel institute vdeh research association was funded by the federal ministry for economic affairs and energy on the basis of a decision by the german bundestag as part of the program to promote industrial collective research (igf). references [1] r. grün.chemische widerstandsfähigkeit von beton, berlin, springer, 1924. [2] j. stark, b. wicht. dauerhaftigkeit von beton von beton vol 2, berlin, springer, 2013. [3] verein deutscher zementwerke e. v. zement-taschenbuch, düsseldorf: verlag bau+technik, 2002. https://www.vdz-online.de/fil eadmin/wissensportal/publikationen/zementindus trie/zement-taschenbuch_2002.pdf. [4] j. kiekbusch. säureangriff auf zementgebundene materialien. berichte aus dem bauwesen, 2007. [5] r. d. hooton. canadian use of ground granulated blast-furnace slag as a supplementary cementing material for enhanced performance of concrete. canadian journal of civil engineering 27(4):754-60, 2000. https://doi.org/10.1139/l00-014. [6] c. rieck, r. hüttl. beton mit erhöhtem säurewiderstand für rohre und schächte-prüung, entwicklung, anwendung. betonw intern (bwi) 3:140-6, 2011. [7] j. gerlach, l. lohaus. sachstandsbericht verfahren zur prüfung des säurewiderstandes von beton vol 620, berlin: beuth, 2016. [8] m. jalal, a. r. pouladkhan, a. a. ramezanianpour, et al. effects of silica nano powder and silica fume on rheology and strength of high strength self compacting concrete. journal of american science 8(4):270-7, 2012. [9] din 66145-4. darstellung von korn-(teilchen-)größenverteilungen rrsb-netz, 1976. 299 https://www.vdz-online.de/fileadmin/wissensportal/publikationen/zementindustrie/zement-taschenbuch_2002.pdf https://doi.org/10.1139/l00-014 my title https://doi.org/10.14311/app.2022.36.0090 acta polytechnica ctu proceedings 36:90–98, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague influence of the structural integrity management on the levelized cost of energy of offshore wind: a parametric sensitivity analysis marko kinnea,∗, muhammad farhana, ronald schneidera, sebastian thönsa,b a bundesanstalt für materialforschung und -prüfung (bam), unter den eichen 87, 12205 berlin, germany b lund university, faculty of engineering, division of structural engineering, department of building & environmental technology, faculty of engineering, box 118, 221 00 lund, sweden ∗ corresponding author: marko.kinne@bam.de abstract. the levelized cost of energy (lcoe) is an important measure to quantify the macro-economic efficiency of an offshore wind farm and to enable a quantitative comparison with other types of energy production. the costs of the structural integrity management which is required to ensure an adequate lifetime reliability of the turbine support structures are part of the operational expenditures of an offshore wind farm. an optimization of the structural integrity management may reduce the operational expenditures and consequently the lcoe. however, the effect of the structural integrity management on the lcoe is hardly known. to investigate this effect, this paper presents a sensitivity analysis of the lcoe of a generic offshore wind farm. the probabilistic models of the parameters influencing the lcoe are based on a literature study including an explicit model for the structural integrity management. the analysis reveals that lcoe may potentially be reduced if an optimization of the structural integrity management enables a service life extension. keywords: levelized cost of energy, sensitivity analysis, structural integrity management. 1. introduction in the context of the energy transition in germany and the european union, the relevance of renewable energy is steadily increasing in contrast to conventional energy sources such as coal-fired power plants. besides hydropower and solar energy, offshore and onshore wind energy is becoming a major part of renewables energies and is likely to be expanded in the future. over the past 20 years, the total number of wind turbines increases from 9400 to 29600 in germany, whereof 1500 wind turbines with a total capacity of 7.7 gw are currently installed offshore [1, 2]. according to the eu strategy presented by the eu commission in 2020, €800 billion will be invested in offshore renewable energy over the next 30 years to increase the capacity of offshore wind energy from 12 gw to 60 gw by 2030 with an aim to reach a total capacity of 300 gw by 2050 [3]. in addition to the investments in the expansion of renewable energy, however, the efficiency of renewable energy sources measured in terms of the levelized cost of energy (lcoe) must also be considered to ensure that these energy sources are competitive. as an example, in 2018 the lcoe of offshore wind energy in germany was still higher than the lcoe of other main energy carrier such as brown coal [4]. besides the reduction of the investment costs and the increase of the power production, wind farm operators optimize structural integrity management (sim) procedures to reduce the lcoe. such optimizations are constrained by different rules and standards (e.g. requirements set out in germany by the bundesamt für seeschifffahrt und hydrographie (bsh) or in united kingdom by the department for business, energy & industrial strategy and the marine management organisation). the challenge is to optimize the operation of an offshore wind farm including the sim in compliance with the governing rules and standards. in the existing literature, the overall costs related to the sim are low in contrast to other costs such as the investment costs and the costs related to the turbine integrity management [5, 6]. based on this information, the question is whether it is worthwhile to optimize the sim for the support structures in an offshore wind farm to reduce the lcoe. in this paper, a sensitivity analysis is performed to quantify the influence of the sim on the lcoe of a generic offshore wind farm and to investigate the opportunity of upgrading/optimizing the sim to reduce the lcoe (i.e., an optimization of the inspection and maintenance strategy as well as monitoring systems). to investigate the influence of the sim on the lcoe, the operational expenditures are divided in a part related to the structures and a part related to the turbines. the lcoe is calculated on the basis of current scientific literature, in which the operational expenditures related to the support structures in an offshore 90 https://doi.org/10.14311/app.2022.36.0090 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 36/2022 levelized cost of energy of offshore wind figure 1. illustration of a global sensitivity analysis, adapted from [5]. wind farm are determined based on the requirements defined by the bsh. to determine the effect of the sim and other influencing parameters, the first order sensitivity index (sobol index) is calculated based on a parametric model of the lcoe. 2. levelized cost of energy the lcoe quantifies the average net costs for generating electricity with a certain type power plant. it is defined as the ratio between (a) the sum of the costs consisting of the capital expenditures (capex) and accumulated operational expenditures (opex) and (b) the sum of the annual energy production (aep) over the service life l (equation 1) [6]. lcoe = capex + l∑ t=1 opex (1 + i)t l∑ t=1 aep (1 + i)t (1) the capex contain the investment costs of an offshore wind farm, while the opex include the costs related to the operating of the support structures and turbines including the costs for monitoring, inspection and maintenance. opex and aep are discounted based on the discount rate i. the aep of an offshore wind farm is computed as the product of the nominal (turbine) capacity nomcap, the nominal capacity availability factor nomcapava, the turbine availability turbavafac, the number of wind turbines nwt and the feed in tariff feedin ((equation 2) [7]. aep = nwt · nomcap · 365.25 · 24 · nomcapava · tubravafac · feedin (2) the capital expenditures are calculated in function of the number of wind turbines nwt in a particular wind farm and the investment costs per wind turbine turbinvest (equation 4) capex = nwt · turbinvest (3) the operational expenditures depend on the number of wind turbines nwt and the operational costs per wind turbine turboperation (equation ??). opex = nwt · turboperation (4) 3. sensitivity analysis a sensitivity analysis provides information on how input parameters x and their uncertainties influence the output y of a deterministic model. a distinction is made between local and global sensitivity analyses [5]. a local sensitivity analysis studies the influence of variability of the input parameters on the model output around a point x0. generally, the influence of small changes in input parameters on the model output is investigated. a global sensitivity analysis determines the influence of the input parameters by varying them over their entire domain. the basic procedure of a global sensitivity analysis is illustrated in figure 1. the input parameters may have the same or different marginal probabilistic distributions. the variance of the model output y depends on the variance of the input parameters and the relations implemented in the deterministic model. in a global sensitivity analysis, the share vi of the variance of model output y that is caused by the input parameter xi is determined. a sensitivity analysis can pursue different goals [8, 9]: • robustness: understanding the model’s robustness with regard to the input parameters • ranking: rank input factors according to their importance • screening: identifying input factors with minor importance, which can be omitted • mapping: identifying the areas of input parameters, which lead to extreme model outputs • decision support: quantify the effect of decision variables on the model output. in this contribution, the sensitivity analysis is performed to rank the parameters influencing the lcoe according to their importance. to this end, the variance-based sensitivity analysis is applied which defines an input parameter’s importance in terms of its contribution to the variance of the model output v ar[y ] = v ar[g(x)]. in this approach, the influence of the uncertainty in an input parameter xi on the variance v ar[y ] is quantified by a first order measure vi (equation 5) [9] vi = v arxi {ex−i [ g (x) |xi]} (5) 91 m. kinne, m. farhan, r. schneider, s. thöns acta polytechnica ctu proceedings wind turbine capexstruc/k€/mw capexturbine/k€/mw reference 4-a-14 677 2021 [10] 4-d-14 861 2131 [10] 8-a-14 689 1997 [10] 8-d-14 722 2180 [10] 6.1-mw fixed-bottom offshore wind turbine 723 2823 [11] generic offshore wind turbine 1065 2891 [12] table 1. capex of different offshore wind turbines divided into a structural and a turbine part. in which ex−i [ g (x) |xi] is the expected value of y = g(x) with respect to all input variables except xi, which is fixed. vi is zero, if xi has no effect on y and the expected values is constant with regards to xi. if xi is the only random variable with an effect to the model output, vi is equal to the full variance of the model output v ar[y ]. the first order measure vi can be estimated using a monte carlo simulation, which results in nmcs samples of the model output [9]. for every input parameter xi, the sample pairs (xi, y ) are ordered according to the value of xi. the samples are divided into blocks of size nb and the mean value µyi of each block is an estimate of ex−i [ g (x) |xi]. the variance of the block means µyi is an estimate of vi. a more efficient way for determining the first order measure vi is the sobol sequence [13]. based on vi, the first order sensitivity index si can be computed as follows [14] (equation 6). si = vi v ar[g(x)] = v arxi {ex−i [ g (x) |xi]} v ar[g(x)] (6) 4. numerical study to study the influence of the structural integrity management on the lcoe, the capital and operational expenditures are divided into a part related to the structure and a part related the turbine (equation 7 and 8). capex = nwt · (capexstruc + capexturbine) (7) opex = nwt · (opexstruc + opexturbine) (8) the capital expenditures capexstruc and capexturbine for one offshore wind turbine are obtained from the data listed in table 1. the values provided in table 1 are normalized by the nominal turbine capacity. based on the data listed in table 1, a range of [677, 1065]/k€/mw is assumed for the capital expenditures related to the structure capexstruc, while the capital expenditures related to the turbine capexturbine exhibit a range of [1997, 2891]/k€/mw. the operational expenditures related to the structure are derived based on a study documented in [15]. the characteristics as well as the environmental and operational conditions of the wind farm considered in [15] are summarized in table 2. based on this wind farm, the operational expenditures are estimated for an inspection and monitoring strategy with three different settings (optimistic, average, pessimistic) [15]. the inspection and monitoring strategy is in accordance with the requirements defined by the bsh as summarized in table 3. according to the requirements set out by the bsh, general visual inspection (gvi) of the primary and secondary steelwork of the support structures in the wind farm above water is performed every year to provide a general overview on any obvious mechanical damage, fatigue or corrosion. typically, such an inspection is performed by inspectors from a crew transfer vessel. in addition, 25% of the support structure are inspected using close visual inspection (cvi) and detailed visual inspection (dvi). the aim of cvi is to identify fatigue or corrosion and determine whether non-destructive testing (ndt) would be necessary to investigate the welded connections. in contrast to gvi, such an inspection is carried out closer to the support structure. according to [15], the objective of dvi is to determine the extent of detected damage. for this purpose. methods of non-destructive testing (ndt) may also be used. below water, the number of support structures considered for gvi is reduced to 25% every year, while the aim of obtaining a general overall overview on the condition of the support structures remains the same. however, the difference is that a remotely operated vehicle is used to perform the inspection. cvi below water is performed to identify corrosion or fatigue damage and to determine whether ndt would be necessary to inspect the structural components [15]. for this type of inspection, good environmental conditions and visibility as well as marine growth cleaning are required. dvi is used to determine the extent of detected damage using ndt methods [15]. the number of support structures inspected by cvi and dvi per 92 vol. 36/2022 levelized cost of energy of offshore wind characteristic unit value number of owts − 100 turbine capacity mw 5 wf area km2 50 average distance to port km 50 average water depth m 30 foundation type − monopile number of offshore substations − 1 average wind speed m/s 10 (at hub height) tidal conditions s 0.5 (hat to lat) 50-year wave m 6.5 current m/s 1 number of export cables − 1 table 2. wind farm characteristics and environmental and operational conditions [15]. activity shms in 10% of wts inspection frequency during service life above water gvi of primary and secondary steelwork 100% every year 25 cvi of primary and secondary steelwork 25% every year 6.25 dvi of primary and secondary steelwork 25% every year 6.25 seabed scour survey 100% the 2 first years and then 25% every year 7.75 subsea marine growth survey 25% every year 6.25 cathodic protection potential survey 100% the 2 first years and then 25% every year 7.75 cvi of the grouted connection 25% every year 6.25 below water gvi of primary and secondary steelwork 25% every year 6.25 cvi of primary and secondary steelwork 25% every year 6.25 dvi of primary and secondary steelwork 25% every year 6.25 table 3. inspection strategy [gvi: general visual inspection; cvi: close visual inspection; dvi: detailed visual inspection] [15]. boundary conditions optimistic average pessimistic opexstruc 1.2% 1.6% 1.9% opexturbine 98.8% 98.4% 98.1% table 4. relative contribution of op exturbine and op exstruc to the op ex. 93 m. kinne, m. farhan, r. schneider, s. thöns acta polytechnica ctu proceedings figure 2. illustration of the distribution types for a given value range defined in terms of the min. and max. value. year below water is the same as the number of support structures inspected by cvi and dvi above water. a seabed scour survey, a subsea marine growth survey and an inspection of the cathodic protection are performed for 100% of the support structures in the first two years. thereafter, this inspect effort is reduced to 25% of the support structures in each year. the objective of the marine growth survey is to determine the coverage, thickness and type of marine growth on the support structures and sacrificial anodes to guide decisions on removing the marine growth. in a scour survey, the seabed around the support structures is inspected to monitor changes in the topology (local and global). an inspection of the cathodic protection determines if there is adequate global cathodic protection of the submerged part of the support structures. according to the requirements defined by the bsh, continuous sensor-based monitoring systems (structural health monitoring systems and conditional monitoring systems) have to be installed on 10% of the wind turbines. the operational expenditures related to the structures opexstruc and turbines opexturbine relative to the opex are given in table 1. these values are determined in [15] based on the inspection and monitoring scenario described above in conjunction with optimistic, average and pessimistic estimates of the associated costs. from table 4 it can be seen that the opexstruc ranges between 1.2% to 1.9% of the overall opex. the accumulated discounted operational expenditures l∑ t=1 opexstruc + opexturbine (1 + i)t of an offshore wind farm correspond to approximately 25% of the total costs consisting of capital and operational expenditures [21]. to estimate the ranges of the absolute values (min./max. values) of opexturbine and opexstruc, a simple monte carlo simulation is performed. in this analysis, the capex is modelled as a uniform distributed random variable. furthermore, the service life and the discount rate are also modelled as uniform distributed random variables. in combination with the knowledge on the relative values of opexturbine and opexstruc (modelled as uniform distributed random variables) (table 4), the ranges of the absolute values of opexturbine and opexstruc can be estimated. from this analysis it was found that the operational expenditures opexturbine exhibit a range of [64, 135]/k€/mw/year, while the operational expenditures opexstruc are [0.8, 2.6]/k€/mw/year. in the current case study, it is assumed that the service life l of the wind farms may be extended from 20 years up to 30 years [20]. the nominal capacity availability factor nomcap ava describes the ratio between the average power output of a wind turbine (usually one year) to its maximum output and exhibits a range of [0.4, 0.5] [16]. this factor is affected by a variety of influencing factors and conditions including the windspeed. the turbine availability factor tubrava fac specifies the expected average availability over the service life. considering only downtime the turbine itself. the factor accounts for the loss of energy associated with amount of time, when the turbine is not available to 94 vol. 36/2022 levelized cost of energy of offshore wind variable unit uniform distribution normal distribution ref. lower bound upper bound mean value standard deviation (99.7%) standard deviation (90%) number of wind turbines nwt 100 100 100 0 0 nominal capacity nomcap [mw] 6 6 6 0 0 nominal capacity availability factor nomcap ava 0.4 0.5 0.45 0.02 0.03 [16] turbine availability factor turbava fac 0.857 0.996 0.93 0.025 0.04 [17] feed in tariff feedin [€/kwh] 0.14 0.16 0.15 0.003 0.006 [18] turbine invest: structure capexstruc [k€/mw] 667 1065 871 70 118 table 1 turbine invest: turbine capexturbine [k€/mw] 1997 2891 2444 162 271 table 1 operational expenditures: structure capexstruc [k€/mw/year] 0.8 2.6 1.7 0.3 0.54 [5, 17] operational expenditures: turbine part capexturbine [k€/mw/year] 64 135 99.5 12.2 21.6 [5, 17] discount rate i 0.06 0.08 0.07 0.003 0.007 [19] service life t [year] 20 30 25 1.8 3 [20] table 5. parameters of the probabilistic models applied in the sensitivity study. produce energy, e.g. due to failure or maintenance. the availability of the turbine per year is in the range of [0.857, 0.996] [17]. the feed in tariff feedin describes the state fixed remuneration for electricity to subsidize certain types of electricity production, e.g. electricity generated by offshore wind. it is in the range of [0.14, 0.16]/€/kwh for offshore wind [18]. the discount rate i is in the range of [0.06, 0.0825]/year for offshore wind in germany [19]. often, only the minimal and maximal value of the parameters are available in the literature, but not the distribution type. first, it is assumed that the parameters influencing the lcoe are normal distributed. the calculation of the sensitivity indexes is performed for two different standard deviations of the input parameters. it is assumed that the input parameter ranges account for 99.7% and 90% of the values, whereby both ranges are symmetric to the mean value. in addition, the sensitivity indexes are also calculated 95 m. kinne, m. farhan, r. schneider, s. thöns acta polytechnica ctu proceedings figure 3. first order sensitivity indices of the parameters influencing the lcoe. parameter first order sensitivity index normal distributed parameter range: 90% normal distributed parameter range: 99.7% normal distributed parameter range: 90% and nomcap ava as weibull distributed uniform distributed nominal capacity availability factor 0.25 0.25 0.54 0.25 turbine availability factor 0.11 0.12 0.08 0.12 feed in tariff 0.09 0.09 0.06 0.09 capital expenditures: structure 0.04 0.05 0.03 0.05 capital expenditures: turbine 0.2 0.2 0.13 0.2 operational expenditures: structure 0.01 0.01 0.01 0.01 operational expenditures: turbine 0.16 0.17 0.11 0.16 discount rate 0.11 0.11 0.07 0.11 service life 0.09 0.08 0.05 0.08 table 6. first order sensitivity indices of the different input parameters. for uniform distributed input parameters (figure 2). moreover, in an additional calculation, the nominal capacity availability factor is modelled as a weibull distributed random variable to reflect the large influence of the wind speed [22]. the parameters of the weibull distribution are determined by assuming that 5% of the values are smaller than 0.4 (the lower value of the parameter range) and 5% are larger than 0.5 (the upper value of the parameter range). this results in a scale parameter of 0.47 and a shape parameter of 18.23. max. value in the current case study, realizations 96 vol. 36/2022 levelized cost of energy of offshore wind of the turbine availability factor tubrava fac which are larger than one are set to be equal to one. note that, in the current example, the probability that the nominal capacity availability factor nomcap ava is smaller than zero or larger than one is zero regardless of whether it is modelled as a weibull or normal distributed random variable. in the calculation of the sensitivity indices, several simplifications are made. first, the input parameters are modelled as independent random variables. however, in reality dependence among some of the parameters exist, e.g. between the turbine availability factor and the costs due to maintenance / repair or investment costs. another simplification is made in that the lcoe is determined without considering failure of the support structures, which are typically associated with large costs. this simplification is reflected in the model of the opexstruc, which describes only the inspection and monitoring costs without taking into account any failure costs. further simplifications are made in the probabilistic modelling of the input parameters. as an example, the feed in tariff feedin and the discount rate i are modelled as time-invariant random variables even though they typically vary with time. the influence of the adopted simplifications has to be investigated in future research. the parameters of the probabilistic models applied the sensitive analysis of the lcoe are provided in table 5. according to figure 3 and table 6, the indices depend on the distribution types. when modelling all the input parameters with the same distribution type (normal or uniform distributed), almost the same sensitivity indices are obtained. when modelling the nominal capacity availability factor as a weibull distributed random variable, its influence increases, while the influence of the remaining parameters decreases. however, the trend in the relative importance of the parameters remains approximately the same. it can be seen that the nominal capacity availability factor has the highest (first-order) sensitivity index, followed by the capital and operational expenditures related to the turbines. the discount rate, the turbine availability factor, and the feed in tariff have sensitivity indices in the same order of magnitude. the operational expenditures related to the support structures have always the smallest sensitivity index. the sensitivity index of the capital expenditures related to the support structure is smaller than the sensitivity index of the service life. 5. summary and concluding remarks in this contribution, a sensitivity analysis is performed to quantify the influence of the structural integrity management (sim) on the levelized cost of energy (lcoe) of an offshore wind farm. the lcoe of a wind turbine/ farm describe the average net present cost, which arise from the conversion from wind to electrical power. to quantify the influence of the structural integrity management, the operational expenditures are divided into a part related to the structures and a part related to the turbines. the input parameters and their uncertainties are derived based on a literature study and characterize the actual situation/ data. the sensitivity analysis is performed for different probabilistic models of the influencing parameters. to quantify the relative influence of the parameters, their first order sensitivity index is calculated. based on the computed sensitivity index of the operational expenditures related to the support structures, it can be concluded that an optimization of the structural integrity management (sim) aiming to reduce the operating costs may have negligible influence on the lcoe. a similar result has been obtained in a case study considering the effect of d-strings on the lcoe of offshore wind turbine blades [23]. however, a significant influence of a service life extension on the lcoe is found. it can thus be concluded that the structural integrity management should be directed towards a service life extension. this may be achieved by means of monitoring the support structural integrity, i.e. by optimized monitoring systems to be utilized to optimized the time and location of measures such as repairs, which ensure the reliability of the wind turbine support structures beyond the original service life. it should be noted that a service life extension of the support structures makes only sense if the turbines also operate for the extended service life. this may be achieved by exploiting existing differences in the design service life of the turbines and the structure (e.g., 30 years for the turbines and 25 years for the structures) and/or turbine service life extension measures. in the latter case, the extension investments would need to be balanced with the monetary gain of extended production. references [1] bundeverbandwindenergie. windenergie in deutschland zahlen und fakten, 2020. https://www.wind-energie.de/themen/zahlen-und -fakten/deutschland/. [2] gmbh iwrei. windparks in deutschland, 2021. https://www.offshore-windindustrie.de/windpark s/deutschland. [3] european commission. communication from the commission to the european parliament, the council, the european economic and social committee and the committee of the regions an eu strategy to harness the potential of offshore renewable energy for a climate neutral future, 2020. https://eur-lex.europa.eu/leg al-content/en/txt/?uri=com%3a2020%3a741%3afin. 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under a cc-by 4.0 licence published by the czech technical university in prague estimating running safety factor of ballastless railway bridges using tail modelling reza allahvirdizadeh∗, andreas andersson, raid karoumi kth royal institute of technology, division of structural engineering & bridges, department of civil and architectural engineering, brinellvägen 23, 100 44 stockholm, sweden ∗ corresponding author: rezaal@kth.se abstract. excessive vertical acceleration of ballastless railway bridges subjected to vibrations induced by passing trains is one of the governing design criteria for bridges in high-speed lines. however, to the authors’ knowledge, the corresponding design limit is not based on a solid theoretical or experimental background. moreover, the traditionally applied safety factor also suffers from these concerns. therefore, in the present study, a crude probabilistic approach is adopted to evaluate the consistency and reliability of this safety factor. for this purpose, deterministically designed bridges (using conventional methods) with short to medium spans are considered. then, their reliability is evaluated using simulation-based techniques and extreme value theory, i.e., tail approximation. then, the existing safety factor is calculated to evaluate the consistency of the current approaches, and possible new values are proposed based on the desired target reliabilities. keywords: ballastless bridges, high-speed railway bridges, running safety, safety factor, structural reliability, tail modelling. 1. introduction the increase in operating speed of trains in recent decades leads to excessive vibration of infrastructure, especially bridges, which raises new safety concerns. such vibrations can affect the structural performance of bridges, jeopardize the safe passage of trains on bridges (denoted here as running safety), and disrupt passenger comfort. ensuring running safety requires preventing loss of contact between wheels and rail, the occurrence of which increases the risk of train derailment. this phenomenon can be formulated as an unloading ratio, which reads as eq.(1) [1]. ∆p pst = pst − pmin,dyn pst (1) where pst is the static vertical wheel load and pmin,dyn is the minimum dynamic vertical wheel load. theoretically, contact loss occurs at a unloading ratio of 1, but [1] limits this to 0.6. considering the formulation presented, the study of the running safety problem must take into account various subsystems, namely the passing train, the rail, the track, the bridge components, the boundary conditions, and the interaction between all these subsystems. the train and the track are coupled to each other by the interactive wheel-rail force, which is transmitted to the bridge through the track. it is worth noting that there are a variety of track types in the world, which can generally be categorized into two classes: ballasted and ballastless (also referred to as non-ballasted or slab track). for the first type, a granular material is formed between the sleepers and the bridge deck. for the latter, however, there are different construction methods, about which the interested reader can find a detailed overview in [2], but in general they consist of two concrete layers and a bituminous mixture in between. clearly, the use of such complicated computational models requires a high level of expertise and involves expensive computational costs. therefore, for practical purposes, moving load models are often used, which are computationally very efficient. in addition, it has been shown that such reduced models can lead to acceptably accurate answers when the train weight is much smaller than the bridge (which seems to be the case for passenger trains) and also in cases where the study of the dynamic behavior of trains is not sought [3]. this approach cannot model the contact loss between wheels and rail. therefore, the design rules implicitly control the ride safety criterion by instead limiting the vertical acceleration of the bridge deck [4] or its deflection [5]. this assumes that contact loss occurs at accelerations greater than 1.0 g; however, the authors found no theoretical or experimental justification for this assumption. the relationship between this criterion and the unloading ratio (wheel-rail contact loss) was previously studied using train-track-bridge interaction (ttbi) models, where it was found that the acceleration criterion almost always dominates the design [6, 7]. in addition, an arbitrary safety factor of 2 is applied to the above threshold. therefore, the maximum allowable vertical acceleration of ballastless railroad bridges should be limited to 5 m/s2. this safety factor is borrowed from the phenomenon of ballast instability [4]; however, it should be emphasized that the consis25 https://doi.org/10.14311/app.2022.36.0025 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en r. allahvirdizadeh, a. andersson, r. karoumi acta polytechnica ctu proceedings tency and reliability of this safety factor is questioned even for bridges with ballast [8]. considering this situation, a rough probabilistic approach is taken in this article to evaluate the reliability of the conventional safety factor for ballastless railroad bridges. the rest of this article is divided into the review of the applied reliability assessment method in 2, the description of the constructed computational models in 3, the formulation of the reliability problem in 4, and the estimation of the corresponding safety factors in 5. the article concludes with a summary of the main results. 2. reliability assessment using tail modelling the probability of failure (pf ) is generally estimated by calculating the presented multiple integral in eq. (2): pf = p (g(x) ≤ 0) = ∫ d fx(x)dx (2) where x ∈ rm is the vector of basic random variables, fx(x) is their joint distribution probability, and g(•) is the limit state function. the latter is basically a decision boundary separating the failure domain (d) from the safe region. these regions are distinguished based on the sign of the limit state function, i.e., failure occurs when g(x) = r − s ≤ 0; where r is the capacity corresponding to the failure mode and s is the demand (e.g., load, displacement, stress, or strain). at this point, it should be mentioned that reformulating the reliability problem as a ratio between action and corresponding capacity, i.e., g(x) = s/r instead of subtracting them, changes the failure domain to the region where g(x) ≥ 1. the calculation of this integral is usually not tractable in real applications. this is because the joint probability distribution of the basic random variables and the failure domain are generally not available. therefore, a variety of methods have been developed to overcome these problems. the best known of these are crude monte-carlo simulation (mc) and first and second order reliability methods (form /sorm). the crude mc gives the most accurate estimate of the failure probability, making it the most widely applicable approach. it can be expressed as eq. (3): pf = ∫ i[g(x) ≤ 0]fx(x)dx = e [ i[g(x) ≤ 0] ] ≈ 1 n n∑ i=1 i[g(x̂i) ≤ 0] (3) where i(•) is an indicator function equal to one if true and zero otherwise. also, n is the number of realizations (samples) of the basic random variables. although it is generally applicable, crude mc is not an efficient approach for the majority of reliability problems. this is because its accuracy (represented by the coefficient of variation of the estimated probability of failure) is inversely proportional to the square root of the number of samples (∝ n−1/2) and also to the order of the probability of failure. considering that the failure probability in structural engineering is usually in the range of 10−4 − 10−3, performing mc incurs significant (and usually unaffordable) computational costs for reliability assessment tasks. moreover, form is basically an optimization-based solution that finds the smallest distance between the point with the largest likelihood on the transformed joint probability distribution in the standard normal space and the most probable point (mpp). the latter should be on the limit state function approximated by the first taylor expansion around the mpp.therefore, form reads as eq. (4). β = arg min t 1 2tt t s.t. g(t) = 0 (4) where β = −φ−1(pf ) is the safety index, t = t (x) is the transformed basic random variables transformed into the standard normal space, and g(•) is the reformulated limit state function based on the transformed variables. it is worth noting that basic random variables can be transformed into standard normal space, for example, using the nataf transformation [9]. an application of form to evaluate the running safety of ballasted high-speed railway bridges is presented in [8]. as mentioned earlier, this method is computationally efficient but may not be accurate enough for highly nonlinear limitstate functions [10, 11]. considering the discussed issues regarding the efficiency and accuracy of classical reliability assessment techniques, they were later improved by the introduction of more advanced methods. these include important sampling (is) [10, 12], subset simulation (ss) [13], surrogate-based approaches [14, 15], and tail estimation (approximation). the explanation of these methods is beyond the scope of this article; therefore, the interested reader is referred to the references provided. therefore, only the latter approach used in this study is explained here. the failure event is usually a rare event among all possible scenarios of a designed system; therefore, the failure probability is located at the tails of the cumulative distribution function (cdf) of the limitstate function. thus, the basic idea of tail modeling is to approximate the tail of the true distribution by another distribution with an equivalent tail by running a limited number of simulations. the tail equivalence property reads as [16]: lim g(x)→∞ 1 − t [g(x)] 1 − f [g(x)] = 1 (5) where f [g(x)] and t [g(x)] are cdf of the limit state function and that of the tail model, respectively. this equivalence holds for the tails of the true cdf (upper 26 vol. 36/2022 running safety factor of railway bridges using tail modelling or lower ones); therefore, a threshold (denoted here as u) should be chosen to fit the tail model for values larger (or smaller) than this. this results in the fitted tail being formulated based on a transformed variable called the exceedance (z = x − u). the concept of tail approximation, threshold, and exceedance is shown schematically in figure 1. it should be noted here that the limit-state function in figure 1 has been reformulated to show the region of failure at the upper end for illustrative purposes. 0 g(x) f[ g( x) ] safe failure z = g(x)-u cdf of g(x) fitted tail u figure 1. concept of tail modelling. the most widely used tail model is the generalized pareto distribution (gpd), given as eq.(6). in addition, the sigmoid (eq.(7)) and exponential functions have been used for reliability assessment of high-speed railway bridges in [17], where it was concluded that the sigmoid function can lead to a more accurate estimate of the probability of failure. tz(z) = 1 − 〈 1 + kz/ψ 〉−1/k + k ̸= 0 1 − exp(−z/ψ) k = 0 (6) tz(z) = c1 + c2[ 1 + exp (z/c3) ]c4 (7) where k is the shape parameter of gpd, ψ its scale parameter, ci, i = 1, ..., 4 are constants of the sigmoid function, and ⟨a⟩+ = max (0, a). these parameters can be calculated using either the maximum likelihood method (mle) or the least squares method (regression). in [18], it was shown that using a single-tail model does not guarantee that the failure probability is always estimated accurately. therefore, they proposed to calculate the probability of failure based on a linear regression on the safety index of the top 10% of the data (betalt), a quadratic polynomial fit to the top 50% of the safety index data (betaqh), and a quadratic fit to the logarithmic transformation of the safety index (lnbetaqt) [18]. then they proposed to use the median value of the estimated failure probability from the discussed approaches, showing for benchmark problems that it is almost always at least the second best estimate. therefore, the same approach is followed in this study, using the sigmoid function as the tail model as well. in addition to the assigned parameters of the tail model, its performance also depends on the threshold considered. in [19], it was proposed to consider 1.5 √ n (where n is the total number of data) to fit the tail model (hereafter referred to as hasofer’s method). similarly, [20] suggests using 0.02n and 0.1n for cases of 50 < n < 500 and 500 < n < 1000, respectively (hereafter referred to as boos’ method). furthermore, in [21], they discussed that thresholds close to the central data cause the fitted tail to suffer from high bias, while very large thresholds can lead to tail approximations with high variance ( trade-off between bias and variance). in this context, they proposed to select the optimal threshold by calculating the mean square error (mse) of the estimated safety index (β) at all thresholds considered. the mse can be calculated as the sum of the squared bias and the variance of the estimated safety index at a given threshold (see eq. (8)). given the definition of bias (see eq.(9)) and variance (see eq.(10)), the expected value of the estimated safety index is needed. therefore, they proposed to find the expected value of the safety index using the bootstrap method. an illustrative example of the latter approach is shown in figure 2, where the horizontal axis represents the threshold value as the quantile of all data. it is worth noting that for illustration purposes, the well-known ishigami function is used in the form of eq. (11) to represent the limit state function [22]. threshold selection using this approach is more robust than other methods, although it increases the computational cost. since this additional computational cost is only at the level of post-processing existing data, it would be affordable. mse(β|u) = bias2(β|u) + var(β|u) (8) bias(β|u) = e[β̂(u)] − β (9) var(β|u) = e [ (β̂(u) − e[β̂(u)])2] (10) f(x) = 16.5 − [ sin(x1) + 7 sin2(x2) + 0.1x4 3 sin(x1) ] xi ∼ u(−π, π) i = 1, 2, 3 (11) 27 r. allahvirdizadeh, a. andersson, r. karoumi acta polytechnica ctu proceedings 0.88 0.9 0.92 0.94 0.96 quantile 0.6 0.65 0.7 0.75 0.8 0.85 0.9 0.95 m se mse best threshold 1 figure 2. threshold selection using mse estimator. 3. computational model as mentioned earlier, this study is concerned with the running safety of ballastless high-speed railway bridges. this problem consists of four subsystems, namely the passing train, the rail, the slab track and the bridge, and their interactions. the used 2d computational model of the simply supported bridges is shown in figure 3. the rail, slab track and bridge are modeled as eulerbernoulli beams. in addition, the rail pads between the rail and the slab track are modeled using lumped springs and dashpots, while the sub grades are accounted for by distributed springs and dashpots. the structure of the train (i.e., the corresponding degrees of freedom) is neglected, resulting in the train being modeled as a series of moving loads. the trains under consideration share a bogie between two adjacent coaches (known as articulated trains) and it is assumed that the axle loads are identical for each realization. since the loads move along the rail, the components of the load vector that do not relate to the rail are zero. therefore, the equation of motion can be expressed as eq. (12) using partitioned matrices. the interested reader can find detailed information about the computational model in [6, 23].mr 0 0 0 ms 0 0 0 mb   ÿr ÿs ÿb +  cr crs 0 csr cs csb 0 cbs cb   ẏr ẏs ẏb  +  kr krs 0 ksr ks ksb 0 kbs kb  yr ys yb  = fr 0 0  (12) where m, c, and k represent the mass, damping, and stiffness matrices of each subsystem, respectively. the indices r, s, and b represent rail, slab track, and bridge, respectively. each subsystem is represented by diagonal terms and the off-diagonal partitioned matrices couple the subsystems (rail to slab and slab to bridge). the rail to slab coupling matrices include the lumped spring and the dashpots of the railpads. similarly, the rail to bridge coupling matrices include the distributed spring and the dashpots of the subgrade. in addition, y is the vector of subsystem displacements and fr is the force vector. note that the force vector is time dependent and must be reconstructed at each step based on the position of the train. next, the equilibrium equations of motion are solved using the hilber-hughes-taylor (hht) direct time integration method. then, the calculated responses are filtered using a low-pass filter whose cut-off frequency corresponds to the minimum of the third vibration frequency of the bridge or 30 hz [24]. the constructed computational model cannot capture the effect of unevenness in the rail geometry (known as rail irregularities) on the responses of the bridge. rail irregularities are random in nature and can be modeled by a stationary gaussian process to generate different rail profiles [25]. then they are treated as an additional force term using the wheelrail contact stiffness [23]. in [26], it was shown that rail irregularities can significantly amplify the maximum vertical accelerations of the bridge. therefore, they proposed random amplification factors as an alternative method for modeling rail irregularities. a similar approach is taken by conventional codes, but they consider it as a deterministic variable. this codified amplification factor is considered here; it reads as eq.(13) [24] for the rails with good quality. ria = 1 + α 200 [ 56e−(l/10)2 + 50(lf1 80 − 1)e−(l/20)2 ] (13) where α = min (v/22, 1), v is the train speed, l is the length of the bridge span, and f1 is the fundamental frequency of the bridge. it should be noted that the use of ria does not necessarily lead to conservative response predictions [26]; however, this possible drawback is neglected in this study. since the main objective of the article is to evaluate the consistency of the conventional safety factor, the speed of passing trains is limited to the allowable operating range, which is assumed to be 240 km/h here. this is achieved by increasing the operating speed of high-speed trains on the swedish network by 20%, which is about 200 km/h [27]. moreover, only the maximum response of the bridge is needed to estimate the corresponding failure probability. therefore, the maximum vertical acceleration of the deck is calculated only for critical speeds (denoted here as vcr). the critical speed corresponds to the situation where the excitation frequency of the passing train coincides with the frequency of the bridge (resonance phenomenon); this is obtained from eq.(14) [28]. it should be noted that the maximum speed is taken into account for the cases where the critical speed was greater than the allowable speed. 28 vol. 36/2022 running safety factor of railway bridges using tail modelling p p v d pp p p dba s slab-subgrade kss & css slab track l krs & crs rail pad mb ,eib ,zb rail ms ,ei s ,zs mr ,eir ,zr figure 3. 2d computational model of the ballastless railway bridge. vcr = dfj i j = 1, 2, 3, ... i = 1, 2, 3, ..., 1/2, 1/3, 1/4, ... (14) where, d is the characteristic length of the coach and fj is the jth frequency of the bridge. 4. reliability problem as mentioned earlier, the running safety is implicitly controlled by limiting the vertical acceleration of the bridge deck. therefore, the limit state function is formulated as eq. (15), where failure occurs for g(x) ≥ 1. g(x) = amax(x)(1 + χm ) alim (15) where amax(x) is the maximum vertical acceleration of the bridge deck for x realization of the basic random variables, alim is the maximum allowable vertical acceleration of the bridge deck (here considered as 10 m/s2), and χm is the model uncertainty. the basic random variables considered are listed in table 1, with those related to the bridge and train properties taken from [29–31]. note that the target bridges have spans of 10 m, 20 m, and 30 m, and the values given in table 1 are in the same order. the mean values of the cross-sectional moment of inertia and mass per length correspond to the minimum allowable values satisfying conventional deterministic design methods [29]. the nominal values of the other parameters are taken from [6, 7]; the assigned distribution function and standard deviations are based on recommendations from [32] and the authors’ engineering judgment. 5. safety factor estimation the objective of the article is achieved by first calculating the maximum vertical acceleration of the bridges for passing trains with critical speeds. in this context, the sensitivity of the estimated safety indices is first evaluated with respect to the number of samples considered (see figure. 4). it should be emphasized here that the analyzes for this task were performed only for trains with a speed of 240 km/h. as can be seen, the method almost converges when the number of samples exceeds 10,000; however, 20,000 samples are considered in this study. 0 1 2 3 # sample 104 3 3.2 3.4 3.6 3.8 4 figure 4. sensitivity to the number of samples. then, the statistical parameters (i.e., mean and standard deviation) of demand are estimated by calculating the 99% quantile of these parameters using the bootstrap method. then, the system’s probability of failure is calculated using the tail approximation method. this is done by calculating the median of the probabilities of failure resulting from the methods discussed. these methods include fitting gpd by the maximum likelihood method using the hasofer, boos, and minimum mse thresholds, fitting the sigmoid function by the regression method using the minimum mse threshold, betalt, betaqh, and lnbetaqt. some examples of such approaches obtained for the 30 m span bridges are shown in figure 5. then, eq. (16) is used to estimate the existing safety factor of the conventionally designed bridges. this equation is developed in [10] for linear limit-state 29 r. allahvirdizadeh, a. andersson, r. karoumi acta polytechnica ctu proceedings variable dist.∗ mean/min./scale std/max./shape truncation bridge ib (m4) moment of inertia n [0.4, 1.5, 5.9] 0.01µib mb (kg/m) mass per length n [19000, 29000, 39000] 0.03µmb ξb (%)† damping ratio ln max(1.5 + 0.07(20 − l), 1.5) 0.3 eb (gpa) modulus of elasticity n 29.7 3.56 train d (m) coach length u 17 28 dba (m) axle distance u 2 3.5 p (kn) axle load w 194.93 9.14 ≤ 120 rail ir (m4) moment of inertia n 2 × 61.1e-6 0.01µir mr (kg/m) mass per length n 7850 × 15.38e-3 0.03µmr er (gpa) modulus of elasticity n 205 5 slab track ts (m) thickness n 0.3 0.01 bs (m) width n 2.5 0.005 ρs (kg/m3) mass density n 2500 100 ξs (%)† material damping ln 2 0.3 rail pad krp (mn/m) stiffness n 2 × 22.5 0.01µkrp ξrp (%)† damping ratio ln 10 0.3 slab subgrade ksg (mn/m3) stiffness n 100 0.2µkrp ξsg (%)† damping ratio ln 2 0.3 model uncertainty χm (-) model uncertainty n 0 0.086 ∗ n , ln , u and w indicate normal, lognormal, uniform and weibull distributions, respectively. † parameters are in physical space. table 1. considered basic random variables. functions using the separation function to linearize its standard deviation. γ = 1 + θβccova 1 + kacova (16) where θ = 0.75 ± 0.06 for 1/3 < σ/µ < 3 is the separation constant linearizing the standard deviation of the linear limit state function, i.e. σz = (σ2 r + σ2 s)1/2 changes to σz ≈ θ(σr + σs) for limit state functions in the form of g(x) = r− s. also, βc is the safety index corresponding to the limit state under consideration, cova is the coefficient of variation of the demand (here the vertical acceleration of the bridge deck), and ka is the standardized characteristic value (95% quantile denoted here as ak) of the demand, which reads as ka = (ak − µa)/σa. next, the new safety factor is calculated using the same data, replacing the existing safety index with a target safety index, which is βt = 3.719 (i.e., pf,t = 10−4). the calculated values are given in table 2; where γexist and γnew indicate an approximation of the safety factor based on conventional design methodology and the potential values to be adopted in the future, respectively. it should be emphasized here that minimum allowable mass values have been assigned to the bridges considered; therefore, the values presented represent an approximate lower bound of the possible safety indices. as can be seen, the conventional design method does not result in a consistent safety factor; although an identical factor was used for the design of all bridges. this conclusion underscores the point that safety factors in the future should probably be a function of span length, rather than being constant. in addition, bridges with shorter spans are more susceptible to higher accelerations, resulting in a lower safety factor compared to longer bridges. considering the safety factor obtained for bridges with a span of 20 m, it is even possible that some scenarios are neoconservative. on the contrary, criteria other than running safety may dominate the design of bridges with larger spans, e.g., displacement-related criteria such as passenger comfort [30]. therefore, a very large safety factor is estimated as a consequence of 30 vol. 36/2022 running safety factor of railway bridges using tail modelling 0.15 0.2 0.25 0.3 g 0.9 0.92 0.94 0.96 0.98 1 1p f emprical cdf gpd-mle-boos (a) . gpd-mle-boos threshold 0.1 0.2 0.3 g 0 0.2 0.4 0.6 0.8 1 1p f emprical cdf sigmoid-reg-best u (b) . sigmoid-regression-minimum mse threshold 0.1 0.2 0.3 g 0 1 2 3 4 emprical beta-lt (c) . beta-lt 0.1 0.2 0.3 g 0 0.5 1 1.5 l n( ) emprical lnbeta-qt (d) . lnbeta-qt figure 5. example of fitted tail models for bridges with span length of 30 m. l (m) γexist γnew 10 2.91 1.18 20 1.29 1.37 30 6.46 1.17 table 2. existing and proposed safety factor for running safety of high-speed ballastless railway bridges. a very small probability of failure. considering this, the reliability of the estimated existing safety factor for bridges with a span of 30 m is questionable. on the other hand, the proposed new values for the safety factor appear to be more consistent with a conservative value of about 1.4. however, as mentioned above, the use of a constant safety factor for all bridges with different spans needs to be revised. 6. conclusions in this study, the reliability of the conventional safety factor for the running safety of ballastless high-speed railway bridges was investigated. it was found that despite the use of a constant safety factor for bridges with different spans, the resulting designs may not have consistent safety factors. therefore, it seems necessary to revise the current safety factor by proposing a set of new factors that depend on the span length of the bridge. in this regard, rough proposals based on an arbitrary target safety level were recommended, showing the potential of using smaller values in future design regulations. acknowledgements this project has received funding from the shift2rail joint undertaking under the european union’s horizon 2020 research and innovation programme under grant agreement no 101012456. 31 r. allahvirdizadeh, a. andersson, r. karoumi acta polytechnica ctu proceedings references [1] cen.en 14363. railway applications testing and simulation for the acceptance of running characteristics of railway vehicles running behaviour and stationary tests. european committee for standardization, 2016. 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[30] r. allahvirdizadeh, r. karoumi, a. andersson. surrogate-assisted versus subset simulation-based stochastic comparison between running safety and passenger comfort design criteria of high-speed railway bridges. in 31st european safety and reliability conference (esrel 2021). 2021. https://doi.org/10.3850/978-981-18-2016-8_577-cd. [31] r. allahvirdizadeh, a. andersson, r. karoumi. minimum design requirements of high-speed railway bridges using reliability-based optimization. unpublished manuscript 2021. [32] jcss. jcss probabilistic model code. joint committee on structural safety (jcss), 2001. 32 https://doi.org/10.1080/23248378.2018.1503975 https://doi.org/10.47964/1120.9282.18654 https://doi.org/10.1016/j.engstruct.2012.02.024 https://doi.org/10.1016/j.strusafe.2009.09.002 https://doi.org/10.1016/s0167-4730(97)00036-2 https://doi.org/10.1007/s00419-007-0122-4 https://doi.org/10.1016/s0141-0296(00)00057-2 https://doi.org/10.3850/978-981-18-2016-8_577-cd acta polytechnica ctu proceedings 36:25–32, 2022 1 introduction 2 reliability assessment using tail modelling 3 computational model 4 reliability problem 5 safety factor estimation 6 conclusions acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.36.0099 acta polytechnica ctu proceedings 36:99–108, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague artificial intelligence finite element method hybrids for efficient nonlinear analysis of concrete structures michael a. krausa,b,∗, rafael bischofa, walter kaufmanna,b, karel thomaa a eth zürich, institute of structural engineering (ibk), chair for concrete structures and bridge design, stefano-franscini-platz 5, ch-8093 zürich, switzerland b eth zürich, center for augmented computational design in architecture, engineering and construction, design++ initiative and immersive design lab, stefano-franscini-platz 5, ch-8093 zürich, switzerland ∗ corresponding author: kraus@ibk.baug.ethz.ch abstract. realistic structural analyses and optimisations using the non-linear finite element method are possible today yet suffer from being very time-consuming, particularly in case of reinforced concrete plates and shells. hence such investigations are currently dismissed in the vast majority of cases in practice. the "artificial intelligence finite element hybrids" project addresses the current unsatisfactory situation with an approach that combines non-linear finite element models for reinforced concrete shells with scientific machine learning algorithms to create hybrid ai-fem models. the ai-based surrogate material model provides the material stiffness as well as the stress tensor for given concrete design parameters and the strain tensor. this paper reports on the current status of the project and findings of the calibration of the ai-based reinforced concrete material model. we successfully calibrated and evaluated k-nearest-neighbour, lgbm and resnet algorithms and report their predictive capabilities. finally, some light is shed on the future work of integrating the ai surrogate material models back into the finite element method in the course of the numerical analysis of reinforced concrete structures. keywords: concrete material model, machine and deep learning, nonlinear finite element method, surrogate modeling, uncertainty quantification. 1. introduction digital design and manufacturing methods, such as those to be developed at the new immersive design lab (idl) at eth zurich, offer great potential for significantly more efficient and sustainable construction. in order to ensure the structural safety, economic efficiency and sustainability of complex structures, reliable and powerful models for automatic analysis, optimisation and design are essential. however, such models are largely lacking to date. this is especially true for concrete structures, as their behaviour is highly non-linear. realistic structural analyses and optimisations using the non-linear finite element method (fem) are possible today, yet being very time-consuming even for the case of extraordinary computational capacities. for reasons of temporal and monetary efficiency, established traditional and in many cases excessively conservative design methods without structural optimisation are still used in the vast majority of projects in practice today. the increased public awareness of and demand for a sustainable built environment however urges civil engineers to make use of structural efficiency to the maximum level possible. to that end, a two-phased research program [1, 2] is proposed to address this unsatisfactory situation. in the first phase, non-linear fem for reinforced concrete plates and slabs developed at eth zurich [3–8] are combined with scientific machine learning (sciml) algorithms to create hybrid ai-fem models, which are expected to be much more efficient compared to established analysis methods both in terms of the computing power required and the reliability of predicting the load-bearing behaviour. in the second phase of this project, the ai-fem-hybrids will be used within a novel generative design process for accelerated yet realistic conceptual design of bridges [1, 2]. within a fe analysis, a material model has to provide on the one hand the material stiffness matrix and on the other hand the current stress state. in the novel hybrid artificial intelligence finite element (aifem) model implementation for material-nonlinear reinforced concrete slabs and plates elements, the mathematical description of the material model is performed using scientific machine and deep learning algorithms, which act as functional approximators of the stress-strain relationship and the stiffness-strain relationship based on numerical simulations. the basic data set for training, validation and testing of the ai algorithms is generated by extensive simulations of strain states for different reinforced concrete configurations with the material model for reinforced 99 https://doi.org/10.14311/app.2022.36.0099 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en m. a. kraus, r. bischof, w. kaufmann, k. thoma acta polytechnica ctu proceedings concrete as implemented in the mentioned usermat in ansys mechanical apdl on the basis of the cracked membrane model (cmm) in combination with a reissner-mindlin layer model. this paper describes the process of data generation and calibration of the ai-based reinforced concrete material model for different machine and deep learning algorithms for stress and stiffness tensor prediction. this paper is organised as follows: we first provide background on the materials and methods from computational mechanics and scientific machine learning used in this paper in sec. 2. sec. 3 reports and presents selected numerical results of the sciml-based substitute concrete material models w.r.t. their approximation accuracy and statistical qualities. sec. 4 presents a discussion of current findings and sec. 5 sheds light on the future steps of incorporating the sciml-based substitute concrete material models into the fem software ansys. 2. materials and methods this section reports on materials and methods for database generation, development of machine and deep learning (ml/dl) models, and the evaluation criteria for the performance of the developed models as a surrogate for a fem. 2.1. concept this research project explores the potential of using ml/dl surrogate models of mechanically consistent nonlinear finite element material models for reinforced concrete plates and shells, developed at eth zurich, as a data-driven yet physics-informed ai method within a fem workflow. the project is divided into two parts. the ai-fem-hybrids of phase one provide predictions of the material stiffness tensor as well as the stress tensor for a current strain state, where the sciml algorithms act as functional approximators of the stress-strain relationship and the stiffness-strain relationship, which was found to be applicable to a number of materials [9]. the basic data set for training, validation and testing of the ai algorithms is generated by an extensive simulation of strain-stressstiffness states for different reinforced concrete configurations with the material model for reinforced concrete as implemented in the mentioned usermat in ansys mechanical apdl on the basis of the cracked membrane model (cmm) in combination with a layer model based on reissner-mindlin plate kinematics. the training, validation and testing of the sciml algorithms for stress and stiffness tensor prediction are presented and discussed in the following, where special consideration is given to model uncertainty quantification to allow for a future eurocodecompliant use within the semi-probabilistic design philosophy. with the completion of the first phase, a validated software demonstrator of ai-fem-hybrids for the efficient non-linear analysis and design of reinforced concrete structures is available. in the second project phase, the ai-fem-hybrids approach will be extended to structural optimisation of concrete bridges. the pilot project in this phase focuses on concrete bridges with a geometry defined by a few parameters. while the case study is specific, the developed optimisation methodology is kept as general as possible to allow its application beyond parametric concrete bridge structures. at the end of the research project, a validated software demonstrator of the aife-hybrids will be available for the efficient non-linear analysis, design and optimisation of reinforced concrete structures. in the future, this will then allow for (i) realistic structural analyses taking into account non-linearities and (ii) structural optimisation to be carried out much more efficiently. the outcomes of this research are potentially of high interest to industry and engineering practice, as structural concrete is the most widely used construction material worldwide and incorporation of these ideas allows for more economic, yet sustainable and reliable design. to achieve the objectives for phase one of the research project, the following sequence of steps was conducted: (1.) define relevant feature x and target variables y for a reinforced concrete material model (cf. tab. 1) (2.) create the database {x; y} of fem simulations (3.) calibrate different ml and dl algorithms given the database (4.) assess different metrics for accuracy and predictive capabilities of the ml resp. dl models (5.) implement a selection of ml resp. dl models for implementation in ansys (6.) verify efficiency and accuracy at industry scale via computation of example problems from reinforced concrete design 2.2. fem material model data generation the dataset generation within the ai-fem-hybrid project was performed by nonlinear finite element analysis of a reinforced concrete structure utilising the cmm-usermat [3], a mechanically consistent material model for reinforced concrete implemented in ansys mechanical apdl, cf. figs. 1 and 2. combined with a layer element (shell181), the non-linear fem analysis of reinforced concrete structures as girders or shells are possible. as presented in [3–5], an excellent agreement between experimental results and fem analysis utilising the cmm-usermat is reported. in order to provide the necessary dataset for the training of the machine learning algorithms, a dummy girder bridge (cf. figs. 1 and 2) was arbitrary loaded incrementally until failure, which enables the export of 100 vol. 36/2022 ai-fem-hybrids for nonlinear concrete materials aa aa 45.00 m aa 5.00 m aa 2.25 m aa 250 mm aa 250 mm aa aa 400 mm aa 1'200 mm aa 495 mm aa 480 mm aa 437 mm qn qn a b c d b0 l/2 l/2 qn y xz x z y 8'600[kn] [knm] vz my [kn] [knm] nx tx a z bz l/2 l/2 cz dz (a) (b) x z y w (x = l/2, s1) w (x = l/4, s1) w (x = l/2, s2) w (x = l/4, s2) qn = 1'911 kn/m zy x q1 = 200 kn/m 11'250 -11'250 -4'500 4'500 -50'625 aa aa = aa = aa = aa = aa = aa aa = aa = aa = aa = aa = z x yq1·b0/2 q1 parameter: l b0 h0 tinf tsup parameter: tw tend ax ay az figure 1. dummy girder bridge used within ansys to generate the database. features xi targets yi reinforcement layer (rela) normal stress in x-direction σx cmm usermat model (cmm) normal stress in y-direction σy reinforcement area as shear stress in xy-direction τxy reinforcement diameter ds stiffness component k11 effective reinforcement ratio for tcm model ρs,eff stiffness component k12 reinforcement yield stress fy stiffness component k13 reinforcement ultimate stress fu stiffness component k21 reinforcement ultimate strain ϵsu stiffness component k22 reinforcement angle θs stiffness component k23 concrete compressive strength fcc stiffness component k31 concrete ultimate strain ϵcu stiffness component k32 normal strain in x-direction ϵx stiffness component k33 normal strain in y-direction ϵy shear strain in xy-direction ϵxy table 1. list of features and targets for data generation and calibration of ai-fem-hybrids. note that ϵxy is the tensorial component of the shear strain γxy, i.e., ϵxy=γxy/2. all necessary features (input parameters x: strain tensor, reinforcement properties, material constants, etc. acc. to tab. 1) and results (stress and stiffness tensors acc. to tab. 1) in every integration point (gausspoints) for all converged load steps. this approach enables the collection of a considerable number of data points within one non-linear fem computations, as in almost every integration point an individual data set exists. for simplicity, a dummy girder bridge was chosen to minimise the number of parameters, e.g. the diameter of the rebars or the material constants. as soon as the general framework of the ai-fem-hybrids is established (proof of concept), the results of any dummy experiment and the results of any fem analysis using the cmm-usermat can be used to train the machine learning algorithms. in total the data set consists of: • ns,wor = 10, 636, 800 samples of concrete elements without reinforcement (configuration "cf 0") • ns,wr = 443, 200 samples of concrete elements with reinforcement (configuration "cf 1") 2.3. data pre-processing the data generated in the previous section is used to develop the ml resp. dl models. the dataset consists of over 11 million data points with 14 feature and 12 target variables, cf. tab. 1. some dimensions of the feature dataset are categorical (reinforcement layer, cmm-usermat model), while the remaining features are all of numerical nature. for the categorical features we use one-hot-encoding. all dimensions of the target dataset are of numerical nature. none of the attributes has any missing data since the data is generated via fem analyses. the range of values 101 m. a. kraus, r. bischof, w. kaufmann, k. thoma acta polytechnica ctu proceedings x z y integration poinst shell element shell181 l l b 0 h 0 a y a z a x a x z x y y x z (a) (b) (c) figure 2. detail of the cmm finite element within ansys to generate the database. of the features as well as the targets are extremely different, hence we employ normalization of the data. for each numerical variable vi we deduce its mean m and divide it by its standard deviation s (yielding its z-score) using: zi = vi − µi σi (1) for sake of brevity of this paper, we omit reporting further statistical properties or histograms of the dataset, which would usually be delivered in the exploratory data analysis step. 2.4. machine learning model development in this study, three ml resp. dl algorithms for regression are investigated for predictive capabilities for stress and stiffness target tensors given the strain tensor together with concrete and reinforcement features: • k-nearest-neighbours (knn) • lightgbm • artificial neural networks: residual neural networks (resnet) while knn is a strictly data-driven approach and hence needs to store some of the simulation data instances, lightgbm as well as resnet are surrogate functions without the need to store the training data. due to reasons of brevity of this paper, only selected results and implementation details can be reported. the ml models are developed using sklearn, keras, and xgboost (extreme gradient boosting) python libraries and run on the eth euler cluster1. 2.4.1. knn the k-nearest-neighbours algorithm is a decision method originally developed for classification tasks [10]. at inference time, it compares the previously unseen samples to all instances in a database and assigns classes by means of majority voting of the k nearest neighbours, where k and the distance metric are hyperparameters. the algorithm was later extended to support regression tasks [11] by interpolating between the values of the k nearest neighbours (e.g. through inverse distance weighted average) to obtain values for new, unseen samples. 2.4.2. lightgbm lightgbm is a gradient boosting decision tree framework [12] that has found application in many different data mining, classification and regression tasks. the lightgbm algorithm variant used in this paper integrates a number of regression trees to approximate the dataset. it contains many of the advantages of other common gradient boosting decision tree algorithms, such as sparse optimization, parallel training, regularization, bagging, and early stopping. however, it grows trees leaf-wise by greedily choosing the leaf that will lead to the largest improvement. this tree-growth method, in combination with a histogram-based memory and computation optimization, make lightgbm considerably more computationally efficient than other frameworks. 2.4.3. artificial neural networks fully-connected feed-forward neural networks (ffnn) consist of one or more layers, where each node is connected to every node in the following layer [13]. despite being very straight forward architectures, ffnns are universal function approximators [14] and have the advantage of being easy to implement and efficient to train. on the other hand, the lack of implicit bias limits their expressiveness and capability to generalise. furthermore, they are prone to pathologies like vanishing gradients [15]. we therefore extend the ffnn by adding skip connections [15] as well as batch normalisation [16] and dropout [17] , cf. fig. 3. these additions allow us to build deeper architectures, which were shown to generalise better in practice [18]. we will hereafter be referring to this model by resnet. 2.4.4. hyperparameter tuning we employ bayesian optimization in order to avoid running extensive grid-search over the entire hyperparameter space [19][20]. at first, 25 random points 1https://scicomp.ethz.ch/wiki/euler 102 vol. 36/2022 ai-fem-hybrids for nonlinear concrete materials fu ll yc o n n ec te d b at c h n o r m a li sa ti o n d r o po u t fu ll yc o n n ec te d + figure 3. resnet: neural network with skip connections on the hyperparameter space are evaluated. gaussian processes then serve as prior distribution in order to approximate the unknown function and a posterior distribution is maintained as 75 more observations are made, where expected improvement (ei) is used as exploration strategy [21]. the hyperparameters together with optimization intervals are reported in tabs. 2, 3, and 4. hyperparameter range number of neighbours k [1, 10] power of minkowski metric p [1, 3] table 2. hyperparameters for training settings together with ranges as used for bayesian optimisation of knn hyperparameter range log-scaling learning rate [10−5, 10−1] yes max depth [3, 50] no min child weight [0, 10] no number estimators [30, 300] no number leaves [10, 100] no min child samples [10, 30] no table 3. hyperparameters for training settings together with ranges as used for bayesian optimisation of lightgbm hyperparameter range log-scaling learning rate [10−5, 10−1] yes depth [3, 50] no width [0, 10] no activation {relu, no leakyrelu} table 4. hyperparameters for training settings together with ranges for bayesian optimisation of resnets for the optimization procedure, we split the dataset into a training, validation and test set at ratios of (50; 25; 25)% of the whole dataset. the regressor is trained on the training set using the hyperparameters chosen by the bayesian optimization algorithm. once the model loss converged, it is evaluated on the validation data set and the result serves as feedback for the bayesian optimization step to refine its posterior and select new hyperparameters. finally, the test set is used to estimate the model’s capability of generalizing. 2.4.5. model quantitative performance metrics and ai surrogate model selection model selection refers to the process of seeking for a model in a set of candidate models, which delivers the best balance between model fit and complexity [22]. for regression modeling, the quantitative performance metrics used with this research are the mean absolute error (mae), root mean squared error (rmse), rsquared (r2) value as well as the data variance v ar (which is the square of the standard deviation (sd)), cf. tab. 5. the mae shows the average difference between the actual values of the output variable in the original data vs. the predicted output values via the ml models. the lower the mae, the more precise the performance of the model is in predicting future occurrences of the output. the rmse is defined as the standard deviation of the response variable. values of r2 range from 0 to 1, where 1 is a perfect fit, and 0 means there is no gain by using the model over using fixed background response rates. it estimates the proportion of the variation in the response around the mean that can be attributed to terms in the model rather than to random error. for comparing different models, we provide taylor diagrams [23] and additionally report r2, rmse and mae values. taylor diagrams are used to quantify the degree of correspondence between the modeled and observed data using the pearson correlation coefficient, rmse, and standard deviation. a model with the highest r2 and the lowest rmse and mae is preferred. the details of how mae, rmse, and r2 are calculated are shown in tab. 5. mae 1 n ∑n i=1 |ŷi − yi| rmse √ 1 n ∑n i=1(ŷi − yi)2 r2 1 − ∑n i=1 (ŷi−yi)2∑n i=1 (ŷi−ȳ)2 , ȳ = 1 n ∑n i=1 |yi| table 5. regression performance evaluation metrics. where n is the sample size, ŷi is the predicted and yi is the true target value, sse is the sum of the square of error and sst is the total sum of squares. 3. results this section describes the results obtained with knn, lightgbm, and resnet and evaluates their performance as a surrogate for the fem data generated and used within this study, cf. sec. 2.3. all three ml models were trained, validated and tested using the data from the total of 12 million 103 m. a. kraus, r. bischof, w. kaufmann, k. thoma acta polytechnica ctu proceedings data points. comparing the r2 values, all models have high values of around 0.98. this indicates that the ml algorithms are able to describe over 98% of variations in the data, and are highly predictive of the output based on the feature variables used in the study. regarding the mae and rmse, the knn model performs one order of magnitude better than the other two ml models for the cf 0 configuration (without reinforcement) at a level of 0.3% resp. 0.05%, while for the cf 1 configuration (with reinforcement) the resnets has the lowest rmse value of 2.4% and the lgbm has the lowest mae value of 1.8%. the results suggest that all ml models developed for predicting the stress (all in unit: mpa) and stiffness (all in unit: mnm2) tensor components are promising, with resnet being the most predictive model amongst the three. the model performances, described by the mean absolute error mae, root mean squared error rmse and r-squared have converged to a stable minimum during training. 3.1. knn model results fig. 4 provides selected results for the knn algorithm in the two configurations "cf 0" (without reinforcement) and "cf 1" (with reinforcement). it is important to note, that the size of the trained knn model is of around 1.13 gb, as it needs to store all training samples in order to make new predictions. 3.2. lgbm model results fig. 5 provides selected results for the lgbm algorithm in the two configurations "cf 0" (without reinforcement) and "cf 1" (with reinforcement). 3.3. resnet model results fig. 6 provides selected results for the resnet algorithm in the two configurations "cf 0" (without reinforcement) and "cf 1" (with reinforcement). 3.4. overall model comparison results the performances of different ai models according to the mentioned criteria are reported in tab. 6 for configuration "cf 0" (without reinforcement) and in tab. 7 for configuration "cf 1" (with reinforcement). model rmse mae r2 v ar knn 0.00045 0.00310 0.99955 0.00044 lgbm 0.00530 0.01837 0.99440 0.00560 resnet 0.00212 0.01576 0.99760 0.00238 table 6. performance of different models on test set without reinforcement for model selection purposes, we provide selected results for the taylor diagrams in the two configurations "cf 0" (without reinforcement) and "cf 1" (with reinforcement) in fig. 7. model rmse mae r2 v ar knn 0.06678 0.01579 0.93789 0.06678 lgbm 0.02783 0.02329 0.97390 0.02783 resnet 0.02417 0.03301 0.97715 0.02416 table 7. performance of different models on test set with reinforcement 4. discussion the choice for the three investigated ai algorithms was done under three aspects: modeling bias, computing efficiency and prediction precision. the knn and lgbm belong to the family of non-parametric models, whereas resnet is a parametric model. nonparametric ml algorithms promise greater performance at the cost of higher data requirements and training times together with a risk of overfitting. parametric ml/dl models on the other hand come with high modeling bias towards the functional relation, yet neural networks provide a great enough expressiveness for modeling the database. concerning computational efficiency in the prediction stage (which is called a lot of times during fem analysis in each iteration step) knn requires to store the dataset and call it at prediction time, leading to impractical lengthy procedures. lgbm and resnet however are much more time efficient and hence to be preferred for an implementation into the fem analysis. the ai models perform better in cf 0 (without reinforcement) than in cf 1 accross the different quality measures. looking at the r2 values, all models have high values of approx. 0.99 (cf 0) resp. 0.97 (cf 1). this means, that the ml algorithms are able to describe over 97% of variations in the data, and hence are highly predictive for the outputs. for cf 0 rmse is around 0.35% and mae is around 1.5%, while for cf 1 rmse as well as mae lie about 3% and are thus one magnitude bigger than for cf 0. it is noteworthy that on average, every target is predicted well, while the prediction’s standard deviation is dependent on the respective target quantity. given all model results, especially the stiffness tensor terms k22 and k23 show severe model predictive deviations. for model selection, inspection of the taylor diagrams in fig. 7 suggests that all described ai models possess great approximation quality with slight differences, where knn outperforms resnet and lgbm (in the order of decreasing approximation quality). the results in summary suggest that the ai models developed for predicting the stress and stiffness responses of the cmm user-mat are promising and may be used within a fem. given the outlines in the beginning of this section towards computational efficiency, the resnet and lgbm are chosen for further implementation into the fem analysis process. another interesting thought is to use the calibration of ai algorithms on such numerical datasets for non-intrusive verification purposes of the implementa104 vol. 36/2022 ai-fem-hybrids for nonlinear concrete materials (a) (b) (c) (d) figure 4. knn results for: (a) stress component σx in cf 0, (b) stiffness component k11 in cf 0, (c) stress component σx in cf 1, and (d) stiffness component k11 in cf 1. (a) (b) (c) (d) figure 5. lgbm results for: (a) stress component σx in cf 0, (b) stiffness component k11 in cf 0, (c) stress component σx in cf 1, and (d) stiffness component k11 in cf 1. 105 m. a. kraus, r. bischof, w. kaufmann, k. thoma acta polytechnica ctu proceedings (a) (b) (c) (d) figure 6. resnet results for: (a) stress component σx in cf 0, (b) stiffness component k11 in cf 0, (c) stress component σx in cf 1, and (d) stiffness component k11 in cf 1. (a) (b) (c) (d) figure 7. taylor diagrams for: (a) stress component σx in cf 0, (b) stiffness component k11 in cf 0, (c) stress component σx in cf 1, and (d) stiffness component k11 in cf 1. 106 vol. 36/2022 ai-fem-hybrids for nonlinear concrete materials tion of the ground truth data generating mechanism. especially the data points in far distance to the diagonal line in figs. 4, 5,6 give rise to outlier detection methods. it is currently under investigation whether the identified outliers are truly faulty data (i.e. the generating fem material model is not correct) or the ai algorithm with its current state is not able to approximate the respective quantities well. 5. conclusions and outlook this paper presented intermediate results of the first phase of a two-stage research project conducted currently at eth zürich. it reports on calibrating aibased surrogates for a highly nonlinear reinforced concrete plate and shell model upon the cmm. all three ai algorithms have proven to furnish as valid candidates for surrogate models to predict the stress and stiffness tensors used within a fem. the datadriven knn algorithm performed slightly better that the lgbm and resnet model for prediction standard deviation and rmse, whereas on average all models show almost full correlation. future research is concerned with further improving predictive capabilities by training an ensemble model using the knn, resnet and lgbm models. finally, the non-intrusive verification analysis of the ansys user-mat by ai surrogate modelling will shed light on generalisation of this idea of physics informed ml [24]. acknowledgements the authors would like to acknowledge the facilities at eth zürich as well as the design++ resp. immersive design lab for providing computational resources for the machine and deep learning parts of this research project. this research was funded through the eth foundation grant no. 2020-hs-388 (provided by kollbrunner/rodio). references [1] m. kraus, w. kaufmann, k. thoma. 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[24] r. bischof, m. kraus. multi-objective loss balancing for physics-informed deep learning. arxiv preprint arxiv:211009813 2021. 108 https://doi.org/10.1177/0049124104268644 acta polytechnica ctu proceedings 36:99–108, 2022 1 introduction 2 materials and methods 2.1 concept 2.2 fem material model data generation 2.3 data pre-processing 2.4 machine learning model development 2.4.1 knn 2.4.2 lightgbm 2.4.3 artificial neural networks 2.4.4 hyperparameter tuning 2.4.5 model quantitative performance metrics and ai surrogate model selection 3 results 3.1 knn model results 3.2 lgbm model results 3.3 resnet model results 3.4 overall model comparison results 4 discussion 5 conclusions and outlook acknowledgements references https://doi.org/10.14311/app.2022.33.0357 acta polytechnica ctu proceedings 33:357–362, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague high-performance concrete containing waste vitrified tiles diana mariakováa,b,∗, kristina fořtováa, zuzana jirkalováa,b, tereza pavlůa,b, petr hájeka,b a university centre for energy efficient buildings of czech technical university in prague, třinecká 1024, 273 43 buštěhrad, czech republic b department of architectural engineering, faculty of civil engineering, czech technical university in prague, thákurova 7, 166 29 prague 6, czech republic ∗ corresponding author: diana.mariakova@cvut.cz abstract. this contribution deals with the possibility to use waste vitrified tiles (vt) in high-performance concrete. silica powder normally used in high-performance concrete is replaced by waste vitrified tiles in 25, 50, 75 and 100 %. the basic experiments are tested, such as water absorption, flexural strength and compressive strength. the durability is examined, freeze-thaw resistance is tested in 25, 50, 75 and 100 freeze cycles. the results are evaluated and compared with the reference high-performance concrete, containing silica powder. the recipe of concrete is optimized based on these results. keywords: durability, high-performance concrete, waste vitrified tiles. 1. introduction the production of construction and demolition waste (cdw) raised almost up to 42 % in 2019 in czech republic. from 2017, the numbers are similarly high, ranging around 40 % [1]. however, in recent years there appears increasing tendency of recycling and reusing cdw. there are studies dealing with the possibilities of the cdw utilization and management of processing it [2–4]. building materials create almost a half of worldwide produced materials. due to the rapid industrial development the problems are appearing all over the world such as decreasing sources of non-renewable resources and environmental impact connected with the extraction, manufacturing and transporting these materials. at the same time, it is necessary not to omit the associated processes, which release exhaust gases such as carbon dioxide, or processes associated with the end of life of these raw materials and materials produced from them [5]. the problem of the necessity of landfilling these used materials is connected as well. sustainable development and the entire global problem with environmental impacts is related with the volume of produced concrete and the buildings made from this material [6]. replacing of primary raw materials needed to create concrete mixtures is convenient because of saving the primary sources and the energy resources required to obtain these raw materials. concrete is the most used produced material in the world. the high production is the reason of the studies searching for alternative components such as natural aggregates substitutes or cement replacement. there are studies dealing with the possibility to replace natural aggregate in concrete, due to the decreasing applicable sources of this material. the standards require the fulfillment of strict criteria for the selection of suitable aggregates for use in concrete, especially for road aggregates. the studies are examined the utilization of bricks, glass or used concrete as recycled aggregate in concrete [7–11]. the main problem is the possibility of an undesired alkali-silica reaction which needs to be discussed and evaluated [12]. the possibility of replacing (fully or partially) cement or cementitious materials are examined as well. researches are testing the appropriate amount of replacement or the convenient grain size[13–15]. verification of the selected material properties is essential for proper interaction of the components in concrete mixture. basic chemical composition, pozzolanic properties or toxicity [16] should be observed to valorize pertinency of future use. this paper deals with the use of waste vitrified tiles in concrete as a partial of full replacement or silica powder (cement replacement). vitrified tiles were selected as a waste to recycle with pozzolanic properties. the aim of this research is to verify basic properties and find the suitable amount of the replacement to preserve the special properties of high-performance concrete, such as low water absorption or high durability of the material. this paper describes behavior of concrete mixtures containing waste vitrified tiles compared with the reference high-performance concrete in the view of durability and mechanical properties, such as flexural and compressive strength. the testing methods are described and explained, the results are measured, 357 https://doi.org/10.14311/app.2022.33.0357 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en d. mariaková, k.fořtová, z. jirkalová et al. acta polytechnica ctu proceedings calculated and evaluated. based on the results, the recipe of concrete mixture is optimized and adapted for future use in construction industry. 2. materials and methods the mixtures were designed based on the original hpc recipe developed in department of architectural engineering [17]. silica powder was replaced by powder from waste vitrified tiles which is a ceramic material composed of feldspar and clay, almost without any pores. the material is produced under high pressure (350 − 450 kg / cm) and fired at temperatures above 1200 degrees celsius. the replaced part of silica powder was 25 %, 50 %, 75 % and 100 % by weight. original hpc recipe was used as the reference sample, needed during the evaluation and data comparison in the next chapters. 2.1. materials materials according the basic recipe were chosen from local sources and suppliers for preparation 5 types of high-performance concrete mixtures. these materials are: portland cement cem i 42.5 r (cem), natural aggregate in two fractions (na fine and coarse aggregate), silica powder (sp), microsilica, superplasticizers and water. silica powder was replaced by the waste vitrified tiles (vt) in the mixtures, the replacement is described in table 1. amount of cement, natural aggregate, microsilica and water remains same in the mixtures, the variable segment is the amount of used waste vitrified tiles as a silica powder replacement. more detailed information about the properties of used materials are not crucial for the experiments and the results of this work. based on the properties of vitrified tiles, there was an assumption of positive impact on the results. the pozzolanic properties of used waste material can have a positive effect on the resulting strength of concrete samples. this work aims to find the ideal percentage part to replace the silica powder. 2.2. methods basic experiments were made to analyse properties of high-performance concrete containing waste vitrified tiles. due to the vitrified tiles characterization water absorption, compressive strength and flexural strength were examined. the durability was verified as well in 25, 50, 75 and 100 freeze cycles. 2.2.1. water absorption another measured parameter was the absorption of the samples. water absorption of concrete samples is a necessary parameter measured in construction due to its use in structures exposed to external influences, such as water. after 28-days concrete hardening, the samples were weighed, stored in water, then reweighed and the water absorption rate was evaluated based on the measured parameters. 2.2.2. compressive strength there were two types of tested samples. first type was cube 150 × 150 × 150 mm. the tested surface ac was 150 × 150 mm. second type was the fragments with the tested surface ac 40 × 40 mm. fragments from the samples after flexural strength testing were used. in this contribution, the destructive method of compressive strength test was used. the procedure was followed the czech standards čsn en 12350-1, čsn en 12390-1, čsn en 12390-2, čsn en 125041 and čsn en 12390-4 [18–22]. the samples were tested under the speed 2400 n/s, peak 20 kn and the test is completed when the sample breaks, after that the maximum measured force is recorded. five samples from each mixture were tested and the average value was then calculated from the results. whole method is described in čsn en 12390-3 [23]. 2.2.3. flexural strength the tested samples were specimens with dimension 40× 40× 160 mm according to čsn en 12390-5 [24]. the flexural strength is defined in čsn en 1015-11 [25] by three-point press. the samples were tested under the speed 50 n/s, peak 2 kn and the test is completed when the sample breaks. three samples from each mixture were tested and the average value is calculated from the highest measured force. this work is dealing with three-point bending, it is possible to use another method four-point bending. 2.2.4. freeze-thaw resistance the tested samples were 40×40×160 mm. this test was made according to čsn 73 1322 [25]. the samples were cooling by water for four hours to -20 ◦c and then warming up for two hours to 20 ◦c. preparation before freeze thaw resistance test includes data recording. the samples were tested in 25, 50, 75 and 100 cycles, three specimens from each concrete mixture. the compressive strength and the flexural strength are measured after freeze-thaw cycles and effect of the cycles (25, 50, 75, 100) is examined and evaluated. 3. results 3.1. water absorption the water absorption was tested on the cube samples 50×50×50 mm on three specimens from each concrete mixture. the results of water absorption test are shown in the table below (table 2). due to similar character of the concrete mixtures it was expected the similar value of water absorption. all the mixtures values are ranging around 3 %, more detailed results are in table 2. 3.2. compressive strength compressive strength was tested on two types of samples. first type was cube specimen 150 × 150 × 150 358 vol. 33/2022 concrete containing waste vitrified tiles component (g) cem na sp ms h2o vt ref 650 1200 240 175 180 0 vt25 650 1200 180 175 180 60 vt50 650 1200 120 175 180 120 vt75 650 1200 60 175 180 180 vt100 650 1200 0 175 180 240 table 1. recipe of the tested hpc mixtures. mixture weight of wet sample (g) weight of dry sample (g) water absorption ref 294.9 285.9 3.124% vt 25 300.4 292.7 2.642% vt 50 293.8 285.7 2.823% vt 75 292.8 283.7 3.220% vt 100 294.7 285.5 3.199% table 2. water absorption of tested high-performance concrete samples. mixture strength after 28 days (mpa) σ percentage relation to ref ref 119.32 4.75 100.00% vt 25 121.64 3.51 101.94% vt 50 112.89 3.47 94.61% vt 75 122.42 3.72 102.60% vt 100 116.34 6.19 97.50% table 3. compressive strength (cubes 150 × 150 × 150 mm). mixture strength after 28 days (mpa) σ percentage relation to ref ref 128.50 4.64 100.00% vt 25 122.96 5.42 95.70% vt 50 125.49 4.54 97.66% vt 75 131.29 3.66 102.17% vt 100 129.39 2.35 100.70% table 4. compressive strength (fragments of blocks 40 × 40 × 160 mm). mixture strength after 28 days (mpa) σ percentage relation to ref ref 12.31 0.59 100.00% vt 25 11.05 0.13 89.76% vt 50 12.60 0.28 102.24% vt 75 12.14 0.76 98.64% vt 100 12.54 0.34 101.84% table 5. flexural strength (blocks 40 × 40 × 160 mm). 359 d. mariaková, k.fořtová, z. jirkalová et al. acta polytechnica ctu proceedings mm, the tested surface was 150 × 150 mm. the results are shown in table 3. the values are raging 112.89 − 122.42 mpa, highest results were achieved by the mixture containing 75 % and 25 % of vitrified tiles. the reference samples hpc showed slightly lower values, the percentage relation to the ref samples is also shown in the table below (table 3). the lowest values have the concrete mixture containing 50% of waste vitrified tiles. however, the value is still high up to 94.61 % compared to the ref hpc sample. the second tested type of sample were fragments of blocks 40×40×160 mm. the used specimens were tested on the flexural strength before, the tested surface was 40 × 40 mm. summarized results are in table 4. the results were ranging 122.96 − 131.29 mpa and were similar as the results from the cube compressive strength testing. the best results were measured on mixture vt 75 in both cases (on cubes 102.6 % compared to the reference sample, on block fragments 102.17 % compared to the reference sample. 3.3. flexural strength flexural strength was tested on specimens 40 × 40 × 160 mm. the blocks were made in sets of three samples from each concrete mixture. the average values were calculated based on the maximum measured force and are shown in table 5. the highest values were measured on the mixtures containing 50 % and 100 % of waste vitrified tiles substitution. in the comparison with the reference samples, the lowest value has the mixture vt 25 which is observed in the most measured parameters (compressive strength on the fragments of blocks and water absorption). 3.4. freeze-thaw resistance the samples were tested in the sets of three samples. each sample was tested in the flexural strength and compressive strength. samples were exposed to 25, 50, 75 and 100 freeze-thaw cycles to confirm the ability of high-performance concrete to resist freeze-thaw cycles. the results validate that there is no negative effect to this quality caused by the addition of new material waste vitrified tiles and all the values are summarized in figure 1 and figure 2 below. the flexural strength results of reference sample (ref) showed increasing tendency depending on the increasing number of cycles. the values went from 11.54 mpa (strength after 28 days) up to 17.64 mpa (strength after 100 cycles). this tendency was observed on the samples from mixture vt100 and vt25. the increasing tendencies did not occur on samples vt50 and vt75. however, the samples did not show any cracks or signs of failure and after all the minimal measured strength is still 82 % (vt50) compared to the ref. on the other hand, the average values measured on mixture vt100 achieved 102 % compared to the ref. the compressive strength was measured on the fragments of samples 40 × 40 × 160 mm. the tested surface was 40 × 40 mm. in the compressive strength experiment, mixture vt75 showed the best results, where the strength achieved 105 % compared to ref. on the other hand, the lowest result was measured on vt100, where the strength stayed on 93% compared to ref. nevertheless, the result is satisfying in the relation with the other average values raging 95 − 98 % after 100 freeze-thaw cycles. the compressive strength after 25 cycles is ranging from 98 − 104 % compared to the ref, 92.5−105 % after 50 cycles and 91 − 98 % after 75 cycles. it is possible to say, that the results met the expectations. 4. conclusion four mixtures containing different amount (25 %, 50 %, 75 % and 100 %) of waste vitrified tiles as a substitutional material were designed and tested (mixtures vt25, vt50, vt75 and vt100). all the results were compared with the reference sample ref (high-performance concrete made from primary raw materials). the water absorption was experimentally tested, the absorption of mixtures with tiles was similar to the ref. the absorbency values of all tested samples were around 3 %. the flexural strength was tested and vt50 and vt100 showed the highest results. the measured force and calculated strength were higher than the values of the ref. similar numbers were evaluated on the samples after compressive strength experiment. however, the highest results were provided by vt25 and vt75 (cubes) and vt50 and vt100 (fragments of blocks). all these results were higher than results of ref. the freeze-thaw resistance was verified. the freezing cycles don’t have the impact to the results. the compressive strength of the samples made from the mixtures containing 25, 50, 75 and 100 % substitution of silica powder by waste vitrified tiles was raging 91 − 105 % compared to the reference sample. the flexural strength achieved 116 % compared to the reference sample after 50 freeze-thaw cycles, rest of the samples were ranging from 82 % up to mentioned 116 %. when omitted maximal and minimal, the average value was 99 % compared to the ref. based on the experiments and the results, it is possible to say that the proposed mixtures are suitable for further testing and verification of their properties, which so far confirm the possibility of their use as hpc. the similarity of the resulting properties of the tested mixtures containing waste tiles with the reference mixture is given mainly by the composition of used waste vitrified tiles. therefore, the resulting values correspond to the assumptions made at the beginning of this experiment and forms a solid basis for further research. 360 vol. 33/2022 concrete containing waste vitrified tiles figure 1. flexural strength values after freeze-thaw resistance experiment. figure 2. compressive strength values after freeze-thaw resistance experiment. acknowledgements this research was funded by ministry of education, youth and sports ltain19205 design of performance based concrete using sand reclaimed from construction & demolition wastes. this work has been supported by the grant agency of the czech technical university in 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[24] sn en 1015-11: methods of test for mortar for masonry part 11: determination of flexural and compressive strength of hardened mortar, 2020. [25] čsn 73 1322: determination of frost resistance of concrete, 1968. 362 https://doi.org/10.4028/www.scientific.net/msf.866.58 https://doi.org/10.1016/j.conbuildmat.2008.07.023 https://doi.org/10.1016/j.jksues.2020.04.013 https://doi.org/10.3390/su12010250 https://doi.org/10.28991/cej-2020-03091620 https://doi.org/10.2139/ssrn.3577292 https://doi.org/10.1016/j.matpr.2020.09.164 https://doi.org/10.3390/ma14020463 https://dspace.cvut.cz/handle/10467/54346 acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.34.0032 acta polytechnica ctu proceedings 34:32–37, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague microstructure reconstruction via artificial neural networks: a combination of causal and non-causal approach kryštof latkaa,∗, martin doškářb, jan zemanb a gymmázium nový porg, pod krčským lesem 25, 142 00 prague 4, czech republic b czech technical university in prague, faculty of civil engineering, department of mechanics, thákurova 7, 166 29 prague 6, czech republic ∗ corresponding author: latka@novyporg.cz abstract. we investigate the applicability of artificial neural networks (anns) in reconstructing a sample image of a sponge-like microstructure. we propose to reconstruct the image by predicting the phase of the current pixel based on its causal neighbourhood, and subsequently, use a non-causal ann model to smooth out the reconstructed image as a form of post-processing. we also consider the impacts of different configurations of the ann model (e.g., the number of densely connected layers, the number of neurons in each layer, the size of both the causal and non-causal neighbourhood) on the models’ predictive abilities quantified by the discrepancy between the spatial statistics of the reference and the reconstructed sample. keywords: microstructure reconstruction, neural network, causal neighbourhood, non-causal neighbourhood. 1. introduction multi-scale modelling is a powerful predictive tool that bypasses the need for complex constitutive laws by performing auxiliary calculations at lower scales, using a characteristic sample of a material microstructure [1]. such a sample can be easily extracted when analysing a material with regular, periodic arrangement of material’s phases; in case of a material with stochastic microstructure, the representative sample is typically constructed artificially such that it matches selected spatial statistics of the material microstructure [2]. while originally the representative samples were generated via optimization approaches, e.g. [3], recently, reconstruction methods relying on machine learning have started to emerge, using various frameworks including markov random fields [4], deep adversarial neural networks [5, 6], or supervised learning using classification trees [7]. several papers include references to causal and non-causal neighbourhood which both prove to be effective ways of extracting input data for the chosen framework. however, the causal approach generally seems to be the preferred one, with one of these papers even claiming that their model cannot generate valid results if based on a non-causal neighbourhood [4]. the objective of our work is to reconstruct an image of a microstructure from an almost random noise with microscopic properties as similar as possible to the original image. while many of the aforementioned proposed approaches are implementation-complex, we present a simple method using the tensorflow framework with the keras sequential api [8]. we closely follow the methodology of bostanabad and coworkers, [7]; however, instead of using classification trees, we use two distinct artificial neural networks (anns), where the first network reconstructs the general pattern of the microstructure and the second network denoises and smoothes out the previously reconstructed pattern. for simplicity, we study only two-phase materials, i.e. we test the framework with black and white images. our implementation and data are publicly available at a gitlab repository [9]. 2. methodology the proposed reconstruction method comprises two distinct steps: (1.) the reconstruction of a general shape of the microstructure from a random noise with margins of the reference image used as a seed. (2.) the smoothing procedure of the reconstructed image that improves the local features of the microstructural geometry. 2.1. reconstructing microstructural geometry in step 1, the overall material distribution should be outlined in the reconstructed sample, rendering the key features of the microstructural geometry. we opted for a sequential approach, in which new values of individual pixels in the discrete, pixel-like representation of the newly generated microstructural geometry are predicted from the values previously determined at antecedent positions (thus the causal approach), because such an approach has already proven its merits in microstructural reconstruction, cf. [7]. 32 https://doi.org/10.14311/app.2022.34.0032 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 34/2022 microstructure reconstruction via ann hr figure 1. illustration of the causal neighbourhood with hr = 2 pixels around the central pixel highlighted in dark grey. the red pixels represent the input data for the ann trained in step 1; however, in order to extract an input structure of a rectangular shape, the black and yellow pixels are also included in the inputs but their values are discarded and replaced by random binary values. to this end, we define a rectangular causal neighbourhood of (2hr+1)× (hr+1) pixels with parameter hr being the given neighbourhood radius; see fig. 1 for an illustration. note that the positions highlighted in red constitute the real input data; the dark grey and yellow pixels are only a padding (without value) such that the whole input features a regular 2d shape and, consequently, pooling layers can be easily applied. the actual value of the dark grey pixel serves as a label during an extraction of the training data from a reference image. the neural network for predicting the pixel values in step 1 is designed such that the rectangular input is first subsampled using a nrp × nrp pooling layer, flattened and passed through several densely connected layers. based on our numerical experiments, the maximum pooling layer consistently delivered better results than the average pooling layer. the setup of the training procedure and the effect of the remaining network parameters, namely the number of densely connected layers nr` , the number of neurons in each layer nrn, the size of the pooling layer nrp and the neighbourhood radius hr, are discussed later in section 4. once the network is trained, a new microstructural realization is generated by iterating over pixels of a to-be-reconstructed image in a raster scan order. consequently, an initial microstructural geometry must be provided in a margin of width hr at the left, top, and bottom edge of the image; the initial values in the remaining part of the image are irrelevant and we generate them as a random binary noise; see fig. 2b. 2.2. smoothing procedure outputs of the model trained in step 1 usually contain the main features of the trained microstructural geometry; however, local details are typically polluted by random noise; compare figs. 2a and 2c. for step 2 we train an additional neural network to smooth out the image and correct irregularities in the image generated in the step 1. this time, the model works with a complete, i.e. non-causal, square neighbourhood around the central pixel (illustrated in figure 3), usually of a smaller neighbourhood radius hs than in the causal model in step 1. again, the two-dimensional structure of the input is needed to facilitate a subsampling/pooling layer, which is then followed by flattening and passing through two densely connected layers. the impact of the actual choice (i.e. average vs maximum pooling) of the subsampling layer is discussed in section 4). to increase robustness of the trained model and prevent if from learning to simply copy the value of the central pixel, we introduce two errors: (i) the value of the dark grey pixel, which is used as a label in the training, is always randomized in the inputs, and (ii) we also randomly choose value for ξ × 100% of red pixels from fig. 3. 3. error quantification in order to assess the performance of the proposed models beyond a visual inspection, we compare generated microstructural samples to the reference images in terms of spatial statistics. the most straightforward spatial statistics is a volume fraction φ of a chosen phase (the white phase, i.e. pixels with value 1, in our case). we define error εφ as an absolute value of the difference between the volume fraction φref in the reference sample and the volume fraction φgen in the generated microstructure, εφ = |φref − φgen| . (1) the second spatial statistics considered in our work is the two-point probability function s2(x), which states the probability of finding two points separated by x in a given phase. since all our data are represented as a regular grid of values, the discrete version of s2(x) can be easily computed using the fast fourier transform [2]. consequently, we quantify the discrepancy in the reference and a generated microstructure by means of their discrete two-point probability functions sref 2 ∈ rni×nj and sgen 2 ∈ rni×nj as εs2 = ‖s gen 2 − sref 2 ‖f ninj , (2) where ‖a‖f is the frobenius matrix norm [10] ‖a‖2 f = ni∑ i=1 nj∑ j=1 (ai,j)2 . (3) finally, to quantify the effect of our smoothing noncausal model, we add the third error metric εd that captures the level of local heterogeneity. assuming a two-phase medium, a microstructure can be represented with a boolean matrix m ∈ {0, 1}ni×nj . for each pixel we can computed a local quantity di,j as a sum of the averaged absolute differences between the 33 k. latka, m. doškář, j. zeman acta polytechnica ctu proceedings (a) (b) (c) figure 2. microstructure reconstruction process using the causal model. after training the neural network model on input data from the initial microstructure (a), the trained model is used to sequentially predict pixel values in a raster scan order using previously predicted pixel values from the causal neighbourhood. hs figure 3. illustration of the non-causal neighbourhood of a neighbourhood radius of hs = 2 pixels. the black pixel represents the central pixel and the red pixels represent non-causal neighbourhood. when extracting the neighbourhood, the central black pixel is represented as a random binary value. value of the central pixel and its neighbouring eight pixels, di,j = 1 8 1∑ k,l=−1 |mi+k,j+l −mi,j | . (4) the error εd is then computed again as an average over the image excluding the one-pixel wide margin, i.e. εd = 1 (ni − 2)(nj − 2) ni−1∑ i=2 nj−1∑ j=2 di,j . (5) the reasoning behind this error measure is that if we consider the phases of pixels in a very small neighbourhood around the central pixel, the number of pixels whose phase is different to that of the central pixel will be lower if the edges are properly smoothed out. even though this might not necessarily be true for the pixels which form the edge of the reconstructed pattern (and thus, the black and white phase must switch), it will apply on a larger scale (hence, we compute the sum of the values of dij, for all pixels in the image). therefore, in theory, the lower the value of εd is for an image, the more smoothed out the image should be. 4. results we report the effect of parameters on the quality of the microstructural reconstruction quantified with the error measures introduced in the previous chapter. first, we focus on parameters of the reconstruction model. tables 1 and 2 illustrate the impact of altering number of neurons nrn 9 16 25 nr` = 2 0.046 0.031 0.036 nr` = 3 0.031 0.049 0.029 nr` = 4 0.002 0.010 0.021 table 1. values of the volume fraction error εφ, depending on the number of layers nr` and the number of neurons in each layer nrn. number of neurons nrn 9 16 25 nr` = 2 1.36×10−4 9.36×10−5 1.05×10−4 nr` = 3 9.32×10−5 1.46×10−4 7.57×10−5 nr` = 4 2.96×10−5 3.71×10−5 6.70×10−5 table 2. values of the two-point probability error εs2 , depending on the number of layers nr` and the number of neurons in each layer nrn. the number of densely connected layers nr` and the number of neurons in each layer nrn, while keeping the neighbourhood radius hr and the pooling size nrp constant, on the volume fraction error εφ and the twopoint probability error εs2 , respectively. in particular, we set hr = 15 and nrp = 3 as these values produced the most visually appropriate reconstructions in early tests of the model. the next two tables, tabs. 3 and 4, summarize the sensitivity study investigating the influence of the neighbourhood radius hr and the pooling size nrp on the reconstruction. this time, all the tests were performed with nr` = 2 densely connected layers with nrn = 16 neurons in each layer. 34 vol. 34/2022 microstructure reconstruction via ann (a) (b) figure 4. effect of the smoothing network based on the non-causal model: (a) reconstructed image serving as an input, (b) microstructure corrected by the smoothing model. both images are of size 200 x 200 pixels. neighbourhood radius hr 10 12 15 nrp = 2 0.491 0.298 0.006 nrp = 3 0.058 0.046 0.031 table 3. values of the volume fraction error εφ, depending on neighbourhood radius hr and pooling size nrp. neighbourhood radius hr 10 12 15 nrp = 2 2.50×10−3 1.24×10−3 4.05×10−5 nrp = 3 1.81×10−4 1.36×10−4 9.36×10−5 table 4. values of the two-point probability error εs2 , depending on neighbourhood radius hr and pooling size nrp. the next three tables, i.e. tables 5, 6, and 7, summarize the parametric study for the smoothing model with non-causal neighbourhood. we chose the reconstructed image obtained by the first model using nr` = 2 layers, each with nrn = 16 neurons, a neighbourhood radius of hr = 15 and a pooling size of nrp = 3 as an input to the model and compared the resulting smoothed-out image to the original microstructure (before reconstruction), recall fig. 2a, in terms of the error measures introduced in section 4. we carried out two sets of test: one for the average and one for the maximum pooling 2d layer. in each set, we altered the radius of the non-causal neighbourhood hs as well as the magnitude of the artificially introduced noise ξ. each set rendered three tables as we inspected also the level of local heterogeneity εd, in addition to the errors εφ and εs2 already reported for the generative model. 5. discussion first, we add an observation regarding the three error measures defined in section 3 and our visual perception of the reconstructed microstructures. the first error measure, εφ, served as a coarse check that the volume fraction in the reconstructed image is similar neighbourhood radius hs 5 7 10 m ax ξ = 0.05 0.041 0.002 0.040 ξ = 0.10 0.004 0.036 0.033 ξ = 0.15 0.051 0.027 0.033 av er ag e ξ = 0.05 0.058 0.000 0.033 ξ = 0.10 0.037 0.003 0.030 ξ = 0.15 0.044 0.029 0.036 table 5. values of the volume fraction error εφ, depending on the magnitude of artificially introduce noise ξ, neighbourhood radius hs, and the type of pooling layer used (max or average). neighbourhood radius hs 5 7 10 m ax ξ = 0.05 1.22×10−4 3.15×10−5 1.20×10−4 ξ = 0.10 3.24×10−5 1.06×10−4 9.88×10−5 ξ = 0.15 1.23×10−4 7.21×10−5 9.88×10−5 av er ag e ξ = 0.05 1.76×10−4 3.08×10−5 1.01×10−4 ξ = 0.10 1.12×10−4 3.25×10−5 8.98×10−5 ξ = 0.15 1.31×10−4 8.90×10−5 1.08×10−4 table 6. values of the two-point probability error εs2 , depending on the magnitude of artificially introduce noise ξ, neighbourhood radius hs, and the type of pooling layer used (max or average). to the original; however, it could not assess how similar the reconstructed pattern is to the original. for this purpose, we adopted εs2 , based on the two-point correlation function, as we excepted it ot be better suited to compare the reconstructed pattern to the original. nevertheless, we noticed a significant discrepancy between the values of εs2 for each model and the visual similarity of the reconstructed pattern to the original one. for example, the generative model using nr` = 2 layers, each with nrn = 16 neurons, neighbourhood radius of hr = 15 and a pooling size of nrp = 3, could probably be considered as the most accurate in terms of the visual pattern (the image reconstructed using this model is in figure 2c), but only seventh 35 k. latka, m. doškář, j. zeman acta polytechnica ctu proceedings neighbourhood radius hs 5 7 10 m ax ξ = 0.05 0.130 0.133 0.123 ξ = 0.10 0.135 0.129 0.124 ξ = 0.15 0.123 0.130 0.127 av er ag e ξ = 0.05 0.113 0.120 0.112 ξ = 0.10 0.114 0.119 0.112 ξ = 0.15 0.114 0.115 0.113 table 7. values of the local heterogeneity error εd, depending on the magnitude of artificially introduce noise ξ, neighbourhood radius hs, and the type of pooling layer used (max or average). for comparison, the value of εd for the reference image is 0.097. best according to εs2 . we conclude that other spatial statistics such as two-point cluster function or lineal path should be added to the suite of error measures as well to capture both the global distribution of a microstructure and its local characteristics. on the other hand, the assessment of the smoothing model by εd values was generally in accordance with the quality of the visual appearance of the reconstructed images. our results show that taking more layers with less neurons in each was favourable to the approach with less but more populated layers. perhaps as expected, the larger neighbourhood radius was considered during training of the generative model, the better the results were. surprisingly, nrp = 2 pooling size was better for the largest neighbourhood radius, while larger pooling was preferential in all other cases. in the case of the smoothing model, considering larger neighbourhood radius hs beyond certain threshold (in our case hs = 7) did not improve its performance. this can be attributed to the different purpose of both models; while the generative model needs information from distant points to properly distribute the material with the sample, the smoothing model is by its nature local. on the other hand, larger pooling layer was consistently outperforming the smaller one in all tests. most importantly, we noticed that the values of εd were significantly lower for the smoothing models using an average pooling layer instead of a maximum pooling layer. this is probably due to the fact that the average pooling layer ignores sharp features in the image (e. g. individual pixels whose phase was not correctly identified in the part 1 of the reconstruction), which allowed us to smooth out the edges of the reconstructed pattern in the image. however, it is important to emphasize that these observations are specific for the considered microstructure. 6. conclusions despite the simplicity of the proposed ann-based model, accompanied by the ease of implementation facilitated by the tensorflow framework and the keras sequential api, the model generates meaningful microstructural geometries. the combination of a causal model used for reconstruction and a noncausal smoothing model in particular yielded satisfying results, cf. figure 4, considering the fact that the models knew only a limited local information. we believe that even better results can be obtained by, e.g., incorporating the considered errors directly in the loss function during the training process of individual networks. yet, this remains to be done in our future work. the need for the margin of initial values during reconstruction might be seen as a limitation restricting the model to generating microstructural samples only as large as the reference one, from which the margin can be easily copied. however, a possible solution is to take the reference sample, dismember it into pieces and reorder the pieces so that they form the margin of desired size. alternatively, starting from different parts of the reference microstructure, a set of smaller samples can be generated; these samples can be then assembled together while blending the microstructure in their overlaps using, e.g., image quilting [6]. acknowledgements m. doškář and j. zeman gratefully acknowledge support by the czech science foundation, project no. 19-26143x. references [1] m. g. d. geers, v. g. kouznetsova, k. matouš, j. yvonnet. homogenization methods and multiscale modeling: nonlinear problems. in e. stein, r. de borst, t. j. r. hughes (eds.), encyclopedia of computational mechanics second edition, pp. 1–34. john wiley & sons, ltd, chichester, uk, 2017. https://doi.org/10.1002/9781119176817.ecm107. [2] s. torquato. random heterogeneous materials: microstructure and macroscopic properties. no. 16 in interdisciplinary applied mathematics. springer, new york, 2002. [3] j. zeman, m. šejnoha. from random microstructures to representative volume elements. modelling and simulation in materials science and engineering 15(4):s325–s335, 2007. https://doi.org/10.1088/0965-0393/15/4/s01. [4] l.-y. wei, m. levoy. fast texture synthesis using tree-structured vector quantization. in proceedings of the 27th annual conference on computer graphics and interactive techniques siggraph '00. acm press, 2000. https://doi.org/10.1145/344779.345009. [5] z. yang, x. li, l. c. brinson, et al. microstructural materials design via deep adversarial learning methodology. journal of mechanical design 140(11), 2018. https://doi.org/10.1115/1.4041371. [6] d. fokina, e. muravleva, g. ovchinnikov, i. oseledets. microstructure synthesis using style-based generative adversarial networks. physical review e 101(4), 2020. https://doi.org/10.1103/physreve.101.043308. 36 https://doi.org/10.1002/9781119176817.ecm107 https://doi.org/10.1088/0965-0393/15/4/s01 https://doi.org/10.1145/344779.345009 https://doi.org/10.1115/1.4041371 https://doi.org/10.1103/physreve.101.043308 vol. 34/2022 microstructure reconstruction via ann [7] r. bostanabad, a. t. bui, w. xie, et al. stochastic microstructure characterization and reconstruction via supervised learning. acta materialia 103:89–102, 2016. https://doi.org/10.1016/j.actamat.2015.09.044. [8] m. abadi, a. agarwal, p. barham, et al. tensorflow: large-scale machine learning on heterogeneous systems, 2015. software available from tensorflow.org. [9] k. latka, m. folwarczny, m. doškář, j. zeman. aibased microstructure reconstruction. https://gitlab. com/martindoskar/ai-based-reconstruction, 2021. [10] g. h. golub, c. f. van loan. matrix computations. the johns hopkins university press, baltimore, 4th edn., 2013. 37 https://doi.org/10.1016/j.actamat.2015.09.044 https://gitlab.com/martindoskar/ai-based-reconstruction https://gitlab.com/martindoskar/ai-based-reconstruction acta polytechnica ctu proceedings 34:32–37, 2022 1 introduction 2 methodology 2.1 reconstructing microstructural geometry 2.2 smoothing procedure 3 error quantification 4 results 5 discussion 6 conclusions acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.38.0262 acta polytechnica ctu proceedings 38:262–268, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague low rents and low operating costs in social housing – an example from germany marc grossklos∗, andré müller institut wohnen und umwelt gmbh, rheinstraße 65, d-64295 darmstadt, germany ∗ corresponding author: m.grossklos@iwu.de abstract. the social housing project “passivehousesocialplus” in germany with 42 residential apartments demonstrates how low rents and low operating costs can be combined. different concepts for reducing operating costs were implemented and tested in one refurbished and one new apartment building. high energy efficiency for heating (passive house standard) and domestic hot water preparation were complemented by efficiency measures for household electricity (e.g. pre-installed energy-efficient kitchen appliances, led lighting) and water savings through gray water usage for flushing toilets. additionally, a flat rate billing scheme for most of the ancillary costs was established. due to the overall low level of consumption, space heating and domestic hot water are also included in the flat rate with defined budgets. displays in all apartments are informing the tenants about the used-up budget and show a forecast for the consumption of water and electricity until the end of the year. results from more than two years of measuring are showing a low consumption of heat, which is about in the calculated range. the budgets for drinking water and electricity are slightly exceeded on average, nevertheless, the consumption levels are significantly below average consumption in germany. the operating costs are around 30 % below the costs of comparable apartments in the same city. keywords: social housing, operating costs, flat rate, budget, monitoring results, heating, water, electricity. 1. introduction affordable housing is a major challenge in the housing market, in germany as well as worldwide. inexpensive housing must also be combined with the requirements for climate protection and the economical use of resources. most of the time, the two aspects seem incompatible, as ambitious efficiency measures often increase construction costs. as a result, lowincome households and recipients of support services for housing have often no access to climate-friendly and resource-saving dwellings. in addition, rents in germany have generally risen steadily in recent years, which poses financial problems for low-income households in particular. in addition to the net rents, however, the operating costs are also increasing, which today can represent a “second rent” that reduces the disposable income of the households. large parts of the population are affected – especially the unemployed, single parents, low-income earners, pensioners and families with many children. in the group of unemployed people, the share of housing costs of the disposable income increased from 35 % to over 47 % between 2004 and 2017 [1]. for them it is essential to reduce the living and operating costs. the general public also has a great interest in reducing costs, because if state transfer payments are paid, minimizing housing costs from rent, operating and ancillary costs is also in the interest of society as a whole. figure 1. refurbished old building (west view). 2. building concept the “neue wohnraumhilfe ggmbh” in darmstadt initiated and implemented the “passivehousesocialplus” to provide people with difficulties in accessing the housing market with inexpensive living space. this project, which was implemented in 2018/19, intended a combination of low rents and low operating costs in a modern and climate-friendly subsidized housing. the two buildings of the passivehousesocialplus were implemented on a former us-army barracks site in darmstadt, germany. two of the three building parts from 1955 were kept and refurbished to an energy efficient building standard (with 22 apartments), to minimize the manufacturing energy expenditure for a new building structure (see figure 1). for barrier262 https://doi.org/10.14311/app.2022.38.0262 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 38/2022 low rents and low operating costs in social housing . . . figure 2. new building (view from south-west). free access one third of the building was replaced by a barrier-free new building (20 apartments in total) with also 6 wheelchair-accessible apartments (see figure 2). the two buildings have 42 apartments in total and 3 186 m2 of heated living space for about 138 residents. concepts for reducing ancillary costs during operation were implemented in both buildings. the focus was on increasing energy efficiency in the areas of heating, domestic hot water (dhw) preparation as well as on electricity consumption in the flats and for auxiliary services. in addition, measures to reduce costs for drinking water and household electricity were implemented. these are e.g.: • passive house standard of the building envelope for the new building, respectively refurbishment with passive house components for the existing building to reduce heating costs; windows have triple thermal insulation glazing with a ug-value from 0.50 to 0.54 w/(m2 · k) in plastic frames (uf = 0.94 w/(m2 · k)), the u-values of the opaque building envelope are in the range of 0.10 to 0.15 w/(m2 · k). • ventilation system with heat recovery in every apartment for increasing indoor air quality while reducing ventilation heat losses. • minimization of distribution heat losses through improved thermal insulation of the distribution pipes (doubled insulation thickness compared to the minimum legal requirements). • energy-efficient kitchen appliances (a++ and a+++) were pre-installed as well as led lighting in all rooms to reduce household electricity consumption; energy-efficient system technology (lift, led lighting, pumps). • water saving measures. the passivehousesocialplus is heated with a common district heating connection (in the basement of the existing building). two tanks with 3 m3 volume in total are buffering peak loads during dhw preparation and reduce the commissioned district heating power, which reduces the ancillary costs for the tenants. the maximum rated power was limited to 60 kw (17.5 w/m2 in total for heating and dhw preparation). a primary energy factor of 0.5 kwhpe/kwhfinal for district heating is guaranteed by the supplier. the heating system of the refurbished building furthermore consists of the existing distribution system and the existing radiators. the dhw is supplied via home stations in each flat. the maximum flow temperature in the 4-pipe-system is 48 °c for heating and dhw. in contrast, the new building has a 2-pipesystem also feeding home stations in each flat but delivering the heat to an air supply heating and only one single radiator in the bathroom. the maximum flow temperature in the 2-pipe-system is 53 °c for dhw in summer and 58 °c for heating and dhw in winter. each apartment in the passivehousesocialplus has its own ventilation system with heat recovery, that allows the tenants to choose among four levels of ventilation. in order to minimize heat losses, the building envelope is sealed against air leaks very well. the new building achieved an airtightness test value according to din en 13829 n50 of 0.24 1/h, the existing building has a mean value of 0.52 1/h. thus, all building parts are well below the limit for passive houses of 0.6 1/h. both buildings have monocrystalline pv systems installed on the flat extensive green roof in an eastwest orientation, the refurbished building 40.9 kwp, the new building 43.3 kwp. there are also lithium-ion storage systems for each dwelling with a net storage capacity of 17.5 kwh in the existing and 43.8 kwh in the new building. the power produced by the pv systems is used in the buildings first, then stored in the battery storage in case of a surplus production and, finally the remaining power is fed to the grid. flushing the toilet needs a lot of drinking water. to minimize this water consumption, the buildings use gray water from a gray water treatment plant in the basement. slightly polluted waste water from showers and hand basins in the bathrooms is collected separately and fed to the shared gray water treatment plant in the basement of the existing building (wastewater from the kitchen was not used because of higher fat loads to be expected there). the gray water system consists of a filter for the gray water, a treatment tank with biological cleaning and a storage tank for the treated process water. the cleaned water is used via a separate water installation exclusively for flushing toilets. if the filling of the process water tank is to low, drinking water is fed in to ensure toilets can be operated at any time. both buildings were finished in 2019 and let between august and december 2019. 3. flat rate billing one objective of the project was to minimize the operating costs. in particular, these are the consumptionrelated costs for heat, electricity, drinking water and 263 marc grossklos, andré müller acta polytechnica ctu proceedings figure 3. visualisation of the budget for household electricity and drinking water on a display in each apartment. sewage. to examine the possibilities of reducing the different types of operating costs, an investigation was carried out beforehand [2]. to simplify the administration by the landlord, it was also assessed which types of operating costs can be billed at a flat rate. usually, the energy supply for space heating and dhw preparation forms a major part of the ancillary costs. in the passivehousesocialplus the consumption for heating was reduced to a very low level of 13.0 kwh/(m2 · yr) needed for both building parts in average (at standard conditions). because this value is below the limit value of 15 kwh/(m2 · yr), which is defined in the heating costs ordinance in germany, a flat rate billing for heating and dhw was legally permitted in contrast of a usage-bound billing. the total costs of consumption for heat are therefore calculated based on the living area of the flat, even if measures to reduce the energy consumption for dhw are difficult. however, the dhw volume is measured and considered for the available budget for drinking water. the drinking water consumption and thus the additional costs should be reduced with the following measures: through the usage of water-saving taps, by the replacement of drinking water with treated gray water for flushing toilets and by the visualization of the remaining budget that is calculated for the drinking water. the amount of the budget depends on the number of persons in the household and should be sufficient if the tenants are frugal. the budget is included in the flat rate billing and was set at 25 m2/yr for the first, 18 m3/yr for the second and 17 m3/yr for each additional person. additional credit balance for drinking water must be purchased, if the budget is exceeded. the aim is to motivate the people to use resources economically. household electricity is also included in the flat rate for operating costs via a budget, which is unusual in germany because electricity is normally billed by an energy supplier. the budget is calculated according to the lowest consumption category a of the german household electricity index [3] and requires frugal behavior of the tenants. the budget is as follows: 850 kwh/yr for the first person, 350 kwh/yr for the second person and 300 kwh/yr for each additional person. the buildings have photovoltaic systems and electricity storage in order to reduce the electricity costs per kwh. since the legal hurdles for the landlord to supply electricity to the tenants are very high in germany [4], a service provider (energy cooperative) was commissioned to supply the tenants. the displays in the apartments show the consumption of the budget for water and electricity as well as a forecast up to the end of the year and historical values (figure 3). an additional feature is the free wifi in the apartments, which the tenants can use with limited bandwidth within the service charge flat rate. 4. measurement results in the passivehousesocialplus more than 600 meters and sensors with more than 1 100 measuring points are installed to measure the consumption and usage parameters over a period of nearly three years. the following results are from the period october 2019 until september 2021 (refurbished building) and march 2020 until september 2021 (new building), respectively. the heated living space of 1 662 m2 (refurbished building) and 1 578 m2 (new building) were used for the area-related parameters. 264 vol. 38/2022 low rents and low operating costs in social housing . . . figure 4. heat consumption for heating, dhw and distribution losses of the refurbished (left and middle column, two heating periods) and the new building (right column, one heating period) as well as heating degree days on site. 4.1. heating and dhw for the first heating period the refurbished building had a heat energy consumption of 20.6 kwh/(m2 · yr) including distribution losses between the buffer tanks and the flat (figure 4 left). according to the phpp, the heating demand was 19.2 kwh/(m2 · yr) including distribution losses. although the values match well, the interpretation must take into account that the outside climate in 2019/20 was very mild (the heating degree days were only 810 kd compared to a long-term average of 1018 kd). at the same time, the average room temperature was around 22 °c, which is often measured in energy-efficient buildings. if these factors and the actual household electricity consumption (internal heat sources) are considered in the phpp, the result is a heating requirement of 19.8 kwh/(m2 · yr). consumption and adjusted heating requirements therefore also go well together. in addition, a consumption of 17.9 kwh/(m2 · yr) for hot water preparation and 9.4 kwh/(m2 · yr) for distribution losses (only distribution for dhw and storage losses) was derived from the measuring data. these values agree very well with the planning values for an adapted outside climate. the total heat consumption reached 47.9 kwh/(m2 · yr) in 2019/20. in 2020/21 the consumption for heating was 27.1 kwh/(m2 · yr) (figure 4 middle). this increased consumption can be explained very well by the lower outside temperature (982 kd heating degree days april until march). due to the longer heating period, the distribution losses are somewhat lower and the hot water consumption is almost unchanged. the total heat consumption reached 53.2 kwh/(m2 · yr). in the new building, 20.4 kwh/(m2 · yr) were measured for heating (figure 4 right) in 2020/21, whereby the distribution losses are not included here. the heat consumption for dhw preparation is 13.5 kwh/(m2 · yr), which is lower than in the existing building, but fits well with the planning values due to the lower occupancy density in the new building. the total heat consumption reached 42.9 kwh/(m2 · yr) in the new building. 4.2. household electricity and total power consumption for household electricity 23.4 kwh/(m2 · yr) where measured 2020/21 in the refurbished building and 25.9 kwh/(m2 · yr) in the new building (table 1). to classify the values, the smaller area per person in the buildings must be considered, which is 43 % below the average in germany for apartment buildings. general power, e.g. for lightning of corridors, is 2.0 or 1.2 kwh/(m2 · yr) respectively. the total power consumption is 31.9 kwh/(m2 · yr) for the refurbished and 34.6 kwh/(m2 · yr) for the new building. the total budget for household electricity was 65 950 kwh/yr, the actual consumption was 79 652 kwh/yr, i.e. the budget was exceeded by 21 %. the following figures are showing for each flat the power consumption over the power budget 2020/21. if the marked rhombus is above the dashed diagonal, the flat has consumed more household electricity than provided with the budget. in the refurbished building (figure 5) approximately as many flats are below the dashed line as are above. in mean, the consumption is 9 % above the calculated budget. 41 % of the flats are reaching electricity efficiency class a, 23 % class 265 marc grossklos, andré müller acta polytechnica ctu proceedings refurbished building new building [kwh/(m2 · yr)] [kwh/(m2 · yr)] household electricity 23.4 25.9 general power 2.0 1.2 auxiliary power 6.5 7.5 total 31.9 34.6 table 1. electricity consumption of the refurbished and the new building in july 2020 – june 2021. figure 5. power consumption per flat over power budget per flat for the refurbished building 2020/21. b, 32 % class c and only one flat (5 %) the class d, which represents the mean power consumption in germany [2]. in figure 6 (new building) the budget is exceeded by 35 % on average. there, only 20 % are reaching class a, 25 % class b, 20 % class c, 15 % class d and 10 % class e and f each. the reasons for the different adherence to the budget between refurbished and new building are not known. 4.3. power generation in 2020/21, a pv yield of 24.0 kwh/(m2 · yr) was measured in the refurbished building, which is in the same order of magnitude as household electricity consumption. of this yield, 11.6 kwh/(m2 · yr) were used directly, 1.9 kwh/(m2 · yr) were used to charge the battery and 11.4 kwh/(m2 · yr) were fed into the grid (table 2). the pv yield in the new building was slightly higher with 25.5 kwh/(m2 · yr) and with 12.1 kwh/(m2 · yr), more electricity was consumed directly in the building. 5.2 kwh/(m2 · yr) were used to charge the batteries and only 9.7 kwh/(m2 · yr) were fed into the grid. the pv yield in the new building is higher than the annual household electricity consumption. the share of direct consumption in the refurbished building was 49 % of the pv power generation, the total self-consumption including battery storage was figure 6. power consumption per flat over power budget per flat for the new building 2020/21. 56 % (table 3). 39 % of the total electricity consumption of the existing building was covered physically by the pv system. the efficiency of the battery storage 2020/21 was 70 % in the refurbished building and 76 % in the larger storage unit in the new building, which is below the expected value of approx. 80 %. after commissioning, a number of problems (cell charging, software) with the battery storages occurred. these were fixed during operation. from march to august 2021 the efficiency of the battery storage reached 75 % in the refurbished building and 78 % in the new building. 4.4. drinking water an important item in the reduction of costs was the economical use of drinking water, since the costs of 3.75 €/m3 for drinking water including wastewater fee in darmstadt are relevant for the ancillary costs. the total budget for drinking water in all apartments, that does not contain any water for flushing the toilet (see below), was 2 663 m3/yr. the consumption of 2 810 m3/yr or 20.4 m3/(person · yr) in the evaluation period 2020/21 was around 5.5 % above budget. in the new building, the budget was even slightly undercut; in the refurbished building, consumption was 10 % above the budget. the average drinking water consumption in germany is 44.9 m3/(person · yr) [5] with a share of 27 % (that means 12.1 m3/(person · yr)) 266 vol. 38/2022 low rents and low operating costs in social housing . . . refurbished building new building [kwh/(m2 · yr)] [kwh/(m2 · yr)] pv-power generation 24.0 25.5 of that •direct consumption 11.6 12.1 •charging of battery 1.9 5.2 •grid supply 11.4 9.7 table 2. power generation and feed-in of the refurbished and the new building in 2020/21. refurbished new building building ratio of direct consumption of pv power generation 49 % 45 % ratio of self-consumption (incl. battery) of pv power generation 56 % 71 % total coverage ratio of household consumption 39 % 43 % table 3. characteristic proportions for electricity in 2020/21. accounted for flushing the toilet. without this proportion, a comparative value of 32.8 m3/(person · yr) could be calculated. this means that the apartments are below the average consumption by 38 %. for flushing toilet 16.7 m3/(person · yr) were measured in the two buildings in mean, the value is thus around 37 % above the national german average. nevertheless, the drinking water consumption could be significantly reduced to 5.57 m3/(person · yr) by the gray water system. the cover ratio has reached a mean value of 67 % from august 2020 until july 2021 which means a reduction in drinking water consumption by 54 % compared to the german mean value. 5. rents and auxiliary costs the construction costs for the refurbished building were 1 485 €/m2 (including building envelope, system technology, pv system and battery storage), for the new building it was 1 763 €/m2 and reached the originally planned amount [6]. as a result, the targeted height of rents, including public funding, could be achieved. the rents in the apartments are 6.50 €/(m2 · month) and thus well below the comparable rent, which in this area is 10.04 €/(m2 · month), while the best energy standard has been implemented at the same time. the billed ancillary costs in 2020 were 1.14 € for consumption costs for heating and drinking water, 0.87 € for other ancillary costs such as property tax, building cleaning or waste disposal and 0.52 € for household electricity, which is normally not included in ancillary costs in germany. the agreed flat rate for ancillary costs and the billed costs were in the same range in 2020, so that the landlord’s expenses were covered. however, the necessary security surcharges are missing, so that the flat rate for ancillary costs (excluding household electricity) had to be increased from 2.10 €/(m2 · month) at the start of the rental to 2.50 €/(m2 · month). nevertheless, the ancillary costs are still well below the comparative value of 3.59 €/(m2 · month) in social housing in the same city for 2016 [2]. 6. conclusions in the social housing sector, 22 apartments were created in a highly efficient refurbished building and 20 in a new building meeting the passive house standard. a special feature of the project is the flat rate for ancillary costs and the budget for drinking water and household electricity, which are also included in the flat rate. the household’s consumption and the remaining range of the budget are shown on a display in each apartment. the measurement results show that the heating consumption is very low, despite flat rate billing, and is only slightly above the expected values if the actual room temperatures and outdoor climate are taken into account. the total heat consumption for heating, dhw preparation and distribution losses is between 45 and 51 kwh/(m2 · yr) and therefore also quite low – especially when the high density of people in the apartments is considered. household electricity and drinking water consumption are slightly higher than the budgets agreed. nevertheless, 81 % of households use less electricity than the average in germany and only 10 % are above the average. the pv systems provide about as much electricity over the year as is consumed in the households in total. physically, 39 resp. 43 % of the electricity consumption of the buildings could be covered with solar energy. drinking water consumption is 38 % below the average consumption in germany. for toilet flushing the average drinking water consumption was 54 % lower than the mean value in germany, despite high individual consumption and due to the gray water treatment and reuse. 267 marc grossklos, andré müller acta polytechnica ctu proceedings the results of the monitoring are predominantly positive and a significant reduction in ancillary costs could be achieved, even if there is still room for improvements. the presented concepts should now be widely applied. in the case of consumption feedback, further comparisons between the use of displays and inexpensive online access would be useful. acknowledgements the research project was funded by the german federal ministry of economics and energy within the project “mobasy” (“solar building” funding initiative, grant number fkz 03sbe0004a). furthermore, the authors are thanking petra grenz and folkmer rasch of faktor10 and wolfgang bauer-schneider and doreen petri of neue wohnraumhilfe. references [1] statistisches bundesamt. fachserie 15/1 wirtschaftsrechnung – einnahmen und ausgabe privater haushalte, volumes for 2004, 2007, 2017, wiesbaden, 2017. [2022-02-10]. https://www.statistischebibliothek.de/mir/ receive/deserie_mods_00000152 [2] m. großklos, m.-c. krapp, c. von malottki, b. stein. ansätze zur reduktion der nebenkosten im sozialen wohnungsbau am beispiel des vorhabens “passivhaussozial-plus” in darmstadt. institut wohnen und umwelt, darmstadt, 2018. [2022-02-10]. https://www.iwu.de/fileadmin/publikationen/ energie/neh_ph/2018_iwu_grossklosetal_ reduktion-der-nebenkosten-am-beispielpassivhaussozialplus.pdf [3] co2online. stromspiegel für deutschland 2019, berlin, 2019. [2022-02-10]. https://www.stromspiegel.de/fileadmin/ssi/ stromspiegel/broschuere/stromspiegel-2019web.pdf [4] i. behr, m. großklos. praxishandbuch mieterstrom. springer vieweg verlag, wiesbaden, 2017. [5] bdew bundesverband der energieund wasserwirtschaft e.v. trinkwasseranwendung im haushalt, berlin, 2020. [2022-02-10]. https://www.bdew.de/service/daten-undgrafiken/trinkwasserverwendung-im-haushalt/ [6] m. großklos, g. behem, a. müller, et al. passivhaussozialplus – konzept, umsetzung, kosten und ergebnisse des ersten messjahres. institut wohnen und umwelt, darmstadt, 2021. [2022-02-10]. https://www.iwu.de/fileadmin/publikationen/ energie/mobasy/2021_iwu_grossklosetal_ passivhaussozialplus-konzept-umsetzung-kostenergebnisse-erstes-messjahr.pdf 268 https://www.statistischebibliothek.de/mir/receive/deserie_mods_00000152 https://www.statistischebibliothek.de/mir/receive/deserie_mods_00000152 https://www.iwu.de/fileadmin/publikationen/energie/neh_ph/2018_iwu_grossklosetal_reduktion-der-nebenkosten-am-beispiel-passivhaussozialplus.pdf https://www.iwu.de/fileadmin/publikationen/energie/neh_ph/2018_iwu_grossklosetal_reduktion-der-nebenkosten-am-beispiel-passivhaussozialplus.pdf https://www.iwu.de/fileadmin/publikationen/energie/neh_ph/2018_iwu_grossklosetal_reduktion-der-nebenkosten-am-beispiel-passivhaussozialplus.pdf https://www.iwu.de/fileadmin/publikationen/energie/neh_ph/2018_iwu_grossklosetal_reduktion-der-nebenkosten-am-beispiel-passivhaussozialplus.pdf https://www.stromspiegel.de/fileadmin/ssi/stromspiegel/broschuere/stromspiegel-2019-web.pdf https://www.stromspiegel.de/fileadmin/ssi/stromspiegel/broschuere/stromspiegel-2019-web.pdf https://www.stromspiegel.de/fileadmin/ssi/stromspiegel/broschuere/stromspiegel-2019-web.pdf https://www.bdew.de/service/daten-und-grafiken/trinkwasserverwendung-im-haushalt/ https://www.bdew.de/service/daten-und-grafiken/trinkwasserverwendung-im-haushalt/ https://www.iwu.de/fileadmin/publikationen/energie/mobasy/2021_iwu_grossklosetal_passivhaussozialplus-konzept-umsetzung-kosten-ergebnisse-erstes-messjahr.pdf https://www.iwu.de/fileadmin/publikationen/energie/mobasy/2021_iwu_grossklosetal_passivhaussozialplus-konzept-umsetzung-kosten-ergebnisse-erstes-messjahr.pdf https://www.iwu.de/fileadmin/publikationen/energie/mobasy/2021_iwu_grossklosetal_passivhaussozialplus-konzept-umsetzung-kosten-ergebnisse-erstes-messjahr.pdf https://www.iwu.de/fileadmin/publikationen/energie/mobasy/2021_iwu_grossklosetal_passivhaussozialplus-konzept-umsetzung-kosten-ergebnisse-erstes-messjahr.pdf acta polytechnica ctu proceedings 38:262–268, 2022 1 introduction 2 building concept 3 flat rate billing 4 measurement results 4.1 heating and dhw 4.2 household electricity and total power consumption 4.3 power generation 4.4 drinking water 5 rents and auxiliary costs 6 conclusions acknowledgements references https://doi.org/10.14311/app.2022.33.0309 acta polytechnica ctu proceedings 33:309–315, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague innovative strengthening of rc columns using a layer of a fibre reinforced concrete peter koteša,∗, martin vavruša, wioletta raczkiewiczb a university of zilina, faculty of civil engineering, department of structures and bridges, univerzitná 8215/1, 010 26 zilina, slovakia b kielce university of technology, faculty of civil engineering and architecture, department of concrete construction and geotechnics, al. 1000 pp 7, 25-314 kielce, poland ∗ corresponding author: peter.kotes@uniza.sk abstract. reinforced concrete structures that are influenced by degradation, overloading, the thawing and freezing cycles, abrasive damage and corrosion of reinforcement, should be repaired or strengthened. each of those mentioned situations lead to decreasing the load-carrying capacity of the construction. the damage of a structure or its member causes exceeding the serviceability limit states and ultimate limit states. for this reason, for further use of the structures, they have to be strengthened to increase the load-carrying capacity and to extend the remaining lifetime. a new method for strengthening the vertical members, e.g., columns, is using the fibre reinforced concrete layer and its increased tensile strength in comparison to common rc concrete. this article deals with the theoretical design of dimensioning the columns using a layer of fibre concrete around a column (wrapping). in this case, it is necessary to calculate the residual strength of fibre concrete, which is a crucial factor for the tensile strength of the element. keywords: columns, experimental measurements, fibre reinforced concrete, numerical modelling, reinforced concrete, strengthening. 1. introduction typical way how to strengthen the member, as the beams and columns, is e.g. using the frp materials [1–5]. the fibre reinforced concrete (frc) is mostly used on concrete floors [6], but recent researches are also focused on use of the frc to strengthen columns [7, 8]. the type of fibres has a huge influence on the load-bearing capacity of the fibre reinforced concrete. therefore, a parametric study of that effect on column strengthening is presented in the article. the theoretical calculation is experimentally and numerically verified on a group of columns. use of the fibre reinforced concrete changes not only its load-bearing capacity, but its behaviour and the nature of failure, as well [9]. an iteration diagram is the most often used for assessment of members subjected to combination of normal force and moment like columns, which is derived from selected material (concrete, reinforcement) and geometric assumptions. the curve of the iteration diagram represents points, which are derived from the limit deformations, based on conditions of the balance of forces and deformation of the corresponding design value of the relative deformation of a material. the cross-section is stressed by the bending moment and a normal force; these forces must be in balance. the internal cross-sectional forces refer to the centre of gravity of the element. it is also assumed that the compatibility condition of relative deformation ε, depending on the used working diagrams, is fulfilled [10–12]. the design bearing capacity of the cross-section is expressed by ultimate force nrd and ultimate moment mrd: nrd = fc + ! fsi (1) mrd = fc · zc + ! fsi · zsi (2) where: • fc is the compression force in the concrete [kn], • fs is the tensile force in the reinforcement [kn], • zc is the length of the internal force distance from the centre of gravity of the cross-section to the compressive force [m], • zsi is the length of the internal forces distance from the centre of gravity of the cross-section to the forces in reinforcement. to construct an iteration diagram of a strengthened cross-section using the fibre reinforced concrete (frc), it is necessary to know the tensile force of the fibre reinforced concrete. since frc has a higher tensile strength than the plain concrete, this material can be applied to strengthening the column. in the cases, where we are limited by the cross-section and it is not possible to use the concrete reinforcement due to non-compliance with requirements for covering the 309 https://doi.org/10.14311/app.2022.33.0309 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en p. koteš, m. vavruš, w. raczkiewicz acta polytechnica ctu proceedings figure 1. experimental program, a) load of a sample, b) sample after loading. reinforcement, it is a suitable way to use a layer of fibre reinforced concrete [13]. by adding a new layer of fibre concrete and its increased strength, it is possible to expand the iteration diagram and thus achieve the required load-bearing capacity, which would comply with the ultimate limit state (uls). 2. experimental program to construct and verify the analytical calculation, it was necessary to start from a real experimental measurement. basic dimensions of the reinforced concrete column cross-section 160 × 160 mm were chosen, which was used for strengthening by the fibre concrete wrapping. the column was reinforced with 4 !10 mm. the class of concrete c16/20 and the reinforcement b500b were used. axial distance of stirrups was 100 mm and diameter of stirrups was 8 mm. detailed reinforcement and description of materials’ characteristic are presented in [14]. for the strengthening was used the dramix 3d fibres of a length 35 mm. the volume of fibres in the concrete mixture was 0.5 %, which represented 40 kg/m3 of fibres. concreting of the reinforced column was carried out horizontally by placing the column in the formwork and consisted of two steps. the first step was the concreting of the lower part, which had a thickness of 35 mm and spacer washers were used to ensure an even thickness of the cladding. the second step was to insert the unreinforced column into the fresh concrete mix from step one and then concrete the fibre concrete. two samples of columns were tested. the contact between the original and the newly added frc was made on a smooth surface and was not specially treated. the samples were loaded eccentrically, the value of eccentricity was 100 mm. reinforced columns strengthening with the frc carried 410.62 kn (s3_sv_01) and 359.34 kn (s3_sv_02). figure 2 shows the relation between the load and deformation in the middle of the column (sensor s3). sample sv_02 carried about 51.28 kn less than sample sv_01, which may be due to uneven distribution of fibres in the concrete mix. the bending moment was determined from the normal force, namely 41.06 knm and 35.93 knm. from these values, point 2 was determined, which are ultimate bending moment and ultimate normal force. more information is given in [14]. figure 2. relationship load deformation, sensor s3. to verify the shear resistance, three samples of push-tests were made. the samples consisted of a core and encasement. dimension of samples of crosssection were the same as dimension of a column. the surface was considered smooth, which was cleaned and moistened before concreting [15]. samples after hardening, i.e., after 28 days, were tested by the axial force. the deformation between the core and the encasement was measured. after dismantling the sample, it was clear that in some places the new concrete "overgrown" with the old concrete, which could be caused by hydration of the concrete. a large number of air cavities can also be seen in the samples, which weakened the shear surface. using the fibre concrete, one or two major longitudinal cracks were formed at the time of the shear resistance. after reaching the maximum shear force, cracks were formed and opened. detailed information is provided in [16]. in order to construct an iteration diagram, it was necessary to determine the material characteristics of the fibre reinforced concrete. a 3-point bending test was performed (figure 3c). the size of the test beam was 150×150×700 mm with a notch. three samples were tested, where the average value was calculated from the 3-point bending test. the average force of the crack mouth opening displacement (cmod) was 10.8 kn. 3. analytical calculation an interaction diagram is a great tool for designing vertical elements. using the iteration diagram, it 310 vol. 33/2022 rc columns strengthening figure 3. push test and 3-point bending test, a) dimension of a push test, b) push test after the loading, c) sample after the 3-point bending test. is possible to calculate the cross-sectional resistance. when designing the reinforcement of the column, it is necessary to combine the bearing capacity of the original cross section and the bearing capacity of the new layer of fibre concrete. however, in order to determine the load-bearing capacity of the reinforced column, using the fibre-reinforced concrete, it was necessary to implement the knowledge from the calculation of fibre-reinforced concrete and combine it with the calculation of the interaction diagram. the basics of calculation, are given in stn en 1992-1-1 [17]. the calculation of the residual strength of the fibre reinforced concrete can be performed according to the fib model code [18] or according to rilem tc 192 [19]. in this article, the calculation was done according to rilem tc. the work procedure according to rilem is based on the results of the 3-point bending by determining the values at certain widths of the opening. from the width of the cracks, it is possible to determine the stresses that describe the tensile behaviour of the fibre reinforced concrete at certain degrees of the frc (fig. 4). to calculate the stresses [20], it is necessary to determine the residual strength of the fibre reinforced concrete for the crack opening displacement of 0.5 mm, 2.5 mm and 3.5 mm according the equation fri = 3 · fi · lz 2 · b · h2 sp (3) according to löfgren [21], it is necessary to compensate the random location and orientation of the fibres by the dimensional factor kh: kh = 1.0 − 0.6 h − 12.5 47.5 , (4) where h is the height of members [mm]. calculation of individual stresses of the diagram from the tensile strength of fibre reinforced concrete was done as follows, [? ]: σ2 = 0.45 · fr1, b · kh (5) σ3 = 0.37 · fr3, b · kh (6) σ1 = 0.7 · fctm · (1.6 − d) (7) the neutral axis xtot (ratio factor) for the tensile stress area is given as: xtot = a1 x1 + a2 x2 + a3 x3 + a4 x4 + a5 x5 atot [−] (8) where: • ai is the individual areas of the frc tensile stress blocks [mm2], • xi is the length to center of gravity of individual tensile areas of frc [−], • atot is the total stress of the tensile stress block of frc [m2]. the resulting stress in the fibre reinforced concrete is given by: σm = atot (9) in calculation of the iteration diagram, for point 2 of the resistance line, it is assumed that the limit relative deformation εcu3 is achieved in the compressed fibres (edge) of concrete and the yield strength is reached in the steel reinforcement (figure 4) [17]. it follows that the height of the compressed part x of the concrete is equal to the limit height xlim. the calculation of the reinforcement deformation is based on assumptions of achieving the limit deformation in concrete εs2 = εcu3 xlim (xlim − d2a) > εyd = 2.5" (10) εs1 = εcu3 xlim (da − xlim) > εyd = 2.5" (11) the following equations are used for calculation of tensile forces fsi, the compressive force of the concrete fc, the compressive force of the fibre reinforced concrete ffci and the tensile force of the fibre reinforced concrete ffti, respectively: a) fs1 = as1 · σs1 b) fs2 = as2 · σs2 (12) 311 p. koteš, m. vavruš, w. raczkiewicz acta polytechnica ctu proceedings figure 4. iteration diagram point 2. figure 5. iteration diagram point 2. fc = b · λ (xlim − a) · σcf (13) ffa2 = 2 · a · λ · xlim · σcf (14) a) fft1 = a · b · σ3 b) fft2 = a · ba · σcf (15) ffct2 = 2 · σm (ha − xlim) · a (16) where: • a is the thickness of strengthening, • b is the width of original column, • ba is the width of new cross-section (strengthened), • ha is the height of new cross-section (strengthened), • σc is the compressive stress in concrete, • σcf is the compressive stress in the fibre concrete. the internal forces arms for fibre reinforced concrete are calculated as follows a) zc = h 2 − λ (xlim − a) 2 b) zcf = ha 2 − λ · xlim 2 c) zft = """" ha 2 − xlim """" + xtot (ha − xlim) (17) the resulting internal forces are calculated according to the equilibrium conditions nrd, 2 = −fc − ffc2 − ffca2 − fs2 + fft1 + fs1 + ffct2 (18) mrd, 2 = fc zc + ffc2 zf2 + ffca2 zcf + fs2 zs2 + fft1 zf1 + fs1 zs1 + ffct2 zft (19) according to the calculation presented above, the other points of the iteration diagram were derived (figure 5). a detailed calculation is given in [22]. 312 vol. 33/2022 rc columns strengthening figure 6. iteration diagrams for individual methods of calculation comparison. rilem tc-162 fib model code plain concrete rilem -pc fib -pc nrd mrd nrd mrd nrd mrd nrd mrd nrd mrd 0 −1354.2 0 −1354.2 0 −1354.2 0 0.00 % 0.00 % 0.00 % 0.00 % 1 −794.07 41.59 −801.19 41.748 −801.19 40.90 0.90 % 1.67 % 0.00 % 2.03 % 2 −412.22 44.32 −417.99 43.52 −430.91 42.77 4.53 % 3.49 % 3.09 % 1.73 % 4 −198.22 31.39 −193.52 31.08 −190.3 29.87 4.00 % 4.85 % 1.67 % 3.92 % 5 176.18 0 197.89 0 157 0 10.89 % 0.00 % 20.66 % 0.00 % table 1. comparison of bending moment and normal forces for various methods of calculating. 4. analytical parametric study in both experimental and analytical studies, an iteration diagram of the strengthened column was constructed using the fib model code [19] and rilem tc-162 [20]. to determine the utilized of fibres compared to plain concrete, an interaction diagram of the strengthening column by using the plain concrete was also created. the reason for comparison of these methods is to determine the contribution of fibres in the strengthening for different areas of the bending moment and the normal force combination (figure 6). in table 1, is presented a comparison of the individual points calculated according to rilem and fib model code and the associated increase in internal forces. the results of the work can be applied to strengthening of the substructures of bridges and footbridges, such as columns of pillars [23–29]. in that case, it would certainly be necessary to investigate the effect of the chloride ions penetration into the cross section and its influence on degradation of concrete and corrosion of steel fibres [30–33]. 5. conclusions when comparing these methods, it is necessary to take into account that the effect of the fibre reinforced concrete (frc) has a great influence on the loadbearing capacity, namely, the frc softening or hardening and how many fibres bridge the crack. in the performed experiment, the composite has softened, i.e., the main crack is formed and the fibres are pulled out due to increase of the load. the given experiment shows that the calculation methods differ from each other only minimally. the largest increase in fibres is observed in the area of plain tension where in comparison to the unreinforced concrete by 10.89 % for the calculation procedure according to rilem and by 20.66 % for the calculation procedure according to fib. a comparison of the calculation procedures in point 2, which is also verified by the experimental program, shows that the increment from the frc for rilem was 4.53 % for normal force and 3.49 % for bending moment. the analytical calculation according to fib in point 2 transferred smaller values for a normal force of 3.09 % and a bending moment of 1.73 %. as can be seen from the experiment, the increase in the load-bearing capacity, due to the frc, is influenced by the number of fibres, the orientation of the fibres in the crack and also by the length of the fibre. using the dramix 3d, only slight increase of the load-bearing capacity was achieved. however, the given analytical calculation was confirmed by an experiment, as well as by a numerical model, where it correctly captured the achieved bearing capacity of the given cross-section. 313 p. koteš, m. vavruš, w. raczkiewicz acta polytechnica ctu proceedings acknowledgements this research was supported by the slovak research and development agency under contract no. apvv-14-0772 and by research project no. 1/0306/21 and project no. 1/0045/19 of the slovak grant agency. references [1] k. gajdosova, v. borzovic, a. valasik, et al. application of gfrp reinforcement in the design of concrete structures 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[30] w. kubissa, r. jaskulski, j. chen, et al. evaluation of ecological concrete using multi-criteria ecological index and performance index approach. architecture, civil engineering, environment 12(1):97-107, 2019. https://doi.org/10.21307/acee-2019-009. 314 https://doi.org/10.2478/sjce-2018-0015 https://doi.org/10.2478/v10189-011-0010-3 https://doi.org/10.2478/sjce-2019-0007 https://doi.org/10.1063/1.5019140 https://doi.org/10.2478/sjce-2018-0027 https://doi.org/10.4028/www.scientific.net/amr.1054.48 https://doi.org/10.3390/ma13235432 https://doi.org/10.1515/cee-2016-0014 https://doi.org/10.21307/acee-2019-009 vol. 33/2022 rc columns strengthening [31] w. raczkiewicz, p. g. kossakowski. electrochemical diagnostics of sprayed fiber-reinforced concrete corrosion. applied sciences 9(18), 2019. https://doi.org/10.3390/app9183763. [32] w. raczkiewicz, a. wojcicki, w. grzmil, et al. 3rd world multidisciplinary civil engineering, architecture, urban planning symposium (wmcaus 2018), iop conference series-materials science and engineering 471:032048, 2019. [33] w. raczkiewicz. influence of the air-entraining agent in the concrete coating on the reinforcement corrosion process in case of simultaneous action of chlorides and frost. advances in materials science 18(1):13-9, 2018. https://doi.org/10.1515/adms-2017-0023. 315 https://doi.org/10.3390/app9183763 https://doi.org/10.1515/adms-2017-0023 https://doi.org/10.14311/app.2022.33.0300 acta polytechnica ctu proceedings 33:300–308, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague suggestion of ultimate strength formulas of partial frame pile cap composed of exterior column, foundation beam and pile shinji kishidaa,∗, tomohisa mukaib a shibaura institute of technology, 3-7-5 toyosu koto-ku tokyo 135-8548, japan b building research institute, 1 tachihara, tsukuba-shi, ibaraki-ken 305-0802, japan ∗ corresponding author: skishida@sic.shibaura-it.ac.jp abstract. currently, there is no research and few valid experiments of pile caps. and shear failure mechanism of a pile, exterior column and foundation beam and pile caps in rc structure is not resolved yet under bi-lateral loading. therefore, a performance evaluation method based on mechanical behaviour for pile caps has not been established. first, the fracture type was specified from the experimental results. secondary, the ultimate strength formula of the pile cap was proposed based on the previous experimental results. it is a theoretical formula based on the truss-arch theory. it was confirmed that this formula can accurately evaluate the ultimate strength of the pile cap. keywords: failure mode, pile-cap, ultimate strength formulas. 1. introduction although the seismic performance of buildings after a large earthquake is ensured under the current seismic standards, measures to ensure continuous use after a large earthquake have not been established. prevention of building collapse at the time of a major earthquake is secured by current earthquake resistance standards but plans to ensure continued use after the earthquake have not been established. it is necessary to develop a method for conducting performanceoriented seismic design with "sustainability of buildings after earthquake" as the required [1]. from the viewpoint of continuous usability, it is conceivable that the pile cap will be damaged and deformed in the axial direction will occur and the building will not be able to continue to use due to the inclination of the building. pile cap is an important structural joint member. its function is to transfer the stresses occurring on the columns through a group of piles to the ground, taking place the complex stresses under earthquake loading. it is very important to clarify pile cap shear failure mechanism of reinforced concrete (rc) structures. however, shear failure mechanism of a pile, exterior column-beam pile cap in rc structure is not resolved yet under bi-lateral loading. therefore, in this study, the frame experiment of the pile cap was carried out using two types of hoops arranged in the pile cap as experimental factors. the purpose of this study was to clarify the effect of columns and pile cap hoops on pile caps. we evaluated the pile cap shear strength formula proposed in the previous study [2, 3] and proposed a pile cap shear strength formula based on the truss-arch mechanism. 2. outline of test 2.1. specimens fourteen half-scale reinforced concrete pile caps assembled a precast pile, an exterior column and aăfoundation beam, those specimens modelled actual middle-high buildings, were tested. aăconfiguration of specimens, section dimensions and reinforcement details are shown in figure 1. specific properties of specimens are summarized in table 1. material characteristics of concrete and steel are listed in tableă2, respectively. the constant axial load in compression was applied at the top of the column for all specimens. the depth and width of the column section were 300mm and 300mm, respectively. the depth and width of the foundation beam section were 200mm and 600mm, respectively. the length from the center of the column to the loading point on a beam end was 1500mm. the height from the center of the beam to the supporting point on the top of the column or to the bottom support was 1200mm and 1275mm, respectively. steel pile (diameter is 190.7mm, thick is 45mm) was used as a precast pile, the embedment length was 100mm, 8-d19 bars were arranged as anchor dowel bars. the grout was filled into the hollow part of the steel pile for all specimens. all specimens were designed to form shear failure mechanism. for the specimen a-7a and a-8, the pile cap hoop ratio (pcpw) was arranged in 0.22%, the column hoop ratio in pile cap (cpw) was arranged 0.47 times more than the specimen a-7a. for the specimen a-7b and a-9, the pile cap hoop ratio (pcpw) was arranged in 0.10%, the column hoop ratio in pile cap (cpw) was arranged 0.23 times more than the specimen a-7a. for the specimen a-8 and a-9, the column hoop ratio 300 https://doi.org/10.14311/app.2022.33.0300 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 33/2022 frame pile ultimate strength formulas �� � �� � �� � �� �� ���� �� � �� � ��� ��� �� �� �� �� �� �� �� � �� � 3rvlwlyh�orglqj1hjdwlyh�orglqj � 㻌�3loh�fds ���� ���� �� �� � �� � �� �� � ��� &rpprq�vhfwlrq ����� �� ��� �� �� �� �� � �� �� �� � ��� 㻼㼕㼘㼑 㻭㼚㼏㼔㼛㼞㼟 㻤㻙㻰㻝㻥㻔㻿㻰㻠㻥㻜㻕 㻯㼛㼘㼡㼙㼚㻌㼙㼍㼕㼚 㼞㼑㼕㼚㼒㼛㼞㼏㼑㼙㼑㼚㼠 㻤㻙㻰㻝㻟 㻌㻌㻌㻔㻿㻰㻟㻠㻡㻕 㻯㼛㼘㼡㼙㼚 㻲㼛㼡㼚㼐㼍㼠㼕㼛㼚㻌㼎㼑㼍㼙 &roxpq�odwhudo�uhlqirufhphqw '��6'����#�� �,q�ehdp�froxpq�mrlqwv 䚷6'���$#���� 0dlq�uhlqirufhphqw ri�irxqgdwlrq�ehdp ��'���3%6'���� /dwhudo�uhlqirufhphqw ri�irxqgdwlrq�ehdp 8����8uxerq�#�� 㻌㻌�%dvnhw�w\sh �6shflphq�$��� $[ldo�irufh��&roxpq�d[ldo�irufh�udwlr�������� figure 1. derails of specimens. figure 2. details of specimens. 301 shinji kishida, tomohisa mukai acta polytechnica ctu proceedings specimen a-7a a-7b a-8 a-9 axial force (axial force ratio : 0.2) 500 kn 514 kn 462 kn 467 kn column width × depth 300 mm × 300 mm main reinforcement 8-d13(sd785) hoop d6(sd785)@50 hoop in pile cap (hoop amount : cpw[%]) d6(sd295a)@100 (0.15) d6(sd295a)@50 (0.30) d6(sd295a)@300 (0.07) d6(sd295a)@300 (0.07) foundation beam width × depth 200 mm × 600 mm main reinforcement upper and bottom 3-d22(pbsd930) stirrup u9.0(1275mpa)@50: high-strength shear reinforcement spacing bar 2-d6(sd295a) pile steel pile s45c ϕ190.7 t-45mm anchor bar 8-d19(sd490) pile cap width × depth × height 500 mm × 500 mm × 770 mm vertical reinforcement 4-d6(sd295a) 4-d10(sd295a) hoop (hoop amount : pcpw[%]) d6(sd295a)@50 (0.22) d6(sd295a)@100 (0.10) d6(sd295a)@50 (0.22) d6(sd295a)@100 (0.10) 10 0 20 0 20 0 25 4, 5 50 50 ���� 12 0 80 12 0 80 12 0 80 15 4, 5 50 50 ���� 11 7 80 12 0 80 12 0 12 0 13 3 ��� 12 0 10 0 10 0 10 0 10 0 10 0 15 0 ��� table 1. properties of specimens details. in pile cap (cpw) was arranged in 0.07%. cpw = caw/ (b × l) (1) pcpw = pcaw/ (b × l) (2) where, b: pile cap width, l: distance between the centers of gravity of the main beams of the foundation beam, cpw, pcpw: the total of each cross-sectional area of the column and the pile cap hoop arranged in the cross section (b × l). 2.2. loading apparatus and instrumentation a loading apparatus is shown in figure 2. the foundation beam end was supported by horizontal roller, while the bottom of pile was supported by a universal joint. the reversed cyclic horizontal load and the constant axial load in compression (an axial load ratio of 0.20 in all specimens) were applied at the top of the column through a tri-directional joint by three oil jacks. the jack orthogonal to a horizontal loading direction prevented an out-of-plane overturn for specimen. all specimens were controlled by a story drift angle for one loading cycle of 0.25%, two cycle of 0.5%, 1%, 2%, one cycle of 3% respectively, and two cycle of 4%. the story drift angle was defined as a story drift divided by height of the column and pile; 3400mm and 2475mm. lateral force, column axial load and foundation beam shear forces were measured by load-cells. story drift, foundation beam and column deflections, and local displacement of a pile cap panel were measured by displacement transducers. strains of foundation beam bars, column bars and pile cap bars, anchors and hoops were measured by strain gauges. 3. test results 3.1. story shear drift relationship relationships between the story shear force and the story drift angle are shown in figure 4. the story shear force was obtained from moment equilibrium between measured beam shear forces and the horizontal force at a loading point on the top of the column. the pile cap hoop yielded before the story shear force achieved the maximum strength. for specimen a7a and a-7b, the maximum strength was 1.06 times larger on the positive loading than specimen a-7a, 1.11 times larger on negative loading, respectively. after the maximum strength, the decreasing rate was 17% at positive loading for specimen a-7a, 25% at negative loading for specimen a-7b. when the pile cap hoop is larger than the column hoop, the effect 302 vol. 33/2022 frame pile ultimate strength formulas specimen reinforcing bar parts used yield stress! n/mm2" yield strain [µ] a-7a a-7b d6(sd295a) column, pile cap 451.2 2246 d6(sd785)∗ column 900.4 6684 d10(sd295a) pile cap 361.0 1989 d13(sd785) column 816.1 5331 d19(sd490) anchor 530.0 3027 d22(pbsd930/1080)∗ main reinforcement of foundation beam 999.4 6933 u9.0(sbpd1275/1420)∗ reinforcing bar of foundation beam 1319.5 8672 a-8 a-9 d6(sd295a)∗ column, pile cap 378.7 4079 d6(sd785)∗ column 928.3 6985 d10(sd295a) pile cap 362.7 1936 d13(sd785) column, pile cap 900.4 6735 d16(sd785)∗ column 879.1 6716 d19(sd490)∗ anchor 543.5 3538 d22(pbsd930/1080)∗ main reinforcement of foundation beam 1001.6 6990 u9.0(sbpd1275/1420)∗ reinforcing bar of foundation beam 1450.8 8507 table 2. material properties of steel. specimen compressive strength! n/mm2" modulus of elasticity! ×104 n/mm2" strain at compressive strength [µ] split tensile strength! n/mm2" a-7a, a-7b 28.4 2.08 2652 2.16 a-8 25.7 2.01 2767 2.21 a-9 26.0 2.02 2755 2.21 table 3. material properties of concrete. on the maximum shear strength and the ductility capacity were increased. for specimen a-8 and a-9, the smaller the total hoop mass(pcpw +c pw) in the pile cap, the lower the maximum shear strength. 3.2. crack patterns crack patterns at the maximum strength are shown in figure 3. for specimen a-7a and a-8, the pile cap vertical bar and the hoop yielded before the story shear force achieved the maximum strength and the column base was crushed at maximum strength. after the maximum strength, the pile cap shear crack width did not increase so much, and the damage to the column base became larger. the maximum strength was determined by pile cap shear failure, and after the maximum strength, it was judged that the column was destroyed by crushing. for specimen a7b and a-9, before the maximum shear strength, the pile cap hoop, the vertical bar and the column hoop yielded. due to the width of the pile cap shear cracks has increased after the maximum shear strength, it was judged that pile cap shear failure was destroyed in these two specimens. 4. consideration of the pile cap hoops 4.1. pile cap crack properties specimens a-7a and a-7b in which the total hoop amount (pcpw +c pw) in the pile cap is almost the same and the ratio of pcpw and cpw are different are compared. comparing the pile cap cracks at the time of the final failure of the two specimens shown in figure 3, specimen a-7a, which had many pile cap hoops, had dispersed pile cap shear cracks. on the other hand, the specimen a-7b, which had many column hoops, showed a different characteristic that the cracks did not disperse, and the width of several shear cracks increased. this suggests that, of the two types of hoops, the pile cap hoops arranged on the outside contribute a greater shear force and are more effective in preventing brittle shear failure. 4.2. strain distribution of pile cap hoop 4.2.1. pile cap hoop figure 5 shows the strain distribution of pile cap hoops for specimens a-8 and aŋ9. the two specimens have different pcpw, and cpw and have the same amount of reinforcement. at the time of positive 303 shinji kishida, tomohisa mukai acta polytechnica ctu proceedings ௪௣௖݌ ൅ ௪௖݌ � $��d䚷䚷䚷䚷�5 ��� $��e䚷䚷䚷䚷�5 ��� $��䚷䚷䚷䚷�5 ��� $��䚷䚷䚷䚷�5 ��� 㻯㼛㼙㼜㼞㼑㼟㼟㼕㼛㼚㻌㼒㼍㼕㼘㼡㼞㼑 㼍㼠㻌㼏㼛㼘㼡㼙㼚㻌㼎㼍㼟㼑 㻼㼕㼘㼑㻌㼏㼍㼜㻌㼟㼔㼑㼍㼞㻌㼒㼍㼕㼘㼡㼞㼑 㻯㼛㼙㼜㼞㼑㼟㼟㼕㼛㼚㻌㼒㼍㼕㼘㼡㼞㼑 㼍㼠㻌㼏㼛㼘㼡㼙㼚㻌㼎㼍㼟㼑 㻼㼕㼘㼑㻌㼏㼍㼜㻌㼟㼔㼑㼍㼞㻌㼒㼍㼕㼘㼡㼞㼑 figure 3. failure mode at end of test. � ͳϭϯϭ ͳθϭ ͳϰϭ ϭ ϰϭ θϭ ϭϯϭ ͳϰ ͳϯ ͳϯ ͳϭ ϭ ϭ ϯ ϯ ϰ 㻿㼠㼛㼞㼥㻌㼐㼞㼕㼒㼠㻌㼍㼚㼓㼘㼑㻔㻑㻕 㻭㻙㻥 ͳϭϯϭ ͳθϭ ͳϰϭ ϭ ϰϭ θϭ ϭϯϭ ͳϰ ͳϯ ͳϯ ͳϭ ϭ ϭ ϯ ϯ ϰ 㻿 㼠㼛 㼞㼥 㻌㼟 㼔 㼑 㼍 㼞㻌 㼒㼛 㼞㼏 㼑㻌 㻔㼗 㻺 㻕 㻿㼠㼛㼞㼥㻌㼐㼞㼕㼒㼠㻌㼍㼚㼓㼘㼑㻔㻑㻕 㻭㻙㻤 ͳϭϯϭ ͳθϭ ͳϰϭ ϭ ϰϭ θϭ ϭϯϭ ͳϰ ͳϯ ͳϯ ͳϭ ϭ ϭ ϯ ϯ ϰ 㻿㼠㼛㼞㼥㻌㼐㼞㼕㼒㼠㻌㼍㼚㼓㼘㼑㻔㻑㻕 㻭㻙㻣㼎 ͳϭϯϭ ͳθϭ ͳϰϭ ϭ ϰϭ θϭ ϭϯϭ ͳϰ ͳϯ ͳϯ ͳϭ ϭ ϭ ϯ ϯ ϰ 㻿 㼠㼛 㼞㼥 㻌㼟 㼔 㼑 㼍 㼞㻌 㼒㼛 㼞㼏 㼑 㻌㻔 㼗㻺 㻕 㻿㼠㼛㼞㼥㻌㼐㼞㼕㼒㼠㻌㼍㼚㼓㼘㼑㻔㻑㻕 㻭㻙㻣㼍 dădžŝŵƶŵ ɛƚƌğŷőƚś ^śğăƌ đƌăđŭ ,žžɖ�ljŝğůě� ŝŷ�ɖŝůğ�đăɖ ,žžɖ�ljŝğůě�ŝŷ�đžůƶŵŷ 4pd[ �����n1� 4plq �����n1� 4pd[ �����n1� 4plq �����n1� 4pd[ ����n1� 4plq �����n1�4pd[ ����n1 4plq �����n1� î��0d[lpxp�vwuhqjwk��������ᶭ��6khdu�fudfn�lq�sloh�fds� �ࠐ�+rrs�\lhog�lq�sloh�fds��ƒ��&roxpq�+rrs�\lhog�lq�sloh�fdsۍ����$qfkru�\lhog� figure 4. details of specimens. � ^ƚƌăŝŷ;ʅϳ ϭ͘ϱй ϯй ϭй wŝůğ�đăɖ�ƶɖɖğƌ�ɛŝěğ� wŝůğ�đăɖ�ďžƚƚžŵ� ɛŝěğ &žƶŷěăƚŝžŷ�ďğăŵ�ƶŷěğƌ�ďğě zŝ ğů ě� ɛƚ ƌă ŝŷ ^ƚƌăŝŷ;ʅϳ ϭ͘ϱй ϯй ϭй wŝůğ�đăɖ�ƶɖɖğƌ�ɛŝěğ� wŝůğ�đăɖ� ďžƚƚžŵ�ɛŝěğ &žƶŷěăƚŝžŷ�ďğăŵ�ƶŷěğƌ�ďğě zŝ ğů ě� ɛƚ ƌă ŝŷ ^ƚƌăŝŷ;ʅϳ ϭ͘ϱй ϯйϭй wŝůğ�đăɖ�ƶɖɖğƌ�ɛŝěğ� wŝůğ�đăɖ�ďžƚƚžŵ�ɛŝěğ� &žƶŷěăƚŝžŷ�ďğăŵ�ƶŷěğƌ�ďğě zŝ ğů ě ɛƚ ƌă ŝŷ ϭ ϭϭϭ ϯϭϭ ϯϭϭ ϰϭϭ ϱϭϭ ϲϭϭ ϳϭϭ θϭϭ ϵϭϭ 'ă ƶő ğ� ɖž ɛŝƚ ŝž ŷ; ŵ ŵ ϳ ^ƚƌăŝŷ;ʅϳ ϭ͘ϱй ϯйϭй wŝůğ�đăɖ�ƶɖɖğƌ�ɛŝěğ� wŝůğ�đăɖ�ďžƚƚžŵ�ɛŝěğ� &žƶŷěăƚŝžŷ�ďğăŵ�ƶŷěğƌ�ďğě zŝ ğů ě� ɛƚ ƌă ŝŷ ����������������������������������������� ����������������������������������������� ���������� ����������������������������� ����������������������������������������� $���� �qhjdwlyh�ordglqj�� $��� ��qhjdwlyh�ordglqj��$���� �srvlwlyh�ordglqj�� $��� �srvlwlyh�ordglqj�� figure 5. strain distribution of pile cap hoop. 304 vol. 33/2022 frame pile ultimate strength formulas ^ƚƌăŝŷ;ʅϳ ϭ͘ϱ й ϯ й ϭ й zŝ ğů ě� ɛƚ ƌă ŝŷ wŝůğ�đăɖ� ƶɖɖğƌ�ɛŝěğ� wŝůğ�đăɖ�ďžƚƚžŵ�ɛŝěğ� &žƶŷěăƚŝžŷ�ďğăŵ�ƶŷěğƌ�ďğě &žƶŷěăƚŝžŷ�ďğăŵ�ƶɖɖğƌ� ďğě ^ƚƌăŝŷ;ʅϳ ϭ͘ϱй ϯ й ϭ й zŝ ğů ě� ɛƚ ƌă ŝŷ wŝůğ�đăɖ� ƶɖɖğƌ�ɛŝěğ� wŝůğ�đăɖ�ďžƚƚžŵ�ɛŝěğ� &žƶŷěăƚŝžŷ�ďğăŵ�ƶŷěğƌ�ďğě &žƶŷěăƚŝžŷ�ďğăŵ�ƶɖɖğƌ�ďğě ^ƚƌăŝŷ;ʅϳ ϭ͘ϱй ϯй ϭй zŝ ğů ě� ɛƚ ƌă ŝŷ wŝůğ�đăɖ� ƶɖɖğƌ�ɛŝěğ� wŝůğ�đăɖ�ďžƚƚžŵ�ɛŝěğ� &žƶŷěăƚŝžŷ�ďğăŵ�ƶŷěğƌ�ďğě &žƶŷěăƚŝžŷ�ďğăŵ�ƶɖɖğƌ�ďğě ϭ ϭϭϭ ϯϭϭ ϯϭϭ ϰϭϭ ϱϭϭ ϲϭϭ ϳϭϭ θϭϭ ϵϭϭ ϭϭϭϭ ϭϭϭϭ 'ă ƶő ğ� ɖž ɛŝƚ ŝž ŷ; ŵ ŵ ϳ ^ƚƌăŝŷ;ʅϳ ϭ͘ϱй ϯй ϭй zŝ ğů ě� ɛƚ ƌă ŝŷ wŝůğ�đăɖ� ƶɖɖğƌ�ɛŝěğ� wŝůğ�đăɖ�ďžƚƚžŵ�ɛŝěğ� &žƶŷěăƚŝžŷ�ďğăŵ�ƶŷěğƌ�ďğě &žƶŷěăƚŝžŷ�ďğăŵ�ƶɖɖğƌ�ďğě �������������������������������������������������������������������������� �������������������������������������� �������������������������������������� $��e��srvlwlyh�ordglqj� $��e� ��qhjdwlyh�ordglqj�� $����srvlwlyh�ordglqj�� $����qhjdwlyh�ordglqj� figure 6. strain distribution of column hoops in pile cap. loading, the strain at the lower end of the foundation beam was large in both specimens. for specimen a8, the strain did not reach the yield to r = 1%, but for specimen a-9, the strain had reached the yield to r = 1%. it is considered that the tensile force acting one hoop was dispersed in the specimen a-8 with a large amount of pcpw, and the hoop did not yield until r=2%. on the negative loading, the strain amount of specimen a-8 was smaller than that of the positive loading until the maximum shear strength (r = −1%). also, the strain at the top of the pile cap was larger than that at the bottom of the foundation beam, and the difference depending on the loading direction was observed. in specimen a-9, the strain became smaller as compared to the positive loading. compared with test specimen a-8, the strain increased up to r = −1% as in the case of positive loading. as described above, by arranging a large amount of pcpw, it is possible to reduce the ratio of a single hoop to the shear force acting on the pile cap. 4.2.2. column hoop in pile cap figure 6 shows the strain distribution of the column hoops in the pile cap of specimen a-7b and specimen a-4. in test specimen a-4, cpw was arranged 0.15%, and pcpw was recombined in the same amount as specimen a-7b. at the time of positive loading, the strain at the lower end of the foundation beam tended to increase as in the pile cap hoops in both specimens, but the column hoops did not yield even at the maximum strength (r = 2%). when the strain values of the two specimens were compared, the strain value of the column hoop was almost unchanged even if cpw was increased. the ratio of the shear force acting on the pile cap differs between the pile cap hoop and the column hoop arranged in the pile cap. it is considered that pile cap hoops contribute more effectively to shear resistance because the tensile force of pile cap hoops decreases with increasing. 4.3. relationship between the pile cap input shear force and the amount of hoop figure 7 shows the relationship between the maximum pile cap input shear force, the amount of pile cap hoop, and the pile cap hoop in the test specimen of this study and specimen a-4. the tensile strength of the main bar of the foundation beam was calculated from the strain at the critical section position of the foundation beam, and the input shear force of the pile cap was calculated by the following equation. vj = t − qc (6) where, t : tensile force of foundation beam bar, qc: story shear force. when cpw was less than 0.15%, the input shear force increased as cpw increased, but when cpw was more than 0.15%, the input shear force became almost constant. the effect of cpw on pile cap strength was considered to be limited to cpw ≤ 0.15% in this study. even when pcpw increased, the input shear force showed almost the same value or a tendency to slightly increase. at the time of positive loading, the input shear force generally tends to increase as the total hoop amount in the pile cap increases. however, the relationship between the total hoop amount and the input shear force became constant under negative loading. 4.4. carrying of the pile cap hoop figure 8 shows the ratio of the average stress and the yield stress of the pile cap hoop at the maximum strength in the specimen of this study and specimen a-4. in the case of pile cap hoops, the stress at the maximum shear strength was large in all specimens at positive loading. on the other hand, at the time of negative loading, the stress varies greatly for each specimen. for the column hoops, the stress at positive loading was smaller than that of the pile cap hoops. furthermore, the carrying stress of the column hoop was almost constant even if cpw was arranged more than 0.15%. this is consistent with the 305 shinji kishida, tomohisa mukai acta polytechnica ctu proceedings � ��� ��� ��� ��� ��� ��� ��� ��� ��� f3z�sf3z��� sũ;ŭeϳ � ��� ��� ��� ��� ��� � ��� ��� ��� sf3z��� sũ;ŭeϳ 䕕䠖đwǁсϭ͘ϭϳй 䕧䠖đwǁсϭ͘ϭϱй � ��� ��� ��� ��� ��� � ��� ��� ��� f3z��� sũ;ŭeϳ 䕕䠖ɖđwǁсϭ͘ϭй 䕧䠖ɖđwǁсϭ͘ϯϯй �d��vj ��cpw �e��vj ��pcpw �f��vj ��cpw�pcpw� figure 7. relationship between maximum input shear force and two kinds of hoop in pile cap. ϭ ϭ͘ϯ ϭ͘ϰ ϭ͘ϲ ϭ͘θ ϭ ϭ͘ϯ ϭ ϭ͘ϭ ϭ͘ϯ ϭ͘ϯ t žƌ ŭŝ ŷő �ɛƚ ƌğ ɛɛ ͬz ŝğ ůě �ɛƚ ƌğ ɛɛ ɖđwǁ;йϳ wŝůğ�đăɖ�śžžɖ ϭ ϭ͘ϯ ϭ͘ϰ ϭ͘ϲ ϭ͘θ ϭ ϭ͘ϯ ϭ ϭ͘ϭ ϭ͘ϯ ϭ͘ϯ t žƌ ŭŝ ŷő �ɛƚ ƌğ ɛɛ ͬz ŝğ ůě �ɛƚ ƌğ ɛɛ đwǁ;йϳ �žůƶŵŷ�śžžɖ sfqf ����� sfqr ����� fqr ����� fqf ����� fqf ���f3z� &rorxu���� &orvlqj�gluhfwlrq�� 2shqlqj�gluhfwlrq� figure 8. ratio of the average stress and the yield stress of the pile cap hoop. 1. when ν0σb −c σt < 0 vu is smaller of the follow. vu = # $% $& λc ν0 σb + cn cpwe cσwy 3 cbe cje λc ν0 σb 2 cbe cje (3) 2. when ν0σb −c σt ≥ 0 and ν0σb −c σt −pc σt < 0 cvt = 2 cn cpwe cσwy cbe cje pcvt is smaller of the follow. pcvt = # $$$% $$$& λpc ( ν0 σb −c σt) + pcn pcpwe pcσwy 3 pcbe pcje λpc ( ν0 σb −c σt) 2 pcbe pcje (4) vu =c vt +pc vt 3. when ν0σb −c σt −pc σt ≥ 0 cvt = 2 cn cpwe cσwy cbe cje pcvt = 2 pcn pcpwe pcσwy pcbe pcje va = (ν0σb −c σt −pc σt) bxn 2 sin 2θ (5) vu =c vt +pc vt + va note: xn = d 4 (1 + 2η), θ = tan−1 d−xn l , ν0 = 2.3 σ−0.33 b . symbols are explained in the nomenclature. table 4. pile cap shear strength formula based on previous investigation. 306 vol. 33/2022 frame pile ultimate strength formulas loading direction closing opening closing opening carrying ratio pcnc pcno cnc cno 0.96 0.64 5.4 cpw (cpw < 0.15 %) 0.81 (cpw > 0.15 %) 0.59 table 5. carrying stress coefficient. �d��rshqlqj�gluhfwlrq� �e��forvlqj�gluhfwlrq� ϭ ϯϱϭ ϱϭϭ ϳϱϭ ϭϭϭϭ ϭ ϯϱϭ ϱϭϭ ϳϱϭ ϭϭϭϭ sƶ͗ ɖŝůğ�đăɖ�ŝŷɖƶƚ ɛśğăƌ�ĩžƌđğ;ŭeϳ yƶ͗ �ăůđƶůăƚŝžŷ�ƌğɛƶůƚ�;ŭeϳ ăǀğƌăőğ͗ϭ͘ϭϭ �žğĩĩŝđŝğŷƚ�žĩ�ǀăƌŝăƚŝžŷ͗ϭ͘ϭϲϲ ϭ ϯϱϭ ϱϭϭ ϳϱϭ ϭϭϭϭ ϭ ϯϱϭ ϱϭϭ ϳϱϭ ϭϭϭϭ sƶ͗ ɖŝůğ�đăɖ�ŝŷɖƶƚ ɛśğăƌ�ĩžƌđğ;ŭeϳ yƶ͗ �ăůđƶůăƚŝžŷ�ƌğɛƶůƚ�;ŭeϳ ăǀğƌăőğ͗ϭ͘ϭϲ �žğĩĩŝđŝğŷƚ�žĩ�ǀăƌŝăƚŝžŷ͗ϭ͘ϭϱϰ figure 9. relationship between experimental values and proposed calculations. fact that the input shear force became almost constant in the range of cpw over 0.15% in the relationship between vj and cpw. based on the above results, the values obtained by approximately calculating the ratio of the carrying stress applied to the hoops of the pile cap and the column are shown by the broken line in figure 8. and table 5 shows the carrying stress coefficient obtained from the figure 8. table 4 shows pile cap shear strength formula applied carrying stress coefficient. the stress of the hoops under positive loading was approximated in the range of cpw < 0.15% so that the proportion of the hoops increased in proportion to the increase of cpw. in the range of ăcpw ≥ ă0.15%, based on the fact that the strength did not change even if cpw was increased, using the ratio of the material strength of specimen a-7a and the carrying stress of 0.81, the upper limit of cpw ·c σy was set to 0.55 (n/mm2). 5. 5. compatibility of pile cap ultimate shear strength formula the calculation was performed by substituting the coefficients obtained in table 5. the target specimens were the specimens of this study and the specimens judged to be pile cap shear failure in past experiments [2–4]. figure 9 shows the comparison between the calculation result by the proposed formula and the experimental value. the input shear force was determined from the value of the strain gauge by defining the position at which the strain of the main bar of the foundation beam was maximum as the critical section. as shown in figure 9, the experimental / calculated values obtained by the proposed formula were generally within ±20% (dotted line in the figure), and the average and coefficient of variation were 1.01-1.06 and 15.4-16.6%, respectively. the average (the dashed line in the figure) and the standard deviation were also considered to be valid as experimental values. however, only the standard type specimen was greatly underestimated on the side where the column-foundation beam opened. the cause is considered to be that the effective reinforcement ratio of the pile cap is extremely low at 0.03%, and the truss mechanism has not been formed. from this experiment, the minimum reinforcement amount is set to 0.07% as the applicable range of the pile cap effective reinforcement ratio. 6. conclusions 1. among the reinforcements arranged in the pile cap, the pile cap stirrups contributed more to the shear load. by arranging many pile cap hoops, an increase in pile cap shear crack width was suppressed. 2. it was confirmed that the relationship between the total amount of hoops in the pile cap and the input shear force to the pile cap was different depending on the loading direction. 3. the shear strength formula proposed in the past 307 shinji kishida, tomohisa mukai acta polytechnica ctu proceedings was able to be evaluated safely by considering the load ratio of the hoops at the time of pile cap shear failure. acknowledgements this work was supported by jsps kakenhi grant number jp23560679. this experiment was carried out in structural lab. at bri. authors would like to express their gratitude to japan pile co., ltd. for providing steel piles, neturen co., ltd. and tokyo tekko co., ltd. for providing steel bars, we are deeply grateful to them. list of symbols cbe the truss effective width in column cje the truss effective depth in column cvt column shear strength in truss mechanism pcpwe the effective ratio of shear reinforcing bar in the pile cap pcvt pile cap shear strength in truss mechanism cσt column compression stress in truss mechanism pcσt pile cap compression stress in truss mechanism cpwe the effective ratio of shear reinforcing bar in the column cσwy the yield stress in column reinforcing bar cσwy truss effective coefficient in column pcσwy the yield stress in pile cap reinforcing bar pcbe the truss effective width in pile cap pcje the truss effective depth in pile cap b pile cap effective width d pile cap effective depth l member length va shear strength in arch mechanism xn the neutral axis position in arch mechanism η axial force ratio θ the angle of compression strut in the arch mechanism λc the yield stress in column reinforcing bar λpc the truss effective coefficient in pile cap ν0 effective coefficient of concrete compression stress σa compression stress in arch mechanism σb concrete compression stress references [1] mukai t., kikitsu h., morita k., and fukuyama h.: factor analysis of barriers to post-earthquake functionality for buildings (part.1-5), summaries of technical papers of annual meeting, architectural institute of japan pp. 445-446, 2014. [2] s. kishida, t. mukai. experimental study on the reinforced concrete pile-cap with a pil, exterior column and foundation beam, fib 2018 congress in australia, id302, 2018. [3] s. kishida s, t. mukai, h. watanabe. study on structural performance evaluation for concrete pile system with post-earthquake functional use (part.29), summaries of technical papers of annual meeting, architectural institute of japan pp. 221-222, 2019. [4] s. kishida, t. mukai, y. maida. a study on failure mode of pile caps on the exterior frame with precast pile, summaries of technical papers of annual convention of japan concrete institute 42(2):271-276, 2019. 308 https://doi.org/10.14311/app.2022.33.0316 acta polytechnica ctu proceedings 33:316–321, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague railway reinforced concrete infrastructure life management and sustainability index seyed mohammad sadegh lajevardi∗, paulo b. lourenço, hélder s. sousa, josé c. matos university of minho, isise, department of civil engineering, campus of azurém, 4800-058, guimarães, portugal ∗ corresponding author: sadegh.lajevardi@gmail.com abstract. infrastructure healthy enhancement for saving resources in operation procedures is one of the most important objectives for owners on their decision support system based on cost management. in this manner, finding the intervention action priority, as well as the inspection method and maintenance system for each component, with regard to a limited resources amount is investigated in this paper. defects on infrastructure components create data and these data are undoubtedly useful to increase the knowledge in decision making in practice. in that sense, risk analysis and value of information can be applied using decision trees together with bayesian networks. for data filtering and noise reduction, a principal component analysis may also be applied to manage a database and prepare useful input variables for the decision tree system. this paper presented an approach for the maintenance managers to prepare their infrastructure available with a sustainable index with minimum cost. this index would be a tool for decision-makers with regard to the cost management and sustainability aspects. keywords: life management, maintenance planing, sustainability index. 1. introduction most of the transportation modes are made through railway networks and road infrastructures. passengers’ expectation is directly affected by the quality of structures and equipment during the operation. saving quality level requires planning and predicting resource consumption based on the status of their materials in their degradation model and maintenance approach. the maintenance approach would be renewed or retrofitted during the operation. quality will explain by performance indicator which has been affected by several attitudes, especially peripheral climate such as strong corrosion processes in the coastal area or freezes and thaw in deserts that result from temperature degree changes or moisture percentage changes in terms of sustainability and environments [1]. therefore, there is a wide range of alternatives and conflicting criteria are involved as the root of failure on infrastructure. therefore, multi-criteria decision making (mcdm) has been applied to estimate the infrastructure’s element performance during the operation [2]. recent research, for finding the sustainability index has been focused on several subsets of mcdm to prepare a decision support system such as preference selection index (psi) [2, 3]. this method is typically chosen when there are no significant weights for attributes. in other research, in order to mine the effective parameters for the indexing model based on pareto concept and making pair-wise comparison questionnaire serving analytical hierarchy process (ahp) requirements to find the weights of them [4]. ahp method has been adopted for the calculation of the sustainability assessment index (sai) based on normalized key performance indicators and their weight, which has been calculated by ahp [5]. the theoretical framework developed for extracting the sustainability index (si) is a function that comprises wellbeing, resource, compliance, and resources [6]. for qualitative assessment in the ahp process, recent research has been used the likert scale and extract sustainability level [7]. the results led to the development of a sustainability index model (sim) useful to assess manufacturing performances or maintenance procedures during the operation with rank alternatives [8]. also, these results have been developed with respect to the evaluation criteria and the weights of the criteria to prioritize the network systems [9]. these indexes could be extracted for prediction by neural network method to decision making with considering probable scenarios [10]. since, future generations are important for these studies, prediction, and monitoring of the results with any scenario is necessary for decision-makers. therefore, sustainable development is a comprehensive solution for the present and future of humans [11]. based on recent research, the sustainable index is an important index for the bridge management system and green infrastructures and green cities. 2. aim of the research this paper presents an approach to developing a practical indexing model in terms of resource management 316 https://doi.org/10.14311/app.2022.33.0316 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 33/2022 railway reinforced concrete infrastructure subject extracting index method subset method coastal climate adaptation planning and evolutionary governance: insights from homer, alaska. [1] interviews with key informants development of sustainable performance index (spi) for self-compacting concretes (scc) [2] mcdm spi application of preference selection index method for decision making over the design stage of production system life cycle [3] mcdm psi sustainability index for highway construction projects [4] mcdm ahp development of sustainability assessment index for machine tools [5] mcdm ahp development of a multidisciplinary approach to compute sustainability index for manufacturing plants singapore assessing the feasibility of using the heat demand-outdoor perspective temperature function for a long-term district heat demand forecast [6] mcdm ahp groundwater sustainability assessment framework: a demonstration of environmental sustainability index for hanoi [7] mcdm ahp developing a sustainability index for mauritian manufacturing companies [8] rank correlationordinal association kendall coefficient life cycle aggregated sustainability index for the prioritization of industrial systems under data uncertainties [9] lcc net present value (npv), internal return rate (irr), and payback time (pt)) . predicting subjective measures of walkability index from objective measures using artificial neural networks [10] ann comparison of sustainability models in development of electric vehicles in tehran using fuzzy topsis method [11] mcdm fuzzy topsis the lisbon ranking for smart sustainable cities in europe [12] feature selection principal component analysissensitivity analysis table 1. research method and index extract. and optimization. it will prepare a decision support system for managers that leads to wider uptake welfare during the infrastructure life cycle for their real owner, people. since this plan optimizes resource consumption, therefore it will keep the environment safe and prevent nature from contamination. based on this framework, the manager will make an optimized decision for the material and shape of their structures’ element during the design. this framework would also be useful for maintenance managers to decide about their maintenance plan with a comparison between the elements and components of the railway network. 3. research contribution this paper is structured as follows: • data collection from overall monitoring, such as visual inspection. • variance measurement and extracting the data value • defining the reliability index • define the effective parameters in terms of sustainability with a questionnaire form • correlation analysis between the reliability index and sustainable parameters • prepare a sustainable model for maintenance managers 317 s. m. s. lajevardi, p. b. lourenço, h. s. sousa, j. c. matos acta polytechnica ctu proceedings ��������� �� ��� �������� �� �� �� ��� � �� ��� �������� ��� � �� ��� �� ����� � �� �� ������ ��� �� � � ���� ������ � ��� ��� ���� � ��� �� ���� � ��� ��� ������ � �� ��� �� ����� ���� ��� ���� ������ ����� ���� ��� ��� !� � �� ��� ������� �� !�!��� � �� ��� �� �� � �� �� �� ��� ��� ����!�� ����� �� ������ ���� ������ �������� "��� �#��� figure 1. flow diagram of extracting sustainable index. 4. methodology this research would establish a comprehensive mathematical method for extracting the sustainability index for infrastructure as follows. • defect detection and extracting the probability of failure • principal component analysis (pca) and the value of information for reducing the useless data • environment features and extracting the sustainability parameters • replacing the roots of the defect (environment features) by the defect and extracting the sustainability index for case study • the weight of each parameter shows the importance of the index elements. based on these weights, a new construction plan and materials regarding their maintenance method will be updated. the framework of extracting sustainable index has been shown as follows diagram. 5. data collection this research would be establishing a comprehensive mathematical method for extracting the sustainability index to assess infrastructure in terms of sustainability. this research has been focused on the tehran subway bridge for monitoring and quality assessment during the operation. based on the inspection checklist, five bridges are determined and inspected as a sample by expert inspectors. these bridges have been assessed by the items of the national code [14]. parameters with subjective attitudes have been ranked based on their environmental status and the other geotechnical features between 1 to 4 or 1 to 5. some of these parameters would be determined according to the observed defects in each element of the bridge. since these parameters are not the same range, they will be normalized by formula as follows. z = x − µ σ (1) in this formula, µ is the arithmetic mean and σ is the standard deviation of the distribution. row data after normalization process transfer to the principal component analysis (pca) framework. based on the scree plot and their eigenvalue, three components have the most value of information. observation from inspection determines the status of the bridge’s elements as follows. some of these elements related to defects and they were affected by environments’ features according to the last table. sustainable structures during their life cycle have to fulfill the thresholds regarding their components and the environment’s features based on their weight. by considering these tools, it will be possible to manage assets along their lifetime in a more sustainable and efficient way [15]. therefore, pca has been used 318 vol. 33/2022 railway reinforced concrete infrastructure component 1 2 3 liquefaction (li) .825 .448 -.084 land slide (ls) -.012 -.681 .258 rock fall (rf) -.811 -.252 .031 thunderstorm (th) .313 .034 .931 flood (fl) .675 -.475 .484 climate (cl) .825 .448 -.084 density of soil (ds) -.558 .648 .306 underground void distance (ud) -.553 .506 .508 table 2. component matrix. defects type dependent element cl fl th ls li ds ud rf deck stability deck • • drainage drainage/pier/ deck/ foundation • • • foundation/embankment settlements foundation/abutment • • • • • • railway track geometry defect (gage widening, alignment, twist, longitudinal level [13]) railway track • bridge thermal movement deck/railway track • temperature stress cycling all bridge’s element • deterioration all bridge’s element • scour pier/foundation • • table 3. impact of environment features on bridge. bridge number defect on railway track defect on deck defect on abutment defect on pier defect on foundation 1 6 0 0 0 0 2 3 1 0 0 0 3 7 4 1 0 0 4 1 3 0 0 0 5 0 4 1 0 0 6 1 3 0 0 0 7 1 0 0 0 0 table 4. number of registered defects based on bridge’s elements. bridge number number of defect on element [table 4] defect type approximate volume (m3) probability of defect [table 4] (%) impact [table 2 & 3] sustainability index 4 3 instability of deck 709.8 0.42 0.931rf+.031th 0.42(0.931 rf+.031th) table 5. sustainability index for a bridge in case study. 319 s. m. s. lajevardi, p. b. lourenço, h. s. sousa, j. c. matos acta polytechnica ctu proceedings figure 2. scree plot for component’s eigenvalue based on pca analysis. and 3 components have been extracted by the pca method for 8 environment features with their weights as follows. based on table 2, li represents the probability of liquefaction and also other environmental features such as ls and rf and the other. this method will keep the valuable data and reduce the main by ranking the parameters in each component [16]. it is necessary for monitoring the mean value and variance changes for each feature, which would be important for sustainable structures. for example, increasing the mean value for flood shows the increasing probability of the scour surrounding the piers of the bridge. since climate change would be effective in increasing the crack growth of concrete elements, these features would be related to these types of defects. these relations are shown as follows. for finding the sustainability index, it is necessary to find the risk according to weight for their defects. the probability of failure will be extracted as the following formula [17]. for each bridge (see table 4), v is an approximate measure representative of their volume regarding length (l) and width (w). the number of defects in each element has been determined by visual inspection [18] (f) based on table 4. this shows that a larger bridge has more capacity for a defect in its components when compared with a smaller one. probability of failure (p-f) are normalized by dividing them with the average of all failure densities. pf = ! f" v dv (2) v ≂ l × v (3) based on table 3 and table 2, the sustainability index has been presented as follows. bridge number 4 in this step is a sample for calculating the sustainability index for the instability of the deck. with this result, it is necessary to consider the risk of reinforced concrete elements and their defects during the operation with regard to rockfall and thunder. it is obvious for other bridges in the same area, impact factor is the same but the probability of defect will change based on their structural features. therefore, the sustainability index will change for each bridge, and the final grade will determine by their environment features and structures. 6. conclusion the sustainability index would be a combination of environmental features to make a decision during the operation or designing of the structure. in this approach after putting up the probability of environment features such as rockfall or climate, finding the location, material and bridge structure type would be possible by benchmarking and expert judgments. with this index it is possible to locate the bridge with a minimum life cycle cost and maximum sustainability. finding the optimum plan with regard to climate change and seasons is also possible during the operation. for example, in winter railway track face with a wide range of changes in some area and this issue consequently effects on increasing the railway track failure such as rail and fastening system breakage. therefore, finding the proper place, the best material and optimum maintenance plan would be possible with the sustainability index. acknowledgements acknowledgments this work was partly financed by fct / mctes through national funds (piddac) under the r&d unit institute for sustainability and innovation in structural engineering (isise), under reference uidb / 04029/2020. this project has received funding from the european union’s horizon 2020 research and innovation programme under grant agreement no769255. the sole responsibility for the content of this publication lies with the authors. it does not necessarily reflect the opinion of the european union. neither the innovation and networks executive agency (inea) nor the european commission are responsible for any use that may be made of the information contained therein. references [1] s. jeff birchall. coastal climate adaptation planning and evolutionary governance: insights from homer, alaska. marine policy 112, 2020. https://doi.org/10.1016/j.marpol.2018.12.029. [2] a. s. reddy, p. r. kumar, p. a. raj. development of sustainable performance index (spi) for self-compacting concretes. journal of building engineering 27, 2020. https://doi.org/10.1016/j.jobe.2019.100974. [3] attri, r., & grover, s. (2015). application of preference selection index method for decision making over the design stage of production system life cycle. journal of king saud university engineering sciences, 27(2), 207-216. https://doi.org/10.1016/j.jksues.2013.06.003. 320 https://doi.org/10.1016/j.marpol.2018.12.029 https://doi.org/10.1016/j.jobe.2019.100974 https://doi.org/10.1016/j.jksues.2013.06.003 vol. 33/2022 railway reinforced concrete infrastructure [4] a. h. ibrahim, m. a. shaker. sustainability index for highway construction projects. alexandria engineering journal 58(4):1399-411, 2019. https://doi.org/10.1016/j.aej.2019.11.011. 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[13] i. soleimanmeigouni, a. ahmadi, a. nissen, et al. prediction of railway track geometry defects: a case study. structure and infrastructure engineering 16(7):987-1001, 2019. https://doi.org/10.1080/15732479.2019.1679193. [14] transportation research center. technical identification of bridge, iranian national code 367, department of strategic planning, 2007. [15] j. matos, a. solgaard, c. santos, et al. life cycle cost, as a tool for decision making on concrete infrastructures. high tech concrete: where technology and engineering meet2018. p. 1832-9. https://doi.org/10.1007/978-3-319-59471-2_210. [16] c. hanley, j. c. matos, d. kelliher, et al. integrating multivariate techniques in bridge management systems. journal of structural integrity and maintenance 2(3):143-51, 2017. https://doi.org/10.1080/24705314.2017.1354280. 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[18] m. moore, b. phares, b. a. graybeal, et al. reliability of visual inspection for highway bridges, volume i: final report, research, development, and technology turner-fairbank highway research center, 2001. 321 https://doi.org/10.1016/j.aej.2019.11.011 https://doi.org/10.1016/j.procir.2019.01.018 https://doi.org/10.1016/j.egypro.2017.12.692 https://doi.org/10.1016/j.jenvman.2019.02.117 https://doi.org/10.1016/j.ecolind.2018.09.003 https://doi.org/10.1016/j.compchemeng.2018.03.015 https://doi.org/10.1016/j.scs.2019.101560 https://doi.org/10.1016/j.scs.2019.101912 https://doi.org/10.1016/j.scs.2018.10.009 https://doi.org/10.1080/15732479.2019.1679193 https://doi.org/10.1007/978-3-319-59471-2_210 https://doi.org/10.1080/24705314.2017.1354280 https://doi.org/10.14311/app.2022.33.0546 acta polytechnica ctu proceedings 33:546–551, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague carbon dioxide binding ability in concretes: methodology and modeling lucie schmitta, jena jeongb, jean-marc potierc, laurent izoretd, jonathan mai-nhua,∗, nicolas decoussera, thomas pernina a cerib: centre d’etudes et de recherche de l’industrie du béton, 1 rue des longs reages, 28230 épernon, france b estp: école spéciale des travaux publics, 2-4 rue charras, 75009 paris, france c snbpe: le syndicat national du bêton prêt à l’emploi, 32 allées d’orléans, 33000 bordeaux, france d atilh: association technique de l’industrie des liants hydrauliques, la defense, 7 place de la défense, 92974 paris-la-défense cedex, france ∗ corresponding author: j.mai-nhu@cerib.com abstract. carbonation of concretes is a natural physico-chemical process that can be described as a reaction between the carbon dioxide contained in the air and the cement matrix. carbonation concerns all concretes types in contact with the ambient air but also concretes in ground, from production stage to use and end-of-life stages. the amount of carbon dioxide bound varies according to the type of binder, the compacity of concrete and the environmental conditions during the use and the end-of-life stages. to consider the re-carbonation of concrete, works have been carried out within the framework of the european standardization group cen/tc229/wg5 and in cen/tc104. a methodology to consider the re-carbonation of concrete structures has been proposed in the nf en 16757 standard on environmental product declarations for concrete and concrete elements. in addition, fd cen/tr 17310 provides detailed recommendations regarding carbonation and absorption of carbon dioxide in concrete and give some precisions for application of nf en 16757. this is an important topic towards a sustainable development in the current context of circular economy and co2 uptake related to the french energy labelling (e+c-). in this paper, numerical and analytical carbonation models are used to estimate the co2 binding ability of concrete structures. the obtained results are compared to the methodology proposed in appendix bb of nf en 16757 standard. they confirm that the methodology described in the nf en 16757 standard leads to estimated degree of carbonation of the same order of magnitude. the advantage of using more advanced models lies in a better consideration of environmental parameters, the possibility to simulate the behaviour of crushed concrete, its reuse in new concrete as recycled aggregate and the possibility to simulate the carbonation of concretes in ground. this is an immediate perspective in the ongoing work in the french national project fastcarb on accelerated carbonation of recycled concrete aggregates. keywords: concrete products, concrete structures, modeling, re-carbonation, standardization. 1. introduction concrete is the most widely used building material in the world because the raw materials are generally considered lowly exhaustible and ubiquitous on the globe. in addition, its manufacturing process is well controlled and its cost price low. however, due to the use of cement and its success in construction works, concrete production can generate considerable co2 emissions. today the cement sector is among the main emitters with between 5 and 7% of global emissions [1]. how can this situation be reconciled with, on the one hand, the need for countries to continue to offer an efficient, robust and affordable solution and, on the other hand, the need to fight effectively against global warming? the building sector is actively working on solutions to reduce its co2 emissions. current research considers many aspects: • to reduce the carbon intensity of cement production operations; • to control the environmental impact of concrete products on their entire life cycle; • to optimize the processes of concrete production and construction; • to better recycle concrete into concrete; • to work in partnership with the construction industry to jointly contribute to green building systems. this study is part of a national research project: the fastcarb project. the aim of the fastcarb project is firstly to store co2 in the recycled con546 https://doi.org/10.14311/app.2022.33.0546 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 33/2022 binding ability of carbon dioxide in concretes figure 1. carbon footprint of 1kg of clinker. type of cement carbon footprint* cem i 765 kg of co2/ton cem ii/a-s cem ii/a-l 671 676 kg of co2/ton cem ii/b-l or ll cem/b-m 579 585 kg of co2/ton cem iii/a 400 kg of co2/ton cem iii/b 274 kg of co2/ton cem v/a 468 kg of co2/ton * not including carbon dioxide emissions from the combustion of secondary fuels table 1. carbon footprint of the different types of cement in france (in average) [2]. crete aggregates (rca), in order to reduce the environmental impact of concrete in the structures and secondly to improve the quality of these aggregates by reducing their porosity. today one of the main issues for the concrete profession is to better evaluate the carbon footprint of concrete and concrete structures. to solve this question, the quantity of carbon dioxide that could be stored by a building during its life time has to be correctly calculated. the objective of this study is to quantify the co2 stored by a building during its life time using different methods and models. 2. context and objectives of the study cement is a widely used building material whose main hydraulic constituent is clinker. to manufacture the clinker, the active "raw material" of the cement, the calcination of the limestone and the clay in the kilns at very high temperature (1,450 ◦)c generates carbon dioxide due to decarbonation of limestone and to combustion reactions during pyroprocessing. this decarbonation step represents about 60% of co2 emissions of the production process [1, 3, 4]. the figure 1 presents the carbon footprint of the production of clinker and the table 1 gives the footprint of the different types of cement. the carbon dioxide emitted during the manufacturing of cement can for a part be stored again by the natural phenomena of carbonation during the concrete life time. during the life of the structure, the concrete could store carbon dioxide at a level of 10 to 15% of the co2 emitted during the decarbonation of the limestone necessary for the manufacture of cement [1]. it is important to note that this percentage is average on the scale of the building. some parts carbonate more than others (exposed/not exposed parts, porous masonry compared to civil engineering parts. . .). these numbers take into account decarbonation and combustion of primary fuels but do not consider co2 emissions due to the combustion of "wastes" like tyres or waste oils for example. this is this value which is used for the lca. the standard nf en 16757 presents a methodology and a model to evaluate the quantity of carbon dioxide that can be stored during concrete lifetime in function of the environment and the materials as concrete strength classes. in this study, the numerical model sdream-crete is used to estimate the capacity of co2 storage of concrete. the results are compared to those obtained with the nf en 16757 model. then the co2 storage capacity of a r+5 building is estimated and compared to the amount of carbon dioxide emitted during its construction. 3. presentation of the models 3.1. model of the standard nf en 16757 [5] the nf en 16757 standards model is an analytic model which considers environment as well as material parameters. the equation 1 gives the carbonation depth. the equation 2 presents the model to 547 l. schmitt, j. jeong, j. m. potier et al. acta polytechnica ctu proceedings material environment xc: carbonation depth (mm) (equation 1) x x k: carbonation rate (mm/year0.5) x x kk: correction to k−factor x − t: time (year) − − utcc: maximum theoretical uptake in co2/kg of cement (0.49 for cem i) x − c: cement content (kg/m3) x − dc: degree of carbonation (−) − x table 2. parameter of the nf en 16757 model. figure 2. schematic presentation of the sdream-crete model (only carbonation and hydric parts). properties value considered resistance class c25/30 cem i 52.5 n [kg/m3] 280 aggregates [kg/m3] 1 922 efficient water [l/m3] 168 weff /c 0.60 relative humidity [%] 80 ± 10 co2 concentration [%] 0.04 table 3. properties of studied concrete and environment. estimate the co2 stored and the table 2 presents the parameters. xc = k · √ t (1) co2 uptake = k · kk ! √ t 1000 " utcc · c · dc (2) each of these factors is detailed in appendix bb of the standard. the appendix bb also gives values for the parameters that depend on the material and the environment. 3.2. model sdream-crete the sdream-crete durability model was developed at cerib with lmdc [5, 6]. it is a numerical model based on mass conservation equations. this model can simulate the chloride ingress, the carbonation, the moisture transfers as well as the development of the corrosion of the rebars in concretes exposed to wetting-drying cycles. it can be used in a deterministic context or in a probabilistic context to achieve reliability calculations for concrete products or concrete works. in this study, it is used to calculate the carbonation depth and then the amount of co2 absorbed by the concrete during its service phase. the quantity of stored co2 is calculated with the carbonation depth and the trapezoidal rules. the figure 2 presents a schematic representation of the sdreamcrete durability model. 4. results 4.1. comparison between analytical method and numerical method the first phase of this study consisted of estimating the amount of co2 that can be absorbed per square meter of exposed concrete with both models presented in the precedent section. the properties of the considered concrete and environment are presented in the table 3. 548 vol. 33/2022 binding ability of carbon dioxide in concretes properties value considered k: carbonation rate (mm/year0.5) 1.6 kk: correction to k-factor 1 (cem i) utcc: maximum theoretical uptake in co2/kg of cement 0.49 (cem i) dc: degree of carbonation (-) 0.85 (outdoor exposed to the rain) table 4. details of the considered value for the calculation with the nf en 16757 model. figure 3. carbonation depth of the studied concrete calculated with sdream-crete model and nf en 16757 model. figure 4. absorbed carbon dioxide calculated with two models and two methods. the figure 3 presents the carbonation depth calculated with sdream-crete model and with the equation 1 (en 16757 on the figure). the two models (sdream-crete et eq. 1) provide results slightly different. that can be explained by the different parameters taken into account by the two models. the standard model is based on the law at the root of time while the numerical model considers more precisely the formulation and the environment of the concrete. that will be discussed later. considering that the carbonation reaction can be summarized as (eq. 3): ca(oh)2 + co2 ↔ caco3 + h2o (3) the equation 3 means that the consumption of one mol of co2 leads to the production of one mol of calcite. with sdream-crete model, the amount of calcite in the concrete can be calculated and, by extension, the amount of carbon dioxide consumed according to the equation 3. the figure 4 presents the quantity of stored co2 for one square meter of the considered concrete. the orange curve was obtained using the equation 2. the blue curve was obtained using the depth of carbonation calculated by sdream-crete and then using the mathematical method of the trapezoids. these results show that two different models and methods, that consider very different parameters, are leading to very close results in this specific scenario. both method and model have advantages and disadvantages: the analytical model has the advantage of being quick and easy to use. in fact, the standard nf en 16757 details each of the parameters and proposes values to be considered depending on the resistance of the concrete and its environment. the numerical model seems more complete than the standard model because of the phenomena it considers (water cycle, detailed composition of concrete...). however, 549 l. schmitt, j. jeong, j. m. potier et al. acta polytechnica ctu proceedings figure 5. illustration of the building modelized for the study. outdoor indoor k: carbonation rate (mm/year0.5) 1.6 (exposed to rain) 4.6 (with cover) kk: correction to k-factor 1.05 1.05 t: time (year) 50 50 utcc: maximum theoretical uptake in co2/kg of cement 0.41 0.41 c: cement content (kg/m3) 300 300 dc: degree of carbonation (−) 0.85 (exposed to rain) 0.4 (dry climate) quantity of stored co2 (according to eq. 2, kg/m2) 1.24 1.68 table 5. values considered for the study. the calculation times can be longer and obtaining the input data can sometimes be complicated in an operational context. in this study, the standard model will be used (eq. 2). 4.2. application to a five-storey building [7] in this paragraph, the nf en 16757 model is applied to a r+5 building made with cem ii/a (case of an exposed architectural concrete, we only take into account the facade, external and internal) and the result is compared to the amount of co2 emitted during the decarbonation of the clinker needed for the building construction. the figure 5 presents an illustration the fivestorey building studied. the first step of the study is to determine the quantity of carbon dioxide emitted during the building construction (only considering decarbonation of cement). the cement used for the construction of this building is cem ii/a-l and contains 80% clinker. the amount of clinker required for this building is therefore 48 384kg. then it is possible to calculate the quantity of carbon dioxide emitted during the decarbonation phase (0.54kg for 1kg of clinker): 26ă127kg of co2 emitted. then we can calculate the quantity of carbon dioxide stored by the building with the model of the nf en 16757 standard (eq. 2). the table 5 presents the values of the parameters used for the simulation. all these values were determined using annex bb of standard nf en 16757. after calculating the amount of carbon dioxide that can be absorbed per square meter of concrete, we can calculate the amount of co2 that can be stored by this building after 50 years of life (eq. 4). total co2 uptake = # outdoor co2 uptake + indoor co2 uptake $ · surface of exposed concrete (4) so, the quantity of carbon dioxide that can be stored by the building is 2ă943.4 kg co2. if we compare this value to the quantity of co2 emitted during the construction (by the decarbonation phase, 26 127kg) we can highlight that the ratio is around 11.3%, which corresponds to the value of the literature [1] (eq. 5). total co2 uptake emitted co2 decarbonation = 2943.4 26 127 = 0.113 (5) this study was carried out on a fictitious and heavily simplified building at first. a single concrete for the whole building has been considered but in reality, we can observe significant divergences due to the possible variety of concretes encountered, e.g., porous masonry concrete or more efficient and dense concretes in some cases. the direct following will be to do the same work on an existing building for which we have all the material and environmental data. both models can be used. 550 vol. 33/2022 binding ability of carbon dioxide in concretes 5. conclusion as an integral part of the national project fastcarb, this study made it possible to carry out a bibliographic study on the ability of concrete to capture co2 during its service phase. several models of different levels (analytical, numerical ...) exist today. nf en 16757 is the product category rules for concrete and concrete elements. one model to take into account the carbonation phenomena in the context environmental product declaration (epd) for the calculation methodology is presented in the nf en 16757 standard. this standard is the reference document when performing epds for concrete elements following en 15804+a1 standard. moreover fd cen/tr 17310 [8] provides technical and scientific elements to support the carbonation treatment part of the nf en 16757. in the first part of this study, two different models and methods for calculating the quantity of stored co2 were compared. the results show that the model of standard nf en 16757, although analytical and "simple", allows to obtain results close to those obtained with a numerical model taking into account many different parameters. then, the model proposed in the standard was used to assess the amount of carbon dioxide captured by a five-storey building during its service phase (only the facade). the calculations highlight the ability of the concrete to capture part of the co2 emitted during the decarbonation of the clinker. with approximately 11.3% of the emitted co2 which can be reabsorbed, the results confirm the literature. it is important to note that this value could be increased significantly if carbonation continued after the demolition of the structure. this last point is the direct perspective of the work reported in this article. recent results show that it is possible to recapture up to 50 to 60% of the co2 emitted during the decarbonation of limestone for traditional concrete [1]. this value including the percentage of carbonation during the service life of the building, this means that the recycling phase alone stores around 40% to 45% of the co2 emitted during the decarbonation of limestone. investigations and calculations are currently underway to precise the co2 storage capacity of a recycled concrete aggregate (rca) and the best condition to optimize the carbonation of rca. acknowledgements the research and results reported herein are supported by the french ministry in charge of construction under the fastcarb research program. references [1] n. roussel. carbonatation des bétons et piégeage du co2, solution béton (hors série), p. 1-8, 2012. [2] atilh. ciments : déclaration environnementale, inventaire & analyse du cycle de vie, 2018. [3] r. andersson, h. stripple, t. gustafsson, et al. carbonation as a method to improve climate performance for cement based material. cement and concrete research 124, 2019. https: //doi.org/10.1016/j.cemconres.2019.105819. [4] gcca. gnr global database on co2 and energy information. https://gccassociation.org/. [5] schmitt l, mai-nhu j, duprat f, de larrard t, rougeau p, 2019, interest of using a model combining carbonation/chloride ingress and depassivation to better anticipate the rehabilitation of concrete structures, international conference on concrete repair, rehabilitation, and retrofitting, cape town, south africa. [6] j. mai-nhu, p. rougeau, f. duprat, et al. probabilistic optimization of the concrete cover for structures exposed to chlorination and carbonation according to climate change. safety, reliability, risk and life-cycle performance of structures and infrastructures 199:1535-42. 2013. https://doi.org/10.1201/b16387-225. [7] l. schmitt, j. jeong, j. m. potier, izoret l, et al. carbon dioxide binding ability in concretes: methodology and modeling, international workshop co2 storage in concrete co2sto2019 france marne la vallée ifsttar, 2019. [8] afnor. carbonation and co2 uptake in concrete, fd cen/tr 17310, 2019. 551 https://doi.org/10.1016/j.cemconres.2019.105819 https://gccassociation.org/ https://doi.org/10.1201/b16387-225 https://doi.org/10.14311/app.2022.33.0457 acta polytechnica ctu proceedings 33:457–462, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague research focused on low carbonation of concrete under old cement-based render peter paulíka,∗, jakub gašpáreka, michal bačuvčíkb, ivan janotkab a slovak university of technology, faculty of civil engineering, department of concrete structures and bridges, radlinského 2766/11, 810 05 bratislava, slovakia b building, testing and research institute, studená 967/3, 821 04 bratislava, slovakia ∗ corresponding author: peter.paulik@stuba.sk abstract. in-situ research and laboratory study of the concrete of old bridges shows that despite the low strength classes of concrete and the long time of exposure to co2, it is possible to moderate the depth of their carbonation. many old bridges were found during the in-situ survey in slovakia, which showed negligible carbonation under an old cement render (prc) even after more than 100 years of direct exposure to co2. at the same time, it was found that if this protective layer was significantly damaged or missing in some places, the depth of carbonation of the same concrete reached considerable depths, locally 70-80 mm. the article presents and summarizes the findings from in-situ and laboratory research with a possible explanation of this phenomenon. keywords: carbonation, cement render, concrete. 1. introduction the carbonation of concrete is a slow process during which the atmospheric co2 reacts with the compounds of the cement matrix, especially with portlandite. speed of carbonation is governed mainly by diffusion. it depends on the quality of concrete (the amount and type of cement used, water to cement ratio, used admixtures, etc.), environmental conditions (relative humidity, wetting and drying cycles, temperature, etc.) and amount of co2 concentration in the surrounding air [1–5]. based on many research projects and experience, it could be stated that the better is the concrete quality, the lower is the speed of carbonation [6]. however, during the study of 100 years old bridges in slovakia, it was observed that in some cases, also the poor quality concrete could have negligible carbonation. it has been found that a dense prc applied 100 years ago was able to prevent the underlying concrete from carbonation. this unexpected result was observed on several bridges from the beginning of the 20th century when a prc applied to the concrete surface was a common practice to enhance the aesthetic properties. however, this prc has proven a feature to be also an effective carbonation barrier [7]. during in-situ research and residual life analyses of 125 years old monier type concrete bridge in 2014, it was observed that a thin layer of cement render (2-4 mm thick) could protect the underlying concrete from carbonation, although it is a low-strength concrete. subsequent repeated measurements in 2015 were reaffirmed by laboratory measurements (chemical and thermal analysis). with ongoing research, other bridges were found from the beginning of the 20th century in the following years, with similar results. 2. measurement methods and results in-situ measurements consisted of the following steps: • the surface with the prc was cleaned, and permeability measurements with the torrent method were performed [8]. • permeability was measured directly at the places from where drilled core samples were taken. by this means, it was possible to correlate the permeability measurements with measurements of carbonation depth. • after drilling the core sample, it was cleaned correctly with water pouring in the direction from the top of the cylinder to avoid contamination of the carbonated layer by non-carbonated concrete dust particles. the sample was wiped, and a 1 % solution of phenolphthalein was applied to its surface in the direction from the outer surface of the cylinder. by this procedure, the carbonation depth was measured in-situ at all drilled core samples, which were then taken to the laboratory for further chemical analyses by the tg-dta method. in-situ research showed a significant dependence between the depth of the carbonation of the underlying concrete and its permeability, which is primarily moderated by the protective render coat (prc). the graphs in figures 1 and 2 show this dependence. according to the quality of the prc, it can be seen that increasing the permeability of the surface also increases the depth of carbonation. in the case of 100-year-old concrete, the 457 https://doi.org/10.14311/app.2022.33.0457 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en p. paulík, j. gašpárek, m. bačuvčík, i. janotka acta polytechnica ctu proceedings figure 1. the results of in-situ measurements of the depth of carbonation and permeability at the bridge in sládkovičovo.. figure 2. the results of in-situ measurements of the depth of carbonation and permeability at the bridge in sládkovičovo. permeability is relatively high. if it is not protected by the prc or this layer is considerably damaged, the carbonation reaches a considerable depth (dark bars of the diagrams). in contrast, if a 100-year-old concrete is protected by this layer (light bars in the diagrams), although this concrete has a high permeability, the prc moderates the co2 ingress and the depth of carbonation. it might be kept in mind that no chemical admixtures or additions were used at the manufacture of the prc at the time of the bridge construction 100 years ago. it is stated that the observed low carbonation depth of underlying concrete can be explained by the protective effect of a thin cement render only, which is at some places almost non-permeable for co2. at the sites where the prc was of good quality (figures 4 and figures 6), the carbonation of the underlying concrete was negligible (less than 2 mm). even at the areas where the concrete was covered by a lower quality cement-based render, it had much lower carbonation depth than concrete at the places where the protective render coat spalled over time (figures 5 and 7). figure 3. places where drilled core samples were taken from the bridge abutment on the left: the site with the protective render coat and on the right: the place where the render spalled over time (bridge in sládkovičovo). 458 vol. 33/2022 carbonation of concrete under old cement-based render figure 4. the carbonation depth of concrete at the places with a thin layer of prc at the surface sample selected from the measurements performed in 2018 (bridge in sládkovičovo). figure 5. the carbonation depth of concrete at the places where the render was missing sample selected from the measurements performed in 2018 (bridge in sládkovičovo). figure 6. the carbonation depth of concrete at the places with a thin layer of prc at the surface sample selected from measurements performed in 2019 (bridge in rimavská sobota). figure 7. the carbonation depth of concrete at the places where this render was missing sample selected from measurements performed in 2019 (bridge in rimavská sobota). figure 8. the amount of caco3 (% wt.) in the surface layer of the unprotected concrete sample rs2b. figure 9. the amount of caco3 (% wt.) in the internal layer of the unprotected concrete sample rs2b. the chemical and tg-dta analysis was performed on the concrete samples to prove the in-situ results of the depth of carbonation. concrete cylindrical specimens taken from the bridge were analyzed at depths of 0-20 mm (designated as surface) and at 280-300 mm (defined as internal) deep from the surface of the cylinder. the amount of caco3 was calculated for each layer by chemical and tg dta analysis. the degree of carbonation (cd) and the extent of co2 attack, characterized by the carbonation stages (i iv), was calculated for each layer according to [7]. as can be seen from figure 8, the surface of unprotected concrete is specific for the increased amount of both the total amount of caco3 and the increased ratio of coarse-grained and fine-grained caco3 compared to the prc-protected ones in figures 10 and 12. its high co2 attack also characterizes carbonation stage iv of the concrete surface sample compared to the internal part of the concrete in figure 9. in contrast, prc-protected concrete (figures 10 and 12) is characterized by a reduced amount of caco3 in the surface zone, compared to the unprotected sample rs2-b, and approximately the same ratio of coarse459 p. paulík, j. gašpárek, m. bačuvčík, i. janotka acta polytechnica ctu proceedings figure 10. the amount of caco3 (% wt.) in the surface layer of the protected concrete sample rs2-c by prc. figure 11. the amount of caco3 (% wt.) in the internal layer of the protected concrete sample rs2-c by prc. figure 12. the amount of caco3 (% wt.) in the surface layer of the protected concrete sample rs2-d by prc. figure 13. the amount of caco3 (% wt.) in the internal layer of the protected concrete sample rs2-d by prc. figure 14. the amount of caco3 (% wt.) in the prc sample over rs2-c. figure 15. the amount of caco3 (% wt.) in the prc sample over rs2-c. 460 vol. 33/2022 carbonation of concrete under old cement-based render figure 16. optical microscope image of cement renders (prc) at various magnifications. grained and fine-grained caco3 for the surface and internal part of the sample (figures 11 and 13). this is due to the efficiency of the prc to moderate the co2 ingress into the concrete below it. the effectiveness of pcr to protect the concrete beneath it lies in the absorption of co2 into its structure to such an extent that the resulting carbonates gradually clog the entire system until it becomes impermeable.it can be seen in figures 14 and 15 that the prc samples are characterized by a large amount of caco3 in a layer only a few millimetres thick. this corresponds to the low permeability values in figure 2. if such a clogged structure is damaged by cracks observed in the prc over rs2-a sample (figure 2), there is an increase in the permeability of the prc, which is reflected in the increased depth of carbonation of the concrete. despite the prolonged exposure to co2, the depth of carbonation is significantly lower than in the case of unprotected concrete. the surface layer of the prc was examined with an optical microscope. figure 16 shows a structure formed predominantly of fine-grained carbonates, which is particularly noticeable at higher magnification. crystallized coarse-grained carbonates (shining needles) are compressed in render thickness (2-4 mm) and, together with fine-grained carbonates, form a very dense structure. this finding is in good agreement with the chemical and tg-dta analysis demonstrated in figures 14 and 15. 3. conclusions the possible explanation of the extremely low permeability of the cement render and thus its superior protection of the underlying concrete could be summarized as follows: • the in-situ measurements of carbonation depth, which were also demonstrated by thermal (tgdta) and chemical analysis showed that a good quality thin cement-based render (prc with a thickness of only 2-4 mm) can sufficiently protect the old concrete against carbonation in case if its integrity is maintained. • if the prc layer was damaged at an unknown time during the entire life of the structure, the permeability of the surface was still lower than in the case of unprotected concrete as well as its depth of carbonation. • during the study, a dependence of the permeability of the prc and the depth of carbonation of the base concrete was observed, which consists in the gradual clogging of the prc structure with carbonates. • one of the possible explanations for this phenomenon could be the self-sealing effect. the pore structure of prc is gradually clogged during the chemical reaction that occurs between co2 and hydration products in a cement matrix. the selfsealing of the prc open pore system is caused by 461 p. paulík, j. gašpárek, m. bačuvčík, i. janotka acta polytechnica ctu proceedings the accumulated fine-grained carbonates compressing the coarse-grained carbonates in the process, which gradually reduces permeability to the point where co2 is no longer able to penetrate its structure. in this way, the amount of coarse-grained carbonates is reduced because they are not able to grow in such a small space formed by the dense structure of the prc. as a result, the remaining clearance in the structure is filled with fine-grained carbonate, which can cause the observed significant reduction in the permeability of the prc. acknowledgements this work was supported by the slovak research and development agency under contract no. apvv-17-0204 and the university science park (usp) of the slovak university of technology in bratislava (itms: 26240220084). references [1] c. f. chang, j. w. chen. the experimental investigation of concrete carbonation depth, cement and concrete research 36(9):1760-1767, 2006. doi:10.1016/j.cemconres.2004.07.025. [2] v. g. papadakis, c. g. vayenas, m. n. fardis. experimental investigation and mathematical modeling of the concrete carbonation problem. chemical engineering science 46(5-6):1333-8, 1991. https://doi.org/10.1016/0009-2509(91)85060-b. [3] physical and chemical characteristics affecting the durability of concrete. aci materials journal 88(2), 1991. https://doi.org/10.14359/1993. [4] l. j. parrott, d. c. killoh. carbonation in a 36 year old, in-situ concrete. cement and concrete research 19(4):649-56, 1989. https://doi.org/10.1016/0008-8846(89)90017-3. [5] g. villain, m. thiery, g. platret. measurement methods of carbonation profiles in concrete: thermogravimetry, chemical analysis and gammadensimetry. cement and concrete research 37(8):1182-92, 2007. https: //doi.org/10.1016/j.cemconres.2007.04.015. [6] s. matthews. design of durable concrete structures. ihs bre press, glasgow, 2014. [7] i. janotka, m. bačuvčík, p. paulík. low carbonation of concrete found on 100-year-old bridges. case studies in construction materials 8:97-115, 2018. https://doi.org/10.1016/j.cscm.2017.12.006. [8] r. torrent, g. frenzer. study on methods to determine and judge characteristic values of the coverconcrete on site report no. 516, bern, 1995. 462 10.1016/j.cemconres.2004.07.025 https://doi.org/10.1016/0009-2509(91)85060-b https://doi.org/10.14359/1993 https://doi.org/10.1016/0008-8846(89)90017-3 https://doi.org/10.1016/j.cemconres.2007.04.015 https://doi.org/10.1016/j.cscm.2017.12.006 https://doi.org/10.14311/app.2022.33.0337 acta polytechnica ctu proceedings 33:337–343, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague influence on permeability and pore structure of polyolefin fiber reinforced concrete containing slag wei-ting lina,∗, an chenga, kinga korniejenkob, michał łachb a department of civil engineering, national ilan university, taiwan b institute of materials engineering, faculty of mechanical engineering, cracow university of technology, poland ∗ corresponding author: wtlin@niu.edu.tw abstract. the purpose of this study is to assess the mechanical and microscopic properties of concrete containing different dosages of polyolefin fibers and slag through tests of compressive strength, resistivity, water absorption, mercury intrusion porosimetry and scanning electron microscopy. test results indicate that the specimens containing slag have higher compressive strength, lower absorption, lower resistivity and denser porestructures than the control and specimen made with fibers. the specimens containing slag and polyolefin fiber demonstrated better performances in fiber reinforced concrete. scanning electron microscopy illustrates that the polyolefin fiber acts to arrest the propagation of internal cracks. still, there are cracks and weaknesses between fiber and paste that cause harmful ions penetrated easier. keywords: critical pore size, mercury intrusion porosimetry, slag. 1. introduction concrete is a porous and cracked brittle material. it has low tensile strength, poor toughness, and small ultimate tensile strain, so it is easy to crack. the existence of cracks in the concrete would reduce the structural capacity and increase the deflection. at the same time, external harmful ions would penetrate into the concrete through the cracks, eroding the concrete or corroding the internal rebars, which would seriously affect the durability of the concrete structures. especially in modern concrete engineering, buildings are developing in the direction of large volume, large area, and complex and diverse shapes, and then concrete is developing in the direction of high strength and superior fluidity. therefore, it was required that the strength and slump of the concrete continue to enhance, and the amount of cement continues to increase. the negative impacts were that the hydration heat, shrinkage, shrinkage deformation stress and the number of cracks of concrete would be increased [1]. in order to solve the cracking problem, researchers have adopted the method of adding fibers to concrete to improve its cracking, thereby increasing the durability of the concrete. on the other hand, by reducing the water-tobinder ratio of concrete or adding pozzolanic materials, such as fly ash, ground-granulated blast-furnace slag (slag), silica fume, etc., it can help improve the pore structures in concrete [2–4]. shannag was suggested to use a certain type of fly ash mixed with silica fume and replace partial cement to improve the compressive strength, splitting strength and elastic modulus of concrete. relevant research has also pointed out that good concrete materials should have several excellent material properties, including workability, strength, corrosion resistance and high resistance to ion penetration [5–7], and the addition of pozzolaic materials can meet the above requirements, providing better macroscopic and microscopic properties of concrete. the most effective method for evaluating the macroscopic properties of concrete was the compressive strength test. mercury intrusion porosimetry (mip) and scanning electron microscopic (sem) observation were needed to understand the microscopic part of the concrete. it can be assessed the strength properties and transmission characteristics of durability on concrete structures. studies in recent years have found that to understand the pore-structures and transmission characteristics of concrete, porosity and pore size distribution are important research parameters. porosity and pore size distribution are the most direct parameters for the permeability and pore structures of cement pastes. permeability is directly related to the continuity of fluid passage (pore diameters of 120 or 160 nm) [8]. porosity is a measure of the ratio of pores to the total volume of the pastes. if the specimens had a high porosity and the pores were connected, the cement-based materials also had higher permeability. on the contrary, if the pores were not connected, the permeability of the materials was tended to decrease. it can be seen that the connectivity of the pores had a considerable relationship between permeability and durability [9, 10]. as far as the pore structures of concrete were concerned, even if there 337 https://doi.org/10.14311/app.2022.33.0337 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en w. lin, a. cheng, k. korniejenko, m. łach acta polytechnica ctu proceedings chemical compositions (%) materials cao sio2 al2o3 fe2o3 mgo so3 k2o l.o.i. cement 63.9 20.7 5.4 3.2 2.0 − 0.7 0.05 slag 41.6 33.5 14.1 0.4 6.9 0.7 2.2 0.78 table 1. chemical compositions of slag and cement. mix no. water cement slag fine aggregates coarse aggregates fibers a 217 395 0 908 780 0 ap1 395 0 902 774 3.6 ap2 395 0 897 769 7.3 ap3 395 0 892 764 10.9 ap4 395 0 887 759 14.5 ap5 395 0 882 754 18.2 ag4 237 158 908 780 0 ag4p1 237 158 902 774 3.6 ag4p2 237 158 897 769 7.3 ag4p3 237 158 892 764 10.9 ag4p4 237 158 887 759 14.5 ag4p5 237 158 882 754 18.2 table 2. mix design of concrete (kg/m3). test target specimen dimensions (mm) referenced standard mechanical properties compressive strength test !100 × 200 astm c39 permeability absorption !100 × 200 astm c642 micro-structure observations mip 10 × 10 × 3 astm d4404 sem observation 10 × 10 × 3 astm c1723 table 3. mix design of concrete (kg/m3). were more other characteristics that would affect the behavior and other engineering characteristics. the porosity and pore size distribution were important pore-structure parameters, which would directly affect the strength, durability and permeability of the concrete. this study is aimed to evaluate the influence on permeability and pore-structures of polyolefin fiber reinforced concrete containing slag using compressive strength, absorption, mip and sem observations. 2. experiments 2.1. materials in this study, it used type i portland cement with the specific gravity of 3.15 and the fineness of 3310 cm2/g. the fine aggregate was natural river sand with ssd specific gravity of 2.56, the absorption was 2.25% and the fineness modulus was 2.82. the coarse aggregates had an ssd specific gravity of 2.65 and absorption of 1.44%. the ground-granulated blast-furnace slag (g) with a white powder was produced by chc resources corporation in taiwan and used as constituents of cement replacement. slag had the specific gravity of 2.88 and fineness of 4000 cm2/g. the pozzolanic activity index of slag at the age of 28 days was 127%. the chemical compositions of slag and cement were summarized in table 1. inclusion of polyolefin fibers in composites can enhance the tensile strength and volume stability of cement-based materials containing slag. the length and aspect ratio of the fiber is 25 mm and 200 (d = 0.125 mm), respectively. the specific gravity, tensile strength and young’s modulus of fiber is 0.90, 275 mpa and 2647 mpa, respectively. 2.2. mix design and test methods the water/cement ratio (w/c) of the concrete specimens was maintained at a constant 0.55 in accordance with the aci 211.1 specification. table 3 lists the mix design of mixtures for the concrete specimens. the specimens were numbered using three letters and numbers to indicate the slag replacement of cement and dosage of the polyolefin fibers. a denotes ordinary portland concrete; p1, p2, p3, p4 and p5 refers to specimens containing 0.4 vol.%, 0.8 vol.%, 1.2 vol.%, 1.6 vol.% and 2.0 vol.% fibers, respectively. g4 refers to specimens containing 40% slag. table ?? presents the tests performed, the dimensions of the specimens and the standards used in this study. 338 vol. 33/2022 analysis of concrete and cement epd figure 1. compressive strength development curves of polyolefin fiber concrete without slag. 3. results and discussion 3.1. compressive strength figures 1 and 2 presents the compressive strength of polyolefin fiber concrete made with and without slag at the age of 7, 28, 56 and 91 days, respectively. the compressive strength increased significantly with the increase in the amount of fibers at 7 days of test age. it indicated that the addition of fibers could reduce the autogenous shrinkage or fine cracks of the specimens and the fibers were played a very important role in the early stage of crack resistance. the strength development and growth rate gradually flattens at the age of 28 days and the effect of age on compressive strength gradually decreases, and the amount of fibers had no significant effect on compressive strength (increasing trend was the same for 56 to 91 days). comparison of different fiber additions, the specimens containing 2.0% fibers had lower compressive strength than that of the specimens containing 1.6% fibers at different ages. it might be due to the addition of a higher fiber addition amount and it might affect the workability of the specimens. the worse workability was caused the fiber to be difficult to be uniformly dispersed in the specimens, which resulted in a lower compressive strength [11]. test results of the specimens with and without slag found that the compressive strength of the specimens increased with the increase of curing age, and the specimens containing slag had higher compressive strength than that of specimens without slag. the compressive strength was tended to increase with the increase of the inclusion of fibers. comparing test specimens at different ages, the specimens containing 2.0% had a slightly higher compressive strength than the specimens containing 1.6% fibers. it indicated that the inclusion of slag in concrete could increase the compressive strength of fiber concrete. the inclusion of slag in the specimens was reflected better workability and enabled the fibers to be uniformly dispersed during the mixing process, resulting in a figure 2. compressive strength development curves of polyolefin fiber concrete with slag. better compressive strength. at the same curing age, the compressive strength of the polyolefin fiber reinforced concrete with 40% slag (ag4p5 specimens) could be 12% higher than the ap5 specimens. fibers also can improve the bonding strength of the fiber interface, and then increase the compressive strength of the concrete. 3.2. absorption absorption is an important indicator for judging the permeability and quality of concrete. lowpermeability concrete can resist the penetration of water, and it also made other harmful ions difficult to penetrate into the concrete. a comparison of the influence on the absorption of the addition of 40% slag on polyolefin fiber reinforced concrete at various age is shown in figures 3 and 4, respectively. it indicated that the absorption was decreased with the increase of curing age, resulting in denser pore-structures. however, the absorption of each group of specimens without slag was remained at about 6%, which did not vary with the inclusion of fibers in concrete. this might be caused by the addition of polyolefin fibers in the specimens and the fibers could reduce the connectivity of the pores. in addition, the weak interface of the fibers had a greater impact on the absorption, resulting in varied values of absorption [12]. the results found that the specimens mixed with 40% slag and 0.4% polyolefin fibers were reflected in lower absorption of various ages. as the amount of fiber increased, the absorption was started to increase. it was obvious that even if slag was added to improve the adhesion between the pastes and the polyolefin fibers. as long as the fibers were slightly unevenly dispersed, it would affect the results of absorption. it was necessary to ensure that the blended polyolefin fibers were evenly distributed in the specimens and prevented the fiber from agglomerating to ensure that it had no agglomerate seriously, it makes poor mechanical properties and permeability of the 339 w. lin, a. cheng, k. korniejenko, m. łach acta polytechnica ctu proceedings figure 3. absorption development curves of polyolefin fiber concrete without slag. figure 4. absorption development curves of polyolefin fiber concrete with slag. concrete. compared with the control specimens, the combination of 0.4% polyolefin fibers and 40% slag reduced the absorption of the specimens. however, the maturity of the specimen had a limited effect on absorption. the polyolefin fiber-reinforced concrete containing 40% slag had lower absorption, which was 50% and 35% lower than the a and ap1 specimens. the fiber could reduce the connectivity of the pores, and the slag could strengthen the degree of adhesion between the fiber and the paste to achieve the double enhancement effect. figure 5 is illustrated the relationship between the absorption and the polyolefin fiber content of each mix of polyolefin fiber-reinforced concrete containing slag at the age of 91 days (testing results were averaged by three specimens for each mixture and the coefficient of variation was controlled under 5%). from the regression equation, the absorption of fiber-reinforced specimens mixed with 40% slag was changed with the increase of fiber addition. when the figure 5. relationship between absorption and fiber comtent (40% slag). figure 6. relationship between the pore size and cumulative mercury intrusion of polyolefin fiberreinforced concrete specimens (without slag). fiber amount reached 1.55%, there was the highest absorption; however, the fiber amount reached 0.45%, there was the lowest absorption. its tendency was similar to that of the specimens without slag. the results also indicated that the specimens containing 40% slag had a lower absorption than that of specimens without slag. 3.3. mip results figures 6 and 7 show the relationship between the pore size and cumulative mercury intrusion of the polyolefin fiber-reinforced concrete specimens with and without slag at the age of 91 days. the test results found that the cumulative mercury intrusion and total porosity of the specimens decreased with the increase of polyolefin fibers. the cumulative mercury intrusion of ap5 specimens was 0.0654 ml/g, which was lower about 43% than the control specimen (a specimen was 0.1147 ml/g). the lowest cumulative mercury intrusion was 0.0288 ml/g of ap2 specimens, although it was lower than that of other 340 vol. 33/2022 analysis of concrete and cement epd figure 7. relationship between the pore size and cumulative mercury intrusion of polyolefin fiberreinforced concrete specimens (with slag). groups. the ap3 specimen had a high cumulative mercury intrusion (0.1288 ml/g), which might also be caused by the weak interfacial transition zone between the pastes and the fibers described in the previous section, but overall it has appeared that the addition of polyolefin fibers was helped to reduce the internal porosity of specimens. from figure 6, the pore distribution curve of the specimen had a relatively gentle trend with the increase of fibers (especially when the pore size is less than 50 nm). it can be seen that the addition of fibers can also help reduce the pore size to a certain extent. it was also verified that the fiber can reduce the connectivity between the pores and achieve the effect of improving the permeability of the concrete [13–15]. figure 7 shows that the cumulative mercury intrusion and total porosity of the specimens had a tendency to decrease significantly with the increase of the slag. the cumulative mercury intrusion of ag4 specimens was 0.0534 ml/g, which was reduced by about 53% than that of control specimens. however, the cumulative mercury intrusion and total porosity of the specimens mixed with slag were tended to decrease with the increase of polyolefin fibers. it was still lower than that of various groups of polyolefin fiber reinforced concrete specimens without slag. it can be seen that the inclusion of slag was more effective in reducing pores and reducing pore size than the addition of polyolefin fibers. except for the ag4p1 specimens, the pore distribution curve of all specimens had a gentler trend with the increase of fibers (especially when the pore size was less than 50 nm). it was found that the addition of fibers can reduce the pore connectivity. however, the effect of the addition in fiber-reinforced specimens on increasing compactness and reducing porosity was significant. it has been verified that the addition of slag in concrete can cause secondary hydration reaction with cement from ca(oh)2 to produce denser c-s-h colloid, thereby reducing the amount of free ca(oh)2, and using cfigure 8. sem photo of the control specimens. s-h colloid to reduce the volume and size of pores, thereby achieving the effect of improving the permeability of concrete [16]. the specimens combined with slag and polyolefin fibers might reflect lower pore characteristics than the control group or the specimens containing polyolefin fibers singly. however, the enhancements of the specimens combined with slag and polyolefin fibers were also lower than those of the specimens containing slag singly. it can be seen that although fibers can reduce the connectivity between pores, the formation of the interfacial transition zone was prone to generate new connected pores [17]. the addition of slag did not completely improve this weak interfacial transition zone. compared with previous studies [4, 12, 17, 18], if this weak interfacial transition zone can be mixed with finer silica fume, it might be effectively improved and reduced the porosity and pore size significantly. 3.4. sem observations figures 8 to 9 are shown the sem observations of control specimens and the specimens containing 40% slag, respectively. figure 8 indicated that more pores can be observed on the surface of the control specimens. the unevenness of the surface on pore structures was reflected in looser pore structures, which in turn results in concrete with lower mechanical properties and higher absorption. it was obvious that the microscopic properties had a considerable effect on the properties of the concrete including strength and permeability. the inclusion of slag in concrete can effectively increase the compressive strength of the concrete and improve permeability. the main reason is that the secondary hydration reaction produced by slag can provide the denser microstructures and the results can be seen from figure 9. it found that the pores had a significant decrease and were filled by hydration reactants. it was also verified that the addition of slag had a positive effect on concrete. figure 10 shows a surface observation diagram of 341 w. lin, a. cheng, k. korniejenko, m. łach acta polytechnica ctu proceedings figure 9. sem photo of the specimens containing 40% slag. polyolefin fiber. it can be found that the fiber surface is not as smooth as the original appearance. it is found that the surface was covered with fine hydration reactants and it was also showed an irregular surface of the fiber. these hydrates can increase the bonding force between the fiber and the pastes, resulting in an increase in the mechanical properties and permeability of the concrete. figure 11 shows the sem photo of the specimen containing 1.2% polyolefin fiber reinforced concrete and 40% slag. it was found that the inclusion of slag in fiber-reinforced concrete had a great benefit for the microstructures. the specimens containing slag had fewer capillary pores and were filled with the hydration reactants between the pores and the pastes. the findings correspond to the test results described in the previous section. 4. conclusions in terms of the results of compressive strength and absorption, the effect of the inclusion of fibers in concrete on the absorption and strength was limited. with the increase in the amount of fiber, the absorption was maintained at about 6%. the inclusion of slag in concrete can increase the strength and reduce the absorption of the specimens. comparing the specimens mixed with fiber and slag, there was no obvious trend in absorption. it is because of the interface between the fiber and the pastes caused by the weak interfacial transition zone. the weaker interfacial transition zone resulted in higher absorption; however, the ag4p1 and ag6p2 specimens still had lower absorption. regression analysis was performed on the compressive strength and the amount of fiber of the specimen, and it was found that there was a good trend relationship between them. it indicated that the best compressive strength was obtained from the specimens containing 1.35% fiber without considering the slag addition; however, the specimens containing 1.70% fiber had the best strength at the group of 40% slag specimens. figure 10. sem photo of the polyolefin fiber. figure 11. sem photo of the specimens containing 1.2% fiber and 40% slag. the sem and mip results can confirm that the addition of slag provided a denser microstructure of concrete, which improves the mechanical properties and permeability. fibers can provide the effect of inhibiting crack extension, and the interfacial transition zone between the fiber and pastes was prone to cause water or harmful ions to penetrate into concrete. the fibers were liable to produce a hydration reaction with the hydrates. the inclusion of slag in concrete can also reduce the weak interfacial transition zone, thereby achieving the effect of enhancing concrete performance and compactness. acknowledgements this work has been financed by polish national agency for academic exchange under the international academic partnership programme within the framework of the grant: e-mobility and sustainable materials and technologies emmat. this work also has been supported by the ministry of science and technology (most-109-2221e-197-001-my2) in taiwan. references [1] c. cao, m. m. s. cheung, b. y. b. chan. modelling of interaction between corrosion-induced concrete cover crack and steel corrosion rate. corrosion science 342 vol. 33/2022 analysis of concrete and cement epd 69:97-109, 2013. 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https://doi.org/10.1016/j.cemconcomp.2004.09.010 https://doi.org/10.14311/app.2022.33.0377 acta polytechnica ctu proceedings 33:377–382, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague sustainable concrete production with recycled concrete wash water beneficiated with co2 sean monkmana,∗, vincent meyerb a carboncure technologies, 42 payzant av, dartmouth ns, canada b holcim innovation center, 95 rue du montmurier, 38070 saint-quentin-fallavier, france ∗ corresponding author: smonkman@carboncure.com abstract. a significant amount of wastewater is generated through the cleaning of equipment utilized within the ready mixed concrete production cycle. the reuse of concrete wash water as mix water is limited by the negative material performance impacts associated with the suspended solids; the effects are exacerbated with increasing solids contents and water aging. a novel carbon dioxide treatment to allow the use of high solids wash water (specific gravity 1.10) as mix water was examined. seven batches of concrete were produced and compared: a reference mix, two batches with untreated wash water and four batches with co2 treated wash water. the carbon dioxide treatment mineralized co2 at 28% by mass of the treated solids. acceptable concrete was produced through adjusting admixtures for workability. the compressive strength at 1, 7, 28 and 56 days was improved relative to both the reference and the concrete produced with untreated wash water. the suspended solids containing mineralized co2 served as a viable cement replacement. the avoided cement and bound carbon dioxide served to lower the carbon impact of the concrete by about 14%. the approach allows three waste streams (co2, wash water and wash water solids) to be reused to produce more sustainable concrete. keywords: co2 utilization, concrete, sustainability. 1. introduction awareness of the environmental impacts of the cement and concrete industries, combined with increasing global infrastructure demands, has resulted in a greater emphasis on improving concrete sustainability. strategic roadmaps and technology assessments have recognized the shortcomings of traditional levers for reducing the carbon impact of the industries and the need for emerging technologies [1]. action is require throughout the supply chain; cement producers, concrete producers, structural engineers and construction companies each have roles to play in the pursuit of a net zero carbon vision [2]. within the context of emerging technologies lies the concept of co2 utilization [3]. in this approach, carbon dioxide is used directly or as a feedstock to produce valuable carbon-containing products. one use is for the manufacture of construction materials; in contrast to other utilization pathways, such as the production of fuels, the carbon dioxide is mineralized and permanently removed from the atmosphere [4, 5]. construction has been identified as the largest market opportunity for the co2 utilization field with technologies in concrete and aggregate production offering a potential to utilize 5.0 gt co2 and generate $550b per year by 2030 [6]. the present work concerns an investigation into the use of carbon dioxide to beneficially treat concrete wash water for its reuse as mix water. the production of ready mixed concrete involves the washing of equipment whether it be a central mixer or individual trucks. after the concrete has been discharged the mixer may be washed out to prepare it for a new load of concrete. the water that is washed from the truck is laden with very fine sand and cementitious materials. the handling and disposal of this wastewater represents a significant operational, regulatory and logistical burden for the concrete producer. the most common approaches to address wash water are clarification and discharge, reclaimers, hydration stabilizing admixtures or no action at all. ideally the wash water can be recycled into new concrete but performance issues often arise when it is used as mix water in lieu of fresh water [7]. the approach can lead to concrete with increased water demand (and lower compressive strengths) and undesirable set acceleration. both of these impacts worsen with the age of water meaning that the performance outcomes are sensitive to the "freshness" of the wash water. in practice, the solution has been to specify that mix water must have less than 50,000 ppm solids [8]. in cases where wash water is used in new concrete it is often merely diluted with fresh water to reduce the negative impacts. while the practice of clarifying water and disposing solids is the current status quo, the industry is being directed towards zero discharge practices and ready mixed producers are looking to decrease their water usage and carbon footprints [9]. the present study concerns the co2 treatment of 377 https://doi.org/10.14311/app.2022.33.0377 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en sean monkman, vincent meyer acta polytechnica ctu proceedings batch control nt nta ct cta ctp ctap ww treatment n/a none none co2 co2 co2 co2 cement (kg/m3) 267 267 267 267 267 240 240 limestone(kg/m3) 63 33 33 30 30 57 57 ww solids (kg/m3) 0 30 30 33 33 33 33 total powder (kg/m3) 330 330 330 330 330 330 330 pce (%/b) 0.34 0.34 0.34 0.34 0.34 1.15 0.77 dispersant (%/b) 0 0 0.66 0 0.74 0 0.74 relative binder 100 91 % 91 % 90 % 90 % 90 % 90 % table 1. mix design of concrete produced during test program. ww refers to wash water. ww solids are the suspended solids present in the wash water that are incorporated into the concrete mix. pce refers to a polycarboxylate plasticizing admixture. admixture dosages are in % with respect to the binder content. high solids concrete wash water for reuse, without dilution, as concrete mix water. carbon dioxide is readily mineralized into calcium carbonate upon reaction with tricalcium or dicalcium silicate [10, 11] or calcium hydroxide [12], wherein the carbon dioxide is permanently removed from the atmosphere it was studied what impact the carbon dioxide would have on the use of wash water as mix water and whether performance-enhanced concrete could be produced. 2. experimental a simulated concrete wash water was produced by mixing 7.7 kg of cement with 18 litres of water. the slurry was mixed in a drum mixer at 24 rpm for 3 hours to simulate a production/delivery/washing cycle. the slurry was then added to a treatment vessel. the total water in the reactor vessel was brought up to 50 litres by adding 7 litres of water through wash out of the drum mixer and a final 25 litres of water. the slurry was designed to have a specific gravity of 1.10 or more than three times the nominal optional limit for concrete mix water that appears in many specifications such as astm [8]. the supply of slurry was maintained under agitation in a barrel adapted as a reactor vessel. an immersible pump served to circulate the slurry through a conduit that was routed up and out of the slurry before bending back down to the vessel bottom whereupon the pumped slurry was ejected. wash water subjected to a co2 treatment received an injection of carbon dioxide gas at 600 litres per hour. the injection of co2 gas was integrated into the base of the conduit where it was connected to the exit of the pump. the conduit, approximately 1.2 meters in total length, served to increase the potential residence time of gas inside the slurry given that any bubbles would rise through the slurry via buoyancy. three different batches of slurry were made (one untreated, two treated) and each batch of slurry was used to make two batches of concrete. both treated and untreated wash water slurries were kept agitated in a minimally rotating drum mixer through to 24 hours prior to use in concrete. samples of the treated slurry were taken periodically throughout the treatment cycle which concluded when the ph had declined from an originally elevated level to a neutral level of about 7.0. the carbon uptake and bound water of the cement slurry solids was determined through thermogravimetric analysis. a second assessment of the bound carbon was completed using an eltra cs 800 carbon sulphur analyser (induction furnace). a series of seven concrete mixes were produced and are summarized in table 1. the batch size was 1 m3. the default (reference) mix design had a w/b of 0.55. the binder content totalled 330 kg/m3 and was comprised of 81% cement and 19% limestone. two batches were produced with untreated wash water comparing concrete excluding and including a retarder as a dispersant to improve slump (marked nt and nta for not treated and not treated plus dispersant admix). likewise, two analogous batches were produced with co2-beneficiated wash water (marked ct and cta for co2-treated and co2-treated plus dispersant admix). lastly, an additional two batches were produced with co2-beneficiated wash water wherein the level of the superplasticizing admixture was increased and either excluded or included the dispersant addition (marked ctp and ctap for co2treated plus plasticizer and co2-treated plus admix plus plasticizer). adjustments were made to the binder loadings of batches containing wash water in order to compensate for the additional solids present in the wash water slurry. the total powder content (cement + limestone + wash water solids) was maintained at 330 kg/m3. reductions were made to the limestone in the first four cases (nt, nta, ct, cta) while a proportional reduction to both cement and limestone was investigated in the final case (ctp, ctap). in the final case the admixture loading was changed with respect to the reference and the adjustments were determined with respect to what was required to achieve the target slump. the dispersant loadings were in proportion to the wash water solids. the concrete was measured in terms of slump and compressive strength at 24 hours, 7 days, 28 days and 378 vol. 33/2022 sustainable concrete production ϭй ϱй ϭϭй ϭϱй ϯϭй ϯϱй ϯϭй ϯϱй ϰϭй ϰϱй ϱ ϲ ϳ θ ϵ ϭϭ ϭϭ ϭϯ ϭϯ ϭϰ ϭ ϯϭ ϲϭ ϵϭ ϭϯϭ ϭϱϭ ϭθϭ �k ϯ� ăŷ ě� ,ϯ k �đ žŷ ƚğ ŷƚ �ď lj� žƌ ŝő ŝŷ ăů �ŵ ăɛ ɛ�ɛ žů ŝě ɛ ɖ, dƌğăƚŵğŷƚ�dŝŵğ ɖ, �kϯ�ǀŝă�d'� �kϯ�ǀŝă�ŝŷěƶđƚŝžŷ �žƶŷě�,ϯk figure 1. carbon uptake in wash water solids and solution ph with treatment time of wash water slurry treated with co2. 56 days with the exception of the batches containing increased plasticizer (pce) which were assessed only at 7 and 28 days. 3. results 3.1. co2 treatment of the slurry the co2 uptake and ph data over time for the two treated slurries closely agreed. one case is presented in figure 1 and discussed herein. the ph of the treated slurry was 12.7 after three hours of prehydration. the ph declined upon addition of the co2 and was only a modest decline at first, down to 12.1 after 60 minutes, before a more rapid drop leading to a ph of 7.1 at 150 minutes. the injection of the co2 into the agitated slurry functionally changed when the treatment neared completion; whereas the initial injection of carbon dioxide did not result in any bubbles of carbon dioxide gas reaching the surface of and exiting from the slurry, the slurry displayed extensive bubbling around the time of neutralization. the carbon content of the slurry solids was observed to increase with treatment time in a broadly linear fashion over the first 120 minutes of treatment. ultimately the treatment was complete after 150 minutes at which time the solids were 28.1% co2 by mass. the carbon contents, as presented in the figure, have been normalized to the same ignited mass to express the uptake as a (percentage) mass of carbon dioxide per mass of the original anhydrous cement. this allowed for a simple comparison against the amount of co2 delivered, which can be described in the same terms. the uptake at the conclusion of the treatment was 41.4% co2 by weight of cement under the tga analysis and 37.4% with the induction furnace. the uptake can be used to calculate process efficiencies. the injection of co2 gas at 600 lph with a density of 1.98 g/l over 2.5 hours means that 2.97 kg of co2 was delivered. where the initial slurry contained 7.7 kg of cement, the uptake rate via the tga translates into 3.19 kg of bound co2 for an efficiency of 107% (a more robust tga analysis that includes unattributed weight losses and a true loi would serve to decrease this calculated value). using the uptake rate via induction furnace there were 2.88 kg of co2 mineralized for a conversion efficiency of 97%. the bound water content was normalized in the same fashion as the bound carbon dioxide. it was observed that the bound water increased over the first 90 minutes of the treatment before declining. in particular the decline in ph may have changed the equilibrium of any hydrate phases. for example, ettringite is destabilized below a ph 10.7 while monosulphate disappears below 11.6 [13]. the change in rate of the uptake curve between 120 and 150 minutes attests to a slowing down of the reaction. the delivery of the carbon dioxide was not changed so at this time the entering carbon dioxide is tending to change the ph of the solution moreso than mineralize with the slurry solids. 3.2. fresh properties the slump measurements of the seven batches of concrete are presented in figure 2. the slump of the control was 19 cm after the conclusion of mixing (5 minutes after the addition of water) and was used as a target in later batches that were adjusted for slump. the untreated wash water served to reduce the slump by about 25% while an addition of the dispersant in turn increased the slump by 10%. the slump of the batch with the co2 treated slurry had 379 sean monkman, vincent meyer acta polytechnica ctu proceedings ϭ ϭ͕ϯ ϭ͕ϰ ϭ͕ϲ ϭ͕θ ϭ ϭ͕ϯ ϭ͕ϰ ϭ ϱ ϭϭ ϭϱ ϯϭ ϯϱ �žŷƚƌžů ed ed� �d �d� �dw �d�w �ě ŵ ŝdž �;й ͬď ŝŷ ěğ ƌϳ ^ů ƶŵ ɖ� ;đ ŵ ϳ w�� �ŝɛɖğƌɛăŷƚ figure 2. workability (slump) of concrete batches produced with wash water slurry. admixture loadings of dispersant and pce are indicated as symbols read from the secondary vertical axis. batch control nt nta ct cta ctp ctap 24 h strength (mpa) 10.2 10.8 10.0 14.6 13.4 n/a n/a 7 d strength (mpa) 25.1 26.0 29.9 30.0 31.0 26.4 29.7 28 d strength (mpa) 32.6 32.3 35.8 36.0 36.9 33.0 36.2 56 d strength (mpa) 35.0 n/a 36.9 38.9 39.9 n/a n/a relative strength 24 h 100 % 106 % 98 % 143 % 131 % − − relative strength 7 d 100 % 104 % 119 % 120 % 124 % 105 % 118 % relative strength 28 d 100 % 99 % 110 % 110 % 113 % 101 % 111 % relative strength 56 d 100 % − 105 % 111 % 114 % − − table 2. compressive strength of concrete mixes from 24 hours to 56 days. a major reduction (75%) in workability and, as in the untreated slurry case, an improvement in slump was observed when the dispersant was used. however, in either case, it was observed that the use of the treated slurry resulted in a workability loss. when the addition of a superplasticizer was explored the slump was improved from 9.5 cm to 18.5 cm to effectively match the target. the addition of the dispersant before the suplerplasticizer allowed the same flowability to be achieved albeit with 33% less pce. the workability issues can be solved through the use of conventional admixtures. 3.3. compressive strength the summary of the compressive strength data is presented in table 2. reported compressive strengths are averages of three specimens. the untreated wash water had a small positive effect at the two earliest ages but was neutral at 28 days. the addition of the dispersant resulted in improved performance at ages of 7 days and beyond, albeit diminishing relative to the control from 19% better at 7 days to 5% better at 56 days. the carbonation treatment served to improve the concrete strength at all ages of batches so produced. the benefit was greatest at the earliest age with a 43% benefit at 24 hours. as with the untreated slurry, the relative strength benefit declined with increasing age but remained at 11% at 56 days. the batch that included the dispersant was not quite as strong at 24 hours (a 31% benefit) but showed higher strengths than the batch without the dispersant at all subsequent ages concluding at a 14% benefit at 28 days. while the batches with the treated slurry were stronger than the control they also showed a reduced workability that would render them unviable. the batches with the treated slurry and the pce had a 10% lower cement content than the control. even so, the 7 and 28 day strengths were acceptable and slightly above the control. as with the other examples of using the dispersant, the strength was increased further with an 11% improvement observed at 28 days, despite the cement reduction. 4. discussion the compressive strength results are best compared in terms of the binder efficiency. the comparisons between the control and the batches with acceptable workability and using the treated slurry are outlined in table 3. the cementing efficiency was calculated in terms of the cement clinker (assuming a clinker factor in the cement of 92% as per the cembureau epd 380 vol. 33/2022 sustainable concrete production batch control ctp ctap clinker efficiency 28d (kg clinker m−3 mpa−1) 7.5 6.7 6.1 cement efficiency 28d (kg cement m−3 mpa−1) 8.2 7.3 6.6 binder efficiency 28d (kg binder m−3 mpa−1) 10.1 9.0 8.2 powder efficiency 28d (kg powder m−3 mpa−1) 10.1 10.0 9.1 carbon efficiency (kg co2 m−3 mpa−1) 7.4 6.3 5.7 anticipated strength via cement loading (mpa) 32.6 29.3 29.3 actual strength (mpa) 32.6 33.0 36.2 change in strength (mpa) n/a 3.7 6.9 equivalent units of cement (kg) n/a 30.0 56.2 actual units of ww solids (kg) n/a 33 33 units ww solids/unit cement n/a 0.91 1.71 table 3. cementing efficiency summary of control and batches with treated slurry. [14]), the cement content, the overall binder content (cement + limestone), the overall powder content (cement + limestone + wash water solids) and the carbon impact [15] (relevant to the binder mixture of cement + limestone + wash water solids). the carbon intensity of the cement is taken to be 0.898 t co2/tonne cement [14] while that of the limestone is taken as 0.008 t co2/t cement [16]. it was observed that the clinker, cement and binder efficiencies of the batches produced with wash water were improved 11% over control in the ctp batch. whereas the control produced concrete with a 28 day strength of 32.6 mpa and 267 kg cement/m3 concrete, the ctp batch was 33.0 mpa with 240 kg of cement. if the cement was to provide strength with the same efficiency in the latter as it had in the former then the expected strength was 29.3 mpa. the difference between the actual strength and the expected, +3.7 mpa, can be attributed to the wash water since the overall powder loading was unchanged. whereas 3.7 mpa of strength would be associated with 30 kg/m3 of cement the ctp batch contained 33 kg of wash water solids. the powder efficiency was unchanged in comparison to the control while the carbon the efficiency was improved 15%. there appeared to be a synergy upon adding both the dispersant and the pce (batch ctap). all of the clinker, cement, and binder efficiencies were improved 19% as compared to the control. in this instance the actual strength was +6.9 mpa greater than the expected strength as per the cement efficiency of the control and cement loading of the batch. the strength difference is equivalent to 56.2 kg of cement. the incorporation of 33 kg of wash water solids meant that the solids had a cement replacement efficiency of 1.7 units of cement per unit of solids. the powder efficiency was improved 10% in comparison to the control while the carbon the efficiency was improved 22%. the sustainability impacts can be achieved with respect to the mineralized carbon dioxide in the wash water solids, the avoided cement as part of the mix design modification, and recycling of waste cement and avoided potable mix water. the direct co2 impacts are estimated from the uptake of carbon dioxide into the wash water. the uptake of 28% co2 by weight of solids determined that the 33 kg of wash water solids/m3 concrete contained 9.2 kg of mineralized co2. in accordance with the cembureau epd for portland cement, the 27 kg of cement avoided were associated with 24.2 kg of carbon dioxide. the combined carbon reduction was 35.4 kg of co2 of which the avoided was 75% and the mineralized was 25%. the carbon impact of the binder was reduced 14% from 240.3 kg co2/m3 concrete in the reference to 206.8 kg co2 m3 concrete in the two batches with co2 treated slurry. the replacement of the fresh mix water meant that 182 kg fresh water/m3 concrete was saved. the untreated wash water produced acceptable concrete with workability better than in the co2 treated case and acceptable strength albeit not as high as in the co2 treated case. the performance of the concrete produced with the untreated wash water would be expected to vary with age of the wash water. it would be less predictable and burdensome to the producer given the evolving properties and condition of the wash water slurry itself. on the other hand, the treatment of the slurry with co2 not only mineralizes co2 permanently into waste cement, but it stabilizes the wash water such that its performance is consistent and predictable. thermal analysis data characterizing % bound co2 and % bound water showed that for an untreated slurry the total mass loss (loi) was increased from 1.2% in the anhydrous case to 3.9% for the slurry hydrated 3 hours to 22.6% for the slurry hydrated 26 hours. conversely, a test of slurry solids treated to an uptake of 27.4% by mass co2 and 8.9% by mass bound water (an loi of 36.3%) at the conclusion of the treatment was observed to be 27.9% co2 and 7.7% h2o (loi of 35.6%) after holding for an additional 24 hours. a successful adoption of the approach could realize several operational benefits. the operating expense for solids disposal could be greatly reduced or eliminated. both costs related to neutralization of 381 sean monkman, vincent meyer acta polytechnica ctu proceedings wash water prior to discharging and permitting costs associated with discharging could be avoided. the process would come with costs associated with the merchant carbon dioxide and increased admixture usage but these costs could be offset by savings of any admixtures used in an incumbent wash water management approach and through cement savings. the future may bring monetizable carbon offsets for co2 utilization that would improve the economic motivation to implement the approach. 5. conclusions the use of carbon dioxide to treat concrete wash water allowed the treated slurry to be used as mix water in the creation of performance enhanced concrete. a high specific gravity wash water slurry was shown to bind co2 at around 40% by weight of cement. the complete replacement of potable water with treated slurry lead to concrete with a reduced workability, but modification of the admixture loading addressed the issue. a mix design adjustment to reduce the binder loading in direct proportion to the suspended solids contained within the slurry did not compromise the compressive strength performance. the wash water solids contributed to the performance of the binder and thereby displayed a latent cementitious property. the approach aligns with industry sustainability goals to reduce the clinker content in concrete mixes, reduced wastes generated by concrete plants, to increase recycled materials usage and to reduce the water impacts of concrete production. an environmental benefit is realized through both mineralized co2 and avoided co2 from replaced cement and amounted to a 14% reduction of the overall binder carbon impacts. the proposed approach represents a significant operational, resource efficiency, cost saving, regulatory, and logistical opportunity for sustainable concrete production. acknowledgements the authors acknowledge that the work was completed within the first edition of the lafargeholcim accelerator program. technical work was completed at the lafarge centre de recherche (lcr) in lyon, france. the contributions of sandra boivin, sébastien rols, yves francomme, vincent briaud, and julien nawrot are greatly appreciated. references [1] iea and wbcsd. technology roadmap: low-carbon transition in the cement industry, paris and geneva: oecd/iea and wbcsd, 2018. 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[16] s. a. miller, v. m. john, s. a. pacca, et al. carbon dioxide reduction potential in the global cement industry by 2050. cement and concrete research 114:115-24, 2018. https: //doi.org/10.1016/j.cemconres.2017.08.026. 382 https://doi.org/10.1088/1755-1315/225/1/012009 https://doi.org/10.1038/s41586-019-1681-6 https://doi.org/10.1016/j.conbuildmat.2016.05.080 https://doi.org/10.1016/j.jcou.2017.07.003 https://rmc-foundation.org/wp-content/uploads/2017/07/scp-guidelines-version-1.1.pdf https://doi.org/10.1038/physci240016a0 https://doi.org/10.1111/j.1151-2916.1979.tb19046.x https://doi.org/10.1016/0008-8846(86)90044-x https://doi.org/10.1016/0008-8846(91)90062-m https://doi.org/10.1016/j.cemconcomp.2010.07.009 https://doi.org/10.1016/j.cemconres.2017.08.026 acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.38.0138 acta polytechnica ctu proceedings 38:138–144, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague review and analysis of existing methods to assess the indoor environmental quality (ieq) for office buildings cláudia de castro jacinto, adilson campos de paula junior, sandra maria gomes monteiro silva, ricardo filipe mesquita silva mateus∗ university of minho, institute for sustainability and innovation in structural engineering (isise), department of civil engineering, 4800-058 guimarães, portugal ∗ corresponding author: ricardomateus@civil.uminho.pt abstract. population in developed countries spend most of their time indoors, whether in their homes, workplaces, stores or leisure areas. due to the covid-19 pandemic, this situation worsened and now, more than ever, the importance of a high indoor environmental quality (ieq) is highlighted. the ieq is very important in building performance since it is directly related to its occupants’ comfort, health, wellbeing, and productivity and the sick building syndrome (sbs) concept. therefore, it is essential to develop tools to support designers’ decision-making in the materialization of indoor environments with higher quality. from the state-of-art analysis, it is possible to conclude that the methods to assess the overall building performance already consider the ieq. still, most use an approach that does not cover all relevant indicators. in this context, this paper presents the first milestone of a research work that aims to develop a new method to rate the overall ieq of office buildings in portugal. the main objective of the present study is to propose a list of ieq indicators for office buildings, adapted to the portuguese context, based on the analysis of existing rating methods for buildings and the recommendations of national and international standards. keywords: indoor environmental quality (ieq) indicators, decision-support tools, office buildings. 1. introduction since the first civilizations, humanity has been concerned with building shelters and creating a safe interior space to develop their activities, whether productive or leisure, under protection from the weather [1, 2]. as knowledge about building environments, both indoors and outdoors, improved over time, other conditions were added to the basic building requirements [2, 3]. technological advances allowed the creation of new materials and construction techniques. until a few decades ago, both the design and construction of buildings paid attention only to characteristics related to engineering, economy and aesthetics [3, 4]. the indoor environmental quality (ieq) only started to be considered after the appearance of pathological symptoms presented by occupants, associated with the construction [4]. currently, one of the main objectives of buildings is to provide healthy and comfortable environments for human activities, providing shelter, light, space, and sufficient amenities to work, live, learn, heal, among others [5]. before the covid-19 pandemic, a study by klepeis et al. [6] showed that people spend about 90 % of their time indoors, whether in their homes, workplaces, shopping, or leisure areas. thus, it is clear the importance of a high ieq since it is directly related to its occupants’ comfort, health, and productivity [7]. in the path of raising awareness about the rational use of energy, reducing consumption and avoiding waste, both the architectural pattern and the building materials were modified to improve thermal insulation [8]. this resulted in buildings with more efficient thermal insulation, without external openings that would allow natural ventilation, creating the so-called “airtight buildings” [8, 9]. the limitation of air exchange through ventilation had negative consequences, such as increased relative humidity and condensation phenomena [2]. thus, in the eighties, the first cases of sick building syndrome (sbs) were reported, characterized by situations of work discomfort and/or acute health problems presented by workers, which were possibly related to the permanence inside some buildings [7]. this syndrome is classified by the world health organization (who) as a public health problem. in the usa alone, the annual cost attributed to it in commercial companies is estimated at between us$10 billion to us$70 billion [10]. in other words, in addition to the health problem, an economic problem is generated, as expenses on health and disability pensions increase and there is a loss in workers’ productivity and in the repairs needed to overcome the problems of the sick building [11]. several studies [12–16] demonstrate that improving 138 https://doi.org/10.14311/app.2022.38.0138 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 38/2022 review and analysis of existing methods to assess the ieq . . . methods country methods country methods country active house belgium fitwel usa lider a portugal beam plus interiors china green globes si usa nabers ie australia breeam united kingdom green star interiors australia sbtool pt portugal casbee japan hqe france well usa dgnb germany leed usa table 1. methods analysed and their origin. the ieq results in increased team productivity and concentration, based on greater comfort, health, and wellbeing of workers. the australian property council estimated that a 1 % improvement in productivity would be equivalent to the total energy cost of the entire building [15]. moreover, the influence of a poor ieq can generate employee health expenses twice as high as energy costs [16]. recent data reveal that portugal is the seventh country in the european union with the lowest productivity per hour of work [17] and that the tertiary sector in the country is expanding, being responsible for employing 69.8 % of the active population in 2020 (in 2010, it was 61.5 % and in 2000, 52.6 %) [18]. given this scenario, it is essential to invest in practices aimed at the ieq in this sector to provide a good ieq for employees to create competitive businesses. several building evaluation methods adopt the quality of the interior environment as one of their indicators. however, their analysis is often superficial and generic. therefore, the main objective of this work involves raising and categorising the main factors that affect the ieq in office buildings, based on existing classification methods that consider it in their analysis and on national and international reference standards. thus, the indicators and categories that best suit the portuguese building stock will be listed, which will serve as a basis for creating a method to assess the overall quality of the interior environment for office buildings. 2. materials and methods the study was developed based on the analysis of 14 building assessment methods (table 1), six european and eight non-european, which include ieq indicators as part of their criteria. each of them differs from the other since the design guidelines of each location are unique, which is the main reason each country sought to develop a system based on its specific context [19]. thus, the ieq indicators and categories were identified within each tool, using information contained in documents available on the official websites. in some of them, such as leed and the dgnb, there are specific indicators for different buildings (commercial, retail, hotels, among others). however, for this study, only those applicable to offices were considered. to approach the ieq in a general way, the designations of the indicators that evaluate the same criterion were standardised, as well as the categories. then, 9 categories were stipulated, with 38 indicators in total. based on this, it was possible to perceive the scope of each method regarding the ieq criteria and to select those that presented a more significant number of categories evaluated as the primary references of the study. for these, the weights of each category were evaluated within a global score, presenting the result using pie charts. in addition, the most mentioned indicators among the tools were identified, with the support of the vosviewer software. a comparative analysis was carried out between the proposed list of indicators and the standard for sustainability of construction works, en 16309:2014+a1 [20], was carried out, highlighting the relevant differences. based on the results obtained, a list of categories and indicators best suited for the portuguese office buildings context was developed. 3. results and discussion the standardized categories stipulated in this study were: indoor air quality, thermal comfort, acoustic comfort, visual comfort, layout, water quality, materials and emissions, nourishment, occupant and community. and its indicators were divided as follows: • indoor air quality: ventilation, air pollution, ventilation user control. • thermal comfort: air temperature, humidity, air speed, hvac automation systems, occupant temperature control. • acoustic comfort: sound insulation, reverberation, internal noise, noise emissions. • visual comfort: light comfort, glare management, colour rendering, views, occupant lighting control. • layout: interior design, flexibility and adaptability, external areas, biophilic design. • water quality: water quality parameters, water quality management. • materials and emissions: material restrictions, waste management, biological control, maintenance. • nourishment: responsible food sourcing, food production, nutrition education. • occupant and community: active life, hygiene support, accessibility, mobility infrastructure, 139 c. c. jacinto, a. c. de paula junior et al. acta polytechnica ctu proceedings methods indoor air quality thermal comfort acoustic comfort visual comfort layout water quality materials and emissions nourishment occupant and community active house x x x x x x beam plus interiors x x x x x x x x breeam x x x x x x x casbee x x x x x x x dgnb x x x x x x x fitwel x x x x x x x green globes si x x x x x x green star interiors x x x x x x x hqe x x x x x x x leed x x x x x x x lidera x x x x x nabers ie x x x x x x sbtool pt x x x x x x well x x x x x x x x x table 2. categories presented by the studied methods. amenities, security, occupant well-being, neighbourhood development. it is important to note that not all tools cover the nine categories defined here. table 2 shows a comparison between the certifications, indicating which criteria they address. it is noticeable that most methods consider the main themes that contribute to the ieq: indoor air quality, thermal comfort, acoustic comfort and visual comfort [21, 22]. however, it should be highlighted that all certifications studied consider materials and emissions, reflecting the global concern with the abusive consumption of raw materials and with pollutants from the construction industry. although many of the certifications address the consumption and, mainly, the efficient use of water, encouraging the economy of this good, this study considered the indicators directly related to the occupant of the building. thus, only four tools, beam plus interiors, fitwel, hqe and well, have criteria that consider this aspect. it is also noted that only three methods, fitwel, lidera and well, comprise parameters that refer to occupant nutrition. in addition, of the indicators that make up the occupant and community, the mobility infrastructure is the one that is present in most of the tools that involve this category. this exposes the fact that social sustainability criteria, which are the ones aimed at the occupant’s wellbeing and health, are undervalued compared to traditional classifications of building-related aspects such as energy, water, or use of materials [23, 24]. to illustrate the different weightings attached by the tools to the categories, figures 1 (a, b and c) show, respectively, the weight by category of the beam plus interiors, hqe and well certifications. these evaluation methods were selected because they presented the largest number of categories evaluated, all for well and one less (nourishment) for beam plus interiors and hqe. when observing the graphs, well has a more balanced distribution of weights than the other two assessment methods. since this assessment is focused on the quality of life of the occupant, the highest percentage of weight is dedicated to this category (occupant and community). in beam plus interiors, most of the weights are associated with other categories, not included in this study, such as energy and management, with only 34 % of the total evaluated destined for the rating of the ieq. the hqe, in turn, has 51 % of the weight attributed to the assessment of the ieq, with 16 % associated with the evaluation of materials and emissions. however, it is worth mentioning that for the hqe, the third and fourth largest portions of the score (thermal comfort and layout) are related to the physical comfort of the occupant, evaluating the temperature and ergonomics of the workplace, among others. for beam plus interiors, indoor air quality and visual comfort represent the second and third largest share of the total score. looking at globally, the vosviewer software was used to build a semantic diagram based on the standardized indicators of this study, in order to identify the most mentioned among the certifications. thus, figure 2 shows the indicators considered by all methods network, where the circles diameter expresses the frequency of the indicators are covered by the considered methods. analysing the diagram, the indicators that had the highest occurrence were air pollution and light comfort, with thirteen occurrences each, followed by material restrictions, sound insulation and ventilation, with twelve. once again, this demonstrates that most of the evaluation methods analysed are more focused on constructive than social aspects. the different colours of the diagram represent groups of indicators that present simultaneous occurrences more frequently. most indicators that evaluate constructive characteristics appear in the red group, while those that evaluate social characteristics appear in the green one. this demonstrates that, in general, there are some methods that majority evaluate constructive characteristics and others that mostly 140 vol. 38/2022 review and analysis of existing methods to assess the ieq . . . figure 1. weight by category of certifications: a) beam plus interiors, b) hqe and c) well. figure 2. semantic diagram of certification documents keywords. 141 c. c. jacinto, a. c. de paula junior et al. acta polytechnica ctu proceedings categories indicators indoor air quality air pollution ventilation ventilation user control thermal comfort air temperature air speed humidity hvac automation systems occupant temperature control acoustic comfort internal noise noise emissions reverberation sound insulation visual comfort colour rendering glare management light comfort occupant lighting control views layout biophilic design external areas flexibility and adaptability interior design water quality water quality management water quality parameters materials and emissions biological control maintenance material restrictions waste management nourishment food production nutrition education responsible food sourcing occupant and community accessibility active life amenities hygiene support mobility infrastructure neighbourhood development occupant well-being security table 3. categories and indicators selected for the method to be developed. evaluate social characteristics. the european standard for sustainability of construction works, en 16309:2014+a1 [20], in its first version, considers the aspects and impacts of the use phase of a building, providing methods and requirements for evaluating its social performance. for this, its indicators are divided into six categories: • accessibility: accessibility for people with additional needs, access to building services. • adaptability: ease of potential for adapting to other use. • health and comfort: thermal characteristics, thermal comfort, characteristics of indoor air quality, indoor air quality, acoustic characteristics, characteristics of visual comfort, visual comfort, spatial characteristics. • impacts on the neighbourhood: noise, emissions, glare/overshadowing, shocks/vibrations. • maintenance and maintainability: maintenance operations. • safety and security: resistance to climate change, accidental actions (earthquake, explosions, fire, and traffic impacts), personal safety and security against intruders and vandalism, security against interruptions of utility supply, security against intruders and vandalism in an overall analysis, all categories of the standard are addressed in the standardized categories of this study. comparing the indicators, those presented in the “health and comfort” criteria were all considered in the categories “thermal comfort”, “indoor air quality”, “acoustic comfort”, “visual comfort” and “layout”. the adaptability category of the en is compatible with the standardized indicator “flexibility and adaptability”, while the “accessibility for people with additional needs” and “access to building services” indicators refer to “accessibility” and “amenities”, respectively. the criteria evaluated in “impacts on the neighbourhood” can be related to the parameters defined in the “air pollution”, “noise emissions” and “glare management”. the “maintenance and maintainability” category is consistent with the “maintenance” indicator of the “materials and emissions” category. finally, the category “safety and security” is almost entirely encompassed in the standardized indicator “security”, except for the “resistance to climate change”. this will not be addressed in the method to be developed, because, even if it is related to the safety of the occupants, its assessment is made exclusively considering the structure of the building. however, en does not present any analysis on important issues such as water quality, nourishment or occupant and community well-being. this ends up corroborating the fact that most evaluation methodologies analysed in this study also do not include these topics in their requirements. based on these results, the method of valuation of office buildings in the portuguese context to be developed should adopt the set of requirements presented in table 3. 142 vol. 38/2022 review and analysis of existing methods to assess the ieq . . . 4. conclusion this study proposed a list of indicators and categories for the assessment of office buildings in portugal. for this, fourteen certification tools of sustainability or ieq of buildings were analysed. in addition, the legislation and standards in force in the country and in the european union were considered in order to guarantee that the chosen requirements were relevant in the current context. the results presented and discussed reinforce that the ieq is already considered in several building evaluation methods. however, most of them do not cover all relevant indicators, especially those related to social aspects and occupant and community well-being. this reflects the absence of regulation that encourages the adoption of these characteristics. even the norms that are related to the social performance of the building often fail to address important criteria such as water quality, nourishment, among others. the analysis of the selected methods showed that there is still no international agreement on the list of ieq aspects to be considered in the sustainability assessments, since it differs from method to method. this can be explained from the regional, sociocultural, environmental and economic differences of the countries of origin of each certification. despite these differences, it was also noticed that several tools evaluate some similar parameters, although they have different nomenclatures. in view of what has been identified, this study will serve as a basis for the creation of a new method of evaluating the ieq for office buildings in the portuguese context. to this end, a list was prepared with 9 categories and 38 evaluation indicators that will be used in the method to be developed. in creating this list, care was taken to ensure that the number of indicators was sufficiently extensive to include the aspects that most influence the ieq, health and well-being of the occupant, but also as small as possible to facilitate their use in practice. references [1] j. f. s. afonso. estudo do comportamento térmico de edifícios antigos. master’s thesis, universidade nova de lisboa, 2009. https://hdl.handle.net/10362/2528 [2] s. m. silva. a sustentabilidade e o conforto das construções. ph.d. thesis, universidade do minho, 2009. http: //repositorium.sdum.uminho.pt/handle/1822/10245 [3] a. t. carmo, r. t. a. prado. qualidade do ar interno, escola politécnica da usp, 1999. 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[19] i. o. consoli, c. a. r. andrade, n. l. tabalipa, et al. análise dos indicadores de qualidade do ambiente interno nas ferramentas de certificação ambiental de edifícios. in 5th ibero-american congress on entrepreneurship, energy, environment and technology, vol. 5, pp. 431–436. 2019. isbn 978-84-17934-30-9. 143 https://hdl.handle.net/10362/2528 http://repositorium.sdum.uminho.pt/handle/1822/10245 http://repositorium.sdum.uminho.pt/handle/1822/10245 https://repositorio-aberto.up.pt/handle/10216/105843 https://repositorio-aberto.up.pt/handle/10216/105843 https://doi.org/10.1038/sj.jea.7500165 http://hdl.handle.net/10451/30896 https://doi.org/10.1590/s0100-40421999000100013 https://doi.org/10.1016/j.buildenv.2020.107480 https://hdl.handle.net/1822/27606 https://doi.org/10.1289/ehp.1510037 https://doi.org/10.1016/j.apergo.2015.07.010 https://doi.org/10.1080/09613210701529518 https://doi.org/10.1016/j.proeng.2017.04.189 https://doi.org/10.1016/j.buildenv.2016.11.020 c. c. jacinto, a. c. de paula junior et al. acta polytechnica ctu proceedings [20] en 16309:2014+a1 – standards publication sustainability of construction works – assessment of social performance of buildings – calculation methodology. european standards, 2014. 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[23] t. s. larsen, l. rohde, k. trangbækjønssonn, et al. ieq-compass – a tool for holistic evaluation of potential indoor environmental quality. building and environment 172:106707, 2020. https://doi.org/10.1016/j.buildenv.2020.106707 [24] w. wei, p. wargocki, j. zirngibl, et al. review of parameters used to assess the quality of the indoor environment in green building certification schemes for offices and hotels. energy and buildings 209:109683, 2020. https://doi.org/10.1016/j.enbuild.2019.109683 144 https://doi.org/10.1016/j.buildenv.2020.107155 https://doi.org/10.1016/j.buildenv.2020.106707 https://doi.org/10.1016/j.enbuild.2019.109683 acta polytechnica ctu proceedings 38:138–144, 2022 1 introduction 2 materials and methods 3 results and discussion 4 conclusion references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.38.0025 acta polytechnica ctu proceedings 38:25–30, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague quantifying the impact of external and internal factors and their interactions on thermal load behaviour of a building christoph matschia,∗, isabell nemethb a hochschule ansbach, campus feuchtwangen, an der hochschule 1, 91555 feuchtwangen, germany b technische hochschule rosenheim, fakultät für angewandte naturund geisteswissenschaften, hochschulstraße 1, 83024 rosenheim, germany ∗ corresponding author: christoph.matschi@hs-ansbach.de abstract. for the energy-efficient design of district heating networks, knowledge about the neighborhood heat load behavior, through heating load profiles in high temporal and spatial resolution, is crucial. due to the high effort required for transient calculations, a less complex method is needed at the neighborhood level. for this reason, a method is developed, which identifies the relevant parameters influencing the building heating load behavior. taking these parameters into account, a simple method for heating load profiling is developed using a machine learning algorithm. for this purpose, a parameter study is conducted using dynamic thermal building simulation software. different parameters influencing the building heating load behavior are varied. to determine the strength of the influence of the individual parameters on the building heating load, to check whether the influence of the parameters is constant or varies over the year and whether parameters are missing here, the results of the parameter study are evaluated statistically. first results show promising results in the detection of the significant parameters, for the creation of a model based on a machine learning algorithm, and the possibility of quantifying their impact on building heating load behaviour. keywords: energy-efficient building design, sector coupling, thermal load behaviour, standardised and parameterised thermal load curves. 1. introduction globally, the building sector accounts for a large share of total energy demand and greenhouse gas emissions. in the eu, the building sector is estimated to be responsible for 40 % of total energy demand and about 36 % of greenhouse gas emissions [1]. in the united states, the building sector accounts for 38.9 % of energy demand, with heating, cooling, and ventilation of buildings already accounting for 34.8 % [2]. these facts highlight the great potential for energy and greenhouse gas savings in the building sector. in order to exploit this potential, not only an energetically optimized building envelope and an optimization of energy generation and transmission technology are required, but also a targeted establishment of technical building equipment, since local and district heating networks are being expanded more and more worldwide. most research on the optimization of local and district heating networks is still based on energy generation and only a few consider the consumer side [3]. however, a suitable prediction of the load behavior on the load side is a fundamental prerequisite for the development and optimization of local and district heating networks. in particular, for an energy-efficient design of the networks and their components, the knowledge of the temporally high-resolution course of the load as well as the simultaneity of the heat demand is of central importance. these parameters, the coincidence factor and the heat load curve, are currently determined using static methods and assigned safety factors due to the resulting uncertainties [4]. as a result, systems are not operated in terms of optimal energy efficiency. in practice, local and district heating networks are designed and optimized on the basis of previous findings. historical data from other local and district heating networks are used to predict the load behavior of an area supplied. since different areas may differ greatly in their underlying conditions, the inherent data characteristics must be adapted for the new area. how well this transfer fits, depends heavily on the experience of the planner [5]. in order to enable a differentiated and quantitative prediction of the heat load demand for the planning and optimization of local and district heating networks, some research has been done in the last years [3]. on the one hand, so-called top-down models have been developed, which are based on historical data of existing local and district heating networks. however, these models have proven to be too coarse and too general for network planning and optimization. bottom-up models, on the other hand, which consider individual nodes in detail as a basis for operation, for example, can be divided into four categories: empirical models, statistical models, physical-statistical hybrid models, and engineering models. while models in the first three categories have a high level of detail, 25 https://doi.org/10.14311/app.2022.38.0025 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en christoph matschi, isabell nemeth acta polytechnica ctu proceedings stage experimental plan climate try 2 try 4 try 14 wind city centre in the countryside sea building tightness tight medium tight leaky window size 10 % 100 % 200 % orientation north south west east table 1. experimental plan. compared to top-down models, they still have disadvantages, especially in the early stages of planning, because they are also based on historical data and are therefore not transferable. it is not possible to implement effects that are not included in the observed data in these models. the engineering models allow the implementation of all required physical effects and user behavior and are based on thermal building simulations. this makes them highly adaptive and also quite accurate in predicting heat demand. however, this approach is very complex and thus expensive to create, since each unit of a neighborhood to be supplied must be simulated individually. to facilitate this, general load profiles can be developed for specific building types, e.g. office buildings, residential buildings, etc. while this simplification speeds up the development of the heat load profiles, it comes at the expense of accuracy as it relates to average building data. for a more detailed overview of heat load profile modeling, the reader is referred to [3], where the authors provide a comprehensive review of current assessment methods. following the latter approach, the goal of this research is to produce heat demand profiles with reasonable accuracy and the advantage of a less timeconsuming setup, compared to methods, where all units are assessed individually. for this purpose, a method is developed to identify the relevant parameters that influence the heating load of a building. on the basis of these identified influencing factors, a simple method for the creation of heating load profiles will be developed with the help of a machine learning algorithm. in this work, the method for identifying the influencing variables is tested. 2. method according to the need to gain insight into the load behavior of the building without a complex transient simulation, the method aims at reducing the building analysis to the crucial parameters of the heat load profiles. in contrast to a coarse simulation for the whole building with only one node, in this work a method is developed and tested making an identification of the most important parameters is possible. with the help of these identified parameters, a method will be developed at a later stage which, with the help of machine learning algorithms, can quickly generate precise heating load profiles. the approach is induced by the task to provide profiles for a long-term simulation and a linear optimization of a district heating network and its heat generators and to develop its future-oriented energy concept. 2.1. principle procedure the individual simulations of the test series for this work are very time-consuming. for this reason, only one building and only one user profile are used to test the method. the basis of the investigations is the small residential building (efh_klein) from the research report “entwicklung einer datenbank mit modellgebäuden für energiebezogene untersuchungen, insbesondere der wirtschaftlichkeit” [6]. using this building, the variables generally considered as relevant, such as storage mass, window size, building orientation, etc., are varied and simulated with the help of a parameter study. subsequently, the simulation results from the parameter study are statistically evaluated with the help of an anova (analysis of variance). in order to find out which parameters have a significant influence on the heating load behavior of a building, the small residential building from the research report “entwicklung einer datenbank mit modellgebäuden für energiebezogene untersuchungen, insbesondere der wirtschaftlichkeit” [6] was first implemented in the dynamic thermal building simulation software ida ice [7]. in this first step, a residential use from din v 18599-10 [8] was assumed as the user profile and user behavior and was not varied further. thus, the results are based on a small residential building and may differ for a large building and/or a different use to these. the varying parameters were changed in a two to four step experimental design. the parameters climate, taken from din 4710 [9], wind, building tightness and window size were varied in a three-stage test plan. the parameter building orientation was varied in a four-stage test plan, see table 1. the building insulation and storage mass were varied in a two-stage experimental design. for the building insulation, the tabula database [10] was used as a basis for the varying component structures. the uvalues and the corresponding storage mass are shown in table 2. all parameters are varied and simulated using the experimental plan table 1. 26 vol. 38/2022 quantifying the impact of external and internal factors . . . heavy lightu-value storage mass [w/m2k] [kj/m2k] exterior wall new construction 0.136 484 24 exterior wall not refurbished 1.52 352 16.6 interior wall 74 16.1 false ceiling 370 176 base plate new construction 0.21 531 348 base plate not refurbished 0.95 495 215 roof new construction 0.14 393 29 roof not refurbished 1.81 281 18.1 table 2. characteristic values of the building components. for these investigations, as mentioned above, anovas were performed to find out to what extent the influencing parameters and their interaction have an influence on the building heating load. for this purpose, the average heating load was determined for each month of the year and used as a dependent variable. the varied influence parameters from the test plan form the independent / explanatory variables in the analysis. thus, 12 anovas were performed, one for each month. subsequently, for each independent variable and interaction among the variables, the partial η2 was used to determine the influence strength on the building heating load within the anova. in a further step it was examined whether the influence strength changes over the year. finally, it was examined, whether the coefficient of determination r2 of the individual anovas changes over the year. 2.2. model set up the software “ida ice” of the software manufacturer equa provides the possibility of parameterized simulations. this makes it comparatively easy to carry out parameter studies with several varying parameters. following the parameterized simulations, the generated data can be relatively easily merged in excel for further analysis. at the beginning of a series of tests, the parameters within this series are permanently set according to the experimental setup in table 1. for example, before a parameter simulation, it is determined whether it is a heavy or light building. furthermore, the quality of the building envelope is defined. here it is distinguished, whether it is a “not refurbished” building envelope or whether it is a “new” building. the components for the category “not refurbished” would correspond to buildings of the age class 1816 to 1918, the components of the category “new construction” to the age classes 2016 to today. the characteristic values of the building components are shown in table 2. the parameters to be varied are defined in the ida ice software at the beginning and automatically varied during the parameter simulations. for the climate, the test reference years try 2, try 4 and try 14 of din 4710 [9] were used. the test reference years contain hourly values of, for example, outdoor temperature, solar radiation, etc. for more detailed information see din 4710 [9]. for the wind profiles (wind), the ashrae profiles “in the countryside”, “sea” as well as “city centre” stored in the software ida ice were used. the n50 air change rate was used to characterize the building tightness. this defines how often the air exchanges in one hour at a pressure difference of 50 pa. in the “tight” category, an n50 value of 0.35/h was assumed, in the “medium tightness” category an n50 value of 3.675/h was assumed, and in the “leaky” category an n50 value of 7/h was assumed. window sizes were simulated at 10 %, 100 % and 200 % starting from the initial model. from the research report “entwicklung einer datenbank mit modellgebäuden für energiebezogene untersuchungen, insbesondere der wirtschaftlichkeit” [6], the small residential building has a volume of 465 m3 in its basic configuration and has 26.5 m2 of window area, a usable floor area of 148.8 m2 and an a/v ratio of 0.98. the building orientation was varied in four steps starting from a south orientation to a west, north and east orientation. 3. results the following section presents the results for testing the method of quantifying the effects of external and internal factors and their interactions on the thermal load behavior of a building and interprets the results statistical analysis on the parameter studies of selected influencing factors and their interactions, as well as the change in model quality over the course of the year. 3.1. difference in the strength of influence of individual parameters and their interactions over the course of the year the first step was to investigate the extent to which the influence of the individual independent variables and their interactions change over the year. due to the abundance of independent variables and especially 27 christoph matschi, isabell nemeth acta polytechnica ctu proceedings data series anova-model parameter (variable and interaction) 1 wind 2 building tightness 3 climate * building tightness 4 window size * wind 5 wind * building tightness 6 wind * average u-value building 7 climate * window size * wind 8 climate * wind * building tightness 9 window size * orientation * average u-value building 10 climate * window size * wind * building tightness table 3. selected data series with model parameters. figure 1. influence of data series 1 and 2 over the year. figure 2. influence of data series 4 and 6 over the year. their interactions, which leads to a total of 1585 model parameters (variables and their interactions) within an anova, the 10 most important data series are shown as examples. the presented data series with their model parameters are listed in table 3. in order to quantify the strength of influence or effect of the individual variables and their interactions on the overall model, the partial eta-squared η2, which results from the quotient of the explainable sum of squares to the total sum of squares, was calculated figure 3. influence of data series 5 and 8 over the year. figure 4. influence of data series 3, 7 and 9 over the year. for each data series. according to cohen [11], a η2 of 0.01 indicates a small effect, whereas a η2 of 0.14 and higher indicates a large effect. the following figures show the influence of the individual variables and their interactions over the year. as shown in figures 1, 2, 3 and 4, the influence of the individual variables and their interactions change over the year, except for data series 2, where the influence on the heating load behavior remains relatively constant over the year. with the data series and the variables and interactions contained therein 1 as well 28 vol. 38/2022 quantifying the impact of external and internal factors . . . figure 5. coefficient of determination of the anovas over one year. as 3 to 10 it is to be recognized that their influence of the winter months to the summer months decreases and in the connection again rises. if we look at the variable in data series 2 in combination with the variable climate, which corresponds to data series 3, we can see that the building tightness, which corresponds to the variable in data series 2, also loses importance from the winter months to the summer months and vice versa gains importance again. it seems to be possible to determine the influence of individual parameters and their interactions with the help of this method. in addition, this analysis indicates that the significance of the influencing variables and their interactions change over the year. 3.2. coefficient of determination of the anovas next, it was checked whether the coefficient of determination r2 of the anovas remains constant over the year or whether there are differences. for this purpose, the coefficient of determination r2 was calculated for each anova and compared with each other. as shown in figure 5, the quality r2 of the anovas decreases from the winter months to the summer months and then increases again. in the winter months the r2 is about 0.91 which indicates a very good predictability of the building heating load by the anova. in the summer months, the r2 decreases to about 0.6, indicating only moderate predictability. overall, this decrease in r2 from the winter months to the summer months indicates that explanatory variables are missing here to be able to predict the building heating load well in summer. this means that more variables have to be included and not all explanatory variables are included in the regression model to predict the building heating load behavior in summer. this means that not all variables that have an influence on the heating load behavior of a building have been identified yet. 4. discussion the results from section 3 show that it is possible to determine the strength of influence of individual parameters and their interactions. furthermore, the analyses show that these vary strongly over the year. also, it seems to be possible to determine with this method whether all important parameters for the prediction of the heating load behavior of buildings have been found. as shown in section 3.2, the decrease of r2 from winter to summer and vice versa indicates that not all important parameters have been included in the analysis yet. in a next step, further analyses will be performed to determine the last missing parameters that have an influence on the building heating load behavior. afterwards a principal component analysis will be performed to check if variables can be combined. this should reduce the input effort for a later heat load prediction model. with the principal component analysis, the influence of the individual parameters on the building heating load model can be determined and quantified even more precisely. the influential parameters will ultimately serve as input variables for a machine learning algorithm, which will then be used to predict the building heating load behavior with similar precision as with a dynamic thermal building simulation, but with considerably less effort. with this algorithm, the heating load behavior of entire neighborhoods should also be precisely presentable without great effort. references [1] european commission. 2009 statistics & market observatory. [2014-07-26], http://ec.europa.eu/energy/observatory/. [2] r. j. de dear, t. akimoto, e. a. arens, et al. progress in thermal comfort research over the last twenty years. indoor air 23(6):442–461, 2013. https://doi.org/10.1111/ina.12046. [3] w. ma, s. fang, g. liu, r. zhou. modeling of district load forecasting for distributed energy system. applied energy 204:181–205, 2017. https://doi.org/10.1016/j.apenergy.2017.07.009. 29 http://ec.europa.eu/energy/observatory/ https://doi.org/10.1111/ina.12046 https://doi.org/10.1016/j.apenergy.2017.07.009 christoph matschi, isabell nemeth acta polytechnica ctu proceedings [4] bwk gleichzeitigkeit – der unterschätzte faktor. [2018-11-21], https://www.rehau.com/download/ 1475518/fachartikel-gleichzeitigkeit.pdf. [5] t. nussbaumer, s. thalmann, a. jenni, j. ködel. planungshandbuch fernwaerme. energieschweiz, 2017. isbn 3-908705-30-4. [6] entwicklung einer datenbank mit modellgebäuden für energiebezogene untersuchungen, insbesondere der wirtschaftlichkeit. dipl.-ing. swen klauß zentrum für umweltbewusstes bauen e.v. verein an der universität kassel prof. dr. anton maas (vorstand) gottschalkstraße 28a 34127 kassel. [7] ida ice 4.8 sp2, equa solutions ag, untermüli 3 6300 zug schweiz. [software]. [8] din v 18599-10:2011-12. [din-standard]. [9] din 4710:2003-01: statistiken meteorologischer daten zur berechnung des energiebedarfs von heizund raumlufttechnischen anlagen in deutschland. [10] tabula. [2021-10-01], https://webtool.building-typology.eu/?c=all#bm. [11] j. cohen. statistical power analysis for the behavioral sciences. taylor and francis, hoboken, 1988. 30 https://www.rehau.com/download/1475518/fachartikel-gleichzeitigkeit.pdf https://www.rehau.com/download/1475518/fachartikel-gleichzeitigkeit.pdf https://webtool.building-typology.eu/?c=all#bm acta polytechnica ctu proceedings 38:25–30, 2022 1 introduction 2 method 2.1 principle procedure 2.2 model set up 3 results 3.1 difference in the strength of influence of individual parameters and their interactions over the course of the year 3.2 coefficient of determination of the anovas 4 discussion references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.38.0247 acta polytechnica ctu proceedings 38:247–254, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague prvok – issue on 3d printing concrete building kateřina novákováa,∗, jiří velea, jiří litošb, vladimír šánab a czech technical university in prague, faculty of architecture, thákurova 9, 166 34 prague 6, czech republic b czech technical university in prague, faculty of civil engineering, thákurova 7, 166 34 prague 6, czech republic ∗ corresponding author: katerina.novakova@fa.cvut.cz abstract. prvok is the first 3d printed concrete floating house in the czech republic. additive manufacturing – 3d printing became a synonym of sustainable building of the 21st century. its experimental manner and lack of world’s standardisation iso approvals hold the 3d printing concrete method on the edge of usability and applicability and stop a broader spread of application in practice. furthermore, the used material was newly developed cement composite prefabricated mixture mady by master builder solutions with polypropylene plastic micro-fibres, which was not previously tested in large structures. what we achieved, was a practical realisation of a 3d printed fully equipped and functioning concrete house as a habitable statue for a public event. in order to fulfil the request on insulation and avoiding heat bridges together with investing least material possible, we parametrically designed and implemented a wall system of construction. in order to be able to open the structure to the public, we tested it on the universal loading machines at the faculty of civil engineering ctu prague in scale 1:1. testing fragments of the walls were also part of the research goals, which led us to the final design. in this paper, we present the results of the experiment together with the experimentally obtained data. keywords: 3d printing, concrete, prefabricated cement composite, parametrically designed self load-bearing structure, static loading tests. 1. introduction we can follow the evolution of 3d printing concrete structures since 2014 [1], when the centennial challenge [2] for 3d-printed habitat was announced by the nasa in the usa. this competition started a race for technological principles and optimal material mixtures for the use in additive manufacturing method of constructing a habitat. the winners of various stages of different phases of the challenge including pennsylvania state university team [3], foster and partners [4] and al space factory [5] published the results throughout 5 years long lasting challenge. we could see that these structures had had initially a circular geometry with the thickness of 6 layers generating at least 30 cm thick walls. it took two years till first inhabitable commercially 3d printed structure called “office of the future” made by winsun – yingchuang suzhou factory appeared in dubai [6]. still, combination of robotic and manual work is inevitable [7] together with experiments on the site during the building process. the inner structure consisting of two walls being laterally supported by inner zigzag was adopted by further realisations. although structural tests proved its stability, this system still had the problem of heat bridges. in 2017 an article about a first 3d printed house made by american company apiscor was published [8]. while previous examples were mounted from prefabricated panels, the construction system introduced by nikita cheniuntai was focused on printing on the site. this minimised required construction space and transportation demands. a different attitude was shown in yhnova house in 2017 [9], where the structure of foam insulation was 3d printed, filled with concrete and reinforced with classical steel caging column structure. although after 2018 many realisations appeared, this did not lead to astm standardisation of mixtures, structures, thicknesses or reinforcement of the walls. mechanical properties of 3d printed concrete specimen were tested and compared with monolithic specimen [10]. anyway, these samples were cut into small standard cubes. furthermore, the 3d printed samples could be negatively influenced by the process of cutting. firstly, this article is investigating the problem of difference between virtual model (digital twin) and robotically manufactured prototype. next, his paper is describing the structure and testing of the load bearing wall, which has different geometry than above mentioned walls known so far. finally, the article mentions possible wall stabilisation method during the printing process. 2. differences between 3d printed object and its digital twin we noticed discrepancies between printed object – prvok (figure 1) and its digital twin. here we understand the digital twin as a real-time digital counterpart of a physical object. our hypothesis expected the real object to be almost precise copy of its virtual model. 247 https://doi.org/10.14311/app.2022.38.0247 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en k. nováková, j. vele, j. litoš, v. šána acta polytechnica ctu proceedings figure 1. prvok 1:1 3d printed structure. figure 2. robot optimization of the path. figure 3. the principle of buckling. 2.1. buckling it is well known [10] there are certain threats of buckling, which is understood as deformation of a structural component of 3d printed objects (figure 3). in our case, on a 3 m tall wall buckling caused 3 cm difference from 3d model (figure 4). interior of prvok was designed as very curvy space and it required e.g. special shaped kitchen surface, shelves, bed, etc. in order to save construction time, curves of this equipment were exported from rhino model and its parts were cut in advance on the cnc machine. but finally the 3d printed walls had different curvature than cnc-cut wood. so the 3d printed walls had to be scanned and new wooden parts were cut. it slowed down the manufacturing process and disapproved the advantages of automated construction, which made us investigate the problem. figure 4. buckling of the 3 meter high wall. 2.2. digital model versus manufacturing process changes happened in data preparation. we used grasshopper for converting rhino model to robots move l instructions. grasshopper derived a curve from rhino model. this curve performed as robot toolpath, but robots instructions were linear movements, so curve was transformed into poly-line of multiple straight segments. if two segments of poly-line had sharp angle, robot readjusted its movement to keep continuous speed and did not follow the exact path of the poly-line. instead, the robot went through the 248 vol. 38/2022 prvok – issue on 3d printing concrete building figure 5. optimal consistent material (left) and inconsistent material (right). figure 6. printed segment of the prvok. zone around desired point. the zone is a circular area around all points on the poly-line. in our case, the zone diameter was 15 mm. average width of each layer is 4.5–5 cm, but if a printing mixture is inconsistent due to fluctuating liquidity, width may differ from 4 to 6 cm. the more liquid mixture is the easier the pumping is and more material is being extruded, which is resulting in layer width around 6 cm. more liquid layers have longer setting time and collapse may happen. less liquid mixture is difficult to pump, so less material is extruded, resulting in layer widths around 4 cm. dry layers crack during the printing and they are not properly bonded with other layers. these are weak spots of structure and if cracks appear, they usually follow these weak layers (figure 5). plus there are other problems with manufacturing process, which cause the different geometry of the prototype (figure 2). for example, the 3d printing mixture master flow 100 is very sensitive to temperature and humidity changes. if uncontrolled temperatures happen to be below 15 °c, the accelerant does not work and it takes too long time for concrete to get solid. thus, under 15 °c any object collapses during the printing. furthermore, freshly printed structures sometimes deform under its self-weight. the scale of deformation is affected by the shape of printed object and by printing speed, ideally higher that 60 s. the longer it takes to print one layer, the more time is given to the fresh concrete to bond the layers in order to gain strength. generally, the conclusion is that objects with small layout have to be printed slowly (60 s speed +), or in segments. complexity of geometry also plays significant role. our observation claims, that straight walls or walls with big radius in plane view tend to deform. this is the reason why external walls of prvok are double curved (figure 6) and interior ones are straight, but supported by small circular shapes (figure 7). we printed hidden column each 500 mm of the path in order to stabilize the wall during the printing process. after the construction dried, steel 249 k. nováková, j. vele, j. litoš, v. šána acta polytechnica ctu proceedings figure 7. construction of the wall during the printing. figure 8. load distribution system. rebars with liquid concrete mixture were installed into these spaces in order to reinforce the walls against deformation. 3. geometry of the walls and compressive tests of the prototype part of the whole building made of cement composite was printed at the experimental centre of the ctu in prague, faculty of civil engineering. static loading test of compressive load capacity and a stability test of the structure were performed. the subject of the static loading test was a 3d printed part of the building (figures 7 and 9). it was 3d printed using a robotic arm. a specially developed cementitious composite material master flow 100 was used as the printing medium. the wall of the structure was divided into two types of walls. firstly, an external self-supporting part, which gave the building characteristic wavy illusion. secondly, an internal load-bearing part reinforced with ribs (figure 7), on which the roof trusses were placed. the subject of the static load test was the internal load-bearing part of the wall. the height of the printed structure was 2.8 m and its enclosed area was 10 m2. the thickness of the printed shell ranged from 50 to 60 mm and the thickness of the whole wall ranged from approximately 350 to 450 mm. figure 9. measuring points of vertical and horizontal deformations. using a system of spreader arms, the load from one load hydraulic cylinder was distributed to 16 locations on the structure (see figure 8), with each of these locations being loaded with the same amount of force. the load locations were the positions of the future roof trusses. during the static loading test, vertical and horizontal deformations of the structure were measured. furthermore, the load force and the path of the load cylinder, was monitored. vertical deformations (compression along the height of the structure) were recorded 250 vol. 38/2022 prvok – issue on 3d printing concrete building load steps load description load force – spreading elements and hydraulic system [kn] surface load [kn/m2] 0 spreading elements 12 1,2 1 roof sheathing 21 2,1 2 increase by snow load – (hidden ref.) area 29 2,9 3 increase by accidental load 41 4,1 4 roof sheathing 21 2,1 5 increase max value by 10 kn 52 5,2 6 +10 kn 62 6,2 7 +10 kn 72 7,2 50 limit of the spreading elements 500 50 table 1. the load stages. at four locations labelled from s5 to s8. the horizontal deformations were measured at points located at the mid-height of the structure labeled from v1 to v4 (see figure 9). verification of the load capacity of the printed structure was carried out successively in three loading steps. the loading of the structure was controlled by gradually increasing the loading force at a rate of 5 kn/min. the first step represented the dead load of the roof structure (21 kn). in the second loading step, the force was increased by the load of snow for the central czech area (+8 kn) and the last step increased the loading force by the value of the accidental load (+12 kn). subsequently, the structure was unloaded to the first step load value. at each loading step, the load was maintained for two minutes to allow the measured values to settle. after this part of the load test, the structure was progressively loaded up to 500 kn in 10 kn increments and held at each load for one minute. after this maximum value was reached, the structure was fully unloaded. the load on the hydraulic load cylinder had to be increased by the load from all the spreading elements, which was 12 kn. the individual load stages are summarised in the table 1 and figure 10. the table shows the values of the hydraulic load cylinder and also the values converted to the surface load of the structure, where the area of the printed part was 10 m2. the results of the static loading test can be seen in the graphs (figure 10 and 11). graph on figure 11 shows the vertical deformations during loading of the structure. graph on figure 12 then shows the horizontal wall deflections. the deformations are then shown in table 2. the values marked from v1 to v4 are the horizontal wall deflections, where a positive sign of the value indicates buckling of the structure towards the living space. the values marked from s1 to s4 indicate compression of the load-bearing part of the structure. almost no vertical or horizontal deformations were measured during the static load test that was intended to verify the load-bearing capacity of the structure (table 2). permanent deformations after unloading are negligible (see step 4). the next load steps show deformation values where the structure was progressively loaded to a value exceeding ten times the required load capacity of the printed structure. in this case, the values were already measurable, but as in the previous case and due to the size of the structure, negligible. load step 51 then indicates the permanent deformation of the structure after the load force has been fully relieved. 4. compressive strength of specimen in the last test session, 1×1 m specimen made of mbs material [11] were generated from the big structure. the measurement was performed on samples printed by a 3d printer and cut from a 1:1 scaled model after static loading test. at first, we had measured the quantity and thickness of individual connections of printed layers on 8 samples (marked in the table with numbers from 1–8) with average dimensions 127.55 × 74.96 mm and a weight with an average of 0.91 kg (figure 13). the result of measuring the thickness of the connection was used to determine the narrowest location, which we assumed to be the weakest point on each sample. subsequently, we performed a destructive test on all samples at this point on a pressure press and calculated the individual compressive strengths (mpa) according to the appropriate well-known formula. 5. discussion and conclusion in lee at al. [10] several contradictory statements from previous research are reported. for example, wolfs et al. [12] states, that the direction of the printed layers has minimum effect on the mechanical properties of the specimen [12] but nerella et al. [13] reports, that the effect is significant. in our experiment, we measured almost same values of compressive strength as in lee at al. [10] 21.4 mpa versus 23.5 mpa is within standard deviation of ±5.4 mpa. because of the nature of the experiment (top-bottom stress), only one direction of specimen-load was measured with positive result. 251 k. nováková, j. vele, j. litoš, v. šána acta polytechnica ctu proceedings figure 10. the loading diagram. figure 11. the vertical deformation. figure 12. the load stages. 252 vol. 38/2022 prvok – issue on 3d printing concrete building load step load force [kn] v1 v2 v3 v4 s1 s2 s3 s4 1 21 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 2 29 0.00 0.00 0.00 0.00 0.11 0.08 0.00 0.00 3 41 0.00 0.00 0.00 0.00 0.32 0.09 0.00 0.00 4 21 0.00 0.00 0.00 0.00 0.31 0.09 0.00 0.00 10 100 0.00 0.00 -0.22 0.00 0.61 0.54 0.11 0.19 20 200 -0.02 0.00 -0.57 0.03 1.00 1.08 0.44 0.48 30 300 -0.04 0.09 -0.78 0.11 1.32 1.58 0.66 0.79 40 400 -0.03 0.08 -1.05 0.05 1.72 1.97 0.99 1.17 50 500 -0.03 0.06 -1.29 -0.01 2.03 2.36 1.22 1.56 51 12 -0.05 0.06 -0.45 -0.01 0.52 1.12 0.1 0.19 table 2. measured deformations during individual load steps [mm]. figure 13. the compressive strength of experimentally investigated specimen. µ s [mpa] [mpa] the compressive strength 21.4 ±5.4 table 3. summarization of evaluated compressive strength. the symbol µ denotes the mean value and s is the standard deviation. the conclusion says, that physical result of digital fabrication process differs from the virtual model. we observed, that the digital twin has two main sources of imperfection: the logic encoded in data transfer from a model to robots’ nozzle and the irregular and hardly controllable consistency and flow of the actual mixture in the process of layering. nevertheless, we can see, that the compressive strength of our samples is comparable with the results referenced below [10]. also the 1:1 stress test has proven satisfactory resilience for the conditions of czech republic. therefore, even though the actually 3d printed structure deviates from the original digital model it is structurally very solid and exceeds many times the required limits of safe structures. this implies the conclusion, that the effect of imperfect printing and inaccurate digital twin does not influence the overall strength of the building. acknowledgements this research has been supported by the ctu in prague and by the ministry of education, youth and sport of the czech republic within inter excellence program, under project no. ltausa19018. references [1] m. rešetár. architektura 3d tlače. batchelors work, ctu prague, 2021. [2] t. prater, t. kim, m. c. roman, r. p. mueller. nasa’s centennial challenge for 3d-printed habitat: phase ii outcomes and phase iii competition overview. in 2018 aiaa space and astronautics forum and exposition. 2018. https://doi.org/10.2514/6.2018-5405 [3] n. d. watson, n. a. meisel, s. g. bilén, et al. large-scale additive manufacturing of concrete using a 6-axis robotic arm for autonomous habitat construction. in solid freeform fabrication 2019: proceedings of the 30th annual international solid freeform fabrication symposium – an additive manufacturing conference. 2019. 253 https://doi.org/10.2514/6.2018-5405 k. nováková, j. vele, j. litoš, v. šána acta polytechnica ctu proceedings [4] techmer pm. branch technology and foster + partners win first prize in nasa’s 3-d printed habitat challenge using materials designed by techmer pm. https://www.inda.org/membership/membernews/branch-technology-and-foster-partners-winfirst-prize-in-nasas-3-d-printed-habitatchallenge-using-materials-designed-by-techmerpm/ [5] r. p. mueller, t. j. prater, m. roman, et al. nasa centennial challenge: three dimensional (3d) printed habitat, phase 3. in 70th international astronautical congress (iac), washington, d.c. 2019. 13 p. https://ntrs.nasa.gov/api/citations/ 20190032475/downloads/20190032475.pdf [6] b. o‘neal. dubai inaugurates first 3d printed office building, constructed in 17 days. https: //3dprint.com/126426/3d-printed-museum-office/ [7] m. alawneh, m. matarneh, s. el-ashri. the world’s first 3d–printed office building in dubai, 2018. https://www.pci.org/pci_docs/papers/2018/32_ final_paper.pdf [8] o. bari. build your own 3d printed house, all in one day, 2017. [2020-11-03]. https://www.archdaily.com/806742/build-yourown-3d-printed-house-all-in-one-day [9] b. furet, p. poullain, s. garnier. 3d printing for construction based on a complex wall of polymer-foam and concrete. additive manufacturing 28:58–64, 2019. https://doi.org/10.1016/j.addma.2019.04.002 [10] c. joh, j. lee, t. q. bui, et al. buildability and mechanical properties of 3d printed concrete. materials 13(21):4919, 2020. https://doi.org/10.3390/ma13214919 [11] master builders solutions cz. masterflow 3d 100. https://assets.master-builders-solutions.com/ cs-cz/tl_masterflow_3d_100_mbs.pdf [12] r. j. m. wolfs, f. p. bos, t. a. m. salet. hardened properties of 3d printed concrete: the influence of process parameters on interlayer adhesion. cement and concrete research 119:132–140, 2019. https://doi.org/10.1016/j.cemconres.2019.02.017 [13] v. n. nerella, s. hempel, v. mechtcherine. effects of layer-interface properties on mechanical performance of concrete elements produced by extrusion-based 3d-printing. construction and building materials 205:586–601, 2019. https: //doi.org/10.1016/j.conbuildmat.2019.01.235 254 https://www.inda.org/membership/member-news/branch-technology-and-foster-partners-win-first-prize-in-nasas-3-d-printed-habitat-challenge-using-materials-designed-by-techmer-pm/ https://www.inda.org/membership/member-news/branch-technology-and-foster-partners-win-first-prize-in-nasas-3-d-printed-habitat-challenge-using-materials-designed-by-techmer-pm/ https://www.inda.org/membership/member-news/branch-technology-and-foster-partners-win-first-prize-in-nasas-3-d-printed-habitat-challenge-using-materials-designed-by-techmer-pm/ https://www.inda.org/membership/member-news/branch-technology-and-foster-partners-win-first-prize-in-nasas-3-d-printed-habitat-challenge-using-materials-designed-by-techmer-pm/ https://www.inda.org/membership/member-news/branch-technology-and-foster-partners-win-first-prize-in-nasas-3-d-printed-habitat-challenge-using-materials-designed-by-techmer-pm/ https://ntrs.nasa.gov/api/citations/20190032475/downloads/20190032475.pdf https://ntrs.nasa.gov/api/citations/20190032475/downloads/20190032475.pdf https://3dprint.com/126426/3d-printed-museum-office/ https://3dprint.com/126426/3d-printed-museum-office/ https://www.pci.org/pci_docs/papers/2018/32_final_paper.pdf https://www.pci.org/pci_docs/papers/2018/32_final_paper.pdf https://www.archdaily.com/806742/build-your-own-3d-printed-house-all-in-one-day https://www.archdaily.com/806742/build-your-own-3d-printed-house-all-in-one-day https://doi.org/10.1016/j.addma.2019.04.002 https://doi.org/10.3390/ma13214919 https://assets.master-builders-solutions.com/cs-cz/tl_masterflow_3d_100_mbs.pdf https://assets.master-builders-solutions.com/cs-cz/tl_masterflow_3d_100_mbs.pdf https://doi.org/10.1016/j.cemconres.2019.02.017 https://doi.org/10.1016/j.conbuildmat.2019.01.235 https://doi.org/10.1016/j.conbuildmat.2019.01.235 acta polytechnica ctu proceedings 38:247–254, 2022 1 introduction 2 differences between 3d printed object and its digital twin 2.1 buckling 2.2 digital model versus manufacturing process 3 geometry of the walls and compressive tests of the prototype 4 compressive strength of specimen 5 discussion and conclusion acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.38.0553 acta polytechnica ctu proceedings 38:553–558, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague lifelong learning of property managers in enabling sustainable and energy efficient residential buildings: experiences of care project in tampere region kalle tammia,∗, pekka väisäläa, petri murtomaaa, jussi-pekka juvelaa, jani hietakangasa, timo lähteenmäkia, silva vuopponenb a tampere university of applied sciences, kuntokatu 3, 33520 tampere, finland b ecofellows ltd., valssipadonraitti 3, 33100 tampere, finland ∗ corresponding author: kalle.tammi@tuni.fi abstract. the existing building stock causes 40 % of the total energy consumption in the european union. the high energy consumption together with ageing buildings put strong pressure on renovation of buildings and lowering of co2 emissions. the directives on energy performance of buildings (epbd) and energy efficiency (eed) were established to urge and steer the activities in the eu member countries. there are about 90 000 housing companies in finland. they consist of 1.7 million apartments and 2.7 million residents, which makes housing companies a significant energy consumer. around 50 % of the building stock in finland was built between 1960s and 1980s. the average level of technical building systems is decent. measures are still needed to tackle the increasing maintenance backlog and energy consumption objectives. management of housing companies consists of the general meeting of apartment owners and a board elected by the general meeting. in addition, a vast majority of housing companies have a professional property manager whose role corresponds to ceo. the property manager’s expertise plays an essential role in planning and execution of maintenance and energy efficiency. the role of the property manager calls for multidisciplinary competencies and continuous learning. the care – resource efficient caretaking of residential buildings project implemented flexible courses and training for property managers to enhance their technical understanding of energy efficient renovations and retrofit of new energy solutions. keywords: low-carbon buildings, property management, retrofitting, housing company, lifelong learning. 1. introduction finland has set a target to be carbon neutral by the year 2035 and so to become the world’s first fossil-free welfare society. this requires accelerated emission reductions in all sectors and the strengthening of carbon sinks. the measures include new climate policy decisions, near-zero-emission electricity, and heat production already by the end of the 2030s, as well as a reduction in the carbon footprint of construction [1]. city of tampere has its own target to become carbon neutral already by the year 2030 and the roadmap for carbon neutrality as part of the sustainable tampere 2030 programme is the key policy instrument influencing on the city’s sustainability activities. according to the roadmap 10 % share of the reduction target should come from the residential buildings [2]. most of the residential buildings are privately owned limited liability housing companies which have their own decision-making process concerning for example renovations. the existing old building stock is seen as a highly potential target for improving energy efficiency and reducing climate emissions. property managers managing limited liability housing companies look after the homes of 2.7 million people living in terraced houses and apartment buildings and they have the expert role in guiding the decisions concerning maintenance and renovations of the buildings. property management services are provided by approximately 1 200 companies in finland [3]. thus, there is a need for life-long learning opportunities for property managers if energy efficiency of housing companies needs to be improved. 1.1. national long-term strategy for renovations based on the article 2a of the epbd (2010/31/eu) [4], as amended by directive 2018/844/eu [5], finland has specified a national long-term strategy for renovations covering years 553 https://doi.org/10.14311/app.2022.38.0553 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en k. tammi, p. väisälä, p. murtomaa et al. acta polytechnica ctu proceedings 2020–2050 [6]. the strategy aims for achieving the energy efficient and decarbonised building stock by 2050. strategy focuses on the building stock constructed before 2020. it has been identified three primary means to transform the existing building stock into highly energy efficient and decarbonized: (1.) building loss and space utilization efficiency, (2.) improvement of energy efficiency in connection with renovations and maintenance, and (3.) replacing fossil energy sources with renewable energy sources in heat production [6]. the loss of building stock has been estimated to be ca. 30 % during the strategy period. the estimation covers building stock built before year 2000. partially the loss can be replaced with space and energy efficient buildings. the energy efficiency improvement through repair and maintenance actions is an obvious, but limited solution because of the good condition of the building stock. the structural energy efficiency improvements are usually implemented in connection with other repair and renovation actions, e.g., the windows can be replaced, or more thermal insulation can be added during the renovation of external cladding. the heat energy decarbonization is the third way to enhance the sustainability of the buildings, and for a single property the decarbonized heating can be achieved for example by replacing the expired oil heating system with a heat pump or a biofuel system. in finland the most common form of heating buildings is district heating. there are targets set to reduce carbon-intensive fuels in producing energy for district heating. as an example, the usage of coal shall be abandoned in heat production by 2029. the building stock can be divided into two main categories: residential and non-residential buildings. the residential buildings consist of three kind of buildings: single-family and semidetached houses, terraced houses and block of flats (i.e., apartment building). if considering the number of housing units, terraced houses and apartment buildings are forming most residential buildings. terraced houses include a total of 0.4 million housing units in a total of 82 000 buildings. blocks of flats include a total of 1.4 million housing units in a total of 62 000 buildings. according to energy performance certificates, a total of four per cent of terraced houses and ten per cent of blocks of flats have low energy efficiency. the terraced houses are responsible for approximately 10 % and the apartment buildings for 30 % of the heating emissions of the entire housing stock. vast majority, 77 % of the apartment buildings in finland have been built before 21st century [6]. the energy consumption of such an old apartment building is divided to the following segments [7]: • ventilation (36–37 %) • windows (19–21 %) • water heating (17–19 %) • walls (13–17 %) • base (5–6 %) • roof (4–6 %) ventilation of old buildings is possessing a high potential for energy savings because the heat recovery ventilation has been obligatory for new buildings only after 2003. the development of heat pumps has enabled solutions to transfer the energy from exhaust air to heating that further improves the energy efficiency of ventilation. if exhaust air heat pumps are utilized the reduction in building’s heat energy consumption can reach level of 30–50 %. water heating is another major energy sink in residential buildings. the water consumption is significantly reduced by making it visible to residents. the eed initiated legislation change in 2020 has made the apartment-specific measurement and invoicing of water consumption compulsory in finland. according to motiva (sustainable development company) studies the apartment-specific measurement reduces the water consumption by 10–30 % and brings 3–9 % savings in energy consumption of a building [8]. 1.2. housing companies in finland in finnish legislation a housing company is defined as a limited liability company whose purpose is to own and manage at least one building or part of a building in which at least half of the combined floor area of the apartment or apartments is reserved for residential use possessed by the shareholders [9]. each housing company shall have its own articles of association that defines the details of the company and its operation. the housing companies are very popular form to manage residential buildings in finland. there exist ca. 90 000 limited liability housing companies in finland [10] with approximately 1.7 million apartments and 2.7 million residents [11]. this makes the housing companies a significant stakeholder managing large portion of property. maintaining the value of the housing companies’ property and ensuring the long lifecycle of the buildings is very important both for the apartment owners and the society in general. 1.2.1. management of housing companies management of the limited liability housing company is taken care by the board of directors and the property manager whose role corresponds to ceo. property manager is chosen by the board of directors, and the board of directors is chosen in the annual meeting by the shareholders. the finnish real estate management federation and the finnish real estate federation (fref) [12] conducted a survey on the quality of property management in 2020. according to the survey the most important quality factor of the property manager’s work was general expertise and overall knowledge [13]. 554 vol. 38/2022 lifelong learning of property managers . . . decision making process of the housing company is challenging. the board of directors and the property manager prepare and present the renovation plans in the general meeting, and shareholders vote about the project implementation. most of the shareholders and members of the board of directors are not experts in the construction and the real estate business. that is the reason why the property manager has an important role in the administration of the housing company and must have knowledge of property management, ensuring the energy efficiency of the building and timing of necessary renovation projects. 1.2.2. role of property manager eligibility requirements of the property manager are prescribed in the limited liability housing companies act [9]. there are no expressly defined requirements for qualification or training for property manager. even if there are no requirements for professional ability in the law, working as a property manager requires various competences. task field of property manager is extremely interdisciplinary, and multidisciplinary expertise is required, for example technical, financial, and legal expertise. typically, knowledge of property manager is based on bachelor’s degree in business administration or technology which they have supplemented with additional training. most professional property managers have a professional degree in property management or an its or ait degree of the real estate training foundation, which qualifies for property management tasks. to maintain professional skills there are available various training courses on different topics to property managers which provide information of topical themes. this kind of training courses are usually organized in helsinki metropolitan area in finland. it can be challenging to participate for a property manager living in some other areas of finland. 1.3. lifelong learning today, the pace of work has changed significantly, and lifelong learning has become part of the work life. technologies are developing rapidly, and it is crucial for professionals to update their know-how. adult learning is also a way to remain competitive on the labour market. a resolution adopted by the eu on a renewed european agenda for adult learning highlights the need to significantly increase adult participation in formal, non-formal and informal learning whether to acquire work skills, for active citizenship, or for personal development and fulfilment [14, 15]. one priority is to offer more flexible opportunities for adults to learn, and improved access through more learning at the workplace and increase the use of ict and so-called ‘second chance’ qualification programmes. in finland workforce is ageing, and it is important to make sure everyone’s skills are up to date so that people can remain employable. the reform of continuous learning [16] examines especially the potential for upskilling, reskilling, and developing competence over the course of people’s careers. the vision and objectives of the reform of continuous learning: • everyone develops their skills and competence during their careers. • everyone has the knowledge, competence and skills required for employment and a meaningful life. • competence renews the world of work and the world of work renews competence. higher education in finland is expected to create new models for supplement training to develop professional skills during different phases through working years. challenge is to correspond to competence needs of a rapidly changing working life and create new methods to educate professionals. one recommendation also in national long-term strategy for renovations is to increase open, digital education in the fields of renovation and energy efficiency to support lifelong learning [6]. 2. care project improving the deployment of energy efficiency solutions the changing climate, tighter energy efficiency requirements and increasing number of technical solutions influence directly on the maintenance and renovations of residential buildings. this causes a demand of new information and education, and that has been noticed also in property management companies. based on the negotiations with property management companies tampere university of applied sciences and ecofellows ltd. made an initiative to authorities for improving the property managers’ competence on energy and renovations related matters. there was started a two-year project called care – resource efficient caretaking of residential buildings at tampere region in 2019 which offered lifelong learning possibilities for property managers and increased their competence on the job market. another goal of the project was to decrease the housing companies’ environmental impact by enhancing the property managers’ and housing company boards’ knowledge on the new technical systems and energy efficiency related solutions for the housing companies. the care project was funded by the european social fund (esf) [17]. 2.1. collecting the requirements the learning demand was verified with the questionnaire made among the ca. 160 property managers in december 2019. it was received 54 responses to the questionnaire. the results of the questionnaire gave a solid overview of the target group’s background. the acquired background data is shown in the figure 1. the responses showed small majority, 61 % 555 k. tammi, p. väisälä, p. murtomaa et al. acta polytechnica ctu proceedings figure 1. property managers’ background based on questionnaire (n = 54). figure 2. survey results on learning theme prioritization among property managers. against 39 %, of male participants. the age division of respondents was clearly biased to ages over 50 years. however, the work experience of property managers was quite uniformly divided from short (less than 5 years) to very long careers (over 20 years). the survey results also strengthened the fact that majority of property management companies are small size in terms of number of employees (less than 10 persons). the survey results were also clearly confirming the need for the care project. based on the answers, the top three most interested learning themes included renovations, energy efficiency and new energy sources, and retrofits. the given responses are shown in the figure 2 and they indicate obviously that over 50 % of the respondents feel they will need additional education in the top ranked themes. 2.2. planning and implementing the courses in early 2020 care project team analysed the questionnaire results and selected four main themes for more careful planning. the selected themes included (1.) energy efficiency and renewable energy sources, (2.) renovations, (3.) digital systems and (4.) climate change impacts on buildings. 17 course topics were recognised to be implemented for these themes in the beginning. during the selection and planning process it was considered especially the differences in the background of potential participants. the level of the selected courses was targeted to cover both the basics and more advanced topics to also satisfy the needs of more experienced property managers. the targeted course participants were all in working life and easy participation was important factor while implementing courses. each course was designed to contain two half day sessions of lectures and group studies, and in between there were four weeks period for self-study and homework. the scope of each course was estimated to 556 vol. 38/2022 lifelong learning of property managers . . . correspond one ects credit. the participation fee of the courses was set to very low level to enable easier access and wider participation especially among the self-employed property managers. in early 2020 the care project was marketed in different occasions like trade fairs and the potential participants were approached also via professional magazines and social media. the local organisations of finnish real estate federation and finnish real estate management federation were also supporting by informing their members. multiple information sessions were organised for marketing the courses to property managers. the registration of the courses started in march 2020 and the first courses were held in autumn 2020. the first eight courses in august and september 2020 were arranged at campus of tampere university of applied sciences enabling physical meeting of the participants. after october 2020 all courses were conducted entirely on digital online platforms like microsoft teams and moodle. 2.3. building passport – model one of the tasks in the care project was to create building passport operating model to help the property managers and board of directors in the housing companies in timing and planning of energy renovations. building passport is a part of the european commission’s energy performance of the buildings directive article 2a (epbd). operating model was created by means of service design and it supports housing companies in renovation process. product is a web page [18] where property manager or housing company can get the information what should be considered when planning renovations and how to proceed. 3. results and discussion during two years of operation care project arranged 32 courses consisting 17 different topics. the courses were conducted in three sets: 13 courses during the second half of 2020, 14 during the first half of 2021 and 5 courses during autumn 2021. the topics covered mainly renovation projects and retrofits related to mep, hvac systems, but also, for example, charging systems for electric vehicles, solar power systems, demolitions and building extensions, sourcing of design services, and contracting. each course had minimum five participants and the most popular course topics gathered 20–30 participants. there were in total 70 participants from 22 companies attending in the courses. the number of participants was slightly lower than estimated in the beginning of the project. the reduction was caused partially by the pandemic situation and the restrictions to arrange on-site courses. on average each participant were attending more than 3 courses totally producing 223 course participations. course-specific, voluntary, and anonymous feedback was collected after each course and it was used for improving the course implementations. the generic feedback from participants was mainly positive and 32 of 34 responses indicated the participants had got useful information for renovation projects. in september 2021 project team interviewed representatives of four attended property management companies to get a deeper understanding of their opinions on the impacts of care project. according to the interviews the property managers are facing more and more projects aiming to energy efficiency improvements. especially renovations, but also retrofits of the heating systems like replacing oil heating with heat pumps are popular among housing companies. furthermore, solar energy and charging of electric vehicles have become new topics within the housing companies. therefore, the interviewed persons found the care project and its courses very valuable and welcome. in their opinions the courses supported their work most in renovation projects management. some of the participants commented the course contents could have been technically more demanding and including more specific examples. 4. conclusions new requirements on buildings and deployment of alternative energy sources and technologies require lifelong learning from people working with the residential buildings. this has been recognised also in the national renovation strategy promoting measures to increase the offering of education in the fields of renovation and energy efficiency to support lifelong learning and the acquisition of new competencies. in an ideal case, the buildings are managed and renovated systematically throughout their whole life cycle. bases is the execution of the housing company’s own strategy, which ensures that there are always upto-date condition surveys made by professionals and long-term planning and budgeting based on them. property managers can provide help to the housing company, but it means they must keep their know-how updated. the short adult educations enable professionals to update their knowledge and keep up in the rapidly changing working life. there is need for short educations but at the same time they are quite challenging for educational institutions to organize. the training model created in care project was designed together with the property managers by interviews, but it was still very challenging to get participants to the trainings. fostering the energy efficiency and mitigating the climate change requires latest information on technical solutions. care project is an example of a measure targeting to increase the availability of continuous learning opportunities for professionals. it tackled the regional competence development needs regarding housing companies and property managers by arranging short courses on multiple topics. care project also developed the building passport model 557 k. tammi, p. väisälä, p. murtomaa et al. acta polytechnica ctu proceedings for housing companies and fostered the energy efficiency awareness among them. although it is difficult to estimate the long-term influence of a project like care, the number of participants and the received good feedback are indicators to verify the short-term positive impact. the courses developed for care project are creating a foundation for new open uas course. acknowledgements the article was supported by care resource efficient caretaking of residential buildings project, european social fund (esf) project code s21778. references [1] ministry of the environment. government’s climate policy: climate-neutral finland by 2035. 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[2021-10-07]. https://www.isannointiliitto.fi/wp-content/ uploads/2020/10/isannoinnin-laatu-2020.pdf [14] european commission. eu policy in the field of adult learning. [2021-10-07]. https://ec.europa.eu/education/policies/ eu-policy-in-the-field-of-adult-learning_en [15] the council of the european union. council resolution on a renewed european agenda for adult learning oj c 372. (approved 2011, december 20). https://eur-lex.europa.eu/lexuriserv/ lexuriserv.do?uri=oj:c:2011:372:0001:0006:en: pdf [16] ministry of education and culture, finland. reforming continuous learning. [2021-10-07]. https://okm.fi/en/continuous-learning-reform [17] ecofellows ltd. kare – resurssitehokasta asuinkiinteistöjen ylläpitoa, 2019. [2021-10-07]. https://ekokumppanit.fi/kare-hanke/ [18] ecofellows ltd. taloyhtiön energia-apuri, 2021. [2021-10-07]. https: //ekokumppanit.fi/taloyhtion-energia-apuri/ 558 https://ym.fi/en/climate-neutral-finland-2035 https://www.tampere.fi/sites/default/files/2022-06/carbon_neutral_tampere_2030_roadmap.pdf https://www.tampere.fi/sites/default/files/2022-06/carbon_neutral_tampere_2030_roadmap.pdf https://www.isannointiliitto.fi/in-english/ http://data.europa.eu/eli/dir/2010/31/oj http://data.europa.eu/eli/dir/2018/844/oj https://ec.europa.eu/energy/sites/default/files/documents/fi_2020_ltrs_en.pdf https://ec.europa.eu/energy/sites/default/files/documents/fi_2020_ltrs_en.pdf https://www.motiva.fi/koti_ja_asuminen/taloyhtiot_-_yhdessa_energiatehokkaasti/korjaushankkeet_ja_energiatehokkuuden_huomioiminen https://www.motiva.fi/koti_ja_asuminen/taloyhtiot_-_yhdessa_energiatehokkaasti/korjaushankkeet_ja_energiatehokkuuden_huomioiminen https://www.motiva.fi/koti_ja_asuminen/taloyhtiot_-_yhdessa_energiatehokkaasti/korjaushankkeet_ja_energiatehokkuuden_huomioiminen https://www.motiva.fi/koti_ja_asuminen/taloyhtiot_-_yhdessa_energiatehokkaasti/korjaushankkeet_ja_energiatehokkuuden_huomioiminen https://www.finlex.fi/fi/laki/kaannokset/2009/en20091599_20100547.pdf https://www.finlex.fi/fi/laki/kaannokset/2009/en20091599_20100547.pdf https://www.prh.fi/en/kaupparekisteri/tilastot/lkm.html https://www.prh.fi/en/kaupparekisteri/tilastot/lkm.html http://www.stat.fi/til/asas/index_en.html https://www.kiinteistoliitto.fi/en https://www.isannointiliitto.fi/wp-content/uploads/2020/10/isannoinnin-laatu-2020.pdf https://www.isannointiliitto.fi/wp-content/uploads/2020/10/isannoinnin-laatu-2020.pdf https://ec.europa.eu/education/policies/eu-policy-in-the-field-of-adult-learning_en https://ec.europa.eu/education/policies/eu-policy-in-the-field-of-adult-learning_en https://eur-lex.europa.eu/lexuriserv/lexuriserv.do?uri=oj:c:2011:372:0001:0006:en:pdf https://eur-lex.europa.eu/lexuriserv/lexuriserv.do?uri=oj:c:2011:372:0001:0006:en:pdf https://eur-lex.europa.eu/lexuriserv/lexuriserv.do?uri=oj:c:2011:372:0001:0006:en:pdf https://okm.fi/en/continuous-learning-reform https://ekokumppanit.fi/kare-hanke/ https://ekokumppanit.fi/taloyhtion-energia-apuri/ https://ekokumppanit.fi/taloyhtion-energia-apuri/ acta polytechnica ctu proceedings 38:553–558, 2022 1 introduction 1.1 national long-term strategy for renovations 1.2 housing companies in finland 1.2.1 management of housing companies 1.2.2 role of property manager 1.3 lifelong learning 2 care project improving the deployment of energy efficiency solutions 2.1 collecting the requirements 2.2 planning and implementing the courses 2.3 building passport – model 3 results and discussion 4 conclusions acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.38.0456 acta polytechnica ctu proceedings 38:456–462, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague sustainability and resilience in building design: discussion on two case studies licia felicionia,∗, antonín lupíšeka,b, jacopo gasparic, ernesto antoninic a czech technical university in prague, faculty of civil engineering, thákurova 2077/7, 166 29 prague, czech republic b university centre for energy efficient buildings, czech technical university in prague, třinecká 1024, 273 43 buštěhrad, czech republic c university of bologna, department of architecture, viale del risorgimento 2, 40136 bologna, italy ∗ corresponding author: licia.felicioni@cvut.cz abstract. designing sustainable and, at the same moment, resilient buildings is a necessity to reach the un sustainable development goals by 2030. however, these two building design approaches – sustainability and resilience – are usually treated separately. typically, resilience-improving strategies are placed only after a disruptive event and not at the design stage. it is clear that there is a substantial intersection between sustainability and resilience and this manuscript aims to determine more precisely the commonalities and contradictions seen in building design sustainable and resilient approaches as crucial elements for improving their cooperation in buildings. to accomplish this, the authors qualitatively analysed two case studies – respectively claiming to be sustainable and resilient – to understand if a sustainable building can also be considered resilient and vice versa. this paper is addressed to the private and public sectors that have a decisive role in building design and are determined to take tangible steps to influence decision-making and resilience-based solutions already at the design phase. in conclusion, once the commonalities of resilience and sustainability are highlighted, a building designed as sustainable or resilient will be in line with both long-term perspectives. keywords: building design, sustainable building, resilient building, synergies, contradictions, future threats. 1. introduction in the 6thassessment report of the intergovernmental panel on climate change (ipcc) [1], it is highlighted that earth is experiencing irreversible impacts and unprecedented warming, including more frequent and more extreme weather events; their consequences will continue to get more intensive for every bit of warming [2]. the frequency and severity of floods, wildfires, heat and cold waves, and droughts in the last decade was increasing, causing remarkable economic costs and life losses [3, 4]. the ipcc’s report shows that emissions of greenhouse gases from human activities are responsible for approximately 1.1 °c of warming since 1850–1900 and significantly contributes to the alteration of the local climatic conditions in the built environment (i.e. urban heat islands) [2]. in europe, buildings are the largest energy consumer, responsible for approximately 40 % of greenhouse gas emissions; indeed, the built environment represents a crucial sector in terms of saving potential and, at the same time, one of the most vulnerable and densely inhabited places affected by climate change effects. 1.1. synergies between sustainability and resilience developing the built environment sustainable and resilient to climate change is a pressing global need, as outlined by the 2015 paris agreement [5] and the recent 2021 ipcc assessment report [2]. sustainability has been a trend since the ’90s, when different protocols aimed at assessing sustainability in buildings were developed, such as leadership in energy and environmental design (leed) [6] in the united states, building research establishment environmental assessment method (breeam) [7] in the united kingdom, and deutsche gesellschaft für nachhaltiges bauen (dgnb) [8] in germany. the current concept of resilience is instead less concrete and much more recent than sustainability. there is no fixed definition, but it is starting to experience a global increment in interest due to the climate change impacts [9]. however, the most accepted and common explanation defines resilience as the ability of a system to maintain or recover functionality in the event of disruption or disturbance [10, 11]. the need for effective strategies to face these interconnected challenges arises in the 2015 un sustainable development goals (sdgs), where adequate mitigation and adaptation measures are expected to be introduced by 2030 [12]. 456 https://doi.org/10.14311/app.2022.38.0456 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 38/2022 sustainability and resilience in building design . . . in particular, the targets of sdg 13 are aimed at increasing resilience from natural hazards, while others, such as sdg 7 or 11, are more focused on sustainability. however, sdgs always share benefits and synergies and are directly connected to sustainability and resilience for the built environment. thus, it is highlighted that both resilience and sustainability can have commonalities at the building level and should be considered already at the design stage. accordingly, green buildings should be designed to be resilient to extreme events (loads connected to heavy rains, floods, or other hazards) to keep the occupants safe and reduce the environmental impacts associated with post-event adjustments [13]. 1.2. scope of the paper using two case studies, the paper aims to analyse sustainability and resilience in building design via a qualitative assessment. qualitative analysis is suited to the initial phases of planning, object programming, and designing at every stage of the design process [14]. the first case study is a leed-certified building ranked platinum (leed new construction v3) among the very new generation of sustainable construction in the czech republic; the second is a resilient building completed in new york city following a new protocol developed as a response to the 2012 sandy hurricane consequences. after preliminary identification of the distinctive criteria associated with sustainability and resilience, the buildings were assessed qualitatively to understand if the claimed as sustainable is also resilient and vice versa. thus, the main scope is to understand the relationship between sustainability and resilience to design principles. 2. materials and methods the adopted method includes a qualitative assessment for two case studies considering different criteria both for sustainability and resilience. the qualitative assessment approach is quite consolidated in the scientific literature as a source of universal methods for investigating whole objects and selected elements of the building [14]. in the case of sustainability, the criteria were selected analyzing the most known and worldwide sustainable protocols for new construction: leed v4.1 [6], breeam international 2016 [7] and dgnb 2020 [8]. once the criteria and the benchmarks were picked, four levels of accomplishment (poor, sufficient, good and excellent) were selected to assess the specific criterion’s level of achievement (table 1). a similar method has been considered in the case of resilience where reli [15] and envision v3 [16], two of the most known resilience assessment tools worldwide, have been chosen as reference tools for selecting the criteria (table 2). the benchmarks for the distribution of the grades were established using different methodologies aligned with benchmark values found in the literature that are explained case by case [7, 15, 17, 18]. dependent on fulfilment of each criterion, points were given according to this scale: 2 points for “poor”, 5 points for “sufficient”, 8 points for “good” and 10 points for “excellent”. it is assumed that buildings in which poor positive pro-environment or resilient measures were applied for the assessed criteria receive a satisfactory grade of 2. consequently, the maximal value is set as 130 (respectively 70 for sustainability and 60 for resilience), which means that the building has a very excellent performance in terms of qualitative aspects. for research purposes, we considered two case studies located in flood-prone areas: • main point karlin (prague, czechia) (figure 1a) – leed-certified office building (leed v3); it was the first platinum-certified building in czechia and even central europe. it was built in 2012 in the karlin neighborhood, an area really close to the vltava river. it has been chosen because one of the first examples of a new generation of buildings in the czech republic – it achieved great results, particularly in the fields of energy utilization, indoor environmental quality and innovations. • dock 72 (brooklyn, new york city) (figure 1b) – class a office building claimed to be flood-resistant. it was built in 2019 in the brooklyn navy yard neighborhood, an area in front of the navy yard bay. it has been chosen since it is a response building to the 2012 sandy hurricane damages; the building hovers above the floodplain on v-shaped columns – it achieved great performance, particularly in the fields of energy back-up and passive systems. finally, a table with grades for every criterion gives a picture of the sustainability or resilience of both case studies. 2.1. buildings descriptions 2.1.1. sustainable building case study the building chosen as a case study for representing sustainability is the main point karlin office building, located in prague (czechia) (figure 1a), in the neighbourhood of karlin, in front of the vltava river. the building was awarded the title “best office building in the world” at the mipim awards 2012 and the first holder of the highest leed platinum certificate in central europe (certified under leed new construction 2009). main point karlin is a technologically equipped 10-floors office building with a leasable area of 22 000 m2. the distinctive facade panels have the function of sun breakers, ensuring optimal workplace lighting by direct and indirect components. the coloured material used in the facade is fiberc, which was considered one of the greenest concrete panels available in 2012 with low embodied environmental impacts (202 mj/m2 for primary energy and 14 kg co2eq/m2 for global warming potential). moreover, a 1 200 mm diameter flushing channel runs through the basement of the building and in 457 l. felicioni, a. lupíšek, j. gaspari, e. antonini acta polytechnica ctu proceedings criteria poor sufficient good excellent connectivity and transport public traffic connection > 1 km public traffic connection within 1 km public traffic connection within 800 m public traffic connection within 400 m land use and ecology using native vegetation green roof and native vegetation previous plus installed rainwater collection system all the techniques before mentioned and other innovative solutions reduction of indoor water consumption > 0 % > 30 % > 40 % > 50 % improvement of energy performance (compared to the baseline building performance) > 5 % > 20 % > 30 % > 40 % resource-efficient and circular material life cycles surface area reused > 0 % surface area reused > 25 % surface area reused > 50 % surface area reused > 75 % renewable energy procurement > 2 % (on-site) > 20 % (off-site) > 5 % (on-site) > 30 % (off-site) > 10 % (on-site) > 40 % (off-site) > 20 % (on-site) > 50 % (off-site) daylight < 55 % of occupied floor area 55 % of occupied floor area 75 % of occupied floor area 90 % of occupied floor area table 1. compilation of criteria for the evaluation of qualitative aspects related to sustainability. criteria poor sufficient good excellent building surroundings protection reduced run-off develop naturebased solutions that protect the surrounding plan system for 100-year floods for the building protect below ground system vents and entrance from floods and 100-year floods for the surrounding passive heating only active solutions direct gain via glazing direct gain via storage + glazing all the strategies mentioned + indirect gain via sunspace passive cooling orientation orientation, cross ventilation solar shading, building facades all the strategies mentioned passive lighting minimum daylight daylight from multiple sides intermediate light shelves and skylights all the strategies mentioned water harvesting none < 50 % of the roof area > 50 % of the roof area > 50 % of the roof area and parking areas for reuse resilience to climate/natural hazards none identification of regional hazards location hazards assessment + passive solutions location hazards assessment + passive solutions and resilience emergency plan table 2. compilation of criteria for the evaluation of qualitative aspects related to resilience. 458 vol. 38/2022 sustainability and resilience in building design . . . (a). (b). figure 1. (a) main point karlin, prague (czechia), designed by dam architeki. details of the façade that works as sun shading. (b) dock 72 at the brooklyn navy yard (nyc), designed by s9 architecture. which the vltava flows from the těšnov weir to the libeň docks. this technology is used to cool the building passively. the quality of the building’s indoor environment, corresponding to a higher standard, is ensured by ceiling induction units. the under-ceiling placement of the units is ideal in terms of natural airflow – the cooled air falls by its weight and does not “blow” on the workers from anywhere. however, even if it is located in proximity to the vltava river, it does not consider any strategies against flood-related events since its ground floor, where restaurants, bars and even the main entrance are placed, is below the street level. 2.1.2. resilient building case study the building chosen as a case study for representing resilience is the dock 72 office complex, located in brooklin (nyc) (figure 1b), on the waterfront of the east river. the 62 700 m2 structure, whose base uses steel frames and steel braced cores, has 16 floors looking out to brooklyn and manhattan, situated 0.3 m above the floodplain. the city of new york experienced the devastation of the hurricane sandy in 2012 that led to a full collapse of the electric system and flooding all around the five neighbourhoods, mainly in manhattan and brooklyn. this class-a office building is designed to withstand potential hazards, such as flood events and sea-level rise, with the 20 v-shaped columns allow water to flow under the building and sloping ramps that provide access to the elevated main floor. the building’s mechanical systems are raised above the first level, ensuring that the building functional level can be preserved or reloaded easier in the recovery phase. the dock 72 indoor environment is characterized by open, flexible, and light-filled workspaces. the stepped massing and gridded, glazed façade maximizes the views on the manhattan bay and allows a direct gain of sunlight. 3. results and discussion 3.1. qualitative assessment of the two case studies 3.1.1. sustainability qualitative evaluation as it was previously mentioned in section 2, each case study was analysed to understand the general sustainability of the building. the qualitative sustainability evaluation considers seven main categories that are listed in table 3. main point karlin presents great outcomes in the categories of connectivity and transport, since public transport services are very close to the building, and in land use and ecology, since the building is placed on a former brownfield, successively redeveloped, and a rainwater collection system that allows to a 100 % reduction in potable landscape water use. in contrast, the category that presents the lower grade is resourceefficient and circular material life cycles due to only 20 % recycled content building materials. dock 72 in brooklyn showcases an excellent result in connectivity and transport since the proximity of various available transport services and good results in energy performance and daylight thanks to energy-saving measures that include increased roof and exposed floor insulation, reduced fan power and variable frequency drives, and high-performance building envelope and reduced lighting power centre. quite the reverse, two of the seven selected sustainability categories are graded as “poor” because there are 459 l. felicioni, a. lupíšek, j. gaspari, e. antonini acta polytechnica ctu proceedings main point karlin dock 72 connectivity and transport excellent excellent land use and ecology excellent sufficient reduce indoor water consumption good sufficient improvement of energy performance good good resource-efficient and circular material life cycles poor poor renewable energy procurement sufficient poor daylight good good table 3. qualitative assessment for sustainability criteria. main point karlin dock 72 building surroundings protection poor excellent passive heating sufficient excellent passive cooling excellent sufficient passive lighting excellent sufficient water harvesting good sufficient resilience to climate/natural hazards poor excellent table 4. qualitative assessment for resilient criteria. no renewables on-site and no intentions of building material reuse. 3.1.2. resilience qualitative evaluation as it was previously mentioned in section 2, each case study was analysed to understand the general resilience of the building. the qualitative resilience evaluation considers six main categories that are listed in table 4. main point karlin presents, on one side, great outcomes in the categories of passive cooling thanks to the 1 200 mm pipe that runs through the entire building from the vltava river, and in passive lighting, the orientation of the building and the characteristic coloured pillars that also work as sun breaker. on the other side, two categories present the lower grade: transportation system protection and resilience to climate/natural hazards. dock 72 was designed with a special focus on floodresilient features to potentially face another catastrophic event like the 2012 hurricane. the ground floor was built 0.3 meters above the 100-year floodplain grade. a 1 500 kw emergency generator ensures that tenants, even in case of extreme events and consequent black-out, would not experience a lack of light or heating. 3.1.3. comparison figure 2 shows the two graphs related to the qualitative sustainability assessment (7 categories) and resilience (6 categories) of the two case studies. as it was preannounced, the building claimed to be sustainable, main point karlin, has greater results than the one claimed as resilient, dock 72, because more categories are under the grades “excellent” or “good”, such as land use and ecology and reduce of indoor water consumption thanks to run-off reduction and rainwater harvesting system. however, dock 72 presents the same great results for connectivity and transport as the prague building, thanks to the closeness of different available public transportation (ferry, bus, metro, and bicycle lanes), while it lacks in reusing of building materials and recycling. focusing on the qualitative resilience assessment, dock 72 presents excellent outcomes for three of the six categories, mostly due to its design thought to be flood-resistant. indeed, placing the main entrance and the mechanical systems at higher levels allow the building’s functional level to be preserved or reloaded easier in the recovery phase in the presence of heavy rain or other climate-related hazards. in 2002, a 100-year flood event caused by over a week of continuous heavy rains ravaged central europe. prague received significant damage; karlin was one of the most severely affected capital city neighbourhoods, with a risk of building collapse. however, as shown in figure 2, main point karlin, built in 2012, precisely ten years after the catastrophic flood, does not present any strategy to face such a possible event that is even more potentially dangerous due to river proximity. however, it showcases great results for passive strategies for cooling, thanks to the tube of vltava water that runs on the second-unground floor, and for lighting, due to the particular façade panels. 3.2. limit of the study the limit of this study is related to the subjectivity of the adopted criteria used for assigning the grades. the list of indicators is limited and simplified but inclusive and clusters some sub-indicators that, otherwise, would expand the comparative process without focusing on the core point of finding a balance between sustainability and resilience. that allowed even 460 vol. 38/2022 sustainability and resilience in building design . . . figure 2. analysis of the two case studies for sustainability (left) and resilience (right) according to the defined criteria. to keep complexity manageable, as (1) other assessment frameworks (and even those that inspired the selection) consider other variables that could change the overall assessment result and (2) some of them are highly criticized “attribute-oriented” rather than “performance-oriented” criteria. however, these clusters address sustainability and mostly environmental resilience while not considering other aspects of resilience, such as economic or social. still, these aspects will be studied in further studies. 4. conclusion the approach that has been adopted serves to identify common elements and distinctive attributes for sustainability and resilience so that these specific solutions can be exported or imported between the two domains to design a new generation of buildings. as the results highlight, it is possible that if in a building only sustainability is considered, some resilience aspects may be neglected and/or when only principles of resilience are taken into consideration, some fundamental sustainability values may be ignored. the primary outcome of this work is that it is necessary to think about a new generation of buildings where both domains are considered to find a proper balance between them. finally, it is possible to conclude that some indicators must be identified and become irreplaceable in guiding the design so that a building, in addition to having minimal elements of sustainability, also contextually can give a minimal response to the concept of resilience since they share common roots. in practice, the final design will depend on the building objectives, which might include more or less severe resilience and/or sustainability requirements. this is in the logic of adapting the principles around which direct the construction sector, particularly the construction of new buildings, with respect to the tomorrow’s challenges, so clearly evoked by the sdgs. acknowledgements this work has been supported by the ministry of education, youth and sports within project interexcellence no. ltt19022. references [1] intergovernmental panel on climate change ipcc. [2021-09-01]. https://www.ipcc.ch/ [2] intergovernamental panel on climate change. climate change 2021: the physical science basis, 2021. [3] c. lomba-fernández, j. hernantes, l. labaka. guide for climate-resilient cities: an urban critical infrastructures approach. sustainability 11(17):4727, 2019. https://doi.org/10.3390/su11174727 [4] l. felicioni, a. lupíšek, p. hájek. major european stressors and potential of available tools for assessment of urban and buildings resilience. sustainability 12(18):7554, 2020. https://doi.org/10.3390/su12187554 [5] united nations. paris agreement, 2015. [6] usgbc. leed v4.1 building design and construction, 2021. [7] breeam. breeam international new construction 2016 – technical manual, 2016. [8] dgnb. dgnb system new construction, buildings criteria set – version 2020, 2020. [9] s. nop, a. thornton. urban resilience building in modern development: a case of phnom penh city, cambodia. ecology and society 24(2):23, 2019. https://doi.org/10.5751/es-10860-240223 [10] the rockefeller foundation; arup. city resilience framework, 2015. [11] n. b. rajkovich, y. okour. climate change resilience strategies for the building sector: examining existing domains of resilience utilized by design professionals. sustainability 11(10):2888, 2019. https://doi.org/10.3390/su11102888 461 https://www.ipcc.ch/ https://doi.org/10.3390/su11174727 https://doi.org/10.3390/su12187554 https://doi.org/10.5751/es-10860-240223 https://doi.org/10.3390/su11102888 l. felicioni, a. lupíšek, j. gaspari, e. antonini acta polytechnica ctu proceedings [12] s. dobie, j. schneider. development of a process improvement matrix to measure community level sustainability and resilience: a collaborative case study of rochester, ny, usa. in 2017 ieee international symposium on technologies for homeland security (hst), pp. 1–5. 2017. https://doi.org/10.1109/ths.2017.7943447 [13] s. j. welsh-huggins, a. b. liel. evaluating multiobjective outcomes for hazard resilience and sustainability from enhanced building seismic design decisions. journal of structural engineering 144(8):04018108, 2018. https: //doi.org/10.1061/(asce)st.1943-541x.0002001 [14] k. fross, d. winnicka-jasłowska, a. gumińska, et al. use of qualitative research in architectural design and evaluation of the built environment. procedia manufacturing 3:1625–1632, 2015. 6th international conference on applied human factors and ergonomics (ahfe 2015) and the affiliated conferences, ahfe 2015. https://doi.org/10.1016/j.promfg.2015.07.453 [15] usgbc. reli 2.0 rating guidelines for resilient design + construction, 2018. [16] institute for sustainable infrastructure. envision v3: what you need to know, 2018. [17] pacific northwest national laboratory. ansi/ashrae/ies standard 90.1-2010 performance rating method reference manual, 2016. [18] usgbc. leed v4.1 building design and construction, 2020. 462 https://doi.org/10.1109/ths.2017.7943447 https://doi.org/10.1061/(asce)st.1943-541x.0002001 https://doi.org/10.1061/(asce)st.1943-541x.0002001 https://doi.org/10.1016/j.promfg.2015.07.453 acta polytechnica ctu proceedings 38:456–462, 2022 1 introduction 1.1 synergies between sustainability and resilience 1.2 scope of the paper 2 materials and methods 2.1 buildings descriptions 2.1.1 sustainable building case study 2.1.2 resilient building case study 3 results and discussion 3.1 qualitative assessment of the two case studies 3.1.1 sustainability qualitative evaluation 3.1.2 resilience qualitative evaluation 3.1.3 comparison 3.2 limit of the study 4 conclusion acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.38.0288 acta polytechnica ctu proceedings 38:288–294, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague interact integration of innovative technologies of peds into a holistic architecture helmut brucknera,∗, markéta adamcováb, albana iloc, andrea wernerd a sonnenplatz großschönau gmbh, sonnenplatz 1, 3922 großschönau, austria b leef technologies s.r.o., forum karlín, pernerova 676/51, 186 00 prague 8, czech republic c tu wien, institute of energy systems and electrical drives, gußhausstraße 25/370-1, 1040 vienna, austria d university of applied science technikum wien, research area renewable energy systems, giefinggasse 6, 1210 vienna, austria ∗ corresponding author: h.bruckner@ped-interact.eu abstract. interact is an international research and innovation project which boosts the emergence of energy communities as one crucial building block to achieve positive energy districts. it is designing an optimal organization and structure for energy communities based on success-factors of existing ped/pen approaches, stakeholder needs and motivation, the available technologies, and a holistic link -based architecture. technological and market-related solutions are delivered that maximize the benefit for the environment and society, considering the current legal framework and available business models. the project will develop a roadmap for the energy community’s secure and reliable embedding into the power system structure, focusing on two pilot regions, a green-field project in sweden and an existing municipality in austria. the holistic architecture is used to ensure the integrity of the solution by harmonizing all interactions within the energy community itself, between it and the market, and the european power system. its standardized and flexible structure allows the straightforward application of the roadmap to perfectly meet the diverse necessities of local communities, thus supporting the large-scale roll-out of the new control paradigms. keywords: positive energy districts, energy communities, link solution, stakeholder involvement, distributed energy resources. 1. introduction the set-plan, adopted by the european union in 2008 and revised in 2015, is a first step to establish an energy technology policy for europe [1]. the set plan focuses on 10 key actions fields, of which action 3.2 on “smart cities and communities” aims to support the planning, deployment and replication of 100 positive energy districts (peds) by 2025 for sustainable urbanization [2]. the reference framework of positive energy districts (ped) / positive energy neighbourhoods (pen) is defined in the “white paper on ped reference framework” [3] as “energy-efficient and energy-flexible urban areas or groups of connected buildings which produce net zero greenhouse gas emissions and actively manage an annual local or regional surplus production of renewable energy. they require integration of different systems and infrastructures and interaction between buildings, the users and the regional energy, mobility and ict systems, while securing the energy supply and a good life for all in line with social, economic and environmental sustainability.” [3]. climatic conditions worldwide force the comprehensive integration of renewable and distributed energy resources into the power system, as well as the effective use of energy and of the existing infrastructure. business models in the energy sector are changing and becoming more customer-centric: the customers play an active role in energy supply [4]. the way in which the power grid should be managed is changing, as also the rate of change in the electric power industry accelerates annually [5]. driven by reduced costs of solar pv and storage energy communities are on the rise and disrupte the energy value chain, pushing energy retailers and dsos towards deep transformation [6]. energy communities have been currently formally acknowledged and defined within europe as “renewable energy communities” and “citizen energy communities”. at the same time a further revision of the renewable energy directive was brought forward in july 2021, proposing to raise the target of renewables to 40 % by 2030, further fostering the transformation of the energy system [7]. the european commission is describing energy communities as follows: “energy communities organise collective and citizen-driven energy actions that will help pave the way for a clean energy transition, while moving citizens to the fore. [. . . ] by supporting citizen participation, energy communities can moreover help in providing flexibility to the electricity system through demand-response and 288 https://doi.org/10.14311/app.2022.38.0288 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 38/2022 interact integration of innovative technologies of peds . . . storage.” [8]. with the increase of renewables in the energy system as stated above, and the planned increase of electrification of energy (direct use of low-emissions electricity) as one of the most important drivers of emissions reductions [9], the pressure and demands on electricity grids are increasing. the implementation of energy communities is bringing a new actor into the electricity markets, driven by investments in sustainable infrastructure, production of green electricity and heat, social and environmental sustainability or self-sufficiency, bringing positive impacts on the energy transition [10]. all of these positive effects are clearly supporting the build-up of peds/pens. furthermore, and in line with the interact approach [11], holistically structured energy communities can also support the operation of distribution networks, helping to increase the usable capacity of the grid and to avoid congestion by improving the overall performance of reactive power control [12]. the necessary ict structure for a reliable operation of the electricity grid might therefore be one of the ict systems to be integrated in a ped, enabling direct and indirect electricity services needed for an optimized operation of local resources. a holistically designed energy community following the interact project and the link -architecture [13] increases the installed capacity of distributed energy resources (der) in the given power grid, offering energy services like flexibility trading, and therefore supporting the ambitions of specific districts of the neighbourhood in becoming ped/pens. not surprisingly there as, the goal of the interact project is the emergence of energy communities as one crucial building block to achieve peds. the project will design an optimal organization and structure for energy communities based on success-factors from a competence-network of existing ped/pen approaches, stakeholder needs and motivation, the available technologies, and a holistic architecture [11]. the project will conclude with the development of a roadmap for the secure and reliable embedding of the energy community into the structure of the power system, focusing on two pilot regions in sweden and austria. 2. methodology energy communities are in the early stages of consolidation within the peds. therefore, interact has developed a comprehensive methodology to consider the results of different projects. it creates a solid foundation for the development of energy communities in all their complexity. the objectives of the project will be achieved by combining the results of four investigation steps as follows: (1.) assessing and evaluation of existing successful ped-approaches – this step aims to assess the state of the art of current ped approaches and to establish a network of international ped communities to foster the knowledge exchange and learning opportunities. it is focusing on assessment criteria for success of ped projects, based on existing project experience. (2.) characterization of current & expected future state of the specific energy communities – a common methodology for the inventory of the current stock of renewable energy resources, the documentation of the automation used, and the existing ict infrastructure are being defined. based on this methodology and the project data management plan, all new technologies already existing and installed are recorded. (3.) design of the link -based energy-community with respect to stakeholder needs the interact energy community according to the holistic link -solution will be designed by defining workflows (use cases), as well as technical and market related interfaces. (4.) contracting models and regulatory framework – based on the relevant european directives, the legal framework for the future design of the electricity market and grid is defined and the implementation of energy communities is being prepared. regional differences among these national regulatory frameworks will be analysed regarding main barriers for establishing local and regional energy communities. (5.) design of the roadmap for the implementation of the interact energy communities define a set of instructions or suggestions needed for the implementation of the interact energy communities all over europe. 3. first results, outlook, and discussion the project is still at an early stage of development, but there are already results and initial successes to report. during cesb conference in july 2022 the current status of interact will be presented. 3.1. assessing and evaluation of existing successful ped-approaches. we identified about 60 european ped approaches via desk research pilots with the aim of adding new ped approaches to already existing repositories like the ped-booklet by jpi urban europe [14]. based on criteria showing relevance to energy communities (local energy markets, peer-to-peer trading, flexibility management, etc.), and/or implementation stage (in implementation or already implemented), 16 pedapproaches from the project partner countries have been identified for deeper analyses and personal interviews. 3.1.1. competence mapping part of the deeper analysis of existing selected pedapproaches was competence mapping, where a coding 289 h. bruckner, m. adamcová, a. ilo, a. werner acta polytechnica ctu proceedings figure 1. code – relations – model on competence level vs. project function. system was created, which is categorized in project function, competence level, competences, project and country. project function defines different roles of ped project partners. competence level is categorized by 3 levels: formal, informal and institutional level. formal competences are qualifications employees learned in their academical education. informal competences comprise non educated competences, e.g., social competences, communication. institutional competences can be understood as every competence which brings institutions themselves. coding, analysis, and visualization was conducted with the qualitative data analysis software maxqda, especially using the code relations models. the frequencies of connections of selected codes were compared. hence the line width of all figures is relative to the number of relations, not the importance, of the extracted competences. figure 1 shows the code-relations-model derived for the competence level on project functions. seven out of eleven project functions are related to each competence level. “planners” and “project development” can be highlighted in high connection with both formal and informal competence level. “competence centre” and “energy supplier” tend more towards the formal competence level. taking a closer look at the competences of the analysed ped-approaches, the below within figure 2 shown competence network was visualized: it is noticeable that many thick lines start from “energy” and “planners”. the competence “energy” is well connected with the project functions “academia”, “competence centre”, “energy supplier” and especially with “planners”. many connections to other competences are “academic research”, “project development” and to energy subcodes such as “power storage” and “renewable energy”. based on more in-depth information on success-factors a normative level associated with the identified competences can strengthen the mapping process substantially and support the establishment of good-practice in ped/pen initiatives. 3.1.2. stakeholder analysis in parallel, all relevant stakeholders within the two focus regions have been identified and a stakeholder mapping approach has been applied: 15 stakeholders of the municipality in austria have been selected for interviews, as well as 3 stakeholders of the green-field project in sweden. the goal was to identify stakeholders’ roles and their perspective on other stakeholders’ roles regarding e.g., knowledge, benefits, burdens as well as potential impact on the success of an energy community, see below table 1. criteria for active stakeholder participation, identification of relevant subgroups and strategies for information and implementation are now being derived from the empirical assessments within the focus regions. main motivation is attributed to the role of being a pioneer, and to financial benefits. this is followed by climate protection. the findings show similar drivers for participation in an ecs as found by caramizaru and uihlein, only missing the goal of a common investment in infrastructure [10]. 290 vol. 38/2022 interact integration of innovative technologies of peds . . . figure 2. code – relation – model on competences vs. project function. stakeholder assessment of motivation by others municipality increase renewables; climate protection; energy autonomy; financial benefits; security of supply communal organizations increase renewables; climate protection; financial benefits, savings; pioneer, role model; security of supply infrastructure grid stability, relieve associations pioneer; climate protection; electricity from renewables; financial benefits private sector pioneer; financial benefits; security of supply multipliers pioneer; regionality and added value; financial benefit; continuation of work, projects; security of supply citizens pioneer; “green” thinking; community spirit; financial benefits; security of supply; role model table 1. stakeholders assessment of motivation by others. 291 h. bruckner, m. adamcová, a. ilo, a. werner acta polytechnica ctu proceedings figure 3. harmonization of the market structure with a local energy community on level 4 in the grid link arrangement [13]. 3.2. characterization of current & expected future state of the specific energy communities we established a common inventory methodology for recording available technologies. described are data categories, data sources, data types, data purpose in the project, data management, and the inventory workflow itself based on the project type. it enables the project to record the existing technologies at the focus community in austria and allows future green-field projects to base on the same standard. based on the recorded data the necessary requirements to upgrade to the primary control as well as to the secondary control will be defined and described. furthermore, the communication protocols between market, grid, ec, and customer plant will be described, together creating the link -ict-structure. 3.3. design of the link-based energy-community with respect to stakeholder needs the energy community must work in harmony with the power grid and needs flexible interaction with the market, which should enable transparent and nondiscriminatory energy trading between different interest groups. figure 3 illustrates the harmonization and coordination of the market structure with the grid-links arrangement. the market structure that incentivizes grid operators, energy communities, and customers to contribute to energy services shall be specified by employing a review of the existing competitive electricity markets. further development of the general lines of the link -market structure [13] that motivates customers to contribute to the energy services (local market) will be specified based on the electricity market design of the “clean energy for all europeans package” [15]. the market-based control must take into account the technical behaviour of the smart grids and support their safe, reliable, and resilient operation while at the same time attracting the demand response bids. the necessary information exchange will be determined, and the use cases will be described. this organizational structure and the resulting management of the ec can be done by using an existing organization or establishing a new organization. the chosen organizational structure depends on the national possibilities and should be in line with the aims and size of the energy community operated by the organization, including its mid-term strategic goals regarding growth, size, and responsibilities. 3.4. contracting models and regulatory framework the member states of the european union are implementing the formal legal framework for the establishment of energy communities based on the european electricity market directive and the renewable energy directive. this is one cornerstone of the regulatory framework which energy communities are based. interact will analyse the regional differences in terms of main barriers for establishing local and re292 vol. 38/2022 interact integration of innovative technologies of peds . . . figure 4. schematic presentation of the harmonized operation of the interact holistic energy community with the grid. gional energy communities in austria, belgium, czech republic, and sweden. furthermore, possible business cases based on the interact energy community will be specified. the defined technological solution will be analysed from the point of its economic feasibility with regards to its implementation by all relevant stakeholders. based on the actual energy behaviour of the interact focus regions, scenario planning for the development of energy demand, energy production and energy storage will be done. this enables us to estimate and evaluate the market potential of the designed ideal energy community, and the implications of it on the various stakeholders involved. 3.5. roadmap for the implementation of the interact energy communities under the vision of prosperous energy communities with the strategy of the holistic approach of the link solution, the roadmap will include a set of instructions or suggestions needed for the implementation of the interact energy communities. five large, different areas that are needed to implement an energy community in practice are being addressed. technique; market; regulation; legislation and organization. there are foreseen three cases: a common case, an upgrade case on the example of großschönau, austria, and a green-field case on the example of fyllinge, sweden. the roadmap will support the planning, deployment, and replication of the targeted 100 positive energy districts (peds) by 2025 for sustainable urbanization, shall foster the creation of energy communities operating in harmony with the grid, and increase the potential for installing der at a given grid-infrastructure. the technical problems are the most persistent barriers to making energy communities a reality. the establishment of energy communities should not only figure 5. active and reactive power flow in a lowvoltage feeder with pv penetration that injects with cos(φ) = 1, the l(u) control strategy is employed, and q-autarky is applied. not disturb the reliable operation of the grid but should bring new benefits to them. there is a vacuum in the literature about the details of the technical barriers that lead to the resignation of the energy communities. therefore, suggestions for overcoming the technical barriers are being given a special place on the roadmap. link -solution figure 4 shows a schematic presentation of the harmonized operation of the interact holistic energy community with the grid. there are included the high-, mediumand low voltage grid as well as prosumers, batteries and e-mobility. the last ones are organised and operated under the umbrella of the interact energy community. the possible voltage challenges appearing in radial structures of distribution grid by too high pv penetration are solved using the link -solution for the volt/var control process. the voltage will be kept within the limits by using the control ensemble l(u) local control at the low voltage level and q-autarky at the customer plant level [16], figure 5. 4. conclusions interact stands for integration of innovative technologies of positive energy districts into a holistic architecture. based on the analysis of competences of 60 existing ped-approaches with the help of 16 personal interviews, the needs, and requirements of around 20 different interviewed stakeholders, recorded existing (and missing) technologies and the link -based holistic architecture, an optimal organization and structure of energy communities is being designed. the technical approach is based on link technical solution. market, regulation, legislation, and organization are being analysed, current barriers identified, and improved solutions suggested. at the end of the project (january 2023) a roadmap for the implementation of the designed interact energy community in general and for the specific local perspectives will be available, fostering the creation of energy communities operating in harmony with the grid all over europe. all current results and related scientific work can be found at https://www.ped-interact.eu. acknowledgements this project has received funding in the framework of the ped programme, which is implemented by the joint pro293 https://www.ped-interact.eu h. bruckner, m. adamcová, a. ilo, a. werner acta polytechnica ctu proceedings gramming initiative urban europe and set plan action 3.2. the project is supported by the austrian ministry of climate action, environment, energy, mobility, innovation, and technology (bmk), technology agency of the czech republic (tačr) and viable cities, a research program funded by the swedish energy agency, formas and vinnova, sweden. references [1] setis. set plan information system, 2021. [2021-1007]. https://setis.ec.europa.eu/what-set-plan_en [2] setis. set-plan action n°3.2 implementation plan. europe to become a global role model in integrated, innovative solutions for the planning, deployment, and replication of positive energy districts, 2018. [2021-10-04]. https://setis.ec.europa.eu/system/files/202104/setplan_smartcities_implementationplan.pdf [3] r. hinterberger, c. gollner, m. noll, et al. white paper on ped reference framework, published by jpi urban europe and set-plan 3.2 programme on positive energy districts, 2020. [2021-10-04]. https://jpi-urbaneurope.eu/ped/ [4] e. stoverinck. why customer-centricity is key to succeeding in the energy business, 2020. [2021-09-21]. https://futurice.com/blog/why-customercentricity-is-key-to-succeeding-in-the-energybusiness [5] m. i. henderson, d. novosel, m. l. crow. electric power grid modernization trends, challenges, and opportunities. ieee advanced technology for humanity, 2017. 17 p. [6] r. héliot. energy communities are reshaping the energy value chain, 2017. [2021-10-07]. https://medium.com/astercapital/energycommunities-are-reshaping-the-energy-valuechain-14e72b3c0381 [7] european commission. timeline for renewable energy in eu, 2021. [2021-09-15]. https://ec.europa.eu/energy/topics/renewableenergy/directive-targets-and-rules/renewableenergy-directive_en#directive-2018-2001-eu [8] european commission. energy communities, 2020. [2020-12-14]. https://ec.europa.eu/energy/topics/markets-andconsumers/energy-communities_en [9] international energy agency. net zero by 2050 – a roadmap for the global energy sector. international energy agency, paris, 2021. 224 p. https://doi.org/10.1787/c8328405-en [10] european commission, joint research centre, a. uihlein, a. caramizaru. energy communities : an overview of energy and social innovation. publications office, luxembourg, 2020. isbn 978-92-76-10713-2. https://doi.org/doi/10.2760/180576 [11] interact – integration of innovative technologies of peds into a holistic architecture. [2021-10-15]. www.ped-interact.eu [12] d.-l. schultis, a. ilo, c. schirmer. overall performance evaluation of reactive power control strategies in low voltage grids with high prosumer share. electric power systems research 168:336–349, 2019. https://doi.org/10.1016/j.epsr.2018.12.015 [13] a. ilo. design of the smart grid architecture according to fractal principles and the basics of corresponding market structure. energies 12(21):4153, 2019. https://doi.org/10.3390/en12214153 [14] c. gollner, r. hinterberger, s. bossi, et al. europe towards positive energy districts. a compilation of projects towards sustainable urbanization and the energy transition, first update. urban europe, 2020. [2021-10-04]. https://jpi-urbaneurope.eu/wp-content/uploads/ 2020/06/ped-booklet-update-feb-2020_2.pdf [15] european commission. electricity market design, 2021. [2021-10-12]. https://ec.europa.eu/energy/topics/markets-andconsumers/market-legislation/electricitymarket-design_en?redir=1 [16] a. ilo, d.-l. schultis. low-voltage grid behaviour in the presence of concentrated var-sinks and var-compensated customers. electric power systems research 171:54–65, 2019. https://doi.org/10.1016/j.epsr.2019.01.031 294 https://setis.ec.europa.eu/what-set-plan_en https://setis.ec.europa.eu/system/files/2021-04/setplan_smartcities_implementationplan.pdf https://setis.ec.europa.eu/system/files/2021-04/setplan_smartcities_implementationplan.pdf https://jpi-urbaneurope.eu/ped/ https://futurice.com/blog/why-customer-centricity-is-key-to-succeeding-in-the-energy-business https://futurice.com/blog/why-customer-centricity-is-key-to-succeeding-in-the-energy-business https://futurice.com/blog/why-customer-centricity-is-key-to-succeeding-in-the-energy-business https://medium.com/astercapital/energy-communities-are-reshaping-the-energy-value-chain-14e72b3c0381 https://medium.com/astercapital/energy-communities-are-reshaping-the-energy-value-chain-14e72b3c0381 https://medium.com/astercapital/energy-communities-are-reshaping-the-energy-value-chain-14e72b3c0381 https://ec.europa.eu/energy/topics/renewable-energy/directive-targets-and-rules/renewable-energy-directive_en#directive-2018-2001-eu https://ec.europa.eu/energy/topics/renewable-energy/directive-targets-and-rules/renewable-energy-directive_en#directive-2018-2001-eu https://ec.europa.eu/energy/topics/renewable-energy/directive-targets-and-rules/renewable-energy-directive_en#directive-2018-2001-eu https://ec.europa.eu/energy/topics/markets-and-consumers/energy-communities_en https://ec.europa.eu/energy/topics/markets-and-consumers/energy-communities_en https://doi.org/10.1787/c8328405-en https://doi.org/doi/10.2760/180576 www.ped-interact.eu https://doi.org/10.1016/j.epsr.2018.12.015 https://doi.org/10.3390/en12214153 https://jpi-urbaneurope.eu/wp-content/uploads/2020/06/ped-booklet-update-feb-2020_2.pdf https://jpi-urbaneurope.eu/wp-content/uploads/2020/06/ped-booklet-update-feb-2020_2.pdf https://ec.europa.eu/energy/topics/markets-and-consumers/market-legislation/electricity-market-design_en?redir=1 https://ec.europa.eu/energy/topics/markets-and-consumers/market-legislation/electricity-market-design_en?redir=1 https://ec.europa.eu/energy/topics/markets-and-consumers/market-legislation/electricity-market-design_en?redir=1 https://doi.org/10.1016/j.epsr.2019.01.031 acta polytechnica ctu proceedings 38:288–294, 2022 1 introduction 2 methodology 3 first results, outlook, and discussion 3.1 assessing and evaluation of existing successful ped-approaches. 3.1.1 competence mapping 3.1.2 stakeholder analysis 3.2 characterization of current & expected future state of the specific energy communities 3.3 design of the link-based energy-community with respect to stakeholder needs 3.4 contracting models and regulatory framework 3.5 roadmap for the implementation of the interact energy communities 4 conclusions acknowledgements references https://doi.org/10.14311/app.2022.33.0511 acta polytechnica ctu proceedings 33:511–517, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague effect of cracks on the service life of rc structures exposed to chlorides nicoletta russoa,∗, matteo gastaldia, luca schiavib, alberto strinib, riccardo zanonib, federica lollinia a politecnico di milano, department of chemistry, materials and chemical engineering "giulio natta", piazza leonardo da vinci 32, 20133 milano, italy b consiglio nazionale delle ricerche, istituto per le tecnologie della costruzione, viale lombardia 49, 20098 san giuliano milanese, italy ∗ corresponding author: nicoletta.russo@polimi.it abstract. to move towards a more sustainable concrete, the enhancement of its durability is strongly encouraged and, dealing in particular with reinforced concrete (rc), this mainly means to prevent the damage due to environmental actions, e.g. due to chloride-induced corrosion. therefore, there is the need of models aimed at designing durable structures. usually the service life design models consider concrete in uncracked condition. in real structures, however, several phenomena can generate cracks on concrete surface, leading to an acceleration of the corrosion of steel rebar. a number of studies have been recently carried out in order to evaluate the influence of cracks on reinforced concrete durability in chloride-contaminated environment, however the knowledge of the effect of cracks on the initiation and propagation periods is still lacking. furthermore, few studies have considered additional protection strategies, such as the use of stainless steel rebar. in this work, experimental results are presented concerning the influence of cracks on the service life of reinforced concrete structures in order to evaluate if cracks lead to an earlier corrosion initiation induced by chlorides. prismatic specimens, reinforced with carbon steel and 304l stainless steel bars, were longitudinally cracked and exposed to ponding with 3.5% nacl solution. the monitoring of corrosion behaviour showed that when cracks reached the steel surface corrosion initiated immediately. keywords: corrosion initiation, corrosion propagation, cracked concrete, chlorides, stainless steel. 1. introduction nowadays the need of reducing the environmental impact of the construction industry, to increase its sustainability, is one of the main factors which guides the research and the innovation in the field of construction materials. an improvement in sustainability can be achieved by increasing the durability of rc structures, since the design stage. to design durable structures several models have been developed, that allow to estimate the evolution in time of deterioration processes, and to evaluate the service life of a structure, as function of several parameters related both to material characteristics and to the environmental exposure conditions [1–3]. however, usually these durability models refer mainly to uncracked concrete, a condition that rarely occurs in practice. in real structures cracks on concrete surface can be generated by several phenomena: shrinkage, chemical reactions, mechanical loads, reinforcement corrosion, etc. cracks are expected to inevitably form in rc structures, which in some cases develop as deep as the reinforcement, from the beginning of the life of the structure. the presence of cracks provides a more direct path for the ingress of chloride ions and oxygen into concrete, possibly leading to a reduction of the initiation time of steel corrosion and to an acceleration of the propagation of corrosion [4]. some design codes provide, for structures subjected to aggressive exposure conditions, limitations in terms of crack opening at the concrete surface, in most cases in the order of magnitude of hundreds of micrometers, while studies on the durability of rc structures seem to show that there are effects even for much narrower cracks [5]. what is widely accepted from a structural point of view, therefore, may have significant consequences from the durability point of view, and, more in general, sustainability in the construction industry. the objective of the studies on the subject is to try to correlate the geometrical parameters of the cracks, in terms of crack surface opening and more rarely crack depth, with the consequences that these may have on corrosion initiation and propagation times, and therefore on the service life of structures. relative few studies, however, provide really useful information on the effects of cracks on chlorideinduced corrosion of carbon steel bars and even fewer analyse more corrosion resistant bars such as stainless steel rebar, that can be used to increase the durability of rc structures exposed to chloride-bearing environment. almost all these works refer to cracks formed in transverse direction with respect to the reinforcement and not in longitudinal direction [6]. nevertheless, longitudinal cracks can be present in rc structures 511 https://doi.org/10.14311/app.2022.33.0511 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en n. russo, m. gastaldi, l. schiavi et al. acta polytechnica ctu proceedings steel name mechanical properties main alloy elements [%] us [mpa] ys0.2 % [mpa] agt [%] c cr ni mo mn n cu si s p v 304l 760 543 24 0.021 18.51 8.61 0.15 1.31 0.146 cs 622 524 12.2 0.145 0.12 0.17 0.03 0.77 0.010 0.43 0.18 0.050 0.017 0.002 table 1. mechanical properties (us = ultimate strength; ys0.2% = yield strength at 0.2% of deformation; agt = total elongation at maximum force) and main alloy elements of the stainless steel bars and carbon steel bars. (e.g. in reinforced concrete bridge girders, in reinforced concrete pavements, due to loading or thermal shock, in steel reinforced concrete pipes, parking garage slabs [7]). even structural cracks that tend to occur in transverse direction with respect to the flexural reinforcement, are parallel to the stirrups (shear reinforcement). as far as the authors have been able to ascertain, the only studies that investigated the effect of natural longitudinal cracks (intended as induced by mechanical or thermal loading, as opposed to artificial notches) on the corrosion behaviour of carbon steel and stainless steel bars are [7, 8]. according to poursaee and hansson [7], the presence of longitudinal cracks, about 0.1 mm wide and as deep as the reinforcement, on concretes made with opc (ordinary portland cement) and hpc (high performance concrete), led to a shortened initiation time in comparison to sound concretes. in particular, after 128 weeks of exposure to salt, corrosion occurred on carbon steel bars embedded in opc and hpc cracked concretes, whilst bars were still passive after 132 week of exposure when embedded in sound concretes. in van niejenhuis et al. [8], even with the use of stainless steel reinforcement, the initiation time was significantly reduced in presence of longitudinal cracks, characterized by average crack width between 0.13 mm and 0.53 mm (crack depth was not considered in this study). since the presence of longitudinal cracks seems to strongly affect the durability of a rc structure, there is the need to carry out further studies on this topic. an experimental research has been carried out in order to evaluate the effect of longitudinal cracks on concretes reinforced with carbon steel bars and 304l stainless steel bars. this paper reports the preliminary results obtained on the specimens exposed for about 13 months to ponding with a 3.5% sodium chloride solution. 2. experimental procedure concretes were made with 422 kg/m2 of portlandlimestone cement (plc), type cem ii/a-ll 42,5r (according to en 197-1), 190 kg/m2 of water, w/c equal to 0.45, and 1731 kg/m2 of calcareous aggregates, subdivided into five grain size fractions with a maximum diameter of 9 mm. an acrylic-based superplasticizer was also added to the mix, to increase workability. a s3 consistency class (according to en 206) and an average compressive strength, on cubic specimens after 28 days of moist curing (r.h. > 95%), of 51 mpa were obtained. as reinforcement, 16-mm rebar in carbon steel (cs) and austenitic stainless steel (ss), of grade 304l, were used, mechanical properties and chemical compositions are reported in table 1. the stainless steel bars were subjected to sand blasting and pickling from producers, in order to remove the oxide scale created during hot forming, whilst carbon steel bars were subjected to sand blasting in the laboratory. before testing, after degreasing with acetone, the two ends of each bar were masked with a styrenebutadiene-modified cement mortar and coated with epoxy; a length of the bar of 80 mm was exposed to the concrete. prismatic specimens, with dimensions of 120×90× 50 mm, were reinforced with one steel rebar, with a concrete cover thickness of 15 mm, and equipped with an activated titanium wire (reference electrode) and a titanium wire (counter-electrode) for electrochemical measurements (figure 1a). to account for the variability of the results, ten replicate specimens were made with each type of steel rebar. five specimens for each type of steel were provided with a v-shaped notch, 5-mm depth, in the longitudinal direction on the casting surface, to promote the formation of a longitudinal crack. as a result, for each type of steel, five replicated specimens were tested in uncracked conditions and five in cracked conditions. to obtain load-induced cracks, the day after casting, a sort of tree-point bending test was performed on specimens presenting the longitudinal notch, by means of a universal testing machine. the cracking procedure was not a crack width-controlled test, and, for some specimens, cracks were observed at the end of the exposure to chlorides. for further details, see [9]. all the reinforced specimens were wet cured (r.h. > 95%) for 7 days. after the curing, a pond was fixed on the casting surface and a 3.5% nacl solution was poured inside for approximately 13 months, with specimens kept in air-conditioned laboratory (t ∼ 20◦c). the ponding was kept continuously for the first 6 months, with a monthly solution replacement, and subsequently wet and dry cycles were alternated 512 vol. 33/2022 rc structures exposed to chlorides � � �a�� (b� )ljxuh� ���*hrphwu\� ri� wkh� frqfuhwh� vshflphqv�� glphqvlrqv� lq�pp� �a��� dqg� frqiljxudwlrq� ri� wkh� 5hihuhqfh�hohfwurgh� 6whho�uhedu� &rxqwhu�hohfwurgh �����1d&o� figure 1. geometry of the concrete specimens, dimensions in mm (a), and configuration of the ponding test and, in dotted line, concrete core taken for crack and corrosion observation (b). (2 weeks dry, 1 week wet) to accelerate corrosion initiation, until about 400 days of total exposure. before starting the wet/dry cycles, in order to estimate the chloride content at the rebar surface, powder samples at different depths from the exposed surface, 5 mm spaced and up to a depth of 25 mm, were collected by dry-drilling one of the uncracked specimens, and the chloride profile was measured by analysing the total chloride content (acid soluble). during the whole exposure period, the corrosion of embedded steel bars was monitored by means of electrochemical measurements. half-cell potential (ecorr) versus a saturated calomel electrode (sce) was measured, placing the electrode on the specimen surface in correspondence of the central part of the rebar. corrosion current density (icorr) was measured through linear polarization resistance measurements (rp) as icorr = b rp · a (1) where a was the bar surface exposed to concrete and b was assumed equal to 26 mv. the measurements were periodically carried out at least with weekly frequency, except for the four months corresponding to the sanitary emergency related to covid-19, during which the specimens were left at dry conditions in the laboratory. in two out of five of the cracked specimens reinforced with carbon steel rebar, a reinforced concrete core (45 mm in diameter, parallel to the direction of the steel reinforcement) was drilled out of the specimen soon after corrosion initiation (figure 1b). the core was then sawn, perpendicular to the direction of the steel reinforcement, in different segments. each segment was polished and observed with a stereomicroscope, to investigate the crack pattern and to detect the presence of corrosion attack at the steel/concrete interface. moreover, also the casting surface of the cracked specimens was observed with the stereomicroscope, after six months of exposure, to quantify the crack width. 3. results and discussion the monitoring of corrosion potential (ecorr) and corrosion current density (icorr) of the bars started from the beginning of the exposure to ponding with a chloride solution. figures 2 and 3 show the trend in time of corrosion potential and corrosion current density of carbon steel and 304l stainless steel bars embedded in all replicate uncracked and cracked concrete specimens. in the figures, the black symbols indicate the corrosion current density, whilst the grey symbols indicate the corrosion potential. with regard to the carbon steel bars embedded in uncracked concrete specimens (figure 2a) at the beginning of the exposure to ponding, ecorr around −150 mv vs sce and icorr around 1 ma/m2 for all the bars were measured, suggesting the passivation of the steel bars. during the continuous ponding exposure period, a slight increase of ecorr and a decrease of icorr occurred on all the carbon steel bars embedded in uncracked concretes, reaching values around −30 ÷ −50 mv vs sce for the corrosion potential and 0.7 ÷ 0.8 ma/m2 for the corrosion current density after about 180 days of exposure. these values suggested that passive conditions were maintained in time on all the rebar despite the penetration of chloride ions due to the exposure to ponding. in order to accelerate corrosion initiation, wet/dry cycles with the same solution were started. during the cycles, the electrochemical conditions changed for two out of five rebar. for one bar after the first wet cycle (218 days of exposure) and for the other during the third wet cycle, performed after the covid-19 suspension (384 days). a sharp decrease of ecorr to −250 mv vs sce and increase in icorr, to about 1.5 ma/m2 were experienced, clearly indicating the initiation on corrosion. for the other three cs bars in uncracked specimens, after about 400 day of exposure ecorr was around −100 mv vs sce (up to around −20 mv vs sce soon after the covid-19 suspension) and icorr around 1 ma/m2 suggesting that passive conditions were maintained. as it concerns the cracked concrete specimens reinforced with carbon steel bars (figure 2b), at the beginning of the exposure to ponding the corrosion potential of 4 out of 5 bars was around −150 mv vs 513 n. russo, m. gastaldi, l. schiavi et al. acta polytechnica ctu proceedings �a�� �b� figure 2. evolution in time of the corrosion potential (grey symbols) and corrosion current density (black symbols) of carbon steel bars embedded in the five replicate uncracked (a) and cracked (b) concretes and exposed to ponding with a 3.5% nacl solution; red symbols in (b) corresponding to the interruption of exposure for corroding specimens. �a�� �b� figure 3. evolution in time of the corrosion potential (grey symbols) and corrosion current density (black symbols) of 304l stainless steel rebars embedded in the five replicate uncracked (a) and cracked (b) concretes and exposed to ponding with a 3.5% nacl solution. sce and the corrosion current density was around 1 ma/m2, whilst ecorr of one bar was -250 mv vs sce and icorr was 1.5 ma/m2. few hours after the beginning of ponding, ecorr of this bar dropped to values lower that -300 mv vs sce and icorr increased up to values higher than 2.5 ma/m2. also the corrosion potential of two other bars experienced a drop after few hours from the beginning of the exposure, as well as the corrosion current density increased. the corrosion behaviour observed on these three bars suggested that corrosion initiated immediately. the ponding was then interrupted in these specimens (last measurements in red in figure 2b). in the other two cracked specimens (figure 2b), after the first six months of continuous ponding ecorr resulted around −50 mv vs sce and icorr around 0.8 ma/m2, in accordance with values detected on the uncracked specimens, indicating that passive conditions were maintained despite chloride penetration. both the remaining specimens, however, started corroding during the exposure to wet/dry cycles, one during the second dry cycle (236 days of exposure), the other during the third wet cycle (384 days), experiencing a drop in ecorr, to values around −350 mv vs sce and a rise in icorr, up to 2.7 ma/m2. the corrosion behaviour of stainless steel bars embedded in uncracked and cracked specimens is shown in figure 3a and 3b, respectively. it can be observed that the corrosion potential slightly increased and the corrosion current density slightly decreased during the continuous ponding in all the bars embedded both in uncracked and cracked concrete. the corrosion potential was around −50÷0 mv vs sce after about six months of exposure, whilst the corrosion current den514 vol. 33/2022 rc structures exposed to chlorides �a�� �b� figure 4. average values of the corrosion potential (a) and corrosion current density (b) before and after initiation of corrosion for all the replicate specimens. figure 5. microscopic image of the exposed surface on a cracked specimen after six months of exposure to ponding. sity was of about 0.1 ma/m2 without any significant difference among replicate specimens and in presence of longitudinal cracks. also the exposure to wet/dry cycles did not significantly affect the electrochemical measurements on stainless steel rebar. in spite of the penetration of chlorides in time due to ponding and wet/dry cycles with nacl solution and the expected increase of the amount of the chlorides near the bars, corrosion did not occur on the 304l stainless steel rebar, neither in sound nor in cracked conditions. to summarize, as it concerns sound specimens, no corrosion occurred during the six months of continuous exposure to ponding with a sodium chloride solution neither in specimens reinforced with cs bars nor in those with ss bars. with the starting of wet and dry cycles, two specimens reinforced with cs started corroding, whilst other three maintained passive conditions up to 387 days of exposure. therefore, initiation time (ti) can be considered quite variable, included in the range 218 days and > 387 days. this high variability is typical of chloride induced corrofigure 6. microscopic image of polished cross section from a cracked specimen reinforced with cs bar, after corrosion initiation. sion, since for the same concrete cover thickness, initiation time is related to the critical chloride threshold (clth), which is characterized by a stochastic nature, and to the variability of the apparent diffusion coefficient of chlorides for different specimens. the influence of the latter parameter, however, is of minor extent, since tests performed to evaluate the resistance to chloride penetration on specimens with the same geometry and composition showed a good repeatability of results [9]. as it concerns the clth, in order to have an indication of the expected chloride content at the steel surface after about six months of exposure to continuous ponding and just before the initiation of wet/dry cycles, powder samples were collected from one of the uncracked specimens. from titration of powder samples, the estimation of the chloride content at the bars depth, resulted 1.2% and 0.08% vs mass of cement, in correspondence of the top and bot515 n. russo, m. gastaldi, l. schiavi et al. acta polytechnica ctu proceedings tom side, respectively. according to [3], for carbon steel bars, clth is included in the range 0.2 − 2% by mass of cement, and in fact, one cs rebar started corroding soon after the evaluation of chloride content, another one after two further wet/dry cycles. on the other hand, clth of 304l stainless steel, according to the literature, is higher than 3.5% by mass of cement [10], therefore it is reasonable to consider that the amount of chlorides at rebar depth is still well below clth even after about 400 days of exposure. for cracked specimens, initiation time is influenced also by the presence of the crack, a discontinuity in the concrete cover that promotes the penetration of chlorides towards the rebar. the geometrical parameters of the crack, therefore, should be considered as further variables affecting ti. in this study, in cracked specimens reinforced with cs bars, corrosion initiated as soon as the ponding started in three out of five specimens, while corrosion initiated after months of exposure to chlorides (236 < ti < 384 days) in the other two replicate specimens. this difference in behaviour could be attributed to differences in the obtained crack in different specimens, therefore an investigation on crack geometrical parameters, in terms of crack width and crack depth, was performed. crack width was observed on the exposed surface and after 6 months of exposure on the two cracked specimens still under testing. a clear detection of the crack along the whole specimens was difficult, maybe due to the presence of crystallized salt on the surface, or to self-healing phenomena that might have occurred during the ponding exposure. where the crack was clearly visible, a width around 20 µm was measured (figure 5). it is worth to mention however that, even if in most literature studies only crack width is considered (typically during the cracking procedure, by means of linear variable displacement transducers, lvdts), this could be misleading. in previous studies, in fact, a clear correlation between crack width and crack depth was not clearly defined and variation of cracks parameters along the specimens was detected [9]. therefore, also crack depth was investigated in this study, through destructive tests on the three specimens that started corroding immediately (as described in paragraph 2). figure 6 shows, as an example, a wide field image obtained combining three different microscope images at 6× of magnification. it can be observed that the crack was characterized by a depth higher than the concrete cover thickness. in several observed sections the crack was at least equal to the concrete cover. a crack width in correspondence of the exposed surface of the order of 15-20 µm was measured on these specimens (comparable with values measured on cracked specimens where corrosion had not initiated immediately). in the section where the crack arrived until rebar depth, corrosion was detected. in cs specimens where the crack developed until the rebar, corrosion started almost immediately after the exposure to a chloride solution, therefore ti was almost equal to zero. on the other hand, in the cracked specimens where corrosion initiated months after the exposure to chlorides, it may be supposed that the crack did not develop until rebar depth. in this case, the results obtained seem to indicate that ti was statistically reduced by the presence of the crack, since up to 387 days of exposure, both cracked samples showed corrosion initiation, while in the uncracked conditions, corrosion initiation started only on two out of five samples). however, the number of samples tested was limited, and therefore further investigation of this aspect is required. moreover, it is reasonable to assume that also in the case of cracked specimens reinforced with ss rebar crack developed until rebar depth in some of the specimens. from results of this study, however, ss rebar remained in passive conditions, independently from crack parameters, for about 400 days after the exposure to chlorides. on uncracked specimens and cracked specimens reinforced with cs that experienced corrosion during the wet and dry cycles with nacl, the exposure was not interrupted as soon as corrosion initiated, in order to investigate the propagation period. figure 4a and 4b, show, respectively, the average values of corrosion potential and corrosion current density, before and after initiation time. it can be noted that no particular differences in corrosion potential were detected, for cs in uncracked and cracked configurations, both before and after corrosion initiation. on the other hand, corrosion current density resulted affected by the presence of the crack in the case the crack developed until rebar depth, being almost doubled with respect to sound and slightly cracked configurations. it must however be noted that chloride induced corrosion is characterized by localized attacks (pitting), and that the linear polarization resistance technique gives a corrosion rate averaged on the whole steel surface. a higher corrosion current density, therefore, may be representative of a wider surface of steel affected by corrosion, but not the most severe condition in terms of loss of resistant cross section in time. further investigation will be required on carbon steel bars to have better statistical results related to initiation time in the different configurations, and destructive test will be performed some months after corrosion initiation for both the steel types in cracked configuration, to observe the crack and the morphology of corrosive attacks (also in uncracked samples to evaluate the corrosion propagation). 4. conclusions corrosion tests were carried out on concrete specimens reinforced with carbon steel and 304l stainless steel bars, in sound and longitudinally cracked configurations, with crack width at the exposed surface 516 vol. 33/2022 rc structures exposed to chlorides around 20 µm. specimens were exposed to a 3.5% nacl solution, by means of continuous ponding for the first six months and subsequently to wet and dry cycles up to about 400 days of exposure. in uncracked configuration, only two out of five specimens reinforced with carbon steel bars started corroding, with variable initiation time (>218 days) and the variability was reasonably attributed to the stochastic nature of pitting corrosion initiation. in cracked configuration, all the carbon steel rebar started corroding, showing two different behaviours. for three specimens out of five, where the crack was observed to develop until rebar depth, corrosion started immediately after the exposure to ponding (corrosion initiation time equal to zero). a higher initiation time (236 < ti < 384 days) was detected for the other two specimens, for which it was reasonably supposed that the crack did not develop until rebar depth. thus, corrosion initiation seemed to be statistically shortened with respect to sound configuration, likely due to the promotion of chloride penetration into the crack. nevertheless, the average values of corrosion potential and corrosion current density before and after corrosion initiation were comparable, suggesting that cracks did not affect corrosion process. finally, stainless steel bars in uncracked and cracked specimens were still in passive conditions after about 400 days of exposure, due to the higher critical chloride threshold. therefore, stainless steel resulted to have a high resistance to corrosion also in cracked configuration, even in the possible case in which crack developed until rebar depth. references [1] fib.durable concrete structures bulletin no. 183, 1992. [2] duracrete. probabilistic performance based durability design of concrete structures. final technical report of duracrete project, 2000. https://cordis.europa.eu/project/id/brpr950132. [3] fib. model code for service life design bulletin no. 34, 2016. [4] a.c. boschmann käthler, u. m. angst, m. wagner, et al.. effect of cracks on chloride induced corrosion of steel in concrete a review. npra reports norwegian public roads administration, 2017. [5] i.-s. yoon, e. schlangen. experimental examination on chloride penetration through micro-crack in concrete. ksce journal of civil engineering 18(1):188-98, 2013. https://doi.org/10.1007/s12205-014-0196-9. [6] f. u. a. shaikh. effect of cracking on corrosion of steel in concrete. international journal of concrete structures and materials 12(1), 2018. https://doi.org/10.1186/s40069-018-0234-y. [7] a. poursaee, c. m. hansson. the influence of longitudinal cracks on the corrosion protection afforded reinforcing steel in high performance concrete. cement and concrete research 38(8-9):1098-105, 2008. https: //doi.org/10.1016/j.cemconres.2008.03.018. [8] c. b. van niejenhuis, s. walbridge, c. m. hansson. the performance of austenitic and duplex stainless steels in cracked concrete exposed to concentrated chloride brine. journal of materials science 51(1):362-74, 2015. https://doi.org/10.1007/s10853-015-9387-0. [9] n. russo, m. gastaldi, p. marras, et al. effects of load-induced micro-cracks on chloride penetration resistance in different types of concrete. materials and structures 53(6), 2020. https://doi.org/10.1617/s11527-020-01580-y. [10] f. lollini, m. carsana, m. gastaldi, and e. redaelli. corrosion behaviour of stainless steel reinforcement in concrete. corrosion reviews 37(1):3-19, 2019. https://doi.org/10.1515/corrrev-2017-0088. 517 https://cordis.europa.eu/project/id/brpr950132 https://doi.org/10.1007/s12205-014-0196-9 https://doi.org/10.1186/s40069-018-0234-y https://doi.org/10.1016/j.cemconres.2008.03.018 https://doi.org/10.1007/s10853-015-9387-0 https://doi.org/10.1617/s11527-020-01580-y https://doi.org/10.1515/corrrev-2017-0088 https://doi.org/10.14311/app.2022.33.0329 acta polytechnica ctu proceedings 33:329–336, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague correlation of porosity and properties of recycled fine aggregate concrete with fly ash hangwei lina,∗, koji takasub, hidehiro koyamadac, hiroki suyamac a the university of kitakyushu, faculty of environmental engineering, department of architecture, 1-1 hibikino, wakamatsu, kitakyushu city, fukuoka, 808-0135 japan b the university of kitakyushu, faculty of environmental engineering, 1-1 hibikino, wakamatsu, kitakyushu city, fukuoka, 808-0135 japan c the university of kitakyushu, department of architecture, faculty of environmental engineering, 1-1 hibikino, wakamatsu, kitakyushu city, fukuoka, 808-0135 japan ∗ corresponding author: 615299569@qq.com abstract. in this paper, we presented the experimental discussion of samples of recycled aggregate concrete with replacement of natural fine aggregate by recycled fine aggregate. three mix kinds were produced and, for each of these three kinds, two levels of water to blinder ratio were used with 0%, 10% and 20% of fa. the result of the tests of drying shrinkage and compressive strength of recycled concrete were used for comparison with tests of mercury intrusion porosimetry (mip), in which the cumulative pore volume and different intervals of pore volume were studied at ages of 28 and 91 days. correlation graph of compressive strength and cumulative pore volume might be predicted with given w/b, days and fa of the concrete or none given and drying shrinkage might be predicted with given fa. keywords: fly ash, drying shrinkage, porosity, recycled aggregate concrete. 1. introduction at present, the decrease in strength and increase in drying shrinkage are major problems for recycled aggregate concrete, and it was considered that managing these will lead to an increase in the use of recycled aggregate concrete (rac) to building structures. many studies have shown that using recycled aggregates in concrete suffered from durability problems. some people reported that, compared with the reference concrete, the shrinkage of recycled aggregate concrete was increased by up to 60% [1]. in past research, the replacement rate for replacing natural aggregate with recycled aggregate (ra) and replacing cement with fly ash (fa) were considered roughly or simply. in this research, i focused on the compressive strength and drying shrinkage of concrete both with fine recycled aggregate and fly ash. then i made correlation graphs between porosity and properties of the concretes. then the correlation process was carried out on the properties obtained from the mercury intrusion porosimetry (mip) tests, and shrinkage and compressive strength. finally carrying out the graphs that gave rise to the correlations of the shrinkage and compressive strength and the porosity obtained in the mercury intrusion porosimetry mip tests. 1.1. experimental outlines table 1 showed the physical properties of the materials used in this experiment. as cement, ordinary portland cement was used, and sea sand and recycled fine aggregate as the fine aggregate (m standard of jis a 5022) and crushed stone as coarse aggregate were used. table 3 showed the mix proportions. in the mix proportions, the unit water amount was 180kg/m3, the unit cement amount was 327kg/m3, the water to binder ratio was 55% and 44%, and the replacement rate of the regenerated fine aggregate was 2 levels of 50, 100%. in this experiment, recycled aggregate was used only for fine aggregate, and natural aggregate crushed stone was used for coarse aggregate in all mix proportions. regarding the compounding symbols, n was the mix proportions using natural aggregate for both the coarse aggregate and the fine aggregate. the replacement rate of the recycled fine aggregate was indicated in this order, m50 was a mix proportions using 50% recycled fine aggregate class m and 50% sea sand. the replacement rate of the fly ash was indicated in this order, n-fa10 was a mix proportions replacing cement with 10% fly ash. the compressive strength test was carried out according to jis a 1108. the sample for the mip rests were extracted from the center of the cylinder (! = 0.05 m × h = 0.1 m) which were dried by putting them in curing room for 28 and 91 days. the cylinder was then cut with a vise to obtain particles from 2 mm to 5 mm in diameter. the samples were dehydrated by submerging them in acetone for 2 h. after this stage, the samples were dried by putting them in an oven at 105 ◦c for 1 h. finally, the samples were put into a vacuum pump for 48 h. the mip tests were done on autopore v which subjected the samples to maximum pressure of 329 https://doi.org/10.14311/app.2022.33.0329 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en h. lin, k. takasu, h. koyamada, h. suyama acta polytechnica ctu proceedings property coarse aggregate fine aggregate fine recycled aggregate contents cement fa density (g/cm3) 2.69 2.59 2.23 loss of ignition (%) 2.40 2.1 fineness modulus 6.9 2.58 4.3 density (g/cm3) 3.16 2.2 water absorption (%) 1.14 1.04 7.38 specific surface area (cm2/g) 3310 3270 table 1. materials. w/b (%): 55 n m50 m100 n-fa10 m50-fa10 m100-fa10 n-fa20 m50-fa20 m100-fa20 fa 0 0 0 45 45 45 90 90 90 s 731 350 0 723 346 0 716 342 0 rfa 0 350 672 0 346 665 0 342 658 w/b (%): 40 n m50 m100 n-fa10 m50-fa10 m100-fa10 n-fa20 m50-fa20 m100-fa20 fa 0 0 0 33 33 33 65 65 65 s 832 398 0 826 396 0 820 392 0 rfa 0 398 765 0 396 759 0 392 754 g 945 w 180 table 2. unit mass of mix proportions. up to 60,000 pisa. 2. experimental results and discussion 2.1. compressive strength 2.1.1. influence of fine ra on the compressive strength figure 1 showed the compressive strength at 28 and 91 days for concrete mixes with various levels of fine ra. for 28 days, it showed that incorporating fine ra in concrete mixes decreased the compressive strength in fa10% w/c 0.40 series and fa 20% w/c0.55 series. but other four series showed that incorporating fine ra in concrete mixes almost had no decreased the compressive strength. for 91 days, it showed that incorporating of fine ra in concrete mixes decreased the compressive strength in w/b-0.40 fa-10% series and w/b-0.55 fa-20% series. and the incorporating of fine ra in concrete mixes made no effect on the compressive strength in w/b-0.40 fa-0% series and w/b-0.55 fa-10% series. this might be explained by two main factors, namely the rca being more porous and weaker than the na [2] and/or due to the increase of the effective w/c ratio in rca concrete mixes to maintain the workability of an equivalent conventional concrete [3]. 2.1.2. influence of fa on the compressive strength figure 2 showed the compressive strength at 28 and 91days for concrete mixes with various levels of fa. for 28 days, it showed that the incorporation of fa in concrete mixes was detrimental in terms of compressive strength, but slightly decreased. for 91 days, it showed that the incorporating of fa in concrete mixes made no effect on increasing the compressive strength on w/b-0.40 ra-100% series, w/b-0.55 ra-100% series and w/b-0.55 ra-0% series. generally, the incorporation of a low volume of fa filled the voids between cement particles, thus decreasing the voids, and slightly increased the fresh density. this was likely to happen because fa particles are finer than cements, so that fa not only can improve the particle size distribution, but also filled the micro pores in the interfacial transition zones around the aggregates [4]. 2.2. drying shrinkage 2.2.1. influence of fine ra on the drying shrinkage figure 3 showed the drying shrinkage at 182 days for concrete mixes with various levels of fine ra. each presented value was the average of three measurements. the results showed that incorporating fine 330 vol. 33/2022 porosity and properties of fly ash concrete figure 1. effect of fine ra on the compressive strength. figure 2. effect of fa on the compressive strength. ra in concrete mixes increased the drying shrinkage of the concretes in w/c-0.40 fa-0% series and w/c-0.40 fa-10% series. on w/c-0.40 fa-0% series, 50% fine ra replacement ratio made more effect on increasing concrete’ durability than 100% fine ra replacement ratio. this was probably the result of the combination of half recycled aggregate and half normal aggregate. the mortar adhered to the recycled aggregate contributed to an increase in the volume of the paste (old + new), thus increasing the drying shrinkage of the resulting concrete [5]. 2.2.2. influence of fa on the drying shrinkage figure 4 showed the drying shrinkage at 182 days for concrete mixes with various levels of fa. the results showed that incorporating fa in concrete mixes decreased the drying shrinkage in w/c-0.40 ra-0% series and w/c-0.40 ra-50% series. both in two series, 10% fa replacement ratio made more effect on decreasing concrete’ durability than 20% fa replacement ratio. this was probably the result of high fa replacement ratio. according to atis et al. [6], the refigure 3. effect of fine ra on the drying shrinkage. figure 4. effect of fa on the drying shrinkage. duction in the drying shrinkage was attributed to the dilution effect of the fly ash particles. on w/c-0.40 ra-100% series, 10% fa replacement ratio would decrease concrete’s durability. this was same as other two series. when 20% fa was replaced, the fa made no effect on decreasing the drying shrinkage of the concrete. it might the that fly ash and recycled aggregates will inhibit each other when they both in high replacement ratio. figure 6 had water to binder ratio 0.40. at 28 days, only 100% fine ra replacement ratio cause the increase of the pore volume when pore diameter was in 0.003 ∼ 0.03 µm in 0% fa replacement ratio series. in 10% fa replacement ratio series, 50% fine ra replacement ratio had the biggest influence on pore volume in 0.003 ∼ 100 µm pore diameter interval. at 91 days, the results showed that the cumulative pore volume increased with the increase of the fine ra replacement ratio when pore diameter was in 0.003 ∼ 100 µm except 0% fa replacement ratio series. 2.2.3. correlation of porosity and properties the pore diameter was divided into many different intervals by its different changed points of the cumulative curve. in figure 7, for example, each line 331 h. lin, k. takasu, h. koyamada, h. suyama acta polytechnica ctu proceedings figure 5. influence of fine ra replacement ratio on cumulative pore volume(w/b0.55). figure 6. influence of fine ra replacement ratio on cumulative pore volume(w/b0.40). has two changed points which were named after point a and b. the compressive strength has relationship with pore diameter interval which left endpoint was after point a. the drying shrinkage has relationship with pore diameter interval which right endpoint was before point b. 2.3. porosity 2.3.1. result of porosity the tests were carried out at 28 and 90 days. figure 5 showed the influence of recycled aggregate replacement ratio on cumulative pore volume with water to binder ratio 0.55. at 28 days, the results showed that the pore volume, when pore diameter was in 0.003 ∼ 100 µm, increased with the increase of the fine ra replacement ratio. at 91 days, the results showed that the pore volume, when pore diameter was in 0.003 ∼ 100 µm, increased with the increase of the fine ra replacement ratio, except 0% fa replacement ratio series. 50% fine ra replacement ratio made more effect on increase of pore volume when pore diameter was in 0.2 ∼ 100 µm in fa 0% replacement ratio series. table ?? showed correlation coefficients (r2) when using different interval division methods (w/b, days and fa) and after the use of different interval division methods, it seemed that w/b, days and fa had no influence on correlations of the compressive strength and pore volume. and the best interval’s endpoint for the pore diameter were 0.05 µm and 100 µm. figure 8 showed the graph that gave rise to the correlations of the compressive strength and the 0.05 ∼ 100 µm pore volume obtained in the mip tests (28 and 91days). y = −3.94x + 54.60, r2 = 0.45 (1) • y: compressive strength (mpa). • x: 0.05 ∼ 100 µm pore volume (×10−2 cc/cc) y = −2.48x + 57.28, r2 = 0.32 (2) • y: compressive strength (mpa) • x: cumulative pore volume (×10−2 cc/cc) the correlation process was carried out on the drying shrinkage of the recycled concrete (91 days). the correlation between porosity and drying shrinkage of the concretes can be divided two degrees: recycled concrete without fly ash and with fly ash replacement. 332 vol. 33/2022 porosity and properties of fly ash concrete figure 7. ’changed points’ of the cumulative curve. figure 8. coefficients of determination (r2) of compressive strength and cumulative pore volume at interval division: (a) a−b µm; (b) a−36 µm; (c) 0.02 − 0.2 µm; (d) 0.02 − 36 µm. 333 h. lin, k. takasu, h. koyamada, h. suyama acta polytechnica ctu proceedings pore volume w/b, days, fa 0.55, 28, 0% 0.55, 91, 0% 0.55, 28, 10% 0.55, 91, 10% 0.55, 28, 20% 0.55, 91, 20% a ∼ b µm r2 − 0.08 0.87 0.07 0.99 0.91 w/b, days, fa 0.40, 28, 0% 0.40, 91, 0% 0.40, 28, 10% 0.40, 91, 10% 0.40, 28, 20% 0.40, 91, 20% r2 0.99 0.58 0.66 0.54 0.01 0.64 w/b, fa 0.55, 0% 0.55, 10% 0.55, 20% 0.40, 0% 0.40, 10% 0.40, 20% r2 0.53 0.31 0.29 0.00 0.49 0.02 a ∼ 2 µm r2 0.10 0.25 0.26 0.00 0.08 0.04 a ∼ 100 µm r2 0.12 0.22 0.15 0.05 0.09 0.03 table 3. correlation coefficients (r2). y = 52.38x + 371.79, r2 = 0.89 (3) • y: drying shrinkage (µ) (with fa) • x: 0.003 ∼ 0.1 µm pore volume (×10−2 cc/cc) y = 48.62x + 257.28, r2 = 0.71 (4) • y: drying shrinkage (µ) (with fa) • x: cumulative pore volume (×10−2 cc/cc) the researchers studied the porosity and pore structure of cement paste. z.w. wu thought pores have four intervals: < 20nm which is named after harmless pore, 20-50nm which is named after lessharm pore, 50-200nm which is named after harmful pore, > 200nm which named after much-harm pore [7]. capillary pores with a diameter greater than 0.011 µmaffect the strength, and gel pores with a diameter of fewer than 0.01 µm affect the durability of mortar [8]. consequently, the correlation between the properties of the rac and the pore volume can be evaluated by dividing various pore intervals. wu’s theory was used for comparison; and the left and right points of the pore diameter intervals were defined as 0.02 and 0.2 µm, respectively. simple linear regression also was performed to evaluate the correlation between the results of mip and the results of compressive strength or drying shrinkage tests under four interval division methods according to the object is strength or dry shrinkage. figure 8 shows the coefficients of determination between the pore volume and compressive strength at the following interval divisions: a−b µm, a−36 µm, 0.02 − 0.2 µm, 0.02 − 36 µm. the pore size range of the mip test was 0.003 − 36 µm. the symbols "all", "fa", "0.55fa" and "0.40fa" shown in figure 8 indicate that the fitted line used the data of the compressive strength and cumulative pore volume of all mixes, mixes with fa, mixes with fa at w/b 0.55 and mixes with fa at w/b 0.40, respectively. for rfa concrete incorporating fa, there was no interval with a high correlation coefficient between the compressive strength and the cumulative pore volume because the correlation coefficients of the four types of intervals ranged between 0 and 0.6 (figure 8). in other words, there was no significant correlation between cumulative pore volume and compressive strength. moreover, simple linear regression cannot use cumulative pore volume in a specific interval to determine or predict the strength of recycled concrete. some researchers use quadratic or exponential regression equations to predict concrete strength. concrete with fa should be considered separately when considering the correlation between compressive strength and cumulative pore volume because the correlation coefficients of the "fa" lines are higher than that of the "all" line for the four intervals. figure 9 shows the coefficients of determination between the cumulative pore volume and drying shrinkage at interval divisions: a−b, a − 36 µm, 0.02 − 0.2 µm, 0.02 − 36 µm. the symbols "all" and "fa" shown in figure 9 indicate that the fitted line used the data of the compressive strength and cumulative pore volume of all mixes and mixes with fa, respectively. concrete with fa should be considered separately when investigating the correlation between drying shrinkage and cumulative pore volume because there are no liner fits with high correlation coefficients in figure 9 (a) and (c), whereas the "fa" line shows high correlation coefficients in figure 9 (b) and (d). the drying shrinkage of the concrete incorporating fa is related to the cumulative pore volume of micropores due to the pore diameter of interval divisions shown in figure 9 (a)and (c), both of which start from 0.003 µm. in other words, there is a correlation between cumulative pore volume and drying shrinkage due to the incorporation of fa in concrete. this can be explained by the fact that fa can reduce both the pore size of capillary pores and gel pores in concrete and because the shrinkage rate depends on the amount of water lost in the mesopore, in which the resistance to water loss was determined by the size of the macropore [9]. hence, the drying shrinkage of concrete with fa is relatively correlated with the pore volume of micropores and mesopores. 0.003 − b µm is the optimal interval between the drying shrinkage and the cumulative pore volume of concrete incorporating fa because the "fa" line in figure 9 (b) has the highest correlation coefficient of 334 vol. 33/2022 porosity and properties of fly ash concrete figure 9. coefficients of determination (r2) of drying shrinkage and cumulative pore volume at different interval divisions: (a) a−b µm; (b) 0 − 0.03 µm; (c) 0.02 − 0.2 µm; (d) 0.03 − 0.2 µm. 0.82. 3. conclusions both fly ash and recycled aggregates in concrete mixes were detrimental but at moderate rates; using fa in rac production was advisable, especially for percentages of fa exceeding the standard limit, since some of the particles of fa work as a pozzolanic binder ratio instead of a filler. cumulative pore volume increases with an increase of rfa replacement ratio and decreases with a decrease of the w/b ratio. concrete incorporating fa should be considered separately when investigating the correlation between cumulative pore volume and compressive strength or drying shrinkage. there is no linear correlation between compressive strength and cumulative pore volume in all interval divisions. there is a linear correlation between cumulative pore volume and drying shrinkage result from the incorporation of fa in concrete and the optimal interval division for investigating the correlation between drying shrinkage and the cumulative pore volume of concrete incorporating fa is 0.003 − b µm. the smallest pore diameter that can be measured by the mip test is 0.003 − b µm and point b is the right change point of pore diameter in a plot of the cumulative pore volume. references [1] t. c. hansen, e. boegh. elasticity and drying shrinkage of recycled concrete aggregate. journal proceedings 82(5):648-52, 1985. [2] s.-c. kou, c.-s. poon, h.-w. wan. properties of concrete prepared with low-grade recycled aggregates. construction and building materials 36:881-9, 2012. https: //doi.org/10.1016/j.conbuildmat.2012.06.060. [3] r. kurad, j. d. silvestre, j. de brito, et al. effect of incorporation of high volume of recycled concrete aggregates and fly ash on the strength and global warming potential of concrete. journal of cleaner production 166:485-502, 2017. https://doi.org/10.1016/j.jclepro.2017.07.236. [4] k. kim, m. shin, s. cha. combined effects of recycled aggregate and fly ash towards concrete sustainability. construction and building materials 335 https://doi.org/10.1016/j.conbuildmat.2012.06.060 https://doi.org/10.1016/j.jclepro.2017.07.236 h. lin, k. takasu, h. koyamada, h. suyama acta polytechnica ctu proceedings 48:499-507, 2013. https: //doi.org/10.1016/j.conbuildmat.2013.07.014. [5] m. tavakoli, p. soroushian. drying shrinkage behavior of recycled aggregate concrete. concrete international 18(11):58-61, 1996. [6] c. d. atiş, a. kiliç, u. k. sevim. strength and shrinkage properties of mortar containing a nonstandard high-calcium fly ash. cement and concrete research 34(1):99-102, 2004. https://doi.org/10.1016/s0008-8846(03)00247-3. [7] z. w. wu, h. z. lian. high performance concrete. beijing: china railway publication house, p. 5-13, 1999. [8] h.-l. ho, r. huang, w.-t. lin, et al. pore-structures and durability of concrete containing pre-coated fine recycled mixed aggregates using pozzolan and polyvinyl alcohol materials. construction and building materials 160:278-92, 2018. https: //doi.org/10.1016/j.conbuildmat.2017.11.063. [9] f. collins, j. g. sanjayan. effect of pore size distribution on drying shrinking of alkali-activated slag concrete. cement and concrete research 30(9):1401-6, 2000. https://doi.org/10.1016/s0008-8846(00)00327-6. 336 https://doi.org/10.1016/j.conbuildmat.2013.07.014 https://doi.org/10.1016/s0008-8846(03)00247-3 https://doi.org/10.1016/j.conbuildmat.2017.11.063 https://doi.org/10.1016/s0008-8846(00)00327-6 https://doi.org/10.14311/app.2022.33.0322 acta polytechnica ctu proceedings 33:322–328, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague enhanced service life of existing infrastructure buildings by use of concrete screw anchors as post-installed shear reinforcement johannes lechnera,∗, jürgen feixb a prof. feix ingenieure gmbh, nymphenburger straße 5, 80335 munich, germany b university of innsbruck, unit for concrete structures and bridge design, technikerstraße 13, 6020 innsbruck, austria ∗ corresponding author: johannes.lechner@feix-ing.de abstract. in the last decades, traffic on railways and roads has increased significantly in central europe. on the other hand, the existing infrastructure mainly origins from the late 1950s to the 1970s. this applies also for the existing bridge and tunnel infrastructure which is now up to 60 years old. due to this age and new standards in the current codes, quite often the amount of shear reinforcement does not fulfil the current design code rules. as a replacement of existing structures is often highly unsustainable, methods to improve the shear strength are needed. therefore, a new system for postinstalled shear reinforcement by the use of concrete screws was developed at the university of innsbruck in the last years. the idea is to use concrete screw anchors already known as anchoring element as post-installed reinforcement. in over 60 shear tests with girders and slabs the excellent applicability of concrete screw anchors as post-installed shear reinforcement has been proven. also shear tests with cyclic loading were performed prove the use for dynamically loaded structures, such as bridges. based on these test results a design approach was determined to meet the eurocode-2 assessment standards.several pilot-projects with concrete screws have been realised in austria and germany. for example a rail-road-bridge over a german highway with a remaining service life of zero years was strengthened to a new service life of over 20 years. also a city-centre tunnel in munich which was erected below a historic building is currently strengthened under ongoing traffic in the tunnel using concrete screw anchors as post-installed reinforcement. keywords: bridge strengthening, concrete screw anchors, shear strengthening. 1. introduction most of the existing infrastructure in central europe was erected between the late 1960s and the early 1980s. according to [1] nearly 60 % of the german bridges date from the decades between 1960 and 1990 and therefore are now between 30 and 60 years old. this applies not only to germany but also to nearly all countries of central europe, where most of the railroad and road infrastructure was built after world war ii. since the erection of these structures the basis for the structural design has changed significantly. in particular the design against shear failure in the european standards has become more restrictive in the last decades, as different publications [2, 3] show. several different investigations [4–6] show the economic impact on the reliability of the transportation network of the developed countries. while factors such as in-time production are becoming more and more important, the reliability of the transport infrastructure is decreasing with growing age. at the same time, evasion traffic and congested traffic routes represent a significant burden on the environment. for this reason, disruption of traffic for maintenance measures or the replacement of structures should be avoided or kept as short as possible. a significant contribution to the avoidance of such closures can be made by keeping existing infrastructure in service. therefore, in recent years a lot of research has been put into developing new and more realistic design models at the shear ultimate limit state, such as detailed in references [7–9]. on the other hand, there is also a need for new, post-installed strengthening systems to keep existing infrastructure in service. hence, at the university of innsbruck a new strengthening system was developed with special attention paid to fast installation and robust load bearing characteristics. 2. concrete screw anchors 2.1. general the new idea is to use concrete screw anchors with nominal diameters of d0 = 16 mm and d0 = 22 mm as post-installed reinforcement for concrete structures. these easily installable anchors can fulfil the requirements of an efficient strengthening system, such as installation during ongoing use, installation from only one side of the structure and robust load bearing characteristics. 322 https://doi.org/10.14311/app.2022.33.0322 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 33/2022 enhanced service life of existing infrastructure buildings figure 1. concrete screw anchor with anchoring thread which creates a form-locked connection and the reanchoring at the concrete surface of the structure. figure 2. test setup and measuring systems of the shear tests with concrete screw anchors at the university of innsbruck. to prove the suitability of this new idea, 67 shear tests in total with screw anchors as post-installed shear reinforcement have been performed at the university of innsbruck’s unit of concrete structures and bridge design. 2.2. concrete screw anchors concrete screw anchors are known as anchoring elements with easy installation in concrete structures and are more frequently used nowadays. with growing demand, anchors with diameters of 16 mm and 22 mm were developed to anchor higher loads and to achieve a wider field of application such as the anchoring of poles of noise barrier walls on bridges. therefore these screws are installed with an additional adhesive to obtain higher anchor loads and stiffer anchor behaviour. these screws have a technical approval by the german railroad authorities for the use under dynamic loading up to 5 mio. load-cycles, for example for the use as anchor for noise-barrier walls near railroad traffic. concrete screw anchors are installed into predrilled holes with a defined diameter. figure 1 shows a screw with a nominal diameter of d0 = 22 mm. it can be seen that the first part of the screw is provided with a special thread which is larger in diameter than the nominal diameter of the drilled hole. therefore, during installation this thread cuts itself into the concrete surface of the borehole which can be made by hammer-drilling or core-drilling. a mechanical load transfer is thus generated on basis of undercut. this develops a very robust and resistant load-carrying action compensating for irregularities during installation such as insufficiently uncleaned drill holes. the type of screw shown in figure 1 can carry loads of up to 200 kn using a glued installation procedure and concrete with a mean compressive strength of fcm.cube ≥ 40 n/mm2. for this case the breaking strength of the steel is reached when a pull-out test with close supports is performed (see [10]). the same tests showed failure loads of up to 120 kn without the use of adhesive. 2.3. concrete screw anchors for the use as post-installed reinforcement for the use as post-installed shear reinforcement the screws have been slightly modified. also re-anchoring elements have been developed as can be seen in figure 1. these elements re-anchor the acting axial forces in the concrete screw on the surface of the structure with a large washer, a wedge-locking washer to avoid loosening of the standardised nut during cyclic loading. as the concrete screw anchor is equipped with a standardised thread (e.g. m24 in figure 1) the re-anchoring can be adjusted to the necessary length and a prestressing force can be applied easily by turning the nut. all of these screws are available with a special zincflake coating which guarantees a corrosion resistance of class c5 high according to din en iso 12944-6. for example, this is also the class demanded by the 323 johannes lechner, jürgen feix acta polytechnica ctu proceedings � figure 3. test results of concrete test girders with heights of 320 mm and 440 mm and with concrete screws of different diameters d0 (22 mm and 16 mm) and different installation depth ds. german codes for the use of steel elements affected by de-icing salt spray on bridges. concrete screw anchors as post installed shear reinforcement have many advantages compared to other shear reinforcement systems such as the easy installation from the underside of the bridge without disruption of the ongoing use of the bridge and the mechanical load transfer. 3. experimental research at the university of innsbruck 3.1. first tests at the university of innsbruck to show the usability and enhancements of the new method, several test series with in total 67 single tests at concrete girders and slabs have been performed. between 2013 and 2016 32 shear tests in 3 test series at concrete beams have been performed. the tests were made as four-point-bending tests as can be seen in figure 2. these tests were equipped with concrete screw anchors of different diameters d0 (22 mm and 16 mm) and carried out in different geometrical arrangements using glued and non-glued installation to proof the applicability of the new system. the method was experimentally analysed with concrete beams of 3.5 m in length. in several different variations of screw diameters and installation geometry all tests resulted in a remarkable increase in shear load capacity of up to 120 % using glued installation. also in un-glued installation a remarkably load increase was determined. in the third test series the influence of the installation depth ds was investigated. the results showed an influence on the maximum load when the anchoring end of the screws was located at the upper side of the upper flexural reinforcement and the end of the screws was located under the upper flexural reinforcement respectively. more detailed information on the relevant test results can be found in [11], a summary of the basic outcome can be found in [12]. three tests were made with dynamic loads of 1.25 million and 5 million load cycles between 70 and 140 kn with 2 hz. this means a dynamic load of between 1/3 and 2/3 of the failure load of the statically loaded comparative test. in comparison, all three tests showed no failure during the dynamic loading and even higher failure loads at static loading afterwards compared to the comparative tests. the results showed that this system achieves a significant increase in shear strength of concrete structures under dynamic load conditions such as bridges in a very robust and economically efficient way (see [12–14]). 3.2. tests for the technical approval an application for a national technical approval by deutsches institut für bautechnik dibt for the new system was made in 2018. therefore, additional tests were performed in fall 2018 to achieve more detailed results for special parameters such as the installation depth ds. tests on girders with heights of 320 mm and 440 mm were made at the laboratory of the university of innsbruck with concrete screws of d0 = 22 mm and d0 = 16 mm in diameter. furthermore, tests with concrete slabs with a height of 320 mm and a width of 880 mm were performed at the university of the german armed forces in munich with a hydraulic testing machine generating testing forces up to 10 mn. to achieve information about the deviation of the failure loads, up to four tests for every configuration were carried out. figure 3 shows the test results of the beam tests with a height of 320 mm compared to the tests with a height of 440 mm. the span of all beams was 2.5 m, which means a variation of the shear span ratio a/d. it can be seen that at the beams with a height of 320 mm no influence of the screw diameter can be identified when using the same number of screws. the maximum shear force differs from 157 kn to 153 kn. this means a load increase of 88 % and 83 % compared to reference tests respectively. the tests with a beam height of 440 mm were made with the same number of concrete screw anchors as the tests with a height of 320 mm. figure 5 shows that these tests achieved load increases of up to 140 % compared to reference tests. this higher strengthening effect results from a better load transfer when 324 vol. 33/2022 enhanced service life of existing infrastructure buildings figure 4. additional flexural reinforcement on both sides of the hollow box girder and additional shear reinforcement drilled through the hollow box. �� figure 5. strengthening of the railway bridge over the highway a70 under ongoing use on and under the structure. anchoring higher into the flexural compression zone with the screws. while there was no significant difference between the d0 = 22 mm and d0 = 16 mm screws at the beams with a height of 320 mm, the tests with 440 mm show a difference in load increase of 23 %. also, it can be seen that for these tests an influence of the anchoring depth ds of screws is significant when anchoring under and at the same height as the upper flexural reinforcement. also tests at slab specimen were performed. for the concrete slabs the shear reinforcement ratio was identical to the beam tests. so the test results can be compared to them and are in good accordance with the performed beam tests. the variation of the installation depth also showed a reduced maximum shear force when using a reduced installation depth ds in accordance to the beam tests. 3.3. conclusion of the performed shear tests all tests showed that with the use of the new system significant load increases can be achieved. therefore and due to the fast and easy installation, the system is highly suitable for the application in strengthening projects with special boundary conditions such as installation under ongoing use. the tests showed that the new system can be used for the strengthening of existing structures also with dynamic loads such as bridges. therefore this new system can contribute strongly in maintenance and strengthening of existing infrastructure and can contribute to avoid road closure due to replacement constructions or other complex strengthening systems. 4. pilot-projects 4.1. railway crossing over the german motorway a70 the railway crossing over the german motorway a70 near bamberg is a two span concrete bridge which was built in 1967. the girder bridge has a maximum span of 17.5 m and two separate superstructures with a hollow box cross-section, as can be seen in figure 4. the bridge is prestressed with tendons manufactured from sigma oval 40 (st 145/160) steel which is known for its tendency to stress corrosion. a recalculation of this bridge was performed in 2011 revealing a remaining service life of zero years. due to the possibility of a sudden failure through a rupture of tendons no ductile failure of the superstructure could be ensured. this is why a bending and shear strengthening was planned using concrete screw anchors. one requirement of this strengthening was the installation under ongoing traffic on the bridge. to raise the shear load capacity concrete screw anchors with a length of about 125 cm from underneath the bridge were drilled through the hollow boxes. to increase the flexural bending failure load steel plates as external reinforcement were attached at both sides of the girder, as can be seen in figure 4 and 5. these steel plates were prefabricated in small elements and fixed with concrete screw anchors to the structure. 325 johannes lechner, jürgen feix acta polytechnica ctu proceedings figure 6. planned shear and flexural reinforcement by the use of concrete screw anchors as shear reinforcement and threaded rods of high tensile steel as new flexural reinforcement. this resulted in short delivery and assembly time. the short elements of the steel plates facilitated the closure of just single lanes for the installation. this led to a very small traffic disruption of the motorway as can be seen in figure 5. especially at both ends of the bridge strong deficits of existing flexural reinforcement was given. therefore the full force of the steel plates had to be anchored into the end cross girder. six screws d0 = 22 mm with a length of 2.3 m were used to transfer this load into the concrete. the whole installation time of the reinforcing system was four weeks. the total cost was about €370 000 whereas a rebuilding of the structure was calculated with around 2.5 million euros. furthermore, no disruption of the railroad traffic was necessary. 4.2. segment 34 of the altstadtring tunnel, munich the altstadtring tunnel in munich is one of the main east-west traffic axes in the centre of munich. the daily traffic through the tunnel is about 60 000 cars on four lanes. the tunnel was built at the end of the 1960s mostly in open type construction and was opened in 1972 for the olympic games in munich. some segments of the tunnel were erected by topdown construction method. one of these is segment 34, which was built underneath the prinz-carl-palais. therefore, the basements of the building were broken off and in total 15 girders with a height of 3.5 m were built section by section underneath the palais. afterwards the tunnel cross section was excavated under the new slab. therefore, the complete dead load of the prinz-carl-palais is carried off by the tunnel slab. the 15 girders which form the tunnel ceiling slab of segment 34 were designed as single span girders, with a maximum span of up to 35 m. the girders were built with a box cross section and prestressed with tendons. again, pre-stressing steel type sigma oval steel st145/160 was used. because of the high number of tendons in both directions of the tunnel slab, the amount of conventional reinforcement is very low. in 2013 a recalculation was executed on basis of the german assessment rules for structures containing prestressing steel known to be susceptible to stress corrosion cracking. it was concluded that for 13 out of the 15 girders a ductile failure cannot be guaranteed. therefore, the authorities decided to strengthen segment 34 of the tunnel. the planned reinforcement is a combination of concrete screw anchors to raise the shear load capacity and high tensile threaded rods as post-installed flexural reinforcement at the underside of the slab, as shown in figure 6 for one girder. the additional shear reinforcement is drilled through the hollow boxes near the supports. in total more than 1000 concrete screws with a length of 325 cm are needed as additional shear reinforcement. the flexural reinforcement is planned with a diameter of 43 mm and 63.5 mm respectively. the additional external reinforcement will be cladded in a 30 cm layer of shotcrete. to ensure the correct bond between the existing concrete structure and the shotcrete and to activate the existing stirrup reinforcement, 5000 concrete screws with a diameter of 16 mm are therefore planned to be installed in the whole area of 1236 m2. this is also shown in the right picture of figure 6. in spring 2019, the strengthening measures for the 326 vol. 33/2022 enhanced service life of existing infrastructure buildings tunnel reinforcement were started, whereby in a first step the screws were installed on the north side of the tunnel. for this, traffic was rerouted, whereby at least two lanes had to remain open at all times. after a detailed non-destructive detection of the existing tendons, the manholes necessary for the installation of the screws could be produced into the hollow bodies. afterwards the installation of the reinforcing screws could be started. the installation of the bolts in the northern half of the tunnel was completed in october 2019. in the next step, the high-strength threaded rods were installed on the underside of the slab and traffic was transferred to the north side so that work could continue on the south side of the slab. the work on the south side is currently being carried out. after the installation of the remaining bolts (a total of approx. 7300 bolts), the new bending reinforcement will be mounted on the south side and connected to the reinforcement on the north side with joints. finally, a shotcrete layer of 30 cm will be applied to the underside of the slab, thus restoring the level ceiling soffit. 5. summary a significant number of concrete structures lack of shear reinforcement. therefore, several research projects aim to develop a new shear strengthening system to increase their shear strength. such systems aim at easy installation without disruption of the use of the structure. the new idea presented in this paper is to use concrete screw anchors as post-installed shear reinforcement system. in several pilot projects the easy and fast installation of the new retrofitting system was proved. a railroad bridge over the german motorway a70 was reinforced with concrete screw anchors as shear reinforcement and external flexural reinforcement within 4 weeks under ongoing traffic on and under the bridge. thus it was possible to extend the life span of the bridge from zero to 20 years. also the tunnel slab of the altstadtring-tunnel in munich is currently reinforced by the use of concrete screw anchors. as this tunnel was erected under the historic prinzcarl-palais and represents the central east-west route through the city of munich, a total closure or replacement construction was not possible. summarising, it can be seen that the new system, the usage of concrete screw anchors as post-installed reinforcement can raise the load capacity of existing structures significantly. because of the easy and fast installation of the system, it is very well-suited for the strengthening of structures under ongoing use. using the new system, the lifetime of existing structures can be increased significantly and disruptions to traffic by road closures or rerouting can be avoided or reduced. in [15] it was shown, that strengthening of existing structures with minimal road-closures on and under the bridge reduces the impact on the global warming potential, the acidification and on non-renewable ceds up to 200 % on one day compared to a replacement of the bridge. a new and quickly installable reinforcement system can help reduce emissions such as co2. acknowledgements the authors want to thank toge dübel gmbh, nuremberg for the current and ongoing excellent collaboration during the development of the new strengthening system. also, the authors want to thank the german railroad company (db netz ag) and the authorities of the capital city of munich for the possibility to carry out the pilot projects shown. references [1] bundesanstalt für straßenwesen. brücken an bundesfernstraßen brückenstatistik 09/2019, 2019. https://www.bast.de/bast_2017/de/statistik/ brueckenbrueckenstatistik.pdf;jsessionid= b8402c5dc792b15352d4823e5f45fef4.live11291_ _blob=publicationfile&v=13. [2] o. fischer, a. müller, t. lechner, et al. ergebnisse und erkenntnisse zu durchgeführten nachrechnungen von betonbrücken in deutschland. betonund stahlbetonbau 109(2):107-27, 2014. https://doi.org/10.1002/best.201300084. [3] p. huber, a. schweighofer, j. kollegger, et al. vergleich der rechnerischen querkrafttragfähigkeit von bestandsbrüüken nach eurocode 2 und fib model code 2010. betonund stahlbetonbau 107(7):451-62, 2012. https://doi.org/10.1002/best.201200023. [4] a. j. nicholson. road network unreliability: impact assessment and mitigation. international journal of critical infrastructures 3(3/4), 2007. https://doi.org/10.1504/ijcis.2007.014115. [5] f. xie, d. levinson. evaluating the effects of the i-35w bridge collapse on road-users in the twin cities metropolitan region. transportation planning and technology 34(7):691-703, 2011. https://doi.org/10.1080/03081060.2011.602850. [6] b. kim, s. shin, d. kim. scenario-based economic impact analysis for bridge closures due to flooding: a case study of north gyeongsang province, south korea. water 10(8), 2018. https://doi.org/10.3390/w10080981. [7] shear strength of members without transverse reinforcement as function of critical shear crack width. aci structural journal 105(2), 2008. https://doi.org/10.14359/19731. [8] j. hegger, s. görtz. querkraftmodell für bauteile aus normalbeton und hochleistungsbeton. betonund stahlbetonbau 101(9):695-705, 2006. https://doi.org/10.1002/best.200600498. [9] a. marí, a. cladera, j. bairán, et al. shear-flexural strength mechanical model for the design and assessment of reinforced concrete beams subjected to point or distributed loads. frontiers of structural and civil engineering 8(4):337-53, 2014. https://doi.org/10.1007/s11709-014-0081-0. 327 https://doi.org/10.1002/best.201300084 https://doi.org/10.1002/best.201200023 https://doi.org/10.1504/ijcis.2007.014115 https://doi.org/10.1080/03081060.2011.602850 https://doi.org/10.3390/w10080981 https://doi.org/10.14359/19731 https://doi.org/10.1002/best.200600498 https://doi.org/10.1007/s11709-014-0081-0 johannes lechner, jürgen feix acta polytechnica ctu proceedings [10] j. lechner, n. fleischhacker, c. waltl, et al. zum verbundverhalten von betonschraubdübeln mit gro(em durchmesser. betonund stahlbetonbau 112(9):589-600, 2017. https://doi.org/10.1002/best.201700043. [11] j. lechner. ein neues verfahren zur nachträglichen querkraftverstürkung von stahlbetonbauteilen phd-thesis university of innsbruck, 2017. [12] j. lechner, j. feix. first experiences with concrete screw anchors as postinstalled shear reinforcement in concrete bridges. civil engineering design 1(1):17-27, 2019. https://doi.org/10.1002/cend.201800004. [13] j. feix, j. lechner. concrete screws as post installed reinforcement proceedings in 3rd international symposium on connections between steel and concrete, stuttgart, germany, 549-59. https://www.feix-ingenieure.de/wp-content/ uploads/07_consc_proceeding.pdf, 2017. [14] j. feix, j. lechner. development of a new shear strengthening method for existing concrete bridges proceedings in 10th japanese-german bridge symposium, münchen, germany, 2014. [15] f. gschösser, r. schneider, a. tautschnig, et al. retrofitting measure vs. replacement-lca study for a railway bridge. sustainable built environment (sbe) regional conference zurich 2016, 2016. 328 https://doi.org/10.1002/best.201700043 https://doi.org/10.1002/cend.201800004 https://www.feix-ingenieure.de/wp-content/uploads/07_consc_proceeding.pdf acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.37.0024 acta polytechnica ctu proceedings 37:24–30, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague modelling of fission gas release in uo2 doped fuel using transuranus code jheffry gonzaleza,b,c,∗, martin ševečeka a czech technical university in prague, faculty of nuclear sciences and physical engineering, jugoslávských partyzánů 1580/3, 160 00 prague 6 – dejvice, czech republic b royal institute of technology, division of nuclear engineering, kungliga tekniska högskolan, 100 44 stockholm, sweden c grenoble inp – phelma, school of engineering in physics, applied physics, electronics & materials science, 3 parvis louis néel, 38016 grenoble cedex 1, france ∗ corresponding author: jheffrygonzalez@gmail.com abstract. the expected benefits from cr-doped fuel are improved retention of fission gases within the pellets due to its large grain size. to demonstrate this, several experiments have been carried out by halden reactor and studsvik. these experiments are now being used to benchmark several fuel performance codes among them transuranus code. all this as part of a coordinate research project (crp) by iaea named testing and simulation for advanced technology and accident tolerant fuels (atf-ts). this work is introducing a novel fission gas diffusivity model for doped fuel in transuranus code. it is observed the benefits of introducing this new model when comparing to the standard model already existing in transuranus. nevertheless, more work needs to be carried out to fully understand all the phenomena involved in adding dopant in uo2 due to change of thermo mechanical properties. keywords: doped fuel, transuranus, atf, fission gas release, diffusivity. 1. introduction the study of fission gas release, fgr, is an important area within the fuel rod performance analysis that needs to be covered because these gaseous fission products in the uo2 fuel pellet are detrimental to the thermo-mechanical properties of the fuel [1]. fission gas release depends on several mechanisms that occur within the fuel pellet, more specifically within the grain (see figure 1). few of these mechanisms are, for instance, thermal contributions: grain boundary diffusion, bubble migration, trapping and resolution; and there are also athermal contributions: recoil, knock-out and sputtering. figure 1. basic mechanisms of fission gas release [2]. because in this work we are dealing with doped fuel, it is vital to understand how the properties and characteristics of this fuel will impact on the fgr mechanisms. although there are other properties that might need to be investigated this work focuses on the selection of a suitable gas diffusivity model for doped fuel. the selected diffusivity model will be implemented in transuranus code. the new implementation done in the source code will be validated against experimental results performed carried out at the halden reactor (ifa 716 experiment) and the studsvik cladding integrity program (scip-ii). this work is also part of a coordinate research project (crp) by iaea1 named testing and simulation for advanced technology and accident tolerant fuels (atf-ts) and the results will be also compared with other members that are simulating this novel fuel in other fuel performance codes. in section 2, the methodological details of this work are explained, a brief explanation of the experiments to compare to and considerations taken when performing the simulations. section 3 shows the results obtained and their comparison against the tests. finally, in section 4, it will be shown all the conclusions and also recommendations for future improvements of the modelling of this fuel in transuranus. 2. methods the methodology used for the development of this work is as follows i) study the models related to fis1https://www.iaea.org/projects/crp/t12032 24 https://doi.org/10.14311/app.2022.37.0024 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en https://www.iaea.org/projects/crp/t12032 vol. 37/2022 modelling of fission gas release sion gas release in transuranus (i.e. fission gas diffusivity, thermal conductivity, etc.); ii) select the gas diffusivity model from transuranus or introduce a new model; iii) verify that the new implemented model works as expected; iv) identify all the fuel properties and reactor conditions required in the input file to run the simulation (type of fuel and cladding, flux, linear power, temperature of the coolant, grain size, etc); v) data collection of the experimental analysis performed by halden and studsvik; vi) validate the implemented model by benchmarking the results obtain from transuranus with the experimental data. fgr is strictly dependent on temperature; therefore, choosing a good thermal conductivity model for this type of fuel is essential. there is no thermal conductivity model available for doped fuel; however, according to arborelius et al. there is only a small change in thermal conductivity of fresh fuel due to doping and this small difference is expected to be negligible after the burnup increase [3].although it would make sense to use standard undoped uo2 thermal conductivity during simulations the reality is that in this work the correlation used is the one proposed by ohira et al. for for high burnup uo2 pellets [4]. the reasoning for this is explained in section 2.4. transuranus uses a mechanistic model for fission gas behaviour and release from doctoral thesis of [5]. for the intragranular diffusion coefficient, transuranus has different models; nevertheless, the model that is most widely accepted for fission gas diffusivity is from [6] which will be named as a standard during this article. 2.1. selection and modelling of fg diffusivity turnbull model for the single gas atom diffusion coefficient can be seen from equations 1 to 4. deff = d1 + d2 + d3 (1) d1 = 7.6× 10−10e− 3500 t (2) d2 = 3.22× 10−16 √ re− 13800 t (3) d3 = 3.22× 10−16r, (4) where deff is the effective diffusion coefficient, d1 is the intrinsic high-temperature component, d2 is the irradiation enhanced thermal component, d3 is the athermal term and r is the rating in w/gu. because che et al. showed a good agreement with halden fgr experiments when using an empirically tuned enhanced gas diffusion, cooper et al. decided to investigate in deep the influence of dopant on fission gas diffusivity. in that work, cooper et al. proposed a diffusion coefficient for uranium doped fuel [7]. equation 5 shows enhancement of the diffusivity, using the undoped uo2 diffusivity model in equation 1 as a baseline. ddoped = exp ( h1 kb [ 1 t − 1 t1 ]) dundoped 1 + exp ( h2 kb [ 1 t − 1 t2 ]) dundoped 2 + dundoped 3 , (5) where kb is the boltzmann constant. h1, h2, t1 and t2 are parameters derived for the multiscale analytical enhanced diffusivity model (see table 1). dundoped 1 , dundoped 2 , and dundoped 3 are calculated with equations 2 to 4. because there are a number of uncertainties when obtaining these parameters cooper et al. examined two cases namely case a and case b. the first case has minimal alterations migration energy and sink strength given by the oxygen potential. in contrast, case b overestimates this migration energy and sink strength based on the experience from [8]. parameter case a case b t1 = t2 [k] 1773 1773 ∆h1 [ev] 0.3198 0.3282 ∆h2 [ev] -0.3345 -0.6998 table 1. parameters for the enhanced diffusivity model derived for cr-doped fuel [7]. now, it is important to understand the distinction (if any) between the models before implementing the proposed fg diffusivity for the doped fuel in transuranus. hence, by plotting the fg diffusion, it is clearly depicted the difference between them (see figure 2). for the model of the fg diffusivity of doped fuel, the two cases proposed by [7] are considered and they are shown case a and case b. figure 2. gas diffusivity in undoped uo2 fuel using turnbull et al. model, and gas diffusivity in cr-doped uo2 fuel applying cooper et al. model. from figure 2, it is observed that from 700 to 1600 k there is a major difference between case a of cooper model and the standard diffusion model, whereas for the case b the range is wider from 500 to 25 jheffry gonzalez, martin ševeček acta polytechnica ctu proceedings 1600 k. the ratio between the doped fuel model and the standard model can be as high as 3.4 and 17.5 for case a and b respectively, at around 1000 k. this ratio difference is significant; however, if we consider the range of temperature from 500 to 2000 k the mean ratio will be for case a a factor of 1.63 and for case b a factor 4.66. 2.2. implementation and verification in this work both cases of cooper model are considered. therefore, it was necessary to have two new versions of transuranus that includes case a and b in which the only difference is the parameters in table 1. the implementation done in transuranus was revised by transuranus developers to ensure that the coding is up to their standards and minor modifications of the code were proposed by them in terms of clarity of the code. as evidenced in figure 2, the value of diffusion coefficient of doped fuel is significantly larger over the range of from 500 to 1600 k, this would mean that if the size of the grain remains the same one would expect a greater fgr when comparing to standard uo2 fuel. hence, for the verification of the modelling, several tests were performed. in these tests the grain size was kept the same but using the new model implemented in tu. table 2 shows the percentage of fission gas release inside the fuel rod and the internal rod pressure difference when using turnbull model and cooper model in an another halden case (ifa 429). test cases parameter model value ifa420cd fgr turnbull 12.34 % cooper 23.17 % pressure turnbull 6.45 mpa cooper 7.27 mpa ifa420ch fgr turnbull 17.79 % cooper 42.95 % pressure turnbull 6.95 mpa cooper 11.18 mpa table 2. model verification using ifa 429 case. for both cases, there is a considerable increase in fgr and pressure when using the diffusivity model for doped fuel. this could confirm that the implementation was done correctly and that the benchmark of the code with experimental data can be carried out. 2.3. experimental data the implemented model in transuranus is validated with experimental data performed by halden and studsvik. from the experimental data and fuel characteristics all the input data required is gathered to run the simulation. some of the input data are unchanged overtime while other data are constantly changing as it is the case of the the power, fast flux and temperature of coolant. for this variable data, an additional processing is needed because transuranus have a maximum number of record for the macro-step that can be included in the input file. this limit is up to 5000, however, the data steps provided is around 160000 for ifa 716 and 24000 for scip-ii. hence, a software called fuel rod analysis toolbox (fratoolbox) is used to merge the quantities of the large number of data sets [9]. 2.3.1. halden experiment ifa 716 the main objective of the ifa 716 experiment is to investigate the effect of dopant concentration and grain size on the fgr [10]. the experiment was carried out between january 2011 and may 2015 for a total operation time of 842 days divided in 15 operation cycles. ifa 716 assembly consisted in six rods each with different type of fuel. few of these characteristics for the ifa 716 case (rod 1 and 2) are reported in table 3; clearly, the main differences between these two fuels are the cr2o3 content and the fuel average grain diameter. moreover, there are some pellets that have a hollow section this only applies for the pellets on the top where instrumentation was inserted to obtain, for instance, the on-line measurement of the fuel temperature center line. figure 3 shows the average linear heat rate measured for the rods 1 and 2 versus time [10]. evidently, rod 1 has a total operation time of 840 days while rod 2 only has over 700 days. this difference is due to a malfunction in rod 2 that caused this rod to be removed from the reactor, in contrast, rod 1 (and the others) remained until the end of the test. the average linear heat rate (lhr) of both rods are roughly the same throughout the experiment; therefore, the average lhr of rod 1 will be used during the simulation in transuranus. figure 3. ifa 716 average linear heat rate measured during the test. 2.3.2. studsvik experiment scip-ii the main objective of scip-ii experiment is to investigate the effect of power ramps on doped fuel with 26 vol. 37/2022 modelling of fission gas release ifa 716 rod 1 ifa 716 rod 2 cladding material zircaloy-4 zircaloy-4 fuel material uo2 + cr2o3 uo2 fill gas he he pellet outer diameter [mm] 9.12 9.13 pellet inner diameter, hollow section [mm] 1.8 1.8 cladding outer diameter [mm] 10.75 10.75 cladding thickness [mm] 0.725 0.725 active fuel length [mm] 399.5 399.4 free volume [m3] 5.8 5.95 fuel density [g/cm3] 10.50 10.55 fill gas pressure [mpa] 1 1 fuel cr2o3 content [ppm] 1580 n/a fuel u-235 enrichment [wt %] 4.9 4.89 fuel average grain diameter [µm] 71 11 table 3. fuel characteristics of ifa 716 rod 1 and 2 simulated in this work [11]. a based irradiation. this based irradiation was carried out between september 2006 and august 2009 at the okg2 reactor divided in 3 cycles. similarly, as the ifa 716 case, the fuel characteristics for the scip-ii case are reported in table 4. rods l707 and l708 are fundamentally the same, there is no major distinction between the two according to the given data. what makes these rods different is the base irradiation that they underwent before being subjected to power ramps in a test reactor. both rods have a total operation time of 1095 days as based irradiation with an average lhr of 14 and 13 kw/m for rod l707 and l708, respectively. this led to a mean bu of 28 mwd/kgu in the case of l707 and 26 mwd/kgu for l708. the ramp test for each rod was carried out differently for both, with rod l707 having a total time of 21.33 h while rod l708 with a total time of 14.32 h. the ramp was hold for 12 h and around 1 minute for rod l707 and l708, respectively. figure 4 depicts the average lhr during the ramp test for each rod. in contrast to the halden experiment, there is no in-situ measurement of fuel center line temperature or plenum pressure which will make the implemented model harder to validate against this experiment. the only measurement that is given comes from the postirradiation examination where the data for fgr is obtained. 2.4. additional considerations the only criteria for selection thermal conductivity model is how close the fuel center line temperature (fclt) obtained from transuranus is to the experimental data. transuranus has a set of thermal conductivity models for different fuels including a correlation for the standard uo2. when using the lwr standard correlation (model already implemented in transuranus) proposed by [12], it results in the figure 8 (top) where evidently there is not a good agreement between this model and the experimental figure 4. scip ii ramp test. data. therefore, exploring other thermal conductivity models is necessary to continue to the validation of the fgr. the model that best fit to the experimental results is surprisingly a correlation used for high burnup uo2. consequently, the correlation for high burnup uo2 pellets found in transuranus is the thermal conductivity model of choice for the study of fgr in this work. in addition to using a thermal conductivity for high burnup uo2, it is considered that the gas diffusivity model for doped fuel is the same independently of the chromia concentration in the fuel because this diffusivity model does not have a correlation as function of the amount of chromia content. 3. results the parameters to benchmark are fuel center line temperature, upper plenum pressure, and fission gas release. however, no all data is available for all the rods tested. for instance, for scip-ii, the only data to compare is the fgr obtained in the post-irradiation examination. 27 jheffry gonzalez, martin ševeček acta polytechnica ctu proceedings scip-ii rod l707 scip-ii rod l708 cladding material zircaloy-4 zircaloy-4 fuel material uo2 + cr2o3 uo2 + cr2o3 fill gas he he pellet outer diameter [mm] 8.67 8.67 cladding outer diameter [mm] 10.05 10.05 cladding thickness [mm] 0.605 0.605 active fuel length [mm] 480 480 fuel density [g/cm3] 10.50 10.55 fill gas pressure [mpa] 1 1 fuel u-235 enrichment [wt %] 3.0 3.0 fuel average grain diameter [µm] 49 49 table 4. fuel characteristics of scip-ii rod l707 and l708 simulated in this thesis. 3.1. benchmark of halden ifa 716 because pressure transducer of rod 2 was damaged, there is no benchmark of this for rod 2. on the other hand for rod 1, the pressure of case a and b for rod 1 is the same during the first 300–350 days of operation and for both cases transuranus predicted well the pressure when compared to the experimental data. however, after 350 days of operation transuranus underestimates the pressure when using diffusivity model case a while when using case b the pressure is overestimated (see figure 5). figure 5. predicted vs. measured rod pressure for rod 1, including the two cases for the diffusivity of doped fuel. figure 7 shows the result of the comparison of the predicted fclt versus the measured fclt taken from the test for rods 1 and 2. it can be observed that for rod 1 (see figures 7a and 7b), there is a good agreement between the experimental and simulation when the burnup is low enough (< 17 mwd/kguo2) while at higher burnup there points that fall out of the established uncertainty zone of ±9 % and yet close to this zone. around 100 % of agreement is found if we accept this uncertainty. in case of rod 2, in figure 7c at low burnup the predicted temperature fall within the uncertainty opposite to what occurs at higher burnup with several data points falling far from this zone. this over-prediction of the temperature, after approximately 400 days of operation, is also observed in cooper et al. work; however, the temperature difference between predicted and measured is larger [7]. finally, for the fission gas release, the given data did not contain the evolution of fission gas release versus operation time; instead, what is available is the fission gas release at different stages of the experiment. nevertheless, this is not true for rod 2, where no data was collected at any point and the rod presented signs of leakage at around 700 days of operation. hence, no puncture was performed to obtain the fission gas release. figure 6 shows the predicted fission gas release for rod 1 for both diffusivity cases, the predicted fission gas release for rod 2 and the measures taken by halden at four different times with their uncertainty for rod 1. despite having a good agreement between measured and predicted for fclt and pressure, the fission gas release shows a wide difference between the measured and simulation. this is more evident for rod 1 case a where the prediction of fgr is 1.6 % while case b gives a better results with 3.8 % at 625 days. the measured value for the fission gas release at the same time was 5.71 % ± 1.36 %. surprisingly, the fission gas release of rod 2 is lower than the rod 1 when using diffusivity case b. figure 6. predicted fission gas release for rod 1 and 2. 28 vol. 37/2022 modelling of fission gas release (a) . rod 1 uo2 doped fuel diffusivity case a (b) . rod 1 uo2 doped fuel diffusivity case b (c) . rod 2 standard uo2 fuel small grain figure 7. predicted vs. measured fuel center line temperature for ifa-716 rods 1 and 2. for burnup lower than 17 mwd/kguo2 (shown as ’5’) and burnup higher than 17 mwd/kguo2 (shown as ’?’). 3.2. benchmark of studsvik scip-ii unfortunately, there is no data to compare fclt or pressure as it was done with ifa-716. table 5 shows the fission gas release obtained during the post irradiation examination and the fission gas release predicted by transuranus using fission gas diffusivity case b. for this experiment, the prediction overestimate the experimental data. this is more evident for the 1 minute ramp (rod l708) where there is almost no release according to pie but the prediction is 2.5 %. rod l707 rod l708 fgr / pie 3.0 % 0.04 % fgr / transuranus 5.8 % 2.5 % table 5. fission gas release comparison at the end of ramp 4. conclusions the fission gas diffusivity model was compared to two different experiments, one for ramp test and the other for long period operation. the results show that there are still uncertainties in the prediction using this model in transuranus. the halden experiment, however, was surrounded by some failures in sensors during the experiment. as a result, it is unclear whether these experimental results can be trusted. similarly, there was a lack of data to make a better comparison against the scip-ii experiment. it will be important to continue the validation, perhaps against other experiments, to draw a final conclusion on this model with the transuranus code. additionally, defining the model exclusively for doped fuel can be beneficial to improving the accuracy of the predictions. acknowledgements this project is financed from the state budget by the technology agency of the czech republic and the ministry of industry and trade within the theta programme. a support from the iaea crp atf-ts t12032 is acknowledged. references [1] m. tonks, d. andersson, r. devanathan, et al. unit mechanisms of fission gas release: current understanding and future needs. journal of nuclear materials 504:300–317, 2018. https://doi.org/10.1016/j.jnucmat.2018.03.016. [2] european commission. transuranus handbook, 2019. [3] j. arborelius, k. backman, l. hallstadius, et al. advanced doped uo2 pellets in lwr applications. journal of nuclear science and technology 43(9):967–976, 2006. https://doi.org/10.1080/18811248.2006.9711184. 29 https://doi.org/10.1016/j.jnucmat.2018.03.016 https://doi.org/10.1080/18811248.2006.9711184 jheffry gonzalez, martin ševeček acta polytechnica ctu proceedings figure 8. fuel center line temperature measured by thermocouples during the ifa 716 test compared to simulated fclt using standard thermal conductivity correlation (top) by hading et al. [12] and high burnup thermal conductivity correlation by ohira et al. [4]. [4] k. ohira, n. itagaki. thermal conductivity measurements of high burnup uo2 pellet and a benchmark calculation of fuel center temperature. in proceedings 1997 international topical meeting on light water reactor fuel performance, pp. 541–549. united states (usa), 1997. isbn 0-89448-616-0. [5] g. pastore. modelling of fission gas swelling and release in oxide nuclear fuel and application to the transuranus code. department of energy, politecnico di milano, 2012. [6] j. turnbull, c. friskney, j. findlay, et al. the diffusion coefficients of gaseous and volatile species during the irradiation of uranium dioxide. journal of nuclear materials 107(2):168–184, 1982. https://doi.org/10.1016/0022-3115(82)90419-6. [7] m. w. cooper, g. pastore, y. che, et al. fission gas diffusion and release for cr2o3-doped uo2: from the atomic to the engineering scale. journal of nuclear materials 545:152590, 2021. https://doi.org/10.1016/j.jnucmat.2020.152590. [8] c. matthews, r. perriot, m. w. cooper, et al. cluster dynamics simulation of uranium self-diffusion during irradiation in uo2. journal of nuclear materials 527:151787, 2019. https://doi.org/10.1016/j.jnucmat.2019.151787. [9] k. lassmann, j. van de laar, a. schubert, p. v. uffelen. ‘mergequantities’: a program for adding axial profiles to a {x,y} data set and for merging quantities. joint research centre of the european commission, 2014. [10] k. fuglesang. in-pile results from the fission gas release mechanisms study in ifa-716 after final unloading. tech. rep., organisation for economic co-operation and development halden reactor project, norway, 2016. [11] m. sobieska. production of six fuel rods for fission gas release mechanisms test rig ifa 716. tech. rep., organisation for economic co-operation and development halden reactor project, norway, 2009. [12] j. harding, d. martin. a recommendation for the thermal conductivity of uo2. journal of nuclear materials 166(3):223–226, 1989. https://doi.org/10.1016/0022-3115(89)90218-3. 30 https://doi.org/10.1016/0022-3115(82)90419-6 https://doi.org/10.1016/j.jnucmat.2020.152590 https://doi.org/10.1016/j.jnucmat.2019.151787 https://doi.org/10.1016/0022-3115(89)90218-3 acta polytechnica ctu proceedings 37:24–30, 2022 1 introduction 2 methods 2.1 selection and modelling of fg diffusivity 2.2 implementation and verification 2.3 experimental data 2.3.1 halden experiment ifa 716 2.3.2 studsvik experiment scip-ii 2.4 additional considerations 3 results 3.1 benchmark of halden ifa 716 3.2 benchmark of studsvik scip-ii 4 conclusions acknowledgements references https://doi.org/10.14311/app.2022.33.0140 acta polytechnica ctu proceedings 33:140–145, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague circular concrete: validating new technologies in the lab and on-site bram doomsa,∗, jeroen vrijdersb a belgian building research institute, division of geotechnics, structures and concrete, rue du lombard 42, 1000 brussels, belgium b belgian building research institute, division of intelligent installations and sustainable solutions, rue du lombard 42, 1000 brussels, belgium ∗ corresponding author: bram.dooms@bbri.be abstract. the growing importance of a circular economy also holds possibilities for concrete. some of the principles of circularity, like the re-use of "waste" as secondary raw material and the optimisation of the design with regards to lifespan, environmental impact and re-use, can be applied. several technological innovations in this domain are in an advanced stage of development. however, the transition from research to full commercial applications is held back, mostly due to the absence of documented experience and the lack of a technical framework (prescriptions). in order to accelerate the adoption of new technologies, the belgian building research institute (bbri) started the project "circular.concrete". in this paper, the results are presented of validation tests regarding 1) smart crushing technology, 2) the carbonation of recycled aggregates, 3) concrete using only recycled aggregates and 4) the use of alternative binders and supplementary cementitious materials. keywords: carbonatation, concrete recycling, pilot projects, recycled concrete aggregates (rca), supplementary cementitious materials. 1. introduction substantiated by the scientific consensus of global warming and climate change and propelled by the increased public awareness, environmental clauses are frequently added to the specifications for buildings for major clients. concrete has a dubious reputation in this matter. this is mostly due to the enormous volumes of cement that are produced worldwide on a yearly basis (∼ 4.5 billion tons, 2019), the co2emissions during cement production (∼ 800 kg of co2 per tonne of cement for cem i), the extraction, processing and transportation of the raw materials and the important waste streams that are produced when concrete structures are demolished [1, 2]. the growing importance of a circular economy also holds opportunities for concrete. some of its principles like the re-use of "waste" as secondary raw material and the optimisation of the design with regards to lifespan, environmental impact and re-use, can be applied. several technological innovations in this domain are in an advanced stage of development. however, the transition from research to real applications is held back, mostly due to the absence of documented experience and of a technical framework, i.e. technical prescriptions, standards etc. in order to accelerate the adoption of new technologies, the belgian building research institute (bbri) started the project circular.concrete. the aim of this project is to translate research results into practice. 2. validating innovative technologies in a first stage of the circular.concrete project, different technological innovations were identified and their trl (technology readiness level) were estimated. the results have been summarized in a stateof-the-art report, which is available on the project website (https://www.circular-concrete.be/). in a second stage a selection was made of technological innovations for further validation on laboratoryscale, based on their trl, their scientific potential and their relevance for the belgian market: 1) smart crushing technology, 2) the carbonation of recycled aggregates, 3) concrete using only recycled aggregates and 4) the use of alternative binders and supplementary cementitious materials. it should be noted that the scale of this validation was very limited and should be considered as exploratory. 2.1. technology 1. smart crushing using the smart crushing technology, concrete construction waste can supposedly be crushed and processed into its original components, i.e. the original fine and coarse aggregates and the binder, of which a part is non-hydrated (and still reactive) [3]. since the obtained recycled aggregates don’t have any mortar adhered, they can be used in new concrete as natural aggregates. this technology was tested on four different belgian concrete waste samples: 140 https://doi.org/10.14311/app.2022.33.0140 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en https://www.circular-concrete.be/ vol. 33/2022 circular concrete: new technologies figure 1. sample 4: low quality concrete waste fraction 20-56 mm. water absorption [%] particle density [kg/m3] micro-deval [−] sample 1 3.8 2440 23 sample 2 6.1 2370 28 sample 3 4.5 2400 23 sample 4 4.3 2420 21 limestone aggregate 0.5 − 1.0 2660 − 2800 15 table 1. characterization of the treated aggregate samples. 1. concrete waste fraction 2-20 cm, originating from railway sleepers, with strength class c50/60 and no reinforcements or contaminants. 2. concrete waste fraction 2-20 cm, originating from the precast industry, with strength class c45/55 and no reinforcements or contaminants. 3. low quality concrete waste fraction 20-56 mm, composed of concrete (70 wt%), natural stone (25 wt%), and masonry and contaminants (5 wt%). 4. low quality concrete waste fraction 20-56 mm, of which individual aggregates can be composed of concrete (80 wt%), fibre reinforced concrete (10 wt%), or asphalt (10 wt%) (figure 1) 500 kg of each sample was sent to a smart crushing facility in the netherlands to be processed. after processing, about 300 kg of material from each sample was returned. 2.1.1. aggregate testing each treated sample was assessed visually and the grain size distribution was determined. all treated samples presented a very similar distribution curve (0/22). the water absorption and particle density (on the fraction +4 mm, in accordance with en 1097-6) and resistance to wear (on the fraction +10 mm, in accordance with en 1097-1) was determined for the treated samples. the results are presented in table 1. the values for the treated samples are very different from the typical values for crushed natural limestone aggregates. these differences can be explained by the presence of mortar on the treated samples, which hasn’t been removed by the smart crushing treatment. belgian concrete is generally made with crushed limestone as coarse material, whereas dutch concrete generally contains natural (round) gravel. possibly this technology is more effective for concrete composed of rounded natural gravel. 2.1.2. fines testing from each treated sample, the fines (< 0.125 mm) were extracted and tested, to determine the potential presence of non-hydrated cement. x-ray diffraction analysis (xrd) of the fines demonstrated that this fraction is dominantly composed of quartz representing aliquots of the silt and sand aggregate fraction of the primary concrete. mortar bars were made (in accordance with en 196-1) using standard sand and cem i 52.5 n, and the fines as cement replacement (40 %). the fresh properties and the mechanical properties of the mortar bars after 7, 28 and 91 days were compared to those of reference mortar bars without cement replacement. the results of the mechanical tests (executed in accordance with en 196-1) are presented in table 2. the results indicate that the fines do not contribute to the strength development of the mortar. possibly the evaluated fraction should be even finer (< 0.063 mm) to exclude as much as possible the presence of silt and sand. 2.1.3. concrete testing using only the treated samples as received from the smart-crushing facility, concrete was prepared with each sample by adding 300 kg/m3 cem iii/a 42.5 and water in a w/c ratio of 0.55. due to an error in 141 bram dooms, jeroen vrijders acta polytechnica ctu proceedings rf,7d rc,7d rf,28d rc,28d rf,91d rc,91d [mpa] [mpa] [mpa] [mpa] [mpa] [mpa] sample 1 5.1 27.1 6.2 31.8 6.9 33.8 sample 2 5.6 27.4 6.4 33.7 6.3 34.7 sample 3 5.6 26.5 5.7 31.3 7.0 33.4 sample 4 5.3 26.9 5.9 32.3 6.2 33.2 reference mix 10.0 63.9 9.4 71.8 10.0 79.3 table 2. mechanical properties of the mortar mixtures with treated fines. rc,28d rc,91d carbonation depth 56 d [mpa] [mpa] [mm] sample 1 21.9 28.8 13.5 sample 2 22.2 23.5 15.5 sample 4 20.3 24.9 17.3 reference mix 30.5 45.8 11.3 table 3. compressive strength and carbonation resistance of the concrete mixtures with treated aggregates. the mix design, a deviant concrete composition was obtained with sample 3, which lead to the rejection of this concrete mixture. without adding superplasticizer but taking into account the water absorption of the treated samples, a very good workability was obtained of the concrete mixtures. the compressive strength after 28 and 91 days (determined in accordance with en 12390-3), and the resistance to carbonation (determined in accordance with en 13295) were determined for the mixtures and compared with a reference concrete mixture, using only crushed natural limestone aggregates. the composed grain size distributions of the different concrete mixtures differ slightly. the results are presented in table 3. a clear influence can be observed of de total replacement of natural aggregates with the treated samples. after 28 days a strength reduction of about 30 % can be observed, after 91 days of about 40-50 %. the concrete with treated sample 3 shows deviant behaviour. the carbonation depth of the concrete after 56 days of exposure to 1.0 % co2 was about 20-50 % higher for the treated samples in comparison to the reference mix. 2.2. technology 2. carbonation of recycled aggregates with the carbonation of recycled aggregates, their physical and mechanical properties can be enhanced and thus their applicability in concrete improved. at the same time co2 can be ’captured’ inside the aggregates. recycled aggregates of different origins (concrete ’c’, high quality concrete ’c a+’, mixed ’m’) and of different sizes (0/6.3, 6/22, 8/20, 0/40 and 10/40) were treated during 5 days in an autoclave at an increased pressure (2 bar) and a very high co2 concentration (100 %). before and after the treatment, the water absorption and particle density (in accordance with en 1097-6) and resistance to wear (in accordance with en 1097-1) were determined for the aggregates. the results are presented in table 4. the co2-treatment clearly affected the water absorption and the particle density of the aggregates. as only the mortar fraction, adhered to the recycled aggregates, is influenced by the carbonation, the aggregate properties are affected proportionally in relation to the mortar content of the aggregates. 2.3. technology 3. maximum aggregate replacement by replacing the natural aggregates in concrete by recycled or artificial aggregates, the need for primary raw aggregates can be reduced. the influence of the presence of recycled aggregates (ra) and artificial aggregates on the properties of fresh and hardened concrete was evaluated by replacing a maximum quantity of natural aggregates in concrete mixtures (% vol). this maximum quantity was determined by combining the recycled and artificial aggregates as received with natural aggregates, while fitting the combined grain size distribution of the concrete to a reference grain size distribution. the following aggregate combinations were tested: 1. 10 % 0/1 natural sand + 90 % ra type 1 (0/8 + 8/22) 2. 100 % ra type 2 (0/8 + 6/20) 3. 10 % 0/1 natural sand + 90 % ra type 3 (0/8 + 6/20) 4. 10 % 0/1 natural sand + 26 % 0/4 treated polluted sand + 64 % crushed limestone (4/20) 5. 20 % 0/1 natural sand + 20 % 0/4 copper slag + 60 % crushed limestone (4/20) 142 vol. 33/2022 circular concrete: new technologies water absorption particle density micro-deval [%] [mg/m3] [−] before after before after before after 0/40 c 6.8 5.4 2.20 2.29 25 27 10/40 m 7.6 7.4 2.04 2.08 42 47 0/6.3 c 6.3 5.9 2.29 2.33 / / 8/20 c a+ 4.3 3.7 2.42 2.44 16 13 8/20 c 4.4 3.5 2.37 2.43 24 16 6/22 c 4.3 3.8 2.34 2.37 11 12 table 4. characterization of the treated aggregates. type 1 concrete type 2 concrete rc,28d rc,91d carbonation depth 56 d rc,28d rc,91d carbonation depth 56 d [mpa] [mpa] [mm] [mpa] [mpa] [mm] combination 1 30.3 33.8 14.8 40.0 36.0 9.8 combination 2 26.6 25.9 17.5 32.8 32.3 14.0 combination 3 29.7 31.4 13.8 37.2 37.8 10.5 combination 4 40.9 53.1 11.0 51.3 58.0 6.3 combination 5 32.7 44.4 11.3 43.2 61.8 8.8 combination 6 29.9 41.4 12.0 57.6 61.7 7.5 reference mix 30.5 45.8 11.3 55.9 62.8 7.3 table 5. compressive strength and carbonation resistance of the concrete mixtures with maximum aggregate replacement. 6. 10 % 0/1 natural sand + 50 % inox steel slag (0/2 + 2/10) + 40 % crushed limestone (10/20) two types of concrete were made with those aggregate combinations: type 1 with 300 kg/m3 cem iii/a 42.5 and a w/c ratio of 0.55, and type 2 with 340 kg/m3 cem iii/a 42.5 and a w/c ratio of 0.45. superplasticizer was added to the fresh concrete to obtain an initial consistence class s3 s4 (in accordance with en 206). the results were compared to a reference concrete mixture using only crushed natural limestone aggregates. it is important to note that, while all concrete mixtures were designed using the same reference grain size distribution for the aggregates, differences exist between the actual grain size distributions. in addition, all the aggregates were applied in oven-dried state, and extra water was added to take into account their total water absorption. considering the differences in the kinetics of the water absorption of the aggregates, the effective w/c ratio of the mixtures may be higher than the targeted values. this is particularly the case with combinations 1 to 3, given the high replacement rates and high water absorption of recycled aggregates. this should be taken into account when considering the results presented in this article. the compressive strength after 28 and 91 days (determined in accordance with en 12390-3), and the resistance to carbonation (determined in accordance with en 13295) were determined of the concrete mixtures. the results are presented in table 5. for type 1 concrete, the influence of the maximum aggregate replacement on the compressive strength after 28 days is very low. only the concrete with combination 4 seems to perform significantly better. after 91 days however, a clear influence on the compressive strength can be observed in the case of maximum aggregate replacement by ra (combinations 1 to 3), with reductions of about 25-45 %. the carbonation resistance of these ra concrete mixtures is also clearly lower than that of the reference mix. for type 2 concrete, the maximum replacement of aggregates by ra (combinations 1 to 3) leads to a reduction of the compressive strength after 28 days of about 30−40 % and after 91 days of 40-50 %. the carbonation resistance of these ra concrete mixtures is also significantly lower than that of the reference mix. the concrete with combinations 4 to 6 behaves similar to the reference concrete mix. the freeze-thaw resistance was determined in accordance with cen/ts 12390-9 of the hardened concrete mixtures (only type 2 concrete). the results are presented in table 6. overall, the influence of the maximum aggregate replacement on the freezethaw resistance does not seem to be important. only the concrete with combination 2 (absence of natural sand) clearly performs worse than the other concrete mixtures. the air content of this concrete mixture (1.6 %) was lower than that of the other mixtures (2.0 − 4.8 %), which probably contributed to this re143 bram dooms, jeroen vrijders acta polytechnica ctu proceedings mean cumulative scaled material after 7 cycles 14 cycles 28 cycles 42 cycles 56 cycles [kg/m2] [kg/m2] [kg/m2] [kg/m2] [kg/m2] combination 1 0.74 1.02 1.30 1.58 1.82 combination 2 1.30 2.08 3.04 4.38 5.54 combination 3 0.28 0.68 1.28 1.76 2.28 combination 4 0.14 0.50 1.06 1.50 1.98 combination 5 0.66 0.96 1.60 2.42 3.00 combination 6 0.50 0.80 1.18 1.70 2.18 reference mix 0.58 0.94 1.42 1.90 2.46 table 6. freeze-thaw resistance of the concrete mixtures with maximum aggregate replacement. sult. the resistance to chloride migration was determined in accordance with nt build 492 of the hardened concrete mixtures (only type 2 concrete). the results are presented in table 7. the maximum replacement of aggregates by ra (combinations 1 to 3) seems to lead to a reduction of the resistance to chloride migration. again, the concrete mixture with combination 2 (absence of natural sand) clearly performs worse than the other ra concrete mixtures. the mixtures with slag aggregates (combinations 5 and 6) seem to perform slightly better than the reference mix. non steady state chloride migration coefficient dnssm [kg/m2] combination 1 6.43 combination 2 10.17 combination 3 8.46 combination 4 6.57 combination 5 4.90 combination 6 3.98 reference mix 5.98 table 7. resistance to chloride migration of the concrete mixtures with maximum aggregate replacement. 2.4. technology 4. alternative binders and supplementary cementitious materials by replacing the traditional portland cement in concrete completely by alternative binders or partially by supplementary cementitious materials, the co2 emissions related to concrete production can be reduced substantially. four alternative binders (’c’, replacing 100 % of traditional cement) and five supplementary cementitious materials (’f’, replacing 40 % of the traditional cement) were selected. c1 and c2 are sulphoaluminate cements, currently available on the market. c3 is a cem iii/a 42.5 cement and c4 a cem iii/b 32.5 cement. f1 is a copper slag, f2 and f3 are inox slags and f4 is a granulated blastfurnace slag. f5 consists of calcinated clay. with cem i 52.5 n as reference cement type and using standard sand, mortar mixtures conforming to en 196-1 were made with the alternative binders and supplementary cementitious materials. the setting times of the mortar mixtures were evaluated conform en 480-2. the results are presented in table 8. depending on the binder type, the alternative binders present a faster (c1 and c2) or a slower (c3 and c4) setting than the reference portland cement. the supplementary cementitious materials all have slower setting times than the reference cement. initial setting time final setting time [minutes] [minutes] reference mix 214 278 c2 103 145 c3 297 403 c4 358 584 f1 293 398 f2 321 418 f3 312 420 f4 267 382 f5 279 388 table 8. resistance to chloride migration of the concrete mixtures with maximum aggregate replacement. the mechanical properties of the mortar mixtures with the alternative binders and supplementary cementitious materials were evaluated in accordance with en 196-1. the results are presented in table 9. all the alternative binders and supplementary cementitious materials lead to lowered mechanical properties of the resulting mortar. 3. conclusions the circular.concrete project aims to identify interesting innovative technologies in the domain of circular economy and enable or broaden their practical application. four different innovative technologies were 144 vol. 33/2022 circular concrete: new technologies rf,7d rc,7d rf,28d rc,28d rf,91d rc,91d [mpa] [mpa] [mpa] [mpa] [mpa] [mpa] reference 10.0 63.9 9.4 71.8 10.1 79.3 c1 5.7 39.5 6.3 43.4 7.0 45.2 c2 8.6 45.3 7.4 50.1 6.5 50.1 c3 6.2 30.4 7.5 42.8 9.6 56.6 c4 5.6 23.4 7.2 34.1 7.3 40.1 f1 6.3 33.0 5.9 35.0 5.6 37.1 f2 5.9 27.2 6.4 35.3 7.0 37.1 f3 4.9 21.4 5.3 26.7 6.0 31.8 f4 5.8 34.4 8.1 49.5 6.5 54.7 f5 6.0 29.0 6.7 40.0 7.7 47.3 table 9. mechanical properties of the mortar mixtures with alternative binders and supplementary cementitious materials. selected and validated in laboratory-scale tests in order to increase their trl. it has to be noted that the scale of this validation was very limited and should be considered as exploratory. the smart crushing technology, which supposedly crushes the concrete into its original components, doesn’t seem very effective for belgian concrete, made with crushed limestone aggregates. the crushed secondary aggregates still contained a lot of adhered mortar. possibly this technology is more effective with concrete with rounded natural gravel. the returned fines (< 0.125 mm) did not show hydraulicity as they are dominantly composed of quartz. the carbonation of recycled aggregates has a clear positive, albeit not that important, influence on their physical and mechanical properties. replacement rates of natural aggregates in concrete of 90-100 % vol by recycled aggregates logically lead to reduced mechanical properties of the concrete (this effect becomes more important for higher concrete strength classes). the durability properties of the concrete are also reduced, although this cannot be concluded for the freeze-thaw resistance (the air content of the concrete seems to be the determining factor for this property). the partial replacement by artificial aggregates did not strongly impact the concrete properties. all the evaluated alternative binders (replacing 100 % of the traditional cement) and supplementary cementitious materials (replacing 40 % of the traditional cement) lead to lowered mechanical properties of the resulting mortar. this research demonstrates the importance of validating innovative technologies by independent laboratories before their application in concrete projects. one the one hand, claims can be validated, on the other hand, extra experience is being built to create the best application conditions in real scale projects. acknowledgements the authors would like to thank the vlaams agentschap innoveren en ondernemen (vlaio), sim (strategic initiative materials) and a great number of industrial partners for their financial support of the circular.concrete project. (www.circular-concrete.be). references [1] cembureau. environmental product declaration (epd) according to en 15804 and iso 14025 portland cement (cem i) produced in europe, 2020. https://cembureau.eu/policy-focus/sustainable-c onstruction/sustainability-standards/. [2] j. lehne, f. preston. making concrete change innovation in low-carbon cement and concrete, chatham house report, 2018. https://www.chathamhouse.org/sites/default/fil es/publications/2018-06-13-making-concrete-cha nge-cement-lehne-preston-final.pdf. [3] smartcrusher bv. from design of the next generation of economical and ecological equipment to delivery and maintenance, 2020. https://www.slimbreker.nl/. 145 www.circular-concrete.be https://cembureau.eu/policy-focus/sustainable-construction/sustainability-standards/ https://www.chathamhouse.org/sites/default/files/publications/2018-06-13-making-concrete-change-cement-lehne-preston-final.pdf https://www.slimbreker.nl/ https://doi.org/10.14311/app.2022.33.0350 acta polytechnica ctu proceedings 33:350–356, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague comparing the environmental performances of new and renovated school buildings oscar mancinelli∗, alessandro pasquale fantilli, bernardino chiaia politecnico di torino, department of structural, building and geotechnical engineering (diseg), corso duca degli abruzzi, 24, 10129 torino, italy ∗ corresponding author: oscar.mancinelli@polito.it abstract. evaluating which is the best choice between renovating an existing construction or building a new structure in countries like italy, where a huge post-war un-listed building heritage does not satisfy the current standards and the economic resources are limited, is not trivial. several parameters come into play, such as such the extent of the construction work, the environmental cost of disposing old materials, the carbon footprint and volume of new materials. this paper is devoted to the analysis of two projects. the first consists of a renovation of a multi-storey existing school built in 1960s having total area of about 9900 m2. the second is a new construction of a three-story school having a total area of about 14000 m2 and made with timber. the results show that the existing school building, although having a lower embodied carbon related to materials, has a higher overall carbon footprint due to the co2 emissions related to operational energy. keywords: environmental sustainability, existing building, reinforced concrete, retrofitting, school buildings, timber. 1. introduction 1.1. state of art according to the latest report on the italian social situation [1], 30% of the italian school buildings were built before 1960, and 44% between 1960 and 1970. this implies that three-quarters of the schools have not been designed according to anti-seismic and energy-saving rules. besides, many of them show significant signs of degradation due to an insufficient maintenance. hence, actions aimed at improving and renovating the italian school building heritage are of fundamental importance and should be rapidly adopted. on the other hand, the environmental sustainability of the construction sector is a problem as well, as it needs a huge volume of raw materials and releases large quantities of greenhouse gases in the atmosphere. specifically, it accounts for 36% of global final energy use and 39% of energy-related carbon dioxide emissions, when upstream power generation is included [2]. also, construction and demolition waste (c&dw) is one of the most important waste streams, accounting for approximately 25% 30% of all the waste generated in the eu [3]. to solve all these problems related to the construction and management of school buildings, some strategies are necessary. the demolition of old and energy-intensive buildings and the construction of new low-energy (or even passive) building, made with materials with low carbon footprint, is definitely an efficient solution. in compliance with this strategy, in recent years, concrete has been replaced by more eco-friendly structural materials, such as wood [4], although studies proved that wooden structures show lower thermal performance than concrete, especially in long-term analyses [5], [6]. nevertheless, the demolition of old constructions, combined with the construction of new buildings, leads to the production of waste and to the consumption of natural resources to produce new materials. on the contrary, rehabilitation, refurbishment and renovation of old buildings make stream waste and the use of new materials limited. regrettably, in many cases, it is not always economically convenient to attain the same thermal and structural performances of a new building by adapting old building to the current code requirements. to provide guidance tools for policymakers and stakeholders, a large number of studies have been carried out, in which environmental performances of new buildings made of different materials are compared. as a result, in terms of global warming potential (gwp), timberframed residential buildings tend to have a lower environmental impact than those made with concrete and masonry [7]. regarding school buildings, studies on structural materials and on envelope systems have shown that concrete and masonry buildings have better thermal performances, because of the heavyweight materials [8]. although manufacture, construction and demolition of masonry and concrete buildings require larger energy and show higher global warming potential, these buildings exhibit lower annual energy consumption and environmental impact in service, which sometimes makes them more sustainable than those made with timber and steel [9]. based on these studies on refurbishment and newly constructed 350 https://doi.org/10.14311/app.2022.33.0350 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 33/2022 environmental performances of new and renovated school buildings case study id gross surface (m2) estimated students type of school cost of construction (€) school building #1 sb #1 13990 1479 science school 23,500,000 school building #2 sb #2 9930 1002 technical school 10,600,000 table 1. main properties of the two schools. buildings, it is still not possible to conclusively determine which of the two alternatives has the best environmental performance over the entire lifespan [10]. besides, studies using the lca methodology mainly focus on energy refurbishment when the environmental impacts before and after intervention are compared. conversely, there are few lcas on the environmental impact of system reparation [11]. also, these approaches have to be reviewed, because the system boundaries of lca are not systematically explicit [12]. the lack of univocal procedures, as well as of the standardised methods of visualising lca results [13], leads researchers to apply methods and assumption that are appropriate for the single case study, making results difficult to compare. therefore, further investigations are needed both to finetune the assessment criteria and to broaden the assessment scenarios. 1.2. research significance is it more convenient to refurbish an existing building or to build a new school? to answer to this question, a benchmarking analysis on two case studies is proposed herein: • school building #1: a newly timber school building. • school building #2: an old school (built in 1960) in which energy efficiency and structural rehabilitation works have been carried out. 1.2.1. the two buildings the school building #1 is a 4-storey precast timber construction (one of which is the basement) with a total living area of about 14000 m2. this type of building has been chosen for the lower carbon footprint of the wood, as well as for the speed of construction given by the prefabrication. the building components (structures, envelope, internal partitions, etc.) fulfil the structural [14] and energy [15, 16] performances required by the current code rules for new buildings. besides, this school has been designed by implementing solutions aimed at optimising the use of climatic conditions and solar radiation, such as avoiding windows on the south side of the buildings and providing shading systems and large overhangs. accordingly, the building consists of a i-shaped main part, where classrooms, offices, laboratories and parking spaces are located. in a separate rectangular block, a sport hall is present. the school building #2 is a 6-storey building (including the basement) with a reinforced concrete frame and a total gross area of about 9900 m2. it consists of a multi-storey building, where mainly offices and classrooms are located, a two-storey building containing two sports halls and some classrooms, and a third single-storey block dedicated to teaching laboratories. refurbishment works, completed in 2013, aimed at complying the structural safety requirements provided by code rules for existing buildings [14]. specifically, the structure has been strengthened by applying steel cage systems for the columns (with l profiles and transverse plates), introducing steel braces, and strengthening the foundation by means of steel pipe piles. to meet the energy performances [15, 16], an extensive renovation of the building envelope was performed, by providing an insulation layer, installing ventilated facades and by substituting old windows and doors. table 1 summarises the main data of the two school buildings. due to confidentiality reasons, the name of the schools and the place where they are located are undisclosed. however, as they are in the same city, some boundary conditions are equal. these include the climatic conditions, the distance between the material production sites and the construction site, as well as the distance to landfill facilities. the life cycle assessment (lca) methodology is carried out to assess the environmental performance of both buildings. even if the environmental impact of the existing school before the renovation and the embodied carbon of the demolished materials are excluded. 2. materials and methods 2.1. methods the life cycle analysis carried out in this study is based on the jrc technical report [3]. the analysis is performed at production level, according to en 15804 [17], and at building level, in accordance with en 15978 [18]. in these standards, a modular approach for the definition of system boundaries is adopted, which enables to allocate the greenhouse gas emissions (ghgs) over the entire life of a building, i.e. from cradle-to-grave. it includes the building materials production (modules a1 to a3), the construction stage (modules a4 and a5), the use phase (modules b1 to b7) and the end-of-life phase (modules c1 to c4). lastly, module d considers the possible benefits and loads beyond the system boundary, namely those provided by the recycling, recovery and reuse of materials. the lifespan of both the buildings investigated herein is assumed to be 50 years, dur351 o. mancinelli, a. f. fantlli, b. chiaia acta polytechnica ctu proceedings figure 1. models created in designbuilder concerning: (a) sb #1; (b) sb #2. ing which the emissions related to maintenance and replacement of components are neglected. although assuming the same lifespan for both the refurbished and the construction of new building could not take into account the longer life expectancy of new school, it can be considered reasonable within the italian context. in fact, 50 years is the average life expectancy that italian building code assumes for ordinary structures [19], and it is also the average time that elapses between major refurbishment works [20]. 2.1.1. life cycle assessment both the schools have been modelled with revit software by implementing the stratigraphy and the geometric dimensions of the projects. in this way, the volumes of materials are computed and then used as input data to estimate the embodied carbon. the unitary impact of each material, expressed in accordance with the climate change indicator, is an input data. the global warming potential (gwp) indicator was selected for the assessment of the environmental impact of the two buildings, because it is consistent with the scope of the study. moreover, it is the most used indicator in the world, as it provides the result in terms of co2 equivalent, which is the main cause of global warming [21]. basically, there are two types of data sources from which the carbon footprint of materials can be obtained: generic databases (i.e., secondary data) and primary data, so-called environmental performance declarations (epds), provided by producers. results coming from the two categories of data can differ even of 25% for the gwp indicator [22]. this is due to the fact that secondary data are frequently unreliable, because they are based on average local data. conversely, epd is nowadays associated to all the new building material. thus, using product-specific primary data is recommended. in the current analyses, when no specific design indications were available, the same epd is associated with the common building components and materials (openings, insulation layers, etc.), in order to make the results of the two buildings comparable. 2.1.2. thermal analysis the evaluation of the co2 eq. emitted by the two buildings in 50-years of use (module b) has been carried out through the thermal analysis of the building, developed within designbuilder [23]. these models are shown in figure 1. as a result, the energy for heating and cooling the schools, and that used for domestic hot water (dhw) over the years is computed and converted in terms of co2 eq. by means of suitable conversion factors [24]. specifically, an electric heat pump system (eer = 1,75) is used to cool both the schools, whereas the needs for heating and dhw are produced by a heating system (cop = 0.85) powered by natural gas. system losses are not assessed. the emissions related to internal furniture and lighting system are neglected as well, because they are not comparable to the energy intensive laboratories present in sb #2. the overall impacts of the two buildings are calculated by summing the contributions from the shortterm (i.e. construction phase, in section 2.1.1) and the long-term (i.e., use phase, in section 2.1.2), assessments. to compare the results, two functional units are considered herein: • co2 eq. emissions per unit of gross building area (kg co2 eq/m2); • co2 eq. emissions per student (kg co2 eq/pers.) 2.2. materials for both the schools, information on the materials is provided, even if suppliers and location are unknown. thus, epd was selected for each material, assuming the same producer when it is present in the two buildings. on the other hand, the embodied carbon of the materials demolished in the school building #2 has not been taken into account, because the type, the amount, and the percentage of possible recycling, renewal or reuse are not included within the project. building materials are aggregated into three categories, namely skin, space plan, and structure, following the layered division (the so called 352 vol. 33/2022 environmental performances of new and renovated school buildings figure 2. embodied carbon of building materials grouped into macro-layers: (a) total amount; (b) amount per unit of gross building area (u.a.); (c) amount per student. figure 3. comparison of operational energy of buildings grouped by districts: (a) total amount; (b) amount per unit of gross building area; (c) amount per person. figure 4. comparison of overall co2 eq. emissions marked by sources (material and energy needs): (a) total amount; (b) amount per unit of gross building area; (c) amount per person. 353 o. mancinelli, a. f. fantlli, b. chiaia acta polytechnica ctu proceedings shearing layers) originally suggested by brand [25], and updated by other authors [26, 27]. through this approach, the contributions to the overall embodied carbon of the materials given by building envelope (skin), interior building components (space plan), and structure, are computed. as expected, sb #1 included a quantity of materials larger than that of sb #2. in fact, the existing materials of sb #2 are not included in the assessment. a significant volume of reinforced concrete has been used in sb #1 because only concrete provides the strength and durability performances required for some structural elements such as foundations, retaining walls, ground slabs and staircase envelope walls, whilst the structure is mainly made of wood. 3. results the histograms of figure 2 depict the results in terms of co2 eq., calculated by multiplying the quantities of materials by the unit emissions reported in the corresponding epds. in figure 2(a) the total amounts of co2 eq. of two buildings are reported and divided into the percentages of macro-layers. sb #1 accounts for the largest embodied carbon of materials, as ghg emissions are 40% higher than in sb #2. this is due to the greater amount of materials used in new constructions. on the other hand, as shown in figure 2(b) and figure 2(c), this difference shrinks when the embodied carbon is referred to the unit of gross floor area and to the number of students, respectively. the incidence of the materials of the building envelope is higher in sb #1 than in sb #2, whereas materials belonging to space plan and structure have a greater impact in sb #2. indeed, the envelope of both the buildings, including expanded polystyrene, glass and aluminium of the windows, etc., which generally have a high environmental impact. on the other hand, the structure and the internal partitions of sb #1, made of wood (which is a biogenic source of carbon storage and highly recyclable [28]), have negative values of co2 emissions (modules a1-a3 and d) and compensates the co2eq. of the high-impact materials such as concrete, steel etc. in the histograms reported in figure 3a, the three types of energy needs (heating, cooling and dhw) are compared. their incidence as a percentage of the total required energy, computed over one year of use of the two buildings, can also be observed. if on the one hand the heating requirement is the same in both the school (i.e., 76% of the required energy), on the other hand the energy for summer cooling is higher in sb #1 than in sb #2. in fact, the lower thermal inertia of wood has a marginal effect. due to the large number of labs and sport halls, hot water consumption in sb #2 is larger than in sb #1. however, the most interesting result is the higher overall energy consumption of sb #2, which is about 6% higher than that of sb #1. this percentage rises to 50% and 57% when related to the gross building area (figure 3b) and the number of students (figure 3c), respectively. such result is due to the intrinsic difficulties of making an existing building as efficient as new constructions, because some technical solutions cannot be always put into practice. for instance, in sb #2, some thermal bridges cannot be removed, and it is not possible to modify the orientation of the glazing or to install shielding systems to exploit solar radiation optimally. multiplying the energy needs by the proper emission factors, the co2 eq. emissions produced by energy requirements over the 50-year lifespan of the building can be calculated. figure 4a summarizes the overall emissions, and the related percentages, due to both materials and energy systems. however, these results are strictly dependent on the assumptions made and boundary conditions, and therefore cannot be generalised. furthermore: • as shown, global co2 eq. emissions are strongly affected by winter heating demand (both sb#1 and sb#2 are located in an alpine area). therefore, different results can be obtained if the same analysis are performed in a warmer climate scenario, where the effects of thermal inertia on the thermal behaviour of the building prevails over the transmittance of the building envelope. • with respect to the embodied carbon of materials, reference has been made to the primary data contained in epds. therefore, the result is strongly influenced by the scenarios assumed by the producers, in particular related to the end of life of the material. for example, the company producing the wood elements assumed their complete recycling at the end of life. • a lifespan of 50 years has been assumed for all building components in both schools. actually, they usually differ according to their function [25– 27], yet it is difficult to reliably estimate their lifespan since it depends on the intrinsic characteristics of the material and the local practices. besides, emission factors, used to convert operational energy into co2 eq. emissions, are assumed to be constant over the lifespan, yet increasing the use of renewable energy sources and improving efficiency of energy production plants is expected in the future, which might lead to a decrease in the emissions related to energy needs. • the environmental impact of the excavation of foundations in sb#1, along with materials demolished in sb #2 and sent to landfill or to the recycling, renovation, reuse chain was not considered due to lack of information. however, it is reasonable to assume that their influence on the total co2eq. computation is small. also, the consumption of undeveloped land, due to the construction of the new school on a vacant lot, is not taken into account. 354 vol. 33/2022 environmental performances of new and renovated school buildings 4. conclusions and limits of the study based on the above results, the following conclusions can be drawn: • the total embodied carbon of the materials of sb #1 is higher than in sb #2, both in absolute value and also related to gross floor area and number of students. this is due to the larger amount of materials used in the new building, although a significant quantity of co2 eq. is compensated by the negative values of the timber structures (wood is a biogenic source of carbon storage). • as far as energy requirements are concerned, sb #2 has higher needs. even though energy efficiency improvements are implemented on sb #2, it is not possible to reach the standards of a new building, especially with regard to the reduction in heating energy needs. however, the same analysis performed in warmer climates, where energy demand mainly relies on the thermal inertia rather than the transmittance of the envelope, gives opposite results. • the overall emissions of the two buildings are very similar. nevertheless, as sb #1 has a larger area and more students, it can be stated that sb #1 performs better than sb #2 from an environmental point of view. in fact, emissions calculated over the lifespan of 50 years override the embodied carbon of the building materials and penalize the global performances of sb #2. this study shows that, in a specific climatic context, characterized by harsh winters and mild summers, the solution of building a new school building with eco-friendly materials, such as wood, should be preferred to the refurbishment of an existing school in reinforced concrete. thus, the outcomes of the research can be implemented into a decision-making process that compare benefits and drawbacks from the two possible alternatives, within the public building sector. references [1] c. lombardi, m. baldi. cinquant’anni di economia marittima in italia : 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[28] g. guest, r. m. bright, f. cherubini, et al. consistent quantification of climate impacts due to biogenic carbon storage across a range of bio-product systems. environmental impact assessment review 43:21-30, 2013. https://doi.org/10.1016/j.eiar.2013.05.002. 356 https://doi.org/10.1126/science.213.4511.957 https://doi.org/10.1007/s11367-015-0938-z http://oz.berkeley.edu/users/breiman/using_random_forests_v4.0.pdf https://doi.org/10.1016/j.scitotenv.2016.03.098 https://doi.org/10.1088/1755-1315/225/1/012019 https://doi.org/10.1016/j.eiar.2013.05.002 acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.38.0410 acta polytechnica ctu proceedings 38:410–416, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague exploring the state of knowledge and gaps regarding sustainability within viennese construction industry aída santana-sosa∗, martin aichholzer, elena mitrenova, martin schachenhofer vienna university of applied sciences campus wien – fh campus wien, department building and design, favoritenstraße 226, 1100 vienna, austria ∗ corresponding author: aida.santana_sosa@fh-campuswien.ac.at abstract. different approaches to assess sustainability goals are found in construction sector, mainly related to energy efficiency through building technology and high-tech components, to low-tech strategies based on passive planning methods, the use of renewable raw materials, and building certification programs, which, however, are mainly an evaluation of already built objects. austrian construction industry is extremely fragmented and heterogeneous, making research, development and innovation an issue for companies. the core of this project is to bridge the specific gaps in knowledge within viennese construction companies about sustainable architecture and respond to these urgent needs by developing a customized postgraduate course. within the first phase of the project, team meetings, a workshop and an online survey were run to assess the state of knowledge of viennese companies. by doing so, information deficits were identified, companies’ need for knowledge was determined and topics to be transferred were raised. keywords: sustainability, knowledge gap, expertise, industry needs, customized modular course. 1. introduction in 2016, construction industry in austria was responsible for 71.8 % of the total waste generated, being also the largest producer of co2 [1, 2]. strategies towards circular economy and the end of the fossil fuel era must be urgently implemented. use of renewable materials, improvement of thermal capacities of envelopes and reduction of operative carbon emissions among others, are imperative topics in the agenda of the city of vienna [3]. the assessment of such requirements is critical and need especial expertise of the companies involved. according to few surveys involving austrian construction companies [4, 5], the most relevant areas willing to be explored in this field are those related to low-tech approaches, energy efficient construction, ecological materials, and particularly life cycle assessment of buildings. based on that, it can be stated that there is a willingness of further qualification in sustainable construction among the professionals of the sector. besides, circular economy and building large-volume construction with renewable materials are relative ground-breaking issues, whereby targets and contents have not been widely explored, nor integrated in the curriculum of universities. for this reason the required expertise is rarely found within construction companies and seldom extended within the industry. main goal of this project is to bridge this gap between crosscutting issues in sustainable architecture and high education training, looking holistically at all sub-areas and serving like a platform for exchange, transfer and expansion of expertise, as well as for permanent networking and regional cooperation between companies, experts, students, alumni and those interested on sustainable planning. this project aims to extract, process, structure and offer specific know-how from different disciplines to professionals and interested public. for doing that, three phases are followed. the first phase consists on the analysis and representation of the current situation of the planning offices in vienna regarding the assessment of sustainability, their average existing level of knowledge and expertise and the specific topics of qualification willing to be explored. by doing so, knowledge gaps are identified. on this analysis relies the second phase, wherein customized public talks and interactive workshop series are developed, where the reported needs of qualification are intensively and interdisciplinary discussed with the purpose to identify enablers and barriers, and support the possible application of sustainable strategies. the third and last phase is focused on the further development of the findings reported in form of a post-gradual course, and the processing of collected knowledge to be offered to professionals and interested public within an open source platform. this paper describes the work done during the first phase, focused on the identification of knowledge gaps. 2. background sustainability aims to offer next generation at least same or higher quality of life than at current situation. it can be roughly divided into the pillars of economic, 410 https://doi.org/10.14311/app.2022.38.0410 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 38/2022 exploring the state of knowledge and gaps regarding sustainability . . . ecological and socio-cultural quality. besides, technical, process and location quality play a major role in building sector. there are many different approaches to assess sustainability in terms of: (i) energy efficiency, mainly controlled by building technology and high-tech components; (ii) low-tech strategies, based on passive design concepts to maximize solar gaining and use of natural resources without high technological expenditure; (iii) renewable raw materials and timber construction; (iv) building certification programmes, which, however, mainly represent an evaluation of already planned objects; and (v) flexibility in floor plan design and participatory planning processes. a strongly arising aspect is the circular economy, which in construction means driving from the current linear system (cradle to grave) to a circular material and energy system (cradle to cradle), in which waste production, energy consumption and emissions are reduced by refusing, reducing and closing material and energy loops. austrian construction industry is scattered, small-scaled and heterogeneous, where 89 % of all austrian companies in the construction sector have 0–9 employees [6]. this situation makes research and development and thus innovation complicated. in addition, sustainable construction is currently struggling with the situation that every new building represents a prototype, meaning considerable additional work for planners, contractors and authorities. issues concerned to fire protection and sound insulation are rarely assessed with tested products. for this reason, several companies have already developed own individual solutions for partial and detailed problems. such in-house solutions could be shared, reducing complexity and increasing efficiency in design and construction. the existing certification and training courses found in austria are on the one side usually focused on specific and separated sub-areas like life cycle costs, straw bale construction, energy performance certificates and passive houses, or on the other side are extensive and expensive. workshops that can be booked individually covering different scales and areas of construction do not exist to date. main goal of this project therefore, is to work as knowledge hub to drive an exchange, transfer and development of know-how between viennese planning companies aiming to bundle the existing and dynamic expertise and to make it available to the public. 3. analysis of current situation in order to develop a customized curriculum and an adequate exchange platform adapted to the needs of the companies, an exhaustive analysis of the current situation was run. goal was to estate the average level of knowledge and knowledge requirements about sustainable planning within the sector. information deficits and learning needs were identified and key topics for deeper analysis and further exchange were uncovered. by doing so, the content of the curriculum and the exchange platform was assessed with the further willingness of collecting and discussing existing knowledge from different disciplines. besides, didactic methods and dedication intensity were also asked aiming to also meet the requirements of the industry, raising awareness and disseminating information on sustainable building. 3.1. data collection and target group aiming to assess correctly the industry needs from different disciplines, a three steps procedure was run when colleting the data: (i) collection and selection; (ii) discussion and development; and (iii) analysis and assessment. firstly, regular team meetings were run to collect ideas, best practices examples and measures for sustainable planning concepts, and select main topics and relevant actors. based on that, a professional workshop was organized where external experts on the defined topics were invited to enhance discussion, unfold sub-areas, fix boundaries and support further development. all inputs were documented and structured stating the basis for the formulation of an online survey, which settled the fundamentals of this study. its analysis and assessment determined the relevance of the selected topics and opened the repertory of new themes. primary target group for the data acquire was planning offices, evolving different stakeholders from spatial and landscape planners, to developers, architects, structural engineers and other specialist planners. secondary target groups were students, alumni, teachers and researchers in the field of sustainable planning. nevertheless other professional groups and construction trades were invited to take part implementing a bottom-up and a top-down approach. 3.1.1. collection and selection main objective of this step was to collect and filter existing knowledge and information on sustainable construction and select the essentials to make it available. literature and online review, together with regular team meetings and brainstorming served to define the key themes under the following discourse: sustainable architecture throughout the entire life cycle of a building englobes resource-related factors, socio-cultural issues and economic aspects. main strategy to ensure sustainability is the implementation of renewable, regional and less energy intensive resources, combined with a comprehensive use of technical equipment. on the socio-cultural level, the inclusion of all individual citizens is key for an active participation and co-determination to design responsible, resourcesaving and resilient cities. on the economic side, 411 a. santana-sosa, m. aichholzer, e. mitrenova, m. schachenhofer acta polytechnica ctu proceedings figure 1. definition of sustainability (grouped by rated values 1 to 2, 3 to 4 and 5 to 6, where 1 means not applicable at all and 6 fully applicable). affordability and circular economic system need to be assessed. 3.1.2. discussion and development main aim of this step was to work out the challenges and potential of sustainable buildings together with experts. an intensive workshop was held, where a total amount of 19 participants from various sectors of construction industry took part led by an experienced moderator. it also served as an official kick-off event for the public presentation of the project, where background, goals and methodology were introduced. the workshop was run in two rounds separated with a presentation of a project that served as best practices example in terms of ecological, economic and social sustainability. within the first round of the workshop, all participants were organized in groups of 4 or 5 according to their expertise and experience enhancing interdisciplinary exchange. groups were asked to discuss about their own concept of sustainable buildings and then asked to picture their developed concept, wherein the main essential characteristics were to be included and defined. each concept was presented and discuss with all participants, what generated and consolidated a common picture of a sustainable building. focus of the second round was the assessment of industry demands under the main concept of “needs and wants”. a brain-walk-method was performed, where four main topics were explored on four different flipcharts, englobing each thematic one to three key questions. all participants were encouraged to move freely through the panels and give answers and inputs to the themes prescribed. the intensive and interdisciplinary exchange performed served as a basis to the development of the online questionnaire, while settling the fundamentals of the upcoming curriculum. at the same time, concrete needs regarding further education and research were defined and existing information sources were identified. 3.1.3. analysis and assessment based on the discussion and inputs reported and documented by previous steps, an online questionnaire was developed following a 3-stage structure: • collection of personal data (profession, position, affiliation); • identification of the level of knowledge (own definition of sustainability and challenges when implementing sustainable strategies); and • analysis of further training demands (knowledge gap, needs and prerequisites). email addresses and linkedin profiles from potential interested professionals were selected and grouped. the survey was sent to a total amount of 15 500 contacts. by doing so, the project was also promoted within the sector, arising the awareness of the thematic. 3.2. findings 423 people were reached and a total of 121 participants took part in the survey. the results are representative with the respective range of fluctuation within a confidence interval of 95.5 %. 3.2.1. participants’ professional background almost half of the participants taking part on the questionnaire are architects (43.8 %), being consequently the most representative group, followed by researchers and lecturers (20.6 %), and advisers (19.0 %). further professionals took part in the survey representatively such as developers (16.5 %), specialist planers (12.4 %) and decision makers (9.1 %). a deeper analysis regarding to specific job positions showed that half of respondents are managing directors (48.6 %), while one fourth employees (27.9 %) and team leaders (23.4 %) respectively. on the question of how often participants deal with sustainability in their daily activities, 40 % said very often to often, while 20 % said little or very little, and another 40 % said only sometimes. these data represent a higher participation of stakeholders involved in the process of decision-making, what brings relevance to the survey and the project as they can work as enablers having relevant impact on further steps in production and construction processes. 3.2.2. definition of sustainability and barriers to its implementation for the definition of the multifaceted concept of sustainability in construction, almost two thirds of the respondents answered that a holistic approach with a responsible use of resources and an intensification of reuse, renovation and re-densification is fully applicable with an average of 5.38, 5.37 and 5.31 respectively, where 1 means not applicable at all and 6 fully applicable. in figure 1 these most suitable definitions 412 vol. 38/2022 exploring the state of knowledge and gaps regarding sustainability . . . figure 2. ability to implement sustainable strategies. of sustainability are represented, where the 86.7 % of all respondents scored a holistic approach with the highest values 5 and 6, followed by the responsible use of resources with 85.6 %, and renovation or restoration with 84.4 %. by these three definitions just around 4.4 % up to 10.0 % of all participants scored them with the lower values of 1 and 2, and 3 and 4. in further stage, the optimisation of buildings physics with an average of 4.91 was scored by the 71.1 % with the highest values 5 and 6, by the 21.1 % with 3 and 4, and by the 7.8 % with 1 and 2. next, the use of renewable energy towards zero-energy and plus-energy got a value of 4.82, where the 72.7 % of participants rated them with 5 and 6, 18.2 % with 3 and 4, and 9.1 % with 1 and 2. last, participation in design process was also highlighted as key aspects with an average score of 4.40, being valued the highest by the 51.1 %, with 3 and 4 by the 40.0 %, and with 1 and 2 by the 8.9 % of respondents. the current situation regarding the capacity or ability of implementing sustainable strategies is rated just as sufficient by the participants, who stated an average of 3.7 on a scale from 1 to 6, where 1 means they cannot implement nothing at all and 6, everything. that means, more than half of participant reported that they are able to only implement an enough amount of strategies, while the other half meant many and very few strategies almost equally. figure 2 represents the quoted answers. main reasons quoted as barriers for a successful implementation of sustainable strategies in design and construction were the lack of: (1.) awareness of the client (5.18 to 6); (2.) willingness to invest time and personnel resources (4.75 to 6); (3.) tax approaches and subsidies (4.72 to 6); (4.) marketing or public information (4.51 to 6); (5.) know-how of the executing companies (4.47 to 6); (6.) cost certainty (4.25 to 6); (7.) expertise of the planners (4.20 to 6); (8.) regulatory or normative basis (4.15 to 6); (9.) willingness to cooperate and share knowledge (4.10 to 6); (10.) specific education and training (3.87 to 6). access to specific knowledge and training opportunities play a comparatively small role, in contrast to the lack of awareness of the client. therein lies the need to provide suitable accompanying measures, which depending of target groups are to be offered in different forms. in order to arise awareness among clients, dissemination, press work or events could be promote its consideration. regarding to architects, trainings for effective advisory talks could be developed, based on conducting discussions, with argumentative discourses emphasising objective advantages and personal benefits, together with opposite foundations to debate, with the aim to bring out decision and generate positive conclusions. 3.2.3. information sources the sources of information consulted by the participants when aiming to design and build more sustainably are mainly specific literature and reports (70.2 %) and personal contacts (67.0 %). second main information sources are further education formats and trainings like seminars or workshops (47.8 %), conferences, messes, and congresses (45,7 %), and online platforms (45.7 %), followed closely by contacting directly planners with experience and representative projects (43.6 %). certification tools, guidelines, building regulations and standards were rated just by 33.0 % and 29.8 % respectively. consulting personal contacts can be considered ambivalent since real sources of knowledge like certification tools, guidelines, etc. as well as building regulations and standards are not extensively used and therefore the level of knowledge of consulted contacts cannot be stated. 3.2.4. knowledge gap and demand by analysing the knowledge gaps, the life cycle assessment of constructions as shown in figure 3 was reported by 59.3 % of the participants as the area with highest score, wherein the 19.8 % and 20.9 % considered there is enough to much available knowledge. since the lifecycle assessment under a holistic approach was also quoted as an accurate definition of sustainability, it should be considered a fundamental aspect when developing a curriculum towards sustainable construction. planning optimized life-cycle costs buildings was also considered by the 43.0 % of the respondents as an area with remarkable knowledge gap, wherein the 38.4 % stated there is enough knowledge, 413 a. santana-sosa, m. aichholzer, e. mitrenova, m. schachenhofer acta polytechnica ctu proceedings figure 3. knowledge gaps (grouped by rated values 1 to 2, 3 to 4 and 5 to 6, where 1 means high gap of knowledge and 6 no gap at all). figure 4. demand of knowledge (quoted from 1 to 6, where 1 means not applicable at all and 6 fully applicable, and starting by 3.00). and 18.6 % declared there are no relevant knowledge gaps. these statements make clear that a compromise between ecological and financial assessment need to be widely explored taking into account the whole lifecycle. specific expertise on sustainable planning approaches and confidence on using tools and software for this purpose were reported by 32.5 % and 35.3 % respectively as aspects with high knowledge gaps willing to be assessed. however, 43.0 % and 50.6 % of the respondents considered that there is enough expertise, where the 24.4 % and 14.1 % respectively considered there are no need for further knowledge building in this regard. these two aspects are strongly related with the two previous ones, since they englobe the required competences and tools for achieving the desired lifecycle assessment rated with the highest scores. finally, building services, building physics and structural performances were pointed out by 27.1 %, 24.4 % and 16.7 % each, meaning that even though being significant areas to consider when planning sustainably, enough expertise is already extended within the sector. that becomes evident with the 52.9 %, 38.4 % and 51.2 % which considered there is enough expertise, and with the 20.0 %, 37.2 % and 32.1 %, which stated there is no knowledge gap. these last aspects can also be grouped into a category which refers to the performance of the building. by doing so, three main groups are recognised where all key knowledge gaps can be embedded. the first and more relevant refers to the correct and accurate planning regarding the whole lifecycle of the building including its ecological and economic assessment. the second group relates to the required expertise and tools to be able to achieve those in the design. the third describes the structural, acoustic and thermal performance of a construction and its correlated building services. by assessing the specific project phases, wherein to focus the implementation of strategies, all phases were rated as important, from basic analysis, to preliminary design, design planning, submission planning, detailed design, tendering, supervision of construction work and documentation, and project management, wherein basic analysis and submission planning was respectively rated with the maximal value (5.18) and the minimal one (4.12) with little difference between. that means all contents are above average in importance, having received the highest value for answer 6 (from 55 % to 28 %), except for submission planning, which highest percentage at answer was 4, what is still above average. the survey also highlighted areas represented in figure 4 as key topics for desired further training, where almost all contents are important to an above-average extent, but without peak. the top demand is formed by a sextet with values between 4.94 and 4.48, namely: (i) deconstruction and disposal, englobing urban mining and dismantling and reuse and recycling of building components (4.98); (ii) life-cycle management including economic and ecological assessment (4.78); (iii) low-tech planning strategies and climate-friendly 414 vol. 38/2022 exploring the state of knowledge and gaps regarding sustainability . . . figure 5. effective strategies (quoted from 1 to 6, where 1 means not applicable at all and 6 fully applicable, and starting by 4.00). architectural concepts (4.75); (iv) building with renewable materials, especially large volume timber constructions, assessing integral planning, structural performance, technical building services and building physics (4.71); (v) sustainable renovation, revitalisation and redensification of existing buildings (4.65); and (vi) innovative materials and developments (4.48). these data reflect a clear trend towards passive and responsible architectural design with low-tech planning strategies that use innovative environmentally friendly, renewable and low-carbon materials, addressing the full life cycle of the building and encouraging the extension of its lifespan through renovation, revitalisation or transformation (change of use) and re-densification, as well as through deconstruction and recycling of its building elements and components. on the other side, the respondents rated with the relatively lowest value, the need for theory and history (3.15), the use of certification tools and systems (3.71), and the social participation in planning processes (3.86). the low rate of the first aspect could be justified through the presumed remoteness from practice and questionable applicability and practicability. less comprehensible is the low interest in certification systems, which actually ease remarkable improvements within the overall process. these certification tools and systems are normally used after the project is planned and built in order to gain recognition, so that they do not impact the design development. with this in mind, it could be of relevant importance to implement these certification systems and tools proactively in order to manage different solutions within design and quantify their repercussions, helping in the decision-making process. low rated were also social aspects like participation in planning processes, what does not fit with one of the main pillars of sustainability, namely the social sustainability, and is an issue that need to be assessed, having architects the chance to work as advisors to clients and society communicating benefits of building responsibly and arising the awareness regarding sustainability. in between we find a number of topics that were scored between 3.96 and 4.33, which englobe strongly related aspects like design concepts towards passive house, building physics and energy concepts, and software tools and simulation. other aspects found within these scores relate to the development of urban and rural areas, and to promotional approaches and fiscal policies. the low scores for these aspects could be justified by considering that they are competencies that are beyond the reach of the planning offices, which are the group with the highest participation in the survey. regarding the methodological design of the customized programme was answered by the majority with the preference of part-time continuing education courses as evening or weekend programmes (67.1 %), as well as the possibility of the completion of individual modules (59.5 %). getting a certificate (44.0 %) was still rated as considerable relevant, while an academic degree (11.4 %) as low important. undesirable was the fact of completing a large course of about 2 years (6.3 %). 3.2.5. possible effective strategies all given strategies were considered by the participants to be more effective than average with a punctuation between 4.28 and 5.50. nevertheless, as shown in figure 5, a top quintet can be recognized, formed by: (i) raising awareness for sustainable building (5.50); (ii) networking and cooperation (5.22); (iii) tax approaches and subsidies (5.16); (iv) easy access to practical solutions (5.12); and (v) (re)use of existing buildings (5.00) instead of new construction with average values between 5.50 and 5.00 of 6.00. three of those topics, namely networking and cooperation, easy access to practical solutions and (re)use of existing buildings, are central concerns of this project, while the fact of raising client’s awareness about sustainability was mentioned in the previous section as important topic to be addressed despite being scored 415 a. santana-sosa, m. aichholzer, e. mitrenova, m. schachenhofer acta polytechnica ctu proceedings low, eventually through architects acting as advisor. the other theme requires a rethink about the inclusion of other stakeholders, especially politicians with regard to tax approaches and subsidies. besides, owners and investors could be also key stakeholders to be included as target group in order to explore the possibilities towards (re)use of existing buildings rather than new construction. on the other side, even the three strategies considered as least effective, namely: (i) promotion of participation, co-determination and other democratic strategies (4.28); (ii) transfer of self-generated knowledge (4.30); and (iii) mandatory monitoring of the entire life-cycle of buildings (4.37), were rated by almost 50 % with 5 and 6 (average values around 4.30), meaning that they are also considered of highly relevance. more transparency eventually through public benchmarks (4.99) was also stated as remarkably important, followed by a clear and common definition of sustainable construction (4.87), alternative forms of financing (4.72), by regulatory or normative obligations (4.71) and basic business knowledge (4.62). 4. conclusions and next steps having explored and analyzed the knowledge gaps, demands and desired approaches of the planning offices during the first phase of the project, a series of interdisciplinary and interactive workshops will be organized within the second stage of the project with the aim of fostering tailor-made and diversity-adapted mutual knowledge transfer for design and planning firms. the main objective of this exchange of different viewpoints and professional approaches is to force a fruitful discussion in order to identify facilitating and constraining aspects of the implementation of the proposed strategies. in this way, the topics chosen for a more detailed analysis are grouped and will be discussed in thematic blocks. experts on each topic will be invited to take part not only in the presentation of the contents, but also to actively interact in the discussion with the participants. in the same way, a building already realized and recognized for its sustainable quality will be chosen and gradually analyzed according to the topics relate to each thematic block. in this way, the participants together with the invited experts and the planners and builders of the building themselves will evaluate the implemented strategies by applying the acquired knowledge. parallel to the analysis, new strategies will be proposed and their applicability will be jointly evaluated. besides these interactive workshops, and according to the analysis run within the first stage of project, public lectures and expert talks are to be organized in order to raise general awareness and disseminate information about sustainability to interested public. both methods of knowledge transfer will be evaluated through questionnaire and discussions to form the basis for the development of the customized postgraduate modules within the third stage of the project, which will be incorporated in the existing university training format. furthermore, all knowledge gained throughout the whole project will be processed and offered to the public, so that they can serve as a basis for new services, research areas and product developments. in further steps within the third stage of the project, an extended literature review on educational theory and construction management will be performed to propose a suitable and functioning learning framework, where concepts like competence will be defined including its types, orders and domains of learning [7], to better address the didactic methods and contribute to the sector. acknowledgements authors want to thank everyone involved in this project, from those who took part into the online survey, to the experts working in the workshops. a special thank goes to the founder agency ma23 for enhancing sustainability on construction and education. references [1] bundesministerium für nachhaltigkeit und tourismus. die bestandsaufnahme der abfallwirtschaft in österreich. (statusbericht 2018). bundesministerium für nachhaltigkeit und tourismus, wien, 2018. [2] international energy agency, united nations environment programme. 2018 global status report: towards a zero-emission, efficient and resilient buildings and construction sector, 2018. [2022-04-06]. https://wedocs.unep.org/20.500.11822/27140 [3] magistrat der stadt wien. klimaschutzprogramm der stadt wien: fortschreibung 2010-2020, 2009. [4] kompetenzzentrum bauforschung. mitgliederbefragung österreichisches baugewerbe: spezialauswertung wien, 2017, 2017. [2022-04-06]. https://www.zukunft-bau.at/sites/default/files/ dateien/inhalt/180227_folder-umfrage_wien.pdf [5] s. wirth. die größten herausforderungen werden noch wichtiger, 2016. in: expertenbefragung zukunft bauen, [2022-04-06]. https://www.expertenbefragung.com/index.php/ zukunft-bauen/zukunft-bauen-2016/groessteherausforderungen-werden-noch-wichtiger [6] statistik austria. bundesanstalt statistik. ergebnisse im überblick: statistik zur unternehmensdemografie 2007 bis 2019 nach wirtschaftsbereichen, 2017. [2022-04-06]. https://www.statistik.at/fileadmin/ pages/178/1_ergebnisse_im_ueberblick_statistik_ zur_unternehmensdemografie_2007_bis_2020.ods [7] g. killip. a reform agenda for uk construction education and practice. buildings and cities 1(1):525–537, 2020. https://doi.org/10.5334/bc.43 416 https://wedocs.unep.org/20.500.11822/27140 https://www.zukunft-bau.at/sites/default/files/dateien/inhalt/180227_folder-umfrage_wien.pdf https://www.zukunft-bau.at/sites/default/files/dateien/inhalt/180227_folder-umfrage_wien.pdf https://www.expertenbefragung.com/index.php/zukunft-bauen/zukunft-bauen-2016/groesste-herausforderungen-werden-noch-wichtiger https://www.expertenbefragung.com/index.php/zukunft-bauen/zukunft-bauen-2016/groesste-herausforderungen-werden-noch-wichtiger https://www.expertenbefragung.com/index.php/zukunft-bauen/zukunft-bauen-2016/groesste-herausforderungen-werden-noch-wichtiger https://www.statistik.at/fileadmin/pages/178/1_ergebnisse_im_ueberblick_statistik_zur_unternehmensdemografie_2007_bis_2020.ods https://www.statistik.at/fileadmin/pages/178/1_ergebnisse_im_ueberblick_statistik_zur_unternehmensdemografie_2007_bis_2020.ods https://www.statistik.at/fileadmin/pages/178/1_ergebnisse_im_ueberblick_statistik_zur_unternehmensdemografie_2007_bis_2020.ods https://doi.org/10.5334/bc.43 acta polytechnica ctu proceedings 38:410–416, 2022 1 introduction 2 background 3 analysis of current situation 3.1 data collection and target group 3.1.1 collection and selection 3.1.2 discussion and development 3.1.3 analysis and assessment 3.2 findings 3.2.1 participants' professional background 3.2.2 definition of sustainability and barriers to its implementation 3.2.3 information sources 3.2.4 knowledge gap and demand 3.2.5 possible effective strategies 4 conclusions and next steps acknowledgements references https://doi.org/10.14311/app.2022.33.0363 acta polytechnica ctu proceedings 33:363–369, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague properties of concrete using crushed stone powder with various specific surface areas takeju matsukaa,∗, mariko suzukib, haruhiko matsushitac, hiroshi yokotad, koji sakaie a national institute of technology, kumamoto college, 2627 hirayama-shinmachi, yatsushiro, kumamoto, 866-8501, japan b kobe university, 1-1 rokkodai-cho, nada, kobe, 657-8501, japan c chuosaiseki co., ltd., 856-4 hara, takatsuki, 569-1051, japan d hokkaido university, kita 13, nishi 8, kita-ku, sapporo, hokkaido, 060-8628, japan e japan sustainability institute, sapporo, hokkaido, 065-0021, japan ∗ corresponding author: matsuka@kumamoto-nct.ac.jp abstract. many countries use crushed stone and crushed sand as the aggregate for concrete. however, a large amount of crushed stone powder is produced as by-products in the manufacturing process. unless effective utilization of crushed stone powder is considered, it is not a sufficient measure in terms of sustainability. in this study, the properties of concrete using crushed stone powders with different specific surface areas are investigated. as the results, a strong correlation was found between the unit water content of concrete and the total surface area of the crushed stone powder contained in the concrete. furthermore, it was concluded that the correlation between the strength index, the co2 index, and the virgin resource index of concrete using crushed stone powder depends on the types of crushed stone powder and crushed sand. keywords: crushed stone powder, fine aggregate, fresh concrete, hardened concrete, specific surface area. 1. introduction crushed stone and crushed sand are often used as the aggregate for concrete in many countries. in the future, concrete aggregates would rely on crushed stone and crushed sand because natural aggregate, such as sea sand is probably prohibited from collecting. the production methods of crushed stone and crushed sand are classified into two types, such as a dry process type and a wet process type. in the dry process type, a large quantity of crushed stone powder is simultaneously produced as by-products. the japan crushed stone association estimates that 12 million tons of crushed stone powder are produced annually. in the wet process type, although crushed stone powder and clay fraction can be removed, sludge water and a dehydrated cake must be disposed. in recent years, the dry process type has become more popular because of its production efficiency and ease of handing waste disposal [1]. however, unless effective utilization of crushed stone powder is considered, a part of the natural resources will be unused, which is not a sufficient measure from the perspective of sustainability. in the world, studies on the effective use of crushed stone powder in concrete have been carried out, but they are few [2, 3]. in 2009, the quality requirements of crushed stone powder were standardized by the japanese industrial standards, jis a 5041. however, no specification is available on the specific surface area of the crushed stone powder. the authors studied the fluidity of mortar using crushed stone powders with various specific surface areas [4]. they found that the watercement ratio of mortar has a high influence on the fluidity of mortar with the crushed stone powder and, if the crushed stone powder with a large specific surface area is used, the fluidity of the mortar decreases. however, these findings are limited to mortar. the properties of concrete using crushed stone powders are not clear. in this study, the properties of the fresh and hardened concrete using crushed stone powders with various specific surface areas were examined. furthermore, co2 emission and the amount of virgin resource input for each concrete mix were estimated. 2. manufacturing of crushed sands and crushed stone powders figure 1 shows the simplified flow chart of the production processes of crushed sand and crushed stone powder in this study. in the dry process type, crushed stone powder type a and crushed sand ds are produced at first. then crushed stone powder type a is sorted by specific surface area as type b and type c. ds contains crushed stone powder type d. ds150 is produced by removing crushed stone powder of 150 µm or less contained in ds. in the wet process 363 https://doi.org/10.14311/app.2022.33.0363 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en t. matsuka, m. suzuki, h. matsushita et al. acta polytechnica ctu proceedings figure 1. production processes of crushed sands and crushed stone powders. figure 2. grain size accumulation curve of crushed sand. type, crushed sand ws and a dehydrated cake are produced. the dehydrated cake is, however, not the target of this study. ws contains crushed stone powder type e. ws150 is produced by removing crushed stone powder of 150 µm or less contained in ws. finally, the crushed sand cs is produced by mixing ws and crushed stone powder type b. in this study, the effect of the combination of crushed stone powder and crushed sand on the properties of concrete is examined. table 1 gives the types and physical properties of crushed sand and crushed stone powder. 3. outline of experiment 3.1. materials table 1 gives the types and physical properties of concrete materials used in this study. as mentioned earlier, in this experiment, two types of dry type crushed sand, three types of wet type crushed sand, and five types of crushed stone powder were used. the specific surface area of crushed stone powder was measured using a laser diffraction particle size analyzer. the specific surface area of the crushed stone powder was figure 3. grain size accumulation curve of crushed stone powder . in the range from 6360-22337 cm2/g. figures 2 and 3 show the grain size accumulation curves of crushed sand and crushed stone powder, respectively. the particle size of crushed stone powder is less than 150 µm as per jis a 5041. 3.2. production and mix proportions of concrete a total of 12 mixtures were planned to produce. concrete was produced in a laboratory at a temperature of 20 řc with a 60 l twin-shaft batch mixer. cement, fine aggregate, crushed stone powder, and coarse aggregate were first dry mixed for 15 seconds, and then further mixed for 90 seconds after adding water and chemical admixture. the water-cement ratio (w/c) was 60% for all the mixtures. the amount of air entraining (ae) water reducing agent was 1.2% to cement mass. table 2 lists the mix proportions of the concrete. when mixing concrete, the slump target was 12.0 ± 2.0 cm and air content target was 4.5 ± 1.0%; the slump and air content were adjusted based on the unit quantities of water and ae agent, respectively. 364 vol. 33/2022 crushed stone powder concrete materials type symbol physical properties water tap water w density: 1.00 g/cm3 cement ordinary portland cement c density: 3.16 g/cm3, specific surface area: 6315 cm2/g (3300 cm2/cm3) arithmetic average particle diameter: 19.4 µm fine aggregate crushed sand (dry type) ds density: 2.62 g/cm3, absorption: 1.80%, finess modulus: 2.85 content of 75 µm or less: 4.0% content of 150 µm or less: 8.0% ds150 density: 2.66 g/cm3, absorption: 1.48%, finess modulus: 2.97 content of 75 µm or less: 0% content of 150 µm or less: 0% crushed sand (wet type) ws density: 2.65 g/cm3, absorption: 1.76%, finess modulus: 2.84 content of 75 µm or less: 2.3% content of 150 µm or less: 7.8% ws150 density: 2.66 g/cm3, absorption: 1.42%, finess modulus: 2.99 content of 75 µm or less: 0% content of 150 µm or less: 0% cs density: 2.63 g/cm3, absorption: 1.79%, finess modulus: 2.80 content of 75 µm or less: 5.0% content of 150 µm or less: 9.1% crushed stone powder type a density: 2.684 g/cm3 specific surface area: 7120 cm2/g arithmetic average particle diameter: 35.2 µm type b density: 2.682 g/cm3 specific surface area: 6460 cm2/g arithmetic average particle diameter: 31.9 µm type c density: 2.687 g/cm3 specific surface area: 22337 cm2/g arithmetic average particle diameter: 3.6 µm type d density: 2.783 g/cm3 specific surface area: 16825 cm2/g arithmetic average particle diameter: 17.2 µm type e density: 2.772 g/cm3 specific surface area: 12914 cm2/g arithmetic average particle diameter: 25.8 µm coarse aggregate sand stone g1 maximum size: 20 mm, density: 2.70 g/cm3 absorption: 0.47%, finess modulus: 7.08 g2 maximum size: 15 mm, density: 2.69 g/cm3 absorption: 0.76%, finess modulus: 6.35 chemical admixture ae water reducing agent − complex of lignin-sulfonic-acid compound and polycarboxylic acid ether ae agent − denatured rosin acid compound-system anionic surface active agent table 1. constituent materials of concrete. 365 t. matsuka, m. suzuki, h. matsushita et al. acta polytechnica ctu proceedings quantity of material per unit volume of [kg/m3] no. w/c w c ds 150 ws ds 150 ws cs type b type c g1 g2 ae water reducing agent ae agent [%] ! kg/m3" ! kg/m3" 1 60 162 270 872 410 613 c×1.2% 0 2 162 270 872 0 3 165 275 846 0.275 4 168 280 844 0.280 5 165 275 850 0.138 6 165 275 830 26 0.413 7 170 283 777 51 0.992 8 165 275 805 52 0.668 9 168 280 787 51 0.980 10 173 288 766 50 1.009 11 170 283 786 51 0.992 12 180 300 668 118 1.500 table 2. mix proportions of concrete. figure 4. amount of crushed stone powder of 150 µm or less for each mix proportion. figure 4 shows the amount of crushed stone powder of 150 µm or less for each mix of concrete. the amounts of crushed stone powder for each mix proportion were in the range of 0 − 170.5 kg/m3. they are included the amount of crushed stone powder in the crushed sand as well. figure 5 shows the unit water content of each mix proportion. in addition, this figure shows the crushed stone powder ratios of 150 µm or less and 75 µm or less. the crushed stone powder ratio is defined in this study as the ratio of the absolutely dry mass of crushed stone powder mass to that of the fine aggregate. 3.3. test methods for properties of concrete for fresh concrete, slump, air content, and bleeding were measured as per jis a 1101, a 1128, and a 1123, respectively. for hardened concrete, compressive strength and youngs modulus were measured as per jis a 1108 and a 1149, respectively. the shape figure 5. unit water content of each mix proportion. of the specimens used for the test of hardened concrete is a cylinder with a diameter of 100mm and a height of 200mm. the number of samples in the test at each age is three. 4. results and discussions 4.1. unit water content figure 6 shows the relationship between the crushed stone powder ratios of 150 µm or less and 75 µm or less and the unit water contents. a correlation exists between the crushed stone powder ratios and the unit water content. the unit water content is specified as 175 kg/m3 or less in the standard specifications for concrete structures (japan society of civil engineers (jsce)). the fine particle content of 75 µm or less is specified as 9% or less in jis a 5005. these results indicate that the crushed stone powder ratio of 75 µm or less should be less than 15% to satisfy the jsce specifications. therefore, it has been suggested 366 vol. 33/2022 crushed stone powder concrete figure 6. relationship between crushed stone powder ratio and unit quantity of water. figure 7. relationship between surface area of crushed stone powder and unit quantity of water. that the specification of jis a 5005 (9%) of the fine particles content can be moderated for concrete. in this study, the total surface area of the crushed stone powder with particle sizes of 150 µm or less was calculated, and the relationship between the total surface areas of the crushed stone powder and the unit water content is shown in figure 7. it was found that the total surface areas of crushed stone powder have a stronger relationship than the crushed stone powder ratio because the determination coefficient is improved. therefore, it can be concluded that the specific surface area of the crushed stone powder is an effective index for evaluating the fluidity of concrete. 4.2. amount of ae agent figure 8 shows the relationship between the amount of ae agent and the crushed stone powder ratio. the amount of ae agent increases with increase in the crushed stone powder ratio. it is well known that air is difficult to be entrained when concrete has fine particles of 0.15 mm or less. the results of this study confirms it. figure 8. amount of ae agent. figure 9. amount of bleeding. 4.3. amount of bleeding figure 9 shows the relationship between the amount of bleeding and the crushed stone powder ratio. to ensure the water-tightness of concrete, the amount of bleeding is specified as 0.3 cm3/cm2 or less by the japanese architectural standard specification 5 (jass 5) [5]. the experimental results indicate that the amount of bleeding is 0.3 cm3/cm2 when the crushed stone powder is 8.5%. based on this result, controlling the amount of crushed stone powder is required to reduce bleeding. 4.4. compressive strength and young’s modulus figure 10 shows the relationship between the crushed stone powder ratio and the compressive strength of concrete. this figure indicates that the compressive strength decreases with increase in the crushed stone power ratio. when the crushed stone powder is used, the compressive strength decreases by up to 21.4% and by 20.0% at 7 days and 28 days, respectively. figure 11 shows the relationship between the compressive strength and the young’s modulus of concrete. this figure also shows the calculated results using the equation specified in the jsce standard 367 t. matsuka, m. suzuki, h. matsushita et al. acta polytechnica ctu proceedings figure 10. compressive strength. specifications for concrete structures [6]. the ratio of the experimental value to the calculated value of the young’s modulus of concrete were in the range 0.99 to 1.15. it can be said that the calculated results estimate the experimental results well almost appropriately. 5. environmental assessment 5.1. outline the importance of sustainability has recently been emphasized in the concrete industry [7]. sustainability is considered from the following three aspects: social, economic, and environmental. in the future, all technologies in the concrete industry will be evaluated from the perspective of sustainability; thus, all technologies in the concrete industry must be advanced based on the above-mentioned three sustainability aspects. in particular, reducing co2 emissions is currently an urgent issue in the world. in this study, the effect of using the crushed stone powder is evaluated from the perspective of sustainability, such as safety in terms of compressive strength and environmental performance in terms of co2 emissions and virgin resource input. 5.2. calculation methods the amounts of co2 emissions and virgin resource input for each mix proportion were calculated. the inventory data relating to the co2 emissions are obtained from "environment text of concrete (draft)" [8]. table 3 gives the unit-based co2 emissions in this study. this study additionally considered co2 emissions associated with kerosene consumed in the mixing and classification of crushed stone powder. however, the co2 emissions associated with electricity was not considered, because the breakdown of the quantity of electricity has not been clarified. the amount of virgin resource input was determined by multiplying the virgin resource input ratio of each material by a unit amount per 1 m3 of concrete. the virgin resource input ratio of cement figure 11. relationship between young’s modulus and compressive strength. items unit-based co2 emissions water 0.348 kg-co2/m3 cement 769 kg-co2/t crushed sand 3.9 kg-co2/t kerosene 2.50 kg-co2/l table 3. co2 emission unit. is 0.811 [9], and that of crushed stone powder a, b, and c is 0 because they are by-products that can be discarded. the virgin resource input ratios of the other materials are set at 1.0. 5.3. amounts of co2 emissions and virgin resource input figure 12 and 13 show the amounts co2 emissions and the virgin resource input, respectively. the co2 emissions increase with increase in the crushed stone powder, because kerosene is consumed to produce the crushed stone powder. in contrast, the amount of virgin resource input decreases with increase in the crushed stone powder. the amount of co2 emissions and the virgin resource input were in the range of 215 − 244 kg-co2/m3 and 2115 − 2276 kg/m3, respectively. 5.4. sustainability evaluation figure 14 shows an example of the correlation between the strength index, co2 index, and virgin resource index. each index was defined based on the respective values of the reference concrete (no.1 mix proportion). the larger the area of the triangle, the higher the overall performance. the results indicate that the relationships between the strength index, co2 index, and virgin resource index of concrete using the crushed stone powder depend on the types of crushed stone powder and crushed sand. it is concluded that evaluation from the perspective of environmental sustainability is also important for the mix proportion design of concrete using crushed stone powder. in addition, further data 368 vol. 33/2022 crushed stone powder concrete figure 12. co2 emissions. figure 13. amount of virgin resource input. accumulation is needed to generalize the various performances of the combinations of the specific surface area of crushed stone powder and the type of crushed sand. 6. conclusions the following conclusions were drawn from this study: • the specific surface area of crushed stone powder greatly affects the fluidity of concrete. • increasing the mixing ratio and the surface area of the crushed stone increase the unit water content of the concrete. however, controlling the amount of crushed stone powder is necessary for reducing the bleeding of concrete. • using crushed stone powder decreases the compressive strength of concrete up to 21.4 • the crushed stone powder reduces, the virgin resource input, but increases the co2 emissions. • the relationships between the strength index, co2 index, and virgin resource index of concrete using crushed stone powder depend on the type of crushed stone powder and crushed sand. figure 14. sustainability evaluation. references [1] t. mano, y. tsuji, f. tomosawa, et al. properties of fresh and hardened concrete using crushed stone powder as part of fine aggregate. concrete journal 46(11):18-24, 2008. https://doi.org/10.3151/coj1975.46.11_18. [2] t. h. song, s. h. lee, b. kim. recycling of crushed stone powder as a partial replacement for silica powder in extruded cement panels. construction and building materials 52:105-15, 2014. https: //doi.org/10.1016/j.conbuildmat.2013.10.060. [3] s. p. s. rajput. an experimental study on crushed stone dust as fine aggregate in cement concrete. materials today: proceedings 5(9):17540-7, 2018. https://doi.org/10.1016/j.matpr.2018.06.070. [4] suzuki m, matsuka t, yokoyama m and matsushita h 2019 influence of crushed stone powder with different specific surface area on fluidity of crushed sand mortar irrigation, drainage and rural engineering journal 87 pp 43-48. [5] architectural institute of japan. japanese architectural standard specification jass 5 reinforced concrete work, 2018. [6] japan society of civil engineers. standard specification for concrete structures -design-, 2018. [7] k. sakai, h. yokota.sustainability design of concrete structures gihodo shuppan co., ltd, 2016. [8] japan concrete institute. environment text of concrete (draft), 2016. [9] h. saito, k. sakai, y. suzuki, et al. proposal of evaluation method of low carbon concrete using fly ash and ground granulated blast-furnace slag. proceedings of the japan concrete institute 36:1852-1857, 2014. 369 https://doi.org/10.3151/coj1975.46.11_18 https://doi.org/10.1016/j.conbuildmat.2013.10.060 https://doi.org/10.1016/j.matpr.2018.06.070 acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.34.0026 acta polytechnica ctu proceedings 34:26–31, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague benchmark vercors 2022: mechanical response of the prestressed concrete containment wall to ambient conditions štěpán krátký∗, petr havlásek czech technical university in prague, faculty of civil engineering, department of mechanics, thákurova 7, 166 29 prague 6, czech republic ∗ corresponding author: stepan.kratky@fsv.cvut.cz abstract. the vercors benchmark was developed to provide a solid experimental basis for numerical modeling of concrete containment buildings (ccbs) to extend their lifespan. the goal of the first phase of the third benchmark is to present a blind prediction of ccb’s behavior across the whole lifespan and under periodic pressure tests. this paper summarizes the calibration of the material model based on the microprestress-solidification (mps) theory according to the provided laboratory experimental data. a special emphasis is given to the influence of the ambient conditions on drying creep and transient thermal creep of concrete and the calibration of the corresponding material parameters. these phenomena are studied on a representative section of the containment wall using weakly coupled thermo-hygro-mechanical analyses. keywords: concrete, creep, shrinkage, drying, cyclic temperature, modeling. 1. introduction in the present days, the main portion of electricity in france (up to 75%) comes from nuclear power plants. nowadays, a considerable number of atomic reactors (a little less than 20%) approach the end of their design lifespan. if this matter is not properly addressed in the near future, it might result in a vast energy shortage in france, not mentioning the increased risk of accidents. the main factor of the limited service life is the concrete containment building (ccb). when upgrading to more powerful 1300 mw reactors, electricité de france (edf) decided to enhance the safety of nuclear power plants and thereby modify the old ccb design. the new design (as shown in fig. 1) consists of two one-meter-thick prestressed concrete walls between which a constant vacuum slightly below the atmospheric pressure is maintained. such a measure ensures that no contamination escapes from the interior. to shut down old reactors and to build new ones proves to be enormously expensive, inconsiderate to the environment, and politically inadmissible. the other possibility is to extend the service life of existing containments by 20 years (from 40 to 60 years). that brings edf to a complicated task to accurately predict more than half a century of ccb’s lifespan. with the associated governing phenomena (creep and shrinkage of concrete and steel relaxation), such prediction becomes even more challenging. vercors mock-up is a diminished model of a ccb in a scale 1:3. according to the diffusion theory, the drying rate is proportional to the square of the representative depth. therefore, at the present scale, the drying process becomes accelerated 9× which means that the extended lifetime of 60 years can be analyzed and assessed in no more than 7 years. the vercors mock-up (fig. 1) was completed in 2014 and is 28 meters tall, 21 meters in diameter and with hundreds of sensors and thousands of meters of optical cables became the most studied concrete structure in the world. the wall of the inner containment is prestressed with steel tendons in both vertical and horizontal directions, which leads to the compression of 14 mpa and 8 mpa, respectively. every 12 months, the pressure test is conducted to verify the air tightness of the containment. in the ccb, the imposed 12-hour-long overpressure can reach up to 6 bar (0.6 mpa) needs to be considered. in addition to structural loading, the walls are exposed to varying ambient conditions. this paper is organized as follows. the first section summarizes the standard laboratory experiments which supplement the mock-up with suitable data for calibration of the constitutive models which are briefly outlined afterwards. next, with the identified material parameters, the representative periodic section of the inner containment wall is analyzed. this reduced model was created to cut the computational time of the full model and at the same time to easily verify the functionality of the most important features. a special attention is given to the sensitivity of the model to the variable ambient conditions and the associated material parameters. the response is evaluated in terms of the stresses and strains in the hoop (tangential) and vertical directions, and the evolution of the prestress. this article incorporates and extends the findings obtained within the final thesis of the first author [2]. 26 https://doi.org/10.14311/app.2022.34.0026 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 34/2022 benchmark vercors 2022: response to ambient conditions figure 1. section of the ccb vercors mock-up [1], dimensions in [mm]. 2. materials and methods 2.1. laboratory experiments the composition of the concrete used in the mock-up and laboratory experiments is summarized in table 1 and is identical to the concrete of nogent sur seine nuclear power plant. the high value of water-toingredients qtty [kg/m3] cement cem i 52.5n 320 water 167.9 aggregates (fine 830, coarse 1005) 1835 superplasticizer 2.6 table 1. concrete composition [3]. cement ratio, w/c = 0.525, predestines this concrete to a larger magnitude of drying shrinkage and creep. apart from the conventional short-term measurements at the age of 28 days (fcm = 48.7 mpa, e = 34.3 gpa, ft = 4.4 mpa), the laboratory data set comprised the following experiments: • basic creep and autogenous shrinkage • total creep and drying shrinkage at henv = 0.5 • moisture loss • porosity and aging sorption isotherm every experiment was conducted under both room (20◦c) and elevated temperature (40◦c). 2.2. material models 2.2.1. heat and moisture transport in the present study, the heat and moisture transport are for simplicity treated as independent processes without cross-coupling. heat conduction is idealized by using a linear transport model characterized by its heat conductivity and capacity. concrete drying is described by a widely accepted model proposed by bažant and najjar [4]. under the assumption of linear desorption isotherm, the governing equation for the diffusion of water vapor reads ∂h ∂t = ∇ · (c(h)∇h) (1) where∇h is the gradient of relative humidity and c(h) is the humidity-dependent diffusivity. for cementitious materials, this dependence is highly nonlinear and can be approximated as c(h) = c1 α0 + 1− α0 1 + ( 1−h 1−hc )n  (2) where c1 is the maximum diffusivity at h = 1, α0 determines the ratio between minimum diffusivity at h = 0 and c1, and parameters hc and n describe the relative humidity threshold and the steepness of the transition. the ambient relative humidity is prescribed using a mixed boundary condition which relates the humidity flux jh with the humidity difference at the boundary via surface factor f . 2.2.2. structural analysis the mechanical behavior of concrete is described by a modified constitutive model based on the microprestress-solidification (mps) theory [5]. under sealed conditions and constant room temperature, the behavior is defined by the basic creep compliance function of the b3 model [6] which is entirely captured by 4 parameters q1 − q4. yet, in concrete, changes both in relative humidity and temperature give rise not only to volume changes (shrinkage/swelling or thermal dilation), but also to further creep (pickett effect or transitional thermal creep). this additional creep is primarily controlled by the parameter k3 which is different from the original mps model and which has been introduced [7] to 27 štěpán krátký, petr havlásek acta polytechnica ctu proceedings minimize the size effect on drying creep. shrinkage strain and relative humidity are linearly linked via their rates, ε̇sh = kshḣ (3) where ksh is a material parameter usually treated as a humidityand age-independent constant. in the analysis, the stresses in steel reinforcement are far below its yield stress, therefore it is described by an isotropic linear elastic material with young’s modulus set to 200 gpa. the behavior of prestressing tendons is characterized by a generalized model from eurocode 2 for class 2 steel with reduced relaxation characteristic strength 1620 mpa and e = 190 gpa. the influence of elevated or variable temperature on the rate of relaxation and prestress losses due to friction or slip at the anchors are not considered. 2.3. computational models and calibration strategy the experimental data sets which served for the calibration of the material models for concrete (rheological and diffusion properties) were provided by edf at the beginning of the benchmark. at room temperature the experiments were done on cylinders 160×1000 mm while at the elevated temperature the dimensions were 100× 200 mm. the experimental data had to be compensated for the spurious shrinkage strains caused by unintended moisture leaking. the basic creep parameters q1− q4 of the b3 model were determined first. the deformation of the creep experiment which started at the age of 90 days was compensated for the autogenous shrinkage measured on the companion specimen. (since the measurements began at the age of 90 days, the recorded value and was very small, ≈ 50 × 10−6.) owing to the homogeneous stress state, the calibration was done using an analytical expression for the compliance function of the b3 model. the identified values which were slightly different from the prediction given by the b3 model are summarized in table 3. in the remaining experiments, the stress state in the specimen is no longer uniform, which explains why a more advanced computational model was necessary to be developed. the resulting axisymmetric models with suitable boundary conditions reflect the behavior of a thin representative section from the mid-height. in this region, the stress distribution can be considered as a function of the distance from the axis of the specimen and entirely free from the boundary effects. the numerical problem was solved using a staggered scheme. in every time step, the heat and moisture transport subproblems were solved first and subsequently were followed by the structural analysis which utilized the computed fields of temperature and relative humidity. calibration procedure continued with the parameter controlling the magnitude of drying shrinkage and the parameters of the bažant-najjar diffusion model. the suitable values were determined by hand fitting and the response of the models was checked against the evolution of drying shrinkage and moisture loss at room temperature. with these parameters set, the parameter k3 was identified from the drying creep experiment, and finally, the effect of elevated temperature was tuned by adjusting parameter kt c of the modified mps theory. the summary of all material parameters is listed in table 3. visual outputs of the calibration can be found in the final thesis of the first author [2]. parameter value q1 9.0× 10−6 mpa−1 q2 70.0× 10−6 mpa−1 q3 25.0× 10−6 mpa−1 q4 6.0× 10−6 mpa−1 ksh 1.0× 10−3 k3 10 kt m 6.5 table 2. identified parameters of material model mps. parameter value c1 28.2 mm2day−1 α0 0.055 hc 0.7 n 10 f 1.08 mm day−1 table 3. identified parameters of material model bažant–najjar for moisture diffusion. figure 2. computational model of the representative section for the structural sub-problem. despite the impressive extent of the vercors experimental program, the laboratory data do not provide sufficient evidence for proper calibration of the material model under general ambient conditions. the obstacle is that the investigated inner containment of the mock-up is subject to cyclic temperature and humidity which produce an additional increase in the creep rate of concrete. in the literature, this subject is covered rather insufficiently, although the 28 vol. 34/2022 benchmark vercors 2022: response to ambient conditions scarce existing data indicate that the creep rate is most significant during the first cycle (both thermal and humidity) and with subsequent cycling gradually decreases. another computational model was created to investigate the influence of the cyclic ambient conditions on the behavior of the containment wall and to estimate a suitable value of the parameter kt c which is responsible for the damping of the transitional thermal creep under temperature cycles this model is a representative section (figure 2) of the inner containment wall. as shown in the figure, in addition to both vertical and horizontal prestressing cables, the model comprises conventional reinforcement. the dimensions of the computational model are determined by the spacing of the cables. in the tangential (horizontal) direction of the wall, the model corresponds to an angle of 2π/160 as there are 160 equally spaced vertical cables. the lateral sides are not parallel. the horizontal cables have two different radii, and for this reason the height of the model is double of the vertical spacing, 264 mm. in the transport subproblems the spatial discretization is refined towards the inner and outer surface. due to the zero flux in the vertical direction, only one element per height of the model is used. in the hoop direction, the discretization is uniform and is identical to the structural subproblem. the boundary conditions of the structural analysis restrain the displacement normal to the lateral sides of the model. in the vertical direction, the periodic conditions are defined which permits not only overall elongation or shortening, but also warping of the horizontal surfaces. with these boundary conditions, the model should realistically capture the behavior at mid-height of the containment inner wall where the influence of the stiffener at the top and the massive foundation at the bottom is negligible. the self-weight is not considered since the effect would be very subtle in contrast to vertical prestressing. the reinforcement is discretized by linear truss elements which are connected to the linear hexahedral fe mesh of concrete (shown in figure 2) by means of hanging nodes. the slip between steel and concrete can be neglected due to the rotational symmetry of the structure and the resulting uniformity of the loading. 3. results and discussion 3.1. overall behavior the figures presented in this section document the evolution of stresses or strains computed with different combinations of ambient conditions and/or material behavior of concrete and prestressing steel. the notation in the legends is consistent and is summarized in the following list. • jb + relax: basic creep of concrete (henv = 0.98), relaxation of prestressing steel, t = 20◦c throughout the simulation • jb + no relax: ditto + no steel relaxation • henv1: creep and shrinkage at constant ambient temperature t = 20◦c and humidity from table 4, steel relaxation • henvi + ti: creep and shrinkage at ambient conditions specified in tables 4 and 5, steel relaxation unaffected by temperature • + test: additional internal overpressure notation description henv1 constant after 250 days henv2 cyclic changes, piecewise constant henv3 cyclic changes, piecewise linear henv1 = 0.26 on the inner side and 0.58 on the outer side with cyclic periods of henv = 0.98 on both faces. table 4. definition of ambient relative humidity. notation description t1 constant after 90 days t2 cyclic changes, piecewise linear t = 28◦c on the inner side and 25◦c on the outer side with cyclic periods of t = 10◦c on both faces. table 5. definition of ambient temperature. in the following figures, time t = 0 corresponds to the concrete age of 90 days. until t = 45 days when the prestressing is applied, the structural model of ccb wall is subject only to ambient conditions as specified by the keyword in the legend. figure 3 shows the evolution of strain in the hoop and vertical directions computed with different setup of the computational model. the results suggest that drying plays a fundamental role in the behavior of the containment wall. in the hoop direction, the strain after 2500 days of drying doubles the response of the models with basic creep only (red and purple curves), in the vertical direction the increase is even higher. temperature cycling (blue and green curves) causes a dramatic increase in strain whose rate is almost linear during the first 4 years. the computed evolution of the hoop stress in the middle and close to the inner surface is shown in fig. 4. prior to prestressing, the hoop stress close to the inner surface (fig. 4 right) induced by the drying shrinkage reaches almost 5 mpa. such a high tensile stress would have caused cracking which is not captured by the present material model. however, the crack depth would have been insignificant in contrast to the wall thickness and in addition to this, soon afterwards the containment wall becomes prestressed in both vertical and hoop directions which restores compression in the entire cross-section. the hoop stress in the middle of the wall is induced primarily by prestressing and only partially by the internally restrained drying shrinkage and nonuniform thermal strains. therefore, the decrease in stress magnitude in the middle of the wall can be attributed to the relaxation of the prestress caused by both creep 29 štěpán krátký, petr havlásek acta polytechnica ctu proceedings -300 -250 -200 -150 -100 -50 0 50 0 500 1000 1500 2000 2500 ta n g e n ti a l st ra in , ε q [ 1 0 -6 ] time, t [day] jb + relax henv1 henv1 + t1 jb + no relax henv3 + t2 henv2 + t2 -150 -120 -90 -60 -30 0 30 0 500 1000 1500 2000 2500 v e rt ic a l st ra in , ε z [ 1 0 -6 ] time, t [day] jb + relax henv1 henv1 + t1 jb + no relax henv3 + t2 henv2 + t2 figure 3. evolution of hoop (left) and vertical (right) strain in the center of the wall computed for different histories of ambient conditions, with basic creep only and with no steel relaxation. -15 -10 -5 0 0 500 1000 1500 2000 2500 s tr e ss , σ θ [ m p a ] time, t [day] jb + relax henv1 henv1 + t1 jb + no relax henv3 + t2 henv2 + t2 -15 -10 -5 0 5 10 0 500 1000 1500 2000 2500 s tr e ss , σ θ [ m pa ] time, t [day] jb + relax henv1 henv1 + t1 jb + no relax henv3 + t2 henv2 + t2 figure 4. evolution of hoop stress σθ in the middle (left) and close to the inner surface (right) of the ccb concrete wall. and shrinkage of concrete. steel relaxation leads to an initial yet considerable drop in prestressing which occurs during the first day. afterwards, this effect becomes negligible (see comparison in fig. 5). it is apparent that the highest decrease of prestress is associated with cyclic temperature and humidity loading (blue and green lines). 800 900 1000 1100 1200 1300 1400 1500 1 10 100 1000 10000 s tr e ss i n t h e t e n d o n s, σ p [ m p a ] time, t-45 [day] jb + relax henv1 henv1 + t1 jb + no relax henv3 + t2 henv2 + t2 figure 5. decrease in prestress in horizontal tendons due to steel relaxation, concrete creep and shrinkage. 3.2. response to ambient conditions and pressure tests -300 -250 -200 -150 -100 -50 0 50 0 500 1000 1500 2000 2500 ta n g e n ti a l st ra in , ε q [ 1 0 -6 ] time, t [day] henv1 + t1 henv2 t2 t2, ktc = ktm/20 figure 6. calibration of material parameter ktc responsible for the cyclic transient thermal creep. the analysis was performed with different histories of the ambient conditions which differed by the degree of simplification. the aim was to estimate the influence of this simplification which can significantly reduce the computational time. the results unanimously show that the temperature defined by a cyclic history t2 gives rise to an enormous increase in compliance which is only slightly affected 30 vol. 34/2022 benchmark vercors 2022: response to ambient conditions by the chosen history of relative humidity. having identified the source of this unrealistic response, a parameter kt c was introduced and based on previous experience set to kt m/20. with this approach, the response of the computational model can be damped in further temperature cycles provided that the temperature does not exceed its maximum previously attained value. the responses obtained with the original and modified formulation are shown in black and green color in fig. 6. -300 -250 -200 -150 -100 -50 0 50 0 500 1000 1500 2000 2500 ta n g e n ti a l st ra in , ε q [ 1 0 -6 ] time, t [day] jb + relax henv3 + t2 henv2 + t2 jb + test henv1 + t1 henv1 + t1 + test figure 7. evolution of hoop strain in the center of the wall pressure tests. -15 -10 -5 0 0 500 1000 1500 2000 2500 st re ss , σ θ [m pa ] time, t [day] jb + relax henv3 + t2 henv2 + t2 jb + test henv1 + t1 henv1 + t1 + test figure 8. evolution of hoop stress in the center of the wall pressure tests. response to regular pressure tests is presented in figure 7 and 8. with the simplified model, the overpressure affects only the hoop stress which rises by ≈ 8.5 mpa, see figure 8. in contrast to the sustained loading by prestressing and drying, the pressure tests provide an interesting insight into the evolution of the incremental stiffness which increases with concrete aging and decreases with temperature cycles. 4. conclusions this paper briefly outlined a procedure used for the calibration of constitutive models for the rheological properties and moisture diffusion of vercors concrete. with the basic material parameters set, attention was paid to the behavior of the inner containment wall subject to ambient conditions and regular pressure tests. the conclusions can be summarized as follows. • a computationally efficient model was developed to study the influence of cyclic temperature and relative humidity on concrete creep. such an extensive study could not have been executed and analyzed using a full computational model of the containment. • the original version of the mps model is not suitable for simulating concrete subject to cyclic temperature. based on the data from the literature, the computed unrealistically high compliance was reduced by incorporating the parameter kt c set to 0.05 kt m. • the values of the most important material parameters were determined based on the provided experimental data. a simplified history of the ambient conditions suitable for the blind prediction of the containment was selected. the developed simplified model of the containment wall will serve for checking the plausibility of the structural response of the full computational model. acknowledgements the authors gratefully acknowledge financial support from the czech science foundation (ga čr), project number 19-20666s, and from the grant agency of the czech technical university in prague, project number sgs21/037/ohk1/1t/11. references [1] direction production ingeniere, edf. project maquette vercors, plan general, 2012. [2] štěpán krátký. benchmark vercors 2022 blind prediction of mechanical response of reinforced concrete containment, bc. thesis (in czech). czech technical university in prague, 2021. [3] direction production ingeniere, edf. specification of vercors benchmark 3, 2021. [4] z. p. bažant, l. j. najjar. nonlinear water diffusion in nonsaturated concrete. materials and structures 5:3–20, 1972. https://doi.org/10.1007/bf02479073. [5] z. p. bažant, a. p. hauggaard, s. baweja, f. j. ulm. microprestress solidification theory for concrete creep. i: aging and drying effects. journal of engineering mechanics 123:1188–1194, 1997. [6] z. bažant, s. baweja. creep and shrinkage prediction model for analysis and design of concrete structures: model b3. adam neville symposium: creep and shrinkage structural design effects 2000. [7] z. bažant, p. havlásek, m. jirásek. microprestress-solidification theory: modeling of size effect on drying creep. in n. bicanic, h. mang, g. meschke, r. de borst (eds.), computational modelling of concrete structures, pp. 749–758. crc press/balkema, eh leiden, the netherlands, 2014. 31 https://doi.org/10.1007/bf02479073 acta polytechnica ctu proceedings 34:26–31, 2022 1 introduction 2 materials and methods 2.1 laboratory experiments 2.2 material models 2.2.1 heat and moisture transport 2.2.2 structural analysis 2.3 computational models and calibration strategy 3 results and discussion 3.1 overall behavior 3.2 response to ambient conditions and pressure tests 4 conclusions acknowledgements references https://doi.org/10.14311/app.2022.33.0344 acta polytechnica ctu proceedings 33:344–349, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague extent of corrosion damage for rc structures exposed to chloride-bearing environment federica lollini politecnico di milano, department of chemistry, materials and chemical engineering "giulio natta", piazza leonardo da vinci 32, italy correspondence: federica.lollini@polimi.it abstract. in industrialized countries, most of the reinforced concrete (rc) structures and infrastructures have more than 40 years, since they were built around 1960 − 1980, and were designed for a service life of 50 years. consequently, in the next years, the number of structures and infrastructures that will need to be repaired will steeply increase. several techniques, characterized by a different durability, environmental impact and economic impact, are available for the repair of a reinforced concrete structure damaged by the corrosion of reinforcement. to help the designers in the choice of the most suitable one, a preliminary assessment of the condition of the structure is essential, aimed at diagnosing the causes of deterioration, the extent of damage, i.e. the extent of corroding reinforcement, and its evolution in time. an approach, with a wide consensus, for the evaluation of the extent of corroding reinforcement is available for carbonation-induced corrosion, whilst it is still lacking for chloride-induced corrosion. in this paper two approaches for the evaluation of the extent of corroding reinforcement for a rc structure subject to chloride-induced corrosion are presented and compared, showing similar results. these approaches can be useful to properly plan a restoration intervention as well as to assess the reliability of the recently proposed model for service life design. keywords: assessment, corrosion, chloride. 1. introduction in industrialized countries, most of the reinforced concrete (rc) structures and infrastructures, were built around 1960 − 1980 and, in general, were designed to last for a period of 50 − 100 years [1]. more and more of these structures are approaching the end of their designed service life, and due to their exposure conditions, i.e. typical urban or rural environment or marine exposure, often suffer damage due to carbonationor chlorideinduced corrosion of reinforcement, such as concrete spalling or cracking or rust stains that may affect their serviceability. a proper maintenance and rehabilitation of reinforced concrete structures could extend their service life, by restoring the structural safety and preventing the future damage. this would avoid to demolish them and build new ones, leading to a lower social, environmental and economic impact, due, for instance, to a less hindrance to traffic and waste of materials [2]. several techniques are available for the rehabilitation, which vary from keeping the conditions of corrosion and serviceability under control by some form of monitoring, when the extent of damage is limited or the remaining period of use is short, to the conventional repair technique, which requires the replacement of carbonated concrete cover or chloride contaminated cover with a cementitious mortar, to electrochemical techniques or hydrophobic treatments, that can control corrosion of steel bars without requiring the removal of the original concrete cover, except the cracked concrete [3–5]. each of these strategies is characterizes by a different durability, environmental impact and economic impact, and the choice of the most suitable protection system should be a compromise among the different requirements [4]. hence, it appears essential to choose and plan the most suitable restoration intervention, a careful condition investigation that cannot be limited to a visual inspection by an expert, but should include, beside the visual inspection and the examination of design documents, field tests as well as sampling for laboratory tests, i.e. carbonation depth, chloride profile and cover depth. the inspection should be aimed at estimating if, both at the time of inspection and at the end of the designed lifetime, the reinforcement is still passive, i.e. corrosion has not initiated since carbonation or a critical amount of chlorides has not reached the steel surface, or the reinforcement is corroding but the propagation is in the early stages, e.g. concrete cover is not cracked and reduction in cross section of rebars is negligible [6]. since concrete quality, mainly the permeability, nature and intensity of cracks, and concrete cover are spatially variable over the concrete surface, due to different concrete batches and the heterogeneity of workmanship, the corrosion conditions will vary accordingly [7–9]. an evaluation of the extent of corroding reinforcement, i.e. of the amount of steel bars that are no more longer passive, and its evolution in time, is, then, needed. for carbonation-induced corrosion the extent of corroding reinforcement can be assessed by comparing measured carbonation depth with measured cover depths 344 https://doi.org/10.14311/app.2022.33.0344 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 33/2022 corrosion damage for rc structures d�� e�� � ��� � ��� � ��� � ��� ����������������������������� c l wk �� �p dv v� ri �f hp hq w� 3uredelolw\ pfruu � ��� � ��� � ��� � ��� � �� �� �� �� �� �� �� c & o� � �p dv v� ri �f hp hq w� 'hswk��pp� 3uriloh�� 3uriloh�� 3uriloh�� 3uriloh�� 3uriloh�� figure 1. cumulative distribution function of the critical chloride content (a) and imaginary chloride profiles measured on a rc structure (b). in the structure and in the literature a statistical approach with a wide consensus was proposed for its evaluation [10]. for chloride-induced corrosion, to evaluate the extent of corroding reinforcement, the chloride content at the depth of the bars needs to be compared to the critical chloride content, since corrosion occurs when at the bar depth a chloride content equal to the critical chloride content is reached. the variability of these parameters makes it difficult to assess the extent of corroding reinforcement and, unfortunately, an approach with a wide consensus is still lacking. this paper presents two mathematical approaches for the evaluation of the extent of corroding reinforcement for a reinforced concrete structure subjected to chloride-induced corrosion. a fictitious example is provided to compare the results of the two approaches and to discuss their real application. 2. mathematical approaches for the evaluation of the extent of corroding reinforcement 2.1. statistical approach in order to assess the extent of corroding reinforcement, rdep, of an existing structure, the critical chloride content, clth, the chloride content, ccl, and the depth of the bars, c, need to be known. the critical chloride content depends on several factors, and, due to its stochastic nature, can be properly defined only through a probability density function. according to the model code for service life design, proposed in 2006 by the international federation of concrete, fib [11], clth for ordinary carbon steel bars, can be described by a beta distribution, lower and upper limited. figure 1a shows the cumulative distribution function of the critical chloride content, assuming that the average value and the standard deviation are respectively 0.6 % and 0.15 % vs mass of cement (values of 0.2 and 2 % vs mass of cement have been assumed for the lower and upper limits). the chloride content at the different depths and the concrete cover thickness can be determined during the inspection of the rc structure, through field and laboratory tests. in order to assess rdep, and to increase its reliability and accuracy, a significant number of measurements of these parameters is required. as an example, figure 1b shows imaginary chloride profiles measured on five concrete samples taken from the structure after 10 years from the construction. in the example the total chloride content was evaluated at depth intervals, ∆c, of 5 mm. from the concrete cover depth measurements, carried out in situ, a frequency analysis can be performed (the depth intervals should be equal to those of the chloride profiles) (figure 2). � � � � � � �� �� �� �� �� �� )u ht xh qf \� �� � &rqfuhwh�fryhu�wklfnqhvv��pp� figure 2. imaginary frequency analysis (grey symbols) and probability density function (black line) of the concrete cover thickness. in order to evaluate the extent of the depassivated bars, initially the variability of the concrete cover 345 federica lollini acta polytechnica ctu proceedings profile ccl (% mass of cement) pcorr, i 1 0.44 0.14 2 0.98 0.99 3 1.33 1 4 1.41 1 5 1.44 1 table 1. chloride contents at the depth c = 22.5 mm and calculated values of pcorr, i. ∆c [mm] 0 − 5 5 − 10 10 − 15 15 − 20 20 − 25 25 − 30 30 − 35 35 − . . . fc [%] 10.2 5.6 6.5 4.6 13 15.7 17.6 26.9 pcorr [%] 100 100 99.9 97 82.5 73.2 55 41 rdep [%] 10.2 5.6 6.5 4.5 10.7 11.5 9.6 3.2 table 2. proportion, fc, of imaginary cover depth measurements falling within the range ∆c, probability of occurrence of corrosion, pcorr, and extent of corroding reinforcement, rdep, in each depth ∆c. thickness, c, is neglected and it is considered that the bars have the same concrete cover (i.e. the concrete cover of all the bars falls within the same cover depth interval, ∆c). moreover, it is initially supposed that, at the bars depth, c, a unique value of the chloride content, ccl, is available. the probability that corrosion initiated, pcorr, corresponds to the probability of occurrence that the variable clth is equal to the considered value of chloride content, ccl. the probability that corrosion occurred is an estimation of the extent of corroding reinforcement. this means that, if at the bars depth a chloride content lower of 0.2 % mass of cement is measured (that corresponds to the lower limit of the beta distribution), pcorr should be null and hence all the bars would be passive, conversely, if the chloride content were higher than 2 % mass of cement (that corresponds to the upper limit of the beta distribution), pcorr would be equal to 100 % and, hence, all the bars would be actively corroding. as an example, a concrete cover thickness equal to 22.5 mm (as indicated by the vertical dotted line in figure 1b) and the first profile, i.e. profile 1, can be considered; at this depth a chloride content of about 0.44 % mass of cement can be evaluated (horizontal dotted line in figure 1b). at this chloride content corresponds a probability pcorr (= rdep) equal to 14 %, given by the intersection between the horizontal dotted line and the clth cumulative distribution function, as shown in figure 1a. considering that more measurements of chloride content at the bars depth, c, are available (as in the example of figure 1b), pcorr can be evaluated through the equation: pcorr = n! i=1 pcorr, i n (1) where n is the number of chloride profiles and i represents the i−th profile. table 1 reports the chloride contents at the concrete cover c = 22.5 mm, shown in figure 1b, and pcorr, i evaluated for each profile. applying equation 1 it can be evaluated that, for this depth interval, pcorr is about 83 %. however, in a real structure, it has to be taken into account also the variability of the concrete cover thickness, as shown for instance in figure 2 and reported in table 2 in terms of percentage of measurements, fc, falling within different depth intervals. hence, pcorr has to be calculated, through equation (1), for all the concrete cover depth intervals, ∆c, as shown in table 2. rdep, j can be, then, evaluated, for each ∆c, as: rdep, j = pcorr, j · fc (2) where fc is the proportion of cover depth measurements falling within the range ∆c and j is equal to the given interval depth, ∆c. hence, the extent of corroding reinforcement can be calculated by summing up corrosion proportions over all the depth intervals: rdep = m! j=1 " #fc, j · $ n! i=1 pcorr, i n % j & ' (3) where n is the number of chloride profiles and i represents the i−th profile, m is the number of cover depth intervals, ∆c, and j represents the j−th depth. by applying equation 3 to the given example, a value of rdep about 62 % can be evaluated. 2.2. probabilistic approach to evaluate the extent of corroding reinforcement, a probabilistic approach, similar to that used for the design of new rc structures, can be also used. in the probabilistic approach for the design of new structures, the extent of corroding reinforcement corresponds to the probability, pdep, that the limit state 346 vol. 33/2022 corrosion damage for rc structures profile 1 2 3 4 5 dapp [10 − 12 m2/s] 0.4 0.7 1 1.2 1.6 cs [% mass of cement] 2.8 3.45 3.6 3.4 3 table 3. diffusion coefficient, dapp, and surface chloride concentration, cs, of imaginary chloride profiles. parameter unit type of distribution value m σ dapp [10 − 12 m2/s] normal 0.98 0.46 cs [% mass of cement] normal 3.25 0.33 c [mm] normal 27.9 14.3 clth [% mass of cement] beta 0.6 0.15 t [year] 10 table 4. values of the parameters and types of statistical distribution used as inputs in the probabilistic approach (m = mean value, σ = standard deviation). equation for the initiation of corrosion, g, is not satisfied, i.e. that pdep{g} < 0. for chloride-induced corrosion, the limit state equation for the initiation of corrosion can be described by means the solution of fick’s second law: pdep = {clth − ccl (x, t)} = p ( clth − cs ) 1 − erf c 2 * dapp t +, < 0 (4) where: clth is the critical chloride threshold, cs is the surface chloride concentration, c is the concrete cover depth, dapp is the diffusion coefficient and t is the time. the variables clth, cs, c and dapp are stochastic variables, whilst the time, t, is a deterministic variable. to evaluate the actual corrosion conditions of an existing structure, reliable values for the input parameters need to be provided. as far as the time, t, is concerned, it corresponds to the time between the year of construction and the considered time (in the example, 10 years). the critical chloride threshold, as previously observed, can be described for carbon steel bars through a beta distribution, with a mean value of 0.6 % mass of cement. the other parameters are related to the considered existing structure and to its exposure conditions and can be determined, if results of a detailed inspection are available, from the experimental measurements. the concrete cover distribution can be evaluated by interpolating the experimental measurements and determining the distribution parameters, i.e. the mean value and the standard deviation. from the imaginary concrete cover thickness shown in figure 2, a normal distribution, with a mean value of 27.9 mm and a standard deviation of 14.3 mm can be determined (black line in figure 2). the interpolation of the chloride content at different depths, at time, t, by means of the second fick’s law, allows to calculate, for each profile, a value of dapp and cs. from the chloride profiles shown in figure 1b, data summarised in table 3 can be evaluated. from these values, both for the diffusion coefficient and the surface chloride concentration, the probability density function, with its characterizing parameters, can be determined. assuming for both parameters a normal distribution, for the diffusion coefficient a mean value and a standard deviation respectively equal to 0.98 and 0.46×10−12 m2/s can be calculated, whilst for the surface chloride concentration a mean value and a standard deviation equal to 3.25 % and 0.33 % mass of cement can be determined. table 4 summarizes the values of the parameters involved in equation 4, to be applied for the evaluation of the extent of corroding reinforcement of the example. solving equation 4 through a probabilistic technique, as the montecarlo method, with the values presented in table 4, a pdep equal to 61 % can be evaluated. 3. discussion both mathematical approaches allow to evaluate the extent of corroding reinforcement i.e. the percentage of reinforcement likely to be non-passive, for a reinforced concrete structure subjected to chlorideinduced corrosion, by comparing the chloride content at the depth of the reinforcement, measured during an inspection, with the critical chloride threshold, taking into account the variability of each involved parameter. the extent of corroding reinforcement could be evaluated both at the time of the inspection and in the future, by estimating the evolution in time of the chloride content at different depths. the approaches provide only indications on the amount of actively corroding bars, but do not give any indication on their localization. furthermore they might overestimate the percentage of bars where initiation of corrosion really occurred. because of the mechanism of chloride-induced corrosion, in fact, in the surrounding zones of corroding bars that usually have a chloride content higher than the chloride threshold, 347 federica lollini acta polytechnica ctu proceedings the steel benefits of the formation of macrocell that provides cathodic polarization, preventing the initiation of corrosion [3]. the statistical approach appears to be easily applicable since the punctual value of the chloride content is directly compared with the critical chloride threshold, whilst the probabilistic approach requires the elaboration of the data obtained through experimental measurements, i.e. the concrete cover thickness and the chloride profile, to determine the probability density function of the input parameters. however, in principle, the probabilistic method could be used for a preliminary estimation of the extent of corroding reinforcement even in absence of any experimental data derived from an inspection. indeed, the input parameters involved in equation 4 could be determined from the design data and the environmental exposure conditions. as concrete cover thickness, the design concrete cover could be considered, whilst from the information related to the construction materials, i.e. the w/b ratio and the type of cement, and the exposure condition, the diffusion coefficient and the surface chloride concentration could be estimated. of course, in this case, the reliability of outcomes will depend on the uncertainty in the parameters chosen for the calculations. from their application, values of 62 and 61 % of extent of corroding reinforcement were respectively obtained with the statistical approach and the probabilistic approach, suggesting that the two approaches are comparable. the accuracy and reliability of the outcomes of both approaches depends on the available number of experimental measurements of concrete cover thickness and chloride content at different depths and their representativeness of the whole structure. the reliability of these methods could be validated by comparing their results with a careful corrosion potential mapping that allows to detect the depassivated area. the availability of a methodology that allows to evaluate the percentage of corroding reinforcement, provided that it is representative of the corrosion conditions of the whole structure, could help the designer in the choice of the most suitable repair technique. the percentage of depassivated area and its evolution in time will represent the amount of concrete, even that sound, that must be removed in the conventional repair, since the concrete must be removed in all areas where the chloride threshold is reached at the depth of the reinforcement or it is expected to be reached during the design life of the repair. this repair technique, that also requires the careful cleaning of the surface of the reinforcement to remove all chloride contaminated rust, appears to be suitable when the amount of concrete to be removed and replaced is limited. conversely when large amount of sound concrete should be removed, i.e. when the damage is extensive, other repair methods, such as cathodic protection or electrochemical chloride removal, that require the removal of the only cracked concrete, may be considered. finally such kind of approaches can be also useful in validating the outcome of the recently proposed probabilistic models for service life design as, for instance, the fib model code for service life design. in these models the probability of failure, i.e. the probability of occurrence of corrosion, is evaluated as the probability that the initiation limit state function reaches negative values and, in case of chlorideinduced corrosion, it is defined as the difference between the critical chloride threshold and the chloride content at the depth of the bars. the probability of failure has the same meaning of the percentage of corroding reinforcement. hence, the application of the models for service life design to existing structures and the comparison of the results to the real corrosion conditions of these structures might allow a preliminary understanding of their reliability. usually in these models the concrete performances in relation to the resistance to chloride penetration are evaluated through accelerated tests. the results of accelerated tests are then modified through a series of corrective factors that are directly provided by the models themselves. these accelerated tests were usually not carried out for structures built in the past and, hence, to apply these models to an existing structure, attention should be paid in their definition. an inaccurate estimation of the parameters chosen for the calculation could compromise the comparison with inspection results. 4. conclusive remarks this paper presented two approaches to evaluate the extent of corroding reinforcement i.e. the percentage of reinforcement likely to be non-passive, of a reinforced concrete structure exposed to chloride-induced corrosion, from the results of an inspection, provided that its results are representative of the whole structure. in the statistical approach the extent of corroding reinforcement is calculated by summing up corrosion proportions that correspond to the probability that the critical chloride content is equal to the chloride content at a certain depth interval, over all the considered depth intervals. in the probabilistic approach the extent of corroding reinforcement, corresponds to the probability that the difference between the critical chloride threshold and the chloride content at the depth of the bars, both considered as stochastic parameters, reaches negative values. their application to a fictitious example showed similar results, suggesting that the two approaches are comparable. these approaches can be useful to properly plan a restoration intervention, since the outcomes represent the amount of concrete, even sound, that must be removed in the conventional repair, providing then indications on the suitability of such kind of repair technique rather than other treatments, such as the 348 vol. 33/2022 corrosion damage for rc structures cathodic protection or the electrochemical removal of chlorides, that require the removal of the only cracked concrete. finally, these approaches can be used to assess the reliability of the recent proposed models for service life design, by comparing the outcomes of their application to existing structures with the results to the real corrosion condition of these structures. references [1] r. b. polder, w.h.a. peelen, w.m.g. courage. nontraditional assessment and maintenance methods for aging concrete structures technical and non-technical issues. materials and corrosion 63:1147-1153, 2012. https://doi.org/10.1002/maco.201206725. [2] e. schlangen. eco-efficient repair and rehabilitation of concrete infrastructures, cambridge: woodhead publishing, 2018. [3] l. bertolini, b. elsener, p. pedeferri, et al. corrosion of steel in concrete: prevention, diagnosis, repair. 2nd edition. weinheim: wiley-vch, 2013. [4] m. a. el-reedy. steel-reinforced concrete structures: assessment and repair of corrosion. crc press taylor & francis group, 2007. [5] e. redaelli, m. carsana, m. gastaldi, et al. electrochemical techniques for the repair of reinforced concrete suffering carbonation-induced corrosion. corrosion reviews 29(5-6), 2011. https://doi.org/10.1515/corrrev.2011.008. [6] l. bertolini, m. carsana, m. gastaldi, et al. corrosion assessment and restoration strategies of reinforced concrete buildings of the cultural heritage. materials and corrosion 62(2):146-54, 2011. https://doi.org/10.1002/maco.201005773. [7] m. g. stewart. workmanship and its influence on probabilistic models of concrete compressive strength. materials journal 9(4):361-372, 1995. [8] s. lim, m. akiyama, d. m. frangopol, et al. experimental investigation of the spatial variability of the steel weight loss and corrosion cracking of reinforced concrete members: novel x-ray and digital image processing techniques. structure and infrastructure engineering 13(1):118-34, 2016. https://doi.org/10.1080/15732479.2016.1198397. [9] u. m. angst. challenges and opportunities in corrosion of steel in concrete. materials and structures 51(1), 2018. https://doi.org/10.1617/s11527-017-1131-6. [10] j. mattila. corrosion of steel in reinforced concrete structures, final report, european communities, luxembourg, p. 207-209, 2003. [11] c. code. model code for service life design. lausanne: federation internationale du beton, fib. bulletin (34), 2006. 349 https://doi.org/10.1002/maco.201206725 https://doi.org/10.1515/corrrev.2011.008 https://doi.org/10.1002/maco.201005773 https://doi.org/10.1080/15732479.2016.1198397 https://doi.org/10.1617/s11527-017-1131-6 acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.34.0127 acta polytechnica ctu proceedings 34:127–132, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague influence of microstructure on rate-dependent response of unidirectional fibrous composites soňa valentová, michal šejnoha∗, jan vorel, zdeněk prošek czech technical university in prague, faculty of civil engineering, thákurova 7, 166 29 prague 6, czech republic ∗ corresponding author: sejnom@fsv.cvut.cz abstract. this paper outlines prediction of the macroscopic response of unidirectional fibrous composites made either from basalt or carbon fibers impregnated by a polymeric epoxy matrix. the viscoelastic response of the matrix phase was represented by the maxwell chain model. a series of creep tests performed at several stress levels served as a stepping stone for the model calibration. the macroscopic behavior of both composites was first examined via computational homogenization. attention was accorded to computational cells with variable size extracted from large representative images. we observed that selecting the computational model as a sufficiently large test window should be approached with caution. because initial designs often need a large parametric study to test various material and geometrical patterns, this study was then supported by computationally much more effective mori-tanaka averaging scheme, clearly showing its potential even if loading the composite beyond its elastic limit. keywords: viscoelasticity, fibrous composites, mori-tanaka method, periodic unit cell, generalized leonov model, maxwell chain model, creep. 1. introduction fibrous composites with a polymeric matrix are used in many engineering applications. this type of material is often favored owing to, e.g., high strength, stiffness, thermal or chemical resistance, while the weight and volume are relatively low. presently, also an ecological point of view plays a significant role and both basalt and carbon fibers, used in the examined composites, appear convenient. thus the resulting system may well substitute more traditional materials, such as steel or concrete, in many industrial fields. on the other hand, prediction of the macroscopic behavior of such a material, where the matrix is viscoelastic, calls for combining a comprehensive, relatively time demanding, laboratory program and computational analysis to calibrate the selected nonlinear viscoelastic model. having the calibrated model, the determination of macroscopic response then builds upon the application of either computational homogenization [1, 2] or application of one of the macromechanical model [3, 4] such as the mori-tanaka method [5]. we begin with the first-order computational homogenization, but unlike in [6, 7] we abandon the approach based on the statistically equivalent periodic unit cell (sepuc) and formulate the representative volume element (rve) as a certain test window taken from randomly from a large image of a real microstructure [8]. despite random distribution of fibers, for unidirectional fibrous composite a periodic unit cell (puc) with the periodic hexagonal arrangement (pha), shown in the fig. 1, is often assumed sufficient. the comparative results shows some drawbacks of both models when loading the present composites beyond elasticity. (a) (b) figure 1. a) puc with pha, b) example of mesh. as already pointed out the finite element element simulations are generally computationally expensive. it is therefore useful to have more efficient, yet reliable, method at hand especially if the averages of local fields are sufficient. in this regard we attempt to reproduce the finite element simulations with the modified moritanaka method, here presented in the framework of dvorak’s transformation field analysis [3, 9]. 2. theoretical background 2.1. generalized leonov model as the epoxy matrix influences the macroscopic response time and rate-dependently, a reliable model describing its nonlinear viscoelastic behavior is indispensable. herein the generalized leonov model is used. a brief description is provided next while the details can be found in [4, 10, 11]. the volumetric response is assumed elastic σm = kεv, (1) 127 https://doi.org/10.14311/app.2022.34.0127 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en s. valentová, m. šejnoha, j. vorel, z. prošek acta polytechnica ctu proceedings whereas the deviatoric response is described by the generalized maxwell chain model as dsij dt = m∑ µ=1 2gµ(deij dt − dep,µij dt ), (2) sij = m∑ µ=1 sµij . (3) the eyring flow model written as dep dt = 1 2a sinh(τ/τ0) , (4) represents the plastic shear rate of deformation and determines the creep strain. the model parameters a and τ0 are obtained from laboratory experiments, see [10–12]. the description of the required creep tests to calibrate the maxwell model for the adopted epoxy matrix is available in [13]. because of space limitation we present in table 1 only the resulting parameters (jµ, τmu) of individual chains necessary for the construction of creep compliance function of the matrix phase. the laplace transform is used next to get the necessary stiffnesses gµ entering eq. (2). 2.2. mori-tanaka method the classical micromechanical mori-tanaka method belongs to frequently used two-point averaging schemes. unlike the finite element approach exploiting periodic unit cells, this method brings a significant computational simplification, as just the knowledge of the shape and orientation of reinforcements, and the volume fraction and material properties of both phases, fibers and matrix, are needed. we start the description from the local stress and strain increments provided by ∆σf = lf∆εf , ∆σm = l̂m(∆εm −∆µm), (5) where l̂m represents the matrix stiffness dependent on the viscoelastic modulus. according to dvorak’s transformation field analysis, see [9], the local strain increments are written as ∆εf = âf∆e + d̂fm∆µm, (6) ∆εm = âm∆e + d̂mm∆µm. (7) the strain localization factors âr (r = f,m stand for the fiber and matrix phase, respectively), attain the form âm = [ cmi + cf t̂f ]−1 , âf = t̂f âm. (8) with reference to [5], the transformation influence matrices d̂rm are given by d̂rm = ( i− âr )( l̂m − lf )−1 l̂m. (9) it was pointed out in [14] that if the nonlinear matrix influences the macroscopic behavior, the mori-tanaka method gives a noticeably stiffer response in comparison to numerical predictions provided by the finite element method. thus in our recent paper, see [13], we introduced a certain modification to the original mori-tanaka formulation which showed potential in arriving at results being reasonably close to those provided by fem. 2.3. homogenization using fem the first-order homogenization assumes periodic fields. the periodic hexagonal array, seen in the fig. 1, can be selected as a suitable candidate. in the present study, however, other rves, generally non-periodic, will also be examined while enforcing the periodicity on the fluctuation part of the displacement field u∗(x). the local displacement and strain increments can be then split into homogeneous and fluctuation parts as ∆u(x) = ∆e · x + ∆u∗(x), (10) ∆ε(x) = ∆e + ∆ε∗(x). (11) if the macroscopic strain e is prescribed, with arrive at a particular form of the hill lemma〈 δεt∆σ 〉 = 0, (12) where 〈·〉 stands for the volume averaging. the solution of the resulting system of algebraic equations [4] is searched in terms of fluctuations u∗. the local stress increment can be expressed as ∆σ(x) = l(x)(∆ε(x)−∆µ(x)). (13) their volume averages then allow us to construct the desired macroscopic stress-strain diagrams. further details can be found for example in [14]. 3. results two types of the unidirectional fibrous composites reinforced by basalt and carbon fibers were investigated. for both systems, the same epoxy resin was used. figure 2 presents binary images of large cross-sections taken from corresponding yarns clearly suggesting a random nature of fiber distributions. the fiber and matrix volume fractions were estimated via standard image analysis. the corresponding values are listed in table 2 together with the material properties of fibers taken from [15]. the two images in fig. 2 also served to construct the computational models of variable complexity while attempting to match the corresponding volume fractions as close as possible. twenty such models, henceforth denoted as unit cells (uc), were generated by taking a random cut from the original images allowing us to cover a sufficiently large range of cross-sectional areas. the unit cells are numbered in the ascending order from the lowest to the highest area. two particular examples identifying the cells with the lowest and 128 vol. 34/2022 viscoelastic response of ud fibrous composites µ τµ [s] jµ [mpa−1] θµ [mpa·s] eµ [mpa] 1 0.001 2.606512×10−4 9.927397×10−3 2.787166×101 2 0.01 1.905071×10−6 9.966502×10−2 1.278184×101 3 0.1 8.808431×10−7 9.815126×10−1 7.056602×101 4 1 4.934025×10−6 9.543319×10+0 1.711529×102 5 10 1.276165×10−5 9.344254×10+1 2.334448×101 6 100 1.969419×10−5 9.580883×10+2 1.418353×102 7 1000 1.290521×10−5 8.275395×10+3 5.659977×102 8 10000 6.291266×10−5 9.647045×10+4 1.586346×102 9 100000 7.887707×10−6 2.005373×10+5 1.944645×103 10 1000000 1.577867×10−3 4.168654×10+5 5.096147×102 table 1. parameters of maxwell chain model. ea et ga gt νa cf [gpa] [gpa] [gpa] [gpa] [-] [-] carbon fibers 294 13 12 5 0.24 0.568 basalt fibers 69.68 64.82 28.10 26.14 0.4 0.563 table 2. material properties of carbon and basalt fibers and its volume fraction in yarn. (a) (b) figure 2. cross-sections of examined fibrous composites. a) carbon fiber composite, b) basalt fiber composite. (a) (b) figure 3. a) basalt cell with the lowest volume, b) basalt cell with the highest volume. (a) (b) figure 4. a) carbon cell with the lowest volume, b) carbon cell with the highest volume. highest area plotted for each system in figs. 3 and 4 for illustration. both materials were subjected to transverse shear loading in the strain control regime and the macroscopic response was recorded. at the initial stage the composite was subjected to the prescribed shear strain rate of 0.001 s−1 until the maximum value of 0.1 was reached. then the shear strain was held constant for the next 100 s to monitor relaxation. the resulting macroscopic response is plotted in figs. 5 9. figures 5 and 6 show the shear stressstrain diagram and the time variation of the shear stress, respectively, for the basalt composite (bi uc). similar graphs are displayed for the carbon composite (ci uc) in figs. 7 and 8. the bold lines correspond to the cells with the maximum and minimum area (vmin, vmax), stiffness (smin, smax), and volume fraction (cimin, cimax), respectively. 129 s. valentová, m. šejnoha, j. vorel, z. prošek acta polytechnica ctu proceedings the results corresponding to the pha model and those derived from the mori-tanaka method are also provided for the sake of comparison. it should be pointed out that the mt results correspond to the standard formulation with no particular adjustments as presented in [13] to match the results of the pha model. figure 5. shear stress strain diagram for basalt composites. figure 6. time evolution of shear stress for basalt composites. figure 9 then compares the overall behavior of the two systems. the graphs corresponding to the basalt and carbon uc represent averages obtained from all unit cells. for both systems we notice a relatively wide spread of results with no particular trend from the uc area point of view. the expected convergence with increasing size of uc as shown, e.g., in [8] for the nonlinear response of random masonry walls has not been observed for the viscoelastic response of the examined material systems. but it is interesting to point out that essentially all curves fall within the limits set by smin and smax cells. the response of the pha model appears considerably more compliant when compared figure 7. shear stress strain diagram for carbon composites. figure 8. time evolution of shear stress for carbon composites. figure 9. time evolution of shear stress for carbon and basalt composites. to all unit cells. surprisingly, this is also true even for the original format of the mt method. this may suggest the need for a rigorously defined statistically equivalent periodic unit cell as proposed, e.g., in [6]. 130 vol. 34/2022 viscoelastic response of ud fibrous composites but this goes beyond the present scope. it is also clear that at some applications the pha model recommended for random composites with a sufficiently high volume fraction of fibers may not always provide reliable results. (a) (b) figure 10. a) basalt cell with minimal stiffness, b) basalt cell with maximal stiffness. (a) (b) figure 11. basalt composite with minimal stiffness, time = 50 s: a) shear stress [mpa], b) creep strain. (a) (b) figure 12. basalt composite with minimal stiffness, time = 100 s: a) shear stress [mpa], b) creep strain. (a) (b) figure 13. basalt composite with maximal stiffness, time = 50 s: a) shear stress [mpa], b) creep strain. the evolution of stresses and creep strains is further demonstrated on surface plots in figs. 11 14 and (a) (b) figure 14. basalt composite with maximal stiffness, time = 100 s: a) shear stress [mpa], b) creep strain. (a) (b) figure 15. carbon cell: a) minimal stiffness, b) maximal stiffness. (a) (b) figure 16. carbon composite with minimal stiffness, time = 50 s: a) shear stress [mpa], b) creep strain. (a) (b) figure 17. carbon composite with minimal stiffness, time = 100 s: a) shear stress [mpa], b) creep strain. figs. 16 19 for the basalt and carbon fibers, respectively. given the results in figs. 5 8 we limited our attention to cells with the lowest and highest stiffness illustrated in fig. 10 for the basalt fiber based composite and in fig. 15 for the carbon fiber based system. 131 s. valentová, m. šejnoha, j. vorel, z. prošek acta polytechnica ctu proceedings (a) (b) figure 18. carbon composite with maximal stiffness, time = 50 s: a) shear stress [mpa], b) creep strain. (a) (b) figure 19. carbon composite with maximal stiffness, time = 100 s: a) shear stress [mpa], b) creep strain. we see that for the smin cell the creep strain is more localized when compared to the smax cell where the corresponding distribution is more diffuse. this partially explains deviations of the macroscopic response pertinent to individual unit cells. 4. conclusion viscoelastic response of the epoxy matrix based basalt and carbon fiber composites loaded in in-plane shear was examined. the option of generating the computational models applicable with the first-order homogenization as test windows, taken randomly from large binary images, in place of statistically equivalent periodic unit cell was examined. this approach, however, seems inappropriate given the large scatter of the predicted stress-strain curves. the present results also suggested that application of the pha model [1], generally accepted for random fibrous composites with high volume fraction of fibers, should be approched with caution. the same holds for the mori-tanaka method when applied to complex nonlienar systems. acknowledgements the financial support provided by the czech technical university in prague within sgs project with the application registered under the no. sgs21/037/ohk1/1t/11 and by the gačr grant no. 21-28525s is gratefully acknowledged. references [1] j. l. teplý, g. j. dvorak. bound on overall instantaneous properties of elastic-plastic composites. journal of the mechanics and physics of solids 36(1):29–58, 1988. [2] j. fish, q. yu, k. shek. computational damage mechanics for composite materials based on mathematical homogenization. international journal for numerical methods in engineering 45(11):1657–1679, 1999. [3] g. dvorak. micromechanics of composite materials. springer dordrecht heidelberg new york london, 2013. [4] m. šejnoha, j. zeman. micromechanics in practice. wit press, southampton, boston, 2013. [5] y. benveniste. a new approach to the application of mori-tanaka theory in composite materials. mechanics of materials 6:147–157, 1987. [6] j. zeman, m. šejnoha. numerical evaluation of effective properties of graphite fiber tow impregnated by polymer matrix. journal of the mechanics and physics of solids 49(1):69–90, 2001. [7] j. zeman, m. šejnoha. from random microstructures to representative volume elements. modelling and simulation in materials science and engineering 15(4):s325–s335, 2007. [8] s. tiberti, g. milani. 3d homogenized limit analysis of non-periodic multi-leaf masonry walls. computers and structures 234:106253, 2020. https://doi.org/10.1016/j.compstruc.2020.106253. [9] g. j. dvorak, y. benveniste. on transformation strains and uniform fields in multiphase elastic media. proceedings of the royal society of london series a mathematical, physical and engineering sciences 437(1900):291–310, 1992. [10] t. a. tervoort. constitutive modeling of polymer glasses: finite, nonlinear visocelastic behaviour of polycarbonate. ph.d. thesis, eindhoven university of technology, eindhoven, 1996. [11] r. valenta, m. šejnoha, j. zeman. macroscopic constitutive law for mastic asphalt mixtures from multiscale modeling. international journal for multiscale computational engineering 8(1):131–149, 2010. [12] r. valenta, m. šejnoha. hierarchical modeling of mastic asphalt in layered road structures based on the mori-tanaka method. acta polytechnica 52(6):48–58, 2012. [13] s. valentová, m. šejnoha, j. vorel, et al. application of the mori-tanak method to describe the ratedependent behavior of unidirectional fibrous composites. acta polytechnica ctu proceedings 30:114–120, 2021. [14] s. valentová, m. šejnoha, j. vorel. comparing mori-tanaka method and first-order homogenization scheme in the viscoelastic modeling of unidirectional fibrous composites. acta polytechnica ctu proceedings 26:133–138, 2020. https://doi.org/10.14311/app.2020.26.0133. [15] j. vorel, e. grippon, m. šejnoha. effective thermoelastic properties of polysiloxane matrix based plain weave textile composites. international journal for multiscale computational engineering 13(3):181–200, 2015. 132 https://doi.org/10.1016/j.compstruc.2020.106253 https://doi.org/10.14311/app.2020.26.0133 acta polytechnica ctu proceedings 34:127–132, 2022 1 introduction 2 theoretical background 2.1 generalized leonov model 2.2 mori-tanaka method 2.3 homogenization using fem 3 results 4 conclusion acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.34.0116 acta polytechnica ctu proceedings 34:116–121, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague steel elements with timber fire protection experiment and numerical analysis stanislav šulca,∗, jakub šejnab, vít šmilauera, františek waldb a czech technical university in prague, faculty of civil engineering, department of mechanics, thákurova 7, 166 29 prague 6, czech republic b czech technical university in prague, faculty of civil engineering, department of steel and timber structures, thákurova 7, 166 29 prague 6, czech republic ∗ corresponding author: stanislav.sulc@fsv.cvut.cz abstract. steel structural elements are sensitive to elevated temperatures, while timber elements have good thermal insulation properties. timber material can fulfill the role of fire protection of steel members. the effect of the protection is demonstrated on an experiment with three beams with different levels of the protection, placed into a horizontal furnace. the experimental task was also numerically analysed with standard computational approach given by the eurocode [1, 2], which leads to an interesting comparison, as the calculation is supposed to provide higher temperatures and larger deformations compared to the experimental data. keywords: timber, fire protection, fire resistance, furnace experiment. 1. introduction steel structural elements are very sensitive to elevated temperatures [3, 4], which leads to protecting them with methods as fire protective paints or panel lining [5]. the protection can also fulfill the visual design demands, which sometimes leads to using timber panels or beams to cover the steel elements. although timber itself can serve as a fuel, its thermal insulating properties and low charring rate can effectively delay the effect of the surrounding fire load, and for some time it can be effectively taken as a fire protection [6, 7]. figure 1. beams in the furnace during the experiment. this article presents an experimental comparison of beam elements with different levels of fire protection using timber panel lining made of osb panels. the beams, placed in a horizontal furnace, were also mechanically loaded to represent a real example of use and to study the mechanical behaviour. this article presents a comparison of the experimental data with simulations based on material models defined by the eurocode [1, 2]. 2. description of the experiment figure 2. cross sections of the beams. the subjects of study were three steel beams with outer dimension 100×100 mm and thickness of 6 mm, placed in a horizontal furnace. the steel type was s235. beam #1 had no protection against fire, beam #2 was protected with one layer of 22 mm osb panels, and beam #3 was protected with two of these layers, 116 https://doi.org/10.14311/app.2022.34.0116 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 34/2022 steel elements with timber fire protection figure 3. geometry of the task, depicted unprotected beam. see the cross sections in fig. 2 and the photo in fig. 4. the span length was 3.5 m and the beams were loaded with 100 kg in the middle. see the geometry in fig. 3. the osb panels were connected to each other with 50 mm narrow crown staples, which are commonly used for osb panels, see fig. 4, so the integrity of the beam should be the same as in a real application. the furnace was heated with eight gas burners and the temperature regulation followed the iso 834 temperature curve given by the eurocode 5 [2]. figure 4. photo of the beam #2 with one layer of osb protection (left) and the 50 mm narrow crown staples (right). the temperatures of steel were measured with thermocouples and the adiabatic surface temperatures were measured by plate thermometers positioned close to the beams. each beam’s vertical displacement was measured in the middle of its span. 3. results of the experiment the temperatures from the middle of the span of each beam are presented in fig. 5. it clearly shows the effect of the timber protective layer, which causes slower temperature increase according to the thickness of the layer. approximately at the time of 25 minutes the timber panels fell off completely from beam #2, which causes the rapid temperature increase. this happened for beam #3 approximately at the time of 45 minutes. the falling off of the panels is partially caused by the mechanical loading of the beams. the time of falling off cannot be predicted precisely, as it is rather a probability task to define how long time the panels keep protecting the beams, which cannot be defined by one experiment. figure 5. temperature comparison of the unprotected and protected steel beams. figure 6. displacement comparison of the unprotected and protected steel beams. 117 s. šulc, j. šejna, v. šmilauer, f. wald acta polytechnica ctu proceedings fig. 5 presents that adding one layer of the osb panels causes approximately 20 minutes delay of the temperature increase, which is a very satisfying effect of the protection. the difference is also clearly expressed by displacements in fig. 6. 4. numerical simulations we present numerical simulations of the experiment to show the capabilities of standard computational approach based on material properties and models from the ec3 [1] and ec5 [2] documents. 4.1. thermal task the thermal task calculates temperature of the steel beam in the middle of its span, as there the data is least affected by the beam endings. the boundary conditions are defined by the adiabatic surface temperature, measured by plate thermal sensors, which provided almost identical values as the prescribed iso 834 time-temperature curve. the surface emissivity and heat transfer coefficient were assumed as 0.95 and 20 w/m/k. the temperature-dependent functions of timber thermal conductivity, specific heat capacity and bulk density are depicted in figs. 7, 8 and 9. the values conservatively assume 0 % initial moisture content. figure 7. ec5 timber thermal conductivity. figure 8. ec5 timber specific heat capacity. figure 9. ec5 timber bulk density. bulk density of steel was considered as 7850 kg/m3. temperature-dependent functions of thermal conductivity and specific heat capacity are depicted in figs. 10 and 11. figure 10. ec3 steel thermal conductivity. figure 11. ec3 steel specific heat capacity. 4.1.1. results fig. 12 shows that in case of beam #1 the simulation results match the experimental data very well, which confirms that the computation is properly defined and the task without osb protection is very well predictable. 118 vol. 34/2022 steel elements with timber fire protection figure 12. comparison of experimental and simulated temperatures for the unprotected beam. the results on beams #2 and #3, depicted in figs. 13 and 14, show a comparison of results with basic timber model and this model with modification for the osb material. as the ec5 defines the temperaturedependent functions of thermal conductivity, specific heat capacity and bulk density of plain timber, these function are not defined for osb material. but it provides the charring rate for both timber (0.65 mm/min) and osb (0.9 mm/min) for equal conditions. the conductivity function was multiplied by the ratio of these two values, which lead to the results denoted by blue lines, which are closer to the experimental data, but still underestimating the temperature. figure 13. comparison of experimental and simulated temperatures for beam protected with 22 mm osb panels until the time when the protection fell off. figure 14. comparison of experimental and simulated temperatures for beam protected with two layers of 22 mm osb panels until the time when the protection fell off. the ec5 timber thermal model is working well for the charring rate propagation through the timber itself under the conditions of the iso-834 time-temperature curve, but it seems that when we change some boundary conditions, the accuracy of the model is not that good, as it changes the task definition significantly. 4.2. mechanical task the key input for the mechanical task is the temperature field, which reduces the mechanical properties. here we consider the temperatures which were measured during the experiment to avoid negative influence coming from the thermal task imperfections. the mechanical behavior given by the ec3 document is defined by the diagram in fig. 15. the effective yield strength function σy,θ is depicted in fig. 16 and the proportional limit function σp,θ is depicted in fig. 17. the starting linear part is given by the young’s modulus function ea,θ=tan(α), which is depicted in fig. 18. the strain parameter εy,θ = 0.02, εt,θ = 0.15 and εu,θ = 0.2. figure 15. considered strain-stress diagram of steel, given by ec3. 119 s. šulc, j. šejna, v. šmilauer, f. wald acta polytechnica ctu proceedings the curved part of the stress function is defined as σ(ε) = σp,θ − c+ b a √ a2 − (εy,θ − ε)2 (1) where c = (σy,θ − σp,θ)2 (εy,θ − εp,θ)ea,θ − 2(σy,θ − σp,θ) (2) b = √ c(εy,θ − εp,θ)ea,θ + c2 (3) a = √ (εy,θ − εp,θ)(εy,θ − εp,θ + c ea,θ ) (4) figure 16. temperature-dependent effective yield strength of steel, given by ec3. figure 17. proportional limit of steel, given by ec3. figure 18. temperature-dependent young’s modulus of steel, given by ec3. 4.2.1. results the displacement results are presented and compared with experimental data in figs. 19, 20 and 21. it shows that the mechanical behaviour of steel changes significantly after reaching the temperature 600 ◦c. figure 19. comparison of the experimental and calculated displacements for beam #1. figure 20. comparison of the experimental and calculated displacements for beam #2. figure 21. comparison of the experimental and calculated displacements for beam #3. the results also show that during all the time the displacements calculated by the ec3 methodology are much smaller than those measured during the experiment, which is caused by the high-temperature steel creep behaviour. this time-dependent phenomenon is missing int the ec3 mechanical model, which corresponds with the data summarization by wald et. al. [8]. ec3 defines the material behaviour for the temperature range up to 1200 ◦c, but it seems to be valid only for short-time applied load. 120 vol. 34/2022 steel elements with timber fire protection it is more complicated to predict the mechanical behaviour above 600 ◦c, the behaviour is often studied only below this temperature, as the ec3 also limits the fire-resistance by temperature. fig. 22 presents the displacement on beam #1 before the temperature reached 600 ◦c, which is not well visible in fig. 19. it shows that the creep behaviour for these lower temperatures and time of several minutes is also significant in the relative comparison. figure 22. comparison of the experimental and calculated displacements for beam #1 until reaching the temperature of 600 ◦c. 5. conclusions the experiment proved that it is reasonable to use timber as a fire-protection, as it provides good thermal insulation and is present long enough to work as a retardant of the thermal load. the comparison of the calculated and experimental data showed underestimating of temperatures, as the thermal material model of timber from ec5 doesn’t work perfectly for our task where timber serves as a fire-protective layer in combination with the steel material, although it performs well in tasks where the whole domain is made of timber. it also showed that the steel material behaviour defined in the ec3 doesn’t take steel creep time-dependent behaviour into account, as there occurs a significant difference for the displacements, which agrees with other existing studies. this article brings experimental data for high temperatures above 600 ◦c, which will be used for development and validation of creep material models in our future work, as there is the need to find an easy-usable model which could be safely used for calculations to estimate the conservative fire resistance of structures where the time plays a non-negligible role. acknowledgements this research was funded by czech science foundation, grant 19-22435s “performance of structures with timber fire protection—multi-physics modelling”, and also by czech technical university in prague, grant number sgs21/037/ohk1/1t/11. references [1] en 1993-1-2:2006, eurocode 3: design of steel structures part 1-2: general rules structural fire design, 2005. [2] en 1995-1-2:2006, eurocode 5: design of timber structures part 1-2: general structural fire design, 2006. [3] g. li, p. wang. properties of steel at elevated temperatures. advanced analysis and design for fire safety of steel structures 2013. https://doi.org/10.1007/978-3-642-34393-3_3. [4] z. nie, y. li, y. wang. mechanical properties of steels for cold-formed steel structures at elevated temperatures. advances in civil engineering 2020. [5] s. gnanachelvam, a. ariyanayagam, m. mahendran. fire resistance of light gauge steel framed wall systems lined with pcm-plasterboards. fire safety journal 108:102838, 2019. https://doi.org/10.1016/j.firesaf.2019.102838. [6] d. h. l. truong, t. meng-ting. experimental assessment of the fire resistance mechanisms of timber–steel composites. mdpi materials 2019. https://doi.org/10.3390/ma12234003. [7] f. riola-parada, k. tavoussi, a. fadai, et al. fire performance of prefabricated timber-steel-concrete ribbed decks. in conference: wcte 2018 world conference on timber engineering. seoul, south korea, 2018. [8] f. wald, i. burgess, l. kwasniewski, et al. benchmark studies experimental validation of numerical models in fire engineering. ctu publishing house, czech technical university in prague 2014. 121 https://doi.org/10.1007/978-3-642-34393-3_3 https://doi.org/10.1016/j.firesaf.2019.102838 https://doi.org/10.3390/ma12234003 acta polytechnica ctu proceedings 34:116–121, 2022 1 introduction 2 description of the experiment 3 results of the experiment 4 numerical simulations 4.1 thermal task 4.1.1 results 4.2 mechanical task 4.2.1 results 5 conclusions acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.34.0122 acta polytechnica ctu proceedings 34:122–126, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague new possibilities for recycling of mineral wool separated from thermal insulation waste jan trejbala,∗, ondřej zobala, mária domonkosa, zdeněk prošeka,b a czech technical university in prague, faculty of civil engineering, thákurova 7, 166 29 prague 6, czech republic b university centre for energy efficient buildings of czech technical university in prague, třinecká 1024, 273 43 buštěhrad, czech republic ∗ corresponding author: jan.trejbal@fsv.cvut.cz abstract. it has become increasingly clear, that the recent consumption of mineral resources is unsustainable. the czech republic produces more than 217,000 tones of mineral wool annually, while its recycling stays unacceptably limited. this paper presents new possibilities how to recycle mineral wool from building insulation systems. cladding of 30 years old building composed of fos 125 facade was disassembled and its mineral wool boards were subjected to micro-milling in order to get filler and shortly chopped fibers to be the multifunction micro-aggregate in new design of plaster mortars. it was shown that mortars containing up to 1.2 wt. % of such waste exhibited decrease in pressure strength in the order of tens of percent than those made from reference mixture. on the other side, their bending strength was decreased slightly or oven improved. other improvement was detected in thermal conductivity. keywords: construction and demolition waste, etics, mineral wool, recycling. 1. introduction 1.1. mineral wool waste production there is still increasing interest to save material resources and environment across all industries. such a phenomenon is also clear in civil engineering which is strongly dependent on mineral resources. it is estimated that civil engineering consumes as many of these resources as it corresponds to the consumption of all other industrial fields together. therefore, building stone and gravel have become critical row material. such a situation is further complicated by depreciation of society to open new mines and quarries due to economical and aesthetic reasons. it is worth mentioning that no new quarries were opened in the czech republic during last 30 years [1]. mineral insulations thus may become an endangered product. these materials occupy ca. 60 % of the market with insulations globally [2]. according to data collected by eurostat of the european commission, the czech republic produced more than 217,000 tones of wools from mineral fibers during yr. 2020. in other countries, the data are following: germany, 615,472; france, 351,010; uk, 250,000 tones [3]. it has to be stressed out that such data do not contain glass wools since these are classified into a different group together with glass fibers for textile production. it is therefore hard to determine which part of them is made for insulations. however, some scientists estimate that glass fiber insulations create approx. 30 % of that made from basalt fibers [4]. 1.2. recycling although the mineral wool waste (mww) presents only a small share of construction and demolition waste (c&dw) production in the czech republic, as stated in visoh system administered by ministry of the environment of the czech republic, it is necessary to find ways how to recycle such material [5]. a number of buildings insulated by systems with ending service life are still growing. thus, it can be expected that know-how in mww recycling will be in demand [6]. as most of insulated building envelope are made from composite system (including building adhesives, mechanical fasteners, reinforcing meshes, base-coats, finish-coat plasters, etc.), it is hard to separate individual materials from each other. that is the reason why these materials are rather landfilled than recycled [4]. it has to be also considered that european union requires the c&dw be recycled, while its landfilling becomes more charged which also raises interest in recycling. it is worth noting that the charge for mineral wool landfilling is 1,750 czk per 1 tone in the czech republic [7]. despite the aforementioned difficulties, some researches dealt with recycling of mineral wool from c&dw. as wäntsi and kärki summarized in their review paper [4], the mww can be briquetted and thus returned into the wool manufacturing process. adediran et al. [8] tested feasibility of incorporating the milled mww into ceramic materials as a fluxing agent in the production of clayand waste-based building ceramics. gebremariam et al. [9] used the milled mww at the form of ultrafines as cement substitution or as supplementary additives in concrete mixtures 122 https://doi.org/10.14311/app.2022.34.0122 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 34/2022 recycling of mineral wool wastes obtaining material with increased modulus of elasticity. ramirez et al. [10] reinforced cement pastes with residues of fibers from the recycled mww. it was shown that such modified material exhibited improved mechanical properties and lower porosity. in this study, we present technology which is capable to produce both fiber residues and filler from the mww. such a technology uses a micro-mill and a separator that are together able to disintegrate individual materials in mineral wool based external thermal insulation composite system (etics). the mww was applied into plaster mortar in order to improve its thermal properties and tensile strength while keeping pressure strength, if possible. 2. materials and methods 2.1. mineral wool waste the mww was provided from 34 meters high building in kralupy nad vltavou where the original external etics was replaced with new materials in 8/2020 due to tightening requirements for heat transfer through the building envelope. the etics was realised in yr. 1991 as the fos 125 type which was composed from basalt mineral wool boards (density 50 kg/m3) covered with sheet metal cladding, see figure 1. given that the insulation was anchored to the wall using steel frames, it was free of contamination with adhesive mortars, plaster, and other building materials. figure 1. building facade under reconstruction. 2.2. recycling after the metal cladding was disassembled from the building facade, insulation boards were removed from the steel frames and subjected to further processing using the stationary recycling line lav/k 350 tex (lavaris, s. r. o., czech republic), which is shown in figure 2. the whole line starts with an input module (1) where the mww is inserted. the module is equipped with the high-speed grinder lav/k 350 tex. the crushed mixture is then transported into a cyclone separator (2) which divides the mixture into two parts. the first is composed primarily of the chopped fibers which can be further reused for production of recycled insulation boards or as blown insulation. such a material is stored into the big bag (3). the second part is composed from mineral dust particles – fillers – and short fibers that are captured in a cyclone double-filter (4). the study deals with utilization of the second part and tries to find its effective application in civil engineering. figure 2. scheme of recycling line lav/k 350 tex. 2.3. specimens the filler and short fibers captured on the double-filter of the recycling line exhibit potential to be used as micro-aggregate with other beneficial properties for production of plaster mortars (defined in čsn en 9981 ed. 3), as the fibers can play the role of randomly dispersed and oriented micro-reinforcement and thus increase tensile strength and ductility of the plasters. moreover, heat insulation of such mortars can be increased due to presence of clumps of fibers that create air voids. composition of mixtures was designed within our previous research (see [11, 12]). it was aimed to use as much recycled material as possible while keeping mortar mechanical properties similar to reference material (conventional mixture with no additives). six mixtures were designed differing from each other in the amount of applied recycled mww (from 0 to 1.0 wt. % of the whole mixture). their composition is summarised in table 1. water to cement ratio was increased with increasing amount of the recycled mww in order to keep still the same mixture workability (classified using the flow test according to čsn en 1015-3). the last column of the table 1 presents density of hardened mortars after 28 days of curing. three specimens with dimensions of 40×40×160 mm were made from each mixtures. after 123 j. trejbal, o. zobal, m. domonkos, z. prošek acta polytechnica ctu proceedings casting the mixtures in metal moulds, the specimens were stored in water bath for 7 days. next, they were removed from water and left for another 21 days under standard laboratory conditions (temperature 22±1 ◦c, relative humidity approx. 55 %). 2.4. testing of mechanical properties basic mechanical properties – compressive and bending strength – were examined following standard čsn en 1015-11. destructive three-point bending strength test was carried out using loading frame mts 40 (mts usa). the loading rate was set to 1 mm/min. both parts of broken specimens were subsequently subjected to pressure strength test using press eu 40 (veb werkstoffprüfmaschinen). in such a case, the loading was controlled with force at the rate of 0.1 mpa/sec. 2.5. testing of thermal properties testing of heat properties was focused on determination of thermal conductivity coefficient λ. heat transfer analyzer isomet 2104 (applied precision) equipped with surface probe api210412 with measuring range 0.3–2.0 w/m.k and accuracy of ±5–10 % was used. such a device applies a dynamic method which is based on response monitoring of examined material on heat flow impulses. the measurement itself was carried out 28 days after the specimens were made. their temperature was equal to 22±1 ◦c (the same as ambient air). each specimen was measured three-times in a different position in order to obtain statistically relevant data. 3. results and discussions 3.1. mechanical properties the results of compressive strength test are shown in figure 3. it is clear that with increasing amount of the recycled mww incorporated into the mixture, compressive strength significantly decreased. the specimens marked as “i5” (those that contained the largest amount of the recycled mww) reached only on one third of the strength when compared to reference mixture ref. on the other side, even the i5 mixture can be considered as plaster mortar of the highest category cs iv (required compressive strength is at least 6 mpa) as defined in čsn en 998-1. it can be therefore said that the decrease in compressive strength as a result of the recycled mww use is massive, however still acceptable. a slightly different trend can be observed in the case of bending strength test. as figure 4 shows, specimens i1 overcome by ca. 13 % those made from reference mixture. after addition more amount of the recycled mww (all specimens i2–i5), bending strength decreased but significantly slower than in case of compressive strength. the highest detected decrease was equal to ca. 37 %. such a phenomenon can be attributed to the function of short fibers that are randomly distributed and oriented through the matrix, as assumed in chapter 2.3. these fibers are able to reduce creation of drying and shrinking cracks and thus contribute to the achievement of higher bending (tensile) strength. these findings are in accordance with our previous research, see [13, 14]. based on the cited researches, it can be assumed that matrices containing the milled mww are able to exhibit ductile behaviour because of short fibers bridging the cracks after the matrix fails in tension. it will be the subject of our further research. 3.2. thermal properties figure 5 shows that thermal conductivity of tested specimens decreases with increasing amount of the recycled mww. while the reference mixture reached on 1.82 w/m.k, those mixtures that contain the mww exhibited improvement from ca. 10 to 40 %. their thermal insulation properties are thus improved. such a finding corresponds to changes in density of hardened mortars (see table 1). as the amount of the mww is increased, the density decreases by up to 17 % as a consequence of increasing porosity. 4. conclusions the presented study deals with recycling of the mineral wool waste (mww) and thus tries to contribute in issues connected with a global deficit of mineral resources and their unsustainable consumption. recycling line lav/k 350 tex developed by lavaris, s. r. o., czech republic, showed to be very effective in processing of the mww from external thermal insulation composite systems. a building in kralupy nad vltavou older than 30 years was insulated with the new insulating system since the old one (fos 125) does not meet current requirements. old insulation boards from basalt fibers were removed from the sub-construction (steel frames) and subjected to the recycled line in order to remove all impurities. the original fibers were chopped with a high-speed mill to achieve (i) fibers for recycled blown insulation and (ii) short fibers with dust particles – filler. the study focuses on the second mentioned materials that are considered to be hardly usable in technical practise. it was shown that they can play the role of multifunction micro-aggregate in plaster mortars. mortar mixtures containing besides standard materials also recycled aggregate at the amount of 0– 1.2 wt. % of the whole mixture were designed for production of laboratory specimens with dimension of 40×40×160 mm. these were after 28 days of curing and hardening tested in order to find out their pressure and bending strength and thermal conductivity. the finding are as follows: • pressure strength massively decreased (over 60 %) with increasing amount of the recycled mww. however, even those specimens that contained the highest amount of the mww exhibited still acceptable 124 vol. 34/2022 recycling of mineral wool wastes mixture ref i1 i2 i3 i4 i5 0 35 c om pr es si ve st re ng th [m pa ] 5 10 15 20 25 30 trend line standard deviation figure 3. compressive strength of tested specimens. mixture ref i1 i2 i3 i4 i5 0 1 2 3 4 5 6 b en di ng st re ng th [m pa ] trend line standard deviation figure 4. bending strength of tested specimens. mixture ref i1 i2 i3 i4 i5 0 0.5 1.0 1.5 2.0 c oe ff ic ie nt o f t he rm al c on du ct iv ity [w /(m .k )] trend line standard deviation figure 5. thermal conductivity of tested specimens. 125 j. trejbal, o. zobal, m. domonkos, z. prošek acta polytechnica ctu proceedings mixture cement sand 0-2 mm concrete fines water mww density [g] [g] [g] [g] [g] [kg/m3] ref 225 900 375 270 0 2,053 i1 270 4.6 1,932 i2 300 9.2 1,909 i3 320 13.8 1,839 i4 345 18.4 1,803 i5 370 23.0 1,705 table 1. composition of plaster mortar mixtures. strength (more than 10 mpa) to be considered plaster mortars according to relevant technical standards. • bending strength at first increased (by 12 %) with increasing amount of the mww, however, after reaching the amount of 0.3 %, the strength decreased very slightly (from ca. 19 to 35 %). • coefficient of thermal conductivity decreased up to 40 % with increasing amount of the mww. thermal resistance of the material was thus improved. aforementioned findings prove that the recycled mww can be processed into the form of microaggregate to be a part of plaster mortar mixtures. based on achieved results, it can be concluded that such material acts as randomly oriented reinforcement (short fibers), insulation aggregate (clumps of fibers), and substitution for fine fraction of virgin aggregates (dust particles). acknowledgements this work was supported by the technology agency of the czech republic (research project no. tj04000208) and the support is gratefully acknowledged. thanks go to ing. vladimír vymětalík, ph.d., visco, s. r. o, ing. milan pokrivčák, mba, knauf insulation, s. r. o., and ing. george karra’a, ph.d., lavaris, s. r. o. for their technical and material support within the research. references [1] m. škopán. recycled construction and demolition waste as one oh the holders of the circular economy in construction industry. in recycling 2020: circular economy in civil engineering, recycling and application of secondary building materials, pp. 9–17. arsm, brno, 2020. [2] a. m. papadopoulos. state of the art in thermal insulation materials and aims for future developments. energy and buildings 37:77–86, 2005. doi:https://doi.org/10.1016/j.enbuild.2004.05.006. [3] eurostat, european commission. prodcom annual data, 2020. https://ec.europa.eu/eurostat. [4] o. wäntsi, t. kärki. mineral wool waste in europe: a review of mineral wool waste quantity, quality, and current recycling methods. journal of material cycles and waste management 16:62–72, 2014. doi:https://doi.org/10.1007/s10163-013-0170-5. [5] ministry of the environment of the czech republic. information system of waste management, 2021. https://isoh.mzp.cz/visoh. [6] a. mueller, b. leydolph, k. stanelle. recycling mineral wool waste-technologies for the conversion of the fiber structure, part 1. international ceramic review 58(6):378–381, 2009. [7] d. průša, s. štastník, k. šuhajda, j. polášek. recycling of thermal insulation materials in the building industry. in recycling 2020: circular economy in civil engineering, recycling and application of secondary building materials, pp. 82–88. arsm, brno, 2020. [8] a. adediran, p. n. lemougna, j. yliniemi, et al. recycling glass wool as afluxing agent in the production of claywaste-based ceramics. journal of cleaner production 289, 2021. doi:https://doi.org/10.1016/j.jclepro.2020.125673. [9] t. a. gebremariam, a. vahidi, f. di maio, et al. comprehensive study on the most sustainable concrete design made ofr ecycled concrete, glass and mineral wool from cd wastes. construction and building materials 273, 2021. doi:https://doi.org/10.1016/j.conbuildmat.2020.121697. [10] c. p. ramírez, e. a. sánchez, m. del rio merion, et al. feasibility of the use of mineral wool fibres recovered from cdw for the reinforcement of conglomerates by study of their porosity. construction and building materials 191:460–468, 2018. doi:https://doi.org/10.1016/j.conbuildmat.2018.10.026. [11] v. nežerka, p. havlásek, j. trejbal. mitigating inclusion-induced shrinkage cracking in cementitious composites by incorporating recycled concrete fines. construction and building materials 248, 2020. doi:hhttps://doi.org/10.1016/j.conbuildmat.2020.118673. [12] z. prošek, j. trejbal, j. topič, et al. utilization of the waste from the marble industry for application in transport infrastructure: mechanical properties of cement pastes. in international conference on building up efficient and sustainable transport infrastructure (bestinfra), pp. 82–88. ctu, prague, 2017. [13] j. trejbal. mechanical properties of lime-based mortars reinforced with plasma treated glass fibers. construction and building materials 190:929–938, 2018. doi:https://doi.org/10.1016/j.conbuildmat.2018.09.175. [14] r. hlůžek, j. trejbal, v. nežerka, et al. improvement of bonding between synthetic fibers and a cementitious matrix using recycled concrete powder and plasma treatment: from a single fiber to frc. european journal of environmental and civil engineering 190, 2020. doi:https://doi.org/10.1080/19648189.2020.1824821. 126 http://dx.doi.org/https://doi.org/10.1016/j.enbuild.2004.05.006 https://ec.europa.eu/eurostat http://dx.doi.org/https://doi.org/10.1007/s10163-013-0170-5 https://isoh.mzp.cz/visoh http://dx.doi.org/https://doi.org/10.1016/j.jclepro.2020.125673 http://dx.doi.org/https://doi.org/10.1016/j.conbuildmat.2020.121697 http://dx.doi.org/https://doi.org/10.1016/j.conbuildmat.2018.10.026 http://dx.doi.org/hhttps://doi.org/10.1016/j.conbuildmat.2020.118673 http://dx.doi.org/https://doi.org/10.1016/j.conbuildmat.2018.09.175 http://dx.doi.org/https://doi.org/10.1080/19648189.2020.1824821 acta polytechnica ctu proceedings 34:122–126, 2022 1 introduction 1.1 mineral wool waste production 1.2 recycling 2 materials and methods 2.1 mineral wool waste 2.2 recycling 2.3 specimens 2.4 testing of mechanical properties 2.5 testing of thermal properties 3 results and discussions 3.1 mechanical properties 3.2 thermal properties 4 conclusions acknowledgements references my title https://doi.org/10.14311/app.2022.36.0001 acta polytechnica ctu proceedings 36:1–5, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague reliability of a structure stricken by a tornado milan holický czech technical university in prague, klokner institute, šolínova 7, 166 08 prague, czech republic correspondence: milan.holicky@cvut.cz abstract. a tornado may become a very dangerous climatic action for any structure, depending on the tornado intensity and extent of the structure. the origin of a tornado, its range and further activity appear to be unpredictable and completely random. the strike of a tornado to a structure depends on the tornado movement and the structure extent. the tornado intensity is specified by the wind speed, following observed damages and consequences. the probability of a tornado strike on a structure depends on the extent of the structure itself, on the tornado range, and the extent of the considered reference area, for which data concerning tornado appearance are available. the probability of failure of a structure is given by the product of the probability of contact of a tornado with the structure and the probability of exceeding the design speed of the tornado wind. the probability of exceeding the design wind speed depends on the assumed distribution of wind speed and the frequency of tornadoes during the required lifetime of the structure. the target failure probability of common structures presented in available documents is 10−6 per year, and 10−7 per year of structures in power plants. however, it is not clear how these criteria have been derived. further development of reliability theory of tornado-stricken structures is recommended to be focused on the risk analysis of appropriate systems, of which the considered structures are elements, on the target failure probability, on detailed analysis of the probability distribution of tornado wind speed and their frequency during the required lifetime of the structures. keywords: design wind speed, power plant, probability, structure, target failure probability, tornado. 1. introduction reliability analysis of a structure stricken by a tornado is treated in several documents and prescriptions [1–3] that are focused primarily on nuclear power plants. recent document [2] provides detailed recommendations for reliability verification of power plants using probabilistic methods considering random properties of structures and tornado effects. all the available documents include analysis of two inseparable factors of tornado action affecting the reliability of a structure: (1) tornado strike on structure, (2) the intensity of tornado effects. both factors have random character depending on several local circumstances and conditions that are often assessed based on observed consequences and available experience. the factor (1) depends on the frequencies of the tornado in the reference territory where the tornado actions are observed. specification of such a territory depends on social and economic aspects, on longtime observation of tornado, and empirical meteorological information. tornado strike on the considered structure depends surface area of the reference territory. it is a random variable that can be only assessed. the factor (2) the intensity of tornado effects is commonly described by wind speed in the air twirl. the wind speed is again a random variable having a large variability of unknown parameters that can be assessed based on observed destructive consequences of tornado action. the design value of the wind speed can be estimated assuming usually weibull distribution and approximate parameters. because of uncertain parameters (including the skewness) the estimated design wind speed may be imprecise. the tornado intensity is usually classified using graduated tables for ground wind speeds. two scales indicated in table 1 are commonly applied: fuji and torro scales. fuji scale (having 6 grades) is primarily used in usa, torro scale (refine fuji scale having 10 grades) is used in europe. the lowest grade of both scales starts with the wind speed of 17 m/s, the highest grade from 121 m/s. 2. probability of structural failure the probability of structural failure pf due to tornado action can be assessed considering probability pa of tornado strike on a given structure, and simultaneous exceedance of wind speed of the design value vd, when the structure may fail with the probability pv. the resulting failure probability pf is therefore given as the product of two probabilities: pf = pa × pv (1) probability pa of tornado strike depends [2] on area of reference territory a0, for which records of tornado actions are available for n years, on extend of structure ak and also on the total tornado extent ator. above 1 https://doi.org/10.14311/app.2022.36.0001 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en milan holický acta polytechnica ctu proceedings fuji torro damage wind detailed description of damage f0 t0 t1 slight damage 17-24 m/s 25-32 m/s accidentally demolished chimneys and wooden fences, minor damage to the roofing, damaged advertisements and road signs next to roads, broken tree branches, sporadically uprooted trees with shallow roots, traces of a tornado are visible in the field. f1 t2 t3 moderate damage 33-41 m/s 42-51 m/s partially destroyed roofing, moving cars are pushed off the roads, mobile homes are shifted from the foundations, overturned or heavily damaged, flimsier buildings and shelters are badly damaged to destroyed, larger trees with stronger roots are sporadically uprooted and broken f2 t4 t5 moderate damage 52-61 m/s 62-72 m/s the roofs of less rigidly built buildings are completely torn down, mobile homes and flimsier structures are completely destroyed, in brick and stronger wooden houses, the side and front walls are not yet seriously damaged. light cars float, small and light debris become projectiles, most isolated large trees are uprooted or broken. f3 t6 t7 significant damage 73-83 m/s 84-95 m/s roofs and some walls are completely torn from the structure even in well-built buildings, heavier cars are buoyed, trains and locomotives are overturned, most trees in continuous forests are uprooted and broken, standing trees and tree stumps are partially debarked by flying debris. f4 t8 t9 severe damage 96-107 m/s 108-120 m/s reinforced concrete buildings are significantly damaged, brick (brick) and stone buildings are severely (mostly irreparably) damaged, less solid buildings are completely levelled, the ruins of flimsy buildings are scattered at considerable distances from their foundations, cars are drifted by the air (just above the ground). ) or towed over long distances, large and heavy "projectiles" are formed from flying debris, tree stumps are completely stripped of bark. f5 t10 complete destruction > 121 m/s reinforced concrete buildings are badly damaged, other buildings are completely destroyed, cars are carried through the air like projectiles over considerable distances, fields are completely devoid of vegetation, crops are mostly torn out even with roots. table 1. tornado intensity scales of fuji and torro. mentioned variables a0, ator and ak are random variables of considerable variability that require statistical and probabilistic analysis. in addition, tornado extent ator may be assessed as product of the length l and width w (ator ≈ l × w) [2]. assuming that the tornado actions occur within the reference territory a0 uniformly, then the probability pa for one year can be estimated from the mean values of involved variables ak, ator and a0 using the formula: pa ≈ (ak + ator)/a0/n, (2) where n denotes the number of years when tornados are observed in the reference territory a0. more detailed probabilistic analysis concerning the specific location of a structure can be provided on the basis of the local conditions within the reference territory a0. an example. the probability pa of a tornado strike per year is strongly dependent on the extent of reference area a0. in south moravian region a0 = 7188 km2 and area ak + ator = ak + l × w = 1 + 4 × 0, 5 = 3 km2. then the probability pa determined from one observation is about ≈ 3/7188 ≈ 4 × 10−4. however, when a0 is considered as a circle with the centre at the structure and the radius of 100 km, then a0 ≈ 31400 km2, and pa ≈ 3/31400 ≈ 1 × 10−4. 2 vol. 36/2022 reliability of a structure stricken by a tornado design values vd [m/s] for pv parameters vinf [m/s] µ [m/s] σ [m/s] α 10−5 10−6 10−7 weibmin 30 [m/s] 42 10 1,5 127 142 157 weibmin 7 [m/s] 42 10 1,0 108 118 127 weibmin 21 [m/s] 42 10 0,5 92 98 104 weibmin 10 [m/s] 42 10 0 80 84 87 normal ± inf 42 10 0 85 90 94 table 2. design values of wind speed vd for given skewness α and exceedance probability pv. 3. design wind speed the design value vd of the wind speed v is defined for the specified exceedance probability pv when the structural failure may occur, so: pv ≈ p (v > vd), (3) weibull distribution of wind speed v is considered in available documents [1–3]. it is the lower bound limited distribution characterized by three parameters: the mean µ, standard deviation σ a and skewness α. in the professional literature and software products (mathcad, excel, easyfit) another three parameters are also used: for example c1 (also α or b), c2 (also b or a) and the lower bound vinf (for standardized variables uinf ). the parameter c1 depends solely on the skewness α, the parameter c2 depends on the parameter c1 (skewness α) and standard deviation σ, the lower bound of the standardized variable uinf depends only on the parameter c1 (on skewness α), the lower bound of the real variable vinf depends on three parameters: on the mean µ, standard deviation σ and also on the skewness α (on parameter c1) [4]. the skewness α affects therefore the lower bound vinf and shape of the distribution for a given mean and standard deviation σ, and significantly affects resulting probability of tornado effect. appendix provides a copy of mathcad sheet enabling application of the weibull distribution. an example of determining the design values of wind speed vd for given skewness α and exceedance probability pv is indicated in table 2. it follows from table 1 and figure 1 that the relatively high skewness α = 1, 5 leads to high design values vd of wind speed, by 50 % greater than for the skewness α = 0 (normal distribution). however, it is not clear how the actual skewness α can be determined from available data. it may be also recommended to utilize experience from the distribution of usual wind speed or the information concerning lower bound vinf indicated in table 1. 4. number of tornado strikes during the required lifetime of a structure several tornados, in general, say n , may occur [3]. if the distribution function of a singular tornado is φ(x, 1), then the overall distribution function of all n tornados is given by power function φ(x, 1)n [4, 5]: figure 1. weibull and normal distribution for µ = 42 m/s and σ = 10 m/s. φ(x, n) = φ(x, 1)n (4) then the design value (upper fractile) vd,n of the distribution function φ(x, 1)n may be greater the design value determined from a single distribution φ(x, 1). the occurrence number n may be in the case of less frequent tornados equal to the number of years n used in equation 2. variation of the design value vd,n of tornados for given exceedance probabilities with the occurrence number n is indicated in table 2. probability density functions and partly distribution functions for occurrence numbers n = 1 and 10 are shown in figure 3. an example. table 3 demonstrates that the resulting design values vd,n for given number of tornados n > 1 are greater than the values for one tornado n = 1. with increasing n the design value increases up to 35 %. consequently, when designing a structure the number of tornados n during the assumed lifetime is necessary to consider. when during the assumed lifetime of a structure the number of tornados is n , then the probability pv,n , that the wind speed vn exceeds the design values vd with the probability: pv,n = p(vn > vd) (5) 3 milan holický acta polytechnica ctu proceedings occurrence number of tornados lower bound mean value standard deviation skewness design value vd,n for exceedance probability n vinf [m/s] µ [m/s] σ [m/s] α 10−5 10−6 10−7 1 30 42 10 1,5 127 142 157 10 30 61 10 1 142 157 172 100 30 80 10 1 157 172 187 1000 30 98 10 1 172 187 201 table 3. variation of the design value vd,n of tornados with the occurrence number n . figure 2. distributions of tornado wind speed for n = 1 and 10. table 4 indicates the probability p(vn > vd) for design values vd determined for a single tornado (n = 1) when structural failure may occur. design values vd are determined as before for the probabilities 10−5, 10−6 a 10−7. the probability p(vn > vd) for the design value vd determined from one observation (n = 1) is therefore increased by the order of tornado occurrence n . 5. failure probability failure probability of a structure due to tornado strike is generally given by equation1 1 as the product of the probability pa, of tornado strike, and the probability pv, that wind speed exceeds design value vd when structural failure may occur. in the case of one isolated tornado the failure probability pf,1 is given as: pf,1 = pa × pv = p(a0, ak, ator) × p(v > vd) (6) in the case of n tornados during the required lifetime the failure probability pf,n is given by the product: pf,n = pa × pv,n = p(a0, ak, ator) × p(vn > vd) (7) it is obvious that when several tornados (n > 1) are assumed, the failure probability is greater than when only one tornado (n = 1) is considered. an example. consider as before the reference area a0 = 7188 km2 and extent ak + ator = 2 + 1 = 3 km2. then the probability pa was determined from one observation ≈ 3/7188 ≈ 4×10−4. when the structure is design assuming the probability p(v > vd) = 10−5, then the expected probability is ≈ pf,1 ≈ pa × pv = 4 × 10−9. however, when during the assumed lifetime number of tornados is n = 10, then the failure probability is about ≈ pf,n = pa × pv,n ≈ 4 × 10−8. thus, it is the failure probability ten times greater. 6. target probability of failure target probability of failure of a structure during the lifetime t denoted ptar,t can be specified based on probabilistic optimization (maximization) of structural utility (cost) ctot(t) during its lifetime t. this can be analyzed in a simplified way considering the total benefit of functioning structure cben(t) during the lifetime t, and the possible losses cf (t) caused by structural failure (within the relevant system) with the target probability ptar,t. the total utility ctot(t) can be then symbolically expressed as [4, 5]: ctot(t) = max{cben(t) − ptar,t × cf (t)}. (8) the cost cben(t) includes all kind of benefits during the life time t. consequences of structural failure cf (t) cover all the material, ecological and social losses caused by the structural failure. these losses cf (t) may be extremely large, and could be very difficult to assess. a detailed analysis is outside this contribution and it is recommended to accept a very small target probability, for example, ptar,t = 10−7 per year as indicated in documents [1–3]. note that the target probability for common structures is usually accepted as ≈ 1.30×10−6 for one year (that is reliability index ≈ 4, 7). this corresponds to ≈ 0, 72 × 10−4 per 50 years (and to the reliability index 3,83), per 100 years the target probability is ≈ 1, 31 × 10−4 and reliability index ≈ 3, 65. variation of the target probabilities ptar,t with reference time t given in years for selected basic probabilities ptar,1 = 10−5, 10−6 and 10−7 is shown in figure 3. 4 vol. 36/2022 reliability of a structure stricken by a tornado number of tornados lower bound mean value standard deviation skewness the probability p(vn > vd) for design value vd n vinf [m/s] µ [m/s] σ [m/s] α 127 142 157 1 30 42 10 1,5 1.0×10−5 1.0×10−6 1.1×10−7 10 30 61 10 1 9.4×10−5 1.0×10−5 1.1×10−6 100 30 80 10 1 9.3×10−4 1.0×10−4 1.1×10−5 1000 30 98 10 1 9.3×10−3 1.0×10−3 1.1×10−4 table 4. probability p(vn > vd) of exceedance the design values vd. figure 3. variation of the target probabilities ptar,t with reference time t in years for basic probabilities for one year ptar,1 = 10−5, 10−6 and 10−7. 7. concluding remarks (1) a tornado may be for any structure very dangerous depending on the extent of the structure and tornado and its intensity. tornado is unpredictable and of random time behaviour. (2) the probability of a tornado strike can be assessed taking into account the extent of a structure and tornado and the extent of the reference territory where tornados are observed. (3) tornado intensity expressed by wind speed is determined after the tornado strike following observed consequences. (4) probability structural failure is given by a product of the probability of a tornado strike and the probability of wind speed exceedance of design value. (5) estimation of wind speed exceedance of design value is strongly dependent on assumed wind speed distribution and the number of tornado occurrences during the lifetime of the structure. (6) available documents provide for target probability for structural failure value 10-7 per year, however, it is unclear how this criterion has been derived. (7) further reliability analysis of structures stricken by tornados is to be focused on detailed analysis of tornado consequences, on wind speed distribution and the number of tornado occurrences during the lifetime of the structures. references [1] iaea safety guides 50-sg-s11a. extreme meteorological events in nuclear power plant siting excluding tropical cyclones. international atomic energy agency, vienna, austria, 1981. [2] pacific northwest national laboratory. tornado climatology of the contiguous united states. nureg/cr-4461, rev. 2, u.s. nuclear regulatory commission office of nuclear regulatory research, washington, 2007. [3] h. zhu, j. chen, f. li, et al. tornado hazard assessment for a nuclear power plant in china. energy procedia 127:20-8, 2017. https://doi.org/10.1016/j.egypro.2017.08.091. [4] m. holický. reliability analysis for structural design. sun press, stellenbosch (south africa), 2009. [5] m. holický. introduction to probability and statistics for engineers. springer, heidelberg, 2013. 5 https://doi.org/10.1016/j.egypro.2017.08.091 acta polytechnica ctu proceedings 36:1–5, 2022 1 introduction 2 probability of structural failure 3 design wind speed 4 number of tornado strikes 5 failure probability 6 target probability of failure 7 concluding remarks references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.34.0050 acta polytechnica ctu proceedings 34:50–55, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague determination of water absorption coefficient of unfired earth materials different in used clay and ratio of components barbora mužíková∗, tereza plaček otcovská, pavel padevět czech technical university in prague, faculty of civil engineering, thakurova 7, 166 29 prague 6, czech republic ∗ corresponding author: barbora.muzikova@fsv.cvut.cz abstract. the paper is focused on unfired rammed earth and its water absorption properties. increasing the utilization of raw natural materials can be one of the approaches towards sustainable development. different prescriptions were designed and specimens were rammed. then they were put in the covering of nylon and settled in the box with soft foam that was moistened. the level of moistening was constant. the specimens were regularly weighted. specimens with montmorillonite clay have the highest values of the water absorption coefficient. montmorillonite clay has a higher binding capacity. the values are compared to values that were found in the literature. then the maximal capillary water capacity by area was determined. keywords: rammed earth, unfired earth, water absorption coefficient, clay. 1. introduction nowadays the price of building material is steeply rising and it begins to be lack of it. stone quarries have stocks for the next seven years in the czech republic and no new ones are opening. one way out of this problem is recycling and the second one is using natural local materials. this leads us to unfired earth that is local material, minimizing the cost for transportation. increasing the utilization of raw natural materials can be one of the approaches towards sustainable development. natural materials have a high potential of environmental quality for example in criteria: embodied co2 and so2 emissions, embodied energy, using renewable sources or easy recycling [1–3]. of course, using this material is not without any problems. since building with them was minimal in the last years, there are no sufficient data about its mechanical and physical data. these data are necessary for designing and building. figure 1. process of making rammed earth specimen. one of the problems for the unfired earth is water. the water inside or outside the earthen construction usually leads to problems. any time earth is to deal figure 2. schema of the water absorption test. with water needs to be controlled thoroughly: water content in mixture, vapor condensation in construction, or effects of weather. these are widely recognized issues of civil engineering. another big issue that is drawing unfired earth back from wide use is a strongly varying composition of raw material. the aim of this paper is to settle the water absorption coefficient for different clay mixtures and describe the influence of used clay and composition of the mixture on its water absorption characteristics [4–8]. 50 https://doi.org/10.14311/app.2022.34.0050 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 34/2022 water absorption coefficient of unfired earth figure 3. prescription of used specimens – graph shows percent procuration of clay, sand and water in the mixture. mixture sand/clay – w/c bulk density [%/%] – [-] [kg/m3] agl 6 85/15 – 0.400 2063 kr 3 80/20 – 0.400 2180 agl 1 80/20 – 0.370 2191 kr 1 80/20 – 0.370 2212 kr 8 80/20 – 0.290 2127 kr 7 80/20 – 0.250 2093 kr 11 75/25 – 0.400 2102 agl 2 75/25 – 0.370 2159 kr 2 75/25 – 0.370 2138 gem 3 75/25 – 0.295 1905 kr 13 70/30 – 0.370 2064 table 1. composition of mixtures and bulk density of specimens. 2. water in unfired earth water is very important for rammed earth it activates the binding properties of clay and thanks to it the material holds together. on the other hand, water is very dangerous to unfired earth. if loam becomes wet, it swells and changes from a solid to a plastic state [3]. mechanical properties are firmly affected by the moisture of the material. the unfired rammed earth is very sensitive to moisture and its structures can even collapse if the volume of water is high and affect the construction for a longer time period. 3. preparation of specimens the earth material is very sensitive to moisture and water that is why it is an important characteristic that needs to be examined. the structure of the earth is widely un-homogeneous, porous, and open. it is capable to absorb water and transport it in the material itself. rammed earth is a mixture of sand, clay, and water. clay works as a binder and water activate the binding forces of the clay. sand is a filler. the mechanical properties are depending on the composition of these components and their ratio and type of used clay. there are three basic kinds of clay – illite, kaolinite, and montmorillonite. eleven prescriptions were designed and made, they differ in the ratio of components and type of clay. prescriptions are shown in tab. 1. the prescriptions are arrayed by a rising amount of clay. the prescriptions are described as a ratio of sand and clay and the water ratio in the table t. water ratio is a relation between binder and water, in this case between clay and water. bulk densities for each prescription are shown in the table. the prescription are also shown in the fig. 3. the compounds are expressed by the percent. sand, clay, and water give together 100 %. in the tab. 1 just sand and clay give 100 % (and water is expressed by the water-clay ratio). for prescription agl the illite clay was used, for gem the montmorillonite clay and for kr mix of illite-kaolinite clay. the reciprocal ratio between the components is in percent. the orange color represents the clay, the yellow color represents sand and the blue color represents water. the first prescription agl 6 had the least amount of clay (14 %), the second group are specimens with clay containing around 19 % (kr 3, agl 1, kr 1, kr 8, kr 7). the third group is specimens with clay containing around 23 % (kr 11, agl 2, kr 2, 51 b. mužíková, t. p. otcovská, p. padevět acta polytechnica ctu proceedings figure 4. water absorption coefficient. figure 5. water absorption coefficient for earth mixture in literature [3]. gem 3) and the prescription kr 13 with the highest amount of clay 27 %. the production of specimens was as follows. firstly, the sand was mixed with two-thirds of water and mixed up. then the clay and the rest of the water were added. it was stomped by a drilling machine with a special ending. specimens of size 40×40×160 mm were prefabricated for the testing in the laboratory. the production was made by ramming into steel molds (shown fig. 1) by hand and by the drilling machine. the molds were wiped by oil and the earth was rammed in four to five layers. the last one was always made by hand. comprehensive strength and tensile strength in bending can be found in [9–11]. figure 6. specimens in nylon cover during testing. 4. method of determining water absorption coefficient specimens of measurement 40×40×160 mm were used for the test of water absorption. the absorbing area of specimens is 40×40 mm. specimens were covered in nylon to protect them from eroding. specimens were measured and weighted. a box with foam was filled with water until the foam was fully waterlogged. specimens were weighted up every hour, data were recorded and the condition of the specimen is judged. it means that a breakup of the specimen or magnitude of soaked up is observed. loam is a material with an open porous structure and it can transport water in capillaries. the water moves from the area with higher humidity to the area with lower humidity. capillarity means the capacity of water that can move such as was described. the 52 vol. 34/2022 water absorption coefficient of unfired earth figure 7. maximal capillary water capacity by area. figure 8. specimens after testing. process is called capillary action. the coefficient w is calculated by the formula: w = w√ t [ kg m2 · hod0.5 ] , (1) where w is the increase in weight per unit surface area and t is the time of measurement. w is calculated: w = m−md a [ kg m2 ] , (2) where m is the weight in the moment of measuring and md is the weight at the beginning of the test, the weight of the dry sample. the parameter w is also called the water movement and it represents the figure 9. type of clay – kaolinite, illite and montmorillonite. quantity of water that can be absorbed by a time period [3]. evaluated data of water absorption coefficient are shown in the fig. 4. the averaged value and its standard deviation are shown. 53 b. mužíková, t. p. otcovská, p. padevět acta polytechnica ctu proceedings 5. method of determination maximal capillary water capacity area the specimens were settled in the water bath as was described above. they were in the water until they broke up. the maximal weight was recorded and then the maximal capillary water capacity was determined by the formula: wmax = mmax −md a [ kg m2 ] , (3) where mmax is the maximal weight until the specimen was broken. the maximal weights are not measured at the same time, the time differed for prescriptions. 6. evaluation of the results 6.1. water absorption coefficient calculated values of water absorption coefficients are shown in the fig. 4. the prescriptions are arrayed by a rising amount of clay. water absorption coefficient w says how quickly can the material absorb the water. the evaluation is after one hour of testing and the absorbing area of specimens was 40×40 mm. the highest value has the prescription gem 3 with montmorillonite clay. this type of clay has the highest absorption capabilities. the value is 21± 7 kg/m2hod0.5. similar value have solid brick 25.5 kg/m2hod0.5 according to [3]. the minimum value had prescriptions with a mixture of illitekaolinite and illite clay. prescription agl 6, kr 8 and kr 7 have higher values than others, they have less water just 5 % and 6 %. other prescriptions had similar value around 3 kg/m2hod0.5. the value responds to the values that were found by g. minke in [3] for his earth mixture. there were also values around 3 kg/m2hod0.5. these values were for different types of loams. these values are shown the fig. 5 with bulk densities of specimens. they can be compared with measured values in the tab. 4. the main influence has the type of used clay, the amount is not so important as the type. clay minerals are three basic ones – kaolinite, illite and montmorillonite. they differ in structure. clays have hexagonal lamellar crystalline structures. these lamellas consist of different layers that are usually formed around silicon or aluminum cores [3]. this is shown in the fig. 9. the difference between clays can be seen in this picture. kaolinite is a two-layered mineral and has a lower ion-binding capacity. on the other hand, montmorillonite is a three-layered mineral and has a higher ion binding capacity [4]. 6.2. maximal capillary water capacity by area calculated values of maximal capillary water capacities by area are shown in the fig. 7. the prescriptions are arrayed by a rising amount of clay again. the maximal capillary water capacity by area is determined from the maximal weight at the end of testing before the specimens collapsed. the time is not the same at all prescriptions, some of them collapsed sooner than others. the results were divided in three groups the first one with values around 30–35 kg/m2 (agl 1, kr 1, kr 8, agl 2, kr 2, gem 3). the second group are prescriptions with value 20–25 kg/m2 (agl 6, kr 3, kr 7 and kr 11) and the minimal value had kr 13 108 kg/m2. gem 3 with montmorillonite clay was specifed. the volume of the specimen extended a lot in the wet bottom and then it brooked. no other mixture had expansion like the mixture with montmorillonite clay. 7. conclusions the price of building material is steeply rising and it begins to be lack of it. one way out of this problem is recycling and the second one is using natural local materials. this leads us to unfired earth that is local material, minimizing the cost for transportation. increasing the utilization of raw natural materials can be one of the approaches towards sustainable development. the earth material is very sensitive to moisture and water that is why it is an important characteristic that needs to be examined. the structure of the earth is widely un-homogeneous, porous, and open. it is capable to absorb water and transport it in the material itself. it is important to search the properties connected with water. different prescriptions were designed and specimens were rammed. then they were put in the covering of nylon and settled in the box with soft foam that was moistened. the level of moistening was constant. the specimens were regularly weighted. the highest values of water absorption coefficient have the specimens with montmorillonite clay that has the higher binding capacity. the values are compared to values found in the literature. then the maximal capillary water capacity by are was determined. the results of maximal capillary water capacity by area were divided into three groups the first one with values around 30–35 kg/m2 (agl 1, kr 1, kr 8, agl 2, kr 2, gem 3). second group are prescriptions with value 20–25 kg/m2 (agl 6, kr 3, kr 7 and kr 11) and the minimal value had kr 13 18 kg/m2. list of symbols a area [ m2] m weight of specimen in the moment of measuring [kg] md weight of specimen in the begging of the test (dry sample) [kg] mmax maximal weight of specimen [kg] t time of testing [hod] w water absorption coefficient [kg/m2hod0 5] 54 vol. 34/2022 water absorption coefficient of unfired earth w capillary water capacity [kg/m2] wmax maximal capillary water capacity [kg/m2] acknowledgements the research was financially supported by the czech science foundation (gacr no.18-0884s) and by the faculty of civil engineering at ctu in prague (sgs project no.sgs19/148/ohk1/3t/11). references [1] f. havlik. development and experimental verification of mechanical-physical properties of pre-formed rammed earth wall panel, vol. 1. cvut, prague, 2017. [2] j. norton. building with earth, a handbook, vol. 1. intermediate technology development group limited, london, 1986. [3] g. minke. building with earth design and technology of sustainable architecture, vol. 2. de gruyter, berlin, 2009. [4] i. zabickova. hlinené stavby. era 21, brno, 2002. [5] m. rauch. refined earth construction and design with rammed earth, vol. 1. detail, munich, 2015. [6] kianfar, ehsan, v. toufigh. reliability analysis of rammed earth structures. construction and building materials (127):884–895, 2016. [7] h. araki, j. koseki, t. sato. tensile strength of compacted rammed earth materials. soils and foundation (56(2)):189–204, 2016. [8] l. miccoli, d. v. oliveira, r. a. silva, u. muller. static behaviour of rammed earth: experimental testing and finite element modelling. materials and structures (48(10)):43–56, 2015. [9] t. otcovska, p. padevet. dependence of tensile bending strength of rammed earth on used clay composition and amount of mixture water. modern methods of experimental and computational investigations in area of construction ii pp. 48–53, 2017. [10] t. otcovska, p. padevet. dependence of compressive strength of rammed earth on used clay composition. experimental stresss analysis 2016 pp. 107–108, 2016. [11] i. zabickova, t. otcovska, p. padevet. compressive strength of unburned clay masonry. modern methods of experimental and computational investigations in area of constructions pp. 31–34, 2016. 55 acta polytechnica ctu proceedings 34:50–55, 2022 1 introduction 2 water in unfired earth 3 preparation of specimens 4 method of determining water absorption coefficient 5 method of determination maximal capillary water capacity area 6 evaluation of the results 6.1 water absorption coefficient 6.2 maximal capillary water capacity by area 7 conclusions list of symbols acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.34.0038 acta polytechnica ctu proceedings 34:38–42, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague the effect of sampling volume size on the apparent stiffness of jointed rock mass martin lebeda∗, petr kabele czech technical university in prague, faculty of civil engineering, thakurova 7, 166 29 prague 6, czech republic ∗ corresponding author: martin.lebeda@fsv.cvut.cz abstract. overall mechanical properties of a jointed rock mass are strongly affected by discontinuities – fractures – that naturally occur in rocks. stochastically-generated discrete fracture network (dfn) modeling, which uses a probabilistic approach to describe the spatial distribution of fractures, such as position, size, or orientation, offers an explicit way to describe geometry of the fracture system. many in-situ measurements and analyses presented in literature indicate that fractures’ sizes can be adequately represented by the power law probability distribution. the parallel plate model of individual fractures combined with an averaging technique makes it possible to estimate the overall compliance or stiffness of jointed rock mass (oda et al. [1]). in the present study, a series of numerical simulations of jointed rock mass modeled by dfn and oda’s approach were conducted to analyze the effect of different sizes of the sampling volume on the overall elastic moduli. the results of the numerical study show that the variance as well as the average of the apparent stiffness decrease as the size of sampling element grows. keywords: jointed rock mass, discrete fracture network (dfn), volume averaging, representative volume element (rve), apparent elastic modulus. 1. introduction brittle structures, such as joints and faults, are found in almost every rock mass body. they vary greatly in quantity, dimensions, and arrangement. the presence of these fractures strongly affects the deformational response of the rock mass to stress and needs to be taken into account in geotechnical calculations. the finite element method (fem) formulated on the basis on the governing equations of solid mechanics is often used for these calculations. in such a case, it is feasible to model discretely (using, e.g., interface elements) only the less-frequent largest fractures, while the smaller ones, which occur in a large quantity, can be represented by the means of so-called equivalent continuum. in the latter approach, a solid containing discontinuities is treated as a homogeneous continuum, whose stiffness or compliance is determined so as to correspond to the relations between the overall stress and overall strain of the fractured body. to this end, homogenization or averaging [2] over a certain sampling volume of the fractured body can be used. the smallest sampling volume element of the fractured body “for which the usual spatially constant ‘overall modulus’ macroscopic constitutive representation is a sufficiently accurate model to represent mean constitutive response” [3], is called the representative volume element (rve). if it is possible to identify an rve that is small enough to be idealized as a material point on the scale of the structural problem analyzed by the fem, then the jointed rock mass can be modeled as a uniform equivalent continuum. if, however, the size of the rve is larger than the resolution required for the structural problem, it is necessary to employ the concept of the statistical volume element (sve). the overall properties of the sve are, again, determined by homogenization or averaging. however, they need to be evaluated over a sufficient number of sampling volumes of the fractured solid and interpreted statistically. the rock mass in the structural problem solved by the fem is then modeled as a nonuniform equivalent continuum. to this end, random fields respecting the statistics of the sves can be used. stochastically-generated discrete fracture network (dfn) models, which uses a probabilistic approach to describe the spatial distribution of fractures, such as position, size, or orientation, offers an explicit way to describe geometry of the fracture system in rock mass. many in-situ measurements and analyses presented in literature indicate that fractures’ sizes can be adequately represented by the power law probability distribution [4], while various distributions for spherical data can model their orientations [5]. one appealing aspect of the dfn modeling is that the parameters of the probability distributions can be identified based on data from structural-geological survey, e.g. [6]. it follows that stochastic realizations of the dfn may serve as the geometrical description of the fractures in rock mass to be used for the earlierdiscussed sampling and determination of the rve or the sve properties [7–9]. in this article, we present results of a numerical case study, in which a body of a jointed rock modeled by stochastic dfn realizations is sampled by volume elements of different sizes and the apparent overall young moduli are evaluated and 38 https://doi.org/10.14311/app.2022.34.0038 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 34/2022 the effect of sampling volume size on the apparent stiffness of jointed rock mass analyzed. the dfns were generated using the power law probability distribution of fractures’ sizes and uniform probability distribution of fractures’ location and orientation. the parallel plate model of fractures and the averaging scheme based on the crack tensors proposed by oda [1] were employed to calculate the overall moduli. in contrast to the previous studies [7– 9] we carried out the analysis in 3d, adopted some different assumptions about the probabilistic distributions of fractures’ sizes and orientations or used a different averaging approach. 2. dfn model of joints’ geometry geometry of individual fractures and the whole fracture system in the modeled rock mass are described by 3d stochastically generated dfn with the following assumptions. 2.1. assumptions on fractures’ orientation, spatial distribution, and shape fractures in natural rock can be usually grouped into sets (populations) based on their geological origin and dominant orientation. however, for the sake of simplicity, to avoid the issue of overall anisotropy of the rock mass, we assume the distribution of fracture orientations as uniform on the sphere (fisher et al. [5]). fractures’ centers are generated by poisson random process, so the probability distribution of center coordinates in space is also uniform. the total number of generated fractures is controlled by a given volumetric density p30 (number of fractures’ centroids over a unit volume). individual fractures are idealized as squares. 2.2. power law distribution of the fracture sizes fractures’ sizes are assumed to follow the power law distribution, whose probability density function is expressed as: p(x) = α− 1 xmin ( x xmin )−α for x > xmin and α > 1, (1) where x is the radius of the circle circumscribed to the fracture, xmin is the minimum fracture size (location parameter), α is the law’s exponent (shape parameter). the parameters used for generating stochastic dfns and for subsequent calculations are listed in table 1. the volumetric density and power law exponent are adapted from our previous work [10], where they were estimated for a highly jointed metamorphic rock based on data from structural-geological mapping of fracture traces on tunnel walls. thus, the range of fracture sizes covered by the model is appropriate for the scale of an underground structure (e.g. tunnel). the dfn models used in this paper were generated using dfram software (kabele et al. [11]). parameter value volumetric density p30 [ 1 m3 ] 2.56 power law exponent α [-] 3.4 minimum fracture size xmin [m] 0.3 volume of the rock mass [m3] 100·100·100 table 1. parameters of dfns. 3. the model of the overall stress-strain behavior of jointed rock mass oda [1] proposed the following formula for overall apparent elastic stress-strain relation of jointed rock mass: εij = 1 e [ (1 + ν)δikδjl − νδijδkl + ( 1 kn − 1 ks ) fijkl + 1 4ks (δikfjl + δjkfil + δilfjk + δjlfik) ] σkl = cijklσkl, (2) where e and ν are young’s modulus and poisson’s ratio of the intact rock, respectively, kn and ks are nondimensional parameters related to the fracture (joint) normal and tangent stiffness, respectively, δij is kronecker’s delta, and fij and fijkl are so-called second and fourth rank crack tensors, respectively: fij = 1 v m∑ k=1 s(k)r(k)n (k) i n (k) j , (3) fijlm = 1 v m∑ k=1 s(k)r(k)n (k) i n (k) j n (k) l n(k) m . (4) here v is the volume of the sample, s(k) is the area of k-th fracture inside the sampling volume, r(k) is a typical size of the fracture equal to a diameter of a circle with the same area and n(k) i are the components of a unit vector normal to the fracture. note that the typical size r is assumed as a property of the fracture independent on the size of the sampling volume and it is calculated using the original area of the fracture generated in stochastic dfn. individual fractures are in oda’s approach represented by the so-called parallel plate model as a set of parallel flat plates connected by two springs that symbolize normal and shear stiffness. the fractures’ stiffnesses influence the evaluation of the compliance tensor through the non-dimensional stiffness parameters kn and ks, as seen in eq. (2). the values of the rock’s and joints’ mechanical parameters used in this paper are listed in tab. 2. the joint’s stiffnesses were determined assuming a small stress change in a rock at the depth of 550 m and the other parameters 39 martin lebeda, petr kabele acta polytechnica ctu proceedings parameter value normal stiffness kn [-] 1.356 shear stiffness ks [-] 1.188 poisson’s ratio of intact rock ν [-] 0.164 elastic modulus of intact rock e [gpa] 44.0 table 2. parameters of oda’s parallel plate model. (e and ν) were determined with the basis on in-situ measurements on the same rock as described in [10]. 4. methodology of the analysis 4.1. general considerations we consider that the overall stiffness of the rock mass evaluated by oda’s method on the basis of the dfn would be eventually used for a finite element analysis of stress and deformation in the vicinity of an underground structure, e.g. a tunnel or deep geological repository of radioactive waste. in this scenario, it is plausible to assume that the equivalent continuum model would be employed to account for joints only up to a certain maximum size amax, while larger fractures would be represented as discrete discontinuities with, e.g., interface elements or extended fem. in the context of the underlying dfn it means that only the abundant smaller joints need to be present in the stochastically generated dfn, while the less frequent larger fractures would be mapped and implemented in a deterministic way. to set the threshold value amax, we used a sufficiently low value of the power law’s complementary cumulative distribution function (ccdf), which is defined as the probability of occurrence of a fracture with size greater than amax. with the parameters listed in tab. 1 and ccdf equal to 1·10−5, the corresponding limit fracture size is amax = 51.4 m·51.4 m = 2642 m2. this value seems acceptable also from the geological survey’s point of view, as brittle geological structures of this size are likely to be mapped deterministically by boreholes and trial tunnels. when the dfn is generated by the dfram program, centers of individual fractures are placed by a random process inside a given prismatic bounding box. thus, there is an inner region along the bounding box boundary, which could be intersected by fractures from outside of the box, but these fractures are not generated as their centers lie outside the box. the “thickness” of this boundary zone corresponds to the radius of the circle circumscribed to the largest fracture, that is 51.4 m 2 · √ 2 = 36.3 m. to avoid distortion of results by this effect, the sampling volumes must be taken from a subdomain of the dfn bounding box excluding these boundary regions. 4.2. generation of dfn models and sampling volumes to obtain a statistically relevant set of sampling volumes for the evaluation of the apparent overall stiffness, 10 different stochastic realizations of dfn model with the same probabilistic parameters and the same parameters of rock mass (tab. 1 and tab. 2) but different random seed were generated. the bounding box of the dfn fractures’ centroids was a cube with side length of 100 m. cubic sampling volumes of different sizes were created by clipping the dfn models, as shown in fig. 1. the samples were always concentric with the dfn bounding box. considering the dfn box size and the elimination of the boundary effect, the side length of the largest sampling volume was 27.3 m. the edge of the smallest sampling volume was chosen 0.5 m to be slightly less than the diagonal of the smallest fracture (2·xmin). in total, 10 sampling volumes (with edge sizes 0.5, 1.0, 2.0, 3.0, 4.0, 5.0, 10.0, 15.0, 20.0, 27.3 meters) were extracted from each realization of the stochastic dfn. figure 1. sample volume. 4.3. averaging procedure – calculation of overall strain and apparent stiffness averaging procedure – calculation of overall strain and apparent stiffness for each sampling volume, the overall apparent young moduli were calculated using eq. (1). although uniform probability distributions were used for the location and orientation of the fractures in the underlying dfn model, which would suggest isotropy, the apparent moduli were calculated for three orthogonal directions: ex = 1/c1111, ey = 1/c2222, ez = 1/c3333. this way of calculation allowed us to analyze not only the magnitude of the stiffness but also its directional dependence. 5. results figures 2–4 present the dependence of apparent deformation moduli calculated in three orthogonal di40 vol. 34/2022 the effect of sampling volume size on the apparent stiffness of jointed rock mass rections on the size of sampling volume. figure 2. apparent deformation moduli – x axis. figure 3. apparent deformation moduli – y axis. figure 4. apparent deformation moduli – z axis. the results show the same trends for moduli in all three directions. as the sampling volume grows, the mean of the apparent moduli obtained from different realizations of the dfn decreases and converges to a certain value. the maxima have the same tendency, while the minima grow. the dependence of the mean values can be well approximated by an exponential curve, as shown by the dotted line. the graphs also show that the smaller are the sample volumes, the greater are the deviations of the moduli obtained from the different realization of the dfn. 6. preliminary analysis of anisotropy of apparent stiffness from figures 2–4 it is obvious, that in spite of the uniform statistical distribution of fractures’ orientation in the underlying dfn, the apparent overall moduli of the rock mass in three orthogonal directions are not equal. this implies that the overall stiffness of the rock mass predicted by the model is not isotropic. hereafter, we present the results of a preliminary analysis that will allow us to gain some insight into how the detected anisotropy is related to the sampling element size. various measures of anisotropy have been proposed in the literature, especially in connection with crystal elasticity – see a review e.g. in [12]. however, as in the present study we examine only the equivalent deformation moduli, we define the index of anisotropy ad-hoc as the coefficient of variation of the apparent moduli ex, ey, ez evaluated in three orthogonal directions: α = std dev(ex, ey, ez) mean(ex, ey, ez) . (5) it is obvious that for an isotropic material, where all moduli are equal, is zero, whereas anisotropy, manifested by a larger variation among the moduli, results in a larger value of α. figure 5. sample size dependence of the index of anisotropy α. figure 5 shows the dependence of the index on the sampling volume element size. as in the case of the elastic moduli, we can see that with increasing size of the sampling volume, the mean and maximum values as well as the scatter of the index decrease. the minima do not exhibit a clear trend and fluctuate close to zero for all volume sizes. the mean value of α for the largest volume is approaching, but is not equal to, zero. this is understandable, since the index α can be only nonnegative and zero mean value would require that in all model realizations a perfectly isotropic response was achieved. these observations indicate that the model converges to an isotropic response (in the sense of ex ≈ ey ≈ ez). however, we point out that these are only preliminary conclusions and a more systematic analysis involving the complete apparent stiffness or compliance tensor and appropriate index of anisotropy should be carried out. 7. conclusions the concept of using oda’s [1] averaging procedure in conjunction with 3d stochastic dfn has been examined with the aim to determine the dependence of apparent rock mass stiffness on sampling volume size. the results of the case study allow us to draw some interesting conclusions: • with growing sample sizes, the mean value of the apparent stiffness is not constant but decreases and converges to a certain value. the minima and maxima converge to the same value. 41 martin lebeda, petr kabele acta polytechnica ctu proceedings • in spite of assuming directionally and spatially uniform probability distributions of fractures, the power law variability of fracture size is sufficient to produce anisotropic overall deformation moduli. the measure of anisotropy decreases with increasing sampling volume size. • the great variance of stiffnesses as well as anisotropy calculated on smaller sampling volumes (up to about 15 m large side of the sample cube with the parameters used in the present study) means that these volume elements cannot be regarded as representative (rve), but stochastic (sve). the corresponding overall properties must be then considered not as effective, but apparent [13]. should these overall properties be used to define equivalent continuum for finite element analysis of an underground structure, the variance must be taken into account, e.g. by using the concept of random fields. it is noted that this may be relevant for analysis of many structures in underground engineering, such as tunnels, where, due to the spatial variability of the stress field and the size of the structure, model resolution on the order of 100 m or even 10−1 m may be required. • on the other hand, if the required finite element model resolution is on the order of 101 m or larger, using the mean stiffness obtained by the averaging over rve as uniform effective property of the equivalent continuum may be acceptable. this could be the case, for example, when variations of the geostatic stress field need to be estimated for hydrogeological simulations on a larger scale. acknowledgements this paper was financially supported by czech technical university in prague under no. sgs project sgs2/037/ohk1/1t/11. references [1] m. oda, t. yanabe, y. ishizuka, et al. elastic stress and strain in jointed rock masses by means of crack tensor analysis. rock mechanics and rock engineering 26(2):89– 112, 1993. https://doi.org/10.1007/bf01023618. [2] m. hori, s. nemat-nasser. on two micromechanics theories for determining micro–macro relations in heterogeneous solids. mechanics of materials 31(10):667–682, 1999. https://doi.org/10.1016/s0167-6636(99)00020-4. [3] w. j. drugan, j. r. willis. a micromechanics-based nonlocal constitutive equation and estimates of representative volume element size for elastic composites. journal of the mechanics and physics of solids 44(4):497–524, 1996. https://doi.org/10.1016/0022-5096(96)00007-5. [4] e. bonnet, o. bour, n. e. odling, et al. scaling of fracture systems in geological media. reviews of geophysics 39(3):347–383, 2001. https://doi.org/10.1029/1999rg000074. [5] n. i. fisher, t. lewis, b. j. j. embleton. statistical analysis of spherical data. cambridge university press (cup), 1993. https://doi.org/10.1017/cbo9780511564345. [6] p. kabele, o. švagera, m. somr, et al. mathematical modeling of brittle fractures in rock mass by means of the dfn method, 2017. [7] k.-b. min, l. jing. numerical determination of the equivalent elastic compliance tensor for fractured rock masses using the distinct element method. international journal of rock mechanics and mining sciences 40(6):795–816, 2003. https://doi.org/10.1016/s1365-1609(03)00038-8. [8] m. gutierrez, d.-j. youn. effects of fracture distribution and length scale on the equivalent continuum elastic compliance of fractured rock masses. journal of rock mechanics and geotechnical engineering 7(6):626–637, 2015. https://doi.org/10.1016/j.jrmge.2015.07.006. [9] h. ni, w. xu, w. wang, et al. statistical analysis of scale effect on equivalent elastic modulus of jointed rock masses. arab j geosci 9(3):206, 2016. https://doi.org/10.1007/s12517-015-2190-z. [10] m. lebeda. určení tuhosti porušeného horninového masivu s využitím dfn modelu, 2020. [11] p. kabele, o. švagera, m. somr, et al. mathematical modeling of brittle fractures in rock mass by means of the dfn method, 2017. [12] s. i. ranganathan, m. ostoja-starzewski. universal elastic anisotropy index. physical review letters 101(5):055504, 2008. https://doi.org/10.1103/physrevlett.101.055504. [13] t. kanit, f. n’guyen, s. forest, et al. apparent and effective physical properties of heterogeneous materials: representativity of samples of two materials from food industry. computer methods in applied mechanics and engineering 195(33-36):3960–3982, 2006. https://doi.org/10.1016/j.cma.2005.07.022. 42 https://doi.org/10.1007/bf01023618 https://doi.org/10.1016/s0167-6636(99)00020-4 https://doi.org/10.1016/0022-5096(96)00007-5 https://doi.org/10.1029/1999rg000074 https://doi.org/10.1017/cbo9780511564345 https://doi.org/10.1016/s1365-1609(03)00038-8 https://doi.org/10.1016/j.jrmge.2015.07.006 https://doi.org/10.1007/s12517-015-2190-z https://doi.org/10.1103/physrevlett.101.055504 https://doi.org/10.1016/j.cma.2005.07.022 acta polytechnica ctu proceedings 34:38–42, 2022 1 introduction 2 dfn model of joints’ geometry 2.1 assumptions on fractures’ orientation, spatial distribution, and shape 2.2 power law distribution of the fracture sizes 3 the model of the overall stress-strain behavior of jointed rock mass 4 methodology of the analysis 4.1 general considerations 4.2 generation of dfn models and sampling volumes 4.3 averaging procedure – calculation of overall strain and apparent stiffness 5 results 6 preliminary analysis of anisotropy of apparent stiffness 7 conclusions acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.38.0044 acta polytechnica ctu proceedings 38:44–49, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague applying life cycle assessment with minimal information to support early-stage material selection matthew roberts∗, valeria cascione, stephen allen, barrie dams, daniel maskell, david coley university of bath, faculty of engineering and design, department of architecture and civil engineering, claverton down, ba2 7ay bath, united kingdom ∗ corresponding author: mar90@bath.ac.uk abstract. traditional life cycle assessment (lca) is too data intensive and time consuming to be used during typical building design processes. conducting an lca during the building design process therefore requires simplifications and assumptions. such “screening lcas” are quicker and can be used with less data but introduce greater uncertainty. unfortunately, uncertainty is not reflected in standard deterministic lca calculations, which produce single-point values in lca results. thus, in this study, data quality scoring has been incorporated into a screening lca to produce probabilistic predictions of environmental performance based on limited data. the approach has been applied during the design process of a bio-based wall panel designed for a circular economy. a combination of ecoinvent and material data sheets were used to analyse a wide range of novel bio-based insulation materials. the screening lca analysed global warming potential and identified a short-list of promising materials that were then subjected to a detailed lca for further consideration in the design. the method uses publicly available information and can be applied at material or building-element level. the method thus helps designers estimate environmental impacts without hindering the design process. keywords: life cycle assessment, bio-based materials, circular economy, uncertainty. 1. introduction the timing of emissions associated with construction is significant when considering how to mitigate climate change [1]. the production of materials, needed for building construction and renovation, contributed 11 % of global energy-related co2 emissions in 2017 [2]. unlike operational impacts, which occur over an extended period, upfront embodied impacts have already occurred by the time a building has been constructed and, therefore, represent the immediate impacts of a building [3]. decisions that occur during the design process greatly influence the magnitude of these embodied impacts. within the uk, the building design process is standardised by the royal institute of british architects (riba) plan of work [4]. the riba plan of work classifies the design process into 8 stages, including: two pre-design stages, three design stages and construction, handover, and use stages [4]. as the design progresses, the ability to influence the design with ease diminishes [5]. this is due to the elimination of design variants and the selection of characteristics and materials. early-stage design decisions have a large influence on the overall environmental performance of a design. when appropriate considerations are taken, the design process can be guided to low-impact solutions that produce the most environmentally beneficial outcome [1, 6]. frequently, designers rely on past experience to evaluate between alternatives [7]. this approach does not guarantee that the best solution is taken and can perpetuate the selection of sub-optimal design solutions; thus, producing environmental impacts that would be avoidable. life cycle assessment (lca) is an internationally recognized means of assessing the environmental impacts that occur throughout all stages of a product’s, or system’s, life cycle [8]. when implemented effectively, lca can be used to support the selection of environmentally beneficial solutions [9]. unfortunately, lca is typically perceived to be too time consuming and data intensive to be compatible with the nuances of early-stage design [10, 11]. therefore, lca is often only implemented late in the design process when little is expected to change [12, 13]. this relegates lca to being an accountancy tool that, generally, is used for green building certification schemes [14, 15]. screening lcas (slcas) have emerged as a means of providing a relatively quick assessment [16]. slcas are, however, characterised by elevated levels of uncertainty that can devalue their conclusions [17]. slcas can be used to simplify the level of detail in the life cycle inventory and the scope of the impact assessment method [18]. the challenges of assessing materials, or design variants, in the design process are amplified when unconventional materials or materials with limited information are considered. the use of bio-based materials to substitute traditional, more impactful, materials has been proposed as one means of lowering the impacts attributed to material use in the built environment [19]. bio-based ma44 https://doi.org/10.14311/app.2022.38.0044 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 38/2022 applying life cycle assessment with minimal information . . . terials sequester atmospheric carbon as they grow and, typically, have low processing impacts when compared to other construction materials [20]. the sequestered carbon is stored within the bio-based materials until end-of-life when a proportion of the sequestered carbon is emitted to atmosphere [20, 21]. the use of bio-based materials provides additional benefits within the context of a circular economy [22]. the concept of a circular economy is focused on the retention of materials in the value chain to minimise raw material extraction and eliminate the production of waste [23]. for bio-based materials that are designed to be circular, the sequestered carbon is stored within the material for the duration that it is retained in the value chain [20, 21]. therefore, the more circular a bio-based product is, the longer the release of sequestered carbon is postponed. the present study uses data quality to incorporate uncertainty into a slca to improve the confidence in the conclusions that can be drawn from this type of assessment. the approach has been applied to the design of a circular economy bio-based external wall assembly. this study has been conducted to demonstrate how lca can be used as a decision support tool while dealing with inadequate levels of information needed for a traditional lca. 2. methods the assessment discussed in this paper is categorised by two distinct phases: (1) an initial slca; (2) a detailed lca of specific materials identified from the slca. the slca was done when there was a lack of available data for the assessed materials, a common obstacle faced during the design process, whilst the detailed lca was performed for materials once whole processes and required data was available. the goal of the slca was to identify which materials should be subjected to a detailed lca based on what their environmental performance is likely to be. the slca did not try to quantify the exact environmental impacts of each material. data quality has been considered in the slca to visualise the uncertainty associated with the calculated impacts. the slca was limited to raw material supply (a1) and product stage transport (a2) [24] due to the availability of information for the assessed materials. product stage manufacturing (a3) was not included within the scope of the slca due to the lack of available data for the assessed materials. the materials were compared based on their anticipated ranges of impacts. after the slca, two materials were identified and were subjected to a detailed lca for a1–a3. 2.1. data quality for the purposes of this slca, only publicly accessible information was used. the use of publicly accessible information was chosen to enable life cycle thinking to support the design process even if information is suboptimal and/or has significant gaps. a combination of publicly available material specifications and articles were used for this study. the use of sub-optimal data introduces various sources of uncertainty and makes data quality very relevant to the types of conclusions that can be made. data quality assessment provide a means to visualise and communicate uncertainty, aiding the process of comparing multiple materials when the level of detail for each varies. the ecoinvent 3.0 pedigree matrix, as presented in ciroth et al. [25], has been used to assign data quality indicators for each of the assessed materials based on reliability; completeness; temporal correlation; geographic correlation; and, further technological correlation. the pedigree matrix is used to assign a value between 1–5, with 1 having the least uncertainty and 5 having the most uncertainty, to each indicator that reflects how well the information represents the assessed system. the data quality indicators are converted into uncertainty factors following table 10.5 from weidema et al. [26]. these uncertainty factors have been combined with a basic uncertainty of 0.04 (table 10.3, weidema et al. [26]) to get the standard deviation for each material by using equation (1). the basic uncertainty is used to capture the variances associated with representing the values as a normal distribution [26]. it is important to note that equation (1) functions under the assumption that each variance is normally distributed and independent. σ = √√√√ 6∑ n=1 σ2 n, (1) where σ2 1 represents the basic uncertainty and σ2 2−6 represent the variance for each indicator score 2.2. impact assessment the eugeos 15804+a2 v4.1, an extension to the ecoinvent version 3.6 database, has been used to determine the environmental impacts of the constituent components of each assessed material. however, any database that includes detailed environmental impacts for individual materials could be used to conduct a similar assessment. for this study, the global warming potential evaluated over a 100-year time horizon (gwp100) has been used to compare the materials against one another. the total gwp100, including biogenic carbon storage, is reported for the slca results of each assessed material. 3. case study the presented approach has been applied to the selection of insulation materials for the design of an external wall assembly. the design of the wall assembly in question is described in cascione et al. [35]. a slca was conducted to compare multiple materials under consideration for improved design iterations and identify materials that would likely provide the most favourable environmental performance based on 45 m. roberts, v. cascione, s. allen et al. acta polytechnica ctu proceedings material density [kg/m3] λ [w/mk] constituent material breakdown references mycelium 95 0.08 pleurotus ostreatus, straw, flour, corn, wheat [27], [28] recycled cotton 20 0.039 cotton, recycled fibre, polymer binder [29] compressed reed 275 0.052 typha, magnesite [30] agriculture fibre 30 0.038 fibres (cotton, flax, hemp), polyethylene (pe) binder, fungicide [31] cellulose wadding 45 0.04 cellulose wadding, hemp, pe binder [31] grass fibre 40 0.04 grass fibre, recycled fibres, polyester [32] flax 23 0.035 flax, polyester binder, salts [33] sheep’s wool 25 0.035 sheep’s wool, bicomponent polyester [34] table 1. key characteristics for the assessed bio-based insulation materials. material reliability completeness temporal correlation geographic correlation further technological correlation standard deviation (σ) mycelium 0.002 0.002 0 2.5 · 10−5 0.008 0.228 recycled cotton 0.0006 0 0.0002 2.5 · 10−5 0.008 0.221 compressed reed 0.002 0.002 0.002 2.5 · 10−5 0.008 0.232 agriculture fibre 0.0006 0.0006 0.0002 2.5 · 10−5 0.008 0.222 cellulose wadding 0.0006 0.0001 0 2.5 · 10−5 0.04 0.284 grass fibre 0 0 0 2.5 · 10−5 0.0006 0.202 flax 0 0.0001 0 2.5 · 10−5 0.0006 0.202 sheep’s wool 0.0006 0.0006 0.0002 2.5 · 10−5 0.008 0.221 table 2. assigned variances (σ2) based on pedigree matrix scores with calculated standard deviation. a limited amount of information. the density, thermal conductivity (λ) and material breakdown of each assessed insulation are included in table 1. the references included in table 1 were used to gather the constituent material composition information needed to conduct the slca. as the study considers novel biobased materials, there were no environmental product declarations (epds) available. epds are typically prepared for established and mass-produced materials to describe the environmental impacts that are anticipated to occur throughout the material’s life cycle. the slca was carried out for all materials under consideration for future design iterations. data quality indicator scores were assigned, following the pedigree matrix [25], based on the quality of information available for each material. these data quality scores were used to assign variances for each data quality indicator which were then combined with the basic uncertainty to acquire the standard deviation based on equation (1). table 2 summarises the assigned variances and standard deviations for each of the assessed materials. based off the pedigree matrix method used, higher variances are associated with more uncertain results and correspond to higher standard deviations. variances with an assigned value of 0 demonstrate the highest level of confidence in the information used to conduct the assessment. 4. results the assessment has been conducted in two parts to capture the thermal resistance and assembly thickness of the first design iteration [35]. the first comparison is conducted for the desired r-value of 6.3 m2k/w and an unrestricted insulation thickness. table 3 presents the expected ranges of a1–a2 gwp100 for each material when the desired r-value is achieved. most materials meet the desired r-value with a thickness of ∼300 mm. mycelium and compressed reed insulation require thicknesses of 510 mm and 365 mm, respectively, to provide the desired thermal resistance. it is important to note that the required thickness of mycelium will have further implications on the wall panel design as it would require additional stud framing and fasteners to house the insulation, thus increasing material usage. these knock-on impacts of increased insulation thicknesses are not discussed within the scope of this paper. a second comparison was completed with an insulation thickness limited to a maximum of 300 mm, matching the wall assembly thickness of the first design iteration [35]. the ranges of expected a1–a2 total gwp100, depicted by probability density func46 vol. 38/2022 applying life cycle assessment with minimal information . . . figure 1. slca (a1–a2) total gwp100 results for 300 mm insulation thickness (r-values vary). material required thickness [mm] a1–a2 total gwp100 [kg co2e] -3 standard deviations median value +3 standard deviations mycelium 510 -416.1 -351.1 -286.1 recycled cotton 300 23.2 36.5 49.8 compressed reed 365 -68.8 123.0 314.7 agriculture fibre 300 -90.0 -70.0 -50.0 cellulose wadding 310 -32.3 6.1 44.4 grass fibre 310 -52.1 -27.9 -3.7 flax 280 -134.8 -120.8 -106.9 sheep’s wool 280 -0.3 16.3 32.9 table 3. expected slca gwp100 ranges and required thickness for thermal resistance of 6.3 m2k/w. tions, are presented in figure 1 for each material. under the 300 mm wall thickness, mycelium and the compressed reed insulations did not meet the required thermal resistance as they were only able to reach thermal resistances of 4 m2k/w and 5.3 m2k/w, respectively. 5. discussion table 3 and figure 1 clearly convey how the certainty of gwp100 results is affected by variance in the underlying data quality of each material. this is much more transparent than the typical approach of presenting single-point estimates and rankings, which implies all values are equally certain. a negative total gwp100 indicates that the amount of sequestered carbon outweighs the fossil impacts of acquiring the constituent materials in a1–a2. the total gwp100 for mycelium and compressed reed display the greatest difference when their expected ranges are compared between table 3 and figure 1. based on table 3 and figure 1, the materials that are most likely to provide environmentally beneficial results are mycelium and the flax based insulations. the distributions for agricultural fibre mix and grass fibre insulations have some overlap when compared at a 300 mm thickness, but the expected ranges for both are separate from other considered materials. the expected range for the cellulose wadding insulation encompasses that of the sheep’s wool insulation in its entirety. from the slca, it would be impractical to make conclusions between these materials due to the significant overlaps present in their respective distributions. the compressed reed insulation presents a very wide range of possible values, most of which are at the highest end of gwp results. the compressed reed insulation should not be included within the design considerations based on the information available. 5.1. comparison against detailed lca following the slca, a detailed lca was conducted for a1–a3 for both the mycelium and flax based insulations. a combination of manufacturer information, material specifications and published literature was used to complete the detailed assessment. the full life cycle inventories (lcis) for the detailed lcas of mycelium and flax based insulations are included within the supplemental materials of the study by cascione et al. [36]. the results for the slca are 47 m. roberts, v. cascione, s. allen et al. acta polytechnica ctu proceedings material required thickness [mm] slca [a1-a2] total gwp100 -3 σ median +3 σ [a1-a3] gwp100 total biogenic fossil luluc mycelium 510 -446.1 -348.5 -250.9 -181.2 -738.8 557.6 0.89 flax 280 -131.4 -120.3 -109.2 -105.5 -155.4 50.0 0.06 table 4. comparison between slca (a1–a2) and detailed lca (a1–a3) results in kg co2e. compared against that of the detailed lca in table 4. the values presented in table 4 represent the environmental impact needed to produce enough material to meet the desired thermal resistance of 6.3 m2k/w. to meet the desired thermal resistance of 6.3 m2k/w, 510 mm of mycelium or 280 mm of flax would be required as previously mentioned. the results from the detailed lca are higher than expected slca ranges presented in figure 1 due to the inclusion of process impacts needed in the manufacturing (a3) life cycle stage. since the sequestered carbon may be emitted at end-of-life, it is important to consider the fossil gwp100 as it indicates an immediate emission of greenhouse gases. in order to reduce the environmental impacts of the built environment, materials with minimal fossil gwp100 impacts should be prioritised. as shown in table 4, the fossil gwp100 impacts for mycelium are 557.6 kg co2e for the required thickness due to the energy required during the manufacturing processes [37]. the fossil gwp100 for the mycelium insulation is 11 times higher than the fossil gwp100 for the flax based insulation. based on the detailed lca, the flax insulation provides a high performing bio-based insulation alternative with minimal a1–a3 fossil gwp100 impacts and therefore would be more environmentally favourable than the mycelium insulation. 6. conclusions the use of lca during early-stage material selection is often hindered by insufficient information and constrained project timelines. the use of screening lcas (slcas) can reduce the time needed to conduct an assessment but can introduce an increased level of uncertainty. in this study, a pedigree matrix approach is used in a slca methodology to calculate a probability distribution for global warming potential and estimate the uncertainty caused by variations in data quality. this is more transparent than the typical approach of presenting single-point estimates and rankings, which implies all values are equally certain. the approach has been applied during the design process of a bio-based wall panel for a circular economy. this was done to identify insulation materials that were likely to result in a low environmental impact while mitigating challenges associated with information gaps during early design material selection. the slca only considered a1–a2. there is scope to incorporate additional life cycle considerations into the assessment, including transportation, product lifespan, and end-of-life. the inclusion of uncertainty highlighted that there was no clear ranking among some materials since their probability distributions of global warming potential exhibited significant overlaps. for the materials subjected to the detailed lca, the slca scope (a1–a2) gave a reasonable indication of a1–a3 impacts for flax based insulation, since it did not have large processing impacts in a3. the mycelium insulation was found to have significant a3 impacts. slcas enable a multitude of materials to be considered prior to the completion of a detailed lca for materials that perform favourably in the slca. acknowledgements this work was completed as part of the circular biobased construction industry (cbci) project funded by the european union regional development interreg 2 seas mers zeen (2s05-036). the work was supported by a leveraged university research studentship in affiliation with the uk engineering and physical science research council funded active building centre [ep/n020863/1]. references [1] l. strain. time value of carbon. carbon leadership forum, seattle, 2017. [2] international energy agency. material efficiency in clean energy transitions. paris, 2019. https://doi.org/10.1787/aeaaccd8-en [3] worldgbc. bringing embodied carbon upfront. london; toronto, 2019, [2019-11-15]. https://www.worldgbc.org/sites/default/files/ worldgbc_bringing_embodied_carbon_upfront.pdf [4] riba. plan of work 2020. riba, london, 2020. 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[36] v. cascione, m. roberts, s. allen, et al. integration of life cycle assessments (lca) in circular bio-based wall panel design. journal of cleaner production 344:130938, 2022. https://doi.org/10.1016/j.jclepro.2022.130938 [37] e. dorr, m. koegler, b. gabrielle, c. aubry. life cycle assessment of a circular, urban mushroom farm. journal of cleaner production 288:125668, 2021. https://doi.org/10.1016/j.jclepro.2020.125668 49 https://doi.org/10.1016/j.buildenv.2018.02.016 https://doi.org/10.1007/s11367-016-1065-1 https://doi.org/10.1016/j.scs.2019.101879 https://doi.org/10.1080/09613218.2017.1356126 https://doi.org/10.1016/j.buildenv.2020.107274 https://doi.org/10.1016/j.jclepro.2017.01.173 https://doi.org/10.1016/j.procir.2017.11.127 https://doi.org/10.1177/1478077120947996 https://doi.org/10.1016/j.jclepro.2017.11.148 https://doi.org/10.3390/f10020160 https://doi.org/10.1016/j.buildenv.2020.107399 https://doi.org/10.1016/j.jclepro.2021.128122 https://doi.org/10.5334/bc.46 https://doi.org/10.1007/s40725-017-0067-y https://doi.org/10.1007/s11367-013-0670-5 https://criticalconcrete.com/producing-mycelium-insulation/ https://criticalconcrete.com/producing-mycelium-insulation/ https://doi.org/10.1016/j.matdes.2019.108397 https://doi.org/10.26168/icbbm2015.104 https://www.greenspec.co.uk/green-products/timber-frame-floor-joist-zone-thermal-insulation/details/isovlas/humans.txt https://www.greenspec.co.uk/green-products/timber-frame-floor-joist-zone-thermal-insulation/details/isovlas/humans.txt https://www.greenspec.co.uk/green-products/timber-frame-floor-joist-zone-thermal-insulation/details/isovlas/humans.txt https://doi.org/10.1016/j.jclepro.2022.130938 https://doi.org/10.1016/j.jclepro.2020.125668 acta polytechnica ctu proceedings 38:44–49, 2022 1 introduction 2 methods 2.1 data quality 2.2 impact assessment 3 case study 4 results 5 discussion 5.1 comparison against detailed lca 6 conclusions acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.38.0527 acta polytechnica ctu proceedings 38:527–532, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague adaptive reuse of factory chimneys – industrial heritage symbols and urban landmarks jana hořická∗, jan pustějovský czech technical university in prague, faculty of civil engineering, department of architecture, thákurova 7, 166 29 prague 6 – dejvice, czech republic ∗ corresponding author: jana.horicka@fsv.cvut.cz abstract. industrial heritage buildings and sites are seen as an important part of urban regeneration and sustainable development strategies over the last two decades for a number of reasons. of particular note is the genius loci that accompanies them, but also the potential to attract artists and creative industries. in this sense, factory chimneys are a strong visual element and also an important symbol. at the same time, thanks to their distinctive proportions, they have also become an unmissable part of the urban structure of cities, in which they can assume a compositional and orienting role (a landmark), comparable with church spires or belfries with all due respect. in order to design adaptive reuse of the chimney, it is therefore necessary to place it in a context that is not only spatial but also symbolic, both in relation to its immediate surroundings and in a wider context. often, however, the subject of the new use of the chimney is only raised when at least the material context of the chimney was irreversibly altered; in extreme situations, the chimney is the only surviving element of the original structure. however, the lost authenticity – the originality of the preserved building structure or technological flow – also opens up new meanings and other possibilities for the use of a solitary chimney in a transformed environment. the contribution introduces adaptive reuse possibilities of the industrial chimneys within the sustainable urban development, considering both industrial heritage values and specific chimney construction limits. keywords: factory chimneys, industrial heritage, adaptive reuse, sustainable urban regeneration. 1. industrial heritage and sustainable urban regeneration in general, a number of arguments was made as to why the adaptive reuse of industrial heritage is an essential part of sustainable urban regeneration, and as factory chimneys are an integral and visually distinctive part of industrial heritage, it is worth retrieving the most important of the arguments for an introduction. considering society as a whole, industrial heritage carries cultural and historical values, that are of undisputable importance for understanding the history, continuity and social identification of the community. considering particular cities, industrial heritage brings diversity to the urban fabric and contributes to the orientation and identification of the place. the architectural quality and uniqueness, unrepeatability that industrial heritage provides to projects developed within its context is important in terms of long-term sustainability. quality of architecture is a basis for long-term resilience to moral obsolescence and need for improvements. last but not least, in the case of tangible industrial heritage, it is about energy efficiency in terms of the life cycle of buildings, taking into account the embedded energy consumption and greenhouse gas emissions. 2. factory chimneys in wider context 2.1. learning from industrial heritage the contribution is based on research focused on factory chimneys, conducted in the czech republic, but considering european experience as well. theoretical basis was drawn from international knowledge on industrial heritage in general, both because specific sources on the subject of factory chimneys were very limited and because chimneys are an integral and inseparable part of industrial heritage. the chimney never stood alone, it was always part of a wider whole, both in sense of material structure of the site and in sense of intangible links of energy or technological flow as well. adaptive reuse principles are therefore based on experience with adaptive reuse of industrial heritage in general and in a broader context, while specific characteristics of chimneys are also considered. compared to the first spontaneous attempts to reuse former production and storage buildings, which in the middle of the 20th century revealed the values of industrial heritage to the world and were realized almost without intervention in the original building structure, the range of current approaches to reuse is very diverse. in principle each project is different depending on the set of conditions and circumstances in which a particular building or site is located. adaptive reuse process is affected with values of industrial heritage 527 https://doi.org/10.14311/app.2022.38.0527 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en jana hořická, jan pustějovský acta polytechnica ctu proceedings as well as the structural and technical condition of buildings and site. 2.1.1. adaptability adaptability, i.e. ability to accept a new function, different from the original one, is also important for the consideration of adaptive reuse of industrial buildings. czech theoretical literature apply categorization of industrial buildings into two types – versatile and single-purpose [1]. spatial structure of versatile buildings allows adaptation for almost any type of new use. these are represented typically with multistorey textile mills or machinery production halls. spatial characteristics of single-purpose buildings, on the contrary, fully correspond to the requirements of the original function for which they were built, and their adaptation for other uses is difficult or impossible. usually, these are only envelopes of technological equipment or, in the extreme case, building forms the production technology itself. in the sense, chimneys are typical representatives of this type, where building is a part of technological equipment rather then spatial structure in the true sense of the word. the professional community agrees on the aim of sustainable transformation of an industrial heritage building is a convenient combination of the requirements for the new use with its original spatial characteristics. the natural adaptive reuse represents a reasonable degree of intervention that does not damage the genius loci and preserves space for other future transformations that we cannot yet see today [2]. applying a certain level of simplification, factory chimneys, which can only be adapted problematically by conventional methods, require a greater degree of creativity in the approach to adaptive reuse design. however, the creativity of architectural design in industrial heritage adaptation is seen in its sophistication and humility rather than in its expressiveness. however, presented research describes a wide range of possibilities of chimneys new use, which of course includes different approaches working with different degrees of intervention in the mass of the chimney body. 2.2. factory chimneys in urban structure industrial heritage buildings and sites are seen as an important part of sustainable urban regeneration strategies over more than last two decades for a number of reasons. the genius loci that accompanies them is of particular note, same as the potential to attract artists and creative industries [3]. in this sense, factory chimneys are a strong visual element and also an important symbol. at the same time, thanks to their distinctive proportions, they have also become an unmissable part of the urban structure of cities, in which they can assume a compositional and orienting role (a landmark), comparable with church spires or belfries with all due respect. in order to design adaptive reuse of the chimney, it is therefore necessary to place it in a context that is not only spatial but also symbolic, both in relation to its immediate surroundings and in a wider context. often, however, the subject of the new use of the chimney is only raised when at least the material context of the chimney was irreversibly altered; in extreme situations, the chimney is the only surviving element of the original structure. however, the lost authenticity – the originality of the preserved building structure or technological flow – also opens up new meanings and other possibilities for the use of a solitary chimney in a transformed environment. 3. initial conditions analysis the first part of the research was focused on collecting the data and formulating a complex analysis of initial condition preceding the design process itself. as a result, initial condition analysis describes: locality, context and relations, site and surroundings, historical and architectural values, chimney parameters [4]. 3.1. locality experience in the field of industrial heritage revitalization shows that the possibilities of new use are strongly influenced by where the building or site is located. as an example, consider the limited possibilities of using a hard-to-reach chimney in a vast industrial area on the outskirts of a large city. the potential of the site can thus significantly influence the potential for new use. from this point of view, we describe urban sites, where the site is part of a compact town or village, is directly connected to the surrounding buildings and can be linked to its surroundings. a specific site is an industrial zone, which may be part of a city or outside the urban structure. this is a larger area where there are usually several industrial estates and accessibility of the chimney is complicated. sites and chimneys in the open countryside deserve special attention, as their accessibility and continuity with the surrounding area may not be straightforward, but they have the added value of an atmosphere of place. 3.2. context and relations as well as location, context also fundamentally influences new use of the particular chimney, specifically its relation to the immediate surroundings. the chimney may be closely connected to or directly part of the building, which must then necessarily be involved in the consideration of the new use project. stand-alone chimneys are usually either part of the site, so the new use of the chimney is part of the overall concept, or one of the few reminders of the production history after the demolition of the site, or in the extreme case, the last reminder. 3.3. site and surroundings significance of the chimney within the site will inevitably affect project of the new use. not infrequently chimneys dominate the landscape, a town or a neighborhood, but there are also chimneys to be 528 vol. 38/2022 adaptive reuse of factory chimneys – industrial heritage symbols . . . find hidden in the grip of surrounding buildings, for example within a city block. a special situation arises in some locations where the chimney was dominant, but lost its role as a landmark due to the development of new higher surrounding buildings, eg. pompeu fabra university poblenou campus in barcelona. an integral part of the consideration of the chimney new use is the space under the chimney, because the chimney often remains untouched and preserved in the status quo, but the immediate surroundings are subject to change. the space under the chimney may or may not be as important for the interpretation of the production history as the chimney itself. the surroundings of the chimney may be preserved in a clearly legible form, as inanimate fragments of the original structure or only in trace. it is not an exception that nothing of the original context of the chimney remains, then the cultural and historical significance of the chimney is crucial. 3.4. historical and architectural values the shape, the structural and material design of the chimney, i.e. its typological characteristics and the closely related historical development of chimneys [5], are also important for the approach to the design of the new use. simply put, a brick chimney in its historical form from the second half of the 19th century offers different possibilities and inspiration compared to a chimney from the second half of the 20th century. however, not only czech practice also shows that the decision to preserve the chimney itself is usually not motivated by its historical or architectural value, but primarily by its symbolic significance, which is why even an ordinary chimney is often preserved. 3.5. chimney parameters as already mentioned above, the adaptability of the original spatial structure – i.e. its ability to accept a new use different from the original one, to be adapted, to be modified – is absolutely essential for the implementation of an adaptive reuse. due to their original function, particularly flue gas extraction, chimneys have very specific spatial parameters that are not comprehensible for conventional adaptive reuse design. in particular, lack of usable spaces with a scale and proportions compatible with human scale, and also limited accessibility, are problematic. the basic internal space available is the chimney flue, whose properties can be described by two basic dimensions height and diameter. the internal diameter, especially at the base of the chimney, plays a crucial role when searching for new uses. if it is sufficient, it allows the design to enter the interior of the chimney and offer an unusual spatial perception. the space open for new use is then offered outside the chimney body. the possibilities for new use can be greatly expanded by the presence of related spaces and structures, especially the boiler house itself if it is in contact with the chimney (sometimes including figure 1. former sugar factory in prague – modřany (cz), the chimney became a symbol of community activities preceded the site development process. photo © martin vonka. underground spaces). the external space under the chimney can then be seen as a separate spatial unit. the exterior space defined by the presence of the chimney plays an unquestionable role in composing the related spatial (but also broader urban) structure and can become an important support or crystallization point of development – a carrier of identity (figure 1). last but not least, possibilities for reuse are also determined by the structural and technical condition of the chimney and its structural parameters. these affect how the mass of the chimney can be intervened in or whether it is possible to put more loads on the chimney structure. 4. new use possibilities the second part of the research resulted in formulation of an overview of new use options to inspire not only owners but all those who make decisions about the future fate of factory chimneys [4]. the new use of factory chimneys can take many forms, beginning with sole preservation of a chimney body without intervention, through completely utilitarian functions or reversible and artistic interventions, to irreversible changes in form and use, i.e. adaptive reuse. the level of intervention in the original building structure, 529 jana hořická, jan pustějovský acta polytechnica ctu proceedings which is the intersection of the requirement to preserve authenticity, cultural value, and the possibilities of adaptability that chimneys have quite narrow, is an important issue, as in case of other objects of industrial heritage. a proposal for a new use can therefore be interpreted as a search for an optimal, appropriate level of intervention in a specific context. therefore, the overview was organized in scale of the intervention. most of the new use options presented in the contribution do not require significant intervention in the body of the chimney or represent reversible intervention that can be removed in the future. 4.1. no intervention the most common and legitimate solution is to preserve the chimney in a more or less authentic form without assigning it a new function and without significant intervention in the building structure. the city of tampere presents its industrial history also with preserving the silhouette of chimneys in public spaces, especial those close to tamerkoski rapids banks. an example of this approach is the preservation of the torso of the chimney and the preservation of the so-called controlled ruin. of course, the chimneys that are listed are also preserved without assigning a new function and without significant intervention in the building structure. however, it is not always the case that the chimney alone is subject of conservation; most chimneys are part of listed buildings or sites. as the conservation of industrial heritage is usually aimed at preserving the technological flow as authentically and completely as possible, chimneys are therefore an integral part of the protected whole. 4.2. utilitarian use – necessary intervention the new use also includes the restoration of the original use, i.e. the state in which the chimney is used again for its original purpose. in such a case, there may not even be significant intervention in the building structure, but only possible modifications to ensure the restored function, for example by lining. another possibility is represented with various forms of utilitarian use, in which the specific parameters of the chimney are utilized. a viable solution is the use of the chimney flue in a figurative form, e.g. as ventilation for garages or gastronomic facilities, which necessarily requires technical modification of the flue. there are several such examples in the czech republic, but this form of use was not always the primary reason for the preservation of the chimney, or rather, the decision to preserve it was preceded and the assignment of this function only supplemented it. 4.3. utilitarian use – reversible intervention the situation is quite different in the case of the utilitarian use of a chimney as a carrier of technical equipment, i.e. typically technology for receiving figure 2. the flowering chimney in česká skalice (cz), an original advertisement of the manufacturer of substrates and fertilizers. photo © martin vonka. and transmitting signals, which is unfortunately not infrequently the only accepted reason for preserving a chimney. such an intervention typically takes a form of a steel gallery suspended from the chimney shaft at the required height, which is not very beneficial to the aesthetics of the chimney. however, both european and a few czech examples show that such a device can be made less conspicuous without affecting the aesthetic appearance of the chimney, even if the cabling is hidden inside the chimney. among other things, advertising can also be placed on the chimney, which, depending on the creativity of the authors, may take a different form than a simple sign (figure 2). 4.4. creative new use – reversible intervention other forms of chimney reviving is the creative use of the chimney’s interior space, either for various activities or again for artistic intervention. this includes simply making the interior of the chimney accessible, for example as part of a guided tour or the installation of a periscope, as well as use for sporting activities such as abseiling. in addition, the accessible space inside the chimney can be occupied by an art installation, physical or light. a specific creative event, limited in time but repeatable, is the smoking of the 530 vol. 38/2022 adaptive reuse of factory chimneys – industrial heritage symbols . . . chimney. chimneys can also be used outside their mass in a similarly creative way. art installations, physical or light, that are suspended from the chimney structure are one example. the base climbing on the steel ladders with which chimneys are often equipped (including belay, of course). similarly, the chimney body can become part of the structural system of related architectural interventions, such as lightweight roof structures, staircases, footbridges, etc. an interesting example could be seen and/or experienced in szczecin (pl), were the 250 m high chimney became a bungee jumping facility. last but not least, there can be an artistic shift in form of colorful paint or graffiti on the surface of the chimney body. 4.5. intervention to the incomplete chimney body the new use may also involve a chimney in a significantly altered, typically partly demolished chimney body. this can take the form of an artistic intervention or an artistic modification, for example. alternatively, the structure could be rebuilt – replicated – and presented in its original state. a more significant artistic shift is the addition of a new construction, visually different from the original material, to the original contour of the chimney, e.g. hamburg hafencity information center or nedaclo distillery chimney in bergen op zoop. 4.6. adaptive reuse – irreversible intervention as the level of intervention in the chimney structure increases, we are reaching a new use of chimneys that can be described as adaptive reuse. a logical variant of adaptive reuse is the conversion of a chimney into a look-out tower, which can take a number of architectural forms. depending on the characteristics of the particular chimney, an access can be fitted to the outer casing of the shaft or to the interior in case of a sufficient dimension; similarly, the design of the viewing platform depends on the parameters (especially the diameter) of the chimney. sufficient spatial parameters of the interior of the chimney may also allow its active use. as already mentioned above, the specific characteristics of the chimney require a special and individual approach when looking for a new use, and thus conventional adaptive reuse solutions cannot be applied. ensuring the accessibility of the interior can also be problematic. the use of flues, flue pipes and inspection openings is a natural option, but these do not always have a sufficient profile. in addition, depending on the original technological design, they are most often located below the surrounding ground level. fortunately and surprisingly, the design of most chimneys allows rather dramatic interventions – new openings – in the chimney shaft. access to the interior of the chimney at only one (ground) level is relatively simple. if the entire chimfigure 3. collage of architecture (ensa), paris – val de seine (f), the staircase inside the chimney body. ney space is to be used “at height”, the situation is complicated by the need for vertical communication – staircase, ladders or lifts. due to the compatibility between the spatial characteristics of chimneys and the spatial requirements of vertical communications, this is a so-called naturally compatible use (figure 3). however, if the interior is to be actively used and does not have sufficient dimensions, it is necessary to extend the communication (staircase, lift) to the exterior. there are also few examples of various adaptive reuse projects, rather marginal both in czech republic and in europe, such as atypical gallery, office, warehouse or an air raid observation tower that is quite exceptional. 4.7. adaptive reuse – additive intervention of course, the adaptability of factory chimneys can be increased by extending the spatial structure – by adding new volumes, for example in the form of an extension to the base of the chimney or by hanging new volumes on the chimney body. 4.8. site and context intervention specific examples are represented with projects where the chimney is reused as part of an adaptive reuse of a building or a site. one possibility is that the chimney was a physical part of the building and is either retained as is or given a new function as part 531 jana hořická, jan pustějovský acta polytechnica ctu proceedings of the adaptive reuse project. in this way, it is integrated into the usable spatial structure of the related buildings – thus its interior can be accessed from adjacent spaces or, conversely, it can serve as a circulation for related spaces. another possibility is when the space under the chimney, originally an exterior space, becomes part of the interior of the new building as part of the adaptive reuse of the site, and the chimney is physically connected to it (figure 4). slightly outside the new use categories is the restoration of the demolished chimney, but this should not go unmentioned. it is not quite common in the czech republic; more examples of such projects can be found in european context. the restoration of a chimney can take various forms, from the most descriptive replacement with a replica, to a simple relocation of the chimney from another site, to a new, equally strong construction or installation, to an artistic reminder of the existence of the chimney in a less physically determined form, such as water, greenery, light. 5. conclusions the results of the research are by no means surprising. although chimneys can be classed as attractive objects of industrial heritage, primarily because of their proportions, they were long the subject of interest to more amateur enthusiasts, unlike the classic typologies – textile mills, breweries, lime kilns, railway stations, etc. in terms of urban development, they are landmarks, aesthetically attractive and inspiring elements of the urban landscape and symbols, often preserved as the only reminders of the industrial history of the locality. their preservation has a symbolic significance that is often considered more important than their historical, architectural or structural value. motivations for preservation or even legal protection are, at least in the czech republic, often quite banal, e.g. a stork’s nest on the top or a mobile communication antenna suspended on the chimney body. for many of the reasons mentioned above, heritage listing or other means of protection does not play a very significant role in the decision on new use, and in extreme cases it may be considered by the owners as a complication in the process of adaptive reuse. rather, the physical surroundings, but also the immaterial context of the chimney, are essential to the new use, as is a creative approach to design. the range of options for new use is wide, but the representation of each option in practice is uneven, with utilitarian and no-intervention forms predominating, and fewer examples of creative and adaptive reuses. a number of chimneys are still in a various state of decay, one could say they are suffered, and are awaiting their fate, which among other things motivated us to create the overview of possibilities of new use (above), which should serve as an inspiration not only for owners but also for all those who make decisions about chimneys. figure 4. breda shopping centre in the former brewery estates in opava (cz), the chimney has become part of the shopping mall interior. photo © martin vonka. acknowledgements the research results presented in this paper were achieved with financial support of naki ii project dg16p02b011 – documentation, registration, presentation and proposals of smokestacks conversions as an endangered group of heritage of industrial monuments in the czech republic. references [1] e. hlaváček. architektura pohybu a proměn: minulost a přítomnost průmyslové architektury. odeon, praha, 1985. 167 p. [2] b. fragner. adaptive re-use. in j. douet (ed.), industrial heritage re-tooled: the ticcih guide to industrial heritage conservation, pp. 110–117. routledge, taylor & francis group, 2016. [3] m. baum, k. christiaanse (eds.). city as loft: adaptive reuse as a resource for sustainable urban development. gta verlag, 2012. [4] j. hořická, j. pustějovský. nové využití továrních komínů. in tovární komíny. nové využití ikon průmyslového věku, pp. 52–70. ctu prague. isbn 978-80-01-06682-9. [5] m. vonka. tovární komíny: funkce, konstrukce, architektura. ctu prague, 2014. 224 p. isbn 978-80-01-05566-3. 532 acta polytechnica ctu proceedings 38:527–532, 2022 1 industrial heritage and sustainable urban regeneration 2 factory chimneys in wider context 2.1 learning from industrial heritage 2.1.1 adaptability 2.2 factory chimneys in urban structure 3 initial conditions analysis 3.1 locality 3.2 context and relations 3.3 site and surroundings 3.4 historical and architectural values 3.5 chimney parameters 4 new use possibilities 4.1 no intervention 4.2 utilitarian use – necessary intervention 4.3 utilitarian use – reversible intervention 4.4 creative new use – reversible intervention 4.5 intervention to the incomplete chimney body 4.6 adaptive reuse – irreversible intervention 4.7 adaptive reuse – additive intervention 4.8 site and context intervention 5 conclusions acknowledgements references https://doi.org/10.14311/app.2022.33.0489 acta polytechnica ctu proceedings 33:489–496, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague prestressed ultra-high performance concrete (uhpc) beams for reusable structural systems: design and testing dario redaellia,∗, jonathan moixa, alex muresana,b, jan brüttingb, corentin fivetb a institute of construction and environmental technologies, university of applied sciences western switzerland (hes-so), fribourg, switzerland b structural xploration lab, swiss federal institute of technology (epfl), fribourg, switzerland ∗ corresponding author: dario.redaelli@hefr.ch abstract. with the aim to reduce the environmental footprint of buildings, this paper presents an original structural system concept for two-way slabs in residential and office buildings. the proposed system extends the best practice in terms of modularity, versatility, demountability, reusability, and durability. a finite set of elements can be used to form the main load-bearing system of multiple successively constructed buildings having different and unpredicted layouts and static systems. ultra-high performance concrete (uhpc) was identified as one of the most promising materials for this application because of its high strength, extreme durability and the opportunities it opens for shape optimization and material consumption reduction. in the first part of this contribution, the main features of the new structural system and preliminary design assumptions for uhpc modules are briefly outlined. then, the authors present and discuss the results of an experimental campaign on prestressed uhpc beams that were tested in bending and shear, under service and ultimate load conditions. beams with transversal openings in the web were also tested to assess the influence of these openings on the cracking behavior and shear strength of the beams. experimental results provide useful information for the subsequent modeling of the structural system. thanks to the presence of fibers, transversal openings in the web only have a limited influence on the response of the beams, thus allowing horizontal technical shafts to pass through the structural thickness of the slab. keywords: building, reuse, slab, uhpfrc. 1. introduction today, the global resource consumption is 1.7 times higher than what the planet can sustain [1]. the construction industry alone is responsible for approximatively one third of material usage [2], energy consumption [3] and waste production [4] worldwide, with load-bearing systems (e.g. slabs) contributing the major share of these impacts. existing strategies to face these issues are mainly based on the use of construction materials with a low environmental impact (e.g. natural, locally available, or recycled materials) or on the design of material-efficient systems (e.g. weight minimization). these approaches, however, do not provide a long-term solution since they are not addressing in a decisive way the building’s end-of-life: costs and energy required to demolish, transport and transform materials to be recycled, and the inevitable degradation (down-cycling) of their properties. an alternative strategy to reduce environmental impacts [5, 6] consists in conceiving building’s components in order to allow their reuse with minimal interventions in successive buildings and for multiple building lifetimes. this strategy, even though not yet widely established from a technological point of view, has the potential to radically transform the construction industry towards a more sustainable circular economy. 2. a reusable structural system for two-way slabs the fundamental requirements that a building system must fulfil in order to ensure open-ended reusability are: durability, versatility, modularity, reversibility and adaptability [5]. a new type of structural system for two-way slabs, which complies with all the above requirements, has been conceived within the framework of a collaborative research project between the swiss federal institute of technology (epfl) and the university of applied sciences western switzerland (hes-so). the system (figure 1.a) consists of quadrilateral slab modules that can be interconnected in a plane to create slabs with different ground plans. each module (figure 1, right) is conceived as a grid of orthogonal and regularly spaced beams, creating an array of transversal openings that define possible locations both for columns (with the addition of a head capital 489 https://doi.org/10.14311/app.2022.33.0489 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en d. redaelli, j. moix, a. muresan et al. acta polytechnica ctu proceedings figure 1. schematic representation of the structural system (a) and of individual modules realized with gluelaminated timber (b) or uhpc (c). shear strength bending strength stiffness weight vrd [kn] mrd [knm] ei [knm2] [kg/m] 1 layer 11.5 8.8 719 8.7 2 layers 23.1 34.0 5750 17.3 table 1. geometric and mechanical properties of glue-laminated timber beams. figure 2. glue-laminated timber beams. to support the slab) and for vertical technical shafts. slab modules can also be stacked vertically to locally increase the static height of the system, thus allowing to tailor the strength and stiffness distribution of the slab to meet specific static requirements [7]. extremely versatile, the proposed structural system allows for multiple re-adaptations in successive buildings with unpredicted uses, geometries and support positions. for practical applications, the system was designed to resist vertical loads of residential and office buildings with spans varying between 4 and 12 meters (with multiple layers). a first implementation is presented here as a reference and was designed in gl32h glue-laminated timber. bolted steel plates fixed to the ends of the beams with epoxy glued steel rods were chosen as connections between modules (figure 1.b). the geometric and mechanical properties of timber beams are given in table 1 for one-layer and two-layer configurations (figure 2). each beam is made of eight 80 × 30 mm laths. due to the crossing of orthogonal beams, however, only half of the laths are continuous in each direction ((figure 2, left), which limits the structural performance of this solution. shear strength, bending strength and bending stiffness given in table 1 were calculated according to swiss standards [8] by assuming that only continuous laths contribute, which was confirmed by laboratory experimental tests. despite the lightness, the positive environmental properties and the good mechanical strength of timber, stronger and stiffer alternative solutions were searched. ultra-high performance fiber-reinforced concrete (uhpc) was identified as a promising alternative due to its very high strength, extreme durability and the opportunities it opens for shape optimization and material consumption reduction. the rest of the paper focuses on design, testing and preliminary modeling of the uhpc implementation. 490 vol. 33/2022 prestressed uhpc beams vr mr ei weight [kn] mrd [knm] [knm2] [kg/m] uls design values 46.5 36.2 6105 41.8expected average values 69.7 46.3 required average values 34.7 43.5 table 2. geometric and mechanical properties of uhpc beams. figure 3. uhpc beams. 3. conceptual and detailed design of the uhpc implementation with respect to timber, uhpc is heavier, more expensive and its production requires more energy and non-renewable materials. to provide a competitive solution with respect to timber, the following criteria were set as basis of uhpc modules design: • provide with a single uhpc layer (24 cm height) strength and stiffness equal or higher than two layers of glue-laminated timber (48 cm, see table 1) • optimize the geometry of the cross-section to reduce self-weight and material consumption • explore the potential to create transversal openings in the webs for horizontal technical shafts, which is not possible for glue-laminated timber beams detailed design of the beams was carried out according to swiss standard sia 2052 [9] assuming the mechanical properties of a relatively common tensilehardening uhpc mixture (class ua, [9]): compressive strength fuck = 150 n/mm2, elastic limit in tension futek = 7.5 n/mm2, maximal tensile strength provided by the fibres futuk = 9.5 n/mm2 and modulus of elasticity eu = 55′000 n/mm2. table 2 gives the calculated uls design values of shear and bending strength, as well as the corresponding average values (that might be expected in case of testing). also given are the estimated average values of shear and bending strength required in case of testing so that the corresponding design values (uls) are at least equal to those of two layers of timber (see table 1 for comparison). uhpc beams are pre-tensioned with 12 mm diameter y1570c pre-stressing wires directly fixed to steel plates at the extremities of the beams (figure 1.c). since the position of a specific slab module within the structural systems is a priori unknown, each module must be able to resist both positive and negative moments. consequently, symmetrically prestress was designed, with one prestressing wire per flange. flanges were sized to resist the compressive force generated by bending moments at ultimate, while their shape was chosen to allow placing longitudinal and transversal prestressing wires with a minimal cover according to [9]. a web thickness of 30 mm (figure 3) was chosen to facilitate the casting of uhpc and to obtain a design value of the shear strength (vrd = 46.5 kn, table 2) significantly higher than the target design value corresponding to two layers of glue-laminated timber beams (vrd = 23.1 kn, table 1). thanks to this shear strength surplus, the creation of transversal openings in the web of uhpc beams can be explored. despite the presence of few experimental and modeling approaches in the literature [10–13], no recognized design approach exist to the authors’ knowledge for the shear strength of uhpc beams with transversal web openings. 4. experimental campaign on pre-stressed uhpc beams with or without web openings to acquire a better knowledge on the influence of web openings on the behavior of the studied system and to provide basic information for further analytical and numerical modeling, an experimental campaign on prestressed uhpc beams with and without web openings was carried out. 4.1. description of the test series four beams were fabricated for testing (figure 2). beams s1 and b1 have no web openings, while beams s2 and s3 have openings with different geometries. beams s1, s2 and s3 were designed to fail in shear and were tested in three-point bending with 1.5 m span. these beams were fabricated with a length of 1.5 times the test span, two test regions in the outer thirds and a strong region in the central part. by changing the position of load and supports (black and red configurations, figure 2), it was possible to carry out two tests per beam, named "side a" and "side b" hereafter. to avoid any risk of bending failure, the 491 d. redaelli, j. moix, a. muresan et al. acta polytechnica ctu proceedings figure 4. geometry of the four tested beams. beam / batch compressive strength fucm* [n/mm2] elastic modulus eucm* [n/mm2] elastic limit in tension futem** [n/mm2] tensile strength futum** [n/mm2] hardening strain in tension εutum** [!] b1 177.2 (3) 60’100 (3) 8.25 (6) 8.90 (4) 1.87 (4) s1 167.2 (3) 55’300 (3) 6.97 (3) 9.82 (2) 5.21 (2) s2 169.1 (3) 56’567 (3) 6.17 (6) 13.53 (6) 3.08 (6) s3 177.1 (3) 58’900 (3) 6.48 (6) 10.83 (6) 2.30 (6) * measured on 70 × 140 mm cylinders. ** calculated by inverse analysis of bending tests on 30 mm thick plates according to [9]. note: (2), (3), (4), (6) indicate the number of tests used to calculate the average value. table 3. measures mechanical properties for uhpc at 28 days central part of these beams was reinforced with an additional " 20 b500b steel rebar. beam b1 was designed to fail in bending and was tested in four-point bending with 3.0 m span. each beam was fabricated with a separate uhpc batch. prestress forces of 115 kn per wire were freed three days after casting. instantaneous and delayed shortening of the beams were monitored until the day of testing for a precise evaluation of prestress losses. for "s" elements, longitudinal strains on "sides b" were monitored while testing "sides a": strains very close to zero were detected, showing that the prestressing level on "sides b" was not influenced by cracking and failure that occurred on sides "a". tests on "side a" were stopped relatively early after the peak force, while tests on "side b" were pursued much further in the post-peak to verify the ductility of the beams. 4.2. materials the commercial product ductal fm was used as uhpc mixture for the test series, with selfcompacting consistency and 270 kg/m3 (3.4% in vol.) high-strength straight steel fibers (length lf = 14 mm, diameter df = 0.2 mm). table 3 summarizes the average 28 days mechanical properties for each batch. measured values are comparable but slightly higher than those assumed for preliminary design. however, relatively low values of the elastic limit in tension, fute, were measured for some specimen. this could be partly explained by the surface grinding procedure which was applied to 30 mm thick plates according to [9], which may have damaged these specimens. 4.3. test results all load tests were carried out at an age of uhpc comprised between 28 and 34 days. table 4 summa492 vol. 33/2022 prestressed uhpc beams beam/ batch web opening maximal measured force numerically predicted max. force maximal measured shear maximal measured moment expected strength (see tab. 2) required strength (see tab. 2) shear/moment at 1st detectable* crack b1 no 86.4 kn 43.2 knm 46.3 knm 43.5 knm 39.0 knm s1-a no 146.3 kn 158.8 kn 73.2 kn 69.7 kn 34.7 kn 54.9 kn s1-b 174.3 kn 87.2 kn 69.7 kn 34.7 kn 75.2 kn s2-a "96 149.6 kn 151.0 kn 74.8 kn 34.7 kn 35.0 kn s2-b 151.3 kn 75.7 kn 34.7 kn 44.9 kn s3-a 96 × 150 139.7 kn 127.8 kn 69.9 kn 34.7 kn 40.1 kn s3-b 135.4 kn 67.7 kn 34.7 kn 45.2 kn * cracks detected either by visual inspection or by digital image correlation. table 4. summary of test results. figure 5. measured force-displacement curve and crack pattern at failure for beam b1. rizes the maximal applied force and the corresponding maximal shear force and bending moment for each test. these values are compared to expected and required average strength values according to preliminary design (see table 2) and to strength predictions obtained with non-linear numerical modeling (see section 5). figures 3, 5, 6 and 7 present force versus mid-span displacement curves (at two different displacements scales) and pictures of the crack patterns at failure for each beam. based on observations and measured values data, the main experimental findings are summarized: • all beams showed the expected failure mode • for all beams, including beams with web openings, the measured strength is equal or higher than the required values (see tables 1 and 2). for beams "s", the location of the critical shear crack was strongly influenced by the position where the additional fă20 rebar ends • test results of sides "a" and "b" are practically identical for beams s2 and s3, but show aăsignificant difference for beam s1 (figure 5). this difference could be explained by the accidental presence of a failure plane with particularly poor number or orientation of fibers on side "a". more detailed analyses are however required to explain this behavior • by taking as a reference the average strength of beams s1-a and s1-b, the reduction of shear strength due to presence of transversal web openings is in any case less than 20% • for all beams, ultimate behavior is extremely ductile, with the progressive formation of a collapse mechanism showing very significant postpeak residual strength • cracks were only detected (tableă4, note 1) at relatively high load levels, indicating that uhpc beams will be uncracked or micro-cracked in service. 5. non-linear finite element modeling to gain a deeper understanding on test results and to carry out further parametric analyses, the structural behavior of type "s" beams was modeled with a non-linear finite element software [14]. the software solution implements a coupled tensile fracture, compressive plasticity model and allows to define strain hardening/strain softening constitutive laws within a smeared crack modeling approach. for the sake of simplicity, a unique strain-hardening tensile law was used for all the beams, with fute, futm and εutu calculated as the average of all the bending tests carried out for batches s1, s2, s3 and b1 (figure 4, black). for the softening part, a stress-crack opening softening law according to [15] was assumed, with zero stress for a crack opening equal to lf /2. to consider 493 d. redaelli, j. moix, a. muresan et al. acta polytechnica ctu proceedings figure 6. tensile stress-strain and stress-crack opening laws for modeling. figure 7. measured vs simulated force-displacement curve and crack pattern at failure for beam s1. 494 vol. 33/2022 prestressed uhpc beams figure 8. measured vs simulated force-displacement curve and crack pattern at failure for beam s2. figure 9. measured vs simulated force-displacement curve and crack pattern at failure for beam s3. 495 d. redaelli, j. moix, a. muresan et al. acta polytechnica ctu proceedings the less favorable alignment of fibers in beams than in thin bending plates, the post-cracking part of the tensile law was scaled on the stress axis by aăfactor 0.8 (figure 4, red). young’s modulus and compressive strength were set according to values given in table 3, while other parameters of the three-dimensional mechanical model were set according to state-of-the-art knowledge and are not detailed here. numerical solutions are compared to experimental ones in figures 5 to 7: despite some numerical instabilities in the post-peak and the need to validate the sensitivity of the numerical solution to variations in the numerous mechanical parameters, the experimental response is quite well reproduced both in terms of crack pattern and forces vs mid-span displacements. 6. conclusions and future work this paper presented the design, testing and preliminary modeling of uhpc beams that were conceived as a possible technical implementation of a new modular, reusable and extremely versatile structural system for two-way slabs of residential and office buildings. experimental and modeling results demonstrate that uhpc beams can be designed to meet the strength and serviceability requirement of the new modular structural system with the imposed limitations of height and with a relatively light crosssection. thanks to the tensile contribution of the fibers, the presence of web openings of significant size only has a limited influence on the mechanical performances of the beams, thus allowing horizontal technical shafts to pass through the structural thickness of the slab. in the next steps of the research program, the economic and environmental impact of the system will be evaluated and compared to other possible implementations of the new modular system (e.g. timber, steel or mixed-construction) and to traditional, nonreusable constructive systems for two-way slabs. the sensitivity of finite element modeling with respect to variation of the input mechanical parameters will be checked, then the model will be used as a tool for parametric analysis and for further optimization of the proposed concept. acknowledgements the authors are very grateful to the whitbybird foundation for funding j. a.’s initial work on the original paper [7]; and to the open-oxford-cambridge arts and humanities research council (ahrc grant number ah/r012709/1) doctoral training partnership for funding the continued project. funding for a. m. came from the school of engineering and 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acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.32.0001 acta polytechnica ctu proceedings 32:1–6, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague influence of new construction projects on vinohrady tunnels jiří barták czech technical university in prague, faculty of civil engineering, depatment of geotechnics, thákurova 7, 166 29 prague, czech republic correspondence: bartakj@fsv.cvut.cz abstract. after demolition of the brutalist architectural style transgas building in vinohradská street, prague, (all buildings of which were located above the historic vinohrady tunnels ii and iii), there will be built the vinohradská administration centre with the addition of a building no. 345 called "above the museum". both buildings have the foundation joint immediately above the lining of the existing tunnels. this paper describes the main phases of construction and the character and size of the deformation affecting the stone block lining of the tunnels. keywords: cracks, deformations, influence on tunnel lining, new buildings, vinohrady tunnels. 1. brief history of construction of vinohrady tunnels the vinohrady tunnel i was brought into service in 1871 as a substantial part of the franz joseph i single track rail line leading from vienna, through the lower austrian city of gmünd, to prague. the 1146m long tunnel was built over a period of two years (1869 – 1871) using a classical austrian tunnelling system, and in difficult conditions under the relatively dense development of the then vinohrady district. it did not proceed without damaging a number of buildings and structures. over the next twenty years, a number of defects (leaks, cracks) appeared on the tunnel lining. for that reason the reconstruction of the tunnel was already under consideration by about 1900. the reconstruction did not actually take place until 1945-1948, when sandstone blocks in the lining vault were replaced by granite keystones and the external waterproofing of the vault was repaired. until the opening of tunnel ii in 1944, the southheading rail line was led through tunnel i – via vršovice, tábor and gmünd to vienna. at present, the rail line leading west in the direction of smíchov and plzeň passes through tunnel i. crossing the state border with germany is possible at several places. the vinohrady tunnel ii was built in 1940-1944 concurrently with tunnel i, again using a classical austrian tunnelling system, with a length of 1149m. the upper vault of tunnel ii was carried out using trapezoidal high-quality granite blocks 1.0 to 1.3m thick; the side walls were made of unreinforced concrete [1]. a 6m wide and 290m long intermediate abutment wall was built during the course of the tunnel ii construction between tunnel ii and a tunnel iii, reaching approximately up to anglická street. the abutment wall has a 2m wide corridor in the middle, which is connected with tunnel ii and tunnel iii by transverse galleries, also 2m wide (see figure 1). tunnel iii was directed according to this design subsequently. figure 1. vinohrady tunnels ([1] – modified) at present the line from prague heading south to české budějovice runs through tunnel ii, with a turnoff in veselí nad lužnicí, and heads in the same historic direction of české velenice, gmünd and vienna. the vinohrady tunnel iii was built to the above-mentioned length of 290m in the period from 1940 to 1944. the handling tracks were placed into the incomplete double-track tunnel; the tunnel was additionally used as a wine cellar. the construction continued later, in 1983-1989, in connection with the expansion of the main railway station and the construction of the prague-south rail storage yard. the original double-track tunnel was followed up with 722m and 794m long tunnels respectively, beginning in a bifurcation chamber, and carried out in a 30m deep shaft in anglická street. the single-track tunnels were driven using the ring method, which was applied at that time even to the construction of the prague metro. at present, the line heading south from prague to české budějovice, or in the historic direction of české velenice, gmünd and vienna, is also led through the single-track tunnels of the vinohrady tunnel iii. 1 https://doi.org/10.14311/app.2022.32.0001 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en jiří barták acta polytechnica ctu proceedings 2. buildings in the area of low tunnel overburden 2.1. characteristic of old buildings old buildings along vinohradská avenue (1922-1939 fochova avenue, 1940-1945 schwerinova avenue, 19481969 stalinova avenue.) above the future tunnels ii and ii were demolished by the ministry of posts and telegraphs in 1939-1940, with the exception of building no. 343, "above the museum", under which the construction of tunnel ii was in preparation. the building was underpinned classically before the tunnel ii excavation, that is by reinforced concrete beams supporting the bearing walls of the structure. shaft pillars forming the supports of the underpinning beams transferred loads induced by the building to the formation level of tunnels ii and ii (see figure 2). figure 2. diagram of underpinning the existing above the museum building no. 343 [1]. four other houses on the corner of římská and rubešova streets were demolished in 1965, so an extensive gap was created next to the cs radio building, located also above tunnel iii. in connection with the construction of the transit gas pipeline from the then ussr to the countries of central europe, this vacancy was used for the construction of a complex of transgas buildings. the construction took place in 1972-1978. it was one of several other important buildings of the so-called brutalist architectural style. the most important part of the complex was the relatively low building of the transit gas pipeline control centre, which faced vinohradská avenue, and two large tower office buildings were built in the rear part of the vacant space (see figure 3). deliberations on the demolition of the entire transgas complex and the creation of a modern administration centre started in 2014. as the ministry of culture rejected the inclusion of this important category "a" building given status as a cultural monument, a new plan was instigated for implementation and, on 15 february 2019, the demolition began. regarding the superstructure, it was practically completed in the spring of 2020. figure 3. a view of the transgas complex (www. google.com – modified). 2.2. characteristics of new buildings the vinohrady administration centre (hereinafter referred to as vac – see figure 4) was built in the vacated space and a relatively narrow building will be added to the existing building no. 343 "nad muzeem" ("above the museum") by its private owner (hereinafter referred to as aam – see figure 7). figure 4. vinohrady administration centre complex ([2] – modified). 2.3. the nature of the influence on tunnels ii and iii by the completed demolition and the new buildings a detailed condition survey of the lining of tunnels ii and iii was carried out prior to the commencement of demolition work on the transgas complex. no damage to the tunnels related to their previous loading was documented. relieving the loads acting on the lining of tunnels during demolition of buildings and excavation of construction pits, and these will also have an asymmetric character, will cause unusual deformations and the formation of cracks in the lining. these undesirable effects had to be analysed in detail by advanced structural analysis. 2 www.google.com www.google.com vol. 32/2022 influence of new construction projects on vinohrady tunnels the construction of new vac and aam buildings will logically have a significantly positive effect on the deformations originating during the demolition and excavation of the construction pits – as the deformations are significantly reduced and the cracks close. this condition was also analysed in detail by structural analysis. during both fundamental phases of the current construction – the demolition of transgas buildings, excavation of construction pits and construction of new vac and aam buildings – it is necessary to conduct geotechnical monitoring according to the approved design [3]. geotechnical monitoring, in addition to checking the results of static assessments, has great safety significance, as it allows for monitoring of real deformations of the operated tunnels. the administrator of the tunnels and all participants in the construction will undoubtedly have a special interest in assessing this integral part of the construction, as it is being carried out in close proximity to the tunnel structures, i.e. in the protection zone of the railway line. 3. assessment of the influence of demolition and buildings on tunnels ii and iii 3.1. engineering geological conditions the engineering geological conditions in the area of the future construction of the new vac and aam buildings and the relevant section of the vinohrady tunnels were described in detail on the basis of archival documents, nine new cored boreholes and the completed analyses in the final report of the eg survey from 2018 [4]. the vinohrady tunnels are located in a rock environment throughout the whole area of interest. the rock environment was assessed by eg survey [4] in terms of faulting on the basis of the gsi index and all strength-related and deformational parameters of the rock types and superficial deposits encountered all required for a structural analysis of the vinohrady tunnels to be affected by the future buildings were thus determined. the vinohrady tunnels were driven through the prague ordovician libeň formation. both lithological facies of this formation, i.e. moderately to heavily weathered silty-clayey shale, strength classes r4 (515mpa) and r5 (15-50mpa) and weathered to fresh quartzite, strength class r2 (50-150mpa), are evident. a substantial part of the rock environment in the area of interest of the new buildings is formed by a quartzite layer (see figure 5) dipping at 45°toward sse; shale is found in the peripheral parts of the construction site. five geological profiles are processed in the exploratory report [5], of which the following one is presented here. this was essential for the assessment of tunnels ii and iii due to the asymmetric load and the formation level of the new buildings located in close proximity to the external surface of the tunnel lining. this geological profile 1-1’1(see figure 6) is found under the facades of buildings adjacent to vinohrady avenue. figure 5. distribution of rock types encountered in the area of interest ([4] – modified). figure 6. geological profile 1 – 1’ ([4] – modified). 3.2. static assessment of the influence of new buildings on tunnels ii and iii the static assessment was carried out repeatedly by various agencies [2, 5–7]. the following text describes the calculation [2], which was carried out by the finite element method using the program sofistik 2d (planar solution), which models the lining of tunnels in interaction with the surrounding environment 1the foundation base level of the new buildings was additionally changed in the designs so that incursion of the bottom of the construction pits to the excavation of tunnels ii and iii was prevented. the minimum distance from the external surface of the upper vault of tunnels ii and iii was set at 0.75m taking into consideration the magnitude of the overcut required for the external waterproofing. 3 jiří barták acta polytechnica ctu proceedings and identifies the places of possible cracking. the behaviour of soils was simulated using the hardening-soil non-linear elastic-plastic model. it takes into account the increase in soil stiffness after unloading (e.g. the condition after the excavation of the construction pit). the drucker-prager non-linear constitutive model was applied to the simulation of the behaviour of the tunnel lining. the model parameters were derived from the compressive strength of the homogenised stone masonry plus mortar material by means of conversion relations. the calculations were carried out in exceptional detail for four chosen sections i’, i” , ii and iii’ (see figure 7) selected to cover the construction progress and changes in the loads inducing asymmetric stresses on the tunnels. the conditions in the remaining sections i, ii’ and iii are sufficiently represented by the calculations for the selected sections. figure 7. cross-sections through the new vac and aam buildings ([2] – modified). the calculations were divided into six construction phases and four sub-phases, namely the primary state of stress (phase 100), construction of the tunnels (phases 200 and 300), current condition (phase 400 – surcharging of tunnels by existing buildings), construction pit (phase 500 – demolition of transgas buildings, excavation of soil from the construction pit, sub-phase 510 – characteristic loading; sub-phase 520 – design loading), new buildings (phase 600 – loading induced by the vac and aam buildings, sub-phase 610 – characteristic loading, sub-phase 620 – design loading). phases 400 to 600 with the respective sub-phases are relevant for assessing the influence of existing buildings and new buildings on the vinohrady tunnels. the results of the calculations are graphically displayed in [2] for the calculation phases 400 to 600 and for all monitored sections i”, i’, ii, iii’ in the form of principal stresses in the lining, radial loads acting on the lining, contact stress between the lining and rock, places of formation of tensile cracks in the lining – in all cases for both the characteristic and design loads. the deformations of the lining are documented only for standard loading. the following text only presents the assessment of construction phases 500 and 600 (demolition, construction pit, new buildings) in section iii’, which were the most important for the assessment of tunnels ii and iii, with respect to the effect of asymmetric loading acting on the tunnel iii and the fact that the foundation base of the new buildings is in close proximity to the external surface of the lining of tunnel iii. phase 500: the diagram for the assessment in phase 500 demolition and construction pit in section iii’ is presented in figure 8. the excavation of the construction pit is considered only for the vac building. given the unfavourable asymmetric loading by the unexcavated construction pit for the aam building, the calculation is made on the safe side. in phase 500, a more significant asymmetric stressing of the upper vault of tunnel iii with the maximum deformation of 4.1mm occurs in the left part of the vault, whilst stressing of the tunnel ii is moderately asymmetric with a maximum deformation of 0.6mm (see figure 9). tensile stressing of tunnel iii (figure 10) occurred on the external surface of the left part of the upper vault, and on the internal surface of the upper vault in its middle and in the right part. the width of cracks is around 0.1mm. tensile damage occurs even on the internal surface of the left side wall. this, as well as the tensile damage in the footings of the side walls, can be considered of little significance in terms of the bearing capacity of the whole lining. regarding tunnel ii, small tensile stressing is obvious in the middle of the internal surface of the upper vault and on the left external surface of the vault. the width of cracks is under 0.05mm. even here, the damage to the footings of side walls can be assessed as insignificant. figure 9. deformation of tunnels ii and iii in phase 500 ([2] – modified). 4 vol. 32/2022 influence of new construction projects on vinohrady tunnels figure 8. construction phase 500 demolition and construction pit in section iii’ [2]. figure 10. tensile damage to tunnels ii and iii by fissures in phase 500 ([2] – modified). phase 600: the chart for assessing the new buildings in section iii’ in phase 600 for concurrent action of the vac and aam buildings is presented in figure 11. results of calculations in phase 600, vac and aam buildings, showed that after surcharging by both new buildings, the asymmetric deformations are eliminated in the negatively affected areas of the vault cross-sections and the tensile stress changes to compressive stress, which then leads to the closing of cracks in the upper vaults of tunnels ii and iii and also in the side walls and their footings. figure 11. new construction in section iii’ – phase 600 the vac and aam buildings [2]. acknowledgements the demolition of the four transgas buildings was carried out above the vinohrady tunnels ii and iii in 2019, and without interrupting the railway services. the construction pits will be excavated and the construction of the new buildings of the vinohrady administrative centre and the above the museum addition will be also excavated with operations still running. the next phase of construction did not take place during 2020 or in the first quarter of 2021. the completed static calculations [2, 5–7] confirmed that the unfavourable effects of the demolition and excavation of construction pits are almost completely eliminated by loads induced by the new buildings, so that the static function and safety of tunnel linings are still maintained. the audit of static calculations [8], ordered by the administrator of the tunnels, confirmed the results of the calculations with a requirement for modification of the design so that the foundation base of the new buildings is at least 0.75m above the top of the external surface of the tunnel linings, where the overcut for the external asphalt waterproofing amounts to 40 to 50cm. references [1] j. sedláček. expert description and assessment "construction of the vinohrady tunnels ii and iii in 1940/1945". prepared for the administration of the prague railway junction, 02/1980. [2] vin consult s.r.o. vinohradská administration centre, vinohradská avenue no. 325/8, prague 2. assessment of influence of the new building on the structure of existing tunnels of czech railways, part of statis assessment. prepared by vin constult s.r.o., 04/2019. [3] fg consult s.r.o. vinohrady administration centre. stabilisation of construction pit, cross-section part ii. prepared by fg consult, s.r.o., 04/2019. [4] sg geotechnika a.s. final report from the engineering geological survey for the vinohradská administration centre in prague 2. prepared by sg geotechnika a.s., 02/2018. [5] červenka consulting s.r.o. vinohrady tunnels. nonlinear assessment of bearing capacity of tunnel linings. prepared by červenka consulting, s.r.o., 04/2019. [6] sudop. vinohradská administration centre, current condition survey of vinohrady tunnels. static assessment of the lining after the new construction. prepared by sudop praha, 08/2015. 5 jiří barták acta polytechnica ctu proceedings [7] amberg engineering brno, a.s. static assessment of the influence of the addition to the above the museum apartment building, vinohradská street no. 6, prague 2. prepared by amberg engineering brno, a.s., 06/2018. [8] j. barták. checking on static assessments and professional audit of the inluence of the construction of the administration centre and the addition of the above the museum building on the vinohrady tunnels. prepared by prof. ing. jiří barták, drsc., 09-11/2019. 6 acta polytechnica ctu proceedings 32:1–6, 2022 1 brief history of construction of vinohrady tunnels 2 buildings in the area of low tunnel overburden 2.1 characteristic of old buildings 2.2 characteristics of new buildings 2.3 the nature of the influence on tunnels ii and iii by the completed demolition and the new buildings 3 assessment of the influence of demolition and buildings on tunnels ii and iii 3.1 engineering geological conditions 3.2 static assessment of the influence of new buildings on tunnels ii and iii acknowledgements references https://doi.org/10.14311/app.2022.33.0585 acta polytechnica ctu proceedings 33:585–590, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague environmental assessment of non-metallic reinforcement for concrete structures as an alternative to steel reinforcement nadine stoiber∗, mathias hammerl, benjamin kromoser research group for resource-efficient structural engineering, university of natural resources and life sciences vienna, peter-jordan-straße 82, 1190 vienna, austria ∗ corresponding author: nadine.stoiber@boku.ac.at abstract. the concrete industry accounts for a significant amount of co2 emissions worldwide. one approach to counter this issue includes material reduction of structural components via the use of nonmetallic reinforcement, such as carbon, glass and basalt fibre reinforced polymers. on the one hand, non-metallic reinforcement. however, as its environmental impact has not been sufficiently investigated yet, a life cycle assessment of the production phase is presented within this paper. in a first step, the environmental impact of the sole various reinforcement components and types is compared to each other per mass, per tensile or rather yield strength as well as density unit, at which an environmental disadvantage of especially carbon-fibre reinforced polymers is apparent in most cases. in a further step, a focus is put on applying the environmental data of carbon-fibre reinforced polymers to a pedestrian bridge, which is finally compared to a conventionally reinforced concrete bridge and a steel bridge with similar boundary conditions. the latter results indicate that an adequate application of carbon-fibre reinforcement in structural components has the potential to lead to designs of less environmental impact in comparison to conventionally reinforced pendants. keywords: concrete structures, environmental impact, frp reinforcement, life cycle assessment, non-metallic reinforcement. 1. introduction: ’concrete footprint’ according to [1], in 2018 the construction industry was responsible for approximately 10% of co2 emissions worldwide. on a global level, the report further identifies concrete as most widely used building material type. what is more, it is predicted by [2] that the building material concrete will account for around 12% of worldwide greenhouse gas emissions in 2060. besides emissions regarding the production phase and respective required fuels, concrete is characterised by so-called process emissions due to its intrinsic chemical reduction from calcium carbonate to calcium oxide, which represent around two thirds of total emissions [3]. these numbers outline the urgency as well as the potential of the concrete industry in pushing attempts to decrease its emission-intensity forward. several options exist to counter the above outlined issue, at which three major approaches, according to [4], are briefly listed: reducing the amount of cement via an optimization of the concrete mixture, a reduction of concrete material used via a structural optimization of the design as well as the use of high-strength materials such as ultra-high performance concrete (uhpc) and fibre-reinforced polymers (frp) made out of basalt, glass or carbon fibres. the latter approach is pursued within this paper, at which the environmental performance is subject of investigation. firstly, the method and materials are outlined. secondly, the investigated reinforcement types are compared with each other, solely on material level. here, several matrix as well as fibre types are analysed. subsequently, the application of the environmental data of carbon fibre-reinforced polymers to pedestrian bridges as conducted in [5] is presented. 2. methodology: life cycle assessment according to en iso 14050, life cycle assessment (lca) is described as an assessment of the inputs, outputs and the potential environmental impacts of a product system throughout its life cycle. lca is divided into several phases, including life cycle inventory (lci) analysis, where the inputs and outputs of a product system are determined, as well as life impact assessment analysis (lcia), where the environmental impacts of a product system are evaluated and assessed. several characterisation models exist regarding the latter step of an lca, at which cmlia from the university leiden is mentioned within this context [6]. the underlying standardisation of conducting an lca comprises en iso 14040 (fundamentals) and en iso 14044 (instructions). en iso 14025 outlines the use of life cycle assessment to establish an envi585 https://doi.org/10.14311/app.2022.33.0585 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en j. anderson, m. hammerl, b. kromoser acta polytechnica ctu proceedings figure 1. comparison of gwp values extracted from epds with the european average of [7]. ronmental product declaration (epd). the national standard önorm en 15804 has to be highlighted in this context, which regulates the development of epds for the product category of construction products. life cycle phases of a product system range from product, construction process to the end of life stage. the product stage is compulsory to be covered when developing an epd, at which this stage is divided into three categories: raw material supply (a1), transport (a2) and manufacturing (a3). this type of lca is described as cradle-to-gate. commonly, a cradle-to-gate lca represents the most adequate type, as the inputs and outputs are often characterised by satisfying data completeness and quality. other life cycle stages, such as the construction process stage, are accompanied by comparatively more underlying assumptions. at best, all life cycle stages are covered within an epd. due to limited data availability, the production stage of the considered construction products is assessed within this paper. furthermore, several impact indicators are selected to be evaluated, such as the global warming potential (gwp) in kg co2-eq., the acidification potential (ap) in kg so2-eq. as well as the abiotic deletion potential of fossil fuels (adpf) in mj. commonly, due to public popularity, the gwp is paid the most attention to. in order to establish a wider picture of the environmental impact of the considered products, further impact indicators should be assessed. consulted sources within this paper include epds and life cycle inventory data. details about the respective sources as well as the chosen declared or rather functional units are outlined in the sections below. it is crucial to note that irregularities go hand in hand with the comparison of environmental data from varying sources. for example, the lcia phase of the life cycle assessment can be based on varying characterisation models. the authors aimed at maintaining maximum transparency within the process to enable traceability and reproducibility. 2.1. excursion: data bias to briefly outline the phenomenon of data bias and raise awareness regarding data quality within life cycle assessment, the example of steel reinforcement is consulted. extracted gwp values of steel reinforcement (cradle-to-gate lca, declared unit: 1 kg) as done by [5] are illustrated in figure 1. the first five, grey bars represent extracted gwp values from european epds, at which the last, green bar outlines the european average of steel production in 2019 provided by [7]. for further detailed information on data sources of the epds, the reader is directly referred to [5]. given the gwp values extracted from the epds in figure 1, one would assume to have an average value of around 1.0 kg co2-eq., nevertheless, the european average is higher. when analysing the manufacturing methods of the various epds it becomes apparent that, except for ukraine, all epds are built up on electric arc furnace. nevertheless, [8] shows otherwise, at which not electric arc furnace but basic oxygen steelmaking represents the major manufacturing method of steel products in europe. the average of the given data extracted from the epds is therefore not representative for the global warming potential of steel reinforcement in europe. 3. results on material level: comparison of reinforcement components and types figure 2 and figure 3 show the normalized results of the gwp of various fibre and matrix types. gwp values for carbon fibres are extracted from [9], for glass fibres from [10], for basalt fibres from [11], for styrene butadiene rubber and acrylate from [12], for polyester resin, vinylester resin and epoxy resin from [13] as well as from [14]. impact indicators, like the gwp, were directly extracted from these sources. only the last source, [14], represents lci data, which was extracted and subjected to selfevaluation according to the specifications of önorm en 15804:2020. in all cases, the production stage (cradle-to-gate) was considered. regarding figure 3 it is apparent that carbon fibres are characterised by the highest global warming potential in comparison to glass and basalt fibres. as reason the energy-intense production process up to 3,000řc is mentioned. the differences between the various matrix materials in figure 2 are not that significant, nevertheless, a row in dependence of the gwp is possible. the normalized gwp values of carbon fibre-reinforced polymers (cfrp), glass fibre-reinforced polymers (gfrp) and basalt fibre-reinforced polymers (bfrp) are illustrated in figure 4, at which epoxy resin was chosen as matrix material. the results are presented in a normalized 586 vol. 33/2022 non-metallic reinforcement for concrete structures figure 2. comparison of the gwp of different matrix materials (normalized values). figure 3. comparison of the gwp of different fibre types (normalized values). figure 4. comparison of the gwp of different frps (normalized values). manner to enhance readability. the respective components of the frps were combined according to an analysis of data sheets, at which the respective fibre share ranges from 75 to 90% and the matrix material share ranges from 10 to 25%. the gwp of carbon fibres of 26.4 kg co2-eq. per kg was extracted from [9] and is given as example. comparing figure 3 with figure 4 it can be seen that the trend of carbon being the least favourable material choice in case of environmental impact continues, nevertheless the difference between the varying fibre types becomes smaller. in this context, the study of [15] is mentioned, who conducted a cradle-to-grave lca for conventional steeland textile-reinforced (glass, carbon and basalt) façade elements. their findings indicate that all textile versions show comparatively less environmental impact. the carbon version showed a comparatively higher impact than the glass or basalt solution amongst most of the investigated impact indicators (gwp, ap and eutrophication potential). 3.1. in detail: cfrp reinforcement the environmental impact of cfrp is further investigated based on evaluations done in [5], at which the analysis is extended to impact indicators such as the acidification potential in kg so2-eq. (ap) and the abiotic depletion potential of fossil fuels in mj (adpf). furthermore, the environmental impact of cfrp reinforcement is directly compared to the one of conventional steel reinforcement (european average, compare with figure 1). the results are illustrated in figure 5 to figure 7. the properties of a cfrp rebar product from solidian gmbh are consulted [16]. in figure 5, the results are solely based on masses or rather per declared unit of 1 kg of the product system. this figure shows the environmental advantage of conventional steel reinforcement in comparison to cfrp reinforcement amongst all considered impact indicators. figure 6, which is based on masses as well as on the performance of the reinforcement type, still shows environmental benefits of steel reinforcement. nevertheless, an additional consideration of the performance, more precisely the tensile strength of 2,100 mpa of cfrp reinforcement as well as the yield strength of 550 mpa of steel reinforcement shows results more on favour of cfrp reinforcement than a sole consideration of the masses as illustrated in figure 5. figure 7 shows otherwise, at which an environmental benefit of cfrp reinforcement amongst the impact indicators gwp and adpf is visible. here, the performance (tensile and yield strength) as well as the related densities (7,850 kg/m3 for steel reinforcement, 1,500 kg/m3 for cfrp reinforcement) were considered. figure 7 already shows the potential of cfrp reinforcement as an environmentally friendly alternative to steel reinforcement on the material level. 587 j. anderson, m. hammerl, b. kromoser acta polytechnica ctu proceedings figure 5. comparison of steel and cfrp reinforcement per kg product system (normalized values). figure 6. comparison of steel and cfrp reinforcement per kg product system as well as per mpa tensile respectively yield strength (normalized values). 4. results on system level: comparison of pedestrian bridges the environmental data of cfrp were further applied to pedestrian bridges of varying building types by [5], whose results will be outlined as a subsequent step. the consulted bridges are illustrated in figure 8. the first bridge is a carbon concrete bridge in albstadt ebingen, germany and was built in 2015 (abbreviated with b1-ccb), which is characterised by a thin cfrp reinforced cross section and a galvanized steel railing. the second bridge is a conventional steel reinforced concrete bridge, built in 2009 in vienna, austria (abbreviated with b2-rcb). the last bridge, a mild steel bridge, was built in 1999 in vienna, austria (abbreviated with b3-sb). the masses for the quantity survey were extracted from [17] for b1-ccb and provided by the municipal department 29 in vienna austria in case of b2-rcb and b3-sb. further details can be found in [5]. the functional unit for the subsequent comparison is a pedestrian bridge with a span length of ∼ 15 m, an effective width of ∼ 3 m and an imposed load of ∼ 5 kn/m2. the materials of the superstructure were considered. the results of the environmental assessment of the pedestrian bridge types are illustrated in figure 9. the impact indicators gwp and adpf are clearly in favour of the cfrp reinforced concrete bridge due to figure 7. comparison of steel and cfrp reinforcement per mpa tensile respectively yield strength as well as per density unit (normalized values). significant savings in material masses when comparing b1-ccb with b2-rcb. to get a deeper understanding of the allocation of the environmental impacts, the results are furthermore divided per building material and outlined in figure 10 to figure 12. the consideration of the gwp as well as of the adpf shows that the galvanized steel railing of b1-ccb has a significant share in the whole environmental impact. regarding adpf, the reinforcement is responsible for the majority of the environmental impact. regarding ap in figure 11, it becomes apparent that the cfrp reinforcement is mainly responsible. the environmental data are characterised by high limits of variation, at which [9] gives the consultation of different reaction equations for the combustion of ammonia and hydrocyanic acid during the production process of carbon fibres as reason for these variabilities. this circumstance outlines room for higher data precision regarding the ap in the future. furthermore, the authors want to indicate, that a more comprehensive consideration of the environmental impact was initially intended, with further impact indicators such as ozone depletion potential (odp), eutrophication potential (ep) and photochemical oxidant creation (pocp) being considered. unfortunately, the respective environmental data of cfrp were either not available nor valid. updates in the data availability are constantly checked and re-evaluated by the authors. 5. conclusion the intention of this paper was to evaluate, whether fibre-reinforced polymers are a possible environmentally friendly alternative to conventional steel reinforcement in concrete structures or not. in a first step, the varying fibre and matrix material types were solely evaluated per unit weight, at which carbon fibres and cfrp reinforcement showed by far the highest global warming potential. subsequently, a more detailed environmental assessment of cfrp reinforcement on material level showed that steel reinforcement is more favourable when considering solely the declared unit of 1 kg as well as the declared unit 588 vol. 33/2022 non-metallic reinforcement for concrete structures figure 8. pedestrian bridge examples, from top to bottom: b1-ccb (solidian gmbh), b2-rcb and b3-sb (mathias hammerl). figure 9. comparison of impact indicators of various pedestrian bridge types (normalized values), extracted from [5]. figure 10. comparison of the gwp of various pedestrian bridge types (normalized values) divided by the used building materials, extracted from [5]. figure 11. comparison of the ap of various pedestrian bridge types (normalized values) divided by the used building materials, extracted from [5]. figure 12. comparison of the adpf of various pedestrian bridge types (normalized values) divided by the used building materials, extracted from [5]. 589 j. anderson, m. hammerl, b. kromoser acta polytechnica ctu proceedings in combination with the tensile or rather the yield strength of the respective material. nevertheless, the consideration of the reinforcements’ performance as well as the density of the material showed a lesser environmental impact of cfrp reinforcement in comparison with conventional steel reinforcement. in a second step, an assessment and comparison of the superstructure of a carbon concrete, a reinforced concrete and a mild steel pedestrian bridge outlined the environmental benefits of the carbon concrete bridge type. the potential of cfrp as an environmentally friendly alternative to steel reinforcement in the case of pedestrian bridges could be shown. it has to be mentioned that overall conclusions of this application example to other structures are not acceptable: an individual profound analysis is always necessary. furthermore, only the production stage of the reinforcement components and pedestrian bridge materials was considered. a more holistic consideration of further life cycle stages with an extension to other structural components is highly recommended as an optimal continuation of this study. acknowledgements the authors would like to thank dirk neuburg from the municipal department 29 in vienna and dr. sergej rempel from solidian gmbh for the provision of the original bridge documents and additional information used for the bridge examples. they would like to thank clare broadbent from the world steel association for the provision of environmental data as used within the cradle-togate lca. the authors also want to thank dr. andrea hohmann for her support in the early stage of the assessment. a part of the data collection was done within the scope of the vif project 2019 "potentiale von nichtmetallischer bewehrung im infrastruktur-betonbau". the authors want to thank the customers austrian railways (öbb), the austrian autobahn and highway financing stock corporation (asfinag), the bundesministerium für klimaschutz, umwelt, energie, mobilität, innovation und technologie (bmk) as well as the austrian research promotion agency (ffg) for the management. references [1] international energy agency and the united nations environment programme. 2018 global status report: towards a zero-emission, efficient and resilient buildings and construction sector, 2018. https: //wedocs.unep.org/handle/20.500.11822/27140. [2] oecd. global material resources outlook to 2060: economic drivers and environmental consequences (paris: oecd publishing), 2019. https: //www.oecd.org/environment/waste/highlights-glo bal-material-resources-outlook-to-2060.pdf. [3] g. mauschitz.emissionen aus anlagen der österreichischen zementindustrie, 2018. [4] b. kromoser, p. preinstorfer, j. kollegger. building lightweight structures with carbon-fiber-reinforced polymer-reinforced ultra-high-performance concrete: research approach, construction materials, and conceptual design of three building components. structural concrete 20(2):730-44, 2018. https://doi.org/10.1002/suco.201700225. [5] n. stoiber, m. hammerl, b. kromoser. cradle-togate life cycle assessment of cfrp reinforcement for concrete structures: calculation basis and exemplary application. journal of cleaner production 280, 2021. https://doi.org/10.1016/j.jclepro.2020.124300. [6] cml. department of industrial ecology, cml-ia characterisation factors, 2016. https://www.universi teitleiden.nl/en/research/research-output/scie nce/cml-ia-characterisation-factors. [7] world steel association. life cycle inventory (lci) request form, 2019. https://www.worldsteel.org/steel-by-topic/lif e-cycle-thinking/lca-lciform.html. [8] world steel association. world steel in figures 2019, brussels, 2019. [9] a. hohmann. life cycle assessment of manufacturing processes for cfrp structures to identify optimization potentials systematic methodology for estimating the environmental impact of manufacturing process chains, dissertation: technical university of munich, 2018. [10] pwc. life cycle assessment of cfgf continuous filament glass fibre products, 2016. https://www.gl assfibreeurope.eu/wp-content/uploads/2016/11/lc a-report-cfgf-products_20161031_pwc.pdf. [11] k. azrague, m.r. inman, l-i. alnaes l-i, et al. life cycle assessment as a tool for resource optimisation of continuous basalt fibre production in iceland, 2016. https://dc.engconfintl.org/cgi/viewcontent.cgi ?article=1009&context=lca_waste. [12] pwc. epdla life cycle inventory of polymer dispersions, 2015. https://specialty-chemicals.eu/ wp-content/uploads/2017/07/4_epdla-life-cycle -assessment-lca-summary-report.pdf. [13] eucia. eco impact calculator for composites, 2021. https://ecocalculator.eucia.eu/account/log in?returnurl=%2f. [14] plasticseurope. eco-profiles, 2005. https://www.plasticseurope.org/de/resources. [15] l. laiblová, j. pešta, a. kumar, et al. environmental impact of textile reinforced concrete facades compared to conventional solutions lca case study. materials 12(19), 2019. https://doi.org/10.3390/ma12193194. [16] solidian. technical data sheet solidian rebar 04-cce, 2020. https://solidian.com/wp-content/u ploads/solidian-rebar-tds.pdf. [17] t. helbig, k. unterer, c. kulas, et al. fußund radwegbrücke aus carbonbeton in albstadt-ebingen. betonund stahlbetonbau 111(10):676-85, 2016. https://doi.org/10.1002/best.201600058. 590 https://wedocs.unep.org/handle/20.500.11822/27140 https://www.oecd.org/environment/waste/highlights-global-material-resources-outlook-to-2060.pdf https://doi.org/10.1002/suco.201700225 https://doi.org/10.1016/j.jclepro.2020.124300 https://www.universiteitleiden.nl/en/research/research-output/science/cml-ia-characterisation-factors https://www.worldsteel.org/steel-by-topic/life-cycle-thinking/lca-lciform.html https://www.glassfibreeurope.eu/wp-content/uploads/2016/11/lca-report-cfgf-products_20161031_pwc.pdf https://dc.engconfintl.org/cgi/viewcontent.cgi?article=1009&context=lca_waste https://specialty-chemicals.eu/wp-content/uploads/2017/07/4_epdla-life-cycle-assessment-lca-summary-report.pdf https://ecocalculator.eucia.eu/account/login?returnurl=%2f https://www.plasticseurope.org/de/resources https://doi.org/10.3390/ma12193194 https://solidian.com/wp-content/uploads/solidian-rebar-tds.pdf https://doi.org/10.1002/best.201600058 acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.37.0010 acta polytechnica ctu proceedings 37:10–15, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague the characterization of the collimated beams of fast neutrons with the clid detection system martin ansorgea,b,∗, jan novákb, mitja majerleb, ján kozica,b a czech technical university in prague, faculty of nuclear sciences and physical engineering, v holešovičkách 747/2, 180 00 prague 8, czech republic b nuclear physics institute of the czech academy of sciences, 250 68 řež, czech republic ∗ corresponding author: ansorge@ujf.cas.cz abstract. a new detection device for the measurements of light ions (p, d, t, α) emitted as the products of the nuclear reactions induced by fast neutrons (5-33 mev) was recently developed at the nuclear physics institute of the czech academy of sciences. the main objective of the chamber-forlight-ion-detection (clid) is to produce new differential nuclear data of high interest for the material applications related to fusion and aerospace technologies and to potentially test and validate models of nuclear reactions. hereby the experimental set-up for the measurements with the clid is described in detail. the experimental characterization of the collimated fast neutron beams produced by the cyclotron-driven converter (p(35 mev)+be(2.5 mm)) is presented. in particular, the implementation of the proton-recoil-telescope technique used for neutron energy spectra determination with the clid is described. keywords: npi, collimated fast neutron beams, cyclotron u-120m, clid, proton-recoil-telescope. 1. introduction a new 1.6 m long collimator coupled to a new neutron converter (based on the p+9be reaction) driven by the isochronous cyclotron u-120m was constructed at the nuclear physics institute of the czech academy of sciences (npi-cas). collimated beams of fast neutrons are bringing new experimental possibilities, especially for on-beam measurements with semiconductor detectors which need to be shielded from intense neutron fields. the new vacuum chamber with the telescopes composed of silicon detectors allows us to perform precise measurements of double-differential cross sections of (n,cp charged particle) reactions induced by fast neutrons with kinetic energies from 5 to 33 mev. motivated by the development of future fission and fusion energy projects, the nuclear data measurements of cross sections for nuclear reactions induced by fast neutrons are of high interest. in recent years, the clid detection system was commissioned and it delivered its first experimental data. this paper describes the characterization of the collimated neutron fields available at the u-120m facility at the npi-cas. the neutron fluxes and energetic spectra were measured in the irradiation position of the clid device using the experimental set-ups with the vacuum chamber as well as with the scintillation probes. the development of the clid system follows the previous development of similar devices used at several nuclear research facilities worldwide. during past decades, the ddcs data for (n,cp) reactions induced by fast neutrons (10-200 mev) on several basic elements (c, li, o, al, si, fe, co, bi, u, etc.) were measured and published at lansce in usa [1], at cyclon in belgium [2–4], at crocker nuclear laboratory in the usa [5–7] or at svedberg laboratory in sweden where the medley chamber was constructed and used for several campaigns of ddcs measurements [8–11]. recently, the medley was moved for the new set of experimental campaigns to the ganil laboratory in france where the new linac accelerator is used for neutron production mainly in the energy region from 1 to 40 mev [12]. in 2022, new data on α-particle production double differential cross-sections on carbon were published by the chinese group from the csns facility for the energy range from 6 to 102 mev [13]. 2. general set-up for clid experiments the initial proton bunches are accelerated by the cyclotron u-120m to the kinetic energy of 34.8±0.5 mev. pulsed beams of primary protons are delivered through few meters of ion beam-line to a thin beryllium-based neutron converter where fast neutrons are produced. the neutron converter is placed very close to the 1.6 m long circular-shaped collimator (3 cm in diameter). behind the collimator, the evacuated reaction chamber is placed at an approximate distance of 2.5 m (from the neutron converter to the irradiated sample). the representative schematics of the experimental set-up is shown in figure 1. 2.1. cyclotron-driven neutron source the new neutron converter consists of 2.5 mm thick beryllium slab. the neutrons with quasi10 https://doi.org/10.14311/app.2022.37.0010 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 37/2022 clid detection system figure 1. the top-view schematics of the experimental set-up used with the reaction chamber. monoenergetic (qm) spectrum are produced by 9be(p,n) reactions. the beryllium is backed by 10 mm thick slab of graphite which is used to stop the protons. the beryllium/graphite slabs are cooled by the flow of de-mineralized water. the primary proton beam current is measured by the integration of the charge deposited by protons in the beryllium/graphite slabs. historically, the qm neutron spectra produced by beryllium targets were already studied at the npicas. a 0.5 mm thick beryllium target was bombarded by ≈30 mev protons and the neutron energy spectra were measured by the time-of-flight (tof) technique using the ne-213 scintillator [14]. two monoenergetic peaks separated by 2-3 mev were recorded on the neutron continuum. based on these results, five times thicker beryllium slab was chosen to produce only one broader peak of monoenergetic neutrons on continuous neutron background. the target station with the neutron converter is placed close to the collimator wall as shown in figure 1. a hollow steel cylinder was implemented in this wall. to shape the beam of fast neutrons, several hollow cylinders made of polyethylene and steel pieces were inserted in the inner part of the collimator cylinder. the rest of the collimator wall consists of the mixture of iron and water. 2.2. clid device inspired by the medley setup [8], a new vacuum chamber for the detection of light ions (p, d, t, 3he, α) produced in nuclear reactions induced by fast neutrons (up to 33 mev) was constructed at the npicas. the chamber allows the measurements of doubledifferential cross sections for reactions (n, cp) with respect to the angle and incident energy of the interaction. the time-of-flight method is used for the energy determination of the primary neutron and remotely controlled rotating table is used for angle differentiation. the central sample holder can be loaded with up to four different targets which can be switched during the on-beam measurement remotely. using the clid, the neutron flux and energy spectrum were measured by the proton-recoil-telescope (prt) method. the polyethylene foil (pe,ch2 250 µm thick and 2 cm diameter) mounted on the strip of 2.5 µm thin mylar foil was placed at the center of the chamber and irradiated. the elastically scattered protons were recorded using the telescopes. a carbon sample of the same diameter and half of the thickness was measured in the same position in order figure 2. the photography of the internal set-up of the clid placed behind the collimator. 11 m. ansorge, j. novák, m. majerle, j. kozic acta polytechnica ctu proceedings to subtract the background proton events from (n,xp) reactions on carbon in pe. at last, an yttrium sample was irradiated and analyzed using the activation technique. the four si-based telescopes were placed on the rotating table around the irradiated sample, see the figure 2. each telescope consisted of two planar si detectors, the thin one to record the energy loss (de) of the passing charged particle and the thick one to absorb the rest of the particle energy (e). in the case of the prt measurement with the pe foil, the telescope consisted of the 150 µm and 5 mm thick detectors. the solid angle between the pe sample and the telescope was restricted by the circularly shaped aperture with the inner diameter of 8 mm. this solid angle was calculated by numerical integration technique based on the method presented by s. pommé [15] and the resulting absolute value of 0.00255 sr was obtained. the telescope was placed at the laboratory angle of 45° with respect to the neutron beam axis. the data acquisition chain consisted of the caen preamplifiers and v1730 digitizer. the power supply modules delivering the highand low-voltage were manufactured by caen as well. 3. prt measurement of the neutron spectrum during the approximately 2-hour-long irradiation of the ch2 target at the beam current of approx. 20 µa the light charged particles were recorded by the prt at the laboratory angle of 45° and proton events were discriminated by the de-e method which is graphically represented by the 2-d plot in figure 3. the energy calibration of the si-detectors has been performed using the radionuclide α-sources with kinetic energies of α-particles up to 8 mev. figure 3. particle type separation for prt data. the background runs were measured with the carbon and empty-holder targets. the background and natc(n,p) proton events were correctly subtracted and the proton spectrum from h(n,p) elastic scattering was reconstructed, see the blue data-set in figure 4. to obtain the energy spectrum of protons produced in the pe sample, the unfolding procedure (based on roounfold package [16]) was implemented to correct the measured spectra for energy spreading and shifting figure 4. proton energy spectra measured by prt (blue) and corrected proton energy spectra (red). due to the energy loss in the pe sample and due to the variations of the recoil angle within the solid angle of the detector. the response function used for the unfolding process was generated using the montecarlo code geant4 [17] and the previously created clid model [18]. the simulated response matrix is plotted in figure 5 and represents the response of the prt set-up of the clid to the theoretical irradiation by the mono-energetic neutrons which are elastically scattered on hydrogen nuclei in pe foil. figure 5. geant4 simulated response function for the set-up with ch2 target and prt. the kinetic energy of protons is recalculated into the energy of interacting neutrons based on the welldefined kinematics of elastic scattering. the following equation is used to calculate the single neutron spectral point in each energy bin of the final neutron spectrum: nn = np ωlabσdiff habwc [n/cm2/mev/c], (1) • np – number of protons detected in the energy bin, • ωlab – solid angle (in lab system) from ch2 target to telescope, • σdiff = ∂σ ∂ωlab∂en – endf/b-vi double differential cross-section for h(n,p), • ha – number of hydrogen atoms in the ch2 sample, • bw – the width of energy bin for which the spectral point is calculated, • c – the charge of primary proton beam integrated over the period of measurement. 12 vol. 37/2022 clid detection system 4. activation measurement two 12-hours-long irradiations of the yttrium foil were performed. yttrium is commonly used as activation detector material and has a well-known (n,xn) reaction cross sections. the parameters for two relevant neutron-induced reactions are listed in table 1. the reaction rates for the production of the 88y isotope were successfully measured (with hpge), while the production of 89y(n,3n)87y could not be measured due to the long cooling times of the yttrium foil compared to the relatively short half-life period. reaction q-value [mev] threshold [mev] half life [days] 89y(n,2n)88y -11.48 11.61 106.65 89y(n,3n)87y -20.83 21.07 3.33 table 1. basic reaction parameters relevant for activation technique assessment. the overview of experimental and evaluated crosssection data for the 89y(n,2n)88y reaction plotted by janis [19] is shown in figure 6. the measured figure 6. experimental and evaluated cross-section data for 89y(n,2n)88y reaction plotted by janis [19]. reaction rates can be used to control the correctness of the absolute values in the neutron spectra measured by the prt technique as will be shown in the following section 5. 5. results the neutron energy spectrum at the irradiation position of the clid was measured by the prt technique and calculated using the eq. 1 together with the measured proton data presented in figure 4. the resulting neutron energy spectrum is plotted in figure 7. the reaction rates (number of produced nuclei per 1 g of the sample and per 1 primary particle) for the production of 88y isotope were measured by the hpge detectors. besides the activation technique described in the previous section 4, the reaction rate was also calculated by the integration of the known cross-section σ(e) (figure 6) with the prt-measured figure 7. spectral neutron flux measured by the prt method with the clid. neutron flux ϕ(e) over the energy range of the neutron energy spectrum in figure 7: rr = ∫ emax emin σ(e)ϕ(e)de, (2) where e is the neutron kinetic energy. the neutron cross-section data for σ(e) were taken from the eaf2010 [20] data library. the absolute values for the rr quantity calculated by both approaches (activation method, prt method) are listed in the table 2. the resulting normalization factor is listed as well and the disagreement between the two measured values is further explained in the conclusion of this article. rr by activation method rr for prt method normalization factor [/g/c] [/g/c] [-] 6.71e+08 4.85e+08 1.3844 table 2. the reaction rates calculated for 89y(n,2n)88y reaction and their ratio. in the figure 8 the final neutron energy spectrum measured by the prt technique and corrected by the normalization factor deduced from activation measurement with yttrium sample is shown. an independent measurement with the ne213 scintillator in the tof mode was performed to obtain the neutron spectral flux at the irradiation position of the clid. the methodology of the tof measurement is beyond the scope of this paper [21, 22]. the resulting tof neutron spectrum is shown in figure 8 for comparison. furthermore, the mcnpx [23] calculation was performed to simulate the neutron flux in the clid irradiation position. the complete geometry with the collimator was used in the mcnpx simulation, the beryllium target backed with the carbon slab and the layer of coolant in the target station was modelled. the neutron production was simulated for the p(34.8 mev)+9be(2.5 mm thick) reaction using the jendl4.0/he data library [24, 25]. comparison between experimentally measured neutron fluxes and mcnpx simulation is shown in figure 8 as well. 13 m. ansorge, j. novák, m. majerle, j. kozic acta polytechnica ctu proceedings figure 8. spectral neutron fluxes measured by prt method and normalized by 88y production rates (red), measured by the ne213 (green) and simulated by the mcnpx code using the jendl4.0/he data library (blue)[24, 25]. for all three energy spectra plotted in figure 8, the absolute values of the qm-peak neutrons measured or simulated at the clid irradiation position were integrated, see the table 3. the lower integration limits were 20 mev, 21 mev and 23 mev for data sets from ne213, prt and mcnpx respectively. both measurements (ne213, normalized prt) are in very good agreement with each other. on the other hand, the mcnpx simulation gives roughly 30 % lower values for the qm-peak integral compared to the measurements. this disagreement with the mcnpx results is probably due to the poor precision of the implemented cross-section data for the production of neutrons in the jendl4.0/he library. the jendl4.0/he has however better description of the neutron production than the other existing library endf7prot [26] and therefore was chosen for this comparison. ne213 prt mcnpx tof normalised simulation peak neut. [n/cm2/c] 1.10e+11 1.11e+11 8.067e+10 table 3. integral values of qm neutrons at the clid irradiation position. 6. conclusions this paper describes the overall experimental set-up with the new vacuum chamber. the implementation of the prt technique for neutron flux determination with the clid was described in detail. during the one weekend-long on-beam experiment with the clid, the ch2, 89y and natc targets were irradiated and the measured data were used for the characterization of the fast neutron spectrum in the collimated beam. the detailed analysis of the experimental data has shown that some events were randomly lost during the data acquisition or their energy deposition values were falsely recorded. the evidence for this effect can be found around the proton band in figure 3. this issue was already addressed and solved for future experiments. the experimental prt data presented in this paper were however compromised and consequently had to be normalised. the normalisation factor was deduced from the activation measurement of the irradiated yttrium sample. the 89y(n,2n)88y reaction has very well known cross section (figure 6) in the energy range from 11 mev to 36 mev. the correctness of the activation measurement was confirmed by the tof measurements performed with the organic scintillator ne213. the rr = 6.711e08 88y/g/c value calculated for the neutron energy spectrum (green in figure 8) measured by the ne213 matches perfectly with the rr value presented for activation in table 2. the tof spectrum agrees with the normalized prt neutron energy spectrum. the integral values for qm-peak neutron fluxes (as presented in table 3) are of the key importance for every experiment on collimated fast neutron beams produced on 2.5 mm thin beryllium target by the cyclotron u-120m at energies around 34.8 mev. the overall relative uncertainty of the measured data in table 3 is expected to be below 10 %. the accuracy of the mcnpx/jendl4.0/he predicted values is lower because of the worse implementation of the neutron production in the jendl4.0/he evaluation. the broad qm peak from the p+be reaction on 2.5 mm thin beryllium target is composed of two narrow peaks which originate from the ground and excited states of the remaining 9b nucleus [14]. the absolute values of cross section and the intensity ratio between these two inner peaks is not implemented correctly into the evaluated data libraries. the production of neutrons is however better described in jendl4.0/he than in the other existing libraries generally used with mcnpx. acknowledgements this project has received funding from the european union’s horizon 2020 research and innovation programme under grant agreement no 847552. this work was supported by op rde, meys, cz under the project canam, ef16_013/0001812. computational resources were supplied by the project "einfrastruktura cz" (e-infra cz lm2018140) supported by the ministry of education, youth and sports of the czech republic. this work was supported by the grant agency of the czech technical university in prague, grant no. sgs18/145/ohk4/2t/14. references [1] p. lisowski, c. bowman, g. russell, s. wender. the los alamos national laboratory spallation neutron sources. nuclear science and engineering 106(2):208– 218, 1990. https://doi.org/10.13182/nse90-a27471. 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https://doi.org/10.11484/jaea-conf-2016-004 acta polytechnica ctu proceedings 37:1–6, 2022 1 introduction 2 general set-up for clid experiments 2.1 cyclotron-driven neutron source 2.2 clid device 3 prt measurement of the neutron spectrum 4 activation measurement 5 results 6 conclusions acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.38.0190 acta polytechnica ctu proceedings 38:190–195, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague advanced modelling of concrete structures for improved sustainability jiří rymeš, jan červenka∗, radomír pukl červenka consulting s.r.o., na hřebenkách 55, 150 00 prague 5, czech republic ∗ corresponding author: jan.cervenka@cervenka.cz abstract. the safe and long-term serviceability of concrete structures is one of the methods how to improve the sustainability of the concrete industry. this study presents a pilot application of an integrated system for online monitoring and service life prediction of concrete bridges. the system consists of strain gauges measuring the structural response coupled with a laser rangefinder for detection of the bridge-crossing traffic. the measured data were used for the development of a computational model of the bridge. next, the deterioration models were applied to the model to assess the long-term mechanical behaviour. in this study, we considered chloride-induced reinforcement corrosion. the numerical data are given for 100-years-long service life prediction. keywords: ageing management, bridge monitoring, finite element method (fem), non-linear analysis, durability assessment. 1. introduction the concrete industry is responsible for approximately 5–7 % of the human-produced emissions of co2, mainly due to the manufacturing process of the cement clinker [1]. among replacing the cement clinker with supplementary cementitious materials [2], carbon storage technologies [3], and structural design optimization [4], the long-term service life of concrete structures is one of the means how to reduce the ecological footprint of the concrete industry. on the other hand, the ageing infrastructure may represent a risk since degradation processes may compromise the structure’s serviceability as in the case of the libeň bridge, prague or, in the worst case, the structure may collapse as in the case of the trója bridge, prague or the morandi bridge, genoa. to prevent such scenarios, regular structural health inspections and repair works need to be conducted. together with physical inspections, numerical tools may aid the ageing management of the structures. the combination of non-linear finite element analysis with structural monitoring can considerably improve the prognosis of structural behaviour, deterioration, damage, and sustainability. recently, a digital twin concept is often utilized: a digital replica of the real structure is developed, and simulation of the model behaviour under service conditions is performed. based on the comparison with data from measurements on the real structure, the most significant model properties are identified using an advanced probabilistic approach. these appropriate model properties are consequently used for assessments of safety, reliability, durability, and sustainability of the investigated structure under service as well as limit conditions. application of such a methodology allows reliable and accurate recognition of the structural damage and prediction of the remaining lifespan and structural sustainability, based on the deeply identified system, parameter sensitivity simulation, and advanced probabilistic methods. in this study, a pilot application of an integrated system for bridge monitoring and life cycle prediction is presented using the results of the wonka bridge, czech republic. first, the online monitoring system is briefly introduced and then details about the numerical model are given with a focus on the chloride ingress and reinforcement corrosion models. the numerical results are given for chloride attack assumed on the surfaces of the concrete box girder as well as the case when a sudden leak into the ducts of the unbonded pre-stressed cables occurs. 2. wonka bridge description for the pilot study, the wonka bridge over the elbe river located in pardubice, czech republic was selected. it is a pre-stressed box-girder concrete bridge, which consists of three arches with spans of 50 + 70 + 50 m. the cross-section depth is up to 3.5 m. the mid-span cross-section and side views of the bridge are shown in figure 1 and figure 2, respectively. the bridge was constructed between 1956 and 1959. during the service life, the bridge is loaded by road transport and pedestrians. furthermore, the bridge is subjected to the deterioration mechanisms originating from the external environment, such as penetration of the de-icing agents and carbonation of the concrete cover. further details about the wonka bridge and the safety format for evaluation of the structural resistance are given in reference [5]. 3. bridge monitoring system 3.1. description of the system in august 2018, a monitoring system was installed on the bridge to monitor the passing traffic and 190 https://doi.org/10.14311/app.2022.38.0190 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 38/2022 advanced modelling of concrete structures for improved sustainability figure 1. cross-section view in the mid-span of the wonka bridge. figure 2. longitudinal view of the wonka bridge. the load it induces. the ibwim (bridge-weigh-inmotion) technology (pec – petschacher consulting, zt-gmbh) was used. the system consists of strain gauges mounted on the underside deck of the bridge paired with a laser rangefinder, which detects the wheels of the traffic. the strain gauges were placed both in the longitudinal and transversal directions. the data from the monitoring system were collected for 60 days from august until october 2018 within the framework of the european cyberbridge project (https://www.cyberbridge.eu/). except for the wonka bridge, the monitoring data were acquired from another bridge with reinforced concrete girder bridge structural scheme, and the comparative study for both bridges was presented by the authors in [5]. the sensitivity of the system is suitable for vehicles of gross weight above 3.5 t. based on the coupled information, the vehicle’s speed, weight, and load distribution over the vehicle’s axles can be obtained. before the measurements, the system was calibrated by crossing the bridge with trucks of known weights. 4. digital twin development 4.1. model digital twin refers to a computational model, which is upon calibration used to replicate the structural behaviour. using the measured data from the monitoring system, the simulation model can be calibrated and further used for the prediction of the structural performance during the service life of the structure. in the presented study, the computational model was developed in the atena software [6]. atena is a tool for non-linear analysis of reinforced concrete structures using the finite element method. the mechanical behaviour of concrete is simulated using the elasto-fracture-plastic model of červenka et al. [7], and červenka and papanikolaou [8]. in the material model, the plasticity approach is used to model the failure in the compression branch while smeared crack approach with a crack band is used to simulate the tensile softening. the fe model of the bridge was created using layered shell elements with smeared reinforcement as shown in figure 3. two types of pre-stressing cables are in the model; unbonded cables inside of the girder box and the bonded cables in the webs of the box girder. to model the pre-stressing cables, one-dimensional compression/tension elements were used. the reinforcement geometry together with the deviators of the unbonded pre-tension cables is shown in figure 3. the supports and pre-tension deviators were modelled using the solid wedge and hexahedral elements. to estimate the design load-bearing capacity, the partial factor method according to fib model code 2010 [14] was used. according to this method, the structural resistance from a non-linear numerical model is obtained using the design material parameters. applying the safety factors from eurocode [15] and excluding the model uncertainty, the material partial safety factors give γc = 1.46 and γs = 1.20 for concrete and steel, respectively. 4.2. chloride ingress and corrosion models the durability assessment was conducted using the durability-related features of the atena software. it consists of a chloride ingress model and a model for the evaluation of reinforcement corrosion. combining these two methods, the reduction of the reinforcement cross-section due to chloride attack can be evaluated. previously, it has been shown that these features can be used to accurately predict the long-term deterioration of concrete bridges [16]. before the results for the wonka bridge are given, this model for durability assessment is briefly summarised. after casting, the reinforcement bars are protected against corrosion by highly alkaline conditions in the concrete matrix [17]. however, during the structure’s service life, the protective role of the concrete may be compromised mainly due to chloride-ingress or carbonation. in the case that a structure is subjected to the de-icing agents or salts from the seawater, the 191 https://www.cyberbridge.eu/ j. rymeš, j. červenka, r. pukl acta polytechnica ctu proceedings figure 3. model of the wonka bridge during pre-processing showing the fem mesh, pre-stressing cables in the webs of the girder, and the unbonded pre-stressing cables with the deviators. the cross-section view schematically shows the application of the chloride ingress boundary condition. parameter symbol value reference chloride diffusion coefficient dref 0.1005 mm2/day [9] decay rate factor m 0.37 [10] surface chloride content cs 2.57 % ∗ [10] critical chloride content cs,crit 0.4 % [9] pitting factor rcorr 3.0 [11] corrosion rate after spalling ẋcorr,env 37.5 µm/y [12] corrosion rate of unbonded cables due to leak in ducts ẋcorr,duct 541.0 µm/y [13] ∗ half and quarter values were used for outer and inner surfaces, respectively. table 1. inputs for the chloride ingress and corrosion models. chloride ions are transported through the porous system towards the reinforcement and once the critical chloride concentration (cs,crit) is reached, the reinforcement corrosion is initiated [18]. this moment is referred to as depassivation of the reinforcement and the period from the beginning of the chloride attack to the moment of corrosion initiation is called the induction phase. its duration generally depends on the structure’s exposure conditions through the surface (boundary) chloride content (cs) and the concrete mixture characteristics such as water-to-cement ratio or cement type, which affect the diffusion properties of the concrete matrix (dref ) and the chloride binding ability of the material (m) [17]. furthermore, the process is accelerated in the presence of microcracks [19]. the process of the chloride penetration and thus the duration of the induction phase is commonly estimated using a diffusion equation [10]. once the critical chloride concentration is reached at the depth of the reinforcement, the corrosion products start to reduce the cross-section area. from this moment, the propagation phase begins. the corrosion rate depends on the total chloride content, temperature conditions, and the duration of the corrosion process [20]. since the corrosion products have a larger volume than steel, internal pressure builds up in the concrete cover. once it exceeds the material strength, spalling of concrete cover occurs. after that, it can be assumed that the reinforcement corrosion continues and is driven by the conditions of the external environment (ẋcorr,env). further details about the models used for the evaluation of the chloride attack can be found for instance in reference [16]. the parameters of the chloride ingress and corrosion models are summarised in table 1. the parameters of the durability model were estimated based on the available literature data, mainly the reports of the duracrete project [10] and long-term data collection by the rise research institutes of sweden [9]. apart from the corrosion of the reinforcement placed inside the concrete elements, a case when a sudden leak occurs through the ducts of the unbonded cables was considered in this study. the rate of corrosion was estimated directly based on the available experimental data [13]. it was assumed that each wire corrodes separately and the combined uncorroded area 192 vol. 38/2022 advanced modelling of concrete structures for improved sustainability figure 4. (top) plot of the reinforcement corrosion of the smeared reinforcement in the shell elements and (bottom) corrosion of the pre-stressed cables in the webs of the girder box after 100 years of chloride ingress. of all wires gives the cross-section of the pre-tensioned cable. the deterioration of the structure due to chloride attack was considered under the loading by dead loads. next, the design load combination according to eurocode [21] was applied to the model and this state was taken as the reference unit load-bearing capacity of the structure. the loading by the design load combination was further gradually increased to determine the ultimate load-bearing capacity of the bridge. the duration of the deterioration process was varied to find the development of the load-bearing capacity in time. the durability analysis was conducted for 0, 30, 63, 100 years of the chloride ingress, which corresponds to the years 1959, 1989, 2022, and 2059, respectively. furthermore, it was considered that a sudden leak into the protective duct of the cables of the unbonded pre-tension occurs in 2022 and the cables cross-section area corrodes for 5 and 8 years. this corresponds to the years 2027 and 2030. 4.3. mechanical model calibration for the initial calibration of the model, the deflection measured during a load test after the reconstruction works in 2006 was used. the mid-span deflection of 14.23 from the numerical model is in good agreement with the measured value of 14.36 mm. next, the data from the monitoring system was used to compare the strain induced by the crossing vehicle of 28.46 t. this comparison gave strain values at the bottom deck of 8.88 µ and 8.35 µ for the numerical and measured data, respectively. the calibration of the model was also used for the determination of the pre-stressing force in the cables since this value was not known in advance. 4.4. results and discussion figure 4 shows the degree of reinforcement corrosion for the smeared reinforcement in the shell elements and the pre-stressed cables in the webs of the box girder after a 100-years-long chloride attack. at maximum, approximately 54 % of the cross-section area is lost due to corrosion in the most exposed sections of the prestressed cables. apart from the pre-stressed cables, the maximum corrosion degree (i.e., the portion of corroded cross-section), reaches up to approximately 48 %. regardless of the duration of the chloride-induced reinforcement corrosion, the results showed a consistent failure mechanism suggesting concrete crushing near the bridge support due to the concentration of shear/compression stresses. the regions with compression strains exceeding -0.0035 are shown in figure 5. the moment when the strain reaches this value was taken as the design load-bearing capacity of the structure for comparison of the results with different durations of the chloride attack. examples of load-deflection (l-d) diagrams for a structure intact by chlorides, for 63 and 100 yearslong chloride attach are plotted in figure 6 together with the case, where 8-years-long corrosion of the unbonded pre-stressing cables due to a leak into the cable ducts was considered. furthermore, the load level, when the compressive strain of -0.0035 is reached in concrete, is indicated for the l-d diagrams. it can be observed that both the stiffness and the load-bearing capacity are reduced because of the chloride attack. on the other hand, the design load-bearing capacity exceeds the required design load level in all analysed cases. this is apparent from figure 7, which plots the development of the design load-bearing capacity in time. furthermore, figure 7 also shows how the load193 j. rymeš, j. červenka, r. pukl acta polytechnica ctu proceedings figure 5. deformed shape of the wonka bridge after the peak load and the detail of the failure region near the left support. figure 6. typical load-deflection (l-d) diagrams for the wonka bridge with the indication of the load level when the minimum principal strain in concrete exceeds the value of -0.0035. bearing capacity of the bridge is reduced if the chlorides from the de-icing salts leak through the protecting ducts to the cables placed in the box girder. in such a case, the bridge structural performance is reduced at a much higher rate. however, as the cables are pre-stressed only to 64 % of the material’s yield strength, the cables still have sufficient capacity for potential stress increase. therefore, the numerical model predicts that the bridge can still transfer the design loads even after 8 years of corrosion of the cables of the unbonded pre-stressing. the numerical results suggest a failure due to the crushing of concrete, which is brittle by nature. on the other hand, the l-d diagrams show that the failure is preceded by the mid-span deflection of the bridge of approximately 0.4 m. generally, the long-term deteriofigure 7. prediction of the development of the design long-bearing capacity of the bridge affected by chlorideinduced reinforcement corrosion and potential leak in the cable ducts of the unbonded pre-tension. ration study showed a good resistance of the structure towards chloride attack. this resistance is mainly provided by a robust structural design, which provides sufficient capacity for the redistribution of the tensile stresses even if the cross-section area of the steel reinforcement is reduced due to corrosion. 5. summary this study presents a durability analysis of the wonka bridge located in pardubice, czech republic. the long-term assessment is based on the integrated system, which is composed of an online system for bridge monitoring and a numerical model. using the measured data, the numerical model is calibrated and then used for evaluation of the structural deterioration due to de-icing agents. the effect of the chloride 194 vol. 38/2022 advanced modelling of concrete structures for improved sustainability attack was evaluated using the chloride transport and reinforcement corrosion models implemented in the framework of non-linear fe analysis. this allowed for the prediction of the time development of the design load-bearing capacity of the bridge. the results revealed that although the mechanical performance characteristics are continuously reduced due to the chloride-induced corrosion, the bridge shows sufficient load-bearing capacity within the examined service period. this is mainly due to the robustness of the initial structural design. on the other hand, the results showed that in the case of a potential leak into the ducts of the unbonded pre-stressed cables, the structural performance is rapidly compromised. therefore, it is suggested to regularly inspect the state of the cable ducts in the box girder. acknowledgements this study was conducted with the financial support of the czech technological agency under the project ck03000023 “digital twin for increased reliability and sustainability of concrete bridges”. the 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1998. https://doi.org/10.1016/s0008-8846(98)00259-2 [21] european committee for standardization. eurocode 1: actions on structures – part 1-1: general actions – densities, self-weight, imposed loads for buildings, 2002. 195 https://doi.org/10.1617/s11527-013-0114-5 https://doi.org/10.1016/j.cemconres.2010.12.001 https://doi.org/10.3390/app11146571 https://doi.org/10.1016/0045-7949(91)90402-8 https://doi.org/10.1007/978-3-031-07746-3_8 https://doi.org/10.1016/j.ijplas.2008.01.004 https://doi.org/10.1016/0008-8846(95)00006-2 https://doi.org/10.1016/j.engstruct.2018.08.006 https://doi.org/10.1680/cc.25929 https://doi.org/10.1016/b978-0-12-409547-2.13435-3 https://doi.org/10.1016/b978-0-12-409547-2.13435-3 https://doi.org/10.4334/ijcsm.2011.5.1.011 https://doi.org/10.1016/s0008-8846(98)00259-2 acta polytechnica ctu proceedings 38:190–195, 2022 1 introduction 2 wonka bridge description 3 bridge monitoring system 3.1 description of the system 4 digital twin development 4.1 model 4.2 chloride ingress and corrosion models 4.3 mechanical model calibration 4.4 results and discussion 5 summary acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.38.0097 acta polytechnica ctu proceedings 38:97–103, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague decision-support tool for prioritizing retrofitting actions on social housing stock in italy jacopo gaspari∗, ernesto antonini, lia marchi university of bologna, department of architecture, viale del risorgimento 2, 40136 bologna, italy ∗ corresponding author: jacopo.gaspari@unibo.it abstract. housing is the main environmental impact generator (62 %) of the whole building sector, but it also has the greatest reduction potential. enhancing its performance is thus crucial to sustainable development. social housing (sh) represents a critical asset within the residential segment, due to the recurrent investment shortage and several environmental, social, and economic related implications. in italy, sh is held by around one hundred public agencies facing endemic resource constraints for both maintenance and retrofitting, which are limited further by a diffused lack of information regarding the conditions and features of the buildings they manage. in cooperation with an italian sh agency (acer bologna), we developed a speedy tool to compare the technical and economic effects of different refurbishment scenarios on a case-by-case basis. this is not a tool to manage retrofitting works, as the many already available, but a means to help large housing managers overcome the intention-action gap that limit their capacity to properly prioritize interventions based on reliable information. the research focuses on the validation of the fast procedure for estimating the baseline energy scenario, arguing that the relatively small inaccuracies are irrelevant for the scope of the tool and are compensated for by the time saved. keywords: retrofitting, social housing, decision-support tool, energy efficiency. 1. introduction the built environment is estimated to contribute 30 % of global greenhouse gas (ghg) emissions and 40 % of total energy final consumption [1]. within it, the main generator of environmental impact is the residential sector (62 %), which accounts for 25 % of total final energy consumption, primarily due to the operational phase of buildings [2]. large energy saving campaigns have therefore been launched globally, as well as actions to accomplish considerable emissions reductions in the building and construction industry by 2050, based on new net-zero carbon buildings, deep renovations of existing ones, green energy supply, and low-emissions materials [3]. households are estimated to have a reduction potential of 40 %, mainly due to the flexibility of their demand for energy and resources [4]. extraordinary energy and carbon savings can be achieved in new constructions, downing up to 70 % reduced energy demand compared to traditional constructions. however, due to the slow turnover rate of the stock [5], the largest potential lies in retrofitting the existing asset. in fact, taking europe as example, in forty years the vast majority (90 %) of the built environment will consist of buildings that are already in use today, with a large portion of them constructed before energy efficiency standards were introduced [6]. for this reason, in europe, where the household sector was responsible for 26 % of final energy consumption in 2018 [7], several policies and measures to increase the building sustainability have been introduced, requiring a deep enhancement of their performance. the energy efficiency directive 2012/27/eu (eed) [8] and the energy performance of buildings directive 2018/844/ec (epdb iii) [9] promote the energy efficiency improvement of both new and existing buildings stock, aimed at reducing 80–95 % of the building related ghg emissions by 2050. the recent eu green deal [10] also highlights the importance of renovating the stock in an energy and resource efficient way. in spite of the huge effort eu has been spent in the last decades, likewise several other countries worldwide, enormous emissions reduction potential remains untapped due to persistent use of fossil fuel-based resources, lack of both effective energy-efficiency policies and investments in sustainable buildings [2]. according to unep, the main barriers preventing a higher pace in interventions for energy efficiency in housing are: economic/financial (e.g., high investment cost compared to energy saving return), due to hidden costs/benefit ratios (e.g., misleading perception of cost focussing on first cost rather than reduced operational costs, and split economic interest between owners and tenants – who spends the money is not who directly benefits from the intervention), market failures, behavioural and organizational obstacles, low level of awareness (e.g., lack of information provided on energy saving potential) and political/structural constraints (e.g., ownership fragmentation) [1]. these barriers especially affect the social housing (sh) stock that, despite representing a relatively small share of the whole residential built stock, is among the harder asset to renovate. this is mainly because of 97 https://doi.org/10.14311/app.2022.38.0097 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en j. gaspari, e. antonini, l. marchi acta polytechnica ctu proceedings its poor baseline energy performance, socioeconomic fragility of users, peculiar tenure and management status, and availability of financial resource for the purpose. in figures, sh accounts for more than 28 million dwellings and about 6 % of the total housing stock in oecd and non-oecd eu countries. in most european states it makes up 10 % of the whole residential sector, reaching up 20 % in some northern eu countries [11]. despite some differences across the members states, most of the current sh stock in eu has been built between 1960s and 1980s as public answer to housing needs of the working classes. in many cases, it consists of high rise residential blocks made by industrial construction systems and new building materials that have proved to be limited in durability (e.g. façades, joints, windows, roofs ) and soon have showed severe technical deficiencies, in addition to their poor energy performance [12]. this not only results in high environmental impacts of the sh stock, but also affect the building operational costs, and in turn economic and health status of its final users. in fact, this has a strong impact on social housing tenants, most of whom are low-income and socially disadvantaged, thus highly exposed to the risk of energy poverty and the inability to access fair energy policies [13, 14]. furthermore, technical obsolescence increases maintenance costs, which are already affected by the heterogeneity of the stock managed by each agency, often large in size but made up of very different assets in terms of age, size and state of conservation [15]. regional and municipal authorities hold on average half of the sh stock in eu, while the rest is divided among non-profit, limited-profit or cooperative housing associations (15 %), national governments (14 %), for-profit providers (11 %) [16]. almost all of them struggle with scarcity of resources and a lack of adequate knowledge of their assets. the combination of these impacts the capacity of agencies to effectively undertake renovation programs. hence, even though the huge effort of many countries in renovating their built park, a significant share of this stock still needs interventions and proper support tools. this is the case of most shs, whose refurbishment and energy efficiency enhancement not only would contribute to reduce environmental impact, but also to lessen social fragility of tenants and improve the management effectiveness. this is the mission of the housing europe network, which collects public, cooperative and non-profit agencies sharing the aim of “considering climate without pricing people” (i.e., retrofitting for lessening building’s environmental impact while reducing operational costs) [15]. 1.1. social housing stock in italy the sh stock in italy is critical to manage and renovate, far more than in other eu countries with greater public housing share, larger resource availability and a wider variety of subjects operating within the sector in addition to public authorities. in fact, the italian sh stock accounted for less than 4 % of total residential asset in 2020 [16], it consisted of 836 000 dwellings hosting 2.2 billion inhabitants [17]. the historic shortage of these subsidised dwellings – which have been heavily reduced after a massive selling campaign started in the 1990s [18] – is largely inadequate to cover the demand, which national association for public housing [17] estimated to increase by 36 % (300 000 additional dwellings). as no financial resources are available for increasing the building offer, it becomes critical for owners and managers to handle their assets as efficiently as possible and to make them fully useable through intensive maintenance and retrofitting interventions. the technological features of italian sh are quite similar to those of other eu countries [19]: most of these buildings were built before the early energy regulation being enforced in 1976 [20]. this usually results in poor energy performances, mainly because of low thermal insulation levels and inefficient technical systems, heavily impacting both the tenants and owners’ finances. this suggests that basic but effective interventions would greatly help, such as thermal insulation of envelopes, window replacement with higher-performing ones, and systems upgrading. however, the pace of refurbishment is very slow, and even decreased in recent years: 11 423 dwellings were deeply refurbished in 2014, while three years later (2017) the annual share was reduced to almost half (6 578 dwellings, that is less than 1 % of the whole stock) [15]. the main reason of this is that social housing agencies find it difficult to successfully retrofit their assets, owing to a lack of investment resources, insufficient information on the status of their properties, and tenure fragmentation [17]. in fact, while there are only 74 territorial agencies (originally, one in each province) managing the sh dwellings, they belong to different public owners (the agencies themselves, municipalities, other bodies) and they are often located within the same building together with some, if not dozens, of family property flats. on the other hand, a lack of information on the status of the property risks misleading the agency in assigning priorities to the interventions: in contrast to what currently happens in the management of private real estates, the implementation of advanced information systems is still not widespread in the publicly owned buildings and often does not include an accurate survey of the physical state of the building. without having properly organized information on buildings, whose lack is typically due to data retrieval costs, sh owners are often unable to implement appropriate management practices. as a result, the already limited resources allocated to social housing refurbishment and energy retrofitting are not optimised [21]. in addition to the difficulty of managing thousands 98 vol. 38/2022 decision-support tool for prioritizing retrofitting actions . . . of accommodations at a time, these agencies often deal with condominium buildings (resulting from the massive sale of the late 90s) which further hinder the possibility for many different owners to agree on renovations. not to mention the resistance of tenants to accept planned actions due to a false perception of cost-benefits ratios or the presumed opposite interests of landlord and tenant [1, 6]. this clearly highlights how crucial it is for owners to carefully select properties to include in their frequent but limited maintenance or renovation programs. in this context, having quick and cheap pre-evaluation tools could be of great help for the owners themselves, in order to maximize the effectiveness of each initiative [19, 22, 23]. several tools are currently available on the market with the aim of supporting the building retrofitting process. however, very few of them have been developed targeting sh and its peculiarities. among those there is the free tool “condomini+4.0”, developed by the italian national energy agency (enea) for multipropriety buildings, which aims to assess their energy demand and seismic vulnerability and, on this basis, suggests a set of intervention to be implemented. the tool has been tested on six sh cases [23]. while the application “investimmo” directly targets the sh, having been developed by the regional housing association aler lombardia to survey its asset and detect maintenance priorities on 320 social dwellings [24]. other similar applications have been developed at regional scale, but restricted in scope and extent, while very few implementations have been performed of tools for quick simulating the effect of maintenance and strategic retrofitting combined [21]. this is also because available tools for this purpose usually require detailed input data that sh managers hardly have on their vast assets. given that the inability to prioritize interventions based on reliable information, investigation and diagnostic activities represents a serious intention-action gap that limit agencies’ operational capacity, it appears that a tool is required to assist them. on this premises, the paper presents a study aimed at developing a tool to support decision-making for retrofitting the sh stock in a timely but effective manner. 2. method the overall methodology can be broken down into four main steps, which almost match with the implementation flow of the tool: (1.) determining a procedure to obtain quick and homogeneous pieces of information about the current energy behaviour of buildings within a certain stock (baseline scenario); (2.) defining a set of suitable retrofitting actions based on the most recurrent interventions in sh; (3.) estimating the benefit of those actions on the overall energy demand of the building; (4.) calculating and comparing the cost-energy benefits ratios of different retrofitting options. the overall procedure has been developed within a study called integrated technologies for smart buildings and predictive maintenance (inspire, 09/201909/2021), supported by acer together with the emilia-romagna region and carried under the umbrella of a broader cooperation between the department of architecture and the interdepartmental centre for industrial research (ciri) of the university of bologna. the study worked on the more frequent retrofitting measures used within the observed stock over the last decade, resulting in the identification of a set of standard intervention schemes and their expected performance improvements. the detected ones have been grouped as follows: (i) application of insulation layer on the building envelope; (ii) application of insulation layer on horizontal closures, such as flat and sloping roofs; and (iii) replacement of windows. for each of them, two increasingly higher performance levels have been determined: (i) basic refurbishment, by applying the most used measures to reach the minimum legal requirements regarding energy transmittance (u-value) of the building envelope elements; or (ii) advanced refurbishment, by implementing the best available technologies to obtain high energy performance, or an improvement of about 30 % compared to the legal minimum requirements. a standard cost of the intervention has then been associated to each. on this basis, the architecture of the digital tool has been completed with a section that compares different refurbishment scenarios on an iterative basis, resulting in user friendly visualisation. the procedure and the general scope of the study are reported in [25, 26] while this paper is focused on reporting the implications of the first research phase and its validation on a sample pool of buildings belonging to acer bologna, which is the local agency for social housing that has funded the project. as sh agencies have often heterogeneous and inadequate data about the status of their assets, the first step was to find how they can get the necessary information timely and in reliable conditions, so to evaluate the baseline performances and rationalise interventions accordingly. to this end, the study identified three alternative ways to calculate a building status in terms of energy performance. the first (a) is to analytically calculate – or just retrieve if already available to the manager – the overall building energy need (i.e., energy performance certificate – ep). this, in general, requires a thorough understanding of the building and its systems, as well 99 j. gaspari, e. antonini, l. marchi acta polytechnica ctu proceedings figure 1. implementation workflow of the tool. as the capacity to operate specialist software with analytic data, or to hire an expert to do this job. the second (b) is to access third-part well-diffused and recognised tools such as the tabula webtool [27] that assigns a building an energy performance class based on parametric data, according to the average consumption of buildings of similar type, location, and construction period. this procedure is especially useful when only cadastral data are available, and no specific knowledge is required to the user. then, the study identified a third option that is an intermediate way to be used when managers already have or can quickly retrieve a set of simple data. this is to prevent more detailed knowledge from being lost by merely applying solution (b). to this end, a minimum dataset (c) was defined, including those basic physical and technical features of the building required to perform the energy simulation (e.g., building volume, net floor area, envelope surface, envelope area to volume ratio, and average thermal transmittance of technical units). however, when parametric and analytic data are used together, the risk of discrepancy arises, as well as the possibility of severe estimation mistakes. therefore, in order to validate the results obtained from the speedy method (b) considering its accuracy to justify the potential approximation, a test on a group of buildings with available eps has been performed by comparing the results and ensuring that their maximum deviations were acceptable to the scope. in particular, the non-renewable primary energy demand for heating and domestic hot water (epgl,nren) was assumed as the most precise value to assign an energy class to a building. it is expressed in total energy used by the building per square meter of surface every year [kwh/m2y]. if (b) is validated, it goes without saying that the intermediate way (c) is acceptable too. then, the average time that is needed to collect information and complete the three procedures has been calculated in order to highlight the potentiality of the tool in saving time and resources while improving the prioritization process, thus in ranking buildings for renovation. 2.1. application case study: acer bologna as it happens to many other agencies in italy, acer bologna runs a large asset (18 000 dwellings across 55 municipalities) on which it has a scarce level of information. the datasets owned by acer have been gathered during multiple campaigns over different time periods indeed and, therefore, it is very heterogeneous and frequently structured according to different criteria. as already mentioned, this prevents the agency from having a thorough understanding of the condition of its asset and properly prioritizing, optimising, and implementing retrofitting interventions. hence, after the methodology was developed, acer explicitly suggested the tool to be validated on a sample pool of buildings belonging to its asset. a first test was undertaken on six buildings for which the agency already had the ep certificate, with the scope of determining the suitability of the speedy method to calculate the baseline scenario. the buildings for the sample have been selected to be representative of the whole stock, balancing construction periods to cover different construction techniques and conservation status (from 1930s to 1990s). then their 100 vol. 38/2022 decision-support tool for prioritizing retrofitting actions . . . building typology∗ (a) ep (b) tabula error error [%] epgl,nren [kwh/m2y] epgl,nren (b − a) 1. via f. albani 1946–1960 211,55 218,2 6,65 3 % 2. via p. tibaldi 1921–1945 220,64 217,2 -3,44 -2 % 3. via a. di vincenzo 1946–1960 173,22 218,2 44,98 26 % 4. via w. goethe 1976–1990 125,94 174,1 48,16 38 % 5. via a. gandusio 1976–1990 195,89 174,1 -21,79 -11 % 6. via verne 1961–1975 221,29 211,9 -9,39 -4 % ∗ building typology for tabula requires the type of building, the period, and the climatic zone: since all these buildings are apartment block (ab), in climatic zone e, only the construction period is reported. table 1. comparison of results from the validation phase. eps have been estimated by means of the tabula webtool and compared with the ones from ep certificates. then, discrepancy between the two values have been calculated to assess the method reliability and accuracy level. 3. results and discussion the comparison between the results from the two methods applied on the 6 buildings from acer sample pool is reported in table 1. errors range from 2 to 38 kwh/m2 per year. for 4 buildings (2/3 of the sample) the ep from tabula differs of less than 11 %, which is assumed as an acceptable error with relation to the scope and for buildings with an average energy demand of 215 kwh/m2 per year, largely justified by time and effort savings. two buildings presented a significative discrepancy (26 % and 38 %) that could not simply be explained as calculation errors. thus, after a site visit emerged that they were involved in some interventions for improving accessibility (this not registered as retrofitting actions) which partially modified the building behaviour influencing the performance level. these circumstances confirmed the need for a more coherent and organised approach to data collection, but also the necessity for a speedy way to solve current discrepancies and proceed with renovation implementation consistently with urgency and according to funding availability. looking at the results of the process it can be easily observed that, assuming the heterogeneity of available knowledge and data, detailed pieces of information must be gathered before implementing the retrofitting process. at the same time, a comprehensive view of intervention priorities is required to schedule actions and possibly consider the logistic and management benefit of operating on narrowing buildings in the same plot. if a detailed level of information is to be achieved for the whole, the process would require a huge amount of time and resources, so impeding retrofitting until the overall picture is fully completed. this would be probably the worst option considering the urgency affecting some buildings and their living conditions. otherwise, a more rapid but less precise approach could be considered. the proposed speedy methodology provides acceptable (tested) results in a quite short time. compared to an average time of 12 hours (1.5 working days) for a detailed calculation of energy performance, only 10 minutes are required for the speedy estimation by tabula. thus, in the same time frame, the manager can obtain the baseline scenarios of around 72 buildings. 4. conclusions the outcome of this study is a speedy digital tool for detecting the baseline performance of a building (or a group of buildings) and then comparing the effects of different retrofitting actions based on criteria such as energy efficiency, intervention costs, or a combination of the two. the paper focused on the need for sh agencies to have homogeneous information on the status and energy performances of their stock to start the retrofitting process and thus prioritize interventions effectively. on this basis, the idea was to investigate if less precise but more expeditious tools to calculate buildings ep, compared to traditional ep certificate procedure, were accurate enough to take adequate decisions. two alternative methods were proposed (accessing tabula web tool and measuring the ep by a minimum dataset) and were proved to be quicker and easier to perform. hence, a validation of their accuracy was performed and resulted that the maximum error is around 10 %. this can be considered acceptable for the scope of the tool, also considering the manager’s abilities, knowledge, and time availability, rather than delegating the assessment to timeconsuming and costly consultations of experts. the main novelty of the study lies in this point. in addition, the fact of having heterogeneous data on the stock also allow to take advantage of mass economy mechanism. for example: if acer found that three close buildings would require the thermal insulation of the envelope, the intervention could be planned to be undertaken simultaneously, reducing transportation cost and test of machineries installation like crane. moreover, the intermediate method to calculate energy performance allow the sh manager not to lose important information when available, representing an added value to accuracy of the tool. in the 101 j. gaspari, e. antonini, l. marchi acta polytechnica ctu proceedings case of acer bologna, for example, about 20 % of dwellings had sufficient info to apply the minimum dataset calculation. hence, the proposed approach may effectively support the decision-making process accelerating the expenditure rate according to a more finalised and priority-based way. to conclude, despite the encouraging results, the method will certainly benefit from additional validations on a larger number of buildings in different situations. the tool’s accuracy can be tested in other acer bologna buildings, but also a national campaign could be launched to this purpose now that considerable funds have been set for building retrofitting under the umbrella of recovery plan. in addition, the tool can be easily transferred in other countries since both tabula webtool and energy performance certificates are already in use, at least in the eu. this may lead to some changes to the tool, mainly concerning the energy retrofitting options and costs not the procedure for calculating the baseline scenario outlined in the paper. acknowledgements the authors thanks acer bologna, ciri ediliziacostruzioni of university of bologna and regione emiliaromagna for supporting the study. references [1] g. 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[2021-10-01]. http://www.alerbg.it/ [25] j. gaspari, m. de giglio, e. antonini, v. vodola. a gis-based methodology for speedy energy efficiency mapping: a case study in bologna. energies 13(9):2230, 2020. https://doi.org/10.3390/en13092230 102 https://www.iea.org/reports/tracking-buildings-2020 https://www.iea.org/reports/tracking-buildings-2020 http://www.bpie.eu/publication/europes-buildings-under-the-microscope/ http://www.bpie.eu/publication/europes-buildings-under-the-microscope/ https://ec.europa.eu/eurostat/statistics-explained/index.php?title=energy_consumption_in_households https://ec.europa.eu/eurostat/statistics-explained/index.php?title=energy_consumption_in_households https://ec.europa.eu/eurostat/statistics-explained/index.php?title=energy_consumption_in_households https://www.oecd.org/social/social-housing-policy-brief-2020.pdf https://www.oecd.org/social/social-housing-policy-brief-2020.pdf http://www.fuelpoverty.org https://doi.org/10.1016/j.enpol.2012.02.035 https://www.stateofhousing.eu https://www.oecd.org/housing/data/affordable-housing-database/ https://www.oecd.org/housing/data/affordable-housing-database/ https://www.federcasa.it/appello-di-federcasa-e-federcostruzioni-in-italia-servono-300mila-nuove-case-popolari/ https://www.federcasa.it/appello-di-federcasa-e-federcostruzioni-in-italia-servono-300mila-nuove-case-popolari/ https://www.federcasa.it/appello-di-federcasa-e-federcostruzioni-in-italia-servono-300mila-nuove-case-popolari/ https://doi.org/10.3390/su9101869 https://doi.org/10.3390/su11092507 https://doi.org/10.12910/eai2020-075 http://www.alerbg.it/ https://doi.org/10.3390/en13092230 vol. 38/2022 decision-support tool for prioritizing retrofitting actions . . . [26] v. vodola, e. antonini, j. gaspari, l. marchi. a methodology for fast simulation of energy retrofitting scenarios of social building stock. in j. r. littlewood, r. j. howlett, l. c. jain (eds.), sustainability in energy and buildings 2021, pp. 147–157. springer singapore, singapore, 2022. https://doi.org/10.1007/978-981-16-6269-0_13 [27] tabula webtool. [2021-03-20]. https://webtool.building-typology.eu/#bm 103 https://doi.org/10.1007/978-981-16-6269-0_13 https://webtool.building-typology.eu/#bm acta polytechnica ctu proceedings 38:97–103, 2022 1 introduction 1.1 social housing stock in italy 2 method 2.1 application case study: acer bologna 3 results and discussion 4 conclusions acknowledgements references https://doi.org/10.14311/app.2022.33.0473 acta polytechnica ctu proceedings 33:473–479, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague lateral load bearing characteristics of light gauge steel and lightweight concrete shear walls chunri quana,∗, qiang huangb, dongbin lic, yong chend a osaka institute of technology, department of architecture, 5-16-1 omiya, asahi-ku, 535-8585 osaka, japan b china bim union, department of management, no.30 beisanhuandonglu, 100013 beijing, china c china building technique group co., ltd., engineering technology research center, no.30 beisanhunadonglu, 100013 beijing, china d arse building engineering technology co., ltd., design department, no.365 dongsihuannanlu, 100023 beijing, china ∗ corresponding author: chunri.quan@oit.ac.jp abstract. in china, there is a new structural system named light gauge steel and lightweight concrete (lslc) structure, which used lightweight concrete as structural material in composite way with coldformed steel. here, the shear walls are the main structural members for the lslc structure, which are assembled with the light gauge steel lattice columns and horizontal braces, and filled with lightweight concrete. in this study, the lslc shear walls are experimentally investigated to evaluate their failure mechanism and lateral load bearing capacity. for this purpose, several specimens with different shear span ratio are designed and tested under static cyclic loading. this paper presents the damage state and hysteresis loops of the specimens detailly. then, the lateral load bearing characteristics of the lslc shear walls are discussed according to the failure mechanism, such as shear and flexural failure. finally, the calculation methods of lateral strength for the lslc shear walls are proposed based on the diagonal strut mechanism and sectional force equilibrium. keywords: failure mechanism, lateral load bearing characteristic, light gauge steel and lightweight concrete structure, shear wall. 1. introduction cold-formed steel structure has been widely used in the low-rise buildings due to its high strength, ease of construction, and low cost [1]. on the other hand, due to low self-weight, good workability, excellent performance on thermal insulation, fire resistance and sound absorption, lightweight concrete was primarily utilized as nonand semi-structural material in the past building construction [2]. motivated by the industrialized performance of cold-formed steel structure, good integrity of cast-in-situ concrete structure and advantages of the lightweight concrete, a new structure system named light gauge steel and lightweight concrete (lslc) structure, which used expanded polystyrene concrete or foamed concrete as the structural material in composite way with the cold-formed steel, was proposed and applied to the building construction in china [3]. compared with the traditional reinforced concrete structure, the lslc structure can reduce seismic load significantly based on the use of lightweight concrete to decrease the self-weight of overall structure. compared with the cold-formed steel structure, the lslc structure has great advantages in features such as fire protection, thermal insulation and sound absorption. in the past several years, the studies of the lslc structure were focused on the members such as shear walls and slabs, furthermore their design method has been developed based on the considerable tests. this paper describes the design details and testing method for the lslc shear wall specimens, with different shear span ratio as the experimental parameters. then, the damage state and hysteresis loops of specimens are presented detailly, and the seismic capacity of the lslc shear wall is evaluated based on the cyclic lateral loading test results. finally, the lateral load bearing characteristics of the lslc shear walls are discussed, and the calculation methods of lateral strength for the lslc shear walls are proposed. 2. outline of experiment 2.1. configuration of the lslc shear wall shear wall is the main structural member of the lslc structure system. figure 1 shows the standard configuration of the shear wall with thickness of 180mm and concrete cover thickness of 20mm. light gauge steel frame is assembled with the light gauge steel lattice columns and horizontal braces, then filled with lightweight concrete. the lattice columns composed of two or four square steel tubes, which were combined and fixed by batten plates and bolts with 600 mm spacing as shown in figure 2. steel strips with 473 https://doi.org/10.14311/app.2022.33.0473 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en c. quan, q. huang, d. li, y. chen acta polytechnica ctu proceedings self-drilling screws brace two-tube lattice column four-tube lattice column lightweight concrete figure 1. configuration of the lslc shear wall. figure 2. installation of batten plates in the lattice column. specimen height × width × thickness (mm) shear span ratio axial force ratio s0.8-a0.4 1650 × 2062.5 × 180 0.8 0.4 s1.5-a0.4 2250 × 1500 × 180 1.5 0.4 s2.5-a0.4 2250 × 900 × 180 2.5 0.4 table 1. experiment parameters. w-shaped cross section were installed as horizontal braces with spacing of 600mm, which were connected to the lattice columns by self-drilling screws. 2.2. design of specimen the test specimens are designed according to the standard design method of lslc shear wall mentioned above. in this study, five full-scale specimens with different shear span ratio and axial force ratio as the experimental parameters are tested under inplain cyclic loadings. table 1 and figure 3 show the experiment parameters and design details of specimens. the light gauge steel lattice columns of each specimen are composed of four and two square steel tubes at the side and the middle of the wall, respectively. then, the lattice columns are anchored in the reinforced concrete rigid beam (stub). the horizontal braces are installed with spacing of 600mm, which are connected to the lattice columns by three self-drilling screws. 2.3. material characteristic table 2 and table 3 show the material test results, where the values represent the mean value of 3 samples in each test. the light gauge steel is designated as s350gd conform to the chinese national standard gb/t2518 [4]. which requires the yield strength and tensile strength not less than 350mpa and 420 mpa, respectively. in addition, no.4.8 self-drilling screw given in the iso15481 [5] is adopted as fastener. the expanded polystyrene concrete with designing density of 1000kg/m3 is used as lightweight concrete. here, test samples with dimensions 100 mm × 100 mm × 100 mm are prepared in casting process of each specimen, then the compressive strength and density are measured according to the chinese national standard gb/t50080 [6]. 474 vol. 33/2022 lateral load bearing characteristic of shear walls 水平拉条墙体 底梁 顶梁 自攻钉 矩形钢管 矩形钢管拼装扣板 brace stub stub wall screws batten plate steel tube figure 3. details of specimens (unit: mm). member yield strength tensile strength young’s modulus cross-section [mpa] [mpa] [mpa] [mm] lattice column 361.5 484.3 2.02 × 105 horizontal brace 352.1 462.2 1.98 × 105 table 2. mechanical properties and cross-section size of light gauge steel. density compressive strength young’s modulus [kg/m3] [mpa] [mpa] 1058 6.36 0.72 × 104 table 3. mechanical properties of lightweight concrete (expanded polystyrene concrete). 2.4. loading program the loading system and history are shown in figure 4 and figure 5, respectively. the lateral cyclic loading is performed by load control system until the yielding of light gauge steel. then, it is switched to displacement control, and peak drift angles (the ratio of lateral deformation to wall height) are planned by the times of displacement (∆) when the light gauge steel is yielded. here, two cycles for each peak drift are imposed. the axial load is applied to each specimen based on the axial force ratio. 3. test results 3.1. failure patterns figure 6 shows the crack patterns in each specimen at the safety limitation, where it is defined as the moment in which the maximum lateral strength of the lslc shear wall deteriorates to its 85%. 3.1.1. s0.8-a0.4 specimen a shear crack at the middle of wall is observed at the drift angle of 0.11% with the width of 0.15mm. loaded to 0.25%, some vertical cracks occur at the upper of wall along the light gauge steel lattice columns. at the drift angle of 1.88%, clear shear cracks are observed at the diagonal of wall. 3.1.2. s1.5-a0.4 specimen a shear crack at the bottom of wall is observed at the drift angle of 0.11% with the width of 0.1mm. loaded to 0.36%, some vertical cracks and flexural cracks occur in succession. at the drift angle of 3.02%, the vertical cracks and flexural cracks continue to extend and the width increases. 475 c. quan, q. huang, d. li, y. chen acta polytechnica ctu proceedings 45 5056 50 3740 positive direction negative direction figure 4. test setup (unit: mm). 7 6 5 4 3 2 1 0 1 2 3 4 5 6 7δ δ δ δ δ δ δ δ δ δ δ δ δ δ load control 1cycle 2cycles 2cycles 2cycles 2cycles 2cycles 2cycles displacement control yielding of steel figure 5. loading history. 3.1.3. s2.5-a0.4 specimen there are some flexural and flexural-shear cracks are observed at the drift angle of 0.21% with the maximum width of 0.15mm. loaded to 0.34%, vertical and horizontal cracks occur along the light gauge steel lattice columns and braces. at the drift angle of 3.04%, the width of flexural cracks increases, and local crush of lightweight concrete cover is observed at the bottom of wall. 3.2. hysteretic characteristics figure 7 shows the relationship between lateral strength and drift angle. 3.2.1. s0.8-a0.4 specimen the maximum lateral strength of 295.5kn is recorded at the drift angle of 0.59%. then, remarkably rapid strength deterioration is observed until the drift angle of 1.53%, which is the safety limitation of the specimen. it shows typical shear failure characteristics. 3.2.2. s1.5-a0.4 specimen the maximum lateral strength of 146.2kn is recorded at the drift angle of 1.15%. then, relatively slow strength deterioration is observed until the drift angle of 2.10%, which is the safety limitation of the specimen. it shows flexural-shear failure characteristics. 3.2.3. s2.5-a0.4 specimen the maximum lateral strength of 73.8kn is recorded at the drift angle of 0.46%. then, slow strength deterioration is observed until the drift angle of 1.23%, which is the safety limitation of the specimen. it shows typical flexural failure characteristics. 4. seismic capacity evaluation 4.0.1. lateral load bearing capacity figure 8 shows the relationship between maximum lateral strength and shear span ratio. the maximum lateral strength decreases with the increase of the shear span ratio. compared with the s0.8-a0.4 specimen, the maximum lateral strength of the s2.5-a0.4 specimen decreases by 75.0%. it can be considered that the flexural failure characteristics of shear wall gradually dominates with the increase of the shear span ratio. 4.1. deformation capacity figure 9 shows the relationship between ductility coefficient and shear span ratio. herein, the ductility coefficient (µ) of the lslc shear wall is calculated as the ratio of failure displacement (∆u, displacement of safety limitation) to yield displacement (∆y), which is defined as shown in figure 10. it is difficult to determine the internal relationship between the shear span ratio and the ductility coefficient of the lslc shear wall. however, the ductility coefficient of each specimen is larger than 4, showing better deformation capacity compared with the reinforced concrete shear walls [7]. 4.2. energy absorption capacity since the absolute value of dissipated energy depends on the scale of the specimen such as the crosssectional area, the normalized equivalent damping ratio (heq) is applied to evaluate the energy absorption capacity of the lslc shear wall. herein, the equivalent damping ratio is calculated from the energy absorbed in one cycle (∆w ) and equivalent potential energy (we, strain energy) as shown in figure 11. figure 12 shows equivalent damping ratio of each specimen. the maximum value of the equivalent damping ratio for each specimen is recorded 17.19 ∼ 19.27%. after loading to the drift angle of 0.5%, the equivalent damping ratio of the lslc shear wall shows the relatively stable values as 12 ∼ 20%. compared with the reinforced concrete shear walls [8], the lslc shear walls show lower energy absorption capacity in this experimental study, and it can be considered that the severe slip of the lslc shear wall reduced the hysteretic dissipated energy. 5. calculation methods of lateral strength in this paper, the past experimental results of eleven specimens are employed to calculate the lateral 476 vol. 33/2022 lateral load bearing characteristic of shear walls 正向加载 负向加载 positive loading negative loading figure 6. crack patterns. -5 -4 -3 -2 -1 0 1 2 3 4 5 -400 -300 -200 -100 0 100 200 300 400 θ=0.25%，第一条竖裂缝 荷 载 p/ kn 位移角θ/% θ=1.53%，85%最大荷载 θ=0.59%，最大正向荷载 θ=0.11%，第一条斜裂缝 -80 -60 -40 -20 0 20 40 60 80 位移δ/mm -5 -4 -3 -2 -1 0 1 2 3 4 5 -400 -300 -200 -100 0 100 200 300 400 位移角θ/% θ=0.11%，第一条斜裂缝 θ=0.20%，第一条竖裂缝 θ=0.36%，第一条横裂缝 θ=1.15%，最大正向荷载 θ=2.10%，85%最大荷载 -90 -60 -30 0 30 60 90 位移δ/mm -5 -4 -3 -2 -1 0 1 2 3 4 5 -400 -300 -200 -100 0 100 200 300 400 θ=0.21%，第一条斜裂缝 位移角θ/% θ=0.34%，第一条竖裂缝 θ=0.46%，最大正向荷载 θ=1.23%，85%最大荷载 -90 -60 -30 0 30 60 90 位移δ/mm drift angle (r, %) displacement (mm) 30 40 50 lo ad (k n ) r=0.25%, vertical crack r=0.11%, shear crack r=0.59%, maximum load r=1.53%, safety limitation drift angle (r, %) displacement (mm) r=2.10%, safety limitation r=0.20%, vertical crack r=0.11%, shear crack r=0.36%, horizontal crack r=1.15%, maximum load drift angle (r, %) displacement (mm) r=1.23%, safety limitation r=0.34%, vertical crack r=0.21%, shear crack r=0.46%, maximum load figure 7. lateral strength and drift angle relation. 0 50 100 150 200 250 300 0 0.5 1 1.5 2 2.5 3 la te ra l s tre ng th (k n ) shear span ratio s0.8-a0.4 specimen s1.5-a0.4 specimen s2.5-a0.4 specimen figure 8. maximum lateral strength. 0 3 6 9 12 15 0 0.5 1 1.5 2 2.5 3 d uc til ity c oe ffi ci en t shear span ratio s0.8-a0.4 specimen s1.5-a0.4 specimen s2.5-a0.4 specimen figure 9. ductility coefficient. 0 δy δu δ a1 a2 a1=a2 p pmax py 0.85pmax μ = δu / δy figure 10. characteristic points of p − ∆ curve. heq = (1/4π)×(δw /we) p δ δw we figure 11. definition of equivalent damping ratio. 477 c. quan, q. huang, d. li, y. chen acta polytechnica ctu proceedings 0 4 8 12 16 20 24 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 s0.8-a0.4 s1.5-a0.4 s2.5-a0.4 drift angle (%) eq ui va le nt d am pi ng r at io (% ) figure 12. equivalent damping ratio. figure 13. concept and calculation results of diagonal strut mechanism. figure 14. concept and calculation results of sectional force equilibrium. 478 vol. 33/2022 lateral load bearing characteristic of shear walls strength for the lslc shear walls, which are showed shear failure [9]. 5.0.1. diagonal strut mechanism the lateral strength qsu for the lslc shear walls could be calculated based on the diagonal strut mechanism, as shown in equation 1, 2, and 3 [10]. here, fc is the compressive strength of lightweight concrete, weq is the equivalent strut width, t is the thickness of lslc wall, θ is the strut angle to lslc wall length, lw is the length of lslc wall, nc is the axial force of lightweight concrete, ac is the sectional area of lightweight concrete, σn is the compressive stress of lslc wall, ft is the tensile strength of lightweight concrete, respectively. as shown in figure 13, the calculation results of the lateral strength for the lslc shear wall based on the diagonal strut mechanism show good agreement with the experimental results. qsu = fc weq t cos θ (1) weq = ! 0.25 + 0.85 nc ac fc " lw (2) θ = 90◦ − cos−1 #! σn σn + 2 ft " $ 2 % (3) 5.1. sectional force equilibrium the lateral strength qsu for the lslc shear walls could be calculated based on the sectional force equilibrium, as shown in equation 4, 5 [3]. here, qc is the strength shared by lightweight concrete, qsv is the strength shared by horizontal brace, qn is the increased strength by axial force, λ is the shear span ratio of lslc wall, ft is the tensile strength of lightweight concrete, fa is the tensile strength of light gauge steel, ac is the sectional area of lightweight concrete, aah is the sectional area of horizontal brace, s is the space of horizontal brace, hw0 is the equivalent length of lslc wall, n is axial force to lslc wall, γre is the seismic adjustment coefficient (here, 0.85), respectively. as shown in figure 14, the calculation results of the lateral strength for the lslc shear wall based on the sectional force equilibrium show conservative evaluation, compared with the experimental results. qsu = qc + qn + qsv (4) qsu = # 1 λ − 0.5 (0.3 ft ac + 0.06 n) + 0.2 fa aah s hw0 %$ γre (5) 6. conclusions seismic performance of the light gauge steel and lightweight concrete shear wall was experimentally investigated under in-plane cyclic loadings. the major findings can be summarized as follows. 1. the effects of shear span ratio on the lateral load bearing capacity of the lslc shear wall were grasped quantitatively. 2. the ductility coefficient of each specimen was larger than 4, showed better deformation capacity compared with the reinforced concrete shear walls. however, the equivalent damping ratio of the lslc shear wall was calculated the values of 12 ∼ 20%, showed lower energy absorption capacity compared with the reinforced concrete shear walls. 3. the lateral strength of the lslc shear wall calculated based on the diagonal strut mechanism showed good agreement with the experimental results. however, the calculation results based on the sectional force equilibrium showed conservative value compared with the experimental results. acknowledgements the financial support of the self-funded project of china academy of building research co., ltd (for applied technology research, grant no. 20151802330730055) is gratefully appreciated. references [1] n. balh, j. dabreo, c. ong-tone, et al. design of steel sheathed cold-formed steel framed shear walls. thin-walled structures 75:76-86, 2014. https://doi.org/10.1016/j.tws.2013.10.023. [2] m. r. jones, a. mccarthy. preliminary views on the potential of foamed concrete as a structural material. magazine of concrete research 57(1):21-31, 2005. https://doi.org/10.1680/macr.2005.57.1.21. [3] ministry of housing and urban-rural development. technical specification of lightweight steel and lightweight concrete structure china, 2016. [4] chinese national standard. continuously hot-dip zinc-coated steel sheet and strip, beijing: standards press of china, 2008. [5] international standards organization. cross recessed pan head drilling screws with tapping screw thread, 1999. [6] chinese national standard (gb/t50080). standard for test method of performance on ordinary fresh concrete, beijing: architecture industrial press of china, 2001. [7] t. shimazu, y. fukuhara, t. satoh t, et al. new reinforced concrete structure japan, 2002. [8] h. tomatsuri. proceedings of the japan concrete institute 31(2):409-14. [9] china building technique group co., ltd. an experimental report of light gauge steel and lightweight concrete shear walls beijing, 2015. [10] s.-j. hwang, w.-h. fang, h.-j. lee, et al. analytical model for predicting shear strengthof squat walls. journal of structural engineering 127(1):43-50, 2001. https://doi.org/10.1061/(asce )0733-9445(2001)127:1(43). 479 https://doi.org/10.1016/j.tws.2013.10.023 https://doi.org/10.1680/macr.2005.57.1.21 https://doi.org/10.1061/(asce)0733-9445(2001)127:1(43) acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.39.0016 acta polytechnica ctu proceedings 39:16–22, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague the interaction between a driver and intelligent transport systems alica kalašová∗, kristián čulík, simona skřivánek-kubíková university of žilina, faculty of operation of transport and communications, department of road and urban transport, univerzitná 1, 010 26 žilina, slovakia ∗ corresponding author: alica.kalasova@fpedas.uniza.sk abstract. a road traffic system is a complex system which includes four main parts – human, vehicle, road, and the natural environment. road safety in slovakia is based on the strategy of the government, as well as on european policies. it is important to implement complex strategic measures in the fields of traffic accidents, emergency services, and definitions of classifying injuries and deaths to decrease the number of injuries. the national plan’s measures must be included in strategic documents, which discuss road safety. it is necessary to combine several strategic goals in the transport process, to provide transport services according to users’ needs. the increase in the safety on the roads is a process that tries to mitigate negative impacts of traffic accidents. in recent years, intelligent transport systems (its) have reduced several road traffic fatalities, particularly amongst passenger deaths. various types of intelligent transport systems (its) are currently implemented in passenger cars and provide a higher level of road safety. however, there is a need to assess the effects of these systems on traffic safety. drivers must perceive technologies as useful, effective, easy to use, affordable, and socially acceptable. there is also a need for drivers’ willingness to use its. this paper discusses the benefits of using intelligent transport systems (its) in urban conditions. they can enable the safety of road traffic. we verified it with a questionnaire. we have gathered 519 answers. the results were processed and evaluated in this article. keywords: traffic accidents, intelligent transport systems, safety of road traffic. 1. introduction intelligent transport system (abbreviation its) has been developing since the 1970s. it helps to maintain an efficient, accurate and real-time management system for humans, vehicles, and roads in the natural environment [1, 2]. intelligent transport system uses information and communication technologies as sensors, controllers, high technology devices and mathematical methods in combination with existing infrastructure [3]. the literature [4] describes how intelligent transport systems became a generic concept that covers a wide range of systems. within this context, the concept is applied to automotive systems and comprises systems generally as advanced driver assistance systems (adas/das), in-vehicle information systems, and road telematics. behavioural adaptation and risk compensation are regarded as core problems, which must be addressed in terms of safety [5, 6]. one aim of the proposed driver behaviour model is to explain and predict risk compensation that might be associated with a given its [7]. the main objectives of slovak transportation policy are to decrease traffic accidents and improve road safety. to achieve the goal for 2011-2020, the slovak republic’s national plan for road safety should generate support for transportation policy. the use of intelligent transport systems in road transportation figure 1. comparison of road fatalities per million inhabitants in eu countries [5]. is also in this proposal. a comparison of road fatalities between the slovak republic and the other eu member states is shown in figure 1 [8]. at present, the direction of european policy of gradually increasing the level of safety is defined by the strategy of the single european transport area. its central idea is global transport, which requires international cooperation for effective actions. this statement creates a significant imperative for security to [9]: • develop and harmonize technologies in the field of transport safety – driver assistance systems, coop16 https://doi.org/10.14311/app.2022.39.0016 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 39/2022 the interaction between a driver and intelligent transport systems eration systems and vehicle-infrastructure connections, etc., • develop a comprehensive strategy for action in the field of road accidents and emergency services, • focus on training and education of road users, • target vulnerable road users, including safer vehicle infrastructure and technology. one of the basic principles of the current safety strategy at the eu level is to achieve higher safety standards in the transport system – creating a safe transport area. an integrated approach used to create a secure transport area aims to integrate the security component into other eu economic and social policies. the current eu strategy and the resulting activities look for synergies with the requirements of a functioning transport system. primary topics aimed at the reduction of environmental burdens, promotion of multimodality and reduction of individual car transport shares have come to the forefront of eu transport policy. to deliver transportation services that meet consumers’ needs, it is required to integrate a number of strategic goals into the transport process. the process of making roads safer aims to lessen the detrimental effects of traffic accidents. in recent years intelligent transport systems (its) have assisted in the decrease in road traffic fatalities, particularly amongst passengers. the advantages of using intelligent transport systems (its) in urban conditions which enable secure road traffic, reduce the number of road traffic accidents and the death rate on roads are described in this article. drivers must perceive technologies as useful, effective, easy to use, affordable, and socially acceptable. 2. materials and methods recent research indicates that driver error contributes to up to 90 % of all roadway crashes. despite this, the problem of human errors is very complicated. we still do not have complete information on what mistakes drivers make and what are the causal factors that contribute to these errors [10]. intelligent transport systems (its) substantially reduce the number of crashes caused by human errors on traffic roads. such systems, however, will influence driving behaviour only if a driver accepts them. our study is aimed at the assessment of driver acceptance of different its interventions designed to enhance driver behaviour. thanks to recent developments in its technologies, autonomous vehicles are becoming a reality. leading technology companies and automotive manufacturers have invested enormous resources in autonomous car technology. however, several technical and nontechnical issues must be specified to achieve this goal: software complexity, real-time data analytics, and testing and verification are among the technical challenges; consumer stimulation, insurance management, and ethical/moral concerns are among the non-technical concerns. to address these issues, intelligent solutions must be developed that meet the needs of consumers, businesses, and government regulations and policies [11]. intelligent transport systems can enhance safety and reduce traffic accidents. this can be ensured when drivers accept and use them. acceptance of drivers can be different. it is defined as a predisposition towards using some system. the system should be reliable and easy to use [12]. at least that’s how drivers must perceive it. if people do not trust systems or they do not correspond to their needs and expectations, they will not be widely used [13]. therefore, the design must focus on the user and not only on the technology itself. the goal or motive of a person to use the device is a key element in determining its utilisation. intentions are the closest predictor of behaviour, according to [14]. a new technology’s actual use and a person’s behavioural intention to utilise it are strongly correlated, according to research [15]. in numerous significant explanatory models of human behaviour, intention is viewed as a crucial antecedent of behaviour. in the transport process, it is necessary to combine several strategic goals to provide transport services according to the users’ needs. increasing safety on the roads is a process that tries to mitigate the negative impacts of traffic accidents [16]. the application of intelligent transport systems is important for keeping sustainable development and increasing traffic safety, which is based on collecting, processing, evaluating, and distributing traffic information. the information and communication technologies create the base of transport telematics systems, which include information about the transport process and transport users. the decrease in traffic accident rates can be achieved by informing drivers and other users of transport sufficiently. the drivers’ voluntary acceptance of information is important for traffic management, using advanced traveller information systems and because of this fact, the credibility of the provided information is important. variable message signs comprise a suitable solution to traffic flow management. it is possible to provide information on queues, traffic delays and works on pavement, traffic accidents and weather conditions. it is also possible to provide information to drivers directly inside the car. with these systems, it is possible to reduce traffic accident rates and to increase drivers’ comfort during travel. we decided to survey the acceptance of intelligent technologies using questionnaires from drivers, particularly how and whether they use them. we wish to ascertain if they are understandable and easy to use. 3. results in general, slovak drivers are not informed sufficiently about the possibilities of obtaining traffic information 17 a. kalašová, k. čulík, s. skřivánek-kubíková acta polytechnica ctu proceedings figure 2. the distribution of each age and gender category. and using its. this must become known to the public. since this research aims to find out how drivers perceive the establishment and the development of intelligent transport systems, we carried out a questionnaire survey. we collected data from april to may 2021 in electronic form. the minimum sample was determined with the help of a “sample size calculator” [17] which is used to determine how many people you need to interview to get results that reflect the target population as precisely as needed. this calculator is available on the internet. we used the following parameters: • confidence level – usually 95 %, therefore we have used this typical value. • confidence interval – in statistics, it is a type of interval estimation of an unknown parameter. we have chosen 5. • population – the total number of statistical units of the basic set. in this case, it is the number of drivers registered in žilina (112,257). the result of these parameters is that 383 samples are needed at minimum. however, we were able to reach 419 respondents. from all respondents participating in the survey, there were 192 women and 327 men, so the results better reflect the target population. figure 2 shows the distribution of each age and gender category. we asked a total of 10 questions. the respondents answered mainly electronically, but we also contacted them in person. the results were evaluated and described in the following pages. the next step was to select respondents according to trip purpose. figure 3 shows that most drivers use their motor vehicles to commute to work. some also use them to travel to school and for healthcare. each driver, when choosing the route, prefers the one which best suits their needs and criteria. fuel consumption, travel time, the distance between the start and the destination, the likelihood of traffic accidents, and other factors are the primary considerations when choosing the best route. the weighted arithmetic averages of the relative weights of the various criteria, figure 3. the division of motor vehicle trips by trip purpose. figure 4. criteria evaluation for route choice. which are also represented graphically in figure 4, are shown in table 1. the standard of road infrastructure is the most crucial factor for men in both age groups. travel time to the destination is the most crucial factor for women. the values in table 1 were calculated as a weighted arithmetic average according to equation (1): c = ∑5 i=1 piri m , (1) where: c – criterion from table 1, pi – number of points given by respondents, ri – number of respondents that used i-points, i – point scale from the range 1; 2; . . . ; 5, m – total number of respondents. the rating in table 1 is as follows: the higher the number, the more significant the given factor. in the case of the traffic accident risk factor, it is obvious that men of active age perceive this factor as less important. on the contrary, they realize the importance of their time and consider the risk of traffic congestion. figure 4 better presents the results from table 1. the next question is related to the respondents’ acceptance of an alternative route suggested by information systems when a traffic accident or other obstacle occurs (figure 5). as shown in figure 6, the most common response was that respondents would 18 vol. 39/2022 the interaction between a driver and intelligent transport systems criteria men 18-26 men 27-65 women 18-26 women 27-65 fuel concumption 3.59 3.52 3.10 3.75 travel time 3.79 3.95 3.96 4.13 travelled distance 3.27 3.39 3.40 3.75 quality of road infrastructure 3.80 3.97 3.90 4.13 risk of congestion 3.48 3.66 3.42 3.50 risk of a traffic accident 3.23 2.71 3.52 3.69 table 1. weighted arithmetic averages of criteria evaluation for choosing route. figure 5. use of an alternative way offered by information system. figure 6. acceptance of alternative according to detour length. be willing to adopt an alternative route, even if the distance to their destination would be longer than 2 km (detour route 2 km longer than the original) and the travel time increases more than 16 minutes (figure 7). current traffic information, which the driver can acquire before or during the trip via information technology, is helpful while planning a trip. radio and the internet are the two most mentioned sources of traffic information, with 48 % and 24 % of respondents, respectively, using each (figure 8). figure 9 displays the proportion of women who use various systems. abbreviation vms means variable message signs. there are only shortcuts in the graph – gps includes all applications that allow navigation of the vehicle or pedestrians (google maps, sygic, etc.). we especially asked about waze as the most used navigation. the following query concerned whether or whether drivers would anticipate intelligent transport systems to alert them to potential dangers, as well as which dangers they deemed to be the most significant ones (figure 10). analysis was done on the value of notifying people about various dangerous driving scenarios from both the women’s and the men’s perspectives. according to the report, women rate blind roads as having the least importance and black ice as having figure 7. acceptance of alternative route according to travel times. figure 8. sources of information about traffic according to respondents. the greatest importance. men rank black ice and traffic accidents as being the most vital things to be wary of, while a blind road and a one-way street are considered the least significant (table 2). figure 11 displays the survey results graphically. our research confirmed that respondents are open to new possibilities offered by its. those its applications that, in addition to comfort, increase safety during the journey appear to be particularly important. 4. conclusions it is described in the literature [18, 19] that human or driver error contributes to as much as 75 % of all roadway crashes. despite this [data], there hasn’t been much research focused on the kinds of driving errors that people make. as a result, very little is now understood about the various mistakes that drivers 19 a. kalašová, k. čulík, s. skřivánek-kubíková acta polytechnica ctu proceedings males males females females traffic situations 18-26 27-65 18-26 27-65 blind junction 3.01 2.97 3.33 3.38 one-way street 2.69 2.89 3.06 3.06 changed traffic organization 3.56 3.87 2.62 4.19 sharp bend 2.99 3.08 3.35 3.13 blind road 2.35 2.71 3.69 2.75 traffic accident 4.20 4.37 4.04 4.25 congestion 3.96 4.24 3.71 4.63 black ice 4.18 4.53 4.29 4.94 foggy weather 3.17 3.61 3.83 4.69 windy weather 2.76 3.42 3.40 4.44 rainy weather 3.15 3.68 3.83 4.50 table 2. the evaluation of warnings’ importance of traffic situations. figure 9. usage of advanced traveller information systems by women. figure 10. the evaluation of the importance of warnings of dangerous traffic situations. make or the causal elements that contribute to these mistakes being made. this is because there aren’t many structured techniques for gathering data on human mistakes in the transportation system, and even when there are, there aren’t many reliable taxonomic systems for correctly categorising driver errors and their contributing causes. literature [20] on ‘cognition and reality’ describes how human thought is closely coupled with a person’s interaction with the world. it is argued in the literature that knowledge of how the world works leads to the anticipation of certain kinds of information, which in turn directs behaviour to seek out certain figure 11. dangerous traffic situations in which respondents consider warnings important. the importance rating is graded according to age and gender. kinds of information and provide a ready means of interpretation. the in-depth reported driver error study was carried out by [21], who aimed to distinguish between violations and driver errors. errors are instances in which a driver intended to perform well, but their actions fell short (for example, meaning to drive within the speed limit but inadvertently pressing the accelerator pedal too far, which is called a slip, or forgetting the speed limit, which is called a lapse, etc.). in contrast, deliberate violations, such as going over the speed limit on purpose, might be described as situations when the driver intended to commit the offence. the authors in [21] created the driver behaviour questionnaire (dbq), a 50-item survey with five categories of aberrant driving behaviour (slips, lapses, mistakes, inadvertent violations, and deliberate violations). they selected 520 drivers from a sample in nine age groups, ranging from 20 to 56 years old. drivers were asked to report how frequently they made different kinds of driving mistakes and offences. the classification of the contributing elements involved in traffic accidents was also studied in the literature [22, 23]. data on errors were gathered from accident evaluations, on-site accident investigations, and incident cases that were documented. 20 vol. 39/2022 the interaction between a driver and intelligent transport systems there were found to be four main categories of incident causation factors. these included human traits (physical/physiological, mental/emotional, experience/exposure), direct human causes (identification errors, choice errors, performance errors), environmental factors (road-related, ambient condition), and vehicular factors. when examining the psychological factors that contribute to errors, authors in the literature [24] estimated that roughly 40 % of errors were caused by attentional problems (such as carelessness, distraction, failing to see out of the car, and lack of attention), roughly 25 % by perceptual problems (such as looking but failing to see, misjudging speed and distance), and roughly 15 % by judgmental problems (e.g., lack of judgement and wrong decision). nowadays, we can use new technologies to reduce traffic accidents or mitigate their consequences. of course, the drivers must use them efficiently. these systems include adaptive cruise control, navigation systems that use gps (global positioning system) or intelligent speed adaption systems. their main advantage is that they make wider tolerance to human mistakes and errors. from the view of traffic psychology, it is necessary to examine and investigate drivers’ errors and mistakes made by other traffic users such as cyclists or pedestrians. but what is the purpose of this difficult investigation? we need to know the impact of drivers’ mistakes on the design of intelligent transport systems and on the intelligent vehicle. we can observe human errors in laboratory conditions, but focusing on accidents is essential, too. thanks to high-quality reports from police forces, it is possible to make detailed statistical analyses of traffic accidents. after this research, we can develop a generic model for driver errors. acknowledgements this paper has been developed under support of the project: mšvvš sr vega no. 1/0178/22 kalašová, a.: basic research of the sharing economy as a tool for reducing negative externalities. references [1] l. qi. research on intelligent transportation system technologies and applications. in 2008 workshop on power electronics and intelligent transportation system, pp. 529–531. 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[2021-10-25], http://www.surveysystem.com/sscalc.htm. [18] n. a. stanton, p. m. salmon. human error taxonomies applied to driving: a generic driver error taxonomy and its implications for intelligent transport systems. safety science 47(2):227–237, 2009. https://doi.org/10.1016/j.ssci.2008.03.006 [19] w. wierwille, r. hanowski, j. hankey, et al. identification of driver errors: overview and recommendations. tech report fhwa-rd-02-003, 2001. [20] a. fu. cognition and reality. science & philosophy – journal of epistemology, science and philosophy 7(2):49– 58, 2019. https://doi.org/10.23756/sp.v7i2.475 [21] j. reason, a. manstead, s. stradling, et al. errors and violations on the roads: a real distinction? ergonomics 33(10-11):1315–1332, 1990. https://doi.org/10.1080/00140139008925335 [22] j. treat, n. tumbas, s. mcdonald, et al. tri-level study of the causes of traffic accidents: final report. executive summary 1979. [23] e. nedeliaková, m. hudáková, v. štefancová. application of risk management methods in transport companies. in transport means-proceedings of the international conference, pp. 237–241. 2017. [24] i. brown. drivers’ margins of safety considered as a focus for research on error. ergonomics 33(10-11):1307–1314, 1990. https://doi.org/10.1080/00140139008925334 22 https://doi.org/10.1111/j.1540-5915.2001.tb00978.x https://doi.org/10.1111/j.1540-5915.2001.tb00978.x https://doi.org/10.1515/ttj-2015-0012 http://www.surveysystem.com/sscalc.htm https://doi.org/10.1016/j.ssci.2008.03.006 https://doi.org/10.23756/sp.v7i2.475 https://doi.org/10.1080/00140139008925335 https://doi.org/10.1080/00140139008925334 acta polytechnica ctu proceedings 39:16–22, 2022 1 introduction 2 materials and methods 3 results 4 conclusions acknowledgements references my title https://doi.org/10.14311/app.2022.36.0059 acta polytechnica ctu proceedings 36:59–65, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague cost-optimization based target reliability for fire design of insulated steel columns ranjit kumar chaudharya,∗, thomas gernayb, ruben van coilea a ghent university, department of structural engineering and building materials, technologiepark-zwijnaarde 60, 9052 ghent, belgium b johns hopkins university, department of civil and systems engineering, latrobe hall, 3400 n charles st #205, baltimore, 21218 maryland, united states ∗ corresponding author: ranjitkumar.chaudhary@ugent.be abstract. adequacy of a structural fire design can in theory be demonstrated through a probabilistic risk assessment (pra) where the compliance with tolerability limits and the alarp requirement are explicitly ascertained. however, explicit assessment of alarp requirement is challenging and impractical for day-to-day designs, due to the burden of estimating uncertain future costs and current safety investment costs. for normal design conditions, the use of target reliability indices has been recommended instead. these target reliability indices however have not been defined for structural design under fire events. to address this gap, the current study demonstrates a method to derive target reliability indices for a fire-exposed structure. as a case study, an insulated steel column (with varying levels of iso fire rating) exposed to parametric natural fires is considered. the target reliability indices are derived for the steel column for varying fire exposure scenarios, considering different fire load densities and opening factor, relating to the building occupancies. this study thus investigates the important issue of adopting target reliability indices in fire design that are cost-optimized from quantitative analyses considering natural fire exposures, which has significant implications for fire safety and rational use of resources in the construction industry. keywords: life-time cost optimization, probabilistic risk assessment, reliability, reliability indices, steel columns. 1. introduction traditionally, structures are assumed to perform adequately in the event of fire if designed based on prescriptive guidelines. these prescriptive guidelines rely on the experience of the profession, gained over time in response to the fire disasters. however, given the variety of structures and development of new materials and systems, prescriptive designs can be inefficient in certain situations and unconservative in others [1]. this observation led to an increased use of performance-based design (pbd), where the structural design is tailored to meet the specific objectives of a building. within a pbd approach, a probabilistic structural fire design can be adopted to consider explicitly the effects of the uncertainties associated with the structural system [2]. the probabilistic structural design should be compliant with the as low as reasonably practicable (alarp) requirement [3]. the evaluation of alarp requirement involves balancing the cost of safety investment and the cost of benefits associated with the safety investment [4]. yet, a project specific evaluation of these costs to assess the alarp requirement is time and resource expensive. for ambient structural design, this challenge is generally avoided by the specification of a target reliability index (βt) [5–7], which is underpinned by cost-benefit considerations. for example, en 1990:2002 [5] recommends a target reliability index of 3.8 for a reference period of 50 years (incorporated in partial safety factors) for a building with medium failure consequences. en 1990 [5] further specifies the target reliabilities considering different level of structural failure consequences. a more detailed recommendation for the target reliability can be found in [7], where target reliabilities have been proposed considering relative cost of safety measure in addition to the consequences of structural failure. van coile et al. [8] argue that these reliability targets derived for normal design cannot be directly transposed for structural design under fire. however, the cost optimization approach to derive these target safety levels remains applicable [9]. in this study, the concept of cost optimization to develop target reliabilities is revisited in the context of structural fire design. as a case study, a fire protected steel column is considered, similar to the steel beam investigations presented by [10]. the cost of the insulation material serves as the safety investment cost for fire exposure, while the structural failure cost is evaluated considering the ultimate limit state. the minimization of the life-time cost allows achieving an optimum insulation and thus determining the reliability target. the reliability target is derived considering 59 https://doi.org/10.14311/app.2022.36.0059 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en r. k. chaudhary, t. gernay, r. v. coile acta polytechnica ctu proceedings natural fire scenarios with varying fire load density and compartment opening factors. 2. reliability targets for fire exposed structures in [7, 8], the target reliabilities are derived by balancing the cost of the added safety measure and expected failure consequences. this concept of lifetime costoptimization to derive target reliabilities relies on works by [11]. the lifetime cost optimization in general is done through the maximization of the life-time utility [11, 12], y , which is given by: y = b − c − a − d = b − k (1) where, b is the benefit associated with the structure’s existence, c is the total cost of the structure, a is the obsolescence cost, and d is the damage cost associated with the structural failure. since the expected benefit b over the structural life-time is assumed independent of the safety investments, the maximization of life-time utility y reduces to the minimization of lifetime costs k. the components of k are further elaborated in equation 2. the evaluations are done considering an infinite time horizon. c = c0 + c1 a = c ω γ = (c0 + c1) ω γ (2) where, c0 is the cost of structure, c1 is the added cost of the safety measure, ω is the obsolescence rate (0.02/ year adopted as in [12]), γ is the discounting factor (0.02/year adopted for societal decision-maker). d is the structural failure cost associated with the limit state z = r − e taking negative values (where r is resistance effect and e is the load effect). the structural failure cost directly relates to the consequences of structural failure and mostly comprises cost of structural reconstruction, cost relating to loss of building functionality, societal costs related to injuries and fatalities, and cost related to environmental effects. these costs depend on many societal parameters, making the evaluation of structural failure cost challenging and subject to a significant uncertainty. the available studies generally simplify the structural failure cost by considering the different levels of structural failure consequences expressed as factor of initial structural cost (ξ c0 considered here as in [12]). this structural failure cost for an adverse event such as fire is evaluated as the expected failure cost for the entire life-time of the structure [12, 13]. equation 3 gives the failure cost during life-time structure and is evaluated as the present net value of the future expected annual failure costs over life-time (infinite time horizon). in equation 3, ξ c0 is the average structural failure cost and λ is the annual probability of fire event occurrence. e [d] = λ pf (p) γ ξ c0 (3) where, pf (p) is the structural failure probability considering the occurrence of a fire event (evaluated as pf (p) = p [r − e < 0]), p is the design parameter. on substituting the components of lifetime cost from equations 2 and 3 in equation 1, the minimization of the life-time cost can be achieved through equation 4: minp [ k = c0 (1 + ε(p)) ( 1 + ω γ ) + λ pf (p) γ ξ c0 ] (4) where, ε(p) = c1/c0 investment cost factor for the design. since, c0 is constant and thus has no effect in the minimization of k, equation 5 further results in: min p [ k = c0 ε(p) ( 1 + ω γ ) + λ pf (p) γ ξ c0 ] (5) 3. case study: fire protected steel columns a fire protected steel column is considered as a case study to derive target reliabilities for a fire exposed structure. for fire protection of the steel column, spray applied fire resistant (sfrm) material is used. the sub-sections below discuss the application of the concept of cost-optimization to derive reliability targets for the steel column. 3.1. structural model a steel column (w14 × 109) of height 5.486 m is considered. the column is from the first story of a nine-story office building designed based on us guidelines [16, 18]. the column is considered here as a reference section for further study, while the structural loads are re-calculated considering safety factors from eurocode [5]. this is done to meet the ambient design criteria as in eurocode (e.g., β of 3.70 is obtained). the characteristic permanent load on the column is estimated as 2903 kn, while the imposed load is estimated as 1244 kn (considering a load factor of 0.3 and structural buckling resistance at ambient temperature). the column is exposed to the fire from three sides (i.e., assuming a peripheral column with wall along one of the flanges). the thermal properties for the steel are taken from [19]. the steel yield strength (characteristic) is 345 mpa with temperature-dependent retention factors for yield strength and elastic modulus from the probabilistic model by [15]. the temperature-dependent thermal properties of sfrm can be found in table 1 and are in accordance with those adopted in [18]. the temperature distribution is assumed to be uniform in the steel cross-section. it is determined based on the lumped mass model from [19]. the column is assumed to be in pure compression and is not susceptible to local buckling. the global buckling resistance thus governs the compression resistance of such column and is calculated as: 60 vol. 36/2022 cost-optimization of fire design of insulated steel columns (a) (b) figure 1. (a) cost of sfrm insulation of steel column. (b) relationship between sfrm insulation thickness and standard fire-rating of the steel column (based on iso 834 criteria) stochastic variables distribution mean cov material properties, sfrm [14] thickness, di lognormal nominal + 1.6 mm 0.2 quantile parameter, e normal standard normal distribution (µ = 0 and σ = 1) density, ρi ρi = exp(−2.028 + 7.83 t −0.0065 + 0.122e) specific heat, ci ci = 1700 − exp(6.81 − 1.61.10−3t + 0.44.10−6 t 2 + 0.213e) conductivity, ki ki = exp(−2.72 + 1.89.10−3t − 0.195.10−6 t 2 + 0.209e) material properties, steel [15] retention factor for steel yield strength, kfy logistic model temperaturedependent temperaturedependent mechanical loading [16] permanent load normal 2903 kn 0.1 imposed load gamma 0.2 × 1244 kn 0.95 fire loading [17] fire load density, qf (en 1991-1-2:2002) gumbel nominal (100 − 1600 mj/m2) 0.3 opening factor, o normal nominal (0.04 − 0.20 m1/2) 0.1 model uncertainties [16] resistance estimation, kr lognormal 1.1 0.1 load estimation, ke lognormal 1.0 0.1 table 1. stochastic parameters for the probabilistic analysis of steel columns. nbr = κt acol fy,t (6) where, κt is the temperature dependent buckling resistance factor, acol is the cross-sectional area of column and fy,t = ky,t fy is temperature-dependent yield strength of steel, in which ky,t is the yield strength retention factor. the buckling resistance factor, κt is evaluated based on en 1993-1-2:2005. 3.2. life-time cost optimization the life-time cost optimization for the steel column involves determining the cost of the sfrm 61 r. k. chaudhary, t. gernay, r. v. coile acta polytechnica ctu proceedings protection and the cost of structural failure for the fire event. the cost optimization is carried out considering sfrm thickness, di varying between 6-60 mm. a factor of ξ = 3.0 is considered for the relative cost of structural failure due to fire event (indicates minor failure consequence as reported in [13]. the reliability targets are later derived in section 4 for failure costs with both a factor of 3 and 20 to assess the sensitivity to this assumption. investment cost the investment cost is the cost of sfrm protection of the steel column. it is obtained from the rsmeans (2019) database [20], which is a database comprising cost of structural building materials. sfrm costs $132 / inch /m2. figure 1(a) plots the cost of sfrm application for the steel column as a function of the thickness, which is the determining factor for fire performance. based on rsmeans (2019), the cost of the steel column is determined as $3618. the cost of the steel column represents the structural cost, co. figure 1(a) also shows the marginal safety investment cost, ε(p), which can directly be implemented in equation 5 for cost optimization. commonly, prescriptive guidelines report the fire protection of steel members in terms of standard fire-rating (e.g., iso834, astm e119). figure 1(b) plots the iso834 fire-rating for the considered thicknesses of sfrm protection, where the critical temperature is determined based on the column’s buckling resistance (here, tb,cr = 536◦c as defined by the accidental limit state for fire design). failure cost the failure cost in equation 3 involves determining the failure probability, pf , of the steel column, considering occurrence of a fire event. the limit state for the column here relates to the exceedance of buckling resistance (assuming the column is in pure compression). the failure probability of the column is represented as: pf = p [kr nbr − ke ne < 0] (7) where, nbr is the column’s buckling resistance, ne is the load on column and kr and ke are the model uncertainties for resistance and load estimation. the estimation of nbr for fire exposure scenario comprises two steps: (i) calculating the maximum temperature reached in the steel section under the considered fire scenario, and (ii) estimating the buckling resistance for this maximum temperature using equation 6. table 1 lists the stochastic parameters identified for the probabilistic analysis of the steel column. the failure probability is estimated considering di of sfrm varying from 6 − 60 mm at increment of 1 mm. the application of sfrm with di < 6 mm is assumed to be difficult in application. thus, the cost optimization approach evaluates an optimum value of di within the range of 6 − 60 mm. the eurocode parametric fire curve is adopted here to define the fire scenarios for the steel column [17]. a wide range of fire scenarios is considered by varying the fire load density (100-1600 mj/m2) and the compartment opening factor (0.04, 0.08, 0.12,0.16 and 0.20 m1/2). the target reliabilities for fire design are then derived for each of these fire scenarios. the considered fire load densities can be related to the building occupancies as discussed in annex e of [17]. considering 80 cases of fire scenarios and 54 cases of di, the probabilistic analysis involving 105 monte carlo (mc) simulations each to determine the maximum steel temperatures is computationally demanding (approximately 120,000 core hours needed). to reduce the computational expense, a neural network (nn) based surrogate model is adopted [21]. the optimum nn model has 4 hidden layers with 300 neurons each and has a r2 of 0.99 (3000 samples used for training and 500 as test data set). to carry out the probabilistic analysis, 105 mc samples are applied. nn model gives the maximum temperature of the steel section, while equation 6 evaluates the buckling resistance of the column for the samples. since the model to evaluate the mechanical response is less computationally expensive than that for the thermal response, 108 mc samples are developed for the load and model uncertainties. a higher number of samples allows developing more precise fragility curves. figure 2 presents a sample of fragility curves for the steel column for the considered scenarios of parametric fire exposures. the opening factor is selected as the intensity measure. for illustration, only the cases with fire load density of 500 and 1500 mj/m2 are presented. in the figure, the pf is 0.12 for a steel column with 10 mm sfrm in case of fire with a fire load of 500 mj/m2 and opening factor of 0.04 m1/2. this pf is reduced to 1.7 × 10−4 with 60 mm sfrm. similarly, the pf is reduced from 0.99 to 0.10 for fire load density of 1500 mj/m2 as shown in figure 2b. thus, sfrm is observed to decrease the failure probability of steel column significantly for given fire inputs. failure costs are evaluated from the computed probability of failure using equation 3. life-time cost figure 3 demonstrates the life-time cost optimization procedure to determine the optimum insulation thickness of the steel column for a particular case of fire exposure (qf = 800 mj/m2 and o = 0.04m1/2). the annual probability of fire occurrence is assumed to be 2.5 × 10−3. this value corresponds to the mean yearly ignition frequency in office buildings (in finland estimated for year 1995 as studied by [22]). this fire frequency (λ) is adopted considering that building relies only on insulation thickness for fire safety and that there are no fire suppression systems or nearby fire brigades. this value can be considered high and is thus considered to result in a high estimation of the 62 vol. 36/2022 cost-optimization of fire design of insulated steel columns figure 2. fragility curves for steel columns with varying sfrm thickness under parametric fire exposure. figure 3. investment, failure and life-time cost for a specific fire scenario case of steel column (qf = 800 mj/m2 and o = 0.04 m1/2). optimum sfrm thickness, di,opt of 32 mm obtained for the case. benefit of fire protection. in absence of data on the failure cost factor for fire hazard, a factor of 3.0 representing the minor failure consequences for ambient structural design [6, 7] has been considered here. for this case, an optimum insulation thickness of 32 mm is obtained. figure 4(a) shows the optimum thicknesses of insulation determined based on life-time cost optimization for all the considered cases of parametric fire exposure. the optimum sfrm thickness required for a mean fire load of 420 mj/m2 (average value for office buildings) can be interpolated from the figure and is found to be 8 mm for o of 0.04 m1/2. a 6 mm insulation is the lowest practical sfrm insulation as mentioned above. however, the optimum di in case of higher consequences of structural failure (ξ > 3.0) will be higher. similarly, if the building is used as residential (i.e., qf = 780 mj/m2), the optimum insulation thickness required is 31 mm for an opening factor of 0.04 m1/2. based on figure 1(b), the sfrm thickness of 31 mm corresponds to 158 min of iso fire rating. in other words, the optimum fire protection for steel column in a residential building needs to be designed for a fire rating of 158 min (practical values are 120 min and 180 min). the cost optimization approach thus enable recommending an optimum design parameter, demonstrating a cost-effective and a reliable structural design. figure 4b shows the optimum failure probability corresponding to the optimum insulation thicknesses of the steel column, as a function of the fire load density. as the fire load density increases, it is more cost-efficient to allow a higher probability of failure should a fire occur, compared with occupancies with lower fire load densities. indeed, it is costly to reduce pf in buildings with large fire loads, while it is assumed here that neither the probability of occurrence of a fire nor the cost of the failure depend on the fire load. the evaluation of optimum failure probability allows further generalization of the concept of cost optimization. in the figure 4b, the optimum failure probability for the steel column considering office building with compartment opening factor of 0.04 m1/2 is 0.053. the optimum failure probability can easily be interpolated for all other parametric 63 r. k. chaudhary, t. gernay, r. v. coile acta polytechnica ctu proceedings figure 4. optimum (a) sfrm thicknesses and (b) failure probability for considered cases of fire scenarios of steel column. figure 5. reliability targets for steel column for parametric fire exposure of steel column (constant annual fire probability of 2.5 × 10−3 and, two cases of relative failure costs, ξ = 3.0 and 20.0 considered). natural fire scenarios. 4. reliability targets for fire protected steel columns the optimum failure probability in figure 4(b) can be translated into optimum reliability index by evaluating the inverse standard normal cumulative density function (φ) and is given by: β = −φ−1(pf ) (8) these evaluated optimum reliability indices represent the target reliabilities for the structure. in this study, the target reliabilities are developed for natural fire exposure of steel column. figure 5 presents the target reliabilities for the steel column for natural fire exposure (ξ = 3.0 and 20.0). the target fire reliability for steel column (part of office building i.e., qf = 420 mj/m2) is 1.65, 2.15, 2.83, 3.36, and 3.53 for compartment opening factors of 0.04, 0.08, 0.16, 0.20 m1/2, respectively, considering ξ = 3.0. for the structure considering higher relative failure cost, i.e., ξ = 20.0 the target reliabilities are 2.47, 2.64, 2.96, 3.36 and 3.48. similarly, the target reliabilities for other possible fire scenarios can be obtained from figure 5. the benefit of deriving such target reliability is the possibility of generalizing the reliability value as a target for similar design situations. the structural fire design based on these derived target reliabilities allows a prompt and economical structural design without a need of re-applying the cost-optimization approach developed in this study. the target reliabilities however depends on the considered structural models and the probabilistic modeling assumptions, which one should be cautious about while reapplying for structural fire designs. this will be considered as part of follow up research targeting the generalization described above. 5. conclusions in this study, reliability targets based on life-time cost optimizations are derived for fire-exposed steel columns. in the first part of the study, optimum thickness of insulation are determined through life-time cost optimization as a function of the fire inputs, which are found in standards and codes based on building 64 vol. 36/2022 cost-optimization of fire design of insulated steel columns occupancies. for example, considering a mean fire load of 420 mj/m2 as recommended in eurocode for office buildings and an opening factor of 0.04 m1/2, an optimum insulation thickness of 8 mm is found. the methodology proposed in the first part of the paper can serve to evaluate the optimum of a design parameter (e.g., insulation thickness) for a specific project. however, application of such methodology is inconvenient and time consuming for practical applications. therefore, in the second part, the study describes how to evaluate general reliability targets based on the obtained optimum values of the design parameter. design based on such pre-derived reliability targets for a fire-exposed structure allows a prompt, economical, and reliable structural fire design. the method was illustrated on the case study of a protected steel column, but the method will be applied in the future to other and more general configurations. the study thus constitutes a next step toward proposing target reliability values for the fire design of structures. references [1] g. spinardi, l. bisby, j. torero. a review of sociological issues in fire safety regulation. fire technology 53(3):1011-37, 2016. https://doi.org/10.1007/s10694-016-0615-1. [2] r. van coile, d. hopkin, d. lange, et al. the need for hierarchies of acceptance criteria for probabilistic risk assessments in fire engineering. fire technology 55(4):1111-46, 2018. https://doi.org/10.1007/s10694-018-0746-7. [3] m. jones-lee, t. aven. alarp what does it really mean? reliability engineering & system safety 96(8):877-82, 2011. https://doi.org/10.1016/j.ress.2011.02.006. [4] bsi. pd 7974-7:2003. application of fire safety engineering principles to the design of buildings part 7: probabilistic risk assessment. british standards institution. https://doi.org/10.3403/30362214u. [5] en 1990:2002. eurocode basis of structural design. european standard. [6] jcss. 2001. probabilistic model code: part 1 basis of design. the joint committee on structural safety. [7] iso 2394:2015. general principles on reliability of structures. international standard. [8] r. van coile, d. hopkin, l. bisby, r. caspeele. the meaning of beta: background and applicability of the target reliability index for normal conditions to structural fire engineering. procedia engineering, 210:528-536, 2017. https://doi.org/10.1016/j.proeng.2017.11.110. [9] k. fischer. societal decision-making for optimal fire safety. bericht ibk, 357, 2014. [10] d. hopkin, i. fu, r. van coile. adequate fire safety for structural steel elements based upon life-time cost optimization. fire safety journal 120, 2021. https://doi.org/10.1016/j.firesaf.2020.103095. [11] r. rackwitz. optimization the basis of code-making and reliability verification. structural safety 22(1):27-60, 2000. https://doi.org/10.1016/s0167-4730(99)00037-5. [12] r. van coile, t. gernay, d. hopkin, n. elhami khorasani, n. resilience targets for structural fire design: an exploratory study. in 17th international probabilistic workshop 2019, edinburgh, uk, 2019. [13] r. d. j. m. steenbergen, m. sýkora, d. diamantidis, et al. economic and human safety reliability levels for existing structures. structural concrete 16(3):323-32, 2015. https://doi.org/10.1002/suco.201500022. [14] n. e. khorasani, p. gardoni, m. garlock. probabilistic fire analysis: material models and evaluation of steel structural members. journal of structural engineering 141(12), 2015. https: //doi.org/10.1061/(asce)st.1943-541x.0001285. [15] r. qureshi, s. ni, n. elhami khorasani, et al. probabilistic models for temperature-dependent strength of steel and concrete. journal of structural engineering 146(6), 2020. https: //doi.org/10.1061/(asce)st.1943-541x.0002621. [16] b. jovanovic, r. van coile, d. hopkin, et al. review of current practice in probabilistic structural fire engineering: permanent and live load modelling. fire technology 57(1):1-30, 2020. https://doi.org/10.1007/s10694-020-01005-w. [17] en 1991-1-2:2002. eurocode 1: actions on structures. general actions-actions on structures exposed to fire. european standard, 2002. [18] t. gernay, n. elhami khorasani, m. garlock. fire fragility curves for steel buildings in a community context: a methodology. engineering structures 113:259-76, 2016. https: //doi.org/10.1016/j.engstruct.2016.01.043. [19] en 1993-1-2. eurocode 3: design of steel structures-part 1-2: general rules-structural fire design. european standard, 2005. [20] gordon group inc. square foot costs with rsmeans data 2019. 40th ed. rockland: rsmeans co, 2018. [21] r. k. chaudhary, r. van coile, t. gernay. potential of surrogate modelling for probabilistic fire analysis of structures. fire technology 57(6):3151-77, 2021. https://doi.org/10.1007/s10694-021-01126-w. [22] j. rahikainen, o. keski-rahkonen. statistical determination of ignition frequency of structural fires in different premises in finland. fire technology 40(4):335-53, 2004. https://doi.org/10.1023/b: fire.0000039162.63876.03. 65 https://doi.org/10.1007/s10694-016-0615-1 https://doi.org/10.1007/s10694-018-0746-7 https://doi.org/10.1016/j.ress.2011.02.006 https://doi.org/10.3403/30362214u ttps://doi.org/10.1016/j.proeng.2017.11.110 https://doi.org/10.1016/j.firesaf.2020.103095 https://doi.org/10.1016/s0167-4730(99)00037-5 https://doi.org/10.1002/suco.201500022 https://doi.org/10.1061/(asce)st.1943-541x.0001285 https://doi.org/10.1061/(asce)st.1943-541x.0001285 https://doi.org/10.1061/(asce)st.1943-541x.0002621 https://doi.org/10.1061/(asce)st.1943-541x.0002621 https://doi.org/10.1007/s10694-020-01005-w https://doi.org/10.1016/j.engstruct.2016.01.043 https://doi.org/10.1016/j.engstruct.2016.01.043 https://doi.org/10.1007/s10694-021-01126-w https://doi.org/10.1023/b:fire.0000039162.63876.03 https://doi.org/10.1023/b:fire.0000039162.63876.03 acta polytechnica ctu proceedings 36:59–65, 2022 1 introduction 2 reliability targets for fire exposed structures 3 case study: fire protected steel columns 3.1 structural model 3.2 life-time cost optimization 4 reliability targets for fire protected steel columns 5 conclusions references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.38.0210 acta polytechnica ctu proceedings 38:210–215, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague mold growth on spruce and pine samples in steady state conditions: time of germination and maximum coverage pavla ryparováa,∗, kamil staněka, jan richterb, pavel kopeckýb a czech technical university in prague, faculty of civil engineering, department of building structures, thákurova 7, 166 29 prague 6, czech republic b czech technical university in prague, university centre for energy efficient buildings, třinecká 1024, 273 43 buštěhrad, czech republic ∗ corresponding author: pavla.ryparova@cvut.cz abstract. wood is a material that is environmentally friendly and has minimal carbon impact. the durability of wood depends on ambient environmental conditions. this article deals with mold growth on wood samples under constant relative humidity (75 %; 87 %; 95 % rh) and temperature (23 °c). pine and spruce samples with surfaces oriented along the three principal anatomical directions of wood were prepared. the occurrence of mold was studied by regular microscopic observations. the mold consortium was a mixture of penicillium sp., aspergillus sp., and alternaria sp. the first signs of mold growth on pine were observed after 7 days for 87 % rh and 12 days for 95 % rh, regardless the surface orientation. molds on spruce began to grow in 12 to 26 days at 95 % rh in dependence on surface orientation: at first on tangential surface, then on transversal surface, and finally on radial surface. the lower relative humidity of 87 % resulted in a shorter germination time: 7 days for tangential and radial surfaces, and 21 days for transversal surface. keywords: wood, mold, germination, spruce, pine, penicillium sp., aspergillus sp., alternaria sp. 1. introduction timber construction elements become more common and popular in building sector. people in many cases prefer the use organic materials instead of more energy and carbon intensive mineral-based materials. they also appreciate the benefits of fast and variable construction of wooden houses. wood is composed of two major elements: lignin (18–35 %) and carbohydrates (cellulose and hemicellulose, 65–75 %). the cellulose microfibrils are organized in a matrix of amorphous hemicellulose and lignin [1]. the chemical composition of wood is approximately 50 % carbon, 44 % oxygen, and 6 % hydrogen [2]. the chemical composition predetermines the mold risk of wooden material. another factor is the nutrient content that is related to a wood specie [3]. typical mold fungi found in moisture-damaged wood are alternaria alternata, aspergillus sp., aureobasidium pullulans, cladosporium cladosporioides, chaetomium globosum, paecilomyces variotii, penicillium sp., and trichoderma viride [4]. molds on the surface of a construction element cause its aesthetic degradation. it may also allow colonization of other biodegrading agents, such as bacteria and wood-decaying fungi, and cause human health problems. the occurrence of mold also points to moisture-related problems within the construction. the live cycle of fungi consists of three main stages: germination of spores, growth, and sporulation of hyphae. it necessitates the availability of nutrients, water, and the right temperature. nutrients are usually available by minor contamination of the surface by dust or other pollutants. the optimum relative humidity for mold growth is in the range of 75–98 % rh and the optimum temperature is between 10 and 40 °c, depending on the mold specie. humidity conditions suitable for mold growth can also be expressed on the basis of moisture content of the substrate material. moisture content is linked to the surrounding relative humidity via the sorption (hygroscopic range), but it is also able to reflect the presence liquid water (over-hygroscopic range), and the moisture history of the substrate material. water can be present in wood in three forms: as a bound water within the cell walls (hydrogen bonding with accessible hydroxyl groups), and as a water vapor or liquid water in the pore system (lumina and vessels). under steady state conditions (constant temperature and relative humidity), and in the absence of liquid water, an equilibrium occurs between the bound water and water vapor [5]. a set of these equilibrium states forms a sorption isotherm. hygrothermal equilibrium with ambient air is not always achieved in reality of building structures. therefore, humidity conditions suitable for mold growth should be better expressed by moisture content of the substrate material instead of relative humidity of ambient air. this paper presents the first results of the project on fungal growth on wood exposed to changing boundary conditions. as a starting point, this paper presents and discusses the results of steady-state mold growth 210 https://doi.org/10.14311/app.2022.38.0210 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 38/2022 mold growth on spruce and pine samples in steady state conditions . . . figure 1. the description of experiment procedure. experiments. the germination time and time for maximum mold coverage are presented for three levels of relative humidity. wood samples prepared with respect to the principal anatomical directions were used. 2. methods and materials the procedure is described in figure 1. the procedure followed the steps: sample preparation, moisture equalization, inoculate preparation, inoculation, and fungal growth observation. 2.1. preparation of samples the samples were prepared from spruce and pine wood of commercial origin and without additional biocidal treatment. the samples (15 × 15 × 3 mm3) were cut so that the surfaces were oriented along the principal anatomical directions of wood. the samples were labeled using the first letter of wood species (spruce – s; pine – p), and the letter representing the observed surface (transverse surface – x; radial surface – r; tangential surface – t), see examples in figure 1. the particular types of samples were prepared in three replications. the samples were sterilized by uv radiation – each surface was exposed to uv for 30 min. the desiccators were sterilized with ethanol, glutaraldehyde, and uv radiation. the sterile samples were placed in a sterile desiccator with the defined rh created by saturated salt solutions (75 % by nano3, 87 % by na2so4, 95 % by kno3). the samples were kept there for 14 days to reach the equilibrium moisture content. prior to the sterilization and conditioning, the samples were stored in a stable environment with rh lower than 60 %. 2.2. preparation of inoculum the inoculum fungi were selected from common occurrence genera in the czech republic. the most common molds identified in the czech republic are alternaria sp., cladosporium sp., aureobasidium sp., penicillium sp., fusarium sp., and aspergillus sp. [6]. the particular molds chosen for the experiments were aspergillus niger (ccm 8155), penicillium brevicompactum (ccm 8040), and alternaria alternata (ccm f-397). these genera were selected because of their ability to grow on wood samples, good sporulation, and easy microscopy observation. the molds gained from czech collection of microorganism were cultivated triple on czapek dox agar at 23 ± 2 °c for seven days. the spores were collected by the following procedure: the agar plate, which was covered with sporangia mold, was poured over with 1 ml of sterile saline buffer. the spores were loosened with a sterile glass rod. the solution with spores and debris of hyphae was taken by sterile pipette and filtrated by sterile gauze with a suitable mesh size. the particular mold genera were collected separately and the spore concentrations were calculated by buchner chamber under a microscope with magnification from 10× to 40×. the suspension of the particular mold spores was adjusted to 1.1 · 105 cfu/ml. the inoculum was prepared as a mixture of particular spores in the ratio (1 : 1 : 0.5; penicillium purpurogenum : alternaria alternata : aspergillus niger). 211 p. ryparová, k. staněk, j. richter, p. kopecký acta polytechnica ctu proceedings figure 2. first sign of mold growth and maximal coverage. label p-x p-t p-r rh [%] 75 87 95 75 87 95 75 87 95 first sign [day] 7 12 7 12 7 12 max coverage [day] 26 34 46 34 68 21 table 1. time in days required for the first signs of mold growth and the maximum coverage of pine samples. 2.3. inoculation of samples and observations after initial 14-day long exposure with the selected rh, the samples were inoculated by spraying a volume of 200 µl containing the mixture of molds as prepared by the procedure described in section 2.2. the inoculated samples were placed in sterile desiccators maintaining 75 %, 87 %, and 95 % rh at 23 °c. the mold growth was observed by olympus bx41 microscope with the magnification of 40× to 400×. at the beginning of the experiment the observation was performed daily. the observation period was extended to 3 and 7 days after 14 days from the beginning. two significant states were defined and times of their achieved since the beginning of the experiment (inoculation) were monitored: the first sign – is the time when hyphae or sporangium was first observed under microscope with 100× magnification maximum coverage – time since which no more molds have grown for one month. one month was set as an opportunity to observe the germination of spores newly formed during the experiment; see figure 2. the total duration of the experiment was 3 months, and subsequently, the experiment had been repeated twice to obtain more dates. 2.4. the determination of germination on agar the control of viability and size of hyphae and sporangium was made on agar by follow procedure. the agar samples were made on a microscope slide following the procedure. the nutrient agar was made from a commercial dry mixture (czapek dox agar, sigma-aldrich, usa) according to their protocol on petri dishes. nutrient agar was cut with a sterile scalpel to size approximately 74 mm × 50 mm and placed on a microscope slide (76 mm × 52 mm). the inoculate solution was transferred to the agar surface by spraying (twice) to obtain uniform coverage of the surface by mold spores. the particular inoculate mold suspension was used in two concentrations. the “plus” concentration was 1.1 · 105 cfu/ml as described in section 2.2 and the “minus” concentration was diluted in a 1 : 1 ratio with sterile water. the mold mixture suspension was not used, we used only particular species (penicillium purpurogenum, aspergillus niger). the inoculated agar samples were placed in humid conditions above water (rh close to 100 %) at 23 ± 1 °c for 14 days. 3. results 3.1. the preparation samples the samples were made by the procedure described above and named by wood species, type surface, and surrounding rh. control of uv sterilization was performed by agar test. the wood specimens placed an on agar plate (czapek dox agar) for 5 days and there was no evidence of mold growth under a microscope with magnification 40–100×. 3.2. germination and grow on wooden samples 3.2.1. pine samples the environment with 75 % rh was not sufficient for mold growth. there was no evidence of hyphae or sporangia. the environments with 87 % rh and 95 % rh led to mold growth, see table 1. for all the samples, the first signs of mold growth were visible after 7 days in the environment with 87 % rh. the environment with 95 % rh prolonged the onset of 212 vol. 38/2022 mold growth on spruce and pine samples in steady state conditions . . . figure 3. the microscopic images with magnification 100× after 18 h; the first row aspergillus plus and aspergillus minus, the second row penicillium plus and penicillium minus. label s-x s-t s-r rh [%] 75 87 95 75 87 95 75 87 95 first sign [day] 21 15 7 12 7 26 max coverage [day] 54 61 21 34 47 34 table 2. time in days required for the first signs of mold growth and the maximum coverage of spruce samples. mold growth to 12 days. samples p-r-95 reached the maximum coverage in the shortest time. samples px-87 needed 26 days, the samples p-x-95 and p-t-95 both needed 34 days. the p-t-87 achieved maximal coverage on the 46th day, whereas p-r-87 needed 68 days. 3.2.2. spruce samples the environment with 75 % rh was not sufficient for mold growth. there was no evidence of hyphae or sporangia. the environments with 87 % rh and 95 % rh led to mold growth, see table 2. samples s-t-87 and s-r-87 showed the first signs of mold growth after 7 days in the environment with 87 % rh. samples s-t-95 showed the first signs of mold grow after 12 days. samples s-x-95 showed the first signs of mold grows after 15 days. samples s-x-87 needed the longest time for the first molds (21 days). samples s-t-87 reached maximum coverage in the shortest time (21 days). samples s-t-95 and s-r-95 needed 34 days, whereas samples s-r-87 needed 47 days. s-x-87 achieved maximum coverage after 54 days and s-x-95 after 61 days. 3.3. germination on agar samples aspergillus niger formed visible sporangia on agar after 18 h. samples with a higher initial spore concentration showed a higher frequency of sporangia than samples with a lower initial concentration. penicillium purpurogenum formed visible hyphae. hyphal coverage was found to be greater in samples with higher initial spore concentration (figure 3). 4. discussion the samples were made from pine and spruce and the sterilization was proceeded by uv radiation to minimization of structure change in wood. the method of sterilization may has influence to mold growth by damaging of wood structure or releasing of nutrient [7], we used the sterilization with the least known influence on chemical and structural properties of wood. the control experiment showed that used sterilization time is sufficient for the killing of all debris of microorganisms, the same for spores of bacteria and mold [8–10]. the further input key factor is the compose of mold inoculum. in terms of species representation, life stage (spores, hyphae) and concentration of individual molds. the mold species were chosen due on literature revue [11–13] and based on previous projects in the laboratory [14–16]. fungal growth was monitored between 75 and 95 % rh, which is the most commonly reported humidity for fungal growth in the literature [12, 17–20] under constant optimal temperature (23 °c). the samples 213 p. ryparová, k. staněk, j. richter, p. kopecký acta polytechnica ctu proceedings were kept in particular humidity for achieved moisture balance for 14 days before main experiment. the samples had equal mount of moisture content after equalization phase regardless of origin wood species and principal anatomical directions. growth of the genera penicillium sp. and aspergillus sp. was observed on wood samples. there was no evidence of alternaria sp. germination in our experiments for 3 months. the experiments used clean wood without pollutants or dust and it could be an unsuitable nutrient for growing alternaria sp. although this genus is known for them occurring on cellulose-based materials [1, 2, 18] wood materials may not be their nutrient. the literature lacks of data on the dependence of mold growth on principal anatomical directions. and generally there is little information about the mold growth for spruce. the time of first sign of mold growth is dependent on visibility or/and observability hyphae and sporangium. the observation of hyphae on wood samples is almost very difficult or even impossible for colourless and small size of hyphae. therefore, we called this stage the first sign of mold growth instead of mold germination. this values is quite clear and carries only a small error of one or two days at most. the coverage rate as a mold index [21] is determined by the concentration of spores used for inoculation, which is the term in the mold growth model. for our research, the term time to reach maximum coverage is the time from which no further fungal growth increased. it is not a total coverage of the sample. determining the exact time from when no more growth occurred is very problematic because it is a subjective assessment. the degree of uncertainty in determining this value is on the order of a week. the tendency of first signs of mold growth is more dependence on used rh then on principal anatomical directions and wood species besides spruce samples with traversal direction. johansson et al. [11] reported that the start of mold growth on pine is around one week for 85 % rh and two weeks for 95 % rh. this corresponds to our results which showed mold growth on the 7th day for 87 % rh and 12th day for 95 % rh for all principal anatomic direction. in our experiment, we observed a faster onset of mold growth then johansson [11] and it can be caused by the higher used humidity, smaller samples, and aspergillus sp. used in the inoculum. the onset of mold growth on spruce had same tendency for tangential and radial principal anatomic direction as pine except that for the radial direction at 95 % rh the growth was observed later. the traversal direction for spruce showed other results. this may be due to the sporangium starting to grow later or being hidden between the filaments for a longer period of time. we determined that the pine samples achieved maximal coverage at 21st (p-r-95) to 34th (p-x-95 and p-t-95) day for 95 % rh. our results are shorter than reported johansson el al. [11]. the lower rh (87 % rh) increased time required to achieved maximal coverage on pinewood. it is 26th day for p-x-87; 46th day. it may be due that the researchers made mainly experiments with samples which had similar principal anatomic directions as p-t, for the simplest preparation of a smooth sample surface. other reasons may be the higher humidity we used, the smaller sample size and the aspergillus sp. used in the inoculum. for spruce wood, the time required to achieve maximum coverage varied widely across the principal anatomic direction. the samples with the tangential and radial directions have similar leaf ring densities we assumed that new spores would be released from the sporangia and new fungi would grow from them and therefore the coverage rate would increase in waves, but this assumption was not confirmed. it seems that the newly formed spores did not come into contact with the wood, i.e. the nutrient soil. 5. conclusion this paper presents results from an experiment that tracked mold growth on spruce and pine in relation to principle anatomical directions. our assumption that the growth tendency would be similar for the same directions in both tree species was not confirmed. the apparent onset of fungal growth in pine depends only on ambient rh and in spruce depends not only on rh used but also on principle anatomical directions. determining the time required for maximum coverage is very problematic and its determination is burdened with large errors. in order to be more accurate, it would be necessary to increase the number of samples on which mold growth wood be recorded on the whole sample with a resolution of at least 40 times every three days. and then to do an image analysis. acknowledgements this work was supported by the grant 20-12941s the grant agency of czech republic and we are extremely grateful to our technical worker ivana loušová a petra šutová. references [1] a.-m. olsson, l. salmén. the effect of lignin composition on the viscoelastic properties of wood. nordic pulp & paper research journal 12(3):140–144, 1997. https://doi.org/10.3183/npprj-1997-12-03p140-144 [2] r. c. pettersen. the chemistry of solid wood, chap. 2 the chemical composition of wood, pp. 57–126. american chemical society, washington, 1984. https://doi.org/10.1021/ba-1984-0207.ch002 [3] r. g. de paiva, m. a. de moraes, f. c. de godoi, m. m. beppu. multilayer biopolymer membranes containing copper for antibacterial applications. journal of applied polymer science 126(s1):e17–e24, 2012. https://doi.org/10.1002/app.36666 [4] a. hukka, h. a. viitanen. a mathematical model of mould growth on wooden material. wood science and 214 https://doi.org/10.3183/npprj-1997-12-03-p140-144 https://doi.org/10.3183/npprj-1997-12-03-p140-144 https://doi.org/10.1021/ba-1984-0207.ch002 https://doi.org/10.1002/app.36666 vol. 38/2022 mold growth on spruce and pine samples in steady state conditions . . . technology 33(6):475–485, 1999. https://doi.org/10.1007/s002260050131 [5] p. a. van meel, s. j. f. erich, h. p. huinink, et al. moisture transport in coated wood. progress in organic coatings 72(4):686–694, 2011. https://doi.org/10.1016/j.porgcoat.2011.07.011 [6] p. ryparová. stanovení antimikrobiálního charakteru materiálů s ohledem na nové trendy ve stavebnictví. ph.d. thesis, fce ctu, prague, 2017. 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https://doi.org/10.7567/jjap.53.05fr03 https://doi.org/10.6090/jarq.40.157 https://doi.org/10.1016/s0022-0248(02)00918-1 https://doi.org/10.1016/j.ibiod.2012.05.014 https://doi.org/10.1016/j.jobe.2021.102301 https://doi.org/10.1016/j.ibiod.2013.08.004 http://www.cesb.cz/cesb13/proceedings/3_materials/cesb13_1294.pdf http://www.cesb.cz/cesb13/proceedings/3_materials/cesb13_1294.pdf https://doi.org/10.4028/www.scientific.net/amr.649.89 https://doi.org/10.4028/www.scientific.net/amr.649.89 https://doi.org/10.1016/j.proeng.2016.07.402 https://doi.org/10.1128/am.1.6.287-293.1953 https://doi.org/10.1016/j.ibiod.2004.05.002 https://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.524.9841&rep=rep1&type=pdf https://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.524.9841&rep=rep1&type=pdf https://doi.org/10.1016/j.buildenv.2010.01.023 https://doi.org/10.1007/s00107-010-0450-x acta polytechnica ctu proceedings 38:210–215, 2022 1 introduction 2 methods and materials 2.1 preparation of samples 2.2 preparation of inoculum 2.3 inoculation of samples and observations 2.4 the determination of germination on agar 3 results 3.1 the preparation samples 3.2 germination and grow on wooden samples 3.2.1 pine samples 3.2.2 spruce samples 3.3 germination on agar samples 4 discussion 5 conclusion acknowledgements references https://doi.org/10.14311/app.2022.33.0263 acta polytechnica ctu proceedings 33:263–270, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague durability evaluation of surface softened steel bar for prestressed concrete kengo iwanagaa,∗, shigeru mizoguchib, yu matsumotoc, kenichi takaid, mikiyuki ichibae a neturen czech s.r.o., jizni 1079 bitozeves prumyslova zona triangle 438 01 bitozeves czech republic b neturen co., ltd., 2-17-1 higashi-gotanda, shinagawa-ku, tokyo 141-8639, japan c sophia university, 7-1 kioi-cho, chiyoda-ku, tokyo 102-8554, japan d department of engineering and applied science, faculty of science and technology, sophia university, 7-1 kioi-cho, chiyoda-ku, tokyo 102-8554, japan e r&d dept., tokyo electric power company holding, inc., 4-1 egasaki-cho, tsurumi-ku, yokohama 230-8510, japan ∗ corresponding author: k-iwanaga@neturen-czech.com abstract. the effects of si addition and surface softening on delayed fracture susceptibility were evaluated in tempered martensitic steels with tensile strength of 1450 mpa. the delayed fracture susceptibility was reduced when si content was increased from 0.2 mass% to 1.88 mass%. the delayed fracture susceptibility of steel containing 1.88 mass% si was reduced further when the tensile strength of its surface was lowered to around 1150mpa. based on above results, the durability of these steel bars was evaluated by exposure test of prestressed concrete poles (cp). the bending load of the pc pole was 1.3 times of the design load. as a result of dismantling investigation after 10 years exposure test, there were no breakage of the steel bars in the pc poles and the soundness was maintained. in this paper, we will also introduce some comparison results of exposure tests and acceleration tests with aiming to establish the evaluation methods for the long-term durability. keywords: delayed fracture, durability, prestressed concrete pole, surface softened steel bar. 1. introduction in recent years, due to the arising problems of climate change and resources depletion, sustainable social and economic activities are being pursued. one of the basis of such social and economic activities is construction works, which requires huge amount of natural resources as well as energy, and this usage is estimated to increase in coming years. in this study, we aimed at the following two purposes. 1. manufacture of high-strength, high-durability "prestressed concrete" (hereinafter referred to as pc) steel bar by "high-frequency induction heating quenching and tempering" (hereinafter referred to as iqt) with low co2 emissions. 2. reducing lifetime costs by using this pc steel bar to improve the durability and safety of buildings. the iqt-type pc steel bar was developed in 1956 and has since been in use until today [1], but there is concern about delayed fracture, and overcoming it is an issue for improving durability. it is known that the following two points are the basic ideas for improving durability against delayed fracture [2]. 1. suppression of the "hydrogen concentration from the environment" (hereinafter referred to as "he") that intrudes into pc steel bar. 2. increasing the "critical hydrogen concentration which causes fracture" (hereinafter referred to as "hc") in pc steel bar. based on this viewpoint and aiming for the improvement in the delayed fracture durability, optimized of chemical composition for high temperature tempering as well as developed surface softening tempering treatment. the developed iqt type 1420mpa class pc steel bar has the same mechanical properties as the conventional material and has remarkably excellent durability against delayed fracture [3]. the pc steel used in this study was evaluated by various delayed fracture evaluation methods [4], and the mechanism for improving its delayed fracture was discussed. furthermore, it was subjected to an exposure test using a "concrete pole" (hereinafter referred to as "cp") as specimens using the developed pc steel bar. the cps which had been exposed for more than 10 years were investigated and their durability was examined in terms of the effects of damage and the possibility of delayed fracture. 2. experimental procedure 2.1. chemical composition of materials the materials used in this study are iqt type pc steel bars which are manufactured according to jis 263 https://doi.org/10.14311/app.2022.33.0263 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en k. iwanaga, s. mizoguchi, y. matsumoto et al. acta polytechnica ctu proceedings c si mn p s cu ti b jis g3137 ≤ 0.030 ≤ 0.035 ≤ 0.03 iqt-conventional 0.30 0.21 0.72 0.019 0.002 0.01 0.03 0.0016 iqt-improved 0.32 1.80 0.74 0.018 0.005 0.01 0.04 0.0023iqt-surface softening table 1. chemical composition of test materials(mass%). figure 1. schematic diagram of heat-treating cycle of iqt-c, iqt-i and iqt-s. specimen 0.2% proof strength [n/mm2] tensile strength [n/mm2] elongation [%] uniform elongation [%] reduction of area [%] jis g3137 ≥1275 ≥1420 ≥5 iqt-c 1316 1426 8.5 1.8 67 iqt-i 1430 1458 12.9 4.6 68 iqt-s 1396 1448 12.3 3.8 65 table 2. mechanical properties. g3137 (small diameter steel bar for prestressed concrete). table 1 shows the chemical composition of conventional material (iqt-c), improved material (iqt-i) and improved material with surface-softening temper treated (iqt-s). with the aim of increasing the tempering temperature, the composition of si is increased in improved materials, contributing to increase in resistance towards softened due to tempering. 2.2. manufacturing methods rolled wire rods with chemical composition shown in table 1 were shaped by deformed wire drawing process and subjected to continuous high frequency induction quenching and tempering. by increasing the amount of si compared to iqt-c, the maximum tempering temperature for iqt-i was able to be raised until 880 k. the surface-softening tempering process was conducted for iqt-s using the same equipment as to iqt-i, in which the material was first subjected to high temperature and short time tempering and then the cooling time was adjusted so that the rate of tempering progression throughout the materials surface was maximized. it was possible for the maximum tempering temperature of this material to reach until 1028 k. the schematic diagram for the heat treatment cycles are shown in figure 1. 2.3. mechanical properties the mechanical properties for the materials are shown in table 2. the tensile properties were measured in accordance with jis g3137. the tensile strengths for iqt-c, iqt-i and iqt-s are almost the same and their mechanical properties satisfy jis g3137. figure 2 shows the measurement result of vickers hardness from the surface to the core in radial direction as well as tensile strength calculated from the vickers hardness. there are relatively no changes in the hardness from the surface to the core of iqt-c and iqt-i. the vickers hardness for iqt-s gradually increases from 350 hv at the outermost surface towards the same hardness as common iqt material at around 1 mm from the outermost surface. then, the hardness remains relatively unchanged at 1.5 mm from the surface until the core. 264 vol. 33/2022 surface softened steel bar durability re and fracture surface 'xulqj�suhsdudwlrq�iru�wkh�plfurvwuxfwxuh�revhuydwlrq�� wkh�vshflphqv�zhuh�iluvw�fxw�xvlqj�zhw�zkhwvwrqh�fxwwhu� wkhq� wkh� fxw� vhfwlrqv� zhuh� pluuru� srolvkhg� dqg� �qlwdo�hwfklqj�vroxwlrq�wr� foduli\� wkh� plfurvwuxfwxuh�� 7kh� plfurvwuxfwxuhv� ri� pdwhuldov� zhuh� revhuyhg� xvlqj� rswlfdo�plfurvfrsh� ru� vfdqqlqj� hohfwurq� plfurvfrsh� �6(0��� 7kh� pdwhuldov� zhuh�pdfklqhg� lqwr� wklq� ilop�vshflphqv�dqg�xvhg� iru� revhuydwlrq� ri� fduelghv� suhflslwdwlrq� xvlqj� wudqvplvvlrq�hohfwurq�plfurvfrsh��7(0���7kh�iudfwxuh� ��� ��� ��� ��� ��� ��� ��� ��� ϭ ϭ ϯ ϯ 9 lf nh uv + du gq hv v� + 9 �� �� 'lvwdqfh iurp vxuidfh� /�pp ,47�6 ,47�, ,47�& ���� ���� ���� ���� ��� 7hqvloh�vwuhqjwk��0 3d ���� 6xuidfh &ruh )ljxuh �� 9lfnhuv kdugqhvv iurp wkh vxuidfhfigure 2. vickers hardness from the surface to the core of iqt-c, iqt-i and iqt-s. 7r� lqfuhdvh� wkh� uhsurgxflelolw\� ri� ),3� whvw� zklfk� zdv� v�zhuh�frqgxfwhg�dffruglqj�wr� phqw�whvw�iru�vwhho�iru�suh� vwuhvvhg�frqfuhwh�lq�d�����dpprqlxp�wklrf\dqdwh�vroxwlrq´� �-6&(� 6� ������ hvwdeolvkhg� lq� ����� e\� -dsdq� 6rflhw\� ri� +rw�zdwhu� $ssolhg�vwuhvv � ı%�������࣭���� � ����1�pp�� 6shflphq� ���1+�6&1�vroxwlrq figure 3. schematic diagram of fip test method. 2.4. microstructure and fracture surface during preparation for the microstructure observation, the specimens were first cut using wet whetstone cutter then the cut sections were mirror polished and subsequently etched using 3% nital etching solution to clarify the microstructure. the microstructures of materials were observed using optical microscope or scanning electron microscope (sem). the materials were machined into thin film specimens and used for observation of carbides precipitation using transmission electron microscope (tem). the fracture surfaces were cleaned using adhesion tape or ammonium citrate depending on necessity before being observed on sem. 2.5. delayed fracture durability evaluation test 2.5.1. fip (fédération internatioale de la précontrainte) test-1980 to increase the reproducibility of fip test which was endorsed by fip in 1980, tests were conducted according to "method of hydrogen embrittlement test for steel for pre-stressed concrete in a 20% ammonium thiocyanate solution" (jsce s 1201) established in 2012 by japan society of corrosion engineering [5]. )ljxuh���vkrzv�wkh�fruurvlrq�slwv�ghswk�glvwulexwlrq�ri� whqgrq�lq�&3�diwhu�frpsohwlrq�ri�vhuylfh���7kh�pd[lpxp� ghswk� ri� fruurvlrq� slw� phdvxuhg� lv� derxw� ���ȝp�� 9� ���pp�zhuh�dssolhg�rq�wkh� vshflphqv�wr�prgho�wkh�fruurvlrq�slwv�irxqg�lq�vhuylfh� pdwhuldov�� 7kh� udglxv� ri� wkh� qrwfk� wls� zkhuh� vwuhvv� frqfhqwudwlrq� rffxuv� zdv� pdfklqhg� wr� ����� pp� iru� )ljxuh� ��� 3lwwlqj� fruurvlrq� ghswk� � � � � � �� �� �� 䡚�� ��䡚��� ���䡚��� ���䡚��� ���䡚��� ���䡚��� ���䡚��� )u ht xh qf \ &ruurvlrq�ghswk��ȝp� 1 �� figure 4. pitting corrosion depth distribution of tendon in cp after completion of service. figure 5. v-notch shape of specimen. figure 3 shows the schematic diagram of fip test. to maintain the temperature inside the reaction cell at 323 ± 1 k, test containers equipped with hot water circulation were used in the test. the reaction cell was filled with 20% nh4scn aqueous solution at 323 k and smooth bar specimen is set up so that it passed through the cell. hydrogen entry into the specimen occurs due to chemical reaction between the specimen’s surface and the test solution. the test was conducted in a way where the reaction cell was firstly filled up with test solution and then immediately tension stress of 0.7 times of standard strength (1420 × 0.7 = 994 n/mm2) was applied to the specimen. the time until fracture was measured and test was stopped at 200 h even when no fracture occurs. 2.5.2. critical diffusible hydrogen measurement figure 4 shows the corrosion pits depth distribution of tendon in cp after completion of service. the maximum depth of corrosion pit measured is about 350µm. v-notches of depth 0.3 mm or 0.4 mm were applied on the specimens to model the corrosion pits found in service materials. the radius of the notch tip where stress concentration occurs was machined to 0.12 mm for evaluation of materials with harsher condition than the ones in service environment. the dimensions of the specimen are shown in figure 5. delayed fracture tests with continuous hydrogen charging of the v-notched specimens in different concentrations of nh4scn solution at 323 k were conducted using 2 conditions stated below. assuming a condition in which corrosion pits occur under normal tension load in "slightly harsh environment" condition, load stress of 0.7 times the magnitude of tensile 265 k. iwanaga, s. mizoguchi, y. matsumoto et al. acta polytechnica ctu proceedings figure 6. exposure test of "cp500" utility poles at the choshi test site in japan (started in august 2010). strength was applied towards specimens with notch depth of 0.3mm. meanwhile, in "harsh environment" condition, load stress of 0.85 times the magnitude of tensile strength was applied towards specimens with notch depth of 0.4 mm to assumptive a condition in which corrosion pits with maximum depth as well as load higher than the limit cp in service. the maximum concentration of nh4scn solution during which no fracture occurs in 100 h test time is determined. different specimens than the ones in delayed fracture tests are immersed for 100 h in nh4scn solutions of concentration determined earlier and then the diffusible hydrogen concentration measured using thermal desorption method from room temperature to 573 k is classified as hc. 2.5.3. cp exposure test following jis a5364 and jis a5373, cp (cp500) with length 14 m and design load 4.9kn as well as cp (cp350) with length 13 m and design load 3.4kn were manufactured using iqt-c and iqt-s. the specifications of the arrangement of pc steel bar are same as to the ones used in real utility poles. cp500 were set up at choshi, chiba in japan (about 4km from southern coast) and exposure tests were started from august of 2010. the tests were set up such that design load (4.9kn) which is the instantaneous abnormal high load such as from typhoon, as well as 1.2 times the design load (5.9kn) were always applied towards the horizontal direction of the cp’s upper part. figure 6 shows the condition of the exposure tests. cp350 were set up at muroran, hokkaido in japan (about 20 m from sea) and exposure tests were started from july of 2013 by always application of design load (3.4kn) or 1.3 times the design load(4.4kn) in the horizontal direction of the upper part of the cp. the tests condition is shown in figure 7. figure 7. exposure test of "cp350" utility poles at the muroran test site in japan (started in july 2013). 3. results 3.1. delayed fracture durability evaluation results 3.1.1. fip test result figure 8 shows the result of fip test. each data was sorted using mean rank method and then cumulative failure probability was calculated and presented as lognormal probability plot. at 50% cumulative failure probability, fracture is recorded at 18.8 h for iqt-c, while no fracture is recorded at stop time of 200 h for iqt-i which has higher si composition. moreover, surface-softening tempered iqt-s also experiences no fracture at 200 h, showing substantial improvement in delayed fracture durability that is not possible to be evaluated by fip test alone. 3.1.2. critical diffusible hydrogen (hc) measurement result the concentration of hydrogen that enters from environment (he) is found to be around 0.15 ppm from previous research [6] and it is suggested that the higher hc is from 0.15 ppm, the better the durability of the material against delayed fracture. figure 9 shows the measurement result of hc. in "slightly harsh environment" condition (0.3 mm notch-0.7σb), hc was higher than he regardless of the types of material. however, in "harsh environment" condition (0.4 mm notch-0.85σb) hc decreased until below 60 percent than that of in "slightly harsh environments" and hc from iqt-c became smaller than he. regardless of the environment model, the pattern for hc was iqt-c<iqt-i<iqt-s. the effectiveness of the methods for improving durability against delayed fracture was confirmed from this result. 3.2. cp exposure test results 3.2.1. cp500 choshi exposure test figure 10 shows the crack width measurement result of the cracks found on the surface of cp at the third 266 vol. 33/2022 surface softened steel bar durability � �� ��� ���� 7l p h� wr �iu df wx uh �k � &xpxodwlyh�idloxuh�suredelolw\ ��� ,47�& ,47�, ,47�6 � �� �� �� �� �� �� �� �� �� �� figure 8. delayed fracture resistance evaluation using quasi-fip test. ��� ��� ��� ��� ��� ,47�& ,47�, ,47�6& ul wlf do �g lii xv le oh �k \g ur jh q� fr qw hq w㸸 + f� >p dv v�s sp @ ���ੈ1rwfk����ı% ���ੈ1rwfk�����ı% +h figure 9. measurement result of critical diffusible hydrogen (hc) without delayed fracture after 100 h. figure 10. crack width measurement result of cp500 utility poles using iqt-s as tendon. orqjlwxglqdo�gluhfwlrq�ri�wkh�irupzrun�mrlqwv�srvlwlrq�ri�&3�zkhuh�lv�whqgv�wr�orz�ghqvlw\�frqfuhwh�e\� )ljxuh ��� 5hvxowv� ri� h[whuqdo� ,47�&� ,47�,� ,47�6� figure 11. results of external observation of cp500 utility poles exposed to bending for 5.9kn for 10 years. 5hvxowv� ri� h[whuqdo� )ljxuh� ����5hvxowv� ri� h[whuqdo� ,47�6,47�&㻌 figure 12. results of external observation of cp350 utility poles exposed to bending for 4.4kn for 5 years. year of exposure test. the cracks occurred immediately after load application and expanded until as large as 0.2 mm at around the first year. it was observed that the cracks’ enlargement slowed down following the first year. figure 11 shows the external observation of utility poles after 10 years of exposure test. the result of using leakage magnetic flux method to detect any fracture in the steels shows that there were no major abnormalities nor any fractures detected for all iqt-c, iqt-i and iqt-s. thus, the exposure test is still ongoing in the present time. 3.2.2. cp350 muroran exposure test figure 12 shows the results of external observation of iqt-c and iqt-s utility poles after 5 years of exposure test. several cracks were found on the surface of cp. the cracks were after enlargement until about 0.2 mm, there were no remarkable changes in the width of cracks on the cp surface. there was no fracture detected by using leakage magnetic flux method at the fifth year of exposure test. the iqt-c and iqts utility poles were dismantled and the condition of 267 k. iwanaga, s. mizoguchi, y. matsumoto et al. acta polytechnica ctu proceedings figure 13. observation results of cracks and spacers during dismantling of the utility pole. figure 14. observation of corrosion state of pc steel bar at the joint position of the concrete formwork during dismantling cp. figure 15. microscopic fracture surfaces near the notch tip after delayed fracture test; (a) a iqt-c specimen shows intergranular (ig) fracture, (b) a iqt-i specimen shows quasi-cleavage (qc) fracture and (c) a iqt-s specimen also shows quasi-cleavage (qc) fracture. �0lfurvfrslf� iudfwxuh� vxuidfhv� qhdu� wkh� qrwfk� wls diwhu� ghod\hg� iudfwxuh� whvw�� �d�� d� ,47�&� vshflphq� vkrzv� e��d�,47�,�vshflphq�vkrzv�txdvl� fohdydjh��4&��iudfwxuh�dqg��f��d�,47�6�vshflphq�dovr�vkrzv ϭ ϭ ϯ ϯ ϰ ϱ 㻜 㻝㻞 㻞㻠 㻟㻢 㻠㻤 㻢㻜 㻣㻞 �ŝ ĩĩƶ ɛŝď ůğ �ś ljě ƌž őğ ŷ� ;ŵ ăɛ ɛ�ɖ ɖŵ ϳ /ŵŵğƌɛŝžŷ�ƚŝŵğ�;śϳ /ydͳ� /ydͳ/ /ydͳ^ figure 16. relationship between immersion time and diffusible hydrogen content in 323 k-20 % nh4scn solution. fracture, cracks as well as rusts of the steel parts was examined. after dismantling, it was found that no fracture occurred in the pc steel bars of both utility poles. figure 13 and figure 14 show some examples of the crack parts during dismantling of the utility poles. local rusts were spotted on the surface of the steel bar located directly under the cracks. several cracks were located in the vicinity of the areas where the bond strength is weak between the reinforcing bar and the concrete due to locally no contact existing between them, such as at the spacers put before the pouring of concrete into the formwork. rusts also observed along the longitudinal direction of the formwork joints position of cp where is tends to low density concrete by centrifugal molding method. 4. discussion 4.1. the mechanisms of improvement of delayed fracture durability 4.1.1. effects of adding si and b previous results show that the delayed fracture durability in iqt-i and iqt-s has improved compared to iqt-c, and the reasons are as follows. figure 15 shows the observation results of the fracture surfaces from the materials. intergranular fracture was observed in iqt-c which has short time to 268 vol. 33/2022 surface softened steel bar durability figure 17. microstructures of specimens obtained by a scanning electron microscopy and a transmission electron microscopy. the micrographs of iqt-s specimen are observed in surface-softened area at 0.4 mm apart from the surface. figure 18. observation results of steel mark in concrete test piece after neutralization test. fracture in fip tests as well as small hc. meanwhile, intergranular fracture was not observed in iqt-i and iqt-s in which the composition of si in the materials was increased. this correlates to the effects of si in suppressing the formation of intergranular fracture as reported by previous study [7]. it is also thought that the addition of b in the materials suppressed the formation of intergranular fracture due to the effects of b in increasing the strength of grain boundaries as reported before [8]. 4.2. effects of surface-softening tempering the results in 3.1.2 show that hc for iqt-s is remarkably higher than iqt-i even though iqt-s is just a surface softening tempered iqt-i. the reasons for the increase of hc for iqt-s despite having the same chemical composition and tensile strength are as follows. figure 16 shows the relationship between immersion time and diffusible hydrogen content when the specimens were immersed in 20% nh4scn solution. the rate of hydrogen diffusion is in the order of iqt-s< iqt-i <iqt-c. as the rate of hydrogen diffusion in iqt-s is slower than iqt-i, this indicates that the surface-softening layer has certain resistance towards hydrogen entry into the material. in order to investigate this point, sem and tem observations of the microstructures of each material were conducted, and the results are shown in figure 17. the observation results are summarized in order of increasing tempering temperature (iqt-c, iqt-i, iqt-s) as follows. 1. as the tempering temperature increases, the carbide precipitations become smaller and the precipitation sites change from the grain boundaries to the inner grain parts. these are also the characteristics of materials that have undergone high frequency induction tempering (short time heating). 2. as the tempering temperature increases, the shape of the carbide precipitated on the grain boundaries changes from plate-shape to film-shape and lastly to divided sphere-shape. 3. the areas with high dislocation density are shown as black contrast areas in the tem observation results. as the tempering temperature increases, the white areas which represented areas with low dislocation density increase, confirming the progression of tempering. 269 k. iwanaga, s. mizoguchi, y. matsumoto et al. acta polytechnica ctu proceedings hydrogen diffusion is closely related to the dislocation motion in materials. as such, the amount of hydrogen transported from the steel surface into the inner matrix increases when dislocations become more mobile or dislocation densities increase. therefore, the amount of diffusible hydrogen decreases because the fine distribution of carbides into the inner grain hinders the dislocation mobility as well as the dislocation densities decrease. based on the observation results above, it is conjectured that the main focus for the mechanisms of the delayed fracture durability improvement lies on the high temperature and short time tempering which is possible due to si addition and surface-softening tempering. the process transforms the coarse plateshape carbides on the grain boundaries into fine sphere-shape carbides, which results in the increase of grain boundaries strength as well as suppressing the hydrogen diffusion through dislocation. 4.3. improvement of the durability of cp in this section the factors behind no fractures occurring for iqt-c and iqt-s reinforcement bars at the cp350 muroran exposure test are discussed. after dismantling of cp350, neutralization test using phenolphthalein was conducted towards the concrete parts which included the crack parts. figure 18 shows one example of the test results. the observed surface changed pink colour which indicated that neutralization did not occur whether on the crack part nor the steel mark part. thus, it can be inferred that even though rusts form on the steel bars, since either the rust progression was slow or the corrosion environment was not sustained, entry of hydrogen exceeding hc did not occur even in iqt-c with low hc value. furthermore, it should also be noted that the cold climate and low humidity at the muroran test site may affect the corrosion’s rate of progression and sustainability. however, fractures were also not recorded at the cp500 (choshi test site) where it is warmer and more humid than muroran. thus, further plans for the verification of durability in real environment involve the continuation of tests while also recording environmental data. 5. conclusions high frequency induction quenched and tempered pc steel bars with improved durability were developed by increasing the tempering temperature due to si addition, as well as conducting surface-softening tempering process. the use of this pc steel bars can be expected to improve pc durability. cp were manufactured using these pc steel bars and subjected to excessive bending load while exposed to real environment. the results showed that no fracture occurred in both conventional material and improved material even for test that has been ongoing for 10 years. with this, we will continue to strive in the future to reduce environmental loads and contribute to society through the development of high quality products utilizing technologies with little environmental loads such as high frequency induction heat treatment. references [1] m. yamada. recent topics on prestressing steel. concrete journal japan concrete institute. 47(11):3-8, 2009. http://www.jci-net.or.jp/j/publish/bulleti n/e_pdf/e_outline_200911.pdf. [2] t. tarui, m. kubota. approaches for fundamental principles 1: method of hydrogen embrittlement and improvement techniques of delayed fracture, nippon steel technical report 391(101):155-157, 2011. https://www.nipponsteel.com/en/tech/report/nsc /pdf/nstr101-23_tech_review-3-4.pdf. [3] y. matsumoto, k. takai, m. ichiba, et al. reduction of delayed fracture susceptibility of tempered martensitic steel through increased si content and surface softening. isij international 53(4):714-22, 2013. https: //doi.org/10.2355/isijinternational.53.714. [4] t. tarui, s. yamasaki. evaluation method of delayed fracture property and overcoming techniques of delayed fracture of high strength steels. tetsu-to-hagane 88(10):612-9, 2002. https: //doi.org/10.2355/tetsutohagane1955.88.10_612. [5] jsce s 1201-2012. method of hydrogen embrittlement test for steel for prestressed concrete in a 20% ammonium thiocyanate solution, jsce, tokyo, 2012. [6] t. kushida. hydrogen entry into steel by atmospheric corrosion. isij international 43(4):470-4, 2003. https: //doi.org/10.2355/isijinternational.43.470. [7] r. hobbs, l. gw. effect of silicon on the microstructure of quenched and tempered medium-carbon steels. 1972. [8] y. ohno, h. fuji, s. sekino.effect of b on delayed class high fracture property of high strength steels. tetsu-to-hagane 83:42-47, 1997. 270 http://www.jci-net.or.jp/j/publish/bulletin/e_pdf/e_outline_200911.pdf https://www.nipponsteel.com/en/tech/report/nsc/pdf/nstr101-23_tech_review-3-4.pdf https://doi.org/10.2355/isijinternational.53.714 https://doi.org/10.2355/tetsutohagane1955.88.10_612 https://doi.org/10.2355/isijinternational.43.470 acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.37.0001 acta polytechnica ctu proceedings 37:1–9, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague subsals: a subchannel thermal-hydraulic code for irt type fuel analysis tomáš adámeka,b a czech technical university in prague, faculty of nuclear sciences an physical engineering, department of nuclear reactors, břehová 7, 115 19 prague, czech republic b research centre řež, hlavní 130, husinec-řež, czech republic correspondence: tomas.adamek@cvrez.cz abstract. the lvr-15 research reactor is operated with a tube type fuel irt-4m. due to fuel’s unique concentric square annular shape with, coolant flow is subject to significant pressure driven crossflow. detailed calculation of such flow patterns is beyond capabilities of standard system codes used for the thermal hydraulic safety analysis. to assess safety of core designs consisting of the irt-4m fuel assemblies, a new subchannel code is under development at research centre řež and czech technical university in prague. computer program subsals is single phase, steady state, subchannel solver in development. rigid computational mesh describes irt-like geometries, and it is also suitable for other specific doubly connected ducts. in this contribution, general description of calculation procedure and computational capabilities is presented along with code-to-code comparison. it could be used as proof of concept for future improvements in thermal hydraulic calculations of research reactors. keywords: subsals, subchannel analysis, thermal-hydraulics, irt-4m, lvr-15, crossflow. 1. introduction thermal-hydraulic analysis of nuclear reactors is a necessary part of safety assessment. to maintain the integrity of physical barriers during the operation, appropriate cooling of all fuel assemblies must be ensured. various sophisticated computational tools have been developed for the simulation of water cooled reactors behaviour. over the past decades increasing computational power of the computers allowed for a great development of one-dimensional system codes. nuclear reactor core thermal-hydraulic safety assessment based on a hot channel theory, adopted to use in system codes, may be unnecessarily conservative. a more realistic approach to evaluate a core performance is accomplished using the subchannel analysis techniques. computational models can be used to represent either an entire core or an individual sub-assembly. the subchannel approach has been used extensively in light water reactor applications. it provides a higher degree of resolution than traditional system code approach and allows for a fast calculation of the flow, temperature, and void distributions with a reasonable accuracy. main domain of application of the traditional subchannel codes like subchanflow are core designs consisting of the rod bundles. moreover, the efforts are made to develop advanced computational tools for the thermal-hydraulic analysis of the reactors operated with plate type fuel. for the utilization in reactors using tube type fuel, especially with significant pressure driven crossflow, however, modelling capabilities of the standard thermal-hydraulic simulation tools are limited. 2. general description of the subchannel solver subsals computer program subsals (subchannel square annular layout solver) is a single phase, steady state subchannel solver under development at research centre řež and czech technical university in prague. its main purpose is the thermal-hydraulic analysis of the lvr-15 research reactor, which main goal is to determine cladding temperature distribution in the irt-4m fuel assemblies under the normal operation. 2.1. irt type fuel and computational mesh the irt type fuel is a series of sandwich, tube-type fuel assemblies with a square cross section, originally designed in 1963 as the irt-m consisting of three fuel tubes. later, further improvements in the fabrication process were achieved which led to four tube design irt-2m. in 1979 a new generation, the irt-3m was developed with an enhanced heat transfer area. finally in 2004, the irt-4m (figure 1) design motivated by the rertr program was completed [1]. currently the irt type fuel is used in two research reactors in czech republic (lvr-15 and vr-1), two research reactors in russia (ir-8 and irt-t) and one research reactor in uzbekistan (vvr-sm) [2]. 1 https://doi.org/10.14311/app.2022.37.0001 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en tomáš adámek acta polytechnica ctu proceedings figure 1. geometry of the irt-4m fuel assemblies: standard 8-tube fuel assembly (top) and control 6-tube fuel assembly (bottom) [3]. 0 1 2 3 4 5 6 7 8 0 1 2 3 4 5 6 7 8 1 20 15 23 23 22 22 14 21 21 5 4 3 6 9 9 10 10 7 8 12 11 11 1213 131415 1415 91011 17 17 16 16 18 12 1819 19 20 20 13 figure 2. radially-azimuthal view of the computational mesh. in it’s current state of development, for the sake of modularity, subsals is designed to analyse one coolant channel between the fuel tubes. the interaction between multiple coolant channels will be achieved by the external iterator module in later stages of development. the computational mesh (figure 2) is rigid in radial and azimuthal direction and user defined in axial direction. radially one coolant channel azimuthally divided into 16 unique subchannels is presented surrounded from both sides by fuel tubes, both azimuthally divided into 24 unique fuel elements. 2.2. basic conservation equations due to multiple simplifications in presented problem and lack of experimental data for precise validation on various specific phenomena, subsals code uses a simple marching procedure as described in [4]. combination of the rigid radially-azimuthal computational mesh and the marching procedure makes it possible to use a third-party numerical solver. gnu scientific library (gsl) [5] is implemented in the program to solve a non-linear systems of the conservation equations, hence no linearization of the equations is needed. for an axial node n for the subchannel i the basic conservation equations in finite difference form are the following. mass conservation: 0 ≡ si ∆z ( ρni w n i − ρn−1 i wn−1 i ) +wn i→(i−1) (1) +wn i→(i+1), where s is subchannel flow area, ∆z is length of axial node, ρ is water density, w is water velocity, wx→y is diversion crossflow from subchannel x to subchannel y. energy conservation: 0 ≡ si ∆z ( ρni w n i h n i − ρn−1 i wn−1 i hn−1 i ) − lin.q n i + w′ n i→(i−1) ( hni − hni−1 ) (2) + w′ n i→(i+1) ( hni − hni+1 ) +wn i→(i−1)h ∗n i−1 +wn i→(i+1)h ∗n i+1, where h is specific enthalpy of water, lin.q is linear heat rate transferred to subchannel, w′x→y is turbulent crossflow from subchannel x to subchannel y, h∗ is specific enthalpy of water in donor subchannel. axial momentum conservation: 0 ≡ si ∆z ( ρni (wni )2 − ρn−1 i ( wn−1 i )2) + fw′ni→(i−1) ( wni − wni−1 ) + fw′ni→(i+1) ( wni − wni+1 ) +wn i→(i−1)w ∗n i−1 (3) +wn i→(i+1)w ∗n i+1 + ρni gsi + si ∆z ( pni − pn−1 i ) + 1 2 fni dhi ρni si (wni )2 , where f is correction factor to help account for the difference between energy and momentum turbulent transport, g is acceleration due to gravity, p is water pressure, f is darcy-weisbach friction factor, dh is hydraulic diameter of subchannel, w∗ is water velocity in donor subchannel. 2 vol. 37/2022 subsals: a subchannel thermal-hydraulic code transverse momentum conservation: 0 ≡ si→(i−1) li→(i−1) ( pni − pni−1 ) + ktrans. ∣∣wi→(i−1) ∣∣wi→(i−1) 2 ( si→(i−1) )2 ρ∗ni−1 si→(i−1) li→(i−1) , (4) where sx→y is effective width of control volume for transversal flow from subchannel x to subchannel y, lx→y is effective length of control volume for transversal flow from subchannel x to subchannel y, ρ∗ is density of water in donor subchannel, ktrans. is transverse flow pressure loss coefficient. 2.3. constitutive equations to solve the conservation equations, a state equation is needed to obtain thermophysical properties of water. for this purpose the external coolprop [6] library for calculation of the thermodynamic properties of water and steam in form of iapws-if97 formulation is used. beside the physical properties,a power distribution, constitutive relations and boundary conditions are needed to avoid underdetermination of the system of equations. boundary conditions, as well as power profile, are implemented via user’s inputs. the power distribution is assumed to be spatially separable, hence total heat pi transferred to coolant from fuel element i is pi = p0φ rad. i φaz. i φax. i ϑ, (5) where p0 is total power generated in fuel assembly. coefficients φrad. i , φaz. i , φax. i are normalized relative powers of fuel element i in radial, azimuthal and axial direction. coefficient ϑ denotes fraction of the heat transferred to the coolant from whole fuel tube, allowing not only to use adiabatic boundary condition ϑ ≡ 1, but also the simple approximation of the uniform power split boundary condition ϑ ≡ 0.5. in the program subsals, various correlations for the friction factor and the heat transfer coefficient calculation are implemented. frictional pressure drop in turbulent regime is calculated using darcy-weisbach friction factor. widely used formula proposed by blasius [7] for calculation of the friction factor in hydraulically smooth pipes reads: f = 0.3164re−0.25, (6) where re is reynolds number. to include a surface roughness ε in the calculation, zigrang-sylvester’s approximation [8] to colebrook-white correlation, is implemented as: 1√ ft. = −2 log10 { ε 3, 7dh + 2, 51 re[ 1, 14− 2 log10 ( ε dh + 21, 25 re0,9 )]} , (7) which is the same form as implemented in the standard thermal-hydraulic codes, e.g. system code relap5/mod3 [9] or subchannel code ctf [10]. due to lack of experimental data, simplified models of various parameters from the conservation equations are used. turbulent transport correction factor f is set to be zero, similarly as in an input example in [11]. this assumption makes turbulent mass transport effect on pressure equalization between subchannels negligible. transverse flow pressure loss coefficient is set to be 0.5, as suggested in various textbooks similarly as in an input example in [11]. in the rod bundles, particular attention is paid to describe a turbulent crossflow mixing. great review of the experimental studies and correlations on this topic is available in the study [12]. for a turbulent crossflow the simplest formula was implemented: w′x→y = βsḡ, (8) where ḡ is an average mass flux of neighbouring subchannels x and y, β is turbulent factor. one of the most common formulae suggested in [12] for turbulent factor proposed by beus reads: β = 0.0035re−0.1 d̄h s , (9) where d̄h is average hydraulic diameter of neighbouring subchannels x and y. cladding temperature of given fuel element is calculated using newton’s law: surf.q = α (tclad. − t∞) , (10) where surf.q is heat flux from given fuel element to adjacent subchannel, tclad. is cladding temperature, t∞ is bulk water temperature in adjacent subchannel and α is heat transfer coefficient defined as: α = nuλf dh , (11) where nu is nusselt number and λf is water conductivity. heat transfer coefficient is function of nusselt number, hence another constitutive relation is required. great review and validation of nusselt number correlations for high aspect ratio rectangular geometry is available in the thesis [13]. four different correlations for nusselt number calculation are implemented in the program subsals. one of the most commonly used correlations proposed by dittus and boelter [14] reads: nu = 0.023re0.8pr0.4, (12) where pr is prandtl number. another implemented correlation proposed by popov and petukhov [15] with siman-tov’s correction for rectangular geometry as suggested in [13]: 3 tomáš adámek acta polytechnica ctu proceedings nu = 1 8frepr k1(f)k2(pr) √ f 8 ( pr 2 3 − 1 ) k1(f) = (1 + 3.4f) (13) k2(pr) = ( 11.7 + 1.8pr− 1 3 ) , where friction factor f is calculated using filonenko: f = 1.0875− 11.25a 1.82 log10 re− 1.642 , (14) where a is subchannel aspect ratio. from popov-petukhov correlation, gnielinski [13] obtained another formula, which is often regarded as one of the most accurate: nu = 0.5f (re− 1000) pr 1 + 12.7 √ 0.5f ( pr 2 3 − 1 ) , (15) where friction factor is calculated according to formula: f = (1.58 ln re− 3.28)−2 . (16) finally, correlation proposed by sieder and tate [13] is adopted: nu = 0.027re0.8pr 1 3 . (17) to account for the wall temperature effect on boundary layer, nusselt number is multiplied by cladding temperature dependent correction factor throughout the iterative procedure. different correction factors are used for individual correlations as listed below: • dittus-boelter correlation is corrected by factor( µ∞ µclad. )0.11 as suggested by [15]. • popov-petukhov correlation is corrected by factor( µ∞ µclad. )0.11 as suggested by [15]. • gnielinski correlation is corrected by factor( µ∞ µclad. )0.11 as suggested by [15]. • sieder-tate correlation is corrected by factor( µ∞ µclad. )0.14 as suggested by [13]. similar correction of value ( µclad. µ∞ )0.11 is used for friction factor [15]. 3. code-to-code comparison according to the iaea safety standards series [16] code validation should be performed on all computer codes that are used for the deterministic safety analysis of nuclear reactors. the purpose of code assessment is to provide confidence in the ability of a code to predict the values of parameters of interest and quantify the accuracy of prediction. the code qualification process should be based on the information 0 1 2 16 24.60 mm12.30 mm 12.30 mm 24.60 mm 9.10 mm figure 3. radially-azimuthal view of the one-eighth of the pin type square annular geometry geometry (with adjacent subchannels). from analytical solutions, experimental data, nuclear and benchmark calculations (code-to-code comparisons). to address basic verification, reference pin type square annular geometry (figure 3) was proposed as the compromise between a standard subchannel code problem and limited modelling capabilities of the code subsals. geometry consists of 16 subchannels arranged to square annular shape and 32 fuel pins. each subchannel is neighbouring with exactly two other subchannels. using this geometry, a flow and temperature distribution in coolant can be compared using various boundary conditions described below. throughout all test cases turbulent mixing was disabled. 3.1. friction flow split using the geometry from figure 3, basic flow split problem can be addressed. due to the symmetry, only two subchannels, e.g. 0 and 1, are essentially needed to be examined. the inlet velocity is uniform in all subchannels, power is set to be zero, and water is highly subcooled. this initial conditions create a difference in frictional pressure drops in the two subchannels, which create a lateral pressure gradient. flow is driven from the higher resistance subchannel to the lower resistance subchannel, hence the name diversion or pressure driven crossflow. flow distribution changes along the axial direction. velocity increases in the low-resistance subchannel, which increases frictional pressure drop in that subchannel and vice versa. flow exchange continues until the frictional pressure losses are equal in both subchannels and subchannels are in mechanical equilibrium. an analytical solution can be calculated for mechanical equilibrium point. for friction flow split comparison, following boundary conditions were used: • inlet temperature is set to 50 °c. • outlet pressure is set to 1 · 1.8 · 105 pa. • inlet velocity is set to be 2, 3, 4 and 5m·s−1. typical axial velocity distribution comparison is shown in figure 4. results for the inlet velocities 2, 3, 4 and 5 m/s are presented in table 1. these results demonstrate that subsals predicts expected flow split in the ptsa-ver-1 geometry and axial velocity 4 vol. 37/2022 subsals: a subchannel thermal-hydraulic code 1.85 1.9 1.95 2 2.05 2.1 0 1 2 3 4 5 0 0.2 0.4 0.6 0.8 1 v e lo ci ty ( m /s ) r e la ti v e d if fe re n ce ( % ) axial position (m) subchanflow subsals analytical equilibrium relative difference figure 4. comparison of axial velocity distribution calculation with analytical solution. inlet maximal analytical velocity error equilibrium error [m·s−1] [%] [%] ch. 0 ch. 1 ch. 0 ch. 1 2 0.149 0.245 0.01 0.04 3 0.170 0.270 0.03 0.06 4 0.170 0.282 0.03 0.07 5 0.175 0.279 0.05 0.09 table 1. friction flow split comparison results. distribution is in great agreement with subchanflow calculation. 3.2. pressure loss friction flow split calculations were also used to evaluate frictional pressure drops using both subsals and subchanflow, see figure 4. comparison demonstrates that subsals predicts frictional pressure loss in ptsa-ver-1 geometry consistently with subchanflow calculation. 3.3. axial temperature profile and cladding temperature one of the main tasks of each thermal-hydraulic code is to predict coolant enthalpy profile, and thus the temperature profile, under different power distribution boundary conditions. to compare cladding and water temperature profile computed using subsals and subchanflow, four test cases were defined, all with uniform radially-azimuthal power profile: (1.) uniform axial power profile. (2.) linearly increasing axial power profile. (3.) linearly decreasing axial power profile. (4.) sine axial power profile. throughout all test cases turbulent mixing was disabled. results of water velocity, water temperature and cladding temperature distributions calculations were compared from both codes with uniform radiallyazimuthal power profile, hence total heat-flux to each subchannel in individual axial nodes was the same. 10 20 30 40 50 60 70 2 2.5 3 3.5 4 4.5 5 0 1 2 3 4 5 pr es su re d ro p (k pa ) r el at iv e d iff er en ce ( % ) inlet velocity (m/s) subchanflow subsals relative difference figure 5. comparison of pressure drop calculations. other boundary conditions were unchanged from previous tests. overall power transferred to coolant was set to 0.7mw. a typical axial velocity distribution, axial cladding and water temperature distributions comparisons are shown in figures 6, 7, 8 and 9. calculations results are summarised in table 2. maximal pressure drop difference between corresponding calculations is 2.1 % with average error only 0.8 %. 3.4. maximum cladding temperature maximum cladding temperature is one of the most important safety parameters, thus capability of subchannel code to correctly predict this value is of great importance. furthermore, being independent on the axial nodalization inconsistency, peak cladding temperature is a valuable parameter for code-to-code comparison. maximal peak cladding temperature difference between both calculations is 1.02 °c, see figure 10, which is significantly less than maximal cladding temperature error 2.23 °c. almost all predictions of peak cladding temperature calculated using code subsals are conservative, maximal non-conservative error being only 0.36 °c. 4. conclusions and outlook in this work new subchannel thermal-hydraulic computer code for analysis of the light water research reactors operated with the irt type fuel assemblies is presented. the main objective of this study is codeto-code comparison of the program subsals against the standard validated subchannel code subchanflow using the reference test geometry. overall 19 different calculations divided into five groups of tests were performed with and without power, with different inlet velocities and different axial power profiles. it is shown, that subsals code provides reasonable predictions of the coolant flow and enthalpy distribution, consistent with reference calculations provided by subchanflow. subsals can predict pressure drop within ±2.5 % error from subchanflow calculations. maximal coolant temperature and velocity rmse is 0.54 °c and 0.020m·s−1 respectively. maximal cladding temperature rmse 5 tomáš adámek acta polytechnica ctu proceedings 1.9 1.92 1.94 1.96 1.98 2 2.02 2.04 2.06 2.08 2.1 2.12 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 0 0.005 0.01 0.015 0.02 0.025 0.03 ve lo ci ty ( m /s ) ab so lu te d iff er en ce ( m /s ) axial position (m) subsals subchanflow relative difference 50 55 60 65 70 75 80 85 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 0 0.5 1 1.5 2 w at er t em pe ra tu re ( °c ) r el at iv e po w er ( % ) axial position (m) subsals subchanflow axial relative power 50 60 70 80 90 100 110 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 0 0.5 1 1.5 2 cl ad di ng t em pe ra tu re ( °c ) r el at iv e po w er ( % ) axial position (m) subsals subchanflow axial relative power figure 6. comparison of axial velocity (top left), water temperature (top right) and cladding temperature (bottom) distribution calculations with uniform axial power distribution. 2.8 2.85 2.9 2.95 3 3.05 3.1 3.15 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 0 0.005 0.01 0.015 0.02 0.025 0.03 ve lo ci ty ( m /s ) ab so lu te d iff er en ce ( m /s ) axial position (m) subsals subchanflow relative difference 50 55 60 65 70 75 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 0 0.5 1 1.5 2 w at er t em pe ra tu re ( °c ) r el at iv e po w er ( % ) axial position (m) subsals subchanflow axial relative power 50 60 70 80 90 100 110 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 0 0.5 1 1.5 2 cl ad di ng t em pe ra tu re ( °c ) r el at iv e po w er ( % ) axial position (m) subsals subchanflow axial relative power figure 7. comparison of axial velocity (top left), water temperature (top right) and cladding temperature (bottom) distribution calculations with linearly increasing axial power distribution. 6 vol. 37/2022 subsals: a subchannel thermal-hydraulic code 1.9 1.92 1.94 1.96 1.98 2 2.02 2.04 2.06 2.08 2.1 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 0 0.005 0.01 0.015 0.02 0.025 0.03 ve lo ci ty ( m /s ) ab so lu te d iff er en ce ( m /s ) axial position (m) subsals subchanflow relative difference 50 55 60 65 70 75 80 85 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 0 0.5 1 1.5 2 w at er t em pe ra tu re ( °c ) r el at iv e po w er ( % ) axial position (m) subsals subchanflow axial relative power 70 75 80 85 90 95 100 105 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 0 0.5 1 1.5 2 cl ad di ng t em pe ra tu re ( °c ) r el at iv e po w er ( % ) axial position (m) subsals subchanflow axial relative power figure 8. comparison of axial velocity (top left), water temperature (top right) and cladding temperature (bottom) distribution calculations with linearly decreasing axial power distribution. 1.9 1.92 1.94 1.96 1.98 2 2.02 2.04 2.06 2.08 2.1 2.12 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 0 0.005 0.01 0.015 0.02 0.025 0.03 ve lo ci ty ( m /s ) ab so lu te d iff er en ce ( m /s ) axial position (m) subsals subchanflow relative difference 50 55 60 65 70 75 80 85 0 1 2 3 4 5 0 0.5 1 1.5 2 w at er t em pe ra tu re ( °c ) r el at iv e po w er ( % ) axial position (m) subsals subchanflow axial relative power 50 60 70 80 90 100 110 0 1 2 3 4 5 0 0.5 1 1.5 2 cl ad di ng t em pe ra tu re ( °c ) r el at iv e po w er ( % ) axial position (m) subsals subchanflow axial relative power figure 9. comparison of axial velocity (top left), water temperature (top right) and cladding temperature (bottom) distribution calculations with sine axial power distribution. 7 tomáš adámek acta polytechnica ctu proceedings inlet velocity water temperature cladding temperature velocity max. error rmse max. error rmse max. error rmse case [m·s−1] [m·s−1] [m·s−1] [°c] [°c] [°c] [°c] 1 2 0.009 0.007 0.90 0.50 1.51 0.83 1 3 0.016 0.013 0.57 0.33 0.63 0.33 1 4 0.021 0.016 0.43 0.25 0.41 0.21 1 5 0.026 0.016 0.35 0.20 0.29 0.14 2 3 0.015 0.011 1.24 0.43 1.71 0.61 2 4 0.022 0.015 0.92 0.32 0.92 0.30 2 5 0.030 0.018 0.74 0.26 0.50 0.18 3 2 0.010 0.008 0.67 0.46 0.99 0.53 3 3 0.013 0.012 0.48 0.30 1.18 0.68 3 4 0.020 0.017 0.33 0.22 1.27 0.61 3 5 0.022 0.019 0.27 0.17 0.97 0.46 4 2 0.012 0.009 1.00 0.54 2.21 0.80 4 3 0.014 0.011 0.67 0.35 1.66 0.75 4 4 0.023 0.017 0.51 0.26 2.23 1.51 4 5 0.028 0.020 0.41 0.21 1.15 0.51 table 2. code-to-code comparison for different power profiles. 60 70 80 90 100 110 120 60 70 80 90 100 110 120 m ax im um c la dd in g te m pe ra tu re ( su bs al s) maximum cladding temperature (subchanflow) ±1°c figure 10. comparison of maximum cladding temperature calculations. is 1.51 °c and maximal peak cladding temperature error is 1.02 °c. temperature rmse decrease as the inlet velocity grows larger. one of the most important steps in the qualification process of a new the thermal-hydraulic code is code-to-code comparison, especially when very few experimental data are available. applicability of presented conclusions to the irt type geometry, however, must be further investigated. attention needs to be paid to closure relations assessment, mainly turbulent mixing model. furthermore, efforts are underway to develop experimental infrastructure for the thermal-hydraulic measurements and code validation at research centre řež. list of symbols a subchannel aspect ratio dh hydraulic diameter of subchannel [m] f darcy-weisbach friction factor f turbulent transport correction factor g mass flux [kg·m−2·s−1] g acceleration due to gravity [m·s−2] h specific enthalpy of water [j·kg−1] h∗ specific enthalpy in donor subchannel [j·kg−1] ktrans. transverse flow pressure loss coefficient l effective length of control volume for transversal flow [m] p power [w] p0 total power generated in fuel assembly [w] p water pressure [pa] lin.q linear heat rate transferred to subchannel [w·m−1] surf.q heat flux to subchannel [w·m−2] s subchannel flow area [m2] s effective width of control volume for transversal flow [m] tclad. cladding temperature [k] t∞ bulk water temperature [k] w diversion crossflow [kg·m−1·s−1] w water velocity [m·s−1] w∗ water velocity in donor subchannel [m·s−1] w′ turbulent crossflow [kg·m−1·s−1] ∆z length of axial node [m] α heat transfer coefficient [w·m−2·k−1] β turbulent factor ε surface roughness [m] ϑ fraction of heat transferred to coolant from whole fuel tube λf water conductivity [w·m−1·k−1] µ water dynamic viscosity [pa·s] ρ water density [kg·m−3] ρ∗ water density in donor subchannel [kg·m−3] φ normalized relative power ax. referring to axial nodalization. az. referring to azimuthal nodalization. i subchannel i n axial node n rad. referring to radial nodalization. nu nusselt number pr prandtl number re reynolds number 8 vol. 37/2022 subsals: a subchannel thermal-hydraulic code references [1] v. nasonov, e. ryazantsev, a. taliev, a. yashin. water velocity determination in the gaps of irt-3m, -4m fuel assemblies. atomic energy 110(389), 2011. https://doi.org/10.1007/s10512-011-9439-8. [2] international atomic energy agency. research reactor database. [2022-06-30], https://nucleus.iaea.org/rrdb/#/home. [3] f. huet. lvr-15 fuel assembly qualification plan. tech. rep., technic atome, 2020. [2022-06-30], https://ec.europa.eu/research/participants/ documents/downloadpublic?documentids= 080166e5d575034a&appid=ppgms. [4] l. s. tong, j. weisman. thermal analysis of pressurized water reactors. american nuclear society, la grange park, illinois usa, 1996. [5] m. galassi, et al. gnu scientific library reference manual (3rd ed.). [2022-06-30], isbn 0954612078, http://www.gnu.org/software/gsl/. [6] i. h. bell, j. wronski, s. quoilin, v. lemort. pure and pseudo-pure fluid thermophysical property evaluation and the open-source thermophysical property library coolprop. industrial & engineering chemistry research 53(6):2498–2508, 2014. https://doi.org/10.1021/ie4033999. [7] h. blasius. das aehnlichkeitsgesetz bei reibungsvorgängen in flüssigkeiten. 1913. in: mitteilungen über forschungsarbeiten auf dem gebiete des ingenieurwesens, 131:1–41, isbn 978-3-662-01944-3, https://doi.org/10.1007/978-3-662-02239-9_1. [8] d. j. zigrang, n. d. sylvester. a review of explicit friction factor equations. journal of energy resources technology 107(2):280–283, 1985. https://doi.org/10.1115/1.3231190. [9] information systems laboratories, inc. relap5/mod3 code manual. volume 4, models and correlations. tech. rep., 1995. https://doi.org/10.2172/106466. [10] r. k. salko, t. s. blyth, et al. ctf validation and verification manual. tech. rep., 2016. https://doi.org/10.2172/1342678. [11] d. s. rowe. cobra iiic: digital computer program for steady state and transient thermal-hydraulic analysis of rod bundle nuclear fuel elements 1973. https://doi.org/10.2172/4480166. [12] a. liu, b.-w. yang, b. han, x. zhu. turbulent mixing models and other mixing coefficients in subchannel codes—a review part a: single phase. nuclear technology 206(9):1253–1295, 2020. https://doi.org/10.1080/00295450.2020.1792753. [13] r. pegonen, s. bourdon, c. gonnier, h. anglart. an improved thermal-hydraulic modeling of the jules horowitz reactor using the cathare2 system code. nuclear engineering and design 311:156–166, 2017. https://doi.org/10.1016/j.nucengdes.2016.11.029. [14] f. dittus, l. boelter. heat transfer in automobile radiators of the tubular type. international communications in heat and mass transfer 12(1):3–22, 1985. https://doi.org/10.1016/0735-1933(85)90003-x. [15] b. petukhov. heat transfer and friction in turbulent pipe flow with variable physical properties. vol. 6 of advances in heat transfer, pp. 503–564. elsevier, 1970. https://doi.org/10.1016/s0065-2717(08)70153-9. [16] deterministic safety analysis for nuclear power plants. no. ssg-2 (rev.1) in specific safety guides. international atomic energy agency, vienna, 2019. 9 https://doi.org/10.1007/s10512-011-9439-8 https://nucleus.iaea.org/rrdb/#/home https://ec.europa.eu/research/participants/documents/downloadpublic?documentids=080166e5d575034a&appid=ppgms https://ec.europa.eu/research/participants/documents/downloadpublic?documentids=080166e5d575034a&appid=ppgms https://ec.europa.eu/research/participants/documents/downloadpublic?documentids=080166e5d575034a&appid=ppgms http://www.gnu.org/software/gsl/ https://doi.org/10.1021/ie4033999 https://doi.org/10.1007/978-3-662-02239-9_1 https://doi.org/10.1115/1.3231190 https://doi.org/10.2172/106466 https://doi.org/10.2172/1342678 https://doi.org/10.2172/4480166 https://doi.org/10.1080/00295450.2020.1792753 https://doi.org/10.1016/j.nucengdes.2016.11.029 https://doi.org/10.1016/0735-1933(85)90003-x https://doi.org/10.1016/s0065-2717(08)70153-9 acta polytechnica ctu proceedings 37:1–9, 2022 1 introduction 2 general description of the subchannel solver subsals 2.1 irt type fuel and computational mesh 2.2 basic conservation equations 2.3 constitutive equations 3 code-to-code comparison 3.1 friction flow split 3.2 pressure loss 3.3 axial temperature profile and cladding temperature 3.4 maximum cladding temperature 4 conclusions and outlook list of symbols references https://doi.org/10.14311/app.2022.33.0480 acta polytechnica ctu proceedings 33:480–488, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague microstructural characterization of high-performance steel fiber reinforced geopolymer concrete zoi g. rallia,∗, stavroula j. pantazopouloua, vladimiros g. papangelakisb a york university, department of civil engineering/lassonde school of engineering, 4700 keele street, toronto, m3j 1p3, canada b university of toronto, department of chemical engineering and applied chemistry, 200 college street toronto, m5s 3e5, canada ∗ corresponding author: zoiralli@yorku.ca abstract. due to growing environmental and economic concerns associated with conventional building materials, research interest gravitates towards the development of novel environmentally friendly materials as alternatives to conventional portland cement concrete. geopolymer concrete is a class of novel advanced and sustainable structural materials that hold promise for the future of infrastructure. its synthesis comprises industrial by-products (fly ash and slag among others) in the role of binder and thus reduces the demand in portland cement leading to a significant carbon footprint reduction. in the present study a high-performance fiber reinforced geopolymer concrete (hpfrgc) is synthesized from first principles and is subsequently characterized, with particular emphasis on its microstructural and mineralogical properties. the study explores the linkage between the microstructure and mineralogy of the precursors, and the properties of the final product. both fresh and hardened hpfrgc are studied. experimental results illustrate the correlation between microstructure, mineralogy and final mechanical properties can be used as an indicator of suitability of industrial by-products for geopolymer precursors. the effect of these choices on stability and physical properties of the material is also explored in the study. keywords: advanced materials, geopolymer concrete, microstructure, mineralogy, sustainability. 1. introduction there is no doubt that nowadays the need for construction building materials has been intensified due to the increase of urbanization rates and the acceleration of global population growth. a commensurate increase in demand for ordinary portland cement (opc) conventional concrete is expected since construction is a basic leverage of economic development and cement is the cornerstone of the modern infrastructure. however, despite of its widespread use, conventional concrete is not a sustainable solution as it impacts the environment with a potential of considerably high cost for mitigating its implications. production of cement, as the main component of opc concrete, is a high-energy consuming process and a major contributor to global warming. note that for every 1 ton of produced opc, an equal amount (i.e., 1 ton) of co2 is generated. additional consumption of energy relating to aggregate production and crushing, contributes to the unsustainable depletion of natural resources. it is expected that the green taxes imposed on cement industry as penalty for not reducing the co2 footprint will skyrocket the cement price. consequently, environmental, and economic concerns emerging from the energy intensive industry of building materials impose a need for reconsideration of every step in their production; one important mitigating strategy is the utilization of innovative alternative cements that have much smaller, or zero co2 footprint. such materials are geopolymer binders, encompassing a variety of industrial wastes in the role of a binder and filler. geopolymers represent the latest advancement in alumino-silicate chemistry and they were first developed by davidovits in the 1970s [1]. alternative binders are used for their production with activation mechanism, chemistry, and composition fundamentally different from that of cementitious materials. the term geopolymers refers to mineral compounds, or blends thereof, consisting of repeated units, such as silico-oxide (-si-o-si-o-) and silico-aluminate (si-o-al-o-) units, generated through a bonding process called geopolymerization. in this process, many small atomic structures, also known as oligomers, are linked together to form a covalently bonded threedimensional network. these geo-chemical syntheses occur in the presence of a reactive aluminosilicate material, also known as precursor, either in alkaline medium in the presence of alkali ions such as na+, k+, li+, ca++ and cs+, or in acidic medium in the 480 https://doi.org/10.14311/app.2022.33.0480 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 33/2022 microstructure of fiber reinforced geopolymer concrete length diameter aspect ratio density tens. strength elastic modulus lf df lf /df ρf ft ef 13 mm 0.2 mm 81.25 7.9 (g/cm3) 2500 mpa 200 gpa table 1. properties of steel fibres. figure 1. particle size analysis of dry materials. presence of phosphoric and humic acids. due to the high cost of the latter method, the most common route for geopolymer concrete is the alkaline route especially in the presence of sodium or potassium silicate solutions, which are also known as hardeners [2]. generally, geopolymer binders demonstrate superior performance and have great advantages over conventional concrete. the development of strength and is not based on a hydration mechanism, but rather on a rigid three-dimensional network formation and is rapid compared to conventional concrete. contrary to conventional concrete, geopolymer concrete does not possess hydrates in its microstructure; this leads to a higher stability at elevated temperatures [3]. regarding sustainability and life cycle cost, its production is associated with considerable reduction of the carbon footprint as a result of the absence of cement. in addition, the utilization of industrial waste by-products such as fly ash, blast furnace slag or red mud as source materials provides an additional benefit. it adds value to wastes, while the use of such materials instead of disposal is beneficial for the environment. the most critical aspect of developing a geopolymer concrete is the right selection of the aluminosilicate source materials. the selection criteria such as high performance and sustainability are determined through deeper understanding of the physicochemical properties of the material. a suitable source material (i.e., precursor) should be highly amorphous, possess the ability to release aluminium easily and contain sufficient reactive glassy content with low water demand. several studies have been conducted on different types of binders and how they affect the properties of the geopolymer concrete. in literature, a variety of aluminosilicate raw or processed natural materials, such as metakaolin or industrial byproducts such as fly ash, as well as granulated blast furnace slag, have been used alone or in combinations as binders for geopolymer concrete and their mechanical and physical properties along with their microstructure have been studied. despite the growing interest of the research community on the characterization of geopolymer concrete, there is a limited number of investigations that explore the effect of precursor’s properties on the properties of the geopolymer concrete. in the present study, a combination of the most commonly used supplementary cementitious materials, namely metakaolin, fly ash type with cao less than 10% (type f), silica fume and blast furnace slag were used for the production of a geopolymer matrix with enhanced early strength and high performance. industrially produced potassium silicate solution was used as the alkaline reagent, whereas brass coated steel fibres were used for the main reinforcement. subsequently, the geopolymer was cured at ambient temperature and was then characterized in terms of fresh, hardened, physical, microstructural, and mineralogical properties. a relation between the latter and the raw materials used was also identified. 481 z. g. ralli, s, j. pantazopoulou, v. g. papangelakis acta polytechnica ctu proceedings % chem. ffa sf mk-750 ggbfs % chem. compos. ffa sf mk-750 ggbfs sio2 56.5 94.5 52.51 35.2 k2o 0.84 0.59 0.19 0.59 al2o3 23.5 0.41 43.3 8.37 tio2 0.63 < 0.01 1.42 0.34 fe2o3 3.55 0.16 0.47 0.68 p2o5 0.09 0.08 0.06 0.02 mgo 1.08 0.3 0.05 10.7 mno 0.06 0.04 < 0.01 0.27 cao 9.1 0.49 0.08 38.9 cr2o3 0.02 0.02 0.03 0.02 na2o 2.13 0.15 0.35 0.31 v2o5 loi 0.01 1.89 < 0.01 3.5 0.03 2.01 < 0.01 2.01 table 2. chemical composition of precursors. 2. experimental 2.1. materials fly ash type f (ffa), ground granulated blast furnace slag (ggbfs), silica fume (sf), metakaolin750 (mk-750) were used as geopolymer precursors. basalt (bs) and granite (gs) were chosen as partially reactive sands due to their enhanced bonding with the binder [4]. a commercially available potassium silicate with molar ratio (mr) of 1.7 was selected as the hardener (ps) on account of its high performance and stability. preliminary studies conducted prior to this project, had shown that an industrially produced hardener with a fixed mr performs better than a laboratory made hardener. the better performance is in terms of fluidity and repeatability, while temperature changes do not affect its properties. straight steel fibers (f) with length 13 mm and diameter 0.2 mm were utilized for main reinforcement at a 2.5% volume fraction (table 1). finally, a polycarboxylate-based high-range water reducing admixture (hrwra) was used to obtain the desired fluidity. for the best quality control and understanding of the geopolymer concrete, the physical, chemical, and mineralogical properties of the studied precursors were analysed through a series of characterization techniques. 2.1.1. particle size analysis the particle size analysis was performed on a laser diffraction particle size analyser (beckman coulter ls 13 320). for very fine particulate materials, surfactants were used for deagglomeration. figure 1 shows the particle size distribution of the analysed materials. 2.1.2. chemical analysis x-ray fluorescence (xrf) was performed to get the chemical composition of the precursors. the data given in table 2 confirm that the aluminosilicate source materials could be potential candidates as geopolymer binders as they contain elevated amounts of alumina and silica in combination with the low magnesia [5]. however, the cao content in the ffa (close to 10%) poses a concern regarding the speed of setting. 2.1.3. mineralogical analysis the mineralogy of the aluminosilicate source materials is also significant in geopolymerization. for this reason, measurements were made to determine through x-ray diffraction (xrd) the mineralogy and degree of crystallinity of the raw materials. table 3 illustrates the results of the analysis. the presence of mullite and quartz in ffa in combination with the relatively lower amorphous content (compared to the rest) suggests that the combustion temperature of ffa was higher than 850 ◦c but lower than 1500 ◦c [6]. ggbfs was found highly amorphous with the presence of gehlenite to be favourable in terms of geopolymerization. however, previous studies have shown that it cannot be used alone as a geopolymer precursor. during alkalination, gehlenite depolymerizes into hydrates and precipitates alkalis. the resulting product is not a geopolymer, but an alkali activated slag, which despite its high strength, there are problems of stability and durability in case the material comes in contact with water. this is because geopolymerization is incomplete and the alkali cation from the hardener is not chemically bonded to the structure [2]. 2.1.4. reactivity index another reactivity and suitability index was determined through ph measurements. this measurement determines whether there is a risk of flash setting [5]. the ph of aqueous solutions consisting of 5 g of raw material in 50 ml of distilled water was measured after 5 minutes of preparation. generally, values lower than 8 do not indicate danger for instant hardening, whereas fast hardening occurs for values between 8 and 11 and flash-set is highly possible for values above 11 [5]. the recorded values of ph of the materials are listed in table 4. 2.2. methods using the data from the characterization steps and taking into consideration cost and sustainability factors, three different mixtures of geopolymer concrete were studied with different ratios of ggbfs to total binder (0%, 5% and 10% of the precursors). the mixture design (table 5) is based on the fundamental concepts for geopolymerization [7]. 482 vol. 33/2022 microstructure of fiber reinforced geopolymer concrete mineral mineral formula ffa sf mk-750 ggbfs quartz sio2 11.1 1.4 mullite 2al2o3 · sio2 19.4 calcite caco3 0.5 silicon carbide sic 2.8 gehlenite 2cao · al2o3 · sio2 3.2 alite 3cao · sio2 1.2 larnite 2cao · sio2 1.2 amorphous 69.0 97.2 98.6 95.4 table 3. mineralogical composition of precursors in wt.%. ph value after 5 ffa (aq.) sf (aq.) ggbfs (aq.) mk(aq.) 11.5 8.1 11.8 7.1 table 4. ph of aluminosilicate materials. materials (kg/m3): ffa sf mk-750 ggbfs k-silicate h2o basalt granite hrwra mix design solution sand sand gs10 907 145 75 94 355 71 574 203 18 gs5 907 145 80 47 355 71 574 203 18 gs0 907 145 92 355 71 574 203 18 steel fibres: all mixes contained 160 kg/m3 table 5. mixture designs. 3. results & discussion 3.1. fresh properties flowability tests were conducted to evaluate the fresh properties of high performance fiber reinforced geopolymer concrete (hpfrgc). the test was performed in accordance with astm c1856 [8]. the flowability of each mixture was measured immediately after mixing and right before casting. during this measurement, the conical mould of the flow table was filled with a single layer of fresh geopolymer concrete. then, the mould was lifted and after one minute, the reported flow value was calculated as the average of maximum and minimum diameter of the spread. the flow values were 157, 172 and 181 mm for the mixtures gs10, gs5 and gs0, respectively. the increase in flowability with the decrease of ggbfs even in such small proportions was evident and linked to the fineness and high cao content of ggbfs. the final setting time was also measured empirically as the time from the start of casting to the time the geopolymer completely hardens. the final setting times were 15, 22 and 30 min for the mixtures gs10, gs5 and gs0, respectively. the short setting time was attributed to the high ph of the precursors and particularly the high content of cao in both ggbfs and ffa. 3.2. compressive strength a force-controlled compression-testing machine was used to test the 50 mm cubes under compression at a loading rate of 0.259 mpa/s in accordance with astm c39 [9]. to study the age effect on compressive results, the specimens were tested at the age of 1, 7 and 28 days. the compressive strength results of each mixture at different age is presented in figure 2. note that the reduction in the ggbfs did not affect the strength at early ages owing to the substitution by mk-750. however, the absence of ggbfs had a more prominent effect on the final strength of gs0. for the mixing of geopolymer concrete, a 5 l hobart mixer was used. the mixing process comprises of dry mixing of ffa with sf for 3 minutes, followed by the addition of the alkaline reagent which was mixed with water and high-speed mixing (200 rpm) for 10 minutes. next, mk-750 is added and mixed for 5 minutes. the last binder material added was ggbfs and was mixed for another 3 minutes. subsequently the sand mixture was added to the system and the mixing continued for 2 min. at this point hrwra was introduced to lower the viscosity and prepare the mix for fibre impregnation. when the mix gained the desired fluidity, fibres were added and mixing continued for 2 more minutes. the concrete was then casted in cubic moulds of 50 mm size (minimum dimension had to be about 4-5 times the fibre length). mild vibration was then performed through the use of tamping rods. finally, the specimens were 483 z. g. ralli, s, j. pantazopoulou, v. g. papangelakis acta polytechnica ctu proceedings figure 2. compressive strength development of hpfrgc. figure 3. qemscan images of hpfrgc thin sections. (a) phase assemblage maps (b) total mineral contents of each sample by mass. figure 4. schematic of thin section qemscan images and the number of fibres per cross section. 484 vol. 33/2022 microstructure of fiber reinforced geopolymer concrete figure 5. ternary diagram of hpfrgc. sealed with a plastic sheet to prevent water evaporation. the specimens were demoulded after 24 h and cured in water at ambient temperature until the day of testing. 3.3. qemscan analysis in order to study the microstructure and mineralogy of the produced geopolymer concrete a qemscan (quantitative evaluation of materials by scanning electron microscopy) analysis was conducted. the system is widely used in the minerals industry as it provides phase and mineral identification at micrometer-scale. polished thin sections were taken from each hpfrgc cube sample. the qemscan results are illustrated in figure 3a. the resolution of the produced images is 15 µm. figure 3b also shows the quantitative composition of the geopolymer mixtures. the mineral maps of all mixtures contain the same major mineral, k-feldspar. the second prevailing mineral is albite. the results agree with what is expected from a geopolymerization process as the k-feldspar and albite are frameworks with 3-dimensional networks. more specifically the produced concrete is a k, na-poly(sialate-disiloxo) with atomic ratio si/al=3 whereas its geological analogue is sanidine and albite [6]. the white parts of the images represent the porosity. the blue parts correspond to fibres with their brass coating intact while the black represent the steel fibres that have lost their coating during hardening. 3.4. fiber distribution qemscan images are used as a tool to interpret the distribution of fibres in the thin sections. a schematic (figure 4) is used to assess the number of fibres crossing an arbitrary plane, a parameter that is critical for assessing their contribution to the mechanical properties of the solid material. the particles with black and blue colour having a dimension in their short axis at least equal to the fibre diameter df = 0.2 mm were identified as fibres (the typical fibre is a cylinder with a diameter of 0.2 mm oriented randomly with respect to the plane of the cross section, therefore in general the intersection has an elliptical shape). in a random arrangement of fibres, the number of fibres, nf , crossing a unit area through hpfrgc is estimated from the cross-sectional area of a single fibre having a diameter (df ) and the volumetric ratio of the fibres, vf [10]. nf = 0.64vf d 2 f (1) equation 1 has been derived assuming only 50% of the fibers to be effective on account of random spatial orientation; for preferential arrangement of the fibers effected by the casting procedure this factor may be higher. for vf = 2.5% and df = 0.2 mm, the nf should be 0.4 per mm2. based on the aforementioned data, ideally the number of fibers at the given randomly cut 11 × 11 mm cross-sections should have been around 54; on average the three samples shown in figure 5 agree quite well with this estimation. 3.5. ternary mix design the microstructure of the studied system was assessed by a ternary diagram (figure 5), plotted from qemscan of the most relevant minerals reported in 485 z. g. ralli, s, j. pantazopoulou, v. g. papangelakis acta polytechnica ctu proceedings figure 6. (a) porosity distribution in hpfrgc measured with qemscan analysis ; (b) fractured hpgc specimen with bright core at 28 days. figure 3b. the corners of the triangle correspond to 100% stoichiometric content of the main oxides, i.e., sio2, al2o3, k2o, na2o & cao. the sides represent axes, depicting weight fractions that range from 0 to 1. any point within the triangle has coordinates on the system of the three axes that represent the fractional ratio of mass content of the standardised oxides; the combination characterizes the stability of physical and durability performance of the resulting composite and therefore may be used to distinguish products that could be classified as alkali activated materials (aam) from geopolymers. in the present study all the systems show very similar phase assemblage which is relatively close to that of the stoichiometric geopolymer [7]. 3.6. porosity qemscan images were further used to provide a rough estimation of porosity and its distribution at the analysed thin sections. figure 6 shows the porosity as a function of size and surface area. the porosity of the hpfrgc can be attributed to three different mechanisms: (a) the production of gas, (b) the fast increase of viscosity with time during casting, and (c) the compaction of the material resulting from simple casting rather than extrusion [11] the first mechanism is a result of incorporation of sf in the mix design. it is worth noting that studies have shown that even small content (< 1 %) of elemental silicon in sf could generate gas approximately 1 to 11 ml/gr of sf [12]. more specifically, this elemental silicon is oxidized in an alkaline environment (equation 2). h2 is generated as a result of water reduction and acts as a foaming agent (equation 3). the overall reaction forms si(oh)4 as shown in equation 4. the chemically induced porosity seems to yield micropores in the range of 0 − 50 µm. this is clearly shown in figure 6 where the porosity distribution is depicted demonstrating that all mixtures have similar percentages of 0 − 50 µm micropores. si0 → si4+ + 4e− (2) 4h2o + 4− e → 2h2 + 4oh− (3) 4h2o + si0 → 2h2 + si(oh)4 (4) the increased viscosity due to the addition of very fine and high-density particles such as sf, mk-750 and ggbfs and the incorporation of the fibres also contributes to the pore formation as the diffusion of the generated gas is enhanced [11]. finally, compaction also affects the porosity, as it depends on the viscosity of the mixture. in case of g10s, the mixture with the highest amount of ggfbs was difficult to compact as much as the other two. this is evident from the high content of pores with maximum size greater then 500 µm in g10s, as shown in figure 6(a). owing to the micropores generated as described above the geopolymer materials are generally lightweight with an apparent density lower than 2 kg/l. for example, the self-weight of the three mixtures tested was 1.9 kg/l. after testing the cubes in compression, and upon examination of fragments from the core of the failed specimens, a whiteish 486 vol. 33/2022 microstructure of fiber reinforced geopolymer concrete discoloration was observed. since this phenomenon could not be studied thoroughly in the hpfrgc broken cubes, as they maintained their integrity on account of the embedded fibres, a series of unreinforced high performance geopolymer concrete cubes were tested in compression to study their fracture surface. although the specimens were cured in water, the core of the specimen is brighter than the rest because it is dried. this is illustrated in figure 6(b). the extent of the dried core area is related to the age of testing, as at the early ages is relatively small while it increases with the time. the reaction of geopolymerization starts from the core and expands through the volume of the material. in the white regions of the core the pores have smaller size compared to the dark regions and with the expansion of the white region (development of geopolymerization) the porosity decreases in the specimen. with the progress of geopolymerization the size of pores becomes gradually smaller, whereas water is expelled as hardening propagates. 4. conclusions from the above experimental investigation, the following conclusions are drawn. 1. a full characterization in terms of physical, chemical, and mineral properties is needed to evaluate the suitability of locally available aluminosilicate source materials as geopolymer binders. the determination of the main precursor should be determined taking into consideration the results of the characterization tests. 2. the phase assemblage of the hpfrgc indicates successful geopolymerization similar to natural rocks such as k-feldspar and albite. the produced hpfrgc is k,na poly(sialate-disiloxo). 3. examining the fresh and hardened properties of hpfrgc, it is concluded that ffa with cao content close to 10% can be used as main precursor in a geopolymer concrete that can harden in ambient temperature and yield high compressive strengths. however, this is accompanied by progressively shorter setting times as ggbfs is added to enhance the final strength. ggbfs has a negative effect in viscosity too, and thus handling of the material. finally, the studied hpfrgc is an excellent candidate for cases where early strengths are needed, as the geopolymer concrete developed in 24 h after casting almost half strength it would have developed in 28 days. 4. all the three mixtures have acceptable number of fibers as proven by examining randomly cut sections. 5. the porosity of the studied hpfrgc is induced by chemical reactions, viscosity, and consolidation. for the chemically induced porosity, pretreatment of aluminosilicate materials to eliminate the free silicon is needed, while workability or viscosity modifying admixtures could be used to decrease the porosity generated from high viscosity and poor consolidation. acknowledgements the authors duly acknowledge the contributions of dr. grammatikopoulos and sgs, canada in carrying out the characterization and microstructural analysis. the support of wöellner, and drain bros. excavating ltd. by donating the alkaline reagent and the basalt and granite sands, respectively, is also acknowledged. references [1] j. davidovits. solid phase synthesis of a mineral blockpolymer by low temperature polycondensation of alumino-silicate polymers, 1976. [2] z. g. ralli, s. j. pantazopoulou. state of the art on geopolymer concrete. international journal of structural integrity 12(4):511-33, 2020. https://doi.org/10.1108/ijsi-05-2020-0050. [3] [4] a. reggiani. types of automatic mixing systems for geopolymer mortar/concrete production and 3d printing geopolymer camp 9-11 july 2019, saint quentin, france, 2019. [5] j. davidovits. geopolymer chemistry & applications, 4th ed, geopolymer institute, saint quentin, france, 2015. [6] j. davidovits, m.izquierdo, x.querol, et al. the european research project geoash: geopolymer cement based on european coal fly ashes, technical paper 22, geopolymer institute library, 2014. https://www.geopolymer.org/wp-content/uploads/g eoash.pdf. [7] w. m. kriven. geopolymer-based composites, ceramic and carbon matrix composites, comprehensive composite materials ii,ăelsevier, oxford, 8(5): 269-279, 2018. https: //doi.org/10.1016/b978-0-12-803581-8.09995-1. [8] american society for testing and materials. c1856/c1856m: fabricating and testing specimens of ultra-high-performance concrete, 2017. [9] american society for testing and materials (astm). c39: standard test method for compressive strength, 2005. [10] n. d. archontas, s. j. pantazopoulou. microstructural behavior and mechanics of nano-modified cementitious materials adv. concr. constr, technopress, 3(1):15-37, 2015. doi:https://doi.org/10.12989/.2015..3.014. [11] e. prud’homme, p. michaud, e. joussein, et al. silica fume as porogent agent in geo-materials at low temperature. journal of the european ceramic society 30(7):1641-8, 2010. https: //doi.org/10.1016/j.jeurceramsoc.2010.01.014. 487 https://doi.org/10.1108/ijsi-05-2020-0050 https://www.geopolymer.org/wp-content/uploads/geoash.pdf https://doi.org/10.1016/b978-0-12-803581-8.09995-1 https://doi.org/10.12989/.2015..3.014 https://doi.org/10.1016/j.jeurceramsoc.2010.01.014 z. g. ralli, s, j. pantazopoulou, v. g. papangelakis acta polytechnica ctu proceedings [12] m.-h.zhang, v. m. malhotra, j. wolsiefer. determination of free silicon content in silica fume and its effect on volume of gas released from mortars incorporating silica fume. aci materials journal 97(5), 2000. https: //www.researchgate.net/profile/min-hong-zhang/p ublication/250613568_determination_of_free_sili con_content_in_silica_fume_and_its_effect_on_v olume_of_gas_released_from_mortars_incorporati ng_silica_fume/links/5dcff98492851c382f43f4e9/ determination-of-free-silicon-content-in-silic a-fume-and-its-effect-on-volume-of-gas-release d-from-mortars-incorporating-silica-fume.pdf. 488 https://www.researchgate.net/profile/min-hong-zhang/publication/250613568_determination_of_free_silicon_content_in_silica_fume_and_its_effect_on_volume_of_gas_released_from_mortars_incorporating_silica_fume/links/5dcff98492851c382f43f4e9/determination-of-free-silicon-content-in-silica-fume-and-its-effect-on-volume-of-gas-released-from-mortars-incorporating-silica-fume.pdf acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.38.0368 acta polytechnica ctu proceedings 38:368–374, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague a review of the fiva project: novel windows employing vacuum glazing products ulrich j. ponta,∗, peter schoberb, magdalena wölzla, matthias schussa, jakob haberlb a tu wien, department of building physics and building ecology, karlsplatz 13, a-1040 vienna, austria b austrian forest products research society (hfa-ögh), franz-grill-straße 7 a-1030 vienna, austria ∗ corresponding author: ulrich.pont@tuwien.ac.at abstract. vacuum glazing products have been in development for the past decades. such glazing products regularly feature two parallel glass panes that have a small, evacuated gap in their interstitial space. to maintain the vacuum and the form of the glass product, regularly vacuum tight edge seals and a grid of distance pillars are integrated. during the years of development, the major focus was set on the production and durability aspects of the glass products, but relatively few efforts had been conducted towards the integration of such glass products into window constructions. employing typically used doubleand triple glazing windows’ frames does not represent a feasible option. this is due to the specification given by vacuum glazing products, such as their small thickness and the requirement for sufficient glass edge coverage due to the major thermal bridge adjacent to the edge seal. the authors, together with major players from the window producing industry, started a r&d effort that targeted disruptive new concepts for vacuum glass windows. four different designs were developed that not only integrated vacuum glass products, but also featured unusual opening patterns, the latest generation of electrically driven fitting products, and specific seals. the thermal and acoustical performance of the prototypes was improved during the development via employment of numeric thermal simulation and lab testing. the present contribution illustrates the four prototypes and their performance, which – for instance – pertaining to the uw-value is down to 0.6–0.7 w · m−2 · k−1 at a glass thickness of less than 1 cm. keywords: vacuum glazing, windows, thermal performance, functional prototypes, numeric thermal bridge simulation. 1. introduction the present contribution reports on the outcome of a recently conducted research project, which focused on the development of highly-insulating windows of contemporary design, which employ vacuum glass products. such vacuum glass products became available during the past decade, but have been under constant development for over one century, if one accepts the patent of zoller from 1913 [1] as a first approach for such glazing products. to the knowledge of the authors, zoller was the first one to suggest an “evacuated space” between two panes of glass. major challenges in his and all further development efforts included the upkeep of the vacuum in the glass product, which required lots of development effort in view of principal concepts of constituents, industrial production processes, and material science. currently, different vacuum glass products can be found on the market [2– 5], which differ not only in their region of origin, but also majorly in used technologies and glass availability. independently of the manufacturer, vacuum glass panels regularly consist of a set of constituents. these are: (i) (at least) two parallel glass panes, (ii) a small, interstitial gap between the panes, which is evacuated, (iii) a set of distance pillars, maintaining the parallel position of the two glass panes against the air pressure load; (iv) a gas/vacuum tight edges seal that covers the perimeter of the overall product and is essential for keeping up the vacuum; (v) a so-called getter unit, which chemically binds left over particles with in the evacuated space; (vi) an evacuation opening that is used to evacuate the interstitial space and is closed after evacuation with a vacuum tight seal. figure 2 illustrates the major constituents of a vacuum glass product schematically. figure 1 illustrates different vacuum glass products from different commercial providers, and some key data about them. in view of thermal building performance aspects, vacuum glass products feature very low thermal transmittance values (ug-values), which is due to the widely eliminated conductive and convective heat transfer processes. these values can be reached at very thin system thicknesses of 1 cm or less. while the overall thermal insulation of vacuum glass products show convenient performance indicators, it has to be con368 https://doi.org/10.14311/app.2022.38.0368 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 38/2022 a review of the fiva project: novel windows . . . (a). synergy / china distance of pillars: 40 mm seal thickness: 10 mm getterpoint diameter: 14 mm ug: 0.58 w · m−2 · k−1 (b). eagon / korea distance of pillars: 40 mm seal thickness: 10 mm getterpoint diameter: 18 mm ug: 0.48 w · m−2 · k−1 (c). guardian / usa distance of pillars: 40 mm seal thickness: 10 mm getterpoint diameter: 16 mm ug: 0.47 w · m−2 · k−1 (d). agc interpane / belgium distance of pillars: 20 mm seal thickness: 5 mm getterline: 2 mm ug: 0.7 w · m−2 · k−1 figure 1. from left to right: different vacuum glazing products (partly [2–5], partly own illustrations). figure 2. terminology of the major constituents of vacuum glass products. sidered that both distance pillars and the tight edge seal pose thermal bridges. it has been proven that the distance pillars – as punctual thermal bridges – do not significantly affect the performance of the vacuum glass panes [6], due to their small dimension and used material. this means that neither condensation risk nor large heat losses can be expected based on the distance pillars. however the tight edge seals are linear thermal bridges of significant contact area with both glass panes, and thus require consideration in window constructions. given these aspects, it seems clear that both in the case of new windows and in the case of existing window constructions require in-depth analysis and adaptation to the characteristics of vacuum glass: in the case of the new windows the overall window construction (including window wing, frame, and wall-connector detailing) should be adapted to the specifications of the vacuum glass. consequently, the overall operation and construction principles of such new windows should be overthought. aspects to be optimized should be building physics aspects (overall thermal performance, mitigation of thermal bridges, acoustical performance, etc.), aesthetics, integration in fast and clean mounting processes, and – last but not least – user comfort and usability. in the case of existing windows the improvement of the thermal performance at major upkeep of the overall appearance of the window might be focal. the authors conducted a set of projects that focused on new and existing windows equipped with vacuum glass. figure 3 illustrates the relation of these projects in view of new and existing windows and technical development and monitoring. note that the cooperative r&d projects included contributions by the industry and domain experts. the present contribution wants to highlight the method and results of the project fiva. in this project four different, innovative, vacuum-glass equipped windows have been developed. the starting point for this research and development project was to reconsider classic window constructions in view of technical innovations such as vacuum glass but also beyond: the four different window constructions have been designed, virtually constructed and subjected to different assessment methods based on numeric thermal simulation. moreover, mock-ups were constructed and evaluated by laymen and domain experts of the window construction industry. 2. methodology figure 4 illustrates the general methodological approaches of the research project fiva as work packages. based on the findings of the previous projects [7, 8], a convenient requirement analysis was conducted in wp2. moreover four design principles which had been envisioned during the previous project 369 u. pont, p. schober, m. wölzl et al. acta polytechnica ctu proceedings figure 3. landscape of r&d projects pertaining to vacuum glass integration into new and existing windows. figure 4. work packages (wps) of the project fiva. motive as drawings and small-scale models were selected for further development in fiva. these were a turn window opening to the inside (i), a turn window opening to the outside (a), a swing window (k), and a sliding window (s). these principles are illustrated in figure 5. the work packages 3, 4, and 5 were conducted in parallel and under iterative information exchange. this was necessary due to the strong interdependencies between these domains. tools and instruments used in these work packages included numeric thermal simulation for work package 4, conducted with a state of the art tool [9], small physical construction models and comprehensive testing on normative test beds. figure 6 illustrates some early stage construction physical function-models. the work package dealing with aspects of nonthermal and non-acoustical technical aspects such as airand water-tightness, resistance against winddriven rain and durability, were conducted via functional mock-ups on test beds. the user-relevant aspects included an evaluation of the window operation and aspects such as universal design. fully functional mock-ups were constructed and presented and exhibited at the annual convention of window and door constructing companies in austria in 2020. thereby, peers of the relevant domain were asked to fill a questionnaire about the different prototypes. likewise, non-domain experts were confronted with the prototypes at a later stage to also harvest laymen opinion. 3. results and discussion 3.1. window prototypes i, a, k and s in an early stage of the development the decision was made to not only generate highly-insulating windows with low thermal transmittance, but also to motorize the window operation at least in part. the idea behind was to integrate a ventilation position in each of the prototypes that allows user-independent operation and automation. thereby, a basic requirement was defined as a fully automated change from locked and closed position to the ventilation position. the further opening (that is from ventilation position to fully opened position) was foreseen as manual operation in 370 vol. 38/2022 a review of the fiva project: novel windows . . . (a). turn window opening to the inside (i). (b). turn window opening to the outside (a). (c). swing window (k). (d). sliding window (s). figure 5. selected operational window concepts elaborated in the fiva project. figure 6. early functional prototypes for sliding windows and turn windows. all but one of the prototypes. in the following subsections, the different prototypes are briefly presented and illustrated. 3.1.1. window prototypes i: turn window opening to the inside turn windows that open to the inside are a wellestablished operation scheme in central europe, and thus come with high level of user acceptance. this well-established operation concept was adapted to the necessities of the vacuum glass. in the design, the glass surface was intended to be in line with surrounding inner wall surfaces. this was reached by situating the wing-frame of the window on the outer side of the vacuum glass pane via structural sealing. thereby, the required glass edge cover necessary to mitigate thermal bridge effects of the glass seal is exceeded. the operation works as follows: the wing is de-locked and moved to the ventilation position (about 10 cm offset) via the integrated electromotor. from this position the window can be opened to a full 90° opening angle. due to the turn movement to the inside, a moveable shading device can be deployed in the outer perimeter of the window aperture. another aspect of this window design is the positioning of all motors and moving fitting parts in the fixed part of the window frame. figure 7 illustrates the window prototype i. 3.1.2. window prototypes a: turn window opening to the outside turn windows that open to the outside are not a wellestablished concept in central europe in contemporary building construction despite the fact that this has been part of the architectural portfolio in past epochs and the undeniable advantages regarding space utilization of the adjacent interior space. however, many buildings in northern europe are equipped with windows that wings that can be turned to the outside. the window prototype a was designed to feature a rather homogeneous appearance by keeping the window clearance and glass clearance in the same dimension. the window has a motor-driven ventilation position, which is a 60 mm offset to the closed position. this still allows the usage of an external moveable shading device that can be lowered before the offset window wing. the window can be unlocked in the offset position to allow a 90 degree turn to the outside. figure 8 illustrates the prototype of window a. 3.1.3. window prototypes k: swing window in contrast to the previous two window prototypes, the swing window features the movement of 1960s and ’70s car port doors. two parallel, fixed rotation points allow a motor-driven swing-movement until the fully open position that stores the window wind closely under the room ceiling. the kinematics of the window can be considered as complex, but allows for very large windows. challenges connected with this window typology encompass the potential lack of user acceptance, the complex sealing detailing around the rotation points, and the difficult implementation of external shading devices. nonetheless, the window offers a set of advantages, such as large opening apertures, full motorization that allows to fix the wing in any opening position and the space-saving handling of the wing. figure 9 illustrates the prototype of window k. 3.1.4. window prototypes s: sliding window this window prototype combines offset and sliding movement. after being offset by motorized actuators, the window can be slid open manually. assets of the window include the aesthetics, as no sliding rails 371 u. pont, p. schober, m. wölzl et al. acta polytechnica ctu proceedings (a). view from inside. (b). view from inside. (c). view from outside including moveable shading membrane. (d). simulation model and false-color temperature distribution. figure 7. window prototype i. figure 8. window prototype a. different views and perspectives of prototype a. figure 9. window prototype k. different photographs of the prototype illustrating appearance in closed and open states as well as details. 372 vol. 38/2022 a review of the fiva project: novel windows . . . figure 10. window prototype s. from left to right: thumb cinema of the opening process, window and fitting detailing. prototype average grade (subjective evaluation) uw-value [w · m−2 · k−1] (ug-value = 0.7 w · m−2 · k−1) uw-value [w · m−2 · k−1] (ug-value = 0.4 w · m−2 · k−1) frsi [-] i 1.8 0.91 0.71 0.74 a 2.3 0.85 0.66 0.77 k 2.5 0.76 0.58 0.75 s 1.7 0.75 0.55 0.87 table 1. evaluation results (average grade), uw-values, and frsi-values of the different prototypes. or other hints unravel the operation, and the easy integration in any opaque wall system. equipped with vacuum glass, the window performs fine in view of thermal bridge aspects and overall heat transfer reduction. moreover, the offset movement allows for the use of rather noncomplex rubber seals. challenges include the uncommon and thus nonuser-proven operation, the cleaning of the transparent parts of the windows, and the size limitations of apertures by the telescope rails used in this prototype. never the less, we believe that this window prototype possess a high degree of innovation that might have disruptive effects for the window building industry. figure 10 shows prototype s. 3.2. evaluation by domain experts and thermal performance of the shown window prototypes we asked domain experts to evaluate the window mock-ups in an exhibition during the fenster-türentreff 2020 in salzburg, austria. evaluation criteria included aesthetics, innovation, technical aspects, mounting, and usability comfort and user acceptance. the results of this evaluation (conducted by nearly 50 experts) can be found in table 1 (austrian school grade system: 1 = excellent, 5 = insufficient). moreover, table 1 illustrates the uw-values and frsi-values that could be reached with two different vacuum glass products (glass i: ug = 0.7 w · m−2 · k−1, glass ii: ug = 0.4 w · m−2 · k−1). a full discussion of the thermal performance of the presented window prototypes can be found in [10, 11]. all prototypes show excellent u-values (given the glass pane size of less than 1 cm) and fulfil the condensation and mold-growth criteria (frsi ≥ 0.71, following en iso 10211 and önorm b 8110-2) [12, 13]. 4. conclusion & future research the present contribution highlighted the main outcome of the project fiva, which focused on generation of new, highly-insulating window prototypes that employ vacuum glazing products. both the construction generation process and insights in the performance analysis were provided. all four developed window prototypes can be considered to be in a rather high level of technology readiness. however, until such windows will emerge on the market, still a long way has to be gone, including further assessment and improvement of industrial construction processes, availability of vacuum glass on the european market, finalizing of the technology development regarding frames and glass pane integration, as well as supplementary parts (fittings and sealing). nonetheless, the project did prove the high potential of vacuum glass for innovative window products. 373 u. pont, p. schober, m. wölzl et al. acta polytechnica ctu proceedings acknowledgements research presented in this contribution was generously funded by the austrian research funding agency (fiva. p.no: 867352, 2018–2020). several companies contributed to the project (see [11]). references [1] a. zoller. hohle glasscheibe, 1913. patent-nummer: no 387655. [2021-06-17]. https://patents.google.com/patent/de387655c/de [2] fineoglass by agc / interpane. [2021-06-17]. https://www.fineoglass.eu [3] eagon – passive house certified glass / vacuum glass. [2021-06-17]. https://www.eagonusawd.com/product/12 [4] guardian vacuum glass ig. [2021-06-17]. https://www.guardianglass.com/content/dam/ guardianindustriesholdings/technical/europe/c2_ 1584_l1_glasstimehandbook_en_0222.pdf [5] synergy. beijing synergy vacuum glass. [2021-06-17]. http://en.xinliji-vg.com/aboutus/ [6] u. pont, a. mahdavi. a comparison of the performance of twoand three-dimensional thermal bridge assessment for typical construction joints. in m. baratieri, v. corrado, a. gasparella, f. patuzzi (eds.), building simulation applications proceedings. 2017. http: //www.antherm.at/antherm/c75_postreview.pdf [7] u. pont, p. schober, m. schuss, et al. motive – modellierung, optimierung, und technische integration von vakuumglas-elementen: sondierung über die detaillierung von vakuumgläsern in neuen holz(alu)fenster-konstruktionen (detaillierung, bau und simulation), 2018. berichte aus energieund umweltforschung 24/2018. 103 pages. https: //nachhaltigwirtschaften.at/resources/sdz_pdf/ berichte/schriftenreihe-2018-24-motive.pdf [8] u. pont, e. heiduk, p. schober, et al. vig-sys-reno sondierung von fenstersystemen mit innovativen gläsern, speziell vakuum-isoliergläsern, zur gebäudesanierung – berichte auf energieund umweltforschung, 2017. report for ffg – programm stadt der zukunft / bmvit. 218 pages. [9] antherm. [2021-06-17]. www.antherm.eu [10] u. pont, m. wölzl, m. schuss, et al. exploring novel solutions for incorporating vacuum glazing in new and existing window constructions. e3s web of conferences 172:24006, 2020. 12th nordic symposium on building physics (nsb 2020). https://doi.org/10.1051/e3sconf/202017224006 [11] u. pont, m. wölzl, m. schuss, et al. fensterprototypen mit integriertem vakuumglas: fiva, 2020. berichte aus energieund umweltforschung 47/2020. 85 pages. https://nachhaltigwirtschaften.at/resources/ sdz_pdf/schriftenreihe-2020-47-fiva.pdf [12] önorm en iso 10211 wärmebrücken im hochbau – wärmeströme und oberflächentemperaturen – detaillierte berechnungen 2008. (iso 10211: 2017). [13] önorm b 8110-2 wärmeschutz im hochbau – teil 2: wasserdampfdiffusion, -konvektion und kondensationsschutz (önorm b 8110 – 2020-01-01). 374 https://patents.google.com/patent/de387655c/de https://www.fineoglass.eu https://www.eagonusawd.com/product/12 https://www.guardianglass.com/content/dam/guardianindustriesholdings/technical/europe/c2_1584_l1_glasstimehandbook_en_0222.pdf https://www.guardianglass.com/content/dam/guardianindustriesholdings/technical/europe/c2_1584_l1_glasstimehandbook_en_0222.pdf https://www.guardianglass.com/content/dam/guardianindustriesholdings/technical/europe/c2_1584_l1_glasstimehandbook_en_0222.pdf http://en.xinliji-vg.com/aboutus/ http://www.antherm.at/antherm/c75_postreview.pdf http://www.antherm.at/antherm/c75_postreview.pdf https://nachhaltigwirtschaften.at/resources/sdz_pdf/berichte/schriftenreihe-2018-24-motive.pdf https://nachhaltigwirtschaften.at/resources/sdz_pdf/berichte/schriftenreihe-2018-24-motive.pdf https://nachhaltigwirtschaften.at/resources/sdz_pdf/berichte/schriftenreihe-2018-24-motive.pdf www.antherm.eu https://doi.org/10.1051/e3sconf/202017224006 https://nachhaltigwirtschaften.at/resources/sdz_pdf/schriftenreihe-2020-47-fiva.pdf https://nachhaltigwirtschaften.at/resources/sdz_pdf/schriftenreihe-2020-47-fiva.pdf acta polytechnica ctu proceedings 38:368–374, 2022 1 introduction 2 methodology 3 results and discussion 3.1 window prototypes i, a, k and s 3.1.1 window prototypes i: turn window opening to the inside 3.1.2 window prototypes a: turn window opening to the outside 3.1.3 window prototypes k: swing window 3.1.4 window prototypes s: sliding window 3.2 evaluation by domain experts and thermal performance of the shown window prototypes 4 conclusion & future research acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.38.0522 acta polytechnica ctu proceedings 38:522–526, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague the new industrial revolution: finding life for the buildings left behind dawn bilobran 313 historic preservation, united states of america correspondence: dbilobran@313hp.com abstract. from large generating facilities to modest neighbourhood substations, public power structures are an exercise in dichotomy. captivating yet mysterious, designed with both powerful function and beauty in mind. quietly playing a role in the development of cities and supporting the activities of home worldwide, the magic of heritage power plants, pumping houses and substations is often hidden behind metal gates and pressed brick facades punctuated with oversized steel windows and carved decorative ornamentations. efforts to achieve global goals of carbon-neutrality paired with advancements in infrastructure, utility distribution and alternative energies now forces the reconsideration of many unique historic resources. brimming with astounding potential, power generation facilities present unique challenges that can be deterrents to redevelopment. adaptive reuse celebrates the contributions of those who designed, constructed and operated the architectural and engineering marvels that powered the world while deterring exceptional building materials from languishing in landfills. as an increasing number of sites are decommissioned how can they be positioned to power new experiences for generations to come? what redevelopment tools are available to incentivize the adaptive reuse of industrial heritage, specifically public utility architecture? how do government-led approaches to adaptive reuse differ? keywords: power plant, adaptive reuse, rehabilitation, heritage, industrial, decommissioning. 1. introduction worldwide goals to achieve carbon-neutrality and promote sustainable energy production are driving advancements in the generation and distribution of power. past reliance on non-renewable and polluting energies are being replaced by wind, solar and biofuels. concurrently, heritage power facilities are reaching a predictable end of life and face replacement by cleaner and more efficient technologies. as the energy sector repositions, the futures of many historic power and utility buildings are called into question. the industrial revolution ushered in a new era of production and growth. reliable delivery of power became a paramount necessity for manufacturing centres and burgeoning municipalities. power plants, substations, gasworks, and waterworks housed massive mechanical operations but also received sophisticated architectural treatments. like public institutions of the period such as post offices, libraries and schools, the design of power buildings emphasized function and proportion without abandoning aesthetic enhancements. for decommissioned heritage power buildings, adaptive reuse of these unique sites is the only alternative to demolition and relegation to landfills teeming with construction waste. distinctive architectural design, high grade materials and robust community affinity are all arguments for retaining and reusing historic generating facilities. the primary challenges to adaptive reuse of utility architecture are overcoming perceived obsolescence and financing. while funding is an inherent issue in historic building rehabilitations, the building scale and potential site contamination of power plants can be daunting adversaries to embracing preservation and reuse. government agencies must support project success by administering financial incentives while fostering partnerships created to activate sites. each are essential to promoting adaptive reuse of legacy power generation and service buildings that have reached end of service. 2. public utility architecture: why adaptive reuse? 2.1. the municipal plant: civic duty and beauty rapid expansion of manufacturing and population required cities around the world to provide massive amounts of power demanded by such unprecedented growth. engineering advancements enabled the manipulation of impressive amounts of water, gas, and heat to generate substantial outputs while intricate systems were installed to deliver power directly to the consumer. no longer reserved for only the largest cities or the elite, soon lamp-lit streets, electric streetcars, and new home comforts became the norm. the expansion of public utilities as a civic amenity resulted in the construction of captivating buildings dedicated 522 https://doi.org/10.14311/app.2022.38.0522 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 38/2022 the new industrial revolution: finding life for the buildings . . . to powering the populace. world expositions, fairs and architectural design contests fostered an exchange of ideas and competitive atmosphere amongst nations and states and the resulting influence on architectural design included power structures. in the united states, the 1876 centennial celebration in philadelphia sparked a period of growth and a new commitment to following established principles of design when planning cities and buildings. the chicago exposition in 1893 further cemented this philosophy and became a launch point for the city beautiful movement, american architect daniel burnham’s response to the dirty and chaotic urban areas resulting from rapid industrial growth. burnham advocated that beautification and order of public buildings and spaces were necessary for a functioning city and resulted in a positive behavioural impact upon the masses [1]. european approaches to architecture had long emphasized aesthetics for civic projects. the paris exposition of 1900 paid special attention to public power innovations, with a palace to electricity featuring state of the art technology as well as the competitive construction of two artfully designed chimney stacks. the french entry to the design competition was a masterpiece, with large shield representing industry and decorative brick patterns terminating in impressively stepped caps and relief of half rosettes, showing “what france could do in the way of making art out of a brick pile” [2]. in germany, the design of utility buildings “were given architectural expression which does not belie their true nature” while still expressing an “appearance of quaintness and charm” [3]. inspired by expositions and planning movements that encouraged enhancement of public buildings and spaces, municipalities commissioned architects with portfolios of museums, libraries, and universities to partner with engineers on the design of new power facilities. styles such as beaux-arts, neoclassical, renaissance revival and later art deco and moderne, were used to elevate plants from modest undertaking to canvas for architectural expression. despite being unnecessary for the facility’s primary functions, the classical treatment and adornment of municipal power buildings was embraced. the orderliness of activities occurring inside the plant was replicated in the classical balance and proportion applied to exterior elevations, punctuated by rows of oversize steel windows to maximize daylight and ventilation. the deliberate application of decorative elements transformed utilitarian plants and substations into captivating works of art. architects designed new facilities with decorative tile, ornamental brickwork and attractive lighting. decorative embellishments occasionally played directly on the building function, with the integration of lightning bolts or patterns mimicking waves or the ombre effect of a chemical reaction. doorways were formalized with pediments and sculptural ornamentation and prominently featured the municipal crest or year of construction. publicly accessible spaces were handsomely trimmed in marble and stone and provided the same interior appointments one would expect visiting a post office or city hall. ample exterior lighting assured that even the night-time onlooker would be awed by the plant’s magnificence. large-scale energy generation and distribution required that power plants, substations, gasworks, and waterworks be constructed in nearly every town and state. as cities grew in population and prominence, pressure increased to construct more noteworthy structures and the application of classical techniques to public buildings enjoyed a resurgence. these facilities played a vital role in supporting metropolitan development, manufacturing, and activities of daily life. municipalities regarded their ability to provide reliable power as a source of civic pride and a way to attract new businesses and residents. privatelyheld utility companies considered power plants to be testaments of corporate success, necessary to build legitimacy and attract new investors. both regarded new facilities as investments worthy of careful planning and consideration, including architectural treatments. in 1916 james n. hatch, a consulting engineer to power plant construction in the united states advocated to the western society of engineers that artistic touches to a power plant should not be overlooked, as the stability and success of any enterprise is judged consciously or unconsciously by the appearance of its physical property [4]. power plants and substations were provided the same architectural considerations as other prominent municipal institutions. with nearly every opening of a new power facility, the plant was proclaimed as “world’s largest” in some capacity. the design, engineering and construction of new power plants became its own competition amongst towns and utility companies. records for highest output, gallons pumped, or new technologies were routinely eclipsed as more facilities were completed and competed for such illustrious distinctions. grand openings were celebrated with ribbon cuttings and lauded with newspaper articles detailing every aspect of the design. equipment suppliers placed advertisements featuring crisp images of brand-new turbines, boilers and engines and noting the impressive performance capacities of machinery. municipalities and private utility companies alike commemorated new additions with postcards and ephemera that artistically depicted the facility and its surroundings. power plants symbolised achievement and signalled a strong future. by refusing to underestimate the importance of architectural design when planning hydroelectric plants or electric substations, decision-makers achieved mechanically superior yet beautiful facilities. generation plants and distribution substations received a uniform architectural treatment favoured by the company or were influenced by the existing architectural styles of the area. california’s pacific gas and electric company (pg&e) placed great emphasis on the de523 dawn bilobran acta polytechnica ctu proceedings sign and engineering of its facilities, recognizing that power stations could be objects of beauty without diminishing the brute forces concealed inside. this approach resulted in a series of substations with stunning stone ornamentation that appear plucked from paris or rome but were instead tucked into san francisco neighbourhoods. other pg&e facilities throughout california are spanish renaissance, with red-tiled roofs and stucco plaster exterior walls, to compliment the prevalent use of that style throughout the region [5, 6]. existing architecture and popular materials routinely influenced design choices at new facilities, especially for distribution substations sited in neighbourhoods or prominent districts. in 1925 plans for a new municipal rapid sand filtration plant in washington d.c. were required to pass examination by the fine arts commission of the national capitol due to the water filtration facility’s prominent setting along a busy highway near the federal seat of government. after multiple rounds of scrutinization, the team selected a style for the complex that complemented the existing buildings in the district [7]. michigan’s detroit edison company’s substations took stylistic direction from current residential building stock in the neighbourhood, using reclaimed timbers to elevate the design of its tudor revival stations. the emphasis on architectural treatment and materials juxtaposed with massive power and force is an exercise in dichotomy and the primary reason historic power facilities are deserving of preservation and reuse. 2.2. material assets: leverage not landfill world leaders are committed to reducing waste and pollutants to promote sustainability and combat climate change. the european union is proposing carbon-neutrality by 2050, supplemented with a fastapproaching goal to drastically reduce emissions by 2030 [8]. the united states aims to cut its greenhouse gas pollution by 2030 [9]. to facilitate these goals, large coal and gas-fired energy generation plants must either convert to cleaner processes or face decommissioning. at the same time, advancements in energy generation and distribution call into question the futures of aging municipal utility facilities, once postcard-worthy beacons of pride. in the absence of a plan that includes preservation, the site either proceeds directly to demolition or languishes in vacancy. removing legacy power plants from service does not end their negative environmental impact if demolition and disposal is the only path forward. demolition undertakings produce an enormous amount of construction waste and risk the release of harmful substances into the environment. while some building materials such as asphalt, concrete and certain metals provide an economic incentive to recover and recycle, others will persist in landfills. a 2018 study by the united states environmental protection agency (epa) estimated that the united states generates 600 million tons of construction and demolition debris with demolition activities responsible for more than ninety percent. of this total, the majority of waste was relegated to landfills [10]. in the european union, construction and demolition debris is the largest single waste stream, responsible for 374 million tons in 2016 [11]. diverting construction waste to secondary recycling markets to lessen the environmental impact of demolition remains an industry priority but barriers remain to a uniform embrace of recycled construction materials. at legacy power plants, demolitions are weeks-long undertakings, often involving controlled explosives and transporting waste to specialised landfills. encapsulated hazardous materials are disturbed and disbursed into the air, ground and water. the massive amounts of building materials used to construct power generation and distribution facilities, including reinforced concrete, steel, and millions of bricks better serve the environment as a repurposed building rather than conveyed to landfills. adaptive reuse is an inherently sustainable solution for heritage utility buildings that are removed from service and deters valuable materials from contributing to an already overwhelming burden of construction waste. 3. overcoming challenges to reuse 3.1. opportunity not obstacle classification as industrial site need not be a death knell for the preservation and adaptive reuse of power and utility buildings. adaptive reuse must be presented early as a viable redevelopment option so that stakeholders understand the opportunities embodied in these unique locations and financial tools to assist with rehabilitation. assessing project feasibility, assembling financial incentives and marshalling local support are critical steps to position adaptive reuse projects for success. leveraging community affinity is exceedingly valuable when assembling support for saving heritage assets, particularly with utility owners and political representatives. strong citizen attachment to a utility facility can be a differentiating factor in the decision by community leaders and private developers to repurpose or demolish the asset. owners seeking to reactivate decommissioned utility facilities without transfer of ownership may enter into publicprivate partnerships or create long-term leases with non-profit organizations. these agreements can remove maintenance and operational burdens, often costly even for a decommissioned site, while enabling adaptive reuse to move forward. in seattle, washington a collaboration to preserve the 1906 georgetown steam plant exemplifies the potential of such municipally driven partnerships. decommissioned in 1977, the generating facility is owned by seattle city light, the municipal electric provider. it was named 524 vol. 38/2022 the new industrial revolution: finding life for the buildings . . . a national historic landmark in 1984, the highest distinction for a historic site bestowed by the national park service. after extended dormancy, the complex reopened in 2014 to great fanfare for guided tours and art-based installations. sustained public interest in the site led seattle city light to seek a partner to maintain and operate the shuttered steam plant. after a proposal process, the newly formed non-profit georgetown steam plant community development authority assumed fundraising, rehabilitation and maintenance responsibilities while continuing the public access and artistic spirit synonymous with the site. the group entered a twenty-five-year lease with plans to reinvigorate the steam plant as a historic, scientific, artistic, and cultural resource [12]. strategic partnerships can catalyse rehabilitation and reuse while removing obligations from local governments or utility providers who are unable to devote necessary resources to preserve historic facilities or undertake ambitious development projects. perceived challenges to the rehabilitation of power plants must instead be embraced during adaptive reuse undertakings. the embodied industrial grit and grime is a valuable commodity which distinguishes energy generation buildings from other architectural heritage. rather than removal, character-defining features and exceptional architectural elements should be preserved and showcased during reuse. multistory ceiling heights create dramatic vertically oriented spaces and equipment such as turbines and boilers can remain in situ as visual links to the building’s history. exposed catwalks, overhead rigging and pully systems create interesting visual features once cleaned and secured. large complexes that offer multiple buildings and auxiliary structures can be activated with a multitude of complementary uses. facilities sited along water should embrace this amenity in rehabilitation plans. the features which differentiate power buildings from other historic resources must not be viewed as deterrents but rather as fascinating elements worthy of a sustainable second act. 3.2. empowering reuse heritage building rehabilitation can be financially daunting and the cost to repurpose power structures fluctuates widely depending on exiting conditions and end use. to deter demolition, it is essential for government agencies to provide financial tools to offset the remediation and rehabilitation of historic power facilities. these initiatives can transform preservation projects from insurmountable to viable economic development undertakings. for maximum impact, stakeholders must be effectively informed and prepared to take advantage of such programs. in the united states, government programs play a large role in encouraging the redevelopment of historic resources. sites of significance designated in the national register of historic places can receive a twenty percent tax credit for eligible building rehabilitation expenditures. undertakings utilizing the credit must follow the secretary of the interior standards for rehabilitation to ensure character-defining features are retained during the project and provide for the modernization of mechanical and plumbing systems as well as accessibility and seismic performance. as of 2022 thirty-nine states administer individual historic tax credit programs that can be twinned with the federal incentive to increase the return. grants supporting adaptive reuse undertakings are also deployed at government and institutional levels but are largely reliant on budget allocations and highly competitive [13]. addressing potential site contamination is important to determining next steps during the adaptive reuse of public utility buildings. as different historical uses created varying degrees of environmental pollution, in some cases the facility may not present stakeholders with as significant of a burden an initially anticipated. for projects where sites are polluted, there are remediation techniques to clean contamination, but they are costly endeavours. to encourage reclamation of polluted industrial sites the u.s. government offers multiple incentive programs to assess, clean and sustainably reuse contaminated properties, also called brownfields. multiple programs and federal tax incentives are administered by the environmental protection agency to underwrite the elimination of contaminants and revitalization of eligible locations [14]. it is common for adaptive reuse projects at industrial sites such as power plants to utilize both the historic tax credit and brownfield programs to counteract the costs of rehabilitation. despite existing initiatives there remains a disconnect between decommissioned property owners recognizing adaptive reuse as an attainable alternative to demolition. government agencies must do more to incentivize the reuse of legacy power plant facilities, dissuade the kneejerk reaction to demolish and encourage thorough site remediation. 4. conclusion global commitments to carbon-neutrality and advancing energy technologies are relegating heritage power facilities to decommissioned status with demolitions occurring at unprecedented rates. these sites are massively undervalued in site planning and the perceived obsolescence of discharged power buildings must instead be viewed as an exceptional development opportunity. character-defining elements that received thoughtful consideration by architects and engineers during design must receive similar contemplation by stakeholders determining the fates of decommissioned facilities. to combat the existing disconnect between disposition planning and adaptive reuse, owners and stakeholders must be empowered to envision the site outside its industrial functions and be informed of best practices and financial incentives that support adaptive reuse undertakings. 525 dawn bilobran acta polytechnica ctu proceedings government support for the adaptive reuse of heritage power facilities is a critical component to project success. financial programs that incentivize remediation and rehabilitation are vital and affirm a commitment to environmental stewardship and preserving industrial heritage but must be buoyed with additional funds. proponents must advocate for the expansion of incentive programs with additional monies specifically directed toward the redevelopment of legacy power plants which present enchanting adaptive reuse potentials but often present with daunting remediation realities. changes to eligibility requirements and construction timelines for federal rehabilitation incentives may be necessary to encourage the phased reactivation of large complexes. collaborations between public and private sector stakeholders are critical to developing sustainable new uses for historic utility buildings. global affinity for power heritage and the inherent community attachment to power generating facilities is often overlooked, underestimated, and can make a significant impact during redevelopment conversations. as we boldly commit to sustainable energy generation and building practices, we must resist the pervasive belief that heritage power plants, gasworks and substations are obsolete and beyond any capable service. with the proper fuel and partnerships, these majestic monuments to power can continue serving generations to come. references [1] c. moore. daniel h. burnham: architect planner of cities. houghton mifflin, boston, massachusetts, 1921. [2] r. grimshaw. the paris exposition chimneys. steam engineering 10(10):289–291, 1900. [3] i. k. pond. modern german architecture. the brickbuilder 36(10):243–247, 1915. [4] r. c. miller. the force of energy: a business history of the detroit edison company. michigan state university press, east lansing, michigan, 1971. [5] f. c. ayars. architecture of utility structures. southwest contractor 16(8):10–11, 1915. [6] c. f. piatt. the new san mateo substation. pacific service magazine 5(1):9–12, 1913. [7] p. o. macqueen. new rapid sand filtration plant, washington d.c. – part ii. american city magazine 35(5):645–648, 1926. [8] european commission. 2030 climate target plan, brussels, belgium, 2020. [9] white house fact sheet. president biden sets 2030 greenhouse gas pollution reduction target, washington dc, 2021. [10] united states environmental protection agency. advancing sustainable materials management: 2018 fact sheet. epa 530-f-20-009, washington dc, 2020. [11] european environmental agency. construction and demolition waste: challenges and opportunities in a circular economy, copenhagen, denmark, 2020. briefing no. 14/2019. [12] n. macdonald. seattle city light leases georgetown steam plant to local non-profit. powerlines, seattle, washington, seattle city light, 2021. [13] national park service. federal tax incentives for rehabilitating historic buildings annual report for fiscal year 2021, washington dc, 2022. [14] united states environmental protection agency. 2021 brownfields federal programs guide, washington dc, 2021. epa 560-b-21-003. 526 acta polytechnica ctu proceedings 38:522–526, 2022 1 introduction 2 public utility architecture: why adaptive reuse? 2.1 the municipal plant: civic duty and beauty 2.2 material assets: leverage not landfill 3 overcoming challenges to reuse 3.1 opportunity not obstacle 3.2 empowering reuse 4 conclusion references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.38.0050 acta polytechnica ctu proceedings 38:50–56, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague toolkit for the efficient articulation of sustainable strategies with design and management methodology used by aeco industry in latin america carolina valbuena-bermúdeza,∗, claudia bengoa-alvarezb a pontificia universidad javeriana, faculty of architecture and design, department of architecture, edificio 16 / carlos arbeláez / cra. 7 no. 40–62, 110231 bogotá, colombia b politecnico di torino, faculty of architecture, department of architecture and design, corso duca degli abruzzi 24, 10129 turin, italy ∗ corresponding author: valbuenas@javeriana.edu.co abstract. digital transformation is changing the world, and with it the construction and architecture industry, however, although there are many collaborative work frameworks for digitization, and agile philosophies among different professionals, the methodologies of creative processes throughout the entire life cycle of a project that involves sustainability strategies have been greatly reduced and relegated over time. latin america has the highest levels of biodiversity, a wide variety of climates, a range of implementations and sustainable opportunities that have been sidelined versus the speed of innovation in construction projects, in this context, the design philosophy based on nature must be implemented as a premise from leaders to developers, regardless of the management method used. this document studies the sustainable construction progress in five latin american countries, analyzing climatic, regulatory and socioeconomic aspects, with the aim of increasing processes efficiency towards better sustainable constructions. consequently, it proposes a methodology for the creation of an effective sustainable tool which works hand in hand with information systems and synchronous digital collaboration (bim, vcd or ipd), in order to improve the design and management of sustainable projects throughout their life cycle through specific design strategies and design guidelines for each city, using the cities of buenos aires, brasilia, santiago, bogotá and lima as a case study. keywords: sustainable design, digital transformation, aeco, design and management methodologies, efficiency, building information management, sustainable certifications. 1. introduction the potential that technologies grant for the development of design and construction projects is enormous if articulated with the sustainability requirements to be implemented in different buildings. the construction sector has become progressively interested in the creation of high energy and performance buildings with a reduced carbon footprint and environmentally friendly architecture, but these projections are mainly made in the final phase of the design stage where many of the parameters, components and elements have already been chosen under generalised parameters without specificities, especially for the latin american context. it is therefore essential to develop a sustainable methodology, integrated throughout the life cycle of projects, that allows conscious decision making from the outset, the selection of different design alternatives that respond appropriately to the site context, and efficient responses to all the studies involved for the settlement site. even today it has not yet been possible to fully integrate the evaluation of environmental parameters in parametric bim models throughout the entire construction life cycle, therefore this article focuses on the use of environmental parameters that are not generalised but rather specific topographic, environmental and climatic as necessary criteria for the latin american context and to give a much more precise response to the buildings to be constructed in this region. 2. sustainable strategies and its relation with design and management methodologies in latin america 2.1. articulation between digital transformation and sustainable design the architecture, engineering, construction and operations (aeco) industry projects are characterised by low technological innovation and little improvement in productivity rates, and seem to lag far behind the latest technological trends in other industries. most production processes in the construction industry are traditionally manual or craft-based, organised with simple management tools and little incorporation of technology [1]. this lack of automation and limited digital transformation explain not only the low productivity performance compared to other industries and the limited 50 https://doi.org/10.14311/app.2022.38.0050 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 38/2022 toolkit for the efficient articulation of sustainable strategies . . . development of countries in terms of gross domestic product, but also the high environmental impact due to the production of large amounts of raw materials, low carbon footprint offsetting and a strong impact on environmental sustainability and the circular economy and thus on the quality of spaces such as homes and offices and people’s well-being [2]. this assessment is common in the analyses carried out by associations that evaluate the performance of the sector in different latin american countries, where the problems in the planning and execution processes are demonstrated. thus, in recent years, the sector has undergone significant changes driven mainly by the implementation of new technologies and collaborative frameworks that have enabled the optimisation of various on-site processes, improving the management of communication and planning based on data analysis and information capture. bim, for example, has become the articulating hub of the building project life cycle for performance analysis requirements, planning, scheduling, costing, data organisation and the provision of construction documents, in addition to design, visualisation and sustainability parameters. by providing a three-dimensional (3d) representation of a project that includes all its component parameters, building information modelling (bim) facilitates integration, interoperability and collaboration in the construction industry [3]. this is how resource scarcity, sustainability challenges, stricter regulations and decrees for recycling and efficiency of all resources in buildings motivate the architecture, engineering, construction and operation (aeco), facility management (fm) and deconstruction sectors to manage resources efficiently [4]. 2.1.1. design and management methodologies used by aeco industry policies, laws and regulations around the world are now requiring the sector to embrace sustainable innovation in terms of products and processes to foster more sustainable outcomes [5, 6] and more productive outcomes for the aeco industry. there are several project management methodologies that today have evolved and provided greater interoperability throughout the project lifecycle [7]. bim is ideal for providing information to improve the design and performance of buildings in relation to sustainable building design with integrated project delivery (ipd) and design optimisation [8] efficiently in the early stages of the process, and produce an improved and compatible solution between all specialties involved. information exchange requires a software environment in which software can exchange data automatically, independent of the software and the location of the data. the trend in the construction industry has been to focus not only on time and cost savings, but also on environmentally friendly buildings that can provide high performance, in order to maintain the level of achievements so far requires the development of more sophisticated and robust platforms that address not global and generalised parameters, but specific climatic and site contexts [9]. 2.2. sustainable construction in latin america in latin america, buildings are estimated to consume 22 % of the region’s total final energy demand. estimates for the region suggest that energy demand will increase by at least 80 % by 2040, compared to current demand, driven by an expanding middle class [10]. considering also that 80 % of latin america’s population lives in cities, where the largest increases in energy demand and growth in built-up area are expected, it is necessary to ensure that this does not result in an overall growth in carbon emissions, therefore it is crucial to take into account that buildings, as major carbon emitters, play a key role in mitigating the effects of climate change from the construction stage, throughout their lifespan and until the end of their life cycle. in the following, the progress of sustainable construction in 5 latin american countries with different climates is analysed, reviewing their laws, regulations, and socio-economic aspects; to determine which deficits do not allow the efficient articulation of sustainable strategies with the methodologies used by the construction industry. 2.2.1. sustainable regulations: case study of argentina, brazil, chile, colombia and peru latin american countries are situated in a context of population growth, economic crisis and lack of planning, this has been reflected in the sustainable construction aspect, however, norms and regulations are beginning to be implemented [11]. each country has a different progress in terms of sustainable construction, in the region, brazil stands out as the country with the fourth highest number of leed-certified buildings in the world. brazil has also developed a new certification for residential buildings and single-family homes, through a course based on laws, climatic and construction conditions specific to the country’s locations [12]. in argentina, the government developed public policies in favour of sustainable construction through the ministry of environment and sustainable development, and also incorporated a sustainable housing manual, which addresses issues from project location to construction and energy consumption, providing recommendations to help improve environmental performance. since 2012, chile has had a housing energy rating system in place, enacted by the ministry of housing and urban development, which functions as a voluntary tool to assess the energy efficiency of a new 51 carolina valbuena-bermúdez, claudia bengoa-alvarez acta polytechnica ctu proceedings home during its period of use. this system, allows to evaluate from individual houses to residences and compare the percentage of energy savings compared to the standard house. chile has positioned itself as a benchmark in comparison to other countries in the region as it has promoted improvement in regulations in relation to sustainability in construction, however, public policy was not adapted to local regulations and a large part of the infrastructure was built under standards of countries with different social and geographical conditions [12]. in colombia there are regulatory documents that support sustainable construction, such as the macro project for social housing, which proposes the use of thermal materials, efficient lighting and official appliances. the macro project is implemented by the national government through the ministry of energy and mining, the ministry of environmental and sustainable development, as well as the ministry of housing, city and territory, and private institutions. nevertheless, the lack of supervision mechanisms means that it is up to the builder to decide whether to apply sustainability practices in their construction projects or not [12]. peru is still at an early stage in the incorporation of energy efficiency measures; in terms of regulations, there is the national environmental management plan 2011–2021 of the ministry of housing, construction and sanitation, which establishes mechanisms for environmental monitoring and control of activities related to construction, urban planning and sanitation. in conjunction with the ministry, the national directorate of construction is in charge of approving environmental impact studies and issuing environmental certifications [12]. in general, in many latin american countries there is lack of interest in the construction sector in the field of sustainability, as the interventions executed respond more to the pressure of international norms and agreements rather than to their own commitment. there is also a lack of strategy to develop a clear policy defining the concepts of sustainable development. 2.2.2. social study: social housing programs vs sustainable certified buildings to understand the social study, it is necessary to analyse the sustainable social housing programmes used in low social class buildings, and the sustainable certifications usually used in middle or upper social class buildings. in terms of sustainable social housing programmes, it can be mentioned that, in argentina, the global environment facility project, called “energy efficiency and renewable energies in argentinean social housing” has the main objective of contributing to the reduction of greenhouse gas emissions by reducing energy consumption in social housing [13]. brazil has made significant progress in the development of voluntary sustainable building certification standards adapted to the local context, specifically for the social housing sector, the certification tools are “selo casa azul da caixa” and “aqua process” [13]. chile is one of the most advanced countries in terms of sustainable social housing, the energy efficiency projects implemented and the housing energy rating system position the country as a regional pioneer [13]. in colombia, the colombian council for sustainable construction created the casa colombia certification system for sustainable social housing, this system highlights projects that, beyond the rational use of energy and water resources, are committed to well-being, social responsibility, sustainable management of construction processes, understanding of the environment and the specification of materials with big attributes regarding their environmental and social impact [14]. finally, peru is still behind both in the development of sustainable construction policies for the social sector and in the implementation of energy efficiency measures in the sector; however, it is possible to mention some strategies that have been developed, such as the sustainable construction code whose implementation will be gradual over time, as well as the social housing programme “mi vivienda” in which a green bond is granted [13]. regarding sustainable certifications used in middle and upper social class buildings, the sustainable certifications analysed will be leed, edge, breeam and well, of which argentina and peru have leed and edge certified buildings, brazil has leed, edgge, breeam and well certified buildings, chile has leed, edge and breeam certified buildings, while colombia has leed, edge and well certified buildings. 3. materials and methodology according to what has been analysed, the articulation between design and management methodologies and sustainable design is produced through the plugins that are installed in design programmes. nonetheless, as these programmes are used by different construction professionals, on many occasions there is a range of options for sustainable analysis and it is not known which are the best strategies to use for each city. on the other hand, it has been observed that sustainable regulations, as well as those for social housing in latin america, are still in the process of being drawn up, which means that there is no comprehensive analysis of sustainable design, but only of the points indicated in the regulations, generating a partial analysis. in terms of sustainable certifications, the building is often designed and then adapted through a consultancy to achieve certification, generating one of the main problems that have been analysed, which is not considering sustainability from the first moment of the building’s design. taking this problems into consideration, the answer is that it is necessary to improve the sustainable plugins used in design and management programmes, creating a new climate analysis tool/plug-in that allows us to select the city where 52 vol. 38/2022 toolkit for the efficient articulation of sustainable strategies . . . argentina brazil chile colombia peru b. aires brasilia santiago bogotá lima data elevation m 25 1172 570 2640 0–1550 latitude ° 34.82° s 15.87° s 33.38° s 4.7° n 12.0° s longitude ° 58.53° w 47.93° w 70.78° w 74.13° w 77.1° w design strategies (ashrae standard 55) comfort % of hours in the year 10.51 15.05 12.56 1.49 9.57 sun shading of windows 7.92 16.16 8.79 8.31 high thermal mass 5.00 high thermal mass night flushed 2.17 6.81 natural ventilation cooling 1.69 internal heat gain 31.03 24.3 27.45 49.31 44.13 passive solar direct gain high mass 14.99 11.89 17.87 10.41 8.19 wind protection of outdoor spaces 0.08 dehumidification only 9.38 21.26 0.12 27.97 cooling, add dehumidification if needed 4.32 3.69 0.10 1.83 heating, add humidification if needed 17.91 0.84 28.11 38.79 table 1. design strategies (ashrae standard 55) for 5 latin american cities. sustainable design is being worked on and that automatically teaches us the design strategies and design guidelines to follow, allowing us to design the building having the information from the beginning, considering all the sustainable strategies that benefit it. to this end, the following methodology is proposed: “creation of a more effective sustainable plugin for latin america” (figure 1): in order for the proposed plug-in to provide design guidelines, the following steps were followed: using http://climaplusbeta.com/ developed by christoph reinhart (mit sustainable design lab), the climatic data of each city is analysed, for this to happen it is necessary to enter an energy plus weather (epw) file on the website; the file, obtained from the opened database of the u.s. department of energy, has hourly measured data in iwec format. the epw file creates a psychrometric chart with the hourly measured data graphically printed. the chart contains a temperature-humidity cartesian coordinate system, so each hourly condition of a year is represented by a point. each point is plotted by its xcoordinate (temperature) and y-coordinate (humidity ratio) [15]. after that, to determine the thermal comfort area, the thermal comfort model ashrae 55 and current handbook of fundamentals is used, this tool can be applied in developed and developing countries [16], the model defines the thermal comfort area on its standards, and the area is graphically represented in the psychrometric chart. the hourly measured data outside the thermal comfort area, can become comfortable, using the givonimilne bioclimatic chart strategy, where different design strategies are represented by specific zones in the psychrometric chart. for this to happen the hours that are located in the comfort zone and in each design strategy zone must be identified, the percentage corresponding to each zone must be calculated, where 100 % are the 8766 hours in a year [15], design strategy zones with the highest percentage must be prioritized in order to get a higher number of comfortable hours. finally, the writing of design guidelines is focuses on the work of ligget-milne, they developed design guidelines that respond to the design strategies zoned by givoni-milne; however, we have adapted the design guidelines to the latin american context, considering their location in another hemisphere, their diverse topography, and the different social context of its application. the design guidelines adapted to the context have been numbered from 1 to 18 (the first having the highest priority) and are correlated with design strategies, in such a way that the first design guideline will be a response to the design strategy with the highest percentage. 4. discussion and results as results, table 1 “design strategies” of five latin american cities is presented, obtained from the analysis of the psychrometric chart of each city, as can be seen in figure 1, each strategy has been given a percentage that is related to the importance of the implementation of that strategy. the percentages in table 1 are translated into table 2 “design guidelines” where a set of guidelines are numbered from 1 to 18, with the number 1 corresponding to the most important design guideline. with this example of five latin american cities to which methodology has been applied, it is demonstrated that this analysis can be performed for any city in latin america, providing design strategies and design guidelines that allow for a more accurate and user-friendly sustainable plugin. 5. conclusion sustainable strategies are commonly implemented in the final design phase by the latin american aeco industry. however, many of the building parameters and elements have already been defined in previous stages and cannot be modified. the use of the methodology proposed by the climate analysis tool/plug-in complementary to bim, which automatically provides 53 http://climaplusbeta.com/ carolina valbuena-bermúdez, claudia bengoa-alvarez acta polytechnica ctu proceedings design guidelines argent. brazil chile colom. peru b. aires brasilia sant. bogotá lima heat gain from lights, people, and equipment greatly reduces heating needs so keep home tight, well insulated. 1 3 3 1 1 for passive solar heating face most of the glass area north (south in bogotá) to maximize winter sun exposure, but design overhangs to fully shade in summer. 2 4 1 4 12 provide double pane high performance glazing (low-e) on west, south, and east, but clear on north for maximum passive solar gain. 3 17 2 traditional passive homes in temperate climates used light weight construction with slab on grade and shaded outdoor spaces. 4 1 6 2 use high mass interior surfaces like slab floors, high mass walls, to store winter passive heat and summer night “coolth”. 5 14 4 6 sunny wind-protected outdoor spaces can extend living areas in cool weather (seasonal sun rooms, courtyards, or verandahs). 6 7 5 9 traditional passive homes in cool overcast climates used low mass tightly sealed, well insulated construction to provide rapid heat buildup in morning. 7 8 3 5 organize floorplan so winter sun penetrates into daytime use spaces with specific functions that coincide with solar orientation. 8 5 11 in this climate, shade to prevent overheating, open to breezes in summer, and use passive solar gain in winter. 9 5 10 4 trees should not be planted in front of passive solar windows but are ok beyond 45 degrees from each corner. 10 9 15 low pitched roofs with wide overhangs works well in this climate. 11 12 14 7 window overhangs or operable sunshades (awnings that extend in summer) can reduce or eliminate air conditioning. 12 7 17 16 keep the building small (right-sized) because excessive floor area wastes heating and cooling energy. 13 11 8 long narrow building floorplan can help maximize cross ventilation in temperate and hot humid climates. 14 10 15 good natural ventilation can eliminate air conditioning if windows are well shaded and oriented to prevailing breezes. 15 9 14 screened porches and patios can provide passive comfort cooling by ventilation in warm weather. 16 2 3 locate garages or storage areas on the side of the building facing the coldest wind to help insulate. 17 13 17 high efficiency furnace (energy star) should prove cost effective. 18 12 7 use plant materials (bushes, trees, ivy-covered walls) especially on the west to minimize heat gain. 6 8 traditional passive homes in hot humid climates used light weight construction, and shaded outdoor porches, raised above ground. 8 use light colored building materials and cool roofs (with high emissivity) to minimize conducted heat gain. 11 18 shaded outdoor buffer zones (porch, patio, lanai) oriented to the prevailing breezes can extend living and working areas in warm or humid weather. 13 11 on hot days, indoor air motion can make it seem cooler by 2.8 °c or more. natural ventilation can store nighttime “coolth” in high mass interior surfaces, to reduce or eliminate air conditioning. 15 minimize or eliminate west facing glazing to reduce summer and fall afternoon heat gain. 16 in wet climates well ventilated attics with pitched roofs work well to shed rain and can be extended to protect entries, verandas, outdoor work areas. 18 10 if a basement is used it must be at least 0.46 meters below frost line and insulated on the exterior or on the interior. 15 small well-insulated skylights (less than 3 % of floor area in clear climates, 5 % in overcast) reduce daytime lighting energy and cooling loads. 16 13 high mass interior surfaces (stone, brick or adobe) feel naturally cool on hot days and can reduce day-to-night temperature swings. 18 glazing should minimize conductive loss and gain (minimize u-factor) because undesired solar radiation gain has less impact in this climate. 2 6 extra insulation might prove cost effective, and will increase occupant comfort by keeping indoor temperatures more uniform. 9 traditional passive homes in cold clear climates had a central heat source, south facing windows, and roof pitched for wind protection. 10 super tight buildings need a fan powered hrv or erv (heat or energy recovery ventilator) to ensure indoor air quality while conserving energy. 12 windows can be unshaded and face in any direction because any passive solar gain is a benefit. 14 insulating blinds, heavy draperies, or operable window shutters will help reduce winter night time heat losses. 16 use vestibule entries (air locks) to minimize infiltration and eliminate drafts, in cold windy sites. 18 raise the building high above ground to minimize dampness and maximize natural ventilation underneath the building. 13 high efficiency air conditioner or heat pump (at least energy star) should prove cost effective in this climate. 17 table 2. design guidelines for 5 latin american cities. 54 vol. 38/2022 toolkit for the efficient articulation of sustainable strategies . . . figure 1. methodology “creation of a more effective sustainable plugin for latin america”. a set of site-specific, easy to understand and apply design guidelines, would ensure the implementation of sustainable strategies from the first design phase, allowing conscious decision making, obtaining a better sustainable design, and optimising the other stages of the life cycle, such as execution, operation and demolition. most of the plug-ins that integrate sustainable strategies and parametric bim models perform analyses of general characteristics and provide extensive information without the user knowing which one to use. the proposed plug-in analyses specific parameters, such as climate data, latitude, longitude and altitude of latin american cities, it has an integrated database of epw weather data files. additionally, it automatically displays the data of the searched city and processes it according to the established comfort parameters thus generating a guideline of effective design strategies for that city. bim is a methodology of integrated project delivery and design optimization, and professionals from different specialties work on it. the proposed plug-in, being simple to understand, can be used by experts and by any type of professional in the aeco industry, allowing them to easily recognize the main design guidelines and apply them to the project throughout its life cycle, which is currently a limit, given that the existing tools require significant analytical expertise in the field of sustainability. sustainable construction laws and regulations are on the process of development or do not exist in latin america; many of the current regulations are optional, focused on the analysis of a single aspect, or are created under the standards of other countries, which do not allow the effective design of sustainable buildings. having taking this into consideration, the proposed plug-in allows each professional to independently analyse what other strategies would be effective to apply in each city to complement and improve the strategies proposed by the regulations. the sustainable social housing programs for the low-income population focused on the fulfilment of specific sustainable strategies, such as reducing energy consumption. the proposed plug-in allows the recognition of passive design guidelines that do not generate extra cost in the design, thus complementing the strategies proposed by the social programs. to achieve sustainable certifications in middle or upper class buildings, usually the project is carried out and then a specialised company is hired as a consultant in order to make adjustments to achieve the certification; with the proposed user-friendly plugin, the user can implement the sustainable design according to the certification standards from the very first stage. references [1] a. a. yeganeh, m. azizi, r. falsafi. root causes of design-construction interface problems in iranian designbuild projects. journal of construction engineering and management 145(12):05019014, 2019. https: //doi.org/10.1061/(asce)co.1943-7862.0001727 [2] h. håkansson, m. ingemansson. industrial renewal within the construction network. construction management and economics 31(1):40–61, 2013. https://doi.org/10.1080/01446193.2012.737470 [3] r. vanlande, c. nicolle, c. cruz. ifc and building lifecycle management. automation in construction 18(1):70–78, 2008. https://doi.org/10.1016/j.autcon.2008.05.001 [4] a. akbarnezhad, k. c. g. ong, l. r. chandra. economic and environmental assessment of deconstruction strategies using building information modeling. automation in construction 37:131–144, 2014. https://doi.org/10.1016/j.autcon.2013.10.017 [5] s. e. haagenrud. ework and ebusiness in architecture, engineering and construction, chap. eu-project stand-inn-integration of performance based building standards into business processes using ifc to enhance innovation and sustainable development, pp. 179–184. 1st edition. crc press, 2006. https://doi.org/10.1201/9781003060819-29 [6] t. hellström. dimensions of environmentally sustainable innovation: the structure of eco-innovation 55 https://doi.org/10.1061/(asce)co.1943-7862.0001727 https://doi.org/10.1061/(asce)co.1943-7862.0001727 https://doi.org/10.1080/01446193.2012.737470 https://doi.org/10.1016/j.autcon.2008.05.001 https://doi.org/10.1016/j.autcon.2013.10.017 https://doi.org/10.1201/9781003060819-29 carolina valbuena-bermúdez, claudia bengoa-alvarez acta polytechnica ctu proceedings concepts. sustainable development 15(3):148–159, 2007. https://doi.org/https://doi.org/10.1002/sd.309 [7] b. ilhan, h. yaman. green building assessment tool (gbat) for integrated bim-based design decisions. automation in construction 70:26–37, 2016. https://doi.org/10.1016/j.autcon.2016.05.001 [8] k. wong, q. fan. building information modelling (bim) for sustainable building design. facilities 31(3/4):138–157, 2013. https://doi.org/10.1108/02632771311299412 [9] a. jrade, f. jalaei. integrating building information modelling with sustainability to design building projects at the conceptual stage. building simulation 6:429–444, 2013. https://doi.org/10.1007/s12273-013-0120-0 [10] c. zepeda-gil, s. natarajan. a review of “green building” regulations, laws, and standards in latin america. buildings 10(10):188, 2020. https://doi.org/10.3390/buildings10100188 [11] p. montilla moreno. la construcción de edificaciones sostenibles. perspectivas, estrategias y retos en latinoamérica. eco diseño y sostenibilidad (2):181–204, 2010. https://www.researchgate.net/publication/ 277831752_la_construccion_de_edificaciones_ sostenibles_perpectivas_estrategias_y_retos_en_ latinoamerica [12] p. flores. la construcción sostenible en latinoamérica. limaq (007):161–173, 2021. https://doi.org/10.26439/limaq2021.n007.5183 [13] united nations environment programme. state of play of sustainable building in latin america, 2014. 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[2021-10-03]. https://www.cccs.org.co/wp/2018/07/31/casacolombia-el-sistema-de-certificacion-para-lavivienda-social-sostenible/ [15] a.-t. nguyen, s. reiter. a climate analysis tool for passive heating and cooling strategies in hot humid climate based on typical meteorological year data sets. energy and buildings 68(part c):756–763, 2014. https://doi.org/10.1016/j.enbuild.2012.08.050 [16] s. schiavon, t. hoyt, a. piccioli. web application for thermal comfort visualization and calculation according to ashrae standard 55. buildings simulation 7(4):321–334, 2014. https://doi.org/10.1007/s12273-013-0162-3 56 https://doi.org/https://doi.org/10.1002/sd.309 https://doi.org/10.1016/j.autcon.2016.05.001 https://doi.org/10.1108/02632771311299412 https://doi.org/10.1007/s12273-013-0120-0 https://doi.org/10.3390/buildings10100188 https://www.researchgate.net/publication/277831752_la_construccion_de_edificaciones_sostenibles_perpectivas_estrategias_y_retos_en_latinoamerica 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https://doi.org/10.1016/j.enbuild.2012.08.050 https://doi.org/10.1007/s12273-013-0162-3 acta polytechnica ctu proceedings 38:50–56, 2022 1 introduction 2 sustainable strategies and its relation with design and management methodologies in latin america 2.1 articulation between digital transformation and sustainable design 2.1.1 design and management methodologies used by aeco industry 2.2 sustainable construction in latin america 2.2.1 sustainable regulations: case study of argentina, brazil, chile, colombia and peru 2.2.2 social study: social housing programs vs sustainable certified buildings 3 materials and methodology 4 discussion and results 5 conclusion references https://doi.org/10.14311/app.2022.33.0391 acta polytechnica ctu proceedings 33:391–397, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague analytical simulation on experimental seismic response of headed anchors embedded in reinforced concrete seiji nagataa,∗, toyofumi matsuoa, hironori morozumib, keizo ohtomoa a central research institute of electric power industry, abiko, abiko-shi, chiba-ken, japan b kansai electric power, co., inc., nakanoshima, kita-ku, osaka-shi, osaka-fu, japan ∗ corresponding author: n-seiji@criepi.denken.or.jp abstract. this paper deal with a series of dynamic response analyses, carried out to give numerical correlation of hysteretic behavior of cast-in-place anchorages obtained by the static loading and shake table tests. specimens for the analyses are reinforced concrete rectangular blocks with embedded headed anchors. failure modes expected are the steel bolt failure and the concrete cone failure, associated with the respective embedment depth. initial flexural cracks presumed seismic damage on the concrete block is developed with bending loading. in the analytical model, nonlinear hysteretic behavior of the anchorage is idealized by translational and rotational springs in which properties estimated from the hysteretic loops of the static loading tests. it was shown that the overall force and displacement performance under dynamic loads are well simulated based on the analytical models presented herein. keywords: dynamic response analyses, headed anchor bolt, nonlinear hysteretic behavior. 1. introduction design and construction of seismically resilient reinforced concrete (hereinafter, rc) structures will contribute toward the sustainable society in an earthquake prone country like japan. structural resiliency for a rc structure supporting industrial equipment can be attributed to its global as well as local behavior. for instance, in rc structures of the cooling system facilities at the thermal or nuclear power plants, the equipment and piping are generally installed on the rc members using anchors [1], [2]. in the meantime, the current seismic design allows rc structures to sustain partial damage such as cracks of concrete and yielding of reinforcements, unless they exceed the ultimate state [3]. however, if such seismic damage develops around the anchors, it is necessary to evaluate the seismic safety of the rc structure as well as the effect of the crack on the anchorage strength. from such background, the authors have carried out a series of the experiments related to the strength and the hysteretic behavior of the cast-in-place headed anchor bolts during an earthquake event considering the effects of the flexural cracks [4, 5]. in the present paper, the dynamic response analyses are performed to give numerical correlation of the nonlinear hysteretic behavior of the anchorages obtained by the past experiments (the static loading and the shake table tests). in the analyses, the hysteretic behavior of the anchorage is idealized as translational and rotational springs. the properties of the springs such as stiffness and damping ratio are identified based on the static loading tests results. then the dynamic response analyses are conducted to simulate the force and displacement hysteresis obtained by the shake table tests. 2. experimental conditions and results 2.1. experimental conditions the test specimens and test cases for this analytical study are presented in figure 1 and table 1, respectively. the specimens are rc rectangular brocks with four headed anchors welded to a baseplate. the material properties of the anchors and reinforcing bars are listed in table 2. the experimental methods for assessing the strength and the hysteretic behavior are the static loading tests and shake table tests. the main parameters of these tests are the failure modes of the anchorages and the initial flexural cracking around the anchors in the rc blocks. the failure modes expected in the anchorages are the steel bolt yielding and the concrete cone failure (hereinafter, bolt yield type and concrete failure type, respectively), associated with the bolt length (250 mm and 100 mm respectively). the initial flexural cracks presumed seismic damage on the concrete block is developed by bending test preliminary conducted as illustrated in figure 2. the bending moment is applied until the main reinforcing bars yield and the maximum residual crack width exceeds 2.0 mm. figure 3 presents the experimental condition and the loading pattern in the static loading test. in these tests, a steel support and a weight (20 kn) are attached on the rc specimen, and a horizontal load 391 https://doi.org/10.14311/app.2022.33.0391 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en s. nagata, t. matsuo, h. morozumi, k. ohtomo acta polytechnica ctu proceedings � �d��%row�\lhog�w\sh��� �e��&rqfuhwh�idloxuh�w\sh� figure 1. test specimens (unit: mm). specimen failure mode bolt length initial cracks test method strength of concrete [mm] ! n/mm2" s-b0 bolt yield 250 without static loading test 39.1 s-c0 concrete failure 100 without static loading test 39.2 s-b1 bolt yield 250 with static loading test 39.6 d-b0 bolt yield 250 without shake table test 42.5 d-c0 concrete failure 100 without shake table test 42.6 d-b1 bolt yield 250 with shake table test 43.1 table 1. test cases. diameter young’s modulus yield strength tensile strength [mm] ! kn/mm2" ! n/mm2" anchor bolt 22 208.5 330.2 464.1 reinforcing bar 19 190.7 401.9 582.9 table 2. development of initial flexural cracks. �d��/rdglqj�frqglwlrq� � �e��0rphqw�dqg�fxuydwxuh�uhodwlrq� �f��)oh[xudo�fudfnv��6�%��� 㻜 㻠㻜㻜 㻜 㻜㻚㻜㻝 㻯㼍㼘㼏㼡㼘㼍㼠㼕㼛㼚 㻿㻙㻮㻝 㻰㻙㻮㻝 㻮 㼑㼚 㼐㼕 㼚㼓 㻌㻹 㼛 㼙 㼑㼚 㼠㻌 㻔㼗 㻺 䞉㼙 㻕 㻯㼡㼞㼢㼍㼠㼡㼞㼑㻌㻔㻝㻛㼙㻕 㼅㼕㼑㼘㼐 㻯㼞㼍㼏㼗 figure 2. development of initial flexural cracks. � �d��([shulphqwdo�frqglwlrq� �e��/rdglqj�sdwwhuq� figure 3. loading conditions in the static loading test. 392 vol. 33/2022 seismic response of headed anchors (a) experimental condition (b) input wave -800 0 800 0 10 20 30 a cc el er at io n (g al ) time (s) maximum: 709.5 gal time history of input motion 101 102 103 104 10-1 100 101 102 a cc el er at io n (g al ) frequency (hz) natural frequency band of specimen acceleration response spectrum (damping ratio: 2%) figure 4. loading conditions in the shake table test. specimen magnification of input motions [%] d−a0 20 40 60 80 100 120 140 160 180 200 d−c0 20 40 60 80 100 120 140 160 180 − d−a1 20 40 60 80 100 120 140 160 180 200 table 3. magnification of input wave in the shake table tests. � �d��6�%��dqg�'�%�� � �e��6�&��dqg�'�&�� � �f��6�%��dqg�'�%�� ��� � �� ��� � �� /d wh ud o�) ru fh ��n 1 � /dwhudo�'lvsodfhphqw��pp� 6�%� '�%������ '�%������ '�%������ ��� � �� ��� � �� /d wh ud o�) ru fh ��n 1 � /dwhudo�'lvsodfhphqw��pp� 6�&� '�&������ '�&������ '�&������ ��� � �� ��� � �� /d wh ud o�) ru fh ��n 1 � /dwhudo�'lvsodfhphqw��pp� 6�%� '�%������ '�%������ '�%������ figure 5. lateral force and lateral displacement relationships based on the static and dynamic tests. is applied to the gravity center of the weight. the lateral displacement is stepwisely increased until the specimen will suffer from significant damage. in the shake table test, the rc specimen is set on the horizontal uniaxial shake table, and then the steel supports and weights are attached on the specimen (see figure 4). the floor response acceleration is used as an input wave, obtained through the seismic response analysis on the rc head race (duct) equipped with seawater pipes [2]. as shown in table 3, the input magnification is initiated with 20 % and then it is increased by 20 % increment until the shaking table capacity (200 %). 2.2. experimental results figure 5 compares the hysteresis curves associated with the lateral force and displacement derived from the static loading as well as the shake table tests. the lateral force in the shaking table test was calculated by multiplying the measured lateral acceleration with the mass of the superstructure. in the case of the bolt yield type without the initial crack (s−b0 and d−b0), the restoring force in the post peak region keeps stable forces compared to the concrete failure types (s−c0 and d−c0). the effect of the initial flexural cracks on the anchorage strength appears insignificant in both bolt yield type specimens (s−b1 and d−b1). these results clearly show that the nonlinear hysteretic loops under the shake table tests well cover the ones under the static loading test. 3. analytical conditions and results 3.1. analytical conditions in this study, numerical analyses are performed with respect to an equivalent linear analysis and a step-bystep nonlinear analysis. the analytical model for the 393 s. nagata, t. matsuo, h. morozumi, k. ohtomo acta polytechnica ctu proceedings figure 6. analytical idealization for the specimen. figure 7. components in lateral displacement. test specimen is shown in figure 6. the steel weight, the steel support and the anchorage part of the rc specimen are idealized as the lumped mass, the beam element and the spring element (translation and rotation), respectively. to evaluate the stiffness and the damping ratio required in the spring elements, the lateral displacement of the static test is decomposed into the translation and the rotation components as shown in figure 7. the equivalent stiffness and the equivalent damping ratio, defined in figure 8, are identified for the equivalent linear analysis, while the nonlinear slip model (see figure 9) is used for the nonlinear analysis. figure 10 shows the equivalent stiffness of the translational and rotational components based on the static loading tests. the equivalent stiffness decreased as the lateral displacement increase and the equivalent stiffness decreased in accordance with the degree of initial damage. in all the specimens, the equivalent stiffness of the translation component is some ten times higher than that of the rotation component. this indicates that the lateral displacement during the experiments is mainly caused by the rotation component at the anchoring. the nonlinear hysteresis model is introduced to the rotational spring because of relatively larger deformation appeared in the rotation component. the equivalent damping ratios based on the lateral force and displacement loops of the static loading tests are presented in figure 11. these damping ratios of the concrete failure type tend to be slightly lower than that of the bolt yield type. the initial figure 8. equivalent stiffness and damping ratio. figure 9. nonlinear model for rotational spring [6]. damage may bring slightly lower equivalent damping ratio. in the both equivalent linear and nonlinear analysis, the stiffness-proportional damping is applied and the damping ratio is assigned to the natural frequency obtained based on the eigenvalue analysis for each model. 3.2. analytical results figure 12 shows the analytical results of the shake table test based on the linear model, comparing with the experimental results. in the specimen d-b0 (the bolt yield type without the initial damage), the numerical analyses well simulate the dynamic behaviours associated with restoring forces as well as displacement response under considerably large nonlinearity states brought by input magnification. on the other hand, a good numerical correlation is observed on the specimen d-c0 (concrete failure type without initial damage) up to the 160 % input. however, this correlation deteriorates under the 180 % input where the significant nonlinearity appears due to significant concrete damage. the equivalent analysis, which employs equivalent stiffness and damping ratio based on the static loading hysteresis, demonstrates a well numerical correlation on the overall dynamic hysteresis behaviour on the bolt yielding type (the specimen d-b1) that involves the effect of initial flexural cracks nonlinearity. 394 vol. 33/2022 seismic response of headed anchors � �d��6�%�� � �e��6�&�� �f��6�%�� � ��� � �� 7udqvodwlrq 5rwdwlrq 6w lii qh vv ��n 1 �p p � /dwhudo�'lvsodfhphqw��pp� � ��� � �� 7udqvodwlrq 5rwdwlrq 6w lii qh vv ��n 1 �p p � /dwhudo�'lvsodfhphqw��pp� � ��� � �� 7udqvodwlrq 5rwdwlrq 6w lii qh vv ��n 1 �p p � /dwhudo�'lvsodfhphqw��pp� figure 10. equivalent stiffness based on the static loading tests. � �d��6�%�� � �e��6�&�� � �f��6�%�� � ��� � �� ' dp sl qj �5 dw lr ��� � /dwhudo�'lvsodfhphqw��pp� � ��� � �� ' dp sl qj �5 dw lr ��� � /dwhudo�'lvsodfhphqw��pp� � ��� � �� ' dp sl qj �5 dw lr ��� � /dwhudo�'lvsodfhphqw��pp� figure 11. equivalent damping ratios based on the static loading tests. as discussed previously, the nonlinear hysteresis model (figure 9) is introduced to the rotational spring because of relatively larger deformation appeared in the rotation component. then, the numerical analysis for the shake table test using this model is performed, resulting in hysteresis curves as plotted in figure 13. these analysis and experiment curves demonstrated that use of the nonlinear hysteretic model improves the degree of numerical correlations, especially in strong nonlinearity observed in the specimen d-c0 and d-b1. 4. conclusions in this paper, the dynamic response analyses on the cast-in-place headed anchor bolts are conducted to simulate the lateral force and displacement performance under the shake table tests. the main parameters are the failure modes of the anchorage and the initial flexural cracks in the rc brock. in the analytical model, the hysteretic behavior of the anchorage is idealized as translational and rotational springs whose mechanical properties are identified based on the static loading tests. based on these analytical simulations conducted herein, the hysteretic behavior under dynamic loads simulate well to those under the static loads. insignificant nonlinear response of the anchors allows to employ the equivalent linear stiffness and damping ratio in the simulations. on the other hand, significant nonlinearity on the anchorage under relatively excessive seismic loads will require the nonlinear hysteretic model for the anchorage to provide more narrow numerical correlations. acknowledgements this research is a part of the project research ’advanced study on the verification method of seismic performance of underground reinforced concrete structures in nuclear power plants’ which was jointly carried out by kansai electric power co. inc., hokkaido electric power co. inc., tohoku electric power co. inc., tokyo electric power company holdings, inc., chubu electric power co. inc., hokuriku electric power co. inc., chugoku electric power co. inc., shikoku electric power co. inc., kyusyu electric power co. inc., the japan atomic power company, electric power development co., ltd. and japan nuclear fuel limited. the authors are grateful for the interest and advice by the above power companies, as well as to the members of the evaluation committee, which was organized by the japan society of civil engineers and chaired by professor k. maekawa of yokohama national university. 395 s. nagata, t. matsuo, h. morozumi, k. ohtomo acta polytechnica ctu proceedings � �d��'�%�� � �e��'�&�� � �f��'�%�� )ljxuh�����$qdo\wlfdo�uhvxowv�edvhg�rq�wkh�olqhdu�prgho� ��� � �� ��� � �� ([s� $qdo�/d wh ud o�) ru fh ��n 1 � /dwhudo�'lvsodfhphqw��pp� $psolwxgh����� ��� � �� ��� � �� ([s� $qdo�/d wh ud o�) ru fh ��n 1 � /dwhudo�'lvsodfhphqw��pp� $psolwxgh����� ��� � �� ��� � �� ([s� $qdo�/d wh ud o�) ru fh ��n 1 � /dwhudo�'lvsodfhphqw��pp� $psolwxgh����� ��� � �� ��� � �� ([s� $qdo�/d wh ud o�) ru fh ��n 1 � /dwhudo�'lvsodfhphqw��pp� $psolwxgh����� ��� � �� ��� � �� ([s� $qdo�/d wh ud o�) ru fh ��n 1 � /dwhudo�'lvsodfhphqw��pp� $psolwxgh����� ��� � �� ��� � �� ([s� $qdo�/d wh ud o�) ru fh ��n 1 � /dwhudo�'lvsodfhphqw��pp� $psolwxgh����� ��� � �� ��� � �� ([s� $qdo�/d wh ud o�) ru fh ��n 1 � /dwhudo�'lvsodfhphqw��pp� $psolwxgh����� ��� � �� ��� � �� ([s� $qdo�/d wh ud o�) ru fh ��n 1 � /dwhudo�'lvsodfhphqw��pp� $psolwxgh����� ��� � �� ��� � �� ([s� $qdo�/d wh ud o�) ru fh ��n 1 � /dwhudo�'lvsodfhphqw��pp� $psolwxgh����� figure 12. analytical results based on the linear model. � �d��'�%�� � �e��'�&�� � �f��'�%�� )ljxuh�����$qdo\wlfdo�uhvxowv�edvhg�rq�wkh�qrqolqhdu�prgho�� ��� � �� ��� � �� ([s� $qdo�/d wh ud o�) ru fh ��n 1 � /dwhudo�'lvsodfhphqw��pp� $psolwxgh����� ��� � �� ��� � �� ([s� $qdo�/d wh ud o�) ru fh ��n 1 � /dwhudo�'lvsodfhphqw��pp� $psolwxgh����� ��� � �� ��� � �� ([s� $qdo�/d wh ud o�) ru fh ��n 1 � /dwhudo�'lvsodfhphqw��pp� $psolwxgh����� figure 13. analytical results based on the nonlinear model. 396 vol. 33/2022 seismic response of headed anchors references [1] the japan electric association. the technical code for seismic design of nuclear power plants jeac4601-2015, 2015. [2] the nuclear civil engineering committee. guideline and recommendation for seismic performance verification of underground reinforced concrete structures in nuclear power stations, japan society of civil engineers, 2018. [3] [4] s. nagata, t. matsuo, h. akira, et al. study on structural performance of anchors embedded at the base of equipment and piping proc. japan society of civil engineering annual meeting, 2017. [5] s. nagata, t. matsuo, h. morozumi, et al. study on structural performance of anchors embedded at the base of equipment and piping considering seismic damage of rc structure, proc. japan society of civil engineering annual meeting, 2018. [6] ark information systems, inc. tdap iii user’s guide, ver.3.11. https://www.ark-info-sys.co. jp/jp/product/tdap/english/. 397 https://www.ark-info-sys.co.jp/jp/product/tdap/english/ acta polytechnica ctu proceedings doi:10.14311/app.2018.20.0001 acta polytechnica ctu proceedings 20:1–4, 2018 © czech technical university in prague, 2018 available online at http://ojs.cvut.cz/ojs/index.php/app high temperature cyclic test rig – a new rig for hot gas parts lifetime validation wieland ammera,∗, mario raddatza, uwe gampea, julien nussbaumb, uwe ruedelb, rudolf kellererb a tu dresden, institute of power engineering, faculty of mechanical engineering, chair of thermal power machinery and plants, 01062 dresden b ansaldo energia switzerland, roemerstrasse 36, 5401 baden ∗ corresponding author: wieland.ammer@tu-dresden.de abstract. flexible gas turbine operation requires adapted hot gas parts design and lifetime assessment methods. the high temperature cyclic test rig (htctr) is developed to cyclically test internally cooled components from combustor and turbine sections of gas turbines in close-to-engine conditions for a wide range of operating conditions in order to improve component lifetime management. these advanced tests should serve as basis for new model development and validation for conditions that are more representative of those in an engine than the standard material specimen tests. keywords: turbine, cyclic, lifetime. 1. introduction the european commission’s broader energy and climate objectives focus on several cost-efficient measures to make the european economy more climate-friendly, more energy efficient and less energy consuming. for the integration of renewable energy sources (res) into the total energy mix for power generation, the targets foresee a reduction of the greenhouse gas emissions by 2030 to 40%, by 2040 to 60% and by 2050 to 80% below 1990 levels, respectively [1]. however, due to the intermittent nature of the res, and in the absence of large scale energy storage technologies with a sufficiently high technological and economical maturity level, the existing conventional thermal power plants are not capable of offering the required operational flexibility without impeding their efficiency and engine lifetime through increased wear and damage induced by the higher number of (shorter) operating/loading cycles. for the introduction of new technologies to balance demand peaks with renewable output fluctuations at minimal fuel consumption and emissions without negative effects on cycling operation, the flexturbine consortium is part of the eu horizon2020 activities and aims to develop medium-to-long term solutions for the implementation into existing and new power plants. the consortium consists of the main european turbine manufacturers, renowned research institutes and universities. 2. aims the present work describes the strategy and the concept for the development of a high temperature cyclic test rig. the purpose of this rig is the validation of hot gas parts at engine relevant conditions to increase the overall confidence level in the lifetime predictions. the test rig will be used for the cyclic testing of novel cooling designs, thermal barrier coatings (tbc), materials and for the validation of lifetime methods. new technologies can be tested in the test rig like: • optimized geometries allowed by manufacturing techniques like selective laser melting process • thermal barrier coatings where both cyclic life and erosion can be tested • new sealing concepts at relevant temperature and pressure the target is to apply cyclic thermal loads to test samples that correspond to current and future gas turbine design targets. the temperature difference between cold and hot steady state has to be high enough to achieve the sample failure (or at least crack initiation) within a reasonable number of cycles/testing time. target value for crack initiation failure is after 500 cycles, corresponding to approximatively 2 weeks. htctr will enable the possibility to test components of current and future gas turbines at a trl level (trl5) which could not yet be tested. as in the engine, thermal loading of the samples is generated by a hot gas flow which overflow the sample surface. the temperature of the hot gas side as well as the through-the-wall temperature have therefore to be reproduced as close as possible to the engine conditions. in order to avoid the wrong interpretation of results in case of crack initiation, the transient damaging effects have to be negligible compared to hot to cold steady states damages. in figure 1 a qualitative course of a cycle is illustrated by plotting the hot gas temperature thg and the heat transfer coefficient htchg on the hot gas surface. at the beginning of each cycle both variables increase for around three minutes until a maximum 1 http://dx.doi.org/10.14311/app.2018.20.0001 http://ojs.cvut.cz/ojs/index.php/app w. ammer, m. raddatz, u. gampe et al. acta polytechnica ctu proceedings figure 1. qualitative sketch of hot gas temperature and heat transfer coefficient on hot gas side. value is achieved. this state is maintained several minutes to realize sufficient steady state conditions in the test sample. following this, hot gas temperature and heat transfer coefficient are decreased to a minimum level with a subsequent second cold steady state. the measurements of boundary conditions via the proper instrumentation will be used to calibrate the finite-element calculations. in a second stage, a low cycle lifetime assessment will be performed to estimate the number of cycles until crack initiation based on current lifing models. the cyclic test will be performed with regular borescope inspection to monitor expected life limiting location and detect cracks. the end of the test is defined as the failure of the test sample, which is not possible in a real turbine. the observed number of cycles to crack initiation and sample failure will then be compared to numerical results and lifing models will be adapted accordingly. 3. concept and validation 3.1. concept in order to fulfil the hot gas generation requirements, an enhanced flox burner [2] is employed adapted and provided by the institute of combustion technology of dlr stuttgart. the target is to avoid the cyclic operation of the burner since the burner is expected to survive several test campaigns. therefore the concept of the test rig htctr architecture has been developed in the form to enable the continuous operation of the burner. downstream of the air-cooled combustion chamber, the hot gas path is split into two channels containing one test sample each, meaning that two samples per campaign can be tested in parallel and leading to a significant costs decrease (the test samples can be different). downstream to the test section, a rotating valve will be used to generate the cycles: during the heating phase, the valve is kept open whereas during the cooling phase the valve is almost closed and purge air is injected upstream to the specimen. while one sample is heated, the other one is cooled down. during the course of the test period both hot gas paths are alternately opened and closed to realize thermal cycles. the angle of the control valve and their rotating velocity are selected in the form to prevent negative impacts on the flox burner stability. all described components belonging to the inner housing are thermal highly loaded by the internal hot gas flow. to prevent an additional loading caused potential by the pressure requirements the inner housing is surrounded by a pressure vessel. the selected concept is presented in figure 2. 3.2. detailed design the detailed design of the test rig components has almost been completed. as a first step, several 1d thermodynamic simulations have been performed in order to estimate the flame temperature required to achieve the relevant thermal boundary conditions at test section and the different heat loss to the components along the hot gas path. the burner used in the test rig is a modified version of the flox burner from dlr stuttgart. the design has been slightly modified to achieve the operating point required to generate the cycles. the validation of the new design has been supported by experiments in the hbk-s test facility from dlr stuttgart. since the pressure in the hbk-s test cell is limited to a lower value than the one required in the htctr, cfd calculations were used to 1. calibrate the model at low pressure based on experimental results 2. extrapolate the operation of the burner to higher temperatures. different configurations have been tested and/or simulated (flame temperature, air/fuel ratio, combustion chamber length, hot gas path geometry) in order to better characterize the behavior of the burner. the main uncertainty remains in the stability of the burner which is difficult to capture with cfd calculations. the cfd results have been then used as boundary conditions for the design of all other hot gas path components. the combustion chamber consists of an inner liner made of hastelloy-x material and an outer liner made of sts18/8 material. the chamber is air-cooled and specific focus on cooling features like cooling ribs have been made. due to the high heat inputs expected from the combustion, the inner liner will be tbc coated. all other hot gas path components are made of cucr1zr alloy and are intended to be intensively water-cooled. for each component, hot gas side boundary conditions have been extracted from the main cfd calculations of the hot gas path. then an iterative process has been used to optimize the cooling structure of the components, mainly by implementing cooling holes or ribs in the components, with the support of cfd calculations. in order to validate the design of the components, a fe calculation has been 2 vol. 20/2018 high temperature cyclic test rig figure 2. test section concept. performed to get the temperature and stress distributions in the components as well as at the interfaces. a lifetime assessment has been then performed to insure the integrity of the components for at least five test campaigns. the pressure vessel is also designed based on cfd and fe calculations. during test operation a slightly higher pressure will be maintained in the outer housing than in the hot gas path in order to avoid hot gas leakage. since this component will be under pressure during operation and in direct contact with the ambient atmosphere, the design will be cross-checked by a technical supervisor. in order to guarantee the proper operation of the test rig, each components have been designed using conservative assumptions on boundary conditions and material properties. figure 3 illustrates the structure of the test rig in form of an axial cut. 3.3. instrumentation intensive instrumentation is planned in the test rig (1.) to get relevant scientific information of the temperature distribution and information about the boundary conditions of the test samples (2.) to guarantee the safety of the test rig in case of unexpected events like seal failure (3.) to operate the test rig according to the operating concept and cyclic loading in total, approximately hundred measurement points are planned in the whole test rig. 40 of them are dedicated to the test samples and their boundary conditions. the main difficulty is to measure the hot gas temperature at test section, where the conditions (temperature and mass flow) are extreme. a concept with correlated measurements from thermocouples and pyrometer is under development to get such information, at least during the hot commissioning. for the first validation test, only metal temperature on both hot gas and cooled sample sides will be measured. no stress or strain measurements are planned, but thermal strains will be estimated based on calibrated thermal field. in the combustion chamber, a dynamic measurement sensor is required to monitor the pulsation of the burner, since the stability of the burner has been identified as high risk. during hot commissioning, it is planned to test different operating points in order to determine the best set of parameters. the other measurement points are mainly metal temperature for protection system, as well as inlet/outlet temperature, pressure and/or mass flow for each medium supply line. 3.4. adaptation to hbk-3 test cell the target is to ship and assemble the test rig in the hbk-3 test cell at the institute of propulsion technology of dlr cologne. hbk-3 provides the sufficient infrastructure to operate the test rig: (pre-heated or not) air and water, fuel, and data acquisition system. adaptation of the hbk-3 facility to the test rig is ongoing in cooperation with dlr cologne. it has been scheduled that test infrastructure design is completed in october 2018 and a draft data acquisition system with few measurements points from hbk-3 ready. 3.5. validation strategy the first milestones are the cold assembly and leakage tests, currently planned for autumn 2018. all the components will be shipped to dlr cologne (germany) where the test rig will be assembled in the hbk-3 facility. for this first step, the tbc and the instrumentation won’t be applied to the parts. cold leakage tests at relevant pressure levels will be performed and the rig component design will be adjusted if required. 3 w. ammer, m. raddatz, u. gampe et al. acta polytechnica ctu proceedings figure 3. axial cut of htctr design. after the disassembly of the test rig, the coating will be applied on the hot gas components and the instrumentation will be implemented. the second milestone is the final validation campaign in may 2019 where the hot commissioning with the entire instrumentation and the test campaign on a stator heat shield will be performed. post-processing will follow and results become available before the end of the project in june 2019. 4. discussion the high test rig parameters for engine close conditions are representing a real challenge for rig design and operation. the difference between hot and cold steady states has to be high enough in order to achieve crack initiation at the test sample within a short number of cycles. cfd and fe calculations are used to evaluate the two steady states and the associated expected lifetime. in case the measurements done during the hot commissioning don’t show sufficiently high temperature differences, the cooling medium supply parameters will be adapted by decreasing cooling in some components and increasing cooling at test sections. due to thermal expansion, sealing of two neighboring components is challenging. currently, different sealing concepts are under investigation depending on the interface: • metal seals like e-seals or strip seals • polymer seals with higher ductility cold leakage test is planned to validate the concepts in cold state. in case some seals fail, the design will be improved before the final validation test campaign in may 2019. of course the hot operation will lead to larger deformations so that remaining uncertainties will be clarified with the first test campaign. finally, the control valve is a critical component. it is supposed to generate the cycles by opening and closing the hot gas path and its failure (e.g. due to sticking) will lead to the end of test campaign and the replacement of the control valve housing and/or rotating cylinder. the design of the control valve is still under progress and two options have been selected, either with an intensively cooled design based on copper alloy or with an uncooled design based on high refractory ceramic material. 5. conclusions the htctr is aimed to support the turbine’s hot gas parts design with novel, easier and cheaper testing capabilities on component level for a wide range of current and future close-to-engine conditions. design solutions of the rig components based on extensive detailed heat balance calculations and fluid-structure interaction finite element analyses. particular challenges for all partners were ensuring compliance with infrastructural limits regarding cooling water and compressed air. htctr developement is still ongoing and the first cold assembly is planned before the end of the year. the first test campaign is expected during spring next year and results will be available for the middle of 2019. acknowledgements this project has received funding from the european unions horizon 2020 research and innovation programme under grant agreement no. 653941. this work was supported by the swiss state secretary for education, research and innovation (seri) under contract number 18.00008. the opinions expressed and arguments employed herein do not necessarily reflect the official views of the swiss government. references [1] the european commission. the european commission’s priorities, energy union and climate, 2050 low-carbon economy, 2018. https://ec.europa. eu/clima/policies/strategies/2050_en. [2] o. lammel, h. schütz, g. schmitz, et al. flox® combustion at high power density and high flame temperatures. journal of engineering for gas turbines and power 132(12), 2010. doi:10.1115/1.4001825. 4 https://ec.europa.eu/clima/policies/strategies/2050_en https://ec.europa.eu/clima/policies/strategies/2050_en http://dx.doi.org/10.1115/1.4001825 acta polytechnica ctu proceedings 20:1–4, 2018 1 introduction 2 aims 3 concept and validation 3.1 concept 3.2 detailed design 3.3 instrumentation 3.4 adaptation to hbk-3 test cell 3.5 validation strategy 4 discussion 5 conclusions acknowledgements references https://doi.org/10.14311/app.2022.33.0591 acta polytechnica ctu proceedings 33:591–596, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague use of recycled concrete fines in cement and as aggregate simone stuerwald∗, ronny meglin, susanne kytzia, sabrina gilg open university, walton hall, milton keynes, keynes mk7 6aa, uk ∗ corresponding author: simone.stuerwald@ost.com abstract. the research project focused on investigating and optimizing the processing and use of recycled crushed sand 0/4 from concrete demolition waste, as an alternative raw material in the cement and concrete industry. crushed sand is produced during the processing of concrete demolition waste. the goal was to identify the optimum way of using the processed material along the entire process chain so that greenhouse gas emissions, waste volumes are reduced, and natural resources are conserved. different samples of laboratory and real crushed concrete fines were collected and examined in relation to various possible applications in accordance with the applicable standards. results highlight, that crushed concrete fines can be used in various applications in the concrete value-chain. however, for an optimal usage, additional processing is needed. keywords: cement, concrete, construction and demolition waste, lca, recycling. 1. introduction more than 10 years ago, a swiss standard was introduced that regulated the use of recycled concrete demolition waste as an aggregate for structural concrete [1]. nowadays the use of such aggregates is very common in europe and switzerland [2–4]. in the coming decades, the amount of construction waste in switzerland will increase especially concrete demolition waste from structures built in the 1950s to 1970s [5]. however, most of the processed construction waste is "down-cycled" into materials with subordinate technical requirements such as lean concrete or as road subbase material [6]. for this reason, various studies have recently been carried out to optimize the use of concrete demolition waste [7–9]. however, most studies focus on the complete concrete demolition waste. we focus on the crushed concrete fines because it has a significant impact on the possible use of the demolition material due to its less favourable properties [10]. we think, that the usage can be optimized, especially as it is assumed that recycling of concrete fine from demolition could further reduce co2 emissions in the concrete value chain [11–13]. we aim to optimize the usage of concrete demolition waste, especially the crushed concrete fines as a by-product. the economic efficiency of concrete recycling will be significantly improved if this crushed sand can be used profitably in cement and concrete production. we evaluate three economically viable uses of recycled crushed sand 0/4 according to sustainability criteria such as climate protection, resource conservation and waste avoidance. possible applications would be (i) use as a low-co2 substitute in cement kiln, (ii) use as an additive in cement mill or concrete production, or (iii) use as an aggregate in concrete production (figure 1). the aim is to identify the optimum place of use, the quality of the crushed sand required and the requirements for processing methods. in this way, crushed sand is to be optimally returned to the material cycle, natural raw materials are conserved, and co2 is saved. 2. methodology we estimate the potential for the use of crushed concrete fines by using various test methods and calculations to determine the properties of real and laboratory specimens. three samples of crushed sands (bs-l) were produced in the laboratory with different cement paste contents. another three samples (bs-r) were taken from recycling plants and a sample of sand from natural resources was used as a reference. based on chemical analyses of the samples, the use of ground crushed fines in clinker production is evaluated in simulations and the possible degree of substitution as a raw material substitute is estimated. subsequently, mortar samples produced with the ground crushed fines as an admixture are tested according to sn en 196. the results are then compared with the current standards and allow predictions on the possible use of the crushed sand as an additive and the possible amount to be added. in a further step, crushed fines samples are used as a substitute for primary sand in the production of concrete. the concretes produced are then subjected to extensive testing according to sn en 206. a comparison with the standards will allow us to estimate the maximum degree of substitution of the primary sand. finally, the results will be evaluated and discussed. 591 https://doi.org/10.14311/app.2022.33.0591 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en s. stuerwald, r. meglin, s. kytzia, s. gilg acta polytechnica ctu proceedings figure 1. in the process chain, raw materials are used at various processes: in the cement kiln (green), in the cement mill/concrete production (blue) and in concrete production (red). figure 2. thin section to determine the content of cement paste. 3. results 3.1. substitution of raw material in cement production recycled crushed fines contains sand and hardened cement paste and can be used as an alternative raw material in cement production [6, 10]. in particular, the cement paste portion contains the raw materials that can be reactivated in the cement kiln. crushed fines samples (bs-l) from the laboratory were specifically made with 40%, 55% and 70% cement paste. the cement paste content of real crushed fines (bsr) was determined on prepared thin sections by polarized light microscopy (figure 2). it was observed that the content of cement paste in the real crushed fines samples was 14% 15%, which is lower than the expected typical content of about 30% (figure 3). it is assumed that parts of the cement paste were lost in the demolition and separation process. material analysis by x-ray fluorescence spectroscopy (xrf) was performed on each of the real crushed fines samples (bs-r). the cao content was between 24 % and 29 % and the lsf (limestone saturation factor) was between 16.9 and 27.8. this reflects the low cement paste content, which is decisive for the maximum substitution of natural raw material. calculations showed that only small amounts of the raw material can be substituted. to raise the substitution rate, the cement paste of crushed sand must be separated completely. hardened cement paste by itself could replace more than 90% of the natural raw materials (figure 4). 592 vol. 33/2022 use of recycled concrete fines in cement and as aggregate figure 3. samples of crushed sand and their content of hardened cement paste. figure 4. maximum substitution rate depending on lime-saturation-factor (lsf). 3.2. scms in cement mill or concrete production to evaluate the possible use of crushed sand as an admixture, selected samples of crushed sand (bs-r1, bs-l-40, bs-l-55 and bs-l-70) were milled and mixed in a ratio of 40:60 % with cem i 52.5 r. these cements were then used to prepare cement paste for further testing of strength (according to en 196-1), setting time and room stability (both according to en 196-3). results of cem ii/b-ll were used as reference. the results of the tests (table 1) indicate, that a lower compressive strength compared to a cem ii/bll must be expected. since the strengths of cements with crushed fines do not differ significantly, it can be assumed that the crushed fine is inert. the results of the setting time do not show a clear picture. while the cement with bs-r-1 crushed fine sets significantly faster than a cem ii/b-ll, the cement with bs-l-40 shows a somewhat slower setting time. the room stability test does not indicate any significant differences between the cements tested. the processing and preparation of the crushed sand as an additive is more complex than for the other two types of use since high demands are made on purity and homogeneity of the material. 3.2.1. substitution of sand in concrete production using the crushed fines as a substitute for natural sand is a good method to reduce the consumption of primary resources. however, sand is often segregated from demolition material because the fresh and hardened concrete properties may deteriorate. first, the particle size distribution (figure 5) and the properties of the crushed sands (table 2) were determined. in a test series on concrete mixtures, it was investigated whether samples of crushed sand are suitable as a substitute for sand and which concrete properties are derived. in each mixture, the entire aggregate fraction 0/4 was replaced by recycled crushed sand. 593 s. stuerwald, r. meglin, s. kytzia, s. gilg acta polytechnica ctu proceedings cem bs-l-70 cem bs-l-55 cem bs-l-40 cem bs-r-1 cem ii/b-ll water [%] 30.1 30 30 28 28 compressive strength [n/mm2] 2 d 21.3 21.6 20.9 20.8 25 7 d 30.8 34.9 32.4 33.3 37 28 d 36.9 41.4 39.4 39.2 47 flexural strength [n/mm2] 2 d 4.8 4.5 4.4 4.9 n/a 7 d 5.7 5.9 6 6.3 n/a 28 d 6.2 6.3 5.9 6.3 n/a setting time [min] begin 157 187 216 127 200 end 204 231 256 207 240 volume stability [mm] 0.5 1 1 1.5 1 table 1. results of cement paste tests compared to cem ii b/ll. figure 5. particle size distribution of crushed sands. a mixture with natural sand was used as a reference. the different water absorption of the crushed sand samples was considered, so that a constant effective w/c ratio of 0.6 should be achieved. the compressive strength of concrete with recycled crushed sand was only reduced by about 10-15%. however, this reduction may also be related to differences in the effective water content and need not be related to the mechanical properties of the crushed sand. crushed sand with high content of cement paste (bs-l-70) show a young’s modulus that is up to 17 % lower, while a mixture with low content of cement paste shows a young’s modulus that is about 6 % lower compared to the reference. natural sand can be completely replaced by recycled crushed sand. if the cement paste is separated during processing, the remaining sand can be used to produce concrete, which is technically equivalent to concrete made from primary material. 4. discussion the results show that the crushed fines can, with certain restrictions, be used for the investigated application possibilities in the value chain of concrete production. the results of the mortar tests (section 3.2.) indicate that a complete substitution of limestone powder is possible without any significant loss of quality. the use as a substitute for natural aggregates is also possible according to the results in 3.3. to do this, however, further concrete technology measures must be taken to prevent any significant loss of quality. the use as a raw material substitute in cement production (3.1.), however, does not seem to be possible without further processing steps. this is due to the low cement content found in the real samples, which leads to a low lime saturation factor and thus prevent a high degree of substitution. an aspect not presented here in detail is the co2uptake of the concrete waste. the cement paste absorbs co2 from the air through natural carbonation 594 vol. 33/2022 use of recycled concrete fines in cement and as aggregate bs-r-1 bs-l-40 bs-l-70 ratio of cement paste [%] 14 40 70 dry density [kg/m3] 2332 2175 1940 water absorption wa24 [%] 6.2 9.7 12.4 table 2. properties of the crushed sands. reference bs-l-70 bs-l-40 bs-r-1 cement [kg/m3] 300 300 300 300 water [kg/m3] 180 180 180 180 w/z (eff.) [−] 0.6 0.6 0.6 0.6 aggregate [kg/m3] 1887 1605 1678 1738 density [kg/m3] 2367 2085 2158 2218 air content [vol. %] 1.5% 1.5% 1.5% 1.5% table 3. concrete mix and properties of fresh concrete mixtures. reference bs-l-70 bs-l-40 bs-r-1 density concrete [kg/m3] 2400 2310 2300 2320 strength 28d [n/mm2] 40.5 36.0 37.0 34.5 e-modulus [n/mm2] 38ă419 32ă883 31ă867 35ă976 table 4. properties of hardened concrete mixtures. and binds this in the formation of calcium carbonate (caco3). this effect has already been investigated by various studies, although there is still no generally valid assumption about the exact maximum possible degree of carbonation [14–16]. carbonation occurs particularly when the cement stone is ground and therefore has a very large surface. the co2 uptake is particularly useful in the production of clinker, as the co2 emissions of the rotary kiln can be kept in the "cycle" by means of forced carbonation. in this way, we could achieve a certain degree of co2 neutrality in clinker production. based on these results, we would like to suggest the following approaches: • the greatest possible separation and recovery of the cement paste when processing concrete demolition. when developing processing methods, the aim should be to separate the cement stone as far as possible and to hold it back as a separate fraction. this gives you (i) a high-quality and climateneutral secondary raw material for cement production and (ii) a high-quality rc aggregate for concrete production, which enables the further optimization of recipes to produce rc concretes. • as far as possible carbonation of the cement paste in the entire process chain of the preparation of the cement paste in the concrete demolition (i.e. the preparation and storage in the various process stages). we assume that concrete demolition in switzerland has a significant potential for rebinding co2, since it is mostly concrete that was produced with portland cement and only absorbed limited amounts of co2 during use. in the development of processing methods, the aim should be to increase the carbonation of the cement paste in the concrete demolition. this is achieved above all by increasing the surface area and designing the storage conditions in a targeted manner. this allows co2 to be extracted from the air and stored in the building material. this corresponds to the "carbon capture and storage" approach that is much discussed today, with which one would like to reduce net co2 emissions by 2050. 5. conclusions the demolition of concrete is not yet considered as a problem and the potential of its recycling is not yet sufficiently recognised. this will change in the coming years or decades, because: (i) the amount of concrete demolition will increase once the concrete structures from the second half of the last century have reached the end of life. therefore, the proportion of mixed demolition in mineral construction waste will decrease in the future and the proportion of concrete demolition will increase accordingly. (ii) the possible uses of rc concretes in structural engineering will increase and with it the demands on the building material. today, qualitative deficiencies in the concrete granulate are compensated by adjustments in the concrete mix design (more cement, more admixtures). this leads to higher costs and environmental pollution. in the future, attempts will be made to improve the quality of concrete demolition. (iii) the pressure on the cement industry to make its contribution to climate protection is increasing (e.g. 595 s. stuerwald, r. meglin, s. kytzia, s. gilg acta polytechnica ctu proceedings through higher co2 fees and measures taken by public clients). many representatives of this industry see a promising approach to solving this problem by rebinding co2 in concrete. for this, the greatest potential lies in the rebinding capacity in the crushed sand of the concrete demolition. for these reasons, efforts to develop new processes for the separation and processing of c&dw will increase significantly in the coming years and further investigations for the usage of c&dw will be necessary. the here described research project improves the decision-making basis for the development of new technologies and possibilities to use a material which, until now, is down-cycled or landfilled. acknowledgements supported by the federal office for the environment foen (utf 591.03.19) and the gebert rüf foundation (grs-049/18). references [1] sia. recyclingbeton, sia 2030, 2010. zürich. [2] j.-l. gálvez-martos, d. styles, h. schoenberger, et al. construction and demolition waste best management practice in europe. resources, conservation and recycling 136:166-78, 2018. https: //doi.org/10.1016/j.resconrec.2018.04.016. [3] a. müller. baustoffrecycling: entstehung aufbereitung verwertung. wiesbaden: springer vieweg, 2018. [4] c. hoffmann, f. jacob. recyclingbeton aus betonund mischabbruchgranulat. dübendorf, 2007. [5] n. heeren, s. hellweg. tracking construction material over space and time: prospective and geo-referenced modeling of building stocks and construction material flows. journal of industrial ecology 23(1):253-67, 2018. https://doi.org/10.1111/jiec.12739. [6] c.-t. galbenis, s. tsimas. use of construction and demolition wastes as raw materials in cement clinker production. china particuology 4(2):83-5, 2006. https://doi.org/10.1016/s1672-2515(07)60241-3. [7] m. menegaki, d. damigos. a review on current situation and challenges of construction and demolition waste management. current opinion in green and sustainable chemistry 13:8-15, 2018. https://doi.org/10.1016/j.cogsc.2018.02.010. [8] s. guignot, s. touzé, f. von der weid, et al. recycling construction and demolition wastes as building materials: a life cycle assessment. journal of industrial ecology 19(6):1030-43, 2015. https://doi.org/10.1111/jiec.12262. [9] a. di maria, j. eyckmans, k. van acker. downcycling versus recycling of construction and demolition waste: combining lca and lcc to support sustainable policy making. waste management 75:3-21, 2018. https://doi.org/10.1016/j.wasman.2018.01.028. [10] c. müller, j. reiners, s. palm. closing the loop what type of concrete re-use is the most sustainable option, 2015. [11] cembureau. the european cement association. cementing the european green deal: reaching climate neutrality along the cement and concrete value chain by 2050. brussels, 2020. [12] r. andersson, h. stripple, t. gustafsson, et al. carbonation as a method to improve climate performance for cement based material. cement and concrete research 124, 2019. https: //doi.org/10.1016/j.cemconres.2019.105819. [13] j. schneider. decarbonizing construction through carbonation. proceedings of the national academy of sciences 117(23):12515-7, 2019. https://doi.org/10.1073/pnas.1913867116. [14] n. zhang, h. duan, t. r. miller, et al. mitigation of carbon dioxide by accelerated sequestration in concrete debris. renewable and sustainable energy reviews 117, 2020. https://doi.org/10.1016/j.rser.2019.109495. [15] e. possan, e. f. felix, w. a. thomaz. co2 uptake by carbonation of concrete during life cycle of building structures. journal of building pathology and rehabilitation 1(1), 2016. https://doi.org/10.1007/s41024-016-0010-9. [16] w. ashraf. carbonation of cement-based materials: challenges and opportunities. construction and building materials 120:558-70, 2016. https: //doi.org/10.1016/j.conbuildmat.2016.05.080. 596 https://doi.org/10.1016/j.resconrec.2018.04.016 https://doi.org/10.1111/jiec.12739 https://doi.org/10.1016/s1672-2515(07)60241-3 https://doi.org/10.1016/j.cogsc.2018.02.010 https://doi.org/10.1111/jiec.12262 https://doi.org/10.1016/j.wasman.2018.01.028 https://doi.org/10.1016/j.cemconres.2019.105819 https://doi.org/10.1073/pnas.1913867116 https://doi.org/10.1016/j.rser.2019.109495 https://doi.org/10.1007/s41024-016-0010-9 https://doi.org/10.1016/j.conbuildmat.2016.05.080 acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.35.0023 acta polytechnica ctu proceedings 35:23–26, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague railway track deflection analysis by using evolutionary algorithms pavel kulich∗, otto plášek brno university of technology, antonínská 548/1, 601 90 brno, czech republic ∗ corresponding author: pavel.kulich@vutbr.cz abstract. in contrast to numerical methods, analytical modelling of the railway track is one of the less time-consuming and computationally demanding methods which, in combination with the computing power commonly available today, can form an effective tool for analysing the behaviour of the railway track. this paper deals with the use of iterative methods of evolutionary algorithms, together with analytical modelling, for the purpose of reverse analysis of the measured deflection caused by moving loads acting on the railway track. the theoretical assumptions of the analytical model used, the data collection methodology and the method used for the reverse analysis are presented. the results of the analysis are also presented. keywords: railway track deflection, reverse analysis, evolutionary algorithms. 1. introduction the analytical model presented in [1] was used for the reverse analysis. the model consists of two infinitely long beams lying on the pasternak foundation, which, in contrast with the winkler foundation, considers the shear interaction of adjacent elements. graphical model interpretation is shown in figure 1. the model can be interpreted so that the first layer represents the rail; the second represents the sleeper with substructure. the influence of dynamic variables is taken into account in the same way as in the frýba model [2]. the model can be described by the following differential equations: ei1 d4w1(x, t) dx4 + m1 d2w1(x, t) dt2 + c1 dw1(x, t) dt + k1[w1(x, t) − w2(x, t)] = 0, (1) ei2 d4w2(x, t) dx4 − ga dw2(x, t)2 dx2 + m2 d2w2(x, t) dt2 + c2 dw2(x, t) dt + (k1 + k2)w2(x, t) − k1w1(x, t) = 0, (2) where ei1 is upper beam bending stiffness, m1 is the mass of one meter of the upper beam. according to the paragraph above, the upper beam represents the rail, but the values of ei1 and m1 do not necessarily correspond to the mechanical parameters of the rail itself due to the interaction of the rail with other elements of the railway track. the parameters k1 and c1 are parameters that act between the first and second layers of the structure and can be interpreted as properties of the fastening system. the bottom layer figure 1. schematic of the two-layer model on pasternak foundation. of the model corresponds to the sleepers and partly to the substructure with parameters ei2 bending stiffness of the second layer, m2 mass of one meter of the second layer, ga shear stiffness of the bottom layer. sleepers are not capable of transferring bending loads in the longitudinal direction of the track, and therefore the bending stiffness ei2 can be understood as the residual ability of the sleeper layer to transfer (when the axle passes) the bending load by horizontal action on the trackbed. this stiffness, therefore, takes on a non-zero but negligible value. the parameters k2 and c2 represent the properties of the substructure. it is necessary to introduce the relative coordinate s to solve the above differential equations. the effect of the wheel load q is introduced during the solution through boundary conditions. by substituting the parameters ei1 = 4500 knm2, ei2 = 0.1 knm2, ga = 6000 kn , k1 = 250000 knm−2, k2 = 40000 knm − 2, v = 30 ms − 1, c1 = 90 knsm − 2, c2 = 120 knsm − 2, m1 = 60 kg, m2 = 300 kg, q = 100 kn into the solution matrix, we obtain the deflection curve for the upper layer w1 and the deflection curve for the bottom layer w2. 23 https://doi.org/10.14311/app.2022.35.0023 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en pavel kulich, otto plášek acta polytechnica ctu proceedings figure 2. output of the two-layer model. the horizontal axis of both deflection curves is in the dimensionless relative coordinate s. 2. methods 2.1. track vertical displacement measuring method the input data for the reverse analysis are the track vertical displacements measured in-situ. vertical displacement of railway track is measured relatively to a plain of substructure. the displacement sensors were positioned in the monitored section in such a way that it was possible to detect the pushing of the rails and sleepers. the rail foots and the sleepers in four points along longitudinal axis were monitored. the sensors were positioned as follows: • s0 – right sleeper head; • s1, s2 – right rail; • s3, s4 – sleeper in thirds; • s5, s6 – left rail; • s7 – left sleeper head. using this sensor set, it is possible to obtain the deflection value of the rail unloaded by the twisting of the cross-section. the rail deflection is easily calculated as the average of the values measured by the pair of sensors s1, s2 and s5, s6. using sensors placed on the sleeper, the sleeper deflection curve can be calculated for each measured moment using an interpolation polynomial (cubic spline). it is also possible to calculate the deflection value of the sleeper under the rail at a point that is otherwise inaccessible for sensor placement. figure 3. schematic of sensor placement [3]. 2.2. evolutionary algorithm method reverse analysis of measured data can be classified as an optimization problem. the quantities to be optimized are the parameters of the analytical model. when solving the problem, we look for a combination of model parameters that leads to the best possible solution. the quality of the solution is assessed by the so-called fitness function, which is a mathematical expression of the similarity of the measured data and the output of the analytical model for the given parameters. in general, the higher the quality of the solution, the higher the value of the fitness function. for reverse analysis, algorithms have been developed that use as input a set of random combinations of parameters of the analytical model, successively test each set of parameters, evaluate their quality and from the best combinations create (by precisely defined methods) combinations of new parameters, which test repeatedly. this method of evaluation 24 vol. 35/2022 railway track deflection analysis by using evolutionary algorithms figure 4. measured in-situ deflection leads to better quality solutions with each subsequent iteration. the number of iterations and the number of elements of the parameter set are the input parameters of the algorithm. the optimisation is completed when the required number of iterations is reached. 3. model results the optimization algorithms were tested on data measured in planá nad lužnicí, km 74,978 section without under sleeper pads. track superstructure consists of 60 e1 rails, w14 fastening system and b 91s/1 sleepers. assessed data trace is measured during the passage of the passenger train. data traces from sensors s2 and s6 were selected for further processing, and their values were averaged at the corresponding time points. the resulting data trace was used for comparison with the deflection w1 of the analytical model. all axles of the passenger train were considered. the analytical model presented above gives the deflection for only one axle. to obtain a total deflection line for multiple axles is necessary to introduce the superposition principle for several deflection lines. figure 4 shows that the axles exert different forces on the superstructure, which is also taken to account during superposition calculation. the displacement line obtained by the presented analytical model captures the main characteristics of the measured signal, such as the travel wave in front of the first axle, the travel waves between axles and the steepness of the drop of the track under the passing axle. parameters of the analytical model are shown in table 1. the track critical speed is also calculated. 4. conclusion this article discussed reverse analysis theoretical presumptions, data collection methodology, data processing methodology. the algorithm output is also compared with in-situ measured data. from the text above it can be concluded that the presented analytical model can be used for further analyses of the rail track dynamic behaviour. despite the convincing qualitative results, the evolutionary algorithms, because of its stochastic character, can lead to misleading quantitative outputs. examination of the applicability of evolutionary algorithms for the purposes of reverse analysis, or the incorporation of other methods, will be the subject of the author’s further work. list of symbols ei1 upper layer bending stifness [n m2] ei2 bottom layer bending stifness [n m2] ga bottom layer shear stifness [n] k1 fastening system spring stifness [n m−2] k2 foundation spring stifness [n m−2] c1 fastening system damping [ns m−2] c2 foundation damping [ns m−2] m1 upper layer mass [kg m−1] m2 foundation layer mass [kg m−1] q wheel force [n] v vehicle speed [km h−1] vcr critical speed [km h−1] x longitudinal coordinate [m] t time [s] s relative coordinate [–] w1 rail layer deflection [m] w2 sleeper layer deflection [m] 25 pavel kulich, otto plášek acta polytechnica ctu proceedings figure 5. analytical model comparison with measured data ei1 ei2 ga k1 k2 c1 c2 [kn m2] [kn m2] [kn ] [kn m−2] [kn m−2] [kns m−2] [kns m−2] 5428 0,739 549 403287 66903 17 190 m1 m2 v vcr [kg m−1] [kg m−1] [km h−1] [km h−1] 292 515 90 1018 table 1. analytical model parameters acknowledgements the paper was prepared within the project "affordable railroad smart sensing system 4.0", project number tm01000016, which is co-financed with the state support of the technology agency of the czech republic within the 1st public competition of the support programme for applied research, experimental development and innovation delta 2 2019. references [1] p. kulich. dynamic analysis of track. diploma thesis, brno university of technology, 2017. [2] c. esveld. modern railway track. mrt productions, zaltbommel, second edi edn., 2001. [3] o. plášek, j. smutný, r. svoboda, m. hruzíková. analysis of railway track dynamic parameters. in evaces 09. experimental vibration analysis for civil engineering structures, pp. 239–240. wroclaw university of technology, wroclaw university of technology, 2009. 26 acta polytechnica ctu proceedings 35:23–26, 2022 1 introduction 2 methods 2.1 track vertical displacement measuring method 2.2 evolutionary algorithm method 3 model results 4 conclusion list of symbols acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.38.0323 acta polytechnica ctu proceedings 38:323–331, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague a guideline to support the use of off-site solutions for façade retrofitting through bim-enabled processes marco cucuzza∗, andrea giuseppe di stefano, giuliana iannaccone, gabriele masera politecnico di milano, department of architecture, built environment and construction engineering, via ponzio 31 – 20133 milano, italy ∗ corresponding author: marco.cucuzza@polimi.it abstract. the architecture, engineering and construction sector requires an intense innovation process to reduce costs, intervention times, and improve energy performance. this is particularly true to increase the rate of deep renovations of the existing european building stock and therefore support the eu’s 2050 decarbonisation targets. in this scenario, prefabrication can be considered a game-changer for the construction industry: on the one hand, it enables the adoption of an industrial mindset to the design, manufacturing, and installation of façade components, with the related advantages in terms of time, cost and quality; on the other, it allows for the customisation of components thanks to digital, bim-based design and fabrication tools. however, its adoption in retrofit operations is still limited due, among other factors, to a lack of practical experience and a limited number of actual demonstration cases. this paper introduces an operative guideline to support a more widespread use of prefabricated thermal insulation components for the energy retrofit of existing buildings in the framework of a bimenabled design and construction process. the guideline highlights the information flow and the role of each actor at every stage of the design and delivery process. the proposed guideline is finally tested through its application to a case study to show the feasibility of the process and the advantages deriving from the adoption of an industrialised approach to façade retrofit in terms of faster installation times. keywords: energy retrofit, mmc, dfma, off-site, bim. 1. introduction as widely discussed in literature, the construction sector needs an intense innovation process to reduce costs, intervention times, and energy needs [1]. achieving a significant energy renovation of the existing building stock would lead to an 80 % reduction in energy demand in 2050 compared to 2005 levels [2]. in this scenario, the european union faces a double challenge: to at least double the annual energy renovation rate by 2030, and to foster deep energy renovations (i.e., renovations that reduce energy consumption by at least 60 %) [3]. modern methods of construction (mmc) are widely regarded as a promising solution to these challenges and, more generally, to many shortcomings of the construction industry. mmc are a broad term that encompasses different dimensions of innovation, such as off-site construction and the related digital tools and techniques [4]. off-site construction is an approach to construction projects that seeks to move the construction process away from the site and take advantage of manufacturing approaches and standardisation efficiencies. the advantages of the mmc adoption are mainly the increase of quality (+ 30 %) and safety of workers (+ 80 %) together with a reduction of costs (20 %), time (20 to 50 %) and energy consumption (30 %) due to better management of resources in offsite activities instead of the on-site ones (70 %) [4–6]. the uncertainty of a dynamic working environment, the highly fragmented supply chain of smes, the need for skilled workers, the limited digitalisation, the shortterm thinking, and a political/governmental/cultural opposition are the main barriers to the mmc adoption. in contrast, horizontal integration of networks, endto-end digital integration, vertical integration, and networked manufacturing systems are the three keys to implement industry 4.0 in buildings [7]. the energy retrofit of existing buildings requires a tailored approach for each situation, because of the variety, among others, of construction periods, building technologies, materials, local climatic conditions, and site constraints. additional differentiating issues are local regulations and budget concerns. in parallel to this complex task, construction companies have a limited propensity to innovation, resulting in a productivity gap. in fact, their production rate grows only 1 % per year, compared to the medium growth rate of 2.8 % of other sectors. according to literature [2], productivity can be boosted by 50–60 % by implementing regulation, contract, design, supply chain management, and improving on-site execution 323 https://doi.org/10.14311/app.2022.38.0323 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en m. cucuzza, a. g. di stefano, g. iannaccone, g. masera acta polytechnica ctu proceedings by technology innovations and reskilling workers. these considerations show the market opportunities laying in the development of off-site prefabricated components for envelope retrofitting, reducing building energy demand and simplifying installation procedures while minimising the discomforts for the occupants. from this point of view, many research projects [8– 12] focused on developing new retrofit systems based on totally off-site or hybrid on-site/off-site technologies, while others, such as bim4eeb [13], worked on the related bim-based information structure. a core aspect for the large-scale implementation of mmc is their integration in a bim approach and the consequent adoption of rich digital communication that can increase interoperability levels, quality of information, and collaboration between stakeholders [14]. off-site automated activities, facilitated by a bim approach, have the potential to deliver the most significant growth in productivity in the construction sector [6]. for these reasons, various guidelines, design and assessment tools were developed specifically for new construction [15–17], but their application to existing buildings has been less explored so far. some research projects aim to streamline the decisionmaking process and integrate bim-based toolkits to facilitate the early-stage selection of retrofit technologies [18, 19]. this paper presents a general-purpose guideline on the bim implementation for prefabricated thermal insulation components, developed in the framework of the european research project bim4eeb. the guideline aims to go beyond the specific aspects of each construction technology, while focussing instead on the information exchange across the design, fabrication and delivery process of the off-site panels and between stakeholders (among others, architect, engineer, façade designer, manufacturer, supplier and general contractor). 2. methodology to develop the guidelines, the first step was an analytical review of previous research projects and off-site products already available on the market. the design, production, installation process, and the information exchange were investigated through literature, reports, available technical information and interviews. in the second step, the information gathered was used to produce a scheme of the steps and information exchange taking place across a general, technologyneutral, off-site process. the information flow defines the path of data through each stage, the actors involved and the necessary tools. in the third step, an analytical guideline was developed, in a form that would be useful both to designers interested in using prefabricated façade panels and to companies developing such solutions. the guideline is presented in the form of a process matrix, highlighting the information flow at each stage of the process and providing information about the actions required from the relevant actors. finally, the guideline has been tested on a real-life case study building model to assess the feasibility of the information flow and the applicability of the different process stages. 3. guidelines the guidelines support designers in identifying the information required at each stage of the process, streamlining the information exchange and enabling more effective communication with the other stakeholders involved (e.g., manufacturer and client). on the other hand, a guideline is also useful for manufacturers who plan to enter, or already are in, the field of off-site panels for the retrofit market, which is often characterised by an ad-hoc approach to each application because of its relatively short history. in this case, manufacturers can organise their technical information in a way that is compliant with the guideline and can also activate early engagement strategies to support the adoption of their products in retrofit operations. the information gathered from existing prefabrication technologies, eu-funded projects [8–10, 20] and interviews with european manufacturers made it possible to arrange the whole design-to-construction process of façade panels according to phases with general applicability, independent from the specific construction technology. the following diagram (figure 1) presents each phase with its purpose (task) and its outcome, i.e., the information it produces for the following steps of the process. this articulation will constitute the basis of the proposed framework. the framework (table 1, appendix a) shows the inputs, actions, and outputs in a clear, step-by-step process matrix, particularly suitable to highlight the correlations between the various steps and the related project phases. the steps column (second column) describes the general scope for each stage. then, the main objectives (third column) are identified, together with the required input activities (fourth column) carried out by other design team members; these activities are preparatory and necessary for those more strictly related to the panelisation activities. finally, under the output activities column (fifth column) and the data/model development column (sixth column), the required actions are summarised both from the process and the modelling point of view. the process matrix is discussed among european manufacturers and design teams to suggest a step ahead in the knowledge-based engineering [21] adoption compared to the existing professional procedure and the scientific literature [22, 23]. compared to other workflows, the proposed one started from the adoption of “panels place orders” during the concept design, allowing parametrical tools to generate various possible solutions based on the façade designer expertise, the manufacturer limitation in the production phase 324 vol. 38/2022 a guideline to support the use of off-site solutions . . . figure 1. information flow for panelisation process. and the site-specific constraints. the proposed solutions are meta-technological products – underpinned on real ones – avoiding the risk of redesigning the entire façade after the manufacturer engagement and, simultaneously, allowing the progressive information enrichment throughout the process thanks to the bimbased design process. stages 1 and 2 refer to the acquisition of information about the existing building, while the design phases (stages 3–5) concern the project of the panels increasing the level of details thanks to the iterations that the parametrical instruments enabled (design optioneering). finally, construction, building use and end of life (stages 7–9) describe the transmission of the data-rich model between the designer and the manufacturer, avoiding the potential loss of information with consequences in the management and disposal phases. at the same time, returning an informed model from the manufacturer to the designer and contractor makes it possible to integrate the assembly sequences smoothly into the site activities. this holistic perspective can significantly reduce construction time, allowing parallel activities on the actual construction site and in the factory. 3.1. case study the framework for the design of prefabricated thermal insulation components was tested on a case study to verify that the steps of the process and the information flow can be used in practice. a simplified version of one of the demonstration sites of the bim4eeb research project – a multi-storey multi-owner building located in monza (italy) – was used for this purpose. the exercise focuses on the first stages of the framework (from 1 to 5), demonstrating its effectiveness and how to select the configuration that best meets the requirements set by the client. the output (design of the prefabricated panels fitting the specific case study) will contain the necessary information to interface downstream with production and site operations (stages 6 to 9). • stages 1, 2: the data flow was defined and the as-is bim model was produced based on a survey. • stage 3: the first phase of the optioneering process was performed about the geometrical aspects of the façade panels. meta-technological objects were used as placeholders to reproduce different panelling hypotheses evaluated with the stakeholders. each time one panelling option was analysed, an automated optimisation (considering specific dimensional constraints) was conducted using a bespoke script, following the goal of maximising the number of equal panels, minimising the total number of panels, and reducing or eliminating unique panels. the software then provided a datasheet identifying individual elements (figure 2). • stage 4: starting from the first optioneering iteration results, several automated parametrical tools performed detailed analyses. once the dimensional and technological information were included, it was possible to evaluate construction details, such as corner solutions. these are crucial topics to be assessed in the early stage because they influence the whole retrofitting operations behaviour in terms of heat loss, thermal bridges, and aesthetics. each alternative was fully defined, and spreadsheets were exported for client evaluation and selection of the technology. • stage 5: the chosen panels were exported back into the bim environment, and a complete ifc model was shared with the manufacturer for validation (early-stage contractor and supply chain engagement). after an early-stage discussion with the manufacturer, the panel layers were designed in detail, defining the specific materials, thickness and other properties, anchors positioning, and dimensions. a detailed structural analysis was also performed in the bim environment, automatically defining the positions of anchors in the bim model through a dedicated script. once the model included the panels and the anchors, it was possible to proceed with the detailed energy analysis, eval325 m. cucuzza, a. g. di stefano, g. iannaccone, g. masera acta polytechnica ctu proceedings figure 2. as-is bim model and first panelisation phase with meta-technological panels. the total number of panels and the number of different panels considered is automatically displayed. figure 3. perspective view of the bim model with the selected panelisation. uate thermal bridges, and check compliance with local regulations. at the end of stage 5, a fully verified and integrated bim model was ready to be shared with all the stakeholders, particularly downstream with the manufacturer. in this whole process, and particularly in stage 5, the manufacturer’s ability to use and return an information-rich model is fundamental to ensure the smooth application of the guidelines. the capability to receipt, manage, transform and transmit the information required for each actor during each stage was verified thanks to the check of the final ifc model and the other additional documents (bill of panels, manufacturing drawings, installation drawings and technical documentation), although it was not possible to test the guideline with the actual production and installation of the panels. 4. conclusions the paper aims to propose a guideline to support the design of prefabricated thermal insulation components for facades of existing buildings within a bim-based approach. this guideline can encourage wider market uptake of off-site systems for retrofit operations, contributing to maximising efficiency in building renovation, reducing the renovation working time, enabling more accurate building quality control, and implementing a bim-based renovation business for construction companies as part of the european transition towards a digital built environment. the guideline presents, in a systematic way, the digital information flow necessary to support the design and delivery of prefabricated façade panels, with the specific data and involved actors correlated to the various stages of the process. thanks to the abstraction of current practices and procedures, the guideline has broad applicability and is not technology-dependent and country-specific. finally, it was tested on a case study to assess the practical feasibility of the process, including aspects such as the possibility to retrieve the necessary data and the information flow between different software and tools. the guideline represents a relevant tool to support designers who intend to adopt prefabricated façade systems in their retrofit projects, clarifying what information is required at each step of the process, from/to whom it should be provided, and what decision-making tools can be used. furthermore, the guideline also supports construction com326 vol. 38/2022 a guideline to support the use of off-site solutions . . . panies/manufacturers of façade systems, anticipating what information should be made available to the other actors of the process to ensure a smooth integration of the prefabricated panels in the bim-based workflow early-stage engagement of the industry when possible. the integration of rules and constraints provided by all the actors – including manufacturers, architects, construction site managers and façade designers – at the beginning of the process, together with the bim willingness to increment information across the stages, can be a step ahead in the adoption of knowledge-based engineering in the design for manufacture of prefabricated façades. acknowledgements this paper builds upon results from the bim4eeb project, which has received funding from the european union’s horizon 2020 research and innovation programme under grant agreement no 820660. it reflects only the author’s view and that the commission is not responsible for any use that may be made of the information it contains. references [1] r. hammond, n. o. nawari, b. walters. bim in sustainable design: strategies for retrofitting/renovation. in computing in civil and building engineering (2014), pp. 1969–1977. 2014. https://doi.org/10.1061/9780784413616.244 [2] mckinsey global institute. reinventing construction through a productivity revolution, 2017. [2022-12-01]. http://www.mckinsey.com/industries/capitalprojects-and-infrastructure/ourinsights/reinventing-construction-through-aproductivity-revolution [3] european commission. communication from the commission to the european parliament, the council, the european economic and social committee and the committee of the regions: a renovation wave for europe – greening our buildings, creating jobs, improving lives. brussels, be, 2020. com(2020) 662 final. https://eur-lex.europa.eu/legalcontent/it/txt/?uri=celex:52020dc0662 [4] m. horner, m. el-haram, d. vitali. advanced industrialised methods for the construction of homes (aimch) – work package 2: productivity mapping and literature review, 2019. 131 p. [5] buildoffsite property assurance scheme (bopas). recognised assurance for innovative or non-traditional methods of construction, 2018. [2022-12-01]. https://www.buildoffsite.com/content/uploads/ 2018/10/bopas-brochure-2018.pdf [6] d. sinclair, a. tait, l. carmichael. riba plan of work 2020 overview. in health and safety. 2020. https://doi.org/10.4324/9780429346637-2 [7] t. d. oesterreich, f. teuteberg. understanding the implications of digitisation and automation in the context of industry 4.0: a triangulation approach and elements of a research agenda for the construction industry. computers in industry 83:121–139, 2016. https://doi.org/10.1016/j.compind.2016.09.006 [8] easee, envelope approach to improve sustainability and energy efficiency in existing multi-storey multi-owner residential buildings, 2016. grant agreement id: 285540. [9] bresaer, breakthrough solutions for adaptable envelopes for building refurbishment, 2019. grant agreement id: 637186. [10] more connect, development and advanced prefabrication of innovative, multifunctional building envelope elements for modular retrofitting and connections, 2019. grant agreement id: 633477. [11] buildheat, standardised approaches and products for the systemic retrofit of residential buildings, focusing on heating and cooling consumptions attenuation, 2020. grant agreement id: 680658. [12] renozeb, accelerating energy renovation solution for zero energy buildings and neighbourhoods, 2021. grant agreement id: 768718. [13] b. daniotti, c. m. bolognesi, s. lupica spagnolo, et al. an interoperable bim-based toolkit for efficient renovation in buildings. buildings 11(7):271, 2021. https://doi.org/10.3390/buildings11070271 [14] bryden wood. data driven infrastructure from digital tools to manufactured components, 2017. [15] e. alfieri, e. seghezzi, m. sauchelli, et al. a bim-based approach for dfma in building construction: framework and first results on an italian case study. architectural engineering and design management 16(4):247–269, 2020. https://doi.org/10.1080/17452007.2020.1726725 [16] e. seghezzi, g. masera, f. r. cecconi. decision support for existing buildings: an lcc-based proposal for facade retrofitting technological choices. iop conference series: earth and environmental science 296(1):012032, 2019. https://doi.org/10.1088/1755-1315/296/1/012032 [17] z. yuan, c. sun, y. wang. design for manufacture and assembly-oriented parametric design of prefabricated buildings. automation in construction 88:13–22, 2018. https://doi.org/10.1016/j.autcon.2017.12.021 [18] a. g. cabrera, d. ntimos, n. purshouse, s. gallagher. impress bim methodology and software tools (ibimm) for façade retrofitting using prefabricated concrete panels. international journal of 3-d information modeling (ij3dim) 6(4):57–84, 2018. https://doi.org/10.4018/ij3dim.2017100104 [19] impress, improving preparedness and response of health services in major crises, 2017. grant agreement id: 608078. [20] f. lattke, k. e. larsen, s. ott, y. cronhjort. tes energyfaçade – prefabricated timber based building system for improving the energy efficiency of the building envelope, 2009. [2022-12-01]. http://www.tesenergyfacade.com [21] j. montali, m. overend, p. m. pelken, m. sauchelli. knowledge-based engineering in the design for manufacture of prefabricated façades: current gaps and future trends. architectural engineering and design management 14(1-2):78–94, 2018. https://doi.org/10.1080/17452007.2017.1364216 327 https://doi.org/10.1061/9780784413616.244 http://www.mckinsey.com/industries/capital-projects-and-infrastructure/our-insights/reinventing-construction-through-a-productivity-revolution http://www.mckinsey.com/industries/capital-projects-and-infrastructure/our-insights/reinventing-construction-through-a-productivity-revolution http://www.mckinsey.com/industries/capital-projects-and-infrastructure/our-insights/reinventing-construction-through-a-productivity-revolution http://www.mckinsey.com/industries/capital-projects-and-infrastructure/our-insights/reinventing-construction-through-a-productivity-revolution https://eur-lex.europa.eu/legal-content/it/txt/?uri=celex:52020dc0662 https://eur-lex.europa.eu/legal-content/it/txt/?uri=celex:52020dc0662 https://www.buildoffsite.com/content/uploads/2018/10/bopas-brochure-2018.pdf https://www.buildoffsite.com/content/uploads/2018/10/bopas-brochure-2018.pdf https://doi.org/10.4324/9780429346637-2 https://doi.org/10.1016/j.compind.2016.09.006 https://doi.org/10.3390/buildings11070271 https://doi.org/10.1080/17452007.2020.1726725 https://doi.org/10.1088/1755-1315/296/1/012032 https://doi.org/10.1016/j.autcon.2017.12.021 https://doi.org/10.4018/ij3dim.2017100104 http://www.tesenergyfacade.com https://doi.org/10.1080/17452007.2017.1364216 m. cucuzza, a. g. di stefano, g. iannaccone, g. masera acta polytechnica ctu proceedings [22] e. voss, q. jin, m. overend. a bpmn-based process map for the design and construction of façades. journal of facade design and engineering 1(1-2):17–29, 2013. https://doi.org/10.3233/fde-130006 [23] s. k. chandrasegaran, k. ramani, r. d. sriram, et al. the evolution, challenges, and future of knowledge representation in product design systems. computer-aided design 45(2):204–228, 2013. https://doi.org/10.1016/j.cad.2012.08.006 328 https://doi.org/10.3233/fde-130006 https://doi.org/10.1016/j.cad.2012.08.006 vol. 38/2022 a guideline to support the use of off-site solutions . . . a. appendix – table 1 stage step(s) objective(s) input mmc-related panelisation activities data/model development 1 in iti at iv e • define project scope and objectives (e.g.: facade, mep, new volumes). • establish retrofitting strategy; • identify renovation areas; • verify regulatory feasibility; • agree on data extraction requirements. • develop bim implementation strategies and incorporate them into bim execution plan. • capture rules for as-is modelling and mmc adoption. • no data yet. time for data-flow definition. 2 in iti at io n • site survey and building(s) 3d modelling. • analyse the existing building and set a common base for future work. • accurate survey of the building (laser scanner or other methods); • collect technological, historical and occupancy information; • 3d model development and information collection; • develop as-is model of the building, including basic information (structural grid, thermal transmittance, etc.). • early assessment of opportunities for mmc adoption. • as-is bim model. 3 c on ce pt d es ig n • develop early-stage design coherently with the project scope and objectives. • explore feasibility of panelisation strategy; • define geometrical alternatives. • as-is bim model; • knowledge-based constraints. • geometrical and design exploration; • use meta-technological objects to generate design optioneering; • develop parametric placeholders panels to understand possible issues. • include information such as massing (panels placeholders), meta-technological information and basic information (structural grid, thermal transmittance). 329 m. cucuzza, a. g. di stefano, g. iannaccone, g. masera acta polytechnica ctu proceedings stage step(s) objective(s) input mmc-related panelisation activities data/model development 4 pr el im in ar y d es ig n • develop and test the prefabricated solution; • early supply chain involvement. • early evaluation of technical alternatives. • client feedback from stage 3 • acquire performance targets, in particular u-value. • technological alternatives exploration for reaching project target; • generate datasheets from objects for approval of functional, environmental and finishes requirements; • identify supply chain and producers for technology evaluation. • develop multiple models with data sheets about technical alternatives. 5 d ev el op ed d es ig n • design analysis and calculations. • confirm full conformity to scope and objectives of the project; • early engagement of contractors and supply chain. • client feedback from stage 4; • perform energy simulations of the existing building to understand possible issues for the retrofitting; • perform structural simulations of the existing building to understand possible issues for the retrofitting; • perform lca simulations of the existing building to understand possible issues for the retrofitting. • add more details to bim objects (both geometry and data); • validate panelling technological solutions through early contractor and supply chain engagement; • use analytical tools and calculations to compare technological alternatives; • generate detailed part and whole models for different disciplines for early coordination; • final choice of the panel technology; • parametric time and cost validation. • include details of the chosen technical solution in the bim model. 6 d et ai le d d es ig n • production design for fabrication; • digital prototype of the panels. • develop full model with information from the manufacturer. • integrate supply chain information about resources and time. • develop overall construction programme schedule and assembly sequencing; • incorporate inputs from supply chain into the bim model; • develop fabrication and installation sequences, resource management plan, etc.; • generate digital prototypes of the panels to verify the construction process; • detailing of panels (connections, special parts, etc.). • construction model including highly accurate information from supply chain. 330 vol. 38/2022 a guideline to support the use of off-site solutions . . . stage step(s) objective(s) input mmc-related panelisation activities data/model development 7 7. 1 pr ec on st ru ct io n • accurate fabrication model. • enable fabrication of components. • accurate bim model from stage 6. • generate shop drawings for fabrication from models or integrate fabrication information from the manufacturer into the bim models. 7. 2 o nsit e c on st ru ct io n • training; • virtual building. • enable construction in line with scope and objectives of the project; • inform users about construction activities (time and location). • construction model updated with pre-construction information. • track construction activities and resources based on planned programme and planned assembly sequence; • validate installation on-site and update as-built model. • interface and communicate with users about construction activities. • as-built model incorporating information from previous stages and construction activities. 8 b ui ld in g u se • life cycle costs; • facility management attributes; • maintenance program. • represent the as-built asset; • produce ordinary maintenance plan. • maintenance information from supply chain; • as-built model from stage 7. • integrate as-built model with the facility management system; • integrate information about maintenance in the as-built bim model. • as-built models are up-to-date and accessible to the property management and tenants; • represent the building in use, including dynamic feedback information from bms and fm (digital twin). 9 en d of li fe • program end of life. • produce end of life actions plan. • digital twin model from stage 8. • develop end of life support documentation. • the model is up-to-date and will continue evolving. table 1. process matrix for energy retrofit with off-site façade panels. 331 acta polytechnica ctu proceedings 38:323–331, 2022 1 introduction 2 methodology 3 guidelines 3.1 case study 4 conclusions acknowledgements references a appendix – table 1 acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.38.0104 acta polytechnica ctu proceedings 38:104–109, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague reliability approaches affecting sustainability of existing steel structures miroslav sýkora∗, vitali nadolski czech technical university in prague, klokner institute, department of structural reliability, šolínova 7, prague, czech republic ∗ corresponding author: miroslav.sykora@cvut.cz abstract. steel structures are second most numerous in the stock of existing buildings. in contrast to dominating concrete buildings, they are typically lightweight and are more sensitive to alterations in use or loads. while the sustainability principles require to maintain and keep using these structures, structural assessments often indicate insufficient reliability and need for replacements. the submitted contribution shows that the most important reliability considerations affecting the sustainability of existing steel structures consist of specifying (1) appropriate target reliability level, (2) verification methods, and (3) intervention procedures. the study focuses on the first two aspects. (1) optimum target reliability can be specified by probabilistic optimisation considering sustainability aspects including structural costs, and expected consequences of replacement and of possible failure. it is shown that lower reliability levels might be considered for the assessment of existing structures than for the design of new structures, with benefits for sustainability in construction. regarding (2), the most efficient verification methods are based on advanced probabilistic approaches. it is demonstrated that sustainability may be significantly affected by the selection of assessment methods. advanced reliability approaches commonly reduce assessment requirements by 10–15 %. sustainability indicators are mostly related to the key aspects (1) and (2). using the advanced methods may bring a significantly positive contribution to sustainability, particularly when an upgrade of the existing structure is associated with high economic cost and significant environmental impact. keywords: existing structures, adjusted partial factors, probabilistic approaches, reliability. 1. introduction steel structures are second most numerous in the stock of existing buildings. in contrast to dominating concrete buildings, steel structures are typically lightweight and are more sensitive to changes in use and adjustment of loads. some of the existing structures are more than 100 years old and protected for their heritage value. while the sustainability principles require to maintain and keep using these structures, structural assessments often indicate insufficient reliability and need for replacements. this situation may be solved by applying advanced reliability assessment methods that mitigate the conservativeness of simplified methods utilised in engineering practice. in agreement with this, the global consensus on sustainability in the built environment [1] requires facilitating and rewarding the use of advanced analyses and methods of structural reliability to achieve sustainability in construction. applications of advanced assessments of existing structures may contribute to achieving the sustainable development goals (sdgs). in october 2015 the united nations adopted resolution 70/1 transforming our world: the 2030 agenda for sustainable development to balance the three aspects of sustainable development: economic, social, and environmental. improved assessments of existing structures can particularly help contribute to reach sdg 12 ensure sustainable consumption and production patterns. relevant targets presented in the resolution include (a) achieving the sustainable management and efficient use of natural resources by 2030, and (b) substantially reducing waste through prevention, reduction, recycling and reuse. the assessment may positively contribute to sustainability in construction, facilitating to keep existing structures in service. the assessment of existing steel bridges may be improved by specifying • appropriate target reliability level, • verification methods, and • intervention procedures. this study focuses on the first two aspects: (1.) optimum target reliability can be specified based on probabilistic optimisation considering sustainability aspects including structural costs, expected consequences of replacement and of possible failures. regarding (2.), the most efficient verification methods are based on advanced probabilistic approaches, considering actual load conditions and properties of the structure and related failure consequences. this study investigates benefits of applying advanced methods in the reliability assessment of an existing steel structure, critically comparing the obtained results with those based on the partial factor method for structural design. 104 https://doi.org/10.14311/app.2022.38.0104 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 38/2022 reliability approaches affecting sustainability . . . basic variable x dist. µx/xk vx yield strength fy ln 1.09 5 % geometry a n 1.0 3 % resistance model uncertainty θr ln 1.15 6 % permanent load g n 1.0 5 % ground snow (1-year maxima) q1 gum 0.4 50 % snow load – time-invariant component c0 ln 0.8 20 % load effect model uncertainty θe ln 1.0 7.5 % µx – mean, vx – coefficient of variation, n – normal distribution, ln – lognormal distribution with the lower bound at the origin, gum – gumbel distribution (max. values), xk – characteristic value of basic variable. table 1. probabilistic models of basic variables considered in the case study. 2. adjusted partial factors general guidelines for adjusting and updating partial factors are provided by the basic eurocode en 1990 [2]. partial factors may be adjusted considering structure-specific (information about materials, dimensions, permanent actions, system behaviour etc.) and site-specific conditions (e.g. information about variable loads). the assessment values are obtained as fractiles corresponding to probability from generalized values of sensitivity factors and a selected target reliability level. for more details see [3]. 3. probabilistic reliability analysis a generic limit state function for members of steel structures may be written as follows: g(x) = θrr − θe [g + c0qref ], (1) where the notation of the basic (random) variables is as follows: θr and θe uncertainties in resistance and load effect models respectively, r resistance of the cross-section or of a structural member, g permanent load, c0 time-invariant component (e.g shape factor for the roof snow loads), and qref time-variant component of the variable load related to a reference period tref (e.g. maxima of the ground snow loads). probabilistic models for basic variables given in table 1 are selected taking into account in situ measurements and data in jcss probabilistic model code [4] and previous studies [5]. the results of numerous studies indicate that a gumbel distribution is often an appropriate model for annual maxima. the background report for eurocodes [6] proposes the generalised values for annual maxima that are adopted here (table 1). the statistical parameters for different reference periods are recalculated using general equations for gumbel distribution. snow load on the roof is obtained from the ground snow load by using shape, thermal and exposure factors. uncertainties related to these coefficients are described here by the timeinvariant coefficient c0 according to [6]. the model for load effect uncertainty, θe is based on the jcss probabilistic model code [4]. 4. case study – reliability analysis of roof girder of existing steel building in this section, reliability requirements following from the fixed partial factors (fpf) provided in en 1990, adjusted partial factors (apf) (section 2), and probabilistic method (pm) (section 3) are critically compared. reliability assessment is performed considering a 10-year remaining service life (equal to a considered reference period). target reliability index is recommended according to en 1990 [2]. however, these recommendations are intended to be used primarily for the design of members of new structures. in general, lower reliability levels can be accepted for existing structures in comparison to structural design as follows from the general principles of structural reliability provided in iso 2394:2015 [7]. optimisation of the target reliability for existing structures by implementing cost optimization procedures and criteria for human safety is presented in [8]. two reliability levels are recommended – the minimum level below which the structure is considered unreliable and should be upgraded – reliability index β0; and the target level indicating an optimum upgrade strategy – βup. for middle consequence class (cc2) βup = 3.3 and β0 = 2.8 are considered [8]. en 1990 [2] is the basic document that suggests the load combinations and relevant partial factors. the following partial factors are recommended for structural design for permanent loads: γg = 1.35 and ξ = 0.85 and for variable loads γq = 1.5 and ψ0 = 0.5 (snow). the load combination rule 6.10(a,b) is applied; for the considered load ratios (see below) relationship (6.10b) with a reduced permanent action effect is dominating. 105 miroslav sýkora, vitali nadolski acta polytechnica ctu proceedings apf apf∗ apf apf∗ pm pm (β0,10 = 2.8) (β0,10 = 2.8) (βup,10 = 3.3) (βup,10 = 3.3) (β0,10 = 2.8) (βup,10 = 3.3) γg 1.07 1.10 1.08 1.11 1.12 1.13 γq 1.10 1.40 1.24 1.64 1.42 1.65 γm0 0.97 0.84 1.00 0.85 0.85 0.86 ∗ adjusted partial factors calculated with the actual values of the sensitivity factors (αr = 0.2, αg = 0.2, and αq = 0.95). table 2. comparison of partial factors (χ = 0.8). figure 1. variation of sensitivity factors with χ. using the adjusted partial factors and the probabilistic method (the first order reliability method form), partial factors are derived to provide for the target reliability index. to cover a wide range of load combinations, load ratio χ is introduced. the load ratio χ denotes the ratio of characteristic variable loads to the total characteristic load. the load ratio may vary within the interval from nearly 0 (underground structures, foundations) up to nearly 1 (local effects on crane girders). for steel structures, 0.5 ≤ χ ≤ 1 is expected [9]. the values of the partial factors are presented in table 2 for χ = 0.8. the main deficiency of the apf is that the generalised sensitivity factors are applied. more precisely, it is possible to determine the values of the partial factors using the actual values of the sensitivity factors obtained by form. figure 1 displays variation of the sensitivity factors with the ratio χ. for adjusted partial factors the sensitivity factor could be recommended αe = -0.95 for the snow load, αe = -0.2 for the permanent load and αr = 0.2 for resistance. the results the apf and pm become close when using these values of the sensitivity factors. the geometrical characteristic (hereinafter referred to as a reliability requirement) of a cross-section wi, such as section modulus, required to satisfy the limit state in accordance with a particular approach to reliability verification the selected system of partial figure 2. variation of wi with χ (apf based on the actual values of sensitivity factors – αr = 0.2, αg = 0.2, and αq = 0.95). factors is calculated from limit state function: g(x) = wfyk/γm0 − [γggk + γqc0sk]. (2) figure 2 displays variation of the standardised ratio wi = wi/wen with χ, where wen is the reference value based on the partial factors recommended in eurocodes for structural design. when wi < 0, the reliability requirements according to approach “i” are lower than those according to eurocodes for structural design. figure 2 shows that the adjusted partial factors (apf) and probabilistic method pm lead to the reliability requirements lower than en. the decrease in requirements is attributed to the use of the lower target reliability level for existing structure β0 (lower than in en) and case-specific probabilistic distributions for basic variables that reduces the conservativeness of fixed partial factors. in contrast, the requirements for upgrades according to apf and pm (considering βup) are close to those based on en. the area between the curves for assessment (β0) and upgrade (βup) in figure 2 is associated with the situations when the application of the advanced methods is expected to provide sustainability benefit. in these situations, en assessment requires an upgrade with economic and environmental impacts while the advanced methods authorise a continued use of the structure “as it is”. for the structures designed according to the czech standards valid before eurocodes has been introduced, ratio wi is expected to range approximately from 0.75 (χ close to unity) to 0.85 (χ close to 0.3) when the 106 vol. 38/2022 reliability approaches affecting sustainability . . . roof snow load is the leading variable action. these estimates are based on the results of detailed reliability analysis of existing steel roofs exposed to snow loads in the czech republic [10]. such low wi-values are attributed to increased design roof snow loads as introduced by eurocodes. 5. discussion on appropriate sustainability indicator for assessment of existing structures this example provides first insights into the sustainability benefits possibly gained by applying advanced reliability methods and considering the target reliability levels optimised for existing structures. the fundamental decision in reliability assessments – whether the existing structure can be used without upgrade or upgrade is needed – is analysed focusing on the roof girder investigated in section 4. various sustainability indicators have been proposed to quantify the effects of decisions about structures on sustainability. for instance, müller et al. proposed a simplified measure – building material sustainability potential (bmsp) [11]. to focus on the main aspects and allow for analysing a range of the assessment situations of practical relevance, this simple indicator is considered: bmsp = p × sl/ei, (3) where p performance; sl service life; and ei environmental impact. it is assumed that the existing structure under consideration fully provides its function if an ultimate limit state (uls) criterion is fulfilled, p = 100 %. when the uls condition is violated, the structure should be closed, p = 0 %. service life is measured in years with a reference level considered here as 50 years, and then sl = 100 %. if a service life is estimated as 25 years, then sl reduces to 50 %. very small environmental impact is assumed when the existing structure is continuously used without upgrade, ei ≈ 0, while ei increases proportionally with the level of strengthening, ei > 0. when comparing two alternatives, bmspa > bmspb should indicate alternative a being preferable. however, it can be argued whether bmsp is an appropriate indicator for comparing decision alternatives about existing structures since the environmental impact is close to zero for a “no upgrade” alternative and bmsp converges to infinity. to further illustrate the need for modification of bmsp, let us assume that: • benefit (p × sl) and environmental impact related to maintenance and upgrade (ei) can be both expressed in monetary terms; typically the former as a gain and the latter as a loss. • the benefit from using the structure, (p0 × sl0), may be much larger than the environmental impact ei0 that is now related to maintenance only; as an example (p0 × sl0) = 100 units and ei0 = 1 unit; the total gain from using the structure is thus 99 units over service life. • in the case of upgrade, performance level may be retained, service life may be doubled and likewise related benefit, p0 × slup = 200 units. the upgrade may have significant environmental impact; for instance 10-times increased compared to “no upgrade”, eiup = 10ei0 = 10 units. the total gain is then 190 units, nearly doubled in comparison to “no upgrade”. in this example, bmsp for the structure “as it is” and upgraded would be: bmsp0 = 100/1 = 100 > bmspup = 200/10 = 20 and the “no upgrade” strategy should be preferred. however, a comparison of the total gains clearly points to the opposite. based on these arguments, it is thus proposed to modify the sustainability indicator for decision making about existing structures when benefits and losses can be expressed in the same units: si = p × sl − (ei + c). (4) the term in brackets denotes the expected losses. newly introduced cost c should cover all expenses related to maintenance and possible upgrade. when the owner (particularly the society) saves financial resources, these may be utilised to implement measures positively contributing to sustainability. focusing on the girder analysed in section 4, figure 3 displays variation of sustainability indicator si, benefit expressed as sl, and losses c with ratio w that is the property of the girder “as it is”. the trends of si, sl, and c are estimated on the basis of the assumptions discussed below. as a reference level, the maximum benefit is assumed to be related to service life of 50 years, slmax = 100 units. no distinction between performance levels is made – the girder just needs to comply with reliability requirements and then the building can be fully used; p is thus disregarded hereafter. assumptions related to losses, c: (1.) based on the detailed analysis in [9], upgrade cost is assumed to correspond to about 40 % of the total benefit, 40 units, out of which 50 % is fixed cost independent of w (costs of surveys, assessment, administration and management, economic losses due to business interruption or replacement of users, etc.). maintenance is disregarded for simplification. environmental impact was ignored in [9]. 107 miroslav sýkora, vitali nadolski acta polytechnica ctu proceedings figure 3. variation of sustainability indicator si, benefit sl, and loss c with ratio w (neglected ei). (2.) focusing initially on application of the partial factor method according to eurocodes (“en”), upgrade is needed for w < 1 while no structural intervention takes place otherwise. full upgrade is assumed to be associated with w = 0.6; upgrade cost then linearly decreases with increasing w and drops to zero for w = 1 when the existing girder complies with the en requirements. (3.) for 0.6 ≤ w < wβ0 = 0.8 (figure 2), similar assumptions for upgrade cost apply when the advanced methods are used. reliability of the girder is below β0 and an upgrade is necessary. as the optimum upgrade level is slightly below to that required by en, wadv,upgrade ≈ 0.9–0.95 (figure 2), cadv(w) is slightly lower than cen (w). for w ≥ wβ0, no upgrade is needed. assumptions related to benefit, sl: (4.) for w < 1, en requires upgrading. the upgrade is assumed to provide for a service life of 50 years. when w ≥ 1, the existing girder meets the en requirements and it is again assumed to have a service life of 50 years. (5.) for 0.6 ≤ w < wβ0, sladv is also 100 units as upgrade provides for a 50-year service life. for w = wβ0 the girder exactly complies with β0 for tref = 10 years (section 4) and thus a 10-year service life is guaranteed, yielding sladv(wβ0) = 20 units. when w increases above wβ0, tref can be increased to comply with the β0-requirement. for w = 0.98, a 50-year service life is reached, sladv(w ≥ 0.98) = 100 units. comparison of si-values – evaluated according to equation (4) and plotted in figure 3 – indicates that: • for 0.6 ≤ w < wβ0 and w ≥ 1, the use of advanced methods has a small effect on the si-values. • for wβ0 ≤ w < 0.96 (marked in figure 3 as “a”), figure 4. variation of sustainability indicator si, benefit sl, and losses (ei+c) with ratio w (including ei). the use of advanced methods seems to lead to lower si-values as a service life lower than 50 years would be authorised. in this case with a relatively low upgrade cost, upgrading seems beneficial. • for 0.92 ≤ w < 1 (marked as “b”), the advanced methods provide benefit as upgrade is unnecessary while the en-based assessment indicate otherwise. in [9] no account for environmental impact was taken. during upgrade, environmental impact may cover for instance material consumption, transportation of materials and equipment and related fuel consumption and emissions etc. to illustrate the effect of environmental impact, let us consider that (ei + c) for full upgrade is 2.5-times higher than in the previous case, (ei + c) = 100. all other assumptions remain unchanged. figure 4 portrays variation of si, sl, and (ei + c) with w. it appears that increased upgrade cost (ei + c) significantly change the obtained si-values: • while for 0.6 ≤ w < wβ0 there is again a small difference between siadv and sien , for wβ0 ≤ w < 1 area “a” nearly vanishes and area “b” remains – the use of the advanced methods is beneficial. • for w ≥ 1, there is again no difference between using the advanced methods or en. it is emphasised that the example presented in this section is intentionally simplified focusing on the main aspects of decision making and implications for sustainability. situations when decision making may be more complex include: • for very low resistance (w < wβ0), the girder is considered unreliable and decision should be made whether it should be replaced or should be upgraded; the example indicates that environmental impact plays a significant role in this decision making. 108 vol. 38/2022 reliability approaches affecting sustainability . . . • for low but possibly acceptable resistance (wβ0 < w < 1), the girder may be considered reliable with a reduced service live. it should be then decided whether the girder can be preserved, upgraded or replaced by a new structure. • even for sufficient resistance (w ≥ 1), upgrading or replacement may be considered to reach a longer service life of the structure. however, this decision should be made with caution as useful service life of buildings is mostly affected by a number of factors causing obsolescence that are beyond the control of civil engineers (economic, functional or technological obsolescence, failure to meet legal requirements) [12]. large investments in the attempt to achieve long service life from the reliability perspective may then be in vain. within further research, the obtained results will be verified considering a wide range of factors to quantify the overall sustainability impact of various assessment strategies by a full probabilistic approach as proposed by webb and ayyub [13]. also, the use of surveys results should generally reduce uncertainties in basic variables and increase reliability estimates for existing structures, making it possible to avoid or minimise structural interventions. 6. conclusions while the sustainability considerations require to maintain and keep using existing structures, structural assessments often indicate insufficient reliability levels and need for replacements. the submitted contribution investigates how this situation may be solved by applying advanced reliability assessment methods. it is demonstrated how the advanced methods may bring a significant positive contribution to sustainability, particularly when an upgrade of the existing structure is associated with high economic cost and large environmental impact. it is newly proposed to modify the sustainability indicator for decision making about existing structures considering associated benefits and losses expressed in the same units. case studies show that the application of advanced probabilistic approaches reduces the assessment requirements by 20–25 % when the minimum reliability level is accepted, and by 5–10 % when the optimum reliability level for upgrades is considered. it is demonstrated that the application of advanced reliability methods may allow continued use of existing structures when conservative methods of structural design may indicate needs for upgrading. acknowledgements this study has been supported by the ministry of culture of the czech republic under grant dg18p02ovv033 “the methods for achieving the sustainability of industrial heritage steel bridges”. references [1] m. h. faber, w. schmidt. globe – global consensus on sustainability in the built environment, 2020. (adopted by jcss and sup-ported by rilem, iabse, fib, cib, eccs and iass), [2022-11-15]. https://www.rilem.net/globe [2] en 1990, eurocode – basis of structural design, 2002. cen, brussels, p. 87. [3] r. caspeele, m. sykora, d. l. allaix, r. steenbergen. the design value method and adjusted partial factor approach for existing structures. structural engineering international 23(4):386–393, 2013. https: //doi.org/10.2749/101686613x13627347100194 [4] joint committee on structural safety. jcss probabilistic model code, 2022. (periodically updated, online publication), [2022-11-15]. https://www.jcsslc.org/jcss-probabilistic-model-code/ [5] v. nadolski, a. rózsás, m. sýkora. calibrating partial factors – methodology, input data and case study of steel structures: methodology, input data and case study of steel structures. periodica polytechnica civil engineering 63(1):222–242, 2019. https://doi.org/10.3311/ppci.12822 [6] cen tc250/ad hoc group reliability of eurocodes (convenor – ton vrouwenvelder) technical report for the reliability background of eurocodes, 2021. p. 165. [7] iso 2394, general principles on reliability for structures, iso, geneve, switzerland, 2015. [8] fib c. tg3.1, partial factor methods for existing structures (fib bulletin 80), fib, 2016. [9] m. sykora, d. diamantidis, m. holicky, k. jung. target reliability for existing structures considering economic and societal aspects. structure and infrastructure engineering 13(1):181–194, 2017. https://doi.org/10.1080/15732479.2016.1198394 [10] m. holický, j. marková, m. sýkora. reliability of light-weight roofs exposed to snow load [in the czech original: spolehlivost lehkých střech zatížených sněhem]. stavební obzor 16(3):65–69, 2007. [11] h. s. mueller, m. haist, j. s. moffatt, m. vogel. design, material properties and structural performance of sustainable concrete. procedia engineering 171:22–32, 2017. https://doi.org/10.1016/j.proeng.2017.01.306 [12] d. diamantidis, m. sykora, e. bertacca. obsolescence rate: framework, analysis and influence on risk acceptance criteria. in proceedings ialcce 2018, pp. 379–386. 2019. [13] d. webb, b. m. ayyub. sustainability quantification and valuation. ii: probabilistic framework and metrics for sustainable construction. asce-asme journal of risk and uncertainty in engineering systems, part a: civil engineering 3(3):e4016002, 2017. https://doi.org/10.1061/ajrua6.0000894 109 https://www.rilem.net/globe https://doi.org/10.2749/101686613x13627347100194 https://doi.org/10.2749/101686613x13627347100194 https://www.jcss-lc.org/jcss-probabilistic-model-code/ https://www.jcss-lc.org/jcss-probabilistic-model-code/ https://doi.org/10.3311/ppci.12822 https://doi.org/10.1080/15732479.2016.1198394 https://doi.org/10.1016/j.proeng.2017.01.306 https://doi.org/10.1061/ajrua6.0000894 acta polytechnica ctu proceedings 38:104–109, 2022 1 introduction 2 adjusted partial factors 3 probabilistic reliability analysis 4 case study – reliability analysis of roof girder of existing steel building 5 discussion on appropriate sustainability indicator for assessment of existing structures 6 conclusions acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.35.0027 acta polytechnica ctu proceedings 35:27–36, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague the railway 4.0 concept – the vision of a digital railway in the czech republic (cr) martin leso czech technical university in prague, faculty of transportation sciences, konviktská 20, 110 00, prague, czech republic correspondence: lesomart@fd.cvut.cz abstract. the paper presents a new alternative approach to the modernization of control and safety technologies on the railway network in the czech republic, referred to as the "the railway 4.0 concept". the presented concept is based on the comprehensive introduction of digital technologies that fulfill the vision of intelligent transport systems on railways (its-r). the paper describes the architecture and basic components of the system based on the distributed technologies with the centralization of logical elements. the paper also places the given issue in the context of current developments in the implementation of the etcs system in the czech republic. keywords: etcs, frmcs, gsm-r, intelligent transportation systems on railway, railway digitalization. 1. introduction the long-planned and long-term implementation of the ertms/etcs system is currently underway, not only on the czech railways. this technology brings, in particular, increased safety in controlling the movement of rail vehicles, i.e. the elimination of error on the part of the train driver. however, the ertms/etcs cannot be seen only as a safety enhancement system, or as a way of meeting one of eu’s interoperable requirements – a requirement to implement a single train protection system. the ertms/etcs technology can clearly be seen as a fundamental element enabling change in the approach to more efficient utilization of the railway system and its implementation must not only follow the safety parameter but must also lead to increased capacity and usability of the railway by the czech transport system. therefore, the question today is not whether to introduce such a system, but how to introduce these new technologies so that they bring real benefit to the railway function and overall transport in the czech republic. 2. history of the railway 4.0 concept the railway 4.0. concept was created as a technological elaboration of the vision "intelligent transport systems on railways" (its-r) 1, which was defined by the sdt working group. the vision of its-r 2 defined the fundamental vision of railway technology 1the its-r vision was added into the strategic materials of the ministry of transportation in 2016 within the implementation plan of the action plan for the development of intelligent transport systems (its) in the czech republic by 2020 (with an outlook to 2050) as project no. 63, zd-20, pilot verification of its-r technology 2http://www.sdt.cz/dokumenty/2015_pozicni_dokument_ sdt_rozvoj_its_na_zeleznici.pdf development. it should clearly lead to the concept of intelligent vehicles, i.e. vehicles which, thanks to the on-board technologies etcs, ato, gsm-r/frmcs, 4g/5g, will enable the full safety of their operation, including automation and optimization of their driving. at the same time, they will enable their full involvement in logistic transportation systems in both freight and passenger transport. finally, they will also provide services to passengers in the form of voice and data communication. however, the infrastructure aspect of these technologies, which must supply adequate technologies in the form of, in particular, radio networks and an appropriate level of etcs train control technology based on at least application level 2 with the possibility of using application level 3, must adapt to this concept of intelligent vehicles. at present, it is not a question of whether this vision of intelligent transport systems will be realized. current global trends, based on the present level of technology and the generally increasing demands for higher railway usability, clearly confirm this direction. similar projects addressing the issue of comprehensive digitalization and modernization of railways can be found in most of the eu’s neighboring countries. the digitales stellwerk 3 project in germany can be mentioned, which is a part of a comprehensive plan for digital railways 4. the traditional swiss railways sbb 5 are dealing with a similar program in the smartrail4.0 project. the comprehensive etcs and gsm-r deployment program is already being implemented by the norwegian railways bane nor 6. the main issue concerning the modernization of 3https://de.wikipedia.org/wiki/digitales_stellwerk 4https://digitale-schiene-deutschland.de/en 5https://www.smartrail40.ch/ 6https://www.banenor.no/prosjekter/prosjekter/ ertms/ 27 https://doi.org/10.14311/app.2022.35.0027 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en http://www.sdt.cz/dokumenty/2015_pozicni_dokument_sdt_rozvoj_its_na_zeleznici.pdf http://www.sdt.cz/dokumenty/2015_pozicni_dokument_sdt_rozvoj_its_na_zeleznici.pdf https://de.wikipedia.org/wiki/digitales_stellwerk https://digitale-schiene-deutschland.de/en https://www.smartrail40.ch/ https://www.banenor.no/prosjekter/prosjekter/ertms/ https://www.banenor.no/prosjekter/prosjekter/ertms/ martin leso acta polytechnica ctu proceedings the railways in the czech republic should therefore be a discussion on how to manage this necessary generational and comprehensive change of technologies in a sufficiently short time, with reasonable financial resources, so that we can take advantage of the benefits that these technologies undoubtedly bring as soon as possible. the aim and the evaluation criteria of modernization strategies should therefore be an increase in the usability of the railway in terms of the speed requirements, infrastructure capacity as well as the accuracy and availability of railways, enabling effective involvement in integrated transport systems throughout the czech republic. in this context of a comprehensive approach, the safety parameter that is most closely linked to the etcs system needs to be seen as a necessary and indispensable condition that is a natural part of the technologies being implemented. in this paper, the 1st of the areas defined in the itsr vision is discussed and further elaborated – control and command technology of railway transport. this technology can be considered the most essential for the function of rail transport. without its systematic introduction, full-fledged railway digitization can no longer be considered. the proposed approach define basic technological elements that should be use by all digital railway applications. subsequent applications or supporting tools forming the overall "digital world" of the railway will then be addressed on this technological infrastructure. however, the issue of related other worlds defined in the its-r vision and its integration into the overall architecture of the "digital world" of the railway is already outside the scope of this paper, so it will not be discussed further here. the railway 4.0. concept was created while the author of this article was active in the safety commission of the ministry of transport of czech republic (mdcr), which was established by the minister of transport of the czech republic doc. ing. karel havlíček, ph.d. mba in 7/2020, in response to a tragic accident at pernink railway station. within the framework of negotiations, a mdcr-o130 vision of the implementation of the etcs and gsm-r systems to all railway lines and vehicles by 2040 was introduced. from the outlined material 7 it is clear that by 2040 all railway lines in the czech republic should be equipped with etcs and gsm-r systems. this political demand, emphasizing in particular the requirement to increase the safety of the entire railway network of the czech republic, was the subject of a solution proposed to address this issue by experts from the správa železnic, státní organizace (hereinafter "sz"). the proposal 8 envisages possible solutions through comprehensive modernization of some major railway lines (most also intended for electrification) via etcs technology at application level 2, on less busy lines it 7https://www.mdcr.cz/media/media-a-tiskovezpravy/bezpecnostni-komise-hlavni-i-lokalni-trati-jiz-zn 8the proposal for the implementation of the etcs system in the czech republic is presented today by the document "plan of modern safety of the czech railways" proposes implementation by repairing existing safety devices and supplementing them with etcs at application level l1, and for the least busy regional lines it proposes supplementing with simplified variants etcs l1 ls and etcs l1 stop. in the case of the etcs l1 version, the gsm-r line radio is also not introduced, on the contrary, the functional use of the trs analogue radio system, which is installed due to the repair of existing srv radios, is practically expanded. however, these simplified variants of the solution only respond to the requirement for accelerated implementation of a safety function preventing trains from an unauthorized passing through a prohibiting signal. nevertheless, they cannot be considered as technologies that would make the railway more usable, including new technologies, as defined by the its-r concept. the railway 4.0. concept brings an alternative proposal for the possibility of a comprehensive approach to the introduction of digital technologies fulfilling the its-r vision on the entire railway network in the czech republic, respecting both the requirements of increasing railway safety and the requirements for higher usability and connectivity of railways for the czech transport system. this paper provides a basic description of the architecture and key components of this concept. when dealing with the complex concept of technology alteration, it can be assumed that the network-wide introduction of these technologies can be managed in a sufficiently short time and at a reasonable economic cost. however, the prerequisite is a sufficiently timeless conceptual planning for the replacement of current oldies technologies, and the subsequent rapid and systematic introduction of digital technologies into real practice. 3. aspects of introducing modern technologies to the czech railway when deploying new technologies based on etcs and gsm-r/frmcs systems, it should be had in mind that the implementation of etcs, which in principle deals exclusively with train safety, as a train safety device having both track-side and mobile parts, is directly dependent on the technology of infrastructure safety devices. these devices are not part of etcs and are therefore do not fall under interoperable specifications and must be addressed specifically by each infrastructure manager. these technologies can thus constitute a significant part of the infrastructure investment that etcs technologies can build on. due to the nature of the etcs system, there are application conditions (functional and safety) that determine which infrastructure safety equipment it is possible or appropriate to apply the etcs system to. when determining the concept of modernization, it is therefore essential to first address the question of the initial situation in the czech republic. unfortunately, 28 vol. 35/2022 the railway 4.0 concept even representatives of the sz in the mdcr safety commission did not provide detailed information on the current parameters of infrastructure technologies. therefore, it is necessary to describe this topic rather in the context of the current approach to the modernization of these technologies. from the current publicly available information and experience of the author of this article, it can be stated that in the czech republic there have traditionally been massive investments into control and safety technologies, especially on corridor networks of approx. 1500 km, which are the backbone railway system included in the ten-t network. nevertheless, there are systems in this part of the infrastructure dating back to the turn of the 19th and 20th centuries, specifically electromechanical safety devices. many railway stations and sections also have installations of relay technology, a track-side safety device, which have been installed since the 1950s. however, the implementation of etcs, application level 2, which is preferred on corridor networks, requires the replacement of these old technologies, as the deployment of etcs would be too complicated and in some aspects also unfeasible for safety reasons. the connection of the etcs system to the newer technologies of electronic interlocks installed at the turn of the millennium can also be complicated due to the moral outdating and unpreparedness of the technology to be connected to the etcs system. specifically, these are, for example, electronic and hybrid interlocking cores, which require data communication with the radio block centre (rbc) and sufficient computing power. as these technologies currently have more than 20 years of active life and are technically and morally outdated, they are currently being replaced by new electronic systems that allow connection to the etcs system. it is evident that the modernization of infrastructure and safety equipment is not easy, it is difficult to solve in the range of corridor lines, and it is very time-consuming and financially demanding. in the czech republic there are currently about 9700 km of tracks in operation. so far, no systematic and comprehensive change of safety technologies has taken place on a larger segment of the off-corridor lines. many of them have so far been modernized due to a lack of funds from their generated profits. nevertheless, a number of national and regional lines, on which most of the necessary comprehensive modernization of the infrastructure has also taken place, have new electronic safety equipment installed. some electronic safety devices are also installed as part of the repairs of outdated safety devices. however, it can be stated that most off-corridor lines (national and especially regional) in the czech republic are burdened by considerable historical underinvestment in control and safety technologies, where mechanical and electromechanical equipment from the turn of the 19th and 20th centuries, as well as relay technologies, are located. 1700 km of regional lines are not equipped with any safety equipment and operation on them is controlled according to the regulation for simplified management of rail transport sz d3 with a minimum of infrastructure technologies. it is therefore a fundamental strategic question how to implement new technologies in a sufficiently short time and as efficiently as possible, which will enable the extension of new etcs and gsm-r technologies to the entire railway network in the czech republic. the key question is certainly also whether it "pays off" to install relatively expensive technologies on lowcapacity regional lines, with current traffic up to tens of trains per day, or whether simpler and cheaper technologies should be applied instead. in this context, it is important to realize that the main role of railways should be primarily the implementation of backbone transport, which forms a unique alternative transport system to the road network, providing both passenger and heavy goods transport. the construction and operation of this railway infrastructure brings relatively high costs both for the infrastructure manager as well as for the carriers of rail vehicles. it is therefore necessary to consider the issue of efficient use of this infrastructure and to consider whether the absence of safety equipment significantly (unduly) restricts the utilization of this infrastructure, and the installation of simplified safety technologies will not significantly increase its utility in terms of capacity (supply offer), as well as its speed. all this can devalue investments in infrastructure and rail vehicles. consequently, this will inevitably lead to increased investment and external costs for higher use of the road network, so savings from the cancellation or reduction of rail transport will certainly not bring overall savings from the point of view of the state or regions. meanwhile, more intensive use of railway transport is currently clearly defined in long-term policies both in the czech republic and in the eu. the inhabitants of the czech republic are currently seeking housing outside large cities for socio-economic and environmental reasons, and it is clear that rail transport can ensure a sustainable way of connecting rural and urban agglomerations. in the area of freight transport, an increase in demand for local freight services in connection with the collection of municipal waste or individual package shipments can also be expected in the coming years. therefore, it is necessary to perceive investments in railways in the context of the transport needs of the whole czech republic, not separately as a solution only to the issue of safety or the introduction of an interoperable etcs system. the introduction of new technologies on regional lines can bring the potential to increase speed and usability even on such underused lines. however, it is an important prerequisite that transport on such lines be implemented as an overall transport concept, where the regional railways will form a fast capacity backbone of the region’s transport system. a specific problem in the czech republic in this context may be regional lines, which to an increased 29 martin leso acta polytechnica ctu proceedings extent face many crossings in a density of up to approx. 1 crossing per 1 km. in addition to the safety problem (road-rail collision), crossings often also limit the speed of rail vehicles, thus limiting the usability of the railways. within the mdcr safety commission, the task of a comprehensive solution for safety at crossings was also assigned. in practice, this means that approx. 150 crossings per year were identified for the implementation of a crossing safety device, typically a separate island crossing safety device. the installation of such crossing facilities is relatively expensive, amounting to around 10 million czk per installation, and in most cases does not lead to an increase in line speeds, due to the existence of other crossings with insufficient safety among other things. from the above description of the initial state of current technologies and the current method of alteration, it is clear that the introduction of new etcs and gsm-r/frmcs technologies in the current railway environment in the czech republic is an extremely complex problem and is a major technical and economic challenge, which must be promptly implemented in the coming years. the railway 4.0. concept brings an alternative comprehensive approach to solving this issue, reflecting the current state of technology in the czech republic based on current modern trends in the field of safety devices and communications. 4. a description of the railway 4.0 concept the railway 4.0. concept proposes a possible technological solution, which is based on the current knowledge of digital technology and reflects developments in this area abroad. the architecture of railway 4.0. is shown in figure 1. the architecture can be divided into the following groups of technologies, which form integral elements of the system: • line transmission and power supply networks – they consist mainly of optical communication and metallic power cables, which form the backbone infrastructure • voice and data communication – enables data and voice communication with on-board technologies, staff, and passengers on board the train • object controllers – a universal programmable element, ensuring the execution of input/output commands controlled by an integrated safety device • integral safety device – centralized logic of all control and safety functions, principally integrating the function of pzz, tzz, szz, rbc • superior operational and dispatch control of traffic – connection to systems enabling remote traffic control and technology management presented architecture of the railway 4.0 concept. cannot be considered final. it is necessary to assume that the number of elements and their applications will continue to develop dynamically according to current system requirements. the technologies discussed in this paper are the basic principal parts necessary for the implementation of the issue of control and command of railway traffic, which is crucial for ensuring rail transport. however, the basic technology defined in the concept (optical line transmission and power supply networks, communications and object controllers) forms the basic infrastructure technology for the implementation of all related digital technologies. it is therefore appropriate that the construction of this basic infrastructure respects the necessary synergies of all technologies, thus enabling the sharing of basic infrastructure elements. as part of the full digitalization of the railways, it will be a matter of supplementing a number of diagnostic and telemetry systems in the infrastructure in order to ensure high availability and safety of the entire railway system. 4.1. line transmission and supply networks the creation of the fundamental infrastructure enabling the transmission of control and diagnostic information and the supply of electricity to technologies is a key part of the implementation of digital technologies. railway is a line system and it requires solution to these technologies along its whole length otherwise it would not be able to ensure the system functionality as defined in the its-r vision. therefore, there are vast and thus high-cost technologies from the point of view of investment and operation. the digital technology being currently implemented requires quality line networks. it is therefore necessary to project these networks with maximum synergy with infrastructural technologies to use investment costs as effectively as possible. when making a long-term conceptual planning of technology modernization at all railways in the czech republic we can assume that the current cable routes are – or in the nearer future will have to be – fixed with optical cables around all railways because the existing railway cable network in most cases consists of outdated communication cables. due to their construction they are mostly at the end of their lifecycle and it is not worth repairing them. currently the standard is that along railways optical transmission networks are laid optical cables. optical routes are thanks to their properties (price, quality of data transmission) an essential part of solution to safety systems and communications. therefore, the railway 4.0 concept is based on building these backbone data routes along all railways in the form of multifibre optical cables. optical data connections serve for connecting object controllers (ocx), bts-r system gsm-r, bts-p of public operators or for further telematics technologies (cameras, detectors of obstacles occupancy, radio beacons etc. . . ) which for their 30 vol. 35/2022 the railway 4.0 concept operation require to gain/provide the railway system with corresponding data. the railway 4.0 concept proposes distributed technologies and therefore at each spot of external technology (crossing, switch, signal, points machine, axel counter point device) the relevant number of optical fibers will be ducted for connecting these components through an object controller. transmission elements (industrial switches) system will be a part of the line transmission network, through which a reliable transmission layer both in inter-station and route sector will be implemented. optical fibers can be also used for other functions mainly of diagnostics and telemetric feature. the application that is currently being discussed and within pilot projects verified is diagnosis of infrastructure faults (typically broken tracks or other defects) or detection of rolling stock failures (typically flat wheels). optical fibers can be also used as elements for diagnosis, monitoring and telemetric of rock or trees falls on superstructure or possibly as a protective perimetric system for protecting railway buildings from unauthorized enter. as a further part of the line routes the railway 4.0 concept suggests supplementing these routes with a power supply line as well. this should serve for supplying all exterior elements and technologies. those are particularly object controllers and peripheral devices of signaling installations controlled by them. it is also necessary to provide electric supply for sufficient support of technologies of radio transmission systems bts-r or public operators bts-p. the current solution to the supply of external technologies lies mainly in connecting public electric distribution systems. for this purpose, long power lines, which have an independent supply line, have to be built, consumption sub-measurement has to be set up. due to the reliability of electric power supply each object has to be provided with a back-up accumulator with corresponding technology of chargers and air condition and heating if necessary. the railway 4.0 concept comes with the possibility of line supply by a route cable 9 that is ducted in excavation together with optical routes within the entire inter-station sector. compared to the solutions so far, the railway 4.0 concept foresees the use of 400v dc supply. this supply technology is currently wide-spread in the systems of photovoltaic power plants and electric vehicles. the prerequisite of such an attitude is the use of low wattage technologies, particularly object controllers and their control peripheral devices. as low wattage technologies we can consider also the independent bts both gsm-r and public operator networks that without control and transmission peripheral devices do not exceed the wattage of 150w to 300w. the object controllers themselves and transmission elements 9such a solution is known from older corridor technologies where the route supply system was made of the line 6kv/75hz dealing with both tract circuits supply and decentralized safety technologies as for example automatic block or crossings. however, this was very expensive and complex solution. in most cases also do not exceed the wattage of 300w. the dimension of such supply networks has to be laid down in detail on the basis of the number of particular supplied objects. according to this, the number and placement of supply and back-up powers of ups is set. the choice of this technology seems to be favorable because of the following reasons: • the supply voltage 400v dc does not require special cables, the technology of electric distribution systems, back-up and protections of similar nowa-days built buildings can be used, for example photovoltaic systems or electric vehicles. • supply back-up can be favorably implemented at railway station buildings where in most cases there is not any problem with connection to a public power line. • the use of dc fitting enables connection of power boosting supply powers anywhere along the supply route which increases acceptable wattage of technologies. from the point of view of costs for acquisition and operation of line networks it is appropriate to build such routes not only for the purpose of technological railway systems but also these networks to enable sharing cable routes for example for the internet within the area or coverage of the area by public operators. current optical cables provide a large number of optical fibers for reasonable prices. compared to the cost of earth-moving – excavation and cable lying (the price of labor-intensive services and machinery) the price of optical cable is negligible. it is, therefore, desirable to lay a sufficiently large number of cables and optical fibers. railway is attractive for lying optical routes for commercial purposes of area coverage with data services. particularly regional lines can enable cheaper and easier implementation of data cables in the so-called white places of the czech republic than lying optical cables in the landscape including the necessity of negotiating with many landowners. for a railway the investments in building and running such networks may become – under the condition the legislative and economic rules are unambiguously set – potentially profitable. 4.2. voice and data communication the railway 4.0 concept introduces the principle of a complex coverage of all lines with data and voice transmissions of mobile radio as a basic means for safe railway traffic control. for control and securing the railway the gsm-r system is currently being built, which in the forthcoming years (appr. after 2030) will have been gradually replaced by the frmcs system. the railway 4.0 concept assumes that the gsm-r system (and frmcs in the future) will be carried out by the system of distributed technologies which is based on the principle shown in figure 2. in an appropriate object of the infrastructure, for example at 31 martin leso acta polytechnica ctu proceedings figure 1. the architecture of railway 4.0. 32 vol. 35/2022 the railway 4.0 concept figure 2. an example westermo data switch. a station, a unit – the so-called digital module (dm) – will be installed that via an optical cable enables to control other up to 6 remote transmitters, the socalled radio remote head (rrh) which create the base station bts-r. the maximum distance you can control with one dm module is up to 30 km. in most cases of terminal regional lines this distance will be fully sufficient though from a practical standpoint at many lines it will be necessary to increase the number of rrh and dh modules due to expected worse radio signal coverage of the area. the supply of rrh would be solely from the line supply cable. as for providing sufficient back-up we can consider elimination of local battery elements which would make the fitting simpler and cheaper. this solution enables minimalization of technology in infrastructure comparing the current state of the gsm-r technology at the corridor network where each bts contained all transmitting elements, accumulators of a power supply and air-conditioning. this coverage of tracks by the gsm-r system will be integrated in a standard way into the already built gsm-r network whose capacity is sufficient to cover all tracks in the czech republic. in the case of the system gsm-r (frmcs) installation we can also – in a synergic way – use the technologies (line transmission and data networks, columns, switch boards) for coverage a track and its adjacent area by public operators’ signal. it is assumed that this technology will become a distributed one that will not require local expensive and demanding technologies. figure 3. architecture of distributed technology gsm-r, source: kontron transportation s.r.o. 4.3. object controllers an object controller ensures the function of input/output interface toward outdoor technologies (crossing scanner, railway barrier, signal, point machine, etc.) on the basis of data communication with integral safety device. in principle it is the so-called smart periphery that carries out the transmission of data reports onto physical inputs/outputs. the object controller can also contain basic logical and diagnostic functions. for connection of specific technologies (for example control of the lights of level crossing signals pzz, axel counter, electric motor of a railway barrier or points machine) in the object controller specific control interface can be fitted that is adjusted to the requirements of external technologies. from the standpoint of a railway safety device this is technology that has to meet the requirements for electronic safety device placed outdoors that meets the requirements for safety of electronic systems with hw and sw solution on sil=4 level. the technologies of the object controllers themselves have to meet the conditions of outdoor environment as they will be placed in an outdoor closet by a track. it is appropriate to design an object controller as a device that is universal and independent of a concrete product. generally, for such devices we use sign cots (commercial off-the shelf) or mots (modifiable off-the-shelf). those are products that are offered to more producers as commercial, mass-produced ones. in contemporary technological development dealing particularly with automation and robotization of industry, also known as industry 4.0, the market with such products has been developing fast and we can assume that it will also include producers that make products meeting railway requirements. the communication interface between oc and izz should be user-open (the owner/operator should get entire product documentation package). this should ensure the independence of oc technology suppliers. currently 33 martin leso acta polytechnica ctu proceedings for example the european initiative eulynx 10 is dealing with such general specification. this initiative associates 13 managers of infrastructure in order to prepare standardization of interfaces and elements of signaling systems. figure 4. an example himatrix controller hw that can be used as object controller. 4.4. integral safety system (izz) the integral safety system is used to implement all logical functions necessary for control and safety of railway operation. current technologies require the installation of classical safety devices, such as pzz – level crossing safety devices, tzz – line safety devices, szz – station safety devices in local objects (usually relay rooms in station buildings), where all the technology is concentrated and connected by cabling to external elements. the railways 4.0. concept is based on distributed object controller technology with the centralization of logical functions into one center. it thus works with a so-called integral safety system that concentrates all logical functions. it is, therefore, a universal computing hw equipped with specific software integrating pzz, szz, tzz functions. it must also integrate the functions of the etcs rbc system at application level l2 or l3. as the izz will use etcs technology of application level l2 or l3, enabling the transition to direct vehicle control via gsm-r (in contrast to the current method of vehicle control via signals, which also uses etcs at application level l1), we can apply more advanced approaches to train control in rbc, where trains are allowed to leave in direct "successions" with dynamically assigned travel authorisation according to real train movement conditions, without having to wait for the release of an entire intermediate block section or a long track section. this can be achieved either by equipping the infrastructure with a larger number of axle counters 11 or by introducing the so-called "floating block" rbc function within the etcs application level l3. the contribution of the etcs l3 application level is that 10https://www.eulynx.eu/ 11typically every 200m, especially at a track junction before entering a station it will allow this train control function to be implemented with a minimum of infrastructure detection features, and also more efficiently in the context of ’flexible traffic control’, without the constraints of infrastructure detection technologies. etcs application level l3 requires an additional train integrity check 12, which can bring investment and operational savings for the infrastructure at the cost of increasing the equipment especially of freight trains 13. however, it is required that exclusive operation with etcsequipped trains and continuous gsm-r / frmcs track coverage are in place. figure 5. an example himatrix controller hw that can be used for izz functions. more efficient traffic control can also be achieved by massive introduction of the ato system 14, which will enable further optimization of train speeds with the positive effect of maximum infrastructure utilization, stabilization of operations and significant energy savings. in order to increase capacity, especially on single-track lines, it is possible, using the abovementioned technologies and the principle of traffic control, to set up approx. 2km long passing tracks with two tracks, e.g. in the intermediate block section, which can very effectively ensure the so-called "fast passing" of trains. these modern control approaches, 12the requirement to check the integrity of the train is addressed in tsi subset026-3v360 in chapter 3.6.5.2 and can be provided by technical means on the train (a unit train is indivisible or the train is equipped with end of train detection technology) or by the operating staff. 13end of train systems are implemented around the world under the name "end of train device" (eod) or end of train (eot) and are massively operated for example on the canadian pacific railway system in the usa. in the eu, this system has not yet been uniformly specified. 14ato – automatic train operation – a system of automatic train guidance on the optimum travel trajectory. the system is currently ready for approval as part of the new tsi subset. 34 https://www.eulynx.eu/ vol. 35/2022 the railway 4.0 concept conditioned by the introduction of etcs technologies at application level l2 or l3 together with gsm-r / frmcs, allow a significant increase in infrastructure capacity and train speed on both corridor lines and single track lines compared to the current situation. the izz concept should allow for easy sw configuration of the system so that in case of a change in the sw configuration (typically the addition of another controlled area or modification of an existing one), only the relevant part of the sw configuration is changed and subsequently tested without affecting the remaining functional part of the izz. according to findings from abroad, izz can also be considered as a so-called "cloud" solution, providing very high computing and data space and very high availability and reliability of the solution. it goes without saying that the requirements for this part of the safety system will also be at sil=4 level for hw and sw functions; and it is absolutely necessary to ensure adequate protection of the entire computer solution against cyber-attacks. the railway 4.0 concept assumes use of the applicable interoperability specifications and the relevant "subset" for ertms / etcs system, with which the technologies used must be in full compliance. therefore, standardly implemented certified technologies such as gsm-r components, etcs mobile parts, balises and rbc functions are assumed. the discussed technology, especially concerning transmission and power supply elements, object controllers or the technology of integrated security devices, is not directly controlled by interoperable requirements. therefore, compliance with the target requirements for safety and reliability, which are controlled mainly according to the requirements of the set of cenelec standards of the en 5012x series, and compliance with the target requirements of safety and reliability management for the given system are expected. these detailed parameters are not the subject of this paper, and can be analyzed in the phase of specifying the requirements for the relevant technological units. 4.5. master operational and dispatching traffic control master operational and dispatching technologies are used to control traffic on a given track or in an area where distributed technology will be deployed according to the railways 4.0. concept. it is assumed that the current approaches and technologies that are built and operated today within the central dispatching departments will be used. however, given the assumption that etcs application level l2 or l3 technology will also be used, it can be assumed that this technology will also be adapted to these new rail traffic control options. it can also be assumed that master technologies for automatic train operation (ato systems) will also be used and should be integrated into this part of the operational traffic control system. significant developments in this area can thus also be expected. 5. practical implementation of technologies according to the railways 4.0 concept when implementing distributed technologies with centralization of logical functions according to the rail 4.0. concept, the construction must start with the building of the backbone line infrastructure, including both optical and power cables. when building this network, the location of the outdoor elements of the tracks, where optical and power lines will be routed to object controllers, and the location of bts radio networks should already be known. this basic infrastructure must, therefore, be built robustly enough and with sufficient consideration to ensure that any further modifications to the superstructure will not disrupt it and will enable it to continue to function flawlessly even if the infrastructure is upgraded over time. the next step is to build local object controllers. their construction is not too demanding as it is an industrial technology that can be installed in industrial distribution boards for outdoor installation. poles for the installation of bts for gsm-r and gsm public operators will also be built. a great advantage of the implementation of distributed technologies can be seen especially in solving the problem of level crossing safety, which is a significant source of danger and slowdown of traffic on regional lines. the railways 4.0. concept practically allows every level crossing to be equipped with an adequate level of external features that will ensure crossing safety pursuant to legal requirements. the benefit of this solution, apart from the increase in safety, is above all the possibility of running trains at the maximum line speed without any restrictions resulting from the currently insufficient level of level crossing safety along the entire track section. at the same time, the investment and operating costs of building such level crossing safety system will locally be significantly lower, because the costs of most of the functions implemented locally at the level crossing (level crossing logic, power supply and data communication, etc.) are already solved by means of line communication and power supply networks and centralization of logic functions in the izz. last but not least, a backup power supply must also be built. buildings that already have or allow for the new installation of a power connection from the public power grid will be given priority. station buildings generally meet this requirement without the need for extensive internal occupation of the building as is necessary in the case of conventional installation of station safety equipment. when using a 400v dc supply voltage distribution system, it is also possible to reinforce the lines in other buildings on the track if necessary; however, calculations to date show that at 35 martin leso acta polytechnica ctu proceedings standard station distances of up to approx. 7km, it will be sufficient to supply this track power line only bilaterally from adjacent station buildings. with a sufficient high-quality implementation of this network, a very long service life of at least 50 years or more can be expected. when connecting object controllers to external peripherals, a unified type method will be used, which will be specified for each type of external peripheral separately. the final activation of the technology is realized only by configuring the izz, where specific inputs/outputs of object controllers are assigned to relevant sw objects, which are further handled by the izz software. this is expected to greatly simplify and speed up not only the design of these technologies but also the final wiring and testing of the entire system. it is evident that with regard to the size of the railway network which must be covered by the etcs and gsm-r / frmcs system in relatively short time, the construction and expansion of this technology cannot be combined with complex modernization activities on the railway construction part, which progress at a significantly slower pace (for technological and economic reasons) than that of line cabling and bts radio networks construction. on the contrary, the approach based on the railways 4.0. concept significantly accelerates the coverage with modern control and safety technologies of even those parts of the infrastructure where no heavy medium repair work or possibly complex modernization of the infrastructure has been carried out so far. a system with distributed technologies has the conceptual requirements to be significantly more adaptive and modifiable in its ability to implement configuration changes or renewal of individual parts than is usual in the case of static locally centralized installations. this is because only partial segments of the technology are replaced when a change or renewal is implemented, while the costliest segments related to power, data networks and central logic remain unchanged or undergo partial configuration changes. 6. economic parameters of the railways 4.0 concept the costs for the implementation of the railways 4.0. concept were processed by the author of the article in the form of an estimate of investment costs according to the sfdi rules 15 for the purposes of the discussion of this proposal in 2020 in the safety commission of the ministry of transport and communications working group infrastructure. this estimate based on a model example of a typical regional line, suggests that the cost of the presented comprehensive view of modernizing railway control and safety is not significantly higher than the currently planned costs, even in case of application of significantly simplified 15https://www.sfdi.cz/pravidla-metodiky-a-ceniky/ cenove-databaze/, sborník pro oceňování železničních staveb ve stupni studie proveditelnosti a záměr projektu etcs l1 ls or stop installations without gsm-r track-side radio. in the case of the railways 4.0. concept, the scope of technology, functions and usability of the railway is comprehensively addressed at the highest possible level of current knowledge, with the possibility of also meeting future requirements, e.g. autonomous operation of trains without drivers. the costs of track safety may range from 5 to 7 million czk/km 16 of single-track line and thus respect the estimated amount of financial resources defined in the vision of the ministry of transport o130 for the introduction of the etcs system until 2040. in addition, lower operating costs can be expected when this technology is implemented. this is due both to the use of highly reliable technologies with detailed diagnostics and lower operating costs ensuring a reliable and secure power supply and centralized logical parts of the system. 7. conclusion the presented railways 4.0. concept proposes a new alternative approach to the modernization of control and safety technologies on the railway network of the czech republic, which should enable a comprehensive introduction of digital technologies on the railway network in the czech republic. the railways 4.0. concept is technological elaboration of the its-r vision, while it not only pursues the issue of modernization of safety systems, introduction of the etcs system enabling the increase of the level of safety of railway operation in the czech republic, but also sets the technological environment enabling the increase of the usability of the railway and it readiness for greater inclusion in the integrated transport systems of the regions in the czech republic. the concept was discussed at the level of the mdcr and the sz in 9/2020. based on the decision of the director general of sž bc. jiří svoboda, mba, sz took over the official patronage of this railways 4.0. concept and will further develop this solution within the framework of joint work group railways 4.0. comprising experts from sž, ctu in prague, faculty of transportation. the work group is currently developing this concept to the level of implementation project documentation and is working towards the subsequent implementation of a pilot verification of this concept on the real infrastructure of the sž test track. 16approximately 200 k euros to 270 k euros per km of installation 36 https://www.sfdi.cz/pravidla-metodiky-a-ceniky/cenove-databaze/ https://www.sfdi.cz/pravidla-metodiky-a-ceniky/cenove-databaze/ acta polytechnica ctu proceedings 35:27–36, 2022 1 introduction 2 history of the railway 4.0 concept 3 aspects of introducing modern technologies to the czech railway 4 a description of the railway 4.0 concept 4.1 line transmission and supply networks 4.2 voice and data communication 4.3 object controllers 4.4 integral safety system (izz) 4.5 master operational and dispatching traffic control 5 practical implementation of technologies according to the railways 4.0 concept 6 economic parameters of the railways 4.0 concept 7 conclusion acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.32.0032 acta polytechnica ctu proceedings 32:32–36, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague verification of the bearing capacity of piles peter turček∗, monika súľovská slovak university of technology, faculty of civil engineering, deparment of geotechnics, radlinského 11, 810 05 bratislava, slovakia ∗ corresponding author: peter.turcek@stuba.sk abstract. the location of the buses in the basement of the new bus station in bratislava, supplemented by two further parking floors below, shifted the foundation joint of the new bus station to a depth of approximately 14 m. the maneuvering space for the buses required unusually large distances between the supporting columns. in extreme cases, it was necessary to transfer a vertical load of more than 30,000 kn from the column to the subsoil. only a relatively thin layer of quaternary gravel remained between the footing bottom and the neogene soils. the whole area was designed to be based on deep foundations. as this was a construction of the 3rd geotechnical category, it was necessary to verify the bearing capacity and quality of the piles. the applied test methods (especially the dynamic load tests) confirmed the correct design and implementation of piles. keywords: dynamic load tests, extreme foundation demands, integrity tests, pile control calculations, pile design. 1. introduction the complex, including the bus station and commercial and administrative areas, is approximately rectangular in shape with ground plan dimensions of 420 x 175 m (see figure 1). the majority of buildings have an average of 5 floors. the dominant feature of the complex is a 125 m high administrative building with 32 floors. the construction site is located on a flat area with a range of altitudes of 136.40 m a.s.l. up to 137.20 m a.s.l. and an average height of 136.80 m a.s.l. [1] figure 1. ground plan of a construction pit with dimensions 420 × 175 m. 2. geological conditions on site the engineering geological investigation included 40 boreholes (some of which reached a depth of up to 37 m) and a large number of dynamic penetration tests. from a comprehensive evaluation of the investigation works, several subsoil models with a comparative level of ± 0.00 = 137.50 m a.s.l. were created for the characteristic parts of the area. a typical geological model for pile design had the following sequence of layers: 0.0 – 1.1 m fill; 1.1 – 6.2 m clay with intermediate plasticity, stiff (f6); 6.2 – 14.6 m gravel badly grained (g2); 14.6 – 22.2 m sand clayey (s5); 22.2 – 23.1 m clay with intermediate plasticity, mostly solid (f6); 23.1 – 23.5 m clay with high plasticity, solid (f8); gwl – 6.6 m below the surface. according to the penetration probes, the quaternary gravel layers were evaluated as medium compacted with a scattering of values id = 0.36 to 0.65 and modulus of deformation edef = 55 to 147 mpa. however, in deeper positions, loose layers with id = 0.21 to 0.34 and edef = 22 to 49 mpa were also found in the gravel. the upper parts of neogene sediments are comprised of sand with an admixture of fine-grained soil also to sandy clay. sands with an admixture of fine-grained soil were evaluated by dynamic penetration tests as medium compacted (id = 0.41 to 0.50 and edef = 23 to 29 mpa). in laboratory tests, neogene layers were evaluated as soils with a predominant solid consistency (ic = 1.04 to 1.31; mean ic = 1.16). given the summary evaluation of neogene soils, it is necessary to point out the average values: φ′ = 27◦; c′ = 19 kpa; eoed = 37 mpa. more precisely, the oedometric modules for the three depth zones that were adopted for the design of the piles were evaluated as follows: • for depth zones 14 – 18 m below ground eoed = 21.1 mpa; • for depth zones 18 – 26 m below ground eoed = 33.2 mpa; 32 https://doi.org/10.14311/app.2022.32.0032 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 32/2022 verification of the bearing capacity of piles • for depth zones 26 – 35 m below ground eoed = 40.1 mpa. for various soil classes, the bulk density and shear strength of soils were also elaborated by laboratory analysis: • for clay with high plasticity (f8-ch): γ = 20.3 kn/m3; φ′ = 19.8◦; c′ = 21 kpa; • for clay with medium plasticity (f6-ci): γ = 20.7 kn/m3; φ′ = 24.7◦; c′ = 16 kpa; • for silt with medium plasticity (f5-mi): γ = 20.2 kn/m3; φ′ = 28.6◦; c′ = 16 kpa; • for sandy clay (f4-cs): γ = 20.2 kn/m3; φ′ = 29.9◦; c′ = 10 kpa; 3. verification of piles design in the predominant part of the ground plan area, there are clayey soils just below the foundation joint, or indeed clayey sand. all these soils are characterized by a relatively smaller deformation modulus, which would cause a large settlement of the foundation plate. in addition, in the floor plan of the construction pit, a zone with worse soil properties was discerned in the middle part. this area caused significant problems in the early stages of design. the whole building had to be based on a foundation plate to protect against the groundwater level. in the larger part of the floor plan, a 0.7 m thick foundation plate was designed; in the places of the pillars, strip-shaped depressions with a thickness of 1.1 to 2.2 m were formed under concentrated loads, and which were supported by piles. taking into account technological possibilities, cfa type piles were selected. the support of the foundation plate with piles made it possible to transfer large loads to greater depths. reducing the settlements was only achieved at the cost of relatively long piles. altogether, more than 600 piles with very high shape variability have been designed. the required short production deadlines placed high demands on the organization of work. in terms of assortment, there were 32 types of piles with a variable length of 6.0 to 19.0 m, with a diameter of 900 or 1200 mm. by taking into account different load conditions and fluctuations in the groundwater level, some piles can also be stressed by tensile effects. due to the large volume of piling work, a detailed audit of the design of piles was carried out, aimed not only at reaching the required bearing capacity but also at minimising the expected settlement [2]. for the given combinations of loads taken from the structural engineers of the upper structure, the vertical bearing capacity of the piles under all columns was recalculated by the classical theory using čsn 73 1002 and the assumed settlement by the nonlinear theory as according to masopust. in the case of piles with a diameter of ø 900 mm, 6.0 to 15.5 m long, the settlement in the range of mobilisations of mantle friction was between 7.5 – 9.6 mm, in the case of piles diameter ø 1200 mm, 14.5 to 16.5 m long there was a settlement corresponding between the mobilisations of the mantle friction in the interval 11.6 – 11.8 mm. for the assumed loading of the piles with vertical forces, the settlement of piles ø 900 mm long 6.0 to 15.5 m in the range of 3 – 24 mm and similarly piles of ø 1200 mm long 14.5 to 16.5 m length of 14 – 20 mm was calculated. the designers of the upper structure determined the permissible settlement to be 25 mm. the foundation designs had to be verified by a load test for the construction of the 3rd geotechnical category. 4. verification of pile manufacturing with an excavation depth of up to 14.7 m, the buoyancy effects of groundwater in some places reached more than 8 m. to this must be added unusually high point loads. therefore, after the audit of the pile design, it was necessary to verify whether the implementation of the foundation met the requirements of the project. as part of the quality control of the construction work, the following piles inspections were carried out: • inspection records of fabrication of piles; • control of pile heads; • integrity check (so-called pit tests); • dynamic load tests of piles. all piles had their production records checked, and from which any anomalies could be detected. no insufficiency was found during this inspection. similarly, the pile heads were inspected, ready for connection to spread foundations. during this stage, harm heads were found in three piles [3]. the unsatisfactory part of the head was excavated to a healthy full profile of the pile (it was approximately 600 mm) and concreted with a concrete grade of strength to a class one degree higher than the original pile. a pile integrity test (pit) was carried out on systemic piles [4]. a hammer blow weighing 1 kg on the pile head will cause acoustic waves to propagate to the foot of the pile. changes in the shape and quality of the material of the pile body create reflections, which are observed as returning waves to the pile head. during the inspection, the projected length of the piles was compared with the determined length and also at the same time with the age of the piles. piles with a diameter of ø 900 and 1200 mm, 6.0 to 18.0 m long, were verified, while the piles had a test time of 5 to 69 days. 58 pit tests were performed on the site. in only one case was a non-standard acoustic wave (at a depth of 9 m) detected [5]. therefore, the test was repeated the following day with the participation of the investor’s technical supervision, and without confirming an anomaly that would indicate a violation of the homogeneity of the pile shaft. the initial non-standard 33 peter turček, monika súľovská acta polytechnica ctu proceedings result was explained by the existence of a working joint under the pile head, which was concreted to the projected level later in the second phase. on a complete evaluation of the pit tests, it was possible to state that all tested piles had the prescribed length and none of the piles showed a narrowing of the diameter or a break in the shaft. 4.1. dynamic load tests the most important form of piles verification were dynamic load tests. compared to static load tests, they are simpler, cheaper, but above all faster. their disadvantage is that they do not allow a direct measuring of the working diagram of the piles. the magnitude of the dynamic force (impact from the free fall of the weight), the magnitude of the instantaneous deformation of the pile head, and also the rate of this deformation were all recorded during the test. the test was evaluated by the capwap method (case pile waves analysis program). the requirement of strict adherence to the construction work schedule did not make it possible to create space for verification of the bearing capacity of piles by classical static load tests. in countries where piling technologies and methods for monitoring the achievement of design parameters are being developed, dynamic load tests are widespread. a pulse wave of voltage is induced in the pile by a specially adjusted weight (figure 2). the strain gauges located at the top of the pile measure the force that causes the pile to move after the impact. another way to measure force is to record the acceleration of the shock wave with an accelerometer. the course of speed is determined by integration. the results of this impact velocity and the impedance of the pilot are the second way to measure force. unlike pure force measurement, this calculation contains an acceleration vector. by comparing these two impedance measurements, two waves of forces are obtained; at the same time, one wave of forces is directed downwards towards the base of the pile and the other force is directed upwards, made manifest by friction on the casing and resistance on the base of the pile. the maximum bearing capacity of the pile is calculated from the measured skin friction and the resistance of the pile base. dynamic load tests (pda) were performed in two terms on 4 non-systemic piles. the tests were performed by the company duba testing cz, plzeň [6]. from the detailed evaluation by the capwap program, in table 1 and 2 there are presented the summarized outputs. figure 2. dynamic load test of the pile and location of strain gauges table 1: permanent settlement of test piles after 5 blows with a load pile no. length of pile (m) fall height (m) settlement of the pile (mm) a4/sp1 12.5 1.76 3.0 b3/sp4 15.0 1.96 3.5 a3/sp2 11.25 1.76 2.5 b2/sp3 13.3 1.96 3.5 table 2: evaluation of test piles by pda test date pile no. age of piles. (days)  of pile (mm) length of pile. (m) max. s (mm) bearing capacity (kn) skin friction pile base total 4.7.18 a4/sp1 35 900 12.5 8.0 4770 1160 5930 4.7.18 b3/sp4 35 1200 15.0 7.0 6550 2650 9200 25.7.18 a3/sp2 28 900 11.25 6.5 5300 1300 6600 25.7.18 b2/sp3 28 1200 13.3 6.5 6630 2100 8730 notes:  pile length is given as length below ground level.  the total length of the piles with the extension was: o pile a4/sp1 – 14.0 m; o pile b3/sp4 – 17.1 m; o pile a3/sp2 – 13.25 m; figure 2. dynamic load test of the pile and location of strain gauges. notes: • pile length is given as length below ground level. • the total length of the piles with the extension was: . pile a4/sp1 – 14.0 m; . pile b3/sp4 – 17.1 m; . pile a3/sp2 – 13.25 m; . pile b2/sp3 – 15.8 m. • max. s in table 2 is the maximum settlement of the pile. • comment on the bearing capacity of piles: . for the a4/sp1 pile, the bearing capacity of the pile base was not fully mobilised; . for the b3/sp4 pile, the bearing capacity of the pile base and the skin friction were not fully mobilised; . for the a3/sp2 pile, the bearing capacity of the pile base and the skin friction were not fully mobilised; . for the b2/sp3 pile, the bearing capacity of the pile base was not fully mobilised. from the evaluated load-dependence curves, it was possible to determine the expected settlement of the tested piles under static load. on some construction works, where static and dynamic load tests of piles were performed in parallel, very good similarities of both tests were confirmed. this attests to the sufficient reliability and credibility of the results of the dynamic load tests. from the evaluation of dynamic tests, it is also possible to determine the expected settlement of a particular pile for different intensity of static load (see table 3). a very important positive circumstance that results from the dynamic load test should be considered the 34 vol. 32/2022 verification of the bearing capacity of piles pile no. length of pile (m) fall height (m) settlement of the pile (mm) a4/sp1 12.5 1.76 3.0 b3/sp4 15.0 1.96 3.5 a3/sp2 11.25 1.76 2.5 b2/sp3 13.3 1.96 3.5 table 1. permanent settlement of test piles after 5 blows with a load. test date pile no. age of piles (days) ø of pile (mm) legth of pile (m) max. s (mm) bearing capacity (kn) skin pile total friction base 4.7.18 a4/sp1 35 900 12.5 8.0 4770 1160 5930 4.7.18 b3/sp4 35 1200 15.0 7.0 6550 2650 9200 25.7.18 a3/sp2 28 900 11.25 6.5 5300 1300 6600 25.7.18 b2/sp3 28 1200 13.3 6.5 6630 2100 8730 table 2. evaluation of test piles by pda. evaluation that for all 4 tested piles the full value of the bearing capacity at the pile base, and in two cases the full value of the mobilised skin friction, was not reached. this means that the test piles still held out the possibility of increasing the load capacity. at the same time, however, it should be noted that this increased bearing capacity will be achieved at the expense of a relatively more intensive settling. the prognosis of bearing capacity, but especially of the expected settlement by exceeding the tested values, is not based on explicitly verifiable data. extrapolation of the evaluated measurements should be considered risky and it was not recommended to consider it. pile bearing capacity load assumed no. after (kn) settlement capwap (kn) (mm) 2800 2.2 a4/sp1 5930 4000 3.1 5600 8.1 4500 1.6 b3/sp4 9200 6300 2.7 9000 6.8 3000 2.2 a3/sp2 6600 4000 3.0 6000 6.8 4500 2.0 b2/sp3 8730 6300 2.9 8500 5.1 table 3. assumed settlement from static load. experience with dynamic tests of piles placed in clayey soils has shown that, under static load, a final pile settlement of two times more than the one calculated from the dynamic test can be expected. data in the last column table 3, even with such an increase, shows they met the required maximum permissible settlement for the upper structure. the data obtained from the dynamic load tests were finally compared with the audit of the pile foundation project, in which control calculations of the bearing capacity and settlement of 32 piles of various dimensions placed in variable geological conditions were performed. for all tested piles on the construction site, it was possible to confirm a considerable margin not only in bearing capacity but especially in the assumed installation of piles in comparison with the control calculations. 5. conclusion dynamic load tests are not yet considered in slovakia as tests the results of which can be accepted without discussion. in western europe, these tests are accepted as routine tests. the spatial distribution of dynamic load tests, as well as pit tests (integrity), could be considered sufficiently representative for the establishment of the entire bus station area. all performed verifications and tests confirmed the fulfilment of the project assumptions. the performed dynamic load tests confirmed the higher bearing capacity of the tested piles than that required by the project for load transfer from the upper structure. given this fact, it was recommended to continue making piles according to the project. interiors are currently being completed throughout the building. the covid-19 pandemic caused a slight time adjustment for the completion of the whole site (figure 3). no anomalies have been detected by monitoring so far. it has been shown that dynamic load tests can also be used in our given conditions as equivalent to static load tests, while their implementation is significantly faster and cheaper. 35 peter turček, monika súľovská acta polytechnica ctu proceedings figure 3. view of the completed area of the bus station. acknowledgements the paper is a part of results from a research project supported by slovak agency vega no. 1/0530/19 "analysis of drainage efficiency in the rehabilitation of unstable slopes". references [1] project documentation for the establishment of a multifunctional complex of the nivy bus station. (in slovak). [2] p. turček. forecast of settlement of th twin city complex in bratislava (area b3.1 and b3.2). expert assessment. t-g, bratislava, 04.04.2018, 6 p. (in slovak). [3] m. balucha. opinion of the designer on the rehabilitation of pile heads. spai, bratislava, 1 p. (in slovak). [4] f. kout. multifunctional building bus station mlynské nivy. pile foundation. accompanying report for checking the integrity of piles using the pit method. zakládání staveb, a.s. praha. 19.09.2018. (in czech). [5] j. šperger. the technologist’s statement on the results of the pit test of the a2-10 pile. zakládání staveb, praha, 05.10.2018, 1 p. (in czech). [6] collective. report on measuring the bearing capacity of piles using the pda method on the twin city construction site, bus station mlynské nivy, bratislava. duba testing cz, s.r.o., plzeň, 04.-09.07.2018 and 25.07.-06.08.2018. 2x18 pp. 36 acta polytechnica ctu proceedings 32:32–36, 2022 1 introduction 2 geological conditions on site 3 verification of piles design 4 verification of pile manufacturing 4.1 dynamic load tests 5 conclusion acknowledgements references https://doi.org/10.14311/app.2022.33.0398 acta polytechnica ctu proceedings 33:398–403, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague interfacial bond behavior of adhesively-bonded timber/cast in situ concrete (wet bond process) ali nemati giva, qiuni fua, libo yana,b,∗, bohumil kasala,b a technische universität braunschweig, department of organic and wood-based construction materials, hopfengarten 20, 38102 braunschweig, germany b fraunhofer wilhelmklauditz-institut, center for light and environmentally friendly structures, bienroder weg 54e, 38108 braunschweig, germany ∗ corresponding author: l.yan@tu-braunschweig.de, libo.yan@wki.fraunhofer.de abstract. the goal of this research was to study the strength of the interfacial bond between cast-in-situ concrete and engineered timber (cross-laminated timber (clt)). double lap specimens were manufactured using fresh concrete that was cast between two clt blocks. polyurethane and epoxy adhesives were used to bond the wet concrete with the clt blocks. the shear strength of wet-bond specimens was compared with the specimens prepared under dry conditions (prefabricated concrete cube glued to clt blocks). the statistical analysis (t-test) of bond strength showed that the shear strengths of wetand dry-bond specimens using epoxy and polyutrthane adhesives were no significantly different for the tested c25 plain concrete and the clt. the failure mode of dry-bond specimens were concrete failure near the interface, however, debonding at interface was the dominant failure for the wet-bond specimens. keywords: cast-in-situ concrete, dry bond, shear strength, timber-concrete composite structures, wet bond. 1. introduction timber-concrete composite system is a construction technique to strengthen and stiffen the existing timber floors and new construction parts such as decks in short-span bridges and multi-story buildings. this technique requires the connection system to transfer shear stress between timber and the concrete composite system. rigid and strong connection system maximize the composite action. rigidity of connection system will increase when notches or holes [1] are cut/drilled in the timber or a continuous connector like adhesive is used [2]. application of adhesive has recently been studied in bonding timber and concrete due to its significant advantages including: a) uniform stress distribution over the bond area, b) removal of cutting and drilling in the wood substrate and c) reduction of workmanship and cost [3]. satisfactory stiffness and strength is of importance for timber-concrete structures. besides the stiffness and strength, other aspects should be considered such as quality, simplicity and speed of manufacturing and erection. two approaches can be taken to manufacture adhesively bonded timber-concrete elements: • adhesively bonded timber-prefabricated concrete composite (dry bond). • adhesively bonded timber-cast-in-situ concrete composite (wet bond). in a dry bond, adhesively bonded timber-concrete beam/slab is fabricated in a workshop under the controlled environment, tranported to cosnstrution site, lifted up and placed on a desired location. in case of wet bond, the wet concrete is directly pumped to wood surface covered with glue. the wet bonding has its own merits such as: • easy transport of wet concrete to desired location making time and cost savings especially for multistory buildings (removal of crane application). • elimination of gap and discontinuities in the adhesive interface area. there is a concern regarding the insufficient bond strength between timber and cast-in-situ concrete when it is compared with dry bond. when the fresh concrete is poured on the wet adhesive, it can cause the adhesive movement, incomplete adhesive curing and consequently bond strength reduction [8, 9]. few researchers studied the bond behaviour of timbercast-in-situ concrete connected with glue. brunner et al [9] investigated the wet bond behaviour of glue laminated timber (gl24h) and cast-in-situ concrete (c25/30) glued with epoxy adhesive. they concluded that the production of wet bond in timber-concrete composite structure is delicate due to danger of adhesive movement during pouring fresh concrete. clouston et al [10] experimentally studied the influence of epoxy adhesive on strength and stiffness improvement of full-scale wood plank-cast-in-situ concrete composite floor decks. the strength and stiffness were increased by about 4 and 2.2 times compared to woodcast-in-situ concrete composite with no connection 398 https://doi.org/10.14311/app.2022.33.0398 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 33/2022 adhesively-bonded timber & cast in situ concrete clt bending strength (mpa) 18 modulus of elasticity in fiber direction (gpa) 12 shear modulus (mpa) 690 density (kg/m3) 527 table 1. mechanical and physical properties of clt [4, 5]. mass percentage cement water fine aggregates (sand) medium aggregates coarse aggregates normal concrete 1 0.6 2 1.27 2.73 aggregates size (mm) − − 0 − 2 2 − 8 8 − 16 table 2. designed mixing ratio for concrete. figure 1. concrete mixing process. epoxy [6] polyurethane [7] commercial name sikadur 300 semparoc 12 nv density 1.16 g/cm3 1.25 g/cm3 viscosity 700 mpa.s 8000 − 12000 mpa.s pot life (at 23◦ c) 4 hours 15 minutes curing time 7 days 7 days table 3. basic information of epoxy and polyurethane adhesives used in this study. in the interfacial area. additionally, glue as a continuous connector in the interface area changed the composite failure mode from concrete cracking and debonding between wood and concrete components to interfacial shear failure by sliding concrete over the wood planks. this paper compares the shear bond strength of two possible manufacturing techniques of adhesivelybonded timber-concrete composite namely wet and dry bonds. two wood blocks were glued with the prefabricated and cast-in-situ concrete cubes using epoxy and polyurethane adhesives. after adhesive and concrete curing, static shear tests up to failure were carried out up to failure. fracture surfaces of failed specimens were analysed to investigate possible correlation with shear bond strength. 2. experimental work 2.1. material to fabricate timber-concrete glued joints, cross laminated timber (see table 1) with a commercial product name of bbs 125 3-s was used. the clt wood was made of norway spruce and was composed of three layers with thicknesses of 20,40 and 20 mm, respectively (totally 80 mm). prefabricated and cast-in-situ concrete have the same mixing ratios (see table 2) and were fabricated using the same mixer. portland cement cem i 42.5 n was used for the concrete mixing. in production of normal concrete, the water was added into two steps (see. figure 1) to ensure aggregates were wet enough causing stronger interfacial transition zone between concrete aggregates and mortar matrix. one-component polyurethane and two component epoxy adhesives applicable to wood and concrete substrates were used in this study. some parameters of these products are shown in the table 3. 2.2. double lap joint geometry double lap joint is symmetrical about the mid plane of a specimen, therefore, bond rotation and the amount of peel stress is considerably reduced compared to equivalent single lap joint [11]. figure 2 shows the designed double lap joint with dimensions and material location. 2.3. double lap joint preparation 2.3.1. dry bond preparation for the dry bond fabrication (see figure 3), fresh concrete was poured into mold, vibrated and cured for 24 hours. afterwards, the concrete blocks were immersed in the water for 6 days at temperature of 23◦ c. finally, they were taken away from water tank and placed in the climate chamber (65% 399 a. nemati giv, q. fu, l. yan, b. kasal acta polytechnica ctu proceedings figure 2. double lap joint geometry. pouring fresh concrete in the mold placement of concrete blocks in the water tank storing concrete blocks in the climate chamber after 1 day after 6 days after 21 days clt clt gluing first concrete block to first wood substrate gluing wood-concrete joint to second wood substrate adhesive layer concrete wood-concrete joint after adhesive consolidation after adhesive consolidation timber-concrete glued joint clt clt clt figure 3. procedure of dry bond preparation. rh and 23◦ c) for completion of concrete curing (for the rest of 21 days) based on the german concrete standard (din en 12390-2 [12]). then, the concrete and clt wood blocks were bonded together and cured for seven days according to adhesives technical datasheets [6, 7]. 2.3.2. wet bond preparation in the wet bond fabrication, the wood blocks were placed next to each other separated by plywood plates. the surfaces of wood blocks were covered by epoxy and polyurethane adhesives (see figure 4a), turned around, fixed by horizontal and vertical c-clamps on their positions and fresh concrete was poured in the middle of the wood blocks (see. figure 4-b). timber-concrete double lap joints were released from mold after 24 hours and then the concrete cubes were wrapped up by damp fabrics to avoid incomplete concrete curing process. finally, the specimens were stored in the climate chamber (65% rh and 23◦ c) for the rest of 27 days. 2.4. shear test setup the shear test of wet bond and dry bond specimens were carried out using din-en 392 standard [13] proposed for timber glued joints. wood blocks were placed onto the steel plates and constrained from rotation and horizontal movement using two steel plates and c-clamps (see. figure 5). the load was forcecontrolled with a loading rate of 30 kn/min through the steel plate positioned on the concrete. the double lap joint specimens were tested up to failure and shear strength of wet and dry bonds was calculated according to eq. 1: 400 vol. 33/2022 adhesively-bonded timber & cast in situ concrete figure 4. manufacturing process of wet bond a) pouring and spreading glue on the wood surface b) pouring fresh concrete between wood blocks covered with glue. figure 5. shear test setup for measurement of shear strength of wet and dry bonds. τu = fu 2a (1) in which, τu, f and a are shear strength, ultimate shear load and adhesive bond area, respectively. τu thus represents the average shear strength for both faces of the concrete block. 3. shear test results the average values and standard deviations of wet and dry bond specimens are shown in table 4. the shear strengths of each adhesive type in wet and dry bond were compared using t-test (two sample assuming unequal variances) to reveal whether their expected mean values (µ) were equal (h0 = µwet bond = µdry bond). the two-tail p-values of wet and dry bonds for epoxy and polyurethane adhesives are shown in table 5. this value indicates the probability of rejecting or accepting the zero hypothesis (equal mean value for wet and dry bonds). according to table 5, the two tail p-values of epoxy (p = 0.88) and polyurethane (p = 0.071) adhesives are higher than the level of significance (α = 0.05) indicating no significant difference between mean values of wet and dry bonds by acceptance of zero hypothesis. it should be pointed out that the significance level, α, is set somewhat arbitrarily but the value of 0.05 represents a reasonable probability of the error in conclusions. as it is evident from the table 4 that the epoxy adhesive had higher shear strength compared to polyurethane adhesive in wet and dry bonds so that the shear strength increased about 98.2% and 58% in wet and dry bonds, respectively. it is worth mentioning that the increase of shear strength achieved by epoxy adhesive does not diminish the application of polyurethane adhesive. in real application when timber-concrete composite beam/or deck is subjected to flexural loading, 401 a. nemati giv, q. fu, l. yan, b. kasal acta polytechnica ctu proceedings type of joint epoxy polyurethane dry bond τu (mpa) 2.26 1.43 sd (mpa) 0.72 0.43 number of specimens 8 11 wet bond τu (mpa) 2.22 1.12 sd (mpa) 0.49 0.27 number of specimens 8 8 table 4. shear strength (τu) and standard deviation (sd) of timber-prefabricated and timber-cast-in-situ glued joints. type of joint epoxy polyurethane two tail p-value 0.88 0.071 level of significance (α) 0.05 0.05 reject or accept the zero hypothesis (h0) accept accept table 5. two-tail p-value results from wet bond versus dry bond. wet bonddry bond polyurethane epoxy 50 mm 50 mm 50 mm 50 mm figure 6. fracture surfaces of timber-prefabricated concrete and timber-cast-in-situ concrete glued with polyurethane and epoxy adhesives. polyurethane adhesive as a ductile adhesive (i.e. adhesive can undergo plastic deformation before failure) can well perform against adhesive peel stress, which may cause premature failure in the adhesive bondline. furthermore, polyurethane is cost saving and compatible with wood substrates [14–16], therefore, there is a motivation for further investigation on the polyurethane adhesive in wet and dry fabrications. 3.1. fracture surface of wet and dry bond specimens the fracture surfaces of failed specimens of wet and dry bonds are illustrated in figure 6. as it is evident from figure 6 that both polyurethane and epoxy adhesives, the amount of concrete failure remained on the wood blocks was significant in dry bond specimens. however, debonding at interface was the dominant failure mode in the wet bond specimens. the failure mode change from concrete failure to debonding at interface is a clear indication of weaker shear strength in wet bond application [17–19]. 4. conclusion in this paper, the influence of two main types of concrete fabrication including cast-on and -off sites on shear strength of timber-concrete glued joints was studied. two types of adhesive polyurethane and epoxy were used in this work. in timber-prefabricated concrete glued joint as a dry bond, the concrete was precast and adhered to wood blocks by its own weight while for timber-cast-in-concrete glued joint (as a wet bond), fresh concrete was added between two wood blocks covered with wet adhesive. the results showed that the shear strength of clt-prefabricated concrete and clt-cast-in-situ concrete bonded with epoxy adhesives had no significant difference whereas for case of polyurethane adhesive, the wet bond shear strength reduced by about 21.6% compared to dry bond. in addition, fracture surfaces of wet and dry bond specimens for epoxy and polyurethane adhesives were investigated. the change of failure mode was observed from concrete failure in dry bond to debonding at interface in wet bond which was an evidence in reduction of wet bond shear strength. 402 vol. 33/2022 adhesively-bonded timber & cast in situ concrete in manufacturing wet bond (especially timberconcrete composite deck/beam), it is recommended that the fresh concrete is poured at several locations of timber beam. this is due to distribute adhesive amount along the timber beam/deck. brunner et al [8] suggested that the height of pumping fresh concrete would be low enough to avoid the adhesive movement. increasing the height of pumping/or pouring fresh concrete may cause to adhesive splashing and movement in the location of pumping/or pouring fresh concrete. therefore, no adhesive remains for gluing timber and concrete. moreover, gentle use of vibrator is approved for debubbling fresh concrete, however, vibrator should not have any contact with adhesive layer. acknowledgements the research was financially supported by fachagentur nachwachsende rohstoffe e. v. (fnr, agency for renewable resources) founded by bundesministerium für ernährung und landwirtschaft (bmel), under the grant award no.: 22011617, and by bundesministerium für bildung und forschung (bmbf) (grant no.: 031b0914a). references [1] a. heiduschke, and b. kasal, composite cross sections with high performance fiber reinforced concrete and timber, forest products journal 53(10):74-78, 2003. [2] d. e. c. yeoh, behaviour and design of timber-concrete composite floor system, university of canterbury. department of civil and natural resources, ph.d. thesis, 2010. [3] t. tannert, b. endacott, m. brunner, et al. long-term performance of adhesively bonded timber-concrete composites. international journal of adhesion and adhesives 72:51-61, 2017. https://doi.org/10.1016/j.ijadhadh.2016.10.005. [4] binderholz-gmbh. binderholz brettsperrholz bbs, 2017. https: //www.binderholz.com/fileadmin/pdf/services_ko ntakt/videos_download/prospekte/bbs_d_web.pdf. [5] mestä-wood, kerto-q brochure, 2018.https: //www.metsawood.com/global/tools/materialarchi ve/materialarchive/kerto-manuallvl-q-panel.pdf. [6] sikadur-300. 2-komponentiges epoxid-imprägnierharz, 2018. https://che.sika.com/pds_sikadur-300_dech.pdf. [7] semparoc 12nv. ik pur-klebstoff, hohe beständigkeit, 2018. [online]. available:https://www.collano.com//downloads/dat enblaetter/de/semparoc_i_12_nv_de.pdf. [8] r. goyal, a. mukherjee, s. goyal. an investigation on bond between frp stay-in-place formwork and concrete. construction and building materials 113:741-51, 2016. https: //doi.org/10.1016/j.conbuildmat.2016.03.124. [9] m. brunner, m. romer, m. schnüriger. timberconcrete-composite with an adhesive connector (wet on wet process). materials and structures 40(1):119-26, 2006. https://doi.org/10.1617/s11527-006-9154-4. [10] p. l. clouston, c. p. quaglia. experimental evaluation of epoxy based wood plank-concrete composite floor systems for mill building renovations. the international journal of the constructed environment 3(3):63-74, 2013. https: //doi.org/10.18848/2154-8587/cgp/v03i03/37394. [11] b. duncan. developments in testing adhesive joints, in advances in structural adhesive bonding. 389-436, 2010. https://doi.org/10.1533/9781845698058.3.389. [12] testing hardened concretepart 2: making and curing specimens for strength test, 2009. [13] din-en392. glued laminated timber shear testing of glue joints, 1996. [14] q. fu, l. yan, t. ning, et al. interfacial bond behavior between wood chip concrete and engineered timber glued by various adhesives. construction and building materials 238, 2020. https: //doi.org/10.1016/j.conbuildmat.2019.117743. [15] q. fu, l. yan, b. kasal. testing methods for shear strength of bond line between concrete and different types of engineered wood. international journal of adhesion and adhesives 102, 2020. https://doi.org/10.1016/j.ijadhadh.2020.102671. [16] a. nemati giv, q. fu, l. yan, et al. interfacial bond strength of epoxy and pur adhesively bonded timber-concrete composite joints manufactured in dry and wet processes. construction and building materials 311, 2021. https: //doi.org/10.1016/j.conbuildmat.2021.125356. [17] l. zhang, w.-w. wang, k. a. harries, et al. bonding behavior of wet-bonded gfrp-concrete interface. journal of composites for construction 19(6), 2015. https: //doi.org/10.1061/(asce)cc.1943-5614.0000550. [18] p. zhang, g. wu, h. zhu, et al. mechanical performance of the wet-bond interface between frp plates and cast-in-place concrete. journal of composites for construction 18(6), 2014. https: //doi.org/10.1061/(asce)cc.1943-5614.0000472. [19] l. li, y. shao, z. wu. durability of wet bond of hybrid laminates to cast-in-place concrete. journal of composites for construction 14(2):209-16, 2010. https: //doi.org/10.1061/(asce)cc.1943-5614.0000060. 403 https://doi.org/10.1016/j.ijadhadh.2016.10.005 https://www.metsawood.com/global/tools/materialarchive/materialarchive/kerto-manuallvl-q-panel.pdf https://che.sika.com/pds_sikadur-300_dech.pdf https://www.collano.com//downloads/datenblaetter/de/semparoc_i_12_nv_de.pdf https://doi.org/10.1016/j.conbuildmat.2016.03.124 https://doi.org/10.1617/s11527-006-9154-4 https://doi.org/10.18848/2154-8587/cgp/v03i03/37394 https://doi.org/10.1533/9781845698058.3.389 https://doi.org/10.1016/j.conbuildmat.2019.117743 https://doi.org/10.1016/j.ijadhadh.2020.102671 https://doi.org/10.1016/j.conbuildmat.2021.125356 https://doi.org/10.1061/(asce)cc.1943-5614.0000550 https://doi.org/10.1061/(asce)cc.1943-5614.0000472 https://doi.org/10.1061/(asce)cc.1943-5614.0000060 my title https://doi.org/10.14311/app.2022.36.0033 acta polytechnica ctu proceedings 36:33–46, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague flood risk assessment for road infrastructures using bayesian networks: case study of santarem portugal erica arangoa,∗, monica santamariaa, maria nogalb, hélder s. sousaa, josé c. matosa a university of minho, institute of science and innovation for bio-sustainability, department of civil engineering, guimarães, 4710-057 braga, portugal b delft university of technology, department of materials, mechanics, management & design., gebouw 23, stevinweg 1, 2628 cn delft netherlands ∗ corresponding author: ericaaarango@gmail.com abstract. assessing flood risks on road infrastructures is critical for the definition of mitigation strategies and adaptation processes. some efforts have been made to conduct a regional flood risk assessment to support the decision-making process of exposed areas. however, these approaches focus on the physical damage of civil infrastructures without considering indirect impacts resulting from social aspects or traffic delays due to the functionality loss of transportation infrastructures. moreover, existing methodologies do not include a proper assessment of the uncertainties involved in the risk quantification. this work aims to provide a consistent quantitative flood risk estimation and influence factor modelling for road infrastructures. to this end, a flood risk factor (frf) is computed as a function of hazard, vulnerability, and infrastructure importance factors. a bayesian network (bn) is constructed for considering the interdependencies among the selected input factors, as well as accounting for the uncertainties involved in the modelling process. the proposed approach allows weighting the relevant factors differently to compute the frf and improves the understanding of the causal relations between them. the suggested method is applied to a case study located in the region of santarem portugal, allowing the identification of the sub-basins where the road network has the highest risks and illustrating the potential of bayesian inference techniques for updating the model when new information becomes available. keywords: bayesian networks, decision-making, flood risk assessment, road networks. 1. introduction the efficient and rational management of natural hazards is a concern that has become of increasing importance for decision-makers, generating the need for a strategy that explicitly addresses the uncertainties and consequences involved in the occurrence of these extreme events. a concurrent event of importance for management is the flooding risk of road networks, which is related to a variety of natural, economic, and social factors such as climate change, use and coverage soil, deficient drainage systems, population explosion, among others [1]. therefore, exploring the relationship between these factors and how they directly or indirectly influence the flood risk of road networks is of great relevance to this study. however, each factor constitutes a complex system, and they also interact with each other. on the other hand, the flood events occurrence leads to social insecurity, undermining the economic development of the impacted regions and affecting ecosystems [2]. not to mention that its frequency is expected to increase more in the future [3]. due to that, global economic risks are also increasing, along with the need for proper potential flood risk assessment and management tools to avoid or manage the level of disaster and minimize the potential damage. existing research on risk assessment can be classified into two main categories. on the one hand, qualitative assessment methods. for example, the analytical hierarchy process (ahp) or multi-criteria method [4–7] allows integrating multiple factors (e.g., hazard, vulnerability, and exposure) for flood risk assessment. on the other hand, the quantitative evaluation methods, normally based on historical data [8] and scenarios simulation based on the hydrologicalhydraulic model [9] or hydrodynamic [10], as well as flood risk simulation models, such as random forest, decision trees, genetic algorithms, among others [11– 14]. qualitative methods are normally criticized for their subjective nature, which results from excessive dependence on empirical knowledge. therefore, they are considered high uncertainty and low credibility models in risk assessment. on the other hand, quantitative evaluation methods require a large amount of data with high precision, which limits their application in some cases due to the reduced availability of data [15]. additionally, none of the aforementioned methods define the mutual relationships between the 33 https://doi.org/10.14311/app.2022.36.0033 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en e. arango, m. santamaria, m. nogal et al. acta polytechnica ctu proceedings figure 1. geographical location and specification of the study area. factors that influence risk, which is often interdependent in a complex and uncertain way [1]. therefore, it is evident the need for simplifications to establish a model that adequately evaluates the flooding risk in the road network, but that effectively considers the relationship between the different risk factors without requiring much data availability. a bayesian network (bn) is considered a simplified model that allows uncertainty estimation during risk assessment, contemplating the probabilities of the random variables and their joint probability distribution [16]. one of its advantages is that it enables the identification of relevant factors and their interrelationships with flood risk using previous knowledge, which can be inferred and reasoned from real data [17]. bns have been implemented in urban flood risk assessment [1], river basins [18], coastal hazards [19], early warning of floods [20], among others. on the other hand, the geographic information system (gis) that has powerful spatial data processing techniques allows greater data accessibility. therefore, there are studies in which the gis and bn models have been integrated to estimate the probability of hazards, for instance [21–23]. however, there is not enough research on the chains of disaster-causing factors based on the bn model to explore the path through which the factors affect flooding in the road network. this work aims to provide a consistent quantitative flood risk estimation and influence factor modelling for road infrastructures. therefore, a flood risk factor (frf) is computed as a function of hazard, vulnerability, and infrastructure importance factors, using a relative flood risk classification method as a practical and measurable process in terms of inputs and application. moreover, a bayesian network (bn) is constructed to consider the interdependencies between the selected input factors, as well as to consider the uncertainties involved in the modelling process. the proposed approach allows the relevant factors to be weighted differently to calculate the frf and improves the understanding of the causal relationships between them, as shown in the case study application, located in the region of santarem, portugal. in this case study, it was possible to identify and quantify the sub-basins where the road network is most at risk and illustrates the potential of bayesian inference techniques to update the model when new information becomes available. 2. methodology the proposed approach in this study consists of four main steps: (1) identification of the main factors that may affect or influence (directly or indirectly) flood disaster occurrence and the data collection from the different data sources associated with the factors previously identified; (2) assessing flood risk of road infrastructures (frf index) and (3) development of the bn structural graph and calculation of probability distribution tables based on the bn structure, and (4) sensitivity analysis. 34 vol. 36/2022 flood risk assessment for road infrastructure in santarem figure 2. factors affecting the risk of floods on road infrastructures. the case study is located in portugal, between 7◦57’ and 9◦9’ eastern longitudes and between 38◦42’ and 39◦54’ northern latitude, which comprises an area of approximately 7044.4 km2. the system boundaries that were specified for the implementation of the framework consist of 32 municipal councils from the santarem district, as shown in figure 1. the case study covers a section of one of the most important rivers in portugal, the tagus river; therefore, the closest sub-basins are low elevation and tend to be a flood-prone area. these lowlands are characterized by an average altitude of 103.5 m and an average slope of 8.4%. the region has an average population of 40.842 inhabitants, with densely populated regions such as santarem municipality. the tagus hydrographic basin presents significant differences in annual precipitation through its spatial distribution, with low rainfall in the upper tagus and higher rainfall in the middle. thus, in this latter region, e.g., in the sorraia sub-basin, flood events occur more frequently. according to the historical flood records dated from 1865 to 2015, the sub-basin with the highest number of flood records are "rio tejo (hmwb jusante bs. castelo do bode e belver)" and "tejo (hmwb jusante b. belver)" (figure 3), with 21 records and more than 2000 people affected. 2.1. factors affecting the flood risk of road infrastructures for flood risk assessment of road infrastructures, it is important to consider the main causing factors, such as the likelihood of the flood occurrence, the structure vulnerability to flooding, and the infrastructure importance. additionally, each factor is constituted by a series of sub-factors, which directly or indirectly influence flood risks on road infrastructures. for instance, among the causes of flood occurrences, the magnitude of rainfall, the number of storms days, or the rain intensity is usually considered. moreover, the environment nature where the flooding occurs is focused on the combination of climatic variables and exposure surfaces (i.e., factors such as land cover, topography, rivers distribution in regions). therefore, elevation, slope, river network, and road density should be considered as some of the most important factors for flood risk assessment. on the other hand, to consider the damage caused by floods in social terms, factors such as population density are required because the higher the population density, the more serious economic and social damage. road density is also an important factor, due to regions with greater transport having greater adaptability to disasters. in addition to the factors that reflect the exposure characteristics (buildings, people, or others), past flood records including information on damages and losses is a relevant factor to take into consideration. the evaluation factors considered in this study are related to the danger of flooding by sub-basin and it is important to clarify that the factors are decided based on the results of previous studies such as [1, 15, 24, 25] and expert judgment. summarizing, the parameters and sub-parameters that affect the flooding risk of road networks and that are considered in this study are summarized in figure 2. 35 e. arango, m. santamaria, m. nogal et al. acta polytechnica ctu proceedings figure 3. data collection for the likelihood of flood occurrence: epsi and historical records. large values of epsi indicate larger susceptibility. 2.1.1. factors affecting the likelihood of the flood risk occurrence the data collection and processing are discussed below. extreme precipitation susceptibility index (epsi) epsi is regarded in this study as a principal factor for considering flood hazards. this index which ranges from 1 to 4 was proposed by santos, fragoso, and santos [26], and considers the following extreme precipitation indices: • r× 1day and r× 5days: annual highest daily precipitation and annual highest 5 consecutive precipitation days, respectively. describe the precipitation associated with periods of extreme rain; • sdii: annual total precipitation divided by the number of wet days (precipitation ≥ 1 mm) in the year, which reflects daily intensity; • r20: number of heavy precipitation days ≥ 20 mm, i.e., moderate to high amounts of daily precipitation; • cwd: number of consecutive wet days (daily precipitation ≥ 1 mm), and • r95ptot: fraction of annual total precipitation exceeding the 95th percentile. it is used to evaluate the contribution of extreme daily precipitation to the total precipitation. epsi considers a return period of 10 years, (i.e., it has a 10% probability of occurring within any year) which makes it a suitable and balanced indicator of extreme precipitation. its calculation involves gis technology justified by the strong seasonality of the precipitation regime, and it was developed for both annual data and the meteorological season considering records between 1950 and 2003. epsi was calculated from four main stages: (1) selection of the most relevant indices chosen from a sensitivity analysis using different combinations; (2) normalization process for all variables; (3) epsi calculation using the previously normalized indices, and (4) definition of four classes of susceptibility, based on a classification by quantiles. for more details about epsi see santos, fragoso, and santos [26]. epsi indices for each sub-basin are shown in figure 3. it should be noted that sub-basins are identified in correspondence with the codes used by the portuguese environmental agency [27]. historical flood records in addition to epsi, it is also important to consider the areas where floods occurred in the past, giving greater relevance to basins that have historical records and therefore greater probability of flood risk. the data was obtained from the national database of hydro-geomorphologic-related disasters (landslides and floods) in portugal, built 36 vol. 36/2022 flood risk assessment for road infrastructure in santarem and maintained since 2009 as part of the disaster project [28]. the data of the factors affecting the likelihood of flood risk occurrence are shown in figure 3. 2.1.2. vulnerability factors topographic characteristics flood occurrences and redistribution are heavily influenced by topography aspects such as elevation and slope. the elevation is characterized by the vertical distance between a given surface and the reference basement, and the slope is an average rate of elevation change in a particular domain. the elevation and slope data were calculated using a digital elevation model (dem), derived from the shuttle radar topographic mission (srtm) in a resolution of 3 arc-seconds, processed for portugal [29]. block statistics and grid algebra calculations were used to divide both components, elevation, and slope, into four levels which were then reclassified (as shown in appendix a figure 9a and figure 9b, respectively). river network the distribution of the river network has an important role in the flood risk of road networks, especially the river density and the distance to the river. in this work, the study area is mainly focused on the tagus river, which has its origin in the sierra de albarracín, spain, with a length of 1,100 km, of which 230 km are in portugal. in the national territory, its main tributaries are the zezere river and the sorraia river. information regarding the river network was collected from the portuguese environmental agency [30]. river density was calculated as the length of rivers per unit area, using line density function, and river proximity as the closest distance to a river, which was obtained using the multiple buffer operator on rivers of order 3 onwards, as shown in appendix a figure 10 (a, b) respectively. land cover and toil type environmental factors influencing flood risks are considered in terms of land cover and soil type. the land cover information was obtained from the global map of land use/land cover (lulc), acquired from esa sentinel-2 imagery at a 10-meter resolution [31], available in [32] (figure 11). from this map, the impervious area percentage was calculated, assuming three categories of importance: buildings and paved surfaces as areas with more flooding risk; tree/wooded areas as medium flooding risk; and other surfaces (such as water (w), grass (g), flooded vegetation (fv), crops, (c), scrub(s) and bare ground (bg)) as lowrisk flood areas. regarding soil type, the information was obtained from the topsoil physical properties for europe, based on the lucas topsoil database [33]. the data were classified according to nrcs soils triangle and discretized using the hydrologic soil group (hsg) classification according to soil properties proposed by zeng et al. [34], as shown in appendix a figure 11. 2.1.3. infrastructure importance factors when a high-traffic-density road collapses or becomes blocked, it can affect the whole traffic system due to the use of alternate routes, resulting in social and economic losses. the higher the traffic density, the greater the road importance and the higher the repair cost due to lack of availability. a high road density usually allows alternative routes in case of disaster. on the other hand, the higher the population density, the greater the damage caused by floods. therefore, in this study, the road traffic density, expected repair cost, road density, and population density are considered sub-parameters for measuring the infrastructure’s importance in social and economic terms. the traffic demand and road data of the case study were obtained from infraestruturas de portugal (ip), the portuguese transportation infrastructures manager, and the population density data were collected from the national statistical institute [35]. daily traffic density was divided into light and heavy traffic, and road density was calculated as the length of roads per unit area, using the line density function. regarding the repair cost, the average construction costs per kilometer of the european court of auditors were taken into account, which depends on the road type and the country, referred to in [36]. these factors are presented in appendix a figure 12. the following step consists in discretizing all the considered sub-factors due to each node of the bn model being assigned to a finite set of state values based on discrete probabilities. according to current studies [1, 15], and expert knowledge, the sub-factors impacting flood risk were categorized into three groups, as shown in table 1. 3. determining the flood risk of road infrastructure expert knowledge and the approach implemented by kim et al. [37] were used to assign weights according to each factor’s importance in the flood risk occurrence. in this case, each evaluation sub-factor was assigned a weight to represent the degree of related flood risk. the weights are assigned using a scale of 1 to 3 through intuitive judgment, where a weighting factor of 1 represented the lowest risk and a weighting value of 3 the highest flood risk. the final risk of each category (hazard, vulnerability, and infrastructure importance) was obtained by adding the weighted average of the total of sub-factors that compose it, also assuming a scale of 1 to 3. finally, the flood risk factor (frf) values are assumed as follows: a final risk value less than 2 is a low-risk degree, high risk is considered from a final value greater than 2.5, and medium risk between 2 to 2.5. 37 e. arango, m. santamaria, m. nogal et al. acta polytechnica ctu proceedings categ. factors classes low medium high hazard epsi 1 2 > 2 historical records 0 0 3 > 3 vulnerability elevation (m) > 150 76 150 ≤ 75 slope (%) > 10 5 10 ≤ 5 river density (km/km2) ≤ 0.15 0.15 0.25 > 0.25 river proximity (m) > 1500 500 1500 ≤ 500 soil typea co, s, ls or sl l, sil, si, scl cl, sicl, sc, sic or c land cover build area trees other surfaces structure importance light traffic (vehicles) ≤ 40.000 40.000 150.000 > 150.000 heavy traffic (vehicles) ≥ 5.000 5.000 20.000 ≥ 20.000 road density (km/km2) ≤ 1 0.5 1.0 ≤ 0.5 repair cost (€) ≤ €500 m €500 m €1.500 m ≥ €1.500 m population density ≤ 20.000 20.000 35.000 > 35.000 a co (coarse), ls (loamy sand) sl (sandy loam); l (loam) sil (silt loam) si (silt) scl (sandy clay loam); cl (clay loam) sicl (silty clay loam) sc (sandy slay) sic (silty clay) or c (clay) table 1. classification of different risk factors. figure 4. frf results by sub-basin. 3.1. flood risk calculation the average weights for the three main categories (λi), namely the likelihood of flood occurrence, vulnerability, and infrastructure importance, were calculated using weights for their sub-factors. essentially, λi can be calculated as equation 1. λi = ∑nk k=1 fk nk , k = 1, 2, . . . nk (1) where, fk represents the corresponding sub-factor for each category; nk represents the number of subfactors for each category, and the subscript k is the index of each sub-factor. the final frf is obtained by calculating the average weights of the categories as equation 2. frf = ∑ni i=1 ηi λk ηi , i = 1, 2, . . . ni (2) where, ni represents the number of categories and, ηi are the weights of each category. in this case, the weights were set equivalently to 1, assuming that all have equal importance for the final probability of flood risk. the weights were applied using the classification 38 vol. 36/2022 flood risk assessment for road infrastructure in santarem figure 5. distribution of the flood risk in the case study. figure 6. bn model for flood risk assessment of road infrastructures y. proposed in section 2.2.3, specifically table 1. for example, a weighting factor 3 was assigned if the epsi exceeded 2, i.e., it was classified as high; a weighting factor 1 was assigned if the epsi value was less than or equal to 1, i.e., classified as low; and a weighting factor of 2 was assigned if the epsi was between 1 and 2, i.e., classified as medium. flood risk assessment was conducted on 67 subbasins in the center and south of portugal. figure 4 illustrates the results of the evaluation, and figure 5 39 e. arango, m. santamaria, m. nogal et al. acta polytechnica ctu proceedings figure 7. frf sensitivity analysis: a) high, b) medium and c) low. 40 vol. 36/2022 flood risk assessment for road infrastructure in santarem illustrates the distribution of the results. from the assessment results, it can be observed that 9% of the sub-basins received a high-risk rating, which suggests that the road network in these regions has a greater probability of suffering large impacts due to flooding events. on the other hand, 61% of the sub-basins (41) received a low-risk rating, while approximately 30% of the total study area is likely to have medium-risk flood events. 4. bn model the structure of a bayesian network consists of two components. on the one hand, the directed acyclic graph (dag) where the random variables are denoted as nodes of eigenvectors and the arc represents the probabilistic dependency between nodes. the other component is conditional probability tables (cpts) used to specify dependency relationships encoded in a dag [22], [38]. the bn model construction for the flood risk evaluation of the road network is based on the defined hazardous factors in section 2.2, which are used as nodes in the bn structure graph. the network construction procedure consisted of three main steps: (1) review of the structure and potential relationships between the different factors (nodes), (2) parameter learning to obtain conditional probability table (cpt), and (3) sensitivity analysis. for the first step, the potential relationships between different factors were first determined based on the scientific literature and expert knowledge and were subsequently verified using learning algorithms such as bayesian estimation and maximum likelihood estimation (mle). regarding the parameter learning, the training was made from the sample data collected from the factors in the study area, once the respective discretization was used to determine the conditional probability distribution for each sub-factor. for this methodology, the genie software [39] was utilized, which allows importing data directly from csv or txt format. additionally, genie software has a great variety of useful algorithms in the learning structures and parameters process, as well as facilitating sensitivity analysis in simple graphs to calculate their impact on the results. for this study pc and bayesian search structure learning algorithm was implemented . the developed bn model is presented in figure 6, which consists of all nodes (sub-factors) with their corresponding states. the categorical classification employed in section 2.2 can be also visualized, i.e., flood risk is represented with the white node, hazard factors with blue nodes, vulnerability factors with orange nodes, and factors to consider the infrastructure importance with green nodes. the nodes states (discretization) are established according to the classes shown in table 1 (e.g., the "elevation" node has different altitudes, such as low, medium, or high) and each square represents the state probabilities in terms of percentage. figure 6 also shows that the 13 sub-factors considered affect directly or indirectly the occurrence of flood risk in the road network. the nodes connected with arrows represent a potential relationship between sub-factors, in which the initial node (arrow beginning) is the cause, while the pointed node (arrow end) is the effect. each potential relationship is expressed through conditional probabilities calculated from the data collected. the developed bn model can objectively evaluate links among diverse influencing factors and statistically infer the likelihood of flood risk based on information obtained from qgis, presenting a novel method for assessing flood risk for road networks. 4.1. sensitivity analysis the sensitivity in the flood risk assessment was evaluated to identify the influence of sub-factors on the obtained results. for this, another tools available in genle was used, which performs a simple sensitivity analysis of bayesian networks using the algorithm proposed by kjaerulff and van der gaag [40]. which efficiently calculates a complete set of derivatives of the posterior probability distributions on the target nodes on each of the numerical parameters of the bayesian network from a set of target nodes. it means, the bn model learning and recording the relative importance of the input variables to predict the output. the sensitivity value of the 10 most dominant subfactors influencing each of the three states of the target node "flood risk" are shown in the tornado graph in figure 7 a, b, c, for high, medium, and low levels, respectively. it is worth mentioning that the bar color indicates the change direction in the target node; red indicates a negative change, while green indicates a positive change. for instance, according to the sensitivity values of different factors, the greatest impact on the high frf is the epsi factor (related to the causes of the hazard) with a state "medium" and the light traffic (low), relative to the importance of the infrastructure, and soil (medium), related to vulnerability. meanwhile, variables such as population density and repair cost have a lower influence on the high flood probability. on the other hand, in figure 7b, the sensitivity analysis for frf with state "medium" is shown, presenting a different order of the sensitive factors, in which the most sensitive factor is light traffic (low), followed by epsi (medium) and soil type (medium). the same sub-factors order is present for the more influents of frf (medium) but not for the less sensitive, which are population density (low) and light traffic (high) as shown in figure 7c. once the network has been established and validated, it is possible to evaluate cases. for instance, the road network of the vala de azambuja basin presents a final probability of 17% of low flood risks, 17 % moderate, and 67 % probability of high risk (rounded values), based on the data evidence introduced in the bn as shown in figure 8. in the analyzed scenario, the flood risk of the road network is most likely to be 41 e. arango, m. santamaria, m. nogal et al. acta polytechnica ctu proceedings figure 8. bn analysis frf results of vala da azambuja road network . "high" given that this state has obtained the highest probability. if additional information regarding any sub-factor is collected, this new evidence can be easily used for updating the model and reassessing the flood risk. 5. conclusions this work developed a consistent quantitative flood risk estimation and influence factor modelling for road infrastructures. several factors affecting the flood risks were identified, and weights were assigned to each factor based on expert knowledge and relevant literature in order to quantify a flood risk factor (frf). moreover, a bn model was introduced to examine the potential relationships between the factors. it was constructed based on the strength of the relationships found and the probability distribution tables were obtained employing parameter learning algorithms. by using the bn model, it is possible to account for the uncertainties in the input factors and their relationships and propagate them throughout the model to obtain a probability of flood risk. this characteristic is distinct from some previous works where the final risk estimation is computed deterministically. moreover, the proposed approach has the advantages of enhancing the understanding of the interdependencies among the influencing factors and being flexible for updating the model when new information becomes available. this framework may provide a basis for a decision analysis and decision support with the quantification of expected categorized consequences. the flood risk in road infrastructures was evaluated based on the proposed methodology in the region of santarém, portugal. the case study showed that 61% of the sub-basins present a low flood risk, 30% present a medium flood risk, and 9% present a high flood risk. moreover, the sensitivity analysis revealed that the most influential sub-factors on a high flood risk were the epsi factor, light traffic vehicles, and soil type. thus, the obtained results enabled the identification of the sub-basins where the road infrastructures are heavily exposed to the risk of flooding and provided insights to decision-makers on how the network can be improved to prevent high impacts during flood events and prioritize available resources. the proposed approach can be implemented in other case study areas with similar data availability and can be revised for those with less information available. there are some limitations to the proposed methodology. first, the established bn model accounted for the most influencing factors of flood risks in road infrastructures, but it was constrained by data availability. in other words, more influencing factors can be included for a more comprehensive risk assessment. for instance, only the reconstruction cost of the roads is included, while indirect impacts originated due to the lack of availability of the roads (e.g., additional travel time) are not accounted for. additional limitations from the proposed approach are data quality and the fact that certain steps from the method involve subjectivity. for the former issue, the focus of future research will be on how to gather data at a higher spatial resolution to improve the accuracy of the obtained results. for the latter, future work can be oriented towards performing subsequent analy42 vol. 36/2022 flood risk assessment for road infrastructure in santarem ses regarding the effect of the decisions made by the experts in the model. acknowledgements this work was partly financed by fct / mctes through national funds (piddac) under the r&d unit institute for sustainability and innovation in engineering structures (isise), under reference uidb / 04029/2020. the first author would like to thank fct portuguese scientific foundation for the research grant 2020.05755.bd. the second author would like to thank fct portuguese scientific foundation for the research grant sfrh/bd/144749/2019. this project has received funding from the european union’s horizon 2020 research and innovation program under grant agreement no 769255. this document reflects only the views of the author(s). neither the innovation and networks executive agency (inea) nor the european commission is in any way responsible for any use that may be made of the information it contains. references [1] z. wu, y. shen, h. wang, et al. assessing urban flood disaster risk using bayesian network model and gis applications. geomatics, natural hazards and risk 10(1):2163-84, 2019. https://doi.org/10.1080/19475705.2019.1685010. 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of vulnerability factors a) b) figure 9. data collection of vulnerability factors: a) elevation and b) slope. a) b) figure 10. data collection of vulnerability factors: a) river density and b) river proximity. a) b) figure 11. data collection of vulnerability factors: a) land cover and b) soil type. 45 e. arango, m. santamaria, m. nogal et al. acta polytechnica ctu proceedings a.2. data collection of infrastructure importance factors a) b) c) d) figure 12. data collection of infrastructure importance factors: a) heavy traffic vehicles, b) road density, c) repair cost and d) population density. 46 acta polytechnica ctu proceedings 36:33–46, 2022 1 introduction 2 methodology 2.1 factors affecting the flood risk of road infrastructures 2.1.1 factors affecting the likelihood of the flood risk occurrence 2.1.2 vulnerability factors 2.1.3 infrastructure importance factors 3 determining the flood risk of road infrastructure 3.1 flood risk calculation 4 bn model 4.1 sensitivity analysis 5 conclusions acknowledgements references a appendices a.1 data collection of vulnerability factors a.2 data collection of infrastructure importance factors https://doi.org/10.14311/app.2022.33.0277 acta polytechnica ctu proceedings 33:277–283, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague a study on properties of concrete with dry fly ash and fly ash slurry stored with stirring xiangnan ji∗, koji takasu, hidehiro koyamada, hiroki suyama the university of kitakyushu, faculty of environmental engineering, department of architecture, japan ∗ corresponding author: b0dbb410@eng.kitakyu-u.ac.jp abstract. in the evaluation of concrete sustainability, what constitutes "sustainable" to one region may vary from another. this often leads to methodological forms of uncertainties that makes the evaluation process more complex. as such, this paper aims to quantify the effect of uncertainties in the regional context on the sustainability evaluation of concrete materials. this is carried out by quantifying the regional context through establishing a weighting scheme and then integrating the obtained weights into the sustainability analysis of concrete materials in tandem with uncertainty analysis. japan is used as a case study because although it relatively appears as a homogeneous country, its prefectures possess unique characteristics that may make the sustainability evaluation of concrete materials vary across prefectures. cluster analysis is carried out in the 47 prefectures of japan using a set of regional context indicators. five clusters are identified with varying characteristics and these are translated into different weighting schemes. the established weights are used in the sustainability evaluation of concrete materials using multi-criteria decision-making analysis. the results showed that one mix is the most sustainable for four of the clusters and a different mix is the most sustainable for the remaining cluster. when uncertainty analysis is conducted, the effect of the weights in the sustainability evaluation is explained by examining the average scores of the concrete mixes and the variance of the scores across the five clusters. this investigation facilitated the understanding of how regional differences and the uncertainties associated with it impact the evaluation of concrete sustainability. keywords: dry powder, flow, fly ash, fresh properties, slump, slurry. 1. introduction as a previous study, we invented a device to remove unburned carbon by flotation method. since the flotation method immerses the fly ash in water for treatment, it is necessary to verify the influence of the slurry of the fly ash on the concrete. therefore, in this study, fly ash from three different sources was stirred and stored for 1-28 days, and the physical changes of the three substances stored under this condition were recorded, and the freshness of the concrete using dry powder was confirmed. and check the usefulness of the slurry. 2. experimental summary 2.1. material used table 1 lists the different properties of the three fly ash. in this experiment, fly ash a, b and c from three different sources were used. they all conform to the class ii specified in jisa6201. the dry powder and the slurry are respectively used as a mixture in the concrete, and the preparation method of the slurry is: the concentration was adjusted to 60 ± 1 % using an infrared moisture meter, placed in a 90 l vessel, and stirred at a rotation speed of 60 rpm to prevent sedimentation. for the samples used for the slurry, the b and c ash were respectively stirred for 7 days, stored and reused, a ash was added on the 7th day, and samples were taken at 1, 3, 14 and 28 days. when used as a mixed material, after measuring the slurry concentration using an infrared moisture meter before putting it in, in order to meet the unit water volume of the preparation, add insufficient water, than start stir. in addition, the slurry concentration before the input showed some change after the initial adjustment, but there was no particularly significant change. table 2 shows the materials used. we use ordinary portland cement as cement, sea sand as fine aggregate and gravel as coarse aggregate. concrete mixing was carried out in a mixing chamber at a temperature of 20◦ c ± 1 using a forced biaxial mixer with a target air volume of 4.5 ± 1.0 % and a target slump of 18.0 ± 2.5 cm. in addition, regarding the external weight of 120 kg/m3 , high fluidity concrete was used, and the target slump flow amount was 50 ± 5 cm. table 3 shows the mix ratio of concrete. in this experiment, the unit water volume is 180 kg/m3, the fine aggregate rate is fixed at 45%, and the fly ash is used as an internal substitute for cement replacement, replacing 10, 20, 30%, using 120 kg/m3. ash was used as a substitute for fine aggregates and experiments were performed by external replacement. in addition, in the b and c ash, the slump and compressive strength tests were carried out at an internal replacement rate of 20%. 277 https://doi.org/10.14311/app.2022.33.0277 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en x. ji, k. takasu, h. koyamada, h. suyama acta polytechnica ctu proceedings s y m b ol t ap e s ta te b ef or e th e ex am sl u rr y co n ce n tr at io n (% ) p er io d l os s on ig n it io n (% ) d en si ty (g / cm 3 ) b ra in sp ec ifi c su rf ac e ar ea b e t sp ec ifi c su rf ac e ar ea (m 2 / g) w or k u p te m p er at u re (◦ c ) s lu m p fl ow (r ig h t af te r) s lu m p fl ow (3 0m in ) s lu m p fl ow (6 0m in ) s lu m p fl ow (9 0m in ) s lu m p fl ow (1 20 m in ) a ir vo lu m e (i m m ed ia te ly af te r) (% ) a ir vo lu m e r ed u ct io n ra te (% ) a d m ix tu re (% ) p − − − − − − − − 18 .2 17 .2 17 .0 14 .4 13 .5 5. 0 50 .0 0. 4 a a − − − 1. 3 2. 3 40 00 − − − − − − − − − a 1d -2 0i n a s 60 .5 1d 2. 0 − − 3. 2 20 .4 19 .0 18 .8 17 .0 15 .9 4. 1 46 .3 0. 4 a 3d -2 0i n s 60 .4 3d 2. 2 − − 3. 1 20 .3 19 .7 18 .5 18 .0 18 .2 3. 5 40 .0 a 7d -1 0i n d − 7d − − − − 18 .4 18 .2 17 .9 15 .3 13 .3 4. 0 52 .5 s 59 .6 2. 3 − − 4. 4 18 .2 13 .7 14 .6 12 .0 8. 2 5. 0 38 .0 a 7d -2 0i n d − − − − − 18 .7 17 .5 18 .7 16 .6 18 .3 3. 9 35 .9 s 60 .4 2. 3 − − 4. 4 19 .3 16 .8 17 .3 15 .5 15 .5 4. 1 36 .6 a 7d -3 0i n d − − − − − 19 .8 19 .6 18 .5 17 .5 16 .5 3. 5 40 .0 0. 6 s 60 .2 2. 3 − − 4. 4 20 .3 19 .1 18 .0 17 .0 16 .0 3. 8 47 .4 a 7d -1 20 ou t d − − − − − 50 .1 50 .0 44 .0 41 .3 38 .0 5. 4 42 .6 0. 6 (s p ) s 61 .2 2. 3 − − 4. 4 52 .6 51 .9 44 .1 39 .8 39 .6 5. 5 40 .0 a 14 d -2 0i n s 60 .7 14 d 2. 3 − − 2. 8 18 .8 16 .7 14 .9 9. 4 9. 0 3. 5 37 .1 0. 4 a 28 d -2 0i n s 61 .8 28 d 2. 4 − − 3. 0 20 .5 19 .5 18 .2 15 .7 15 .2 4. 1 53 .7 b b − − − 1. 6 2. 3 41 40 − − − − − − − − − b 7d -2 0i n d − 7d − − − − 17 .7 16 .7 14 .5 13 .3 12 .1 3. 5 54 .3 0. 4 s 60 .6 − − − − 19 .5 18 .2 17 .8 15 .9 14 .2 3. 5 40 .0 c c − − − 3. 0 2. 2 35 20 − − − − − − − − − c 7d -2 0i n d − 7d − − − − 20 .0 20 .0 19 .2 17 .3 16 .6 3. 6 30 .6 0. 4 s 60 .8 − − − − 19 .0 19 .0 16 .6 15 .9 15 .0 3. 6 36 .1 t ab le 1. p hy sic al pr op er tie s of fly as h an d fr es h pr op er tie s of co nc re te . 278 vol. 33/2022 a concrete with dry fly ash and fly ash slurry figure 1. a ash s age every particle size distribution. the dry ash content of fly ash is d, the amount of slurry is s, the internal is replaced by in, and the external is replaced by out. the storage period is 1 day, 3 days, 7 days, 14 days, 28 days, respectively 1d, 3d, 7d, 14d, 28d. 2.2. experimental project the measurement items in this experiment are slump and flow. the concrete slump test was carried out in accordance with jisa1101. in order to confirm the change of the measured amount with time, the slump was recorded every 30 minutes for the first 2 hours after the start of the experiment. use fresh concrete at the beginning and end of the measurement of slump. in addition, the fresh concrete is required to be carried out in a mixing chamber at a temperature of 20◦ c ± 1◦ c. 3. experimental results and consideration 3.1. the physical properties of fly-ash as shown in table 1, for the a ash, the bet specific surface area and the strong heat loss were measured according to the age of the different preservation materials, and the former tends to increase as the material aged. previous studies have shown that the rising trend of jis ii ash has not changed and a similar trend has emerged. however, the latter was the largest at 7d and no change was observed at ages other than 7d. figure 1 shows the particle size distribution of the slurry of a ash. previous studies have reported that particles in the range of 0.1 to 1 µm tend to increase by storing the slurry, but there is no similar tendency in the jis ii seed ash of this experiment, and there is no change in particle size. figure 2. a ash slump. 3.2. concrete slump-flow table 1 shows the results of the experiment, and figure 2 shows the change in the slump of d of a and the change of s according to time. it can be seen that in addition to s10 in, the slump value at the time of placement was reduced to about 80% after 2 hours. it can be seen that within the scope of this experiment, the quality of the fresh property did not change even when used in d, s, although s10 had been reduced to about 50%. jass5 (building standard specification) stipulates that if the external temperature is lower than 25 ◦ c, it will be 120 minutes from mixing to completion, but during this period, the slump of concrete mixed with p and fly ash has the same timedependent change. as confirmed, it can be considered that the concrete containing the fly ash mixed with s can be appropriately placed. in addition, since it is impossible to clarify the time-dependent change of s10 in slump in this experiment, it will be a future research topic including re-experiment. the a, b, and c ash drop as a function of time as shown in figure 3 as a function of time. the b and c ash showed the same trend as the a ash 2 hours after the slump value. within the scope of this experiment, it was found that when the ash type equivalent to the jis ii type is used, the fresh performance of the concrete mixed with fly ash in d and s does not change. figure 4 shows the change in 20 in slump over time for s, which was stored for 1, 3 and 14 days with stirring, except for the 7 ash a ash. as a result, the slump value of s using 14d was significantly lowered. from table 1, the bet specific surface area of each s material age was confirmed, but the s value of 14d was the lowest. generally, the larger the bet specific surface area, the higher the adsorption rate of the mixture, and the fluidity becomes worse. further, in the previous study 3), fly ash having a bet spe279 x. ji, k. takasu, h. koyamada, h. suyama acta polytechnica ctu proceedings materials type physical property symbol cement ordinary portland cement density: 3.16 g/cm3 c water tap water − w fine aggregate sea sand dead density: 2.59 g/cm3 swater absorption rate: 0.76 % coarse grain rate: 2.4 performance rate: 61.2 % coarse aggregate crushed stone dead density: 2.59 g/cm3 gwater absorption rate: 1.41 % coarse grain rate: 6.9 performance rate: 56.7 % admixture high performance ae water reducing agent polycarboxylic acid ether system sp ae agent alkyl ether anionic surfactant ae table 2. materials used. figure 3. a ash, b ash, c ash, 20in slump. cific surface area of 3.6 m2/g or more was considered to be caused by deterioration of fluidity. however, in the 14d having the lowest bet specific surface area, the fluidity deteriorated due to aging. further, the value of the bet specific surface area fell within the range of the previous study, and the păof 7d was 4.4 m2/g, which showed other s ages have the same slump change. from this, it can be considered that the deterioration of the fluidity of s 14d is the most serious, and it is necessary to study this in the future. 3.3. compressive strength figure 5 shows the d and s concrete compressive strengths of a-ash. for 10in and 20in concrete, the initial strength is lower than that of p, at 91 days strength, compressive strength equivalent to p figure 4. a ash s age every 20in slump. was shown by the pozzolanic reaction of fa. 120out showed greater compressive strength than p due to fine aggregate replacement. a-ash showed a slight difference at 91 days of age, but the compressive strength tended to be higher when it was mixed with concrete in s. figure 6 shows the compressive strength of 20inch concrete with b and c ash in addition to a ash. in the case of b ash, the compressive strength was higher when mixed with d than when mixed with s, and the results were opposite to those of a and c ash. figure 7 shows the compressive strength of a ash for each age of slurry agitation storage. by compressing and storing the slurry, the compressive strength increased up to 3 days, and slightly decreased from 7 days, but the value tended to decrease as the storage period became longer. however, 280 vol. 33/2022 a concrete with dry fly ash and fly ash slurry preparation s/a unit mass (kg/m3) % w c s g fa p 45 180 360 773 978 0 fa10in 324 768 971 36 fa20in 288 763 965 72 fa30in 252 758 958 108 fa120out 360 635 978 120 table 3. mix proportion. acceleration symbol condition w/b (%) neutralization rate coefficient (mm/ √ week) p − 50 3.70 a7d-10in d 4.76 s 4.24 a7d-20in d 4.72 s 4.69 a7d-30in d 7.13 s 6.96 a7d-120out d 38 4.98 s 4.45 table 4. neutralization rate coefficient. figure 5. a ash compressive strength. it showed almost the same compressive strength as p at all ages. as in the case of the slump test, it was clarified that, even within the range of this experiment, the performance was equivalent to that of d even when stored under stirring with s. 3.4. accelerated neutralization figure 8 shows the results of the accelerated neutralization test for concrete mixed with d and s in aăash, and table 4 shows the neutralization rate coefficients figure 6. a, b, c ash 20in compressive strength. for each. compared with p, the carbonation of concrete containing fa was more likely to increase in both d and s. it is considered that the alkali component in the concrete decreased due to the decrease in the amount of cement, and the progress of neutralization was accelerated. further, in the accelerated neutralization test, a long-term increase in strength due to a volcanic ash reaction was observed in high concentrations of carbon dioxide from an early age, this is probably because the densification of the or281 x. ji, k. takasu, h. koyamada, h. suyama acta polytechnica ctu proceedings figure 7. 20-in compressive strength for each age of a-ash s. figure 8. a ash accelerated neutralization. ganization was not considered. 3.4.1. drying shrinkage figure 9 shows the results of the concrete drying shrinkage test mixed with d and s of the ash. both d and s showed the same tendency in all formulations from the initial drying shrinkage value to about 4 months. although slightly smaller than p, the dry shrinkage value tended to be smaller when fa was mixed in all the formulations, indicating the effect of reducing the dry shrinkage of fa. 3.5. pore structure figure 10 shows the change in pore size distribution of a ash 20in, 120out, b ash and c ash 20in with time, divided into four ranges: 0.003µm 0.05µm, 0.05µm 0.5µm, 0.5µm 5µm, 5µm 100µm. it can figure 9. a ash drying shrinkage. be seen that the total pore volume tends to decrease with age for most formulations and ash types. in addition, it can be seen that in all mixtures and ash types, the value of the number of pores in the range of 0.5 to 5 µm and 5 to 100 µm hardly changes as the material ages. in the range of pore diameters from 0.003 to 0.05µm, a-ash 20in showed a slight increase tendency, but b-ash and c-ash did not show any increase or decrease. it can be considered that due to the hydration reaction, large pores with a diameter ranging from 0.05 µm to 0.5 µm transition to small diameters, and the number of pores has not changed significantly, and a similar trend has been observed in previous studies. in addition, it can be seen that the pore volume decreases most in the range of 0.05ăµm to 0.5 µm with the age of the wood, depending on the composition and ash type. from the above, it is inferred that the pore volume in the range of 0.05 µm to 0.5 µm is related to the compressive strength. figure 11 shows the relationship between the amount of pores in the range of 0.05 µm to 0.5 µm and the compressive strength. it can be seen that the relationship between the range of 0.05 µm to 0.5 µm and the compressive strength is high in all cases, regardless of the state, type, and composition of the fa. there is a possibility that the compressive strength is increased by decreasing the amount of pores in this range due to the hydration reaction and making the structure denser. previous studies have shown aăcorrelation with compressive strength in the range of 0.03 µm to 0.3 µm, confirming a similar relationship with pore volume. also, when comparing the amount of pores with the same level of compressive strength, if the amount of pores is the same, the compressive strengths are almost the same. therefore, it was shown that the pore volume could be inferred from the compressive strength even if the state of fa was different. 282 vol. 33/2022 a concrete with dry fly ash and fly ash slurry figure 10. pore size distribution. figure 11. compressive strength and pore volume (0.05 0.5µm). 4. summary concreteslumpsshowednodifferencewithin2hoursregardless of the fa condition. it is considered that there is no difference in slump properties within the range of this experiment even when s is continuously stirred and stored. regardingthecompressive strength, regardless of the state of fa, the long-term strength of 10in and 20in showed the same compressive strength as that of p, and it was shown that the compressive strength manifestation may be higher when s is mixed. regarding the neutralization rate coefficient, the result was larger whenfawasmixedregardlessof thefastate. theneutral acceleration coefficient tended to be smaller when s was mixed. from the relationship between the compressive strength and the amount of pores, it was shown that the compressive strength could be increased by decreasing the amount of pores in the range of 0.05 to 0.5 µm regardless of the state of fa. the drying shrinkage shows almost the same tendency regardless of the fly ash condition. in addition, it was confirmed that the drying shrinkage reducing effect of fly ash showed the same tendency regardless of the state. 283 acta polytechnica ctu proceedings doi:10.14311/app.2019.25.0048 acta polytechnica ctu proceedings 25:48–51, 2019 © czech technical university in prague, 2019 available online at http://ojs.cvut.cz/ojs/index.php/app in-situ compression test of artificial bone foams in controlled environment using x-ray micro-computed tomography jonathan glinza,∗, daniel kytýřb, tomáš fílab, jan šleichrtb, andreas schrempfc, david fürstc, johann kastnera, sascha sencka a university of applied sciences upper austria, stelzhamerstraße 23, 4600 wels, austria b czech academy of sciences, institute of theoretical and applied mechanics, prosecká 809/76, 190 00 prague 9, czech republic c university of applied sciences upper austria, garnsionstrasse 21, 4020 linz, austria ∗ corresponding author: jonathan.glinz@fh-wels.at abstract. in this study, we investigated specimens of artificial bone foams, developed by the research group for surgical simulators at the uas linz, which are used to mimic the haptic feedback of physiologic and pathologic bone for more realistic surgery training. specimens with two kinds of mineral filler material as well as different amounts of foaming agent were tested in an environmental in-situ loading stage developed by the itam cas and scanned via x-ray micro-computed tomography. in this in-situ stage, specimens can be immersed in liquid and tested under temperature-controlled conditions. consequently, a total amount of 12 specimens was subjected to compression loading; half of them immersed in water at 36.5 ◦c and half in dry condition. results showed that there is no significant influence of liquid immersion to the compression outcome. however, foams with less amount of foaming agent appeared to have smaller pores resulting in higher compression strength. keywords: x-ray micro-computed tomography, artificial bone foams, in-situ testing. 1. introduction the hybrid patient simulator devices developed by the research group for surgical simulators (ressl) at the uas linz use artificial bone foams to recreate the haptic feedback of human bone. these devices should thereby provide the possibility to train surgical procedures such as the application of pedicle screws and bone cement in a safe environment [1]. since excessive hours of training are essential to build up the experience necessary for surgeons, with this approach of a simulated patient, the use of actual specimens from human or animals can be reduced and cost as well as ethical questions satisfied [2]. however, because of the complex structure of bone and bone replacement materials, pore morphology and mechanical properties have to be investigated and visualized in a three-dimensional way in order to guarantee for characteristics comparable to actual human bone [3, 4]. e.g. simulation experiments by means of finite element methods can be performed to evaluate material parameters in a non-destructive (ndt) way. however, the reliability of such experiments highly depends on the accuracy of the acquired volume data. x-ray micro-computed tomography (xct) has established as a standard ndt method, which offers sufficient accuracy for the investigation of such materials [5]. for an in-depth characterization of histomorphometric features, digital volume data acquired by xct can be processed and visualized three dimensionally to determine bone volume fraction, cortical thickness and porosity. similar specimen of artificial bone foam developed by have already been characterized precisely in [6, 7]. however, these specimens were tested only in a dry state. since physiological bone typically is in a wet state, the main purpose of this study was the investigation of the influence of environmental conditions on artificial bone foams of varying composition. to this end, non-destructive xct in combination with destructive compression tests were carried out. 2. materials and methods 2.1. sample preparation in the course of this study, artificial bone foam specimens made from polyurethane resin with two different kinds of mineral filler material (caco3, 3ca3(po4)·cao) were produced. this mineral fillers were added in order to increase the density of the foams to match that of physiological bone more closely. to start the foaming process, water from 0.5 to 1.0 % by weight was added as blowing agent. the ingredients were mixed and filled into casts where the foam structure developed by chemical reaction. the test specimens were cut out of these casted foams using hollow-core drills. the final dimensions of the specimens are ∅12 × 20 mm as visible in fig. 1. 2.2. mechanical testing the specimens were tested in an in-situ loading stage, shown in fig. 2, which has been developed by the itam cas. the device consists of aluminum main 48 http://dx.doi.org/10.14311/app.2019.25.0048 http://ojs.cvut.cz/ojs/index.php/app vol. 25/2019 compression of artificial bone foams using µct figure 1. examples of cap (red) and cac (yellow) foams as casted (left) and the final specimens after hollow-core drilling (right). frame, hollow shaft stepper motor, harmonic gearbox (with standard accuracy about 1.5 arcmin) and carbon tube equipped with a upper support. in this stage, specimens can be immersed in liquid and tested under temperature-controlled conditions also. consequently, a total amount of 12 specimens were subjected to compression loading; half of them immersed in water at 36.5 ◦c and half in dry condition. for all displacementdriven tests, a uniform value of the loading rate was set to 1 mm/min and the experiments were performed up to ≈ 70 % of deformation. the position of the moving support was declared by a magnetic linear encoder with the resolution of 20µm. values of the actual force, performed by a loadcell with nominal capacity of 500 n (hbm u9b, germany) was recorded by a multifunction data acquisition device. the motion of the system and all other peripherals are controlled using a custom developed control software based on the open-source project linuxcnc running on the real-time kernel [8]. the control hardware consists of high-performance motion control components (mesa electronics, u.s.a.), sensor readout electronics (labjack t7, u.s.a.) and custom electronics. furthermore, a time-lapse in-situ investigation with xct scans in-between the load steps was performed for one specimen immersed in water to investigate behavior during load. 2.3. micro-computed tomography the xct scans were performed on a rayscan 250e system equipped with a 225 kv and 450 kv x-ray source and a 2048 × 2048 pixels flat panel detector as visible in fig. 3. for this study only the 225 kv source was utilized. to investigate the morphological changes caused by the compression of a specimen, a total amount of 6 xct scans was performed. the scanning parameters was set according to maximize signal to noise ration and to obtain reliable statistics for proper volumetric dataset reconstruction [9]. the first scan figure 2. environmental in-situ stage in closed carbon tube (left) and open state (right). figure 3. image of the rayscan 250e system with dual sources on the left and flat panel detector on the right. an exemplary specimen is positioned on the rotary stage in-between. was recorded during the initial, unloaded state of the specimen, followed by a scan after every load step applied. to prevent motion artefacts, 30 minutes of relaxation time was scheduled after every load step before starting the next scan to allow for sufficient stabilization of the sample. with the diameter given by the environmental stage, we were able to achieve a minimum voxel size of 10 × 10 × 10µm3. a tube voltage of 90 kv at a current of 115 kv was chosen to penetrate the samples inside the carbon tube of the in-situ stage. the xct scans were performed at a total amount of 1260 projections with an exposure time of 666 ms each. no additional pre-filter plates or averaging was applied. this sums up to a total scan time of 54 minutes for each scan. 2.4. image processing for the visualization of water immersion, a 5 × 5 × 5 median filter was applied to get rid of the most severe image noise. however, a quantification of water filled pore volume was not possible because of the low contrast between water and the artificial bone foam. 49 j. glinz, d. kytýř, t. fíla et al. acta polytechnica ctu proceedings figure 4. resulting stress-strain diagram of the time-lapse in-situ investigation performed on a caco3 filler specimen with 0.5 % foaming agent. the first two loading steps were performed with 0.6 % strain increment followed by 1.2 % steps. figure 5. cross section images of the time-lapse in-situ investigation. pores that filled with water between the load steps are indicated with red dots. therefore, the investigation of permeability was performed by measurement of remaining air inside the specimen. pores filled with air were segmented by an iso50 threshold between the air and bone foam material peak in the grey value histogram [10]. eventually detected bubbles of air outside of the bone foam material were removed manually from the data. 3. results and discussion the force resistance resulting from the time-lapse insitu experiments is visible in fig. 4. it can be seen that the maximum force response increases during the early load steps while steps from position 2 to 5 result in an approximately constant maximum forces. in accordance with these results, fig. 5 shows cross sectional images of the specimen at position 0 (initial state), 1 and 4. despite the open porous morphology of the foam, water immersed only partially into the foam, leaving pores closer to the center unfilled. pores indicated with red dots are empty in the shown position but fill with water in the course of the experiment. it can be seen, that during the first and last load steps a higher amount of water ingresses into the pores than from position 1 to 4. this indicates that more time for the intrusion of water into the pores is necessary at the beginning of the investigations. furthermore, the higher intrusion of water during the final load step might indicate that at this state deformation has already exceeded the elastic range and cell walls are being destroyed. in numbers, the volume of air filled pores was reduced by 15.82 % after the first load step and only by additional 0.73 % until position 4. during the final load step, air volume again decreased by 13.56 %. 50 vol. 25/2019 compression of artificial bone foams using µct the resulting forces during the continuous compression experiments performed on 12 samples are shown in fig. 6. each material composition (filler material and amount of foaming agent) was tested in dry condition and immersed in water. since both, dry and wet conditions, lead to comparable force response, no significant influence of liquid immersion to the compression outcome could be observed. however, foams with less amount of foaming agent show higher forces. this can be explained by a reduced average pore size resulting in higher compression strength. although one might argue that the cap filled sample with 0.75 % foaming agent shows slightly higher forces in comparison to the cac filled, a general trend of cap filler material leading to higher compression strength cannot be confirmed figure 6. stress-strain diagram of all tested specimens. dry specimens are shown in solid and specimens immersed in water as dashed lines. 4. conclusions concluding, the usage of the artificial bone foams investigated is despite their physiologically wet condition also valid in a dry state since environmental differences are nonessential for their mechanical properties. however, to confirm the influence of different filler materials on the mechanical properties of the foams, additional measurements with a larger sample size for statistical evaluation are necessary. furthermore, it must be considered that the influence of water immersion to the compression force results might be altered since not all pores were filled with water during the tests. for future studies, longer waiting times for the immersion of water or a pressurized intrusion should be considered. acknowledgements the research has been supported by the european regional development fund in frame of the projects competence center for high-resolution 3d x-ray imaging (atcz38) in the interreg v-a austria czech republic programme and by operational programme research, development and education in project inafym (cz.02.1.01/0.0/0.0/16_019/0000766). references [1] d. fuerst, m. hollensteiner, a. schrempf. a novel augmented reality simulator for minimally invasive spine surgery. vol. 46, pp. 200–204. 2014. http: //dl.acm.org/citation.cfm?id=2685617.2685645. [2] m. hollensteiner, d. furst, p. augat, et al. procedurespecific validation of artificial vertebrae. ieee transactions on biomedical engineering 65(8):1852– 1858, 2018. doi:10.1109/tbme.2017.2782797. [3] s. gomez, m. vlad, j. lopez, et al. characterization and three-dimensional reconstruction of synthetic bone model foams. materials science and engineering c 33(6):3329–3335, 2013. doi:10.1016/j.msec.2013.04.013. [4] h. chen, s. shoumura, s. emura, y. bunai. regional variations of vertebral trabecular bone microstructure with age and gender. osteoporosis international 19(10):1473–1483, 2008. doi:10.1007/s00198-008-0593-3. [5] t. fila, j. sleichrt, d. kytyr, et al. deformation analysis of the spongious sample in simulated physiological conditions based on in-situ compression, 4d computed tomography and fast readout detector. journal of instrumentation 13(11), 2018. doi:10.1088/1748-0221/13/11/c11021. [6] m. thompson, i. mccarthy, l. lidgren, l. ryd. compressive and shear properties of commercially available polyurethane foams. journal of biomechanical engineering 125(5):732–734, 2003. doi:10.1115/1.1614820. [7] k. calvert, k. trumble, t. webster, l. kirkpatrick. characterization of commercial rigid polyurethane foams used as bone analogs for implant testing. journal of materials science: materials in medicine 21(5):1453–1461, 2010. doi:10.1007/s10856-010-4024-6. [8] v. rada, t. fila, p. zlamal, et al. multi-channel control system for in-situ laboratory loading devices. in 16th youth symposium on experimental solid mechanics, vol. 18 of acta polytechnica ctu proceedings, pp. 15–19. 2018. doi:10.14311/app.2018.18.0015. [9] i. jandejsek, j. dammer, j. jakubek, et al. contrast optimization in x-ray radiography with single photon counting imagers of medipix type. journal of instrumentation 7(12), 2012. doi:10.1088/1748-0221/7/12/c12011. [10] a. kraemer, g. lanza. assessment of the measurement procedure for dimensional metrology with x-ray computed tomography. vol. 43, pp. 362–367. 2016. doi:10.1016/j.procir.2016.02.018. 51 http://dl.acm.org/citation.cfm?id=2685617.2685645 http://dl.acm.org/citation.cfm?id=2685617.2685645 https://doi.org/10.1109/tbme.2017.2782797 https://doi.org/10.1016/j.msec.2013.04.013 https://doi.org/10.1007/s00198-008-0593-3 https://doi.org/10.1088/1748-0221/13/11/c11021 https://doi.org/10.1115/1.1614820 https://doi.org/10.1007/s10856-010-4024-6 https://doi.org/10.14311/app.2018.18.0015 https://doi.org/10.1088/1748-0221/7/12/c12011 https://doi.org/10.1016/j.procir.2016.02.018 acta polytechnica ctu proceedings 25:48–51, 2019 1 introduction 2 materials and methods 2.1 sample preparation 2.2 mechanical testing 2.3 micro-computed tomography 2.4 image processing 3 results and discussion 4 conclusions acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.37.0016 acta polytechnica ctu proceedings 37:16–23, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague degradation of cr-coated cladding under the simulated loss-of-coolant accident phenomena petr červenkaa,b,∗, jakub krejčíb, ladislav cvrčeka, františek manochb, adéla chalupovác, patricie halodovád, petra gávelovád a czech technical university in prague, faculty of mechanical engineering, department of material engineering, karlovo náměstí 293/13, 120 00 prague, czech republic b ujp praha a.s., nad kamínkou 1345, 156 00 praha-zbraslav, czech republic c czech technical university in prague, faculty of mechanical engineering, department of energy engineering, technická 4, 166 07 praha 6, czech republic d research centre řež s.r.o., hlavní 130, husinec – řež, 250 68 husinec, czech republic ∗ corresponding author: p.cervenka@fs.cvut.cz abstract. the present study focuses on the evaluation of the mechanical properties degradation of cr-coated zr-alloy fuel cladding. the main objective of the work is to find a suitable methodology to evaluate the mechanical properties degradation of coated cladding by performing several separate effects experiments. apart from the many positive effects of protective coatings on the overall cladding properties, coatings’ general disadvantage is their reduced ability to tolerate plastic strain. therefore, coating cracks might occur in the first stage of the hypothetical loss of coolant accident (loca). the study is unique because of the consideration of coating cracks. prior to the high-temperature (ht) oxidation, samples were subjected to either a scratch test or burst test, resulting in the creation of coating defects. the subsequent evaluation of the obtained data consisted of wavelength dispersion spectroscopy (wds) and optical microscopy analysis and hydrogen content measurements. keywords: atf, loca, coating, oxidation, secondary hydriding, cladding embrittlement. 1. introduction following the events at the fukushima daiichi (2011) power plant, substantial effort has been made studying the idea of accident tolerant fuel (atf), which would significantly enhance nuclear safety. in the past decade, a large variety of atf concepts has been proposed and tested worldwide [1–9]. however, the most manageable and short-term approach to enhance the cladding performance is surface modification by specific technologies and various coatings. in particular, metallic cr has attracted many researchers due to its high melting point, superior corrosion resistance, good ht oxidation resistance, and similar thermal expansion coefficient of 6.2·10−6 k−1 compared to zr (i.e. 6·10−6 k−1) [10]. cr-based coatings’ good performance under both normal and accident conditions is based on the successful growth of a protective cr2o3 scale. cr coatings might significantly increase the fuel coping time by prolonging the oxidation time, which would lead to severe cladding embrittlement or even failure. it was confirmed as a promising atf concept mitigating the main limitations of zr cladding, such as fast degradation and an accelerated exothermic reaction with steam at high temperatures associated with hydrogen generation. however, the ht oxidation experiments performed so far have focused mainly on the performance of the ideal coating without defects [11–14]. according to existing studies, the ht steam oxidation process of cr-coated zr-based alloys can be divided into three groups. based on the duration and temperature of ht oxidation, the coating is protective, transition or no longer protective [15]. the protective coating features nearly parabolic oxidation kinetics with the growth of the outer protective cr2o3 layer. the loss of protectiveness is characterized by the slow acceleration of oxidation kinetics, which is due to zr diffusing outward along the cr grain boundaries and in the opposite direction the cr diffusing deeper into the β-zr phase. following the formation of the zro2 layer along the cr grain boundaries on the residual cr scale, the inward diffusion of oxygen through the zr network is induced. in the end, the coating loses its protective ability and the oxidation kinetics rapidly accelerates. this work aims to evaluate the behavior of the coating in case of cracks present in the protective coating. cracks in the coating may be formed, for instance, during fabrication, transportation, storing, normal operation (mostly grid-to-rod fretting, axial offset anomalies, etc.), or during a loca. in the event of loca in a pressurized water reactor, depressurization of the primary circuit may cause the fuel cladding to balloon as a result of internal pressure in 16 https://doi.org/10.14311/app.2022.37.0016 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 37/2022 degradation of cr-coated cladding under the loca phenomena figure 1. the schematic overview of lb-loca accident. the fuel rods. the limited ability of the coating to tolerate plastic deformation leads to the formation of coating cracks. details of the ballooning and burst phenomena were studied for various coatings [16]. regarding accident conditions, massive hydrogen uptake should also be considered as a possible cause of cladding failure [17–20]. the secondary hydriding phenomenon might occur in ht transients (e.g., loca) due to the possible formation of burst openings after the ballooning phase (see figure 1). the water steam flows through the burst area into the cladding. since the flow is impeded by nuclear fuel pellets, there is steam starvation inside the cladding. as long as the steam flow remains sufficient, the inner oxidation is present, and the oxide layer forms a barrier against hydrogen ingress. further from the burst opening, steam starvation might result in massive hydrogen uptake. hydrogen concentrates in β-zr phase (β-stabiliser) and can lead to cladding embrittlement due to the oxygen solubility increasing in β-zr phase [21, 22]. it is important to note that the thin oxide layer formed inside the cladding during normal operation is not sufficient to prevent the cladding from hydrogen uptake [20]. at high temperatures, the oxygen diffusion inward to the zr substrate can be assumed because of the high oxygen diffusion coefficients. in other words, the dissolution of the preexisting oxide layer is expected [23]. the preparation of the specimens and experimental procedures was carried out at ujp praha in collaboration with ctu in prague and research centre řež. the main goal of this study is to verify additional mechanical properties degradation phenomena that might occur under accident conditions and can lead to sever cladding embrittlement. 2. materials and methods 2.1. preparation of specimens all experiments were performed using standard zrbased alloy as a reference material. the as-received fully recrystallized cladding rods had an outer diameter of 9.1 mm and a wall thickness of 0.57 mm. figure 2. the chamber of the hauzer flexicoat 850 coating machine. the non-irradiated segments used for the burst tests were 90 mm long, whereas the specimens used for the scratch tests were 45 mm long. all specimens were deburred and ultrasonically cleaned according to the standard procedure (3 × 10 minutes in acetone, ethanol and distilled water, respectively). moreover, the surface of the specimens was cleaned in the first stage of the deposition process by ion etching in an argon plasma to remove all impurities and thin oxide layers. 2.2. coating deposition pure chromium coatings were deposited on the outer surfaces of tubular samples. the deposition was carried out using an industrial flexicoat 850 (hauzer) system, see figure 2. the facility may utilize different deposition techniques, such as sputtering, hipims, pacvd, and others. however, the unbalanced magnetron sputtering was used to obtain a thin and fully dense microstructure free of defects. the deposition process was carried out using a target of 99,6 % cr which was sputtered in the ar atmosphere at 250 °c. the temperature of the process was lower than the temperature during normal operating conditions in the core, therefore thermal modification of specimens was avoided. the thicknesses of the as-deposited coatings were estimated using a calotest (csm instruments), see figure 3. the resulting coating thickness was approx. 20 µm depending on the particular batch. 2.3. experimental methods in studying the effect of scratches on cladding degradation, two different surface damage techniques were used. the first method consisted of creating artificial defects, that is, scratches through the protective scale, 17 p. červenka, j. krejčí, l. cvrček et al. acta polytechnica ctu proceedings figure 3. calotest. on a scratch test device. this test allowed the creation of individual defects, which set optimal conditions for the detailed study of diffusion processes through the damaged coating during subsequent ht oxidation. the second batch of specimens was damaged naturally by the ballooning process due to internal pressure. afterward, each specimen was subjected to ht oxidation in ar + steam to simulate the loca scenario. the subsequent analyses focused on the diffusive process of cr into the zr substrate and the secondary hydriding phenomenon. 2.3.1. scratch test the first batch of cr-coated specimens was artificially pre-damaged to simulate cracking of the coating layer during the ht transient. using the revetest xpress device (csm instruments) with a standard rockwell diamond indenter, several scratches through the coating layer were made. scratches 40 mm in length were made at the speed of 20 mm·min−1 using the constant load of 50 n. the scratch test process is shown in figure 4. the resulting u-shaped scratches were 200 µm wide. 2.3.2. ballooning and burst test the ballooning was performed in a burst test facility consisting of a resistance furnace, temperature controller, and argon containing reservoir to pressurize the specimen. the specimens were sealed using endplugs and connected to the argon-containing reservoir. each specimen was equipped with a pair of k-thermocouples – one attached next to the specimen’s surface and the second welded directly to the specimen. various methods may be chosen to perform the burst test. a ramp test, the course of which is visualized in figure 5, was selected. first, the specimen was preheated at 360 °c and pressurized to an inner pressure of 4 mpa. then, the pressurized specimen was heated at a heating rate of 7 °c·s−1 until its failure (899 °c). a constant flow of argon was induced to the retort throughout the experiment to prevent ht figure 4. scratch test revetest xpress. figure 5. time course of the burst test. oxidation. after the test, the specimens were visually analyzed and photographed. subsequently, the time to burst, the deformation, and the creep rate were evaluated. 2.3.3. high-temperature oxidation the experiment of ht oxidation was performed in the electric resistant furnace in the ar + steam mixture according to the scheme shown in figure 6. the steam atmosphere is typical for dbas such as loca. in addition to that, steam oxidation might be followed by secondary hydriding, a phenomenon that might occur in the cladding after burst due to inner oxidation and 18 vol. 37/2022 degradation of cr-coated cladding under the loca phenomena figure 6. schematic of the facility for the ht oxidation experiment. steam starvation. the experiment was conducted at the temperature of 1200 °c, which is considered the highest temperature of design basis accidents. the temperature was measured by the thermocouple placed inside the cladding tube in case there were no end caps on the specimen (i.e., specimens after scratch test, figure 4). specimens predamaged by the burst test were tested for oxidation and secondary hydriding, thus they were filled with zro2 pellets and sealed with end plugs to impede the steam flow inside the cladding. the schematic overview is shown in figure 7. the temperature in these experiments was measured by the thermocouple placed next to the outer cladding surface. experiments were performed following this procedure. first, the atmospheric gases were removed from the preheated furnace by a constant flow of ar for 5 minutes. then, the flow of ar + steam stabilized the homogeneous conditions inside the quartz tube for 5 minutes. finally, the specimen was inserted into the furnace within the predefined temperature zone (±3 °c). the duration of ht oxidation ranged from 90 seconds to 9 minutes. each experiment was terminated by iced water quenching. the reason behind this is that quenching directly from the oxidation temperature to 0 °c is a less conservative approach, more demanding on the mechanical properties. 2.3.4. material characterization before and after each experiment, the specimen was weighed with mettler toledo xs105 semi-micro balfigure 7. schematic overview of the secondary hydriding after burst test experiment. figure 8. ring segments cut for microscopy analysis. ance with the readability of 0,01 mg. furthermore, the longitudinal dimensions were measured using an electronic caliper with an accuracy of 0,02 mm. the diameter was measured with a micrometer with an accuracy of 0,002 mm. after evaluation in the whole state, the specimens were cut into smaller pieces (rings) to perform additional analyzes. specimens with single cracks made on the scratch test device were observed using the scanning electron microscope (sem) with a field emission gun operated at an accelerating voltage of 15 kv. the test aimed to observe and evaluate diffusive processes through the damaged coating and the distribution of chemical composition in the affected area. the distribution profiles of zr, cr, o, n, and fe were obtained from wavelength dispersion spectroscopy (wds) analysis. the spectra were automatically processed by incawave software. the wds chemical composition profiles were measured in two directions – perpendicular to the coating surface in the area not directly affected by the defect and parallel to the coating surface at the distance of 25 µm. regarding the ht oxidation after the burst experiments, the specimens were cut according to the following procedure. the first segment was cut in the middle of the burst, the second segment was located 10 mm from the burst opening and the third segment was located 20 mm from the burst opening, see figure 8. moreover, the hydrogen content was measured on a small part of each of these segments using the g8 galileo (bruker) analyzer. the inert gas fusion (igf) technique is based on fusing the tested macroscopic sample and thus is limited to obtaining only the peak hydrogen content, not its distribution. 3. results in this section, the results are shown and commented. first, wds profiles of artificially predamaged (scratched) specimens are presented. then the results of the ht oxidation after the burst test 19 p. červenka, j. krejčí, l. cvrček et al. acta polytechnica ctu proceedings figure 9. wds composition profile perpendicular to the coating, 4,5 min ht oxidation. figure 10. wds profile measured parallel to the coating at a distance of 25 µm, 4,5 min ht oxidation. are presented; first, the optical microscopy structure is discussed and then the measured hydrogen content is shown. 3.1. chemical analysis – ht oxidation after scratch test the chemical composition measured perpendicular to the outer surface (blue line, figure 9) clearly shows the diffusion of cr from the coating inwards the zr substrate. after a 4,5-minute exposition at 1200 °c, the cr diffused approx. 150 µm in an inward direction. as long as there was some amount of cr diffused, the measured content of oxygen remained stable yet increased, i.e. approx. at 2–2,5 %. once the zero value of cr is reached, the content of diffused oxygen drops significantly to 1,5–2 %. the same, i.e., stable yet increased, oxygen content (∼2–2,5 %) was also measured in a parallel line to the outer surface (yellow line, figure 10). there was no noticeable gradient of oxygen concentration along the entire length of the measured β-zr section. 3.2. metallography – secondary hydriding the detailed microstructure of all 3 segments is shown in figure 11. cracking of the coating formed within the ballooning and burst test is apparent. the number of coating cracks depended on the size of the deformation. therefore, the segment 1 with the highest deformation had the largest number of coating cracks. furthermore, part of segment 1 near the burst opening consisted of a noticeably thinned cladding wall due to deformation in the ballooning phase. finally, segment 3 located 20 mm from the burst opening featured substantially fewer uniformly distributed small cracks. in the microstructure of segment 1, a massive growth of the α-zr(o) can be seen. the cr scale is massively cracked and there are no β-zr sections directly under the coating. on the other hand, segments 2 and 3 contained significantly fewer coating cracks, which contributed to the formation of β-zr segments in between. the growth of α-zr(o) grains from defects was apparent in the inward direction. due 20 vol. 37/2022 degradation of cr-coated cladding under the loca phenomena figure 11. results of secondary hydriding experiments. to the cr stabilizing the β-zr phase, no growth of αzr(o) grains was observed in the tangential direction, in other words, there was no α-zr(o) grains growth under the cr scale. the cr enriched β-zr phase itself can be slightly seen, however, the true evidence of the cr diffusion inwards the substrate remains the wds analysis. regarding the internal oxidation, the decreasing thickness of the oxide layer further from the burst opening was evident. furthermore, the growth of α-zr(o) grains + α-zr(o) grain incursions inwards was less evident further from the burst area, and additionally, in segment 3 was not observed at all. 3.3. hydrogen content – secondary hydriding following the expected consequences of the secondary hydriding phenomenon, the hydrogen content grew with increasing distance from the burst opening (see figure 11). while a very low hydrogen content (122 wppm) was measured at the area of the burst (i.e. segment 1), it considerably increased in segments 2 and 3. particularly, the hydrogen content measured at a distance of 20 mm from the burst opening reached almost 2000 wppm. 4. discussion while the positive effect of undamaged cr coating on oxidation performance has been widely presented and indeed remains undeniable, additional phenomena must be taken into account when evaluating coated cladding embrittlement. it was observed that the cr content led to increased oxygen solubility in the substrate (figure 9). since the oxygen content was at the same level throughout the measured β-zr section (figure 10), it can be concluded that loss of protectiveness of the coating and therefore diffusion of oxygen through the coating would not be the only mechanism of degradation of the coating. apart from the inward growth of α-zr(o) grains from the coating defect, which can be evaluated using optical microscopy methods and seem to be very local (as discussed, e.g. in brachet’s study [24]), the considerable amount of oxygen rapidly diffused into the cr-enriched substrate. the reason behind this is cr stabilizing the β-zr phase. the diffusion coefficient of oxygen in β-zr is much higher compared to α-zr(o) phase [25, 26] resulting in the accelerated 21 p. červenka, j. krejčí, l. cvrček et al. acta polytechnica ctu proceedings oxygen dissolution in the cr-stabilized β-zr phase. therefore, the impact of the coating defects cannot be excluded as the oxygen rapidly diffuses along the whole cr scale. on the basis of the results presented, it can be assumed that the impact of oxygen diffusion through the coated cladding upon accidents such as loca cannot be described only by looking at the protectiveness of the coating itself. the methodology must take into account the coating cracks that occur in the ballooning phase. they will lead to an accelerated diffusion of oxygen over the ht transient. the β-zr phase will be affected. as a result, the load-bearing ability can be significantly reduced even under conditions (temperature, time) so far believed not to result in the loss of coating protectiveness. the separate-effects-combining study confirmed the secondary hydriding phenomenon occurring in crcoated zr-based cladding exposed to the ballooning and burst experiment first and then ht oxidized in ar + steam. the hydrogen content measured in segment 2 and especially in segment 3 was high enough to increase the solubility of oxygen. especially in the load-bearing prior β-zr phase, such hydrogen uptake can result in cladding failure and must be taken into account. 5. conclusion to summarize the observations, using the cr-coated standard zr cladding alloy, separate effects experiments were performed. specifically, ballooning and burst, ht ar + steam oxidation, and secondary hydriding (ht oxidation after burst). the study was unique considering the cracking of the coating that occurred in the first stage of the hypothetical accident. the wds analysis proved the cr diffusion into the β-zr substrate leading to accelerated diffusion of oxygen and its increased concentration even under the coated (thus so far believed to be protected) areas. accelerated diffusion of oxygen, as well as its increased solubility, can affect mechanical properties. in particular, the ductility of the residual prior β-zr phase, which is essential to maintain the integrity of the cladding, can be reduced. therefore, it is necessary to consider the cracking of the coating when evaluating the degradation processes of the coated cladding. the secondary hydriding phenomenon was observed when ht oxidation was performed using the specimens after ballooning and burst test. massive h uptake (2000 wppm) occurred at a distance of 20 mm from the burst opening. the larger the deformation, the more cracks occurred in the cr scale. additionally, significant clad thinning was observed near the burst opening area. the hydrogen content measurement has been limited to macroscopic samples. its detailed distribution across the cladding wall remains a matter of concern, since absorbed hydrogen can increase the oxygen solubility in β-zr and have a huge impact on mechanical properties, i.e., on the cladding embrittlement. while uncoated zr-based claddings and their postaccident properties can be evaluated using standard methodology, such testing is not suitable for cr-coated claddings due to more phenomena that must be taken into account. as the coating cannot withstand high plastic strain, various cracks in the cr scale are likely to occur during heat-up. moreover, in the case of a burst, local massive hydrogen uptake must be considered. it can be concluded that evaluating cr-coated zr cladding embrittlement requires complex testing that includes all of the loca phenomena, respectively. acknowledgements the authors wish to thank the whole zirconium alloys team at ujp praha a.s. for sample preparation and post-experimental analyses. special thanks to p. halodová and p. gávelová (research centre řež s.r.o.) for performing the sem analysis, and ladislav cvrček (ctu in prague) for chromium coating deposition. the financial support of this research through čez a.s. company is gratefully acknowledged. also, this work was supported by the institutional support by ministry of industry and trade, technology agency of the czech republic grant no. tk03020169. references [1] s. zinkle, k. terrani, j. gehin, et al. accident tolerant fuels for lwrs: a perspective. journal of nuclear materials 448(1):374–379, 2014. https://doi.org/10.1016/j.jnucmat.2013.12.005. 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[26] r. perkins. oxygen diffusion in β-zircaloy. journal of nuclear materials 68(2):148–160, 1977. https://doi.org/10.1016/0022-3115(77)90234-3. 23 https://doi.org/10.14311/app.2020.28.0001 https://doi.org/10.1016/j.net.2017.12.004 https://doi.org/10.3390/coatings10090835 https://doi.org/10.1016/j.jnucmat.2019.03.039 https://doi.org/10.1016/j.surfcoat.2018.05.088 https://doi.org/10.1016/j.jnucmat.2015.06.016 https://doi.org/10.1002/maco.201810156 https://doi.org/10.1016/j.corsci.2020.108537 https://doi.org/10.14311/app.2019.24.0009 https://doi.org/10.1520/stp159720160063 https://doi.org/10.1520/stp159720160031 https://doi.org/10.1016/j.nucengdes.2012.10.024 https://doi.org/10.1016/j.jnucmat.2017.03.009 https://doi.org/10.1016/j.jnucmat.2015.11.019 https://doi.org/10.1520/stp37540s https://doi.org/10.1016/j.jnucmat.2019.02.018 https://doi.org/10.1149/1.2129227 https://doi.org/10.1016/0022-3115(77)90234-3 acta polytechnica ctu proceedings 37:16–23, 2022 1 introduction 2 materials and methods 2.1 preparation of specimens 2.2 coating deposition 2.3 experimental methods 2.3.1 scratch test 2.3.2 ballooning and burst test 2.3.3 high-temperature oxidation 2.3.4 material characterization 3 results 3.1 chemical analysis – ht oxidation after scratch test 3.2 metallography – secondary hydriding 3.3 hydrogen content – secondary hydriding 4 discussion 5 conclusion acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.38.0642 acta polytechnica ctu proceedings 38:642–648, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague circular design in the global south pekka huovilaa,∗, usha iyer-ranigab, christina cheongc, riya malhotrad, yatin choudharyd, guillermo penagose a green building council finland, lapinrinne 3, 00100 helsinki, finland b rmit university, 124 latrobe street, melbourne vic 3000, australia c global green growth institute, 19f jeongdong building, 21-15 jeongdong-gil, jung-gu, 04518 seoul, republic of korea d the energy and resources institute (teri), darbari seth block, ihc complex, lodhi road, 110 003 new delhi, india e càtedra unesco de sostenibilitat, universitat politècnica de catalunya, eseiaat – campus terrassa, c/colom, 1, 08222-terrassa, barcelona, spain ∗ corresponding author: pekka.huovila@figbc.fi abstract. one planet sustainable buildings and construction programme (sbc), led by the ministry of the environment, finland and co-led by rmit university and un environment programme was initiated in 2015. work on circular built environments commenced in the second iteration of the programme’s work plan. sbc was the first global programme that worked on circularity and responsibly sourced materials in the buildings and construction sector. in 2020, the sbc programme published a range of reports focusing on the state of play for circular built environments across specific regions, tied together with a global report. the present focus of the programme is on africa, asia and latin america, where case studies are collected following a common framework. these case studies together with a global survey provide reliable performance data for responsibly sourced building materials in the global south. the underpinning premise through this process is to support related sdgs across the social, environmental, and economic considerations and enable countries to achieve their targets under the paris agreement. this paper presents key findings from this study, largely derived through case studies in the global south. the results show that not all stages of the building life cycle are addressed through local examples. keywords: case studies, circular built environment, life cycle, sustainable buildings and construction, sustainable development goals (sdgs). 1. introduction the buildings and construction sector has an important impact on the planet. the sector is often a test of how well economies are performing, as this sector contributes to local employment and revenue generation, yet, also influences environmental impacts, being quite resource intensive. global material use has been increasing and expected to increase in the near future due to the increased urbanisation in the african, asian and latin american regions. material use expected to increase are steel, cement and glass with concrete alone contributing to about 12 % of global emissions in 2060 [1]. this paper builds on work undertaken by the one planet sustainable buildings and construction programme [2] in 2019-20 on circular built environment and launched at the beyond 2020 conference [3]. the global report presents ten recommendations [4]. these are integrating circular principles into planning and design, developing and applying circular built environment indicators, considering building and material lifecycles, developing innovative and new circular building materials and techniques, planning for resilience, drawing on local and indigenous solutions, developing new circular business models, developing a culture of collaboration, supporting skills and capacity building, and creating new financing and procurement models that support sustainability. even though the un 2030 sustainable development goals (sdgs) do not specifically refer to circular economy, the principles underpinning them are aligned with circularity. circular practices are directly linked to sdg 12 on responsible consumption and production. circularity can support the move to a less resource intensive building and construction sector. circularity can be applied to different phases of the building lifecycle. these are: (1.) design: site planning that influences design of the building (e.g. modular design) as well as of the building products, (2.) construction: technical know-how of construction, (3.) building operation: use of the buildings and maintenance, (4.) renovation: refurbishing, renovating or changing 642 https://doi.org/10.14311/app.2022.38.0642 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 38/2022 circular design in the global south the building for adaptive reuse, (5.) post-life or deconstruction: reuse as well as recycling of materials and building elements, and (6.) manufacturing: both of the products and the construction of buildings that are needed for construction and operational maintenance or for renovation/reconstruction. these steps are derived from iso standards (international organisation for standardisation) [5]. the objective of the research was to consider the different life cycle phases of buildings and construction using iso standards [6] alongside the triple bottom line (tbl) approach [7] capturing environmental, economic, and social considerations in the form of the sdgs [8]. in doing so, the paper focuses on highlights from three regions in the global south: africa, asia and latin america. the underlying intent of this study is to share knowledge on circular practices in these regions, support place-based knowledge and practices, and support local demonstrations of circular practices. analysis of the case studies are undertaken for different building life cycle phases, and across the tbl represented through selected sdgs. the analysis is followed by conclusions. 2. research approach case study research has been used commonly in the built environment and business disciplines to analyse specific issues within a narrow and well-defined scope [9]. the approach taken in this research is an in-depth investigation of circularity approaches and outcomes across particular types of buildings to understand the drivers and challenges in achieving these outcomes across the selected regions. top-down and bottom-up initiatives were considered. data was collected by the regional authors through a process of selection through literature reviews (grey and white) and selective interviews with the stakeholders involved in design, construction, and operation of buildings. a template was developed, so there is consistency in the information collected across the three regions. researchers across the regions were guided by the template in data collection. 3. highlights from africa africa, as a region, is one of the least developed in the world. 29 out of the lowest 32 countries ranked low in human development index are from africa [10]. the region is, however, the fastest growing in the world, with the united nations estimating the population to reach 2.5 billion by 2050 [11]. the rapid population increase, coupled with increasing wealth and urbanization, drive the demand for new buildings and infrastructure in every country across the continent. the building and construction sector of africa is worth usd 5.4 billion, and it is expected to increase at a compound annual growth rate of 6.4 % by 2024 [12]. in 2019, buildings used 57 % of the total energy consumption and accounted for 32 % of the process-related co2 emissions in africa [12]. this may partly be ascribed to the linear operation of the built environment where materials are extracted, used in buildings and discarded as waste. 3.1. green building minimum compliance system, rwanda the rwanda green building minimum compliance system [13] has been developed as a holistic system with a set of green building indicators to guide building designers and practitioners to integrate energy and water efficiency, environmental protection, better indoor environmental quality and green innovation in new buildings during the design, construction and operational stages. compliance with gbmcs is mandatory for new and larger commercial and residential buildings and major refurbishments and serves as. an annex-3 to the rwanda building code 2019. it has been recognized as an exemplar adopted by african government to promote green buildings. gbmcs was developed with technical support local and overseas partners. implementation of the gbmcs is led by the rwanda housing authority (rha) with support from 30 district one stop centers located throughout the country. since its adoption in 2019, several new developments have applied the gbmcs in the design and construction phases of the projects. over time, the rha plans to integrate the gbmcs into an online portal for building permit applications. at present, resource limitations to country-wide sensitization and roll out of the gbmcs result in a more gradual implementation of the new system. the rha plans to extend the gbmcs to cover all building typologies in rwanda in the long term. the same approach of developing a customized policy for green buildings can be replicated for different countries. 3.2. upcycle africa (sina village), uganda upcycle africa is a social enterprise based in uganda. they aim to address the plastic pollution problem while creating employment opportunities for disadvantaged women and youth in uganda. the enterprise conducts initiatives to sensitise high school students to the problems caused by plastic pollution and instil good waste management practices. over 20 000 students have been trained since the founding of upcycle africa in 2015. the organization also operates a waste picker’s program, employing women and youth from marginalized communities. the program focuses on plastic waste collection from beaches, streets and public spaces. the waste collected is sorted, with some sold to recycling companies and suitable plastic bottles being upcycled for construction. upcycle africa estimates over 3 million plastic bottles have been removed from the waste stream through their program [14]. 643 p. huovila, u. iyer-raniga, c. cheong et al. acta polytechnica ctu proceedings figure 1. house in sina village, uganda constructed by upcycle. credit: gggi. suitable plastic bottles retrieved from the environment are used for construction of affordable housing. the bottles are filled with compacted soil by hand, increasing their compressive strength and thermal insulating properties. these bottles replace bricks as walling material to produce houses which are robust and affordable. the structures are durable, comfortable and earthquake resistant (see figure 1). upcycle africa has constructed houses in different parts of africa while creating employment opportunities for rural women and youth, bringing economic empowerment for them. this livelihood opportunity has a further benefit of enabling low-income families to pay the school fees for their children to access education. 4. highlights from asia the urban population of south asia is expected to increase by nearly 250 million people by 2030, leading to a shortage of affordable housing and resulting in unplanned urbanization with 130 million people living in slums [15]. the lack of affordable housing affects not only the poor but also many middle-income households. it is estimated that 203 million housing units will be needed in south asia from 2010 to 2050 to accommodate the projected growth of the urban population [15]. asian nations have begun to recognize the benefits of circular models for their economies and are taking steps to implement circular economy principles and closed-loop systems through regulatory measures and national policy frameworks. two examples from pakistan and india demonstrate this trend. 4.1. denso hall rahguzar walking street for regeneration of karachi’s historic core, pakistan urban disarray in developing countries, such as pakistan, is a growing problem due to poor governance and lack of funding for urban services. in response to this issue, the denso hall rahguzar street project represents an effort in low-carbon, climate-smart ecourban regeneration in karachi’s historic core. this figure 2. before and after images of the denzo hall rahaguzar street. credit: heritage foundation, pakistan. area has suffered from high-density urban issues, including an ineffective waste management system, deterioration of historic buildings and public spaces, and overcrowding. the project is a collaborative effort between civil society organizations, local administration, and the community. it was conceived and designed by architect yasmeen lari, and was financially supported and implemented by the heritage foundation of pakistan in partnership with the karachi government departments [16]. the project adopts a sustainable approach that incorporates locally sourced and hand-crafted biodegradable materials, minimized water usage and waste, and a focus on refurbishing historic structures to reduce energy consumption (see figure 2). the project features a landscaped walking street that promotes commerce, recreation, creative and cultural activities. it also provides a healthier environment through the planting of 600 trees to absorb carbon, restricting vehicle entry to prevent air and visual pollution, and using permeable pavements to mitigate urban flooding and heat. other sustainable features include the use of recycled wood for lamp posts and cast-iron elements from discarded materials in entry gates, aquifer pits for rainwater harvesting, and the reuse of grey water for sustaining vegetation. the project has had a positive impact on the local community, with increased safety and job opportunities, and has become a source of pride and ownership for shopkeepers located in the street. this initiative demonstrates the potential for urban regeneration through the application of sustainable and eco-friendly approaches. 4.2. c&d recycling waste plant, surat, india india generates approximately 150 million tons of construction and demolition waste each year, however, only a small portion, around 1 %, is recycled, with a daily rate of 6 500 tons [17]. a state-of-the-art mobile recycling facility was established in surat, in644 vol. 38/2022 circular design in the global south dia through a public-private partnership to process 300 tons of waste daily, in compliance with the c&d waste management rule 2016 of the ministry of environment, forest, and climate change. the waste is collected from 10 collection centers and processed using reurban compact technology, resulting in the recovery of high-quality sand (50 %), aggregates (28 %), and soil (18 %). these recycled materials are then utilized in construction and for producing certified products, such as tiles and blocks, which are sold at a 30 % discount with a buyback option by the local government. this circular waste recycling model offers significant environmental benefits compared to landfilling and is being replicated in other cities in india. although the capital and operational costs are high (usd 1 481 730), it will reduce 508 tons of co2 emissions, the equivalent of burning 250 000 kg of coal annually, save 9100 giga joules of energy, enough to power 3 000 homes in india for a year, free up 30 000 yd2 of land from landfills, and unlock 100 crore (or ten million) worth of real estate annually [18]. 5. highlights from latin america and the caribbean (lac) current population in lac region is 659 million inhabitants [18]. despite declining rates of population in the region generally, by 2050, the population is expected to reach 759 million. currently, 81 % of its population live in cities being the most urbanized developing region in the world [19]. although highest rates of urban population growth in lac region occurred during the 1950s to 1970s, driven by ruralurban migration, urbanization will continue growing and will reach 90 % in 30 years [20, 21]. although there has been a decline in overall population, the region has been affected by increased rural-urban migration. consequently, population has been increasing in the cities. the two case studies chosen from latin america are focused on a material use and an urban regeneration project. 5.1. cemex circularity strategy cemex is a global building material manufacturer focusing on concrete and cement products. this case study covers multiple countries in the region: colombia, costa rica, dominican republic, el salvador, guatemala, jamaica, mexico nicaragua, panama, peru, puerto rico, and trinidad and tobago. the company’s mission is to “create sustainable value by providing industry-leading products and solutions” [22] which is expressed through their priority sdgs focusing on sdgs 8 (economic growth), 9 (industry, innovation and infrastructure), 11 (sustainable cities and communities), 13 (climate action) and 15 (life on land). these sdgs are relevant to their overall goal of increasing their economic revenue while creating innovative solutions to reduce impact on local air, water and waste and conserving biodiversity. thus, their sustainability model covers environmental, economic and social governance. cemex’s environmental focus is on a net zero co2 emission strategy by creating a circular economy through use of zero carbon concrete and water, implementing biodiversity conservation, and preserving water on site. their work in zero carbon concrete is achieved through clinker plants using biomass and non-recyclable waste as a source of energy; use of alternative raw materials and industrial waste/by-products to manufacture cement and use of recycled aggregates from construction and demolition waste and ensuring transport emissions are offset via a planned afforestation program. from a water perspective, nearly all their water is recirculated and recycled in the production process. a concrete plant in colombia is treating water from the food sector and reusing it for concrete production in a joint initiative with a food company “meals de colombia s.a”. rehabilitation of quarry sites post extraction is part of the company’s commitment to ensure regeneration where biodiversity is monitored. in addition, health and safety considerations, respect of human rights and diversity, equity and inclusion are all important social considerations that underpin cemex’s approach to sustainability. figure 3. moravia before intervention. credit: unesco chair on sustainability. 5.2. moravia urban renovation project. medellin, colombia from 1970-80, “morro (hill) moravia” was medellin’s municipal dump. the poor used this hill for recovering waste as a source of income. the dump was eventually closed, and the closure attracted informal settlement in the area. by the turn of the 21st century, the population increased leading to large informal settlement in the same area. urban regeneration and social intervention supported by spatial and physical planning resulted in de-risking the space. in 2007, a project was developed by the medellín mayor’s office, with the cooperation of the government of catalonia and the participation of the unesco chair of sustainability, 645 p. huovila, u. iyer-raniga, c. cheong et al. acta polytechnica ctu proceedings figure 4. moravia after intervention. credit: unesco chair on sustainability. with the aim of improving life conditions for the local community [1] while reducing environmental risks such as fires, landslides and environmental pollution. the main achievements of this project resulted in improvement in the life of poor parts of moravia, due to the focus on environmental and landscape recovery of such a degraded urban area as shown in figures 3 and 4. the stakeholders involved are medellin mayor office and community action boards. the main outputs as a result of this project are land remediation and relocation of are over 2 220 families. in total, 4.4 hectares of land was remediated by phyto-remediation, using buffer-strips and constructed wetlands. hundreds of jobs were created on community gardening and flowering production, where compost from organic waste is used as substrate. the rate of urbanisation in the latin american region has shown an increase of 73 % since 1990 [22]. the urban sprawl has been driven by two main factors: informal urban development, and sub-urbanization produced by formal development of new residential areas occurring at the urban peripheries [23–25]. 6. analysis and discussion through literature review and stakeholder interviews, the case studies presented in this paper are quite varied. to promote circularity in the built environment, mapping and promotion of best practices are crucial. these practices are context dependent and need to consider a range of localised considerations such as skills and capacities, business practices and culture, manufacturing capacities, local regulations and the like. focusing on the capital cities of burkina faso, rwanda, senegal and uganda, the africa research team found an encouraging number of initiatives promoting circularity in buildings. these initiatives span a range, including bottom-up and top-down approaches, with direct benefits of reducing construction and demolition waste, as well as the demand for raw building materials. many of the projects also produce other social and environmental co-benefits. due to the high level of informality of the building sector in many african countries, the transfer of building and construction materials between parties is often casual and undocumented. government policies are gradually catching up to provide an operational framework within which material and water circularity can be promoted. building and construction stakeholders in asia have a long history of incorporating sustainable practices, such as reusing, renovating, sharing, and upgrading products. this results in an advanced form of circular economy. the push towards sustainability in this region is largely driven by government regulations, policies, and technological advancements in the construction industry. however, these policies need to be further advanced. the latin american case studies focused on building materials manufacture across several countries in this region and on an urban development project. the collaboration and leadership shown across both these projects clearly demonstrates replicability and scalability across not just the latin american region but also into other parts of the world. the case studies have been analysed from a life cycle perspective using iso life cycle approach, and following the approach undertaken by iyer-raniga and huovila [4]. the analysis is across the main goals underpinning the sdgs rather than considering the indicators in each of these goals. table 1 shows that while most of the life cycle phases have been considered across the case studies, renovations and end of life considerations need more attention. 7. conclusion this paper focused on a snapshot study to understand what if any, and how circularity practices in the built environment in africa, asia and latin america existed. the presence of initiatives as shown from the eclectic samples presented from urban design solutions to building code development and material considerations show a willingness and clear opportunities to demonstrate circular practices across many 646 vol. 38/2022 circular design in the global south africa asia lac africa asia lac africa asia lac africa asia lac environmental impact sdg6 sdg7 sdg12 sdg15 life cycle stage water energy materials and waste biodiversity design x x x x x x x construction x x x x x x x operation and use x x x x x x x x renovation x x deconstruction/end of life x x x manufacturing x x x x africa asia lac africa asia lac social impact sdg3 sdg8 life cycle stage health and well-being green jobs operation and use x x renovation deconstruction/end of life manufacturing x design x x x x construction x x x x africa asia lac africa asia lac africa asia lac economic impact sdg11 sdg12 sdg9 life cycle stage capital cost operational costs new businesses deconstruction/end of life manufacturing x x x x x x design x x x x x x construction x x x x x x x operation and use x x x x x x renovation x x table 1. analysis of the case studies. of the sdgs. while there are some examples available, it is clear that considerable barriers exist to the institutionalization and mainstreaming of such initiatives and substantial investment in institutional and capacity development will be required. generally, the transition to circularity in the built environment offers opportunity for countries in the identified regions to improve waste management, reduce cost of buildings, create new value chains and jobs and significantly reduce the dependence on material imports to fulfil the demand for housing and other buildings spaces. references [1] organisation for economic co-operation and development (oecd). global material resources outlook to 2060: economic drivers and environmental consequences – highlights. oecd publishing, paris, 2018. [2] one planet network (opn). sustainable buildings and construction (sbc) programme, 2021. [2021-09-21]. https://www.oneplanetnetwork.org/sustainablebuildings-and-construction [3] beyond 2020 special track. circular built environment – a global perspective, world sustainable built environment online conference, 2020. [2021-09-20]. https: //www.oneplanetnetwork.org/circular-transitionglobal-south-learnings-north-east-and-west [4] u. iyer-raniga, p. huovila. global state of play for circular built environment. a report on the state of play on circularity in the built environment across africa, asia, europe, gulf cooperation council countries, latin america and the caribbean, north america and oceania, united nations one planet network sustainable buildings and construction programme, 2020. [2021-0920]. https://www.oneplanetnetwork.org/resource/ global-state-play-circular-built-environment [5] international organisation for standards (iso). about us, 2021. [2021-10-05]. https://www.iso.org/about-us.html [6] iso 14001. iso/tc 207/sc1 environmental management systems. 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[2021-10-08]. https://www.worldometers.info/world-population/ latin-america-and-the-caribbean-population/ [20] a. arsht. urbanization in latin america. atlantic council, 2014. [2021-10-08]. https://www.atlanticcouncil.org/commentary/ article/urbanization-in-latin-america/ [21] cemex. sustainability, building a more sustainable future. [2021-10-08]. https://www.cemex.com/sustainability/overview [22] celade – population division of eclac, united nations, department of economic and social affairs, population division. world population prospects, 2019. [2021-10-08]. https://www.cepal.org/es/temas/proyeccionesdemograficas/estimaciones-proyecciones-excel [23] l. inostroza. informal urban development in latin american urban peripheries. spatial assessment in bogotá, lima and santiago de chile. landscape and urban planning 165:267–279, 2017. https: //doi.org/10.1016/j.landurbplan.2016.03.021 [24] inter-american development bank. evaluación de la iniciativa ciudades emergentes y sostenibles del bid. banco interamericano de desarrollo. oficina de evaluación y supervisión. 1350 new york avenue, n.w. washington, d.c. 20577, 2016. [2021-10-08]. https://www.iadb.org/evaluacion [25] j. c. duque, n. lozano-gracia, j. e. patino, p. restrepo. urban form and productivity what is the shape of latin american cities? world bank group, washington, dc, 2019. policy research working paper; no. wps 8697. 648 http://hdr.undp.org/en/composite/hdi https://population.un.org/wpp/publications/files/wpp2015_databooklet.pdf https://population.un.org/wpp/publications/files/wpp2015_databooklet.pdf https://globalabc.org/sites/default/files/inline-files/buildings-gsr-2020_report_24-03-21.pdf https://globalabc.org/sites/default/files/inline-files/buildings-gsr-2020_report_24-03-21.pdf https://globalabc.org/sites/default/files/inline-files/buildings-gsr-2020_report_24-03-21.pdf https://www.rha.gov.rw/fileadmin/user_upload/documents/general_documents/laws_of_construction/green_building_minimum_compliance_system-official_gazette.pdf https://www.rha.gov.rw/fileadmin/user_upload/documents/general_documents/laws_of_construction/green_building_minimum_compliance_system-official_gazette.pdf https://www.rha.gov.rw/fileadmin/user_upload/documents/general_documents/laws_of_construction/green_building_minimum_compliance_system-official_gazette.pdf https://www.rha.gov.rw/fileadmin/user_upload/documents/general_documents/laws_of_construction/green_building_minimum_compliance_system-official_gazette.pdf http://upcycleafrica.org/ https://openknowledge.worldbank.org/handle/10986/22549 https://openknowledge.worldbank.org/handle/10986/22549 https://www.heritagefoundationpak.org/hf https://www.cseindia.org/another-brick-off-the-wall-10325 https://www.cseindia.org/another-brick-off-the-wall-10325 https://www.worldometers.info/world-population/latin-america-and-the-caribbean-population/ https://www.worldometers.info/world-population/latin-america-and-the-caribbean-population/ https://www.atlanticcouncil.org/commentary/article/urbanization-in-latin-america/ https://www.atlanticcouncil.org/commentary/article/urbanization-in-latin-america/ https://www.cemex.com/sustainability/overview https://www.cepal.org/es/temas/proyecciones-demograficas/estimaciones-proyecciones-excel https://www.cepal.org/es/temas/proyecciones-demograficas/estimaciones-proyecciones-excel https://doi.org/10.1016/j.landurbplan.2016.03.021 https://doi.org/10.1016/j.landurbplan.2016.03.021 https://www.iadb.org/evaluacion acta polytechnica ctu proceedings 38:642–648, 2022 1 introduction 2 research approach 3 highlights from africa 3.1 green building minimum compliance system, rwanda 3.2 upcycle africa (sina village), uganda 4 highlights from asia 4.1 denso hall rahguzar walking street for regeneration of karachi's historic core, pakistan 4.2 c&d recycling waste plant, surat, india 5 highlights from latin america and the caribbean (lac) 5.1 cemex circularity strategy 5.2 moravia urban renovation project. medellin, colombia 6 analysis and discussion 7 conclusion references https://doi.org/10.14311/app.2022.33.0271 acta polytechnica ctu proceedings 33:271–276, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague study for re-asr behaviour of recycled concrete using asr generated concrete and considering countermeasure technology takeshi iyodaa,∗, nobuhiro matsudab a shibaura institute of technology, 3-7-5 toyosu koto-ku, tokyo, japan b tokyo techno company, 3343 onoji-machi machida city, tokyo, japan ∗ corresponding author: iyoda@shibaura-it.ac.jp abstract. it is predicted that the concrete volume to be discarded will increase due to the renewal of the concrete structures and the increase in the returned concrete from construction site. the use of this concrete block as recycled aggregate is very important in terms of sustainability. however, recycled aggregate concrete is difficult to use due to large problems such as drying shrinkage and freeze-thaw action. on the other hand, the aggregate used for the raw concrete is often unknown, suggesting the danger of asr. there is no previous research about what kind of danger is caused when an aggregate having an asr risk is used as a recycled aggregate. therefore, in this study, recycled aggregate was made from raw concrete where asr occurred, after that recycled concrete was manufactured using this aggregate. after this, asr tests were conducted again for the recycled concrete. in addition, we examined whether recycled concrete using aggregates with properties improved by using the previously reported carbonation technique could be an asr countermeasure technology. as a result, it was found that the recycled concrete using the asr-generated raw concrete as aggregate can suppress asr by using recurring asr aggregate treated by carbonation technology. keywords: carbonation technology, re-asr behavior, recycled concrete. 1. introduction it is predicted that the concrete volume to be discarded will increase due to the renewal of the concrete structures and the increase in the returned concrete from construction site. it is important to use the recycled aggregate from this concrete to achieve sustainability society. further, in order to spread recycled aggregate concrete, it is desired to use lowquality recycled aggregate that also generates few byproducts such as fine powder and can be manufactured reducing energy and cost. however, recycled aggregate concrete has significant problems such as low strength, drying shrinkage and freeze-thaw. furthermore, the recycled aggregate reactiveness is often unknown for the danger of alkali-silica reaction (asr). it should be noted that in asr, alkali supply from attached mortar on recycled aggregate is also assumed. there has been little discussion about the dangers that would occur if concrete with reactive aggregates were used as recycled aggregate. in order to promote the use of low-quality recycled aggregate in the future, it is necessary to get more knowledge about asr. on the other hand, in past works, the authors have proposed a modification technique by carbonation for the purpose of improve low-quality recycled aggregate [1–4]. this technology focuses on the carbonation mechanism of concrete, it applies high-concentration carbon dioxide on the recycled aggregate to carbonate the mortar in the recycled aggregate, thereby modifying the recycled aggregate itself. it has been confirmed that recycled aggregate modified by this technique meliorate concrete strength, length change by drying shrinkage and freeze-thaw resistance. however, the impact on asr has not been studied. carbonation was expected to have the effect of lowering the alkali concentration of the mortar, and was thought to be helpful in suppressing asr. therefore, in this study, the raw concrete was prepared by using the reactive coarse aggregate in order to develop asr. the recycled coarse aggregate was manufactured from this raw concrete with confirmed asr, and the recycled concrete was prepared. at that time, recycled coarse aggregates low (l) and high (h) quality were produced. the recidivism of asr (re-asr) of this recycled concrete were experimentally investigated. in addition, the effect of asr suppression using modified recycled aggregate by carbonation was also investigated. 2. outline of the experiments 2.1. preparation of raw concrete and asr characteristics table 1 shows the types and physical properties of the aggregates. regarding the rock types of each coarse aggregate, og is crushed hard sandstone, kg is andesite, and sg is gravel. the asr characteristics of the aggregate used were confirmed by the chemical method (jis a 1145), the mortar bar method (jis a 1146) and the accelerated mortar bar method (astm 271 https://doi.org/10.14311/app.2022.33.0271 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en takeshi iyoda, nobuhiro matsuda acta polytechnica ctu proceedings kinds dry density water absorption ration f.m. sc rc classify of asr! g/cm3" [%] [mol/l] [mol/l] os fine agg. 2.55 2.18 2.82 28 84 harmless og coarse agg. 2.63 0.89 6.62 40 78 harmless kg 2.72 1.72 6.41 388 94 not harmless sg 2.60 1.18 7.41 73 107 harmless table 1. the types and physical properties of the aggregate. figure 1. mortar bar test on jis. c 1567). the test results of the chemical method are shown in table 1. the os (fine aggregate), og and sg were judged as "harmless" and kg was judged as "not harmless". figure 1 shows the results of the mortar bar test using jis. all the coarse aggregates were judged to be "harmless" with less than 0.1% of expansion ratio at the age of 26 weeks. however, for kg, the expansion increased after 13 weeks of age, and it is estimated that the expansion further increased after 26 weeks. figure 2 shows the results of the accelerated mortar bar test by astm. the kg and sg were judged "harmful", and og was "harmless". it can be seen that the expansion amount of kg greatly exceeds 0.2% at 14 days of age, and the reactivity is high. the sg gradually expanded from around 5 days of age and exceeded 0.2% at 14 days of age, indicating that it is an aggregate with low to moderate reactivity. based on the above results, kg and sg were determined to be reactive aggregates, and og was determined to be harmless aggregates. using these aggregates, raw concrete was produced with the planned composition shown in the table 2. the cement used was ordinary portland cement with a na2o equivalent of 0.57%. in order to promote asr, naoh was added during mixing so the amount of alkali in concrete increased to 7.0 kg/m3. table 2 also shows the fresh properties of the raw concrete and the compressive strength when cured in tap water for 28 days. regarding the fresh properties and compressive strength of the raw concrete, there figure 2. accelerated mortar bar test on astm. was no significant difference between the slumps, although it varied slightly. however, the compressive strength was slightly lower due to on the effect of adding naoh. next, a rilem aar-3 (38◦c concrete prism test) test was performed to evaluate on the asr characteristics of the raw concrete. figure 3 shows the results of the rilem aar-3 test. the criteria for rilem aar-3 is described as those samples with a swelling above 0.04% at the age of 52 weeks are judged to be harmful. the kg aggregate concrete (kc) increases the swelling from around 10 weeks of age and 0.04% at the age of 52 weeks. cracks were confirmed at 14 weeks. the swelling of sc gradually increased around 10 weeks, and slightly exceeded 0.04% at 52 weeks. on the other hand, oc was determined to be "harmless." these results show the same tendency as the asr characteristics of the raw aggregate on the accelerated mortar bar test as astm. in order to prepare recycled coarse aggregate, cylindrical specimens of ! 100 × 200 mm were prepared. the raw concrete specimens were stored in an outdoor environment for one to three weeks to further promote asr, and then cured in water at 40◦c. to check the amount of expansion of the raw concrete specimen, a 100×100×400 mm prismatic sample was prepared. figure 4 shows the amount of expansion of the monitored specimens. the swelling behaviour of these samples were slower when compared with rilem aar-3 test. this is because the monitoring specimens were cured under the same conditions as 272 vol. 33/2022 re-asr behaviour of recycled concrete kinds of unit weight fresh concrete properties compressive coarse agg. w/c c w s g slump air temp. strength [%] [kg/m3] [cm] [%] [◦c] [n/mm2] oc og 981 11.0 4.7 25.0 29.4 kc kg 50 350 165 823 1021 6.5 4.3 24.0 28.7 sc sg 962 16.0 4.1 23.0 27.9 table 2. mix proportion, fresh properties and compressive strength of concrete at 28 days. figure 3. asr expansion test on rilem. the concrete specimens. however, the amount of expansion of kc increased rapidly from the age of 13 weeks, showing a similar tendency to the raw aggregate and raw concrete. at 20 weeks of age, cracks were confirmed in the test specimen. on the other hand, in sc, although the amount of expansion was slightly small, there was no significant difference from the expansion behaviour in the rilem aar-3 test. recycling of the original concrete specimens was performed at about 20 weeks for kc and 26 weeks for oc and sc at the age of the monitoring specimens. 2.2. production of recycled coarse aggregate recycled coarse aggregates l and h and modified recycled aggregate lc were produced from oc, kc and sc raw concrete specimens. at this point, kc has confirmed expansion and cracking due to asr. recycled coarse aggregate l was produced only by crushing treatment (jaw crusher / impact crusher). recycled coarse aggregate h was produced by further grinding (ball milling) the recycled coarse aggregate l. in the modified recycled aggregate, the coarse aggregate l was carbonated. the forced carbonation condition was 1 week long under the of temperature of 20◦c and relative humidity of 60% in the accelerated carbonation chamber, as in the previous study [1, 3]. all the produced recycled coarse aggregates were washed with water to remove fine particles. figure 5 shows the density and water absorption of the recycled coarse aggregate. in all types of modified recycled aggregate, the density and water absorption figure 4. expansion test on monitoring specimens. figure 5. density and water absorption ratio on recycled aggregates. properties had improved. there are also improvements for the quality of recycled coarse aggregate m. although the alkali content was separately measured, no significant difference was found in the total alkali content of the recycled coarse aggregates l and h in this study. 3. asr characteristics of recycled aggregate concrete 3.1. outline of the experiment concrete was prepared using the recycled coarse aggregate shown in table 3 and the asr characteristics were confirmed. the materials used the same as preparing recycled aggregate concrete. the composition of recycled aggregate concrete was w/c = 0.5, 273 takeshi iyoda, nobuhiro matsuda acta polytechnica ctu proceedings kinds of concrete coarse agg. compressive strength coarse agg. non-naoh adding naoh ol oc og 37.1 24.2 olc 38.5 27.5 oh 37.1 38.5 kl kc kg 39.3 25.5 klc 40.3 30.7 kh 40.1 27.3 sl sc sg 35.8 24.5 slc 38.5 26.7 sh 39.9 27.8 table 3. results of compressive strength on different concrete. s/a = 0.46, and unit water volume of 165 kg/m3. for the purpose of promoting asr, two types were prepared, one with naoh added and one without naoh, so that the total alkali content in recycled aggregate concrete including alkali from cement was 7 kg/m3. in addition, the calculation of the total alkali content of the sample to which naoh was added did not include the alkali content from the aggregate. a 100 × 100 × 400 mm prism was prepared, and after demolding, it was cured for 1 week in an environment at a temperature of 20◦c and humidity of 60%. table 3 shows also the compressive strength on recycled concrete. the compressive strength tends to be larger for h class aggregate. it can be said that the amount of attached mortar affects the strength development. also, the modified recycled aggregate has increased compressive strength compared to l, as in previous studies [2, 4]. when naoh was added, the compressive strength was reduced. 3.2. experimental results figure 6 shows the expansion of recycled aggregate concrete. comparing the types of raw aggregates, those using the non-reactive raw aggregate og showed almost no expansion regardless of whether naoh was added or not. it is clear that there is no danger of asr occurring again when recycled coarse aggregate made from non-reactive raw aggregate is used. in the case of using highly reactive kg, the amount of expansion was significantly increased with the addition of naoh. silica remains at the time of production of recycled coarse aggregate, and it is considered that asr occurred when recycled aggregate concrete was used. in addition, those without addition of naoh showed almost no swelling. this is probably because the total alkali content of recycled aggregate concrete using kg and not containing naoh was 2.9 kg/m3, indicating that the alkali content did not reach asr. on the other hand, when sg was used, the aggregate had low to moderate reactivity as the asr characteristics of the original aggregate, but almost no expansion was observed as in og. comparing l and h, h increased the rate of expansion when naoh was added and using reactive aggregate. this is considered to be due to the fact that the amount of attached mortar with h was small, so that the elution of silica and the penetration of alkali were easily performed. on the other hand, in the case of using kg with high reactivity without addition of naoh, l slightly expanded. it is possible that the alkali in the attached mortar affects asr. in addition, the amount of expansion is greatly suppressed by using the modified recycled aggregate. in particular, those using highly reactive kg as the raw aggregate were very remarkable, showing a high suppression effect on asr. figure 7 shows an image of this asr suppression mechanism. in addition to the presence of attached mortar, the modified recycled aggregate become denser by carbonation, which suppresses the penetration of alkali and further dissolution of silica in the raw aggregate. it is thought that asr is reduced by suppressing it. however, in this experiment, the expansion of the original concrete due to asr was considered to have calmed down to a certain extent, and the scope of the experiment was limited. in the future, it is necessary to continue studying the raw concrete before asr progresses, and also to confirm the suppression mechanism by the modified recycled aggregate. 4. summary the following findings were obtained in this study. 1. the asr characteristics by the accelerated mortar bar method in raw aggregate and rilem aar-3 in raw concrete showed the same tendency. 2. it was found that there was no danger of asr occurring again when recycled coarse aggregate made from non-reactive raw aggregate was used. in the case of using highly reactive kg, the amount of expansion significantly increased with the addition of naoh. this suggests that even when the asr of the raw aggregate has progressed to some extent, the silica remains when the recycled coarse 274 vol. 33/2022 re-asr behaviour of recycled concrete figure 6. results of re-asr on recycled concrete. figure 7. the mechanism of prevented asr on using carbonated recycled aggregate. 275 takeshi iyoda, nobuhiro matsuda acta polytechnica ctu proceedings aggregate is made, and when the recycled aggregate concrete is used, the asr occurs again. 3. when l and h quality recycled aggregate were compared, the expansion was higher for h with the addition of naoh and using reactive aggregate. 4. the swelling behaviour was greatly suppressed by using the recycled aggregate modified by carbonation, and for the one using highly reactive kg as the original aggregate the decrease was very remarkable, showing a high inhibitory effect on asr. acknowledgements this experiments were carried out by mr. abdulkareem abdulkadeer abdullahi and the all members of our laboratory. references [1] n. matsuda, t. kameyama, m. matsuda, et al. study on the production method of low-energy recycled aggregate by forced adsorption of co2 gas. proceedings of the japan concrete institute annual convention 36:1732-1737, 2014. [2] n. matsuda, s. suzuki, t. iyoda. proceedings of the japan concrete institute annual convention 38:1791-1796, 2016. [3] t. iyoda, n. matsuda. ii international conference on concrete sustainability, pp. 132-141, 2016. [4] a. a. abdulkadeer, n. matsuda, t. iyoda. ialcce2018 the sixth international symposium on life-cycle civil engineering, pp. 575-582, 2018. 276 acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.38.0450 acta polytechnica ctu proceedings 38:450–455, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague implication of climate changes on design of structures jana marková∗, miroslav sýkora, vitali nadolski czech technical university in prague, klokner institute, šolínova 7, 166 08 prague 6, czech republic ∗ corresponding author: jana.markova@cvut.cz abstract. the climatic data on which the current generation of the eurocodes are based are mostly about 20 years old, with some exceptions of recent updates at a national level. the second generation of the eurocodes for structural design is expected to be nationally implemented within next few years and operational national annexes should be subsequently developed and the climatic maps revised. some models for extreme climate actions are still missing within eurocodes including wind action effects due to non-synoptic storms, which are common in most of the world and are of increasing importance in europe. the aim of this contribution is to analyse how the impact of anticipated changes in european climate could affect the assessment of design weather parameters, including the partial factor design approach for structures according to eurocodes, based on current knowledge concerning projection models of future climate in europe. keywords: climatic actions, design weather parameters, climate change, probabilistic assessment, partial factors, target reliability. 1. introduction for the completion of the second generation of the eurocodes for structural design and selection of the national determined parameters for new national annexes, the potential impact of climate changes on various types of construction works should be evaluated, mainly for structures with a longer design life as significant buildings or bridges. the aim is to estimate how foreseen changes in european climate could affect the extreme and design weather parameters for the design of structures, based on current knowledge concerning the models of future climate in europe. the largest contributors to uncertainty in the estimation of climate actions on structures include: • natural variations in climate due to solar activity and other influences. • some essential properties of the climate models, their spatial and temporal characteristics. • the appearance of phenomena that was not previously characteristic for a certain region (such as tornadoes), and therefore there is no collected data on this phenomenon and there are no available reliable models that can be used in the design of structures. • uncertainties with the future emissions of greenhouse gases and other resources including volcanic activities or gas releases from sea. • political or economic decision on effective reduction of emissions of greenhouse gases in the coming decades. though these uncertainties make it difficult to give substantial recommendations on impact of expected climate changes on a regional scale, it is possible to indicate trends of selected basic variables which influence the models of climatic actions on structures, environmental actions or degradation of materials including steel corrosion or concrete carbonation. 2. trends during the past decades in europe the temperature has increased in europe since the 1960s and considerable differences exist between the regions as well as seasonal variations, with the highest increase in northern and north-eastern parts of europe during the winter season. the number of warm days has increased during the period 1971–2000. there are regional variations, with up to 10 days increase in parts of scandinavia, central europe and southern europe. observed trends in number of warm days are illustrated in figure 1 where warm days are defined as days with temperatures exceeding the 90 percentile of daily maximum temperatures during the period 1971–2000 [1]. a consequence of rising temperatures is the reduction of snow cover and snow mass over the northern hemisphere. for europe the decrease is estimated to 30 % for the period 1980–2015 but there are considerable variations. although a decrease in mean snowfall is expected in most regions due to the decrease of snowfall frequency with increasing temperatures, a contrasting response may be experienced for snowfall extremes in other regions, where temperatures still remain below the freezing level during precipitation events, due to the intensification of precipitation rate associated with the temperature growth. due to more 450 https://doi.org/10.14311/app.2022.38.0450 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 38/2022 implication of climate changes on design of structures figure 1. observed trends in number of warm days during the period 1971-2000. precipitation, the snow mass at the highest altitudes in norway has increased. this is further evidenced by observed exceptional snowfall events occurred, e.g. in the winter season of 2005/2006 in central europe where many roofs of buildings and stadiums were seriously damaged or collapsed due to extraordinary long-term winter snow accumulation without its melting; this has triggered a revision of the czech national snow map. studies of extratropical cyclones and wind data from the past decades show high variability but no significant trends, and hence it is impossible to find any clear trends for extreme wind speeds in europe. a raise in atmospheric greenhouse gas concentrations may cause some of the atmospheric conditions conducive to tornadoes such as atmospheric instability due to increasing temperature and humidity. development of monthly average temperatures based on measurements of the oldest czech meteorological station in klementinum in prague in the past three 50-year periods is illustrated in figure 2 [2]. an increasing trend with time is apparent for each month. 3. trends in europe towards 2100 all parts of europe are expected to become warmer towards the end of the century with the strongest increase in areas at high altitude such as the alps and parts of scandinavia during summer and north-eastern parts of europe during the winter, where the increase in average temperature could exceed 6 °c under the highest emission scenario of rcp8.5. such increase is figure 2. development of average temperatures (in °c) based on records from the klementinum meteorological station in prague since 1861. expected to increase the frequency and magnitude of heat waves [3] while the number of periods with cold temperatures is likely to decrease. the climate models indicate a future decrease in annual precipitation between 10 to 20 % in part of southern europe and an increase between 10 to 20 % in northern europe under the medium emission scenario of rcp4.5. the signals for rcp8.5 are similar but stronger with up to 30 % decrease in southern europe and 30 % increase in northern europe. the frequency and intensity of heavy precipitation events are expected to increase, especially in scandinavia and eastern europe, but also southern europe could experience an increase of such events, despite the reduction in annual precipitation. this may increase frequency and/or magnitude of local and possibly also of large floods. 451 jana marková, miroslav sýkora, vitali nadolski acta polytechnica ctu proceedings increased temperatures in the winter would lead to a decrease in snow cover and snow mass and in the length of the snow-covered season. the snow mass is expected to decrease between 10 % and 30 % towards the end of the century according to eea report [3] but the trend is probably nonlinear and regional variations should be expected. increased precipitation could lead to higher snow loads in inland areas at high altitudes, and consequently the snow mass could increase to a maximum in such areas before they decrease towards the end of the century. wind in future climate is highly dependent on tracks of extratropical cyclones as well as their intensity, and the signals are unclear as different model studies show some divergence in the results. however, there are indications of increasing intensity of strong thunderstorms, especially for northern and central europe [1, 4]. 4. national climatic maps current national maps in eurocodes which are used as a reference for the design of structures have a number of limitations including: • they are mostly 10 to 15 years old. • they are sometimes based on a limited number of records of measurements and some measurement techniques are deemed to be obsolete (commonly associated with much larger measurement uncertainty in comparison to available modern techniques). • they were developed at a national level and in some cases discontinuities across borders appear (for instance croce et al. [5] revealed common differences along borders above 1 kn/m2 for the characteristic ground snow load in europe). • the processing of the data is inconsistent among countries and often limited to standard techniques. the potential effects of climate changes are not reflected in the present national climatic maps, and an adaptation may thus be needed to consider relevant observed and future changes in extreme temperatures. the relevancy of these changes needs to be carefully examined considering the type of climatic action and specific climatological conditions of a particular region. 5. climate change coverage 5.1. wind the approach in en 1991-1-4 for wind actions assumes that the strongest wind actions arise from synoptic storms. this could be considered for the case for well behaving wind climate and simplified in the present generation of eurocodes. it is well known that winds have a rather heterogeneous nature. three major wind circulation classes can be distinguished: a global circulation, a synoptic circulation and a local circulation. winds belonging to the first class are mild and do not represent any threat for the construction works. winds of the second class can be very strong, and among them are extratropical cyclones (also synoptic storms), tropical cyclones (hurricanes or typhoons) and monsoons. these winds each manifest themselves in a limited part of the earth surface. winds belonging to the third class have a wide variety, from mild thermal breezes to disruptive tornados, and their possibility and probability of occurrence is strongly related to the geographic location and to orography maps which are used as a reference information for structural design. in the last decades the european wind climate has shown an apparent change, making winds other than those arising from synoptic storms stronger than they used to be. both changes raise the question whether or not it is still appropriate that en 1991-1-4 gives provisions only for wind pressures derived for synoptic storms, the answer to which is difficult to give and its potential impact is high. therefore, among non-synoptic winds of interest for structural design are tropical cyclones, monsoons, tornados, downburst outflows, and some local thermal winds. high-intensity tornados commonly strike outside europe, and until few years ago only a limited number of small to moderate intensity ones usually stroked the coasts. however recently significant tornados appeared in some european countries including tornado in the south moravia where 5 people died and about two hundred people were injured, more than 200 houses completely destroyed. with climate change, the number of such rare phenomena as tornadoes may increase. the main characteristic of a tornado is the intensity class, which characterizes the degree of damage from a tornado. the main characteristics of the tornado are determined on the basis of intensity class. the influences of tornadoes for structural design can be characterized by the following parameters: • the probability of appearance a tornado of a certain intensity class in a certain region. accordingly, climate change can affect both the intensity class (degree of damage) and the number of tornadoes in the region. • the probability of the impact of a tornado on a specific object. it depends on the intensity class, in particular on the width and length of the tornado. for a certain intensity class, this probability depends more on the density and the size of the object in a region under consideration. downburst outflows, which are common in most of the world, are also recently becoming stronger and stronger in europe, and this could be potentially an effect of climate changes. wind-induced damage occurred to small structures in the last years has in most cases to be ascribed to downburst outflows, and 452 vol. 38/2022 implication of climate changes on design of structures the nature of their meteorological features as well as of the action they exert on structures is currently a hot research topic in the field of wind engineering. a mature loading model, for incorporation in design codes has been unavailable yet, even though it is clear that this should be a separate one from that used for synoptic winds, and not merely a modification of design speeds. 5.2. snow although a decrease in mean snowfall is expected in most regions due to the decrease of snowfall frequency, a contrasting response may be experienced for snowfall extremes, e.g. in central parts of europe in the winter period 2005–2006. analysing daily precipitation extremes, the increase of surface air temperature due to global warming leads to a decrease of snowfall fraction but also to an increase of precipitation rate during extreme events. therefore, the temperature rises and facilitates the melting of snow and increases the fraction of precipitation falling as rain, however it can increase the intensity of the single snow events, leading to higher ground snow loads in some regions. in respect of snow patterns, european regions belong to various climates such as maritime, continental, or cold, often with significant dependence of the ground snow loads on altitude of the site. the climate in lowlands of western-central europe and in the mediterranean is characterised by an intermittent snow cover, i.e. single of a few snowfalls followed by often complete melting. in contrast, the climate in mountains such as in the alps and in the cold northern regions is characterised by a sustained and accumulating snow cover. kvande et al. [6] investigated the predicted effects of climate change on the ground snow loads in norway. they concluded that the expected global temperature increase would, in the majority of the country, lead to decreasing ground snow loads in 2070–2100. yet, in some inner areas, an increase of ground snow loads is predicted with the expected impact on structural reliability. regarding milder snow climate regions, the statistical analysis of recorded ground snow loads in central europe has revealed statistically significant decreasing trends in annual maxima for the swiss alps and the carpathian region. however, an increase in the variability of the meteorological effects might result in more frequent heavy snowfalls especially in some mountainous regions as for example deemed to be demonstrated by recent roof failures in bavaria; see [7] for an overview. further, snow load on the ground is a basic variable, which is nowadays based on the statistically evaluated measurements of the snow water equivalent (swe). in the past the snow height and density were measured, providing largely uncertain estimates of ground snow loads in comparison to swe: verification and site-specific re-calibration of old measurements is commonly needed to have consistent time series for revisions of snow maps. in summary, unambiguous recommendations regarding the modelling of ground snow load extremes affected by the climate change cannot be provided considering the state-of-the-art knowledge. this is why it is widely accepted that the models for climatic loads should be continuously developed and periodically revised, at minimum each 15 years. expected trends of significant time-dependent parameters should be given. 5.3. thermal actions it is evident that the temperatures are increasing over several previous decades and this trend is expected to continue towards the end of this century. both summer average temperatures and winter average temperatures increase. according to the cen/sc1.t5 report on climate changes [4], the winter mean temperature is expected to rise more in northern europe than in central europe or mediterranean, whereas summer warming will likely be less intense in northern europe. the trends in temperature can be summarized as follows: • a number of cold days over most parts of europe seems to be decreasing since the mid-20th century. • hot days and heat waves are becoming more frequent. • most places in europe will very likely experience more hot extremes and fewer cold extremes as global temperature increases. • the magnitude of hot extremes is expected to increase faster and more severely than mean temperatures over large parts of europe. in fact, observed variations of temperature extremes results already in being much more significant than variations of mean temperature values [2, 8]. several factors influence the magnitude of resulting temperatures and their effects on structures including structural materials and their thermal properties, colour and type of surface, geometry of the structure, its exposition to sun, shading by surrounding objects and vegetation, air humidity, geographical and geomorphological position of the site etc. it is also necessary to consider the accuracy of measurements (selected cross-sections, instrumentation), time period and procedure of evaluation. daily temperatures (instantaneous part) and seasonal temperatures (long-term part) influence the thermal action effects on the structure. the increase in frequency and magnitude of heatwaves due to climate changes have impacts on temperature sensitive structures, mainly having a longer design life such as bridges. increasing temperature demands should be considered in the design of these structures or their parts. 453 jana marková, miroslav sýkora, vitali nadolski acta polytechnica ctu proceedings anticipated increase of shade air temperature due to climate changes will magnify the uniform component of the thermal action, resulting in volume changes for unrestrained structures (including elongations of bridges) and increase of internal forces for restrained structures (e.g. for frame or arch bridges). en 1991-1-5 gives the relationship between the shade air temperature and the uniform temperature component for bridges made from different materials. the relationship given is valid for a daily range of 10 °c which could be also influenced by changing trend of temperatures. ranges of daily temperatures should be nationally checked for the potential updating of the relationship [9]. expected increase of solar radiation will mainly amplify the temperature difference component, which affects temperature profiles in structures, leading to additional stresses e.g. for continuous multi-span bridges or for joints of structures made of different materials. the recommendation on periodically statistically re-evaluation of the characteristic values of shade air temperatures is given in revised en 1991-1-5. 6. estimation of factor of change in order to cover climate change influence on snow loads, the factors of change (fcs) can be applied. factors of change can be derived analysing long series of high-resolution climate projections in moving time windows. the procedure can be summarised in the following steps based on cen/sc1.t6 [1]: • collection of high-resolution climate projections of the investigated climate variables for the observation period (to be compared with recorded data for the same period) and for the future period. • extraction of annual maxima considering time periods t of fixed length shifted by e.g. ten years each other of past observations. • extreme value analysis in each time period to derive characteristic values of the investigated climate variables. the factor of change in terms of ratio for snow loads can be determined fck,cm (n) = ck,cm (n) ck,cm (n = 1) (1) and for temperatures fck,cm (n) = ck,cm (n) − ck,cm (n = 1). (2) the multiplicative format seems to be more appropriate for snow loads while an additive format is applied for temperatures [4]. future trends of the investigated variable are finally determined applying the factors of change to the characteristic values, obtained from the real measurements in the observed period (ck,observed). assuming that the first-time window corresponds to the observation period used for the definition of the actual design value, the future characteristic can be specified for snow ck(n) = fck,cm (n)ck,observed (3) and for temperatures ck(n) = fck,cm (n) + ck,observed (4) similar procedure can be given for precipitations or icing. the factors of change could be used for the development of maps, which can be compared with the current climatic load maps in the eurocodes, providing guidance for potential amendments and improvements. however, the same approach is less physically consistent and therefore less representative in the case of wind actions. it was observed that different phenomena giving rise to wind actions (synoptic storms, thunder storms, tornadoes, downbursts etc.) are changing following different trends. recent observations have revealed a reduction of extreme winds coming from synoptic storms, and an increase of extreme winds coming from downburst outflows and tornados. it appears that effects of climate change could decrease structural reliability over time. consequently, the required target reliability may not be reached for the design working life of the construction. to maintain the required reliability level, climatic actions would need some adaptation to consider the effect of climate change. note that the implementation of the ck-factor may need to be accompanied by adjustment of the partial factor for the climatic load as design values can be more sensitive to uncertainties increasing due to predicting climate change effects than the characteristic values. however, unlike the characteristic values based solely on the ground snow loads, the design values account also for large uncertainties in time-invariant components of the climatic load effects and thus relative influence of uncertainties due to climate change may be small. 7. concluding remarks both average temperature and temperature extremes are expected to increase towards the end of the century. a consequence is that heat waves can be expected to occur more frequently and be longer and warmer in the future. in southern europe the annual precipitation is expected to decrease while it is expected to increase in the north. annual precipitation decreases also in the central europe. more events with heavy precipitations are expected over most of europe. historical trends and climate models indicate a reduction of snow mass but there are large regional differences. the trends are non-linear, and inland areas at high altitudes may experience increasing snow loads due to increased precipitation. however, these increases are of marginal interest for structural design – most of buildings are concentrated in lowlands where 454 vol. 38/2022 implication of climate changes on design of structures no or statistically insignificant decreases are often predicted. similar developments could be expected for atmospheric icing. wind records exhibit considerable variability and no clear long-term trends. it is impossible to draw clear conclusions regarding future wind climate but there are indications of increasing wind speeds associated with most severe extratropical cyclones over northern and central europe. associated increases of design or assessment requirements may be critical for a number of existing structures as wind pressures depend on the quadrate of wind speed. further research is needed on the effects of downburst outflows and other extreme winds including tornados on civil and industrial structures, with the expectation that in a reasonable time this could provide suitable tools for updating en 1991-1-4. the ongoing changes in the climate are expected to continue during the coming decades towards 2100 and become even more relevant due to the delayed impacts of past and current greenhouse gas emissions. this will potentially affect also structural design since the basic assumption of stationary climate for the determination of climatic loads will be not satisfied in a changing climate. cen/sc1.t5 report [4] recommended revision of relevant climatic variables at minimum 10 to 15 years intervals. although data from climate models provide information about future trends for climatic parameters, there is considerable dispersion in the data depending on parameter as well as emission scenario. as stated in the report, quantification of future extremes with accuracy required for design purposes is not feasible. regular re-examination of weather parameters considering uncertainties in extremes of climate actions should be used for verification and updating of related partial factors. beyond the scope of this contribution is flooding that is a major natural hazard in most of europe and that is strongly affected by global temperature changes. numerous studies have indicated that risks related to river flooding increase in many european regions (such as north-western and northern europe) while in some regions both increases and decreases are locally expected, depending on specific conditions of various sub-regions. though first attempts have been already made to reflect the climate change effect in designing safety measures against river flooding, considerable research efforts are still needed to provide the recommendations for optimum adjustments of design procedures with respect to the climate change effect on river flood frequencies and magnitudes [7]. climate change studies often focus on areas with predicted increases in climatic loads. however, in some regions the climate change seems to decrease the load effects (reducing probability of occurrence and/or magnitude) and thus less strict design or assessment requirements might be considered, providing for economic savings. however, large uncertainty related to the climate change effects often surpasses the positive effect of decreased hazards. monitoring of the structure may reduce these uncertainties. acknowledgements this study has been conducted at the ctu in prague, klokner institute within the framework of the grant ltt18003 supported by the ministry of education, youth and sports of the czech republic. references [1] s. m. fikke, j. markova, r. höffer, b. wichura. climate change, cen/sc1.t5 final report, cen/tc 250 n 2654, 2018. pp. 64. [2] j. markova. design of structures in the changing climate. in proceedings of the 31st european safety and reliability conference, pp. 1101–1107. 2021. https: //doi.org/10.3850/978-981-18-2016-8_468-cd [3] european environment agency. climate change, impacts and vulnerability in europe 2016 : an indicator-based report. publications office, 2017. https://doi.org/doi/10.2800/534806 [4] cen/tc 250/sc1.t6 interdependence of climatic actions, probabilistic basis for determination of partial safety factors and load combination factors (background document, final draft), report no. sc 1/ n 1735, 2021. pp. 320. [5] p. croce, p. formichi, f. landi, f. marsili. harmonized european ground snow load map: analysis and comparison of national provisions. cold regions science and technology 168:102875, 2019. https: //doi.org/10.1016/j.coldregions.2019.102875 [6] t. h. t. t. tayet, h. o. hygen, t. kvande. present and future of snow loads in norway, 2013. 13th ems/11th ecam, reading. [7] a. orcesi, a. o’connor, d. diamantidis, et al. investigating the effects of climate change on structural actions. structural engineering international 32(4):563–576, 2022. https://doi.org/10.1080/10168664.2022.2098894 [8] european commission, joint research centre, m. sousa, et al. thermal design of structures and the changing climate. publications office, 2020. https://doi.org/10.2760/128894 [9] g. könig, m. emerson, m. froli, et al. background document to env 1991-2-5: thermal actions, cen (european committee for standardization), 1997. 455 https://doi.org/10.3850/978-981-18-2016-8_468-cd https://doi.org/10.3850/978-981-18-2016-8_468-cd https://doi.org/doi/10.2800/534806 https://doi.org/10.1016/j.coldregions.2019.102875 https://doi.org/10.1016/j.coldregions.2019.102875 https://doi.org/10.1080/10168664.2022.2098894 https://doi.org/10.2760/128894 acta polytechnica ctu proceedings 38:450–455, 2022 1 introduction 2 trends during the past decades in europe 3 trends in europe towards 2100 4 national climatic maps 5 climate change coverage 5.1 wind 5.2 snow 5.3 thermal actions 6 estimation of factor of change 7 concluding remarks acknowledgements references https://doi.org/10.14311/app.2022.33.0098 acta polytechnica ctu proceedings 33:98–104, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague trc sandwich solution for energy retrofitting isabella giorgia colombo, matteo colombo, marco di prisco∗ politecnico di milano, department of civil and environmental engineering, p.za l. da vinci, 32 20133 milano, italy ∗ corresponding author: marco.diprisco@polimi.it abstract. concerning energy improvement of existing façades, a favourable system involves prefabricated multilayer panels, made of internal insulation core and outer textile reinforced concrete layers. it is a convincing alternative to external thermal insulation composite systems (etics) and ventilated façades, and it meets all the requirements for façade systems. the main advantage is the possibility to apply the panel using a crane, without any scaffolding. the paper considers two solutions: the former uses expanded polystyrene (eps) as insulating material; the latter substitutes eps with an innovative green insulation material made of inorganic diatomite. the paper aims at comparing the solutions in terms of mechanical properties of the components and behaviour of the composite sandwich at lab-scale level. numerical models, previously calibrated, will be instrumental for the discussion. keywords: energy performance, sandwich panels, textile reinforced concrete. 1. introduction in europe, building sector uses a large amount of energy (40% of the total) and produces relevant co2 emissions (36%). for this reason, the focus of many european policies is to improve energy efficiency [1]. each year, only approximately 1% of buildings are energy renovated, in the framework of a building stock that is old and energy inefficient, with the half of residential buildings dated before 1970, year in which the first european thermal standard was emanated [2]. hence, improvement in energy efficiency of buildings is a crucial topic. in multi-storey buildings, façades cover a large amount of the total outer envelope surface. hence, solutions devoted to the energy retrofitting of existing façade result particularly effective in terms of building energy saving. a convenient solution is represented by multilayer precast sandwich panels made of an inner insulation core and thin external textile reinforced concrete (trc) layers. trc sandwich panels were proposed about 15 years ago by german researchers [3] and constituted a new idea in the framework of cement-based sandwich elements: trc thickness (10-15 mm) is smaller than that of traditional r/c external claddings (>40 mm) [4], leading an important decrease in the weight of the panel. other conveniences are durability and the good appearance, obtained using fine-grained mortar. although connectors were introduced between the layers for a durable connection [5], advantage is taken from adhesive bond between trc and the insulation layer to transfer the shear stresses. 2. trc sandwich solutions the research concerns prefabricated sandwich panels constituted by thin external trc layers and an internal insulation core. large size panels (3 × 1.5 m2) have been designed to reduce the manufacturing and installation costs. the installation is performed through a crane, without scaffolding, thus minimizing installation timing and inconveniences for inhabitants. other advantages are: the integration of the insulation and finishing systems; the durability and possibility to obtain different colour and texture as finishing; the protection of the insulation core through the external, waterproofed layer of trc; the good resistance to concentrated loads. only the inner trc layer is connected to the existing r/c frame; the shear transfer between the two trc layers is entrusted to the insulation material (for fire safety, some supplementary connectors could be inserted). 2.1. panel with expanded polystyrene core (easee project) the consortium of the european project easee [6] developed a prefabricated trc sandwich panel with expanded polystyrene (eps) as insulation material. a thickness of 100 mm was considered for the core. all the details concerning the panel design, testing at different scale levels, durability, and modelling can be found in [7–12]. easee main output is represented by a demonstration building [11] in cinisello balsamo, italy, where a 70% decrease in the thermal transmittance of the wall was achieved [13]. one of the main criticisms of this panel is that eps may melt during fire, leading to the loss of mechanical performances of the composite solution. 2.2. panel with diatomite core (smart p.i.qu.e.r.) smart p.i.qu.e.r. project [14] was aimed at solving the criticisms of the previous project. among the 98 https://doi.org/10.14311/app.2022.33.0098 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 33/2022 trc sandwich solution for energy retrofitting figure 1. test results: trc tensile behaviour (a) and core material compressive behaviour (b). goals, particular relevance is assumed by the development of a green insulation material, with suitable mechanical properties and good fire and insulating capabilities. at the same time, trc layers need to be optimised, reducing the shrinkage and weight of the mortar, and enhancing the fabric fire behaviour and sustainability. 2.3. materials 2.3.1. textile reinforced concrete trc developed in the easee project is made by high strength fine grain mortar reinforced with one alkali-resistant glass fabric. the warp is aligned in longitudinal direction. this fabric [7] (leno-weave production; sbr coating: water resin based on styrene butadiene rubber) is characterised by a nominal tensile strength in warp direction of 820 mpa (computed on the equivalent cross-sectional area of the glass). the cementitious matrix has a water to binder ratio equal to 0.225, a superplasticiser to cement ratio of 9.3% and 1 mm maximum aggregate size. the average cubic compressive strength (fcc) of mortar, measured as specified in [15], is equal to 71.89 mpa on 12 specimens, with a coefficient of variation of 9.13%. concerning smart p.i.qu.e.r. project, several mortar mix-designs and alkali-resistant glass fabric reinforcements were investigated [14]. in this paper, a high-performance mortar that includes shrinkage reducing admixtures and is reinforced with one alkaliresistant glass fabric coated with a thermosetting epoxy coating is considered (see "hpc+sra_ap1" in [14]). the epoxy coating is particularly suitable in case of fire exposure. the fabric is characterised by a nominal strength in the warp direction of 1041 mpa. the cementitious matrix used is characterised by a maximum aggregate size of 1 mm, a water to binder ratio of 0.20 and a super-plasticiser to cement ratio of 5.1%. the average cubic compressive strength (fcc) of mortar measured according to [15] is equal to 97.53 mpa on 32 specimens, with a coefficient of variation of 8.33%. in both cases, three 400×70×10mm3 trc samples were tested in uniaxial tension using an electromechanical press instron 5867 with a maximum load capacity of 30 kn, imposing a constant crosshead displacement rate of 0.02 mm/s. the same test apparatus and modalities described in [7] are used. the test results are shown in figure 1a in terms of nominal stress (σ = p/a, with p=load and a=specimen cross section) vs. normalised displacement (δ/l, with δ = imposed displacement and l = free length) curves. 2.3.2. core material in the framework of the easee project, expanded polystyrene was used as insulation material in sandwich panels. the product eps250, characterised by a nominal density of 35 kg/m3, was selected. as resulting from experimental tests [10], it is characterised by a uniaxial compressive yield stress of 0.188 mpa, an estimated young’s modulus in compression of 13.7 mpa, a uniaxial tensile yield stress of 0.392 mpa, a shear yield stress of 0.160 mpa and an estimated shear modulus of 5.04 mpa. as already anticipated, one of the main aims of the smart p.i.qu.e.r. project was to develop a core material that could be more sustainable and more suitable in case of fire with respect to eps. hence, a novel lightweight sustainable insulation core (dhcf) was developed by verdolotti and other authors [16] using diatomite (natural source) as matrix. short fibres were added to guarantee a ductile behaviour of the insulation material both in compression and in bending, in order to guarantee a proper global behaviour of sandwich panels. these fibres also contribute to the enhancement of thermal performances of the material, by reducing the thermal conductivity. polypropylene (pp) fibres were selected among different kind of reinforcement tested (jute, waste synthetic, mixed and hemp fibres, as such and chopped). 99 i. g. colombo, m. colombo, m. di prisco acta polytechnica ctu proceedings figure 2. four-point bending test set-up (smart p.i.qu.e.r.). this choice was related not only to the proper thermal and mechanical performances of the foam in which pp fibres are included, but also because they are the only ones that allow the production at industrial level by means of an industrial plant. considering that the level of the plastic plateau is related to specimen density, a target density of 300 kg/m3 was fixed to guarantee comparable performances of the core material with those of expanded polystyrene. the core mix design is collected in table 1. diatomite (wt%) 19.57 polysilicate (wt%) 65.24 si (wt%) 0.046 catalyst (wt%) 8.39 vegetable (wt%) 5.82 pp fibres (wt%) 1 table 1. dhcf_pp mix design. compressive tests were performed on eps specimens with a size of 100 × 100 × 150 mm3, and dhcf specimens with a dimension of 40 × 40 × 40 mm3. the tests were displacement controlled by imposing a constant displacement rate of the machine crosshead equal to 1e-3 mm/s and 2e-3 mm/s respectively. the compressive behaviour of both expanded polystyrene and diatomite-based insulation material is shown in figure 1b where the nominal stress σ vs. nominal strain ε curves are plotted (σ = p/a, p = load, a = unloaded specimen cross-section; ε = δ/h, δ = top displacement, h = specimen height). 3. experimental behaviour of lab-scale sandwich beams with diatomite-based core a wide experimental campaign on lab-scale sandwich specimens were developed within the easee project. a four-point bending set-up was adopted for both deep (550 × 150 × 120 mm3) and slender (1200 × 300 × 120 mm3) sandwich beams, with eps 100 mm thick. detailed description of experimental procedures and results can be found in [8]. hence, this section is focused on the experimental tests performed at lab-scale level within smart p.i.qu.e.r. on beams in bending. 3.1. test set-up four-point bending tests were performed on sandwich beams using the same electromechanical press instron 5867 already mentioned. two nominally identical specimens with size of 600 × 150 × 80 mm3 and the warp of the glass fabric aligned in the beam longitudinal direction were tested. the specimen geometry and the test set-up are shown in figure 2. the insulation panel densities are collected in table 2. specimen ρ1st_measure ρ2nd_measure (kg/m3) (kg/m3) piquer1 330 (31 days) 303 (125 days) piquer2 348 (28 days) 297 (122 days) table 2. density of the insulation core. the insulation layer of full-scale panels designed in the smart p.i.qu.e.r. project is obtained placing side by side different core panels with size of 600×600×100mm3. for this reason, some cuts are introduced in the insulation layer also in the lab-scale beams (figure 2). with the aim of reducing stress concentration, 50 mm wide steel plates are glued on the specimen over the cylindrical supports and under the loading-knives. the loading and the supporting cylinders are free to rotate on their axis and in the plane perpendicularly to the longitudinal axis of the specimen. during the initial phase of each test, specimens were instrumented with displacement transducers in order to measure: the specimen deflection under the loading knives, the deflection of the upper trc layer under the loading knives, the superior longitudinal displacement on the compressed side inside the constant bending moment region, and the crack opening displacement on the tense side. however, these aspects are not treated in this paper. tests were displacement-controlled, considering the machine crosshead displacement (stroke) as feedback parameter. an initial stroke rate of 2e-3 mm/s 100 vol. 33/2022 trc sandwich solution for energy retrofitting figure 3. experimental results (smart p.i.qu.e.r.): load vs. stroke curves (a) and pictures of specimen piquer 2 during the test (b) and at failure (c). was imposed; then, after crack formation, the rate is increased up to 5e-3 mm/s. 3.2. experimental results the test results are shown in figure 3a in terms of load (p) vs. stroke curves. looking at the results, it is interesting to note that specimen piquer1 experienced a premature shear failure of the core, thus leading to a localised failure and preventing the multicracking of trc layers that guarantees the hardening behaviour of the sandwich solution. in fact, while performing a check before testing, core panel used to cast specimen piquer1 presented to the touch some softer regions at the edges. on the contrary, specimen piquer2 showed a ductile hardening global response, experiencing multi-cracking of trc faces. a picture of this specimen during testing is shown in figure 3b. the final failure is related to the debonding between the beam core and the lower trc layer (figure 3c). 4. numerical modelling of the tests and comparison between the two sandwich solutions 4.1. geometry and constraints abaqus /cae 6.14-5 was adopted for the modelling of the panel. yz symmetry was exploited, and half of the beam was reproduced, thus minimising the numerical effort (figure 4a). as shown in the figure, the panel layers (trc and core), and the steel plates are modelled as solid and homogeneous. perfect bond is assumed at core/trc interfaces and between the panel and steel plates. the interaction between adjacent core panels is characterized by a hard contact in the normal direction and by a frictionless contact in the tangential direction. concerning constraints, displacement orthogonal to the symmetry plane are prevented, the lower steel plate is constrained in the vertical direction on the bottom face and a vertical displacement δ is imposed on the loading line. the mesh is shown in the same figure: eight-node linear hexahedral elements (c3d8r continuum, 3-d, 8-node reduced integration) are used. 4.2. constitutive laws in this section, a description of the constitutive laws adopted for each material is proposed. the elastic phase of trc is defined introducing a poisson’s ratio of 0.2 and a young’s modulus of 30 gpa (according to literature results [17] on mortar characterized by equal maximum aggregate size and compressive strength). the plastic behaviour of trc is modelled through abaqus concrete damage plasticity model. no damage curve is inserted; hence, the model behaves as a plasticity model. in compression, an elasticperfectly plastic behaviour is assumed, imposing a yield strength of 97 mpa according to the values shown in section 2.3.1. the stress-irreversible strain relationship (table 3) adopted in tension is extracted from the experimental results of piquer specimens shown in figure 1a, considering strains computed basing on the measurements of displacement transducers applied on trc. trc tensile behaviour is assumed homogeneous over the layer thickness; this assumption is reliable according to [10]. point yield stress cracking strain t1 2.5 0 t2 5 0.00275 t3 20.81 0.017 table 3. trc plastic tensile constitutive law. concerning the core material, the elastic phase is defined introducing a young’s modulus of 20 mpa 101 i. g. colombo, m. colombo, m. di prisco acta polytechnica ctu proceedings figure 4. fe model: geometry and constraints (a) and validation of core material model considering three-point bending (b). figure 5. fe model results: numerical response compared with experimental results (a) and with easee bending behaviour (b); failure modes for specimen piquer2 showing core strut yielding at point c1 (c) and trc multicracking at point t3_sup (d). 102 vol. 33/2022 trc sandwich solution for energy retrofitting (according to experimental results in compression shown in figure 1b) and a poisson’s ratio of 0.2. the plastic behaviour of the core material is introduced using abaqus concrete damage plasticity model. also in this case, no damage curves are introduced. an elastic-perfectly plastic behaviour is assumed both in tension and in compression: a compressive yield strength of 0.30 mpa is used according to experimental results shown in figure 1b (average value for specimens of the same batch of core sample piquer2); a tensile yield strength of 0.053 mpa is adopted according to average experimental results of tensile tests performed on core specimens. with the aim of checking the validity of the material model adopted for the core, a three-point bending test performed on core prismatic specimen with size of 160×40×40mm3 was modelled. experimental and numerical curves are compared in figure 4b, demonstrating that the adopted material model is reliable. steel is assumed elastic with a young’s modulus of 210 gpa and a poisson’s ratio of 0.1. 4.3. numerical results the numerical response is shown in figure 5a in terms of load (p) vs. stroke. a good correlation is found with respect to the behaviour of specimen piquer2 that showed a hardening behaviour. a perfect overlapping is obtained in the initial phase of the test; the divergency of the two curves while the test is progressing could be related to a bond weakness at trc-core interfaces, leading to the final failure due to debonding already underlined in figure 3c. in order to investigate the failure mechanisms occurred for a specimen characterised by a good trccore bond, critical points of material constitutive laws are underlined on the numerical curve. in particular: t points refer to the trc layers (table 3), with subscript "inf" related to lower trc layer and subscript "sup" related to the upper trc layer; point c1 indicates the yielding of the strut in the core material. it is worth noting that the change in the slope of the global response corresponds to point c1 (figure 5c), when both trc layers are already cracked (figure 5d). in figure 5b the numerical response of piquer specimens, in which the core thickness was enhanced to 100 mm, is compared with the experimental behaviour of easee sandwich beams characterised by the same cross-section and size. 5. conclusions the bending behaviour of sandwich solutions developed within the easee and smart p.i.qu.e.r. projects was compared at lab-scale level. even though the response of the piquer sandwich beam showed lower maximum load and final displacement, a ductile hardening behaviour and a reasonable sustained load are guaranteed by the solution. it is worth noting that a good bond at trc-core interfaces must be guaranteed to obtain a proper behaviour of the composite. acknowledgements the research was developed within the smart p.i.qu.e.r. project, funded by regione lombardia (smart living; decree n. 14782 24/11/2017; id siage: 379284). authors would like to thank the partners; in particular, ipcbcnr and isoltech involved in the development of the core material. references [1] eur-lex. directive 2018/844/eu of the european parliament and of the council of 30 may 2018 amending directive 2010/31/eu on the energy performance of buildings and directive 2012/27/eu on energy efficiency, 2018. https://eur-lex.europa.eu/l egal-content/en/txt/?uri=celex:32018l0844. [2] european commission. data from eu building database, 2019. https: //ec.europa.eu/energy/en/eu-buildings-database. [3] j. hegger, m. horstmann. light-weight trc sandwich building envelopes. excellence in concrete construction through innovation 187-194, 2008. https://doi.org/10.1201/9780203883440.ch27. [4] a. einea, d. c. salmon, g. j. fogarasi, et al. state-of-the-art of precast concrete sandwich panels. pci journal 36(6):78-98, 1991. https://doi.org/10.15554/pcij.11011991.78.98. [5] a. shams, m. horstmann, j. hegger. experimental investigations on textile-reinforced concrete (trc) sandwich sections. composite structures 118:643-53, 2014. https: //doi.org/10.1016/j.compstruct.2014.07.056. [6] european commission. european project easee (2012-2016) envelope approach to improve sustainability and energy efficiency in existing multi-storey multi-owner residential buildings, grant no.: 285540, 2019. [7] i. g. colombo, m. colombo, m. di prisco. tensile behavior of textile reinforced concrete subjected to freezing-thawing cycles in un-cracked and cracked regimes. cement and concrete research 73:169-83, 2015. https: //doi.org/10.1016/j.cemconres.2015.03.001. [8] i. g. colombo, m. colombo, m. di prisco. bending behaviour of textile reinforced concrete sandwich beams. construction and building materials 95:675-85, 2015. https: //doi.org/10.1016/j.conbuildmat.2015.07.169. [9] i. g. colombo, m. colombo, m. di prisco. trc precast façade sandwich panel for energy retrofitting of existing buildings. aci sp. pub. sp 305, 2015. https://doi.org/10.14359/51688590. 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[12] i. g. colombo, m. colombo, m. di prisco. precast trc sandwich panels for energy retrofitting of existing residential buildings: full-scale testing and modelling. materials and structures 52(5), 2019. https://doi.org/10.1617/s11527-019-1406-1. [13] g. salvalai, m. m. sesana, g. iannaccone. deep renovation of multi-storey multi-owner existing residential buildings: a pilot case study in italy. energy and buildings 148:23-36, 2017. https://doi.org/10.1016/j.enbuild.2017.05.011. [14] i. g. colombo, m. colombo, m. di prisco, et al. lightweight trc sandwich panels with sustainable diatomite-based core for energy retrofitting of existing buildings. advances in building energy research 15(2):231-52, 2019. https://doi.org/10.1080/17512549.2019.1697752. [15] en 196-1:2005. standard for mortar. methods of testing cement part 1: determination of strength, 2015. [16] l. verdolotti, b. liguori, i. capasso, et al. synergistic effect of vegetable protein and silicon addition on geopolymeric foams properties. journal of materials science 50(6):2459-66, 2014. https://doi.org/10.1007/s10853-014-8801-3. [17] w. brameshuber. report of rilem technical committee 201-trc rilem publications, 2016. 104 https://doi.org/10.14359/51711059 https://doi.org/10.1617/s11527-019-1406-1 https://doi.org/10.1016/j.enbuild.2017.05.011 https://doi.org/10.1080/17512549.2019.1697752 https://doi.org/10.1007/s10853-014-8801-3 acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.38.0488 acta polytechnica ctu proceedings 38:488–494, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague increasing thermal protection with use of green roof peter juráš university of zilina, faculty of civil engineering, department of building engineering and urban planning, univerzitna 8215/1, 010 26 zilina, slovakia correspondence: peter.juras@uniza.sk abstract. the use of green roof is a great choice in case of climate change mitigation and reduction of urban heat islands. positive aspects of green roofs during winter or the whole year round balance are often overlooked. the surface of highly insulated flat roof is overcooled during the night by the long wave sky radiation. this radiative cooling increases the thermal losses that are reduced by the existence of additional layers. the green roof composition layers also have their thermal resistance, which is not usually included within the calculation of thermal resistance using the en iso 6946. the presence of snow on the roof can also increase the resistance. this paper analyzes the measurement results of various experimental green roof fragments in central europe. keywords: green roof, temperature, experimental, measurement, winter, snow. 1. introduction hanging gardens of babylon are considered as the first use of green roof, dating back to around 600 years b.c. the riding school building has the oldest green roof in the former czechoslovakia constructed in 1911. in 2005, most of the green roof layers were replaced by the new ones, but the waterproof membrane remained original [1]. in slovakia, terraces of the nitra castle are one of the oldest examples of green roofs. therefore, the use of the green roof is not a new idea. but in the slovakia, they are nowadays in the center of public interest. green roofs have many benefits, for example: reducing the urban heat island intensity, reducing the summer overheating of interiors, rainwater retention and evapotranspiration, dust particles collection, fire safety, different architectural and aesthetic aspects etc. state of the art is provided by authors in [2, 3]. measurements worldwide in this area differ in the aim and scale [4, 5]. summer benefits are more often analyzed [2, 3, 5–7]. different simulation approach [8], boundary conditions [9] and material parameters are analyzed in [10]. the winter regime is analyzed in these works [11–13]. usual energy saving can be between 10–15 % depending highly on the thermal insulation thickness. higher insulation has lower saving [14]. water retention regime is also important part of the green roof research [15, 16]. research in this area in slovakia is limited to the experimental roof in kosice [17]. similar research is conducted in the czechs uceeb with more types of green roofs [18]. with its new measuring platform of green roofs, the university of zilina was added to the international effort to mitigate the climate change [19, 20]. in this paper, part of the initial results of winter regime of the green (g) and non-green flat roof (r) figure 1. aerial view during renovation work of the roof. is analyzed. the additional thermal resistance of the green roof layers and its influence on the thermal loss are also analyzed. also, the planned and possible outcomes in the future are mentioned. 2. test site and methodology in the year 2019, one of the flat roofs in the campus of the university of zilina was completely reconstructed (figure 1) the whole structure up to the load-carrying concrete slab was removed. this enable the possibility for creation of testing platform (figure 2). during the reconstruction, temperature/humidity sensors sensirion sht21 were incorporated within the roof structure in two places (figure 3). these sensors are connected to the raspberry pi with one-minute recording interval. 488 https://doi.org/10.14311/app.2022.38.0488 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 38/2022 increasing thermal protection with use of green roof figure 2. front row for measuring the rainwater retention, the second one for temperature measurement (picture from autumn 2020). marked position of compared compositions in this paper. figure 3. incorporation the sensirion sht21 sensors during the reconstruction work. calibration measurement made between these two places was done with and without green roof and showed matching courses with slight differences caused by the shading of the higher part of the building showed also in figure 1 and the fact that the pitched insulation layer is about 20 mm thick [19]. during the year 2020, four different green roof compositions were constructed. each composition is made twice. the first one is for monitoring the temperatures, because it is placed on the original waterproof membrane. the second one is slightly elevated with additional pitched layer to increase the angle and add the possibility to use the tipping bucket rain gauge for measuring the amount of rainwater [19]. the complex view of both segments is shown in figure 2. the winter period was with either total snow cover (figure 4) or with partial powder snow cover. the differences between the individual compositions are up to the manufacturers of the compositions: they differ from each other by the use of mineral wool for water retention and filtration or the use of different drainage layer. all compositions are commercially available. in this paper, only one composition is anafigure 4. roof covered with snow. lyzed (marked in figure 2) and compared to non-green roof. in the green roof compositions, there are used pt100 sensors and sheathed thermocouples with capability of immersion into the water or for usage in humid environment. the recording time interval of fluke hydra datalogger is one minute. as it was stated before, monitoring of the outdoor climate is also important. there is a weather station on the site, on the roof of another building within the campus [21– 23]. the sketch of the both roof compositions is in figure 5 with marked positions of sensors used for comparison in this paper. the comparison is made for the temperature on the membrane, where also in the results graphs is this position named membrane (either only membrane for roof r or green roof membrane for roof g). also, the temperature bellow the vegetation surface is measured, where the sensor is below the vegetation and covered with the layer of substrate as thin as possible. the impact of the green roof on the thermal properties and thermal loss is analyzed in a simple steady state calculation with the use of measured outdoor climate temperature and also with the impact of surface temperatures, which were taken into account as air temperatures. properties of individual materials within the regular roof and from the green roof composition are summarized in table 1. material properties of the snow differ [24] according to the state of the snow; if it is powder, or has several melt-froze cycles. based on the observation after snowing two days before, there were properties of powder snow used in this case. 3. results and discussion 3.1. in-situ temperature measurement results of initial temperature measurement of the samples are in figures 6 and 7. from the longer time period, there were two 2 days periods chosen, which can show and demonstrate the impact of the green roof. compared are the temperatures on the waterproof membrane show the selected temperature 489 peter juráš acta polytechnica ctu proceedings material thickness [m] λ [w/(m · k)] ρ [kg/m3] snow (powder, chosen) 0.15 0.029 100 soil (wet) 0.03 2.0 2000 green roll substrate (wet state) 0.05 0.2 50 mpvc membrane 0.0015 0.35 1470 eps 150 0.035 30 vapor barrier 0.0035 0.21 1200 reinforced concrete 0.25 1.74 2500 table 1. material properties of used materials example [24]. figure 5. detail of compared roof compositions with installed sensors – color-marked ones are used in the results graphs. figure 6. temperature courses for two days without snow on the roof. daily difference on the membrane is 37 °c, green roof membrane 0.4 °c. air temperature difference is 15.5 °c. 490 vol. 38/2022 increasing thermal protection with use of green roof figure 7. temperature courses for two days with roof covered by snow. daily difference on the membrane is 9.2 °c, green roof membrane 0.5 °c. air temperature difference is 15.3 °c. courses in the green roof compared to the non-green roof. the difference between membrane temperature and outdoor air temperature is important and can be almost the same or much bigger during the clean night sky. the first comparison is for the roof without snow (figure 6) and two winter days with freezing and with sunny weather during the day. although the air temperature during the day barely reached zero, the non-covered membrane had temperature higher than 15 °c. so also during winter, the daily variance of temperature on the membrane is higher than 30 °c, which can influence the durability. however, the more important fact in this case is that the surface of the roof is highly overcooled during the night. night radiative cooling overcooled the surface of the nongreen roof by around 5 °c. benefits of the green roof can be clearly seen. the daily course for the green roof is without significant peaks. daily variations affect the durability of membrane. the green roof also protects the membrane against uv radiation. there is also time shift in the temperature peaks, which is not such significant nowadays with highly insulated structures [25–27]. the temperature course for the g membrane is supported by the other findings [12, 13]. the measured temperature below the vegetation is close to the membrane temperature, which can be influenced by the thermal inertia of the substrate, insulation effect of the plants or partly by shading of the elevated part of a building next to the samples (figures 1 and 2). however, the overcooling of the r membrane will also be influenced in this case. there is a correlation between the air temperature course and the vegetation surface course, but the difference is up to 10 °c. this finding will be evaluated in later research. in the case of the roof covered with snow, the surface is not overcooled as in the case without snow (figure 7), where the non-green roof surface is much cooler than the outdoor air. powder snow has a very good thermal conductance coefficient [24], so it creates relatively good insulating layer of the non-green roof (figure 7). snow also serves as the protective layer against the long-wave radiation from the cold sky during the night. snow on the roof can effectively reduce the surface temperature of r membrane. the difference in this case is up to 9.5 °c. in the g roof case, the membrane temperature course is without any visible change or correlation with the outdoor air temperature. 3.2. thermal loss calculation based on the measurement findings, the measured air temperatures were used as boundary temperatures for numerical thermal loss calculation. calculated u-values for the analyzed roofs based on the material properties from table 1 are in table 2. direct comparison is made for the same thickness of thermal insulation (the pitch of the roof is made from polystyrene; the difference between the two samples in situ is 30 mm). therefore, the calculation is made for the same thermal insulation thickness and only with added green roof layers, based on the thermal insulation thicknesses of position 2. with the measured outdoor air temperature (-12.8 °c), the thermal loss through the fragment is 8 % lower. table 3 gives the calculated values with the influence of the vegetation and night sky radiation – the temperature was measured below the surface of the 491 peter juráš acta polytechnica ctu proceedings roof type u [w /(m2k)] thermal loss without snow [w/m2] thermal loss with snow [w/m2] ∆θ = 30.8 k ∆θ = 35.9 k non-green roof (position 1) 0.127 3.92 4.56 non-green roof with the same thermal insulation thickness as green roof – theoretical (position 2) 0.116 3.57 4.16 green roof (position 2) 0.107 3.30 (-8 %) 3.84 (-8 %) table 2. calculated u-values and thermal loss based on climate temperatures (θai and θae) . roof type u thermal loss without snow [w /(m2k)] ∆θ [k] [w/m2] non-green roof (position 1) 0.127 37.4 4.75 non-green roof with the same thermal insulation thickness as green roof – theoretical (position 2) 0.116 37.4 4.34 green roof (position 2) 0.107 19.5 2.09 (-52 %) table 3. calculated thermal loss without snow based on the measured surface temperatures (figure 8). vegetation during the same night -1.5 °c and for nongreen membrane -19.4 °c. these two temperatures were considered as outdoor air temperature and the indoor – outdoor air temperature difference was calculated with them. the high surface temperature differences showed extremely high thermal loss, with the difference up to 52 %. calculated temperature courses within the compositions are in figure 8. left picture is for the roof without snow. the red line represents results from table 2; the other two lines represent results based on table 3. table 4 shows calculated results for snow-covered roofs. steady state temperature courses are again in figure 8. because there were no surface temperatures on the snow measured, only outdoor air temperature can be taken into account. snow with the thickness of 150 mm was added for u-values of the regular compositions measured on 18th january in the morning (figure 5). thermal conductance is used from [24]. heat loss is 5 % lower for the green roof. snow decreases the sky radiation cooling potential for both roofs. 4. conclusions experimental setup for measuring the properties of the extensive green roof was briefly described in this paper with a closer look at the chosen two different days. as this is considered as the initial evaluation results, potential of the green roof during winter is drawn. results of the two days winter periods showed a clear benefit of the green roof. the green roof can decrease the peaks of the thermal loss through the fragment up to 50 %, which are during the freezing weather with the clear night sky. also during the other times, the additional layers decrease the loss. normal or usual benefit could be around 8 %. as stated in the introduction, the difference is influenced by the u value of the roof and nowadays, required values in slovakia are lower than 0.15 and recommended lower than 0.1. the analyzed roof is between these values. the reduction of the losses supported the findings by [12–14]. the correlation between the courses of the membrane temperature and outdoor air course is almost none, which is also confirmation of [13]. if the roof is covered with snow, the difference is not so high. the waterproofing membrane has other crucial benefits such as protection against uv radiation. the temperature variances are also very low; they are almost constant during the analyzed days. this is due to the higher thermal capacity of the vegetation, soil etc. compared to the flat roof with eps. this also minimizes the radiation overcooling in the night. the snow layer more influences the thermal loss of the non-green roof, where the snow increases the membrane temperature by 9 °c in this case by thermal resistance of the snow layer itself and also by protection against the night sky radiation. this is valid for the case of powder snow, as presented in this paper. the measurements are suited as long-term and are constantly running. therefore, these initial results will be supported by more measurements and more g roof compositions. some results, such as big difference between the air and vegetation surface differences will 492 vol. 38/2022 increasing thermal protection with use of green roof figure 8. steady state calculations of temperature courses within the roof not covered by snow (left) and with snow (right). roof type u thermal loss with snow [w /(m2k)] ∆θ [k] [w/m2] non-green roof (position 1) 0.073 35.9 2.62 non-green roof with the same thermal insulation thickness as green roof – theoretical (position 2) 0.069 35.9 2.48 green roof (position 2) 0.066 35.9 2.37 (-5 %) table 4. calculated thermal loss with snow based on the measured surface temperatures (figure 8). be analyzed. in the future, the summer season and the rainwater retention will be analyzed. other roof compositions are also upon completion. heat flux measurement will be added (nowadays it is only in one composition). measurement of the water content is 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https://doi.org/10.1016/j.enbuild.2021.110800 https://doi.org/10.1016/j.apenergy.2016.05.079 https://doi.org/10.1016/j.jenvman.2021.112682 https://doi.org/10.1016/j.ecoleng.2018.10.006 https://www.uceeb.cz/cz/zelene-strechy-a-fasady/ https://www.uceeb.cz/cz/zelene-strechy-a-fasady/ https://doi.org/10.1007/978-3-030-86001-1_13 https://doi.org/10.3390/en15041493 https://doi.org/10.2478/cee-2018-0016 https://doi.org/10.2478/cee-2020-0037 https://doi.org/10.4028/www.scientific.net/amr.1041.129 https://doi.org/10.4028/www.scientific.net/amr.1041.129 https://doi.org/10.1051/matecconf/201711700140 https://doi.org/10.1016/j.proeng.2015.07.062 https://doi.org/10.3390/ma15062040 acta polytechnica ctu proceedings 38:488–494, 2022 1 introduction 2 test site and methodology 3 results and discussion 3.1 in-situ temperature measurement 3.2 thermal loss calculation 4 conclusions acknowledgements references https://doi.org/10.14311/app.2022.33.0610 acta polytechnica ctu proceedings 33:610–616, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague effect of using biomass fly ash on the concrete sustainability elisabete rodrigues teixeiraa,∗, aires camõesb, fernando g. brancoc a university of minho, isise, department of civil engineering, guimarães, portugal b ctac, department of civil engineering, school of engineering, university of minho, campus de azur’em, 4800-058 guimarães, portugal c university of coimbra, isise, department of civil engineering, coimbra, portugal ∗ corresponding author: b8416@civil.uminho.pt abstract. biomass fly ash has been studied has a partial cement material, since this material has a positive effect on concrete properties. even though, some mixes with bfa presents a positive benefit, they need to be economically competitive and present a good environmental performance. so, the analyse and comparison of concrete that uses bfa as raw material substitution in terms of environmental impacts related to the production of conventional concrete. one of the best approaches to develop this type of study is to use the life cycle assessment (lca) method. this method quantifies both the input flows, such as energy, water and materials, as well as the output flows, such as co2 emission, solid wastes and liquid wastes. based both on the abovementioned context and methodological approach, a quantification and comparison of potential environmental impacts resulting from the production of 1 m3 of concrete was made, using different types of binder and quantities of cement substitution. keywords: life cycle analysis, sustainability, wastes. 1. introduction biomass fly ash (bfa) has an effect on concrete properties, presenting similar results to concrete with traditional pozzolanic materials such as coal fly ash [1]. moreover, the analysis of the potential environmental impacts related to the production of a concrete that uses bfa as raw material substitution is mandatory. this analysis needs to be compared with the production of plain cement concrete. one of the best approaches to develop this type of study is using the life cycle assessment (lca) method [2]. this method enables the quantification of the potential environmental impacts of products or services. it quantifies both the input flows, such as energy, water and materials, as well as the output flows, such as co2 emission, solid wastes and liquid wastes [3]. lca allows estimating the potential impact on humans and nature and enables identifying areas with improvement potential [3]. based both on the abovementioned context and methodological approach, a quantification and comparison of potential environmental impacts resulting from the production of 1 m3 of concrete was made, using different types of binder: i) portland cement; ii) portland cement and cfa or/and bfa, iii) portland cement and cfa and hl; and iv) portland cement, cfa, bfa and hl. this work was presented for the defence of the first author phd degree and it is presented in [4]. 2. methods and methodology 2.1. goal and scope the main goal of this study was to evaluate the environmental performances of various concrete formulations using biomass fly ash as a cement replacement material (table 1) [4]. the method used in this study followed the phases of an lca. the use of lca is done according to the iso 14040 standard, which provides a consensual framework, terminology and methodological phases [3]. the implementation of this method is based on four major phases: i) goal and scope definition; ii) inventory analysis; iii) impact assessment; and iv) interpretation [3]. the goal and scope express the purpose, objectives, product system, boundaries and functional unit. in the inventory analysis, the data necessary to analyse the life cycle of the product is collected. in the impact assessment, the life cycle inventory (lci) flows are classified, characterized and normalized, using one of many possible life cycle impact assessment (lcia) methodologies to estimate the potential environmental impacts. the last phase, interpretation, is very important to : i) identify, quantify and evaluate the information that results from the last two phases; ii) communicate the information in a correct way; and iii) recommend improvements within the analysed system [3, 4]. the comparative analysis and the aggregation of indicators were developed using the multicriteria decision support methodology for the relative sustainability assessment of building technologies (mars-sc) [5]. the mars-sc methodology is 610 https://doi.org/10.14311/app.2022.33.0610 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 33/2022 biomass fly ash on the concrete sustainability concrete mix %wt w/b cement bfa cfa hl ref 100.0 0.0 0.0 0.0 0.50 cfa20 80.0 0.0 20.0 0.0 0.50 bfa20 80.0 20.0 0.0 0.0 0.50 cfa40 60.0 0.0 40.0 0.0 0.50 bfa40 60.0 40.0 0.0 0.0 0.50 cfa50 50.0 0.0 50.0 0.0 0.50 cfa50b 50.0 0.0 50.0 0.0 0.35 bfa50 50.0 50.0 0.0 0.0 0.5 cfa60 40.0 0.0 60.0 0.0 0.5 bfa60 40.0 60.0 0.0 0.0 0.5 cfa49.5hl0.5 50.0 0.0 49.5 0.5 0.35 cfa49.5bfa0.5 50.0 0.5 49.5 0.0 0.35 cfa48.8bfa1.3 50.0 1.3 48.8 0.0 0.35 cfa45bfa5 50.0 5.0 45.0 0.0 0.35 cfa48.8bfa0.6hl0.6 50.0 0.6 48.8 0.6 0.35 table 1. binder fraction and water/binder ratio used in the concrete formulations [4]. based on three groups of sustainability categories: environmental, functional and economic [6]. since this research aimed at assessing the environmental performance of different concrete formulations, only the environmental category of mars-sc was considered. the mars-sc methodology is processed in five steps: i) definition of the sustainability indicators; ii) quantification of the indicators (including the life cycle inventory); iii) normalization of the indicators; iv) aggregation of the indicators; and v) sustainable score calculation and global assessment [5]. 2.2. system boundaries and functional unit the boundaries of this work mark the embodied environmental impacts (cradle-to-gate) of the different concrete compositions as well as the environmental impacts that result from the transportation of the materials to the concrete plant and their mixing. the choice of limiting the study to the cradle-to-gate stage is justified by the fact that, in the studied compositions, the use and disposal of concrete will result in similar environmental impacts [4]. the declared functional unit is dependent on the goal of life cycle analysis and therefore constitutes 1 m3 of concrete. 2.3. inventory analysis to quantify the sustainability indicators, it is necessary to first develop the inventory analysis [6]. the inventory is used to quantify the inputs (e.g. energy, materials and chemical) and outputs (e.g. emissions and wastes) of the product system [7].) table 2 shows the inventory of the materials and transportation considered for each concrete formulation [4]. this inventory took into consideration the specific context of the portuguese concrete industry. the life cycle analysis software simapro 7.3.3 was used to facilitate the quantification of the impact categories. in this study, the specific consumption of raw materials, energy and fuels and the emissions released to air, water and soil during the cement production of an important portuguese cement plant, located in the south of portugal, was considered. the data used are described in the public environmental declaration [8] of this cement plant. for this research, it was considered that this plant supplied the cement used for the preparation of the different concrete formulations. it was necessary to quantify the impact categories, since the environmental declaration did not cover all impact categories necessary for this study, being limited to those mandatorily declared according to portuguese environmental legislation [4]. using the inventory listed in the environmental declaration, the simapro software was used to assess the potential environmental impacts of the used portland cement. regarding each type of fly ashes, it was necessary to make the allocation of flows of the power plant according to the place of production. allocation is necessary in the case of joint coproduction, where the processes cannot be sub-divided, as is the case in fly ashes production [9]. allocation shall respect the main purpose of the processes studied, appropriately allocating all relevant products and functions. since the main purpose of a thermal power plant is to produce electricity and since the difference in revenue between the electricity and the fly ashes is high, it is not possible to use an allocation process based on physical proprieties (e.g. mass and volume). therefore, the allocation process used in this research was based on economic values [4]. due to the environmental report [10] from a major portuguese coal power plant (located in the centre 611 e. r. teixeira, a. camões, f. g. branco acta polytechnica ctu proceedings formulations pc* gravel sand water sp* cfa bfa hl material input (kg) ref 350.0 969.2 738.1 200.0 0.0 0.0 0.0 0.0 cfa20 280.0 982.1 756.7 149.5 0.0 70.0 0.0 0.0 bfa20 280.0 985.3 759.1 149.4 0.0 0.0 70.0 0.0 cfa40 210.0 973.0 749.6 149.7 0.0 140.0 0.0 0.0 bfa40 210.0 979.3 754.5 149.5 0.0 0.0 140.0 0.0 cfa50 175.0 968.4 745.1 149.8 0.0 175.0 0.0 0.0 cfa50b 175.0 1044.3 804.5 88.4 8.8 175.0 0.0 0.0 bfa50 175.0 976.3 752.2 149.6 0.0 0.0 175.0 0.0 cfa60 140.0 963.8 742.5 149.9 0.0 210.0 0.0 0.0 bfa60 140.0 973.3 749.9 149.7 0.0 0.0 210.0 0.0 cfa49.5hl0.5 175.0 1049.0 768.6 91.5 5.3 173.5 0.0 1.9 cfa49.5bfa0.5 175.0 1049.0 795.2 91.5 11.4 173.5 1.9 0.0 cfa48.8bfa1.3 175.0 1050.1 842.8 35.5 11.4 170.6 4.4 0.0 cfa45bfa5 175.0 1049.5 838.9 42.4 3.9 160.0 17.8 0.0 cfa48.8bfa0.6hl0.6 175.0 1067.2 832.9 36.9 2.5 170.6 2.2 2.2 transportation (tkm) ref 14.5 357.6 272.4 − − − − − cfa20 11.6 362.4 279.2 − − 12.4 − − bfa20 11.6 363.6 280.1 − − − 10.2 − cfa40 8.7 359.0 276.6 − − 24.8 − − bfa40 8.7 361.4 278.4 − − − 20.3 − cfa50 7.3 357.3 274.9 − − 31.0 − − cfa50b 7.3 385.3 296.9 − 2.9 31.0 − − bfa50 7.3 360.3 277.6 − − − 25.4 − cfa60 5.8 355.6 274.0 − − 37.2 − − bfa60 5.8 359.1 276.7 − − − 30.5 cfa49.5hl0.5 7.3 387.1 283.6 − 1.7 30.7 − 0.2 cfa49.5bfa0.5 7.3 387.1 293.4 − 3.8 30.7 0.3 − cfa48.8bfa1.3 7.3 387.5 311.0 − 3.8 30.2 0.6 − cfa45bfa5 7.3 387.3 309.6 − 1.3 28.3 2.6 − cfa49.5bfa0.6hl0.6 7.3 393.8 307.3 − 0.8 30.2 0.3 0.3 pc portland cement; sp superplasticizer table 2. binder inventory results of the material and transportation inputs for each concrete (per m3 of produced concrete) [4]. of the country), it is possible to known how many tons of coal are consumed to produce 1 kwh of electricity as well as the quantity of cfa produced during coal combustion. in portugal, the commercial value of cfa is about 18e/ton and the value of the electricity is 0.16e/kwh. therefore, the economic allocation coefficient of 0.17 % is applied to the impacts of the extraction, transportation and combustion of the coal from that power plant. as with the cement plant, this environmental report only covered the impact categories that are mandatory according to portuguese environmental legislation. as a result, all the flows (inputs and outputs) declared in this report were introduced in the simapro software, taking into consideration the quantified economic allocation coefficient of 0.17 %. regarding bfa, it is important to highlight that in portugal this kind of fly ash is considered a waste product and therefore they do not provide an economic value. because of this fact and according to the allocation rules presented in iso 14040, no flows from the thermal power plant are allocated in the production of bfa. with respect to the life-cycle inventory of the other used materials (gravel, sand, water and superplasticizer), generic data was used. since the development of specific environmental information for products is very time and cost consuming, initial lca studies, whose main goal was to compare different design scenarios, are normally based on generic (average) data [4,6]. for this reason and due to the lack of public available specific data for the abovementioned materials, this information was gathered from one of the most internationally accredited generic environmental databases of the ecoinvent report v2.2 [11]. the nearest context to the portuguese one was considered for this study. since the energy consumed during the manufacturing process is the parameter that most influences the life-cycle environmental impact [12] and 612 vol. 33/2022 biomass fly ash on the concrete sustainability since the portuguese energy mix is different from the european average [13], a contextualization of the energy used in each process was developed. this means that all used processes from the ecoinvent database were edited, and all energy input flows were changed to consider the portuguese energy mix. in the inventory of the transportation processes, the study considered the distances between the portuguese places of raw material extraction or raw materials storage facilities and the concrete mixing plant in question [4]. the distance between the raw materials production facilities and concrete production was considered for this study. the kilometres considered as the transportation distance of cement (secil group), sand and gravel (mibal minas de barqueiros, s.a.), coal fly ashes (pegop-energia eléctrica sa), biomass fly ash (altri), superplasticizer (basf) and hydrated lime (lusical-companhia lusitana de cal sa) to the concrete industry considered in this study were 41.5, 369, 177, 145, 329 and 117 km, respectively. the inventory related to the production of concrete was quantified taking into account the environmental product declaration (epd) of a specific portuguese concrete plant [14], where the different concrete formulations are supposedly produced. from this epd, only the flows related to the concrete mixing phase were considered. 2.4. impact assessment the life cycle inventory data was converted into potential environmental impact, using the lifecycle impact assessment (lcia) methods. in mars-sc, the definition of the sustainability indicators depends, above all, on the type of analysed product or building element and on the aims of the study. in this method, the environmental performance assessment is based on the following six environmental impact categories: i) global warming; ii) ozone depletion; iii) acidification of soil and water; iv) eutrophication; v) photochemical ozone creation; and vi) depletion of abiotic resources-fossil fuels. compared with the list of impact categories found in the en15804:2012 [15] standard, mars sc does not consider the depletion of abiotic resources-elements as an impact category. 2.5. normalization, aggregation and global assessment to avoid the scale effects in the aggregation of parameters of the different indicators and in order to minimize the possibility that some of the parameters are of the type. higher is better and lower is bad, it was necessary to normalize the indicators [15]. the normalization was done using the diaz-balteiro [16]. after normalization, is important to calculate the aggregation of each environmental indicator in terms of a global indicator, describing the overall environmental performance (nda). the global indicator nda results from the weighting average of each normalized. the sum of all weights must be equal to 1. for aggregation purposes, this study considers the weights (table 3) set in a study developed by the us environmental protection agency’s science advisory board (sab) [17]. in each sustainable profile, the global performance of a respective concrete with fly ash was monitored and compared with that of the reference concrete. indicator weight (%) gwp 38 odp 12 ap 12 ep 12 pocp 14 adp_ff 12 table 3. weight for each environmental indicator [17]. 3. results and discussion table 4 summarizes the values obtained in the quantification of the environmental impact categories, related with the production of the different concrete mixes. analysing the results, when it used just cement as binder, concrete presents the highest values of co2 emission. the high emission of co2 is a result of the chemical reactions (calcination) that occur during clinker production [4,18]. the incorporation of the bfa blended with cfa allows a reduction in all environmental impacts, moreover the environmental impacts decrease with increasing of cement substitution. at this stage, it is necessary to highlight the effect of the allocation step on the obtained results. in portugal, bfa are considered a waste product without economic value [18] and therefore there are no flows from the biomass power plant allocated to its production [4]. the same does not happen with cfa, as they have a market value and consequently a percentage of the power plant’s flows is allocated to their production [19]. concrete mixes 0.5%wt and 1.3%wt of bfa decreased the environmental impact related with the co2 emissions but an increase on the values of the others environmental indicators was observed [4]. this is a not result of the incorporation of bfa, since this result is not noted in the mixes with 5, 20, 40 and 60%wt, but it is due to the fact of these two mixes presented a higher superplasticizer content. it was concluded in other studies that superplasticizer had high influence on e.g. odp and adp_ff impact categories, but did not have a significant influence on gwp that most influences the overall environmental impact concrete with ashes, which confirm the results achieved in this study [4]. the normalization of the values obtained for each environmental impact category was obtained and the 613 e. r. teixeira, a. camões, f. g. branco acta polytechnica ctu proceedings formulations gwp100 odp ap ep pocp adp_ff kg (×102) kg (×10−5) kg kg (×10−1) kg (×10−2) kg (×103) ref 4.02 2.67 1.00 2.53 4.00 2.71 cfa20 3.44 2.55 0.91 2.32 3.64 2.56 bfa20 3.44 2.55 0.91 2.27 3.64 2.53 cfa40 2.83 2.38 0.80 2.08 3.23 2.36 bfa40 2.83 2.38 0.80 1.98 3.22 2.30 cfa50 2.53 2.30 0.75 1.96 3.02 2.26 cfa50b 2.88 2.69 0.90 2.41 3.97 3.10 bfa50 2.53 2.29 0.75 1.83 3.01 2.18 cfa60 2.23 2.21 0.69 1.84 2.82 2.16 bfa60 2.23 2.21 0.69 1.69 2.80 2.07 cfa49.5hl0.5 2.75 2.53 0.84 2.22 3.55 2.77 cfa49.5bfa0.5 2.96 2.77 0.94 2.51 4.20 3.31 cfa48.8bfa1.3 2.99 2.85 0.95 2.54 4.26 3.36 cfa45bfa5 2.75 2.59 0.85 2.22 3.57 2.73 cfa48.8bfa0.6hl0.6 2.73 2.57 0.83 2.18 3.46 2.64 table 4. values obtained for the different environmental impacts. formulations gwp100 odp ap ep pocp adp_ff [kg co2 eq] [kg cfc-11 eq] [kg so2 eq] [kg po4 eq] [kg c2h4 eq] [mj eq] ref 0.00 0.24 0.00 0.01 0.18 0.50 cfa20 0.32 0.44 0.30 0.26 0.42 0.62 bfa20 0.32 0.44 0.31 0.32 0.43 0.65 cfa40 0.66 0.72 0.65 0.54 0.71 0.78 bfa40 0.66 0.72 0.65 0.66 0.71 0.82 cfa50 0.83 0.86 0.82 0.68 0.85 0.85 cfa50b 0.64 0.20 0.32 0.16 0.20 0.20 bfa50 0.83 0.86 0.83 0.83 0.86 0.91 cfa60 1.00 0.99 0.99 0.82 0.99 0.93 bfa60 1.00 1.00 1.00 1.00 1.00 1.00 cfa49.5hl0.5 0.71 0.47 0.52 0.37 0.49 0.46 cfa49.5bfa0.5 0.59 0.09 0.21 0.04 0.04 0.03 cfa48.8bfa1.3 0.58 0.00 0.17 0.00 0.00 0.00 cfa45bfa5 0.71 0.37 0.50 0.38 0.47 0.49 cfa48.8bfa0.6hl0.6 0.72 0.41 0.54 0.42 0.55 0.56 table 5. normalized values of the studied environmental impact categories [4]. results are presented in table 5. this enables a better perception of which of the concretes has a better environmental performance. it is observed that, concrete in which 60% bfa had the best environmental performance, being ref and cfa48.8bfa1.3 the mixes that presented the lowest environmental performance. figure 1 present the sustainability profiles and the overall environmental performances. at the level of each impact category, the best concrete is the one that has the value closest to one. it is verified that the bfa60 concrete presented the best environmental performance (nda = 1.00) and plain cement concrete (ref) presented the worst performance (nda = 0.12) [4]. therefore, these results allow the conclusion that using a high content of bfa significantly increases the environmental performance of concrete production, since the overall environmental performance of concrete is improved. additionally, the usage of these materials contributes to a better compatibility between the construction sector and the goals of sustainable development [4]. 4. conclusions bfa used alone or blended displayed a capability to reduce the environmental impacts of concrete, when compared to conventional concrete. the results showed that 60% of bfa presented the best results. besides the good results observed, it is important to refer that the concrete formulations with 0.5 and 1.3%wt of bfa presented a low environmental performance, but better than that of plain cement concrete. the environmental issues related with 614 vol. 33/2022 biomass fly ash on the concrete sustainability figure 1. normalized values that described the sustainability profile. these two concrete formulations are due to the use of a higher content of superplasticizer when compared with the other formulations. the effect of superplasticizer on the environmental indicators is known, but the results showed that it is possible to slightly reduce the content of sp, without having a significant effect on the workability, maintaining the values similar to those of the concrete formulation with 5%wt of bfa [4]. the decrease of the content of sp has an important contribution on the improvement of environmental performance of these two concrete mixes. acknowledgements the authors wish to thank the portuguese foundation for science and technology (fct) and the eco-construction and rehabilitation doctoral program for supporting the phd scholarship (reference pd/bd/ 52661/2014). this work was partly financed by fct/mctes through national funds (piddac) under the r&d unit institute for sustainability and innovation in structural engineering (isise), under reference uidb/04029/2020. references [1] r. rajamma, l. senff, m. j. ribeiro, et al. biomass fly ash effect on fresh and hardened state properties of cement based materials. composites part b: engineering 77:1-9, 2015. https: //doi.org/10.1016/j.compositesb.2015.03.019. 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[14] concretope fabrica de betão pronto s.a, environmental product declaration for ready-mixed concrete, (2005). [15] cen en 15804:2012. sustainability of construction works environmental product declarations core rules for the product category of construction products, cen:brussels, 2012. [16] l. díaz-balteiro, c. romero. in search of a natural systems sustainability index. ecological economics 49(3):401-5, 2004. https://doi.org/10.1016/j.ecolecon.2004.02.005. [17] epa science advisory board, toward integrated environmental decision-making, washington, dc, united states. epa-sab-ec-00-011, 2000. [18] e. r. teixeira, r. mateus, a. f. camões, et al. comparative environmental life-cycle analysis of concretes using biomass and coal fly ashes as partial cement replacement material. journal of cleaner production 112:2221-30, 2016. https://doi.org/10.1016/j.jclepro.2015.09.124. [19] d. garcía-gusano, d. garraín, i. herrera, et al. life cycle assessment of applying co2 post-combustion capture to the spanish cement production. journal of cleaner production 104:328-38, 2015. https://doi.org/10.1016/j.jclepro.2013.11.056. 616 https://repositorium.sdum.uminho.pt/bitstream/1822/7542/1/artigo%204.pdf https://doi.org/10.1016/j.ecolecon.2004.02.005 https://doi.org/10.1016/j.jclepro.2015.09.124 https://doi.org/10.1016/j.jclepro.2013.11.056 my title https://doi.org/10.14311/app.2022.36.0191 acta polytechnica ctu proceedings 36:191–197, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague structural reliability of existing rc beams strengthened with uhpfrc tensile layers lenganji simwanda∗, nico de koker, celeste viljoen, adewumi john babafemi stellenbosch university, department of civil engineering, private bag x1, matieland 7602, south africa ∗ corresponding author: 24429902@sun.ac.za abstract. a methodology for reliability analysis of reinforced concrete (rc) beams strengthened with ultra high-performance fibre reinforced concrete (uhpfrc) tensile layers is presented. the proposed methodology includes stochastic stress-block analysis of a section, assuming a perfect bond between the rc beam and the uhpfrc layer. annual reliability analysis of the rc beam before and after the strengthening operation is conducted. deterioration induced by chloride corrosion is incorporated into the analysis via a chloride induced corrosion model based on fick’s law of diffusion and described stochastically to account for the epistemic uncertainty in the time to corrosion initiation and rate of corrosion. a plot for determining the required thickness of the uhpfrc tensile layer to upgrade to the required reliability level is also given, considering the time from construction to when the strengthening operation is conducted. the proposed approach is easy to apply for routine practice. keywords: corrosion damage, rehabilitation of existing structures, reliability analysis, uhpfrc tensile layers. 1. introduction addition of ultra high-performance fibre-reinforced concrete (uhpfrc) layers on the tensile side is a newly developed technique for strengthening reinforced concrete (rc) constructions, such as floor slabs, beams and bridge decks [1]. other conceptual ideas include additional layers on the compression side or even including on the sides (referred to as jackets) to improve the bending resistance of rc beams. efforts have been made in the recent past to investigate the efficacy of the uhpfrc strengthening techniques for retrofitting and rehabilitation of existing rc structures. for example, farhat et.al [2] observed an increase in the load carrying capacity and reduction in autogenous shrinkage strains of damaged rc beams retrofitted with uhpfrc and proposed a load-carrying prediction model based on fracture mechanics. habel et.al [3] investigated, through a parametric study, the flexural response of composite uhpfrcrc elements via the traditional analytical crosssectional approach and observed that addition of steel reinforcement in the uhpfrc layer is the most effective approach to increase the moment capacity. a most recent work by lampropoulos et.al [4] assessed, experimentally and numerically, the efficacy of uhpfrc layers and jackets for the strengthening of existing rc beams, and observed that the three side uhpfrc jacket approach was superior in terms of performance. with efforts to apply these techniques to routine practice, analytical and numerical models [1–4] have been proposed for the estimation of bending moment capacity of rc beams strengthened with uhpfrc layers. despite several uncertainties present in the formulations of the present analytical approaches, the application of structural reliability method has received very little attention in this research area. the objectives of this paper are two-fold. the first being to assess the viability of the stress-block section analysis in the estimation of nominal bending moment capacity of rc beams strengthened on the tensile side with uhpfrc layers, and the second being to present and apply a statistical methodology for conducting reliability analysis of existing rc beams strengthened with uhpfrc tensile layers. 2. stochastic modelling of deterioration of rc beams prior to strengthening 2.1. corrosion modelling a significant factor to the structural deterioration of rc beams exposed to saline conditions is the corrosion of steel reinforcements. therefore, their remaining service life when strengthened with uhpfrc is considered in this study in the context of chloride induced corrosion. the effect of corrosion-induced deterioration on the cross-section area of steel rebars is discussed while ignoring its effects on other geometrical and material properties of steel and concrete. for simplicity, a model that assumes uniform corrosion and proposed in [5, 6] is considered here, but more detailed corrosion models may be applied in appropriate cases following the methodology presented in this paper. in [5, 6], a model is presented such 191 https://doi.org/10.14311/app.2022.36.0191 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en l. simwanda, n. de koker, c. viljoen, aj. babafemi acta polytechnica ctu proceedings that the evolution in time of the diameter ϕ of the corroded rebars is computed via equation (1) ϕ(t) = { ϕinitial t ⩽ t0 ϕinitial − 0.0232icorr(t − t0) t > t0 (1) where ϕinitial is the initial bar diameter, t0 is the time (in years) to corrosion initiation, and icorr is the corrosion ratio (in µa/cm2) computed empirically via equation (2) icorr = 37.8(1 − w/c)−1.64 c′ (2) where w/c and c′ is the water to cement ratio of concrete and concrete cover to reinforcement, respectively. with this model, the reduction of the cross-section area of the steel reinforcement as(t) as a function of time can be computed from the time varying diameter ϕ(t). 2.2. corrosion stochastic simulation it is worth noting that equations (1) and (2) contain parameters associated with given levels of uncertainty, that needs to be quantified and accounted for in the reliability analysis of rc beams exposed to marine environments. specifically, the espistemic uncertainty in the time to corrosion initiation t0 and corrosion ratio icorr needs statistical description. following a previous work [7], the epistemic uncertainty in t0 and icorr were reported to be well described by weibull and uniform probability distributions, respectively. for the problem considered in this work, t0 was estimated to have weibull parameters (α = 11.3, βw = 4.81), whereas icorr was taken to be uniformly distributed between the closed interval icorr ∈ [2, 3]. figure 1 shows the mean time series and its corresponding 10% confidence interval of the stochastic process describing as and estimated via monte carlo sampling with 106 samples. 3. analysis of rc beams strengthened with uhpfrc 3.1. moment capacity: proposed approach the moment capacity of non-strengthened rc beams can readily be computed by analytical models reported in literature [8–10]. the eurocode model [9] of singly reinforced concrete beams is used in this work. building on the assumptions of the eurocode model [9], a procedure for estimating the moment capacity of rc beams strengthened with tensile uhpfrc layers is proposed. the approach is based on the stress-block analysis of a balanced section is, assuming a perfect bond between the rc beam and the uhpfrc layer and that the section cracks in the rc zone of the strengthened beam transferring the section forces to the longitudinal reinforcements and the fibres in the uhpfrc layers[11]. 10 20 30 40 50 0 50 100 150 200 250 300 figure 1. uncertainty quantification of deterioration of steel cross-sectional area in the rc beam over its lifetime. the analysis of the cracked section assumes strain compatibility across the cross-section (see figure 2), maximum strain in concrete, and yielding of steel fibres in the uhpfrc layer is reached first. with this assumption the characteristic moment capacity of the rc beam strengthened with layers may be estimated via the following equations. the stress σf and strain εf in the extreme steel fibres of the uhpfrc layer may be computed via equations (3) and (4) σf = 2τf lf df (3) εf = σf ef (4) so that the position of the the neutral axis of a balanced section can be obtained via a = fykas + ftkbtγ fckb (5) c = a/0.80. (6) the position where the force in the steel rebars and the force in the steel fibres effectively act (denoted as ζ) is ζ = fykas(h − c′ − c) + ftkbtγ(h − c − btγ) fykas + ftkbtγ (7) and finally, the characteristic moment capacity mc of the beam strengthened with a tensile uhpfrc layer is mc = ζ(fykas + ftkbtγ) (8) 192 vol. 36/2022 reliability of rc beams strengthened with uhpfrc as b d c′ tγ uhpfrc layer h c na εst εfs εcc 0.85fc fstas ft a fcc fst ffs a 0.85fc fcc ffs + fst c ζ z 0.85fc figure 2. stress block geometries for the for the proposed approach considered with tensile capacity of uhpfrc layer due to steel fibres considered 3.2. comparison of proposed approach to test results 12 rc test beams strengthened with tensile uhpfrc layers from previous experimental protocols [3, 12, 13] are used to validate the proposed stress-block approach. figure 3 shows a comparison in moment capacity prediction between the proposed analytical procedure and what was experimentally reported. a significant correlation between the proposed approach and test results is observed. 10 20 30 40 50 60 70 80 0 10 20 30 40 50 60 70 80 figure 3. comparison of a proposed analytical procedure with experimentally predicted moment capacity. 4. reliability assessment of rc beams strengthened with uhpfrc 4.1. basic concepts the current set of recommendations for the assessment of existing concrete structures given by the iso 13822:2010 [14], probabilistic model code by the joint committee on structural safety (jcss pmc 2010 [15]), and the iso 2394:2010 [16] are strongly based on reliability concepts. the fib bulletin 80 [17] gives a codified reliability-based framework that is consistent with iso 2394:1998 [18] for the upgrading of existing concrete structures. generally, reliability targets of βup,50 = 3.3 and β0,50 = 2.3 are given in fib bulletin 80 [17] for upgrading and assessing existing structures related to the ultimate limit states for reliability class 2 and 50 years reference period, respectively. for a reference period of 1 year, these values correspond to reliability indices of about βup,1 = 4.3 and β0,1 = 3.5, respectively. considering the ultimate limit state of flexure, for a rc beam strengthened with a uhpfrc tensile layer and originally designed to take an annual peak moment mapp, the performance function g required for reliability analysis can therefore be cast as g = θmc − kemapp (9) where θ and ke are the uncertainties in moment capacity and applied moment respectively (see table 1). statistical properties of θ were assessed by comparing the present stress block approach with the test results in figure 3. using similar reliability formulations as stated in [27], annual failure probability pf,1 and reliability index β1 can readily be computed by first order reliability method (form) and monte carlo simulation (mcs). 193 l. simwanda, n. de koker, c. viljoen, aj. babafemi acta polytechnica ctu proceedings parameter statistical model char mean cov ref. b, mm gaussian 150 152.50 0.01 [19] h , mm gaussian 250 258.20 0.02 [20] 1)as, gaussian as 1.021as 0.0125 [21] efs, gpa lognormal 200 202 0.01 [21] fy, mpa lognormal 500 573.15 0.083 [20] fc, mpa lognormal 39.50 52.25 0.17 [22] ft, mpa lognormal 12 19.66 0.3 [22] ρc, kg/m3 gaussian 2268 2315 0.012 [21] 2)qk, kn gumbel 0.682qk 0.25 [23] c′, mm gamma 25 26.20′ 0.03 [23] l, m constant 2.2 [23] 3)tγ, mm constant tγ [23] τf , mm lognormal 4.15 4.72 0.06 [24, 25] lf /df , m constant 6.5 θ, [-] lognormal 1.08 0.16 ke , [-] lognormal 1.01 0.1 [26] notes: 1) the values of as are updated using the stochastic model in figure 1. 2) qk is adjusted for each beam so that the strengthened beam is critical when analysed. and designed via the present stress block approach. 3) tγ is varied to determine its variation with β1 (section 4.3). table 1. statistics of stochastic parameters in the load, geometric, and resistance models 4.2. reference beam and data collection the rc beam strengthened with a tensile uhpfrc layer to be considered in this work is based on a previous study [4]. the cross sectional dimensions of the beam were 150 mm by 250 mm, with a span of 2.2m. the rc beam was strengthened with 50mm thick layer of uhpfrc on the tensile side. the beam was reinforced with two 500mpa high strength steel rebars with a diameter of 12mm, at a cover of 25mm. the 28 days characteristic compressive strength of the beam and of the uhpfrc layer were 39.5 mpa and 164 mpa, respectively. the average tensile strength of uhpfrc was reported as 12 mpa. to account for the uncertainty in each of the parameters of the beam describing the performance function, a stochastic description of each parameters in terms of probability distribution, mean value and coefficients of variation were defined based on previous literature (table 1). the rc beam strengthened with the uhpfrc tensile layer is analysed and designed with characteristic imposed load qk adjusted for the beam to be critical. specifically, the characteristic imposed load needed to obtain the strengthened beam via the present stress block approach is determined, after which the corresponding mean imposed load is obtained using the assumed coefficient of variation. in addition, the dead load is considered as the self-weight of the beam with its uncertainty accounted for via uncertainties in the geometric properties of the strengthen beam and density of concrete. 5 10 15 20 25 30 35 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 figure 4. required upgrading reliability increase β1,upgrade as a function of the thickness of the uhpfrc layer tγ and the year when the strengthening operation is performed. 4.3. data analysis to apply the proposed methodology for assessing and upgrading the reliability of rc beams strengthened with uhpfrc layers, lets consider a rc beam under the effect of corrosion whose annual reliability index at year n has been assessed and say deteriorated to βup,1 = 4.3. techniques presented in [28] may be applied for this purpose. the evolution of the reliability 194 vol. 36/2022 reliability of rc beams strengthened with uhpfrc 10 20 30 40 50 3 3.2 3.4 3.6 3.8 4 4.2 4.4 4.6 4.8 5 figure 5. evolution of the reliability index of the rc beam prior to strengthening strengthening (blue solid line) and after addition of 14mm thick uhpfrc tensile layer at 28 years old (black dotten line). index of the rc beam prior to strengthening up to the time β1 reaches a value of βup,1 = 4.3 is given by the blue curve in figure 4. now, regarding the requirement of upgrading this rc beam back to βtarget,1 = 4.7 by strengthening with uhpfrc tensile layers, the thickness of the uhpfrc layer tγ that guarantees a reliability upgrade (β1,upgrade = βtarget,1 − βup,1 = 4.7−4.3 = 0.4) needs to be determined. by computing the increase in reliability index for increasing tγ of the rc beam considered to be upgraded at year n, figure 4 was obtained and may be used for estimating tγ given the required increment in the reliability (∆β). assuming continuation of the corrosion process, the reduction of the reliability index after the strengthening operation can be computed to estimate the remaining service life of the beam (shown by a dotted black curve in figure 5). to decipher the relative importance of the stochastic basic variables on the computed reliability indices, a form-based sensitivity analysis is conducted (see figure 6). the live load q has the highest influence on the computed β values followed by the model uncertainties in the resistance and load models, respectively. as expected, the variables with the lowest variability (table 1) have insignificant influence of the computed β values. most importantly, the α value of model error θ serves as an indication of the required partial resistance factors for the stress-block formulation presented in this work. detailed investigation on the statistical characteristics of θ based on a larger database remains an avenue for future studies q a s f y h c b f c f t k e 0 0.2 0.4 0.6 0.8 1 figure 6. first order reliability method (form) sensitivity factors of the stochastic basic variables. deterministic basic variables have form α values of zero. 5. conclusions a methodology for assessing and upgrading the reliability of rc beams strengthened with uhpfrc tensile layers has been presented. the applied methodology includes stochastic stress-block analysis of a section, assuming a perfect bond between the rc beam and the uhpfrc layer, and then conducting life-time reliability of the rc beam before and after the strengthening operation. deterioration induced by corrosion can be incorporated into the analysis via stochastic corrosion models. a plot for determining the required thickness of the uhpfrc tensile layer to upgrade to the required reliability level is also given, considering the time from construction when the strengthening operation is conducted. 195 l. simwanda, n. de koker, c. viljoen, aj. babafemi acta polytechnica ctu proceedings the present work has only considered the timedependent deterioration model in terms of the corrosion of steel reinforcing area. consideration of timedependent behaviour of concrete, uhpfrc and steel strength, other geometric properties of concrete and the uhpfrc layer, and the time-dependent correlations in reliability analyses should be considered in future studies, with more detailed models and availability of information on the time-dependent correlation of deteriorating rc beams strengthened with uhpfrc layers. 6. list of symbols a, depth of a rectangular stress block as, area of steel rebar in tension b, width of the beam c, depth to the neutral axis d, depth from extreme compressive fiber to centroid of rebar steel efs, modulus of elasticity of fibers fc, compression strength of uhpc ft, the tensile stress of uhpc fy, yield strength of steel rebar fst, stress in steel rebar ρc, density of concrete h, height of the beam c′ cover to steel rebar lf , length of fibers df , diameter of the fibers ρs, rebar percentage σfy, fiber yielding stress τf , fiber-concrete bond strength β1, stress block parameter ρc, density of uhpfrc ρst, density of steel vf , percentage fibre content tγ, thickness of the uhpfrc layer. acknowledgements reliability analysis computations were performed using the uqlab reliability analysis library [29]. references [1] t. noshiravani, e. brühwiler. analytical model for predicting response and flexure-shear resistance of composite beams combining reinforced ultrahigh performance fiber-reinforced concrete and reinforced concrete. journal of structural engineering 140(6):04014012, 2014. https://doi.org/10.1061/(asce)st.1943-541x.0000902. 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[29] s. marelli, b. sudret. uqlab: a framework for uncertainty quantification in matlab. in vulnerability, uncertainty, and risk, pp. 2554–2563. american society of civil engineers, reston, va, 2014. https://doi.org/10.1061/9780784413609.257. 197 https://doi.org/10.1007/s11367-011-0297-3 https://doi.org/10.35789/fib.bull.0080 https://doi.org/10.1007/s11367-011-0297-3 https://doi.org/10.1016/j.engstruct.2013.07.015 https://doi.org/10.1111/j.1559-3584.2002.tb00128.x https://doi.org/10.1016/j.cemconcomp.2017.02.006 https://doi.org/10.1016/s0140-6736(06)69720-1 https://doi.org/10.6028/nbs.sp.400-64 https://doi.org/10.1016/j.engstruct.2006.02.005 https://doi.org/10.1080/15732479.2016.1198394 https://doi.org/10.1016/j.engstruct.2021.112767 https://doi.org/10.1016/j.strusafe.2007.02.002 https://doi.org/10.1061/9780784413609.257 acta polytechnica ctu proceedings 36:191–197, 2022 1 introduction 2 stochastic modelling of deterioration of rc beams prior to strengthening 2.1 corrosion modelling 2.2 corrosion stochastic simulation 3 analysis of rc beams strengthened with uhpfrc 3.1 moment capacity: proposed approach 3.2 comparison of proposed approach to test results 4 reliability assessment of rc beams strengthened with uhpfrc 4.1 basic concepts 4.2 reference beam and data collection 4.3 data analysis 5 conclusions 6 list of symbols acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.39.0006 acta polytechnica ctu proceedings 39:6–15, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague psychosocial aspects of the use and provision of carsharing services in the czech republic lucie hartmannováa,∗, martin schlossarekb, ondřej kročilc, matúš šuchaa a palacký university olomouc, faculty of arts, department of psychology, křížovského 511/10, 779 00 olomouc, czech republic b palacký university olomouc, faculty of science, department of development and environmentla studies, 17. listopadu 12, 771 46 olomouc, czech republic c palacký university olomouc, faculty of arts, department of economic and managerial studies, křížovského 511/10, 779 00 olomouc, czech republic ∗ corresponding author: luciehartmannova03@upol.cz abstract. the concept of shared mobility involves a type of transportation based on advanced digital technologies which make it possible to approach mobility as a service. this concept seeks to reduce the need for individual ownership of means of transport and enhance the effectiveness of their use. the aim of this paper is – to identify the reasons for the use of carsharing services among czech users and to reveal the perceived barriers and benefits from the perspective of both providers and users of carsharing services. in line with the objectives of the paper, qualitative research was chosen. the study is expected to describe the psychosocial aspects of the use of carsharing, and report experience with the provision of such services in the czech republic. based on an interpretative phenomenological analysis, we found several reasons, benefits and barriers to the use and provision of b2c carsharing among 11 users and 4 providers. the contributions of this paper include the fact that it focuses on the experiences of both parties – carsharing providers and carsharing users, and it is the first study of this type that has been conducted in the czech republic. this deliverable was co-financed by the grant projects iga_ff_2021_001 and iga_ff_2021_018 (upol). keywords: carsharing, czech republic, users, providers, benefits, barriers. 1. introduction the concept of shared mobility is one of the sectors of the sharing economy. against the backdrop of global environmental, quality of life or social issues, the sharing economy has significant potential to contribute to more sustainable consumption, taking into account all social, economic or environmental impacts [1]. shared mobility can be seen as modes of transport that are based on advanced digital technologies that allow to approach mobility as a service. it focuses on increasing the efficiency of transport use and reducing the need for individual ownership. shared mobility can positively influence the development of the urban transport system, which faces many challenges due to the significant increase in the number of privately owned vehicles or the rate of motorization [2]. one component of shared mobility is carsharing, which provides citizens with a new travel option that influences their travel behaviour. it is also a factor influencing transport and consequently the environment in cities [3]. carsharing thus represents a system that allows people to use locally available cars at any time and for any duration [4]. carsharing is still a relatively new and pioneering concept for czech citizens. although the use of this service has increased rapidly in recent years, it is still not used in sufficient quantities, yet there is great potential for its expansion. the czech carsharing association (2021)1 reports that there were only 30 carsharing cars on czech streets in 2014, in 2018 the number of carsharing cars was around 490 and today in 2021 carsharing services boast a total of 1250 carsharing cars. although there are many foreign studies (see below) dealing with carsharing, the truth is that there are not many for the czech republic. in the domestic context, for example, foltýnová and vejchodská [5] have tried to map the typology of carsharing users, matowicki and colleagues [3] have looked at understanding the purposes of carsharing from the users’ perspective, and kimbrell [6] has looked at shared electric cars. we do not know much about the motivations to use carsharing and the experience of providers with carsharing users in the czech environment. therefore, we set out to describe the experiences of both parties – providers and users of carsharing – in more detail. this is the first study of this type that has been conducted in the czech republic. the next part of the paper is devoted to empirical data obtained from semistructured interviews with providers and users of b2c (business-to-consumer) carsharing. in this form of carsharing, individual providers (companies, coopera1https://ceskycarsharing.cz 6 https://doi.org/10.14311/app.2022.39.0006 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en https://ceskycarsharing.cz vol. 39/2022 psychosocial aspects of the use and provision of carsharing services tives or non-profit organisations) own a fleet of cars that customers can use. the cars are mostly located in cities. users can pick up the cars in a self-service mode and pay for the distance travelled or the time of use. insurance, maintenance and repairs are usually paid by the providers [7]. 2. research design and methods the research problem of the present study lies in the lack of empirical data that would help to understand the psychosocial aspects of the use and provision of the relatively newly emerging carsharing services in the czech republic. therefore, we set out to (1) identify the reasons for the use of carsharing services among czech users and (2) uncover perceived barriers and benefits from the perspective of both providers and users of carsharing services. the starting point will be a description and evaluation of carsharing users’ personal experiences with the service itself and providers’ personal experiences with their clients. for both objectives, we set several research questions: rq1: how do providers respond to feedback from b2c carsharing users? rq2: how do b2c carsharing providers rate the perceived benefits and barriers associated with implementing the service? rq3: how do b2c carsharing users rate the perceived advantages and disadvantages of using these services? rq4: for what reasons do b2c carsharing users use these services? in line with the focus of our research, we chose a qualitative design, similar to svennevik [8, 9] or svennevik, julsrud, and farstad [10], because it allows us to explore the phenomenon in depth in several cases. qualitative research was chosen mainly because there are no such focused studies conducted in the czech republic and we need to get to the core of the phenomenon under investigation. the method of data collection was a semi-structured interview. interpretative phenomenological analysis (ipa) was chosen to process the collected data as in the study by quyen [11]. we chose non-probabilistic methods of participant selection – simple purposive sampling, self-selection and snowball sampling [12]. for the purpose of this paper, respondents are divided into two categories, according to whether they use or provide carsharing services (b2c type). as part of the preparatory phase of the interview, detailed research was conducted to develop the questions. the interview was divided into six main parts depending on whether the interviewee was a carsharing provider or user. the length of the interviews averaged around 50 minutes. the video interviews were conducted online from march to august 2022 via the zoom and skype platforms or by telephone. advertising was conducted through two main communication channels – the social network “facebook” and email correspondence. 2.1. description of participants 15 respondents answered the advertisement and met the criteria, including 4 providers (autonapůl, car4way, karkulka, greengo) and 11 b2c carsharing users. all respondents agreed to record an audio version of the interviews. the reader can find individual providers under codes p1-p2 and individual users under codes u1-u11. in the case of the b2c carsharing providers, we conducted semi-structured interviews with people in relevant positions (p1 and p4 are directors, p2 is a manager and p3 is the head of the carsharing section) so as to have as much insight as possible into the phenomena under study. the length of experience of the providers in their subject ranged from 1 and a half years (p4) to 13 years (p1), with an average of 6 years (p2 and p3 were around that time). providers p2 and p4 have their main location in prague, p1 in brno and p3 in plzeň. next, we come to b2c carsharing users. the average age of users was around 39 years (range 31-50 years). except for the respondent (u11), all of them consistently stated that they use b2c carsharing in and around their city of residence. user u11 lives in susice, but there are no carsharing parking zones, so she has to commute to plzeň or české budějovice. the highest level of education was higher education, with only one case of secondary education with a high school diploma. the following table 1 describes the basic data about the research participants (users). the above description corresponds with the profile of a typical client (user) that we obtained from the mentioned providers. they agreed that the most frequent users of their service are individuals of working age, university educated and from a larger city in the czech republic. the predominance of men in the use of carsharing, as reported by the users, is also in line with our surveyed set of users. all users also emphasized that they had no need to own a private car. which in turn is consistent with the providers’ description of typical clients. in addition to the description of the typical user, it should not be forgotten that they most frequently reported that they use carsharing primarily for private purposes and rather occasionally. similarly, this purpose was consistently reported by all providers. 2.2. ethical considerations prior to the interview, an electronic informed consent was sent to all participants, where they were instructed on all ethical aspects of their participation in the research (all in accordance with the provisions of act no. 110/2019 coll. [13], gdpr). subsequently, when they agreed to participate in this research, they were asked to scan and forward the signed informed consent electronically. both providers and users agreed 7 l. hartmannová, m. schlossarek, o. kročil, m. šucha acta polytechnica ctu proceedings gender educational residence length of service used carsharing attainment (in years) companies/associations in the czech republic u1 man uni∗) brno 6 autonapůl u2 man uni brno 2 autonapůl, ajo u3 man uni praha 6 anytime, car4way, greengo u4 man uni praha 7 autonapůl, car4way u5 man uni praha 8 autonapůl, car4way u6 man uni liberec 7 autonapůl u7 man high school brno 2 autonapůl u8 man uni praha 4 car4way u9 man uni praha 5 autonapůl u10 man uni praha 7 autonapůl u11 man uni sušice 5 autonapůl ∗) uni = university table 1. basic characteristics of b2c carsharing users. in advance to the audio recording of the interviews and were assured that the data collected would be anonymised. to preserve ethical considerations, we do not provide their real names or the names of the carsharing entities. 3. analysis of the data in analyzing the data we followed the ipa (interpretative phenomenological analysis) methodology. it is a great method if we want to study a person holistically, we do not know much about the phenomenon and we do not want to reduce it to numbers or disembodied patterns, as is the case in quantitative research [14]. in the previous section we commented on researcher self-reflection. the following process consisted of six stages: (0) self-reflection of the researcher, (1) reading the interview transcripts, (2) initial notes, (3) developing themes, (4) finding connections across themes, and (5) cases [10, 11]. common themes, interrelationships between themes, and schemas were processed in word processing and spreadsheet software (microsoft word and excel). as the number of participants is quite high, we do not have space here to delve too much into describing the individual experiences of the participants (we will only mention selected specific themes), so we will directly focus on finding common themes and their interrelationships [15] on the side of both users and carsharing providers. 3.1. common themes in the following paragraph we will focus on the resulting synthesis of the themes. the analysis of user and provider responses identified 16 main themes and 8 sub-themes that are common to most participants and offer insights into the issues. each theme is illustrated with specific examples of participants’ accounts. the division of the text adheres to the following logic: a) we list the main areas according to the research questions, b) we list the common themes and sub-themes for each area, and c) in some cases we also list selected specific themes. 3.1.1. experience with clients – feedback and its solution first, we look at how providers evaluate and respond to feedback from their clients. in particular, we will note common themes that providers agree on, while illustrating them with common themes from users who feel similarly. all providers agreed that they have primarily positive experiences with clients. reckless handling of the car may occur, but these issues are rare. some example issues are outlined by provider p4: “...users who frequently leave the car dirty inside – whether from mud, pet dogs making a mess on the seats, hair removal, etc.”. provider p2 states, similar to p4, that “what annoys us the most is that some people treat it badly because it’s a rental.”. provider p4 adds that “dirty car, that happens only with a few of the users”. thus, we can see that there may be a disregard for shared property on the part of users. these little things, if not noticed and acknowledged by providers, can then lead to clients picking up a car after someone who is not very tidy and noticing it understandably so. this may show up negatively in their feedback of the providers. it is these little things that are also mentioned by users u2, u4, u5, u6, u8. as an example, user u4 mentions that “... you don’t know who you are renting the car from, like i said, the car might be dirty, it might be damaged etc.”. as the client is the main priority for all providers, their feedback is very important for them to be able to adjust their service offer according to their needs and the current market, ideally to the satisfaction of both parties, thus increasing the flow of new users and the retention of existing ones. feedback from users is generally positive and constructive, as stated by e.g. provider p1: “the vast majority of our client 8 vol. 39/2022 psychosocial aspects of the use and provision of carsharing services reviews are positive or constructive.” or provider p4: “... mostly good. so we have 98-99 % positive feedback”. in the feedback, providers consistently reported that they encounter negative feedback from users when there is insufficient coverage of their carsharing services both in and around cities and outside cities. providers are aware of this and are doing their best to expand in the country. insufficient coverage was also a common theme for all users. they would like to see services more widespread both in the city but also around the train stations. so that for the car they find in their area, it would be within walking distance. user u1 suggests that it would be “ideal to keep those parking zones as small as possible and make them a commuter area where a lot of people live and work”. how do users respond to feedback from their clients? by making acceptance of the client’s needs a central concern, providers try to listen to and accommodate their clients. this is represented by phrases such as “we try to accommodate those people” (p2) or, “we really listen to our clients and register their insights, ...” (p4). related to this is the fact that providers try to address concerns proactively and quickly, most often through compensation, i.e. by compensating or minimising client harm. for example, provider p1 states that they try to “... do everything possible for the user so that everything goes as it should. and if there are any inconveniences, to feel them as little as possible or to be spared the inconvenience.” and compensation typically takes place by providing the user with “... a replacement car, probably a car in a higher category than he had booked, and by the fact that he is not at fault, he will get the car at a better price than the cheaper car.”. similarly, clients also perceive quick problem solving by providers, i.e. an effort to accommodate users. this is also one of the reasons why they like to use the service. 3.1.2. barriers to the provision and disadvantages of carsharing how do providers rate the barriers to provision and users rate the disadvantages in using car valeting services? we look at this in the following paragraphs. one of the main barriers mentioned by all providers relates to the mindset of society. for example, provider p4 communicated that, “it’s about the overall mindset of society...”. this theme took different forms, whether it was emphasising the habit of owning a car and the associated conformity even though the owner does not use the car and the car unnecessarily occupies public space. the reluctance of the habit of owning cars (but also the overall need to own) to step back when it is not necessary. unwillingness to learn new things or not understanding the principles of carsharing services. a second common theme among providers in the area of barriers was the financial literacy of the citizenry, as illustrated by the p1 provider’s statement that, “... not being able to calculate well that all the costs of running a car are still insurance, that’s 5-20k per year”. this brings up another common theme that carries the potential to change this situation – through educating society about shared mobility. to illustrate, the p2 provider stated that, “... there’s still a lot of room in educating society on how it works, where it can help them, where it can work.”. or p3: “i guess the only thing i see is education. i think that’s what’s missing here.”. as we can see, the barriers were assessed by the providers more with regard to the whole society and the citizens of the czech republic. users, in the context of evaluating the disadvantages of carsharing, naturally described their individual point of view. as we have noted above, there are some aspects (themes) of disadvantages of using carsharing services on which users agree with providers, or providers are all aware of, such as: lack of coverage and minor breakdowns or messiness in exceptional cases. users also agreed that planning ahead (logistics) sometimes seems to be a disadvantage. in order to rent a car, it is indeed important to decide in advance and book a vehicle as it might not be available from minute to minute. as user u9 states, “... there’s that barrier that you have to think about it like really a little bit in advance, book it at least, like, i don’t know, two days in advance, you can’t do it like instantly.”, or user u6: “... a person has booked a certain time, and if someone after them has booked it, they can’t change it if they don’t leave enough time in reserve...”. under this theme can be included the statement of user u11, who is uncomfortable with the fact that a smartphone is needed to operate as she does not own one: “... they rely heavily on having smartphones nowadays, i just don’t have one and sometimes i come across it...”. lastly, we consider the financial costs associated with carsharing. in this case, users rate carsharing as financially advantageous (see benefits) and add that it depends on the frequency and duration of use or kilometres travelled. thus, for long journeys or for daily use, carsharing is not advantageous. this could be summarized as a higher cost of operation for frequent and long trips and nicely summarized by user u3: “... for example if i want to go for a whole day and drive a lot of miles i would say it’s a bit more expensive, but again in the relationship that i would buy a car, it’s probably not that expensive.”. 3.1.3. benefits and reasons for using carsharing from the perspective of users and providers in the following section we include the evaluation of the benefits of carsharing. here, too, is a thematic overlap between the two sides, which is understandable, because, as we have seen, feedback and response to it works effectively. thus, providers adjust their offer according to the needs of their clients. moreover, 9 l. hartmannová, m. schlossarek, o. kročil, m. šucha acta polytechnica ctu proceedings these are declared benefits that come from the very idea of shared mobility, so it is important that they are perceived by both parties. it should be noted that what the provider perceives as a benefit of their service, then the user also evaluates these perceived benefits as the very reason why they use these services. it is therefore not possible to separate the evaluation of the benefits from the actual reasons for using b2c carsharing. among other things, the providers themselves use their services, so they can empathise with the users and speak from their own experience. in terms of benefits and reasons, on what topics do users and providers clash? one of the most frequent themes was saving resources. here we can include financial savings (compared to owning a car) and time savings (compared to other modes of transport such as public transport). from the providers’ perspective, theses such as, “...it can save them a lot of money.” (p2) or “further saving their own capacity...” or “economically sustainable” (p1). users added what these savings hinge on, e.g., “the first (reason is) financial, simply owning a big family car costs a lot of money.” (u2), “... the ability to instantly use that car without owning it and really only paying for the time i really need that car.” (u3) or “... it’s for the reason that it’s a time saver, so that i can be somewhere quickly, sort things out and be back quickly. because it’s quicker than public transport...” (u4). eco-friendliness, this was another topic that was mentioned in various ways, both by users and providers of carsharing services. environmental friendliness was not a conviction for everyone, but at least they considered this reason as a nice added value. reduced fuel consumption, lower emissions and occupying less of the public space are associated with this theme. this theme is supported by provider statements such as, “...environmentally, eco-friendly.” (p1), “we have new vehicles every year so they meet the most stringent emission standards.” (p2), or “but once the car leaves the factory, it’s already running emission-free and it’s environmentally friendly.” (p4). similarly, users reported that, “...the main motivation is probably the environmental aspect, probably quite simply put, that is to have fewer cars on the streets.” (u7), “...the reasons stem from environmental beliefs...” (u11), or “it makes sense to me environmentally...” (u5). a theme that did not appear for all users, but is close to the previous one, is that users find carsharing more convenient than using public transport. that is, they use carsharing even in places, where it is difficult to get around by public and intercity transport (u1, u2, u3, u6, u8, u9, u11), such as public transport, buses, trains, etc. there were also concerns about the use of public transport, buses or trains in the context of the covid-19 pandemic and therefore a greater sense of security when using carsharing (u3, u4, u8, u9) as our users do not want to own a car. this is illustrated by users’ statements such as, “... when we want to get somewhere that would be too complicated by public transport.” (u2) or “now because of the coronavirus i would say maybe more safety...” (u4). at the end of this section, as an interesting reason for using carsharing, user u10 stated that carsharing: “... used to use now in that pandemic instead of going on dates, like, instead of a café.”. as we can see, everyone finds some reason for using carsharing and it can be used for socializing. 4. discussion the aim of this paper was to identify the reasons for the use of carsharing services among czech users and to reveal the barriers and benefits from the perspective of both providers and users of carsharing services. several research questions are related to this aim. based on an interpretative phenomenological analysis, we found several reasons, benefits and barriers to the use and provision of b2c carsharing among 11 users and 4 providers in the czech republic. the following will be devoted to answering the set research questions and the results will be compared with previous studies. the first research question serves to provide context and refers to how providers respond to feedback from b2c carsharing users. feedback from users is predominantly positive and constructive, which is also related to the fact that providers have primarily positive experiences with their clients. although providers report that there are occasional instances of reckless handling of cars by users, which can be explained by the fact that some people do not know how to value other people’s (shared) property, in most cases everything is fine. the risk of damage to shared property can be seen, for example, in the study by weber [16] or puschmann and alt [17], who suggest that concerns about shared property on the part of providers could be reduced by special insurance for shared cars. although these are mainly minor obstacles, this could promote greater mutual trust. one of the reasons why mutual satisfaction works may be that the client is seen as the top priority, their needs are accepted and providers try to accommodate their clients. for providers, feedback from their clients is very important so that they can adjust their service offering according to the needs of their users and the current market offer. when constructive criticism arises, providers try to address comments proactively and quickly, most often through compensation and loss minimization. this approach to users is typical for the strategies of the so-called smart services that use shared mobility services. smarter services are generally more user-oriented. they allow users not only to use the relevant service offerings, but also to configure the service according to their individual needs and preferences [18]. this approach has been positively evaluated by users and, as providers have indicated, the number of users is increasing. this can also be seen on the individual websites of b2c 10 vol. 39/2022 psychosocial aspects of the use and provision of carsharing services carsharing providers (mentioned in the theoretical section) or in commercial articles [19, 20]. the second and third research questions focused on assessing the perceived benefits and barriers to provision and the perceived advantages and disadvantages of using b2c carsharing. we summarise these two research questions in this paragraph as the analysis has shown that they are interrelated. from the providers’ perspective, the biggest barrier to providing their services is the mindset of the society. according to them, the citizens of the czech republic still consider owning (not only) cars as a common phenomenon, even though they do not use this car for their everyday needs. often it can be a comfortable habit that is not easy for people to change. society’s mental attitude towards the excessive use of private cars compared to other modes of transport is rooted in the history and culture of our society (traditions, customs, upbringing and media). the perception of ownership of things, specifically cars, varies considerably across countries or cultures. as kuhnimhof and colleagues [21], for example, report that the generation born after 1980 (the so-called millennials) in western europe have a lower interest in owning and driving a car compared to their parents. they appear to be much more inclined towards alternative modes of transport. on the other hand, in central and eastern europe, after the fall of socialism in the late 1980s and early 1990s, there was an increase in the use and ownership of private cars [22]. car ownership was supposed to symbolize higher socio-economic status and freedom. this is gradually changing. but it is also the reason why carsharing came to the czech republic later and is spreading slowly in society. not only in the czech republic, but also in other countries, the development towards sustainable mobility is slow. several recent international studies show that even today, for many young people, owning a car is associated with a sense of achievement or pride. such a positive attitude towards car ownership often leads to car ownership and increased car use. conversely, individuals who have positive attitudes towards other modes of transport, such as walking, using public transport or cycling, then use active modes of transport more frequently and seem to be discouraged by car ownership [23–25]. providers see the low financial literacy of some citizens as another barrier. not everyone can calculate that it is more profitable for them to use a shared car than to own a private one when they do not use a car for everyday needs. they see educating society about the benefits of carsharing as a way out of this situation. indeed, countries where carsharing is more widespread have higher financial literacy scores than the czech republic [26]. these countries are germany and china in particular [27]. in europe, germany is the biggest leader of carsharing services. as an example, in 2021 carsharing is available and operating in 855 locations in germany, with around 2,874,400 registered users. this shows an increase in users of 25.5 % compared to the previous year [28]. if we look at the current state of carsharing in asian countries, the number of users in shanghai was over 1.2 million in 2017 [29]. in 2017, the estimated number of carsharing vehicles in metropolitan cities in china exceeded 26,000 and the carsharing fleet in china is expected to grow by 45 % per year until 2025 [30]. another disadvantage of carsharing from the perspective of users is the lack of coverage of shared vehicles, especially in and around cities. the providers themselves are aware of this and are trying to expand the supply of vehicles in cities. this is in line with the findings of the czech carsharing association, who note a significant increase in carsharing in the czech republic (as mentioned in the theoretical section) and one of the visions is further expansion. users also see the need to plan their trips in advance (logistics) as a disadvantage, as the car might not be available at the required time, so it is better to book in advance. this disadvantage was also noted by strnadová [31] in the czech context. from the user’s perspective, scheduling trips may seem like a disadvantage and discourage him/her from using carsharing. however, from an ecological perspective, this attribute can be seen as advantageous, as it reduces the need to drive and use, for example, more active and sustainable modes of transport, or to combine modes of transport. in the case of the last research question, which focuses on the reasons for using b2c carsharing, there was a link to the benefits of carsharing from the perspective of both providers and users. the benefits underlying the carsharing service offering are the main reason for using the service. these reasons are also perceived by users as benefits. thus, from the perspective of both users and providers, this includes saving resources, where financial savings (compared to owning a car) and time savings (compared to other modes of transport) fall. in particular, the financial savings compared to owning a private car are visible if the carsharing user drives up to approximately 10,000 km·year−1 [32, 33]. thus, a noticeable advantage of carsharing is access to a vehicle without having to pay acquisition costs or high fixed costs associated with car ownership [34]. similarly, there is also a time saving if one travels to places that are not easily accessible by other modes of transport such as public transport or active modes of transport and one has to count in the time lost to get to, for example, a train station, bus stop, etc. [35]. it is therefore ideal if as many czech citizens as possible are aware of these benefits. from the users’ perspective, it was also important that they found it convenient to use the carsharing service as they did not have to worry about servicing and other costs associated with owning a car. they can conveniently transfer things or people (family etc.). this benefit is closely related to the previous one. by using carsharing, users are relieved of the 11 l. hartmannová, m. schlossarek, o. kročil, m. šucha acta polytechnica ctu proceedings hassle of acquiring a car, paying for breakdown and compulsory insurance, maintenance or repairs. all of these responsibilities are transferred to the professional carsharing service provider. convenience as an advantage or reason for using carsharing can be found in many studies [36–39]. when one becomes aware of the hassles associated with acquiring a car, one may find it more convenient to use a carsharing service that does not need to devote this care, thus saving one’s energy, time and money. users and providers alike cited eco-friendliness as another reason and advantage of carsharing. however, in terms of the environmental impact of carsharing, there are two main streams of opinion. earlier studies primarily evaluated carsharing as an environmentally friendly mode of transport [40] and emphasized the positive impact of carsharing on reducing the number of kilometres travelled [41]. later opinions become slightly more critical, as studies point out that carsharing can also be environmentally unfriendly. this is particularly because people will prefer to use carsharing instead of more sustainable modes of travel such as public transport, walking, cycling or scooter [42]. as we have seen, our respondents mainly use carsharing for private purposes and only occasionally, such as for transporting heavy materials or family (e.g. on a trip). only one respondent gave a not very sustainable reason for using carsharing – for socialising (a date), but this was rarely done. the eco-friendliness was linked to a reduction in consumption, as a shared car is used much more often, by different users, than a private car, which is parked most of the time. the implication for respondents is that shared cars take up less of the public space. precisely, an undeniable advantage of carsharing is that it allows for better use of each car [43]. shared vehicles are much more frequently used on roads than private users’ vehicles, i.e. they spend less time parked and occupying public space. moreover, interviews revealed that our providers offer new cars to meet the most stringent emission standards. according to users’ opinions, reasons for preferring carsharing to using public transport include the fact that some places are difficult to reach by public transport and intercity public transport, and carsharing (in the context of the covid-19 pandemic) seemed safer. restrictions associated with the covid-19 pandemic, as well as fears associated with the outbreak, slightly altered modes of transport during this period. for example, the transport yearbooks for the capital city of prague suggest that, compared to previous years, residents’ overall confidence in using public transport has declined [44, 45], which may have contributed to the use of shared mobility after the crisis measures (lockdown). this is also in line with studies from other european countries [46, 47]. it appears that after the mitigation measures, the interest in carsharing in the czech republic continues to grow. for example, the autonapůl cooperative reported that the number of users is growing 73 % faster this year compared to 2019 [19]. however, an analysis of the state of carsharing services during the covid-19 pandemic is beyond the scope of this study. 4.1. benefits and limits of research as mentioned earlier, one of the benefits of this research is that it offers a complementary view of the benefits, barriers and reasons for the use and provision of b2c carsharing in the czech republic. not only has such a study not been conducted in this country, but similar research has not been traced abroad. if we want to support the development of sustainable mobility, we need to understand the needs of both sides – users and providers. it is also important that these needs, benefits or obstacles are reflected in the planning of the public administration, the municipality or the city council. as in other studies, we find several limitations in ours. one of them is the use of a non-probabilistic method of participant selection, since only one woman was included in this study. although carsharing is used to a greater extent by men, it would still be preferable to recruit more women next time. in addition, the survey mainly recruited users of the autonapůl cooperative. this may be because autonapůl has a long tradition in the czech republic and their users are used to being included in various research. this is also related to the smaller representativeness of the research sample, although we managed to get people from the crowd of typical clients. representativeness is also more associated with quantitative research design. in this context, it should be emphasized that the results of our study cannot be generalized since we used qualitative research and the number of participants is 15 people. however, the advantage of the study is that in the future we can create a questionnaire that could be administered to a much larger number of users. in the future, we plan to include carsharing p2p and potential users in the research. 5. results and conclusion in the czech republic, we do not have sufficient empirical data on the benefits and barriers to the provision and use of b2c carsharing and the reasons for using this particular shared mobility service from the perspective of both users and providers. therefore, we set out to (1) identify the reasons for the use of the carsharing service by czech users and (2) uncover the barriers and benefits from the perspective of both providers and users of the carsharing service. in line with the objectives of the paper, qualitative research was chosen. the main method of data collection was semi-structured interviews with users and providers of carsharing services. the data collection and analysis took place from march to august 2021. four providers (namely autonapůl, car4way, karkulka, greengo) and 11 users participated in this research, of which only one was female, the average age was 12 vol. 39/2022 psychosocial aspects of the use and provision of carsharing services around 39 years and the predominant educational level was university education. although this is not many participants, the research nevertheless included people who also represent, as the providers state, a typical b2c carsharing client. a non-probabilistic method of participant selection was chosen. interpretative phenomenological analysis was chosen to process the data. based on the data analysis, several common themes were identified in the context of barriers associated with the provision of b2c carsharing, such as: the mindset of czech society or the lack of financial literacy of czech citizens. from the users’ perspective, the following themes emerged as dominant in the context of disadvantages of carsharing (b2c type): lack of carsharing coverage in cities and surrounding areas, planning travelling well in advance (logistics) or higher cost of operation for frequent and long-term trips. in the case of the benefits of using and providing this service, an overlap was identified with the reasons for using b2c carsharing services. thus, the advantages and reasons for using carsharing services according to users and providers include: saving resources, namely financial savings (compared to owning a car) and time savings (compared to other modes of transport, such as public transport, etc.) or environmental friendliness, comfort (users do not have to worry about the service and operation of a shared car). whether it is communication, feedback, feedback handling and access to clients, both users and providers reported mutual satisfaction. the contribution of this paper is that it focuses on the experience of both parties – providers and users of carsharing and is the first study of this type to be conducted in the czech republic. in the future, the market value of companies operating in the sharing economy can generally be expected to grow. as stated by ganapati and reddick [48], the sharing 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[2021-10-02]. https://arxiv.org/abs/2108.02324 [48] s. ganapati, c. g. reddick. prospects and challenges of sharing economy for the public sector. government information quarterly 35(1):77–87, 2018. internet plus government: advancement of networking technology and evolution of the public sector. https://doi.org/10.1016/j.giq.2018.01.001 15 https://doi.org/10.1109/scsp.2016.7501022 https://www.tsk-praha.cz/static/udi-rocenka-2019-cz.pdf https://www.tsk-praha.cz/static/udi-rocenka-2019-cz.pdf https://www.tsk-praha.cz/static/udi-rocenka-2020-cz.pdf https://www.tsk-praha.cz/static/udi-rocenka-2020-cz.pdf https://doi.org/10.1186/s12544-021-00476-4 https://arxiv.org/abs/2108.02324 https://doi.org/10.1016/j.giq.2018.01.001 acta polytechnica ctu proceedings 39:6–15, 2022 1 introduction 2 research design and methods 2.1 description of participants 2.2 ethical considerations 3 analysis of the data 3.1 common themes 3.1.1 experience with clients – feedback and its solution 3.1.2 barriers to the provision and disadvantages of carsharing 3.1.3 benefits and reasons for using carsharing from the perspective of users and providers 4 discussion 4.1 benefits and limits of research 5 results and conclusion references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.38.0396 acta polytechnica ctu proceedings 38:396–401, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague critical analysis of monitoring indoor air quality in education centres anna figueroa-lopez∗, xabat oregi, alexander martín-garín, rufino javier hernández-minguillón university of the basque country, department of architecture, caviar research group, plaza oñati 2, donostia-san sebastián, 20018, spain ∗ corresponding author: anna.figueroa@ehu.eus abstract. indoor air quality perception in education centres has being a special concern based on their high occupancy and lack of ventilation. this study aims to present an overview of 20 relevant previous studies carried out in the last 5 years regarding monitoring and evaluating indoor air quality in education centres. this analysis focuses on four specific aspects: general description of each study, ventilation typology, indicators measured and the number of locations measured. the results show that 60 % of the studies were located in an urban context, 75 % included primary schools and 75 % included naturally ventilated buildings. indoor and outdoor was measured in 60 % of the studies. the most measured indicators were co2 (60 %), pm2.5 (55 %), temperature (50 %) and relative humidity (40 %) all indoor. in 60 % of studies were carried in more than five centres, mostly two rooms were measured and 35 % of studies placed one sensor per room. this can be a major limitation, as the monitored data may differ significantly from the actual situation. in conclusion, the greater correlations found relating to what indicators have been measured, were the ventilation typology and the location of the building, which influences what parameters and concentration can be expected. keywords: indoor air quality, monitoring, school, ventilation. 1. introduction indoor air quality (iaq) is a subject that has been extensively studied, especially because of its impact on health. most studies focus on office buildings or dwellings, with fewer studies being carried out in educational centres. some different standards and guidelines limit indoor pollutant concentrations, especially in collective buildings. several studies have found that the concentration of pollutants in educational centres is very high compared to the recommendations. most of these studies implement monitoring in multiple classrooms as a methodology to quantify the concentration of pollutants and other variables such as thermal comfort. this study aims to relate air quality monitoring to the main characteristics of the case study. to find if there is a correlation between different characteristics. in recently published studies [1, 2], the most critical aspects when characterising the influence of iaq in educational institutions are (figure 1). external conditions outside the building have a major influence, such as the climate [3] or the location of the school, urban, rural, industrialised, etc. pollution levels will vary especially concerning road traffic and industry [4, 5]. the ventilation schedule also influences the penetration of pollutants through ventilation inside the building if it is located close to major roads [4]. the permeability of the building is related to the penetration of pollutants inside the building. the more permeable the building, the more energyefficient it is and, generally, the lower the operating costs are. if the emissions of pollutants are mainly produced outdoors, the higher the permeability, the lower the penetration of pollutants into the building. conversely, when the emission source is indoors and the permeability is high, this can lead to high indoor concentrations caused by inadequate ventilation. there are multiple types of ventilation in buildings, which can be classified into three main groups: natural ventilation [6, 7], hybrid [3] and mechanical [4, 8]. these typologies have a major influence on the concentration of indoor pollutants. in the european union, naturally ventilated classrooms represent the vast majority [3], where it has been shown that can be achieved a good iaq as with mechanical ventilation systems. the perception of air quality in mechanically and hybrid ventilated classrooms is worse than in naturally ventilated classrooms [1]. the change of seasons also contributes to the concentration of indoor and outdoor pollutants, indirectly in the iaq because of the different actions performed by the occupants of the centre to achieve mainly thermal comfort [1]. occupancy is one of the aspects that has the greatest influence on iaq in educational centres, mainly due to their high occupancy density [3]. also influences secondary activities such as cooking or cleaning, which without proper ventilation increase the concentration of co2, pm2.5 and vocs, they can last until the next day when the teaching activity starts. the maintenance and correct condition of the building is also important [3], so when renovating or improving 396 https://doi.org/10.14311/app.2022.38.0396 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 38/2022 critical analysis of monitoring indoor air quality in education centres main characteristics external conditions permeability indoor/ outdoor ratio ventilation rates seasonality occupancy buildings' characteristics figure 1. main schools’ aspects that may influence iaq. • building general aspects • ventilation system typologycase studies • indicators measured • number of measurementsmonitoring • data visualization • ability to control indoor conditionsend-user implication figure 2. characteristics analysed in the study. schools, aslo influences the chosen materialson energy performance, iaq and occupant comfort in the post-occupancy phases, not only in the design phase. several articles have studied iaq in schools, focusing on the influence of different improvement strategies. they are of particular interest for this research to understand what results can be expected in the different case studies in terms of the implementation of different strategies that use monitoring as part of their methodology. in the study carried out by wang et al. [8], is analysed the improvement of air quality and its perception by students in multiple schools in sweden. the study is conducted over a period of two years and evaluated after the implementation and improvement of mechanical ventilation systems. the study found that the perception of air quality, subjective indoor air quality, improved when the ventilation ratio, lighting levels and the implementation of new ventilation systems were increased. on the contrary, it worsened with increasing co2 or tvocs concentration, or when the age of the students increased. in another study, carried out by pacitto et al. [9], air purifiers are installed in the gymnasiums of two naturally ventilated educational centres located in the urban area of barcelona. the hourly renewals achieved and the concentration of pm1−10, black carbon, co2 and no2 are measured. as a result, when the windows were completely closed and only air purifiers were applied, the indoor/outdoor ratio decreased the most, up to 95 %. in conclusion, the implementation of air purifiers in school gymnasiums located in environments with high road traffic density is an excellent solution to reduce indoor pollutant concentrations. another more accessible strategy is the introduction of a live visualisation system of the monitored data inside the classroom, with a clear protocol for action according to the values displayed. in the study carried out by wargocki and da silva [10], 4 classrooms were monitored for 2 weeks during heating period and 2 weeks in non-heating period, where the data display system was introduced one week and removed the next week. a protocol was established for opening windows and doors according to the co2 concentration. before the intervention, during the non-heating season, there was already adequate window opening to achieve the thermal comfort provided by adequate levels of ventilation. therefore, the biggest change occurred during heating season, as ventilation was increased which led to an increase in heating consumption. 2. methods the aim is to establish whether there is a correlation between different characteristics in previous studies that can be replicated to establish a monitoring strategy in future case studies. after studying the most critical aspects that influence iaq according to previous literature, those considered to be of special interest have been grouped and selected, relating the construction scale and location of the building with the monitoring strategy carried out. for the proposed methodology, the first step was to search for scientific publications that dealt with air quality in educational centres and used monitoring as part of their methodology. subsequently, these articles were classified according to different characteristics (figure 2) and finally, possible relationships between the different characteristics were analysed. the following aspects have been classified: at the scale of the study, the year and season in which it was carried out and the country. at the scale of each case study, a general classification of the environment in which the case study is located (urban or rural), what type of educational centre it is (nursery, school or institute) and the ventilation system it has. per397 a. figueroa-lopez, x. oregi, a. martín-garín et al. acta polytechnica ctu proceedings 0 2 4 6 8 10 12 14 16 18 20 other ufp (i) co (i) temperature (o) pm1 (i) pm10 (i) vocs (i) pm2,5 (o) co2 (o) relative humidity (i) temperature (i) pm2,5 (i) co2 (i) figure 3. variables monitored. (i) indoor; (o) outdoor. haps the most relevant feature is which indicators have been measured, especially the most representative ones, and whether they have been measured indoors or outdoors. to categorise the number of measurement points the number of schools, the number of classrooms or locations within the school and the number of sensors per classroom were considered for those that contained this information. finally, to understand how this monitoring affects the end-user, it has been added whether this data was visible to the users and whether the users can control or change the environmental conditions inside the space they occupy. 3. results and discussion 3.1. search for articles the search for articles was carried out in the scopus database in selected journals with the following combination of keywords: “indoor air quality” and “school” and “monitoring”. it was also limited to those publications published after 2015. the evolution of the number of publications on this subject has been stable, reaching a higher number of articles in 2020 due to the influence that the pandemic has had on this subject. from a total of 88 articles found related to the search terms, 20 were chosen, those considered most interesting and relevant to this study [4–8, 11–25]. in terms of the motivation of these articles, most of them are part of an international research project [5, 7, 11, 15, 16, 18, 19, 23, 24]. it is worth noting the influence that the health aspect has in this field, as in several articles different health departments have subsidised part of the research [5, 6, 12, 14, 20, 21]. in terms of research content, 6 of these studies are a continuation of previous research done by the same authors [6, 15]. some of them deal with very innovative topics, such as the study of new technology for ventilation [22] or the study of the impact of shoeless classrooms [16]. 3.2. analysis of the different case studies the selected articles have been produced in countries with cold or temperate climates, which is a limitation to understand the results that would be obtained in warmer climates. also, the most monitored period has been winter ([6, 8, 11–15, 17, 18, 20, 21, 23, 25]), being the most vulnerable season, especially in those buildings with natural ventilation, since in general, their users ventilate less to achieve thermal comfort. concerning the type of building and age of the pupils, researches are more focused on schools ([4, 7, 8, 11–17, 19, 20, 22, 23, 25]), whose activity is very different from that carried out in kindergartens ([6, 18, 22]) or institutes ([5, 8, 21, 24]). analysing the location of the schools, 60 % of the studies were located in an urban environment ([4, 5, 7, 11–13, 15, 17, 21, 23, 24, 26]), while the other 40 % were in urban and rural environments ([6, 8, 14, 16, 18, 19, 22, 25]), none studies were found exclusively in rural environments. this may lead to a lack of information and examples for future research that wishes to monitor in rural settings, as the literature comparing urban and rural settings shows a large difference in pollutants from outdoor sources. in 55 % of the studies analysed, they focus their research on schools with a natural ventilation system ([6, 7, 13–15, 17–21, 25]), while those that study only mechanical ventilation are 25 % ([4, 5, 16, 23, 24]). these can be considered representative values since in the european union most schools have a natural ventilation system [3]. in 60 % of the investigations, they have monitored both indoors and outdoors ([4–7, 11–15, 17, 18, 23]), most of them using the i/o ratio as an indicator of iaq. this ratio is a good indicator because the indoor concentration is influenced among others by outdoor sources, but also by indoor sources [7]. as can be seen in figure 3, the most monitored variables were co2 concentration ([4, 7, 8, 13, 17–19, 21–25]), pm2.5 ([4–8, 12, 15–18, 22]), temperature ([4, 7, 8, 13, 17, 19, 21, 22, 24, 25]) and relative humidity ([4, 7, 8, 17, 19, 21, 22, 24]), all monitored indoors. through these variables, information on thermal comfort (temperature and humidity), calculation of classroom air renewals and occupancy (co2) and the concentration of pollutants (pm2.5) can be obtained. the most commonly used sensor type was the ndir 398 vol. 38/2022 critical analysis of monitoring indoor air quality in education centres 1 2 to 5 6 to 15 > 15 number of educational centres 4 4 6 6 [5, 13, 21, 24] [4, 6, 18, 23] [7, 14, 15, 17, 20, 25] [8, 11, 12, 16, 19, 22] number of locations within the school 4 11 0 0 [7, 12, 13, 23] [4, 8, 11, 12, 14, 17, 19– 21, 24, 25] number of sensors within the classroom 7 3 0 0 [4, 7, 17, 20, 22–24] [12–14] table 1. number of measurements. (non-dispersive infrared sensors) for the measurement of co2 [4–6, 11, 12, 14–16, 21, 25]. for the assessment of the suspended particles, gravimetric analysis was used in 6 articles [7, 18, 20, 23–25]. followed by negative temperature coefficient (ntc) sensors for the measurement of temperature [4, 5, 11, 15]. in terms of connectivity, the most commonly used were wireless sensors with data storage [4, 5, 12, 14–17, 19, 25]. the frequency of measurement has varied from 6 articles where it has been measured every minute, being this the most abundant frequency [5, 12, 14–16, 18, 19], up to where it has been measured every 15 minutes [6, 21]. in some of the articles, this information on which sensors were used was missing. this is an important aspect that will influence the decision-making process as to which technologies are the most widely used and which may be the most suitable for each study. about the number of measurement points carried out, they have been classified according to the number of schools, the number of locations inside the school and the number of sensors in the classroom (table 1). the studies in which the number of schools analysed is high, with 3 studies analysing more than 30 schools. and more than half of the studies monitored more than 5 schools. on the other hand, within the same school, the number of classrooms and the number of sensors in the classroom are much lower. 3.3. users’ implication none of the studies had monitoring data visible, this is because they were studies on the usual behaviour of their users. previous studies [10] have shown a positive influence on the improvement of air quality when indoor iaq values are represented with an action protocol according to co2 concentration. nor have the articles referred to the subsequent use of the data obtained to act in schools or possible strategies for action to improve their iaq. with most case studies being naturally ventilated, the end-user has limited ability to change indoor conditions solely based on outdoor conditions and the ability to open windows and doors. whereas, in most mechanical ventilation systems the ability to change indoor conditions is often null because the control is centralised. 3.4. relation between different aspects we have studied whether there is a relation between some of the aspects to be taken into account for future monitoring strategies based on the characteristics of the school. when the environment of the school changes (urban or rural) there is no great difference in which variables are monitored. the numbers are similar except when measured both indoors and outdoors, with most schools located in urban environments. this may be generally caused by the fact that road traffic is one of the major emitters of pollutants in urban areas, being lower in rural locations if there are no major roads or industries in the immediate surroundings of the school. the variables monitored were also related to the type of ventilation in schools. nor is there a great difference, except in those that monitor co2 and temperature outdoors, and co indoors, with far fewer or even no studies with mechanical ventilation monitoring these variables. the rest of the variables monitored indoors do not vary for each type of ventilation. although the ventilation system greatly influences the penetration of some pollutants. mainly due to the filtration process in mechanical ventilation systems, or the number of renewals that can be achieved in uncontrolled ventilation such as natural ventilation. regarding the number of measurements and the characteristics of the centre, it has been found that when the case study was schools, the number of sensors is much higher than when the study was carried out in nursery or institutes, varying from an average of 39 measurement locations in schools compared to 4 and 2 in nursery and institutes respectively. as for the number of sensors and the monitored station, no relationship was found between the number of sensors and the monitored station. 399 a. figueroa-lopez, x. oregi, a. martín-garín et al. acta polytechnica ctu proceedings 3.5. limitations throughout the previous literature overview, different limitations have been detected. firstly, in the number of sensors used in monitoring, as there have been few sensors in classrooms, and their accuracy and interpretation can vary greatly depending on their location and use. in the last year, because of the pandemic, information and concern about iaq have increased. it has influenced classrooms because of their high occupancy density and the vulnerability of their occupants. this has led to the publication of scientific articles, legislation and guidelines to facilitate the incorporation of different measures to reduce the viral load in these indoor spaces. to this end, indoor co2 concentration has been commonly used as an indicator of indoor air renewal, trying to prioritise greater ventilation over other parameters such as thermal or acoustic comfort. as well as the correct use of masks in indoor spaces and limiting activities inside classrooms that increase the emission of aerosols. other limitation found has been the contradiction between different articles on whether to use co2 as an indicator. most studies have used co2 concentrations and thermal comfort as an indicator to measure air quality. as reviewed in previous literature [9] these indicators can be considered insufficient and an oversimplification as most pollutants cannot be predicted based on co2 concentration. the origin of these pollutants must also be taken into account, since those that are produced indoors, as is the case of radon and vocs, their behaviour can be similar to that of co2, but those gases whose origin is outside, mainly produced by road traffic such as no2 or o3, penetrate the building according to infiltration and the ventilation system used in the building [4]. although it may be considered limited and insufficient, in the current covid situation where air renewal is more prioritised than other comforts, co2 is a good indicator that allows the calculation of air renewals that occur in the interior space (according to occupancy and physical properties of the building) being easy to implement and easy to take improvement measures according to the concentration values. 4. conclusions in conclusion, once the relationships between different variables have been studied when monitoring educational centres, the type of educational centre, its ventilation system and the environment in which it is located have had a special influence. these variables will indicate which pollutants are most likely to be found and the control that can be exercised over indoor conditions, as well as the number of measurements to be taken. this monitoring can be used to engage users through live visualisation of the values obtained, which is an easily replicable strategy and has been shown to be effective in improving iaq. it 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https://doi.org/10.1016/j.egypro.2016.11.033 https://doi.org/10.1016/j.envres.2019.108534 https://doi.org/10.3390/ijerph14020145 https://doi.org/10.1016/j.scs.2016.12.014 https://doi.org/10.1007/s11869-017-0469-9 https://doi.org/10.1016/j.buildenv.2015.11.031 acta polytechnica ctu proceedings 38:396–401, 2022 1 introduction 2 methods 3 results and discussion 3.1 search for articles 3.2 analysis of the different case studies 3.3 users' implication 3.4 relation between different aspects 3.5 limitations 4 conclusions acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.39.0032 acta polytechnica ctu proceedings 39:32–40, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague on applications of brain-computer interface roman mouček∗, lukáš vařeka, petr brůha, pavel šnejdar university of west bohemia, faculty of applied sciences, department of computer science and engineering, technická 8, 306 14 plzeň, czech republic ∗ corresponding author: moucek@kiv.zcu.cz abstract. brain-computer interface (bci) applications implement a direct communication path between the brain and the computer. this paper deals with the fundamentals of bci systems and the experience of the neuroinformatics team with the design and implementation of various bci applications. their advantages, drawbacks and suitability are discussed in multiple contexts. keywords: brain-computer interface, electroencephalography, event-related potentials, steady-state visual evoked potentials. 1. introduction jonathan r. wolpaw formalized a brain-computer interface (bci) system [1] as a communication or control system in which the user’s messages or commands do not depend on the brain’s normal output channels. nerves and muscles do not carry messages, and neuromuscular activity is not needed to produce the activity that does carry the message. in other words, bci systems mediate direct communication between the (human) brain and computer. apart from invasive bcis, its non-invasive forms utilize surface electroencephalography (eeg) and event-related potential (erp) methods; the scalp-recorded electrical activity of the human brain is acquired to control an application or environment. research on bcis has a long history, but its ultimate result, bci systems working successfully and in the long-term in everyday life, has not been achieved so far. bci paradigms, techniques and workflows focused on signal detection, off-line preprocessing and processing of the signal have been developed. however, standardized implementation, actual online deployment, testing, and customization of bci systems on target users have not achieved the expected results. the persistent problems such as low classification accuracy and information transfer bit-rate still prevent these systems from becoming more widespread and competing with other communication solutions such as eye-tracking or voice recognition systems [2]. on the other hand, there are no other usable means of communication for locked-in people. current bci systems rely on several paradigms such as detecting the brain frequencies, event-related components, steady-state visual evoked potentials (ssveps), visual evoked potentials (veps) or motor imagery signals. a comprehensive review of eegbased bci paradigms is provided in [3]. current signal sensing technologies and computational intelligence approaches (including machine learning and deep learning algorithms) in bci applications are reviewed in [4]. the goal of this paper is to acquaint readers with the fundamentals of bci systems and the experience of the neuroinformatics team at the university of west bohemia with the design and implementation of various brain-computer interface applications, shortly present the outcomes of these applications and debate the advantages, drawbacks and suitability of bci systems in multiple contexts. the paper is organized as follows. the materials and methods section deals with the fundamentals of the electroencephalography method, event-related potentials and steady-state visual evoked potentials broadly used in bci paradigms and applications. bci design is shortly presented, related work is included, the neuroinformatics lab at the university of west bohemia is introduced, and the methodology of presenting individual bci experiments and applications is provided. the section bci experiments presents five designed and implemented bci applications. while the section results provides individual results of and experience with the bci applications, the section conclusions summarizes experience from all bci experiments/applications and gives prospects for the future. 2. materials and methods this section provides an essential insight into the fundamental concept of bci. the basics of collecting the scalp-recorded electrical activity of the human brain using the eeg/erp methods and techniques are described. it is extended by explaining erp components, steady-state visual evoked potentials (ssveps), and bci design. then a short overview of related bci research is provided. finally, the neuroinformatics laboratory at the university of west bohemia in the czech republic is introduced. here, the bci applications presented further have been developed, and related experiments have been carried out. 32 https://doi.org/10.14311/app.2022.39.0032 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 39/2022 on applications of brain-computer interface 2.1. electroencephalography the methods and techniques of electroencephalography (eeg) and event-related potentials (erps) are essential for designing and developing bci systems. they monitor human brain electrical activity by measuring voltage changes on a scalp surface. in general, the brain electrical activity oscillates, and these oscillations are termed as frequencies that include the alpha (8-13 hz), beta (13-30 hz), gamma (30-60 hz), delta (≤4 hz), and theta (4-8 hz) bands. the eeg signal amplitude varies in the tens of millivolts. at the same time, erps, as changes time-locked to particular events, have a very small amplitude (up to tens of microvolts) and can be assessed in small time windows (tens or hundreds of milliseconds). the eeg method has many advantages: affordability, non-invasiveness, routine examination protocols, and the opportunity to measure spontaneous activity. however, it also has a significant disadvantage; the resulting picture of brain activity (the eeg signal) is rough since it represents many sources of neuronal activity. then it is challenging to derive individual neurocognitive processes from the measured brain activity. this significant limitation must be considered in the design of bci systems. 2.2. brain oscillations brain oscillations (also called brainwaves), a rhythmic and repetitive brain electrical activity generated spontaneously or in response to stimulation, play a key role in brain sensory-cognitive processes and neural communication and create a prevalent feature of brain recordings. these oscillations (frequencies) are associated, e.g. with relaxation/concentration states, and their detection is utilized in simple bci systems and neurofeedback applications. however, two basic paradigms are used for more complex bci applications: event-related potentials and steady-state visual evoked potentials. 2.3. event-related potentials the event-related potential (erp) method initially focused on identifying and understanding erp components (these are explained later) and was used as an alternative to measurements of the speed and accuracy of motor responses in paradigms with discrete stimuli and responses. later, erp research changed from identifying and recognizing individual components to answering questions of broader scientific interest, such as processing visual and auditory information in the brain, and research into human attention and human behaviour when performing parallel tasks in the brain. this experience has been subsequently used in bci research. with the development of imaging methods and techniques (fmri – functional magnetic resonance imaging and pet – positron emission tomography), a gradual attenuation of the erp method was predicted. however, this method has become an essential part of bci and cognitive neuroscience experiments due to the relative affordability and high temporal resolution of the eeg signal that imaging methods lack. erps have two advantages compared to behavioural methods. they help determine which stage or stages of processing are influenced by a given experimental manipulation; for a detailed set of examples, see [5]. they can also provide an online measure of stimuli processing, even when there is no behavioural response [6]. as in the case of the eeg method, the erp method does not require insertion of the electrodes directly into the brain, as the change in potential is recorded directly on the scalp. based on the information above, we can state that the erp method is suitable for solving issues such as ‘which neurocognitive process is affected by a given experimental protocol’ [6]. in erp research, erp components must be observed in the eeg signal. an erp waveform consists of a series of peaks and troughs, but these voltage deflections reflect the sum of several relatively independent underlying or latent components [5]. these latent components are complicated to isolate, as the maximum and minimum amplitude values and their latencies in the observed signal may not necessarily be the right guide for determining them. limited possibilities of their independent measurement are the single biggest roadblock to designing and interpreting erp experiments [5]. it is, therefore, necessary to distinguish between observable maximum signal values and latent components. the erp component is then defined as scalp-recorded neural activity generated in a given neuroanatomical module when a specific computational operation is performed [5]. erp components are obtained from the eeg signal by averaging the epochs around the events. thus, observable local positive and negative maxima of voltage values found in the resulting erp signal are referred to as components. the designation of the component consists of a character indicating the polarity of the local maximum (p – positive local maximum of voltage value, n – negative local maximum of voltage value) and the order of local maxima (for positive and negative local maxima separately, e.g., p1, n1, p3). instead of indicating the order of the local maximum, a timestamp is sometimes used to indicate the latency of the component in milliseconds, e.g., p300 or n400. the order of erp components reflects the flow of information through the brain. the p300 (also denoted as p3) component depends entirely on the task performed by the subject and is not directly influenced by the physical properties of the stimulus. it is sensitive to various global factors, such as time since the last meal, weather, body temperature, and even the time of day or year [5]. although thousands of experiments related to the p300 component have been published, we still do not know what the p300 component really means. the proposal 33 r. mouček, l. vařeka, p. brůha, p. šnejdar acta polytechnica ctu proceedings that the p300 component is related to a process called ‘context updating’ seems approximately correct. however, there are known factors that influence the amplitude and the latency of the p300 component. the p300 component is sensitive to the probability of the target stimulus. p300 amplitude increases when the probability of the target stimulus class decreases and when more non-target stimuli precede the target stimulus. p300 amplitude is also larger when the subject pays more attention to a task. on the other hand, p300 amplitude is smaller if the subject does not know whether a given stimulus is/is not a target. it means that more complex tasks can increase p300 amplitude because the subject pays more attention to these tasks and simultaneously decrease it because the subject is not certain of the stimulus category [5]. as can be seen, the experimental design of eeg/erp and bci experiments is a challenging and critical step that influences other technical issues related to bci research. the p300 component is widely used in bci experiments. 2.4. steady-state visual evoked potentials steady-state visually evoked potentials (ssvep) are natural oscillatory cortical responses to visual stimulation at specific frequencies. as an alternative to erps and p300 bcis, ssveps can be used for bci systems because they are easy to detect (they have an excellent signal-to-noise ratio and resistance to artifacts) and stable across participants. the relative stability of ssveps under different perturbations, such as speaking, listening, or thinking, is also often highlighted [7]. on the contrary, a significant disadvantage is a certain discomfort towards the subjects because the flickering of visual stimuli is disturbing. 2.5. bci systems design all of the paradigms presented above (brain oscillations, p300 erp component and ssveps) require little to no training and can easily fit the purpose of bci systems. they all need to use an appropriate acquisition system and an experimental protocol that allows collecting, analyzing, and interpreting the eeg/erp recordings to establish communication between the brain and computer. one of the challenges bci systems face is their ability to classify the eeg signal in real-time. bci pipelines (workflow management systems) and technical solutions, such as cloud environments in the training phase, have been proposed and tested. the bci experiments section presents five various bci applications, from simple to more complex ones, to demonstrate their opportunities and drawbacks. 2.6. related studies bci systems are designed and developed at many workplaces around the world. although they are still considered more as research topics, there is already a market dominated by non-invasive bci systems. wireless transmission of the eeg signal in bci systems is widespread; it can provide more comfort to endusers. various eeg electrodes (gel, semi-dry, dry), eeg caps, headsets and headbands as a part of bci acquisition systems are designed and developed. neurosky eeg headset [8] and muse headband [9] detect and analyze brain frequencies and provide neurofeedback platforms. a complete bci research system that uses eeg and ecog (electrocorticography) signals and supports all common bci approaches (p300, ssvep/sssep, motor imagery, vep slow waves) is promoted by g.tec [10]. this company also provides complete matlab-based research and development systems. bci systems focusing on the p300 component and ssveps paradigms are developed by the braintech company [11]. a framework of hardware and software components for bci research is developed by the brainproducts company [12]. the emotiv company offers bci devices paired with emotivbci software [13]. mobile wireless eeg devices are produced by the mbraintrain company [14]. finally, we need to also mention the neuralink project [15] that is on the border of invasive and non-invasive brain data collection. when focusing on scientific studies, we need to mention, in particular, a 10-year update of a review of classification algorithms for eeg-based bcis [16]. at this time, deep learning methods still had not shown convincing improvement over state-of-the-art bci methods. another systematic review of hybrid deep learning approaches in bci systems is provided in [17]. bci paradigms, signal processing, feature extraction methods, hybrids bcis, and design of the synchronous/asynchronous bcis are reviewed in [18]. progress of eeg-based bcis from the perspective of encoding paradigms and decoding algorithms is summarized in [19]. finally, as the result of the ieee working group, a first version of the standardized bci glossary for a community review is presented in [20]. 2.7. neuroinformatics laboratory when performing bci applications/experiments, operating a laboratory with appropriate bci infrastructure is important. this includes mainly: • eeg acquisition system to collect brain recordings, • equipment/tools for stimuli presentation, in case of the p300 paradigm, accurate time synchronization with the eeg recording system is required, • a computer (computers) for processing eeg recordings (and stimuli) and establishing the communication path to the brain, • devices that are controlled by the bci application, • a pipeline (workflow management system) for processing/managing the whole bci communication. not all parts of this infrastructure are always used in the bci applications presented below. 34 vol. 39/2022 on applications of brain-computer interface figure 1. smart train a model train controlled with brain oscillations. 2.8. methodology five various bci experimental applications were selected to demonstrate the opportunities and experience from the real use of such systems. the bci experiments section briefly provides specific goals, application design, used equipment, and basic information about experiments. while the smart train application used a simple acquisition headset and was developed to show bci to the public, the following four applications were designed and developed to achieve specific goals in research projects. the results section then provides experience with designing and using these bci applications to collect bci data; i.e. specific scientific or educational findings are not focused on in this paper. 3. bci experiments this section introduces five bci experiments/applications that use various bci paradigms described above. 3.1. smart train a simple bci system smart train was developed to demonstrate to students and the interested public the basic principles and practical utilisation of brain oscillations (figure 1). the system is based on acquiring the participant’s eeg signal, finding its frequency, and controlling the model train depending on the frequency. brain signal is collected, and brain frequencies are analyzed with the headset; the train goes faster with a higher concentration level (higher frequency of the brain signal). the eye blinking signal is processed to change the direction of the model train. the users can observe their brain signals and interpretation in the graphical user interface. the neurosky headset (as the eeg acquisition system and signal processing unit) is used to control the speed of the model train (a controlled device). 3.2. bci for developmental coordination disorder experiment a bci system was designed and developed to investigate developmental coordination disorder (dcd). dcd is described as a motor skill disorder characterized by a marked impairment in the development of motor coordination abilities. the main research goal was to contribute to a diagnosis of this disorder using the erp method. the next goal included annotating the raw data with relevant metadata and providing them publicly for further analysis. the experimental protocol was based on the erp paradigm; a combined auditory and visual stimulation was used. visual stimuli were represented by pictures of animals. the corresponding auditory stimuli were represented by the sounds of animals that occurred in synchronization with the visual stimuli. participants were asked to respond to various stimuli combinations by pressing two different buttons during the experimental session. standard gel eeg caps, the international 10-20 system (that describes the location of eeg electrodes on the scalp) and the brainamp dc amplifier were used to collect eeg data (the eeg acquisition system). the sampling frequency was set to 1 khz. the raw eeg signal was filtered using an analogue band-pass filter with cut-off frequencies of 0.1 and 250 hz. the experiments were performed in a soundand electrically shielded booth (figure 2). the data were collected from 32 school children (16 with possible dcd and 16 in the control group). the experimental work, collected data and data validation process were described in detail and published in [21]. 3.3. bci for guess the number experiment guess the number is a simple p300-based bci experiment [22]. it aimed to demonstrate another simple bci application to school-age children and students. in parallel, it aimed to collect a large amount of bci data in a noisy environment, annotate them properly, create a large publicly available bci dataset and use machine/deep learning methods for p300 component classification. visual stimuli (the numbers between 1 and 9) were presented on the screen, the participants picked one of these numbers, and experimenters tried to guess the number thought while they were observing erp waveforms online (figure 3). eeg data were collected from three midline eeg channels (frontal, central and parietal electrodes of the 10-20 international system). the common gel electrodes and the brainproducts v-amp amplifier were used. the experiment was carried out in elementary and secondary schools in the czech republic on 250 participants. 35 r. mouček, l. vařeka, p. brůha, p. šnejdar acta polytechnica ctu proceedings figure 2. a participant during the dcd experiment [21]. figure 3. researchers are observing erps during the guess the number experiment [22]. 3.4. bci for drivers’ attention investigation about twenty experimental protocols (some experiments were published), each performed on 10 participants on average, were designed and run to investigate drivers’ attention during simulated driving. the driving conditions varied and included, e.g., heavy workload put on the drivers, various types of disturbance or, on the other hand, driving on a monotonous track. different stimuli (both visual and auditory) were used; the p300 component paradigm mainly was applied. the stimuli were presented either using a custom programmable hardware stimulator (for visual stimulation with leds) placed on the car windshield or the presentation software tool (for figure 4. a participant during simulated driving in a drivers’ attention experiment. auditory stimulation) produced by neurobehavioral systems, inc [23]. the drivers were asked to respond to stimuli by pressing the wheel buttons. the car simulator (figure 4, a front part of a real škoda octavia car with the logitech g27 wheel, accelerator, and brake connected to the control computer via the usb port) was used. the tracks for driving simulations were prepared mainly using the world racing 2 game produced by the synetic company. the track was projected on the wall in front of the car simulator. gel eeg caps (the 10-20 international system) and the v-amp amplifier were used to collect eeg data. the amplifier also served as an input unit for collecting additional biosignals. 3.5. bci for people with limited mobility the bci application for people with limited mobility focused on designing, developing, and testing an open and affordable prototype of a bci system built on lowcost hardware and open-source software components. the prototype allows people with limited mobility to control their basic home environment. the project added the concept of the cloud for remote bci computations and relied on testing and customization of the whole bci system. experimental protocols were based on the p300 component and ssvep paradigms. in the case of the p300-based experiment, the pictures corresponding to the needs/activities of the end-user were presented sequentially or in a matrix. in the case of the ssvep experiment 6, three pictures corresponding to endusers needs/activities (turn on the radio – 15 hz, turn on the light – 12 hz, make a phone call – 10 hz) were selected. at first, we used common gel electrodes and the v-amp amplifier within this bci system. later, hardware components for signal acquisition, designed and developed by sensorik-bayern gmbh, were tested. these components included dry electrodes (figure 5), a head-mounted device, and a base station. wireless eeg signal transmission via bluetooth connection was implemented. the hardware components were supple36 vol. 39/2022 on applications of brain-computer interface figure 5. dry electrodes (made by the sensoricbayern company) developed and tested during the basil project. figure 6. a participant while testing the ssvep protocol. mented with software components for local execution of online bcis; these included mainly an analysis library and workflow designer. the application was tested on 10 participants in laboratory conditions, but its parts (mainly a variety of p300 paradigms) were also tested in hospitals on 20 people. 4. results this section summarizes the experience with designing and using the bci applications and performing individual experiments described above. in general, we can state that the neuroinformatics laboratory has been working successfully, and the conducted experiments have contributed both to scientific results and the everyday operation of the laboratory by including advances achieved in the bci data lifecycle (data collection, annotation, storing, preprocessing, analysis, visualization, interpretation and publication). while experimenting with the smart train bci application (approx. five years of experimenting during excursions, science and technology days, and exhibitions), we found that most people could control the model train after a short training. there have been occasional technical difficulties with the headset setfigure 7. mean scalp potentials distributions of the p3b response in dcd (two left columns) and ndc children (two right columns). adopted from [24]. tings, but this simple bci system can be successfully used for such a purpose. it turned out to be very important and beneficial that the basics of bci can be simply explained to bci non-professionals (usually students and their teachers). when designing and developing the bci application for the dcd experiment, we experienced difficulties obtaining clean data, even in laboratory conditions and soundand electrically shielded booth. this experiment also set a comparison base to evaluate various eeg acquisition systems (mainly various types of eeg electrodes) to collect clean eeg data. the used gel electrodes require skillful personnel to apply them, but at the same time, their impedance remained consistently low during the whole experiment when compared to dry and semi-dry electrodes. the experiment data, stored in the custom eeg/erp portal, were collected and annotated respecting the outcomes of the international neuroinformatics coordinating facility (incf) program on standards for data sharing and the group developing the ontology for experimental neurophysiology. finally, the responses to auditory stimuli measured by erps at pre-attentive and attentive levels between children with dcd and children with normal motor development (ndc) were compared. the child’s crying (a stimulus) evoked a significant p300 response, composed of the early p3a peak and late p3b peak (p3a and p3b are p300 subcomponents). the p3a component had its maximum amplitude in the central region, while the p3b component had maximum amplitude in the parietal region of the brain. no significant differences were observed between the dcd and ncd groups in the amplitude and latency of the p3a component. in the case of the p3b component, significantly lower amplitude in the parietal region was found in the dcd group (figure 7). in contrast, no significant difference between the groups was found in the component latency [24]. during the guess the number experiment, we experienced that bci applications can also be run, with some limitations, in a noisy environment. midline electrodes were used to analyze erps and the p300 component was used to guess the number thought. 37 r. mouček, l. vařeka, p. brůha, p. šnejdar acta polytechnica ctu proceedings figure 8. comparison of target and non-target epoch grand averages. there is a large p300 component following the target stimuli. adopted from [26]. the number of participants also showed a variety of p300 representations; researchers highly improved their skills in identifying the p300 component. moreover, an online p300 component classifier (used in the following bci applications) based on machine learning methods was trained on the obtained recordings. the accuracy of up to 79.4 % was achieved when using stacked autoencoders for classification, while the researchers guessed the numbers manually by observing gradually averaging erp waveforms with 64.4 % accuracy [25]. when using convolutional neural networks to classify the p300 component, the accuracy was 62-64 % for single trials and 76-79 % for averaged trials [26]. the accuracy achieved for averaged trials was comparable with other state-of-the-art methods used for this dataset. a comparison of target and non-target epoch grand averages is shown in figure 8. the most important experience we gained during driver’s attention experiments was properly designing the experimental protocol since obtaining clear and interpretable data was difficult. besides that, we had to cope, e.g. with the troubles related to participants’ willingness to wear an eeg cap for a longer time or the necessity to reduce drivers’ movements. we verified that the principles and rules for designing bci experiments based on the erp paradigm had to be rigorously followed, although many other scientific studies often violate them. an example result [27] of driver’s erp data validation (based on the objective that target and non-target trials are expected to be associated with differently shaped erp components) from a successful experiment using a stacked autoencoder (as an important step preceding data analysis) is provided in figure 8. if the classification of a specific dataset from a participant yields low error rates, the objective of the odd-ball paradigm is considered to be fulfilled. the error rate was calculated as: err = fp + fn tp + tn + fp + fn , (1) where tp is the number of correctly classified targets, tn is the number of correctly classified non-targets, fp is the number of misclassified non-targets, and fn is the number of misclassified targets. as a result, error rates indicate the extent to which the classifier was unable to separate target and non-target single trials. in the case of the bci application for people with limited mobility, we experienced that a bci system can be built on low-cost devices for eeg signal acquisition and amplification. eye blinking and alpha activity were clearly observable, especially when gel and dry electrodes with long pins were used to collect data. ssveps were clearly observable, independently of frequency (frequencies between 8.5 hz and 20 hz were evaluated). using the ssvep paradigm brought more reliable results, but end-users could not be exposed to ssveps for a long time. a trainingless classifier for online ssvep classification was developed. six out of ten participants could control the system online, achieving more than 70 % accuracy. table 1 shows the results of ssvep online detection for each participant when the spectral difference (sd) method and canonical correlation analysis (cca) were used. on the other hand, we failed to evoke an observable p300 component when dry electrodes were tested. it can be explained by a generally low p300 amplitude and a relatively low signal-to-noise ratio. 5. conclusions this paper presented the fundamentals of bci systems and five specific bci experiments/applications designed and performed in the neuroinformatics laboratory of the university of west bohemia. we experienced the advantages and disadvantages of several bci paradigms, coped with bci design principles and their limitations, used various kinds of eeg acquisition systems, and brought advances to the whole lifecycle of bci data. this experience accompanied scientific and educational goals achieved. we experienced that simple eeg devices (headsets, headbands) work reasonably well when brain frequencies are evaluated and controlling any end device is not a critical step. following design principles while building bci systems is essential; their violation leads to uninterpretable results. we found out that gel electrodes generally worked better than dry electrodes, but the latter ones worked better in the case of the ssvep paradigm used. based on our experience, we are convinced that bci systems and applications are promising for future use, although they seem beneficial for a very limited group of people (especially those in the locked-in state) when used as a primary communication path. however, some issues such as low information-transfer bit-rate and lower performance persist. we also believe that 38 vol. 39/2022 on applications of brain-computer interface figure 9. results of validation. the error rates for each subject are depicted in bars. higher error rates mean lower amplitudes of p3s and/or more distortion in the eeg/erp signal. adopted from [27]. accuracy [%] subject id number of trials sd cca combination 1 53 35.9 49.1 45.3 2 35 68.6 74.3 80 3 60 61.7 73.3 78.3 4 60 58.3 35 45 5 60 78.3 78.3 88.3 6 60 80 95 96.7 7 60 36.7 46.7 50 8 60 78.3 73.3 85 9 60 38.3 40 38.3 10 60 86.7 78.3 88.3 summary 568 62.3 64.1 69.4 table 1. the results achieved for ssvep online detection for each participant are depicted. more than half of the participants were able to control the bci with a relatively low error rate. sd – spectral difference method, cca – cca-based method. publicly affordable bci applications will soon provide not only entertainment but also help people improve their mental health. list of symbols bci brain-computer interface cca canonical correlation analysis cnn convolutional neural network dcd developmental coordination disorder eeg electroencephalography eog electrooculography erp event-related potential fmri functional magnetic resonance imaging pet positron emission tomography p300 p300 component p3a p300 subcomponent – originates from stimulus-driven frontal attention mechanisms during task processing p3b p300 subcomponent – originates from temporal-parietal activity associated with attention and appears related to subsequent memory processing sd spectral difference method ssvep steady-state visual evoked potential vep visual evoked potential 39 r. mouček, l. vařeka, p. brůha, p. šnejdar acta polytechnica ctu proceedings acknowledgements this work was supported by the university specific research project sgs-2022-016 advanced methods of data processing and analysis (project sgs-2022-016). references [1] j. wolpaw, n. birbaumer, w. heetderks, et al. brain-computer interface technology: a review of the first international meeting. rehabilitation engineering, ieee transactions on 8(2):164–173, 2000. https://doi.org/10.1109/tre.2000.847807 [2] s. amiri, a. 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[24] i. holeckova, l. cepicka, p. mautner, et al. auditory erps in children with developmental coordination disorder. activitas nervosa superior 56(1):37–44, 2014. https://doi.org/10.1007/bf03379606 [25] l. vařeka, t. prokop, p. mautner, et al. application of stacked autoencoders to p300 experimental data. in proceedings of the 16th international conference on artificial intelligence and soft computing, icaisc 2017, pp. 187–198. 2017. https://doi.org/10.1007/978-3-319-59063-9_17 [26] l. vařeka. evaluation of convolutional neural networks using a large multi-subject p300 dataset. biomedical signal processing and control 58:101837, 2020. https://doi.org/10.1016/j.bspc.2019.101837 [27] r. moucek, l. hnojskỳ, l. vareka, et al. experimental design and collection of brain and respiratory data for detection of driver’s attention. in healthinf, pp. 441–450. 2017. https://doi.org/10.5220/0006249504410450 40 https://doi.org/10.1109/tre.2000.847807 https://doi.org/10.5772/56135 https://doi.org/10.1088/1741-2552/aaf12e https://doi.org/10.1109/tcbb.2021.3052811 https://doi.org/10.1016/ s1364-6613(00)01545-x https://doi.org/10.1371/journal.pone.0191673 https://doi.org/10.1101/2020.07.14.202085 https://doi.org/10.1088/1741-2552/aab2f2 https://doi.org/10.3390/brainsci11010075 https://doi.org/10.26599/bsa.2018.9050010 https://doi.org/10.1007/s11571-021-09676-z https://doi.org/10.1080/2326263x.2021.1969789 https://doi.org/10.1093/gigascience/gix002 https://doi.org/10.1038/sdata.2016.121 https://doi.org/10.1007/bf03379606 https://doi.org/10.1007/978-3-319-59063-9_17 https://doi.org/10.1016/ j.bspc.2019.101837 https://doi.org/10.5220/0006249504410450 acta polytechnica ctu proceedings 39:32–40, 2022 1 introduction 2 materials and methods 2.1 electroencephalography 2.2 brain oscillations 2.3 event-related potentials 2.4 steady-state visual evoked potentials 2.5 bci systems design 2.6 related studies 2.7 neuroinformatics laboratory 2.8 methodology 3 bci experiments 3.1 smart train 3.2 bci for developmental coordination disorder experiment 3.3 bci for guess the number experiment 3.4 bci for drivers' attention investigation 3.5 bci for people with limited mobility 4 results 5 conclusions list of symbols acknowledgements references acta polytechnica ctu proceedings doi:10.14311/app.2020.28.0001 acta polytechnica ctu proceedings 28(0):1–7, 2020 © czech technical university in prague, 2020 available online at http://ojs.cvut.cz/ojs/index.php/app experimental study of damaged cr-coated fuel cladding in post-accident conditions petr červenkaa,b,∗, jakub krejčíb, ladislav cvrčeka, vojtěch rozkošnýb, františek manochb, david radab, jitka kabátováb a czech technical university in prague, faculty of mechanical engineering, department of material engineering, karlovo náměstí 293/13, prague, 12000, czech republic b ujp praha a.s., nad kamínkou 1345, praha-zbraslav, 15610, czech republic ∗ corresponding author: cervenka@ujp.cz abstract. to enhance the safety of nuclear power, the focus of researchers all around the world has recently mainly objected on the development of accident tolerant fuels. especially the chromium coating of current zirconium based cladding has been widely suggested and discussed for its immense positive effect on overall cladding properties. nevertheless, it was observed that during the first stage of the loss of coolant accident, cracks appear in the cr coating due to its inability to tolerate higher plastic strain. therefore, experimental methodology used in this article focuses on testing fuel cladding with damaged cr coating after the high-temperature transient. the impact of cracks on degradation of cladding mechanical properties was observed using optical microscopy, ring compression test, microhardness, and evaluating hydrogen content and weight gain. keywords: atf, cladding embrittlement, coating, loca, oxidation. 1. introduction over the years, many concepts of accident tolerant fuels (atfs) have been suggested [1]. the main objective of atf is to mitigate the possible negative consequences occurring in accident conditions. this article deals with the design basis accident (dba), namely, the large break loss of coolant accident (lb-loca). during the lb-loca, the peak cladding temperature (pct) might reach 1200 °c. the process of oxidation is accelerated at temperatures over 800 °c which is also a temperature when zirconium undergoes a phase transformation from its α phase to the β phase. the oxide dissolved in metal stabilizes the α phase which grows at the expense of the β phase. part of oxygen then dissolves in the β phase. if the amount of oxygen absorbed is too high, the cladding becomes brittle. since the structural integrity is essential, the load-bearing prior β phase must be thick enough to withstand various stresses occurring in and after the quench phase of lb-loca, otherwise, it might end up in a rupture of the cladding. the most promising atf concept is the fuel cladding made of zr-based alloy with a protective cr coating of outer wall [2, 3]. coating slows down the absorption rate of hydrogen as well as the process of oxidation in normal operation [4–6]. moreover, calculations show that a thin layer of cr entails only a minor decrease in a fuel cycle length [7]. regarding the accident conditions, the positive impact of cr coating has been presented. reduced deformation leading to a decelerated ballooning process as well as prolongation of time to burst was observed [8]. the crucial hypothesis is that in the first stage of the loca accident, in the process of ballooning, cracks appear in the protective chromium layer due to its inability to tolerate higher plastic strain. therefore, the cladding is locally not being protected from the accelerated oxidation process when high temperatures are reached. this contribution focuses on evaluating the effect of cracks on the overall mechanical properties of fuel cladding after the high-temperature transient. 2. methods 2.1. preparation of specimens all experiments were performed using non-irradiated zr-1%nb (e110) alloy, which is the main cladding material used in water-cooled water-moderated power reactors (wwers). it was observed that the negative effects of irradiation are annealed when the temperature is rising and the material recovers before reaching pct, therefore testing of non-irradiated cladding is possible [9, 10]. the geometry of tested specimens: • length 45 mm • outer diameter (od) 9.1 mm • inner diameter (id) 7.93 mm • wall thickness ~0.6 mm specimens were divided into three branches: reference cladding, cr coated cladding, and damaged cr cladding. first, the specimens were ultrasonically cleaned in acetone, ethanol, and distilled water (in that order). then, a chromium layer was deposited on the outer 1 http://dx.doi.org/10.14311/app.2020.28.0001 http://ojs.cvut.cz/ojs/index.php/app p. červenka, j. krejčí, l. cvrček et al. acta polytechnica ctu proceedings figure 1. the chamber of hauzer flexicoat 850 system. surface of the specimen using the magnetron sputtering process with 99.6 % cr sputtering targets in the protective argon atmosphere of the hauzer flexicoat 850 device (figure 1). the process lasted 16 hours at the temperature of 250 °c, which is even lower than the normal operation cladding temperature in the core, thus the cladding was not thermally affected. finally, the coating thickness of 18.6 µm was determined using the calotest method (figure 3). to simulate the coating defects (cracks) occuring during the ballooning process, several scratches through the chromium layer were made using the scratch test device csm instruments revetest xpress (figure 2). the constant force of 50 n was required to make the scratch go through the whole chromium layer without affecting the zr substrate more than necessary by adding additional local plastic deformation. the resulting u-shaped scratches (figures 6, 7) had a width of 200 µm and the length of 40 mm. 2.2. high-temperature oxidation the process of ballooning might result in the burst of cladding, hence the the double-sided high-temperature oxidation experiments were performed as it is more conservative approach. the cr-coated outer surface is expected to be protected from oxidation, whereas the oxygen is supposed to be absorbed through the inner wall. in addition to that, such experiments might clearly show the difference in oxidation rate through the whole (inner) surface and locally through the scratched (damaged) coating. the experiment of high-temperature oxidation was performed in an electric resistance furnace (figure 4) figure 2. process of creating scratches. figure 3. calotest. in an argon-steam mixture according to the scheme shown in figure 5. the temperature of the experiment was set up to 1200 ± 3 °c since it is generally accepted as the highest possible temperature within the acceptance criteria for dbas [2]. a thermocouple was located inside the cladding tube and it was recording the temperature every 2 seconds. specimens were oxidized for 1.5; 3; 4.5 and 9 minutes. at the end of the experiment, the specimens were directly quenched in cold water (water + ice) and dried out at the temperature of 90 °c. 2.3. weight-gain the non-destructive testing of oxidized samples was performed on mettler toledo xs 105 semi-micro balance with the readability of 0,01 mg. specimens were weighed both before and after the high-temperature oxidation, thus the experimental value of weight gain wg-exp [mg/dm2] could have been calculated. in addition to that, the estimated value of weight gain wg-cp [mg/dm2] was calculated using the experimental temperatures and the cathcart-pawel correlation. after the process of weighing, the specimens were cut into smaller rings for the purposes of destructive testing. using buehler isomet 5000 linear precision 2 vol. 28/2020 experimental study of damaged cr-coated cladding figure 4. the apparatus for high-temperature oxidation. saw rings of the following lengths were cut: • 1.5 mm – hydrogen content analysis • 7 mm – optical microscopy, microhardness • 4 × 7 mm – ring compression test 2.4. ring compression test to evaluate the residual ductility, a ring compression test (rct) was performed on instron 1185 machine. for the purposes of this test, small rings of a width of 7 mm were cut from the oxidized specimens. those rings were pressed at a constant speed of 1 mm/min. the temperature of the experiment was 135 ± 1 °c. the ductile enough specimens oxidized for 1.5 minutes were not a subject of this test. the compression was stopped if the displacement of 2 mm was reached since the measured ductility was not relevant to determine the ductile-brittle transition area. 2.5. hydrogen content the analysis of hydrogen content was performed on g8 galileo brucker machine by using the inert gas fusion (igf) method. short rings of a width of 1.5 mm were cut from the oxidized specimen. afterward, those rings were fused in a vacuum atmosphere at the temperature of 1300 °c. together with other gases, hydrogen was extracted from the fused specimen and carried by an inert gas. the evaluation of hydrogen content is based on the differences of the measured thermal conductivity of evolved gases. figure 5. the scheme of high-temperature oxidation experiment. 2.6. optical metallography metallographic specimens were analyzed using an optical microscope nicon eclipse ma200. the images were captured and enhanced using nis-elements software. in the figure 6 a typical microstructure of high temperature oxidized specimen can be seen. the oxygen-stabilized α-zr(o) phase grows from the unprotected surface at the expense of load-bearing prior β-zr phase. in addition to that, the process of oxygen absorption through the coating defects (scratches) locally leading to the growth of the α-zr(o) phase can be seen, whereas there is no oxidation through the undamaged coating (figure 6). since the e110 tested cladding contains nb, there is a layer of both α-zr(o) and β-zr grains. the structure of zr cladding without nb (such as zry-4) is shown in figure 6, the difference in the presence of the layer containing both α-zr(o) and β-zr grains is evident. the local microstructure close to the scratch is very similar to the microstructure in the area of the inner surface. 2.7. microhardness the hardness across the cladding wall was analyzed on buehler micromet 5114 device by using the vickers microhardness test. the pyramidal vickers-type indenter was used and the test load was 0,1 kg. on each specimen, 10 square indents were produced in 5 lines across the cladding wall as shown in figure 7. afterward, every indent was precisely located using getdata graph digitizer software to get the information about the position and appropriate hardness. as a result, the hardness profiles across the wall in 3 p. červenka, j. krejčí, l. cvrček et al. acta polytechnica ctu proceedings figure 6. typical misrostructure of high-temperature oxidized zr-based cladding: with nb (top) and without nb (bottom). figure 7. microhardness indents in 5 vertical lines. both longitudinal and transverse directions as well as surface maps of hardness were generated. 3. results 3.1. weight-gain as it can be seen in figure 8, the coating effectively protects the cladding from oxidation. even in the case of a 9-minute exposition, the weight gain is relatively small. on the contrary, the reference cladding oxifigure 8. results of weight-gain. figure 9. results of rct with theoretically obtained value (green) for double-sided oxidation (=uncoated). dized from both sides features a significantly higher weight gain, which is rather similar to the expected rate calculated according to the cathcart-pawel correlation [11]. furthermore, figure 8 shows the weight gain comparison of the coated cladding with and without a scratch (red and yellow dots). the presence of scratch did not have a measurable impact on the overall weight gain. 3.2. ring compression test the residual ductility of 2 % is generally considered as the ductile-brittle transition. all specimens seem to undergo the transition at the time of high-temperature oxidation lasting around 9 minutes. the presence of the scratch does not seem to affect the ductile-brittle transition as shown in the figure 9. according to the cathcart-pawel correlation (equivalent cladding reacted ecr-cp 18 %), the limit time value of hightemperature oxidation at 1200 °c for double-sided oxidation is 5 minutes. therefore, regardless of the scratch being present or not, the coated specimens feature a significantly longer time to undergo the ductile-brittle transition, thus the positive impact of cr coating is apparent. as all specimens had inclined to crack on the side, specimens with scratches were tested twice to determine the impact of the crack rotation. first, the cladding ring was rotated featuring a scratch on the top, second, the scratch was on the side. the results are summarized in figure 10. 4 vol. 28/2020 experimental study of damaged cr-coated cladding figure 10. comparison of differently rotated scratches. figure 11. ring compression test with the position of the scratch. 3.3. hydrogen content the hydrogen pickup is lower than 50 ppm for all specimens. regardless of the presence of the scratch, the coating protects the cladding from accelerated hydrogen pickup. according to current knowledge, there is no impact of low hydrogen concentration (< 70 ppm) on cladding ductility [12], therefore hydrogen pickup cannot be the reason behind the tested cladding embrittlement. 3.4. microhardness the hardness increases sharply with the phase transition. the ductile load-bearing prior β-zr is characterized by the hardness of < 300 hv0,1, whereas the α-zr(o) typically shows the hardness of 450 – 600 hv 0,1. as the solubility of oxygen in β-zr is limited, by extending the time of high-temperature oxidation, the deceleration of increasing hardness rate is observed (figure 12). once the values of around 350 hv0,1 are reached, the amount of oxygen possibly dissolved in β-zr is close to its limit. accordingly, by prolonging the time of the high-temperature oxidation, hv0,1 figure 12. comparison of singleand double-sided oxidation. figure 13. transversely measured microhardness: 3 min exposition (top) and 9 min exposition (bottom). values remain nearly steady until the sharp increase of hardness values is measured due to the growth of very hard α-zr(o) grains. the increase in the hardness values at the area near the uncoated surface is the result of oxygen absorption. against the odds, the hardness values increase also close to the coated surface as shown in figure 13. the diffusion of cr coating into the zr substrate is clearly present. the longer is the time of the hightemperature oxidation, the larger is the affected area with increased values of hardness. the results in the form of surface plots are presented in figure 14. 4. discussion the study of mechanical properties of damaged coated cladding in post-accident conditions is still unique and similar results have not been widely observed yet. 5 p. červenka, j. krejčí, l. cvrček et al. acta polytechnica ctu proceedings figure 14. surface plots of microhardness. therefore, the suitable methodology remains a subject of further research. scratches made on the scratch test device allow us to customize the geometry of the scratch. on the other hand, the actual cracking would be more precisely reached by performing burst tests first. in the figure 15, the residual ductility in relation with the microhardness values measured in the middle of the cladding wall can be seen. the critical microhardness value for uncoated zr-1%nb (m5) cladding for ductile-brittle transition (dbt) is 300 350 hv0,1 [13]. presented results (red and yellow dots) are in accordance with the existing results. moreover, according to the following studies [1, 14], the crcoated specimens performing double-sided oxidation at various temperatures (blue dots) feature similar values for dbt, thus the obtained results seem to be relevant. the impact of scratch on the measured hardness seems to be very local. the reason behind this might be the cr diffusing into β-zr and stabilizing it, which leads to cr enriched prior β phase. as there is no cr in the area of damaged coating, the area below the scratch is not affected by diffusing cr. the growth of α-zr(o) through the scratch is therefore impeded. that would explain the rather local impact of scratches where the α-zr(o) grows practically only in a radial direction. accordingly, cr stabilized β-zr absorbs diffusing oxygen. the diffusion process of oxygen into β-zr figure 15. comparison of obtained results with the ones presented in other studies might be accelerated due to the concentration of cr dissolved. regarding the diffusion kinetics, cr seems to dissolve in the cladding metal faster than oxygen thanks to its high diffusion coefficient. dissolved cr might cause the accelerated diffusive process of oxygen. such results would be in accordance with existing knowledge [6, 15]. nevertheless, the possible (and rather likely present) interaction of diffusing cr and oxygen needs to be observed. currently, the negative interaction among distant scratches can not be excluded. 6 vol. 28/2020 experimental study of damaged cr-coated cladding 5. conclusions the mechanical properties of the potentially damaged cr coating of zr-based nuclear fuel cladding in post-accident conditions were studied. cracks of the coating occurring in the first stage of lbloca were simulated by creating scratches using the scratch test device. the oxidation through the coating was not observed. it was proved that the coating thickness of 18,6 µm is sufficient to prevent the specimens from oxidation for times of high-temperature oxidation of up to 9 minutes. presented results show the positive impact of cr coating on the overall mechanical properties after the high-temperature transient. the impact of scratch on the ductile-brittle transition was studied as well. the presence of the scratch did not cause a decrease in residual ductility. by using the standard methodology, i.e., weight gain, residual ductility, hydrogen pickup, microhardness in the middle of the cladding wall, the cr coating features an unequivocally positive impact. a more detailed measurement of microhardness through the whole cladding wall revealed diffusion of cr coating into the zr substrate. this diffusion process has to be considered and further observation is needed. acknowledgements the authors would like to express gratitude to the whole zirconium alloys team at ujp praha a.s. led by v.vrtílková for running the experiments and their help with the post-experimental analysis. references [1] j. krejčí, j. kabátová, f. manoch, et al. development and testing of multicomponent fuel cladding with enhanced accidental performance. nuclear engineering and 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[13] j.-c. brachet, v. vandenberghe-maillot, l. portier, et al. hydrogen content, preoxidation, and cooling scenario effects on post-quench microstructure and mechanical properties of zircaloy-4 and m5® alloys in loca conditions. journal of astm international 5(5):1–28, 2008. [14] analysis of options and experimental examination of fuels for water cooled reactors with increased accident tolerance (actof). no. 1921 in tecdoc series. international atomic energy agency, vienna, 2020. [15] w. xiang, s. ying. reaction diffusion in chromium-zircaloy-2 system. tech. rep., china nuclear information centre, 2001. 7 https://doi.org/10.1016/j.net.2019.08.015 https://doi.org/10.1787/9789264308343-en https://doi.org/10.14311/app.2018.14.0027 https://doi.org/10.14311/app.2019.24.0009 acta polytechnica ctu proceedings 28(0):1–7, 2020 1 introduction 2 methods 2.1 preparation of specimens 2.2 high-temperature oxidation 2.3 weight-gain 2.4 ring compression test 2.5 hydrogen content 2.6 optical metallography 2.7 microhardness 3 results 3.1 weight-gain 3.2 ring compression test 3.3 hydrogen content 3.4 microhardness 4 discussion 5 conclusions acknowledgements references my title https://doi.org/10.14311/app.2022.36.0084 acta polytechnica ctu proceedings 36:84–89, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague maximizing information obtained from standardized fire tests using a bayesian framework balša jovanovića,b,∗, ruben van coilea, edwin reyndersb, geert lombaertb, robby caspeelea a ghent university, department of structural engineering and building materials, technologiepark-zwijnaarde 60, 9052, ghent, belgium b katholieke universiteit leuven, department of civil engineering, kasteelpark arenberg 40, 3001 leuven, belgium ∗ corresponding author: balsa.jovanovic@ugent.be abstract. current practice is mostly focused on prescriptive design approaches where the performance of the structure in case of fire is assessed based on its performance in standardized fire tests. those tests indicate whether the structural member can withstand standardized iso 834 fire exposure for a certain code specified time. this method however does not provide an explicit safety level. this issue is enlarged even more by the fact that the iso 834 fire exposure does not represent a natural fire exposure, but a pseudo-worst-case exposure, making the correlation between the standardized fire test results and real-life behaviour of structural members exposed to fire questionable. however, there has been a century-old tradition of standardized fire tests with a lot of experience and infrastructure based on it. for that reason, here, a methodology is presented to obtain more information on the behaviour of structural members exposed to a natural fire from the standardized fire test results by using a bayesian framework. as an example structure, a simply supported concrete slab is considered. its failure during the standardized fire test is modelled and the parameters affecting the time when it fails (i.e., parameters affecting the nominal fire resistance time) are determined. the model is then used in a markov chain monte carlo procedure to update parameter distributions based on the measured fire resistance time. using these updated distributions, a full probabilistic calculation of the performance of the slab considering a natural fire exposure is then conducted to assess the failure probability. keywords: bayesian analysis, concrete, fire, safety level, uncertainty. 1. introduction structural stability of a building in case of fire is of utmost importance both for the safety of its occupants as well as for other fire safety objectives such as property protection. therefore, it is essential to provide a building design with an adequate safety level. in the structural sense, it is important to mitigate the risk of a collapse due to the fire effects. a common way of representing a structure’s performance in the case of fire is through its fire-resistance rating. the fire-resistance rating represents how long a structural member can withstand a furnace test before collapsing. during the furnace test, the structural member is exposed to a standardized fire exposure, a universal temperature-time curve that in its essence should represent a conservative worst-case fire scenario. this approach has a lot of benefits: it is standardized, it provides the same temperature exposure for a range of different materials and structural members, and therefore can be used to compare different solutions. however, it also has a few shortcomings. first of all, the standardized temperature-time curve does not represent any real fire scenario. this temperaturetime curve is monotonically increasing and therefore does not include the cooling phase of the fire. secondly, even though it should represent an envelope case for multiple possible fire scenarios, there are cases where this approach is not conservative [1]. despite these shortcomings, the standardized approach is highly popular. there is an abundance of data and research focused on the material and structural behaviour of members exposed to the standard temperature-time curve. unfortunately, the test does not provide any direct information about how the structure will behave in a real fire situation. all it provides is a time of failure for a structural member exposed to a fire scenario that, as mentioned earlier, might be conservative, but does not represent a real fire. this problem is even enhanced by the fact that a lot of current building codes allow the use of this value as proof of safety by prescribing the minimal fire resistance rating a member must achieve in the standardized test. in order to overcome this issue, in this paper, a bayesian updating procedure is applied. the bayesian framework in fire safety engineering has been successfully used in the past in a variety of fire safety problems. for example, wang and zabaras [2] used them in order to determine the magnitude of a heat source from the temperature data, guo et al. [3] used a zone 84 https://doi.org/10.14311/app.2022.36.0084 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 36/2022 standardized fire tests using a bayesian framework figure 1. temperature field of a 20 cm thick concrete slab with the concrete cover of 35 mm after 120 min of standardized fire exposure. model as a forward model for a bayesian inversion to obtain the fire size and its origin within a multicompartment structure based on the measured gas temperature. overholt and ezekoye [4] managed to invert the fire’s heat release rate based on the measured temperature data from room-scale experiments. the bayesian updating is used to predict the behaviour in a more realistic fire for the case of a simply supported concrete slab, considering the fire resistance as measured in the standardized test. using numerical heat transfer analysis, a bayesian framework and full probabilistic calculation, the failure time determined during the furnace test is used to compute the failure probability in case of a natural fire. 2. methodology 2.1. overview the procedure presented in this paper consists of two main parts. first, the prior distributions of the material properties of the slab are updated considering the measured fire resistance time. second, these posterior distributions are used to determine the failure probability of the slab in case of the natural fire. for the updating procedure, a model that calculates the failure time during the furnace test must be created. that is done by calculating the time instant when the capacity of the slab is lower than the loads applied. afterwards, this model is used in the bayesian framework to update the material properties based on the test failure time. using the updated material properties based on the test failure time the failure probability is calculated for the slab exposed to a natural fire. a monte carlo calculation is conducted taking into account uncertainties in the fire exposure, loads, geometry and updated material properties. finally, the failure probability is calculated for different values of test failure times and compared. 2.2. thermal calculation in order to determine the capacity of the concrete slab in the case of fire, the first step is to determine the temperate distribution inside of it. this requires the solution of a heat transfer problem which is commonly solved using the finite difference method. in this study, a 20 cm thick reinforced concrete slab exposed to the heating from the bottom side is considered. a fine mesh with the size of 0.1 mm and a time step of 0.1 s is used. the boundary condition on the bottom (heated) side is defined using a gas temperature-time curve and heat transfer through convection (convective heat coefficient hconv = 25 w/(m2.k) and radiation (emissivity ε = 0.7). on the top (cold) side, an ambient temperature of tamb = 20◦c is imposed. the density of concrete in the function of temperature is defined by en 1992-1-2 [5] with a moisture value of 3 %. a common approach of calculating for a concrete only section and assuming that the reinforcement temperature is equal to the concrete at the same position is employed. figure 1 shows the temperature distribution of the slab after 120 min of iso 834 fire exposure. the uncertainty associated with the parameters in the heat transfer problem is the focus of this paper. thus, probabilistic models for the thermal properties, thermal conductivity k and specific heat cp of concrete, are used. the models’ origin and details are presented in [6] and the main characteristics are summarized here. the main assumptions are that each of these properties is a continuous function of temperature, and at each temperature the values are distributed according to the gamma distribution. in order to obtain a random curve, the same quantile is used for all temperatures. for example, if the specific heat has a 5 % quantile value at 20 ◦c, it will also have a 5 % quantile value at 100 ◦c, 200 ◦c and so on. this way it is possible to define one continuous function for a thermal property at any temperature based only on the quantile value considered. this enables a quite easy implementation in numerical models and its use is presented in the following paragraph. first, a quantile value for each of the thermal properties is chosen independently, next mean and standard deviation at each needed temperature are calculated using the following formula (temperature t in ◦c), 85 b. jovanović, r. v. coile, e. reynders et al. acta polytechnica ctu proceedings parameter symbol unit distribution mean standard deviation ref. thermal conductivity k [w/(m.k)] gamma temperature dependent temperature dependent [6] specific heat cp [kj/(kg.k)] gamma temperature dependent temperature dependent [6] reinforcement yield strength fy [mpa] lognormal 560 30 [7] table 1. prior distribution for the parameters used in the bayesian framework. valid in the range 20 − 1200 ◦c [6]: kmean [w/(m.k)] = 6.627 · 10−7 · t 2 − 1.458 · 10−3 · t + 1.772 cp,mean [kj/(kg · k] = −2.953 · 10−7 · t 2 + 6.498 · 10−4 · t + 0.872 kstd [w/(m · k)] = 3.19 · 10−7 · t 2 − 0.691 · 10−3 · t + 0.434 cp, std [kj/(kg · k)] = −3.500 · 10−7 · t 2 + 7.700 · 10−4 · t + 0.042 (1) afterwards, using these values and given quantile, well-known equations defining the gamma distribution are used to calculate both thermal properties for each needed temperature. finally, the same moisture peak should be added to the specific heat curve as defined in en 1992-1-2 [5]. 2.3. bending capacity and failure time once the temperature inside the slab is calculated, its bending capacity can be calculated using the following formula [11]: mr = as · kfy(t ) · fy · ( h − c − ϕ 2 ) − (as · kf y(t ) · fy)2 2 · b · kfc(t ) · fc (2) where as is the reinforcement area, kfy(t ) is the temperature-dependent strength retention coefficient for the yield strength of steel, fy is the yield strength of steel in normal design conditions, h is the slab height, c is the concrete cover, ϕ is the reinforcement diameter, b is the width of the slab, kfc(t ) is temperature-dependent strength retention coefficient for the compressive strength of concrete and fc is the compressive strength of concrete in normal design conditions. in order to calculate when the slab would fail during the furnace fire resistance test, first the temperature distribution needs to be calculated for each point in time of the test. that is done using the described numerical heat transfer model and the iso 834 fire curve as boundary temperature-time curve. then, using equation 2 the slab’s capacity can be calculated for each point in time and from it consecutively the failure time can be calculated by determining the point in time when the capacity is lower than the applied loads. the loads during the test are assumed to be equal to 50 % of the slab’s capacity in normal conditions, (i.e., 50 % utilization in normal design conditions). this process is computationally expensive for later probabilistic calculations. hence, for 10 000 different combinations of the stochastic variables (table 1), a failure time is calculated and a polynomial regression model is fitted on the results (with the coefficient of determination being r2 = 0.99). all the other parameters in equation 2 have been regarded as constants instead of variables. 2.4. updating of uncertain parameters based on standardized fire test the updating procedure is a standard bayesian procedure using markov chain monte carlo simulations. the parameters that are updated are the quantiles of the thermal properties’ models and the yield strength of the reinforcement. all the priors are presented in table 1. the likelihood is defined under the assumption that the failure time error is normally distributed with a mean of 0 and standard deviation σerr and is presented in equation 3: l (y|x) = 1 σ exp ( −1 2 (y − m(x))2 σ2 err ) (3) where y is the measured failure time, m(x) a model output for the input parameters x = (k, cp, fy). the measurement uncertainty σerr is modelled by a normal distribution with a mean of 2 min and a standard deviation of 1 min, these values are chosen based on the model error and measurement precision during the furnace test. 2.5. natural fire exposure the end goal is to obtain the probability of failure in the case of a real fire using the measured test failure time. for that, the thermal gradient for a natural fire exposure also has to be calculated. a parametric 86 vol. 36/2022 standardized fire tests using a bayesian framework parameter symbol unit distribution mean standard deviation ref fuel load gf [mj/m2] gumbel 780 234 [8] concrete cover c [mm] beta [µ ± 3σ] 40 5 [9] dead load g [knm] normal 27 2.7 [10] live load (qk = 16) q [knm] gamma 3.2 3.04 [10] load model uncertainty ke [−] lognormal 1 0.1 [10] resistance model uncertainty kr [−] lognormal 1 0.15 [10] table 2. distributions of the parameters used to calculate the failure probability. figure 2. posterior distributions for thermal conductivity (left) and specific heat of concrete (right) at different temperatures for different values of failure time with the shaded 90 % highest density intervals (hdi). fire curve from en 1991-1-2 [8] is used to represent a natural fire exposure. in order to use it, some parameters defining the compartment where the slab is located must be defined. in this study, a compartment with dimensions of 10 m × 10 m × 3 m, opening factor o = 0.05m1/2 and wall linings with thermal inertia of 1450 j/(m2ks1/2) is considered. these parameters can be considered as easily determined and constant. however, to define the parametric fire curve, the fuel load in the compartment also needs to be defined. there is a lot of uncertainty on this parameter and therefore it will be considered stochastically (see table 2). similarly, as with the analysis of the furnace test, a heat transfer calculation is needed to determine the temperature gradient and the slab’s bending capacity. again, polynomial regression is used as there is a need for a large number of calculations. for 1000 different combinations of the thermal conductivity quantile, the specific heat quantile and the compartment fuel load, the temperature gradient is calculated and that data is used for the regression. the final step is the calculation of the conditional failure probability given a structurally significant fire. this is done using the updated material properties’ distributions and additional stochastic parameters (presented in table 2). the failure probability is calculated using equation 4 which describes the limit state z. z = kr · mr − ke · (g + q) (4) 3. results and discussion the bayesian updating was performed multiple times for different assumed values of the failure time during the test. updated posterior probability density functions of both thermal conductivity and specific heat at different temperatures for a few failure times are presented in figure 2. from these results, it is observed that the failure time is not particularly sensitive to the value of the thermal conductivity and the updating procedure results in only small changes to the prior distribution. for example, at ambient temperature, the standard deviation reduces from 0.42 to approximately 0.29 w/(m.k). however, the failure time is highly dependent on the value of the specific heat. the standard deviation at ambient temperature reduces from 0.57 for the prior to roughly 0.13 kj/(kg.k) for the posterior. furthermore, an almost linear relation between the posterior mean value of the specific heat and the failure time can be seen, as shown in figure 3 (left). using the first bayesian method (mcmc) the yield strength of reinforcement steel was also updated, how87 b. jovanović, r. v. coile, e. reynders et al. acta polytechnica ctu proceedings figure 3. posterior mean value of the specific heat of concrete at ambient temperature in function of the failure time (left); posterior distributions of the reinforcement steel yield strength at ambient temperature for different values of the failure time (right). figure 4. failure probability (left) and reliability index (right) in the function of the failure time. ever, according to the results in figure 3 (right), it is evident that the failure time sensitivity to this parameter is relatively low. it must be mentioned that the mcmc sampling method might produce unrealistically low posterior uncertainty as in theory the update would only be specifically valid for the tested slab and not account for the possible variations between similar slabs that are going to be used for the same structure (which are not tested). in the case where more than one test is used, a procedure similar to the one presented in [12] should be used where the mechanical properties of reinforcement are not updated in order to focus on the thermal properties. to determine how much the failure probability changes with the updated posteriors, the failure probability is first determined based on the prior distributions. this is calculated using 106 monte carlo realizations and the obtained value is equal to 4.4 · 10−3. . the probability of failure is then also calculated using the posterior distributions using the posteriors for thermal conductivity and specific heat instead of their respective priors. the results are presented in figure 4 in the function of the failure time. in order to demonstrate how different test failure times affect the probability of failure in the case of the real fire, the failure probability has been calculated for the range of test failure times from 110 to 150 min and it ranges from 8.35 − 2.03 · 10−3. as expected, a higher value of the failure time leads to a lower value of the failure probability. however, what is interesting is that for failure times lower than 88 vol. 36/2022 standardized fire tests using a bayesian framework 120 min, the failure probability is higher than the one calculated using the prior distributions. when analysing the distributions of the moment capacity of the slab, it is observed that the reduction of the variability compared to using prior distributions is almost negligible. this can be attributed to the fact that the uncertainty on the resistance of the slab is mainly governed by the uncertainty on the fuel load. however, the posteriors of the thermal properties still have an effect, as it is observed that the mean value of the capacity changes for different measured failure times. 4. conclusions a procedure to obtain a more precise failure probability of a structural member in case of fire using bayesian updating based on the standard furnace test result was presented. this procedure focuses on maximizing information from the highly used furnace test in relation to the performance-based assessment of the structural capacity under a real fire condition. as the furnace test provides the failure time in case of standardized fire exposure, a clearer connection from its results to the structure’s behaviour in real fire is explored. the procedure is demonstrated on the case of a reinforced concrete slab. a standard markov chain monte carlo simulation approach was used and showed that the failure time during the test is most sensitive to the variation of the specific heat. for example, the updating procedure resulted in a reduction of the standard deviation of specific heat at ambient temperature from the prior value of 0.57 to a posterior value of approximately 0.13 kj/(kg.k). this also has an effect on the calculated failure probability under a real fire condition. using the updating procedure and monte carlo simulations, the failure probability under a real fire condition was calculated for the range of test failure times from 110 to 150 min and it ranges from 8.35 − 2.03 · 10−3. it was also observed that the variability of the concrete slab capacity in the case of a natural fire is mainly determined by the uncertainty of the fire exposure and that a reduction of the uncertainty of material properties obtained with this method has a small but distinctive effect. it must be noted that this study’s results are limited to the behaviour of the reinforced concrete slab, where the failure is mostly a consequence of strength reduction of the reinforcement due to the elevated temperature. in the cases of other materials and structural members, the effectiveness of this approach might differ. nevertheless, this presents an improvement in the use of the furnace test results as this approach tackles one of its biggest flaws, correlation to the real fire behaviour, and shows that they can be used to quantitatively assess the safety level of a structure in case of a real fire. acknowledgements the authors wish to thank the research foundation of flanders (fwo) for the financial support on the research project (grant number 3g010220) "vibration-based postfire assessment of concrete structures using bayesian updating techniques". references [1] a. law, l. bisby. the rise and rise of fire resistance. fire safety journal 116, 2020. https://doi.org/10.1016/j.firesaf.2020.103188. [2] j. wang, n. zabaras. a bayesian inference approach to the inverse heat conduction problem. international journal of heat and mass transfer 47(17-18):3927-41, 2004. https://doi.org/10.1016/j. ijheatmasstransfer.2004.02.028. [3] s. guo, r. yang, h. zhang, et al. new inverse model for detecting fire-source location and intensity. journal of thermophysics and heat transfer 24(4):74555, 2010. https://doi.org/10.2514/1.46513. [4] k. j. overholt, o. a. ezekoye. characterizing heat release rates using an inverse fire modeling technique. fire technology 48(4):893-909, 2012. https://doi.org/10.1007/s10694-011-0250-9. [5] cen, en 1992-1-2:2004: eurocode 2: design of concrete structures part 1-2: general rules. structural fire design. european standard, 2004. [6] b. jovanović, n. e. khorasani, t. thienpont, et al. probabilistic models for thermal properties of concrete. proceedings of the 11th international conference on structures in fire (sif2020), p. 342-352, 2020. https://doi.org/10.14264/363ff91. [7] jcss, jcss probabilistic model code. part 3.2 static properties of reinforcing steel. joint committee on structural safety, 2001. [8] cen, en 1991-1-2:2002 actions on structures part 1-2: general actions actions on structures exposed to fire. european standard, 2002. [9] jcss, probabilistic model code. part 3.10 dimensions. joint committee on structural safety, 2001. [10] b. jovanović, r. van coile, d. hopkin, et al. review of current practice in probabilistic structural fire engineering: permanent and live load modelling. fire technology 57(1):1-30, 2020. https://doi.org/10.1007/s10694-020-01005-w. [11] t. thienpont, r. van coile, r. caspeele, et al. burnout resistance of concrete slabs: probabilistic assessment and global resistance factor calibration. fire safety journal 119, 2021. https://doi.org/10.1016/j.firesaf.2020.103242. [12] w. botte, e. vereecken, l. taerwe, et al. assessment of posttensioned concrete beams from the 1940s: large-scale load testing, numerical analysis and bayesian assessment of prestressing losses. structural concrete 22(3):1500-22, 2021. https://doi.org/10.1002/suco.202000774. 89 https://doi.org/10.1016/j.firesaf.2020.103188 https://doi.org/10.1016/j.ijheatmasstransfer.2004.02.028 https://doi.org/10.1016/j.ijheatmasstransfer.2004.02.028 https://doi.org/10.2514/1.46513 https://doi.org/10.1007/s10694-011-0250-9 https://doi.org/10.14264/363ff91 https://doi.org/10.1007/s10694-020-01005-w https://doi.org/10.1016/j.firesaf.2020.103242 https://doi.org/10.1002/suco.202000774 acta polytechnica ctu proceedings 36:84–89, 2022 1 introduction 2 methodology 2.1 overview 2.2 thermal calculation 2.3 bending capacity and failure time 2.4 updating of uncertain parameters based on standardized fire test 2.5 natural fire exposure 3 results and discussion 4 conclusions acknowledgements references https://doi.org/10.14311/app.2022.33.0604 acta polytechnica ctu proceedings 33:604–609, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague the status of sustainable concrete codes in the united states andrew warren taylor kpff consulting engineers, 1601 fifth avenue, suite 1600, seattle, wa 98117, usa correspondence: andrew.taylor@kpff.com abstract. as the focus on embodied carbon within the built environment has grown in the u.s., local jurisdictional codes have increasingly proposed and implemented code language that requires the use of concrete mixtures that have reduced environmental impacts. this paper provides details about the various ways that localized sustainable concrete provisions have been proposed and implemented in the u.s., including limits on cement content and global warming potential (gwp). these laws and code revisions vary in their measurement of low-carbon concrete and their exemptions and incentives. while the current version of the aci building code, aci 318-19, does allow for the consideration of sustainability in concrete design, specifics about how the relative sustainability of concrete mixtures and systems are to be measured and compared are not included. this paper addresses how the changing landscape of local jurisdictional codes is driving the development of future aci 318 provisions to play a part in providing more consistency in the application of sustainable design practices in the united states. keywords: codes, concrete, sustainability. 1. introduction in the last decade engineers, architects, local and national government agencies, and national building code committees in the united states have started to evaluate and take measures to mitigate the environmental impacts of building construction and operations. as these efforts mature, new programs to encourage more sustainable design and operation of buildings, and new regulations that require the reduction of carbon emissions, have been established. regarding concrete structures, it is most common for new programs and regulations to focus on the carbon emissions associated with the initial construction of a building: atmospheric carbon released by the manufacturing of portland cement, by the mining and transportation of constituent materials, and by the construction process itself. less often, consideration is given to the carbon emissions associated with the complete life-cycle of the building: the construction period; the operating life of the building; and the demolition and disposal or recycling of the building materials. when viewed in this broader life-cycle sense, the carbon emissions associated with portland cement manufacturing make up a smaller proportion of the total atmospheric carbon impact than when only the initial construction emissions are considered. this paper describes the most recent programs and regulations implemented in the united states that promote the reduction of carbon emissions associated with concrete buildings. programs and regulations that consider only the atmospheric carbon emissions related to initial construction, as well as one program that consider the impacts of the entire life cycle of the building, are summarized 2. metric for global warming potential the de facto international standard for assessing the global warming potential (gwp) of construction materials is the environmental product declaration (epd). an epd is prepared in accordance with iso 14025 [1], and each epd provides a quantitative measure of gwp in terms of kilograms of co2 per cubic meter of material (kg co2e/m3). an epd is required for each variation of a construction material; thus a unique epd is necessary for each concrete mixture that is incorporated into a building project for which a carbon emissions assessment is performed. the existence of epds, and gwp ratings provides a consistent basis for evaluating the overall gwp of a project. epds and gwp ratings also enable government agencies and code-writing authorities to establish objective frameworks for quantifying the greenhouse gas (ghg) emissions associated with a particular building project, group of projects, type of construction, or regional or national construction activity. the gwp of a particular concrete mixture depends on many factors, including portland cement content, supplementary cementitious materials content, coarse and fine aggregate sources, and the density of aggregates (normal weight vs. light weight aggregates). in general, the gwp of most concrete mixtures commonly produced in the united states varies between about 200 and 700 kg co2e/m3 604 https://doi.org/10.14311/app.2022.33.0604 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 33/2022 sustainable concrete codes in the usa 3. organizations promoting reduced gwp of concrete in the united states several organizations have emerged as leaders of the movement to reduce the environmental impacts of concrete. generally, these are not-for-profit organizations, professional societies, or industry consortia. four organizations that support a reduction in gwp of concrete are described below. 3.1. carbon leadership forum the carbon leadership forum (clf) is a not-forprofit forum operated within the college of built environments at the university of washington, in seattle. the clf was founded in 2009 with the objective of developing and promoting quantitative and standardized measures of embodied carbon in construction materials. a primary focus of the clf has been embodied carbon of construction materials, including the promotion of standardized and verified epds, which will allow valid comparisons between various materials and between various suppliers of a given material. a major accomplishment of the clf has been to foster development of computer software that provides a standardized method for calculating the embodied carbon in construction materials. this software was released on november 19, 2019 and is called the embodied carbon in construction calculator, or ec3. the ec3 software is available without charge. it relies on a database of over 19,000 epds from the united states and canada. the vast majority of the epds in the database are for concrete mixtures; epds for other materials as well as additional epds for concrete are continuously added as they become available [2]. 3.2. structural engineers 2050 challenge the structural engineers 2050 challenge (se 2050) was conceived in 2016 as a way to encourage the structural engineering profession to actively participate in a movement towards buildings with greatly reduced, or zero, net embodied carbon. since structural engineers are responsible for the selection of structural materials, structural engineers are in the best position to implement structural materials with the lowest possible embodied carbon, and to report embodied carbon of individual projects to a centralized database. it is envisioned that such a database will be used to track annual total embodied carbon for buildings in the united states. membership in the challenge is voluntary. as of this writing, signatories to the se 2050 challenge include 19 structural engineering consulting companies, 62 individual structural engineers, and 32 other individuals or companies affiliated with the building design and construction industry. 3.3. architecture 2030 challenge the non-profit group architecture 2030 was founded in 2002 to address the need for reducing operating carbon emissions from the built environment. in 2006 architecture 2030 launched the 2030 challenge, which defines quantitative goals for reducing the greenhouse gas emissions associated with the ongoing operations of buildings. emphasis is placed on reducing the consumption of energy generated by fossil fuels, with the goal of reducing operational greenhouse gas emissions of all new or renovated buildings to zero by 2030. "these targets may be met by implementing innovative sustainable design strategies, generating on-site renewable energy, and/or purchasing (20% maximum) off-site renewable energy." [3]. to support the architecture 2030 challenge, the american institute of architects (aia) has created the "2030 commitment." this is a program under which signatory design firms voluntarily report project data, in a standardized format, which quantifies the operational greenhouse gas emissions over the life of the building. reported data is collected in a central database called the design data exchange (ddx). the ddx can be searched using multiple project attributes (like building type, size, and environmental conditions) to help designers create buildings with reduced operational carbon emissions. currently over 600 design firms have signed on to the 2030 commitment, and each year project reports are added to the ddx for hundreds of millions of square meters floor area of new and renovated buildings. 3.4. the american concrete institute in recent years the american concrete institute (aci) has sought to address the challenge of reducing greenhouse gas emissions through a range of initiatives and publications. these include more than 40 standards and guides containing information on sustainability in concrete construction; over 60 journal articles related to sustainability; 200 practiceoriented papers and articles on sustainable design and construction methods; more than 100 online learning tools; and sponsorship or co-sponsorship of over 25 events that have supported the advancement of sustainability in concrete design and construction. in 2008, the american concrete institute (aci) initiated a focus on mitigating the environmental impacts of concrete with the formation of technical committee aci 130 "sustainability of concrete." the committee currently has over 100 voting and associate members, and maintains eight subcommittees: • materials • production/transportation/construction • structures in service • rating systems/sustainability tools • design/specifications/codes/regulations • education 605 andrew warren taylor acta polytechnica ctu proceedings figure 1. local government and state initiatives related to sustainability of construction materials. • climate change impacts on the sustainability of concrete • liaison subcommittee in 2019 committee 130 published report on the role of materials in sustainable concrete construction [4] which provides comprehensive background on sustainability topics, and describes available practices for design, production, and construction that reduce the environmental impacts of concrete. aci formed innovation task group 10 (itg-10), which in 2019 published two important reports that provide guidance on the characteristics and implementation of alternative cements (i.e. alternatives to portland cement): report on alternative cements [5], and practitioner’s guide for alternative cements [6]. these reports present practical approaches to incorporating alternative cements in concrete mixtures, as a means of reducing greenhouse gas emissions caused by production of portland cement. it should be noted that in the 2019 edition of the aci 318 building code [7], section 26.4.1.1.1 was added with the intention of providing increased flexibility to engineers and contractors for incorporating alternative cements in concrete construction: "alternative cements shall be permitted if approved by the licensed design professional and the building official. approval shall be based upon test data documenting that the proposed concrete mixture made with the alternative cement meets the performance requirements for the application including structural, fire, and durability." in order to further develop sustainability provisions in the aci 318 code, in 2019 a new technical subcommittee of the building code committee was established: subcommittee n, "sustainability." this new subcommittee is currently working on revisions to the 2025 edition of the aci 318 code, including an appendix with expanded provisions regarding sustainability. 4. local and state initiatives to quantify and reduce gwp of concrete in the united states within the last two years several local and state government initiatives have been either proposed or enacted to reduce the environmental impacts of concrete construction. these initiatives are identified in figure 1. in addition, the author is aware of preliminary discussions within other jurisdictions that may lead to further concrete sustainability initiatives, but it is still too early to report on the details of those efforts. for example, the marin county ordinance (figure 1, and section 4.3) is likely to serve as a model ordinance for adoption by other san francisco bay area governments. 4.1. the buy clean washington act on january 8, 2018 a proposal was introduced to the washington state house of representatives, house bill 2412, titled the "buy clean washington act." [7] this proposed legislation recognizes that "climate change will have devastating global impacts," and that "all scientific evidence points to the need for washington and the world to reduce greenhouse gas emissions to avert the worst effects of climate change." to reduce greenhouse gas emissions, the legislation proposes "incorporating emissions information from throughout the supply chain and product life cycle into procurement decisions, and using that information to help direct expenditure." in other words, if adopted, the legislation would make global warming potential a key factor in the selection of construction materials and suppliers in washington state. this act would apply to new construction with a floor area greater than 500 square meters and a building renovations greater than 500 square meters where 606 vol. 33/2022 sustainable concrete codes in the usa the cost of the renovation is greater than 50 percent of the value of the building. the list of construction materials that this act would apply to includes • carbon steel concrete reinforcing bars • flat glass • mineral wool board insulation • structural steel • cement • structural timber • solar panels • refrigerants in new equipment • aluminum • gypsum • concrete the proposed legislation was not adopted during the 2018 washington state legislative session, but it is likely the proposal will be re-introduced in 2020. 4.2. city of portland ordinance the city of portland, in the state of oregon, enacted an ordinance in january 2020 that is intended to be the starting point of regulations to limit the greenhouse gas emissions of concrete purchased by the city. at this time the ordinance requires reporting of a product-specific type iii environmental product declaration that is verified by a third party and is less than 5 years old for every concrete mixture supplied for city-funded projects. the ordinance does not, however, currently set limits on embodied carbon or global warming potential of concrete. such limits will be published by the city by april 1, 2021, when the city will issue limits on the maximum global warming potential of any concrete mixtures on city-funded projects. the ordinance does not apply to small projects that require less than 38 cubic meters of concrete. 4.3. marin county ordinance one of the most significant recent developments in regulatory rules regarding the global warming potential of cement and concrete is the enactment of an ordinance in marin county, california. this ordinance, titled "low carbon concrete requirements," forms section 19.07 of the marin county code [9]. it was adopted in november, 2019. in establishing the rationale for implementing this ordinance, the county board of supervisors stated, ". . . marin county experiences climatic seasonal reduction in vegetative moisture content, combined with our heavily populated steep terrain, which presents increased wildfire risk to our residents from carbon-induced global warming; and is also bordered by sea water on three sides and subject to direct adverse local impact from sea-level rise as the result of construction related contributions to climate change, including significant carbon emissions from cement production." to accomplish the goal of reducing greenhouse gas emissions by controlling embodied carbon in concrete, marin county has taken the approach of providing two alternatives: all concrete used in marin county must either meet maximum limits on cement content, or it must meet maximum limits on embodied carbon. these limits are dependent on the minimum specified compressive concrete strength (f ′ c). table 1 below is adapted from table 19.07.050 of the marin county ordinance [9]. an exception to the limits in table 1 is permitted for concrete that must attain a high early strength, such as concrete used in precast or pre-stressed construction, concrete used in above-ground beams and slabs, and pneumatically placed concrete (shotcrete). in those cases the values in table 1 may be exceeded by 30 percent. another exception is allowed to the limits in table 1 if a pre-approved cement is used that has an environmental product declaration with embodied carbon less than 1,040 kg co2e per metric ton. the marin county regulations permit compliance with the table 1 limits on the basis of either a) the concrete mixture design, or b) based on the total volume of concrete used in the project. the latter method provides a way to use a variety of concrete mixtures on a project, some of which may not meet the table 1 limits, provided the total cement usage or the total embodied carbon of concrete used on the project does not exceed the hypothetical total cement usage or the total embodied carbon of concrete calculated based on the actual volume of materials and the upper limits of table 1. the regulations define a "hardship or infeasibility exemption" which may be invoked if a) there is a lack of commercially available material necessary to comply with the regulations; b) the cost of complying with the regulations is disproportionate to the overall cost of the project; or c) compliance with the regulations would impair the integrity of a recognized historic building. since passage of the marin county ordinance, there has been considerable interest from other county and city governments in considering similar regulations, particularly in the state of california. 4.4. the buy clean california act the california state legislature passed the "buy clean california act" in 2017 [10]. this legislation requires that, for state-funded projects, information about the global warming potential of the materials used in the project must be reported. the act also mandates that embodied carbon of construction materials must be considered as one of the decision factors when selecting materials, contractors and materials suppliers. 607 andrew warren taylor acta polytechnica ctu proceedings minimum specified concrete strength f ′ c a maximum ordinary portland cement contentb maximum embodied carbon, from epd [mpa] [kg/m3] ! kg (co2e) /m3" ≤ 17.2 215 260 20.7 243 289 27.6 271 313 34.4 298 338 41.3 315 356 48.2 352 394 > 48.2 390 433 ≤ 20.7 light weight 304 578 27.6 light weight 339 626 34.4 light weight 373 675 a for concrete strength between the stated values, use linear interpolation to determine cement and/or embodied carbon limits. b portland cement of any type meeting the requirements of astm c150 [8]. table 1. marin county cement and embodied carbon limits. certain construction materials are addressed by the act, but at this time cement and concrete are not among them. steel reinforcing bars are currently covered by the act, and it is reasonable to expect that cement and concrete will eventually be included. any construction material that is listed under the act is assigned an upper limit on global warming potential; subsequently, when that material is purchased by the state a version of that material must be supplied that meets the global warming potential limit. because the state of california has a large economic and political influence in the western united states, it is likely that other western states will consider adopting similar "buy clean" rules for state-funded construction projects. 4.5. new york state legislation on september 12, 2019, a proposal was introduced to the new york state legislature that would require the use of concrete with low embodied carbon on new york state projects (i.e. all state funded projects that incorporate concrete as a construction material). this legislation would also promote the use of concrete that implements co2 capture technologies. in addition, the proposal introduced a tax credit system that would promote the preparation of epds by allowing concrete suppliers to deduct the cost of preparing epds from their tax bill. although this proposed legislation was not adopted, a revised version will likely be re-introduced to the new york state legislature in the near future. 5. discussion given the recent surge of interest in the global warming potential of construction materials in general, and cement and concrete in particular, it is incumbent upon codes and standards writing organizations to develop frameworks for quantifying and reporting the global warming potential of construction projects. because each construction material is unique in terms of its manufacture, transport, and implementation, it naturally falls upon the professional and industrial organizations related to each material to develop such guidelines. complementary to the work of professional and industrial organizations to understand the global warming potential of specific construction materials, there is the work of local, state, and national government organizations to develop guidelines and regulations that will lead to reductions in greenhouse gas emissions. these government agencies must balance the technical, economic, and political factors that influence the creation of environmental policies. it is the job of structural and materials engineers to ensure that government agencies have the technical data and analytical tools necessary to make informed decisions about the environmental impacts of construction materials. this is why the american concrete institute has embarked on a multi-faceted program to study, quantify, and develop engineering solutions that will reduce the environmental impacts of cement and concrete. section 3.4 above describes aci’s current activities in this area. 6. conclusions this paper provides an overview of current activities in the united states related to building codes and regulations governing the environmental impacts of cement and concrete. this is a societal issue that is evolving rapidly, not only in the united states, but worldwide. some of the pending regulations described in this paper may soon be enacted into law, and other new initiatives will soon be introduced in local, state, and national jurisdictions. in an effort to support this trend towards improved control of greenhouse gas emissions, various groups in the united states are developing metrics for quantifying 608 vol. 33/2022 sustainable concrete codes in the usa the global warming potential of construction materials. this presents an opportunity for technical organizations in the united states, such as the american concrete institute, to provide leadership in matters related to quantifying and reporting the global warming potential of cement and concrete. acknowledgements the author wishes to thank the american concrete institute (aci) for travel funding provided through the aci ambassador program. references [1] so 14025:2006. environmental labels and declarations type iii environmental declarations principles and procedures, geneva: iso, 2006. [2] n. post, j. gonchar. embodied carbon tool premieres engineering news record, new york: bnp media ii, .p 8-9, 2019. [3] architecture 2030, 2020. http://architecture2030.org. [4] aci committee 130. 130r-19: report on the role of materials in sustainable concrete construction, farmington hills: american concrete institute, 2019. https://cecollection2.files.wordpress.com/2020/ 05/130r-19-report-on-the-role-of-materials-i n-sustainable-concrete-construction.pdf. [5] aci committee itg-10. report on alternative cements, farmington hills: american concrete institute, 2018. https://www.concrete.org/portals /0/files/pdf/previews/itg-10.1r-18_preview.pdf. [6] aci committee itg-10. practitioner’s guide for alternative cements, farmington hills: american concrete institute, 2018. https://www.concrete.org/portals/0/files/pdf/pr eviews/itg-10r-18_preview.pdf. [7] state of washington. house bill 2412: an act relating to creating the buy clean washington act; and adding a new chapter to title 39 rcw, olympia: washington state house of representatives 65th legislature, 2018. [8] american society for testing and materials international 2019 astm c150/c150m 19a: standard specification for portland cement, west conshohocken: astm international, 2019. [9] marin county california. section ii: marin county code chapter 19.07 added to marin county code title 19, marin: marin county board of supervisors, p. 1-6, 2019. [10] state of california. public contract code division 2, part 1, chapter 3, article 5: buy clean california act, sacramento: california state legislature, 2017. 609 http://architecture2030.org https://cecollection2.files.wordpress.com/2020/05/130r-19-report-on-the-role-of-materials-in-sustainable-concrete-construction.pdf https://www.concrete.org/portals/0/files/pdf/previews/itg-10.1r-18_preview.pdf https://www.concrete.org/portals/0/files/pdf/previews/itg-10r-18_preview.pdf acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.38.0606 acta polytechnica ctu proceedings 38:606–612, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague oldest concrete vaulted and arch bridges in the czech republic – evaluation and preservation of their historical value roman šafář∗, vladislav hrdoušek, helena včelová, vendula hlavničková czech technical university in prague, faculty of civil engineering, department of concrete and masonry structures, thákurova 7, 166 29 praha 6, czech republic ∗ corresponding author: roman.safar@fsv.cvut.cz abstract. the paper is related to a research project focused on tools for an evaluation and preservation of the historical value and function of arch and vaulted road bridges. it deals with the beginning of use of concrete vaulted and arch bridges in the area of the czech republic from the first structures built at the end of the 19th century up to the time just before the wwii. the paper includes examples of built structures as well as a general summary and development of the used technical solutions. important part of the paper is a proposal of criterions for evaluation of their structural and historical value. keywords: concrete bridge, arch bridge, vaulted bridge, existing structure, repair of bridges, strengthening of bridges, historical value of bridges, structural value of bridges. 1. beginning of use of concrete for bridges in the area of the today’s czech republic the first structures using cement concrete were built here principally in the second half of the 19th century, when this area was a part of the austria-hungarian empire (up to 1918) and it can be said a part quite highly economicly developed. nevertheless, thanks to relatively cheap and easily accessible other construction materials, use of cement concrete for bridges and other load-bearing structures was not too needed from economical reasons and concrete was used more for such structures as water buildings, sewerage systems and foundations. for the first time, cement concrete was applied for a bridge here probably in the town of poděbrady – it was for watertight layers in anchoring chambers of main chains of a chain bridge, built over labe river in 1840–1842. the chambers were several times flooded and damaged by high water and these watertight layers were found as really effective [1]. the first known concrete bridge in this area was a vaulted structure of a clear span of 8.000 m and a rise of 2.000 m, built in 1892 over bobrava creek in tetčice. the next proven concrete bridge was built here for the ethnographic exhibition held in prague in 1895. this exhibition included besides others also quite a large number of timber-made maquettes of typical buildings from various cities and villages. one of such buildings was a maquette of the castle of kokořín and on the access ramp to this castle, the concrete bridge was built. it was a vaulted structure of a clear span of 12.956 m and of a rise of the vault of 2.450 m. the bridge was designed by a. v. velflík and built by company “hrůza and rosenberg”. after the exhibition, it was removed (figure 1). the first concrete bridge built for transport reasons was the bridge over rokytka river in prague, built in 1896. it was built within only 35 days according to the design of antonín loss. it is a plain concrete vaulted structure of a clear span of 13.300 m and a rise of 2.750 m. the width of the bridge was during next years and decades enlarged by subsequently added structures – vaulted as well as made of girders – up to the form of a small “square”. the bridge exists up to our days. the first bridge made of reinforced concrete was built over a rocky gorge in the castle veveří near brno at the place of an older structure made of stone masonry. the bridge was built in 1898 according to a design of josef melan with reinforcement made of internal embedded steel i-shaped girders. josef melan elaborated and patented a whole system of concrete structures reinforced by this kind of reinforcement (so-called melan system). clear span of the bridge is 17.550 m and it has a three-cell box concrete superstructure fixed to abutments made of stone masonry (figure 2). in the next years, quite large number of concrete vaulted and arch bridges were built, so only a few of them can be mentioned here. in 1900, a skew two-span vaulted bridge of span lengths of 2×16.260 m was built over tichá orlice river at lanšperk near ústí nad orlicí, which was the largest concrete bridge in bohemia after its completion. the bridge was built by company “uhlíř – construction of concrete buildings at velim” and after a repair in 1996 it is still in service. in 1901, a vaulted bridge of two spans of 2 × 20.0 m 606 https://doi.org/10.14311/app.2022.38.0606 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 38/2022 oldest concrete vaulted and arch bridges in the czech republic . . . figure 1. concrete bridge at the ethnographic exhibition in prague 1895. figure 2. bridge over rocky gorge at the castle veveří near brno, 1898 (left – former masonry bridge [2], right – concrete bridge at present). was built at leština near zábřeh. in 2010, after 109 years of service, the bridge was replaced by a new one with a single-span arch (“bow-string”) superstructure with a span length of approximately 40.0 m. in 1902, a single-span vaulted bridge of a span of approximately 20 m was built in olomouc over former mlýnský potok (“mill creek”), removed in 1951. another single-span vaulted bridge was built in 1902 at srbská kamenice near děčín – its span length is 18.0 m [3]. the bridge was built at the place of an older structure, destroyed by floods in 1897. one more single-span vaulted bridge of a span length of 26.900 m was built in 1902 at písečné – modletice near jindřichův hradec. both of these structures exist so far. in the same year, a vaulted footbridge over tracks in the railway station at sokolov was built, with a span length of 16.5 m. in 1903, a three-span vaulted bridge over bečva river was built in přerov on the place of a former timber structure. span lengths were 3 × 22.400 m. this bridge was destroyed in may 1945 and then a temporary timber and then a steel footbridge were built there. since 2012, a new reinforced concrete cast-in-situ girder bridge with a noticeable artistic decoration is at that place. in near hranice, a new bridge was built in 1905 on the place of an older structure (figure 3). the final solution with two reinforced concrete arch spans of a length of 32.500 m was chosen of several steel as well as concrete proposals. this bridge was also destroyed in may 1945; then in 1960, a new two-span reinforced concrete arch bridge with a similar arrangement but not the same structure was built at this place. span lengths of the new bridge are 34.150 + 34.020 m. in 1910, the first “bow-string” (with a deck carried by ties below the arch) reinforced concrete bridge was built within the area of a paper-producing factory at hostinné near hradec králové (with so called melan 607 r. šafář, v. hrdoušek, h. včelová, v. hlavničková acta polytechnica ctu proceedings figure 3. arch bridge at hranice, 1905 (left – during construction, right – after completion) [4]. figure 4. vaulted bridge at peřimov – dolní sytová, 1912 (left – during construction [5], right – at present). system), of a clear span of 24.000 m. the arch of the bridge is rigidly fixed to the abutments, while its deck is separated from the abutments by expansion joints. the bridge was strengthened by external steel elements end exists so far. in 1912, a single span vaulted bridge of the span length of 45.0 m was built at peřimov – dolní sytová near liberec (figure 4). its vaulted superstructure had three hinges – two at springers and one at the top of the vault. the structure was repaired and strengthened (the hinges were canceled during that repair) and exists so far. several vaulted and arch bridges of the span length of approximately 30 m to 50 m and even more were built during next years – e.g. hlávka bridge in prague (39.0 m; 1912), hradec králové – plácky (34.0 m; 1913), nymburk – velké valy (40.0 m; 1913), mánes bridge in prague (41.800 m; 1914), masaryk bridge in plzeň (55.600 m; 1918), libeň bridge in prague (42.8 m; 1928), the bridge at hradec near stod (63.860 m; 1928), the bridge at kolín (30.400 m; 1928), masaryk bridge at kralupy nad vltavou (85.0 m; 1928), mohelnice nad jizerou (40.0 m; 1930) and ostrov bridge at karlovy vary (77.500 m; 1931) – the both with a three-hinge arches, švehla bridge at tábor (81.600 m; 1935), the bridge at loket (62.0 m; 1936) etc., as well as reinforced concrete bow-string bridges and bridges with a lower deck or an intermediate deck – for example the bridge at hořepník (26.6 m; 1912), the bridge at besednice – pořešín near český krumlov with the superstructure fixed to the abutments (30.0 m; 1913), sokolov (34.0 m; 1920), skew bridge at dřevohostice (19.70 m; 1922) and at horky nad jizerou (29.40 m; 1923), česká třebová (41.0 m; 1924), mladá boleslav – debř (46.50 m; 1924), svijany – podolí (48.10 m; 1924), tuřice (51.50 m; 1924), karviná – darkov (56.0 m; 1925), hradec králové – malšovický (43.20 m; 1925), the bridge at brno – husovice with a three-hinge arch, uherský ostroh (52.80 m; 1928) etc. in 1928, an arch bridge over lužnice river was built at bechyně in southern bohemia (figure 5). the bridge with a length of the main span of 90,0 m is used both for road traffic as well as for a single-track railway line. the bridge was repaired in 2004 and is still in use. at štěchovice near prague, an arch bridge with an intermediate deck and with a pair of box arch ribs for a span length of 113.80 m was built in 1939. at the end of construction, the bridge was flooded by high water reaching up to the handrail. the bridge sustained that situation without a serious damage and after another repair in 1995 it is still in service (figure 9). between podolsko and temešvár near písek in southern bohemia, an arch bridge of a total length of 510.0 m and a length of the main span of 150.0 m was built from 1939 to 1942 (figure 6). the bridge was built in the place of an older chain bridge constructed in 1848 because of the rise of the water level due to 608 vol. 38/2022 oldest concrete vaulted and arch bridges in the czech republic . . . figure 5. arch road – railway bridge at bechyně, 1928 (left – construction [6], right – at present). figure 6. arch bridge at podolsko, 1939–1942 (left – during construction still with the former chain bridge [7], right – at present). construction of the orlík dam reservoir. the chain bridge was disassembled, moved to stádlec and built again over lužnice river, where it still exists. the arch in the main span of the new arch bridge is fixed at the both ends, while the other arches (in the other spans and above the main arch) have two hinges at springers. the design of the bridge was in 1930’s awarded at the architectural exhibition in paris by the gold medal and was called “le beau pont de l’europe”. another award the bridge gained in 1939 at an exhibition in liège. although this paper and the related project are focused primarily on road bridges and footbridges, we would like to mention here also at least two railway bridges – the bridge at jizerní vtelno with three arch spans of the length of approximately 3×50 m, built in 1924 and the arch bridge at loučky with a main span of 110.0 m, built with some interruptions in 1940–1953. 2. principles of structural arrangement the structures considered in this paper were built as vaulted structures, i.e. covered by soil embankment or masonry filling, which carries the running surface for traffic, or arch bridges with a deck, which can be supported by struts above the arch (an “upper deck”), carried by ties below the arch (so called bowstring structures or bridges with a “lower deck”) or arches with a deck in an intermediate position passing through the arch and supported partly by struts and partly by ties. the vaults and arches with the upper deck and intermediate deck are in the most cases fixed to supports, but also structures with three hinges (two at springers and one at midspan) were built in relatively higher number of cases (figure 7). also existing, but rather exceptional (in concrete) are structures with just two hinges at springers. the hinges are made as concrete hinges, or steel hinges can also be used, similar to fixed bearings. special arrangements of supports and bearings can be found at some multiple-span structures. bow-string bridges are usually supported by a manner, which enables displacements of the structure due to thermal changes and volume changes of concrete. for that, steel bearings are used in most cases at such older structures, which are sometimes replaced by new types of bearings during repairs of these bridges. in 609 r. šafář, v. hrdoušek, h. včelová, v. hlavničková acta polytechnica ctu proceedings figure 7. three-hinge arch bridge at brno – husovice, 1928 (left – overall view, right – steel hinges). figure 8. pendulum supports (left – bow-string bridge at bačkovice, 1929; right – end of the deck of the arch bridge at přibyslav, 1940). some cases, also concrete hinges and pendulum supports were used; pendulum supports have hinges at the both ends – concrete hinges or steel hinges can be used for that (figure 8). in some cases, special arrangement of supports was used, for example supporting the structures just by thin elastomeric layers. not very often, but in some cases the superstructures of arch bridges with a lower deck are fixed into abutments and then it can be said by now-days terminology, that these bridges perform as integral structures. exceptionally the deck is fixed at one end and supported by a movable manner at the other end. also an arch bridge with a lower deck and with hinges at springers of the arch itself exists (at brantice). vaults and arch structures are usually perpendicular, but also skew structures have been built. in the case of bow-string bridges, special arrangement of cross beams is then adopted in the support area of the deck. also structures with horizontally curved carriageway can be found – then, the vault or the arch are usually straight and the curvature is obtained by a varying width of lateral cantilevers and/or by a change of position of struts in the transvers direction, exceptionally also arches are curved. struts of arch structures with the upper deck may be fixed at the both ends, or fixed at the lower end and hinged (usually with a concrete hinge) at the upper end or hinged at the both ends. decks can be continuous over the whole length of the structure (expansion joints are then usually at the ends of the bridge), or they are divided by expansion joints above supports of the arch and in some cases also near the top of the arch. depending on required displacements and on the overall arrangement of the structure, the deck at its ends may be supported by an easy way e.g. with use of bituminous or metal sheets, or it can be supported by bearings or pendulum supports. sometimes struts are not used and the upper deck is supported only at piers and at midspan of the arch. according to the deck span length the deck itself may be done as a slab or as a girder structure with longitudinal beams and transvers cross-beams. decks of bow-string bridges and arches with an intermediate and lower deck are usually composed of a deck slab, cross-beams in the places of ties, relatively large longitudinal edge beams (acting at bow-string structures also as a tie in the longitudinal direction) and of smaller longitudinal intermediate girders. if the 610 vol. 38/2022 oldest concrete vaulted and arch bridges in the czech republic . . . figure 9. expansion joints in the deck (left – expansion joint near abutment, uherský ostroh, 1927; right – expansion joint between the ends of arches, štěchovice, 1939). figure 10. ribbed uhpfrc layer around existing abutments and wing walls (left – preparation of the shape and reinforcement, right – casting the uhpfrc layer) – meziboří at litvínov. arches of such structures are fixed to their foundations, decks are usually separated by expansion joints; they can be placed near springers of arches from the side of the span (at one or at the both supports of the bridge), they may be lead between the ribs of the arches, they may be at the both of these places or a joint can be placed at midspan (figure 9). 3. principles of repairs, strengthening and widening for a repair of vaulted and arch concrete structures, quite standard procedures can be used. nevertheless, special methods of repairs and strengthening may be also needed and convenient in some cases (see e.g. figure 10). a brief survey of methods will be added at the conference. 4. principles of determination of historical and structural value of structures for the final decision “what to do with an existing structure”, technical, economical and environmental parameters should be principally considered. nevertheless, at old structures, some other parameters should be considered too to evaluate their structural and historical value. for that, the questions below are proposed: • was the structure in the time of its construction innovative by its material and/or structural solution? • was its span length and/or overall dimensions in the time of construction exceptional? • has the structure an exceptional aesthetical solution? • has the structure an interesting (extraordinary) history? • how many similar structures (of a similar arrangement and span or dimensions) have been built? • how many similar structures is still existing? • are the other still existing similar structures in a better technical state or at a better place (from the point of view of their protection – e.g. at a path with a less intensive traffic) – if yes, how many? • others – for example, is the bridge a part a pro611 r. šafář, v. hrdoušek, h. včelová, v. hlavničková acta polytechnica ctu proceedings tected urban or natural area etc.? depending on the number of points obtained from answers to these questions, final solution how to handle historical structures can be adopted – for example it is possible to use a repair of the structure by a less or more standard way, bridging an existing vault by a new horizontal superstructure (steel, concrete etc.) or embedding the existing structure by new load-bearing and protective layers, inside which the existing structure can be preserved. if possible and in needed cases, the existing structure can be further used only for a light traffic (pedestrians and cyclists), while heavier traffic can be deflected to a new path with a newly built structure. valuable structures should be protected by such a way, while in many other – mostly standard – cases, existing structures will be probably replaced by new ones at the end of their service life. if a highly-valued structure has to be removed because of its poor technical state, it is possible to build its replica at the same place. otherwise, modern structures will be probably the best solution, although even they may keep “genius loci” of the place by a similar principal concept, if it is convenient. acknowledgements the paper was written thanks to the support of the research project “tools for a preservation of the historical value and function of arch and vaulted road bridges”, no. dg20p02ovv001, granted by the ministry of culture of the cech republic. we would like to very thank to all who helped us to gain information needed for this paper – ms. and mr. mgr. barták, mgr. natalie belisová, hana bendová, karel beran, tomáš borolič, ing. ivana brdičková, mgr. martin burian, phdr. jaroslav čáp, mgr. čížek, ondřej daněk, dis, mgr. petra dohnalová, miroslav dostál, vladimír hanousek, bronislava havelková, bc. václav havlan, ing. petr hejduk, zdeněk hetzendorf, bc. zuzana hiklová, ing. pavel hojgr, mgr. jaroslav horák, vlastimil horák, ing. jana horáková, mgr. petr houzar, věra hrabovská, jaroslava chmelíčková, paní infanovičová, mgr. jana janáčková, t. janda, lucie jandíková, martina jarošová, petr joza, ing. markéta karbanová, mgr. vlasta kauerová, jiří klecker, mgr. rené klimeš, silvie klimešová, marek kny, jaroslav kocanda, pavel kočí, petr koleta, mgr. jana konvičná, jan kopecký, ing. jiří koutník, ing. vladimír kudrnáč, františek kužel (1), františek kužel (2), phdr. jiří lapáček, ing. petr masařík, mgr. simona mátlová, jiří medek, renata medunová, ing. dušan merta, mgr. tomáš mihalik, pavlína mocová, ing. aleš moskal, ing. tomáš navrátil, ivana yael nepálová, bc. ludmila otiepková, lucie paurová, ing. ondřej plášil, ing. zdeněk podráský, ing. jiří polák, ing. petra pospíšilová, ing. zuzana procházková, mgr. miroslava přikrylová, renáta purnochová, ing. václav rajchot, bc. libuše rückerová, ing. jan rund, ing. michael rund, gisela salibová, jiří satránský, taťána skokánková, ing. arch. eva strejčková, mgr. marek suchánek, ing. jan šesták, jan šon, dana štěpánová, phdr. františek šuman, bc. pavel uedl, ing. dagmar včeláková, drahomíra večeřová, mgr. jana viktorínová, ing. lenka vítová, stanislav volf, marcela vychodilová, hana winklerová, libor zámečník, kamila živná. references [1] i. seidlerová, j. dohnálek. dějiny betonového stavitelství v českých zemích do konce 19. století [history of concrete construction in the lands of bohemian crown until the end of the 19th century]. čkait, praha, 1999. isbn 80-86364-01-1. [2] archives of the national heritage institute. valdštejnské náměstí 162/3, 118 01 praha 1. [3] n. belisová. osud má jméno dolský mlýn [name of the destiny is dolský mill]. polart, 2016. isbn 978-80-87286-21-0. [4] museum of the city of hranice. masarykovo náměstí 71, 753 01 hranice. [5] information banner at the bridge, city of peřimov. 50.6240689n, 15.4422336e. [6] city museum of bechyně. náměstí t.g.m. 140, 391 65 bechyně. [7] archives of the department of concrete and masonry structures, faculty of civil engineering of the czech technical university in prague. 612 acta polytechnica ctu proceedings 38:606–612, 2022 1 beginning of use of concrete for bridges in the area of the today's czech republic 2 principles of structural arrangement 3 principles of repairs, strengthening and widening 4 principles of determination of historical and structural value of structures acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.38.0183 acta polytechnica ctu proceedings 38:183–189, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague digitalising cities: a methodology to map evaluation requirements into robust and feasible data collection approaches josé l. hernándeza, ana quijanoa,∗, pierre noailleb, mikko virtanenc, rubén garcíaa a cartif technology centre, energy division, parque tecnológico boecillo 205, 47151, boecillo, spain b cerema, direction territoriale ouest, man, rue rené viviani, 44000, nantes, france c vtt technical research centre of finland ltd., p.o. box 1000, 02044, espoo, finland ∗ corresponding author: anaqui@cartif.es abstract. the sustainable development of cities relies on the implementation of multi-sectoral actions towards carbon neutrality, reducing the air pollutants emissions. the actions’ decision-making process for cities transformation should be supported by lessons learnt from previous interventions and kpis (key performance indicators). to do so, gathering real data becomes pivotal, complementing simulation tools (currently used), solving the inherent uncertainties due to assumptions. data collection methodologies are then necessary, being the main driver for digital cities and providing better mechanisms for informed decision-making. most of the cities still operate in silos and do not always implement the strategic plans supported with a digitalization of the municipal processes. within this perspective, this paper presents a methodology to support cities in the preparation of monitoring programmes to collect real data in a robust and feasible manner. taking the kpis and the smart cities urban strategies into account, this paper concludes with some lessons learnt within cities to deploy monitoring approaches. from the city challenges to the review of the plans, all the process is driven by real data and kpis. the methodology has been applied in the mysmartlife project (grant agreement #731297) and deployed into the cities of nantes (france), hamburg (germany) and helsinki (finland). keywords: digitalisation, sustainable cities, kpis-based assessment, monitoring, decision-making, data. 1. introduction cities population is growing and nowadays, more than two thirds of the european population (the 70.9 % [1]) is living in urban areas [2], where the 29.1 % of people live in rural areas [1] with expectations to increase the statistic in the next years. this is partially because of the perception of cities as centres of economic growth that provide opportunities for study, innovation and employment [2]. additionally, cities are responsible for more than 60 % of greenhouse gas emissions, a figure that will increase with the population growth. for these reasons, there is a need for a transition, not only from current cities into smart cities, but also, as stated in the eu green deal and the horizon europe climate-neutral and smart cities mission, towards a more ambitious target that aims at the creation of climate-neutral cities. these are cities that optimize the resources, enhance the citizens’ participation, increase the satisfaction of living, as well as quality of live and sustainability [3]. however, the major challenge lies in how to assess the level of smart cities and the achievement of the sustainable goals. many cities implement subjective criteria to evaluate themselves; hence, objective methodologies and indicators are necessary, with certain level of uniformity [4], so as to be able to determine the strengths and weaknesses of the municipality. these methodologies and indicators should be driven by real data, but cities fail in the definition of proper strategies to collect real data [5, 6]. on the one hand, they lack of expertise in the application of technologies like iot [5], being usually supported by external parties. on the other hand, cities are split into verticals (e.g. energy or mobility), without relationships between councillorships [6]. cities are currently working on new urban transformation or regeneration models to transform current practices towards a smart city [4]. these require the dependency between various perspectives of the city, e.g. impacts on the energy demand due to the electrification of the mobility. then, smart cities of the future are a combination of multiple pillars such as energy, mobility, nature, economy or water management, among others, but always with the citizens at the core. all of these are supported by the integration of the information and communication technologies (ict) [7]. within this perspective, mysmartlife project [8] aims at the transformation of the cities of nantes, hamburg and helsinki into more environmentally friendly, as well as more liveable cities, by reducing greenhouse gas emissions, supported by the digital183 https://doi.org/10.14311/app.2022.38.0183 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en j. l. hernández, a. quijano, p. noaille et al. acta polytechnica ctu proceedings isation process. more than 150 actions have been implemented across the three cities following the concept of “inclusive cities”, where citizens play a key role through the smart people concept. furthermore, smart economy is the driver for the urban transformation, attracting talent and providing goods and offering services accompanied by innovative business models to satisfy the requirements and creates a solid business case. the final impacts of these actions need to be calculated to extract conclusions. under this objective, mysmartlife has defined a holistic evaluation procedure [9], where key performance indicators (kpis) are set to facilitate the impact assessment. the mysmartlife framework merges multiple pillars (energy, mobility, social, economy, ict and governance) of the city to provide a holistic analysis of the smart city. the implementation and deployment of this procedure relies on data, which is presented as one of the main challenges [5]. to overcome it, this paper presents a methodology that has been applied in nantes, hamburg and helsinki to define robust and feasible monitoring programmes with the aim of collecting real data that support the calculation of the kpis. it should be noted that the paper is focused on the digitalisation of the cities under the pillars of energy and mobility, whose assessment is based on real data, leaving out of the scope the analysis of qualitative data for other verticals. this is an iterative process for the implementation of smart city strategic plans, which can be adapted in each step to support better informed evaluation and, thus, decision-making processes. as results, this paper extracts examples and lessons learnt of the cities of nantes and helsinki. the rest of the paper is organised as follows. section 2 provides a background of current practices. section 3 presents the methodology that has been proposed within mysmartlife to approach the urban plans and data-driven evaluation. finally, section 4 shows the examples for nantes and helsinki. section 5 extracts the conclusions and lessons learnt of the proposed methodology. 2. background and current practices in smart cities as it was introduced before, the smart city concept merges multiple pillars, such as energy and environment, sustainable mobility, smart people, smart economy and transparent government, among others. all of them are supported and enabled by the implementation of ict solutions for digitalisation and data management. however, there is no consensus about standard procedures for assessing the achievement of the city goals within these pillars [4]. for instance, the authors in [10] studied 16 evaluation frameworks, including more than 950 indicators, concluding the lack of sustainability assessment. also, the ranking of indicators is highly conditioned by the data availability [4], which complicates the application of data-driven approaches for smart cities evaluation. open initiatives, like the one presented in [11], offer new frameworks under which impact and sustainable evaluation can be carried out. the result of [11] is mainly theoretical, but fosters the open data accessibility to calculate indicators. nevertheless, the reality of a city is different. the challenge is not the exposition of data, but the collection from field, requiring the definition of robust monitoring programmes to be able to ingest data before being openly shared. this is of special importance when private companies are involved. apart from data availability, it should be considered the heterogeneity of cities and its complexity in the governance aspects, being one of the causes the involvement of different actors (i.e. the councillors of the municipality, urban planners, citizens, private companies, investors . . . ). the convergence is pursued by the methodology that is presented in [12]. within this analysis, building and district renovation, urban mobility, icts and non-technical actions are considered under the umbrella of a data collection approach. the authors in [13] presented a survey, which includes smart nature, smart living, smart mobility, smart governance, smart people and smart economy, without a clear data gathering approach to evaluate the aforementioned verticals, similar to the case in [11]. the examples of current initiatives for city impacts assessment converge in the need for kpi-driven evaluation framework. however, no one considers the mature level of the city in terms of digitalization or, in other words, data to calculate the indicators. while some cities are highly digitalised, with a wide deployment of sensors across the city, others that are still using analogue data. this paper then presents a methodology to support cities in the digital transformation to overcome the barriers between data collection and kpi calculation. as far as it is known by the authors, there are not researches regarding this line of work. literature is focused on the analysis of evaluation frameworks, such as [10], without caring about the procedure to assure real data to apply them. 3. mysmartlife methodology: translating kpis into monitoring this section provides an overview of the methodology that has been applied within mysmartlife project, which is depicted in figure 1. it consists of four stages, described below, which are cyclical and iterative. cities need to be continuously transforming and adapting to changes; therefore, this methodology allows dynamic and continuous evaluation. the starting point of the approach is the availability of any assessment framework with well-established kpis, which will be the reference for the city. in this case, mysmartlife counts on its own framework [9]. the 184 vol. 38/2022 digitalising cities: a methodology to map evaluation requirements . . . figure 1. roadmap for a data-driven sustainable environment evaluation. four stages of the proposed methodology are explained in the next bullets: (1.) analysis of the city. being a cyclic and iterative methodology, this phase represents at the same time the initial and final stage of each one of the iterations. it aims at quantitatively determining the city challenges (i.e. by using the kpis from the evaluation framework). the outcome of this stage is to obtain the city audit. in the first iteration, this analysis is made at macro level (i.e. city as a whole), meanwhile, in the rest of iterations it is micro level, i.e. evaluating the actions that have been implemented in the smart city plans. this quantitative analysis, at the end of the iteration, allows planning the next steps by extracting the lessons learnt and knowledge from real data. (2.) smart city plans. from the previous analysis of the city, where the challenges and needs are gathered, the actions and interventions for the sustainable transformation are determined in this stage. these actions and interventions contain clear objectives and targets to be achieved, e.g. reducing of energy demand in 30 %. the selection of the most suitable actions is based on benchmarking through estimated values from the indicators, such as the case of the morgenstadt initiative [14]. it is normal that the estimations were performed with simulation tools, which make some assumptions that are not representing the real situation. within the methodology being described in this paper, real data support the estimation thanks to the digitalisation strategy and icts. the main advantage or benefit is the capability of selecting actions under quantitative evidences, instead of experiences, which are usually biased by subjective perceptions. (3.) kpi selection for actions’ evaluation. within this stage, the most suitable kpis from the framework [9] are extracted according to the objectives of the actions. the framework [9] considers five axes: smart environment (that covers energy and greenhouse emissions), smart mobility, smart people, smart economy and smart governance, as well as icts in the form of urban platforms for the digitalisation of the city. more than 150 indicators are available. (4.) monitoring stage. this is the main challenge of the cities. current decisions are based on simulation tools that make some assumptions, introducing errors. data are crucial to calibrate the models or make data-driven decision-making. this phase establishes the requirements about what to measure according to the kpis and where to measure (location of sensors), as well as how (e.g. iot pollution sensors or heat meters), when (frequency) and who (responsible). many of the studies do not include this crucial stage, which is in charge of the data collection for assuring data-driven strategic plans for sustainable transformation. this methodology is complemented with the urban data platforms, with the goal of storing data in a persistent way that is defined by the monitoring programmes [15]. the urban data platform makes data available for a later analysis and data-driven decision making. the monitoring programmes are a theoretical definition of the datasets and data-points that are required from the kpis perspective, but its deployment is not covered by the proposed methodology. in this case, the cities, together with their local innovation ecosystems and through the deployment of sensors and the integration of the related datasets 185 j. l. hernández, a. quijano, p. noaille et al. acta polytechnica ctu proceedings figure 2. kpi-driven assessment framework. smart smart smart smart smart environment mobility people economy governance city (macro-) level 56 22 16 16 15 project (micro-) level 32 51 5 22 7 table 1. total number of kpis defined for each of the pillars. in the data platforms, are the responsible for managing the physical interventions. the main advantage of using the urban data platforms is the availability of data in open formats for further treatment and kpi calculation. the automation of the calculation algorithms and decision-making dashboards is out of scope of this paper, although it is recommended for the analysis [15]. by following the proposed methodology, one of the main benefits is to keep aligned the objectives and challenges of the city, which are driven by quantitative evidences coming from data. data collection and knowledge extraction support the analysis of the actions impact and lessons learnt to enhance sustainable development based on better-informed decisionmaking. data are the complement for the simulation tools in order to calibrate them and extract real performance metrics. in this way, the assumptions taken when creating the simulation models and uncertainties are demonstrated or solved. all in all, this methodology presents a holistic approach for the cities to continuously improve the sustainable transition. on one hand, by updating for each cycle the indicators of sustainable development; on the other hand, by aligning objectives, kpis and real data. 3.1. kpi-driven assessment framework the methodology relies on the existing kpi-based evaluation framework. this section summarises the evaluation framework under which the methodology is supported. the concept is represented in figure 2 [9]. as it can be observed, there are two levels: city (providing the smart city vision) and project (the mysmartlife actions). the city level extrapolates the results of the actions to the whole city to determine the benefits under the assumption of implementing these interventions across the city. both levels cover the same six categories (named fields in the city level and pillars in the project level). although the aim of this paper is not the definition of the kpis, which are available on [9], table 1 summarises the kpis account for each pillar, excluding the digitalisation metrics. all are included within the overall framework of the, but only the selected kpis (as per step 3) will be evaluated in the iterative analysis of the city, being chosen considering the strategic plan objectives and actions. 3.2. mapping kpis into monitoring requirements the key for the success of the application of this datadriven procedure is the capability for collecting real data from the different city resources. some cities fail in the collection of data so as to achieve a specific objective and they just acquire data [16] that is not exploited. then, a comprehensive and robust data collection approach needs to be applied. for that end, a common template has been provided to support cities in the mapping of the objectives, kpis and monitoring requirements. this template is focused on energy and mobility, which require the deployment of physical devices, while social, economic, governance and ict (digitalisation) aspects do not require such monitoring schemas, although definition of data collection methods like surveys and questionnaires are also used. 186 vol. 38/2022 digitalising cities: a methodology to map evaluation requirements . . . figure 3. monitoring template schema for energy monitoring in cities. figure 3 [17] represents the schematic view for the energy monitoring. on right side, it shows the thermal and electrical generation systems that could be the grid, renewables, district heating or any other supplying energy. in the middle of the schema, there is the assessment boundary, where the interventions apply, which is pivotal to determine the area of intervention [9]. in the example, the boundary covers both the buildings (e.g. retrofitting) and energy distributions elements (e.g. heat exchangers). this virtual boundary is like a geographical or physical one that splits the systems/interventions to be analysed (those that are part of the smart city plan for sustainability) from the existing elements (e.g. the electricity grid). this boundary is flexible and can be completely adapted to the strategic plan of the city. finally, on the left side, it is shown the demand side, e.g. the buildings that are retrofitted or new construction, which are the final “users” of the energy. also, the monitoring template allows cities to identify the energy flows and the connection between elements. while a grey arrow points the thermal energy flows, a black arrow identifies the electricity flows. in this way, the elements of the physical connections are easily identified and where to measure by placing directly in the schema the icon representing the measurement sensor (e.g. wattmeter). each of these sensors represents one measurable parameter of the kpis. it should be noted that this schematic definition is flexible enough to be adapted to the needs of various cities. it aims at following the same concept of boundaries, energy flows and so on. nantes and helsinki have already adapted this concept to their specific contexts. moreover, it should be remarked that zoom-in is possible to specific interventions, such as district heating details. 4. examples of application in the cities this section aims to show direct examples of the application of this methodology to define monitoring schemas in nantes and helsinki lighthouse cities within the framework of mysmartlife project. figure 4. monitoring programme for district heating in nantes. 4.1. nantes case nantes is a city committed with the sustainable transformation for climate change adaptation. for that end, sustainable energy action plan (seap) and sustainable urban mobility plans (sump) were developed as initial strategic plans based on macro-indicators. these plans have driven the calculation of the city audits and baseline to establish the targets of the actions defined in mysmartlife [18]. the main interventions defined in the smart city plans are the renovation of the building stock, renewable district heating, electro-mobility, citizens’ involvement and engagement. under this perspective, the evaluation framework defined during the first stage of the project was adapted to these plans by selecting the significative kpis for the nantes evaluation. following these kpis, the monitoring programme was defined as depicted in figure 4 [17]. it reflects the flexibility of the template, where a zoom is made to the district heating, which has to assure 80 % of renewable contribution, according to the smart city plans. in this sense, the contribution for each source is measured, both renewable and non-renewable to obtain the final ratio. with the results, at the end of the project, the fulfilment of the plan will be evaluated (by means of the kpis) to be reviewed. 4.2. helsinki case helsinki started from initial seap and sump [19], whose baseline and targets were established based on the selected actions. kpis adapt the evaluation 187 j. l. hernández, a. quijano, p. noaille et al. acta polytechnica ctu proceedings figure 5. monitoring example for the viiki building in helsinki. framework to the necessities of these strategic plans. throughout the process, existing data sources for the kpi calculation were mapped and possible blind spots in terms of monitoring identified and, thus, obtaining the monitoring needs for the impact calculation. by applying this data-driven methodology, the monitoring schema for high-performance office building (equipped with a rather sophisticated energy system consisting of both thermal and electrical grid connections, renewable energy generation and storage) is detailed in figure 5 [17]. the template is able to encase the core needs for monitoring in terms of final calculation of the kpis. 5. conclusions and lessons learnt in this paper it has been described a methodology to support cities in the digitalisation process to deploy impact assessment frameworks when evaluating sustainable plans for climate change adaptation. the methodology provides guidance in the data collection strategies to supply quantitative evidences in the decision-making processes, while qualitative analysis is out of the scope of this paper. the presented mysmartlife methodology benefits cities in the capabilities to deploy monitoring programmes focused on well-established kpis and evaluation frameworks. the iterative nature of the methodology grants an adaptative capacity in the smart city strategic plans for sustainable transition. the methodology has been deployed in the cities of nantes, hamburg and helsinki, although this paper presented the cases of nantes and helsinki. the main lesson learnt or conclusion, extracted from the cities, is the support provided by a visual tool to determine the monitoring programmes. these help to calculate audits and baselines and apply better-informed decision-making processes. in contrast, another lesson learnt is the complexity in the management of the completion of the schemas because they require the interaction among municipality stakeholders, which is not always easy. acknowledgements the authors would like to thank, first, the ec for funding and supporting mysmartlife project under ga#731297. secondly, the authors would like to thank the rest of consortium of mysmartlife for the contributions in the definition of the framework and roadmap. references [1] eurostat. urban and rural 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[2022-02-06], https://www.mysmartlife.eu/fileadmin/user_ upload/deliverables/d4.1_baseline_report_of_ helsinki_demonstration_area.pdf. 189 https://doi.org/10.3390/environsciproc2021011021 https://doi.org/10.1016/j.cities.2016.09.009 https://doi.org/10.1016/j.cities.2020.102860 https://doi.org/10.3390/su13179683 https://doi.org/10.1109/seaa.2018.00084 https://www.morgenstadt.de/en.html https://doi.org/10.3390/s20082402 https://doi.org/10.3390/s17081834 https://www.mysmartlife.eu/fileadmin/user_upload/deliverables/d5.3_monitoring_programs_and_deployment_in_the_three_lighthouses_stamped.pdf https://www.mysmartlife.eu/fileadmin/user_upload/deliverables/d5.3_monitoring_programs_and_deployment_in_the_three_lighthouses_stamped.pdf https://www.mysmartlife.eu/fileadmin/user_upload/deliverables/d5.3_monitoring_programs_and_deployment_in_the_three_lighthouses_stamped.pdf https://www.mysmartlife.eu/fileadmin/user_upload/deliverables/d5.3_monitoring_programs_and_deployment_in_the_three_lighthouses_stamped.pdf https://www.mysmartlife.eu/fileadmin/user_upload/deliverables/d2.1_baseline_report_of_nantes_demonstration_area.pdf https://www.mysmartlife.eu/fileadmin/user_upload/deliverables/d2.1_baseline_report_of_nantes_demonstration_area.pdf https://www.mysmartlife.eu/fileadmin/user_upload/deliverables/d2.1_baseline_report_of_nantes_demonstration_area.pdf https://www.mysmartlife.eu/fileadmin/user_upload/deliverables/d4.1_baseline_report_of_helsinki_demonstration_area.pdf https://www.mysmartlife.eu/fileadmin/user_upload/deliverables/d4.1_baseline_report_of_helsinki_demonstration_area.pdf https://www.mysmartlife.eu/fileadmin/user_upload/deliverables/d4.1_baseline_report_of_helsinki_demonstration_area.pdf acta polytechnica ctu proceedings 38:183–189, 2022 1 introduction 2 background and current practices in smart cities 3 mysmartlife methodology: translating kpis into monitoring 3.1 kpi-driven assessment framework 3.2 mapping kpis into monitoring requirements 4 examples of application in the cities 4.1 nantes case 4.2 helsinki case 5 conclusions and lessons learnt acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2023.41.0015 acta polytechnica ctu proceedings 41:15–19, 2023 © 2023 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague vision zero application on czech railways jan kruntorád∗, lukáš týfa czech technical university in prague, faculty of transportation sciences, department of transportation systems, konviktská 20, 110 00 prague, czech republic ∗ corresponding author: kruntjan@fd.cvut.cz abstract. the railway is perceived as a modern, safe and environmentally sustainable way of transporting people and goods. in the past, appropriate innovations in the field of infrastructure and vehicles, combined with an attractive operational concept, have gradually become the backbone of public transport in the country. even in this system, similar to other transport systems, there are accidents (events of emergency) or potentially risky situations. the paper places the issue of railway safety in the legislative context and uncovers the points of view from which the safety of railway traffic can be viewed. keywords: failure modes and effect analysis (fmea), key player, rail sector, railway emergency, railway stakeholder, railway safety, railway security, risk analysis, risk priority number (rpn), vision zero (0). 1. introduction the paper presents selected passages of the study of safety and security issues in the rail sector [1], which was completed in 2022 at the czech technical university in prague (ctu), faculty of transportation sciences, with the participation of experts from other faculties of ctu in prague (faculty of civil engineering and faculty of mechanical engineering) and the university of defense (faculty of military leadership and faculty of military health sciences) [1]. správa železnic, státní organizace, (the infrastructure manager of railway lines owned by the czech republic) is the orderer (and therefore also the owner) of the study. to preserve commercial secrecy, only partial passages of the study are published in the paper. if the reader is interested in more detailed output from the study, he can contact the správa železnic with his request. the study focuses on safety issues from the point of view of emergencies and the implementation of the vision zero concept in the railway sector with a focus on the railway transport route (with a partial overlap on vehicles, traffic management, and also on transport processes). the measures to reduce risks that the study proposes should have come primarily from the area of intelligent transport systems (its). 2. vision zero 2.1. vision zero in road transport human errors can never be eliminated. the idea of vision zero was first presented in sweden in the 1990s. in principle, it is an innovative approach to the issue of road safety. vision zero means that eventually no one will be killed or seriously injured within the road transport system. the basic premise of vision zero is the idea that mistakes must not be punished with death [2]. therefore, it is necessary to integrate the design elements to minimize human errors and their consequences into technical standards and regulations for road design, which will increase road safety [3]. the vision zero concept fundamentally changes the original way of looking at responsibility for road safety. it is not only aimed at the individual road user (driver), but at all key players, or at the system as a whole. those who design the traffic safety system and its elements bear the main responsibility for the final level of road safety. this responsibility is jointly shared by vehicle manufacturers, transporters, road managers, politicians, legislators, public sector employees, or components of the integrated rescue system. however, this in no way relieves road users of their responsibility. it is the responsibility of each individual to comply with the relevant provisions of the law. ultimately, this is a diametrical change in the way a traffic accident is viewed, which is no longer considered the result of an individual failure of a road user, but rather a failure of the entire system. currently, the idea of vision zero in the czech republic is part of the strategy of development of intelligent transport systems 2021–2027 with a view to 2050 issued by the ministry of transport of the czech republic, and its goals are implemented in the czech road traffic safety strategy 2021–2030 [4, 5]. 2.2. vision zero application on railways the railway is perceived as a modern, safe and environmentally sustainable way of transporting people and goods. in the past, appropriate innovations in the field of infrastructure and vehicles, combined with an attractive operational concept, have gradually become the backbone of public transport in the country. even in this system, similar to other transport systems, there are accidents (events of emergency) or 15 https://doi.org/10.14311/app.2023.41.0015 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en jan kruntorád, lukáš týfa acta polytechnica ctu proceedings figure 1. study [1] procedure. potentially risky situations. the purpose of the study of safety and security issues in the rail sector is to answer some questions regarding railway safety and security issues related to the achievement of vision zero so that the railway system strengthens its position as the backbone transport system in the czech republic, which will implement the most modern technological trends to increase the competitiveness of the railway compared to other modes of transport, travel comfort, and last but not least, safety and security [1]. 3. methods an overview of the individual steps in the study is shown in figure 1. 3.1. first step the first step represents an initial analysis that prepares the conditions for further work. thematic areas of rail safety and security are defined and categories of key players are identified. 3.1.1. thematic areas “railway safety and security” is a relatively broad term, therefore, it was classified by the research team into categories as follows: • damage to health and property due to the transport process (safety), • resistance to illegal actions (security), • cyber security, • resistance to natural disasters, • chemical and biological threats. due to the extensiveness of the entire study, the paper will focus mainly on the first area (safety). 3.1.2. stakeholders’ categories to completely cover the entire railway system, the classification of railway entities was carried out as follows: • state administration and self-government (including subordinate organizations), • owners and operators of the railway infrastructure, • railway carriers, • customers of carriers and providers of specific services to carriers and railway operators, • the human factor in rail transport, • investors, manufacturers, suppliers and contractors of infrastructure and vehicles, • manufacturers, suppliers and operators of it and its systems, • entities providing power electricity supplies (traction and non-traction), • scientific research and testing institutions, • international institutions, • affected entities from other modes of transport. 3.2. second step the second step contains another analytical part of the study, specifically focusing on the identification and evaluation of risks. 3.2.1. list of risks this creates a list of risks that identifies people who may die in connection with the transport process on the railway and the places/activities where and when they may die. ten vulnerable people were classified, for example: • people in a railway vehicle, • people managing and ensuring railway traffic, • people building or maintaining infrastructure, • people who construct or maintain vehicles, • other people not classified, for example, suicide. the list contains 14 risk groups at the first hierarchical level, with a total of 79 risk factors (rf). examples of risk groups: 16 vol. 41/2023 vision zero application on czech railways input values from the list of risks 0 1 2 3 4 values used for the calculation of the rpn according to equation (1) 0 1 2 4 8 table 1. conversion matrix of input values and values for rpn calculation. • derailment of a railway vehicle while in motion, • collision of railway vehicles, • collision of a railway vehicle with a foreign object on a level crossing or platform, • non-compliance with instructions when organizing rail transport, • risky situations caused by improper cargo handling. examples of risk factors: • the cause of the derailment is a technical fault on the railway track (for example, rail fracture, released rail fastening), • the cause of the collision is a failure of railway signalling and control systems – human influence (including train driver), • the cause of the situation is the leakage of a dangerous substance during handling (loading, unloading) – contamination of the surroundings, • the cause of the situation is the fall of a natural object on the track (trees, rocks). the causes of the risks themselves are evaluated by expert qualitative analysis. the qualitative evaluation uses predefined value scales 0–4; the meaning of these value scales is explained below: probability of cause of death: • value 0: there is no cause-and-effect link for a specific group of vulnerable persons, • value 1: almost zero (practically zero) probability that people will be killed, • value 2: the low probability that people will be killed, • value 3: the medium probability that people will be killed, • value 4: the high probability that people will be killed. the severity of consequences (if someone dies, typically how many people): • value 0: there is no cause-and-effect link for a specific group of vulnerable persons, • value 1: one person is killed, • value 2: 2 to 9 people are killed, • value 3: 10 to 99 people are killed, • value 4: 100 or more people are killed. partial evaluation of individual experts was later converted to consensus values with the participation of the entire team. the consensus corresponded to the arithmetic mean of the ratings or their nearest integer values. 3.2.2. risk priority number risks, as an eventuality (probability) of an undesirable event, can be assessed by the methods of qualitative and quantitative risk analysis in any system. qualitative methods describe risks, fault modes and scenarios of the possible consequences of undesirable states based on the opinions of experts, and the outputs in these cases are presented on a relative scale. on the contrary, quantitative methods are used to determine the probability that an event will occur and its consequences in measurable units [6]. the chosen evaluation method is based on failure modes and effect analysis (fmea), one of the most powerful methods used for risk assessment and maintenance management. the output evaluation criterion is a quantitive index, risk priority number (rpn), which is given by the multiplication of the occurrence (o), severity (s) and detection (d) of a failure. since the security of the entire system was assessed at a theoretical level, it is not considered the detection parameter, the relationship for rpn depends only on the occurrence and severity quantities [7]: rpn = ∑ o · ∑ s (1) • o – occurrence is the sum of probabilities that people may die, • s – severity defines the strength of the impact of failure, expressed by an estimate of the number of dead. a non-linear scale according to table 1 was used to convert between a value scale and a probability or severity rating. it is a method of qualitative risk assessment to highlight significant identified relationships. the higher value rpn according to equation (1) is the: • more probable cause of the risk is, • greater number of persons may die in connection with the risk, • more vulnerable groups are concerned with the risk. the highest values of rpn (more than 250) were evaluated for these risky situations caused by: • the leakage of dangerous substances and contamination of the surroundings, • the transmission of diseases from people or animals, • the collision of a railway vehicle with a foreign object at a crossing with insufficient visibility. 17 jan kruntorád, lukáš týfa acta polytechnica ctu proceedings the lowest rpn values (below 50) were evaluated for risky situations caused by a sudden and unexpected change in movement of a railway vehicle, for example, due to rapid braking. 3.3. third and fourth steps the third step provides the first suggestions for finding methods that lead to risk reduction. the methods are expertly searched in the processing teams and presented as specific measures for specific risks. in this phase, possible opportunities for eliminating or mitigating risks are formulated, including “high-level measures”, relationships with stakeholder groups, the relationship with the railway infrastructure manager and an estimate of the time required. the fourth step presents the evaluation of the chosen risk reduction methods, which consists of consulting the chosen methods with stakeholder groups and further a mutual consensus among the expert team. the purpose is to clarify which of the methods the railway infrastructure manager is already using or at what level of development it is going to use them. risk catalogue sheets (fiches) are a key tool for working in these steps of the study of safety and security issues in the rail sector [1]. the catalogue sheet was prepared for each risk (risk factor), which had a standardized form and content. for risks of little significance (according to the rpn value), the catalogue sheet was easily filled out. a key part of each catalogue sheet (fiche) is the identification of opportunities and proposed measures to reduce the given risk. the processing of the first drafts of the catalogue sheets was divided among the members of the research team according to their specialization. subsequently, the drafts of the catalogue sheets were revised by other members of the research team and then discussed in a workshop attended by representatives of stakeholders. content of the risk catalogue sheet (fiche): • risk factor code and risk name, • partial specification of the risk situation (description of the causes of the risk situation), • rpn value (see above) and ranking in the evaluation of the risk significance, • risk assessment – opportunities for change and risk mitigation (comprehensive view), • proposal of high-level measures for change and risk mitigation from the its area, which is at least partially within the competence of the správa železnic (the study orderer), • specification of the link of high-level measures to the správa železnic (the degree of implementation of measures by the správa železnic; proposal of the organizational units of the správa železnic that should address the measures), • specification of the link of high-level measures to other stakeholders in the railway sector, • positives (advantages, benefits) of the proposed measure (strengths and opportunities in the sense of swot analysis), • negatives (limitations, difficulties) of the proposed measure (weaknesses and threats in the sense of swot analysis), • the general time requirement of the implementation of the proposed measure (length of preparation and implementation of the measure, division into stages, continuity with other measures), • recommended technical, technological and organizational means for implementing the proposed measure, • any additional notes. 3.4. fifth step the fifth step represents the final design part. 3.4.1. sorting of measures into groups as part of the fifth step of the study work, the measures that were designed separately for individual risks in the previous steps are organized into a single system. on the one hand, similar measures are merged, and, on the other hand, the measures are sorted into groups that represent to a certain extent separate fields mainly from the its area, alternatively the it area. a total of 52 proposed measures were divided into the following six groups: • new data inputs and work with them, • work with data already obtained, • new technological systems, • new educational systems, • buildings and structures, • amendment of legislation. subsequently, a catalogue of methods and measures to reduce risks was compiled. for comparison and, above all, the ranking of individual measures according to their importance, a measure significance index (msi) was defined. the msi (2) takes into account the number of reduced risks and their importance. the msi is based on the highest rpn value among the risk factors, which is the reduction to which the given measure contributes, and at the same time, it takes into account the amount and importance of all risk factors, which is the reduction to which the given measure contributes. msij = c ·max i∈aj {rpni}+ (1− c) ∑ i∈aj rpni (2) • msij – measure significance index no. j, • j – measure order number: j ∈ {1, . . . , 52}, • c – weighting coefficient of the maximum rpn value: c = 0.8 (the coefficient value was determined for this study), 18 vol. 41/2023 vision zero application on czech railways • rpni – risk priority number of the risk factor no. i, • i – risk factor order number: i ∈ {1, . . . , 79}, • aj – set of risk factors to which the reduction of which the given measure contributes, no. j. the order of importance of the proposed measures is then compiled according to the msi values. 3.4.2. implementation of the scale of measures in the final phase of the study, a schedule was proposed for the implementation of the measures, with an emphasis on the tasks of the study orderer. risk reduction measures were timed in terms of their importance, difficulty and feasibility. the schedule for the implementation of individual measures is considered at the following three levels: • framework schedule for complete implementation that is implementation of a complete measure within a 25-year horizon, when the overall length of the measure and the division into rough stages are particularly important. • a more detailed schedule of the initial short-term implementation that is a more detailed implementation of measures for risks of the highest group of significance within a maximum of five years. • action plan for the start of implementation that is gradual steps for the period 2022–2023. 4. conclusions the paper focused mainly on the fulfilment of vision zero, that is, reducing the number of people killed on the railway. according to the study cited, it is possible to take gradual steps in the form of ensuring the necessary measures to reduce risks, which can be solved in the short, medium, and long term. some measures are dependent on legislation; some must be solved in coordination with the state administration, but also with carriers, railway technology suppliers, and other stakeholders so that it is possible to gradually fulfil the goals of vision zero. the study should serve as a summary and supporting document to reflect on and expand other possible approaches and perspectives to reduce potential risks in the railway system. although the study’s outputs are focused on the railway administration, its conclusions also apply to other institutions and key players that were identified in the study. director of the strategy department of the správa železnic, commented on the implementation of specific measures recommended in the study at the last workshop. the results of the project will need to be reflected in the internal regulations of the správa železnic. some conclusions of the study also lead to the initiation of changes in generally applicable legislation. at the same time, it will be necessary to separately address complex security on the high-speed railway system that is being prepared in the czech republic. references [1] t. tichý, l. nouzovský et al. study of safety and security issues in the rail sector. czech technical university in prague, faculty of transportation sciences. praha, 2022. [2] r. johansson. vision zero – implementing a policy for traffic safety. safety science 47(6):826–831, 2009. https://doi.org/10.1016/j.ssci.2008.10.023 [3] j. kocourek, t. padělek. human (driver) errors. perner’s contacts 6(4):123–128, 2011. [4] ministry of transport of the czech republic. strategy of development of intelligent transport systems 2021-2027 (with a view to 2050). praha, 2015 [2022-08-15], https://www.mdcr.cz/getattachment/do kumenty/strategie/its/akcni-plan-rozvoje-intel igentnich-dopravnich-syste/akcni-plan-rozvojeits-prilohy.zip.aspx. [5] ministry of transport of the czech republic. czech road traffic safety strategy 2021-2030. praha, 2021 [2022-08-15], https://www.ibesip.cz/pro-odborniky/ narodni-strategie-besip/aktualni-strategie. [6] t. javorik, l. tyfa. assessment of railway station modernisation combining risk analysis with multicriteria analysis. communications 17(2):60–65, 2015. https://doi.org/10.26552/com.c.2015.2.60-65 [7] l. ciani, g. guidi, g. patrizi. a critical comparison of alternative risk priority numbers in failure modes, effects, and criticality analysis. ieee access 7:92398–92409, 2019. https://doi.org/10.1109/access.2019.2928120 19 https://doi.org/10.1016/j.ssci.2008.10.023 https://www.mdcr.cz/getattachment/dokumenty/strategie/its/akcni-plan-rozvoje-inteligentnich-dopravnich-syste/akcni-plan-rozvoje-its-prilohy.zip.aspx https://www.mdcr.cz/getattachment/dokumenty/strategie/its/akcni-plan-rozvoje-inteligentnich-dopravnich-syste/akcni-plan-rozvoje-its-prilohy.zip.aspx https://www.mdcr.cz/getattachment/dokumenty/strategie/its/akcni-plan-rozvoje-inteligentnich-dopravnich-syste/akcni-plan-rozvoje-its-prilohy.zip.aspx https://www.mdcr.cz/getattachment/dokumenty/strategie/its/akcni-plan-rozvoje-inteligentnich-dopravnich-syste/akcni-plan-rozvoje-its-prilohy.zip.aspx https://www.ibesip.cz/pro-odborniky/narodni-strategie-besip/aktualni-strategie https://www.ibesip.cz/pro-odborniky/narodni-strategie-besip/aktualni-strategie https://doi.org/10.26552/com.c.2015.2.60-65 https://doi.org/10.1109/access.2019.2928120 acta polytechnica ctu proceedings 41:15–19, 2023 1 introduction 2 vision zero 2.1 vision zero in road transport 2.2 vision zero application on railways 3 methods 3.1 first step 3.1.1 thematic areas 3.1.2 stakeholders' categories 3.2 second step 3.2.1 list of risks 3.2.2 risk priority number 3.3 third and fourth steps 3.4 fifth step 3.4.1 sorting of measures into groups 3.4.2 implementation of the scale of measures 4 conclusions references https://doi.org/10.14311/app.2022.33.0571 acta polytechnica ctu proceedings 33:571–577, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague mechanical and durability performance of cementitious matrices with low cement and different additions eliana soldadoa,∗, ana antunesa, hugo costaa,b, ricardo carmoa,b, eduardo júliob,c a isec-polytechnic institute of coimbra, department of civil engineering, rua pedro nunes quinta da nora, 3030-199 coimbra, portugal b ceris-ist, universidade de lisboa, av. rovisco pais, 1049-001 lisboa, portugal c instituto superior técnico universidade de lisboa, department of civil engineering, architecture and georesources, av. rovisco pais 1049-001 lisboa, portugal ∗ corresponding author: eliana.soldado@isec.pt abstract. nowadays, the decarbonization of the concrete sector is a priority to counteract the global environmental harmful. knowing that portland cement is responsible for significant anthropogenic emissions worldwide, the optimization of cement dosage on concrete mixtures is a key issue, combined with the mechanical and durability performances, allowing to improve its eco-efficiency. the goal of the present work consists in optimizing the formulation of concrete matrices, used to produce reinforced concrete poles for the electric lines, by increasing the packing density, reducing the cement dosage and incorporating different additions, in order to promote the performance. two different formulations of matrices were considered, fresh moulded concrete with dry consistency and formwork moulded concrete with plastic consistency. for each one, different dosages of cement and various additions (limestone filler, fly ash, natural pozzolan, electric furnace slag) were considered to replace the cement, which were used single or combined. the mortar matrices of the concrete formulation were characterised regarding: fresh properties; mechanical properties (flexural and compressive strengths and young’s modulus); shrinkage and durability properties (capillary water absorption, carbonation and chloride ingression resistances). the influence of the additions on those properties was analysed and compared with reference cement-based mixtures and the main conclusions are presented. keywords: additions, durability, formulation optimization, low cement concrete, mechanical performance. 1. introduction eco-efficient and economical concrete production has been achieved, among other strategies, by partially replacing portland cement with by-products from other industries. the great potential for the impact reduction of the environmental footprint related to cement production has been seen especially for concretes with current strengths, but also for high and ultra-high strengths. the granulometric optimization, combined with the use of cement with high reactivity and additions as cement substitutes, and the use of powerful superplasticizers together with the reduction of water dosage, allow a significant reduction of the clinker present in portland cement, which enables the production of concretes with low cement dosages and good mechanical performance [1, 2]. this evolution, from conventional mixtures to eco-efficient concrete, which is expected to be proved by studies of life cycle analysis, should not diminish the performance of cementitious materials and must take into account their workability, strength and durability performances. optimizing the formulation of this concrete has a potential positive influence on mechanical properties at longer ages due to pozzolanic effect, as well as also on time dependent and on durability properties. although the development of the initial strengths is slower compared to traditional concretes, it is possible to obtain concrete with higher mechanical strengths at older ages, because the pozzolanic reactions are slow but prolonged [2–4]. optimizing packing density is not only beneficial for reducing the water amount, but can also reduce the space between cement particles. to optimize the packing density of the concrete, these should be selected to fill the voids between large particles with smaller particles and so on to obtain a dense and stiff structure of concrete matrix, reducing the space that needs to be filled by hydration products, which results in higher strengths. the volume of water required to ensure workability can also be reduced by adding superplasticizers in the matrix [3–5]. the published works on this subject indicates that, in the production of eco-efficient concrete, a signifi571 https://doi.org/10.14311/app.2022.33.0571 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en e. soldado, a. antunes, h. costa et al. acta polytechnica ctu proceedings mixture binder cement fly ash lim. filler pozz slag sand mix. coarse aggr. mix. w/b compactness (kg/m3) (kg/m3) (kg/m3) (kg/m3) (kg/m3) (kg/m3) (kg/m3) (kg/m3) (−) (−) bt ref 400 400 − − − − 930 902 0.390 0.824 bt200 fa 350 200 150 − − − 804 1144 0.312 0.865 bt250 350 250 50 50 − − 829 1129 0.323 0.860 bt200 poz 350 200 − − 150 − 797 1138 0.326 0.860 bt250 slag 350 250 − 50 − 50 861 1120 0.323 0.860 mt ref 360 360 − − − − 778 1046 0.472 0.808 mt250 fa 350 250 100 − − − 902 992 0.409 0.840 mt+200 fa 400 200 200 − − − 888 920 0.370 0.835 mt250 poz 350 250 − − 100 − 902 992 0.408 0.840 mt250 slag 350 250 − − − 100 957 982 0.408 0.840 table 1. formulation of concrete mixtures used to produce the mortar matrices. cant reduction in the amount of cement is achieved without changing the properties of the mixtures, thus reducing co2 emissions per cubic meter of concrete in 25% [3, 6]. filer and fly ash, for example, have been identified as viable additions with visible improvements in workability, mechanical properties and durability [1, 2]. fly ash is a by-product with moderate cost and an efficient addition to replace cement in concrete mixtures, with good results to assure strength and durability. however, its growing demand and uncertain availability in developed countries, also due to environmental concerns, demands alternative additions, such as natural pozzolans or other by-products, like slags. some studies point to more than 65% reduction in environmental impact when blast furnace slag is used for partial cement replacement [2]. industrial concrete, both in precast and ready-mixed production, is not optimized in terms of formulation or cost and efficiency. dosages of portland cement should be minimized to reduce production costs and to be more concerned with ecoefficiency, by including more additions in formulation. also, by maximizing the packing density and densification of the binder matrix, it makes the matrix more closed and provides greater durability and high mechanical strength with lower cement content. in this sense, mixtures in mortar matrix of the concrete, considering the partial replacement of cement by different additions such as limestone filer, fly ash, natural pozzolans or ground electric furnace slag, were studied in the present work, promoting sustainable concrete solutions. two types of mixtures were studied: one with dry consistency for fresh moulded concrete bt and another with plastic consistency concrete mt, for formwork moulding concrete. 2. materials and methods 2.1. materials to study the mixtures, the mortar matrix of the concrete was considered, consisting of binder paste and sand fractions. cem ii-a/l 42.5r (c) was used, as well as f-type fly ash (fa), limestone filler (lf), cape verde pozzolans (poz) and electric furnace slag (slag) additions, with the following density values in kg/gm3: 3.08; 2.30; 2.70; 2.30; 3.83, respectively. since sand has a great influence on the grain size of the mixtures, particularly on packing density and workability, a mixture of fine sand 0/1 mm and medium 0/4 mm sand was used. two types of admixtures were used: sikament he200p, a water reducer for dry concrete with density of 1.17 kg/gm3, favouring a better dispersion and utilization of cement for the bt dry mixtures; glenium sky526, ether-polycarboxylate based superplasticizer with a density of 1.06 kg/gm3, to increase plasticity and reduce the amount of water for the mt mixtures. the dosage set for each mixture was adjusted to achieve the desired target workability and the set air content. 2.1.1. mixtures the two types of mortar matrix mixtures, bt-dry consistency and mt-plastic consistency, were formulated according the proportioning of corresponding concretes, being those presented in table 1, together with water/binder ratio (w/b) and the compactness of each mixture, and compared with two reference mixtures, bt_ref and mt_ref, which have current design parameters of precast concrete. compactness is a packing density volumetric ratio between the volume of solid particles and the total volume of con572 vol. 33/2022 mechanical and durability performance of cementitious matrices figure 1. flexural strength at 28 and 56 days. crete, which is firstly estimated using the formulation parameters and then controlled by the measured values of density, air content and compacted volume. the air content was targeted to 2.0% in bt and 1.5% in mt mixtures, which corresponds to 3.5% and 2.4%, respectively in mortar matrices. 2.2. experimental tests the tests to measure the consistency through spreading and air content of the mixtures, in order to adjust the dosages of the constituents, were performed according to the procedures of en 1015-3 [7] and en 1015-7 [8], respectively. admixtures’ dosages were adjusted to achieve the target consistency of each mixture, whose dosage in relation to the weight of cement varied between 29 and 34.8 ‰ in bt and between 7.2 and 10.5 ‰ in mt. the tests to access flexural and compressive strengths on mortars were performed on prismatic specimens (40 × 40 × 160 mm3), according to en 1015-11 [9], and the results were obtained by averaging three specimens for flexural and six half specimens for compressive, at each age (28 and 56 days). shrinkage was assessed at several ages (1, 3, 7, 14, 28, 56, 90, 120 and 180 days) by the average of three prismatic specimens (40 × 40 × 160 mm3) according to en 1015-13 [10]. the young’s modulus test was performed after a 56-day of curing and followed the standard lnec e397 [11], using a prismatic specimen of 50 × 50 × 300 mm3. capillary water absorption was evaluated by averaging three specimens of 40 × 40 × 160 mm3 according to the lnec e393 [12]. the determination of carbonation resistance was assessed according nt build 357 [13] standard by averaging two specimens of 40 × 40 × 80 mm3 with 5% of co2 at different ages (7, 14, 28, 56 and 90 days). the chloride diffusion coefficient by non-steady state migration was obtained according to the procedure of nt build 492 [14], at 28 and 56 days, by the average of three specimens per age. figure 2. compressive strength at 28 and 56 days. 3. results and discussion 3.1. mechanical properties and shrinkage 3.1.1. flexural and compressive strengths the results of average strengths at 28 and 56 days, for bt and mt mixtures, are shown in figure 1 and 2, respectively for flexural and compressive strengths. both bt and mt formulations, regarding flexural strength, do not tend to evolve from 28 to 56 days, in which minor reductions were registered. despite the high reduction of cement, several bt mixtures present better performance than the reference, being that improvement even higher in mt. bt250 slag and mt 250 slag and mt+200 fa mixtures stands out, since they reach higher values than those of the references, with increases of 39%, 85% and 100%, respectively at 28 days, being those relations similar at 56 days. in compressive strength, for both formulations (bt and mt), the evolution from 28 to 56 days is significant, until 15%, due to pozzolanic effect. despite the cement reduction, mixtures present similar or higher performance than the references, by increasing packing density and incorporating additions. mixtures with fly ash and with slag in cement replacement revealed improvements until about 11%, and 6% respectively. 3.1.2. shrinkage evolutions of mortar shrinkage over time are shown in figure 3, for bt and mt mixtures. in the first type, bt250 mixture (with filler and fly ash additions) and bt200 slag stands out for its lower shrinkage results, being the shrinkage about 20 to 25% below reference; bt200poz mixture stands out for having a very high initial shrinkage, which stabilizes after the first month, being the value about 20% superior than reference; the other mixtures of this type present a smoother evolution, stabilizing after 90 days. in mt mixtures, all present lower values (from 20 to 50%) than reference, being mt250 fa and mt250 slag those with the lowest shrinkage; the evolution of those mixtures tend to stabilize between 28 and 56 days, while reference reaches that point at 120 days. 573 e. soldado, a. antunes, h. costa et al. acta polytechnica ctu proceedings � � ��� ��� ��� ��� ���� � �� �� �� �� ��� ��� ��� ��� ��� i̇ f v� �î �� � � w��gd\v� %7�uhi %7����)$ %7��� %7����3r] %7����6odj � ��� ��� ��� ��� ���� � �� �� �� �� ��� ��� ��� ��� ��� i̇ f v� �î �� � � w��gd\v� 07�uhi 07����)$ 07�����)$ 07����3r] 07����6odj figure 3. shrinkage evolution of bt (left) and mt (right) mixtures with time. ��� � ���� ���� ���� ���� ���� ���� ���� ���� ���� ���� � � �� �� �� �� �� �� �� %7������������������������������������07 (� �* 3d � %7�uhi %7����)$ %7��� %7����3r] %7����6odj 07�uhi 07����)$ 07�����)$ 07����3r] 07����6odj figure 4. young’s modulus test (left) and the values for bt and mt mixtures at 56 days (right). 3.1.3. young’s modulus the obtained average values for the young’s modulus at 56 days are shown in figure 4. by analysing the graph, the additions and increasing of packing density is evident in increasing young’s modulus compared to reference, being that increase of 8 to 16% in bt mixtures and until 6% in mt mixtures. pozzolans, fly ash, and slag additions revealed significant increases, which may have an even higher effect on concrete, since high packing density and coarse aggregates will increase that property. 3.2. durability 3.2.1. water absorption through capillarity figure 5 shows the values of capillary water absorption of the bt and mt mixtures. since the concrete quality of this parameter is considered "high" below 0.1 mg/mm2 ×min1/2, "average" between 0.1 and 0.2 mg/mm2 ×min1/2 and "low" above 0.2 mg/mm2 × min1/2 [15], it is possible to verify that in bt mixtures, only the bt250 can be considered of medium quality, while the others are well rated. as for mt mixtures, only mt250 poz and mt250 slag remain throughout the test in the "high quality" classification. the rest start from being medium quality and acquire a better rating over time. this proves a nonlinear behaviour of this parameter, since the evolutions are more compatible with power functions, tending to improve with age by the pozzolanic effect. by analyzing the results of this test, performed on three consecutive days, it can be concluded that the mixtures with higher packing density, both bt and mt, combined with slag addition present the best results in the evaluation of this parameter, absorbing about 45% less water than the reference in bt mixture and less 60% in mt mixture. fly ash and pozzolan addition also revealed good performance in reducing water absorption. 3.2.2. carbonation resistance the analysis of carbonation depth, cdepth, of the studied mixtures (figure 6) proved to evaluated with a linear tendency with square root of time (figure 7), as expected. it is evident that the amount of cement is highly influent in carbonation resistance; despite the increase of packing density and the replacement of cement by additions, the optimized mixtures revealed in general higher values then the references, until 160% in bt200fa and 95% in mt+200fa. in bt mixtures, this phenomenon is more noticeable since the difference of cement dosage is higher and 574 vol. 33/2022 mechanical and durability performance of cementitious matrices � � � � � � � � �� �� �� �� �� 6d ��p j� p p � � w��plq���� 6 ��� 6 ��� %7�uhi %7����)$ %7��� %7����3r] %7����6odj � � � � � � � � �� �� �� �� �� 6d ��p j� p p � � w��plq���� 6 ��� 6 ��� 07�uhi 07����)$ 07�����)$ 07����3r] 07����6odj figure 5. capillarity water absorption through square root of time of bt (left) and mt(right) mixtures. ���gd\v������������������gd\v������������������gd\v� figure 6. chamber for the carbonation test and evolution of carbonation mt250fa with time. � � � � �� �� �� � � � � � � � � � & gh sw k �p p � w��gd\v���� %7�uhi %7����)$ %7��� %7����3r] %7����6odj � � � � �� �� �� � � � � � � � � � & gh sw k �p p � w��gd\v���� 07�uhi 07����)$ 07�����)$ 07����3r] 07����6odj figure 7. carbonation depth of bt (left) and mt (right) mixtures. � � � � � �� �� �� ���gd\v ���gd\v '��î�������p��v� %7�uhi %7����)$ %7��� %7����3r] %7����6odj 07�uhi 07����)$ 07�����)$ 07����3r] 07����6odj ���gd\v���������������������������gd\v� figure 8. chloride migration (left) and d coefficient at 28 and 56 days for bt and mt mixtures (right). 575 e. soldado, a. antunes, h. costa et al. acta polytechnica ctu proceedings the reference shows the best results, followed by mixtures with more cement, bt250 slag and bt250, although it contains partial substitution by additions. mt mixtures show the same trend, although with a smaller difference from the reference; in those, it is also possible to verify that the increase in the amount of binder replacing cement is harmful to the carbonation resistance, while mt250fa present good resistance, with a carbonation depth similar to the reference. mt250 slag present a significant reduction (about 30%) in comparison to the reference, proving the ability of slag to increase carbonation resistance. 3.2.3. chloride migration the results shown in figure 8 represent the chloride diffusion coefficient (d) by non-steady state migration test at 28 and 56 days. in this test (figure 8), as lower is d coefficient as higher is the resistance to chloride migration, being the durability also higher. the graph prove that mixtures improve its results at 56 days, comparing to 28 days, due to pozzolanic effect, in which in most cases this difference is substantial (reduction until near 65% for mixtures with fly ash). mixtures with pozzolan additions (bt200 poz and mt250 poz) have very low d results compared to the others and to the reference, which makes them stand out positively, being the coefficient until 85% and 80% lower than references, respectively. in contrast, mixtures with slag additions (both bt250 slag and mt250 slag) show the higher chloride migration, being until about 25% and 60% higher than reference in mt and bt mixtures, respectively; in slag addition does not promote apparent improvement with age, contrarily to the remaining mixtures. 4. conclusions from the analysis of the obtained results in the experimental study, regarding the effect of combining the increase of packing density with adding different cementitious supplementary materials in two types of mortar mixture (bt, with dry consistency; mt, with plastic consistency), it was possible to draw the following conclusions: 1. despite the reductions in cement content (from 30 to 50%), flexural strength increases significantly with different additions up to 100% comparing to reference mixtures, being the flay ash and slag mixtures those with higher strengths. 2. compressive strength of mixtures with additions tend to increase from 28 to 56 days up to 15% due to pozzolanic effect and, despite the cement reduction, it increases up to 6% and 11% with slag and fly ash, in comparison to references. 3. the replacement of cement by additions promotes a significant decrease in the shrinkage between 20 to 50%, being mixtures with slag and fly ash those with major reductions. 4. adding eco-efficient supplementary binders (pozzolan, slag and fly ash) increases young’s modulus of mortar matrices up to 16% (mainly in mixtures with dry consistency, bt), comparing to references, being expected to have a higher effect on corresponding concrete, due to the stiffness of coarse aggregates and to high packing density. 5. capillary water absorption allow to consider all mixtures as high quality, but the mixtures with slag addition promotes high reduction of water absorption compared to references, from 45 to 60%, followed by fly ash mixtures, also with reductions up to 45%. 6. in carbonation resistance, the cement dosage has an important positive influence on reducing carbonation depth, being reference mixtures those with less carbonation; however, mixtures with slag addition have similar or even better effect in reducing carbonation than references. 7. chloride diffusion coefficient of mixtures with additions proves a high influence of pozzolanic effect and the corresponding maturity of the matrices with age, since chloride migration reduce until 65% from 28 to 56 days of age; the additions of fly ash and of pozzolans are very advantageous for resistance to chloride migration, being the latter the best, since it reduces the chloride coefficient by 80 to 85%; contrarily, slag addition proves a different effect on tested mixtures, since it increases the chloride coefficient from 25 to 60% in comparison to references, and it does not improve with age. overall, the evolution of conventional cementitious mixtures to eco-efficient mixtures with less cement and addition does not decrease the performance of materials, on the contrary, depending on the additions, it may result in significant improvements of the mechanical, time dependent and durability performances of concrete mixtures. acknowledgements this work is co-financed by the european regional development fund (erdf), through the partnership agreement portugal2020 operational program for competitiveness and internationalization (compete2020), under the project poci-01-0247-feder033523 "postejo 4.0: inovação para a diversificação e a exportação". references [1] l. carvalho. experimental characterization of the deferred properties and durability of concrete with low cement dosage, master thesis, isec-polytechnic institute of coimbra, 2016. [2] t. proske, s. hainer, m. rezvani, et al. eco-friendly concretes with reduced water and cement content mix design principles and application in practice. construction and building materials 67:413-21, 2014. https: //doi.org/10.1016/j.conbuildmat.2013.12.066. 576 https://doi.org/10.1016/j.conbuildmat.2013.12.066 vol. 33/2022 mechanical and durability performance of cementitious matrices [3] s. fennis, j. walraven, j. uijl. the use of particle packing models to design ecological concrete, heron, 54(2):185-204, 2009. http://heronjournal.nl/54-23/5.pdf. [4] h. costa, h. alves, e. freitas, e., et al. mechanical performance of concrete with low cement dosage. in national meeting of structural concrete be2016, coimbra, 2016. [5] s. a. a. m. fennis, j. c. walraven, j. a. den uijl. compaction-interaction packing model: regarding the effect of fillers in concrete mixture design. materials and structures 46(3):463-78, 2012. https://doi.org/10.1617/s11527-012-9910-6. [6] s. fennis. design of ecological concrete by particle packing optimization. phd thesis, delft university of technology, 2011. [7] en 1015-3. methods of test for mortar for masonry, part 3: determination of consistence of fresh mortar (by flow table), 1999. [8] en 1015-7. methods of test for mortar for masonry, part 7: determination of air content of fresh mortar, 1998. [9] en 1015-11. methods of test for mortar for masonry, part 11: determination of flexural and compressive strength of hardened mortar, 1999. [10] en 1015-13. methods of test for mortar for masonry, part 13: determination of dimensional stability of hardened mortars, 1995. [11] e 397. hardened concrete. determination of the modulus of elasticity of concrete in compression (in portuguese). lnec, lisbon, 1993. [12] e393. concrete. determination of the absorption of water through capillarity (in portuguese). lnec, lisbon, 1993. [13] nt build 357. concrete, referring materials and protective coating: carbonation resistance, 1989. [14] nt build 492. concrete, mortar and cement-based repair materials: chloride migration coefficient from non-steady-state migration experiments, 1999. [15] browne, r. d. field investigations: site & laboratory tests: maintenance repair and rehabilitation of concrete structures, ceec, lisbon, 1991. 577 http://heronjournal.nl/54-23/5.pdf https://doi.org/10.1617/s11527-012-9910-6 acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.38.0241 acta polytechnica ctu proceedings 38:241–246, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague acoustical aspects of replacing traditional materials in building elements with renewable and recycled ones jiří nováčeka,b,∗, jaroslav hejla a czech technical university in prague, university centre for energy efficient buildings, třinecká 1024, 273 43 buštěhrad, czech republic b czech technical university in prague, faculty of civil engineering, department of architectural engineering, thákurova 7, 166 29 prague 6, czech republic ∗ corresponding author: jiri.novacek@fsv.cvut.cz abstract. building elements, especially partitions, floors and external walls significantly affect indoor acoustic comfort. their ability to reduce noise transmission from neighbouring rooms or from outdoors depends on the element composition and the building materials used. in central europe, the heavyweight masonry or concrete walls and slabs are typical elements both for family and residential buildings. however, increasing popularity of lightweight multi-layered structures is noticeable. this creates new opportunities for the gradual replacement of traditional materials with renewable and recycled ones, both for load-bearing components and for fillings and other layers of building elements. this paper introduces such design changes in relation to acoustics, particularly airborne sound insulation. the greatest attention is paid to the replacement of masonry and mineral wool insulation with timber and wood fibres. the overview is supplemented by examples of low-energy house external wall and timber wall with recycled infill whose sound insulation has been determined by measurements in the acoustic laboratory. keywords: building acoustics, sound insulation, sound reduction index, building elements. 1. introduction in building acoustics, as in other fields of building design, there has been an effort in recent years to gradually replace traditional materials with more environmentally friendly ones. regarding this, multilayered lightweight building elements provide more opportunities. due to their complex character, material substitution can be made either in the loadbearing construction, sheathing or filler layers. since the requirements for sound insulation are constantly increasing, such changes should not impair the acoustic properties. this can be verified by comparative laboratory tests of original and new solutions. this paper focuses on airborne sound insulation of interior and exterior walls. for interior wall, the aim was to improve poor sound insulation at low frequencies which is typical for lightweight structures and to increase the overall weighted sound reduction index with regard to new acoustical requirements described in section 2.1. the composition of exterior wall is usually driven by reaching the target value of the heat transfer coefficient. the sound insulation is often not so determining because the weakest parts of building envelopes are windows. therefore, only the overall effect of material substitution on rw value is of interest in this paper. 2. sound insulation requirements for dwellings different descriptors are used to express requirements for sound insulation in buildings in various eu countries. therefore, great efforts have been made to harmonize them in the form of a new classification scheme in recent years, see section 2.2. however, for laboratory measurements determining the airborne sound insulation of building elements, it is common to use the same quantities throughout europe, the weighted sound reduction index rw and relevant spectrum adaptation terms c, ctr, c50−3150, etc. 2.1. national requirements according to new čsn 73 0532 in december 2020, after ten years of using the older version, the new czech standard specifying requirements for sound insulation in buildings was issued. the new requirements, listed in table 1, are 0–2 db higher for walls and floors between apartments, but 2 db lower for walls between habitable rooms within the same flat. however, this second requirement, which is not typical for other eu countries, has been extended to all habitable rooms in the apartment (not just one as it was before). the requirements for sound insulation of building envelopes depend on external noise levels and range between 30 db and 48 db in extreme cases. the quantity r′ w is used for individual parts (e.g. windows or 241 https://doi.org/10.14311/app.2022.38.0241 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en jiří nováček, jaroslav hejl acta polytechnica ctu proceedings floors walls type of space r′ w, dnt,w l′ n,w, l′ nt,w r′ w, dnt,w between all habitable rooms within the same flat ≥ 47 ≤ 58 ≥ 40 between habitable room and all rooms of other apartments ≥ 54 ≤ 53 ≥ 53 table 1. required sound insulation between habitable rooms in family houses and residential buildings [1]. type of space class a class b class c class d class e class f between habitable rooms in dwellings and other dwellings dnt,50 ≥ 58 dnt,50 ≥ 54 dnt,a ≥ 52 dnt,a ≥ 48 dnt,a ≥ 44 dnt,a ≥ 40 table 2. classes for airborne sound insulation [2]. walls) and dnt,w for building envelope as a whole. 2.2. classification scheme according to iso/ts 19488:2021 in 2021, iso/ts 19488:2021 introducing new acoustic classification scheme for dwellings was issued. this new technical specification is based on the work of cost action tu0901 “integrating and harmonizing sound insulation aspects in sustainable urban housing constructions” between 2009 and 2013, summarized in [3]. this new document uses six quality classes a to f (the best acoustic comfort ensures class a) based on the evaluation of various acoustic aspects (the airborne sound insulation is one of them), for details see table 2. classification scheme for airborne sound insulation uses weighted standardized level differences dnt,a and dnt,50 (dnt,a = dnt,w + c and dnt,50 = dnt,w + c50−3150). since this paper is focused on acoustical behavior of building elements (in laboratory conditions), corresponding quantities rw + c and rw + c50−3150 are used instead. acoustic classification of façades is based on descriptor dnt,a,tr (dnt,a,tr = dnt,w + ctr). from the same reasons as for internal walls and floors, rw + ctr is used instead in this paper. 3. timber wall with crushed brick rubble infill the original idea was to use the recycled loose-fill in a timber frame interior wall and to compare the acoustical properties of such structure with the same wall with mineral wool infill and with a traditional hollow brick partition of approximately the same thickness and weight. the goal was to achieve equal or better weighted sound reduction index and to improve poor sound insulation at low frequencies, which is typical for lightweight building elements. schemes of all three walls are shown in figure 1. the tested timber wall with crushed brick rubble infill had following composition (thickness 140 mm, mass per unit area ca. 134 kg/m2): • structural plasterboard 12.5 mm (11.5 kg/m2), figure 1. schemes of tested walls: masonry (left), timber with recycled infill (center), timber with mineral wool (right). • timber frame from kvh profiles (spruce wood) 60× 100 mm, axial distance of studs 625 mm, filled with recycled brick rubble 4–8 mm, • oriented strand board 15 mm (9 kg/m2), • structural plasterboard 12.5 mm (11.5 kg/m2). the process of filling the wall with crushed brick rubble is shown in figures 2 and 3. for the lightweight variant, mineral wool boards, 100 mm thickness (≥ 15 kg/m3 and ≥ 5 kpa · s/m2) were used instead of rubble infill. the approximate total weight for this wall was 43.5 kg/m2. the hollow block masonry partition was 145 mm thick and weighted ca. 137 kg/m2. the measured laboratory sound reduction index vs. frequency is shown in figures 4 and 5. there are several interesting observations coming from the frequency response: • the sound reduction index of timber wall with rubble infill is higher compared to the other variants almost in all frequency bands below 1 600 hz, • sound insulation is well improved especially in low frequency region (in some 1/3-octave bands by more than 10 db), • the acoustical behaviour of the wall is rather com242 vol. 38/2022 acoustical aspects of replacing traditional materials . . . figure 2. crushed brick rubble in the cavity between sheathing and studs (photo by k. staněk). figure 3. filling the wall with crushed brick rubble (photo by k. staněk). plex and the wall has probably two critical frequencies; the first around 315 hz is possibly caused by bending stiffness of the wall as a whole (it is higher than the critical frequency of single hollow brick wall approx. 160 hz); the second around 2 500 hz, associated with plasterboard sheathing, is the same for both dry wall variants, it comes from the fact that plasterboards are not rigidly fixed with the core (recycled infill). for the overall rating of walls the single number quantities presented in table 3 are the most important. based on the rw values, wall with recycled infill is by 2 db better than the other walls. compared to the national requirement r′ w ≥ 40 db on walls between all habitable rooms within the same flat, the laboratory value rw = 44 db is high enough even with regard to correction for flanking sound transmission. the parameter rw + c50−3150 is by 3 db better compared to dry wall with mineral wool. figure 4. sound reduction index of walls: timber with recycled infill – the solid blue curve, timber with mineral wool – the dashed red curve. figure 5. sound reduction index of walls: timber with recycled infill – the solid blue curve, masonry – the dashed red curve. 243 jiří nováček, jaroslav hejl acta polytechnica ctu proceedings type of wall rw [db] c [db] ctr [db] c50−3150 [db] ctr,50−3150 [db] recycled infill 44 -1 -3 -1 -3 mineral wool 42 -2 -3 -2 -8 solid 42 -1 -3 -1 -4 table 3. single number results for internal walls. type of wall rw [db] c [db] ctr [db] c50−3150 [db] ctr,50−3150 [db] wood fibre – basic element 40 -2 -6 -2 -10 mineral wool – basic element 44 -1 -2 -3 -13 wood fibre – with etics 42 -1 -5 -1 -7 mineral wool – with etics 51 -1 -6 -5 -17 table 4. single number results for external walls. figure 6. schemes of tested external walls: basic part of the wall (left), basic part with etics (right). 4. external wall for low energy houses the external wall with pre-insulated i-joists is an environmentally friendly building system for low energy and passive houses. the heat transfer coefficient varies between ca. 0.15 and 0.18 w/(m2 · k), depending on the thickness of thermal insulation. the external wall shown in figure 6 was tested with two types of thermal insulation: wood fibre boards and mineral wool of the same thickness. the composition of wall with wood fibre insulation was as follows (thickness 255 mm, mass per unit area ca. 55.8 kg/m2), the layers of the basic element (thickness 190 mm, mass per unit area ca. 32.4 kg/m2) are in italics: • 5 mm base coat with reinforcing fibre mesh (7.5 kg/m2), • 60 mm thermal insulation wood fibre boards 265 kg/m3 fastened with anchors (15.9 kg/m2), • vapour-permeable wood fibre board 15 mm (9 kg/m2 ), figure 7. wood fibre boards between i-joists steico wall. • pre-insulated i-joists steico wall 60/160 mm, stud centres 625 mm, with 2 × 80 mm wood fibre boards 60 kg/m3 and ≥ 5 kpa · s/m2 (14.4 kg/m2 ), • oriented strand board 15 mm (9 kg/m2 ). the composition of wall with mineral wool was following (thickness 252.5 mm, mass per unit area ca. 46.5 kg/m2), the layers of the basic element (thickness 187.5 mm, mass per unit area ca. 30 kg/m2) are in italics: • 5 mm base coat with reinforcing fibre mesh (7.5 kg/m2), • 60 mm thermal insulation mineral wool boards 150 kg/m3 fastened with anchors (9 kg/m2), • vapour-permeable wood fibre board 15 mm (9 kg/m2 ), • pre-insulated i-joists steico wall 60/160 mm, stud centres 625 mm, with 160 mm mineral wool 20 kg/m3 and ≥ 8 kpa · s/m2 (8 kg/m2 ), • plasterboard 12.5 mm (13 kg/m2 ). the measurement results are listed in table 4. the test elements are shown in figures 7 and 8. 244 vol. 38/2022 acoustical aspects of replacing traditional materials . . . figure 8. mineral wool boards of etics fastened with anchors. figure 9 shows that the sound reduction index of the basic element with mineral wool is higher across the frequency spectrum compared to the element with wood fibres. since the structural parts of the wall were not changed (except osb replaced with plasterboard), it can be assumed that sound transmission via studs is almost the same in both cases and the difference in the sound reduction index is therefore caused by increased sound transmission through the cavity for wall with wood fibres. this is also indicated by different slope of r between 100 hz and 1 000 hz (for wood fibres 8 db/octave and for mineral wool 4 db/octave). smaller slope is typical for the effect of sound bridges (wooden studs) while steeper slope is common for cavity effect (usually due to weakly attenuated cavity). the difference between walls is also significant if we look at the weighted sound reduction index rw which differs by 4 db. the difference for rw + ctr (recommended descriptor for traffic noise spectrum) is even 8 db. in contrast, when the low frequencies below 100 hz are taken into account, the difference for rw + ctr,50−3150 is only 1 db. this is due to the absence of a drop in sri curve for the wall with wood fibres. for walls with etics the observations are almost the same with even greater differences, see figure 10 and table 4. for masonry external walls, typical values of rw are between 45 db and 50 db. this is similar to the wall with mineral wool. taking into account the spectrum adaptation term ctr, quantity rw + ctr for masonry wall will be approximately 5 db higher. this can be compensated with independent interior plasterboard lining in case of timber wall, which is often used for fire protection and for electrical installations. 5. conclusions the use of renewable and recycled materials in building elements can significantly change their acoustical properties. it was shown in two examples that this figure 9. sound reduction index of basic element: wood fibre – the solid blue curve, mineral wool – the dashed red curve. figure 10. sound reduction index of walls with etics: wood fibre – the solid blue curve, mineral wool – the dashed red curve. 245 jiří nováček, jaroslav hejl acta polytechnica ctu proceedings change can mean either improvement or deterioration of airborne sound insulation. the idea of using recycled crushed brick rubble infill for increasing the sound reduction index of walls, especially at low frequencies, was found correct. although the application to a vertical structure is probably not so acoustically efficient as in the case of floors (since the rubble infill increases bending stiffness of the wall), the measured weighted sound reduction index rw = 44 db was nevertheless higher than for the double wall with mineral wool or for the hollow brick partition. this value is fully sufficient also with regard to the czech requirement for sound insulation between the habitable rooms of the same apartment. outstanding sound reduction index at low frequencies predetermines the use of such a wall between bedrooms and livings rooms where sources with strong low frequency components are common (e.g. home cinema, reproduced music) [4]. it can be also successfully used as a part of a wall between different dwellings if supplemented by independent acoustic lining. interpretation of achieved results for tested external wall is more complicated. in general, the wood fibre infill was found less acoustically efficient than mineral wool in studied case, probably because it provides less sound attenuation in the cavity. however, the observed differences in sound reduction index can be also affected by different type of structural boards used at one side of the wall, since the wood based boards are lighter than plasterboards or gypsum fibre boards. the total weight must be considered in acoustic design and if necessary, the wall should be provided with independent lining on interior side. acknowledgements this work has been supported by the ministry of education, youth and sports within national sustainability programme i, project no. lo1605. the measurements of interior timber wall with crushed brick rubble infill were realized within the nck – mts, tn01000056/08 – centre for advanced materials and efficient buildings. references [1] čsn 73 0532 acoustics – protection against noise in buildings and evaluation of acoustic properties of building elements – requirements, 2020. [2] iso/ts 19488:2021 acoustics – acoustic classification of dwellings. [3] b. rasmussen, m. machimbarrena (eds.). cost action tu0901 – building acoustics throughout europe. volume 1: towards a common framework in building acoustics throughout europe. discript preimpresion, 2014. 257 p. isbn 978-84-697-0158-4. https://www.cost.eu/uploads/2018/07/part_i.pdf [4] j. h. rindel. sound insulation in buildings. crc press, 2017. 476 p. isbn 978-1-4987-0041-2. 246 https://www.cost.eu/uploads/2018/07/part_i.pdf acta polytechnica ctu proceedings 38:241–246, 2022 1 introduction 2 sound insulation requirements for dwellings 2.1 national requirements according to new čsn 73 0532 2.2 classification scheme according to iso/ts 19488:2021 3 timber wall with crushed brick rubble infill 4 external wall for low energy houses 5 conclusions acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.38.0579 acta polytechnica ctu proceedings 38:579–584, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague dynamics of apartment building renovation investment costs based on estonian reovation grant scheme grete miller, lauri lihtmaa∗, targo kalamees tallinn university of technology, school of engineering, department of civil engineering and architecture, ehitajate tee 5, 19086 tallinn, estonia ∗ corresponding author: lauri.lihtmaa@taltech.ee abstract. while energy prices have increased substantially recently, we also observe that renovation prices have increased twofold wihtin last decade. this could seriously affect the motivation to invest in building renovations because of negative yield and in turn jeopardise the carbon reduction plans. therefore, we attempt to show some evidence of renovation cost dynamics based on sample of 112 apartment buildings which received state support for deep energy renovations during 2010 and 2017. we found that investments started out very cautiously in 2010. eventually renovations were embraced as rational investment and renovations ambitions grow with prices. construction price index grow during the study period 20 percent and renovations over two times. we could not confirm the popular hypothesis that demand pressure will also increase prices. however, most significant increase of renovation price was due to the additional construction works and quality improvement. this was especially evident after the redesign of grant rules in 2015 which allowed more indirect construction works to be eligible for the subsidy. keywords: energy renovation, construction cost, procurement, apartment buildings. 1. introduction as buildings are one of the largest sources of energy consumption in europe, deep energy renovation of buildings is the most feasible method towards decarbonised economy. european commission strategy “a renovation wave for europe – greening our buildings, creating jobs, improving lives” [1] objective is to at least double the annual energy renovation rate of buildings by 2030 and to foster deep energy renovations. to meet decarbonisation targets all eu member states have prepared national long-term renovation strategies. the estonian renovation strategy [2] aims to renovate 22 % of the building stock by 2030, 64 % by 2040, and the rest by 2050. current goal is to renovate apartment buildings to energy performance level, which was equivalent to a new building in 2018 which classifies as near zero energy building’s standard for renovations. active renovation promotion, subsidy programme and other supportive measures have resulted in deep energy renovation of 150 large apartment buildings in average per year. however, the annual need for renovation is approximately 500 apartment buildings [3], with a total area of 600 000 m2 per year in order to achieve carbon neutrality targets by 2050. it is evident that there is an urgent need to speed up the renovation pace. however, there are several renovation barriers that hider the renovation adoption [4]. the investment capability is found to be one limiting factor to achieve deep energy renovation targets [5, 6]. investments are evaluated by comparing costs with benefits. while home improvements are not usually addressed as investment projects, deep energy efficiency renovations are so expensive and complex that the decisions to renovate are always related to profitability of the costs. investing in energy efficiency should be a simple dilemma whether to pay the high energy bills or service the bank loan for renovations. however, deep energy renovations usually are not selfsufficient as construction of proper ventilation system would increase the investment and operation costs. therefore, increasing construction prices could become another substantial barrier which in turn might have significant influence on achieving wider climate goals, and securing the wellbeing and safety of the residents in severely depreciated buildings. we observe that during the estonian renovation grant programme which started in 2010, construction costs for renovations have been continuously increasing. in 2010 the annual average cost for net area of deep energy renovation was about 160 €/m2, however a decade later, in 2020, the average cost rate has increased to 380 €/m2. this large increase of 2.4 times significantly exceeds the construction price index for new build which grew 20 % during the same period. such phenomenon proposes several difficulties for adoption of long-term renovation strategy. on the one hand, growing prices decrease the attractiveness of energy renovations and render the incentive measures less efficient. on the other hand, uncer579 https://doi.org/10.14311/app.2022.38.0579 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en g. miller, l. lihtmaa, t. kalamees acta polytechnica ctu proceedings tainty of renovation prices can hinder forecasting of long-term planning of renovation funds and evaluation of financial capacity of homeowners who are expected to invest in energy renovations with increasing rate within the next three decades. so far analysts and planners have been operating with an average value of renovation price in calculating the forecast of renovation investment and incentives projections. however, there is little knowledge why renovation prices can exceed the price growth of the new build and more importantly whether such trends will extend to the near or long-term future. preconditions for carbon neutral economy require the supply side of renovations system to cope with the increasing demand [7]. therefore, if the renovations costs are increasing due to the greater demand, regulators need to address such issues explicitly by focusing on the increasing the capacity of supply side. based on the data of estonian renovation grant programme the current study aims to explain why renovation prices have been increasing more than expected by construction price index for new build. we also provide discussion in which we address the implications of such price trajectory for planning next iterations of renovation strategy. doing so, we focus on three research questions: • how typical renovation solutions have changed over time? • how technical and quality requirements have affected renovation solutions? • why have costs changed within procurement biddings for renovations? 2. methodology 2.1. data collection in the period 2010–2017, 561 apartment buildings in estonia were deeply renovated using state renovation grant mediated by fund kredex. although the average renovation price continued to increase also after the year of 2017, we were unable to extend the study period due to the limited of the data availability. still, we can provide meaningful results because we cover two different grant periods within the 8 years. first, we examine the overall renovation dynamics using the information provided form kredex databases for the entire data. the relationships between building size, age and construction material are then analysed. for more elaborate enquiry we used the sample of 112 projects which is about 20 % of the overall data. buildings were chosen randomly, and sample’s representativeness was evaluated by the number of apartments, construction time, location, and renovation cost. contractors included in sample had renovated at least seven large apartment buildings within the study period and were therefore expected to provide comparable cost information mainly because of their similar in size, management, and salary system. all prices include value added tax. 2.2. analysis methods to eliminate the effect of general increase of construction prices and inflation, effect of construction price index was removed from the observed renovation cost, normalising all prices to the level of january 2018. in the sample analysis, used construction solutions were mapped and three types of work were examined in more detail. at first, the most common solutions for additional insulation of facades are compared by the type of facade, insulation and finishing materials. secondly, the reconstruction of the heating system of different apartment buildings is compared by the depth of construction works. thirdly, the most common solutions for the construction of a mechanical ventilation system are identified and costs are compared. the cost per net area was compared for all three work types to identify how technological changes have affected the renovation cost. second part of the study seeks to determine to what extent the grant requirements changes have impacted the renovation cost. this approach is based on the regulation analysis which describes the work financed by the grant. the cost of observed components is deducted from the cost of the closed net area of the renovation and the extent to which the impact on the cost of the renovation has been directly added is assessed. third part of this study examines the energy efficiency ambitions of apartment association. the information about pre-construction 3-year period average energy consumption was collected and compared with after-renovation 3-year period average energy consumption. accomplished energy savings were compared with renovation cost, to recognize possible relations. 3. results 3.1. the dynamics of apartment buildings renovation costs the average annual cost of deep renovations, in the period 2010–2018, have increased from the level of 160 €/m2 to 330 €/m2 (approx. 2 times). figure 1 shows the average annual deep renovation cost. largest price increase is in the middle of the period of the two grant measures, in the transition from one period into another. since the conditions for the payment of support changed then, this work supports the hypothesis that the change in the support regulation is a major factor influencing renovation cost. we found three cost-influencing aspects. first, construction cost is higher for smaller buildings. management costs and the cost of mechanisms, scaffolding etc. play a bigger role in smaller construction projects, resulting in higher cost per net area. secondly, old wooden buildings are more expensive to renovate than 580 vol. 38/2022 dynamics of apartment building renovation investment costs . . . figure 1. renovation costs in 2010–2018. figure 2. common facade reconstruction technologies. prefabricated concrete buildings. it is most advantageous to renovate a prefabricated house (better developed standard solutions and less effort is needed to strengthen the load-bearing structures). wooden buildings are usually located in historically valuable areas and therefore require more personal approach. thirdly, renovation of older buildings has higher costs and price variability is also higher. both can be related to the performance of previous reconstruction works (some of the renovation works have already been done before). the result is also affected by the fact that among older buildings there are primarily wooden apartment buildings, as the construction of reinforced concrete large-panel buildings started a maximum of 60 years ago. and, as described in previous analysis, renovating wooden houses is more expensive. 3.2. detailed analysis of mandatory renovation works the increase in the unit price of additional facade insulation is influenced using increasingly common ventilated facade solutions, which are twice as expensive as plastered systems (average prices 60 €/m2 and 100 €/m2, respectively). the technology of plaster facades has changed. in addition to mineral plaster, silicone plaster is more often used, and wool insulation is used more often in facades than before, which is 1.4 times more expensive than expanded polystyrene insulation (figure 2). it can be stated as a positive sign of increased awareness of residents and in the know-how of specialists that decisions are not only cost-driven, but technological aspects are also calculated. the average cost of mechanical ventilation systems has increased from 32 to 43 €/m2 in second renovation period (figure 3). this due to the reason that natural ventilation systems and room-based ventilation solutions were banned as such systems do not offer proper ventilation and indoor climate. the construction of a mechanical ventilation system accounts for about 5 % of the price increase of renovation. in addition, the construction of the ventilation system involves 581 g. miller, l. lihtmaa, t. kalamees acta polytechnica ctu proceedings figure 3. common mechanical ventilation solutions. figure 4. cost of added renovation works during second grant scheme period. additional works that were not previously performed or were performed less invasively (primarily interior finishing). the cost of building an apartment-based 2-pipe heating system largely depends on the condition of the existing system and the renovation work carried out previously. price variability is high, as the scope of work can be very different. the statistical t-test resulted in p value of 0.178, which indicates that there is a small probability of a statistically significant difference between prices in the total population, but this did not emerge due to high price variability when examining the sample. the more comprehensively the heating system needs to be renovated, the more expensive is the construction cost of this solution. 3.3. mapping of added renovation works figure 4 describes which components the price increase consists of and how large the effect of these components have on the cumulative price increase. the exponential regression line well describes the significant increase in water supply and sewerage prices (r2 = 0.37), slightly less, but also the prices of interior finishing works have increased exponentially. added to that, the significant price increase comes from the addition of projects that have decided to reconstruct electricity systems in public areas. in other words an increase in the integrity of the works results in higher construction prices. to summarize the share of the above-mentioned works in the price increase of renovation, in the first period the works related to interior finishing averaged 1 €/net area, in the second period already 8.3 €/m2 (growth of 830 %, share in the total renovation cost 2.3 %). water and sewerage work cost raised form 1.6 €/m2 in the first period to 15.3 €/m2 in the second period (growth of 956 %, share in the total renovation cost 4.4 %). high-current works costs surged form 2.5 €/m2 to 4.5 €/m2 (growth 180 %, share in the total renovation cost 1.7 %). 582 vol. 38/2022 dynamics of apartment building renovation investment costs . . . 3.4. energy ambitions the energy savings found based on the weighted specific energy use of residential buildings in the second period range from 15-55 % and the correlation (r = 0.0302) shows that there is only a minor positive relationship between construction costs and energy savings. there is also some increase in the energy efficiency ambition within the second period, but it is so small that it can be statistically random. on average, renovation works achieve energy savings of 38 %. which is below the 50 % energy savings stated in the long-term renovation strategy[2]. we could not confirm that based on actual post-renovation energy uses such high energy savings were achieved on average. 4. discussion and conclusions the starting point of the search was the observation that renovation prices have increased 100 percent between 2010 and 2018 while construction price index for new build only grow 14 percent during that time. therefore, it was evident that there are specific reasons for renovation prices to grow. for understanding the causes for price change, we observed a sample (n = 112) of renovations that were supported by state renovation grant (n = 561) and were therefore carefully documented by grant manager. while the price information was obtained from procurement bidding documents, technical renovation solutions were gathered by analysing project designs of the sample buildings. we observed that housing associations were quite modest at investment size during the beginning of the grant programme. therefore, those constructions were significantly cheaper than renovations in later stages. energy conservation ambitions also were mediocre and contractors lacked sufficient experience to deliver deep energy renovations. we must also note that enterprises that started to offer the renovation services were mostly newly established while experienced new builders were cautious about entering the renovation market. gradually the deep energy renovations established itself as rational and working concept and more ambitious energy saving decisions were made. this, however, increased the project prices. to run for state grant, one had to comply with the technical and quality requirements that were established for achieving energy efficiency targets. we observed that the changes in grant requirements in 2015 added several eligible construction tasks and allowed additional indirect energy saving works to benefit from the grant programme. more importantly, the highest ambitions in efficacy were rewarded with even higher share of subsidy, mainly to boost integrated renovations. this in turn resulted with the adoption of more expensive solutions and additional works pushed the average prices around 40 % higher compared to the previous stage in 2010–2014. at the beginning of the study, there was speculations that renovation grants could over-stimulate the market and induce price rally because of the significant increase of supply. while this is quite valid and obvious notion, however, we could not establish such cause in our case, because price growth was mostly explained by additional works and unit prices of separate elements did not show significant price increase during the study period. informal interviews furthermore demonstrated how supply chains shift profit from general contractor to subcontractors in hot market conditions where there is pressure from demand side for additional construction volume. the latest average prices in 2020 for deep energy renovations are about 380 €/m2. the performance and quality that is available for this price is significantly better than the renovations a decade ago and fulfilling the renovation ambitions for colder climates like eastern europe and scandinavia. in terms of planning state funds and estimating overall renovation investment need, there are still some studies necessary. for example, we observe trends form onsite renovations to prefabricated elements. however, currently such methods are more expensive than traditional fully onsite renovation. some latest pilots in estonia indicate for about 500 €/m2 for prefabrication. it is expected as larger volumes and automation could eventually lower the costs but this is yet to be observed. this, however, calls for new inquiries for understanding the price impact of emerging renovation technologies. on the other hand, we see some examples of ambitious pilot project to test the limits of current renovation technology. renovation prices in innovation projects of smartencity [8] and moreconnect [9] reached almost 800 €/m2. if there is a trend to move towards such projects, renovation prices will most certainly exceed the level of growth of the new build again. as the planning of renovation strategy is influenced by renovation costs, we suggest analyst and planners to have a deeper look into the causes of price change in their region. by understanding the price change, one reduces the uncertainty in planning for carbon neutrality. acknowledgements this work has received funding from the estonian centre of excellence in zero energy and resource efficient smart buildings and districts, zebe (grant no. 20142020.4.01.15-0016) funded by the european regional development fund; by the estonian research council (grant no. prg483) moisture safety of interior insulation, constructional moisture and thermally efficient building envelope; by the european commission through the h2020 project finest twins (grant no. 856602); and european regional development fund and the estonian ministry of education and research (finest centre for smart cities project resto). 583 g. miller, l. lihtmaa, t. kalamees acta polytechnica ctu proceedings references [1] eu com/2020/662 final. a renovation wave for europe – greening our buildings, creating jobs, improving lives. brussels, 2020. [2021-07-09]. https://eur-lex.europa.eu/legal-content/en/txt/ ?uri=celex:52020dc0662 [2] ministry of economic affairs and communications. long-term strategy for building renovation. tallinn, 2020. 89 p. [3] k. kuusk, t. kalamees. estonian grant scheme for renovating apartment buildings. energy procedia 96:628–637, 2016. https://doi.org/10.1016/j.egypro.2016.09.113 [4] s. d’oca, a. ferrante, c. ferrer, et al. technical, financial, and social barriers and challenges in deep building renovation: integration of lessons learned from the h2020 cluster projects. buildings 8(12):174, 2018. https://doi.org/10.3390/buildings8120174 [5] p. tuominen, k. klobut, a. tolman, et al. energy savings potential in buildings and overcoming market barriers in member states of the european union. energy and buildings 51:48–55, 2012. https://doi.org/10.1016/j.enbuild.2012.04.015 [6] k. kuusk, t. kalamees. retrofit cost-effectiveness: estonian apartment buildings. building research & information 44(8):920–934, 2016. https://doi.org/10.1080/09613218.2016.1103117 [7] l. lihtmaa, t. kalamees. preliminary assessment of preconditions to deliver carbon neutrality in apartment buildings by 2050. e3s web conferences 172:18004, 2020. https://doi.org/10.1051/e3sconf/202017218004 [8] r. ahas, v. mooses, p. kamenjuk, r. tamm. housing estates in the baltic countries. the urban book series, chap. retrofitting soviet-era apartment buildings with ‘smart city’ features: the h2020 smartencity project in tartu, estonia, pp. 357–375. springer international publishing, cham, 2019. https://doi.org/10.1007/978-3-030-23392-1_17 [9] p. pihelo, t. kalamees, k. kuusk. nzeb renovation with prefabricated modular panels. energy procedia 132:1006–1011, 2017. https://doi.org/10.1016/j.egypro.2017.09.708 584 https://eur-lex.europa.eu/legal-content/en/txt/?uri=celex:52020dc0662 https://eur-lex.europa.eu/legal-content/en/txt/?uri=celex:52020dc0662 https://doi.org/10.1016/j.egypro.2016.09.113 https://doi.org/10.3390/buildings8120174 https://doi.org/10.1016/j.enbuild.2012.04.015 https://doi.org/10.1080/09613218.2016.1103117 https://doi.org/10.1051/e3sconf/202017218004 https://doi.org/10.1007/978-3-030-23392-1_17 https://doi.org/10.1016/j.egypro.2017.09.708 acta polytechnica ctu proceedings 38:579–584, 2022 1 introduction 2 methodology 2.1 data collection 2.2 analysis methods 3 results 3.1 the dynamics of apartment buildings renovation costs 3.2 detailed analysis of mandatory renovation works 3.3 mapping of added renovation works 3.4 energy ambitions 4 discussion and conclusions acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.32.0025 acta polytechnica ctu proceedings 32:25–31, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague remediation of a railway embankment using geocells and deep soil mixing a numerical analysis jan štefaňáka,∗, josef vašinab a brno university of technology, faculty of civil engineering, institute of geotechnics, veveří 331/95, brno, czech republic b waltec gds, s.r.o., masarykova 1355, blansko, czech republic ∗ corresponding author: stefanak.j@fce.vutbr.cz abstract. progressive settlement of the embankment under the railway track near žďár nad sázavou, the czech republic, has been observed for many years. immediate improvement was needed to secure the safety of the traffic and to increase the travel speed of the track. the reconstruction work consisted of deep soil mixing. also, a load spreading geocell reinforced mattress was designed. the evaluation of the proposed solution by means of numerical modelling and associated laboratory testing is described in the paper. keywords: geocell, deep soil mixing, remediation, numerical modelling. 1. introduction the purpose of the remediation work is the reconstruction of both an embankment and the railway track near the railway station žďár nad sázavou, czech republic. the railway of interest is a national railway included in the european transport network ten-t. the height of the embankment is cca 5.5m. the unsatisfactory condition of about 320 metres of the embankment was identified during the preliminary walk-over survey. horizontal deformation of the crowns of the embankment body was observed. tilting of the traction line columns were observed in some places also, as can be seen in figure 1. the owners of the regional railway infrastructure proved the necessity of repeated tamping of the track ballast under railway tracks due to the disintegration of the geometrical position of the track rails [1]. hence, the main purpose of both remediation and increasing the width of the embankment is to increase stability and to ensure the safety of railway transport. a consequent effect is an increase of the railway traffic speed. the measures designed to achieve the stated goals are described in detail later in the paper. the area concerned is built of metamorphic rocks of the moravian branch of the moldanubian in terms of geological structure. metamorphic rocks in the subsoil of the railway embankment are heavily weathered and to a considerable depth. the rock eluvium has the character of clay and dusty sand. the eluvium is covered with a layer of fill and or quaternary clay of variable thickness. clayey eluvium and clay are insulators from hydrogeological point of view. the clayey sandy and sandy eluvium is only slightly waterlogged. the slope of the groundwater level and the direction of groundwater flow is to the n, nne and ne, towards the sázava river. climatic conditions in the area of interest are characterized by freezing depth hpr =1.1m. 1.1. results of the geotechnical survey of the embankment evidence of instability, visible to the eye, was found in both the railway tracks and the embankment. tilting of traction line columns and deformation of the embankment crown were observed during the preliminary investigation and walk-over survey. based on the results of the preliminary investigation, the strategy for a detailed survey of the embankment body was established. the survey was conducted in three typical cross-sectional profiles. combinations of trial pits, core boreholes, and dynamic penetration tests were performed in each profile. the results of penetration tests were evaluated according to [2]. the probes were evenly distributed at the base of the embankment body, at the top (in the axis of the two parallel rail tracks) and in the crowns on both sides of the embankment. based on the results of dynamic probing, the relative density of soil into the depth of 3.3m was recognized as loose to medium-dense. the soil sample was classified according to en iso 14688-2 as sisa. the percentages of the individual soil fractions were 4% of fine grains, 21% of sand and 62% of sand and 13% gravel. an admixture of slag by-products was found in the samples also. the material ranges from poorly permeable to permeable. probes at the foot of the embankment body and in the axis of the embankment crown could not be performed due to an abundant presence of cable routes. 1.2. proposed remediation work the following sequence of remediation measures was proposed, based on results of the conducted surveys: 25 https://doi.org/10.14311/app.2022.32.0025 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en jan štefaňák, josef vašina acta polytechnica ctu proceedings figure 1. condition of the railway before reconstruction: tilting of the traction line columns (top) and temporary measures against disintegration of the track rail geometrical position(bottom). (1.) removal of the top part of the embankment along its entire length to a depth of approximately 1.6m below the current sub-ballast layer. (2.) construction of the deep soil mixing (dsm) columns evenly distributed in the embankment. the purpose of dsm columns is to increase the stiffness of the embankment and to help transfer the traffic load from the rail through the embankment body to the subgrade formed by competent weathered gneiss rock. (3.) building of a gabion wall situated at the foot of the embankment body. the height of the wall varies between 1.0m and 3.0m, according to the actual height of the embankment in specific sections. the purpose of the gabion walls is to improve a currently too steep geometry of embankment slopes and to eliminate the vertical deformation of the embankment due to the traffic loading. (4.) construction of the geocell mattress filled with crushed stone. the mattress is constructed above the hi-strength single-axial geogrid and geomembrane that cover the heads of dsm pillars. 2. numerical analysis a relatively high number of technologies and structural elements is combined in the above described remediation measures. hence, it was decided to use the finite element method (fem) as a tool for assessing the effect of designed measures on the stability and deformation of the embankment. the plaxis 2d package was used for simulation of the development of the stress-strain state in the embankment, with respect to the sequence of the remediation work phases. 15node finite elements were used for modelling the soil, which approximates the deformation by quartic polynomials. "plastic" and "safety" types of calculations were performed. the input values of the properties of the soil materials forming the embankment and the subgrade were determined on the basis of the results of conducted laboratory tests and in-situ tests. approaches for transforming each component of the remediation measures into a plane strain numerical model are described in the following paragraphs. 2.1. modelling of deep soil improvement the installation of deep soil mixing (dsm) pillars with a diameter of 0.6 meters with axial distance 1.2 metres in the longitudinal direction is one of the key components of the proposed remediation measures. the process of column mixing is captured in figure 6. adequate data from the literature [3] were used to determine the strength and deformation properties of the final product of the dsm technique, the soil–cement composite. the line of columns is represented by the virtual wall in the 2d numerical model. the equivalent modulus of elasticity of the wall was calculated as the weighted average of the modulus of elasticity of the column ecol=2000mpa [3] and of the modulus of the soil between the pillars in the longitudinal direction esoil=6mpa. the principle of calculation of the equivalent modulus of elasticity 26 vol. 32/2022 remediation of a railway embankment – a numerical analysis of the wall summarised in [4] is described by equations (1)–(3) and schematically illustrated in figure 2. symbol acol represents the cross-sectional area of the dsm column and the ar the area of the virtual wall. rcol = acol ar = acol acol +asoil (1) rsoil = 1− rcol (2) eeq = (rcolecol) + (rsoilesoil) (3) the uniaxial compressive strength of the dsm column composite material was estimated according to [3] as σc=3.5mpa. the methods proposed by [5] were used then for the transformation of the uniaxial compressive strength on the strength parameters of mohr-coulomb failure criterion ϕ and c. the input value of angle of internal friction is ϕ = 35 °. the value of cohesion was determined as c = 187 kpa using equation (4). c = √ σc 100 (4) contact between the outer surface of the virtual wall and the surrounding soil was modelled by a contact element with the value of parameter rinter =0.75. 2.2. modelling of the geocell mattress the geocell filled with crushed stone was modelled as a layer with equivalent strength and stiffness parameters. the input values of the strength and stiffness parameter of equivalent material were determined according to [6],[7], and [8], considering the beaviour of crushed stone to be governed by the duncan–chang non–linear constitutive model [9]. increasing of the values of the strength and deformation parameters of the filling material is considered due to the 3d stress state in the individual cell pocket [10]. the development of the apparent cohesion cr depends on the increased lateral stress ∆σ3 arising in the soil inside the cell due to the membrane stress induced by the wall of the cell. the relationship between the apparent cohesion cr and the lateral stress ∆σ3 is illustrated in figure 3. the smallest circle illustrates the failure stress state of the unreinforced filling material. the lateral stress σ3 increases to the value σ3 + ∆σ3 due to the confinement of the filling material by the cell wall. the value of major principal stress σ1 increases also from the value σ1u. this stress state is illustrated by the circle named "with confinement" in figure 3. however, the same stress state can also be described by the largest circle, which is determined by the lateral stress σ3 and the apparent cohesion cr. the relationship between cr and σ3 can be derived using mohr–coulomb failure theory. the major principal stress σ1 is stress that the soil is able to sustain before failure, and can be described by equation (5). σ1 = 1 + sinϕ 1− sinϕσ3 + 2cr √ 1 + sinϕ 1− sinϕ = kpσ3 + 2cr √ kp (5) where kp is the passive earth pressure coefficient of the filling material. if the confining pressure σ3 + ∆σ3 is applied to such material, then the σ1 is given by the equation (6) σ1 = kp(σ3 + ∆σ3) (6) however, the value of σ1 stays unchanged. the increment in the apparent cohesion cr due to the increase of the confining pressure can be expressed from the comparison of equations (5) and (6), which results in formula (7). cr = ∆σ3 2 √ kp (7) increase in the confining pressure ∆σ3 on the soil due to the presence of geocell is given by equation (8), which originates in the membrane theory proposed by [12]. ∆σ3 = 2m d0 [ 1− √ 1− ξa 1− ξa ] (8) where m is the secant modulus of the geocell material at axial strain ξa, which is obtained from the tensile load–strain response of the geocell material at 2% axial strain, and d0 is the equivalent diameter of the geocell pocket. the stiffness of the composite material depends on the stiffness of the filling material and on the secant modulus of the geocell material. the nonlinear relationship between these two parameters and between the parameter kr can be expressed by the equation (9). kr = ke + 200m0.16 (9) in the above ke is the modulus number, which is a dimensionless parameter that represents young’s modulus of the filling material [9]. the elastic modulus of the duncan–chang model in loading conditions ei is given by equation (10), where n is a modulus exponent and pt is the atmospheric pressure which is used to normalise the stress inputs. ei = krpa ( σ3 pa )n (10) 27 jan štefaňák, josef vašina acta polytechnica ctu proceedings dsm column wall width soil between columns (walls) dsm column wall figure 2. the principle of replacing dsm pillars in a plain strain model by a virtual wall. adapted from [4]. σ1σ3 δσ3 cr σ1,u φ φ unreinforced reinforced composite with confinement unreinforced σ τ figure 3. mohr circles for unreinforced, reinforced and composite (geocell filled with crushed stone) material (adapted after [11]) the values m =450mpa, d0 =0.24m, ke=725, n=0.43 and kp=3 were used in this presented analysis. the contact between the equivalent geocell layer and the surrounding soil was modelled by a contact element with the value of parameter rinter =1.0 according to the results of experiments published in [13]. the gabion walls constructed on both sides of the embankment were also included in the model. the gabion baskets were modelled by the "geogrid" type of finite elements. the mechanical parameters of the welded gabion basket net made of steel wire of diameter= 3.0mm and wire spacing of 75 mm were determined according to [14]. the value of axial stiffness of the net was ea=19.0e3 kn/m. the contact between the basket net and the filling material and the wall backfill material was modelled by a contact element with the parameter rinter =0.75 . the geogrid element with stiffness ea=12.9e3 kn/m was used also for modelling the hi–strength geogrid placed above the heads of the dsm columns. the 60.0m×36.0m model with 48 172 finite elements was used for analysis. the detailed cutout of the fe model with all proposed remediation measures is shown in the figure 4. the elastic–perfectly plastic mohr coulomb (mc) constitutive model was used to describe the behavior of soil and composite materials. input values of the mc model are summarised in table1. 3. laboratory testing of dsm composite material some of the design assumptions stated above were evaluated by execution of laboratory tests. preproduction laboratory tests were conducted due to the uncertainities in the chemical reactions in the cementsoil mixture, because the slag by-products was added into the embankment during its construction. mixtures for these tests were prepared according to the recommended procedures described in detail in [15], [16]. three variants of the mixture were prepared from the soil taken from the embankment during investigation and from a cement suspension. cement content in suspension varied between 250 kgm−3 to 350 kgm−3. water to cement ratio was varied between w/c=0.85 to 1.3. the uniaxial compressive strength of the composite material produced by the dsm technology was measured. the compressive strength was determined by placing cylindrical specimens of known dimensions in a test press and loading them at a rate of 1% of the sample height per minute until failure. the reduction of the resulting strength due to the ratio of the length of the speciment to its height was performed according to [15] to normalise the results of tests performed on samples with a different geometry. the laboratory testing programme follows with an evaluation of the uniaxial compressive strength of the samples taken in-situ. these control tests were performed on the samples prepared from the mixture taken on site directly from the fresh column and cured in laboratory conditions. the second type of samples for control test were prepared from the borehole cores taken from the colums which had already reached a certain strength. the uniaxial compressive strength was evaluated on the 7, 21 and 28 days old specimens. the mean value of a uniaxial compressive strength of 28 vol. 32/2022 remediation of a railway embankment – a numerical analysis 5 2 3 11 7 8 1 9 10 6 4 dsm column geocell mattress gabion wall hi-strength geogrid figure 4. cutout of the fem model detail of the embankment. (the numbers in circles refer to the material characteristics summarised in table (1)). material unit weights strength par. deformation par. γunsat γsat c´ ϕ´ e´ ν´ [kn/m3] [kn/m3] [kpa] [◦] [mpa] [-] 1 eluvium 18.5 20 4 30 11.89 0.3 2 gabion infilling rock 17.9 19.6 100 35 20 0.2 3 geocell mattress 17.5 19.0 33 30 80 0.3 4 silty gravel 19.00 20.50 3 35 26 0.3 5 ballast 20.00 22.00 1 40 135 0.25 6 fills 17.00 18.00 4 30 8.9 0.3 7 embankment upper part 17.50 19.00 1 35 80 0.25 8 embankment lower part 17.50 19.00 2.8 30 3.13 0.28 9 weathered rock in subgrade 28.00 28.00 5600 39.5 600 0.25 10 dsm columns 20 21 187 35 844 0.3 11 backfill 17.50 19.00 4 32 10.4 0.3 table 1. input values of mc constitutive model parameters the composite was σc=13.8mpa. hence, it can be said that the results of numerical analysis are on the safe side. 4. results the calculated value of the factor of safety was f =1.09, when only the self-weight of the embankment was applied to the model of current state of the embankemnt without any proposed remediation measures. this result confirms the in-situ observed situation the permanent increase of deformations. the implementation of remediation measures increased the calculated factor of safety to the value f =1.57. the calculated total deformation of the embankment body will not exceed 13.4mm after the reconstruction. the calculated field of total deformations is presented in figure 5. 5. conclusions the paper describes the evaluation of the design of remediation action at defective railway embankment. the remediation measures consisted of deep soil mixing columns covered by a geocell mattress. the effect of the proposed measures was verified by numerical analysis. the process of building the 2d numerical model is presented. results of numerical analysis proved, that the proposed remediation measures leads to a redistribution of the stress field in the embankment body. the stress in the embankment body improved by dsm technique is concentrated in the dsm columns, because they have significantly higher stiffness than the surrounding soil. the largest portion of stress is transmitted by two pillars located closest to center of the embankment body. the laboratory tests are also described, erformed for the purpose of the design and evaluation of the dsm composite material. 29 jan štefaňák, josef vašina acta polytechnica ctu proceedings figure 5. isolines of computed total deformations caused by a traffic loading of intensity 80 kn/m figure 6. construction of dsm columns at the žďár nad sázavou building site list of symbols hpr freezing depth [m] ecol modulus of elasticity of dsm column [mpa] esoil modulus of elasticity of soil [mpa] eeq modulus of elasticity of virtual wall [mpa] acol cross-sectional area of dsm column [m2] ar cross-sectional area of virtual wall [m2] σc uniaxial compressive strength [mpa] ϕ angle of internal friction [◦] c cohesion [kpa] cr apparent cohesion [kpa] ∆σ3 lateral stress increment [kpa] σ3 minor principal stress [kpa] σ1 major principal stress [kpa] σ1u major principal stress in unreinforced material [kpa] m secant modulus of the geocell material [mpa] d0 equivalent diameter of the geocell pocket [m] ei elastic modulus in loading conditions [mpa] pt atmospheric pressure [kpa] rinter interface strength kp passive earth pressure coefficient ξa axial strain kr modulus number of reinforced material ke modulus number of unreinforced material n modulus exponent acknowledgements this work was funded by the research project no. ta04031092 supported by the technology agency of the czech republic. references [1] j. vašina. kolejové úpravy v žst. žďár nad sázavou technická zpráva (remediation of the railway near the station žďár nad sázavou technical report, in czech). waltec gds, s.r.o., blansko, czech republic, 2019. [2] p. řezníček, k. zdražil, d. relich, l. miča. metodika použití zkoušek statické penetrace cpt a těžké dynamické penetrace dph při inženýrskogeologických průzkumech pro dopravní stavby a při jejich výstavbě. (methodics of implementation of the cone penetration test cpt and heavy dynamic penetration test dph in engineering-geology investigation and building process of 30 vol. 32/2022 remediation of a railway embankment – a numerical analysis traffic structures; in czech). technological agency of the czech republic, brno, czech republic, 2017. [3] p. svoboda. hloubkové zlepšování zemin v české praxi (practical experience with deep ground improvement methods in the czech republic; in czech. akademické nakladatelství cerm, brno, 1st edn., 2009. [4] c. m. tatarniuk. deep soil mixing as a slope stabilization technique in northland allochthon residual clay soil phd thesis. university of canterbury, christchurch, new zealand, 2014. [5] d. h. ardiaca. mohr-coulomb parameters for modelling of concrete structures. plaxis bulletin pp. 12–15, 2009. [6] a. hedge, s. g. thallak. experimental and numerical studies on geocell reinforced sand beds. in proceedings of geosynthetics 2013, pp. 119–126. long beach, california, usa, 2013. [7] s. k. dash, g. m. latha, k. rajagopal. numerical modelling of geocell reinforced soil foundations. in proceedings of international congress on computational mechanics and simulation, iccms 06at, pp. 1–4. guwahati, india, 2006. [8] g. m. latha, s. k. dash, k. rajagopal. equivalent continuum simulations of geocell reinforced sand beds supporting strip footings. geotechnical and geological engineering 26(4):387–398, 2008. https://doi.org/10.1007/s10706-008-9176-5. [9] j. m. duncan, c. y. chang. nonlinear analysis of stress and strain in soils. journal of soil mechanics & foundations div 96(5):1629–1653, 1970. [10] p. punetha, s. nimbalkar, h. khabbaz. evaluation of additional confinement for three-dimensional geoinclusions under general stress state. canadian geotechnical journal 57(3):453–461, 2020. https://doi.org/10.1139/cgj-2018-0866. [11] k. rajagopal, n. krishnaswamy. behaviour of sand confined with single and multiple geocells. geotextiles and geomembranes 17(3), 1999. https://doi.org/10.1016/s0266-1144(98)00034-x. [12] d. j. henkel, g. d. gilbert. the efect of the rubber membrane on the measured triaxial compression strength of clay samples. geotechnique 3(1):20–29, 1952. [13] l. g. madhavi, r. karpurapu, n. r. krishnaswamy. interfacial friction properties of geocell reinforced soil. in isrm international symposium, 19-24 november, pp. 1–6. international society for rock mechanics and rock engineering, melbourne, australia, 2000. [14] m. ćwirko, m. jastrzębska, s. kwiecień. the analysis of the usefulness of welded meshes to embankment reinforcement. studia geotechnica et mechanica 39(3):3–9, 2017-09-1. https://doi.org/10.1515/sgem-2017-0024. [15] b. c. m. ellen, r. r. berg, j. g. collin, et al. federal highway administration design manual. office of transportation management federal highway administration, 400 7th street sw washington, dc 20590, 2013. [16] m. kitazume, m. terashi. the deep mixing method. crc press/balkema, eh, leiden,the netherlands, 2013. 31 https://doi.org/10.1007/s10706-008-9176-5 https://doi.org/10.1139/cgj-2018-0866 https://doi.org/10.1016/s0266-1144(98)00034-x https://doi.org/10.1515/sgem-2017-0024 acta polytechnica ctu proceedings 32:25–31, 2022 1 introduction 1.1 results of the geotechnical survey of the embankment 1.2 proposed remediation work 2 numerical analysis 2.1 modelling of deep soil improvement 2.2 modelling of the geocell mattress 3 laboratory testing of dsm composite material 4 results 5 conclusions list of symbols acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.38.0031 acta polytechnica ctu proceedings 38:31–37, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague options of modelling refurbishment in lca: continuous improvement strategy and next life cycle approach maria balouktsi∗, thomas lützkendorf karlsruhe institute of technology (kit), centre for real estate, kaiserstraße 12, 76131 karlsruhe, germany ∗ corresponding author: maria.balouktsi@kit.edu abstract. compared to other goods, buildings have a long lifespan. it is therefore the norm and not the exception that adaptation will be needed at some point in their life cycle to changing environmental conditions, technical progress and new/additional user requirements. the adaptation of existing buildings to future challenges in the form of refurbishment contributes to sustainable development. the proportion of refurbishments is increasing in both absolute and relative terms in the building sector in europe. it is therefore surprising that the modelling particularities of refurbishment in the context of a life cycle-based environmental performance assessment are still neglected in the enstandards. the paper deals with these particularities. specifically, it presents a typology of approaches starting with a distinction between planned refurbishment as b5 and unforeseen refurbishment as an independent design and construction task / next life cycle, as well as the status of the discussion in selected countries. recommendations for action for the life cycle assessment are given for specific decision-making situations. keywords: sustainable development, buildings, refurbishment, re-purposing, life cycle assessment. 1. introduction compared to other goods, buildings have a long lifespan. this can be between 20 and over 100 years, depending on the building type and pattern of use. maintenance during a building’s lifetime is necessary to keep the building in a usable condition. this includes activities such as maintenance, repair work, as well as replacement of components whose service life is shorter than the service life of the building. in a life cycle assessment (lca), these processes correspond to modules b2, b3 and b4, respectively. the aim of these processes is to maintain the technical and/or functional performance of the building in use. however, over a comparatively long building service life, changes may arise in social, legal, climatic and/or economic boundary conditions, changes in user requirements and/or technical progress. in addition to maintaining the technical and/or functional features, there is a need to explore whether a building will be usable, rentable and marketable also in the future. the need may arise to go beyond maintenance at some point in a building’s life cycle and apply technical and/or functional improvements to adapt to technical progress (better products are entering the market) and/or to new user requirements. such improvements can be realized by refurbishment measures. major building refurbishments usually take place after a 20–30 year’s period of use of a building and are possibly combined with maintenance, repair and/or replacement measures. refurbishment may extend the service life of buildings. a longer building lifespan can slow down the material cycles, reduce the energy and mass flows as well as negative impacts on environment. while many new buildings will have to be constructed in parts of the world in the coming years, in europe adapting and further developing the existing building stock is a major priority [1]. to establish environmental performance assessment methods of refurbishment options would be needed to support design and decisionmaking processes in this context. the application of the environmental performance assessment is therefore becoming more important on the one hand, but has not yet been sufficiently methodologically penetrated, on the other, especially in contrast to lca methods for new design / new buildings [2]. this impedes the effective use of lca to quantify and understand the environmental benefits of refurbishments in practice. the aim of this paper is to increase understanding of the subtleties of the use of lca for modelling refurbishment, either as (1) a scenario in a new building design or as (2) an option for a further use phase (expressed as a next life cycle / next reference study period (rsp)) of an already existing building. this discussion is useful for clarifying the methodological context, especially given the fact that life-cycle-based benchmarks for refurbishment projects will be needed in future, although not yet the norm. it is important to note that within the professional and scientific literature, the terms refurbishment, renovation, retrofit and modernisation are often used interchangeably. the international standard iso 219311:2010 and european standard en 15978:2011 both use the term refurbishment, which represents a “large scale (substantial) modification and improvements to 31 https://doi.org/10.14311/app.2022.38.0031 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en maria balouktsi, thomas lützkendorf acta polytechnica ctu proceedings figure 1. timeline for planned and unforeseen refurbishment. an existing construction work to bring it up to an acceptable condition”. to be in line with the standards, the term refurbishment is used throughout the rest of the paper as an umbrella term. furthermore, the term re-purposing is here also used to denote a special form of refurbishment where an existing building of particular type of use is transformed to a new kind of use (i.e. change of function), e.g. an office building to an apartment block. re-purposing involves the same type of system boundaries with refurbishment, but with a changed function and – as a result – a new functional equivalent as object of assessment. 2. options for modelling “refurbishment” 2.1. refurbishment as part of a first or start of a next reference study period in this paper a distinction is made between: “planned” (anticipatory) refurbishment and “unforeseen” / not according to initial plan (re-active) refurbishment (figure 1). in a planned refurbishment a new building still under design becomes the object of assessment. in this option refurbishment is represented by module b5 as part of the life cycle model of a new building design and the related impacts are calculated and assessed. in such a case the option for planned refurbishment becomes part of the functional equivalent. examples are the repurposing of exhibition and olympic game buildings, the refurbishment of a building to make it net zero in a prepared second step, etc. in the case of building concepts without refurbishment already planned at the time of design, module b5 becomes irrelevant, but modules b2, b3 and b4 must still be processed. in an unforeseen refurbishment, at first an existing building, later the design for refurbishment and at the very end the refurbished building become the object of assessment. this means that the type and time of refurbishment was not yet known during the initial design and the need for improvement (and/or adaptation to a new type of use) emerged during use. 2.2. reflections in existing standards standards (e.g. en 15978:2011, which is currently under revision) already offer module b5 (refurbishment) to serve as an option to document the impacts of a planned future refurbishment of buildings during the initial reference study period (rsp) in the sense of an upgrade of their technical and/or functional performance at some point during their lifetime. what distinguishes planned replacements according to the initial plan (b4) from a planned refurbishment (b5) is that, in the present authors’ interpretation, b4 only corresponds to replacements of like-for-like materials, building components and technical systems. finally, refurbishment may also include the installation of entirely new components which are not present from the start, such as pv panels in the case of a planned net zero refurbishment. this approach comes with implications. in future standards (and/or national guidelines complying with the standards) how to forecast technical progress of building components shall be addressed. less problematic is the variant in which the best available technology is not yet used in the new building, e.g. for cost reasons, but is already known. in this case, the impacts and benefits associated with the re-placement of components with better ones in the context of b5 can be estimated without any problems. a particularity of the case of addition of new types of components is that the building must be designed from the start to include them. this also may involve additional impacts, which are present before the refurbishment (b5) itself occurs. for example, in the case of solar systems planned to be installed at a later stage, empty pipes may be already defined and incorporated during design that can then accommodate components of the system to be installed at a later stage. impacts associated with the pipes should be part of overall lca. therefore, it would make sense to document any additional efforts taking place at the time of refurbishment/repurposing as part of b5, while additional efforts in relation to the future use incorporated already in the initial design are part of a1-5, later also b4, c1-c4 and, in addition, b1. this is also an aspect not currently mentioned in the standards. as seen, the inclusion of b5 necessitates complex assumptions. furthermore, depending on the depth of the refurbishment/adaptation and type of building, its magnitude in the whole life embodied impacts can be nearly as important as a1-3 impacts [9]. it also significantly influences post-refurbishment operation 32 vol. 38/2022 options of modelling refurbishment in lca . . . country iso code / new retained components removed components method guidance components initial new initial current [a–c3] [a–b] [b–c3] [a–b] [c3] fi/national method [3]1 x x x uk/professional guide rics [4]1 x x de/certification bnb [5] x x nl/addendum to national x x2 x x2 x method [6] fr/certification hqe [7] x x2 x x2 x ch/sia approach 1 (main) 2032 [8] approach 24 approach 34 x x x2 x x x x2 1 document applicable to both new buildings and refurbishments (only generic rules for refurbishment). 2 proportional to the remaining service life. 3 the same considerations apply to recycling, recovery, reuse potential (i.e. d1), if considered. 4 approaches proposed for existing buildings, but some principles can also be applied to refurbishments. table 1. overview of selected methods. processes (b6 and b7) if one carries out an energyefficient and water-saving refurbishment. therefore, when included in an assessment the question naturally arises as to the credibility of the assumptions made and the probability of their actual realization. there is a risk of manipulation. it would be helpful to develop the basis for voluntary commitments or contractual guarantees. for example, level(s) system in its rules for scenario development of future refurbishment/adaptability requires that, if lca results are to be publicly reported, an independent critical review of the assumptions shall be carried out by a property market specialist with knowledge of the local and regional conditions and his or her opinion appended to the reporting [10]. it is expected that the call for extending the life of buildings through adaptable/flexible designs will lead to upgrade the importance of module b5. when it comes to unforeseen refurbishment (action of its own), en 15978:2011 standard states that “if a building is refurbished and the refurbishment was not taken into account at the outset, i.e. in any previous assessment, a new assessment should be carried out, particularly where the refurbishment changes the functional equivalent, . . . in the new assessment of the refurbished building, the environmental impacts and aspects of the refurbishment materials and reconstruction/installation processes are allocated to modules a1 to a5”. although the standards resulting from iso tc 59 sc17 and cen tc 350 deal in principle with both new building and refurbishment projects, the peculiarities of the assessment of refurbishments have hardly been discussed in detail so far. despite the reference above it is not made clear how to treat the environmental impacts of the initial building, whose parts are being further used in the next life/rsp, among other peculiarities of applying lca to refurbishments of existing buildings. special rules and guidance are needed. such rules are expected to be added in the new version of en 15978-1, as well as the results of the work of the cen tc 350/ wg 8 sustainable refurbishment remain to be seen. 2.3. reflections in existing national methods up to now only a few countries provide special guidance on how to calculate the life cycle environmental performance of refurbishment projects (table 1), and even less provide benchmarks for their assessment. however, the inclusion of mandatory calculations and legal binding benchmarks also for refurbishments, along the requirements for new buildings, is currently examined in some countries (e.g. sweden [11]). table 1 shows the difference of existing methods surrounding the treatment of (a) new components installed in the building (replacing old components or not); (b) the retained components of the initial structure continued to be used post-refurbishment; the removed components. what all methods include are the life cycle impacts of the new components and the life cycle impacts of retained components (i.e. maintenance, replacements, etc) occurring during the next life cycle/rsp (post-refurbishment). this already necessitates the determination of an additional parameter compared to a method for new buildings: the residual / remaining service life of retained components so that to be able to define the next replacement in the next building life. most methods do not yet provide calculation rules for this. what makes the methods differ is how to treat the “past” impacts of all initial building, either retained or removed. this leads to various modelling approaches as defined in the next section. 33 maria balouktsi, thomas lützkendorf acta polytechnica ctu proceedings 3. typology of options in modelling of refurbishment activities in the case of refurbishment of an existing building as a starting point of a next service life / rsp, different modelling and inclusion/exclusion possibilities exist for: (1) parts of the initial building structure that are removed and processed (for recycling, reuse, recovery) or disposed and (2) parts of the initial building structure that remain for further use. the implications are discussed below. 3.1. treatment of initial structure further used there are three main approaches. their appropriateness is dependent on the decision-making situation. 3.1.1. approach “zero” this approach represents a time-related allocation of environmental impacts / resource consumption; impacts occurred in the past are allocated to the past, i.e., embodied impacts of the initial structure are neither calculated nor considered in the lca of refurbished building. up to now, in literature studies and in lca tools and methods, this has been the most used approach to determine the life cycle-based environmental performance of major refurbishments (see table 1). this approach makes particular sense in the context of setting a specific allowable budget for environmental impacts (including ghg emissions) for a time period. all building construction and maintenance activities carried out in the past have already been charged to a budget of a past time period. according to budget-approach, assigning effects that occurred in the past to future time periods (here to a current, subsequent life cycle) in the sense of a remaining “load” could be a double-counting. furthermore, in practice, what has already been “spent” cannot be influenced retrospectively. 3.1.2. approach “remaining load” this approach involves the determination of the “remaining environmental load” at the time of refurbishment, with straight-line depreciation hypothesis. this means that the environmental impacts of the initial parts of the building are divided over their planned service life and then allocated proportionally to the service life that has already taken place. the remainder corresponds to the remaining load at the time of consideration. this allocation approach acknowledges that when observing a building at a specific point of time during its life cycle, some materials are already at (or close to) the end of their service life and some still have a large remaining service life, because either they have a very long service life, or they were recently exchanged. this is a perspective towards the past. this approach is more complex as it requires the definition of at least two additional parameters/inputs: the materialisation of the components retained (type and quantity of each product/material) and their environmental impacts (lca data). the following variants occur based on the type of lca data this is calculated: (1.) historical data – this presupposes that the original environmental impact data during the construction of the initial building as well as past repair and replacement measures are still known or can be reconstructed. in most countries, the application of lca and the provision of life cycle-based environmental information of products has not been around for more than twenty years, which makes the retracing of the “environmental” history of already long-standing buildings very difficult or unfeasible. however, this will be less of a problem long-term with increasing digitalisation of environmental information and calculations in the construction sector. (2.) present-day data – original components can be calculated as they were built today (i.e. by choosing the environmental values of equivalent products in current databases), although material efficiency in production and use significantly improve decade after decade. (3.) dynamically modelled data – there are examples of already-existing lca datasets being remodelled to account for the influence of the dynamic development of electricity mix models and an increase in production efficiencies on environmental impacts of products to get the “future” environmental impacts [12]. in the same way, re-modelling of current lca datasets can be done to correspond with the electricity mix used at the time of the production of the products constituting the existing building. the methods that currently follow the remaining load approach exclusively (table 1) do not clearly specify what type of lca data shall be used for the original structure. it can be assumed that the data found in their national tools apply (present-day-data). in general, from a practical point of view, variants 1 (when feasible) and 2 make sense and can be part of an official approach. variant 3 is a theoretical possibility for science. 3.1.3. approach “fictive replacement of components” structural parts continued to be used can be assessed based on current lca data. this results in a “fictive replacement of building components” if existing components continue to be used in the sense of “as if newly constructed”. in other words, this approach measures the impacts that are assumed as avoided by continuing to use existing parts of the building, i.e. not entirely constructing a new identical building (same functionality, size, operational energy efficiency, etc.). this is one of the approaches in sia 2032 in the case existing buildings (table 1). while the primary intention of 34 vol. 38/2022 options of modelling refurbishment in lca . . . variant for removed products with considerable residual life approach zero approach remaining load ib rb ab ib rb ab 1 – removed and finishes its eol full allocation zero n/a % allocation % allocation n/a 2 – removed, sold & reused in another building full allocation n/a zero % allocation n/a % allocation 3 – reused in the same building full allocation zero n/a % allocation % allocation n/a table 2. different ways of allocating removed elements with residual service life. note: ‘ib’ = initial building; ‘rb’ = refurbished building; ‘ab’ = another building outside the system; ‘%’ = proportional to remaining service life; ‘n/a’ = not applicable. this approach in early studies has been to understand the environmental value of existing buildings, this can also be seen as additional information/metric to demonstrate the benefit of refurbishment compared to building entirely new (i.e. additional information to a zero approach a-c result). for example, if one maintains only the structural system of an existing building for further use post-refurbishment, while the rest is demolished and replaced by new components, then the “fictive replacement . . . ” equals the impacts of demolishing the existing structural system (if c1-4 is included at all) + rebuilding a comparable structural system under the precondition of fulfilling same technical requirements for the next rsp. this approach is useful when the main objective of an assessment is to decide whether a refurbishment project gives enough environmental benefits to balance the perhaps higher costs. however, this “benefit” is a hypothetical representation of the reality; developers do not often replace an existing structure with a similar one in terms of size and basic design. therefore, other options and perspectives are possible. this will be discussed in next research activities. 3.2. treatment of removed components there are different reasons for refurbishing or repurposing buildings. there may be cases in which building parts are removed that have not yet reached the end of their useful life or of wear margin (early removal), i.e. they still have a potential for residual life. like the maintained existing components for the second life, the treatment of removed components can follow a zero or a remaining load approach. one difference is that the residual service life of these products is not used in the specific next building cycle, but potentially in other construction activities. after removal, different courses of life for these products exist which lead to different modelling variants (table 2). this can be discussed in relation to circularity of building components. variant 1: an advantage of remaining load approach over zero approach is that it “punishes” premature removal of products without possibility or intention of reuse. this can be a significant effect in the case of long-lived products and undesirable in view of circular economy [6]. on the other hand, this effect can be negligible for short-lived products since the moment of refurbishment will often not deviate that far from the regular replacement moment. not all country methods have set rules of what early discarding/removal means, and therefore for what removed components the impacts can be overlooked, e.g. would a component replaced just two years before its eol be considered early discarding? country methods following the remaining load approach should clarify such issues. for example, hqe [7] allows overlook the remaining load of existing components when rsl ≤ 10 % technical service life (tsl). variant 2: according to the standards (e.g. en 15978:2011) net benefits of potential reuse shall be considered under module d (renamed to d1 in latest versions). however, in the specific case of a (partial) deconstruction of an existing building, reuse is not far in the future but takes place (almost) at the same time as the refurbishment interventions. therefore, issues arise on how to proceed with all processes concerning modules c1-4 and d1 of initial building parts being removed. does the demolition of an existing building in the sense of clearing the property form the end of a previous or the beginning of a subsequent life cycle/object of assessment? the two life cycles are overlapping in reality; partial demolition, deconstruction, transport, etc. are activities happening during a5 of the refurbished building. for example, anink [6] allocates impacts of module c2 to module a4, while assigns the impact of modules c1, c3, c4 and d1 to module a5. here further discussions are needed, like in current activities of iea ebc annex 72. 3.3. complete overview of differences in approaches below (figure 2) the scope of the different approaches is visualized along the typical refurbishment timeline. 4. discussion and recommendations table 3 provides an overview of options for action and assessment and creates a connection with typical decision-making situations. on how to deal with an existing building, three possible options exist: (a) further preservation/maintenance (in this case, it is decided to not carry out any major intervention that deviates from the regular replacements and maintenance, but no action is not an option); (b) refurbishment (with the same or changed function); (c) deconstruct it and build a new similar one. 35 maria balouktsi, thomas lützkendorf acta polytechnica ctu proceedings figure 2. visualisation of the three modelling approaches. note: newly added is denoted as “new”, removed is denoted as “rem.” and retained as “ret.”. typical cases for the application of lca approach approach approach to major refurbishment “zero” “remaining load” “fictive replacement . . . ” guidance for a choice between refurbishment versus maintenance x (x) (x) guidance for a choice between refurbishment versus demolition/deconstruction and rebuilding x∗ (x) (x) support of design optimisation of the refurbished building x (x) (x) sustainability assessment of the refurbished building for the purpose of certification x (x) (x)∗∗ support of research to improve the modelling of the dynamics of changes in the building stock x (x) (x) ∗ to reduce the risk of manipulation, a minimum retention/use period of 5 years should be guaranteed. ∗∗ recommended as additional information. table 3. suitability of approaches for different typical decision-making cases. 36 vol. 38/2022 options of modelling refurbishment in lca . . . the first two decision-making situations express these investigations. if refurbishment has been decided, the next two cases come into play. it becomes clear that all approaches described are practically applicable for the respective decisionmaking situations. the authors recommend orienting in any case to the “zero” approach. this maps the points in time the energy and material flows occur more precisely, including the resulting impacts on the environment, and can thus be better combined with sectoral considerations. in an environmental performance assessment, it is possible and sensible to additionally (and separately) indicate the impacts avoided by continuing to use parts of the existing building fabric. this enables to communicate the advantages of slowing down material cycles. the quality and residual service life of the still-in-use structural parts must always be checked. the refurbished building rsp should correspond to the residual service life of the load-bearing structure. if not the case, there would only be a temporal shift in environmental impacts. 5. conclusions the refurbishment of the building stock to adapt to climate change and new user needs, is an essential element of sustainable development in europe. it is necessary to develop methodological bases for the life cycle-based environmental performance assessment of refurbishment measures that lead to reliable results. these must also find their way into standardization in the short term. scientific work must be intensified, and consensus-building accelerated. too many variants inhibit transparency and traceability of the chosen approach. one solution is to assign specific models to specific decision-making situations. the present authors will contribute to iea ebc annex 72 in this direction. acknowledgements the authors would like to thank the operating agent (r. frischknecht), and all participants of the ongoing international project iea ebc annex 72 (see: https: //annex72.iea-ebc.org) for useful discussions. references [1] european commission. a renovation wave for europe – greening our buildings, creating jobs, improving lives, 2020. com/2020/662 final, brussels, belgium. [2] survey conducted within the project iea ebc annex 72 (publication forthcoming). [3] ministry of the environment, m. kuittinen. method for the whole life carbon assessment of buildings, 2019. 60 p. isbn 978-952-361-030-9, [2021-10-05]. https:// julkaisut.valtioneuvosto.fi/handle/10024/161796 [4] royal institution of chartered surveyors (rics). whole life carbon assessment for the built environment, london, rics, 2017. 41 p. isbn 978-1-78321-208-8. [5] bmub. bnb: büround verwaltungsgebäude komplettmodernisierung, 2017. [2021-10-05]. https://www.bnb-nachhaltigesbauen.de [6] d. anink. bepalingsmethode milieuprestatie verbouw en transformatie, 2020. [2021-10-05]. https://milieudatabase.nl [7] alliance hqe-gbc. renovation lca. practical guide, 2020. [2021-10-05]. http://www.hqegbc.org [8] sia. sia 2032: graue energie: ökobilanzierung für die erstellung von gebäuden, zürich, sia, 2020. 44 p. [9] f. n. rasmussen, m. birkved, h. birgisdóttir. low-carbon design strategies for new residential buildings – lessons from architectural practice. architectural engineering and design management 16(5):374–390, 2020. https://doi.org/10.1080/17452007.2020.1747385 [10] n. dodd, s. donatello, m. cordella. level(s) indicator 1.2: life cycle global warming potential (gwp), 2021. 37 p., [2021-10-05]. https://susproc. jrc.ec.europa.eu/product-bureau//sites/default/ files/2021-01/um3_indicator_1.2_v1.1_37pp.pdf [11] boverket. regulation on climate declarations for buildings. proposal for a roadmap and limit values. 154 p. isbn 978-91-7563-731-0, [2021-10-05]. https://www.boverket.se/globalassets/ publikationer/dokument/2020/ regulation-on-climate-declarations-for-buildings. pdf [12] m. alig, r. frischknecht, l. krebs, et al. lca of climate friendly construction materials, 2020. uster, treeze ltd., [2021-10-05]. https://treeze.ch/ fileadmin/user_upload/downloads/publications/ case_studies/building_and_construction/670_lca_ constructionmaterials_1.5c_v1.4.pdf 37 https://annex72.iea-ebc.org https://annex72.iea-ebc.org https://julkaisut.valtioneuvosto.fi/handle/10024/161796 https://julkaisut.valtioneuvosto.fi/handle/10024/161796 https://www.bnb-nachhaltigesbauen.de https://milieudatabase.nl http://www.hqegbc.org https://doi.org/10.1080/17452007.2020.1747385 https://susproc.jrc.ec.europa.eu/product-bureau//sites/default/files/2021-01/um3_indicator_1.2_v1.1_37pp.pdf https://susproc.jrc.ec.europa.eu/product-bureau//sites/default/files/2021-01/um3_indicator_1.2_v1.1_37pp.pdf https://susproc.jrc.ec.europa.eu/product-bureau//sites/default/files/2021-01/um3_indicator_1.2_v1.1_37pp.pdf https://www.boverket.se/globalassets/publikationer/dokument/2020/regulation-on-climate-declarations-for-buildings.pdf https://www.boverket.se/globalassets/publikationer/dokument/2020/regulation-on-climate-declarations-for-buildings.pdf https://www.boverket.se/globalassets/publikationer/dokument/2020/regulation-on-climate-declarations-for-buildings.pdf https://www.boverket.se/globalassets/publikationer/dokument/2020/regulation-on-climate-declarations-for-buildings.pdf https://treeze.ch/fileadmin/user_upload/downloads/publications/case_studies/building_and_construction/670_lca_constructionmaterials_1.5c_v1.4.pdf https://treeze.ch/fileadmin/user_upload/downloads/publications/case_studies/building_and_construction/670_lca_constructionmaterials_1.5c_v1.4.pdf https://treeze.ch/fileadmin/user_upload/downloads/publications/case_studies/building_and_construction/670_lca_constructionmaterials_1.5c_v1.4.pdf https://treeze.ch/fileadmin/user_upload/downloads/publications/case_studies/building_and_construction/670_lca_constructionmaterials_1.5c_v1.4.pdf acta polytechnica ctu proceedings 38:31–37, 2022 1 introduction 2 options for modelling ``refurbishment'' 2.1 refurbishment as part of a first or start of a next reference study period 2.2 reflections in existing standards 2.3 reflections in existing national methods 3 typology of options in modelling of refurbishment activities 3.1 treatment of initial structure further used 3.1.1 approach ``zero'' 3.1.2 approach “remaining load” 3.1.3 approach ``fictive replacement of components'' 3.2 treatment of removed components 3.3 complete overview of differences in approaches 4 discussion and recommendations 5 conclusions acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.38.0116 acta polytechnica ctu proceedings 38:116–123, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague integrated sustainability assessment using bim maria teresa henriques alves ferreiraa,∗, antónio aguiar costab, josé dinis silvestrea a university of lisbon, instituto superior técnico, civil engineering, ceris, av. rovisco pais 1, 1049-001, lisbon, portugal b built colab, collaborative laboratory for the digital built environment, rua de álvares cabral 306, 4050-041 oporto, portugal ∗ corresponding author: teresaferreira@tecnico.ulisboa.pt abstract. the construction industry is responsible for 40 % of the energy consumption and 36 % of the co2 emissions, and buildings are responsible for a significant part of energy consumption in europe. thus, a growing concern regarding environmental impacts in the construction sector is in place. reducing these impacts and optimise the design process is a major priority, and technology needs to be integrated along with the design to allow for better buildings performance. building information modelling (bim) methodology is one of the technologies that is revolutionising how the supply chain delivers the construction projects, allowing for an overview of the whole life cycle, keeping track of the data along the process, and potentiating more advanced simulations and supported decisions. the tool proposed in this paper aims to integrate different types of sustainability analysis, namely streamlined life cycle assessment (lca), carbon footprint, life cycle cost (lcc) and level(s) framework with bim. this involves defining adequate product data templates (pdt) and a database structure for bim objects, including the necessary parameters to enable designers to do holistic and dynamic assessments from early design stages to a complete lca. also, considering the importance of using bim to visualise different scenarios, a graphical interface will be developed to show the key sustainability indicators and support decision-making for more sustainable buildings. the results achieved show that technology must be taken to meet climate most ambition targets and reduce the impact of construction. keywords: building information modelling, sustainability, technology, life cycle assessment, life cycle cost. 1. introduction in line with the most ambitious targets established in paris agreement in 2015, eu green deal has an ambition of reducing carbon emissions by 55 % or more until 2030 and for europe to be a climate-neutral continent by 2050 [1, 2]. construction and retrofit of buildings cause substantial environmental impacts [3] due to their significant consumption of energy (40 %) and materials and energy-related greenhouse gas emissions (36 %) [4]. the application of environmental (lca, including global warming potential impact category) and economic (or life cycle cost – lcc) life cycle assessment (lca) [5] to construction are methodologies that: are recognized and standardized at the european level [6, 7], are becoming more and more important, and which are being increasingly used in this sector by experts and researchers. the life cycle (lc) paradigm emerges, and construction materials and buildings now represent the sum of all impacts and costs in the respective lc (no longer seen as individual impact and cost). the construction industry plays a major role in the decarbonization process [3]. however, the lack of interoperability between eu targets for sustainability assessment and supply chain tools is a major issue in archive such important goals on climate issues. in figure 1 is shown a diagram of circular economy principles as a circle of interaction covering all lc stages of its analysis. applying circular economy principles (figure 1), current research aims to use sustainability indicators established in level(s) framework in a new construction case study. this is made by using a bim environment to complete both economic and environmental assessment analyses. circular economy principles and bim (figure 2) have a common ground in their full lc approach, and this is seen by the authors as a huge advantage that should be merged contributing to digitalization and circularity development. level(s) is a common framework for sustainable buildings assessment across europe, that has a whole lc approach. it considers measurement and improvement from design to end-of-life, covering both renovation and new construction [8]. level(s) framework provides a common methodol116 https://doi.org/10.14311/app.2022.38.0116 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 38/2022 integrated sustainability assessment using bim figure 1. circular economy principles. ogy for assessing the sustainability of buildings based on six macro-objectives. it contributes therefore to achieving eu and member states policy goals in energy, material use and waste, water, and indoor air quality in an lc perspective [8]. intending to bring buildings into the circular economy, level(s) comprises a set of indicators, scenario tools, a data collection tool, checklists, and rating systems that allow professionals and project actors to measure buildings’ performance [9]. the aim of this research is to aggregate green ambitions with technology by using building information modelling (bim) software to perform building environmental impacts and help supply chains to make their decisions through the full lc of the building, covering from early design stages decisions to the end of life. this was done by developing an lca plugin for revit software to perform lca and lcc calculations, the plugin creates a set of parameters, each one representing an environmental or economic indicator, and does the calculation using the information and quantities from the bim model. 2. materials and methods in order to answer research questions and meet the defined goals, the plugin proposed integrates with bim different types of sustainability analysis in its functionalities, namely streamlined lca, carbon footprint, and lca in level(s) framework. economic analysis is also important and the parameter to measure lcc is also integrated into the plugin. to do so, it was necessary to develop an adequate product data template (pdt) for bim objects and construction elements, including the necessary parameters to enable designers to do holistic and dynamic assessments from early design stages to a complete lca at the end of the design process. the objective is to give the user a set of options to perform environmental and economic calculations, nowadays there are different ways to archive so, the aim is to give the user different functionalities that correspond to different ways to figure 2. bim – full life cycle approach. get the lca results for the user to choose the best one that fits its project objectives. to perform an lca based on level(s) framework analysis, the corresponding indicators were studied to find which ones can be read within a bim environment, in bim object or elements, and possibly converted into parameters of analysis in the scope of lca studies. from the six level(s) macro objectives, the ones necessary for an lca analysis were selected: objective 1 – greenhouse gas emissions along building life cycle, covering lc global warming potential; and objective 6, optimized lcc and value. from the analysis of these macro-objectives, the following indicator was considered to do level(s) lca: global warming potential (gwp) measuring the co2 emissions. to go further on the cradle to grave lca analysis as recommended by eu [10] on the standard en15804:2012+a2:2019 [11], the following indicators were also addressed: abiotic depletion potential for fossil fuels (adpe), abiotic depletion potential of materials (adpm), acidification potential (ap), eutrophication potential (ep), ozone depletion potential (odp), photochemical ozone creation potential (pocp), water (user) deprivation potential, deprivation-weighted water consumption (wdp). to reach macro-objective 6 optimized life cycle cost and value indicator, product and construction stages, use stage, and end of life stage, containing parameters for initial costs, annual costs, and periodic costs, were considered to measure lcc. 2.1. plugin functionalities mentioned indicators are read in the construction bim elements/objects and can be analyzed directly within a bim software. the bim-based environment chosen to apply the plugin was autodesk revit. plugin functionalities were divided into four main topics, following the areas of research; (i) streamlined lca; (ii) carbon footprint; (iii) level(s) lca; (iv) cost. 117 m. t. h. a. ferreira, a. aguiar costa, j. d. silvestre acta polytechnica ctu proceedings figure 3. plugin functionalities with indicators. in figure 3 is possible to find core indicators per category of analysis. those indicators will be converted to parameters to be read by revit shared parameters. 2.2. parametrization and product data template (pdt) the definition of an adequate pdt [12] structure for bim objects was necessary to correctly convert indicators to parameters to be read in a bim environment. table 1 shows the proposed pdt including the new shared parameters with the objective of allocating the data need to perform the lca calculations and analysis. 2.3. framework for bim lca lcc analysis tables 2–4 show lc stages for production, construction, use and end of life from a to d modules. the analytical calculation of the environmental impacts of the project on the production and construction phase (a1–a5 modules) is presented in equation (1) [13]: streamlined lca a1–a3︷ ︸︸ ︷ eimc x = i∑ a=1 (qm a × eim a ) + complete lca︷ ︸︸ ︷ a4︷ ︸︸ ︷ j∑ b=1 (dv b × n × eiv b ) + a5︷ ︸︸ ︷ k∑ c=1 (qcon c × eicon c ) + l × [a1 − a3], (1) eimc x environmental impact of category x resulting from the manufacturing and construction phase (a1–a5 modules); i, j, k number of existing materials i, transportation j, and construction utilities k; qm a quantity of material a; eim a environmental impact (of category x) of material a; dv b distance from the supplier to the construction site, multiplied by the number of travels (n); eiv b environmental impact (of category x) of transportation b; qcon c consumption of utility c throughout the construction (e.g., electricity [kwh], gas [mj], water [m3]); eicon c environmental impact (of category x) of utility c; l percentage of wasted materials during the construction phase. the analytic calculation of the economic impacts of the project, on the production and construction phase 118 vol. 38/2022 integrated sustainability assessment using bim indicator/ parameter name streamlined lca complete lca level(s) lca carbon footprint level(s) lcc bim objects/ elements project details units adpe x x x x mj adpm x x x x kg sb eq pe-re x x x mj pe-nre x x x mj ap x x x x kg so2 eq ep x x x x kg po3− 4 eq odp x x x x kg r-11 eq pocp x x x x kg c2h4 wdp x x m3 world eq deprived gwp x x x x x kg co2 eq lcc-ic x x €/m2/yr lcc-ac x x €/m2/yr lcc-pc x x €/m2/yr lcc-eol x x €/m2/yr durability x x yr density x x ρ = m/v building’s service life x x yr transportation type x x transportation distance x x km utilities used x x x waste generated x x x table 1. proposed pdt. raw material supply transport manufacturing construction installation production a1 a2 a3 construction a4 a5 table 2. lc stages a modules/production & construction stages. use maintenance repair replacement refurbishment operational operational energy use water use use stage b1 b2 b3 b4 b5 b6 b7 table 3. lc stages b modules/use stage. deconstruction, demolition transport waste processing disposal reuse, recycling, or energy recovery potentials end of life stage c1 c2 c3 c4 d table 4. lc stages c/d modules / end of life. 119 m. t. h. a. ferreira, a. aguiar costa, j. d. silvestre acta polytechnica ctu proceedings (a1–a5 modules) is presented in equation (2): streamlined lcc a1–a3︷ ︸︸ ︷ cmc = i∑ a=1 (qm a × acm a ) + complete lcc︷ ︸︸ ︷ a5︷ ︸︸ ︷ j∑ b=1 (qm b × cc2b b ) + k∑ c=1 (qcon c × ccon c ) + l × [a1 − a3], (2) cmc costs resulting from the manufacturing and construction phase (a1–a5 modules); i, j, k number of existing materials i, j and construction utilities k; qm a , qm b quantity of materials a or b used in the construction; acm a the acquisition cost of material a; cc2b b cost to build/assemble construction elements (e.g., cost to apply 1 m2 of mortar); qcon c consumption of utility c throughout the construction; ccon c cost of utility c; l percentage of wasted materials during the construction phase. 3. results in this article was developed an innovative approach for a level(s) lca methodology analysis and its integration into a bim plugin. in this way, designers are enabled to perform a holistic and dynamic assessment from early design stages to a complete lca, and rapidly visualize different scenarios, using core sustainability indicators. the innovative nature of the approach of the current plugin is that level(s) lca framework is integrated into bim and gives the users the possibility to choose which type of analysis they will want to perform, including streamlined lca, carbon footprint, level(s) lca and cost (lcc) (figure 4). also, considering the importance of using bim to visualize different scenarios, a graphical interface was developed to show the key sustainability indicators and support decision-making for more sustainable buildings. 3.1. buildings assessment information an example of environmental information read in revit regarding exterior double brick walls with insulation is illustrated in figure 5. it is possible to see the shared parameters for the lca analysis inserted in revit. figure 7 explains the workflow of the lca plugin, per analysis, and shows the connections, once one analysis is finalized, to go further/ deeper in the lca analysis. using the case study of a new family swelling, the authors ran the plugin for exterior walls. according to the workflow in figure 7, the first step is to choose the stage of the project development, either conceptual design (level 1) or detail design (level 2) stages. if level 2 is chosen, then the user is asked which functionality of the plugin he would like to perform (figure 4). if he chooses, for example, level(s) lca, the shared parameters corresponding to the environmental indicator of analysis, table 1, are loaded into the bim model and the data from the database is loaded to the project to be assembled to the correspondent elements, for example, a wall, figure 5. the environmental data is stored in the properties of the bim family and read also in the plugin. the next step is to perform the corresponding calculations, which are done automatically by the plugin following analytic models presented in section 2.3; for this example equation (1) is being used. results can be exported both to excel and 3d visualization with a colour scheme, as presented in figure 6 for the exterior walls sample. 4. discussion the potential of disruptive technologies, such as building information modelling (bim), changes how the assessment of its sustainability is conducted [14]. lc existing tools/platforms for bim available in the market that integrate sustainability analysis for building design were already analyzed by the authors, i.e. tally and one click lca. from that analysis, several gaps, lack of interoperability between tools, limited parameters of analysis, limited databases, and lack of bim libraries were found. regarding the digitalization of construction, the development of the proposed methodology and tool is fundamental to supporting the performance-driven architectural design of buildings [15]. following the results presented, the authors believe that a user-friendly tool to make simulations of different sustainability analyses helps designers and supply chains to make choices with lower environmental impact. the time and effort of level(s) lca are reduced, and a graphic user interface would provide the decision-making support for the design of sustainable buildings. for future research, the idea is to build a plugin that is flexible enough to address new indicators and analysis in its functionalities and also remove the 120 vol. 38/2022 integrated sustainability assessment using bim figure 4. plugin functionalities options. figure 5. environmental impacts incorporated in shared parameters of a wall in revit. figure 6. level(gwp) analysis complete 3d results for a wall. 121 m. t. h. a. ferreira, a. aguiar costa, j. d. silvestre acta polytechnica ctu proceedings figure 7. workflow scheme of the lca plugin. ones that with the following developments will be made redundant. it is also possible to expand the proposed methodology to assess other methods such as leed, bream, wells, or others but some of them have methods of analysis not quantitative and that is a limitation to where this plugin can automatically grow and specified engineering consultants is required to fulfill all the requirements of specified schemes. 5. conclusion the application of bim can simplify level(s) implementation by systematizing the approach and accelerating access to data. the integration of different types of sustainability analysis, namely streamlined lca, carbon footprint, lcc, lca/ level(s) framework, and lcc with bim enable designers and the construction industry to easily integrate and see the environmental impacts of their buildings, and possible study better alternatives and actively collaborate to reduce the impact of the construction industry on the important climate change issues. acknowledgements the authors acknowledge the civil engineering research and innovation for sustainability (ceris) research unit, funded by fundação para a ciência e a tecnologia (fct) and the scholarships attributed to the first author with the references ui/bd/153398/2022. the authors would also like to thank the support through the circular ecobim project, funded by eea grants within the environment programme. finally, the authors would like to demonstrate their gratitude to the design office “atelier dos remédios” and prof. dr. francisco t. bastos for providing a case study. references [1] ec. communication from the european commission: a renovation wave for europe – greening our buildings, creating jobs, improving lives, 2020. com(2020)662. brussels, belgium. [2] ec. renovation wave, 2021. [2021-03-11]. https://ec.europa.eu/energy/topics/energyefficiency/energy-efficientbuildings/renovation-wave_en [3] ec. communication from the european commission: the european green deal, 2019. com (2019) 640, brussels, belgium. [4] ec. energy efficiency – revision of the energy performance of buildings directive, 2010. [2020-03-11]. https://ec.europa.eu/info/law/betterregulation/have-your-say/initiatives/12910revision-of-the-energy-performance-ofbuildings-directive-2010-31-eu [5] r. santos, a. a. costa, j. d. silvestre, et al. bim-based life cycle assessment and life cycle costing of an office building in western europe. building and environment 169:106568, 2020. https://doi.org/10.1016/j.buildenv.2019.106568 122 https://ec.europa.eu/energy/topics/energy-efficiency/energy-efficient-buildings/renovation-wave_en https://ec.europa.eu/energy/topics/energy-efficiency/energy-efficient-buildings/renovation-wave_en https://ec.europa.eu/energy/topics/energy-efficiency/energy-efficient-buildings/renovation-wave_en https://ec.europa.eu/info/law/better-regulation/have-your-say/initiatives/12910-revision-of-the-energy-performance-of-buildings-directive-2010-31-eu https://ec.europa.eu/info/law/better-regulation/have-your-say/initiatives/12910-revision-of-the-energy-performance-of-buildings-directive-2010-31-eu https://ec.europa.eu/info/law/better-regulation/have-your-say/initiatives/12910-revision-of-the-energy-performance-of-buildings-directive-2010-31-eu https://ec.europa.eu/info/law/better-regulation/have-your-say/initiatives/12910-revision-of-the-energy-performance-of-buildings-directive-2010-31-eu https://doi.org/10.1016/j.buildenv.2019.106568 vol. 38/2022 integrated sustainability assessment using bim [6] cen. sustainability of construction works – assessment of buildings part 2: framework for the assessment of environmental performance. en 15643-2., 2011. comité européen de normalisation, brussels, belgium. [7] cen. sustainability of construction works – assessment of buildings part 4: framework for the assessment of economic performance. en 15643-4, 2012. comité européen de normalisation, brussels, belgium. [8] ec. level(s), putting whole life carbon into practice. publications office of the european union, 2021. https://doi.org/10.2779/79139 [9] ec. level(s), putting circularity into practice. publications office of the european union, 2021. https://doi.org/10.2779/19010 [10] n. dodd, s. donatello, m. cordella. level(s) indicator 1.2: life cycle global warming potential user manual: overview, instructions and guidance (publication version 1.0), 2020. [11] en15804:2012+a2:2019, sustainability of construction works – environmental product declarations – core rules for the product category of construction products. [12] m. signorini, s. frigeni. development of a bim-based product data template for sustainability. master’s thesis, politecnico di milano, arc i – scuola di architettura urbanistica ingegneria delle costruzioni, 2018. http://hdl.handle.net/10589/143041 [13] r. santos, a. a. costa, j. d. silvestre, l. pyl. integration of lca and lcc analysis within a bim-based environment. automation in construction 103:127–149, 2019. https://doi.org/10.1016/j.autcon.2019.02.011 [14] r. santos, a. a. costa, j. d. silvestre, l. pyl. informetric analysis and review of literature on the role of bim in sustainable construction. automation in construction 103:221–234, 2019. https://doi.org/10.1016/j.autcon.2019.02.022 [15] x. shi, w. yang. performance-driven architectural design and optimization technique from a perspective of architects. automation in construction 32:125–135, 2013. https://doi.org/10.1016/j.autcon.2013.01.015 123 https://doi.org/10.2779/79139 https://doi.org/10.2779/19010 http://hdl.handle.net/10589/143041 https://doi.org/10.1016/j.autcon.2019.02.011 https://doi.org/10.1016/j.autcon.2019.02.022 https://doi.org/10.1016/j.autcon.2013.01.015 acta polytechnica ctu proceedings 38:116–123, 2022 1 introduction 2 materials and methods 2.1 plugin functionalities 2.2 parametrization and product data template (pdt) 2.3 framework for bim lca lcc analysis 3 results 3.1 buildings assessment information 4 discussion 5 conclusion acknowledgements references https://doi.org/10.14311/app.2022.33.0105 acta polytechnica ctu proceedings 33:105–111, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague rc deck stiffened arch existing bridges: simulated design and structural analysis giovanni criscia,∗, francesca ceronia, gian piero lignolab, andrea protab a university of naples parthenope, department of engineering, centro direzionale c4, 80143 naples, italy b university of naples federico ii, department of structure for engineering and architecture, via claudio 21, 80125 napoli, italy ∗ corresponding author: giovanni.crisci@uniparthenope.it abstract. the 20th century is known as the age that gave birth to the largest reinforced concrete structures. many applications of this new material were realized at that time, both from a theoretical and practical point of view. with reference to bridges, the engineer robert maillart achieved a new concept of arched bridges, characterized by very stiff deck beams and slender and wide vaults, i.e., the "deck-stiffened arch". the paper deals with the study of such bridge typology, particularly widespread in italy around the 50s of the 20th century. while, nowadays, calculation tools allow developing very refined structural modelling, in the past very simple structural schemes were adopted in the design phase in order to simplify the calculation effort. the study starts from a "simulated design" of such a bridge typology adopting a reliable geometry and following the design rules and the simplified structural schemes of the time and, then, by means of a refined three-dimensional model, the performance of a typical "maillart-type arch" bridge is analysed. keywords: maillart-type arch bridges, reinforced concrete arch bridges, structural analysis. 1. introduction the arch bridges are one of the oldest type of structures and are still today widely used to cross deep ravines in mountain areas. reinforced concrete (rc) arch bridges represent a suitable alternative to the classic "girder bridges", both on the aesthetical and functional points of view. the arch is, indeed, able to optimize the compressive strength of concrete or masonry materials by transferring the gravitational loads through a distribution of axial compressive forces. due to the development of new building techniques and high performance materials, the diffusion of rc arch bridges increased considerably. rc arch bridges can have different forms such as "tied arch bridges", "deck arch bridges" and "through-arch bridges". generally, the "deck arch bridge" is the most widely preferred typology because the arch sustains the gravity loads by means of compressive stresses. the key components of an rc deck arch bridge are: the deck beam (i.e., the superstructure), the piers (i.e., the piers that connect the deck to the arch), and the arch. within the "deck arch bridge" typology, the paper focuses the attention on the "deck-stiffened arch bridges", also known as "maillart-type arch bridges", aăparticularly widespread typology in italy, built between the 40s and the 60s of 20th century. in large span bridges, the contribution of the deck beam to the bearing capacity of the whole structure is completely ignored, but it is assumed that the deck beams only bear and transfer gravitational and accidental loads over the arch. on the other hand, in medium span bridges, the contribution of the deck beam, as long as it is continuous, is considerable and it would be uneconomical not to take its contribution into account. the "maillart-type arch" bridges (see figure 1a and 1b) are an example of the so-called collaborative deck structures, in which the deck is not only a simple element for transferring the loads, but it contributes to the bearing capacity of the whole structure. for typical rc arch bridges, the inertia of the deck beam is very small in comparison with the inertia of the arch cross section and, thus, the arch bears most bending moments (and shear). the swiss engineer robert maillart, expert of bridges and rc structures, at the beginning of 20th century, reversed this idea, since he conceived a new bridge typology, which took his name, made of a slender and wide vault, characterized by a low moment of inertia, and a very stiff deck beam. the two elements were connected by means of slender piers, often wall-like and lightened by central windows. the inversion of the classic ratio of arch and deck stiffness was a very important expedient, since it led to the significant reduction of the bending (and consequently shear) forces in the arch, which worked mainly in compression (i.e. a funicular arch). this has the advantage of simplifying the scaffoldings for the construction of the arch, which was usually built in deep ravines and ended with a structural scheme that is an inverted suspended bridge. 105 https://doi.org/10.14311/app.2022.33.0105 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en g. crisci, f. ceroni, g. p. lignola, a. prota acta polytechnica ctu proceedings � d�� � e�� figure 1. example of maillart-type arch bridge. several viaducts operating in italy and inserted in the national highway or motorway network have now reached the so-called "service life". surely these structures have been designed and built with a very different philosophy from the current design principles and with different design actions. recent collapses, such as the one involving the polcevera viaduct in italy, have highlighted the need of a good balance between social, economic, and environmental evaluations in order to attain a sustainable development. for bridges, sustainability can be interpreted as the resilience that the structure has to offer against the ever increasing environmental and loading conditions. thus, for a suitable sustainable approach, it is necessary to understand the structural concept that governed the design of the bridges in order to catch their resilience over the time. within the rc deck arch bridge, in this paper, the attention is focussed on the "maillart-type arch" typology. firstly, the design methodology of the time for such a bridge typology was investigated, then a "simulated" design of a reliable geometry was carried out according to simplified models and assumptions in agreement with the methods of the time. the elements of the simulated bridge have been designed and verified according to the "allowable stress method (asm)" or "working stress method (wsm)" [1] and under the design loads of the time. lacking high computational capacity, the typically adopted design schemes at the time were easier than those currently adopted. two-dimensional models have been, indeed, adopted for the design of structural elements and some simplifications have been assumed about their constraints and working conditions. in turn, the real geometry of the case study has been implemented in a finite element (fe) software and, under the loading conditions of the time, the structural elements have been verified again according to the wsm. 2. design of structural elements for the case study 2.1. definition of geometry, materials and loading conditions a simple model of a "maillart-type arch" bridge, i.e. a rc deck arch bridge, was assumed according to the scheme of figure 2a as case study for the analysis presented in this paper. the bridge is made of a slender vault with a cross section having constant dimensions of 6m × 0.2ăm. the shape of the arch was designed in order to be funicular of the gravitational loads corresponding to the self-weight of the entire structure. this means that it is subjected to axial compression only, without any bending moment and shear force. to this aim, an iterative procedure was adopted where, taking into account the gravitational loads of the deck and the piers, a funicular arch was generated starting from the shape of a parabolic arch [2]. this was a common practice in the past, mainly in order to facilitate the construction of the bridge; if the vault was built first and, later, the entire superstructure was built on it, it is possible to use very light scaffolding for the vault during the hardening of concrete. in fact, after this phase, the arch alone is able to bear the loads for the next phases of the construction. the length of the bridge, at the imposts of the arch, is l = 60 m, while the rise is f = 24 m; the deck is 9 m wide (figure 2b) and it consists of one driveway with two lanes (3.5 m each) and with two lateral sidewalks 1 m wide. the structure of the deck is made of a regular grid of main and secondary rc beams (figure 2b) with rectangular cross sections with dimensions of 0.3 m × 1.2 m and 0.3 m × 0.6 m , respectively, and of a slab 0.2 m thick. a total of 11 triplets of piers with a constant 0.3 m × 0.5 m cross section connects the vault to the deck (see figure 2a). typical concrete and steel usually used for this type of structure in ’50s of 20th century were assumed. based on historical sources and on several consulted projects of the time, tor 60 steel was largely used: it was characterized by an average yield 106 vol. 33/2022 rc deck stiffened arch existing bridges d�� e�� /� ����p i� �� �� p ��p ��p ��p ��p ��p ��p ��p ��p ��p ��p 3� 3� 3� 3� 3� 3� 3� 3� 3� 3�� 3�� d �p �p �p ���p / 0dlq�ehdpv 6hfrqgdu\�ehdpv figure 2. a) longitudinal view of the deck arch bridge assumed as case study; b) plan view of the deck. point of about 600 mpa and was classified as semihard or hard steel. a characteristic cylindrical compressive strength fck = 30 mpa was adopted (i.e. about an average cubic strength rcm = 45.8 mpa), since semi-hard steel was usually adopted with more performant concretes. the structural elements of the bridge were designed replicating the common practice of the past. at that time, very simple structural schemes were adopted in comparison with current design approaches. for the purposes of this work, a simulated design was performed only for the beams, the piers and the arch. based on the design rules of that time and after the examination of old projects of rc deck arch bridges too [3], the typical design procedure was replicated in order to define the dimensions of elements and amount of steel reinforcement. after the category of the bridge is fixed depending on the road typology, the expected traffic load was defined. thus, the analysis of load combinations was carried out with reference to the mandatory codes of the time, in particular the royal decree of 1939 [4] and the circular n. 6018 on the loads on bridge structures of 1945 [5]. in particular, assuming a bridge of first category (high traffic roads), two different types of loading schemes were adopted: 1. two or more undefined trains of trucks weighing 12 tons (about 120 kn) side by side; 2. two compressor rollers weighing 18 tons each (about 180 kn) side by side. in addition, a compact crowd was expected on the sidewalks next to the driveway. the second load scheme was typically considered for local verifications, since it locally provides higher stresses (a concentrated force of 6 tons per wheel was, indeed, assumed). conversely, the design of the main structural elements i.e. piers, arch, and beams was carried out with reference to the first load scheme, which globally engages the entire structure. it is worth to note that in the following, the simulated design will be carried out according to a simplified model and will be, thus, focussed only on the beams, piers and arch as load carrying elements, while the slab of the deck will be considered only with its weight. 2.2. assessment of loading schemes for the main beams there are many simple structural schemes that can be used for a preliminary design. the approach adopted in this paper considers an equivalent two-dimensional structure, characterized by a continuous deck beam supported by the piers, a fully fixed arch at its imposts and axially rigid piers connected to the deck and the arch with hinges at both ends, as commonly adopted by the designers of that time. such an assumption yields to zero shear and bending actions on the piers, while the real constraints and the detailing allows for the development of some shear and bending actions; however usually this mismatch has negligible impact on the structural behaviour. moreover, in this phase, the main beams are considered rectangular instead that t shaped, hence neglecting the structural contribution of the slab, but with the height equal to the whole deck thickness. for the main beams, the assessment of maximum stress was carried out by placing the moving loads, given by the loading condition 1 [4, 5], in the most 107 g. crisci, f. ceroni, g. p. lignola, a. prota acta polytechnica ctu proceedings figure 3. influence line of bending moment and position of the moving loads able to maximize the positive bending moment. unfavourable position on the deck, which was found by means of the influence lines. figure 3 shows the influence lines of the bending moment for moving vertical loads in some cross sections of the deck (red lines). the envelope of the peaks (blue line in figure 3) provides the diagram of the maximum and minimum moments from which it is possible to identify the cross section and, therefore, the position of the moving loads able to maximize the bending moment. it was found that the most stressed section is at about 1/4 of the bridge span and the loads associated to its development should be applied on about one third of the bridge span. given the maximum moment (positive and negative), it has to be divided among the individual main beams of the deck. the division can follow different approaches: for the examined case, it was assumed that the load solidly affects all the elements of the deck so that each beam is burdened by the same percentage of load. it is worth to note that the maximum bending moment induced by the moving loads must be combined with the actions produced by the gravitational loads; to take into account the dynamic effects, the moving loads have been amplified by 35%, which is obtained by the following formulation: ϕ = 1 + 16 l + 40 (1) where l is the span of the main beams to be designed. with regard to the royal decree of 1939 [4], the design of steel reinforcement was made according to the "allowable stress method (asm)", also known in literature as "working stress method (wsm)" [1]. for the main beams, the steel bars have been designed with the simplified formula for rc cross section with single bars layer: as = m 0.9 d σy, k (2) where d is the effective height and σy, k is the steel working stress, which is established by means of a safety factor 2 with respect to the characteristic yielding stress and, thus, is 300 mpa, being σy, k = 600 mpa. moreover, for the main beams, a double steel bars layer has been placed, a common solution found in several old projects [3]. details of steel reinforcement for the main beams are reported in figure 5. 2.3. assessment of loading schemes for the arch, the piers and secondary beams the position of the moving loads giving the maximum bending moment on the deck does not yield the maximum horizontal thrust in the arch too. in fact, in order to maximize the horizontal thrust, h, and the compression force, n , in the arch as well, the moving loads must be applied on the whole deck. since the arch is designed to be funicular of self-weights and even under moving loads it remains funicular due to the high flexural stiffness of the deck (because bending moments in the arch remain negligible), the compression force in the arch can be calculated by knowing the value of the horizontal thrust h and inclination of cross section, α: n = h cos α (3) the piers were studied under the same load condition considered for the arch. it is worth to note that both the piers and the arch, under the simplified design assumption, were designed to carry compression force only. under such an assumption, the area of the steel bars was designed as 0.8% of the ideal concrete 108 vol. 33/2022 rc deck stiffened arch existing bridges / 0rylqj�/rdg 'hdg�/rdg'hdg�/rdg / 0rylqj�/rdg figure 4. different constraint conditions for the secondary beams. ��i���#�p ��i�� �i�� ����fp �� �� fp 6hfrqgdu\�ehdp ��i�� ��i�� ����fp �� �� fp 3lhu ��i�� ��i�� ����fp �� �� �f p ��i��0dlq�ehdp �� �� fp ����fp $ufk�9dxow ��i���#�p figure 5. details of cross section and steel reinforcement of different structural elements. area, i.e. the minimum area of concrete related to the maximum compressive force, n [4]: aci = n σc, c (4) where σc, c is the working stress for the concrete under compression which was assumed equal to 70% of σc, f . since the compressive cylinder strength of concrete was assumed as fck = 30 mpa (i.e. characteristic cubic compressive strength rck = 36 mpa), the working stresses under flexural action, σc, f , is: σc, f = 7.5 + rck − 22.5 9 = 9 mpa (5) and the maximum working stress in the concrete under compression forces is, thus, σc, c = 6.3 mpa. the secondary beams were considered appropriately fixed to the main beam. in particular, the constraint conditions at the ends have been assumed as ranging between a simple supported beam and a fully fixed beam, in order to maximize the positive and negative bending moments along the element (figure 4a). the secondary beams have been designed with reference to their tributary area to define the entity of the gravitational loads (uniformly distributed loads) and with reference to the worst condition for the moving loads, i.e. when the axis of the vehicle is just on the secondary beam (loading condition 2 according to [4, 5]). the steel reinforcement was designed according to eq. 2 too. figure 5 reports the details of cross sections and steel reinforcement for the arch, the piers and the beams. 3. analyses and structural verifications based on the bridge geometry discussed in the previous section, a three-dimensional model has been implemented in the sap2000 software [6]; two-node frame elements are used to model the beams of the deck and the piers of the bridge. the eccentricity of the piers with respect to the axis of the arch is taken into account with rigid offset and massless links. differently from the simplified schemes adopted in the simulated design, the piers are fixed at the ends and are not connected, thus, by hinges. figure 6. 3d fe model of the rc deck arch bridge assumed as case study. 109 g. crisci, f. ceroni, g. p. lignola, a. prota acta polytechnica ctu proceedings d�� e�� ��� ��� ��� ��� ��� : ru nl qj �6 wuh vv �f rq fu hw h� >0 3d @ 3lhuv $ufk 7kuhvkrog ydoxh�����03d � �� �� �� �� �� : ru nl qj �6 wuh vv �>0 3d @ 3lhuv $ufk 7kuhvkrog ydoxh�����03d figure 7. structural verifications of compression stresses in the arch and the piers: a) compressed concrete; b)ăcompressed steel reinforcement. d�� e�� � � � � � �� �� : ru nl qj �6 wuh vv �f rq fu hw h� >0 3d @ 3rv��0rp��0dlq�ehdpv 1hj��0rp��0dlq�ehdpv 3rv��0rp��6hf��ehdpv 1hj��0rp��6hf��%hdpv7kuhvkrog ydoxh���03d � �� ��� ��� ��� ��� : ru nl qj �6 wuh vv �>0 3d @ 3rv��0rp��0dlq�ehdpv 1hj��0rp��0dlq�ehdpv 3rv��0rp��6hf��ehdpv 1hj��0rp��6hf��%hdpv 7kuhvkrog ydoxh�����03d figure 8. structural verifications for bending moment in the beams: a) compressed concrete, b) steel reinforcement in tension. based on the fe model shown in figure 6, the simple rc deck arch bridge assumed as case study was analysed under the loading conditions prescribed by circular n. 6018 of 1945 [5]. in order to carry out a global analysis of the structure, the undefined trains of trucks weighing 12 tons were considered, i.e. the loading condition 1 provided by [5]. such a loading scheme was converted into a uniform distributed load by means of appropriate tables [5]. each main beam, in addition to its own weight and the loads relative to the pavement, was loaded by the same amount of moving loads, according to a common assumption particularly widespread in ’40s and ’50s years. the compact crowd, on the other hand, was considered only for the two main edge beams. the structural verifications, as done for the simulated design, have been performed according to the wsm [1]. in addition to the allowable normal stresses under bending actions for the concrete given by eq. 5, the allowable shear stresses, for a concrete with a good performance, were equal to: τc, 0 = 0.6 mpa, τc, 1 = 1.6 mpa (6) note that if τ < τc, 0, it is not necessary to design an adequate shear reinforcement and usually 3 or 4 stirrups per meter were adopted. if τc, 0 < τ < τc, 1 an adequate shear reinforcement is required; otherwise, it is necessary to change the rc cross section. for the steel, the same allowable stress of 300 mpa has been considered. according to the wsm, the structural verifications are satisfied when the stresses in all the cross sections of the element are lower than the allowable thresholds, assuming a linear elastic behaviour for materials and a homogenization coefficient n = 10 [4]. figure 7 summarizes the working stresses in compression in the concrete and in the steel reinforcement distributed along the horizontal axis for the cross sections of the arch and the piers under compression forces. the working stresses are significantly lower than the threshold values (6.3 mpa for concrete and 300 mpa for steel). moreover, the working stresses in the beams, both main and secondary ones freely distributed along the horizontal axis, under bending moment (positive and negative) and shear forces are shown in figure 8 and 9, respectively. figure 8 highlights that the stresses in every cross section due to bending moments are always lower than the allowable ones (9 mpa for concrete and 300 mpa for steel). moreover, figure 9 shows that in every cross section the shear stresses are always lower than τc, 0, meaning that it was not 110 vol. 33/2022 rc deck stiffened arch existing bridges ���� ���� ���� ���� ���� : ru nl qj �6 kh du �6 wuh vv �>0 3d @ 0dlq�ehdpv 6hfrqgdu\�ehdpv7kuhvkrog ydoxh�wc0 ����03d figure 9. structural verifications for shear in the beams.. necessary [4] to design a specific shear reinforcement, but it was sufficient to consider a minimum amount, i.e. 3 or 4 stirrups per meter. it is worth noting that the shear verification was carried out only for the beams since the arch, designed as funicular of the self-weight of the whole structure, and the piers, designed as pendulums, are expected to work mainly in compression. the fe model in sap2000 has, indeed, highlighted the presence of bending moments and shear actions on the arch as well, although of very small entity if compared to the dominant axial compression load. although starting from a very simple structural scheme, it seems that the actions on the structural elements calculated with a more refined fe model are consistent with the simplified approaches used in the past. it is worth to remember that the wsm entails the designer to check that the maximum working stresses in the structural elements under the service loads do not exceed the allowable stress. therefore, although some values are close to the reference thresholds, it can be reasonably supposed that they are however far from the effective failure values. future analyses will be, indeed, focussed on the verification of the same case study under the actions provided by current italian building code and adopting the current methodology, i.e. the limit states approach. 4. conclusions several rc deck arch bridges were built around the 1950s in italy. the present work analyses the simulated design of a rc "maillart" arch bridge, in particular, replicating a typical approach adopted in the past. starting from simple design schemes, a simulated design was performed accounting for the load conditions required by the past codes. the response of the case study was, then, examined by a 3d finite element model under the same load conditions adopted in the simulated design phase, where real shape and constraints have been simulated. the structure was verified according to the working stress method (wsm), typical of the 1950s. all the verifications resulted adequate. further studies will investigate several modelling strategies and will concern the analysis of the case study under the actions provided by new italian building code and performing the verifications according to the current verification methodology, i.e. the limit states approach. references [1] w. lin, t. yoda. bridge engineering classifications design loading and analysis methods, butterworth heinemann, 2017. :https: //doi.org/10.1016/b978-0-12-804432-2.00001-3. [2] v. franciosi. il ponte ad arco (liguori editore), 1994. [3] w. d. ponti. italiani in cemento armato. santarella and miozzi (book review). town planning review 15(3), 1933. https: //doi.org/10.3828/tpr.15.3.hv41614521888l41. [4] regio decreto 16/11/1939 n. 2229 norme per la esecuzione delle opere in conglomerato cementizio semplice ed armato (in italian), 1939. https://www.edilportale.com/normativa/regio-dec reto/1939/2229/norme-per-la-esecuzione-delle-o pere-in-conglomerato-cementizio-semplice-od-a rmato_15060.html. [5] ministero dei lavori pubblici: direzione generale dei servizi tecnici. circolare 9/06/1945 no. 6018 carichi da considerare nel calcolo di ponti per strade ordinarie (ministero dei lavori pubblici direzione generale dei servizi tecnici) (in italian), 1945. [6] computer & structures inc. (csi) sap2000 integrated software for structural analysis and design computers and structures inc., berkeley, california, 2013. https://www.scirp.org/(s(351jmbntvnsjt1aad kposzje))/reference/referencespapers.aspx?refe renceid=961255. 111 https://doi.org/10.1016/b978-0-12-804432-2.00001-3 https://doi.org/10.3828/tpr.15.3.hv41614521888l41 https://www.edilportale.com/normativa/regio-decreto/1939/2229/norme-per-la-esecuzione-delle-opere-in-conglomerato-cementizio-semplice-od-armato_15060.html https://www.scirp.org/(s(351jmbntvnsjt1aadkposzje))/reference/referencespapers.aspx?referenceid=961255 my title https://doi.org/10.14311/app.2022.36.0198 acta polytechnica ctu proceedings 36:198–205, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague full probabilistic design of a submerged floating tunnel and format for partial safety factors franka e. m. swaalfa,b,∗, raphaël d. j. m. steenbergenb,c a delft university of technology, faculty of civil engineering and geosciences, stevinweg 1, 2628 cn delft, the netherlands b the netherlands organization for applied scientific research tno, p.o. box 155, 2600 ad delft, the netherlands c gent university, department of structural engineering, technologiepark-zwijnaarde 60, 9052 gent, belgium ∗ corresponding author: lisa.swaalf@tno.nl abstract. a submerged floating tunnel (sft) can be a promising solution for crossing a deep or wide waterway. this innovative concept however lacks research into its probabilistic design. in this research, the reliability of the tether-stabilized sft is assessed. a first-order reliability method (form) and a monte carlo simulation (mcs) are performed for the limit state functions of the most important failure mechanisms. stochastic variables are chosen so that a target reliability index of 3.8 for a reference period of 50 years is met. the calculated factors from the full probabilistic design are compared with the general recommended partial factors for strength and resistance from eurocode en1990. for the strength mechanisms, the calculated factors are smaller than the factors from eurocode. however, for the equilibrium mechanism, the calculated factor for the unfavorable loading is larger than the factor from eurocode and should be increased by 10% in order to design a safe enough sft. keywords: eurocode en1990, first-order reliability method, submerged floating tunnel, monte carlo simulation, partial safety factors, tether-stabilized, tether slackening, tether yielding. 1. introduction a solution for crossing a deep and wide waterway can be a floating tunnel under the water level: a submerged floating tunnel (sft). this type of tunnel consists of an immersed concrete tube, either attached with anchor cables to the seabed or attached to pontoons floating on the water surface. these two variants can be seen in figure 1. the concept of an sft was patented in the united kingdom in 1886, and the first norwegian patent on the subject was issued in 1923. in the 1960’s a small group of norwegian experts started to evaluate the potentials of the sft concept. more recent, the government of norway made plans for the highway route e39, crossing many fjords, to be ferry-free. this goal could be achieved by constructing sfts crossing the fjords on this route [1]. in this paper, we focus on the reliability of the sft and the applicability of the recommended general partial safety factors from eurocode en1990 [2]. previous research focused mostly on the structural lay-out of the sft. few researchers have addressed the safety of the structure and no probabilistic design has been made before. the experience gained from other civil structures can only partly be used, since sfts differ from common civil structures. in general, sfts have different relevant limit states, different magnitude of consequences and costs involved with failure of relevant limit states, different accuracy of the models that predict the structural response and different load scenarios [3]. the forcing on underwater structures differs from the forcing on structures on land, since the equilibrium of the system is determined by the buoyancy-weight ratio. the general partial safety factors for strength and resistance from eurocode en1990 are calibrated on forces acting on buildings and bridges. it is questionable whether these factors can be applied to the sft [2]. the aim of this paper is to perform a full probabilistic calibration of partial factors for most relevant limit states of the sft. a level iv probabilistic analysis is not performed, since costs are difficult to determine and can vary significantly. accidental loading types are out of scope for the limit state functions, as well as the loads on the land-bored part of the tunnel and the end-joints. this paper is structured as follows: first, the methodology is presented (section 2), in which a typical sft structure is shown, a target β-value is set and the calculation of partial factors is explained. consequently, in section 3, the failure mechanisms are described and all stochastic variables are explained. in section 4, the resulting reliability indices and partial safety factors are discussed, followed by the overall conclusions (section 5) [4]1. 1this paper is based on the authors thesis, reliability analysis of submerged floating tunnels, 2020 [4] 198 https://doi.org/10.14311/app.2022.36.0198 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 36/2022 submerged floating tunnel design for safety figure 1. two variants of an sft figure 2. simple model of sft-element with loading types 2. methodology 2.1. procedure first, a typical sft structure is investigated (section 2.2). in order to assess the reliability of the sft, a decomposition of this system is made (section 2.3). the governing failure mechanisms are determined. consequently, the reliability requirements on element level are set. a target β-value is selected for a certain reference period and limit state functions are formulated (section 3). python’s pyre module and prob2b are used to perform a form and mc analysis. when the failure mechanisms are tuned to the target β-value, the partial factors can be calculated (section 4). [4] 2.2. typical sft-structure the design made for the bjørnafjorden in norway concerns the typical failure mechanisms of an sft structure, with a certain depth and size, and is therefore chosen as reference in this paper. the structure consists of two identical concrete tubes with steel reinforcement. between the two tubes, there are crossbeams and diagonals. the diameter of the tube is not the same along its length, since there are emergency lanes every 250 m. the tube elements are 200 m, and the total length of the sft is approximately 5400 m. the tube is stabilized by steel tethers attached to the seabed with drilled and grouted rock anchors. for this structure, typical loading types were defined, e.g. the permanent downward loads, the buoyancy force, the traffic loading and the wave-current loading (figure 2). for axial loading, the hydrostatic load is considered. the vortex-induced vibrations, temperature loading and creep and shrinkage turned out to be negligible. [4] 2.3. decomposition of the system the entire structure of the sft can be seen as one system. we need to make a decomposition of this system in order to assess the reliability on elementlevel, conform eurocode en1990 [2]. figure 3 shows a decomposition in multiple sft-sections, where each section can fail separately. one sft-element is considered here as one single tube. the framework in between the tubes is not 199 franka e. m. swaalf, raphaël d. j. m. steenbergen acta polytechnica ctu proceedings figure 3. fault tree for an sft figure 4. fault tree for an sft-element included in the calculations, since we only look at the most important force actions. the tube is attached to the bottom of the sea with tethers [1]. we assume four tethers per mooring for our model input. on element level, multiple relevant failure mechanisms can be defined, as can be seen in figure 4. from design experience, the following four mechanisms can be assumed as most important: yielding (1), slackening (2), failure of the tube in longitudinal direction (3) and shear failure of the tube in transverse direction (4). more about these mechanisms will be explained in section 3. compression in transverse direction was first assumed to be an important failure mechanism as well. however, it is proven that the compressive stress is small compared to the concrete resistance. within the scope and planning of this study, it has been assumed that the additional failure mechanisms of corrosion, fatigue, geotechnical failure and accidental failure are not governing due to their complexity and expected research time needed. for geotechnical failure extensive research into the geolocation should be performed [4]. 3. reliability of main failure mechanisms in this section, the four most important failure mechanisms are explained and their limit state functions are formulated. the input parameters are defined by their mean values and standard deviations, and can all be found in table 1. a β-value of 3.8 for a reference period of 50 years is chosen for the individual failure mechanisms, mainly since general partial factors from eurocode en1990 are also determined on a β-value of 3.8 with a reference period of 50 years [2]. looking at the fault trees in figure 4 and figure 3, we expect the system β-value to be lower than the element β-value. however, this subject is not further studied in this paper [4]. 200 vol. 36/2022 submerged floating tunnel design for safety 3.1. yielding of the tethers when the stress in the cross-section of the tether reaches the yield point, the steel begins to deform plastically. the yield point is the point where nonlinear (elastic and plastic) deformation begins. yielding of the tether can occur when the buoyancy force increases or the downward force decreases. this change of loading can take place due to erosion, temporary replacements, shortage of ballasting, or due to waves. the yield strength of steel and the cross-sectional area determine the strength of the tether. for this study, not all tethers need to be investigated separately because equal properties are assumed. both the models for resisting forces and load contain uncertainty, because the used mathematical models are not a full representation of reality. this is included in model factors for uncertainty (θr and θs). z = θr·fresistance,steel−θs ·(fbuoyancy+fwave−current − fconcrete − fballast − fasphalt− fequipment − fmarine) (1) the resistanceand strength parameters can be plotted in a joint probability density function (5, left). the failure point is the point where the line z = 0 and the contour plot of the function intersect (5, right). from this contour plot, the design points and α-values can be derived. for all four mechanisms, these plots have been assessed [4]. 3.2. slackening of the tethers slack means that there is no tension in the cable anymore, and the stiffness is zero. as a result of slackening, snap forces occur, which can lead to structural failure of the system. due to marine accumulation, extra ballasting, or leakage, slackening can occur. loosening of the anchorages can also occur due to waves, and often lasts for one to two seconds during each wave run. in order to prevent slack, an equilibrium should be maintained between upward and downward forces. in the governing situation for slackening, lift force acts as a downward force. the accompanying drag force has no vertical component, so this force is not included in the formulation [4]. z = θr·fbuoyancy+θs ·(−fwave−current−fconcrete− fballast −fasphalt −fequipment −ftraffic −fmarine) (2) 3.3. longitudinal failure of the tube in the longitudinal direction, the tube can deform due to tidal differences or excitation due to waves or currents. furthermore, sea level rise and extreme water levels can cause larger hydrostatic loads on the cross-section, which increases cross-sectional forces and moments. the point along the cross-section with the largest bending moment needs to be further investigated. for this point, two cases need to be investigated: • lower limit: the elastic moment with zero tension in cross-section • upper limit: the plastic moment capacity for an elastic stress state, all stress distributions are considered to be linear. the stress at the bottom of the cross-section can be calculated according to equation 3. σc,total = −next ac − np ac − np · e(x) w + mext(x) w (3) w = π · (d4 − d4) 32 · d (4) where: x = longitudinal coordinate [m] σc,total = total stress in the cross section [kn/m2] mext = applied bending moment [knm] next = normal force due to hydrostatic pressure [kn ] w = section modulus [m3] np = normal force due to prestressing [kn ] ac = concrete area of the tube [m2] e = assumed eccentricity of the post-tensioning [m] d = outer diameter [m] d = inner diameter [m] it is assumed that the resulting compressive stress in the concrete cross-section is within the acceptable limit. the cross-section should not be subjected to tensile stresses. the limit state function for this mechanism is stated in equation 5. z = θσr · ( np ac + np · e w ) −θσs · ( next ac + mext w ) (5) in reality, a larger external moment can be applied to the cross-section before it fails in terms of cracking and transmitting the load. the cross-section can be seen in figure 6. in any case, the structure fails when the moment exceeds the plastic moment capacity. for the maximum plastic moment capacity, it can be assumed that a third of the cross-section is yielding with an inner arm of 0.8·d [6]. mplastic,max = 1 3 · ac · fy · 0.8 · d (6) the plastic moment at this point is: mplastic = 1 16 · qtotal · l2 (7) the following limit state results in the upper limit for longitudinal failure: [4] z = mplastic,max − mplastic (8) 2norm= normal distribution, log= lognormal distribution, gum= gumbel distribution, det= deterministic value 201 franka e. m. swaalf, raphaël d. j. m. steenbergen acta polytechnica ctu proceedings figure 5. joint probability density function (left) and contour plot (right) for yielding parameter distr. 2 mean st.dev. cv source cross-section tether (at) [m2] norm 0.67 0.017 0.025 [1] yield strength s235 (fy) [n/mm2] log 285 20 0.07 [5] diameter of the tube (d) [m] norm 15.0 0.375 0.025 [1] thickness of the tube (t) [m] norm 0.85 0.021 0.025 [1] length of the tube (ltube) [m] det 150 [1] gravitational acceleration (g) [m/s2] det 9.81 unit weight of concrete (γc) [kn/m3] norm 24.5 1.7 0.07 [6] unit weight of water (γw) [kn/m3] norm 10.035 0.4 0.04 [1] drag coefficient (cd) [-] log 0.7 0.2 0.3 [7] lift coefficient (cl) [-] log 0.1 0.02 0.2 [7] wave-current velocity for 50 years (uc) [m/s] gum 1.5 0.15 0.1 [1] structural weight of asphalt (qasphalt) [kn/m] norm 28 2.8 0.1 [1] weight of permanent equipment (qequipment) [kn/m] norm 10 1 0.1 [1] weight of average solid ballast (qballast) [kn/m] norm 100 10 0.1 [1] traffic load for 50 years (qtraffic) [kn/m] gum 50 7.5 0.15 [8] marine load (qmarine) [kn/m] norm 10 2 0.2 [1] area of strands (astrand) [mm2] det 150 [9] initial strength of prestressing y1860 (fs) [n/mm2] log 1300 90 0.07 [5] concrete compressive strength c45/55 (fc) [n/mm2] log 53 8 0.15 [6] model uncertainty for shear capacity θ1 [-] log 1 0.2 0.2 [6] model uncertainty for resistance θr [-] log 1 0.05 0.05 [7] model uncertainty for load effects θs [-] log 1 0.05 0.05 [7] model uncertainty for the capacity of the cross-section θσr [-] log 1 0.05 0.05 [7] model uncertainty for stresses in the cross-section θσs [-] log 1 0.05 0.05 [7] table 1. input parameters for reliability analysis parameter yielding slackening longitudinal failure shear failure β 3.8 3.8 3.8-6.7 5 table 2. the β-values per failure mechanism 3.4. shear failure of the tube a large shear force can occur at the attachment points of the tethers to the tube, due to wave-current forcing. this can lead to shear failure. it turned out that the limit state for shear failure resulted in a β-value of 5. the β-value of shear failure could be tuned near 3.8 by changing the intermediate distance of the tethers. however, it is better to have longitudinal failure first, since this is a ductile mechanism. since the probability of failure due to shear is now significantly lower 202 vol. 36/2022 submerged floating tunnel design for safety figure 6. lay-out of reinforcement: normal reinforcement in green and eccentric reinforcement in red than failure due to one of the other mechanisms, this mechanism is not taken into account for the partial factor analysis [4]. 4. results and discussion 4.1. interpretation in terms of β’s and α’s the resulting β-values from the four failure mechanisms can be found in table 2. the β-value for yielding is a slight underestimation of the real β-value. this is caused by two factors: the traffic load was not taken into account (1) and the wave-current force was only taken as unfavorable loading (2). the β-value for slackening resulted in 3.8, which is also a slight underestimation, since the lift force was only taken as unfavorable loading. the β-value for longitudinal failure turned out to have a value between 3.8 (lower limit) and 6.7 (upper limit). the sensitivity factors (α) are presented in table 3. only the parameters with an α-value larger than 0.1 are shown. the diameter and thickness have two α-values, due to the uncorrelated and correlated calculation respectively. the diameter is dominant over the thickness, and the parameters serve as loading parameters for yielding and as resistance parameters for slacking. the α-value of d decreases significantly for the correlated case. the largest α-values are found for the concrete density, water density, yield strength of the steel tethers, the diameter of the tube and the model uncertainties. the variable loads, i.e. traffic load and wind-current load, have relatively low α-values. most α-values are smaller than either 0.8 (dominant resistance parameter) and −0.7 (dominant loading parameter) or 0.28 (other resistance parameters) and −0.32 (other loading parameters) from eurocode en1990 [2]. this means that the α-value multiplied with β-value results in a smaller distance between the design point and the mean value than eurocode en1990, which consequently leads to a smaller partial factor. in a form analysis, no distinction is made upfront between dominant and non-dominant parameters. furthermore, when more parameters are added, the lower the α-values become, because the sum of the α-values squared always adds up to one [4]. 4.2. interpretation in terms of γ’s for the assessment of partial factors, the parameters with α-values above 0.1 are chosen stochasts, and the rest of the values as deterministic parameters. consequently, the design points of the stochasts are calculated. parameters are divided into favorable load, unfavorable load and variable load. then, characteristic values according to eurocode en1990 were determined [2]. the partial factors of the full probabilistic design can be calculated by using this design value and characteristic value. eurocode en1990 distinguishes in types of calculations [2]. for a strength calculation, the strconditions can be used. these factors can thus be compared with the calculated factors from tether yielding and longitudinal tube failure. for an equilibrium calculation, the equ-conditions can be used. these factors can be used for slackening of the tethers. the factors according to the strand equ-conditions and the calculated partial factors, according to the probabilistic design, can be found in table 4. for the str-criteria, the favorable permanent load factor and the material resistance factor fit well. however, the general partial factors for the unfavorable permanent load and for the variable load seem to be very conservative. based on this case study, the general partial factors for variable loading can be decreased with at least 20 %. since the influence of variable loads on the structure is small, the decrease 203 franka e. m. swaalf, raphaël d. j. m. steenbergen acta polytechnica ctu proceedings parameter α yielding α slackening α longitudinal failure fy [n/mm2] 0.50 γw [kn/m3] -0.48 0.43 -0.52 γc [kn/m3] 0.47 -0.52 0.52 fs [n/mm2] 0.55 d [m] -0.46, -0.04 0.27, 0.11 -0.35, -0.11 t [m] 0.13, -0.34 -0.19, 0.08 0.14, -0.21 θr [-] 0.32 0.48 θs [-] -0.29 -0.48 θσr [-] 0.32 θσs [-] -0.32 table 3. the α-values from form analysis parameter str equ yielding slackening longitudinal failure resistance 1.15 1.07 1.08 unfavorable permanent load 1.35 1.1 1.23 1.19 1 favorable permanent load 1 0.9 0.98 0.82 0.86 unfavorable variable load 1.5 1.05 1.18 1 1.04 table 4. summary of the resulting partial factors from the models of this factor will not result in significant changes in the design. thus, the economic advantage is small. in contrast, a change in water density will have a large influence on the design. thus, decreasing the general partial factor for unfavorable permanent load will have a significant effect on the design. according to this case study, the factor of 1.35 could be decreased with almost 10 %. however, conclusions cannot be too firm, since this calculation was performed for one case only. for the equ-criteria, partial factors from eurocode en1990 are not safe to be applied. the partial factor for unfavorable permanent loading is insufficient. this factor should be increased by 10% in order to design a sft [4]. 5. conclusions this paper showed that the reliability requirements of the sft can be met in the design, and that the design can be optimized by a full probabilistic calibration of partial factors. the following conclusions were drawn: • the conventional reliability methods (i.e. form and mc) can be adopted in a reliability based design of an sft. permanent loads proved to be dominant, i.e. concrete and water density, and thus significantly influence the design of the sft. the same reliability methods can be applied to other sfts, but they might result in different designs based on geolocation specific circumstances. • a target reliability is required to perform the reliability analyses. the general partial factors from eurocode en1990 are based on a β-value of 3.8 for a reference period of 50 years [2]. therefore, in this research, a target β-value of 3.8 for 50 years was chosen as starting point for the individual failure mechanisms. design parameters were determined so that this target value was met. the actual βvalues were computed using simplifications that are by definition safe. on system level, it is expected that the β-value is lower than 3.8, but this is not studied further. • four important failure mechanisms for the sft were derived using a decomposition of the system. these important mechanisms are yielding of the tethers, slackening of the tethers, longitudinal failure and transverse shear failure of the tube. it has been assumed within the scope and planning of this study that the additional failure mechanisms of corrosion, fatigue, geotechnical failure and accidental failure are not governing due to their complexity and expected research time needed. for geotechnical failure extensive research into the geolocation should be performed. slackening proved to be the governing failure mechanism over the other three mechanisms. the resistance of slackening depends on the force equilibrium, whereas the resistance of the other mechanisms depends on structural strength. • the influence factors (α-values) from the form analysis indicated that the most dominant parameters were the concrete density, water density, yield strength of the steel tethers, the diameter of the tube and the model uncertainties. the resulting α-values from form do not only depend on the coefficient of variation, but also on the absolute value of the mean of the parameter. the permanent loading parameters turned out to be dominant, because their relative contribution to the total load is large. the variable loads, i.e. traffic load and wind-current load, have relatively low α-values. • the general partial factors from eurocode en1990 204 vol. 36/2022 submerged floating tunnel design for safety were never calibrated on sft-type of structures [2]. they seem nevertheless safe enough to apply for the strength (str) cases (table 4). the general partial factors for the unfavorable permanent load and for the variable load turned out to be very conservative. a decrease of 20% for the variable loading factor and 10% for the factor of unfavorable permanent loading is proposed. since the influence of variable loads on the structure is small, the decrease of this factor will not result in significant changes in the design. in contrast, a change in one of the permanent loading parameters will have a large influence on the design. thus, decreasing the factor for unfavorable permanent load will have a significant effect on the design and is economically attractive. • the equilibrium (equ) criteria from eurocode en1990 should be applicable to the slackening mechanism. however, the general partial factor for the unfavorable permanent loading turned out to be insufficient (table 4). this factor should be increased by 10% in order to be able to design a safe enough sft [4]. references [1] reinertsen, o. dr. techn. olsen, norconsult. bjørnafjord submerged floating tube bridge 2016. [2] en1990. eurocode 0: basis of structural design 2011. [3] m. baravalle, j. köhler. risk and reliability based calibration of design codes for submerged floating tunnels. procedia engineering 166:247–254, 2016. https://doi.org/10.1016/j.proeng.2016.11.547. [4] l. swaalf. reliability analysis of submerged floating tunnels 2020. [5] j. melcher, z. kala, m. holický, et al. design characteristics of structural steels based on statistical analysis of metallurgical products. journal of constructional steel research 60(3-5):795–808, 2004. https://doi.org/10.1016/s0143-974x(03)00144-5. [6] en1992. eurocode 2 part 1-1: design of concrete structures: general rules and rules for buildings 2005. [7] jcss. probabilistic model code. joint committee on structural safety 2001. [8] en1991-2. eurocode 1 part 2: traffic loads on bridges 2015. [9] tensabv. multi-strand post-tensioning system 2007. 205 https://doi.org/10.1016/j.proeng.2016.11.547 https://doi.org/10.1016/s0143-974x(03)00144-5 acta polytechnica ctu proceedings 36:198–205, 2022 1 introduction 2 methodology 2.1 procedure 2.2 typical sft-structure 2.3 decomposition of the system 3 reliability of main failure mechanisms 3.1 yielding of the tethers 3.2 slackening of the tethers 3.3 longitudinal failure of the tube 3.4 shear failure of the tube 4 results and discussion 4.1 interpretation in terms of 's and 's 4.2 interpretation in terms of 's 5 conclusions references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.34.0015 acta polytechnica ctu proceedings 34:15–19, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague a modular extension of a finite element contact implementation ondřej faltus∗, martin horák czech technical university in prague, faculty of civil engineering, department of mechanics, thákurova 7, 166 29 prague 6, czech republic ∗ corresponding author: ondrej.faltus@cvut.cz abstract. the structural mechanics module of the oofem finite element software has been developed to include various algorithms for solving contact mechanics problems. after developing small strain contact algorithms in the 2d domain and their extension to large strain applications [1, 2], the present contribution extends the existing framework to the 3d domain. after reviewing the current code and comparing existing solutions found in the literature, we identify the common ground between 2d and 3d applications, then propose and implement the necessary changes and additions to smoothly integrate the 3d support into existing code. tests and example problems are presented to confirm the functionality of the resulting implementation. keywords: contact, large strains, penalty method, 3d contact, node-to-segment, oofem. 1. introduction contact mechanics, as a specific subdiscipline of structural mechanics, lend themselves very handily to computational implementations. analytical solutions to contact mechanics problems are scarcely found. since the end of the 19th century, when heinrich hertz studied the contact of convex elastic bodies without friction [3], progress in this originally fringe discipline had been very slow until the advent of computational methods, mainly the finite element method (fem) in the latter half of the 20th century [4, 5]. today, more and more complicated problems can be studied with the help of more and more involved algorithms, owing to the technological advances in computational hardware. in our past work [1, 2], we have developed the basis of contact algorithms in the oofem software. oofem [6] is a finite element computational software package developed at the czech technical university in prague. from a programming perspective, oofem is built as a strictly object-oriented c++ codebase. in our past implementations, we have paid close attention to this fact, adapting some of the commonly in literature presented programmatic solutions to suit the object-oriented philosophy better. so far, contact algorithms in oofem can handle contact problems in the 2d domain, using the node-to-node and node-tosegment discretizations and alternatively the penalty method or lagrangian multiplier method of contact condition enforcement. of these options, the penalty method node-to-segment approach is adapted for use with geometrically nonlinear problems as well. this work aims to extend our previous implementation to the 3d domain. keeping in with the idea of object-oriented code, we try to achieve this by extending the existing objects and algorithms they represent instead of coding everything anew. chiefly, we would like to avoid the "contact element" approach often presented in literature [4, 7, 8] and create instead a universal contact condition object able to interact with every possible contact segment type. 2. theoretical basis at the core of our modular code extension lies the desire to adapt the existing geometrically nonlinear penalty contact formulation for 2d domains to fully work in the 3d domain as well. for this purpose, we would like to derive a universal formulation for the forces and stiffness applicable to all cases equally. 2.1. formulation of penalty contact the penalty formulation counts as one of the most straightforward possible implementations of contact conditions into the fem framework. its governing idea is the introduction of an arbitrary penalty parameter to the force vector, aimed at penalizing unwanted states of penetration among the contacting bodies. in other words, we express the force on the contact boundary as h(−gc)pgc, where p is the chosen penalty parameter and gc the contact gap (negative in penetration) [1, 4, 5, 7]. h is the heaviside function. to this end, the energy contribution of the contact process can be expressed as w c(u) = ∫ γc 1 2h(−gc(u))pg2 c (u) dγ (1) where u is the global vector of displacements and γc is the contact boundary. note that for a node-tosegment contact discretization, the integral morphs into a sum over a finite set of discrete contact points. with the first variation of the contact energy, we obtain the internal force vector as 15 https://doi.org/10.14311/app.2022.34.0015 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en ondřej faltus, martin horák acta polytechnica ctu proceedings fc = ∫ γc pgc ( n∗t n ||n|| ) dγ (2) where n is the normal vector to the contact boundary in the deformed configuration and n∗ is the extended n-matrix: a matrix serving to distribute the forces to the according degrees of freedom of both the external node and the nodes of the element forming the contact surface (see [1] for a more detailed discussion of the concept). under the assumption of small strains, the normal vector and the extended n-matrix would remain independent of u, giving with the second variation of contact energy a simple formula for the algorithmic stiffness contribution [1, 2] kc = ∫ γc p ( n∗t n ||n|| ) ( n∗t n ||n|| )t dγ (3) however, those assumptions no longer hold with nonlinear geometry, and a more complex stiffness formula has to be derived. keeping in mind the desired universality of the solution for both 3d and 2d domains, we shall do that in the following paragraph. 2.2. a universal stiffness formula in expression (2) for the internal forces of contact, the gap function gc depends on the current deformation state of the domain, with the variation δgc = nt v δd (4) with nv = n∗t n/||n|| and δd being the variational displacements of the related degrees of freedom. moreover, the extended n-matrix n∗ and the unit normal vector ν = n/||n|| depend on the convective natural coordinates ξ denoting the contact point on the element surface. the β-th natural coordinate ξβ has the variation δξβ = a−1 αβ (tv,α − gcbv,α) δd = dv,βδd (5) where: bv,α = b∗t α ν tv,α = n∗t tα (6) the matrix b∗ α being the derivative of n∗ with respect to ξα, the vector tα being the tangent vector to the surface in the α-th direction and a being an extended metric tensor of the surface, composed of the covariant metric tensor m and the curvature tensor κ [4] aαβ = mαβ − gn καβ (7) summation convention over α and β (parametric surface coordinate indicies) is implied wherever applicable. after substitution of these variations into the known expressions and exploitation of the orthogonality of the normal and tangent vectors, we can express the desired tangential stiffness as kc = ∫ γc pnt v nv (8) + pgc ( bv,αdt v,α + dv,αbt v,α + καβdv,βdt v,α + gcmαβb̄v,αb̄t v,β ) dγ where b̄v,α = bv,α + καβdv,β (9) and mαβ is the contravariant metric tensor (i.e. m−1). note that the first term in equation (8) corresponds perfectly to the geometrically linear formula (3). this formula has been derived with the express aim to make no assumptions about the dimension of the domain space or the number of nodes on the element boundary. it is, therefore, a universal formula for all cases of nonfrictional contact. letting α and β only assume the value of 1, setting m to element length squared and κ to 0, an expression for a linear element in 2d space is obtained, which reduces perfectly to the stiffness formula found for this case in [4] and used in [2]. 3. implementation this section shall discuss the precise intricacies of the chosen approach to implementing the contact algorithms. 3.1. the contact condition and contact segment objects contact usually occurs among two bodies within the domain space, of which at least one is deformable and described in fem by a mesh of finite elements. by definition, when the contact gap is open, no solid matter occupies the area between the two bodies, and therefore no mesh is present. from a practical programmatic perspective, an essential part of any fem algorithm is the assembly process. in this process, all the elements within the domain space contribute to the global stiffness matrix and the global vector of internal forces, assembling their contributions to their proper places. in the case of contact, it is unclear how to introduce contact forces and resulting algorithmic stiffness into this process. in literature [4, 5, 7], it is customary to introduce so-called contact elements. those are finite elements occupying the physically empty space in between the contacting objects. those elements perform the aforementioned force and stiffness assembly function. therefore, the gap area in the reference configuration has to be meshed with those special elements describing the contact algorithm. this approach has several noticeable drawbacks. firstly, in the node-to-segment approach, for example, a separate contact element has to be formulated for 16 vol. 34/2022 a modular extension of a finite element contact implementation figure 1. a uml diagram describing the concept of object relationships within oofem. each possible case of element edge or surface forming the segment (in the segment-to-segment approach, matters would be even much worse). for a codebase with a reasonable claim to universality, this could shortly result in a significant increase of necessary code. secondly, the formulation of those elements may be restrictive in terms of the possible contact cases. a contact element usually only pairs one node with one boundary segment. each node deemed possible to come in contact with a given segment, therefore, requires a separate element defined, and this has to be decided already at the time of meshing. already in the existing implementation in oofem, we have diverged from this standard practice. in line with object-oriented programming principles, the aforementioned assembly process in oofem is handled with the use of objects. apart from object classes representing elements, another class capable of contributing to the global stiffness matrix and force vectors is the activeboundarycondition class. we have chosen to handle contact algorithms through subclasses descended from that. the principle is illustrated diagrammatically in figure 1. all oofem objects are collectively stored in the overarching domain class. a node2segmentpenaltycontact object is a subtype of an activeboundarycondition, which itself is a subtype of a boundarycondition. this contact condition class maintains a generalized reference to a contactsegment object, which, in the case it is of the boundarycontactsegment subtype, can reference element objects relevant for its task of describing an element boundary. this way of structuring the code offers us a great potential of generality in large parts of the code and presents a more intuitive way of modeling the real physical idea of the contact phenomenon. 3.2. universality of the contact condition object in our previous work, four boundary condition objects have been created, namely node2nodepenaltycontact, node2nodelagrangianmultipliercontact, node2segmentpenaltycontact, and node2segmentlagrangianmultipliercontact [1, 2]. in the case of the node-to-node approach, the contact condition object itself handles the references to the nodes concerned, requiring no interaction with any other objects. its universality and extension to the 3d domain is therefore trivial and not a subject of this paper. in the node-to-segment case, however, the operations within the code rely heavily on an interaction between the condition and an object representing the contact segment. as described by figure 1, we aimed to maintain the connection of the boundary condition class only to the generalized contactsegment parent class, rather than differentiating among its subtypes, which include formulations as diverse as an analytical polynomial function in 2d space and a segment representing multiple 3d element surfaces. the formulas necessary to this universal approach have already been discussed in section 2.2. the only necessary part of the implementation part is to ensure the ability of the code to handle all possible matrix sizes of the various contributions. extensive use has been made of the std::list array object in the c++ language, which allows for an unspecified number of objects to be passed between c++ functions. to this end, the functions in the 2d contact segment have been altered to return an array of size one (of, i.e., tangent vectors) as opposed to a single object. on the level of the node2segmentpenaltycontact class, the code is written to always loop through those arrays regardless of their size. key to the computations in the node2segmentpenaltycontact class are the computetangentfromcontact() and computeexternalforcesfromcontact() methods, which compute the stiffness and force contributions, respectively, for a given node-segment pair. in addition to those, auxiliary methods exist to compute the various expressions seen in formula 8, e.g. the computenvmatrixat() method for the nv matrix or the computemodifiedbvmatricesat() method for the (one or multiple) b̄v matrices. those then poll the contact segment for the relevant information, such as gap value, metric and curvature tensor, normal and tangent vector(s), the n matrix etc. 3.3. the universality of contact search algorithm another issue with code universality arises specifically in the 3d domain. contrary to the 2d case, in 3d, even the limitation to finite elements with linear formulation does not necessarily ensure the linearity of their boundary [9]. more specifically, only surfaces of triangular shape can be described linearly. we have come to the conclusion that limiting our implementation to those would severely hamper the usability of the resulting code; it was, therefore, necessary to take the possible nonlinearity of the contact surface into account when conducting a contact point search. the solution 17 ondřej faltus, martin horák acta polytechnica ctu proceedings (a) (b) (c) figure 2. node-to-segment contact with penalty formulation in the 3d domain: tests on a linear wedge and a linear brick element. displayed geometries correspond to the 0th (undeformed configuration), 40th, and 60th steps of the computation. coloring represents the vertical normal stress σyy. a) a test on a linear wedge element. b) a test on a linear brick element, with the initial contact point at an axis of symmetry of the top surface. the deformed state remains symmetric along this axis. c) a test on a linear brick element with an arbitrary initial contact point. the apparent gap is only a consequence of the imprecise rendering of the resulting bilinear surface. is an implementation of a closest-point-projection algorithm in the computecontactpoint() method of the linear3delementsurfacecontactsegment class. the parametric coordinates of the desired contact point (i.e., the point on the surface closest to the contacting node, regardless of whether contact occurs) can be expressed as a vector ξ minimizing the distance function f (ξ) = 1 2(r − ρ(ξ)) · (r − ρ(ξ)) (10) where r are the global coordinates of an external point (the node), and ρ are the global coordinates of the contact point (on the surface). solving this minimization problem with the newtonraphson scheme leads to a convenient expression for the iterative increment δξ in the form δξ = −a−1 df dξ (11) where a is the already well-established extended metric tensor of the contact surface (see (7)). 4. computational examples in light of the original aim of this paper, the extension of oofem’s contact formulations to the 3d domain, we shall demonstrate the functionality of the described implementation on several 3d test examples. the tests concern the node-to-segment formulations for geometrically nonlinear cases. in one case, the contact segment is made of a triangular surface of a linearly formulated wedge element, while in the other two cases, the used element is a linear brick. therefore, the first element represents an example of a fully linear surface without curvature, while the latter concerns a bi-linear surface, where the curvature tensor is generally nonzero. on the brick element, two different tests are performed; in the first one, the initial contact point is positioned on one of the axes of symmetry of the surface, in the other one, an arbitrary point is chosen. the elements in question are the oofem elements lwedge and lspace, respectively. above them, a single ltrspace element is positioned. this is a linear tetrahedral element, which serves, however, only to provide the contacting node. all its nodes are fixed, and the lower node is loaded by a prescribed vertical displacement. this node is designed to come into contact with the top surface of the lower element and push it downwards. all nodes of the contact surface are free; the lower element is fixed by dirichlet boundary conditions on its lower base. all elements in these test simulations have a linear 18 vol. 34/2022 a modular extension of a finite element contact implementation elastic material model assigned, with young’s modulus and poisson’s ratio e = 500 mpa and ν = 0. in oofem, such a material model translates to the saint venant-kirchhoff law in the case the geometrical nonlinearity of the element is turned on [6]. loading is performed in 60 steps in each case. the total displacement prescribed for the external node is 0.3 m, i.e. 0.005 m per loading step. the results of the test are pictured in figure 2. for the visualization of the results, paraview, an open-source software package, has been used [10]. we can observe that the deformation of the surface follows the positioning of the contact point. for the brick element, in the first case, the contact point remains on the axis of symmetry, and the rotation of the contact surface only follows the other axis, while in the second case, the entire surface twists and assumes a bi-linear deformed shape. note that the somewhat apparent gap between the element surface and the contacting node is only a result of the rendering software’s inability to portray this bi-linear surface properly. 5. conclusions the paper describes an object-oriented approach to handling contact mechanics in fem. as a result of this object-oriented approach, a universal contact algorithm is necessary. this is derived on a theoretical basis, and the implementation in an open-source finite element code oofem is then discussed in detail. to verify the practical usability of the chosen approach, tests are performed on contact in the 3d domain, which is a feature previously unavailable in the existing oofem contact implementation. acknowledgements the authors would like to acknowledge the support received for work on this project from a ctu grant no. sgs21/037/ohk1/1t/11. references [1] o. faltus. object-oriented design and implementation of contact mechanics into finite element code oofem. master’s thesis, czech technical university in prague, 2020. [2] o. faltus, m. horák. finite element implementation of geometrically nonlinear contact. acta polytechnica ctu proceedings 30:18–23, 2021. [3] h. hertz. on the contact of elastic solids. z reine angew mathematik 92:156–171, 1881. [4] p. wriggers. computational contact mechanics. springer, new york, 2nd edn., c2006. [5] v. a. yastrebov. numerical methods in contact mechanics. wiley, hoboken, nj, 2013. [6] b. patzák. oofem home page, 2000. http://www.oofem.org. [7] a. konyukhov, r. izi. introduction to computational contact mechanics. wiley, chichester, west sussex, 2015. [8] t. a. laursen. computational contact and impact mechanics: fundamentals of modeling interfacial phenomena in nonlinear finite element analysis. springer science & business media, 2003. [9] o. c. zienkiewicz, r. l. taylor. the finite element method. butterworth-heinemann, boston, 5th edn., 2000. [10] u. ayachit. the paraview guide: a parallel visualization application. kitware, 1st edn., 2015. 19 acta polytechnica ctu proceedings 34:15–19, 2022 1 introduction 2 theoretical basis 2.1 formulation of penalty contact 2.2 a universal stiffness formula 3 implementation 3.1 the contact condition and contact segment objects 3.2 universality of the contact condition object 3.3 the universality of contact search algorithm 4 computational examples 5 conclusions acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.38.0509 acta polytechnica ctu proceedings 38:509–515, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague verification of window properties after 10 years of exploitation: results of measurements in the pavilion laboratory and the climate chamber marek bartko∗, pavol ďurica university of žilina, faculty of civil engineering, department of building engineering and urban planning, univerzitná 8215/1, 010 26 žilina, slovakia ∗ corresponding author: marek.bartko@uniza.sk abstract. the article will deal with the analysis of measured data on a plastic window with thermal insulating triple glazing, which is suitable for low-energy or passive houses. the window was installed in 2011 in the test laboratory of the department of building engineering and urban planning, faculty of civil engineering, university of žilina (slovakia), where it was tested under standard indoor climate conditions and real outdoor climate conditions. surface temperatures on the frame friezes and glass system and heat flux density were recorded at a five-minute time step. in 2020, the window was removed from the laboratory and subsequently tested in a climate chamber. this paper will present the results of these measurements in terms of heat flow density waveforms, heat transfer coefficient, and total solar transmittance through the glazing. subsequently, a simulation model of this window will be created in the environment of a computational program and its verification based on the measurements will be carried out. a series of calculations will be performed on the tuned model and analyses of the results and comparisons will be presented under the same climatic conditions as during the real measurements recorded by the meteorological station. keywords: window, triple glazing, climate chamber, pavilion measurement, building physics. 1. introduction windows are an integral part of the envelope of almost all buildings. they are the most used transparent system in building envelopes. the main function of windows is to protect the indoor environment from the external climate. other important functions are daylighting and insolation of the space [1], fresh air supply by natural ventilation, contact with the outdoors, etc. with increasing demands for thermal protection, window constructions are constantly evolving. physical criteria such as thermal-technical, acoustic, lighting, and hygiene are important. another important aspect of the proper functioning of windows is their installation in envelope construction. window constructions are the most problematic point of the building envelope in terms of thermal insulation and construction design. therefore, continuous research and development of window constructions are essential. a significant problem that needs to be addressed more than in the past is overheating in summer [2]. another problem is the elimination of thermal bridges in window construction. for example, by using triple glazing we limit the formation of water vapor condensation at the bottom of the glazing. in slovakia, thermal protection is dealt with in stn 730540:2019 [3], which follows the ec energy performance of buildings directive (epbd) 2010/31/eu. valid values for window constructions are uw = 1.0 w/(m2 k) from 2016 and from 2021 the recommended value is uw = 0.65 w/(m2 k). since 2011, three different windows have been monitored in the laboratory of the department of building engineering and urban planning. it is a so-called pavilion laboratory. the indoor environment is controlled by a heating system and an air conditioning unit. the outdoor environment is represented by the real climate, which is recorded by a weather station [4]. from the research conducted so far, several results related to temperature, heat flux, and u-value measurements have been published. comparisons have been made with the model in the fem therm software and result from different years were analyzed. during autumn 2020, the laboratory building envelope was renovated and insulated with a new etics system, while one of the measured windows was dismantled and taken for measurement in the climate chamber [5, 6]. a comparison of the measured results from the pavilion laboratory and the climate chamber after ten years of window exploitation at the same boundary conditions is described in this paper. 2. methods of measurement the analyzed plastic window has been installed in the department testing laboratory since 2011 (figure 1). the window was oriented to the south with a slight inclination to the west (15°). from the exterior side, the window was exposed to real outdoor climate conditions, which are monitored and recorded by a mobile experimental weather station [7], located on the roof of a nearby building. the indoor climate is provided 509 https://doi.org/10.14311/app.2022.38.0509 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en marek bartko, pavol ďurica acta polytechnica ctu proceedings figure 1. external and internal view of the built-in plastic window in the test laboratory. material number of glazing gas total solar energy uw uf ug chambers transmittance g [%] [w/(m2 · k)] [w/(m2 · k)] [w/(m2 · k)] plastic 6 triple ar 36 0.80 1.00 0.50 table 1. properties of the measured plastic window given by the manufacturer. by an air-conditioning unit, which maintains it based on the slovak standard boundary conditions: 20 °c and 50 % humidity [8]. the sensors used for the measurements consist of nicr-ni [9] thermocouples and heat flux density plates (hfp), also equipped with a correction thermocouple (standard 120 × 120 mm) and a half-sized (120×60 mm) for window frames and sashes [10]. the monitoring points are the frame, sash, and glazing. the datalogger and both types of sensors are from ahlborn. the data logging interval in this case was five minutes. the heat flux density was only recorded at the glazing, as measuring the heat flux density through the window frame requires different input conditions – the glazing must be replaced by a full panel. for this reason, the degradation of the glazing system will be mainly evaluated in this paper. subsequently, the window was dismantled in 2020 during the reconstruction of the facade of the pavilion laboratory. however, it is not at the end of its life. after dismantling, the window was removed and installed in the climate chamber, where it has been and will continue to be tested and measured. the purpose was to compare the parameters of the window measured in the pavilion laboratory with those measured in the climate chamber. for the measurements in the climate chamber, two variants of indoor climate modeling were used, one with a constant indoor temperature and a measurement with a hotbox, the other without the application of a hotbox and with the same temperature waveform as was recorded in the pavilion. for the measurement of the climate chamber was chosen 1st march 2018. the outside temperature ranges from -3.4 °c on a sunny day to -16.7 °c at night. the indoor air temperature was also used. this method was chosen to compare the behavior of the window under the same boundary conditions in the pavilion laboratory and the climate chamber. instead of thermocouples, pt100 and ntc sensors were used to measure temperature. the hfp used is of similar types to those used in the pavilion measurements. when the window was initially installed in the pavilion laboratory, not all the parameters that we now measure in the laboratory were measured. for this reason, the measured values from the pavilion laboratory and the climate chamber are compared with the values set and declared by the manufacturer. the declared values of the window parameters from the manufacturer are summarized in table 1. 2.1. u-value the heat transfer coefficient u of a structural element describes the amount of heat energy that passes through it from one side to the other per second in a square meter of the area at a constant ambient temperature difference of 1 °c. the relation for calculating the heat transfer coefficient u is shown in equation (1): u = 1 r0 = 1 (rsi + r + rse) = 1 1 hi + 1 λ + 1 he = q θai − θae , (1) u heat transfer coefficient [w/(m2 k)], r0 structure resistance to heat transfer [(m2 k)/w], rsi internal surface resistance [(m2 k)/w], 510 vol. 38/2022 verification of window properties after 10 years of exploitation . . . figure 2. detail of the window in the wall of the laboratory. marked positions for comparison of surface temperatures. surface transfer stn 73 0540 glazing glazing glazing coefficient [w/(m2 · k)] pavilion clim. chamber clim. chamber with hotbox without hotbox hi 7.62 10.47 10.92 16.51 he 25 14.56 15.92 15.66 table 2. the surface transfer coefficient is defined in stn 73 0540 and calculated using equations (2) and (3). rse external surface resistance [(m2 k)/w], r thermal resistance of the structure [(m2 k)/w], hi surface transfer coefficient at internal surface [w/(m2 k)], he surface transfer coefficient at external surface [w/(m2 k)], λ thermal conductance [w/(m2 k)]. the surface transfer coefficient can be calculated according to the equations (2) and (3): hi = q θsi − θai , (2) he = q θse − θae . (3) 2.2. solar transmittance for transparent and translucent constructions such as window constructions, besides the thermal quantification, the optical properties, especially the solar transmittance, are equally important parameters [11]. there are several ways, to measure solar transmittance. in our case, measuring devices in the form of two pyranometers were used, one of which was mounted on the interior side of the measured window and the other on the exterior side as part of a mobile meteorological station. from the values measured in this way, the solar transmittance is finally determined as the ratio of the observed solar radiation intensities behind (from the interior side) and in front (from the exterior side) of the measured window located in the pavilion laboratory and subsequently in the climatic chamber. 2.3. window simulation in simulation software in the design and planning process of a building, thermal bridges are analyzed for several construction fragments, including the window frame detail and the window fitting into the opening. the analysis can be carried out using various simulations, however, modeling different window construction is quite timeconsuming [12]. in the field of two-dimensional modeling of heat transfer in buildings, several software is available. this article used therm software, developed by lawrence berkeley national laboratory. the simulation results in the form of surface temperatures were compared with the actual measured surface temperatures in the pavilion laboratory. a detailed view of the modeled window is shown in figure 2. these windows were placed in the same composition as in the laboratory to achieve the best possible temperature match. the simulation was carried out in the lining and the sill. 3. results of measurements for the calculation of the surface transfer coefficient were used equations (2) and (3). results are summarised in table 2. the results show in some cases (especially outdoors) a large difference between the 511 marek bartko, pavol ďurica acta polytechnica ctu proceedings heat transfer specified by the glazing glazing glazing coefficient [w/(m2 · k)] manufacturer pavilion clim. chamber clim. chamber with hotbox without hotbox ug 0.5 0.92 0.94 0.96 uw 0.8 1.12 1.13 1.15 table 3. heat transfer coefficient window uw and glazing ug. figure 3. temperature courses for the centre of glazing. very good match of courses at the night (without solar radiation). measured values and the standard values. the difference could be due to the measurement method itself, a non-stationary state, lower accuracy of the thermocouples, or imperfect contact between the sensor and the surface. in the case of the outdoor coefficient, the non-stationary state of the external environment: solar radiation, wind, and rain. the calculated values of the surface transfer coefficient based on the measured heat fluxes are given in table 2 and the heat transfer coefficient glazing in table 3. the measured surface temperature values at each position are shown in figures 3–6. in this case, the temperature waveforms for a selected time interval were compared (from 20:00 28th february 2018 to 23:59 1st march 2018). although there are 6 (10) positions displayed within the window, only 3 specific positions are compared. in other positions the results are similar. figure 3 shows the temperature waveforms at the center of the glazing from the inside. temperatures were measured using hfp. figure 4 and figure 5 show a comparison of the different internal temperatures. in the first case, the steady – constant air temperature is measured with a hotbox in the climate chamber. in the second figure, the non-stationary temperature was used as the boundary condition in the chamber. figure 6 shows the surface temperatures at the window sash location. in table 4 we can see the solar transmittance calculated as the ratio of the observed solar intensities behind (from the interior side) and in front (from the exterior side) of the measured window. measurements were carried out in the pavilion laboratory and the climate chamber. in table 5 we can see a comparison of the surface temperatures measured in the climate chamber and pavilion laboratory with the surface temperatures from the therm simulation software (figure 7). the boundary conditions in the simulation were set approximately according to the real conditions from the measurements in the pavilion. outdoor temperature -10 °c and indoor temperature 20 °c. 4. conclusion this article deals with the comparison of measured parameters of a plastic window after its ten-year exploitation. the measured parameters were compared only with the data given by the manufacturer. the measured window was installed in the pavilion laboratory where it was exposed to the real outdoor climate. it was later dismantled and used for measurements in the climate chamber with the same boundary conditions set as on the selected winter day in 2018. for comparison, a simulation was also created but in the stationary environment of the therm software. the results showed a very good agreement in the shape of the temperature waveform and for the position in the center of the glazing also in the values where the difference is relatively small. in other positions, there is a different temperature approx. 3–4 °c. daily temperatures in the pavilion laboratory are strongly influenced by solar radiation, wind, and rain. discrepancies in the sill results require further analysis, such as the impact of the masking panel compared to the wall. the glazing heat transfer coefficients calculated from the measured heat fluxes in the pavilion laboratory and the climate chamber show similar values but differ significantly from the values given by the manufacturers. the heat transfer coefficient glazing 512 vol. 38/2022 verification of window properties after 10 years of exploitation . . . figure 4. temperature courses for the bottom of glazing. variant 1 with constant indoor temperature. difference of about 3 °c in surface temperatures. figure 5. temperatures courses for the bottom of glazing. variant 2 with non-steady indoor temperature. difference of about 4 °c in surface temperatures. figure 6. temperature courses for the bottom of windows sash. difference of about 3 °c in surface temperatures. specified by the pavilion climatic chamber manufacturer total solar energy transmittance g [-] 0.36 0.34 0.31 table 4. total solar energy transmittance g. 513 marek bartko, pavol ďurica acta polytechnica ctu proceedings position temperature [°c] therm pavilion chamber pl as tic w in do w 1 o ut do or -1 0 in do or 20 17.3 15.1 15.0 2 16.5 14.9 13.3 3 -9.1 -8.3 -8.9 4 -8.5 -8.3 -9.0 5 17.5 9.9 15.4 6 15.9 12.5 16.9 7 -8.7 -10.6 8 -8.9 -10.6 table 5. comparison of surface temperatures measured in the pavilion and climate chamber with temperatures from the therm simulation software. figure 7. temperature waveforms according to the therm software simulation at the lining and sill location. shows a difference compared to the manufacturer’s values of about 90 %. the values of the total solar energy transmittance compared to the manufacturer’s value is quite similar. the value measured in the pavilion laboratory compared to the manufacturer’s value show a deterioration of only about 5.5 %. for the values measured in the climate chamber, a deterioration of 14 % occurs. based on these findings, we can say that the thermal-technical properties of the plastic window after 10 years of exploitation show deterioration. this deterioration of the parameters is influenced by possible leakage of filler gas from the glazing system, non-stationary conditions, especially from the exterior side (solar radiation, wind, rain), and airflow in the vicinity of the construction. the surface temperatures obtained from the simulation were then compared with the surface temperatures measured in the pavilion and the climate chamber. the results showed a slight overestimation of the temperatures in the lining area, however, there is a large difference between the temperatures in the sill area, which may be due to the airflow in the sill area and the stationary environment of the therm software. acknowledgements the research is supported by the grant project vega no. 1/0673/20. references [1] m. glória gomes, a. j. santos, a. moret rodrigues. solar and visible optical properties of glazing systems with venetian blinds: numerical, experimental and blind control study. building and environment 71:47–59, 2014. https://doi.org/10.1016/j.buildenv.2013.09.003 [2] r. ponechal. increasing thermal mass in low carbon dwelling. procedia engineering 111:645–651, 2015. https://doi.org/10.1016/j.proeng.2015.07.062 [3] stn 73 0540:2019: tepelná ochrana budov. tepelnotechnické vlastnosti stavebných konštrukcií a budov. [4] p. juras, d. jurasova. influence analysis of climate data time-step on the accuracy of ham simulation. matec web of conferences 196:02029, 2018. https://doi.org/10.1051/matecconf/201819602029 [5] p. juras, p. durica, m. bartko. long-term window evaluation: comparison of pavilion laboratory and climate chamber measurement. in p. akimov, n. vatin (eds.), xxx russian-polish-slovak seminar theoretical foundation of civil engineering (rsp 2021), pp. 132–140. springer international publishing, cham, 2022. https://doi.org/10.1007/978-3-030-86001-1_16 [6] k. banionis, j. kumžienė, a. burlingis, et al. the changes in thermal transmittance of window insulating glass units depending on outdoor temperatures in cold 514 https://doi.org/10.1016/j.buildenv.2013.09.003 https://doi.org/10.1016/j.proeng.2015.07.062 https://doi.org/10.1051/matecconf/201819602029 https://doi.org/10.1007/978-3-030-86001-1_16 vol. 38/2022 verification of window properties after 10 years of exploitation . . . climate countries. energies 14(6):1694, 2021. https://doi.org/10.3390/en14061694 [7] p. juras, d. jurasova. outdoor climate change analysis in university campus: case study with heat-air-moisture simulation. civil and environmental engineering 16(2):370–378, 2020. https://doi.org/10.2478/cee-2020-0037 [8] p. juras. comparison of triple glazed windows based on long-term measurement. iop conference series: materials science and engineering 415(1):012020, 2018. https://doi.org/10.1088/1757-899x/415/1/012020 [9] nicr-ni thermocouple t190-0. [2022-04-20]. https://www.ahlborn.com/download/pdfs/kap07/ 05draehted.pdf [10] thermal flux plates. [2022-04-20]. http://www. ahlborn.com/download/pdfs/kap13/wflplatten.pdf [11] m. čekon, r. slávik. total solar transmittance quantifying of transparent insulation building materials based on real climate outdoor measurements. energy procedia 132:243–248, 2017. https://doi.org/10.1016/j.egypro.2017.09.694 [12] p. juras. analysis of window model accuracy and its influence on the results compared to the measurement. matec web of conferences 196:02028, 2018. https://doi.org/10.1051/matecconf/201819602028 515 https://doi.org/10.3390/en14061694 https://doi.org/10.2478/cee-2020-0037 https://doi.org/10.1088/1757-899x/415/1/012020 https://www.ahlborn.com/download/pdfs/kap07/05draehted.pdf https://www.ahlborn.com/download/pdfs/kap07/05draehted.pdf http://www.ahlborn.com/download/pdfs/kap13/wflplatten.pdf http://www.ahlborn.com/download/pdfs/kap13/wflplatten.pdf https://doi.org/10.1016/j.egypro.2017.09.694 https://doi.org/10.1051/matecconf/201819602028 acta polytechnica ctu proceedings 38:509–515, 2022 1 introduction 2 methods of measurement 2.1 u-value 2.2 solar transmittance 2.3 window simulation in simulation software 3 results of measurements 4 conclusion acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.34.0011 acta polytechnica ctu proceedings 34:11–14, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague influence of oxygen and argon plasma treatment on wettability and surface morphology of polypropylene microfibers jakub ďurejea,∗, zdeněk prošeka, jan trejbala, štěpán potockýb, pavel tesáreka a czech technical university in prague, faculty of civil engineering, department of mechanics, thákurova 7, 166 29 prague 6, czech republic b academy of sciences of the czech republic, institute of physics, cukrovarnická 10, 162 53 prague 6, czech republic ∗ corresponding author: jakub.dureje@fsv.cvut.cz abstract. the surface treatment of polypropylene microfibers by plasma processing in oxygen and argon was studied with the motivation to increase their cohesion with the cement matrix. by plasma treatment, it is possible to modify the surface of the microfibers mechanically, chemically or with a combination of both. the microfibers were modified using the reactive ion etching plasma system. the surface of the microfibers was modified by oxygen and argon plasma. the wettability of the microfibers was measured using the packed cell method. furthermore, the weight of the microfibers before and after plasma modification was measured. finally, the surface of the modified microfibers was examined by scanning electron microscopy. almost all modifications of the microfiber surfaces were able to increase their wettability with water. the wettability of the oxygen plasma treated microfibers increased on average by about 11%, the wettability of the argon plasma treated microfibers increased on average by about 6% compared to the fibers without modification. the mechanical effect of plasma treatment was proven only for microfibers modified by oxygen plasma. keywords: oxygen plasma, argon plasma, plasma treatment, plasma modification of microfibers, polypropylene, sem, wettability. 1. introduction the interfacial transition zone significantly influences the properties of cement composite materials containing fibrous reinforcement at the interface between the fiber and the cement matrix [1, 2]. to achieve suitable mechanical properties of the fibers reinforced cement composite material, it is desirable to increase the wettability by water and mainly mechanically modify the fiber’s surface [3]. by plasma modification, it is possible to modify the surface of the fibers both chemically and mechanically. previous experiments proved that plasma modification by oxygen plasma could sufficiently increase the wettability of polypropylene (pp) fibers with water [4, 5]. however, the mechanical effect of plasma modification, realized by inductively coupled plasma, was insufficient on the surface of microfibers [4, 5]. the increased physical effect of plasma modification can be achieved by the impact of energetic ions i.e., with plasma system generally used for ion etching which utilizes ion bombardment process [6]. a typical representative of such systems is the reactive ion etching (rie) system intensively etching the material’s surface [7]. to increase the physical modification of pp microfibers, we present a study of oxygen and argon pp microfibers modifications by the rie system to their wettability and surface morphology. 2. materials and methods polypropylene microfibers fibrifor multi (contec fiber ag, switzerland) were used. the microfibers are made in bundles (type 127) with the diameter of each individual microfiber around 32 µm and the length of 12 mm ± 5 %. the properties of the microfibers are summarized in table 1. the surface of the fibers was modified using the rie system (phantom iii, trion technology inc., usa). microfibers weighing approximately 5 g were inserted into a petri dish. three petri dishes with microfibers were inserted into the rie system and pumped down to the pressure <1·10−3 pa. after that, the chamber was filled (50 sccm) by selected gas (oxygen or argon) and maintained at pressure 13.3 pa. the plasma modification time was set from 30 to 480 seconds. the influence of rf power on microfibers was tested in the range of 100 w to 500 w for a fixed treatment time of 60 seconds. the influence of working gas atmosphere was studied at fixed rf power 100 w and various treatment times were studied. after the plasma modification process, the chamber was evacuated, vented with nitrogen and the microfibers were removed. the microfibers were stored in airtight bags. to determine the treatment stability, part of the samples was exposed to ambient conditions in a laboratory environment at a temperature of about 22 ◦ c and relative 11 https://doi.org/10.14311/app.2022.34.0011 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en j. ďureje, z. prošek, j. trejbal et al. acta polytechnica ctu proceedings humidity of about 50 % for 12 hours. the sample description with relevant process parameters of the rie is in table 2. fibers fibrofor multi material polypropylene diameter [µm] 32 length [mm] 12 tensile strength [mpa] 270 melting point [°c] 160 bulk density [g/cm3] 0.91 table 1. properties of selected fibers. designation input power [w] modification time [s] gas a200w 200 60 argon a300w 300 60 argon a400w 400 60 argon a500w 500 60 argon a30 100 30 argon a60 100 60 argon a120 100 120 argon a240 100 240 argon a480 100 480 argon o30 100 30 oxygen o60 100 60 oxygen o120 100 120 oxygen o240 100 240 oxygen o480 100 480 oxygen table 2. designation and device settings during plasma treatment of microfibers. all microfibers with the petri dishes were weighed on a laboratory scale denver instrument s-64 (denver instrument company, usa) before and after plasma modification. the wettability of microfibers was measured by the packed-cell method. the microfibers were in a perforated container and loaded by a lead weighing 53.55 g. the weight of the container was 9.45 g. subsequently, the sample (microfibers, container, and lead) was weighed on a kern abs (kern & sohn gmbh, germany) laboratory scale. weight values were rounded to tens of mg. next, the sample was dipped into the water for 10 seconds, removed from the water and placed in the holder for 120 s, during which time excess water could drain from the container. finally, the sample was weighed again. from the measured weights, the percentage weight of water compared to the weight of microfibers was calculated according to equation 1 [8]. mv = (mm −mn) − (ms −mn) (ms −mn) · 100, (1) where: mv – weight of water to weight of microfibers [%], mm – weight of wet microfibers with container and lead [g], ms – weight of dry microfibers with container and lead [g], mn – weight of a wet container with wet lead [g]. the surface morphology of the microfibers was examined by electron scanning microscopy (sem, phenom xl, usa). to observe objects using sem, the surface of these objects must be conductive. for this reason, a thin layer of platinum was applied to the surface of the microfibers using a mini sputter coater quorum sc7620 (quorum technologies, uk). platinum was sputtered at a pressure of 8 pa for 120 seconds resulting in approx. 10 nm thin pt layer. the sem was set to high resolution (10 kv) with low current intensity. a sed detector was used. the high vacuum option on the sem was chosen, this option set pressure in the chamber of the device to 1 pa. images of fiber surfaces from sem were magnified 10,000×. 3. results and discussion the surface of the fibers was modified by oxygen plasma and by argon plasma. a more pronounced effect of mechanical modification was expected with argon plasma treatment because argon ions have a higher mass than oxygen ions [9]. in the preliminary runs we tested different rf power inputs from 100 w to 500 w at constant time of 60 s in argon atmosphere. as the power input increases, the temperature loading also increases. at a power input of 300 w and higers (samples a300w, a400w and a500w), the microfibers fused together by heat and started to melt. the temperature of the microfibers during the modification has to reach their melting point, which is around 160 ◦c. due to this fact further testing was performed at a fixed rf power of 100 w. at a fixed rf power of 100 w, we tested different times from 30 s to 480 s in argon or oxygen atmosphere. the weight loss of the microfibers during the plasma modification process was almost negligible, the maximum weight loss was about 0.12 % (sample o480). for microfibers modified by oxygen plasma for 240 s and 480 s, the weight loss was significantly higher compared to other types of plasma modification. weight loss is mainly caused by etching of the microfiber surface during modification. therefore, we can conclude that oxygen plasma changes the morphology of the microfiber surface more than argon plasma. the summary of microfibers weight losses after the plasma modification is shown in figure 1. the wettability of the microfibers was slightly higher after almost all performed plasma modifications. the weight of water between the microfibers in the samples after argon plasma treatment increased on average by about 6 % compared to the microfibers without modification. the weight of water between 12 vol. 34/2022 influence of oxygen and argon plasma treatment the microfibers in the samples after oxygen plasma treatment increased on average by about 11 % compared to the microfibers without modification. the microfibers that were left in the laboratory environment for 12 hours (12h added to sample description) after the plasma modification had similar results of wettability as measured immediately after the plasma modification. the weight of water between the microfibers to weight of microfibers is given in figure 2. morphological changes were observed on the surface of microfibers modified by oxygen plasma for 60 seconds or more by the sem. with increasing time, the mechanical effect of plasma modification due to the ion bombardment was increased. in the sem images we can observe that the mechanical modifications occurred preferentially in one direction. the most pronounced changes were observed on top of the samples (microfibers) placed in the chamber of the rie system. the typical sem images are shown in fig. 3. no morphological changes were observed by sem on the surfaces of the argon modified fibers, so not only the ion bombardment process but also the chemical effect of plasma modification is important to achieve morphological changes on the surface of microfibers. according to the results, to achieve more significant morphological changes, it may be appropriate to extend the modification time, increase the rf power of the rie system or use a mixture of gases (oxygen and argon) during the modification. figure 1. percentage weight loss of microfibers after argon and oxygen plasma treatment (a, o) and respective time in seconds (30, 60, 120, 240 and 480). 4. conclusion plasma modifications of polypropylene microfibers was performed using the rie system and compared the wettability and mass changes depending on plasma process parameters. we studied the influence of, plasma modification times (30–480 s), and working gases (argon or oxygen). based on the performed measurements and results, we can conclude: • polypropylene microfibers begin to melt when the power input of the plasma modification device is set to 300w or more for ar plasma and tested time of 60 s. figure 2. wettability measured by the indirect packed cell method – reference microfibers (ref) microfibers after argon and oxygen plasma treatment (a, o), respective time in seconds (30, 60, 120, 240 and 480) and samples that were tested after 12 hours in a laboratory environment (12h). figure 3. sem images of treated microfibers: a) reference (ref), b) o60, c) o120, d) o240, e) o480. • plasma modifies only the microfiber surfaces. the highest weight loss did not exceed 0.12 % of the initial weight, so that the cross-sectional area of the microfiber almost did not decrease. • at a rf power of 100 w, modification with oxygen plasma for 240 s and 480 s is most suitable – the greatest weight losses occurred during the 13 j. ďureje, z. prošek, j. trejbal et al. acta polytechnica ctu proceedings modification, while the reduction of the microfiber cross-sectional area is negligible. • a slight increase in wettability occurred in almost all samples after plasma treatment. there was on average a more significant increase in wettability of the microfibers comparing oxygen to argon plasma modification. leaving the microfibers in a laboratory environment for 12 hours after plasma modification had no significant effect on their wettability. • significant mechanical changes were observed on the surface of the oxygen plasma treated microfibers at 10,000× magnification. with increasing plasma modification time, the changes were more pronounced. the first mechanical changes have already been observed on fibers treated at 100 w after 60 s. • no morphological changes were observed on the surface of the argon plasma treated microfibers at 10,000× magnification. treatment of the microfiber surface only by argon plasma appears to be inappropriate. • the morphological modification of the microfibers by oxygen plasma was very different on each section of the fiber surface, the fibers were mostly modified only from one direction. to achieve a uniform surface treatment, it would be necessary to carry out the plasma modification several times and to mix the fibers between each modification or to use a mixer during the plasma processing. the results show that the mechanical effect of the plasma modification depends not only on the mass of ions, but also significantly depends on the chemical processes that occur during the plasma modification. maybe it could be achieved better results by using a mixture of oxygen and argon gas during the plasma modification process. better results could be also achieved by increasing the rf power or extending the modification time. 5. acknowledgements this work was financially supported by the czech technical university in prague – the project sgs19/148/ohk1/3t/11 and tačr ss03010302. the authors also thank the center for nanotechnology in civil engineering at the faculty of civil engineering, czech technical university in prague. references [1] k. scrivener, a. crumbie, p. laugesen. the interfacial transition zone (itz) between cement paste and aggregate in concrete. interface science (12):411–421, 2004. https://doi.org/10.1023/b:ints.0000042339.92990.4c. [2] v. machovic. microstructure of interfacial transition zone between pet fibres and cement paste. acta geodynamica et geomaterialia pp. 121–127. https://doi.org/10.13168/agg.2013.0012. [3] a. bentur, m. sidney. fibre reinforced cementitious composites. new york: taylor, 2007. [4] r. hlůžek, z. prošek, j. trejbal, et al. plasma treatment impact on physical and chemical properties of polymeric fibers. acta polytechnica ctu proceedings 13:49–54, 2017. https://doi.org/10.14311/app.2017.13.0049. [5] j. ďureje, z. prošek, j. trejbal, et al. plasma treatment impact on physical and chemical properties of polymeric fibers. acta polytechnica ctu proceedings 13:49–54, 2021. https://doi.org/10.14311/app.2021.30.0007. [6] m. collaud, s. nowak, o. küttel, et al. surface modifications of polypropylene after in-situ ar and n2 plasma treatments: an xps study. applied surface science 72(1):19–29, 1993. https://doi.org/10.1016/0169-4332(93)90038-d. [7] triontech. phoenix. https://triontech.com/. [8] j. ďureje. recycling and its use in concrete waste processing by high-speed milling, 2018. diplomathesis. [9] p. slepička, a. vasina, z. kolaská, et al. argon plasma irradiation of polypropylene. nuclear instruments and methods in physics research 268(11–12):2111–2114, 2010. https://doi.org/10.1016/j.nimb.2010.02.012. 14 https://doi.org/10.1023/b:ints.0000042339.92990.4c https://doi.org/10.13168/agg.2013.0012 https://doi.org/10.14311/app.2017.13.0049 https://doi.org/10.14311/app.2021.30.0007 https://doi.org/10.1016/0169-4332(93)90038-d https://triontech.com/ diploma thesis https://doi.org/10.1016/j.nimb.2010.02.012 acta polytechnica ctu proceedings 34:11–14, 2022 1 introduction 2 materials and methods 3 results and discussion 4 conclusion 5 acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.38.0463 acta polytechnica ctu proceedings 38:463–469, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague innovation and experimentation of adaptive model for curtain wall martino milardi, evelyn grillo∗ mediterranea university, department of architecture and territory, via dell’università 25, 89124, reggio calabria, italy ∗ corresponding author: evelyn.grillo@unirc.it abstract. the entire construction sector is the protagonist, in recent years, of continuous research in terms of innovation of strategies and technical solutions leading to the progressive modification in the structure of the systems that make up the building envelopes, also to cope with climate change. working on the envelopes today means, in fact, correctly adapting their performance reactions to dynamic climatic conditions. in this scenario, lies the current research, whose goal is the development of an adaptive model, using innovative materials based on the biomimetic approach, which can be applied to curtain wall systems. the investigation conducted through experimental evaluations in the laboratory, the tclab testing section of the bfl of the mediterranea university of reggio calabria, will provide the formulation of solutions to achieve an adaptive envelope that improves overall performance through new technological strategies. the methodological approach has been prepared on sequential phases, organized in thematic sections, related to a study phase and another application phase, which constituted a real added value to the research, realizing actions of applied experimentation with high-reliability results. keywords: climate change, building envelope, testing. 1. introduction the current environmental and energy issues that affect the scenario of this time, accentuated by the effects of climate change, such as global warming [1] and the increase in co2 emissions, highlight the pressing need to define concrete and measurable responses, moving from a phase considered transitional to one of transformation of the built environment. from many sides of the contemporary cultural debate, it is clear how this urgency causes an acceleration towards a transformative systemic change, with consequences beyond reach. according to the latest report of working group 1 of the ipcc, climate change 2021 – the physical and scientific basis [2], on average over the next twenty years, the global temperature is expected to reach or exceed 1.5 °c of warming, of which human activities are responsible for about 1.1 °c of warming compared to the period 1850–1900. the construction industry is involved in this process and it is called to define new processes and application methods that can read the dynamic responses of the built environment. today, building envelopes are understood as a pivotal element of the construction, defining the performance quality and the regulation of internal conditions independently of transient external ones. at the same time, it is becoming increasingly clear that climate change requires a substantial change in approaches to building design to make urban systems more adaptive to climate change. in particular, buildings face the greatest risk of damage caused by the expected impacts of climate change including more frequent strong wind, increased heat, especially in cities (urban heat island effect), floods, and fires that accompany certain extreme weather events. in recent years, the climatic conditions of the mediterranean areas are characterized by rising temperatures, water bombs, micro-typhoons increasingly frequent and known as medcane (cyclones formed in the mediterranean), becoming the new quality requirements that building envelopes must certify. the evolution of construction processes and the use of advanced technologies to raise the performance qualities of buildings have enabled the creation of building envelopes that interact with the external environment and adapt to specific climatic conditions. in view of the above, it follows the importance of the new adaptive functioning of the envelope in the responses to stress phenomena, to limit the impacts and stress conditions on the built environment. therefore, an envelope can adapt to different needs as a dynamic epidermal layer, emulating from nature the adaptive response strategies [3]. the multidisciplinary approach (inherent in the adaptive requirement of building envelopes), known as biomimetic, is inspired by the forms present in nature and the mechanisms that govern it to apply them to design solutions and experiments, contributing to an increase in the performance offered to buildings today [4]. biological systems are seen as the result of complex procedures of refinement and improvement that are proposed to the culture of the project through strategies and design solutions from which to draw inspiration. the research carried out in this field is characterized by a strong scientific component and an interdisciplinary approach [5]. 463 https://doi.org/10.14311/app.2022.38.0463 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en martino milardi, evelyn grillo acta polytechnica ctu proceedings the contribution is placed in this scenario and is related to research in progress, financed by funds por calabria fesr/fse 2014-2020 (axis 12, action 10.5.6), with the ultimate aim of addressing a design methodology related to the concept of adaptivity of the building envelope. the research activities aim at defining scenarios/models for simulation and application experimentation; the objective is the development of an adaptive model for curtain wall systems through innovative materials, elements, and components based on the biomimetic approach. recent applications of curtain walling in the building sector show that these technologies are favoured by architects and system manufacturers, especially for tall buildings, due to their advantages in terms of ease of construction, lightweight, weather tightness, quality of detailing, and thermal performance. these systems allow easy processing through mass production and pre-assembly of individual façade units [6]. the deductive methodological approach of the research allows to structure the investigation work by sequential and iterative moments: passing from a study and design phase to finally reach the application phase of implementation and subsequent testing of the adaptive model, through testing activities at the tclab section of the building future lab1 (bfl) of the mediterranea university of reggio calabria. the instruments and machines of the tclab section reproduce on mock-up, extreme climatic stresses to study the performance responses and to measure their resilient characteristics. in this paper, the authors present parts of the research results, in which a necessary step was the recognition of adaptive materials of biomimetic approach for the development of the adaptive model and the testing of the dynamic behaviour of two curtain walls for future application. 2. biomimetic approach for adaptive model the interest in understanding the behaviours of nature as well as the ability to adapt a species within a given ecosystem, become the necessary prerequisite for interventions capable of maintaining a balance between the ability to react, which opposes the change, and the ability to maintain functionality after the change. for this reason, it is useful to adopt biomimetic concepts in architecture, as it allows to identify direct solutions and strategies that respond to external stresses organically and passively. most organisms have evolved morphological, physiological, and behavioural adaptations to survive in their habitats. for example, plants exchange water vapour to decrease their internal temperature internal temperatures during hot periods through their stomata [7]. 1large infrastructure of advanced testing, which bases its activities on a set of standardised and experimental tests (unien, astm, aama), having as its object the envelope in its technological and material assets. source: https://www.unirc. it/ricerca/laboratori.php?lab=69 in line with these assumptions, the research wants to investigate the complex concept of adaptivity of biomimetic matrix of building envelopes, leading to the creation of a model that can be used as a serial component for curtain wall systems and aimed at achieving high-performance standards. from a review of the state of the art, it highlights how adaptive materials can be divided into two macro-categories: those whose variability is linked to the intrinsic properties of the material of which they are made and those whose variability is linked to the variability of the shape and configuration, using sensors and handling mechanisms classifiable in pneumatic actuators, hydraulic actuators, and drives based on electric motors. these devices can read external variations and send commands to allow parts of the structure to be moved without adversely affecting the stability of the entire building [8]. the study activities focused on the first macro-category, to identify systems that can respond to external stresses, such as rising temperatures and the resulting heat island effect. within this category, belong electro-active polymers (eap), whose molecular compositions control and produce performance at the level of the material’s shape capable of stretching, bending or folding, depending on the environmental stimulus, without the need for complex electromechanical systems or energy supply [9]. the dynamic behaviour of electroactive polymers is activated when an electric current, the stimulus, flows through the laminate, increasing the electrostatic forces that generate a contraction of the elastomer. an example of the application of this material is homeostatic facade in new york, a glass façade system that self-regulates itself based on light and temperature variations. both sides of the dielectric material are coated with silver electrodes: this layer reflects light and also distributes the electric charge through the material, causing it to deform. when environmental conditions change, the charge in the silver layer causes movement using a sensitive actuator, which is created by wrapping the dielectric material around a flexible polymer core. the increased charge causes the elastomer to expand, bending the core and pulling the elastomer material to one side. the effect is that the facade closes, with the opaque construction blocking the light [10]. another application of electro-active polymers can be found in the media tic building in barcelona. eap strips help to operate and regulate the outer façade, consisting of etfe cushions. protection from external heat is achieved by using the so-called “diaphragm” configuration of the etfe cladding, whereby three layers of plastic are fixed inside the triangular frame and inflated like a cushion. the resulting bubble contains up to three air chambers that together create a shadow effect and provide thermal insulation for the building [11]. however, the biomimetic projects are linked to individual cases of architectural design, with large-scale limitations of innovative envelope systems contribut464 https://www.unirc.it/ricerca/laboratori.php?lab=69 https://www.unirc.it/ricerca/laboratori.php?lab=69 vol. 38/2022 innovation and experimentation of adaptive model for curtain wall ing significantly to climate change control. indeed, the technologies currently on the market are scarcely applicable or used only in buildings of particular use or of size or value such as to justify the adoption and the cost of those systems [12]. in addition, there are gaps of knowledge in terms of market share of adaptive facades, including the main concepts, technologies plus more promising technologies, their categorization, their best use, and the distinction between short-term and long-term structural trends of adaptive facades [13]. this activity was necessary for the acquisition of knowledge about innovation and experimentation of materials and the identification of the most congruent material of the adaptive model. the direct impact that is examined in this research concerns the solar radiation reflected directly from the surface of the curtain walls towards the urban microclimate. an adaptive improvement of the facade could, therefore, reduce the environmental temperature and the impact of the building and, consequently, decrease the magnitude of the phenomenon. given the complex characteristics of innovation, unified testing protocols also require new methods and equipment capable of offering investigation spectra in line with the aspects traced by innovation [14] to test the levels of adaptability of the model by performing simulations of climatic conditions in the urban environment and the effects related to them. 3. methodology and instruments in accordance with the illustrated scenario, the research and its activities are organized in theoreticaloperational sections: from the identification and study of advanced technologies and materials for the adaptive management of curtain walls to an idea of a model and the verification of its technical feasibility, through laboratory experimentation. the first phase relates to the in-depth analysis of the movement and variability systems of the closing-opening system, the study of innovative materials, the analysis of project technologies already underway, the investigation of process innovation and industrial production, the performance aspects and quality controls necessary both to assess the critical nodes of the state of the art and to propose new solutions for the architectural integration of adaptive components. the aim is to define the modalities of possible transfers of adaptivity of biomimetic origin to curtain wall systems, through the use of electro-active polymers. we would like to underline how the complexity of the factors underlying the development of a component in the construction sector is also characterized by long testing times and trials in specialized laboratories, in order to test the real conditions of use, so it often happens that new materials are used but born from other industrial sectors and already tested in other production contexts [15]. according to the subsequent steps, the focus is on the performance of the curtain walls compared to extreme events, choosing representative mock-ups to be tested (provided by partner companies of the laboratory for experimental purposes). the comparison and overlapping of the results obtained from laboratory tests on continuous facade systems, without and with the adaptive model have the ultimate aim of developing and updating test protocols, both for subsequent experimental activities and for regulatory certifications. the testing phase starts thanks to the support of the tclab section, with the possibility of simulating the climatic conditions in urban environments and the effects related to them in order to test the levels of adaptability of the model applied on mock-continuous facade up. for this reason, compared to the phenomena of climate change in the mediterranean area to be examined, the following procedure has been identified for the experimental activities: • heatwave: the experiment is carried out by reproducing a flow of wind, through a fan aama/astm, able to verify the performance behavior of the materials of the outer facade of the mock-up under high pressure and in accordance with astm e 283-04 (2012); • pluvial flooding: the experiment is carried out by reproducing a constant rain directly on the outside of the mock-up, through a sprinkler system, compared to three water jet simulations, through a sprinkler network with calibrated nozzles: in the absence of wind, wind and extreme wind conditions (hurricane power) and following astm e 331-00 (2009), aama 501.1-05-00 (2007) and astm e 330-02 (2010). the advances produced in these first phases of study, represent the bases for the successive activities of development of the model, of experimentation and simulation from life, through the use of the machinery and the equipment2 of the section tclab [16]: a “chamber of test” (test lab, 15 × 12 × 4 m) and a climatic simulation “cell” for indoor and outdoor accelerated tests (test cell, 360 somebody/lat/β). the test chamber, the test lab used for the experimental phase, consists of a steel frame structure 17 × 12 × 4.50 m, closed on three sides, while the fourth side is open to ensure the assembly of mockups in scale 1 : 1, of curtain walls (defined by en 13830:2015 – curtain walling), or similar elements, to be tested according to standardized test procedures. the test lab consists of an aav system (air, water and wind, pressure-depression), a thermal chamber (dimensions 7×5×1.50 m), seismic and mobile beams, for the performance of displacement tests and elastic 2other instruments to support large equipment: uni fan (∼ 60 km/h), aama / astm fan (≳ 190 km/h), fan pressure (+ and -) 6 000 pa, mobile sprinkler grid for watertightness tests, 8 beams for seismic tests of non-structural elements (2 seismic beams on x-y-z axes + 6 mobile beams), in addition to the normal tests involving the standard “facade-building” connection. 465 martino milardi, evelyn grillo acta polytechnica ctu proceedings figure 1. air permeability test on mock-up 01. balances. it is also equipped with 50 lasers for measuring front deflections during wind load tests and a sprinkler system (subject to international patent) which aims to generate a uniform film of water on the surface of the specimen with sprays of different intensities, according to the flow rates required by the uni and astm regulations. the test cell is a structure for the thermodynamic characterization of building envelope systems in full scale and is equipped with a hot-heat flow evaluation box [17]. to carry out the partial results obtained, the first experimental activities are described in the following section. 3.1. the experimental actions of testing according to the “report on the italian building envelope market”, the building envelope component production sector, despite the economic downturn, is aiming towards an increasingly advanced innovation that aims at the production of components resulting from modelling, testing, and improved feedback, increasingly configuring the “new role of the envelope” [18]. the instrumental and operational opportunity of the research, in its different scientific implications, is realized in the strategic possibility to support the prediction of the expected results – or to better calibrate the adaptive configuration of the model – through particular conditions able to read in advance future outcomes, thus enriching the process of a new ability to control the performance dynamics. following the objectives of the research, we wish to refer to the routines of the experiments carried out figure 2. air permeability test on mock-up 02. in the laboratory in a simulated regime. we report the experimental activities carried out on 2 mock-ups of curtain walls without the adaptive model, to verify the performance behavior to the phenomena of heatwaves and pluvial flooding. this activity was necessary to orientate the technological solutions for the development of the adaptive model. the mock-up of façade 01, consisting of aluminum mullions and transoms (stick wall system) with one opening to the outside, is by the aluk group, instead of the mock-up of façade 02, consisting of aluminum mullions and transoms (stick wall system) with a curved corner and three opening windows (one window and two doors), including a balcony (not considered for the overall performance of the façade system), is by glasbilt llc. in accordance with the test protocols and for these types of façade, the following test methods were planned and carried out: • air permeability, astm e 283-04 (2012), consists of the application of incremental and decremental, positive or negative (6.24 psf, 300 pascal) pressure steps, with airflow measurements at each test pressure, to determine the air permeability of curtain walls, both fixed and opening portions expressed in (m3/h). the test is divided into three different phases: measurement of the air permeability of the test chamber, measurement of the permeability of fixed sample and test chamber joints and measurement of the overall of the sample and test chamber (figures 1, 2); • watertightness performance under static pressure was performed in accordance with astm e 331-00 (2009) by applying a constant flow of water to the exterior façade and exposed edges simultaneously with a uniform static pressure of 15 lbf/ft2 (pounds per square foot), 720 pa for a time equal to 15 minutes (figure 3]); • watertightness performance under dynamic conditions was performed in accordance with aama 501.1-05-00 (2007), by applying a dynamic pressure 466 vol. 38/2022 innovation and experimentation of adaptive model for curtain wall figure 3. watertightness performance under static pressure on mock-up 01. of 31.5 lbf/ft2 (pounds per square foot), 1508 pa and an amount of water equal to 5 u.s. gallons per square foot in an hour (= 3.4 l/min m2) to the specimens for 15 minutes through a series of nozzles placed on bars placed horizontally and at a normed distance from the exterior face of the specimens (figure 4); • structural performance was performed in accordance with astm e 330-02 (2010), applying positive and negative test pressures of 50 % and 100 % of the design wind load, for which measurements and checks are made to verify that, under these effects, the two specimens exhibit allowable deformation and retain their stability characteristics. after conducting the tests, a synthetic comparison framework was developed to highlight the potentialities and criticalities of the technological system of the mock-ups. in summary, no disadvantageous behaviour was identified, to the simulation of the heatwaves and pluvial flooding in the absence of wind, revealing a positive and flexible response behaviour of the facades. the results of the air infiltration tests for mockup 01 (table 1), and mock-up 02 (table 2) are reported. it should be noted that the acceptance limit for fixed modules is 0.06 cfm × ft2 and the acceptance limit for the openable module is 0.10 cfm × ft2. test is validated for positive pressure values of fixed modules with the value 0.0116 cfm × ft2 < figure 4. hurricane test on mock-up 02. 0,06 cfm × ft2 and openable modules with the value 0,0121 cfm × ft2 < 0,10 cfm × ft2. test is validated for negative pressure values of fixed modules with the value 0,0211 cfm × ft2 < 0,06 cfm × ft2 and openable modules with the value 0,0442 cfm × ft2 < 0,10 cfm × ft2. it should be noted that the acceptance limit for fixed modules is 0.1 × ft2 and the acceptance limit for the openable module is 0.30 × ft2. test is validated for positive pressure values of fixed modules with the value 0.028 × ft2 < 0.1 × ft2, openable modules (n.1 window) with the value 0.03 × ft2 < 0.30 × ft2 and openable modules (n.2 doors) with the value 0.0095 × ft2 < 0.30 × ft2. test is validated for negative pressure values of fixed modules with the value 0.008 × ft2 < 0.1 × ft2, openable modules (n.1 window) with the value 0.12 × ft2 < 0.30 × ft2 and openable modules (n.2 doors) with the value 0.098 × ft2 < 0.30 × ft2. although the tests are considered successful, it is pointed out that a slight wind pressure on the façade can change the façade behaviour. therefore, as an adaptation strategy for curtain wall systems, it is necessary to develop adaptive models as components to be applied in the façade to increase the performance quality to external phenomena and stresses. 4. conclusion the research has concentrated its efforts on the assumption that the current curtain walls guarantee 467 martino milardi, evelyn grillo acta polytechnica ctu proceedings positive pressure negative pressure air infiltration of facada 3.7 mc/h (0.01 cfm × ft2) 6.7 mc/h (0.02 cfm × ft2) air infiltration of openable 0.3 mc/h (0.01 cfm × ft2) 1.1 mc/h (0.04 cfm × ft2) max value of facada 19.07 mc/h (0.06 cfm × ft2) 19.07 mc/h (0.06 cfm × ft2) max value of openable 2.4 mc/h (0.10 cfm × ft2) 2.4 mc/h (0.10 cfm × ft2) table 1. air infiltration test for the mock-up 01 in positive and negative pressure. positive pressure negative pressure air infiltration of facada 8.47 mc/h (0.028 × ft2) 2.23 mc/h (0.008 × ft2) air infiltration of openable (n.1 window) 0.41 mc/h (0.03 × ft2) 1.53 mc/h (0.12 × ft2) air infiltration of openable (n.2 doors) 0.29 mc/h (0.0095 × ft2) 3.06 mc/h (0.098 × ft2) max value of facada 50.00 mc/h (0.06 × ft2) 50.00 mc/h (0.1 × ft2) max value of openable n.1 6.4 mc/h (0.3 × ft2) 6.4 mc/h (0.3 × ft2) max value of openable n.2 15.9 mc/h (0.3 × ft2) 15.9 mc/h (0.3 × ft2) table 2. air infiltration test for the mock-up 02 in positive and negative pressure. constant performance thresholds calibrated to the average values required by the regulations, resulting in envelopes that have the same performance even in different contextual conditions. therefore, this research, even if still in progress, is oriented towards the design of the optimal combination of the adaptive model, through the use of electro-polymer materials aiming at the most appropriate adaptive efficiency, calibrated to specific environmental contexts. the theme intended to fit into the context of studies conducted on the new dynamic performance of the architectural envelope, which is offered not only by mechanical elements or typological, stratigraphic, and material passivity but above all by the panorama of biomimetic strategies. incorporating these principles into the prototyping of the adaptive model, to be applied and experimented on the mock-ups studied, becomes an essential activity for an architecture that intends to follow the laws that regulate vital processes and cycles. from this point of view, the prototype to be developed represents the physical element of mediation between the external and internal environment, capable of regulating and reacting to the signals that qualify the external environment, contributing to becoming an element of transformation and control. it can be affirmed that the activities and prospects of research and experimentation, can outline concrete solutions, at a theoretical and operational level, referring to the current and future challenges, moving towards an increasingly controlled vision in the relationship between innovation and architectural design. references [1] noaa – national centers for environmental information. state of the climate: global climate report for january 2021, 2021. 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[15] m. mandaglio. from the airplane to the building: the transfer of technology for innovation and experimantation with new materials in the facade. in 40th iahs world congress: sustainable housing construction, 16-19 december 2014, funchal portugal. 2014. isbn 978-989-98949-1-4. [16] m. milardi. un laboratorio di ricerca applicata per i processi di innovazione tecnologica in edilizia/an applied research laboratory for technological innovation processes in building. techne – journal of technology for architecture and environment (11):113– 118, 2016. https://doi.org/10.13128/techne-18409 [17] m. milardi. strumenti e protocolli per il testing avanzato nelle attività di ricerca. testlab testroom testcell. in m. lauria, c. trombetta (eds.), building future lab. ricerca sperimentale per la sostenibilità nel settore delle costruzioni. maggioli editore, rimini, 2016. isbn 8891612830. [18] c. garzia, (a cura unicmi). rapporto sul mercato italiano dell’involucro edilizio. numeri e prospettive del mercato delle facciate, dei serramenti 2020-2021. n. 1, 2020. [2021-12-09]. http://www.unicmi.it/in_evidenza/in_evidenza/ serramenti-e-facciate-continue.html 469 https://doi.org/10.13128/techne-22965 https://doi.org/10.1002/ese3.725 https://doi.org/10.13128/techne-18409 http://www.unicmi.it/in_evidenza/in_evidenza/serramenti-e-facciate-continue.html http://www.unicmi.it/in_evidenza/in_evidenza/serramenti-e-facciate-continue.html acta polytechnica ctu proceedings 38:463–469, 2022 1 introduction 2 biomimetic approach for adaptive model 3 methodology and instruments 3.1 the experimental actions of testing 4 conclusion references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.38.0302 acta polytechnica ctu proceedings 38:302–309, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague transformation and acceleration of the construction supply chain through smart factories bennet zandera,∗, kerstin langea, yanaika decorteb, marijke steemanb, christian struckc, chris gielingc a jade university of applied sciences, department of maritime and logistics studies elsfleth, weserstraße 52, 26931 elsfleth, germany b ghent university, department of architecture and urban planning, jozef plateaustraat 22, 9000 gent ghent, belgium c saxion university of applied sciences, sustainable building technology, maarten harpertszoon tromplaan 28, 7513 ab enschede, netherlands ∗ corresponding author: bennet.zander@jade-hs.de abstract. the european building stock is in high need of refurbishment due to its contribution to excessive global energy consumption. in the north-sea region (nsr) alone there are 22 million houses built between 1950 and 1985 with an annual co2 emission of 79 mton. current deep retrofits are carried out on a limited-scale production, which may result in climate targets not being met in time. to tackle the need for rapid renovations, prefabricated insulation elements with integrated intelligent technologies, manufactured in novel smart factories using mass customization, could offer a solution. this approach is also followed by the interreg project indu-zero. the project examines a far-reaching automated production and develops a blueprint for a smart construction factory in the nsr that can produce 15 000 renovation packages per year. this paper aims to quantify the acceleration potential of the supply chain by improving its production, logistics, and on-site mounting processes for dutch single-family terraced houses. first, the design of the renovation packages and smart construction factories are introduced. then, the procedure is elaborated on how the supply chain can be abbreviated. the results show that the renovation cycle time can be completed within two weeks through coordinated efforts between production, logistics, and mounting. keywords: smart factory, renovation cycle time, supply chain. 1. introduction industry 4.0 still inspires the manufacturing industries and leads to the introduction of new technologies every day. driven by the fourth industrial revolution, companies are investing in the development of smart factories with the aim of integrating digitalization into fabrication structures [1]. by creating an autonomous production and logistics environment in which machines, systems and vehicles communicate independently with each other, companies hope to remove humans from manufacturing processes in order to operate more efficiently [2]. undoubtedly, the building industry seems trivial with its rather stereotypical processes and analogue techniques compared to the pioneering power of the mechanical engineering industry [3]. in the near future, a major restructuring has to take place to tackle the high need for renovation of the building stock as here caused co2 emissions contribute to around 36 % of the european co2 emissions [4, 5]. to achieve the eu’s climate targets, about three million buildings per year should be renovated in europe in the period 2022–2030. however, the renovation speed is too low to deal with these numbers. today, there are only a few production facilities that can offer renovation solutions for a maximum of 500 dwellings per year. in this case, each dwelling is still supplied with specially tailored proposals and the work is mostly done on-site. as a result, the costs of renovations to energyneutral homes, which often exceed €110 000 per unit, is too high to convince homeowners of an energetic renovation [6]. therefore, six nsr countries collaborate in the interreg project indu-zero “industrialization of house renovations toward energy-neutral” to upscale the renovation process with a focus on the renovation of the nsr housing stock and to design a smart factory blueprint to produce 15 000 fully integrated prefabricated renovation packages per year per factory. these packages consist of facade and roof panels, which are complemented by photovoltaic systems as well as heat pumps and ventilation systems. the aim of the design is to achieve energy-neutral dwellings in one-step. the project definition of energy-neutral implies that the energy demand is fully compensated by renewables on an annual basis. the energy demand for household appliances, lighting, material extraction, production and distribution is not included [7]. the facade and roof panels are developed considering several design boundaries such as production 302 https://doi.org/10.14311/app.2022.38.0302 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 38/2022 transformation and acceleration of the construction supply . . . figure 1. assembly of the wall component design: sandwich panel. speed, environmental impact on co2 reduction and total cost of ownership. they are designed for semidetached houses, terraced houses and apartment buildings built between 1950 and 1985, representing 30 % of the housing stock in the nsr. to meet the market demand, all steps are planned in cooperation with social housing corporations and house owners. the structure of the developed renovation packages is independent of a specific project or building. the aim of this masscustomized approach is to eliminate construction as well as on-site complexities and constructability issues, as the panels always follow the same production and mounting processes. moreover, the packages have to be pre-assembled as much as possible before being transported to the construction site, to cut down labour time, total costs and nuisance on-site [7]. figure 1 shows the build-up of the indu-zero facade sandwich panel, which largely consists of hd and ld eps, held together by polyurethane foam glue and complemented by mesh, fire resistant mortar bonding and the outer facade layer. the difference between a facade and a roof panel is that the roof panel is equipped with pv systems instead of fire-resistant stone strips, which are also already installed during the automated production process. in five previous papers, published as part of the project, the moisture assessment, environmental impacts and global warming potential of the shown panel and prefabricated renovation systems, energy demands of the supply chain, as well as the data chain of the smart factory were analysed [6–10]. in contrast, this paper focuses on the acceleration potential of the construction supply chain through smart factories by improving its production, logistics and on-site mounting processes exemplarily illustrating the renovation cycle of a single-family terraced house in the netherlands and comparing it with the current duration. in course of this paper, times of current and future supply chain processes are explained collectively and at the end compared in a resuming table. the outline is as follows. the applied methodology is introduced in section 2. section 3 presents the current supply chain processes and its future transformation through the implementation of smart factories. the renovation concept as well as acceleration potential is shown in section 4. finally, section 5 discusses the results, summarizes the recommendations for future research and concludes the paper. 2. methodology the methodical procedure is based on the method of structured, focused comparison developed by george and bennett, as their approach allows case studies to be compared with previous published work [11]. as presented in their research, the case study developments must explore the same phenomenon, pursue the same research goal, adopt equivalent research strategies and select the same focus [11]. to follow the approach in a proper manner, this paper is being carried out by the three steps illustrated in figure 2. first, a comprehensive literature review of the current processes in the construction supply chain is conducted to show how these are changing through the implementation of smart factories. the first step and its results are complemented using the indu-zero case study to calculate and illustrate the acceleration potential of production, logistics and assembly. in this course, individual methods are used to measure the future times in the three areas of observation. these are explained in each subsection in section 4. subsequently, the specifically identified times of the current and future supply chain are compared and paired. 303 b. zander, k. lange, y. decorte et al. acta polytechnica ctu proceedings figure 2. research process. 3. transformation of the construction supply chain through smart factories the following chapter examines the impact of the implementation of smart factories on the supply chain compared to current industry processes and presents the changes that will result. 3.1. overview of the current industry processes generally, architecture and construction have a low level of industrialization and automation compared to other industries, as only buildings with low technical complexity, such as parking garages, hall buildings or supermarkets, have been recommended for prefabrication. in addition, the following reasons for the low level of automation and digitization in the retrofit market can be identified [12]: • manufacturers and suppliers of products are mostly small companies. there is a lack of original equipment manufacturers and production structures on an industrial scale that can make the necessary investments to build up autonomous production facilities. • due to the predominantly artisanal production as well as rigid organizational structure, trade unions exist that defend the market and thus prevent an overarching approach. • due to the products and materials used in the popular solid construction, it is difficult to automate the production processes, as the raw materials require analogous approaches. • workload in the industry is too high. the construction industry is struggling with a serious labour shortage due to demographic change and simultaneous boom in the real estate market. it is not uncommon to hear that the current supply chain in the construction industry is different from other industries, since there are no factories at its centre. in most instances, raw materials (e.g. clay) are either transported to producers for building materials (e.g. bricks) and after further processing to the construction site, or directly to it, where they are used for building or renovating the house or apartment. most projects are realized with a design-to-order or engineering-to-order approach. each building is supplied by means of tailor-made offers from scratch and most of the work is carried out manually on-site. considering current standards and norms, such as the dutch building decree or german building code, and application of conventional renovation technologies, the energy renovation of a building to a zeroenergy house here takes up to four weeks. in scandinavian countries, this can take even longer, as timber construction is not as easy to refurbish as solid construction, since the wooden facade incl. vinyl tarpaulin must be dismantled, and the humid climate also negatively affects construction site processes [8]. the most time-consuming parts are spent on measuring and design, which takes around three days, and the energy retrofit itself, which takes around seven days for a single terraced house. on the one hand, this is due to the fact that the share of prefabrication has been very small so far. on the other hand, production, transportation and construction processes are only partially coordinated, because cooperating companies do not have necessary capacities. in this way, these procedures are completed one after the other and not simultaneously. this shows the result of the qualitative surveys conducted among three dutch housing corporations. 3.2. smart factories and their impact on construction supply chains the expectations for industry 4.0 are augmented transparency, traceability and process optimizations to reduce repair as well as maintenance costs while increasing machine availability and efficiency [3]. in the future, it will be possible to produce entire house units in a decentralized manner in smart construction factories that aim for a standardized, cost-effective and faster production of residential properties compared to conventional construction on-site processes [13, 14]. standardisation is highly developed to have a more cost-effective and faster production due to a configureto-order approach. products no longer have to be designed from scratch, only slight adjustments need to be configured. by manufacturing according to an industrial scale, housing shortage can be counteracted. furthermore, these factories can target the market for old buildings and renovation projects as the production of customized renovation packages is also possible [15]. 304 vol. 38/2022 transformation and acceleration of the construction supply . . . figure 3. future indu-zero construction supply chain process overview. figure 3 shows the future indu-zero construction supply chain process overview. the smart factory adds another value adding process step to the supply chain, as materials are now no longer transported directly to building sites. in particular, these factories rely on created point cloud and bim models of houses being renovated. based on the derived number of windows and doors and the square meterage of the facade, they can manufacture panels to the millimetre. furthermore, the factory, as the centre of the supply chain, continues to have all necessary data of procurement and distribution. based on digital connections in industry 4.0, the factory knows at what time which materials have to arrive and when production has to take place in order to transport the panels to the construction sites on time [16]. it can coordinate production, logistics as well as mounting process simultaneously without having to consider individual priorities from other companies. in this context, production planning has to consider how the processes of incoming goods, warehousing, production and outgoing goods can be controlled by cyber-physical systems [17]. for independent architects and designers, in particular, this means they can be bypassed in some tasks in the new, transformed value chain. where previously customers consulted architects to design their home retrofit, they now contact the factory or their agents directly. once the house has been measured and digitized via 3d scan, customers configure the new design themselves. afterward, engineers and architects, who work for the factory, only need to check that structural requirements have been met and that the new design adapts to the neighbourhood. the results of indu-zero show that the realization of smart factories in the construction industry is possible. regarding a potential market acceptance, the project identified that not only housing associations, but also governments, industry and investors are increasingly interested in the implementation, since, in addition to the reduction of renovation costs, numerous jobs are created in the factory and on the construction sites [18]. 4. acceleration potential of renovation processes the time advantages resulting from smart factories are explained in detail in the following chapter. 4.1. production the aim of smart construction factories is the standardized, cost-effective and faster production of residential properties compared to conventional construction processes, in which buildings are constructed manually on-site [13]. the production of the facade element in figure 1 requires a number of manufacturing processes shown in figure 4: • a wire cutter slices the ld eps panels to the right length; • hd and ld eps panels are glued together and pressed until bonded firmly; • the cnc milling machine accurately shapes the panels outer dimensions and cuts out the apertures for windows and doors; • hd eps fixation blocks are then attached to the sandwich panel; • the panel is then reinforced with a fiberglass mesh; • the external finishing is attached to the sandwich panels, e.g. plaster or wood strips; • windows and doors are mounted into the panels; • electrical cabling is integrated in the panels; • the quality of the panel is controlled using latest vision-based technologies; • panels are sorted and packed on racks by bundles; • the racks with the panels are then stored before the shipment. all factory processes were adjusted in detail with the software tecnomatix plant simulation and thus provide reliable process times that can be used as a reference for a later reality. the above shown production process takes five hours to complete. moreover, before the materials are brought to the production lines, 305 b. zander, k. lange, y. decorte et al. acta polytechnica ctu proceedings figure 4. smart construction factory production process and used machinery. trucks have to be unloaded, materials have to be separated and assigned to the panels, and a quality check in the incoming goods department has to be carried out, which takes two hours. after production, the glue has to cure for another 24 hours before the panels can be transported to the construction sites. 4.2. logistics just as production can be simulated in the future by digital twins or buildings by bim, logistics can also be planned, replicated and controlled in real time by digital supply chains. in conventional logistics, the parties involved in a value chain tend to appear like rigid members acting separately along a linear supply chain. indu-zero aims to break up this chain and replace it with a flexible network. the focus is on the development of open, digital ecosystems that support the ability to communicate on the market. through them, incompatible interfaces resulting from communication between people, machines, and companies can be closed. thus, the normative planning area as well as the operational machine area are now decoupled from the physical material flow and decisions are made decentrally on basis of the available real-time information. indu-zero serves not only as an example for the autonomous production of 15 000 renovation packages per factory, but also for the design of an intelligent supply chain. the transport of the building materials from suppliers to the smart factory and subsequent to the construction sites play a decisive role, as the logistics processes are also time consuming and can considerably contribute to the overall co2 emissions. in contrast to the current production of individual materials and their use in renovation, indu-zero requires additional transport distances in the supply chain, since raw materials first have to be brought to the smart factory, where they are integrated into the renovation packages. consequently, it is aimed to also offset the here caused extra time as well as co2 emissions. this can be achieved by digital supply chains and an appropriate location planning. by using digital supply chains, procurement, production and distribution logistics can be planned automatically on the basis of predefined criteria. for example, any inventories held by different suppliers are linked online. if one supplier is unable to deliver the required quantity, capacities are compared and other suppliers are automatically commissioned with the supply. in distribution, building sites that are in close proximity to each other can be combined, so that empty runs or additional transports can be avoided. since the supplier locations have already been identified, the future factory can be located in their vicinity to avoid long transport distances. on average, the current transport distance from the suppliers to the manufacturers of the products is 130 km per route in the european construction industry. this is followed by another 100 km to the construction sites. the regular transportation time is four days and the procurement of materials takes two days [19]. due to the sophisticated location planning, the average transport distance can be reduced to around 60 km per route in indu-zero. this results in time savings of over in 60 minutes per way [7]. as roughly four trucks, transporting the raw materials for the production, per dwelling are needed, the location planning saves over eight hours of transportation time. in the end, the additional transport in the supply chain is offset by the mentioned measures, because the total time of the distribution process can thus be reduced to three days. 306 vol. 38/2022 transformation and acceleration of the construction supply . . . figure 5. process and planning of a six-dwellings terraced house. process current time consumption future time consumption measurement and design 3.5+ days 1.5 days procurement 2 days 2 days production 0 days 1.25 day distribution 4 days 3 days on-site mounting 7+ days 3 days total 16.5 days 10.75 days table 1. current and future time consumption for renovating a terraced single-family dwelling. 4.3. on-site mounting on-site mounting covers, depending on the project, about half of the total cost and most of the time of a renovation. the purpose is, equally to the main target, to reduce the current cost and time by around 50 %. furthermore, the duration of the nuisance for the residents should also be avoided as far as possible. innovative machinery (i.e. suction mechanisms for sand and rooftiles, “chimney grabbers” or drones for drilling holes) can be a solution to replace as much human labour as possible, dismantle the current dwelling components as well as mount the new renovation packages. therefore, simulation tools with gamification are used to train machine operators. figure 5 represents the on-site mounting process of a six-dwellings terraced house according to induzero. the times are based on a real-life energetic renovation of a dutch single-family terraced house following the indu-zero concept as well as the experience of dutch housing corporations and their general building contractors interviewed as part of the qualitative survey, which have previously refurbished houses with prefabricated facade elements [20]. the process shows that a block of dwellings can be renovated in 14 days by five workers. in the first day, typical preparations are made and end facade elements are mounted, while precaution measurements are executed for mounting the elements during the second day. day three and four cover the mounting of the facade elements. in turn, the roof panels are mounted from day five till seven. after seven days, the first three dwellings are finished, but remain part of the building site because there are still safety issues for the finished dwellings since they may experience nuisance of heavy machinery like cranes. from day eight till 13, the same cycle repeats for the next three dwellings. day 14 is dedicated to finish the renovation. in the end, the renovation time amounts three days per dwelling on average. 4.4. time saving potential of the renovation process based on the previous results, table 1 can be generated, which summarizes the current and potential future timeline for renovating a dutch terraced singlefamily dwelling. the insights of the current time consumption were gathered from the mentioned surveys and overall renovation experience. the future time consumption results from indu-zero’s research in section 4. the implementation of smart factories can shorten the duration of the supply chain processes by at least six days to a total of 10.75 working days. in the measurement and design phase, two days can certainly be saved, since the renovation packages no longer have to be redesigned from the beginning, but only adapted. typically, the design can take several weeks, depending on the individual wishes of the residents and housing corporations. smart factories make use of predefined components and designs, similar to an automobile configurator, and reduce the required time 307 b. zander, k. lange, y. decorte et al. acta polytechnica ctu proceedings to less than two days. intelligent location planning can further shorten the distances from suppliers to the factory and from there on to the construction site by one day. however, the production process adds another day here. the procurement time of two days is hardly affected and can only be made more flexible, as the process times continue to be influenced by the suppliers. another reason for the reduction in duration is the digital supply chain, as it creates a digital image of all real processes. consequently, these can be planned in a timed manner several weeks in advance in order to simulate any processes and possible incidents. one of the biggest time differences can lastly be seen in the installation processes onsite. here, prefabrication, smart machinery and welldesigned work processes can save around four days. 5. discussion and conclusions this paper showed how the construction supply chain processes can be transformed and accelerated through the integration of smart factories following the method of structured, focused comparison by george and bennett. in total, the entire process can be completed within two weeks through coordinated efforts between production, logistics, and mounting. within ten days after being measured, buildings can be renovated in a climate-neutral way with less waste production on site. the study is limited to single-family terraced house dwellings in the netherlands. however, it can be assumed that the renovation times for other types do not differ greatly from the result, as production and logistics processes remain identical. nevertheless, there may be positive or negative deviations in measurement and assembly, depending on the structure and environment of the building, such as the dismantling of a dilapidated wooden facade including vinyl sheeting in the nordic regions. as there has been some previous investigations [6– 10], the jit supply of prefabricated panels to construction sites or the in-depth investigation of different installation processes can be proposed for future research. there remains also the fundamental question of how home and energy neutrality offset the cost of refurbishment, since housing associations bear the costs but residents save on energy costs. in the end, the construction companies and housing corporations are being forced to push themselves ahead with digitization in order to meet the specified climate guidelines on-time whether with or without smart factories. acknowledgements the described object of research was carried out as part of the project “indu-zero”. the realization is co-financed by the european commission in the interreg north sea region programme 2014–2020 under grant agreement no.: 38-2-10-18. references [1] h. ruile. vom internet der dinge zum geschäftsmodell. in i. göpfert (ed.), logistik der zukunft – logistics for the future, pp. 139–155. 2019. https://doi.org/10.1007/978-3-658-23805-6_4 [2] a.-w. scheer. industrie 4.0: von der vision zur implementierung. in r. obermaier (ed.), industrie 4.0 als 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[12] j. albus, h. drexler. prefab max – das potential vorgefertigter konstruktionssysteme im kostengünstigen wohnungsbau. in b. schönig, j. kadi, s. schipper (eds.), wohnraum für alle?!: perspektiven auf planung, politik und architektur, pp. 301–331. 2017. https://doi.org/10.14361/9783839437292-019 [13] m. tetik, a. peltokorpi, o. seppänen, j. holmström. direct digital construction: technology-based operations management practice for continuous improvement of construction industry performance. automation in construction 107:102910, 2019. https://doi.org/10.1016/j.autcon.2019.102910 308 https://doi.org/10.1007/978-3-658-23805-6_4 https://doi.org/10.1007/978-3-658-16527-7_2 https://www.mckinsey.com/business-functions/operations/our-insights/imagining-constructions-digital-future https://www.mckinsey.com/business-functions/operations/our-insights/imagining-constructions-digital-future https://www.mckinsey.com/business-functions/operations/our-insights/imagining-constructions-digital-future https://ec.europa.eu/energy/topics/energy-efficiency/energy-efficient-buildings_en https://ec.europa.eu/energy/topics/energy-efficiency/energy-efficient-buildings_en https://www.eea.europa.eu/soer/2015/europe/mitigating-climate-change https://www.eea.europa.eu/soer/2015/europe/mitigating-climate-change https://doi.org/10.1088/1755-1315/323/1/012155 https://doi.org/10.1088/1755-1315/588/3/032041 https://www.sir.nl/media/afbeeldingen/nieuws/sintef-report-indu-zero-moisture-asessment.pdf https://www.sir.nl/media/afbeeldingen/nieuws/sintef-report-indu-zero-moisture-asessment.pdf https://doi.org/10.1088/1755-1315/855/1/012007 https://doi.org/10.15480/882.3992 https://doi.org/10.14361/9783839437292-019 https://doi.org/10.1016/j.autcon.2019.102910 vol. 38/2022 transformation and acceleration of the construction supply . . . [14] a. vestin, k. säfsten, m. löfving. on the way to a smart factory for single-family wooden house builders in sweden. procedia manufacturing 25:459–470, 2018. https://doi.org/10.1016/j.promfg.2018.06.129 [15] b. zander, k. lange, h.-d. haasis. impacts of a smart factory on procurement logistics. in hamburg international conference of logistics (hicl) 2020, pp. 459–485. 2020. https://doi.org/10.15480/882.3137 [16] c. rausch, r. lu, s. talebi, c. haas. deploying 3d scanning based geometric digital twins during fabrication and assembly in offsite manufacturing. international journal of construction management 0(0):1–14, 2021. https://doi.org/10.1080/15623599.2021.1896942 [17] x. yao, j. zhou, y. lin, et al. smart manufacturing based on cyber-physical systems and beyond. journal of intelligent manufacturing 30(8):2805–2817, 2017. https://doi.org/10.1007/s10845-017-1384-5 [18] indu-zero. letter of intent signed by 14 parties, 2021. [2021-10-09]. https://northsearegion.eu/indu-zero/background/ news/letter-of-intent-signed-by-14-parties/ [19] seals consortium. statistical coverage and economic analysis of the logistics sector in the eu (seals), 2008. [2021-10-09]. https://transport.ec.europa.eu/ system/files/2016-09/2008_12_logistics.pdf [20] domijn. indu-zero | showcase, 2022. [2022-02-05]. https://www.domijn.nl/overons/duurzaamheid/indu-zero-showcase/ 309 https://doi.org/10.1016/j.promfg.2018.06.129 https://doi.org/10.15480/882.3137 https://doi.org/10.1080/15623599.2021.1896942 https://doi.org/10.1007/s10845-017-1384-5 https://northsearegion.eu/indu-zero/background/news/letter-of-intent-signed-by-14-parties/ https://northsearegion.eu/indu-zero/background/news/letter-of-intent-signed-by-14-parties/ https://transport.ec.europa.eu/system/files/2016-09/2008_12_logistics.pdf https://transport.ec.europa.eu/system/files/2016-09/2008_12_logistics.pdf https://www.domijn.nl/over-ons/duurzaamheid/indu-zero-showcase/ https://www.domijn.nl/over-ons/duurzaamheid/indu-zero-showcase/ acta polytechnica ctu proceedings 38:302–309, 2022 1 introduction 2 methodology 3 transformation of the construction supply chain through smart factories 3.1 overview of the current industry processes 3.2 smart factories and their impact on construction supply chains 4 acceleration potential of renovation processes 4.1 production 4.2 logistics 4.3 on-site mounting 4.4 time saving potential of the renovation process 5 discussion and conclusions acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.38.0065 acta polytechnica ctu proceedings 38:65–71, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague lightwell in residential building: architectural solutions to the daylighting performance through parametric simulation joão pedro de melo souzaa,∗, klaus chaves albertoa, sabrina andrade barbosaa,b a federal university of juiz de fora, faculty of architecture and urbanism, graduate program of built environment, josé lourenço kelmer street, 36036-110 – juiz de fora, brazil b state university of rio de janeiro, department of architecture and urbanism, ipiranga avenue, 544, centro, 25610-150 – petrópolis, brazil ∗ corresponding author: joao.pedro@arquitetura.ufjf.br abstract. the lightwell is an architectural solution to promote daylight into the internal spaces from the core of a building. the shaft configurations, such as geometry and reflectance can affect the performance of the daylighting entering the adjoining rooms under different conditions. this study aims to indicate the adequate architectural solutions to improve the daylighting performance of buildings with lightwells at three different southern latitudes. from a base model of 6-stories building, alternative cases were parametrically simulated using rhinoceros, grasshopper and climatestudio software. from variations in the lightwell geometry and walls reflectance, the cases were analyzed considering the lighting metrics udi and sda300. results demonstrate that the daylighting is adequate on the floors near the top of the building and weakens towards the base of it. in macapá, são paulo and chile, the sda values reach 100 % on the top floor, but only 3 % on the first floor. it was also observed that materials with high diffuse reflectance on the shaft is more efficient in improving the daylighting performance than increasing its geometry. this research presents early-design guidance to inform architects and policymakers when considering the exploitation of daylight by the use of the lightwell. keywords: lightwell, daylighting, wall reflectance, built environment, parametric simulations. 1. introduction the lightwell, a shaft designed to admit daylight through the core of a building, is recognized as an important resource for transmitting lighting to the interior rooms positioned at lower levels [1]. natural light not only benefits people’s comfort and health [2] but can also contribute to the reduction in energy demands [3]. in brazil, for instance, the lightwell geometry is often dimensioned following general guidelines, without accounting for architectural and climatic variables that may influence the lighting availability of the spaces connected to it. thus, when design solutions that can influence the lighting conditions, such as adequate openings dimensions, are neglected by normative and building codes, it may result in environments without adequate daylight [4]. as a result, a poorly designed lightwell can lead to excessive natural light on the upper floors and insufficient natural light on the lower floors [3]. the amount of daylighting reaching the deeper floors depends mainly on the geometry of the lightwell and the reflectance of the internal surfaces. lighter surfaces tend to better distribute light in the deepest portion of the building, while the amount of light received on upper floors is indifferent of wall’s reflectance, since a considerable amount of light received comes directly from the sun [5]. white coloured surfaces can reach values above 80 % of reflectance, while coloured or grey surfaces can have a reflectance of 50 % [6]. joudi et al. [7] pointed out that highly reflective surfaces can generate greater heating of the internal environment when they reflect direct sunlight. however, it is necessary to consider that inside the lightwells this reflection will depend on the solar altitude. in conditions where the sun’s trajectory is lower, direct sunlight reaches only the upper floors, resulting in a significant reduction of daylight on the lower floors of the shaft [4], which tend to be cooler than the top [8]. bugeat et al. [5] indicated that increasing the reflectance of surfaces to up to 85 % does not promote visual and thermal discomfort in the environment. some studies have measured the loss of illuminance in the lower rooms served by lightwells in residential buildings. in the city of tehran, the average annual illuminance was 88 % lower on the first floor compared to the upper floor in interior rooms connected to a 4 m diameter lightwell in a 7-story building [8]. in barcelona [5], this reduction reached 90 % in a building with 12 m high lightwell of 3 m diameter. additionally, it was found that increasing the wall reflectance in 10 % was sufficient to double the illuminance on the first floor. freewan et al. [9] identified that for buildings with three floors, lightwells with a geometry of 1 × 1 m or 2 × 2 m do not provide sufficient illumi65 https://doi.org/10.14311/app.2022.38.0065 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en j. p. m. souza, k. c. alberto, s. a. barbosa acta polytechnica ctu proceedings figure 1. simulated model settings. nance levels on the internal rooms, requiring a shaft of at least 3 × 3 m to provide adequate illuminance throughout the year. however, those studies did not consider the relationship between wall reflectance and shaft geometry. in addition, they focused on the analysis of cities in the northern hemisphere, making it necessary to also consider the use of such solution in the southern latitudes. thus, this study aims to indicate the adequate architectural solutions to improve the daylighting performance of buildings with lightwells at three different southern latitudes. for this, different lightwell geometry, wall reflectance and solar orientations were compared through the use of parametric computational simulations. 2. methods building models were tested through the combination of rhinoceros (version 6.11.18348.17061), grasshopper (version 2019.01.00) and climatestudio (version 1.5.7955.28487) software. a base model of a lightwell building was created using grasshopper software. it has six floors, and the shaft has a square geometry of 4 m and 17.1 m height. for the external wall and floor surfaces, a 50 % reflectance was fixed, which is equivalent to a coloured wall [6]. on each floor, four rooms with a dimension of 2 × 4.5 × 2.85 m (w × l × h) were positioned connected to the lightwell under the main solar orientations. each room has an aperture to the lightwell with a window/wall ratio of 22 %. the diffuse reflectance of the rooms’ surfaces was fixed at 80 % for ceiling and wall, and 50 % for flooring. in each room, 551 sensors were placed at a height of 0.75 m from the floor, a distance of 0.15 m from the walls and spaced 0.15 m between them. illuminances in each room were calculated using dynamic sky models based on three latitudes: macapá, brazil (0° 2′ n, 51° 3′ w), são paulo, brazil (23° 32′ s, 46° 38′ w) and santiago, chile (33° 26′ s, 70° 40′ w). alternative 5 m, 6 m and 7 m square lightwell models were also considered. for each case, five reflectance indices (50 %, 60 %, 70 %, 80 %) were simulated (figure 1). room configurations remained unchanged in the alternative models. the metrics used to evaluate the lighting performance were: sda (spatial daylight autonomy) and udi (useful daylight illuminance). the sda (sda300.50 %) defines the percentage of the floor that receives at least 300 lux for at least 50 % of the busy hours – between 8 a.m. and 6 p.m.; and above 75 %, it is considered satisfactory [10]. the udi determines the percentage of the floor area that receives natural light in the range of 300 to 2000 lux. below this range, the light is considered insufficient to carry out work activities, and above this range, the light is considered excessively blinding [11]. the data obtained through the udi identifies whether the adjustment of architectural variables would promote an increase in uncomfortable natural lighting, especially at the top floors. thus, the combined use of metrics enables a more accurate analysis of the daylighting availability at the length of the building. results are presented and discussed in the following item. it presents the findings of the base models under different latitudes (macapá, são paulo and santiago). the results of daylight under different shaft geometries and reflectance are presented with a focus on the city of mapacá due to the similarity of the cases performances among the different latitudes. finally, it indicates the models with adequate natural lighting among the cases tested. this may help the early-stage design of lightwells. 3. results and discussion 3.1. base model under different latitudes results demonstrated that the daylighting entering the rooms in all latitudes tested is adequate on the floors near the top of the building and weakens towards the base of it. as seen in figure 2, in macapá, the sda values are reduced from 100 % to 48 % from the 6th to the 5th floor, reaching 10 % on the 4th floor. on the first floor, the floor area percentage that receives at least 300 lux for at least 50 % of the busy hours is as low as 3 %. it is important to note that those values were similar for the other latitudes, indicating that for a lightwell with 4 m depth and 50 % of reflectance, only the top floor would reach adequate natural lighting. udi values were also significantly reduced from the 6th to the 1st floor. in macapá, they ranged from 77 % to 3 %; in são paulo, this variation was from 73 % to 3 %; and in santiago, from 81 % to 3 %. thus, the performance of adequate daylighting 66 vol. 38/2022 lightwell in residential building: architectural solutions . . . figure 2. sda values for the base model at different latitudes. figure 3. sda values in the macapa’s model at different lightwell geometries. was similar in the cities analysed. in all cases, the 6th floor receives more than 2000 lux on at least 20 % of the floor during the year. this dazzling lighting remains on the 5th and 4th floors, although to a lesser extent. however, it is important to highlight that high daylight values occur near the windows, which reduces its uncomfortable effect within the environment. it is also interesting to note that the daylight availability is similar on the faces tested. this can be due to the fact that the illuminance corresponds to an average value throughout the year. 3.2. daylight availability for different lightwell geometries in macapá, the enlargement in the lightwell geometry resulted in improvements in the daylighting performance. by increasing the shaft square from 4 m to 5 m, the 5th floor achieved the recommended sda. as can be seen in figure 3, in all solar orientations, this percentage rises from around 48 % to over 75 %, reaching 95 % on the west face. on the lower floors, although the sda values have a considerable increase from the 4 m to the 5 m case, they remained with insufficient adequate natural lighting, reaching less than 25 % on the 4th floor and 4 % on the first floor. in the 6 m geometry model, the 4th floor still has insufficient sda values, with a maximum of 43 %, while the first floor does not exceed 5 %. in the largest lightwell case (7 m), the sda reaches 66 % on the 4th floor and 10 % on the first floor. in all simulated geometries, the udi and sda values were similar at the top floors of the cases tested, indicating that these floors are well lit. increasing the depth of the shaft improves lighting in the intermediate floors, but these improvements are not extended to floors near the first floor, which remain receiving little daylight. 3.3. daylight availability under different shaft reflectance modifications in the reflectance index of the shaft walls resulted in an increase in the sda and udi values in the rooms. by increasing the reflectance from 50 % to 80 %, the 5th floor achieved ideal sda values, jumping from 48 % to 93 %. in those cases, the udi values have also increased, going from around 48 % to 65 %, indicating that daylighting is useful. on the 4th floor, daylighting improves with the increase in reflectance, but do not exceed 33 % of sda. on the first floor, 67 j. p. m. souza, k. c. alberto, s. a. barbosa acta polytechnica ctu proceedings figure 4. sda values in the macapa’s models at different lightwell reflectance. sda and udi double, but still do not reach significant values, remaining at 8 % and 3 %, respectively. figure 4 shows the sda values for the case with 4 m lightwell under different reflectance applied in the shaft. analysing all cases, it was observed that the narrower well (4 m) is the one that benefits the most from the increasing in the reflectance levels. this can be due to the intensity on the reflections rays within the shaft. it can be seen that for the 4 m lightwell case with 80 % of reflectance, the 5th floor showed sda values similar to those on the top floor. on the 4th floor, these values drop, reaching about 30 %. thus, it can be said that increasing the reflectance of the shaft walls proved to be an efficient alternative in improving daylighting performance. in the model with 4 m geometry and 80 % reflectance, the daylight availability is higher than in the 5 m model with 50 % reflectance. however, it is important to note that increasing wall reflection does not vary natural lighting penetration on the floors at the top of the building. in these rooms, the main source of light comes directly from the sun, so reflection contributes little. however, as udi values remains similar in the cases tested, this suggests that there is no increase in lighting above 2000 lux, which would result in uncomfortable lighting. 3.4. best case scenario for each latitude the case that presents the best results is the lightwell with 7 × 7 m geometry and 80 % reflectance (table 1). in the macapá model, the sda values reach 100 % from the upper floor up to the 4th floor and drops to 68 % on the 3rd and 29 % on the first floor. half of the building achieved a satisfactory level of daylight, with sda300 index above 75 % (figure 5). however, they are located on the top three floors, which demonstrates that the improvements lose efficiency as the lightwell gets deeper. the best scenario for the ground floor has a sda of 29 %, which is 14.5 times more than the base model with 4 × 4 m geometry and 50 % reflectance. the improvements in daylighting performance are due to the better light capture by increasing the geometry of the well, and then to the better distribution of light to the bottom of the building due to the increase in reflectance. thus, the potential effect of the combination of architectural variables to improve the performance of daylight is evident. from the analysis of the udi (table 1) in the 7 m model, it was observed it reached 78 % and 78 % on the upper floor and 21 % and 38 % on the first floor of the cases with 50 and 80 % of reflectivity, respectively. thus, increasing the reflectivity enhances the udi values at the bottom of the building, although it remained similar at the top of it. thus, it is possible to conclude that the use of highly reflective surfaces does not contribute to excessive daylighting in the lightwell, reducing the risk of glare and may lowering the heat gains into the building. at the latitudes corresponding to the cities of são paulo and santiago, the results are comparable to those of macapá, due to the similarity in the performance of daylight in the lightwell. 4. conclusion this study aimed to indicate the adequate architectural solutions to improve daylighting performance in buildings with lightwells at three different southern latitudes. for this, different lightwell geometry, wall reflectance and solar orientations were compared. results indicated that using materials with a high diffuse reflectance index to cover the walls of the lightwell is the more efficient solution for improving the daylighting performance of the rooms connected to it that increasing the shaft geometry. it is important to highlight however the potential effect of the combination of architectural variables to improve the performance of daylight. in a narrow well (with 4 m side), a diffuse reflectance index above 80 % results in sda300 values similar to the values of a wider lightwell, such as 6 and 7 m. this indicates that modifying the material that covers the well wall can contribute to improve the daylighting 68 vol. 38/2022 lightwell in residential building: architectural solutions . . . li gh tw el l4 × 4m li gh tw el l7 × 7m r -5 0% r -6 0% r -7 0% r -8 0% r -5 0% r -6 0% r -7 0% r -8 0% sd a u d i sd a u d i sd a u d i sd a u d i sd a u d i sd a u d i sd a u d i sd a u d i 6thfloor e 10 0% 74 % 10 0% 74 % 10 0% 74 % 10 0% 74 % 10 0% 74 % 10 0% 74 % 10 0% 74 % 10 0% 74 % n 10 0% 77 % 10 0% 77 % 10 0% 77 % 10 0% 77 % 10 0% 78 % 10 0% 78 % 10 0% 78 % 10 0% 78 % w 10 0% 76 % 10 0% 77 % 10 0% 77 % 10 0% 77 % 10 0% 78 % 10 0% 78 % 10 0% 78 % 10 0% 78 % s 10 0% 76 % 10 0% 77 % 10 0% 77 % 10 0% 77 % 10 0% 78 % 10 0% 78 % 10 0% 78 % 10 0% 78 % 5thfloor e 53 % 52 % 67 % 58 % 83 % 62 % 98 % 66 % 10 0% 71 % 10 0% 73 % 10 0% 74 % 10 0% 74 % n 48 % 48 % 60 % 55 % 74 % 60 % 93 % 65 % 10 0% 71 % 10 0% 73 % 10 0% 75 % 10 0% 76 % w 55 % 53 % 68 % 58 % 85 % 63 % 98 % 67 % 10 0% 73 % 10 0% 74 % 10 0% 75 % 10 0% 75 % s 48 % 49 % 58 % 55 % 74 % 60 % 94 % 65 % 10 0% 71 % 10 0% 73 % 10 0% 75 % 10 0% 76 % 4thfloor e 13 % 21 % 16 % 27 % 23 % 32 % 32 % 39 % 66 % 56 % 84 % 62 % 97 % 66 % 10 0% 69 % n 12 % 22 % 17 % 27 % 21 % 32 % 32 % 39 % 55 % 53 % 72 % 59 % 93 % 64 % 10 0% 69 % w 13 % 22 % 17 % 27 % 22 % 32 % 33 % 39 % 66 % 57 % 87 % 63 % 99 % 66 % 10 0% 70 % s 12 % 22 % 17 % 27 % 23 % 32 % 33 % 39 % 55 % 53 % 71 % 59 % 92 % 64 % 99 % 69 % 3rdfloor e 5% 10 % 7% 12 % 9% 16 % 13 % 21 % 28 % 38 % 40 % 45 % 52 % 52 % 68 % 59 % n 5% 10 % 7% 13 % 9% 16 % 12 % 21 % 28 % 36 % 38 % 43 % 49 % 50 % 64 % 57 % w 6% 10 % 7% 12 % 9% 16 % 12 % 21 % 30 % 39 % 40 % 46 % 52 % 52 % 68 % 59 % s 6% 10 % 7% 13 % 9% 16 % 13 % 21 % 26 % 37 % 38 % 44 % 49 % 50 % 63 % 57 % 2ndfloor e 3% 5% 4% 6% 4% 8% 6% 11 % 15 % 26 % 20 % 33 % 30 % 38 % 41 % 46 % n 3% 5% 4% 7% 4% 8% 6% 11 % 14 % 27 % 20 % 32 % 29 % 38 % 41 % 45 % w 4% 5% 4% 6% 4% 8% 6% 11 % 14 % 26 % 19 % 32 % 30 % 39 % 43 % 46 % s 3% 5% 4% 6% 4% 8% 6% 11 % 13 % 27 % 19 % 32 % 29 % 38 % 42 % 45 % 1stfloor e 2% 4% 2% 4% 2% 6% 3% 8% 10 % 21 % 13 % 26 % 17 % 31 % 28 % 37 % n 2% 4% 2% 4% 2% 6% 3% 7% 9% 22 % 12 % 26 % 18 % 31 % 27 % 37 % w 2% 3% 2% 4% 2% 6% 3% 7% 9% 21 % 13 % 26 % 18 % 31 % 28 % 38 % s 2% 3% 2% 5% 2% 5% 3% 7% 9% 22 % 12 % 26 % 18 % 31 % 29 % 37 % table 1. comparison between the metrics values under different reflectance levels at macapá city. 69 j. p. m. souza, k. c. alberto, s. a. barbosa acta polytechnica ctu proceedings figure 5. sda values in the model at macapá city. 70 vol. 38/2022 lightwell in residential building: architectural solutions . . . performance in situations where it is not feasible to increase the shaft geometry. despite this, the lightwell with 4 m geometry showed the best results with increased illuminance, especially on the fifth floor. it is also important to note that there were no substantial differences in results among the cities studied or among the cardinal orientations. the use of highly reflective surfaces does not contribute to excessive daylighting in the lightwell. on the 7 m model with 80 % and 50 % reflectance, the udi remains at 78 %. this indicates that most of the illuminance is below 2000 lux. thus, it reduces the risk of glare and heating of the building. not consider the reflections of furniture in the light distribution inside the rooms is one of the limitations of this research. it should also be considered that the dynamic simulations were made using annual climate data. it is possible that in a static simulation, with data referring to a certain time, the values in specific face or latitude may change. it is still necessary to evaluate the contribution of lighting to the heat gains into the interior spaces. this research can provide a better understanding of the functioning of the lightwell, so it is designed to ensure adequate daylighting performance in residential environments. it is suggested that future studies analyse the contribution of natural ventilation in the lightwell of the studied cases. references [1] g. c. bolssoni, a. c. laranja, c. e. alvarez. disponibilidade de iluminação natural em ambiente interno orientado para poço de iluminação. proarq cadernos 31:101–117, 2018. https://cadernos.proarq. fau.ufrj.br/public/docs/proarq31%20art%2005.pdf [2] a. wirz-justice, d. j. skene, m. münch. the relevance of daylight for humans. biochemical pharmacology 191:114304, 2021. https://doi.org/10.1016/j.bcp.2020.114304 [3] i. l. wong. a review of daylighting design and implementation in buildings. renewable and sustainable energy reviews 74:959–968, 2017. https://doi.org/10.1016/j.rser.2017.03.061 [4] m. s. c. albuquerque, c. n. d. amorim. iluminação natural: indicações de profundidade-limite de ambientes para iluminação natural no regulamento técnico da qualidade do nível de eficiência energética de edifícios residenciais – rtq-r. ambiente construído 12(2):37–57, 2012. https://doi.org/10.1590/s1678-86212012000200004 [5] a. bugeat, b. beckers, e. fernández. improving the daylighting performance of residential light wells by reflecting and redirecting approaches. solar energy 207:1434–1444, 2020. https://doi.org/10.1016/j.solener.2020.07.099 [6] j. a. jakubiec. building a database of opaque materials for lighting simulation. in proceedings of the 36th international conference on passive and low energy architecture (plea 2016). 2016. [7] a. joudi, h. svedung, m. cehlin, m. rönnelid. reflective coatings for interior and exterior of buildings and improving thermal performance. applied energy 103:562–570, 2013. https://doi.org/10.1016/j.apenergy.2012.10.019 [8] a. alah ahadi, m. r. saghafi, m. tahbaz. the optimization of light-wells with integrating daylight and stack natural ventilation systems in deep-plan residential buildings: a case study of tehran. journal of building engineering 18:220–244, 2018. https://doi.org/10.1016/j.jobe.2018.03.016 [9] a. a. freewan, a. a. gharaibeh, m. m. jamhawi. improving daylight performance of light wells in residential buildings: nourishing compact sustainable urban form. sustainable cities and society 13:32–40, 2014. https://doi.org/10.1016/j.scs.2014.04.001 [10] iesna – illuminating engineering society of north america. lm-83-12: ies spatial daylight autonomy (sda) and annual sunlight exposure (ase), illuminating engineering society, 2012. [11] a. nabil, j. mardaljevic. useful daylight illuminances: a replacement for daylight factors. energy and buildings 38(7):905–913, 2006. https://doi.org/10.1016/j.enbuild.2006.03.013 71 https://cadernos.proarq.fau.ufrj.br/public/docs/proarq31%20art%2005.pdf https://cadernos.proarq.fau.ufrj.br/public/docs/proarq31%20art%2005.pdf https://doi.org/10.1016/j.bcp.2020.114304 https://doi.org/10.1016/j.rser.2017.03.061 https://doi.org/10.1590/s1678-86212012000200004 https://doi.org/10.1016/j.solener.2020.07.099 https://doi.org/10.1016/j.apenergy.2012.10.019 https://doi.org/10.1016/j.jobe.2018.03.016 https://doi.org/10.1016/j.scs.2014.04.001 https://doi.org/10.1016/j.enbuild.2006.03.013 acta polytechnica ctu proceedings 38:65–71, 2022 1 introduction 2 methods 3 results and discussion 3.1 base model under different latitudes 3.2 daylight availability for different lightwell geometries 3.3 daylight availability under different shaft reflectance 3.4 best case scenario for each latitude 4 conclusion references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.34.0043 acta polytechnica ctu proceedings 34:43–49, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague comparison of feti-based domain decomposition methods for topology optimization problems tomáš medřickýa,∗, martin doškářa, ivana pultarováb, jan zemana a czech technical university in prague, faculty of civil engineering, department of mechanics, thákurova 7, 166 29 prague, czech republic b czech technical university in prague, faculty of civil engineering, department of mathematics, thákurova 7, 166 29 prague, czech republic ∗ corresponding author: tomas.medricky@fsv.cvut.cz abstract. we critically assess the performance of several variants of dual and dual-primal domain decomposition strategies in problems with fixed subdomain partitioning and high heterogeneity in stiffness coefficients typically arising in topology optimization of modular structures. our study considers total feti and feti dual-primal methods along with three enhancements: k-scaling, full orthogonalization of the search directions, and considering multiple search-direction at once, which gives us twelve variants in total. we test these variants both on academic examples and snapshots of topology optimization iterations. based on the results, we conclude that (i) the original methods exhibit very slow convergence in the presence of severe heterogeneity in stiffness coefficients, which makes them practically useless, (ii) the full orthogonalization enhancement helps only for mild heterogeneity, and (iii) the only robust method is feti dual-primal with multiple search direction and k-scaling. keywords: total feti, feti dual-primal, topology optimization, heterogeneous problems, k-scaling, simultaneous feti. 1. introduction topology optimization has became an indispensable tool in the design process, allowing for optimal distribution of a material within a provided space with respect to given criteria (such as minimization of compliance, maximization of output response of mechanisms, or increasing the lowest eigenfrequency) under given constraints (typically limiting the available material volume). in solid isotropic material with penalization (simp) method [1]—one of the most common approaches to topology optimization—the material distribution is parameterized with a scalar relative density field ρ(x), with 0 ≤ ρ ≤ 1, which consequently governs the distribution of stiffness parameters in a domain such that e(x) = emin + ρp(x) (e0 −emin) , (1) where p is a given penalization coefficient (usually p ≥ 3), helping the optimization procedure to achieve the desired “0-1” design without regions of intermediate densities, and e0 and emin are stiffness tensors of the solid material and voids, respectively. note that while the stiffness tensor of the bulk material is physical, void stiffness serves only numerical purposes of avoiding an indefinite hessian matrix. on one hand, it should be small enough in order to model voids properly, on the other hand too small values lead to ill-posed state problem. the optimal design is then usually sought-for in an iterative manner, alternately solving the state equations with a fixed densities and updating design density variables according to the sensitivities computed for a current design. being a matured technology, topology optimization is now widely used in industrial applications with millions of unknowns, and first applications reaching billions of unknowns are emerging [2]. consequently, such scales of optimization problems often prohibit the use of direct solvers in favour of iterative ones, which are further combined with parallelisation. domain decomposition (dd) techniques thus seem promising candidates; however, the presence of high contrast in coefficients due to the stiffness parameterization, recall eq. (1), may cause slower convergence and consequently poor performance of iterative solvers based on dd, as has been already shown in [3]. to make the task even more challenging, in this contribution, we focus in particular on modular topology optimization problems, recently introduced in [4, 5], in which the main domain is composed of a limited number of square modules. the multiple occurrence of individual modules can thus be readily exploited to accelerate calculations; on the other hand, the modularity automatically introduces fixed, regular partitioning, which—as will be shown later—is detrimental for the performance of dd-based solvers. admittedly, the modular grid constitutes only the coarsest partitioning and more refined divisions of each module following e.g. the distribution of a material with the module can be performed, but we did not follow this possibility in our current study. after recalling the fundamentals of two finite element tearing and interconnecting methods in section 2, we review several modifications aimed at improving the performance and robustness of the solu43 https://doi.org/10.14311/app.2022.34.0043 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en t. medřický, m. doškář, i. pultarová, j. zeman acta polytechnica ctu proceedings tion strategy; see section 3. finally, in section 4, the performance of the original methods and their modifications is assessed on two test sets: three academic problems from the literature and three geometries arising from three distinct snapshots of modular topology optimization of the emblematic messerschmitt–bölkow–blohm beam. 2. dual and dual-primal domain decomposition methods characteristically for domain decomposition methods, we assume that the original domain ω is partitioned into ns mutually disjoint subdomains ωs with s = 1 . . . ns. the solution to the system of linear equations kωuω = fω , (2) which arises from e.g. numerically discretized partial differential equations valid in ω, is then sought for via a series of subdomain-wise problems ksus = fs for s = 1 . . . ns , (3) with an additional constraint ensuring continuity of the solution {us}s=1...ns across subdomain boundaries. two major branches of domain decomposition methods can be distinguished based on the manner in which the aforementioned continuity is enforced: primal decomposition methods, such as the schur complement method [6], satisfy the continuity by construction of an approximation space, while the dual methods, e.g. finite element tearing and interconnecting (feti) method [7], impose the continuity requirement as an equality constraint enforced via lagrange multipliers λ. the multipliers λ then also contribute to the right-hand side of eq. (3); see the first row of eq. (6). finally, as its name suggests, the finite element tearing and interconnecting dual-primal (feti-dp) method [8] combines both approaches. in the following two subsections, we briefly summarize the essentials of both dual and dual-primal methods. 2.1. total-feti the total feti method (t-feti), introduced by dostál and co-workers [9] and adopted here as a representative of the dual domain decomposition methods, is closely related to the original feti method [7] with the main difference being the way the dirichlet boundary conditions (bc) are imposed. in t-feti, these bc are handled similarly to enforcing the continuity across subdomain boundaries, i.e. the constraints posed on the domain-wise displacements us read as ns∑ s=1 bsus = bu = c (4) where u collects the domain-wise unknowns such that u = [ u1t , u2t , . . . , uns t ]t (5) and b combines the individual bs’s accordingly. thus, in contrast to the standard feti, constraint (4) features a right-hand side vector c that is in general nontrivial; in addition to the zero entries pertinent to the cross-boundary continuity, it also contains values related to the dirichlet bc. as a result, the number of rigid body modes of each domain is only problem specific (e.g. three for two-dimensional mechanics problems) and factorization can be recycled for all occurrences of one module irrespective of whether the occurrence is supported or floating. combining eqs. (3) and (4) leads to[ k bt b 0 ] [ u λ ] = [ f c ] (6) where k is a block-diagonal matrix composed of ks and f arises from fs assembled similarly to u in eq. (5). assuming the λ is known and provided that f−btλ is orthogonal to the nullspace of k, u can be uniquely determined up to rigid body modes of individual domains expressed as r α, where r is a matrix containing the nullspace vectors of each domain and α is a vector of rigid body modes coefficients. without diving into technical details, for which we refer an interested reader to e.g. [10], the original formulation can be recast into the dual form[ f gt g 0 ] [ λ α ] = [ d e ] (7) with f = bk†bt , d = bk†f − c , g = −rtbt , e = −rtf , where k† denotes the moore-penrose pseudo-inverse of k. problem (7) is traditionally solved with a projected preconditioned conjugate gradient method. 2.2. feti-dp in finite element tearing and interconnecting dualprimal (feti-dp) method [8], the domain-wise displacement field us is decomposed into two parts: (1.) boundary degrees of freedom us c whose continuity is directly enforced in the primal unknowns, and (2.) remaining degrees of freedom us r that contain both internal degrees of freedom, which do not directly communicate with other domains, and boundary degrees of freedom whose continuity is enforced via equality constraints, similarly to eq. (4). without loss of generality, we assume that us is ordered such that us = [ us r us c ] , ks = [ ks rr ks rc ks cr ks cc ] , and fs = [ fs r fs c ] . (8) furthermore, we consider a global vector uc that stores the shared boundary degrees of freedom (dofs) and a boolean matrix bs c such that us c = bs cuc . (9) 44 vol. 34/2022 feti methods for topology optimization problems the equality constraint ensuring continuity of the remaining us b then reads ns∑ s=1 bs rus r = brur = 0 , (10) where—analogously to eqs. (4) and (5)—ur and br are concatenations of us r and bs r, respectively. expressing the equilibrium (3) and the constraint (10) in terms of the quantities introduced in eqs. (8)–(10) leads tokcc kcr 0 krc krr bt r 0 br 0  uc ur λ  = fc fr 0 , (11) with kcc = ns∑ s=1 bs c tks ccbs c , fc = ns∑ s=1 bs c tfs c , kcr = kt rc = [ b1 c tk1 cr b2 c tk2 cr . . . bns c tkns cr ] , krr =  k1 rr 0 . . . 0 0 k2 rr . . . 0 ... ... . . . ... 0 0 . . . kns rr  , and fr =  f1 r f2 r ... fns r . finally, due to the domain-wise nature of the second row in eq. (11), us r can be computed in parallel from the known uc and λ; hence, plugging the expression for us r in terms of the remaining quantities into the first and the third rows of eq. (11) leads to the final dual-primal form[ frr frc ft rc −k̄cc ] [ λ uc ] = [ dr −f̄c ] , (12) where frr = brk−1 rr bt r , frc = brk−1 rr krc , k̄cc = kcc − kcrk−1 rr krc , dr = brk−1 rr fr , f̄c = fc − kcrk−1 kk fr . note that dofs in uc are chosen such that there is no need for a pseudo-inverse of krr in contrast to t-feti. in feti-dp, dofs related to corner nodes in a regular subdomain partitioning typically constitute uc. problem (12) is traditionally solved iteratively with a preconditioned conjugate gradient method with uc being condensed out, resulting in a problem dependent solely on λ [8]. 3. enhancements for heterogeneous domains 3.1. k-scaling the merit of both formulations described above is that they can be conveniently parallelized thanks to their additive structure. in both, the solution is sought with an iterative conjugate gradient method considering only the first block of the corresponding equations (7) and (12), while the remaining blocks are incorporated either via projection (t-feti) or by condensating out the primal unknowns (feti-dp). distinctively for domain decomposition methods, the iterative procedure also benefits from a coarse problem preconditioner f̄−1 of either f, recall eq. (7), or frr, eq. (12), constructed as a sum of local inverses f̄−1 = ns∑ s=1 ( b̃s [ 0 0 0 s̃s ] ( b̃s )t ) (13) where s̃s either • represents the schur complement ss = ks bb − ks biks ii −1ks ib if the optimal dirichlet preconditioner is used, or • s̃s = kbb or s̃s = diag kbb if a computationally cheaper approximations of ss is used, denoted as a lumped or super-lumped preconditioner, respectively, where diag • extracts only the diagonal part of •. note that in feti-dp the unknown index sets i and b are subsets of r only, since the unknowns in set c are handled directly. in the simplest setting, one can assume b̃s = bs; however, b̃s can be arbitrarily scaled provided that [11] ns∑ s=1 ( bs(b̃s)t) bj = bj , ∀1 ≤ j ≤ ns . (14) based on a mechanical reasoning, rixen and farhat [12] proposed k-scaling, which compensates for different stiffness coefficients across subdomain boundaries. in their approach, b̃s = wsbs with the entries of the diagonal matrix ws obtained as w s ii = kr ii ks ii + kr ii , (15) where ks ii denotes the component of the stiffness matrix pertinent to the ith row of bs and, similarly, kr ii is the corresponding coefficient of the stiffness matrix of the adjacent ωr. in the case of multi-domain constraints (which typically happen for corner nodes in t-feti), the authors of [12] advocate for the use of pair-wise constraints despite the introduced redundancies. consequently, only the denominator in eq. (15) changes such that it sums the stiffness contributions from all related domains; see [12, eq. 68]. also note that eq. (15) naturally falls back to multiplicity scaling in the case of domains composed of the same homogeneous material. 3.2. full orthogonalization for practical applications, f-orthogonalization of the current (projected) preconditioned search direction 45 t. medřický, m. doškář, i. pultarová, j. zeman acta polytechnica ctu proceedings w, which appears in the classical preconditioned conjugate gradient method used to solve problems (7) or (12), with respect to the search directions w from the previous iterations is often recommended, e.g. [11]. consequently, this modification necessitates storing the previous directions and their f-norms. however, the faster convergence caused by this modification usually compensate for both the increased memory requirements and the orthogonalization-related calculations. 3.3. rank-revealing simultaneous feti finally, the last enhancement investigated in this work, is the simultaneous variant of both t-feti and fetidp [11]. this modification was particularly developed to handle heterogeneous problems by exploiting the additive structure of the local preconditioners. instead of considering only one preconditioned search direction w in each iteration, it defines an individual search direction for each subdomain. note that the original search direction can be restored with w = w 1, where 1 = [ 1, 1, . . . 1 ]t ∈ rns and w collecting these individual directions as its columns. as a result, each iteration i requires solving a system of ns × ns equations related to minimization and conjugation steps; however, the matrix ∆i as defined in algorithm 1 may be ill-conditioned, e.g. due to a linear dependence of the search directions, and its pseudo-inverse might need to be constructed. here, we employ the rank revealing cholesky factorization [13] with permutation matrix n to extract only the linearly independent search directions, which are subsequently f-orthonormalized; see lines 9–12 in algorithm 1, which summarizes the strategy for t-feti. with this modification, we also automatically employ the full orthogonalization described in the previous subsection; lines 18–21 in algorithm 1. note that in algorithm 1, p is the projection matrix p = i − gt ( ggt)−1 g enforcing the increments of λ to satisfy g λ = e. 4. numerical tests 4.1. academic problems we test both approaches on two sets of illustrative examples. the first suite comprises three test problems studied, e.g., in [11] and shown in fig. 1. unlike fig. 1, however, we keep the regular partitioning and do not investigate the effect of irregularity and bad aspect ratios of the decomposition. we included these problems primarily to verify our implementation against the convergence reported in [11] and also to compare the performance of all considered variants in these academic problems against the problems arising in topology optimization. the first test is a laminated beam consisting of nine repeating square subdomains, each composed of seven alternating layers of stiff (blue) and compliant algorithm 1: rank-revealing simultaneous feti [11] 1 r0 = pt(d− fλ0) 2 z0 = [ . . . , b̃s s̃s(b̃s)tr0, . . . ] ,∀s ∈ {1, . . . , ns} 3 w0 = p z0 4 λ̃0 = 0 5 i← 0 6 while √ rt i zi 1 > ϵ do 7 qi = f wi 8 ∆i = qt i wi 9 factorize n ∆i nt = l lt s.t. l = [ l̃ 0 × 0 ] 10 wi ←wi nt [ l̃−t 0 ] 11 qi ← qi nt [ l̃−t 0 ] 12 ∆i ← i 13 γi = wt i ri 14 λ̃i+1 = λ̃i + wi γi 15 ri+1 = ri − pt qi γi 16 zi+1 = [ . . . , b̃s s̃s(b̃s)tri+1, . . . ] 17 wi+1 = p zi+1 18 for 0 ≤ j ≤ i do 19 ψj = qt j wi+1 20 wi+1 ←wi+1 −wj ψj 21 i← i + 1 (green) material. the second test problem is a square grid of 3 × 3 subdomains, again, with alternating layers. this problem features the corner cross-point and hence different variants of enforcing the continuity of solution is expect to deliver different performance results. finally, the last test problem is a 4× 4 grid of subdomains with a stiff inclusion immersed in each subdomain. unlike the previous two problem, there are no jumps in stiffness coefficients and hence the convergence should be the fastest. all structures were clamped on the left-hand side and subjected to tangential and tensile normal traction on their righthand sides. the two solution strategies (t-feti vs. feti-dp) in combination with the scaling options (stiffness vs. multiplicity) and the option for full orthogonalization or simultaneous search directions yield 12 variants in total. figure 2 plots the history of the residual error defined as εr = √ rt i zi , (16) where the ri and zi are the projected residual and its preconditioned counterpart from algorithm 1, respectively. in accordance with expectations, the third problem with stiff square inclusions was indeed the easiest to solve and the performance of all variants did not vary significantly except for the most advanced si46 vol. 34/2022 feti methods for topology optimization problems (a) (b) (c) figure 1. three academic problems taken from [11]: a) the laminated beam, b) the 3 × 3 grid of periodic, layered subdomains, and, c) the 4×4 grid of peridodic subdomains with immersed stiff square inclusion. multaneous variants of both t-feti and feti-dp which converged in fewer than half of the iterations needed by the other variants. on the other hand, once the heterogeneity in stiffness coefficients reached subdomain boundaries (the first and the second test problems), the performance of the unmodified variants deteriorated rapidly. however, the importance of k-scaling (solid lines) in contrast to the simple multiplicity scaling (dotted lines) arises only in the second example due to the presence of the cross-points; in the third problem for instance, the k-scaling falls back to multiplicity scaling as mentioned in section 3.1. 4.2. topology optimization problems the second test suite comprises several snapshots from the modular topology optimization [4] of a beam fixed at both bottom corners and loaded by a unit force acting at the midspan of the top edge; see fig. 3. the beam is composed of 96 square modules of 16 types (depicted with distinct colours in fig. 3). the modular composition was directly reflected in the beam’s partitioning into 96 subdomains. each subdomain was discretized with 30 × 30 bilinear quadrilateral finite elements, which resulted in 184,512 dofs in total and over 10,000 dofs in the interface problem of both t-feti and feti-dp. in particular, we tested the described solution strategies on the 4th, 8th, and 30th iteration snapshots of the topology optimization. distribution of the base material, parameterized by the relative density ρ (recall eq. (1)), in these snapshots is shown in fig. 4. as the topology optimization progresses, the initially uniform density gradually concentrates to the most efficient locations, which leads to a highly heterogeneous problem with significant jumps in stiffness coefficients along subdomain boundaries, making the problem difficult to solve with iterative methods. similarly to section 4.1, figure 5 summarizes the convergence history of all 12 variants for the three topology optimization snapshots from fig. 4. the later stages of the topology optimization in particular (a) (b) (c) figure 2. convergence history of 12 variants of the solution strategies for the three academic problems shown in fig. 1: a) a laminated beam, b) a 3 × 3 grid of laminated subdomains, and, c) a 4 × 4 grid of subdomains with stiff square inclusions. fo denotes full orthogonalization enhancement, rrs stands for the simultaneous variant with rank-revealing decomposition. in all graphs, results with the k-scaling enhancement are plotted in solid lines while the dotted lines represent the multiplicity scaling. the dash-dotted line mark the desired residual threshold ϵ from algorithm 1. render challenging problems for t-feti and feti-dp solvers; while in the first snapshot, all methods except for t-feti with full orthogonalization successfully converged, in the last snapshot, only the simultaneous feti-dp with rank-revealing factorization succeeded. 5. discussion the numerical tests clearly demonstrate that the linear systems of equations arising in modular topology optimization are indeed challenging for domaindecomposition based iterative methods. the main culprit is the potentially unfavorable partitioning into 47 t. medřický, m. doškář, i. pultarová, j. zeman acta polytechnica ctu proceedings figure 3. an illustration of the beam problem arising in modular topology optimization. the distinct colours represent 16 different types of 96 modules, which constitute the beam’s subdomains ωs. (a) 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 r el at iv e de ns ity (b) (c) figure 4. three snapshots of modular topology optimization constituting the second suite of test problems: a) 4th iteration, b) 8th iteration, and c) 30th iteration. subdomains, which leads to coefficients jumps across subdomain boundaries, and most importantly the high contrast in stiffness coefficients. these effects are partly present in the academic test problems, however, they are significantly more pronounced in the topology optimization problems up to the point at which most of the considered solution variants break or do not converge in a reasonable time. the only variant successful in all tests was the simultaneous feti-dp with rank-revealing cholesky decomposition. with incorporated k-scaling, this variant was also the fastest to converge, even though the convergence rate of t-feti with rank-revealing was usually comparable when it converged. on the other side of the convergence spectrum were the original variants of t-feti and feti-dp. however, while featuring the slowest convergence, those methods proved to be relatively robust, unlike the modifications with full orthogonalization, which reduced the needed iterations only for mildly heterogeneous problems and broke completely for the final test problem. we conjecture that the rapid increase in the (a) (b) (c) figure 5. convergence history of 12 variants of the solution strategies for the three snapshots of modular topology optimization from fig. 4: a) 4th, b) 8th, and c) 30th iteration of the topology optimization. similarly to fig. 2, fo denotes full orthogonalization enhancement, rrs stands for the simultaneous variant with rank-revealing decomposition. in all graphs, results with the k-scaling enhancement are plotted in solid lines while the dotted lines represent the multiplicity scaling. the dash-dotted line mark the desired residual threshold ϵ from algorithm 1. monitored residual in fig. 5c is caused by the loss of numerical accuracy due to round-off errors during the orthonormalization. 6. summary in this work, we investigated twelve variants of dual and dual-primal domain decomposition methods with particular emphasis on applications to highly heterogeneous problems with predefined partitioning which arise in modular topology optimization problems [4, 5]. we considered total-feti [9] and feti-dp [8] as the base methods, for which we tested two scaling 48 vol. 34/2022 feti methods for topology optimization problems approaches (k-scaling and multiplicity scaling), full orthogonalization, and simultaneous search directions with rank-revealing decomposition. we tested the variants on two suites of two-dimensional elasticity problems: first one composed of three academic problems, the second suite comprising snapshots of modular topology optimization. all variants were compared in terms of the number of iterations needed to reach a desired residual ϵ. we are aware that such a comparison does not paint the whole picture and comparing the total wall-clock time would be optimal; however, such a comparison would be heavily biased by the implementation and available hardware. based on our results, we conclude that the k-scaling should be a default setting when dealing with nonhomogeneous problems thanks to its significant impact on the convergence rate and negligible computational overhead. our results also show that the original formulations of both t-feti and feti-dp exhibit very slow convergence in the presence of severe heterogeneity in stiffness coefficients and hence performance enhancements are needed. the simultaneous search directions with the rank-revealing cholesky decomposition reduced the number of needed iterations the most and should be therefore preferred to full orthogonalization alone. finally, feti-dp proved to be more robust than t-feti as it was the only method capable of reaching the desired residual in all test problems. following these conclusions, our current research interest lies in (i) an adaptive choice of the nodes whose compatibility should be enforced directly in primal variables (i.e., not necessarily choosing only the corner nodes for the primal part of feti-dp) and (ii) identification of boundary modes responsible for the bad convergence and eliminating from the iterative process in the spirit of [14]. acknowledgements the authors would like to thank prof. jaroslav kruis and dr. petr mayer, both from the czech technical university in prague, for invaluable discussions and suggestions. tm, md, and jz acknowledge support by the czech science foundation, project no. 19-26143x. ip acknowledges the support by the center of advanced applied sciences, the european regional development fund, project no. cz 02.1.01/0.0/0.0/16 019/0000778. references [1] m. p. bendsøe, o. sigmund. topology optimization. springer berlin heidelberg, berlin, heidelberg, 2004. https://doi.org/10.1007/978-3-662-05086-6. [2] n. aage, e. andreassen, b. s. lazarov, o. sigmund. giga-voxel computational morphogenesis for structural design. nature 550(7674):84–86, 2017. https://doi.org/10.1038/nature23911. [3] s. arul, t. brzobohatý, t. kozubek. feti based domain decomposition methods for large-scale topology optimization. in aip conference proceedings, p. 330002. rhodes, greece, 2020. https://doi.org/10.1063/5.0026486. [4] m. tyburec, j. zeman, m. doškář, et al. modular-topology optimization with wang tilings: an application to truss structures. structural and multidisciplinary optimization 63:1–19, 2021. https://doi.org/10.1007/s00158-020-02744-8. [5] m. tyburec, m. doškář, j. zeman, m. kružík. modular-topology optimization of structures and mechanisms with free material design and clustering, 2021. prerint, 2111.10439. [6] j. kruis. domain decomposition methods for distributed computing. saxe-coburg publications on computational engineering. saxe-coburg publications, stirling, scotland, 2006. [7] c. farhat, f.-x. roux. a method of finite element tearing and interconnecting and its parallel solution algorithm. international journal for numerical methods in engineering 32(6):1205–1227, 1991. https://doi.org/10.1002/nme.1620320604. [8] c. farhat, m. lesoinne, p. letallec, et al. feti-dp: a dual-primal unified feti method–part i: a faster alternative to the two-level feti method. international journal for numerical methods in engineering 50(7):1523–1544, 2001. https://doi.org/10.1002/nme.76. [9] t. kozubek, v. vondrák, m. menšík, et al. total feti domain decomposition method and its massively parallel implementation. advances in engineering software 60-61:14–22, 2013. https://doi.org/10.1016/j.advengsoft.2013.04.001. [10] z. dostál, d. horák, r. kučera. total feti-an easier implementable variant of the feti method for numerical solution of elliptic pde. communications in numerical methods in engineering 22(12):1155–1162, 2006. https://doi.org/10.1002/cnm.881. [11] p. gosselet, d. rixen, f.-x. roux, n. spillane. simultaneous feti and block feti: robust domain decomposition with multiple search directions. international journal for numerical methods in engineering 104(10):905–927, 2015. https://doi.org/10.1002/nme.4946. [12] d. j. rixen, c. farhat. a simple and efficient extension of a class of substructure based preconditioners to heterogeneous structural mechanics problems. international journal for numerical methods in engineering 44(4):489–516, 1999. https://doi.org/10.1002/(sici)1097-0207 (19990210)44:4<489::aid-nme514>3.0.co;2-z. [13] l. bastos, a. hagan. pivoting cholesky decomposition applied to emulation and validation of computer models. tech. rep., university of sheffield, 2007. [14] n. spillane, v. dolean, p. hauret, et al. abstract robust coarse spaces for systems of pdes via generalized eigenproblems in the overlaps. numerische mathematik 126(4):741–770, 2014. https://doi.org/10.1007/s00211-013-0576-y. 49 https://doi.org/10.1007/978-3-662-05086-6 https://doi.org/10.1038/nature23911 https://doi.org/10.1063/5.0026486 https://doi.org/10.1007/s00158-020-02744-8 2111.10439 https://doi.org/10.1002/nme.1620320604 https://doi.org/10.1002/nme.76 https://doi.org/10.1016/j.advengsoft.2013.04.001 https://doi.org/10.1002/cnm.881 https://doi.org/10.1002/nme.4946 https://doi.org/10.1002/(sici)1097-0207 (19990210)44:4<489::aid-nme514>3.0.co;2-z https://doi.org/10.1002/(sici)1097-0207 (19990210)44:4<489::aid-nme514>3.0.co;2-z https://doi.org/10.1007/s00211-013-0576-y acta polytechnica ctu proceedings 34:43–49, 2022 1 introduction 2 dual and dual-primal domain decomposition methods 2.1 total-feti 2.2 feti-dp 3 enhancements for heterogeneous domains 3.1 k-scaling 3.2 full orthogonalization 3.3 rank-revealing simultaneous feti 4 numerical tests 4.1 academic problems 4.2 topology optimization problems 5 discussion 6 summary acknowledgements references https://doi.org/10.14311/app.2022.33.0058 acta polytechnica ctu proceedings 33:58–62, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague durability of gfrp connectors under sustained compression load for use in sandwich walls stefan carstens∗, matthias pahn technische universität kaiserslautern, department of civil engineering, erwin-schrödinger-straße 52, 67663 kaiserslautern, germany ∗ corresponding author: stefan.carstens@bauing.uni-kl.de abstract. precast concrete sandwich panels are used to fulfil the rising thermal requirements. the sandwich walls consist of three layers, a facing, a thermal insulation layer, and a load bearing layer. the two outer layers are coupled by connectors made of glass-fibre reinforced polymer. a lack of knowledge about load-bearing behaviour prevents the removal of sustained compressive loads. in the context of this article, tests under sustained compressive load are presented. to represent closely the in service-conditions of sandwich walls, the examined connectors were subjected to a saturated alkaline concrete environment as well as to a specified stress level till failure occurs. thus, the experimental setup combines alkaline resistance and creep rupture tests into one comprehensive testing. by using temperature effects as an accelerating factor, reasonable test durations were enabled. the obtained time to failure line was determined to extrapolate the characteristic values of the long-term strength for a service life up to 50 years. the test results are compared and evaluated with existing test results under a sustained tensile load. keywords: durability, gfrp connector, sandwich wall sustained compression load. 1. introduction precast concrete sandwich panels have been used as exterior wall systems for many years and have proven themselves in practice [1]. they are characterised by numerous design options (fig. 1) as well as high resistance to weathering and ageing [2]. the prefabricated wall elements have a three-layer structure consisting of a facing layer, a factory-installed thermal insulation layer, and a load-bearing layer. in standard cases, the facing and load-bearing layers are made of normal concrete [3] and in individual cases of lightweight concrete [4] or high-performance concrete [5]. the facing layer serves as an architectural design element that is at least 70 mm thick [6] and fulfils the function of weather protection for the thermal insulation material. the load-bearing layer performs load-bearing functions and is at least 140 mm thick. the thermal insulation materials used are expanded polystyrene (eps) according to din en 13163 [7], extruded polystyrene (xps) according to din en 13164 [8], and insulation materials based on polyurethane (pur) according to din en 13165 [9]. usual insulation thicknesses are between 60 to 240 mm. discrete connectors made of corrosion-resistant materials are used to connect the facing and the load bearing layer [6]. these must allow deformations of the facing layer due to thermal effects as free as possible from constraining forces and simultaneously transfer wind and dead-loads across the thermal insulation [10]. to avoid thermal bridges, metallic connectors such as stainless-steel connectors are increasingly being replaced by connectors made figure 1. precast concrete sandwich panels for use as a façade [5]. out of fibre-reinforced plastic (frp). the connectors usually consist of a thermosetting plastic matrix and unidirectional reinforcing glass fibres. the individual components perform different tasks within the composite [11]. the fibres used are thin (! 3 . . . 25 mm), unidirectional oriented continuous glass fibres, which are completely enclosed by a synthetic resin matrix. due to their mechanical properties, the inorganic fibres have a decisive influence on the stiffness and strength of the composite material [12]. thermosetting polymers such as epoxy resins (ep), vinyl ester resins (ve), and vinyl ester urethane resins (veu) are mainly used as materials for the synthetic resin matrix. the matrix fixes the fibres in the desired geometric position, transfers the stress between the fibres, prevents the fibres from buckling under a compressive load, and protects the fibres from external mechanical and chemical stress. the advantages of the individual components are combined by the interaction of the glass fibre and the polymer matrix. in addition to the properties of the components and their proportions, pultrusion 58 https://doi.org/10.14311/app.2022.33.0058 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 33/2022 durability of gfrp connectors name isolink thermopin tm-mc anchor matrix material veu ep ve fibre material wt. % 87.1 n/a 74.0 vol. % 74.3 n/a 53.0 surface ripped wound flat min. tensile strength n/mm2 1.000 1.500 700 tensile young’s modulus n/mm2 60.000 60.000 40.000 section dimensions mm !c 12.0; !e 13.5 !e 7.3 h/w 9.8/5.7 table 1. characteristics of gfrp connectors acc. to [13]. [12] and the specific interaction of matrix resin and reinforcing fibre are decisive for the properties of the composite material. connectors made of glass-fibre reinforced plastic differ significantly in terms of material composition and cross-sectional geometry. table 1 gives an overview of the material and geometry characteristics of different grp connectors. 30° ø �1 2� ø �1 3. 5� figure 2. the geometry of the examined connector’s head made out of glass fibre reinforced polymers. in the further course of this paper, the isolink [14] shown in figure 2 will be considered as the connector for sandwich wall elements. the connector has a core diameter of 12 mm made out of glass fibre reinforced polymers. the total diameter of the connector, core, and ribs, is 13.5 mm. the ribs are made by indented trapezoidal thread with a profile depth of 0.6 to 0.75 mm and a pitch of 8 mm. the ends of the connector are bevelled at an angle of 30◦. in addition to temporary loads due to wind and temperature effects, the connectors must also be withstanding permanent loads due to the dead load of the facing layer. loads acting vertically on the facing layer are carried by horizontally arranged anchors. the dead load of the facing layer is transferred using load-bearing anchor systems consisting of horizontal and diagonal anchors. the support anchor system consisting of horizontal and diagonal anchors and the associated static system for calculating the distribution of forces in the support anchor system under the dead load of the facing layer are shown in figure 3. as a result of the force-deflection, tensile forces arise in the diagonal connector nt,sust and compressive forces in the horizontal connector nc,sust which have a permanent effect. 2. load-bearing behaviour under short-term compression load while the load-bearing behaviour of the connectors under short-term [15] and long-term tension load [16] is known, there is a lack of knowledge about the loadbearing behaviour under compressive load, especially the stability load-bearing behaviour under consideration of exposures (humidity, temperature and alkalinity). in the context of this paper, therefore, the results of experimental investigations into the loadbearing behaviour under compressive load with large insulation thicknesses are presented. during the test, the connectors are subjected to short-term and longterm pressure loads. in evaluate the load-bearing behaviour under short-term compressive loading, compression tests were carried out at insulation thickness of 160 mm, 250 mm, 300 mm and 350 mm [17]. the specimen geometry is shown in figure 4. to ensure a realistic anchorage of the connectors in the facing and load bearing layer, the connectors are embedded in normal concrete. the thickness of each concrete layer is 120 mm, the other cross-sectional dimensions are 300 × 300 mm. the core layer consists only of the connector, no thermal insulation is used. the test specimen is loaded during the test procedure using a servo-hydraulic test cylinder controlled by displacement. the transverse speed is 0.5 mm/min. simultaneously, the force is measured using a load cell and the deformation through inductive displacement transducers. the determined loads are shown in figure 5. with an insulation thickness of 160 mm, the connectors fail locally in the clamping area combined on thrust and pressure. with other insulation thicknesses, an interlaminar shear crack occurs over the entire length of the connector. furthermore, figure 5 shows the recalculation of the stability tests according to equation 1. a doublesided clamping of the connector is assumed (euler case iv, k = 0.5). the second moment of inertia is using the core diameter !c = 12 mm without considering a stabilizing effect of the ribs. pcr = π2ei (kl)2 (1) 59 stefan carstens, matthias pahn acta polytechnica ctu proceedings figure 3. support anchor system (left) and static system (right) to calculate the distribution of forces in the support anchor system due to the dead load of the facing layer ved. figure 4. geometry of the test specimens for shortand long-term tests under compression load. it can be shown that the results of the experimental investigations for the insulation thicknesses 250 mm, 300 mm and 350 mm correspond very well with the recalculation. with an insulation thickness of 160 mm, the load-bearing capacity is overestimated by the recalculation. the reason for this is the change in the failure mode described above. 3. experimental investigation under sustained compression load the mechanical and physical properties of the composite material can degrade over time due to physical effects from e.g. creep and environmental influences. since the transfer of the dead load of the facing layer must be ensured over the entire service life of the façade, these influences must be taken into account when determining permissible load-bearing capacities. the connectors are particularly exposed to temperature and humidity. furthermore, the alkalinity of the concrete can damage the matrix. figure 5. experimentally and numerically determined the load-bearing capacity of the connector under compression load as a function of the insulation thickness. to evaluate the load capacity of the connectors under compressive load, creep tests to failure are carried out to determine the respective service life. 3.1. experimental set-up and execution of experiments under sustained load the test setup for the creep tests is shown in figure 6. the test specimens described above are placed in a tempered water bath. the water level reaches to just below the upper edge of the lower concrete abutment. the water is held at 60 řc to simulate increased surfaced temperatures in the area of the façade. the increased temperature causes water vapour to form, which settles on the connector and loads to a moderate moisture load. the force is applied by a centrally arranged hydraulic cylinder. the hydraulic pressure in the cylin60 vol. 33/2022 durability of gfrp connectors 1 23 4 figure 6. experimental set-up for compression tests under continuous load and exposure to humidity, temperature and alkalinity; 1 hydraulic jack and oil pressure sensor; 2 test specimen; 3 temperature regulation; 4 stainless-steel profiles for guiding the specimen. der is applied by a manual hydraulic pump and measured by an oil pressure sensor. the deformations of the specimen are measured simultaneously by an inductive displacement transducer. the concrete abutments of the test specimens are guided in parallel by a stainless-steel construction to prevent later deflections. 3.2. results of experimental investigations under sustained load the time-deformation diagram in figure 7 shows the course of the deformation over the load duration. the three technical creep ranges are marked. the elastic deformation of the test specimen can be seen at the beginning. in the initial area, the test specimen shows an increase in linear deformation (primary creep area). the test specimen is then subjected to constant creep (secondary creep). this is followed by an increasing creep deformation (tertiary creep) which is finally followed by the failure of the test specimen. compared to the load capacity of 18.9 kn shown in figure 5, a reduction of the load capacity to 16 kn can be determined for a service life of approximately 100 h. this reduction is probably due in particular to the influence of temperature and humidity. 3.3. evaluation of the results of experimental investigations further service lives are achieved by varying the loads. figure 8 shows the previous service lives of running and finished tests and the associated loads. with decreasing load, an increase in service life is expected. from the service lives of the individual tests figure 7. deformation of the test specimen over the test period at an insulation thickness of 350 mm. and loads, a permissible load for the intended service life of 50 years is then to be derived by extrapolation. the extrapolation should be performed according to [18]. the calculated load capacity for an insulation thickness is on the safe side as insulation thicknesses range from 60 to 350 mm. figure 8. load time to failure diagram of creep tests at an insulation thickness of 350 mm. 4. conclusions multilayer concrete sandwich panels have been used as exterior wall systems for many years and have been proven themselves in practice. connectors made of glass fibre reinforced polymers are increasingly being used to couple the concrete layers. to transfer the dead load of the facing layer through a support anchor system consisting of connectors, the connectors must permanently absorb compressive loads. in this paper, experimental investigations under short-term and long-term compression load are presented. it can be shown that the tests with insulation 61 stefan carstens, matthias pahn acta polytechnica ctu proceedings thicknesses between 250 and 350 mm can be recalculated very well using the euler equation. in the longterm tests, an increased influence of temperature and humidity on the failure load can be derived from the initial results. finally, a concept for extrapolation is proposed for the determination of permissible load capacities. acknowledgements the authors thank schöck bauteile gmbh for the financial and technical support provided. references [1] m. pahn. beitrag zur ermittlung von schnittund verformungsgrößen bei mehrschichtigen stahlbetonwandtafeln mit verbindungsmitteln aus glasfaserverstärktem kunststoff kaiserslautern germany, ph.d. thesis, kaiserslautern, deutschland, 2011. 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[6] din en 1992-1-1/na 2013. nationaler anhang national festgelegte parameter eurocode 2: bemessung und konstruktion von stahlbetonund spannbetontragwerken teil 1-1: allgemeine bemessungsregeln und regeln für den hochbau, 2013. [7] en 13163 2016. thermal insulation products for buildings factory made expanded polystyrene (eps) products specification, 2016. [8] en 13164 2015. thermal insulation products for buildings factory made extruded polystyrene (xps) products specification, 2015. [9] en 13165 2016. thermal insulation products for buildings factory made rigid polyurethane foam (pu) products specification, 2016. [10] p. bindseil. stahlbeton fertigteile nach eurocode 2 konstruktion, berechnung, ausführung, werner verlag, 2012. [11] a. schumann, m. may, m. curbach. carbonstäbe im bauwesen. betonund stahlbetonbau 113(12):868-76, 2018. https://doi.org/10.1002/best.201800077. [12] h. schürmann. konstruieren mit faser-kunststoff-verbunden, springer verlag, 2017. [13] dafstb 615 (en). commentary to en 1992-4 design of fastenings for use in concrete, 2019. [14] zulassung z-21.8-1894. schöck isolink für mehrschichtige betontafeln, 2018. https://www.dibt.de/de/service/ zulassungsdownload/detail/z-218-1894. [15] f. micelli, a. nanni. mechanics of composite materials 39(4):293-304, 2003. https://doi.org/10.1023/a:1025638310194. [16] m. l. keller, m. pahn. durability of gfrp bars with different bar diameters. iabse symposium, guimaraes 2019: towards a resilient built environment risk and asset management, p.603-10, 2019. https://doi.org/10.2749/guimaraes.2019.0603. [17] bericht 06046pa/528. untersuchungen zur eignung von combar hohlwandankern aus glasfaserverstärktem kunststoff als verbindungselement von kerngedämmten wänden mit aufstehender vorsatzschale unreleased bericht 10012pav/14511 2015 versuche zur bestimmung des widerstandes gegen stabilitätsversagen von schöck thermoankern (unreleased), 2008. [18] a. weber. prüfkonzepte für bewehrungsmaterialien mit zeitabhängigen widerständen bauingenieur, 2018. 62 https://doi.org/10.1002/best.200301750 https://doi.org/10.1002/best.201100040 https://doi.org/10.1002/best.201500038 https://doi.org/10.1002/best.201800077 https://www.dibt.de/de/service/zulassungsdownload/detail/z-218-1894 https://doi.org/10.1023/a:1025638310194 https://doi.org/10.2749/guimaraes.2019.0603 my title https://doi.org/10.14311/app.2022.36.0185 acta polytechnica ctu proceedings 36:185–190, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague a procedure to derive partial safety factors in textile-reinforced concrete members jan philip schulze-ardeya,∗, tânia feirib, marcus rickerb, josef heggera a rwth aachen university, institute of structural concrete, mies-van-der-rohe-str. 1, 52074 aachen, germany b hochschule biberach university of applied sciences, institute of structural engineering, karlstraße 9-11, 88400 biberach an der riß, germany ∗ corresponding author: jschulze@imb.rwth-aachen.de abstract. in the last decades, modern technological and research developments of textile-reinforced concrete have led to extensive applications in building and civil engineering structures all over the world. examples of textile-reinforced concrete can be found in retrofitting of existing buildings, facade slabs or bridges. despite its potential, the widespread use of textile-reinforced concrete remains still limited. this is partly explained by the lack of a consistent design framework since conventional design methods used for other materials (e.g., steel reinforced concrete) cannot be directly applicable to textile-reinforced concrete. thus, procedures to derive partial safety factors for textile-reinforced concrete would be a major step forward towards a regular procedure for the design of structural members made of this material.this paper offers a general procedure to determine safety factors. the approach is illustrated with a bending design example of a textile-reinforced concrete facade slab. the example is calculated in a recently developed software package for structural reliability analysis built in the statistical programming language r. for the derivation of safety factors, initial data is required, which can be obtained from experimental or numerical tests or from literature. the paper includes the basics of data evaluation as well as the statistical characterisation of data extracted from literature. keywords: partial safety factor, r-package, reliability-based methods, textile reinforced concrete. 1. introduction in the last decades, encouraged by modern technological and research developments, textile-reinforced concrete (trc) has emerged as a promising alternative to conventional steel reinforced concrete (e.g., [1–5]). examples of trc can be found in multiple structural applications as in retrofitting and rehabilitation of existing reinforced concrete structures, facade slabs or bridges. in albstadt-ebingen (germany), an innovative bridge with a span of 15 meters is a well-known example of a structure made of trc [6]. besides the well-defined material properties, high strength-to-weight and stiffness-to-weight ratios, nonmetallic reinforcements, such as carbon or glass, are insensitive to corrosion [7]. consequently, cover requirements of the reinforcement can be reduced to minimum static values, allowing to decrease the overall thickness of trc elements to 10-30 mm [8]. in addition, since no passivation of the reinforcement is required, a low-clinker content cement can be also used allowing to reduce the environmental footprint of the material regarding its co2 emissions [8]. despite the potential of trc, its widespread use remains still limited. this is partly explained by the lack of a consistent design framework since conventional design methods used for other materials, such as steel reinforced concrete, cannot be directly applicable to trc due to its distinct mechanical behavior. normally practical applications of trc require a proof of usability or a specific approval. to this, building authorities may request extensive load-bearing tests that tend to be complex, time-consuming, costly, and can even lack consistent specifications (e.g., [8–11]). to overcome such difficulties, structural designers have long needed a suitable safety format not only for the design of trc, but also for the reliability assessment of structural systems and components, as previous research studies have showed (e.g., [4, 10, 12]). this paper offers a general procedure to determine partial safety factors based on structural reliability methods for structural members made of trc. the procedure is illustrated using a bending design example of a facade slab made of trc. the example is calculated in a recently developed software package for structural reliability analysis built in the statistical programming language r [14]. this open-source package has been developed in the context of an ongoing research initiative promoted by the german federal ministry for economic affairs and energy (bmwi) with the goal to develop a general standard for deriving partial safety factors based on probabilistic methods (i.e., levels ii and iii) and to establish it in the form of a guideline. ultimately, this guideline aims to facilitate and promote the use of reliability-based approaches among structural engineering practitioners and scientific communities [15, 16]. 185 https://doi.org/10.14311/app.2022.36.0185 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en j. p. schulze-ardey, t. feiri, m. ricker, j. hegger acta polytechnica ctu proceedings figure 1. relation between individual partial factors (adapted from en1990 [13]). 2. deriving partial safety factors (psf) for trc 2.1. brief overview on the code safety format for structural components it is widely acknowledged that the traditional engineering approach to deal with uncertainty and risk in structural engineering has been to apply safety factors in design calculations [17]. modern structural design codes, such as eurocodes [13, 18] and fib model code [19], incorporate probabilistic reasoning in the selection of safety elements (i.e., resistance, actions or partial material factors) with the goal to ensure that the reliability levels for representative structures in design do not exceed an acceptable target (or threshold) reliability level. to this, partial safety factors for different resistance and action variables, γm and γf respectively, need to be calibrated (e.g., [13, 20, 21]). in addition, limit state verifications can be made with the design values of basic variables (see figure 1). current design and examination procedures for the ultimate limit state are based on the verification of the inequality given by equation (1): rd ≥ ed (1) where rd is the corresponding design value of the resistance and ed is the design value of the internal force at the cross-section being analysed. typically, the rd value depends on partial factors applied to material strength variables γm which is influenced by the factor for model uncertainty in structural resistance γrd and by the factor for uncertainty in material properties γm. the ed value depends on partial factors applied to action variables γf which is influenced by the factor for model uncertainty in actions and action effects and the factor for uncertainty in representative values of actions γf (see figure 1). in principle, safety factors for resistance variables shall account for uncertainties in the modelling of material properties, geometric variables and uncertainties related to the model under consideration [22]. recently, yu et al. [8] highlighted that in addition to the basic uncertainties, these safety factors should also account for approximations and uncertainties in the safety format calibration. safety factors for action variables should consider uncertainties in the values of actions and uncertainties related to the models of actions and action effects. like the considerations adopted for conventional steel reinforced concrete, also for trc, the design values of basic variables shall be defined through partial safety factors for resistance variables (in the design of trc these factors are commonly expressed as γt ) and action variables γf . as it is explained in [8], to define a safety format for trc structures, only the partial safety factors related to the resistance need to be calibrated, while the partial safety factors for action variables γf from eurocodes [13, 18] can theoretically be maintained. 2.2. brief overview on code calibration procedures code calibration refers to that particular activity that is exercised when some superior method is applied to assign values to the variables of a code format such that a specific design code is formulated [23]. in the past, for example in eurocodes [13, 18], calibration procedures were based on the so-called level i methods (i.e., deterministic, historical and empirical methods) where the code calibration was also deterministic, historical and empirical [21]. nowadays, due to several pioneering calibration efforts, there is broad agreement on the procedure to be followed for code calibration. for example, inspired by the work of lind [24], baker [25], hawrenek & rackwitz [26] and ellingwood et al. [27] among others, melchers [20] recently presented a general formulation for probabilistically-based code calibration (see figure 2) where a set of essential steps are proposed. these are: (1) define scope, (2) select calibration points, (3) (analyse) existing design code, (4) define limit states, (5) determine statistical properties, (6) apply method of reliability analysis, (7) select target safety index (or reliability index), (8) 186 vol. 36/2022 partial safety factors in textile-reinforced concrete members figure 2. flowchart for calibration of code safety-checking format [20]. observe partial factor format implicit in existing code, and (9) select partial factors. the iterative nature of this general formulation seems to be in line with the principles adopted in modern structural codes. in current eurocodes [13, 18], the proposed safety factors are based on the so-called level ii methods using first-order second moment methods. as explained in [21], the primary assumption of these methods is the independent load combination where a load reduction occurs. this is implemented by sensitivity factors of actions and resistance (i.e., αe and αr, respectively), which decrease the target safety index βt and cause a load reduction. it is highly unlikely that, in this load combination, the highest permanent load and the highest variable load occur simultaneously. here, the partial safety factors are set by minimizing equation 2 in all feasible design cases (i.e., in all materials and in permanent-variable load proportion). in other words, for a given range 1, . . . , m of calibration points, the safety factors that best approximate a uniform target safety level βt can be obtained, in principle, by minimising the value of a measure of ’closeness to a target safety level. the approach aims to minimize the weighted least-squares error in βt : s = m∑ i=1 (βt − βci)2 wi (2) where βt is the target safety level (with nominal failure probability pft = φ(−βt )) and βci is the nominal reliability index for a given calibration point i (with target failure probability pfci = (−βci)). the term wi denotes the weighting factor accounting for the importance of the calibration point relative to design practice, which should result in ∑m i=1 wi. as melchers [20] emphasised, the safety factors are not constant for a given code safety-checking format and given value of βt . for normal design, it is convenient for the safety factors in the code safety-checking format to be constant, at least over large groups of design-checking situations. to achieve this, some deviation from βt is to be expected. hence, the selection of appropriate partial factors involves a certain amount of subjective judgement. it should be noted that to determine the partial factors for the new design code-checking format, trial values of the safety factors are used to calculate βci for each calibration point i [20]. by repeated trial and error, and perhaps by (arbitrarily) assigning values to one or more partial factors, the set of partial factors minimizing equation 2 can be obtained. these values will be then the safety factors in the new generation code safety-checking format. finally, it should be also mentioned that the use of full probabilistic methods − the so-called level iii methods − has not been implemented in modern structural codes yet; however, the use of these meth187 j. p. schulze-ardey, t. feiri, m. ricker, j. hegger acta polytechnica ctu proceedings ods is being considered in the second generation of eurocodes being under development since 2015 [28]. 2.3. proposed methodology to derive partial safety factors for trc considering the existing approaches for code calibration (see sections 2.1. and 2.2.), a general methodology to derive partial safety factor for trc is proposed: 1. define the basic random variables that are relevant for the investigated design case and identify all the parameters that are deterministic. 2. define all the statistical properties for all the basic random variables and specify all the deterministic values. 3. specify the limit state function for the investigated design case. it should be noted that normally the following verification is defined med = mrd. 4. assign a set of possible values for the partial safety factor γt being investigated. note that this factor shall be higher than 1. 5. specify the target safety index βt (e.g., βt ≥ 3.8). 6. calculate the failure probabilities and the corresponding reliability indexes βi for the set of values specified for the partial safety factor (step 4). to this, different reliability assessment methods (i.e., level ii or level iii methods) can be used. 7. evaluate the reliability indexes βi obtained in step 6. the goal is to attain a βi−value close to the selected target safety level βt (e.g., βt ≥ 3.8). if the reliability index βi is smaller than the target level βt , the partial safety factor γt being investigated shall be increased. otherwise, if the reliability index βi is higher than the target level βt , the partial safety factor γt shall be decreased. repeat step 6 until the βi-value is close to the target level βt . here it is important to evaluate if the partial safety factor γt being investigated for a specific design case (e.g., partial safety factor γt for bending) has an influence on the value of mrd. for example, in the case of bending, it should be evaluated if there is a (theoretical) failure mode in the reinforcement (tension zone) or in the concrete (compression zone). if there is a failure mode in the concrete, certain parameters can lead to values with no influence on the partial safety factor γt . 3. practical example 3.1. methodology in this numerical example a facade slab under bending is chosen to demonstrate the derivation of a partial safety factor in a trc member. as described in the previous section, step 1 entails the selection of basic random and the identification of all the variables and parameters. the static system and the respective geometric parameters were selected from [10] and are illustrated in figure 3: then, in step 2 all the basic random variables as well as the deterministic parameters were specified. for reasons of simplicity and inspired by [10], the following basic variables were selected for the characterisation of the resistance side: • model uncertainty for flexural bending for trc members θr • flexural depth d • concrete compression strength fc,cyl • strain of concrete εc • textile reinforcement strength ft • modulus of elasticity of textile reinforcement et follows the definition of the statistical properties for the investigated design case. as well as in step 1 the chosen values are chosen in comparison to those which are used in [10]. the unknown partial safety factor is γt . to specify the design case (step 3), the limit state function was also taken from [10] with the constant approach for stress-strain curve of the concrete regarding to eurocode 2 [18]. med ≤ mrd (3) in step 4, the set of values for the partial safety factor γt was chosen between 1.15 (safety factor of steel rebars γs = 1.15.) and 1.5. then, the target safety level βt was chosen so that βt ≥ 3.8 (step 5). next, in step 5, the failure probabilities pf and the corresponding reliability indexes βi for the specified design case were calculated with suitable probabilisticbased methods. for this numerical example, a level ii method was selected − the first-order reliability method using the classic algorithm of rackwitz and fiessler [29, 30]. finally, step 6 entails the evaluation of the βi−values. this evaluation is discussed in the following section. 3.2. results of parameter study as described above, the goal of this exercise is to find a suitable partial safety factor which satisfies the requirements for safety in codes. normally, this requires a parameter study with variation of the unknown partial safety factor. the results of this parameter study give an indication of the value of the unknown partial safety factor (see figure 4). figure 4 shows that the reinforcement ratio ρl has an influence on the reliability level. for reinforcement ratios of ρl ≥ 0.6 % there is no influence of the chosen value for γt . furthermore, there is a difference if the (theoretical) failure mode is in compression (concrete) zone or in the tensile (textile reinforcement) zone. for a failure mode in reinforcement, a partial safety factor γt of 1.3 or higher, reach the defined target safety level (βt ≥ 3.8). for a failure mode in concrete, a smaller γt of 1.15, satisfies the defined target safety level (βt ≥ 3.8). 188 vol. 36/2022 partial safety factors in textile-reinforced concrete members l q g h d at = ρl ∙ b ∙ d b figure 3. chosen boundary conditions for [10] a) static system and b) geometric parameters. . figure 4. parameter study for the bending design case for different reinforcement ratios and with target safety level βt (red line): a) failure mode of reinforcement and b) failure mode of concrete. despite not being a generalised design case, the investigated example illustrates the procedures required to derive a partial safety factor for a structural member made of trc. for general partial safety factor, broader parameter studies shall be conducted. 4. conclusions despite the potential of trc as an alternative to conventional steel reinforced concrete, its widespread use is still limited due to the lack of a consistent design framework for code calibration. to overcome such limitation, this study proposes a generic methodology to derive partial safety factors for structural members made of trc. the feasibility of the methodology was demonstrated by means of a generic bending design example. despite being applied to a specific failure mode, the generic methodology can be further extended to derive partial safety factors of trc members for different failure modes. the proposed methodology will be further addressed and extended in a new guideline being currently under development in germany. finally, this study has highlighted that there is a need for more universal parameter studies to derive partial safety factors in european codes. this study can give a high-level methodology consisting of six steps. further research work shall be done towards a more detailed methodology for the calibration of code safety-checking. this is particularly relevant for the universal derivation of new partial safety factors in structural engineering. in addition, further parameter studies with a major range of parameters (e.g., different ranges of concrete strength, different ranges of flexural depth, among others) shall be conducted to derive such universal partial safety factor for codes. references [1] s. rempel, n. will, j. hegger, et al. filigrane bauwerke aus textilbeton. betonund stahlbetonbau 110(s1):8393, 2015. https://doi.org/10.1002/best.201400111. 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[28] cen. draft pren 1990:2020 eurocode-basis of structural design; cen: brussels, belgium, 2020. [29] a. m. hasofer, n. c. lind. exact and invariant second-moment code format. journal of the engineering mechanics division 100(1):111-21, 1974. https://doi.org/10.1061/jmcea3.0001848. [30] r. rackwitz, b. flessler. structural reliability under combined random load sequences. computers & structures 9(5):489-94, 1978. https://doi.org/10.1016/0045-7949(78)90046-9. 190 https://doi.org/10.1002/suco.201700225 https://doi.org/10.3390/ma13184210 https://doi.org/10.1002/best.201600058 https://doi.org/10.1016/j.proeng.2013.04.004 https://doi.org/10.1016/j.engstruct.2021.113306 https://doi.org/10.1016/j.engstruct.2021.113306 https://doi.org/10.37544/0005-6650-2017-06-76 https://doi.org/10.3390/app10134625 https://doi.org/10.3390/app10207328 http://www.r-project.org/ https://doi.org/10.1007/978-3-030-73616-3_30 https://doi.org/10.1016/0167-4730(94)90024-8 https://doi.org/10.1680/dgte4.31517 https://doi.org/10.3390/app11125474 https://doi.org/10.1617/s11527-014-0409-1 https://doi.org/10.1061/jsdeag.0005959 https://doi.org/10.1061/jmcea3.0001848 https://doi.org/10.1016/0045-7949(78)90046-9 acta polytechnica ctu proceedings 36:185–190, 2022 1 introduction 2 deriving partial safety factors (psf) for trc 2.1 brief overview on the code safety format for structural components 2.2 brief overview on code calibration procedures 2.3 proposed methodology to derive partial safety factors for trc 3 practical example 3.1 methodology 3.2 results of parameter study 4 conclusions references https://doi.org/10.14311/app.2022.33.0383 acta polytechnica ctu proceedings 33:383–390, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague an experimental study of reinforced concrete beamcolumn joint with partially high strengthened longitudinal bar yoshiyuki murataa,∗, kazuhiro kitayamab, shinji kishidac, ken murakamic a neturen co., ltd., specialty steel & wire products division, tokyo, japan b tokyo metropolitan university, division of architecture and urban studies, japan c shibaura institute of technology university, department of architecture, japan ∗ corresponding author: y-murata@k-neturen.co.jp abstract. it is necessary to generate the yield position away from the column face to minimize damage to the beam-column joint during a large earthquake in a reinforced concrete building. the same efficacy can be realized using partially high strengthened rebar. the number of longitudinal bars can be calculated for a bending moment smaller than the column face, reducing their number compared to the conventional bar-arrangement method. this paper describes reinforced concrete an interior beamcolumn subassemblage tests using this rebar as the longitudinal bars of beams and a column. the beam yield hinge was formed at a position apart from the column face, and the damage to the beam-column joint was less than the conventional bar-arrangement method. additionally, the good performance was obtained if the bending strength of the column was large, even if the shear capacity margin of the beam-column joint was small. the column-beam flexural strength ratio and shear capacity margin at the beam-column joint need to be set with consideration of their relationship. keywords: induction heating, partially high strengthened longitudinal bar, reinforced concrete beam-column joint. 1. introduction the seismic design of high-rise buildings prevents their collapse during large earthquakes by creating yield hinges on the first-floor column bases and the column face of the beam. it is necessary to minimize the damage of the beam-column joint ensure the assumed collapse mode. one of the methods for suppressing damage to the beam-column joint and obtaining a hysteresis loop with excellent seismic resistance is to increase the number of longitudinal bars near the beam-column joint and move the hinge position away from the column face in reinforced concrete beams. these are called hinge relocations. however, the method for increasing the number of longitudinal bars has not been widely used because the arrangement of the longitudinal bars is overcrowded and the number of the longitudinal bars is limited. therefore, partially strengthened rebar was developed as a construction method capable of realizing member performance with excellent seismic resistance while eliminating the overcrowding arrangement. this rebar is arranged on the longitudinal bar of the beam so that the vicinity of the beam-column joint has high-strength (the design yield strength was set to 700 mpa). the hinge relocation is realized by yielding at the strength boundary part. the number of longitudinal bars can be reduced to aămaximum of 56% (= 390/700) compared to using 390 mpa longitudinal bars in the conventional method. this paper describes the experiment of an interior beam-column joint using partial high-strength rebar as the longitudinal bars of the beam and column to confirm the effectiveness of this partial high-strength rebar and obtain design data. 2. manufacture of rebar 2.1. heat treatment method the heat treatment method is shown in figure 1. the heat treatment of the rebar was performed by induction heating. the partially high strengthened rebar was manufactured by fixing the heating coil and moving the rebar in the heating coil. energization by high-frequency current for quenching was performed, when the part to be strengthened passed through the coil. it is difficult to partially increase the strength of one rebar using the furnace heating. however, induction heating allows for easy partial heating, and it is possible to manufacture partially high-strength rebar. 2.2. strength distribution the schematic diagram of the strength distribution of one rebar is shown in figure 2. the actual hardness distribution measurements are shown in figure 3. the raw material was a commercially available rebar with a standard yield strength of 390 mpa, and 383 https://doi.org/10.14311/app.2022.33.0383 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en y. murata, k. kitayama, s. kishida, k. murakami acta polytechnica ctu proceedings +ljk�iuhtxhqf\�fxuuhqw +hdwlqj�frlo 5hlqirufhg�edu figure 1. heat treatment method. the high-strength part was set so that the design yield strength was 700 mpa. the heat treatment was performed on the rebar at a constant speed. as a result, the temperature gradient occurs in the heating coil when the current is turned on and off at the start and end of the high-strength part. the part where the intensity changes at the boundary between the normalstrength part and high-strength part is formed. the hardness at the center, half point, and outer circumference was confirmed by cutting the rebar every 30 mm since the strength of the strength changing part cannot be confirmed by a tensile test. additionally, iron does not transform until approximately 720 degrees. therefore, the strength rapidly increases at the strength boundary part. initially, there was concern that the strength was reduced at the strength boundary part. however, it was confirmed that there was no weakened part as a result of the measurement. 3. test program 3.1. test parameters test results from nine specimens [1–3] are selected and reported here. table 1 provides characteristics of the specimens. the specimen shape and rebar arrangement are shown in figure 4. the test specimens are an interior beam-column joint of about 1/2 scale. each specimen was designed to be a flexural yield type. seven are specimens using partially highstrength rebar. however, specimen a-1 and b-1 are test specimens of the conventional bar-arrangement method for comparing ordinary rebar for both the longitudinal bars of the column and beam. the experimental factors of the specimen using the partial high-strength rebar were the column-beam flexural strength ratio and the joint shear capacity margin to confirm the effect of each value on the damage properties and hysteresis loops. the strengthening length of a-3 and b-3 was set to 320 mm, and the flexural strength of the beam was equal to that of a-1 and b-1 for comparison with the conventional method. the purpose of a-4 was to confirm the effect of the high-strength length. the purpose of a-5 and a-6 was to confirm the effect of reducing the longitudinal bars of the column compared to a-3. the difference between a-5 and a-6 1rupdo�vwuhqjwk� 6wuhqjwk�fkdqjh�sduw� +ljk�vwuhqjwk� 1rupdo�vwuhqjwk� 6wuhqjwk�fkdqjh�sduw� ���03d� ���03d� ���03d6wuhqjwk�erxqgdu\�sduw� figure 2. schematic diagram of the strength distribution. figure 3. actual hardness distribution measurements. was the strength of the longitudinal bars of the column. the column longitudinal bars of a-5 were a normal strength rebar, but a-6 was a partially highstrength rebar, and the purpose is the effect of highstrength rebar. b-1 is the specimen using ordinary rebar like a-1, but the flexural strength of b-1 was greater than a-1. the beam and column longitudinal bars of ma-1 and ma-5 used partially highstrength rebar as in a-6. the strengthening length of the beam longitudinal bars in ma-1 and ma-5 was set to 400ămm, and the stress generated on the column face of the beam was greater than that of the other specimens. the results of the ma-1 experiment showed that the beam-column joint had a relatively severe failure, so ma-5 was used of increase the flexural strength of the column by increasing the total longitudinal bars. the longitudinal bars of a-1, a-3, b-1, and b-3 beams buckled during a large deformation. the distance between the shear rebar of the other specimens was narrowed. furthermore, the distance of the beam shear rebar in the high strengthened part of a-5 and a-6 was narrowed in order to confirm the confinement effect of the concrete by rebar. the shear rebar of the column and beam in ma-1 and maŋ5 was normal strength. 3.2. loading methods the specimen was supported by a roller at the beam end and by a pin at the lower column. the specimen was loaded horizontally by a horizontal jack after the introduction of the axial force of the column. the loading was applied once at the story drift angle of 384 vol. 33/2022 ... specimens a-1 a-3 a-4 a-5 a-6 b-1 b-3 ma-1 ma-5 concrete compressive strength [mpa] 34.7 34.7 42.6 47.4 39.2 40.3 37.8 36.8 beam width × depth [mm] 250 × 400 longitudinal tensile rebar -d16 4+2 2-d16 2-d16+4-d13 -d16 4-d13 4-d16 strengthening length [mm] − 320 200 320 − 320 400 beam stirrups 2-u7.1 @150 2-u7.1 @150 2-u7.1@50 2-u7.1@150 2-d6.1@50 column width × depth [mm] 350 × 350 longitudinal total rebar 10-d16 6-d16 16-d19 6-d19 10-d19 strengthening length [mm] − 350 − 350 axial force ratio 0.16 0.13 0.10 0.16 0.10 column hoops 2-u7.1@100 2-d6@50 joint joint hoops (ratio) 2-u7.1/4set(0.32%) 2-d6/6set (0.35%) yield point strength [mpa] normal part / high strength part d13 371 366/1145 379/972 388/997 − 366/1145 − − d16 − 371/1116 396/964 427/850 399(a366 371/1116 402/823 409/870 d19 − 366 411/893 562/920 u7.1 1309 1434 1393 1309 − d6 − 420 438 column-beam flexural strength ratio 1.42 1.42 1.52 1.13 1.42 2.10 2.13 1.58 2.41 joint shear capacity margina 1.10 1.13 1.38 1.28 1.28 1.19 1.25 1.16 1.12 ∗ the equations adopts aij guidelines for rc buildings [4]. table 1. outline of the specimens. � figure 4. specimen shape and rebar arrangement. 385 y. murata, k. kitayama, s. kishida, k. murakami acta polytechnica ctu proceedings 0.25%, twice at story drift angles of 0.5%, 1.0%, 2.0%, 3.0%, and 4.0%, and then to 5.0-6.0%. the loading setup are shown in figure 5. � )ljxuh��� figure 5. loading setup. 4. test results 4.1. outline of the test results the story shear-story drift angle relations are shown in figure 6. photo 1 shows crack pattern at the end of the second story drift angle of 4.0%. the beam longitudinal bar yielded and the maximum strength reached a cycle of the story drift angle of 2.0% in the conventional bar-arrangement methods a-1 and b-1 using only normal strength rebar. thereafter the strength gradually decreased. the damage of the beam-column joints of a-1 and b-1 was greater than that of the other specimens, and the hysteresis loop was slightly a slip type. the beam longitudinal bars of b-1 was 2.3 times greater than a-1. however, the crack pattern and hysteresis loop were nearly the same, and the effect of increasing the amount of the longitudinal bars was not observed. the specimens using partial high-strength rebar as the beam longitudinal bar had less damage to the beam-column joints than the conventional bararrangement methods a-1 and b-1, and the cracks in the beam were concentrated outside the strength boundary. all specimens using partial high-strength rebar as the beam longitudinal bar showed a tendency to increase in strength even after yielding of the beam longitudinal bar. the hysteresis loop was spindle-shaped until the maximum strength, and the crack pattern was different due to the difference in the experimental factors. the column longitudinal bar of a-3 yielded at the cycle of the story drift angle of 3.0%, and the strength decreased from the second cycle at the story drift angle of 4.0%. this decrease in strength was due to buckling of the longitudinal bar near the strength boundary part. the column longitudinal bar of a-4 yielded on the way to the 6.0% story drift angle. however the hysteresis loops of a-4 and b-3 were spindle-shaped until the end, and the strength reduction was small. a-5, which has the lowest column-beam flexural strength ratio, showed relatively large damage at the beam-column joint. the column longitudinal bar also yields after the beam longitudinal bar yields, and the increase in strength was small. a-6, with longitudinal bars of a-5 with partially high-strength rebar, yielding of the column longitudinal bar did not occur, so the hysteresis loop became more spindle-shaped than a-5. the damage of the beam-column joint of a-6 was less than a-5, but the hysteresis area was smaller than a-3, a-4, and b-3. the effect of densely arranging the shear rebar in the high strengthening length was not particularly recognized. ma-1 with the high strengthening length of the beam longitudinal bars of 400 mm and normal strength rebars for the shear rebar of columns and beams showed severe damage to the beam-column joint, and the shear crack of the beam-column joint extended to the column side at maximum strength. the strength did not reach the calculated flexural strength on the negative side. ma-5, in which the longitudinal bar was greater than ma-1, suppressed the failure of the beamcolumn joint more than ma-1. however, the strength decrease after the maximum strength was greater than that of the a series and b-3. 4.2. comparison with the calculated maximum strength the comparison between the maximum strength experimental values and calculated values are shown in table 2. the flexural strength was calculated by a cross-sectional analysis of the fiber model assuming that plane sections remain plane. a bilinear model was used for the longitudinal bar, and an e-function model was used for the concrete. the beam-column joint shear strength was calculated by the equations of aij guidelines for rc buildings [4]. the maximum experimental strength on the positive side was greater than the calculated flexural strength for all the specimens. however, the maximum experimental strength on the negative side of the specimens where the shear failure of the beamcolumn joint was relatively severe (a-1 and b-1 using only normal-strength rebar as the longitudinal bars, 386 vol. 33/2022 ... � &rqyhqwlrqdo� phwkrg� &rqyhqwlrqdo phwkrg� figure 6. story shear-story drift angle relations. $��� $�� $�� $��� $�� %��� 0$��0$�� &rqyhqwlrqdo� phwkrg� &rqyhqwlrqdo phwkrg %�� figure 7. crack pattern. 387 y. murata, k. kitayama, s. kishida, k. murakami acta polytechnica ctu proceedings specimens a-1 a-3 a-4 a-5 a-6 b-1 b-3 ma-1 ma-5 attained maximum + 120.5 121.4 119.9 133.6 141.8 125.0 145.2 119.3 124.0 story shear qexp [kn] − 112.8 119.8 120.9 129.6 132.4 116.7 125.5 110.9 123.6 beam flexural strength qmb [kn] 115 115 110 128 128 116 116 117 119 column flexural strength qmc [kn] 163 163 167 144 182 244 247 185 287 qexp/qmb + 1.05 1.06 1.09 1.04 1.11 1.08 1.25 1.02 1.04 − 0.98 1.04 1.10 1.01 1.03 1.01 1.08 0.95 1.04 column-beam flexural strength ratio 1.42 1.42 1.52 1.13 1.42 2.10 2.13 1.58 2.41 joint shear capacity vju [kn] 855 855 987 1064 1064 931 948 908 891 joint shear strength vj [kn] 780 756 717 833 833 780 756 781 794 vju/vj 1.10 1.13 1.38 1.28 1.28 1.19 1.25 1.16 1.12 note: + : positive loading, − : negative loading. table 2. maximum strength experimental and calculated values. �+ljk�vwuhqjwk�sduw� figure 8. strain at outer longitudinal bar position calculated from the displacement sensor. a-5 with low column-beam flexural strength ratio, a3 with small shear capacity margin, ma-1 and ma-5 with small shear capacity margin), was less than, or slightly greater than the calculated value. 4.3. increase in the strength after beam yielding the relation between strain at the outer longitudinal bar position calculated from the displacement sensor and story shear in a-3 showing the spindle-shaped hysteresis loop is shown in figure 8. it is assumed that the calculated strain is different from the value of the strain gage attached to the longitudinal bar. however, the longitudinal bar had been deformed in the region of strain hardening at the strength boundary position. therefore, the increase in strength after yielding of the beam longitudinal bar is caused by the strain hardening of the beam longitudinal bar. the inside of the strength boundary part is elastic especially in this method, and the outside expands plasticization. the increase in strength due to strain hardening is greater than that of the conventional bar-arrangement method. 4.4. longitudinal bar strain of beam and column the relationship between the strain of the beam longitudinal bar and the column longitudinal bar according to the experimental results and analysis results are shown figure 9. the strains of the beam and column by the analysis were calculated by separate cross-sectional analysis, and values at the same story shear were plotted. the cross-sectional analysis was performed using a fiber model assuming that plane sections remain plane. the analysis results well reproduced the experimental results. in addition, the strain of the column longitudinal bar was at most 2000 µas a result, and it is assumed that the design may not always be appropriate even if the flexural strength is sufficient due to the high-strength rebar. 4.5. beam-column joint shear deformation the relationships between the shear deformation of the beam-column joint and the story shear are shown in figure 10. the column-beam flexural strength ratio and beam-column joint shear capacity margin are shown in the figure. 388 vol. 33/2022 ... 6wudlq�ri�%hdp +ljk�vwuhqjwk�sduw 6wudlq�ri�&roxpq figure 9. relationship of the strain of beam longitudinal bar and column longitudinal bar. � ����� ����� ���� ���� ���� ���� ���� ���� ����� ����� ���� ���� ���� ���� 8sshu�qxphulfdo�ydoxh���&roxpq�%hdp�ioh[xudo�vwuhqjwk�udwlr��%rwwrp�qxphulfdo�ydoxh���-rlqw�vkhdu�fdsdflw\�pdujlq� ( ) � � � �� b c b c j j j j ! " " + = # $ # # figure 10. relationship of the shear deformation of the beam-column joint and story shear. even if the shear capacity margin of the beamcolumn joint exceeds 1.1, the shear deformation of the beam-column joint isn’t necessarily suppressed. furthermore, if the column-beam flexural strength ratio is large as in the case of a-3 to some extent, the shear deformation can be suppressed to a relatively small value. therefore, it is necessary to set the columnbeam flexural strength ratio and the shear capacity margin at the beam-column joint with consideration of their relationship. 5. conclusions 1. a yield hinge was formed at a position away from the column face using a partially strengthened rebar for the beam longitudinal bars. 2. the damage to the beam-column joint can be mitigated as with other hinge relocation methods. 3. excellent member performance was obtained with fewer longitudinal bars than the conventional bararrangement method, which reduces construction work. 4. the increase in the strength after tensile yielding of the beam longitudinal bar was considered to be due to strain hardening of the beam longitudinal bar. 5. the relation between the strains of the column and beam longitudinal bars was reproduced by crosssectional analysis assuming that plane sections remain plane. 6. the column longitudinal bars may not require high strength at the time of yielding of the beam longitudinal bar, even if a high-strength rebar is used as the column longitudinal bar. it is necessary to set the column-beam flexural strength ratio by appropriately evaluating the strain condition of the column longitudinal bar in order to obtain a good hysteresis loop. 7. the good performance was obtained if the bend389 y. murata, k. kitayama, s. kishida, k. murakami acta polytechnica ctu proceedings ing strength of the column was large, even if the shear capacity margin of the beam-column joint was small. the column-beam flexural strength ratio and shear capacity margin at the beam-column joint need to be set with consideration of their relationship. 8. the good performance was obtained at least if the shear capacity margin of the beam-column joint was 1.2 or more and the column-beam flexural strength ratio was 1.4 or more when a normalstrength rebar was used as the column longitudinal bar. the design data on the relationship between the column-beam flexural strength ratio and shear capacity margin of the beam-column joint was obtained. acknowledgements this experiment was performed in a large laboratory at tokyo metropolitan university. i thank for permission to use the facility. references [1] n. nakamura, s. kishida, t. fukuyama, et al. study on ultimate strength of reinforced concrete beam-column joint using partially high strengthened reinforcing bar in beam and column longitudinal bar proceedings of the japan concrete institute, pp. 319-32, 2016. [2] k. kitayama, k. murakami, s. kishida, et al. and oda m 2018 seismic performance evaluation of reinforced concrete beam-column sub-assemblage with partially high strengthened reinforcing bar (part.1, part.2) summaries of technical papers of annual meeting, architectural institute of japan, pp. 639-642, 2018. [3] k. murakami, t. ishikawa, y. yamaya, et al. seismic performance evaluation of reinforced concrete beam-column subassemblage with partially high strengthened reinforcing bar (part.4, part.5) summaries of technical papers of annual meeting, architectural institute of japan, pp. 135-138, 2019. [4] design guidelines for earthquake resistant reinforced concrete buildings based on inelastic displacement concept. architectural institute of japan, 1999. 390 my title https://doi.org/10.14311/app.2022.36.0127 acta polytechnica ctu proceedings 36:127–134, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague establishing model uncertainty of sls reinforced concrete crack models applied to load-induced cracking christina helen mcleod university of kwazulu-natal, school of engineering, discipline of civil engineering, mazisi kunene road, glenwood, durban 4041, south africa correspondence: mcleodc@ukzn.ac.za abstract. although some research has been performed, serviceability limit state (sls) concrete crack models have yet to be calibrated fully in probabilistic terms in structural design standards. this is partly due to the fact that the sls is generally not the critical limit state in structural design. however, in some specialist structures such as water retaining structures, the sls such as cracking is the limiting design criterion, specifically the crack width required to control leakage and thus requires a proper probabilistic analysis. in probabilistic crack models, the reliability of the crack model is determined by the performance function whereby the design limiting crack width is greater than the estimated crack width calculated using the appropriate design crack model. as cracking in concrete is a random mechanism with a high degree of variability and crack models tend to be at least semi-empirical with inherent uncertainty, model uncertainty of the crack model is significant and is applied in the reliability model as a random variable. a database was established of both short and long term cracking experimental data for the tension load case to quantify model uncertainty. however, the data for long term cracking is limited which meant that the model uncertainty in this case was not definitively established. this paper discusses the determination of model uncertainty for tension cracking, which could be extrapolated to other models where data is limited. keywords: concrete crack models, model uncertainty, reliability. 1. introduction serviceability limit state (sls) concrete crack models are treated nominally in probabilistic terms in structural design standards. this is partly due to the fact that slss are generally not the critical limit states. however, in some specialist structures such as water retaining structures, sls cracking is the limiting design criterion, specifically the crack width required to control leakage. in addition, crack models are mostly empirically or semi-empirically derived so have not been assessed to any extent in reliability terms. however, cracking in concrete is a random mechanism with a high degree of variability indicative of a significant level of model uncertainty which, in turn, influences the level of treatment of model uncertainty in probabilistic analyses. retief (2015) [1] and holický et al (2016) [2] outlined a framework to assess model uncertainty. the relative influence of model uncertainty can be divided into model uncertainty classes [1] classified by an increasing level of treatment, as having nominal effect, significant effect or dominating effect, with, respectively. when model uncertainty is at least significant, it is deemed to be justified to treat model uncertainty as a random variable in the probabilistic model. this contribution discusses the quantification of model uncertainty for shortand long-term tension cracking, which could be extrapolated to other models where data is limited and where sls governs. the selection of the theoretical model for the general probabilistic model (gpm) is also important as model uncertainty is specific to the chosen model. load-induced tension cracking is controlled in structural concrete design standards by limiting the crack width for a given loading, section and reinforcement configuration. the maximum crack width is considered at the tension surface of the cross section. in probabilistic assessments of sls crack models, the reliability of the crack model is determined by the performance function g, expressed as g = wlim − θ. wpredict where wlim is the design limiting crack width, θ is the model uncertainty as a random variable and wpredict is the estimated crack width calculated using the appropriate design crack prediction model. as model uncertainty is specific to the prediction model concerned, suitable load-induced crack models were investigated, applicable to structures where serviceability cracking governs the design, such as reinforced concrete water retaining structures (wrs). the south african industry utilises the now withdrawn british standard bs 8007 (1987) [3] and is at present updating the standard sans 10100-1 (2004) [4] for the design of reinforced concrete structures by adopting the corresponding eurocode [5], concurrently with the development of sans 10100-3 (draft) (2015) [6] for the design of wrs. mcleod (2013) [7] in a deterministic analysis of typical wrs wall configurations, showed that the eurocode crack model was 127 https://doi.org/10.14311/app.2022.36.0127 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en christina helen mcleod acta polytechnica ctu proceedings conservative considering the reinforcement ratio compared to bs 8007’ especially for the tension loading case (> 38 % increase), for a limiting crack width of 0,2 mm. this has significant economic consequences if en 1992 adopted in south africa. the release of the fib model code mc 2010 [8] included an update of the fundamental crack model of fib mc 90 on which the eurocode crack model is based and allows for long term effects. the load-induced crack models for pure tension of bs 8007 (1987) [3], bs en 1992-1 (2004) [5], fib mc 2010 [8] were thus considered and compared to the results obtained for flexural cracking, as reported in [9] and [10]. the results of this research are also of relevance to researchers developing eurocode-related crack models and are applicable to structures other than wrs where serviceability cracking has a significant level of importance in the design thereof, such as bridges. the results of this research will also be utilised in the probabilistic assessment of sls cracking in existing structures such as wrs and bridges. 2. model uncertainty treatment uncertainty exists in all models and requires quantification as far as possible for proper reliability assessment and may be divided into two main types of uncertainty [1]. inherent random variability (aleatory) exists in model parameters such as material properties and loads. as these parameters are treated as random variables in the reliability model, this inherent variability is measured by their coefficients of variation. epistemic uncertainty (that due to incomplete knowledge and statistical uncertainty) may be measured by a model factor treated as a random variable in the reliability model. this model factor is defined as the ratio of measured to predicted values, in this case, crack widths. this ratio, or model factor (mf), is then quantified in terms of its statistical parameters of mean, coefficient of variation (cov) and probability distribution function (pdf). model uncertainty is a measure of the performance of the prediction model where the mean of model factors indicates bias in the model. thus, a mf mean of 1 indicates an unbiased model. a mf mean greater than 1 represents underprediction; conversely, a mean less than 1 indicates an overprediction by the model. the variation of model factors is indicative of the level of uncertainty in the model a higher variation signifying a higher uncertainty. for a model to be suitable over a reasonable range of design applications, it should behave consistently in terms of model factor bias and uncertainty over the range of application. interdependence between model uncertainty and parameters results in drift in the mf mean and an inconsistent model. with a large degree of randomness in concrete crack mechanisms and multiple influencing variables, crack models have high degree of variability (cov well in excess of 0,1 which is the reference value for general structures). model uncertainty (θ) is thus a significant variable in the reliability crack model, as shown by mcleod (2019) [9], requiring full probabilistic treatment. some research (e.g. [11]) has been done in establishing model uncertainty as a model factor but mostly for flexural short term cracking. the mean and cov of parameters were reported but not the pdf of model uncertainty necessary for reliability assessment. 3. deterministic analysis of predicted crack width in determining the predicted crack width for the mf, typical configurations of wrs and thus critical load cases for cracking were investigated. load-induced pure tension cracking occurs in the wall of circular water retaining structures (wrs) due to hoop stresses in the horizontal plane induced by water load, which is considered as a quasi-permanent load by both eurocode and sans design standards. the main reinforcement is thus placed in the horizontal direction with any tension cracks running perpendicular to this reinforcement. as tension cracks in this case tend to be through the full cross section, this is a critical load case for water tightness. a brief summary of the chosen design standard formulations used to calculate the predicted crack width for the critical pure tension load case in a wrs follows. a more comprehensive discussion may be found in [9]. crack mechanism philosophies differ in the modelling of the development and transfer of the stresses between the steel and concrete around a crack in the section, and the resulting strain incompatibility. as cracking is a serviceability limit state, linear elastic stress-strain theory applies. the predicted maximum crack width corresponds to a 5 % probability of exceedance by most design code formulations [9],[10]. this may not probabilistically match the measured maximum crack width, and therefore contributes to model uncertainty. 3.1. bs 8007 crack model the bs 8007 crack model [3] is a no bond-slip empirical model. the maximum surface crack width, w, for tension is calculated using: w = 3 acr εm (1) crack spacing is assumed to be a function of acr, as the distance from the crack considered to the nearest longitudinal reinforcing bar. the mean strain at the surface, εm, is calculated from: εm = ε1 − ε2 (2) where ε1 is apparent steel strain (εs) at the surface and ε2 is the concrete tension-stiffening effect. the equations to calculate tension stiffening strain for tension were derived empirically, calibrated to the specified maximum crack width limit (wlim) as follows: 128 vol. 36/2022 sls reinforced concrete crack models uncertainty ε2 = 2 bt h 3 esas for wlim = 0, 2 mm (3) ε2 = bt h es as for wlim = 0, 1 mm (4) es is the steel modulus of elasticity, h is section depth, bt is the width of the section in tension and as is the area of the tension reinforcement. interpolation for other crack widths is not possible which limits the application of this crack model. 3.2. en 1992 crack model the en 1992 crack width design equation [5] for pure tension cracking is a semi-analytical bond-slip model, developed from the compatibility relationship for cracking and assuming the stabilised cracking phase has been reached, namely: wm = srm · εm (5) where wm is the mean crack width, srm is average crack spacing and εm the mean strain. this crack width prediction model is based on the more fundamental model of fib mc 90. the mean strain is: εm = εsm − εcm (6) where εsm is the mean strain in the reinforcement under loading calculated using linear elastic theory. the mean concrete strain, εcm, also known as tension stiffening, is calculated using: εcm = kt fct,eff ρct,eff (1 − αe ρpeff ) / es (7) where αe is the modular ratio es/ec, ρp,eff is the effective reinforcement ratio between the reinforcement area and the effective area of concrete in tension, fct,eff is the mean tensile strength of the concrete at the time of cracking, ec is the concrete modulus at the time of cracking, and kt is a factor dependent on the duration of load. a minimum limit of 0, 6 σs/es (where σs is the steel stress) is placed on the mean strain. the design (or "maximum") crack width is required rather than the mean width. this maximum crack width is related to the mean crack spacing by the equation: wk = (βw srm) εm (8) where (βw srm) is the maximum crack spacing (sr,max) and the factor βw (the ratio of sr,max to srm) has a value of 1,7 corresponding to 1, 64× standard deviations from the mean (normal distribution of crack widths). the en 1992 maximum crack spacing is: sr,max = k3 c + k1 k2 k4 φ/ρp,eff (9) where φ is the bar diameter, c is the concrete cover to the longitudinal reinforcement and k1 is a coefficient taking into account of the reinforcement bond properties, having a value of 0,8. the values of the factors k3 and k4 are determined by european union individual member countries’ national annexes, with recommended values of 3,4 and 0,425 given for k3 and k4, respectively. the factor k2 allows for stress distribution and has a value of 1 for pure tension loadinduced cracking. all k-values are empirical factors, therefore would contribute to model uncertainty. 3.3. mc 2010 crack model the fib mc 2010 crack model [8] is an update of the fib mc 1990 model. the design crack width, wd, as a maximum (assumed to be the 95th percentile, as en 1992) is determined from: wd = 2 ls,max(εsm − εcm − η εsh) (10) where ls,max is the transfer length over which slip occurs (equal to half the crack spacing) and εsh is the free shrinkage strain over time. the factor η is zero for short-term cracking, and 1,0 for long-term cracking. the minimum limit on mean strain of en 1992 is also specified by mc 2010. the transfer length is determined using: ls,max = k · c + 0.25 fctm τcms · φs ρp,eff (11) where k is an empirical parameter to account for the influence of the concrete cover (k = 1, 0 can be assumed), τcms is the mean bond strength between steel and concrete (considered to be evenly distributed between two cracks) and φs is the nominal diameter of reinforcing bars. the ratio between the concrete tensile strength and mean bond strength (fctm/τcms) is 1/1,8 for stabilised cracking for both short and long-term loads. mc 2010 allows for crack width determination over both the crack formation stage and the stabilised cracking phase. the mc 2010 crack model is valid for c ≤ 75 mm. 4. quantification of crack model uncertainty the model factor for the chosen concrete crack models was defined as the ratio between measured crack width and predicted crack width, wexp/wpred. a database of experimental values for load-induced tension cracking was thus compiled to establish the stochastic parameters of model uncertainty for the crack models of en 1992-1-1, mc 2010 and bs 8007. the database was that from mcleod (2019) [9] updated to include more recent testing by gribniak et al (2020) [13]. the statistical parameters and probability distribution necessary for probabilistic analyses were determined for the model factors of each crack model using standard statistical test methods to a 95 % confidence level. the same procedure of analysis was followed 129 christina helen mcleod acta polytechnica ctu proceedings researcher element type test duration no. of samples** farra & jaccoud [12] ties with single reinforcing bar, square cross section. short-term 71 gribniak [13] ties with 4 no. reinforcing bars, cover varied. 10 samples total, repeat samples. short-term 4 hartl (1977), upm data [14], [15] ties with single or 2 no. reinforcing bars, square cross section. short-term 48 hwang [16] slab elements reinforced in both directions, axial tension in one direction. variation of cover and reinforcement short-term 34 wu [17] ties with single reinforcing bar, square cross section. short (7) & long-term (4) 7 + 4 eckfeldt [15] ties with 1or 2 no. reinforcing bars, square cross section. repeats 2 × 4 no. ties & 3 × 1 ties long-term 11 *final load steps considered only, en 1992 minimum strain complied with. table 1. sources of experimental data direct tension load-induced cracking. as detailed in [9] and [10]. non-parametric normality tests such as kolmogorov-smirnov (using lilliefors significance correction) and shapiro-wilks (corrected), as appropriate, were performed to a significance p of 0,05 to establish the estimated probability distribution of the model factor. graphical methods such as probability plots and box plots were used to validate the estimated probability distribution. through the statistical analysis of model factors, a first assessment of the crack models’ performances was done. pearson’s correlations were used to investigate the relationship between significant parameters and model uncertainty, and thus assess the consistency of each crack model. 4.1. experimental database with data for existing wrs lacking and challenging to obtain, experimental data was used to establish model uncertainty of the crack models of en 1992, bs8007 and mc2010. both shortand long-term data was considered, given the quasi-permanent nature of the water load in wrs. however, the data for long term tension cracking [9] is limited. sources for the database are summarised in table 1. given that the first filling of a wrs only occurs once concrete has reached at least its 28-day strength and considering the quasi-permanent nature of the water load, only data from the stabilised cracking stage was considered. where steel stress is small, crack widths may be underpredicted resulting in an overestimation of model uncertainty, unduly increasing the upper tail of the model uncertainty distribution. in addition, the applied loads and section geometry of water retaining structures typically result in a mean strain well above the minimum strain limit. therefore, data was selected where the calculated mean strain was at least the specified minimum limit of 0, 6σs of en 1992 and mc 2010 where σs is the steel stress determined using linear elastic theory. crack widths measured on the final load step only were considered to ensure independence of samples. results for any repeat samples were averaged to prevent undue sample bias. analyses were performed for en 1992 and mc 2010 using the updated database, whilst the results for bs 8007 were taken from [9]. 5. discussion on model uncertainty quantification 5.1. short term tension the final sample size for short-term tension was 86 after repeats were averaged, which is sufficient to obtain a reasonable estimate of model uncertainty. the statistical parameters for model uncertainty are summarized in table 2. the values obtained for the statistical parameters are similar to those reported on in [9]. there were some small differences on including the research of [13] which consisted of ties reinforced with multiple bars rather than the mostly single bars or uniform samples of the database of [9]. the statistical analysis showed that the en 1992 crack model tends to be conservative, overestimating the predicted short-term tension crack widths, with a mf mean of 0,75. the mf mean of around 1 obtained for the mc 2010 crack model shows little bias in this model. the mc 2010 mf cov of 0,32, however, is higher than that of the en 1992 model (cov of 0,25). the bs 8007 crack model underpredicts crack widths with mf means over 1,27. this bias increases significantly when tension stiffening is determined using the smaller limiting crack width, which is problematic when developing a gpm as the model is not consistent over crack widths away from the given limiting crack width (either 0,1 or 0,2 mm). there is also some uncertainty in the determination of acr in ties reinforced with a single bar as the formulation for acr results in underprediction of the crack spacing and width. 130 vol. 36/2022 sls reinforced concrete crack models uncertainty statistical parameter en 1992 mc 2010 bs 8007 wlim 0.2 mm [9] bs 8007 wlim 0.1 mm [9] mean 0.747 0.996 1.271 1.430 standard error 0.020 0.034 0.032 0.041 median 0.722 0.956 1.225 1.398 standard deviation 0.183 0.319 0.290 0.369 cov 0.245 0.321 0.228 0.258 sample variance 0.033 0.102 0.084 0.136 kurtosis 0.051 -0.581 -0.056 1.234 skewness 0.540 0.427 0.441 0.777 range 0.927 1.408 1.516 2.139 minimum 0.374 0.416 0.582 0.657 maximum 1.301 1.824 2.097 2.796 pdf ln ln n n count 86 86 82 82 table 2. model uncertainty statistical parameters short-term tension cracking. figure 1. probability plots for en 1992 and mc 2010 short-term tension cracking. referring to table 2, both the en 1992 and mc 2010 crack models exhibit a positive skewness which suggests that the distribution is not normal. for the en 1992 model uncertainty, the shapiro-wilks nonparametric test rejected the null hypothesis, whilst for the mc 2010 crack model, the shapiro-wilks significance factor was just greater than 0,05. thus, it is estimated that both models have a non-normal distribution for model uncertainty. curve-fitting was done for both crack models, considering normal and lognormal distributions. this indicated that both en 1992 and mc 2010 tends towards a lognormal distribution, as shown in figure 1. considering that a lognormal distribution produces lower reliability estimates than a normal distribution (so more conservative), and that the distribution of the mc 2010 crack model has a positive skewness and is not clearly normal, a lognormal distribution is assumed for both models. bs 8007 model uncertainty appears to have a normal distribution. 5.2. long term tension statistical analyses for long-term tension cracking are summarised in table 3, and as reported in [9]. the sample size given is the final one after all repeats were averaged. comparing the long-term model uncertainty statistical parameters [9] to the short-term case, crack models that do not take long term shrinkage into account (bs 8007 and en 1992) do not have a consistent model uncertainty as load duration increases. the en 1992 model uncertainty mean increases from 0,75 to 0,90 for short to long-term tension loading. however, when considering the mc 2010 crack model, the mf means for short and long-term loading are very similar. the cov for all models for long-term loading is lower than that of short-term tension, but still demonstrate the significant influence that model uncertainty has on the crack models. however, some restraint is required on interpreting the data given the small sample size. the sample size is too small to give a good indication of skewness, nevertheless, 131 christina helen mcleod acta polytechnica ctu proceedings statistical parameter en 1992 mc 2010 bs 8007 w 0,2 mm bs 8007 w 0,1 mm mean 0.895 0.988 1.318 1.603 standard error 0.078 0.076 0.211 0.280 median 0.860 0.946 1.089 1.353 standard deviation 0.220 0.214 0.597 0.793 cov 0.246 0.216 0.453 0.495 sample variance 0.048 0.046 0.356 0.629 kurtosis -1.539 -0.059 3.779 2.151 skewness 0.442 0.806 1.882 1.447 range 0.571 0.618 1.807 2.328 minimum 0.656 0.764 0.836 0.935 maximum 1.227 1.382 2.643 3.262 pdf (estimated) ln ln n n count 8 8 8 8 table 3. model uncertainty statistical parameters for long-term tension cracking [9]. a lognormal rather than normal distribution is suggested from the positive skewness values obtained for both the en 1992 and mc 2010 crack models. as found for short-term tension, the bs 8007 model uncertainty statistical parameters are not consistent for the different tension stiffening models. 5.3. evaluation of crack models pearson’s correlations the interdependence between selected parameters and model uncertainty for each model were evaluated, together with further assessment of the performance of the en 1992, mc 2010 and bs 8007 crack models by means of scatter plots, regression analyses and pearson’s correlations. the selected parameters were reinforcing ratio (as % as), steel stress, concrete tensile strength (fctm), section depth (h), concrete cover (c) and section width (b). the pearson’s correlation factors between the model factor and selected parameters are summarised in table 4 for each load case and crack model, respectively. the small sample size means that the long-term tension cracking correlations of [9] need to be viewed with caution. little variation in the sample configurations for long-term tension loading [9] also results in correlations relating to section geometry being overestimated for all models. this applies to some extent to short-term tension loading, as although the sample size is larger, many samples were limited to ties with a single reinforcing bar or have little variation. adding the experimental programme of [13] (which consisted of repeat samples with multiple bars as opposed to much of the more uniform database of [9] with ties reinforced with single bars) resulted in differences to the short-term tension values obtained for the pearson’s analyses reported in [9] for mc 2010 and en 1992 even though the sample size increase was small. further research using different configurations/real structures is required for tension cracking for a more accurate evaluation of interdependence between these parameters and the model factor. referring to table 4 and short-term tension, with the exception of the section depth to width ratio (h/b) for mc 2010, model uncertainty for the en 1992 crack model has low to moderate correlations with all parameters. as would be expected, model uncertainty has a negative correlation with concrete tensile strength (fctm) for both en 1992 and mc 2010. the inconsistency in the bs8007 crack model is clearly demonstrated as the pearson’s correlations vary substantially between the tension stiffening formulations. a larger sample size is required for a true reflection of the pearson correlations for the long-term case [9], as can be evidenced by the extremely varied values between shortand long-term loading. 5.4. comparison to flexure model uncertainty as the database for long-term tension cracking was very limited, comparisons were made to analyses to quantify model uncertainty as reported in [9] and [10] for shortand long-term flexure loading applied to the en 1992, mc 2010 and bs 8007 crack models. the statistical parameters for the mf for flexural cracking are summarised in table ??. further detail on the flexural crack models analyses can be found in [9, 10]. comparing the short-term model uncertainty for tension to that of flexure, the means for the mc 2010 crack model are similar at approximately 1, although the variation for the tension case is less than that of flexure. this is probably partly due to the greater uniformity of sample configurations of the tension database compared to flexure. lognormal distributions are indicated for the en 1992 and mc 2010 crack models for both flexural and tension cracking. however, model uncertainty parameters are similar enough to conclude that it is not necessary to distinguish between the flexural and tension load cases if the mc 2010 crack model is considered for the gpm. although the dataset for long term flexural cracking 132 vol. 36/2022 sls reinforced concrete crack models uncertainty load case parameter correlation with model factor en 1992 mc 2010 bs8007 w = 0, 2 mm [9] bs8007 w = 0, 1 mm [9] short-term tension steel stress 0.052 0.204 -0.284 -0.364 h/b 0.579 0.722 0.049 0.206 c 0.286 0.420 0.332 0.230 bar dia, φ 0.518 0.570 -0.068 -0.241 fctm -0.294 -0.503 -0.030 0.080 % as 0.352 0.390 -0.036 -0.019 long-term tension [9] steel stress 0.825 0.641 0.840 0.777 h/b 0.490 0.335 0.602 0.479 c -0.825 -0.641 -0.840 -0.777 bar dia, φ -0.414 -0.209 -0.544 -0.407 fctm -0.826 -0.724 -0.792 -0.773 % as -0.349 -0.156 -0.426 -0.334 table 4. pearson’s correlation matrix between model uncertainty & model parameters. load case statistical parameter en 1992 mc 2010 bs 8007 w = 0.2 mm bs 8007 w = 0.1 mm short term mean 1.107 1.052 1.185 1.112 cov 0.397 0.376 0.380 0.459 pdf ln ln ln ln count 164 164 164 164 long term mean 1.443 1.127 1.502 1.514 cov 0.331 0.380 0.336 0.357 pdf ln ln ln ln count 30 30 30 30 table 5. summary of statistical parameters of mf for flexural cracking [10]. [9, 10] is larger than that of the tension load case, further research for long-term loading in general is recommended, including expanding the database and application to real conditions, for a more accurate estimation of the statistical parameters of model uncertainty. pearson’s correlations were performed for flexural cracking, as reported by [10]. low to moderate correlations were found for all parameters except for % as for long-term flexure (r of 0,58). it is noted that the dataset for flexural cracking had more variation in terms of section and reinforcement configurations than that of tension cracking, which supports the suggestion that the uniformity of the dataset for tension cracking results in apparent dependence between these parameters and the mf. 6. summary and concluding remarks in quantifying model uncertainty for pure tension crack models, the significant degree of uncertainty was confirmed, which justifies a full probabilistic treatment of model uncertainty and sls crack models. the model uncertainty statistical parameters were determined for short and long-term tension cracking, although the latter case does require further investigation to confirm values obtained. however, considering the mf statistical parameters determined for short and long-term flexural cracking ([9, 10]), the shortterm mf values for tension cracking may be used for long-term mf’s when considering crack models such as mc 2010 which take long-term shrinkage into account. indications are also that for the mf, it is not necessary to distinguish between flexural and tension load-induced cracking. this simplifies the reliability assessment of load-induced cracking and the development of the gpm. in terms of crack model performance, the bs 8007 crack model is shown to be inconsistent as its tension stiffening model is dependent on the specified limiting crack width. any variation in crack width away from this limit results in a high variation in the mf, and thus an inconsistent model. this is a point to note for any empirically-developed model which may be considered for a gpm. it was also noted during the analyses that the point on the cross section where the crack was measured was generally not given in the experimental 133 christina helen mcleod acta polytechnica ctu proceedings records, introducing some further uncertainty into the predicted crack widths due to potential mismatches between measured and estimated crack positions. using acr based on the geometry of the cross section, rather than a crack spacing formulation, for ties with single bars resulted in underprediction of the measured crack widths. this effect would be mitigated in structures under real conditions. crack models that omit shrinkage strain (en 1992 and bs 8007) underpredict crack widths in the long-term loading case. mc 2010 was the most consistent crack prediction model and will thus be the basis for the gpm. further to this research, data will be compiled for existing structures such as bridges and water retaining structures for use in the probabilistic analysis of the gpm. the experimental database was compiled and refined with wrs in mind, where sls cracking is dominant in design, so focused on small crack widths. for long-term tension cracking in particular, it must be noted that as many of the elements tested were mostly ties reinforced with single reinforcing bars, the crack widths recorded may not necessarily be representative of real conditions in structures such as wrs where there are obviously multiple reinforcing bars in both the main and transverse directions. this was noted in [9] and [10], borne out in the further analyses. this requires further research to properly assess the influence of section and reinforcement configurations on tension cracking, and which will include data from existing structures. references [1] j. v. retief. contributions to the implementation of the principles of reliability to the standardised basis of structural design. dr of eng thesis, stellenbosch university, south africa. supervisor gap van zijl, co-supervisor c viljoen, 2015. [2] m. holický, j. v. retief, m. sýkora. assessment of model uncertainties for structural resistance. probabilistic engineering mechanics 45:188-97, 2016. https: //doi.org/10.1016/j.probengmech.2015.09.008. [3] bs 8007. design of concrete structures for retaining aqueous liquids. british standards institute, 1987. [4] sans 10100 -1 (2004): the structural use of concrete: part 1: design. south african standards division. pretoria, 2004. [5] bs en 1992-1-1: 2004. eurocode 2: design of concrete structures part 1-1: general rules and rules for buildings. british standards institute, 2004. [6] sans 10100 part 3 (draft): design of concrete liquid retaining structures. sabs-tc98-sc2 working group on implementation of south african national standard for liquid retaining structures. south african standards division, pretoria., 2015. [7] c. h. mcleod. investigation into cracking in reinforced concrete water-retaining structures. msc eng thesis, stellenbosch university, south africa. supervisors profs ja wium & jv retief, 2013. [8] g. l. balázs, p. bisch, a. borosnyói, et al. design for sls according tofibmodel code 2010. structural concrete 14(2):99-123, 2013. https://doi.org/10.1002/suco.201200060. [9] c. h. mcleod. model uncertainty in the prediction of crack widths in reinforced concrete structures and reliability implications. phd thesis, stellenbosch university, south africa. supervisor prof c viljoen, 2019. [10] c. h. mcleod, c. viljoen. quantification of crack prediction models in reinforced concrete under flexural loading. structural concrete 20(6):2096-108, 2019. https://doi.org/10.1002/suco.201900036. [11] m. lapi, m. orlando, p. spinelli. a review of literature and code formulations for cracking in r/c members. structural concrete 19(5):1481-503, 2018. https://doi.org/10.1002/suco.201700248. [12] b. farra, j.-p. jaccoud. influence du béton et de larmature sur la fissuration des structures et béton. rapport des essais: de tirants sous deformation impose de courte durée. institut de statique et structures beton armé et preconstraint. publication no. 140, 1993. [13] v. gribniak, a. rimkus, a. pérez caldentey, et al. cracking of concrete prisms reinforced with multiple bars in tension-the cover effect. engineering structures 220, 2020. https: //doi.org/10.1016/j.engstruct.2020.110979. [14] a. pérez caldentey. upm experimental data. personal communication via email, 2016. [15] l. eckfeldt. schlussbeicht verbesserug de vorhersagequalitat um sehr kleinen vissbreiten. phd thesis, tu dresden, germany. supervisor prof ehm curbach, 2009. [16] l. s. hwang. behaviour of reinforced concrete in tension at post-cracking range. msceng (civil) thesis. university of manitoba, canada, 1983. [17] m. h. q. wu. tension stiffening in reinforced concrete: instantaneous and time-dependent behaviour. phd thesis, university of new south wales, australia. supervisor prof ri gilbert, 2010. 134 https://doi.org/10.1016/j.probengmech.2015.09.008 https://doi.org/10.1016/j.probengmech.2015.09.008 https://doi.org/10.1002/suco.201200060 https://doi.org/10.1002/suco.201900036 https://doi.org/10.1002/suco.201700248 https://doi.org/10.1016/j.engstruct.2020.110979 https://doi.org/10.1016/j.engstruct.2020.110979 acta polytechnica ctu proceedings 36:127–134, 2022 1 introduction 2 model uncertainty treatment 3 deterministic analysis of predicted crack width 3.1 bs 8007 crack model 3.2 en 1992 crack model 3.3 mc 2010 crack model 4 quantification of crack model uncertainty 4.1 experimental database 5 discussion on model uncertainty quantification 5.1 short term tension 5.2 long term tension 5.3 evaluation of crack models pearson's correlations 5.4 comparison to flexure model uncertainty 6 summary and concluding remarks references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.38.0295 acta polytechnica ctu proceedings 38:295–301, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague optimization model of solar cooling system with latent heat storage lana migla∗, kristina lebedeva riga technical university, latvia, faculty of civil engineering, department of heat, gas and water technology, k, ı̄psalas street 6a/6b, riga, latvia, lv-1048 ∗ corresponding author: lana.migla@rtu.lv abstract. today, the fastest growing end-use in the buildings sector is cooling demand, notes the international energy agency (iea). in the last few years, the interest in solar cooling systems has been growing globally. solar cooling systems are the next solution to meet the growing demand for air conditioning. the literature analysis shows that solar cooling systems mainly use single-stage absorption chillers, with water as the working fluid, and only a small fraction of them use phase change materials to improve heat storage. based on previous studies on solar-assisted cooling systems, the use of phase change materials for thermal storage should be investigated. the paper will present a study of a solar cooling system with the optimized pcm thermal storage, with the aim of stabilizing the operation of heat storage, taking into account the volatility of solar energy, the impact of short-term operations and peak hours on the amount of heat produced. this means that the amount of electricity consumed to heat up the accumulation tank will be significantly reduced. the optimization model in simulation software will be performed to test solar cooling system with latent heat storage, with aim to investigate the efficiency of the developed latent energy storage and provide technical guidance for the implementation of such system in the practice. all indicators, including environmental impact and economic calculations, will be identified in order to determinate the specific systems for foresight market uptake. keywords: solar cooling, thermal energy storage, phase change materials. 1. introduction the indoor climate in buildings has a major impact on occupant productivity, especially during the hot summer period when large amounts of energy are used to cool rooms. in 2020, global demand for space cooling continued to grow, in part due to greater home cooling with more people spending more time at home. space cooling accounted for almost 16 % of the final electricity consumption of the building sector in 2020 (approximately 1 885 twh) [1]. as solar cooling systems (scs) use solar thermal energy as a source of energy, they have the potential to substantially reduce the proportion of conventional energy used to cool rooms and the negative impacts they generate. however, there is a fundamental shortcoming of solar energy, a mismatch between available energy and consumption, which makes it necessary to store the energy. phase change materials (pcms) are effective for storing heat in thermal energy storage because they can store not only thermal energy beyond temperature change as sensible heat storage but also through melting and solidification, as latent heat storage, which occurs over a narrow temperature range allowing 5 to 14 times more thermal energy to be stored than sensible thermal energy storage of the same size [2]. latent heat storage uses the phase change of matter between different aggregate states – gas/liquid, solid/gas, solid/liquid and solid/solid [3]. based on a literature analysis [4, 5], the scs working in a temperature range of 60–120 °c were determined to be most efficient, depending on the thermal heat supplier used in the chiller and the capacity of the cooler. it is therefore necessary to look in depth at the possibility of integrating the various pcms into the energy storage to stabilize its functioning, thereby increasing the cop (coefficient of performance) of solar thermal driven air-conditioning system. 2. methods 2.1. solar thermal cooling systems in solar cooling systems a mixture of ammonia, lithium bromide (libr) and water can be used in the absorption cycle [6]. a solar assisted absorption cooling system using a mixture of 60 % libr and 40 % water has been investigated and the performance of the system has been optimized using various thermodynamic analysis methods. among other sorption cooling systems, absorption cooling circuits are the mainstream in the context of high cop, in particular at temperatures above 80 °c [7]. however, when comparing absorption cycle cop (0.5 to 0.8) to traditional cooling systems with cop up to 3.0, the absorption cycle efficiency optimization becomes significant [8]. the generator of the chiller is warmed up using heat from solar thermal panels. the heat is collected and stored in a thermal storage tank, after which the required amount of heat is supplied to the generator. it is 295 https://doi.org/10.14311/app.2022.38.0295 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en lana migla, kristina lebedeva acta polytechnica ctu proceedings figure 1. scheme of solar cooling system. estimated that the system heat carrier could reach 85–90 °c using the solar thermal panels [9]. the heat supplied to the generator will be used to evaporate the water from the libr-water solution mixture returning from the absorber. the simulation model of solar cooling system was developed with absorption chiller nominal power −8 kw and maximal cooling capacity of 11 kw. heat is mainly produced by solar collectors and used for thermal driving chiller. in the presented case, heat output of solar thermal system is 15 kw [10]. the solar thermal system includes one cubic meter thermal energy storage tank with an additional 8 kw electric heater. solar thermal cooling system model has the following parts: 1 – solar collectors, 2 – thermal storage tank with integrated pcm encapsulate tubes and auxiliary heater, 3 – chiller (evaporator, condenser) with cooling tower, 4 – cold storage and 5 – consumer part. basic configuration of solar thermal absorption cooling system technologies is shown in figure 1. a simulation model was developed and validated to assess the impact of the pcm on the operation parameters of the solar assisted cooling system. during the day solar available energy is volatile. it also has impact on the temperature in the thermal storage. this is shown in the case study where an experimental solar pilot plant was developed to verify the pcm behavior under realistic conditions, in which following stratification, the pcm modules were integrated at the top of the hot water storage tank. in addition, pcm module is designed to use multiple cylinders, notes luisa f. cabeza et al. [11]. the scientist concludes that the introduction of the pcm module into thermal water storage tanks for dhw supply is highly prospective technology. this will allow hot water to be supplied for a longer period, even without an auxiliary source, or use smaller tanks for the same application. as shown in maher shehadi et al. study [9], stable temperature for heating the generator has great importance in order to raise absorption cooling systems at cop. it can be seen in the figure 2. the additional cop of solar cooling system shall be determined as follows: copthermal = useful power [kw] input power [kw] . (1) when using pcm in a thermal energy storage tank, the additional useful energy will be the energy of the pcm storage. the cop with additional energy from hot water heating will be calculated by following equation: copthermal,modified = co + sho + who + upfpcms aspfc , (2) where co chiller output [kw], sho space heating output [kw], who water heating output [kw], upfpcms usefull power from pcm store, aspfc available solar power from collector [kw]. 2.2. pcm selection criteria in selecting pcms to be applied as lhtes materials for thermal storage, there are some important aspects to consider. the main criteria [12] for selecting pcms are: • thermophysical properties: pcm, within the required temperature range, has high latent heat of fusion, high conductivity, high specific heat, cyclic stability, high density, low phase transition volume change, low vapor pressure at operating temperature, and consistent melting. • kinetic properties: high nucleation rate and high crystal growth rate (achieving optimal recovery from storage system). 296 vol. 38/2022 optimization model of solar cooling system with latent heat storage figure 2. cop of the absorption cycle depending on the generator temperature, accounting for various absorber temperatures [9]. pcm/parameter mgcl2 · 6h2o rt100 erythritol pcm melting point, tm [°c] 116.7 99 117.7 pcm heat of fusion, lp cm [kj kg−1] 168.6 168 339.8 specific heat of pcm, liquid, cp,l [j kg−1°c−1] 2.6 (120 °c) 2.4 2.8 specific heat of pcm, solid, cp,s [j kg−1°c−1] 1.4 (20 °c) 1.8 1.4 thermal conductivity of pcm, liquid, kl [w m−1°c−1] 0.6 (120 °c) 0.2 0.3 (140 °c) thermal conductivity of pcm, solid, ks [w m−1°c−1] 0.7 (20 °c) 0.2 07 (20 °c) density of pcm, liquid, rl [kg m−3] 1450 (120 °c) 770 (130 °c) 1300 (140 °c) density of pcm, solid, rs [kg m−3] 1570 (20 °c) 940 (20 °c) 1480 (20 °c) table 1. main parameters of the selected pcm. • chemical properties: fully reversible cycle, chemically stable without deterioration after multiple freeze/melt cycles, chemically compatible with encapsulating materials, non-corrosive, non-toxic, nonexplosive, non-flammable. • economical characteristics: cost effectiveness and commercial availability for large-scale practice. • environmental characteristics: environmentally friendly, non-polluting over a lifetime, and has the possibility of recycling. based on the above criteria, pcm – mgcl2 · 6h2o, paraffin rt100 and erythritol to enhance the efficiency of a libr-h2o solar cooling system was selected. similar findings are also in place for other researchers [13]. their main parameters of selected pcm [14–16] are summarized in the table 1. highest energy density for a specified mass of all pcms was calculated using following equation: q = m[cp,l(t − tm) + λ + cp,s(tm − tref )], (3) where m [kg] is the mass of material, cp [j kg−1°c−1] is a specific heat of material, l [kw] is the latent heat, and t [°c] is the temperature. the thermal conductivity λ is calculated from the measured thermal diffusivity a, density r and heat capacity cp. 2.3. storage tank design for most latent heat applications, methods to improve heat transfer are required, as majority of phase change materials have inappropriately low thermal conductivity [17, 18] resulting in slow charging and discharging rates. series of methods have been used to optimize the heat transfer properties of pcm. among the most widely used methods are the finned tubes [19], rings and bubble mixing techniques [20], the application of carbon fibre and multitubes [21]. most of the studies report significant improvements in the thermal conductivity of phase change materials but have not compared them to other improvement techniques. from this it can be concluded that the design of the thermal storage tank is in charge of efficiently charging/discharging the pcm as needed. shell-and-tube heat exchangers are the most widely used type of storage technique, with pcm on the shell side. for example, francis agyenim [22] conducted a study 297 lana migla, kristina lebedeva acta polytechnica ctu proceedings on a variety of pcm tube configurations, where the heat transfer improvement for three different modifications – circular fins, longitudinal fins and multitube systems was compared. the findings indicate that the use of a heat transfer mechanism increases the amount of the charge/discharge energy. the quantity of charged/discharged energy depends on the effectiveness of the heat transfer technique used in the pcm modules. equally important is that the multi-tube system has the highest stored energy, followed by the longitudinal and circular finned systems in that order. the longitudinally finned system with the highest discharge efficiency, followed by the multi-tube circular finned systems. as the author [22] points out, the multi-tube system was most effective for charging, but exhibited significant subcooling during discharging process. multitube systems will require a minimum container size, followed by a system with vertical and circular fins. the combination of longitudinal fins and multitubes in each configuration is the optimal combination to boost the pcm charge and discharge efficiency. cylindrical geometries are said to be the best performing storage systems with high efficiency and cylindrical containers with minimal dimensions [23]. based on effective heat capacity method, heat capacity is considered to be a function of temperature due to phase changes in the temperature interval between melting and solidification, so the calculations are done on pcm, using both – temperature and latent energy. when charging (pcm melting) – the hot water from the solar collector exchanges heat with the solid pcm and the melting of pcm starts. whereas when discharging (pcm solidification) – the cold heat carrier flows in the inner tube and the pcm modules exchange heat with it to initiate the solidifying process. based on the mentioned above, two experimental configurations of pcm modules were studied. in the first experiment, pcm is filled in the copper shell tube with an inner diameter of 146 mm and the heat carrier flows into the embedded 54 mm tube without a heat increase. this served as a starting point for evaluating the efficiency of the other advanced configurations. in the second experiment, circular fins were welded in the surface of the heat transfer tube. the amount of cumulative energy charged and discharged, heat lost to the environment and storage efficiency were calculated using mathematical model [22] which contains the following equations: • cumulative energy charged/discharged qchar [j]: qchar = t∑ 0 ṁcp,h(∆t )∆t, (4) where ∆t = t0 − ti for charged energy, ∆t = ti − t0 for discharged energy, ṁ mass flow rate [kg/s], ∆t change in time [s], ∆t change in temperature [°c], cp,h specific heat capacity of heat carrier [kj/kg°c]. • cumulative energy heat lost qloss [j]: qloss = t∑ 0 ti,ins − to,ins ln(ro,ins/ri,ins) 2rkinsl + 1 h0a0 ∆t, (5) where ri,ins inner radius [m], ro,ins outer radius [m], h0 heat transfer coefficient [w/(m2 °c)], a0 area [m2], l length [m], kins thermal conductivity of material [w/m°c], ti,ins inner temperature [°c], to,ins outer temperature [°c]. • charging efficiency ηchar: ηchar = qchar − qloss qchar . (6) • discharging efficiency ηdischar: ηdischar = qdischar qchar − qloss . (7) • storage efficiency based on energy charged: ηstore,input = ηchar × ηdischar. (8) the efficiency of the heat transfer mechanism used in the pcm determines the cumulative amount of energy charged/discharged. 3. results after equation (3), it is calculated that the energy storage capacity using erythritol is the highest. erythritol results are 20 % higher than magnesium chloride hexahydrate (mgcl2 · 6h2o), moreover 32 % higher than rt100. this coincides with other studies [13]. in addition, pure mgcl2 · 6h2o shows supercooling about 37 k, thus the study will continue and seek appropriate modifications. therefore, in the experiment erythritol was used for filling in tubes. the results of two experimental configurations of copper tubes filled with erythritol are shown in figure 3. the temperature curve produced by the experiment shows the temperature spread over the three different phases during the charge/discharge process of experimental tube filled with erythritol. in the first phase the energy was accumulated by the pcm and was used to raise the temperature of the erythritol till it nearly reached melting point temperature 117.7 °c. the temperature increased almost linearly with time. in the 298 vol. 38/2022 optimization model of solar cooling system with latent heat storage figure 3. temperature curves of two experimental tube configurations melting/solidification. next phase temperature curves tended to asymptote due to a rapid change of phases. in the third phase the energy was absorbed in the form of sensible heat and the temperature increased once again, only then the temperature curve started to fall. due to the solidification a subcooling process effect was observed. the circular finned system improved the temperature curve a little during the charging phase of sensible heat addition when conduction controlled the heat transfer, despite the increase in surface area of the circular finned system over the tube without a fin system. in the discharge phase, the tube with fins shows slightly better results, but this difference does not appear to be significant. the cumulative amounts of energy charged and discharged were calculated using equations (4)–(8) and summarized in table 2. the overall coefficient of performance of real solar cooling systems was calculated by equations (1) and (2), the copthermal close to 0.78, but copthermal,modified 0.81 throughout the year. 3.1. energy and economic analysis of the system based on the simulation results, it was concluded that the modified thermal storage tank maintains the temperature of the heat carrier required for the operation of the scs operation for a long period of time, so using the latent storage tank significantly reduces the energy generated by the auxiliary heater. given that, the excess heat is used for dhw needs. it should be added that the modified storage tank simulation results show substantial outcome for providing stable operation of the solar collector system and compensating for short-term variations in available solar energy (eg. for example, cloudy hours) [24]. with the pcm modified storage tank, the cop increased by an average of about 4 % compared to the cop of the scs non-modified storage tank. in addition, the auxiliary heater activation time for additional heating has been reduced by 21 % compared to scs with non-modified thermal storage tank. this is due to more stable thermal insulation/retention which would reduce energy consumption and will increase the lifetime of the scs. the specific economical audit software tool is used to run simulations to determine total energy consumption and co2 emissions over the service life of the system. the main phases of the simulation tool include material and manufacturing process, as well as transportation and end-of-life potential. the cost of capital of the system can be categorized among the individual components, with collectors and the chiller which accounts for most of the system cost (30–40 %). next most expensive part is the cooling tower, then the control costs, and finally the thermal storage unit (about 5 %). the low cost of thermal storage provides the potential to enhance total system performance at a very little additional capital cost. 4. conclusions research investigates the efficiency of the developed latent energy storage for solar cooling system to increase cop of such systems. the main indicators, including environmental impact and economic calculations, were identified. the economic impact of modified thermal storage on solar cooling performance depends on many factors, including the electricity charges used in traditional compressor-based cooling systems. the possibility of using pcm as a storage instead of water also depends on the system properties such as operating temperature set points. this study presents an analytical methodology that allows the selection of the optimal size of thermal storage as well as to quantify the benefits of this thermal storage in terms of additional solar fraction. this can be used to maximize the added economic value of the storage for different 299 lana migla, kristina lebedeva acta polytechnica ctu proceedings parameters/configuration tube tube with circular fins cumulative energy charged [kj] 9555.0 10501 cumulative energy lost [kj] 552.3 589.3 effective energy charge [kj] 8665 9935 cumulative energy discharge [kj] 4966 7591.1 storage efficiency (%) 62.2 75.2 table 2. energy charged and discharged and storage efficiency. solar cooling systems. it has been found that pcm can reduce the demand for an auxiliary heat source during peak loads and low radiation periods. acknowledgements this work has been supported by the european regional development fund within the activity 1.1.1.2 “postdoctoral research aid” of the specific aid objective 1.1.1 “to increase the research and innovative capacity of scientific institutions of latvia and the ability to attract external financing, investing in human resources and infrastructure” of the operational programme “growth and employment” «latent 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2016. https://doi.org/10.1016/j.renene.2015.10.012 [23] m. e. zayed, j. zhao, w. li, et al. recent progress in phase change materials storage containers: geometries, design considerations and heat transfer improvement methods. journal of energy storage 30:101341, 2020. https://doi.org/10.1016/j.est.2020.101341 [24] y. allouche, s. varga, c. bouden, a. c. oliveira. dynamic simulation of an integrated solar-driven ejector based air conditioning system with pcm cold storage. applied energy 190:600–611, 2017. https://doi.org/10.1016/j.apenergy.2017.01.001 301 https://doi.org/10.1016/j.solener.2009.04.007 https://doi.org/10.1016/s0038-092x(98)00128-5 https://doi.org/10.1016/j.renene.2009.03.010 https://doi.org/10.1016/j.renene.2015.10.012 https://doi.org/10.1016/j.est.2020.101341 https://doi.org/10.1016/j.apenergy.2017.01.001 acta polytechnica ctu proceedings 38:295–301, 2022 1 introduction 2 methods 2.1 solar thermal cooling systems 2.2 pcm selection criteria 2.3 storage tank design 3 results 3.1 energy and economic analysis of the system 4 conclusions acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.34.0085 acta polytechnica ctu proceedings 34:85–93, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague adhesion test – testing of selected adhesives on fired clay kristýna richterová∗, pavel heinrich, petr bílý czech technical university in prague, faculty of civil engineering, department of concrete and masonry structures, thákurova 7, 166 29 prague 6, czech republic ∗ corresponding author: kristyna.richterova@fsv.cvut.cz abstract. the following paper deals with the comparison of adhesion of specimens made of fired clay and selected adhesives. the adhesion was tested on specimens at the age of one, two and three days in order to determine the parameters of young masonry. nine different adhesives (adhesives intended for masonry and adhesives for others material) were tested in total. the above mentioned test is not standardized. the obtained results from the adhesion test determine the tensile strength between the adhesive and the fired clay and the adhesion of selected adhesive to the fired clay. the small specimens were made of two ceramic plates and an adhesive. selected adhesives were applied to ceramic plates according to the usual standards. eighty-one small specimens were produced in total – twenty-seven specimens for the adhesion test at each defined age of the specimens (one day, two days, three days). the produced specimens were cured in the climatic chamber with set laboratory conditions until the time of the test. the specimens were tested in the laboratory of the heluz brick plant in dolní bukovsko. this paper deals with the procedure of preparation of the specimens, the course of the test and the comparison of gained results. keywords: fired clay, adhesion test, adhesive. 1. introduction the adhesion test determines the tensile strength between the adhesive and the building material (in our case fired clay) and the adhesion of selected adhesive to the building material (fired clay). although requirement for tensile strength between a fired clay and adhesive is not defined by any regulation, the obtained results of the measurement provide information, which is important for the design of innovative construction methods. the test was performed at the age of the samples of one, two and three days. the measured values of “young” samples give an idea of the behaviour of selected adhesives available on the czech market and indicate, which adhesives reach the required strength in a short time. a similar adhesion test (the centric adhesion test on two-unit test samples with 2c-pur adhesive and thin-layer mortar) was performed in 2013 by the institute of building materials research (ibac) in aachen. the performed tests were a necessary basis for the introduction of a new construction method in germany – see [1]. today´s trend is the construction of structures in the shortest possible time, using suitable and efficient construction methods in compliance with high quality implementation, so the adhesion test is a felicitous tool for determining the required properties of tensile stressed structures. 2. materials and samples eighty-one small samples were produced for the adhesion test – twenty-seven samples for adhesion test at each defined age of the samples. the samples were made of fired clay and selected adhesive. nine adhesives were tested in total. the selected adhesives are listed in the following table. sample description a heluz sidi b heluz sb mortar for a thin joint c heluz tytan pur foam (thin-film adhesive) e pu-contact two-component adhesive f den braven – multi kleber – masonry foam g den braven – low expansion – masonry foam i den braven – mamut glue (high tack) j illbruck – universal chemical anchor ot120 k den braven – wood glue – woodfix d3 table 1. list of used adhesives [2]. the fired clay in the form of plates used for the adhesion test – size 30×30 mm and thickness 15 mm – has the same properties as heluz bricks. small samples for the adhesion test were made of two plates of fired clay and selected adhesive – table 1. heluz sidi is a silicate-dispersion masonry mortar for a thin joint, which is specified for masonry made of clay blocks heluz grinded. heluz sidi was 85 https://doi.org/10.14311/app.2022.34.0085 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en k. richterová, p. heinrich, p. bílý acta polytechnica ctu proceedings launched in the spring of 2020, it is a prepared mixture and is applied evenly crosswise with a structured roller [3]. heluz sb mortar for a thin joint is applied to the ribs of the grinded clay blocks and is applied with heluz applicator roller sb or by direct dipping of the bed joint in a pre-prepared mixture. the mixture is prepared from heluz sb and water – in a ratio of 0.45 liters of water per 1 kg heluz sb. heluz sb was applied to small samples on the whole surface of the bed joint using a spatula [4]. heluz tytan pur foam was applied with the application gun on the whole surface of the bed joint [5]. pu-contact two-component adhesive is composed of two components – adhesive and accelerator – and it was applied on the whole surface of the bed joint of both parts of the sample. the adhesive was applied by a pneumatic spray gun. the application of pucontact two-component adhesive has to be fast due to the presence of the accelerator. the increase of adhesion and application of the adhesive are very fast. den braven masonry foams were selected to compare the results with heluz tytan pur foam and were also applied by application gun [6, 7]. all samples with masonry foam were not loaded fast enough – the foam has swelled up, and the thickness of bed joint was greater than stated in the technical datasheets of adhesives. swelling of the foam negatively affects its declared values, the probability of failure at the site of adhesive increases and therefore the values obtained from the adhesion test are on the side of safety. mamut glue is a one-component adhesive ms polymer-based. this adhesive has a declared high initial adhesion (up to 500 kg/m2) and has been applied by application gun [8]. illbruck – universal chemical anchor ot120 is a two-component resin without styrene. this chemical anchor is intended for anchoring rods, bolts and bushings to concrete and common building materials. this chemical anchor is also suitable for anchoring in hollow masonry. the chemical anchor is applied by an application gun for silicone sealants [9]. the wood glue – woodfix d3 – is an aqueous dispersion glue and is intended for gluing wood [10]. figure 1. the fired clay in the form of plates and the produced samples. the produced samples (9×3×3) for the adhesion test were cured in the climatic chamber until the time of the test. the climatic chamber was set to figure 2. the selected adhesives: b – heluz sb mortar for a thin joint, a – heluz sidi, c – heluz tytan pur foam (thin-film adhesive), e – pu-contact two-component adhesive. figure 3. the selected adhesives: f – multi kleber – masonry foam, g – low expansion – masonry foam, i – mamut glue (high tack), j – illbruck – universal chemical anchor ot120, k – wood glue – woodfix d3. laboratory conditions – constant temperature 23 ◦c, constant relative humidity 50 %. the test samples were loaded by steel plate (m = 280 g) in the chamber. the steel plate is a component for the adhesion test. each sample from a series of test samples was equipped with steel plates with hitches for attachment to the test apparatus before the adhesion test. steel plates measuring 50×50 mm were glued by epoxy glue (sikadur®-31 cf rapid) to the lower and upper surface of the test sample. a small number of samples with the selected adhesive at each defined age (three test samples) were selected due to the total number of test samples (eightyone test samples).the adhesion test was performed mainly for the selection of unsuitable adhesives. adhesives with satisfactory results from this test will be subjected to further testing in the future. 86 vol. 34/2022 adhesion test – testing of selected adhesives on fired clay figure 4. application of epoxy to samples for fixing steel plates. 3. adhesion measurement the adhesion test was performed at the age of the samples one day, two and three days – each series of 27 samples at a defined age. the series of samples at a defined age was transferred from the climatic chamber to the testing laboratory in dolní bukovsko on the day of the test. at first the cross-sectional area ai of each test sample was measured using caliper. then the samples were placed in a test apparatus and after that loaded with a tensile force. the used test apparatus was apparatus named tiratest 2803, which can exert a force of up to 3 kn. this force is sufficient for the purpose of the adhesion test. figure 5. the sample preparation in a test apparatus. at first the sample in a test apparatus was fixed manually. after that the sample was loaded with tensile force – the value of increase of tensile force was set to 10 n/s – until the sample failed. the measured tensile force fti,max and the type of failure of the test sample were recorded in the prepared measurement table. the possible types of failure of the sample are failure in the adhesive, in the fired clay, in a combination of fired clay and adhesive or in the epoxy between the fired clay and the steel plate. the named failure types and scheme of adhesion test are depicted in the following figures. from the tensile force fti,max and from the crosssectional area ai the tensile stress σti, the average stress σtm and finally the characteristic stress σtk were calculated. the characteristic stress σtk was figure 6. the scheme of adhesion test. figure 7. the possible types of failure of the sample in the adhesion test. calculated using the standard deviation s, the coefficient of variation v and the uncertainty coefficient kn (2.01 (n=2), resp. 1.89 (n=3)). the individual parameters were calculated according to the following formulas. ai = a ·b [mm2] (1) σti = fti,max a [mpa] (2) σtm = 1 n · ∑ σti [mpa] (3) σtk = σtm · (1 − kn · v ) (4) s = √ 1 n− 1 · ∑ (σti − σtm)2 (5) v = s σtm (6) 87 k. richterová, p. heinrich, p. bílý acta polytechnica ctu proceedings 4. results and discussion the values and the information obtained from the adhesion test in the following tables state which adhesives performed the best in the adhesion test. the assessment of the adhesion test depends mainly on the measured values of the tensile force at the failure of the sample, but the variance of the results and the character of the failure of the sample are also important. the type of sample failure gives us an idea of what limits the measured value of tensile stress and whether the used adhesive can resist greater tensile stress then was measured. the failure of the sample in the fired clay says that the used adhesive is likely to resist greater tensile stress than the fired clay used to production of the samples; however, this type of failure did not prevail in any set of samples. the samples that demonstrated satisfactory results in the adhesion test are the samples made of adhesives den braven – mamut glue (high tack), heluz sidi, pu-contact two-component adhesive and den braven – wood glue – woodfix d3. mamut glue (high tack) performed the best in the adhesion test and the samples with this adhesive failed in the adhesive or in a combination of fired clay and the adhesive – it can be assumed that the adhesive is limited by tensile stress values close to the values measured by us. the negative factors of mamut glue (high tack) are the unpredictability of sample failure and its price. a typical failure of pu-contact two-component adhesive is failure in the adhesive at different deformation. this adhesive is very malleable, with reduced adhesion when applied unprofessionally and is applied in a thin layer. very surprising results were measured on samples with applied wood glue – woodfix d3. the measured values of tensille stress were higher for wood glue than for some samples with aplicated adhesives intended for masonry. the low expansion foam (den braven) was the worst of the tested masonry foams. the worst of all the samples were those with the adhesive illbruck – universal chemical anchor ot120 and heluz sb mortar for a thin joint. the principle of the chemical anchor consists in its expansion into the borehole; this was not possible in the case of application to the contact area of small samples. the samples with the chemical anchor were broken in the adhesive in all cases, which did not adhere sufficiently and reached low resistance to tensile stress. the performed adhesion test is not completely valid, because the number of test samples for series at a defined age was small (three samples for every adhesive at a defined age). relatively high standard deviations in individual series of measurements (small number of test samples) caused a significant decrease of the characteristic stress of some samples aged two days and three days, respectively. the large variance of the measured values is also the reason for the negative value of the characteristic stress of samples with adhesive named heluz sb mortar for a thin joint at the age of one day. it would be necessary to produce and test more small samples to obtain valid values of the characteristic stress. 5. conclusion this article introduces the results of the adhesion test performed on the small samples, which were made of fired clay in the form of plates and selected adhesive. the obtained results are not completely valid (a small number of test samples), but the results give an idea of the tensile properties of selected adhesives applied to the fired clay. therefore they are useful for the selection of adhesives for a future, more detailed study. the adhesion test was performed at an early age of the samples to determine the adhesion and tensile strength between the fired clay and the selected adhesive. these ascertained properties are important for the progress of innovative construction methods – not only for the construction of brick buildings. surprising results were measured on samples with applied adhesives that are not intended for masonry. we must be careful about these results because we do not know the development of their properties in contact with masonry over a longer period of time. the research will continue by deeper examination of several selected adhesives. list of symbols a,b horizontal dimension of the sample ai cross-sectional area fti,max tensile force n number of measurements σti tensile stress σtm average stress σtk characteristic stress s standard deviation v coefficient of variation kn uncertainty coefficient acknowledgements the acknowledgement belongs to assoc. prof. petr bílý, who was the supervisor of my master thesis and to the company heluz, which helped to perform the adhesion test. the preparation of this paper was financially supported by student grant of the czech technical university in prague sgs19/149/ohk1/3t/11. 88 vol. 34/2022 adhesion test – testing of selected adhesives on fired clay a – heluz sidi day name crosssectional area tensile force tensile stress type of failure nr. of measurements avg. stress std. deviation char. stress ai [mm2] fti,max [n] σti [mpa] n σtm [mpa] s [mpa] σtk [mpa] 1 a1 961.61 543.22 0.565 adhesive 2 0.583 0.025 0.532a2 959.31 575.86 0.600 adhesive a3 951.04 adhesive (when clamping) 2 a1 956.97 958.41 1.002 3/4 adhesive + 1/4 fired clay 3 0.954 0.171 0.630 a2 945.87 1036.55 1.002 fired clay a3 944.93 721.25 0.763 fired clay 3 a1 945.55 779.50 0.824 fired clay 3 0.687 0.219 0.273a2 965.87 773.15 0.801 fired clay a3 960.64 417.10 0.434 4/5 adhesive + 1/5 fired clay b – heluz sb mortar for a thin joint day name crosssectional area tensile force tensile stress type of failure nr. of measurements avg. stress std. deviation char. stress ai [mm2] fti,max [n] σti [mpa] n σtm [mpa] s [mpa] σtk [mpa] 1 b1 953.53 270.47 0.284 adhesive 2 0.341 0.224 -0.082b2 952.96 144.19 0.151 adhesive b3 945.78 555.59 0.587 adhesive 2 b1 951.72 219.49 0.231 adhesive 3 0.347 0.131 0.099b2 942.94 302.15 0.320 adhesive b3 946.13 462.38 0.489 adhesive 3 b1 946.79 when clamping 2 0.275 0.050 0.175b2 966.27 300.20 0.311 adhesive b3 956.36 229.42 0.240 adhesive c – heluz tytan pur foam (thin-film adhesive) day name crosssectional area tensile force tensile stress type of failure nr. of measurements avg. stress std. deviation char. stress ai [mm2] fti,max [n] σti [mpa] n σtm [mpa] s [mpa] σtk [mpa] 1 c1 956.04 325.55 0.341 adhesive 3 0.338 0.163 0.031c2 956.31 477.45 0.499 adhesive c3 935.68 162.82 0.174 adhesive 2 c1 958.21 177.41 0.185 adhesive 3 0.192 0.006 0.180c2 954.40 183.67 0.192 adhesive c3 952.93 188.39 0.198 adhesive 3 c1 955.73 308.77 0.323 adhesive 3 0.297 0.034 0.233c2 956.85 296.97 0.310 adhesive c3 942.39 243.62 0.259 adhesive 89 k. richterová, p. heinrich, p. bílý acta polytechnica ctu proceedings e – pu-contact two-component adhesive day name crosssectional area tensile force tensile stress type of failure nr. of measurements avg. stress std. deviation char. stress ai [mm2] fti,max [n] σti [mpa] n σtm [mpa] s [mpa] σtk [mpa] 1 e1 944.30 adhesive (when clamping) 2 0.724 0.078 0.568e2 964.10 644.85 0.669 adhesive e3 964.41 750.96 0.779 adhesive 2 e1 939.39 526.22 0.560 adhesive 3 0.443 0.103 0.248e2 947.68 382.27 0.403 3/4 adhesive + 1/4 fired clay e3 926.90 338.60 0.365 adhesive 3 e1 980.25 27.68 0.028 adhesive (did not adhere) 2 0.534 0.038 0.457e2 946.78 530.87 0.561 adhesive e3 946.88 479.58 0.506 adhesive f – den braven – multikleber – mansory foam day name crosssectional area tensile force tensile stress type of failure nr. of measurements avg. stress std. deviation char. stress ai [mm2] fti,max [n] σti [mpa] n σtm [mpa] s [mpa] σtk [mpa] 1 f1 991.60 211.40 0.213 adhesive 3 0.211 0.048 0.121f2 968.45 157.38 0.163 adhesive f3 962.23 248.28 0.258 adhesive 2 f1 723.23 368.26 0.509 adhesive 3 0.393 0.137 0.133f2 730.33 176.21 0.241 adhesive f3 948.33 406.53 0.429 adhesive 3 f1 859.60 253.21 0.295 adhesive 3 0.426 0.167 0.110f2 744.07 457.25 0.615 adhesive f3 948.64 350.27 0.369 adhesive g – den braven – low expansion – masonry foam day name crosssectional area tensile force tensile stress type of failure nr. of measurements avg. stress std. deviation char. stress ai [mm2] fti,max [n] σti [mpa] n σtm [mpa] s [mpa] σtk [mpa] 1 g1 969.70 182.85 0.189 adhesive 3 0.165 0.044 0.082g2 956.29 109.63 0.115 adhesive g3 855.50 164.46 0.192 adhesive 2 g1 885.25 94.46 0.107 adhesive 3 0.198 0.084 0.040g2 989.78 267.68 0.270 adhesive g3 755.55 164.08 0.217 adhesive 3 g1 969.96 328.94 0.339 adhesive 3 0.417 0.190 0.058g2 915.55 255.04 0.279 5/6 adhesive + 1/6 fired clay g3 822.25 521.58 0.634 5/6 adhesive + 1/6 fired clay 90 vol. 34/2022 adhesion test – testing of selected adhesives on fired clay i – den braven – mamut glue (hight tack) day name crosssectional area tensile force tensile stress type of failure nr. of measurements avg. stress std. deviation char. stress ai [mm2] fti,max [n] σti [mpa] n σtm [mpa] s [mpa] σtk [mpa] 1 i1 937.27 1098.71 1.172 adhesive 2 1.042 0.118 0.819i2 979.38 922.21 0.942 adhesive i3 980.92 992.69 1.012 adhesive 2 i1 938.44 1076.41 1.147 3/4 adhesive + 1/4 fired clay 3 1.004 0.176 0.672 i2 980.00 791.50 0.808 2/3 adhesive + 1/3 fired clay i3 965.33 1022.11 1.059 adhesive 3 i1 957.52 560.67 0.586 fired clay 3 0.769 0.159 0.468i2 974.93 841.90 0.864 adhesive i3 958.48 823.55 0.859 1/3 adhesive + 2/3 fired clay j – illbruck – universal chemical anchor ot120 day name crosssectional area tensile force tensile stress type of failure nr. of measurements avg. stress std. deviation char. stress ai [mm2] fti,max [n] σti [mpa] n σtm [mpa] s [mpa] σtk [mpa] 1 j1 959.45 158.83 0.166 adhesive 2 0.244 0.110 0.022j2 974.06 adhesive (when clamping) j3 1005.73 323.34 0.321 adhesive 2 j1 1003.29 214.63 0.214 adhesive 3 0.248 0.029 0.192j2 667.99 176.98 0.265 adhesive j3 970.94 256.95 0.265 adhesive 3 j1 942.78 265.29 0.281 adhesive 2 0.317 0.050 0.217j2 962.20 338.38 0.352 adhesive j3 962.54 adhesive (when clamping) 91 k. richterová, p. heinrich, p. bílý acta polytechnica ctu proceedings k – den braven – wood glue – woodfix d3 day name crosssectional area tensile force tensile stress type of failure nr. of measurements avg. stress std. deviation char. stress ai [mm2] fti,max [n] σti [mpa] n σtm [mpa] s [mpa] σtk [mpa] 1 k1 947.99 344.52 0.363 adhesive 2 0.449 0.121 0.206k2 955.68 2.84 0.003 fired clay (when clamping) k3 981.25 524.34 0.534 adhesive 2 k1 974.64 980.99 1.007 adhesive 3 0.878 0.179 0.540k2 961.84 647.83 0.674 fired clay k3 956.02 912.48 0.954 adhesive 3 k1 981.08 440.06 0.449 adhesive 3 0.659 0.210 0.263k2 992.86 656.44 0.661 fired clay k3 946.76 822.06 0.868 3/4 adhesive + 1/4 fired clay table 2. the values and the information obtained from the adhesion test [1]. figure 8. summary of results from the adhesion test at the age of the samples one, two and three days [1]. 92 vol. 34/2022 adhesion test – testing of selected adhesives on fired clay references [1] w. brameshuber, m. graubohm. prefabricated masonry panel system with two-component polyurethane adhesive / vorgefertigte mauertafeln mit zweikomponenten-polyurethanklebstoff. cement and concrete composites 19:3–26, 2015. https://doi.org/10.1002/dama.201500644. [2] k. richterová. precast masonry wall panels. master thesis 2021. [3] heluz. heluz sidi. https://www.heluz.cz/sidi/. [4] heluz. heluz sb malta pro tenkou spáru. https://www.heluz.cz/files/sb_sbc.pdf. [5] heluz. heluz pěna (tenkovrstvé lepidlo). https://www.heluz.cz/files/heluz-trend_m5_p_na_ technicky-list_cz.pdf. [6] d. braven. multi kleber. https://www.denbraven.cz/produkt/multi-kleber/. [7] d. braven. nízkoexpanzní pěna. https: //www.denbraven.cz/produkt/nizkoexpanzni-pena/. [8] d. braven. mamut glue high tack. den braven. https://www.denbraven.cz/produkt/ mamut-glue-high-tack/. [9] illbruck. ot120 univerzální chemická kotva. https://www.illbruck.com/cs_cz/produkt/ ot120-univerzalni-chemicka-kotva/. [10] d. braven. lepidlo na dřevo woodfix d3. den braven. https://www.denbraven.cz/produkt/ lepidlo-na-drevo-woodfix-d3/. 93 https://doi.org/10.1002/dama.201500644 https://www.heluz.cz/sidi/ https://www.heluz.cz/files/sb_sbc.pdf https://www.heluz.cz/files/heluz-trend_m5_p_na_technicky-list_cz.pdf https://www.heluz.cz/files/heluz-trend_m5_p_na_technicky-list_cz.pdf https://www.denbraven.cz/produkt/multi-kleber/ https://www.denbraven.cz/produkt/nizkoexpanzni-pena/ https://www.denbraven.cz/produkt/nizkoexpanzni-pena/ https://www.denbraven.cz/produkt/mamut-glue-high-tack/ https://www.denbraven.cz/produkt/mamut-glue-high-tack/ https://www.illbruck.com/cs_cz/produkt/ot120-univerzalni-chemicka-kotva/ https://www.illbruck.com/cs_cz/produkt/ot120-univerzalni-chemicka-kotva/ https://www.denbraven.cz/produkt/lepidlo-na-drevo-woodfix-d3/ https://www.denbraven.cz/produkt/lepidlo-na-drevo-woodfix-d3/ acta polytechnica ctu proceedings 34:85–93, 2022 1 introduction 2 materials and samples 3 adhesion measurement 4 results and discussion 5 conclusion list of symbols acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.38.0038 acta polytechnica ctu proceedings 38:38–43, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague review of sunlight exposure of buildings in central europe climatic conditions jaroslav vychytila,∗, jitka mohelníkováb a czech technical university in prague, faculty of civil engineering, department of architectural engineering, thákurova 7, 166 29 prague 6 b brno university of technology, faculty of civil engineering, institute of building structures, veveří 512/95, 602 00 brno ∗ corresponding author: jaroslav.vychytil@fsv.cvut.cz abstract. the sunlight exposure represents one of key parameters of indoor climate comfort. the evaluation of sunlight access into buildings is based on a methodology of european standard en 17037. the methodology is focused on specification of an insolation time which would comply with requirements for the sunlight expo sure of permanently occupied spaces like houses and residential buildings as well as schools or hospitals. the sunlight exposure evaluation is recommended to be between 1st february and 21st march. the specific date from the interval can be selected for individual evaluations. this is a relatively long period of days in which the insolation could vary meaningfully. the aim of this study is to evaluate how the sunlight exposure might be changed in the recommended standard period in dependence on the geographic locality and climatic conditions in the central europe region. it depends on the sunlight time and specific design situations as well as shading obstructions. the review of the sunlight exposure is determined. the review results can perform information about design possibilities for the sunlight exposure in real buildings in the given climatic locality. keywords: solar radiation, sunlight exposure, solar altitude, building insolation. 1. introduction sunlight positively influences on indoor climate in buildings. access of sunlight into rooms is one of key parameters of the building design [1, 2]. sunlight time for building insolation performs importance of the insolation for occupied rooms [3, 4]. criteria for evaluation sunlight provision in buildings and basic rules for design of effective insolation of buildings were specified in standard requirements [5]. they are useful for comparable evaluation of sunlight exposure in different european localities. these requirements represent ways of urban development regulation with respect of solar radiation access for sustainable and comfortable built environment. requirement are specified for residential buildings as well as schools and kindergartens and hospital rooms. for the insolation assessment all aspects need to be considered as a window size and its position in the facade and orientation to cardinal points, distance and geometry of shading obstructions. geographic locality and date and time of the evaluation also play key role for the insolation prediction. the insolation is evaluated for the control point positioned on the internal surface of the window glass pane. it is position where the point and the window jamb limit the horizontal angle of the solar radiation access. apart the horizontal angle also a vertical angle is taken into the consideration. this elevation angle represents of minimum solar altitude which can be accepted for the sunlight access into a room. this angle is a parameter for specification of minimum solar exposure changes in different european countries as determined in the standard metric of en 17037 [5]. this minimum angle is lower for northern countries compared to southern localities. this is in a response of sun position. values of minimum solar altitude are for capital of eu countries specified for day of 21st march [5]. the sunlight exposure can be evaluated for any days between 1st february and 21st march. this is a relatively long interval in which insolation can be changed significantly [6]. this paper is focused on a study of the daytime for determination of minimum solar altitude for a specified locality in the eu region. 2. method sunlight access into a building was studied localities of the central europe region with latitudes from 46° to 52.5° which comprises countries slovenia, switzerland, hungary, austria, slovakia, czechia, poland and germany. capitals of the mentioned countries are specified for geographical localities and minimum required solar altitude. the minimum solar altitude was calculated for 1st and 21st february as well as 1st and 21st march for sunlight exposure time minimum 1.5 hour of sunlight exposure for sunny and cloudless conditions. 38 https://doi.org/10.14311/app.2022.38.0038 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 38/2022 review of sunlight exposure of buildings in central europe . . . figure 1. minimum solar altitude in accordance with en 17037 [5] (hs is height of the window sill, hw is height of the window, hh is height of the window head). solar altitude is determined as: γs = arcsin(cos ωη × cos φ × cos δ + sin φ × sin δ), (1) where ωη [°] is hour angle determined by: ωη = 15° × (tst − 12), (2) tst is true solar time in hours, φ [°] is the geographical latitude of the site, δ [°] is the declination of the sun determined by: δ = 0.3948 − 23.2559 × cos(j ′ + 9.1°) − 0.3915 × cos(2 × j ′ + 5.4°) − 0.1764 × cos(3 × j ′ + 26.0°). (3) the parameter j ′ [°] indicates the day number converted to an angular measure and is determined by equation: j ′ = 360° × j 365 , (4) where j is the day number of the year (e.g. for 1st january, j = 1 and for 31st december, j = 365, february is taken to have 28 days). 3. the solar altitude in the considered localities the solar altitude was calculated in all localities according equation (1). the necessary input parameters are listed in the table 1. country capital geographical latitude φ [°] geographical longitude λ [°] by en 17037 [5] of the capital austria wien 48.12 n 16.22 e czechia prague 50.10 n 14.22 e germany berlin 52.47 n 13.25 e hungary budapest 47.48 n 19.05 e poland warsaw 52.17 n 21.00 e slovakia bratislava 48.20 n 17.07 e slovenia ljubljana 46.22 n 14.33 e switzerland bern 46.25 n 7.28 e table 1. considered localities and their characteristics. it was considered with true solar time. the large variable of the geographical longitude in the considered localities was the reason. the declination of the sun determined by equation (3) is: • δ = −17.30° for 1st february, the day number of the year is j = 32; 39 jaroslav vychytil, jitka mohelníková acta polytechnica ctu proceedings • δ = −10, 80° for 21st february, the day number of the year is j = 52; • δ = −7, 83° for 1st march, the day number of the year is j = 60; • δ = −0, 02° for 21st march, the day number of the year is j = 80. the figure 2 shows the dependence of the sun altitude on time during the day. figure 2. dependence of the sun altitude on the time (top left for 1st february, top right for 21st february, bottom left for 1st march, bottom right for 21st march). 4. extreme limit for the position of the sun the standard en 17037 [5] specifies the maximum diversion of the normal line of a window from the south 120°, as shown in figure 3 on the left. this means the maximum position of the window lining from 30° to 210°. figure 3. recommended window orientation to cardinal points (on the left) and marginal condition of calculating the minimum solar altitude (on the right). it can be seen from figure 3 on the right that the effective angle of incidence of the sun’s rays is in the range from 55° to 185° at the limit of the window orientation. it is necessary to know the azimuth of the sun to express the minimum required height of the sun. the topographic azimuth αs [°] given in en 17037 [5] is determined according to the equation: αs = 180° ± arccos sin γs × sin φ − sin δ cos γs × cos φ . (5) 40 vol. 38/2022 review of sunlight exposure of buildings in central europe . . . the use of the astronomical azimuth αast [°], which represents the divergence of the sun’s rays from the south, is more advantageous for the subsequent calculation (with the use of [7] and [8]): tg αast = sin ϖη sin φ × cos ϖη − cos φ × tg δ . (6) all partial quantities mentioned in equations (5) and (6) have already been described above. the equation (6) is advantageous in expressing the required sun exposure time by means of an hour angle ωη [°]. subsequent expression of the height of the sun depending on the hour angle is our effort namely, as the following mathematical adjustments show: tg αast × sin φ × cos ϖη − tg αast × cos φ tg δ = sin ϖη (7) (tg αast × sin φ × cos ϖη − tg αast × cos φ tg δ)2 = 1 − cos2 ϖη (8) tg2 αast × sin2 φ × cos2 ϖη − 2 × tg2 αast × sin φ × cos φ × cos ϖη × tg δ + tg2 αast × cos2 φ × tg2 δ + cos2 ϖη − 1 = 0 (9) (tg2 αast × sin2 φ + 1) × cos2 ϖη − (2 × tg2 αast × sin φ × cos φ × tg δ) × cos ϖη +tg2αast × cos2 φ × tg2 δ + cos2 ϖη − 1 = 0. (10) the solution of the general quadratic equation written in the form ax2 + bx + c = 0 has two roots, which are determined according to the equation: x1,2 = −b ± √ b2 − 4ac 2a . (11) the solution for the value of the hour angle at the boundary of the ineffective angle of incidence of the sun’s rays is obtained substituting equation (10) to equation (11). the 1st root of the equation (the + sign) is important: cos ϖη = 2 × tg2 αast × sin φ × cos φ × tg δ 2 × (tg2 αast × sin2 φ + 1) + √ 4 × (tg2 αast × sin φ × cos φ × tg δ)2 − 4 × (tg2 αast × sin2 φ + 1) × (tg2αast × cos2 φ × tg2 δ + cos2 ϖη − 1) 2 × (tg2 αast × sin2 φ + 1) . (12) the solution of equation (12) is given in table 2 for the considered localities. latitudes were considered according to table 1. the azimuth value was αast = 55° in accordance with the above text. the declination values are given in section 3. the required sun exposure time of 90 minutes, i.e. 1.5 hours, corresponds to an hour angle. after substituting into the modified equation (2): ωη,1.5h = 15 × 1.5 = 22.5°. the limit value of the hour angle is therefore: ωη,req = ωη + ωη,1.5h = ωη + 22.5. (13) country hour angle date min. solar altitude 1st february 21st february 1st march 21st march austria ωη [°] 58.495 53.915 51.913 46.769 γs,min [°] -6.99 0.83 −→ 0.7 4.37 −→ 4.3 13.65 −→ 13.5 czechia ωη [°] 58.703 54.378 52.486 47.624 γs,min [°] -7.73 -0.04 3.44 −→ 3.3 12.58 −→ 12.4 germany ωη [°] 58.889 54.863 53.101 48.568 γs,min [°] -8.55 -1.01 2.41 −→ 2.3 11.38 −→ 11.2 hungary ωη [°] 58.418 53.753 51.715 46.480 γs,min [°] -6.737 1.13 −→ 1.0 4.69 −→ 4.5 14.02 −→ 13.9 poland ωη [°] 58.869 54.806 53.027 48.453 γs,min [°] -8.45 -0.89 2.53 −→ 2.4 11.53 −→ 11.4 slovakia ωη [°] 58.505 53.935 51.937 46.805 γs,min [°] -7.02 0.80 −→ 0.6 4.33 −→ 4.2 13.61 −→ 13.4 slovenia ωη [°] 58.250 53.418 51.308 45.890 γs,min [°] -6.23 1.72 −→ 1.5 5.32 −→ 5.2 14.75 −→ 14.6 switzerland ωη [°] 58.254 53.426 51.318 45.905 γs,min [°] -6.24 1.71 −→ 1.6 5.31 −→ 5.2 14.73 −→ 14.6 table 2. minimum solar altitude γs,min [°]. 41 jaroslav vychytil, jitka mohelníková acta polytechnica ctu proceedings the minimum solar altitude is obtained by substituting the required hour angle ωη,req [°] together with other known data to equation (1). the values of the equation of time are by equation (15) following: −0.226 h for 1st february; −0.232 h for 21st february; −0.212 h for 1st march and −0.122 h for 21st march. the negative values γs,min [°] mean that the sun position is too low and it is not possible to ensure sun exposure therefore. the positive values γs,min [°] apply during simplification that local time is the same as true solar time. the values behind the arrow, which are reduced by 0.1° to 0.2°, are considered more appropriate. they take into the account the tolerance of the results and also apply to the calculation of true solar time according to equation (14). determining the date of sun exposure will be necessary to verify the validity of the calculation. the true solar time required to accurately calculation of equation (2) is determined in dependence on the location and local time: tst = lt + λ − 15 15 + et, (14) where tst is true solar time in hours, lt is true solar time in hours, λ is true solar time in hours, et is equation of time in minutes (need to convert to hours for further use): et = 0.0066 + 7.3525 × cos(j ′ + 85.9°) + 9.9359 × cos(2 × j ′ + 108.9°) + 0.3387 × cos(3 × j ′ + 105.2°), (15) in where j ′ [°] is determined by equation (4). country time∗ quantities 21st february 1st march 21st march austria s requirement / real min. solar altitude γs,min [°] 0.7 / 0.78 4.3 / 4.32 13.5 / 13.51 sunlight exposure [from : to] 6:54 to 8:24 7:02 to 8:32 7:22 to 8:52 d requirement / real min. solar altitude γs,min [°] 0.7 / 0.77 4.3 / 4.35 13.5 / 13.59 sunlight exposure [from : to] 7:03 to 8:33 7:10 to 8:40 7:25 to 8:55 czechia s requirement / real min. solar altitude γs,min [°] — 3.3 / 3.44 12.4 / 12.50 sunlight exposure [from : to] 7:00 to 8:30 7:22 to 8:49 d requirement / real min. solar altitude γs,min [°] — 3.3 / 3.31 12.4 / 12.43 sunlight exposure [from : to] 7:15 to 8:45 7:29 to 8:59 germany s requirement / real min. solar altitude γs,min [°] — 2.3 / 2.32 11.2 / 11.28 sunlight exposure [from : to] 6:57 to 8:27 7:15 to 8:45 d requirement / real min. solar altitude γs,min [°] — 2.3 / 2.36 11.2 / 11.23 sunlight exposure [from : to] 7:17 to 8:47 7:29 to 9:00 hungary s requirement / real min. solar altitude γs,min [°] 1.0 / 1.13 4.5 / 4.66 13.9 / 14.00 sunlight exposure [from : to] 6:55 to 8:25 7:03 to 8:33 7:24 to 8:54 d requirement / real min. solar altitude γs,min [°] 1.0 / 1.01 4.5 / 4.58 13.9 / 13.98 sunlight exposure [from : to] 6:52 to 8:22 6:59 to 8:29 7:15 to 8:45 poland s requirement / real min. solar altitude γs,min [°] — 2.4 / 2.40 11.4 / 11.50 sunlight exposure [from : to] 6:57 to 8:27 7:16 to 8:46 d requirement / real min. solar altitude γs,min [°] — 2.4 / 2.45 11.4 / 11.45 sunlight exposure [from : to] 6:46 to 8:16 6:59 to 8:29 slovakia s requirement / real min. solar altitude γs,min [°] 0.6 / 0,75 4.2 / 4.29 13.4 / 13.48 sunlight exposure [from : to] 6:54 to 8:24 7:02 to 8:32 7:22 to 8:52 d requirement / real min. solar altitude γs,min [°] 0.6 / 0,65 4.2 / 4.23 13.4 / 13.48 sunlight exposure [from : to] 6:59 to 8:29 7:06 to 8:36 7:21 to 8:51 slovenia s requirement / real min. solar altitude γs,min [°] 1.5 / 1.67 5.2 / 5.36 14.6 / 14.68 sunlight exposure [from : to] 6:56 to 8:26 7:05 to 8:35 7:26 to 8:56 d requirement / real min. solar altitude γs,min [°] 1.5 / 1.57 5.2 / 5.30 14.6 / 14.67 sunlight exposure [from : to] 7:12 to 8:42 7:20 to 8:50 7:36 to 9:06 switzerland s requirement / real min. solar altitude γs,min [°] 1.6 / 1.66 5.2 / 5.20 14.6 / 14.67 sunlight exposure [from : to] 6:56 to 8:26 7:04 to 8:34 7:26 to 8:56 d requirement / real min. solar altitude γs,min [°] 1.6 / 1.69 5.2 / 5.26 14.6 / 14.63 sunlight exposure [from : to] 7:41 to 9:11 7:48 to 9:18 8:04 to 9:34 ∗ the letter s indicates simplification that lt (local clock time) = t st (true solar time). the letter d indicates the determination of t st according to equation (14). table 3. sunlight exposure (total sunlight exposure is min. 1.5 hours in all cases). 42 vol. 38/2022 review of sunlight exposure of buildings in central europe . . . 5. results minimum solar altitude is a key parameter for building insolation assessment. results of the minimum solar altitude γs,min [°] for sunlight exposure 1.5 hour are for all localities summarized in table 3. it is obvious that for geographical localities of central europe the minimal solar altitude is applicable from about 21st february and from 1st march for localities with latitude bigger than 50°. however, it must not be forgotten that the sun’s rays at an elevation of up to 5° (see [8]) above the horizon cover a relatively large distance through the atmosphere. namely, the effectiveness of the positive effects of sunlight is reduced. 6. conclusion the paper shows results of sunlight exposure of the central europe localities based on the en 17037 methodology. results show that the method of the building insolation assessment has a potential impact of the building design and its indoor environment in different climatic localities. climate-based evaluation of the solar provision in buildings has utmost importance especially in highly occupied indoor spaces. sunlight exposure has importance for indoor climate in buildings [9]. window position and orientation play key roles for access solar radiation into interiors. it is influencing factor for urban planning and building design [10–12]. minimal solar altitude was calculated for several european localities. results calculated according to the standard en 17037 show relatively bigger differences in minimal solar altitude influenced by the geographic latitude, day and time of the assessment. the minimal solar altitude is applicable parameter from 21st february in the central european region. this region is mostly characterized by partly cloudy and overcast skies. it means that the exposure to sunlight should be considered carefully to ensure insolation in interiors. this is mostly important for places with permanent occupancy like residential buildings, schools and hospitals. acknowledgements this article has been elaborated under the project no. lo1408 “admas up – advanced materials, structures and technologies”, supported by ministry of education, youth and sports under the “national sustainability programme i”. references [1] p. tregenza, j. mardaljevic. daylighting buildings: standards and the needs of the designer. lighting research & technology 50(1):63–79, 2018. https://doi.org/10.1177/1477153517740611 [2] s. darula, j. christoffersen, m. malikova. sunlight and insolation of building interiors. energy procedia 78:1245–1250, 2015. https://doi.org/10.1016/j.egypro.2015.11.266 [3] b. deroisy, a. deneyer. a new standard for daylight: towards a daylight revolution? in lighting for modern society : proceedings of the lux europa 2017, pp. 340–343. 2017. isbn 978-961-93733-4-7. http://www.sdr.si/pdf/le2017_proceedings.pdf [4] b. paule, j. boutiller, s. pantet, y. sutter. a lighting simulation tool for the new european daylighting standard. conference building simulation and optimization 2018 cambridge. [2021-10-11]. https://www.researchgate.net/ publication/329091178_a_lighting_simulation_tool_for_the_new_european_daylighting_standard [5] en 17037 daylight in buildings. cen 2018. [6] j. kaňka. about minimum solar altitude in new european standard. [in the czech original: o minimální výšce slunce v nové evropské normě], tzb-info, 2021. [2021-10-05]. https://stavba.tzb-info.cz/denni-osvetleni-aosluneni/22433-o-minimalni-vysce-slunce-v-nove-evropske-norme [7] čsn 73 0581 insolation of buildings and outdoor areas – the method of assessment the values, 2009. 10 p. [8] j. vychytil, j. kaňka. lighting technology in buildings. lectures. [in the czech original: stavební světelná technika. přednášky]. ctu in prague, prague, 2016. isbn 978-80-01-06060-5, p. 18, 24. [9] m. boubekri. daylighting, architecture and health: building design strategies. 1st ed. elsevier/architectural press, amsterdam, 2008. p. 12-39. [10] r. compagnon. solar and daylight availability in the urban fabric. energy and buildings 36(4):321–328, 2004. https://doi.org/10.1016/j.enbuild.2004.01.009 [11] f. de luca, t. dogan. a novel solar envelope method based on solar ordinances for urban planning. building simulation 12:817–834, 2019. https://doi.org/10.1007/s12273-019-0561-1 [12] c. chatzipoulka, r. compagnon, m. nikolopoulou. urban geometry and solar availability on façades and ground of real urban forms: using london as a case study. solar energy 138:53–66, 2016. https://doi.org/10.1016/j.solener.2016.09.005 43 https://doi.org/10.1177/1477153517740611 https://doi.org/10.1016/j.egypro.2015.11.266 http://www.sdr.si/pdf/le2017_proceedings.pdf https://www.researchgate.net/publication/329091178_a_lighting_simulation_tool_for_the_new_european_daylighting_standard https://www.researchgate.net/publication/329091178_a_lighting_simulation_tool_for_the_new_european_daylighting_standard https://stavba.tzb-info.cz/denni-osvetleni-a-osluneni/22433-o-minimalni-vysce-slunce-v-nove-evropske-norme https://stavba.tzb-info.cz/denni-osvetleni-a-osluneni/22433-o-minimalni-vysce-slunce-v-nove-evropske-norme https://doi.org/10.1016/j.enbuild.2004.01.009 https://doi.org/10.1007/s12273-019-0561-1 https://doi.org/10.1016/j.solener.2016.09.005 acta polytechnica ctu proceedings 38:38–43, 2022 1 introduction 2 method 3 the solar altitude in the considered localities 4 extreme limit for the position of the sun 5 results 6 conclusion acknowledgements references my title https://doi.org/10.14311/app.2022.36.0047 acta polytechnica ctu proceedings 36:47–58, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague practical concepts for the use of probabilistic methods in the structural analysis and reassessment of existing bridges presentation of latest research and implementation thomas braml, christian kainz∗ universität der bundeswehr münchen, fakultät für bauingenieurwesen und umwelttechnik, institut für konstruktiven ingenieurbau, werner-heisenberg-weg 39, 85579 neubiberg, germany ∗ corresponding author: christian.kainz@unibw.de abstract. in structural engineering and bridge construction, probabilistic calculations are only performed in a few exceptional cases. the ability to directly use information from the existing structure is an important advantage of probabilistic methods. geometric data or the position and quantity of the installed reinforcing steel and tendons can be measured. results from monitoring of the traffic can be used to generate structure-specific load models. many clients are still very sceptical about the results of probabilistic calculations, even though there are several examples of successful application. the use of probabilistic verification formats is explicitly permitted by the german guideline for the assessment of existing bridges (german: nachrechnungsrichtlinie). since a high degree of special knowledge is required for the application of probabilistic calculations, it is difficult to establish the potential of this verification method in practice. it is necessary to enable the practical engineer to utilize the advantages of probabilistic calculations and to include the actual structural conditions into the calculation model, but without the need for all the special knowledge. structure-specific partial safety factors include measured data and can be used for the well-known and in the codes established semi-probabilistic design concept. the authors present examples for the successful application of probabilistic methods and of data measurement during the reassessment of two existing bridges in germany. the capabilities of full-probabilistic calculations for the reassessment of existing structures are described and concepts for the calculation of structure-specific partial safety factors are shown. keywords: existing bridges, existing concrete structures, modified partial safety factors, statical recalculation, structural analysis, structural assessment, structural reassessment. 1. introduction in recent years, research activities for the application of probabilistic calculations intensified considerably. however, for the structural analysis of existing bridges the use of probabilistic methods is still limited to few cases. distinct reasons for this limitation can be identified. in general, the clients are often very sceptical about the results of a probabilistic calculation, either due to the lack of experience or because of regulatory limitations in the past. the results of numerical or probabilistic calculations highly depend on the selected models. their usage requires a high level of competence and responsibility [1]. therefore, it is difficult to establish the potential of this verification method in practice. compared to the design of a new structure, the construction process of an existing structure is already completed and documented. typically, the structure has already been used for decades. relevant information can be detected or measured on site [2–4]. this is a major advantage during the assessment of existing structures. most codes are primarily focused on new constructions and include reserves for the load bearing capacity, which are mandatory and important for a safe planning process. during the construction process and the service lifetime of a structure, mistakes or variations of parameters or deviations are possible and must be considered in the design and calculation [5]. but the actual parameters are likely to be more beneficial than assumed during the design process for a structure, that operates over a long time without major damage [5]. existing structures often have more load bearing capacity than currently certifiable, even if they are designed according to historical codes [6]. in most cases, these reserves are not used for the calculations, because information about the actual properties of the structure can hardly be implemented into the semi-probabilistic calculation concept. 2. capability of full-probabilistic calculations for the structural analysis of existing structures full probabilistic methods deliver exact information about the reliability of the examined structure, but they require specialized knowledge. table 1 gives an overview of possible reliability concepts. it is important to differentiate these levels from the stages of the 47 https://doi.org/10.14311/app.2022.36.0047 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en thomas braml, christian kainz acta polytechnica ctu proceedings ac cu ra cy level reliability concept reliability measurement tool co m pl ex ity0 deterministic global safety factor γ 1 semi-probabilistic partial safety factor γm , γf 2 probabilistic approximation reliability index β 3 probabilistic exact failure probability pf 4 economically optimal risk (accepted failure probability pf ) table 1. concepts for the determination of the load bearing capacity of structures [7]. figure 1. possibilities of probabilistic calculations in structural engineering. guideline for the assessment of existing bridges. the deterministic reliability concept uses a global safety factor γ, to obtain a certain safety level. before the introduction of the semi-probabilistic concept into most design codes, the global safety factor method was the most often applied concept. currently, the semi-probabilistic concept is used in most cases and is standardized by european codes [8, 9]. different partial safety factors for loads and resistances allow for a more efficient design, because the variation of each parameter can be quantified separately. another reliability concept is the use of the reliability index β, which is an approximation. the calculation of the failure probability is theoretically exact. figure 1 shows advantages of probabilistic methods for the reassessment process of existing structures. with the use of probabilistic methods, a realistic assessment of the structure is possible. usually bridge access technology is used for the inspection of bridges. information about actual damage and data from insitu measurements can be implemented directly into the calculation model. for example, the self-weight of the construction can be determined with the actual geometry from measurements instead of using the indications from the as-built documents. lifting the superstructure is a pragmatic way to verify and measure the construction weight. if the loads are more specified and uncertainties are reduced, a reduction of the partial safety factors does not cause a reduction of the reliability of the structure. image based damage detection is a current research topic, where image classifiers and damage detectors support the bridge inspector during the evaluation of the on-site documentation [10]. structure-specific traffic load models are often based on traffic counting. if statistical data about the traffic is already available, e.g., in germany there is an archived dtv-sv (average daily traffic intensity of the heavy-load traffic), this information can also be used. in general, possible costs and benefits should be estimated before any examination is performed. a sensitivity analysis informs about the contribution of each parameter to the total uncertainty. after the relevant aspects are known, a cost-optimized planning of the examinations is possible. during the reassessment process of existing structures, present defects are identified and calculation models simulate future degradation, e.g., the progress of corrosion or carbonation. the residual service lifetime can be estimated based on the condition and age of the structure, its planned use in the future and potential repair or maintenance actions. the identification of relevant limit states and parameters is a central aspect of a probabilistic calculation. the available information and measurement data must be preprocessed and classified before it is used in the calculation model. the measurement quality must be high and the data has to be prepared well. with an increasing amount of information and parameters, the susceptibility to errors increases as well. it is vital to validate the calculations with comparative analysis or plausibility checks. other limiting factors for the application of probabilistic methods are their demand for highly specialized knowledge, the detailed computational programs, the need for a solid data basis and the effort of in-situ measurements. regulatory standards for the use of full-probabilistic calculations in the analysis of existing structures further reduce the acceptance of probabilistic methods. although probabilistic verification formats are explicitly permitted by the german guideline for the assessment of 48 vol. 36/2022 probabilistic methods in the structural analysis and reassessment of bridges existing bridges [11], the approval from the building authority is still required for the application in each case. 3. structural reassessment of existing bridges in germany 3.1. concept of the german guideline for the assessment of existing bridges about ten years ago, a systematic analysis of the load bearing capacity of german road bridges started. the increased heavy load traffic and the deterioration of existing bridges raised public doubts about the stability and reliability of german bridges. as a result, the federal department for traffic together with specialists from engineering companies, administration and scientists developed and introduced a special national code in 2011 to achieve consistent and standardized calculation methods. the so called guideline for the assessment of existing bridges (german: nachrechnungsrichtlinie) [11] is a national code, which is especially designed for the standardized reassessment of existing structures. the objective of this code is a more realistic assessment of the reliability of existing bridges, with regards to the increased traffic on one hand and to advancements in calculations methods on the other hand. the current condition of the bridge must be considered as well. in this context the term statical recalculation must be explained. regular inspections are performed every three years in germany. damaged structures are analyzed whenever a major defect is detected. the goal of this maintenance is to preserve or recover the "as-built" condition of the structure. however, a statical recalculation focuses on the theoretical reliability of the structure from a modern perspective and with regards to today’s requirements. characteristic aspects of existing structures are also considered. an additional amendment [12] including further improvements was released in april 2015. the reliability of a few thousand existing bridges was examined according to the regulations of this guideline during the past ten years. many positive experiences about this special assessment of existing bridges are reported [6]. currently, the bem-ing part 2 [13] is in preparation for technical approval. this new guideline will extend the regulations of the guideline for the assessment of existing bridges. the guideline provides a standardized procedure with four stages. figure 2 shows the principal process of a statical recalculation based on [11]. with each step the effort for the calculation and the on-site examinations increases. in the first stage (stage 1), the calculations are performed based on the current regulations of the eurocodes. depending on the intensity of heavy traffic on the bridge, it is also possible to apply the regulations of the former german technical reports (german: din-fachbericht). these national codes were introduced during the preparation of the eurocodes and created especially for bridge design. the technical report 101 (german: din-fachbericht 101 ) [15] specifies loads and actions on bridges. the reports 102 to 104 [16–18] provide rules for the calculation and design of concrete, steel and composite bridges. the intent of the national codes was to facilitate the transition to the europe-wide harmonized codes. most of the now assessed bridges were designed without the use of numerical calculations. present-day calculation models and computer-based methods, e.g., finite element method, can mobilize considerable load bearing reserves compared to the manual calculations of the past. on the other hand, the increased design loads of newer code generations can repeal those profits. in the second stage (stage 2), specific regulations and additional methods can be used. the guideline for the assessment of existing bridges adapts design equations and input variables like loads, resistances and coefficients. for example, predefined and general modifications of the partial safety factors are allowed. a reduction of the factors for the variation of post tensioning forces is possible. a common aspect during the structural analysis of existing concrete bridges is their shear capacity. especially in old concrete bridges, the shear stirrups are designed according to former standards. the number, form and design of the stirrup reinforcement bars is appropriate to modern code standards. adaptions to the verification process of the shear force resistance are necessary to obtain sufficient load bearing capacity and based on new knowledge since the construction of the existing bridges. the guideline defines certain requirements for stirrup forms of the reinforcement. based on a better understanding of the material behavior a variation of the shear compression angle is allowed. a verification of the load bearing capacity based on the principal stress criteria is also possible during the reassessment process. one example for the mobilization of reserves in the load bearing capacity is the so called cutting of the shear force coverage, where local deficits in the shear reinforcement are permitted. this means that the amount of existing shear reinforcement is allowed to be smaller than theoretically required, if there is more shear reinforcement than required in the immediate surrounding area. this gives another small additional reserve, that can be relevant for a decision about the future use of the structure, if the existing reinforcement is otherwise slightly surcharged. by this time, a reassessment according to the stages 1 and 2 is a common task for specialized engineering companies and for federal administrations. if the performed analysis cannot provide the necessary reliability level, and the methods of the stages 3 or 4 seem unlikely to achieve the necessary reliability, usage restrictions and compensatory measures are necessary. possible actions are weight or speed limits for the traffic, changes to the lane markings like closed or 49 thomas braml, christian kainz acta polytechnica ctu proceedings figure 2. flow chart for the process of a statical recalculation. 50 vol. 36/2022 probabilistic methods in the structural analysis and reassessment of bridges figure 3. longitudinal section and view of the heinrich’s bridge [14]. figure 4. cross section of the heinrich’s bridge (left: extract from plan, right: photo under the bridge) [14]. narrowed lanes or other additional rules for heavy traffic. the regulations of stage 3 deliver further options for the reassessment process, like the use of monitoring or proof load tests. measured data, e.g., material properties and deformations of the construction, can be used for the structural analysis [2]. a more precise knowledge about the actual traffic on the bridge or material properties reduces uncertainty and enables more economic calculation results. the results of in-situ measurements can also be used to validate the results of the finite element calculation model. a practical example for the successful use of data measurement during the structural analysis of an existing bridge is presented in the next section. in the last stage (stage 4) of the structural analysis, the use of scientific methods is authorized. these methods are usually not included in the design codes and the calculations exceed the regular level of detail and knowledge. possible methods are analytical models, physical non-linear calculations, non-linear finite element models or non-linear crack simulations. probabilistic methods are explicitly allowed during this stage of the examination. pressure arch models for the shear resistance or a modified compression field theory are further options. at the end of the reassessment process, each bridge is classified with a verification class (german: nachweisklasse). the three verification classes inform about the depth of the examination and if there are any future restrictions necessary (class c). they provide information, if modifications to the calculation were necessary to reach the target load level (class b) or not (class a). 3.2. data measurement during the reassessment process on the example of heinrich’s bridge in bamberg the heinrich’s bridge (german: heinrichsbrücke) is a 270 m long road bridge located in bamberg, germany. it crosses the rhine-main-danube canal as well as a street and the right arm of regnitz river. therefore, the four spans have rather unusual proportions with measures of 18 m 119 m 56 m 79 m. figure 3 illustrates the urban conditions and shows the longitudinal section and a picture of the bridge. this bridge from 1974 is constructed with an orthotropic steel deck. the single superstructure carries two traffic lanes in each direction and bike lanes on both sides, see figure 4. in 2016 a structural analysis with investigations according to the regulations of the guideline for the assessment of existing bridges [11] started. a finite-element model was used during the first and second stage of the reassessment process. the serviceability limit state (sls) as well as the ultimate limit state (uls) and the fatigue limit state showed insufficient load bearing capacity. an examination according to stage 3 of the guideline for the assessment of existing bridges [11] was then used to meet the requirements in the critical limit states. on-site measurements and a calibration of the finite element model were performed during 51 thomas braml, christian kainz acta polytechnica ctu proceedings figure 5. load model lm1 and adjustment factors α according to din-fachbericht 101 [15]. figure 6. gumbel distribution of the measured stresses over the whole measurement period [14]. the reassessment. the analysis of the bridge is part of a research project and the final report [14] was compiled last year. generally, a distinction is made between structural monitoring over a certain time to evaluate statistical data, and structural health monitoring (shm). the heinrich’s bridge was equipped with measurement instruments (13 strain gauges, 2 accelerometers and 3 temperature sensors) over a period of 15 months. to calibrate the results of the finite element model with the measurement data from the strain gauges, a typical truck had to drive on the bridge in predefined schemes. the bridge was closed for other public traffic during this night. it is essential to select a vehicle, where all the relevant data (axle weights, geometric measurements or driving speed) is exactly known [14]. the driving schemes for the calibration include a variation of driving speeds from 5 to 70 km/h and predefined lanes to drive in. the tension stresses are calculated from the measured strains by using hooke’s law of elasticity. the load model for the single truck is applied to the finite element model and the measurement data is then compared to the results of the fe-calculation. based on the measurement data from november 2019 to january 2021, a structure-specific traffic load model is developed [14]. the adjustment factors α of the vertical traffic load model lm1 make reference between the normative values of the codes and the actual traffic intensity on the bridge. the load model lm1 of the technical report 101 (german: din-fachbericht 101) [15] with its regular adjustment factors α is shown in figure 5. the load model lm1 of the technical report 101 is similar to the lm1 of the eurocode, but the tandem loads are smaller and only applied in two traffic lanes. the strain gauge dms1 is located on the bottom of a longitudinal girder in the middle of the main 52 vol. 36/2022 probabilistic methods in the structural analysis and reassessment of bridges figure 7. layout, cross-section and view of the bw 046.1 across the barkauer crossing in kiel, germany [19]. span. figure 6 shows the distribution of the measured stresses from dms1 due to heavy traffic. the stresses on the horizontal axis do not start at zero, because just the distribution of the heavy traffic is relevant for the load-model. the beginning of heavy traffic was defined at a measured stress of 12 n/mm2. as seen in figure 6, a gumbel distribution is very well suited to describe the traffic on the bridge [14]. this distribution function can only be implemented in a full-probabilistic calculation, but in the third stage of the structural analysis the semi-probabilistic reliability concept has to be used. however, the structurespecific definition of the characteristic value can be based on the measurement data. according to din en 1991-2 [9] the recurring interval of the characteristic traffic load values is 1,000 years. this approximates a probability of 0.1% for the characteristic value of the traffic load to be smaller than the weight of the heaviest truck of one year. as a result, the structurespecific traffic load model is defined with a characteristic value equal to the 99.9% quantile of the gumbel distribution. over a period of 15 months, the maximum measured stress was 30.36 n/mm2, see figure 6. the characteristic value from statistical evaluation is 31.02 n/mm2. because the assessment represents only the traffic intensity in 2020 and the heavy traffic increased considerably over the last decades, an additional factor of 1.20 for possible future growth of traffic is implemented [14]. additionally, a model uncertainty factor of γsd = 1.20 hedges simplifications and inaccuracy in the calculation models. the characteristic values should be multiplied with these two factors. the structure-specific adjustment factors α are calculated by comparison of the characteristic stresses from the measured data with the stresses of the finite element model, which implements the uls loading according to the code [15]. the adjustment factors α are different for each location of the strain gauges. in general, the adjustment factors α for the heinrich’s bridge had a high conformity and numbered in a range from 0.19 to 0.39. the choice of α = 0.5 represents an engineer-like and pragmatic approach, which allows a more cost-efficient maintenance work compared to the value of α = 0.8 from [15]. on the other hand, there are additional reserves for bigger traffic loads, in case the results of the measurement data are not exactly representative due to a generally reduced traffic during the covid-19 pandemic [14]. the damage equivalence factors λ for fatigue stresses can also be reduced based on the measurement results. the measured stress ranges are classified by their occurrence probability with the rainflow method. the contribution of each stress range category to the damage is determined with the palmgren-miner rule. the stress range spectrum is then represented by the damage equivalent stress range related to 2×106 cycles ∆σe,2. the damage equivalence factor λ is calculated with division of ∆σe,2 from the measurements with ∆σe,2 from the finite element model, which implements the load model lm3 according to the codes. the additional factors for future growth of traffic and for model uncertainties are applied in the same way as mentioned before [14]. the measured data of this bridge is currently used for further research about intelligent data processing and machine learning techniques [21, 22]. 3.3. use of full-probabilistic methods for the analysis of the barkauer crossing in kiel the bridge bw 046.1 across the barkauer crossing in kiel, germany, is an example for the successful application of stage 4 methods from the guideline for the assessment of existing bridges [11]. the 198 m long concrete bridge has a heavily curved horizontal alignment and a hollow-core superstructure. the seven spans are up to 33 meters long. the superstructure is prestressed in longitudinal and lateral direction. the bridge was constructed in 1970/71 and originally designed for bridge class 60 according to the regula53 thomas braml, christian kainz acta polytechnica ctu proceedings figure 8. finite element model including the reinforcement (left) and possible probability distributions for the height of the cross-section (right) [20]. tions of din 1072:1967-11 [23]. the concrete strength b450 of that time equals the contemporary c30/37. the reinforcement steel of type st iii b has a yield stress fy = 420 mpa. figure 7 shows part of the construction plans and a photo of the bridge. further descriptions and material properties can be found in [19]. in 2013, a reassessment process according to the german guideline for the assessment of existing bridges [11] started. the methodology of the stages one and two delivered satisfying results for almost every aspect of the bridge. however, substantial local shortcomings in the stirrup reinforcement and the torsional longitudinal reinforcement showed. the critical web, where the existing stirrup reinforcement is maximally stretched, is located 1.50 m before axis c and marked red in figure 7. it was necessary to reduce the torsional moments for the superstructure with the installation of a single-lane traffic restriction together with a weight limit of 30 tons for traffic. all construction documents were completely archived and available for the structural reassessment, which is a key requirement for a possible analysis. because the reinforcement was only insufficient in some local areas, the methods according to stage four of the guideline for the assessment of existing bridges [11, 12] can be used to proof sufficient reliability of the structure. a three-dimensional fem shell model with a linearelastic material behavior was created. figure 8 shows a part of the model on the left side. this model implements the reinforcement as well as the tendons in longitudinal and lateral direction. compared to a two-dimensional model, this mobilizes considerable reserves in the load bearing capacity of the torsional longitudinal reinforcement. the amount of necessary stirrup reinforcement remained nearly the same as calculated during the analysis in the second stage of the process [19]. the first amendment to the guideline for the assessment of existing bridges [12] gives the option to proof the shear load bearing capacity based on the main stress criteria and not based on the existing amount of reinforcement. the condition for the use of this method is, that the concrete is not cracked [12]. to analyze the concrete for cracks, an additional non-linear finite element model uses the layer method for the superstructure. the cross-section profile is thereby modeled with multiple, consecutive superimposed layers that include assignments of the reinforcement, material behavior and bond properties. the shell-elements have non-linear material behavior assigned. the redistribution of internal forces as well as cracking and tension-stiffening are implemented. the commercial sofistik -software was used for the physical nonlinear calculations. the mean values of the material parameters are used as input values. the model indicated cracks with widths of 0.2 to 0.3 mm [19]. the stirrup reinforcement is further analyzed with probabilistic methods. the first order reliability method (form) calculations are performed with the program strurel [24]. a detailed on-site investigation of the structure gives precise knowledge about the material properties, i.e., concrete density, and geometry [19]. the limit state equation g is: g(r, e) = nr − ne = 0 (1) the resistance of the stirrup reinforcement nr is determined by the area of reinforcement as and the yield strength fy. the force acting on the stirrups ne is based on the results of the linear-elastic finite element model. it contains the combined effects of permanent load ng, prestressing nv , creepage nc , settlement of supports ns , temperature nt , tandem system of traffic loads nslw and uniformly distributed traffic load nudl. the section width b is included in both sides of the equation. the factors ur (nv , µ = 1.00, vx = 0.05) and us (nv , µ = 1.00, vx = 0.07) regard model uncertainties. substitution of these variables gives the equation of the limit state for the stirrup reinforcement. as a result, the failure probability of the superstructure is calculated to pf = 1.94 × 10−4 [19]. g(r, e) = ur · (as · b · fy)− us · (ng − nv + nc + ns + nt + nslw + nudl) · b (2) 54 vol. 36/2022 probabilistic methods in the structural analysis and reassessment of bridges figure 9. possibilities of the safety-equivalent assessment of road bridges through monitoring (translated and reformatted) [25]. additional rehabilitation work improved the durability of the structure. due to the successful proof of the load bearing capacity with the methods of the guideline for the assessment of existing bridges [11], a continued use of this existing bridge is possible. 4. structure-specific partial safety factors for the assessment of existing bridges compared to the planning of a new building, existing structures require adapted methods for an economic, sustainable and reliable reassessment. structural monitoring is used in an increasing number of cases. a considerable amount of research has been conducted in this field. most of the current code regulations focus primarily on the design of new structures. only few regulations explicitly focus on existing structures. examples for national regulations in germany are the guideline for the assessment of existing bridges [11, 12], the ril805 [26] for railway bridges or the dbv-booklet [5] for buildings. international publications are, for example, iso 13822:2010 [27] and the fib bulletin 80 [28]. probabilistic methods give the possibility to use data from on-site measurement, because the data can be implemented directly into the calculation. however, their infrequent application in the past and their effort restrict a broad commercial use. the large number of necessary reassessments of the ageing infrastructure in the next decades can only be handled with the contribution of the majority of bridge engineers. the implementation of on-site data is essential for a more realistic reassessment and for the mobilization of load bearing reserves. many existing bridges can have a continued use under traffic with the mobilization of these reserves during the structural analysis process. structure-specific partial safety factors combine an individual reassessment of existing bridges, the use of the familiar and efficient semi-probabilistic reliability concept and the implementation of measurement data. results from on-site analysis provide additional information about defects and material properties. a more detailed knowledge about the existing structure reduces uncertainties and justifies the reduction of partial safety factors. because of their simple application, the structure-specific partial safety factors are more suitable for practical engineers than full-probabilistic methods. the calculations are comparable to past assessments, because in most cases the semi-probabilistic concept was used there. with a focus on the measurement of actions and loads, possible methods for the safety-equivalent assessment based on data were presented in [25, 31, 32]. figure 9 shows possible applications for the use of monitoring data in the reassessment process. as mentioned earlier, a reliability based probabilistic analysis can implement the measurement data. the adaption of standard load models or combination factors to the actual traffic on the bridge uses the monitoring results with a semi-probabilistic reliability concept. the load models from the code are either corrected with factors or individual load models are generated [32]. structure-specific partial safety factors are deduced from the results of a full-probabilistic calculation. with focus on the investigation of the structure, 55 thomas braml, christian kainz acta polytechnica ctu proceedings figure 10. application and results of ultrasonic measurements of a prestressed concrete bridge [29]. figure 11. results of a sensitivity analysis for the shear (left) and bending moment load bearing capacity (right) during the reassessment of an existing prestressed bridge by küttenbaum et al. (translated) [30]. practical examples for the use of data in the reassessment process were published in [2, 29, 30]. structural inspection processes are, e.g., the localization of rebars and tendons, the determination of the cross-sectional thickness, the estimation of concrete strengths, the detection of corrosion or the investigation of the injection status and of the applied forces to the tendons. figure 10 shows the use of bridge access technology and the application of scanning equipment on a bridge. the red lines in figure 10 display the longitudinal tendons in the web over a length of about 12 meters. the measurement technique and an approach for the validation of their results were presented in detail by küttenbaum in [33]. methods to quantify uncertainties during the measurement are crucial for the calculated reliability of the structure. parameter studies and sensitivity analysis inform about the influence of each sensitivity factor. figure 11 displays the results of a sensitivity analysis of the load bearing capacity for shear and bending moment of an existing bridge based on measured data. the numbers in brackets display the change compared to the results of the sensitivity analysis without the implementation of the measurement data, see [30]. as seen on the left side, the sensitivity of the distance of shear reinforcement changes significantly with the implementation of the on-site results. the limit state functions should be formulated to include as many data from the measurement as possible. a sensitivity analysis is able to show the influence of the results of the non-destructive testing on the reliability index of the structure. how to quantify the complete inability to measure a certain parameter during the probabilistic calculation is still a key question in this context. possible aspects of the decisionmaking process are shown in [34]. whether the data from individual measurements can have an equivalent significance as the well-known normative load models is another aspect which must be addressed more detailed. requirements to the equipment or the qualification of the personal can ensure accurate data. however, a standardized framework for the calculation of structure-specific partial safety factors is needed and will be topic of future research. the goal is to propose a guideline aimed at practical engineers, that can be used for a more realistic reassessment of existing bridges based on measured data, but without the need for a full-probabilistic calculation. 5. conclusions this paper presents the german concept for the reassessment of existing bridges and the practical implementation of the latest research during the reassessment process. many existing structures have reserves in their load bearing capacity. the guideline for the assessment of existing bridges includes four stages to evaluate the reliability of a bridge. with each step the effort increases and modifications in the calculation 56 vol. 36/2022 probabilistic methods in the structural analysis and reassessment of bridges methods are added. probabilistic calculations have potential and advantages, but they are only used for some special cases because of their complexity. the successful use of probabilistic methods is shown on the example of the barkauer crossing in kiel. the example of heinrich’s bridge in bamberg presents the implementation of data from on-site measurements to calibrate a structure-specific load model. the costs for maintenance work are reduced with the use of a structure-specific load model. structure-specific partial safety factors combine an individual reassessment of existing bridges, the use of the familiar and efficient semi-probabilistic reliability concept and the implementation of measurement data. even though a general modification of partial safety factors is allowed by existing regulations, there is no normative standard to obtain structurespecific partial safety factors. a standardized process is important for the practical use and for the comparability of the results. the structure-specific partial safety factors can be used by practical engineers to perform a reassessment of existing bridges based on the semi-probabilistic reliability concept and including additional information from on-site measurement and testing. with this approach, a more realistic analysis is possible and the service lifetime of existing bridges can be extended after the assessment. proposals for a standardized guideline regulating the data-based reassessment process with structure-specific partial safety factors are therefore aspired for an increased efficiency during the reassessment process and for a better sustainability due to the preservation of the existing infrastructure. references [1] m. keuser. anmerkungen zur nachrechnung von brücken. betonund stahlbetonbau 111(2):57, 2016. [2] t. braml, m. haslbeck, e. hiller. daten im ingenieurbau möglichkeiten der bauwerksspezifischen beurteilung. https://doi.org/10.13140/rg.2.2.27098.41924. [3] m. haslbeck, t. stübing, t. braml. quantification of uncertainties for geodetic observations in the context of bridge surveillance. proceedings of the 1st conference of the european association on quality control of bridges and structures, p. 1-13. https://doi.org/10.1007/978-3-030-91877-4. [4] m. haslbeck, m. a. kraus, t. braml. bayesian reliability assessment and system identification for existing concrete bridge structures elaboration of a template for existing civil engineering structures. proceedings of the 17th international probabilistic workshop, 2019. [5] deutscher betonund bautechnik verein. dbv heft 24 begründung eines reduzierten zuverlässigkeitsindexes und modifizierter teilsicherheitsbeiwerte für stahlbetontragwerke im bestand, 2014. [6] t. braml. erfahrungen nach knapp 10 jahren brückennachrechnung in deutschland gemäß nachrechnungsrichtlinie, 2016. [7] g. spaethe. die sicherheit tragender baukonstruktionen, springer, vienna, 1992. [8] din en 1990:2010 eurocode basis of structural and geotechnical design, 2010. [9] din en 1991-2:2010 eurocode 1: actions on structures part 2: traffic loads on bridges, 2010. [10] j. flotzinger, p. j. rösch, n. oswald, et al. building inspection toolkit: unified evaluation and strong baselines for damage recognition, 2022. https://doi.org/10.48550/arxiv.2202.07012. [11] bundesanstalt für straßenwesen (bast). richtlinie zur nachrechnung von straßenbrücken im bestand (nachrechnungsrichtlinie), 2011. [12] bundesanstalt für straßenwesen (bast). 1. ergänzung zur richtlinie zur nachrechnung von straßenbrücken im bestand (nachrechnungsrichtlinie), 2015. [13] bundesanstalt für straßenwesen (bast). regelungen und richtlinien für die berechnung und bemessung von ingenieurbauten (bem-ing), 2022. [14] t. braml, l. lapidus. abschlußbericht heinrichsbrücke bamberg. [unpublished], 2021. [15] din-fachbericht 101. actions on bridges. berlin: beuth verlag, 2009. [16] din-fachbericht 102. concrete bridges. berlin: beuth verlag, 2009. [17] din fachbericht 103. steel bridges. berlin: beuth verlag, 2009. [18] din-fachbericht 104. composite steel and concrete bridges. berlin: beuth verlag, 2009. [19] k. morgen, s. süfke, m. keuser, et al. hochstraße barkauer kreuz in kiel. bautechnik 93(2):105-13, 2016. https://doi.org/10.1002/bate.201600008. [20] t. braml, a. kopp. nachrechnung von brücken nach stufe 4 der nachrechnungsrichtlinie. fortbildung vsvi schleswig-holstein e. v., 2016. [21] t. braml, j. wimmer, y. varabei, et al. digitaler zwilling: verwaltungsschale bbox als datenablage über den lebenszyklus einer brücke. bautechnik 99(2):11422, 2021. https://doi.org/10.1002/bate.202100094. [22] t. braml, j. wimmer, s. maack, et al. datenablage als grundlage für den digitalen zwilling eines bauwerks verwaltungsschale bbox. 1. fachkongress digitale transformation im lebenszyklus der verkehrsinfrastruktur: fachtagung über planung, bau, betrieb von brücken, tunneln, straßen digital-tagungshandbuch, expert verlag, p. 75-84, 2021. [23] din 1072:1967. road and foot bridges; design loads, 1967. [24] strurel. a structural reliability analysis program-system, 2022. http://www.strurel.de. [25] n. steffens. sicherheitsäquivalente bewertung von brücken durch bauwerksmonitoring. dissertation, technische universität berlin, 2019. [26] ril 805. tragsicherheit bestehender eisenbahnbrücken, 2002. [27] iso 13822:2010. bases for design of structures assessment of existing structures, 2010. 57 https://doi.org/10.13140/rg.2.2.27098.41924 https://doi.org/10.1007/978-3-030-91877-4 https://doi.org/10.48550/arxiv.2202.07012 https://doi.org/10.1002/bate.201600008 https://doi.org/10.1002/bate.202100094 http://www.strurel.de thomas braml, christian kainz acta polytechnica ctu proceedings [28] federation international du béton (fib). fib bulletin 80 partial factor methods for existing concrete structures. recommendation, fib task group 3.1., 2016. [29] s. küttenbaum, s. maack, t. braml, et al. bewertung von bestandsbauwerken mit gemessenen daten. betonund stahlbetonbau 114(6):370-82, 2019. https://doi.org/10.1002/best.201900002. [30] s. küttenbaum, s. maack, t. braml, et al. bewertung von bestandsbauwerken mit gemessenen daten, teil 2. betonund stahlbetonbau 116(3):183-99, 2020. https://doi.org/10.1002/best.202000087. [31] k. geißler, n. steffens, r. stein. grundlagen der sicherheitsäquivalenten bewertung von brücken mit bauwerksmonitoring. stahlbau 88(4):338-53, 2019. https://doi.org/10.1002/stab.201900018. [32] n. steffens, k. geißler. standortspezifisches ziellastniveau und objektspezifische lastmodelle für die nachrechnung auf der basis von bauwerksmonitoring. bautechnik 98(10):720-35, 2021. https://doi.org/10.1002/bate.202100051. [33] s. küttenbaum. zur validierung von zerstörungsfreien messverfahren für die probabilistische beurteilung von bestandsbauwerken mit gemessenen daten. dissertation. universität der bundeswehr münchen, 2021. [34] f. zimmert, t. braml, m. keuser. decision-making for the efficient life cycle management of structures. structural concrete 21(4):1217-26, 2020. https://doi.org/10.1002/suco.201900408. 58 https://doi.org/10.1002/best.201900002 https://doi.org/10.1002/best.202000087 https://doi.org/10.1002/stab.201900018 https://doi.org/10.1002/bate.202100051 https://doi.org/10.1002/suco.201900408 acta polytechnica ctu proceedings 36:47–58, 2022 1 introduction 2 capability of full-probabilistic calculations for the structural analysis of existing structures 3 structural reassessment of existing bridges in germany 3.1 concept of the german guideline for the assessment of existing bridges 3.2 data measurement during the reassessment process on the example of heinrich's bridge in bamberg 3.3 use of full-probabilistic methods for the analysis of the barkauer crossing in kiel 4 structure-specific partial safety factors for the assessment of existing bridges 5 conclusions references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.38.0011 acta polytechnica ctu proceedings 38:11–17, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague sustainability aspects in real estate management – basics and opportunities for an indicator-based target and reporting system in real estate companies matthias buchholz∗, thomas lützkendorf karlsruhe institute of technology, department of economics and management, centre for real estate, chair for sustainable management of housing and real estate, kaiserstraße 12, 76131 karlsruhe, germany ∗ corresponding author: matthias.buchholz@kit.edu abstract. the challenge for real estate companies to assess and communicate sustainability aspects at corporate, portfolio and building level is constantly increasing due to many external and internal influences. corresponding requirements address the sustainability reporting of the company or the environmental performance of the building stock for example. in order to meet legal requirements and to stay competitive, companies use specialisation techniques. one possibility is provided by indicators which can help to gather, assess and communicate sustainability aspects. this paper investigates the information flows between different decision levels of real estate management (rem) that can be established with the help of indicators. the results reveal the relevance of indicators for a horizontal as well as for a vertical integration of sustainability-related information in rem. moreover, synergies for the development and interpretation of indicators across decision levels can be identified by taking into account measuring units, reference units and system boundaries. results from a research that reflects the situation in germany and considers current developments in the eu will be presented. keywords: sustainability indicators, key performance indicators, sustainability assessment, corporate sustainability reporting, information management, portfolio management. 1. introduction researchers, politicians, economists and society agree on the key role that the real estate industry plays in the combat of climate change. the construction, operation, renovation and deconstruction of buildings still draws responsible for vast amounts of energy use, greenhouse gas emissions (ghg emissions), resource consumption and waste creation. since political goals to slow down climate change are ambitious, legal binding requirements on the sustainability of companies, institutional building stocks and buildings are under discussion or have been established in germany and worldwide already. therefore, and due to the growth of institutional real estate companies that can be observed in germany, the information management of sustainability aspects in companies became more complex. in the past these developments led to the existence of supporting tools and sustainability assessment methods that either exclusively address sustainability aspects on a corporate or building level or do not differentiate clearly between the context of decision levels in rem. the result is that real estate companies use various sustainability indicators for business processes, building stocks or single buildings without integrating the relevant flow of information horizontally and vertically in their organisation. this leads to the following research question: how can real estate companies use sets of indicators to support the management and communication of sustainability aspects on a corporate, portfolio and single building level and what kind of information flows go along with that? this paper builds on the results of a master’s thesis at the karlsruhe institute of technology (kit) and further develops them by incorporating new information sources and new perspectives. the thesis can be found here [1]. to answer the research question, the definition of the three levels of action and decision in institutional rem is given first. second, a variety of the most important decision support systems and assessment tools for sustainability in real estate companies that include indicators is characterized. finally, an approach is presented that links sustainability indicators to tasks in rem and considers the relevant information flows, while an example supports the understanding for the correct interpretation of an indicator. the system can be used for different building types by weighting single or multiple indicators according to their relevance. since the research focuses on institutional rem the findings are primarily applicable to the rented building stock. 2. analysis of sustainability indicators in real estate management for a systematic analysis of sustainability-related indicators used in rem the most relevant concepts of rem and the related flow of information will be in11 https://doi.org/10.14311/app.2022.38.0011 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en matthias buchholz, thomas lützkendorf acta polytechnica ctu proceedings figure 1. decision levels in real estate management adopted from [2]. troduced. additionally, possible sources for indicators will be identified and described. 2.1. decision levels in institutional real estate management rem combines many essential tasks within the real estate industry. in specific, it deals with leadership tasks concerning social systems and individuals that are private, corporate or public and that focus on value creation processes within the lifecycle of buildings [2]. rem processes can be characterised by three different dimensions: the management or decision level, the lifecycle phase of a building and the perspective which is either use-oriented, financial-oriented or technologydriven. this research focuses on the different decision levels which are shown in figure [1]. the pyramid reflects the hierarchy of processes and information flows within rem. for detailed information about the basic concept consider [2]. 2.2. types and functions of sustainability indicators there is no general definition for the term “indicator”, but it is generally considered as an aid to support decision-making through the precise presentation of required information. this also includes the abstraction of complex objects of interests to measure, analyse and assess those in a quantitative or qualitative way. further functions of indicators refer to the orientation they can give in the identification of problems, conditions and trends, to their role in controlling as well as to their ability to communicate complex objects of interests effectively and comprehensively [3]. to fulfil their functions, the formulation of scientific, use-oriented and practical-analytical requirements according to the needs of a user is recommended [3]. practical use of indicator sets has primarily been known from politics, for example in form of indicators in the context of the sustainable development goals (sdgs) [4]. in rem, indicators have been developed mostly under the name of criteria or key performance indicators (kpis). the special characteristics of rem not only led to a differentiation between quantitative and qualitative indicators but also to a distinction between processand object-orientation (see figure 1). quantitative indicators define themselves by a measured variable that is set in relation to a specific reference unit. to be able to compare certain quantitative indicators details about the system boundaries as well as the reference units are required. an example will be given in section 3.3. more details about the relevance of reference units can be found in the literature, e.g. [5]. 2.3. sources and frameworks for indicators in real estate management since sustainability aspects affect real estate companies in many ways depending on the object of assessment and the respective system boundaries, indicators can origin from a variety of sources and frameworks. table 1 categorises different types and explains their relevance for the formulation of indicators. an analysis of representative examples of each category delivered a list of 179 possible sustainability indicators for the three different decision levels of rem which can be looked up in [1]. 3. proposal for an overall indicator system to support corporate information management the system of indicators that will be presented is based on the definitions of rem and indicators. identified indicators from the literature can be allocated 12 vol. 38/2022 sustainability aspects in real estate management . . . type description examples references corporate mission since sustainability finds its way more and more into corporate processes and companies need to react on the development of legal requirements, societal pressure and market competition, it is common practice nowadays to integrate the company-wide understanding of sustainability into the corporate mission. the corporate mission reflects sustainability on a normative level which sets the frame for guidelines and rules on a strategic, tactical and operational level and therefore plays a relevant role for the use of sustainability indicators especially on a corporate level. a term that is often used analogously to sustainability in this sense is “corporate social responsibility” (csr). – [6, 7] political requirements political requirements manifest themselves in the case of germany in legislations on a regional, national and eu-wide level. they determine the frame for all corporate activities while requirements in terms of sustainability grew more and more relevant in the past. legislation influences corporate sustainability activities on all decision levels of rem, e. g. in form of requirements for the disclosure of nonfinancial aspects on the corporate level or in form of requirements on the energetic quality on a single building level. since legislation already involves the specification of certain figures at some points, the respective data can also serve as sustainability indicators to a company. legal requirements for csr reporting (csr-rug), german building energy law (geg) [7] stakeholder analysis stakeholder analysis serves companies as a tool to incorporate sustainability requirements of stakeholders and thus to adapt adequately to external influences. the relevance of stakeholder analysis for real estate companies is illustrated through the heterogenous structure of the industry and the differences of the environment on a corporate, portfolio or building level. – [7] industry guidelines industry guidelines published by state institutions, industry federations or research institutions can support companies at different stages of a building lifecycle and in different disciplines of rem. there are guidelines that define principles for a sustainable development in the industry as well as guidelines that specify recommendations. guideline for sustainable building, nuwel [8, 9] standards standardisation is evolving constantly under the premise of sustainability requirements for companies, building stocks and buildings. standards play a universal role in the real estate industry because they set the basis for a common understanding of terms, concepts and methods. relevant standards on a corporate level refer e.g. to social or environmental aspects while on a building level construction-related standards are widespread. iso 26000, din iso 14001, din en 16309 [10–12] lifecycle assessment (lca) methods lca in the real estate industry stands for methods that assess the sustainability of buildings while in the narrow sense the focus lies on ecological impacts on the environment. over time a variety of different methods evolved mainly depending on the target variable, e.g. greenhouse gases or cumulative energy demand. the results of lcas often can serve as aggregated indicators in rem. lca, carbon footprint, cumulative energy demand [13] building certification building certification became a popular instrument to assess and to reflect the sustainability of a building after a predetermined scheme. it builds on an assessment in an aggregated index that can be subdivided in categories and specific criteria. especially single criteria can be taken as example in the selection of sustainability indicators on a building level. due to the lasting covid-19 pandemic, certification systems that focus on the health of occupants are gaining relevance. dgnb, leed, breeam, well, fitwell [13] benchmarks the popularity of benchmarking in the real estate industry increased recently due to the needs of sustainability. systematic approaches for esg-benchmarking have been developed from an investor point of view. they incorporate building-related criteria as well as management criteria in their assessment. gresb, epra sbpr [14, 15] reporting standards large real estate companies are obligated to disclose about nonfinancial aspects of their business in germany and also smaller companies use the opportunities to report about the sustainability of their business and their building stock. reporting standards fulfil legal requirements and provide specific guidance to companies. industryspecific additions address real estate companies separately. gri, the sustainability code (dnk) [16, 17] table 1. sources and frameworks for indicators. 13 matthias buchholz, thomas lützkendorf acta polytechnica ctu proceedings figure 2. horizontal information flows through indicators. to the three decision levels of rem according to their relevance for companies, institutional building stocks and single buildings. for this, the specific tasks and challenges of the decision levels play a decisive role. the allocation of indicators reveals that there exist several similarities between the portfolio level and the building level which can be explained by the building focus on both levels. indicators on a corporate level differ to the point that they reflect the normative and strategic relevance of this decision level. on all decision levels, indicators show the difference that can be observed between financial and non-financial aspects: the performance of financial aspects is represented through economic indicators that are all monetised and of quantitative nature while ecological, social or other types of indicators cannot always be quantified or monetised. after the allocation of sustainability indicators to the specific decision levels, their thematic connections and the resulting information flows can be analysed. according to the presented concept of rem (see figure 1), coordination processes can either be established horizontally on a specific decision level (see section 3.1) or vertically between different levels (see section 3.2). 3.1. horizontal information flows the horizontal integration of information is strongly influenced by the interdependence of economic, ecological and social aspects. relevant indicators can support the prioritisation and weighting of different sustainability goals. moreover, they allow for the controlling and assessment of impacts decisions have on the three pillars of sustainability. an example for the interdependence of indicators is represented by a subsequent thermal insulation of a building which ideally leads to a reduction of energy demand (ecological), thus to a reduction of energy costs (economic) as well as to an increase of thermal comfort (social). another aspect to be considered is the possible representation of ecological, social and other non-financial qualities through aggregated indicators. here, these indicators are called economic substitute indicators and they allow for a comprehensive integration of non-financial aspects into economic decision-making. examples can be the overall technical quality of a building or the results of a lca on a single building level. the analysis of indicators suggested by the literature highlights another relevant aspect for horizontal integration: generally, a clear relation between processand object-oriented indicators can be observed. process-oriented indicators, which are mostly qualitative, stand for the existence of managerial aspects with a supporting function for the implementation of object-oriented indicators, while the latter allow statements about the performance of companies, building-stocks and buildings in sustainability-related aspects. the existence of ecological risk assessments for a building for instance can be an indicator for the process quality, while the results of sustainability assessment methods describe the relative object-oriented indicators. in practice, the use of indicators needs to be adjusted to the company-specific organisation structure and the respective data strategy. the term “enterprise data strategy” stands for the integration of processes and data resources in order to serve corporate goals. there are relevant approaches for data organisation that consider the specific requirements of the real estate industry [18]. the general relation between processand objectoriented indicators described above is given in figure 2. the managerial function of process-oriented indicators becomes clear, while respective values of object-oriented indicators can be directly linked to the controlling of sustainability goals. 3.2. vertical information flows additional to the horizontal coordination, both existing directions of vertical information flows in rem need to be considered: • top-down: top-down information flows refer to the communication of goals and requirements to align normative, strategic, tactical and operational aspects. indicators are supposed to reflect the level of abstraction on the specific decision level. • bottom-up: to evaluate existing goals and requirements that are communicated to the next lower level, decision makers rely on the reporting from 14 vol. 38/2022 sustainability aspects in real estate management . . . figure 3. decision-level specificity of indicators including selected examples. decision level goal object of assessment (includes system boundaries) measuring unit reference unit(s) corporate level ghg neutral company company t co2-equivalent €sales/a €revenues/a portfolio level ghg neutral building stock building stock t co2-equivalent m2 building stock/a building level ghg neutral building single building t co2-equivalent m2 single building table 2. indicator ghg emissions. the respective lower level. here, indicators can serve as a tool to communicate required information effectively, while their controlling function gets emphasised. the specific role decision levels take in the topdown and bottom-up logic of rem gets reinforced by the system of sustainability indicators. decisions on a corporate level are key for the identification of goals and requirements of the whole organisation. on the contrary, the capture of building-related information and effective communication through indicators emphasises the meaning of the single building level. in addition to that, the portfolio level takes in an intermediate role. in practice, there can be more decision levels or sub-levels like a distinction between asset management as a strategic object level and property management as an operational object level. this aspect will not be discussed in more detail here. the addition of the vertical component to the system leads to manifold information flows through indicators between the decision levels. for a further elaboration, a comparison of quantitative object-oriented indicators in specific reveals additional insights. it becomes clear that there are several similarities between economic, ecological, social and other indicators from different decision levels. to identify these similarities, an analysis of the object of assessment, the reference unit and the system boundaries of a quantitative indicator reveals two main types: decisionlevel-overarching indicators are named identically on different decision levels and they follow the same calculation method. they differ due to their object of assessment which leads to diverging reference units and to diverging system boundaries that need to be considered for a transparent specification (see section 3.3). decision-level-specific indicators are typical for a decision level since they deal with aspects that can only be specified on one decision level. however, decision-level-specific indicators can – similarly to decision-level-overarching indicators – follow similar economic, ecological or social strategic sustainability goals. the distinction between the different types of indicators is shown in figure 3. 3.3. example: the role of reference units and system boundaries to illustrate the differentiation between the previously explained types of indicators, an example will be given for the indicator “ghg emissions”. due to the internally and externally motivated need to reduce ghg emissions, the indicator is relevant on all decision levels and thus can be classified as a decision-leveloverarching indicator. table 2 illustrates the differences for the calculation of ghg emissions on different levels but also emphasises the interdependencies that should be considered through vertical communication flows. ideally, actions and decisions aim to support the long-term goal of “ghg neutrality” (sometimes also called climate neutrality or carbon neutrality) which translates into sub-goals for the decision levels. the slight difference between ghg neutrality and 15 matthias buchholz, thomas lützkendorf acta polytechnica ctu proceedings c on st ru ct io n m ai nt en an ce d em ol it io n & r ec yc lin g o pe ra ti on & us e d ir ec t g h g em is si on s of bu si ne ss : co m bu st io n, fle et in di re ct g h g em is si on s of bu si ne ss : en er gy co ns um pt io n t ra ns po rt : b us in es s tr av el , em pl oy ee co m m ut in g p ro du ct io n, pr oc ur em en t an d di sp os al of us ed go od s an d se rv ic es d ir ec t an d in di re ct g h g em is si on s of le as ed as se ts assessment per year company (x) x x x x x building stock (x) x x single building (x) x x assessment per lifecycle (stage) single building x x x table 3. system boundaries for ghg emissions. “climate neutrality” that is often overlooked can be read in the literature, e.g. [19]. the complexity of information management and assessments of sustainability aspects in rem derives mainly from the heterogeneity of companies and the unique character of buildings. on the one hand this eventually complicates internal processes, on the other hand methods and principles are needed to make companies, building stocks and single buildings comparable to one another. indicators that express information about sustainability-related aspects can serve as a tool in both cases. for this purpose, transparency about the included processes and objects of the measurement and calculation of an indicator is key. to reach this, the specification of system boundaries which an indicator is bound to on a specific decision level can be used. an example for system boundaries in the calculation of ghg emissions is given in table 3. the attributes that are used for the definition of system boundaries are retrieved from sustainability reporting conventions [16, 17, 20]. in this sense, the classification of ghg emissions in scopes following the ghg protocol plays a relevant role to companies [20]. table 3 shows two different perspectives on the measurement and calculation of an indicator: on the one hand, ghg emissions can be assessed yearly which serves in particular the fulfilment of sustainability reporting requirements. on the other hand, a lifecycle perspective for single buildings allows the assessment in a broader sense. the illustrated system boundaries are also relevant if ghg emissions are used for the calculation of the global warming potential (gwp). the example emphasises the complexity of a corporate sustainability information management by illustrating the interdependencies between indicators. however, synergies of measurement and calculation methods can be used for the efficient coordination of tasks while indicators serve as a tool to exchange information between decision levels. 4. conclusions the contribution presents an approach that illustrates the possibilities of an allocation of sustainability indicators to the decision levels of rem. relevant indicators need to be identified and defined so that they serve the goals and requirements of the relative decision level as well as the goals of the whole company. the analysis shows that indicators represent an effective aid to quantify, communicate and manage sustainability for companies, building stocks and buildings. the suggested horizontal and vertical integration of sustainability-related information following the definition of rem allows for a holistic system of indicators. the presented system focuses on the representation of sustainability-related information through indicators as well as on the relevant information flows. compared to existing reporting standards, benchmarks or certification systems the approach clearly introduces a distinction between sustainability aspects on different decision levels. moreover, the approach does not represent a new sustainability assessment method but rather emphasises on the difference between managerial and object-oriented aspects which remains vague in many existing systems. in the first place, the proposed view on sustainability indicators addresses decision-makers in rem and belonging disciplines such as portfolio management or asset management. future research in this field should include the examination of integration opportunities for indicators into existing controlling mechanisms and the development of standardised methods for measurements and calculations. references [1] m. buchholz. nachhaltigkeitsaspekte im immobilienmanagement. grundlagen und möglichkeiten eines indikatorgestützten zielund berichtssystems. master’s thesis, karlsruher institut für technologie 16 vol. 38/2022 sustainability aspects in real estate management . . . (kit), 2022. https://doi.org/10.5445/ir/1000143074. [2] a. kämpf-dern. immobilienwirtschaftliche managementebenen und -aufgaben. definitionsund leistungskatalog des immobilienmanagements. technische universität darmstadt, 2009. [3] a. grunwald, j. kopfmüller. nachhaltigkeit. 2nd ed. campus, frankfurt am main, 2012. [4] united nations (un). the 17 goals, 2021. [2021-09-14], https://sdgs.un.org/goals. [5] t. lützkendorf, m. unholzer. kennwerte zur energetischen und ökologischen qualität von bauwerken in deren nutzungsphase. karlsruher institute of technology, 2013. [6] united nations environment programme (unep). sustainabilty metrics. translation and impact on property investment and management. unep finance initiative, 2014. [7] zentraler immobilien ausschuss (zia). nachhaltigkeit in der immobilienwirtschaft. kodex, berichte und compliance. zentraler immobilien ausschuss, 2015. [8] federal ministry of interior, building and community (bmi). guideline for sustainable building. 3rd ed. bmi, 2019. [9] center for corporate responsibility and sustainability an der universität zürich (ccrs). nachhaltigkeit und wertermittlung von immobilien. leitfaden für deutschland, österreich und die schweiz (nuwel). ccrs, 2011. [10] international organization for standardization (iso). iso 26000:2010 guidance on social responsibility, 2010. [11] deutsches institut für normung (din). din en iso 14001:2015-11 environmental management systems – requirements with guidance for use, 2015. beuth. [12] deutsches institut für normung (din). din en 16309:2014-12 sustainability of construction works – assessment of social performance of buildings – calculation methodology, 2014. beuth. [13] l. andes. methodensammlung zur nachhaltigkeitsbewertung. grundlagen, indikatoren, hilfsmittel. karlsruher institute of technology, 2019. [14] global real estate sustainability benchmark (gresb). real estate reference guide, 2020. [2021-09-16], https://documents.gresb.com/ generated_files/real_estate/2020/real_estate/ reference_guide/complete.html. [15] european public real estate association (epra). assessment methodology for the epra sbpr awards, 2018. [2021-09-16], https://www.epra.com/ application/files/7715/6802/2860/2019_epra_ assessment_methodology_final.pdf. [16] global reporting initiative (gri). g4 sector disclosures – contruction and real estate. gri, 2014. [17] gdw bundesverband deutscher wohnungsund immobilienunternehmen. leitfaden zur branchenspezifischen ergänzung des deutschen nachhaltigkeitskodex. gdw, 2015. [18] oscre. functions contained in the oscre industry data model (idm), 2022. [2022-03-01], https://www.oscre.org/industry-data-model/howthe-industry-data-model-is-organized. [19] umweltbundesamt (uba). der weg zur treibhausgasneutralen verwaltung. etappen und hilfestellungen. uba, 2020. [20] greenhouse gas protocol. a corporate accounting and reporting standard, 2004. [2021-09-16], https://ghgprotocol.org/sites/default/files/ standards/ghg-protocol-revised.pdf. 17 https://doi.org/10.5445/ir/1000143074 https://sdgs.un.org/goals https://documents.gresb.com/generated_files/real_estate/2020/real_estate/reference_guide/complete.html https://documents.gresb.com/generated_files/real_estate/2020/real_estate/reference_guide/complete.html https://documents.gresb.com/generated_files/real_estate/2020/real_estate/reference_guide/complete.html https://www.epra.com/application/files/7715/6802/2860/2019_epra_assessment_methodology_final.pdf https://www.epra.com/application/files/7715/6802/2860/2019_epra_assessment_methodology_final.pdf https://www.epra.com/application/files/7715/6802/2860/2019_epra_assessment_methodology_final.pdf https://www.oscre.org/industry-data-model/how-the-industry-data-model-is-organized https://www.oscre.org/industry-data-model/how-the-industry-data-model-is-organized https://ghgprotocol.org/sites/default/files/standards/ghg-protocol-revised.pdf https://ghgprotocol.org/sites/default/files/standards/ghg-protocol-revised.pdf acta polytechnica ctu proceedings 38:11–17, 2022 1 introduction 2 analysis of sustainability indicators in real estate management 2.1 decision levels in institutional real estate management 2.2 types and functions of sustainability indicators 2.3 sources and frameworks for indicators in real estate management 3 proposal for an overall indicator system to support corporate information management 3.1 horizontal information flows 3.2 vertical information flows 3.3 example: the role of reference units and system boundaries 4 conclusions references my title https://doi.org/10.14311/app.2022.36.0006 acta polytechnica ctu proceedings 36:6–14, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague assessment of existing structures under climate change johan v. retief stellenbosch university, department of civil engineering, private bag x1, stellenbosch, 7600, south africa correspondence: jvr@sun.ac.za abstract. assessment of the influence of human activities on recent, current, and future global and regional climate conditions and extremes has advanced sufficiently to provide a reasonable measure of its impact across the globe. the lack of concurrent adaptation of the design base for load bearing structures results mainly from the absence of a clear signal that climate change will have a significant effect on the climate actions that are accounted for in the structural design basis. the recent ipcc assessment of the physical science basis of climate change reports significant advances in observing and projecting changes in weather and climate extremes due to human influences. this provides an opportunity to reassess projections of future climate action conditions. whilst the ipcc assessment confirms previous indications that, for example extreme wind will respond moderately globally, improvements in understanding and projecting changes show that trends will be overshadowed by uncertainties. the implication is that the design base will need to account for increasing uncertainties as climate actions are projected into the future, over the service life of existing structures, as well as those designed to current standards. the design base consequently in advance need to reflect continuous changes of existing structures. keywords: climate actions, climate change, design base, existing structures, projection skills, structures, uncertainties. 1. introduction since early hypotheses that the use of fossil fuels will lead to global warming due to increased atmospheric concentration of co2 as a greenhouse gas, the more general version of global warming due to changes in various greenhouse gases caused by human activities is now considered to be proven [1], [2]. furthermore, the subsequent impact of the additional heat load on global systems is now investigated extensively, with an emphasis on the climate, because of the wide exposure of both human and natural systems, although related systems such as the ocean and the cryosphere also play an integral role in the response to human caused warming [3]. the extensive body of information on climate change compiled by the ipcc series of assessment reports on climate change [3] provides an opportunity to consider the potential impact of anthropogenic climate change on any system or activity exposed to climate and weather conditions. one such case, considered in this review, is the role of extreme climate actions, such as wind and snow loads, or thermal actions, in the design base for load bearing structures, such as buildings, industrial structures, and civil engineering infrastructure. although this class of human systems have a low profile in the assessment of the impact of climate change, it represents vast capital investment and play a fundamental role in the socioeconomic environment. the low profile could be ascribed to the level of specification of climate actions with return periods of the order of millennia for levels of reliability associated with safety, depending on the reliability class of the structure. such specifications are much more severe than the level of event likelihoods considered in climate change investigations. there is nevertheless a realisation that extreme events represent an important component of the impact of human induced climate change on the natural and socioeconomic environment [4, 5]. this has led to extensive consideration of climate and weather extremes in climate change [6–8]. the design base for load bearing structures evolved concurrently with the growing attention being paid to human induced climate change, running in parallel since around the mid-twentieth century. notably the inherent variability of climate actions on structures made a significant contribution to the application of risk and reliability in deriving operational semiprobabilistic design, expressed in limit states format. ironically, the potential impact of climate change has yet to be implemented in general standardized design practice, as reflected for instance as an explicit class of risk [9] or as a prominent design situation [10]. this situation could be the result of climate actions that are specified at several standard deviations from annual extremes. furthermore, pioneering efforts to account for climate change observe limited trends from either historic data or projected values, even under severe scenarios of human caused radiative forcing of global warming [11, 12]. when the difficulties of extracting reliable extreme value models from historic data is considered, such as for wind load in south africa [13–15], the opportunity to benefit from the extended 6 https://doi.org/10.14311/app.2022.36.0006 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 36/2022 assessment of existing structures under climate change assessment of climate extremes [7] should be utilised. less obvious in the somewhat surprising state of matters is the acceptance that climate change will have a relatively mild effect on structural loads, given the growing body of information on substantial changes in future conditions coming out of the ipcc assessment of climate change. some scrutiny of the relevance of climate change to structural loads indicate a lack of credible information on future extreme climate and weather conditions [16]. the objective of this paper is to review the advances in developing the physical sciences basis of climate change as reflected in the latest ipcc assessment [1] as source of information on climate actions on structures, specifically considering climate extremes [7]. assessment of climate extremes rests on information from observations and paleoclimatic histories, which is applied to the identification of human drivers of changes, within processes of natural variability, towards sufficient understanding for attribution to human influences, to changes in the climate and in extremes [6, 16], observed changes, and the application of change indicators and uncertainties [17], projection of future conditions, and their various sources of uncertainty [18], finally input required for risk assessment [8]. the review is done at three levels: firstly, confirmation of climate change, its attribution to human origins, and the current rate of change, provide the background information for decision-making on the necessity, even urgency to implement adaptation measures for the climate action design basis. at the second level the state of knowledge on changing of climate extremes relevant to structural actions provides the background to what information could be extracted from the climate change knowledge base currently available for possible decisions on immediate implementation. thirdly the ultimate check is to review the available skills for projection of climate actions to determine the required resolution of projecting future trends, and the relative dominance of natural variability and projection uncertainties, as determined by projection model resolution. each of these objectives could require an exposition on its own, so this review can only be done at strategic level within the scope of this paper. a secondary objective is to raise the issue of the impact of climate change on the design base for structures and provide an overview of the state of information. the perspective taken in this review is significantly biased towards extracting information to be used in decision-making, see for example [19–21]. 2. climate state and pathways towards extreme climate based on robust understanding of climate system fundamentals, the systematic scientific assessment of climate change has evolved from theory since the 1970s, to now be regarded as an established fact. recent advancements are based on the integration of multiple lines of evidence consisting of observations that include the emergence of a climate change signal, paleoclimatic evidence, the identification of natural and human drivers of the climate, understanding and attributing climate change, and implementation of this information in model development [1]. this provides the platform for projecting possible future climate conditions for representative pathways that depend on mitigation measures taken on a global scale. characterization of the current state of the climate, representative climate futures, with specific reference to climate and weather extremes, serve as background information currently available to inform decisions on future trajectories for climate actions on structures. 2.1. current climate state whilst the complexity of climate change emerges from each successive version of the ipcc series of assessment reports, the progression of the process is concisely represented by the metric of global warming since preindustrial conditions: the observed temperature increase ∆t of 1.09 [0.95 to 1.20] ◦c above the 1850-1900 baseline for the decade ending 2020 can be related to co2 atmospheric concentration, as most important greenhouse gas, reaching 410 ppm, compared to the 270 ppm baseline, with changes over land of 1.59 [1.34 to 1.83] ◦c and the ocean of 0.88 [0.68 to 1.01] ◦c respectively [1]. the trend since 1850 is displayed graphically in figure 1, reflecting significant achievements in simulation of the climate change [1], [3]. the range of natural variability of about 0.5 ◦c is evident, including episodic perturbations due to natural causes. the overall trend is to show a steep incline starting at around the 1970’s towards the ∆t -region of 1 ◦c. climate simulation including natural and human drivers follow the trend remarkably well, though with some lag in the initial increase. the smoother simulated average (brown line) can be ascribed to the multi-model averaging process, however the uncertainty range (shading) that exceeds natural variability, suggests that the observation is just one ’worldline’ of many possible outcomes of a highly complex system. simulation of a counterfactual world without human influence provides a clear indication of emergence of climate change above natural variability at around the 1970s and attribution of the global temperature rise to human causes as indicated by the differences between simulations. attribution is resolved further to indicate the contribution of all greenhouse gases [+1.5 ◦c] and other human causes [−0.4 ◦c], with co2 as the dominant gas [+0.8 ◦c] and so2 as the dominant other cause [−0.5 ◦c]. scaling global temperature change across the global climate and down to its regional impact due to climate extremes is demonstrated by figure 2, confirming human influences on extremes in temperature and precipitation, as representation of different modes of climate extremes. this overview of pervasive impact of human 7 johan v. retief acta polytechnica ctu proceedings figure 1. observed and simulated history of global surface temperature changes (annual average) and causes of recent warming, demonstrating model skills [1]. activities on the environment is based on the systematic implementation of methodologies that apply also to resolving the climate extremes central to this paper, consisting of (i) understanding the climatological processes and drivers of change, (ii) to be able to observe historic trends, (iii) which are applied to validate simulation models, (iv) using the identification of emergence and attribution of the phenomenon under investigation, (v) as basis for establishing confidence in projection of future changes and conditions. this methodology is also used to assess climate extremes [7] and applied here to take measure of extremes relevant to climate actions on structures. 2.2. future climate pathways the main future climate drivers caused by human activities are represented by five scenarios of combined shared socioeconomic pathways (ssp) that is combined with the level of radiative forcing by the end of the century (in w/m2) with trajectories for co2 as the most significant greenhouse gas. global warming scales almost linearly with accumulated atmospheric co2 concentrations (figure 3(a)), resulting in alternative projections of observed warming up to the year 2100 (figure 3(b)). the range of outcomes depending on mitigation policies [ssp1-1.9 to ssp5-8.5] can be compared to the very likely range for simulated pathways (coloured shading) shown for ssp1-2.6 and ssp3-7.0. a significant implication of the relationship between accumulated carbon release and temperature rise is that every tonne of carbon released contributes to global warming. an indication of the geographic distribution of these changes is provided in figure 4, showing the comparison between observed and simulated changes in surface temperature at 1 ◦c, and for projected ∆t -values of {1.5, 2.0.4.0} ◦c. an indication of the regional distribution of climate change extremes is compiled in figure 5, indicating the number of regions for which climate impact drivers (cid) for the set of climate extremes are relevant, where a cid is a measure of a physical climate system condition (mean, event, extreme) that will affect society or an ecosystem, by either increasing or decreasing. confidence levels are indicated by shading. mixed signals and low confidence levels are notable for severe wind and snowstorms. 3. changing weather and climate extremes the occurrence of climate and weather extreme events have a high profile in public interest: it is often connected with climate change and its attribution to human activities. information on such a relationship is therefore of interest to policy decision-making, as a prime objective of ipcc assessment of human induced climate change [1]. differentiation of human related radiative forcing of changes from natural variability, however, poses stringent challenges to climate science skills, particularly to determine attribution for individual events. successful simulation of climate extremes provides confidence in its projection skills. furthermore, extreme weather and climate events evidently make a significant contribution to the impact of climate change. against this background, recent 8 vol. 36/2022 assessment of existing structures under climate change figure 2. synthesis of assessed observed and attributable regional changes for hot extremes and heavy precipitation, demonstrating the diversity of regional and modal changes [1]. advancement in the assessment of changes in extremes is regarded as one of the significant achievements of climate sciences [3]. climate extremes provide a logical point of entry for climate actions on structures. this could be done in two steps: projection of climate extremes in general, and extreme classes associated with extreme wind as assessed in [7] and [8] are reviewed in this section. the mode of presentation of the assessment is to aggregate results to inform general observations on the relevance of extremes to climate change. application of this information to the adaptation of the design base for wind load is discussed in the next section, based on extracted information on trends in climate actions as it is demonstrated by considering wind load from [7, 8]. this section explores the challenges in observing and modelling of climate extremes, with the intrinsic properties of being rare, short-lived, dependent on local conditions, and inherently highly variable. this is aggravated by the extensive extrapolation into the future from an uncertain observation base, particularly for more extreme events. 3.1. assessment methodology for extremes a systematic methodology is employed to relate human influences to changes in extreme weather and climate from historic observation to the prediction of future conditions through validated models [7]. the first step consists of the identification of mechanisms and drivers of extreme events, to reflect the knowledge base of the process. observed trends provide the baseline against which changes are measured. model evaluation considers both the match between models and observation, and an assessment of projection uncertainties. detection of human influences beyond natural variability is used as measure of the level of attribution; considered across the dimensions of the situation under investigation, such as extreme class, global, regional, or local scale; demonstrated for the class of extreme climate. projections ultimately provide trends and uncertainties in changes to extremes as climate change progresses. assessment of the results is expressed in the customary levels of confidence and likelihood { low, medium, high }; in case of high confidence, a likelihood may be assigned as { likely, very likely, extremely likely, virtually certain }; based on skills to simulate observed trends, a measure of model uncertainty based on inter-model comparisons, and judgement on the scientific basis for dominating processes [6]. detection and attribution of human influences are applied at global warming of ∆t of 2 ◦c; with projected changes at {1.5, 2.0, 4.0} ◦c [7]. the success of the process is confirmed by the finding that changes in extremes for temperature, precipitation, droughts, tropical cyclones, and compound fire 9 johan v. retief acta polytechnica ctu proceedings (a) (b) figure 3. pathway dependent global climate futures: (a) near-linear linear relationship between cumulative emissions and global surface temperature (b) global surface temperature changes in ◦c relative to 1850-1900 [1]. weather, would have been extremely unlikely without human causes [1]. recent advances include resolving human influences on individual extreme events, regional scales, and at different warming levels. this is based on improved knowledge, complementary sources of evidence, improved models, and accumulating historic data. 3.2. extreme climate classes the assessment is concerned with occurrence over land, consisting broadly of temperature related extremes, the water cycle, storms, and compound events that are classified as low-likelihood high-impact (llhi) situations. temperature extremes provide the yardstick for observing changes in extremes and their sensitivity to human influences. this results from the good standing of the state of knowledge, observation, modelling, attribution, and projection skills. heavy precipitation, generally and as associated with extreme storms, demonstrates the implications of more complex extremes with interacting and feedback processes, yet advancing in projection and attribution skills. extreme storms, subdivided into tropical cyclones (tc), extratropical cyclones (etc), and severe convective storms (scs), include extreme wind as a component. an extreme climate class dedicated to extreme wind was introduced recently [7, 8], allowing for the aggregation of common characteristics from storm classes; providing for better alignment between climatology and wind engineering application. extension of climate change assessment to extreme climate provides an additional line of scientific proof of the process: it is regarded as an integral component of climate change, manifested by the increased frequency of climate extremes, particularly hot extremes, on global and regional scales, on most continents, even 10 vol. 36/2022 assessment of existing structures under climate change figure 4. geographical distribution of the global climate (a) comparing observation with simulation (b) for simulated changes in global warming at representative levels [1]. for individual events based on case studies [1], [7]. observations made on extreme storms range from high confidence of increases in precipitation rates, average and peak wind for tropical cyclones globally and severe convective storms regionally, but low confidence in past changes of maximum wind speed for extratropical cyclones [7]. low-likelihood high-impact events are assessed primarily from the perspective of concurrent events. notably high likelihood is assigned to the generic observation that historically unprecedented events and surprises can be caused by the rate of global warming. as a general observation on the outcome of the ipcc review, extreme weather and climate events are predominantly characterised by precipitation extremes, (occasionally complemented by wind) as a measure of frequency, intensity, or by implication by the impact of human origins on changing conditions. this approach is used extensively for assessing tropical cyclones. a significant deviation is the conclusion that an increase in the proportion of high intensity tropical cyclones will very likely lead to an increase in average peak wind speed, as well as an increase in rain-rates, despite a decrease in global frequencies of such events [7]. a similar low profile for strong wind can be observed for scs, as opposed to using precipitation as indicator. it is concluded for example with high confidence that average and maximum rain rates associated with such storms will increase with global warming in some regions [7]. yet, wind and tornadoes are included as an outcome of projecting an increase in frequency and intensity of severe thunderstorms with high confidence. an exception to the subdued role of severe wind in extreme storms is the use of near-surface wind speed, together with precipitation, as extreme value indicator of the severity of etcs. still, extreme precipitation events are well established in the identification of extratropical storm conditions. indication of past changes in maximum wind speed associated with etcs is regarded to be observed with low confidence, with medium confidence that projected changes will be small, and with high confidence that precipitation rates will increase with warming. notably, there is medium confidence that the projection of wind speed and precipitation associated with etcs depend on the resolution and formulation of climate models. the aggregated conclusion, made with low confidence, is that the intensity of observed extreme winds is becoming less severe in the lower to mid-latitudes, while becoming more severe in poleward latitudes beyond 60 degrees. there is medium confidence that the frequency and intensity of extreme winds will be associated with the projected changes in the frequency and intensity of associated tropical and extratropical cyclones. although no explicit mention is made of scss in the summary, high confidence is indicated that convective available potential energy increases in response to global warming in the tropics and subtropics, suggesting more favourable environment for scss; though with high confidence that limited application of convection-permitting models lead to significant uncertainty about projected regional changes. 11 johan v. retief acta polytechnica ctu proceedings figure 5. synthesis of the number of regions where climate impact drivers (cid) are projected to change, increasing (top) and decreasing (bottom) with high/medium confidence levels (shading) [1]. 4. adaptation of design base for wind actions the difference between the scientific approach followed in the ipcc assessment of climate change to provide best estimates of the process, even including climate extremes, and the information required for risk-based decision-making that includes the consequences of tailend events, is appreciated by assessors and researchers [22, 23]. conversion of the information on extreme wind, as summarised above, to information serving as background to the adaptation of the design base for wind load, requires best estimates of both trends and their uncertainties. 4.1. assessment of projected wind actions in the absence of any reported observation related to extreme wind changes in [7], expressed at likelihood levels as indicated in the previous section, confidence levels are applied in an inverse manner to reflect uncertainty in the projection qualitatively, as opposed to its intended scientific qualifier, as applied in [7]. climate systems associated with extreme wind for south africa are synoptic scale frontal systems and mesoscale convective storms [24], which are related to etcs and scss respectively. confidence levels for the components of assessment (see section 3.1) for these two extreme storm classes are therefore utilised as basis for qualifying changes in trend and uncertainty of wind load projections. significantly, there is no review of the mechanisms and drivers used for the identification, modelling, and projection of changes due to human causes for etc’s (compare section 2.1). accordingly, it is not surprising that there is low confidence in observed changes recently, and over the past century. this is due to large interannual and decadal variability. ironically, high confidence is assigned to model underestimation of dynamical intensity of events (related to surface wind speed), similarly for linking systematic bias between etc events and rainfall intensity to model limitations. furthermore, there is low confidence in attribution of events to human influence resulting from limited observations. projections of dynamic intensity depend, with medium confidence, to the resolution and formulation of the representation of convection in climate models. given the limited information on trends in etc intensity, also for extreme wind projections, the poor rating obtained for all components of the assessment methodology leads to the conclusion that the current state of information does not provide any 12 vol. 36/2022 assessment of existing structures under climate change guidance on future changes, rather than to indicate small changes. this situation arises from limitations in observation, attribution to human influences, and modelling that reflects drivers of change in etc wind extremes. a rather complex situation emerges from the assessment of scs, or mesoscale convective systems (mcs), with highly skewed information provided across all elements of the assessment steps, ranging from noticeable advances in mesoscale observation networks, complemented by high resolution modelling, mainly in the usa, versus low confidence in resolving regional scale impacts across the balance of regions due to limits in observation and simulation modelling. the specific advances in resolving mesoscale extremes could therefore serve as benchmark to estimate uncertainties in other regions, mostly outside the usa. such comparisons should include the identification of regional climate impact drivers, limits to observed trends due to insufficient coverage and long-term records, model deficiencies of resolution and provision for convection processes, the influence of the fine balance between opposing environmental factors that affect severe storm development. 4.2. integrated basis for design and assessment the adaptation of the design base for wind load on structures is evidently dominated by uncertainties of the changes to the extreme wind climate caused by future climate conditions. an expedient use of the current design base to enhance the robustness of structures against such uncertainties, consists of applying the stipulated wind load in a sensitivity analysis as an accidental design situation for climate change [25]. the extension of such an approach to incorporate methodologies for the assessment of existing structures [26] provides the opportunity to reflect the trajectory of changing wind loads over the service life of the structure. at the fundamental level, such an approach explicitly accounts for the significant conversion from treating climate actions under assumptions of stationarity to transitionary. at an operational level, the progression of assessment methodologies from design value, through reliabilityand risk-based approaches, enables more advanced estimates of reserve capacity of the structure to changing wind loads. strategically, planning for future assessments of the structure as the changing climate pathways evolve can be incorporated into the design process. in addition, performing the first assessment upon completion of the structure would not only ensure that pertinent information for subsequent assessments is captured, but should contribute to proper integration of the bases for design and assessment of structures in general. 5. observations and conclusions the paper reviews the latest set of assessments on the physical science basis for climate change due to human influences from the perspective of its relevance to climate actions on load bearing structures. of particular interest is the advances made in determining the impact of changing climate extremes on human and ecological systems. characterisation of climate change in general provides the context for decisions on timing of any adaptation of the design base, whilst climate extremes inform decisions on climate actions, which are specified at extreme value fractiles to achieve required levels of reliability for structures. both topics are therefore reviewed in the paper. it is notable that initial suppositions and pioneering observations on both climate and extreme changes are substantiated with advancement of its scientific bases, with growth of the information base, arguably over orders of magnitude, predominantly realised as confirmation, but with an appreciation of the complexity of the process, if not its pervasive nature. this is demonstrated by the observation that the initial identification of human caused global warming has progressed not only to measurable global and regional warming, but also to the attribution of human influences on an increasing set of extreme events, and an extension of the set of human influences. the main conclusions of this review are thus related to both the changing climate and extremes. human induced climate change has demonstrably advanced to such an extent that any system that would be impacted over multidecadal scales, need to implement adaptation measures, irrespective of the outcomes of mitigation scenarios. on the question of whether this includes structures exposed to climate actions, the conclusion is that such exposure results not so much from the mild trends observed and projected for climate actions, but predominantly from uncertainties of future changes which might be several standard deviations from expected trend conditions. two main lines of action follows: whilst advances in climate extreme projections should be followed up, sufficient robustness and adaptability should in the meantime (now) be incorporated in design decisions. the methodologies for the assessment and adaptation of existing structures provide useful instruments for the transition from the context of stationarity of the current design base. as a final disclaimer, the review considers an issue which evidently falls within the domain of climate sciences, rather than engineering sciences. however, it is essential for engineering considerations to appreciate the impending conversion of climate actions on structures from an empirical base under stationary conditions, to one requiring at least a reflection of the transient future climate and its impact on an adapted design base that is intimately related to climate sciences. the review therefore also demonstrates an engineer’s perspective of climate change, in preparing to engage with climatologists in divining the future. 13 johan v. retief acta polytechnica ctu proceedings list of symbols cid climate impact driver etc extratropical cyclons ipcc intergovernmental panel on climate change ll-hi low-likelihood high-impact situations mcs mesoscale convective systems scs severe convective storms ssp share socioeconomic pathways tc tropical cyclones references [1] ipcc. summary for policymakers. in: climate change 2021: the physical science basis. contribution of working group i to the sixth assessment report of the intergovernmental panel on climate change [masson-delmotte, v., p. zhai, a. pirani, s. l. et al. 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[12] h. p. hong, q. tang, s. c. yang, et al. calibration of the design wind load and snow load considering the historical climate statistics and climate change effects. structural safety 93, 2021. https://doi.org/10.1016/j.strusafe.2021.102135. [13] j. v. retief, a. m. goliger. development of an updated fundamental basic wind speed map for sans 10160-3. journal of the south african institution of civil engineering 59(4):12-25, 2017. https: //doi.org/10.17159/2309-8775/2017/v59n4a2. [14] j. botha, j. v. retief, c. viljoen. uncertainties in the south african wind load design formulation. journal of the south african institution of civil engineering 60(3), 2018. https: //doi.org/10.17159/2309-8775/2018/v60n3a2. [15] f. p. bakker. characterisation of the south african extreme wind environment relevant to standardisation. phd dissertation, stellenbosch university. 2021. [16] v. eyring, n. p. gillett, k. m. achuta et al. human influence on the climate system. in: climate change 2021: the physical science basis. cambridge university press. in press, 2021. [17] s. k. gulev, p. w. thorne, j. ahn j et al. changing state of the climate system. in: climate change 2021: the physical science basis. cambridge university press. in press, 2021. [18] j. y. lee, j. marotzke, g. bala, et al. future global climate: scenario-based projections and near-term information. in: climate change 2021: the physical science basis, 2021. [19] h. e. auld. adaptation by design: the impact of changing climate on infrastructure. j public works & infrastructure. 1(3), 276-288, 2008. [20] s. hallegatte. strategies to adapt to an uncertain climate change. global environmental change 19(2):240-7, 2009. https: //doi.org/10.1016/j.gloenvcha.2008.12.003. [21] c. helgeson. structuring decisions under deep uncertainty. topoi 39(2):257-69, 2018. https://doi.org/10.1007/s11245-018-9584-y. [22] r. t. sutton. climate science needs to take risk assessment much more seriously. bulletin of the american meteorological society 100(9):1637-42, 2019. https://doi.org/10.1175/bams-d-18-0280.1. [23] c. p. weaver, r. h. moss, k. l. ebi, et al. reframing climate change assessments around risk: recommendations for the us national climate assessment. environmental research letters 12(8), 2017. https://doi.org/10.1088/1748-9326/aa7494. [24] a. c. kruger, a. m. goliger, j. v. retief et al. strong wind climatic zones in south africa. wind and structures journal 13(1):37-55, 2010. [25] j. retief, c. viljoen. provisions for climate change in structural design standards. in engineering for extremes, springer tracts in civil engineering, p. 429-46. 2022. https://doi.org/10.1007/978-3-030-85018-0_21. [26] m. holický, j. v. retief, c. viljoen. reliability basis for assessment of existing building structures with reference to sans 10160. journal of the south african institution of civil engineering 63(1), 2021. https: //doi.org/10.17159/2309-8775/2021/v63n1a1. 14 https://doi.org/10.1016/j.oneear.2020.05.011 https://doi.org/10.3390/app9245416 https://doi.org/10.1016/j.strusafe.2021.102135 https://doi.org/10.17159/2309-8775/2017/v59n4a2 https://doi.org/10.17159/2309-8775/2017/v59n4a2 https://doi.org/10.17159/2309-8775/2018/v60n3a2 https://doi.org/10.17159/2309-8775/2018/v60n3a2 https://doi.org/10.1016/j.gloenvcha.2008.12.003 https://doi.org/10.1016/j.gloenvcha.2008.12.003 https://doi.org/10.1007/s11245-018-9584-y https://doi.org/10.1175/bams-d-18-0280.1 https://doi.org/10.1088/1748-9326/aa7494 https://doi.org/10.1007/978-3-030-85018-0_21 https://doi.org/10.17159/2309-8775/2021/v63n1a1 https://doi.org/10.17159/2309-8775/2021/v63n1a1 acta polytechnica ctu proceedings 36:6–14, 2022 1 introduction 2 climate state and pathways towards extreme climate 2.1 current climate state 2.2 future climate pathways 3 changing weather and climate extremes 3.1 assessment methodology for extremes 3.2 extreme climate classes 4 adaptation of design base for wind actions 4.1 assessment of projected wind actions 4.2 integrated basis for design and assessment 5 observations and conclusions list of symbols references my title https://doi.org/10.14311/app.2022.36.0161 acta polytechnica ctu proceedings 36:161–166, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague comparison of the collapse frequency and failure probability of buildings dirk proskea,∗, michael schmidb a bern university of applied science, school of architecture, wood and civil engineering, pestalozzistrasse 20, 3401 burgdorf, switzerland b rothpletz, lienhard + cie ag projektierende bauingenieure sia, blumenberggasse 50, 3013 bern, switzerland ∗ corresponding author: dirk.proske@bfh.ch abstract. in a previous study, the collapse frequencies and associated mortalities of buildings were compiled based on various publications. the investigation showed a considerable scattering of collapse frequencies depending on the countries. in this paper, the collapse frequencies determined for buildings are compared with the results of probabilistic calculations. such comparisons have already been carried out for bridges, dams, tunnels and retaining walls including the consideration of central estimators and standard deviations. in order to limit the scatter, the probabilities of failure for buildings were subdivided into the different causes of failure, and collapse frequencies were subdivided into different countries and geographical regions. overall, the comparison shows that the probabilities of failure are on overage larger than the observed collapse frequencies. furthermore, the comparison shows are large span, which is unusual for other types of structures. of concern are the high observed collapse frequencies in various developing countries. human error, lack of training, etc. are often cited as the cause. should this correspond to the facts, the basic safety concept of modern building standards, which generally excludes human error, would only be applicable in these regions to a limited extent. however, the investigation includes some limitations, such as different safety targets in different standards and different years of constructions, which are not considered. further work is required. keywords: buildings, collapse, collapse frequency, probability of failure. 1. introduction modern safety concepts for structures are based on probabilistic models [1, 2]. in contrast to these calculation models, in which either directly or indirectly nominal failure probabilities are determined, there are the observations of building collapses. whether and how the calculated and observed values are comparable and related, is the subject of scientific discussion [3, 4]. comparison is common in other disciplines [5, 6], but is often rejected in the construction industry [1, 2]. nevertheless, the calculated failure probabilities and the observed collapse frequencies of bridges [7], dams [8], tunnels [9] and supporting structures [10] were compared within the framework of a series of articles. furthermore, the comparison was also methodologically extended from central estimators to statistical uncertainty parameters [11]. also, the mortality due to structural collapses was estimated, see for example [9, 12]. this document presents the comparison of collapse frequencies and failure probabilities for buildings. in the following section the term building is defined. 2. definition of buildings structures can basically be divided into two classes: • buildings as residential, commercial, and public buildings as well as buildings for the sports and leisure sector. • civil engineering structures as part of the infrastructure such as bridges, dams, retaining walls and tunnels. this paper focuses on buildings. three definitions of buildings are given. the german federal statistical office [14] defines buildings as: "...structures that generally rise substantially above the earth’s surface. for technical reasons, buildings also include independently usable underground structures that can be entered by people and are suitable or intended for the protection of people, animals or property (e.g., ...., underground shopping centres and production facilities, underground car parks)". the swiss confederation [15] defines: "buildings [are] a permanent structure, provided with a roof, firmly attached to the ground, capable of accommodating persons and serving residential purposes or purposes of work, education, culture, sport or any other human activity; ..." the free state of saxony [16] in germany defines: "buildings are independently usable, covered structures that can be entered by people and are suitable or intended to serve the protection of people, animals or 161 https://doi.org/10.14311/app.2022.36.0161 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en dirk proske, michael schmid acta polytechnica ctu proceedings figure 1. annual frequency of building collapses based on [13]. the individual references are given in [13]. property." 3. definition of collapse and failure after the definition of the term building, the terms collapse and failure should be defined. usually, a collapse is a sequence of failures of structural elements leading to the destruction of the building (non-repairable). the sequence is often described as chain reaction and progressive collapse. collapse frequencies are computed based on observed and documented collapses of buildings. another definition considers the building collapse as the entire structure or parts come down. in contrast, failure is the computational exceedance of a single ultimate limit state equation, which may or may not yield to a non-repairable damage. this exceedance of the ultimate load state equation is related to one internal force (moment, shear force, axial force, buckling etc.) and one location at a structural element. probabilistic failure computations are related to numerical models incorporating random variables as well as individual limit state functions. the results of individual probabilistic computations for one structural element and one limit state can be combined to consider the entire structures. 4. collapse frequency of buildings the collapse frequency is computed as follows: f = nc nb (1) the stock of buildings nb , the number of collapses nc and the collapse frequencies of buildings f were investigated and visualized in [13] based on a large number of publications. this reference also includes the investigation of the mortality due to building collapses. however, the study does not include collapses from earthquakes and floods. figure 1 gives an overview of references related to the collapse frequency of buildings and figure 2 shows the mortalities based individual references. the individual references are given in [13]. 5. probability of failure of buildings while only a relatively small number of publications can be used to determine collapse frequencies, there is an almost unmanageable number of publications with calculations of the probability of failure of buildings. in the context of this study, 22 publications were selected, however finally only 21 are used. the publications refer to a wide range of actions, but they are not yet a representative selection of the building stock. the discussion of the assumptions and limitations of each of the used publications is beyond the scope of this article. even further, the study should be extended using more references and distinctions of the building stock. table 1 and figure 3 visualises the probabilities of failure of the used references. furthermore, figure 3 includes the observed collapse frequencies from figure 1 in grey colour. additionally, the references of the collapse frequencies have been excluded. 6. discussion based on table 1 and figure 3, it can be seen, that the probabilities of failure show an extraordinary scatter. the values range 10−19 to 10−1 per year. the scatter 162 vol. 36/2022 collapse frequency and failure probability of buildings figure 2. mortality due to building collapses based on [13]. the individual references are given in [13]. number reference action range of probability of failure 1 [17] structural analysis 3.17 × 10−5 to 9.68 × 10−4, 6.21 × 10−3 to 2.12 × 10−1 2 [18] snow 3.17 × 10−5 to 1.39 × 10−2 3 [19] earthquake 1 × 10−5 to 1.05 × 10−2 4 [20] earthquake 9.87 × 10−10 to 1.07 × 10−2 5 [21] structural analysis 3.47 × 10−3 to 6.87 × 10−4 6 [22] wind 6 × 10−4 to 8 × 10−4 7 [23] earthquake 0.310 × 10−2/0.363 × 10−2 8 [24] human failure 0.3 × 10−3 to 1.3 × 10−3 9 [25] structural analysis 8.275 × 10−6 to 1.285 × 10−5 10 [26] human failure 1.35 × 10−2 to 8.63 × 10−2 11 [27] foundation 1 × 10−6 to 1.3 × 10−4 12 [28] foundation 4.5 × 10−6/4.21 × 10−4/1.86 × 10−8/2.47 × 10−8/1.70 × 10−6 13 [29] wind 1 × 10−5 14 [30] wind, typhoon 1.7 × 10−4/1.69 × 10−3 15 [31] earthquake 3.4 × 10−4 and 2.8 × 10−3 16 [32] wind 1.0 × 10−6 to 5.58 × 10−5 17 [33] foundation 2.72 × 10−8/1.22 × 10−9/1.61 × 10−11/1.89 × 10−16 18 [34] fire 3.8 × 10−7/7 × 10−10 19 [35] human failure 6.4 × 10−6 20 [36] fire 3.3 × 10−9/3.3 × 10−8 21 [37] structural analysis 1 × 10−5 table 1. references and related probabilities of failure. is significantly larger than the scatter of the collapse frequencies. furthermore, the probabilities of failure are on average clearly above the average of the collapse frequencies. of course, the collapse frequencies in figure 1 and 3 do not consider large-scale accidental actions such as earthquakes or floods, which are considered in the probabilities of failure. whereas the median of the probabilities of failure without accidental actions is in the range of 1.06×10−5, the median of all results is in the range of 6 × 10−4 per year. the collapse frequency on a worldwide scale without earthquakes and floods was 3.3 × 10−6 per year [13]. own estimations considering earthquakes and floods yields to a factor 3 to 20 for this collapse frequency, either based on table 2 and 3 or on the average number of fatalities due to earthquake and the number of collapsed buildings [39, 40]. this would yield to a 163 dirk proske, michael schmid acta polytechnica ctu proceedings figure 3. computed probabilities of failure for buildings based on various references given in table 1 and related to actions. year country action number of buildings destroyed 1970 bangladesh flood 400,000 1995 kobe, japan earthquake 46,000 100,000 2001 el salvador earthquake 200,000 2001 peru earthquake 20,000 2004 southeast asia tsunami 300,000 2010 haiti earthquake 250,000 2011 japan earthquake and tsunami 45,000 130,000 destroyed 190,000 240,000 damaged table 2. number of destroyed houses during large-scale accidental actions. year damaged houses destroyed houses collapse frequency ratio to 3.3 × 10−6 2015 660,000 85,000 6.5 × 10−5 19.81 2016 380,000 90,000 6.9 × 10−5 20.98 2017 560,000 115,000 8.8 × 10−5 26.81 2018 450,000 95,000 7.3 × 10−5 22.14 2019 340,000 90,000 6.9 × 10−5 20.98 2020 170,000 30,000 2.3 × 10−5 6.99 table 3. number of damaged and destroyed houses during earthquakes per year worldwide (earthquake impact database). the total stock of buildings has been assumed with 1.3 billion, which is slightly below 1.5 used in [38]. 164 vol. 36/2022 collapse frequency and failure probability of buildings collapse frequency in range from 1 × 10−5 to 7 × 10−4. therefore, the ratio of median probabilities of failure to median collapse frequencies is 3.2 ((1.06 × 10−5)/(3.3 × 10−6)) without considering earthquakes and floods. for the consideration of earthquakes and floods this ratio would be in the range just below 1 to well over 20 with an average value in the upper range ((6 × 10−4)/(1 × 10−5 to 7 × 10−4). the large span of the difference between the calculations and the observations stays in contrast to all other types of structures, such as bridges, dams, retaining structures. for those the values are in the range of 2. on the other hand, buildings form the largest number among the structures. therefore, subgroups have probably to be formed to increase the quality of this comparison, especially with focus on earthquakes. even further, we have not considered the different target values in different standards, such as asce and eurocode, the different consequence classes, and the different construction years of the buildings and the relevant standards. it is well known that newer standards yield to better structural behaviour under extreme loads such as earthquakes. these distinctions can be investigated in future works. on the other hand, this study shows that the computed probabilities of failure in average are conservative. 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https://doi.org/10.1007/s10694-018-0764-5 https://doi.org/10.1016/j.proeng.2017.10.555 https://doi.org/10.1016/j.firesaf.2013.10.002 https://doi.org/10.1002/pse.78 https://doi.org/10.1007/978-3-030-73616-3_24 acta polytechnica ctu proceedings 36:161–166, 2022 1 introduction 2 definition of buildings 3 definition of collapse and failure 4 collapse frequency of buildings 5 probability of failure of buildings 6 discussion 7 conclusion references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.38.0354 acta polytechnica ctu proceedings 38:354–360, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague twin rooms – new experimental test cells for testing advanced facade elements boris bielek∗, daniel szabó, josip klem, kristína kaniková, alžbeta danková slovak university of technology in bratislava, faculty of civil engineering, radlinského 11, 810 05 bratislava, slovak republic ∗ corresponding author: boris.bielek@stuba.sk abstract. nowadays the global trend is the integration of new materials, constructions and technological principles, which are simultaneously implemented in individual scientific and engineering disciplines. reducing the energy intensity of buildings will increasingly resonate in individual political and professional circles. as a result, new fragments of building envelopes in the field of facade engineering are being developed and tested. testing of building envelope is carried out either in static (laboratory) boundary conditions or in dynamic (climatic, real) conditions. the determination of the test method is conditioned by the specific intention or the investigated phenomenon within the construction of the building envelope. currently, we finished the development and realization of the new experimental facility twin rooms for testing advanced elements of building envelopes in dynamic boundary conditions (in the real climate of central europe – bratislava) in terms of building thermal engineering and energy efficiency of buildings. it is based on the concept of pavilion measurement. the essence of the research is that the outdoor climate is modelled by the conditions of the real outdoor climate. test cells consists of a solar laboratory – two-room for a comparative study of the effect of solar radiation and heat transfer on energy consumption and indoor climate. the space of two identical laboratory rooms is situated inside a container – a pavilion, whose climate is a compensating space. only the tested facade element walls are exposed to the outdoor climate. the exchange of energy with the environment is possible only through this measured facade wall. the article brings a detailed description of this experimental equipment, basic technical parameters of its technological circuits and methodology of experimental measurements. keywords: experimental pavilion test cells, advanced facade elements, real outdoor climate. 1. introduction in the current period, humanity is solving one of the most difficult technical and economic problems of ensuring sufficient energy for continual development of society without negative impact on ecology, production and environmental protection. approximately in the 1980s, the economic system expressing human activities exceeded the reproductive value of the earth’s biocapacity (3 to 3.5 billion tons of emissions of greenhouse gases) and the man began to produce the ecological debt on the planet. the world, at the present time, produces environmental burden that exceeds its biocapacity by about 30 %. humanity must find a solution for the repayment of the ecological debt. for a successful solution to this problem, mankind has to reconsider its priorities in investments, which represent the transformation to a sustainable society by transforming the energy sector towards environmentally clean renewable energy sources and their conversion, transformation of material sector towards ecologically clean materials and their production and transformation of the whole economy towards lowenergy and low-emission technologies and environmentally friendly products. sustainability is synonymous with the image of a future world whose aim is to place man, nature and technology in permanently stable equilibrium. this means such a development which allows both current and future generations to meet their basic living needs, preserves the diversity of nature and functions of the ecosystems. from the internal structure of the overall energy balance of non-productive sphere, the human settlements are the second largest consumer of energy. the majority of this energy consumption falls on the operation of buildings (heating, cooling, ventilation, hot water and lighting), coupled with hygienic and ecological comfort in them. there are two major approaches to sustainable development of society in the interaction energy – human settlements: energy saving by its rational use and focus on environmentally clean renewable energy sources. renewable energy sources, as the dominant production technology value of nature, with its simultaneous renewal of ecosystems are becoming determining factors for changes in fundamental concept of energy quantification of buildings. building is becoming a place for the collection and on-site conversion of renewable energy as part of the transformation of organization of the energy market. it is becoming part 354 https://doi.org/10.14311/app.2022.38.0354 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 38/2022 twin rooms – new experimental test cells . . . of the energy distribution networks and it is just in the interaction to these distribution networks that the new quantification of physical-energy demand of the building is being conceived, expressed by the term net zero energy building or nearly net zero energy buildings or net plus energy buildings. facade structures are indispensable and, by their functions, extremely important parts of the building envelopes. besides their physical (thermal, sound and water insulation and air impermeability) and mechanical features (static and dynamic wind resistance) they also have additional specific indispensable functions (natural daylight, natural ventilation, visual contact with the surroundings, fire escape route, architectural expression). with the development of technology in the architecture towards a sustainable society, the function of transparent structures is highlighted as an element of passive solar systems of buildings. given the need to regulate energy flows through transparent constructions within dynamically changing conditions of the external climate, new elements are being integrated to modern transparent building constructions (adjustable ventilation louvers, flaps or units, mobile screening devices...), connected to electronic sensors of physical or chemical parameters of internal or external climate that dynamically respond to climate changes and optimize the energy flows through transparent constructions in order to ensure a comfortable indoor climate while minimizing demands on building environmental technology (i.e. energy). with development of active solar systems based on photo thermal or photoelectric conversion, these elements are then integrated directly into transparent facade structures. these transparent structures then partially take over the function of building environmental technology. with the development of modern climate-adaptive transparent facades capable of dynamically responding to the climatic parameters of the external climate and thus optimizing the energy flows through their construction in order to significantly reduce the heat load in the summer period, the heat losses in the winter period and to maximize the available renewable energy sources, the need to verify their parameters and energy efficiency depending on the conditions of the external climate comes to the fore. 2. overview of experimental equipment for research of advanced facade elements research in the test cell began in 1985 as an effort to increase faith in the application of energy-conscious and passive solar building products and evaluation techniques used to provide practical thermal properties of the products. the passys (passive solar components and systems testing) project focused on test cell equipment as a means of determining the performance of passive solar building components and providing additional information on building design and simulation tools. the advantage of test cells is that they provide a well-controlled, realistic room (cell) environment without occupancy [1]. the passys test cell consists of a well-insulated construction 8 × 2.7 × 2.7 m with two rooms. one room measuring 5 × 2.7 × 2.7 m is a south-facing test cell and an adjacent north-facing area is a service room that contains an air conditioning unit. it is possible to test various building elements on the south facade of the test room. for this purpose, manuals for instruments, operations, calibrations and test procedures have been developed. the cells are able to test vertical and horizontal building elements. the passys test cells are located in european countries: belgium, denmark, france, germany, italy, the netherlands, scotland, greece, portugal and spain [2]. similar test cells in the rest of europe are vliet test building at ku leuven university in belgium [3, 4], test box at technische hochschule rosenheim in germany [5], minibat test cell in lyon, france [6], fact in cea-ines, le bourget du lac in france [7], zeb test cell laboratory in trondheim, norway [8, 9] and the cube in aalborg, denmark [10–12]. the vliet test building research facility is one of the full-scale test cells. the research is solved at the component level and focuses on the combined transfer of heat, air and moisture through walls, roofs and floor systems and their impact on sustainability. the methodology includes modelling, accelerated aging tests, hot-box measurements and “in situ” studies [3, 4]. the test box was provided to the technical university of rosenheim, germany, from the facade construction company josef gartner, for which it acquired all the rights to research, development and training. in addition to evaluating comfort and energy efficiency, they also evaluate the effect of shading, angle of blinds to obtain the most daylight with the least possible glare. therefore, the test cell is rotatable about a vertical axis. thus, it can be adapted to the requirements of the respective test program. of particular interest are comparative studies on energy efficiency, comfort and visual comfort in real conditions [5]. the minibat test cell in lyon, france, consists of two adjacent cells connected by a door located in a compensating container. one side of the first cell opens into a climatic chamber that models the outdoor climate. the walls and interior rooms are equipped with many sensors and both cells can be ventilated, heated, cooled, etc. the aim of this test device is to gain information about heat and air transfers within one room of a building, between two rooms and between a cell and the outdoor climate simulated by a weather generator. the device allows to collect detailed data for validation of numerical models. typical studies include air distribution (indoor air quality, analysis of airflow from inlet openings and their mixing with indoor air), heating, cooling (energy efficiency, humidity and heat comfort for different ventilation systems and heating / cooling elements) and facade systems (facade elements, 355 b. bielek, d. szabó, j. klem et al. acta polytechnica ctu proceedings double skin facades, phase change materials, etc.) [6]. fact (facade tool) is a test facility at cea-ines, le bourget du lac, in south-eastern france. it is an experimental facility consisting of a two-storey building with 10 rooms for testing the perimeter cladding of buildings with different widths and heights of facades, with different geometries of the internal environment. the size of the test cells can be modified so that measurements can be performed in one room or in open space [7]. the zeb test cell laboratory in trondheim, norway, is used to test low-energy integrated building systems with real operating conditions. the test cell can be divided into two smaller chambers, in which different technologies can be used to compare results. in the laboratory, various elements of building materials, building envelopes, energy installations and control systems are jointly developed and optimized in real climate conditions [8, 9]. the university of aalborg uses a test cell, namely the cube, which is one of the full-scale test facilities, for research double skin facades. it is a test cell situated in a compensation space with one side facing south, into which the test sample is inserted. the cell is designed to be able to adapt to operating modes, natural or mechanical flow conditions, various shading techniques and similar. the thermal conditions in the cell can be perfectly controlled, as well as in the room next to it. the test facility is equipped to allow the measurement of any power supplied to the experimental zone in order to maintain the required temperature conditions [10–12]. there are currently several testing facilities at slovak and czech universities in košice [13], žilina [14], brno [15–17] and prague [18]. the laboratories in košice include 3 outdoor experimental cells, which are designed for monitoring, research of physical properties and characteristics of facade structures – thermaltechnical and humidity problems in real conditions of use of buildings and real outdoor climatic conditions [13]. pavilion research has been built in the uniza research centre in žilina, the main part of which consists of two differently oriented (east and south) experimental walls. stable indoor environment parameters are ensured in the test cells. temperature and relative humidity sensors are located in the experimental walls. in the middle of the test cells there are sensors for measuring temperature and humidity. outdoor climate conditions are obtained from a meteorological station located on the roof of the building [14]. in the research centre centrum admas (advanced materials, structures and techniques) in brno, four small test cells were built in 2014 to research the thermal stability of buildings. the exterior walls of the cells have different material bases (brick, aerated concrete and wooden construction). sensors are placed in the cells to measure surface temperature, air temperature, to quantify the amount of solar radiation in the planes on the surface of the glazing. the outdoor climate is recorded by their meteorological station’s prototype. the research centre also includes a mobile experimental full-scale test set of solar-activated facades and their concepts [15–17]. the university centre for energy efficient buildings uceeb at the czech technical university in prague has a climate facade with a controlled indoor environment and 6 monitored fields with samples exposed to the weather [18]. however, it must be stated that none of the above-described test facilities in the territory of slovakia and the czech republic makes it possible to accurately quantify the energy flows of the examined sample. 3. “twin rooms” construction the “twin rooms” test cells are used to determine the thermal properties (u-value, thermal resistance, surface temperatures, temperature fields) and energy flows through advanced facade elements in the range of outdoor climatic conditions. the technological equipment for the “twin rooms” must ensure controllable, stable, homogeneous and reproducible indoor temperature conditions in the two measuring cells and in the compensation room. this will make it possible to perform measurements of heat balances, total and local heat flows as well as surface temperatures simultaneously on two tested structures installed in measuring openings under the influence of non-stationary outdoor climatic conditions. the experimental test cells consist of an officetype mobile container with external dimensions of 6058 × 2990 × 4200 mm, which is placed on pre-built concrete foundations. the supporting structure of the container itself consists of a space frame welded from steel hollow rolled profiles. the envelope consists of lightweight sandwich panels, with an external galvanized shaped sheet, a filling high-efficiency thermal insulation based on mineral wool and an internal lining made of chipboard. the entrance to the container is situated on the rear longer side of the container (6058 × 4200 mm) and is made by a door opening with dimensions of 1000 × 2000 mm with galvanized doors with infill thermal insulation – figure 1. the south-facing front side of the 6058 × 4200 mm container has two holes cut out with 2400 × 3400 mm, within which the test element of the building envelope will be fitted. directly behind the openings, a dividing structure of a light wooden partition creates two rooms (twin rooms) with dimensions of 2400×2000×3400 mm thermally insulated from the interior space of the container – figure 1. 4. “twin rooms” technology the technology of the test cells “twin rooms” consists of several parts, which form one functional unit. using environmental technology, the same constant indoor climate θai [°c] will be maintained in both cells and the compensating space of the container, which will ensure zero heat flow through the dividing structures separating the two cells from the inner space 356 vol. 38/2022 twin rooms – new experimental test cells . . . figure 1. construction schemes of an experimental test cells twin rooms for testing advanced facade elements in real conditions of outdoor climate. of the container. by ensuring this, all energy flows will be only through the tested facade elements of the building envelopes fitted in the measuring openings of the container. in order to maintain a constant temperature θai [°c] in the two measuring cells, we must supply energy (heat or cold) to those cells. the amount of this supplied energy is identical to the energy that was released by the measured facade element. we can measure the energy delivered to individual cells (rooms) and thus we can also measure the energy that was released by through the facade element. in this way we can measure all energy flows through facade elements in conditions of dynamically changing outdoor climate over time. the technological equipment for air conditioning of the compensation room (container) consists of an outdoor air conditioning unit exterior air – room air for the room “container” with a volume of about 25 m3 with an output of about 3 kw and a heating element with an output of 2 kw – figure 2. this ensures a stable temperature in the compensation room to prevent the exchange of thermal energy between the measuring cells and the compensation room. at the same time, it is necessary to ensure even air circulation through the circulating fans in the compensation room, in order to avoid local temperature inhomogeneities of the air surrounding the measuring cells. technological equipment for air conditioning of testing cells (rooms) 1 and 2 consists of air conditioning unit (identical as for container) air exterior – water exchanger for space of cells 1 and 2 with a volume of about 15 m3 with an output of 6 kw and a heater with an output of 2 kw – figure 2. this ensures a stable and homogeneous temperature in the cells. in this case, it is necessary to continuously measure the power supplied to the air-conditioned space of cell, so on the air-conditioned side there is a water exchanger measuring water flow and its inlet and outlet temperature as well as an electricity meter to measure the electrical power of the heater and circulating fans. to evaluate the total heat fluxes by the measured structures, it is necessary to measure parallel in “cell 1” and in “cell 2”: • the amount of energy supplied or taken by the heat exchanger (inlet and outlet water temperature – media flowing through the exchanger, the volume of water flowed through the exchanger), • the amount of electrical energy supplied by the radiator and circulating fans. guaranteed parameters for container space: • cooling circuit power 3 kw, • heat circuit power 2 kw, • adjustable temperature range: ▷ for summer period + 19 °c to + 40 °c, homogeneity in space and time ± 1.5 °c, ▷ for winter period + 10 °c to + 25 °c, homogeneity in space and time ± 1.5 °c, • air velocity range: 0.10 m/s to 1.50 m/s. 357 b. bielek, d. szabó, j. klem et al. acta polytechnica ctu proceedings figure 2. construction scheme of technological equipment of an experimental test cells twin rooms. 1 – cooling source, qc = 16.4 kw, mex ea 117 z c, 2 – plate heat exchanger – part of the supply of the cooling source, 3 – water tank – storage tank galmet 140 l, 4 – pipe fancoil nxkt3-v1000, 5 – sill fancoil nxkh3-v600, 6 – circulating fans – homogeneous thermal field, 7 – electrical switchgear, 8 – opening for mounting a new facade sample. guaranteed parameters for cell space 1 and cell space 2: • cooling circuit power 6 kw, • heat circuit power 2 kw, • adjustable temperature range: ▷ for summer period + 19 °c to + 40 °c, homogeneity in the space in front of the measured sample ± 1.5 °c (at the output of the cold and heat circuit up to 2.0 kw), ▷ for winter period + 10 °c to + 25 °c, homogeneity in the space in front of the measured sample ± 1.5 °c (at the output of the cold and heat circuit up to 2.0 kw), • air velocity range: 0.10 m/s to 1.00 m/s. all sensed physical quantities will be continuously recorded and evaluated by the local control unit. the system enables remote access in real time to all sensed and evaluated quantities as well as remote control of the entire control and regulation system. 5. “twin rooms” measuring equipment in order to be able to evaluate the total heat balance of the measured structures under the influence of nonstationary outdoor climatic conditions, it is necessary to measure these conditions with the help of a meteorological station. the weather station will be located on the roof of the container with measuring cells. the temperature and humidity of the outside air, the speed and direction of the wind, the intensity of sunlight on the vertical and horizontal planes and the amount of precipitation will be measured continuously. to evaluate the local thermal parameters of the measured structures, it is necessary to measure: • local heat fluxes using heat plates, • surface temperatures on the outside and inside of the structure by pt100 sensors, • flow velocities along the measured structures, • air pressure differences on the outside and inside of 358 vol. 38/2022 twin rooms – new experimental test cells . . . the structure, • intensity of solar radiation on the vertical surface of the perimeter wall. all the above-mentioned physical quantities are continuously measured and stored by means of a measuring control panel. at the same time, they are archived by means of a pc and are remotely accessible at any place via the internet. 6. initial measurements on the new experimental test cells twin rooms after the successful delivery of technology to the new twin rooms test cells for research of advanced facade elements in real climate conditions (october 2021) and after its successful test operation, we plan to start experimental verification of a prototype of a new modular double skin transparent facade with a narrow slot cavity with a usage of multi-stage renewable energy of solar radiation, which was developed within the scientific projects apvv-16-0126 and vega 1/0113/19 in cooperation with ingsteel s.r.o. bratislava. the resulting design of the double facade is characterized by: • progressive construction of the completed spatial part – an element mounted from the exterior to the elements of the supporting system of the building, characterized by fast assembly technology and the removal of seasonality, • high thermal-technical quantification without thermal bridges of frame profiles based on aluminium and anchoring systems and high acoustic quantification of the facade, • application of the design solution of a double skin transparent facade with a narrow slot cavity and the height of the section identical to the height of one floor, which significantly eliminates the load from solar radiation in summer and at the same time reduces heat losses during the heating period – winter, • application of the ventilation heat recovery unit in the parapet part of the element, which in winter uses preheated air from the physical cavity of the double skin facade with a positive impact on a significant reduction of heat losses of the building from ventilation, • application of photovoltaics for direct conversion of solar radiation into electrical energy in the parapet part of the outer transparent wall of the double skin facade for direct use in the building, • application of an automated control system of individual structural elements of the element (closing of inlet and outlet openings, movement of shading devices, control of heat recovery unit, etc.) based on measuring outdoor climate parameters and indoor climate requirements. the resulting energy flows of the new double skin transparent facade will be compared with the classic simple transparent facade of similar physical quantification as the inner wall of the double facade, also with the application of the same ventilation heat recovery unit and photovoltaics in its parapet part. we will test both samples of facades by long-term research in the experimental twin rooms test cells during the period of 1 year. 7. conclusion after the construction of the experimental test cells twin rooms for pavilion research of advanced facade elements in conditions of 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[2021-09-29]. https://www.web-old.uceeb.cz/system/files/ souboryredakce/laboratore/experimentalnifasada/rp1_experimentalni-fasada.pdf 360 https://www.zeb.no/index.php/en/test-cell-laboratory https://www.zeb.no/index.php/en/test-cell-laboratory https://doi.org/10.1016/j.egypro.2017.09.718 https://doi.org/10.1016/j.buildenv.2014.09.009 https://doi.org/10.4028/www.scientific.net/amm.861.213 https://doi.org/10.4028/www.scientific.net/amm.861.213 https://doi.org/10.4028/www.scientific.net/amm.824.519 https://doi.org/10.4028/www.scientific.net/amm.824.519 https://www.web-old.uceeb.cz/system/files/souboryredakce/laboratore/experimentalni-fasada/rp1_experimentalni-fasada.pdf https://www.web-old.uceeb.cz/system/files/souboryredakce/laboratore/experimentalni-fasada/rp1_experimentalni-fasada.pdf https://www.web-old.uceeb.cz/system/files/souboryredakce/laboratore/experimentalni-fasada/rp1_experimentalni-fasada.pdf acta polytechnica ctu proceedings 38:354–360, 2022 1 introduction 2 overview of experimental equipment for research of advanced facade elements 3 ``twin rooms'' construction 4 ``twin rooms'' technology 5 ``twin rooms'' measuring equipment 6 initial measurements on the new experimental test cells twin rooms 7 conclusion acknowledgements references https://doi.org/10.14311/app.2022.33.0552 acta polytechnica ctu proceedings 33:552–557, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague methodology for evaluation of the life cycle of buildings with a focus on the private sector renáta schneiderová heralová∗, eduard hromada, jakub holcman, stanislav vitásek czech technical university in prague, faculty of civil engineering, department of construction management and economics, thakurova 7, 166 29 praha 6, czech republic ∗ corresponding author: heralova@fsv.cvut.cz abstract. the paper describes the innovative methodology for the evaluation of the life cycle of buildings, which will be used in construction projects of skanska, or other construction companies, in selecting the optimal solution from the point of view of construction engineering and lcc. the aim is to help lcc to be perceived by construction practice as a topic that needs to be addressed at the beginning of each construction project. the quantification of life cycle costs based on relevant input data on the technical parameters of the construction, structural elements and equipment, the time period of incurring the related costs should be an important basis for the decision of the investor, designer and future user of the building also with regard to environmental and social aspects, and in particular the long-term economic consequences. keywords: building, life cycle, life cycle costing, methodology. 1. introduction this paper deals with innovative methodology in construction company skanska concerning the evaluation of construction’s life cycle costs (lcc). mentioned methodology will be applied to construction projects of blocks. this will lead to optimal choices accounting for technical aspects of construction, and lcc from the very moment of designing a construction. according to the international standard [1], life cycle costing is a valuable technique that is used for predicting and assessing the cost performance of buildings. life cycle costing is one form of analysis for determining whether a project meets the clients performance requirements. life cycle costs (lcc) mean involving all of the costs deriving from the use of buildings during their entire life span. the lcc methodology is a tool for evaluating these costs over time. its main goal is to assess the various designs, where those designs differ not only in their acquisition costs but also in their operational and maintenance costs. assessment of life cycle costs is a valuable tool for both the construction company and the eventual user of construction. both company and end user are clearly informed about total costs from the beginning of construction. therefore, they do not need to concentrate only on investment costs or use them as the single criterion of choice. options of construction may be designed as soon as the designing phase of a project. so the end user obtains higher added value and may compare variant designs to reach optimal value for money. variant designs enable better choices of construction suitable for the end user’s requirements. end user may plan eventual maintenance costs accordingly. this advantage may be the key factor especially for end users who finance the construction with third party resources, such as loans. life cycle planning lowers the amount of risk in the investment. transparency can be reached as to operational costs of construction. with life cycle costing it will be possible to benchmark competing projects. lcc analysis has already been implemented as one of the measures for varied certifications, which serve for project evaluation (breeam, leed, sb tool, etc.). 2. literature overview buildings and constructions are long-term possessions. this is the reason why all decisions connected with a construction project have long-term and significant impact [2]. construction project investors have often focused only on the acquisition costs, when they were about to make decisions on such matters as the building design, facilities, fittings. they often neglected future operation and maintenance costs [3]. life cycle costing is a method assisting an effort to estimate the total cost of ownership. the method is able to help in making decisions within building investment projects [4]. life cycle costing is particularly useful for estimating total costs at the early stage of a project [5]. life cycle costing consists of all costs directly connected to a construction during its whole existence. that is building costs, maintenance and renovation of construction units’ costs, facility equipment costs, facility management and operation costs, demolition costs. [6] many authors also link other factors closely with life cycle costing. among such factors, let us name externalities, non-construction costs and income which arise from 552 https://doi.org/10.14311/app.2022.33.0552 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 33/2022 lcc methodology for private construction sector id category designation description 1 const. mcw1 works and provisions 2 const. 1 ground works 3 const. 2 ground solidification 4 const. 3 vertical and complete constructions 5 const. 4 horizontal constructions 6 const. 6 finishes and flooring 7 const. 60 finishes internal 8 const. 63 flooring and floor construction 9 const. 9 other constructions and works, demolition 10 const. 90 other constructions and works 11 const. 94 scaffolding, construction lifts 12 const. 96 demolition of constructions 13 const. acw2 works and provisions 14 const. 71 insulation 15 const. 711 water, moisture and gas insulation 16 const. 76 constructions 17 const. 763 assembled constructions 18 const. 764 external plumbing 19 const. 766 carpentry 20 const. 767 ironworks 21 const. 768 panel work windows 22 const. 77 flooring 23 const. 771 floors tiled 24 const. 775 floors wooden, frieze, floating 25 const. 776 floors thin finish 26 const. 777 floors cast, synthetic 27 const. 778 floors double 28 const. 78 finishing works 29 const. 781 ceramic tiles 30 const. 784 paintwork and wallpapers 31 const. 788 fire-fighting devices 32 const. 789 facade casing and contact insulation 33 const. 790 roof constructions 34 const. 791 roof vegetational 35 const. 792 roof trafficable 36 const. 793 roof non-trafficable 37 const. 794 roof movable 38 monolith mcw1 works and provisions 39 monolith 2 ground solidification 40 monolith 27 foundations 41 monolith 3 vertical and complete constructions 42 monolith 31 construction walls 43 monolith 33 columns, pier, framework 44 monolith 34 walls and partitions 45 monolith 4 horizontal constructions 46 monolith 41 ceilings and ceiling constructions of buildings 47 monolith 43 stairway constructions and ramps 48 infrastructure 101 sewage connection 49 infrastructure 102 waterway connection 50 infrastructure 103 heating connection 51 infrastructure 105 gas connection 52 infrastructure 106 weak current service line 53 infrastructure 107 outdoor water distribution 553 r. schneiderová heralová, e. hromada, j. holcman, s. vitásek acta polytechnica ctu proceedings 54 infrastructure 108 outdoor sewage 55 infrastructure 109 outdoor gas distribution 56 infrastructure 110 outdoor weak current service line 57 infrastructure 111 outdoor heavy current service line 58 infrastructure 112 outdoor lights 59 infrastructure 113 electricity substation 60 infrastructure 114 ground preparation 61 infrastructure 115 roads, pavements, public equipment 62 infrastructure 117 landscaping 63 infrastructure 118 fencing 64 infrastructure 119 traffic signs 65 infrastructure 120 structural walls 66 air conditioning 240 air conditioning 67 heating / cooling 731 central heating 68 sanitary units / gas conduit 721 sanitary units 69 sanitary units / gas conduit 723 indoor gas conduit 70 heavy current 210 heavy current, lightning rod 71 weak current 211 weak current 72 electric fire alarm 212 electric fire alarm 73 measuring and regulation 213 measuring and regulation 74 fire extinguishing equipment 214 semi-stable fire extinguishing device 75 lifts 330 lifts, escalators 1 major construction works, 2 additional construction works. table 1. structure of construction units and technologies with regard to lcc. the use of a construction in a specific location [7]. other authors are dealing with the topic of nearly zero energy buildings (nzeb) and cost-optimal level [8]. building certification is also closely related to the issues addressed. there is a national certification system sbtoolcz in the czech republic. the certification systems were focused on new construction originally [9]. 3. construction project database the lcc methodology will include a draft of skanska construction project database. the database has already been created with microsoft access. it currently contains rough data of projects. the database will serve as an individual tool as well as data source for other programs. for the sake of clarity, the table 2 shows the structure of construction units and technologies with regard to lcc. decisions concerning the choice of the construction technology and construction materials are not any longer based entirely on technical and economic attitudes, but are becoming increasingly influenced by life cycle cost and environmental considerations. in fact, the capability to influence the outcomes of whole life ownership is enormous during the design phase. the types of material specified, the quality of the design and the contracting method have to be chosen directly upon operation and maintenance costs. for example, choice of materials for horizontal and vertical supporting structures influences the total building time, transportation of materials and construction life span. moreover, it influences the environmental impact of construction. it is well documented that the majority of decisions about operating, maintenance and rehabilitation costs are predetermined at the design stage. basically, it is crucial to establish a device at the design stage that brings together the life cycle costing, service life, environmental life cycle assessment, and risk associated with decisions taken at this stage. the proposed methodology will enable variant solutions of a construction project which will help the construction company minimize environmental impact and life cycle cost. regarding the construction site itself, these may include air pollution caused by transportation and dust fall. the level of dust in the air may be reduced by sprinkling dirt roads within construction sites. however, water costs for sprinkling enter the total building costs, and they are not perceived as either economically efficient or ecological in terms of introducing water to the landscape in appropriate ways. moreover, this example is set in the operational phase which is currently under minimal surveillance when investment decisions are made. the methodology assigns economical life cycle of a construction to be 30 40 years. within this period it is not necessary to renovate carry constructions or stabilize them in any way unless an emergency situation. most construction elements are protected by finishes which need maintenance. this will be taken into account in the application. therefore, carry elements do not appear in the following phases of the life cycle, unlike maintenance and renovation costs of 554 vol. 33/2022 lcc methodology for private construction sector maintenance level (lm) annual costs service life sub-standard maintenance reduction by 20 % reduction by 5 % standard maintenance no change no change above-standard maintenance increase by 20 % increase by 5 % table 2. maintenance level. other construction units. (demolition of construction is not considered in the model.) concrete constructions may only enter the maintenance phase in case there are face concrete units which would need renovation during the building life cycle. these issues can be treated as soon as the design phase the relation of construction demands in operational and maintenance phase. use of concrete suggests variants of monolithic constructions or prefabricated units. monolithic ceiling construction has ultimately different characteristics than prefabricated construction. with prefabricated units, there is higher risk of plaster ruptures or there are higher costs of plaster in such quality as to ensure consistent finish. when considering the eventual life cycle phase demolition waste management would need to be taken into account, as well as transportation and/or recycling materials (such as concrete). environmental issues and health protection should not be underestimated in this phase, as well as costs connected with these issues. an environmental lcc methodology takes into consideration not only the main cost categories (investment costs, operating costs, maintenance cost), but also external environmental costs. the environmental impact may carry consequential costs for society. in general, constructed buildings, materials and products may have environmental impacts. the processes of manufacture, transport, assembly/disassembly, maintenance and disposal linked to buildings result, for example, in emission of greenhouse gases. subsequently, significant investment will be needed to neutralize these consequences in the future. calculation should include the costs of greenhouse gas emissions calculated using cumulated carbon costs for the period which is analyzed (through prices of emission allowances). the lcc methodology evaluates costs of mitigating/reducing environmental impacts. as a result, the best value solution could be identified from both economic and environmental perspectives. 4. lcc calculation tool life cycle cost calculation will be performed by a tool which was created by the authors of this paper. the application sw is used for the calculation of the lcc for ground structures in the microsoft excel operating environment. the lcc calculation includes the costs of acquisition, maintenance, service, renovation and energy consumption for designed development project options. it is a complex tool the operation of which is subject to the entry of input values to several separated topic-related spreadsheets. this separation was chosen for the sake of better clarity of data entered to the sw. the tool offers the possibility of the entry of three options of the design. each option makes it possible to follow up to sixty years period of the project including the possibility to postpone the starting date of its operation. this time shift takes into account a postponement and the lead time of the construction works. the following sections describe individual spreadsheets that are included in the calculation of the lcc in the designed application sw. the spreadsheet recapitulation is used to summarize the basic identifiers of the project and clear evaluation of individual lcc calculation options. the software shows both figures and graphic representation of individual lcc sections and the total lcc of the project including key parameters of the calculation (the nominal discount rate and the monitoring period). the lcc formula components are converted to the current value in individual periods using the discount factor. the additional (comparative) indices include "lcc per 1 m2 of the floor area", and "the ratio (budget costs/lcc)". the spreadsheet acquisition costs is used for the calculation of the total acquisition construction costs of individual project options that are divided into two groups. the first category includes the structuresť acquisition costs included in the maintenance, operation and renovation. the other category includes structures where the maintenance, service and renovation costs are expressed by a lump sum. in contrast to the above, costs of structures included to the maintenance, operation and renovation are calculated on individual basis. the model parameters spreadsheet is used for the entry of key inputs for the calculation of the lcc total net present value such as the monitoring period, inflation rate, actual discount rate, anticipated postponement of the project implementation and anticipated construction period for designed options. the spreadsheet structural element is used for the entry of key inputs for the calculation of the total present net value of the lcc such as the acquisition price, area, service life, maintenance costs, individual structural element service regularity and costs. moreover, the spread sheet makes it possible to enter the maintenance level (lm) to the structural elements. it uses a specific coefficient to modify their maintenance costs and their service life (for details see the table 2 below starting values). 555 r. schneiderová heralová, e. hromada, j. holcman, s. vitásek acta polytechnica ctu proceedings figure 1. evaluation of design options life cycle cost. the spreadsheet operation_energy is used for the calculation of energy price during the monitoring period for individual design options. the spreadsheet includes respective results diagrams for a better presentation. three energy types consumption is monitored: electric power, gas and water. the calculations are based on designed energy consumption for individual technological systems included in the project (usually in the document called building energy performance certificate (bepc). each option specifies eight pre-defined technologies that could be included in the project. moreover, there is a so called optional item available that can be used for the entry of any technology whatsoever that has been added to the project. not all technologies have to be accounted for in all cases. sometimes only the most relevant ones from the point of view costs can be included. on the spreadsheet structure, there is a key calculation tool for the partial parameters of the total net present value of the lcc such as the service life including maintenance level, number of renovation cycles in the monitoring period, acquisition costs, renovation costs, maintenance costs including the maintenance level, the number of service cycles in the monitoring period and costs of service of structural elements of individual design options. the spreadsheet renovation, maintenance and operation is primarily designated for the graphic representation of acquisition costs, renovation costs, maintenance costs, service costs and operation costs broken down to individual years converted to the present value using the discount factor. a summary table is available for the service, renovation and maintenance with three maximum and minimum cost values for individual structural elements. 5. case study a benefit of the lcc calculation tool is the possibility to model the life cycle costs for individual design options with different input parameters. that means that the user can choose the more efficient solution for him/her. for instance, he/she may find out what will be the impacts of a more expensive but energy saving option like not only on energy costs but on the total lcc, too. similarly, if he/she includes "maintenance" free equipment that requires higher acquisition costs, there is an apparent impact residing in the reduction 556 vol. 33/2022 lcc methodology for private construction sector of the maintenance costs but also in the value of the total costs of this project design option. the model includes a graphic processing of data for the presentation of results to all stakeholders. the data recapitulation is the most useful tool for the analysis of the building lcc for any managerial decisions. this is the presentation of results and basis source document for the adoption of respective decisions in the investment project preparation phase. any project can be integrated to the calculation model. having added various design options, we can find out the impact of the given change on the total costs. anticipated changes can be addressed on individual basis or as summary change sets. the given recapitulation example clearly indicate the load of the project when we take into account the maintenance, service and building operation (energy) costs. the key importance of the developed model resided in its capability to define the advantageous combination of the designed technologies for the economical operation. this is one of the most essential aspects from the point of view of the building life cycle costs. the optimal combination of the technological units becomes an essential quantity for the entire building life cycle. the model cannot secure the presence of the complete offer of technologies as may be available on the construction market. properly specialized and authorized designers should be aware of such information. the model can look at the technologies from the framework of the interval of performance, consumption of energy or operating costs. the combination of the most expensive systems does not guarantee the problem-free and low cost operation of the building. chosen technologies have to be able to communicate with each other, i.e. they have to be effective. this is the most demanding part of the lcc calculation. the developed tool is therefore designed as an aid for those company workers who are in charge of the preparatory phase of the project. given the fast development of technologies, it will be necessary to see the future results as just recommending but not binding data that are correct for the entire economic life period. 6. conclusions one of tools use for the assessment of the economic sustainability of designed buildings is the analysis of life cycle costs that is executed based on relevant input data on technical parameters of the building design, structural elements and equipment, related time interval of the costs that incurred. the analysis becomes an important underlying document for the decision of the investor on the identification of the best option of the building design taking into account also environmental aspects and long-term economic consequences. the analysis may be a significant marketing tool focused on the future building user to whom it presents the environment-friendly parameters and economic sustainability of the building. the evaluation of a construction project on the basis of its life cycle is included in certification systems evaluating the quality of buildings. the analysis of life cycle cost is assessed as a pertinent tool for improving the sustainability of building structures. it is advisable that both professional and lay population realize the necessity of exploring both acquisition costs and all combined other costs during the entire life cycle when making decisions about new constructions. acknowledgements this paper was supported by the technology agency of the czech republic, project id tn01000056, title "centre of advanced materials and efficient buildings cameb". references [1] iso 15686-5:2017(e). buildings and constructed assets service life planning part 5: life-cycle costing. international organization for standardization, 2017. [2] m. ryghaug, k. h. sørensen. how energy efficiency fails in the building industry. energy policy 37(3):984-91, 2009. https://doi.org/10.1016/j.enpol.2008.11.001. [3] m. jakob. marginal costs and co-benefits of energy efficiency investments. energy policy 34(2):172-87, 2006. https://doi.org/10.1016/j.enpol.2004.08.039. [4] r. flanagan, a. kendell, g. norman, et al. life cycle costing and risk management. construction management and economics 5(4):s53-s71, 1987. https://doi.org/10.1080/01446193.1987.10462093. [5] u. bogenstatter. prediction and optimization of life-cycle costs in early design. build. res. information 28(5-6):376-86, 2000. [6] r. heralova building’s value assessment using the utility and the lcc. cesb 2007. prague, 1:126-131, 2007. [7] x. meng, f. harshaw. the application of whole life costing in pfi/ppp projects. proceedings 29th arcom 2013, united kingdom. pp. 769-778, 2013. [8] j. karasek, j. veleba j. development of nearly zero energy buildings and application of cost optimum. business & it, vol. vii(2), pp. 18-25, 2017. doi:https://doi.org/10.14311/bit.2017.02.03. [9] d. macek. criteria for national rating system for buildings sbtoolcz. business & it, vol. vi(2), pp. 2-9, 2016. doi:https://doi.org/10.14311/bit.2016.02.01. 557 https://doi.org/10.1016/j.enpol.2008.11.001 https://doi.org/10.1016/j.enpol.2004.08.039 https://doi.org/10.1080/01446193.1987.10462093 https://doi.org/10.14311/bit.2017.02.03 https://doi.org/10.14311/bit.2016.02.01 acta polytechnica ctu proceedings https://doi.org/10.14311/app.2023.41.0020 acta polytechnica ctu proceedings 41:20–27, 2023 © 2023 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague possibilities of using commercial electronics to measure driver physiological functions in a vehicle simulator david lehet czech technical university in prague, faculty of transportation sciences, department of vehicle technology, konviktská 20, 110 00 prague, czech republic correspondence: david.lehet@cvut.cz abstract. the aim of this paper is to explore the potential of using inexpensive, commercial electronics such as a smartwatch or chest belt to measure human physiological functions while driving in vehicle simulator, both to improve safety and to use these devices in other experiments to facilitate their conduct and replace more complex laboratory equipment. in the first part of the paper, the signs of stress and cognitive load through physiological functions are researched, then the possibilities of measuring these signs are explored. this is followed by an experiment in which two commonly available devices are compared in terms of accuracy and usefulness for measurement during the experiment. keywords: ecg, heart rate, vehicle safety, design of experiment, vehicle simulator. 1. introduction for the design of the vehicle, and in particular the interaction of its systems with the driver, it is essential that the systems are properly tested. one of the best tools for this testing is a vehicle simulator, which allows both standard and hazardous situations to be simulated repeatedly in a safe environment and under controlled conditions. another important part of the design process is the ability to record objective data from the participants in the experiment. one part of this objective data can be the driver’s physiological functions, the evaluation of which can be used to objectively assess their stress and overall workload. traditionally, laboratory tools such as ecg or eeg are used for these measurements. although these instruments are very accurate, they carry several disadvantages. in particular, they are more complex to evaluate and also introduce discomfort and thus noise into the measurement. the aim of this paper is therefore to explore the possibility of using commercial electronics in the form of a smartwatch or chest strap to measure the heart rate of a participant in an experiment. 1.1. physiological functions and their measurement this part of the paper describes choosen physiological functions and how they could be measured. 1.2. eeg eeg, or electroencephalography, captures the electrical activity of the human brain [1]. to measure eeg, electrodes that sense electrical activity must be placed on the head (surface of the skull) of the person being measured. the number of electrodes varies according to the specific eeg application [2]. yan et al. [3] conducted an experiment in which they compared the accuracy using 16 and 63 electrodes. the results showed that the reduction in accuracy when using only 16 electrodes is small, which can be used in designing further experiments. much research has been done on the principles of driver distraction monitoring using eeg. of these, for example, lin et al. [4] found experimentally that driver distraction correlates with increased frontal theta and beta wave activity (types of brain activity monitored by eeg). zhang et al. conducted research in which they showed that eeg is useful for observing how a driver notices various stimuli (for example, when interacting with assistance systems). they conducted the experiment in both a simulator and a real vehicle and note that the eeg was more prone to inaccuracies in the in-vehicle experiment [5]. kumar et al. in their research used eeg to capture both visual and cognitive distraction. as a result of the measurements, the eeg was highly accurate at measuring both types of distraction [6]. in recent years, there has been an increasing number of studies on this topic focusing on new ways of processing data obtained by eeg, especially using neural networks, for example [7–10]. 1.3. ecg electrocardiography is another method that measures the electrical signals generated by the human body, in this case the signals produced by the heart. the basic output is activity (electrical signal) over time. this signal can then be used to calculate heart rate, heart rate variability (the evolution of the time differences between successive heartbeats), and possibly even respiratory rate [8]. 20 https://doi.org/10.14311/app.2023.41.0020 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 41/2023 driver physiological functions in a vehicle simulator the basic elements of the ecg curve are the pwave – a smaller deflection indicating depolarization of the atria, the p-r interval – the time between the first deflection of the p-wave and the first deflection of the qrs complex – this represents depolarization of the ventricles, the time difference between the peaks of the r-waves represents the heart rate. the t-wave represents atrial repolarization [11]. electrodes are used to capture the ecg signal, as in eeg, and in this case they are placed on the limbs and chest of the subject [12]. medical ecgs use 12 electrodes, while other devices (such as smartwatches) make contact with the skin differently. in the case of smartwatches, this is a single-lead ecg (e.g. conducted through a circuit connection between the back of the watch, the body of the person wearing the watch and a finger placed on another part of the watch) [13]. ecg is more suitable for measuring driver stress, as shown for example by keshan et al. [14] when they used ecg data to classify three levels of stress with a success rate of 88.24%, detecting high levels of stress with a success rate of 100% (naive bayes classifier with the difference in mean heart rate as a feature) or 98% (decision tree and eight different features). pourmohammadi and maleki [15] in their research compared emg (electrical muscle activity) and ecg in stress detection. again, using ecg, they were able to detect different levels of stress with high accuracy (greater than 95%). it can be seen that when using ecg, the heart rate is used as one of the signs for measurement. heart rate variability is also used quite often. therefore, if there were a way to measure these variables without ecg (for example, the aforementioned smart watches), this would lead to a simplification of the experimental process. 1.4. gsr the principle of the gsr measurement is to apply a weak and constant (undetectable to humans) voltage to the skin and to observe how the skin conductivity changes depending on the stimuli (changes due to perspiration). sikder et al. [16] used gsr measurements to determine the cognitive load of students taking an online exam. there was an attempt to asses stress based on the gsr time domain during thirteen different tasks with three different levels of stress. in a research focusing on comparing cognitive load measurement options, tervonen et al. [17] describe the capabilities of different wearable sensors and the processing of data from them using different lengths of data. among other things, they conclude that cognitive load can be detected from gsr but more slowly than from ecg. borisov et al. [18] used grs as one component to monitor cognitive load using human physiological manifestations. a combination of the use of gsr and eeg was proposed by manikandan et al. [19]. they proposed a system that monitors driver distraction based on these two variables. the measured signal is processed and divided into time and frequency domain. according to the authors, the proposed system is capable of accurately measuring fatigue and auditory and visual distraction. the research shows that from the physiological signals it is possible to monitor distraction (eeg) as well as stress and cognitive load (ecg, gsr). the next section will focus on identifying the possibilities of commercial sensors (smart watches and similar devices) whose application would facilitate the measurement process. advances in wearable electronics (miniaturization of the necessary electronics to enable wearable computing) could allow for easier measurement within an experiment, mainly due to ease of use [20]. today, these electronics are capable of recording user data in real time. the most important part of this sector today is mainly smartwatches [21]. another advantage of smartwatches would be within the measurement of cognitive load, where they would not add additional problems to the participant of the experiment, such as ecg electrodes [21]. 2. material and methods this part of the papes describes the experiment that was carried out to identify capabilities of commerical sensors. 2.1. used sensors several devices were used for the measurements. it was a vehicle simulator used for simulating driving in a superb iii equipped with automatic transmission. the scene used was a combination of an intramural and extramural road, with a maximum speed of 90 km.h−1. next was an ecg sensor – vlv-scope device (faculty of biomedical engineering, ctu in prague). a two-lead ecg sensor was used as a reference for the measurements. the electrodes were placed in the subclavian wells. next was wahoo tickrx chest belt. this was the first sensor used for comparison, it works on the principle of electrodes with contact in the lower chest area, so it is a simple ecg. the second sensor compared was a sports watch with a pulse oximeter function. the sensor is in the form of an elastic sleeve on the thumb. 2.2. pilot measurement the aim of the pilot measurements was to see if it was possible to measure the selected devices at all. for this purpose, a run with four probands was carried out and the data from these runs were subsequently processed. two methods were proposed for evaluation, these were compared in the pilot measurement and one of them was selected. time synchronization was achieved by simultaneous starting and stopping the 21 david lehet acta polytechnica ctu proceedings figure 1. output of the ecg probe. measuring devices. outputs of the sensors: eg probe the output of the ecg probe is a text file (see figure 1) that can be processed in software such as ms office excel or matlab. the output is from a program that allows you to measure several variables at once (in addition to the ecg, skin resistance or muscle activity, for example). since the sensors required for this scan were not active during the experiment, other items are unusable except for the time trace and the ecg recording. chest strap unlike the ecg probe, the output of the chest strip measurement is just a file containing a time trace and an instantaneous heart rate reading (see table 1). hr time 0 2/19/2020 10:02:25 am 115 2/19/2020 10:02:26 am 115 2/19/2020 10:02:28 am table 1. chest strap output. pulse oximeter the output of a pulse oximeter is similar to that of a chest belt (table 2). index hr spo2 motion vibration 0 80 98 0 nepravda 1 80 98 0 nepravda 2 80 98 0 nepravda table 2. chest strap output. again, this is a text file that contains the instantaneous heart rate data, labeled with a serial number. as this is an oximeter, the data also includes a record of blood oxygen saturation. the first problem that had to be solved was the processing of the ecg data. from the ecg recording, the instantaneous heart rate can be calculated using equation 1. hr = 60 r − r (1) where the r−r term represents the time difference of two consecutive r peaks. since the frame rate of the ecg probe is 1 khz, one recording represents 1ms. to calculate the time difference between the peaks, it was necessary to localize the peaks in the signal. for this, the findpeaks function included in matlab was used. a plot of the raw data is in figure 3 (x-axis is time, y-axis is ecg signal strength). after application of findpeaks some parts of the ecg curve were wrongly labelled as peaks. this shortcoming was corrected by applying the minpeakprominence and minpeakdistance function parameters. the first of these arguments modifies the minimum difference that must exist between points for a peak to be identified, and the second argument modifies the minimum distance at which peaks are identified. the value of the first argument was experimentally set to 15000. the value of the second argument was easier to find because it had to roughly correspond to the time between heartbeats. it was set to 400. the result is the identification of peaks on the ecg curve, without false positives and without missing them. the details of the curve are shown in the figure 2 through figure 4. after finding a procedure to correctly identify the peaks, it was possible to calculate the instantaneous heart rate from the ecg using the aforementioned equation. the result is in the figure 4. for comparison, data had to be resampled to the lowest common sampling frequency. comparison of all three devices is in figure 5. based on this data, a method could be selected to compare the different devices. two methods were considered. the first method consisted of summing 22 vol. 41/2023 driver physiological functions in a vehicle simulator figure 2. raw ecg. figure 3. identification of ecg peaks, mpp – 15 000, mpd – 400, detail 1. figure 4. instantaneous heart rate calculated from ecg. 23 david lehet acta polytechnica ctu proceedings figure 5. results of the pilot measurement. absolute distance chest strap pulse oximeter id1 22111.0 11763.0 id2 2177.1 1450.6 id3 3484.6 1345.7 id4 2542.2 3947.1 table 3. absolute distances of the assessed devices from the ecg. the absolute distances of the measured heart rate of the device under consideration from the reference (i.e., the ecg), according to equation 2. hrdistance = numberofrecords∑ i=1 ∣∣hrreference(i) − hrdevice(i) ∣∣ (2) in matlab this function was implemented as: id(:,4)=abs(id(:,1)-id(:,2)); distance = sum(id (:,4)); column 4 contained the calculated data, columns 1 and 2 contained the ecg record and the device under assessment. the result was stored in the variable distance. the results of this method for the 4 pilot measurements are shown in the table 3. this method takes into account the entire measurement process, the closer the sensor is to the standard, the lower the values become. the disadvantage is the poor comparability between different measurements, mainly because the value will be higher the longer the measurement itself. the results are also not very intuitive to look at. the advantage is then taking into account the magnitude of the error with which the sensor is measuring. the second proposed method involves calculating what percentage of the measured values are from a certain distance from the standard. in matlab, the calculation was implemented as follows: for i=1:length(id) if id(i,2)>((id(i,1)*0.9)) && id(i,2)<((id(i,1)*1.1)) id(i,8)=1; else id(i,8)=0; end end kpi_10_pas=(sum(id (:,8))/length(id))*100 the values 0.9 and 1.1 represent the distance from the standard, in this case 10%, at which the sensor must be located. the method was tested for two different distances, 10% and 15%. the value of 10% was chosen based on the “american national standard of cardiac monitors, heart rate meters, and alarms” and the value of 15% was chosen as an additional value [22]. if the checked value of the instantaneous heart rate was within the permissible distance, it was marked as 1, otherwise as 0. after checking the entire record, the data vector was summed and the proportion of records that were sufficiently accurate was calculated. the proportion is expressed as a percentage. the results for all four measurements are shown in the table 4. 24 vol. 41/2023 driver physiological functions in a vehicle simulator chest strap pulse oximeter 10% 15% difference 10% 15% difference id1 39.06 52.03 12.97 24.77 46.53 21.76 id2 82.02 90.35 8.33 78.51 94.74 16.23 id3 60.70 71.14 10.44 69.65 87.56 17.91 id4 63.61 84.59 20.98 41.64 63.93 22.29 table 4. comparison of two parameters for evaluating sensor accuracy. first measurement second measurement id strap [%] oxi [%] strap [%] oxi [%] 1 57.77 73.65 67.3 63.87 2 66.5 81.55 0.39 59.38 3 60.8 57.6 70.64 62.84 4 54.51 63.53 53.31 60.66 5 42.72 16.02 89.04 0.68 6 75 75.55 18.25 65.87 7 69.77 79.07 87.18 52.56 8 75.97 82.47 16.39 63.93 9 50.54 42.93 0.76 50 10 69.68 73.4 8.92 78.57 mean values 62.326 64.577 41.218 55.836 standard deviation 10.40 19.94 34.03 19.80 table 5. measurement results. it is clear from the table that the assessed devices are accurate enough to be assessed for distances from the reference signal (0.9−ekg_reference, 1.1−ekg_reference) (the intervals are open). comparing the two methods, it is clear that although the second method does not reflect the magnitude of the error (it does not distinguish whether the sensor is off by 1 or 50 beats), it is significantly easier to interpret. moreover, the two methods are consistent in their results. therefore, the method based on the distance from the reference signal was chosen for further processing. 3. results for the final comparison, a total of 20 runs were measured using 10 probands, with each proband completing 2 runs. both genders and all somatotypes were represented in the measurements. measurements were processed using the procedures described above. the results are presented in the table 5. the interpretation of the results is complicated by the fact that the chest belt, although more accurate (the highest accuracy of all measurements was achieved by the belt – 89.04%), suffered from signal dropouts, which are marked in red in the table. the most accurate result of the pulse oximeter was an accuracy of 82.47%. interestingly, when comparing measurements in which the chest belt did not experience signal dropouts, the average values for both sensors are similar. for illustration, the best results of both sensors are presented in figures 6 and 7 and their strengths and weaknesses are described on the measurements. figure 6 shows the aforementioned best measurement using a chest strap. however, it can also be seen here that the belt is slower than the reference ecg in responding to changes in heart rate, and some sharp fluctuations are not detected at all. in this measurement, the pulse oximeter readings were very poor, but this was not due to failure. figure 7 shows the best pulse oximeter measurement. it can be seen that although the pulse dynamics are less well tracked by the oximeter than by the chest belt, the tracking of the overall trend is quite good. 4. conclusion and future work in this article, two commonly available sensors – a chest belt and a pulse oximeter in the form of a watch – have been selected for a comparison of usage of common commercial sensors for monitoring driver during a test in a vehicle simulator. using pilot measurements, a suitable methodology was designed and selected to evaluate their accuracy against a reference signal in the form of a two-lead ecg. from the measurements it is clear that while both devices are capable of accurate enough measurement 25 david lehet acta polytechnica ctu proceedings figure 6. measurement, id5_2. figure 7. measurement id 8_1. to be used during experiments, the chest strap is capable of the more accurate measurement. this is to be expected as the technology used by its sensors is more accurate than the photoplethysmography on which the pulse oximeter is based. slower reactions of the oximeter show that it is more suited for long term measurement, while better dynamics of the chest strap make it useful for measuring driver during stressful events. however, the ergonomics of the measurement complicate the choice of a suitable device. for the participant in the experiment, it is obviously better if he or she just puts on a smartwatch instead of a chest strap. however, both options are an improvement against ecg electrodes. further research in this direction should focus on collecting a larger sample of data using the described ones, on which the results of the measurement could be statistically verified. another weakness of the experiment was imperfect way of synchronizing the measuring devices. although it was sufficient for a proof of concept such as this, subsequent experiment should use more precise method. another part of the experiment that should be perfected in subsequent iterations is the problem of sampling. each device had a different sampling rate and even though it was again deemed sufficient for this experiment, devices with higher and more similar sampling rate should be chosen next time. it must also be kept in mind that the commercial electron field is rapidly evolving, testing should be repeated periodically using newer equipment but the same (but updated) methodology. 26 vol. 41/2023 driver physiological functions in a vehicle simulator references [1] e. k. s. louis, l. c. frey. electroencephalography – an introductory text and atlas of normal and abnormal findings in adults, children, and infants. american epilepsy society, chicago, il, usa, 2016. isbn 13-978-0-9979756-0-4. 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reliability and applicability of heart rate and motion measurements. applied ergonomics 105:103855, 2022. https://doi.org/10.1016/j.apergo.2022.103855 [22] f. sartor, j. gelissen, r. van dinther, et al. wristworn optical and chest strap heart rate comparison in a heterogeneous sample of healthy individuals and in coronary artery disease patients. bmc sports science, medicine and rehabilitation 10(1):10, 2018. https://doi.org/10.1186/s13102-018-0098-0 27 https://doi.org/10.1109/ichms53169.2021.9582456 https://doi.org/10.1109/iscas.2008.4541611 https://doi.org/10.1088/1741-2560/12/6/066028 https://doi.org/10.1109/iccmc48092.2020.iccmc-000118 https://doi.org/10.1109/iccmc48092.2020.iccmc-000118 https://doi.org/10.1109/tase.2021.3088897 https://doi.org/10.1016/j.engappai.2022.105309 https://doi.org/10.1051/itmconf/20224403030 https://doi.org/10.1016/j.jsr.2022.05.015 https://www.ncbi.nlm.nih.gov/books/nbk2214/ https://doi.org/10.1016/j.tcm.2019.10.010 https://doi.org/10.1109/bigdata.2015.7364066 https://doi.org/10.1016/j.cmpb.2020.105482 https://doi.org/10.1109/eict54103.2021.9733564 https://doi.org/10.3390/electronics10050613 https://doi.org/10.1016/j.chbr.2021.100116 https://doi.org/10.1109/icces51350.2021.9489076 https://doi.org/10.4071/2380-4505-2019.1.000415 https://doi.org/10.1016/j.apergo.2022.103855 https://doi.org/10.1186/s13102-018-0098-0 acta polytechnica ctu proceedings 41:20–27, 2023 1 introduction 1.1 physiological functions and their measurement 1.2 eeg 1.3 ecg 1.4 gsr 2 material and methods 2.1 used sensors 2.2 pilot measurement 3 results 4 conclusion and future work references https://doi.org/10.14311/app.2022.33.0597 acta polytechnica ctu proceedings 33:597–603, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague fracture behavior of multiple adhesive postinstalled anchors subjected to shear force yuji tajimaa,∗, kazuaki hokib a aichi syukutoku university, 2-9 katahira nagakute-city aichi prefecture 480-1197 japan b the university of kitakyusyu, 1-1 hibikino wakamatu-ku kitakyusyu-city fukuoka prefecture 808-0135 japan ∗ corresponding author: yujita@asu.aasa.ac.jp abstract. in this study, experiments were conducted on post-installed anchors subjected to shear force in order to enhance applicability when post-installed anchors were used for seismic reinforcement. furthermore, since there were a large number of post-installed anchors when they were actually used, an experiment was conducted in which aăshear force was simultaneously applied to one to four anchors. in this study, it was focused on number and pitch of anchors. major findings of this study were summarized as follows: in case of 22.46 (n/mm2) for concrete compressive strength, shear strength of anchors was not double, triple, and quadruple as number of those increased to 2, 3, and 4, and that gradually decreased as number of those increased, the boundary condition between anchor shear rupture and pry-out failure was found that concrete compressive strength was 25.12 (n/mm2) or less and anchor pitch was 30 to 50 mm, and anchor was tensile strength over sd390. keywords: adhesive post-installed anchor, anchor pitch, multiple anchors, shear strength. 1. introduction in japan, reinforced concrete structures built in 1960s and 1970s were reinforced for sustainable use of buildings due to lack of earthquake resistance. steel braces were most often used to reinforce buildings, and many anchors were installed in existing buildings to transmit stress to steel braces. since the stress transmission between the existing building and the steel brace had a high ratio of shear force, when using the postinstalled anchor for seismic reinforcement, the anchor reinforcement should be designed with shear performance. however, for post-installed anchors, structural standards in japan [1] were determined based on tensile performance of one anchor to ensure safety. the above-mentioned problem should be considered in terms of shear performance, but had been limited in effective use due to standards for tensile performance. on the other hand, there were many tensile tests about one anchor, and there were shear tests about it, but in any case, experiment about it had been done according to past studies. therefore, in this study, experiments were conducted on postinstalled anchors subjected to shear force in order to enhance applicability when post-installed anchors were used for seismic reinforcement. furthermore, experiments were conducted in which not only one anchor but also multiple anchors were subjected to shear force simultaneously. for post-installed anchors, various factors such as anchor tensile strength, concrete compressive strength, embedding depth, anchor pitch, and clearance distance should be considered. as mentioned above, although there were various factors, it was necessary to narrow down the parameters. therefore, the embedding depth was constant, and the experiment was conducted focusing on differences in concrete strengths, anchor strengths, and anchor pitches. as an additional note, the anchor bars were installed at a sufficient distance from the edge of the concrete so as not to consider the edge rupture. 2. test program 2.1. specimens properties of specimens were summarized in table 1. there were 23 specimens in total. most of the test results were based on reference [2]. the added specimens were marked with * in the table 1. the anchor was used deformed bar of nominal diameter of 10mm(d10). the post-installed anchor arrangement was made such that only one anchor is arranged, two anchors are arranged along loading direction (dp = 100mm, dp/da = 10, da: nominal diameter), three anchors are arranged (dp = 50mm, dp/da = 5), and four anchors are arranged (dp = 30mm, dp/da = 3). in other words, pitches of two, three and four anchors were arranged along loading direction at 100, 50 and 30mm, respectively. the compressive strengths of concrete were 3 types of 23.12, 25.12 and 29.34 (n/mm2). materials of anchors were used sd390 and sd295. the embedded depth was constant 70mm (7 da). an epoxy resin was used as a fixing agent for the post-installed anchor. 597 https://doi.org/10.14311/app.2022.33.0597 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en yuji tajima, kazuaki hoki acta polytechnica ctu proceedings serial number material of anchor σy σu es n dp σb ec quantity mpa mpa gpa (mm) mpa gpa 1 sd390 d10 428 590 209 1 − 29.34 26.9 12 2 100 3 3 50 4 4 30 5 1 − 25.12 23.1 16 2 100 7 3 50 8 4 30 9 1 − 23.12 24.6 1 10 1,2 2 100 2* 11 3 50 1 12 1,2 4 30 2* 13 sd295 d10 345 479 183 1 − 29.34 26.9 114 2 100 15 3 50 16 4 30 17 1 − 22.46 31.4 1 18 2 100 1 19 1,2 3 50 2* 20 4 30 1 table 1. properties of specimens. 2.2. loading method and instrumentation a loading apparatus was shown in figure 1. the loading method was monotonous loading. shear force was simultaneously applied to a plurality of arranged anchors using a loading frame. shear force was measured by a load-cell installed outside a loading frame. the displacement between tensile tool and concrete was measured by two displacement transducers. the average value of these outputs was taken as the shear displacement. this value was evaluated as a shear displacement, although the flexural deformation of the anchor bar was included. 3. test results 3.1. shear force shear displacement relations shear force-shear displacement relations were shown in figure 2 for each concrete compressive strength and anchor tensile strength. in figure 2, the experimental values of four anchors, three anchors, two anchors and one anchor were shown by a solid line, a two-dot chain line, a one-dot chain line and a dotted line, respectively. although some specimens showed experimental values with different initial stiffness due to construction accuracy, they roughly showed the same initial stiffness even when the number and pitches of anchors were different. as the shear force increased, the local concrete gradually failed in compression and the flexural moment of anchors gradually increased. these states could be clearly seen from the history in figure 2(c) and (e). 3.2. crack pattern and failure mode examples for crack patterns of concrete and shear ruptures of anchors were shown in figure 3 and 4. except for specimens numbered 12-1 and 2, the anchors of the other specimens failed in shear rupture. specimens numbered 12-1 and 2 using four d10(sd390) anchors and a concrete compressive strength of 23.12 (mpa) failed in pry-out. the above mentioned pryout failure was a type of concrete failure that was scraped as shown in figure 4(a). as a result, two of the four anchors did not rupture in shear. for a specimen numbered 8 using four d10(sd390) anchors and a concrete compressive strength of 25.12 (mpa), although the anchors eventually failed in shear rupture, as shown in figure 4(b), diagonal cracks occurred on the back in the loading direction. this could be judged as a sign of a pry-out failure. the cracks for specimens of one and two anchors were isolated as shown in figure 3(a) and (b). on the other hand, cracks for specimens of three and four anchors were connected as shown in 3(c) and (d). in the next chapter, the maximum shear force and the shear displacement at the maximum shear force were discussed based on the hysteresis, crack and anchor fracture properties. 4. discussions 4.1. maximum shear force number of anchors relations maximum shear force-number of anchors relations used with sd295 were shown in figure 5. in figure 5, maximum shear forces with the compressive 598 vol. 33/2022 fracture behavior of multiple adhesive postinstalled anchors /rdglqj�'luhfwlrq� $qfkruv� 2lo�-dfn� 'lvsodfhphqw� 7udqvgxfhuv /rdglqj�'luhfwlrq� $qfkruv /rdg�fhoo� 'lvsodfhphqw� 7udqvgxfhuv $qfkruv� figure 1. (a) framing plan of loading apparatus, loading apparatus. (b) elevation of loading apparatus, loading apparatus. concrete strengths of 22.46 and 29.34 (n/mm2) were shown by triangles and circles respectively. when the average of the maximum shear force for one anchor was calculated without considering the concrete strength, it was 28.04 (kn). the value obtained by multiplying that by the number of anchors was shown by a dotted line in figure 5. maximum shear forces with a concrete strength of 22.46 (n/mm2) tended to be lower than the dotted line. on the other hand, maximum shear forces with a concrete strength of 29.34ă(n/mm2) tended to be larger than the dotted line. maximum shear force-number of anchors relations used with sd390 were shown in figure 6. in figure 6, maximum shear forces with the compressive concrete strengths of 23.12, 25.12 and 29.34 (n/mm2) were shown by squares, diamonds and circles respectively. the maximum shear forces with the compressive concrete strengths of 23.12 and 25.12 (n/mm2) for specimens failed in pry-out were shown by asterisks and a cross respectively. when the average of the maximum shear force for one anchor was calculated without considering the concrete strength, it was 35.96 (kn). the value obtained by multiplying that by the number of anchors was shown by a dotted line in figureă4. maximum shear forces with a concrete strength of 23.12 (n/mm2) tended to be lower than the dotted line. maximum shear forces with a concrete strength of 29.34 (n/mm2) were almost on the dotted line. considering these results and the above-mentioned crack patterns, the boundary condition between anchor shear rupture and pry-out failure was found that concrete compressive strength was 25.12 (n/mm2) or less and anchor pitch was 30 to 50mm, and anchor was tensile strength over sd390. in this study, since the embedded depth of the anchor was constant 70mm (7da), the embedded depth was not considered. when the hysteresis of the four anchors and that of the three anchors in figure 2 were compared again from the above viewpoint, signs could be confirmed that the second stiffness was almost the same. furthermore, by examining the boundary conditions precisely hereafter, it would be possible to install the anchor at a shorter pitch than the current standards [1] in japan when using the anchor for seismic reinforcement. 4.2. displacement at maximum shear force pitches of anchors relations displacement at maximum shear force-pitches of anchors relations used with sd295 were shown in figure 7. in figure 7, displacements at maximum shear force with the compressive concrete strengths of 22.46 and 29.34 (n/mm2) were shown by triangles and circles respectively. linea approximation lines with the compressive concrete strengths of 22.46 and 29.34 (n/mm2) were shown by a dotted and one-dot chain 599 yuji tajima, kazuaki hoki acta polytechnica ctu proceedings � � �� ��� ��� � � �� �� �� 6k hd u�) ru fh �n 1 � 'lvsodfhphqw�pp� ı%������6'��� � �� ��� ��� � � �� �� �� 6k hd u�) ru fh �n 1 � 'lvsodfhphqw�pp� ı%������6'��� � �� ��� ��� � � �� �� �� 6k hd u�) ru fh �n 1 � 'lvsodfhphqw�pp� ı%������6'��� � �� ��� ��� � � �� �� �� 6k hd u�) ru fh �n 1 � 'lvsodfhphqw�pp� ı%������6'��� 2qh�dqfkru 7zr�dqfkruv �������������������7zr�dqfkruvჟ� 7kuhh�dqfkruv �������������������7kuhh�dqfkruvჟ� )rxu�dqfkruv �������������������)rxu�dqfkruvჟ� � �� ��� ��� � � �� �� �� 6k hd u�i ru fh �n 1 � 'lvsdohphqw�pp� ı%������6'��� �d�6huldo�1xpehu�iurp���wr������������������������������������e�6huldo�1xpehu�iurp���wr��� �g�6huldo�1xpehu�iurp����wr������������������������������h�6huldo�1xpehu�iurp����wr���� �f�6huldo�1xpehu�iurp���wr������ figure 2. shear force shear displacement relations. 600 vol. 33/2022 fracture behavior of multiple adhesive postinstalled anchors �d���6huldo�1xpehu�����rqh�dqfkru��� � �e����6huldo�1xpehu�����wzr�dqfkruv��� �f����6huldo�1xpehu�������wkuhh�dqfkruv��� � �g����6huldo�1xpehu�����irxu�dqfkruv��� /rdglqj�'luhfwlrq� /rdglqj�'luhfwlrq figure 3. examples for crack patterns of concrete and shear ruptures of anchors. �d���6huldo�1xpehu�������irxu�dqfkruv��� � �e����6huldo�1xpehu����irxu�dqfkruv��� 'ldjrqdo�&udfn� figure 4. pry-out failure and diagonal cracks (sign of pry-out failure). � �� �� �� ��� ��� � � � � 0 d[ lp xp ��� ��� ��� ��� ��� � 6k hd u�) ru fh �n 1 � 1xpehu�ri�$qfkruv 6'��� ˜ ��ı% ������1�pp�� ʕ ��ı% ������1�pp�� ��1xpehu�7lphv�/lqh figure 5. maximum shear force-number of anchors relations. (sd295). 601 yuji tajima, kazuaki hoki acta polytechnica ctu proceedings � �� �� �� ��� ��� � � � � 0 d[ lp xp ��� ��� ��� ��� ��� 6k hd u�) ru fh �n 1 � 1xpehu�ri�$qfkruv 6'��� ˚ � ı% ������1�pp�� ˘ � ı% ������1�pp�� ʕ ��ı% ������1�pp�� ː ��3u\�rxw�)dloxuh�ı% ������1�pp��� ʹ ��3u\�rxw�)dloxuh�ı% ������1�pp��� ��1xpehu�7lphv�/lqh figure 6. maximum shear force number of anchors relations. (sd390). � � �� �� �� �� � �� �� �� �� ��� ' lvs od fh p hq w�� ��� ��� ��� �� dw �0 d[ lp xp �6k hd u�) ru fh �p p � 3lwfkhv�ri�$qfkruv�pp� 6'��� vorsh����������� ı% ������1�pp�� vorsh����������� ı% ������1�pp�� ˜ ��ı% ������1�pp�� ʕ ��ı% ������1�pp�� ��/lqhdu�$ssur[lpdwlrq� /lqh �ı% ������1�pp��� ��/lqhdu�$ssur[lpdwlrq� /lqh �ı% ������1�pp��� figure 7. displacement at maximum shear force pitches of anchors relations. (sd295) � � �� �� �� �� � �� �� �� �� ��� ' lvs od fh p hq w�� ��� ��� ��� ��� � dw �0 d[ lp xp �6k hd u�) ru fh �p p � 3lwfkhv�ri�$qfkruv�pp� 6'��� vorsh����������� ı% ������1�pp�� vorsh����������� ı% ������1�pp�� vorsh����������� ı% ������1�pp�� ˚ � ı% ������1�pp�� ˘ � ı% ������1�pp�� ʕ ��ı% ������1�pp�� ��/lqhdu�$ssur[lpdwlrq� /lqh �ı% ������1�pp��� ��/lqhdu�$ssur[lpdwlrq� /lqh �ı% ������1�pp��� ��/lqhdu�$ssur[lpdwlrq� /lqh �ı% ������1�pp��� figure 8. displacement at maximum shear force pitches of anchors relations. (sd390) line respectively in figure 7. the anchor pitch was considered appropriate for the horizontal axis when evaluating shear displacement, but one anchor did not have a pitch. therefore, the crack pattern for the two anchors was referred to in figure 3(b). the pitch for the two anchors was 100 (mm), the cracks did not interfere each other. consequently, the pitch of one anchor was considered as 100mm and discussed afterwards. in figure 7, the slopes of the linear approximation line for each concrete strength were shown. these slopes tended to increase the displacement at maximum shear force as the pitch became shorter. displacement at maximum shear force-pitches of anchors relations used with sd390 were shown in figure 8. in figure 8, displacements at maximum shear force with the compressive concrete strengths of 23.12, 25.12 and 29.34 (n/mm2) were shown by squares, diamonds and circles respectively. linea approximation lines with the compressive concrete strengths of 23.12, 25.12 and 29.34 (n/mm2) were shown by a dotted, two-dot chain and one-dot chain line respectively in figure 8. with the same idea as above, the pitch of one anchor was considered as 100mm and discussed afterwards. in figure 8, the slopes of the linear approximation line for each concrete strength were shown. although the slope with sd390 was lower than the slope with sd295, the displacement at maximum shear force tended to increase as the anchor pitch became shorter. these meant that as the anchor pitch became shorter, the compressive failure zone for the concrete was expected deeper from the surface. 5. conclusions the following conclusions can be drawn from the present study about multiple adhesive post-installed 602 vol. 33/2022 fracture behavior of multiple adhesive postinstalled anchors anchors subjected to shear force: 1. except for two specimens, the anchors of the other specimens failed in shear rupture. the two specimens with four d10(sd390) anchors and a concrete compressive strength of 23.12 (mpa) failed in pryout. for the specimen with four d10(sd390) anchors and a concrete compressive strength of 25.12 (mpa), although the anchors eventually failed in shear rupture, diagonal cracks occurred on the back in the loading direction. this could be judged as a sign of a pry-out failure. 2. maximum shear forces with a concrete strength of 22.46 (n/mm2) tended to be lower than the line obtained by multiplying the maximum shear force for one anchor by the number. in other words, in case of 22.46 (n/mm2) for concrete compressive strength, shear strength of anchors was not double, triple, and quadruple as number of those increased to 2, 3, and 4, and that gradually decreased as number of those increased. considering these results, the boundary condition between anchor shear rupture and pry-out failure was found that concrete compressive strength was 25.12 (n/mm2) or less and anchor pitch was 30 to 50mm, and anchor was tensile strength over sd390. 3. considering the pitch of one anchor as 100ămm, the slopes of the linear approximation line for each concrete strength tended to increase the displacement at maximum shear force as the pitch became shorter. as a result, when the anchor pitch was shortened, the compressive failure zone for the concrete was expected deeper from the surface, and the displacement at the maximum shear force was increased. from the above conclusions, it was found qualitatively that the correlation between concrete compressive strength, anchor pitch and anchor tensile strength at the boundary condition between anchor shear rapture and pry-out failure when the embedded depth for anchor was constant. in the future, it is necessary to conduct experiments that can quantitatively evaluate the boundary conditions, and research the relationship between the depth of the compressive zone and the anchor pitch. and, using these results, we plan to conduct an experiment to investigate the sustained shear load performance of adhesive post-installed anchors as an aid to the sustainable use of the building. acknowledgements this work was supported by jsps kakenhi grant number jp18k04447. any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the authors’ organization, jsps or mext references [1] architectural institute of japan. design recommendations for composite constructions, p. 252, 2010. [2] k. hoki, y. tajima. strength evaluation of densely-placed post-installed anchors part 2: shear test of monotonic loading for post-installed anchors used sd390, summary of technical papers of annual meeting of kyusyu chapter, architectural institute of japan, p. 469-472, 2019. 603 acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.34.0098 acta polytechnica ctu proceedings 34:98–104, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague phase-field damage of laminated glass in explicit dynamics jaroslav schmidt∗, tomáš janda czech technical university in prague, faculty of civil engineering, department of mechanics, thákurova 7, 166 29 praha 6, czech republic ∗ corresponding author: jaroslav.schmidt@fsv.cvut.cz abstract. laminated glass is aesthetic but also functional modern material, which has spread from the automotive industry to the civil engineering in the form of design as well as structural elements. proper design of this material requires understanding not only its intact phase, but also its post-breakage behavior. to describe crack initiation and propagation the phase field damage model seems to be suitable tool despite the fact that this approach requires a fine mesh to sufficiently interpolate sharp discontinuity. in this contribution we investigate possibilities of explicit phase field dynamic model with specific use on glass plates, especially applied for dynamic impact load. the dependence of the result on residual stiffness term is examined and incorrect crack speed is observed. usability of phase-field model is still uncertain. keywords: phase field damage model, laminated glass, dynamic damage, glass fracture. 1. introduction the so called phase-field damage model firstly presented in [1] is nowadays very popular approach and it is widely studied. the disadvantage of such a model is the need for a fine mesh to simulate brittle failure, but the main advantage remains the prediction of crack initiation and branching without additional ad hoc criteria. these phenomena are predicted within a variational principle. despite the fact that it was academicly investigated in dynamic regime [2], there is still a few articles focused on application on real structures. this paper focuses on such an analysis by modeling the cracking of glass under dynamic loading. the dynamic model including impact is briefly presented in section 2. the phase-field model formulation follows partially from [3] and [4]. for comprehensive overview see [5]. the implementation details are summarized in section 3 followed by fem model description in section 4. the first numerical results are benchmarked in section 5 and finally the main results are presented in section 6 and concluded in section 7. 2. phase-field model we suppose the elastic damageable material occupying space ω is loaded by external traction forces t on part ωt and the current body deformation is described by displacement field u(x) and damage field d(x). the latter one represents the relative measure of damage as a transition of the intact material d = 0 to a fully developed crack d = 1. for dynamical systems, the common principle of minimum potential is replaced by hamilton’s principle of stationary action and fields u(t) and d(t), t ∈ 〈0, t 〉 are obtained by minimization of action a(u, u̇, d) = ∫ t 0 l(u, u̇, d) dt, (1) where l is so called lagrangian which form is postulated as l(u, u̇, d) = k(u̇)−ψe(u, d)−ψs,l(d) + p(u), (2) where k is kinetic energy defined as quadratic form of displacement k(u̇) = ∫ ω 1 2ρu̇ · u̇ dx. (3) this formulation assumes that kinetic energy is not affected by damage, therefore the functional is dindependent. the second term in equation (2) is elastic potential. it can be additively decomposed on part ψ+ which is degraded by damage through function g(d) and intact part ψ−, i.e. ψe(u, d) = ∫ ω g(d)ψ+(u) + ψ−(u) dx, (4) where ψ• represents elastic energy density of ψ•. in what follows we always assume that g(d) = (1− d)2 + kres, where kres is small (kres << 1) auxiliary residual relative stiffness which stabilizes the calculations. the third part of (2) is dissipated energy ψs,l which is approximated by following regularization ψs,l = gf cα ∫ ω α(d) l + l|∇d|2 dx, (5) where gf is the critical energy release rate and l is length of the process zone. for brittle material, l can be treated as numerical parameter with value as 98 https://doi.org/10.14311/app.2022.34.0098 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 34/2022 phase-field damage of laminated glass in explicit dynamics small as possible, but not smaller than element size. the function α and associated coefficient cα drives the phase-field model type. two common options are commonly used in literature: α = d with cα = 8/3 and α = d2 with cα = 2 however we restrict to the first one. finally, the last ingredient is external work of traction forces t defined as p(u) = ∫ ωt t · udx. (6) minimization of the action integral (1) with additional per partes integration leads to common continous governing equations for displacement field div ( g(d)∂ψ + ∂u + ∂ψ− ∂u ) = ρü (7) supplemented by damage-evolution inequality gf cα ( α′(d) l − 2l∆d ) = −g′(d)ψ+, ḋ > 0 (8) gf cα ( α′(d) l − 2l∆d ) > −g′(d)ψ+, ḋ = 0 (9) and approriate boundary conditions. strong-form equations (7)-(9) have a generally unknown solution, therefore some numerical method must be used. 2.1. plate models the model and its properties are determined by the choice of dissipation function α(d), degradation function g(d) and by active/passive elastic energy densities ψ±. four common choices for the latter one are presented in this subsection. despite the fact that we restrict our attention to model p-vd, others are presented. this is because we intend to investigate them in the future research. 3d spectral decomposition it is assumed that material behaves linearly elastically in pre-breakage stage, therefore it can be described by two independent lame’s coefficients λ and ν. since the damage variable starts evolving, the only positive part of energy must be degraded, therefore ψ±(u) = 1 2λ〈±tr(ε)〉2 + µε± : ε±, (10) where ε is conventional strain tensor defined as symmetric part of displacement field gradient ε = ∇su. positive and negative component of strain tensor follows from the principal strains εi and the dyadic product of eigenvectors pi of ε, thus ε± = ∑ i ±〈±εi〉pi ⊗ pi. (11) we denote this model as 3d− sd 3d volumetric decomposition the second common choice of decomposition is volumetric-deviatoric split, which assumes that only compression part of volumetric strain component remains intact, thus ψ+(u) = 1 2k〈tr(ε)〉 2 + µεd : εd (12) ψ−(u) = 1 2k〈−tr(ε)〉2, (13) where k stands for bulk modolus and εd is deviatoric part of strain ε. this model is denoted as 3d−vd. mindlin-reissner plate besides full 3d model, we investigate the spatialy reduced mindlin-reissner plate model. this model assumes linearly distibuted strain and stress field across thickness and kinematics is driven by middle-surface longitudinal displacement field u, out-of-plane scalar deflection w and vectorial tensor of cross-section rotations ϕ. based on the linearity assumption the longitudinal strain ε and shear strain γ can be established as ε(xs, z) = ∇su(xs) +∇ssϕ(xs)z, (14) γ(xs) = 1 2 (∇w(xs) + sϕ(xs)) , (15) where z is out-of-plane coordinate meanwhile xs = {x, y} is in-plane coordinate system of middle-surface ωs. the s is auxiliary matrix s = [ 0 1 −1 1 ] . (16) mindlin-reissner plate model ignores normal stress across thickness, therefore in each infinitesimal layer plane-stress situation occurs. therefore stress fields are defined as σ(ε) = dps : ε, (17) τ (γ) = 2νi : γ, (18) where dps is plane-stress elastic tensor defined again through lame’s coefficients λ and ν and i is identity tensor. the last ingredient is thickness integration for spatial reduction. it is impossible to perform it in closed form therefore numerical integration is employed and active/pasive elastic energy become ψ+ = 1 2 ∫ ωs g(d) (∑ i ∆ziεi : σ+ i + ξhγ · τ ) dxs, (19) ψ− = 1 2 ∫ ωs ∑ i ∆ziεi : σ−i + (1− ξ)hγ · τ dxs. (20) summation represents dividing thickness into layers i, each represented by in-plane stress σi and strain εi with sub-thickness ∆zi. parameter ξ decides what amount of shear is degraded; ξ = 1 means shear is fully degraded meanwhile ξ = 0 is shear damage-free 99 jaroslav schmidt, tomáš janda acta polytechnica ctu proceedings state. still the stress tensor σ±i must be somehow decomposed. based on this decomposition this model is denoted as p− sd if spectral decomposition is used and is denoted p−vd for volumetric-deviatoric split. the degradation function still remains g(d) = (1 − d)2 +kres, but we found that it is appropriate to select different parameter kres for shear, which is denotes by ks res. 2.2. discrete dynamic model first of all we suppose that the solution is evaluated only in discrete time instants 0 = t0 < t1 < t2 < . . . < tn = t selected from the original time interval 〈0, t 〉. the action integral (1) is approximated based on this time discretization as n−1∑ i=0 ∆tild(ui,ui+1, di, di+1), (21) where common notation ui = u(ti) and di = d(ti) is used and also ∆ti = ti+1 − ti = ∆t is supposed to be constant. function ld is discretized lagrandian, which approximate the action integral on given subinterval. for purpose of this article, relatively simple expression leading to central difference scheme is used, i.e. ld(ui,ui+1, di, di+1) = l(ui, ui+1 − ui ∆t , di). (22) as a result the original minimization in continuous time domain fell apart to individual minimizations with respect to each displacement ui and damage di, where i = 0, . . . , n . the stationarity condition for displacement field ui leads to discrete euler-lagrange equation in form ∂ld(ui−1,ui, di−1, di) ∂ui + ∂ld(ui,ui+1, di, di+1) ∂ui = 0, (23) which can be expressed implicitly in weak form δk(δu)− δψe(δu) + δp(δu) = 0,∀δu (24) where δk = 1 ∆t2 ∫ ω ρ(ui+1 − 2ui + ui−1) · δu dx, (25) δψe = ∫ ω ( g(di) ∂ψ+(ui) ∂ui + ∂ψ−(ui) ∂ui ) · δu dx, (26) δp = ∫ ω t · δui dx. (27) the presented weak form can be further modified to obtained governing equations in strong form, but we employ finite element method for solving spatial distribution of ui which requires weak form as an input. the governing weak form for damage is identical with quasi-static version. the stationary condition δa(u, u̇, d, δd) = 0, ∀δd (28) becomes∫ ω gf cα ( α′(d) l + l∇d∇δd ) + g′(d)ψ+ dx = 0, ∀δd (29) with damage irreversibility condition di(x) ≤ di+1(x), ∀x, i = 0, . . . , n − 1 (30) this approach therefore induces the following explicit non-incremental dynamic procedure: (i) solve explicitly displacement in time ti by linear problem (24), (ii) solve damage field di by variational ineqaulity problem (29)+(30), (iii) increment time. 2.3. nonlinear hertz force the impact of steel impactor with weight mimp is included in the model through hertz-law. the impactor is characterized by scalar displacement uimp, which is binded to plate by nonlinear contact force f (u, uimp) = k〈uz(ximp)− uimp〉3/2, (31) where ximp is impact position on body ω, uz is out-ofplane component of u = (ux, uy, uz) and k is contact stiffness. although the force is nonlinear the pseudo potential of this force can be still founded. this potential phertz = 2 5k〈uz(ximp)− uimp〉5/2 (32) is added to external work (6) and it induces force (31) applied to impact point ximp and negative force (31) applied to impactor. additionaly the kinetic energy must be enhanced by kimp = 1 2mimpu̇impu̇imp (33) with impactor displacement rate u̇imp. this part of kinetic energy is again approximated by discretized lagrangian and discrete euler-lagrange equation induces following additional member of weak form (26), i.e. δkimp = mimp uimp,i+1 − 2uimp,i + uimp,i−1 ∆t2 δuimp (34) 3. implementation for solving spatial distribution of ui, the common finite element method (fem) is used. the displacement field is approximated by nodal values ri through selected basis functions. since the fem is well established, the details of method is not presented here. to handle simultaneously deflection of elastic body and 100 vol. 34/2022 phase-field damage of laminated glass in explicit dynamics l1 = 1.5 m l 2 = 1. 5 m h = 0.02 m e = 72 gpa ν = 0.22 ρ = 2500 kg/m3 x y figure 1. setup of investigated task. impactor position, the all matrices and vectors are extended by one additionaly degree of freedom and final object is marked by tilde. for example displacement vector r̃i or mass matrix m̃ mean r̃i = {ri, uimp}, m̃ = [ m 0 0 mimp ] . (35) with this in hand, the displacement problem (24) after fem assemble process and localization becomes 1 ∆t2 m̃(r̃i+1 − 2r̃i + r̃i−1) + k̃r̃i + f̃ (r̃i) = 0̃, (36) which is linear in parameter ri+1. the matrices k̃ and m̃ are stiffness and mass matrices. presentation of their precise form goes beyond the scope of this paper. the assembling and localization of damage problem (29) leads to linear form hdi + fd = 0, (37) but let us remark several comments. first of all, the discretized damage field di still undergoes irreversibility condition di � di+1 i = 0, . . . , n − 1, (38) where � operator stands for element-wise comparison. from that reason the linear problem (37) must be solved by nonlinear solver based on variational inequalities to satisfy condition (38). the second remark is about matrices k̃, h and damage-driving force fd. these are ui resp. di nonlinearly dependent and must be assembled in each time steps. it significantly slows down the calculations. 4. setup the all simulations are performed on the same task – unsupported square glass plate hit by steal impactor. this setup follows from real experimental arrangement, see [6]. the plate has dimensions 1.5 × 1.5 m and thickness 0.02 m, but only one quarter is modeled due to symmetry, see figure 1, where material parameters are also presented. the contact stiffness is defined as k = 4 3 √ r 1−ν2 e + 1−ν2 imp eimp , (39) where r = 0.05 m is impactor head radius and eimp = 210 gpa and νimp = 0.3. further weight of impactor is mimp = 48.7 kg and initial velocity-free height of impactor is 0.3 m. finally the phase-field parameter l is choosen as element length and gf = (8lf2 t )/(3e), where ft = 55 mpa is tensile strength. 5. benchmarks phase-field damage model requires fine mesh with lower-order elements rather than higher-order elements with lower resolution. it is contrary to plate theory, where lower-order elements exhibit shear locking and must be somehow enhanced to overcome this issue. the plate element enrichment leads to additional degrees of freedom which reduces computability and the ability to do repeated calculations. the finer resolution is more important in this analysis than exact shear implementation, therefore suitability of "classic" low-order plate elements is investigated in this benchmark. the first benchmark is behavior of impacted plate in elastic regime. the task setup presented in section 4 is used and the response is calculated in time interval 〈0; 0.01〉 s with time step ∆t = 4 · 10−7 s. resolutions of mesh 50× 50 and 100× 100 elements are investigated with two different element typologies: triangles and squares. the reduced integration is employed for shear terms, which enhanced rectangle elements, but triangles still locking. the reason for not using square elements is because they exhibits undesirable zero-energy modes. the results of each case is presented in figure 2, where evolution of impactor acceleration is plotted. this is one of the interesting investigated quantity, which can be experimentaly validated in future research. also the different behavior of two cases stands out more than in impactor displacement function. it is evident from figure 2 that behavior is slighly different for triangles in case of mesh 50× 50, but both line are comparably the same for case 100× 100 and shear locking does not cause significant deviation. this statement holds for given time resolution and fraction h/l of course. nevertheless this setup follows from real experiment and we assume that results are adequate. the deflections behaves similarly and do not suffer for larger deviation for mesh 100× 100. however, the usage of triangle elements exhibit parasitic behavior in the damaged state as is presented in second benchmark. the task is the same as in previous benchmark, but initial crack is incorporated 101 jaroslav schmidt, tomáš janda acta polytechnica ctu proceedings 0 0.2 0.4 0.6 0.8 1 ·10−2 −600 −400 −200 0 time t [s] im p. ac ce le ra tio n [m /s 2 ] 50x50 triangles rectangles 0 0.2 0.4 0.6 0.8 1 ·10−2 −600 −400 −200 0 time t [s] 100x100 triangles rectangles figure 2. evolution of impactor acceleration for triangles (solid) or squares (dashed) mesh plotted for resolution 50× 50 (red) and 100× 100 (blue). 0 0.2 0.4 0.6 0.8 0 2 4 6 ·10−3 x coordinate [m] d efl ec tin [m ] rectangles triangles triangles-full figure 3. deflection distribution on line {(x, y)|y = 0.75}. on plate in this case. the initial crack is implemented through dirichlet boundary condition d = 0.99 on the line {(x, y)|ζ > |x−l1/4|}, where ζ parameter is chosen such that the damage d = 0.99 is enforced at least over whole width of one element. the simplified but still sufficient analysis is performed in this benchmark using so called hybrid approach [7] for phase-field. furthermore, the damage d and updated matrix k̃ from (36) is calculated in each third step to speed up analysis. this approach allows some minor wave propagation through crack, but wave dispersion through crack is still significant. the result also depends on the parameter ξ from (19)-(20). first of all, let us investigate case ξ = 0, therefore shear is not degraded. the results are plotted by solid lines in figure 3, where plate deflection along line {(x,y)|y=0.75} is plotted in time instant t = 0.003 s. the square-topology mesh m1 m2 m3 figure 4. three investigated meshes. behaves reasonable, but triangle-topology mesh does not stop the wave on crack interface. it is caused probably due to element locking. on the other hand, triangles with ξ = 1 behaves almost like squares with ξ = 0 as a result of shear degradation by damage. it significantly reduces the shear locking artifacts. based on these conclusions, the triangle-based mesh with shear reduced by degradation function is used in the next section. 6. results the calculations are time-consuming and therefore only preliminary results of model p-vd are presented in this contribution. the model is still under investigation and there are still many unresolved issues. the same setup, defined in section 4, is used, i.e. the steel impactor hit the thin glass plate. three different meshes m1, m2 and m3 are tested to detect possible mesh-dependency, see figure 4. the model with fully degraded kinematic variables, i.e. ξ = 1.0, kres = ks res = 0.0, does not behave correctly. the impactor hit the plate causing damage localization under impactor. the plate has zero stiffness and impactor flies through fully damaged elements and no longer influences the rest of plate. as a result, the cracks are not evolving further. this behavior is illustrated in figure 5 by solid line, where again the distribution of plate deflection along {(x, y)|y = 0.75} 102 vol. 34/2022 phase-field damage of laminated glass in explicit dynamics 0 0.2 0.4 0.6 0.8 0 1 2 3 ·10−3 x coordinate [m] d efl ec tin [m ] ks res = 0.0 ks res = 0.05 figure 5. deflection distribution on line {(x, y)|y = 0.75}. (a) (b) (c) (d) figure 6. damage distribution on meshes m1 (a, c) and m2 (b, d) for resolution 50 × 50 (a, b) and 100× 100 (c, d). the blue represents intact material whereas red represents d close to 1. and in time t = 1.4 ms is plotted. the example is evaluated on mesh m3. the sort of way-out can be shear coupling in damage state, i.e. kres = 0.0, ks res 6= 0. it prevents large slope of deflection curve, see dashed line in figure 5, where ks res = 0.05. the observed behavior is more pleasant and it leads to crack evolving as presented below. moreover the crack is not thickening after additional load. in contrast with that, the case kres 6= 0 causes thickening. mesh dependency the meshes m1, m2 and m3 in two resolutions 50× 50 and 100× 100 was used to investigate mesh dependency. the setup is again the same with constant time step ∆t = 2 ·10−7 s and final time instant tn = 0.01 s. the results of damage fields are plotted in figure 6, where (a) is mesh m1 50× 50, (b) is mesh m2 50× 50, (c) is mesh m1 100× 100 and finally (d) is mesh m2 100× 100 all evaluated in end time. the mesh m3 behaves similarly as m1 and is not presented here. there is evident that crack topologies are different on coarse mesh, however this discrepancy is removed figure 7. visualisation of plate deflection. after refinement. the slight mesh dependency can still remain in phase-field models for unstable solutions, but it still remains more or less objective approach in contrast with other techniques. time refinement the damage solution (d) in figure 6 and its visualisation of deflection in figure 7 is therefore appropriate and objective without any further improvement during time refinement as seen in the graph 8. the red and blue line are almost identical. an interesting parasitic behavior appears when the residual stiffness is prescribed also to the bending members (kres = 0.01), see black line in graph 8. the residual term causes unwanted harmonic oscillations between impactor and plate. 0 2 4 6 8 ·10−3 −400 −200 0 time t [s] im p. ac c. [m /s 2 ] ∆t = 2 · 10−7, kres 6= 0 ∆t = 1 · 10−7, kres = 0 ∆t = 2 · 10−7, kres = 0 0 0.2 0.4 0.6 0.8 1 ·10−3 −350 −300 −250 −200 time t [s] im p. ac c. [m /s 2 ] detail figure 8. evolution of impactor acceleration for two different time steps (red and blue) and for case kres 6= 0 (black) with detail below. crack speed the main drawback of presented approach is its inability to correctly predict crack velocity. it is evident from near-the-failure evolution of 103 jaroslav schmidt, tomáš janda acta polytechnica ctu proceedings t = 0.00492 s t = 0.00522 s 0.5135 m 0.7210 m figure 9. crack development in two different time instants for mesh m1 100× 100. crack in figure 9. the crack length increment divided by time increment gives us the numerically predicted crack speed vpf = 692 m/s. however the crack speed of glass is about vreal ≈ 1500 m/s [[8]], which is still twice as much. 7. conclusion the explicit variationally consistent dynamic phasefield damage model was presented in this contribution with additional implementation details and preliminary results. the plate model with volumetricdeviatoric decomposition was successfully implemented, but it still exhibits some issues – especially wrong crack velocity and no crack branching. this two facts may be related, because crack branching is caused by large crack velocity. it is under further investigation as well as implementation of other models (particularly p-sd). acknowledgements this publication was supported by the czech science foundation, the grant no. 19-15326s and by the grant agency of the czech technical university in prague, grant no. sgs21/038/ohk1/1t/11. references [1] b. bourdin, g. a. francfort, j.-j. marigo. the variational approach to fracture. journal of elasticity 91(1):5–148, 2008. [2] m. hofacker, c. miehe. continuum phase field modeling of dynamic fracture: variational principles and staggered fe implementation. international journal of fracture 178(1):113–129, 2012. [3] y. shen, m. mollaali, y. li, et al. implementation details for the phase field approaches to fracture. journal of shanghai jiaotong university (science) 23(1):166–174, 2018. [4] k. pham, h. amor, j.-j. marigo, c. maurini. gradient damage models and their use to approximate brittle fracture. international journal of damage mechanics 20(4):618–652, 2011. [5] j.-y. wu, v. p. nguyen, c. t. nguyen, et al. phase-field modeling of fracture. advances in applied mechanics 53:1–183, 2020. [6] a. zemanová, p. konrád, p. hála, et al. experimental study on the gradual fracture of layers in multi-layer laminated glass plates under low-velocity impact. arxiv preprint arxiv:210712151 2021. [7] m. ambati, t. gerasimov, l. de lorenzis. a review on phase-field models of brittle fracture and a new fast hybrid formulation. computational mechanics 55(2):383–405, 2015. [8] f. barstow, h. edgerton. glass-fracture velocity. journal of the american ceramic society 22(1-12):302–307, 1939. 104 acta polytechnica ctu proceedings 34:98–104, 2022 1 introduction 2 phase-field model 2.1 plate models 2.2 discrete dynamic model 2.3 nonlinear hertz force 3 implementation 4 setup 5 benchmarks 6 results 7 conclusion acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.38.0502 acta polytechnica ctu proceedings 38:502–508, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague evaluation of thermal and mechanical properties of demonstration wall utilizing phase change cementitious materials jan červenkaa,∗, michaela herzfeldta, antonio caggianob, eduardus koendersb a červenka consulting s.r.o., na hřebenkách 55, 150 00 prague 5, czech republic b technische universität darmstadt, franziska-braun-straße 3, 642 87 darmstadt, germany ∗ corresponding author: jan.cervenka@cervenka.cz abstract. international project poropcm involves partners from germany, czech republic, spain and japan with the objective to develop new multifunctional phase change materials modified porous cementitious nanocomposite (poropcm). such material can be utilized for storing heat energy in the insulation layer of buildings compared to commonly used insulation materials since the phase change increases heat capacity. this enhanced feature reduces the amount of energy necessary for running the heating/cooling system. for the testing of the newly developed phase change cementitious composite a demonstration wall will be developed and tested for its thermal as well as mechanical performance. the topic of the paper is the description of the properties of the new phase change cementitious nanocomposite. the main emphasis of the paper is the description of the demonstration wall behaviour under typical environmental conditions. the wall design is supported by numerical simulation of the wall physical parameters. the numerical modelling involves the definition of suitable numerical models for the simulation of the thermal properties of the new phase change nanocomposite. the numerical model is then used to demonstrate the performance of the wall layer design. the presented pilot results show efficiency increase of the insulation material in the range 15–70 %. also modelling of wind resistance of the layered structure is included. the developed wall design and poropcm material will be tested and verified by a large scale test in the final year of the project. keywords: phase change material, cementitious nanocomposite, insulation performance, thermal analysis. 1. introduction the energy demand for heating and cooling of the global building stock represents a massive part of the total energy consumption around the world (≈ 40 %). in the eu, it accounts for about half its energy consumption [1]. to attenuate demand, thermal insulation of construction and building elements, like walls, roofs and floors, has become the most important measure to enhance energy savings of the new and existing building stock. in order to comply with the european targets on energy savings, the german federal government will merge the rules that currently apply in parallel: (i) the energy saving act (eneg), (ii) the energy saving regulation (enev), and (iii) the renewable energy heat act (eewärmeg), into a new building energy act (geg) [2]. these stricter regulations have led to extreme thicknesses (even up to 40 cm) of the standard insulation layers, mostly attached to the outer surface of the building envelope. possible innovative solutions could include the potential of materials to embody large amounts of thermal energy, which would stabilize the inner thermal comfort and leads to a strong reduction of the insulation thickness, like a thin layer [3]. a maximization of the synergetic interplay between thermal conductivity and the potential to store/release large amounts of solar and environmental heating/cooling energy into components of the building envelope can be achieved by an effective use of phase change materials (pcms), which minimizes the additional need of primary energy for heating/cooling [4, 5]. nowadays, numerous articles on pcms have been reported in literature, in several fields of applications, and most of them of scientific nature. the experimental-based research is predominantly addressing the thermo-hydro-chemo-mechanical properties of cementitious materials with pcms [6–8]. in those works, the investigated pcms are characterized by a melting temperature that varies between 19 °c and 26 °c, which corresponds to a standard temperature range for comfortable living [9]. according to their chemical compositions, pcms can be categorized as organic(o) (paraffin and non-paraffin), inorganic (i) compounds (salt hydrate and metallic) and eutectics (o-o, i-i, i-o) [10]. pcms can also be identified by their mode of phase transition: liquid-gas, solid-gas, solid-liquid and solid-solid. solid-liquid is the pre502 https://doi.org/10.14311/app.2022.38.0502 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 38/2022 evaluation of thermal and mechanical properties . . . (a). samples of the poropcm material. (b). structure in the electronic microscope. figure 1. poropcm material samples and detail of the structure. figure 2. individual layers of demonstration wall with the locations of temperature monitoring points (dimensions in meters). ferred mode for building applications with thermal energy storage (tes) [11]. the poropcm system is proposing a highly innovative insulation system for energy savings of new and/or existing buildings. it is made of ultra-light porous cementitious foam blended with pcm, see figure 1. it is sustainable, non-flammable and fully recyclable. the major difference, with the current practice, is that the proposed insulation system combines the benefits of a minimum thermal conductivity (λ-value [12–14]) with the capability to embody large amounts of thermal energy that will stabilize the inner climate of buildings. by doing this, a very thin insulation layer can be achieved having similar thermal insulation properties as a regular layer, made of eps, xps, or similar. this innovative system has an extremely low co2 footprint, a very low raw materials use, is non-toxics, acoustic absorbing, dimensionally stable, and not affecting the indoor air quality [15, 16]. 2. phase change cementitious material since the development of porous pcm insulator is basically new concept the simulation of insulation performance is a useful tool for developing a suitable wall design. the investigated wall design represents typical system with heavyweight load-bearing part of the wall made of reinforced concrete (figure 2). in a preliminary study, mainly numerical tests of the insulation performance have been carried out on lightweight and heavyweight structures in different locations all over the world. more details on this initial study are available in [17]. those preliminary results helped to distinguish different porous pcm materials and optimal performance at different conditions. the condition that varied were mainly location, internal air-conditioning, heavy or lightweight system and heating or cooling season or the combination throughout whole year. based on the result of this internal study the porous pcm materials with specific thermal properties have been 503 j. červenka, m. herzfeldt, a. caggiano, e. koenders acta polytechnica ctu proceedings (a). heat capacity of materials 748, 336 and eps. (b). thermal conductivity of materials 748, 336 and eps. figure 3. thermal properties of studied materials. material melting point [°c] heat storage capacity [j/g] liquid density [g/ml] rt24 24 160 0.77 pt4 5 187 0.88 table 1. pure pcm thermal properties. chosen. for the analyses presented in this paper, the two materials with large differences in their thermal properties have been chosen, specifically, materials with ids 336 and 748. these materials are part of numerically conceived database developed by cimec containing two types of pcm, puretemp® (pt) and rubitherm® (rt). the id 336 refers to material with zero porosity containing air but 40 % of microencapsulated pcm fraction in cement paste (rt24). contrary to that, the id 748 describes very porous material with porosity 96.6 % and 40 % of microencapsulated pcm fraction (pt4), see table 1. in contrast to the usual structural materials that have constant thermal properties, in the new porous pcm materials the thermal properties vary with the temperature. in the following calculations, the porous pcm materials are going to be compared to standard expanded polystyrene (eps) insulation material. the time dependent thermal properties of pcm materials are depicted in figure 3. regarding the mechanical testing of poropcm, preliminary tests have been carried out using cube samples of size 10 × 10 × 10 cm. the initial portion of pcm within cementitious foam samples is 20 % of volume. bulk density of samples is 200 kg/m3 and the testing shows compressive strength of samples of about 0.2 mpa. 3. demonstration wall description the demonstration wall design used for the initial study of the pcm material performance is shown in figure 2. the wall composition is formed with five layers, namely concrete (200 mm), glue (3 mm), insulation (150 mm), base coat (5 mm) and finish coat (3 mm) (as it goes from inside to outside). the thickness and thermal properties of separate layers are described in table 2 and in figure 2. 4. numerical modelling – thermal analysis in the numerical modelling with atena software [18], the heat transport in the wall structure as shown in figure 2 is simulated considering too scenarios of cold and warm conditions. for the cold case, the outside temperature is assumed to cycle between -5 to -11 °c. the temperature in the interior cycles between 18 to 21 °c. the heat convection coefficient for the interior surface was assumed at 7.7 w/(c m2). for the exterior surface thermal and energy flux condition was assumed as described in figure 4. the heat convection coefficients for the exterior surface were assumed at 20 w/(c m2). the results for the simulation of up to 10 temperature cycles starting from initial constant temperature of 20 °c are shown in the subsequent figures. figure 5 shows the results for the case of cold boundary conditions. the first set of graphs at figure 5 show the comparison of temperature evolution at various distances from the exterior surface for eps and pcm material. figure 6a compares the evolution of thermal flux at the interior surface, which corresponds to the energy (heating) input necessary to keep the optimal interior temperature of 18–21 °c. the similar comparison is provided at figure 6b for the case of warm boundary conditions. 504 vol. 38/2022 evaluation of thermal and mechanical properties . . . material thickness specific heat thermal conductivity [mm] [mj/m3/k] [w/m/k] reinforced concrete 200 2.55 2.1 glue 3 2.55 0.7 insulation 150 varies varies base coat 5 2.55 0.7 finish coat 3 2.55 0.7 table 2. wall composition materials and their properties. figure 4. boundary conditions on the exterior wall surface. (a). (b). figure 5. temperature evolution for eps (a) and pcm-748 (b) insulation at various distances (see figure 2) from outer surface for the heating case with the cold conditions. (a). (b). figure 6. comparison of energy flux on the interior heated surface for eps and pcm-748 material for the cold case with heating and pcm-336 material for the warm case with cooling. 505 j. červenka, m. herzfeldt, a. caggiano, e. koenders acta polytechnica ctu proceedings (a). (b). figure 7. temperature evolution for eps (a) and pcm-336 (b) insulation at various distances from outer surface for the warm case with cooling. (a). (b). figure 8. comparison of temperature distribution at peak outer surface temperature for eps (a) and pcm-336 (b) material for the warm condition case with cooling during day 9 of simulation (9.29 days = 80300 s). the comparison of cold temperature evolution (figure 5) does not show significant differences for the two materials. the differences are more pronounced if the energy input (flux) at the interior wall is compared in figure 6a, which clearly shows approx. 15 % lower thermal energy input for the case of pcm material. similar results can be observed also for the case of warm boundary conditions. in this case significant differences are observed also in the graphs of temperature evolution at various distances in figure 7b. it is possible to observe stronger inertia in the temperature evolution for the pcm material, while large temperature oscillations appear for the standard eps material. this higher thermal inertia can be also observed in the graph of thermal flux at the interior surface namely at the first 2–4 days of the thermal cycles. it shows that in the case of pcm material it is necessary to activate cooling system after almost 4 days of warm temperatures, while in the case of eps material the cooling needs to be activated already after two days. this difference in thermal behaviour is also well documented in figure 8, which compares the temperature profiles through the wall thickness at the peak exterior temperature. it clearly shows the better insulation behaviour of the pcm material. 5. numerical modelling – wind resistance the demonstration wall is also tested on mechanical performance under the wind suction load. due to the selected location (sofia, bulgaria), where future real case experiments are planned, the extreme wind suction is assumed as 1.01 kpa. the demonstration wall is considered with mechanical anchors that are located between poropcm insulation boards within joints as depicted in figure 9. there are three mechanisms that prevent the failure of the structure under wind load conditions: steel anchors, glue between reinforced concrete and poropcm boards and the poropcm material integrity. the anchor pull-out strength is assumed 1 kn and the glue 506 vol. 38/2022 evaluation of thermal and mechanical properties . . . (a). distribution of anchors. (b). deformed structure showing displacement in the wind load direction. figure 9. wall structure with attached insulation tested for wind load. (a). maximal principal stress with crack pattern. (b). the displacement in the wind suction direction. figure 10. structure response to wind load. tensile strength is assumed 0.3 mpa. the poropcm compressive strength was determined by first pilot experiments to be about 0.2 mpa, the tensile strength is assumed as tenth of the compressive strength which results in rather low value of tensile strength 0.02 mpa. the simulation shows the failure of pcm material integrity under the wind load. the design wind load (1.01 kpa) is exceeded more than 65 times by the resistance of the wall (65.7 kpa), as shown in figure 10. 6. conclusions the paper presents the first results from the international project poropcm (https://poropcm.eu/). in this project the first mix designs of optimized porous pcm materials were developed for various climatic conditions. the paper describes in more detail the thermal performance of two selected materials suitable for european conditions. the performance of the two selected materials suitable for warm and cold climates demonstrated in mainly cooling and heating conditions. these pilot results show about 15 % savings in heating energy for the cold conditions. very good performance is observed in the warm/cooling case. the time when cooling needs to be activated when warm weather arrives is extended by about 70 % compared to standard eps material. the performance of the pcm material is especially superior to eps when hot periods are alternating with cooler ones or hot days with cooler nights. if hot periods extent over more than 6 days, the advantage of the pcm material is not so pronounced. also the wind resistance of demonstration wall is tested to prove that the resistance for wind load exceeds at least 65 times the expected wind loads. acknowledgements the presented results were obtained during the project 8i19001 “functional porous cementitious nanocompos507 https://poropcm.eu/ j. červenka, m. herzfeldt, a. caggiano, e. koenders acta polytechnica ctu proceedings ites for heat storage in buildings using phase change materials” under the program eig concert japan. references [1] european commission. an eu strategy on heating and cooling, 2016. com/2016/051 final, [2021-09-06]. https://eur-lex.europa.eu/legal-content/en/txt/ ?qid=1575551754568&uri=celex:52016dc0051 [2] federal ministry for economic affairs and climate action. enhancing energy efficiency in buildings. [202109-10]. https://www.bmwi.de/redaktion/en/dossier/ enhancing-energy-efficiency-in-buildings.html [3] k. o. lee, m. a. medina, e. raith, x. sun. assessing the integration of a thin phase change material (pcm) layer in a residential building wall for heat transfer reduction and management. applied energy 137:699–706, 2015. https://doi.org/10.1016/j.apenergy.2014.09.003 [4] l. f. cabeza. 2 – advances in thermal energy storage systems: methods and applications. in l. f. cabeza (ed.), advances in thermal energy storage systems (second edition), woodhead publishing series in energy, pp. 37–54. woodhead publishing, second edition edn., 2021. https://doi.org/10.1016/b978-012-819885-8.00002-4 [5] p. sukontasukkul, p. uthaichotirat, t. sangpet, et al. thermal properties of lightweight concrete incorporating high contents of phase change materials. construction and building materials 207:431–439, 2019. https: //doi.org/10.1016/j.conbuildmat.2019.02.152 [6] m. kenisarin, k. mahkamov. passive thermal control in residential buildings using phase change materials. renewable and sustainable energy reviews 55:371–398, 2016. https://doi.org/10.1016/j.rser.2015.10.128 [7] a. d’alessandro, a. l. pisello, c. fabiani, et al. multifunctional smart concretes with novel phase change materials: mechanical and thermo-energy investigation. applied energy 212:1448–1461, 2018. https://doi.org/10.1016/j.apenergy.2018.01.014 [8] m. bahrar, z. i. djamai, m. el mankibi, et al. numerical and experimental study on the use of microencapsulated phase change materials (pcms) in textile reinforced concrete panels for energy storage. sustainable cities and society 41:455–468, 2018. https://doi.org/10.1016/j.scs.2018.06.014 [9] a. s. silva, e. ghisi, r. lamberts. performance evaluation of long-term thermal comfort indices in building simulation according to ashrae standard 55. building and environment 102:95–115, 2016. https://doi.org/10.1016/j.buildenv.2016.03.004 [10] f. souayfane, f. fardoun, p.-h. biwole. phase change materials (pcm) for cooling applications in buildings: a review. energy and buildings 129:396–431, 2016. https://doi.org/10.1016/j.enbuild.2016.04.006 [11] c. vélez, m. khayet, j. ortiz de zárate. temperature-dependent thermal properties of solid/liquid phase change even-numbered n-alkanes: n-hexadecane, n-octadecane and n-eicosane. applied energy 143:383–394, 2015. https://doi.org/10.1016/j.apenergy.2015.01.054 [12] x. lu, a. m. memari. comparative study of hot box test method using laboratory evaluation of thermal properties of a given building envelope system type. energy and buildings 178:130–139, 2018. https://doi.org/10.1016/j.enbuild.2018.08.044 [13] a. jiang, a. o’meara. accommodating thermal features of commercial building systems to mitigate energy consumption in florida due to global climate change. energy and buildings 179:86–98, 2018. https://doi.org/10.1016/j.enbuild.2018.08.046 [14] g. woltman, m. noel, a. fam. experimental and numerical investigations of thermal properties of insulated concrete sandwich panels with fiberglass shear connectors. energy and buildings 145:22–31, 2017. https://doi.org/10.1016/j.enbuild.2017.04.007 [15] a. gilka-bötzow. stabilität von ultraleichten schaumbetonen: betrachtung instationärer porenstrukturen. springer-verlag, 2017. 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[17] v. d. fachinotti, f. bre, c. mankel, et al. optimization of multilayered walls for building envelopes including pcm-based composites. materials 13(12):2787, 2020. https://doi.org/10.3390/ma13122787 [18] v. červenka, l. jendele, j. červenka. atena program documentation, 2021. červenka consulting, prague, 2021, [2021-08-19]. https://www.cervenka.cz/ 508 https://eur-lex.europa.eu/legal-content/en/txt/?qid=1575551754568&uri=celex:52016dc0051 https://eur-lex.europa.eu/legal-content/en/txt/?qid=1575551754568&uri=celex:52016dc0051 https://www.bmwi.de/redaktion/en/dossier/enhancing-energy-efficiency-in-buildings.html https://www.bmwi.de/redaktion/en/dossier/enhancing-energy-efficiency-in-buildings.html https://doi.org/10.1016/j.apenergy.2014.09.003 https://doi.org/10.1016/b978-0-12-819885-8.00002-4 https://doi.org/10.1016/b978-0-12-819885-8.00002-4 https://doi.org/10.1016/j.conbuildmat.2019.02.152 https://doi.org/10.1016/j.conbuildmat.2019.02.152 https://doi.org/10.1016/j.rser.2015.10.128 https://doi.org/10.1016/j.apenergy.2018.01.014 https://doi.org/10.1016/j.scs.2018.06.014 https://doi.org/10.1016/j.buildenv.2016.03.004 https://doi.org/10.1016/j.enbuild.2016.04.006 https://doi.org/10.1016/j.apenergy.2015.01.054 https://doi.org/10.1016/j.enbuild.2018.08.044 https://doi.org/10.1016/j.enbuild.2018.08.046 https://doi.org/10.1016/j.enbuild.2017.04.007 https://doi.org/10.3390/ma13122787 https://www.cervenka.cz/ acta polytechnica ctu proceedings 38:502–508, 2022 1 introduction 2 phase change cementitious material 3 demonstration wall description 4 numerical modelling – thermal analysis 5 numerical modelling – wind resistance 6 conclusions acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.38.0375 acta polytechnica ctu proceedings 38:375–381, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague solar shelter: exploring architectural design input for industrially-crafted shading devices ulrich j. ponta,∗, sigrun swobodab, peter schoberc a tu wien, institute of architectural science, department of building physics and building ecology, karlsplatz 13, a-1040 vienna, austria b tu wien, institute of architectural science, department of digital architecture and planning, karlsplatz 13, a-1040 vienna, austria c austrian forest products research society (hfa-ögh), franz-grill-straße 7 a-1030 vienna, austria ∗ corresponding author: ulrich.pont@tuwien.ac.at abstract. european cities are strongly affected by the urban heat island effect and climate change impacts in recent and future summer periods. as such, the integration of passive cooling strategies is of increasing importance for the overall aec (architecture-engineering-construction) domain. amongst other strategies, one of the most prominent strategies is the timely deployment of shading devices. while the shading device constructions connected to the inside, interstitial position or exterior of transparent building envelope components is thus an important part of the building, little change can be observed in the principal design and technology of such shading elements. moreover, target conflicts between shading device deployment and architectural appearance can be observed in european cities. for instance, the ensemble protection of the vienna city morphology was – for a long time – considered to be more important than potent shading devices. the present contribution presents the approach of an effort toward highly-esthetical, functional shading devices that provide – despite their architectural-design approach – a high degree of shading performance. together with the window-building industry, four different shading device designs have been designed, developed and virtually tested. the design process was guided by critical reviews by domain experts and performance simulation efforts. we present the genesis of the four designs and their performance implications as a proof of concept that effective shading systems can be understood as part of the esthetics of a building and practical passive cooling device at the same time. keywords: shading devices, summer overheating, passive cooling strategies, industry-university collaboration, urban heat islands, climate change, cooling demand reduction. 1. introduction summer overheating has become a prominent topic in recent years, which can be seen by the rising number of scientific research, publications, and the action plans of different cities and regions to react to summer overheating. once specific topics reached the attention of governments, regularly it can be assumed that the issue is not any longer just an academic one. while the analysis of the impact of co2 emissions on global temperatures can be traced back to the 19th century [1], the information of the general public of rising temperatures is a matter of the second half of the 20th century [2–5]. moreover, most people – by feeling – would say they did not identify the environmental issue of rising temperatures as a severe one before the millennium change to the 21st century. other environmental issues, however, were a matter of public interest and also international action was taken against these issues: this is true for the hole in the ozone layer [6, 7], and on the forest dieback phenomena [8]. both matters were identified to be majorly caused by emission of certain harmful substances that reacted with atmospheric components to acid or diluting substances. in contrast to the mentioned environmental threats, climate change and urban heat island effects has rather seldomly popped up in the public perception. one of the first occurrences might have been the summer heat wave of central europe in 2003, which showed the high vulnerability of urban citizens against overheating, in detail for people who themselves are part of vulnerable groups due to their age, health situation, or social status [9]. studies and reports reckoned that the heat wave caused up to 70 000 people’s death [10]. it might be clear that fighting global warming by de-carbonization is somehow a half-lost battle, even if politicians proudly announce successes such as the paris treaty [11], and its different follow up international treaties. while the concepts and ideas for reducing emissions, decarbonisation of industry, economy, and society are discussed and memoranda of understanding are signed and declared as path guide by decision makers, the reality is different and still shows rising co2 emissions [12] and thus a clear and present risk of reaching tipping points which would inevitably move mankind and nature into a hot-house earth scenario [13]. urban citizens are stricken not only by the climate change itself, but also 375 https://doi.org/10.14311/app.2022.38.0375 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en u. pont, s. swoboda, p. schober acta polytechnica ctu proceedings by the so called urban heat island effect. this term describes the observation, that urban agglomerations tend to show higher temperatures and also accelerated heating up in heat waves, which is widely due to reduced vegetation, use of materials of low albedo and high energy storing potential, and – a major aspect – anthropogenic heat emissions by buildings, traffic, and systems. between 2011 and 2014, a central european research project was conducted by a large consortium, which intended to identify sources and extent of the urban heat islands in european metropolitan regions [14], and – moreover – to develop strategies in mitigation [15]. mitigation refers to minimizing the uhi effect as such by specific urban interventions, such as a balanced ratio of buildings and greenery, implementing greenery in building facades and roofs, minimizing unnecessary heat emissions, and utilizing the benefits of deployed green-blue infrastructure. if mitigation also refers to minimizing the harmful effects of heat waves, urban heat islands, and climate change onto citizens, the scale of interventions can be described as from “general city master planning” to “small scale impacts at the individual residential unit”. the recently published guidebook on cooling of cities edited by the un [16] clearly defines passive cooling strategies such as adapted design of the buildings regarding orientation, morphology and ratio of transparent components, careful operation of the buildings, and education of the occupants as key elements in fighting overheating. needless to say, architects play a key role in implementation of such passive cooling strategies. previous studies, however, identified that while the selection of appropriate building materials for summer overheating protection is of importance, it is by far outnumbered by the impact of the occupants’ behaviour regarding shading deployment and ventilation [17, 18]. while ventilation via windows can be considered as easy-to-implement passive strategy, effective night-time cooling might require semiautomated windows that employ a ventilation-only position and automated opening and closing, as for instance suggested in [19]. shading devices can be fixed or moveable/deployable. as of today, many architects consider shading device implementation as a severe aesthetical issue for new building designs, but also for adaptation of the existing building stock. this has – in part – to do with the rather “functional” design of deployable shading devices. while architects often dream of performance-oriented architecture far away from today’s built practice (compare [20, 21]), a close-by industry designing of shading devices rarely can be found in academia or in the architectural practices of today. the authors and the windowand shading-device building industry therefore decided to ask graduate students of architecture to rethink today’s available shading devices. the present contribution presents the methodology and the results of these efforts and shall thereby act as a proof that solar shading can be aesthetically pleasing and highly performative at the same time on the one hand, and on the other hand as a showcase of the reached design solutions. 2. methodology the overall efforts were conducted within a 5 ects design studio for graduate students of architecture. the task was to develop non-of-the shelf shading devices for both existing, richly-decorated gründerzeit-buildings as can be found in many european cities, and new (generic buildings). thereby, both design, functionality and performance impact was set for in-depth explorations during the design studio. naturally, architects are not full-fledged engineers, but generalists and designers. as such, the students were supported by three instructors of architectural and building construction engineering background. moreover, during the semester, three review sessions with experts from the windowand shading-construction industries were held, which allowed the students to receive a swot (strengths-weakness-opportunities-threats) analysis of their concepts and inspiration and guidance for the further development. beyond the design and construction integration, the participating students were asked to consider the aspects of kinematics and performance impact. while the kinematics required animation or at least “thumb cinema” illutstrations that show the state of the system in any state during the operation, the performance impact on the summer overheating of buildings was emulated via the generation of virtual buildings that employed the corresponding design and a thermal building performance simulation via the simulation process described in the austrian standard for summer overheating [22]. these simulation efforts were conducted with the software archiphysik in version 17 [23]. students were asked to generate presentations for the austrian community of window constructing companies (plattform fenster), which evaluated the projects and acknowledged the principle necessity of rethinking shading devices. 3. results and discussion in this section the four different designs are presented briefly, including their potential application domain (new windows / new buildings or building stock / existing windows), and performance impact. thereby advantages and potential challenges are highlighted as well. 3.1. project 1: rutatus-blende (“rutatus – shutters”) the defined aims of this design were as follows: (i) integration in window-retroffiting efforts, (ii) development of a casement window upgrading kit, (iii) compatible in aesthetics to traditional gründerzeit facades, 376 vol. 38/2022 solar shelter: exploring architectural design input . . . figure 1. project 1: rutatus-blende: upper left: functional principle, lower left: façade view be integration in a gründerzeit façade, sequence on the right: shading effect / sun penetration reduction (rendering). (iv) inexpensiveness in investment and operation (maintainance). given these goals, some design decisions could be made, such as the positioning of the shading device at the outside of the window plane, the manual operation (no motorization), and a specific identity in the design. the final design was inspired by claws grid systems, which for a very long time have been used as anti-burglary devices in front of doors and windows. while this systematic in its traditional form never was intended as a “shading device”, the key further development of the project was to find possibilities of attaching fabric that does not hamper the kinematic of the system but unfolds for cutting short of sun penetration and glare. figure 1 illustrates the rutatus-blende principles. the strong part of this project is for sure in its easy to implement concept as a frame that can simply be set into the window niche. the aesthetical impact on gründerzeit facades might be an acceptable one, in comparison to traditional jalousies. moreover, the shading device can be deployed independently from the state of the window as opened or closed (given that we have turn windows opening to the inside). this means that sun shading and ventilation would in principle be possible at the same time. an integration in the interstitial space of casement windows can be considered. however, some shortcomings of the design need to be mentioned as well: the shading device is – due to its frame limitation – not useable to fully darken a room, some “open” spots remain. depending on the used canvas simulations showed reductions of 21 to 79 % of transmitted short wave radiation. as a result the amplitude of indoor temperature curves over 24 hours can be significantly lowered, if the shading device is deployed during daytime in summer heat waves. 377 u. pont, s. swoboda, p. schober acta polytechnica ctu proceedings figure 2. project 2: whirlshade: upper row: installation and opening sequence; lower row left: box with mechanics (pulleys, motorization); lower row right: principle construction of the foldable sail. 3.2. project 2: whirlshade this concept – illustrated in figure 2 – was inspired by the folding principles used for solar panels of satellites and space-faring vehicles. utilizing a box structure that can be mounted in line with additional insulation material on the outer perimeter of existing and new buildings’ walls close to the window, the wings of the shading system can be transformed from hidden to visible, to partly deployed and fully deployed. this step-wise deployment allows gradients of light penetration to the inside but also full shading. needless to say, the system of whirlshade offers a high degree of innovation and can – if applied accordingly – also provide a “sense-of-wonder” aesthetics, as for instance the façade of jean nouvels’ centre du monde arabe [24]. however, it shares also the shortcomings, which are a potential high degree of maintenance and thus connected cost and effort. one might argue that different deployable membrane-based shading devices also have a certain degree of complexity, as such the design would require a further technical fine-detailing to allow for a concluding evaluation of its usability. regarding the summer heating mitigation the system – if deployed at the right time with the right extent – provides full relief of summer overheating for typical residential rooms. similar to project one, the deployment of the system is independent of the opening state of the window behind. 3.3. project 3: sonnensterne (“sun and stars”) this concept has been developed based on innovation pertaining to “translucent” timber [25, 26]. this material can be generated by removing major parts of the lignine structure from the wood. the material provides slow bio-degradability (such as any other timber element), and as such will change over time. the design of the shading devices utilizing this material is based on vertical construction axis, which can be moved from a hidden position of the left and right perimeter of the window niche to central positions via a railing system. along these axis a set of rotation points in provided, which – in the basic version have one “part” of the star visible. similar to the concept of the solar flower [27], different parts of the system can be deployed via rotation around the central rotation points. as such gradients of shading are possible. different levels of overlap also form different light and radiative penetration barriers, allowing users to adopt their degree of shading accordingly. figure 3 illustrates the principles of the sonnensterne-system. advantages of the system include – similar to the other projects – the independence from the window open/closed state, the flexibility of the system regarding degree of shading on the one hand and the application possibilities on the other side: the system can also be used as an alternative to photochrome or electrochrome glass panes used for generating privacy at still transmitting light in a translucent fashion. such application could be used in fully glazed conference 378 vol. 38/2022 solar shelter: exploring architectural design input . . . figure 3. project 3: sonnensterne: upper rows: sequenze of different states of deployment; lower row: isometry and horizontal section of the technical principle of sonnensterne. rooms. similar to the other projects, simulations conducted in the framework of the development proofed that summer overheating could be significantly reduced by deployment of the sonnensterne system. disadvantages of the system encompass the complex kinematics, even if this was reduced to “sliding” and “rotating”, and potentially the sensitivity against wind pressure events, as the system is foreseen to be outside of the buildings and window niche perimeter. 3.4. project 4: fenster-krausen (“window neck brace”) this project is inspired by the historic neck braces worn by noble(wo)men of earlier centuries. this idea has been translated into a folding system that can be used as alternative to traditional deployable jalousies. similar to them, the degree of lowering and the position of the individual elements can be changed. however, the relative position of the single blinds is not just rotated but can be folded / unfolded to the neighbouring blinds. as a result the shading structure of this project provides a lively, irregularly pattern, comparable with the leaves of a tree and their diffusing effect on light and radiative transmission. performance-wise, the system is as capable as regular deployable blinds. challenges connected with this system include the rather complex fabric-pull-rope relation in the different parts of the shades. figure 4 illustrates the system schematically, as well as the gradients of shading possible. 4. conclusion the present contribution showcased four different innovative and non-of-the-shelf shading device ideas. two of them – fenster-krausen and rutatus-blende – seem to be widely adaptable in the sensible building stock of gründerzeit-buildings without shock-changing the overall appearance of such facades. given the necessity to provide sun shading also for these buildings, such ideas should be followed up more in detail. the two other projects – sonnensterne and whirlshade – are rather ambitious and innovative in their kinematic patterns but seem to be better suited for new buildings. all of the designs were examined towards their lighting and shading impact. whereas performative aspects in both domains could be satisfactorily fullfiled, open questions, such as potentially disturbing light/shadow patterns in the inside remain a matter to be worked upon. while this contribution on the one hand highlighted the early design phases of such shading devices, it highlighted that architectural designers can contribute to the mitigation of overheating also in a creative fashion, not only by purchasing and implementing the industrial products by now. on the other hand, the industry acknowledged the necessity to supplement the current offer of shading devices by high-performantive, aesthetically satisfying 379 u. pont, s. swoboda, p. schober acta polytechnica ctu proceedings figure 4. project 4: fenster-krausen: left: façade view, middle and right, upper row: fully closed system, middle and right, lower row: nature-inspired light / radiation transmission. and contemporary technology using shading possibilities. future research shall emphasize the collaboration between the industry and academia to intensify the knowledge exchange and thus allow to realize shading devices close to the presented ones of this contribution. acknowledgements the authors thank the participating students, who participated in the design studio solar shelter in winterterm 2020/2021, namely p.n. dettmer, a. iankov, c. ließ, and g. yilmaz. moreover, the authors want to thank the officals and contributors of the plattform fenster, who generously funded a student competition that was the basis for this design studio work. references [1] wikipedia. history of climate change science. 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[2021-06-01]. https://www.wiwo.de/technologie/green/faechermodul-mobile-solarblume-ersetzt-denstromanschluss-/13876018.html 381 https://www.corp.at/archive/corp2013_78.pdf https://www.unep.org/resources/report/beating-heat-sustainable-cooling-handbook-cities https://www.unep.org/resources/report/beating-heat-sustainable-cooling-handbook-cities https://www.glaswelt.de/top-thema/fensteroeffnungsarten-neu-gedacht-einfach-auf-die-seite-geschoben https://www.glaswelt.de/top-thema/fensteroeffnungsarten-neu-gedacht-einfach-auf-die-seite-geschoben https://www.glaswelt.de/top-thema/fensteroeffnungsarten-neu-gedacht-einfach-auf-die-seite-geschoben https://doi.org/10.4028/www.scientific.net/amm.887.227 https://doi.org/10.4028/www.scientific.net/amm.887.227 https://www.archiphysik.at/ https://en.wikipedia.org/wiki/institut_du_monde_arabe https://en.wikipedia.org/wiki/institut_du_monde_arabe https://www.db-bauzeitung.de/produkte/holzbau/transparentes-holz-woodoo/ https://www.db-bauzeitung.de/produkte/holzbau/transparentes-holz-woodoo/ https://en.wikipedia.org/wiki/transparent_wood_composite https://en.wikipedia.org/wiki/transparent_wood_composite https://www.wiwo.de/technologie/green/faecher-modul-mobile-solarblume-ersetzt-den-stromanschluss-/13876018.html https://www.wiwo.de/technologie/green/faecher-modul-mobile-solarblume-ersetzt-den-stromanschluss-/13876018.html https://www.wiwo.de/technologie/green/faecher-modul-mobile-solarblume-ersetzt-den-stromanschluss-/13876018.html acta polytechnica ctu proceedings 38:375–381, 2022 1 introduction 2 methodology 3 results and discussion 3.1 project 1: rutatus-blende (``rutatus – shutters'') 3.2 project 2: whirlshade 3.3 project 3: sonnensterne (``sun and stars'') 3.4 project 4: fenster-krausen (``window neck brace'') 4 conclusion acknowledgements references https://doi.org/10.14311/app.2022.33.0558 acta polytechnica ctu proceedings 33:558–563, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague concrete admixtures sustainable concrete oliver schwoon sika services ag, tueffenwies 16, 8048 zurich, switzerland correspondence: schwoon.oliver@ch.sika.com abstract. durability of concrete, an important part of the sustainability theme, was, is, and will be one of the main topics for large infrastructure projects like the gotthard base tunnel. for such important structures a service life of more than 80 years is expected. the prevailing underground conditions like water ingress, aggressive waters etc. will influence the choice of the concrete system and mix design. therefore, a focus is on the potential damage mechanisms on the concrete like chloride ingress or sulfate attack. a well-designed concrete will be durable concrete which can withstand various external attacks and achieves or even extends the designed service life of the concrete structure. based on the knowledge that the main damaging mechanisms for concrete are based on water penetration into the concrete, various measures can be taken to achieve concrete structures with significantly reduced permeability. these measure ranges from reduction of the water/binder-ratio, increasing the density of the concrete using silica fume, to the use of special admixtures in order to increase the durability of concrete. in an extensive research program, the influence of mix design components e.g. different admixture types, water/binder-ratio on hardened concrete properties like chloride resistance (chloride migration coefficient), sulfate resistance and water conductivity have been tested and analyzed. keywords: admixtures, chloride migration coefficient, mix design, sulfate resistance. 1. introduction the durability of concrete, an important part of the sustainability theme, was, is, and will be one of the main topics for large infrastructure projects like the gotthard base tunnel. for such important structures a service life of more than 80 years is expected. hardened concrete can be damaged by water containing sulfates which can occur in soil or dissolved in ground water. these sulfates reacts with the tricalcium aluminate hydrates (afm) in the cement and forms ettringite (also thaumasite under certain conditions). that leads to increases in volume and to high internal pressure in the concrete structure and therefore cracking and spalling occurs [1]. another aspect is on the protection of the reinforcement against chloride exposure or chloride ingree into the concrete. chloride ions can migrate through the concrete mainly by diffusion, interact with the passivation layer of the steel and leading subsequently to corrosion of the steel reinforcement. other ways of transportation of the chloride ions are capillary absorption as well as hydrostatic pressure [1]. the prevailing underground conditions like water ingress, aggressive waters etc. will influence the choice of the concrete system and mix design. therefore, a focus is on the potential damage mechanisms on the concrete like chloride ingress or sulfate attack. a well-designed concrete will be durable concrete which can withstand various external attacks and achieves or even extends the designed service life of the concrete structure. based on the knowledge that the main damaging mechanisms for concrete are based on water penetration into the concrete, various measures can be taken to achieve concrete structures with significantly reduced permeability. these measure ranges from reduction of the water/binderratio, increasing the density of the concrete using silica fume, to the use of special admixtures in order to increase the durability of concrete. the focus of this paper is on the use of special admixtures to create a durable and therefore sustainable concrete. 2. investigation and experimental set-up in an extensive research program, the influence of mix design components e.g. different admixture types, water/binder-ratio on hardened concrete properties like chloride resistance (chloride migration coefficient), sulfate resistance and water conductivity have been tested and analyzed. 2.1. testing methods the fresh concrete was assessed by the flow table spread (en 12350-5), density (en 12350-6) and air void content (en 123350-7) after mixing (5 minutes) and the flow table spread after 30 minutes. concrete cubes (150 × 150 × 150 mm) have been mad and evaluated after 28 days of curing (en 123902) according to sia 262/1 "betonbau-ergänzende festlegungen" annex a (water conductivity), b (chloride resistance) and d (sulfate resistance). limits of each test can be seen in table 1 and will be referred to in the following chapters. 558 https://doi.org/10.14311/app.2022.33.0558 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 33/2022 concrete admixtures sustainable concrete test limit sulfate resistance lengthening [!] ≤ 1.0 chloride resistance chloride migration coefficient [m2/s] ≤ 10 × 10−12 water conductivity ! g/ " m2.h #$ ≤ 1.0 table 1. limits according to sia 262/1 [2, 3]. mix design a mix design b mix design c mix design d cement type cem i 42.5 cem i 42.5 cem i 42.5 cem i 42.5 cement amount [kg/m3] 300 350 300 350 w/c-ratio 0.58 0.45 0.58 0.45 max. grain size [mm] 32 32 32 32 admixture 1 dosage [%] sika® viscocrete® 0.0-0.2 sika® viscocrete® 0.6-0.8 sika® viscocrete® 0.0-0.2 sika® viscocrete® 0.6-0.8 admixture 2 dosage [%] sikacontrol® crc 1.0 / 2.0 sikacontrol® crc 1.0 / 2.0 sikacontrol® wt 0.5 / 1.0 sikacontrol® wt 0.5 / 1.0 table 2. concrete mix design. 2.2. concrete mix design the mix design has been set for all trials except the needed amount of plasticizer or superplasticizer to achieve the aimed flow table spread (fts). fts has been targeted at around 50 ± 5 cm. trials were conducted at two water/cement ratios (w/cratio): 0.58 and 0.45. the trials have been conducted with two different admixture technologies called sikacontrol® crc and sikacontrol® wt. the first one achieves an improvement of the durability of the concrete by creating a more chemical resistant concrete, whereas the second one aims on the water resistance of the concrete. sikacontrol® crc is an admixture based partly on hydroxyl combinations and one of the main effects is the in reduction in dry shrinkage and therefore shrinkage cracks. sikacontrol® wt is a water resisting admixture which incorporates mainly pore blocking and hydrophobic technologies. these concrete mix designs used in this investigation can be seen in table 2. 3. results concrete mixes with and without the special admixtures has been tested on different w/c-ratios in order to assess the performance and influence of the admixtures. limits of the tests have been listed in table 1. 3.1. influence of the admixtures at a w/c-ration of 0.58 the results of the sulfate resistance, chloride resistance and water conductivity of the concrete cubes are shown in the following figure 1 to 5. it can be observed that a significant increase of the sulfate resistance and be achieved with already 1% of the sikacontrol® crc albeit not enough to stay below the set limits. an increase in dosage up to 2% let the cubes pass the test limits (figure 1). a similar trend is shown by the results of the chloride resistance tests. a reduction of the chloride migration coefficient can be realized with 1% of the sikacontrol® crc. but again, still not good enough to pass the test. as before an increase in dosage up to 2% results in staying below the limits (figure 2). only the concrete with the higher dosage of sikacontrol® wt using the mix design c stays below the limits of the sulfate resistance test (figure 3). the chloride migration coefficient is significantly reduced versus the reference by the use of sikacontrol® wt and both dosages are below the test limits but showing only minor improvement from an increase in dosage (figure 4). sikacontrol® wt displays a significantly reduction in water conductivity versus the reference with both dosages (figure 5). 3.2. influence of the admixtures at a w/c-ration of 0.45 the results of the sulfate resistance, chloride resistance and water conductivity of the concrete cubes are shown in the following figure 6 to 9. the reference concrete with a w/c-ratio of 0.45 passes the sulfate resistance test but leaves not much room for a margin of safety. whereas both dosages of sikacontrol® crc pass the test and provide a healthy margin of safety (figure 6). all cube of the test set of mix design b pass the chloride resistance test. the positive influence of the special admixture sikacontrol® crc can be seen in the results and the reduction of the chloride migration coefficient. further investigations are necessary regarding the higher results at the higher dosage against a lower dosage (figure 7). 559 oliver schwoon acta polytechnica ctu proceedings figure 1. mix design a: sulfate resistance. figure 2. mix design a: chloride resistance. figure 3. mix design c: sulfate resistance. 560 vol. 33/2022 concrete admixtures sustainable concrete figure 4. mix design c: chloride resistance. figure 5. mix design c: water conductivity. figure 6. mix design b: sulfate resistance. 561 oliver schwoon acta polytechnica ctu proceedings figure 7. mix design b: chloride resistance. figure 8. mix design d: sulfate resistance. figure 9. mix design d: water conductivity. 562 vol. 33/2022 concrete admixtures sustainable concrete figure 10. reference concrete with various w/c-ratios: sulfate resistance. figure 11. reference concrete with various w/c-ratios: chloride resistance. the use of mix design d incorporating the admixture sikacontrol® wt will pass the test at both tested dosages and show a significant improvement against the reference concrete (figure 8). as seen before sikacontrol® wt displays a significantly reduction in water conductivity versus the reference with both dosages (figure 9). 4. conclusions the investigation has shown that the use of these special admixtures like sikacontrol® crc and/or sikacontrol® wt improve the durability of a concrete in respect to chloride and sulfate resistance and/or water conductivity. it will allow the concrete designer to choose an appropriate w/c-ratio and still maintain a margin of safety in regard to the named durability tests. furthermore, the results of the many tests conducted showed that a certain variance within the test itself has to be accounted for and a margin of safety is necessary. figure 10 displays the results of the sulfate resistance test of a reference concrete with different w/c-ratios. the results of the trialled admixtures and mix designs are shown included in figure 10 as well. it should be noted that those are only the w/c-ratios of 0.58 and 0.45 connected by a straight line. especially at higher w/c-ratios a significant positive influence can be achieved by the use of the tested admixtures. as for mix designs with lower water w/cratios a higher margin of safety can be accomplished. same conclusion as before can be drawn when it comes to the chloride resistance and the tested mix designs and special admixtures. next to well-known measure like reduction of w/cratio, use of scm etc. in order to increase the durability and therefore the service life of a structure the tested special admixtures will give concrete designers, specifiers, contractors and/or concrete producers another valuable tool to fulfil the challenging requirements of important infrastructure projects or any other needed concrete. references [1] j. schlumpf, b. bicher, o. schwoon. sika concrete handbook, zurich: sika services ag, p. 158-181, 2020. [2] the swiss standards association. sia 262/1:2013 betonbau ergänzende festlegungen, zurich: schweizer ingenieurund architektenverein, 2013. [3] the swiss standards association. sn en 206:2013+a1:2016-c1:2019, zurich: schweizer ingenieurund architektenverein, 2013. 563 acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.36.0216 acta polytechnica ctu proceedings 36:216–223, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague a study on the effects of hidden safety when assessing existing structures max teichgräber∗, daniel straub technical university münich, school of engineering and design, engineering risk analysis grou, theresienstraße 90, 80333 münchen, germany ∗ corresponding author: max.teichgraeber@tum.de abstract. in many instances, the safety of existing structures can no longer be demonstrated by standard code-based assessments. reasons for this include changes in the code, changes in the demands on the structures and deterioration. to address this problem, it is common practice to perform a more detailed assessment utilizing advanced models. in this way, many structures can be shown to comply with safety requirements, even if they cannot be verified by standard assessments. the standard code models are often conservatively biased. this leads to designs which include hidden safety. if the reassessment is performed with more advanced models in lieu of standard models, the hidden safety can vanish. concurrently, the reduced uncertainty of advanced models may compensate this safety reduction. in this paper we investigate this issue on a hypothetical population of existing bridge structures under traffic. we consider that the standard code model is exchanged by an advanced traffic load simulation. keywords: code based design, existing bridge structures, hidden safety, traffic load. 1. introduction structural design codes aim to provide easy-to-use rules that lead to economical and safe structures [1]. today’s structural design codes are based mainly on the semi-probabilistic partial safety factor (psf) concept [2–5]. it contains two explicit safety components: the psfs and the characteristic values. both components are determined via code calibration [6–8]. the calibration is typically based on standard models and the determined safety components are intended to be used with these standard models. in many cases the standard models are biased in a conservative sense (e.g. [9]). this leads to another implicit safety component: the hidden safety. the presence of hidden safety is unproblematic, as long as standard models are applied; however, problems may arise if standard models are exchanged by more advanced models. thereby, two effects are present: first, advanced models are typically not biased; hence, the hidden safety vanishes. second, advanced models typically include less model uncertainty. when it comes to the design of new structures, these two effects influence the structural reliability in opposite directions. if one effect dominates, a recalibration of the explicit safety components of the psf concept is needed. the recalibration ensures that, on average, the standard and the advanced models lead to the same level of safety. we investigate this in detail in [9, 10]. in the assessment of existing structures both effects are present as well. the difference to new designs is that for existing structures the design choices are already made; hence, the advanced models are only used to reassess if a given design is in compliance with the requirements. whether or not the accepted design is safe enough is the question addressed in this paper. we perform a case study on a hypothetical population of existing bridge structures under traffic load. we consider an increase of the traffic load since the construction date of the bridges such that they are no longer in compliance with standard code-based assessments. we investigate how the application of advanced traffic load simulation affects the reliability. 2. design following the partial safety factor concept we follow the approach and the nomenclature of the eurocode [11]. the investigations can be transferred to other semi-probabilistic structural design codes. in a eurocode design, four different models can be identified (figure 1): the load model ml,ec , the structural model ms,ec , the material model mm,ec , and the resistance model mr,ec . we use the subscript ec to stress that these are the standard models provided by eurocode. the load model and the material model are typically probabilistic models. in order to make the design process deterministic, a characteristic load lk,ec and characteristic material properties mk,ec are determined. to consider the uncertainty of the load model and the material model and to ensure a sufficiently safe structure, the characteristic values are modified by the psfs γf and γm. the resulting values are the input of functions ts,ec and tr,ec provided by the structural model and the resistance model. to consider the uncertainty of these models, the outcome is 216 https://doi.org/10.14311/app.2022.36.0216 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 36/2022 a study on the effects of hidden safety load model ml,ec characteristic load lk,ec partial safety factor γf structural model ms,ec partial safety factor γsd design load effect ed,ec characteristic material mk,ec partial safety factor γm resistance model mr,ec partial safety factor γrd design resistance rd,ec material model mm,ec figure 1. overview of the eurocode design appraoch. again modified by psfs γsd and γrd. eventually, the design values ed,ec and rd,ec are obtained. a design is sufficient, if the design values fulfill the following inequality: ed,ec ≤ rd,ec (1) ⇔ γf · ts,ec(lk,ec · γsd) ≤ tr,ec ( mk,ec γm ) γrd for the sake of simplicity, the eurocode merges the partial safety factors of the action and the resistance side: γf = γf × γsd (2) γm = γm × γrd (3) in the subsequent case study we assume ts,ec and tr,ec to be linear functions through the origin. then equation 1 simplifies to: γf · ts,ec(lk,ec) ≤ tr,ec (mk,ec) γm (4) if the design is chosen optimally in regard to resource consumption, equality holds in equation 4. from this, pec is derived: pec = γf · γm · ts,ec(lk,ec) tr,ec (mk,ec) (5) pec can be interpreted as the design choice following the psf concept (e.g. the cross section area of a truss). the corresponding probability of failure can be determined via the following limit state function (lsf): g = pec · tr(m) − ts(l) (6) note that in equation 6 tr, ts , m and l are not subscripted with ec, hence, do not originate from the eurocode. m and l are the "true" – or as we call them: purely aleatoric [9] – distributions of the load and the material property. tr and ts represent the true relationship between the load and the material property to the load effect and the resistance. generally, these are unknown and have to be estimated carefully. the probability of failure pr(f ) results from pr(f ) = ∫ ωf fm,l(m, l) dm dl (7) where fm,l(m, l) is the joint probability density function (pdf) of m and l and ωf = {m, l | g < 0} is the failure domain. 3. case study: existing bridge structures under traffic load in the following we perform a case study on a hypothetical population of existing bridge structures under traffic load. 3.1. setup and assumptions we choose the following setup of the case study: • n = 10 000: number of samples of the population of bridge structures. this is needed, since some of the subsequent calculations are sample-based. apart from the statistical error originating from the sampling, the results are not effected by n. • we choose the material property m to follow a lognormal distribution: m ∼ ln e[m ] = 1 c. o. v.[m ] = 0.1 (8) the coefficient of variation of 0.1 is in the typical range of material properties (e.g. concrete compression strength) [12]. some material properties may have higher (e.g. timber bending strength) or lower coefficients of variation (e.g. structural steel yielding strength). therefore, we vary the coefficient of variation between 0.05 and 0.3 in a sensitivity analysis. • we choose the annual maximum of the traffic load l to follow a standardized gumbel distribution: l ∼ g e[l] = 1 c. o. v.[l] = 0.08 (9) the choice is based on simulated traffic following the approach of [13]: local traffic data from three locations of german highways with light, medium and heavy traffic is analyzed. on this bases, traffic streams (sequences of vehicles) are generated applying numerical traffic simulation. the simulated traffic load is applied on a portfolio of bridges. the portfolio includes bridges with 1-3 spans, each with 10, 15, or 20 m. on each bridge we apply either light, medium and heavy traffic. hence, the portfolio consists of 3 · 3 · 3 = 27 bridges. for each bridge a structural analysis is carried out and the resulting 100-year time histories of the internal forces are evaluated at all locations that are needed for the design. the annual block maxima of the time histories are taken to fit generalized extreme values distributions via the maximum-likelihood-method. the shape parameters of the fitted generalized extreme value distributions are close to 0, in which case the 217 max teichgräber, daniel straub acta polytechnica ctu proceedings generalized extreme value distribution converges to the gumbel distribution. the sample mean of the coefficient of variation of the field moments is 0.06, of the supporting moments 0.08 and of the shear forces at the supports 0.08. this justifies the choice of c. o. v[l] = 0.08. note that l is the purely aleatoric distribution; hence, c. o. v[l] = 0.08 only covers aleatoric uncertainties. epistemic uncertainties are added via the error in the estimation of the characteristic load (see θec and θadv below). we alter the coefficient of variation between 0.05 and 0.3 in a subsequent sensitivity analysis. a coefficient of variation of 0.3 is rather high in the context of traffic loads. by this, we ensure to also cover design situations with high uncertainty (e.g. due to rerouting of roads). • as already mentioned, we assume that the functions ts,ec (translates the loads to the load effects) and tr,ec (translates the load property to the resistance) are both linear and trough the origin. we further assume that both functions are equal to the true functions ts and tr. with this assumptions the functions ts,ec , ts , tr,ec and tr cancel each other out within the lsf (equation 6). • we assume a traffic load increase since the construction of the bridge by a factor of finc = 1.3. this value is not on the bases of any data but should be interpreted as a hypothetical increase. in an actual application case, this value must be derived respectively. we vary finc between 1 and 2 in a subsequent sensitivity analysis. • following the traffic load model of the eurocode [14], we define the characteristic traffic load as the 99.9% quantile of l (1000 year return period which corresponds to the probability of exceedance of 5% in 50 years). we model the error in the estimation of the characteristic value via a relative error: θadv = lk lk,adv (10) θec = lk lk,ec (11) where lk is the characteristic value of the purely aleatoric traffic load, lk,adv is the estimation of lk by means of advanced traffic load modeling and lk,ec is the estimation of lk following the eurocode. we choose θadv and θec to follow lognormal distributions: θadv ∼ ln e[θadv] = 1 c. o. v[θadv] = 0.1 (12) θec ∼ ln e[θec ] = 0.7 c. o. v[θec ] = 0.2 (13) the choices of the distribution-parameters are justified as follows: ▷ e[θadv] = 1: since the simulation is based on real data, it is reasonable to assume that it estimates the characteristic value without a bias. ▷ c. o. v[θadv] = 0.1: the coefficient of variation comprises three different uncertainties: the statistical uncertainty due to a limited simulation time, the uncertainty of the chosen distribution type and the uncertainty of incomplete modeling (e.g. construction sites on the highway may change the traffic load, but are not included in the simulation). the statistical uncertainty is estimated via the multivariate normal distribution of the parameter estimates of the fitted extreme value distributions of the inner forces [15]. the resulting coefficients of variation due to statistical uncertainty are in the range of 0.02-0.04. we – unfortunately – lack any data to estimate the uncertainty of the two other sources; hence, the overall uncertainty of c. o. v[θadv] = 0.1 is based on our subjective assessment. we alter c. o. v[θadv] between 0.05 and 0.2 in a subsequent sensitivity analysis. ▷ e[θec ] = 0.7: within the portfolio of bridge structures the sample mean of the relative error is 0.76 regarding field moments, 0.70 regarding supporting moments and 0.79 regarding the shear forces at the supports. we apply a value of 0.7 and alter e[θec ] = between 0.4 and 1.0 in a subsequent sensitivity analysis. ▷ c. o. v[θec ] = 0.2: θec can be rewritten as θec = lk lk,adv · lk,adv lk,ec = θadv · lk,adv lk,ec . the distribution parameters of θadv is already known/estimated. the distribution of lk,adv lk,ec is also lognormal. the mean coefficient of variation of lk,adv lk,ec is 0.17 regarding field moments, 0.13 regarding supporting moments and 0.14 regarding the shear forces at the supports. we follow the biggest value of 0.17. this results in a a coefficient of variation of θec of approximately 0.2. we alter c. o. v[θec ] between 0.1 and 0.3 in a subsequent sensitivity analysis. • following eurocode [11], we define the characteristic material property as the 5% quantile of m . we assume that mk,ec exactly meets mk, hence, that no estimation error in the determination of the characteristic value is present. • following eurocode [11] we choose the psf γf = 1.35. we further set γm = 1.1. 3.2. numerical investigations our main objective is to investigate the probability of failure of the bridges within the hypothetical population designed by eurocode and the change of the probability of failure when more advanced traffic load models are used. we investigate how parameter changes in the setup influence the mean probability of failure and the acceptance rate of reassessed bridge structures. our secondary objective is to understand how the probability of failure would change if the advances models would have been used to design the bridges. 218 vol. 36/2022 a study on the effects of hidden safety we only cover this issue for the setup defined in section 3.1 and do not investigate parameter changes within the setup. by rearrange equation 10 and 11, we determine the distribution of the characteristic traffic load according to eurocode and according to advanced traffic load modeling by solving equation 11 and 10 for lk,ec and lk,adv. the resulting distributions are: lk,ec ∼ ln µln lk,ec = ln(lk) − µln θec σln lk,ec = σln θec (14) lk,adv ∼ ln µln lk,adv = ln(finc · lk) − µln θadv σln lk,adv = σln θec (15) where µln and σln are the location and the scale parameter of the respective lognormal distribution. we sample lk,ec,i and lk,adv,i from lk,ec and lk,adv simulating the estimation of the characteristic traffic load of i = 1, ..., 10 000 bridges. we sample from lk,ec and lk,adv independently. given one sample lk,i (either from the characteristic value of the eurocode or the advanced traffic load model) the probability of failure is calculated as: pr(f ) = ∫ ωl fm ( finc · l · mk γm · γf · lk,i ) · finc · fl(l) dl (16) first, we calculate the probability of failure for the case of finc = 1. from this our secondary objective (how the probability of failure changes if the advances models would have been used to design the bridge) can be studied. in a next step, we increase the traffic load finc > 1 to investigate how the increase in the load changes the probability of failure and how a reassessment using advanced traffic load modeling affects the probability of failure). the reassessment decision is based on the following train of thought: advanced traffic load modeling may estimate a characteristic traffic load that is lower than the characteristic traffic load according to eurocode (even when an increased traffic load is present). if this lower characteristic value would be used to design the bridge, it would result in smaller resistances; therefore, the reassessment according to advanced traffic load modeling would deem the existing eurocode design to be sufficient. from this, the following acceptance/rejection rule is derived: lk,adv,i ≤ lk,ec,i ⇒ accept (17) lk,adv,i > lk,ec,i ⇒ reject (18) 3.3. results and discussion figure 2 shows the probabilities of failure (annual reference period) of the bridges designed by eurocode loaded with the original traffic load (finc = 1). the mean annual probability of failure is e[pr(f | ec-design, finc = 1)] = 4.9 · 10−8. this probability can be interpreted as the target probability of failure, 10−30 10−25 10−20 10−15 10−10 10−5 100 0 200 400 600 800 e[pr(f | ec-design, finc = 1)] = 4.9 · 10−8 figure 2. histogram of the probability of failure pr(f | ec-design, finc = 1) of bridge structures designed following eurocode. since the original design criteria reaches this probability of failure on average and these designs were accepted by society. the probability is rather low compared to the common range of probabilities of failures of structures or compared to the target probability of failure defined in the eurocode of 8.5 · 10−6. the reasons for the deviation are as follows: we do not consider model uncertainties of the structural model, the material model and the resistance model. moreover, we applied only traffic load on the bridge structures. if also other loads would be considered (with a higher coefficient of variation compared to the coefficient of variation of the traffic load), the probability of failure might be higher. we do these simplifications to keep the issue simple. this way the effects hidden safety are more isolated and more straightforward to study. in case of an actual application these simplifications should not be conducted. due to these simplifications the absolute value of the probability of failure may not be very meaningful. however, a relative comparison to the probabilities of failure of the bridges designed or accepted/rejected by advanced traffic load models is valid, since the probabilities of failure are calculated on the same basis. figure 3 shows the probabilities of failure that would result if the bridges (loaded with the original traffic load finc = 1) would have been designed using advanced traffic load modeling. the resulting mean probability of failure e[pr(f | adv-design, finc = 1)] = 3.1 · 10−7 is below the target of 4.9 · 10−8. this shows, that the negative effect on the structural reliability of the lost hidden safety due to advanced traffic load modeling is stronger than the positive effect of the reduced model uncertainty. an additional safety factor would be needed if advanced traffic load models were to be used for bridge design. here, we do not conduct a calibration of such a factor. a detailed guide is 219 max teichgräber, daniel straub acta polytechnica ctu proceedings 10−30 10−25 10−20 10−15 10−10 10−5 100 0 200 400 600 800 e[pr(f | adv-design, finc = 1)] = 3.1 · 10−7 figure 3. histogram of the probability of failure pr(f | adv-design, finc = 1) of bridge structures designed following advanced traffic load modeling. 10−30 10−25 10−20 10−15 10−10 10−5 100 0 200 400 600 800 e[pr(f | ec-design, finc = 1.3)] = 1.1 · 10−5 figure 4. histogram of the probability of failure pr(f | ec-design, finc = 1.3) of bridge structures designed following eurocode loaded by a 30 % higher traffic load. given [9]. remark: the resulting probabilities of failure of the advanced designs with an increased load (finc > 1) would be exactly the same as in figure 3, as long as the load increase is also considered within the design. finc would appear twice and cancel each other out: once within the design (a factor applied on lk,adv,i) and once as a factor applied on the load l. figure 4 shows how the probabilities of failure of the bridges designed according to eurocode change if the traffic load is increased by 30 % (finc = 1.3). the increased traffic load raises the mean probability of failure from 4.9 · 10−8 to 1.1 · 10−5. figure 5 subdivides the histogram of figure 4 into accepted/rejected cases (according to advanced traffic load modeling). 68.8 % of the bridge structures are accepted and 31.1 % are rejected. the mean probability of failure of the accepted bridges 10−30 10−25 10−20 10−15 10−10 10−5 100 0 200 400 600 800 e[pr(f | ec-design, finc = 1.3), accepted] e[pr(f | ec-design, finc = 1.3), rejected] = 3.5 · 10−5 = 1.7 · 10−8 figure 5. histogram of the probability of failure pr(f | ec-design, finc = 1.3) of bridge structures designed following eurocode loaded by an 30 % increased traffic load divided into accepted (green) and rejected (red) bridges according to advanced traffic load modeling. e[pr(f | ec-design, finc = 1.3), accepted] = 1.7 · 10−8 is smaller than the target probability of failure of 4.87 · 10−8; hence, the reassessment leads to sufficiently safe structures. therefore, the use of advanced models for reassessment of existing structures appears justified without further adjustment of the safety factors. this is in contrast to the design of new structures. the deifference can be understood by comparing the histogram of the probability of failure of the accepted bridge structures (green histogram in figure 5) to the histogram of the probability of failure of bridge structures designed by advanced traffic load modeling (blue histogram in figure 3): following equation 17 a structure is accepted if lk,adv ≤ lk,ec . if equality would holds, the accepted eurocode design and the advanced design are equal, hence both histograms would coincide. the greater the difference lk,ec − lk,adv, the more hidden safety remains, which decreases the probability of failure in the reassessment case. we conclude that the application of advanced models to reassess a given structure with increased load • retains parts of the hidden safety of accepted structures; • decreases the probability of failure by reducing uncertainty. figure 6 shows the changes in the mean probability of failure and the acceptance rate changes, when altering selected parameters of the case study one at a time. the reassessment of bridge structures should be critically considered, if the mean probability of failure of the accepted structures is greater than the mean 220 vol. 36/2022 a study on the effects of hidden safety 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0 10−4 100 10−2 10−10 10−12 10−8 10−6 finc 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0 0.8 1.0 0.2 0 0.4 0.6 finc 0.4 0.5 0.6 0.7 0.8 0.9 1.0 10−4 100 10−2 10−10 10−12 10−8 10−6 e[θec ] 0.8 1.0 0.2 0 0.4 0.6 e[θec ] 0.10 0.15 0.20 0.25 0.30 10−4 100 10−2 10−10 10−12 10−8 10−6 c. o. v.[θec ] 0.8 1.0 0.2 0 0.4 0.6 c. o. v.[θec ] 10−4 100 10−2 10−10 10−12 10−8 10−6 c. o. v.[θadv] 0.8 1.0 0.2 0 0.4 0.6 c. o. v.[θadv] 0.05 0.10 0.15 0.20 0.25 0.30 10−4 100 10−2 10−10 10−12 10−8 10−6 c. o. v.[m ] 0.05 0.10 0.15 0.20 0.25 0.30 0.8 1.0 0.2 0 0.4 0.6 c. o. v.[m ] 0.05 0.10 0.15 0.20 0.25 0.30 10−4 100 10−2 10−10 10−12 10−8 10−6 c. o. v.[l] 0.05 0.10 0.15 0.20 0.25 0.30 0.8 1.0 0.2 0 0.4 0.6 c. o. v.[l] 0.4 0.5 0.6 0.7 0.8 0.9 1.0 0.10 0.15 0.20 0.25 0.30 0.05 0.10 0.15 0.20 0.10 0.15 0.20 0.25 0.30 figure 6. mean probability of failure (left) and acceptance rate (right) of bridge structures altering setup parameters one at a time. the blue lines represent the mean probability of failure when the original load is applied (target probability of failure), the teal dashed lines represent the mean probability of failure when the increased load is applied and the dash doted green and red lines represent the mean probability of failure of the accepted and the rejected bridge structures. 221 max teichgräber, daniel straub acta polytechnica ctu proceedings probability of failure of the bridge structures under the original load (target probability of failure). this is the case if • the load strongly increases (finc > 1.6). however, this case is not critical, since the target probability of failure is not exceeded by much and the acceptance rate simultaneously drops very low. • the coefficient of variation of the material property is rather large (c. o. v[m ] > 0.2). this is also not critical, since the target probability of failure is only exceeded marginally. • the coefficient of variation of the traffic load is rather large (c. o. v[m ] > 0.15). again, this is not critical, since the target probability of failure is only exceeded marginally. • the estimation of the characteristic traffic load according to eurocode is very biased (e[θec ] < 0.65). this is also not concerning, since in this case the eurocode design includes high amounts of hidden safety, which leads to very low probabilities of failure. this hidden safety can serve as a reserve against increased traffic loads. • the estimation of the characteristic traffic load according to eurocode is associated with low uncertainty (c. o. v[θec ] < 0.18). in this case the coefficient of variation in the estimation of the characteristic value of the eurocode model and the advanced traffic load model match closely. hence, the reassessment does not reduce the uncertainty significantly, but reduces the bias (hidden safety). this case can be critical and should be prevented; however, this case is rare, since the uncertainty of advanced models is typically much smaller than the uncertainty of standard models. • the estimation of the characteristic traffic load according to advanced models is relatively uncertain (c. o. v[θadv] > 0.18). as in the previous bullet point the coefficient of variation in the estimation of the characteristic value of the eurocode model and the advanced traffic load model are close. this case can also be critical and should be prevented; however, again is rather rare. overall, the reassessment of bridge trough advanced modeling in most cases leads to sufficiently safe structures. it is only critical, if c. o. v[θec ] and c. o. v[θadv] are very similar. however, the conduced sensitivity analysis only varies one parameter at a time. a case in which multiple parameters are varied simultaneously may lead to a more critical outcome. 4. conclusion we investigated the effects of hidden safety if advanced traffic load models replace the standard eurocode traffic load model. in the design case, the loss of hidden safety increases the probability of failure more than the reduced uncertainty of advanced models decreases it; therefore, advanced traffic load models should not be applied to design new structures without recalibrating the partial safety factors. the reassessment case showed less critical results. in the vast majority of considered cases the accepted bridge structures that are assessed as compliant are sufficiently safe and no recalibration is needed. in this case the hidden safety is only reduced proportional to the load increase and not fully erased (in contrast to the design case). references [1] o. ditlevsen, h. madsen. structural reliability methods. wiley new york, 1996. [2] b. ellingwood, t. galambos, j. macgregor. development of a probability based load criterion for american national standard a58: building code requirements for minimum design loads in buildings and other structures. department of commerce, national bureau of standards, 1980. [3] t. galambos, b. ellingwood, j. macgregor, a. cornell. probability based load criteria: assessment of current design practice. journal of the structural division 108(5):959–977, 1982. https://doi.org/10.1061/jsdeag.0005958. [4] b. ellingwood, j. macgregor, t. galambos, a. cornell. probability based load criteria: load factors and load combinations. journal of the structural division 108(5):978–997, 1982. https://doi.org/10.1061/jsdeag.0005959. [5] deutsches institut für normung. grusibau: grundlagen zur festlegung von sicherheitsanforderungen für bauliche anlagen [grusibau: basic principles for the definition of safety requirements for structures]. beuth verlag, 1981. [6] h. madsen, s. krenk, n. c. lind. methods of structural safety. courier corporation, 2006. [7] m. faber, j. sørensen. reliability-based code calibration: the jcss approach. proceedings of the 9th international conference on applications of statistics and probability in civil engineering 9:927–935, 2003. [8] m. baravalle. risk and reliability based calibration of structural design codes. ph.d. thesis, norwegian university of science and technology, 2017. [9] m. teichgräber, j. köhler, d. straub. hidden safety in structural design codes. engineering structures 257:114017, 2022. https://doi.org/10.1016/j.engstruct.2022.114017. [10] m. teichgräber, j. köhler, d. straub. über den umgang mit versteckten sicherheiten eine fallstudie am windlastmodell des eurocode [on dealing with hidden safeties a case study on the wind load model of the eurocode]. baustatik – baupraxis 14 2020. [11] cen. eurocode 0: basis of structural design, 2002. [12] joint committee on structural safety. the jcss probabilistic model code. 2001. [13] m. nowak, o. fischer. site-specific load models for road bridges – an important tool for advanced evaluation strategies in bridge reassessment. betonund stahlbetonbau 112(12):804–814. https://doi.org/10.1002/best.201700064. 222 https://doi.org/10.1061/jsdeag.0005958 https://doi.org/10.1061/jsdeag.0005959 https://doi.org/10.1016/j.engstruct.2022.114017 https://doi.org/10.1002/best.201700064 vol. 36/2022 a study on the effects of hidden safety [14] cen. eurocode 1: actions on structures, 2005. [15] s. coles, j. bawa, l. trenner, p. dorazio. an introduction to statistical modeling of extreme values. springer, 2001. 223 acta polytechnica ctu proceedings 36:216–223, 2022 1 introduction 2 design following the partial safety factor concept 3 case study: existing bridge structures under traffic load 3.1 setup and assumptions 3.2 numerical investigations 3.3 results and discussion 4 conclusion references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.38.0483 acta polytechnica ctu proceedings 38:483–487, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague specifying boundary conditions for the operation of pipe heating systems with impact on the building energy balance jakub spurný∗, michal kabrhel czech technical university in prague, faculty of civil engineering, department of indoor environmental and building services engineering, jugoslávských partyzánů 1580/3, 160 00 prague 6 – dejvice, czech republic ∗ corresponding author: jakub.spurny@fsv.cvut.cz abstract. unaddressed boundary conditions in the design of heating systems affect the energy balance of buildings, especially in buildings with very low energy consumption. buildings with very low energy consumption are very sensitive to any heat flow and neglecting realistic heating water parameters affects their energy balance. two simulation models of the end part of the heating system have been developed. the first simulation model shows the effect of the inaccuracy of the designed heating element on the calculated room temperature. the second model shows the hydraulic behaviour of the connection pipes of the designed heating element. keywords: radiator, heating system, energy balance, low energy building heating system. 1. introduction in buildings with low energy consumption, we expect high efficiency of energy systems [1]. the use of hot water systems is most common due to the worst environmental impacts of direct electricity use. therefore, it is important not only to fit new efficient heat sources and use modern control valves but also to focus on the most accurate calculations for their correct design. there is a lot of simplification of individual boundary conditions in the usual heating system design procedures, as they were fully suitable for older and simpler systems. 2. heating system effect the output of the heating elements that provide the indoor ambient temperature should be the same as the heat loss of the heated space. in this case, the heating system operates at its highest efficiency. there are several reasons why this condition may not be met. these include incorrect determination of room heat loss, oversized heating element performance, neglect of heat gain from visible or built-in ductwork, or incorrect heating water parameters – temperature or flow rate. at the same time, a different internal room temperature can also cause heat flows within an energy-efficient building that did not occur previously due to temperature equality. the dependence of the heating performance on the indoor temperature can be determined by calculation. 3. model of the panel radiator the room temperature indicates the calculated internal room temperature and is based on the rules specified in the csn en 12831-1 standard [2]. the standard considers radiators to have the same air temperature as the room temperature. the calculation is solved for steady state in accordance with the standard, and therefore the accumulation does not affect the resulting parameters. heat losses and heat gains are solved in the calculation using their own control elements. the results of the behaviour of control elements are always considered after stabilization of the changed parameters. the following iteration formulas (1), (2) are used to calculate the operating room temperature. tiop = mop · c · (tpop − tzop) hop + te. (1) t zop = t pop − mn · (t pn − t zn) mop · ( t pop+t zop 2 − msk·c·(t pop−t zop) hop − t e t pn+t zn 2 − t in )n . (2) subscripts are used in equations (1) and (2): op operating, n nominal. these subscripts are used for the physical parameters: q radiator heating power [w], m mass flow [kg s−1], c specific heat capacity water [j kg−1k−1], n heating element exponent [-], tp supply water temperature [°c], tz return water temperature [°c], ti room temperature [°c], te external temperature [°c], h specific heat flux through building structures [w k−1]. 483 https://doi.org/10.14311/app.2022.38.0483 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en jakub spurný, michal kabrhel acta polytechnica ctu proceedings var. 1 var. 2 var. 3 var. 4 var. 5 var. 6 var. 7 var. 8 supply water temperature tpn [°c] 75 75 75 75 55 55 55 55 return water temperature tzn [°c] 65 65 60 60 44 45 40 40 real mass flow mop [kg s−1] = mn mp mn mp mn mp mn mp table 1. boundary conditions to calculate the internal temperature of the room (mn = mass flow for requested power output, mp = mass flow for room heat loss). figure 1. calculated value qn [%] and t iop [°c]. the panel radiator model contains 8 variants of different heating water temperatures and the resulting mass flow rates (see in table 1). design room temperature tin = 20 °c, external temperature te = −15 °c, heating element exponent n = 1.3 (typical value for flat radiator), room heat loss qp = 350 w. the actual room temperature tiop [°c] and the actual output of the heating body qn [%] were calculated. 3.1. theoretical case study of the panel radiator the results of modelling the given variants are presented in graphical form in the following figure for clarity. from the results in figure 1, it can be seen that the variants with real mass flow mop = mn (nominal value) have larger room temperature deviations tiop than the variants with real mass flow mop = mp when the same % deviation of qn from qp is specified. hence, it appears to be more advantageous to regulate the balancing at mop = mp, as the effect of underrated qn is compensated here, and conversely, overrated qn does not overheat as much again. it can also be seen that the larger deviations of the room temperature tiop are at qn < 100 % than at qn > 100 %, hence more problematic when undersizing the panel radiator. however, in general the variations in room temperature are quite significant for individual qn <,> 100 % with respect to thermal comfort of the occupants in the heated room and the control range of the local heating element control. for qn = 110–120 % the pressure (according to variants tpn and tzn) is out of the range of the thermostatic valve with head. this implies that the valve closes already by design error and is therefore no longer able to react to random heat gains, which must thus legitimately start to overheat the heated space. for the calculated underpressure at qn < 100 % (according to the variants tpn and tzn) it is important whether the valve on the heating element has the possibility of a so-called overflow compared to the balanced kv value e. g. at xp = 2 k and whether this overflow is able to raise the underpressure to the desired state. however, even in this case, the overflow when opening the valve reduces the effectiveness of the subsequent control of the random heat gains. 4. model of the radiator connection pipe today, however, the requirements for systems are more complicated with the growth of many variant solutions, and at the same time computer programs no longer pose obstacles to more demanding mathematical models. one of the boundary conditions addressed is the effect of heating water temperature on the pressure loss in piping systems commonly known calculation for the total pressure loss is the sum of the pressure loss through the inserted resistances and the frictional 484 vol. 38/2022 specifying boundary conditions for the operation of pipe heating . . . tm [°c] ρ [kg/m3] ρ [%] ν [m2/s] ν [%] 75 974,97 100 3,75e-07 100 70 977,92 100,3 4,03e-07 107,4 65 980,69 100,6 4,34e-07 115,7 60 983,28 100,9 4,69e-07 125,0 55 985,69 101,1 5,08e-07 135,5 50 987,92 101,3 5,53e-07 147,4 45 989,97 101,5 6,04e-07 161,0 40 991,84 101,7 6,63e-07 176,6 35 993,53 101,9 7,31e-07 194,7 30 995,04 102,1 8,10e-07 215,9 table 2. comparison of kinematic viscosity ν and water density ρ as a function of mean water temperature tm. pressure loss according to the darcy-weisbach equation (3) [3–5]. table 2 shows a comparison of the deviations of the kinematic viscosity ν and density ρ as a function of the mean heating water temperature tm. from table 2 it can be seen that the influence of ρ is very small within units of %, but the influence of ν is significant. heating systems often operate at variable water temperatures during operation, whether it is the designed temperature gradient, the effect of qualitative regulation [5] or, for example, the temperature drop due to cooling of the heating water in the pipe route [6]. therefore, a comparison of the pressure losses at heating water temperatures from 75 °c to 30 °c at a step of 5 °c was made. other variable inputs for the determination of the pressure loss are the internal pipe diameter d, the wall roughness k and the mass flow rate m with respect to the type of flow. in the following section it is shown how ν and ρ affect the pressure drop through local resistances z and the frictional pressure drop r in the different flow types. pressure loss through the local resistances z depending on the heating water temperature depends only inversely on ρ according to table 2. thus, at tm = 30 °c it is about 98 % compared to tm = 75 °c. for this reason, we will only consider the frictional pressure drop r (3) in the following, where the problem is more complex. r = λ · w2 · ρ d · 2 . (3) in this paper, the flow is considered for analysis in the laminar region according to poiseuille (4). λlam = 64 re . (4) substituting (4) into (3) we get the modified formula for the basic parameters (5). r = 0.04 · m · ν π · d4 . (5) and in the smooth region of flow according to blasius (6). λsmooth = 0.3164 re0.25 . (6) substituting (6) into (3) we get the modified formula for the basic parameters (7). r = 0.0005 · m 7 4 · ν 1 4 π 7 4 · ρ 3 4 · d3 , (7) where: r friction pressure loss [pa m−1], m mass flow [kg s−1], d piping diameter [m], ρ water density [kg m−3], λ friction coefficient [-], ν kinematic viscosity [m2 s−1], re reynolds number [-], w water speed [m s−1]. 4.1. theoretical case study of the radiator connection pipe the following example shows the flow in the laminar region for a cu 15 × 1 pipe up to mass flow m = 31 kg s−1 for variable water temperatures. the connection of a heating element with a power of 550 w at ∆t = 15 °c and 350 w at ∆t = 10 °c is solved. for larger pipe dimensions where mass flows are also larger, laminar flow does not occur. as can be seen from figure 2, the deviation increases at tm = 30 °c up to 216 % compared to tm = 75 °c, which is a lot, but it is necessary to look at the absolute values of the pressure drop. these values are at extremes of r = 4.5–9.6 pa m−1 for m = 30 kg s−1, which are very low for heating systems in energy efficient buildings and the difference does not have a significant effect on the resulting hydraulics. in figure 3 we can see that in the smooth flow zone, the change of friction loss depending on the temperature of heating water is caused by both ν and ρ with corresponding exponents. the deviation of friction loss at tm = 30 °c is 119 % in comparison with tm = 75 °c, which is not negligible. this behaviour 485 jakub spurný, michal kabrhel acta polytechnica ctu proceedings figure 2. relative pressure drop r for different mean heating element temperatures tm (laminar flow). figure 3. relative pressure drop r for different mean heating element temperatures tm (smooth zone). of the deviance in the smooth zone is according to blasius constant for all m, before the flow transforms from smooth to transition zone. in general, for the solved variants with boundary conditions of conventional heating systems, the transition from laminar flow to turbulent flow behaves hydraulically smooth at first, even for values with piping roughness k = 0.2 mm. only by further increasing mass flow does the smooth region variant change to a transition region. the rough hydraulic region is never reached in the boundary conditions for normal mass flow in the pipeline. 5. conclusion for all of the above reasons, it is recommended that the heat loss is calculated accurately, and the heating surface is designed for this loss correctly. if we avoid problems with an incorrect design of heating surface performance, we will have a heating system ready for operation with effective control efficiency and a positive effect on thermal comfort. the results also showed that it is preferable to balance the design of the heating surfaces to the flow rate given by the design output, as it compensates for the problem of underand overheating compared to heaters balanced to the flow rate given by the design output. it was also observed that the internal temperature deviation increases faster for underbalanced heaters than for balanced heaters. an important piece of information is that oversizing a heater by about 15 % above the requirement will already overheat the room beyond the normal local control range, leading to cycling. furthermore, the effect of the heating water temperature was pointed out. in laminar flow, which occurs in the heating surface connections in modern heating systems, a large percentage deviation of r was shown with decreasing heating water temperature. it was also found that in the smooth region of turbulent flow, the pressure drop deviations with decreasing temperature go up to 119 %. this flow region is most commonly represented in today’s heating system. this implies that for today’s heating systems in modern low energy buildings, which are typically low temperature due to the small required heating element capacities and the requirements of heat sources such as heat pumps or gas condensing boilers, the pressure drops are proportionally higher for the same flow rate than for older buildings with high heating water temperatures. this has implications for the design and power consumption of circulators and the design of control and balancing valves. 486 vol. 38/2022 specifying boundary conditions for the operation of pipe heating . . . acknowledgements this research was supported by grant sgs21/009/ohk1/ 1t/1. references [1] european union. directive (eu) 2018/844 of the european parliament and of the council of 30 may 2018 amending directive 2010/31/eu on the energy performance of buildings and directive 2012/27/eu on energy efficiency. official journal of the european union 61(l156):75–91, 2018. http://data.europa.eu/eli/dir/2018/844/oj [2] standard csn en 12831-1 energy performance of buildings – method for calculation of the design heat load – part 1: space heating load, module m3-3, 2021. [3] a. avci, i. karagoz. a new explicit friction factor formula for laminar, transition and turbulent flows in smooth and rough pipes. european journal of mechanics b/fluids 78:182–187, 2019. https: //doi.org/10.1016/j.euromechflu.2019.07.007 [4] c. a. dorao, t. langeland, m. fernandino. effect of heating profile on the characteristics of pressure drop oscillations. chemical engineering science 158:453–461, 2017. https://doi.org/10.1016/j.ces.2016.10.009 [5] r. petitjean. total hydronic balancing: a handbook for design and troubleshooting of hydronic hvac systems. tour & andersson ab, 1994. isbn 9163026260. [6] j. spurný, m. kabrhel. effect of heat distribution losses and cooling of heating water on design of heating system [in czech]. (vliv tepelných ztrát rozvodů a ochlazování otopné vody na návrh otopné soustavy). vytápění, větrání, instalace 26(1):2–5, 2017. 487 http://data.europa.eu/eli/dir/2018/844/oj https://doi.org/10.1016/j.euromechflu.2019.07.007 https://doi.org/10.1016/j.euromechflu.2019.07.007 https://doi.org/10.1016/j.ces.2016.10.009 acta polytechnica ctu proceedings 38:483–487, 2022 1 introduction 2 heating system effect 3 model of the panel radiator 3.1 theoretical case study of the panel radiator 4 model of the radiator connection pipe 4.1 theoretical case study of the radiator connection pipe 5 conclusion acknowledgements references my title https://doi.org/10.14311/app.2022.36.0015 acta polytechnica ctu proceedings 36:15–24, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague structural assessment and expected utility gain sebastian thönsa,b a lund university, faculty of engineering, box 117, 221 00 lund, sweden b bam federal institute for materials research and testing, unter den eichen 87, 12205 berlin, germany correspondence: sebastian.thons@kstr.lth.se abstract. this paper contains an introduction, probabilistic formulation, and exemplification (1) of system state and utility actions, (2) system state and utility action value analysis and (3) a threshold formulation for a predicted information and action decision analysis. the approaches build upon structural condition assessment and provide a basis for the condition management by maximising the expected utility for information and actions before information acquirement and action implementation. following the basic distinction of system state and utility actions, strengthening, replacement, repair, load reduction and consequence reduction actions are formulated. with an exemplary study encompassing an expected utility calculation and an action value analysis, it is demonstrated how expected utility optimal physical system changes can be identified before implementation. the threshold formulation for a predicted information and action decision analysis relies on the equality of the decision theoretical posterior action optimality condition. by extending the exemplary predicted action decision analyses with different structural health information (shi), the threshold formulation is exemplified and the optimal condition management strategies are identified. keywords: structural condition management, action uncertainty modelling, action and information value. 1. introduction condition assessment is usually understood as a structural reliability analysis with additional information. most commonly it is performed with obtained information, i.e., by posterior structural reliability updating. however, condition assessment constitutes a part of the structural condition management, i.e., ensuring the integrity and functionality of a system over time. the structural condition management may require measurements to learn about the structural condition and actions to adapt the structural condition. these two basic means of structural condition management should be performed efficiently, i.e., in a way that measurements and actions are planned to maximise their technological, economic and risk reduction effects. an efficient structural condition management can be performed and planned by the utilisation, adaptation and the application of the bayesian decision theory [1] to structures; or more widely to built environment systems. the decision theoretical context of structural condition assessment and management necessitates (1) a system state model, (2) a utility model, (3) that the information acquirement state is considered in the probabilistic modelling and (4) that the relevant actions can be modelled. whereas points (1) to (3) have been extensively developed over the last decade, see e.g., [2–17], the action modelling remains a challenge. relevant actions for a condition management would encompass e.g., structural modifications and utility changes. further, it would be required to analyse the expected action utility prior to the decision to implement an action. this paper focusses on the introduction and formulation of system state and utility actions as well as the action value analysis building upon [18] and [19] in sections 2 and 3. the action modelling in section 2 includes the introduction of an action uncertainty modelling and the formulation of strengthening, replacement, repair, load reduction and utility actions in conjunction with a limit state function representative for a highly correlated series system including extreme events and deterioration. based on the action uncertainty modelling, predicted and implemented action decision analyses are formulated (section 3). in section 4, the action modelling is exemplified and expected utility theorem consistent action values for various actions are quantified. section 5 contains a decision theoretical formulation of predicted information and action decision analyses (pipa da) and a derivation for continuous and direct structural health information (shi). it is shown how this shi formulation can be discretised and solved. the in section 4 introduced example is further developed in section 6 and the value of the condition management is quantified and separated into measurement information and system state and utility actions. section 7 includes and a summary and concluding remarks. 15 https://doi.org/10.14311/app.2022.36.0015 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en sebastian thöns acta polytechnica ctu proceedings 2. system performance and action modelling this section encompasses the formulation of the system state analysis, i.e., a structural reliability analysis in a decision theoretical context, and the combination of system state reliability and a utility model to quantify the system performance (section 2.1). in section 2.2, a probabilistic action modelling in introduced. 2.1. system state analysis and system performance analysis a generic component limit state function gx1 , describing the component state event x1, may be formulated with the resistance r, the damage d and the loading s, which are subjected to the resistance and loading model uncertainties mr, md and ms , respectively: x1 : gx1 = mr · r (md · d) − ms · s ≤ 0 (1) the resistance may be decomposed in the section property z and the material strength rmat. both may be subjected to the influence of deterioration, i.e., the design variable is subjected to section deterioration (md,z · dz) and the material strength is influenced by degrading material properties (md,rmat · drmat ): x1 : gx1 = mr · z (md,z · dz) · rmat (md,rmat · drmat) − ms · s ≤ 0 (2) in assessing normatively an expected utility-based decision or a decision value, the benefits, costs and consequences of the system states are modelled. based on these models, the preference of decision makers can be described using the concept of a utility function implying the context of a descriptive decision analysis [20]. in engineering decision analysis, the utility function is rather linear and usually based on monetary values, where the utilities of the system states are expressed in terms of costs or, generally, negative consequences. in the following, the utility function is thus omitted and a normative decision analysis performed. the system states are described by xl (see above) resulting in a utility u (xl) . the system performance usp , i.e., the expected system utility, is then calculated as the expected value of a probability mass function (where the system states denote realisations of a discrete random variable), i.e.: usp = exl [u (xl)] = ∑ xl u(xl) · p (xl) (3) 2.2. action modelling the expected system performance (see above) can be influenced by physical changes affecting (1) the utility of the system states and (2) the resistance and/or the load of the system states. this basic distinction leads to the denotation of utility actions and system state actions, respectively. the latter type can be associated with actions performed by an engineer as it includes e.g., design, repair and strengthening actions. beyond the distinction of action types, an action implementation state and uncertainty modelling is required to analyse and to optimise the expected of the utility before action implementation (see section 1). it is thus in the further distinguished between a predicted or implemented action. the explicit modelling of the m implementation states yk,m and the modelling of implementation uncertainties are introduced. a utility action is described with the system state utilities u(xl, ak, yk,m), the implementation states yk,m and the implementation uncertainties p (yk,m) and the costs of the action c(ak). the index m may be used for distinguishing e.g., with m = 1 . . . 3 full, partial or failed action implementation. a system state action ak is described with the system state probabilities p (xl(ak, yk,m)) beside the implementation uncertainties and the costs. in the context of existing structures, examples for system state actions are e.g., repair, replacement, strengthening and loading actions. a repair action can be modelled by reversing deterioration effects due to mechanical reshaping of the component. the repair action including the action implementation uncertainties (ak, yk,m) is then allocated to the damage, i.e., d(ak, yk,m) affecting the material resistance or the section properties: x1 : gx1 = mr · z (md,z · dz) · rmat (md,rmat · drmat (ak, yk,m)) − ms · s ≤ 0 (4) x1 : gx1 = mr · z (md,z · dz(ak, yk,m)) · rmat (md,rmat · drmat ) − ms · s ≤ 0 (5) a replacement action would result in a damage free component with same, similar or new material properties: x1 : gx1 = mr · z · rmat(ak, yk,m) − ms · s ≤ 0 (6) a strengthening action results in an enhanced resistance of a component by e.g., section enlargement, material addition or the replacement with a stronger material, i.e., z(as,k, yk,m, . . .) and rmat(as,k, yk,m, . . .): 16 vol. 36/2022 structural assessment and expected utility gain utilitiesactions system state action chance chancechoice system state and utility management implementationsystem states kalx utilities u (objective) function ( ) lsp x lu e u x= é ùë û ,k my utility action ( ) ( ) ( ) s k ,m l k k ,mk pa y l kx a ,ya u max e e u x c aé ù= -é ùë ûë û ( ) ( ) u k ,m l k pa y x l k ,m k ka u max e e u x ,y ,a c aé ùé ù= -ë ûë û system statesactions ka lx implementation ,k my utility u system performance (sp) analysis predicted system state action (psa) decision analysis predicted utility action (pua) decision analysis figure 1. system performance functions and objective functions for predicted action decision analyses. x1 : gx1 = mr · z(ak, yk,m, md,z · dz) · rmat(md,rmat · drmat ) − ms · s ≤ 0 (7) x1 : gx1 = mr · z(md,z · dz) · rmat(ak, yk,m, md,rmat · drmat ) − ms · s ≤ 0 (8) actions on the load side can be a load application for testing purposes or the change of use leading to a modified loading, s(as,k, yk,m): gx1(as,k,yk,m) = mr · z(md,z · dz) · rmat(md,rmat · drmat) − ms · s(ak, yk,m) ≤ 0 (9) utility actions may include a benefit enhancement due to an enhanced functionality or a consequence reduction in case of failure. a utility action is subjected to action implementation uncertainties dependent on the system states, i.e., u(xl, yk,m, ak). for illustration purposes, the expected utility quantification of an implemented action analysis can be written as: uia = eyk,m [exl [u(xl, yk,m, ak) − c(ak)]] = ∑ xl ∑ yk,m p (xl) · p (yk,m) · u(xl, ak) (10) a utility action can be further detailed to consequence reduction ((c(xl, ak), equation (11)), benefit and functionality enhancement (b(xl, ak) , equation (12)) and (equation (13)) cost reduction actions in conjunction with the (normative) utility model characteristics: uia = ∑ xl ∑ yk,m p (xl) · (b(xl) − p (yk,m) · c(xl, ak) − c(ak)) (11) uia = ∑ xl ∑ yk,m p (xl) · (p (yk,m) · b(xl, ak) − c(xl) − c(ak)) (12) uia = ∑ xl ∑ yk,m p (xl) · (b(xl) − c(xl) − p (yk,m) · c(ak)) (13) 3. predicted action decision analysis and action value analysis the objective function for a predicted action decision analysis (pa da) can be written for system state and utility actions. for both formulations, the action is selected for maximising the expected value of the utility, i.e., for maximising the system performance. 17 sebastian thöns acta polytechnica ctu proceedings upa = max ak (∑ yk,m ∑ xl u(xl, ak, yk,m) · p (xl (ak, yk,m)) − c(ak) ) (14) for the notation of the action types, an index u for a utility action and an index s for a system state action are introduced. the objective function for a predicted utility action (pau) da may be written with the action implementation uncertainty p (yk,m, which is assigned to the utility. the action cost is here considered as deterministic for simplicity. upa = max ak (∑ yk,m ∑ yxl u(xl, ak) · p (yk,m) · p (xl) − c(ak) ) (15) the objective function for a predicted system state action (pau) da is written analogous with the action dependent system state probability p (xl(ak, yk,m)) and without the action influence on utility: variable mean cov characteristic value distribution reference load s with a reference period of 1 year 1.0 0.2 0.98 quantile gumbel e.g., jcss probabilistic model code [21], [22] representative for live and traffic loads load model uncertainty ms 1.0 0.1 logn. e.g., jcss probabilistic model code [21], part 3.9 (2001): model uncertainty for moments in frames, [22] resistance r 1.0 0.05 0.05 quantile logn. generic, jcss probabilistic model code [21], [22] resistance model uncertainty mr 1.0 0.05 logn. jcss probabilistic model code [21], part 3.9 (2001): model uncertainties, [22] deterioration d 0.1 0.02 logn. generic deterioration model uncertainty md 1.0 0.1 norm. generic damage resistance transfer function td td(md · d) = 1 − md · d generic safety factor γs 1.5 for one dominating action between 1.35 and 1.5, according to en 1990 (2010) safety factor γr 1.0 for steel between 1.0 and 1.5 according to material specific eurocodes consequence cf ∼ u(20.0, 40.0) [14] table 1. probabilistic system performance models. upa = max ak (∑ yk,m ∑ yxl u(xl) · p (xl(ak, yk,m)) · p (yk,m) − c(ak) ) (16) the system performance and objective functions for the introduced pa das are visualised with decision trees, generalised and summarised in figure 1. an action value can be quantified in analogy to an information value as the gain of an expected utility by a predicted action, vp a, i.e., as the difference between the maximised expected utility of a predicted action da (up a) and the system performance (usp ): vp a = up a − usp (17) the action value may be normalised (v p a) to the system performance without an action implementation, i.e.: v p a = (up a − usp ) / usp (18) 18 vol. 36/2022 structural assessment and expected utility gain action type and uncertainty space description probabilistic model cost and consequence model system state actions a a0 do nothing − − a1 strengthening of section 1.2 · γ 2.0% a2 replacement with higher strength 1.2 · e[rmat] 2.0% a3 repair md · d = 0 1.0% a4 load reduction s/1.2 0.5% a5 consequence reduction − 0.3% u: uniform distribution, tr: triangular distribution table 2. action type and cost models. action type and uncertainty space description probabilistic model implementation uncertainty y y1 strengthening performance p (y1) ∼ tr(0.95, 1.05, 1.10) y2 replacement performance − y3 repair performance p (y3) ∼ tr(0.9, 0.95, 1.0) y4 load reduction uncertainty p (y3) ∼ n(1.0, 0.1) y5 cons. reduction uncertainty p (y1) ∼ tr(0.85, 0.95, 1.05) n: normal distribution, tr: triangular distribution table 3. action uncertainty models. it should be noted that per definition, the action value analysis includes the cost of the action for consistency with the expected utility theorem. 4. exemplary study the predicted system state and utility action da is studied for an existing and deteriorated structure, for which the optimal risk reduction action is to be found. the system performance model includes the limit state function (2) representative for a highly correlated structural series system subjected to direct and indirect system failure consequences (section 4.1). the system state actions "section strengthening", "component replacement", "component repair" and "load reduction" as well as the utility action "consequence reduction" are probabilistically modelled and exemplified. 4.1. system performance model the system state model is represented with equation (2). for a semi-probabilistic safety concept, it can be shown that the section property can be decomposed to a central safety factor γ (see e.g., [23]), i.e., the product of the resistance and the loading safety factors (γr and γs , respectively), and the ratio rk of the characteristic resistance and loading values (rk and sk, respectively). the influence of the damage on the resistance is modelled with a damage transfer function td: z = γ · rk · td (md · d) = γr · γs · sk rmat, k · td (md · d) (19) the limit state function (2) becomes with focussing on cross section rather than material deterioration: x1 : gx1 = mr · γ · rk · td (md · d) · rmat − ms · s ≤ 0 (20) the probabilistic model and the references are summarised in table 1. 4.2. action model following the introduction of the action type and uncertainty model in section 2.2, a strengthening, a replacement, a repair, and a load reduction and a consequence reduction action are modelled (table 2). the section strengthening leads to an increase of 20% of the section property, whereas the replacement with a higher strength material leads to an increase of the expected resistance by 20% with a constant variability. both have a cost of 2.0%. the repair action sets the deterioration to 1.0 and costs 1.0%. the loading reduction action leads to decrease of the loading by 20% with the cost 0.5%. the consequence reduction sets the direct and indirect consequences in case failure to a uniform distribution between 10.0 and 20.0 with a cost of 0.3%. 19 sebastian thöns acta polytechnica ctu proceedings figure 2. system performance normalised relative action values without the action costs (left side, grey) and with action costs (left side, black) and total expected action cost and risks (right). the action implementation uncertainties are modelled continuous, i.e., with a triangular distribution accounting for a known range and most probable value (table 3). the strengthening action provides most probably a higher strengthening (1.05) than envisaged (table 2) within a range between 0.95 and 1.10. the component replacement is subjected to negligible action implementation uncertainties. the repair performance is subjected to on site execution uncertainties with a mode of 0.95 and a range of 0.9 to 1.0 of repairing the deterioration. the load reduction uncertainty is normal distributed and on average as expected and uncertain with a standard deviation of 0.1. the consequence reducing usage uncertainty is in the range of 0.85 to 1.05 with a mode of 0.95. 4.3. predicted action and action value analysis the results of the predicted action da and the action value analysis are depicted in figure 2. the load reduction action leads to the lowest expected costs and risks due to the comparably high load reduction including the uncertainties and the low action costs (figure 2, left). it also provides the highest action value including its costs, followed by the also relatively cheap consequence reduction action (figure 2, right, black). the load reduction action is identified as the optimal action in consistency with the expected utility theorem. the action values without their costs would be significantly higher for strengthening, replacement and repair (figure 2, right, grey). 5. information and integrity management value analysis the value of predicted information vp i and the value of information and actions are quantified as the gain of an expected and optimised utility by shi or shi and actions, respectively [1, 18]. the value of predicted information is then the difference between the expected and maximised utilities of a predicted information and action da (up ip a) and a predicted action da (up a): vp i = up ip a − up a (21) the difference between the expected and maximised utilities of a pipa da and the system performance (up a) is referred to as the value of the structural condition management encompassing information and actions vp ip a: vp ip a = up ip a − usp (22) the decision values may be normalised by the subtrahend in analogy to equation (18): v p ip a = (up ip a − usp ) / usp (23) it should also be noted that the information value can be calculated relative to system performance. in this case, the information value is divided by the expected system performance utility: 20 vol. 36/2022 structural assessment and expected utility gain v p i = (up ip a − up a) / usp (24) 5.1. predicted information and action da by threshold determination a general form of a pipa da can be derived for utility and system state actions (building upon the formulations in figure 1). for the shi acquirement strategies with the information outcomes zi,j and costs c(ii), the pipa da objective function in extensive form constitutes: up ip a = max ii ezi,j [ max ak eyk,m [ exl(ak,yk,m|zi,j [u(xl, ak, yk,m] ] − c(ak) ] − c(ii) (25) in the following it will be shown that the above formulation (25) can be solved for continuous information outcomes by discretising to two complementary outcomes. continuous information outcomes are basically any measurement of a random variable contained in the system state function. per definition they are referred to direct shi . with consideration of the model uncertainty determination process in a multiplicative formulation [21], the model uncertainty can be understood as the measurable model outcome prediction normalised with the precise measurements of a structural reliability property. for a continuous model uncertainty outcome mm,i of information acquirement strategy with index i, equation (25) is readily rewritten: up ip a = max ii emm,i [ max ak eyk,m [ exl(ak,yk,m|mm,j [u(xl, ak, yk,m] ] − c(ak) ] − c(ii) (26) the extensive form equation (26), can be rewritten with an obtained, i.e. posterior, information and predicted action decision analysis (oipa da) providing uoip a: up ip a = max ii emm,i [uoip a(mm,i)] − c(ii) (27) uoip a(mm,i) = max ak eyk,m [ exl(ak,yk,m)|mm,i [u(xl, ak, yk,m) ] + c(ak) (28) the expectation operator in regard to the information can be written for continuous random variables: uoip a = max ii ∫ mm,i uoip a(mm,i) · fmm,i (mm,i) dmm,i − c(ii) (29) the measurement threshold tm,i,k is now found as the action-wise, i.e., ak dependent, expected posterior utility equality of "do nothing" and the predicted action, i.e.: tm,i,k : exl|tm,i,k [u(xl)] = eyk,m [exl(ak,yk,m )|tm,i,k [u(xl, ak, yk,m]] + c(ak) (30) the integral in equation (29) can then be split up into two sub-integrals and the expected utility maximisation can be performed outside the integrals: up ip a = max ik,ak  mm =tm,i,k∫ −∞ exl|mm [u(xl)] · fmm,i (mm,i) dmm,i + ∞∫ mm =tm,i,k [ eyk,m [ exl(ak,yk,m )|mm [u(xl, ak, yk,m] ] + c(ak) ] · fmm,i (mm,i) dmm,i −c(ii)  (31) the integration can be rewritten for the model uncertainty density functions providing: up ip a = max ik,ak  mm =tm,i,k∫ −∞ exl|mm [u(xl)] · dfmm,i (mm,i) + ∞∫ mm =tm,i,k [ eyk,m [ exl(ak,yk,m )|mm [u(xl, ak, yk,m] ] + c(ak) ] · dfmm,i (mm,i) −c(ii)  (32) 21 sebastian thöns acta polytechnica ctu proceedings the two complementary outcomes zi,1 and zi,2 for action k = 1 are then defined with equations (33) and the integrals can be solved by calculation of the pre-posterior probability for direct information [12]: zi,1 : m ∣∣tm,i,1 −∞ p (zi,1) = fmm,i (tm,i,1) zi,2 : m ∣∣∞ tm,i,1 p (zi,2) = 1 − fmm,i (tm,i,1) (33) it should be noted that there is no further assumptions nor probabilistic models about information outcomes required [24]. 6. exemplary study the application of the threshold determination approach (equations (30), (32) and (33)) lead for the in section 4 introduced example to equation (34): given a realisation of the measurement, the threshold is to be found for which the optimal action changes to action implementation instead of do nothing. the measurement is subjected to a measurement uncertainty , which is normal distributed with a mean of 1.0 and a coefficient of variation of 0.03. the information cost equals 0.001, e.g. according to [11]. tm,k : p ( x1 ∣∣tm,k, mu ) · e[cf ] = p ( x1 ∣∣tm,k, mu , ak, yk,m ) · e [ cf (ak, yk,m) ] + c(ak) (34) with the introduced limit state functions (see section 4.1), the threshold dependent probabilities of the failure state are calculated for damage information (equation (35)), loading information (equation (36)) and resistance information (equation (37)). p ( x1, tm,d,k, mu ) = p ( mr · z(tm,d,k · mu · d) · rmat − ms · s ≤ 0 ) (35) p ( x1, tm,r,k, mu ) = p ( mr(tm,r,k · mu ) · z(d) · rmat − ms · s ≤ 0 ) (36) p ( x1, tm,s,k, mu ) = p ( mr · z(d) · rmat − ms(tm,s,k · mu ) · s ≤ 0 ) (37) the solution of the first integral in equation ( 31), i.e. the calculation of the pre-posterior probability, is readily performed for damage information with the to the interval [0.0, tm,d,k] truncated (index t ) probability distribution of md [12]: p ( x1, tm,d,k, mu ) = p ( mr · z ( md [ 0.0, tm,d,k ]t · mu · d ) · rmat − ms · s ≤ 0 ) (38) the full solution of equation (31) for damage, loading and resistance information is performed analogous. the pipa and value da in threshold formulation (equations (32) and (21) to (24)) result in the relative predicted action and information values for damage, load and resistance measurements (figure 3, left side). the damage measurement has no value for damage repair nor for replacement due to the nature of the actions. for the actions "load reduction" and "consequence reduction", damage monitoring leads regardless of and dependent on the damage realisation to higher expected utility and is thus not predictable. the integrity management strategies with the highest values (38.0% to 63.1%) are load and resistance measurements for the replacement, load reduction system state actions and the "consequence reduction" utility action. when considering separately the system performance related information values (figure 3, right side), it is observed that there are only positive information values for (1) load and resistance measurements in conjunction with the replacement action and (2) for load measurements for the strengthening and repair actions. only for the replacement action, measurement information contributes significantly to the condition management strategy value (see figure 3 and figure 2, right side). the optimal condition management strategies are thus replacement with load measurement (vp ip a = 63.1 % of the system performance), replacement with resistance measurement (vp ip a = 58.1 %of the system performance) and load reduction (vp a = 57.5 % of the system performance). 7. summary and concluding remarks this study builds upon structural condition assessment and provides a basis for the condition management by maximising the expected utility for information and actions before information acquirement and action implementation. the study is intended for progress towards consistent probabilistic, decision theoretical and built environment system relevant modelling of actions, information, expected utility quantification and value. 22 vol. 36/2022 structural assessment and expected utility gain figure 3. relative predicted action and information values for damage, load and resistance measurements. the study encompasses an introduction, probabilistic formulation, and exemplification (1) of system state and utility actions, (2) system state and utility action value analysis and (3) of a threshold formulation for predicted information and action decision analyses. the introduced probabilistic action modelling facilitates to distinguish between system state and utility actions, i.e., between actions influencing the reliability of the structural components and actions influencing the utility model. the action uncertainty modelling includes the distinction of an implemented or predicted action, the action implementation states and the action implementation uncertainties. with an action value decision analysis as introduced, the optimal action with consideration of the action costs can be identified before action implementation in consistency with the expected utility theorem. this is demonstrated for the system state actions strengthening, replacement, repair, load reduction and the utility action: consequence reduction. the threshold formulation for a predicted information and action decision analysis relies on the equality of the decision theoretical posterior action optimality condition, i.e., that the action is implemented according to the higher expected posterior utility. the threshold formulation is derived on the basis of direct shi and the measurability of model uncertainties. the identification of the threshold facilitates the quantification of the shi value for continuous measurement information and provides readily the decision rule, i.e., for which measurement outcomes the action implementation is optimal. the application of the information value threshold formulation and the action value analysis show exemplarily which structural integrity management strategies are optimal and how the action values and information values contribute to each strategy value. acknowledgements the kalix bridge (trv 2021/58089, ef 173793) and the condtionvalue (trv / bbt 2021/51600) projects funded by trafikverket, sweden, are gratefully acknowledged. references 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[20] j. r. benjamin and c. a. cornell, probability, statistics and decision for civil engineers. mcgraw-hill, new york (in english), 1970. [21] jcss. probabilistic model code. jcss joint committee on structural safety, 2001-2015. [22] ad hoc group: reliability of eurocodes, technical report for the reliability background of eurocodes, 2021. [23] k. fischer, c. viljoen, j. köhler, et al. optimal and acceptable reliabilities for structural design. structural safety 76:149-61, 2019. https://doi.org/10.1016/j.strusafe.2018.09.002. [24] s. thöns, a. a. irman, and m. p. limongelli, on uncertainty, decision values and innovation. the international conference on uncertainty in mechanical engineering (icume), darmstadt, germany, june 7 to 8, 2021. 24 https://doi.org/10.3390/infrastructures4030056 https://doi.org/10.1016/j.ress.2020.106814 https://doi.org/10.3389/fbuil.2020.00014 https://doi.org/10.1111/mice.12332 https://www.jcss-lc.org/publicationsjcss/ https://doi.org/10.1016/j.strusafe.2020.101957 https://doi.org/10.1080/15732479.2021.1890139 https://doi.org/10.1186/s43065-021-00027-0 https://doi.org/10.1016/j.marstruc.2020.102888 https://doi.org/10.1177/1475921720981833 https://doi.org/10.1016/j.strusafe.2018.09.002 acta polytechnica ctu proceedings 36:15–24, 2022 1 introduction 2 system performance and action modelling 2.1 system state analysis and system performance analysis 2.2 action modelling 3 predicted action decision analysis and action value analysis 4 exemplary study 4.1 system performance model 4.2 action model 4.3 predicted action and action value analysis 5 information and integrity management value analysis 5.1 predicted information and action da by threshold determination 6 exemplary study 7 summary and concluding remarks acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.34.0056 acta polytechnica ctu proceedings 34:56–62, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague microstructure modification of cement pastes using nanosilica daniel ňachaja,∗, jiří němečeka,b, jiří němečeka,c a czech technical university in prague, faculty of civil engineering, department of mechanics, thákurova 7, 166 29 prague 6, czech republic b ph. d. candidate at ctu in prague c associate professor at ctu in prague ∗ corresponding author: nachadan@fsv.cvut.cz abstract. the paper describes modifications of cement paste mixtures using nanosilica to affect their physical and chemical properties. cement pastes from portland cement cem i 42.5r with three different substitution ratios of nanosilica 1.5, 3 and 4.5 wt.%/cem. were examined. micromechanical response and microstructure were characterized using nanoindentation, statistical deconvolution and scanning electron microscopy. about 400 locations were examined on each sample over 40,000 µm2 area. the results indicate an increase of c-s-h phases by 11 vol.% in samples with 4.5 wt.% nanosilica replacement compared to pure portland cement paste. the increase in c-s-h phases volume was accompanied by the decrease in ch phases by approximately 11 vol.%. keywords: nanosilica, nanoindentation, cement paste, c-s-h, image analysis, microstructure. 1. introduction cement has been used as a hydraulic binder for two hundred years. its composition of inorganic raw materials can solidify, harden and bind other materials. this ability makes it one of the most used building materials. after mixing with water, the cement paste is being formed, which is the basic binding component of concrete. cement paste is a very complex heterogeneous material that differs in a range of nanoscale to macroscopic dimensions. a mixture of cement particles in water triggers a series of chemical reactions that interact with each other and lead to physical, chemical, and mechanical changes in the system. products of these reactions are stable hydrated compounds that bind to each other and provide cement with adhesive and cohesive properties [1]. under various conditions, such as e.g., environmental impacts like carbonation of concrete or impacts caused by the transport of chlorides onto reinforcement’s surface, concrete may lose some important properties such as strength or resistance. it can also be impacted by freezing and thawing or impacts of chemical compounds from aggressive environments. therefore, various additives or admixtures are often used affecting properties, such as: strength, workability, resistance to specific impacts or even the rate of solidification. at present, various admixtures are being used, such as: limestone, fly ash, furnace slag, microsilica, nanosilica (ns), zeolites, and others [2]. the workability of the fresh mixture and the density of cured material are regulated by water-reducing additives. solidification rate and the development of microstructure can be controlled by accelerators, retarders, crystallisation additives and other substances [3]. nowadays, nanoparticles have become popular components of cement composites. their advantages are small size and increased reactivity. due to their small dimensions, it is possible to achieve transport through a porous concrete system. nanoparticles can also be used to repair hardened concrete [4]. dispersion itself might be a problem, but some particles are available in liquid form, which greatly simplifies their usage. ultrafine particles exhibit unique physical and chemical properties compared to conventional materials. due to their unique properties, nanoparticles are gaining tremendous attention and are used in many areas to produce new materials. for example, it has been found that the compressive and flexural strength of cementitious mixes has gained an increasing trend by the addition of nanosilica [2, 5, 6]. the incorporation of nanosilica can also increase the strength of the cement paste due to its hydrating effect and high pozzolanic activity, which leads to a higher amount of c-s-h (calcium silicate hydrate) gels and a denser bulk structure due to the reaction of nanosilica with portlandite. however, the disadvantage of cement paste with the addition of nanosilica is that it adversely affects its workability due to its high specific surface area [3]. to study cement microstructure, nanoindentation can be used to determine material parameters such as modulus of elasticity, hardness, plastic or viscous parameters obtained from the indenter load history and penetration depth. the forces involved are on the scale of micronewton, and the depth is on the scale of nanometers. the main advantage compared to classical mechanical tests is that a very small material volume can be accessed with the tip of the nanoindenter [7]. in cementitious composites, hundreds of 56 https://doi.org/10.14311/app.2022.34.0056 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 34/2022 indents need to be made to cover the spatial variation of the mechanical properties of their small material phases. by default, the individual indents are spaced several micrometers apart not to influence each other, and large sets of indents (grid) are generally required to cover a representative area of the sample, including all its microstructural constituents in sufficient quantity [8]. the research aims to experimentally examine the influence of nanosilica on the final cement microstructure. the most observed effects include the change in volume representation of individual phases their distribution in the mixture. it is also the mechanical properties of cement paste determined by nanoindentation. 2. microstructure characterization portland cement is composed in an approximate ratio of 5.5:2.5:1:1 mainly from c3s (alite), c2s (belite), c3a (aluminate), c4af (ferrite). hydration of portland cement produces cement paste. the basic hydration chemical reactions include the formation of c-s-h gels and ch (crystalline calcium hydroxide) particles, also known as a portlandite which are the main hydration products of cement paste’s microstructure (see fig. 1) and also other phases of cement’s microstructure such as: residual clinker, porosity, afm, aft and more [1]. the c-s-h gel exists in two mechanically distinct phases, which are referred to hereafter as lowand high-density c-s-h gels. the chemical composition of the paste is not completely constant but may slightly differ from place to place, as well as porosity. hydration products c-s-h consist of two variations, where one variation is the so-called inner product rich in high-density c-s-h gel (see fig. 1), developed within the original cement grain boundaries, and outer product created mostly of low-density c-s-h gel mixed with other hydrates, which is formed by storage in an originally water-filled space [9]. to modify the microstructure, it is suitable to use nanosilica, which is chemically compatible with cement and can be produced relatively easily. in addition to creating nucleation sites to produce c-s-h gels arising from reaction with ch, nanosilica can also act as a nano-filler, which reduces the nano and microporosity of concrete [1, 10]. when applying nanoparticles, the problem of particle aggregation is often encountered. nanoparticles easily aggregate due to their high surface tension and large amount, so their even distribution can be a problem [11]. 3. nanoindentation the main advantage compared to mechanical tests at the macro level is that the tip of the indenter can gain access to very small volumes of material and their figure 1. sem photo of cement paste showing phases. properties in the nanometre range. the principle of nanoindentation consists in pressing a very small tip with certain properties (material and geometry) into the surface of specimen, which then creates an indent. this makes it possible to obtain different material parameters such as: modulus of elasticity, hardness, plastic, and viscous parameters. the nanoindentation itself can also be affected by unwanted influences such as imperfectly polished surfaces containing scratches or even not completely clean surfaces where even a small dust particle attached to the tip of the indenter can affect the measurement. errors will be more pronounced for the phase with a lower abundance than for the phase with a higher abundance. oliver and pharr (1992) proposed a method for determining the reduced modulus of elasticity based on the contact depth of the indent [12]. it is based on the general relationships derived by sneddon for loading, unloading and contact surface for an indent, which can be described as a solid of revolution of smooth functions. sneddon proved that the loaddisplacement relationships can be described by the following function[13]: p = αhm, (1) where p is the indenter load, h is the displacement, m is the shape parameter (m = 1 for flat cylinder, 2 for cone and 1.5 for sphere) and α is the material constant. the reduced modulus er is evaluated from the unloading curve by oliver-pharr method [12]. er = 1 β √ π 2 s√ ac , (2) where β is the geometric constant (1.034 for berkovich tip [8]), s is an initial unloading stiffness and ac is 57 d. ňachaj, j. němeček, j. němeček acta polytechnica ctu proceedings the projected contact area of the indenter at the peak of loading. the function is calibrated on fused quartz. the young’s modulus of investigated material can be determined using the relation: 1 er = (1− ν2) e + (1− νi 2) ei , (3) where e is the young’s elastic modulus of the material, ν is poisson’s ratio of the material (0.2 for cement paste [8]), ei is the elastic modulus of the indenter tip and νi is the poisson’s ratio of the indenter tip (ei=1141 gpa and νi=0.07 for diamond [8]). the output of each indent is a curve showing the applied force and the depth of the indent. from this curve, young’s modulus can be calculated using the above relations. 4. image analysis image analysis is used for observation and subsequent preparation of cement paste phases. the observed parameters are e.g. the size of the phases, their representation and, where appropriate, their shape. images are made in a gray range (0-255) of colors, and the colors of pixels vary according to the density of the physical element. the darker colored pixel in the sem-bse image represents elements with a lower proton number, while the lighter colored pixel, on the other hand, represents areas with elements with a higher proton number [14]. it is important to select an adequate gray-level threshold for extracting individual objects when processing an image. various techniques have been proposed in this regard. ideally, the histogram has a deep and sharp valley between two peaks representing objects, so choosing a threshold at the bottom of this valley is possible. however, it is often difficult for most images to accurately detect the bottom of a valley, especially when the valley is flat and wide, full of noise, or when the two peaks are extremely different in height and often without creating a detectable valley. in computer vision and image processing, the otsu method is used to perform automatic image thresholding, whose algorithm in its simplest form returns a single intensity threshold that separates pixels into two classes. using this method, pixels can be separated into several classes. in this method, the image is viewed as two groups of points with different ranges of intensity values. the problem is that these intensity ranges usually overlap, and therefore the method seeks to minimize misidentification of pixels [15]. the color of the pixels varies according to the density of the physical element. a darker colored pixel represents lower density elements, while lighter elements represent a higher density area. the sizes of the individual phases reach dimensions in the order of tens of micrometers [9]. thus, the chosen area should ensure a sufficiently representative volume of samples (rve) [16]. 5. experimental part 5.1. sample preparation four types of cement paste samples were produced, three of them with different proportions of nanosilica and one reference sample without nanosilica. the cement used for this purposes was cem i 42,5r (radotín, českomoravský, a.s.). part of the cement was replaced by nanosilica in three proportions, precisely: 1.5 wt.%, 3 wt.% and 4.5 wt.%. major substitutions have not been made based on the available literature, as in some cases, its have been proved counterproductive in terms of pore growth [5]. the addition of nanosilica was performed using an akzo nobel levasil cb8, which is a colloidal solution of nano-sio2 with the following properties: density = 1.4 g/cm3, solid particle content = 50 %, h2o = 50 %, particle size = 20 250 nm and ph = 9.5. the cement slurry was placed in cylindrical plastic ampules with a base diameter of 27 mm and a cylinder height of 70 mm. the following day, the hardened samples were demoulded, stored in water and labeled as follows (see tab 1), where ns representation is the ratio of ns weight to cement weight in percentage: label ns/cem cem water cb8 w/b [wt%] [g] [g] [g] [-] cns00 0.0 300.0 120.0 0.0 0.4 cns15 1.5 298.1 116.6 8.9 0.4 cns30 3.0 293.7 112.2 17.6 0.4 cns45 4.5 289.5 108.0 26.1 0.4 table 1. labelling of individual samples with the specific weights of the additives. where c represents cement, ns represents nano-sio2, cb8 is the label for akzo nobel levasil cb8, w is water and b is a binder which is the sum of cem and ns. to examine samples at a specific point in the aging process was hydration stopped at 28 days by placing samples in acetone for approximately 30 days. subsequently, samples were dried in a dryer for one day at a temperature 50 °c. for possible observation, the surface of the samples was treated, where the samples were polished with a struers tegramin 20 polisher. the polishing process was the same for all samples. the samples were polished with silicon carbide papers with grit sizes of 1200, 2000 and 4000. the samples were placed in a glass flask with technical alcohol and placed in an ultrasonic cleaner for two minutes between each polishing step. finally, the samples were manually polished with a diamond suspension with a particle size of 0.25 µm on a fine cloth disk struerss md-dac and subsequently cleaned in an ultrasonic cleaner. 5.2. sem analysis prepared samples were observed by sem phenom xl from which outputs (bse images) were subjected to 58 vol. 34/2022 the image analysis necessary to perform representation of individual phases. the size of the examined area of one bse image from which the image analysis was performed is 746× 746 µm for 360× magnification and 384×384 µm for 700× magnification. the magnifications were chosen to cover the phases and their transitions, as the phase sizes reach tens of micrometers [9]. for each sample, 20 images were taken from a 360× magnification and also 20 images from a 700× magnification. thus, a total of 40 images were taken, representing an area of 14.08 mm2. the basis of image analysis is, in addition to the correct preparation of samples, also the correct choice of the thresholds of individual phases. thresholding was performed using matlab software based on the otsu method mentioned above, which is predefined in the program. the threshold value of some images was manually adjusted due to the different contrast and brightness of the images and also due to scratches and an inaccurate amount of pore growth. 5.3. nanoindentation the most suitable area was selected for each sample with the least possible occurrence of defects caused during polishing and with the smallest possible occurrence of pores. the size of the matrix created by indents for each sample was 200× 200 µm. nanoindentation was gradually performed using the csm instruments device. samples were probed by a cube-corner tip. the trapezoidal loading diagram was as follows: with a loading rate 24 mn/min, the maximum force of 2 mn was reached after 5 seconds, where began 20 second holding phase, followed by 5 s linear unloading at the same rate. totally 400 indents (20 × 20 grid) with an axial distance of 10 µm between the indents were made for each sample. 6. results and discussion 6.1. results of image analysis microscope images show us that the addition of ns causes a change in the volume representations of individual phases. it is possible to see scratches in the images caused by imperfect polishing (see fig. 2, 3). these scratches slightly increase the percentage of pores, which, however, should not significantly impact the results from a statistical point of view. statistically, the representation for a magnification of 700× is similar to a magnification of 360×. at both magnifications, a decrease in the incidence of portlandite and an increase in the incidence of c-s-h can be observed in the same way. that was the reason for merging these magnifications into one group. a clear trend can still be seen where 4.5 wt% of nanosilica increases the proportion of c-s-h phases in the samples by about 7%, 3 wt% by approximately 3.5% and 1.5 wt% by about 3% (see tab. 2 and fig. 5). the increase in the incidence of samples with the addition of nanosilica could be attributed to ns, which reacts with ch in the presence of water and thus contributes to the production of c-s-h. the increase in the c-s-h phase was accompanied by a decrease in the ch phase by approximately 5% for 4.5 wt% of nanosilica, 3% for 3 wt% of nanosilica and about 1% for 1.5 wt% of nanosilica, see tab. 2 and fig. 5. regarding pores, a significant reduction can also be observed in samples with a higher nanosilica content. 6.2. results of nanoindentation every sample was subjected to 400 indents where a load-displacement curve was made for each indent, see fig. 4. subsequently, the clinker phases (e > 45 gpa) and the low stiffness phases (e < 17 gpa) were separated due to the great variability in which a large statistical error could occur [7]. the values from the indentations in the clinker region varied widely, which is due both to their composition, where unhydrated clinker residues tend to contain al2o3, which substantially increased the measured values, but also to their position and orientation relative to the tip of the indenter, where the individual particles have different young’s moduli in different directions of the force application. in addition to the orientation of the particle itself towards the indenter tip, young’s modulus is also affected by the compliance of the surroundings. the distribution of elastic moduli was calculated with the aid of deconvolution, where all data whose young’s modulus of elasticity ranges from 17 gpa to 45 gpa were deconvoluted into two phases, namely c-s-h and ch phases [7]. the distribution of the individual young’s modules of certain phases from which the volume representation was subsequently made can be observed in table 3, from which it is possible to observe that the nanosilica with a high probability does not affect young’s modulus as affect volume representation of certain phases. based on this distribution, a table (see tab. 4) representing the percentages of each phase was then evaluated. an increase in the c-s-h phase of approximately 5% was also observed for samples with the addition of 3 wt% of ns with an equal decrease in the ch phase and an increase of approximately 3% for samples with the addition of 1.5 wt% of ns also with an equal decrease in the ch phase (see tab. 4). the finding can be observed for samples with ns addition, where samples with higher ns content usually contain more c-s-h, which is attributed to the reaction of ns with ch. 6.3. mutual comparison on the positive side, the results from nanoindentation copy the results from image analysis, which to some extent confirms the trend of the behavior of the samples with ns addition. a comparison of the results of the percentage volume representations from the two experimental techniques can be observed in fig. 5 where the results from image analysis were converted from four phases (pore, c-s-h, ch, clinker) to two 59 d. ňachaj, j. němeček, j. němeček acta polytechnica ctu proceedings label c-s-h increase ch increase clinker increase pores increase [%] c-s-h [%] [%] ch [%] [%] clinker [%] [%] pores [%] cns00 60.5 ± 1.3 ±0.0 18.3 ± 2.1 ±0.0 10.4 ± 1.4 ±0.0 10.8 ± 2.1 ±0.0 cns15 63.4 ± 2.7 +2.9 17.1 ± 2.3 -1.2 10.8 ± 1.9 +0.4 8.7 ± 2.6 -2.1 cns30 63.5 ± 1.7 +3.5 15.6 ± 1.8 -2.7 13.6 ± 2.0 +3.2 7.3 ± 2.3 -3.5 cns45 67.8 ± 2.7 +7.2 13.1 ± 1.1 -5.2 13.6 ± 1.6 +3.2 5.4 ± 0.9 -5.4 table 2. volume representations of individual phases obtained from image analysis. figure 2. a) sem photo of sample cns00 b) marked phases after image analysis, where blue color represents pores, yellow portlandite, orange clinker, green hd c-s-h and turquoise ld c-s-h. label c-s-h ch e [gpa] e [gpa] cns00 30.3 ± 1.7 39.2 ± 2.5 cns15 30.7 ± 2.7 40.1 ± 2.5 cns30 26.7 ± 3.8 37.8 ± 2.9 cns45 29.6 ± 4.0 41.1 ± 2.0 table 3. distribution of young’s modulus of individual phases. label c-s-h increase ch increase [%] c-s-h [%] [%] ch [%] cns00 74.9 ±0.0 25.1 ±0.0 cns15 77.8 +2.9 22.2 -2.9 cns30 80.2 +5.3 19.8 -5.3 cns45 85.9 +11.0 14.1 -11.0 table 4. volume representations of individual phases obtained from nanoindentation. phases (c-s-h, ch). the reduction from four phases to two phases was made in order to be able to compare the techniques since only two phases were evaluated by nanoindentation. it is also important to note that the area of the sample examined by nanoindentation is 0.04 mm2. the area of the sample examined by image analysis is an order of magnitude higher, namely 14.08 mm2, which may also play a role in comparing the results. 7. conclusions this paper focuses on the modification of the microstructure and nanomechanical properties of cement paste using nanoparticles. the most important conclusions of the work can be summarized in the following four points: • based on the results from image analysis and nanoindentation, it can be said that replacing cement with nano-silica leads to an increase in the volume of c-s-h gels in this experiment. for samples with 4.5 wt% of ns aged 28 days, approximately by 7% compared to the pure cement paste sample according to image analysis and approximately by 11% according to nanoindentation. • the increase in the c-s-h phase was accompanied by a decrease in the ch phase. by nanoindentation, it was found that the decrease of c-s-h phase for samples with 4.5 wt% of ns was approximately by 11%, where by image analysis, a decrease of approximately by 5% was recorded. the difference in the two methods is due to the area studied and the fact that two phases were produced by nanoindentation and four by image analysis. on the positive side, the trend for both techniques is favorable. 60 vol. 34/2022 figure 3. a) sem photo of sample cns45 b) marked phases after image analysis, where blue color represents pores, yellow portlandite, orange clinker, green hd c-s-h and turquoise ld c-s-h. 0 100 200 300 400 500 600 700 0.4 0.8 1.2 1.6 2.0 2.4 displacement [nm] f or ce [m n ] ip op ch clinker figure 4. typical indentation load-displacement curves, where op is outer product and ip is inner product. • the volume fraction of low-stiffness phases and clinker obtained by nanoindentation is difficult to determine because the fraction of these phases is small or easily influenced by other phases. • it can be concluded that the rve for image analysis is large enough, as the results from 360x magnification were very similar to those from 700x magnification. however, the image analysis revealed an unexpected trend of increasing clinker phase in the samples with the ns addition, which may be due to incorrectly determined thresholding. acknowledgements financial support of the czech science foundation (project no. 21-11965s) and the grant agency of the czech technical university in prague (sgs20/107/ohk1/2t/11) are 0 1.5 3 4.5 20 40 60 80 100 ns/cem [%] v ol u m e re p re se n ta ti on [% ] ia c-s-h ia ch ni c-s-h ni ch figure 5. volume representations of a given phases, where ia is image analysis ni si nanoindentation. gratefully acknowledged. references [1] f. sanchez, k. sobolev. nanotechnology in concrete – a review. construction and building materials 24(11):2060 – 2071, 2010. https://doi.org/10.1016/j.conbuildmat.2010.03.014. 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[13] i. n. sneddon. the relation between load and penetration in the axisymmetric boussinesq problem for a punch of arbitrary profile. international journal of engineering science 3(1):47–57, 1965. https://doi.org/10.1016/0020-7225(65)90019-4. [14] a. luis, l. deng, l. shao, h. a. li. triaxial behaviour and image analysis of edmonton clay treated with cement and fly ash. construction and building materials 197:208–219, 2019. https://doi.org/10.1016/j.conbuildmat.2018.11.222. [15] n. otsu. a threshold selection method from gray-level histograms. ieee transactions on systems, man, and cybernetics 9(1):62–66, 1979. [16] j. němeček, v. králík, j. vondřejc. micromechanical analysis of heterogeneous structural materials. cement and concrete composites 36:85–92, 2013. special issue: nanotechnology in construction, https://doi.org/10.1016/j.cemconcomp.2012.06.015. 62 https://doi.org/10.4028/www.scientific.net/kem.760.3 https://doi.org/10.1016/j.conbuildmat.2014.04.063 https://doi.org/10.1016/j.cemconcomp.2013.09.001 https://doi.org/10.1016/j.mtcomm.2019.100806 https://doi.org/10.1016/j.cemconcomp.2004.02.028 https://doi.org/10.1016/j.cemconres.2005.07.004 https://doi.org/10.1016/s1359-8368(03)00052-0 https://doi.org/10.1557/jmr.1992.1564 https://doi.org/10.1016/0020-7225(65)90019-4 https://doi.org/10.1016/j.conbuildmat.2018.11.222 https://doi.org/10.1016/j.cemconcomp.2012.06.015 acta polytechnica ctu proceedings 34:56–62, 2022 1 introduction 2 microstructure characterization 3 nanoindentation 4 image analysis 5 experimental part 5.1 sample preparation 5.2 sem analysis 5.3 nanoindentation 6 results and discussion 6.1 results of image analysis 6.2 results of nanoindentation 6.3 mutual comparison 7 conclusions acknowledgements references my title https://doi.org/10.14311/app.2022.36.0076 acta polytechnica ctu proceedings 36:76–83, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague uncertainty assessment for the bayesian updating process of concrete strength properties matthias haslbecka,∗, robert kroyerb, andreas tarasb, thomas bramla a university of the bundeswehr munich, institute for structural engineering, werner-heisenberg-weg 39, d-85577 neubiberg, germany b swiss federal institute of technology zürich (ethz), institute of structural engineering, stefano-franscini-platz 5, 8093 zürich, switzerland ∗ corresponding author: matthias.haslbeck@unibw.de abstract. reassessment of infrastructure buildings has become an essential approach to deal with increasing traffic loads on ageing infrastructure buildings and to verify the service-life of those structures. good estimation of the actual material properties is highly relevant for reliable structural reassessment. although this holds for all building materials, the importance of good parameter estimation is of special importance for concrete structures, where the strength properties show relatively high variation and where the nominal strength properties tend to be too conservative. modern design guidelines allow to make use of scientific methods such as bayesian updating of material properties to enable a more realistic consideration of the actual material properties in the reassessment of existing structures. however, guidelines for application and experience with those methods are not yet reported much or are rather vague [1]. the presented study focuses on the effect of the bayesian updating process for material parameters with special emphasis on the number and sampling location of test specimens as well as on the accuracy and confidence in the obtained posterior distribution, since sampling also includes a certain margin of uncertainty. the investigation on the methodological potential and on the uncertainty margin in the updating process in this contribution uses a batch of 14 test results on the concrete compressive strength obtained from drill cores along with the inherent measurement uncertainties from the testing procedure. after a short review of bayes’ theorem, the markov chain monte carlo method (mcmc) and the bootstrap methodology, all combinations of subsamples of size 1, 3 and 5 specimens were built and provided to the bayes’ updating procedure via mcmc to determine the posterior distributions. the series of obtained posterior distributions for a certain subsample was used to determine the uncertainty in the bayesian updating process by evaluation of the scatter in the expected value, the standard deviation and the 5 %-quantile of the updated distribution. the simulations show the importance of an adequate sample size and quantify the uncertainties arising from the limited number of observations. keywords: bayesian updating, bootstrapping, burn-in, concrete compressive strength, markov chain monte carlo, mcmc, metropolis algorithm, roding bridge, structural reassessment. 1. introduction continuously increasing life-loads and ageing of infrastructure are the most important reasons for the requirement of reassessment of existing infrastructure buildings. for this purpose, the correct estimation of the actual material properties is of high practical relevance. this is considered specifically difficult for concrete structures, since the material strength depends on various factors and as consequence of this shows considerable scatter. in many cases the nominal values in the design codes turn out to be rather conservative, since they must represent the expected strength values with a certain safety margin. material tests on specimens taken from the structure are worth a lot to verify the expected strength class, but can’t be used directly for the structural reassessment process. as a consequence of this, the scientific motivation strives for a more advanced methodology and consideration of the measurement results for structural reassessment. statistical methods such as bayes’ theorem play an increasingly important role for interpretation and exploitation of data and setup of specifically tailored reassessment of individual structures. more precisely, bayes’ theorem allows to update prior information from literature or other sources by direct or indirect observations. however, spatial variations of material properties within the structure and uncertainties in the measurement process lead to follow-up questions for the application of such methods, especially when the sample sizes are small. in this paper, the methodology of stochastic updating is shown on the example of the compressive strength of concrete using bayes’ theorem via monte carlo approximation. similar studies have been discussed in e.g. [2] using theoretical data sets. for this 76 https://doi.org/10.14311/app.2022.36.0076 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 36/2022 bayesian uncertainty updating concrete strength properties figure 1. (a) sampling of the drilling cores and (b) bird’s eye view of the investigated bridge in roding. purpose, a limited number of destructive material test results from a large-scale structure are used to show the outcome of the updating procedure for different sizes of the data set. for the bayesian updating process, all possible subsamples of size 1, 3 and 5 core samples are composed from the entire data set of 14 measurements. for all those subsamples the posterior stochastic models are calculated. the resulting expectation, the standard deviation and the 5 %-quantile of the subsamples are used to derive a distribution that shows the uncertainties related to the choice of the measurement locations and cope with the aspect of spatial variation. the study is conducted using markov chains which have proven to be an excellent tool for the implementation of bayesian updating in the framework of a monte carlo simulation. as a sampling strategy, the metropolis algorithm has been implemented in matlab to conduct the more than 2,300 simulations. 2. methodology 2.1. evaluation of compressive strength within a research project dealing with the updating of finite-element-models for a refined calculation of existing structures using different kinds of experimental data, 14 core samples were taken at different locations during the demolition of a specific bridge located in roding, bavaria. the three-span box girder bridge dating from 1965 was made up from site concrete of strength class b300 corresponding to the strength class c20/25 in current design standards in europe [3]. for the determination of the sampling locations, a minimum distance of 3.5 m between those locations corresponding to the requirements on the correlation length of compressive strength properties described in [4] was respected. figure 1(a) gives an impression of the drilling process on a bridge part after demolition while figure 1(b) shows a bird’s eye view of the structure. for further information concerning the research project the data set originates from, please refer to [5–7]. the material testing of the cylinder strength of hardened concrete specimens was conducted according to din en 12390-3:2019-10 [8] using cylindrical drill cores with 150 mm in diameter and about 300 mm in height. the results of the material tests are given in table 1. the material tests were carried out on a compression test machine of type mts (max. force 5 mn). the accuracy of the strength testing procedure can mainly be attributed to the measurement of the pressure area with an estimated homoscedastic measurement uncertainty of σ = 0.05 n/mm2 [9]. specimen no. test result [ n/mm2] no. 1 59.6 no. 2 55.1 no. 3 45.7 no. 4 48.2 no. 5 58.0 no. 6 58.0 no. 7 70.4 no. 8 59.9 no. 9 73.4 no. 10 38.0 no. 11 67.9 no. 12 63.6 no. 13 57.2 no. 14 73.0 table 1. data from compressive strength testing. due to additional sources of uncertainty from the reading of the results and the handling of the specimen, the uncertainty in the data shall be assumed as a normal distribution with an estimated standard deviation of ≈ 1 n/mm2. in addition to the cylinder strength testing, the young’s modulus was determined. since this contribution focuses on the methodological approach, the test results are not reported here. 77 m. haslbeck, r. kroyer, a. taras, t. braml acta polytechnica ctu proceedings figure 2. (a) histogram of the observed concrete strength values (b) kernel density estimation of the standard deviation of the bootstrap samples as statistical scatter estimator. 2.2. statistical analysis of the original data set the data set from section 2.1 has been examined by means of descriptive statistics. figure 2(a) shows a histogram of the data set using 6 bins. the mean value of the 14 samplesis 59.1 n/mm2, while the standard deviation in the data set is 10.3 n/mm2. in order to obtain a good estimation on the point estimators of the population from one specific and limited set of test results, the bootstrapping methodology can be used. in this methodology, the available data set is resampled with replacement to generate additional vectors of resampled data that can be compared among each other. the distribution of a certain estimator, in this case the expected value, the standard deviation and the 5 %-quantile of the compressive strength, is subsequently evaluated using the vectors of resampled data. the derived statistics can thus be seen as an evaluation of the uncertainty for the point estimators. the standard deviation of the point estimator for the measurement results can be computed to be ≈ 2.7 n/mm2 (expectation) and ≈ 1.8 n/mm2 (standard deviation), while it is about 4.6 n/mm2 for the 5 %-quantile. this indicates that the gained data give a sufficient approximation, but still leaves space for further research which uses more specimens. further details on the bootstrap methodology can be found, e.g. in [10–12]. in order to visualize the assumed pdf of the bootstrap-samples, kernel density estimation can be employed to smoothen the graph. for the kernel density shown in figure 2(b), a number of 106 resamples have been evaluated. 2.3. bayesian framework the application of bayes’ theorem allows for the updating of information based on knowledge from observations i.e. measurements. as a basis for updating, the distribution p(fc) has to be established which states our belief in the occurrence of a certain compressive strength. in order to formalize our belief in the appearance of a certain observation under uncertainty, the conditional probability p ( d̃fc |fc ) has to be determined which represents the probability to observe a cylindrical compressive strength d̃fc conditional on the value of fc. this expression is also called likelihoodfunction and is abbreviated here l ( fc|d̃fc ) . the formal expression for l ( fc|d̃fc ) for a number of n observations and a measurement uncertainty of σ is given in equation 1. l ( fc|d̃fc ) = n∏ i=1 1√ 2πσ2 exp ( fci − d̃fci 2σ2 ) (1) as input for the simulations, the test data from table 1 has been used and is denominated d̃fc in equation 1. the measurement uncertainty inherent to observations using imperfect measurement devices finds its way into the formula using the standard deviation, which has been determined in section 2.1 to be 1 n/mm2 . for the specification of the applied probability function for p(fc), please refer to chapter 2.5. using these basic relations, bayes’ theorem can be stated when normalized by the probability p(d̃fc). the fundamental equation of bayes’ theorem is given in equation 2. for a more detailed review on bayesian techniques please refer to [13–15]. p ( fc|d̃fc ) = p(fc)p(d̃fc |fc) p(d̃fc) = p(fc)l(fc|d̃fc ) p(d̃fc) (2) 2.4. markov chain monte carlo as the evaluation of the posterior distribution is in many cases not feasible in an analytical way, monte carlo techniques are to be used for the computation 78 vol. 36/2022 bayesian uncertainty updating concrete strength properties figure 3. flowchart of the applied algorithm for sampling from the posterior distribution π(·). of posteriors. the basis of the markov chain monte carlo (mcmc) methodology is the construction of one or more markov chains. in a discrete markov chain the sampled values of the variable only depend on the preceding sample, but not on the states sampled before. the markov chain explores the posterior which is its stationary distribution. however, in order to ensure this property, a certain burn-in sequence of samples has to be cut off from the sample set [16]. in this study, a single-chain algorithm has been implemented in matlab using metropolis sampling. as a proposal distribution, a normal function x ∼ n ( 0, σ2 = 4 (n/mm2)2) has been used and a burn-in of 200 samples has been implemented giving reasonable rates of acceptance. as a starting value 60.0 n/mm2 was applied. the entire length of the markov chain has been determined to be 20,200, which leads to a number of 20,000 samples for the construction of the posterior. the applicability of these parameters is proven in chapter 3.1. a flowchart of the applied algorithm is given in figure 3, where the posterior distribution is abbreviated by π(·). further information for the implementation and a good overview on the subject is given in [17]. 2.5. choice of prior distribution the prior information on the strength property is a basis for the updating procedure and thus has to be determined properly. therefore, a literature review has been conducted in order to determine the type of distribution, the expectation and the standard deviation. the test specimens were drawn from a structure that was built in concrete. according to the structural analysis documentation of the structure, concrete of material strength class b300 according to the former design code din 1045:1959-11 [18] was used in the sampled parts. this former designation for a concrete strength class corresponds to class c20/25 in din en 19921-1 [19] as described in [3], resulting in an expected strength value of fcm = 28 n/mm2. according to [20], a standard deviation of σ ≈ 5n/mm2 shall be assumed. the probability distribution function is approximated with a normal distribution [21]. consequently, the prior can formally be expressed by x ∼ n ( 28 n/mm2, σ2 = 25 (n/mm2)2) . for further insight into the probabilistic modelling of historic concrete structures and the derivation of a truncated distribution function on the basis of historic quality regulations, please refer to [22]. 2.6. workflow of the conducted procedure in order to study the methodological effect, i.e. the potential of the methodology as a measure of how much the characteristic quantile in the updated "posterior" distribution is increased in comparison to the nominal strength value, subsets of different size were built from the original data set with 14 test results. the sizes of those subsamples were chosen at random for studying purposes and contained 1, 3 or 5 test specimens respectively. since all possible combina79 m. haslbeck, r. kroyer, a. taras, t. braml acta polytechnica ctu proceedings figure 4. illustration of the procedure. figure 5. (a) traceplot of the first 200 samples for a simulation using 5 measurements (no. 1,3,4,7,12); (b) histogram of the samples from the posterior distribution using 5 measurements (no. 1,3,4,7,12). tions of test results for the mentioned subsample sizes were built, the uncertainty in the posterior distribution resulting from the sampling pattern is addressed simultaneously. this results in 14 subsamples of size 1, whereas 364 subsamples of size 3 and 2002 subsamples of size 5 were created and analysed. for all subsamples of a certain size, the corresponding posterior distributions are stored and empirical probability density functions for the different point estimators are calculated after completion of the updating process for all subsets. the computed mean values, standard deviations and the 5 %-quantiles of the simulations are presented as a kernel density-pdf and empirical cdf in section 3. the workflow is illustrated in figure 4. 3. results and discussion 3.1. determination of burn-in and convergence to ensure the validity of the computed distributions, a small study is conducted that verifies the burn-in and the number of mc-samples taken for the exploration 80 vol. 36/2022 bayesian uncertainty updating concrete strength properties figure 6. (a) kernel density function and (b) ecdf of the simulation results of the mean value using 1, 3 and 5 drill cores. figure 7. (a) kernel density function and (b) ecdf of the simulation results of the standard deviation using 1, 3 and 5 drill cores. of the posterior. figure 5(a) shows a representative traceplot which indicates that a burn-in-sequence of 200 is more than sufficient to make the samples independent from their starting value. the convergence of the results has been determined differently to the recommendations in [17] via bootstrapping (see section 2.2) using 104 replications. the maximum of the length of the 95 %bootstrap confidence intervals for the applied number of 20,000 samples (without consideration of burn-in) can be determined to be at maximum 0.03 (n/mm2) for the mean values, 0.03 (n/mm2) for the standard deviation and 0.02 (n/mm2) for the simulations of 5 %quantiles. the uncertainty in the recording of this posterior distribution thus can be seen as sufficiently small to ensure validity on the results from section 3. a histogram of one of the chains for 20,000 samples is given in figure 5(b). 3.2. uncertainty in the simulated mean values the uncertainty that can be attributed to the determination of the posterior distributions shows significant differences for different quantities of applied observations. when only one sample is used, the standard deviation of the mean values is at σ ≈ 10 (n/mm2), while a number of 3 or 5 core samples shows variations of σ ≈ 5 (n/mm2) and σ ≈ 3.7 (n/mm2). figure 6(a) and figure 6(b) show a pdf and the empirical cdf of the data. the jumps visible in figure 6(b) stem from the limited number of drawings. the simulation results indicate that from a certain sample size, the accuracy of a simulated expected 81 m. haslbeck, r. kroyer, a. taras, t. braml acta polytechnica ctu proceedings figure 8. (a) kernel density function and (b) ecdf of the simulation results of 5 %-quantiles using 1, 3 and 5 drill cores. value grows subproportionally and reaches reasonable results for a number of 3 core samples. 3.3. uncertainty in the simulated standard deviation similar to the study from 3.2, also the empirical standard deviations of the computed mcmc-samples have been computed. the peak value of the results using only one core sample for updating is at about 1 (n/mm2), which is roughly the assumed measurement uncertainty. for samples of 3 and 5 cores, the simulated values of standard deviation are smaller and show less scatter. figure 7 gives both the kernel density (a) and the empirical cdf (b) for the simulation results of standard deviations across the different densities. 3.4. uncertainty in the simulated 5 %-quantiles the 5 %-quantile is of high interest for practical engineering problems and is defined as the applicable characteristic value in most standards. again, the deviation of the 5 %-quantile is significantly higher for a sample set of size one and reaches more stable values for samples of 3 and 5. the respective values are σ ≈ 10, 5.2 and 3.7 (n/mm2). besides the absolute values and the distribution functions given in figures 8(a) and 8(b), the results also reveal the enormous potential of updating information on concrete compressive strength for the practical application in the reassessment of infrastructure buildings. in this example, the characteristic value of a b300 ≈ c20/25 can roughly be estimated to be at least 35 (n/mm2) instead of 25 (n/mm2). 4. conclusions this contribution reports about application of a bayesian updating framework in civil engineering. more precisely, the compressive strength of concrete is subject to this procedure in order to study the potential and the characteristics of such an algorithm. the studies are carried out using a data set taken from a real large-scale concrete bridge and the aspects of spatial variation as well as uncertainties in the sampling of the compressive strength from the bridge are taken into account explicitly. in order to address those aspects, the analysis was carried out for different subsamples. the framework shown here applies bootstrapping for first statistical evaluation of the available data set. subsequently, the markov chain monte carlo method was chosen and implemented to carry out bayesian updating. by conducting a great number of mcmc simulations for all possible combinations of single measurement values according to the subsample sizes, uncertainties from measurement were included appropriately in the analysis. the results obtained for the example in this contribution revealed considerable scatter in the point estimators of the posterior distributions, especially for the mean value and the 5 % quantile. as expected, the scatter in those parameters was significantly higher for the rather academic subsample of size one. for the subsamples of size 3 and 5 a certain trend of convergence was observed. a minimum of three samples is thus recommended. the effect of the presented methodology is to be measured best by the 5 % quantile of the concrete compressive strength. by comparing the nominal compressive strength of a c20/25 according to the design guidelines with the 5 % quantile after application of the sampling and updating procedure, the algorithm proposes an increase from 25 to about 35 n/mm2 corresponding to nearly 82 vol. 36/2022 bayesian uncertainty updating concrete strength properties 50 % increase in the concrete compressive strength. however, the simulations unveiled considerable scatter margins on the point estimators. with regard to the 5 % quantile of the concrete compressive strength, the uncertainty in the point estimators resulted in an uncertainty range between 16 to 5 % depending on the size of the subsample. the posterior distribution is thus not a unique stochastic model, since the sample values and the sampling process include some uncertainties. the presented paper contributes to uncertainty quantification in bayesian updating processes in consideration of spatial variability of the drawn samples and measurement uncertainty. acknowledgements the author’s team would like to thank company max bögl, sengenthal, germany, for providing and operating the drilling equipment. references [1] f. zimmert, t. braml, m. keuser. decision-making for the efficient life cycle management of structures. structural concrete 21(4):1217-26, 2020. https://doi.org/10.1002/suco.201900408. 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[19] din en 1992-1-1:2011-01. eurocode_2: bemessung und konstruktion von stahlbetonund spannbetontragwerken_teil_1-1: allgemeine bemessungsregeln und regeln für den hochbau; deutsche fassung en_1992-1-1:2004_+ ac:2010. berlin: beuth verlag gmbh, p. 241, 2011. [20] h. rüsch, r. sell, r. rackwitz. statistische analyse der betonfestigkeit. berlin: ernst, 1969. [21] deutscher ausschuss für stahlbeton dafstb. heft 526 erläuterungen zu den normen din en 206-1, din 1045-2, din 1045-3, din 10045-4 und din 4226. berlin: beuth verlag gmbh, 2003. [22] m. haslbeck, t. braml. estimating concrete strength properties for existing structures using historic quality regulations for a monte-carlo-simulation in: fédération internationale du béton [hrsg.] preceedings of the 18th fib symposium: new trends for eco-efficiency and performance, 2021. 83 https://doi.org/10.1002/suco.201900408 https://doi.org/10.1002/best.201500022 https://doi.org/10.1002/best.200800606 https://doi.org/10.1016/j.conbuildmat.2006.08.002 https://doi.org/10.1016/j.conbuildmat.2006.08.002 https://doi.org/10.1007/978-3-030-91877-4_1 acta polytechnica ctu proceedings 36:76–83, 2022 1 introduction 2 methodology 2.1 evaluation of compressive strength 2.2 statistical analysis of the original data set 2.3 bayesian framework 2.4 markov chain monte carlo 2.5 choice of prior distribution 2.6 workflow of the conducted procedure 3 results and discussion 3.1 determination of burn-in and convergence 3.2 uncertainty in the simulated mean values 3.3 uncertainty in the simulated standard deviation 3.4 uncertainty in the simulated 5 %-quantiles 4 conclusions acknowledgements references my title https://doi.org/10.14311/app.2022.36.0206 acta polytechnica ctu proceedings 36:206–215, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague probabilistic models for resistance variables in fib model code 2020 for design and assessment miroslav sýkoraa,∗, wouter botteb, robby caspeeleb, dimitris diamantidisc, aurelio muttonid, raphaël d. j. m. steenbergenb,e a czech technical university in prague, klokner institute, šolínova 7, 166 08 prague, czech republic b ghent university, faculty of engineering and architecture, department of structural engineering and building materials, technologiepark-zwijnaarde 60, 9052 zwijnaarde, belgium c oth regensburg, faculty of civil engineering, galgenbergstrasse 30, 93053 regensburg, germany d école polytechnique fédérale de lausanne, school of architecture, civil and environmental engineering, station 18, ch-1015 lausanne, switzerland e tno delft, department of structural reliability, p.o. box 155, nl-2600 ad delft, netherlands ∗ corresponding author: miroslav.sykora@cvut.cz abstract. the fib model code offers pre-normative guidance based on the synthesis of international research, industry and engineering expertise. its new edition (draft mc 2020) will bring together coherent knowledge and experience for both the design of new concrete structures and the assessment of existing concrete structures. this contribution presents an overview of the main developments related to the partial factors for materials. in the draft mc2020, the partial factors are presented in tables for clusters of cases depending on consequence classes and variability of basic variables. furthermore, formulas and background information are provided to facilitate updating of the partial factors. this contribution discusses the different assumptions adopted in mc 2020 for design and assessment. main changes with respect to the previous version are related to description of the difference between in-situ concrete strength and the material strength measured on control specimens, and to modelling of geometrical variables. the presented comparison of the requirements imposed by eurocodes and mc 2020 for design reveals insignificant differences. the assessment requirements may be decreased by about 25% when the conditions specified in mc 2020 are satisfied. hence, the revised mc 2020 will provide designers and code makers with wider possibilities to utilise actual data and long-term experience in assessments of existing structures. keywords: assessment of existing concrete structures, eurocodes, fib model code 2020, partial factors, probabilistic models, reliability, updating. 1. introduction fib (international federation for structural concrete) has been systematically revising its flagship document fib model code (mc). the mc offers pre-normative guidance, synthesis of international research with industry and engineering expertise, and advanced tools for international code writers as well as industry practitioners. the main aspiration of its new edition (draft mc 2020) is to bring together coherent knowledge and experience for both design and assessment and to provide a single code for both new and existing concrete structures [1]. the fib mc serves as a pre-standard, intended to provide the basis for development of future eurocodes and other international standards. under fib commission 3 existing concrete structures, task group 3.1 is drafting the mc 2020 sections on: • reliability requirements (target reliability levels for various limit states in design and assessment and different consequence classes) and • the partial factor method for design and assessment. this contribution presents an overview of the main developments related to the latter topic, focusing on resistances. it is demonstrated that the revised mc 2020 should provide designers and code makers with wider possibilities to utilise actual data and longterm experience in the assessment of existing concrete structures. 2. probabilistic models for resistances 2.1. general the draft mc 2020 section on the partial factor method is focused on the analysis of basic structural elements such as beams, slabs and columns through simplified (analytical) models. the safety formats 206 https://doi.org/10.14311/app.2022.36.0206 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 36/2022 probability of fib model code 2020 resistance variables fc on control specimens vfc = 0.1 conversion factor for in-situ cast concrete (μη = 0.95, vη = 0.12) concrete section area (μa ≈ 1 and va ≈ 0.04) design resistance model uncertainty – column crushing (μθr = 1.02, vθr = 0.07) fc in-situ vfc,is = 0.08 or 0.15fc in-situ vfc,is = 0.08 or 0.15 concrete section area (μa ≈ 1 and va ≈ 0.01) concrete section area (μa ≈ 1 and va ≈ 0.01) assessmentassessment resistance model uncertainty – column crushing (μθr = 1.02, vθr = 0.07) figure 1. basic variables considered in resistance model for short columns in design and assessment situations. to be applied in conjunction with advance numerical models are also addressed by mc 2020 but they remain beyond the scope of this contribution. mc 2020 emphasises the importance of updating of the models applied in reliability verifications considering all available information about the structure. however, to assist routine applications, the model code also provides the conventional probabilistic models for parameters of resistances and load effects considered to be applicable for common reinforced concrete (rc) structures with a reasonable level of approximation. these conventional models are mostly based on the probabilistic models assumed in the background document to the revised pren 1992-1-1:2021 for design and assessment of concrete structures [3] and they are consistent with those provided by the jcss probabilistic model code [4], and with the recommendations of fib bulletin 80 [5]. focusing on non-deteriorated rc structures, the flexural resistance of members and the resistance of short columns under compression, r, are obtained as the product of a resistance model uncertainty, θr, geometrical property, a, and of material strength, f : r ≈ constant × θr × a × f (1) where the strength f can cover the factor, η, accounting mainly for the difference between the compressive concrete strength measured on control specimens (fc) and the in-situ strength (fc,is), and then fc,is ≈ η × fc. the resistance r is assumed to be lognormally distributed. it is assumed that reinforcement properties are governing bending failures whereas concrete compressive strength dominates in compressive crushing of columns without significant second order effects. figure 1 shows the basic variables considered in the resistance model for short columns in design and assessment situations. the flexural resistance, dominated by reinforcement yielding, is described in a similar way. table 1 provides the probabilistic models for resistance parameters considered in mc 2020. µ denotes the bias of a variable systematic deviation of random values of the variable from its characteristic (nominal) value, considered as the ratio of mean to characteristic (nominal) value; v is a coefficient of variation. 2.2. material properties the models for concrete strengths, fc and fc, is, and the conversion factor, η, provided in table 1 are related to ordinary strength concretes (cylinder concrete strengths approximately up to 100 mpa) and standard quality related to in-situ cast rc structures. for high-strength or precast concretes or other than standard execution quality, other models may apply [6]. the coefficient of variation (cov) of concrete strength commonly decreases with increasing mean strength; see also [7]. for instance, torrenti and dehn [8] proposed for in-situ cast concrete: vfc ≈ 0.1 × (fc/40 mpa)3/2 (2) the value of µη in table 1 represents concretes in tops of columns; for other governing regions the bias may be increased to 1.0 or up to 1.05 for bottoms of columns. similarly, for smaller members (h < 450 mm) lower biases µη = 0.95 may be considered while 207 m. sýkora, w. botte, r. caspeele et al. acta polytechnica ctu proceedings basic variable x situation dist. bias µx coeff. of variation vx note concrete compressive strength measured on control specimens (in-situ cast concrete), fc design ln exp(1.645vfc) = 1.18 0.1 µfc = 1.10 and vfc = 0.06 could be considered for precast concrete. concrete compressive in-situ strength, fc, is assessment ln exp(1.645vfc,is) = 1.14 − 1.28 0.08− 0.15 to be estimated from core tests. conversion factor for in-situ cast concrete, η design ln 0.95 0.12 based on [2]. for precast concrete, lower coefficient of variation can apply. yield strength of steel reinforcement, fy design and assessment ln [exp(1.645vfy)] = 1.08 0.045 no conversion factor is commonly applied (µη = 1 and vθ = 0). concrete section area, ac design n 1 0.04 the case of normal uncertainty in geometry considered as the default case, width of the column ∼ 300 mm. mc 2020 provides guidance for other cases. assessment n 1 0.015 should be based on measurements in the existing structure. normal uncertainty in geometry as adopted for design may be considered in the absence of measurements. see section 3 for details. effective depth, d design n 1 0.04 normal uncertainty in geometry as the default case, effective depth ∼ 200 mm. mc 2020 provides guidance for other cases. assessment n 1 0.01 to be based on measurements in the existing structure. normal uncertainty may be considered in the absence of measurements. flexural resistance model uncertainty, θr design and assessment ln 1.09 0.045 considered to derive γs in both design and assessment situations. resistance model uncertainty for crushing of columns without significant 2nd order effects, θr ln 1.02 0.07 considered to derive γc in both design and assessment situations. table 1. probabilistic models for resistance parameters considered in draft mc 2020. 208 vol. 36/2022 probability of fib model code 2020 resistance variables for larger members (h ≥ 450 mm), more favourable values may be applied (e.g. µη = 1.03). mc 2020 indicates that the conversion factor can be influenced by the effects of non-standard quality of execution, various types of binding materials, seasonal aspects, size of the structure etc. more details including an extensive literature review and the analysis of available test results can be found in the background document to pren 1992-1-1 [3]. note that the conversion factor η is implicitly covered by test results for in-situ compressive strength, fc, is, and µη = 1 and vη = 0 are considered when the model for fc, is is based on core tests. 2.3. geometrical variables regarding geometrical variables, it is assumed that the section area, ac, relates to the failure modes governed by concrete crushing (typically columns under compression) while the effective depth, d, should be taken into account when reinforcement yielding is dominating (typically bending of beams or slabs). the case of normal uncertainty in geometry is considered as the default case for design. for the section area of a square column with fully correlated dimensions b × b, no bias is assumed and the cov of ac can be obtained as: vac = 2vb = 0.04 × (300 mm/b)2/3 (3) when the two widths are statistically independent, the factor of 2 should be replaced by √ 2 and thus: vac = 0.0283 × (300 mm/b)2/3 (4) in the case of doubts about a level of correlation, use of equation 3 is recommended. when considering normal uncertainty in geometry, the statistical characteristics of effective depth can be considered as a function of its nominal value, dnom: µd = 1 − 0.05 (200 mm/dnom)2/3 vd = 0.05 (200 mm/dnom)2/3 (5) it is further considered that uncertainty in geometry may be reduced when a decisive geometrical property is measured in the existing (finished) structure. this is considered as the default case for the assessment of existing structures. µac = 1 and vac = 0.015 may then be considered for the section area, and µd = 1 and vd = 0.01 for the effective depth. further, uncertainty in geometry can also be reduced when an increased execution quality and quality control are required. for instance, when higher execution quality is reached (ensuring that the geometrical deviations of tolerance class 2 according to en 13670 on execution of concrete structures are fulfilled), µac = 1 and vac = 0.02 may be considered for the section area. for the effective depth, the following characteristics may be considered: µd = 1 − 0.025 (200 mm/d)2/3 vd = 0.025 (200 mm/d)2/3 (6) it can be shown that the partial factor for concrete, γc , is insignificantly affected by the variability of the section area in common cases. in contrast, covs of the effective depth, yield strength and model uncertainty for bending are of a similar magnitude and the variability of the effective depth should be adequately considered when determining the partial factor for reinforcement, γs . considering a reference period of 50-years, a recommended value of the sensitivity factor for resistance is αr = 0.8 and the target reliability index for design and consequence class 2 (medium failure consequences) is β = 3.8 in mc 2020. assuming that the three resistance variables (fy, d, θr) have a similar effect on the design or assessment value of the flexural resistance, the sensitivity factor for the effective depth is estimated as αd = 0.8/ √ 3 = 0.46 and its design value should approximately correspond to a 4% fractile, φ(0.46 × 3.8) = 4 % (with φ denoting the cumulative distribution function of the standardised normal distribution). for assessment situations, target reliabilities lower than those for design are considered in mc 2020 and a slightly higher fractile would characterise the assessment value of the effective depth. to provide indications for both situations, figure 2 displays 5% fractiles of the effective depth as a function of its nominal value, dnom (the fractiles are normalised with respect to dnom). the models for effective depth described in this section are applied. figure 2. effective depth 5% fractiles as a function of nominal value. figure 2 shows that the probabilistic models recommended for the effective depth in mc 2020 (for d = 200 mm) yield rather lower estimates for the cases of improved quality and particularly for normal 209 m. sýkora, w. botte, r. caspeele et al. acta polytechnica ctu proceedings uncertainty that are primarily relevant for design situations. in the latter case for dnom = 1000 mm, the design value of effective depth increases by about 8% in comparison to the reference case based on dnom = 200 mm. in contrast, measurements in the existing structure are associated with the smallest uncertainty and the highest design or assessment value of effective depth is obtained. regarding considerations of the variability of effective depth in design, mc 2020 provides an additional simplification. the variability of d may be ignored (µd = 1 and vd = 0) if the design resistance is based on the design value of effective depth obtained as dd = dnom − ∆d where: • dnom is determined on the basis of the nominal cover. • ∆d is the deviation of the effective depth with: ▷ ∆d = 15 mm for reinforcing and post-tensioning steel, ▷ ∆d = 5 mm for pre-tensioning steel 2.4. model uncertainties in the background document to pren 1992-1-1 [3], the statistical characteristics for resistance model uncertainties adopted in mc 2020 (table 1) are based on the comparison of test and model results. the model uncertainty is estimated as the ratio of test resistance to a model estimate [9]. for short columns with negligible second order effects, moccia et al. [6] investigated cases without and with eccentricity for cylinder concrete strengths up to 100 mpa. they found a bias of 1.02 and cov of 0.087. the latter is affected by the test variability and the uncertainties in the test parameters. considering typical variabilities for these effects, the vθr-value reduced to 0.07. the statistical characteristics for flexural resistance, µθr = 1.09 and vθr = 0.045, were derived in the background document [2] in a similar way. bending tests were considered for various reinforcement steel classes and calculated strains around 1-1.5%. both bias and cov tend to increase with increasing calculated strain. in the draft mc 2020, the resistance model uncertainties are assumed to have a lognormal distribution [4]; for further information see also annex a of fib bulletin 80 [5] or the numerical study by sykora et al. [10]. note that slightly less favourable resistance model uncertainty characteristics (bias around unity and higher cov) are indicated in mc 2020 complex situations analysed by advanced numerical models [11]. 3. updating of probabilistic models when specific information about the structure under investigation is available, the probabilistic models for materials and actions and subsequently related partial factors can be updated. particularly in the assessment of an existing structure, uncertainties in resistances and load effects can often be reduced on the basis of inspections, measurements, and tests. the experience from practical applications suggests that it is often highly beneficial when in-situ concrete strength is investigated and hence the important uncertainty in the conversion factor is eliminated. however, it is also possible that uncertainties to be considered in the reliability verification of the existing structure exceed those considered for a relevant design situation and the partial factors adopted for design might be insufficient. the prospective eurocode on the reliability assessment of existing structures, pren 1990-2:2021, makes distinction between the preliminary and detailed assessment; the latter being in the main focus of this contribution. regarding the updating of basic variables, the preliminary assessment is typically conducted assuming the default characteristics of basic variables similar to those for new structures while these characteristics are updated for the detailed assessment or in the case of doubts about an appropriate model for the basic variable. if justified to be relevant for the structure under investigation, prior information can be combined with new information obtained e.g. by measurements and/or tests through bayesian updating. mc 2020 provides no information about the strength of the prior information provided by the presented conventional models. following the background document to the reliability basis in eurocodes [12], it might be assumed that the mc 2020 resistance models are based on prior information associated with the equivalent sample sizes n′ = 1 to 5 for the mean values (biases) and v′ = 3 to 10 for standard deviations. typically, lower values apply for concrete and higher for steel reinforcement. more details can be found in the jcss probabilistic model code [4] and the jcss monograph on assessment of existing structures [13]. 4. recommended values of partial factors in draft mc 2020 the draft mc 2020 elaborates on a partial factor method for the reliability assessment of new or existing structures conforming to iso 2394:2015 for structural reliability principles and en 1990:2004 for basis of structural design. 4.1. partial factors for design for design with respect to ultimate limit states (ulss), the partial factor for materials is obtained according to mc 2020 as: γm = rk/rd ≈ [exp(−1.645 vf )]/[ µθr µa µη exp(−αr β √ v 2 θr + v 2 a + v 2 η + v 2 f ] (7) 210 vol. 36/2022 probability of fib model code 2020 resistance variables cc γc γs γg self-weight γg other permanent loads γq (imposed) γq (wind) cc1 1.4 1.1 1.2 1.3 1.3 1.6 cc2 1.5 1.15 1.25 1.35 1.5 1.85 cc3 1.6 1.175 1.25 1.4 1.7 2.1 table 2. recommended values of partial factors for design according to mc 2020 and various ccs. cc γc γs γg self-weight γg other permanent loads γq (imposed) γq (wind) cc1 1.1 0.975 1.125-1.2 1.25-1.3 1.0 1.05 cc2 1.15 1.0 1.125-1.225 1.275-1.325 1.075 1.15 cc3 1.15 1.0 1.15-1.25 1.3-1.375 1.25 1.3 table 3. recommended values of partial factors for assessment of existing structures according to mc 2020. where a denotes a decisive geometrical property and the subscripts "k" and "d" refer to characteristic and design value, respectively. equation 7 assumes that: • the resistance is obtained as the linear product according to equation 1. • the characteristic value of the material property (commonly strength) corresponds to a 5% fractile of the respective lognormal distribution. • unity characteristic values are considered for resistance model uncertainty and conversion factor, respectively. • statistical uncertainty is reflected in the estimate of characteristic resistance. the same assumptions apply also for relationship (8) in section 4.2. table 2 provides the recommended values of the partial factors for design according to mc 2020 for low, medium and high consequence classes (cc1 to cc3 respectively); γg denotes the partial factor for permanent load effects and γq for variable load effects. a detailed discussion on the partial factors for load effects is beyond the scope of this contribution but selected values are provided in table 2 and in table 3 in section 4.2 to allow for comparisons with the design requirements of eurocodes in section 5. for the sake of brevity, the following discussion is focused on cc2 only. the partial factors for materials in table 2 are derived using relationship (7) and considering: • the probabilistic models provided in section 2. • a 50-year reference period along with the target reliability index β = 3.8 (cc2) and sensitivity factors αr = 0.8 for resistance and αe = −0.7 for load effects. the partial factors in table 2 are in broad agreement with the values provided by eurocodes. important for many concrete structures is that γg for self-weight can be reduced to 1.25. in contrast, the γq-value for wind action effects is increased. note that en 1990:2002 and pren 1990:2021 keep partial factors for materials fixed across ccs and differentiates by adjusting partial factors for unfavourable load effects only. the background documents for mc 2020 further demonstrate that similar values of the partial factors are obtained when the informative annual target reliability indices β given in mc 2020 are taken into account along with the sensitivity factors adjusted for a 1-year reference period αr = 0.7 and αe = 0.8. note that mc 2020 provides partial factors for a number of other design situations such as the cases with higher execution quality, effective depth considerably exceeding 200 mm and effective depth measured in the finished structure. the draft mc 2020 emphasises that the recommended values of the partial factors for design or assessment may only be applied when the conditions of the structure under consideration comply with the assumptions adopted therein. examples of the cases when the γ-values should be updated include: • changes of the target reliability level and/or of the sensitivity factors • decreased or increased variability of a basic variable such as a material property, geometry, load effect or model uncertainty • basic variables having other probabilistic distributions than those assumed when deriving the γ-values in particular for existing structures, the partial factors often need to be updated considering structurespecific information. higher-level methods should be applied when structure-specific conditions deviate from those commonly accepted. the more advanced approaches include the design (assessment) value method (using e.g. equation 7), a full-probabilistic approach (with updated models for basic variables and without making assumptions on the values of the sensitivity factors), or risk analysis (by which the target level can be optimised for the particular structure). 211 m. sýkora, w. botte, r. caspeele et al. acta polytechnica ctu proceedings 4.2. partial factors for assessment in order to identify the need for a safety measure to achieve a required reliability level at uls for the existing structure, a relationship similar to equation 7 is considered for the partial factor for concrete: γm = [exp(−1.645 vf is)]/[ µθr µac exp(−αr β √ v 2 θr + v 2 ac + v 2 f is ] (8) where the in-situ concrete compressive strength evaluated from core tests, fc, is, is considered and the conversion factor is not included. table 3 provides the recommended values of the partial factors for assessment to verify the need of safety measure(s) (termed as "assessment of the existing structure" hereafter for brevity) under the assumptions provided below. the partial factors for materials in table 3 are derived using the equation 8 and considering: • the probabilistic models provided in section 2 along with vfc,is = 0.15 and vfy = 0.045, and various levels of the load effect model uncertainty (on which the γg-values are dependent while the γq-values are affected insignificantly) • an annual reference period along with the target reliability index β = 3.3 (cc2) and modified sensitivity factors αr = 0.7 and αe = −0.8 regarding decisive geometrical parameters, related uncertainties are described in mc 2020 for two cases: a) measurements in the finished structure and b) normal uncertainty. following section 2.2, the partial factors in table 3 are based on µa/anom = 1, vac = 0.015, and vd = 0.01 in case a). in case b), it is assumed that reliability verification adopts information on geometry from documentation that is checked by in-situ measurements, but the geometry of the decisive section is not directly verified. µac/ac,nom = 1, µd/dnom = 0.95, vac = 0.04, and vd = 0.05 are then considered. the draft mc 2020 specifies that the partial factors in table 3 should only be applied when: • concrete compressive strength is verified by adequate destructive tests (following standards on the evaluation of concrete in-situ strength such as en 13791:2019). • the type and amount of reinforcement is verified. • degradation does not affect structural behaviour significantly. additionally to the conditions described in section 4.1, updating of the recommended values may be needed e.g. due to structural damage including deterioration, differences in material properties, detailing provisions or execution tolerances. note that the partial factors for resistance may need to be increased to cover epistemic uncertainties related to the lack of knowledge about important details that may have a significant influence on structural behaviour such as actual geometry or amount of reinforcement in joints of frames. in these cases, a sensitivity analysis can help to explore the effect of such uncertainties on structural behaviour. as an example, assumptions for representative favourable and unfavourable situations might be made and structural analysis can then reveal the effect of this uncertainty on structural resistance. the draft mc 2020 further emphasises that human safety may require reliability levels higher than those provided in the mc for the assessment (such as annual β = 3.3 for cc2) if structural failure is expected to result in human losses. in particular, for special cases e.g. with many persons at risk or with large failure consequences, higher target levels might be more appropriate and the partial factors given in table 3 should be increased. it is seen from table 3 that all mc 2020 partial factors for the assessment are considerably lower than those provided in the mc and in eurocodes for design. as an example of the important change, γs reduces from 1.15 to 1.0. this decrease can be associated with two differences between design and assessment situations of similar importance: 1. the probability of the fractile associated with the design (assessment) value of resistance increases by an order of magnitude from φ(αr × β) = φ(0.8 × 3.8) = 1.18 ‰ for design to φ(0.7 × 3.3) = 1.04% for the assessment. 2. the effective depth is assumed to be measured in the existing structure and the bias and cov becomes more favourable than in design (cf. figure 2 indicating the difference of about 10% for normal uncertainty and measurements in the finished structure). in addition, mc 2020 again provides partial factors for other assessment situations (concrete strengths with low variability when vfc,is = 0.08 or reliability assessments based on information about geometry from verified documentation). 5. comparison of design and assessment requirements in this section, the requirements imposed by the recommended values of the partial factors according to eurocodes ("ec", primarily intended for design), draft mc 2020 for design (mc design), and for assessment (mc assessment) are compared. table 4 provides the overview of the values considered in this section, focusing on cc2 only. the comparison is made on the basis of a fundamental reliability condition, rd = ed, for compression of the short column with a dominating concrete contribution according to equation 9 and for bending of the beam with a dominating reinforcement contribution according to equation 10: 212 vol. 36/2022 probability of fib model code 2020 resistance variables reference γc γs γg permanent loads γq (imposed) γq (wind) ec* 1.5 1.15 1.35 1.5 1.5 mc design 1.3** 1.85 mc assessment 1.15 1.0 1.25**,*** 1.075 1.15 *considered here to be primarily intended for design. **considering self-weight and other permanent actions equally important. ***averaged over low and normal levels of load effect model uncertainty, rounded. table 4. overview of partial factors considered in comparison of design and assessment requirements (cc2). ac fck/γc = γg gk + γq qk = γg(1 − κ) + γq κ = ed(κ) (9) as fyk dnom/γs = γg gk + γq qk = ed(κ) (10) where κ = qk/(gk + qk)is the load ratio based on the characteristic values of load effects; and ed = design (assessment) values of the load effect. the ratio of section or reinforcement area required by mc assessment to the requirement by eurocodes becomes: ρc,ex = ac,ex ac,ec = γc,ex ed,ex(κ) fck,is γc,ec ed,ec(κ) fck ≈ 1.15 γc,ex ed,ex(κ) γc,ec ed,ec(κ) (11) ρc,ex = ac,ex ac,ec = γs,ex ed,ex(κ) fyk dm γs,ec ed,ec(κ) fyk dnom ≈ 1.05 γs,ex ed,ex(κ) γs,ec ed,ec(κ) (12) in equation 11, ec is based on the characteristic value of concrete strength from control specimens, fck = µfc exp(1.645 vfc) ≈ 0.85 µfc considering vfc = 0.1. mc assessment applies the characteristic value of in-situ strength affected by the bias of the conversion factor. when the effect of concrete aging is-mostly conservativelyignored, the characteristic value might be estimated as fck,is ≈ µη µfc exp(1.645 vfc,is) which leads to fck,is ≈ 0.74 µfc for µη = 0.95 and vfc,is = 0.15, and to fck/fck,is ≈ 1.15. for mc design based on fck, this effect plays no role and equation 11 reduces to ρc,des = γc,des ed,des(κ)/[γc,ec ed,ec(κ)]. in equation 12, all three approaches are based on the same characteristic value of yield strength, fyk, and there is no numerical effect on ρs-values. ec is based on the nominal value of effective depth, dnom, while mc assessment assumes that in-situ measurements provide its mean, dm. according to section 2.3, it is then considered dnom/dm ≈ 1.05. for mc design based on dnom, equation 12 reduces to ρs,des = γs,des ed,des(κ)/[γs,ec ed,ec(κ)]. when evaluating ρ-values, the load ratio is a study parameter considered in the range from 0.1 to 0.7 characteristic for rc structures [14] and the values of partial factors are considered according to table 4. figure 3 displays the variation of ratio ρ for mc design. when ρ < 1, mc design leads to lower requirements than ec. in this case, there is no difference between flexural resistance and columns under compression as the same material factors are provided by ec and mc design (cf. table 4). in the case with imposed loads, mc design leads to nearly same levels as ec (the former being lower by 1-3%). for wind pressure (and similarly for snow loads), mc design (50 y.) requires higher resistances (5-15%) as the γw -value is significantly increased in comparison to ec. figure 3. variation of ratio ρ for mc design. figure 4 displays the variation of ratio ρ for mc assessment. in this case, mc assessment is below ec for all situations. in comparison to flexural resistance, 213 m. sýkora, w. botte, r. caspeele et al. acta polytechnica ctu proceedings figure 4. variation of ratio ρ for mc assessment. systematically lower levels are obtained for columns in compression. for flexural resistance, 75-80% of the reference ec level is mostly reached for variable actiondominated or permanent action-dominated structures, respectively. for columns, the level drops to 70-75%. for both failure modes, slightly lower levels are obtained for imposed loads. the ρ-ratios shown in figure 4 can be considered low and consequently the reliability levels associated with mc assessment might be questioned. such reduced requirements are attributed mainly to a lower target reliability adopted in the mc for assessment. as an example, for κ = 0.5 and wind pressure, the γc-value increases from 0.74 (obtained for β = 3.3) to 0.83 (β = 3.7). a smaller effect is related to reduced uncertainty as the measurements in the existing structure are assumed. 6. discussion besides the partial factors for design and assessment discussed in this contribution, the draft mc 2020 provides guidance for a number of other design and assessment situations. while new parts of an upgraded structure are to be designed in a way proposed for new structures, a specific approach for the reliability assessment of existing parts of the upgraded structure is provided. this approach is based on requirements adopted also for assessment in section 4.2, addressing the need of concrete core tests, the verification of the actual reinforcement, and a check of the degradation. however, in contrast to section 4.2, the target reliability levels for design are considered and dimensions are assumed to be measured in-situ in all cases (as is common practice before structural interventions). the draft mc 2020 provides detailed guidance for the specification of the target reliability levels considering annual or 50-year reference periods, somehow recommending the use of the former one. while both reference periods can be considered in design situations, in assessment situations mc 2020 focuses only on annual reference period and recommends annual target levels (requiring that the annual target reliability should be fulfilled in every year of a service life of the structure). for the two reference periods, different values of the sensitivity factors are recommended. 7. summary and concluding remarks the partial factors for materials, permanent load effects, and variable load effects are presented in draft fib mc 2020 in tables for the clusters of cases depending on consequence classes and variability of basic variables. formulas and background information facilitate updating of the recommended values, i.e. adjusting the partial factors for structure-specific conditions. the key inputs for deriving the partial factors include the probabilistic models of basic variables. different assumptions may then need to be adopted in design and assessment situations; the main differences are related to the considerations of the concrete strength measured in control specimens (design) and in-situ compressive concrete strength (assessment) as well as to the models for geometrical properties. the presented comparison of the requirements imposed by eurocodes and mc 2020 for design reveals on average insignificant differences (with somewhat increased partial factors for snow and wind loads and reduced partial factor for permanent actions in the model code). as a key new feature in the draft mc 2020, a lower target reliability is recommended for the assessment and the respective assessment requirements may decrease by about 25%. the conditions specified in mc 2020 should then be satisfied, including in-situ verification of concrete strength, reinforcement, and effects of degradation. an important principle when developing mc 2020 proposals was that the model code should be consistent with eurocodes, primarily with pren 1990 for basis of design and assessment and pren 19921-1 for design and assessment of concrete structures including bridges. regarding specific issues for concrete structures, mc 2020 often offers more detailed guidance than eurocodes. in particular, the revised mc 2020 will provide designers and code makers with wider possibilities to utilise actual data and long-term experience in assessments of existing structures. acknowledgements this study is based on the developments of fib com 3 tg 3.1. it has been partly supported by the czech science foundation under grant 20-01781s; results of grant ltt18003 supported by the ministry of education, youth and sports of the czech republic have been utilised. 214 vol. 36/2022 probability of fib model code 2020 resistance variables references [1] s. matthews, a. bigaj-van vliet, j. walraven, et al. fib model code 2020: towards a general code for both new and existing concrete structures. structural concrete 19(4):969-79, 2018. https://doi.org/10.1002/suco.201700198. [2] f. moccia, x. kubski, m. fernández ruiz, et al. the influence of casting position and disturbance induced by reinforcement on the structural concrete strength. structural concrete 22(s1), 2020. https://doi.org/10.1002/suco.201900562. [3] a. muttoni. background document to 4.3.3 and annex a, partial safety factors for materials. cen/ tc250/ sc2/ wg1/ tg6 (report epfl-ibeton 16-06-r11). epfl ecole polytechnique fédérale de lausanne, lausanne, 2021. [4] jcss. jcss probabilistic model code (periodically updated, online publication). joint committee on structural safety. www.jcss.byg.dtu.dk. [5] fib. fib com3 tg3.1. partial factor methods for existing structures (fib bulletin 80, recommendation). fib, lausanne, 2016. [6] f. moccia, q. yu, m. fernández ruiz, et al. concrete compressive strength: from material characterization to a structural value. structural concrete 22(s1), 2020. https://doi.org/10.1002/suco.202000211. [7] m. weber, c. thiele. correlation between compressive and tensile strength of old concretes: applicability of the relationship described in en 1992-1-1. structural concrete 20(1):483-92, 2019. https://doi.org/10.1002/suco.201700244. [8] j. m. torrenti, f. dehn. on the relation between the mean compressive strength and the characteristic one. structural concrete 21(1):409-12, 2019. https://doi.org/10.1002/suco.201900153. [9] m. holický, j. v. retief, m. sýkora. assessment of model uncertainties for structural resistance. probabilistic engineering mechanics 45:188-97, 2016. https: //doi.org/10.1016/j.probengmech.2015.09.008. [10] m. sýkora, j. krejsa, j. mlcoch, et al. uncertainty in shear resistance models of reinforced concrete beams according to fib mc2010. structural concrete 19(1):28495, 2018. https://doi.org/10.1002/suco.201700169. [11] m. engen, m. a. n. hendriks, g. monti, et al. treatment of modelling uncertainty of nlfea in fib model code 2020. structural concrete 22(6):3202-12, 2021. https://doi.org/10.1002/suco.202100420. [12] cen/ tc250/ sc10/ ahg reliability (chair acwm vrouwenvelder). technical report for the reliability background of eurocodes (draft, june 2021), cen tc250/ sc10, 2021. [13] jcss. probabilistic assessment of existing structures (ed. d. diamantidis). joint committee on structural safety, rilem publications s.a.r.l., paris, 2001. [14] n. e. meinen and r. d. j. m. steenbergen. reliability levels obtained by eurocode partial factor design a discussion on current and future reliability levels. heron 63(3): 243-302, 2018. https://heronjournal.nl/63-3/2.pdf. 215 https://doi.org/10.1002/suco.201700198 https://doi.org/10.1002/suco.201900562 www.jcss.byg.dtu.dk https://doi.org/10.1002/suco.202000211 https://doi.org/10.1002/suco.201700244 https://doi.org/10.1002/suco.201900153 https://doi.org/10.1016/j.probengmech.2015.09.008 https://doi.org/10.1016/j.probengmech.2015.09.008 https://doi.org/10.1002/suco.201700169 https://doi.org/10.1002/suco.202100420 acta polytechnica ctu proceedings 36:206–215, 2022 1 introduction 2 probabilistic models for resistances 2.1 general 2.2 material properties 2.3 geometrical variables 2.4 model uncertainties 3 updating of probabilistic models 4 recommended values of partial factors in draft mc 2020 4.1 partial factors for design 4.2 partial factors for assessment 5 comparison of design and assessment requirements 6 discussion 7 summary and concluding remarks acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.38.0275 acta polytechnica ctu proceedings 38:275–280, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague the road to circularity: a framework for and experiences in collecting road data in a circular renovation process bram entrop saxion university of applied sciences, school of business, building & technology, research group sustainable environment, m.h. tromplaan 28, 7513 ab, enschede, the netherlands correspondence: a.g.entrop@saxion.nl abstract. the construction and transport sectors both have a substantial impact on the environment. the construction, maintaining and renovating of roads involves both these sectors and the environmental impact of this work can be reduced. the basic principle of a circular economy is to close material loops and so retain the highest utility, quality and value of products, components and materials as possible. an important question in this respect is how to qualify and quantify material flows. material and project passports seem to be part of the solution to improve insights and sharing information on quantities and qualities of materials used in construction projects. this paper has used a literature study on material passports and has taken into account current project management software used by a municipality, in order to share a framework for organising and collecting road construction data. furthermore, various scanning equipment and procedures were employed onsite in an experiment in collecting actual road data. this resulted in a large amount of different data files that have been interpreted and incorporated into the existing database structure of the municipality. the insights gained may help other researchers, principals and contractors in the road construction industry in collecting and storing reliable data necessary to renovate roads circularly. keywords: road renovation, circularity, collecting data, material characteristics. 1. introduction the construction industry is under pressure to strive for closed material loops. as part of the european horizon 2020 project cityloops, municipalities are developing and testing tools that help enhance a circular economy. however, before the built environment can be regarded as circular, not only the material loops in buildings need to be closed, but also the loops in infrastructure projects. in cityloops, the municipality of apeldoorn has been collaborating with saxion uas to create a circular road-renovation project. this project can use comparable circular principles as applied to buildings, e.g. reducing the need for materials, designing for disassembly, reusing used components, products and materials (e.g. [1, 2]). experiences with consumer products and buildings show that in order to achieve a circular economy, there needs to be an awareness of the specifications characterising them (e.g. [3]). therefore, this paper starts with a theoretical framework of material characteristics that are necessary to enable others to close material loops. this information can be recorded in material and project passports (e.g. [1, 4]), which seem to be part of the solution to improving insights and sharing information on quantities and qualities of materials used in construction projects. although this literature study might be able to provide insights into which information is required, it is not always possible to collect all the information needed. on the one hand, collecting, organising and storing data takes time and costs apply. on the other hand, data might already be available from the stakeholders involved. with this in mind, six interviews were conducted with experts who were able to give insights into road renovation projects and the collecting and storing of road data. the construction and maintenance of infrastructural projects are often commissioned by public entities. considering these public entities are experienced professional principals, it is likely that they have at least part of the data needed for material passports. however, in moving from a linear to a circular economy it might well be possible that extra data is required to know in more detail what materials in which quantities and with what qualities are at our disposal. therefore, the main research question in this project is: how can material characteristics be obtained for renewing roads circularly in residential areas? the research project, disseminated in this paper, focuses on how to match required and available data around a particular road renovation project in the dutch municipality of apeldoorn. the aim was to renovate the residential paved road at griffiersveld, and adjacent public space, as circularly as possible. 275 https://doi.org/10.14311/app.2022.38.0275 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en bram entrop acta polytechnica ctu proceedings 2. theoretical framework 2.1. circularity and data the concept of a circular economy has gained momentum, as kirchherr et al. [5] aptly put it, when analysing 114 definitions in order to create greater clarity in understanding this concept. they concluded that circular economy within their iteratively developed coding framework, can be defined as “an economic system that replaces the ‘end-of-life’ concept with reducing, alternatively reusing, recycling and recovering materials in production/distribution and consumption processes” [5, p. 229]. when introducing this concept to the construction industry, where large amounts of materials are used, it soon became clear that relatively little seems to be known about which materials are applied where. hart et al. [6] mention that buildings and infrastructure both “are characterised by long lifespans, numerous stakeholders, and hundreds of components and ancillary materials that interact dynamically in space and time”. from this perspective not only buildings can be regarded as material banks, but also infrastructure and other materialised public space. being a set of data, material passports contain information that enables us to understand the background of a particular material and therefore the possibilities to reuse, recycle and recover it. in some cases these material passports are also known as product passports or circularity passports [4]. in 2009 by means of life cycle analysis (lca) huang et al. [7] studied how the impact on the environment of asphalt pavements varies when using different process and pavement parameters. the quality of data for these parameters is of vital importance. heinrich & lang [4] distinguished four categories of information in a material passport, namely process, biological, chemical and physical information. based on interviews and by studying multiple existing methods to store information on materials and products, goselink [8] defined a top five of requirements for a material passport, namely: (1.) it needs to include a bill of materials (bom) with quantities, material composition, and location (gis) of the materials on site; (2.) inspection and maintenance history of the materials on site needs to be recorded in the passport; (3.) it includes technical lifetime expectancy of materials on site, so information on production date, manufacturer’s or contractor’s lifetime expectancy adjusted with information from the field; (4.) renovation or “end-of-life” options of the materials are addressed; (5.) the setup of the material passport complies with a uniform system and clear definitions. although much information might be available, it is seldom easily or freely accessible at one central point. building information modelling (bim) is a potential gamechanger in the construction industry, as honic et al. [9, 10] put it. charef and emmitt [11] also explored how bim can help practitioners to adopt a circular economy approach and identified seven new roles for bim, when it comes to a circular economy. in the netherlands, pavement information modelling (pim) has been introduced for infrastructure projects, specifically asphalt roads [12]. in pim a particular road (section) can be incorporated as an object with multiple underlying layers to store information in. from big to small, one can distinguish object, element, construction part, component, and construction unit. a construction unit might be a mixture containing multiple building materials. at the building material level a different set of qualitative and quantitative characteristics applies than at the level of a complete road section. some of the qualitative and quantitative characteristics might also be attributed to specific gps-locations [12]. 2.2. collecting road data automatically to be able to learn more about the quality and quantity of road materials, visual inspections are commonly used. more recently a broader range of sensors, besides the obvious use of video equipment, can be applied, which may or may not be mounted to a means of transport. this section addresses some of these automated data collection methods, not just for the sake of completeness but with attention to conciseness. data can be collected in the built environment for example, with the assistance of unmanned aerial vehicles (uavs). this enables effective and efficient data collection by means of a protocol to fly camera equipped uavs around objects, for example provided by [13]. regulations may apply and obstacles may be present when flying uavs visible to the naked eye over populated areas with a flight height of 35 m, but biçici & zeybek [14] demonstrated that automated detection of road surface distress through point clouds generated from uavs photogrammetry is within reach. experiences in using ground penetrating radar (gpr) systems to detect cracks in roads have also been recorded (e.g. [15]). the processing technique used here, made it possible to detect cracks larger than 1.3 mm in width. experimental results indicated that the gpr system can reliably be applied to automatically detect road cracking in practice. although this gpr system was moved at walking pace on a trolley across the cracks, it is also possible to mount it in front or at the back of a motorised inspection vehicle. gamma spectrometers can also be attached to inspection vehicles, or again uavs [16], to collect data that provides insights into the composition of a road and the underlying soil texture. due to specific gamma signatures of stones, it is also possible to distinguish differences in asphalt mixtures. some sensors are much smaller than the aforementioned ones, an interviewee suggested considering the 276 vol. 38/2022 the road to circularity: a framework for and experiences in . . . figure 1. screenshot of the gbi asset management system showing griffiersveld in apeldoorn. use of motion sensors in mobile phones that, in combination with gps, can enable researchers to estimate traffic density and the flatness of the road surface. last, but not least stationary sensors can also be used by placing them alongside or even in roads. 3. entering the field efforts of multiple stakeholders are needed to collect and store data appropriately. considering that dutch municipalities were managers of approximately 86 % of all roads in the netherlands in 2019, it is interesting to take a closer look at which data is available to and needed by them. this is over 120 000 km of roads [17], with speed limits from walking pace up to 50 km/h within city limits and up to 80 km/h outside the builtup area. the municipality of apeldoorn aims to close materials loops. in order to be able to facilitate this transition to circularity, it is necessary to be aware of which materials are in use and at which location. applications exist to help municipalities manage and maintain their public works. apeldoorn adopted a gis-based gemeentelijk beheer informatiesysteem (gbi – municipal management information system) of anteagroup to store data relating to the management and maintenance of public works. according to three of the interviewees, a visual inspection takes place to make sure the roads are clean, whole and safe, approximately once every two years. inspections like these make it possible to check if the municipality’s information in gbi still corresponds with the actual situation (see figure 1). when aesthetics, usability and/or safety fall short in real life, an intervention will be planned. an intervention might consist of relatively simple repairs up to a complete renovation of the road and surrounding public space. when taking a closer look at the key in figure 1, information about griffiersveld is also available regarding the different road materials in use. the neighbourhood in which the area under consideration is located, was built in the late seventies. the following road product categories can be distinguished: concrete pavers, concrete paving slabs, other types of paving slabs, semi-paved, dense graded asphalt concrete, clay brick pavers, presence of surface treatment, or other materials. for concrete and clay brick pavers two particular product sizes have their own categories, as these products are very common in this municipality. after becoming familiar with the selected case, griffiersveld, and with the way road information is currently stored at the municipality of apeldoorn, it is now time to take a closer look at the results of the extra data that was collected on site in two scan projects. 4. results an ms excel file extracted from the gbi-system shows that the municipality of apeldoorn distinguishes up to 53 different characteristics for each road section. these characteristics include the road’s identity, location, typology, inspection date, year of origin, maintenance year, appearance, safety level, width, surface and perimeter of the particular road section. a significant number of these 53 characteristics are particularly useful when focusing on the quality of asphalt roads, but are less relevant to roads consisting of concrete pavers or paving slabs. furthermore, it is striking to see that many cells addressing the qualitative characteristics of road sections are empty, due to 277 bram entrop acta polytechnica ctu proceedings figure 2. lines of measurement in griffiersveld. missing data. available data helps assess the quality of a road section by means of pavement unevenness, grout width, appearance and safety. quantities by means of the total number of pavers or paving slabs, their original sizes and original product mass are currently not provided. the main actors involved are not addressed either, so it might not be an easy task to learn more about the product’s manufacturer, the road’s contractor, contracted repairmen, inspector or principal. in order to test if we can easily complete the data automatically, an initial scan of griffiersveld was made on the 7th of april 2020. an ids ris hi-pave ground penetrating radar system at the back and a gamma spectrometer at the front of a van were used. the van drove approximately five times through griffiersveld resulting in a dataset, that seems to consist of lines, but are in fact measuring points in close proximity (see figures 2 and 3). the data collected by the gamma spectrometer offered insights into radioactivity levels (in bq/kg) of pavements. the ground penetrating radar made it possible to assess the type of pavement and its thickness, namely around 5 cm thick concrete paving slabs and 8 cm thick concrete pavers. furthermore, it was possible to get insights into the thickness of the sand package under the pavement, that seems to vary from 1.4 to 22.0 cm. in a second scan process light detection and ranging (lidar) data and panoramic high resolution images were collected on the 17th of june 2020. the scan equipment was mounted on the front of a van. this data, consisting of point clouds and images, gave insights into the location and status of surfaces and obfigure 3. applied pavement materials (red = concrete pavers, green = concrete paving slabs). jects located in and adjacent to griffiersveld. pavers, paving slabs and edge beams can be checked regarding typology, colour, thickness and condition. furthermore, a gis-based inventory was made of inspection chambers (or manholes), road gullies, street poles, light posts and street furniture (figure 4). 5. discussion the adopted asset management system originates from a time in which linear principles prevailed over circular principles. although much data is stored in gbi, items currently available do not necessary align with items that are part of material or project passports or with the actual situation in real life. the first set of scans made it possible to check if the upper surface of the road indeed consists of the materials as laid down in the asset management system, but only by means of its typology and thickness. the thickness of the layer of soil directly underneath the surface layer was also scanned. however, this characteristic could only be assessed for approximately a sixth of the length of the whole street. at other locations the sand base was not present or too thin to be detectable by the applied sensors. furthermore, the pavement was scanned for differences in typologies of crushed stone aggregates in the upper layer by means of their radiation values. it is, 278 vol. 38/2022 the road to circularity: a framework for and experiences in . . . figure 4. results of scanning griffiersveld through collecting images, radar and laser data. however, not clear how these radiation values can be linked to the quality of the road, or the necessity for repairing and replacing the road. due to the data collection method during the first scan process, material differences remained unnoticed by the sensors around two speed bumps consisting of pavers instead of slabs. also multiple footpaths were outside the scope of the data collection. however, in particular places sensors did notice that the thickness of the upper layer of the road changed, while this was not properly recorded in gbi. the asset management system mentioned the presence of concrete paving slabs here, but in fact concrete pavers are in place. these were probably put there after repairs had been made. the second set of scans resulted in point and surface data, where collected images were manually inspected to assess the products present. when comparing the collected information with the information stored in the asset management system, the collected data could be used to complete or update the information in the asset management system for multiple locations in the street. two locations of approximately 1 m2 and 10 m2 had been paved using concrete paving slabs, while this was not stored as such in gbi. also a small footpath consisting of concrete pavers along a parking place was spotted by the sensors, but had not yet been stored in gbi. the sensors also provided extra information regarding a pavement planter that was not properly assessed in gbi. at one location, a cycle path stored in the asset management system and consisting of asphalt, was unnoticed by the sensors. lastly, neither the asset management system, nor the scans picked up a basketball court in griffiersveld. the road, parking space and footpaths surrounding it were scanned and are included in the gbi data, but the municipality was unfamiliar with the court itself and therefore also the materials present. 6. conclusions in a circular economy material loops are closed and materials are used, maintained, repaired and re-used with the lowest environmental impact and the highest user-value possible. to accommodate a circular economy it is necessary to know what materials are currently in use. data that qualifies and quantifies the materials in place, is necessary to prepare materials for future life cycles. in this research project the municipality of apeldoorn was able to discover, whether the available and newly collected information on its asset griffiersveld, meant that it was already futureproof from a circular perspective. data addressing the quality of asphalt roads can be stored in the asset management system. the experiment conducted on a road consisting mainly of concrete with two different road scanning processes, showed that differences exist between stored and actual data. at griffiersveld these differences were in general relatively small, but not insignificant. it is also important to bear in mind that this was only one street in which multiple areas were paved differ279 bram entrop acta polytechnica ctu proceedings ently than expected and a basketball court had been evidently overlooked. hence, false assumptions existed concerning the materials the municipality had in use. although an asset management system is in place and much data has already been stored, the framework of goselink [8] shows that information on mass, exact product dimensions, and the composition of products is lacking. furthermore, lifetime expectancy and re-use options of materials, products and components in public space have not yet been assessed or registered. the consistency in naming elements is also an area for improvement. the most challenging aspect may well be how to assess the remaining technical lifetime expectancy of the materials in place and how to make sure that this remaining circular lifetime rules out linear arguments focusing on the need to use virgin materials. the research also demonstrated that although a ground penetrating radar system and a gamma spectrometer can give additional insights to the asset manager, the lidar system with high resolution imaging currently provides data that is closer to the traditional process of a visual on-site inspection. therefore this process tends to be favoured by the respondents interviewed. the use of a uav equipped with a highresolution camera, when legally acceptable, also appealed to multiple interviewees. acknowledgements the authors would like to express their gratitude to the public servants of the municipality of apeldoorn and respondents for providing valuable insights and data. this project has received funding from the european union’s horizon 2020 research and innovation programme under grant agreement no. 821033. references [1] m. r. munaro, a. c. fischer, n. c. azevedo, s. f. tavares. proposal of a building material passport and its application feasibility to the wood frame constructive system in brazil. iop conference series: earth and environmental science 225(1):012018, 2019. https://doi.org/10.1088/1755-1315/225/1/012018 [2] l. a. lópez ruiz, x. roca ramón, s. gassó domingo. the circular economy in the construction and demolition waste sector – a review and an integrative model approach. journal 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[2021-12-06]. https: //www.cbs.nl/nl-nl/visualisaties/verkeer-envervoer/vervoermiddelen-en-infrastructuur/wegen 280 https://doi.org/10.1088/1755-1315/225/1/012018 https://doi.org/10.1016/j.jclepro.2019.119238 https://doi.org/10.1016/j.jclepro.2019.118482 https://doi.org/10.1016/j.resconrec.2017.09.005 https://doi.org/10.1016/j.procir.2018.12.015 https://doi.org/10.1016/j.jclepro.2008.06.005 https://doi.org/10.1016/j.jclepro.2019.01.212 https://doi.org/10.1016/j.jobe.2019.01.017 https://doi.org/10.1016/j.jclepro.2020.124854 https://doi.org/10.1016/j.egypro.2017.09.636 https://doi.org/10.1016/j.autcon.2020.103475 https://doi.org/10.1016/j.jappgeo.2020.104118 https://doi.org/10.18174/466037 https://www.cbs.nl/nl-nl/visualisaties/verkeer-en-vervoer/vervoermiddelen-en-infrastructuur/wegen https://www.cbs.nl/nl-nl/visualisaties/verkeer-en-vervoer/vervoermiddelen-en-infrastructuur/wegen https://www.cbs.nl/nl-nl/visualisaties/verkeer-en-vervoer/vervoermiddelen-en-infrastructuur/wegen acta polytechnica ctu proceedings 38:275–280, 2022 1 introduction 2 theoretical framework 2.1 circularity and data 2.2 collecting road data automatically 3 entering the field 4 results 5 discussion 6 conclusions acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.38.0090 acta polytechnica ctu proceedings 38:90–96, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague energy performance estimation for large building portfolios with machine learning-based techniques frédéric monteta,∗, alessandro pongellib, jonathan riala, stefanie schwabc, jean henneberta, thomas jusselmeb a university of applied science of western switzerland (heia-fr, hes-so), icosys institute, bd. de pérolles 80, 1705 fribourg, switzerland b university of applied science of western switzerland (heia-fr, hes-so), energy institute, bd. de pérolles 80, 1705 fribourg, switzerland c university of applied science of western switzerland (heia-fr, hes-so), transform institute, bd. de pérolles 80, 1705 fribourg, switzerland ∗ corresponding author: frederic.montet@hefr.ch abstract. building operation is responsible for 28 % of the world’s carbon emissions. in this context, establishing priorities in refurbishment strategies at the scale of a city or a group of buildings is important. such procedures are usually led by experts in energy performance and, therefore, they are rarely carried out due to their long and costly nature. this research aims at the estimation of building energy performance to pave the way towards finding near-optimal refurbishment strategies. thanks to the identification of easily-accessible building characteristics, the method applies machine learning models to scan a building portfolio based on a low level of details. the results show good potential to identify low-performer buildings with simple machine learning methods. it also opens the door for further improvements through the inclusion of supplementary building features at the input of the predictive system. this work includes (a) the integration of a knowledge database thanks to the swiss cecb energy performance certificates, referencing more than 70 000 buildings, (b) the preparation of a training data set through the selection of relevant physical characteristics of buildings (input) and the corresponding energy consumption labels (output), (c) the development of predictive models used in a supervised way, (d) their evaluation on an independent test set. keywords: refurbishment strategies, machine learning, energy performance certificates. 1. introduction in a world where climate change imposes major adjustments in order to slow down the rise in environmental temperature, one important area to improve is the planning of building renovations. thanks to changing construction techniques and equipment, building renovations became a major focus of energy strategies of governments. switzerland, for example, is implementing a strategy called “energy strategy 2050” which aims, among other things, to reduce the energy consumption of buildings through incentives [1]. this, in the coming years, may cause a rush to renovate old buildings and to prioritise the management of renovations due to the costs and timeframes imposed. for building stock managers it means having to priorities and create a ranking of interventions that can lead to energy improvements, but which do not involve high costs. in switzerland there is the cantonal energy certificate for buildings (cecb) [2], which allows the assessment of the current state of the building and the planning of a possible renovation. it allows to attribute an energy label to the efficiency of the building envelope, which describes the quality of the thermal envelope including roof, wall, floor and window insulation, and also takes into account thermal bridges and the shape of the building. a second label is given to the overall energy efficiency including heat demand, electricity demand, own production of electricity as well as the building’s equipment for heat and domestic hot water. the labels are divided into 7 classes: from a, the best class, to g, the worst class compared to a reference. this certificate is compulsory in cantons such as geneva, vaud, fribourg, neuchâtel, nidwald, zug and zürich, in case of sale and/or renovation of the building. this has led to the creation of a database with more than 70 000 certificates describing the physical characteristics and energy performance of each building. however, this certificate oblige experts to travel to the building site in order to collect the various data needed for in-depth analysis, increasing the cost and time required to analyse a large building stock. in recent years it has also become possible, through portals such as registre fédéral des bâtiments et des logements (regbl) [3], cantonal geoportals [4, 5], google street view [6] and google maps [7], to find precise data or to collect it remotely. therefore, in the 90 https://doi.org/10.14311/app.2022.38.0090 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 38/2022 energy performance estimation for large building portfolios . . . figure 1. map of switzerland showing the concentration of building energy performance certificates (cecb) per cantons. following chapters we will try to answer the main question: “how can the available online data be used to quickly classify a building stock energy performance?”. in this paper, we investigate the use of machine learning (ml) approaches as a solution to estimate automatically the less performing buildings according to the cecb methodology. the proposed method is to take as input of the ml systems easy-to-find building characteristics that do not need the intervention of an energy expert. the output of the system is a prediction of the cecb energy label of the building. if functional, such approaches could provide a quick and easy way to rank buildings by priorities of renovations. the cecb association gave us access to his data, under a data agreement for the protection and sharing of sensitive data. we used these data to train and test our ml systems. the paper is organized as follows. section 2 presents the methods used to prepare the data and to select and optimize the best performing models. section 3 presents the obtained results according to specific metrics that we propose to evaluate their performances. finally, sections 4 and 5 present discussions, conclusions and future works. 2. methods this section introduces exploratory and preprocessing phases performed on the data. details regarding the machine learning models selection, their optimization and the process used to assess the results quality are provided. 2.1. data exploration as indicated in the previous section 1, more than 70 000 certificates are in the dataset. to understand the nature of the data, statistics were computed as figure 2. evolution of the average heat transfer coefficient in relation to the year of construction of buildings. a preliminary analysis. knowing that in some cantons the certificate is compulsory, it was verified that this is reflected in the form of certificates in the database. in figure 1 it is possible to see how the dataset is divided according to the different cantons and it is possible to identify the cantons that have a compulsory certificate. a second analysis was carried out to verify data consistency. in figure 2, it is possible to see the average heat transfer coefficient for each year. the evolution of this value is getting lower through time, which correlates with the increase in building insulation from 1970. this conclusion is similar to the findings of the energy and renovation (eren) project [8]. to get a better understanding of the subdivision of the dataset, we then checked which categories are present. they are subdivided according to the cat91 f. montet, a. pongelli, j. rial et al. acta polytechnica ctu proceedings categories cecb qty. percentage single-family building 40 590 55.10 % multi-family building 27 780 37.71 % administration 1 932 2.62 % mixed 1 802 2.45 % school 1 374 1.87 % retail 79 0.11 % hotel 77 0.20 % restaurant 29 0.04 % table 1. representation of the various building categories in the cecb dataset. original data after cleaning a 1620 277 b 10695 4594 c 16036 10056 d 17271 10718 e 11615 7355 f 6950 4374 g 9474 4986 table 2. number of certificates per label before and after cleaning the dataset. egories of regbl [3]. in the table 1, it is possible to see that the two most represented categories are single-family building and multi-family building. the subdivision in classes before the cleaning of the dataset is present in the table 2 where we can see that the most represented class is d followed by other classes and ended by the least represented a. 2.2. data preprocessing the data received was in the form of several tables. to use them, the latter were merged to combine the data in a single structure. subsequently, a data cleaning was carried out. after manual checks, outliers were identified and removed when samples were outside two standard deviations from the mean, thus removing 5 % of the data. in addition, missing values were removed when their percentages were more than 90 % per column or more than 80 % per samples, i.e. per certificate. the result of this part of the method is presented in table 2 as a number of certificates after the cleaning. for all following machine learning (ml) tasks, a balanced dataset is preferred1. to achieve this, subsampling can be used. since the class a is clearly underrepresented with its 277 samples, sub-sampling the data would induce a high loss of samples. for this reason, class a and b are merged before subsampling for all further ml tasks. moreover, as a and b building classes are best performers, they are not targeted by refurbishment plans. 1a dataset is said balanced when the number of samples per class is equal. 2.3. model performance exploration the modelling method includes 3 steps. (1.) pre-processing routines were carried out. the latter include an ordinal encoding of the data as well as a normalization by computing the z-score on all variables. (2.) then, eighteen different models from the scikitlearn library were selected when appropriate for a classification task and trained with a k-fold cross validation with k = 3 [9]. (3.) the evaluation of all models is made based on their f1-scores and accuracy metrics to select the two most promising ones. 2.4. optimization and model selection on the two best models from the previous section, optimization and training is performed to allow for final model selection. since the optimization tasks is memory expensive, the research for optimal parameters has been done on a reduced number of samples per class selected in a random manner. the optimization includes a cross-validated randomized search where k = 3 on a selection of parameters (see section 3). model training is computed as in step 3 from section 2.3. finally, the overall best model is selected given its per-class metrics (accuracy and f1) and confusion matrix. 2.5. special cases identification as a last step of the method, a more in-depth analysis of special cases is performed to identify the reasons behind a high prediction error. a distance between classes is computed with a method taken from ordinal regression problems [10]. to compute this distance, let y = a and ŷ = g be a sample’s class and its estimation. their encoded version would be y′ = (1, 0, 0, 0, 0, 0, 0) and ŷ′ = (1, 1, 1, 1, 1, 1, 1). the distance d between the two classes can then be calculated with d = ∑n i=1 |y′ i − ŷ′ i|. once obtained, all samples were ordered by descending order from their distance d. 3. results 3.1. model performance exploration the eighteen algorithms were trained on a resampled dataset to make it balanced. performances obtained in the table 3 allow for the comparison of all algorithms in order to identify the two best candidates for energy performance certificates predictions. the dummyclassifier performs a classification in a random manner with 0.17 accuracy. the latter result sets a baseline above which an algorithm learns the characteristics of the data. since most algorithms performed with a f1-score and accuracy between 0.3 and 0.5, the problem at hand can be characterized as difficult. 92 vol. 38/2022 energy performance estimation for large building portfolios . . . model accuracy f1-score histgradientboostingclassifier 0.50 0.49 randomforestclassifier 0.50 0.48 extratreesclassifier 0.49 0.48 baggingclassifier 0.49 0.48 gradientboostingclassifier 0.49 0.47 mlpclassifier 0.47 0.46 svc 0.46 0.45 nusvc 0.46 0.45 logisticregression 0.45 0.43 decisiontreeclassifier 0.43 0.43 adaboostclassifier 0.43 0.42 lineardiscriminantanalysis 0.43 0.41 extratreeclassifier 0.39 0.38 kneighborsclassifier 0.39 0.38 linearsvc 0.41 0.37 sgdclassifier 0.40 0.35 ridgeclassifier 0.39 0.34 nearestcentroid 0.35 0.34 perceptron 0.33 0.32 passiveaggressiveclassifier 0.32 0.31 bernoullinb 0.32 0.31 gaussiannb 0.30 0.30 dummyclassifier 0.17 0.05 table 3. model performances exploration. models are ordered by f1-scores and then, by their accuracy metrics. before after gain catboost 0.59 histgradientboosting 0.49 0.58 18 % randomforest 0.48 0.57 19 % table 4. comparison of the best classifiers with their f1-score average across class. the top 5 algorithms include recent classification models performing with similar result around 0.48 f1-score. the mlpclassifier2 performed with moderately good result. the best selected candidates for the next step of the method are the histgradientboostingclassifier and randomforestclassifier. 3.2. optimization and model selection the fine tuning procedure was performed on histgradientboostingclassifier and randomforestclassifier models. in addition, one state-of-the-art gradient boosted tree implementation was added from the catboost library, without fine-tuning [11]. as table 4 shows, all models are reaching performances above or close to 0.5 without optimization, thus already allowing for an educated guess. when parameters from listing 1 are used, substantial gains can be obtain within the order of ∼20 %; thus proving the value of randomized search. 2stands for multi-layer perceptron, i.e. a neural network with the default parameters from the scikit-learn library. best_params_hist_gradient_boosting = { ’ l 2 _ r e g u l a r i z a t i o n ’ : 0 .00081 , ’ l ea rn ing_rate ’ : 0 .11889 , ’ max_bins ’ : 60 , ’ max_leaf_nodes ’ : 11 , ’ min_samples_leaf ’ : 87 } best_params_random_forest = { ’ n_est imators ’ : 800 , ’ min_samples_split ’ : 5 , ’ min_samples_leaf ’ : 1 , ’ max_features ’ : ’ s q r t ’ , ’max_depth ’ : 80 , ’ boots t rap ’ : fa l se } listing 1. best parameters. unbalanced balanced ab 0.76 0.82 c 0.69 0.62 d 0.60 0.54 e 0.45 0.44 f 0.31 0.47 g 0.68 0.67 accuracy 0.60 0.59 f1 avg. 0.59 0.59 table 5. global building efficiency f1-scores. a final f1-score average across classes of 0.59 makes the catboost based model the more performant of the selection. also, both algorithms from scikit-learn library had a notably high performance gain, making them reach a scores comparable to catboost. 3.3. best model performances the catboost model, using gradient boosting on decision trees, is the final model selected. this section introduces it in three phases. first, with explaining an additional data processing task. second, by presenting the model performances in depth. finally, by analyzing the errors produced by the model. 3.3.1. model performance the final model performances are summarized in table 5. in average, the model reaches an accuracy of ∼0.6. this represents an improvements of ∼350 % compared to the baseline of 0.17 given by the dummyclassifier from section 3. on unbalanced data, scores have a great variability, which makes the model quality hard to evaluate with confidence. since unbalanced data has generally more samples, the awaited behavior is a higher score, but this isn’t the case for all classes. a possible explanation lies in the selection of more representatives samples while sub-sampling. 93 f. montet, a. pongelli, j. rial et al. acta polytechnica ctu proceedings (a). unbalanced dataset. (b). balanced dataset. figure 3. confusion matrices for global efficiency. on balanced data, the comparison between classes gives an insight on the difficulty to evaluate each class. from ab to f , scores are decreasing from 0.82 to 0.47. this shows that the lower the efficiency of a building is, the harder it becomes to predict its class. both end of the class spectrum – ab and g – have higher scores. this behavior is probably due to the absence of strict intervals i.e. a certificate made on a highly inefficient building would have a class higher than g, but since it doesn’t exist, it is classed as g. more details about the predictions uncertainty are given by the confusion matrices in figure 3 where the precision of the models are presented visually. a highlighted diagonal is predominant in both the unbalanced and balanced plots; showing the correct predictions. on each side of the diagonal, the predictions made with an error of one class above or below the true label are represented. 3.3.2. prediction reliability to evaluate how the false predictions are spread around the diagonal, the plot on figure 4 shows the cumulative density of the distances between predicted and true value. the accuracy of ∼60 % is visible where the distance is 0. then, in case of a wrong prediction, there is a ∼90 % probability that the true class is only a letter away; ∼100 % for two letters, and so on. this highlights that wrong predictions are generally not far away from their target. 3.4. online variables to assess whether the necessary input variables are available online to speed up the classification process with ml-based techniques, a wide search was carried out on the various portals listed in the section 1. the figure 5 presents the most discriminant variables in the catboost algorithm. for brevity, only the ten firsts are presented. of the most important variables for the operation of the algorithm, we can easily find the year or era of construction of the building on the regbl site through figure 4. cumulative distribution of the distances between true and predicted global efficiency given two two different metrics. the interactive building map. a second variable that can easily be found is the building width through google maps for example as the measurement tool can be used to obtain a value of the desired building. a third and a fourth that can be found on the internet, but is subject to restrictions, are the year of construction of the energy agent and the energy source. they are available in the regbl database, but this type of data is only granted after authorization and verification of credentials. all other variables are not accessible as they are not present on any platform at the moment. 4. discussion the applicability of the methods depends on (1.) the accuracy of the model that is used to predict performance classes, and (2.) the easy online access of inputs of the predicting model describing the physical properties of buildings to be classified. 94 vol. 38/2022 energy performance estimation for large building portfolios . . . figure 5. feature importance of the first ten variables used by catboost. regarding (1.), the catboost model is promising as it has close to 60 % accuracy. moreover, the prediction is only 1 class away in 90 % of the buildings. this seems to be highly acceptable if the method decreases dramatically the time spent to assess the building performance. in order to be able to understand how to further improve prediction, it is necessary to start by analysing the process used step by step, starting with the collection of data for the generation of the cecb certificate. the initial dataset is the result of certificates executed by many cecb experts. several points in the creation of a cecb certificate are decided through visual inspection and thus based on the knowledge of the individual expert. for example, in order to get a u-value for walls, the expert can enter a proposal for the composition of the wall by visual inspection and the program calculates a u-value based on what the expert insert. this means that each expert, according to his knowledge, can enter or omit data, but in the end the program still manages to generate a complete certificate. this, for example, explains the missing data for some certificates. in order to have a more complete dataset, it would be important to reduce the possibility of omitting data for the generation of a certificate to a minimum. regarding the type of certificate, before 2012, a simpler type of certificate with fewer parameters was in place, then a more detailed certificate was adopted. this certainly causes a lack of data on some of the certificates used. filling the gaps and convert less detailed certificates into more detailed ones by entering the missing data could be a solution to improve the base set. during the data cleaning, arbitrary choices were made to eliminate values with the 3 sigma method, to eliminate columns with 90 % of the data missing as well as certificates with 80 % of the data missing. this system can clearly decrease the accuracy by removing variables and especially population from the data set. a more thorough analysis for cleaning the data should be done, so as to be sure that all inconsistent values are removed and that all consistent values are kept. the same can be done by keeping variables considered important as well as certificates to increase the population. finally, when it comes to prediction, changing the settings of the algorithm could lead to further improvement of the dataset. a better tuning can lead to a more precise prediction. moving on to the second point (2.), it was identified in section 3.4 that only two variables are easily accessible. in the next part of the article we will analyse each variable in the figure 5, exploring possible methods of providing representative values of the analysed building for the data required. in order, the most important variable is the heat transfer coefficient of walls, which cannot be found online. one solution would be to send a person on site to do an analysis in order to give a value to this variable. a second possibility that should be explored is to use artificial intelligence to reconstruct this value, for example from an image of the wall of the building and the year of construction, so that it can recognize some key features and then give a value based on other similar buildings. moving on to the second variable in the list is the energy source. as already discussed in the section 3.4, it can be found on the regbl website with restrictions. however, this data may be in the possession of the owners or managers of the building and could therefore be entered easily. the year of construction of the heating, as mentioned in the section 3.4, can be found on the regbl website with restrictions. also this variable should be checked if the owner or manager of the building is in 95 f. montet, a. pongelli, j. rial et al. acta polytechnica ctu proceedings possession of it. the envelope coefficient is not found directly but could be calculated from other variables. online it is possible to measure the building perimeter through google maps. on the regbl website it is possible to find the number of floors in the building and the ground area. using for example an average value for the height of a floor, the various parameters can be combined. multiplying the perimeter by the number of floors and the average height and then adding the floor area twice gives an approximation of the envelope surface. dividing the envelope surface by the number of floors multiplied by the floor surface gives an approximate value of the envelope coefficient. clearly an approximate value that should be checked for potential and especially for possible errors brought with it. the utilisation rate of heat generator for hot water cannot be found online and so there are two possibilities to find this data. the first is that the manager or owner of the building knows this value. the second is to send an expert on site to make an assessment. the energy reference surface can also be estimated from other variables. on the regbl website we find the floor area of the building and the number of floors, multiplying these variables together we find a rough estimate. clearly, the reliability of predicting the correct energy class using this approximation must be checked. the climate station is not available online, but knowing the address of the building it is possible to indicate which weather station is to be used for the calculation. the linear thermal bridge building base lwf is like the heat transfer coefficient of walls, is not available online and it is possible to use the same solution proposed. it must be said that all missing variables could be estimated or found easily with an expert on site. having only 10 variables to find would simplify and speed up the work to be done on site. 5. conclusion in this paper we have highlighted the preliminary reliability results of using a classification algorithm to analyse a building. the result of ranking the building in the good class with 60 % accuracy is a promising result for future developments. it should be noted that there is a 90 % probability of being in the adjacent class, which brings value to the work done. in addition, the work carried out to check the online presence of the most important variables for prediction has shown that it is still premature to find the exact value online. nevertheless, it may be possible to recreate some them by developing further techniques. some recommendations for future work are necessary. for example, a special attention must be taken when merging the different data available during the preparation phase, as this could lead to consecutive errors in the other phases of the work. a special attention must also be taken during the cleaning phase to ensure that the maximum amount of data is available to carry out the work. acknowledgements the authors would like to express their gratitude to ms karine wesselmann and the cecb association for providing the data for this work. they would also like to thank professor mylène devaux and the itec institute for their collaboration. financial support is gratefully acknowledged from the heia-fr smart living lab research program. references [1] swiss federal office of energy. stratégie énergétique 2050, 2020. [2021-11-05]. https://www.bfe.admin.ch/ bfe/fr/home/politik/energiestrategie-2050.html [2] association geak-cecb-cece. le certificat énergétique cantonal des bâtiments (cecb). [2021-11-05]. https://www.cecb.ch/ [3] office fédéral de la statistique. registre fédéral des bâtiments et des logements (regbl). [2021-11-05]. https://www.housing-stat.ch/fr/index.html [4] canton de fribourg. portail cartographique du canton de fribourg. [2021-11-05]. https://map.geo.fr.ch/ [5] canton de neuchâtel. portail cartographique du canton de neuchâtel. [2021-11-05]. https://sitn.ne.ch/ [6] google llc. google street view. [2021-11-05]. https://www.google.com/intl/en_ch/streetview/ [7] google llc. google maps. [2021-11-05]. https://www.google.ch/maps/ [8] s. schwab, l. rinquet, m. devaux, et al. rénovation énergétique. tech. rep., fribourg, suisse, 2018. [9] f. pedregosa, g. varoquaux, a. gramfort, et al. scikit-learn: machine learning in python. journal of machine learning research 12:2825–2830, 2011. [10] j. cheng, z. wang, g. pollastri. a neural network approach to ordinal regression. in 2008 ieee international joint conference on neural networks (ieee world congress on computational intelligence), pp. 1279–1284. 2008. https://doi.org/10.1109/ijcnn.2008.4633963 [11] l. prokhorenkova, g. gusev, a. vorobev, et al. catboost: unbiased boosting with categorical features, 2019. arxiv:1706.09516 96 https://www.bfe.admin.ch/bfe/fr/home/politik/energiestrategie-2050.html https://www.bfe.admin.ch/bfe/fr/home/politik/energiestrategie-2050.html https://www.cecb.ch/ https://www.housing-stat.ch/fr/index.html https://map.geo.fr.ch/ https://sitn.ne.ch/ https://www.google.com/intl/en_ch/streetview/ https://www.google.ch/maps/ https://doi.org/10.1109/ijcnn.2008.4633963 http://arxiv.org/abs/1706.09516 acta polytechnica ctu proceedings 38:90–96, 2022 1 introduction 2 methods 2.1 data exploration 2.2 data preprocessing 2.3 model performance exploration 2.4 optimization and model selection 2.5 special cases identification 3 results 3.1 model performance exploration 3.2 optimization and model selection 3.3 best model performances 3.3.1 model performance 3.3.2 prediction reliability 3.4 online variables 4 discussion 5 conclusion acknowledgements references https://doi.org/10.14311/app.2022.33.0533 acta polytechnica ctu proceedings 33:533–538, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague improvement mechanism of recycled concrete using c-s-h type accelerator and modifying recycled aggregate by carbonation technology akari shibuyaa,∗, nobuhiro matsudab, takeshi iyodac a former graduate school of engineering and science, shibaura institute of technology, tokyo, japan b tokyo techno co., ltd., tokyo, japan c shibaura institute of technology , tokyo, japan ∗ corresponding author: me19047@shibaura-it.ac.jp abstract. in japan, there is concern that concrete blocks due to demolition will increase with the renewal of concrete structures. although it has been used for roadbed materials as recycling, in recent years, aggregates are removed from the concrete mass by crushing and grinding and used again as concrete aggregates. in concrete using recycled aggregates, it is necessary to take account of insufficient strength, drying shrinkage, freeze-thaw resistance. in previous study, the quality of recycled aggregates was improved by adsorbing carbon dioxide to recycled aggregates, and recycled concrete used the modified aggregates has been confirmed to increase strength and reduce drying shrinkage. in this study, we focused on the aggregate interface of concrete such as the interfacial transition zone for the purpose of clearing the quality improvement method of recycled concrete. we attempted to modify the aggregate interface using a c-s-h hardening accelerator using two recycled aggregates of different quality and concrete using ordinary aggregates. we will also compare which is more effective, concrete that tried to modify the aggregate interface and recycled concrete that uses carbonated modified recycled aggregate. we try include strength test, drying shrinkage test, and freeze-thaw test. we clarify the matters concerned about recycled concrete. keywords: c-s-h hardening accelerator, mass transfer resistance, recycled aggregate, recycled concrete. 1. introduction in japan, structures built during the period of high economic growth are increasingly required to be rebuilt or repaired over their life. 30 million tons of concrete are demolished annually. demolition concrete are increasing year by year. in addition, the readymixed concrete left over and returned to the factory is called the return concrete, and the amount of return concrete is increasing. recently, recycled aggregate is attracting attention from the viewpoint of reducing environmental impact. the recycled aggregate is an aggregate obtained by removing the aggregate from demolition concrete or return concrete so that it can be reused. the quality of recycled aggregate varies depending on the condition and processing method of the original concrete demolition material. in japan, there are three levels of quality, l, m and h. standards of recycled aggregate and concrete are specified in jis a 5021 ∼ 5023. the worst quality l is aggregate that has just been crushed and has a lot of mortar attached. on the other hand, l has merits such as lower energy cost than others. it was thought that low-quality recycled aggregate could be modified. various problems due to the large amount of attached mortar. the big problem is the decrease in strength due to many pores. in addition, high water absorption results in large drying shrinkage, causing freeze-thawing and scaling problems. therefore, the purpose of this study is to understand the problems of recycled aggregate and to examine methods that can solve these problems. in previous studies, carbonation of aggregates has been reported as a method of modifying recycled aggregates. it is reported that strength, drying shrinkage, and freezethaw resistance were improved. by forcibly carbonating the aggregate, calcium carbonate is generated in the attached mortar. pores decrease and mortar area becomes denser. on the other hand, previous studies have reported that calcium silicate hydrate type accelerator (acx) is effective in densifying the interfaced at the aggregate interface [1]. acx is a curing accelerator developed in recent years. it is mainly composed of nanoparticles of calcium silicate hydrate. it is reported that adding 5% of acx to water densifies the voids in the transition zone around the aggregate. by using acx, which is said to be effective at the aggregate interface, it is thought it can reform recycled concrete using recycled aggregate. in this study, we try to understand their impact on concrete by comparing recycled concrete with carbonation of aggregate and recycled concrete with acx added. 533 https://doi.org/10.14311/app.2022.33.0533 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en a. shibuya, n. matsuda, t. iyoda acta polytechnica ctu proceedings surface dry density! g/cm3" dry density! g/cm3" water absorption [%] rl 2.42 2.29 5.53 rc 2.46 2.36 4.38 table 1. physical properties of coarse aggregate. 5;:� � � � � � � � � � � � 5;6� figure 1. overview of rxw and rxs. w/b [%] s/a [%] air [%] quantity of material per unit volume of concrete acx [w × %] fresh properties w opc ggbfs s g air [%] slump [cm] r 50 48 4.5 170 187 153 847 rl 847 0 3.5 22.5 rx3 3 3.9 22.2 rx10 10 2.4 22.8 rxw 1* 4.1 23.1 rxs 2* 2.3 23.0 rc rl 861 0 4.2 23.4 1* : pre-wetting with 10% strength acx 2* : spraying the same amount of acx as in rx3 table 2. mix proportion. 2. materials and specimen condition 2.1. the kinds of coarse aggregates it was used recycled aggregate (rl). in addition, the recycled aggregate is carbonated in an environment with a carbon dioxide concentration of 5% (rc). table 1 shows their physical properties. rl has high water absorption, and carbonation has the effect of reducing water absorption. before mixing, rl was pre-soaked for 24 hours and the surface dried because the recycled aggregates have high water absorption. 2.2. mix proportions table 2 shows the mix proportions on this research. concrete with a water binder ratio of 50% was prepared by replacing the ground granulated blast furnace slag by 45%. concrete using the recycled aggregate was similarly prepared (r, rx3, rx10) and in addition, the concrete using the two types aggregate. one is the pre-wetting was pre-soaked with the acx solution of 10% concentration for 24 hours (rxw) and the other is the concrete that acx was sprayed over all sides of the aggregate (rxs) as shown in figure 1. 3. outline of experiment 3.1. bleeding test bleeding was tested to determine the effect of bleeding suppression by the addition of acx and the modification of aggregate in accordance with jis a 1123. concrete was cast into formwork that does not penetrate water, and the formwork size is φ100×200 mm. the test was started by lowering the casting surface by 30mm±3mm. every 10 minutes after the casting, the water that had exuded on the upper surface was sucked with a dropper and transferred to a graduated cylinder for measurement. measurements were taken until no bleeding was observed. the bleeding amount was determined by equation 1. 534 vol. 33/2022 cdw structural concrete figure 2. air permeability test. figure 3. bleeding amount. bq = v a , (1) where • bq: amount of bleeding, • v : total bleeding water volume until the end • a: area of concrete casting surface. 3.2. drying shrinkage test two pieces of concrete of 100×100×400 mm with pins were prepared. the measured value after 7 days of underwater curing was used as the initial value, and after curing, the specimen stands in a constant temperature and humidity room (20◦c, 60%rh). the length was measured after 1,4,14, and 28 days in accordance with jis a 1129. 3.3. compressive strength test and splitting tensile strength test three specimens of φ100 × 200 mm were prepared for each case, and after curing in water for 28 days, a compressive strength was tested in accordance with jis a 1108. also, a splitting tensile strength test was performed in accordance with jis a 1113. 3.4. air permeability test specimens of 150 × 150 × 150 mm was prepared and cured for 28 days. after extracting the core to the size of and leaving it in a completely dry state, an air permeability test was performed. in order to understand the effects of bleeding and the quality of the aggregate interface, air permeability test was performed from two directions as shown in figure 2. in concrete, water rises to the upper surface, so it is considered that there are places where the water rises, and where there is a void after hardening due to water that could not rise and accumulated on the lower surface of the aggregate. the air permeability coefficient was calculated by using equation 2. k = 2 l p1 (p 2 1 − p 2 2 ) · q a , (2) where • k: air permeability coefficient [cm4/(n·s)] • l: sample thickness [cm] • p1: loading pressure ([n/cm2] • p2: outflow side pressure [n/cm2] • q: air permeability [cm3/s] • a: air permeability area [cm2] 3.5. porosity test the porosity was calculated by the archimedes method. the weight saturated with water (w1) and the weight in water (w2) were measured, and then the specimens stand in a chamber at 40◦c. until the weight loss became constant, and the dry weight (w3) was measured. the weight was calculated using the mass, and the weight was calculated by equation 3. 535 a. shibuya, n. matsuda, t. iyoda acta polytechnica ctu proceedings porosity [%] = w1 − w3 w1 − w2 × 100. (3) figure 4. drying shrinkage rate. figure 5. compressive strength and splitting tensile strength. 4. results 4.1. bleeding test in figure 3, the total amount of bleeding is organized for each specimen. rx3 and rx10 showed non-bleeding. it became clear that the addition of acx to concrete has a large inhibitory effect. rxw and rxs almost showed inhibitory effect. the reason that rxs had a smaller bleeding amount than rxw is probably that acx sprayed on the surface diffused during mixing and had the same effect as that added. 4.2. drying shrinkage test figure 4 shows the results of drying shrinkage at 28 days. drying shrinkage was suppressed by adding acx. it became clear that adding 10% was more effective than adding 3%. also, rxw, rxs and rc has almost same shrinkage rate. they did not show a reduction effect more than when acx was added to the concrete itself. therefore, drying shrinkage is best when acx is added to the concrete mixing water. 4.3. results of strength test for concrete using recycled aggregate, rx10 had higher compressive strength, however rxw and rxs did not get stronger. therefore, adding acx to concrete is effective for compressive strength, however figure 6. air permeability coefficient (horizontal). figure 7. compressive strength and air permeability coefficient. figure 8. air permeability coefficient in horizontal and vertical direction. methods such as pre-wetting and spraying that attempt to modify the aggregate itself have no effect. the greater the amount of acx added, the greater the splitting tensile strength. rxw showed no increase in strength, but rxs showed the same degree of increase in strength as the addition of acx. a possible reason for this is that the pre-wetting aggregate was not able to play a role in modifying the aggregate interface by wiping the surface. in addition, rc showed the highest strength among concrete using recycled aggregate. comparing the compressive strength with the split tensile strength, the effect on the split tensile strength is seen in rc. therefore, it was considered that the splitting tensile strength increased because the strength of the aggregate itself increased due to carbonation, but the compressive strength of the aggre536 vol. 33/2022 cdw structural concrete �� �� �� �� �� �� �� �� & rp su hv vl yh �vw uh qj wk � �1 �p p � � 3rurvlw\���� 5 5;� 5;�� 5;: 5;6 5& ��� ��� ��� ��� � �� �� �� 6s olw wlq j� wh qv loh �vw uh qj wk � �1 �p p � � �� 3rurvlw\���� 5 5;� 5;�� 5;: 5;6 5& figure 9. the relationship between porosity and strength. figure 10. the relationship between porosity and air permeability. gate interface and mortar did not change. 4.4. air permeability test 4.4.1. air permeability coefficient figure 6 shows the permeability coefficient calculated from the result of the permeability test in the horizontal direction on recycled concrete. the significant improvement of the air permeability by adding acx is apparent. due to the modification of the recycled aggregate itself, it is considered that rxw and rc had been densified by the mortarattached to the recycled aggregate. 4.4.2. the relationship between compressive strength and air permeability coefficient as shown in figure 7, for concretes other than rxs, there was a correlation that the compressive strength decreased as the air permeability decreased. it was cleared that rx10 had significantly improved concrete. only the permeability was improved with almost the same strength excluding rx10. since the mortar content in the concrete does not change, it is considered that the compressive strength shows approximately the same value. whether or not the improvement in the air permeability coefficient has been observed in that is whether the part which becomes the air pass has been reformed. in the case of rxs, the strength of the aggregate itself was considered to have increased due to the high splitting tensile strength, but it was considered that the concrete was not modified as rx3 and rx10. 4.4.3. air permeability coefficient in horizontal and vertical direction in order to see the effect of the aggregate interface, the horizontal axis shows the air permeability coefficient in the horizontal direction and the vertical axis shows the air permeability coefficient in the vertical direction in figure 8. in r and rc, it was found that the air permeability in the vertical direction was high, and it is considered that acx bleeding water path was formed, and the air became easier to pass because r and rc had high bleeding amount. on the other hand, the values in the horizontal and vertical directions were almost the same for all rx3, rx10, rxw, and rxs using acx. it is considered that the gap at the aggregate interface was improved by acx. in addition, rc was a modification of the mortar attached to recycled aggregate, so the aggregate interface was not modified. 4.5. relationship with porosity 4.5.1. the relationship between porosity and strength figure 9 shows the relationship between porosity and strength. rx10 improved both strengths over rl, however the pore was almost same. on the other hand, the strength of rxs did not increase and the porosity decreased. for this reason, it is considered that rxs with acx closer to the aggregate also contributed to the modification of the aggregate. rc contributed to splitting tensile strength rather than compressive strength. 4.5.2. the relationship between porosity and air permeability coefficient figure 10 shows the relationship between porosity and air permeability. the rc and rxw obtained by modifying the aggregate have almost the same air permeability as rc, however the porosity decreased. on the other hand, those with the addition of accelerators to concrete, such as rx3 and rx10, improved the air permeability, however the porosity was not improved. therefore, it was found that although the modified mortar portion of the recycled aggregate had a significant effect on the porosity, even if the aggregate was modified, air permeability was not improved. 537 a. shibuya, n. matsuda, t. iyoda acta polytechnica ctu proceedings compressive strength splitting strength dry shrinkage air permeability porosity vertical horizontal aggregate ! ! • ! ! • transition zone • • ! ! • ! mortar • ! • ! ! ! table 3. list of contributions by modification. 4.6. consideration based on the above results, it was considered possible to summarize the thoughts and sort out the degree of influence that each part of concrete has on physical properties. table 3 shows the parts that affect each characteristic. in this study, it was clear that the properties affected by the modified part changed. the properties affected by the respective modifications of the aggregate, the transition zone, and the mortar portion are represented by "•". the parts that do not have an impact are marked as "!", and those that are difficult to judge are marked as "!". 5. conclusion • the quality of recycled concrete improved most when calcium silicate hydrate type accelerator was added to water at 10%. • air permeability can be improved by pre-wetting or spraying with calcium silicate hydrate type accelerator. however, it increases in compressive strength cannot be expected. • forced carbonation of recycled aggregate increases the strength of the aggregate itself and increases the split tensile strength. however, it does not improve the air permeability or compressive strength because it does not modify the aggregate interface in the recycled concrete. • in the future, we would like to study methods for promoting the use of recycled coarse aggregate and study of recycled fine aggregate. in addition, we aim to sort out which part has the greatest effect on the physical properties of concrete. references [1] fukazawa y and iyoda t 2019 clarification of pore modification mechanism in concrete by c-s-h type accelerator 3rd acf symposium assessment and intervention of existing structures. 538 acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.34.0105 acta polytechnica ctu proceedings 34:105–115, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague finite element simulation of shear and compact tension tests on timber eliška šmídová∗, petr kabele czech technical university in prague, thákurova 7/2077, 166 29 prague 6, czech republic ∗ corresponding author: eliska.smidova@fsv.cvut.cz abstract. the non-linear finite element simulation of the astm shear and pre-stressed compact tension tests was conducted. tested specimens were cut from the glued laminated timber made of european spruce (picea abies) with the lamina thickness of 10.5 mm and 45 mm. the 2d homogeneous orthotropic constitutive model of the tensile-shear fracture in timber, which has been proposed by the authors, was used. the model calibration was adopted from the authors’ recent experimental study. the numerical results show that the model can adequately reproduce both the experimental response and the crack pattern, which in certain cases comprises of cracking parallel or perpendicular to fibers. furthermore, the results confirm that the value of the parameter fastmxy , obtained from astm shear test as the maximum attained force divided by the shearing area, represents the averaged stress at the failure plane, while the extreme stress experienced by the material is much higher. keywords: astm shear test, pre-stress, compact tension test, glued laminated (gl) timber, tensileshear fracture model, non-linear finite element analysis. 1. introduction the procedure for the determination of shear strength according to the standard astm d143-94 [1], see figure 2, is widely accepted and used for timber and wood products. however, as shown by numerical simulations by sajdlová and kabele [2], the stress distribution along the failure plane in such shear-test configuration is non-uniform with strong concentrations at the edges of the loading platens. as reported in [3], the compact tension (ct) test on pre-stressed specimens with reduced ligament depth was conducted with the primary aim of obtaining the fracture behavior in the crack-normal direction for the fibers’ rupture concentrated around or at the cross-section ahead of the notch. nevertheless, such failure type occurred only in one case out of five accompanied by fiber bundles’ disintegration propagating along the grain. in the rest of the specimens, various failure types with fibers’ rupture or disintegration or their combinations were observed. the authors recommended only qualitative interpretation of the obtained results due to low precision of prescribed strain, presence of disintegration propagating along the grain that accompanied fibers’ rupture, role of lateral compression in fibers’ rupture, or insufficient number of specimens for each failure type. the aim of this study is to conduct a numerical simulation of (i) the astm shear test and (ii) the prestressed ct test on glued laminated timber (glt) specimens and analyse the behavior of the tensileshear crack parallel or perpendicular to the grain. to this end, the material model of the tensile-shear fracture in timber implemented in finite element method is used. 2. constitutive model we briefly review the material model of tensile-shear fracture in timber that the authors’ team has recently developed and implemented in a fem code [4]. timber is considered as a 2d homogeneous continuum. the model aims at capturing fracture under tension, shear, and their combination, while taking into account the phenomena of the elastic and inelastic behavior in a small deformation range, material orthotropy, both in linear and non-linear range, cracking across and along fibers, and the behavior under unloading/reloading. non-linear response under combination of compression and shear is considered as a perfectly plastic. the model for the nonlinear behavior is composed of: • (i) failure criterion defining the stress condition for crack initiation, • (ii) crack-type criterion that distinguishes whether the crack occurs across or along the fibers, • (iii) cohesive (traction-separation) law defining the response of a crack. failure (fracture) is assumed to occur when the following condition is satisfied: f (σx, σy, τxy) = 1 (1) here σx, σy, and τxy are the stress components with respect to the axes of orthotropy. the failure function f is defined by the tsai-hill formulation [5] as: f = σ2 x f2 x + σ2 y f2 y + τ2 xy f2 xy − σxσy f2 x (2) 105 https://doi.org/10.14311/app.2022.34.0105 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en eliška šmídová, petr kabele acta polytechnica ctu proceedings if σx ≥ 0 then fx = ftx else fx = fcx (3) if σy ≥ 0 then fy = fty else fy = fcy (4) where fxy is the absolute value of the shear strength. symbols fx and fy are tensile or compressive strength along and across the fibers, respectively: for the positive values of σx and σy, the strengths fx and fy correspond to the tensile strength values ftx and fty, respectively, otherwise the compressive strength values fcx and fcy are adopted. the crack-type criterion is based on the consideration that when timber is exposed to tension in the direction parallel or nearly parallel to the grain, rupture occurs across the grain and the crack is perpendicular to the principal stress direction. when the angle θ, describing the deviation of the principal tension from the grain, is larger than a certain threshold (θc), then the crack forms along the grain regardless of the principal stress direction. to distinguish these cases, the crack-type function fct , eq. 5, and criteria for cracking across and along the fibers, eq. 6 and eq. 7, are introduced: fct = ∣∣∣∣ 2τxy σx − σy ∣∣∣∣− tan(2θc) (5) crack across the grain: fct < 0 (6) crack along the grain: fct ≥ 0 (7) in the 2d stress space the crack-type function fct defines a plane, which divides the failure surface into two parts. the concept of the cohesive crack model [6] is used to model the material response after failure. the bridging effect of incompletely ruptured or delaminated fibers is represented by a cohesive traction acting between the crack faces. the traction-separation law defines the relation between the normal and tangent tractions tn and tm and the relative normal and tangent displacement between the crack surfaces δn and δm. the exponential form of the traction-separation law originally proposed by hordijk [7] was adapted in the present model as: tn(δn, δm) =  tcin ( 1− √ δ2 n δ2 n,crit + δ2 m δ2 m,crit ) ( 1 + ( c1δn δn,crit )3 ) exp ( c2δn δn,crit ) − exp (−c2) ( 1 + c3 1 ) δn δn,crit for δn ∈ [0, δn,crit) and δm ∈ [0, δm,crit) 0 for δn > δn,crit or δm > δm,crit (8) (a). (b). figure 1. cohesive law in (a) crack-normal direction tn(δn, δm) with fracture energy gf as an integral of tn(δn, δm) and (b) crack-tangent direction tm(δn, δm) [4]. tm(δn, δm) =  tcim ( 1− √ δ2 n δ2 n,crit + δ2 m δ2 m,crit ) 2 π arctan  δm lch 1− ( δn δn,crit )p( δn δn,crit )p  for δn ∈ [0, δn,crit) and δm ∈ [0, δm,crit) 0 for δn > δn,crit or δm > δm,crit (9) where tcin is the crack-normal traction at crack initiation, tcim is the crack-tangent traction at crack initiation, c1 and c2 are fitting parameters defining the slope of the tn function, δn,crit and δm,crit are the values of normal and shear displacement jump at which the cohesive traction diminishes to zero, lch is the characteristic length of the smeared crack model (the width of the fracture process zone), and p is a material parameter that determines the slope of the function tm. the same form of the traction-separation relation is used both for cracks along and across the grain, although the respective values of the parameters may be different. the plot of traction-separation law is shown in figure 1. 3. model calibration the tensile-shear fracture model has already been calibrated within authors’ recent experimental study [3]. to this end, experimental results from off-axis tensile and compressive tests and compact tension test were used. the parameters applicable to the states before failure, such as the elastic moduli (ex, 106 vol. 34/2022 finite element simulation of shear and pre-stressed compact tension tests on timber parameter unit avg ex [mpa] 12418.0 fx [mpa] 77.6 ey [mpa] 371.0 fy [mpa] 3.2 νxy [-] 0.37 νyx [-] 0.01 gxy [mpa] 310.0 ec,x [mpa] 16142.0 fc,x [mpa] 56.3 ec,y [mpa] 310.0 fc,y [mpa] 3.3 fastmxy [mpa] 3.7 table 1. off-axis tension and compression and shear test results for 10.5 mm glt where avg is the mean (values adopted from [3]). ey, gxy) and poisson’s ratios (νxy, νyx), are listed in table 1. in this section, we briefly review the parameters obtained by the calibration of the failurerelated model components. the failure surface is defined by five parameters: the uniaxial tensile and compressive strengths along and perpendicular to the grain (fx, fcx, fy, fcy, resp.) and the shear strength (fxy). the values of tensile and compressive strengths were directly retrieved from the tensile and compressive tests. they are listed in table 1. the parameter fxy was obtained by the leastsquare fitting of the failure criterion, expressed using an uniaxial failure function defining the relation between the axial stress at failure σaxial and the off-axis angle θ, to the failure data from tensile and compressive tests. this procedure yielded fxy = 8.5 mpa [3]. with the crack-type criterion, the parameter of the critical load-grain angle θc is introduced in the model to distinguish the cases when failure occurs across or along the grain. based on visual inspection of tensile specimens that failed due to fibers rupture, it was found that the actual grain deviation from the load direction is in the interval of [0°, 2°]. opposed to the recent numerical simulation analysis [8], we set θc = 1.6° to enable cracking both parallel and perpendicular to the grain. the traction-separation relationship tn vs. δn for the crack along the grain was obtained by the inverse analysis of the compact tension (ct) tests results [9]. the inverse analysis was applied on three load vs. crack mouth opening displacement curves, which corresponded to the minimum, intermediate, and maximum measured response. corresponding values of parameters in table 2 are denoted accordingly. ct test cohesive law fit tcin δn,crit c1 c2 response [mpa] [mm] [−] [−] minimum 3.6 0.52 8.8 21.5 intermediate 3.8 0.18 1 3.7 maximum 5.3 0.20 1.0 3.7 table 2. values of the cohesive law parameters for the crack along the grain calibrated to the results of inverse analysis for the minimum, intermediate, and maximum response of the ct test results (transcribed from [3]). 4. shear test and pre-stressed compact tension tests 4.1. material the specimen for the astm shear test [1] was a notched block saw-cut from larger pieces of glued laminated timber (glt) made of european spruce (picea abies) with the lamina thickness of 10.5 mm. the dimensions and configuration are depicted in figure 2. the compact tension (ct) specimens were cut from larger pieces of european spruce glt with the lamina thickness of 45 mm with the load-grain angle θ = 0°. the specimen size and test set-up are shown in figure 5a. both samples were fabricated to be representative elements of the glt structure and free of defects such as knots or cracks. the tested specimens were conditioned in an indoor environment with a temperature of 21° and relative humidity of 55% for 2 months. during testing, the average moisture content of the samples was 8% and the density was 463 kg/m3. 4.2. equipment and data acquisition the shear and compact tension tests were conducted in mts alliance rt/30 electromechanical testing machine equipped with a 30 kn load cell. force and crosshead displacement were recorded at 10 hz. image data were collected for a subsequent digital image correlation (dic) analysis with snapshots taken every 5 sec using canon eos 70d 20 megapixel digital camera fitted with canon ef 100 mm f/2.8 lens. the camera and data logger records were synchronized so as to link the photographs with the applied load. the ncorr v. 1.2 open-source software [10] with the ncorr_post in-house graphical interface [11] were involved to run the dic analyses. 4.3. shear test the shear test set-up was conducted according to the standard astm d143-94 [1], see figure 2. the specimens were freely laid on the steel base plate without capping or gluing. no extensometer was attached and the vertical relative displacements ∆x,dic depicted in figure 2a were acquired from dic measurements. 107 eliška šmídová, petr kabele acta polytechnica ctu proceedings (a). (b). figure 2. shear-parallel-to-grain test according to the standard astm d143-94: (a) set-up and relative vertical displacement ∆x,dic calculated by dic analysis, (b) a mounted specimen. [3] the load was applied with the crosshead displacement control at a rate of 0.6 mm/min. the valid specimens failed as the plane defined by the inner edge of the base plate and the inner corner of the notch sheared off in the direction parallel to the grain. the specimens with the fracture plane extended back onto the supporting surface e.g. due to material imperfections were ignored, since this failure is governed by the compressive resistance between the load and support plates, rather than by shear. we calculated the shear strength parameter fastmxy according to the standard astm d143-94 [1] as: fastmxy = fmax a (10) where fmax is the maximum attained force, and a is the shearing area. as seen in figure 2 the design shearing area is 2500 mm2. the actual size of this area was calculated using the measured dimensions of each specimen. the figure 3 shows the valid responses as load vs. relative vertical displacement. for the curve s1, the peak value can be clearly identified and it is followed by a decrease and hardening. compared to it, the figure 3. results of the shear-parallel-to-grain test according to the standard astm d143-94 [1]: load vs. dic relative vertical displacement ∆x,dic . [3] curves s2-s4 exhibit plateau. despite not being plotted in this figure, all the curves s1-s4 end with sudden drops, which are associated with an unstable fracture propagation. the table 1 lists the obtained values of the shear strength fastmxy . the specimens failed with the crack parallel to the grain more or less along the expected shearing area. the crack of the specimen s1 was slightly skewed heading from the inner edge of the notch towards the inner edge of the bottom support, see figure 4a. compared to it, the cracks of the specimens s2-s4 were almost aligned with the specimens’ longer edges spreading from the inner edge of the notch, see figure 4b. 4.4. pre-stressed compact tension test the outer dimensions and loading of the ct specimen were based on the astm e1820-09 standard [12]. to induce the fiber rupture propagating along the ligament and obtain the fracture behavior in the crack-normal direction, the test arrangement was modified with (i) pre-stress ∆σya between 2.7 mpa and 3.3 mpa (figure 5a) and (ii) reduction of the ligament depth to 10 mm (figure 5b). the load was applied with the crosshead displacement control at a rate of 0.5 mm/min. all five specimens failed with various failure types. two specimens failed with the failure type f0 disintegration parallel to the grain initiated at or around the notch tip where the crack propagated perpendicular to the ligament, see figure 6a. the failure type f1, in which fibers rupture is concentrated around or at the cross-section ahead of the notch (figure 6b), occurred once. two specimens exhibited the failure type f2, in which fibers’ rupture concentrated around or at the cross-section ahead of the notch was either followed by disintegration along the grain propagating from the cross-section’s middle, denoted as f2-a (figure 6c), or accompanied by disintegration propagating from both the notch tip and the cross-section’s middle, denoted as f2-b (figure 6d). the recorded load vs. crosshead displacement responses are presented in figure 7. their differences 108 vol. 34/2022 finite element simulation of shear and pre-stressed compact tension tests on timber (a). (b). figure 4. the final fracture of the shear-parallel-tograin test according to the standard astm d143-94: (a) the specimen s1 and (b) the specimen s3. (a). (b). figure 5. pre-stressed compact tension (ct) test with the load-grain angle θ = 0°: configuration and dimensions. (a) front view. (b) side view. [3] in shape reflected various failure types that occurred. all five curves were linear at the beginning. two curves with failure type f0 proceeded with hardening branch and a final drop. in one curve exhibiting failure type f1, softening branch with little drops and hardening portions followed. in the curve with failure type f2-a, the softening and then hardening branches followed. after the linear branch, the curve with failure type f2-b captured a decrease in slope, softening, and hardening. (a). (b). (c). (d). figure 6. failure of pre-stressed ct specimens with the load-grain angle of θ = 0°: (a) type f0 disintegration parallel to the grain initiated at or around the notch tip, (b) type f1 fibers’ rupture concentrated around or at the cross-section ahead of the notch, (c) type f2-a fibers’ rupture concentrated around or at the cross-section ahead of the notch followed by disintegration propagating from the crosssection’s middle, and (d) type f2-b fibers’ rupture concentrated around or at the cross-section ahead of the notch accompanied by disintegration propagating from both the notch tip and the cross-section’s middle. [3] 5. numerical simulations of shear test 5.1. introduction the aim of the astm shear test simulations was to demonstrate the capability of the constitutive model outlined in section 2 to reproduce the behavior of timber dominated by shear crack parallel to the grain. as the critical crack-sliding displacement δm,crit for 109 eliška šmídová, petr kabele acta polytechnica ctu proceedings (a). figure 7. load vs. crosshead displacement for prestressed ct specimens with the load-grain angle of θ = 0° that failed in modes f0, f1, and f2. [3] the crack parallel to the grain is not known from the model calibration reviewed in section 3, numerical simulations were run with different values from an estimated range. the mesh used for discretization of the timber shear blocks (figure 8) was regular consisting of four-node quadrilateral plane-stress elements with the size of approx. 2.5 mm by 2.5 mm. only the elements along the shearing line had the width of 2.0 mm, which was used as the characteristic length parameter lch for the crack parallel to the grain. as the material was orthotropic, the material axes were assigned to each element so as the grain direction was parallel to the specimen longitudinal direction. the vertical displacement uy with the unit magnitude of u0 y = 0.1 mm was prescribed in half of the nodes along the horizontal notch edge, except for the inner corner node enabling shearing of the column of elements. this unit load magnitude was scaled by an appropriate incremental loading factor, see section 5.2. the vertical load plate that prevents the specimen from rotation was modelled as prescribed supports ux = 0 mm along the vertical notch edge. similarly, the base plate and the crossbar were considered as the supports uy = 0 mm and ux = 0 mm, respectively. the zero vertical displacement was prescribed only in the nodes placed further from the base plate crossbar corner to minimize the effect of friction. the material-nonlinear calculations were performed using the newton-raphson method. 5.2. numerical solution aspects the element size and loading increment magnitude were determined by the numerical analyses of two models with meshes with typical element size of 2.5 mm and 1.0 mm. the mesh with 2.5 mm element size was used to compare the incremental loading factor of 0.15 and 0.30, that is the actual load increment of 0.015 mm and 0.030 mm, respectively. the results in figure 9 show that the load-relative displacement curves are similar for both models up to the relative displacement of ∆x,dic = 0.175 mm, where the finer figure 8. 2d fe mesh composed of approx. 2.5 mm by 2.5 mm quadrilaterals aligned with the specimen longitudinal direction and lamina. figure 9. the effect of element size and load step size on calculated response in the shear test simulation. mesh calculation was ended due to exceeded convergence criteria. the obtained fracture patterns were also similar: the crack parallel to the grain spread from the notch edge downwards along the shearing line towards the compression-shear zone with non-linear deformation around the edge of the bottom support. a conclusion was drawn to be used for all the subsequent analyses, that the mesh with the 2.5 mm element size provided a converged solution. comparing the calculation results with the incremental loading factor of 0.15 and 0.30, we can see a good agreement in responses and crack patterns. 5.3. results first, in figure 10, we compare the experimental results with the numerical responses obtained with the intermediate parameters in table 2 for different values of critical crack-sliding displacement δm,crit = {0.03, 0.055, 0.100, 0.600} mm. we can see, that the model reproduces well the initial slope, the post-peak branch, and the lowest, intermediate, and the highest level of the load capacity observed in tests. we can notice, that the results with δm,crit = {0.100, 0.600} mm yield almost the same response and the parameter δm,crit ≥ 0.100 mm does not influence the responses. second, in figure 11, we compare the experimental 110 vol. 34/2022 finite element simulation of shear and pre-stressed compact tension tests on timber figure 10. load vs. dic relative vertical displacement ∆x,dic for the results of astm shear test experiments and simulations calculated with the intermediate parameters from table 2 and different values of δm,crit. figure 11. load vs. dic relative vertical displacement ∆x,dic for the results of astm shear test experiments and simulations calculated with the minimum, intermediate, and maximum parameters from table 2 and δm,crit = 0.055 mm. results with the numerical responses obtained with δm,crit = 0.055 mm and the minimum, intermediate, and maximum parameters from table 2. the calculations with the minimum and the intermediate parameters reproduce well the s1 experiment, as for the peak value followed by a decrease, hardening and a final drop. they differ only in the final drops that occur beyond ∆x,dic = 0.2 mm and ∆x,dic = 0.26 mm, respectively. compared to it, the calculation with the maximum parameters reproduce well the s2 and s4 experiment exhibiting the plateau followed by the final drop. as expected, a similar crack pattern occurred in the numerical simulations: the crack parallel to the grain initiated under tensile-shear stress states at the upper edge of the shearing area propagating downwards. a zone, in which the non-linear deformation was developing under compression and shear, spread upward from the lower support along the shear plane until it reached the downward-growing crack, see figure 12a. the final strain states εxx and γxy are shown in figure 12b and figure 12c, respectively. comparing the strain states with the image of cracking at the maximum relative vertical displacement in figure 4, we can see, that the model captures the crack pattern adequately. (a). (b). (c). (d). figure 12. astm shear test simulation result at (ac) ∆x,dic = 0.3 mm and (d) ∆x,dic = 0.16 mm for intermediate parameters from table 2 and δm,crit = 0.055 mm: (a) crack parallel to the grain (red) and non-linear deformation zone (green) developed under compression and shear stress states, (b) normal strain εxx, (c) shear strain γxy, and (d) shear stress τxy. as overviewed in section 4.3, the shear strength according to the astm standard fastmxy is calculated by dividing the maximum attained force fmax and the shearing area a. therefore, it corresponds to the average shear stress on the area at the specimen failure. the figure 12d shows the shear stress along the shearing area around the peak load at ∆x,dic = 0.16 mm. 6. numerical simulations of pre-stressed ct test 6.1. introduction the main objectives of the pre-stressed ct test simulations were to demonstrate the capability of the constitutive model outlined in section 2 to reproduce the behavior of timber dominated by tensile crack parallel or perpendicular to the grain or combination of both, i.e., the failure modes f0-f2. based on the results from section 5.3, the parameter δm,crit = 0.055 mm for the crack along the grain was used in pre-stressed ct test simulations. as the cohesive law parameters for the crack across the grain are not known from the model calibration reviewed in section 3, numerical simulations were run with assumed values of critical crackopening and sliding displacements δacrossn,crit = 1.1 mm and δacrossm,crit = 1.1 mm, and parameters cacross1 = 3.0 and cacross2 = 6.7. the pre-stressed ct specimens were discretized with a regular mesh consisting of four-node quadrilateral plane-stress elements with the size of approx. 111 eliška šmídová, petr kabele acta polytechnica ctu proceedings figure 13. 2d fe mesh composed of approx. 2.0 mm by 2.0 mm quadrilaterals aligned with the specimen edges. 2.0 mm by 2.0 mm (to model the specimen and confinement plattens) and four external cable elements (to induce the confinement), seefigure 13. the axes of the orthotropic timber material were assigned to each element so as the grain direction was perpendicular to the direction of the specimen’s notch. the loading was applied as the vertical displacement uy with the unit magnitude of u0 y = 0.01 mm to the upper loading hub as shown in figure 13. this unit load magnitude was scaled by an incremental loading factor specified in section 6.2. the loading point was placed at a small vertical offset of 4 mm from the loading hub’s centroid to enhance the non-symmetric failure as observed in the experiments. the bottom hub was fixed at its center. the material-nonlinear calculations were performed using the newton-raphson method. 6.2. numerical solution aspects using two models with meshes with typical element size of 2.0 mm and 1.0 mm, the appropriate element size and loading increment magnitude were determined. the mesh with 2.0 mm elements was used to run calculations with the incremental loading factor of 50 and 35, that is the actual load increment of 0.50 mm and 0.35 mm, respectively. as seen in figure 14, the load-crosshead displacement curves were similar for both models up to the displacement of 3.0 mm, where the calculation with the finer mesh ended due to exceeded convergence criteria. the obtained fracture patterns were similar, both of the type f1 fibers’ rupture concentrated around or at the cross-section ahead of the notch. it was concluded that the mesh with 2.0 mm element size provided a converged solution and, thus, can be used for all subsequent calculations. comparing the calculation results with the incremental loading factor of 50 and 35, we can see a good agreement in responses in figure 14 and the same can be said about the crack patterns. the slight difference in slope of the softening branches is attributed to the fact that the disintegration of the fibers’ bundles, that accompanied fibers’ rupture along the ligament within the failure type f1, occured at different locations and had a different extent in figure 14. the effect of element size and load step size on calculated response in the ct test simulation with the pre-stress of 3.3 mpa, θc = 1.6°, and failure type f1. figure 15. the effect of element size and load step size on calculated response in the ct test simulation with the pre-stress of 3.3 mpa, θc = 0.4°, and failure type f0. each calculation. to confirm that the element size of 2.0 mm and the loading increment of 0.50 mm are also suitable for other failure types, numerical analysis similar to the previous one (figure 14) was conducted with the critical load-grain angle of θc = 0.4° which hindered propagation of the main crack across the grain in the notch direction and encouraged failure due to vertical cracking along the grain (type f0) see also the discussion in section 6.4. the results in figure 15 show that both models yielded similar response up to the displacement of 2.5 mm, where the finer mesh calculation ended due to exceeded convergence criteria. the obtained fracture patterns were very similar, both categorized as failure type f0. the comparison of the simulations with the incremental loading factor of 50 and 35 showed a very good agreement in responses and crack patterns. 6.3. results results of the calculations with pre-stress levels of 3.3, 3.0, 2.7, and 2.1 or 1.0 mpa are summarized in figure 16 and figure 17. figure 17 displays the normal strains εxx and εyy. concentrations of these strains occur due to opening of horizontal (εyy) or vertical (εxx) cracks; therefore their contour plots indicate the crack patterns, which can be compared with those observed in experiments and shown in figure 6. 112 vol. 34/2022 finite element simulation of shear and pre-stressed compact tension tests on timber figure 16. load vs. crosshead displacement for the results of ct test experiments and simulations calculated with θc = 1.6° and different pre-stress levels. in the simulation with pre-stress of 3.3 mpa we observed propagation of a major crack across fibers in the direction of the notch (figure 17a top) with multiple short branches along the grain (figure 17a bottom). this pattern corresponds well with the failure type f1 seen in figure 6b. the respective loaddisplacement curve (figure 16) exhibits the highest peak and post-peak softening. in the simulation with 3.0 mpa, crack across fibers propagated in the direction of the notch up to approx. one third of the ligament’s length (figure 17b bottom) from where crack along the grain started to propagate (figure 17b top). such a cracking corresponds to failure type f2-a shown in figure 6c. the calculated load-displacement response (figure 16) reaches the second highest peak, out of the curves for different pre-stress levels, followed by post-peak softening, hardening, and then a decrease at the end. in the simulation with 2.7 mpa, a major crack across fibers (figure 17c bottom), that propagated in the direction of the notch, was accompanied by the first branch along fibers (initiated at the notch tip) and followed by the second branch along fibers (initiated at approx. one half of the ligament’s length), see figure 17c top. this cracking resembled failure type f2-b shown in figure 6d. the obtained load-displacement curve (figure 16) has the third highest peak followed by post-peak softening, hardening, and a decrease at the end. crack along the grain initiated at or around the notch tip dominated in the calculations with 2.1 mpa and 1.0 mpa (figure 17d top). nevertheless, it was accompanied by crack across the grain in a few finite elements at the ligament’s beginning (figure 17d bottom) with crack-normal strain (i.e. εyy) much lower than that of crack along the grain (i.e. εxx). such failure was considered comparable to failure type f0 presented in figure 6a. the obtained load-displacement curves are shown in figure 16 with the fourth and fifth highest calculated peak. for the calculation with 2.1 mpa, the peak is followed by post-peak hardening and a decrease at the end. for the simulation with 1.0 mpa, the post-peak branch is softening. it is obvious that the numerical calculations were able to reproduce the different failure patterns (f0f2) observed in the experiments. the models also (a). (b). (c). (d). figure 17. distributions of normal strains εxx and εyy obtained from the ct test simulations with different pre-stress levels: (a) pre-stress 3.3 mpa (state at uy = 7.0 mm), (b) pre-stress 3.0 mpa (state at uy = 9.0 mm), (c) pre-stress 2.7 mpa (state at uy = 7.0 mm), and (d) pre-stress 1.0 mpa (state at 1.5 mm). captured well the initial slope, the peak, and the post-peak trends of the load-displacement records. 6.4. effect of threshold angle θc the value of the threshold angle θc, which determines if a crack forms across the grain (splinter failure) or along grain, could not be exactly determined from uniaxial tension tests [3]. the tests only indicated that it falls within the range between 0° and 2°. therefore, to investigate how sensitive is the model of the ct test to this parameter, two more sets of calculations were run with different values of θc = 0.4° and θc = 1.4°. results are summarized in figure 18 and figure 19. in both sets of simulations with pre-stress of 1.0 mpa 3.3 mpa, we observed a major crack along the grain initiated at or around the notch tip (figure 19 top), that was accompanied by a short branch, which propagated from the notch tip in the direction of the notch, either as crack along the grain for θc = 0.4° 113 eliška šmídová, petr kabele acta polytechnica ctu proceedings (a). (b). figure 18. load vs. crosshead displacement for the results of ct test experiments and simulations calculated for different pre-stress levels with (a) θc = 0.4° and (b) θc = 1.4° leading to failure type f0. (a). (b). figure 19. ct test simulation result of normal strains εxx and εyy at crosshead displacement of (a) 4.0 mm and (b) 5.0 mm with the pre-stress level of 3.3 mpa and critical load-grain angle of (a) θc = 0.4° and (b) θc = 1.4°. (figure 19a bottom) or crack across the grain for θc = 1.4° (figure 19b bottom). this crack pattern is considered to correspond well with the failure type f0 shown in figure 6a. all the load-displacement curves (figure 18) reach the peak with respect to the pre-stress level. the post-peak behavior exhibits hardening, except for the calculation with 1.0 mpa in which softening occurs. to summarize the ct test simulations with regard to critical load-grain angle θc and pre-stress level, different failures were observed: • (i) type f0 occurred with θc = 1.6° and the prestress level of 1.0 mpa and 2.1 mpa or with θc = 0.4° and 1.4° and all the pre-stress levels of 1.0 mpa to 3.3 mpa, • (ii) type f1 with θc = 1.6° and 3.3 mpa, • (iii) type f2-a with θc = 1.6° and 3.0 mpa, • (iv) type f2-b with θc = 1.6° and 2.7 mpa. the simulations with the pre-stress level of 2.7 mpa to 3.3 mpa and θc = 1.6° reproduced well the peak load, post-peak behavior, and respective failure types f1 f2 (figure 16), that were observed in three experimental results out of five. compared to it, the simulations with the same pre-stress level range and θc = 0.4° or θc = 1.4° reproduced adequately the load capacity, post-peak hardening, and respective failure type f0 (figure 18), that were observed in two experiments out of five. based on experimental and numerical results, the following recommendation was drawn up: to use θc = 1.6° and θc = 1.4° or less in 60% and 40% of pre-stressed ct test simulations, respectively, for a pre-stress level from the range between 2.7 mpa and 3.3 mpa. treshold angle θc, determined as the interval between 0° and 2° by visual assessment of off-axis tension specimens [3], may be regarded as one of timber material characteristics, which are typical for a significant variability. thus, statistically significant results of not only off-axis tension tests but also ct experiments with regard to failure type are needed together with an appropriate statistical approach to numerical models. 7. conclusion the finite element method with an anisotropic constitutive model were used for numerical simulation of astm shear test and pre-stressed compact tension test. the numerical results reproduced adequately both the experimental responses and the crack patterns. it was shown in shear test simulation that the value of the parameter fastmxy , obtained from fastmxy = fmax a , represents the averaged stress at the failure plane, while the extreme stress locally experienced by the material is much higher. it is concluded that, if a finite element model of a structural member has a finer resolution of the stress field variation than is the size of the tested specimen, using the result of the astm shear test as the material strength may lead to underestimation of the member’s load capacity. the pre-stressed ct test simulation demonstrated the capability of the constitutive model to reproduce complex tensile-shear behavior of timber governed by tensile crack parallel or perpendicular to the grain or combination of both. the influence of the treshold principal load-grain angle θc to the results was analysed. 114 vol. 34/2022 finite element simulation of shear and pre-stressed compact tension tests on timber list of symbols a shearing area [mm] c1, c2 parameters of the tn function [–] δn, δm relative normal and tangent displacement [mm] δm,crit shear displacement jump at which the cohesive traction diminishes to zero [mm] δn,crit normal displacement jump at which the cohesive traction diminishes to zero [mm] ∆x,dic relative vertical displacement by dic [mm] ex modulus of elasticity along fibers [mpa] ey modulus of elasticity across fibers [mpa] fcx compressive strength along fibers [mpa] fcy compressive strength across fibers [mpa] ftx tensile strength along fibers [mpa] fty tensile strength across fibers [mpa] fxy shear strength [mpa] fast m xy shear strength according to standard astm [mpa] gxy shear modulus [mpa] lch characteristic length of crack band model [mm] νxy, νyx poisson’s ratios [–] p material parameter of the slope of the tm function [–] fmax maximum attained force [n] σaxial axial stress at failure for the off-axis test [mpa] σx, σy normal stress along and across fibers [mpa] tn, tm normal and tangent tractions [mpa] tci m crack-tangent traction at crack initiation [mpa] tci n crack-normal traction at crack initiation [mpa] τxy shear stress [mpa] θ principal load grain angle [°] θc treshold principal load grain angle [°] acknowledgements this work was supported by the grant agency of the czech technical university in prague, grant no. sgs20/155/ohk1/3t/11. references [1] a. international. astm d143-94, standard test methods for small clear specimens of timber. astm international, west conshohocken pa, usa, 2000. [2] t. sajdlová, p. kabele. finite element analysis of test configurations for identification of interface parameters in layered frcc systems. in hpfrcc-7: proceedings of the 7th rilem workshop on high performance fiber reinforced cement composites, pp. 425–432. stuttgart, germany, 2015. [3] e. šmídová, p. kabele, m. šejnoha. material parameters of european spruce for tensile-shear fracture modeling. construction and building materials [in preparation], 2021. [4] e. smidova, p. kabele. constitutive model for timber fracture under tensile and shear loads. applied mechanics and materials 784:137–144, 2015. https:// doi.org/10.4028/www.scientific.net/amm.784.137. [5] c. b. norris. strength of orthotropic materials subjected to combined stresses. misc pub fpl-1816 madison, wis : us dept of agriculture, forest service, forest products laboratory 1962. [6] a. hillerborg, m. modeer, p.-e. petersson. analysis of crack formation and crack growth in concrete by means of fracture mechanics and finite elements. cement and concrete research 6(6):773–782, 1976. [7] h. reinhardt, h. cornelissen, d. hordijk. tensile tests and failure analysis of concrete. j struct eng 112(11):2462–2477, 1986. [8] e. šmídová, p. kabele, m. šejnoha. finite element simulation of single edge notched timber arch. journal of computational and applied mathematics p. 113676, 2021. https://doi.org/10.1016/j.cam.2021.113676. [9] j. vorel, p. kabele. inverse analysis of tractionseparation relationship based on sequentially linear approach. computers & structures 212:125–136, 2019. https://doi.org/10.1016/j.compstruc.2018.10.005. [10] ncorr downloads, http://ncorr.com/index.php/downloads, accessed 2021-6-13. [11] ncorr_post: dic post-processing tool, http://mech.fsv.cvut.cz/~nezerka/dic/index.htm, accessed 2021-6-13. [12] astm standard e1820-09: standard test method for determination of resistance to stable crack extension under low-constraint conditions. astm international, west conshohocken, 2010. 115 https://doi.org/10.4028/www.scientific.net/amm.784.137 https://doi.org/10.4028/www.scientific.net/amm.784.137 https://doi.org/10.1016/j.cam.2021.113676 https://doi.org/10.1016/j.compstruc.2018.10.005 acta polytechnica ctu proceedings 34:105–115, 2022 1 introduction 2 constitutive model 3 model calibration 4 shear test and pre-stressed compact tension tests 4.1 material 4.2 equipment and data acquisition 4.3 shear test 4.4 pre-stressed compact tension test 5 numerical simulations of shear test 5.1 introduction 5.2 numerical solution aspects 5.3 results 6 numerical simulations of pre-stressed ct test 6.1 introduction 6.2 numerical solution aspects 6.3 results 6.4 effect of threshold angle c 7 conclusion list of symbols acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.38.0436 acta polytechnica ctu proceedings 38:436–442, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague adapting façade performances to climate change in northern europe: analysis of future scenarios for an office building in stockholm matteo costanzoa,∗, andrea giovanni maininib, giuliana iannacconeb, ivo martinacc, david parsmand a wsp in the uk, facade department, 70 chancery lane, wc2a 1af london, uk b politecnico di milano, architecture built environment construction engineering department, 20133 milano, italy c kth royal institute of technology, building services and energy systems department, 11428 stockholm, sweden d wsp sverige, hvac and energy system departement, 7 arenavägen, 12188 stockholm, sweden ∗ corresponding author: matteo.costanzo1995@gmail.com abstract. future climate change will affect many human activities and sectors. among those, the built environment will face several challenges about the varying climate conditions, including increased demand for summer cooling and related heat stress indoor conditions. in this framework, the paper presents the results of a recent study that investigated the global warming impacts on energy demand and indoor climate comfort for an office building in stockholm over the next 50–60 years. the future climate conditions were investigated in 2070 and 2080 with different climate morphing approaches. three different passive cooling solutions to decrease the cooling demand (such as external roller shade, electrochromic glazing, and internally ventilated shading) have been preliminarily assessed about thermal and optical properties, then integrated into the building energy simulation software ida-ice to evaluate the building energy performances regarding different swedish climates, and finally economically estimated with a simplified lcc analysis. the results indicated that an increment of the cooling demand from 3 up to 24 kwh/m2 and a reduction of the heating usage of 20–50 % will be experienced in 50–60 years. the different weather data morphing approaches displayed the inherent uncertainties when future evaluations are performed, although similar weather patterns were found. the improvement of the solar and optical properties indicated a lower cooling and ventilation usage with reductions of about 10–16 %. the electrochromic technology reported the lowest cooling demand (decrease up to 24 %), while the internally ventilated shading option outperformed the others with an annual energy consumption 4–9 % lower and the lowest lcc. keywords: climate change, building design, passive cooling solutions, solar control techniques. 1. introduction world population is constantly increasing, and with it also its needs, consumptions, and emissions. in the european union (eu), research and analyses about energy consumption and greenhouse gases (ghgs) emissions reported how the built environment was responsible in 2018 for approximately 40 % of the energy consumption and 36 % of all the co2 emissions in the eu countries [1]. in sweden, the building and services sector represents the predominant one, accounting for more than 34 % of the overall energy usage [2]. buildings are usually designed and constructed to provide a service life of 50 years, consequently raising the importance of a detailed preliminary study in the building design process to assess its behaviour and response to possible different climatic conditions. the expected air temperature increase will soon promote space cooling as one of the major challenges in the building sector [3], thus design solutions that can decrease the cooling demand of the buildings will come to be of crucial value. among those, passive cooling strategies, which take advantage of natural and renewable resources to reduce the building energy usage, can be a vital contribution. nowadays the fast progress of a worldwide climate change is evident with several environmental alterations. the intergovernmental panel on climate change (ipcc) reported the risks of profound changes in the earth system given by the increase in ghgs emissions [4]. the global rise of average temperature is the foremost and most detectable aspect, with a measured warming of 0.85 °c over the period 1880– 2012 [5]. this increase is mainly connected to the increase in co2 emissions and the related greenhouse effect, directly connected to both human activities and natural causes [6]. the construction sector in sweden is regulated by the swedish board of housing, building and planning (boverket), which determines the constrains in terms of energy demand and indications for the indoor com436 https://doi.org/10.14311/app.2022.38.0436 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 38/2022 adapting façade performances to climate change in northern europe . . . figure 1. typical floor layout. fort together with other national and international authorities. in this research, the building code boverkets byggregler (boverket’s building rules – bbr) with modification up to bfs 2019:2 (“bbr 28”) was taken into consideration for the energy performances evaluation of the case study [7]. considering an office building, the energy performance (expressed as primary energy) is limited to 80 kwh/m2atemp, and the envelope performance (considered as um-value, average heat transfer coefficient) is limited to 0.60 w/m2k. for what concerns the indoor comfort, specific recommendations for thermal comfort in office buildings and workplaces are reported by the swedish work environment authority (swea) based on the provisions given by the en 7730 [8]. the indoor operative temperature ranges are 20–24 °c in winter and 20–26 °c in summer, for an office building. finally, indoor visual comfort is restricted directly by boverket which takes into considerations a minimum level of daylight factor (df) of 1 % for rooms with direct access to sunlight in office buildings [7]. in cooling-dominated buildings external shading devices are efficient tools to block the direct and part of the diffuse solar radiation. according to kunwar et al. [9], the introduction of external roller shades enables an increase in energy savings, with a cooling demand decrease up to 26 %. while an in-situ study carried out by ip et al. [10] showed an annual energy demand reduction of 16 % and a maximum reduction of the air temperature of 3.5 °c when external roller shades have been implemented instead of internal venetian blinds into an office building in brighton (u.k.). cannavale et al. [11] recently analysed different kinds of electrochromic (ec) glazing for a mediumsized office building, obtaining a reduction of the annual energy demand up to 23 % and a cooling usage reduction of 60 %. ajaji & andré [12] reported enhancements of the indoor thermal and visual comfort performances for an office space, limiting the indoor temperature and the risk for over-illumination. in the case of internal shading systems, which are known to be less performing, a promising solution couples the system with an exhaust air extraction system to remove the stored heat before it is re-irradiated into the room. denz et al. [13] considered in their study a similar integration in the gap between window and sd. in this research, the g-value of the system has been reduced by 50 % with a cooling energy demand decrease of 25 %. similarly, gustafsson & säfblad [14] evaluated an internally ventilated system, variating several parameters, such as gap width, shading’s fabric, and air flow rate, obtaining a g-value reduction up to 45 %. 2. methodology 2.1. stockholm case study the selected case study is a typical modern 10-storey office building, with the two main façades facing north and south. the building is located in the central urban area of stockholm city (sweden) and fulfils the latest building regulations and construction standards. the total floor area is 430 m2, with 57 occupants per floor, divided between single offices (10 m2), meeting rooms (26–28 m2) and an open space of 115 m2. the floor height is 2.6 m and the windows have dimensions 1.1 × 2.2 m and 2.2 × 2.2 m, for a total wwr of 23.3 %. a schematic representation of the typical floor is reported below in figure 1. the parameters of the case study model are set into the ida-ice software to represent a typical office usage, in accordance with the prescriptions of the swedish building code (bbr 28). among those, general construction parameters, internal gains settings, and the ahu system are defined also according to the swedish building environmental certification system “miljöbyggnad” [15] and the sveby (standardize and verify energy performance in buildings) “user data for office” [16], when not specified by the building code. afterwards, in order to analyse the ida-ice results, the energy consumption is divided into electric, district, and tenancy demand, as required by swedish building regulations. the purchased energy is multiplied by the primary energy factor per energy carrier (pe), except the consumption due to the tenancy use, 437 m. costanzo, a. g. mainini, g. iannaccone et al. acta polytechnica ctu proceedings glazing units screen properties g1 g2 g3 s1 s2 clear int. 1∗ int. 2∗∗ tinted tvis 70 % 63 % 21 % 6 % 2 % 60 % tvis 22 % 4 % rfext 11 % 11 % 6 % 5 % 5 % 14 % tsol 24 % 4 % rbint 13 % 12 % 10 % 9 % 10 % 17 % rsol 39 % 74 % g-value 0.31 0.47 0.16 0.09 0.06 0.27 o.f. 14 % 2 % u-value [w/m2k] 1.0 1.4 1.4 1.4 1.4 1.0 emisfront 76 % 19 % emisback 76 % 81 % ∗ int. 1 = intermediate state 1. ∗∗ int. 2 = intermediate state 2. table 1. solar control glasses performances and screen fabric properties. and the energy demand for the electric cooling is further increased by the electric cooling factor (1.875) in accordance with section 9:2 of bbr 28, which must be considered in buildings with an installed electric input for space heating and hot tap water below 10 w/m2. 2.2. solar control techniques evaluation in this paper, the performances of triple glazed units with different solar and light properties and under different shading conditions were assessed. the shading strategies consist in an external vertical awning with a roller blind (g1), electrochromic properties (g2), and an interior reflective fabric screen fabric with a ventilated air gap (g3). the external and internally ventilated shading techniques are characterized by a high selectivity solar control glass pane and a low-e pane, respectively on the outer and inner glass panes. the shading devices used are a roller screen made of a composite fabric in a vinyl-coated polyester mesh for the outdoor use (s1), and a semitransparent metallised roller screen with a higher solar reflection and lower emissivity for the indoor case (s2). the electrochromic solution has a dynamic glass pane instead of the high selectivity one. the optical and thermal properties of the insulated glass units and the screen fabric characteristics are reported in the following table 1. the overall optical and thermal properties of the different solutions are determined following the calculation methods provided by the european standards in matter of glazing and energy calculation for buildings ([17–19]). the calculations are carried out with the support of the software lbnl window 7.7 [20]. case g3 requires the assessment of the thermal and optical performances of the glazing-shading system when the exhaust air extraction is active. wis tool [21] is used assess the optimal ventilation rate and the relative thermal and optical performances of the system. a preliminary energy balance of a room is provided in accordance with the european standard en iso 52016-1:2017 [22]. the energy balance for the most critical hour of a winter, summer, and midseason day is analysed for an office room with an airflow varying from 0.18 m3/h to 5.4 m3/h, in accordance with the results derived from the literature review ([13, 14]). then, the passive cooling solutions performances are subsequently introduced into the ida-ice model building performance simulation software, version beta 07, to assess the building energy consumption and the indoor thermal comfort for the office building under investigation [23]. 2.3. climate scenarios the climate conditions investigated in this paper consist of the current climate of sweden and two climate scenarios for the future conditions which consist of hourly data of temperature, relative humidity, wind speed and direction, and solar radiation. the current climate conditions (c1) are provided by the swedish meteorological and hydrological institute (smhi) for city of stockholm [24]. the future climate scenarios are based on the emission models provided by the ipcc, called “representative concentrations pathways” (rcps) [5]. the first future climate scenario (f1) was derived from a previous work by tsaousoglou [25] for the year 2070 and rcp4.5, only considering the variation of the air temperature. the latter was initially increased by 5 °c in winter and 2 °c in summer with a linear variation in the transition months, and then manually modified with increased temperature up to 31.2 °c, to include variations in the amount of tropical nights and heatwaves periods [26]. the second future climate scenario (f2) was alternatively derived from the baseline climate data by energyplus weather database for the location stockholm-arlanda [27]. the climate morphing was carried out with the support of the climate change world weather generator [28]. the climate scenario refers to the year 2080 and the forecasted scenario denoted as b1 [29], which projects the comparable amount of co2 emissions and reflects similar changes in the weather parameters by the year 2100 as for the rcp4.5 previously selected [4], due to limitations of the climate generator tool. in figure 2 is shown the monthly average daily temperature in stockholm for the three climate scenarios under analysis. 438 vol. 38/2022 adapting façade performances to climate change in northern europe . . . figure 2. monthly average daily temperature in stockholm for the three climate scenarios. figure 3. graphical report of the g-value results with respect to the extraction airflow rate. element u.m. g1 + s1 g2 g3 + s2 window sek/m2 3500 8500 3500 shading sek/m2 3000 1100 extraction sek/m2 977.1 total sek/m2 6500 8500 5577.1 table 2. window and shading elements initial investment costs. 2.4. life-cycle cost analysis the economic benefits of the three different solar control techniques are assessed with the support of a life-cycle cost analysis. in this research, the calculation is limited to only the window systems costs in relation to the total annual building energy costs. the initial costs for windows, shading elements, and the exhaust air extraction system are reported in table 2. they were provided by wsp sweden company [30], with reference to internal projects and by the cost list sektionsfakta-vvs 19/20 by wikells [31]. the initial investment and the later maintenance and renovation evaluations are related to the total window area of 750 m2 (27 windows per floor, 10 stories). the maintenance and replacement expenses are evaluated based on a literature review about the different window systems elements. average service life between 20–30 years were considered for the windows and ventilation system, while annual costs between 50–80 sek/m2 were defined for each solution. subsequently, to evaluate the life-cycle cost of the three options a pre-established working sheet provided by wsp sweden is used. the calculation considers average prices for the energy carriers to evaluate the annual energy costs of the building over its service life (district heating 680 sek/mwh, electric cooling and electricity 920 sek/mwh), together with interest rate (3 %), loan rate (2 %), and price increase for the energy (3 %). 3. results and discussion 3.1. solar control techniques in case g3, the introduction of the exhaust air extraction showed a potential reduction of the g-value by 49 % (corresponding to 0.067) for the maximum air flow studied, as presented in figure 3. considering the analyses carried out on the office room model, the selected airflow for the exhaust air extraction was 3.24 m3/h, which corresponded to a gvalue of 0.073 and an air velocity of 0.11 m/s. the latter configuration provided the best trade-off between air flow increment and heat loads reduction, showing the last considerable improvement in the energy balance of the room with heat loads and heat removal by the air conditioning reductions of respectively 43.8 % and 21.7 %. in table 3 are reported the resulting values for the three techniques used in the further ida-ice simulations. finally, to manage the change of state for each of 439 m. costanzo, a. g. mainini, g. iannaccone et al. acta polytechnica ctu proceedings g1 + s1 g2 g3 + s2 u.m. clear drawn clear int. 1 int. 2 tint clear drawn extr. tvis % 63 13.8 55.6 16.2 5.2 0.9 55 2.6 2.6 tsol % 27.3 6 30.4 7.1 2.3 0.4 23.2 1.1 1.1 g-value 0.312 0.083 0.367 0.127 0.078 0.055 0.267 0.131 0.073 table 3. optical and thermal properties of the three window systems. climate energy meter g1 g2 % ∆g1−g2 g3 % ∆g1−g3 scenario [kwh/m2atemp] c1 electric cooling 3.2 2.9 -9.4 % 3.0 -6.3 % electric fans 7.2 6.9 -4.2 % 7.2 0.0 % district heating 23.2 26.2 12.9 % 23.6 1.7 % total 54.5 57.0 4.6 % 54.9 0.7 % f1 electric cooling 24.4 20.6 -15.6 % 21.9 -10.2 % electric fans 8.2 8.1 -1.2 % 8.5 3.7 % district heating 11.9 13.9 16.8 % 11.6 -2.5 % total 67.7 65.4 -3.4 % 64.9 -4.1 % f2 electric cooling 19.1 14.4 -24.6 % 15.2 -20.4 % electric fans 8.9 8.0 -10.1 % 8.2 -7.9 % district heating 18.9 20.2 6.9 % 17.6 -6.9 % total 69.1 63.9 -7.5 % 62.7 -9.3 % table 4. annual building energy consumption in the different years for each solution. these techniques on ida-ice, control macros based on levels of incident solar irradiation (w/m2) with an override connected to the outdoor air temperature have been implemented. different solar irradiation levels have been considered for each façade and climate scenario depending on the characteristics of the window system. 3.2. current and future climates the annual energy demand and ventilation needs results are reported for the three solar control techniques and different climate scenarios in table 4. the results revealed an increment of the cooling consumption in both the future scenarios, efficiently reduced with the introduction of the electrochromic and internally ventilated solution. at the same time, the heating demand decreased, in accordance with the mitigation of the winter periods, showing a deficit of performance for option g2. the results for the climates f1 and f2 take into consideration an optimization of the fans usage and a combine use of room units and ahu system for the summer period. the latter configuration was due to extreme cooling usage peaks and ventilation air flow initially detected for the scenario f1, caused by the coexistence of high outdoor air temperature and high relative humidity due to the manual alteration of the outdoor air temperature and unvaried relative humidity. in the alternative scenario f2, built with the variation of all the weather parameters, these extreme conditions were not present, and the summer consumption was considerably decreased while the heating demand was increased, always remaining lower than the c1 results. the thermal comfort ranges, i.e. max values of ppd, min and max operative and indoor air temperatures, were respected in all the cases and seasonally analysed, reporting lower ppd values for the winter in the current climate, while more homogeneous results for the future climates. 3.3. life-cycle cost analysis in figure 4 are reported the results of the life-cycle cost analysis, expressed in swedish crowns, and divided by investment costs, building energy usage (district heating, electric cooling, electricity), general maintenance, and renovation. the ec technology resulted to be inappropriate for the case study. the costs due to the materials, installation, and renovation overturned the limited benefits in terms of annual energy costs. on the other hand, the internally ventilated solution showed the best results. the initial investment, operational and maintenance costs were improved, remarking the potential advantages of such technique also in a long-term perspective in relation to a changing climate. 440 vol. 38/2022 adapting façade performances to climate change in northern europe . . . figure 4. graphical representation of all the present values and resulting lcc. 4. conclusions following a previous study on the climate change effects on the built environment for a building in stockholm, this research evaluated the energy consumption and indoor thermal comfort variations for an office building equipped with different solar control techniques for the current and future climate. • the internal shading with the extraction offered better performances than without it, for both the current and future climate, especially regarding the cooling demand of the building. • the introduction of the ec technology and the ventilated shading did not considerably modify the energy behaviour of the building in the current scenario, while they successfully improved it in both future climates, decreasing the solar thermal load, lowering the maximum airflow required by the building, and thus reducing the size and related issues of the ventilation system. • the initially installed hvac system resulted inefficient to withstand the future climate conditions derived by the only variation of outdoor air temperature, pointing out the necessity to evaluate and design new active systems for varying weather conditions. • the results regarding the future scenarios were limited by the intrinsic level of uncertainty within the study of forecasted future climate conditions. • finally, in the life-cycle cost analysis the internally ventilated option showed the best results, remarking relevant potential benefits also in a long-term perspective. references [1] european commission. european commission – new 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[2020-02-06]. https://www.researchgate.net/publication/ 331174454_act_facade_-_interior_sun_shading_ for_energy_efficient_fully_glazed_facades [14] e. gustafsson, f. säfblad. decreasing g-value by combining internal solar screen and exhaust ventilation – a numerical study on what parameters affect the possible reduction. master’s thesis, chalmers university of technology, göteborg, 2014. [2020-04-02]. https://publications.lib.chalmers.se/records/ fulltext/215327/215327.pdf [15] miljöbyggnad. miljöbyggnad 3.1 – metodik och. manual ny byggnad. sweden green building council, stockholm, 2020. [2020-06-16]. https://www.sgbc.se/app/uploads/2020/05/milj% c3%b6byggnad-3.1-nybyggnad.pdf [16] sveby. brukarindata kontor, sveby, stockholm, 2013. [2020-07-01]. https://www.sveby.org/wp-content/uploads/2013/ 06/brukarindata-kontor-version-1.1.pdf [17] european standard: en 410:2011 – glass in building – determination of luminous and solar characteristics of glazing, european committee for standardization, brussels, 2011. https://doi.org/10.3403/30219373 [18] european standard: en 673:2011 – glass in building – determination of thermal transmittance (u-value) – calculation method, european committee for standardization, brussels, 2011. https://doi.org/10.3403/02351907u [19] international organization for standardization: en iso 52022-3:2017 – energy performance of buildings – thermal, solar and daylight properties of building components and elements, european committee for standardization, brussels, 2017. [20] lawrence berkeley national laboratory. window & daylighting – building technology & urban systems division. berkeley lab lbnl, 2019. [21] wis. advanced window information system. tno building and construction research, 2006. [22] european standard: en iso 52016-1:2017 – energy performance of buildings – energy needs for heating and cooling, internal temperature and sensible and latent heat, international organization for standardization, brussels, 2017. [23] equa simulation ab. equa – ida indoor climate and energy, equa, 2020. [24] swedish meteorological and hydrological institute. smhi research department ec-earth, smhi, 2020. [25] a. c. tsaousoglou. the impact of climate change on the energy demand and indoor climate of an apartment building in stockholm. degree project, royal institute of technology, stockholm, 2019. [2020-04-22]. http://kth.diva-portal.org/smash/get/diva2: 1388648/fulltext01.pdf [26] swedish government official reports. sweden facing climate change – threats and opportunities. edita sverige ab, stockholm, 2007. [2020-04-23]. https://www.government.se/49b75f/contentassets/ 5f22ceb87f0d433898c918c2260e51aa/sweden-facingclimate-change-sou-200760 [27] energyplus. energyplus weather data, energyplus, 2020. [28] energy and climate change division. ccworldweathergen – energy & climate change. university of southampton, southampton, uk, 2020. [29] intergovernmental panel on climate change. climate change 2001 – synthesis report. ipcc, cambridge, uk, 2001. [2020-07-30]. https://www.ipcc.ch/site/ assets/uploads/2018/05/syr_tar_full_report.pdf [30] wsp sverige. vvs och energi avdelning, wsp sverige, 2020. [31] wikells. sektionsfakta-vvs 19/20, wikells byggberäkningar, 2019. 442 https://doi.org/10.1016/j.egypro.2015.11.230 https://www.researchgate.net/publication/331174454_act_facade_-_interior_sun_shading_for_energy_efficient_fully_glazed_facades https://www.researchgate.net/publication/331174454_act_facade_-_interior_sun_shading_for_energy_efficient_fully_glazed_facades https://www.researchgate.net/publication/331174454_act_facade_-_interior_sun_shading_for_energy_efficient_fully_glazed_facades https://publications.lib.chalmers.se/records/fulltext/215327/215327.pdf https://publications.lib.chalmers.se/records/fulltext/215327/215327.pdf https://www.sgbc.se/app/uploads/2020/05/milj%c3%b6byggnad-3.1-nybyggnad.pdf https://www.sgbc.se/app/uploads/2020/05/milj%c3%b6byggnad-3.1-nybyggnad.pdf https://www.sveby.org/wp-content/uploads/2013/06/brukarindata-kontor-version-1.1.pdf https://www.sveby.org/wp-content/uploads/2013/06/brukarindata-kontor-version-1.1.pdf https://doi.org/10.3403/30219373 https://doi.org/10.3403/02351907u http://kth.diva-portal.org/smash/get/diva2:1388648/fulltext01.pdf http://kth.diva-portal.org/smash/get/diva2:1388648/fulltext01.pdf https://www.government.se/49b75f/contentassets/5f22ceb87f0d433898c918c2260e51aa/sweden-facing-climate-change-sou-200760 https://www.government.se/49b75f/contentassets/5f22ceb87f0d433898c918c2260e51aa/sweden-facing-climate-change-sou-200760 https://www.government.se/49b75f/contentassets/5f22ceb87f0d433898c918c2260e51aa/sweden-facing-climate-change-sou-200760 https://www.ipcc.ch/site/assets/uploads/2018/05/syr_tar_full_report.pdf https://www.ipcc.ch/site/assets/uploads/2018/05/syr_tar_full_report.pdf acta polytechnica ctu proceedings 38:436–442, 2022 1 introduction 2 methodology 2.1 stockholm case study 2.2 solar control techniques evaluation 2.3 climate scenarios 2.4 life-cycle cost analysis 3 results and discussion 3.1 solar control techniques 3.2 current and future climates 3.3 life-cycle cost analysis 4 conclusions references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.34.0094 acta polytechnica ctu proceedings 34:94–97, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague detection of the presence of microorganisms during production of plasterboard hana sekavováa,b,∗, zdeněk prošeka, george karra’ac a czech technical university in prague, faculty of civil engineering, thákurova 7, 166 29 prague 6, czech republic b knauf praha spol. s r.o., mladoboleslavská 949, 197 00 prague 9, czech republic c lavaris s.r.o., areál šroubáren 43, 252 66 libčice nad vltavou, czech republic ∗ corresponding author: hana.sekavova@fsv.cvut.cz abstract. the article introduces possibility how to use special material from plasterboard back to the production of plasterboards. the recycled material was prepared from standard plasterboards knauf white (it means plasterboard type a according to european standard en 520) [1]. plasterboards were crushing, micromilled and separated from the paper on recycling line, which was developed in company lavaris s.r.o. [2]. this material was used as an inert to the standard recipe instead of stucco. there were checked the influence on properties of final plasterboards in comparison with standard plasterboards. keywords: gypsum plasterboard, stucco, recycling, inert material. 1. introduction the massive expansion of use gypsum plasterboards in czech republic started in 1990s. the first years were boards imported from neighboring countries and also came first producers and built their plants there [3]. the gypsum plasterboard plant knauf praha spol. s.r.o. is situated in počerady, direct next to power plant počerady, was opened in year 1994. the capacity of plant is millions square metres per year and the scrap together with customer´s scrap is in thousands tons per year and this amount is sent to landfills. in the world there are sent to landfills more than 15 million tons of gypsum waste from plasterboards each year [4]. the very comprehensive and clear overview of problematic recycled gypsum in eurepean union is in research presented by a.j. rivero et al. [5]. this research shows that recycling of gypsum is neccesary because of hundred thousands tons of waste, which is send to landfills, because of closing stocks of natural and also syntetic gypsum and last but not least the influence for energy consumtion and emissions. in the beginning, the fgd (flue gas desulphurization) gypsum is also kind of waste and m.a. pedrenorojas et al. [6] consider use of fgd gypsum such as use of waste. in their research is presented successfully use of fgd gypsum and waste from production from plasterboard production in plasters. the plasterboards only from recycled material was produced in laboratory condition. the recycling was with calcination, the material, which was produced, was stucco with binding properties – calcium sulphate hemihydrate [7]. there was also watched influence on properties of multiple recycled gypsum from plasterboards. it was processed, that gypsum waste is feasible until the third cycle [8]. the recyled gypsum from plasterboards can be used not only back to production of plasterboards. the essential use is also in portland cement manufacture and as a soil amendment. possibility is use this material as a compost amendment and for animal bedding [9]. the development of new sources and gypsum and recycling of gypsum products is necessary because in the near future is planned the shutdown of coal fired power plants. it means no fgd gypsum and also there is expectation that environmental impacts related to electricity consumption will decrease [10]. the target of this research is find the most safe amount of recycled material from gypsum plasterboards what is possible to use as an inert in plasterboard production. 2. material the material, which was used for this trial, was plasterboards knauf white and knauf green. knauf white is trademark for standard plasterboard type a and knauf green is trademark for standard impregnated boards type h2 [1]. these boards are produced in knauf praha spol. s r.o. in plant počerady. the plasterboards are produced of stucco, which is calcinated of fgd gypsum. the fgd gypsum is produced during the desulphurization process flue gases in power plant, where is burned the brown coal. this fgd gypsum has very high purity (96 % on average). it means there is possibility to use inert to the recipes and don´t lose the binding properties of slurry. the process of calcination (1) and hydration (2) is quite very simple and the energy, which is necessary 94 https://doi.org/10.14311/app.2022.34.0094 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 34/2022 detection of the presence of microorganisms for production of stucco, is very low in comparison with production of cement. the energy consumption for production 1 t of stucco from fgd in rotary kiln is 250–350 kcal depending on the moisture content. however, if we use the inert material instead of stucco, we can save corresponding quantity of heat energy for stucco production. caso4 · 2h2o + heat→ → caso caso4 · 1/2h2o + 1/2h2o 110− 170 ◦c → caso caso4ii + 2h2o > 200 ◦c . . . anhydrit ii (aii) → cao + so2 + o2 + caso4i > 800 ◦c . . . anhydrit i (ai) (1) caso caso4 · 1/2h2o + 1/2h2o→ → caso4 · 2h2o + heat (2) the inert could be also material which could be produced during recycling of gypsum plasterboards. there is a quite big amount scrap, which is formed during the production of plasterboards and also there is a lot of scrap, which is formed during construction (customer scrap). on the other hand, the recycled gypsum can be used as a retarder during rehydration of cement [11]. figure 1. schema of production of plasterboards. schema of production of plasterboards is on the figure 1. for this topis is important point 2, where the all of components are mixed in the mixer and the gypsum slurry is put between face and back paper. the second important place is point 5. it is knife position, where the endless belt is divided to boards and where the wet scrap is formed. after dryer (point 8) are boards sawed and milled to precise dimension (point 10) and here is formed dry scrap. in year 2019 was counted that approximately 5700 tons of scrap was sent to landfill only from plasterboards produced in počerady plant (internal and external scrap). there is solved only influence of recycled material from dry knauf white boards to behavior of production and products in this paper, but there is showed inputs and outputs of recycling process in počerady plant on figure 2. the photo of raw materials and outputs of recycled proces is showed on figure 3 (first line from left: fgd gypsum, stucco, recycled paper; second line from left: recycled material 2–5 mm, recycled material 0–2 mm, recycled material 0–0.05 mm). the plasterboards knauf white were input and output from the trial too. in the beginning was taken boards, which were not fulfill all of the requirements (european and internal standards) and they usually are throw out as a scrap. this material was delivered to company lavaris s.r.o. and there was crushed, milled and it was separated out of the paper too. the process of crushing and milling was with cooling in order to avoid recalcination. output is calcium sulphate dihydrate. there is process of crushing and milling on the figure 4. in the beginning of this project was assumed prepare granulometry of inert same as used stucco (therefore the fgd gypsum – also used as an inert). on the figure 5 is a grain size curve of počerady stucco (purple one) and defined milled material (green one). the maximal size of grain was 1 mm. from the input material was prepared recycled material of three fractions: 2–5 mm, 1–2 mm and 0–1 mm. each fraction formed approximately one third of the original volume. this material (fraction 0–1 mm) was used to the production. it was prepared for the existing equipment. unfortunatelly the dosing system was not able to dose this material fluently. the very fine material made an arches and the material was very often blocked. because of this trial production was stopped and the rest of material was thoroughly mixed with the material grain size 1–2 mm. this material was dosed without any problems. for second trial was prepared material grain size 0–2 mm. there was also milled small amount of knauf green boards for verification assumption, that impregnated boards cannot be used as an inert to the gypsum slurry. 3. experimental methods and results 3.1. production the milled material was used in production in 3 variants. the first one was use recycled material with grain size 0–2 mm which was mixed from two fractions (0– 1 mm and 1–2 mm). this situation was describe in capture material. this variant has three steps (1.) standard recipe with 3 % recycled material. (2.) standard recipe without 3 % stucco with 4.5 % recycled material. (3.) standard recipe without 4 % stucco with 6 % recycled material. edit: percent means % wt. of amount of stucco in standard recipe 95 h. sekavová, z. prošek, g. karra’a acta polytechnica ctu proceedings figure 2. diagram of recycled process in počerady plant. figure 3. raw materials and outputs from the recycled process. figure 4. lavaris recycling babyline. each step took about 5 minutes. there was watched figure 5. diagram of granulometry – stucco and recycled material. only small changes on gypsum slurry. any other check didn´t notice any abnormalities. the second variant was long term trial with material with fraction 0– 2 mm. during second trial was standard recipe included inert (raw fgd), because of that was only fgd gypsum replaced with recycled material 1:1. the amount was 3 % wt. of stucco. this trial took 55 minutes and there was consumed almost 500 kg of recycled material. during the change material and trial production was not noticed any abnormalities on production line. the third one was verification assumption, that impregnated boards cannot be used as an inert to the gypsum slurry. there was used fine milled material 0–1 mm with median under 0.5 mm. there was dosed only 6 g/m2 of this material it means less than 0.002 % wt. of stucco. immediately afterwards there was a massive decline of gypsum slurry on the forming table due to degradation of foam, which is integral part of gypsum slurry. the first assumption was hereby confirmed. 96 vol. 34/2022 detection of the presence of microorganisms 3.2. output quality control output quality control consists of the visual, mechanical, and chemical tests. the set of visual controls consists of checking defects and abnormalities on the surface of boards. the boards produced during trials had no defects like bubbles or blisters under the paper. the porosity of gypsum core was without changes. mechanical properties are tested according to european standard en 520 [1]. the breaking loads was in the same range of values like boards produced with standard recipe. there was not any increase but also any decrese of values. it means all of values were between 610 and 620 n in lengthwise and between 210 and 230 n in crosswise. the most exposed and watched test is paper bonding. there is three tests of paper bonding. the first one is paper bonding direct after drying process, next one is paper bonding after 24 hours of storage and third one is paper bonding after 24 hours storage in moist box (or climatic chamber). all of the test use cross cut method, which is showed on figure 6. there is also the results of paper bonding of trial boards. there was watched no decrease of bonding between paper and gypsum core. figure 6. results of paper bonding cross test. 4. conclusions all of the trials were successful and they fulfill all of the expectation. the safe amount of recycled material is 6 % wt. of stucco. this amount of recycled material had no influence to behavior of production process and quality of product. the benefit of use inert is safe energy for production of stucco. on the other hand the recycled material is more expensive that raw fgd gypsum which is also used as an inert. important is that raw fgd gypsum is better use for production of stucco and from an ecological point of view is better use the recycled material as an inert instead of send it to the landfill. definitely this trial open the possibility to increase the amount of inert in recipes in the future. acknowledgements the research is carried out within the project mpo trio č. fv30359 “recyklace sádrokartonových desek a nová materiálová využití s přidanou hodnotou – gipsrec” (gypsum board recycling and new value-added material uses). references [1] čsn en 520+a1. sádrokartonové desky definice, požadavky a zkušební metody. praha: úřad pro technickou normalizaci, metrologii a státní zkušebnictví 2010. [2] h. sekavová, j. herrmann, z. prošek, et al. the influence of the use of recycled gypsum on the properties of gypsum products in labor conditions. acta polytechnica ctu proceedings 26:81–85, 2020. [3] m. nyč. sádrokarton. grada publishing as, 2005. [4] a. ahmed, k. ugai, t. kamei. investigation of recycled gypsum in conjunction with waste plastic trays for ground improvement. construction and building materials 25, 2011. https://doi.org/10.1016/j.conbuildmat.2010.06.036. [5] a. rivero, r. sathre, j. navarro. life cycle energy and materiál flow implications of gypsum plasterboard recycling in the european union. resources, conservation and recycling 108, 2016. https://doi.org/10.1016/j.resconrec.2016.01.014. [6] m. pedreno-rojas, i. flores-colen, j. d. brito, c. rodríguez-linán. influence of the heating proces on the use of gypsum wastes in plasters: mechanical, thermal and enviromentalanalysis. journal of cleaner production 215, 2019. https://doi.org/10.1016/j.clepro.2019.01.053. [7] a. erbs, a. nagalli, k. q. de carvalho, et al. development of plasterboard sheets exclusively from waste. journal of building engineering 44, 2019. https://doi.org/10.1016/j.jobe.2021.102524. [8] a. erbs, a. nagalli, k. q. de carvalho, et al. properties of recled gypsum from gypsum plasterboards and commercialgypsum throughout recycling cycles. journal of cleaner production 183, 2018. https://doi.org/10.1016/j.jclepro.2018.02.189. [9] čsn en iso 846. plasty hodnocení působení mikroorganismů. praha: úřad pro technickou normalizaci, metrologii a státní zkušebnictví 2019. [10] k. weimann, c. adam, m. buchert, j. sutte. enviromental evaluation of gypsum plasterboard recycling. minerals 2021. https://doi.org/10.3390/min11020101. [11] n. f. medina, f. hernández-olivares, x. arroyo, et al. characterization of a more sustainable cement produced with recycled drywall and plasterboards as set retarders. construction and building materials 124, 2016. https://doi.org/10.1016/j.conbuildmat.2016.08.140. 97 https://doi.org/10.1016/j.conbuildmat.2010.06.036 https://doi.org/10.1016/j.resconrec.2016.01.014 https://doi.org/10.1016/j.clepro.2019.01.053 https://doi.org/10.1016/j.jobe.2021.102524 https://doi.org/10.1016/j.jclepro.2018.02.189 https://doi.org/10.3390/min11020101 https://doi.org/10.1016/j.conbuildmat.2016.08.140 acta polytechnica ctu proceedings 34:94–97, 2022 1 introduction 2 material 3 experimental methods and results 3.1 production 3.2 output quality control 4 conclusions acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.38.0018 acta polytechnica ctu proceedings 38:18–24, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague environmental performances evaluation through building information models ambra barbinia,∗, giada malacarneb, giovanna a. massaria, carmen marcherb,c, dominik t. mattb,c a university of trento, department of civil environmental and mechanical engineering, via mesiano, 77, 38123 trento tn, italy b fraunhofer italia research, via alessandro volta, 13 a, 39100 bolzano bz, italy c free university of bozen, faculty of science and technology, piazza università, 5, 39100 bolzano bz, italy ∗ corresponding author: ambra.barbini@unitn.it abstract. the construction industry produces several negative environmental impacts. to promote solutions that reduce these negative impacts, it is crucial to increase awareness among all parties involved in the supply chain. currently, certification protocols, environmental labels and declarations are the most common tools for promoting sustainability in the construction industry. however, these tools are generally non-mandatory and require specific competencies to thoroughly understand and interpret their outputs, especially for non-specialized users in sustainability assessment. this paper presents a tool for visualizing environmental impacts directly through a building information model, by assigning different colours to model objects according to their environmental performances. understanding environmental performances data is thus more accessible to non-expert users. unlike several bim-lca integration studies that primarily focus on the needs of designers, this contribution considers the perspective of manufacturers. the developed tool focuses on building products and has been carried out with the autodesk revit bim authoring platform and its vpl-based dynamo plug-in. through the combination of 3d models and histograms, the tool facilitates manufacturers to visualize with real-time updates which phase of a product life cycle requires sustainable innovation, which sub-components or process determine the major environmental impacts and calculate the environmental cost indicator of a product. keywords: bim (building information modeling), sustainability, environmental impacts, lca (life cycle assessment), eci (environmental cost indicator). 1. introduction according to international reports and statistics, the construction sector is responsible for about 36 % of waste generation in europe [1], 36 % of world energy consumption [2] and 39 % of global carbon-dioxide emissions [3]. a conscious innovation of all the involved parties in the architecture, engineering and construction (aec) industry is a necessary step in limiting negative environmental impacts, such as nonrenewable resources consumption and pollutant emissions. interest in reducing environmental impacts is steadily increasing, as evidenced by the sustainable developments goals (sdgs 2030), which encourage, among others, actions to combat climate change and to promote responsible production and consumption [4]. within the construction industry there are different tools developed to increase consumer awareness of environmental impacts. these tools, such as certification protocols, labels and declarations, rate and certify the level of sustainability of buildings and their components. building certification protocols are increasingly in demand and appreciated, as they are recognised to bring economic advantage, increasing rental and market value, and decreasing operating costs [5]. similarly, labels and declarations highlight virtuous characteristics of a product, such as the possibility to disassembly it and recycle its sub-components or a low carbon footprint throughout its life cycle. certification protocols are generally based on multicriteria analysis and based on the evaluation of an external committee, while labels and declarations can be self-declared (type i), awarded by a third party after a qualitative (type ii) or a quantitative (type iii) assessment. this third type includes the environmental product declaration (epd), which is a certificate of the environmental performances of a product based on the results of a life cycle assessment (lca). lca isa generally accepted methodology for assessing the sustainability of a product or process. a life-cycle assessment provides values that objectively quantify the impacts of a product on the environment, considering the consumption of energy and natural resources and the release of pollution and waste respectively as input and output flows of a process [6]. the analysis of lca results provides access to important information on a production process and can support manufacturers in identifying the main factors that lead to negative environmental impacts. despite the possibility to objectively quantify environmental impacts, lca presents two main limits for 18 https://doi.org/10.14311/app.2022.38.0018 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 38/2022 environmental performances evaluation . . . common users: (1.) it requires a large amount of input data, and (2.) provides a detailed set of results, which are complex to understand and to compare. both limits can be overcome by taking advantage of building information modelling (bim). as a system able to collect and manage a variety of data related to building components at different levels of details and stages of their life cycle, bim can support life-cycle analysis of buildings and their components. indeed, lca data can be easily integrated or linked to bim objects, representing building components [7]. in addition, considering the ongoing digital implementation of the aec industry, manufacturers and suppliers are also showing increasing interest in developing bim object libraries of their products to use as digital catalogues or to support the optimization of design and production phases. therefore, digital implementation and sustainable development strategies can support each other. this paper presents the possibility of exploiting bim objects of building components to improve the readability of lca results and to support manufacturers in identifying possible opportunities for product innovation, towards the reduction of environmental impacts. 2. state of the art the environmental impacts assessment of products is generally related to its whole life cycle and requires specific expertise to understand the results. by analysing the entire life cycle of a product, as well as, all possible environmental impacts and their aggregations, life cycle assessment (lca) is considered one of the most comprehensive procedures for the evaluation of environmental performances [8]. lca results are generally presented as complex tables reporting the values of several impacts categories for each phase of a product life cycle, thus generally requiring the interpretation of an expert. to support manufacturers and suppliers in identifying and pursuing sustainable innovation strategies, not only motivated by marketing logics but with a real contribution to the sdgs, it is essential to improve the visualization and readability of these technical data. private and public organizations have developed digital tools to facilitate manufacturers in reading and visualizing the environmental performances of their products. some of these tools [9, 10] do not refer to a specific product category (e.g. ecoscan by echochain), others are specifically addressed to the construction sector (e.g. totem by ovam). within the aec sector, several research groups worked on the integration of bim and lca, mainly adopting two approaches [11]: the first one involves the combination of a bim platform with an lca software (e.g. simapro, openlca and gabi) or plug-in (e.g. one click lca and tally), whereas the second one requires the enrichment of the model with lca data. some of these research groups also considered the importance of results visualization through the model both at the building and infrastructure scale. for example, van eldick et al. [12] developed a bim-based system to assess environmental impacts of infrastructure, which includes report generation and model visualization for each phase of the project. röck et al. [13] presented a solution to assess and visualise the impacts embedded in building elements, as a tool to support early stages of design. therefore, bim methods and tools can support not only the automatic and dynamic life cycle analysis of a building and its components but also the visualization of environmental impacts directly through a building information model, making the results much more readable and accessible. however, most of the research, exploring the possibility of integrating lca and bim, consider the perspective of the designer or the client. this paper proposes a solution for the evaluation of building components environmental performances, considering the point of view of a building product manufacturer. the aim of the research is to support building products manufacturers to understand and interpret environmental impacts data of their products. to improve the readability and transparency of the environmental performances of the building component life cycle, three specific solutions for the results visualization have been implemented. through 3d models and histograms, the developed solutions allow manufacturers to visualize: • which phase of a product life cycle can benefit the most from a sustainable innovation, • which factors are crucial to reduce environmental impacts, • on which impact category a specific factor presents the highest influence. 3. methods and tools the research is based on a case study and tests the possibility to evaluate the environmental impacts of a building component through building information models. the fraunhofer italia research institute developed the case study in collaboration with the ecamricert testing laboratory and with the support of the university of trento, periodically discussing potential and limitations of the defined solutions with local stakeholders. the case study is referred to a wooden window. four stages of the window life cycle have been considered, according to en 15978: product, construction process, use and end of life (table 1). as an lca of the considered window has not yet been performed, the ecamricert laboratory selected from a database the lca results of some components as similar as possible to those of the window and considered helpful for the research objectives. in this respect, it is important to note that the presented case study does not aim at 19 a. barbini, g. malacarne, g. a. massari et al. acta polytechnica ctu proceedings phase 1: phase 2: phase 3: phase 4: product stage construction use stage end of life stage process stage fa ct or s wood [m3] silicon [kg] rubber seal [kg] wood [kg] glass [kg] nail [kg] window handle [kg] glass [kg] window handle [kg] transport [tkm] transport [tkm] transport [tkm] rubber seal [kg] transport [tkm] table 1. factors considered for each phase of the window life cycle and relative reference units according to en 15978. figure 1. conversion factors for calculating the environmental cost indicator (eci). defining real and precise lca values for the selected window, but rather at testing the feasibility of the bim-lca integration process and different ways of displaying the results. for the production stage, which includes the raw material supply and manufacturing, lca results were collected for the wood of the frame, the window glass, the rubber seals, and the window handle. the construction process stage mainly refers to the products employed to fasten the window to the wall (nails and silicon). the use stage considers the possibility to replace and repair some components (e.g. window handle, rubber seals) and the end-of-life stage concerns the impacts connected to the dismission of the window components, including the possibility of recycling some of them (e.g. wood elements). for each phase the authors also considered the impact of transport assuming that the window was installed not far from (< 50 km) the production site and that the replacement components and disposal site are accessible within the same region (< 100 km). the output of an lca includes different environmental impacts categories (e.g. ozone depletion potential) and each one has a different fictitious unit of measurement, known as equivalent unit (e.g. cfc-11 eq), used to quantify the possible effect of different factors on the environment. as evidenced by cheng et al. [14], the difference in units prevents from a comprehensive assessment of all the embodied environmental impacts of a building. for this reason, the comparison among different products or among different phase of the life cycle, is only possible considering one environmental impact category at the time or adopting a system of conversion factors. in this research the authors adopted the conversion factors (figure 1), that the dutch government developed to determine the environmental cost indicator of a building product, by converting the values of impact categories into summable costs, expressed in euros [15]. the research workflow (figure 2) includes three main steps: (1.) the collection and organization of geometrical data within the bim model and of environmental data within the spreadsheet; (2.) the definition of the appropriate analysis for the evaluation of the environmental performances of the window; (3.) the development of different progressively detailed options for results visualisation. 3.1. data collection and organization the geometry of the window is based on a prototype of a bim object library. the university of trento, in collaboration with a local manufacturer, developed the library with the bim authoring software autodesk revit 2020, starting from the detail drawings and technical information of the product data sheet. the geometric model of the window is replicated four times and each one represents a different stage of the life 20 vol. 38/2022 environmental performances evaluation . . . figure 2. workflow of the process. cycle: product, construction process, use and end of life. each model representing a phase of the window life cycle is enriched with specific information useful for the evaluation of the environmental performances (table 2). most of this information represents the quantities of the window components (e.g. wood volume). these quantities are calculated through the parameter system of the bim object and depend on both the technical data and the size of the window. the flow of information is the following: the manufacturer supplies the technical data about the used material (e.g. specific weight) and the designer, according to the client wishes, defines the window dimensions. other information related to the environmental impacts of each phase, comes out from the epd, when available, otherwise environmental impacts values can be obtained from an lca database, selecting products with similar characteristics or the average values peculiar of specific categories of materials and components. for this research, ecamricert testing laboratory provided the environmental impacts values using the ecoinvent lca database. furthermore, data about the production site have been included, to consider the impact of transports during the window life cycle. different stakeholders participate in the definition of this information according to the considered life cycle phase: the manufacturer contributes during the product stage; the construction company during the construction process stage; the building manager or the owner during the use stage; the demolition company for the end-of-life stage. as these data were not available for the case study, they were assumed. to avoid modelling all these data within autodesk revit, they have been collected within a spreadsheet using microsoft excel and then linked to the bim environment. the same spreadsheet also includes the conversion factors to calculate the environmental cost indicator (eci). 3.2. analysis definition to analyse the environmental performances of the selected window, the environmental impacts and the eci of each phase have been calculated. to calculate the environmental impacts of each phase, all the values collected in the spreadsheet have been linked to the quantities and dimensions of the modelled window. the environmental impacts selected from the lca database consider one cubic metre [m3] as reference unit for wood, one tonne kilometre [tkm] for transports and one kilogram [kg] for all the other materials. the bim software autodesk revit supports the extraction of information about the volume [m3] of each modelled element. to get the mass of most of the window sub-components, the specific weight [kg/m3] of each material needs to be multiplied for the corresponding volume [m3]. the values of each factor, those contributing to the calculation of the window environmental impacts, have been saved in a new spreadsheet, where summed to obtain the environmental impacts of each lca phase. at this point, using the conversion factors, the eci can be easily obtained. the development of a script supported the automation of all the steps of the analysis facilitating to automate and parametrically control different aspect of the building information model. the script was developed with the visual programming language (vpl) application dynamo, an autodesk revit plugin. the link between the bim environment and the spreadsheet has been also performed through dynamo. it is important to note that when changing the dimensions of the bim model while running the dynamo script, the environmental impact values are updated to the new dimensions in real time. 3.3. results visualization results are displayed both through 3d models and bar diagrams (figure 3): • to support the visualization of different information at different stage of the life cycle (3d model), 21 a. barbini, g. malacarne, g. a. massari et al. acta polytechnica ctu proceedings parameter unit phase value origin window frame hight m 1-2-3-4 designer + client window frame width m 1-2-3-4 designer + client window volume m3 1-2-3-4 model geometry wood specific weight kg/m3 4 manufacturer wood mass kg/m3 1-4 manufacturer glass specific weight kg 1-3-4 manufacturer glass mass kg/m3 1-3 manufacturer window handle mass kg 2-4 manufacturer rubber seal specific weight kg/m3 2 manufacturer nails mass kg 2 manufacturer silicon specific weight kg/m3 1-3 manufacturer silicon mass kg 1-3 manufacturer distance (supply-manufacturing) km 1 manufacturer (assumption) distance (manufacturing-construction site) km 2 building company (assumption) distance (replacement-building) km 3 building manager/owner (assumption) distance (building-disposal site) km 4 demolition company (assumption) ade – abiotic depletion elements kg sb eq 1-2-3-4 lca database/epd adff – abiotic depletion fossil fuel mj 1-2-3-4 lca database/epd gwp – global warming potential kg co2 eq 1-2-3-4 lca database/epd odp – ozone depletion potential kg cfc-11 eq 1-2-3-4 lca database/epd ht – human toxicity ctuh 1-2-3-4 lca database/epd fwae – fresh water abiotic ecotoxicity ctuh 1-2-3-4 lca database/epd mae – marine abiotic ecotoxicity ctue 1-2-3-4 lca database/epd te – terrestrial ecotoxicity kg 1,4-db eq 1-2-3-4 lca database/epd po – photochemical ozone kg 1,4-db eq 1-2-3-4 lca database/epd ap – acidification potential kg c2h4 eq 1-2-3-4 lca database/epd ep – eutrophication potential kg so2 eq 1-2-3-4 lca database/epd table 2. list of parameters included within the bim object according to their phase. figure 3. results visualization options: (a) colour variation of the four life cycle phases, considering the impact category “human toxicity” (ht), (b) bar diagram of the environmental impacts factor of the installation phase and (c) bar diagram of the silicon eco-profile. 22 vol. 38/2022 environmental performances evaluation . . . figure 4. dynamo script to automate the colour variation of the four life cycle phases. • to allow the comparison of the environmental impacts of different components of the same stage and the visualization of the eco profile of a single component (bar diagrams). the environmental impacts of each factor and of each phase and the eci of each phase are stored within the spreadsheet collecting all the results. the eci of each phase is also integrated in the models as parameter value. furthermore, a colour variation of the models has been developed to enhance the readability of the environmental data. the colour variation can be applied both on the values of the impacts of individual categories or on their eci and displays the geometric models of the window according to a colour scale, indicating respectively high, medium or low environmental impacts. the environmental impact values or their eci in each phase are normalized and automatically associated to a colour range that varies from green, for lowest values, to red, for highest values, going through shades of orange and yellow for intermediate value. all the intermediate results stored into the spreadsheet are used to display impact categories for single factors and the environmental profile of each factor through bar diagrams. by changing the window dimensions in the bim environment, both the colour comparison and the diagrams display are automatically updated according to the effective volumes of sub-components. 4. results the developed case study tested the possibility to visualise environmental data through a building information model through the thematic colour variation of the model. furthermore, an automated process has been developed, allowing: • the correct matching of corresponding values between the spreadsheet and the bim object data, • the automatic calculation of environmental performances referred to the window dimension, • the visual comparisons of the analysis results though the colour variation of the geometric model, • the creation of bar diagrams with detailed information on the component environmental performances, • the real time update of the results changing the dimensions of the model in the bim environment. the automatic calculation of environmental performances regards: • the impact categories of each factor (material and transport impacts), • the impact categories of each life cycle phase, • the eci of each life cycle phase according to the dutch conversion factors. for the visual comparison through colour variation, the user can choose to compare a single impact category (e.g. global worming potential) or the environmental cost indicator (figure 4). 5. discussion and conclusion the tool developed aims to support manufacturers and suppliers in increasing their awareness on the environmental impacts of the products they place on the construction market. ideally, the starting points of this solution are the epd and the bim object of the product which has to be evaluated. therefore, this solution is mainly thought for those manufacturers and suppliers interested both in sustainable innovation and in digital implementation. moreover, bim objects containing environmental performances data 23 a. barbini, g. malacarne, g. a. massari et al. acta polytechnica ctu proceedings of products can facilitate the design phase to perform sustainability simulations and analysis and to support decision making processes. it is important to note that in this case the wooden window frame is a solid material in cross-section. this way, the volume of the 3d model corresponds to the real volume of the frame. in case of aluminium or pcv frames there are air chambers, which are usually not in the model due to hight performance demand on the computer, the volume of the 3d model should be adjusted through a formula to obtain the real volume of the frame and avoid small errors that could have high impact on the whole assessment. a similar issue is related to window glazing systems, that are not necessarily three layers of glass. a further development of this study concerns the use of bim data in ifc (industry foundation classes) open format to visualize the results. many building components manufacturers and suppliers do not have a licence for a bim authoring software and therefore the neutral environment of an ifc viewer could be much more accessible [16]. future implementations of the tested prototype could concern the comparison of the window components directly through the 3d model, changing the colour of the exact component of the window to show hot spots. moreover, the prototype could be extended to larger elements, such as systems of components, functional units or entire buildings and considering different points of view. this case of study privileged the point of view of a building component manufacturer, but environmental performances visualization could also support clients, both private and public, as well as designers and construction companies in monitoring and improving project sustainability. references [1] global alliance for building and constructions. 2018 global status report for buildings and construction, 2018. [2021-08-26], https://wedocs.unep.org/bitstream/handle/20. 500.11822/27140/global_status_2018.pdf. [2] eurostat. waste gerenation by economic activities and households. [2021-08-26], https://ec.europa.eu/eurostat/statisticsexplained/index.php?title=file: waste_generation_by_economic_activities_and_ households,_eu-28,_2016_(%25).png&oldid=413561. [3] global alliance for building and constructions. 2019 global status report for buildings and construction, 2019. [2021-08-26], https://globalabc.org/sites/default/files/202003/gsr2019.pdf. [4] united nations. sustainable development goals. [2021-08-26], https://sdgs.un.org/goals. [5] n. leskinen, j. vimpari, s. junnila. a review of the impact of green building certification on the cash flows and values of commercial properties. sustainability 12(7):2729, 2020. https://doi.org/10.3390/su12072729. [6] national italian unification body. uni en iso 14040: 2021 environmental management – life cycle assessment – requirements and guidelines. [7] a. barbini, g. malacarne, k. romagnoli, et al. integration of life cycle data in a bim object library to support green and digital public procurements. international journal of sustainable development and planning 15(7):983–990, 2020. https://doi.org/10.18280/ijsdp.150702. [8] f. asdrubali, c. baldassarri, v. fthenakis. life cycle analysis in the construction sector: guiding the optimization of conventional italian buildings. energy and buildings 64:73–89, 2013. https://doi.org/10.1016/j.enbuild.2013.04.018. [9] ecoscan. website echochain. [2021-10-12], https://ecochain.com/ecoscan/. [10] totem. website ovam (flanders public waste agency). [2021-10-12], https: //www.ovam.be/materiaalprestatie-gebouwen-0. [11] l. álvarez antón, j. díaz. integration of life cycle assessment in a bim environment. procedia engineering 85:26–32, 2014. https://doi.org/10.1016/j.proeng.2014.10.525. [12] m. a. van eldik, f. vahdatikhaki, j. m. o. dos santos, et al. bim-based environmental impact assessment for infrastructure design projects. automation in construction 120:103379, 2020. https://doi.org/10.1016/j.autcon.2020.103379. [13] m. röck, a. hollberg, g. habert, a. passer. lca and bim: visualization of environmental potentials in building construction at early design stages. building and environment 140:153–161, 2018. https://doi.org/10.1016/j.buildenv.2018.05.006. [14] b. cheng, k. lu, j. li, et al. comprehensive assessment of embodied environmental impacts of buildings using normalized environmental impact factors. journal of cleaner production 334:130083, 2022. https://doi.org/10.1016/j.jclepro.2021.130083. [15] stichting bouwkwaliteit (foundation for building quality). determination method. environmental performance. buildings and civil engineering works [update 2017], 2019. [2021-10-12], https://milieudatabase.nl/wpcontent/uploads/2020/02/05-determinationmethod-v3.0-jan2019-en.pdf. [16] a. barbini, g. malacarne, g. a. massari, d. t. matt. environmental impacts visualization through open bim procedures. techne – journal of technology for architecture and environment (23):240–249, 2022. https://doi.org/10.36253/techne-12143. 24 https://wedocs.unep.org/bitstream/handle/20.500.11822/27140/global_status_2018.pdf https://wedocs.unep.org/bitstream/handle/20.500.11822/27140/global_status_2018.pdf https://ec.europa.eu/eurostat/statistics-explained/index.php?title=file:waste_generation_by_economic_activities_and_households,_eu-28,_2016_(%25).png&oldid=413561 https://ec.europa.eu/eurostat/statistics-explained/index.php?title=file:waste_generation_by_economic_activities_and_households,_eu-28,_2016_(%25).png&oldid=413561 https://ec.europa.eu/eurostat/statistics-explained/index.php?title=file:waste_generation_by_economic_activities_and_households,_eu-28,_2016_(%25).png&oldid=413561 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https://milieudatabase.nl/wp-content/uploads/2020/02/05-determination-method-v3.0-jan2019-en.pdf https://doi.org/10.36253/techne-12143 acta polytechnica ctu proceedings 38:18–24, 2022 1 introduction 2 state of the art 3 methods and tools 3.1 data collection and organization 3.2 analysis definition 3.3 results visualization 4 results 5 discussion and conclusion references https://doi.org/10.14311/app.2022.33.0226 acta polytechnica ctu proceedings 33:226–231, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague utilization of recycled and secondary materials in concrete production lca anna horáková∗, alena kohoutková, iva broukalová czech technical university in prague, department of concrete and masonry structures, faculty of civil engineering, thákurova 2077/7, 166 29 praha 6 dejvice, czech republic ∗ corresponding author: anna.horakova@fsv.cvut.cz abstract. the paper describes an assessment of concrete in terms of environmental impacts in relation to the utilization of recycled materials. the article includes a short summary of the literature search on evaluation methods for environmental impacts and on recycled and secondary materials. the environmental impacts of several concrete mixtures were calculated. the reference concrete mixture containing only cement as a binder and only natural aggregate was compared with other mixtures containing recycled and secondary materials as a binder or aggregates. some of these concrete mixtures were then compared within the whole structure. variant design of a simple structure was performed, whereas the variants differed in the concrete mixture. these variants were then evaluated in terms of environmental impacts. keywords: concrete, environmental impacts, life cycle assessment, recycled and secondary materials, variant design. 1. introduction in past years, the issue of sustainable development and the impact of construction activities on the environment are gaining importance. it is desirable to minimize negative environmental impact by suitable design, optimal manufacturing process and material selection. the environmental aspects of sustainable development in the construction industry consist also in the utilization of secondary raw materials in the design and construction of new structures. 1.1. state of the art most often, studies deal with comparison conventional concrete and concrete containing waste or recycled materials such as fly ash, slag or recycled aggregate. the comparison of unit volumes of different concrete mixtures almost always shows, that lower cement dosages make the concrete more environmentally friendly. cement production is responsible for a significant amount of released harmful emissions and its energy consumption is very high. hence, it plays a crucial role in the overall environmental impact of a concrete structure and the cement content in the concrete mixture is a key factor for its environmental assessment. for example, a study [1] reported that a 30% reduction of the cement content leads to a 26.6% reduction in co2 emissions. similar results observed study [2]. the decrease of cement dosage leads also to an energy and raw materials savings. a study [2] reported a 21 % decrease in energy consumption and a 4,3% decrease in raw materials consumption for a 35% reduction of cement content in concrete. the fact, that the environmental impacts of the concrete structure are largely dependent on the cement consumption, led to many research projects dealing with a utilization of latent hydraulic materials as a partial replacement of cement. these supplementary cementitious materials (scm) are capable of hydrating when used together with cement. typical examples are fly ash, blast furnace slag and silica fume. the use of these materials as a partial cement replacement reduces the environmental impacts of concrete production. moreover, the scm are usually waste products, which are generated during the production of other materials or during the energy production. for example, fly ash is a by-product of thermal power plant electricity production and blast furnace slag is a by-product of steel production. thus, there is another environmental advantage, which lies in a utilization of waste products. it should be mentioned that the replacement of cement by scm generally does not leads to a decrease in strength of the concrete. the strength of the concrete containing these materials can be comparable or even higher. cement production harms the environment especially in terms of global warming and climate change and in terms of energy consumption. hence, most studies deal with these environmental impacts and the environmental benefits of using cement replacement materials. the study [3] compares several concrete mixtures with different fly ash replacement levels (25%, 30% and 40%). with the increasing cement replacing level, carbon dioxide emissions decrease. for replacing level of 25%, 30% and 40%, the study reported a decrease in co2 emissions by 6%, 11% and 23%. deterioration of mechanical properties occurred until the replacement level reached 40 %. on the contrary, these mechanical properties improved for lower 226 https://doi.org/10.14311/app.2022.33.0226 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 33/2022 analysis of concrete and cement epd ref fa bfs wpa wtr rca pc cement [kg/m3] 380 280 324 380 410 380 0 water [kg/m3] 190 236 180 200 200 190 0 fine aggregate [kg/m3] 705 1142 681 572 840 705 1580 coarse aggregate [kg/m3] 1100 493 1160 1020 960 0 0 fly ash [kg/m3] 0 95 0 0 0 0 0 blast furnace slag [kg/m3] 0 0 36 0 0 0 0 waste plastic aggregate [kg/m3] 0 0 0 143 0 0 0 waste tyre rubber [kg/m3] 0 0 0 0 40 0 0 recycled concrete aggregate [kg/m3] 0 0 0 0 0 1100 0 waste pet (as a binder) [kg/m3] 0 0 0 0 0 0 472 superplasticizer [kg/m3] 2 1.8 0 0 0.8 2 0 table 1. the composition of the concrete mixtures. cement replacement levels. for example, at 30 % replacement level, the compressive strength increased by 10 %. if the study was carried out for a real structure, the difference in environmental impact would be more significant than when comparing unit quantities of concrete. this is due to possible reduction of the dimensions of supporting structural elements. nevertheless, it should be mentioned, that the difference would not be significant in case of this study, because there is not a significant increase in strength. some studies deal with the utilization of blast furnace slag as a partial replacement of cement. these studies reported, a reduction of environmental burden, when cement is partially replaced. for example, study [4] reported, that for 50 % replacement level, co2 emissions reduced by 39 %. the study [2] reported similar results. the reduction of co2 emissions, and thus the influence on global warming is the most significant environmental benefit. on the other hand, replacing cement by blast furnace slag has only little effect on raw material consumption. many studies deal with comparison conventional concrete and concrete containing recycled aggregate, which is produced by crushing waste concrete or bricks. according to most of these studies, use of this recycled aggregate leads to reduction of released harmful emissions and consumed raw materials and energy. furthermore, utilization of these waste materials is another benefit. for example, the study [5] compares the environmental impacts of the production of conventional concrete containing only natural aggregate and concrete containing recycled aggregate. the natural aggregate was partially or fully replaced by crushed waste concrete. according to this study, the use of recycled aggregate was advantageous for most environmental impact categories. this was the most significant for the consumption of raw materials, which decreased by 47 % when natural aggregate was fully replaced by recycled aggregate. the reduction of energy consumption was less significant, about 30 % for fully replaced natural aggregate. study [6] reported, that the benefit of using recycled aggregate strongly depend on the transport distance of recycled aggregate. if recycled aggregate is transported over a long distance, the environmental burden caused by the transport may outweigh the environmental benefits of using waste material. 2. methods this paper deals with the utilization of recycled and waste materials in concrete production and its advantages in terms of environmental impacts. at first, the assessment in terms of environmental impacts was performed for concrete, which contains only cement as a binder and only natural aggregate (ref) [7]. then, the assessment was performed for other concrete mixtures, which contain secondary or waste materials as a binder or aggregate. in two of these material variants, there was cement partially replaced by fly ash (fa) [8] or by blast furnace slag (bfs) [9]. in some other variants, there was natural aggregate partially or fully replaced by waste plastics aggregate (wpa) [10], crushed waste tyre rubber (wtr) [11] or recycled concrete aggregate, obtained from demolished buildings (rca) [7]. one more material variant was included in this evaluation innovative material called polymerconcrete, which is composed of aggregate and plastic waste, which replaces cement as a binder (pc) [12]. this material is made from fine aggregate (aggregate size 0 − 4 mm) and waste pet, which is cut into small pieces. the production consists in homogenizing the mixture of plastic waste and small aggregate at high temperature. according to the experiments performed in [13], average compressive strength of this material is 22,7 mpa and average flexural strength is 7,91 mpa. all these mixtures were compared with each other. in the next step, a simple reinforced concrete structure reinforced concrete frame was designed from selected concrete mixtures. then, these structures were compared in terms of environmental impacts. the composition of the concrete mixtures evaluated in this paper is shown in the table 1. 227 a. horáková, a. kohoutková, i. broukalová acta polytechnica ctu proceedings environmental impact explanation global warming global warming is a long-term increase in global average temperature caused by excessive production of greenhouse gases. acidification acidification is the ongoing decrease in the ph of the environment. this phenomenon is caused by the presence of acid-forming substances in the atmosphere, which react with water to form acids. eutrophication eutrophication of the environment leads to ecosystem disturbance due to excessive nutrients in water and soil due to excessive fertilization. photochemical oxidant creation photochemical oxidants are air pollutants that are formed under the influence of sunlight by complex photochemical reactions in air that contain nitrogen oxides and reactive hydrocarbons and cause damage to organisms. abiotic depletion abiotic depletion refers to the depletion of abiotic resources such as fossil fuels, minerals, and clay. table 2. environmental impacts. 2.1. life cycle analysis the assessment of materials and structures in terms of environmental impacts was performed using lifecycle assessment (lca) according to relevant standards [14]. the lca approach is usually based on the whole life-cycle of the investigated product or at least its significant part. so, the assessment includes obtaining raw materials, their transport to the place of processing, manufacturing of the final product, use of the product and further maintenance or repairs if necessary, and final disposal of the product. however, the prediction of the course of the phase of use is sometimes not possible. in these cases, the evaluation includes only a part of the life cycle, it includes for example only obtaining raw materials, their transport to the place of processing and manufacturing of the final product ("cradle to site" evaluation). in this paper, the assessment was performed for concrete as a material for further use, and for a concrete structure too. the evaluation of a unit quantity of different concrete types included the phase of obtaining of raw materials, their transport and processing and manufacturing of the final product. the evaluation of a concrete structure included these phases too, and it includes in addition the phases of transport of concrete and steel and manufacturing of final structure. at first, the consumption of raw materials and emissions released into the environment were defined for each interproduct such as cement, aggregate, water, fly ash, blast furnace slag and other materials. within the assessment, the most significant environmental impacts were considered: consumption of raw materials, global warming and climate change, acidification and eutrophication of the environment and photooxidant formation. in lca, these environmental impacts are called impact categories. principles of these environmental impacts are explained in table 2. the environmental impacts of each interproduct were calculated. impacts on the environment were quantified by so-called impact category indicators measurable variables that can be used to observe changes in the environment. the values indicate the extent of environmental damage caused by human activities. usually, the impact category is influenced by various of substances where some substances are very harmful, and some less. thus, all the substances are converted to an equivalent amount of the reference substance (for example carbon dioxide for global warming and climate change or sulfur dioxide for acidification of the environment). the effect of a specific substance to each impact category was determined by so-called characterization models. a characterization model for a specific impact category is a set of values that reflect the ability of various substances damage the environment within the impact category. all of the issued substances are converted to the equivalent amount of a reference substance by using these values (characterization factors cf). this paper used a characterization model which is recommended in product category rules (pcr) for concrete products [1]. the resultant impact category indicator was calculated according to the following relationship: v xy = cf1, xy · ! m1i + cf2, xy · ! m2i + ... + cfn, xy · ! mni (1) where v xy is a result of the impact category indicator (xy indicates the impact category), cf is a characterization factor and m is an amount of a released substance. when evaluating the impact categories, emissions of following substances were considered: carbon dioxide co2, sulfur dioxide so2, nitrogen oxides nox, carbon monoxide co, methane ch4, non-methane volatile organic compound nmvoc, nitrous oxide n2o, hydrochloric acid hcl, hydrofluoric acid hf, hydrogen sulfide h2s, ammonia nh3. in this paper, seven material variants in the amount of 1 m3 were compared (table 1). in addition, several of the above-mentioned material vari228 vol. 33/2022 analysis of concrete and cement epd figure 1. comparison of 1 m3 of concrete mixtures regarding sustainability. ants were compared in designed structure: conventional concrete, concrete with fly ash, concrete with blast furnace slag, concrete with plastic aggregate, concrete with waste tyre rubber and concrete with recycled concrete rubber the structure design was performed for all material variants except polymerconcrete. because of the lack of experience in reinforcing of this material, the load-bearing structure designed from this material was not included in the assessment. a simple reinforced concrete frame was designed from different types of concrete. because of different strength of the concrete types, the volume of concrete needed for the construction depends on the material variant. the amount of steel is the same for all variants. the assessment of the material variants in terms of environmental impacts depends strongly on the volume of concrete, which is used for the structure. therefore, the frame was designed with utilization of the highest load-bearing capacity of the structural elements to make this assessment relevant. the compressive strength of the mixtures and volume of concrete for the variants are shown in table 3. variant the compressive strength [mpa] the volume [m3] ref 32.3 740.00 fa 36.2 728.40 bfs 36.2 728.40 wpa 29.5 749.86 wtr 33.8 735.36 rca 29.2 751.60 table 3. the compressive strength of the concrete and volume ofăconcrete for designed variants. 3. results 3.1. results for 1m3 of concrete the results of the sustainability assessment are related to the specific environmental impact. the following figure shows the comparison of the concrete mixtures for considered impact categories. polymerconcrete was evaluated for all most impact categories as the best material variant. this is probably because this material does not contain any cement, whose production is very burdening for the environment. on the other hand, the energy consumption (the impact category abiotic depletion potential adp fossil) is relatively high, higher consumption was calculated only for conventional concrete. the obvious reason is the high heat consumption for melting the waste polymer. moreover, the use of this material for structures is limited and there is a lack of previous experience with its production. very favourable results have also been obtained for concrete with fly ash. the obvious reason is the reduction of cement consumption due to its partial replacement by fly ash. in the case of concrete with blast furnace slag, cement is also partially replaced, but the replacement level is lower. hence, the results are less favourable for this material variant. material variants, in which recycled materials replaced aggregates, were evaluated as less favourable in terms of environmental impacts. this is because the production of aggregates does not cause such a high environmental burden as the production of cement. another reason is that the waste material (concrete, plastic, tyre rubber) for production of aggregates replacement must be mechanically processed (crushed). this process is energy intensive and reduces the environmental benefits of using waste material. therefore, for reducing the environmental impacts it is preferable to replace cement. 3.2. results for concrete structures for comparison of the whole structures, a simple reinforced concrete frame was designed from different types of concrete. regarding the environmental impacts of concrete frames designed from different material variants, the results did not differ significantly from the previous comparative study (comparison of unit quantities of concrete mixtures). all investigated concrete mixtures have the similar compressive strength. therefore, the dimensions of the supporting elements of the frame do not differ significantly and consumption of concrete is very similar for all designed variants. it is obvious, that a partial replacement of cement by scm is the most advantageous in terms of environmental impacts. the figure 3 shows the comparison of the designed variants for considered impact categories and figure 2 shows the sketch 229 a. horáková, a. kohoutková, i. broukalová acta polytechnica ctu proceedings of the designed frame. figure 2. designed concrete frame. figure 3. comparison of the concrete frames regarding sustainability. 4. conclusion according to this study, in terms of environmental impacts, it is most advantageous to replace a part of cement by supplementary cementitious materials (scm), such as fly ash or blast furnace slag. cement production causes a large environmental burden and many environmental impacts are significantly dependent on cement consumption. it could be reduced by partial replacement of cement by scm. furthermore, supplementary cementitious materials are often waste or secondary materials and their utilization is another environmental benefit. polymerconcrete is also very favourable from the environmental point of view, especially in terms of consumption of raw materials. however, the production of this material is energy intensive, due to the melting of waste plastic. moreover, so far there is no experience with its use in structures. therefore, it is desirable to find a suitable use for this material, for example non-bearing part of structures, such as floors or pavements. when choosing a material for a specific structure, it is necessary to investigate the influence of the transport of recycled materials to the structure site. the transport distance of recycled materials, especially recycled concrete aggregate or waste plastics aggregate, is probably greater than the transport distance of natural aggregates. the utilization of recycled material in concrete usually brings benefits in terms of environmental impacts. on the other hand, the influence of the use of recycled materials on the mechanical properties of concrete must be considered. the use of some waste materials may increase uncertainties in the mechanical behaviour of concrete due to the variability of the properties of these materials. this is typically related to materials that are not commonly used in concrete production, such as recycled concrete aggregate, waste plastics aggregate or waste tyre rubber. hence, it is more appropriate for load-bearing structures to reduce the environmental impacts by using scm. acknowledgements this outcome has been achieved with the financial support of the ministry of education, youth and sports, project: durability of concrete structure and assessment of its life cycle, sgs19/149/ohk1/3t/11. references [1] v. c. li, m. lepech, s. wang, et al. development of green engineered cementitious composites for sustainable infrastructure systems, proceedings of structural engineering world congress (sewc), proc., int’l workshop on sustainable development and 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[14] wbcsd. technology roadmap: low-carbon transition in the cement industry, 2018. https: //www.wbcsd.org/sector-projects/cement-sustain ability-initiative/resources/technology-roadmap -low-carbon-transition-in-the-cement-industry. 231 https://doi.org/10.1016/j.cemconres.2004.09.031 https://doi.org/10.1016/j.conbuildmat.2019.05.132 https://doi.org/10.1016/j.wasman.2007.08.023 https://doi.org/10.1088/1757-899x/583/1/012021 https://doi.org/10.1088/1757-899x/596/1/012033 https://doi.org/10.1088/1757-899x/596/1/012033 https://www.wbcsd.org/sector-projects/cement-sustainability-initiative/resources/technology-roadmap-low-carbon-transition-in-the-cement-industry my title https://doi.org/10.14311/app.2022.36.0167 acta polytechnica ctu proceedings 36:167–174, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague risk based maintenance for swiss railway bridges: concept, implementation and first experiences dirk proskea,∗, jean-baptiste payeura, blaise girardinb, herbert friedlb a bern university of applied science, wood and civil engineering, school of architecture, pestalozzistrasse 20, 3401 burgdorf, switzerland b swiss railway sbb, asset management and infrastructure, hilfikerstrasse 3, 3000 bern 65, switzerland ∗ corresponding author: dirk.proske@bfh.ch abstract. the swiss federal railway (sbb) has an inventory of approx. 6,000 bridges. so far, condition classes of the individual bridges have been used for maintenance management. to improve the efficiency of the maintenance management, a long-term change from the condition class-based approach to a risk-based approach is considered. such a risk-based approach was developed and implemented during a two-stage process into software (excel and python). the software was also linked to the sbb databases to access the relevant data. the python software now includes 12 parameters to adapt the initial failure probability to specific bridge conditions and 15 damage parameters. so almost 30 parameters are used to compute the risk for each bridge. the software is also linked to geographical maps to show the location of the bridges. besides the development of the approach, also the first experience of the application of this methodology will be discussed. for example, the risk-based ranking of the bridges clearly showed that specific bridge types are dominating. also, some original ideas and concepts were not applicable due to difficulties in providing the required input data. however, currently the risk-based bridge ranking complies well with former investigations carried out by hand. keywords: bridges, collapse, condition class, damage, probability of failure, risk assessment. 1. introduction switzerland has one of the densest and busiest rail networks in the world, but still achieves a top international ranking in punctuality. in 2010, rail transport accounted for 17 % of passenger traffic and almost 40 % of freight traffic [1]. the majority of the swiss railway network dates back to the 19th century. the mean age of the bridges of the swiss federal railway (sbb) is 71 years, the median is 64 years and about 1,300 bridges are older than 100 years (figure 1). also, major extensions of the railway network have been implemented in recent years, such as the gotthard base tunnel or the diameter line in zurich [1]. figure 1. age distribution of sbb bridges. the swiss government supports the operation and maintenance of the railway network with approximately 2 billion us-dollar annually. in addition, major extension investments are financed through a special fund. the fund reached a volume of approx. 32 billion us-dollar (1995 prices) [1]. the replacement value of the entire sbb network is given as 105 billion us-dollar and the replacement value of the sbb network’s engineering structures as 46 billion us-dollar [2]. table 1 gives the replacement values for various sbb engineering structures, with bridges alone accounting for approximately 12 billion us-dollar [3]. sbb has set itself the goal of ensuring that mobility remains attractive and affordable for customers, cantons, and the federal government. uncertainties in the management of the railway network have to be considered [4]. for this reason, knowledge of the network condition and thus also of the engineering structures, such as the bridges, is necessary. sbb therefore regularly prepares a network condition report [2]. the network condition report uses so-called condition classes for the evaluation of the engineering structures. today, sbb determines the condition of bridges as part of inspections which include visual and other examinations. based on observed condition classes and target condition classes, the maintenance of bridges can be managed. the network condition 167 https://doi.org/10.14311/app.2022.36.0167 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en d. proske, j.-b. payeur, b. girardin, h. friedl acta polytechnica ctu proceedings engineering structures stock (approx.) replacement value (approx.) retaining walls 11,000 sections 4 billion us-dollar bridges 6,000 12 billion us-dollar tunnel 300 12 billion us-dollar table 1. number and replacement value of various sbb engineering structures [3]. figure 2. potential procedures for risk-based asset management of sbb bridges. report thus forms the basis for the asset management of the bridges. in principle, only the structure itself is considered in this condition-based assessment, not the sbb network. the condition-based planning of maintenance measures can therefore not consider any information on the importance of the bridge structure in the sbb network. in contrast, the risk-based assessment can consider the importance of the structure for the sbb network and thus for the fulfilment of the above-mentioned global goals of sbb [4]. to do so, the risk-based assessment includes numerous performance and functional parameters of the railway. this assessment therefore allows better prioritisation of computational investigations and maintenance measures on the bridges from an overall perspective. based on these considerations, sbb launched a project on the development of risk-based bridge management in 2019. the project consisted of various working steps: • collection and preparation of the state of the art in risk-based maintenance management for possible application at sbb (2019) • implementation of a proof-of-concept for the sbb bridges (2019) • software implementation and linking with the sbb databases (2020) • practical application and feedback of experience to improve the procedure (2021 − 2022). in addition to the implementation for bridges, an implementation for other engineering structures is planned and partly prepared. 2. methodology in order to prioritize bridges and maintenance measures on the given sbb network, a mathematical procedure should be applied. from the point of view of risk-based maintenance management, three procedures are possible: • risk assessments (time-independent), • life cycle cost analyses (time-dependent, includes risk assessments) and • quality-of-life assessments (considering macroeconomic indicators). the goal of all procedures is the evaluation of possible measures based on rational criteria. figure 2 shows the evaluation of measures in these three models. as can be seen in figure 2, the x− and y−axes of the three diagrams are labelled differently. each of 168 vol. 36/2022 risk based maintenance for swiss railway bridges example of sbb data plotting import classes %matplotlib inline import matplotlib.pyplot as plt import pandas as pd import numpy as np load data data = pd.read_csv('20181210_dfa.csv', sep=';', decimal=',' ) data2 = pd.read_csv('globale_auswertung.csv', sep=';', decimal=',') merge data result = pd.concat([data, data2], axis=1, sort= false) select versagenswarscheinlichkeit columns col_contains_versagen = [col for col in result.columns if 'versagenswahrscheinlichkeit' in col] print(col_contains_versagen) ['versagenswahrscheinlichkeit sia 269:2007 (o. korr.)', 'versagenswahrscheinlichkeit sia 269:2007 (mit korr.)', 'versagenswahrscheinlichkeit badewannenkurve (o. korr.)', 'versagenswahrscheinlichkeit badewannenkurve (mit korr.)', 'versagenswahrscheinlichkeit taricska (o. korr.)', 'versagenswahrscheinlichkeit taricska (mit korr.)', 'versagenswahrscheinlichkeit zustandsklasse (o. korr.)', 'versagenswahrscheinlichkeit zustandsklasse (mit korr.)', 'versagenswahrscheinlichkeit konstant (o. korr.)', 'versagenswahrscheinlichkeit konstant (mit korr.)'] compute risiko for col_name in col_contains_versagen: name = col_name.replace('versagenswahrscheinlichkeit', 'risiko') result[name] = result[col_name] * result['schadensfolgekosten'] figure 3. python programming (jupyter notebook). these models takes different parameters into account and can therefore lead to different results. in principle, the models become more and more universal from risk assessment to life cycle analysis to quality-of-life assessment, i.e., they consider an increasing number of parameters. for this reason, quality-of-life parameters are also referred to as the highest form of risk assessment. simplified life cycle cost models are currently applied in sbb’s asset management, but without considering detailed risk assessments. it is planned to embed the risk assessment later in the existing life cycle cost models. the general model for the life cycle cost (lcc) is: lcc = g(d) − c(d) − i(d) − r(d) (1) where g is the operating profit, c the construction costs, i the maintenance costs, r the risk of loss and d the vector of design variables. risk assessment represents a trade-off between traceability of results, number of parameters required and data management. the implementation of quality-oflife assessment models requires additional parameters, which are currently not available. it is therefore not pursued further in this study. in principle, risk parameters r always consist of two factors: probability of occurrence e and extent of damage s: r = e × s (2) the determination of these two factors is explained in the following sections. 3. probability of collapse in order to calculate realistic risk values r, the use of realistic collapse probabilities of bridges is mandatory, otherwise one obtains risk values that are only of limited practical relevance and that may lead to a misallocation of sbb resources in the long run. the term collapse probability e used here symbolises the mixture of calculated failure probabilities and corrections based on statistically evaluated collapse frequencies: eb = pf (a) · ∏ i ki (3) with eb as the collapse probability for each bridge, pf (a) as the age-dependent failure probability and ki as correction factors. the factors k1 to k12 for the calculation of the individualised corrected collapse probability are listed below. the factors consider 1. human error 2. correlation of the limit states 169 d. proske, j.-b. payeur, b. girardin, h. friedl acta polytechnica ctu proceedings figure 4. sensitivity analysis i (the colour indicates the condition class). figure 5. sensitivity analysis ii. 3. static determinacy 4. early failure recognition 5. type of overpass 6. quality of the static calculation 7. bridge type 8. building material 9. construction technology 10. robustness 11. serviceability 12. load tests results available furthermore, different models for the age-dependent failure probability were integrated, such as the bathtub curve, the model according to sia 269, the model according to taricska [5], a model depending on the condition classes and a model without ageing. the sbb degradation curves were also considered. the development of the correction factors was shown in [6] and the approach was implemented for each sbb bridge. the necessary data for determining the age-dependent probability of failure and correction factors are taken from the sbb bridge databases. 170 vol. 36/2022 risk based maintenance for swiss railway bridges 4. damage parameters the calculation of damage costs s of a bridge collapse, failure or closure can become very extensive and detailed. in some cases, up to 50 parameters are considered in the literature, ranging from the number of victims to environmental damage [7]. for the sbb model, however, only 15 damage parameters have been included: sb = ∑ i si fi (4) where sb are the damage costs for each individual bridge and si individual damage cost shares. some of the shares si and the correction factors fi, respectively are listed below: 1. replacement value of the bridge 2. price of a goods train 3. price of a passenger train 4. mean number of fatalities and injuries 5. value to avoid one fatality (vosl) 6. ratio of passenger to goods trains 7. number of trains per day 8. restrictions on the detour route 9. additional expense due to detour route 10. duration of the line closure parts of the development of such factors have also already been published, see for example [8]. most of the damage cost values and the necessary data for determining the factors were provided by sbb 5. software implementation first, the sbb exported the bridge data into excel. in excel, the calculation procedure was implemented as a "proof-of-concept" according to the formulas shown above. however, the calculation must access a large number of dynamic sbb databases, for example detailed train data. for this reason, the entire calculation was implemented into python (jupyter notebook, see figure 3) and migrated to the sbb computer system. as a result, confidential data does not leave sbb. in total, almost 30 parameters are read from the sbb databases per bridge. the python software is also linked to a dynamic gis model. due to licensing restrictions, however, no images of the gis link are shown within the scope of this article. 6. sensitivity analysis for the almost 30 parameters, testable and robust values must be provided. therefore, all parameters should have a significant influence on the calculation. to test the sensitivity of the model and the calculations, various sensitivity calculations were carried out. the sensitivity calculation of the overall model was carried out in two steps. firstly, four parameter combinations were calculated for approx. 4,000 bridges and secondly, approx. 30 parameter combinations were calculated for 30 bridges. figure 4 shows the position of approx. 4,000 sbb bridges in a risk diagram in the context of the first sensitivity calculation. the sensitivity calculations consider four variations of the computation of 11 of the individual bridge input parameters, such as the probabilities of failure, replacement values of bridges, price of freight trains and passenger trains, ratio of freight to passenger trains, number of trains per day, costs of downtime and the number of fatalities and injured. the x−axis shows the extent of damage in us-dollars and the y−axis the probability of collapse. one can see clearly both, the change of the bridge results in y−direction (combination a and b) and in the x−direction (combination c and d) by the different value combinations. figure 5 shows a second sensitivity study for 30 bridges. each column is a bridge and each point per column is a parameter combination. the black dot is the anchor point (base case). as a result of the sensitivity studies, the weighting of the different input parameters is also obtained. figure 6 shows the importance of four damage cost variables for risk determination. these results are in accordance with other works on risk assessment of bridges [7, 9, 10]. figure 6. normalised damage contributions. 7. results the results of the risk assessment of all sbb bridges can be presented either in tabular form as a top-100 risk list or geographically. figure 7 shows a geographical representation of the location of 4,237 sbb bridges and the associated risk class. one can clearly see the sensitivity of the calculation models and thus also indirectly the effects of maintenance measures. however, some bridges are dominating the risk in most models. 171 d. proske, j.-b. payeur, b. girardin, h. friedl acta polytechnica ctu proceedings figure 7. geographical location of the sbb bridges and representation of the respective risk class for two models. the location of the bridges can be shown much more comprehensively in gis maps but is not shown here due to copyright restrictions. 8. practical experience and outlook the practical application of risk-based maintenance management did not take place without intensive discussions and challenges. for example, it became apparent at an early stage that the choice of the number of risk classes effects considerably the acceptance of the procedure among inspectors and civil engineers [11]. furthermore, discussions about the comparability of the condition classes and risk classes also arose. figure 8 shows such a comparison. this figure clearly indicates a difference in the relative frequency of bridges in condition classes and risk classes respectively. whereas the final consequences in terms of required maintenance measures may be clear, the interpretation of the causes of the different frequencies requires detailed inside into the computations. in general, the distribution of risk classes indicates a better overall situation than the distribution of conditional classes. this may indicate that in practice already engineers and managers consider costs and risks for the selection of maintenance measures by a non-mathematical approach. furthermore, the calculation was also extended to bridges over sbb lines. here, however, not all data are available, as other owners are involved. the extension of the bridge population also applies to the sbb itself, since in principle culverts can also be considered. in addition, further effects have been shown in practical application: • certain bridge types (single-span steel girders) and certain constellations (bridges over water) occur more frequently or dominate the top 100 risk list. this observation cannot be independently verified at the moment because all known collapse and damage events have already been used for the modelling (burned data). • individual control calculations showed inconsistencies in the input data. individual case checks are then necessary. for the bridges, however, the error rate in the databases is below 5 %. there are considerations to identify and automatically replace such erroneous entries by means of artificial intelligence 172 vol. 36/2022 risk based maintenance for swiss railway bridges figure 8. comparison of the frequency of bridges in the condition and risk classes (class 1 means either good condition or low risk, class 5 means very bad condition or very high risk). or other advanced software. • a prioritisation list of the bridges, created independently and with a different method, shows an overlap of 50 % with the top 100 risk list. whether this is a good value cannot be assessed at this stage, as different input parameters are considered. • the feedback from the bridge managers was very positive when the bridges relevant to them were confirmed by the risk list. • the ossingen bridge, which was closed in january 2021 due to safety concerns, was ranked 90th on the top 100 risk list. the sbb considers all bridges in the top 100 as "potentially risky" and plans further investigations or measures. • the communication of the results of the risk-based assessments requires an increased effort, since the condition class of a bridge is easier to communicate than the risk value which includes a variety of input variables from different fields, e.g., structural engineering, economy, rail network organisation. therefore assumptions, simplifications, methods, values, and consequences must be explained in detail to achieve acceptance of the method at various organisational levels. • the use of the results for inspection and maintenance depends also on other factors. organisational and technological effects also play a major role, such as possibility of closure, required closure time of a route, the accessibility of bridges, etc. • the procedure is more sensitive to changes in the general conditions, such as the changed traffic volume associated with covid-19 pandemic. here, the condition classes are more stable, but of course, concluded maintenance measures are also subject to budgetary constraints. • for some input parameters, values are not yet available. for example, it is unknown what effects construction technologies have on the collapse probability of bridges. further work is needed here. so far, the factor 1 has been used in the computation. • an extension of the existing model to include timedependent influences is planned. the data have already been prepared. this would be the transition from the solely risk-based to the life-cycle-based models. since such models are available here at sbb, the two models would have to be linked and adapted only. • natural hazard models are available at sbb [12, 13]. these could also be linked with the risk model. with the methodological development and software implementation of the risk-based asset management of sbb bridges, both structural analysis, inspections of bridges and maintenance measures can be prioritised. from the authors’ point of view, the concept of condition classes is retained and thus allows the person responsible for the structure to influence the risk calculation and, under certain circumstances, to veto them (e.g., condition class 5 always beats risk list). the risks calculated serve as an additional information or decision variable to optimise the utilisation of the portfolio and the investment in the infrastructure. the long-term impact of the risk analysis should also allow an appropriate and transparent use of financial resources and control of the risk to society. references [1] uvek. zukunft der nationalen infrastrukturnetze in der schweiz, bericht des bundesrates vom 17. september 2010, eidgenössisches department für umwelt, verkehr, energie und kommunikation uvek, bern, 2010. [2] sbb. netzzustandsbericht 2019, sbb infrastruktur, bern, 2020. [3] h. friedl. herausforderungen und stossrichtungen im anlagenmanagement für brücken der sbb, burgdorfer 173 d. proske, j.-b. payeur, b. girardin, h. friedl acta polytechnica ctu proceedings brückenbautag 2019, berner fachhochschule und bau und wissen, tagungsunterlagen, 2019. [4] sbb. vision und strategie der sbb, https://company.sbb.ch/de/ueber-die-sbb/profil/ strategie.html, 2021. [5] m. r. taricska. an analysis of recent bridge failures in the united states (2000 2012). msc. thesis, the ohio state university, 2014. [6] d. proske, s. hostettler, h. friedl. korrekturfaktoren der einsturzwahrscheinlichkeit von brücken. betonund stahlbetonbau 115(2):128-35, 2019. https://doi.org/10.1002/best.201900072. [7] a. a.r. al-wazeer. risk-based bridge maintenance strategies, dissertation, university of maryland, 2007. [8] d. proske. zur berücksichtigung hypothetischer opferzahlen in lebenszykluskostenberechnungen von brücken. betonund stahlbetonbau 115(6):459-68, 2020. https://doi.org/10.1002/best.201900102. [9] d. imhof. risk assessment of existing bridge structures, university of cam-bridge, dissertation, kings college, 2004. [10] d. saydam. reliability and risk of structural systems under progressive and sudden damage, dissertation, paper 1616, lehigh university, lehigh preserver, 2013. https://citeseerx.ist.psu.edu/viewdoc/download ?doi=10.1.1.947.9919&rep=rep1&type=pdf. [11] d. proske, d. tschan. discussion of the number of risk classes for risk based maintenance. 18th international probabilistic workshop. lecture notes in civil engineering. pp. 281-91, 2021. https://doi.org/10.1007/978-3-030-73616-3_20. [12] bafu. economerailway: risikoanalysen naturgefahren entlang von bahnstecken, methodik, version 1.0, 25. juni 2012, 2021. [13] j. hess, s. lagger, r. manser, h. p. willi. umgang mit naturgefahren in der schweiz, bericht des bundesrats in erfüllung des postulats 12.4271 darbelly, 2012. 174 https://company.sbb.ch/de/ueber-die-sbb/profil/strategie.html https://company.sbb.ch/de/ueber-die-sbb/profil/strategie.html https://doi.org/10.1002/best.201900072 https://doi.org/10.1002/best.201900102 https://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.947.9919&rep=rep1&type=pdf https://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.947.9919&rep=rep1&type=pdf https://doi.org/10.1007/978-3-030-73616-3_20 acta polytechnica ctu proceedings 36:167–174, 2022 1 introduction 2 methodology 3 probability of collapse 4 damage parameters 5 software implementation 6 sensitivity analysis 7 results 8 practical experience and outlook references https://doi.org/10.14311/app.2022.33.0219 acta polytechnica ctu proceedings 33:219–225, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague the role of conceptualization in the evaluation of sustainable concrete michael henrya,∗, joel oponb a shibaura institute of technology, toyosu 3-7-5, koto-ku, tokyo, japan b msu iligan institute of technology, iligan city 9200, philippines ∗ corresponding author: mwhenry@shibaura-it.ac.jp abstract. improvement of the sustainability of concrete materials will be realized through the development of analytical tools that facilitate sustainable design and evaluation. however, these processes may be dependent on how sustainability is conceptualized for concrete. conceptualization is the means by which sustainability is operationalized by creating a structure that connects a qualitative goal to its quantitative indicators. as there exists no established definition of sustainability for the concrete field, conceptualization is a source of uncertainty in the sustainability evaluation of concrete. this paper explores the role conceptualization plays in the evaluation of concrete material sustainability by analyzing its effects using multicriteria analysis and a sustainability indicator framework to quantify sustainability for concrete materials. six analytical scenarios are explored using frameworks based on direct loading, the three pillars of sustainability, and the sustainable development goals, together with two aggregation methods. it was found that the most sustainable concrete mix varied by scenario, but one concrete mix combining blast furnace slag and high grade recycled aggregates could be judged as the most sustainable due to its highest mean score and lowest variance across all analytical scenarios, which suggests it as the mix least sensitive to methodological choices on conceptualization and aggregation. overall, however, the sustainability scores were highly correlated between the different scenarios. keywords: conceptualization, multi-criteria analysis, sustainable design. 1. introduction the use of concrete incurs significant impacts to environmental systems, including climate change, ecosystem degradation, resources depletion, and pollution [1]. to address these issues, it is necessary to adopt more sustainable materials in construction, which requires evaluation of the sustainability of concrete considering the multi-dimensional nature of sustainable development. literature presents a variety of indicators and evaluation methods for tackling the sustainability evaluation of concrete [2]. examples include comparative analysis of the trade-off between environmental impact and other performances, such as safety and cost [3], and the application of life cycle assessment (lca) to quantify the impacts across the life cycle [4]. multicriteria decision-making techniques, such as analytical hierarchy process, have also been explored to identify the most sustainable construction material [5]. however, these approaches are non-equivalent and lead to uncertainty in the evaluation result, as differing conclusions may be reached when adopting different methods. the lack of a formalized conceptual paradigm for concrete sustainability may contribute to the multiplicity of evaluation methods, which introduces uncertainty into the sustainability evaluation. conceptualization is the process whereby a complex system is decomposed into increasingly smaller components until they can be measured by individual indicators. this creates a structure that rationally links quantitative measures to a qualitative objective. one of the most well-known conceptualizations of sustainable development is the "three pillars" model, wherein the nebulous goal of "sustainability" is first decomposed into the three dimensions of society, environment, and economy. these dimensions are then further broken down into relevant themes and sub-themes, which are ultimately measured by specific indicators [6]. the three pillars, however, represent just one conceptual paradigm for sustainability, and there exist other, equally valid conceptualizations derived from alternative perspectives on sustainable development [7]. formalizing a conceptual framework for concrete sustainability is essential to unify sustainability evaluation for the concrete field. in this paper, the effect of conceptualization on the sustainability evaluation of concrete materials is explored through an exploratory evaluation using multicriteria analysis. three conceptual frameworks for sustainable development are examined: direct loading, wherein sustainability is measured directly from individual indicators; the aforementioned three pillars paradigm; and the sustainable development goals (sdgs), which represent the latest global framework for pursuing sustainable development through a set of 17 goals. compensability, or the treatment of trade-offs between indicators, pillars, or sdgs, is also examined. 219 https://doi.org/10.14311/app.2022.33.0219 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en michael henry, joel opon acta polytechnica ctu proceedings � )ljxuh����6hwxs�iru�vxvwdlqdelolw\�hydoxdwlrq�e\�0&$�figure 1. setup for sustainability evaluation by mca. mix opc bfs fa w ng rg s w/b bfs/b fa/b f ′ c−28 [kg/m3] [−] [−] [−] [mpa] opc-ng 325 0 0 184 1063 0 783 0.57 − − 30.6 opc-rg 325 0 0 184 0 1063 783 0.57 − − 30.6 fa-ng 238 0 94 187 950 0 847 0.56 − 0.28 30.2 fa-rg 238 0 94 187 0 950 847 0.56 − 0.28 30.2 bfs-ng 200 133 0 192 965 0 806 0.58 0.40 − 30.4 bfs-rg 200 133 0 192 0 965 806 0.58 0.40 − 30.4 opc: ordinary portland cement, bfs: blast furnace slag, fa: fly ash, w: water, ng: normal coarse aggregate, rg: high grade recycled coarse aggregate, s: fine aggregate, b: total binder materials table 1. mix proportions and 28-day compressive strengths of the concrete mixes. the results and discussion are expected to provide insights into how to address uncertainty in conceptualization for improving the transparency and robustness of sustainability evaluation for concrete. 2. methodology 2.1. multicriteria analysis setup sustainability evaluation was carried out using multicriteria analysis (mca). mca is widely utilized to support decision-making by identifying the most sustainable option among a set of potential alternatives [8]. the mix proportions of a set of concrete mixes were used as the inputs. the conventional stages of mca are: indicator selection (i), data normalization (n), weighting assignment (w), and aggregation (a). the output of this process is typically a sustainability score (s) or rank (r) for each alternative, which can then be used for quantitative comparison and decision-making. in this paper, the selection of conceptual framework (c) for linking the indicators to sustainability is treated as an additional stage in the mca process. the setup for sustainability evaluation by mca is illustrated in figure 1. only a single indicator set, normalization method, and weighting scheme were adopted for their respective stages. however, three conceptual frameworks and two aggregation methods were utilized for their stages for a total of six analytical scenarios leading to six sustainability scores for each mix alternative. these scores are then statistically examined to explore how a plurality of perspectives on concrete sustainability may affect the evaluation result. 2.2. input concrete mix data the concrete mix alternatives were sampled from a database assembled by noguchi et al. [9]. a target compressive strength of 30 mpa at 28 days was set, and three concrete mixes with different binder contents were chosen. an additional three mixes were then generated assuming 100% replacement of normal coarse aggregates with high grade (class h) recycled coarse aggregates. the japan industrial standard (jis a 5021) indicates that high grade recycled aggregates should be equivalent to normal aggregates, so the concrete compressive strengths are assumed to remain unchanged. however, the energy consumption and emissions footprints of high grade recycled aggregates are higher than that of normal aggregates due to the recycling process, so the inclusion of these mixes will explore the tradeoff between increased consumption of recycled materials and increased environmental impacts in the sustainability evaluation. the mix proportions are summarized in table 1. 2.3. sustainability indicators a subset of 14 quantitative indicators was adopted from a comprehensive set of sustainable concrete material indicators (scmis) [2], with descriptions and characteristics given in table 2. measurement was carried out using available inventory data from the japan society of civil engineers (for ng, rg, s) and the life cycle assessment society of japan (for opc, bfs, fa), characterization values from the 220 vol. 33/2022 evaluation of sustainable concrete scmi id description unit loading by concept three pillars sdgs 1 energy consumption, renewable and non-renewable mj all 7 2 raw materials consumption, primary and secondary kg all 12 3 water consumption kg all 6 4 recycled materials consumption, primary and secondary kg all 12 5 co2 emissions kg-co2 env 9 6 sox emissions kg-sox envsoc 11 7 nox emissions kg-nox envsoc 11 8 particulate matter emissions (pm) kg-pm env 11 28 global warming potential (gwp) kg-co2 eq env 13 29 photochemical ozone creation potential (pocp) kg-c2h4 eq env 11 30.2 acidification potential, aquatic (ap) kg-so2 eq env 14 31.1 eutrophication potential, terrestrial (ep) kg-po4 eq env 2 34 human toxicity potential (hp) kg-c6h4cl2 eq soc 3 40 material cost of concrete yen eco 8 all: environmental, social, and economic, envsoc: environmental and social, env: environmental, soc: social, eco: economic table 2. summary of the adopted sustainable concrete material indicators. mix scmi 1 scmi 2 scmi 3 scmi 4 scmi 5 scmi 6 scmi 7 energy raw mats. water recyc. mats. co2 sox nox opc-ng 1213.0 2225.8 184.0 123.8 255.1 0.053 0.513 opc-rg 1563.8 1162.8 184.0 1186.8 270.9 0.053 0.539 fa-ng 956.6 2075.1 186.7 184.8 190.2 0.043 0.379 fa-rg 1270.1 1125.2 186.7 1134.7 204.3 0.043 0.402 bfs-ng 861.8 2005.3 192.0 209.2 162.6 0.038 0.320 bfs-rg 1180.4 1039.9 192.0 1174.6 176.9 0.039 0.344 mix scmi 8 scmi 28 scmi 29 scmi 30.2 scmi 31.1 scmi 34 scmi 40 pm gwp pocp ap ep htp cost opc-ng 0.015 255.1 0.017 0.412 0.067 0.620 5775 opc-rg 0.016 270.9 0.018 0.430 0.070 0.652 5775 fa-ng 0.012 190.2 0.013 0.308 0.049 0.458 5266 fa-rg 0.012 204.3 0.013 0.324 0.052 0.487 5266 bfs-ng 0.010 162.6 0.011 0.262 0.042 0.388 5679 bfs-rg 0.011 176.9 0.011 0.279 0.045 0.416 5679 table 3. mix proportions and 28-day compressive strengths of the concrete mixes. lca database of leiden university (for gwp, pocp, ap, ep, hp), and constituent material costs reported by henry et al [10]. the cost of high grade recycled aggregates was assumed the same as that of normal aggregates due to lack of data. the raw indicator values for the six mixes were calculated as shown table 3. the loadings of the indicators within the three pillars and sdgs concepts, as established by opon and henry [2], are also shown in table 2. the environmental pillar is most represented among the three pillars, with 12 of the 14 indicators loading to the environment. ten of the 17 goals are evaluated under the sdg concept, with sdg 11 (sustainable cities and communities) receiving the most representation with four indicators highly relevant to this goal. 2.4. normalization method as the adopted indicators exhibit differing characteristics and behavior, their data were normalized by standardization. standardization was carried out through a combination of z-scores, which convert the data based on a mean of zero and standard deviation of one (eq. 1), and t-scores, which shift the data to a non-negative scale of zero to 100 (eq. 2). z = x − µ s (1) t = (z × 10) + 50 (2) where z: z-score, t: t-score, x: raw value, µ: average, and s: standard deviation,. in some cases, the t-score is divided by 100 to scale the data from zero to one. 221 michael henry, joel opon acta polytechnica ctu proceedings �d��'luhfw�ordglqj� �e��7kuhh�slooduv�frqfhswxdo�iudphzrun� �f��6'*v�frqfhswxdo�iudphzrun� figure 2. conceptual frameworks for sustainability with indicator loadings. 2.5. conceptual frameworks to represent different conceptual approaches to sustainability, three frameworks linking the individual sustainability indicators to the overall objective of concrete sustainability were constructed (figure 2). the first framework, direct loading, assumes no priority areas or sub-categories; that is, all indicators load directly to the sustainability performance of the concrete materials. the second framework decomposes concrete sustainability into the three pillars of society, environment, and economy, and each indicator then loads to its related pillar, or pillars (for multidimensional indicators), following the loadings given in table 2. the final framework sets the ten relevant sdgs as the intermediate priority areas, with each indicator loading to its respective sdg. 2.6. weighting scheme as no information was available to establish the comparative importance of the indicators, pillars, and sdgs, equal weighting was applied for all loadings between levels of the conceptual framework. this is consistent with the united nations resolution adopting the sdgs, which states that they will give "equal priority" to all goals in their implementation efforts [11]. 2.7. aggregation methods two aggregation methods were used to produce the sustainability scores for the concrete mixes: linear and geometric. linear aggregation is an additive operation (eq. 3), and is frequently the default method in mca, whereas geometric aggregation is a multiplicative operation (eq. 4). sin = n! i=1 (wi × scmii) (3) sgeo = n" i=1 (scmii)wi (4) where s: sustainability score, n: total number of indicators, scmii: sustainability indicator i, and wi: weighting applied to indicator i. the choice of aggregation method represents another perspective on sustainability in the evaluation process. linear aggregation allows for full compensability between indicators; that is, an increase in one performance (such as co2 emissions) can be balanced by an equivalent decrease in another performance (such as cost). geometric aggregation, however, is less compensatory [12], meaning that trade-offs between indicators have a more pronouned effect on the sustainability score. the impact of compensability on the sustainability evaluation result may differ according to the conceptual framework, so these two 222 vol. 33/2022 evaluation of sustainable concrete �d��'luhfw�ordglqj��olqhdu�djjuhjdwlrq �e��'luhfw�ordglqj��jhrphwulf�djjuhjdwlrq �f��7kuhh�slooduv��olqhdu�djjuhjdwlrq �g��7kuhh�slooduv��jhrphwulf�djjuhjdwlrq �h��6'*v��olqhdu�djjuhjdwlrq �i��6'*v��jhrphwulf�djjuhjdwlrq� ���� ���� ���� ���� ���� ���� � �� �� �� �� �� �� �� 23&�1*23&�5* )$�1* )$�5* %)6�1* %)6�5* 6x vw dl qd el olw \� vf ru h ���� ���� ���� ���� ���� ���� � �� �� �� �� �� �� �� 23&�1*23&�5* )$�1* )$�5* %)6�1* %)6�5* 6x vw dl qd el olw \� vf ru h ���� ���� ���� ���� ���� ���� � �� �� �� �� �� �� �� 23&�1*23&�5* )$�1* )$�5* %)6�1* %)6�5* 6x vw dl qd el olw \� vf ru h ���� ���� ���� ���� ���� ���� � �� �� �� �� �� �� �� 23&�1*23&�5* )$�1* )$�5* %)6�1* %)6�5* 6x vw dl qd el olw \� vf ru h ���� ���� ���� ���� ���� ���� � �� �� �� �� �� �� �� 23&�1*23&�5* )$�1* )$�5* %)6�1* %)6�5* 6x vw dl qd el olw \� vf ru h ���� ���� ���� ���� ���� ���� � �� �� �� �� �� �� �� 23&�1*23&�5* )$�1* )$�5* %)6�1* %)6�5* 6x vw dl qd el olw \� vf ru h figure 3. sustainability scores by analytical scenario. aggregation methods were included in the mca process. 3. results and discussion 3.1. sustainability scores figure 3 shows the sustainability scores for the concrete mix alternatives by analytical scenario. for direct loading, the two bfs mixes exhibited the best sustainability performance, with bfs-ng slightly higher for linear aggregation, and bfs-rg slightly higher for geometric aggregation. in the case of the three pillars, fa-rg possessed the highest sustainability score regardless of aggregation method, followed by bfs-rg. finally, for the sdg concept, bfs-ng demonstrated the best sustainability for both aggregation methods, but the gap between the top four mixes was relatively small. overall, the four concrete mixes containing mineral admixtures consistently exhibited higher sustainability scores than the two opc mixes across all analytical scenarios. these mixes tended to have lower raw values for environment-related indicators compared to opc; and, since 12 indicators loaded to the environment, the effect of reduced environmental impacts was amplified through the mca process. while considering the limations of this analysis, these results nonetheless support the general perception that the application of alternative cementitious materials to concrete construction is a critical technological solution for improving sustainability in the concrete industry. 3.2. statistical examination it remains unclear which concrete mix is actually the most sustainable among the alternatives, as the results of each analytical scenario may be considered as equally valid due to the lack of an established stan223 michael henry, joel opon acta polytechnica ctu proceedings mix sustainability scores ranks min. mean max. std. dev. variance mean variance opc-ng 34.3 37.0 38.5 1.6 2.2 6 3 opc-rg 35.8 38.1 42.1 2.6 5.8 5 4 fa-ng 53.4 55.0 56.7 1.3 1.4 4 2 fa-rg 54.6 56.8 61.2 3.1 8.0 2 5 bfs-ng 50.2 55.5 58.2 3.7 11.4 3 6 bfs-rg 56.3 57.6 58.4 1.0 0.8 1 1 table 4. summary statistics of the concrete mixes sustainability performance. analytical scenario (a) (b) (c) (d) (e) (f) (a) direct loading, linear aggregation 1.00 (b) direct loading, geometric aggregation 1.00* 1.00 (c) three pillars, linear aggregation 0.89 0.89 1.00 (d) three pillars, geometric aggregation 0.88 0.89 0.99 1.00 (e) sdgs, linear aggregation 0.99 0.99 0.86 0.86 1.00 (f) sdgs, geometric aggregation 0.99 0.99 0.86 0.86 1.00* 1.00 *actual coefficients were less than 1.00, but appear as 1.00 due to rounding table 5. correlation coefficients for sustainability scores between analytical scenarios. dard for concrete sustainability evaluation. to judge the sustainability under the uncertainty introduced by multiple concepts and aggregation methods, statistical properties of the sustainability score distributions for each material were examined (table 4). the mix with the highest mean sustainability score was bfs-rg; however, per figure 3, this mix was the most sustainable alternative in only one analytical scenario. on the other hand, bfs-ng was the most sustainable material in three scenarios, but its mean sustainability score was ranked third out of the six alternatives. similarly, farg was the most sustainable in two scenarios, but was ranked second by mean score. this result may be explained by the variance of the sustainability scores. the variance of bfs-rg is the lowest among all alternatives, suggesting it is relatively insensitive to the uncertainty introduced by different conceptual frameworks and aggregation methods, whereas the variance of bfs-ng is the highest, and thus the alternative most affected by the choice of concept and aggregation method (and their interactions). considering both the highest mean sustainability score and accompanying lowest variance, bfsrg may be judged as the most sustainable material when there is no consensus on the conceptual framework or aggregation method. finally, the pearson correlation coefficient matrix for the sustainability scores was calculated between analytical scenarios. it can be seen that all coefficients are equal to or greater than 0.86; in particular, the results of the linear and geometric aggregation scenarios were almost perfectly correlated for all three conceptual frameworks. this result suggests a very highly correlated set of sustainability scores regardless of the chosen conceptual framework or aggregation method, and despite the rankings of the mix alternatives shifting between scenarios. 4. conclusion this paper examined the effect of conceptualization on the sustainability evaluation of concrete using multicriteria analysis and a set of six concrete mix alternatives with varying binder contents and aggregate types. it was found that the most sustainable concrete mix varied according to conceptual framework and aggregation method, with three different mixes achieving the highest score among six scenarios. statistical examination of the distributions of the mixes’ sustainability scores revealed that the concrete mix with the highest mean sustainability score, bfs-rg, was only ranked first in one analytical scenario. however, the variance of this mix’s scores was the lowest among the alternatives, suggesting that its evaluation was comparatively less sensitive to methodological choices regarding conceptualization and aggregation. despite the different rankings for each analytical scenario, the sustainability scores were still very highly correlated between all scenarios, indicating that different methodological choices produced closely related results. it should be noted that the results reported here cannot simply be generalized to the greater concrete industry, as they are dependent on the sample and conditions of this specific analysis. however, it is expected that the demonstrated evaluation method will provide concrete industry stakeholders with an example of how to consider methodological uncertainties in sustainability evaluation to increase the robustness of their decision-making. 224 vol. 33/2022 evaluation of sustainable concrete references [1] k. sakai, t. noguchi. the sustainable use of concrete, crc press, pp. 188, 2017. [2] j. opon, m. henry. an indicator framework for quantifying the sustainability of concrete materials from the perspectives of global sustainable development. journal of cleaner production 218:718-37, 2019. https://doi.org/10.1016/j.jclepro.2019.01.220. [3] h. yokota, s. goto, k. sakai. proc. of second international conference on concrete sustainability, cinme, barcelona, p 1046, 2016. [4] h. s. müller, m. haist, m. vogel. assessment of the sustainability potential of concrete and concrete structures considering their environmental impact, performance and lifetime. construction and building materials 67:321-37, 2014. https: //doi.org/10.1016/j.conbuildmat.2014.01.039. [5] p. o. akadiri, p. o. olomolaiye, e. a. chinyio. multi-criteria evaluation model for the selection of sustainable materials for building projects. automation in construction 30:113-25, 2013. https://doi.org/10.1016/j.autcon.2012.10.004. [6] undesa. csd theme indicator framework from 2001, united nations, new york, 2001. https://www.un.org/esa/sustdev/natlinfo/indica tors/isdms2001/table_4.htm. [7] l. janeiro, m. k. patel. choosing sustainable technologies. implications of the underlying sustainability paradigm in the decision-making process. journal of cleaner production 105:438-46, 2015. https://doi.org/10.1016/j.jclepro.2014.01.029. [8] m. cinelli, s. r. coles, k. kirwan. analysis of the potentials of multi criteria decision analysis methods to conduct sustainability assessment. ecological indicators 46:138-48, 2014. https://doi.org/10.1016/j.ecolind.2014.06.011. [9] t. noguchi, f. tomosawa, k. m. nemati, et al. a practical equation for elastic modulus of concrete. aci structural journal 106(5):690, 2009. [10] m. henry, g. pardo, t. nishimura, et al. balancing durability and environmental impact in concrete combining low-grade recycled aggregates and mineral admixtures. resources, conservation and recycling 55(11):1060-9, 2011. https: //doi.org/10.1016/j.resconrec.2011.05.020. [11] undesa. resolution adopted by the general assembly on. a/res/70/1, united nations, new york, 2015. https://www.un.org/en/development/de sa/population/migration/generalassembly/docs/g lobalcompact/a_res_70_1_e.pdf. [12] m. j. dobbie, d. dail. robustness and sensitivity of weighting and aggregation in constructing composite indices. ecological indicators 29:270-7, 2013. https://doi.org/10.1016/j.ecolind.2012.12.025. 225 https://doi.org/10.1016/j.jclepro.2019.01.220 https://doi.org/10.1016/j.conbuildmat.2014.01.039 https://doi.org/10.1016/j.autcon.2012.10.004 https://www.un.org/esa/sustdev/natlinfo/indicators/isdms2001/table_4.htm https://doi.org/10.1016/j.jclepro.2014.01.029 https://doi.org/10.1016/j.ecolind.2014.06.011 https://doi.org/10.1016/j.resconrec.2011.05.020 https://www.un.org/en/development/desa/population/migration/generalassembly/docs/globalcompact/a_res_70_1_e.pdf https://doi.org/10.1016/j.ecolind.2012.12.025 doi:10.14311/app.2021.30.0053 acta polytechnica ctu proceedings 30:53–57, 2021 © czech technical university in prague, 2021 available online at http://ojs.cvut.cz/ojs/index.php/app fracture energy of illitic rammed earth with high water-clay ratio barbora mužíková∗, tereza plaček otcovská, pavel padevět czech technical university in prague, faculty of civil engineering, department of mechanics, thákurova 7, 166 29 prague 6, czech republic ∗ corresponding author: barbora.muzikova@fsv.cvut.cz abstract. the article is focused on design of mixture of rammed earth, producing, testing and determination of fracture energy of unfired rammed earth and its stress-strain curve in tensile bending test. three different mixtures of rammed earth were designed and tested. the amount of water and binder is one of the key properties of the rammed earth, the amount of the water is expressed by the water-clay ratio. mechanical properties of the earth material highly depend on the composition of sand, clay and water. the prescription agl iii with 80 % of sand, 20 % of clay and 0.400 water-clay ratio reached the maximum value of fracture energy 4.858 ± 0.002 j/m2 and set agl v had the minimum value 1.934 ± 0.310 j/m2. keywords: fracture energy, illite, rammed earth, stress-strain curve, water-clay ratio. 1. introduction the rammed earth is nowadays considered as a sustainable way for modern building. the article is focused on illitic rammed earth with high water-clay ratio. the mechanical property that was searched is the fracture energy and stress-strain curve. earth as a building material is coming back thanks to the trend to use this natural and traditional material of our ancestors. an expression “sustainable building” describes this trend and unfired earth perfectly meets all its principles. besides that it is also an environmentally friendly final product which is not synthetically fabricated. moreover, it is recyclable and there is no need after demolition to take this material to the garbage dump. but the main disadvantage for its using is that the mechanical properties needed for a structure design are not well known. this problem is more complicated because the mechanical properties depend on a composition of the used earth [1–5]. 2. composition of rammed earth as written before, the mechanical properties depend on the composition of earth. earth is a mixture of clay, sand and water. from the point of mechanics, the clay is a binder (such as cement for concrete), sand is a filler that makes the structure and water activates the binding forces of clay. 2.1. binder the clay is a binder for rammed earth. unfortunately for searching the mechanical properties, there are three basic types of clay – kaolinite, illite and montmorillonite. they differ in chemical composition. clay minerals are hydrous aluminium phyllosilicates with variable amounts of iron, magnesium, alkali metals, alkaline earths. the structure is made by sheets. the number and size of these sheets and gaps between them have an impact on water absorption of the final product. the clay changes the properties of the final product – mechanical properties and the colour is different as can be seen in the fig. 3. the specimens tested for the fracture ennergy determination were made with illite clay. 2.2. filler the mixture of clay and sand (or sand with bigger parts like gravel) is called a loam. the process of the formation is called an erosion of rocks. it takes a very long time and this process occurs naturally and depends on local weather conditions and surrounding nature. as a result of erosion, the particles of different sizes arise – clay(less than 0.002 mm), silt (0.002 – 0.060 mm), sand(0.060 – 2.000 mm) and gravel (more than 2.000 mm). the smaller parts have the function of a binder and the others have the function of a filler. 2.3. water-clay ratio water is the starter for binding reaction of clay. thus it is very important to have the suitable amount of water. the amount of water in rammed earth is expressed by the water-clay ratio. the meaning is the same as the water-cement ratio at concrete [6–12]. 3. production of specimens the principle of building rammed earth using framework is shown in fig. 1 and the production in laboratory in fig. 2. first of all, the framework is built to create the shape of the future wall. secondly, a small part of the earth mixture is put in the framework 53 http://dx.doi.org/10.14311/app.2021.30.0053 http://ojs.cvut.cz/ojs/index.php/app b. mužíková, t. plaček otcovská, p. padevět acta polytechnica ctu proceedings figure 1. procces of building earth wall. figure 2. prefabrication of the specimens in laboratory conditions. figure 3. specimens with different type of clay as a binder. 54 vol. 30/2021 fracture energy of illitic rammed earth set sand/clay ratio water-clay ratio pcs of specimens bulk density type of clay [%/%] [−] [−] [kg/m3] [−] agl iii 80/20 0.400 3 2103 illite agl v 85/15 0.370 3 2005 illite agl ix 80/20 0.450 3 2107 illite table 1. composition of designed mixture for rammed earth. and compressed. then next layers of rammed earth are put and compressed and together they create the rammed earth wall. as the final step, the framework is removed. in the same way the specimens are made in laboratory, but there is no big framework for the wall but just small ones to create the specimen of exact dimension. the difference is the step before the producing. the prescription has to be chosen. that means to choose a type of clay, then a ratio between clay and filler and a water-clay ratio. the accurate amount of clay, the sand and water are weighed and mixed. for the need of three-point bending test three different prescriptions were designed and tested. the agl sets iii, v and ix are all with illitic clay. the composition of these set is in-depth described in the table 1. for the three-point bending test were made specimens of size 40×40×160 mm. another comperative tests were made in [11–13]. the photograph captured during the test is shown in the fig. 4. figure 4. the three-point bending test on specimen from rammed earth material. 4. method of determining fracture energy to get fracture energy the three-point bending test needs to be done. the force that is apllied on the specimen and the matching displacement are recorded. the force-displacement curve (f-δ) is recorded and the stress-strain curve (σ-ε)can be calculated from these data. the get stress-strain we have to caltulate the strain: figure 5. the stress-strain curve for agl iii specimen numb. 3. ε = δ ÷ l, (1) where l is the length of the specimen between the support points in the test. then the stress can be calculated: σ = f ÷ a, (2) where a stands for the cross area. now we have data to get stress-strain curve (fig. 5). next step is to calculated the fracture energy. the fracture energy is defined as the amount of energy necessary to create one unit area of a crack. firstly, the work of the apllied force wf is calculated (7): wf = ! δmax 0 fdδ , (3) and then the fracture energy is: gf = af ÷ bw, (4) where bw stands for the area of the crack (w is the length of the ligament and b is the width of the specimen). in this case this area is the same as the cross area a of the specimen. the schema of the test can be seen in the fig. 6. 55 b. mužíková, t. plaček otcovská, p. padevět acta polytechnica ctu proceedings figure 6. schema of the three-point bending test. set s/c w type of clay gf ft e [−] [−] [j/m2] [mpa] [mpa] agl iii 80/20 0.400 illite 4.858±0.002 0.059±0.002 68.742±28.224 agl v 75/25 0.370 illite 1.934±0.310 0.028±0.003 49.056±15.171 agl ix 75/25 0.450 illite 4.493±0.521 0.051±0.001 27.038±7.597 table 2. evaluated fracture energy, tensile strength in bending and modulus of elasticity. figure 7. the work to create the crack is the area under f -δ curve. 5. results of the measurements the result of the measurement were the arithmetic mean and the standard deviation for each set from the measured values, they are in table 2. there are the compositions of the mixture, used type of clay, evaluated fracture energy (± standard deviation), their tensile bending stregths (± standard deviation), elasticity modulus (± standard deviation). the values of fracture energy are shown also in fig. 8. set agl iii with water-clay ratio 0.400 has the highest value of the fracture energy 4.858±0.002 j/m2, agl ix with watter-clay ratio 0.450 just a little bit lower value 4.493 ± 0.521 j/m2 and set agl v has significantly figure 8. fracture energy of tested mixtures. lowest value 1.934 ± 0.310 j/m2. it is interesting that set agl iii with 20 % clay and water-clay ratio 0.400 have very simillar result as agl ix with 25 % clay and water-clay ratio 0.450. so the set agl ix have more clay and also more mixing water. also their tensile strengths are very similar 0.059 ± 0.002 and 0.051 ± 0.001. 6. conclusions three different mixtures were designed, producted and tested. measured data were evalueted a compared in relation to its composition. the paper was focused on evaluating fracture energy and strainstress curve of different sets of rammed earth. the mechanical properties of the earth material highly depend on the composition of sand, clay and water. 56 vol. 30/2021 fracture energy of illitic rammed earth the amount of water and the binder is one of the key properties of the rammed earth, it is similar to concrete, so the amount of water is expressed by the water-clay ratio. the water-clay ratio 0.400 seems to be optimal with the mixture agl iii (80 % sand and 20 % clay) and also very good results have the composition with water-clay ratio 0.450 for mixture agl ix (75 % sand and 25 % clay). list of symbols e elascity modulus [mpa] f load force [kn] gf fracture energy [j/m2] fc compressive strength [kpa] ft tensile bending strength [kpa] δ displacement [mm] ε strain [–] # bulk density [kg m−3] σ stress [kpa] l length between supports in the test [mm] a cross area [mm2] acknowledgements this research was financial supported by the czech science foundation (gacr no.18-0884s) and by the faculty of civil engineering at czech technical university in prague (sgs project no.16/201ohk1/3t/11). references [1] b. little, m. morton. building with earth in scotland: innovative design and sustainability, vol. 1 of first printing. scottish executive central research unit, edinburgh, 2001. [2] j. norton. building with earth, a handbook, vol. 1. intermediate technology development group limited, london, 1986. [3] g. minke. building with earth design and technology of sustainable architecture, vol. 2. de gruyter, berlin, 2009. [4] i. zabickova. hlinené stavby. era 21, brno, 2002. [5] m. rauch. refined earth construction and design with rammed earth, vol. 1. detail, munich, 2015. [6] kianfar, ehsan, v. toufigh. reliability analysis of rammed earth structures. construction and building materials (127):884–895, 2016. [7] h. araki, j. koseki, t. sato. tensile strength of compacted rammed earth materials. soils and foundation (56(2)):189–204, 2016. [8] l. miccoli, d. v. oliveira, r. a. silva, u. muller. static behaviour of rammed earth: experimental testing and finite element modelling. materials and structures (48(10)):43–56, 2015. [9] q. liu, t. l. tong. engineering properties of unstabilized rammed earth with different clay contents. journal of wuhan university of technology: materials science (32(4)):914–920, 2017. [10] r. el-nabouch, q. bui, o. ple, p. perrotin. characterizing the shear parameters of rammed earth material by using a full-scale direct shear box. construction and building materials (171):414–420, 2018. [11] q. bui, j. morel, b. v. reddy, w. ghayad. durability of rammed earth walls exposed for 20 years to natural weathering. building and environment (44(5)):912–119, 2009. [12] x. dong, v. soebarto, m. griffith. strategies for reducing heating and cooling loads of uninsulated rammed earth wall houses. energy and buildings (77):323–331, 2014. [13] t. otcovska, p. padevet. dependence of tensile bending strength of rammed earth on used clay composition and amount of mixture water. modern methods of experimental and computational investigations in area of construction ii pp. 48–53, 2017. 57 my title https://doi.org/10.14311/app.2022.36.0135 acta polytechnica ctu proceedings 36:135–141, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague probabilistic assessment and sensitivity analysis of existing concrete bridge via surrogate modeling lukáš novák∗, drahomír novák brno university of technology, faculty of civil engineering, institute of structural mechanics, veveří 331/95, 602 00 brno, czech republic ∗ corresponding author: novak.l@fce.vutbr.cz abstract. the paper is focused on probabilistic assessment and sensitivity analysis of existing prestressed concrete bridge using surrogate model in form of polynomial chaos expansion (pce). the bridge was selected in the framework of the european project interreg austria-czech republic "tcz190 safebridge" focused on advanced numerical analysis of existing bridges represented by non-linear finite element model. in this study, surrogate model in form of pce was created, which represents very efficient type of surrogate model. one of significant advantages of pce is powerful post-processing including sensitivity and moment analysis of the response, which is important part of probabilistic analysis. the obtained numerical results of advanced stochastic analysis consisting of uncertainty quantification and sensitivity analysis of the existing bridge structure are presented in the paper. keywords: concrete structures, polynomial chaos expansion, sensitivity analysis, statistical analysis. 1. introduction the development of computational methods for civil engineering has become more important than ever, since it is often necessary to employ advanced numerical methods for the design of new structures in order to fulfil the significantly increasing economical and safety requirements in the last decades. moreover, there are a lot of structures, especially bridges, built in the last century, which must often be enhanced for higher loads assuming actual conditions of the structures. as a result of these industrial needs, researchers and civil engineers are more interested in advanced numerical methods to solve the mathematical models of structures – typically non-linear finite element method (nlfem). although nlfem is a very accurate numerical method for solving differential equations, there is still a lack of knowledge of material characteristics (e.g. fracture energy), actual geometrical properties (e.g. position of reinforcement) and even mathematical models of some physical phenomena (e.g. fracture mechanics of quasi-brittle materials) collectively called uncertainties. as can be seen from the given examples, uncertainties play an important role, especially in the case of concrete structures. this lack of knowledge may generally lead to inaccurate results and even fatal failures despite the advanced numerical analysis performed by nlfem. in modern structural analysis, uncertainties are represented by random variables described by specific probability distribution, the structural system can then be seen as a mathematical function of a set of random parameters. deterministic numerical analysis of structures must thus be enriched by stochastic analysis. the elementary task of stochastic analysis is to propagate uncertainties through a mathematical model in order to obtain statistical and/or sensitivity information of quantity of interest (qoi). results of statistical analysis are important especially for semiprobabilistic design and assessment of structures, since it is necessary to estimate design quantile of structural resistance, which fulfils given safety requirements. unfortunately statistical analysis of complex mathematical models (e.g. bridges) is highly computationally demanding or even not feasible in industrial applications, since the statistical analysis typically consists of large number of repetitive deterministic calculations. on the one hand, it is possible to reduce the number of simulations as much as possible by simplified design methods such as taylor series expansion or methods for estimation of coefficient of variation (ecov). on the other hand, highly computational requirements per simulation can be significantly reduced by the surrogate model, which can be used as a computationally cheap approximation of the original mathematical model. although there are various types of surogate models such as support vector machine, krigging or artificial neural networks, it is beneficial to use approximations which can be easily used for analytical post-processing such as polynomial chaos expansion (pce) originally proposed by norbert wiener [1] offering efficient post-processing. in this paper pce is employed for sensitivity and statistical analysis of an existing prestressed concrete bridge. the bridge consists of 16 precast posttensioned bridge girders and it is loaded by exceptional load according to national annex in order to estimate design value of resistance. the bridge is represented by nlfem based on theory of non-linear fracture mechanics of quasi-brittle material. the obtained results and applied methodlogy might be interesting 135 https://doi.org/10.14311/app.2022.36.0135 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en lukáš novák, drahomír novák acta polytechnica ctu proceedings for civil engineers as well as for scientist dealing with semi-probabilistic analysis of existing structures. 2. polynomial chaos expansion evaluation of mathematical model of qoi is often highly computationally demanding and thus it is necessary to create an efficient approximation. one of the most popular approach is pce [1], which represents the output variable y as a polynomial expansion gp ce of an another random variable ξ called the germ with given distribution y = g(x) ≈ gp ce(ξ), (1) a set of polynomials, orthogonal with respect to the probability distribution of the germ, are used as a basis functions. the orthogonality condition for all j ̸= k is given by the inner product defined for any two functions ψj and ψk with respect to the probability density function of ξ ⟨ψj , ψk⟩ = ∫ ψj(ξ)ψk(ξ)pξ(ξ) dξ = 0. (2) polynomials ψ orthogonal with respect to a selected probability distributions pξ can be chosen according to wiener-askey scheme [2] or created directly by gram-schmidt orthogonalization. in this paper we use normalized polynomials with inner product equal to the kronecker delta δjk. in the case of x and ξ being vectors containing m random variables, the polynomial ψ(ξ) is multivariate and it is built up as a tensor product of univariate orthogonal polynomials. the quantity of interest (qoi), i.e. the response of the mathematical model y = g(x), can then be represented, according to ghanem and spanos [3], as y = g(x) = ∑ α∈nm βαψα(ξ), (3) where α ∈ nm is a set of integers called the multiindex, βα are deterministic coefficients and ψα are multivariate orthogonal polynomials. naturally, the approximating function given by eq. (3) must be truncated to a finite number of terms p using e.g. total-order truncation by retaining only terms whose total degree |α| is less than or equal to a given p: am,p = { α ∈ nm : |α| = m∑ i=1 αi ≤ p } . (4) in case of high p and m , it possible to use additional “hyperbolic” reduction of the truncated set [4]. 2.1. non-intrusive approach truncated pce can be seen as a linear regression model with deterministic coefficients β, which can be thus obtained by ordinary least square (ols) regression. estimated β thus minimize the sum of the squares of the differences between the results of original mathematical model y corresponding to he input random vector x together called the experimental design (ed) and the results of surrogate model. specifically, the vector of deterministic coefficients β is calculated using data matrix ψ as β = (ψt ψ)−1 ψt y. (5) the number of deterministic coefficients is directly connected to p , generally dependent on the number of input random variables m and the maximum total degree of polynomials p as can be seen in eq. 4. unfortunately, this leads to computationally highly demanding problems in case of large stochastic nonlinear models. in order to reduce p , it is possible to select the best model represented by sparse set of basis functions. the best model selection is a broad scientific topic and several methods were proposed, here we use least angle regression (lar) [5] to find an optimal set of pce terms as proposed by blatman and sudret [4]. 2.2. approximation error estimation naturally, it is necessary to measure the approximation error of pceṡuch a measure can be further used for construction/selection of the best surrogate model. however, it might be highly computationally demanding to create a validation set containing several calculations of the original mathematical model. therefore, it is beneficial to utilize accuracy measures, which do not need any additional simulations. commonly used technique is the coefficient of determination r2, which is well known from machine learning. however, this measure often leads to over-fitting and thus scientists are focused on more advanced techniques. the robust, computationally efficient and generally reliabile estimator is the leave-one-out cross validation error q2. the estimated error is based on residuals between predictions of the surrogate model and the results of original mathematical model measured on ed, while excluding one realization in construction of surrogate model. the errors are calculated for all realizations in ed and further the average error is estimated. it is clear that iterative process of pceconstructions for all realizations in ed can be computationally demanding. fortunately in case of pce, it is possible to get q2 analytically from a single pcebased on all realizations in ed as follows [6]: q2 = 1 − 1 nsim ∑nsim i=1 [ g(x(i))−gpce(x(i)) 1−hi ]2 σ2 y,ed , (6) where σ2 y,ed is a variance of experimental design obtained from results of the original mathematical model and hi represents the ith diagonal term of the matrix h = ψ ( ψt ψ )−1 ψt . 136 vol. 36/2022 probabilistic assessment of existing concrete bridge 2.3. statistical moments the pce is famous for statistical analysis thank to its powerful and efficient post-processing allowing for analytical derivation of statistical moments of the qoi. the mean value is obtained from general formula of the first statistical moments as µy = 〈 y 1〉 = ∑ α∈nm βα ∫ ψα(ξ) pξ ( ξ ) dξ. (7) considering the orthonormality of the polynomials∫ ψα(ξ)pξ ( ξ ) dξ = 0 ∀α ̸= 0, ψ0 ≡ 1, the original integration is reduced to simple postprocessing of the pce deterministic coefficients. namely, the mean value is equal to the first deterministic coefficient of the expansion µy = 〈 y 1〉 = β0. (8) the second raw statistical moment, 〈 y 2〉 , is written as〈 y 2〉 = ∑ α∈a β2 α ∫ ψα (ξ)2 pξ (ξ) dξ = ∑ α∈a β2 α ⟨ψα,ψα⟩ . (9) similarly as in case of the mean value, it is possible to obtain the variance as the sum of all squared deterministic coefficients except the intercept (which represents the mean value), i.e. σ2 y = ∑ α∈a α ̸=0 β2 α. (10) 3. sensitivity analysis once the pce approximation is constructed, it is also possible to obtain sensitivity measure of input random variables. although there are many sensitivity techniques beneficially coupled with surrogate models [7], the following two common and efficient techniques are employed in this paper: non-parametric rank-order correlation obtained by mc simulation and sobol indices derived directly from pce coefficients. results of both techniques represent different information and thus they should be combined in order to correctly investigate the influence of input random variables. 3.1. non-parametric rank-order correlation the traditional sensitivity analysis method in statistics is represented by the correlation between an input variable and the quantity of interest of mathematical model. although standard measurement via the pearson correlation coefficient is simple and efficient enough for linear monotonic dependency, it is necessary to utilize a generalized measure for nonlinear monotonic relationships called the non-parametric spearman rank-order correlation technique [8]. obtained correlation coefficient ρ is in an interval ⟨−1, 1⟩. the higher absolute value of ρ corresponds to the stronger relationship between the two variables. if it is positive, then as an input variable increases, the qoi tends to increase. if it is negative, then as an input variable increases, the qoi tends to decrease. 3.2. sobol indices one of the most important tasks in uncertainty quantification is the analysis of variance "the analysis of the influence of input variables on the variance of a mathematical model. such information may be utilized to practically reduce the uncertainty of important input variables (material characteristics) used in mathematical model by experiments and measurements, which leads to a significant reduction in the uncertainty of the quantity of interest. herein, the well-known anova method represented by sobol indices is employed. the sobol indices method is widely used and well developed method for sensitivity analysis. however, it is still highly computationally demanding to evaluate sobol indices via the classical double loop monte carlo method. fortunately, there is a connection between pce and the hoeffding-sobol decomposition [9] allowing for analytical derivation of sobol indices. pce can be rewritten in the form of the hoeffdingsobol decomposition by a simple reordering of the terms: gp ce (x) = β0 + ∑ α∈au βαψα (ξ) , (11) where the set of basis multivariate polynomials dependent on selected input random variables xu is au = { α ∈ am,p : αk ̸= 0 ↔ k ∈ u } . (12) therefore, the first order sobol indices can be analytically obtained directly from pce as follows [9]: si = ∑ α∈ai β2 α σ2 y , (13) where basis functions are selected as: ai = { α ∈ am,p : αi > 0, αj ̸=i = 0 } . (14) important information about the influence of input variables and all interactions can by expressed by total sobol indices representing the first order influence and influence of all interactions, which can be obtained as st i = ∑ α∈at i β2 α σ2 y , (15) where basis functions are selected as: at i = { α ∈ am,p : αi > 0 } . (16) note that, the above expressions represent just a selection of the specific pce coefficients associated to selected input random variable. 137 lukáš novák, drahomír novák acta polytechnica ctu proceedings 4. post-tensioned concrete bridge the pce is employed for probabilistic assessment of an existing concrete bridge. the pce is utilized as an alternative to semi-probabilistic assessment partly addressed in the simplified case-study of the selected bridge during the european project interreg austria-czech republic "atcz190 safebridge". [20]. the bridge consists of three spans constructed from 16 bridge girders ka-61 in transverse direction. the crucial part of the bridge for assessment is the mid-span: 19.98 m long with total width 16.60 m. the geometry of a typical bridge girder ka-61 is created according to an original documentation describing also positions of reinforcement and tendons. the drawing together with the simplified cross-section is depicted in fig. 1. figure 1. cross-section of a singe bridge girder ka-61. from structural point of view, it was necessary to create numerical model of the whole bridge span. the reason is that although the structure is symmetric, the individual bridge girders are not transversely prestressed, which leads to the different deflection of each girder in dependence on their distance to the loading position. in order to create numerical model reflecting their real connection conditions, the girders are connected by reinforcement according to original documentation together with a concrete mixture between single girders. 4.1. finite element model the cross-sections of girders ka-61 were simplified to regular shapes in order to reduce number of finite elements and to obtain regular mesh, see fig. 1. boundary conditions are assumed to be as a simply supported beam with elastic blocks. the geometry of elastic blocks and positions of loading plates are modeled according to bridge documentation and a national annex of eurocode for load-bearing capacity of road bridges by exclusive loading (by six-axial truck). the nlfem is created in software atena science including theory of non-linear fracture mechanics [10]. in order to reflect complex behavior of the bridge, the numerical model contains three construction phases as illustrated in fig. 2: (1.) prestressing of bridge girders and simultaneous application of the self-weight; (2.) activating of the pavement and concrete among girders connecting bridge girders; (3.) application of a load by a six-axial truck. the major part of nlfem is represented by 13,000 elements of hexahedra type and triangular ‘prism’ elements in the blue-colored parts of the cross-section (see fig. 1). hexahedra elements lead to better numerical stability of simulation and leads to easier construction of mesh compatible between two volumes connected by fixed contact, i.e. nodes of elements in both connected sub-volumes have same coordinates. the advantage of brick elements is that the structured mesh constructed from brick elements leads to a significantly lower number of finite elements in comparison to tetrahedra elements. fracture-mechanical behavior of concrete is described by a non-linear mathematical model [10]. reinforcement together with tendons are modeled as discrete 1d elements with positions and shape according to the original documentation. the numerical model is analysed in order to investigate the following three limit states of the bridge: (1.) the ultimate limit state (uls) (peak of a loaddeflection diagram); (2.) the first occurrence of cracks in bridge girders (cracking); (3.) the serviceability limit state of decompression defined according to eurocode (sls). note that obtained results are further reduced by dynamic amplification factor δ = 1.4 in order to reflect that results are from static analysis. 4.2. stochastic model the stochastic model contains 4 random material parameters of a concrete c50/60: young’s modulus e ; compressive strength of concrete fc; tensile strength of concrete fct and fracture energy gf . characteristic values of e, fct, gf were determined from fc according to formulas implemented in the fib model code 2010 [11] (gf , e) and pren 1992-1-1: 2021 (fct). the last random variable p represents prestressing losses with cov according to jcss: probabilistic model code [12]. the stochastic model is summarized in 138 vol. 36/2022 probabilistic assessment of existing concrete bridge figure 2. three construction phases of the bridge represented by nlfem. tab. 1. mean values and coefficients of variation were obtained according to pren 1992-1-1: 2021 (annex a) for adjustment of partial factors for materials. var. mean cov [%] distrib. units fc 56 16 lognormal [mpa] fct 3.64 22 lognormal [mpa] e 36 16 lognormal [gpa] gf 195 22 lognormal [jm2] p 20 30 normal [%] table 1. stochastic model of the numerical example. in this first numerical study, there is an assumption of uncorrelated random variables.though the correlation might play crucial role in case of concrete structures. 4.3. numerical results the ed contains 30 numerical simulations generated by latin hypercube sampling (lhs) [13] together with corresponding results of nlfea. note that each simulation takes approximately 24 hours. the pce is created with maximal polynomial order p = 5 with the lar algorithm for a selection of the best set of basis functions. the whole algorithm of adaptive construction of pce connects state of art techniques into stand-alone software tool [14]. obtained mean, cov and design values together with pce accuracy measured by q2 are summarized for all limit states in tab 2. the design values of resistance rd for each limit state in tons are determined as a quantile of lognormal distribution with identified statistical moments and target reliability indices βuls = 3.8, βcrack = 3.8 and βsls = 1.5 according to en 1990 [15]. additionally, design values are reduced by global safety factor reflecting model uncertainties γrd = 1.06 introduced originally in fib model code 2010 [11]. once the pce is created, it is also possible to easily obtain sensitivity indices as described in section mean cov [%] q2 rd uls 480 7.2 0.91 365 cracking 400 8.4 0.96 290 sls 150 14.1 0.98 120 table 2. obtained statistical moments and pce accuracy for each limit state. 3.2. obtained sensitivity measures are summarized in tab. 3. 5. discussion and further research as can be seen from obtained accuracy, all three limit states are well approximated by pce and thus one could use obtained statistical moments for derivation of design values of resistance. it can be seen, that a critical limit state (the lowest rd) is represented by limit state of decompression, which is typical for prestressed structures. it is interesting that sls has the highest cov, though it is almost linear limit state. this could be explained by results of sensitivity analysis revealing that p (which has a high cov) has absolutely dominant influence. the uls and cracking limit state have lower cov since there is also a signficant influence of concrete material characteristics and their interactions. it is clear that sensitivity analysis plays important role in the probabilistic analysis of structures, however, it is important to understand obtained information from different types of sensitivity analysis [7]. moreover, correlation among material characteristics may have a significant influence on obtained results [16] and thus further work will be focused on analysis assuming realistic correlation matrix of concrete material parameters similarly as in previous work of authors of this paper [17, 18]. although q2 > 0.9 for all limit states, it should be improved in further work in order to obtain more reliable results, especially for sensitivity analysis. the recently proposed adaptive 139 lukáš novák, drahomír novák acta polytechnica ctu proceedings uls cracking sls si st,i ρ si st,i ρ si st,i ρ fct 0.00 0.01 0.23 0.00 0.15 0.31 0.00 0.05 0.23 fc 0.00 0.10 0.15 0.00 0.01 0.05 0.00 0.01 0.10 gf 0.00 0.10 0.16 0.00 0.01 0.06 0.00 0.01 0.05 e 0.00 0.01 0.07 0.06 0.07 0.45 0.00 0.05 0.27 p 0.90 0.95 -0.90 0.77 0.90 -0.85 0.95 0.95 -0.95 table 3. results of sensitivity analysis via pce. ρ represents non-parametric rank order correlation by spearman coefficients, si first-order sobol indices and st,i total-order sobol indices. sequential sampling will be employed [19] for efficient extension of existing ed. 6. conclusions the paper presents practical application of pce for probabilistic assessment of an existing prestressed concrete bridge. the bridge is represented by highly computationally demanding nlfem reflecting theory of non-linear fracture mechanics of concrete. the stochastic model contains 5 random variables representing concrete characteristics and prestressing losses. the pce is created by adaptive best model selection algorithm lar (non-intrusive approach) with ed containing 30 samples generated by lhs. pce was created for three limit states, which were further analysed in order to obtain statistical moments and sensitivity indices. the obtained statistical moments are used for the estimation of design values of resistance. acknowledgements the first author acknowledge financial support provided by the ministry of education, youth and sports of the czech republic under project no. ltausa19058. references [1] n. wiener. the homogeneous chaos. american journal of mathematics 60(4):897–936, 1938. https://doi.org/10.2307/2371268. 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[20] l. novák, d. novák. semi-probabilistic assessment of prestressed concrete bridge in proc. of the 13th fib international phd symposium in civil engineering, pp. 45-51, 2021, 141 https://doi.org/10.1016/j.cma.2021.114105 acta polytechnica ctu proceedings 36:135–141, 2022 1 introduction 2 polynomial chaos expansion 2.1 non-intrusive approach 2.2 approximation error estimation 2.3 statistical moments 3 sensitivity analysis 3.1 non-parametric rank-order correlation 3.2 sobol indices 4 post-tensioned concrete bridge 4.1 finite element model 4.2 stochastic model 4.3 numerical results 5 discussion and further research 6 conclusions acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.38.0599 acta polytechnica ctu proceedings 38:599–605, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague advancing circular economy in the existing building stock: a methodology to support building characterisation for sustainable refurbishment design joana bastos fernandesa,∗, paulo cadete ferrãoa, josé dinis silvestreb, antónio aguiar costab, verena gösweinc a universidade de lisboa, instituto superior técnico, in+, av. rovisco pais 1, 1049-001 lisboa, portugal b universidade de lisboa, instituto superior técnico, ceris, av. rovisco pais 1, 1049-001 lisboa, portugal c 3 drivers – engenharia, inovação e ambiente, lda., av. conde valbom 6, 6°, 1050-068 lisboa, portugal ∗ corresponding author: joanabfernandes@tecnico.ulisboa.pt abstract. the lack of standard practices and platforms for assessing refurbishment strategies towards circular economy (ce) and their impact in global warming constitutes a challenge for the decarbonization of existing building stock. incorporating data and feedback from designers and practitioners since early design stages is important to feed a multi-criteria dynamic process with multiple dimensions, which must be assessed under a life cycle perspective. to tackle this issue, this paper introduces a new methodology to support the implementation of tailored refurbishment strategies for increased recovery, reuse and recycling of construction materials. the final objective is to build a methodological framework for sustainable refurbishment design in a bim environment, which aims to facilitate standardized practices in the construction sector, regarding ce, with a positive impact in the mitigation of global warming and the decarbonization of the building stock. to test the development of this methodology, a case study building in lisbon, corresponding to a 1919–1945 archetype is analysed, making use of its bim model, where bim standardization criteria and circularity indicators are discussed, in order to be implemented as a plugin for circularity. keywords: bim-based platform; building archetype; building automated characterisation; ce; methodological framework; sustainable refurbishment design. 1. introduction construction industry is responsible for over 30 % of the global extraction of natural resources and 25 % of solid waste generated [1], whereas only 20–30 % of construction and demolition waste is recovered [2]. improving this situation is at the core of circular economy (ce), which is intended to reduce natural resource extraction, by minimizing waste, materials and energy consumption and extending and maximizing the use of materials and existing structures [3]. the ce action plan of the european union manifests these efforts at the european level [4, 5]. a shared digital representation of buildings, the management of complex information in projects, and the improved collaboration and communication among stakeholders in all life cycle stages of the building are ideal processes for ce assessment in architecture, engineering and construction (aec) and can be provided by building information modelling (bim) tools. however, difficulties arise with the implementation of processes for measuring circularity in a bim environment, from early design stages [6]. the characterisation of the existing buildings plays an important role in building refurbishment, since it constitutes the baseline scenario for circularity assessment, including deconstruction strategies and the definition of future design options, which are difficult to determine. to overcome the described difficulties, this paper proposes a new methodology, building automated characterisation (bac), to support the implementation of tailored refurbishment strategies as a strategy for increased recovery, reuse and recycling of construction materials, and to provide quantitative information on buildings materials based on construction systems typification. 2. background and major gaps developments for promoting ce in construction are still in their infancy. typically, buildings are subject to demolition or renovation, with little or no parts that can be reused or recycled, because their different functions, systems, elements and materials are fixed in a closed structure that cannot be separated, for partial changes or disassembly, or simply because this information is not available. moreover, there isn’t yet a common methodology adopted for measuring circularity in the built environment, within its multiple dimensions [7]. the lack of standard practices for assessing refurbishment strategies towards energy efficiency and ce principles [1] constitutes a challenge for the decarbonisation of the existing building stock. incorporating data and feedback from designers and 599 https://doi.org/10.14311/app.2022.38.0599 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en j. b. fernandes, p. c. ferrão, j. d. silvestre et al. acta polytechnica ctu proceedings practitioners starting from the early design stages is also crucial [3] to support a multi-criteria dynamic process with multiple dimensions, which have to be assessed under a life-cycle perspective. the new methodology developed in this paper intends to fill these gaps and it assumes that the starting point of a refurbishment design consists in analysing existing buildings’ architecture and construction systems and materials, on assessing their thermal performance, characterising its uses and their dynamic energy consumption [8]. it should allow the characterisation of existing building stock at a large scale, taking advantage of bim, to create databases for circularity assessment adapted to existing building stock and implementing stock and flow analysis of resources and materials. 3. methodological developments different existing approaches and various advancements are combined in the proposed methodology. it builds upon a method for the generation of archetypes developed by monteiro et al. [9] for energy assessment at an urban scale and provides methodological advances to assess circularity and an information based system to provide information on materials and construction systems. the following sections detail the quantification of circularity from material to building scale and its impact on climate change, the conceptual workflow for circularity assessment in project design and bac, and the characterisation of an existing building corresponding to an archetype. 3.1. circularity indicators the adopted model for circularity indicators, at the core of this research, is based on verberne [10] and cottafava and ritzen [11], which were developed from the material circularity indicator (mci) [12]. the mci methodology was established by the ellen mcarthur foundation in 2015 to measure the circularity potential at the product level, considering the mass of virgin raw material, the mass of unrecoverable waste and the utility factor. mci assumes a score between 0 and 1, where 0 represents that the product is fully “linear” (only using virgin feedstock and its only future scenario is landfill) and where 1 refers to a fully “circular” product (containing neither waste nor virgin material). based on the existing literature, it is suggested to combine the mci with a disassembly index [11, 13, 14], a weighting system that quantifies the connections between construction elements and their disassembly potential to obtain the product circularity indicator (pci). the pci reflects the degree of circularity of a product in a particular system/building. thereafter, the system circularity indicator (sci) assesses the circularity of multiple products in a system based on their mass. it distinguishes between six system layers [15]. each system consists of a collection of products and materials, including their characteristics and interrelated behaviour. finally, the building circularity indicator (bci) assesses the various systems as a whole considering a factor that weighs the level of relative importance for each subsystem. embodied energy (ee) and life cycle global warming potential (gwp) [16] are calculated according to en 15978:2011 [17], to assess the impact of the building on climate change. an important feature of this methodology is that it was designed to be compatible with the eu level(s) framework [18]. 3.2. conceptual process map for bim-based circularity assessment a bim model requires as a first step to “take a picture” of the existing situation by modelling it in the bim software, with the minimum level of detail (lod) 200 [19], as the geometry is already known. in this phase, an assembly code should be assigned for each construction element, to guarantee the necessary standardization for assessment and common outputs. here, the uniclass 2015 classification system is used. simultaneously, construction properties should also be assigned to bim objects in lod 200. a bac (detailed in the next section) can be performed or, instead, specific information on the construction system and materials may be directly assigned to bim objects, if known. in both scenarios, some predefined parameters need to be characterised for the circularity assessment. they are summarized in the product data template [20], according to figure 1. automated bim-based circularity assessment for refurbishment design makes use of a bim-based plugin, connected to external databases, namely the classification system database (uniclass 2015), the building characterisation database (containing information regarding archetypes and their construction systems and materials) and the circularity database (containing additional data for pdt indicators, per product). its conceptual workflow (excluding interactions with other stakeholders) is represented in figure 2, where the new features for bim-based circularity assessment are in black and the bac inside the red rectangle. after introducing the minimum construction information, for lod 200, analysing, discussing, and validating the existing situation with other stakeholders, the software platform will enable the designer to identify, in the bim model, which elements will be maintained, demolished or created. the interconnections between new elements and elements to be demolished/disassembled (figure 2), which constitute the disassembly index, will also be characterised at this stage, making use of predefined alternatives, through the bac or inserted directly by the user. with this information recorded in the information system associated to the bim model, a circularity assessment is performed making use of a bim-based plugin, and a score for bci is quantified. afterwards, 600 vol. 38/2022 advancing circular economy in the existing building stock . . . figure 1. product data template for circularity assessment. figure 2. bim-based circularity assessment conceptual workflow for refurbishment design. 601 j. b. fernandes, p. c. ferrão, j. d. silvestre et al. acta polytechnica ctu proceedings figure 3. building automated characterisation workflow. new refurbishment options can be simulated, resulting in new circularity assessments, which can be compared with previous ones or an optimized design can be suggested by the plugin. as an example, finding the design optimization to improve insulation of a wall considers the trade-off between the impact of operational energy, embodied energy and embodied carbon, as increasing thickness of conventional insulation material reduces operational energy but increases embodied impact. the use of bio-based materials can also be considered as a possible solution for this dilemma. after choosing the optimized design, which should be validated by all stakeholders, the bim model can be detailed for construction (lod 300) and a new circularity assessment can be performed afterwards, if necessary. 3.3. methodology for the characterisation of existing building stock (bac) an archetype-based methodology is proposed to overcome the lack of information to characterise the existing building’s construction systems [9]. the characterisation of the building stock through building archetypes is illustrated with a case study from lisbon, portugal. the tabula project (typology approach for building stock energy assessment) methodological approach is adopted. archetypes are defined according to the buildings’ physical characteristics [9], making use of a top down approach. the detailed characterisation of building stock will be the first level for the analysis and tool development. different parameters are taken into consideration, such as the main use (residential or non-residential), construction period, size-class, roof type and neighbouring conditions, in a total estimate of 56 residential building archetypes and 28 non-residential building archetypes. an existing building representing one archetype was modelled in a bim software, considering its construction elements and the following shearing layers: skin, structure, service and space plan [15]. the bim model includes the exterior envelope, foundations, bearing structural frame, interior walls, partitions and doors, floors, etc.; fittings and fixed furnishings (sanitary fittings, cupboards, wardrobes, etc.) and services (energy, ventilation, sanitary, lifts, etc.). shared parameters for assembly code and disassembly information are to be created for bim objects [21, 22], to assess circularity and impact on global warming, and also to evaluate the elements’ potential of reversibility, derivation points, critical points that can be changed without demolition of other construction elements, etc. to operationalize the bac, the user answers consecutively to a set of questions (the former criteria used for categorizing archetypes), in a graphic user interface (gui) (figure 3) which leads to archetype identification. once the archetype is identified, the plugin 602 vol. 38/2022 advancing circular economy in the existing building stock . . . figure 4. site map (google maps). figure 5. main façade (google maps). returns the standard corresponding composition (materials and construction systems) for skin (exterior envelope: walls, roofs, doors and windows); structure (foundations, bearing elements, stairs) and space plan (interior walls, partitions, doors, ceilings, floors, sanitary fittings and kitchens). the service layer is not considered, because, it varies from one building to another. the information stored in the building characterisation database is obtained by statistical information combined with on-site inspections, building permits and literature review. 4. testing bac with a case study the selected archetype corresponds to a residential building (r), built between 1919 and 1945 (period 2), multi family (“mf”), with sloped roof (“sl”), continuous (“c”), with masonry with slab structure (“cs”) and non-cement exterior finish (“o”). a building, located at rua capitão leitão, 80-82, in marvila, lisbon (figures 4 and 5), provides a real case study corresponding to this archetype. metric information was obtained, in this case, in building’s construction permit (figure 6). after modelling the building in bim and running bac, the results obtained are presented in figure 7. the connections between construction elements (disassembly information) have already been characterised in background, so that they can be added to the model figure 6. original project (lisbon municipal archive). when designing refurbishment options. figure 8 shows the results of disassembly information. bac returns qualitative results (materials / composition of materials and their thickness) for the characterisation of construction elements, corresponding to bim objects, after determining the corresponding archetype for the existing building. this allows the user to introduce construction information when modelling the existing building in bim. to perform the circularity assessment and evaluate the building’s impact on global warming, the designer needs to add further information to bim objects to be demolished and created: disassembly information (adjacent elements, connection type, connection accessibility, crossing and form containment), if there is any existing reused or recycled content (bac assumes, by default, that there isn’t) and the end of life strategy (to be repaired, reused, refurbished, remanufactured, recycled, not modified or not recoverable), which are also results provided by this method. with this qualitative data together with the quantitative data on all the construction elements composition, and the corresponding circularity and environmental data stored in the external database, all indicators from the product data template can be calculated and bci can be determined. 5. discussion and conclusions this paper contributes with a new methodology that supports the characterisation of existing buildings for the implementation of tailored refurbishment strategies for increased recovery, reuse and recycling of construction materials. this bac methodology is tested in a case study building from lisbon, corresponding to a 1919–1945 archetype, making use of its bim model. regarding its results, bac proves to be a useful tool for architects, engineers and other practitioners in aec industry, by providing a reliable basis for data collection and standardization in the early stages of refurbishment projects, which is essential for the circularity assessment of existing building stock. with the automated bim plugin, designers can obtain immediately circularity indicators per square meter, 603 j. b. fernandes, p. c. ferrão, j. d. silvestre et al. acta polytechnica ctu proceedings figure 7. bac – construction system characterisation. figure 8. bac – disassembly information. which can work as reference values for future design refurbishment options. bac provides standard information for clusters of buildings, but does not replace inspection on site, which should be the primary source for building characterisation. on the other hand, bac provides the characterisation of the construction system “as built”, without later modifications. further steps will be made by the authors to improve data collection and organization for circularity assessment in a bim environment, defining the optimal detail level of information for each construction element and its impact on the circularity assessment. the goal is to develop an automatic tool that integrates circularity within a bim software and defines which further information can be added to national legislation, taking advantage of the use of bim, to create building passports regarding circularity. acknowledgements joana fernandes acknowledges the support provided by the doctoral grant sfrh/bd/151363/2021 financed by the portuguese foundation for science and technology (fct), and with funds from portugal 2020 under mit portugal program. this research is also part the circular ecobim project, funded by eea grants within the 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[2021-10-12]. https://www.scribd.com/document/518736487/ lod-spec-2020-part-i-2020-12-31 [20] p. l. martins, a. a. costa. bim product data templates: desafios e oportunidades. in 2º congresso português de building information modelling, p. 79. 2018. [21] r. santos, a. a. costa, j. d. silvestre, l. pyl. integration of lca and lcc analysis within a bim-based environment. automation in construction 103:127–149, 2019. https://doi.org/10.1016/j.autcon.2019.02.011 [22] i. bertin, r. mesnil, j.-m. jaeger, et al. a bim-based framework and databank for reusing load-bearing structural elements. sustainability 12(8):1–24, 2020. https://doi.org/10.3390/su12083147 605 https://doi.org/10.1016/j.jclepro.2020.121046 https://doi.org/10.1016/j.jobe.2019.01.017 https://doi.org/10.1016/j.resconrec.2019.104507 https://www.arcc-journal.org/index.php/repository/article/view/672 https://www.arcc-journal.org/index.php/repository/article/view/672 https://doi.org/10.1007/s12053-018-9707-8 https://doi.org/10.1016/j.jclepro.2020.121134 https://doi.org/10.1088/1755-1315/225/1/012058 https://doi.org/10.1016/j.jobe.2019.100772 https://doi.org/10.5075/epfl-cisbat2015-901-906 https://doi.org/10.5075/epfl-cisbat2015-901-906 https://doi.org/10.1016/j.resconrec.2020.105120 https://doi.org/10.1680/ener.2008.161.2.87 https://standards.iteh.ai/catalog/standards/cen/62c22cef-5666-4719-91f9-c21cb6aa0ab3/en-15978-2011 https://standards.iteh.ai/catalog/standards/cen/62c22cef-5666-4719-91f9-c21cb6aa0ab3/en-15978-2011 https://standards.iteh.ai/catalog/standards/cen/62c22cef-5666-4719-91f9-c21cb6aa0ab3/en-15978-2011 https://www.scribd.com/document/518736487/lod-spec-2020-part-i-2020-12-31 https://www.scribd.com/document/518736487/lod-spec-2020-part-i-2020-12-31 https://doi.org/10.1016/j.autcon.2019.02.011 https://doi.org/10.3390/su12083147 acta polytechnica ctu proceedings 38:599–605, 2022 1 introduction 2 background and major gaps 3 methodological developments 3.1 circularity indicators 3.2 conceptual process map for bim-based circularity assessment 3.3 methodology for the characterisation of existing building stock (bac) 4 testing bac with a case study 5 discussion and conclusions acknowledgements references my title https://doi.org/10.14311/app.2022.36.0175 acta polytechnica ctu proceedings 36:175–184, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague statistical characterisation of reinforcement properties for textile-reinforced concrete: a novel approach marcus rickera, sergej rempelb, tânia feiria,∗, jan schulze-ardeyc, josef heggerc a hochschule biberach university of applied sciences, institute of structural engineering, karlstraße 11, 88400 biberach, germany b hochschule augsburg university of applied sciences, faculty of architecture and civil engineering, an d. hochschule 1, 86161 augsburg, germany c rwth aachen university, institute of structural concrete, mies-van-der-rohe-str. 1, 52074 aachen, germany ∗ corresponding author: feiri@hochschule-bc.de abstract. the potential of textile-reinforced concrete is broad: it can be used in new structures and in the retrofitting of existing structural components. designing textile-reinforced concrete requires knowledge about the mechanical properties of different textile types. to this, a standardised tensile test for fibre strands was used. the test aims to statistically characterise two material properties needed in design: ultimate tensile strength and the modulus of elasticity. to this, the influence of length and number of fibre strands were evaluated. the results show that the ultimate tensile strength can be statistically modelled by a gumbel distribution and the modulus of elasticity can be characterised by a normal distribution. these findings can be used to derive appropriate partial safety factors for the design value of tensile strength using probabilistic methods, or to directly determine the failure probability of textile-reinforced concrete components. keywords: ar-glass reinforcement, carbon concrete, carbon reinforcement, design provisions, standardised tensile test for fibre strands, textile-reinforced concrete. 1. introduction textile-reinforced concrete (trc) is a composite construction material that combines the use of a matrix of fine-grained concrete and mesh-like fibre reinforcements made of alkali-resistant (ar) glass, polymeric, carbon or basalt, among others. as it has been largely demonstrated by the scientific community and practitioners, the construction sector has been showing a growing interested is the use of trc in structures. this is mostly due to favourable mechanical properties of trc, namely the high tensile strength and durability [1–9]. in fact, the range of potential civil engineering applications is not exclusive to new structures, as the carbon concrete bridge in ebingen (germany) [10]; trc is also a prime alternative to retrofit and rehabilitate reinforced concrete structures. yet, the acceptance and utilisation of trc structural solutions depend on the availability of clear design guidelines, installation procedures and construction specifications. to overcome the lack of clear design guidelines, normally, building authorities request proofs of usability [13] by means of individual approvals (e.g., a "zie" in germany) or even general permits (e.g., european technical assessments). consequently, load-bearing tests are needed to evaluate the ultimate and the serviceability limit states, which can be complex, costly and also slow [3, 12, 14]. thus, there is little doubt that alternative design approaches that do not depend on exhaustive experimental campaigns would be valuable to the structural design community. previous investigations have showed that as opposed to steel reinforcement, ar-glass or carbon reinforcement has a linear-elastic behaviour without a pronounced yield plateau and such reinforcement can have three to seven times higher ultimate tensile strengths [3, 12]. these properties have motivated hinzen [11] to propose a standardised tensile test for fibre strands. this standardised tensile test can support the derivation of design values of textile reinforcement (e.g., epoxy resin-soaked ar-glass reinforcement) and has the benefit to consider the impact of the weaving structure on the material parameters of fibre strands, namely damages and distortions during weaving. this means that the material properties of individual fibres are not necessarily needed for the reinforcement design [15]. in the context of this investigation, two relevant textile reinforcement properties were considered – (1) ultimate tensile strength and (2) modulus of elasticity – whose statistical parameters can be determined through the standard tensile test. the statistical characterisation of these properties is vital for the assessment of failure probabilities of textilereinforced concrete members and/or for the calculation of partial safety factors. as numerous scientific 175 https://doi.org/10.14311/app.2022.36.0175 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en m. ricker, s. rempel, t. feiri et al. acta polytechnica ctu proceedings f 16 0 10 0 strain gauge pressure jaws fibre strand jaws [mm] f fibre strand figure 1. (a) standardised tensile test [11]. (b) testing grid of ar-glass reinforcement [12]. studies demonstrate (e.g., [16–18]), probability-based concepts are used during safety level evaluations of structural components. thus, there is little doubt that a probabilistic-based reasoning is essential to derive new design provisions and/or to improve existing ones. in this investigation, the results of a standardised tensile test on an epoxy resin-soaked ar-glass textile are adopted for the calculation of statistical parameters of ultimate tensile strength and modulus of elasticity. nonetheless, a similar approach can be adopted for all epoxy resin-soaked fibre strands. for the sake of this investigation, it should be made clear that multiple fibres form a filament and multiple filaments compose a strand [19]. finally, it is also relevant to mention that the results of the experimental campaign presented in this investigation were partially discussed in another publication [20]. 2. description of the standardised tensile test 2.1. characterisation of the test setup the standardised tensile test proposed by hinzen [11] was used to determine the behaviour of a textile reinforcement (e.g., ar-glass textiles soaked with epoxy resin). individual fibre strands with the lengths of 60 mm, 160 mm, 320 mm, and 640 mm were cut out of the soaked and cured textile. these were used to investigate the influence of the fibre strand length. further, a tension load was directly applied on a reinforced concrete body through pressure jaws to guarantee that the strands were evenly loaded. the strain was registered with two clamp-on strain transducers over a length of 100 mm (figure 1). the strain was also recorded with linear variable differential transformers (lvdts) over a reference length of 450 mm. this experimental setup followed the rilem recommendations [21]. seven tensile test series were conducted on composite members reinforced with different number of fibre strands. 2.2. characterisation of the material parameters the results of the standardised tensile tests are shown in the stress-strain diagram in figure 2a. an idealised stress-strain relationship is derived from the measurements, which can be later used for the cross-sectional design of a component. the textile stress σt is calculated from the measured force f and the accumulated fibre strands cross sectional area ar (equations 1). by using the results of the standardised tensile test, the material behaviour of the fibre strands with a linear-elastic approach can be determined with equations 1 and 2. the parameters are: (i) the mean value of the modulus of elasticity (or young’s modulus) etm, (ii) the ultimate tensile strength ft,u, and (iii) the ultimate strain εt,u. in principle, only two of the parameters are required for the characterisation of the textile reinforcement: σt = f ar = εt · etm ≤ ft,u (1) εt = σt etm ≤ εt,u (2) by assuming a linear-elastic behaviour, the textile stress value of each strain (equation 1) and the strain value of each stress (equations 2) can be determined for each point of the stress-strain diagram by using the mean value of the modulus of elasticity. the relationship between these parameters is illustrated in figure 2b. equations 2a and 2b guide the design approach represented in figure 2a. 2.3. results of the experimental campaign in this experimental campaign, more than 400 standardised tensile tests were conducted to describe the 176 vol. 36/2022 statistical characterisation of textile-reinforcement properties t ftd tmtk td t t etm value 5 %-fractile mean value design value ftk ftm tdesign value 5%-fractile mean value 0 400 800 1200 1600 0,0 0,5 1,0 1,5 2,0 strain εt (%) te ns ile s tre ng th (n m m −2 ) single test value mean test result design approach b)a) figure 2. stress-strain diagrams [12](a) ar-glass reinforcement. (b) design of textile-reinforced components. 0 10 20 30 40 50 60 70 80 1200 1400 1600 1800 ultimate tensile strength (n mm−2) ab so lu te fr eq ue nc y (− ) empirical normal dist. 0.000 0.001 0.002 0.003 1200 1400 1600 1800 ultimate tensile strength (n mm−2) fr eq ue nc y de ns ity h (x ) ( −) empirical normal dist. a) b) figure 3. ultimate tensile strength [12]: (a) histogram for the ar-glass reinforcement (empirical and theoretical values). (b) probability density function of the ar-glass reinforcement distribution functions of the material parameters. the test results were used to evaluate the normality assumption, since previous studies assumed that ultimate tensile strengths follow a normal distribution [22], the measured ultimate tensile strengths were divided into 13 classes (each with a width of 57 n mm−2) and compiled in a histogram (figure 3a). the values were then converted into a frequency density h(x) by generating the ratio of the relative frequency to the class width. through the mean values of each class, a curve of the frequency density was obtained as it is illustrated in figure 3b. note that the shape of this curve seems to mirror a normal distribution. the expected value was approximated by the arithmetic mean value µx ≈ x̄x = 1 590 n mm−2 and the standard deviation was estimated by the empirical standard deviation σx ≈ s̄x = 138 n mm−2. these values were considered in the probability density function of a normal distribution (see equation 3) [23]. f(x) = 1 138 · √ 2 π exp ( − (x − 1 590 2 · (138)2 ) (3) to evaluate the data normality, a goodness-of-fit test shall be adopted due to the fact that a single analysis of the graphical plot is not sufficient to confirm that the ultimate tensile strength follows a normal distribution to this, a chi-square test was applied, which did not reject normality (i.e., the p-value of 0.70 is above the significance level of 0.05); the arithmetic mean value and the empirical standard deviation were used to approximate the normal distribution. 177 m. ricker, s. rempel, t. feiri et al. acta polytechnica ctu proceedings 0 500 1000 1500 2000 0 100 200 300 400 500 600 700 length of fibre strand (mm) u lti m at e te ns ile s tr en gt h (n m m −2 ) experimental mean value experimental 5%−fractile theoretical mean value theoretical 5%−fractile figure 4. ultimate tensile strength of the textiles depending on fibre strand length. 3. investigation of the influence of fibre strand length 3.1. ultimate tensile strength: experimental investigations the influence of the fibre strand length on the ultimate tensile strength was investigated for the four lengths of the ar-glass textile placed in the weft direction (i.e., axial direction of the weft strands). at least seven samples were used to test each fibre length. a normal distribution was assumed to analyse the results. figure 4 shows the influence of the fibre strand length strand length on the ultimate tensile strength. the mean value decreases with a growing fibre strand length in a non-linear fashion. it is also visible that the mean ultimate tensile strength ranges between 1 709 n mm−2 (length = 60 mm) and 1 257 n mm−2 (length = 640 mm). the scale effect, which was previously investigated by griffith [24], can explain such differences, owing to the fact that number of imperfections rises with a growing length of the strand. more recently, bažant zp (e.g., [25–27]) carried on extensive studies on the size effects. also chudoba [28] and rypl [29] concluded that the standard deviation is reduced when the strands have an increased length. 3.2. modulus of elasticity: experimental investigation for the modulus of elasticity, the influence of the strand length was investigated for the four lengths of the ar-glass textile placed in the weft direction (i.e., axial direction of the weft strands). also here, at least seven samples were used to test each fibre length. likewise, a normal distribution assumption was considered for the modulus of elasticity. similarly to the ultimate tensile strength, the frequency density curve obtained mirrors a probability density function of a normal distribution. in this curve, the expected value was approximated by the arithmetic mean value µx ≈ x̄x = 74 618 n mm−2 and the standard deviation was determined by the empirical standard deviation σx ≈ s̄x = 1 610 n mm−2. in the distribution fitting analysis, the chi-square test did not reject the normality assumption (i.e., p-value of 0.07 is above the significance level of 0.05). these values seem to confirm that the modulus of elasticity of fibre strands soaked with epoxy resin can be characterised by a normal distribution function indicating that the statistical parameters can be characterised with the arithmetic mean value and the empirical standard deviation. the entire set of results are available in [14] and in [20]. 3.3. ultimate tensile strength: theoretical investigations this section addresses the estimation of the statistical parameters for any number of strands, n, using the parameters determined from the experimental tests and extreme value theory. to this analysis, it was considered that the strands are linked in series. note that in a series system, the weakest link governs the failure. furthermore, it was considered that a normally distributed random variable x describes the ultimate tensile strength of each strand. the calculation of the expected value and the standard deviation of the ultimate tensile strength of a single fibre strand can be conducted with the support of extreme value theory. this theory also supports the distribution function of multiple fibre strands connected in series. thus, the distribution of the minimum ultimate tensile strength (i.e., governing the series system) – the minimum mn – can be also determined with the extreme value theory. the distribution function of the minimum fmn(x) expressed by equation 4 [30] can be applied to any number of fibre strands n. the results of the standardised tensile tests on fibre strands with a length of 160 mm support the derivation of the cumulative distribution function fx(x) of the ultimate tensile strength. p (mn ≤ x) = fmn (x) = 1 − [1 − fx(x)]n (4) equation 4 is only valid for independent and identically distributed random variables with a cumulative distribution function fx(x) [23]. all the strands linked in series have the same distribution function. by derivating equation 4, the probability density function fmn (x) of the minimum ultimate tensile strength mn can be calculated. equation 5) allows to determine the probability densities of the extreme value distributions for different lengths. fmn (x) = fx(x) · n · [1 − fx(x)]n−1 (5) by rearranging equation 4, the fractiles of the extreme value distribution can be determined: 178 vol. 36/2022 statistical characterisation of textile-reinforcement properties 0.000 0.002 0.004 0.006 1000 1100 1200 1300 1400 1500 ultimate tensile strength (n mm −2 ) f( x ) (− ) n = 100 n = 25 gumbel dist. normal dist. 0.0000 0.0005 0.0010 0.0015 1000 1050 1100 1150 1200 ultimate tensile strength (n mm −2 ) f( x ) (− ) n = 25 gumbel dist. normal dist. figure 5. probability density function of the extreme-value function and approximation with normal and gumbel distribution functions [20]: (a) entire distribution and (b) selected area of the distribution at the tails. fmn(xp) = 1 − [1 − fx(xp)]n = p (6) fx(xp) = 1 − n √ 1 − p (7) equation 8 can be used to determine the fractile values of the extreme value distribution if the tensile strength of each link is to be normally distributed: xp = f −1 x (xp) = µx + σx · φ−1(1 − n √ 1 − p) (8) through equation 5) it is visible that, as the fibre length increases, the expected value and the standard deviation of the extreme value distribution decrease. since the density does not present the characteristics of a normal distribution, a gumbel distribution (i.e., generalised extreme value distribution, type-i) [30] was assumed. this distribution can be easily considered in the calculations of reliability analysis when evaluating of the safety level of structural components or systems. a gumbel distribution is typically characterised by two parameters: a and u and the probability density function (for data minimum) (see equation 9): f(x) = a · ea·(x−u)−ea.(x−u) (9) the ultimate tensile strengths of the 50%-fractile (median) and the 5%-fractile of the extreme value distribution are calculated to approximate the extreme value distribution by a gumbel distribution through equation 5. then, by using a gumbel distribution, these fractiles are assumed for the 50%-fractile and the 5%-fractile respectively. consequently, the parameters a and u of the gumbel distribution can be calculated. for two different theoretical values of fibre strands n (n = 25 and n = 100), the probability density functions of the extreme value distribution were determined values of extr. value normal gumbel x dist. dist. dist. 1 194 0.0011180 0.0012952 0.0009687 1 155 0.0004917 0.0005132 0.0004586 1 128 0.0002604 0.0002322 0.0002688 1 046 0.0000302 0.0000107 0.0000534 974 0.0000034 0.0000003 0.0000128 the values of x correspond to the 5%, 2%, 1%, 0.1%, and 0.01% values of the original extreme value distribution. table 1. extreme value distribution approximated by a normal distribution and a gumbel distribution for n = 25 fibre strands [20]. (figure 5a). it is perceived that the activation of more than 100 fibre strands under a load is highly unlikely. both distributions – the normal and the gumbel – were used to determine the probability density function for each n. it is widely acknowledged that the behaviour of the distributions at the tails of the functions is of major importance (figure 5b). by observing the results for n = 25, it is visible that an approximation by a normal distribution sits slightly below the curve of the extreme value distribution. table 1 shows that for fractile values smaller than 2%, the gumbel distribution is somewhat above the extreme value distribution, whereas the normal distribution presents lower values. figure 5b shows that the normal distribution curve changes its course to below the extreme value distribution curve at an ultimate tensile strength (i.e., roughly below 1 150 n mm−2). note that normal distributions are characterised by thinner tails than extreme value distributions tails. based on these results, it can be argued that a normal distribution can generate underestimated failure probabilities, which can seriously affect the robustness of reliability analyses. contrary 179 m. ricker, s. rempel, t. feiri et al. acta polytechnica ctu proceedings 0 400 800 1200 1600 0.0 0.5 1.0 1.5 2.0 2.5 strain εt (%) te ns ile s tr en gt h (n m m −2 ) fs tension single test value mean test result 0 400 800 1200 1600 0,0 0,5 1,0 1,5 2,0 2,5 strain εt (%) te ns ile s tr en gt h (n m m −2 ) fs tension single test value mean test result figure 6. tension-strain diagram [12]: a) two and b) eight embedded fibre strands. to this, a gumbel distribution seems to be more on the safe side for the assessment of very low failure probabilities. it is widely recognised that in structural design, the 5%-fractile is a governing value [31, 32], which is also used as the characteristic tensile strength of the textile reinforcement f t,k. the design value of the tensile strength f t,d is calculated by dividing the characteristic value f t,k by the partial safety factor γt. by assuming a partial safety factor γt=1.0, the characteristic value would be equal to the design value. 4. investigation of the influence of fibre strand number 4.1. ultimate tensile strength: experimental investigations in this section, the influence of the number of fibre strands on the ultimate tensile strength is investigated. to this, uniaxial tensile tests on composite members (i.e., textile embedded in the concrete matrix ) were used. the results of 40 tests (i.e., eight series with five tests each, beginning with one fibre strand and ending with eight) were considered. the fibre strand tension (i.e., fs tension) of the strands (i.e., tension at the strand without concrete) are represented in figures 6a and 6b alongside the mean and the single test results. equation 1 was used to determine the textile tension σt by means of the measured force f and the accumulated filament cross sectional area ar. figures 6 and 6b show that a textile failure always occurs in the tensile tests of the composite members. the black curve shows the mean course of the individual experiments and the grey curve shows the results of the individual experiments. in figure 6a it is also visible that three cracking states: state i (uncracked), state iia (crack formation) and state iib (stabilised crack phase). in state iib, the curve does not flatten, but runs parallel to the results of the standardised tensile test on the plain fibre strand, which is illustrated as dashed lines. in both tests, the same modulus of elasticity for the textile is achieved in state iib. this behaviour leads to believe that the results of the test setup can be used to assess the influence of the number of fibre strands. additionally, the number of fibre strands do not seem to affect the modulus of elasticity. 4.2. ultimate tensile strength: theoretical investigations a mathematical relationship for any number of fibre strands can be determined by assuming that fibre strands with the length of 160 mm are theoretically and successively connected next to one another. here, each element follows a normal distribution, which was determined with the standardised tensile test for a single strand. note that here, a brittle failure occurs as soon as the end of the linear-elastic range is reached as opposed to steel that follows a ductile failure behaviour. each fibre strand in the system is loaded with the same load during the testing procedures. yet, the strands have distinct ultimate tensile strengths as a result of the material variation. when the ultimate tensile strength of the weakest element is reached, it suddenly fails, and the force is absorbed by the remaining elements. a redistribution can only take place if the remaining fibre strands have sufficient residual load-bearing capacity, which is only possible with a high number of fibre strands, or a large variation of the ultimate tensile strength. considering a system with n identical fibre strands, which ultimate tensile strengths x i follow a cumulative distribution function fx(x), the ultimate tensile strength r can be described as [33]: r = max(n · x̂1, (n − 1) · x̂2, ..., x̂n) (10) with x̂1, ..., x̂n being the ultimate tensile strength of the individual strand sorted in ascending order by 180 vol. 36/2022 statistical characterisation of textile-reinforcement properties 600 800 1000 1200 1400 1600 0 20 40 60 80 100 number of fibre strand (−) u lti m at e te ns ile s tr en gt h (n m m −2 ) theoretical mean value theoretical 5%−fractile mean value (simulation) 5%−fractile (simulation) figure 7. influence of the fibre strand number on the ultimate tensile strength [20]. no. fibre expected value (i.e., mean value) 5%-fractile value strands n mm−2 n mm−2 simulation gumbel dist. diff. (%) simulation gumbel dist. diff. (%) 5 1 435 1 434 -0.07 1 281 1 270 -0.87 10 1 404 1 383 -1.55 1 282 1 236 -3.71 25 1 367 1 325 -3.21 1 272 1 194 -6.57 50 1 339 1 286 -4.13 1 258 1 165 -7.96 75 1 323 1 265 -4.55 1 247 1 148 -8.63 100 1 311 1 250 -4.90 1 241 1 137 -9.10 table 2. ultimate tensile strength: differences between simulated and theoretical values [20]. size. it can be argued that a safe approximation can be made by assuming that the weakest link governs the failure mechanism. thus, a parallel connection can be compared to the behaviour of a series connection due to the nearly ideal brittle behaviour of the components. consequently, equation 4 can be used to determine the cumulative distribution function of the minima fmn(x). 4.3. trade-off between experimental and theoretical investigations in this section, the experimental and theoretical investigations are compared. to this, a chain system of fibre strands was considered. a gumbel distribution was assumed to calculate the theoretical mean value and the characteristic value of the ultimate tensile strengths (5%-fractile). these values were used to characterise the ultimate tensile strength, where the mean value is µx ≈ x̄x = 1 590 n mm−2 and the empirical standard deviation is σx ≈ s̄x = 138 n mm−2. simultaneously, 50 000 simulations were performed in the statistical software r [34] by using the principles of crude monte-carlo. here, it was considered that when the weakest fibre strand fails and the stresses are redistributed to the remaining fibre strands of the system. additionally, a theoretical expected value was determined by means of equation 7 (see figure 7 and table 2). the results of the simulation seem to indicate that as the number of fibre strands rises, the average ultimate tensile strength decreases. at some point, the curves tend to flatten. consequently, the standard deviation and the coefficient of variation also decrease with an increasing number of fibre strands. the extreme value distribution approximated by a gumbel distribution loses expression (i.e., decreases at a very slow pace) for a growing number of fibre strands. the results in table table 2 indicate that the differences between the simulated values and the mathematical approximation through a gumbel distribution can go up to around 9%. a possible explanation is linked to the fact that the gumbel distribution does not consider a redistribution of stresses after the failure of the first fibre strand. thus, a gumbel approximation seems to be on the safe side. 4.4. practical implications as described in [17, 35–37], the design value of the tensile strength f td is the basis for the structural calculations with bending and shear load. yet, a conversion must be made to enable the use of the standardised test results in general structural applications. 181 m. ricker, s. rempel, t. feiri et al. acta polytechnica ctu proceedings 0 10 20 30 0 5 10 15 20 25 30 35 40 45 0 1000 2000 3000 4000 5000 number of fibre strands (−) length of fibre strand (mm) d iff er en ce ∆ σ t ( n m m −2 ) figure 8. differences between the n and n-1 fibre strands (5%-fractile values) [12]. this can be achieved by using the 5%-fractile values; note that the characteristic tensile strength of the reinforcement f tk form the basis for the design values of the tensile strength f td. since the 5%-fractile needs to be used for the design, the problem is not as pronounced, as it can be seen in figure 8, where the different lengths and numbers of fibre strands are illustrated. the difference is generated from the 5%fractile values of the ultimate tensile strength of n and (n − 1) fibre strands. for a small number of fibre strands, the 5%-fractile of the tensile strength is influenced by the number of fibre strands (figure 8). from around five strands, the curve tends to rapidly flatten and the difference between the characteristic values becomes gradually smaller. the gradient is almost constant from a length of 1 600 mm (i.e., around ten strands). in the case of the analysed ar-glass, the value is 4.5 n mm−2, which corresponds to just 2.8h of the mean ultimate tensile strength. thus, a reasonable number of fibre strands is suggested for the calculation of the characteristic value. this corresponds to the area where the curve slope of the 5%-fractile becomes almost constant. yet, a specific number of strands varies with the practical problem. the standardised tensile test needs to be carried out on an individual fibre strand, and then, the ultimate tensile strength must be adjusted by using the extreme value theory. with this approximation, the mean value f tm, the characteristic value f tk , and finally, the design value f td can be determined. the design strain εtd is also required for the design model. it is sufficient to measure the textile tension and divide it by the modulus of elasticity. the tests showed that the modulus of elasticity is not influenced by the number of fibre strands. the mean value from the standardised test on a single fibre strand can be used as an appropriate modulus of elasticity. 5. conclusion in this paper it was demonstrated that the results of a standardised tensile test can be used to derive the statistical values of relevant textile reinforcement properties. this is particularly relevant for the design of components with textile reinforcement impregnated with epoxy resin. by using a reference strand length of 160 mm in the standardised tensile test, only the measurements of the ultimate tensile strength and the modulus of elasticity of a fibre strand are needed. the test results showed that a fibre strand has a linear-elastic behaviour until it fails when subjected to tensile stress. the length and number of fibre strands seem to influence the ultimate tensile strength. the expected value and the scatter of the ultimate tensile strength decrease non-linearly with a growing length and number of fibre strands. yet, once a certain fibre length and number is exceeded, the characteristic ultimate tensile strength are no longer affected. in this investigation, it was demonstrated that the statistical values can be determined for any length and number of strands by using the extreme value theory. in this context, calculations are simplified because an extreme value distribution can be approximated by a gumbel distribution. for the modulus of elasticity, the use of a normal distribution is recommended. acknowledgements the authors thank the two companies: solidian gmbh and fta-forschungsgesellschaft für textiltechnik albstadt gmbh for their support in carrying out the fibre strand tensile tests and providing the textile reinforcements. references [1] s. rempel, n. will, j. hegger, p. beul. filigrane bauwerke aus textilbeton: leistungsfähigkeit und anwendungspotenzial des innovativen verbundwerkstoffs. beton-und stahlbetonbau 110(s1):83– 93, 2015. https://doi.org/10.1002/best.201400111. 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[37] j. bielak, j. hegger, m. schmidt. shear capacity of carbon fibre textile reinforced concrete slabs with planar and c-shaped shear reinforcement. proceedings of the fiber reinforced polymer reinforced concrete structures 14, 2019. 184 https://doi.org/10.1002/best.201900086 https://doi.org/10.3390/app9071382 acta polytechnica ctu proceedings 36:175–184, 2022 1 introduction 2 description of the standardised tensile test 2.1 characterisation of the test setup 2.2 characterisation of the material parameters 2.3 results of the experimental campaign 3 investigation of the influence of fibre strand length 3.1 ultimate tensile strength: experimental investigations 3.2 modulus of elasticity: experimental investigation 3.3 ultimate tensile strength: theoretical investigations 4 investigation of the influence of fibre strand number 4.1 ultimate tensile strength: experimental investigations 4.2 ultimate tensile strength: theoretical investigations 4.3 trade-off between experimental and theoretical investigations 4.4 practical implications 5 conclusion acknowledgements references https://doi.org/10.14311/app.2022.33.0617 acta polytechnica ctu proceedings 33:617–623, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague breakthrough of the resemblances and correspondences between resilience and sustainability in civil infrastructures oscar javier urbina∗, elisabete teixeira, hélder sousa, josé matos university of minho, isise, department of civil engineering, guimars̃es, portugal ∗ corresponding author: oscarj105@gmail.com abstract. sustainable construction has become a growing trend among researchers and stakeholders. simultaneously, resilience and risk assessments for civil infrastructures have flourished in terms of importance among researchers, economic sectors, and society. nevertheless, there is no abundant research that correspond to both approaches, despite that, there are massive similarities and shared characteristics between both investigation branches. distinctively, this year has demonstrated that sustainable development is directly obstructed by different extreme events that trigger risks and vulnerabilities in civil infrastructures. these extreme events require a deep and complex study to minimize the impacts they may cause in society and economy, two main factors considered in the study of sustainability. therefore, when a risk and resilience assessments are conducted, it is already analyzed as a sizable part of sustainability. consequently, there exists a possibility to create a methodology that examines and assesses four categories of civil infrastructure sustainability: technical, environmental, social, and economical. the aim of this paper is to demonstrate the need of a comprehensive approach between sustainability, risk, and resilience assessment, compiling and comparing the existing methodologies for assessing the impacts on civil infrastructures, showing that both present resemblances and none can be omitted, being necessary for the decision-making. keywords: civil critical infrastructures, extreme events, resilience assessment, risk assessment, sustainable construction. 1. introduction world population has increased dramatically in the last decades, with a difference of 6 billion people between only one century. this exponential increment of population results on an excessive use of primary resources in order to fulfill their needs, being this one of the explanations towards the climate change problem we are currently facing [1, 2]. as a result of this worldwide problem, in the late 70s, reports began to appear concluding that if the population growth and its consumption of resources are not controlled, the world would not withstand the pressure on its resources, giving birth to the term of "sustainable development" [3]. nowadays sustainability is a common concept in research that has been the focus of multiple studies, these investigations usually determine the development of humanity in the interconnection of three dimensions, the economic dimension, which is the most widespread over time, the environmental dimension, and the social dimension [3, 4]. also, it is clearer now that the sustainable development is occasionally obstructed by new risks and vulnerabilities, such as terrorist attacks or diseases, as we are currently facing the new sars-cov-2 [5]. as emphasized in the united nations agenda 21, there are some risks, designated as systemic risks, that are arising actively (e.g. diseases, terrorism, natural disasters due to climate change), facing them in a neverending challenge loop [5]. today, critical infrastructure systems face an increased number of hazards, such as: natural (earthquakes, floods, fires), technological (operational failures in systems), and human (fires, cyber-attacks, or terrorism), that can intervene in the functionality of these systems. the malfunction of these systems can cause a cascading effects through the community that produce social, economic, and functional disruption [6, 7]. therefore, there is the need to identify the new upcoming risks that may affect infrastructures, so at the time needed, governments address hazards within a comprehensive management and understanding of the complexity of civil infrastructures. risk management models and resilience assessments are the best solution to assess and find solutions for planning, mitigation, and recovery for civil infrastructures under extreme events, where the consequences of damage and losses are quantified for decision making [7]. in this context, risk and resilience analysis play an important role, as it provides information that help decision makers to develop risk mitigation plans and strategies, taking into account that the economic development of a country strongly depends on its level of infrastructures [8]. in consequence, these concepts should be brought into a comprehensive approach that leads towards a sustainable development 617 https://doi.org/10.14311/app.2022.33.0617 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en o. j. urbina, e. teixeira, h. sousa, j. matos acta polytechnica ctu proceedings of infrastructures [5]. the aim of this paper is to demonstrate the need of a comprehensive approach between sustainability and risk and resilience assessment, compiling and comparing the existing methodologies for assessing the impacts on civil infrastructures, showing that both present resemblances and none can be omitted, being necessary for the decision-making. thus, this paper seeks to establish the theorical foundation for a future approach that combines and evaluates the concepts of resilience, risk, and sustainability of civil infrastructures simultaneously using quantitative and qualitative methods. as this is of high importance for stakeholders, authorities, and industries in the decision making for civil infrastructures, under any hazard and the impacts that it would incur in social, economic, environmental and functionality terms. this paper is mainly developed in 5 chapters. the first chapter consists on a brief introduction, which includes a general framing of the topics, the definition of the objectives of the work and a presentation of the structure of the article. in the second chapter, it is introduced the concept of sustainable development, as it is necessary to understand the context in which it emerged, as well as its historical evolution, then a short display of the most known methods at international level for sustainability in construction are presented, showing their characteristics and some examples of the indicator measures are introduced. in the third chapter it is addressed the beginning of risk and resilience analysis, recalling on some of the worst recent events that impacted infrastructures and society, that at the time, pointed out the necessity of this studies to be born, then a brief explanation is given upon this topic. on the fourth chapter is approached the main topic of this paper, that is the convergence between sustainability and risk and resilience concepts in civil infrastructures, where is presented the current situation of research within these two areas, through a brief state of the art focused towards civil infrastructures, emphasizing on the necessity to fill this research gap and stating some key points that the creation of a methodology that addresses sustainability, risk and resilience for civil infrastructures must have. the last chapter are the conclusions, where the authors summarize the key points obtained from the research. 2. sustainable construction 2.1. history, definition, and application climate change has been increasing in recent years, being the result of the existence and the multiple activities conceived by humanity. among these activities, specifically, there is one of those that produce the greatest impact, which is the construction industry. this industry currently uses about 40% of the fossil fuels, 30% of the raw materials and 25% of the water consumed annually in the world. additionally, it consumes about 30% of electricity and generates 30% of greenhouse gas emissions [4], [9]. however, this industry has the potential to generate large reductions with small changes in the way buildings are designed and used, which could achieve savings of up to 30% in energy consumption, a 35% reduction in co2 emissions, and a reduction in water consumption of up to 50% [3, 4, 9]. this brought the attention to the international community to focus on the construction sector and its impacts began to emerge in the early 1970’s when the first united nations conference on environment and development was held in stockholm, giving rise to the united nations environment program, with the aim to promote the appropriate use and sustainable development [3, 4]. then, in 1988, in the report of the world commission on environment and development named "our common future", defined sustainability as "the attempt to meet the needs of the present without compromising the ability of future generations to meet their own needs". thus, it was only by the year 1992 that international principles for sustainable development were established, during the united nations conference on environment and development in rio de janeiro, where agenda 21 was elaborated, a document that systematized a plan of oriented actions creating the minimum necessary conditions for new constructions, both for developed and undeveloped countries [3, 10]. this rio conference has been continued with the versions of rio+10 in 2002 in johannesburg and rio+20 in 2012 in rio de janeiro, to reaffirm the rio-92 goals and to include clean energy and corporate responsibility in the debate and also to focus on the green economy, being capable of generating jobs with low impact on the environment and efficient use of natural resources [3, 11]. after these world conferences, the need to develop methods and tools to study the sustainable performance of buildings emerged, because the countries that were implementing projects with better environmental performance had no ways to verify their improvements, obtaining situations where green buildings consumed more energy than conventional ones. accordingly, it was necessary to develop methodologies to standardize building sustainability in a global way, allowing the analysis and comparison of various solutions to further improve the environmental performance of buildings [3]. 2.2. methodologies to assess sustainability currently there are tools that are not legally required, but they raise awareness and help to promote sustainability in construction entities. countries such as united states, canada, france, japan, among others have already implemented tools for assessing the sustainability of buildings, as a support for project design and at the same time assessing their postoccupancy. 618 vol. 33/2022 resilience and sustainability in civil infrastructures some of the first certifications of the sustainability level of buildings began to emerge through the use of methods such as breeam in the united kingdom, hqe certification (haute qualité enviromentale) in france, leed certification in the united states and gbtool (green building assessment tool) in some of the eu member states and adapted towards each state’s necessities [3, 4][3]. some of these methods are briefly explained down below. 2.2.1. leadership in energy and environmental design (leed) [3] developed by the u.s. green building council in 1994, in the united states of america. in this method, the environmental performance of the building is evaluated holistically, throughout its life cycle, i.e., in the design, construction, operation and maintenance phases. this tool applies to various types of buildings, such as residential, commercial, and school buildings, among others. for a building to be evaluated it must meet a minimum required criterion, a pre-selection with a series of requirements. after this verification, the building becomes eligible, and the stage of analysis and evaluation of its performance begins. this is a points-based system, awarding points for specific criteria in five different categories: sustainable sites; water efficiency; energy and atmosphere; materials and resources; and indoor environmental quality. these categories in total add up to 100 points, but there are two additional categories as a bonus, the innovation in design category, with 6 points, and the regional priority with 4 points. the points of the evaluation can only be assigned if the building complies with the requirements of the system; in the end, with the sum of these points, a classification can be assigned to the building, among four possible certifications, basic, silver, gold, and platinum. 2.2.2. building research establishment environmental assessment methodbreeam [3] the breeam method was developed in the early 1990s in the uk by researchers. the assessment with this tool considers the following sustainability categories: water, energy, materials, health and wellbeing, management, transportation, waste, contamination, and land use; each performance is verified by comparing them with pre-established benchmarks, thus obtaining the building’s assessment. breeam can be used for virtually any type of building, such as offices, industrial plants, residential buildings, and hospitals. thanks to the multiple existing versions of this tool, each one specifically developed to suit the building under assessment. in each of the categories mentioned above, requirements are defined, giving credits to the building, which are added as the building complies with them. on the other hand, in each category, specific weights are fixed, indicating their relevance, and depending on the type of building being evaluated. in this way, the set of credits and weights of the categories conform an environmental performance index for the building, which can obtain a value between zero and 100 for the environmental performance index (epi). according to the obtained epi, six levels of certification are attributed, "excellent", "great", "very good", "good", "approved", and "no classification", each one depending on the total points obtained in the environmental performance index. 2.2.3. sustainable building tool portugal sbtool pt [3, 4, 11] . this methodology follows four steps: (i) quantification of building performance at the level of each indicator; (ii) parameter normalization; (iii) parameter aggregation; (iv) sustainability score calculation and overall assessment. the overall rating of the building depends on the weighting of the different criteria, considering benchmarking practices reference practices that are set at the national level (portuguese level). this weighting is done by comparing the performance of the building with national reference practices: the best practice, which has a value of 1.0, and the conventional practice, with a value of 0.0. the value of this normalization is within a range between -0.2 and 1.2. subsequently, with the performance of each indicator and its corresponding weight, the performance of the category to which it belongs is calculated. finally, with the performances of each category and their respective weights, the results for each dimension are calculated, thus obtaining in the end an overall sustainability level for the building chosen for the case study. 3. risk and resilience analysis in critical infrastructures the growing number of catastrophic events and their implications, such as 9/11 in the united states, or the terrorist events in madrid on march 11, 2005, have prompted europe and other nations around the world to take steps to prevent these events from leading to high consequences that can be reduced or prevented. in june 2004, the european council called for a strategy to protect critical infrastructure, which had a response on october 20, 2004, in a statement in which the commission described the actions being taken to protect critical infrastructure and proposed additional measures to strengthen existing instruments and meet the mandates of the european council [12]. later, on november 17, 2005, the commission adopted a green paper on a european programme for critical infrastructure protection, stating that the main scope of the european programme for critical infrastructure protection (epcip) is the need to increase the capability to protect critical infrastructures (ci) in europe and to help reduce vulnerabilities related to ci [13]. 619 o. j. urbina, e. teixeira, h. sousa, j. matos acta polytechnica ctu proceedings the first step in protecting ci involves identifying and assessing the factors that may negatively influence its operations, defining a systematic, analytical approach to prioritizing resilience measures for ci. this analysis should include an assessment of the impacts of ci disruption by pre-established criteria. several approaches are used in oecd countries [14]. in terms of criteria, the european commission defines a minimum set for the assessment of critical infrastructure, including public impacts, economic impacts, environmental impacts, interdependence, political impacts, and psychological impacts. identifying weaknesses allows prioritizing where to focus resilience efforts in existing infrastructure systems: on points of failure that would have the most severe consequences. this prioritization can be a decisive variable in decision making, such as which infrastructure should be hardened or relocated, or which ci should receive priority restoration after a disaster to ensure rapid recovery [12, 14]. infrastructures must be designed and built to serve adequately over longer life terms without major deterioration or reaching collapse. with the current technology of materials, analysis, design, and construction, it should be possible to specify a bigger design life in new infrastructure projects. this leads towards a broader perspective which indicates that risk and resilience assessments are part of the general decision support to plan, to design and produce new infrastructures that are economically efficient, reliable, safe, secure, and sustainable [15]. 4. integrated approach for civil infrastructures recent literature has discussed the importance to go beyond the sustainability assessment of single buildings and to enlarge the assessment scale to communities and cities to meet all the different aspects of sustainability [9]. this chapter is divided in two main parts, the first part, where are presented some of the tools used to assess sustainability in infrastructures, and the second part, present diverse studies that integrate risk or resilience assessments with at least one dimension of sustainability’s assessment (environmental, social, or economic). 4.1. sustainability in infrastructures sustainability assessment tools for civil infrastructures are less frequent rather than the existing tools to evaluate buildings, however, some researchers have already progressed in this topic. following are showed some of their studies. • pardo-bosch et. al. [16], presented the multicriteria decision system mives. this decision model is divided into 4 steps. i) identification of the problem; ii) development of the decision tree, a diagram that organizes and structures the concepts that will be evaluated; iii) defining the relative weight of each of the aspects that are to be considered in the decision tree using analytical hierarchical process; and iv) establishment for indicators of value function that in each case reflects the appraisal of the decision-maker [16]. it prioritizes with technical accuracy public infrastructure projects that one administration must finance with only one budget in a developed country, helping to minimize the subjectivity in the entire decision-making process. • rosasco et. al. [8], show a study under a program named "regional strategic intervention program" (p.r.i.s), which objective is to guarantee the social protection of citizens that reside according to the italian law about the expropriation of private real estate for the construction of public work projects. within the framework of this program, the authors developed a mass appraisal estimation model that quantifies the indemnities values through a multi-parameter model of residential and commercial units within the area affected by the public work project. this multi-parameter estimation model uses a survey evaluated on the real estate units that are in the studied area; seven main features were selected, which are: 1. dimension (sqm); 2. age of building; 3. type of building; 4. maintenance state; 5. floor level; 6. lift (or not); 7. accessibility. this estimation model was applied in a new infrastructure project located near the city of genoa, in italy, the results show that the acceptance percentages of the indemnities estimated present a high degree of satisfaction, although the indemnities partially compensate all the inconvenience suffered. the authors conclude that these economic indemnities contribute to the achievement of that social sustainability of the infrastructures, and it can guarantee reasonable transfer alternatives for both residence and economic activities. • jones et. al [17], considered the holistic nature of water infrastructure development in terms of rural areas in developing countries, through an outcomebased assessment method using life-cycle analysis (lca). this method uses the lca framework to supply a holistic set of sustainability indicators, these indicators are grouped into three categories including metrics for technical (performance), environment, and economic (market-based) aspects. all the metrics are employed at community level to indicate the current state of the system. finally, this method should provide the socio-economic impacts generated. the authors applied the method on a generic example of arsenic water-treatment in bangladesh. 4.2. risk-resilience and sustainability in civil infrastructures the evaluation of resilience should not only consider technical but also environmental, organizational, social and economic dimensions [9]. despite this, there 620 vol. 33/2022 resilience and sustainability in civil infrastructures is limited number of previous studies that integrate them with critical infrastructure sustainability. few authors have worked onto developing a comprehensive approach that unites sustainability and riskresilience analysis as it could provide synergies and clearer results about the overall role and needs of civil infrastructures. • markert et. al. [15], presented a method for risk and sustainability analysis of complex hydrogen infrastructures, this model is based on a high level risk assessment, that are complemented with other decision support tools such as gis, lca and life cycle cost (lcc). providing a novel and comprehensive study as it enables the possibility to obtain spatial analysis with the gis system, for both, the risk assessment approach, and the sustainability approach, easing the identification of vulnerable elements and high-risk zones, in addition to the environmental and economic aspects. • bocchini et. al. [18], concluded that sustainability and resilience have a great deal of similarities and common features, as i.e. both integrate structural analysis with social and economic aspects, and both seek to enhance an infrastructure in terms of structural design, used materials, maintenance, management strategies, and impacts on society. therefore, the authors proposed an approach established on a risk assessment, using the concepts of probability of occurrence and risk, to address resilience and sustainability at the same time. despite the differences between sustainability and resilience targets, both converge on seeking to perform a service level to society during and after the occurrence of an extreme event and recover to the optimal functionality at great pace. for this, the proposed approach must be rigorous, quantitative, and unified, being able to assess numerous events and compare them. these events are weighted using eq. 1 to quantify their probability of occurrence. i = ! e∈er peie + ! e∈es peie (1) where i, is expected life-cycle impact of the infrastructure under analysis on the community (in monetary terms), and are the domains of events addressed by resilience and sustainability, respectively; is the probability of occurrence of the event e, and is the predicted impact on the community of the event e, this result is nondimensional. eq. 1 can be used at the individual scale of an infrastructure (e.g. a bridge), or at a global scale. • mejia et. al. [19], defined three performance measures for the long-term investment in energy and transport infrastructures, which are: cost, sustainability and resiliency. the authors defined sustainability as the period where the infrastructure system satisfies operational and environmental requirements, and resilience as the cost from a precontingency state of the infrastructure to a postcontingency due to an occurrence of an extreme event. in addition, the authors used four input factors to quantify the resilience, cost, and sustainability performance of the system: the system of interest; the expected demand; projected events, and the expected actions. this quantitative approach was demonstrated in a case study of an integrated energy and transportation system in the united states. • marinella giunta [20] proposed an integrated approach of resilience and sustainability to identify the most efficient alternative of road infrastructure rehabilitation after an extreme event based on life cycle costs. the author addressed that the main difficulty of this kind of approach is to do an evaluation of the sustainability and resilience using quantitative indicators. to overcome this issue, she articulated the approach in three main steps: 1. identify the rehabilitation alternatives considering technical, economical, and time aspects. 2. quantify the life cycle cost of each alternative. for each alternative it is quantified the life cycle assessment (lca) and the life cycle cost analysis (lcca). 3. resilience assessment of the infrastructure for each rehabilitation alternative, based on economic aspects. it is done by the estimation of the costs to restore functionality, and the time needed for the reconstruction. the result of this unified approach is a best solution of rehabilitation based on the lowest sum of the costs of sustainability and resilience. 4.3. conclusions this paper provided a broad analysis of the characteristics of different tools, methodologies and applications for the identification and evaluation of risks, resilience, and sustainability in civil infrastructures, as an opening towards developing an integrated and multidisciplinary methodology that allows to assess simultaneously to serve adequately over longer life terms without major contingencies. it was observed in the literature from the most relevant articles related to risk and resilience assessment, that there is a possibility to create a multidisciplinary methodology that includes sustainability as it directly involves the economical and societal consequences that any disruptive event might bring. in this way, the aspects associated with sustainability and resilience assessment are considered simultaneously and in a process of mutual interaction, opening the possibility to be used for planning and development of new cities, industries, and facilities as this comprehensive analysis is important for their continues optimization. the development of this kind of 621 o. j. urbina, e. teixeira, h. sousa, j. matos acta polytechnica ctu proceedings research seems to push designers and administrators in a similar direction, as it is a useful source of information for stakeholders at the local level, as the assessment methods and tools may comprehensively support sustainable choices that are also the basis for the sustainable pursuit of civil infrastructures. risk, resilience, and sustainability are complementary characteristics for civil infrastructure. while sustainability addresses the time-continuous impacts on the economy, society, and environment that the infrastructure certainly will distribute over its entire service life, resilience, and risk focus on the big impact that the service failure of the infrastructure can have in the case of extreme events. the combination of these approaches will provide a truly comprehensive assessment of the quality of the infrastructure. a possible big obstacle for the integrated assessment of both sustainability and resilience is the computation of truly quantitative metrics. the resilience research is more advanced in terms of quantitative analyses and indicators for civil infrastructures. instead, sustainability assessment systems have promoted a culture of qualitative assessment. finally, it is essential to continue investigating and improving the way of collecting and processing these methodologies. hence, future studies are needed to conduct to validate the usefulness and reliability of a comprehensive methodology described to evaluate the resilience and sustainability of civil infrastructures. additionally, it is vital to realize applications on case studies embracing different scenarios for each characteristic. acknowledgements this work was partly financed by feder funds through the competitivity factors operational programme compete and by national funds through fct (foundation for science and technology) within the scope of the project poci-01-0247-feder-039555). references [1] j. a. barbosa, l. 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[20] m. giunta. sustainability and resilience in the rehabilitation of road infrastructures after an extreme event: an integrated approach. the baltic journal of road and bridge engineering 12(3):154-60, 2017. https://doi.org/10.3846/bjrbe.2017.18. 623 https://doi.org/10.1061/(asce)is.1943-555x.0000177 https://doi.org/10.1061/(asce)ey.1943-7897.0000067 https://doi.org/10.3846/bjrbe.2017.18 acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.38.0613 acta polytechnica ctu proceedings 38:613–619, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague densification of single and two-family houses considering green space and mobility florian schöpflina,∗, sabine erberb, david madlenerb, thomas prinza,c a research studios austria fg, studio ispace, schillerstrasse 25, 5020 salzburg, austria b energieinstitut vorarlberg, stadtstrasse 33, 6850 dornbirn, austria c university of salzburg, department of artificial intelligence & human interfaces, ida lab salzburg, jakob-haringer-straße 6, 5020 salzburg, austria ∗ corresponding author: florian.schoepflin@researchstudio.at abstract. the question of future energy-efficient creation of living space is becoming more and more important in many cities due to limited space availability and simultaneously increasing settlement pressure. against the background of the desired space, resource and energy efficiency, cities are steadily-focusing on internal development and densification. in order to encourage a sustainable urban development, the existing densification potentials need to be mobilized by addressing the owners directly, to convince them for the measures to mobilize the given potentials. therefore innovative gis methods and databases are used within the research projects “bonsei!” and “bonus” in order to detect these potentials. following is a gradually constructive consultation that helps house owners, depending on their needs and wishes, to find potential construction actions to increase the living space within their parcel. the research project “bonus” further tackles problems and concerns, which come with densification, by maintaining high quality green spaces and at the same increase the usage of sustainable forms of mobility and try to reduce car usage within the districts. keywords: densification, internal development, sustainable development, green space. 1. densification and related problems within cities tackled by research projects the question of future energy-efficient creation of living space is becoming more and more important in many cities due to limited space availability and simultaneous increasing settlement pressure. against the background of the desired space, resource and energy efficiency, cities are therefore steadily-focusing on internal development and densification. especially single and two-family houses, which represent nearly 70 % [1] of all residential buildings in salzburg, offer a great potential for sustainable densification and also show a high level of refurbishment needs. however more than 90 % [1] of these buildings are privately owned. one way to encourage a sustainable urban development, is the mobilization of the existing densification potentials by addressing the owners directly to convince them for the measures to mobilize the given potentials. plans and actions of authorities and the city administration are a key element towards this process. not only is it important to steer and enable a qualitative densification in order to minimize the negative side effects, like compensating the loss of private green space with increasing the quality of public green space, but it is also an impulse towards mobilizing existing potentials. in the case of salzburg, densification is also implemented within the spatial development concept, to minimize land consumption and preserve green land [2]. there are many examples of cities pursuing densification as a method for increasing living space for an increasing number of inhabitants like rotterdam [3]. a lot of projects and research towards densification exists, for an example different gis-based approaches like in the paper of eggimann et al. [4]. other research considers the dynamics that are associated to densification processes like shukla et al. [5] or the social problems coming with this process like afandi [6]. contrary to this paper’s focus, treija et al. [7] examined the densification of large housing estates. in contrary to singleand two-family houses, densification of large housing estates shows more distinct accompanying problems, like gated communities and social segregation [7]. although densification seems to be a sustainable way to create more living space within a city without having to assign new building land, some risks and downsides exist and have to be considered. one problem that comes hand in hand with most densification processes, is not considering green spaces and mobility, although being acknowledged in many studies [8, 9]. these two issues are important to address in order to enable a socially accepted and sustainable densification program within a city. these aspects are also important to bring cities towards a more energy-efficient future and reduce ground densification. they also often might worry neighbouring house owners. raising the social acceptance for den613 https://doi.org/10.14311/app.2022.38.0613 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en f. schöpflin, s. eeber, d. madlener, t. prinz acta polytechnica ctu proceedings sification projects therefore is one of the main goals of the research project “bonus” (optimal use of existing stock – strengthening the environment). the quality and the impact of densification projects have already been proven in the predecessor research project “bonsei!”. within the project “bonsei!” (optimal use of existing stock – energy-efficient implementation of refurbishment!) a corresponding consulting service for addressing post-compression measures within cities was already developed. it has been shown, that qualified and personal advice is an important source of inspiration for owners in order to mobilize post-compression potentials. using innovative gis-methods, the densification potential can be calculated and therefore properties with the possibilities to achieve more living space can be detected [10]. this innovative data and consultation is now extended in the project “bonus” by data and indicators for green space and mobility [11]. because of that, elements of “bonsei!” are reused within “bonus” and selectively further developed. the holistic “bonus” consulting is implemented in pilot areas in feldkirch and salzburg and thus the research results are transferred directly into practice. in addition, a transferable operator model will be developed for long-term establishment of this service. in order to ensure practical suitability and innovation effectiveness, in “bonus” nine loi partners from vorarlberg and salzburg from several specialist domains / infrastructure levels (urban planning, smart city coordination, spatial planning, energy management and consulting, housing promotion, transport planning) are involved. through the initiation of creating energyand resource-conserving housing by the building owners, a sustainable urban development can be enabled and thus an austria-wide transferable significant municipal added value can be generated. the consultation within the project “bonus” and the predecessor “bonsei!” only considers singleand two-family houses. on a larger scale, developments are considered in other research projects like in the “researchand transfercenter alpines bauen” (http: //alpinesbauen.at). here the research concentrates on the development and visualization of gis-based settlement and densification models. 2. sequence of the densification consultation from first approach to first sketches in order to mobilize the reserves, available in stock within a city, a holistic advisory offer is needed, to provide an objective, product-neutral consulting to further develop already existing buildings regarding densification and a high quality energy efficient redevelopment. goal of this consulting is to show the house owner different approaches to develop their homes further in the aspects of energy efficiency and structural development. to get residential owners to participate in densification consulting, they need to be approached directly using different strategies. hereby the collaboration with the city administration has shown to be helpful, to raise awareness for the project and to demonstrate the reputation and importance of the project. the city administration also helps with advertising, by writing an official letter to the residents and also support events where the project is presented [11]. to get into contact with the house owners, different strategies are applied. when developing the approach strategies different target groups were kept in mind and how to get them interested into the densification consultation. one possibility is to use events to hand out flyers with information about the project or using media to advertise the consultation. this can be happen through newspapers or social media, in order to get the attention of the house owners. another different approach is using energy advisor. during their work in the household, they can hand out flyers and speak about the possibility of densification as part of a reconstruction. the most efficient method is an anonymised letter distributed in the whole district of interest. this letter contains information about the densification consultation and a small text, addressed by the administration of the city, to get the reader’s attention. at this occasion, the owners are confronted with general information about densification and if they are interested in this topic and in a consultation, they can make an appointment for a first meetup with a densification advisor [11]. once the property owners are interested in a densification consultation and want to get advised, the procedure written into an operator model takes place. this operator model contains the sequence of the consultation by describing the different steps and their content. it is further used to bring the developed densification consulting towards other municipalities and cities, by providing guidance on how to run the different steps of consulting and how the costs are splitted between the owners and the municipality. with this model, other municipalities should get a manual on how to implement and use the densification consultation on their own [11]. the consultation itself is performed by trained advisors. they built their consultancies upon data sheets including information about the property and the building (densification potential, development plan, building stock, infrastructure and environment) which they are provided with beforehand. based on this information, an efficient consulting service is enabled. because these building and property parameters are known to the advisors before the first talk, they can already develop and sketch different strategies beforehand. before the first meeting, the interested house owner gets send a questionnaire with questions about their wishes and needs, as well as some easy questions about specifications of their house. the first meeting is then used to collect more detailed data of 614 http://alpinesbauen.at http://alpinesbauen.at vol. 38/2022 densification of single and two-family houses . . . figure 1. schematic visualization of the optimal building model (schneider & gadocha 2019). the property, which are not already within the data sheet or the questionnaire and also to further talk about the wishes and needs of the ones who are advised. as it appeared in previous talks within the “bonsei!” project, especially the additional creation of living space for the family is very important for most house owner. basis for the second meetup are individual designed solutions, based upon the wishes and needs of the house owner. according to predefined typologies, different structural measures, like addition of storeys or extension of the building, are possible. these sketches are presented to the owner as part of the second meeting and the implementation possibilities as well as the funding opportunities are discussed. one big concern, when it comes to densification, is the fear of increasing traffic as well as the reduction of green spaces within the districts because of construction extensions and additional people living in the neighbourhood. therefore, it is important to also include aspects like social acceptance, mobility and blue-green infrastructure within the densification consultation. considering mobility, it is important to strengthen more environmental friendly alternatives. within the district, it is useful to build upon on already existing infrastructures (like cycleways and bus stops). this way, the usage of private cars could be reduced and therefore the overall mobility should be more sustainable. it is important to not limit to possibilities for car usage but to show and strengthen alternatives in order to provide new possibilities. this can happen through mobility offers and incentives for walking and biking and a comprehensive mobility management for the whole city. the other important part that is considered in the project “bonus” is green space. especially high quality green spaces on buildings and green spaces next to the properties play an important role in the ecology of a city. ecosystem services like recreation, health and biodiversity are affected the most. another important aspect of green spaces in urban spaces is regarding the regulation of the climate, adaption to climate change and its contribution to quality of life. therefor it is important to include climate ecological aspects within the advisory. for both aspects, green space and mobility, indicators are developed in “bonus” and calculated using innovative gis methods, that help the advisors doing their work. another important tool that helps densification advisors, are appealing cards with selected measures regarding sustainable mobility and green space, which are shown and handed out to the house owners during consultation. they include information on how to support more sustainable mobility and for an example how to raise the quality and the amount of green spaces within their gardens. 3. spatial data and gis as a base for detecting densification potentials using spatial data and innovative methods of geoinformatics plays a crucial role in the mobilization and detection of densification potentials. with the developed gis methods, it is possible to detect the existing densification reserves on a very high parcel-specific resolution. therefor necessary input data are mapped buildings, surface and elevation models, zoning plan and key figures regarding the structural exploitability of a property. to calculate the existing densification potentials, the maximum potential gross floor area is subtracted by the existing one of the building. the result of this calculation offers the theoretical realizable densification reserve within a parcel. to get a better overview of the calculation process, figure 1 shows the process behind the detection of the maximum potential gross floor area for a parcel [12]. how a parcel then can be further used for densification, depends on the already existing building as well as on the available space. because the construction type can vary from parcel to parcel, it is useful to provide the possible type of densification possibilities for each parcel to the advisor. hereby it can be differentiated between: addition of storeys, independent additions on the parcel, demolition and building a new house and the realization of new living space through interior work and apartment division [13]. gis can also help with controlling the densification 615 f. schöpflin, s. eeber, d. madlener, t. prinz acta polytechnica ctu proceedings figure 2. calculated heated and cooled areas within tisis, feldkich (own illustration). progress. to show the long-term effects of the densification advisory within a city, a semi-automatic gis process was developed in the project “nachverdichtungsmonitor stadt salzburg”. this tool enables the user, based on the parcel data, surface and elevation model, digital development plan, to constantly get insight into the densification potential as well as the usage of the potential for creating new living spaces [14]. 4. database and indicators for green space and mobility measures in addition to the “bonsei!” database, within “bonus” the database is now extended with data and calculated indicators regarding green space as well as mobility. to get a deeper insight into the existing amount and quality of green spaces a broad selection of data is needed. for an example, to measure the adaption to climate change, infrared images and a surface model of the investigation areas are needed. with this input data, in combination with gis methods, heat sources and cold sources within the study district can be detected. this indicator, shown in figure 2, further gives a deeper insight into the distribution and amount of green spaces and parks. in the shown map, the areas that have a heating effect as well as the areas with a cooling effect can be seen. houses and concrete areas as well as green areas with trees and grass can be distinguished this way. in conclusion the information gained from this analysis helps to choose appropriate measures for densification. in districts with a lot of heated areas and only few cooled areas, during densification the preservation of trees and high quality green areas should be considered more carefully, to not have too much impact on the climate within the district. in terms of mobility, aspects like public transport stops and their accessibility as well as their quality are crucial. another helpful indicator is the accessibility of cycle ways, as it is an important measure to see how the district is connected to a street network that enables sustainable mobility. to calculate these exemplary accessibilities, data including the public transport stops with their quality, the network of cycle paths and the street network is needed. another example for a calculated mobility indicator, namely the distance to educational centers on a parcel sharp resolution, can be seen in figure 3. in addition, the theoretical shortest ways used by the inhabitants to get there, are accumulated and shown within the map. depending on the amount of their usage, the street segments are drawn with a different thickness. cities can benefit from the implementation of the densification consultation including the innovative database in various forms. one of them is the potential for cities to create further living space without using newly assigned building spaces. this leads to a reduced land consumption and at the same time 616 vol. 38/2022 densification of single and two-family houses . . . figure 3. accumulated everyday walks and accessibility of educational centers in tisis, feldkirch (own illustration). to an increasing renovation rate because of structural measures on the buildings. another point is the inclusion of different quality aspects, namely: blue-green infrastructure, mobility and social acceptance. by including them into the densification consultation, the quality of the densification within the city can be increased. the consideration of blue-green infrastructure further helps to improve the climatic interaction within urban areas. this is achieved by maintaining high quality green spaces or by strengthening them. this results into an increased adaption to climate change as well as in increasing life quality within the districts. next to the blue-green infrastructure, considering the topic mobility is crucial. considering this aspect in the consultation can be an important incentive towards reducing car usage and towards raising awareness and usage of environmental friendly forms of transportation. bringing these elements together into one high quality densification advisory, by using spatial indicators, can therefor lead to a better climate adaption and further towards more socially accepted densification measures within cities. 5. example of a successful densification consultation first densification consultations were already implemented during the research project “bonsei!”. one example for a successfully applied advisory is the building shown in figure 4. during the consultation the need to add living space to enable more family figure 4. example building used for densification consultation during “bonsei!” (source: project “bonsei!”). members to live here, was detected. in more detail, there should be enough additional space for up to three new inhabitants. also, during the construction work, they wanted to reduce noise coming into the house from the surrounding environment. based on the information, the densification advisor, in this case an architect, sketched two possible solutions that would come into question for this specific parcel and also estimated the costs that would come with this construction. for this building an addition of storeys or an addition next to the house were found to be possible densification solutions. the second solution, 617 f. schöpflin, s. eeber, d. madlener, t. prinz acta polytechnica ctu proceedings figure 5. 3d visualization of possible densification process (source: project “bonsei!”). adding some construction next to the building, which was suggested to the owner, is shown in figure 5. this sketch is part of the final step within the consultation, as the aim of the project “bonsei!” and “bonus” is to offer possible actions and to raise awareness for further possible steps. the realisation and construction itself is not part of the project and is left to the owner. 6. conclusion densification within cities is a sustainable way to add more living space without using newly assigned building land. the project “bonsei!” has shown, that it is important to mobilize the potentials by speaking to the owners directly and show them the benefits, which can come with adding more living space to their parcel. the past has also shown, that there are some concerns regarding reducing green space and increasing traffic volume by densification measures and by people that move into the district. to reach socially accepted densification, it is important to consider green space and mobility, while planning possible densification measures. the project “bonus” considers these two aspects, by using indicators regarding green space and mobility, to improve densification measures and at the same time reduce the accompanying concerns. that way, a high-quality consultation and in succession a high quality densification process can be enabled. but it shouldn’t be neglected that densification processes can also have negative side effects beside potential loss of green space and increasing traffic within a neighbourhood. one aspect is the potential increase of social relation problems within the neighbourhood [6]. as a result, a less active and harmonious dynamic within the community can occur. also, the phenomena of gentrification, caused by densification processes must be noted. according to afandi [6] densification and gentrification cannot be separated from each other. it is stated that these processes can trigger conflicts between citizens. as a conclusion, densification can be an efficient method to increase living space for a raising number of inhabitants. ecological and social aspects should nevertheless be considered and minimized. the project “bonus” tries to consider mobility and green space in order to make densification processes more accepted within a neighbourhood. because the focus lays on oneand two-family houses, it has been shown within the project “bonsei!”, the owners mainly focus on creating additional living space for family members. as a result, negative side effects like gentrification and processes aren’t that obvious but should nevertheless be also kept in mind when initiating densification processes within a city. references [1] statistik austria. gebäude 2011 nach überwiegender gebäudeeigenschaft, eigentümertyp und bundesland, 2011. [2] stadt salzburg. grundlagenbericht zum neuen räumlichen entwicklungskonzept der stadt salzburg, 2021. [3] n. n. m. j. d. tillie, m. aarts, m. marijnissen, et al. rotterdam – people make the inner city: densification + greenification = sustainable city. mediacenter rotterdam, 2012. https: //www.researchgate.net/publication/293095884_ rotterdam-_people_make_the_inner_city_ densification_greenification_sustainable_city [4] s. eggimann, m. wagner, y. n. ho, et al. geospatial simulation of urban neighbourhood densification potentials. sustainable cities and society 72:103068, 2021. https://doi.org/10.1016/j.scs.2021.103068 [5] a. shukla, k. jain, r. ramsankaran, e. rajasekaran. understanding the macro-micro dynamics of urban densification: a case study of different sized indian cities. land use policy 107:105469, 2021. https: //doi.org/10.1016/j.landusepol.2021.105469 [6] i. n. afandi. gentrification and densification as determining factors of orientation and quality of neighbors relations in indonesia (a hypothetical study). in proceedings of the 8th international conference of asian association of indigenous and cultural psychology (icaaip 2017), pp. 140–143. atlantis press, 2017. https://doi.org/10.2991/icaaip-17.2018.31 [7] s. treija, u. bratuškins, a. korolova. urban densification of large housing estates in the context of privatisation of public open space: the case of imanta, riga. architecture and urban planning 14(1):105–110, 2018. https://doi.org/10.2478/aup-2018-0014 [8] b. lin, j. meyers, g. barnett. understanding the potential loss and inequities of green space distribution with urban densification. urban forestry & urban greening 14(4):952–958, 2015. https://doi.org/10.1016/j.ufug.2015.09.003 [9] c. haaland, c. k. van den bosch. challenges and strategies for urban green-space planning in cities undergoing densification: a review. urban forestry & urban greening 14(4):760–771, 2015. https://doi.org/10.1016/j.ufug.2015.07.009 618 https://www.researchgate.net/publication/293095884_rotterdam-_people_make_the_inner_city_densification_greenification_sustainable_city https://www.researchgate.net/publication/293095884_rotterdam-_people_make_the_inner_city_densification_greenification_sustainable_city https://www.researchgate.net/publication/293095884_rotterdam-_people_make_the_inner_city_densification_greenification_sustainable_city https://www.researchgate.net/publication/293095884_rotterdam-_people_make_the_inner_city_densification_greenification_sustainable_city https://doi.org/10.1016/j.scs.2021.103068 https://doi.org/10.1016/j.landusepol.2021.105469 https://doi.org/10.1016/j.landusepol.2021.105469 https://doi.org/10.2991/icaaip-17.2018.31 https://doi.org/10.2478/aup-2018-0014 https://doi.org/10.1016/j.ufug.2015.09.003 https://doi.org/10.1016/j.ufug.2015.07.009 vol. 38/2022 densification of single and two-family houses . . . [10] t. prinz, w. spitzer, s. gadocha, et al. bestand optimal nutzen – sanierung energieeffizient implementieren. 2018. berichte aus energieund umweltforschung, 12/2018. [11] t. prinz, f. schöpflin, a. schmidbaur. potenziale der nachverdichtung. nachverdichtung in einund zweifamilienhausgebieten am beispiel der stadt salzburg und deren aktivierung durch qualitative aktive beratung. ögz – das magazin des österreichischen städtebundes (3):28–30, 2021. [12] c. schneider, s. gadocha. bonsei! – bestand optimal nutzen – sanierung energieeffizient implementieren! in tagungsband bauz! wiener kongress für zukunftsfähiges bauen, pp. 40–43. 2019. [13] t. prinz, s. gadocha, w. spitzer. bestand optimal nutzen – gis-gestützte modellierung von entscheidungsgrundlagen zur energieeffizienten nachverdichtung. agit – journal für angewandte geoinformatik (4):326–331, 2018. https://doi.org/10.14627/537647041 [14] w. spitzer, j. reithofer, t. prinz. monitoring der nachverdichtung in der stadt salzburg. agit – journal für angewandte geoinformatik (3):383–389, 2017. https://doi.org/10.14627/537633042 619 https://doi.org/10.14627/537647041 https://doi.org/10.14627/537633042 acta polytechnica ctu proceedings 38:613–619, 2022 1 densification and related problems within cities tackled by research projects 2 sequence of the densification consultation from first approach to first sketches 3 spatial data and gis as a base for detecting densification potentials 4 database and indicators for green space and mobility measures 5 example of a successful densification consultation 6 conclusion references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.38.0162 acta polytechnica ctu proceedings 38:162–168, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague holistic assessment methodology for positive energy districts jakub kvasnica∗, kateřina válková czech technical university in prague, faculty of civil engineering, department of construction management and economics, thákurova 7, 166 29, praha 6, czech republic ∗ corresponding author: jakub.kvasnica@fsv.cvut.cz abstract. globally, cities are responsible for most of the world’s energy consumption and most of the carbon dioxide emissions. therefore, the efficient transformation of urban energy systems is becoming a crucial step towards carbon neutrality and more effective climate protection. the positive energy district (ped) concept has been pointed out as one of the key principles of this path. the positive energy district concept leads towards an integrated and sustainable urban development with an emphasis on the use of renewable energy sources and attention shifting from energy performance at the level of individual buildings to the level of the entire district. however, peds represent a relatively new and still not sufficiently explored area in the field of sustainable development. the aim of this paper is to describe the technical part of the new holistic assessment and innovative process of stakeholder engagement to identify positive energy districts in the early development phase that arises within the ped-id project. the early development phase is crucial since the structure of the urban area is being decided along with its sustainability and climate protection levels. the assessment methodology is based on the creation and optimization of energy balance scenarios. case studies of existing urban areas have been developed in order to verify the methodology. it is concluded that this innovative methodology devised for ped evaluation, together with strengthening of knowledge-based and decision-making framework, will help to accelerate the process of transforming the current urban energy systems into more efficient and renewable systems. keywords: positive energy districts, sustainable urban development, assessment methods. 1. introduction globally, cities are responsible for 60 to 80 percent of the world’s energy consumption and for nearly 75 percent of the carbon dioxide emissions [1], whereas buildings are responsible for 40 percent of the energy consumption and 36 percent of the carbon dioxide emissions [2]. urban agglomerations consist of a large number of non-renovated buildings, three quarters of which are not energy-efficient and therefore waste energy [2]. only 1 % of non-renovated buildings are renovated annually [3]. the concept of positive energy districts implements the objectives of the european union by connecting sustainable urban development with renewable energy production and energy efficiency. the european union aims to increase the renovation rate of buildings [3], increase energy efficiency in the eu to 32.5 % and the use of renewables to at least 32 % by 2030, along with the long-term goal of decarbonisation by 2050 [4, 5]. therefore, the efficient transformation of urban energy systems is becoming a crucial step towards carbon neutrality and more effective climate protection. the positive energy district (ped) concept has been identified as one of the key principles of this path. in 2021, the topic of positive energy districts was already being addressed by a number of projects [6– 8]. the objective is to accelerate the pace of the transformation of the energy system in europe to a more efficient and renewable system by achieving the positive energy district status and to accomplish the goal of 100 ped sites in europe by 2025 [9]. 1.1. introduction of ped-id project the approach to ped assessment presented in this paper was developed under the project “holistic assessment and innovative stakeholder involvement process for identification of positive-energy-districts (pedid)” funded by the technology agency of the czech republic and jpi urban europe. the goal of the ped-id project was to develop an innovative knowledge-based stakeholder process for ped areas and holistic assessment methods to use information for the stakeholder process and to provide decision-makers at an early stage with better information about how to achieve ped status in the best way, but also about possible options and impacts. this process is then tested in living labs, all potential peds, with focus on the participation process and the identification of the data needed. the principle is to create a handbook that will serve as a guide to peds. there are currently many guides and methods to assess the energy performance of buildings, however, in the case of peds, the assessment is also extended to the simultaneous assessment of a larger number of buildings and other sectors such 162 https://doi.org/10.14311/app.2022.38.0162 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 38/2022 holistic assessment methodology for positive energy districts as transport, technical infrastructure and greenery that need to be included and balanced. 1.2. ped definitions and characteristics the ped definition is not yet clear and comprehensive. it has been addressed within a number of studies [10, 11] and documents [12–14]. based on the definitions and descriptions of positive energy districts or similar designs [15], the following elements and parameters of peds have been identified: • peds are composed of groups of buildings or an urban area. • peds require interconnection and interaction of several sectors along with a high degree of system and communication integration. • peds use comprehensive (advanced) energy management. • peds focus on the energy efficiency of buildings and systems and energy balance of all sectors involved. • peds reach at least net-zero energy import and greenhouse gas emissions balances per year with the goal of producing surplus energy. • peds rely exclusively on (local) renewable energy sources and energy storage systems. • peds are not limited by social, material, technical or technological solutions. the seven points represent the main elements in the implementation of peds but also the risks and challenges as positive energy districts form a very comprehensive system. peds aim to create an emissionneutral, energy self-sufficient and sustainable local economy. however, due to the diversity of urban areas, a holistic approach is needed in the development and implementation of peds, which needs to be adjusted to specific local conditions. 2. methods applied the information presented in this paper is based on the data from the ped-id project, primarily on the holistic assessment method and stakeholder involvement process in early development phase of potential ped areas [16]. the method is based on an extensive analysis of documents and studies in the field of energy efficiency. first, research on peds was conducted followed by an analysis of technical and technological solutions, methods and tools for energy performance assessment in the sectors relevant to peds. based on the analysis and synthesis of the data, their adaptation to the ped characteristics and identification of challenges and barriers, a proposal for a holistic assessment method for peds was developed. the proposal is a first step, which was further consulted with stakeholders for its further optimization. in 2022, the method was applied to pilot projects. the results of the implementation process together with the final adjustment of the method are expected by the end of the project. 3. results and discussion this chapter introduces selected outcomes of the pedid project. at first, the position of ped holistic assessment within the early stage of the project is defined. subsequently, the individual assessment phases are presented. 3.1. early stage of the project the earlier the stage of a project is, the more it is possible to influence the outcomes. with a comprehensive and multi-layered project such as ped, involving many different stakeholders [17], cutting edge technologies and with a lifespan of decades, the importance of early stage preparation cannot be underestimated. the ped assessment method basically represents a feasibility study, which is elaborated in a greater detail. two directions meet here – the feasibility study should not be too complicated as its aim is to say whether to implement the project or not. on the other hand, the assessment of peds requires a large amount of information and quite a lot of details, increasing the complexity of the processing and assessment. however, it is possible to perform certain simplifications and aggregations or to use guide values which are then specified in the detailed design, thus reducing the amount of data needed. peds differ in detail. the specific solution depends on local conditions. they nonetheless share some common features – excellent local availability of renewable energy sources, high awareness and engagement of local citizens and wider community, financial support and the method of initiation. 3.2. ped inception when it comes to the implementation of positive energy districts, it is necessary to consider 4 main topics – motivation, barriers & challenges, costs and risks. the last two topics consist in a holistic assessment dealing with economical, technical, environmental, social and political/legal aspects of ped implementation, as described in figure 1. these must be considered and assessed together as they are interconnected and intertwined in many cases, and they affect one another. 3.3. holistic assessment holistic assessment of the positive energy districts implementation consists of several key steps, as shown in figure 2. first, there is a preselection of the area, where the ped in question could possibly be implemented. for this area, the data have to be collected on the basis of which the technical and technological solutions can be selected. the assessment of potential solutions leads to the development of possible implementation scenarios. the scenarios are assessed on 163 jakub kvasnica, kateřina válková acta polytechnica ctu proceedings figure 1. five main aspects of ped holistic assessment. figure 2. key steps in ped holistic assessment. the basis of indicators and benchmarks and then the final solution with the final area is selected. 3.3.1. pre-selection of the area the basis of the positive energy districts is formed by the initial idea of their development which is followed by a core notion of the area where the positive energy district could be implemented. a positive energy district can be part of both rural and urban infrastructure. there are three options for pre-selecting the area type [11]. peds can be built as a new greenfield district, implemented into an extension of an existing urban area as a combination of old (renovated) and new buildings, or rebuilt from an existing urban area through deep renovation. within the third option, the renovation level together with the renewable energy production must balance the final energy consumption of the area. all in all, greenfield development is the easiest way to create peds, while utilization of existing urban areas is the most difficult one. new buildings (districts) can be built as energy efficient as possible. on the other hand, given the average age of the building stock in europe, most of the buildings have to undergo deep renovation in order to comply with increasingly stringent climate policies [3]. peds in built-up areas therefore represent a significant potential to boost the renovation rate along with new renewable energy capacities. when selecting an (preliminary) area for implementation of positive energy districts, the boundaries of the area must be determined and the following points must be considered: • system, technical, topographic and geographical boundaries; • objects to be included in the ped area – built-up area and non-urban area – general identification of objects and infrastructure for disposal; • the scope of ped in terms of feasibility – considering the size of the area; • compactness of the ped area – should be as compact as possible as it is easier for the overall assessment and ongoing management, although it is possible to involve more separated or distance areas as well (especially for the renewable energy sources utilization). however, the fragmentation of the ped area can cause problems when there are changes in its surroundings; • site specificity – adaptation to the specific conditions of the area. positive energy districts should ideally form a coherent area within of which all relevant parameters can be evaluated. this area cannot be selected immediately without prior analysis as it may not be entirely clear where the ped will be implemented. first, it is necessary to identify the potential of the 164 vol. 38/2022 holistic assessment methodology for positive energy districts area, i.e., its possibilities, barriers and limits (data collection). then, a decision on the selection of the final area is made based on individual elements of the area (options available) and in relation to the implementation analysis (what can be achieved after the implementation of the selected measures). 3.3.2. current situation – data collection data collection is one of the crucial steps within the ped preparation as it forms the basis for a holistic ped assessment. it is also one of the most timeconsuming steps because usually many data are missing or are not available at the moment. therefore, a comprehensive survey must be carried out to obtain the data. however, it is often the case that data are not available at the early stage, and then an estimation has to be made based on the available benchmarks and other calculations. better quality data facilitates subsequent assessment and design of suitable solutions for ped implementation. however, with larger ped (preliminary) area, the amount of data needed (and obtained) grows significantly, which makes the data processing more demanding. therefore, data collection efforts must be optimised. estimates based on similar or grouped parameters can also be used to simplify the process. it is also appropriate to apply spatial analysis, e.g. [18], which uses geographic information systems (gis) for data processing. the spatial analysis within peds should address the following topics: • buildings – type of building, ownership, energy consumption and production, building use (load profiles, temperature level, flexibility potential, etc.), technical and technological condition (energy sources – primary heat sources, energy performance, other information). • infrastructure – current heating and electricity systems, other energy sources, current utilities (heat pipes, mv/lv networks, water mains, sewers), water treatment plants, incinerators. • transport and mobility – traffic intensity, traffic routes, energy consumption. • other municipal objects – energy consumption parameters and energy savings potential • res potential – current production from res, potential production from res – water energy (rivers, creeks, reservoirs, ponds, weirs, water canals, irrigation canals), geothermal energy (near-surface geothermal energy potential), wind power, bio power (production of unused biomass in the area or its surroundings, areas suitable for biomass production), solar power (roofs, facades, windows, other – large water bodies, free unused areas, covered parking). • utilization of energy recovery – waste power (municipal waste, bio-waste and sorted waste – focus on unused biological and “combustible” waste figure 3. assessment of key positive energy districts aspects. in the area and its surroundings), heat recovery (significant heat sources in the area – not at the level of individual buildings). • greenery potential – current state (mature freestanding trees, parks, forests, grass areas), unused or underused areas for future potential (flat roofs, parking areas, covered car parks). • restrictions – heritage protection, urban development plan, other (easements . . . ). within the data collection, levels of data detail have been set. the basic level is only sufficient to provide a comprehensive overview of the area and for some basic estimations, the advanced level can already be used for most calculations and evaluations. the expert level will enable an accurate assessment of all parameters, but it is also the most difficult to obtain and process all the required information. 3.3.3. options available following the data collection at least at the basic and advanced level, technical and technological solutions for the implementation should be considered. a technical-technological analysis is therefore the first step, followed and accompanied by social, legal and political assessment (see figure 3). within the peds, an effort is expected for maximum utilization of the area’s potential to reduce the energy consumption, greenhouse gas emissions (ghg), and increase renewable energy production. technical concepts are then used to develop models and scenarios for ped implementation. figure 4 shows the technical relations within the positive energy districts, which are further described below. the issue of energy consumption and its coverage is crucial in the case of peds. positive energy districts are energy self-sufficient locations that provide a secure supply of energy while responding flexibly to changing demand, balancing energy consumption peaks and optimizing energy supply. surplus production of renewable energy is integrated and supplied to the regional or national energy distribution network [9]. the goal of positive energy districts is to minimize 165 jakub kvasnica, kateřina válková acta polytechnica ctu proceedings figure 4. technical relations within positive energy districts. energy consumption, use highly efficient systems and cover energy consumption with local renewables [19]. the emphasis is placed on energy recovery and exploiting the maximum potential of resources including grey water use and black water management, the issue of waste sorting and waste management. the use of different types of energy creates new links, for example between individual owners of smart grids, owners of buildings and production facilities (photovoltaic panels, cogeneration units) and consumers/users. new links are also emerging at the legislative, economic and social levels. a key element of energy positive districts is the interconnectedness of buildings that makes the site energy-plus, even if a certain number of buildings do not and cannot meet the plus-energy standard [13]. these include, for example, historic buildings that cannot undergo a deep renovation for historic preservation reasons, or buildings with high energy demands, such as freezers and cold stores. within peds, the use of smart communication and information technologies as well as energy management itself is essential. also, each building will need to have its own energy performance target, having its own contribution or roadmap to achieve the district’s targets. attention should also be paid to district mobility. an optimal mobility solution is the situation when all means of transport in the ped territory use renewables. the promotion of non-motorized modes of transport as well as utilization of smart mobility solutions and vehicle sharing is a matter of course, which can help reduce ghg emission as well as improve quality of life in the ped area. the technical and technological options within peds offer many possibilities that include increase in energy efficiency of buildings, reduction of energy losses in infrastructure, green transport and mobility, maximization of res potential (including small and micro energy sources), utilization of energy recovery, greenery potential, smart innovations and others. the emphasis should be put on the use of proven technologies as well as on new innovative solutions. 3.3.4. model and scenarios the model and scenarios are based on the available technical and technological solutions proposed in the previous phase. the development of the scenarios consists of three main parts: • technical analysis – calculation of basic technical units, e.g., for energy consumption and production. this should also include an energy balance evaluation of the proposed measures. energy balance is in fact a comprehensive input/output model that can be conveniently depicted by sankey diagrams or other specialized calculation tools (e.g., city energy analyst or district energy concept adviser . . . ). detailed energy flows may be aggregated to show individual buildings within a ped or different energy uses within buildings or the whole ped. • economic analysis – use of payback period, discounted payback period, net present value and internal rate of return for both costs and revenues. basically, the economy is considered in direct relation to investments, energy consumption and production but for larger peds it is appropriate to include also various non-energy benefits. the life cycle assessment method should be used as it can assess all costs over the life cycle [20]. however, it needs to be adapted for the whole ped, which can be quite difficult. also, the preliminary funding options based on stakeholder consultations should be resolved during this phase. • emission analysis – ghg emission balance calculation, environmental impact assessment. within the points mentioned above, the model building scenarios, sectoral scenarios, renewable energy scenarios, and partial and overall energy balances need to be addressed. together these parts should form the overall scenario. the use of modern evaluation methods, approaches, models and tools is an advantage for successful assessment. 3.3.5. selecting a solution determining the final extent of peds depends on how we evaluate the number of inputs. technical and economic constraints as well as preferences of a wide range of stakeholders must be taken into account. therefore, the decision on the ped area needs to follow an evaluation of multiple criteria, both quantitative and qualitative (e.g., ensuring the quality of the indoor environment). combining an energy and financial assessment relying on quantitative methods with stakeholder input proves to be challenging and calls for a holistic approach to support decision making. such an approach has been explored in a study [21] which encouraged stakeholders to scrutinize the quantitative modelling results in a structured manner through a multi-criteria decision analysis (mcda). mcda follows the technical analysis presented in the previous sections and introduces additional criteria – economic, social, urban planning-related etc. the aim 166 vol. 38/2022 holistic assessment methodology for positive energy districts is to benchmark technical scenarios vis-à-vis more subjective criteria beyond simply achieving ped energy balance or not. the evaluation based on the selected criteria can also be used to compare different peds. 4. conclusions this paper presents the basis of the holistic assessment methodology for positive energy districts, which can contribute to a greater uptake of peds and thus accelerate the transformation of the energy system in europe to a more efficient and renewable system. developing peds is a way to achieve carbon neutrality in a given area, which is in line with the long-term goals of climate protection and decarbonization of the economy (and cities) by 2050. however, due to the specific conditions across the municipalities and different areas, there are still barriers to overcome. the first is to clearly define the ped boundaries and the degree of solution complexity, which is crucial within the solution assessment. the interconnectedness of different sectors can be challenging for proper evaluation or application of the life cycle assessment method due to the need for a large amount of data. it is therefore necessary to further develop evaluation procedures or specialized tools for a detailed ped assessment. although the tools for assessing specific individual areas exist, there is currently a lack of comprehensive evaluation tools. the second obstacle lies in political and public support, as the ped implementation represents a significant transformation in the social, economic and environmental areas. stakeholder engagement is essential in the ped development. therefore, it is necessary to develop and apply a comprehensive (visual) framework (e.g., in the form of an overview maps and schemes [10]) to engage various stakeholders to present them the meaning and goals of the positive energy districts as well as to facilitate communication within the development process and within the presentation of results and benefits to maximize the ped implementation potential. obtaining sufficient funds is also a challenge, as it can be expected that ped implementation will have high initial financial costs. various innovative financing methods will need to be used. on the other hand, it can be assumed that the benefits of peds can sufficiently outweigh the initial investment in the long run. acknowledgements this work was supported by the technology agency of the czech republic, project no. tk02010029. special thanks to seven, the energy efficiency center for the cooperation and provided information on the ped-id project. references [1] unep. sustainable development goals: goal 11: sustainable cities and communities. 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polytechnica ctu proceedings 38:162–168, 2022 1 introduction 1.1 introduction of ped-id project 1.2 ped definitions and characteristics 2 methods applied 3 results and discussion 3.1 early stage of the project 3.2 ped inception 3.3 holistic assessment 3.3.1 pre-selection of the area 3.3.2 current situation – data collection 3.3.3 options available 3.3.4 model and scenarios 3.3.5 selecting a solution 4 conclusions acknowledgements references https://doi.org/10.14311/app.2022.33.0181 acta polytechnica ctu proceedings 33:181–187, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague fundamental properties and durability of slope protection spray mortar reinforced with bamboo fibers kazuo fujiyoshia,∗, takao uedab, masayuki tsukagoshic a kankyo bosai co., ltd., 1-57 akui-cho, tokushima 770-0046, japan b inst. of science and engineering, tokushima university, 2-1 minamijyosanjima-cho, tokushima 770-8506, japan c faculty of engineering, fukuoka university, 8-19-1 nanakuma, jonan-ku, fukuoka 814-0180, japan ∗ corresponding author: k-fujiyoshi@kan-bou.jp abstract. in order to enhance resistance against cracking and durability, short fibers such as steel and organic fibers have been conventionally mixed into spray mortar used for slope protection. in this research, fundamental properties of bamboo-fiber-mixed spray mortar were examined by laboratory tests. the mechanical properties of spray mortar containing bamboo fibers were examined under cyclic wet and dry conditions along with its resistance against freezing and thawing by a spray test. it was confirmed that 0.75% mixture of bamboo fibers in spray mortar most successfully improved mechanical properties and durability. these include adhesion strength to the base surface following exposure to cyclic wet/dry conditions and overall resistance against freezing and thawing. in addition, higher compressive strength and adhesion strength to the base surface were achieved by further mixing in vinylon fibers or fly ash in combination with bamboo fibers. keywords: adhesion strength, bamboo fiber, cyclic wet and dry condition, fly ash, resistance against freezing and thawing, spray mortar, vinylon fiber. 1. introduction the spray application method of mortar for slope protection has been widely used because it can help preserve bedrock conditions from weathering and erosion. however, it has been observed at many sites that sprayed mortar cracks, peels and flakes due to aging. sprayed mortar is directly influenced by the external environment immediately after construction and initial cracking occurs in certain weather conditions. therefore, in recent years, the use of spray mortar mixed with short fibers such as organic and inorganic fibers has been increasing for the purpose of reducing initial cracking and improving flexural toughness [1]. bamboo forests are abundant in japan and have been a familiar source of natural materials. however, proper forest management is somewhat being neglected due partly to the lack of successors who take care of the forests recently. when proper management is neglected, the bamboo forests may trigger landslides and other disasters caused by crowded bamboo growth density and shallow rooting system. as part of the efforts to widening the scope of bamboo material applications, experiments have taken place in the use of short bamboo fibers as a reinforcement medium in concrete and mortar. past experiments [2] have found that bamboo fibers mixed in concrete improve flexural toughness and resistance to crack development. based on these findings, the fundamental properties of bamboo-fiber-mixed-mortar for slope protection were tested in laboratory. then, the durability of bamboo fiber reinforced mortar was tested by spraying on formworks in the actual working spray facilities. the durability examination included two mechanical property tests (for compressive and adhesion strength) under cyclic wet/dry conditions (hereinafter referred to as "cwdc"), and a resistance test in freezing/thawing conditions. 2. experimental program 2.1. properties of fibers the bamboo fibers used in this study were prepared using fibers collected from a bamboo tree with a unique rotating knife. bamboo trees used were anywhere from three to five years old and the cultivar is called moso bamboo grown in awaji, japan. prepared bamboo fibers were then classified by fiber length to l = 20 ± 10 mm using 1.2 − 5.0 mm sieves. the classified bamboo fibers were air dried indoors for about three weeks prior to actual experiments. additionally, commercially available vinylon fibers were mixed into spray mortar to facilitate comparison with spray mortar reinforced with bamboo fibers. the material properties of the bamboo and vinylon fibers are shown in table 1. the dry surface state was defined as a state where the fibers absorbed water for 24 hours and then dehydrated until the fiber surface was dry. to achieve this dry surface state, the fibers that absorbed water were dehydrated for three minutes using 181 https://doi.org/10.14311/app.2022.33.0181 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en k. fujiyoshi, t. ueda, m. tsukagoshi acta polytechnica ctu proceedings a centrifugal dehydrator. bamboo vinylon density: air-dry (g/cm3) 0.77 1.09 density: dried surface (g/cm3) 1.27 1.17 water absorption rate (%) 87.2 12.1 fiber length (mm) 19.8 (av.) 18.0 fiber diameter (mm) 0.59 (av.) 0.2 aspect ratio (−) 34 90 tensile strength (mpa) 154 490 table 1. material properties of bamboo fiber and vinylon fiber. 2.2. laboratory test for fundamental properties 2.2.1. mix proportions of mortar and specimen preparation table 2 shows the mix proportions of mortar for fundamental property tests in laboratory, normal mortar with c = 400 kg/m3, w/c = 55% and s/c = 4.2 was named "n" as a standard composition of spray mortar. based on n, compositions with bamboo fibers added with mixing ratios of 0.25%, 0.75% and 1.25% in volume were each named "b0.25," "b0.75" and "b1.25." in addition, 20% in fine aggregate of b0.75 was replaced with fly ash, which was named "b+fa." in b+fa, w/b was 32% when the binder included fly ash. furthermore, a composition n with 0.75% vinylon fibers was named "v," and the n mortar with 0.375% each of bamboo fibers and vinylon fibers was named "b+v." readily available portland cement (density: 3.16 g/cm3) was used. crushed sand manufactured in naruto, japan was used as fine aggregate (density: 2.57 g/cm3, fineness modulus: 2.63). moreover, type ii fly ash (density: 2.33 g/cm3, specific surface area: 3240 cm2/g, ignition loss: 2.8%) specified under the japanese industrial standard (jis a 6201) was used. the mortar was mixed in accordance with the cement physical test method (jis r 5201), and the fibers were immersed in the mixing water. all compositions in this experiment were stiff consistency mortar with low fluidity close to zero slump. in mortar spraying operations on the field, mortar is sprayed on the bedrock by compressed air generated by a compressor. thus, the sprayed mortar not only adheres to the bedrock but is also compacted. since fundamental property tests are conducted indoors, the specimens were prepared by an elaborate compaction to obtain effects similar to spray compaction without spraying. cylindrical specimens (! 50×100 mm) for the compression test were compacted every three layers by tamping down with a thrust bar. 2.2.2. curing and various tests all specimens went through indoor air curing for 28 days (room temperature: 20 ◦c , 75% r.h.), being sealed except for the upper surfaces of the specimens to simulate site conditions. two cases of compression tests were conducted. in the first case, the compressive strength was measured after air curing for 28 days. in the second case, the specimens were subjected to cwdc for 12 days after air curing for 28 days. cwdc were based on the method for the accelerated rock slaking test (jgs 2125) to simulate the environment of natural weathering after spraying on slope surfaces. the specimens were dried at 40 ◦c for 48 hours, then immersed in water at 20 ◦c for 24 hours, and then dried at 110 ◦c for 24 hours. this cycle was repeated three times. a porosity test was performed using cylindrical specimens (! 5 × 10 cm) with a material age of 28 days. the mortar porosity was determined by equation 1, which was consisted of the mass of the specimen dehydrated in a drying furnace at 80 ◦c for 24 hours (wd) and the mass of its surface-dried state immersed in water at 20 ◦c for 48 hours (ws). ε = ws − wd ρ × v × 100 (%) (1) where ε is porosity (%), ρ is density of water and v is volume of specimen. 2.3. spray test using actual plant 2.3.1. mix proportions of mortar and spray method for the spray test, the amount of mixed fibers was determined to be 0.75% as found to perform the best by the laboratory test. five kinds of mortar compositions, n, b, v, b+v and b+fa were selected. it is presumed that the viscosity gets too high due to the influence of the fly ash that replaced fine aggregate in b+fa, and there is a risk of clogging the material hose and nozzle. consequently, the replacement rate of fly ash was adjusted to be 7%, and w/b was 45% when fly ash was used as the binder. mortar spraying was conducted in the facilities normally used for wet spraying. the fibers were mixed in in a small portion at a time to prevent unevenness, and the mixing duration was set to be two minutes per batch. 2.3.2. specimen preparation and various tests figure 1 shows an outline of the formworks sprayed with mortar and the specimens cut out. two types of wooden formworks in the figure were prepared for each mortar composition. three cylindrical specimens (! 50 × 100 mm) for compression test and four specimens for adhesion test were made in a formwork with dimensions of 850 × 300 × 150 mm. in the attempt to take actual spraying on bedrock or existing concrete surfaces into account, two kinds of specimens for the adhesion test were prepared by spraying on rock pieces (! 75 × 25 mm, compressive strength: 207 n/mm2) and on a flat concrete 182 vol. 33/2022 spay mortar reinforced with bamboo fibers fiber type fiber addition rate w/b unit amount [−] [vol. %] [%] ! kg/m3" c w s fiber pa n − − 55 400 220 1679 0 0 b0.25 bamboo 0.25 55 400 220 1676 1.93 0 b0.75 bamboo 0.75 55 400 220 1670 5.78 0 b1.25 bamboo 1.25 55 400 220 1663 9.63 0 v vinylon 0.75 55 400 220 1662 8.14 0 b+v bamboo, vinylon 0.375, 0.375 55 400 220 1674 2.89, 4.07 0 b+fa bamboo 0.75 32 400 220 1356 5.78 280 table 2. mix proportions of mortar in laboratory test. 850 30 0 300 400 150 specimen for compression test (ij50×100) 25 0 550 specimen for freeze/thaw test (ij100×200) specimen for adhesion test to rock piece (ij75×25) specimen for adhesion test to flat concrete plate unit: mm figure 1. top view of formworks for spraying and sampling of specimens. plate (300 × 300 × 60 mm for interlocking block, flexural strength: 5.5 n/mm2). rock pieces were cut out from sandstone with a stone cutter. the cross section of the stones for bonding with mortar became smoother than the surface of flat concrete plate. moreover, two cylindrical specimens for freeze/thaw test (! 100×200 mm) were cut out of mortar sprayed on a wooden formwork with the size of 550×250×250 mm. the sprayed formworks were cured outdoors exposed to natural weather for 28 days from 11/24 to 12/22. during this period, the average temperature was 8.3 ◦c, and it rained for seven days. each specimen except for freeze/thaw test was cut out at a material age of 25 days. when the age reached 28 days, the compression strength test (jis a 1108) and the first case of the adhesion strength test (jscek 561-2013) were conducted. an adhesion strength testing device of building research institute method was used for the adhesion test. after outdoor exposure for 28 days, two specimens for the adhesion strength test were exposed to cwdc for 12 days. then the second case of the adhesion test was performed. three cylindrical specimens (! 50×100 mm) were cut out of mortar after the second adhesion test, and used for the second compression test. 56-day-old specimens were cut out and used for the freeze/thaw test. in this test, 300 cycles of freezing and thawing in water were conducted according to the freeze/thaw test method of concrete (jis a 1148, procedure a), and the relative dynamic modulus of elasticity and the mass reduction rate were obtained. the relative dynamic modulus of elasticity was determined by the ultrasonic propagation velocity with reference to the previous study [3]. 3. fundamental property laboratory test results 3.1. porosity figure 2 shows the results of porosity test at a material age of 28 days. according to this figure, when the mixing amount of bamboo fibers increases from 0% (n) to 1.25% (b1.25), the porosity also increases. this increase is caused by fine bubbles introduced in the mortar with the fibers. the porosity of v mixed with vinylon fibers is the largest. one of the reasons is because the number of vinylon fibers per mortar volume is greater than the number of bamboo fibers. the diameter of vinylon fiber is about 1/3 that of bamboo fiber as shown in table 1. when the same volume of fibers (0.75%) is mixed, the number of vinylon fibers is about 10 times that of bamboo fibers. the porosity of b+v mixed with composite fibers (bamboo/vinylon), is almost the same as b0.75 mixture of bamboo fibers solely. moreover, the porosity of b+fa with the combination of bamboo fibers and fly ash is the smallest because mortar voids are reduced by fly ash. since fine aggregate was replaced with fly ash, voids were filled by the filler effect of fly ash with extra fine particles. in the long term, as the pozzolanic reaction of the fly ash proceeds, the porosity of b+fa will decrease further. 183 k. fujiyoshi, t. ueda, m. tsukagoshi acta polytechnica ctu proceedings 10 12 14 16 po ro si ty (% ) figure 2. porosity of mortar measured in laboratory test. 3.2. compressive strength the results of compressive strength test of 28-day-old specimens are shown in figure 3 "pre wet/dry exposure." the compression strength increased when the addition rate of bamboo fibers increased from 0% (n) to 0.75% (b0.75), but it decreased when the rate further increased from 0.75% (b0.75) to 1.25% (b1.25). in general, the higher the porosity and organic content, the lower the compressive strength. however, when bamboo fiber content increased from 0% to 0.75%, not only the porosity, but also the compressive strength increased. the cause of the increase in compressive strength is inferred to be influenced by the internal curing of bamboo fibers. since the specimens were cured in air, the moisture in the mortar (needed for the hydration reaction) tended to evaporate easily. because moisture contained in bamboo fibers with high water absorption rate (as shown in table 1) is replenished inside the mortar as it dries, the strength of b0.25 and b0.75 is considered to increase from that of n. when bamboo fiber mixing ratio increases to 1.25%, the increased voids (as shown in figure 2) have a more negative effect on the mortar strength. it also becomes difficult to sufficiently disperse the fibers while mixing, so that the compressive strength of b1.25 seemingly decreased. v in which vinylon fibers solely was mixed had a lower compressive strength than n due to its large porosity, and b+v in which bamboo and vinylon fibers were combined had comparable strength to n. b+fa in which bamboo fibers and fly ash were used in combination had the smallest porosity and the highest compressive strength. b0.75 also shows a similarly high-level of strength to b+fa. in terms of strength, approximately 0.75% of bamboo fiber mix ratio proved to be most desirable. when the specimens were exposed to cwdc for 12 days after 28 days of outdoor air curing, the strength of the bamboo-fiber-mixed compositions except b+fa shows a relatively large decrease (see figure 3 "post wet/dry exposure"). the decrease is assumed to be caused by the hollow cells of bamboo 20 30 40 50 c om pr es si ve st re ng th (n /m m 2 ) pre wet/dry exposure post wet/dry exposure figure 3. compressive strength of mortar measured in laboratory test. fibers [4]. it is presumed that shrinkage stress is generated in the mortar around the hollow cells while drying in wet/dry cycles, and defects have occurred inside the mortar. the strength of b+fa mixed with fly ash largely increases. since fly ash used, overall amount of the binder increased. this resulted in increased amount of unreacted binder. it is possible that the hydration reaction of the unreacted binder progressed rapidly during water immersion in wet/dry cycles. 4. the results of spray test using actual plant 4.1. porosity figure 4 shows the results of porosity test of 256-dayold specimens (! 50 × 100) cut out in the spray test, together with the porosity of 28-day-old specimens in the laboratory test. according to figure 4, the values of b and v in the spray test are less than those in the laboratory test, and are similar to the value of n. the reason is because the spraying pressure purged the fine bubbles which were entrained while mixing in the fibers [5]. the value of b+fa in the spray test is drastically lower than that in the laboratory test. the decrease was caused by densification of mortar, in addition to bubbles vanished while spraying. since the specimen material age reached as old as 256 days, the pozzolanic reaction of the fly ash progressed. 4.2. compressive strength figure 5 shows the measurement results of compressive strength and density of cylindrical specimens (! 50 × 100 mm) before and after exposure to cyclic wet/dry conditions (hereinafter referred to as "ecwdc"). before ecwdc, the compressive strength of b was similar to that of n and v, while the density of b mixed with bamboo fibers was the smallest. the moisture within the mortar easily evaporates during outdoor curing. in this situation, it is assumed that the bamboo fibers that have absorbed water gradually supplied moisture (the internal curing effect [6]) and the hydration reaction of b further 184 vol. 33/2022 spay mortar reinforced with bamboo fibers 10 12 14 16 n b v b+v b+fa po ro si ty (% ) laboratory test spray test figure 4. porosity of mortar measured in spray test. developed. that is why the compressive strength of b+v is greater than that of v. the reason why b+fa had the largest strength was that the dense mortar matrix was formed by the lower w/b, the filler effect of fly ash and the internal curing of the bamboo fibers. after ecwd, the density of all compositions increased, but the compressive strength of n and b decreased. the decrease was especially drastic in b than in n. the air-dry density of the bamboo fibers is small as shown in table ??, and the inside of the bamboo fiber cells becomes hollow when drying [4]. accordingly, it is considered that the internal defects of the mortar were caused by the stress of drying shrinkage. the compressive strength of v, b+v, and b+fa increased slightly. these increases are attributed to the hydration reaction of the unreacted binder that was promoted during the wet period of cwdc. v and b+v were also mixed with vinylon fibers which do not have a hollow structure like bamboo fibers. as for b+fa, the addition of fly ash resulted in increased amount of powder element that acted as a binding agent, which led to increased compressive strength. the increased compressive strength is assumed to be further improved by the pozzolanic reaction of fly ash over a long period. 4.3. adhesion strength figure 6 shows the measurement results of adhesion strength between spray mortar and rock/concrete surfaces before and after ecwdc. before ecwdc (material age of 28 days), since the rock surfaces cut with a rock cutter were smooth, the adhesion strength with the rock in all compositions was as small as approximately 1.0 n/mm2. peeling fracture also occurred at the adhered interfaces. as for concrete surfaces, the base concrete or spray mortar was destroyed, showing values exceeding 2.0 n/mm2. these values can be regarded as close to the tensile strength of mortar and concrete. regardless of the 2.1 2.12 2.14 2.16 2.18 2.2 20 30 40 50 n b v b+v b+fa d ensity (g/cm 3) c om pr es si ve st re ng th (n /m m 2 ) compressive strength: pre wet/dry exposure compressive strength: post wet/dry exposure density: pre wet/dry exposure density: post wet/dry exposure figure 5. compressive strength of mortar measured in spray test. base type, the value of b+v using composite fibers was the largest. adhesion strength after ecwdc decreased on rock/concrete surfaces for all compositions. the decrease was caused by the changes in the mortar volume following ecwdc, increasing the shear strain on the adhered interfaces. for the rock surfaces, peeling fracture occurred at the interfaces. the values of b and b+v were about twice the value of n, and greater than v. for the concrete surfaces, the base concrete or spray mortar was destroyed. the values of b+v and b+fa were larger than other compositions. these results show that compositions with bamboo fibers mixed in have excellent adhesion strength. bamboo fibers inside the mortar absorb water during the wet period of cwdc, and gradually supply moisture in the dry period. it means that drying shrinkage strain on the adhered interfaces are suppressed. 4.4. resistance against freezing/thawing figure 7 shows the mass reduction rate and the relative dynamic modulus of elasticity in the freeze/thaw test results. with respect to the three compositions n, v and b+v, relative dynamic elastic modulus dropped sharply before reaching 200 cycles. these reductions were attributed to the peeling fracture of the mortar pieces at the corner portions of the cylindrical specimens. it is possible that the cracks progressed from the partial sand streaks generated during spraying and led to the peeling fracture. the sand streaks also existed on the specimens of b and b+fa mixed with bamboo fibers, but the relative dynamic elastic modulus showed a gentle decrease, with 89% and 57% at 300 cycles, respectively. after the freeze/thaw test, deterioration due to scaling was observed on the specimen surfaces in all 185 k. fujiyoshi, t. ueda, m. tsukagoshi acta polytechnica ctu proceedings 0 1 2 3 n b v b+v b+fa a dh es io n st re ng th (n /m m 2 ) rock: pre wet/dry exposure rock: post wet/dry exposure concrete: pre wet/dry exposure concrete: post wet/dry exposure figure 6. adhesion strength between sprayed mortar and rock/concrete surfaces measured in spray test. compositions. as shown in figure 7, the mass reduction rate moved momentarily to the negative side in all compositions, but then shifted to the positive side. the cause of the negative shift was that fine cracks occurred at the early stage of the freeze/thaw action, then the mass increased due to the penetration of moisture into the cracks. as the number of cycles increased, scaling or peeling fracture occurred and finally the mass decreased. in the previous study on sprayed concrete [5], it has been reported that helpful air bubbles for freeze/thaw resistance are lost by spraying pressure. when alternative measures were taken to obtain the benefits of bubbles (such as adding hollow microspheres to spray concrete mixed with vinylon fibers), fine cracks were suppressed and reduction in relative dynamic elastic modulus was hardly observed. in this freeze/thaw test, no special measures were taken to compensate for the bubbles lost by spraying pressure. thus, v and b+v mixed with vinylon fibers are considered to have shown small resistance against freezing and thawing. in contrast to n, v and b+v, the freeze/thaw resistance of b and b+fa mixed with bamboo fibers was greatly improved. it can be presumed that bubbles remaining in hollow parenchyma cells of bamboo fibers absorbed and relaxed the expansion pressure during freezing. 5. conclusion in this study, fundamental properties and durability of slope protection spray mortar reinforced with bamboo fibers were experimentally examined. the results and findings are summarized as follows. -1.5 1.5 � ��� ��� ��� m as s re du ct io n ra te (㸣 ) 0 20 40 60 80 100 0 100 200 300 r el at iv e dy na m ic e la st ic m od ul us ( 㸣 ) number of cycles n b v b+v b+fa figure 7. variation curves of mass reduction rate and relative dynamic elastic modulus measured during freeze/thaw test in spray test. 1. as far as strength is concerned, 0.75% mixing ratio of bamboo fibers proved to perform the best. compressive strength decreased following exposure to cyclic wet/dry conditions when only bamboo fivers were mixed. however, when bamboo fibers were mixed with vinylon fibers or fly ash, the decreased strength was improved. 2. concerning spray mortar with 0.75% bamboo fibers mixed in, adhesion strength between the base rock/concrete and spray mortar after exposure to cyclic wet/dry conditions was increased by 180% for the base rock and by 120% for the base concrete, compared to mortar without fibers. the performance improvement was more remarkable when vinylon fibers or fly ash was mixed in in addition to bamboo fibers. 3. spray mortar with 0.75% bamboo fibers had great resistance against deterioration caused by freezing and thawing, and relative dynamic elastic modulus remained at 89% after 300 cycles in the freeze/thaw test (mortar without fibers: 0% at 120 cycles). the mortar combining 0.75% bamboo fibers with fly ash remained at 59% at 300 cycles. acknowledgements we would like to express our gratitude to sugimotoshoten co., ltd. for providing bamboo fiber materials for the research and green system co., ltd. for cooperating in spray test. references [1] s. yan, h. jiao, x. yang, et al. bending properties of short-cut basalt fiber shotcrete in deep soft rock 186 vol. 33/2022 spay mortar reinforced with bamboo fibers roadway. advances in civil engineering 2020:1-9, 2020. https://doi.org/10.1155/2020/5749685. [2] d. k. gupta, r. c. singh. an experimental evaluation of compressive strength and flexural strength of bamboo fiber reinforced concrete. international research journal of engineering and technology 5(6):699-708, 2018. https://www.irjet. net/archives/v5/i9/irjet-v5i9126.pdf. [3] r. takada, s. guo, h. ogata, et al. proc. jp. concr. inst. (kochi) vol 26 pp 1911-6, 2004. [4] j. zhu, h. wang, c. wang. study on the swelling characteristics of bamboo based on its graded hierarchical structure. wood and fiber science 51(3):332-342, 2019. [5] f. taguchi, m. takahashi, n. kishi n, et al. improving the frost resistance of short-fiber-mixed shotcrete by hollow microspheres. 6th international conference on concrete under severe conditions (consec’10), merida, yucatan, mexico, pp 273-8, 2010. [6] v. k. ganesh, k. ramanurthy. the influence of superabsorbent polymer beads used as internal curing agent on the compressive strength of mortar. international research journal of engineering and technology 3(2):410-416, 2016. https://www.irjet. net/archives/v3/i2/irjet-v3i272.pdf. 187 https://doi.org/10.1155/2020/5749685 https://www.irjet.net/archives/v5/i9/irjet-v5i9126.pdf https://www.irjet.net/archives/v3/i2/irjet-v3i272.pdf acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.39.0055 acta polytechnica ctu proceedings 39:55–59, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague validation of a longitudinal motorcycle riding dynamic model for a powered two-wheeler interactive simulator josef svoboda∗, přemysl toman, adam orlický czech technical university in prague, faculty of transportation sciences, department of vehicle technology, horská 3, 128 03 prague 2, czech republic ∗ corresponding author: svoboj88@fd.cvut.cz abstract. engine or battery powered two-wheeled vehicles are an essential part of a transport system. its users are however more vulnerable in comparison with personal vehicles. in general, compared to four-wheel vehicles even a small percentage of accidents that include motorcycles causes a significant rate of all fatalities in the czech republic. two-wheelers nowadays face the challenge of electric propulsion and wider user adoption. this fact emphasizes the need to assess rider’s behaviour. one way of examination of rider’s behaviour in safe and scientific environments is an interactive vehicle simulator. in order to develop a complex semi-active motorcycle simulator, a virtual model in unity platform was created. the main purpose of this paper is to verify the longitudinal dynamics of the physical model for emerging interactive simulator. to achieve this goal, crucial parameters of the model were set. for validation of the model we defined several riding scenarios. based on the data measured during a track test experiment in a real environment, model parameters further needed were indirectly defined. the results show that the output of the longitudinal dynamics model closely correlates with the real data gathered on a track. keywords: riding dynamics, powered two wheelers, vehicle simulator, longitudinal dynamics. 1. introduction in the czech republic (based on yearbooks of road accidents in the czech republic and traffic 2017-2020, published by the police of the czech republic) we observe a steady ratio of number of road accidents caused by motorcycle riders and total number of people killed in road accidents. it means that 2 % of accidents caused by motorcycle riders stand behind 10 % of people killed. in comparison with cars there is the ratio circa 40-50 % caused road accidents to 60-70 % people killed. this makes us see more than 3 times higher seriousness of accidents caused by motorcycle riders. compared with the situation in the eu (european union) the average fatalities percentage is 15 % of total fatalities from 2010 to 2018 based on european commission (2020) facts and figures motorcyclists and moped riders. powered two wheelers (ptw) are the way to increase mobility and accessibility for citizens. this statement presents the acem (the european association of motorcycle manufacturers) in the document decarbonisation of transport: powered two-wheelers (ptws) on the road to 2050. towards this approach the question of decarbonization and implementation of electric ptw is also mentioned. the similar forecast is also reported in paper [1]. as for the transition to electric ptw we also have to be aware of different risks coming from riders facing electric propulsion of ptw [2]. the rider style can differ significantly. as an extreme example of potential risk of easily available electric micro-mobility vehicles nowadays we present e-scooters. as reported in [3] the injury rate per million miles travelled was 180 times higher than the overall group of motor vehicles. to assess rider style adaptation to electric propulsion, when using several research methods, it is preferable to perform experiments in driving, respectively riding simulators. as the riding itself could be dangerous and the consequences of small error could lead to several injuries, the simulators are considered a satisfactory way. similar effects arise when assessing potentially dangerous activities of drivers such as fatigue or aggressive driving styles as documented in [4] and [5]. concerning motorcycle simulators, the approach turned out to be a successful tool to assess riders’ behaviour as presented in [6]. the development of a motorcycle simulator is specific for several reasons. if we compare it with conventional four-wheel simulators, we can determine differences. if we focus on motorcycle riding dynamics from the physical point of view, we can observe several specific effects which we cannot neglect. a motorcycle is a spontaneously unstable vehicle. if we consider the weight ratio between the rider and the bike, it is clear that the rider’s movement affects the cog (centre of gravity) of the whole ridermotorcycle system. that is why the manoeuvrability (especially leaning) of powered two wheelers is strongly affected by the rider’s changing body position. all of these conditions have to be taken into account when developing a virtual physical model as well as control 55 https://doi.org/10.14311/app.2022.39.0055 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en j. svoboda, p. toman, a. orlický acta polytechnica ctu proceedings hardware [7]. in addition to physics, there are other technical challenges such as control and command of the whole motorcycle and its hmi (human-machine interface) system. another technical specific relate to the visualization of virtual scenery projected to a rider. the rider compared to a driver has a potentially wider range of view including the view near the rider’s feet, open rear view or the sky view while looking up. in order to improve the feeling of accurate speed adoption and virtual environment perception the visualization is a significant element of the whole simulation and its devices should be designed precisely [8]. the motorcycle riding simulator reveals challenges typical of ptw such as e.g. roll motion height and vehicle motion controlled by a rider [8]. when we design the simulator the motorcycle physical model is an essential part of the whole structure. this physical model can be divided into two main systems of the straight movement and the turning movement system [7]. straight forward movement or the so-called longitudinal riding dynamics includes in general acceleration and braking in the motorcycle riding field even shifting gears, recuperation braking and deceleration as a typical feature of each engine or motor type and settings. based on findings above we see that ptw causes high percentage of fatalities and that they are an integral part of future mobility planning moreover enlarged by the group of electric ptw. it is important to be able to examine accurately rider’s behaviour within the context of electric ptw mobility. it means both the risks and benefits. the safety of testing riders, repeatability and reproducibility is very important. therefore, we decided to develop a motorcycle simulator aiming at problematic issues of electric propulsion of ptw. one of the main sub-systems of such a simulator is the longitudinal dynamics model. the primary motivation for creation of an accurate longitudinal model is the possibility of future accurate assessment of riders’ behaviour in safe environment. one of the key factors of the simulator of electric motorcycle is the authenticity of specific riding propulsion characteristics of longitudinal movement associated with such a motorcycle. 2. method our approach towards accurate longitudinal motorcycle dynamic models combine real test track and virtual computational development in the software unity. the basis for the following work is the architecture (see figure 1) of the whole simulator system which is divided into two parts – “set-up geometry” module, where the basic geometry and mass characteristic of the bike is defined. the second module (see figure 2) provides the communication between the individual blocks of the physical model as shown in figure 1. module 1 – motorcycle input parameters. figure 2. module 2 – motorcycle simulator architecture. the diagram. this step provides the possibility of a complex view on the whole development and testing. as the whole system is developed according to the presented architecture, we created the first draft version of the longitudinal dynamic model able to operate within the system. the following step is to validate the output of the simulations according to the real track test data. for the testing and as the first steps in the development of the simulator we also consider the future possibility of implementation of control hardware and thus we created a simplified handlebar controller (see figure 3) able to communicate with the model. for the validation we defined three motorcycle-rider system movement scenarios. acceleration from zero up to 80 km·h−1, emergency braking from 80 km·h−1. the third scenario describes the straight free movement from the speed of 80 km·h−1 to a complete halt. in this scenario the motorcycle with a rider stops only due to riding resistances of the whole system motorcycle-rider. test scenarios are defined ideally with 0 % slope and wind speed 0 km·h−1. in the first longitudinal model parameters set-up we focus on the road motorcycle similar to the category moto3 of the world road racing championship. 56 vol. 39/2022 validation of a longitudinal motorcycle riding dynamic model figure 3. diagram of the simplified motorcycle handlebar for simulator. figure 4. visualization from the development unity platform. virtual development of environment and visualisation of the bike presents figure 4. the motorcycle is produced by the student team ctu lions of the faculty of transportation sciences, czech technical university in prague. basic parameters are weight of 165 kg, electric motor with peak power up to 42 kw, torque up to 95 nm. the rider has testing weight of 75 kg. this particular motorcycle (see figure 5) was used for the measurement for track tests according to the presented scenarios. for data acquisition we used our self-developed daq (data acquisition system) verified in a series of previous track tests as presented in [2, 9]. for this application the main parameters were the motorcycle velocity and throttle openings. data was collected within closed area. the basis for the validation is the learning algorithm for the exact bike from which the measured data comes from. in the first step we use the scenario of free movement to adjust parameters of riding resistances. then we use acceleration scenario data. throttle openings and velocity curves are input to the simulation model. the model afterwards computes the optimal engine power curve to correlate with the input speed curve. then we tune the parameters of braking to correlate the most with the speed curve in braking scenario. when the learning phase ends we perform the exact same testing scenario in the unity through the simplified handlebar controller. the data from the virtual testing are stored as the simulation output data and then compared with the input velocity curves. for the evaluation of the accuracy of the longitudinal dynamics model we define a final speed difference error. figure 5. motorcycle used for track test and daq. figure 6. free movement scenario speed profiles comparison. 3. results the results chapter is divided into two parts in compliance with the presented method and testing scenario sequence. in each section the results of the final set-up of the longitudinal dynamics model compared to the data from the track test are presented. the first one focuses on the final speed profiles and the second one looks up closely to the absolute speed differences. 3.1. speed profiles following figures 6, 7 and 8 present the speed profile within time in the acceleration, braking and free movement testing scenario. we observe very close correlation of the track test and simulation output data. the correlation coefficient in the all three scenarios is almost equal to 1. the track test data are represented by blue line and the simulation output data by black line. 3.2. speed profiles differences when we look closely (see figure 9, 10 and 11) at the speed profiles or to its differences, we see maximal absolute values 2.8 km·h−1. in all cases the speed difference is calculated as test track data minus simulation output data. the total average speed difference values in order of testing scenarios are 0.78 km·h−1, 0.77 km·h−1 and 0.24 km·h−1. when the speed simulation output values are higher than test track values the maximal differences are 1.61 km·h−1, 0.57 km·h−1 and 2.80 km·h−1. in the opposite situation the max57 j. svoboda, p. toman, a. orlický acta polytechnica ctu proceedings figure 7. acceleration scenario speed profiles comparison. figure 8. braking scenario speed profiles comparison. imal differences are 2.63 km·h−1, 2.30 km·h−1 and 0.76 km·h−1. 4. discussion we performed validation of our motorcycle longitudinal dynamic model. as the result of validation, we see very closely correlating speed profiles of track test and simulation output data. focusing on the results in the form of speed differences we observe relatively small average values lower than 1 km·h−1. the maximum speed difference across all scenarios is 2.8 km·h−1 after our validation process. this result we find accurate and in proper dimensions based on other studies e.g. [10] where the maximum speed difference in comparable speed level reaches 7.46 km·h−1. from this point we achieved a very realistic representation of the longitudinal dynamics motorcycle behaviour. the main limitation is the influence of the rider’s weight and the rider’s aerodynamic position which can cause inaccuracies especially in aerodynamic drag. both will be subject of further analysis and development. the second main limitation is the fact that this process is oriented on a specific motorcycle. on the other hand, we have very precise simulation outputs. from the simulation point of view the validation process is easily repeatable with new data from track tests. figure 9. free movement scenario speed profiles deviation. figure 10. acceleration scenario speed profiles deviation. figure 11. braking scenario speed profiles deviation. 5. conclusion motorcyclists are relatively vulnerable road users. according to the current trends of electromobility, riders are very often facing challenges in form of new propulsion systems of ptw: on the other hand, these new primarily electric ptw are facing unexperienced users. an important fact is that electric propulsion system has many different aspects compared to petrol ones. thus, riders may behave in a different way. rider’s behaviour assessment in scope of new types of ptw is a path towards safe ride. from this point of view an interactive vehicle simulator serves as a tool for safe testing under laboratory conditions. focusing more on motorcycle and ptw in general, they have several specifics such as e.g. spontaneous instability. in this article we validated the longitudinal dynamic model of the interactive motorcycle simulator. our validation process is designed to reproduce precisely the behaviour of a real motorcycle based on data collected during track tests. we created 3 testing scenarios on which the validation was done. the whole process is 58 vol. 39/2022 validation of a longitudinal motorcycle riding dynamic model applicable to wide palette of motorcycles so that an accurate database of virtual vehicles could be created. we assessed speed profiles of a track test and of an output of simulation. afterwards we calculated speed profiles deviations. as the results present, the final virtual model precisely reproduces the longitudinal dynamic behaviour within the simulation with maximal speed deviation of 2.8 km·h−1. the limitation of the study is primarily influenced by rider’s variable positions that will become the subject of our future research. our next steps will also aim to validation of other parts of our physical motorcycle simulator model. references [1] n. haworth. powered two wheelers in a changing world – challenges and opportunities. accident analysis & prevention 44(1):12–18, 2012. safety and mobility of vulnerable road usears: pedestrians, bicyclists, and motorcyclists. https://doi.org/10.1016/j.aap.2010.10.031 [2] p. toman, j. svoboda, j. pinkas. study of riding specifics of the electric and conventional powered two-wheeler users. in 2021 smart city symposium prague (scsp), pp. 1–6. 2021. https://doi.org/10.1109/scsp52043.2021.9447401 [3] k. rix, n. j. demchur, d. f. zane, l. h. brown. injury rates per mile of travel for electric scooters versus motor vehicles. the american journal of emergency medicine 40:166–168, 2021. https://doi.org/10.1016/j.ajem.2020.10.048 [4] p. bouchner, r. piekník, s. novotny, et al. fatigue of car drivers-detection and classification based on the experiments on car simulators. wseas transactions on systems 6(12):2789–2794, 2006. [5] p. bouchner, s. novotny, d. rozhdestvenskiy, et al. drivers aggressive behavior experiments on the driving simulator. in proceedings of the 8th international conference on energy and development, environment and biomedicine, pp. 148–153. 2014. [6] d. crundall, a. stedmon, e. crundall, r. saikayasit. the role of experience and advanced training on performance in a motorcycle simulator. accident analysis & prevention 73:81–90, 2014. https://doi.org/10.1016/j.aap.2014.08.009 [7] y. miyamaru, g. yamasaki, k. aoki. development of a motorcycle riding simulator. jsae review 23(1):121–126, 2002. https://doi.org/10.1016/s0389-4304(01)00147-3 [8] k. nagaseak, a. yamasaki, h. ishii. development of a riding simulator for motorcycles. sae technical paper 2018-32-0031:7, 2018. issn 0148-7191, e-issn 2688-3627. https://doi.org/10.4271/2018-32-0031 [9] p. toman, j. svoboda, a. orlický. assessment of the motorcycle rider behaviour – experimental study on the race track. in 2020 smart city symposium prague (scsp), pp. 1–6. 2020. https://doi.org/10.1109/scsp49987.2020.9133760 [10] l.-t. hieu, n. x. khoa, o. lim. an investigation on the effects of input parameters on the dynamic and electric consumption of electric motorcycles. sustainability 13(13):7285, 2021. https://doi.org/10.3390/su13137285 59 https://doi.org/10.1016/j.aap.2010.10.031 https://doi.org/10.1109/scsp52043.2021.9447401 https://doi.org/10.1016/j.ajem.2020.10.048 https://doi.org/10.1016/j.aap.2014.08.009 https://doi.org/10.1016/s0389-4304(01)00147-3 https://doi.org/10.4271/2018-32-0031 https://doi.org/10.1109/scsp49987.2020.9133760 https://doi.org/10.3390/su13137285 acta polytechnica ctu proceedings 39:55–59, 2022 1 introduction 2 method 3 results 3.1 speed profiles 3.2 speed profiles differences 4 discussion 5 conclusion references https://doi.org/10.14311/app.2022.33.0404 acta polytechnica ctu proceedings 33:404–410, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague thin concrete overlays wit carbon reinforcement on deteriorated concrete pavements julia neumann∗, rolf breitenbücher ruhr university bochum, department of civil and environmental engineering, institute for building materials, universitätsstrasse 150, 44801 bochum, germany ∗ corresponding author: julia.neumann-a4n@rub.de abstract. in many countries, concrete pavements are normally built as jointed plain concrete pavements. due to a lack of alternatives, repairing deteriorated concrete pavements usually requires the replacement of the complete pavement structure and maintaining the joints, which is labourand resourceintensive. to increase the durability of repairs and to save resources concrete overlays with carbon reinforcement are developed. by the application of non-corrosive carbon-textile reinforcement cracks might be distributed so fine, that such an overlay can be executed jointless, unlike in previous repair methods. for a durable repair the bond behaviour between the retained concrete and the overlay as well as between the overlay-concrete and the textile reinforcement have to be considered. in this paper, the basic principles and feasibility of such a repair method are examined. on the one hand, the decisive influencing variables and parameters such as bond behaviour and cracking behaviour within the overlay are pointed out and discussed. on the other hand, the performed special lab tests will be presented. these tests include cyclic loadings on large-scale beams with integrated overlays of such types, evaluating the bond behaviour and the durability after a few millions of load cycles. furthermore, the crack formation in the overlay is determined by means of tensile and flexural tensile strength tests. keywords: carbon reinforced concrete, carbon reinforcement, jointed plain concrete pavements, maintenance, thin concrete overlays. 1. introduction in general, concrete pavements are exposed to considerable thermal and hygric stresses in addition to mechanical stresses. seasonal and diurnal temperature and moisture fluctuations cause temperature or moisture gradients in the cross section of the pavement structure thus resulting in constraint stresses and deformations. these in turn are expressed as tensile or compressive stresses on the topside of the pavement [1, 2] as well as over its entire cross section. overriding traffic, as a mechanical cyclic stress, causes bending stresses as well as degradation of the pavement concrete which can lead to material fatigue, as recent studies indicate [3]. to cope with these stresses, different construction methods are suitable for concrete pavements. in germany, the construction of jointed plain concrete pavements (jpcp) has historically prevailed [4] and is regulated in the german guideline "ztv betonstb 07" [5]. these structures are divided into slabs by joints. on the one hand, contraction joints are cut into the concrete as predetermined breaking points when the slab is still young, in order to avoid wild cracking and to enable the compensation of length changes. on the other hand, longitudinal joints are created due to production flow. in order to allow movement in the longitudinal direction and at the same time ensure the transfer of lateral forces between the pavement slabs and thus secure their height relative to each other, dowel bars are inserted into the green concrete in the contraction joints. the slabs are prevented from moving by tie bars in the longitudinal joints. to prevent dirt and water from penetrating into the substructure, the joints are sealed with suitable joint sealants or profiles. these must be designed in such a way that the longitudinal deformations of the slabs can continue to occur undisturbed. in order to guarantee the functionality of the joint seals and due to their limited service life of approx. 6 to 10 years, they require regular maintenance and servicing [6]. in addition to considerable costs from the maintenance work itself, this causes also massive traffic disruptions. 2. approach for a jointless repair concept regardless of the chosen construction method, concrete pavements will have to be repaired sooner or later. different repair methods are used depending on the nature and cause of the damage [7]. lacking marketable alternatives, in germany the entire superstructure is usually removed and rebuilt, if the serviceability and load-bearing capacity of concrete pavements cannot be restored by conventional repair measures due to damage or external factors, which 404 https://doi.org/10.14311/app.2022.33.0404 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 33/2022 thin concrete overlays with carbon reinforcement figure 1. schematic construction of a jointless carbon textile reinforced concrete pavement (ctrcp) overlay on a deteriorated jpcp bottom-layer (jointed). materials basic composition: exposed aggregate concrete 8 mm cement cem i 42,5 n (sd) 430 kg/m3 w/c-ratio 0.42 sand 0 − 2 mm 487 kg/m3 basalt 2 − 8 mm 1310 kg/m3 grading curve − a/b 8 superplasticizer pce based 0.60 % by mass of cement air-entraining agent highly concentrated 0.15 % by mass of cement table 1. concrete mix design. is neither ecologically nor economically sensible [8]. abroad, on the other hand, there are various methods for large-scale repairs. in general, the damaged concrete layers are milled off and then reprofiled by use of unreinforced or reinforced overlays or precast elements [9–11]. bute these repair methods have limitations. for unreinforced overlays, the joints must be mirrored into the repair layer thus the scope of future maintenance cannot be reduced. precast elements, on the other hand, are mainly suitable for the repair of smaller areas, such as bus stops. reinforced overlays must be installed in relatively thick layers to protect the corrosion-prone reinforcement. the repair concept examined here is intended to avoid these disadvantages. after removing the damaged concrete layer (e.g. by milling or ultra-high pressure water jetting), it is planned to install a thin, jointless and continuously reinforced overlay. in order to achieve the planned layer thicknesses of only 30 to 70 mm, corrosion-resistant carbon reinforcement is used (see figure 1). the basic idea is to transfer the singular relatively large deformations in the joint area of the retained concrete, which are mainly caused by thermal and hygric influences, into multiple deformations in the form of fine cracks in the carbon textile reinforced concrete. 3. laboratory tests the laboratory tests were carried out on large scale beams to gain knowledge about the material behaviour of the carbon concrete overlay in combination with the retained concrete. for this purpose, the bond behaviour as well as the crack formation behaviour of the composite system of old concrete and carbon concrete were investigated in flexural tensile tests under static and cyclic load. 3.1. test specimen the tests were carried out on large beams consisting of a typical pavement concrete mix according to the german guideline "tl beton-stb 07" [12] (c30/37, xf4 or xm2 exposure class, maximum grain size 22 mm), which represents the deteriorated retained concrete, and a carbon textile reinforced concrete overlay (ctrc overlay) (consistency f4, air void content in fresh concrete 4.5 % by volume), which is realized in a concrete mix typically for exposed aggregate concrete with a maximum grain size of 8 mm (see table 1). the decisive advantage that carbon reinforcement offers for use in thin overlays is its corrosion resistance that allows small concrete covers of approx. 5 − 10 mm. in addition, the small diameters of the carbon rovings combined with its high tensile strength (2000 − 6000 mpa [13]) also contribute to thin overlays. here, a carbon mat reinforcement was used [14]. it was equipped with a factory applied polystyrene coat405 julia neumann, rolf breitenbücher acta polytechnica ctu proceedings overlay thickness number of textile layers average of type of failure number of cracks crack spacing [mm] [−] [−] [mm] [−] 70 2 2.5 65.2 delamination of reinforcement / fracture of test specimen 3 5.0 50.0 delamination of reinforcement 50 1 2.0 75.0 delamination of reinforcement / fracture of test specimen2 3.5 79.0 3 4.0 30.0 30 1 2.5 112.5 delamination of reinforcement 2 2.0 150.0 delamination of reinforcement / fracture of test specimen table 2. average number of cracks and crack spacing above bond breaker. figure 2. textile reinforcement made of carbon rovings. ing which is intended to improve the bond between reinforcement and concrete. the textile (see figure 2) had a centre distance of the rovings of 12.7 mm in longitudinal (cross section 141.02 mm2/m) and of 16.0 mm in transverse (28.02 mm2/m) direction, resulting in a mesh size of 9 mm × 14 mm. due to the maximum grain size of the aggregate of 8 mm complete enclosure of the reinforcement mats by the concrete without major air inclusions was only possible if the concrete was placed and compacted in layers between the reinforcement mats. the test specimens obtained had a length of 1800 mm, width of 250 mm and 500 mm and varied in height between 180 mm and 270 mm, depending on the planned tests and configurations of the ctrc overlay (see figure 3). the overlays thickness varied between 30, 50 and 70 mm and it was reinforced with 1 to 3 layers of carbon reinforcement. two large beams were investigated for each parameter combination. before the overlay was applied the retained concrete (150 − 240 mm in height) were roughened by high-pressure water jetting and afterwards the surface was cleaned. the contraction joint between two pavement slabs was simulated by placing a 3 mm thick pvc foil for the cyclic tests and a recess without filler material for the static tests. in addition to this, two dowel bars (250 mm distance, 500 mm length and 25 mm diameter) were placed mid-height of the specimen for the cyclic tests. a bond breaker in form of a pe construction foil of 150 mm width was placed above the contraction joint in the bond joint between retained concrete and ctrc overlay to allow a free expansion length and thus promote the formation of fine cracks in the overlay and prevent reflection cracking [15]. 3.2. test program the following described tests and results represent an extract from the research project. a comprehensive presentation can be found in the final report of the project. 3.2.1. flexural tensile tests the aim of the flexural tensile tests under static load was to gain knowledge about the crack development of the ctrc overlay. for this purpose, the test specimens were subjected to 4-point flexural tensile tests at a loading speed of 1 mm/min according to figure 3. in addition to the documentation of the position, size and order of occurrence of the individual cracks, the strains that occurred during the tests were recorded simultaneously to the applied force. independent of the configuration of the carbon concrete layer, almost all test specimens showed similar crack patterns to figure 4. shear cracks occurred, running from the supports to the load inducing points 406 vol. 33/2022 thin concrete overlays with carbon reinforcement figure 3. specimens for flexural tensile tests. 1: bearing, 2: dowel, 3: simulated contraction joint, 4: bond breaker, 5: dowel bar. figure 4. example of crack pattern. 1: bearings, 2: carbon textile reinforcement layers, 3: joint, 4: bond breaker, 5: shear cracks, 6: horizontal cracks, 7: delamination. 407 julia neumann, rolf breitenbücher acta polytechnica ctu proceedings figure 5. example of the development of the relative cyclic young’s modulus over 2 million load cycles on a 30 mm thick cracked ctrc overlay (2 reinforcement layers, with bond breaker) as a indicator for micro-cracking. (1). in between, horizontal cracks were formed in the areas under compressive stress (2). in many cases, delamination occurred starting from the shear cracks (3), which spread in the direction of the supports and, until failure, also in the direction of the beam centre. depending on the configuration of the carbon concrete layer, different crack patterns formed between and outside the described cracks (4). an influence of the dowels in the bond joint on the resulting crack patterns could not be determined so far. in addition, more cracks tended to occur in the joint area if a bond breaker was used. then up to seven cracks with a distance of 30−150 mm could be observed. without a bond breaker only up to 3 cracks with a distance of 75 − 150 mm could be observed. furthermore, the number of cracks tended to increase with the number of reinforcement layers, although this could not be observed for a carbon concrete layer thicknesses of 30 mm (see table 2). 3.2.2. cyclic flexural tensile tests the bond behaviour of retained and carbon concrete as well as carbon reinforcement and concrete was investigated by flexural tensile tests under cyclic load. here, too, a 4-point flexural tensile test was used to simulate the loads occurring in situ in the joint area. the combination of the load cases "traffic load + rapid cooling" leads to the highest stresses and differences in stress and is therefore used as the design load for the slab edge. the maximum constraint stress of σo = 3.40 mpa results from the superimposition of the two load cases traffic load σqr = 1.2 mpa and rapid cooling. with the relevant temperature gradient of 0.4 k/cm, the latter generates a reduced buckling stress σw ≈ 2.2 mpa as a tensile stress, which represents the lowest stress in the cyclic flexural tests [16]. based on the determined flexural tensile strength of the unreinforced carbon concrete of 5.610 mpa this results in a degree of utilisation of σo 6/fc, t ≈ 0.61. more detailed information on the resulting constraint stresses and the determination of the decisive design load can be found in [16]. the large beams were installed in the hydraulically controlled test stand in such a way that the ctrc overlay was under tensile stress. the test were carried out with a loading frequency of 5 hz. during the test, the concrete surfaces were examined at regular intervals for crack formation and delamination and the change in the relative cyclic young’s modulus, as an indicator of micro-cracking, was determined by means of ultrasonic measurement. at the same time, the strain above the contraction joint was continuously recorded by means of a strain gauge glued to the centre of the carbon concrete surface. after two million load cycles, drill cores were taken from loaded and unloaded beam areas for further laboratory tests, the results of which are not shown here. in addition to shear tests and centric tensile tests to 408 vol. 33/2022 thin concrete overlays with carbon reinforcement determine the effective composite forces, these also included leakage tests, as well as investigations of the freeze-thaw resistance in the loaded joint area. in the tests under cyclic load carried out so far, cracks were observed on some large beams over the entire beam width. after only 10,000 to 50,000 cycles, a significant drop in the relative cyclic young’s modulus of 53−79 % (to 21−47 % of the zero value) was recorded in these cases at the measuring section concerned (example see figure 5). these cracks were macroscopically visible from 50,000 load cycles on, whereby they showed a crack width of approx. 0.1 mm. while in some cases one crack occurred in the middle of the beam, in the remaining cases two further cracks occurred directly at the edges of the bond breaker. cracks tended to occur on beams with thinner carbon concrete layers (30 − 50 mm) and 1 to 2 reinforcement layers, although the crack pattern was not always the same on beams with the same configuration of the overlay. spalling did not occur on the crack flanks. if no cracks formed, the relative cyclic young’s modulus of elasticity dropped by 9−19 % (on average 11.2 %) after 2 million load cycles. the values fluctuated over the length of the beam, but no clear tendencies were discernible. in previous research projects, 10−15 % decrease in the relative cyclic young’s modulus was observed for jpcp [3]. based on these results, structural damage could be determined to an extent which is also to be expected in non-reinforced concrete pavements. in the cracked ctrc overlays similar damage was observed outside the cracks. furthermore, no delamination of concrete and reinforcement was observed. tensile tests carried out on drill cores from loaded and unloaded beam areas also only showed failure in the bonded joint of retained and ctrc overlay. 4. final discussion and conclusion in the flexural tensile tests at 20 ◦c on large concrete beams with ctrcp overlays, it was shown that the contraction joint can be bridged without reflection cracking and at the same time can be transferred into several fine cracks in the overlay. although the number of cracks tends to increase with used reinforcement layers, fewer cracks have formed than desirable. thus a wider bond breaker seems useful and should be investigated. the failure types to be observed are the fracture of the test specimen with associated tearing of the reinforcement and the delamination of the overlay concrete from the reinforcement. no test specimen has shown a failure in the bond joint between retained concrete and overlay. under service load, cracks could be observed on some beams directly above the contraction joint and the edges of the bond breaker. up to 2 million load cycles they remained relatively constant in width as soon as they were macroscopically visible. further investigations of water absorption and frost resistance are being carried out to investigate the effect of the cracks on the durability of the overlay. if no cracks occurred, the structural damage was comparable to the damage of concrete pavements under cyclic load observed in previous projects. to what extent these damages influence the durability of the ctrc overlay should be investigated in the future. furthermore, the behaviour of ctrc overlay and retained concrete under pressure and centric tensile load should be investigated. additionally it is necessary to research the functionality of the repair method in the form of test tracks. acknowledgements the authors would like to thank the federal ministry of education and research (bmbf), germany, represented by forschungszentrum jülich and the committee of the collaborative research project "c3 carbon concrete composite" for their funding of this research project. further, the authors would like to acknowledge the fruitful cooperation with the institute of concrete structures, technische universität dresden. their partner project has added valuable aspects to the findings of this 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[2] s. foos. unbewehrte betonfahrbahnplatten unter witterungsbedingten beanspruchungen, karlsruhe: universität fridericana zu karlsruhe, ph.d. thesis, 2006. [3] r. breitenbücher, s. kunz. betonfahrbahndecken unter zyklischer verkehrsbelastung auswirkungen auf den werkstoff. stra'e und autobahn 6. pp 41-447, 2014. [4] s. maier. durchgehend bewehrte betondecke, munich: tu munich, ph.d. thesis, 2004. [5] ztv beton-stb 07. zusätzliche technische vertragsbedingungen und richtlinien für den bau von tragschichten mit hydraulischen bindemitteln und fahrbahndecken aus beton, forschungsgesellschaft für stra'enund verkehrswesen, 2012. [6] r. oesterheld, m. peck, s. villaret. straßenbau heute band 1: betondecken, erkrath, germany, 2018. [7] werner and hermann r 1997 reparatur von betonbelägen cement bulletin, 65th year of pubilcation 3. [8] l. pfeifer. erneuerung und instandsetzung von betonfahrbahnen stra'e + autobahn volume 43(4):246-251, 1992. [9] s. spalt. sanierung von betondecken mittels der neuen österreichischen betondeckeninstandsetzungsmethode (nöbi) update 42, 2015. https://www.zement.at/downloads/update1_15.pdf. [10] s. villaret. hybrides ertüchtigungssystem für die stra'enerhaltung hester. proceedings of the conference hightechmatbau, berlin, 2014. 409 https://www.zement.at/downloads/update1_15.pdf julia neumann, rolf breitenbücher acta polytechnica ctu proceedings [11] s. tayabji, d. ye, n. buch. precast concrete pavements: technology overview and technical considerations pci journal 58.1, 2013. https://www.pci.org/pci_docs/publications/pci% 20journal/2013/winter/precast%20concrete%20pav ements%20-%20technology%20overview%20and%20tec hnical%20considerations.pdf. [12] tl beton-stb 07. technische lieferbedingungen für baustoffe und baustoffgemische für tragschichten mit hydraulischen bindemitteln und fahrbahndecken aus beton korrekturen, forschungsgesellschaft für straßenund verkehrswesen, 2012. [13] m. krüger. vorgespannter textilbewehrter beton, stuttgart: universität stuttgart, ph.d. thesis, 2004. http://dx.doi.org/10.18419/opus-192. [14] w. kneitz. solutions in textile gmbh, technical data sheet for textile reinforcement (sitgrid 040), 2018. [15] j. orlowsky, m. raupach, a. westendarp. textilbewehrte spritzmörtelschichten zur instandsetzung von wasserbauwerken / shotcrete layers with textile reinforcement for repair of hydraulic constructions. restoration of buildings and monuments 17(3-4):181-90, 2011. https://doi.org/10.1515/rbm-2011-6452. [16] c. sievering. dauerhaftigkeit von betonfahrbahndecken unter besonderer berücksichtigung des externen alkalieintrags, bochum: institute for building materials, ruhr university bochum, ph.d. thesis, 2012. 410 https://www.pci.org/pci_docs/publications/pci%20journal/2013/winter/precast%20concrete%20pavements%20-%20technology%20overview%20and%20technical%20considerations.pdf http://dx.doi.org/10.18419/opus-192 https://doi.org/10.1515/rbm-2011-6452 acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.38.0566 acta polytechnica ctu proceedings 38:566–572, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague from intelligent building to smart city – a case study dennis heung-fu mui alpine capital limited, 200 hennessy road, wanchai, hong kong sar correspondence: dhfmui@netvigator.com abstract. this paper is to provide some insights to professionals on the evolution of smart city, to present its concept and appropriate elements based on the author’s 40+ years of experience in international building development projects. with the oil crisis in the 1970s, the use of alternative energy sources such as solar energy and energy saving measures were explored to combat the threat of fossil fuel supply. the concept of intelligent building to improve the productivity of the occupants and energy saving was then emerged in 1980s, followed by the focusing of the built environment like green building and sustainable construction in the 1990s. various metrics were developed by different countries for assessing green building provisions e.g. leed in a more structure manner. building resilience has recently become popular for abating the increasing natural disasters. the features of intelligent building, green resilient building and smart city will be illustrated by making references to developments which were designed and project managed by the author. on a more macro scale, with the rapid advancement of ict in recent years, the concept of smart city has been advocated. the various definitions of smart city would first be depicted. based on these definitions, the attributes of smart building and the respective features were suggested. the appraisal of the designed provisions of a mixed development project in southeast asia was then illustrated with suggestions on further elements to enhance its smart city characteristics. keywords: intelligent building, green building, building resilience, smart city. 1. introduction with the oil crisis in the 1970s, the use of alternative energy sources including renewable energy such as solar energy and energy saving measures were explored to combat the threat of fossil fuel supply. the concept of intelligent building was then emerged in 1980s, followed by the focusing of the built environment such as green and sustainable building in the 1990s and subsequently building resilience. with the rapid advancement of information, communication and technology (ict) in recent years, the concept of smart city has been advocated. project in south-east asia will be cited to illustrate the salient features of each type of developments. 2. intelligent building 2.1. definition and objectives cityplace in hertford, connecticut completed in 1983 was heralded as the world’s 1st intelligent building. the intelligent buildings institute in washington had proposed that an intelligent building was one which integrates various systems (such as lighting, hvac, voice and data communications, and other building functions) to effectively manage resources in a coordinated mode to maximise occupant performance, investment and operation cost savings and flexibility [1]. the main objectives were to increase the effectiveness of office workers and to create an image for the users’ customers and competitors. it also aimed to avoid major refurbishment as office automation progress and to have the flexibility to meet the future changes and expansion of the office. the building services systems would be equipped with state-of-the-art technologies to conserve energy with energy management facilities and to enable the owners/operators to effectively manage the resources and hence the investment return [2]. 2.2. hang sang bank new headquarters building, hong kong sar the development which comprised of 27-storey enduser bank office building with 3 basement levels and gross floor area of about 44 000 m2 was planned and completed from 1987 to 1991 in the prime site of central district, hong kong island. salient intelligent features [3] covered energy saving tinted glass with external vertical fins façade, flexible air-conditioning and electrical services with spare capacity for future expansion, reliability of services supplies, controlled environment for minimizing sick building syndrome, flexible / user friendly building management system with software for maintenance schedule, early warning, energy management for lighting and air-conditioning control and integration with other systems. communication systems included back566 https://doi.org/10.14311/app.2022.38.0566 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 38/2022 from intelligent building to smart city – a case study bone and structural cabling system, cabd and satellite telecommunication, video teleconferencing facilities, voice system and telephone interface control for office lighting. security system covered cctv, magnetic door contact, watchman tour, car parking access control, access control and individual occupier’s specific security alarm monitoring. other features were multiplexed automatic fire alarm and smoke control system, high performance (i.e. waiting time and handling capacity) vertical transportation and allowance in structural design to cater for additional live load and internal staircase if required in future. most of the features were still widely adopted in the subsequent sustainable/resilient design. 3. from sustainability to resilience 3.1. sustainability sustainable development concepts, applied to the design, construction, and operation of buildings, can enhance both economic well-being and environmental health of communities and around the world and could interact more positively with the environment. benefits acquired include resource and energy efficiency, healthy buildings and materials, ecologically and socially sensitive land use, transportation efficiency, and strengthened local economies and communities. the united states green building council has established the leed assessment rating system, which was used as framework for many green building rating systems around the world including in asia pacific. the assessment aspects cover location & transportation, sustainable sites, water efficiency, energy and atmosphere, materials and resources, indoor environmental quality, innovation and regional priority. 3.2. resilience sustainability is all about protecting nature and the environment from human endeavours. however, there is need to protect humans from both the mother nature (climate change) and human threats by which the built environment has been confronted today. as a result, resilience has become the next evolution of sustainability [4]. hence, the society has a strong desire in advancing the sustainability to resiliency to cope with these vulnerabilities reactively and proactively. rdi [5] described resilience is the capacity to adapt to changing conditions and to maintain or regain functionality and vitality in the face of stress or disturbance. it is the capacity to bounce back after a disturbance or interruption. through resilience, livable conditions in the event of natural disasters, loss of power, or other interruptions in normally available services can be maintained. in the context of built environment, resilience means incorporating into the design of a building, aspects and features that allow the building to carry out its intended functions, now and in the foreseeable future [6]. the principles of resilient building design are: (a) meeting basic human needs including potable water, sanitation, energy, livable conditions, lighting, safe air and occupant health; (b) anticipation of interruptions and a dynamic future – adaptation to changing climate such as higher temperatures, flooding, earthquakes, solar flares and anthropogenic actions like terrorism and cyberterrorism; (c) diverse and redundant systems; (d) use low carbon-input materials systems; (e) maximising the use of day-lighting; (f) design for future flexibility of use e.g. modularity and standardization; (g) durability and robustness including strong building envelope; (h) systems that can be serviced/maintained with local material/parts and labours; (i) low energy inputs for constructability and ongoing building operations; (j) renewable energy for less reliability on grid power; and (k) water capture and storage, and usage reduction [6– 8]. the existing sustainability or green rating tools, such as leed can be used to assess whether new developments address both the adaptation and mitigation demands of climate change. there are overlaps between resilient design principles and the leed certification system and that some of the principles identified had already been incorporated by the existing leed rating system [9]. therefore, it is necessary to integrate resilience into the sustainable building certification system and consider the gaps in light of the local environment to incorporate the necessary resilient features in the development project life cycle. 3.3. pacific century place (pcp) jakarta, indonesia pcp situating in the new cbd area of jakarta with 40storey and 6 basement and gross floor area of about 150 000 m2, is a green, sustainable and prestigious grade a office for international tenants and was completed in 2017 to solicit a higher yield in the city by a hong kong developer. in light of the local context in which there were gaps in culture, technology savvy, body of management knowledge, mind-set and approach, the strategy to cope with the local situation as shown in figure 1 was crafted so as to transcend sustainability to resilience. the development was certified with platinum grade by leed and by greenship which have illustrated that applicable features of sustainability and resilience 567 dennis heung-fu mui acta polytechnica ctu proceedings figure 1. approach to sustainability and resilience for pcp jakarta [10]. such as site connectivity, heat island mitigation, landscaping and biodiversity, recycled materials, regionally manufactured materials, daylighting application, energy conservation, low emitting materials, heat recovery, increased ventilation etc. were incorporated. the more salient resilient features are succinctly described as below [9]. (a) redundancy and reliability – 100 % backup power by diesel generator; dual electrical risers and telecommunication lead in and risers; spare chiller cooling capacity for future increase in cooling load; and 100 % wi-fi coverage in common area and mobile network for connectivity; (b) water – reduction in water use by using efficient water devices and sanitary fitments; and deep wells as backup water supply; (c) flood and drought – zero run-off design; ground floor level at 1 m above the flood plain; installation of water gates to prevent backflow; critical equipment on level 6; long soak pond to control the site run-off; greywater, black water and rainwater recycling; and reducing landscape water use using drip irrigation system and indigenous plants; (d) flexibility – modular and standardized design; high floor to floor (4.5 m) and high false ceiling (3.05 m) with raised flooring system; variable air volume air-conditioning system integrated with lighting system; coordinated ceiling and floor grids, column spacing and façade modular size; and spare electricity and chilled water supply in each tenancy floor for future expansion; (e) energy conservation – low-e glass with external horizontal shading fins façade; waste heat recovery from the toilet exhaust; chillers optimization control; led lighting with daylight sensors in perimeter zone and motion sensors in staircase and toilets; vvvf lift system with destination control and regenerative braking; hence with 25 % and 33 % saving as compared to baselines of leed and greenship respectively; and (f) community – clear goals for sustainable parameters in tenancy lease, fit out guidelines and manual; post occupancy evaluation; measurement and verification plan to mitigate deviation of building performance. 4. smart city 4.1. definition of smart city smart city is a new concept which covers a continually evolving definition. generally speaking, a smart city is a city which capitalizes on new technologies and developments to enhance its systems, operations and service delivery. smart cities share one thing in common, the application of ict to connect and integrate the systems and services of the city [10]. a smart city is an urban area that uses different types of electronic internet of things (iot) sensors to collect data and then use insights gained from that data to manage assets, resources and services efficiently [11]. this includes data collected from citizens, devices, and assets that are processed and analyzed to monitor and manage traffic and transportation systems, power plants, utilities, water supply networks, waste management, crime detection, information systems, schools, libraries, hospitals, and other community services. smart city was defined by eight key aspects: smart governance, smart energy, smart building, smart mobility, smart infrastructure, smart technology, smart healthcare and smart citizen [12]. iso [13] defines 17 key indicators for evaluation of performances of cities from the point of view of ensuring urban services and quality of life i.e. economy, education, energy, environment, finance, fire and 568 vol. 38/2022 from intelligent building to smart city – a case study emergency response, governance, health, recreation, safety, shelter, solid waste, telecommunication and innovation, transportation, urban planning, wastewater, water and sanitation. nonetheless, a more encompassing definition is that the concept is not static, there is no absolute definition of smart city, no end point, but rather a process, or services of steps, by which cities become more “livable” and resilient and, hence, able to respond more quickly to new challenges [14]. 4.2. attributes of smart city against the above backdrop, the attributes and the respective elements of smart city are as follows: (a) smart environment – green and smart buildings, green energy, environmental monitoring (air pollution monitoring), managing environmental challenges, high efficiency air conditioning and ventilation systems, mitigation of the heat island effect, control of the site run-off; (b) smart living – safe, well managed and informed community, public information systems, complaint redressal, efficient management of day-to-day operations, smart retail and shopping experience, smart home; (c) smart community – active and healthy lifestyle, recreational facilities, community centers, public art, disabled accessibility, gaming community, medical and social care; (d) smart infrastructure, utilities and public realm – smart grid and energy management, integration of electrical vehicle, zero waste environment, smart water distribution and waste management (sewage and solid), water, waste and sewage monitoring, smart metering of natural gas, water, electricity consumption on real-time basis, smart logistic supply chain, information communication infrastructure, smart ambient lighting (led-based lamps and use of sensors to switch on/off automatically when necessary), public kiosks with internet, wi-fi hotspots, electromagnetic emission, climate control environment (e.g. landscaping); (e) smart mobility – smart car parking, prioritized clean and non-motorized options (e.g. bicycle, electric vehicle cars), smart traffic management including real time information sharing, smart parking places using sensors, walking path, traffic monitoring and control, multi-modal options, access management, minimizing the impact on the environment; and (f) smart governance – ict enables security, surveillance such as cctv and automatic name plate recognition system (anprs), secured data management and cloud storage. 4.3. purpose, objective and benefits of smart cities the goal for smart cities is to improve city management and residents’ quality of life through the efficient use of resources and service delivery whilst at the same time reducing environmental footprint [10]. the purpose of the smart cities mission is to drive economic growth, provide core infrastructure and improve the quality of life of people, enable a clean and sustainable environment. in the nutshell, smart city technologies improve the efficiency of city operations and services, thereby reducing energy and water consumption, cutting down traffic congestion and in turn carbon emission. more specific benefits include: (a) efficient public utilities reducing the inadvertent waste of water and electricity, and identifying times of peak usage and outages, ensuring proper efficiency and mitigating risks, reducing damages; (b) providing good living conditions for anyone living inside the city; thus increasing the overall morale of a city, leading to an overall increase in happiness and hence, the citizens will care more about the city itself and work to improve it further; (c) safer communities enabled by technologies such as anprs, security cameras in real time give law enforcement an edge while on the job; (d) connectivity which is the fabric keeping everything together in a smart city to create a more equitable environment for citizens and enabling people and their devices connected with public wi-fi hotspots that are strategically placed throughout the city offering reliable internet services to all residents; (e) more inflow of talent because smart cities give their citizens access to safer, happier living conditions, enabling the cities to have access to more talent and have a healthy mix of populous, thus a rising economy and gdp; (f) smart ecosystem, better sustainability because of reduced environmental footprint, air quality monitoring, energy use optimization, and electricity, water, and waste tracking, resulting in fewer ghg emissions, fewer kilograms of solid waste and water saving; (g) smart transportation with improved transportation saving the commuters tons of time and reduce fuel consumption; and also smart parking management, intelligent traffic signals optimizing traffic flow, relieving congestion during peak travel times; (h) smart and green buildings; and (i) big data which can be used for analytical purpose in commerce. in europe, there was a research concluding that smart cities would see an overall economic development of 5 % annually, which translates to almost 569 dennis heung-fu mui acta polytechnica ctu proceedings $20 trillion in a decade [15]. from the developer standpoint, smart city provisions will enhance the value of the investment because of higher rate of return due to sustainability and resilience of the development. smart cities in india offered substantial scope for price appreciation. kochi smart city could expect 30–40 % price appreciation over the next three years. gift development was expecting 10–15 % annual price increase. wave city and palava city have the respective rise of 9 % from june 2012 to june 2014 and 128 % appreciation from june 2010 to june 2014 [16]. 4.4. appraisal of a smart city development in south east asia the development is to be built on a land of 70 ha for a population of 140 000 and comprises of hospital, wellness center, religious church, residential, retail / shopping mall, car parking, office, data centre, duty free shop, hypermarket (diy & festive market), pharmaceutical hub, super automobile show rooms, financial hub, hotels, convention centre, police and fire stations. appraisal on features of the development for each attribute of smart city are depicted in table 1 (appendix a). 5. conclusion intelligent building was emerged in the 1980s because of the aspiration of the society to use technology to maximise the occupant performance, as well as the investment return of the development in terms of the operating cost such as energy efficiency and the flexibility to cope with the future demand of the tenants. green/sustainable building concept was subsequently developed to encompass the features of intelligent buildings but covering wider perspective of environmental protection and conservation as well as the well-being of occupants. to protect humans from the vulnerabilities of the mother nature and human threats reactively and proactively, resilience was evolving from sustainability. with the rapid advancement of ict in recent years, concept of smart city has been strongly advocated and is proliferating worldwide. whatever the development is intelligent building, sustainable and resilient building or smart city, it is important that an integrated approach be adopted to incorporate the features at the early stage of the project life cycle to maximise the benefits while optimising the investment cost. references [1] s. hawkins. the intelligent building and its potential for the uk. cme (february):31–32, 1987. [2] d. mui. building services for intelligent buildings – making a building smart. building journal hongkong china (july):126–128, 1992. [3] d. mui. rising high – hang seng bank. building services the cibse journal (may):42–43, 1992. [4] t. peng, l. lemay, j. hansen. resilience is the new sustainability. concrete infocus (january/february):26–29, 2012. [5] resilient design institute (rdi). what is resilience. [2016-05-12]. http://www.resilientdesign.org [6] y. alfraidi, a. h. boussabaine. design resilient building strategies in face of climate change. world academy of science, engineering and technology international journal of civil, environmental, structural, construction and architectural engineering 9(1):23–28, 2015. [7] resilient design institute (rdi). resilient design principles. [2016-05-12]. http://www.resilientdesign.org [8] c. l. champagne, c. b. aktas. assessing the resilience of leed certified green buildings. procedia engineering 145:380–387, 2016. https://doi.org/10.1016/j.proeng.2016.04.095 [9] d. mui. fostering sustainable building in indonesia by foreign developer for resilience. in conference proceeding of world sustainable built environment conference 2017 hong kong – transforming our built environment through innovation and integration: putting ideas into action, the hong kong convention and exhibition centre, hong kong, 5-7 june 2017, pp. 1979–1985. 2017. [10] central policy unit hksar. research report on smart city, 2015. the government of the hong kong special administration region, [2019-12-22]. https://www.pico.gov.hk/ [11] wikipedia. smart city. [2019-12-20]. https://en.wikipedia.org/wiki/smart_city [12] s. singh. smart cities – a $1.5 trillion market opportunity, 2014. forbes. [2014-11-04]. https://www.forbes.com/sites/sarwantsingh/2014/ 06/19/smart-cities-a-1-5-trillion-marketopportunity/?sh=26c25f916053 [13] international organization for standardization (iso). iso 37120 sustainable development of communities, 2014. [14] department for business innovation & skills, uk. smart cities: background paper, 2013. [2020-01-25]. https://www.gov.uk/government/publications/ smart-cities-background-paper [15] k. vyas. 10 ways smart cities will restructure the economy, 2019. [2020-01-17]. https: //interestingengineering.com/innovation/10ways-smart-cities-will-restructure-the-economy [16] r. yadav. are smart cities a good options for property investors?, 2014. money today. [2020-02-26]. https://www.businesstoday.in/moneytoday/realestate/smart-cities-in-india-as-propertyinvestment-destinations/story/210791.html 570 http://www.resilientdesign.org http://www.resilientdesign.org https://doi.org/10.1016/j.proeng.2016.04.095 https://www.pico.gov.hk/ https://en.wikipedia.org/wiki/smart_city https://www.forbes.com/sites/sarwantsingh/2014/06/19/smart-cities-a-1-5-trillion-market-opportunity/?sh=26c25f916053 https://www.forbes.com/sites/sarwantsingh/2014/06/19/smart-cities-a-1-5-trillion-market-opportunity/?sh=26c25f916053 https://www.forbes.com/sites/sarwantsingh/2014/06/19/smart-cities-a-1-5-trillion-market-opportunity/?sh=26c25f916053 https://www.gov.uk/government/publications/smart-cities-background-paper https://www.gov.uk/government/publications/smart-cities-background-paper https://interestingengineering.com/innovation/10-ways-smart-cities-will-restructure-the-economy https://interestingengineering.com/innovation/10-ways-smart-cities-will-restructure-the-economy https://interestingengineering.com/innovation/10-ways-smart-cities-will-restructure-the-economy https://www.businesstoday.in/moneytoday/real-estate/smart-cities-in-india-as-property-investment-destinations/story/210791.html https://www.businesstoday.in/moneytoday/real-estate/smart-cities-in-india-as-property-investment-destinations/story/210791.html https://www.businesstoday.in/moneytoday/real-estate/smart-cities-in-india-as-property-investment-destinations/story/210791.html vol. 38/2022 from intelligent building to smart city – a case study a. appendix – table 1 designed elements recommended enhancements attributes 1 – smart environment eia to mitigate the impact on environment; ground level of the site 1.6 m above the normal ground level of neighborhood for future flood proof; box culvert across site for drainage discharge of neighboring sites to minimize flooding potential; centralized potable water treatment plant to reduce the reliance on plastic bottle water and plastic bottle waste; reliance on hydro plant to provide green energy. led instead of hps for the street lightings; water-cooled air conditioning; green building certification for each plot; metering of electricity and water consumption to provide real time data to the portal dashboard of development; air pollution monitoring station to provide real time levels of co, no2, o3 and rsp pm 2.5; softscape with area at least 20 % of the total site area to reduce heat island effect; use of pervious/permeable pavers for on grade hardscape to control run-off and minimize flooding potential; formulation/implementation of waste management plan to reduce waste; promoting the use of electrical car by installing ev chargers. attribute 2 – smart living fire services and police station inside the development to enhance peace of mind of the community; site fenced off with controlled access points and equipped with security cctv; central control to monitor the security and fire safety. formulation of mobile app and portal to provide e-services, public information and real time data on weather and air pollution data, parking space availability, traffic information, smart retail and shopping experience; smart home for residential development; 24 hours manned central control to monitor security and fire for enhancing safety and peace of mind of the community; extensive coverage of cctv with future proof quality for incorporating facial recognition and avnpr systems. attribute 3 – smart community mix land usages to achieve a self-sustainable development and community; 24 hours operation european medical center / hospital and wellness center to enhance the health of the community; community centres and religious community halls and bicycle tracks to provide an active and healthy lifestyle; international standard of facilities management service level to the community, proper operation and maintenance of the infrastructure assets, training and educating the local human resources on the best practices. street furniture and jogging path to provide recreational facilities and create active and healthy lifestyle; bicycle parking to promote healthy life style; extensive softscape to provide amenities and healthy environment to community lifestyle; infrastructure and each plot to incorporate barrier free access (disabled accessibility). 571 dennis heung-fu mui acta polytechnica ctu proceedings designed elements recommended enhancements attribute 4 – smart infrastructure, utilities and public realm centralized potable water treatment plant to control the quality of water and monitoring the usage; centralized sewage treatment plant enabling the monitoring of the waste water and ensuring the discharge standard; centralized refuse collection point to allow collection, storage, sorting and disposal of solid waste; new hydro plant to enhance the reliability and capacity of the electricity supply; own step down transformers with spare transformers to meet future demand and dual feed to increase the reliability of supply; telecommunication sub-station to improve the service level and future flexibility to expand the services; street lamp controlled by photo sensors. softscape and hardscape with street furniture, amenities and jogging path to improve public realm and climate control; incorporating 4g (lte) or 5g small cells on the smart lamp pole to enhance the connectivity; cctv at strategic locations to monitor traffic and give real time information in the portal smart metering for energy management and water consumption; smart metering at the centralized sewage treatment plant to monitor the disposal data; centralized refuse collection point in the infrastructure to reduce and recycle waste; ev charger provisions in the infrastructure to promote the use of electrical car. attribute 5 – smart mobility traffic impact assessment on car parking provisions, loading/unloading requirements, capacity of the road network, type of control, configuration at intersections and access locations; road divider, signage, traffic signalization and road hump to smoothen the traffic; clean and non-motorized option i.e. bicycle track network infrastructure; site fenced off with vehicle access control points; delivery to hypermarket planned outside the site to avoid affecting the traffic inside the zone; road designed to be all season ones; close proximity to highways, conventional train stop and high speed train station. smart parking system in each plot to provide real time indication of the spaces availability locally (parking sensors) and remotely at car park entrances and individual floors space panels as well as the portal of the development; bicycle parking to enhance clean mobility. attribute 6 – smart governance data center to enable data security, ict provision and secured storage of the operational data. reliable infrastructure backbone with sufficient capacity to facilitate the current and future smart city attributes. table 1. appraisal of the smart city features in the south-east asia development. 572 acta polytechnica ctu proceedings 38:566–572, 2022 1 introduction 2 intelligent building 2.1 definition and objectives 2.2 hang sang bank new headquarters building, hong kong sar 3 from sustainability to resilience 3.1 sustainability 3.2 resilience 3.3 pacific century place (pcp) jakarta, indonesia 4 smart city 4.1 definition of smart city 4.2 attributes of smart city 4.3 purpose, objective and benefits of smart cities 4.4 appraisal of a smart city development in south east asia 5 conclusion references a appendix – table 1 https://doi.org/10.14311/app.2022.33.0564 acta polytechnica ctu proceedings 33:564–570, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague unique and innovative technology for sustainable and efficient structural concrete made of 100% recycled aggregate from cdw bohuslav slánskýa,∗, pavel zelinkab, jan čermákc a skanska a.s., head of technical research and innovation, křižíkova 682/34a, 186 00 praha 8 karlín, czech republic b skanska transbeton s.r.o., chief technologist, toužimská 664, 190 00 praha letňany, czech republic c eerc-tech, head of r&d, vyšehradská 1349/2, 128 00 praha 2 nové město, czech republic ∗ corresponding author: bohuslav.slansky@skanska.cz abstract. for supporting an environmental and social approach to the construction industry, the authors are bringing a new unique technology for maintaining the production of sustainable and efficient structural concrete made of 100% of recycled aggregate from construction demolition waste (cdw) as a replacement of natural aggregate. a wide range of laboratory testing of concrete with a grade up to c 30/37 using brick and concrete recycled aggregate as well as pilot projects will be presented with very successful results. this has been done by taking a different approach to the concrete mix design and production using new modern materials as well as new methods. then full-scale production testing proved the new concept with only a minor adjustment in concrete plant technology. this new solution has very positive environmental impact and cost optimization shows its great potential for use as a structural concrete supporting a circular economy, saving natural sources, minimizing dumping and landfilling of cdw as well as reducing the co2 footprint. this innovative technology makes the reuse of cdw possible in concrete with higher added value in the construction production chain (upcycling) compared to conservative cdw downcycling. this unique concrete production together with proper cdw management brings with it a quite cost-effective contribution to the building industry and market. keywords: concrete, demolition waste, recycled aggregate. 1. introduction the substitution of natural aggregates in concrete by recycled ones brings both environmental and economic benefits. in terms of environmental protection, it involves the utilization of rarely used, yet recyclable raw materials, which are largely deposited in landfills or dumps, while maintaining these landfilling capacities for materials that cannot be further used. at the same time, it is saving natural aggregate resources for applications where irreplaceable. the reduction of the traffic exposure is another benefit of this method, as the resources of cdw coming from the demolition of old buildings are usually closer to concrete mixing plants than quarries and sand borrow pits usually located far outside of urban areas. in business terms, this method reduces material costs in tens of percent by replacing expensive natural aggregates by what is basically considered a cheap waste. the results of this paper were compared with other research results, where is used 100 % of recycled aggregates. in [1] are compared influences of bricks recycled aggregate on properties of concrete, there is one relevant mixture rmac d with 100 % of recycled aggregates. in mixture rmac d is used 70 kg more cem i 42.5 r than in mixture in table 5. the most significant difference is in used ra, in [1] were used three fractions of ra 0/4, 4/8, 8/16 mm, in this paper is used only mono fraction 0/22 mm, which saves economical and ecological costs during recycling of cdw. the other difference is in parameters of concrete with ra, water absorption is 10 % lower with this technical solution. 2. principles of the technical solution partial substitution of the natural aggregate in concrete by recycled one has been a standard procedure. however, our research and development focused on the production of concrete made solely from the recycled aggregate (ra) including fine fractions of up to 4 mm (100% replacement of natural aggregate including sand). several years of laboratory testing have been finalized with success and new technology has been developed that was patented both in the czech republic as well as internationally. the patented solution used for the production of concrete from cdw has been launched in the czech republic under the registered trademark of rebetongő. the technical solution starts with the proper preparation of ra with demanded granulometry prescribed by technical specifications. the main technical know-how then continues in the production 564 https://doi.org/10.14311/app.2022.33.0564 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 33/2022 cdw structural concrete process of concrete mixing using "nanofiller", the particles of which reinforce the structure of aggregates itself. the final concrete mixture, in general, achieves similar performance properties as conventional concrete made of natural aggregates, with a strength class of up to 30 mpa including waterproof and frost resistant properties. the use of nanofiller, as well as the reactivation of the binding components contained in recycled aggregate, allows to use the same or even lower cement dose as with quality natural aggregate. due to the softer nature of recycled aggregate, the modulus of elasticity is lower which has to be taken into account in structural design. the used additive ensures the durability of concrete products while allowing for their full recyclability thus completely eliminating future environmental burdens and meeting the strictest views of a circular economy in the construction industry [2, 3]. 3. the development process from the laboratory to industrial production in the concrete plant the performance parameters of concrete made of 100% recycled aggregate are the outcome of the long-term development and laboratory testing in the czech republic with final results in table 1. that was followed by the joint development with the construction company for further industrial optimization and upscale. the first laboratory tests in their laboratories made with concrete mix design in 3 basic modifications using 100% concrete recycled aggregate (rc-c), 100% brick recycled aggregate (rc-b), or mixed recycled aggregate (rc-m) showed surprisingly good results see table 2. this confirmed the high potential of ra concrete for commercial use on construction and development projects. therefore, operational tests have been started immediately at the pilot concrete mixing plant, again with very good results (see table 3). the joint development brought among others the further optimization of production processes, the verification of feasibility in the conditions of industrial production at the concrete plant, and the verification of technological as well as rheological properties of concrete from ra during the production, transport, and casting. based on this good experience the production of concrete made of 100% ra has been certified and since september 2019 branded the certified production rebetong has started. it offers ready-mix concrete in strength classes up to c 25/30, exposure class up to xc4, xf1, and prefabricated wall blocks with the reblok trademark. it focuses on the production of the concrete made of 100% ra mono-fraction 0-22 mm from brick cdw, which makes leftovers in the market and the use of which has a greater economic as well as environmental effect. 4. results of the testing within the framework of research and development, laboratory tests have been performed both on fresh and hardened concrete. as a standard, consistency, air content in fresh concrete, compressive strength of concrete, bulk density of fresh and hardened concrete, have all been performed. on selected samples, concrete bending tensile strength and transverse tensile strength, frost resistance, concrete resistance to de-icing solutions, static modulus of elasticity, water absorption, volume changes of concrete, depth of pressure water infiltration and alkali-silica reaction tests have all been tested. the results of laboratory and production tests show good performance properties of concrete with 100% recycled aggregate made of concrete and bricks. table 1 shows final laboratory result during development phase and table 2 indicate the results of laboratory tests made prior to the industrial production. after laboratory testing, industrial production tests were carried out on the bhs dkxs mixing plant with the mixer volume of 1.67 m3. recipes for concrete strength classes c20/25 and c25/30 with 100% recycled brick aggregate were tested during the first production tests. the test results are indicated in table 3. other concrete parameters tested during certification are shown in table 4, the example of the most commonly used concrete recipes is shown in table 5. one of the success factors is the production of the recycled aggregate from cdw. it took some time and several steps to set up optimize production process, select most suitable equipment and proper set up of each machine in the production chain. development of the concrete made of 100% ra started with 2 fractions 0-8 and 8-16 mm. after some steps of optimization of the ra production the mono-fraction 0-22 mm with similar granulometry was managed. this was an important step for efficient production at concrete plants as they usually lacking of storage space for more fractions of aggregate. also a source of the cdw is important and the best possible way is to manage the whole process starting with selective demolition and create a large storage of the recycled aggregate with similar properties. the final recycled aggregate is tested according en12620 + a1 and furthermore a prescribed limited gradation curve for granulometry is specified see figure 1. storage of the recycled aggregate is similar to natural one. the sensitivity test for the moisture in aggregates between 0 % to full saturation have been done with no significant differences. during the operational tests, the workability of concrete made of recycled aggregate, its behavior, transportability and possibilities of placement have been verified. the behavior of recycled fresh concrete has been found to be similar to conventional natural aggregate concrete. 565 b. slánský, p. zelinka, j. čermák acta polytechnica ctu proceedings sample no. type of rca consistency strength class compressive strength [mpa] 7 days 14 days 28 days 1 rc-c s4 c25/30 18.8 28.4 35.0 2 rc-b s4 c25/30 19.8 27.8 35.3 3 rc-m s4 c25/30 22.5 31.0 38.0 bulk density [kg/m3] tensile bending strength [mpa] tensile splitting strength [mpa] water absorption [%] static modulus of elasticity [gpa] depth of penetration of water [mm] 1 2150 4.7 2.8 3.5 18.6 22.0 2 1920 3.7 2.4 3.2 12.9 16.0 3 2100 4.4 2.5 4.4 14.8 18.0 table 1. results of final laboratory tests during initial development. strength class type of rca consistency sample no. compressive strength on cubes [mpa] bulk density [kg/m3] coefficient of frost resistance 2 d 7 d 28 d 92 d c25/30 rc-c s3 1 11.5 27.2 41.1 45.0 2115 − c25/30 rc-b s4 2 12.2 29.7 44.2 48.5 1913 0.98 c25/30 rc-c s4 3 10.6 25.9 40.0 42.0 2132 1.00 c25/30 rc-b s4 4 13.5 29.3 44.7 48.8 1928 − c25/30 rc-m s4 5 14.4 32.7 46.5 51.7 2043 0.99 c25/30 rc-m s4 6 13.3 30.4 44.4 49.8 2027 − table 2. results of laboratory tests prior start of industrial production. strength class type of rca consistency number of samples compressive strength [mpa] bulk density [kg/m3] depth of water penetration [mm] 7 d 28 d c 20/25 rc-b s4 20 22.0 32.4 1990 19 (6 samples) c 20/25 na s3 11 20.5 28.1 2260 − c 25/30 rc-b s4 6 25.6 36.9 1990 18 (4 samples) table 3. average results of operating tests during industrial production. age of samples [day] compressive strength [mpa] flexural tensile strength [mpa] bulk density [kg/m3] tensile splitting strength [mpa] frost resistance (100 cycles) [−] water absorption [%] static modulus of elasticity [gpa] depth of water penetration [mm] shrinkage [%] c 25/30 with 100 % rc-b 7 24.1 4.3 2020 2.7 0.92 4.1 17.6 16 0.428 14 31.8 0.697 28 37.8 0.977 56 41.0 1.188 c 25/30 with 100 % rc-c 7 19.7 3.9 2110 2.4 0.99 3.9 20.9 21 0.558 14 22.8 0.854 28 35.1 1.119 56 37.5 1.307 table 4. results of concrete parameters during certification. 566 vol. 33/2022 cdw structural concrete component quantity [kg/m3] 100% rc-b 100% na cem i 42,5 r 250 270 ground limestone / or fly ash 50 50 nanofiller (microsilica) 25 − natural sand 0-4 mm − 870 natural crushed aggregate 4-8 mm − 250 natural crushed aggregate 8-16 mm − 700 brick recycled aggregates 0-22 mm 1500 − admixture (plasticizer) 5,5 2,0 water 210 165 table 5. comparison of concrete mix design for c 20/25 with ra and na. figure 1. stress diagram of a strengthened beam. source: adapted from silveira [4]. strength class type of ra time [min] air content [%] slump test [mm] temperature of concrete [◦c] c25/30 rc-b 0 4.4 210 23.3 30 4.2 210 23 60 4.0 160 22.5 table 6. results of workability testing in time. �� � )ljxuh����3lfwxuhv�iurp�erqg�vwuhqjwk�whvw��figure 2. pictures from bond strength test. 567 b. slánský, p. zelinka, j. čermák acta polytechnica ctu proceedings )ljxuh���� � � )ljxuh����6kulqndjh�ghyhorsphqw�whvw��ohiw�±������eulfn�5$��uljkw�±������1$�� figure 3. shrinkage development test: left 100% brick ra, right 100% na. ��� � figure 4. concrete placement during the first semi-operational tests and concrete blocks. figure 5. the first ready mix concrete placement. �� � figure 6. residential project devil’s hill in prague and first concrete wall. 568 vol. 33/2022 cdw structural concrete due to the higher fluctuation of the absorption of ra, especially the one consisting of bricks, it is necessary to pay attention to the workability time of the fresh concrete and to use special additives to ensure good workability for at least 60 minutes. within the testing, consistency using the slump test was determined over a time horizon of sixty minutes on brick recycled concrete to verify the workability and transportability of recycled concrete. the results are displayed in table 6. after satisfactory standard test results some additional over-standard tests, according to [5], were carried out for concrete made of 100% brick recycled aggregate. for example reinforcement bars bond test according to [4] with final bond strength of 5,2 mpa compare to reference result of 5,4 mpa for a concrete made fully of natural aggregate (na). pictures from the test see in figure 2. other additional test was the shrinkage development test according to [6]. resulting graphs are in the figure 3. 5. pilot projects, application examples the concrete made from 100% ra has performance properties similar to the conventional concrete made from natural aggregates. therefore, it can be used for common plain and reinforced concrete structures. when designing horizontal load-bearing structures, consider the lower modulus of elasticity (see chap. 4.). customers have become soon interested in the prefabricated wall blocks, of which almost 2000 pcs have already been produced see figure 4. as a presentation of the pilot production, these precast units were used for the construction of the parkour playground at "cukrkandl" in prague. the first pilot project of a ready-mix concrete made of 100% rc-b was executed for a base concrete for a commercial multi storey project in olomouc in a volume of 350 m3 see figure 5. the next complex pilot project was a residential project called devil’s hill in prague, where 2000 m3 of 100% rc-b were placed for base slabs and reinforced concrete walls see figure 6. 6. technical legislation currently, not only within the czech republic but throughout the eu, there are significant barriers in technical regulations that significantly reduce the use of recycled aggregates in concrete. although properties of ra are specified in european standard en12602 + a1, the european standard for concrete en206 + a1 recommends the use of coarse recycled aggregate in concrete only up to a maximum of 50% for types a and b, and only for the x0 exposure class, and even less for other exposure classes. to support wider use of ra in concrete, it is necessary to change this conservative normative approach and focus on proving the final properties of the concrete without limiting the ra content. in the czech republic, this process, thanks to significant support from the ministry of the environment, and ministry of industry and trade, started in september 2019 and its aim is to create a revised national standard for concrete with up to 100% of ra (čsn p 73 2404). at the time being the only possible way to apply 100% recycled aggregate in concrete is the procedure according to the national act no. 163 i.e. certification of the system of production control of concrete from ra and introducing the product to the market on a basis of building technical approval (sto). unfortunately, this procedure cannot be applied to projects where the contractor is contractually bound by applicable national and european standards. at the moment, this applies for the majority of public procurement projects. however the situation is slowly but surely changing as public investors are increasingly becoming aware of the importance of having a sustainable construction in public procurement and a circular economy. recently they are coming up with requirements in their projects for construction products made from secondary, recycled raw materials. 7. conclusion the parameters determined during laboratory and operational testing show a great potential of concrete made from 100% recycled aggregate, since its performance properties are comparable with conventional concrete in most indicators. the patented solution is in full compliance with the principles of a circular economy, where all inert cdw can be converted back into the building material with excellent properties, which is in addition fully recyclable using the same technological process. considering the ever-decreasing stocks of natural aggregates, the use of alternative resources is economically and environmentally beneficial. the use of a patented technological process also results in a reduction in the carbon footprint by more than 10%, as less cement is used for the production of concrete and the otherwise rarely used cdw are used as a replacement of natural aggregates. references [1] t. pavlu, k. fortova, j. divis, et al. the utilization of recycled masonry aggregate and recycled eps for concrete blocks for mortarless masonry. materials 12(12), 2019. https://doi.org/10.3390/ma12121923. [2] j. čermák, j. fiala, f. polak. comparison of utility properties of concrete with different types of recycled aggregates. brno: brno university of technology, faculty of mechanical engineering, 1996. [3] b. slánský, p. zelinka, j. čermák. beton z recyklovaného kameniva. in: betonářské dny: konference s mezinárodní účastí. pardubice: česká betonářská společnost, 1993. 569 https://doi.org/10.3390/ma12121923 b. slánský, p. zelinka, j. čermák acta polytechnica ctu proceedings [4] čsn en 73 2404. concrete: specification, performance, production and conformity additional information. prague: czech normalization institut, 2016. [5] čsn 73 1328. determination of shear adhesion of steel to concrete. prague: czech normalization institut, 1971. [6] önorm b 3329. grout: requirements and test methods. austrian standards, 2009. 570 https://doi.org/10.14311/app.2022.33.0045 acta polytechnica ctu proceedings 33:45–51, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague sustainability of concrete pavements considering traffic and de-icing agents rolf breitenbücher, robin przondziono∗ ruhr university bochum, department of civil and environmental engineering, institute for building materials, universitätsstrasse 150, 44801 bochum, germany ∗ corresponding author: robin.przondziono@ruhr-uni-bochum.de abstract. in concrete pavements, special conditions, that increase the likeliness of damaging reactions, as for example an alkali-silica reaction (asr), prevail. especially to the superposition of microstructural degradation caused by cyclic loading with an external alkali supply can influence the sustainability negatively. concrete pavements are subjected to cyclic loadings by traffic and climate changes. these cause microcracks (about 5 µm) within the concrete matrix during service. additionally, an asr in pavements is promoted by externally supplied alkalis (de-icing agents). by superposition of both effects the alkali capacity close to the asr-reactive aggregate aggregate is increased substantially as the externally supplied alkalis can easily penetrate through the microcracks. thus, both effects intensify the asr in concrete pavements. within cooperative research projects the different interdependent influencing factors for a damaging asr in concrete pavements are studied by experiments as well as by numeric modelling. on the micro-level the asr-related processes within the aggregate, such as gel-formation or ion-transport, are investigated. on the meso-level, the project focuses on the characterization of degradation effects in the concrete microstructure due to cyclic loading. further, special attention is paid to the transport behavior of fluids in such pre-damaged concrete structures. a significant increase of the penetration depth of alkaline solutions could not only be observed at different stages of progressing degradation. it becomes also obvious that the ingress of externally supplied alkalis is enhanced by the overrunning traffic. finally, on the macro-level, the risk of an asr-damage is assessed. keywords: degradation, durability, transport. 1. introduction concrete pavements are subject to static, cyclic and dynamic influences. in order to assess theses loads, different stress frequencies should be considered. due to changing climate conditions, thermally and hygrally-induced deformations are inevitable. if those deformations are restrained, restraint stresses are caused that depend both on the degree of restraint and on the stiffness of the structural component [1? ]. these stresses are changing when exposed to varying influences. in this context, it has to be distinguished between daily temperature changes (day night) and seasonal temperature and moisture changes (summer winter). these effects are characterized as lowfrequent. apart from these low-frequent influences, high-frequent load-dependent stresses due to traffic also act on a concrete structure. especially concrete pavements and likewise wind energy plants are exposed to a high number of load cycles day by day. these effects should be considered as high-frequent. such cyclic influences only rarely cause macrocracking, which would lead to a fatigue failure of the whole structure. however, due to these loads fine cracks might form in the concrete’s microstructure, which may then lead to the gradual degradation of the concrete properties, while, at the same time, the material’s stiffness decreases [2–4]. comparative ultrasonic runtime measurements are an appropriate non-destructive method to assess these effects [5]. the extent of degradation is dependent on two factors: the number of load cycles and the magnitude of stresses. this relation is usually described as the ratio between the stress σo and the adequate strength f. in pavements, in which degradation is caused by compressive as well as tensile stresses, the tensile properties are more relevant (σo/fct, fl). at the same time, it is essential to superpose the actual stresses with those that are already active in the concrete itself, i.e. residual stresses caused by former hydration heat release [1]. as suggested by holmen [6], at a stress-ratio of σo/fct, fl, below approx. 40 %, microcracks will not turn into macrocracks, even after millions of load cycles. at a stress-ratio above 65 %, fatigue failure of the concrete may occur considerably earlier (already after a few 10.000 load cycles). nevertheless, the formation of microcracks due to cyclic loading plays a crucial role for the durability of concrete pavements. as compared to undamaged concrete structures, such microstructural degradation alters the penetration behavior of fluids into concrete 45 https://doi.org/10.14311/app.2022.33.0045 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en rolf breitenbücher, robin przondziono acta polytechnica ctu proceedings figure 1. stresses and cracks in concrete pavements due to negative gradients of deformation [7]. [8]. in general, the transport of water in porous media depends on the size, type and form of the pores or microcracks. in addition, it also depends on the transport mechanisms and the physical laws that are effective in the respective pores / microcracks (i.e. chromatography effect). apart from diffusion, capillary transport is essential when it comes to fluid transport processes, since dissolved alkalis penetrate much deeper into the interior of the concrete structure and may hence increase the asr-damage potential. the penetration of alkalis into undamaged concrete structures and their influence on damaging asr have been sufficiently studied [9, 10]. however, the findings of these studies are not definitive. usually, an increased alkali concentration in the concrete’s nearsurface zone could be observed for specimens stored in nacl-solution. asr as a result of such an increased alkali concentration could only be observed, if the phvalue of the concrete was comparatively high [10, 11]. accordingly, the asr-risk was significantly lower in concretes that used cement which contained ground granulated blast furnace slag (ggbs), because it consumes ca(oh)2 in a latent hydraulic and pozzolanic reaction and thus lowers the ph-value [11, 12]. in this context, it should be noted that exhaustive studies on the interaction of an increased alkali ingress due to microstructural degradation of cracks with a width between 1 and 10 µm in relation to asr progression are yet to be performed; previous studies were conducted on undamaged specimens. related studies have shown, however, that the load of the surface traffic increases the penetration depth of alkalis and correspondingly, the asr-potential is promoted as well. 2. influencing factors on concrete pavements 2.1. degradation due to the continuous construction of concrete pavements, deformations in them are practically completely restrained. in the case of thermal or hygral changes, which extent over the complete crosssection, longitudinal restraint stresses are generated. during the wintertime, tensile stresses are raised while during summertime adequate compressive stresses are induced. although, to some extent a movement is enabled in transverse direction, restraint stresses cannot be excluded completely in this case. such longitudinal restraint stresses, constant over the complete cross-section, can result in throughcracks with nearly constant width. besides these, bending stresses can also be generated by a temperature gradient. in the case of a cooler surface, the slab would tend to warp. this however is prevented by its own weight, so that in the surface zone tensile stresses are provoked, while in the lower zone (next to the subbase) compressive stresses develop. if these bending stresses exceed the concrete flexure strength, uniform cracks arise with a larger crack width at the surface. furthermore, map cracking can be provoked by residual stresses (often also named as "eigenstresses"), when a non-linear temperature gradient arises. in this case, the net-like cracks extend only to a depth of a few millimetres. an overview of stresses acting on concrete pavements and corresponding potential damages is given in figure 1. microstructural degradation as a result of the concrete’s cyclic loading can be assessed on the basis of ultra-sonic runtime measurements of the longitudinal acoustic wave close to the concretes surface. based on the results of the ultrasonic runtime measurements, 46 vol. 33/2022 concrete pavements sustainability figure 2. relative dynamic e-modulus of the service lane (ss) and the driving lanes (1.fs and 2. fs) [13]. the relative dynamic e-modulus can be calculated [13]. thereby, comparative ultrasonic runtime measurements are an appropriate non-destructive method to assess these effects. sievering [13] measured the surface-near ultrasonic runtime on concrete pavements of german highways. herein the service lane was compared to the driving lanes, as it was exposed to significantly less cyclic traffic loads. correspondingly, the ultrasonic runtime was higher in the driving lanes and thus the calculated relative dynamic emodulus significantly lower in those lanes (figure 2). this indicates a higher microstructural degradation within the driving lanes, which is caused by the additional traffic loads compared to the service lane. 2.2. external alkali supply in common concrete structures an asr can be prevented by the limitation of the alkali content in the cement as this is normally the single alkali source. in concrete pavements, however, the external alkali supply by de-icing salts plays an important role on the asr-gel-formation as so, the total amount of reactive alkalis is increased significantly. in this context, also a modification in the application of the de-icing salt is relevant. in former times salt was dispersed, only when ice or snow were already present on the pavement. for some years these agents have been already applied, when such conditions were forecast. that way, the alkalis are in touch with the concrete surface in a stage at which the concrete is well absorptive. the absorption in the concrete is particularly enhanced by the above mentioned microcracks. further the de-icing agents are pressed quite intensely into the concrete microstructure by the overrolling traffic. hence, in combination with the prophylactic application of nacl the external support of alkalis into concrete pavements currently is significantly intensified in comparison with the past. 3. experimental investigation 3.1. degradation in experimental studies, large concrete beams measuring 180 × 50 × 27cm3 were exposed to cyclic stresses. all beams were produced using a typical pavement concrete mix according to the german guideline "tl beton-stb 07" [14]. as for a cement, cem i 42 n with a maximum aggregate size of 22ămm was chosen. finally, the concrete surface was broom finished. the concrete beams were exposed to cyclic stresses at a minimum age of 56 days. the tests were conducted in a four-point-flexural setup, in which the chosen upper and lower stresses represent different stress conditions of superposing loads acting on a concrete pavement. slowly recurring thermal restraint stresses were to be superposed with high-frequent cyclic traffic stresses. thus, an upper stress level and a corresponding lower stress level (σmax and σmin) could be derived from these boundary conditions [6]. the study yielded the following variations: • maximum stress to bending tensile strength (σo/fct, fl = 0.35; 0.50; 0.60; 0.70). • number of load cycles (n = 0; 1.0; 2.0; 5.0; 10.0 million) figure 3 shows the development of the relative dynamic e-modulus for the different stress-ratios as well as over the course of different load cycles as an average of three specimens each. as expected, the rel. dynamic e-modulus decreased stronger with a higher stress-ratio. after five million load cycles, the remaining relative dynamic e-modulus amounted to 47 rolf breitenbücher, robin przondziono acta polytechnica ctu proceedings figure 3. decrease of the rel. dyn. e-modulus of beams after 5 million load cycles and at different stress-ratios. rel. dynamic e-modulus [%] 100 92.7 91.5 86.5 80.2 70.2 number of cracks [-] 12 74 104 128 203 403 average crack width [µm] 6.6 5.2 5.3 4.8 5.7 4.6 average crack length [µm] 1,400 1,600 1,000 1,400 1,600 1,500 average crack cross section [µm2] 13,400 8,100 6,100 6,700 9,200 8,500 total cross section [µm2] 160,000 600,000 630,000 860,000 1,800,000 3,450,000 table 1. crack characteristics. about 91 % for a stress ratio of σo/fct, fl = 0.35 and to about 87 % and 75 % for σo/fct, fl = 0.50 and σo/fct, fl = 0.60, respectively. after completion of the cyclic loading a microscopic investigation of the crack characteristics on thin polished sections (5 × 5cm2) has been conducted. table 1 lists the corresponding crack characteristics. compared to the undamaged reference specimens (rel. edyn = 100%), with decreasing relative dynamic e-modulus, the number of cracks increased significantly. at a relative dynamic e-modulus of about 70.2 % the number of cracks increased by more than factor 30. consequently, the total cross section of cracks within the thin polished sections increased similarly. nevertheless, the average crack width and length stayed rather constant at about 5 µm and 1.5 mm, respectively. it can thus be concluded that the observed increased degradation rather resulted from the formation of new cracks than from the expansion of already existing cracks. 3.2. external alkali supply the described microstructural degradation also influenced the penetration behavior of fluids into the concrete. an investigation of the capillary water uptake in concrete was carried out on specimens previously subjected to cyclic loads with varying degrees of damage. therefore, each three karsten-tubes were applied on the surface exposed to tensile stresses. the water uptake was assessed over time. it showed, that the water uptake (more or less merely through capillary suction) occurred faster and higher with increasing damage (figure 4). in a pre-damaged concrete with a relative dynamic e-modulus of approximately 92.7 % the water uptake already increased by about 60 %. at a damage of rel. edynă = ă70.2ă% the water uptake increased by about 250 %. alkali solutions not only penetrate in in-situ concrete pavements through capillary suction, their ingression is in fact considerably increased by the load of the surface traffic. in a test setup that was developed at the ruhr university bochum to specifically explore these effects, the aforementioned concrete beams were arranged in a circular array, to ensure that they could be loaded with six tires consecutively. the top surface of the beams had previously been exposed to flexural tension in the cyclic fourpoint-flexural setup. while the beams were loaded, their top surface was treated with a sodium-chloride solution of 5%-nacl. the tires that were used to apply the load were additionally equipped with a dead weight of up to one ton in order to achieve near to real-life loading conditions for each beam. since the solution was only pressed into the material within the track width of the tires, a merely capillary solution uptake was to be expected for the adjacent areas. this setup thus allowed the comparative assessment of the two types of liquid penetration into the con48 vol. 33/2022 concrete pavements sustainability figure 4. water uptake in concrete with different relative dynamic e-modulus. figure 5. average chloride penetration depth dependent on the number of tire passes and microstructural damage 49 rolf breitenbücher, robin przondziono acta polytechnica ctu proceedings crete [15]. after the overrolling of the large-format concrete beams up to two million times, two small specimens (10×10×200cm3) were cut from each of the different areas of liquid penetration. the specimens were then split perpendicular to the overrolled surface. the freshly broken surfaces were sprayed with silver nitrate to visualize the chloride penetration depth. the average penetration depths over the cross section of the specimens are visualized in figure 5. the average penetration depth on the specimens with the most damages (rel. edyn = 80.2%) without tire passes amounted to about 23.8 mm already 3ămm above the penetration depth on the undamaged specimens (rel. edyn = 100%). this effect was also observable for the less damaged specimens (rel. edyn = 87.6% and rel. edyn = 93.3%) but less distinctive. the average penetration depth increased only by 0.3 and 0.4ămm respectively. it can further be noticed, that with increasing tire passes, the penetration increases in damaged as well as undamaged specimens. the average penetration depth of an undamaged specimen after 2 million tire passes already amounted to 27.8 mm, whereas for a strongly damaged specimen (rel. edyn = 80.2%) the average penetration depth amounted to 35.2 mm. the influence of the overrolling traffic could not be observed for comparatively little damage (rel. edyn = 93.3%). concludingly, it can be stated, that the influence of the locally induced external pressure through the tire passes is increasing fluid penetration. however, this effect is limited to a depth of about 2 to 4 cm and becomes more significant with increasing microstructural damage. 4. final discussion and conclusion the presented results showed a distinct influence of cyclic mechanical loadings on concrete. due to microstructural degradation, the penetration of liquids into porous concrete is altered and therefore the potential of an intensified damaging asr increases considerably. the dependence of degradation on the number of load cycles and especially on the ratio between the maximum stress acting on the concrete and its bending flexural strength also becomes obvious through the performed ultrasonic measurements. furthermore, the occurred degradation was additionally described through a microscopic analysis of the crack characteristics. previous investigations of the penetration behavior of water towards concrete showed a significant influence of the externally applied pressure on the overall water uptake as well as on the penetration depth. in addition, concrete that had previously been exposed to tensile stresses showed a higher penetration depth, depending on the extent of damage that had been induced before. last but not least, the numerical models that were developed in the context of this research project highly agree with the results of the experimental studies [16, 17]. they can hence be used as a basis for a model, which simulates the entire interaction of influences that may lead to an intensified damaging asr in concrete pavements. acknowledgements the authors would like to thank the german research foundation (dfg) for their financial support of this research project. further, the authors would like to acknowledge the fruitful cooperation with the rub institute for structural mechanics, the federal institute for materials research and testing, berlin, the f.a. fingerinstitute for building materials, weimar and the institute for concrete structures and building materials, karlsruhe. since the foundation of this research group in 2012 outstanding results in the field of asr have been accomplished on the basis of joint projects. references [1] r. springenschmid. die ermittlung der spannungen infolge von schwinden und hydratationswärme im beton. betonund stahlbetonbau 79(10):263-9, 1984. https://doi.org/10.1002/best.198400390. 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[13] c. sievering. dauerhaftigkeit von betonfahrbahndecken unter besonderer berücksichtigung des externen alkalieintrags phd thesis (bochum: institute for building materials, ruhr university bochum), 2012. [14] tl beton-stb 07: technische lieferbedingungen für baustoffe und baustoffgemische für tragschichten mit hydraulischen bindemitteln und fahrbahndecken aus beton korrekturen (forschungsgesellschaft für straßenund verkehrswesen), 2012. [15] r. przondziono, r. breitenbücher. akr unter kombinierten einwirkungen wie beeinflussen zyklische beanspruchungen die degradation und den alkalieintrag in beton tagungsbericht 20. int. baustofftagung ibausil 2:85-92, 2018. [16] r. przondziono, j. j. timothy, m. nguyen, et al. vorschädigungen in beton infolge zyklischer beanspruchungen und deren auswirkung auf transportprozesse im hinblick auf eine schädigende akr. betonund stahlbetonbau 110(1):3-12, 2015. https://doi.org/10.1002/best.201400095. [17] r. przondziono, j. j. timothy, f. weise, et al. degradation in concrete structures due to cyclic loading and its effect on transport processes-experiments and modeling. structural concrete 18(4):519-27, 2017. https://doi.org/10.1002/suco.201600180. 51 https://doi.org/10.1002/best.201400095 https://doi.org/10.1002/suco.201600180 https://doi.org/10.14311/app.2022.33.0631 acta polytechnica ctu proceedings 33:631–637, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague quantifying uncertainties in the sustainability evaluation of concrete materials considering regional characteristics in japan nicole alexis kwan viosa,∗, michael ward henryb, joel galupo oponc a hokkaido university, graduate school of engineering, kita 13 nishi 8, kita-ku, sapporo 060-8628 japan b shibaura institute of technology, department of civil engineering, toyosu 3-7-5, koto-ku, tokyo 135-8548 japan c msu-iligan institute of technology, department of civil engineering, faculty member, iligan city 9200 philippines ∗ corresponding author: nicolealexisv@gmail.com abstract. in the evaluation of concrete sustainability, what constitutes "sustainable" to one region may vary from another. this often leads to methodological forms of uncertainties that makes the evaluation process more complex. as such, this paper aims to quantify the effect of uncertainties in the regional context on the sustainability evaluation of concrete materials. this is carried out by quantifying the regional context through establishing a weighting scheme and then integrating the obtained weights into the sustainability analysis of concrete materials in tandem with uncertainty analysis. japan is used as a case study because although it relatively appears as a homogeneous country, its prefectures possess unique characteristics that may make the sustainability evaluation of concrete materials vary across prefectures. cluster analysis is carried out in the 47 prefectures of japan using a set of regional context indicators. five clusters are identified with varying characteristics and these are translated into different weighting schemes. the established weights are used in the sustainability evaluation of concrete materials using multi-criteria decision-making analysis. the results showed that one mix is the most sustainable for four of the clusters and a different mix is the most sustainable for the remaining cluster. when uncertainty analysis is conducted, the effect of the weights in the sustainability evaluation is explained by examining the average scores of the concrete mixes and the variance of the scores across the five clusters. this investigation facilitated the understanding of how regional differences and the uncertainties associated with it impact the evaluation of concrete sustainability. keywords: regional characteristics, sustainable concrete, uncertainty analysis. 1. introduction sustainable development, commonly defined as "the development that meets the needs of the present without compromising the ability of future generations to meet their own needs" [1], has its implementation dependent on individual governments. each country has varying context on this concept making the means of attaining sustainability different among countries [2]. with the growing awareness of sustainable development, the concrete industry is taking efforts to participate in the practice of sustainability. similar to sustainable development, the implementation of sustainability in concrete also varies by region because concrete is a regional context material itself. henry and opon [3] introduced a means for quantitatively analyzing the regional context of sustainability and linking this regionality to the sustainability evaluation of concrete materials. however, methodological forms of uncertainties exist in this evaluation which make the evaluation process more complex. as such, it is the goal of this paper to quantify the effect of uncertainties in the regional context on the sustainability evaluation of concrete materials. japan was used as a case study for this analysis because although japan appears to be relatively homogeneous as a country, it has 47 prefectures having unique characteristics which may make the sustainability evaluation of concrete materials vary across prefectures. 2. methodology the analytical flow adopted in this study is summarized in figure 1. initially, a set of quantitative regional context indicators was constructed, gathered for all prefectures and processed accordingly. then, an agglomerative hierarchical clustering method was used to explain the regional context by identifying groups of prefectures exhibiting similar characteristics. these clusters form the basis of weighting schemes which represent regionality and the obtained weights were used as inputs in the concrete sustainability evaluation of a set of concrete mix utilizing multicriteria decision-making analysis. finally, the 631 https://doi.org/10.14311/app.2022.33.0631 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en n. a. vios, m. w. henry, j. g. opon acta polytechnica ctu proceedings figure 1. analytical flow. figure 2. sustainability evaluation framework using mca. range of z-score z≤-3 -3<z≤-2 -2<z≤-1 -1<z≤-0.5 -0.5<z<0.5 0.5≤z<1 1≤z<2 2≤z<3 3≤z relative intensity (−) 1/9 1/5 1/3 1/2 1 2 3 5 9 relative intensity (+) 9 5 3 2 1 1/2 1/3 1/5 1/9 table 1. conversion of in-cluster mean z-scores to relative intensities for calculation of weights. effect of methodological uncertainties in the regional context of concrete sustainability evaluation was investigated using uncertainty analysis to understand how these uncertainties influence concrete sustainability evaluation results. 2.1. cluster analysis methodology and extraction of weights in accordance with the guidelines provided by kassambara [4] for conducting cluster analysis, the agglomerative hierarchical clustering method was adopted using the open source statistical analysis software r [5]. in this approach, the prefectural data was first standardized to a mean of zero and a standard deviation of one converting them to z-scores. after which, the distance matrix which summarizes the distance between any two prefectures in an n dimensional space (where n is the number of indicators) was calculated using the euclidean distance method. a hierarchical tree was created using a linkage function to group prefectures based on their distance information. finally, a dendrogram, which is a visualization of the cluster analysis results, was produced and examined for analyzing the regionality of concrete sustainability. the clusters were the basis in the extraction of weights. weighting of indicators is an important area in sustainability evaluation as they reflect the significance of different dimensions in their contributions to the sustainability performance of a system [6]. a weighting scheme based on the analytic hierarchy process (ahp) approach was used to reflect regionality in the evaluation of concrete sustainability similar to the method taken by henry and opon [3]. ahp is a multi-criteria decision-making tool that structures the decision-making process as a hierarchy composed of quantifiable criteria and their relations and alternatives towards a goal [7]. weight determination is dependent upon pairwise comparison of indicators and since pairwise comparison is not applicable to negative or zero values, the z-scores were converted to relative intensities following the scale shown in table 1 depending on the loading direction of the indicator as shown in table 5. 2.2. concrete sustainability evaluation methodology after obtaining the different weights, sustainability evaluation of the concrete mixes was conducted using multi-criteria analysis (mca) shown in figure 2. each step in the framework is explained in the following sections. 632 vol. 33/2022 sustainability evaluation of concrete materials in japan label data source description (units) res1 a total cement consumption in concrete per capita (100kg/capita) res2 a total aggregate consumption in concrete per capita (100kg/capita) res3 b total water withdrawal per capita (m3/person) res4 c usage rate of slag blended cements (0% of total cement consumption) res5 c usage rate of fly ash blended cements (0% of total cement consumption) wst1 d total concrete construction waste generated per capita (100kg/person) eco1 d total construction investment per capita (1000yen/person) env1 e total co2 emissions per capita (tons/person) env2 e total energy consumption per capita (gj/person) data source: a: ministry of economy, trade and industry b: e-stat c: japan cement association d: ministry of land, infrastructure and transportation e: ministry of the environment table 2. indicators for regional context analysis data from 2010. 2.2.1. indicator selection indicators are selected based on their relevance to what is being measured. they are the fundamental structural basis on which the sustainability of a concrete material can be judged [8]. 2.2.2. normalization the indicators are expressed in different units. in order for them to work together in an aggregated composite, normalization is necessary. several methods are available for this purpose but for this paper standardization using z-scores was used. 2.2.3. weighting weighting schemes can sometimes increase or eradicate trade-offs between indicators, which makes weighting a controversial issue in concrete, as current codes are silent about it and still emphasize the hierarchy of mechanical performance [9]. due to this subjectivity, clarifying how much of an impact the weights have in the concrete sustainability evaluation is demonstrated in this analysis. 2.2.4. aggregation geometric aggregation (g) was chosen as the aggregation method in this paper as it involves aggregation by geometric mean using the product values [8] and is appropriate when less compensability is desired [10]. equation 1 shows how this is calculated where w is the weight assigned to each indicator and sci is the indicator normalized value. g = n! i=1 = (sci)wi (1) finally, the aggregated values were standardized using z-scores followed by a transformation to t-scores converting the results to a scale of 0 to 100 assuming a mean of 50 and herein referred to as sustainability score (ss). 3. results and discussions 3.1. regional context indicators for quantifying the regional context of concrete sustainability, a set of 9 indicators was constructed as summarized in table 2 spanning four categories. the resource consumption indicators (total cement, aggregate and water) and the waste generation indicator (total concrete construction waste) focus on the flow of materials from the beginning to end of its life cycle. the distribution of steel manufacturing and coal power generation within japan affects the usage of blended cements leading to the inclusion of two additional resource consumption indicators (usage rate of slag blended and fly ash blended cements). finally, economic and environmental aspects of sustainability were represented with three indicators measuring total construction investment, total co2 emissions and total energy consumption. normalized values (e.g. per capita) were utilized to facilitate comparison of variables that scale with prefectural size. 3.2. clustering analysis with the regional context indicators, cluster analysis was carried out to identify groups of prefectures exhibiting similar characteristics. the linkage function used was the average linkage function because it had the highest correlation coefficient of 0.86 between the euclidean distance matrix of the standardized data and the cophenetic distance matrix among the several linkage functions tested. the dendrogram produced by the clustering result is shown in figure 3. the authors set the number of clusters to five by cutting the tree at the height shown to sufficiently represent regionality without overly simplifying nor overly complicating the analysis. clusters are numbered from one to five from the left. the in-cluster mean z-scores of the five clusters for each regional context indicator are summarized in table 3. clusters 1 and 4 are single-membership clusters 633 n. a. vios, m. w. henry, j. g. opon acta polytechnica ctu proceedings figure 3. dendrogram of agglomerative hierarchical clustering results. cluster res1 res2 res3 res4 res5 wst1 eco1 env1 env2 1 0.38 0.45 -0.52 -1.44 6.04 0.31 1.98 -0.18 -0.16 2 0.38 -0.14 -0.17 0.67 -0.24 0.84 -0.39 3.17 3.19 3 -0.28 -0.26 0.04 0.02 -0.12 -0.27 -0.22 -0.19 -0.18 4 1.34 1.02 -1.58 1.29 -0.24 3.69 2.00 -0.37 -0.55 5 1.91 2.20 0.32 -0.67 -0.15 0.92 1.34 -0.42 -0.47 table 3. in-cluster mean z-scores for regional context indicators by cluster. containing aomori and shimane, respectively, that is the in-cluster mean z-scores of these clusters are the z-scores of the prefecture themselves. the in-cluster mean z-scores explain the distinctive characteristics of each cluster and these were used as inputs in calculating the weights. 3.3. extraction of weights the calculated weights are shown in table 4 and it can be deduced from these results that different clusters put different weights on the indicators. as these values were normalized to have a total sum of 1, it is easier to compare than the in-cluster mean z-scores. cluster 1 consisting aomori puts a very high weight on res5 derived from the high value of the prefecture’s fly ash usage rate. cluster 2 composed of oita, okayama and yamaguchi puts the same emphasis on env1 and env2 attributed to the high value of its co2 emissions and energy consumption. cluster 3 containing 38 clusters have equal weighting for all the indicators and can be said to represent the average conditions in japan. cluster 4 with only shimane has its greatest weight on wst1 associated with its high concrete construction waste. finally, cluster 5 comprising okinawa, toyama, fukui and yamanashi has a number of indicators with high weight including res1, eco1 and especially res2 related to its high total aggregate consumption. these weights are parallel with the in-cluster mean z-scores of the clusters. 3.4. concrete sustainability evaluation a set of 9 indicators for concrete sustainability evaluation was constructed as shown in table 5. each concrete sustainability indicator has its corresponding regional context indicator. the table shows the loading direction of each indicator which indicates the behaviour that contributes to increased sustainability and is employed in the calculation of weights. for example, decreasing the consumption of raw materials improves the sustainability of a concrete mix. a test set of three concrete mixes was drawn from noguchi et.al. [11] and the mix proportions are shown in table 6. to render consistency in the analysis, mixes with 28th day compressive strengths of approximately 30mpa which are generally used for civil engineering infrastructures were chosen. three new alternatives were created containing high-grade recycled aggregates are also shown in the table. highgrade recycled aggregates were assumed to have the exact same properties and cost as normal aggregates 634 vol. 33/2022 sustainability evaluation of concrete materials in japan cluster weight (w) res1 res2 res3 res4 res5 wst1 eco1 env1 env2 1 w1 0.05 0.05 0.02 0.15 0.44 0.05 0.15 0.05 0.05 2 w2 0.04 0.04 0.04 0.07 0.04 0.07 0.04 0.33 0.33 3 w3 0.11 0.11 0.11 0.11 0.11 0.11 0.11 0.11 0.11 4 w4 0.12 0.12 0.01 0.12 0.04 0.35 0.19 0.04 0.02 5 w5 0.16 0.26 0.05 0.11 0.05 0.11 0.16 0.05 0.05 table 4. indicator weights for each cluster. label indicator units loading related regional context indicator cem cement content kg/m3 − res1 agg natural aggregate content kg/m3 − res2 wat water content kg/m3 − res3 bfs blast furnace slag content kg/m3 + res4 fa fly ash content kg/m3 + res5 rca recycled aggregate content kg/m3 + wst1 cst construction materials cost yen/m3 − eco1 co2 co2 emissions kg−co2/m3 − env1 ener input energy mj/m3 − env2 table 5. indicators for concrete sustainability evaluation. mix 28th day f ′ c w/b w c bs fa s na ra (mpa) (−) (kg) (kg) (kg) (kg) (kg) (kg) (kg) ref 30.57 0.57 184 325 0 0 783 1063 0 a 30.57 0.57 184 325 0 0 783 0 1063 b1 30.44 0.58 192 200 133 0 806.2 965.4 0 b2 30.44 0.58 192 200 133 0 806.2 0 965.4 f1 30.23 0.56 186.7 238.1 0 94.1 847 949.9 0 f2 30.23 0.56 186.7 238.1 0 94.1 847 0 949.9 table 6. concrete mix proportions per m3. for simplicity. thus, concrete with high-grade recycled aggregates have the same properties as mixes with normal aggregates, but the inventory data shows that they require more energy to process to go back to the original state which is reflected by higher co2 emissions and energy consumption [12]. table 7 shows the calculated indicator values for each mix wherein the six constituent material indicators were all determined directly from the mix proportions. due to the lack of data on the regional variation of the costs of constituent materials, this analysis uses a single value per mix obtained from henry and opon [3] and calculated to get the unit cost per mix. lastly, the two environmental impact indicators were computed using japanese inventory data [13]. the concrete sustainability evaluation results using mca are shown in figure 4. each cluster’s results represent one implementation of mca. looking at each cluster, varying weighting schemes lead to varying sustainability scores for the different mixes. for clusters 1, 3, 4 and 5, the most sustainable concrete material is only one mix (f2). while for cluster 2 the most sustainable concrete material is a different mix (b1) whose selection is heavily supported by the high weight placed on co2 and ener indicators by this cluster and among the concrete mixes, mix b1 has the lowest co2 emission and energy consumption values. despite the same most sustainable concrete material for four of the clusters, the ranks of the other mixes change. but, a high level of confidence can be presumed on mix f2 considering a general choice for the most sustainable concrete material for the entire country. 635 n. a. vios, m. w. henry, j. g. opon acta polytechnica ctu proceedings mix cem agg wat bfs fa rca cst co2 ener ref 325 1063 184 0 0 0 5,775.04 255.12 1213 a 325 0 184 0 0 1063 5,775.04 270.86 1563.8 b1 200 965.4 192 133 0 0 5,679.39 162.63 861.84 b2 200 0 192 133 0 965.4 5,679.39 176.92 1180.4 f1 238.1 949.9 186.7 0 94.1 0 5,265.96 190.22 956.62 f2 238.1 0 186.7 0 94.1 949.9 5,265.96 204.28 1270.1 table 7. concrete mix characteristics (units are shown in table 5). figure 4. sustainability scores of concrete mixes by cluster. mix average ss rankaveragess variance rankvariance ref 36.00 6 7.27 1 a 41.08 5 29.11 5 b1 50.55 4 32.62 6 b2 57.09 2 18.46 2 f1 54.41 3 27.18 4 f2 60.87 1 21.61 3 table 8. descriptive statistics of uncertainty analysis for each concrete mix. 3.5. uncertainty analysis the subject of uncertainty propagation in concrete material sustainability evaluation that arises from weighting schemes is discussed in this section (i.e. how much the weights impact the sustainability evaluation results). the method undertaken for this analysis is where the ss of each mix for all of the clusters were aggregated and the corresponding average and variance together with their individual ranks were computed as shown in table 8. when choosing a sustainable concrete material in a national level but at the same time being aware of the existence of the regional context, saying that mix f2 is the most sustainable concrete mix because it is ranked 1st by four out of five clusters has uncertainties associated with it. the uncertainty analysis is a way of quantitatively examining how much consideration of the regional context will affect the selection of a sustainable concrete mix. as an example, mix f2 was ranked 1st based on the average sustainability score as it showed to be the most sustainable concrete material for four of the clusters, but considering the variance of this selection, it ranked 3rd which can reasonably be identified as sustainable. however, when looking at mix b2 which ranked 2nd based on the average sustainability score, it showed a lower variance. this then is where the choice between choosing a moderately scored material with low variance versus a highly scored material with moderate variance comes in. generally, a mix with higher average sustainability score and lower variability is desired as this entails a more confident result. based on this idea and referring to figure 5, the bottom right mix is the most desirable decision and is identified to be mix f2. these results cannot be an output by conducting concrete sustainability evaluation alone. viewing the mixes in the perspective of uncertainty analysis gives evidence that even though considering regional context in concrete sustainability evaluation as represented by the weights, the most sustainable concrete material for a national level nonetheless shows to be the same material that displayed the most sustainable character in a cluster level. 636 vol. 33/2022 sustainability evaluation of concrete materials in japan figure 5. scatter plot of average ss vs variance. 4. conclusions in this paper, quantifying the regional context of concrete sustainability was demonstrated that lead to obtaining different weighting schemes. this then was integrated in the evaluation of concrete sustainability. however, uncertainty analysis in this evaluation suggests that those weights, while they can lead to different evaluations of concrete mixes at a mesoscale (cluster level), the effect of the weights potentially may be negligible in a macroscale (national level) considering that the assumptions made in the analysis holds the same and true. this is evidenced by the varied sustainability scores for each mix per cluster, but the rank of the most sustainable concrete material does not change for one mix except in one cluster. with this at hand, it can be concluded that the weights are not leading to a significant difference of the selection of the most sustainable concrete material. therefore, applying a simple weighting scheme, such as equal weighting, may be sufficient for the representative sustainability evaluation of concrete. references [1] g. h. brundtland. our common future-call for action. environmental conservation 14(4):291-4, 1987. https://doi.org/10.1017/s0376892900016805. [2] m. henry, y. kato. sustainable concrete in asia: approaches and barriers considering regional context. construction and building materials 67:399-404, 2014. https: //doi.org/10.1016/j.conbuildmat.2013.12.074. [3] m. henry, j. opon. regional context indicators for the evaluation of concrete sustainability: an analysis at the prefectural level in japan. the 8th international conference of asian concrete federation, china, 2018. [4] a. kassambara. practical guide to cluster analysis in r, edition 1, sthda.com, 2017. [5] r core team. a language and environment for statistical computing r foundation for statistical computing vienna, 2018. http://www.r-project.org/. [6] x. gan, i. c. fernandez, j. guo, et al. when to use what: methods for weighting and aggregating sustainability indicators. ecological indicators 81:491-502, 2017. https://doi.org/10.1016/j.ecolind.2017.05.068. [7] t. l. saaty. decision making for leaders rws publications, rws publications, pittsburgh, 2008. [8] m. j. burgass, b. s. halpern, e. nicholson, et al. navigating uncertainty in environmental composite indicators. ecological indicators 75:268-78, 2017. https://doi.org/10.1016/j.ecolind.2016.12.034. [9] j. opon, m. henry. understanding the propagation of uncertainty in concrete material sustainability evaluation the 8th international conference of asian concrete federation, china, 2018. [10] m. saisana, a. saltelli. rankings and ratings: instructions for use. hague journal on the rule of law 3(02):247-68, 2011. https://doi.org/10.1017/s1876404511200058. [11] t. noguchi, f. tomosawa, k. m. nemati, et al. a practical equation for elastic modulus of concrete. aci structural journal 106(5):690-96, 2009. https://www.researchgate.net/profile/kamran-nem ati/publication/279700549_a_practical_equation _for_elastic_modulus_of_concrete/links/5cbd7a 30a6fdcc1d49a5ea2c/a-practical-equation-for-e lastic-modulus-of-concrete.pdf. [12] m. w. henry. formation and evaluation of sustainable concrete based on social perspectives in the japanese concrete industry, 2010. [13] t. uomoto, c. hashimoto. recommendation for shotcreting (draft) part tunnels by natm by the japan society of civil engineers. shotcrete for underground support x, p. 14-29, 2006. https://doi.org/10.1061/40885(215)3. 637 https://doi.org/10.1017/s0376892900016805 https://doi.org/10.1016/j.conbuildmat.2013.12.074 http://www.r-project.org/ https://doi.org/10.1016/j.ecolind.2017.05.068 https://doi.org/10.1016/j.ecolind.2016.12.034 https://doi.org/10.1017/s1876404511200058 https://www.researchgate.net/profile/kamran-nemati/publication/279700549_a_practical_equation_for_elastic_modulus_of_concrete/links/5cbd7a30a6fdcc1d49a5ea2c/a-practical-equation-for-elastic-modulus-of-concrete.pdf https://doi.org/10.1061/40885(215)3 acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.36.0109 acta polytechnica ctu proceedings 36:109–118, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague on the treatment of measurement uncertainty in stochastic modeling of basic variables stefan küttenbauma,∗, stefan maacka, alexander taffeb, thomas bramlc a bundesanstalt für materialforschung und -prüfung (bam), division 8.2: non-destructive testing methods for civil engineering, unter den eichen 87, 12205 berlin, germany b htw berlin (uas), division building materials sciences and diagnostics, 12459 berlin, germany c universität der bundeswehr münchen, inst. of struct. engineering, 85577 neubiberg, germany ∗ corresponding author: stefan.kuettenbaum@bam.de abstract. the acquisition and appropriate processing of relevant information about the considered system remains a major challenge in assessment of existing structures. both the values and the validity of computed results such as failure probabilities essentially depend on the quantity and quality of the incorporated knowledge. one source of information are onsite measurements of structural or material characteristics to be modeled as basic variables in reliability assessment. the explicit use of (quantitative) measurement results in assessment requires the quantification of the quality of the measured information, i.e., the uncertainty associated with the information acquisition and processing. this uncertainty can be referred to as measurement uncertainty. another crucial aspect is to ensure the comparability of the measurement results.this contribution attempts to outline the necessity and the advantages of measurement uncertainty calculations in modeling of measurement data-based random variables to be included in reliability assessment. it is shown, how measured data representing time-invariant characteristics, in this case non-destructively measured inner geometrical dimensions, can be transferred into measurement results that are both comparable and quality-evaluated. the calculations are based on the rules provided in the guide to the expression of uncertainty in measurement (gum). the gum-framework is internationally accepted in metrology and can serve as starting point for the appropriate processing of measured data to be used in assessment. in conclusion, the effects of incorporating the non-destructively measured data into reliability analysis are presented using a prestressed concrete bridge as case-study. keywords: existing structures, form, measurement uncertainty, nondestructive testing, reliability assessment. 1. introduction considering that absolute certainty is practically not attainable and further not worth striving for, since its theoretical achievement implies the consumption (or dissipation, respectively) of infinite resources [1], every decision is associated with a higher or smaller degree of uncertainty. this also includes decisions about the reliability of existing structures. uncertainties in reliability assessment may be characterized process inherent (such as future wind and seismic loading) and arise generally from the persistently existing lack of knowledge regarding the considered system, which is described by the characteristics, exposition, behavior, condition, etc. of the structure. accordingly, it is vitally important to find a computation model that appropriately represents the structural system and its environment, and, for this purpose, to gather and treat relevant, additional information in a suitable way. this serves to sufficiently fill the knowledge gap regarding the system of interest and to increase the level of approximation of a model used for the assessment. in most cases many types of information are available or at least obtainable that can (and, where appropriate, should) be utilized in reliability and condition assessment of existing structures to reduce uncertainties and to identify both biases and errors in models and assumptions. these include information from design that can be extracted from reports or drawings as well as information from field experience and observed data, respectively [2]. the evaluation of the condition of individual information to be used in assessment is crucial, amongst others because the inclusion of imprecise or even incorrect (in the sense of biased) information may lead to unfavorable decisions on reliability that may have serious consequences. it should be noted that the various conceivable sources of information provide diversely structured data that may not necessarily be treated in the same way. the significance of on-site observations has been shown, e.g., in studies on the appreciation of advanced measurement techniques in condition assessment [3] and on the use of monitoring-data in reliability anal109 https://doi.org/10.14311/app.2022.36.0109 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en s. küttenbaum, s. maack, a. taffe, t. braml acta polytechnica ctu proceedings ysis [4, 5]. the refinement of computation models by including additional information facilitates the targeted planning of actions such as use restrictions, maintenance, and reconstruction, as well as improved life time predictions, the optimization of resource consumption and overall more realistic assessment results that can lead to better decisions. whereas measurements can be seen as a tool to generate knowledge, it should be kept in mind that observed data in most cases only represent a quantity of interest, e.g., a physical characteristic, with uncertainty. this uncertainty can be understood as a measure to quantify the quality of the measurement result and its accuracy, respectively, shall be expressed in a suitable, practically applicable way and has to be principally considered when using measured information in assessment, see i. a. [2, 6]. this paper attempts to outline the necessity and the advantages of measurement uncertainty calculations in measured data-based modeling of random variables to be used as basic variables in reliability assessment. stochastic processes and random fields are delimited. the rules provided in the guide to the expression of uncertainty in measurement (gum) [7], which are suitable for the calculation of measurement uncertainty in many cases in metrology and at the same time computationally simple, are summarized in order to shed light on an internationally accepted approach. this concept could be applied for the comparable modeling of uncertainty that is related to information acquisition and processing in measuring data-supported reliability assessment. how measurement results can then be incorporated into a time-invariant component reliability analysis and what effects this can have on reliability is demonstrated using inner geometrical dimensions, which have been measured non-destructively on a prestressed concrete bridge in northern germany. 2. necessity and calculation of measurement uncertainties the set of basic variables included in a reliability analysis and their mathematical relationship form the "entire input information" to the model used for an assessment [8]. fundamental challenges in stochastic modeling of basic variables to be used to compute small probabilities such as failure probabilities in many engineering problems include the treatment of model uncertainties, the tail-sensitivity problem, and the quantification of correlation [9]. to address, for instance, the tail-sensitivity problem, [8] and others recommend the standardization of types of distribution functions of basic variables for certain groups of structural problems. in addition, all relevant types of uncertainty should be covered in a stochastic model of a basic variable [8]. frequently mentioned types are the intrinsic physical or mechanical uncertainty, the statistical uncertainty, the model uncertainty [8], and furthermore the measurement uncertainty [2], on which this paper focuses. this uncertainty describes the precision of measured information provided that systematic errors have been corrected appropriately and serves to express the measurement result in a comparable way. this is significant in reliability analysis in that calculated values need to be comparable to certain target values. measurement uncertainty can be defined as a parameter to quantify the dispersion of the values assigned to the quantity to be measured (the measurand) based on the incorporated information [10]. "from the metrological point of view, a measured value to which no measurement uncertainty has been assigned is useless. the calculation of measurement uncertainty serves to establish confidence in measurement, to ensure the comparability of measurement results and to express the quality, that is, trueness and precision, of the information measured about a characteristic. in the context of modeling basic variables to be used in assessment, two central requirements on stochastic models can be met by adequate measurement uncertainty considerations: verifiability and comparability. moreover, a measurement result is required to be unambiguously expressed and transparently documented. thus, the objectivity is assured in the sense that the calculated results as well as the models, input quantities, and assumptions underlying the measurement uncertainty considerations are deniable." [11]. "a good or rather useful measured data-based probabilistic model should cover the uncertainty associated with information acquisition and processing besides the uncertainty quantifying the inherent natural variability of the considered characteristic. the measurement uncertainty describes the limits of an interval containing the (generally unknown) true value of the measurand with a certain probability, and is epistemic, provided that an alternative exists to obtain the information (different testing methods, etc.). a stochastic model that has been created based on observations on site and that does not cover the uncertainty to be attributed to the information acquisition and processing appears to be equally useless as a measurement value to which no measurement uncertainty has been attributed to." [11]. in the following, it is shown how measurement uncertainties can be calculated according to the gum. the explanations refer to the main document [7]. the application of monte carlo simulation to the propagation of distributions and the computation of multiple output quantities is treated in the supplements [12, 13]. the basic idea is to find a model of the measurement consisting of different input quantities xi that may be either necessary to compute the measurement result or influence the outcome of the experiments in most cases unfavorably (and hence contribute to measurement uncertainty unless they are modeled deterministically). the functional relationship of these input quantities can be often expressed in form of an explicit model equation and is used to determine the output quantity 110 vol. 36/2022 measurement uncertainty in stochastic modeling of basic variables y , that is, the quantity of interest (the measurand): y = f(xi) (1) the input quantities xi have to be identified using, e.g., the knowledge about the measuring process, and evaluated with respect to their individual relevance. example: a considerable number of planners and owners requested the nondestructive localization of the reinforcement and tendons inside of concrete components in the past. the aim was not necessarily to reconstruct missing or incomplete, but also questioned as-built drawings. an example is the localization of longitudinal tendons inside existing bridge webs to safely drill cores and subsequently pretension anchor blocks transversely against each other for external strengthening actions. let us thus define the depth position of a (posttensioned bonded) tendon in relation to the concrete surface, which serves as the measuring area in ground penetrating radar (gpr) inspection, as the measurand . the principle of gpr is to derive the distance between an antenna and an object of interest using (a) the observed times of flight (tof) needed for an impulse to travel the respective distance forwards and backwards and (b) the propagation velocity of the electromagnetic wave inside the specific investigated volume. the equation y = dsp = f(xi) = v t/2, (2) with t being the observed tof and v being the velocity, can therefore serve as starting point to develop the individual model function in the case of echo arrangement (see figure 2 in section 3) of the gpr antennas. subsequently, the relevant input quantities need to be quantified. since they are considered to be random variables in most cases, quantification means finding a stochastic model appropriately representing the individual phenomena. for this purpose, the evaluation of measuring series with statistical methods, the type a evaluation, or the use of other information such as data from calibration certificates, physical reasoning, and experts’ judgements, the type b evaluation, can lead to suitable distributions of the input quantities. the type a evaluation can be applied if a measuring series consists of a sufficiently large number of identically distributed, independent values that were observed under constant conditions. the distribution parameters of an input quantity can then be estimated with statistical methods. it should be noted that the application of statistical methods may yield less reliable results in comparison to the type b evaluation if the number of observations is too small. particularly precise knowledge about a quantity may in turn obviate the need for experiments and type a evaluation. in type a, the sample mean is often considered the best estimate x̂ = x − b = ( 1 n n∑ i=1 xi ) − b (3) of a directly measurable quantity, provided that the systematic errors b have been estimated and corrected. to this best estimate, a standard uncertainty u(x̂) has to be attributed, that can be interpreted as standard deviation of the mean. it describes, how well x̂ estimates the expected value [7], and equals the sample standard deviation divided by the square root of observed values √ n. u(x̂) = s√ n = √√√√ 1 n(n − 1) n∑ i=1 (xi − x)2 (4) it should be noted that the simple description of observations may not lead to appropriate stochastic models. on the one hand, the frequently used standard deviation σx describes the dispersion of observations and allows for interpretations in a way that, e.g., 68 out of 100 single values observed in the future are expected to be included in an interval (x − σx ; x + σx). on the other, the standard measurement uncertainty, cf. equation ??, quantifies the scattering behavior of the directly measurable input quantity, that is, the characteristic of interest. this measure appears suitable in measured data-based modelling of basic variables since it is usually aimed to characterize (physical) characteristics, but not to predict single future observations. with regard to the distribution type, a convenient justification for choosing a normal distribution in type a evaluation (and also for the measurand y ) can be found in the central limit theorem for many practical cases. example: consider t in equation 2 as a set of random variables which not only consists of the observed tofs ta but also of further input quantities affecting the outcome of the measurement. these quantities ∑ ti among others include unknown processes within the equipment ("black box"), the limited measuring scale resolution, the spacing between transmitting antenna, receiving antenna, and concrete surface, as well as frequencyand travel path dependent changes in pulse shape, and uncertainties in signal processing. t = ta − tv − ∑ ti (5) all identified and individually relevant input quantities such as the lead time tv are usually modelled as random variables. the black box phenomena, for example, can be comparatively easily quantified. in this specific case, 200 tofs have been derived from echoes originating from the backwall of an isotropichomogeneous specimen (polyamide) at the same position to ensure repeatability condition approximately. all the tofs equal 4, 617ns. unknown processes possibly occurring in the equipment therefore do not lead to significant random errors. the related input quantity can thus be individually neglected. 111 s. küttenbaum, s. maack, a. taffe, t. braml acta polytechnica ctu proceedings figure 1. a) 2 ghz gpr antenna, specimen, and measuring line; b) radargram with both tendon indications (diffraction hyperbola); c) radargrams of kirchhoff migrated data gathered on the same line using different propagation velocities. an overestimation leads to v-shaped amplitudes in upper direction (on the right). if underestimated, the diffraction hyperbola remains visible (third radargram from the right). considering that metal layers with thicknesses less than λ/20 can be usually detected in gpr testing, that wavelengths λ = 5 . . . 12 cm are common in concrete, and that the electromagnetic pulses are totally reflected at metallic objects, the measuring series ta = (ta,1, . . . , ta,n)t contains i = 1, . . . , ntime stamps tai describing the time it takes the pulse to travel through the concrete to the tendon duct and back to the antenna. due to the requirement of i.i.d. observations, such a measuring series is only valid to describe the mounting depth of a tendon at one position in tendon length axis (sampling point) and therewith contains the values observed in a certain small area within the measuring plane. the sampling points can then be combined to interpolate the curve of the tendon. the time stamps gathered to describe the tendons mounting depth in the cross-section to be assessed in section 3 are: 2, 81ns, 2, 81ns, 2, 81ns, 2, 79ns, 2, 79ns, 2, 79 ns, 2, 79 ns. thus, equation 4 yields the best estimate t̂a = 2, 80 ns, and equation ?? the standard uncertainty u(t̂a) = 4 × 10−3 ns. the aim of tof measurement is to determine the time span required for a pulse to travel a certain distance within a component. picked time stamps ta,i additionally contain (at least partly) the time span necessary to generate, transmit, and sample the signal. this lead time or offset tv is a systematic error which must be estimated and corrected to define an unbiased time zero. different approaches have been proposed for this purpose (see, e.g., [14]). one option is the lead time estimation using the intercept of a regression line, where the -values describe a successively varied distance between the antenna and a metal plate. the -values are the respective measured tofs. a numerical discussion is delimited. however, the -intercept at x = 0 mm yields the best estimate and the variance of the intercept the (squared) standard uncertainty. in type b evaluation, the standard measurement uncertainty is derived from a distribution function defined using non-statistical methods. the standard uncertainties to be attributed to the best estimates of the input quantities depend on the distribution families chosen. example: consider the individual propagation velocity of the electromagnetic wave inside the concrete v , which (acc. to equation 2) needs to be known to derive mounting depths and depends primarily on the relative permittivity. provided the personnel is appropriately qualified, it can be sufficient to estimate the value of the velocity by analysing the shape of the diffraction hyperbola or focussing level of migrated indications of bar-shaped reflectors with round cross sections. experts judged that a deviation from the physically reasonable shape of the respective indications could be identified in this individual case when the velocity is overor underestimated by about ± 0, 75 cm/ns. the effects are shown qualitatively in figure 1. the subjectively highest focusing level of the tendon indications in the specific case outlined in sect. 3 could be achieved with a set velocity of 12 cm/ns.. if solely the limit values of an interval are known, in which the random variable realizes arbitrarily, then it can be derived from the principle of maximum entropy that the quantity is uniformly distributed. from this it follows, that v ≈ u with a best estimate v̂ = 12cm/ns and a standard uncertainty u(v̂) = (b − a)/(2 √ 3) = (12, 75 − 11, 25)/(2 √ 3) = 0, 433 cm/ns with a, b being the maximum and minimum values. if it turns out that such "rough" modelling is not sufficient to achieve the desired measurement precision, models with large uncertainty contributions can be refined. another input quantity tz that arises frequently in measurements describes the limited resolution of the measuring scale, in this case time axis, and can be 112 vol. 36/2022 measurement uncertainty in stochastic modeling of basic variables abbr. description distr. type best estimate x̂i standard uncertainty u(x̂i) v propagation velocity uniform 12 cm/ns 0,433 ns ta "displayed" tofs normal 2,8 ns 4 × 10−3 ns tv offset / delay normal (0,9 ns) 8, 6 × 10−2 ns tm "black box" ndt-system const. 0 ns 0 ns tz time axis resolution uniform 0 ns 7 × 10−3 ns tab spacing antenna / surface uniform (0,035 ns) 0,02 ns table 1. individual models of the input quantities used to compute the measurement result. the biases regarding offset t̂v , antenna-surface-spacing t̂ab , and transmitter-receiver-spacing were corrected during data migration. evaluated precisely via type b. the spacing ∆t between two sample values equals the inverse of the sampling rate fs. the quantity contributes to uncertainty in that an observed amplitude falls arbitrarily into an interval spanned symmetrically around a sample value. thus, again, the limit values of the random variable can be specified, whereas no information about a weighted distribution of the probabilities within the interval is available. from this, fs = 42, 7 ghz, and thus ∆t = 0, 023 ns it follows a uniform distribution with b = ±∆t/2 and u(v̂z) ≈ 7 × 10−3 ns. modeling should not rely too much on the consideration of perceived or physically reasoned correlations since it is the statistical relationship between (two) random variables and not the dependencies between the associated physical quantities that need to be estimated. it may therefore be sufficient in practice, to solely appreciate the pairwise correlations between type a evaluated input quantities, since empirical covariances could then be easily computed (among other parameters). nevertheless, correlations with at least on type b evaluated input quantity would be disregarded, which is only permissible if the considered input quantities are not significantly correlated [7] or respective information is neither available nor appropriately obtainable. if the input quantities have been identified individually, if suitable models have been defined at least for the relevant quantities, and if the input quantities xi have been brought into a functional relationship, the main issue in measurement uncertainty calculation has been solved. based on this model of the measurement, the calculation formulae provided in the gum can be straightforwardly applied. inserting the estimated values x̂i of the input quantities (in case of type a evaluation acc. to equations 4) into the model equation, cf. equations 1, 2 and 5, yields the estimated value of the output quantity ŷ, which needs to be corrected for systematic errors that have not yet been taken into account: ŷ = f(x̂1, . . . , x̂n) (6) the uncertainty to be covered by the output quantity y is referred to as combined standard measurement uncertainty u(v̂), which expresses the measurement uncertainty as an estimated standard deviation of the measured quantity value ŷ and is calculated by applying the error propagation law to the model equation: u(ŷ) = √√√√ n∑ i=1 c2 i u2(x̂i) + 2 n−1∑ i=1 n∑ j=i+1 ci cj u(x̂i, x̂j) (7) the empirical covariance is denoted by u(x̂i, x̂j), and the sensitivity coefficient of the input quantity xi by ci. it should be mentioned that, in contrast to form , the model equation is partially derived with respect to the input quantities at the coordinates of the best estimates x̂i in order to compute the sensitivity coefficients. example: the model equation used to calculate the measurement result is given in equations 2 and 5. the stochastic models of the input quantities can be found in table 1. a measured value of the mounting depth ŷ = d̂spis calculated in each case for one tendon at one specific point in the longitudinal tendon axis by inserting the best estimates x̂i into the model equation. from this, it follows that: d̂sp = 16.8 cm. the combined standard measurement uncertainty u(ŷ) = u(d̂sp) is computed with equations 7. first, the partial derivatives of the model equation with respect to the single input quantities at the best estimates yield the sensitivity coefficients . second, the uncertainty contributions c u(x̂i) can be easily computed. the squared contributions divided each by the sum of squares indicates the percentage distribution of the combined measurement uncertainty among the input quantities. the sum of [c u(x̂i)]2, in turn, yields the squared combined standard measurement uncertainty in the case of uncorrelated input quantities. with cv = ∂d/∂v ≈ 1, 4, the uncertainty contribution of the propagation velocity, e.g., equals 0,61 cm. the influence of the velocity is thus [cv u(v̂)]2 / ∑ [ci u(x̂i)]2 = 0, 612/0, 66 ≈ 56 %, which appears consistent considering the comparatively "rough" modeling process of v . third, the combined standard measurement uncertainty is the square root of ∑ [c u(x̂i)]2 since correlations do not affect the outcome in this particular case. the measurement result can now be expressed as follows: dsp ∼ n ; 113 s. küttenbaum, s. maack, a. taffe, t. braml acta polytechnica ctu proceedings figure 2. longitudinal view and cross-section of the investigated bridge (top); excerpt of the decisive cross-section with imaged migrated data (bottom left) and perpendicular scan with sketched gpr echo arrangement (bottom right). dsp = 16, 8 cm; u(dsp) = 0, 8 cm. comparative simulations result in slight differences of ∆ < ±0, 01 cm each. the choice of the normal distribution is thus individually suitable, although the propagation velocity, which has been modelled uniformly distributed, can be considered as a dominant uncertainty component as shown above. the interval 16, 8 cm ± 0, 8 cm covers the set of true measured values with a probability of approx. 68 %. if this coverage probability is considered individually too low, the interval can be easily extended. a suitable way to unambiguously express a measurement result acc. to the gum is the specification of the measured quantity value ŷ, the combined standard measurement uncertainty u(ŷ) and the distribution type of the measurand y . it may be helpful to provide information on the considered correlations. another option is to compute the expanded measurement uncertainty in order to derive coverage intervals. further details can be found within the gum framework and schematically, e.g., in [15]. when developing the gum, it was aimed to provide a general method and to enable the further use of the results [7]. the comparability of the results allows for successive calculations of measurement uncertainties in relation to usual boundary conditions, which may be used as orientation in the future. compared to the traditional approaches, the application of the gum yields rather realistic than disproportionately large values for the measurement uncertainty [7]. one point of criticism is that the type a input quantities imply the frequentist interpretation of probability and those evaluated acc. to type b the subjective interpretation [17]. nevertheless, the classical estimators could be interpreted as an approximation of the estimators according to type b evaluation, so that the equal treatment of all input quantities is also justified in terms of probability theory [17]. overall, the gum rules yield exact results for linear model equations and normally distributed input quantities. however, they are in most cases sufficiently accurate for practical applications. simulation techniques can be additionally used to verify the choice of a distribution family for the measured quantity. 3. demonstration measurement uncertainty in structural reliability assessment the investigated structure is a four-span longitudinally and transversely prestressed concrete bridge with a total length of approx. 96 m and a slab-and-beam cross section with a width of more than 23 m (figure 2). the bridge has been assessed in serviceability limit state (sls) decompression in transverse direction. prior finite element analyses revealed the center of a cross-section within the right span highlighted at the top of figure 2 to be decisive. the investigations thus refer to this specific component. the stress analysis was performed using a representative one-meter strip in the longitudinal bridge direction (b = 1 m) at the upper extreme fiber of the cross section. the limit state function contains the normal forces n and bending moments m as well as the dimensions a, h, b of the cross section and an inner lever arm zp describing the eccentricity of the transverse strands 114 vol. 36/2022 measurement uncertainty in stochastic modeling of basic variables figure 3. individual effects of including the non-destructively measured vertical position of the transverse tendons on reliability using form in sls decompression; βhl,init = 5, 18, βhl,ndt = 6, 18; dsp,init ∼ n(16, 3 cm, 1, 0 cm), cf. [8]; dsp,nt d ∼ n(16, 8 cm, 0, 8 cm); extracted from [16]. related to the vertical center of the cross-section. this inner lever arm zp can be written as a function of the spacing between the tendon ducts and the undersurface of the slab dsp. this distance dsp can be efficiently measured with gpr and has been found to be decisive (αr, dsp = 0, 74) during pre-investigations. the initially assumed standard deviation of 1 cm (appendix a) is based on the jcss probabilistic model code [8]. the initial mean µ = 16, 3 cm assumed prior to onsite testing corresponds to the available drawings. the sensitivity of the structural behavior to conceivable geometrical imperfections, cf. [8], motivated ndt on-site. the stochastic models can be found in appendix a. the limit state function is: g(σc) = 0 − ( n a + m w ) = 0 − θe,n ∑ ni h b − θe,m (np zp + ∑ mi) h3 b/12 h 2 ; zp = −h 2 + dsp, y + ϵ (8) chosen measured data are illustrated in figure 2. the measurement uncertainty calculation in section 2 yields the measurement result dsp ∼ n with dsp = 16, 8 cm and u(dsp) = 0, 8 cm. this result describes the vertical tendon position in relation to the measuring plane (the slab undersurface) in the center of the cross section, which is shown at the bottom left of figure 2. thus, the set of population, cf. [8], equals the specific component to be assessed in this particular reliability analysis, i.e. decompression proof. the spatial variability would have to be considered additionally, for instance, in system reliability assessments, and temporal changes (which appear rather unlikely with respect to geometrical dimensions) for time-variant quantities. the combined measurement uncertainty u(ŷ) expresses the inherent variability of the observed quantity as well as the measurement uncertainty itself as standard deviation. the measured quantity value ŷ can be used as expected value of the basic variable especially in the case of a justified choice of a normal distribution. both values serve as starting point for the ndt-based modeling of the basic variable dsp. it can be necessary to cover additional uncertainties such as model and statistical uncertainties. in view of the fact that modeling in particular may significantly influence the computational results, the application of the gum concept seems to be beneficial as the comparability of measuring data-supported modeling processes can be increased. competing models and the basic challenges in stochastic modeling of basic variables mentioned in section 2 should also be taken into account. models can be regarded as competing if, on the basis of the available information, it cannot be decided without arbitrariness which is individually more suitable. however, the measurement result equals the stochastic model of the ndt-supported basic variable in this particular case since the tail-sensitivity problem does not affect the update of this geometrical quantity (the initial model is also normally distributed), the uncertainty of the measurement model remains insignificant (which is not unusual in metrology) and other model uncertainties have been implicitly covered, as appropriate, by entering competing models sequentially and observing the change in reliability. the statistical uncertainty was estimated using the standard deviation of the experimental standard deviation of the mean of type a evaluated input quantities [7]. the additional appreciation of this statistical uncertainty in type a evaluated input quantities yields a difference in the combined measurement uncertainty of individually less than 0,1 mm. finally, the incorporation of prior knowledge would have been necessary, e.g., if the measured data could not describe the characteristic of interest comprehensively. regarding the localization of tendons, this scenario may occur, if the center of a tendon bundle is to be measured, but not all single tendons could be detected. then, the 115 s. küttenbaum, s. maack, a. taffe, t. braml acta polytechnica ctu proceedings quality of the measurement result also depends on the detection frequency. probability of detection analyses may be used to objectively evaluate whether certain objects can be reliably detected. the effects of incorporating the non-destructively measured inner geometrical dimension, that is, the vertical position of the transverse tendons in the crosssection center, on the hasofer-lind-reliability is summarized in figure 3. it can be seen that the slight shift compared to the initially assumed position and the reduction of uncertainty in relation to the probabilistic modeling recommendations [8] yield an increase in reliability of approx. + 19 %. although the value of the reliability index calculated based on the knowledge available prior to any testing has been found to be already comparatively high, it can be deduced that a larger deviation between the as-planned and as-built position of the tendons would affect reliability considerably stronger. in other cases where inspection results are incorporated into reliability assessment, a noticeable reduction in numerical reliability may also be observed certainly in favor of a more realistic structural assessment and to support the engineer in making reasonable decisions. further information about the case-study outlined above can be found in [11]. a second study dealing with ultimate limit states was published in [18, 19]. 4. discussion and conclusions it is vitally important to be aware of the condition, i.e., the relevance, trueness, and precision, of information to be used in reliability assessment. the uncertainty associated with information acquisition and processing should generally be considered; especially if measured data are intended to be incorporated into assessment. an advantage of the gum approach is that the results are verifiable and thus objectively deniable. the rules are internationally accepted and broadly applicable. furthermore, inferences from the calculated distribution to realizations that have not been observed can be more likely drawn by synthesizing the various uncertainty components, cf. [1]. in addition, measurement uncertainty can be taken as a comparable measure expressing the capability of testing methods (or measurement procedures, respectively) and may therewith be used for comparison with the results of pre-posterior-analyses, e.g., to initiate useful inspections. the measurement uncertainty can and probably should also be included when updating information based on, e.g., long-term monitoring data via bayes’ theorem (see, for example, [20]) and in monitoring-based condition assessment particularly when recorded values are close to certain thresholds. a practical limitation is both the relatively large effort and detailed knowledge of the measurement processes required to model a measurement appropriately. for this reason, the authors aim to develop a systematic repository of models and quantified input quantities for specific boundary conditions that are considered common in structural engineering, which can then serve as orientation for future measurement uncertainty calculations performed by the engineers working in practice. in addition, structure-specific partial safety factors are intended to be derived as a function of the type and extent of different on-site measurements in order to increase the practicability of the outlined approach. references [1] p. thoft-christensen., m. j. baker. structural reliability-theory and its applications. berlin: springer, 1982. 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[19] s. küttenbaum, s. maack, t. braml, et al. bewertung von bestandsbauwerken mit gemessenen daten, teil 2. betonund stahlbetonbau 116(3):183-99, 2020. https://doi.org/10.1002/best.202000087. [20] s. thöns. monitoring based condition assessment of offshore wind turbine support structures: dissertation. eth zurich, 2011. [21] t. braml. zur beurteilung der zuverlässigkeit von massivbrücken auf der grundlage der ergebnisse von überprüfungen am bauwerk. dissertation. münchen, 2010. [22] j. thierling. vergleich zwischen semiprobabilistischer und probabilistischer nachweisführung am beispiel des dekompressionsnachweises in querrichtung einer bestandsbrücke. masterarbeit. berlin, 2020. [23] h. bachmann et al. tragwerkszuverlässigkeit. einwirkungen [8]. der ingenieurbau (editor: g. mehlhorn). berlin: ernst, 1996. [24] cen/tc 250. eurocode 1: actions on structures part 2: traffic loads on bridges(en 1991-2). comitée européen de normalisation, 2003-09-01. [25] e. m. eichinger. beurteilung der zuverlässigkeit bestehender massivbrücken mit hilfe probabilistischer methoden (german). dissertation. wien, 2003. [26] a. strauss. stochastische modellierung und zuverlässigkeit von betonkonstruktionen (german). dissertation. wien, 2003. 117 https://www.bipm.org/documents/20126/2071204/jcgm_101_2008_e.pdf/325dcaad-c15a-407c-1105-8b7f322d651c https://www.bipm.org/documents/20126/2071204/jcgm_101_2008_e.pdf/325dcaad-c15a-407c-1105-8b7f322d651c https://www.bipm.org/documents/20126/2071204/jcgm_101_2008_e.pdf/325dcaad-c15a-407c-1105-8b7f322d651c https://www.bipm.org/documents/20126/2071204/jcgm_102_2011_e.pdf/6a3281aa-1397-d703-d7a1-a8d58c9bf2a5 https://www.bipm.org/documents/20126/2071204/jcgm_102_2011_e.pdf/6a3281aa-1397-d703-d7a1-a8d58c9bf2a5 https://www.bipm.org/documents/20126/2071204/jcgm_102_2011_e.pdf/6a3281aa-1397-d703-d7a1-a8d58c9bf2a5 https://www.bipm.org/documents/20126/41373460/jcgm_104_2009.pdf/19e0a96c-6cf3-a056-4634-4465c576e513 https://www.bipm.org/documents/20126/41373460/jcgm_104_2009.pdf/19e0a96c-6cf3-a056-4634-4465c576e513 https://www.bipm.org/documents/20126/41373460/jcgm_104_2009.pdf/19e0a96c-6cf3-a056-4634-4465c576e513 https://doi.org/10.1007/978-3-030-91877-4_58 https://doi.org/10.1088/0026-1394/40/5/305 https://doi.org/10.1002/best.202000087 s. küttenbaum, s. maack, a. taffe, t. braml acta polytechnica ctu proceedings a. appendix a individual stochastic models of the basic variables used to analyze sls decompression with form; cf. [11]. abbr. description distr. type mean std. dev. dim. θe,n model uncertainty (normal forces) (values based on [21]) n (normal) 1,0 σ = 0, 05 cov = 5,0 % − ng normal force due to dead loads n -1,1 [22] σ ≈ 0, 07 cov = 6,0 % [23] kn/m nq,t s normal force due to traffic loads (ts, load model 1 acc. to en 1991-2 [24]) gumbel -1,23 σ ≈ 0, 18 cov = 15,0 % [21] kn/m nq,udl normal force due to traffic loads (udl, lm 1 acc. to en 1991-2) gumbel 1,01 σ ≈ 0, 15 cov = 15,0 % [21] kn/m np normal force due to prestressing n -2036 [22] σ = 203, 6 cov = 10,0 % [25, 26] kn/m nk+s normal force due to creep and shrinkage n 270 [22] σ = 40, 5 cov = 15,0 % [25, 26] kn/m nse normal force due to load case: subsidence const. -0,63 [22] − kn/m nt normal force due to load case: temperature const. 12,80 − kn/m hy=0 height of the cross-section n 0,327 σ = 0, 01 [8] cov ≈ 3,1 % [25, 26] m θe,m model uncertainty (moments) [21] n 1,0 σ = 0, 10 cov ≈ 10,0 % − mg bending moment due to dead loads n 25,98 [22] σ ≈ 1, 56 cov = 6,0 % [23] knm/m mq,t s bending moment due to traffic loads (ts, lm 1 acc. to en 1991-2) gumbel 0,98 σ ≈ 0, 15 cov = 15,0 % [21] knm/m mq,udl bending moment due to traffic loads (udl, lm 1 acc. to en 1991-2) gumbel 4,87 σ = 0, 73 cov = 15,0 % [21] knm/m mk+s bending moment due to creep and shrinkage n -12,0 [22] σ = 1, 8 cov = 15,0 % [25, 26] knm/m mse bending moment due to subsidence const. 0,37 [22] − knm/m mt bending moment due to temperature const. −0, 58 − knm/m dsp,init initial distance between bottom of the slab and the bottom of the tendon duct n 0,163 [22] σ = 0, 01 [8] cov ≈ 6,1 % m ϵ eccentricity of the strands inside the tendon duct n 0,034 [22] σ = 0, 0068 [8] cov = 20,0 % m dsp,gp r measured distance between bottom of the slab and the bottom of the tendon duct n 0,168 σ = 0, 008 cov ≈ 4,8 % m 118 acta polytechnica ctu proceedings 36:109–118, 2022 1 introduction 2 necessity and calculation of measurement uncertainties 3 demonstration measurement uncertainty in structural reliability assessment 4 discussion and conclusions references a appendix a https://doi.org/10.14311/app.2022.33.0578 acta polytechnica ctu proceedings 33:578–584, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague load transfer mechanism of concrete screws florian stocker, oliver zeman∗, michael schwenn, konrad bergmeister university of natural resources and life sciences, department of civil engineering and natural hazards, institute for structural engineering, vienna, austria ∗ corresponding author: oliver.zeman@boku.ac.at abstract. even though the market and development for concrete screws has been increasingly rising in recent years, the load transfer mechanism of concrete screws has not yet been fully investigated. therefore, different tests of concrete screws made of galvanized and stainless steel were performed in concrete c20/25 and c50/60. the main aim is to measure the strain along the embedment depth. this will be achieved by using strain gauges that get placed in a centrically drilled borehole through the concrete screw. to get a comparison to the mechanism of the screws the same process will be executed in threaded rods used as a part of bonded anchors. due to the fact that the threaded cuts of concrete screws have geometrical similarities to bonded anchors, it was examined if the load transfer of both fasteners is related and may be compared. the results of the testing have shown that the load transfer mechanism of both fastener types is similar in low-strength concrete showing a concrete cone failure. in high strength concrete due to the mainly occurring steel failure the maximum measured strains at the maximum load step are not comparable. however, at lower load steps where the steel does not exceed the yield strength the results show a similar load transfer mechanism, too. keywords: anchorage to concrete, concrete screw, deformation behaviour. 1. introduction in fastening technologies there are three basic principles of load transfer mechanisms: friction-based, bond-based and mechanical interlock-based load transfer. the load transfer mechanism of postinstalled concrete screws is based on the principle of the mechanical interlock as shown in [1, 2]. concrete screws typically consist of five different segments: the head, the shaft, the run-out of thread section, the main load-bearing section and the thread cutting section. to provide the form locking function, the specially hardened thread cutting section is liable for cutting the threads for the following ones into the base material. the research presented in this paper is based on the question of how the load transfer behaviour behaves from the tip of the concrete screw to the upper edge of the concrete. this has already partially been examined by lechner et.al. [3] for concrete screws with a larger diameter used as retrofitting for shear force strengthening. therefore, a hole was drilled through the concrete screws along the embedment depth. in this manufactured hole, strain gauges got placed in specified intervals to measure the strain along the screw during a defined load situation. furthermore, to compare the load transfer mechanism of concrete screws with bonded anchors, the same process was done for threaded rods that get placed in the borehole with an injection mortar. 1.1. scope in the european assessment document ead 33023200-0601 "metal anchors for use in concrete" [4], concrete screws have different segments with specified tasks, see figure 1. the head (1) for transferring the load into the thread sections. the shaft (2) is for attaching components. it is dimensioned that it can be completely sunk into the borehole. the runout area where the thickness of the thread is running out to zero. the main load bearing section (4) is supposed to bear the main load into the base material and the thread cutting section (5) cuts the threads into the base material. so, every section has its specific task, however the detailed load transfer behaviour is not fully examined yet. it is assumed that the load transfer mechanism is similar to them from bonded anchors because both systems are transferring the loads into the basement almost over the entire embedment depth. the tests of the concrete screws presented in this contribution have been performed with an unconfined setup for an unrestricted concrete cone failure according to technical report tr 048 "details of tests for post-installed fasteners in concrete", [6]. this paper wants to provide a look into the segments of a concrete screw and their reaction with an applied load. 1.2. load transfer mechanism of concrete screws and bonded anchors the mechanical interlock principle is providing the load transfer between the fastener and the base material. this principle is used by different fastener systems, for example drop in anchors, headed anchors, undercut anchors, concrete screws and any type of 578 https://doi.org/10.14311/app.2022.33.0578 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 33/2022 load transfer mechanism of concrete screws ���� ���� ���� ���� ���� � figure 1. sections of a concrete screw; modified according to [4]. figure 2. load transfer mechanism of concrete screws (left) and bonded anchors (right) based on the approach of [5]. cast-in anchorages. concrete screws in general are cutting the thread in the concrete through the hardened thread cutting section. figure 1 shows a main load-bearing section ((4) in figure 1) in which the main load is supposed to be transferred into the base material. furthermore, it is estimated that every single helix is contributing to the load transfer because all threads are fixed in the concrete. however, it is still not completely clear how much each thread is included in the load transfer mechanism. the question is which threads are contributing more to the load transfer into the base material. the strains of the two concrete screw types and the bonded anchor may differ from each other because of different material characteristics like e-modul, yield strength, tensile strength and fracture strain. the mechanical and the chemical interlocking principle are depicted in figure 2 schematically based on the approach given in [5]. the left picture in figure 2 shows the concrete screw interlocked in the base material loaded with a force and transferring the load through the threads. the picture on the right shows the bonded anchor and the theoretical load transfer through the bond stress. the failure modes are well described in the relevant literature [1, 7]. two of the failure modes for mechanical post-installed fasteners are concrete cone failure or combined failure. in this research there have been used single concrete screws with a nominal embedment depth of hnom = 85 mm and an effective embedment depth of hef = 68 mm. the calculation for the design situation of a single concrete screw follows the concrete capacity design (ccd) approach [8] where the ultimate strength of the anchor is mostly based on the square root of the concrete compressive strength and the embedment depth to the power of 1,5 [7, 9]. the load transfer mechanism of bonded anchors is working with chemical interlock. the different types of bond systems and how to work with them are well described in [1, 7]. the effective embedment depth of the bonded anchors has been hef = hnom = 85 mm in this research. the main bond models are the uniform bond model and the elastic bond model [7]. the uniform bond model is mainly dependent on the strength of the anchor (the anchor itself and the injection mortar), the embedment depth and the diameter [7]. it assumes a constant stress distribution along the embedment depth. if the bond stress surpasses the bond strength, then failure occurs. for high loads and strength design methods this model is more appropriate than the elastic bond model [7]. the bond stress τ can be derived by testing based on the following equation. nu = τ · π · d · hef (1) where • nu . . . ultimate strength of the anchor, • τ . . . bond stress, • d . . . diameter of the fastener, • hef . . . effective embedment depth. cook et. al. [7] showed that for low loads the elastic bond model is more appropriate than the uniform bond model to describe the anchor behaviour. a finite element analysis made by mcvay, cook and krishnamurthy [10] also showed that bonded anchors following the elastic bond model at low load levels and the uniform bond model at peak loads. following calculation can be made based on these results. nu = τ · π · d0 ! tanh (λ · hef ) λ " (2) 579 f. stocker, o. zeman, m. schwenn, k. bergmeister acta polytechnica ctu proceedings figure 3. test samples and position of the strain gauges. type concrete strength n test samples 20−a c20/25 5 galvanized steel concrete screw 20−b 3 stainless steel concrete screw 20−c 3 threaded rod 10.9 50−a c50/60 5 galvanized steel concrete screw 50−c 3 threaded rod 10.9 table 1. overview of the testing program. where • d0 . . . diameter of the borehole, • λ . . . stiffness characteristic of the bond anchor system. davis and cook showed with a finite element analysis of an elastic model that the behaviour of a bonded anchor with a low short term load has a stress distribution relationship like an hyperbolic tangent curve [4]. this hyperbolic tangent curve approaches a distribution of the bond stress like the uniform bond model as the load is increasing or the duration of the test increases at a specific load level [11]. the load transfer behaviour of the bonded anchors that was found in this research seems to reflect the behaviour of the elastic model also approaching a hyperbolic tangent curve. the load transfer principle and the calculation approaches are different from form-locking systems like concrete screws. however, through the threads along the screw it is assumed that bonded threaded rods have a similar bearing behaviour like concrete screws. 1.3. tested concretes the tests shown within this contribution have been performed in non-cracked concretes of strength classes of c20/25 and c50/60 from the same supplier, with a concrete compressive strength measured at cubes of fc,cube = 29, 4 n/mm2 for the c20/25 and fc,cube = 68, 2 n/mm2 for the c50/60 at time of testing. the maximum aggregate size (round aggregate) was defined by dg = 16 mm following the grading curve given in tr 048 [8] which is based on din 1045-2 [5]. the resistance to fragmentation of the aggregates is at a los angeles abrasion value of 28 and an impact value of 21 for the considered low strength concrete, according to [12]. 2. test program and description 2.1. general in standard pull-out tests the result is usually forcedisplacement relationship. but there is a lack of information about the behaviour of the screw segments along the whole anchoring depth and to what extent they are involved in the load transfer process. this paper aims to examine how the load transfer behaves and makes a comparison to the behaviour of bonded anchors. in order to make this behaviour visible, the strains along the concrete screws and the bonded anchors are measured by means of strain gauges. to develop the test rig, a centrically arranged hole with a diameter of 3,5 mm has been drilled through the concrete screw. the strain gauges (short called dms) are placed in the borehole through the testing samples as shown in figure 3. table 1 shows the test program and the test samples. therefore, the testing program consist of five centrically drilled concrete screws made from galvanized steel, three centrically drilled concrete screws 580 vol. 33/2022 load transfer mechanism of concrete screws ��� ��� ��� ��� ��� ��� ��� ��� � � � � � � 7\sh� ���$ �����ordgvwhs ������ordgvwhs ������ordgvwhs ������ordgvwhs džm [‰] figure 4. type 20−a εm[‰] in 25 % loadsteps with concrete screws made of galvanized steel in c20/25; [n = 5]. made from stainless steel and three centrically drilled threaded rods with a steel strength of 10.9 in the concretes of c20/25 and c50/60. the measurements of the concrete screw are 9,4 mm for the core diameter, 12,5 mm for the total diameter including the thread and about 110 mm for the total length. the threaded rod has a nominal diameter of 10 mm. the exact value of the diameter for a m10 without the threads is 8,6 mm. the nominal embedment depth for all tests was set to 85 mm with hnom ≈ 9d with a diameter d = 9, 4 mm. for the hammer a medium drill bit size of the diameter was used. this means for the concrete screws dcut = 10, 25 mm−10, 35 mm and for the bonded anchors dcut = 12, 25 mm − 13, 25 mm . the used cement-based injection mortar has a nominal bond strength of 18 n/mm2, the bonded anchors have been installed following the manufacturerťs product installation instruction. the strength of the screws has been tested on samples out of the middle part of the cross section. the measured tensile strength of the galvanized concrete screws was 960 n/mm2. therefore, to provide aăgood comparison between concrete screws and bonded anchors, threaded rods of strength class 10.9 with a tensile strength of 1000 n/mm2 have been chosen for the bond test sample. the screw made of stainless steel was chosen out of the same product line as the galvanized one, tests have shown that the tensile strength of the inner part is 780 n/mm2. 3. evaluation of the test results 3.1. test results in this section the results of the performed tests are provided. in the evaluation of the results, the measured strains of the 5 strain gauges are depicted. these strain curves are arranged in loadsteps of 25 % in the order of 25 %, 50 %, 75 % 100 %. for the type 20−a and 20−b a descending strain curve is depicted in figure 5 and figure 7. these are showing the strain at a load level of 75 % after the maximum load of 100 % was reached. the analysis is based on mean ��� ��� ��� ��� ��� ��� ��� ��� � � � � � � 7\sh� ���$ �����ordgvwhs ������ordgvwhs�diwhu�pd[��ordg džm [‰] figure 5. type 20−a εm[‰] of 100 % and 75 % loadsteps after max load with concrete screws made of galvanized steel in c20/25; [n = 5]. values of the measured data, not considering the scatter. for further results, see [13]. the scattering of the strain values is wide. though, with an increasing distance from the tip to the head of the test samples the data shows more consistency as a result of lower scatter. 3.2. results of the concrete screw made of galvanized steel in concrete c20/25 in this section the results of the tests of galvanized concrete screws in concrete c20/25 are provided (type 20−a in table 1). figure 5 is showing the strain ε [‰]] along the embedment depth of the screw. on the left side, depicted are the strains in 25 % steps beginning with 25 % up to 100 % of the maximum load. the graph shown on the right is depicting the 100 % step and the 75 % step after the peak load of the fastener. the x−axis is providing the position of the strain gauges. for the positioning, see figure 3. the maximum mean load (n = 5) is fm = 46, 50 kn with a cv = 9,08 %. all fasteners in this test series failed with concrete cone failure. each thread of the concrete screw seems to be included in the load transfer with a specific amount. most of the load transfer appears to take place at positions 3 and 4, therefore, these measurements are consistent with the assumptions in figure 1 that the main load is transferred into the base material at the "main load bearing section". beginning with position 2, it can be stated that there is a linear increase in the strain up to the strain gauge at position 5. 3.3. results of the concrete screw made of stainless steel in concrete c20/25 the strain of the concrete screw (type 20-b) has higher values than that from the galvanized concrete screws (see the following graph shown in figure 6). at the same load level, the stainless steel screw stretches nearly double the size compared to the gal581 f. stocker, o. zeman, m. schwenn, k. bergmeister acta polytechnica ctu proceedings ��� ��� ��� ��� ��� ��� ��� ��� � � � � � � 7\sh� ���% �����ordgvwhs ������ordgvwhs ������ordgvwhs ������ordgvwhs i̇p >å@ figure 6. type 20−b εm[‰] in 25 % loadsteps with concrete screws made of galvanized steel in c20/25; [n = 3]. ��� ��� ��� ��� ��� ��� ��� ��� � � � � � � 7\sh� ���% �����ordgvwhs ������ordgvwhs�diwhu�pd[��ordg i̇p >å@ figure 7. type 20−b εm[‰] of 100 % and 75 % loadsteps after max load with concrete screws made of galvanized steel in c20/25; [n = 3]. vanized version. the maximum mean load at these tests is f = 45, 47 kn with a cv = 9,83 %. based on the characteristics of both steels, the higher strain of the stainless steel screw is a result of the higher fracture strain up to εb = 40 %. the failure mode of this test series was concrete cone failure too. furthermore, the cutting tip failed in the middle of the thread cutting section in 2 out of 3 tests. the graph is showing a test sample n = 3 and after analyzing the data, the peak at position 3 is due to the result of one experiment at a loadstep of 100 %. the other tests show an almost linear behaviour (dotted line in figure 7). with a higher number of tests, the strain curve could be harmonized (like the curves at type 20-a). 3.4. results of the concrete screw made of galvanized steel in concrete c50/60 the results of the concrete screw in a c50/60 concrete (type 50-a) show that up to the 75 % loadstep the distribution of the measured strains have a similarity to the type a tests in low strength concrete c20/25. the strain values are slightly higher, this may be caused due to a higher stiffness of the con��� ��� ��� ��� ��� ��� ��� ��� ��� ��� � � � � � � 7\sh� ���$ �����ordgvwhs ������ordgvwhs ������ordgvwhs ������ordgvwhs džm [‰] figure 8. type 50−a εm[‰] in 25 % loadsteps, galvanized concrete screw in c50/60; [n = 5]. ��� ��� ��� ��� ��� ��� ��� ��� ��� ��� � � � � � � 7\sh� ���$ �����ordgvwhs �����ordgvwhs�diwhu�pd[� ordg i̇p >å@ figure 9. type 50−a εm[‰] of 100 % and 75 % loadsteps after max load with concrete screws made of galvanized steel in c50/60; [n = 5]. crete. after the 75 % loadstep, 3 out of 5 tests failed due to steel failure of the concrete screw. the tensile force exceeded both, the yield strength and the tensile strength, which is the reason for the peak at position 4 in figure 8. the screws with steel failure broke at position 4 with a maximum load of 58,33 kn with a cv = 5,26 %. the 75 % loadstep after the maximum load in the graph shown on the right are the strains of the screws with concrete cone failure (n = 2), which show also a similar behaviour as the tests in low strength concrete. 3.5. comparison to the bond model in this section the results of concrete screws made of galvanized steel and the bonded anchors with threaded rods steel class 10.9 in a concrete c20/25 will be compared. the values of the measured strains of the bonded anchors (type 20-c) are higher than the strains at the galvanized concrete screw (type 20-a), see figure 10. but it turns out that the strain development over the embedment depth can be assumed as quite similar, as shown in figure 10. figure 10 is showing the strain curves of the type 20a and 20-c tests compared at each 25 % loadstep. up to thew 75 % loadstep the strain curves showing a similar behaviour (except the specific strain values 582 vol. 33/2022 load transfer mechanism of concrete screws ��� ��� ��� ��� ��� ��� ��� ��� � � � � � � 7\sh����$�dqg����& ����ordgvwhs i̇p >å@ ����ordgvwhs�w\sh����$ ����ordgvwhs�w\sh����& --��� ��� ��� ��� ��� ��� ��� ��� � � � � � � 7\sh����$�dqg����& ����ordgvwhs džm [‰] ����ordgvwhs�w\sh����$ ����ordgvwhs�w\sh����& -��� ��� ��� ��� ��� ��� ��� ��� � � � � � � 7\sh����$�dqg����& ����ordgvwhs i̇p >å@ ����ordgvwhs�w\sh����$ ����ordgvwhs�w\sh����& --��� ��� ��� ��� ��� ��� ��� ��� � � � � � � 7\sh����$�dqg����& �����ordgvwhs i̇p >å@ --�����ordgvwhs�w\sh����$ �����ordgvwhs�w\sh����& figure 10. comparison of εm[‰] each 25 % of galvanized concrete screws (denominated as type 20-a) and bonded anchors (type 20-c). at positions 4 and 5). at the maximum load (the 100 % loadstep), shortly before the fasteners failed the strain curves of the 20-a and 20-c types differ from each other. but there are two different failure modes developing. the bonded anchors (type 20-c) failed by steel failure with a maximum load of 49,43 kn (cv = 11,13 %) where the maximum load of the concrete screws (type 20-a) was 46,50 kn (cv = 9,08 %) by concrete cone failure. as discussed previously, the elastic bond model is a good approach for low load levels before the peak [7]. this is also confirmed in figure 10. these strain curves increase evenly like in the elastic bond model. regarding the similarity of the steel strength and the yield strength of both anchor systems (960 n/mm2 to approximately 1000 n/mm2), the difference in failure modes can be a reason for the higher strain values in the bonded anchor. however, the graphs in figure 10 compared may represent an indication that the mechanical behaviour and the load transfer behaviour of concrete screws can be assumingly similar to bonded anchors. lastly, the shape of the curves shown in test series type 20-a and 20-c is a confirmation of the elastic bond model discussed before. the comparison between the results of the bonded anchor (type 50-c) and the concrete screw (type 50-a) in a c50/60 will be left out. the strain curve of the bonded anchor (type 50-c) is similar to the bonded anchor (type 20-c), and the strain values are slightly higher. however, the different failure modes of the concrete screw (type 50-a) in a c50/60 does not allow a practical comparison. 4. conclusion in this study, tests with concrete screws and bonded anchors were performed in uncracked concrete c20/25 and c50/60. all tests had a nominal embedment depth of hef = 85 mm. regarding the results of the concrete screws, the main load bearing section is carrying most of the load into the base material. in this test series the main load bearing section of concrete screws made of galvanized steel transferred 90 − 95 % of the load into the concrete, depending on the concrete strength. the higher the strength of the concrete is, the more strains got measured in the main load bearing section. this is confirmed by the measured strains at dms positions 3 and 4. this also reflects the failure mode of the type 50-a tests where steel failure only occurred at dms position 4. the thread cutting section seems to be included in the load transfer mechanism. the values of the measured strains at dms positions 1 and 2 are low compared to the dms positions 3 and 4, but nevertheless this section is also able to bear a part of the load into the base material. (5 − 10 %) the amount seems to be influenced by the screw material. the stainless-steel screw has higher strains at the tip as the galvanized concrete screw. in this test series the concrete screw made of stainless steel was with up to 15 % included in the load transfer mechanism. 583 f. stocker, o. zeman, m. schwenn, k. bergmeister acta polytechnica ctu proceedings the results of the data indicate that the load transfer behaviour of concrete screws in concrete c20/25 are similar to bonded anchors. the strain curves of the tested bonded anchors are comparable to the elastic bond model [7]. therefore, as seen at the strain curves, concrete screws can be assumed to react like a bonded anchor (a threaded rod) with the elastic bond model. the behaviour of concrete screws investigated in this test series is only valid for the parameters used to perform these tests. therefore, the failure modes should be considered. the compared threaded rods have failed with steel failure and the concrete screws with concrete cone failure. however, before the steel failure occurs, the measured strains or at least the form of the curves can assumingly be compliant. the results themselves appear to be satisfactory, but additional tests should be performed to (1) enlarge the number of samples and (2) observe additional load transfer mechanisms of different concrete screw anchors made from different materials. references [1] r. eligehausen r., r. mallée, j. silva. anchorage in concrete construction, berlin: ernst & sohn, p. 370, 2006. [2] j. küenzlein. tragverhalten von schraubdübeln unter statischer zugbeanspruchung dissertation (stuttgart: selbstverlag des autors), p. 223, 2005. [3] j. lechner, n. fleischhacker, c. waltl, et al. zum verbundverhalten von betonschraubdübeln mit gro)em durchmesser. betonund stahlbetonbau 112(9):589-600, 2017. https://doi.org/10.1002/best.201700043. [4] eota. ead 330232-00-0601 mechanical fasteners for use in concrete (brussels: european organization for technical assessment), p. 57, 2016. [5] din 1045-2. tragwerke aus beton, stahlbeton und spannbeton teil 2: beton festlegung, eigenschaften, herstellung und konformität anwendungsregeln zu din en 206-1, berlin: deutsches institut für normung) p. 48, 2008. [6] eota. technical report tr 048: details of tests for post-installed fasteners in concrete (brussels: european organization for technical assessment), p. 26, 2016. [7] r. a. cook, j. kunz, w. fuchs, et al. behavior and design of single adhesive anchors under tensile load in uncracked concrete. structural journal 95(1):9-26, 1998. [8] r. eligehausen, j. küenzlein. zu: tragverhalten von befestigungen mit schraubdübeln. betonund stahlbetonbau 97(4):a16-a, 2002. https://doi.org/10.1002/best.200200980. [9] austrian standards institute. eurocode 2, en 1992-4: design of concrete structures, part 4: design of fastenings for use in concrete (vienna: austrian standards institute), p. 129, 2019. [10] r. a. cook. behavior of chemically bonded anchors. journal of structural engineering 119(9):2744-62, 1993. https://doi.org/10.1061/ (asce)0733-9445(1993)119:9(2744). [11] t. m. davis, r. a. cook. sustained load performance of adhesive anchor systems in concrete. aci structural journal 114(4), 2017. https://doi.org/10.14359/51689723. [12] austrian standards institute. önorm en 1097-2: 2010-03: tests for mechanical and physical properties of aggregates (vienna: austrian standards institute), p. 33, 2010. [13] f. stocker. untersuchungen zum lastabtragungsmechanismus von betonschrauben diplomarbeit, wien: universität für bodenkultur, p. 161, 2019. 584 https://doi.org/10.1002/best.201700043 https://doi.org/10.1002/best.200200980 https://doi.org/10.1061/(asce)0733-9445(1993)119:9(2744) https://doi.org/10.14359/51689723 acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.38.0478 acta polytechnica ctu proceedings 38:478–482, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague durability of latent heat storage systems sylva bantová, milan ostrý∗, karel struhala brno university of technology, faculty of civil engineering, institute of building structures, veveří 95, 602 00 brno, czech republic ∗ corresponding author: ostry.m@fce.vutbr.cz abstract. latent heat storage represents technology with significantly higher energy storage density. the thermal energy storage capacity of building structures and storage units integrated into building services contribute to the energy flexibility of buildings. this paper presents results from laboratory experiments focused on the compatibility of heat storage media represented by phase change materials (pcms) with materials of container. material compatibility of selected pcms with the plastics and metals were tested by a long-term experiment. two organic-based and two inorganic-based phase change materials were selected for tests of compatibility with selected metals (aluminium, copper and brass) and plastics (pp-h, pe-hd, and pvc-u). plastic-pcm compatibility was determined by gravimetric method. for evaluation of metal-pcm compatibility, calculation of corrosion rate was applied. the less mass changes and lower penetration of pcms to the matrix was observed for inorganic-based pcms compared to organic-based pcms. in case of compatibility between metals and pcms, the highest values of corrosion rate were calculated for copper immersed in inorganic-based pcms rubitherm sp25. keywords: phase change materials (pcms), compatibility of materials, metal corrosion, mass changes, container, latent heat storage systems, durability, energy flexibility. 1. introduction the situation on the european energy market in 2021 which is accompanied by rising of energy cost is one of the reasons for the demand of larger energy independence and energy supply safety of energy consumers. when fossil fuels such as natural gas or coal are considered to be replaced by renewable energy sources (res), the sufficient energy storage capacity should be employed. the reason is that energy production from res, such as sun, is not constant in time. if the energy in the form of heat should be stored for short or longer time, then the sensible heat storage or latent heat storage technology should be integrated in energy grid. the amount of stored heat in sensible heat storage technology depends on the weight of storage medium, its thermal capacity and change between initial and final temperature. on the other hand, the latent heat storage represents advanced and much more sophisticated approach to heat storage. materials suitable for latent heat storage are called phase change materials (pcms). latent heat storage technology uses the phase change of storage material and solid-liquid transformation is suitable for building applications. due to the phase change, proper encapsulation of pcms must be designed for all heat storage units with reasonable durability. durability and sustainability of each heat storage unit depends on the compatibility between the material of encapsulation and pcms to avoid undesirable leakages of pcms and their interaction with surrounding environment. this paper presents the results of compatibility tests of pcms selected from organic or inorganic group, and plastics and metals that are suitable materials for their encapsulation. 2. materials and methods 2.1. materials a total of four pcms were selected for testing: salt-hydrate-based rubitherm sp22 and rubitherm sp25 (representatives of the inorganic group) and paraffin-based linpar 17 and linpar 1820 (representatives of the organic group). table 1 shows the selected pcm parameters. as all tested pcms have a mean value of peak temperature around 24 °c, the actual material range values of materials is 22–28 °c, this suggests the suitability of specific pcms for integration into systems inside residential buildings due to similar indoor room temperatures. three common metals and plastics were selected for the experiment as potential encapsulation materials for pcms. the reason for the selection was good market availability, the high value of thermal conductivity of metals and the wide use of plastics in the buildings for distribution systems of rainwater or sewer pipes, for storage tanks, etc. two groups of samples were determined for the experiment: • plastic group representatives: pp-h (polypropylene), pvc-u (polyvinylchloride) and pe-hd (highdensity polyethylene); • metal group representatives: aluminium, copper and brass. table 2 shows the parameters of tested metals. 478 https://doi.org/10.14311/app.2022.38.0478 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 38/2022 durability of latent heat storage systems type latent heat onset temperature peak temperature [j g−1] [°c] [°c] linpar 17 organic 152 21 22 linpar 1820 organic 141 24 27 rubitherm sp25 inorganic 122 18 28 rubitherm sp22 inorganic 145 14 25 table 1. the parameters of selected pcms. composition density thermal conductivity area [g cm−3] [w m−1k−1] [cm−2] aluminium (en aw1050 h111) al99.5 2.70 230 copper (en cw024a) cu-dhp-99.9 8.96 340 41.80 brass (en cw617n) cuzn40pb2 8.73 114 table 2. the selected parameters of the metal samples. 2.2. methods the assessment of compatibility between selected pcms and the metal/plastic samples is based on the weight changes of the samples with the calculation of the evaluation parameters separately for each material. the test procedure is based on the methodology of the american society for testing and materials test g1-03 [1] and consists of the following parts: • preparation of the samples – visual inspection, cleaning (with toluene for metals), dried, clearly labelled; • weighing of samples – the samples were weighed with the analytical balance; • immersion in pcms – the samples were placed into the testing beakers and were poured with liquid pcms, the test beakers were placed into incubator; • thermal cycling in the incubator – the samples were exposed to repeating temperature cycling ranging from 15 °c to 40 °c, the cycle was divided into 4 parts, each lasting for two hours; • removal from pcms: the samples of one set were withdrawn from the testing beakers at the given time: after 7-28-84 days (for the metals) and every seventh day (for the plastic) of exposure to the pcms, one set containing three pieces of samples; • visual check (the changes of appearance) of samples; • cleaning of the samples after removing; • weighing of the samples; • the selection of one sample from each set for subsequent calculations using the median value – to eliminate extreme values of the weight changes; • the calculation of the parameters (the corrosion rate for metal, the percentage change in mass in case of plastic) and evaluation of the samples. 2.2.1. the method for evaluation of the metal-pcm compatibility the corrosion rate (cr) was calculated for each metal sample in contact with the pcms for a specified time, the procedure and the experimental conditions are given in section 2.2. the cr methodology includes monitoring the mass loss of the metal samples relative to the surface area and the immersion time of the samples, cr is defined by equation (1), described in [2, 3]. cr = ∆m a · (t0 − t) , (1) where cr is the corrosion rate in mg cm−2 year−1, ∆m is a mass loss in mg and it is defined by the equation ∆m = m(t0) − m(t), where m(t0) is the initial weight of the sample before immersion to pcms and m(t) is the final weight after the immersion time, a is the surface area of the sample in cm2 and (t0 − t) is the experimental immersion time of the samples in pcms in years. in this study, the calculated corrosion rates of the samples are compared to the classification described in the guide for corrosion weight loss used in the industry [2, 4]. according to the guide, pcms are classified as compatible with the selected metal or incompatible, or compatible for a specific application. the suitability scales with the limit values and the verbal evaluation of cr are shown in table 3. the overall classification of the metal samples is supplemented by the calculation of the cr value defined according to the methodology described in [5, 6]. the calculation of the cr is based on the american society for testing and materials test g-31 [7] as well, which determines the average cr values. the method is based on the weight changes that are caused by exposure of the samples to a corrosive environment during the experiment. this calculation procedure is not possible to use in case of localized corrosion on the surface of samples such as pitting or intergranular corrosion. 479 s. bantová, m. ostrý, k. struhala acta polytechnica ctu proceedings recommendation (verbal evaluation) cr = 50.0 to 999.0 mg cm−2 year−1 not recommended for service cr = 10.0 to 49.9 mg cm−2 year−1 caution recommended, based on the specific application cr = 0.3 to 9.9 mg cm−2 year−1 recommended for long term service table 3. the guide for corrosion weight loss used in industry (the selected limit values) [2–4]. material immersion linpar 17 rubitherm sp25 period (organic group of pcm) (inorganic group of pcm) ∆m cr cr ∆m cr cr [day] [mg] [mg cm−2 year−1] [mm year−1] [mg] [mg cm−2 year−1] [mm year−1] aluminium 7 0.800 1.049 0.00389 0.800 1.024 0.00379 28 0.700 0.219 0.00081 0.900 0.292 0.00108 84 0.500 0.053 0.00020 1.800 0.194 0.00072 copper 7 1.400 1.807 0.00202 4.400 5.400 0.00603 28 1.700 0.525 0.00059 5.800 1.863 0.00208 84 1.600 0.174 0.00019 9.600 1.004 0.00112 brass 7 1.300 1.685 0.00193 2.900 3.882 0.00445 28 2.000 0.621 0.00071 3.200 1.061 0.00121 84 0.400 0.044 0.00005 4.500 0.476 0.00055 table 4. the corrosion rates values of tested metals exposed to effects of selected pcms from the organic and inorganic group for periods 7, 28 and 84 days. cr = k · w a · t · d , (2) where cr is the corrosion rate in mm year−1, k is a constant in mm year−1 that determines the resulting unit of the cr value (in our case 8.76 · 104), w is a mass loss in g, a is an area of a sample in cm2, t is a time of exposure in h and d is the density of materials in g cm−3. 2.2.2. the method for evaluation of the plastic-pcm compatibility the criterion for evaluating plastic-pcm compatibility is the percentage change in mass of the tested samples. this parameter is determined by calculation according to the gravimetric method defined in the international standard [8] iso 175:2000. ∆m = (mt − mt0) mt0 · 100, (3) where ∆m is the percentage change in mass in %, mt is the weight of the sample after removal from pcms in mg and mt0 is the initial weight of the sample before immersion in pcms in mg. 3. results and discussion the material compatibility of the selected four pcms with the plastics and metals was verified by a longterm experiment. the samples of both metals and plastics were immersed in the pcms for the same period with a difference in their extraction frequency. metals were extracted three times throughout the experiment, plastics every 7 days. this time cycle is similar by moreno [3] and óro [2]. in both cases, the evaluation procedure included the visual inspection of the surface changes and the weight changes of the individual samples before and after removal from the pcms, followed by calculations of the evaluation criteria. after removing the metal samples from the pcm, it was found that the samples immersed in inorganic pcm showed visual changes on their surface compared to the samples immersed in organic pcm. all metal samples were tarnished as early as one week after immersion, more pronounced changes were discovered on the surface of the copper samples. the results of the test for metal-pcms pairs are shown in table 4. the table shows only the representatives of the two groups whose weight changes were higher compared to the other material in the group. the evaluation criterion is the corrosion rate that was calculated according to equation (1) and (2). in general, the samples immersed in the organic pcm group showed smaller weight changes and lower cr values (see table 4) than samples immersed in the inorganic group. these results are consistent with our initial assumption and also with the conclusions presented in the previous studies [3, 4]. the most significant mass loss occurred in the samples immersed in pcm rubitherm sp25 (inorganic pcm), the highest values were achieved in the case of the copper, where the cr exceeded 5.4 mg cm−2 year−1 with the value mass loss of 4.4 mg. the cr curves of metal samples immersed in rubitherm sp25 are shown in figure 1b. the cr curves of all metal samples in inorganic pcms (figure 1b) are very similar to those in organic pcms (figure 1a). the difference is in the scale of values, in the case of samples in inorganic pcms (figure 1b), the cr values are much higher. 480 vol. 38/2022 durability of latent heat storage systems (a). linpar 17 (organic pcm). (b). rubitherm sp25 (inorganic pcm). figure 1. comparison of the dependence of cr value (in mg cm−2 year−1) of metals in two types of pcms. figure 2. comparison of the dependence of cr value (in mg year−1) of metals in rubitherm sp25 (inorganic pcm). the cr parameters of the metal samples were compared with the limit values in the guide for corrosion weight loss (in table 3), unlike other study [2], all considered metal-pcm combinations can be classified as recommended for long term service [8]. figure 2 shows the cr values in mm·year−1 from which we determined the estimated service lifetime of these metal samples, which is 100 years for copper, up to 150 years for aluminium, and up to 200 years for brass when immersed in inorganic pcms. this (a). linpar 1820 (organic pcm). (b). rubitherm sp22 (inorganic pcm). figure 3. comparison of the dependence of ∆m value (in %) of plastics in two types of pcms. evaluation will be complemented by a further experiment using the planned interval test according to the methodology in astm [8]. the courses of the values of percentage change in mass of the samples are shown in figure 3, it is a criterion for the evaluation of the mutual compatibility of the plastic-pcm pairs and the calculation is described in section 2.2. the figures are presented for the representatives of the organic and inorganic pcm group as in the case of the metal samples. the results show that samples immersed in the organic group of pcms achieved higher weight changes, where the maximum value of mass change reached 7 %. the organic pcms have a higher ability to penetrate the matrix of the tested plastic samples, the penetration of organic pcms into the plastics materials is demonstrated in figure 3a. in terms of the selected materials, the smallest weight changes were found for pvc-u material and the highest for pp-h immersed in linpar 1820 (organic pcms), similar conclusions are described by castellón [9]. all samples were checked after removal from the pcms and no visual changes were observed on the sample surface, the same results are documented by oró [2]. 4. conclusions in this study, we focused on testing the compatibility of selected plastics and metals with organic and inorganic groups of pcm. according to the results presented in section 3, a significant difference in the weight changes of the samples was found depending 481 s. bantová, m. ostrý, k. struhala acta polytechnica ctu proceedings on the selected group of pcms. it was verified experimentally in the case of immersion of plastics in the inorganic group of pcms, that less mass change and lower penetration of pcms to the matrix of the tested samples occurred. the pvc-u samples did not show any significant weight changes, while a rapid penetration of pcms into the samples of pp-h and pe-hd was clearly visible during the first 4 weeks, but this increase slowed down in the following weeks. the interaction between pcms and metals was verified using the cr value. compared to the plastics, the metal samples in the organic group of pcms showed less weight and visual changes on their surface. the highest values were achieved in the case of the copper immersed in rubitherm sp25, which is the worst variation of the metal-pcm pair in the experiment. the test method for metals used in the experiment was based on the assumption of the normal (constant) corrosion rates during the exposure period of the metals in corrosive environments. in further research, we would focus on the testing metal-pcms combination using the “planned interval test”. this method measures the variation of corrosion rates over time throughout the experiment. acknowledgements this research was funded by czech science foundation, grant number 19-20943s “compatibility of plastics and metals with latent heat storage media for integration in buildings”. references [1] astm g1–03 standard practice for preparing, cleaning, and evaluating corrosion test speciments, astm international, west conshohocken, pa, usa, 2019. [2] e. oró, l. miró, c. barreneche, et al. corrosion of metal and polymer containers for use in pcm cold storage. applied energy 109:449–453, 2013. https://doi.org/10.1016/j.apenergy.2012.10.049 [3] p. moreno, l. miró, a. solé, et al. corrosion of metal and metal alloy containers in contact with phase change materials (pcm) for potential heating and cooling applications. applied energy 125:238–245, 2014. https://doi.org/10.1016/j.apenergy.2014.03.022 [4] g. ferrer, a. solé, c. barreneche, et al. corrosion of metal containers for use in pcm energy storage. renewable energy 76:465–469, 2015. https://doi.org/10.1016/j.renene.2014.11.036 [5] a. j. farrell, b. norton, d. m. kennedy. corrosive effects of salt hydrate phase change materials used with aluminium and copper. journal of materials processing technology 175(1):198–205, 2006. https: //doi.org/10.1016/j.jmatprotec.2005.04.058 [6] p. rolka, j. karwacki, m. jaworski. compatibility tests between three commercially available organic pcms and metals typically used in fin-and-tube heat exchangers. materials 14(18):5172, 2021. https://doi.org/10.3390/ma14185172 [7] astm g 31 standard practise for laboratory immersion corrosion testing of metals, astm international, west conshohocken, pa, usa, 1999. [8] iso 175:2010 plastics – methods of test for the determination of the effect of immersion in liquid chemicals. [2019-02-15]. https://www.iso.org/standard/55483.html [9] c. castellón, i. martorell, l. f. cabeza, et al. compatibility of plastic with phase change materials (pcm). international journal of energy research 35(9):765–771, 2011. https://doi.org/10.1002/er.1723 482 https://doi.org/10.1016/j.apenergy.2012.10.049 https://doi.org/10.1016/j.apenergy.2014.03.022 https://doi.org/10.1016/j.renene.2014.11.036 https://doi.org/10.1016/j.jmatprotec.2005.04.058 https://doi.org/10.1016/j.jmatprotec.2005.04.058 https://doi.org/10.3390/ma14185172 https://www.iso.org/standard/55483.html https://doi.org/10.1002/er.1723 acta polytechnica ctu proceedings 38:478–482, 2022 1 introduction 2 materials and methods 2.1 materials 2.2 methods 2.2.1 the method for evaluation of the metal-pcm compatibility 2.2.2 the method for evaluation of the plastic-pcm compatibility 3 results and discussion 4 conclusions acknowledgements references my title https://doi.org/10.14311/app.2022.36.0142 acta polytechnica ctu proceedings 36:142–148, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague semi-probabilistic assessment of existing bridge using simplified methods for estimation of variance lukáš novák∗, drahomír novák brno university of technology, faculty of civil engineering, institute of structural mechanics, veveří 331/95, 602 00 brno , czech republic ∗ corresponding author: novak.l@fce.vutbr.cz abstract. the paper is focused on assessment of existing prestressed concrete bridge by simplified methods for estimation of coefficient of variation. the bridge was selected in the framework of the european project interreg austria-czech republic "atcz190 safebridge" focused on advanced numerical analysis of existing bridges represented by non-linear finite element model. the key ingredient in semi-probabilistic design and assessment of structures is an estimation of coefficient of variation (ecov). recently, correlation interval approach together with novel eigen ecov were proposed by authors of this paper and theoretically proved to be an efficient and accurate alternative to existing methods. this contribution is focused on practical application of eigen ecov on real example solved by nlfem and its comparison with other existing simplified methods. keywords: concrete structures, statistical analysis, estimation of coefficient of variation. 1. introduction the development of computational methods for civil engineering has become more important than ever, since it is often necessary to employ advanced numerical methods for the design of new structures in order to fulfil the significantly increasing economical and safety requirements in the last decades. moreover, there are a lot of structures, especially bridges, built in the last century, which must often be enhanced for higher loads assuming actual conditions of the structures. as a result of these industrial needs, researchers and civil engineers are more interested in advanced numerical methods to solve the mathematical models of structures – typically non-linear finite element method (nlfem). although nlfem is a very accurate numerical method for solving differential equations, there is still a lack of knowledge of material characteristics (e.g. fracture energy), actual geometrical properties (e.g. position of reinforcement) and even mathematical models of some physical phenomena (e.g. fracture mechanics of quasi-brittle materials) collectively called uncertainties. as can be seen from the given examples, uncertainties play an important role, especially in the case of concrete structures. this lack of knowledge may generally lead to inaccurate results and even fatal failures despite the advanced numerical analysis performed by nlfem. in modern structural analysis, uncertainties are represented by random variables or vectors described by specific probability distribution, the structural system can then be seen as a mathematical function of a set of random parameters. deterministic nlfem numerical analysis of structures must thus be enriched by stochastic analysis [1]. the elementary task of stochastic analysis is to propagate uncertainties through a mathematical model in order to obtain statistical information of outputs. in the semi-probabilistic approach for nlfem [2], the structural resistance r is separated and the design value rd that satisfies safety requirements is evaluated, as a simplification of the direct calculation of failure probability pf = p (z(x) < 0). the typical formula for the estimation of rd, assuming a lognormal distribution of r, is rd = µr · exp(−αrβvr), (1) where µr is the mean value, vr is the coefficient of variation (cov) and αr represents sensitivity factor derived from first order reliability method (form) [3]; the recommended value is αr = 0.8 according to eurocode [4]. the target reliability index is dependent on consequence classes (dependent on a type of structure), e.g. β for the ultimate limit state, moderate consequences of failure and a reference period of 50 years is set at β = 3.8 according to the eurocode. note that, from a probabilistic point of view, the whole process represents the estimation of a quantile satisfying the given safety requirements under the prescribed assumption of lognormal distribution. 2. semi-probabilistic methods there are several safety formats and semi-probabilistic methods for determination of a design value of response in normative documents or scientific papers, which are briefly described in the following paragraphs. these methods assume several simplifications and thus are less computationally demanding in comparison to the fully probabilistic approach. main advantage of safety formats and semi-probabilistic methods is possibility of their usage without complex reliability knowledge. 142 https://doi.org/10.14311/app.2022.36.0142 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 36/2022 semi-probabilistic assessment of bridge using ecov methods 2.1. partial safety factors the commonly known method according to en 1990 works with design values of input random variables and obtained result of nlfem corresponds directly to the r̃d. design values of material parameters, extremely low quantiles, can be obtained from laboratory experiments or directly from en. herein, lognormal probability distribution of material parameters was assumed and design value is estimated as the following quantile: xd = µx exp(−α β vx) (2) where vx is coefficient of variation (cov), α represents sensitivity factor and β target reliability index. however, this approach leads to extremely low design values of input random variables which migh lead to unrealistic behavior of the numerical model. moreover, this method does not estimate statistical moments of the resistance and therefore is not employed in this comparison. 2.2. ecov fib method assuming lognormal distribution of the response variable r, the task of ecov methods is the estimation of the mean value and variance. the first presented method is ecov according to fib model code 2010 [5– 7], the coefficient of variation caused by uncertainty of input parameters vr can be estimated by simplified formula for vr < 0.2 as vr = 1 1.65 ln ( rm rk ) . (3) the simplified formula is based on two numerical simulations rm using mean values of input random variables and rk using characteristic values (5% percentile). the global resistance safety factor is then calculated as γr = exp(αrβvr). (4) this method was adopted in the fib model code 2010, though design value rd was later decreased by additional factor γrd=1.06 to take model uncertainties into account: rd = r(fcm, fym, anom, ...) γrγrd . (5) note that the strong assumption of rk being equal to simulation with characteristic values is not generally applicable for non-linear functions, but it can be justified for engineering applications where the nonlinearity of investigated function is not typically very high. the second significant limitation arises from a fact, that this method is special case of taylor series expansion assuming full correlation among all input random variables [8]. 2.3. taylor series expansion classical method for a statistical analysis of function of random input vector is taylor series expansion (tse). the most significant advantage of ecov based on tse is its versatility and adaptability. it is common in engineering applications to use tse truncated to linear terms and thus µr ≈ rm = r(µx) and cov is obtained as: vr ≈ 1 rm √√√√ n∑ i=1 ( ∂r(x) ∂xi σxi )2 . (6) where the derivatives are evaluated at µx for all n input random variables. the efficiency and accuracy of tse depends on the number of used terms (truncation of infinite tse) and the differencing scheme for the practical computation of derivatives. the simplest form is linear tse with derivatives approximated by one-sided differencing as: ∂r(x) ∂xi = rm − rxi∆ ∆xi . (7) where the response of mathematical model rm is determined by a calculation with mean values, and rxi∆ is the result of a model using mean values of input random variables and a value of the i-th random variable which has been reduced by ∆xi . for the sake of clarity, the difference is calculated as ∆xi = µxi − xi∆. of course, one can use various differencing schemes instead of eq. 7 as originally proposed in [9] and illustrated in fig. 1. assuming linear tse, one of the most promising advanced differencing schemes using nsim = 2n + 1 simulations is ∂r(x) ∂xi = 3rm − 4rxi ∆ 2 + rxi∆ ∆xi , (8) where the middle additional term rxi ∆ 2 is obtained via the evaluation of the original mathematical model with mean values and a reduced i-th variable xi ∆ 2 = µxi − ∆xi /2. the last aspect of adaptivity of tse is represented by step size parameter c used for definition of the difference ∆xi = µxi − xi∆, wherexi∆ = f −1 i (φ (−c)), where f −1 i is an inverse cumulative distribution function of the i-th variable and φ is the cumulative distribution function of the standardized gaussian distribution. schlune et al. [10] proposed step size parameter in dependence on target reliability index as c = (αrβ) / √ 2. however, it brings additional computational burden when analyzing different limit states with different target β, since it is necessary to calculate n + 1 (eq. 7) or 2n + 1 (eq. 8) simulations for each limit state. therefore, it can be recommended to use c = 1.645 independently on type of investigated limit state, which is in compliance to ecov according to fib model code 2010 [7]. 143 lukáš novák, drahomír novák acta polytechnica ctu proceedings note that tse is suitable for uncorrelated input random variables, since the differencing schemes evaluate the influence of each input variable independently. if one needs to include specific correlation coefficient, one can use following formula: v arrtl ≈ n∑ i=1 ( ∂r(x) ∂xi )2 σ2 xi + (9) + ∑ i,j=1,...,n i ̸=j ρi,jσxi σxj ∂r(x) ∂xi ∂r(x) ∂xj . (10) where ρ is the correlation coefficient. however, it is often necessary to use advanced differencing scheme or higher order of tse (such as quadratic depicted in fig. 1) for accurate estimation of variance in case of correlated input random variables [9]. 2.4. eigen ecov method the recently proposed eigen ecov [8] is derived directly from tse. however, there is an assumption of fully correlated input random variables similarly to ecov according to fib model code 2010 and thus number of simulations is dramatically reduced – 3 independent on n . eigen ecov is based on idea of projection of input random vector on 1d eigen distribution θ with variance equal to the first eigenvalue of input covariance matrix σ2 θ = ∑ σ2 xi = λ1 and mean value is simply obtained as: µθ = √√√√ n∑ i=1 (µxi )2 . (11) in the original proposal, there are three levels of eigen ecov corresponding to tse methodology consisting of three increasing levels of complexity and accuracy [9]. the most promising eigen ecov formula for estimation of vr offering a balance between the efficiency and accuracy (derived directly from eq. 8) is in the following form: vr ≈ 3rm − 4rθ ∆ 2 + rθ∆ ∆θ · √ λ1 rm , (12) where a simulation rθ∆ = r (xθ∆) with coordinates of input realization xθ∆ = (x1∆, . . . , xn∆) and rθ ∆ 2 = r ( xθ ∆ 2 ) with coordinates xθ ∆ 2 =( x1 ∆ 2 , . . . , xn ∆ 2 ) are depicted together with illustration of eigen ecov method in fig. 1 for the sake of clarity, the input vectors consisting of reduced values of input random variables are xi∆ = f −1 i (φ (−c)) and the intermediate coordinates are as follows: xi ∆ 2 = µxi − µxi − xi∆ 2 = µxi − ∆xi 2 . (13) finally, the ∆θ represents distance between µθ and desired quantile f −1 θ (φ (−c)). it is generally obtained directly from the inverse cumulative distribution function of lognormal distribution with corresponding statistical moments of eigen distribution θ or one can use the following approximation suitable for calculation without software: ∆θ = µθ − µθ · exp ( −c · √ λ1 µθ ) (14) the eigen ecov combines versatility and adaptability of tse via various differencing schemes and step size parameter c together with efficiency of ecov according to fib model code 2010. note that, more theoretical details can be found in the original proposal of eigen ecov including additional formulas based on another differencing schemes or higher tse, which is suitable for input variables with high cov [8]. 3. post-tensioned concrete bridge the described semi-probabilistic methods are employed for probabilistic assessment of an existing concrete bridge. this study significantly extends the obtained results from the original simplified case-study [17]. the bridge was selected in the framework of the european project interreg austria-czech republic "atcz190 safebridge" focused on advanced numerical analysis of existing bridges. the bridge consists of three spans constructed from 16 bridge girders ka-61 in transverse direction. the crucial part of the bridge for assessment is the mid-span: 19.98 m long with total width 16.60 m. the geometry of a typical bridge girder ka-61 is created according to an original documentation describing also positions of reinforcement and tendons. the drawing together with the simplified cross-section is depicted in fig. 2. from structural point of view, it was necessary to create numerical model of the whole bridge span. the reason is that although the structure is symmetric, the individual bridge girders are not transversely prestressed, which leads to the different deflection of each girder in dependence on their distance to the loading position. in order to create numerical model reflecting their real connection conditions, the girders are connected by reinforcement according to original documentation together with a concrete mixture between single girders. 3.1. finite element model the cross-sections of girders ka-61 were simplified to regular shapes in order to reduce number of finite elements and to obtain regular mesh, see fig. 2. boundary conditions are assumed to be as a simply supported beam with elastic blocks as supports. the geometry of elastic blocks and positions of loading plates are modeled according to bridge documentation and a national annex of eurocode for load-bearing capacity of road bridges by exclusive loading (by sixaxial truck). 144 vol. 36/2022 semi-probabilistic assessment of bridge using ecov methods x x latin hypercube sampling 2 ecov by červenka eigen ecov m+1 3 linear simple differencing taylor series expansion ecov methodnsim ( )m 2 quadratic simple diff. m linear advanced diff. + + x y meancharacteristic ch ar ac te ris tic δ y δx m ea n 2 δx δ y 2 figure 1. representation of semi-probabilistic methods in 2d example. figure 2. cross-section of a singe bridge girder ka-61. the non-linear finite element model is created in software atena science including theory of nonlinear fracture mechanics [11]. in order to reflect complex behavior of the bridge, the numerical model contains three construction phases as illustrated in fig. 3: (1.) prestressing of bridge girders and simultaneous application of the self-weight; (2.) activating of the pavement and concrete among girders connecting bridge girders; (3.) application of a load by a six-axial truck. the major part of nlfem is represented by 13,000 elements of hexahedra type and triangular ‘prism’ elements in the blue-colored parts of the cross-section (see fig. 2). hexahedra elements lead to better numerical stability of simulation and leads to easier construction of mesh compatible between two volumes connected by fixed contact, i.e. nodes of elements in both connected sub-volumes have same coordinates. another advantage of brick elements is that the structured mesh constructed from brick elements leads to a significantly lower number of finite elements in comparison to tetrahedra elements. fracture-mechanical behavior of concrete is described by a non-linear mathematical model [11]. reinforcement together with tendons are modeled as discrete 1d elements with positions, diameters and shape according to the original documentation. the numerical model is further analysed in order to investigate three limit states of the bridge: (1.) the ultimate limit state (uls) (peak of a loaddeflection diagram); (2.) the first occurrence of cracks in bridge girders (cracking); (3.) the serviceability limit state of decompression defined according to eurocode (sls). note that obtained results are further reduced by dynamic amplification factor δ = 1.4 in order to reflect that results are from static analysis. 145 lukáš novák, drahomír novák acta polytechnica ctu proceedings figure 3. three construction phases of the bridge represented by nlfem. 3.2. stochastic model the stochastic model contains 4 random material parameters of a concrete c50/60: young’s modulus e, compressive strength of concrete fc, tensile strength of concrete fct and fracture energy gf . characteristic values of e, fct, gf were determined from fc according to formulas implemented in the fib model code 2010 [7] (gf , e) and pren 1992-1-1: 2021 (fct). the last random variable p represents prestressing losses with cov according to jcss: probabilistic model code [12]. note that prestressing force itself is assumed to be deterministic, since it was measured and controlled during prestressing process. the stochastic model is summarized in tab. 1. the bias factor for determination of mean value and cov of fc were taken from pren 1992-1-1: 2021 (annex a) for adjustment of partial factors for materials. cov of fct was assumed according to pren 1992-1-1 and bias factor is identical to fc. note that there is lack of information about gf in literature and thus it is assumed to be described by identical bias and cov as fct. note that values in tab. 1 determined according to pren 1992-1-1: 2021 (annex a) reflect also geometrical uncertainties by additional amplification of material uncertainty. var. mean cov [%] distrib. units fc 56 16 lognormal [mpa] fct 3.64 22 lognormal [mpa] e 36 16 lognormal [gpa] gf 195 22 lognormal [jm2] p 20 30 normal [%] table 1. stochastic model of the numerical example. in this first numerical study, there is an assumption of uncorrelated random variables. though the correlation might play crucial role in case of concrete structures. 3.3. numerical results the design values of resistance rd for each limit state in tons are determined as a quantile of lognormal distribution with identified statistical moments, target reliability indices are βuls = 3.8, βcrack = 3.8 and βsls = 1.5 according to en 1990 [4]. note that βcrack is not assumed to be serviceability limit state, since a cracking of the post-tensioned bridge leads to corrosion of tendons and ultimately collapse of the structure. moreover, corroded tendons can not be replaced. obtained design values are additionally reduced by global safety factor reflecting model uncertainties γrd = 1.06. obtained statistical moments together with determined rd can be found in tab. 2. note that the reference solution obtained by latin hypercube sampling (lhs) [13, 14] is based on 30 numerical simulations and one simulation takes approx. 24 hours. from the obtained results, it can be seen that all presented semi-probabilistic methods are in agreement with reference solution, though there are some differences leading to lower rd (approx. 10%) in comparison to lhs. however, one can see that ecov methods are both on conservative side while tse leads to accurate estimation near the reference results obtained by lhs. 4. discussion and further research the obtained results summarized in tab.2 are in good agreement among all presented methods. this can be explained by relative low non-linearity of this example failing in bending. in case of shear failure, this difference could be significantly higher as presented in previous work of the authors of this paper [15, 16, 18]. note that the results of simplified methods also show good agreement with their theoretical behavior, since the ecov according to fib and eigen ecov assume fully correlated input random variables (in order to reduce number of simulations to 2 resp. 3) and thus their estimation should be conservative in 146 vol. 36/2022 semi-probabilistic assessment of bridge using ecov methods limit state: uls cracking sls method µ vr rd µ vr rd µ vr rd ecov fib 485 0.098 340 399 0.107 272 154 0.153 121 eigen ecov 485 0.075 364 399 0.097 280 154 0.153 121 tse 485 0.066 375 399 0.085 290 154 0.132 124 lhs (reference) 480 0.061 376 396 0.074 298 154 0.142 123 table 2. obtained statistical moments and corresponding design values of resistance in tons for each limit state. comparison to tse and lhs. the conservative estimation is caused by a fact, that high correlation among input random variables typically leads to higher variance of r. if one needs to obtain accurate results without assumption of full correlation among input random variables, it is necessary to perform higher number of simulations by tse (at least n +1) or lhs (tens-hundreds). for the correct comparison, further work will be focused on reference solution assuming fully correlated variables and also realistic correlation matrix. the two limit cases – uncorrelated input random variables and fully correlated random variables – then define the interval of design values affected only by assumed correlation [8]. unfortunately, there is not any recommendation about correlation in codes and other relevant documents and thus this vague information might lead to significant differences in rd and further research should be also focused on this aspect as already discussed in the previous general pilot comparison of existing semi-probabilistic methods [19]. 5. conclusions the paper presented simplified probabilistic assessment of the existing bridge by selected semiprobabilistic methods for estimation of coefficient of variation. the bridge is represented by highly computationally demanding nlfem reflecting theory of nonlinear fracture mechanics of concrete. the stochastic model contains 5 random variables representing concrete characteristics and prestressing losses. the comparison of selected methods included also recently proposed eigen ecov and corresponding taylor series expansion. from the obtained results, it can be concluded that all selected methods lead to accurate estimation of design value of resistance for selected limit states. moreover results also corresponds to theoretical behavior of the methods with respect to assumed correlation among input random variables. acknowledgements this research was funded by czech science foundation, czech republic under project no. 20-01781s. moreover, the practical part of the research was funded by technical agency of the czech republic under project no. th04010138. references [1] n. bagge. demonstration and examination of a procedure for successively improved structural assessment of concrete bridges. structural concrete 21(4):1321–1344, 2020. [2] d. val, f. bljuger, d. yankelevsky. reliability evaluation in nonlinear analysis of reinforced concrete structures. structural safety 19(2):203 – 217, 1997. [3] a. m. hasofer, n. c. lind. exact and invariant second-moment code format. j engineering mechanics div 100(em1):111–121, 1974. [4] comité européen de normalisation (cen), brussels, belgium. en 1990: eurocode: basis of structural design, 2002. [5] v. červenka. global safety format for nonlinear calculation of reinforced concrete. betonund stahlbetonbau 103(s1):37–42, 2008. [6] v. červenka. reliability-based non-linear analysis according to fib model code 2010. structural concrete 14(1):19–28, 2013. [7] fib federation internationale du beton. fib model code for concrete structures 2010. john wiley & sons, berlin, heidelberg, 2013. [8] l. novák, d. novák. estimation of coefficient of variation for structural analysis: the correlation interval approach. structural safety 92:102101, 2021. https://doi.org/10.1016/j.strusafe.2021.102101. [9] l. novák, d. novák. on taylor series expansion for statistical moments of functions of correlated random variables. symmetry 12:1379, 2020. [10] h. schlune, m. plos, k. gylltoft. safety formats for nonlinear analysis tested on concrete beams subjected to shear forces and bending moments. engineering structures 33(8):2350 – 2356, 2011. [11] j. červenka, v. k. papanikolaou. three dimensional combined fracture-plastic material model for concrete. international journal of plasticity pp. 2192–2220, 2008. [12] jcss. jcss probabilistic model code. joint committee on structural safety, 2001. isbn 978-3-909386-79-6. [13] r. l. iman, w. conover. small sample sensitivity analysis techniques for computer models.with an application to risk assessment. communications in statistics theory and methods 9(17):1749–1842, 1980. [14] d. l. allaix, v. i. carbone, g. mancini. global safety format for non-linear analysis of reinforced concrete structures. structural concrete 14(1):29–42, 2013. [15] o. slowik, d. novák, l. novák. a. strauss. stochastic modelling and assessment of long-span precast prestressed concrete elements failing in shear. engineering structures 228:111500, 2021. https://doi.org/10.1016/j.engstruct.2020.111500. 147 https://doi.org/10.1016/j.strusafe.2021.102101 https://doi.org/10.1016/j.engstruct.2020.111500 lukáš novák, drahomír novák acta polytechnica ctu proceedings [16] d. novák, l. novák, o. slowik, a. strauss. prestressed concrete roof girders: part iii – semi-probabilistic design. in proceedings of the sixth international symposium on life-cycle civil engineering (ialcce 2018), pp. 510–517. crc press, taylor and francis group, 2018. [17] l. novák, d. novák. semi-probabilistic assessment of prestressed concrete bridge in proc. of the 13th fib international phd symposium in civil engineering, pp. 45-51, 2021, [18] l. novák, d. novák, r. pukl. probabilistic and semiprobabilistic design of large concrete beams failing in shear. in advances in engineering materials, structures and systems: innovations, mechanics and applications. taylor and francis group crc press, 2019. [19] m. sýkora, j. červenka, v. červenka, et al. pilot comparison of safety formats for reliability assessment of rc structures. in proceedings of the fib symposium 2019: concrete innovations in materials, design and structures, pp. 2076–2083. 2019. 148 acta polytechnica ctu proceedings 36:142–148, 2022 1 introduction 2 semi-probabilistic methods 2.1 partial safety factors 2.2 ecov fib method 2.3 taylor series expansion 2.4 eigen ecov method 3 post-tensioned concrete bridge 3.1 finite element model 3.2 stochastic model 3.3 numerical results 4 discussion and further research 5 conclusions acknowledgements references my title https://doi.org/10.14311/app.2022.36.0224 acta polytechnica ctu proceedings 36:224–230, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague the role of multi-fidelity modelling in adaptation and recovery of engineering systems rui teixeiraa,∗, beatriz martinez-pastora, maria nogalb, alexandra micuc, alan o’connorc a university college dublin, school of civil engineering, a richview newstead belfield, dublin 4, ireland b technical university delft, department of materials, mechanics, management & design, stevinweg 1, 2628 cn delft, the netherlands c trinity college dublin, school of civil, structural and environmental engineering, college green, dublin 2, ireland ∗ corresponding author: rui.teixeira@ucd.ie abstract. significant research has been conducted in identifying optimal recovery and adaptation decisions in disruptive scenarios using engineering models. in this context, an aspect that has been target of limited research is that of response times. modelling is expected to grow progressively more complex as it becomes more accurate. such complexity increases modelling efforts, and the promise of optimal adaptation and recovery may become hindered. the present work discusses the role of modelling fidelities in adaptation and recovery of systems, and in particular that of using a lower fidelity model that enables zero-time analyses of a system. a framework is proposed for using different fidelities in adaptation and recovery, considering system’s decision time requirements. the relevance of this analysis is researched in two traffic networks and results show that multi-fidelity models should be expected to play a key role in increasing the efficiency of optimal adaptation and recovery decisions. keywords: metamodels, multi-fidelity model, system adaptation, system recovery, systems. 1. introduction adaptation and response of civil engineering systems has become one of the most prominent topics of research in recent years. several methodologies and approaches have been proposed to tackle the need for adaptation and responsiveness, however, most of them fail to discuss one of the most relevant aspects of the system’s responsiveness, the decision time. engineering systems are complex in nature. they depend on multiple variables that can interact together and that originates a need for these to be treated as holistic. it is with no surprise that system analysis is recurrently highly complex, relying on intensive data analytics or costly modelling techniques. due to this optimal decision-making for civil engineering systems can become a resource intensive task. most approaches for adaptation and response to natural or man-made disasters lay their foundation on the premise that under a disruption an optimal adaptation decision making scheme can be found that will maintain the functionality of a system at certain minimum operational level. this can be achieved through intervention, by adapting the system operational variables, or other alternative system approaches [1]. in all cases, some form of technique needs to inform decision making, and current trends of research indicate that there is a growing interest in the application of high-fidelity (hf) modelling as a tool to inform decisions. this is identified in the progressive interest for hf tools such as, finite-element methods, and approaches, such as digital twinning or prediction of complex adaptivity in systems. most of these techniques were introduced in order to enable prediction and decision-making that is as close as possible to real-time response. however, in practice, running a decision-making scheme in such hf models can be effort consuming. in the instance of needing to use real-time information and prediction of the system operation to adapt and prevent any perturbation a form of parametric analysis needs to be performed. as this procedure will still depend on the hf model built to have a accuracy on predicting the system’s response, then performing any analysis that will establish a new point of operation that maintains (or optimizes) its functionality is expected to have large cost. within this context, the present paper discusses the role that multi-fidelity (mf) techniques may have in efficient adaptation of engineering systems. mf, that is, a blend of hf and low-fidelity (lf) modelling, may allow to balance the effort that is required to set optimal adaptation and recovery decisions. in particular, decision-times are expected to have a significant influence on the efficiency of the decision-making schemes. in the case of adaptation and recovery of an engineering system to a disruptive event, stalling a decision for a time longer than strictly necessary may set the boundary between significant or minor impacts in the system. in order to discuss the role of mf models in the context described, section 2 introduces the idea of mf modelling, section 3 discusses a framework 224 https://doi.org/10.14311/app.2022.36.0224 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 36/2022 role of multi-fidelity in system adaptation and recovery figure 1. multi-fidelity, from low-fidelity to high-fidelity models [2]. of application to adaptation and recovery of systems, section 4 presents two examples of application to a traffic network, and finally, section 5 draws the main conclusions of the work developed. 2. multi-fidelity (mf) modelling mf modelling has been widely researched in order to tackle the increasing analysis costs that are imposed by progressively more complex and accurate modelling techniques. the fundamental idea of mf is to combine hf models (commonly characterized as of high-resolution, high analysis cost), with lf models (commonly characterized as of lower resolution, low computational cost), see figure 1. hf models are expected to have better accuracy in practice, however, this does not mean that lf models cannot achieve adequate accuracy. in mf, both hf and lf are applied to balance the amount of effort required to perform an analysis based on the principle that an adequate amount of hf data may suffice an accurate analysis if complemented by lf data. such usage of different levels of fidelity has been previously researched and identified to have an important role in fields that are characterised by large analysis efforts such as, uncertainty quantification and optimization [3, 4]. it should be expected to become relevant in all engineering analyses that depend on effort consuming models, and where the time necessary to inform decision-making schemes has a key role. 2.1. forms of multi-fidelity modelling mf models can appear in different forms, and the level of fidelity depends on a series of factors. it can be said that hf models represent the behaviour of a system with an adequate level of accuracy, and that lf models are frequently a less refined representation of these. lf models can be created by assuming simplified forms for hf models. this can be achieved through dimensionality reduction, linearisation, simpler physics models, coarser domains, partially converged results, among other [5]. [5] highlight also different forms in which a system can be distinguished in terms of levels of fidelity. these may depend on the physical assumptions (e.g. may use different theories), numerical assumptions (e.g. more refined grids or different computational models that use same underlying theory), and also on the simple nature of the information (e.g., experimental versus simulations). one alternative that has captivated interest in the field of mf modelling is metamodelling. it consists in creating surrogates of a function or model by characterizing the relationship between inputs and outputs and relies on metamodels as lf pairs to hf data [6]. an overview of popular metamodelling techniques is provided in [7], and [8] provides insight on the usage of mf paired with metamodelling. different metamodels can be applied in this context. the interested reader is directed to the extensive literature on this topic [7] for a review of some examples. in the present work a kriging model is applied to discuss the relevance of mf alternatives in adaptation and recovery of systems. 3. integration of mf modelling in adaptation and response of engineering systems effective system response or adaptation is expected to rely on quick responses to damaging or disruptive events. when an informative state of the system is acquired, if decisions that are based on this state are not implemented fast enough, then this informative state may be lost as the system evolves. moreover, if an optimal decision of recovery or adaptation that was based on this informative state is later obtained; then there is a large likelihood that it won’t be optimal as the system evolved. to add that when an informative state of a system that is being affected by a disruption exists, from the perspective of the decision-maker, it is very difficult to not act and wait for an optimal evaluation of the decision-making scheme, even if this would be the most beneficial choice to achieve efficient decisions. as a result if an optimization for adaptation or recovery is pursued that aims at optimal decision-making, and if the results are delayed by running optimal searches for the decision variables in hf techniques, then loss of efficiency should be expected. in order to illustrate the importance of these aspects, research on mf applications is implemented in a traffic network. two networks are used to research on the effectiveness of mf in contexts of, respectively, recovery and adaptation. the nguyen-dupuis and 225 r. teixeira, b. martinez-pastor, m. nogal et al. acta polytechnica ctu proceedings sioux-falls traffic networks are used. a more detailed description of the network parameters used in these can be found in [9]. the mf model applied combines as hf the traffic network user equilibrium, a nonlinear model that determines the optimum state of equilibrium for the network users; and as lf model a kriging model, a surrogate of the hf model. 4. discussion on recovery and adaptation of a traffic network to a disruptive event the mf relationship between the hf and lf in the contexts of adaptation and recover is highlighted in figure 2. two essential notions of time under decisionmaking schemes can be highlighted in this context: • slow-time: where time is not key for the effectiveness of overall decision-making response. • fast-time: where time has critical role in the overall effectiveness of the decision-making scheme. a metamodeling discussion that uses a similar idea was previously implemented in the research of digital twinning in [10]. figure 2. framework for mf application in adaptation and response. for most of the operation of a system, decisionmaking can be considered to be in slow-time. any modelling of decision-making in such circumstance can rely in hf models and limited change to steady conditions should be expected. in this phase, hf can predict outcomes for the system at limited cost. effectiveness will be largely unaffected by decisiontimes as the pairing of the system modelling with its operation is continuous. however, under a disruptive scenario the same assumptions about the relationship of decision-making schemes and effectiveness are not expected to hold. in unsteady conditions, the system will be changing, hence, an optimal decision on slowtime is unlikely to be optimal (to adapt or recover the system a decision-making scheme may be needed to adapt the decision variables). in such scenario optimal decisions should have zero-time and be paired with the system unsteady conditions. it is within this idea that mf modelling is framed in system recovery and adaptation. mf allows to control time in optimal decision-making, depending on the system demand. in slow-time the multi-fidelity technique can be improved as a predictor of the system response for fast-time. in this phase, which is representative of normal operational conditions, the hf model responds and the lf model improves using hf data (including potential disruptive scenarios). to construct the mf model for this effect, it is assumed that a hf model f(x) exists such that it is an exact approximation of the system behaviour s(x) for any operational state x, then in slow-time a mf model can be built using metamodelling that includes a lf surrogate g(x) of f(x). this lf can use system inputs under foreseen and unforeseen scenarios of disruption or operation, x. if a global approximation to the space of x can be achieved, then the lf metamodel can act as an accurate predictor of f(x) and s(x) for the whole domain of operation. application of the kriging enables lf improvement in slow-time using its properties to enrich the lf model so that it responds accurately in fast-time. in this phase, it is possible to train the lf model in x using hf data. this relies on operational data and exploits the kriging unique gaussian uncertainty. with kriging as a lf, it is possible to search the operational space x for points of large uncertainty. the lf has then the potential to systematically become a more accurate prediction of the hf model. as a result, in fast-time, the lf swaps to the responding position, and the hf is expected to improve collecting data from unforeseen occurrences. in this phase decision-times are critical, and the lf model, i.e. the kriging, is expected to provide a sufficiently appropriate prediction in order to allow for a quasi-optimum (with relation to the lf-hf error) decision-making. moreover, for any x that does not fulfil the accuracy condition in fast-time a hf sample can be taken to enrich the lf model. in fast-time, samples at hf should be minimized and only extracted when necessary. two examples of application, in recovery and adaptation, are studied as reference examples of the relevance of mf in the context described. these are presented in sections 4.1 and 4.2. 4.1. recovery of traffic network in a scenario of damage in the first example of application, that concerns recovery of a traffic network, the nguyen-dupuis network is used. [11] researched this same traffic network in the context of reliability. it is assumed that a damage affects 7 of its links at 50% of their capacity; links 1,7,8,9,10, 12, 23 as per figure 3. then four recovery teams are to be deployed in order to mitigate the effects of this damage and to recover the network 226 vol. 36/2022 role of multi-fidelity in system adaptation and recovery functionality. this recovery will be as efficient as the decision-making that plans the recovery strategy. in practical terms, an efficient recovery should restore the network operational performance to the a stage of pre-damage as soon as possible. such procedure depends on an efficient allocation of resources, which minimizes the loss of functionality in the network and improves its resilience [12, 13]. a damage in the network will increase the cost for users to travel from one node in the network to another, in particular if this trip passes through an affected link. from a decisionmaker perspective the interest is to distribute the resources that will efficiently decrease the travel cost. if a damage occurs in the system at t = 0 then in this moment the decision-maker should set the optimum strategy in place to restore the network so that in t=1 its functionality starts to be recovered. in the present case the lf model is defined using kriging where for x states of operation, the mf model uses the lf surrogate enriched on the hf model (the network user-equilibrium model) in slow-time. x encloses potential link capacities for the considered links. in the seven-dimensional operational state (i.e., of damaged links), only 28 hf evaluations in slowtime are required to set an accurate lf model with a maximum relative error of prediction of a latin hypercube sample (see example [13, 14]) of 100 points of 5%. in order to discuss the relevance of the approach proposed, figure 4 presents the results of recovery using the mf approach. the mf is used in fast-time to predict allocation decisions. the results of this approach are then compared with the result from decision-making that relies exclusively in the hf technique. in both cases a genetic algorithm (ga) is used to allocate teams for recovery. results show that recovery decisions for the mf approximate with small loss of accuracy the results provided by the hf. mf prediction refers to the value of prediction of the mf model, while mf-dm refers to the effective system recovery after the decision-making takes place, i.e., real evaluation of mf prediction solution in the system. from t = 1 to t = 3 the mf prediction provides almost the same recovery efficiency as the hf (loss is below 0.1% in total travel cost), while at t = 4 there is a slightly larger loss of recovery cost when compared with the ga decision making supported by the hf model. the mf approach, however, consumes a small fraction of the decision time. in figure 4-(c) it is possible to infer that the ga optimal decision for recovery (team allocation) is achieved in mf for each day in less than 2 seconds, regardless of the ga parameters. such small decision-time has a large potential to increase the agility of this decision process. finding the optimal decision with ga relying on the hf technique would impose a delay of the decision-making scheme stalling the recovery process. decision-making that relies on the hf technique would require at least 25 minutes to achieve similar results in the most critical days, even considering that this is a relativity low dimensional problem in a relatively simple network. it is possible to infer that the analysis time increases significantly when the optimisation search in the ga uses more points, which is an important consideration for analyses that include more decision variables and that may demand larger population sizes. when the population size of 100 individuals is applied jointly with the convergence of the fitness function for 50 generations, the effectiveness of the hf dm making decreases substantially (decision times increase by more than 3); while in practical terms the mf model is largely unaffected by this. it is still capable of enabling zero-time responses in practice. this may be an important consideration in more involved applications. 4.2. adaptation of traffic network under incomplete information about the damage scenario the role of mf modelling in a context of adaptation is now discussed in the sioux-falls network, see figure 5. this network includes 76 links and 24 nodes. in the present example, 14 od pairs (with a demand of 200 users/hour for each od) are considered in the modelling, each comprising 4 routes. the capacity of each link is set initially to be 50% larger than the od pair demand. the idea of the current implementation of mf modelling is to find a representative example of the role of different fidelities in adaptation of a system. in this case decision-making for adaptation should be studied considering a scenario of incomplete information. a damage is known to have occurred in the network, however its extent is uncertain. existing information indicates that there is a likelihood of 70% that a scenario of minor damage occurred; where the number of affected links (n(ld)) has 25% probability of being n(ld) ≤ 4, 35% probability of being n(ld) = 5, 20% probability of being n(ld) = 6, 15% probability of being n(ld) = 7, and 5% probability of being n(ld) ≥ 8. in all possibilities the extent of the damage is expected to be 70% of the link capacity. then, there is a 30% probability of a scenario of high damage having occurred; where there is 30% probability of (n(ld)) ≤ 6, 20% probability of n(ld) = 7, a 20% probability of n(ld) = 8, a 15% probability of n(ld) = 9, and 15% probability of n(ld) ≥ 10. in all the cases of this scenario, a damage of capacity of 75% is expected. figure 5 presents potentially affected links (with highlighted link numbers). a decision should be made about how to adapt the network to this disruption. decisions that use the hf model are highly accurate, but involve large computational cost. in this example of disruption, with 18 dimensions, that would cover the decision to adapt potentially disrupted link capacities. so, as soon as the decision-maker commits to search for an optimal decision, he will need to perform an optimization 227 r. teixeira, b. martinez-pastor, m. nogal et al. acta polytechnica ctu proceedings figure 3. perturbation considered in the nguyen-dupuis network. red arrows represent the links that are damaged. figure 4. results of the application of the mf approach to the problem of recovery. (a) results based on a ga team distribution with a population size of 50 and that converge when no improvement in the fitness function occurs for 20 iterations. (b) results based on a ga team distribution that uses a population size of 50 and converges when no improvement in the fitness function occurs for 20 iterations. (c) computational times for (a)-(b). 228 vol. 36/2022 role of multi-fidelity in system adaptation and recovery figure 5. sioux-falls network and uncertain scenario of damage simulated. the (red) rectangles identify the links that are potentially damaged in the uncertain disruption. of the decision criteria on the hf model. there is limited adaptivity to this procedure, as changing the input conditions to it will involve a restart of the optimization calculations. if a mf model is introduced, then decision-making into adaptation can become adaptive; and because running an optimization scheme on a lf model that is representative of the hf model is virtually costfree, then, as new information becomes available it is possible to change decisions. in the case of the sioux-falls with 18 potentially damaged links, the lf model is built in slow-time and runs in tandem with the hf model considering an x that includes the capacities of each link from a reduction of 80% in relation to the initial capacity to an increase of 50% of the initial capacity. it uses 3205 evaluations of the hf model and is able to validate in average a random global sample within 10% of absolute error in prediction. if new points are required these can be added in slow-time or fast-time. figure 6 presents the results for adaptation to the uncertain scenario presented in this example. when the system is damaged it is assumed that a budget of adaptation is available that can add extra capacity to the network links. this budget has a limit 300 users/hour, and any of the potentially damaged links cannot have its capacity enhanced in more than 150 users/per hour. the adaptation decision-making problem involves then finding the optimal usage of this budget to improve the capacity of the potentially damaged links in figure 6, such that the loss in travel cost in the system is minimized. a genetic algorithm is used to allocate the additional capacity in the links considered in order to mitigate the effects of the perturbation. the population size is set to 100 figure 6. results for the adaptation probability distribution function using hf and mf modelling. individuals, complemented with convergence of the best fitness value for 50 iterations. loss of effectiveness for the adaptation decision is compared in terms of the reduction of travel cost that is possible to achieve with the it (i.e. distributing the budget available among links). the black dotted line presents the probability distribution of the response considering predictions using the mf model. because the mf model is virtually costless to run, the optimal operational point can be determined as new information about the extent of the damage enters the decision-making scheme, that is, an optimal adaptation strategy can be defined for all possible scenarios at virtually no cost (zero-response time). this curve is to the left of the remaining adaptation decisions that rely on hf modelling combined with the same decision technique. because these rely on the hf model it is not possible to change the input conditions of the decision-making scheme variables with the same level of flexibility. a single optimization to a specific scenario of damage will demand several hours to run. three scenarios are therefore tested; to optimize the adaptation of the network considering that all links are damaged at 70% of their capacity, optimize the adaptation of the network considering that all links are damaged at 75% of their capacity, and optimize the adaptation of the network considering that no links are damaged. in relation to the case of no adaptation, all contribute to an important improvement of the response; nonetheless, still outperformed by the adaptation with mf. the capability to change ad-hoc the decision with new information means total adaptive capacity. 5. conclusions the present work discussed the role that multi-fidelity modelling may have in the future of system analysis, with explicit relation to decision-making in recovery and adaption. while significant research has been directed to the need for accurate system models that can 229 r. teixeira, b. martinez-pastor, m. nogal et al. acta polytechnica ctu proceedings accurately inform decision-making, limited research has been directed to the practicality of these as a tool to inform decision schemes. while modelling of systems becomes more complex and accurate, so does the cost of it (effort required). in particular if paired with an optimization scheme, achieving an optimal decision can be quite resource and effort-demanding. in practice, and considering that systems modelling is inherently complex, this may not provide an efficient alternative for decision-making in scenarios of disruption. decisions in recovery and adaptation require zero-response times to be effective. as systems are dynamic, non-zero response times imply that at the moment a decision is achieved, there is a likelihood that it won’t be optimal or near-optimal; a consequence of the system’s evolution in time. a metamodel was used to construct a multi-fidelity model, where it fulfilled the role of low-fidelity model, that enables zero-response times in recover and adaptation of a system. the ideas of slow-time and fast-time were introduced and the effectiveness of multi-fidelity models emphasized in two examples of traffic networks. an adaptive system should include zero-time decision-making schemes, so that the optimal decision is enabled in each moment of operation, and any recovery or adaptation of the system is continuous with the system operation. different layers of fidelity have a key role in enabling this system behaviour. references [1] y. shang, m. nogal, h. wang, a. r. m. r. wolfert. systems thinking approach for improving maintenance management of discrete rail assets: a review and future perspectives. structure and infrastructure engineering 0(0):1–19, 2021. https://doi.org/10.1080/15732479.2021.1936569. [2] s. guo. https://towardsdatascience.com/an-intr oduction-to-surrogate-modeling-part-iii-beyond -basics-a60125767271. [3] a. i. forrester, a. sóbester, a. j. keane. multi-fidelity optimization via surrogate modelling. proceedings of the royal society a: mathematical, physical and engineering sciences 463(2088):3251–3269, 2007. https://doi.org/10.1098/rspa.2007.1900. [4] b. peherstorfer, k. willcox, m. gunzburger. survey of multifidelity methods in uncertainty propagation, inference, and optimization. siam review 60(3):550– 591, 2018. https://doi.org/10.1137/16m1082469. [5] m. g. fernández-godino, c. park, n.-h. kim, r. t. haftka. review of multi-fidelity models. arxiv preprint arxiv:160907196 2016. https://doi.org/10.2514/1.j057750. [6] q. zhou, y. wu, z. guo, et al. a generalized hierarchical co-kriging model for multi-fidelity data fusion. structural and multidisciplinary optimization 62:1885–1904, 2020. https://doi.org/10.1007/s00158-020-02583-7. [7] r. teixeira, m. nogal, a. o’connor. adaptive approaches in metamodel-based reliability analysis: a review. structural safety 89:102019, 2021. https://doi.org/10.1016/j.strusafe.2020.102019. [8] d. j. toal. some considerations regarding the use of multi-fidelity kriging in the construction of surrogate models. structural and multidisciplinary optimization 51(6):1223–1245, 2015. https://doi.org/10.1007/s00158-020-02583-7. [9] r. teixeira, b. martinez pastor, m. nogal, a. o’connor. metamodel-based metaheuristics in optimal responsive adaptation and recovery of traffic networks. sustainable and resilient infrastructure 2022. https://doi.org/10.1080/23789689.2022.2029325. [10] s. chakraborty, s. adhikari, r. ganguli. the role of surrogate models in the development of digital twins of dynamic systems. applied mathematical modelling 90:662–681, 2021. https://doi.org/10.1016/j.apm.2020.09.037. [11] r. teixeira, b. martinez-pastor, m. nogal, a. o’connor. reliability analysis using a multimetamodel complement-basis approach. reliability engineering & system safety 205:107248, 2021. https://doi.org/10.1016/j.ress.2020.107248. [12] m. nogal, a. o’connor, b. caulfield, b. martinezpastor. resilience of traffic networks: from perturbation to recovery via a dynamic restricted equilibrium model. reliability engineering & system safety 156:84–96, 2016. https://doi.org/10.1016/j.ress.2016.07.020. [13] b. martinez-pastor, m. nogal, a. o’connor, r. teixeira. transport network resilience: a mapping and sensitivity analysis strategy to improve the decision-making process during extreme weather events. international journal of critical infrastructures 17(4):330–352, 2021. https://doi.org/10.1504/ijcis.2021.120165. [14] r. teixeira, a. o’connor, m. nogal. probabilistic sensitivity analysis of offshore wind turbines using a transformed kullback-leibler divergence. structural safety 81:101860, 2019. https://doi.org/10.1016/j.strusafe.2019.03.007. 230 https://doi.org/10.1080/15732479.2021.1936569 https://towardsdatascience.com/an-introduction-to-surrogate-modeling-part-iii-beyond-basics-a60125767271 https://towardsdatascience.com/an-introduction-to-surrogate-modeling-part-iii-beyond-basics-a60125767271 https://towardsdatascience.com/an-introduction-to-surrogate-modeling-part-iii-beyond-basics-a60125767271 https://doi.org/10.1098/rspa.2007.1900 https://doi.org/10.1137/16m1082469 https://doi.org/10.2514/1.j057750 https://doi.org/10.1007/s00158-020-02583-7 https://doi.org/10.1016/j.strusafe.2020.102019 https://doi.org/10.1007/s00158-020-02583-7 https://doi.org/10.1080/23789689.2022.2029325 https://doi.org/10.1016/j.apm.2020.09.037 https://doi.org/10.1016/j.ress.2020.107248 https://doi.org/10.1016/j.ress.2016.07.020 https://doi.org/10.1504/ijcis.2021.120165 https://doi.org/10.1016/j.strusafe.2019.03.007 acta polytechnica ctu proceedings 36:224–230, 2022 1 introduction 2 multi-fidelity (mf) modelling 2.1 forms of multi-fidelity modelling 3 integration of mf modelling in adaptation and response of engineering systems 4 discussion on recovery and adaptation of a traffic network to a disruptive event 4.1 recovery of traffic network in a scenario of damage 4.2 adaptation of traffic network under incomplete information about the damage scenario 5 conclusions references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.38.0424 acta polytechnica ctu proceedings 38:424–428, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague bringing students closer to the issues of indoor environmental quality and technology using nemos device: an experience in albania francesco salamonea,∗, lorenzo belussia, ludovico danzaa, etleva dobjanib, matteo ghellerea, italo meronia, arben shtyllab, saimir shtyllab a itc-cnr, via lombardia 49, 20098 san giuliano milanese, italy b polis university, department of applied research, rr. bylis 12, autostrada tiranë-durrës, km 5, 1051, tirana, albania ∗ corresponding author: salamone@itc.cnr.it abstract. the article presents the outcomes of a monitoring campaign and a survey performed in a school building in tirana. included in the bilateral agreement between the national research council of italy and the ministry of education and sport of the republic of albania, the research is based on environmental analysis, collection of data from monitoring of environmental variables and students’ feedback. the monitoring of indoor environmental quality (ieq) was carried out with a nearable environmental monitoring system (nemos) for ieq purposes, designed and developed by itc-cnr based on the do-it-yourself (diy) philosophy. the proliferation of the maker movement philosophy has promoted the diffusion of diy-based technologies. the spread of this movement is observed outside schools but there is a growing interest among educators to introduce this philosophy in the students’ education, approaching to steam (science technology engineering arts mathematics) in a different and more stimulating way. the application of two nemos devices in two classrooms (in different locations) aroused the curiosity of students, bringing them closer to the world of technology with a practical example. it also increased their awareness about the overall ieq conditions in the classrooms where they spend a considerable part of their lives. keywords: ieq, albania, school, sustainability, survey. 1. introduction school buildings are fundamental in the education of future generations and to transmit them the desire for research and innovation, too. coding and programming have emerged as the most desired success factors for society [1] where especially for the first one there should be considered its introduction in the didactic curricula in response to the labour market demand. the main difference among them is that coding is a part of programming and involving the translation of a written code by a machine; while the second is the process of creating a program following specific standards and performing a specific task, too. in the last decade, researchers and educators have developed tools to simplify the mechanics of learning computing at school, introducing new programmers to become more fluent and expressive towards new technologies [1]. new technological developments of microcontrollers, low-cost sensors and actuators foster the teaching of these new concepts [2]. countries that are distinguished for their technologically advanced economies and societies are reconsidering their education policies by concentrating their economic efforts in the promotion of the stem education (science, technology, engineering and mathematics) [3]. in this sense, arduino-based projects can stimulate childrens’ computing competences [4] as well 3d-printing approach can be a promising technology to engage students with hands-on interactions [5]. in the last years, the indoor environmental quality (ieq) in schools has attracted the attention of several researchers, since the quality of indoor spaces affects children’s well-being, health, and productivity. scientific literature reports several case studies focused on the evaluation of the ieq in educational buildings, where children and teachers were inquired about their perception of the indoor environment. as planned in the two-year bilateral agreement between the national research council of italy and the ministry of education and sport of the republic of albania, a monitoring campaign was designed and conducted in a school in the outskirts of tirana but in a reduced number of effective days due to covid19. therefore, it can be considered as a small-scale test and a useful reference to be reproduced in similar scenarios with a longer time span. the short three-day monitoring campaign made possible to comprehend whether the methodology based on the use of the nemos device [2], developed in the laboratories of itc-cnr, can be used, on the one hand, to collect data on subjective and objective environmental quality in the classrooms 424 https://doi.org/10.14311/app.2022.38.0424 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 38/2022 bringing students closer to the issues of indoor environmental quality . . . figure 1. school building with sun path for the considered class period (november 16th–18th), classrooms configurations and position of nemos devices (in red) during the campaign. where students spend most of their lives; on the other hand, to raise students’ awareness regarding the importance of assessing the environmental quality in the classrooms thanks to a diy-based and 3d-printed monitoring device. in this case, the pupils are not involved in the production of the devices, but they could be in the future. this pilot project can be seen as an opportunity by stakeholders to integrate new technical disciplines in the educational background. to acknowledge pupils and teachers with the results of the experiment, the research team delivered a final report to the school’s board. 2. case study and methodology the case study is the “kolë jakova” school (figure 1), located in the north-western suburb area of tirana (lat. 41° 21′ 28′′ n, long. 19° 45′ 44′′ e). built in 19831 and with a capacity of 12-classrooms2, it is among the most diffused typologies in rural areas during the period 1945–1990 in albania. nowadays its capacity represents a limit because the demand for classrooms is higher and consequently it constraints the split of the didactic activity for some classes in the morning and for others in the afternoon. indeed, the school hosts 420 pupils for a total of 18 classrooms (elementary and lower secondary cycle) but during the monitoring campaign all classrooms were at half capacity due to the covid-19 restrictions. the classrooms chosen for the campaign were positioned on the opposite sides of the building (east and 1referring site plan file at the central national technical archive: “shkollë në fshatin bregu i lumit dt 10.05.1983” (school in the village of bregu i lumit dated 10.05.1983). 2technical name of the project at the central national technical archive: “shkollë 8-vjeçare me 12 dhe 8 klasa 73.6” (8-year cycle school with 12 and 8 classrooms 73.6). west facing) to comprehend the impact of different orientations on people comfort perception. besides being built in the 80s it has undergone to partial renovations including the installation of a heating system not expected in the original design. the vertical opaque components are uninsulated brickbased walls (u-value = 1.26 w/m2k). the flat roof was insulated with 0.05 m thick polystyrene layer with a waterproof layer on top (u-value = 0.48 w/m2k). the windows (130 × 160 cm) are single-glazed with aluminium frame (u-value of 6.8 w/m2k and shgc = 0.7). the heating system is a centralized two-stage plant with power ranging between 178–391 kw with radiators in each classroom, but inactive during the survey, whereas two separate boilers guarantee the production of domestic hot water (dhw). the monitoring campaign and the survey aiming to collect of subjective and objective data was carried out between november 16th–18th, 2020 in the middle fall/winter school session in albania. the objective monitoring of indoor environmental variables was carried out using nemos [3, 4]. before the campaign, students were introduced to concepts of computing and 3d-printing (the pillars nemos was built) and ieq. the nemos device gained great attention among students in both classrooms. they were not informed regarding any instant value of the acquired data to avoid any eventual influence on their responses. before the start of the campaign, students were informed regarding the purpose of the monitored environmental factors (efs) measured by the nemos device. the monitoring campaign was performed for three consecutive days in each classroom. a simplified survey form was elaborated to acquire the subjective data of pupils’ perceptions in order to avoid any eventual confusion and misunderstanding. as illustrated in the figure 2, the questions were focused on the thermal 425 f. salamone, l. belussi, l. danza et al. acta polytechnica ctu proceedings figure 2. questionnaire structure about thermal, visual and iaq perception (translated from albanian). classroom total [-] age [y] weight [kg] height [cm] iclo [clo] metst [met] total (m–f) avg ± std avg ± std avg ± std avg ± std avg ± std east 13 (6–7) 14 ± 0.5 57.92 ± 14.38 164.32 ± 13.01 0.73 ± 0.11 1 ± 0 west 8 (4–4) 13 ± 0.5 50.42 ± 6.42 166.73 ± 6.62 0.78 ± 0.10 1 ± 0 table 1. data of the 21 students involved in the study. and visual perception, based on the five-point bipolar scale (from “cool” or “dark” = −2 to “warm” or “bright” = +2), while the question about the indoor air quality (iaq) perception was based on a unipolar scale (from “not smelly” = 0 to “very smelly” = 3). as mentioned above, due to the restriction because of covid-19, the classrooms were not at full capacity. the subjective data were collected from the pupils’ perceptions expressed in the specific questionnaire regarding the three parameters of the ieq during the didactic activity. during three consecutive one-hour intervals, between 1.00 p.m. to 3.00 p.m. the pupils had to express in the questionnaire their thermal, visual and iaq perceptions. all the 21 pupils that took part in the campaign, have been grouped for each class in the aggregated data as listed in table 1. students brought the parental consent to take part in the survey. from the analysis of participants’ answers it was possible to calculate the thermal resistance of their clothing in compliance with annex c of the standard en iso 7730. an additional thermal resistance of 0.1 clo for sedentary activities due to the standard school chair is considered [5]. the standard metabolic rate metst was defined in accordance with the value reported in annex b of en iso 7730. 3. results and discussions for a better evaluation of the impact of the orientation and direct daylight exposure, the chosen classrooms are positioned on the opposite sides of the building as illustrated in figure 1. in figure 3, there are reported the environmental data acquired during the monitoring campaign in the chosen classrooms, whereas in figure 4 are reported the subjective responses of students on the considered aspects. the assessed classrooms have similarities regarding the measured levels of air velocity (av) and relative humidity (rh). the av levels highlight lower averages and limited variation, determining ideal conditions for low draught. on the other hand, rh levels highlight moderate variations and mean values slightly higher than 60 %, not distant from the standard design value of 50 %. there is a slight difference regarding the illumination (il) levels between the classrooms: although the mean il values are almost the same, the data variation in west classroom (yellow device) is higher compared to the east classroom (green device). evident differences between the two classrooms emerge from the comparison of the operative temperature (ot) and carbon dioxide concentration (co2) levels in terms of mean values and variability. indeed, in the east classroom (green device) are registered the highest mean values for ot (19.8 °c vs 18.5 °c) and co2 (1800 vs 1200), as well the highest co2 variability, while the west classroom (yellow device) highlights the highest ot variability. the huge variability in co2 concentration is essentially due to the natural ventilation assured in both classrooms [6]. the analysis of subjective responses partially confirms the ieq monitored data (figure 4). analysing visual comfort (vc) perception, 74 % and 71 % of the respondents respectively report a neutral room brightness. in east classroom the percentage of pupils perceiving a “slightly bright” condition is more than three time higher than in the west classroom (18 % and 5 % respectively). and conversely the percentage of pupils perceiving the “slightly dark” condition is three time higher in west than xib (24 % and 8 % respectively). previous results suggest that a mean 426 vol. 38/2022 bringing students closer to the issues of indoor environmental quality . . . figure 3. monitored environmental data: a) east classroom; b) west classroom. figure 4. subjective perception of thermal comfort (tc), visual comfort (vc) and indoor air quality (iaq): a) east classroom; b) west classroom. illuminance value around 300 lx implies a perceived neutral or bright environment for at least 3 out of 4 users, confirming the findings of [7] for similar buildings and students age. the iaq perception partially reflects the analysis of the corresponding monitored data (co2). despite a consistent difference in co2 concentration values (the east value is 600 ppm higher than the west value), only 7 % of east perceive the air as “very smelly”, while the most common perception for both classrooms is “1 – low smelly”. a reason for this could be the low sensitivity of users to air quality, since other ieq aspects are more immediate to assess (i.e., room brightness, temperature level). even though the reduced number of pupils for classrooms, due to the pandemic, the finding related to iaq data, both monitored and perceived, are in line with [8]. finally, the tc analysis confirms the measured data (east classroom warmer than west classroom). the east classroom shows a higher neutral perception (62 % vs 48 %) and a lower “−2 – cool” perception (0 % vs 14 %) than the west classroom. the tc subjective responses also reflect the main ot variability in west classroom as long as it causes a high thermal adaptation body activity of the users. 4. conclusions the monitoring campaign and the survey have been performed in an unusual period because of the ongoing pandemic situation worldwide with didactic activities performed with reduced number of pupils. another important limitation of the study is related to the resolution of the monitoring campaign: only one measuring point for each classroom. in addition, it was not possible to record the status of opening or closing doors and windows. this would have allowed identification, without speculation, of what the effect on co2 concentrations might be. this is an improvement that could be considered in a future monitoring campaign. nevertheless, monitoring based on the use of low-cost, 3d-printed solutions represents a great stimulus to arouse students’ interest in the topic of growing field such as (iot) and increases their awareness regarding the indoor comfort issues. the main differences of monitored data between the two classrooms are identified in ot with the lowest mean value recorded in the west classroom, and co2, with the highest value monitored in east classroom. probably the main differences in ot data can have influenced the temperature comfort (tc) perception, thus allowing to justify 14 % of west classroom perceiving “cool”. the 427 f. salamone, l. belussi, l. danza et al. acta polytechnica ctu proceedings co2 differences are not reflected by iaq perception and this probably may reveal students’ adaptation to this environment condition [7]. these aspects could be considered to improve the overall ieq in classrooms performing participatory activities where students can be engaged with hands-on interactions, using self-developed monitoring devices. in this sense, it might be possible to bring students close to the themes of ieq and technology. measurement performed on a large scale, with devices such as nemos, along with feedback provided by a larger number of participants, will make it possible to identify more accurate differences between objective data and subjective perceptions in relation to the considered efs and the target audience. acknowledgements the authors thank the ministry of education and sport of the republic of albania for the authorization to conduct the monitoring campaign. references [1] p. tuomi, j. multisilta, p. saarikoski, j. suominen. coding skills as a success factor for a society. education and information technologies 23:419–434, 2018. https://doi.org/10.1007/s10639-017-9611-4 [2] f. salamone, l. danza, i. meroni, m. c. pollastro. a low-cost environmental monitoring system: how to prevent systematic errors in the design phase through the combined use of additive manufacturing and thermographic techniques. sensors 17(4):828, 2017. https://doi.org/10.3390/s17040828 [3] f. salamone, l. belussi, l. danza, et al. design and development of nemos, an all-in-one, low-cost, web-connected and 3d-printed device for environmental analysis. sensors 15(6):13012–13027, 2015. https://doi.org/10.3390/s150613012 [4] f. salamone, l. danza, i. meroni, m. c. pollastro. a low-cost environmental monitoring system: how to prevent systematic errors in the design phase through the combined use of additive manufacturing and thermographic techniques. sensors 17(4):828, 2017. https://doi.org/10.3390/s17040828 [5] t. wu, w. cui, b. cao, et al. measurements of the additional thermal insulation of aircraft seat with clothing ensembles of different seasons. building and environment 108:23–29, 2016. https://doi.org/10.1016/j.buildenv.2016.08.008 [6] details about natural and mechanical ventilation. [2022-03-21]. https://www.epfl.ch/labs/hobel/mechanical-ornatural-ventilation/ [7] l. noda, a. v. p. lima, j. f. souza, et al. thermal and visual comfort of schoolchildren in air-conditioned classrooms in hot and humid climates. building and environment 182:107156, 2020. https://doi.org/10.1016/j.buildenv.2020.107156 [8] l. dias pereira, l. neto, h. bernardo, m. gameiro da silva. an integrated approach on energy consumption and indoor environmental quality performance in six portuguese secondary schools. energy research & social science 32:23–43, 2017. https://doi.org/10.1016/j.erss.2017.02.004 428 https://doi.org/10.1007/s10639-017-9611-4 https://doi.org/10.3390/s17040828 https://doi.org/10.3390/s150613012 https://doi.org/10.3390/s17040828 https://doi.org/10.1016/j.buildenv.2016.08.008 https://www.epfl.ch/labs/hobel/mechanical-or-natural-ventilation/ https://www.epfl.ch/labs/hobel/mechanical-or-natural-ventilation/ https://doi.org/10.1016/j.buildenv.2020.107156 https://doi.org/10.1016/j.erss.2017.02.004 acta polytechnica ctu proceedings 38:424–428, 2022 1 introduction 2 case study and methodology 3 results and discussions 4 conclusions acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.37.0054 acta polytechnica ctu proceedings 37:54–57, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague reactor criticals and spent fuel systems similarities matěj šikla,b a czech technical university in prague, faculty of nuclear sciences and physical engineerings, department of nuclear reactors, v holešovičkách 2, 180 00 prague 8, czech republic b nri řež, a. s., department of reactor physics, hlavní 130, 250 68 husinec – řež, czech republic correspondence: matej.sikl@ujv.cz abstract. many conservative assumptions for subcriticality assessment calculations have to be made in current calculations of spent fuel storage systems. this is due to a fact that there is no available database of benchmarks containing spent fuel or materials with isotopic composition like spent fuel. however one potential source of spent fuel system experiments is being omitted – there are hundreds of commercial reactors around the world containing spent fuel on some level of burnup. these reactors have to be tested for safety reasons in the beginning of each cycle and parameters of these test states are well known. in this paper a possibility of using commercial reactor critical states for code validations and for subcriticality assessments of storage systems is discussed and potential approach for criticality safety analysis is given. keywords: reactor criticals, spent fuel storage systems, calculation similarity, criticality safety analysis. 1. introduction spent fuel storage systems subcriticality limit of keff < 0.95 is set in current czech [1] and many other countries legislation. this limit is usually fulfilled, even though some adjustments and estimations not reflecting real state have to be used when creating storage models. firstly, due to a lack of credible benchmarks with spent fuel isotopes, currently there is low knowledge about spent fuel model calculations precision and therefore reliable calculations with fresh fuel are usually performed. this also means that calculated keff s are usually higher than reality, because by using of fresh fuel, falling trend of fuel reactivity with raising level of burnup is omitted. secondly, some conservative assumptions have to be used about neutron absorbers included in system. but if these would be too conservative, not all systems would fulfil the legislative limit. because of these adjustments, calculations for proving subcriticality of spent fuel storage system are performed on models, which are not perfectly alike to the reality of storages. subcriticality calculations of storage systems (and even criticality calculations of systems including spent fuel) would be more reliable, if a calculation code and process could be validated on spent fuel experiments. luckily one possible source of experiments is available – physical and safety start-up tests from beginning of each cycle of commercial reactors in nuclear power plants. states from these tests has sufficiently known parameters to become benchmark experiments, including mainly exactly known keff = 1 (that is why these states are being referred as "reactor criticals") and precisely measured core parameters from instrumentation and control system. as was said, there is potential for development of more trustworthy process for spent fuel systems calculations validation. but the first milestone is to verify that reactor criticals and spent fuel systems are sufficiently similar. this is what will be discussed in this paper. 2. current approach to subcriticality assessment as was already written in the introduction, spent fuel storage systems subcriticality assessments have to be performed based on calculations using fresh fuel isotopic composition and conservative assumptions. brief summary of fresh fuel systems experiments available for code validations and generally used conservative adjustments will be given. 2.1. generally available benchmarks for code validation in an ideal case calculation codes should be validated using benchmarks of same type of systems calculations like the newly projected system it will be applied to. in reality this is impossible for spent fuel storage systems, because benchmarks with spent fuel composition are usually not described in available databases. few artificially prepared experiments with spent fuel 54 https://doi.org/10.14311/app.2022.37.0054 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 37/2022 reactor criticals and spent fuel systems similarities isotopes were described, but these usually has only units of required isotopes and not in atomic density quantities which are typically present in real spent fuel. therefore validation of calculation codes for any type of calculation is usually performed using fresh fuel experiments only. this approach is not ideal and in case of using it, calculations of models of different types should be penalized. main source of benchmarks cases with fresh fuel which can be used for validation of calculation codes is international handbook of evaluated criticality safety benchmark experiments (produced by icsbep) [2]. this database contains sets of various cases of different systems from just uranium sphere in moderator to simplified minicores (for example vver reactors type largest available minicore is with 17 fuel assemblies). isotopic composition for these benchmarks is fresh fuel and closest to real cores of nuclear reactors are systems with fresh mox fuel, because of pu included in it (from comparisons in [3]). summarized from previous text – benchmarks currently used for code validations are not similar to storage systems on sufficient level and therefore not appropriate for spent fuel systems calculations validation. 2.2. conservative adjustment of models fresh fuel isotopic composition is used for currently performed spent fuel storage subcriticality assessment. for that case there are two main generally used adjustments: • omitting of burnable absorbers, • partial boron credit. in newly produced fuel assemblies burnable absorbers (for vver it is gadolinium in gd203 mixed to some pins in fuel assembly) are used for flattening of neutron flux profile in fuel assemblies during reactor operation. these absorbers reduces reactivity and therefore have to be removed from model. technically after few cycles in reactor, their influence is negligible, but since fresh fuel is used in calculation, these absorbers have to be removed to stay conservative. in spent fuel storage pools a boric acid is dissolved to reduce reactivity. conservatively the boric acid should be completely removed from model, since it reduces reactivity. on the other side, with zero concentration of the boric acid and fresh fuel isotopic composition, keff of the pool loaded with currently used fuel assemblies from vver–440 cycles from last years would not fulfil 0.95 keff limit, because of their initial enrichment. rule of partial boron credit means utilization of 1/3 of a nominal boron concentration in storage models and was permitted by state supervisor. with that amount of boron in the pool system, currently used fuel with original enrichment 4.38 % meets limit requirements [4]. 3. theory of similarities as was often said in previous text, fresh fuel and spent fuel systems are not similar, but it was not said, how similarity can be measured. this section will quantify similarity. for purposes of this paper when talking about the similarity, the similarity of calculations between modelled systems is meant. when a model is prepared and calculations are performed, final keff is based on a geometry of model, an isotopic composition and nuclear data. when nuclear data changes (each time individual value from library), it affects keff – value describing size of this change is called "sensitivity" (of keff to variance of nuclear data). set of this values describes computational behaving of calculation. for similarity assessment sensitivities for each isotope, reaction and energy group are needed. set of these sensitivity profiles is unique for each calculation and if compared with another systems profiles it gives information about how similar these two systems are. scale code system [5] includes module tsunami–ip, which is used for comparison of systems based on sensitivities. there are few similarity values produced by tsunami–ip comparison, but only similarity coefficient ck will be used in this paper, because it is most complex coefficient of all coefficients available. ck coefficient describes a total similarity between two systems. this value is normalized, therefore 1 means a total computational similarity, 0 means no similarity. generally value of 0.95 is considered to be sufficient for similarity , but based on [6] ck > 0.8 will be sufficient, if more than 25 experiments with this level of similarity will be available for a comparison and following (sub)criticality assessment (which will not be described in this paper). experiments with ck < 0.7 are considered to be "highly dissimilar" based on [7]. main objective of using a similarity comparison and filtering of experiments dissimilar to newly projected system is that validation of code for final calculations of new system (here for example spent fuel system) should not be performed based on dissimilar experiments or benchmarks. 4. reactor criticals states start–up tests on commercial reactors have to be performed in the beginning of each cycle after loading of new fuel. timepoint in the beginning of cycle before powering up means that there is no xe and other short term decay fission products in core, which would bring in additional uncertainty, because they decay during a shutdown when refuelling. during start–up tests, some of states are so called "hot zero power" (which implies, that power in core is considered to be negligible) and some of these start–up tests are critical (keff = 1, from there comes reactor "criticals") – these states (on figure 1) can be used as experiments and were used in works described in this paper. 55 matěj šikl acta polytechnica ctu proceedings figure 1. reactor critical fullcore model for reactor criticals all parameters of core are known from instrumentation and control systems. therefore reactor criticals are experiments on reliability level of benchmarks, just with no detailed description in generally available database. data for reactor criticals models used for work described in this paper were taken from a database of operation histories of czech commercial reactors in dukovany and temelin, which is stored in department of reactor physics, nri rez near prague. this database contains more than 120 reactor critical states from beginnings of cycles with average core burnups within 16 to 26 gwd/tu. 5. spent fuel storage systems typical spent fuel storage systems are of two types – casks and pools. in this paper, a vver–440 storage pool filled with gd2mp with initial enrichment 4.38 % and a vver–1000 cask castor filled with tvsa–t c44e12 fuel are used for comparison. for calculations of both models are used states when spent fuel in various levels of burnup is loaded and model is filled with water with no boric acid. these states are used conservatively, because these are potentially most reactive states during lifecycle of storage’s intended usage. visualisations of spent fuel storage models are showed in figures 2 and 3. 6. software used in calculations comparison calculations were performed in tsunami–ip module of scale mentioned above, version used was 6.2.3. sensitivities were calculated in serpent 2.1.32, because during works it was found that scale code (namely tsunami–3d–k6) was not figure 2. spent fuel storage pool figure 3. spent fuel storage cask castor able to easily calculate such complex system as a fullcore model of a reactor core. calculations in serpent 2 can produce sensitivity data, but they are written in format incompatible to scale. therefore a software for preparation of sensitivities, which can read sensitivity data from serpent 2 outputs and prepare sensitivity inputs for tsunami–ip comparison calculations, had to be created. 7. results of similarity comparison for this work 19 reactor criticals states from various cycles of dukovany and temelin nuclear power plants start–up tests were taken for sensitivity calculation. storage systems were prepared artificially, with exactly given burnups on various levels. similarity coefficients were calculated in tsunami–ip and amounts of reactor criticals which are more similar to individual spent fuel storage systems than various ck values can be seen in tables 1 and 2. 56 vol. 37/2022 reactor criticals and spent fuel systems similarities fuel burnup pool required ck > [gwd/tu] keff 0.8 0.9 0.95 10 0.890 19 18 16 15 0.862 19 19 16 20 0.836 19 19 19 25 0.814 19 19 19 45 0.720 19 6 0 table 1. amounts of reactor criticals with ck similarity higher than 0.8, 0.9 and 0.95 to pool storage systems in various levels of burnup. from table 1 can be seen, that for the storage pool there are plenty of sufficiently similar available reactor criticals experiments. in future subcriticality assessments of spent fuel storage pool, it will be possible to validate calculation codes based on reactor criticals states. fuel burnup castor required ck > [gwd/tu] keff 0.7 0.75 0.8 5 0.801 5 1 0 15 0.796 9 4 0 25 0.744 13 6 0 45 0.649 10 4 2 table 2. amounts of reactor criticals with ck similarity higher than 0.7, 0.75 and 0.8 to storage cask castor systems in various levels of burnup on the other side castor (as can be seen from table 2) has less sufficiently similar systems. that could be for example due to a pitch between fuel assemblies or support construction itself. current results for these sets of castor and reactor criticals models are not similar enough for reliable calculations for subcriticality assessment. anyhow by comparisons presented in this section is shown that calculating states from the reactor criticals database is possible and that they are more similar to spent fuel systems than usually used fresh fuel systems are. 8. conclusions as was described in the beginning of this paper, current approach to calculations of spent fuel systems is not considered to be optimal. conservative assumptions have to be made, using these it is not automatic to fulfil legislative limits, and credit of burnup lowering reactivity is not used. non-using of burnup credit is mainly due to fact, that there is a lack of spent fuel systems for validation. in this paper the lack is resolved by using reactor critical states from start–up tests of commercial nuclear reactors. similarities between reactor criticals and spent fuel storage systems were compared using tsunami–ip module from scale code system. based on this comparison it was shown that code validated using reactor criticals would be eligible for calculations of storage pools, but not so adequate for castor cask calculations. even though similarities were high enough to storage pool only, possibility of validation of code for spent fuel systems calculation was proved when reactor criticals were used. generally it was shown and is suggested that codes can and should be validated based not only using fresh fuel benchmarks, but also using reactor criticals states. acknowledgements work presented in this report was supported by tačr grant tk02010136. references [1] vyhláška č. 329/2017 sb. o požadavcích na projekt jaderného zařízení, 2017. [2] international handbook of evaluated criciality safety benchmark experiments. nea/nsc/doc(95)03, oecd, nea no. 7382, 2016. [3] m. šikl. stanovení neurčitosti ve výpočtech kritičnosti s využitím metodiky tsunami. master’s thesis, department of nuclear reactors, fnspe, ctu in prague, 2018. http://hdl.handle.net/10467/77139. [4] m. gren, l. marková. průkaz podkritičnosti dolní kompaktní mříže bazénu edu s implementací pbc při zavezení palivem gd-2m. ujv z – 3732, 2013. [5] b. rearden, m. jessee, et al. scale code system, ornl/tm-2005/39, version 6.2.2. oak ridge national laboratory, oak ridge, tennessee, 2017. [6] b. broadhead, b. rearden, c. hopper, et al. sensitivityand uncertainty-based criticalitz safety validation techniques. nuclear science and engineering: 146, pp. 340–366, 2004, https://doi.org/10.13182/nse03-2. [7] b. rearden, m. williams, m. jessee, et al. sensitivity and uncertaintz analysis capabilities and data in scale. nuclear technology, 174:2, pp. 236–288, 2011, https://doi.org/10.13182/nt174-236. 57 http://hdl.handle.net/10467/77139 https://doi.org/10.13182/nse03-2 https://doi.org/10.13182/nt174-236 acta polytechnica ctu proceedings 37:54–57, 2022 1 introduction 2 current approach to subcriticality assessment 2.1 generally available benchmarks for code validation 2.2 conservative adjustment of models 3 theory of similarities 4 reactor criticals states 5 spent fuel storage systems 6 software used in calculations 7 results of similarity comparison 8 conclusions acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.38.0222 acta polytechnica ctu proceedings 38:222–227, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague natural materials in building construction – annual evaluation daniela michálková∗, pavol ďurica university of žilina, faculty of civil engineering, department of building engineering and urban planning, univerzitná 8215/1, 010 26 žilina, slovakia ∗ corresponding author: daniela.michalkova@uniza.sk abstract. the construction industry’s focus on a low-carbon economy will result in the need for a deeper examination of natural-based building materials. from an environmental point of view, the benefits of these materials are undeniable. however, it is necessary to consider their shortcomings in other areas of design in terms of building thermal engineering. this article observes and evaluates a wall designed for a wooden building with almost zero energy demand in year-round operation and subsequent assessment in confrontation with a different composition, seemingly more advantageous in thermal resistance and humidity regime. these assemblies are under long-term examination within the pavilion research of the authors’ workplace, in laboratory conditions from the interior side. at the same time, they are exposed to the realistic boundary conditions of the external environment. the paper includes an environmental assessment of two compositions, variating in the used material. the research shows that the wall composition of natural materials is more advantageous from an ecological perspective and can also show favourable effects in terms of temperature and relative humidity regulation. keywords: timber-framed, natural building materials, temperature, relative humidity, environmental assessment. 1. introduction the acute need to improve the environment is an increasingly trending topic. sustainable development includes many issues, among others, improvement of the building industry [1]. in order to maintain liveable conditions of the earth, it is necessary to reduce the use of materials with great primary energy demand [2]. according to the paris agreement [3], the united nations – 191 countries, which signed the document – are bound to keep the global temperature rise this century below 2 °c and report every five years their actions to ensure the goal. the harmonised requirements for construction products summarises the eu regulation no. 305/2011 of the european parliament and council [4], which significantly supports the use of raw and secondary materials in the building industry. at the same time, the requirements of current legislation regarding the thermal protection of buildings [5] are equally important. 2. experimental wall samples this article aims to present two different wall assemblies containing various materials. both face south orientation with 15 ° inclinations to the west, and both are exposed to the natural exterior climate, measured on the laboratory roof. the interior space uses air conditioning, and its’ temperature and relative humidity are appropriately measured. the research contains overall ten wall samples. all of them are under long-term investigation to monitor the temperature and relative humidity within the timber-framed multi-layered constructions. the monitoring is in three high levels – 0.5 m under the ceiling, in the middle of the structure height, and 0.5 m above the floor – in all material interfaces. in this paper, we present values in the middle of structures’ height, apart from the interface between phenolic foam and osb in s1, bearing in mind that we would have nothing to compare these values to in wall s1. used are ntc thermistors with the accuracy of ± 0.2 °c for temperature and capacity probes with the precision of ± 2 % for the relative humidity. the first wall in this paper, marked as s1 (south 1), is the only one with solely natural materials. the second, marked as s2 (south 2), was chosen for comparison, as it is the most similar to the first assembly. both these wall constructions are diffusely open, enabling air and water penetration and thereby reducing the risk of fungi and mould settling. wall s1 in figure 1 solely consists of timber frame, timber log profiles from exterior and interior and sheep wool within. s2 has the same exterior layer, followed by basalt fibre thermal insulation isover granulate, phenolic foam insulation kingspan kooltherm k5, finished by osb. the probes are in three depths within the wall: inside under the log profile, in the middle of thermal insulation, between wool and interior log profile (s1), and between isover granulate and kingspan kooltherm (s2). table 1 comprises the materials of both wall assemblies with tier main physical characteristics and environmental indicators. among them is d – thickness, 222 https://doi.org/10.14311/app.2022.38.0222 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 38/2022 natural materials in building construction – annual evaluation figure 1. left south wall 1 (s1), right south wall 2 (s2) – layers, probes placement. material d λ µ ρ c gw p p ei ap [m] [ w m3·k ] [-] [kg/m3] [ j kg·k ] [co2eq/kg] [mj/kg] [so2eq/kg] timber profile [6] 0.068 0.180 157 400 2510 0.109 1.959 0.00128 sheep wool [7] 0.220 0.042 1.5 16 1720 0.537 19.324 0.00463 basalt fibre ti [6] 0.220 0.040 1 50 1020 0.346 21.363 0.01413 phenolic foam ti [8] 0.040 0.021 35 35 1400 3.821 96.515 0.01742 osb [6] 0.012 0.130 50 650 1700 0.481 12.506 0.00210 table 1. layer materials of both assemblies and their main characteristics. figure 2. s1 environmental evaluation (baubook.at). λ – thermal conductivity factor, µ – water vapour diffusion factor, ρ – bulk density, c – specific heat capacity, gwp – global warming potential, pei – primary energy intensity, and ap – acidification potential, listed. ti stands for thermal insulation. figure 3. s2 environmental evaluation (baubook.at). 3. environmental assessment the environmental benefits of the first wall are undeniable. on the other hand, the second wall consists of more durable materials and therefore may balance the negatives. thus rose the need to establish the environmental impact of both structures. the assessment stems from the baubook website [9], which is 223 daniela michálková, pavol ďurica acta polytechnica ctu proceedings figure 4. comparison of the temperature in the winter period. figure 5. comparison of the relative humidity in the winter period. an online platform that promotes sustainable building design. this website enables the evaluation of the building structures in terms of the oi – ecology index (ökoindex) [10] that considers the environmental indicators such as gwp , ap , and pei. for comparison, the standard value for common wall assemblies is 70 pts/m2 [11]. the results of the environmental assessment are in figure 2 and figure 3. for comparison, the u -value (heat transfer coefficient) is according to stn en iso 6946 [12] 0.196 w/(m2 · k) for s1 and 0.145 w/(m2 · k) for s2, calculated as inhomogeneous wall structure. as shown, the wall assembly marked as s1 consisting of timber log profiles and sheep wool is classified as a++ according to oi-class, with ∆oi3 value 6 pts/m2. the structure s2 of timber log profile, basalt fibre insulation, phenolic foam insulation and oriented strand board reached value 38 pts/m2 and thus fell to the a+ class. 4. temperature and relative humidity the temperature and relative humidity, measured in three depths of the wall (figure 1), provided large datasets throughout the year. to simplify the comparison, these quantities are in this paper divided into three sections – winter period from 20th january to 2nd february, spring period from 19th march to 1st april, and summer period from 18th june to 1st july. dotted lines present the temperature and relative humidity of exterior air. interior climate was set to 20 °c temperature and 50 % relative humidity. the values in placement s1.1 or s2.1 are further referred to as the interior. in this case, we use this term only to simplify the text slightly. however, we would like to emphasise the layers between the probes placement and the interior, which we are aware of. 4.1. winter period the graph in figure 4 shows the temperature in both structures during two weeks of winter. the yellow dotted line represents the exterior air temperature. the other six lines stand for the temperature, whereas the different placements have different colours. the wall assemblies are distinguished by colour shade. although the temperature of s1 is from the exterior and in the middle of the wall lower than in s2, it reaches almost 5 °c higher values from the interior. that is naturally caused by the phenolic foam from the interior in the case of s2. however, s2 is ex224 vol. 38/2022 natural materials in building construction – annual evaluation figure 6. comparison of the temperature in the spring period. figure 7. comparison of the relative humidity in the spring period. pected to reach a lower temperature under the exterior cladding due to the higher thermal performance. another investigation consequently followed this analysis. unfortunately, we found out that in s2 was created an air cavity, causing an additional thermal bridge. significant is the alignment during the days where the amplitude of exterior temperature was smaller. the temperature these days was in the middle almost identical. figure 5 displays relative humidity in the same period. the walls and probe placement is distinguished the same way as in the case of temperature. the regulation of relative humidity is, in the case of s1, very significant. the difference between exterior and interior relative humidity is almost 50 %, whereas in s2, only 20 %. it may sound unreasonable, but the explanation behind this statement is the following. both assemblies are under constant interior boundary conditions, which means we can see only the influence of exterior climate. although s1 reaches higher relative humidity from the outside, it meets the values of s2 relative humidity already in the middle of the insulation layer. the sheep wool was able to further lower the relative humidity to circa 30 %, whereas in basalt fibre, the value is around 40–50 %. this is the reason why we claim that sheep wool has the ability to regulate relative humidity better. the relative humidity in the middle is very similar in both assemblies. the variations occur under the outside and inside layers of both. also interesting is the course of the humidity itself. the wall s1 reached steadier values throughout the whole period. 4.2. spring period to represent the spring period, we have selected two weeks from 19th march to 1st april. figure 6 shows the temperature comparison. the daily amplitude is naturally more significant. from the outside, the solar impact on the temperature of the exterior layer is evident, where the temperature in the walls is 15 °c higher than the surrounding air temperature. other than that, the course of temperature is similar to the winter period, where the middle temperature is almost identical, and the interior temperature differs the most. figure 7 presents the relative humidity. the difference between exterior and interior humidity is here not so significant and gets even smaller towards the end of selected weeks. however, the internal relative humidity is again lower in the case of s1 compared to the wall s2, creating a difference of nearly 10 %. 225 daniela michálková, pavol ďurica acta polytechnica ctu proceedings figure 8. comparison of the temperature in the summer period. figure 9. comparison of the relative humidity in the summer period. 4.3. summer period the summer period is represented by 14 days from 18th june to 1st july. as expected, the temperature amplitude is large, with a maximum variance of 24 °c. it is displayed in figure 8, where a noticeable temperature shift is apparent, in some cases reaching 6 hours. according to figure 8, wall s1 is capable of higher temperature regulation. to support this statement, notice that the temperature from the outside is, in its case, higher than in s2. in contrast, already in the middle of thermal insulation is vice versa, where the s1 temperature is 3 °c lower than the s2. figure 9 shows the course of relative humidity during the summer period. in this case, the difference is foremost in the interior layer, caused by the air relative humidity rise due to some technical difficulties with the air conditioning. however, the relative humidity in the middle of both walls is relatively consistent, varying from 50 to 60 %. 5. conclusion although both wall assemblies appear very similar, their difference is rather significant. the first structure consists solely of natural materials with minor adjustments to enable their use in buildings. the second wall uses a combination of natural materials – timber frame, exterior timber log profile, and oriented strand board – and synthetic materials, such as basalt fibre thermal insulation and phenolic foam to improve its thermal resistance. nevertheless, the difference is not significant enough to balance the environmental impact. as stated in the section 3 (environmental assessment), both structures fall into the highest categories in terms of ecoindex. however, wall s2 requires almost double primary energy in comparison with s1. moreover, the acidification and global warming potential are also double. in terms of ∆oi3 is for wall s2 is more than six times greater – 38 pts/m2 versus 6 pts/m2 for s1. from the perspective of used materials, wall s2 is expected to show better temperature and relative humidity results. the opposite turned out to be the case, where the first assembly showed its capability of not only competing with s2 in terms of thermal performance but, in some cases, even outstanding it. wall s1 provides more significant temperature decrement towards the interior, obvious foremost in the summer period. the regulation of relative humidity within s1 was in some cases 30 % greater opposite to 226 vol. 38/2022 natural materials in building construction – annual evaluation wall s2. however, it is needless to say that neither of the wall assemblies showed relative humidity values that could initialise the growth of moulds and fungi or cause material degradation. the main conclusion of this paper is not to underestimate such wall assemblies, which can prove their worth not only in terms of environmental sustainability but also in terms of overall performance. future research could contribute by numerical simulation of both wall structures within an actual building and deeper evaluation from an lca point of view. acknowledgements the research of this paper was enabled through the vega project nr. 1/0673/20 theoretical and experimental analysis of energy effective and environmentally friendly building envelopes. references [1] d. a. suhamad, s. p. martana. sustainable building materials. iop conference series: materials science and engineering 879(1):012146, 2020. https://doi.org/10.1088/1757-899x/879/1/012146 [2] w. s. e. ismaeel. material selection for sustainable buildings. in conference paper arcom 2021. 2021. [3] united nations. paris agreement to the united nations framework convention on climate change, 2015. [2022-02-28]. https://unfccc.int/sites/ default/files/english_paris_agreement.pdf [4] european union. regulation (eu) no 305/2011 of the european parliament and of the council of 9 march 2011 laying down harmonised conditions for the marketing of construction products and repealing council directive 89/106/eec. [2022-02-28]. http://data.europa.eu/eli/reg/2011/305/oj [5] stn 73 0540-2+z1+z2 thermal protection of buildings. thermal performance of buildings and components. part 2: functional requirements, 2019. [6] inštitút pre energeticky pasívne domy. ekologické materiály pre energeticky pasívne domy (epd), 2020. [2022-02-28]. https://ozartur.sk/wpcontent/uploads/2020/08/iepd-materialy.pdf [7] u-values for sheep wool insulation, 2014. [2022-02-28]. http://www.isolena.cz/wp-content/uploads/0259_ iso17_architektenordner_2_u-werteproduktdatenbl%c3%a4tterakustikd%c3%a4mmung_cz_iii.pdf [8] kingspan. environmental product declaration – kingspan kooltherm k5, 2019. [2022-02-28]. https://www.kingspan.com/pl/plpl/produkty/izolacje/pliki-dopobrania/certyfikaty/epd-kooltherm-k5 [9] baubook component calculator, 2018. [2022-02-28]. https://www.baubook.at/btr/ [10] oekoindex component evaluation, guideline for the ecological evaluation of component constructions by means of eco-index v1.4„ energy institute vorarlberg, dornbirn. [11] a. iringová. design of envelopes for timber buildings in terms of sustainable development in the low-energy construction. iop conference series: materials science and engineering 415:012010, 2018. https://doi.org/10.1088/1757-899x/415/1/012010 [12] stn en iso 6946 building components and building elements. thermal resistance and thermal transmittance. calculation methods, 2001. 227 https://doi.org/10.1088/1757-899x/879/1/012146 https://unfccc.int/sites/default/files/english_paris_agreement.pdf https://unfccc.int/sites/default/files/english_paris_agreement.pdf http://data.europa.eu/eli/reg/2011/305/oj https://ozartur.sk/wp-content/uploads/2020/08/iepd-materialy.pdf https://ozartur.sk/wp-content/uploads/2020/08/iepd-materialy.pdf http://www.isolena.cz/wp-content/uploads/0259_iso17_architektenordner_2_u-werte-produktdatenbl%c3%a4tter-akustikd%c3%a4mmung_cz_iii.pdf http://www.isolena.cz/wp-content/uploads/0259_iso17_architektenordner_2_u-werte-produktdatenbl%c3%a4tter-akustikd%c3%a4mmung_cz_iii.pdf http://www.isolena.cz/wp-content/uploads/0259_iso17_architektenordner_2_u-werte-produktdatenbl%c3%a4tter-akustikd%c3%a4mmung_cz_iii.pdf http://www.isolena.cz/wp-content/uploads/0259_iso17_architektenordner_2_u-werte-produktdatenbl%c3%a4tter-akustikd%c3%a4mmung_cz_iii.pdf https://www.kingspan.com/pl/pl-pl/produkty/izolacje/pliki-do-pobrania/certyfikaty/epd-kooltherm-k5 https://www.kingspan.com/pl/pl-pl/produkty/izolacje/pliki-do-pobrania/certyfikaty/epd-kooltherm-k5 https://www.kingspan.com/pl/pl-pl/produkty/izolacje/pliki-do-pobrania/certyfikaty/epd-kooltherm-k5 https://www.baubook.at/btr/ https://doi.org/10.1088/1757-899x/415/1/012010 acta polytechnica ctu proceedings 38:222–227, 2022 1 introduction 2 experimental wall samples 3 environmental assessment 4 temperature and relative humidity 4.1 winter period 4.2 spring period 4.3 summer period 5 conclusion acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.32.0015 acta polytechnica ctu proceedings 32:15–20, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague calculation of seismically loaded earth retaining structures jan pruška∗, veronika pavelcová czech technical university in prague, faculty of civil engineering, thákurova 7, 166 29 prague, czech republic ∗ corresponding author: jan.pruska@cvut.cz abstract. eurocode 1998-5 for the design of earth retaining structures in seismic conditions describes calculations using the mononobe-okabe method. this simplified quasi-static procedure is to some extent very conservative. the article describes the limits of this method and brings out the comparison using theoretical numerical examples with other approaches. the first, is a calculation method introduced in the american nhcrp 661, the second, is the chen and liu method and the last one is the mylokanis et al. method. keywords: earth retaining structures, seismicity, seismic coefficient, earth pressure, mononobe-okabe. 1. introduction the theory of coulomb and rankin has formed the basis for current commonly used theories of earth pressure calculation. during the 20th century, it was found that earth pressures during seismic action can be significantly higher than in the case of purely static loading conditions. okabe (1924) and mononobe & matsuo (1929) introduced the first method that could calculate the total earth pressure from static and seismic loading. this method became known as the mononobe okabe method (hereinafter m-o) and is recommended in many geotechnical standards. the eurocode 1998-5 for the design of retaining and gravity walls in seismic conditions also provides a calculation using this method. the m-o method as a simplified quasi-static procedure that is to some extent very conservative. the article describes the limits of this method, and finds comparison with other approaches for the calculation of seismically loaded retaining walls in the american nhcrp 661, also in the method of chen and liu (1990) and in an alternative to the standard m-o method given by mylonakis et al. 2. seismic loading for quasi-static calculations of gravity and retaining walls, the seismic soil load is given by horizontal and vertical forces which are equal to the weight of the active wedge w and the corresponding seismic coefficient kh, kv, see figure 1. the vertical load must be considered to have the most adverse effect. the value of the seismic coefficient is generally determined to be equal to the ratio of the considered acceleration and gravitational acceleration and it is possible to determine the relationships according to the different authors. figure 1. wall load during quasi-static calculation 2.1. according to terzaghi (1950) kh = ah g (1) kv = av g (2) where: ah design acceleration in the horizontal direction [m · s−2] av design acceleration in the vertical direction [m·s−2] g gravitational acceleration [m · s−2] 2.2. according to gutenberg and richter (1956) kh = a0 g (3) log10 a0 = −2.1 + 0.81ms − 0.027m2 s (4) where: a0 earthquake acceleration with magnitude ms [m · s−2] 15 https://doi.org/10.14311/app.2022.32.0015 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en jan pruška, veronika pavelcová acta polytechnica ctu proceedings 2.3. according to eurocode 1998-5 kh = s r (5) kh = ±0.50kh if both av and g > 0.6 (6) kh = ±0.33kh for other cases (7) where: s the subsoil coefficient defined in eurocode 1998-1 [−] r coefficient according to table 1 [−] for walls lower than 10 m, the seismic coefficient must be constant in height. eurocode 1998-5 does not specify a procedure for determining the effect of higher walls on the value of seismic loading. 2.4. according to nchrp 611 and aashto the national cooperative highway research program (nchrp) is a research program of the united states of america. road infrastructure research is funded by the american association of state highway and transportation officials (aashto). report 611 presents current results from the research of seismic design of road infrastructure, including slopes and load-bearing structures. determination of seismic acceleration is not found in the nchrp 611 report itself but in superior document: lrfg bridge design specifications (aashto, 2007 and 2012). report 611 nchrp concerns this report in chapters 5 and 7. it introduces the factor α which takes into account the height of backfill. kh,nchrp = fpga pgaα g (8) where: kh,nchrp horizontal seismic coefficient kh,d as defined in aashto (2007/2012) [−] fpga locality coefficient for the zero period in the elastic response spectrum values in table 3.10.3.2-1 aashto (2012) [−] pga peak soil level acceleration for bedrock (class b) [m · s−2] α backfill height coefficient [−] g gravitational acceleration [m · s−2] the backfill height coefficient α described in the 611 nchrp report is the result of a one-dimensional free field wave propagation analysis. the analysis took into account the fact that the acceleration of the backfill of the supporting structure is not the same at any point. this coefficient can be calculated for soils of classes c, d and e (equivalent to all soils except rock and very soft clay) according to aashto as follows: α = 1 + 0.003h [( 0.5 fv s1 fpga pga ) − 1 ] (9) h height of load-bearing structure or backfill [m] fv locality coefficient for long-term range in the elastic response spectrum according to table 3.10.3.2-3 aashto (2012) [−] s1 horizontal spectral acceleration coefficient for a period of 1 s in rock (class b) [m · s−2] however, equation (9) can only be used if the height of the supporting structure is h > 6m. for the construction of a height h ≥ 30m, the height in equation (9) must be set to h = 30m. the backfill height coefficient corresponds to the function correction coefficient qh according to swiss sia 267. 3. mononobe-okabe method following the 1923 earthquake in kanto, japan, okabe (1924) and mononobe & matsuo (1929) performed a series of experiments on a vibrating table. a pseudostatic method (introducing the principle of equivalent accelerations) based on the obtained experimental results was derived to calculate the total earth pressure from both static and seismic loading. this method (known as the mononobe-okabe method), based on the coulomb’s limit approach and the assumption that the earth pressure is derived from a rigid body that moves along a shear plane, and gives relations for active or passive earth pressure under seismic loading as follows (the sign convention is in figure 2): pae = 1 2γ (1 ± kv)h2kae (10) ppe = 1 2γ (1 ± kv)h2kpe (11) kae = cos2(ϕ− θ − α) cos θ cos2 α cos(δ + θ + α) [ 1 + √ sin(ϕ+ δ) sin(ϕ− β − θ) cos(β − α) cos(α+ θ + δ) ]2 (12) kpe = cos2(ϕ− θ + α) cos θ cos2 α cos(θ − α) [ 1 − √ sin(ϕ) sin(ϕ+ β − θ) cos(θ − α) cos(β − α) ]2 (13) 16 vol. 32/2022 calculation of seismically loaded earth retaining structures type of gravity or retaining wall r free gravitational walls that can deviate up to dr = 300α · s [mm] 2.0 free gravitational walls that can deviate up to dr = 200α · s [mm] 1.5 flexible concrete walls, anchored or spread walls, reinforced concrete walls 1.0founded on vertical piles, embedded foundation walls and bridge abutments table 1. values of the coefficient r for the calculation of the horizontal seismic coefficient (α is the ratio of the design acceleration ad for the foundation soil type a to the gravitational acceleration g, s is the subsoil coefficient defined in eurocode 1998-1) the following notation is used as in the previous equations: γ volumetric density of soil [kn ·m−3] h height of load-bearing structure [m] kae active earth pressure coefficient (static + dynamic) [−] kpe passive earth pressure coefficient (static + dynamic) [−] α inclination of the back of the wall [◦] β backfill inclination behind the retaining wall [◦] δ angle of friction between the soil and the wall structure [◦] ϕ angle of internal friction of the backfill material [◦] kh seismic coefficient of horizontal acceleration [−] kv seismic coefficient of vertical acceleration [−] θ tan−1 ( kh 1 − kv ) [−] figure 2. sign convention (left: passive, right: active) in many cases, the seismic coefficient of vertical acceleration kv in these examples is neglected because the effect of vertical acceleration is relatively small. 3.1. limits of the mononobe-okabe method the relations of the m-o method are based on the following assumptions of the mohr-coulomb theory of soil failure with a criterion according to the coulomb theory: • dry, homogeneous, isotropic backfill material • the soil above the shear plane is solid it creates a wedge • the soil under the supporting structure is loadbearing • seismic load is modelled as a static force (multiple of kh,d) • the wall must be able to move or rotate so that a shear plane can form in the soil • the resulting shear plane is planar • the terrain above the wall is modelled as an infinitely long slope the last two items in this list are responsible for problems using the m-o method if the slope (backfill) behind the wall β reaches or exceeds the inclination of the shear plane, the result is an infinitely large "wedge" of soil. as a result, the calculated active earth pressure is infinitely large and the relations of the m-o method no longer provide a solution for these inclinations. proof of the limited validity of equation (12) can simply be shown by the example of determining the stability of an infinitely long slope with a planar shear plane where the slope is formed by incoherent soil and there is no water in it. the seismic load is given by the horizontal acceleration kh,d = ah,d/g and the vertical acceleration kv,d is neglected. it is known that such a slope is stable if the inclination β is less than the angle (ϕ−θ) or in the extreme case β = ϕ−θ when the shear plane is exactly parallel to the inclination of the surface. when using equation (12) for the active earth pressure coefficient, for kh,d = 0.1; β = 30◦, ϕ = 35◦, the expression sin(ϕ−β−θ) below the square root becomes negative and no natural solution can be found. another limitation is, that the m-o method has only limited suitability for calculating passive earth pressure from seismic loading because in many cases the shear plane at passive earth pressure does not have to be considered horizontal but logarithmic. if we compare the value of passive earth pressure using the logarithmic shape of the shear plane and the horizontal one (which the m-o method considers), it can be seen that a planar plane can cause significant overestimation of the resistance to passive earth pressure, especially when considering the angle between the soil and the wall structure δ = 2/3ϕ. the last disadvantage of the m-o method is the assumption of a constant seismic load over the entire height of the structure (i.e. the soil wedge). 17 jan pruška, veronika pavelcová acta polytechnica ctu proceedings 4. eurocode 1998-5 appendix e gives two formulas for calculating the active earth pressure coefficient kae. for the inclination of the terrain surface behind the wall β ≤ ϕ− θ uses the equation (12) according to m-o. but if the angle of the inclination behind the wall satisfies the condition β > ϕ−θ, the original relationship is modified by complete exclusion of the influence of the inclination of the terrain surface behind the wall β as follows: kae = cos2(ϕ− θ − α) cos θ cos2 α cos(δ + θ + α) (14) thus, for the inclinations of the terrain behind the wall with values of the angle β > ϕ− θ, a limit arises where the original relation (equation (12)) calculates the infinite coefficient of earth pressure kae but equation (14) gives a constant value of the coefficient kae. the inclination of the terrain behind the retaining wall can therefore be arbitrarily increased above the value ϕ − θ according to the equation (14) without increasing the soil pressure (see figure 3). in this way, the fact that the slope above the wall is already unstable and theoretically the whole area must be supported by the supporting structure so that its displacement does not occur can be missed. therefore, the use of the equation (14) is unsuitable in the case of very steep slopes of the terrain above the wall. the calculations for this paper have been carried out in the geo5 program which has, inter alia, a procedure according to eurocode 1998-5 built-in. the advantage of this program is that at the values of the inclination β > ϕ− θ it notifies the user to this fact and possible problems related to it. eurocode 1998-5 uses the equation (13) to calculate the passive earth pressure coefficient kpe. however, it is recommended to use this equation mainly for the vertical front side of the wall. in addition, it requires consideration of the friction between the wall and the soil δp = 0◦ (section 7.3.2.3 (6)) which leads to very conservative results. 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 0 5 10 15 20 25 30 35 40 45 k a e [-] β [°] φ = 25; kh,d = 0,1 φ = 25; kh,d = 0,2 φ = 25; kh,d = 0,3 φ = 30; kh,d = 0,1 φ = 30; kh,d = 0,2 φ = 30; kh,d = 0,3 φ = 35; kh,d = 0,1 φ = 35; kh,d = 0,2 φ = 35; kh,d = 0,3 figure 3. dependence of the inclination of the terrain behind the wall and the coefficient kae according to eurocode 1998-5 5. chen and liu chen and liu’s method is based on chang’s doctoral dissertation (1981). as with the m-o method, it is based on a kinematic approach. however, it considers a logarithmic shear plane instead of a horizontal shear plane for the m-o method. chen and liu (1990) also present tabular values of earth pressure coefficients for the most common cases of retaning walls and backfill soils. this method generally provides lower passive earth pressures than the m-o method (for the same value of friction between wall and soil). 6. nchrp 611 the shamsabadi method (2007), recommended by nchrp 611, is a kinematic method that considers the logarithmic shear plane and the hyperbolic relationship between stress and strain. thus, this method differs from conventional kinematic methods such as chen and liu (1990). standard nchrp 611 (2008) in appendix 5 gives graphs for the passive earth pressure coefficient kpe as a function of variables: soil cohesion c, wall height h , soil volumetric density γ, seismic coefficient kh and soil internal friction angle ϕ. 7. mylonakis et al. in 2007, mylonakis et al. presented a closedform stress plasticity solution for gravitational and earthquake-induced earth pressures on retaining walls. the proposed solution is essentially an approximate yield-line approach, based on the theory of discontinuous stress fields. it is an alternative to the standard m-o equations for seismic earth pressures suffering thus from the same limits as the m-o method. on the other hand this alternative approach works with respect to active and passive conditions, as it can be expressed by a single equation with opposite signs for soil friction angle and wall roughness. according to this method the active and passive presure under seismic loading can be determined by: pe = 1 2γ (1 − kv)h2ke (15) where ke =cos(α− β) cos(β + θ) cos θ cos δ cos2 α · · [ 1 ∓ sinϕ cos(∆2 ∓ δ) 1 ± sinϕ cos [(∆1 ± (β + θ)] ](∓2ψe tanϕ) (16) the caquot angles ∆1, ∆2 are given by mathematical expressions sin ∆1 = sin β sinϕ (17) sin ∆2 = sin δ sinϕ (18) 18 vol. 32/2022 calculation of seismically loaded earth retaining structures and twice the revolution angle of principal stresses in the two regions under seismic conditions 2ψe = ∆2 ∓ (∆1 + δ) + β − 2α− θ (19) the upper signs in the equations refers to active conditions pae and the lower signs to passive ppe and the notation used in equations above is the same as in the standard m-o method (section 3). 8. comparison of methods with the active seismic earth pressure coefficient to compare the methods of chen and liu, mylonakis at el. and m-o with the modification given by the equation (14) for backfill inclinations β > ϕ − θ according to the eurocode 1998-5, a simple case was deliberately chosen: a gravity wall with a vertical back side of the wall (α = 0◦) with a height h = 3m and a wall friction angle δ = 2/3ϕ, inclination of the terrain surface β = 0–30◦, soil friction angle ϕ = 30◦, cohesion c = 30 kpa. the vertical acceleration of the earthquake was neglected, the horizontal acceleration kh,d is 0.1 and 0.2, respectively (see figure 4). � � �a h p ae w khdw figure 4. scheme of solved example the differences between methods are minor for higher inclinations of the terrain surface β (figures 5). the reason for this is the fact that the logarithmic shear plane considered by chen and liu’s method becomes practically flat for the calculations of the active earth pressure coefficient performed here. as mentioned before, it can be observed that mylonakis’s et al. method is not able to give any natural solution after exceeding the critical backfill inclination β. the peculiarity of the chen and liu’s (1990) method is evident from both graphs in figure 5 where it can be seen that the method gives results even for backfill inclinations above the critical inclination β although for very steep slopes these values are already above the physically permissible limit (and the slope would be unstable). signs of chen and liu’s logarithmic shear plane are visible only in figure 5 (bottom) as the values of the active earth pressure coefficient slightly differs from other methods. 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 0 5 10 15 20 25 30 k a e [-] β [°] mononobe-okabe chen & liu mylonakis et al. 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 0 5 10 15 20 25 30 k a e [] β [°] mononobe-okabe chen & liu mylonakis et al. figure 5. comparative calculations of the effect of cohesion on active seismic earth pressure coefficient: kh = 0.1 (top) and kh = 0.2 (bottom). 9. comparison of methods with the passive seismic earth pressure coefficient to compare the values of the passive earth pressure coefficient kpe according to chen and liu (1990) and eurocode 1998-5 (figure 6) a gravity wall with a vertical back side of the wall (α = 0◦), with a height h = 3m and horizontal backfill surface behind the wall β = 0◦ was chosen. not only the standard m-o method, as the eurocode 1998-5 recommends, but also the alternative approach of the m-o method presented by mylonakis et al., (as it should theoretically give exactly the same results as the standard m-o method), was used for the calculation. for the chen and liu (1990) method, the wall friction angle equals δ = 2/3ϕ, for eurocode 1998-5 (m-o and mylonakis et al.) δ = 0◦ (prescribed by the standard), the internal friction angle ϕ was considered from 25◦ to 35◦. the vertical acceleration of the earthquake was neglected, the horizontal acceleration of kh,d took values of 0, 0.1, 0.2 and 0.3. 19 jan pruška, veronika pavelcová acta polytechnica ctu proceedings 0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 25 27 29 31 33 35 k p e [-] ϕ [°] m-o khd = 0.0 m-o khd = 0.1 m-o khd = 0.2 m-o khd = 0.3 ch&l khd = 0.0 ch&l khd = 0.1 ch&l khd = 0.2 ch&l khd = 0.3 m. et al. khd = 0.0 m. et al. khd = 0.1 m. et al. khd = 0.2 m. et al. khd = 0.3 mononobe-okabe δp = 0° chen & liuδp = -2/3 ϕ mylonakis et al.δp = 0° figure 6. comparison of methods for calculating the passive coefficient of seismic earth pressure according to the literature, the procedures recommended by eurocode 1998-5 give (δ = 0◦) (which corresponds to its requirements) the most conservative values of all the methods. in fact the method of mylonakis et al. gives even lower values of passive pressure coefficient than the standard m-o method but taking into account the scale of the values of chen and liu’s method, the difference is insignificant. if the value δ = −2/3ϕ was used for the friction between the wall and the soil using the standard m-o method, higher coefficients of passive earth pressure would be obtained than by following the chen and liu’s method (1990). however, since many derivations of the passive seismic pressure coefficient kpe reported in the literature are controversial, it is recommended to use the calculation involved in the m-o method according to eurocode 8 (δp = 0◦). 10. conclusion it should be noted that the m-o method represents a considerable simplification of reality. in addition to the limitations of all pseudo-static methods, it should be emphasized that the frequency range of excitation or soil stiffness are not taken into account in seismic loading while using the m-o method. in the case of solving the problem with the value of the inclination of the terrain β, we obtain constant values of the coefficient of active earth pressure kae for and above critical inclination angle β using eurocode 19985 and while determinating the passive earth pressure according to eurocode 1998-5, the requirement of friction between the wall and the soil (δp = 0◦) (section 7.3.2.3) has to be satisfied. in both of the mentioned cases, it seems more advantageous to use the chen and liu’s method. another disadvantage is that when calculating walls higher than 10m, eurocode 1998-5 does not specify the procedure for determining the seismic load (unlike the american nchrp 611). it also cannot be overlooked that the classical m-o equations are empirical and hardly derived by physicbased reasoning. on the other hand, the alternative approach presented by mylonakis et al. proposes a simpler solution in which the main features of the mathematical expressions, including signs, can be deduced by such reasoning from physics. moreover, the proposed solution is symmetric with respect to active and passive conditions, as it can be expressed by a single equation with opposite signs for soil internal friction angle ϕ and angle of friction between the soil and the wall structure δ. despite all the limitations and possible shortcomings, the m-o method is still used as an easy-tounderstand and meaningful method for the calculation of seismic loaded earth retaining structures. also eurocode 1998-5: eurocode 8: design of structures for earthquake resistance part 5: foundations, retaining structures and geotechnical aspects explicitly refers to this method in appendix e. acknowledgements the support provided by the sgs project no. sgs21/045/ohk1/1t/11. references [1] aashto. lrfd bridge design specifications, vol. 4th edition. american association of state highway and transportation officials, 2007. [2] eurocode 8. : design of structures for earthquake resistance–part 5: foundations, retaining structures and geotechnical aspects en 1998-5: 2004. brussels: european committee for standardization 2006. [3] fhwa (federal highway administration). seismic retrofitting manual for highway structures. part 1, bridges. tech. rep., turner-fairbank highway research center, 2006. [4] geo 5. user manual. http://www.fine.cz. [5] b. gutenberg, c. richter. magnitude and energy of earthquakes. annali di geofisica 9:1–15, 1956. [6] w.-f. chen, x. liu. limit analysis in soil mechanics. elsevier, 1990. [7] n. mononobe. on determination of earth pressure during earthquake. in proc. world engineering congress, vol. 9, pp. 177–185. 1929. [8] nchrp 611. seismic analysis and design of retaining walls, buried structures, slopes, and embankments. 2008. [9] s. okabe. general theory of earth pressure. journal of thejapanese society of civil engineers-1926-12 (1) 311, 1926. [10] pruška. dynamika geotechnických konstrukcí. čvut fsv, 2017. [11] a. shamsabadi. three-dimensional nonlinear seismic soil-abutment-foundation-structure interaction analysis of skewed bridges. university of southern california, 2007. [12] k. terzaghi. mechanism of landslides. application of geology to engineering practice, geol soc am pp. 83–123, 1950. [13] g. mylonakis, p. kloukinas, c. papantonopoulos. an alternative to the mononobe–okabe equations for seismic earth pressures. soil dynamics and earthquake engineering 27(10):957–969, 2007. 20 http://www.fine.cz acta polytechnica ctu proceedings 32:15–20, 2022 1 introduction 2 seismic loading 2.1 according to terzaghi (1950) 2.2 according to gutenberg and richter (1956) 2.3 according to eurocode 1998-5 2.4 according to nchrp 611 and aashto 3 mononobe-okabe method 3.1 limits of the mononobe-okabe method 4 eurocode 1998-5 5 chen and liu 6 nchrp 611 7 mylonakis et al. 8 comparison of methods with the active seismic earth pressure coefficient 9 comparison of methods with the passive seismic earth pressure coefficient 10 conclusion acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.39.0001 acta polytechnica ctu proceedings 39:1–5, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague augmented reality to support parcel handling in last-mile logistics dragos datcua,∗, jelmer winkelb, leon rothkrantzc a inteliwatch, césar franckstraat 103, 2625 cb delft, netherlands b university of groningen, 9712 cp groningen, netherlands c delft university of technology, mekelweg 5, 2628 cd delft, netherlands ∗ corresponding author: email@dragosdatcu.eu abstract. we have developed an augmented reality (ar) based system which keeps track of events during the parcel handling process for last-mile logistics. the system can retrieve and highlight the location of parcels in large piles with ar on the user’s smartphone. a camera array automatically detects the parcels laid down manually by an operator. new parcels are scanned and parcel fingerprints are generated semi-automatically. the system can detect and track the known parcels by fingerprint and can further highlight the location of the parcel using 3d visual clues, directly on the smartphone of the operator. keywords: augmented reality, parcel tracking, logistics. 1. introduction the rapid growth in e-commerce challenges the lastmile logistics and further demands for more effective technical solutions and procedures [1]. last-mile logistics concerns the transport of goods from the final warehouse to the customers’ home and can be separated into three main phases: (1) loading the truck at the warehouse, (2) transport from the warehouse to the delivery area, (3) the final delivery to the customers [2, 3]. augmented reality is expected to be one of the enabling key technologies in the age of industrial digitalization [4]. recent studies show warehouse operations can benefit from augmented reality technology through increasing the efficiency [5, 6] and reducing the costs [7]. several technical challenges reported in the literature [7, 8] limit the adoption of augmented reality at the industrial level. so far, little work has been done to investigate the barriers of integrating augmented reality into logistics processes adapted to last-mile delivery. this paper presents a technical solution which integrates augmented reality to support indoor as well as outdoor logistics processes for last-mile delivery of parcels. in order to show the field delivery agent the correct location of the parcel, it is important that the sensing system works successfully. the camera system tracks the parcel’s precise location and presents this in augmented reality on the user’s smartphone. the visual tracking uses parcel fingerprints with the characteristics of parcels, such as the dimensions, the shape and the texture or the bar-codes. we have implemented the ar system and further conducted a small-scale targeted experiment for testing the functional system prototype. this is the first approach to couple a camera sensor array which detects parcels in storage racks with a visualization in augmented reality on smartphones, to help localize parcels for parcel picking and delivery. the technical solution is called solid and means self organized logistics in distribution. the research project represents the joint efforts of primevision bv, dpd and the university of groningen in the netherlands. the next section presents relevant research projects and related work on the automatic input sensing algorithms, tracking parcels and displaying information for building situational awareness in operational environments. section 3 discusses the aspects related to the architecture of the parcel tracking system. section 4 presents and discusses the results of the experiments with the first functional system prototype applied in a scaled-down version of the first operational scenario. the last section addresses the conclusions of our research and pinpoints aspects regarding the future work. 2. related work the topic of augmented reality has been explored and found to be an interesting technology with the potential to improve the efficiency in logistics operations [3, 9, 10]. the use of situation-specific information technology generally saves a significant amount of time due to the fact that less paperwork or manual checks have to be performed [2]. augmented reality technologies are usually offered in the form of apps for mobile phones, because these hand-held projectors are the best practice for introducing augmented reality to the market due to the low production costs and the ease of use [10, 11]. 1 https://doi.org/10.14311/app.2022.39.0001 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en d. datcu, j. winkel, l. rothkrantz acta polytechnica ctu proceedings figure 1. system architecture. recent laboratory experiments have shown that augmented reality can boost efficiency and accuracy for order picking in warehouse operations [5]. augmented reality graphic-based user interfaces can reduce job completion time and number of errors by 13 % and 59 % respectively, as concluded from a simulated warehouse job involving order picking and part assembly [6]. also, earlier work reported on the benefit of augmented reality over the traditional routines for warehouse operations [12]. in recent years, several system designs and architectures have been created to support the use of augmented reality. essential elements in those architectures are a smart device (smart glasses or mobile phone), a projecting application system, camera vision, internet connection, privacy gates, command controls, modeling tools, back-end services and databases [13]. this model has been used to develop the system design. over the last years, a large number of open-source augmented reality frameworks (e.g. arcore, arkit, vr juggle, unity 3d) have been developed with tool kits that can be used easily by computer engineers [14, 15]. these tool-kits provide the computer engineers the tools to develop 2d or 3d content in an augmented form (e.g. arrow, text, cross etc.). the aforementioned frameworks help the computer engineers create the front-end part of the system, the part that is visible for the users and where the interaction with the system takes place. there are also scholars that doubt the actual benefits that augmented reality can bring to the logistics operations because of multiple challenges during the implementation. rejeb [8] conducted an extensive literature review of 43 papers from academic journals on the challenges of augmented reality in logistics and 74 % of these papers mentioned that technical challenges are an obstacle for implementing augmented reality systems. one key challenge is object recognition, because the digital world has to be in harmony with the real world in order to achieve high accuracy [16]. robustness of the system is continually improving despite computational costs are still high, but real-time tracking for pure vision-based approaches looks very promising [10]. furthermore, software challenges follow no standardization of the programming language and this makes it difficult for practitioners to experiment and create the correct algorithm [17]. next to the integration of augmented reality as a means to enhance the visualization, the integration of various sensors [18] and vision models [19, 20] has been the focus in order to recognize, validate, and optimize the activities of workers in logistics and for warehouse automation. in our research, we have integrated and built upon previous findings regarding input sensing models for activity recognition with augmented reality oriented user interface for supporting last-mile logistics operators and delivery agents. 3. system architecture the system architecture consists of a front end and back end (figure 1). the user interacts with the frontend part of the system, where the operator loads the truck at the distribution center. the system is using cameras that are installed inside the truck, for registering and tracking the parcels inside the truck by using camera vision. the cameras provide video streams which extract knowledge about the layout of the parcel after loading and for rendering the ar layer. the algorithms for the detection and tracking of parcels are replicated on each camera of the camera array. the back end fuses together all the camera observations at the high semantic level so as to build global situational awareness on the parcels at the storage rack. when the field delivery agent arrives at the customer, he/she runs the augmented reality application and sees a pop-up screen with the details about the customer and the parcel. the 3d user interface displays the animated visual clue in the form of an animated arrow pointing to the parcel to be handled next by the field delivery agent. the 2d user interface displays information about the parcel to be handled. moreover, the interface includes buttons to update the handling status of the parcel (see figure 2). 2 vol. 39/2022 augmented reality to support parcel handling in last-mile logistics figure 2. highlighting in ar the location of the parcel to be delivered next. the detection of 2d markers from one or multiple cameras is used to reconstruct the 3d geometry of the physical shelves and storage rack. following this, the camera parameters are computed. a second calibration relates to a manual alignment and is done through the mobile application by adjusting the 3d position of specific key-points of the shelf. this step is necessary for fine tuning the 3d rendering. the pattern match algorithm has a multi-threaded implementation and uses scale-invariant feature transform (sift) descriptors for visual pattern matching. the bar-code module relies on zxing [21], an open-source, multi-format 1d/2d bar-code image processing library. in scan mode, the sensor component uses all information about the detected parcels to generate parcel fingerprints. the segmentation of the side images of a parcel uses the algorithm of wang and guo [22] which reconstructs the rgb-d indoor scene with plane primitives. the algorithm estimates the entire scene by adaptive planes without losing geometry details and preserves sharp features in the final mesh. in addition to the sensor component, the solid system consists of an augmented reality-based visualization component. this component is a mobile application running on the smartphone of the field delivery agent (android device). figure 3. results of the lab experiment. 4. results primevision and the university of groningen in the netherlands have run a first experiment on a scaleddown version of the first scenario with the fixed storage rack at the premises of the parcel delivery company. figure 2 illustrates a part of the experimental setup together with the ar layer highlighting the position of the parcel to be delivered next. the experiment has shown that the combined accuracy of parcel detection and the precision of 3d parcel presentation in augmented reality for a smallscale scenario with the delivery agent and with tens of parcels in the parcel database, is 95 % (figure 3). a limitation of the current system is the parcel detection by image recognition. the keypoint-based detector fails when a parcel has a few or no key points at all. on the other hand, the key point-based approach for parcel detection and tracking proved to be robust against variation in scale, illumination, rotation or occlusion. two different tracking algorithms were tested in the lab test, namely state-based tracking with one single key frame and state-based tracking with multiple key frames. our findings showed that when the augmented reality system used a multiple key-frame state-based algorithm was more robust compared to a single key-frame state-based algorithm because it was able to track 95 % of the parcels compared to 80 % respectively. increasing the number of key frames per state has a positive impact on the performance of the system. the factors relate to lowered illumination and the associated noise sensed in the video frames. one of the drawbacks of a single state-based tracking algorithm is that it only has one opportunity to find a parcel in contrast to a multi-frame state-based tracking algorithm. the camera vision is subjected to the dynamic environment such as shadows, incidence of light or turbulence, and this creates noise for the algorithm to track the parcel. the system must be robust since it has to operate in a dynamic environment due to the fact that the cameras are installed inside a truck. another reason for the lower success rate, was that parcels were only partially in the active field of view (fov) of the camera. this made it more difficult for the algorithm to recognize the unique fingerprints of a parcel. because the system only uses one key frame it sometimes was not able to find the parcel when it was partially in the active fov. however, the higher 3 d. datcu, j. winkel, l. rothkrantz acta polytechnica ctu proceedings success rate was achieved through a higher number of key frames that the tracking algorithm can use to find the parcels and thus increases the chances to find a parcel. the multiple key-frame state-based algorithm stored ten key frames every time a parcel was loaded into the truck. this increased the chances of finding the parcels by tenfold. the results of the preliminary study can be found in figure 3. 5. conclusions and future work the first results from a small-scale targeted experiment of primevision and the university of groningen in the netherlands has shown very promising results. the research has been carried out on a scaled-down version of the storage rack to comply with the new imposed governmental covid-19 oriented regulations forbidding access to the research facilities. our experiments have revealed that the solid prototype can automatically determine the location of parcels during the parcel handling process in last-mile logistics. a follow-up step of the current research project has already started by testing the one version of the system in a real-life production environment. we want to see the extent at which the system can support the parcel-handling activities in the storage location of a local store which has dedicated service for parcel handling. acknowledgements this work is done as part of project solid (selforganising logistics in distribution) and is supported via a national nwo grant with grant number 439.17.551. the project was facilitated by tno den haag that enabled the collaboration between the different project partners. references [1] w. wang, f. wang, w. song, s. su. application of augmented reality (ar) technologies in inhouse logistics. e3s web of conferences 145:02018, 2020. https://doi.org/10.1051/e3sconf/202014502018 [2] r. gevaers, e. van de voorde, t. vanelslander, et al. characteristics of innovations in last-mile logistics-using best practices, case studies and making the link with green and sustainable logistics. association for european transport and contributors 1:21, 2009. 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[2023-01-12], https://github.com/zxing/zxing. [22] c. wang, x. guo. plane-based optimization of geometry and texture for rgb-d reconstruction of indoor scenes. 2018 international conference on 3d vision (3dv) pp. 533–541, 2018. https://doi.org/10.1109/3dv.2018.00067 5 https://doi.org/10.1145/2968219.2968270 https://doi.org/10.3390/s21041437 https://doi.org/10.3390/computers7010013 https://github.com/zxing/zxing https://doi.org/10.1109/3dv.2018.00067 acta polytechnica ctu proceedings 39:1–5, 2022 1 introduction 2 related work 3 system architecture 4 results 5 conclusions and future work acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.35.0037 acta polytechnica ctu proceedings 35:37–41, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague accident rate of regional railway vehicles at railway crossings for the years 2014 to 2018 jakub seidl czech technical university in prague, faculty of mechanical engineering, department of automotive, combustion engines and railway engineering, technická 4, 160 00 prague 6, czech republic correspondence: jakub.seidl@fs.cvut.cz abstract. from a societal point of view, there are growing demands to increase traffic safety and reduce the risk of accidents of rail and road vehicles. the research to increase the safety of railway vehicles, which is carried out at the faculty of mechanical engineering of the czech technical university in prague, aims to increase the active and passive safety of railway vehicles. components and systems of active safety are designed to reduce the risk of an accident. components and systems of passive safety are designed to minimize the consequences of accidents. one of the parts of the active safety of railway vehicles is the condition and safety of the railway lines on which the railway vehicle is operated. the first part of the article focuses on the evaluation of accident statistics of railway vehicles with other road users at railway crossings for the years 2014 to 2018 with regard to the cause of the accident and the level of security of railway crossings. the conclusion of the article is devoted to proposals for solutions that could lead to a reduction in accidents at railway crossings. keywords: accident, active safety, etcs, railway crossing, railway vehicle, road vehicle. 1. introduction modern rail transport provides acceptable transport speed, low energy consumption, high transport comfort and big transport capacity. for these reasons, rail transport is one of the basic pillars of international, regional and urban transport [1]. regional rail transport enables the quick transport of passengers between cities or the transport of passengers from smaller cities to the centres of larger cities. with the increasing number of inhabitants living in agglomerations around large cities, the need for the development of regional railways is growing. the lines of the regional railways run separately from the roads. railway crossings enable the crossing of a railway line with roads for road vehicles. there are less than 8 000 railway crossings in the czech republic. each of these crossings increases the risk of a train accident with another road user – figure 1 [2]. each accident of a train with another road user causes a disruption of the flow of passenger transport and causes local paralysis of the railway network, as the train involved in the accident must not leave the scene until the accident is recorded by staff of správa železnic. throughout the removal and recording of the accident, the section of the railway line is impassable and the train operator is forced to introduce alternative transport for passengers [1]. from a societal point of view, there are growing demands to increase road safety and reduce the risk of accidents. vehicle safety is divided into two basic groups: active and passive safety. components and systems of active safety are designed to reduce the risk of an accident, while components and systems of passive safety are designed to minimize the consefigure 1. train accident with a road motor vehicle [zpravy.akutalne.cz]. quences of accidents to passengers and vehicles. one of the elements of active safety is the state and level of security of railway crossings (hereinafter only rc) – figure 2 [1]. this article deals with the evaluation of the causes of the occurrence and influence of the rc safety on the number of accidents involving rail and road vehicles and the proposal of an adjustment that could lead to a reduction in the risk of accidents on the rc. 2. accidental statistics for regional railway vehicles správa železnic (hereinafter only sž) provided, for the production of accident statistics, data on extraordinary events (accidents) of regional railway vehicles for 37 https://doi.org/10.14311/app.2022.35.0037 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en zpravy.akutalne.cz jakub seidl acta polytechnica ctu proceedings figure 2. railway crossing for pedestrian, prague 5 – jinonice. the years 2014 to 2018. a total of 853 extraordinary events were recorded during the period under review. 422 accidents of the total number, i. e. 49.4%, of extraordinary events were with another participant in road traffic on rc – table 1. the remaining, i. e. 431, extraordinary events were accidents of a regional train with game, different train, fallen tree or pedestrian in a section of track outside the rc [1]. years 2014 2015 2016 2017 2018 accidents 91 81 78 87 85 total 422 table 1. total number of accidents. sž provided for individual accidents the following information: type of collision vehicle, place of the accident, type of security of the rc, culprit of the accident, cause of the accident and consequences for the health of the accidents participants [1]. three evaluation criteria were established for evaluation of accident statistics. individual accidents were divided into groups according to these criteria [1]: • type of collision vehicle, • cause of the accident, • type of security or rc. all three criteria were chosen so that it was possible to design a best solution to reduce the risk of accidents of trains on the rc with other road users [1]. 2.1. type of collision vehicle the criterion the type of collision vehicle was chosen in order to determine the most common type of collision vehicle in train accidents on the rc. the individual accidents were divided according to the following types of collision vehicles [1]: • bus, • pedestrian, • bicycle, • motorcycle, • passenger motor vehicle (here in after pmv), • lorry, • tractor. 2.2. cause of accident the criterion of the cause of the accident was chosen in order to determine whether the drivers of collision vehicle or, for example, failures of the security equipment at the rc may be more likely to cause accidents. the finding of the most common perpetrator of an accident determines whether the behaviour of drivers or the failure of safety devices has a greater influence on the frequency of accidents [1]. the individual accidents were divided according to the following types of causes: • not giving priority to driving by another road user, • suicide attempt, • environmental failure, • another cause. determining the most common cause of an accident will allow a more appropriate proposal for solutions to reduce accident in the rc. 2.3. type of security of railway crossings the criterion of the type of security of rc was chosen in order to determine whether a higher level of rc security reduces the risk of an accident. individual accidents were divided into the following groups according to the type of rc safety [1]: • rc secured by warning crosses, • rc secured by a light device without mechanical barriers (rc with ld), • rc secured by a light device with mechanical barriers (rc with mb). 2.3.1. railway crossings secured by warning crosses this is the most common type of rc used in the czech republic, there are a total of 3 658. it is a so-called "unsecured rc", which is secured only by vertical traffic signs according to the road laws – figure 3. the rc is not equipped with any warning indicating the arrival of the train at the current time. this type of rc protection is used only on railway lines with a speed of v ≤ 60 km/h. railway vehicles usually use an audible warning signal in the form of a horn when passing through there rcs [1]. 38 vol. 35/2022 accident rate of regional railway vehicles figure 3. rc secured by warning crosses [idnes.cz]. 2.3.2. railway crossings secured by a light device without mechanical barriers (rc with ld) the rc with ld belongs to the so-called "secure crossings" – figure 4. there are a total of 2 388 rc with ld in the czech republic. rc with ld is equipped with light and sound signals, which alert the drivers of road vehicles to the arrival of a railway vehicle. the arrival of the railway vehicle is signalled by an intermittent red light supplemented by the sound of a bell. the rc with ld can be supplemented by a white signal indicating safe crossing of the rc [1]. figure 4. rc secured by a light device without mechanical barriers [ct24.ceskatelevize.cz]. 2.3.3. railway crossings secured by a light device with mechanical barriers (rc with mb) rc with mb are the best secured rc in the czech republic – figure 5. there are a total of 1 475 rc with mb in the czech republic. in contrast to others types of rc are rc with mb also equipped with mechanical barriers, which prevent road vehicles from passing rc when the train passes. drivers of road vehicles are first alerted to the arrival of rail vehicles by light and sound signals and then by lowering the barriers over the road [1]. figure 5. rc secured by a light device with mechanical barriers [zdopravy.cz]. 3. evaluation of accident statistics the results of the accident statistics according to all three selected criteria are shown in table 2 and table 3. cause of accident collision not giving suicide vehicle priority to attempt drive bus 2 0 pedestrian 15 14 bicycle 11 0 motorcycle 6 0 lorry 40 0 pmv 323 0 tractor 11 0 total 408 14 table 2. evaluation of accident statistics, collision vehicle vs. cause of the accident. type of security or rc collision warning with with vehicle crosses ld mb bus 0 1 0 pedestrian 7 14 8 bicycle 3 7 1 motorcycle 3 3 0 lorry 27 10 3 pmv 211 109 5 tractor 9 1 0 total 145 17 table 3. evaluation of accident statistics, collision vehicle vs. type of cr security. 39 idnes.cz ct24.ceskatelevize.cz zdopravy.cz jakub seidl acta polytechnica ctu proceedings the results shown in table 2 show that the most common cause of all accidents in the rc is not giving priority to drive by another road user and violation of §28 and §29 of law 361/2000 on traffic on roads. most of rc accidents could be prevented if road vehicle drivers respected the laws of the road. many educational campaigns have already been targeted at road vehicle drivers, informing them about the consequences of accidents with rail vehicles. therefore, it cannot be expected that the new educational campaign will change the behaviour of drivers on the environment and it is necessary to propose another solution to reduce accidents on the rc. the second most common cause of accidents of pedestrian is suicide attempt. unfortunately, accidents of this type cannot simply be prevented by improving the active safety of the rc, as the pedestrian acts in such a way that an accident with a rail vehicle occurs. from the results given in table 3 is evident that only at the best secured rc with mb is a significant decrease in accidents involving railway vehicles with other road users. there were 71 accidents per 1 000 rc secured by warning crosses. there were 61 accidents per 1 000 rc secured with ld. there were 11,5 accidents per 1 000 rc secured with mb. a similar number of accidents occurred on less secured crossings during the period under review. the effect of the increase security with light device does not have such an effect on the reduction of accidents, compared to the security by warning crosses. for a more accurate evaluation, it would be appropriate to consider the influence of the traffic flow (how many cars pass through the rc per hour). this effect was not evaluated due to lack of information. 4. proposal for reducing accidents on the railway crossings according to the results of accident statistics, the reduction of accidents in the rc could be achieved by equipping all rc with light devices and mechanical barriers. another possibility seems to be the addition of security for lower secured rc with retractable barriers – figure 6, which are used on some crossings in the russian federation. however, the reconstruction of all rc would be very costly, time-consuming and the operation of rail and road vehicles would be limited during the rc reconstruction [3]. for these reasons, it is appropriate to reduce the risk of accidents in the rc through less invasive rc modifications. the interconnection of communication between rail and road vehicles appears to be a possible solution that could lead to a reduction in accidents on the rc. for first step, it would be appropriate to start intensive cooperation between the manufacturers of the european hybrid train protection system etcs figure 6. railway crossings in russia federation [4]. with the manufacturers of navigation and control system for road vehicles. transmitting train position information to the car navigation and displaying this information to the driver and storing this information in the recording equipment would increase the criminal liability of car drivers for their behaviour at railway crossings and could lead to reduction of tragic accident at railway crossings [3]. after the accident of two trains on july 7, 2020 near pernink in the karlovy vary region – figure 7, the debate on the low security of railway traffic in the czech republic and the need to increase traffic safety has revived [5]. figure 7. train accident near pernink [čtk]. the ministry of transport plans to increase the security of the czech railways through the introduction of the etcs (european train protection system). etcs is gradually replacing the various european national train protection and control systems. the extension of etcs to all rail systems in the eu will allow the abolition of costly multiple on-board signalling equipment for individual states and the inefficient exchange of locomotives at national borders [? ]. the main task of the etcs is to ensure the safety of rail transport and to actively intervene in the train control in the event of a driver failure or error. one 40 vol. 35/2022 accident rate of regional railway vehicles of the pioneers of the etcs is the trainguard system from siemens s.r.o. [6]. the trainguard system has different levels of security, depending on the equipment installed on the railway lines and railway vehicles [6]: • level 0, • level 1, • level 2, • level 3. at security levels 2 and 3 – figure 8, railway vehicles are equipped with radio equipment to receiving and sending information about track position, train speed, etc. [6] figure 8. function of etcs at level 2 [mobility. siemens.com]. trains equipped with etcs level 2 and 3 could send a signal to around moving road vehicles that they are arriving to the rc and road vehicles are not allowed to ride through the rc. this information would be received by road vehicles via radio signals antennas, which are standard equipment of currently sold vehicles. the signal would then be evaluated by the on-board computer in the road vehicle. if the driver did not heed the rc’s warning about the arrival of the railway vehicle, the on-board computer would prevent the road vehicle from driving through the rc and prevent a potential accident. 5. conclusions the results of the accident statistics shown in table 2 show that all accidents on the rc during the period under review were caused by drivers of road vehicles and pedestrians by violation of §28 and §29 of law 361/2000. many educational campaigns informing about the consequences of accidents on the rc have already been targeted at road vehicles drivers. therefore the new campaign cannot be expected to lead to a reduction in rc accidents. the results shown in table 3 show that a similar number of accidents occur on the rc secured by warning crosses and light device. the effect of better rc security is not apparent here. the reduction of accidents occurs only at the best secured crossings, which are equipped with light and sound devices supplemented by mechanical barriers. however, the extension of this type of security to all rc cannot be expected due to the high financial and time demands. as a possible solution, that could lead to a reduction of accidents in the rc, is introduction of communication between railway and road vehicles. railway vehicles equipped with etcs trainguard level 2 and 3 from siemens s.r.o. would be able to send a signal of arrival to the rc. the on-board computer of road vehicles would evaluate this signal and not allow the driver to enter the rc in front of the arriving rail vehicle. the proposed solution for reducing rc accidents described in this report must be subjected to more detailed research that would verify the possibility of use in practice. list of symbols sž správa železnic rc railway crossing pmv passenger motor vehicle ld light device mb mechanical barriers etcs european train protection system eu europian union acknowledgements this research has been realized using the support of the technology agency of the czech republic, programme national competence centres, project #tn01000026 josef bozek national center of competence for surface transport vehicles this support is gratefully acknowledged. references [1] j. kolář, j. seidl, v. zelený, o. vincik. statistika nehodovosti tramvají a vozidel regionální železnice. prague: ctu fme, department of automotive, combustion engine and railway engineering, 2020. [2] r. pavelka. dopravní nehody na železničních přejezdech. brno, 2019. [3] j. kolář, j. seidl, v. zelený. zpráva o možnosti zvyšování pasivní bezpečnosti železničních vozidel. prague: ctu fme, 2020. [4] j. schrötter. železniční přejezdy – krizová místa v dopravě. in: vědeckotechnický sborník čd č. 41/2016, 2016. [5] aktuálně.cz. nehody na železnicích přibývají, za červenec jich bylo nejvíc za posledních pět let. [online]. available: https://zpravy.aktualne.cz/. [6] siemens mobility s.r.o. european train control system (etcs). [online]. available: www.mobility.siemens.com. 41 mobility.siemens.com mobility.siemens.com https://zpravy.aktualne.cz/ www.mobility.siemens.com acta polytechnica ctu proceedings 35:37–41, 2022 1 introduction 2 accidental statistics for regional railway vehicles 2.1 type of collision vehicle 2.2 cause of accident 2.3 type of security of railway crossings 2.3.1 railway crossings secured by warning crosses 2.3.2 railway crossings secured by a light device without mechanical barriers (rc with ld) 2.3.3 railway crossings secured by a light device with mechanical barriers (rc with mb) 3 evaluation of accident statistics 4 proposal for reducing accidents on the railway crossings 5 conclusions list of symbols acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.38.0077 acta polytechnica ctu proceedings 38:77–83, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague comparison of thermal bridge calculation methods balázs nagy∗, martin marosvölgyi, zsuzsa szalay budapest university of technology and economics, faculty of civil engineering, department of construction materials and technologies, 3 műegyetem rakpart, h-1111 budapest, hungary ∗ corresponding author: nagy.balazs@emk.bme.hu abstract. we need to consider linear heat losses due to thermal bridges for the accurate calculation of building heat losses. our research examined a whole building, and different thermal bridge calculation methods were compared. the following techniques were included in the study: a simplified method according to the hungarian energy performance regulation where the effect of thermal bridges can be taken into account by multiplication factors applicable to thermal transmittances; a simplified thermal bridge catalogue of iso 14683; a recent national thermal bridge catalogue; two-dimensional thermal modelling as well as a conjugated heat and moisture (ham) simulation considering steady-state and dynamic conditions. overall, we created eight different numerical modelling approaches depending on the type of simulation and boundary condition. the modelling and simulations were carried out using multiphysics software based on the finite element method according to iso 10211 and en 15026. all the relevant details of the building were analysed to get a complete picture. based on the results, we analysed each method’s relative proportions of surface and linear heat losses. the evaluation showed that the hungarian simplified method generated the lowest heat losses for thermal bridges, while iso 14683 produced the highest results, with the numerically simulated results in the middle. the overall heat losses varied by 30 %, depending on the thermal bridge calculation method. linear heat losses were between 12 % to 32 % of the surface heat losses. our study helps to choose the adequate method to perform thermal bridge simulations. keywords: thermal bridges, building constructions, heat and moisture transfer, numerical modelling. 1. introduction as a result of trends and stricter legislations in recent years, our buildings today are built with a significant amount of thermal insulation compared to the past. buildings can be designed with different building constructions, but poorly designed details are implemented in many cases. linear thermal bridges are responsible for a significant percentage of energy losses in buildings, even with a consciously designed thermal insulation, but this percentage increases further with inappropriate joint constructions. a better understanding of thermal bridges is essential for calculating the energy demand of buildings. several studies worldwide have been carried out that compare the calculation of linear heat losses and their share in the total energy balance of buildings. in most studies, there are significant differences between the methods used [1, 2]. this study aimed to compare these methods to investigate the thermal bridge effect using a nearly zero-energy residential building as a case study, which is considered typical for the hungarian market. we compared the hungarian simplified method according to the domestic energy performance regulation [3], the simplified thermal bridge catalogue according to msz en iso 14683:2017 [4], the results of a detailed national thermal bridge catalogue, and the results of complex two-dimensional heat and moisture transfer simulations with steady-state and dynamic environmental conditions. for each thermally relevant building construction joint, the temperature factor at the internal surface (frsi), the average thermal transmittance (u), the linear thermal transmittance due to the thermal bridging effect (ψ), as well as the transmission heat transfer coefficient of the entire building (htr) is analysed. the minimum internal surface temperatures (ts,min) were also determined for the numerical finite element method-based models. 2. methodology the examined building, built with an approximate gross floor area of 80 m2, with a pitched roof and partially a flat roof, provides comfort for 3–4 people and has a flat roof used as a large terrace. the house is simple, 21st-century, and modelled using building information modelling (bim) software archicad 25 to create the envelope and details for energy performance and thermal bridge calculations. the used construction materials are typical for central europe. the building comprises ceramic hollow brick walls with a 15 cm thick expanded polystyrene external thermal insulation composite system (etics) achieving u = 0.167 w/m2k. in comparison, the flat roof slab is monolithic reinforced concrete with 25 cm extruded polystyrene insulation having u = 0.138 w/m2k. the pitched roof includes 30 cm mineral wool insulation between the rafters and has a vapour barrier layer behind the interior gypsum 77 https://doi.org/10.14311/app.2022.38.0077 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en b. nagy, m. marosvölgyi, z. szalay acta polytechnica ctu proceedings figure 1. analysed building construction joints of the examined building. board interior finish, and its thermal transmittance is u = 0.138 w/m2k. in the slab-on-ground, 10 cm load-bearing extruded polystyrene thermal insulation was installed above the bituminous waterproofing on the reinforced concrete base plate. the u -value of the designed layers is u = 0.225 w/m2k; however, since the footing contains perimeter insulation, the u value of the ground contact floor can be reduced to 0.191 w/m2k. in the research, the heat transfer coefficient of the building is calculated using different calculation methods and was compared together. firstly, the simplified calculation procedure was carried out. the transmission heat losses were determined based on the specific length of thermal bridges according to hungarian building energy performance regulation tnm decree 7/2006. [3]. the specific length shall be calculated by dividing the length of the specific joint (e.g., wall corner) by the area of the connecting surfaces (e.g., façade walls). then based on the result, the thermal bridging of the joint is classified as slight, moderate, or large. according to the category, the u value of the envelope element should be multiplied by a correction factor included in tabulated form in the legislation. additionally, the heat loss of ground contact structures was determined according to iso 13370:2017 [5]. in the case of the detailed calculation, the linear thermal transmittances of the thermal bridges can be obtained from the standard iso 14683:2017, which can be used as a simple thermal bridge catalogue. for this purpose, we need to know the type and length of the thermal bridge and its material composition. if the detail can be found in the standard, the value of the linear thermal transmittance is obtained from its tables, which is then multiplied with the length of the thermal bridge to determine the transmission heat loss. the disadvantage of this standard is that it only covers eight different types of thermal bridges and is less varied in terms of insulation material, thickness, use of construction materials and structural design. the building constructions presented in this standard do not comply with hungary’s current energy performance requirements. the heat transfer coefficient of the building was also determined with a new hungarian thermal bridge catalogue containing linear thermal bridges generated by mainly domestic products and building constructions [6]. in contrast to the iso 14683:2017 [5] standard, this is a much more detailed catalogue. in total, the thermal bridge catalogue contains about 21 thousand linear thermal transmittance values for the most typical building detail designs, with a particular focus on building materials from domestic product manufacturers (more than 50 different masonry blocks with different building construction designs, several types of slab constructions, etc.). building materials used to create the catalogue were modelled based on performance data published by manufacturers or distributors. besides simplified calculations and using a different kinds of thermal bridge catalogues, numerical modelling was also deployed to calculate the linear thermal transmittances of the examined building. the geometry for the numerical finite element analyses was modelled according to iso 10211:2017 [7], neglecting details that are not thermally relevant (e.g., roof tiles, roof battens, counter battens, etc.). the model, which includes the central element, is bounded by cutoff planes. the geometric dimensions used to create the model are more extensive than those specified in the standard. the distance between the cut-off plane and the central element is at least three times the structural thickness, or at least 1 metre. where possible, a 1.5 m distance was taken from the central element. a total of 4 surfaces and 15 linear thermal transmittance values were determined using numerical modelling. together with the reference tests, numerical finite element analyses were run on 23 different geometries (see figure 1). however, not only steady-state numerical models were created. seven different studies were considered for each construction joint: three steady-state and four dynamic-state. two different thermal models were investigated and a conjugated heat and moisture transfer model in a steady state. in the thermal models, the design is assumed to be completely dry (0 % relative humidity) in the first condition, while in the second condition, relative humidity of 50 % is assumed. former is test78 vol. 38/2022 comparison of thermal bridge calculation methods building envelope components u-values (with thermal bridge effect) [w/m2k] surface heat transfer coefficient [w/k] linear heat transfer coefficient [w/k] total heat transfer coefficient [w/k] external wall 0.218 23.78 7.13 30.92 flat roof 0.152 4.08 0.41 4.49 pitched roof 0.159 3.50 0.52 4.02 ceiling of pitched roof 0.152 6.04 0.61 6.65 ground contact floor 0.191 12.35 12.35 windows and doors frame:1.1 15.46 15.46glazing: 0.6 total 65.21 8.67 74.18 table 1. heat transfer coefficients based on simplified calculation according to [3, 5]. ing the case when building materials are modelled based on performance data published by manufacturers, who usually publish dry test results only to show the lowest possible thermal conductivity of their products. at the same time, 50 % relative humidity is assumed to be closer to real application cases; however, out of the three steady-state numerical finite element analyses, the conjugated heat and moisture (ham) analysis is considered to be the most accurate method because it takes more physical processes into account. in steady-state conditions, the methodology described in iso 10211:2017 [7] was used to calculate the φ linear thermal transmittance using the l2d thermal coupling coefficient taken from the numerical modelling. in steady-state conditions, the environmental conditions (outdoor and indoor air temperature and relative humidity) were statistically assumed based on the heating season in budapest. the surface resistance values were set by iso 6946:2017 [8]. for dynamic-state thermal models, the evaluation of the results is almost identical to the method used for the steady-state analyses. however, in the case of the dynamic study, an hourly-based simulation was conducted for 4 369 hours (length of the standard heating period between 15th october and 15th april in hungary according to the previously mentioned legislation) and then averaged. however, the definition of thermal transmittance refers to steady-state conditions. therefore, it is essential to consider before the evaluation at least the temperature difference between the different building elements at the thermal coupling coefficient from the numerical investigations. when there is only a slight difference between the internal and external temperature, the magnitude of the heat flux density decreases, but it is important to note that this decrease is not directly proportional. these excursions can be filtered out by choosing the correct minimum temperature difference. our study chose a 10 k temperature difference to analyse the dynamic results. four different dynamic models were created; two used dry constructions, and two used 50 % relative humidity, like the steady-state models; however, each condition was tested with solar radiation and without solar radiation, respectively. the boundary conditions were set using weather data for the dynamic study were determined according to [9]. boundary conditions also depend on temperature, relative humidity, long and short-wave radiation, wind speed, which vary every hour. the modelled period was two years in total, of which only the heating period of the second year was considered during the evaluation. both steady-state and dynamic numerical simulations were carried out using comsol multiphysics. 3. results and discussion 3.1. simplified calculation according to hungarian regulation table 1 shows the heat losses based on the simplified calculation. the additional heat loss due to thermal bridges may contain significant inaccuracies, as this calculation does not consider the design of the joints. the correction factors were determined when the regulation was adopted in 2006, based on the typical designs used at that time, which have undergone significant changes in the last decade for new buildings. the surface heat transfer coefficient of the ground contact floor was calculated using [5]. 3.2. thermal bridge catalogues when using a thermal bridge catalogue, the surface heat transfer coefficient of the building envelope (external wall, flat roof, pitched roof, ground contact floor) is still calculated according to the simplified procedure (total: 65.21 w/k). still, the thermal bridge heat losses are now considered with the values determined from the catalogues (table 2). if a detail was not included in the thermal bridge catalogue, it was not considered in the calculation. the linear heat transfer coefficients clearly show that the standard iso 14683:2017 [4] gives significantly higher values than the newer and more extensive thermal bridge catalogue published in 2020 [6]. this is mainly the reason for the differences. the iso 14683:2017 was published in 1999 and was last updated in 2017, but neither were the values for existing designs revised nor was the standard extended to include more current details. the standard especially seemed incorrect for wall and ground contact floor joint values since it was more than ten times greater than the value 79 b. nagy, m. marosvölgyi, z. szalay acta polytechnica ctu proceedings thermal bridges length [m] linear thermal linear heat transfer transmittance [w/(m · k)] coefficient [w/k] msz en thermal bridge msz en thermal bridge iso 14683 catalogue iso 14683 catalogue wall corner 18.60 0.15 0.05 2.79 0.93 wall – slab joint 20.90 0.10 0.07 2.09 1.46 window lintel – slab joint 16.75 0.19 3.18 wall – window joint 55.15 0.00 0.02 0.00 1.10 wall – ground floor joint 34.80 0.75 0.06 26.10 2.09 wall – flat roof joint 20.20 0.10 2.02 total 30.98 10.79 table 2. heat transfer coefficients based on thermal bridge catalogues [4, 6]. model surface heat transfer coefficient [w/k] linear heat transfer coefficient [w/k] total heat transfer coefficient [w/k] thermal with 0 % rh 59.92 13.81 73.73 thermal with 50 % rh 70.22 18.93 89.15 ham 65.18 18.05 83.23 table 3. heat transfer coefficients based on steady-state numerical analyses . from the hungarian thermal bridge catalogue. this value generated a massive difference between the two catalogues’ linear heat transfer coefficient results. 3.3. numerical finite element analyses the evaluation of the numerical modelling results allows us to determine the heat transfer coefficient of each envelope element (external wall, flat roof, pitched roof, ground contact floor) from the average heat flux density and the difference between the internal and external air temperature (table 3). out of the examined cases, the thermal model at 0 % rh gives a nearly 12 % lower value than the ham analysis, while the joints calculated at 50 % rh provide a 7 % higher value. it is essential to know that most material or building element producer gives their materials’ thermal performance data at 0 % rh since the thermal conductivity of their products could be much lower without moisture content. most thermal calculations and thermal bridge catalogues use data right from the material producers. although there is always moisture in constructions, therefore, ham model is supposed to be closer to the average of the real-life behaviour than a thermal model without moisture content. after the 10 k temperature difference data filtering, four different studies were evaluated in dynamic conditions: (1.) with solar radiation included and with dry conditions, (2.) with solar radiation included and 50 % relative humidity, (3.) without solar radiation and dry conditions, (4.) without solar radiation and 50 % relative humidity. among the four studied dynamic simulations, the highest heat transfer coefficient was obtained for the model with 50 % relative humidity, including the solar radiation in the analysis. in contrast, the lowest was obtained with dry conditions and without considering solar radiation (see table 4). this could be explained by solar radiation heating up the construction, and warmer materials are less insulating, while the evaporation of the moisture was neglected in the thermal models. the overall difference comparing case (2.) to case (3.) is close to 30 %. however, if we compare case (1.) to case (3.) or case (2.) to case (4.) the difference is only 3.5–5.5 % (the cases including solar radiation were higher due to the reason mentioned above), showing that the effect of moisture in the construction is much more dominant than the effect of the solar radiation. the dynamic analysis method has the highest number of physical processes considered, and the calculation method requires the longest time of all the analysis methods presented. the speciality of this calculation method is that it requires a large computational power, which is even more pronounced than for the conjugated thermal and moisture models. 3.4. comparison of the heat transfer coefficient of the building envelope and construction joints in the following section, we compared the heat transfer coefficients of the building envelope and construction joint designs and the differences between numerical analysis methods. the thermal transmittances of the openings (glazing and frame) were determined using 80 vol. 38/2022 comparison of thermal bridge calculation methods figure 2. thermal transmittance of the building envelope under different modelling conditions. model surface heat transfer coefficient [w/k] linear heat transfer coefficient [w/k] total heat transfer coefficient [w/k] 0 % rh w. solar radiation 55.01 16.41 71.42 50 % rh w. solar radiation 65.58 23.99 89.57 0 % rh w/o solar radiation 53.74 15.21 68.95 50 % rh w/o solar radiation 65.30 19.64 84.94 table 4. heat transfer coefficients based on dynamic thermal simulations. a manufacturer’s catalogue. therefore, there is no difference for different environmental conditions in our calculations. observing the calculated thermal transmittances of the building envelope constructions, figure 2 clearly shows no significant difference between the analysis methods for the external wall and flat roof constructions. however, the magnitude of the u values of the pitched roof construction is significantly influenced by the internal moisture content and for the corresponding environmental conditions (0 % or 50 % rh). still, there is no significant difference between the steady-state and dynamic conditions. it is also visible that the steady-state condition gave higher results for the ground contact floor than the dynamic analysis method. this is because we can include the heat storage and the soil under the floor that significantly influence heat transfer in a dynamic state. on this basis, the heat transfer coefficient of the ground contact floor in the simplified case and the steady-state analyses is significantly higher than in the case of the dynamic condition, which is much more realistic. figure 3 illustrates four different building construction joints for the numerical models analysed under different modelling conditions (out of 15 different analysed construction joints in total). in the case of the wall corner joints, the φ value is mainly influenced by the moisture content, like other construction joints with capillary active materials and not connected to the ground (e.g., firewall, openings in walls). for the intermediate slab and wall joint (similarly to the flat roof and external wall, flat roof, and attic wall joint), the dynamic analyses gave higher φ values, while for the external wall and pitched roof joint, the dynamic analyses gave lower results compared to the steadystate conditions. in the analyses for the horizontal section of the openings and the partition or the external wall connection, both moisture and modelling conditions significantly affected the φ value. only the case of the flat roof and external wall joint resulted in almost identical results independently from the moisture or modelling conditions. considering the different conditions for a single design, it can be concluded that in most cases, there can be a difference of ±50 % between the linear thermal transmittances, which is a significant discrepancy. it is important to emphasise that we are talking about relative situations here; we cannot determine whether the dynamic model without moisture transport or the steady-state model with moisture transport is better than reality. the values for all joints are within ±3 % of the values measured in the temperature factor for the internal surface. it is safe to say that condensation and mould growth is not expected on the internal surface since frsi > 0.8 was obtained in every case. 3.5. comparison of the difference of heat transfer coefficients depending on the used methods the results for surface heat transfer coefficients are within approximately ±10 % for all methods, with significant differences in the analysis of the linear heat transfer coefficients, as visible in figure 4. according to the hungarian regulation, the value of the 81 b. nagy, m. marosvölgyi, z. szalay acta polytechnica ctu proceedings figure 3. linear thermal transmittance of construction joints under different modelling conditions. figure 4. comparison of heat transfer coefficients: 1) simplified calculation, 2) msz en iso 14683, 3) thermal bridge catalogue, 4) steady-state and 0 % rh, 5) steady-state and 50 % rh, 6) steady-state ham, 7) dynamic, with effect of radiation and 0 % rh, 8) dynamic with solar radiation, 50 % rh, 9) dynamic without solar radiation and 0 % rh, 10) dynamic, without solar radiation and 50 % rh. linear heat transfer coefficients is the lowest for the simplified method, almost 50 % below the average of the other methods. this is because the correction value only needs to be determined based on the specific surface areas of the building envelope containing thermal bridges. there are different kinds of thermal bridges that cannot be considered. as expected, the highest heat transfer coefficient was obtained using iso 14683:2017 [4], which exceeded the average of the other methods for the linear heat transfer coefficients by almost 90 %. this is due to the obsolescence and lower level of detail mentioned earlier. the new 2020 hungarian thermal bridge catalogue gave higher results than the simplified method but significantly lower than the standard. it also can be said that the use of the thermal bridge catalogue was substantially more straightforward and faster compared to all other methods presented. the results show that including solar radiation increases the result by an average of 5 %, with the additional increase of the heat transfer coefficient occurring mainly at the thermal bridges. at 50 % rh, the thermal performance of the constructions deteriorated by about 20 %. observing the values for the total heat transfer coefficients of the dynamic conditions, it can be concluded that it gave a lower result than the steady-state models with the same moisture conditions. it can be said that with the use of steady-state modelling, the value of the total heat transfer coefficients is mistaken in favour of safety. roughly 22 % of the total heat transfer coefficient of the building used for the study comes from thermal bridges. the building can be considered an average new building in hungary, but the proportion of the surface and linear heat transfer coefficients for other building construction designs may vary. 4. conclusion the research aimed to compare the methods used to investigate the thermal bridging effect of buildings, us82 vol. 38/2022 comparison of thermal bridge calculation methods ing the example of a newly built residential building, which is considered typical on the hungarian market. the results of the simplified method according to tnm decree 7/2006., a thermal bridging catalogue according to iso 14683:2017, a new hungarian thermal bridge catalogue, and detailed two-dimensional thermal and ham simulations in both steady-state and dynamic conditions were compared. the simplified calculation determined the surface heat transfer coefficients with satisfactory accuracy compared to the average of the other methods, but the linear heat transfer coefficient was much lower. this contradicts the general assumption that the simplified method is biased in favour of safety. the simplified method may result in lower heat transfer coefficients for low-energy buildings than the detailed calculations and numerical modelling. the use of the standard iso 14683 is not recommended in any situation. due to its obsolescence and lack of variation, it is recommended to update this standard. in the case of the examined building, the hungarian thermal bridge catalogue provided a much better alternative, which stands out from the other methods for its ease of use and its simplicity and speed of use. at the same time, it approximates the results of the steady-state thermal model with 0 % rh. however, not all building constructions were included in the catalogue. it is not possible to consider every thermal bridge; therefore, this catalogue should be expanded in the future. the 2d steady-state model is excellent for more complex building constructions. although the model construction takes a similar time regardless of modelling conditions, a steady-state calculation is much faster than solving the dynamic simulation. nevertheless, with the development of computational technology and software solutions, it may soon become applicable to professional works. however, due to its long calculation time, dynamic thermal analysis and ham simulations are currently recommended only for research purposes. acknowledgements project fk_128663 has been implemented with the support provided from the national research, development and innovation fund of hungary, financed under the fk_18 funding scheme. first author is supported by the únkp-21-4 new national excellence program of the ministry for innovation and technology from the source of the national research, development and innovation fund. second author is supported by the únkp-21-1 new national excellence program of the ministry for innovation and technology from the source of the national research, development and innovation fund. references [1] h. ge, f. baba. effect of dynamic modeling of thermal bridges on the energy performance of residential buildings with high thermal mass for cold climates. sustainable cities and society 34:250–263, 2017. https://doi.org/10.1016/j.scs.2017.06.016 [2] k. kuusk, j. kurnitski, t. kalamees. calculation and compliance procedures of thermal bridges in energy calculations in various european countries. energy procedia 132:27–32, 2017. https://doi.org/10.1016/j.egypro.2017.09.626 [3] 7/2006. (v. 24.) tnm decree on the determination of the energy performance of buildings. [2022-03-03]. https://njt.hu/jogszabaly/2006-7-20-6f [4] msz en iso 14683:2017 thermal bridges in building construction. linear thermal transmittance. simplified methods and default values (iso 14683:2017). hungarian standards institution, budapest, hungary. [5] msz en iso 13370:2017 thermal performance of buildings. heat transfer via the ground. calculation methods (iso 13370:2017). hungarian standards institution, budapest, hungary. [6] b. nagy. thermal bridge catalogue 1.1, 2020. [2022-03-03]. https://epito.bme.hu/em/emkek [7] msz en iso 10211:2017 thermal bridges in building construction. heat flows and surface temperatures. detailed calculations (iso 10211:2017). hungarian standards institution, budapest, hungary. [8] msz en iso 6946:2017 building components and building elements. thermal resistance and thermal transmittance. calculation methods (iso 6946:2017). hungarian standards institution, budapest, hungary. [9] b. nagy. designing insulation filled masonry blocks against hygrothermal deterioration. engineering failure analysis 103:144–157, 2019. https: //doi.org/10.1016/j.engfailanal.2019.05.005 83 https://doi.org/10.1016/j.scs.2017.06.016 https://doi.org/10.1016/j.egypro.2017.09.626 https://njt.hu/jogszabaly/2006-7-20-6f https://epito.bme.hu/em/emkek https://doi.org/10.1016/j.engfailanal.2019.05.005 https://doi.org/10.1016/j.engfailanal.2019.05.005 acta polytechnica ctu proceedings 38:77–83, 2022 1 introduction 2 methodology 3 results and discussion 3.1 simplified calculation according to hungarian regulation 3.2 thermal bridge catalogues 3.3 numerical finite element analyses 3.4 comparison of the heat transfer coefficient of the building envelope and construction joints 3.5 comparison of the difference of heat transfer coefficients depending on the used methods 4 conclusion acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.38.0585 acta polytechnica ctu proceedings 38:585–591, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague reintegration of karachi port through sustainable adaptive reuse of abandoned and underused industrial buildings naveed iqbal∗, syed hamid akbar, koenraad van cleempoel hasselt university, faculty of architecture and arts, building e, office cs.02, campus diepenbeek agoralaan 3350, diepenbeek, belgium ∗ corresponding author: ar.naveed9@gmail.com abstract. the economic capital and one of pakistan’s most important cities gain popularity due to the reputable karachi port. karachi was referred to as korokala and debal in ancient times; a city conquered by famous conquerors, including alexander the great and muhammad bin qasim due to its seaport importance. however, most of the port’s tangible and material heritage were built by the british during the 19th–20th century. the britishers conquered the city and started developing karachi port because of the seaport. due to the port, the area of karachi grew from a small fishermen’s village to a bustling metropolitan city. during the second industrial revolution in the late 19th century, the britishers developed indian industries so that they depended on british capital goods. to transport the goods, the british empire constructed railways and maritime routes in the mid and late nineteenth century, the port of karachi was one of them. karachi port remains the hub of pakistan’s main logistic transportation, but some of its heritage buildings have been neglected on this site. after the 9/11 tragedy in the us, due to security reasons, karachi port trust restricted the port to the public. now, the security situation is better, and the port can be reintegrated and made accessible to the public through sustainable adaptive reuse of abandoned or underused buildings. karachi is a congested city with few places for recreation; reintegrating this site can open opportunities for tourism while showcasing the city’s industrial heritage. this paper wants to articulate the potential of industrial heritage. but the latter in pakistan is not yet recognized as part of a heritage discourse. the port offers a rich case study due to its layered history and various typologies of buildings on site. keywords: industrial heritage, reintegration, adaptive reuse, seaport. 1. introduction seaports are always associated with the development and history of port cities. they play a vital role in the urbanization and social setup of the cities. similar circumstances have occurred in karachi. different monarchs ruled it at different times and gave it different names, so karachi has a variety of tangible and intangible heritage. karachi can be associated with different ancient empires but the most important one that developed the seaport into one of the busiest ports of the 19th and 20th century in asia and shaped the city of karachi was the british empire. they conquered the city for developing trade routes with eastern and western countries. due to its central strategic location between east and west, the karachi port was the nucleus of the development during british interventions in karachi. they constructed the roads and railway tracks originating or directing towards the port to keep smooth flow of trade from land to sea and sea to land routes. in the process, they constructed several important buildings on and around the port. due to the port activities, people started migrating from different parts of british india resulting in the abrupt growth of the city. after the partition of the indian subcontinent, the seaport remained an important site for the city and the state of pakistan and saw growth and expansion in the early few decades of the newly build pakistan. the seaport is still an important site but some of the buildings on the site of heritage value are neglected and are not taken care of besides its rich historical background and a high potential for adaptive reuse and regeneration of the port in terms of tourism. some buildings on the port site can be reused for new functions to open the site for the public which has a rich history and social value in karachi. currently, the port is restricted for the public after the security protocols changed due to the 9/11 world trade center tragedy which resulted in the invasion of afghanistan. but now the security situation is good, and the abandoned/underused buildings can be reused to reintegrate the site of karachi port in the locality in terms of tourism and urban revival. in this perspective, the paper will shed light on the historical background of the port through a timeline and will investigate the present condition of british time buildings on and around the port to see its potential in the urban uplift of the locality through the strategy of sustainable adaptive reuse of the abandoned or underused buildings. 585 https://doi.org/10.14311/app.2022.38.0585 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en n. iqbal, s. h. akbar, k. v. cleempoel acta polytechnica ctu proceedings 2. archival data collection and literature review the paper is a part of broader phd research on industrial heritage and its adaptive reuse potential in pakistan. the site of karachi port is not only associated with the industrial revolution in british india but also with current-day pakistan. during the british raj, the port of karachi acted as an important node for the trade between india and eastern and western countries. a literature review was done to have a historical overview, based on which a research visit was done by the first author to the city of karachi to collect the archival data, related to the karachi port site and to interact with the port authorities, and users through semi-structured interviews. during the visit, the first author visited different archives departments. ancient maps of karachi port, old photographs, drawings, and published reports were collected from sindh archives, karachi port trust archives, and ned heritage cell archives. besides this, a site survey was also done to map the present situation of the karachi port, during which different buildings were marked which were abandoned or underused at the port. also, some informal interaction was done with port authorities and the persons working on the port, through semi-structured interviews and photographs to gain insight into the port’s history. the buildings that were currently not in use for any function were considered abandoned like berth 16-17a (figure 1) whereas the buildings which were temporarily used for some other functions and had the potential to be used more beneficially were considered underused like berth 15a which is used as praying area by the workers. internationally, abandoned or underused industrial sites are reused in a way that benefits the locality and preserves the heritage value of the site. the concept of adaptive reuse is widely in practice in relation to industrial heritage. adaptive reuse is the process in which an existing building that lives longer than its function is made suitable for another function with clever alteration and interventions [1–5]. numerous sites of industrial character are regenerated and reused internationally, which are attraction points for the local, national and even international communities now [3, 6–8]. according to the “nizhny tagil charter” the industrial heritage consists of all the infrastructure and buildings that are associated with production and transportation including mills factories, housing onsite, religious buildings, bridges, aqueducts, seaports, storage areas, machinery [9]. after the initial literature review, it was clear that some of karachi port buildings have heritage value but are not given attention as compared to other cultural heritage in pakistan. in general, the industrial buildings in pakistan are not considered or recognized as heritage besides their historical value. they are covered in the same cultural heritage category, and no specific sections or clauses are mentioned in pakistan’s figure 1. 16–17a berth east wharf karachi. national heritage legislation. the situation is the same with the karachi port and its british-time buildings. 3. historical overview of karachi and its port over a period of only one hundredand fifty years, karachi has grown from a small town of fifteen thousand people to pakistan’s largest metropolitan city of more than sixteen million inhabitants [10, 11]. the history of the city dates to the 3rd century bc and is linked to the arrival of alexander the great when the city was known by the name of krokala. and the 7th century ad association with the muslim conqueror muhammad bin qasim by the name of debal. [10]. although the identification of karachi with these two ancient cities are under considerable debate since the 19th century, however modern karachi gain popularity during the late 18th century and later due to the interest of the east india company and the british government in the seaport in the area. the relationship between the indian subcontinent and europe dates to ancient times when alexander the great established the greco-indian empire, now called punjab. alexander was interested in establishing permanent links with the region for political and commercial reasons. for this purpose, he planned two ports with naval dockyards, one of which was probably the current-day karachi, on the westernmost mouth of the indus. while some believe that morontobara, a station on the return expedition of alexander’s navy, was probably near the location of today’s karachi harbour. [12]. the cretan admiral nearchus, a close friend of alexander, sailed down the mighty river indus and arrived at the site of new alexandria (probably at 326 bc) called “crocola” or “krokala”, which was an important centre for food supplies for the onward voyage. [13]. the romans also had maritime trade with india’s coastline from their ports in egypt. in the 2nd century ad. the greco-egyptian cartographer, claudius ptolemy, developed a map of india which showed the 586 vol. 38/2022 reintegration of karachi port through sustainable adaptive reuse . . . figure 2. boat basin, passenger wharf karachi port. different ports visited by roman mariners. the second ancient city with which karachi is associated is debal, a port of the great empire of “rai chach”. the arrival and conquest of the kingdom of mighty dahar on the shores of sindh in 711 ad by muhammad bin qasim also marked the entry of the religion of islam into the indian subcontinent. the conquest happened at the port of debal, which is believed to be the location of karachi harbour. [14]. british interest in the indian subcontinent started in the early 17th century during the mughal empire. the first east indian company started at surat in 1612 in a house partly hired and partly gifted by the mughal emperor jahangir. [15]. it was until the early 18th century that the british government got interested in karachi. henry pottinger, a political agent in sindh and a key player in the game, analyzed karachi harbour and its importance. he noted that karachi, situated in the south-eastern extremity of sindh, known for the principal bundar or seaport known by the name of khoor alee (harbour of ali) could be used by fair-sized vessels. [16]. due to the value and location of the karachi harbour, the britishers took over karachi in 1839 and started developing the port. due to the port, the population of karachi started to rise, and in 1873 the manora breakwater was also completed, which resulted in increased trade activities at the port. people from different parts of british india started to migrate to karachi for work. the britishers constructed different organizational, residential, monumental buildings and infrastructure on the port site over the next 100 years since overtaking the port and city in 1839. the important infrastructure and buildings that were built at that time are east wharf, karachi port trust head office, berth 15–17a, native jetty bridge, mule’s mansion, boat wharf gate, manora lighthouse, st. pauls church, boat basin karachi port (see figure 2) among many. after the partition of the indian subcontinent in 1947, karachi became the capital of pakistan, and the seaport remained the backbone of pakistan’s economy and trade. new berths and terminals were constructed after the partition for the rising trade and commercial activities on the seaport. the buildings constructed by the britishers are mostly still in use, with some of them being left abandoned. the port of karachi became an attraction point and remained open for national and international tourists along with port employees until the 9/11 world trade centre tragedy in 2001. after that incident, the us invaded afghanistan, and pakistan was an ally in the process. due to security reasons, sites like seaports, airports, and dams were made restricted for the public. the security situation in pakistan is better now, and the karachi port, which has a rich history and high tourism potential, can be reopened for the public by reusing abandoned and underused buildings on site. 4. the port of karachi: field visits and data collected karachi harbour is one of the most important sites in karachi, both historically and economically. the port of karachi is known as the golden gateway to pakistan [17]. it is one of the largest ports in asia. the intangible and immaterial heritage of the site is quite vast, whereas tangible and material heritage and most of the modern port buildings are related to the british colonial time, which is located on the east wharf along with other infrastructure (see figure 3 last image of the timeline). most of the city roads and railway lines originate from harbour to establish a smooth flow of trade through the sea route (see figure 4). the population of karachi grew abruptly due to the activities and trade at the port. until 1914 the east wharf was constructed on the port, which consisted of berths, godowns, and cranes. karachi port trust building which is one of the most iconic buildings of karachi, was also built in the early 20th century by the british raj (see figure 5). 587 n. iqbal, s. h. akbar, k. v. cleempoel acta polytechnica ctu proceedings figure 3. historical timeline of karachi port. 588 vol. 38/2022 reintegration of karachi port through sustainable adaptive reuse . . . figure 4. karachi harbour 1870 map. figure 5. karachi port trust kpt building. after the partition, karachi port was still the main route for trade for both pakistan and the landlocked country afghanistan; that’s why due to the cargo ships traffic, the pakistan government added a west wharf to the port, thus extending its capacity. until 2001 the port was open for the public to visit and see the hustle and bustle of the site and the people working over there on the port. most of the people in karachi have a relationship with the port in one way or another. the urban setup of the city of karachi originates from the port. people take pride in talking about the port and the memories related to it in terms of work, travelling and trade. in the immediate vicinity of the west wharf, there is the squatter settlement in kemarhi, and this settlement has mostly families of the people working or used to work on the seaport. in the squatter settlement, you see an informal community now with congested bazaars and negligible public amenities, and green areas (see figure 6). after a change in security protocols in pakistan, the karachi seaport was also restricted, and the public or the tourist couldn’t enter most of the sites at the port. figure 7 shows the private areas in red, which figure 6. connection of the port in urban setup. figure 7. restricted, public and semi-public areas on karachi port. are not accessible by the public except the employees, semiprivate regions of orange, which are only accessible by appointment, while the areas in green can be visited by everyone (figure 7). during the interviews with some of the employees of the port of karachi, they also mentioned that the port was accessible to all and was not segregated into different zones. but it all happened after the 9/11 incident in the us. presently the situation in karachi is better now, and the port can be reintegrated into the locality more efficiently than before by reusing the abandoned or underused buildings on site as the security situation is back to normal now in karachi. 5. result and discussion karachi is the economic capital and largest city of pakistan in terms of population. the city of karachi is overly populated, with little to no recreational areas in most of the neighbourhoods. the port of karachi, which was once part of the locality, can play a vital role in the urban regeneration of the area. the 589 n. iqbal, s. h. akbar, k. v. cleempoel acta polytechnica ctu proceedings figure 8. karachi port boat wharf entry gate. abandoned/underused buildings on the site (see figures 8, 1 and 9) can be reused in a way to preserve and display the industrial heritage of the city, along with opening things up for the public to create space for tourism. the reintegration of the port and allowing the public can be planned by taking into consideration the traffic on the port; figure 10 was developed after visiting the site along with a port employee, and it shows the areas in green that have minimum traffic of the seaport and can be open up for the public. the area which is used by ships loading and unloading and parking trucks was considered high traffic area and is shown in light red in figure 10; the map also shows the buildings with heritage status under “the sindh cultural heritage (preservation) act of 1994” and infrastructure on the port site in red. abandoned industrial buildings are reused and reintegrated into the urban setting around the world, especially in western countries, which is a sustainable way to preserve the embodied energy and heritage value of the place instead of just demolishing or leaving it for decay. there are several underused and abandoned buildings on and in the vicinity of the port which can be reused in a way to preserve the heritage value at the same time taking advantage of its potential. passing the early 20th-century entry gate (figure 8) berth 15–17a at the east wharf (figure 1), which once was a storage building, and a lighthouse is lying abandoned, and it’s in the green area (figure 10) in low traffic zone. another multi-story residential building that is associated with the port is the mules mansion (figure 9) is also lying abandoned in the green zone mentioned in figure 10. one of the reasons behind the negligence of these buildings at the port is also the heritage laws, which gave them the heritage status but did not separate them or specify their category. these laws are more related to commercial, residential, or landscape sites but do not say anything regarding the areas which have industrial values [18]. figure 9. mules mansion near karachi port. figure 10. traffic on the port to show the potential in opening areas for the public in low traffic zones, image by the first author. 6. conclusion industrial heritage is a new addition to the heritage realm compared to cultural and natural heritage and is recognized separately in most of the world. but in south asian countries, including pakistan, this area of heritage is under-focused and is mostly neglected [19]. based on the international precedent literature review and the study of the seaport of karachi, there are many buildings with historic and industrial values which are a heritage of the city. these buildings have played a significant role not only in the economic development but also in the social, cultural, and urban development of karachi in the past. through the survey of the port, documenting and presenting the present condition of buildings, it becomes clear these sites have a potential for adaptive reuse and can become part of sustainable redevelopment and urban regeneration. 590 vol. 38/2022 reintegration of karachi port through sustainable adaptive reuse . . . references [1] p. a. bullen. adaptive reuse and sustainability of commercial buildings. facilities 25(1/2):20–31, 2007. https://doi.org/10.1108/02632770710716911 [2] p. a. bullen, p. e. d. love. the rhetoric of adaptive reuse or reality of demolition: views from the field. cities 27(4):215–224, 2010. https://doi.org/10.1016/j.cities.2009.12.005 [3] a. condello, s. lehmann (eds.). sustainable lina: lina bo bardi’s adaptive reuse projects. springer, schweiz, 2016. [4] b. plevoets, k. van cleempoel. adaptive reuse of the build heritage: concepts and cases of an emerging discipline. routledge, london, 2019. https://doi.org/10.4324/9781315161440 [5] b. plevoets, k. van cleempoel. adaptive reuse as a strategy towards conservation of cultural heritage: a literature review. in structural studies, repairs and maintenance of heritage architecture xii, pp. 155–164. 2011. https://doi.org/10.2495/str110131 [6] j. chen, b. judd, s. hawken. adaptive reuse of industrial heritage for cultural purposes in beijing, shanghai and chongqing. structural survey, 34(4/5):331–350, 2016. https://doi.org/10.1108/ss-11-2015-0052 [7] m. robiglio. re-usa: 20 american stories of adaptive reuse, a toolkit for post-industrial cities. jovis verlag gmbh, berlin, 2017. [8] royal australian institute of architects, department of the environment and heritage. adaptive reuse: preserving our past, building our future. department of the environment and heritage, canberra, 2004. [9] ticcih. the nizhny tagil charter for the industrial heritage, 2003. [2022-11-17]. https://hrcak.srce.hr/file/122740 [10] y. lari. the dual city karachi, during the raj. heritage foundation, karachi, 1996. http: //katalog.ub.uni-heidelberg.de/titel/9912451 [11] pakistan bureau of statistics. provisional summary results of 6th population and housing census-2017, pakistan bureau of statistics, 2017. [12] h. t. lambrick. john jacob of jacobabad. oxford university press, 1975. [13] r. lane fox. the search for alexander. 1st ed. little, brown, boston, 1980. [14] h. m. elliot, j. m. dowson. history of india, as told by its own historians: the muhammadan period, volume 5. primary source ed. edition. nabu press, 2013. [15] m. ramsay. the making of british india 1756-1858. book on demand ltd., 2014. [16] h. pottinger. travels in beloochistan and sinde: accompanied by a geographical and historical account of those countries. longman, hurst, rees, orme, and brown, 1816. [17] r. haq, b. tetley. karachi port, golden gateway to pakistan. camerapix publishers international limited, 2009. [18] n. iqbal, k. van cleempoel. adaptive reuse & regeneration as potential for industrial sites in the metropolitan cities in pakistan. in eaae-arcc international conference & 2nd vibrarch: the architect and the city, pp. 784–795. 2020. [2021-05-26]. https: //www.academia.edu/45514508/adaptive_reuse_and_ regeneration_as_potential_for_industrial_sites_ in_the_metropolitan_cities_in_pakistan [19] n. iqbal, s. h. akbar, k. van cleempoel. identification of industrial heritage and a theoretical framework for an industrial heritage inventory system in pakistan. sustainability 14(10):5797, 2022. https://doi.org/10.3390/su14105797 591 https://doi.org/10.1108/02632770710716911 https://doi.org/10.1016/j.cities.2009.12.005 https://doi.org/10.4324/9781315161440 https://doi.org/10.2495/str110131 https://doi.org/10.1108/ss-11-2015-0052 https://hrcak.srce.hr/file/122740 http://katalog.ub.uni-heidelberg.de/titel/9912451 http://katalog.ub.uni-heidelberg.de/titel/9912451 https://www.academia.edu/45514508/adaptive_reuse_and_regeneration_as_potential_for_industrial_sites_in_the_metropolitan_cities_in_pakistan https://www.academia.edu/45514508/adaptive_reuse_and_regeneration_as_potential_for_industrial_sites_in_the_metropolitan_cities_in_pakistan https://www.academia.edu/45514508/adaptive_reuse_and_regeneration_as_potential_for_industrial_sites_in_the_metropolitan_cities_in_pakistan https://www.academia.edu/45514508/adaptive_reuse_and_regeneration_as_potential_for_industrial_sites_in_the_metropolitan_cities_in_pakistan https://doi.org/10.3390/su14105797 acta polytechnica ctu proceedings 38:585–591, 2022 1 introduction 2 archival data collection and literature review 3 historical overview of karachi and its port 4 the port of karachi: field visits and data collected 5 result and discussion 6 conclusion references https://doi.org/10.14311/app.2022.33.0160 acta polytechnica ctu proceedings 33:160–167, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague a new performance test to evaluate the sulfate resistance of concrete by tensile strength measurements volkert feldrappea,∗, johannes haufeb,c, andreas ehrenberga, anya vollprachtb, thomas matscheib a building materials institute, bliersheimer str. 62, 47229 duisburg, germany b institute of building materials research, rwth aachen university, schinkelstr. 3, 52062 aachen, germany c zertplus überwachungsgesellschaft mbh, mühlenweg 11, 06749 bitterfeld-wolfen, germany ∗ corresponding author: v.feldrappe@fehs.de abstract. concrete structures without sufficient durability can be damaged by sulfates in groundwater and from surrounding rock layers. to evaluate the performance of a concrete mixture, precise and performance-oriented test methods are a must. therefore, a new a performance oriented concrete test procedure based on tensile strength measurements was developed considering experiences reported in international literature and recommendations of state-of-the-art reports. a vast parameter study with approx. 3850 tensile tests on astm briquets, 1900 flexural tensile tests on standard prisms and 2100 expansion tests on mortar flat prisms of different ages and with different storage conditions was statistically assessed. based on the results a performance-oriented test method could be defined which considers not only the chemical, but also the physical resistance of a concrete against sulfate attack. the method was verified by 23 concretes with different cements or cement fly ash combinations and additional field tests. it could clearly be demonstrated that the results represent the performance of a practical concrete in case of sulfate attack. furthermore, it leads much faster to an evaluation of the sulfate resistance compared to the most other practical oriented methods. keywords: concrete, performance test, sulfate attack, tensile strength. 1. introduction if concrete structures do not have a sufficient resistance, they can be damaged by sulfates dissolved in groundwater or incorporated in surrounding rock layers. such a sulfate attack is described as exposure class xa in the european concrete standard en 206. to ensure a sufficient concrete resistance, minimum requirements for concrete composition, such as a minimum binder content, a maximum accepted watercement ratio (w/c), accepted types of cement and additives as well as other protective measures, if necessary, are defined in en 206 together with their specific national concrete standards (e.g. din 1045-2 for germany). beside the descriptive measures the mechanisms that trigger damage because of sulfate attack have been extensively investigated in the past, e.g. in [1– 10]. a lot of test methods were developed worldwide. most of them can be used to assess the chemical resistance of cements and binders, respectively. only few, however, allow to test concrete. depending on the method, the test specimens are completely, partially or cyclically immersed in sulfate solutions. the concentration of the sulfate solution varies over a wide range, as does its temperature. in germany, only test methods are applied, which evaluate the chemical sulfate resistance of a binder. these are the socalled sva (sachverständigenausschuss) method of the dibt (deutsches institut für bautechnik), mainly used at present and the older methods by kochsteinegger [11] and wittekindt [12]. despite intensive research and optimization, all of them show considerable scattering of the results and test artifacts. these can be attributed, to extremely high sulfate concentrations of the test solution that are not related to practical applications. furthermore, the physical resistance the structural density of the concrete was deliberately neglected in these procedures. for these reasons, none of the methods has been included in german or european standardization so far [13, 14]. however, for sustainability reasons, it is a must to be able to evaluate the performance of a concrete in a precise and performance-oriented way, because new types of cements, additives and concretes with lower ecological footprints must continue to be developed in future. also, for technical or economic reasons, it is often advisable to deviate from the normative concrete specifications. moreover, there are current activities to shift the classical descriptive concept of concrete standardization to a performancebased concept. all these activities require a reliable test method that allows an unerring evaluation of the sulfate resistance of a concrete mix. 160 https://doi.org/10.14311/app.2022.33.0160 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 33/2022 performance test for sulfate attack on concrete variable unit variation binder type − cem i; cem i-sr3; cem i +fly ash; cem ii/b-s; cem iii/a testing temperature ◦c 5, 12, 20 concentration of sulfate solution mg/l 3000, 6000 type of sulfate solution − na+, mg2+ as cation cement content kg/m3 320, 360, 400 content of cement + fly ash kg/m3 270 + 90, 285 + 94, 300 + 100 equivalent water-cement-ratio w/ceq 1 − 0.45, 0.50 1 w/ceq = w/(c + k · f) with c = cement content, k = 0.4 and f = fly ash content table 1. influencing variables on the sulfate resistance and variation parameters. 2. objectives and realization the main objective of the research project was to develop a practical test method for the precise and selective determination of the sulfate resistance of concrete within an appropriate test period [15, 16]. the following three essential questions were defined to achieve the objective: 1. what are the test constraints that can be used to accelerate the damage mechanism without causing test artifacts? are the test parameters verifiable, and which damage can be recorded reproducibly and accurately? 2. is it possible to validate the results obtained with the new testing procedure with practical construction experience and field tests? 3. is there a limit value to differentiate between concretes with high and insufficient sulfate resistance? the study was based on experience gained from the currently applied methods as well as the findings of the state-of-the-art reports of dafstb (deutscher auschuss für stahlbeton) [13, 17, 18] and cen/tc 51 [14]. furthermore, both chemical resistance of the binder and physical resistance of the concrete structure were considered, since both partial resistances are important for the durability of concrete under practical conditions. therefore, parameters influencing the sulfate resistance of concrete were varied systematically at the beginning of the project. in addition, their effect on various parameters characterizing the microstructure was determined. the systematic statistical evaluation of the test results made it possible to define a test procedure based on the influencing parameters considered. this was followed by verification with another setup of approx. 25 concretes made with both binders known to have high and low sulfate resistance. at the same time, several of these concretes were stored under practical conditions at two different sites for at least one year [19]. the evaluation of all results made it possible to propose an acceptance criterion for the test procedure with which the sulfate resistance of a tested concrete can be evaluated reliably. 3. experimental setup 3.1. statistical design of experiments (doe) statistical methods of design of experiments were used intensively in order to consider a wide as possible test matrix. the influencing parameters considered in the statistical experimental design are summarized in table 1 [16]. in addition, their range of variation is also listed in the table. the statistical software minitab© was used for designing and analyzing the experiments. due to the large number of influencing parameters and the different verification levels, the experimental strategy was to create a full-factorial experimental design first and to select an optimal experimental design from it afterwards. for this purpose, individual combinations were chosen by doe software using methods of sequential optimization and taking into account terms up to the second order. the general full-factorial experimental design contained a total of 180 experiments covering all binders and a constant w/ceq at 0.45. the optimal design selected from it could be reduced to 100 experiments. based on the statistical analysis of the results, the original test design was adapted so that individual influencing parameters that did not show any significance were omitted and additional parameters that were deliberately not considered in the first step (e.g. w/ceq) were included. a total of 120 tests series were considered in the parameter study for the development of the test method [16]. 3.2. concrete production and storage the binders listed in table 1 have been investigated in detail. the four commercially available cements were in line with en 197-1, the hard coal fly ash (fa) complied with en 450-1. their strength development measured according to en 196-1 is shown in table 2. concrete mixtures with those binders and quarzitic aggregate having a maximum grain size of 8 mm were produced. the w/ceq value followed the specifications of the optimal test design. the production of the so-called fine concretes was in accordance with en 12390-2. prisms with dimensions 40 × 40 × 160 161 v. feldrappe, j. haufe, a. ehrenberg et al. acta polytechnica ctu proceedings compressive strength after 2 d 7 d 18 d 91 d n/mm2 n/mm2 n/mm2 n/mm2 (2) cem i 42.5 n 28.5 46.6 61.9 71.0 (5) cem i 42.5 n-sr3 24.5 42.5 50.3 60.8 (14) cem ii/b-s 42.5 n 22.2 41.1 61.8 73.3 (15) cem iii/a 42.5 n 18.7 40.8 61.9 73.9 (21) fa 11 24.4 40.1 51.9 65.4 1 combination of 25 wt.-% fly ash and 75 wt.-% cem i 42.5 r. table 2. strength development of cements and fly ash. figure 1. concrete test specimens acc. to astm c307-03. mm3 for testing the flexural tensile strength and the dynamic modulus of elasticity and briquet specimens according to astm c307-03 (cf. figure 1) for testing the tensile strength were produced as test specimens. all specimens were demolded after one day and then stored in saturated ca(oh)2 solution at 20 ◦c for 27 days. 3.3. execution of tests and test parameters the test started at a concrete age of 28 days. the specimens were stored in sulfate solution at 5, 12 or 20 ◦c for 181 or 273 days according to the conditions defined in the doe. the flexural and tensile strengths as well as the dynamic modulus of elasticity were determined after 119, 181 and 273 days, where applicable. all individual values and no mean values were always used to relate them to respective reference values. the relative values obtained this way can be compared directly with each other. reference values were based on corresponding parameters obtained either on samples of same age but stored in saturated ca(oh)2 solution or determined before the start of sulfate storage. furthermore, a maturity function was used for the calculation of relative bending and tensile strengths. the basis was the function described in the fib model-code [20]. it was adapted to account for the influence of supplementary cementitious materials on the strength development of concrete, as proposed by vollpracht et al. [21]. since the adapted method was originally developed for the prediction of compressive strength, its suitability was tested in advance for bending and tensile strengths [15, 16]. by using single values and not mean values, the number of results for each individual experiment of the statistical design was increased. three single results each were used for the flexural tensile strength and the dynamic modulus of elasticity and even six single values for the tensile strength. this significantly increased the statistical certainty in the evaluation of the experimental designs. 4. results and discussion 4.1. suitable test criterions the different relative test parameters were analyzed in terms of their significance for the test procedure. damage to concrete due to sulfate attack was best characterized by the tensile strength of astm briquets. it was also shown that the common procedure, which uses the strength of specimens of the same age stored in a saturated ca(oh)2 solution as a reference value, gives comparatively wide scattering results. in contrast, tensile strengths based on the adapted maturity formula of the fib model-code show 162 vol. 33/2022 performance test for sulfate attack on concrete figure 2. contour diagram of relative tensile strength ft/ftm of concretes with (2) cem i 42.5 after 182 days storage in na2so4 solution, reference: fib maturation function. figure 3. mean decrease of the rel. tensile strength of the fine concretes with w/ceq = 0.50 with a change of the sulfate concentration of the na2so4 solution from 3000 mg/l to 6000 mg/l. significantly lower test scatter. the necessary test effort is also significantly reduced. therefore, this relative tensile strength ft/ftm is the appropriate test parameter for the new test procedure. 4.2. definition of the test procedure the evaluation of the parameter study provides statistically secured information on the significant influencing parameters and their contribution to the expected relative tensile strength ft/ftm. as an example, the evaluation is visualized in a contour diagram (figure 2) for fine concretes with portland cement (2) cem i 42.5 n after 182 days storage in na2so4 solution. it shows the effect of testing temperature and concentration of sulfate solution on the expected relative tensile strength. as a result, the lowest relative tensile strength can be expected if the concrete is tested at 5 ◦c and 6000 mg/l so 2− 4 concentration. the figure also contains the results of six additional tests, carried out to verify the results of the statistical evaluation. each single significant influencing parameter was analyzed in terms of its effect on accelerating the testing and its potential tendency towards test artifacts. for example, figure 3 illustrates the accelerating effect when the sulfate concentration was increased. an increase of sulfate concentration from 3000 mg/l to 6000 mg/l led to a decrease of relative tensile strengths of comparable fine concretes between 0.10 and 0.15 (10 to 15 %) at a test age of 273 days, if damage occurred. at the same time, no excessive gypsum formation was observed in the pore 163 v. feldrappe, j. haufe, a. ehrenberg et al. acta polytechnica ctu proceedings concrete composition • fine concrete with max. aggregate size of 8 mm • w/ceq ratio 10 % higher than planned for concrete formulation • binder content as planned for concrete formulation test specimens • briquets acc. to astm c307-03 made from one concrete batch storage • 28 d in saturated ca(oh)2 at 20 ◦c test conditions • test solution: na2so4 • so4 2− concentration: 6000 mg/l • storage temperature: 5 ◦c • test duration: 273 days test parameters • relative tensile strength ft/ftm • ft: measured tensile strength at testing • ftm: tensile strength at testing, calculated by maturity function in accordance to fib model code • visual assessment (cracks, spalling, etc.) table 3. definition of the performance-oriented, test method for evaluating the sulfate resistance of concrete. figure 4. storage site (1) in a german gypsum mine. space of the specimens. consequently, the increase of sulfate concentration to 6000 mg/l accelerates the test and does not cause artefacts, especially for less sulfate-resistant concretes. as a result of this evaluation the test procedure could be described. its main features are summarized in table 3. 4.3. storage under practical conditions a large number of fine concretes as well as normal concretes the latter fulfilled the minimum requirements of din 1045-2 for the composition of exposure class xa2 were stored under practical conditions at two sites. figure 4 shows the exposure site in a gypsum mine. the laboratory results obtained with the new method will be verified by long-term tests with this real sulfate attack. many samples were stored over a period of more than one year and inspected in regular intervals. no concrete deterioration was detected during this period as it is shown in figure 5. it illustrates the relative dynamic modulus of elasticity of fine concretes stored in the gypsum mine. as expected, the storage time was too short to induce damage even on concrete that is known to possess insufficient sulfate resistance. the trials under practical conditions will be continued for some years. figure 5. rel. tensile strength of concretes after more than one year of storage under practical conditions. 4.4. proposal of an acceptance criterion after the definition of a new test procedure another task of the research project was to develop a proposal for an acceptance criterion for a reliable evaluation of the concrete sulfate resistance. for this purpose, 23 additional concretes with different cements and cement-fly ash combinations of different manufacturers were tested with the new test method. the relative tensile strength was determined after 119, 182 and 273 days. figure 6 illustrates the relative tensile strengths of the 23 concretes after 182 and 273 days of sulfate storage. after 119 days of storage no reliable statement can be made on the sulfate resistance. the first differences between concretes with different binders did occur after 182 days of storage. however, a definitive differentiation between concretes with known high or low sulfate resistance was not yet possible. some con164 vol. 33/2022 performance test for sulfate attack on concrete figure 6. relative tensile strength ft/ftm of concrete with 23 different binders after 182 and 273 days. cretes produced with portland cement without sr property showed residual tensile strengths comparable to those of some slag cement concretes, for which a high sulfate resistance can be expected in the light of experience. after 273 days (9 months) of sulfate storage, it was possible to make a clear distinction regarding the sulfate resistance of concrete. concretes with blast furnace cements cem iii/a or cem iii/b and portland cement/fly ash combinations obviously showed a high sulfate resistance with relative tensile strengths of 0.97 to 1.02. in contrast, concretes with portland cement including those with sr property and portland composite cements showed low residual tensile strengths and significant damage. the damage of concretes with cem i-sr cements was confirmed by further tests. c3a was determined by xray diffraction for all these cements. presumably, the c3a content is high enough to trigger a damaging ettringite reaction with the sulfate ions in the cement stone structure. considering the discussed results, it can be stated that a concrete has a sufficient sulfate resistance if its relative tensile strength is not lower than 0.70 (70 %) after 273 days of sulfate storage. furthermore, two stop criteria can also be defined for the test after 182 days. firstly, the test can already be stopped at this time if the relative tensile strength ft/ftm is lower than 0.70 (70 %), since the acceptance criterion will definitely not be reached even after 273 days. such a concrete will have a low sulphate resistance. secondly, the test can also be stopped at this point if the relative tensile strength ft/ftm is higher than 0.85 (85 %), because the acceptance criterion defined for a test age of 273 days is then also fulfilled with certainty. such a concrete will have a high sulfate resistance. 5. conclusions the focus of the research project was the development of a concrete test procedure based on tensile strength tests, which allows a clear differentiation between concretes with and without high sulfate resistance. in the development of the test method, the recommendations of the state-of-the-art report [13] 165 v. feldrappe, j. haufe, a. ehrenberg et al. acta polytechnica ctu proceedings were taken into account and the tensile strength was determined as the best test parameter [16]. based on the statistical evaluation of approx. 3850 tensile tests on astm briquets, 1900 flexural tensile tests on standard prisms and 2100 elongation tests on mortar flat prisms of different ages and after different pre-storage conditions, a new performance-oriented test method could be defined which was verified by 23 concretes with different cements or cement fly ash combinations. the concluding assessment of the research project is that the newly developed performance-oriented test method • can represent the performance of a practical concrete in case of sulfate attack, • considers not only the chemical, but also the physical resistance of a concrete against sulfate attack, • leads much faster to an evaluation of the sulfate resistance compared to common methods (current regulation sva test: testing at 3000 mg so4 2−/l and 5 ◦c for 2 years), • represents the damage mechanism more realistically than most conventional test methods and therefore leads to the avoidance of test artifacts, and • could also be carried out as a "binder test" if a fixed concrete formulation is used (e.g. the limit formulation of din 1045-2 for exposure class xa2). acknowledgements the igf project no. 19251 n of the research association vdeh-gesellschaft zur förderung der eisenforschung mbh was funded via the aif as part of the program for the funding of joint industrial research (igf) by the federal ministry of economics and energy based on a resolution of the german bundestag. the authors would like to express their thanks for the support. references [1] f. bellmann, b. möser, j. stark. influence of sulfate solution concentration on the formation of gypsum in sulfate resistance test specimen. cement and concrete research 36(2):358-63, 2006. https: //doi.org/10.1016/j.cemconres.2005.04.006. [2] h.-g. smolczyk, g. blunk. zum verhalten von sehr jungem beton gegen sulfatwässer beton-information 12:1-9, 1972. [3] h. passow. hochofenzement und portlandzement in meerwasser und salzhaltigen wässern verlag der tondindustrie-zeitung gmbh (berlin, germany), 1915. [4] r. grün, h. beckmann. untersuchungen über das verhalten von erhärtetem hochofenzement gegen sulfatlösungen und salzarmes wasser. angewandte chemie 45(48):739-43, 1932. https://doi.org/10.1002/ange.19320454802. [5] j. skalny, j. marchand, i. odler. sulfate attack on concrete spon press (london, uk), 2002. [6] w. müllauer. mechanismen des sulfatangriffs auf beton phasenneubildungen und expansionsdrücke in mörteln unter na2so4 belastung (münchen, germany), 2013. [7] k. lipus. new approach for testing sr cements external sulphate attack menéndez, baroghel-bouny (basel: springer international publishing) pp. 107-120, 2020. [8] f. aguayo, o. j. funez, t. drimalas, et al. an alternative method to evaluate the sulphate resistance of cementitious binders. external sulphate attack field aspects and lab tests. rilem bookseries p. 93-105, 2020. https://doi.org/10.1007/978-3-030-20331-3_8. [9] n. b. tiburzi, j. garcia, t. drimalas, et al. sulfate resistance of portland-limestone cement concrete systems: linking laboratory and field performances. construction and building materials 250, 2020. https: //doi.org/10.1016/j.conbuildmat.2020.118750. [10] f. herding. mikrostrukturelle schädigung von beton durch sulfatangriff unter unterschiedlichen bedingungen, master thesis rwth aachen university (aachen, germany), 2020. [11] a. koch, h. steinegger. ein schnellprüfverfahren für zemente auf ihr verhalten bei sulfatangriff zement-kalk-gips, 13:317-324, 1960. [12] w. wittekindt. sulfatbeständige zemente und ihre prüfung zement-kalk-gips, 13:565-572, 1960. [13] din-fachbericht cen/tr 15697:2008-10, cement performance testing for sulfate resistance state of the art report, 2008. [14] r. breitenbücher, d. heinz, k. lipus, et al. sulfatangriff auf beton sachstandbericht deutscher ausschuss für stahlbeton beuth-verlag (berlin, germany), 2006. [15] j. haufe, a. vollpracht, t. matschei. development of a sulfate resistance performance test for concrete by tensile strength measurements: determination of test conditions. crystals 11(8), 2021. https://doi.org/10.3390/cryst11081001. [16] a. ehrenberg, a. vollpracht. entwicklung eines performanceprüfverfahrens zur bestimmung des sulfatwiderstands von beton, schlussbericht zum igf-vorhaben nr. 19251 n fehs institut für baustoff-forschung (duisburg, germany), 2020. [17] h.-m. ludwig, m. müller, p. nobst. rgänzende untersuchungen zum dafstbverbundforschungsvorhaben vertiefte untersuchungen zum sulfatwiderstand von beton, abschlussbericht zum dafstb-forschungsvorhaben v 469/2 bauhausuniversität weimar (weimar, germany), 2012. [18] d. heinz, l. urbonas. ergänzende untersuchungen zum dafstb-verbundforschungsvorhaben vertiefte untersuchungen zum sulfatwiderstand von beton, abschlussbericht zum dafstb-forschungsvorhaben v 469/1, technische universität münchen (münchen, germany), 2012. 166 https://doi.org/10.1016/j.cemconres.2005.04.006 https://doi.org/10.1002/ange.19320454802 https://doi.org/10.1007/978-3-030-20331-3_8 https://doi.org/10.1016/j.conbuildmat.2020.118750 https://doi.org/10.3390/cryst11081001 vol. 33/2022 performance test for sulfate attack on concrete [19] j. haufe, a. vollpracht, t. matschei. performance test for sulfate resistance of concrete by tensile strength measurements: determination of test criteria. crystals 11(9), 2021. https://doi.org/10.3390/cryst11091018. [20] fib 2013 fib model code for concrete structures 2010 ernst & sohn (berlin, germany), 2010. [21] a. vollpracht, m. soutsos, f. kanavaris. strength development of ggbs and fly ash concretes and applicability of fib model code’s maturity function a critical review. construction and building materials 162:830-46, 2018. https: //doi.org/10.1016/j.conbuildmat.2017.12.054. 167 https://doi.org/10.3390/cryst11091018 https://doi.org/10.1016/j.conbuildmat.2017.12.054 https://doi.org/10.14311/app.2022.33.0638 acta polytechnica ctu proceedings 33:638–643, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague rational and sustainable procedure in the design of bridges using pre-stressed concrete beams a theoretical practical method abel noé xochicale cortés bufer & co. méxico, sa de cv., privada 9 b sur #5117, prados agua azul, 72430 puebla, pue., mexico correspondence: anxochicale@yahoo.com abstract. currently there is not a direct procedure for the design of this bridge system, the procedure is still based on trial and error, there are no studies that give us both the optimum number of beams and other characteristics such as the correct thickness of the slab or its adequate strength. keeping in mind the concept of optimization, which supports the idea of sustainability and environmentally friendly structures, we apply this concept to the design of pre-stressed concrete girder bridges, which are an integral part of the road network in most countries of the world. knowing also that concrete is the most produced material by man and that without it, it would be difficult to understand our current world, due to its own characteristics that make it unique when it comes to sustainability, however, its production requires energy and material consumption, that is why an optimal design of a superstructure will lead to the minimum consumption of materials obtaining a more friendly design with nature. it is a global necessity to obtain and deliver concrete structures that meet the conditions of sustainability. keywords: bridges, girders, sustainability. 1. introduction it is true that after many years in which have been used pre-stressed concrete girders supporting a concrete slab, has not been able to establish the suitable type of girder according to the span, nor to know the maximum performance of each type of girder, giving in this way many solutions to a given problem, but none adopted in a generalized way to provide an appropriate and economical solution to the proposed array of girders, in other words, the most sustainable or optimum solution. it is shown a direct method to determine the number of girders more suitable according to the type to use. 2. review to the technical literature 2.1. relationship between optimization and sustainability recently has emerged the concept of the sustainable design of structures, which by applying it to prestressed concrete girder bridges, could be synthesized in the use of 1. minimum number of girders, 2. optimum thickness of slab over girders, 3. minimum number of columns in the case of piers 4. minimum number of spans in the bridge, which is achieved using the girders to its maximum performance. in 1971, f. jacques, [1] conducted a study motivated by the need to have greater spans with the existing girder sections and to have the most suitable section for local conditions. the study gave the correct importance to the employment of segmented girders spliced in field with the application of a posttensioning, however due to the little studies on this process is opted to find another solution. although we had enough information about the types of girder sections in use, the question remained is, what it is the cheapest and practical section to be used?. it was considered the following criteria; • practicability, realistic concrete strengths 6000 psi (42 mpa) consistently, perhaps up to 7000 psi (50 mpa), realistic size and weight limits for precast girders. • safety, the use of a single unit precast section appears desirable since it can span existing roadway without shoring. this eliminates inherent shoring hazards. • esthetics, it appears desirable to eliminate the stubby end blocks such as on the old aasho sections. also, it seems desirable to eliminate the possibility of texture blemishes that can result from field splices. • economy, the cost of field splicing, in all cases, appears to be an added cost that can be justified only if a single piece section cannot be hauled overroad to the site. in terms of number of girders and their spacing, it is clear that a deep girder is capable of carrying a greater payload moment when compare to a shallow girder for a given span, this allows that greater 638 https://doi.org/10.14311/app.2022.33.0638 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 33/2022 bridges design using pre-stressed concrete beams figure 1. optimum girder spacing cost curves for seven stone concrete girders. height beams are spaced at greater distance than the shallower girders which results in fewer girders for a width of bridge, but this is reflected in a thicker deck slab with more reinforcement, on the other hand, may require greater volume of earthwork for the clearance require, several sections and parameters were defined like, span and girder spacing, as well as, the thickness of the slab deck. two concrete strength in girders were defined, 6000 y 7000 psi. live load aasho hs20-44, intermediate diaphragms at 12 m maximum and 20 cm thick have been proposed and using a computer program, were obtained graphs that relate the span, type of girder and cost per unit of surface, as we see in the figure 1. as a first conclusion of this study we can say that for a certain span, optimum spacing between girders provides more economic bridge, in this case, a spacing equal to 1.25 times the height of the girder. it is noted the concept of balanced section according to the area of lower flange compared to the top flange area. 2.2. advances in cross section optimization in the report by rabbat and russell [2], they wanted to determine optimal designs for common girders and to analyze the potentialities of standardizing these sections as well as make recommendations for practical and economic designs. to do this we studied structural efficiency and cost-effectiveness of the best existing designs and study the impact to make some modifications to aashto girders, it was proposed to study the following sections shown in the figure 2 making the following assumptions • design conforms to aashto specifications. • live loads consists of hs 20-44 loading. • girders are simply supported • a typical interior girder is considered concrete deck is cast in place and acts compositely with the girder. deck formwork is supported on the girder. in calculations of the composite section properties, the transformed area of strands is neglected. concrete compressive strength of the deck is constant and equal to 5000 psi (28 mpa) at 28 days. strands are grade 270 (1,900 mpa) stress relieved with 1/2 in ( 1.3 cm) diameter and have an idealized trilinear stress-strain curve. total pre-stressing looses are constant and equal to 45,000 psi (310 mpa). initial or long term camber or sag do not govern design, because aashto specifications do not specify deflections limits for concrete bridges. as a result of studies in different sections of girders show that the aashto sections are not the most economical for the characteristics of materials, the live load applied and to the design conditions prevailing in the country, and suggests to modify the above sections to reduce cost in the work, also suggests the use of concrete in beams from 35 to 50 mpa to increase the until span about 15%. also suggests that the modified section "bulb-t" is most suitable for spans from 24 to 42 meters, for greater spans it is recommended spliced sections. intermediate diaphragms are not needed, end diaphragms are sufficient. 2.3. maximum structural performance of the cpci girder in 1993, lounis and cohn [3], defined first some of the parameters involved in the design of a bridge with girders, such as: a) live load, b)the type of girder, c) the strength of the concrete slab and the strength of concrete in the girder and 639 abel noé xochicale cortés acta polytechnica ctu proceedings figure 2. modified girder analyzed (aashto, bulb t, colorado and washington). d)existence or not of continuity at intermediate supports and with the use of a computer program defined for spans and common widths, the type of girder, the minimum number for girders and the maximum spacing for girders, maximizing the performance of these elements. 3. a practical-theoretical approach based on the study of the structures previously shown and using basic concepts of optimization, has been related numerically the different parameters involved in the design of this bridge system, defining certain parameters as is mentioned in previous reports, has been obtained an optimization constant involved in the geometry of the superstructure, this constant is an indicator which depends of the type and number of girders, so that, thickness and concrete strength of the slab over girders. at the same time that this constant produces an optimal superstructure, implicitly define the more sustainable superstructure. it has been established the following equation: η × k0 × h0! 0 dy l × b → {k1 → 0.02} (1) where • η . . . represents he number of girders in the superstructure. • k0 . . . is a constant with a value 1.0 approximately for girders with web width of 20 cm. • l . . . represents the span. • b . . . is the width of the superstructure. • k1 . . . is the optimization constant with a value of 0.02 approximately. • h0! 0 dy . . . represents the girder depth in the superstructure. it has been established certain constraints; • concrete strength in girders of f ′ c = 40, 45 and 50 mpa • two concrete strength are considered for the slab 25 and 30 mpa 640 vol. 33/2022 bridges design using pre-stressed concrete beams figure 3. maximum feasible girder spacing, optimum girder spacing and optimum number of girders for single, two and three-span continuous cpci girder bridges. z. lounis and m. cohn. • live loading is that defined by sct t3-s2-r4 (710 kn), which is very similar to the o.h.b.d. live loading. • design specifications is that one given by aashto. • width web is approximately 20 cm. this is summarized in the figure 4, which relates the live loads and the type of girder, obtaining the optimal ratio of the sum of the height of beams on the system, divided by the product of the span and total width of the system. the kxoc factor should be close to the value of 0.02. any system which produce a kxocfactor close to this value, will produce an optimal solution. a very remarkable feature of this factor, is that it is bidirectional, because it can be applied to both directions of the axis of the bearings. design process: • according to the live load and the girder type selected, choose the appropiate constant kxoc. • get the product of the constant kxoc times the span and the width of the superstructure. • choose the more appropriate girder according to the its depth htr. • obtain the number of girders dividing the first product by girder depth, closing it to the next higher integer, obtaining this way, the real value for kxoc. • accommodating the number of girders obtained above, we get the spacing between girders which must be within the acceptable maximum values. • the final design of the girder will be according to the initial conditions. applying the formulation for the following cases, we obtain a-factor kxoc equal or very close to 0.02, so reaching the optimal solution. this formulation perfectly agrees with the results given in figure 4 for the condition of single spans girder bridges. for continuous spans we can take into account only 0.85 of the length between intermediate supports. for the case of the reconstruction of the walnut lane bridge, we can notice that we have a kxoc = 0.0202 (see [4]). it is necessary to mention another determining factor in the cross section of the superstructure called kbr, which can be defined as the result of dividing the sum of the depth of the beams in the superstructure by the width of the same superstructure, as explained below (see xochicale et al. [5]): 0.4 < η × k0 × h0! 0 dy l × b < 0.8 (2) 4. application of the concept of sustainability to this bridge type in summary, a sustainable design is one that is obtained at this time without compromising the ability of future generations to solve their own needs. from this definition, an engineering and sustainable project such as a bridge project based on concrete beams, is one that is conceived, designed, built, operated on, is maintained and eventually is put out of service so that these activities require as little as possible in terms of energy and material consumption in support of the community. under this concept of sustainability, what is sought are engineering projects 641 abel noé xochicale cortés acta polytechnica ctu proceedings figure 4. graphic definition of kxoc, according to live loading and girder type. figure 5. definition of sustainability triple bottom line. that have the greatest positive impact at the intersection between the interests of the people they serve, the planet in its short and long term scale and the utility for anyone involved in the life of that project as defined by the concept of triple bottom line. a fully sustainable bridge is one that strives to serve the population for which it was made, and that its long-term cost is totally environmentally friendly. in this sense, concrete plays an important role in providing solutions to future challenges by contributing to the construction of sustainable infrastructure. with a growing population, concrete’s crucial contribution is required as it supports communities in building their infrastructure to connect cities and transport people and goods. its benefits and sustainable characteristics make concrete a unique material for a more environmentally friendly future. the above according to n. subramanian [6]. 5. application to a current project in mexico applying these concepts to a current project to be built in mexico, in the state of michoacán, we can see the advantage of applying the concepts of optimization and sustainability to this type of structure. this is a road distributor which consists of a series of 13 spans of 34.0 meters, in which 80 aashto type v beams are used, and 19 spans of 29.0 meters, in which 99 aashto type iv beams are used, meaning the following report in terms of material consumption, each span contains 5 beams spaced at 150 cm. and is reduced to 4 beams spaced at 200 cm. in its modified semi-optimized version in which it has a kxoc closer to 0.02. the final version is the optimized one and shows the lowest consumption of materials whose kxoc factor is even lower than 0.02. it is noted that with the application of the optimization criterion it is possible to reduce quantities of materials by 20% and with it the total costs of the work, having this way a structure that fulfills the aspects of sustainability. 6. materials savings in general terms it can be seen that for the mentioned project it is possible to save up to 980 m3 of concrete and up to 214 tons of steel, this represents to avoid up to 340 tons of co2 in the production of the cement, knowing in addition that at the moment up to 75% of the steel is recycled in the construction, it is immediately noticeable that an optimal design leads to a sustainable design since it means savings in materials and the reduction of pollutants, in addition to 642 vol. 33/2022 bridges design using pre-stressed concrete beams figure 6. study case to show the kxoc factor and the relation between the number of girders and the consumption of materials. other expenses as they are the transfer of equipment, personnel and other materials. 7. conclusions the shown method has given very successful solutions and it is recommended in the development of new projects. once defined the type and the number of beams by use, spacing is defined at the same time, respective calculations done can proved the convenience of using each type of girder that fits its height to the closest value to the factor kxoc = 0.02. in addition, as can also be seen, this value leads to economic and rationally designed solutions including sustainability features, among which can be mentioned, the reduction in the consumption of natural resources, the reduction of pollutants emitted into the environment and the possibility of recycling the materials used. references [1] f. j. jacques. study of long span prestressed concrete bridge girders. pci journal 16(2):24-42, 1971. https://doi.org/10.15554/pcij.03011971.24.42. [2] b. g. rabbat, h. g. russell. optimized sections for precast prestressed bridge girders. pci journal 27(4):88-106, 1982. https://doi.org/10.15554/pcij.07011982.88.106. [3] z. lounis, m. z. cohn. optimization of precast prestressed concrete bridge girder systems. pci journal 38(4):60-78, 1993. https://doi.org/10.15554/pcij.07011993.60.78. [4] c. c. zollman, f. depman, j. nagle, et al. a 40-year saga building and rebuilding of philadelphia’s walnut lane memorial bridge. pci journal 37(4):66-82, 1992. https://doi.org/10.15554/pcij.07011992.66.82. [5] a. n. xochicale, reg. no. 03-2017-021011304600-1 optimizacion de super-estructuras y aplicacion de trabes de sección i de concreto presforzado en puentes carreteros de grandes claros para sistemas de losa sobre trabes, 2017. [6] n. subramanian. sustainability challenges and solutions. the indian concrete journal, 2007. https://www.researchgate.net/profile/subramani an-narayanan-2/publication/290228957_sustainabi lity_-_challenges_and_solutions/links/61b4f e5463bbd9324286718a/sustainability-challenges-a nd-solutions.pdf. 643 https://doi.org/10.15554/pcij.03011971.24.42 https://doi.org/10.15554/pcij.07011982.88.106 https://doi.org/10.15554/pcij.07011993.60.78 https://doi.org/10.15554/pcij.07011992.66.82 https://www.researchgate.net/profile/subramanian-narayanan-2/publication/290228957_sustainability_-_challenges_and_solutions/links/61b4fe5463bbd9324286718a/sustainability-challenges-and-solutions.pdf acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.35.0054 acta polytechnica ctu proceedings 35:54–60, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague comprehensive verification of the behavior of the continuous welded rail on the bridge jiří vendela,∗, otto plášeka, jan valehracha, tomáš říhaa, václav machb a brno university of technology, faculty of civil engineering, veveří 331/95, 602 00 brno, czech republic b independent researcher, bílovice, czech republic ∗ corresponding author: xsvendelj@vutbr.cz abstract. the aim of this paper is to verify the real behavior of the continuous welded rail on the bridge with a longer expansion length than allowed by czech national regulations. special attention is focused on the rate of interaction of the continuous welded rail and the bridge, which decisively affects the stress state in the continuous welded rail when the temperature changes. in order to improve the result and increase the accuracy of observation, two measuring methods (geodetic and strain gauge) were chosen while recording the temperature conditions of individual components of the system were simultaneously recorded. keywords: continuous welded rail, longitudinal resistance, thermal expansivity. 1. introduction when designing new bridge structures with a continuous welded rail, it is very appropriate to avoid a solution with expansion devices (or rail joints) in the track for several reasons. this is primarily a benefit of passenger comfort in the case of exclusion crossing of these problem areas, a significant reduction of dynamic effects in transition zone and reduction of maintenance costs. especially when replacing the permanent way on current bridge structures and in adjacent sections of a track is also the need to take into account the financial benefit of establishing a continuous welded rail in comparision with the variant with expansion devices whose acquisition costs are severalfold higher than the costs of a solution with an uninterrupted running edge of the rails. on the other hand the elimination of expansion devices in the bridge zone involve increasing of stress in terms of the combined response of the structure and track dealing with the issue in the design. from the designer’s point of view, however, the current standard provisions may appear as insufficient, especially with regard to the variability of the permanent way system and bridge structures and it happens that certainty is taken into account compromise precautions in the calculations. in aspect of infrastructure serviceability of the continuous welded rail is considered the most important the spacial track stability. its uses on bridges has drawback, therefore it is necessary to take into account especially the span of bridge structures and loads that affect the interacting system. this work solves especially the first part of the problem, which is continuous welded rail and its role in the interaction system with a bridge. it focused particularly on the range of its dependence on a bridge structure movements and phenomena that occur when the state of the whole system changes. the design of the bridge structure and its characteristics deals only marginally, but tries to describe its behavior in detail, especially when temperature conditions changes. a number of research projects focus on determining the combined response of the bridge structure and continuous welded rail from individual types of additional stresses in the rails, which are: • stress from thermal changes in rails, • stress from thermal changes in bridge structure, • stress from traction and braking forces, • stress from classified vertical traffic loads. 2. methodology 2.1. basic assumptions for a theoretical model of a cwr and bridge when calculating the force effects in a track that is only affected by a load from a temperature changes, is based on knowledge of hooke’s law and physical law of thermal expansion material, see equation 1. nx = ea(du dx − α∆t) (1) the system of horizontal springs simplifies the problem of the rigidity of the connection between bridge and rails (see figure 1). the system is divided into individual beams, for which the basic differential equation were defined. the beginnings of the coordinate systems are always on the left sides of the beams, except beam no. 1, which starts on the right side to maintain continuity computational model. differential equations for beams marked with numbers 1, 2, 3, 4 in figure 1 can be written as: 54 https://doi.org/10.14311/app.2022.35.0054 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 35/2022 example of an article with a long title figure 1. a simple girder with a fixed bearing on one side of the bridge. −eiaiu ′′ i + kiui = 0, i = 1, 3, −e2a2u ′′ 2 + k2(u2 − u4) = 0, −e4a4u ′′ 4 + k4(u4 − u2) = 0. (2) and forces in beams are calculated according to equation 1 as: nx = eiai(u ′ i − α∆t). (3) if we want to solve a system of differential equations, it is necessary to determine the boundary conditions. these are compiled based on knowledge of the behavior of the real system, where with growing distance from the ends of the bridge, the influence of the continuous welded rail by the bridge structure decreases. that is, the displacements at a sufficient distance from the bridge are zero and also the displacements and the stresses at the junction of the two beams are the same. then the general solution of the system of differential equations 2 can be written as: ui(x) = bieλix + cie−λix, i = 1, 3, u2(x) = b2eλ2x + c2e−λ2x + α4∆t4x. (4) the exponential term λ takes into account the nonlinear dependence of the longitudinal resistance on the mutual displacement of the track owing to the bridge structure and is calculated according to the following relation: λ2 i = ki u0eiai (5) a graphical representation of this nonlinear phenomenon can be seen in the figure 2. uic code 774-3[1] specifies the value of the displacement u0 at the interface of the longitudinal linear and shear resistance and provides at least a basic overview of when approximately occurs shear behavior between the bridge and the continuous welded rail (cwr) when loaded by external forces: • ballasted-deck bridges: displacement at the interface of longitudinal linear and shear resistance: u0 = 0, 5 mm for the resistance of the rail to sliding relative to the sleeper (frozen ballast), u0 = 2 mm for the resistance of the sleeper in the ballast. current values of the resistance in the plastic zone: k = 12 kn.m−1 resistance of sleeper in ballast (unloaded track), moderate maintenance, k = 20 kn.m−1 resistance of sleeper in ballast (unloaded track), good maintenance, k = 60 kn.m−1 resistance of loaded track or track with frozen ballast. • unballasted bridges: displacement at the interface of longitudinal linear and shear resistance: u0 = 0, 5 mm for the resistance of the rail due to the upper surface of the structure. current values of the resistance in the plastic zone: k = 40 kn.m−1 resistance of the rail due to the upper surface of the structure (unloaded track), k = 60 kn.m−1 resistance of the rail due to the upper surface of the structure (loaded track). in laboratory conditions, a number of research projects have demonstrated the validity of the scope values track resistance on bridge structures. but as other authors prove on experimental measurements directly on bridges, these recommendations may not always be generally valid and each new combination of parameters affecting the interaction cwr and the bridge can be determined by direct measurements that the degree of interaction can deviate from the submitted standards. therefore, even this work tries to set values that will capture the bahavior of a particular interacting track and bridge system by experimental research. 3. experimental measurements 3.1. bridge and cwr parameters bridge due to the fact that these are three independent structures whose central longitudinal joints are covered with a metal plate always welded to only one of them, so that they can move independently, only the monitored construction nk1 with the following parameters that are important to describe the behavior of the whole system will be presented below: • marking: nk1, • description: plate steel girder, • type: continuous with 5 fields, • lenght: 80,3 m, • width: 9,5 m, 55 j. vendel, o. plášek, j. valehrach et al. acta polytechnica ctu proceedings lo ng itu di na l sh ea r f or ce [ n ] displacement of the rail u [m] (3) (2) (4) (1) u0 u0 figure 2. variation of longitudinal shear force with longitudinal track displacenlellt for one track. (1) resistance of the rail in sleeper (loaded track without ballast); (2)resistance of sleeper in ballast (loaded track); (3) resistance of the rail in sleeper (unloaded track without ballast); (4) resistance of sleeper in bal1ast (unloaded track). • span: 79,6 m, • lenght: 80,3 m, • number of tracks: 2. continuous welded rail there are three different tracks on the bridge, of which two are double-track and one single track (see figure 3). cwr is set up in all tracks on the bridge. the length of the supporting structure was adjusted to meet the requirements of regulation [2] for uic 60 rails. however, it is located on the nk1 structure track with rails 49 e1 without any special modifications to the railway superstructure. permanent way in this track is in the following composition in the entire length of the bridge and in the adjoining sections: • rails: 49 e1, • concrete sleepers: b 91s/2, • elastic rail fasteners: w 14, • sleepers spacing: 600 mm, • deep trackbed. 3.2. phases of measurement the whole experiment was divided into two phases, which took place independently of each other: • phase i: geodetic measurements of the position of the bridge structure and continuous welded rail based on geodetic monitoring of bridge structure displacement and continuous welded rail depending on temperature changes. • phase ii: tensometric measurements of the stress state of the continuous welded rail based on monitoring the stress state of continuous welded rail by strain gauges depending on temperature changes. 3.3. phase i: geodetic measurements of the position of the bridge structure and continuous welded rail the method of geodetic monitoring with simultaneous measurement of air, rail and bridge temperature was chosen to monitoring the movement of the bridge structure and the continuous welded rail. the following was monitored on the bridge structure in phase i: • displacement of rail strings depending on rail temperature, • displacement of the bridge load-bearing structure nk1 depending on its temperature. the first step was to select the location of the measuring points, marked on both rail strings a total of 20 measuring profiles (20 points on the left and right rail string, numbered 1 to 20 against the direction of stationing). for monitoring displacements of the bridge structure 8 points were fitted (marked m1 to m8 points m1 and m8 were located on abutments, other points on the bridge deck in bridge piers areas). 56 vol. 35/2022 example of an article with a long title figure 3. view of the track on the bridge in the direction from fixed bearing. for monitoring the expansion behavior of the bridge structure and the permanent way depending on the temperature changes were marked observed points in selected cross-section profiles of rail strings whose positional changes in the direction of the track axis were measured in stages, together with the monitoring of other factors (especially the temperature of the rails and the bridge structure). their stabilization was accomplished a mild hole φ 1 mm punched out on the outer unmoved edge of the rail heads. these points were numbered and marked in color on the rail web. applies to both rail strings. 3.4. phase ii: tensometric measurements of the stress state of the continuous welded rail the aim of phase ii was to supply the geodetic monitoring of bridge and track displacements with knowledge in terms of the stress state of the continuous welded rail. during seven continuous measuring cycles, when one cycle always lasted for 3-4 days, data were collected from the installed resistance strain gauges and temperature sensors. recording period of continuously measured quantities was 1 second. in phase ii, data were collected from installed resistance strain gauges and temperature sensors about: • relative deformation of rail strings depending on temperature change, • temperature of the bridge structure and continuous welded rail. from the data obtained from the individual phases, it was necessary to approximate the set of measured data by the given equation so that the approximation function was best matched to the measured data. using force of the matlab program, the parametric space was searched, discretized and seeks the optimum of the function on it. it was necessary to determine the size of the error that occurred during approximation. this was done by calculating the residue, whose sum was minimized to estimate the parameter of the regression function. the residue calculation was used: ei = yi − ŷi (6) and to estimate the regression function parameter by minimizing the residual sum using the following relationship: s2 e = n∑ i=1 (yi − ŷi)2 (7) where yi are particular displacement values determined by measurement, ŷi are the displacement values obtained when searching for the minimum of the function on the parametric space of an unknown quantity. 4. results and disscusion 4.1. determination of the equivalent coefficient of thermal expansion of the bridge construction as no data were known on the stiffness of the substructure, friction in moving bearings, nor other possible effects that could also affect the longitudinal expansion bridge structure, therefore the whole phenomenon was expressed by an equivalent factor thermal expansion αm (similar to [3]), which will be determined on the basis of data recorded in phase i. all data (longitudinal displacements) obtained by experimental measurements during nine stages (e0 e8) were used to approximate the finding coefficient solution αm. the data were interpolated with a previously known approximation function: um = lmαm∆tm (8) the graph of this function is a straight line. to solve this linear approximation mathematicalstatistical least squares method (lsm) was chosen. 57 j. vendel, o. plášek, j. valehrach et al. acta polytechnica ctu proceedings figure 4. residual sums of control functions with variable αi m on the searched parametric space. figure 5. the resulting value of αm as a minimum of control functions. in general, it was a matter of approximating the set of measured data by the given equation so that the function matched the measured data as best as possible (see figure 4). the resulting value of αm is shown on the figure 5 as the average of the mean square errors of the functions of the individual stages over all iterated αi m. the resulting coefficient of thermal longitudinal expansion of the bridge structure, based on geodetic measurements and mathematical linear approximations using least square method, was determined by the value: αm = 9,66 · 10−6 k−1 . looking into recent experimental research (for example experimental measurements on the znojmo viaduct or on the bridge in kolín) it can be stated that for these three steel structures with a rail bed the bridge structures are comparable and very similar in terms of the determined equivalent coefficient of thermal expansion of the bridge structure (znojmo – αm = 9, 7 · 10−6 k−1, kolín – αm = 8, 5 a 8, 9 · 10−6 k−1). figure 6. residual sums of control functions with variable ki ip on the searched parametric space. figure 7. the resulting kip value from phase i as a minimum of control functions. 4.2. determination of the longitudinal resistance of the track on the bridge based on the principles described in chapter 2.1, approximation was performed on experimental data. in this case, it is a general linear regression using general polynomials defined by differential equations 4. for solutions in this case, too, the lsm was chosen, which helped with sufficient accuracy determine the finding values. in accordance with the requirements [4] the influence of the longitudinal shear resistance ki p according to the figure 2 was taken into account. the approximation was further carried out by the above equations 6 and 7, it means that after the search of the parametric space, the resulting optimized function reached the lowest residual sum. the area where the minimum mean square error of the control equations was expected is shown by a black bar in figure 6. the resulting value of longitudinal shear resistance of the track kip is shown in figure 7 as the average of the mean square errors of the functions of individual stages over all iterated ki ip. the resulting longitudinal resistance of the track 58 vol. 35/2022 example of an article with a long title figure 8. residual sums of control functions with variable ki ip across all recordsets. on the bridge without loading by rail traffic, based on geodetic measurements of the position of the rail strings and mathematical general linear approximation using least square method, was determined by the value: ki = 4,5 · 103 n.m−1 . 4.3. longitudinal resistance cwr on the bridge according to tensometric measurements this chapter will verify the conclusions from chapter 4.2, where the value of the longitudinal resistance cwr was determined on the bridge. the following evaluation was performed on the based of strain gauges measurement of relative deformation of rail string and measurement temperature of the bridge construction and track. a total of 1, 442, 033 values were recorded in 7 measurement cycles from each strain gauge and the same number of temperature sensors. the number of values corresponds to total measurement length in seconds. for a purpose the same method for the evaluation was applied to as a reliable comparison of the results data from geodetic measurements, it means by minimizing the residual sum of deviations measured and approximated by lsm with the validity of equations 6 and 7. the course of residual sums of control functions with the variable ki ip across all recordsets are seen in the figure 8. the approximation functions are the equations of the individual axial forces. parametric space in which the solution of the longitudinal shear resistance value was searched, was limited as follows: • n〈1; 1442025〉 is the number of records, • ki ip〈1; 10000〉 is the range of the vector of the searched longitudinal shear resistance of the track, • i〈1; 14〉 is the number of monitored strain gauges. figure 9. the resulting value of ki ip from phase ii as a minimum of control functions. in this space, based on tensometric measurements of the relative deformation of the left rail string and temperature measurements of the bridge structure and the track, it was found out that there was a good agreement with the results obtained by geodetic measurements. the right rail string is assumed to behave symmetrically in terms of tension. therefore with a degree of simplification, based on the assumption of symmetry, is the resulting value double of the longitudinal shear resistance found out on the left rail string. by tensometric measurement of the left rail string and mathematical general linear approximation using lsm, was found out the value of longitudinal shear resistance of the track on the bridge without load by rail traffic of the size: ki = 4,6 · 103 n.m−1 . 5. conclusions it is apparent that the resulting values from both phases are in area of very low values of longitudinal shear resistance of the track. in other words, the tension in the rails due to thermal expansion of bridge structure is much less affected than might be expected. this is evident by the fact that the track on the bridge, with an expansion length of more than 20 m longer than permitted by national regulation [2] in the relevant part xii, does not show, more than ten years of operation, anomalies in the behavior of continuous welded rail and there are no defects on permanent way of a greater extent than anywhere else on the line. acknowledgements project: interakce koleje a mostů s velkými dilatačními délkami; hs12257021210001 in cooperation sždc, s.o. and brno university in 2013 – 2015. specific research: sledování bezstykové koleje na mostě; fast-j-14-2528 in 2016. 59 j. vendel, o. plášek, j. valehrach et al. acta polytechnica ctu proceedings references [1] uic code 774-3r. track/bridge interaction: recommendations for calculations. 2nd edition. paris – france: international union of railways, 2001. 70 p., isbn 2-7461-0257-9. [2] sždc s3. železniční svršek – regulation. effective from 1. 10. 2008 as amended by dmendment no. 4 (effective from 1. 3. 2021). praha: sždc, s.o., 423 p., 2008. [3] l. frýba. dynamika železničních mostů. praha: academia, 1992. 328 s., isbn 80-200-0262-6. [4] čsn en 1991-2 (73 6203). eurocode 1: actions on structures – part 2: traffic loads on bridges. prague: czech committee for standardization, 152 p., 2005. 60 acta polytechnica ctu proceedings 35:54–60, 2022 1 introduction 2 methodology 2.1 basic assumptions for a theoretical model of a cwr and bridge 3 experimental measurements 3.1 bridge and cwr parameters 3.2 phases of measurement 3.3 phase i: geodetic measurements of the position of the bridge structure and continuous welded rail 3.4 phase ii: tensometric measurements of the stress state of the continuous welded rail 4 results and disscusion 4.1 determination of the equivalent coefficient of thermal expansion of the bridge construction 4.2 determination of the longitudinal resistance of the track on the bridge 4.3 longitudinal resistance cwr on the bridge according to tensometric measurements 5 conclusions acknowledgements references https://doi.org/10.14311/app.2022.33.0624 acta polytechnica ctu proceedings 33:624–630, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague reduction of dissolved oxygen in minimizing corrosion mary grace ventanillaa, jason ongpenga,∗, nishida takahirob, keiyu kawaaic a de la salle university, department of civil engineering, 2401 taft avenue, 0922 manila, philippines b port and airport research institute, nagase 3-1-1 yokosuka-shi, 239-0826 kanagawa, japan c ehime university, bunkyo-cho 3 matsuyama-shi emime 790-0826, japan ∗ corresponding author: jason.ongpeng@dlsu.edu.ph abstract. concrete structures are susceptible to corrosion especially when exposed to marine environment. in order to minimize the corrosion in reinforced concrete, reduction of dissolved oxygen (do) in mixing water is recommended. the do is the measurement of the amount of free oxygen that is dissolved in water which is proportional to the corrosion rate of steel bars inside concrete. in this paper, the use of industrial waste/by-products, agricultural waste, natural minerals, and green inhibitors as additive for cement in reducing the do level of the mixing water was explored. candidate materials from different types of agricultural waste, industrial waste, natural minerals, and green inhibitors. the percentage difference of do were computed for all types of materials as ash or powder or extract in room temperature. this is the percentage difference of do level in distilled water and the solution mixed with the candidate material having a mass ratio of 3 : 2 and/or 3 : 1. results showed that more than 90% reduction of do level were achieved when mixed with ginger extract, ginger powder, aloe vera extract, ginger pulp, and rice hull ash. future experimental studies using the candidate materials producing reinforced concrete specimens with high reduction of do level in mixing water is recommended. keywords: corrosion, dissolved oxygen, waste. 1. introduction concrete is considered as most extensively used construction material globally and each component has contributed to an environmental impact [1]. it is made up of aggregates (coarse and fine) and paste (cement and water). concrete industry is one of the largest consumers of natural resources, therefore attaining reliable, vigorous, sustainable and economical concrete products has been proposed. cost of concrete products has increased over the years resulting to increase in the shortage of aggregates and worsens situation. use of industrial and agricultural byproducts have gained interests as a potential replacement to concrete manufacturing process [2] since they are rich in silica as well as alumina which are suited as supplementary cementitious material and superior reactivity [3]. through the years, there is scarcity of natural resources. to address such issues, substitution to concretes components could be an answer to environmental impacts, however, substitute materials should impart comparable or enhanced attributes compared to the natural concrete [2]. several factors affect the durability of reinforced concrete structures such as change in temperature that could lead to cracks due to shrinkage, amount of moisture that could permeate concrete, physical factors (strength, resistance to wear and tear, and abrasion), chemical factors like aggressive agents that could interfere with the cement paste and biological factors (organisms) that could damage concrete. one way of achieving durable concrete is to study corrosion which can be defined as "destruction of metal by either chemical, electrochemical or electrolytic reaction within its environment" according to american concrete institute. it is a global problem that deteriorates the durability of concrete structures [5]. also, corrosion of steel in concrete is one of the main reasons for premature failure where in marine environment, chloride ingress is the probable cause for breaking the passivating layer and the onset of extreme corrosion [6]. most marine structures are susceptible to corrosion as it is located near marine environment and may trigger chloride intrusion [7]. nowadays, it is necessary to inspect and alleviate corrosion to extend serviceability of rc structures and is accepted worldwide [4]. three parameters that greatly affect corrosion of reinforcing bars in concrete are shown in figure 1. these are oxygen, ph level, and the permeability of concrete. in this research, reduction of oxygen through dissolved oxygen (do) level in mixing water is the main focus to prevent corrosion inside concrete. additives such as industrial waste/by-products, agricultural waste, natural minerals, and green in624 https://doi.org/10.14311/app.2022.33.0624 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 33/2022 reduction of dissolved oxygen in minimizing corrosion figure 1. graph showing relationship of embodied carbon for cements by clinker content, source [4] hibitors are considered as candidate materials in reducing the do level of mixing water that could affect concretes resistivity to corrosion. the selection of these materials is based on the result of related studies utilizing them as replacement to cement or aggregate that enhances concrete’s durability. industrial wastes are known to have high ph level which is being measured under varying temperatures to enhance its reactivity [8]. since these wastes no longer have industrial application, converting them to construction materials could improve its properties. in addition, agricultural waste is also found to contain minerals that could improve various properties of concrete. natural minerals as admixture could enhance resistance of concrete to chloride penetration [9]. plant extracts are also considered as these were proven to be good corrosion inhibitors [10]. shown in table 1 is the process of accumulation of the candidate materials from previous researchers. numerous literatures are available considering corrosion in reinforced concrete structures. however, researches associated with oxygen reducing materials haven’t been thoroughly studied. using keyword searches "oxygen", "permeability", "concrete" and "corrosion" resulted to 34 published journals. it is evident that researches concerning on the effects of do in concrete corrosion has an average of only two publications per year and in which approximately one publication per year under cement and concrete research. this study introduces the use of waste materials, plants and natural minerals to contribute significantly in the performance of reinforced concrete, particularly, in the reduction of oxygen that can inhibit corrosion on the steel bars. 2. apparatus and materials 2.1. dissolved oxygen meter sufficient amount of free oxygen at the initial curing stage is critical in the formation of the passive layer protecting the reinforcing steel bars [26]. there is a relevant connection when aggregates in concrete have open porosity as it could increase the permeability of oxygen [27]. corrosion rate of steel is higher on areas with high oxygen contents. measuring the oxygen level of water at a certain location can be used to foresee the performance of different alloys in the same environment [28]. oxygen permeability in reinforced concrete is high using deformed bars which makes it more susceptible to corrosion than plain bars since cracks begin at the ribs of the rebars and eventually becomes surface cracks [29]. do is the measure of the amount of free oxygen that is dissolved in water. factors such as atmospheric pressure, temperature and salinity determine the amount of oxygen that can be dissolved physically in water. rate of corrosion process on the reinforcing steel bars in concrete can be controlled by do on pore solution, thus, reduced do could enhance corrosion resistance [30]. 2.2. candidate materials materials were chosen based on its effect on the performance of concrete. related studies as presented in table 1, utilized the use of these candidate materials as an additive or replacement to either cement and aggregates. table 2 to 4 summarize the candidate 625 m. g. ventanilla, j. ongpeng, n. takahiro, k. kawaai acta polytechnica ctu proceedings corrosion inhibitors candidate materials process of accumulation source wastes industrial waste blast furnace slag acquired from industry as it is a by-product of steel manufacturing [11] bottom ash acquired from industry as it is a by-product of coal power plants [12] fly ash acquired from industry as it is a by-product of coal power plants [13] granite / marble dust acquired from industry producing marble / granite as tiles [14] waste glass powder crushing, sieving using ball mill [15] wood waste ash incineration, combustion [16, 17] agricultural waste coconut husk ash burning under controlled temperature or incineration, pyrolysis [18] rice husk ash burning under controlled temperature or incineration, through annealing furnace [19] corn cob ash burning under controlled temperature [20] sugarcane bagasse ash burning under controlled temperature or incineration, combustion, acquired from sugar mill [13, 17] seashell cleaning, drying, crushing / pulverizing, furnace [21] natural volcanic ash readily available on areas near volcanoes [22] minerals natural zeolite mining [23] plant extracts water hyacinth extraction, drying, grinding [24] aloe extraction [10] ginger extraction, drying [25] table 1. process of accumulation adapted from previous researches. agricultural wastes specimen ash powder 3 : 2 3 : 1 3 : 2 3 : 1 bagasse ash bagasse ash aw-ba-2 aw-ba-1 coconut husk ash coconut husk ash aw-cha-2 aw-cha-1 corn cob ash corn cob ash aw-cca-2 aw-cca-1 rice hull ash rice hull ash aw-rha-2 aw-rha-1 shells oyster shell aw-ss-a-os-2 aw-ss-a-os-1 aw-ss-p-os-2 aw-ss-p-os-1 yellow cockles aw-ss-a-yc-2 aw-ss-a-yc-1 aw-ss-p-yc-2 aw-ss-p-yc-1 strombus sicad aw-ss-a-sc-2 aw-ss-a-sc-1 aw-ss-p-sc-2 aw-ss-p-sc-1 brown scallop aw-ss-a-bs-2 aw-ss-a-bs-1 aw-ss-p-bs-2 aw-ss-p-bs-1 clamrose big aw-ss-a-cb-2 aw-ss-a-cb-1 aw-ss-p-cb-2 aw-ss-p-cb-1 granular ark aw-ss-a-ga-2 aw-ss-a-ga-1 aw-ss-p-ga-2 aw-ss-p-ga-1 distant scallop aw-ss-a-ds-2 aw-ss-a-ds-1 aw-ss-p-ds-2 aw-ss-p-ds-1 green mussel aw-ss-a-gm-2 aw-ss-a-gm-1 aw-ss-p-gm-2 aw-ss-p-gm-1 table 2. agricultural wastes as candidate materials in reducing do level. 626 vol. 33/2022 reduction of dissolved oxygen in minimizing corrosion industrial wastes specimen ash powder 3 : 2 3 : 1 3 : 2 3 : 1 blast furnace slag blast furnace slag iw-bfs-2 iw-bfas-1 bottom ash bottom ash iw-ba-2 iw-ba-1 fly ash fly ash iw-fa-2 iw-fa-1 marble / granite waste dust marble / granite waste dust iw-wwa-2 iw-wwa-1 table 3. industrial waste as candidate materials in reducing do level. natural mineral specimen ash powder 3 : 2 3 : 1 3 : 2 3 : 1 volcanic pumice volcanic pumice nm-vp-2 nm-vp-1 natural zeolite natural zeolite nm-nz-2 nm-nz-1 natural mineral plants specimen extract powder 3 : 2 3 : 1 3 : 2 3 : 1 aloe vera aloe vera p-av-2 p-av-1 ginger ginger extract 1 p-ge1-2 p-ge1-1 ginger extract 2 p-ge2-2 p-ge2-1 ginger pulp p-gp-2 p-gp-1 ginger powder p-gpw-2 p-gpw-1 water hyacinth water hyacinth p-wh-2 p-wh-1 table 4. natural and plants as candidate materials in reducing do level. materials based on their ratio with water and type whether powder, ash or extract. a reducibility effect in the do was found using 3 : 1 and 3 : 2 mass ratio (water : specimen ratio), therefore, it was adapted and was considered that it may have a possibility in obtaining the same effect when combined with concrete [31]. candidate materials that were burned, powdered and extracted have composition similar to cement and aggregates that may replace them or be added to enhance concrete properties. usually, those materials that were burned and became ash are associated with cement properties, powdered materials are associated with aggregates while extracted materials may serve as coating to metals to reduce corrosion. in this study, candidate materials are used as an additive to mixing water in reducing do level. figure 2 shows the do meter used and the sample candidate materials. 3. methodology candidate materials were gathered from different sources such as industrial plants, farms, quarries and plantation. industrial wastes were readily available since these are by-products being disposed, whereas, agricultural wastes were burned to form ash which can produce properties similar to cement. moreover, minerals are also readily available as they are part of natural resources while plants, on the other side, were gathered from plantation and were extracted. 3.1. mixing procedure 300ml of distilled water was used to determine the baseline value of do to identify the percentage dif627 m. g. ventanilla, j. ongpeng, n. takahiro, k. kawaai acta polytechnica ctu proceedings figure 2. do meter and sample candidate materials. figure 3. candidate materials for do testing. ference of do when mixed with the candidate materials. do of water was found out to be 5.08ppm. do of the mixture were measured by placing 100grams and 200grams of materials into a bottle with 300ml of distilled water (3 : 1 and 3 : 2 ratio), stirred until thoroughly mixed to form a solution. at room temperature (average 26 degrees celsius), do level was measured using do meter. except for the case of garlic powder and rha, the ratio used was 10 : 1 since these materials were observed to be low density, thus, bagasse ash and wood waste ash were tested only in 3 : 1 ratio. figure 3 shows how the specimens were prepared prior to do testing. 3.2. percentage difference of do the equation used to determine the do is shown below. %dodiff = dowater − dosolution dowater · 100 (1) where: • dowater = do level of distilled water at room temperature • dosolution = do level of mixed solution at room temperature 3.3. results and discussion shown in table 5 are the materials with highest do level reduction represented by the percentage difference resulted to a reduction of more than 90% which include ginger extract, ginger powder, aloe vera, ginger pulp, and rice hull ash. materials do diff. (ppm or mg/l) % do diff p-ge-1 4.99 98.23 p-ge-2 4.98 98.03 p-gpw 4.97 97.83 p-av-1 4.96 97.64 p-av-2 4.91 96.65 p-gp-1 4.81 94.69 aw-rha 4.77 93.90 table 5. top percentage do difference of the candidate materials. 628 vol. 33/2022 reduction of dissolved oxygen in minimizing corrosion 4. conclusion corrosion inside concrete is very complex with a lot of factors contributing to it. in this study, the objective is to reduce the do level of mixing water that can reduce the corrosion of steel rebars inside concrete. identification of candidate materials from agricultural waste, industrial waste, minerals, and plants were made in the region. among the candidate materials, it can be observed that plants such as ginger and aloe extracts have the highest capacity to reduce oxygen in a solution due to its inherent properties and constituents that may inhibit corrosion followed by agricultural and industrial wastes. future studies are recommended to test the candidate materials as an additive to mixing water in concrete by conducting permeability test and evaluating through scanning electron microscope sem, to identify its efficiency as part of the binding material in the reduction of corrosion. references [1] b. bhardwaj, p. kumar. waste foundry sand in concrete: a review. construction and building materials 156:661-74, 2017. https: //doi.org/10.1016/j.conbuildmat.2017.09.010. 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[31] t. nishida, n. otsuki, a. saito, et al., editors. influence of reducibility environment around steel bars on steel corrosion in concrete. proceedings of the seminar workshop on the utilization of waste materials e, 2017. 630 https://doi.org/10.1016/j.conbuildmat.2013.02.069 https://doi.org/10.1016/j.jobe.2018.01.002 https://doi.org/10.1016/j.jclepro.2018.12.299 https://doi.org/10.1016/j.cemconcomp.2015.10.022 https://doi.org/10.1016/j.conbuildmat.2017.01.075 https://doi.org/10.1016/s0008-8846(01)00482-3 https://doi.org/10.1016/j.cemconres.2019.105822 acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.38.0649 acta polytechnica ctu proceedings 38:649–655, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague catalogue of construction products containing secondary raw materials from different industries and municipal waste teraza pavlůa, jan peštaa,b,∗, martin volfa, antonín lupíšeka a university center for energy efficient buildings of technical university in prague, třinecká 1024, 273 43 buštěhrad, czech republic b university of chemical technology in prague, faculty of environmental technology, department of sustainability and product ecology, jankovcova 23, 170 00 prague 7, czech ∗ corresponding author: jan.pesta@cvut.cz abstract. the building industry consumes a large amount of primary raw materials and also contributes significantly to the production of waste. applying circular principles in this field to reduce resource consumption and waste production has been investigated in several projects considering the reuse or recycling of construction and demolition waste. however, consumption of primary raw materials can also be reduced by re-targetting waste from different industries and municipal waste to produce new construction products. thus, opportunities for the recycling of industrial and municipal waste were investigated in this project. the main output is the catalogue, which provides an overview of products with recycled content and secondary materials with the potential to be used in the construction industry such as blast furnace slag, ash, and energy by-products. also, it contains a list of valid requirements for the use of recycled materials under specific conditions of the czech republic. in addition, examples of good practice are presented to break the existing behavioral barriers to the use of secondary raw materials in the czech construction industry. this contribution summarizes the findings in the field of industrial and municipal waste recycling and its further use as secondary raw materials in the construction industry. keywords: waste, circular economy, secondary raw materials, primary raw materials, recycled materials, recycled content. 1. introduction sustainable buildings are based on sustainable resources, which are used for construction products. however, how sustainable are the sources of primary raw materials for the building industry? the amount of materials consumed in the global economy is more than ten times higher than the amount of reused or recycled materials [1]. the difference between these two flows is called the circularity gap and is increasing annually. this is significantly affected due to the construction industry. on the one hand, the construction industry consumes more than 30 % of domestic material consumption in the czech republic [2]. on the other hand, construction and demolition waste represents almost one-third of european waste [3]. in an effort to increase circularity in the european economy, the european commission published the circular economy action plan [4] as one of the supporting strategies for the green deal [5]. both of these documents described construction and building as a key sector for reducing the impact on climate change and exploiting resources. meanwhile, the main regulation for high rates of recycling materials and their utilization in the construction industry is defined by regulation (eu) no. 305/2011 of the european parliament and the council, which focuses on sustainable utilization of natural resources [6]. the development of this effort under the conditions of the czech republic is mainly defined by the waste management plan, which defines the minimum recycling rate of construction waste as 70 % [7]. to support the recycling of demolition and construction waste, many activities were carried out. one of them was the publication of a catalogue of construction products containing recycled materials from construction and demolition waste [8]. however, the circularity of the whole economy can also be improved by re-targetting waste from different sources to industries, where it can be recycled or reused. to improve the re-targeting of resources from different industries and sectors, the second part of the previously mentioned catalog will be published at the end of 2021 [9]. the catalogue of construction products containing secondary raw materials from different industries and municipal waste was prepared as a collection of information from experts, producers, and civil engineers and was discussed at round tables in cooperation with czech authorities. in addition, the prioritization of key waste flows with a high potential for re-targetting to the construction industry was performed considering national statistics of secondary raw materials 649 https://doi.org/10.14311/app.2022.38.0649 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en t. pavlů, j. pešta, m. volf, a. lupíšek acta polytechnica ctu proceedings figure 1. production of secondary raw materials in the czech republic [million tons] [10]. production. this catalogue contributes by summarizing possible uses, risks, and barriers for recycling of waste in the building industry. it will be used by architects, designers, civil engineers, construction contractors, and public and private investors who want to improve the circularity of their projects or find new sources for their constructions and products. in the catalogue, an overview of products with recycled content is presented together with the list of valid requirements on the utilization of recycled materials listed in standards and legislation. moreover, the catalogue provides examples of good practices to decrease suspicions about the use of recycled materials. this contribution summarizes the findings on the possible use of secondary raw materials from different industries and municipal waste in the construction industry. 2. methods the preparation of this catalogue followed after publication of the previous catalogue of construction products containing recycled materials from construction and demolition waste. similarly, the new part is based on the evaluation of national statistics describing waste production. in addition, information about the possible utilization of recycled materials in the construction industry was summarized after discussions and roundtables with experts. the group of experts included producers of industrial waste, civil engineers, and producers of construction products. the outputs of this group were consulted with state officials representing perspective focused on the protection of nature and public health. the discussions were organized in collaboration with the ministry of industry and trade and the czech standardization agency. based on evaluation of national statistics and summarization of possible utilization following key waste flows with the highest potential to be recycled were descripted in this catalogue: slag, ashes and fly ashes, energy gypsum, plastic materials, glass, paper, composite packaging, tires and rubber, wood, and metals. prioritizing key waste flows was followed by a broad literature study of available standards, existing legislative documents, and regulation. the possible use of recycled materials was declared in the catalogue on the basis of communication with companies and good examples from construction practice. the next stage of preparation of catalogue was collecting of feedback from experts, companies, and state officials. finally, the results of the catalog will be published as a freely available pdf document on the web http://www.recyklujmestavby.cz/, which was developed for the first part of the catalogue [9]. 3. results the final version of the catalog will be published at the end of november 2021. however, a summary of the main outcomes is provided in the following chapters. 3.1. production of secondary raw materials secondary raw material can be characterized as a product (including certified products) or recovered waste, which stopped being waste after meeting the recovery regulations. in 2018, the production of secondary raw materials was 22.2 million tuns [10]. this production increased in comparison with the production in 2017. in comparison to the extraction of primary materials in the czech republic, it represents approximately one third of the excavated amount of construction materials (more than 64 million tuns) [12]. based on this, the high potential for saving of primary resources can be assumed. the largest amount of secondary materials is produced from energy processes and reached (including by-products) more than 10.2 million tun in the year 2018. the overview of the flows with the biggest amount is presented in figure 1. the flows 650 http://www.recyklujmestavby.cz/ vol. 38/2022 catalogue of construction products containing secondary raw . . . type of secondary raw material from energy processes production [t] percentage [%] fly ash from combustion of coal 7 322 239 60.0 slag 1 287 574 10.5 fly ash from fluid combustion of coal or a coal with biomass 1 452 175 11.9 fly ash from fluid combustion of biomass 118 783 1.0 fly ash from comb. of biomass (except fluidized bed boiler) 5 707 0.0 sda product 70 380 0.6 energy gypsum 1 952 843 16.0 table 1. production of by products from energy sources in 2017 [11]. specification the main risks to reuse and recycling possibilities of utilization slag aggregates – grinding and sorting of crystal slag disintegration of slag in contact with water. backfilling, landscaping. as a layer under construction foundations. aggregates for concrete mixtures. prefabricated construction concrete elements. granulated slag – blast furnace slag produced after cooling with water disintegration of granulated slag is slow process. after milling used in the cement production. hydraulic road binder. boiler slag – the solid waste from combustion of solid fuels different composition. admixture for brick production to optimize properties of ceramic material. table 2. possible use of slag in buildings. of secondary materials are more specified in table 1. the most important energy source for the by-products is coal combustion, in which fly ash is produced [11]. 3.2. possible use of secondary raw materials 3.2.1. slag slag with potential to be used in buildings is produced as a byproduct during steel production in thermal processes and combustion. because of these sources, slag is available in a large amount and with a low price. on the other hand, the variability of sources is connected to variability of properties. for example, slag aggregates can disintegrate when in contact with water. based on this, use of slag in practice is limited by suspicion of practitioners. however, this main barrier can be overcome with a proper description of the original source of the slag. other barriers are described in table 2. 3.2.2. ashes and fly ash several types of fly ashes are described in table 3. fly ashes are used as an admixture for cement production. also, it is produced in high amounts and at low cost as an alternative for the cement industry. however, the main barrier is suspicion to this material, which is produced as a byproduct. however, fly ash must comply with the regulations for the declaration of ecotoxicological safety. 3.2.3. energy gypsum energy gypsum is produced as a by-product in the process of wet scrubbing of flue gas using a solution with limestone. the recycling or reuse of gypsum materials and products is a highly recommended way of waste management because landfilling of gypsum materials can be allowed only under specific conditions, which will reduce the potential contact with organic matter leading to the production of hydrogen sulfide. the possible use of energy gypsum is described in table 4. however, the production of gypsum in the future can be reduced due to the closure of thermal power plants. 3.2.4. plastic materials in table 5, the specific use of recycled plastic materials is described. the main barrier to recycling plastic waste is insufficient plastic waste classification. municipal waste can contain tens of types of platic materials and for their use sorting capacity is needed. moreover, the content of toxic chemical substances in plastic materials should be tested in case this material is used on place, where, for example, there is a high probability of contact with human skin. 3.2.5. glass the glass is a material, which can be recycled multiple times. moreover, using waste glass as an admixture leads to a reduction in the melting temperature in 651 t. pavlů, j. pešta, m. volf, a. lupíšek acta polytechnica ctu proceedings specification the main risks to reuse and recycling possibilities of utilization fly ash – produced in process with high-temperature combustion the utilization is limited by regulation describing technical properties for future use. pozzolanic properties can be reduced during storing in space with high humidity. high difference of properties based on specific sources. replacement of cement. aggregates or filler for concrete and prefabricated concrete elements. production of aerated autoclaved concrete. admixture for brick production. mineral fibers. thermal insulation products for buildings. fluid ash – produced in process with fluid incineration high difference of properties based on specific sources. mortar production. aggregates or filler for concrete. hydraulic road binder. thermal insulation products for buildings. production of aerated autoclaved concrete. admixture for brick production. mineral fibers. table 3. possible use of ash and fly ash in buildings. specification the main risks to reuse and recycling possibilities of utilization energy gypsum the possible use in gypsum plasterboard is limited by technical regulations that define the gypsum parameters. gypsum production. gypsum binders. cement production. gypsum plasterborard. gypsum fibreboard. table 4. possible use of energy gypsum in buildings. specification the main risks to reuse and recycling possibilities of utilization recycled polyvinyl chloride (pvc) insufficient sorting of pvc backfilling. landscaping. as a layer under construction foundations. aggregates for concrete mixtures. recycled plastic materials from municipal waste variable composition of municipal waste. high amount of types of plastic materials. products and accessories for garden. covering of terrace, pavement, fences. recycled polyester insufficient sorting of pe geotextiles recycled polystyrene (eps) insufficient sorting of eps recycled eps recycled plastic insulation for cables insufficient sorting of pvc floor covering recycled materials from automobile interiors inseparable mixture of plastic materials and textile materials. acoustic insulation table 5. possible use of plastic materials in buildings. 652 vol. 38/2022 catalogue of construction products containing secondary raw . . . specification the main risks to reuse and recycling possibilities of utilization flat glass – from deconstruction or as waste from automobile industry an insufficiently sorted batch of glass can be devalued by impurities and different types of glass. window fillings. recycled glass insufficient sorting glass fibers – thermal and acoustic insulations, foam glass. lightweight aggregates for concrete. glass fibers as reinforcement. table 6. possible use of recycled glass in buildings. specification the main risks to reuse and recycling possibilities of utilization recycled paper insufficient sorting blown cellulose fibers as thermal and acoustic insulation. fibers as reinforcement. table 7. possible use of paper in buildings. specification the main risks to reuse and recycling possibilities of utilization recycled composite packaging disintegration of slag in contact with water. boards as construction elements, roof covering. table 8. possible use of recycled composite packaging in buildings. specification the main risks to reuse and recycling possibilities of utilization recycled tires and recycled rubber the health risk should be secured according to the regulations for the use of products containing recycled rubber. coverings of playgrounds. floors for industry, coverings for balconies and terraces. insulation against vibration. table 9. possible use of tires in buildings. specification the main risks to reuse and recycling possibilities of utilization furniture wood possible contamination with dangerous substances (glues, paint) woodfibre boards and insulations wood from wood industry production of pellets for energy recovery. wood from municipal waste wood can be damaged by biological processes or infested by pests. composting. energy recovery. table 10. possible use of recycled composite packaging in buildings. 653 t. pavlů, j. pešta, m. volf, a. lupíšek acta polytechnica ctu proceedings specification the main risks to reuse and recycling possibilities of utilization metal packaging, metal parts of machines and tools, and metal waste from the manufacturing industry insufficient sorting. production of new metal parts and products. table 11. possible use of recycled metals packaging in buildings. batches for the production of glass from primary resources. other risks and possible use are described in table 6. 3.2.6. paper paper material can be contaminated by impurities from municipal waste. however, in case of proper sorting, the paper material has several possibilities for utilization, which are described in table 7. 3.2.7. composite packaging material recycling of composite packaging is limited due to the number of materials used and the technology of their connection. thus, the composite packaging is often just shredded. the possible utilization of this material is described in table 8. 3.2.8. tires and rubbers in table 9, the possible use of tires and rubbers is described. the main barrier to their recycling is the risk that the recycled material contains dangerous substances. therefore, these materials must be tested and fulfil the regulations for their use. 3.2.9. wood taking into account the effort to circularity, wood should be reused or recycled, but, in some cases, incineration with energy recovery can be a more economical way of waste management. however, possible use for recycling or reusing wood waste is described in table 10. the main barrier to the reuse of wood is possible contamination by pests or dangerous substances. 3.2.10. metals the sorting of metals is supported by high value of scrap, which can be easily sold in facility for waste collection. in addition, metals have high potential to be recycled after sorting due to the high cost of the primary resource and the sufficient infrastructure for recycling. the possible use of metal is briefly described in table 11. 4. conclusion the catalogue contributes to increasing the circularity of materials in the czech economy by identifying key material flows with high potential to be used in the building industry. moreover, the catalogue presents the products with recycled content and examples of good practices. furthermore, the possible use of secondary raw materials is described according to the european and czech legislation. for each of the key material flows, risks and barriers in the recycling process were described. one of the main barriers is the suspicion against the use of products with recycled content. nevertheless, the products containing recycled materials must fulfil at least the same regulations on the market as the products made from primary resources. further research is planned on possibilities of how to re-target waste materials and increase circularity. however, research should focus not only on retargetting of materials but also on improving materials, as inputs to the economy, to be recyclable in the future. acknowledgements this work was supported from the grant of specific university research – grant no a1_ftop_2021_003. references [1] the circularity gap summary 2021. [2022-03-03]. https://drive.google.com/file/d/1_dqhx5ztajz_ 4nyp72gjijej_yqchnxw/view [2] zpráva o životním prostředí české republiky 2015. [2022-03-03]. https: //www.mzp.cz/c1257458002f0dc7/cz/news_161129_ zprava_o_zp/$file/zprava_zp_2015_291116.pdf [3] strategy for a sustainable built environment. [2022-0303]. https://www.europarl.europa.eu/legislativetrain/theme-a-european-green-deal/filestrategy-for-a-sustainable-built-environment [4] a new circular economy action plan for a cleaner and more competitive europe. [2022-03-03]. https://eur-lex.europa.eu/legal-content/en/txt/ ?qid=1583933814386&uri=com:2020:98:fin [5] the european green deal. [2022-03-03]. https://eur-lex.europa.eu/legal-content/en/txt/ ?qid=1588580774040&uri=celex:52019dc0640 [6] regulation (eu) no 305/2011 of the european parliament and of the council of 9 march 2011 laying down harmonised conditions for the marketing of construction products and repealing council directive 89/106/eec. [2022-03-03]. https://eur-lex.europa.eu/legal-content/en/txt/ html/?uri=celex:32011r0305&from=cs 654 https://drive.google.com/file/d/1_dqhx5ztajz_4nyp72gjijej_yqchnxw/view https://drive.google.com/file/d/1_dqhx5ztajz_4nyp72gjijej_yqchnxw/view https://www.mzp.cz/c1257458002f0dc7/cz/news_161129_zprava_o_zp/$file/zprava_zp_2015_291116.pdf https://www.mzp.cz/c1257458002f0dc7/cz/news_161129_zprava_o_zp/$file/zprava_zp_2015_291116.pdf https://www.mzp.cz/c1257458002f0dc7/cz/news_161129_zprava_o_zp/$file/zprava_zp_2015_291116.pdf https://www.europarl.europa.eu/legislative-train/theme-a-european-green-deal/file-strategy-for-a-sustainable-built-environment https://www.europarl.europa.eu/legislative-train/theme-a-european-green-deal/file-strategy-for-a-sustainable-built-environment https://www.europarl.europa.eu/legislative-train/theme-a-european-green-deal/file-strategy-for-a-sustainable-built-environment https://eur-lex.europa.eu/legal-content/en/txt/?qid=1583933814386&uri=com:2020:98:fin https://eur-lex.europa.eu/legal-content/en/txt/?qid=1583933814386&uri=com:2020:98:fin https://eur-lex.europa.eu/legal-content/en/txt/?qid=1588580774040&uri=celex:52019dc0640 https://eur-lex.europa.eu/legal-content/en/txt/?qid=1588580774040&uri=celex:52019dc0640 https://eur-lex.europa.eu/legal-content/en/txt/html/?uri=celex:32011r0305&from=cs https://eur-lex.europa.eu/legal-content/en/txt/html/?uri=celex:32011r0305&from=cs vol. 38/2022 catalogue of construction products containing secondary raw . . . [7] plán odpadového hospodářství české republiky pro období 2015–2024, 2014. [2022-03-03]. https://www.mzp.cz/c1257458002f0dc7/cz/poh_cr_ prislusne_dokumenty/%24file/oodp-poh_cr_20152024_2035_vlada-20220511.pdf [8] t. pavlů, j. pešta, m. volf, a. lupíšek. catalogue of construction products with recycled content from construction and demolition waste. iop conference series: earth and environmental science 290(1):012025, 2019. https://doi.org/10.1088/1755-1315/290/1/012025 [9] katalog výrobků a materiálů s obsahem druhotných surovin z průmyslových provozů a komunálních odpadů pro použití ve stavebnictví (2021) – recyklujme stavby!, 2022. [2022-03-03]. http://www.recyklujmestavby.cz/ download/katalog-ii-2021/ [10] produkce, využití a odstranění odpadu a produkce druhotných surovin v roce 2018. [2022-03-03]. https://www.czso.cz/documents/10180/98121450/ 280029-19.pdf/a732554f-bc8f-431a-b41bacd9b89b1f8a?version=1.0 [11] analýza současného stavu vybraných komodit druhotných surovin a jejich zdrojů včetně vize rozvoje daného odvětví, 2018. [2022-03-03]. http://www.asvep.cz/prilohy/51_analyza_ podkladovy-material-pro-pds-cr_1_optimize.pdf [12] statistická ročenka životního prostředí čr 2020, cenia, 2022. [2022-03-03]. https://www.cenia.cz/publikace/statistickarocenka-zivotniho-prostredi-cr/statistickarocenka-zivotniho-prostredi-cr-2020/ 655 https://www.mzp.cz/c1257458002f0dc7/cz/poh_cr_prislusne_dokumenty/%24file/oodp-poh_cr_2015-2024_2035_vlada-20220511.pdf https://www.mzp.cz/c1257458002f0dc7/cz/poh_cr_prislusne_dokumenty/%24file/oodp-poh_cr_2015-2024_2035_vlada-20220511.pdf https://www.mzp.cz/c1257458002f0dc7/cz/poh_cr_prislusne_dokumenty/%24file/oodp-poh_cr_2015-2024_2035_vlada-20220511.pdf https://doi.org/10.1088/1755-1315/290/1/012025 http://www.recyklujmestavby.cz/download/katalog-ii-2021/ http://www.recyklujmestavby.cz/download/katalog-ii-2021/ https://www.czso.cz/documents/10180/98121450/280029-19.pdf/a732554f-bc8f-431a-b41b-acd9b89b1f8a?version=1.0 https://www.czso.cz/documents/10180/98121450/280029-19.pdf/a732554f-bc8f-431a-b41b-acd9b89b1f8a?version=1.0 https://www.czso.cz/documents/10180/98121450/280029-19.pdf/a732554f-bc8f-431a-b41b-acd9b89b1f8a?version=1.0 http://www.asvep.cz/prilohy/51_analyza_podkladovy-material-pro-pds-cr_1_optimize.pdf http://www.asvep.cz/prilohy/51_analyza_podkladovy-material-pro-pds-cr_1_optimize.pdf https://www.cenia.cz/publikace/statisticka-rocenka-zivotniho-prostredi-cr/statisticka-rocenka-zivotniho-prostredi-cr-2020/ https://www.cenia.cz/publikace/statisticka-rocenka-zivotniho-prostredi-cr/statisticka-rocenka-zivotniho-prostredi-cr-2020/ https://www.cenia.cz/publikace/statisticka-rocenka-zivotniho-prostredi-cr/statisticka-rocenka-zivotniho-prostredi-cr-2020/ acta polytechnica ctu proceedings 38:649–655, 2022 1 introduction 2 methods 3 results 3.1 production of secondary raw materials 3.2 possible use of secondary raw materials 3.2.1 slag 3.2.2 ashes and fly ash 3.2.3 energy gypsum 3.2.4 plastic materials 3.2.5 glass 3.2.6 paper 3.2.7 composite packaging 3.2.8 tires and rubbers 3.2.9 wood 3.2.10 metals 4 conclusion acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.37.0048 acta polytechnica ctu proceedings 37:48–53, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague on the ongoing computation of the lvr-15 reactor fuel burnup jan pintaa,b a czech technical university in prague, faculty of nuclear sciences and physical engineering, department of nuclear reactors, v holešovičkách 2, 180 00 prague 8, czech republic b research centre řež, department of neutron physics, hlavní 130, 250 68 husinec-řež, czech republic correspondence: pintajan@fjfi.cvut.cz abstract. isotopic composition of the lvr-15 reactor fuel changes significantly during the operation time. in order to enable monte carlo calculations of the lvr-15 core throughout any reasonable time period, the isotopic compositions of partially depleted fuel assemblies have to be known at the start of the simulation. even though the infinite lattice calculation can represent useful initial approximation, the non-leakage assumption is not necessarily valid for compact and highly heterogenous cores like in case of the lvr-15. therefore, a more advanced calculation process have to be implemented towards appropriate determination of fuel assembly burnup. the main aim of this paper is to describe the current state of the ongoing computation as well as provide an explanation of the chosen calculation scheme of the lvr-15 operation history. currently available results that are affected by profound inaccuracy are discussed in the final part alongside with planned future improvements. keywords: burnup calculation, serpent, lvr-15 reactor. 1. introduction the lvr-15 reactor [1] is a tank-type light water research reactor with 10 mw thermal power output. the reactor utilizes the irt-4m 19.7 % enriched fuel and its main applications are material testing, silicon doping and production of radioisotopes. currently, the serpent [2] computational model of the reactor is being developed with aim to perform its experimental qualification in the near future. one of the most crucial challenges concerning the development is a coverage of fuel burnup. there is an essential question of how to create proper representations of partially depleted fuel assemblies that are present at the start of an operation cycle. in fact, there is a majority of fuel assemblies with nonzero burnup contained inside the lvr-15 reactor core at the beginning of each cycle. therefore, determination of isotopic composition of partially depleted fuel have to be implemented in order to obtain convincing initial conditions that are needed for the serpent calculation of any time step of any lvr-15 reactor cycle. the first obvious possibility is to calculate burnup-wise fuel isotopic concentrations in the infinite lattice approximation. however, it will be shown later that such approach does not have to be necessarily valid for compact and highly heterogenous cores with considerable neutron leakage. for this reason, a more advanced fuel burnup computation procedure has been applied. the idea of this procedure is to take into account two important characteristics of the lvr-15 reactor. the first one is the above mentioned compactness and high heterogeneity of the core. the second one is the complicated operation scheme of the reactor, which involves short-term shutdowns in the run of a cycle operation. by adopting these two features, a more exact approximation in the fuel burnup implementation should be obtained. this work is particularly focused on the introduction and description of the advanced lvr-15 reactor fuel burnup computation. the results presented in this paper are considered rather preliminary due to the fact that the computational process is still in the ongoing state. nevertheless, the profound improvements will be realized on the basis of these first results in the following iteration of the fuel burnup calculation. 2. serpent model the current version of the 3d serpent model of the lvr-15 reactor allows to build the reactor model with any desired core configuration that contains the most frequently used components, i.e. fuel assemblies, beryllium blocks, water displacers and irradiation channels. all components in the core are surrounded with water and placed inside the reactor vessel. an instance of a core configuration geometry plot of the serpent model is provided in figure 1. as for fuel components, both standard 8-tube and control 6-tube irt-4m fuel assemblies can be chosen. control rod positions are adjustable according to the demanded pattern. in addition, both fuel assemblies can be defined with a variety of possible burnup distribution structures. different fuel material compositions can be assigned to corresponding regions of a particular fuel component using these distribution structures. 48 https://doi.org/10.14311/app.2022.37.0048 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 37/2022 on the ongoing computation of the lvr-15 reactor fuel burnup figure 1. serpent geometry plot of the opening lvr-15 core configuration of the k216 cycle; two fuel assemblies with initial zero burnup are highlighted (standard 85a710 – red, control 65c666 – black). the initial fuel burnup implementation is based on burnup-wise isotopic compositions of standard fuel assembly calculated with serpent in axially and radially infinite lattice. the serpent model utilizes predefined burnup library with atomic densities of considered nuclides as functions of burnup (see table 1). 3. computational process the chosen process of the burnup computation is based on the real operation scheme of the lvr-15 research reactor. the complexity of the reactor schedule is emphasized in figure 2 and figure 3, where powers and keff from serpent are visualized, respectively. more than a 2.5 year latest time period of the reactor operation has been considered for the purpose of the computation. this time interval was determined according to the moment when a fuel assembly with the largest burnup in the currently running cycle has been introduced into the reactor core for the first time as a fresh fuel component. this condition is fulfilled by the k216 cycle, where two new fuel assemblies have been added into the operation procedure – one standard (85a710) and one control (65c666). the position of the former is marked with red colour, while the latter is highlighted with black colour in terms of the k216 core layout visualization in figure 1. the whole computational process consists a total of 14 operation cycles starting from the k216 up to the k229. two fresh fuel assemblies in average are introduced into the reactor core at the start of each of the examined cycles. since that point, burnup development of these fuel assemblies can be calculated without the uncertainty in the fuel material composition. this presumption is supported by the fact that the fresh fuel assemblies have known initial isotopic concentrations. overview of fuel assemblies with this attribute is listed in table 2 alongside with the information whether any particular fuel assembly was present in the corresponding operation cycle. the computational process of the latest operation history of the lvr-15 reactor has been designed on the basis of data from the nodal code that is used for cycle designing at the lvr-15 [3]. these data come from nodal calculation of given cycle that has been carried out with genuine operational power data of the reactor. a typical calculation schedule of the lvr-15 reactor cycle contains over a hundred partial critical calculation steps. that reflects the complexity of the reactor operation. the same approach has been adopted in the serpent based computational process in order to achieve authentic operational conditions of the lvr-15 reactor as closely as possible. as for initial step of the entire computation, referred as the step 0 of the k216 cycle, the material compositions of fuel assemblies with nonzero burnup have been obtained from the above mentioned infinite lattice burnup library implemented in the serpent computational model. axial-wise burnup data divided into 5 regions of these fuel assemblies have been taken from the calculation of the previous k215 cycle from the operational nodal code. the scheme of the computational process is expressed in figure 4. a single step of the computation other than the initial step 0 of the k216 cycle can be described with a sequence of the same partial procedures. first, the appropriate operational data of the lvr-15 are loaded and transformed into the input file that is needed for the building of the corresponding lvr-15 reactor serpent model. these data include: • the name of the current cycle, • the calculation step number in the cycle, • time interval length of the step, • reactor power during the calculation step, • identifiers of present fuel assemblies, • core configuration layout within the step, • control rod insertions in the step core layout, • specifics of uranium targets in the core centre. in addition to the input file, the second set of data has to be provided. the second set contains calculated isotopic compositions of fuel assemblies from the previous step. this data stream is referred as the olddepfile and it is sent to the slvr15 together with the input file. the slvr15 procedure represents a software tool designed for creation of the serpent input file of demanded configuration of the lvr-15 reactor 49 jan pinta acta polytechnica ctu proceedings 80160 130270 420950 430990 441010 451030 471090 481130 531350 541350 551330 551340 551350 601430 601450 611470 611490 611510 621470 621490 621500 621510 621520 631530 631550 641550 641570 922340 922350 922360 922380 932370 942380 942390 942400 942410 942420 952410 952430 962430 962440 962450 table 1. list of 42 considered nuclides that are passed on between subsequent computation steps (zai numbers [4]). k216 k217 k218 k219 k220 k221 k222 k223 k224 k225 k226 k227 k228 k229 85a710 × × × × × × × × – × × × × × 65c666 × × × × × × × × × × × × × × 85a711 – × × × × × × × × × × × × × 85a712 – × × × × × × × – × × × × × 65c667 – × × × × × × × × × × × × × 65c668 – – × × × × × × × × × × × × 85a713 – – – × × × × × × × × × × × 85a714 – – – × × × × × × × × × × × 65c669 – – – – × × × × × × × × × × 85a715 – – – – – × × × × × × × × × 65c670 – – – – – × × × × × × × × × 65c671 – – – – – – × × × × × × × × 65c672 – – – – – – × × × × × × × × 85a716 – – – – – – – × × × × × × × 65c673 – – – – – – – × × × × × × × 85a717 – – – – – – – – × × × × × × 85a718 – – – – – – – – × × × × × × 65c674 – – – – – – – – × × × × × × 65c675 – – – – – – – – – × × × × × 85a719 – – – – – – – – – – × × × × 85a720 – – – – – – – – – – × × × × 85a721 – – – – – – – – – – – × × × 65c676 – – – – – – – – – – – × × × 60c834 – – – – – – – – – – – – × × 60c835 – – – – – – – – – – – – × × 85a722 – – – – – – – – – – – – – × 85a723 – – – – – – – – – – – – – × table 2. the presence of initially fresh fuel assemblies throughout considered cycles k216–k229 (× in the cycle, – not in the cycle). 0 5 10 k216 k217 k218 k219 k220 k221 k222 0 14 28 0 5 10 k223 14 28 k224 14 28 k225 14 28 k226 14 28 k227 14 28 k228 14 28 k229 time (days) po we r ( m w th ) figure 2. power sequence of the lvr-15 reactor operation including cycles k216–k229. 50 vol. 37/2022 on the ongoing computation of the lvr-15 reactor fuel burnup 0.9 1.0 k216 k217 k218 k219 k220 k221 k222 0 14 28 0.9 1.0 k223 14 28 k224 14 28 k225 14 28 k226 14 28 k227 14 28 k228 14 28 k229 time (days) k e ff figure 3. development of keff calculated with serpent throughout cycles k216–k229. lv r -1 5 op er at io n da ta input slvr15 olddepfile mainserpent newdepfile step n step n+1 figure 4. scheme of the computational process. computational model. the main input file is prepared for the serpent calculation of particular step of the computational process after the slvr15 execution. one of the step calculation outputs is the serpent depletion file indicated as the newdepfile. this output file alongside with appropriate operational data is used as the input for the calculation of the following step. the introduced process is repeated n times according to the number of steps in the whole computation. the isotopic composition of fuel assemblies between subsequent steps is passed on for 42 selected nuclides. the list of considered nuclides is written in table 1. the decision of the usage of the 42 nuclide approximation was made based on the burnup credit isotopes [5] with consideration of 135xe. the simulation of the xenon poisoning problematics was achieved by the elimination of 135i and 135xe nuclei before the beginning of each cycle, i.e. k216, k217, k218, etc. this behaviour was assumed to be sufficient enough to simulate actual operational conditions in the lvr-15 reactor core. parameters for neutron population in criticality source mode of each individual serpent calculation were set to 100 000 neutrons per generation, 200 active and 40 inactive generation cycles. standard deviation of keff did not exceed 27 pcm for each calculation step. all serpent calculations were carried out using the endf/b-viii.0 [6] based cross section library. 4. results the collection of three entangled types of results is presented in this paper. the first type of results covers calculated atomic densities of chosen nuclide compared to the infinite lattice approximation. second type of results represents verification of the calculated atomic densities with aim to determine whether the results are credible or not. the final part of the collection supports the statement that has been gained from the previous type of the results. 4.1. calculated burnup qualification visualization of 239pu atomic density burnup dependence has been chosen for the purpose of the calculated fuel burnup qualification. the axially averaged 239pu concentrations in the fuel as functions of burnup are provided in figure 5. these results are taken from the advanced computational process described in section 3. the visualization of the data is shown for all fuel assemblies with initial zero burnup at the moment of their first introduction into the lvr-15 reactor core during studied cycles k216–k229. the comparison with the infinite lattice calculated 239pu atomic densities is also included in figure 5. 51 jan pinta acta polytechnica ctu proceedings 0 20 40 60 80 100 burnup (mwd/kgu) 0.0e+00 1.1e-06 2.3e-06 3.4e-06 4.5e-06 at om ic de ns ity (1 e2 4/ cm 3) 0.0e+00 1.1e-05 2.3e-05 3.4e-05 4.5e-05 in fin ite la tti ce at om ic de ns ity (1 e2 4/ cm 3) infinite lattice 85a711 65c666 85a710 65c667 85a712 85a713 85a714 65c668 85a715 65c669 65c672 65c671 65c670 85a716 85a717 65c673 65c675 65c674 85a718 85a719 85a720 65c676 85a721 60c835 60c834 figure 5. axially averaged atomic densities of 239pu calculated for all fuel assemblies with initial zero burnup (left axis). the calculated values are compared with atomic densities from the infinite lattice approximation (right axis). 0 50 100 150 1.1e-04 7.5e-04 1.4e-03 u-235 0 50 100 150 5.3e-03 5.4e-03 5.5e-03 u-238 0 50 100 150 0.0e+00 2.3e-05 4.5e-05 pu-239 0 50 100 150 0.0e+00 3.8e-06 7.6e-06 pu-241 burnup (mwd/kgu) at om ic de ns ity (1 e2 4/ cm 3) infinite lattice 85a710 65c666 figure 6. comparison of assembly-wise atomic densities between the infinite lattice approximation and the unbroken burnup simulation of the opening step of the k216 cycle. two fuel assemblies, standard 85a710 and control 65c666, has been taken into the account in the latter type of the calculation. values for 235u, 238u, 239pu and 241pu are included. a considerably profound discrepancies between the calculated results and the infinite lattice approximation can be observed in figure 5. it is apparent that the amount of 239pu in case of the infinite lattice is one order of magnitude higher than in the results from the lvr-15 reactor operation history computation. the result of such kind raises the immediate demand for a validation of the entire computational process and for a verification of the data as well. the attempt to clarify these incompatibilities is the point of interest of the following type of the results collection. 4.2. computational process validation the validation of the actual computational process has been carried out based on the previous results. therefore, two kinds of problems have been compared. first, the burnup calculation of the infinite lattice approximation that was used as a prerequisite for the considered burnup library mentioned in section 2. second, an uninterrupted burnup calculation of the lvr-15 reactor core in the initial point of the whole computational process, i.e. the step 0 of the k216 cycle configuration. this comparison also represents the difference in the non-leakage approach against the genuine core configuration calculation with emphasis on high heterogeneity and compactness of the lvr-15 core. the data from the unbroken calculation of the opening step of the k216 cycle involves axially averaged atomic densities of the fresh standard and control fuel assemblies with identifiers 85a710 and 65c666, respectively. altogether, atomic density plots of 4 nuclides, 235u, 238u, 239pu and 241pu, are illustrated in figure 6. the greatest similarity of the atomic density data is observable in case of 235u. still, figure 6 shows that the depletion of 235u is slightly more significant in case of both fuel assemblies from the unbroken cycle simulation. however, the differences for 238u and for both pictured isotopes of plutonium are more extensive. the production of plutonium is directly linked to the neutron capture on 238u nuclei. the figure 6 indicates that there is a noticeably higher amount of 238u capture in the infinite lattice approximation and therefore a lesser concentration of the plutonium isotopes in the fuel assemblies of the comparison k216 cycle calculation. to proceed to the main point of this validation analysis, there is a fundamental assumption that the 239pu atomic densities should be in the same order of magnitude as the atomic densities determined from the unbroken real core burnup calculation. such accordance, however, has not been achieved. this outcome gives an evidence that some presumption in the designed computational process has been applied inaccurately. the investigation of the cause of the expressed discrepancies in the computational process is carried out in the following type of the results collection. 4.3. the cause of discrepancies the transfer of fuel composition data between particular steps of the computational process has been investigated as a potentially problematic part in the process design. more accurately, in the determination of which nuclides should be chosen for the conservation of atomic densities from one step to another. originally, there were 42 chosen nuclides as stated in table 1. a new calculation concerning a first few steps of the carried out computational process was executed with the change in the number of chosen nuclides. this time, atomic densities of all nuclides in the serpent cross section library from the end of a particular calculation step have been written into the input data of the following step. due to this decision, the expected results have changed significantly 52 vol. 37/2022 on the ongoing computation of the lvr-15 reactor fuel burnup 0.0 0.2 0.4 0.6 0.8 1.0 burnup (mwd/kgu) 0.0e+00 6.5e-08 1.3e-07 2.0e-07 2.6e-07 3.3e-07 3.9e-07 at om ic de ns ity (1 e2 4/ cm 3) infinite lattice k216 unbroken 42 nuclides all nuclides figure 7. short-term development of 239pu atomic densities for various calculation types. towards the results that were observed in figure 6. this behaviour for the case of 239pu atomic densities is shown in figure 7. 5. discussion according to the presented results, a misleading assumption in the design of the advanced computational process has been made. it has been investigated that the 42 nuclide representation does not reflect realistic conditions in the core during the lvr-15 reactor operation and thus a more comprehensive attitude has to be implemented. clearly, the consideration of all nuclides available in the serpent cross section library should be involved in the next generation of the lvr-15 reactor burnup computation. even though such upgrade of the computational process will require a larger amount of space for the output data storage, running time of the serpent calculation of a single step will not be affected. this indicates the feasibility of the improvement. on the other hand, the obtained results in figure 5 shows the differences in the fuel composition based on the individual operation scheme of each fuel assembly. therefore, the data can be used for analysis of the influence of the various schemes on the final form of the atomic density that will be represented in the burnup library of the lvr-15 reactor serpent model. 6. conclusions the serpent computational model of the lvr-15 reactor has been introduced in this work with focus on the problematics of fuel burnup implementation. the advanced burnup computational process has been described using the most important aspects and operational characteristics of the lvr-15 reactor. currently available results have been presented and analyzed according to the additional calculations. the analysis has shown that the number of nuclides considered in the transfers between calculation steps have been chosen inaccurately. this misleading nuclide interpretation has caused significant discrepancies in the atomic density values between the computation and the fundamentally anticipated results from the infinite lattice approximation and from the unbroken burnup calculation of the realistic lvr-15 core. the outcome of this paper will be used in the next generation of the advanced burnup computational process in order to determine a more realistic burnupwise fuel composition of the lvr-15 reactor. list of symbols keff effective multiplication factor endf evaluated nuclear data file references [1] m. koleška, z. lahodová, j. šoltés, et al. capabilities of the lvr-15 research reactor for production of medical and industrial radioisotopes. journal of radioanalytical and nuclear chemistry 305:51–59, 2015. https://doi.org/10.1007/s10967-015-4025-5. [2] j. leppänen, m. pusa, t. viitanen, et al. the serpent monte carlo code: status, development and applications in 2013. annals of nuclear energy 82:142–150, 2015. https://doi.org/10.1016/j.anucene.2014.08.024. [3] j. ernest. abstract of computational program noder 2006. [4] j. leppänen, et al. serpent – a continuous-energy monte carlo reactor physics burnup calculation code. vtt technical research centre of finland 4, 2013. [5] nea, oecd. spent nuclear fuel assay data for isotopic validation 2011. state-of-the-art report, https://www.oecd-nea.org/science/wpncs/adsnf/ soar_final.pdf. [6] d. brown, m. chadwick, r. capote, et al. endf/b-viii.0: the 8th major release of the nuclear reaction data library with cielo-project cross sections, new standards and thermal scattering data. nuclear data sheets 148:1–142, 2018. https://doi.org/10.1016/j.nds.2018.02.001. 53 https://doi.org/10.1007/s10967-015-4025-5 https://doi.org/10.1016/j.anucene.2014.08.024 https://www.oecd-nea.org/science/wpncs/adsnf/soar_final.pdf https://www.oecd-nea.org/science/wpncs/adsnf/soar_final.pdf https://doi.org/10.1016/j.nds.2018.02.001 acta polytechnica ctu proceedings 37:48–53, 2022 1 introduction 2 serpent model 3 computational process 4 results 4.1 calculated burnup qualification 4.2 computational process validation 4.3 the cause of discrepancies 5 discussion 6 conclusions list of symbols references https://doi.org/10.14311/app.2022.33.0411 acta polytechnica ctu proceedings 33:411–416, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague physical properties of portland cement based concrete exposed at a depth of 3520 m in the nankai trough shun nomuraa,∗, takafumi kasayab, yuki sakoic, hisashi fukadad, akira matsumotoe a japan agency for marin-earth science and technology, research institute for value-added-information generation, center for mathematical science and advanced technology, 3173-25 showa-machi kanazawa-ku yokohama-city kanagawa, 236-0001, japan b japan agency for marin-earth science and technology, research institute for marine resources utilization, submarine resources research center, 2-15 natsushima-cho yokosuka-city kanagawa, 237-0061, japan c hachinohe institute of technology, faculty of engineering, department of civil engineering and architecture, 88-1, obiraki, myo, hachinohe, aomori, 031-8501, japan d fudo tetra corporation, geo-technical division, 7-2 nihonbashi komai-cho chuou-ku, tokyo, 103-0016, japan e fudo tetra corporation, technical research institute, 2-7 higashi nakanuki, tsuchiura, ibaraki, 300-0006, japan ∗ corresponding author: nomura.shun@jamstec.go.jp abstract. concrete is widely used in large-scale construction of submarine infrastructure because of its high strength, durability, and ease of handling. however, knowledge of its durability in deep seawater is lacking. in the deep sea, materials are exposed to high pressures and low temperatures, which may cause early deterioration of concrete over time. concrete materials may also be affected by the chemical composition of seawater, which induces the leaching of calcium. in situ exposure tests are therefore important for understanding degradation processes in the deep sea. in this study, portland cement based concrete specimens were placed at a depth of 3520 m on the northern edge of the nankai trough in 2018 and retrieved in 2019, in the deepest exposure testing conducted to date. here we provide an outline of the tests, describe the physical properties of materials exposed to deep seawater, freshwater, and air, and discuss possible concrete degradation mechanisms. keywords: deep sea, exposing test, physical property. 1. introduction the deep sea is regarded as the ’last frontier’ and has attracted much attention in earth science [1–3]. recent studies have indicated that the deep sea has potential for the production of natural resources and oceanic energy. there is also interest in the monitoring of ocean-bottom dynamics and management of subsea disasters [4]. carbon dioxide capture and storage under the seafloor may lead to further utilization of sea-bottom areas [5]. submarine infrastructure with long-term stability is essential for increasing resource productivity and improving monitoring accuracy. concrete is the most preferred material for infrastructure construction because of its cost-effectiveness and rigidity. however, there may be problems with its use in the deep sea, with existing field data being available for depths of up to only 200 m, and with concerns over the durability of concrete in deep-sea environments. concrete strength may decrease due to the effects of extremely high hydrostatic pressures and low temperatures. chemical reactions in the deep-sea environment must also be considered. the carbonate compensation depth (ccd) of the pacific ocean is of the order of 3000 m. below that depth, calcium-one of the main components of concrete-may be leached from concrete, promoting its degradation. there may also be other factors, which together affect the durability of concrete in the deep sea. however, the field data concerning concrete properties in the deep sea are lacking, and there are concerns in extrapolating from shallow-water data. deep-sea exposure testing was therefore undertaken in this study. here we describe the testing of ordinary portland cement based concrete specimens at a depth of 3520 m in the nankai trough, pacific ocean, in the deepest concrete testing yet undertaken. 2. method research cruises were undertaken in 2018 and 2019 as described below. 411 https://doi.org/10.14311/app.2022.33.0411 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en s. nomura, t. kasaya, y. sakoi et al. acta polytechnica ctu proceedings figure 1. field location. figure 2. concept (left) and image (right) of the platform. 2.1. field location the concrete exposure area lies on the northern edge of the nankai trough, off sagami bay, japan, at 34◦1.65’n 138◦31.50’e (figure 1). multi-beam echo-sounder (mbes), bathymetric, and bottom-imaging data were obtained by the research vessel ’yokosuka’ at locations satisfying the following conditions: depth > 3000 m (below the ccd); ease of access (within a two-day cruise), maximum eight-hour dive, solid seabed, and stable tidal systems. 2.2. specimen placement a stainless-steel platform (2.2 × 2.2 × 1.5 m; 600 kg weight) was installed to mount specimens on and to reduce effects of benthic organisms and loss in the strong bottom current (figure 2). the platform was based on four 200-mm diameter discs to prevent sinkage in the sediment. on 18th july 2018, the platform was lowered from the ’yokosuka’ by winch at 45 m.min−1. concrete specimens (36) were placed in six plastic baskets (400×272×300 mm; figure 3). specimen size was 100 mm diameter by 200 mm length (to fit basket pockets). on july 19th, the specimens, in the baskets, were installed on the platform using the deep-sea submersible research vehicle, ’shinkai 6500’, as shown in figure 4. 2.3. retrieval of specimens on 29th september 2019, the study area was accessed by the research vessel ’kairei,’ and the remotely operated vehicle ’kaiko’ was used to retrieve the specimens after 437 days of exposure. the overview of the research cruises in 2018 and 2019 is presented in figure 5. 3. materials and exposure results 3.1. preparation of concrete specimens the concrete mix used for the specimens is described in table 1. ordinary portland cement (density 3.15 g cm−3) was used with a water/cement ratio of 0.55. natural sand (density 2.64 g.cm−3) and crushed limesand (density 2.66 g.cm−3) were mixed and used as a fine aggregate. coarse aggregate was prepared from crushed limestone (5 − 15 mm particles, density 2.67 g.cm−3; 15 − 20 mm particles, density 2.70 g.cm−3). admixtures included an air entraining agent (i.e., 412 vol. 33/2022 physical properties of portland cement figure 3. concept (left) and image (right) of the plastic basket the for concrete specimen. figure 4. installation of the basket by the manipulator (left) and top view of the exposure system (right). figure 5. overview of placement and retrieval processes in 2018 and 2019. 413 s. nomura, t. kasaya, y. sakoi et al. acta polytechnica ctu proceedings unit content [kg.m−3] admixture w/c s/a w c s g ae agent water reduction agent [%] [%] lime sand natural sand 5 − 15 mm 15 − 20 mm [a*] [c × %] 55.0 41.0 157 285 230 532 553 559 3.5 0.05 ∼ 0.2 * 1a = 0.001% table 1. the concrete mix. figure 6. specimens retrieved from the deep sea. deep sea water salty water exposing term 437 days number of specimens 3 2 2 compression strength fc (mpa) 35.4 33.0 39.3 maximum strain ea (1e-3) 1.89 1.62 1.70 young’s modulus es (kn/mm2) 22.1 30.5 35.4 table 2. averaged compression test values. ae) agent and a water reduction agent. the slump and air content of the fresh concrete were 8.5 cm and 4.7%, respectively. the cast specimens were solid cylinders(!100×200 mm) left in the formwork for 1 day before curing in a water bath for 28 days. the water-cured specimens were covered with polypropylene sheet to prevent drying and stored at room conditions pending use. exposure tests were also undertaken in fresh water and salty water under atmospheric pressure. 3.2. description of degraded specimens recovered specimens are shown in figure 6. there were no obvious cracks affecting the overall specimen structure. exposed aggregate was evident on specimen surfaces. there were several spreading white circular spots of 1 − 3 cm diameter. the mass density increased by ∼ 1% during exposure, but specimen dimensions were almost the same as after curing for 28 days. 3.3. compression testing one-dimensional compression test results obtained by a hi-acits-3000 (marui & co. ltd., japan) automatic compression testing apparatus are shown in figure 7, where profiles represent results of deepsea, fresh, and salt water. averaged physical data for the tests are given in table 2. the deep-sea specimen plot has a lower gradient than that of the other cases, indicating a lower elastic modulus. compression strengths ranged from 33.0 to 39.3 mpa with small variations, regardless of exposure state, with the maximum strain at the compression strength being the highest for the deep-sea specimens. the young’s modulus values of the deep-sea specimens were ∼ 67% of that of the specimen exposed to salt water. the relationship between compression strength and young’s modulus is depicted in figure 8 where solid and dotted lines represent the upper and lower criteria for materials used in the construc414 vol. 33/2022 physical properties of portland cement figure 7. results of compression testing. figure 8. relationship between compression strength and young’s modulus. tion of buildings and bridges, among others, set by the architectural institute of japan [6]. all deep-sea specimens plot below the lower limit, meaning these specimens were sufficiently degraded to fall short of architectural criteria. 3.4. travel velocity of ultrasound ultrasound travel velocity, vp, was determined using a pangit pl-200 (fts ltd., japan) instrument, with results as shown in figure 9. the deep-sea specimens exhibited a decrease in vp relative to the other specimens (18% less than that of salty-water specimens). based on elastic wave theory, poisson’s ratio ν is expressed as: ν = 1 − a + ! |9a − 1||a − 1| 4a , where a = v 2 p ρ ed (1) where, ρ[kg.m−3] is density, and ed " kg.m−1.s−2# is the dynamic viscosity coefficient. figure 10 shows the ν values of specimens exposed to fresh and salt water, representing the general values for concrete materials (ν = ∼ 0.25) [7]. in contrast, the lower vp value of deep-sea specimens (ν = ∼ 0.39) is similar to that of non-compressive material (ν = ∼ 0.50). figure 9. ultrasound travel velocity. x-axis represents the exposure conditions and number of specimens. figure 10. poisson’s ratio ν. x-axis represents the exposure conditions and number of specimens. 4. discussion the compression and ultrasonic tests confirm that the mechanical states of concrete specimens exposed in the deep sea were different to those exposed to fresh and salt water, with the deep-sea specimens being more ductile and softer. a conceptual representation of the deep-sea exposure environment is provided in figure 11, illustrating how specimens underwent rapid isotropic loading and unloading (0-35 mpa) during emplacement and retrieval. the specimens were also exposed to high pressures and low temperatures (1−2◦c) after installation. it, therefore, seems that the specimens were degraded by short-term mechanical and long-term chemical actions. to elucidate high loading and unloading effects, cui et al. [7] performed compression tests after exposure of deep-sea samples to high hydrostatic pressures of 30-500 mpa. they confirmed that as hydrostatic pressure increases, the compressive strength decreases, the strain at compressive strength increases, and the young’s modulus decreases. it was also confirmed by scanning electron microscope (sem) observations that, even in an isotropic environment, anisotropy develops inside the concrete specimens due to extremely high pressure, with fine cracks developing in the damaged concrete, which had a poisson’s ratio of almost 0.4. these results are consistent with 415 s. nomura, t. kasaya, y. sakoi et al. acta polytechnica ctu proceedings figure 11. conceptual representation of the physical and chemical environments around in the concrete specimens. those for the deep-sea specimens, suggesting that differences in mechanical history contribute to the deterioration of concrete. 5. conclusion concrete specimens were exposed at a depth of 3520 m for 437 days and their physical properties studied. the specimens deteriorated in the deep-sea environment. in this study, we introduced only those physical properties shown by the compression and ultrasonic tests, whereas the deterioration mechanisms are currently being elucidated by mercury injection tests, sem, electron microprobe analysis, and x-ray diffraction studies. these studies will aid the understanding of physical and chemical processes that affect concrete in deep-sea environments more precisely. further samples are scheduled to be collected over the next five years, and the scientific reasons for their deterioration may become clearer. acknowledgements we thank dr hide sakaguchi and mr masahiko kamei at japan agency for marine-earth science and technology and prof. kenji kaneko of the hachinohe institute of technology for their useful discussion, research assistance, and encouragement. references [1] e. araki, d. m. saffer, a. j. kopf, et al. recurring and triggered slow-slip events near the trench at the nankai trough subduction megathrust. science 356(6343):1157-60, 2017. https://doi.org/10.1126/science.aan3120. [2] r. sharma. environmental issues of deep-sea mining. procedia earth and planetary science 11:204-11, 2015. https://doi.org/10.1016/j.proeps.2015.06.026. [3] k. a. miller, k. f. thompson, p. johnston, et al. an overview of seabed mining including the current state of development, environmental impacts, and knowledge gaps. frontiers in marine science 4, 2018. https://doi.org/10.3389/fmars.2017.00418. [4] w. o. mccarron. deepwater foundations and pipeline geomechanics, florida: j. ross publishing, pp. 336, 2011. [5] s. m. benson, f. m. orr. carbon dioxide capture and storage. mrs bulletin 33(4):303-5, 2011. https://doi.org/10.1557/mrs2008.63. [6] reinforced concrete structure calculation standard and explanation, 9th edition, (tokyo: architectural institute of japan). [7] j. cui, h. hao, y. shi, et al. experimental study of concrete damage under high hydrostatic pressure. cement and concrete research 100:140-52, 2017. https: //doi.org/10.1016/j.cemconres.2017.06.005. 416 https://doi.org/10.1126/science.aan3120 https://doi.org/10.1016/j.proeps.2015.06.026 https://doi.org/10.3389/fmars.2017.00418 https://doi.org/10.1557/mrs2008.63 https://doi.org/10.1016/j.cemconres.2017.06.005 https://doi.org/10.14311/app.2022.33.0175 acta polytechnica ctu proceedings 33:175–180, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague bending behavior of adhesively-bonded engineered wood-concrete composite decks qiuni fua, libo yanb,∗, bohumil kasalb a technische universität braunschweig, institute of building materials, concrete construction and fire safety, department of organic and wood-based construction materials, hopfengarten 20, 38102 braunschweig, germany b fraunhofer insitute for wood research wilhelm-klauditz-institut wki, bienroder weg 54e, braunschweig 38108, germany ∗ corresponding author: l.yan@tu-braunschweig.de, libo.yan@wki.fraunhofer.de abstract. four-point bending tests were conducted on five medium-sized (i.e., 2300 mm in length and 215 mm in width) engineered timber (laminated veneer lumber (lvl) and cross-laminated timber (clt)) concrete (wood chip concrete and plain concrete) composite decks. the concrete was glued to the wood substrate with epoxy and polyurethane adhesives. the observed failure modes of the composite decks were concrete crushing or wood failure in tension or shear. no failure of the adhesive interface was observed and the decks behaved linearly until failure. in the subsequent analysis, the authors quantified the shear flexibility of transverse layers (stressed perpendicular to the fiber direction) in clt and lvl boards and its effect on the bending stiffness on the composite decks using γ-method described in en 1995-1-1 (ec5). the analytical predictions of the effective bending stiffness were verified via experiments, showing consistently good agreement. keywords: γ-method, adhesive bond, bending behaviour, timber-concrete composite, wood chip concrete. 1. introduction timber-concrete composite (tcc) structural elements have advantages of higher stiffness, fire resistance and better acoustic performance compared with massive timber. connection between concrete and wood is usually achieved by conventional mechanical dowel-type fasteners (shear studs) or notches. mechanical fasteners are not rigid and there is always a slip between the wood and concrete. thus, full composite action of tcc connected by mechanical fasteners is hardly achievable. adhesive bonding is often considered as rigid. the application of glued wood-concrete connections is not fully explored and the knowledge in this area is limited [1–7], let alone the relevant specifications in standards. studies [1–7] demonstrated that glued tcc systems had higher stiffness (e.g. slip modulus of connections and bending stiffness of tcc beams) compared to mechanically connected ones, and sufficient adhesion strength for structural applications under short-term loading and ambient-temperature. the wood products involved in these studies [1–7] were solid timber, glulam or vertically used laminated veneer lumber (lvl) assuming the rigid characteristics of the used adhesives, the behavior of these kinds of tcc beams was characterized by assuming continuous strain at the wood-concrete interface and using the known transformed cross-section procedure [5, 8– 10].the low shear modulus of transverse layers in the clt and lvl boards subjected to bending can cause significant shear deformation (or slip) and hence increase the deflection. in the present work, the authors studied the shear behavior of glued cltand lvl-concrete connections [11]. they further experimentally investigated the bending behavior of cltand lvl-concrete composite decks. specifically, clt and lvl boards were bonded wood chip concrete (wcc) and with plain concrete (pc) using epoxy and polyurethane (pur) and adhesives. the addition of wcc [12] resulted in reduced concrete density and increased thermal insulation properties of plain concrete. the polyurethane (pur) used in the present study was stiff, its modulus of elasticity was as high as that of the used epoxy (around 3.5 gpa), showing nearly no deformability in the adhesive layer. the pur in this study was different from the pur in ref. [13] (modulus of elasticity: 0.008 gpa) that resulted in visible slip at bond line (over 1 mm). to describe the slip in transverse layers and its effect on the bending stiffness of composite decks, the authors used the shear analogy method [14] for quantifying shear deformation of the transverse layers in combination with γ-method of ec 5 [15]. the predictions of the effective bending stiffness were compared with the experimental results, showing a good match. 175 https://doi.org/10.14311/app.2022.33.0175 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en q. fu, l. yan, b. kasal acta polytechnica ctu proceedings clt flatwise lvl thickness (mm) 80 (20 + 40 + 20) 69 (thickness of each veneer: 3 mm) shear modulus (mpa) 50 (rolling shear, gr) 22 (perpendicular to grain, g90) elastic modulus in grain direction (e0) (gpa) 12 11.4 table 1. mechanical properties of the engineered wood products [16]. 2. experimental studies 2.1. overview of test plan engineered timber, clt and lvl, and concrete, wcc and pc used in this study were identical to those in the previous study [11]. epoxy and pur were selected due to their bonding performance [11]. the authors constructed five mediumsized specimens, wcc-epoxy-lvl, wcc-pur-lvl, wcc-epoxy-clt, wcc-pur-clt and pc-epoxylvl, and tested them under bending. figure 1 (e) shows the configurations of the specimens. 2.2. materials the mechanical data and configurations of clt and lvl are shown in table1. moisture content of the used wood was about 10%. the beech wood chips used in wcc were in a relatively flat shape, having around 20 mm side length and 2.4 mm thickness. the density of the wood chip was 0.68g/cm3. the mass ratio of the constituents of wcc was cem ii/b-s 42.5 r cement: water: 0-2 mm sand: 2-8 mm gravel: 8-16 mm gravel: wood chip = 1: 0.60: 2.00: 1.27: 2.32: 0.11. the mass ratio of the constituents of pc was cem ii/b-s 42.5 r cement: water: 0-2mm sand: 2-8 mm gravel: 8-16 mm gravel = 1: 0.60: 2.00: 1.27: 2.72. the compressive strength of the wcc and pc cylinders were 25.8 mpa (sd = 3.79 mpa) and 27.9 mpa (sd = 1.73 mpa) based on five samples, respectively. the respective elastic modulus of pc and wcc were 29.9 gpa and 12.3 gpa. according to the product of primetm 20lv from [17] datasheet, the elastic modulus of epoxy is 3.5 gpa. the pull-off tests in authors previous study [11] showed that the load-displacement curves of the used epoxy and pur almost overlapped at the initial loading stage, so the elastic modulus of the used pur is similar to that of the epoxy. 2.3. specimen preparation when the concrete was cured for 28 days, the concrete surfaces were cleaned and glued to shaped wood decks with wood decks on the top for the first 3 days. the specimens were turned upside down since the fourth day and tested after the seventh day. the detailed processes can be found in ref. [16]. 2.4. experimental setup a load (f ) was quasi-statically applied on the tcc decks with a rate of 0.35 mm/s through a two-point spreader, as shown in figure 2. it also shows the locations of the support and loading points. a deflection at the mid span was measured by two lvdts (linear variable differential transducers). 3. results and discussion 3.1. load-carrying capacities the load versus mid-span deflection curves of all the tcc decks showed linear behaviour until the ultimate failure. figure 3 shows the response of wccpur-clt as an example, and the ultimate loads, the ultimate mid-span deflections and bending stiffness (ei)test of all the specimens are summarized in table 3. the calculation of (ei)test was based on the ascending slope of a load versus mid-span deflection curve as follows [16]: (ei)ef = f w a ! 3l2 − 4a2" 48 (1) where w is the average reading value of the two lvtds; a is the shear span; and l indicates the span of the deck. from table 3, it follows that the bending stiffness of a beam was not affected by adhesive type. the reason was that a rigid connection was achieved by using either epoxy or pur, which was also demonstrated by the absence of slippage between concrete and wood decks (see section 3.2 and table 3). the specimens comprising clt boards were stiffer than the counterparts made of lvl (table 3). the inherent reasons are discussed in section 4. on the contrary, the ultimate load-bearing capacities of the lvl-specimens were higher than that of clt-specimens., due to different failure modes (table 3). as the pc had higher elastic modulus and strength compared with wcc, pc-epoxy-lvl had the largest flexural stiffness (4.12 × 1011n.mm2) and ultimate load-carrying capacity (63.2 kn) [16]. the average bending stiffness and load-bearing capacity of wcc-epoxy-lvl and wcc-pur-lvl was around 78.9% and 70.0% of those of pc-epoxy-lvl, respectively. meanwhile, the thermal conductivity of wcc could be only 68.9% of that of pc. [18]. besides, wide application of wcc can reuse wood waste, and reduce the demand for natural aggregates. the load-bearing capacity of the weakest specimen, wccepoxy-clt, was 39.1 kn, equivalent to a uniformlydistributed load of 85.4 kn/m2. considering serviceability limit design, ec 5 [15] allows the maximum 176 vol. 33/2022 bending of woodconcrete composite deflection of a slab to be l/300 (i.e. 7.1 mm). this deflection corresponds to an equivalent slab load of 34.7 kn/m2 in the case of wcc-epoxy-clt. 3.2. failure modes the failure modes of all the specimens are summarized in table 3. neither of the specimens was failed by adhesive bonding interface, indicating that the used adhesives can provide satisfactory bonding performance. the different mechanical properties of clt and lvl caused different failure modes of the corresponding tcc decks. -see [16] for details. one can also have a deeper appreciation of the structural behaviour of lvl and clt in tcc decks in section 4. 4. prediction of bending stiffness in building slab design, deflection is one of the important serviceability considerations. structural engineers usually calculate deflections with eulerbernoulli beam theory that does not take shear deformation into account. however, when engineered wood (e.g. clt) is under bending, transversely stressed layers have significant shear deformation, because of low rolling shear modulus (gr) and low shear modulus in the perpendicular direction (g90) (table 1). to consider the shear flexibility of transverse layers, the authors regarded it as sources of partial composite action and calculated the effective bending stiffness (ei)ef for all the specimens based on γmethod in ec 5 [15]. the γ-method [15] allows a composite cross-section to be comprised of three parts with two connecting interfaces featured with the respective degree of composite action (γi) relative to the middle layer (referred as "part 2" in figure 4 (a)). a term γi ranges between 0 (no interaction) and 1 (rigid connection). then (ei)ef of a composite deck is calculated with equation (2) [15]. (ei)ef = 3# i=1 ! ei ii + γi ei ai a2 i " (2) where ei is the modulus of elasticity of part i in the longitudinal direction; the term ii means moment of inertia of part i; the term ai denotes the cross-section area of part i; the term ai indicates the distance between the neutral axis (i.e. n.a. in figure 4 (a)) and the centroid of part i. according to ec5 [15]: γi = $ 1 + π2 ei ai si % (ki l2) & −1 (i = 1 and 3) (3) where ki is the slope of a shear force slip curve (i.e. slip modulus) of a connection, and si denotes the equidistance between shear fasteners connecting part 2 and part i. for a continuous adhesive connection, ki can be defined based on a given length, e.g. 1 mm. then, si is equal to 1 mm. as explained in ref. [16], the clt board (part 2) and concrete (part 1) were rigidly bonded based on the experimental observations (i.e. k1 = ∞ γ1 = 1). part 2 is the reference layer, so γ2 is equal to 1. in this paper, the slip modulus k3 relates to the shear connection between the top (part 2) and bottom (part 3) longitudinal layers of clt. therefore [16], k3 = gr · b · (1000 mm) / htr (4) where gr denotes the rolling shear modulus of the clt ( refer to table 1), htr denotes thickness of the transversal layer, and b is the width of the decks. according to ec [15]: a2 = γ1 e1 a1(h1 + h2) − γ3 e3 a3(h2 + h3) 2 3' i=1 γi ei ai (5) the terms a1 and a3 can be obtained based on the geometric relationship in figure 4 (a). to model the shear flexibility of dispersed transversal veneers in lvl, the longitudinal and transversal veneers were gathered into the respective two layers for simplification. the full concept is illustrated in figure 4 (b). for details regarding the modelling and analysis, please refer to the authors’ previous work in ref. [16]. in simple terms, equations (2) through (5) are applicable for tcc decks using lvl, with h1 = 0 and g90 replacing gr. all the detailed derivations can be found in ref. [16]. for comparison, γi and (ei)ef of tcc decks comprising the respective clt and lvl boards are presented in table 2. the degree of composite action in the tcc decks comprising lvl (0.52) was lower compared with that of the tcc decks comprising clt (0.70). the reason was that the transversely stressed veneers have a very low shear modulus (22 mpa). the (ei)ef of cltinvolved tcc decks was slightly higher than that of lvl-involved tcc decks, which agrees with the test results (table 3). the higher (ei)ef of clt-involved tcc decks stemmed from a larger γ3 and a larger a3 (see figure 4). the specimen made of pc had the largest stiffness due to the larger elastic modulus of the plain concrete. the larger elastic modulus of pc led to the largest stiffness of pc-epoxy-lvl. as shown in table 2, the (ei)ef calculated in this study conservatively predicted the corresponding test results with an error of around 10% [16]. ignoring the shear deformation (i.e. γ3 = 1), ei was overestimated by around 34% on average. with the latter kind of calculation, another undesired situation is that the errors of predictions of ei varied significantly (cov = 14.1%), depending on different types of engineered wood products having different degrees of shear flexibility. this problem was solved by this study through quantifying the shear flexibility of transversal wood layers [16]. the errors of predicted (ei)ef of cltand lvl-involved tcc decks had small variations 177 q. fu, l. yan, b. kasal acta polytechnica ctu proceedings figure 1. dimensions of fabricated tcc decks. concrete lvdtȭ1ȱ(2) 710ȱmm 710ȱmm 710ȱmm timber f/2 f/2 f355ȱmm 355ȱmm roller figure 2. setup of 4-point bending test. specimen id γ1 γ3 eia (n.mm2) ei/(ei)test (ei)ef (n.mm2) (ei)ef/(ei)test wc-epoxy-clt 1 0.70 4.13×1011 1.14 3.21×1011 0.89 wc-pur-clt 1 0.70 4.24×1011 1.13 3.33×1011 0.89 wc-epoxy-lvl \ 0.52 4.62×1011 1.42 2.90×1011 0.89 wc-pur-lvl \ 0.52 4.90×1011 1.50 3.05×1011 0.93 pc-epoxy-lvl \ 0.52 6.15×1011 1.49 3.82×1011 0.93 average 1.34 0.91 coefficient of variance 14.0 % 2.6 % a the term ei denotes the flexural stiffness of tcc decks, ignoring the shear flexibility of transvers layers. table 2. predictions of flexural stiffness. figure 3. load vs. displacement responses of wccpur-clt at mid-span. (cov = 2.6%) (table 2). thus, this study provides an effective solution for the calculation of (ei)ef (or deflection) of tcc decks using engineered wood. 5. conclusions from this study, some conclusions can be drawn, as follows: 1. the purand epoxy-bonded wood and concrete could form a strong and rigid interfaces to transfer the shear in tcc decks subjected to quasi-static bending, without failure and visible slip. 2. the failures of tcc decks in this study were brittle, which were governed by wood fracture and concrete crushing. 3. wood chip concrete is feasible to be used with the engineered wood as composite structural slabs. engineered wood could be used together with wcc to realize a sustainable solution, with satisfactory structural performance. 4. the method in this study provides an effective solution for the calculation of (ei)ef (or deflection) of tcc decks under bending. acknowledgements the research was financially supported by fachagentur nachwachsende rohstoffe e. v. (fnr, agency for renewable resources) founded by bundesministerium für ernährung und landwirtschaft (bmel), under the grant award no.: 22011617, and by bundesministerium für bildung und forschung (bmbf) (grant no.: 031b0914a). 178 vol. 33/2022 bending of woodconcrete composite (a) concrete-clt composite decks concreteȱ(ec) adhesive* 40 20 1000 ȱlongitudinalȱlayer ȱlongitudinalȱlayer transversȱlayer gr 20 e0 e0 3ȱ (5 ) 50 h t r h 2 h 3 h 1 model n.a. a1 a3 h2 /2 h1 /2 h3 /2 a 2 *ȱtheȱmomentȱofȱinertiaȱofȱtheȱadhesiveȱlayerȱisȱignored,ȱbutȱitsȱthicknessȱisȱconsideredȱwhenȱdeterminingȱgeometricȱpositionsȱforȱotherȱmaterialȱlayers. construction concreteȱ(e1) adhesive 40 20 1000 ȱlongitudinalȱlayer ȱlongitudinalȱlayer k3=gr*b*(1000mm)/htr partȱ1 partȱ2 partȱ3 20 k1=infinite e3 e2 3ȱ (5 ) 50 (b) concrete-lvl composite decks partȱ3 k3=g90*b*(1000mm)/htr h 3 =5 4 h 2 3ȱ (5 ) ȱrigidȱadhesiveȱlayeradhesive:ȱrigidȱbond htr=15ȱmm model construction concreteȱ(ec) ȱlvlȱ(e0,ȱg90) ȱlongitudinalȱlayertransversȱlayer 3 3 ȱlegends 69 1000 3ȱ (5 ) 50 concreteȱ(e2) ȱlvlȱ(e3) 34 ,5 1000 50 34 ,5 27 27 centroidȱline ofȱwood partȱ2 figure 4. analytical model of composite decks (unit: mm) quoted from ref. [16]. references [1] m. brunner, m. romer, m. schnüriger. timberconcrete-composite with an adhesive connector (wet on wet process). materials and structures 40(1):119-26, 2006. https://doi.org/10.1617/s11527-006-9154-4. [2] j. kanócz, v. bajzecerová, m. mojdis. experimental and numerical analysis of timber-lightweight concrete composite with adhesive connection. advanced materials research 969:155-60, 2014. https://doi.org/10.4028/www.scientific.net/amr .969.155. [3] p. l. clouston, c. p. quaglia. experimental evaluation of epoxy based wood plank-concrete composite floor systems for mill building renovations. the international journal of the constructed environment 3(3):63-74, 2013. https: //doi.org/10.18848/2154-8587/cgp/v03i03/37394. 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[16] gurit. primetm 20lv-epoxy infusion system, 2018. https://www.gurit.com/sitecore/content/old-pro duct-pages/other-products/laminating-infusio n-systems/prime-20lv. [17] en 1995-1-1, eurocode 5 design of timber structures part 1-1: general common rules and rules for buildings. european committee for standardisation, brussels, 2004. [18] m. bederina, l. marmoret, k. mezreb, et al. effect of the addition of wood shavings on thermal conductivity of sand concretes: experimental study and modelling. construction and building materials 21(3):662-8, 2007. https: //doi.org/10.1016/j.conbuildmat.2005.12.008. 180 https://doi.org/10.1016/j.conbuildmat.2019.117743 https://doi.org/10.1016/j.compositesb.2020.108042 https://doi.org/10.1016/j.conbuildmat.2020.118578 https://www.gurit.com/sitecore/content/old-product-pages/other-products/laminating-infusion-systems/prime-20lv https://doi.org/10.1016/j.conbuildmat.2005.12.008 acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.36.0286 acta polytechnica ctu proceedings 36(0):286–295, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague new or existing, does it matter? peter tannera,b,∗, ramon hingoranic, carlos laraa a eduardo torroja institute for construction science, serrano galvache 4, 28033 madrid, spain b cesma ingenieros, villa de marín 6, 28029 madrid, spain c norwegian university of science and technology, richard birkelands vei 1a, trondheim, norway ∗ corresponding author: tannerp@ietcc.csic.es abstract. both when designing new and when evaluating existing structures, methods of analysis must be based on established engineering theory and practice. also, the general principles of structural reliability regarding the treatment of uncertainties when verifying the established requirements apply to both, design and assessment. taking further into account that new or existing civil engineering works are usually unique, either because they are prototypes or because they are exposed to specific conditions, from an engineering point of view, structural analysis and verification must be carried out, both in the design and in the assessment, under case-specific conditions according to the same principles. however, important differences exist between assessment and design, for example regarding the state of information and its updating through different types of information, the structural condition, reliability requirements, verification methods or decision options. such differences, which are briefly summarized in the contribution, can often lead to a high level of conservatism when using design-oriented methods for assessment purposes. there is therefore a need to develop a generally recognized, coherent and harmonized set of rules for the assessment of existing structures. the cen technical specification for assessment and retrofitting partially closes this gap, but only establishes principles on assessmentspecific reliability aspects. the consistent application of these generic assessment rules in practice requires further assumptions, or even case-specific developments. practical applications are therefore of paramount importance in identifying relevant issues to consider in the future development of assessment methods and codes. keywords: existing structures, deterioration, risk, reliability, robustness, acceptance criteria, assessment, uncertainty, updating, probabilistic methods, partial factors, codes. 1. introduction current and future activities related to the development of cities, industrial areas and infrastructures should be determined by sustainability goals [1]. changing needs are therefore not simply answered by adding new buildings and infrastructures to the existing built environment or by replacing existing with new engineering works. rather, ways are being explored to modify existing systems to meet new demands, or simply to extend their service life. for this purpose, the performance of existing structures must be evaluated, activity that is usually denominated as assessment [2, 3]. in general, an assessment of an existing structure may be required in the case of [4]: • a change in the purpose of the structure compared to that for which it was originally designed or previously assessed; • deviations in the properties of the structure or the environment from those adopted in the original design or in the previous assessment. society expects the failure of any engineering structure, whether new or existing, to be an extremely rare event. as with the design of new structures, the assessment of existing structures must therefore demonstrate their ability to meet specified requirements throughout their remaining service life, with the aim of achieving an acceptably low probability of failure. it is this ability that is understood as reliability, a notion covering structural performance related to safety, serviceability, durability and other requirements that could be affected by uncertainties. reliability analysis is therefore a fundamental part of any structural engineering activity. both in the design of new and in the assessment of existing structures, the effects of the relevant actions and influences on the load-bearing system and its response are determined based on recognized engineering theory and practice, taking into account all relevant parameters for the system under consideration [5, 6]. the general principles of structural reliability regarding the treatment of the uncertainties associated with these parameters also apply to both, design and assessment, when using different methods to verify the specified requirements. inasmuch as all engineering structures, new or existing, are fundamentally different as they are mostly prototypes that are exposed to specific conditions, the question could arise as to whether an assessment is basically equivalent to structural design, since in both cases structural analysis and verification must be carried 286 https://doi.org/10.14311/app.2022.36.0286 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 36 no. 0/2022 new or existing, does it matter? out on a case-by-case basis according to the same principles. nonetheless, there are important differences between assessing existing and designing new structures. the use of design-oriented methods to assess existing structures therefore often leads to a high degree of conservatism with serious economic, ecological and socio-political consequences if resources are invested in the unnecessary strengthening or replacement of existing structures with the associated disruptions [1]. against this background and focusing on reliability issues, relevant differences between assessment and design are briefly reviewed in section 2 of this article. widely divergent approaches exist to deal with these differences. such approaches differ according to their historical stage of development and are characterized not only by national choices and preferences, but also by differences depending on the type of structure being assessed [7]. there is therefore a need to merge the various options into a harmonized set of rules for the assessment of existing structures that complement those for the design of new structures. this was the main impetus for including the working item of assessment and retrofitting of existing structures as a high priority in the mandate for the evolution and extension of the eurocodes to develop their second generation [1], assigned to the european committee for standardization, cen, by the european commission and the european free trade association. the status of eurocode developments on these topics is summarized in section 3. it should be emphasized that the codes currently available mainly set out general principles of assessment-specific reliability aspects. the consistent application of such rules in practice therefore requires further, often case-specific assumptions and developments, as the contribution shows using a practical example (section 4). in this vein, there is a need for further research and evolution in the area of assessment methods and codes. before choosing the direction of such developments, the reasons for the sparse practical application of reliabilityand risk-based methods should be identified, as suggested in the concluding section 5. 2. assessment versus design 2.1. information 2.1.1. state of information the main difference between assessing the performance in existing and design phase structures is that many characteristics whose values are merely anticipated in the latter can be measured in the former. in the assessment of an existing structure, the effects of the construction process and subsequent service life, during which it may have undergone alteration, deterioration, misuse and other changes to its, respectively, as-designed and as-built states, should also be taken into account. the accuracy of the assessment results obtained by applying load and strength models can usually be improved by collecting more data about the structure under analysis and about the actions and influences to which it is exposed (see 2.1.2). this does not mean, however, that the uncertainties can be completely resolved: in-service inspection and testing are also associated with uncertainties [4]. therefore, assessment is conducted by stages [3], raising the quality of the information available from stage to stage. 2.1.2. updating the acquisition of new data about an existing structure by means of inspections, measurements or tests is intended to supplement the available prior information, which may often be vague, with respect to parameters such as geometrical properties, actions and influences, construction material and geotechnical properties, as well as the actual condition of the structure, its behaviour or deformation capacity [3]. when new information becomes available, all relevant data need to be evaluated, taking into account the existing prior information. this process is known as updating of information, which is one of the main tasks of any assessment. two complementary approaches can be considered to update information about the properties of a structure and its performance under the actions and influences to which it is exposed [4]: • the updating of the probability of structural failure by using information from load testing or about the performance of the structure in the past; • the collection of data on individual basic variables by performing on-site inspections to update previously available uncertain information. 2.2. deterioration deterioration due to environmental influences, repeated actions, or use-induced wear, is typically a cumulative process that can adversely affect the reliability of existing structures. when designing new structures according to the rules of most codes, the possible combination of cumulative deterioration and extreme action effects is often neglected: durability design is usually considered separately from the design for ultimate and serviceability limit states. in some material-oriented codes, the design to prevent deterioration is based on verifications of well-defined and controllable limit states without direct negative consequences. these are often approximations to real limit states, with direct consequences, that are difficult to quantify and are therefore referred to as condition or proxy limit states [4]. such simplifications are not appropriate for the assessment of existing structures that are already affected by deterioration mechanisms: • reliability requirements should be verified for the combined effects of cumulative deterioration and the relevant actions and influences likely to occur during the remaining service life; 287 p. tanner, r. hingorani, c. lara acta polytechnica ctu proceedings figure 1. quantitative and qualitative methods to verify the reliability of existing structures. • condition limit states intended to prevent deterioration from affecting the performance of a new structure may not apply to existing structures that are affected by ongoing deterioration; • indicators that are not based on measurable quantities cannot be used for inspection and maintenance planning. any verification of the reliability requirements for the combined effects of deterioration and the relevant extreme actions assumes that models are available that adequately describe the propagation of deterioration as a function of time, as well as resistance models for deteriorated structures. in this context it is also important to acknowledge that in deteriorating structures the reliability requirements should be based on a shorter reference period than the design of new structures, e.g., one year, and that verifications should be carried out for the final year of the intended remaining service life. 2.3. reliability requirements although the principles and parameters to consider are the same, another relevant difference between the design of new and the assessment of existing structures manifests itself in the required level of reliability. the requirements for existing structures can often be lower than for new structures. economic, social and sustainability considerations can justify such a choice [8]: • the relative cost of safety measures to increase the reliability of an existing structure can be very high, while the additional cost of increasing the reliability in the design phase of new structures is generally low; • the strengthening or replacement of existing structures can lead to the resettlement of residents, the interruption of activities or may influence the values of cultural heritage, circumstances that normally do not play a role in the design of new structures; • extending the life of existing buildings and infrastructures or adapting such systems to new needs implies a reduction in resource consumption compared to replacing them with new structures. because of these circumstances, the required reliability levels for new structures can be considered conservative in most cases when used for the assessment of existing structures. in more detailed investigations, requirements, expressed in terms of acceptable failure probabilities, pft, or required reliability indices, βt, can be derived based on an explicit risk analysis or economic optimization, meeting acceptable human safety levels, e.g. with regard to current best practice [8]. when selecting required reliability levels, it should be taken into account that the intended remaining service life of an existing structure often may be shorter than the design service life of new structures. this is particularly relevant for cases in which the consequences of failure are dominated by economic losses: lower reliability requirements make sense for shorter periods of time, as investments in structural reliability are more cost-effective when the benefits can be realized over a longer period of time [8]. it should be noted that the reference period to which the target reliability is related can be selected regardless the anticipated remaining service life. if annual reliabilities are used, the corresponding structural performance should be achieved in each year of the remaining service life (see 2.2 for deteriorating structures). 2.4. verifications similar to the design of new structures, when assessing existing structures, it should be verified that with an appropriate reliability level no limit state is exceeded for all relevant assessment situations [6]. verifications can be performed using different methods (figure 1): • quantitative assessment based on calculations; • qualitative assessment based on past performance; • combination of both types of methods. if qualitative methods are used to assess a structure at the relevant limit states based on satisfactory past performance over a sufficiently long period of time, a detailed structural investigation should be carried out to show that several specific conditions (e.g., no damage nor deterioration, no changes to the structure or the environment, etc.) are met [6]. note that there is no reliability background for qualitative assessment methods. on the other hand, the available information about the past performance of a structure can also be used in a quantitative assessment, for example, for direct updating of failure probabilities or for resistance updating based on action effect histories (see 2.1.2). when elaborating the results of proof load tests or track records, it is important to distinguish between conclusions related to the structure (or structural 288 vol. 36 no. 0/2022 new or existing, does it matter? member) that was actually proof loaded and other (similar) structures or members [8]. for verification purposes (figure 1), the probability, p , of a structural failure event represented by a limit state, f , updated by new data, i, for example in case of load survival, p (f |i), is compared with the acceptable probability of failure, pft. the most accurate way of assessment would be to explicitly consider updated load and strength variables through the use of reliability methods or risk-based decision procedures. in the case of the former, the requirements are met if the reliability index, β, determined by using updated basic variables, reaches or exceeds the corresponding required value, βt, for all relevant assessment situations. alternatively, reliability can also be demonstrated in terms of failure probabilities [6]. when applying risk-informed methods [6, 8], the reliability of an existing structure with significant human, economic and environmental consequences of failure is demonstrated when the expected risk, er, also determined using updated basic variables and considering relevant assessment situations, does not exceed the corresponding acceptance criteria, ert. reliabilityand risk-based methods and procedures are time-consuming, however, calling for a specific operational knowledge of probabilistic methods, and are only used in special cases. to verify whether an existing structure meets the relevant reliability requirements for all assessment situations, the partial factor format is normally used, equivalent to the format specified for structural design [5]. the difference is that the relevant parameters can be modified based on updated information [6]. this applies in particular to the characteristic values of the basic variables, while partial factors can be fixed, valid across a range of cases, or adjusted in individual cases. 2.5. conclusions from the assessment although the detailed design of any engineering structure requires multiple iterative steps, the ultimate result is a clear definition of the type, layout and dimensions of the load bearing system and its individual members and details, as well as an appropriate selection of construction materials. this can be fundamentally different in an existing structure in which, after completing the assessment, different approaches may be advisable. the staged assessment process (see 2.1.1) of an existing structure is usually completed if clear conclusions can be drawn from the findings regarding the assessment objectives, or if an additional assessment step is unlikely to provide relevant new knowledge. depending on the assessment findings, the structure or a structural member may, within the scope of the assessment [6, 8]: • achieve the reliability required, assuming adequate inspection and maintenance during the remaining service life; • achieve the reliability required at the time of the assessment, but not for the complete period of time during which the existing structure is intended to remain operational, taking into account the anticipated development of its condition and the planned level of maintenance; • fail to achieve the reliability required; • need immediate correction of the existing condition by means of urgent risk mitigation measures. an intervention is required for each but the first of the possible assessment outcomes, i.e., if the required degree of reliability is not achieved with regard to structural safety, serviceability or durability, or if damage propagation after local failure is possible due to lack of robustness. interventions can include constructional or operational measures that belong to different categories [6]. appropriate interventions should be defined case-specifically, taking account of [6, 8]: • the type and importance of the structure; • the type of basic requirement (i.e., robustness, structural safety, serviceability, durability) that is not met; • possible cause and mode of attaining a limit state; • expected consequences of failure; • options of interventions that are available. 3. existing structures and the eurocodes 3.1. general the first generation of en eurocodes, published between 2002 and 2007, includes rules primarily intended for the design of new structures. additional provisions are needed to allow assessment of existing structures considering the differences between both tasks as described in section 2. new european technical rules for the assessment and retrofitting are planned for all types of existing structures and construction works, including geotechnical aspects, exposed to all sorts of actions. as a first working item, the material independent general rules, on the basis of assessment and actions on structures, should be developed [1]. however, there are two exceptions to this seemingly logical priority: • since the first generation of eurocodes, rules for seismic assessment and retrofitting of buildings have been part of the seismic structural design code [9]. a revised assessment part of this eurocode is also being developed for the second generation of eurocodes [10], including rules on the basis of seismic assessment. • among the second-generation material-specific eurocodes, that for concrete structures [11] includes 289 p. tanner, r. hingorani, c. lara acta polytechnica ctu proceedings an annex with specific provisions for existing structures. this document was prepared prior to the general rules on the basis of assessment [6]. cen’s standardization work programme envisages that work for all new parts of the eurocodes, including the new rules relating to existing structures, should follow a step-by-step approach. new prenormative documents are first published as science and policy reports (e.g., [1], related to assessment and retrofitting). subject to the relevant approval, these documents are transformed and published as technical specifications (e.g., [3], see 3.2). after a period of trial use and comments, cen decides whether such specifications should be converted into new eurocodes or new parts of existing eurocodes (e.g., [6], see 3.3). this process allows progressive development to consider observations not only from cen members but also from national experts and users. 3.2. technical specification the technical specification [3] is intended to provide, based on the recommendation of the pre-normative document [1], those additional or modified provisions which can allow the general principles of structural reliability, specified in the eurocode for structural design [5, 12], to be applied to the structural assessment of existing structures. the document thus addresses the reliability-related differences between assessment and design specifications (section 2). it also provides general principles regarding actions for assessment, complementing those for actions on structures used for design [13]. on the other hand, the document does not contain: • specific rules for the initiation of an assessment; • supplementary provisions for assessment and retrofitting in case of seismic actions; • resistance models that depend on the structural system and constituent materials; • rules on how to undertake interventions that can be carried out as a result of an assessment; • design of new members to be integrated into an existing structure. according to cen’s internal regulations, the technical specification [3] is a normative document made available at national level. however, it is important to realize that this type of document does not have the status of a eurocode and conflicting national standards may still exist. 3.3. eurocode on basis of assessment and retrofitting 3.3.1. general rules and actions the starting point for converting the technical specification [3] into a eurocode is that, as in the former, the rules for existing structures are not self-standing. consequently, they only address the differences between the design of new structures and the assessment and retrofitting of existing structures and are being developed as a new part [6] of the eurocode on basis of design [5]. its scope includes principles related to: • using updated data for basic action effects and strength variables, as well as for updated structural analysis and resistance models; • structural assessment in case of retrofitting, including the assessment of retained parts from existing structures. 3.3.2. material-specific provisions calculation models used for quantifying structural resistance in the design of new structures can implicitly or explicitly rely on specific requirements for material properties, detailing arrangements and execution tolerances being satisfied. structures that were designed and built to withdrawn standards may not meet these requirements. in such cases, design strength models cannot be used for assessment purposes, even if the existing structure being assessed is not affected by any deterioration mechanism. to allow the assessment of the strength of structures that do not comply with the design standards, the material-specific eurocodes need to be supplemented with additional provisions that must be consistent with the basis of assessment and retrofitting [6] and harmonized across materials. the background document [14] establishes recommendations and guidance for consideration by the code committees for the development of material-specific assessment and retrofitting provisions in the relevant future generation eurocodes, after the end of the current mandate. the document does not contain any requirements or instructions, but it seeks to achieve a consistent approach throughout the eurocode suite. 4. stadium refurbishment and extension 4.1. context structures should not only meet specified safety, serviceability and durability requirements with the aim of achieving an acceptably low probability of failure over a specified period of time (section 1). load-bearing systems are also expected to be robust [5, 6]. to reach an adequate level of robustness, many modern codes, such as the eurocode on basis of design [5], require that the consequences of damage to structures due to an unforeseen adverse event must not be disproportionate to the original cause. although the relevance of this feature of structures is well recognised, the clauses in structural codes and standards that seek to achieve this design goal are usually vague [15] and are mainly limited to general statements and deem to satisfy rules. to improve structural performance in terms of robustness, most known strategies require the adoption of measures in the conceptual design phase. a major problem in this regard is the lack of a general design philosophy for robustness. therefore, any conceptual solution used may improve the 290 vol. 36 no. 0/2022 new or existing, does it matter? a) b) figure 2. grandstand of the chapín stadium: a) as originally built; b) after refurbishment and expansion (courtesy of cesma ingenieros and cruz y ortiz arquitectos, © duccio malagamba). structural performance for some hazard scenarios and worsen it for others [16]. given this situation, even for apparently robust solutions, it is of the utmost importance to unequivocally identify all relevant hazards and hazard scenarios and to take them into account appropriately in the analysis. this is particularly relevant in the context of existing structures, where the adoption of measures related with the conceptual layout is normally not possible without constructional interventions. in addition, operational rules are needed to check structural robustness, beyond a list of general strategies such as those included in current codes [5, 6, 17]. these rules should comprise quantitative criteria for decisionmaking. a case study serves to illustrate a possible way to overcome the lack of practical rules to demonstrate sufficient robustness of an existing construction. 4.2. structure and intervention the existing chapín stadium in jerez, spain, a purely functional construction with no particular architectural or structural significance, was completed in 1988. the grandstands consist of in-situ concrete frames spaced 6m apart and prefabricated concrete platforms for the spectator seats, with only the main grandstand being covered. a typical frame is constituted by an inclined beam, reaching a maximum height of about 11m above the ground and having a standard crosssection of 0.85m × 0.5m, and two supporting columns about 9.25m apart, which transfer the loads to the foundations (figure ??). the effective cross-sections of the outer and inner columns are 1.1m × 0.5m and 0.8m × 0.5m, respectively. the stadium’s refurbishment and expansion included the addition of a new roof (figure ??) and gallery to the existing grandstands. access for the public has also been reorganized. in addition, a sports centre and a hotel were integrated into the stadium. working areas for sports officials have been set up on two levels under the access to the main grandstand. a section through one of the standard frames after the stadium’s expansion shows the new cantilevered roof with integrated catwalk for maintenance purposes, as well as the new gallery with a 2.25m wide slab for the circulation of the spectators (figure ??). the cross-section also shows the new concrete wall surrounding the entire stadium, with integrated doors giving access to the new stairs, which in turn lead to a new slab and new walkways for spectator access to the stands. in the case of the main grandstand (figure ??), the solution is essentially the same. the main differences are the presence of a basement and that the access to the grandstand is formed by a solid slab instead of walkways. steel frames are connected to the existing concrete frames to form the new roof, implying an important change of the structural system and leading to completely different internal forces and moments in the existing members than those for which they were originally designed and built. gravity loads and positive wind pressure result in the flow of forces in the steel structure as shown in figure ??, and in important action effects, in particular bending moments, in the existing concrete frames. wind tunnel tests have shown that negative wind pressure can reverse the action effects, leading to inverted bending moments in the existing concrete structure (figure ??). the assessment of the existing frames for situations at sports and other events, when persons are present on and below the grandstand, is not discussed here as setting reliability requirements for structures under temporary use conditions implies specific considerations due to reduced risk exposure times [18]. irrespective of the above situations, the office space under the main grandstand (figure ??) is subject to normal use conditions. the question arises whether the risks associated with the existing frames are still at an acceptable level after the change in the static system. indeed, in particular, extreme wind loads acting on the new roof could lead to frame failure with important consequences including fatalities when 291 p. tanner, r. hingorani, c. lara acta polytechnica ctu proceedings a) b) figure 3. cross-sections after refurbishment and expansion (courtesy of cesma ingenieros): a) standard frame; b) frame of main grandstand. persons are present in the office areas. it must therefore be investigated whether the stadium conversion might lead to insufficient robustness (see 4.1). 4.3. operational procedure 4.3.1. reliability requirements the target reliabilities, which may be associated with indicative consequence classes according to some codes, βt,code, often pursue economic optimisation [4, 19]. when failure is assumed to entail possible human death or injury, life safety risk should also be embraced by an additional reliability requirement, βt,lr. this typically occurs when considering robustness problems. in such cases, the target reliability index should therefore be taken as the greater of βt,lr and βt,code. under an operational criterion suggested [16], reliability requirements for human safety, βt,lr, are inferred from current best practices and represented in figure 4 for a reference period of one year, varying with the area affected by the collapse, acol, and the respective consequence class, cc1 to cc3, as defined in the eurocode [5]. 4.3.2. assessment of robustness inasmuch as the expected consequences of failure and the relative cost of safety measures may differ depending on the failure mode envisaged, a distinction should be drawn among reliability requirements for: • member failure due to identified design situations: βt,code; • key member failure due to identified design situations: max(βt,lr; βt,code); • collapse of the remaining system still standing during and after key member failure due to an unidentified accidental situation: βt,rs. in this context, key members are those on which the strength and stability of the structural system or subsystem depend. they may be identified, then, as members in whose absence an anticipated load-bearing mechanism will develop only if suitable measures are adopted, i.e., if the corresponding structural behaviour can be verified with sufficient reliability. in terms of the remaining structural system left standing after key member failure, both the relevant design situations and the performance requirements depend on the design goal, e.g., rescuing any endangered people [16]. given that in robustness verifications the possible failure of a key member is assumed to be due to an unidentified accidental situation, the required reliability for the remaining system, βt,rs, may be lower than calculated for persistent and identified accidental situations, since conditional reliability is envisaged [5]. the required reliability, βt,rs, can therefore be found by factoring in the likelihood of scenarios that might induce key member failure, a parameter for which exact values cannot be determined and discussion of which lies outside the scope of this paper. approximate values can be drawn from empirical estimates of the likelihood of specific natural and man-made hazards, which are available in the literature [20–22]. 4.4. robustness of existing frames in the event of a frame failure, induced for instance by unexpected extreme wind loads acting on the new roof 292 vol. 36 no. 0/2022 new or existing, does it matter? figure 4. annual reliability requirements for human safety, βt,lr, depending on the collapse area, acol, and the consequence class, cc [5], recommended for the assessment of existing structures; βt,eco [19]: tentative target reliabilities based on economic optimization for large relative cost of safety measures and different consequence classes, cc. connected to the existing frames, collapse propagation would be prevented as the existing prefabricated transverse platforms are statically determinate without being anchored to the adjacent frames, while the transverse steelwork needed for the new gallery and for the out of plane stability of the steel frames of the new roof allow for large deformations without pulling down adjacent frames. regardless of this collapse stop mechanism, the frames over the office areas (see 4.2) should be considered key members (see 4.3) as persons may be at risk. a potential beam, column, joint or foundation failure of any of these existing frames, for which reliability requirements need to be established, could affect an office area of approximately acol ≈ 24·6·2·2 ≈ 580m2. with such an area possibly affected by a failure mechanism of a frame, for whatsoever assessment situation, more stringent reliability requirements may need to be imposed on frame members than on other grandstand members, in persistent and also in identified accidental situations. if the grandstand with office space is classified under standard eurocode [5] consequence class cc2 (buildings where people normally enter, e.g., residential and office buildings), the requirement to reliably ensure human safety, βt,lr,cc2 (figure 4), that must be used in frame assessment in persistent situations is higher than the relevant requirement based on economic optimization, βt,eco,cc2 [19]. the reference period used for comparison is tref = 1 year. on the other hand, the human safety requirement, βt,lr,cc2, is considerably lower than the target value specified in the eurocode, βec,1 [5]. reasons for this difference are that in the eurocode [5]: • target values are intended for new structures where the relative cost of increasing safety is rather low, making them conservative for existing structures (see 2.3); • no distinction is made between target and minimum requirements [23]; • annual requirements do not take into account correlation in failure events in each year of the lifetime: βec,1(cc2) = 4.7 > βt,lr,cc2(acol = 580m2; 1 year) = 3.7 > βt,eco,cc2(1 year) = 3.3 (1) this result calls for partial factors for verifying frame structural safety in persistent situations that are higher than the values that would result from purely economic optimization, but lower than the values recommended in the eurocode for design [5]. analogously, the assessment action effects and respective strengths may have to be adjusted to verify structural safety in identified accidental situations. 5. conclusions individuals and society expect civil engineering structures to be safe for users and people in their sphere of influence, and to protect the relevant tangible and intangible assets. the failure of a load bearing system should therefore be an extremely rare event. accordingly, the ability of structures must be ensured to meet specified safety, serviceability and durability requirements with the goal of achieving an acceptably low probability of failure over a specified period of time. in this context, the methods of analysis based on established engineering theory and practice and the general principles of structural reliability with regard to the treatment of uncertainties when verifying the specified requirements are the same, both for the design of new structures and for the assessment of 293 p. tanner, r. hingorani, c. lara acta polytechnica ctu proceedings existing ones. that notwithstanding, there are important differences between assessing existing and designing new structures. such differences, which are summarized in the paper along with their treatment in the eurocodes, can often lead to a high degree of conservatism when designoriented methods are used for assessment. unnecessary strengthening or replacement of existing structures can be the result. to avoid such an unsatisfactory outcome, both from an economic and a sustainability point of view, reliabilityand risk-based methods and procedures can be applied to demonstrate the sufficient reliability of existing structures. such methods, which have been developed over the past decades, are well documented in the literature and a growing number of codes, including the eurocodes, allow their use. however, although they represent the state of the art and despite their advantages compared to standard verifications and their recognition by current regulations, these more advanced methods of demonstrating structural reliability are rarely applied in practice. the question arises why? beyond aspects like the fact that reliabilityand risk-based methods are time-consuming, e.g., to collect valid and sufficient input data, and require specific knowledge and software (see also 2.4), is it because of: • insufficient training at engineering schools, i.e., educational problems? • insufficient knowledge of these methods among stakeholders, i.e., communication problems, outside of their circle, of the specialists who advocate the use of probabilistic and risk-based methods? • engineers’ preference for hard-and-fast rules over decision-making processes, e.g., as a result of actual or perceived liability reasons? • lack of time and financial resources for advanced analysis, possibly because engineering services in the construction sector are viewed by owners, promoters and contractors as an expense rather than an investment? • in connection with the previous aspect, the perverse business model in the construction sector, which links budgets and fees to material consumption? • low relative costs for structural interventions compared to the other costs for conversions of existing buildings or infrastructure, in cases of change of purpose or needs, where an assessment is required? • availability of other, more efficient methods and tools to assess the full capacity of structural systems, e.g., nonlinear finite element analysis? there may be other reasons, or a combination of reasons, for the sparse practical application of reliabilityand risk-based methods. that notwithstanding, or perhaps because of it, operational procedures and tools are essential for the efficient assessment of existing structures, against the background of dwindling resources and increasing awareness of sustainability issues. there is therefore a need for further research and development in the area of assessment methods and codes. in this sense, answers to the questions raised should be sought and considered when choosing the most promising way forward. in addition, it should also be considered that recent work on the reliability background of the eurocodes [8] has shown that on certain issues there are discrepancies between current codes and the state of the art, or that on other aspects there is a lack of sufficient scientific knowledge. most of these issues are relevant to both the design of new and the assessment of existing structures. they are not listed here, but also represent topics for future research. references [1] jrc. new european technical rules for the assessment and retrofitting of existing structures. european commission, joint research centre, ispra, 2015. isbn 978-92-79-46022-7. [2] iso 13822. basis for design of structures – assessment of existing structures, international organization for standardization, geneva, 2010. [3] cen/ts 17440. technical specification – assessment and retrofitting of existing structures, european committee for standardization, brussels, 2020. [4] iso 2394. general principles on reliability for structures, international organization for standartization, geneva, 2015. [5] pren 1990. eurocode – basis of structural and geotechnical design, n2885, 2nd enquiry draft, european committee for standardization, brussels, 2021. [6] pren 1990-2. eurocode – basis of assessment and retrofitting of existing structures: general rules and actions, final document, european committee for standardization, brussels, 2021. [7] fib bulletin 80. partial factor methods for existing concrete structures. fédération internationale du béton, lausanne, 2016. isbn 978-2-88394-120-5. [8] cen/tc250/sc10/ad-hoc group reliability of eurocodes. technical report for the reliability background of the eurocodes, draft, european committee for standardization, brussels, 2021. [9] en 1998-3. eurocode 8 – design of structures for earthquake resistance – part 3: assessment and retrofitting of buildings, european committee for standardization, brussels, 2005. [10] pren 1998-3. eurocode 8 – design of structures for earthquake resistance – part 3: assessment and retrofitting of buildings, european committee for standardization, brussels, 2021. [11] pren 1992-1-1. eurocode 2 – design of concrete structures – part 1-1: general rules – rules for buildings, bridges and civil engineering structures, document prepared by subcommittee cen/tc250/sc2, european committee for standardization, brussels, 2021. [12] en 1990. eurocode – basis of structural design, european committee for standardization, brussels, 2002. [13] en 1991. eurocode 1 – actions on structures, european committee for standardization, brussels, 2002–2006. [14] cen/tc250/wg2.t2/project team assessment of existing structures. report on requirements and guidance for the development of material-specific assessment and retrofit provisions, final document, european committee for standardization, brussels, 2021. 294 vol. 36 no. 0/2022 new or existing, does it matter? [15] cen/tc250/wg6.t2. robustness rules in material related eurocode parts, report of project team wg6.t2, european committee for standardization, brussels, 2020. [16] p. tanner, r. hingorani. approach to robust and elegant bridge design. structural engineering international 31(4):638–648, 2021. https://doi.org/10.1080/10168664.2021.1938345. [17] en 1991-1-7. eurocode 1 – actions on structures – part 1-7: general actions – accidental actions, european committee for standardization, brussels, 2006. [18] r. hingorani, p. tanner. risk-informed requirements for design and assessment of structures under temporary use. risk analysis 40(1):68–82, 2020. https://doi.org/https://doi.org/10.1111/risa.13322. [19] jcss. probabilistic model code, joint committee on structural safety, jcss, 2001. [2022-08-04], https://www.jcss-lc.org/jcss-probabilisticmodel-code/. [20] b. r. ellingwood. mitigating risk from abnormal loads and progressive collapse. journal of performance of constructed facilities 20(4):315–323, 2006. https://doi.org/10.1061/(asce)08873828(2006)20:4(315). [21] m. g. stewart. life-safety risks and optimisation of protective measures against terrorist threats to infrastructure. structure and infrastructure engineering 7(6):431–440, 2011. https://doi.org/10.1080/15732470902726023. [22] t. vrouwenvelder, b. j. leira, m. sykora. modelling of hazards. structural engineering international 22(1):73–78, 2012. https://doi.org/10.2749/101686612x13216060213356. [23] j. köhler. reliability targets for semi-probabilistic design standards. in life-cycle analysis and assessment in civil engineering: towards an integrated vision, pp. 387–395. taylor and francis group, london, 2019. 295 https://doi.org/10.1080/10168664.2021.1938345 https://doi.org/https://doi.org/10.1111/risa.13322 https://www.jcss-lc.org/jcss-probabilistic-model-code/ https://www.jcss-lc.org/jcss-probabilistic-model-code/ https://doi.org/10.1061/(asce)0887-3828(2006)20:4(315) https://doi.org/10.1061/(asce)0887-3828(2006)20:4(315) https://doi.org/10.1080/15732470902726023 https://doi.org/10.2749/101686612x13216060213356 acta polytechnica ctu proceedings 36(0):286–295, 2022 1 introduction 2 assessment versus design 2.1 information 2.1.1 state of information 2.1.2 updating 2.2 deterioration 2.3 reliability requirements 2.4 verifications 2.5 conclusions from the assessment 3 existing structures and the eurocodes 3.1 general 3.2 technical specification 3.3 eurocode on basis of assessment and retrofitting 3.3.1 general rules and actions 3.3.2 material-specific provisions 4 stadium refurbishment and extension 4.1 context 4.2 structure and intervention 4.3 operational procedure 4.3.1 reliability requirements 4.3.2 assessment of robustness 4.4 robustness of existing frames 5 conclusions references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.35.0019 acta polytechnica ctu proceedings 35:19–22, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague construction 4.0 in the concept of the railway pavel krupík czech technical university in prague, faculty of civil engineering, department of construction management and economics, thákurova 2077/7, 166 29 prague 6, czech republic correspondence: pavel.krupik@fsv.cvut.cz abstract. the digital and technological innovations that have been developing massively in recent years are often referred to as the 4th industrial revolution or industry 4.0. the construction industry seems to be somewhat on the sidelines. the big topic is bim (building information modelling), but this is only one component of the contemplated construction 4.0. other topics, if not completely neglected, are minimally neglected in the czech republic. yet, and perhaps for this very reason, it is very important to address them. this article looks at the description of the different elements of the construction 4.0 concept and their possible application in transport, through the overall digitisation of the railways. keywords: construction 4.0, digitalisation, railway. 1. introduction the construction industry has innovative solutions and visions at its disposal. however, the key problem is that most construction companies do not consider themselves part of the evolutionary process and behave as if digital innovation has no real impact on them. this means that there is not yet a consensus among most construction executives about the reasons for change and therefore they are not adapting to it. the dominant uncertainties are productivity, costs, quality of work, workforce and skills management, and health and safety, which are just some of the opportunities that construction 4.0 could address. these need to be factored into a longer-term strategy to adapt in a way that will allow society to thrive in the next phase of digital evolution that will change many aspects of society [1]. as these innovative technologies make their way into different phases of the construction project lifecycle, there is growing concern about the future of jobs. while the use of technology, and robotics in particular, has the potential to increase productivity and safety, it should not necessarily reduce overall employment in the construction industry in the long term. it is expected that existing roles will evolve and new roles will emerge (e.g. employees with digital skills will be needed in addition to designers) [2]. construction 4.0 in the context of this article includes bim, virtual and augmented reality, 3d printers, advanced building materials, prefabricated and modular construction, big data and predictive analytics, 3d scanning and photogrammetry, autonomous vehicles and drones, cloud collaboration (real-time collaboration), the internet of things, or wireless monitoring and device connectivity [3]. construction 4.0 in railways is somewhat confronted with the fact that transport systems are complex in terms of technology and operation, and involve a range of human actors, organisations and technical solutions. for the operation and management of such complex environments, a viable solution is the use of intelligent computer systems, such as computerised traffic management systems for air traffic coordination or advanced monitoring and diagnostic systems in vehicles. in addition, the means of transport cannot compromise the safety of passengers by applying operational and maintenance activities. in fact, safety is becoming a more difficult objective to achieve using traditional maintenance strategies and computerised solutions are coming into the picture as the only option to deal with complex systems that interact with each other and try to balance the increase in technical complexity together with stable and acceptable reliability indicators [4]. the importance and significance of railway development through the construction 4.0 concept is evidenced by efforts to apply it abroad. these include: 1.1. smartrail 4.0: the swiss railway modernisation programme in the next few years, a number of technical production systems that are important for train travel will be replaced due to their age. at the same time, technologies are being developed at an ever-increasing pace, offering rail companies new opportunities to improve efficiency and punctuality [5]. sbb, bls, schweizerische südostbahn ag (sob), rhb, transport public fribourgeois (tpf) and the public transport union (ptu) would like to take advantage of this opportunity together. together, these companies are preparing the railway for the future as part of the smartrail 4.0 industrial programme. this is the first programme that covers the entire railway production, from timetable planning to train control [5]. as a first step, the concept of this overall architecture has been consolidated and refined in the period 19 https://doi.org/10.14311/app.2022.35.0019 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en pavel krupík acta polytechnica ctu proceedings up to 2020, and a more detailed roadmap has been drawn up. the smartrail 4.0 programme is divided into five sub-programmes and covers technological development for the next 20 years. the participating groups are modernising the rail system to ensure the robustness of future services, increase the capacity of existing infrastructure, improve the safety of rail workers and stabilise system costs [5]. 1.2. the construction 4.0 concept goes hand in hand with industry 4.0 thanks to digitisation, it is possible to use data obtained from traffic, for example, when planning the development of new railway lines. recent innovation projects announced by european rail operators deutsche bahn (db) and sncf are a perfect example of how railways are using frmcs to become a leader in industry 4.0. db, one of the world’s largest and most important railways, has announced that it will work with railway experts from nokia on the "digitale s-bahn hamburg" project, which will use 5g-based frmcs to deploy – as early as 2021 – highly automated trains and shunting operations on its s-bahn line in hamburg [6]. 2. aims the aim of this article is to describe the possibilities in terms of construction 4.0 in the planning and implementation of railways. therefore, building 4.0, in the concept of this paper, has the task of preparing the infrastructure to be ready for operation and maintenance. 3. methods the article mentions the possible use of construction 4.0 in the railway concept. the research methodology consists in searching the scopus open access database for relevant articles with innovative solutions and visions. furthermore, solutions that have been used in the recent past or are yet to be planned to be used in the future are also presented. 4. results and discussion 4.1. frmcs communication standard the latest standard for rail communications is moving from the widely adopted gsm-r to the frmcs (future railway mobile communications system) standard, with 5g as a key enabling technology. frmcs sets the stage for total connectivity and seamless endto-end operations with smarter automation and the ability to leverage new internet of things technologies for increased productivity and reliability breakthroughs. once frmcs is in place, railways can deliver – in the near future – key improvements that will increase operational speed and deliver a superior ride and travel experience for passengers and goods [6]. 4.2. human factor the human factor in innovation and development is in many cases one of the slowers of construction 4.0 transition and application. the natural human reaction to change is fear, rejection and even negativity. in the process of industry 4.0 development, it becomes therefore necessary and important to involve employees and students in these processes in an early and important way. their involvement can significantly accelerate the processes, so the development of it should take place in advance. similarly, the acquisition of technology knowledge or practical experience with machines, software or intelligent programs that they will encounter or actively use in the future to perform their work in the workplace will significantly accelerate and streamline adaptation, leading to increased time efficiency , reduced costs and time-consuming further training of employees [7]. the social (human) factor was identified as the most critical factor that has the greatest impact on the successful implementation of construction 4.0 in general, but other contributing factors (pestels: political, economic, social, technological, environmental, legal and security factors) suggest that these factors are interrelated and should be addressed simultaneously. the introduction of industry 4.0 in the construction sector through the principles of construction 4.0 would put the sector on a par with similar sectors such as manufacturing and automotive, where industry 4.0 is already widely used [8]. 4.3. bim currently, great efforts are being made to achieve a more general application of the bim methodology in the field of transport infrastructure, beyond specific elements such as tunnels, viaducts and bridges, as evidenced, for example, by the case study of the renewal of a railway line [9]. this highlighted the fact that in the field of transport infrastructure systems, the railway presents specificities and complexities that lend more interest and importance to the implementation of bim. the highest demands on railway safety (e.g. high-speed train) require more frequent monitoring and maintenance. in addition, the complexity of railway components and systems (infrastructure and vehicles) requires better tools for data analysis and processing. in this sense, bim will be a useful methodology with huge technical, economic and environmental benefits throughout the life cycle of railway infrastructure. one of the benefits of bim is the possibility to implement a track inspection system that includes all the usual geometric parameters in order to integrate maintenance planning linked to quality control tests. 4.4. artificial intelligence possible applications of artificial intelligence in building 4.0 are: 20 vol. 35/2022 construction 4.0 in the concept of the railway figure 1. railways 4.0 with 5g [6]. • design based on artificial intelligence for topology analysis of prefabricated elements – configuration, segmentation and optimization, • optimisation of supply chain management to reduce costs, • predicting various potential project risks based on historical and real-time data [10]. 4.5. worker 4.0 this is a concept that materializes the main principles and behaviors of workers in the construction 4.0 scenario. precisely because the productivity of the worker 4.0 movement is designed to manage and continuously improve processes, the four-stage pdca management method is appropriate for its implementation. first, an innovation project based on the worker 4.0 movement’s productivity methodologies for craftspeople needs to be planned. the next step is to complete data collection and processing. this means checking process performance to assess the level of mechanization. the last step is to act on productivity in the workplace, implementing control measures and setting up continuous improvement procedures. such an analysis should be based on the quantitative and qualitative values found in the measurements and the subsequent possibility of improving production indicators [11]. 5. conclusion the article dealt with the use of construction 4.0 elements that can be used in the construction and downstream operation of railways. some of these are (fortunately) very close to being used in practice. during the research it was found that one of the main factors why industry 4.0 is not as developed in the construction sector as in other sectors is the human factor, whether it is the reluctance to use innovation in practice, the lack of legislation or the need to train employees. the author believes that it makes sense to continue with the set direction of research in this area, but considers it appropriate to address other parts of the contemplated construction 4.0 in the railway sector than just bim. in this vein, the author welcomes the planned establishment of the "construction 4.0" platform, whose ambition is to bring together major research institutes, especially at technical universities, with progressive construction companies and to offer a space for mutual collaboration. the aim is to introduce new technologies and the use of industry 4.0 principles into construction and to increase the competitiveness of the czech republic in this area on a european scale. acknowledgements this work was supported by the grant agency of the czech technical university in prague, grant no. sgs20/100/ohk1/2t/11. references [1] r. woodhead, p. stephenson, d. morrey. digital construction: from point solutions to iot ecosystem. automation in construction [online]. 2018, 93, 35–46. issn 09265805, https://doi.org/10.1016/j.autcon.2018.05.004. [2] b. garcía de soto, i. agustín-juan, s. joss, j. hunhevicz. implications of construction 4.0 to the workforce and organizational structures. international journal of construction management [online]. 2019, 1-13. issn 1562-3599, https://doi.org/10.1080/15623599.2019.1616414. [3] p. krupík. systems for intelligent parking of trucks at motorway rest areas as part of construction 4.0. iop conference series: materials science and engineering [online]. 2021, 1203(3) [cit. 2022-01-18]. issn 1757-8981, https://doi.org/10.1088/1757-899x/1203/3/032052. 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[7] z. gerhátová, v. zitrický, v. klapita. industry 4.0 implementation options in railway transport. transportation research procedia [online]. 2021, 53, 23–30. issn 23521465, https://doi.org/10.1016/j.trpro.2021.02.003. [8] w. alaloul, m. liew, n. zawawi, i. kennedy. industrial revolution 4.0 in the construction industry: challenges and opportunities for stakeholders. ain shams engineering journal [online]. 2020, 11(1), 225–230. issn 20904479, https://doi.org/10.1016/j.asej.2019.08.010. [9] j. neves, z. sampaio, m. vilela. a case study of bim implementation in rail track rehabilitation. infrastructures [online]. 2019, 4(1). issn 2412-3811, https://doi.org/10.3390/infrastructures4010008. [10] m. wang, c. wang, s. sepasgozar, s. zlatanova. a systematic review of digital technology adoption in off-site construction: current status and future direction towards industry 4.0. buildings [online]. 2020, 10(11). issn 2075-5309, https://doi.org/10.3390/buildings10110204. [11] d. calvetti, p. mêda, m. chichorro gonçalves, h. sousa. worker 4.0: the future of sensored construction sites. buildings [online]. 2020, 10(10). issn 2075-5309, https://doi.org/10.3390/buildings10100169. 22 https://doi.org/10.1007/978-3-319-27064-7_30 https://www.smartrail40.ch/index.asp?inc=en/program.asp https://www.smartrail40.ch/index.asp?inc=en/program.asp https://www.nokia.com/blog/railways-set_-pace-industry-40-transformation-frmcs5g https://www.nokia.com/blog/railways-set_-pace-industry-40-transformation-frmcs5g https://doi.org/10.1016/j.trpro.2021.02.003 https://doi.org/10.1016/j.asej.2019.08.010 https://doi.org/10.3390/infrastructures4010008 https://doi.org/10.3390/buildings10110204 https://doi.org/10.3390/buildings10100169 acta polytechnica ctu proceedings 35:19–22, 2022 1 introduction 1.1 smartrail 4.0: the swiss railway modernisation programme 1.2 the construction 4.0 concept goes hand in hand with industry 4.0 2 aims 3 methods 4 results and discussion 4.1 frmcs communication standard 4.2 human factor 4.3 bim 4.4 artificial intelligence 4.5 worker 4.0 5 conclusion acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.38.0389 acta polytechnica ctu proceedings 38:389–395, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague ecopool+++: developing a sustainable outdoor heated swimming pool miguel josé oliveiraa,∗, armando invernoa, fátima farinhaa, jânio monteiroa, cristiano cabritaa, maria seguraa, francisco do carmob, adelino venturinhac, gina matiasd a university of algarve, campus da penha, 8005-139 faro, portugal b cristal construções – materiais e obras de construção civil, sítio da mesquita, mexilhoeira grande, 8500-132 portimão, portugal c itelmatis control systems, r. da fonte velha, 8700-178 olhão, portugal d itecons, rua pedro hispano, s/n, 3030-289 coimbra, portugal ∗ corresponding author: mjolivei@ualg.pt abstract. the university of algarve, in consortium with “cristal construções – materiais e obras de construção civil”, “itelmatis control systems” and “itecons – instituto de investigação e desenvolvimento tecnológico para a construção, energia, ambiente e sustentabilidade”, is developing a new concept of outdoor heated sustainable swimming pool, having a higher energy efficiency and lower environmental footprint. the main objective is to minimise energy and water consumptions, while assuring a comfortable temperature for the user, extending the utilization of the pool in the algarve region, from the summer months, to at least 8 months of the year. in order to do it, different systems will be tested, including solar thermal and photovoltaic panels, inverted underfloor heating, heat accumulator exchanger with phase change materials and energy dissipating pipes. an innovative thermal insulation system will be included in the interior of the pool tank, together with a new system for the covering of the water plane. all these systems will be monitored and controlled by an industrial automation system that will communicate, via a programmable logic controller, using industry standard communication protocols, with a smart platform, that will be in charge of the global operation’s optimisation. this platform will support an intelligent and predictive control and monitoring module, that controls the automation system to maximize the energy usage of the renewable sources, while assuring the user preferences. one of the main outputs of this project is the construction of a smaller scale prototype of a swimming pool, with the characteristics mentioned above, in order to test and validate the proposed developments. keywords: outdoor heated swimming pool, solar energy, pcm heat accumulator, smart energy and water management system. 1. introduction most existing swimming are largely inefficient due to heat and water losses, having a substantial impact in the associated environmental footprint. tyler et al. [1] calculated the environmental impact of residential swimming pools in arizona (usa) and other warmer regions, pointing out, as the main impacts per pool the following numbers: a water footprint from 45 to 185 m3/year; an energy footprint between 2 400 and 2 800 kwh/year; a carbon footprint of 1 400 ± 50 kg co2e/year. in terms of heat losses, outdoor heated pools require very high energy consumptions, due to: (1.) evaporation of water into the surrounding air, (2.) radiation to the sky; (3.) convection near the surface of the water and (4.) conduction through the walls and floor to the ground. the water in swimming pools is an excellent medium for heat accumulation, which is the reason why, when there is greater availability of solar radiation, particularly in the summer period, the conditions for its use improve. outside this period, the environmental conditions tend to no longer be favourable, causing lower free heat gains and simultaneously, greater thermal losses. the large energy losses in outdoor swimming pools, as previously mentioned, are due to evaporation, radiation and convection, in descending order of importance. as the water surface is the pool area where the greatest heat exchanges take place, it is essential that it is efficiently and safely protected. a conventional roof with thermal insulation can reduce energy losses by between 50 % and 70 % [2]. muleta [3] considers that it is possible to reduce evaporation by about 95 % using composite material coverings. by reducing, for example, the effect of evaporation, using an adequate coverage, benefits are not only associated with the 389 https://doi.org/10.14311/app.2022.38.0389 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en m. j. oliveira, a. inverno, f. farinha et al. acta polytechnica ctu proceedings reduction of water replacement needs, but also in the subsequent reduction in the consumption of chemicals for water treatment. thermal losses due to conduction through the walls and floor are usually neglected. there are some thermal insulation solutions on the market for the swimming pool tank, which are based on the application of extruded polystyrene sheets on the outside of the tank, in contact with the ground [4], or on modular systems that integrate the structural and insulating function in the panel [5] or in the block [6]. this type of solution is, however, aimed at new pools, being difficult to implement or even impractical to apply in existing pools. rose [7] proposes that the thermal insulation of the walls and floor in existing swimming pools is done from the inside of the tank, based on rigid phenolic panels (high pressure laminates) or cellular glass, together with a rigid waterproof mortar and pvc coating. in addition to the innovative construction system for reducing thermal losses, highly efficient solutions from an energy point of view are required, such as heat storage systems. systems integrating phase change materials (pcm) were studied by silva et al. [8] in building structures of brick masonry walls. the pcms that have greater interest in buildings and civil construction in general, are well characterised in [9], where 150 of these materials are carefully described. zsembinszki et al. [10] simulated the thermal behaviour of an outdoor swimming pool with 50 m2 and 1.65 m depth, in three locations in spain using local climatic data such as temperature and relative humidity of the outdoor air, solar radiation and wind speed. the bottom and walls of the pool were composed of a heat exchanger, coated with 3 cm of pcm for heat accumulation, allowing the control of the water temperature in advance (about 20 hours) in relation to the foreseeable period of use of the pool. salgueiro [11] presents a good characterisation of mortars based on light materials with pcm mixtures. it describes the methods used to determine the mechanical and thermal properties of different specimens, having different compositions of mortars. the energy consumed in pumping water, for circulation through the filtering systems, is not negligible, even though it depends on the size of the pool. as suggested by hunt and easley [12] and zhao et al. [13], the replacement, in swimming pools, of simple pumps that work only at nominal speed by pumps equipped with variable speed drives, which enable the motor to run at different speeds of rotation, can allow a 75–80 % energy reduction. in terms of water losses, severine [14] refers the importance of optimising the regeneration operations of swimming pool filtering systems to reduce water consumption. this author also highlights the possibility of reusing the water from washing filters for watering green spaces. water losses also result from leaks in the hydraulic circuits. at this level, the most used detection method is the evaluation of pressure variation. thus, the existence of an intelligent automatic control system for the detection of leaks in swimming pools is relevant, allowing the minimisation of human intervention, in similar way to what is already done in free surface drainage channels [15] or in fuel tanks [16]. active measures can be used to reduce the impact of water losses and to increase the utilization of locally installed renewable energy sources, reducing the energy requirements of swimming pools. at this level, the best market practices point to the implementation of industrial automation systems, made-to-measure using programmable logic controllers for the supervision and control of equipment in an integrated manner for the interconnection of hydraulic systems, water treatment, and water heating systems. these systems already exist in some hotels pools and spas, normally integrated in building management systems, called bms (building management system) through industrial communication buses. complemented with alarmist software modules, they promote the optimisation of energy and water consumption as well as the reduction of response times in the detection and resolution of occurrences, with their advantages being widely recognised [17]. these intelligent control systems that allow the monitoring of parameters, imposing user-defined limits and generating alarms when they occur, are already used today in a wide variety of situations. for instance, in the context of swimming pools, the low-cost solution proposed in [18], aims to reduce the requirements of human intervention in the maintenance of the pool. in terms of intelligent control algorithms, to improve the energy efficiency of indoor swimming pools, marín in [19] proposes the use of predictive control in a hybrid control system that involves solar collectors and a boiler (possibly powered by biomass) for the thermal regulation of temperature. using this methodology, it is possible to reduce the pool’s energy demand significantly (in the order of 18.76 %) and consuming 42.64 % less fuel compared to a conventional proportional integral derivative (pid) control. still in the context of optimising energy efficiency and specifically with the aim of reducing response time, machine learning techniques such as random forest [20] or predictive control based on models [21, 22] can be considered. 2. ecopool+++ project the ecopool+++ project aims to develop a new heated pool solution, more efficient, in terms of energy and water requirements, integrating an intelligent management of the various systems. the main system to be developed will consist of a thermal insulation solution inside the tank’s envelope, which will reduce energy losses and, consequently, the heating needs. an attempt will also be made to develop a new system for covering the water surface, made up of multilayer panels of glass and air, which will reduce evaporation 390 vol. 38/2022 ecopool+++: developing a sustainable outdoor heated . . . losses, which are both heat and mass, as well as heat losses by convection and radiation. regarding water heating needs, efforts will be made to ensure that these are fully met using solar energy, maximizing solar gains through the water plan and integrating solar energy capturing systems through the use of solar panels and a heat collector floor. the possibility of heat storage through the use of reserve tanks containing pcms will also be analyzed. the possibility of “diverting” the excess heat from the water body of the tank for other purposes will also be analyzed, either directly for the production of domestic hot water or for the heating of the ventilation air of changing rooms, in periods when the water temperature exceeds comfort needs. regarding water efficiency, systems will be studied and designed to contribute to the reduction of water consumption, either through the early detection of leaks, or through the minimization of water consumption in the regeneration operations of the filtration and eventual reuse of the resulting effluents. finally, a control and monitoring system will be designed, with intelligent and predictive functions, capable of interconnecting the various technologies described above. through the comprehensive monitoring of all systems in operation in a real context, it will be possible to evaluate their respective performances (individual and in collective operation). this information will make it possible to decide which is the best operational set of recommended systems for each situation in view of its geographic location. the main market objectives of this project, are: (1.) to ensure the existence of an aggregated final solution, enhancing water and thermal savings, supporting with greater thermal comfort (water temperature with an adequate level of comfort for most of the year using only solar energy) and (2.) provide the leading company’s product portfolio with a disruptive pool solution. within the scope of this project, a small-scale prototype will be built, incorporating all the recommended solutions, in order to test and validate the intended developments, in the technical, environmental and economic aspects, allowing the final solution to be commercially valuable and with a potential to be exportable. the solutions developed, in part or in a whole, will be able to be applied in existing swimming pools, in order to improve their energy and water efficiency. 3. ecopool+++ systems in the following a description is made of the developments occurring in the aim of ecopool+++ project to implement the prototype of a sustainable swimming pool. 3.1. thermal insulation system the functions, requirements and properties of the new thermal insulation systems inside the pool were studied, and the main requirements that the layers of the system must meet have already been established. the system must ensure not only the limitation of heat losses by the interior lining, but also a good mechanical performance. the thermal insulation layer must not only have high thermal resistance but also have resistance to deformation. the base layer must present protection characteristics of the insulating material (impact resistance and deformation), waterproofing and support capacity of the coating, deformation capacity balanced with rigidity and resistance to blistering under negative pressure. the bonding mortar, which will support the final coating, must comply with good adhesion to the final coating (generally ceramic), high deformability (low susceptibility to cracking and chemical resistance (joint mortar). 3.2. performance requirements of other technological solutions the technical specifications of the equipment to be installed will be defined, in order to obtain the performance output of the ecopool+++ system and respective subsystems. a holistic analysis of the interaction between the different project specialties was already carried out. for this purpose, the study was segmented into 4 different areas of expertise, namely: (a) constructive solutions; (b) hydraulic and thermal systems; (c) electrical installation and industrial automation system; and (d) the smart platform. in the following we describe each of these areas in more detail. (a) constructive solutions the constructive solutions encompass all the studies carried out in the definition of functional and performance requirements of construction materials and thus proceeded to validate the possibility of implementing the test bed. after validation by the university of algarve of the test bed installation site, a summary specification of the constructive solutions was created, namely, with the definition of materials, dimensions of the pool and of the engine room. the first studies were also carried out to verify the hydraulic and thermal performance of the system, using the solidworks software, based on the first draft of the architectural project. (b) hydraulic and thermal systems there are two different hydraulic systems, one dedicated to water filtration, common in every swimming pool, and another for the water heating purpose. the latter is schematically represented in figure 1. 391 m. j. oliveira, a. inverno, f. farinha et al. acta polytechnica ctu proceedings figure 1. solar heating based systems. although a common design option is to have only one circuit to perform both functions (filtration and heating), this solution is sometimes found in swimming pools. the choice for this singularity in the ecopool+++ project comes mainly from the flexibility it allows in terms of individual control of each one of the heat recovering systems. this could be also done with one hydraulic circuit, but at expenses of more valves and controls. furthermore, the other reason for the choice is that the water flows are significantly different; those used in recirculation and filtration are much higher than those used in heating systems. finally, the measurement of both flow rates and temperatures, in this way, makes monitoring of each heating sub-system easier. the pool’s heating systems focus on the use of solar energy as an energy source through solar collectors, collecting floors and heat accumulation in pcms, whenever generation exceeds the needs, to keep the water temperature in comfortable values, i.e. between 24–26 °c. the storage in pcm materials allows shifting its use in periods when the water temperature drops, by known atmospheric reasons, as for example one or more days with a sky cloud’s coverage or extreme heat losses at night. these solutions will be evaluated individually and their value verified in terms of contribution to the heating of the swimming pool, which will later be done in an integrated manner. figure 1 presents the operating diagram. currently, the simulation has started and there is not yet any results of it that could be exhibited to show in a satisfactory way the performance of the outlined systems. (c) electrical installation and industrial automation system several sensors will be integrated in the swimming pool, allowing the measurement of local variables such as the temperature at different points, the ph of the water and the chlorine level. it will also contain a set of devices, such as: (1.) water pumps that will be automatically controlled, (2.) electric valves in order to select if the water flows in the thermal production and retainment systems, (3.) automatic compensation of water tank refilling according to hydrostatic level proves, (4.) pressure sensors in different parts of the hydraulic circuit to check for pressure alarms and (5.) automatic backwash systems according to differential pressure on the filters, among others to be determined during the project execution. all the systems’ controls will be made recurring to industrial plc project, and all the digital/analogue system input/output will recur to low voltage and dry contact actuation on equipment and sensors, and all the analogue probes will communicate via generic 4–20 ma and 0–10 vdc, in order to simplify equipment’s maintenance and replacement routines, according to the best practices of automation systems. a local controller, called swimming pool control and monitoring system (spms) will be responsible for the communication with the set of sensors and actuators, including interfaces that will allow the integration with internet protocol (ip) sensors, but also modbus or knx automation devices. the controller will also be able to connect directly with the ip network of a local area network, in the house using wi-fi or ethernet interfaces, or connect directly to the internet using a separate access router. the architecture also considers the communication between the local controller and internet based cloud services, through a websocket protocol or a web application program interfaces (web api). the user will be able to select and configure its preferences using a web human machine interface (web hmi), made available at the local controller. the draft of the “point map” of the automation system, will result from the p&id draft and the generic specifications of the hydraulic, heat production and storage equipment. a study and definition of the architecture of communication protocols between the industrial automation system will be required, including the subsystems to be supervised and controlled, and the interconnection between the control devices and the smart platform. as a final product the idea is that all the monitoring and controlling systems will be integrated in the local automation system, so 392 vol. 38/2022 ecopool+++: developing a sustainable outdoor heated . . . figure 2. intelligent monitoring and control system. (a). ecopool+++ prototype (option a). (b). ecopool+++ prototype (option b). figure 3. ecopool+++ prototypes. that all relevant data can be provided to the smart platform in order for it to run algorithms that allow the optimisation of water and energy consumption, while detecting malfunctions and water breaches. (d) smart platform the integration of sensors, hmi and controllers enabling an intelligent control system is shown in figure 2. it illustrates the optimised monitoring, activation and control architecture responsible for managing all pool equipment in order to reduce consumption and maximise the use of renewable energy sources. the system should place the user as a central element, preventing him from having to perform individual/detailed control functions. thus, in addition to responding to the on/off request, the system should only require the specification of the value that the user wants for the water temperature. the control of all other parameters will be done automatically. the system must also learn from the users’ actions, enabling an intelligent mode, adapting to the user’s habits, minimising the need for entering parameters. the control and monitoring system will work autonomously using an external smart server, placed on the internet/cloud, only when it is necessary to change usage patterns. all measured variables will be sent to a cloud server, as represented in figure 2. the local controller, represented in figure 2 as spms, must collect the measurements from the sensors and modbus devices, sending them later to the cloud server. the user will always have the possibility to access the local controller through a web interface, which allows him to control or monitor system data. 4. ecopool+++ design the prototype of the ecopool+++ project will be located in the university of algarve, at campus da penha, faro, portugal. it was established that the pool will be 10 m long by 5 m wide. the average depth should be 1.20 m with a maximum depth that can reach 1.40 m. the engine’s room will be located on a wall adjoining the pool and must allow simultaneous access to several people, with a minimum area of approximately 10 m2 and a minimum height of 2 m. the heat storage tank must have a minimum volume of 2.5 to 3.0 m3, with easy access, and must allow the replacement of all materials, if necessary. figures 3a and 3b illustrate 2 drafts of the prototype’s architecture (still under discussion). 393 m. j. oliveira, a. inverno, f. farinha et al. acta polytechnica ctu proceedings 5. conclusions in the ecopool+++ project, an innovative solution for heated outdoor pools is being developed with the aims of reducing thermal losses and integrating smart water and energy management systems. the project started in 2021 and ends in june 2023. in this article, a bibliographical review was made about the various solutions that are being considered. a summary of the work already carried out was presented as well as some basic layouts already established, which will be developed in the next months. specifically, the ongoing ecopool+++ systems as presented, includes a thermal insulation system, an hydraulic and thermal systems, electrical installation and industrial automation systems, together with a smart platform. two solutions were also presented for the proposed prototype, which will be implemented at the university of algarve and which will allow testing and validation of all the solutions developed. it will also constitute an excellent teaching model that can be used by students of this institution. acknowledgements the project ecopool+++ is funded by feder, projetos de i&dt empresas em copromoção, programa operacional: programa operacional regional do centro, programa operacional regional do algarve, contact n. alg-01-0247-feder-047231. the professional expertise and valuable cooperation of all the partners are gratefully acknowledged. references [1] t. gallion, t. harrison, r. hulverson, k. hristovski. estimating water, energy, and carbon footprints of residential swimming pools. in s. ahuja (ed.), water reclamation and sustainability, pp. 343–359. elsevier, boston, 2014. https://doi.org/10.1016/b978-0-12411645-0.00014-6 [2] us doe. swimming pool covers. 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ecopool+++ project 3 ecopool+++ systems 3.1 thermal insulation system 3.2 performance requirements of other technological solutions 4 ecopool+++ design 5 conclusions acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.39.0049 acta polytechnica ctu proceedings 39:49–54, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague eye-tracking technology in automotive industry jan novotnýa,∗, jaroslav machanb, adam orlickýb a czech technical university in prague, czech institute of informatics, robotics and cybernetics, jugoslávských partyzánů 1580/3, 160 00 prague 6, czech republic b czech technical university in prague, faculty of transportation sciences, department of vehicle technology, horská 3, 128 03 prague 2, czech republic ∗ corresponding author: jan.novotny@cvut.cz abstract. the research paper “eye-tracking technology in automotive industry” describes the implementation of eye-tracking technology into the development of new vehicles and their decisionmaking process. this objective technology tracks the eye movement of a participant, detects where the subject is looking during an interval of an experiment, and thus offers a wide range of applications. it can be used to detect distractions while driving on a simulator under a certain workload or to record objective data during an otherwise subjective customer-oriented clinic. in this topic, we evaluate areas of the vehicle which are the most interesting for customers, and therefore provide support and recommendations for new vehicle designs. keywords: eye tracking, customer interest, driver distraction, eye fixation. 1. introduction the eye-tracking technology offers nearly limitless possibilities of applications in research and commercial sectors. its applications might vary from marketing research studies, medicine (diagnosis of psychological conditions), to design assessment of vehicle design. in the automotive industry, eye-tracking technology can be used to gather objective data to verify subjective results, and eventually influence the decision-making process of developing or modifying new vehicle design. because this methodology can evaluate areas of the vehicle which are the most interesting for customers, it serves as an objective source of customer voice. besides customer clinics and design related topics, eye tracking is a great tool to assess distraction levels while driving on a simulator or in any vehicle on the road under a certain workload. thus, it can evaluate the safety of given tasks or the infotainment system’s architecture. for instance, while driving in a car, subjects were told to carry out several tasks, one of them was to turn on the wi-fi hot-spot. this task number 7 proved to be one of the most difficult as illustrated in figure 1. it took on average 37.8 seconds to accomplish. in those time intervals, 72.9 % of total eye fixations were on central display (infotainment)! only a quarter of eye fixations remained on the road in front of drivers. therefore, we can conclude that the driver’s attention isn’t where it should be (on the road). certain tasks and functionalities in modern vehicles have a high ability to distract a driver and potentially create a dangerous situation. eye tracking was used in various studies focused on the topic of driver distraction. the study by ma et al. used gaze monitoring to compare size and positioning of displays on 13 mass produced vehicles [1]. figure 1. gaze map of eye fixations while turning on hot-spot wi-fi. 2. eye tracking applications in related studies as stated in the introduction, eye-tracking technology in the automotive industry is used mostly in the safety area. another example is a study by lange et al., where visual demand of in-vehicle touchscreen displays was measured [2]. kapitaniak et al. performed eye movement recordings and analyses to understand the nature of the complex task of driving a vehicle [3]. a study by orlický et al. focused on external hmi (human-machine interaction) communication between the pedestrian and autonomous vehicle. to assess different types of communication on the front of the vehicle (led panels, pictograms, etc.) they used eye-tracking technology [4]. however, the automotive industry has been missing the design or marketing application of this technology, although design and eye tracking are closely related. eye tracking is widely used to assess customer interest and attraction of various products. it is evident from a study by djamasbi et al. that eye tracking 49 https://doi.org/10.14311/app.2022.39.0049 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en j. novotný, j. machan, a. orlický acta polytechnica ctu proceedings figure 2. illustration of basic eye movements. is applicable also in web design and assessing its visual esthetics. according to this study, generation y prefers pages that include a large main image, images of celebrities and a search feature [5]. according to khalighy et al., this technology proved to be valid as a qualitative data source for product design. a conducted study compared the subjective opinion of 300 participants and 200 participants measured with eye tracking. the comparison confirmed the capability of eye tracking to quantify visual aesthetics, beauty and product design [6]. according to goyal et al. it is even possible to predict a customer’s behaviour and, to a large extent, a purchasing decision by using eye-tracking data [7]. 3. eye movement vision is a very sophisticated sense that employs a great deal of brain power. the brain processes images on the retina on which light falls. however, for the image to be sharp, the light must hit the retina at the right angle. therefore, the eyes (along with their pupils) are constantly moving towards the observed area [8]. the eyes can perform a variety of movements. in terms of eye tracking, we are interested in a gaze, fixation and saccade. when the eye stops for a fraction of a second, it is called a gaze. if multiple gazes occur in close proximity in short succession, this cluster can be considered a fixation. a fixation is defined as a period of approximately 100-600 milliseconds when the eyes are relatively still, focusing on one area, holding the central foveal vision in place so that the visual system can take in detailed information about what is being looked at [8]. as visualized in the figure 2 below, saccades are transitions, trajectories between fixations. a saccade is always one-directional and lasts for a very short time, between 20 and 40 milliseconds for a healthy individual. the last concept is smooth pursuit, which is when the eye follows an object in motion [8]. all these states of the eyes can be detected and tracked by the eye-tracking technology, which allows the acquisition of a lot of objective data. 4. eye-tracking technology the tobii pro glasses 2 50 hz wireless eye-tracker is used for the experiments. it is a binocular eye tracker functioning on the principle of videooculography. this method senses the position of a pupil using the reflection of infrared light shining on the surface of the eye, using videodetection. based on the respective figure 3. the tobii pro glasses 2. locations of these two points, the participant’s gaze direction vector is calculated, which is then mapped onto the image captured by the eye tracker’s front camera. eye tracker consists of 4 cameras monitoring the eyes (2 cameras per eye) and one fullhd camera for monitoring the environment in front of the participant. an immense advantage of these glasses is their unobtrusiveness and light structure, weighing only 45 grams. comfortable glasses help the participants forget that they even have glasses and are being measured. because of that, it is easier to combat the so called “hawthorne effect” which implies that a participant might change their behaviour, because they are aware of being observed [8]. the output data of eye tracking are gaze points, which are locations of the visual axis landing on the stimulus. as such, they have an (x, y) coordinate and a timestamp corresponding to its measurement. that is then analyzed in a dedicated software [9]. the software tobii pro lab is used to evaluate raw data recorded by the eye tracker. an algorithm is used to evaluate the recording, which processes raw data from the eye tracker and classifies the individual states of the eye (saccade, fixation, blinking). the i-vt classification algorithm is used in the ciirc laboratory for standard experiments. this algorithm classifies data based on the velocity of the eye’s sampled movement. the velocity threshold is the most important parameter of this classification algorithm as it is the primary parameter used to classify the states of eye movement. eye movements below the velocity threshold are classified as a fixation, while eye movements at higher speeds are considered saccades. in the ciirc laboratory, the value of the velocity threshold parameter was set for the experiment to 30◦·s−1, because it proved to have the best reliability and success rate of accurate assessment of eye movements [10, 11]. the accuracy, precision and gaze detection success rate vary according to the situation and conditions in the experiment area. the most important conditions are distance, observation angle and light. the gaze detection success rate is typically between 90 % and 98 %. it can’t be 100 % gaze detection, because there is no detection while blinking. according to the table below (figure 5), eye-tracking functions best in lighting conditions of around 300 lux and when the 50 vol. 39/2022 eye-tracking technology in automotive industry figure 4. gaze data and velocity graph [9]. figure 5. results of binocular accuracy, precision and gaze detection [9]. observing gaze angle is below 15◦. the data was published by the eye tracker manufacturer tobii after a testing measurement with 20 (n) participants in each condition. for this measurement, 300 lux was considered an optimal lighting condition and was fixed for angle measurement [9]. the accuracy of the gaze vector is around 0.6◦ in these conditions, precision is 0.05 rms◦. as illustrated in figure 6, the accuracy is the angle ai difference between the observed stimuli point and the average gaze point location, which is the measured and reported location by the eye tracking. the precision defines the reliability of the technology to measure and detect the same gaze point repeatedly with the same result or only marginal variance [9]. 5. the design and methodology of experiments the analysis of vehicles based on eye-tracking measurements requires robust methodology. when preparing the experiment, it is necessary to determine which locations of the examined vehicle are of importance. the exterior locations of a vehicle can be the front, side and back of a vehicle. in an interior-based experiment, it is usually the dashboard area. however, an interior-based experiment can be divided into several phases – getting into the vehicle, observing the interior with the engine off, observing the interior with the engine on, getting out of the vehicle. the observed locations such as the front of the vehicle or its dashboard are photographed and difigure 6. illustration of the accuracy calculations [9]. figure 7. example of areas of interest (tesla model 3). vided in the software into areas of interest (aoi). as shown in figure 7, this divides the observed image into individual areas. this can then be analyzed in post-processing to determine which areas have seen the most or least fixations. when working with a participant, a moderator must also ensure that they are not biased, that they do not lead participants into certain areas and that there aren’t any distracting or moving objects in the background. we also want to use the glasses to record their first views and impressions to really see what attracts attention at the first moment. the second look would be slightly different, because they already saw the “eye-catching” element. for that reason, participants come to a mark on the floor with their head down so they don’t see the vehicle before the start of experiment. once everything is prepared, the participant can look freely at the observed object. 6. the eye-tracking application the experiment, which is one of the essentials of this article, is a design assessment of the front masks of selected vehicles. at the customer clinic there were mid-size vehicles from several manufactures including ford, volvo, mercedes, vw and peugeot. some of them also brought a prototype which was actually the point of the customer clinic – to get opinions of potential customers before the prototype cars even start their production. this “voice of a customer” is essential when developing a new product. the areas of interest (aoi) for these cars were divided as shown in the illustrative drawing (figure 8). obtained data from approximately 50 participants revealed interesting findings. as the following graphs 51 j. novotný, j. machan, a. orlický acta polytechnica ctu proceedings figure 8. universal areas of interest of the exterior based study. and heat maps in figure 9 show, the examined time intervals are always first 5 seconds and first 20 seconds. this interval has been established by past experiments. when participants have too much time to observe certain objects, they either start to look at the background and surroundings, they start to repeat the previous observing pattern, or they simply get bored and stop. therefore, a longer interval is not necessary. when examining the first 5 seconds, we get the participants’ initial fixations, possible “wow effect”, what really drew attention. during the first 20 seconds, participants already have time to visually explore the whole vehicle, focus on several details and thus offer more comprehensive data. the results showed a lot of data from which it is important to select relevant interpretations. the results of all individual vehicles would take up too much space, so the paper will focus on individual specifics and overall results. an example evaluation is represented by mercedes-benz c180. the heat map on the left (figure 9) shows concentrations of fixations during the first 20 seconds of observing the vehicle. it is clearly noticeable that major clusters of fixations are located on the logo, lights and windshield. however, from the graph we can see that the large areas such as the bumper and hood have more fixations in total, 20 % and 24 % respectively. it is due to their size and also because of the observers’ transitions between other surrounding areas of interest. nevertheless, the hood is a very relevant area. for instance, ford mondeo has very distinctive lines on its hood and the heat map indeed shows increased concentrations on that area, as illustrated in figure 10. the differences between 5 and 20 second intervals were generally not significant. the attention of participants during this observed interval usually steered from large areas to smaller details. therefore, we can observe only slightly increasing percentages on logos, mirrors and lights. overall, the findings for all eight vehicles outlined some interesting facts. with exceptions, a similar pattern for observing areas (or elements) is seen for all vehicles. the most observed areas and thus most relevant in terms of design are the bumper, hood, lights and logo. moreover, the windshield tends to figure 9. graphs of both intervals and heat map of eye fixations during the first 20 seconds. have a high observing percentage, however, part of the reason is its transparency into the vehicle interior. the interior draws attention which is unwanted in this particular exterior-oriented measurement. distinctly different elements attract high attention, for example a newly shaped logo. also observed was axis symmetry of tracking – left and right side of the tracked vehicle. significant tracking of primarily hidden elements occurred very often, mainly exploring the location of the acc sensor. lower observing times were recorded for vehicles with the logo placed in the hood area (ford, škoda). logos on a bumper (e.g. mercedes) recorded higher observing time – more attention. 52 vol. 39/2022 eye-tracking technology in automotive industry figure 10. heat map of ford mondeo during first 20 seconds. figure 11. areas of interest of enyaq model. as in the mercedes-benz example, overall results determine bumpers, hoods and windshield as the most observed areas. however, compared to their size, lights and logos recorded high fixation rate and customer interest. 6.1. interior based study another study is measuring customer interest in the interior of the škoda enyaq vehicle with help of 15 participants. due to the nature of the study, age and gender division of the participants wasn’t considered. observing parts of the interior were again divided into areas of interest. the experiment consisted of 4 phases described in the previous chapter. the areas of interest were divided as illustrated in figure 11. observation measurement during “getting in” phase was evaluated from the first contact with the outer door handle to driver door closure. based on the data and heat map in figure 12, it is apparent that the attention is spread across more areas of interest, though the driver door received the most attention (23.1 %) jointly with the steering wheel (21.4 %). the average observation time (i.e. the time of the process of getting in) was 5.51 seconds. interestingly, only a few participants actually looked at the driver seat (less than 7 % of total fixations). majority didn’t even look at the area where they were about to sit. figure 12. getting in phase. on the heat map visualization, the fixation distribution is clearly displayed. the fixation on the heat map corresponds to an observation of unfamiliar surroundings. this phenomenon can be caused by the fact that the enyaq model was a brand-new addition to the škoda auto model line-up, and as such was seen for the first time by the majority of participants. while “observing the interior with the engine off”, the gauge cluster had the highest fixation duration (40.3 %) during the first five seconds, together with the steering wheel (29.9 %) and the central display (15.8 %). other elements had nearly no fixations. in longer observing intervals of 20 seconds the mentioned elements with high fixation duration remained dominant, but were joined by the central tunnel and driver door. “observing the interior with the engine on” had very different results. the number of active areas of interest further decreased. the gauge cluster, which lit up during the ignition and drew more attention, received the biggest fixation time percentage (65.4 %). with ongoing observation time, the attention shifted dramatically towards the central infotainment display (65.1 %), without affecting other areas of interest. as expected, during the phase “getting out” attention switched mainly to the driver door. there are also high percentages for dashboard and gauge cluster. this may be due to the fact that the participants wanted to reflexively make sure to turn off the ignition when getting out of the car. in an electric vehicle such as enyaq it is unrecognizable by noise. 7. conclusion the aim of this paper was to introduce the technology of eye tracking and to demonstrate its possible applications in the automotive industry. this technology offers a wide range of possibilities to obtain valuable objective data about potential customers or drivers while operating a vehicle. the paper described the function of the technology, its accuracy and limitation, eye movement and methodology of the experiment. furthermore, two studies concerning exterior and interior of vehicles were introduced. the exterior-based study shows the 53 j. novotný, j. machan, a. orlický acta polytechnica ctu proceedings main frontal areas of vehicles that are the most relevant in terms of design. from previous research, interior data were available also for the škoda kodiaq, measured using the same methodology as for enyaq. therefore, it was possible to compare these two models. it is the comparison of vehicles with each other that brings the most valuable information about the interior of vehicles. from this evaluation, it is clear that during the entry and exit phases, the distribution of views is very similar. however, during the interior observation phases, a significant difference can be observed in the fixation distribution among the areas of interest. the biggest difference is the length of observation of the infotainment during the “engine on” phase. both vehicles recorded a majority of fixations on both screens, which means an overwhelming hmi supremacy. but during this phase, the enyaq model dominates with its 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https://www.tobiipro.com/siteassets/tobii-pro/learn-and-support/analyze/how-do-we-classify-eye-movements/tobii-pro-i-vt-fixation-filter.pdf?fbclid=iwar1_em_u-tgzzlsndhjboemnycualvdt9khcbcp1pug3emhxoqbfsdgvj6y https://www.tobiipro.com/siteassets/tobii-pro/learn-and-support/analyze/how-do-we-classify-eye-movements/tobii-pro-i-vt-fixation-filter.pdf?fbclid=iwar1_em_u-tgzzlsndhjboemnycualvdt9khcbcp1pug3emhxoqbfsdgvj6y https://www.tobiipro.com/siteassets/tobii-pro/learn-and-support/analyze/how-do-we-classify-eye-movements/tobii-pro-i-vt-fixation-filter.pdf?fbclid=iwar1_em_u-tgzzlsndhjboemnycualvdt9khcbcp1pug3emhxoqbfsdgvj6y https://www.tobiipro.com/siteassets/tobii-pro/learn-and-support/analyze/how-do-we-classify-eye-movements/tobii-pro-i-vt-fixation-filter.pdf?fbclid=iwar1_em_u-tgzzlsndhjboemnycualvdt9khcbcp1pug3emhxoqbfsdgvj6y https://www.tobiipro.com/siteassets/tobii-pro/learn-and-support/analyze/how-do-we-classify-eye-movements/tobii-pro-i-vt-fixation-filter.pdf?fbclid=iwar1_em_u-tgzzlsndhjboemnycualvdt9khcbcp1pug3emhxoqbfsdgvj6y https://www.tobiipro.com/learn-and-support/learn/eye-tracking-essentials/how-do-tobii-eye-trackers-work/ https://www.tobiipro.com/learn-and-support/learn/eye-tracking-essentials/how-do-tobii-eye-trackers-work/ https://www.tobiipro.com/learn-and-support/learn/eye-tracking-essentials/how-do-tobii-eye-trackers-work/ acta polytechnica ctu proceedings 39:49–54, 2022 1 introduction 2 eye tracking applications in related studies 3 eye movement 4 eye-tracking technology 5 the design and methodology of experiments 6 the eye-tracking application 6.1 interior based study 7 conclusion references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.38.0332 acta polytechnica ctu proceedings 38:332–339, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague influence of heated facade air on the results of climate data measurement peter kysela∗, radoslav ponechal, ján rybárik university of zilina, faculty of civil engineering, department of building engineering and urban planning, univerzitna 8215/1, 010 26 zilina, slovakia ∗ corresponding author: peter.kysela@uniza.sk abstract. measuring climate data is a lengthy and technically challenging task. to record temperature data, small meteorological stations are located on the facade of the research centre building. due to the position of the meteorological stations, which are mounted directly on the facade of the building, the temperature measurement sensor is not only affected by the solar radiation falling on the sensor housing, but also by the solar radiation falling on the facade of the building. the illuminated surface of the facade gradually heats up during the day and warms the air flowing near the facade. the temperature readings during the day may therefore be significantly distorted. to avoid this phenomenon, better-quality radiation shields have been purchased. the new radiation shield is characterized by a design that resembles eddy currents. the relationship between the shape of the outer and inner spiral is optimized, allowing vortices to form even in virtually windless conditions. in this paper, we would like to point out the differences in the measured air temperature data, according to the radiation shield used and the distance of the temperature sensor from the building facade. keywords: climate measurement, facade, radiation shield. 1. introduction – measuring climate data the need for climate data measurement has been increasing in recent years as it is proving to be an integral part of the proper design of the indoor environment of buildings. simulation programs that are used in building design, either for the correct design of the indoor environment or for energy calculations, work with climate data sets. climate datasets come in different sizes and details, depending on the intended use and the design of the indoor environment. the most used is the test reference year, which is defined as the year of hourly data of selected meteorological parameters that characterize the long-term climate. such a dataset is suitable for some simple energy calculations or the assessment of long-term climate change [1]. but for many simulation tasks, more accurate temperature data characterized by smaller time steps or distances between the measurement site and the future building are needed for the proper design of technical equipment and indoor microclimate in buildings [2]. since november 2016, small meteorological stations have been placed on the facade of the research centre building for this purpose, shows in figure 1. the meteorological stations measure basic climatic parameters such as air temperature and humidity, solar radiation intensity, and wind speed and direction using a powerful anemometer. these are placed on the walls of the building, shows in figure 2, facing the four cardinal directions at different heights and positions on the facade [3]. figure 1. view of the west facade of the research centre. measuring air temperature is a technically challenging task because the measured data can be significantly distorted by various extraneous factors. for representative results, it is also necessary to consider factors that influence the measurement results, such as the influence of solar radiation, shading technology, urban heat islands, or the heating of the ambient air from the building facade. it is the heating of the ambient air from the facade that has been shown to be a significant factor influencing the temperature data from the small meteorological stations located on the facade of the research centre building. the surface heated by solar radiation 332 https://doi.org/10.14311/app.2022.38.0332 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 38/2022 influence of heated facade air on the results . . . figure 2. meteo-stations located on the west facade of the research centre. can influence the airflow along the facade of a highrise building during windless periods. heated air enters the interior when windows are opened and can cause lower passive cooling efficiency [4]. to refine the measured temperature data, new radiation-resistant shields for the temperature sensors were purchased to make them more resistant to overheating from solar radiation and to overheated air that heats and flows near the building facade. the shields were placed on the facades of the research centre building, which are most affected by solar radiation, and at varying distances from the facade surface. comparison of temperature data during the hottest days of the year (june, july) showed a significant difference according to the cover used [5]. a less significant difference lies in the distance of the shield from the façade. the use of the new better-quality shields gives more accurate temperature data during the hottest summer days, which will be used for simulation calculations concerning the assessment of overheating of buildings during the summer period. this can be assessed by many indicators where the impact can be demonstrated over a longer time. the assessment would thus be further affected by high temperatures introduced by inaccurate measurements, the influence of heated air from the building facade [6]. figure 3. illustration of airflow in the old shield. 2. methods of measurement – the importance of radiation shields 2.1. facade meteorological station (fms) the fms itself consists of a metal frame that is mounted on the facade under the window and is connected to the power supply by a cable. sensors measuring temperature, relative humidity, and atmospheric air pressure are mounted on the frame. for measurement used capacitive digital sensor aht10 for humidity and temperature, accuracy typical ±0.3 k at 5 to 60 °c, operative range −40 to +80 °c. it is protected by a radiation shield, shows in figure 3. this is followed by a pyrgeometer to measure longwave solar radiation, a pyranometer to measure and monitor global solar radiation, and an ultrasonic anemometer to measure wind speed and direction. the data collection is wireless. measured data with a one-minute write interval is collected and stored on a remote data storage server located in the building [4, 7, 8]. 2.2. radiation shield the new radiation shield features a design that is reminiscent of the eddy currents that occur in nature. the unique continuous 27-piece double helix design serves the designed purpose in achieving superior measurement and sensor protection characteristics. figure 4. shows how air can pass freely through the enclosure wall to the interior of the enclosure, where it is formed into a vortex shape by the spiral shape that flows around the sensor. the hot air flows upwards and exits the enclosure through the top. figure 5 shows how the vortex from the inner spiral lead’s particles heavier than air particles naturally away from the sensor, such as dirt, sand, snow, and water. the relationship between the shape of the outer and inner spiral allows vortices to form even in virtually wind333 p. kysela, r. ponechal, j. rybárik acta polytechnica ctu proceedings figure 4. illustration of airflow in the new shield [9]. figure 5. schematic illustration of the eddy flow in the shield [9]. less conditions. this ensures that the air is constantly flowing around the sensor and prevents the stagnant air in the enclosure from overheating [9, 10]. 3. installation and positioning on the facade, selection of representative days 3.1. installation of new radiation shields replacement of shields on individual fms was carried out as part of the meteorological station inspection. for the replacement of the radiation covers, it was necessary to modify not only the fitting of the new shield but also the location of the sensor itself, since the outer diameter of the radiation shield is larger than the original type, shows in figures 6 and 7. the new position of the sensor is therefore approximately 15 cm further away from the facade of the research centre building, shows as figure 8. the positioning of the new enclosures has therefore also affected the distance of the sensor from the facade of the building. for a relevant assessment of the effectiveness of the new shield compared to the previous shield, the figure 6. fms with the previous type of shield on the facade. figure 7. installation and replacement of the new type of shield. sensors with the previous shield type were placed in the same position on the two fms as for the new shields. this means that it is possible to compare how not only the type of shield used but also the distance of the sensor from the building facade affects the measurement results. 334 vol. 38/2022 influence of heated facade air on the results . . . figure 8. fms with the new type of shield on the facade. 3.2. location of fms and used shields the location of the new shields was chosen so that at least one fms with a new type of shield was placed on the west, south and east sides of the building. no new shields were placed on the north side of the building as it receives minimal glare in the morning hours. the results shown in the graphs present the temperature data from four fms with new radiant shields, one on the east and south sides of the building and two on the west side of the building. the fms with the previous type of shield, placed in the new position, were mounted on the west and south sides of the building. a schematic of the location of each fms is shown in figure 9. the installation of new radiation shields on the fms took place during the months of march, april during the fms inspection. for the best assessment of the heated air factor from the building facade, the period when air temperatures reach the highest values is suitable. according to the observed data, it was evident that the new type of shield is significantly more resistant to this factor. due to the size of the new radiation-resistant shields, the sensors and the shield mounting were more distant from the facade surface. thus, not only the new type of radiation shield influenced the measured data, but also the displacement of the sensor position further away from the building facade. to evaluate this factor, the sensors with the previous cover were mounted in the same position as the sensors with the new type of cover on two fms. by comparing the data, it is thus possible to evaluate what effect the position or distance of the sensor from the building facade has. 3.3. meteorological control station for evaluating the measured data from the individual fms on the facade of the research centre building, the temperature from the meteorological station located on the roof of the adjacent building is also shown in the graphs. this automatic meteorological station is used for all the experimental research carried out by the department. for all the research tasks carried out, monitoring of climatic parameters with very accurate and high resolution is required [11]. the temperature sensor is placed in an open space, far enough away from the roof surface, thus avoiding heating from the roof surface. 4. evaluation of measured results the difference in the resistance of the shields used is especially noticeable on warm sunny days [10]. the graphs present measurements from a series of warm days in early july 2021, namely from 0:00 on 6 july 2021 to 23:59 on 9 july 2021, recorded in minute intervals. the graph in figure 10 presents the results from the west façade of the building where the two fms with the new shield (t17-n, t24-n) are located, shown by the solid blue and red lines, for temperature comparison the dashed line shows the temperature data from fms with the original type of shield (t21, t23, t28), located close to the fms with the new shield. for reference, the temperature data is shown in red solid bold line with the air temperature from the meteorological station (t m37) located on the roof of the adjacent building, in figure 10. to compare the effect of the sensor location or distance from the building facade, the solid green line shows the temperature from the fms with the previous type of shield and the sensor at the same position from the facade as the new shield types. the deviation of the individual sensors from the air temperature sensed by the meteorological station on the adjacent building (m37) can be observed from the temperature course during the daily maximum. in figure 11, which shows the deviation of the sensors with the new type of shield is about 2 to 3 °c, which represents a measurement error of approximately 5 to 6 % compared to meteorological station m37, during the maximum temperature during the day. the deviation of the sensors with the previous type of shield is significantly higher, by 8 to 10 °c, which represents a measurement error of approximately 26 to 27 % compared to meteorological station m37. the deviation of the sensor with the previous type of shield placed in the same position as the sensors with the new shield (t26-s), i.e., 15 to 335 p. kysela, r. ponechal, j. rybárik acta polytechnica ctu proceedings figure 9. schematic layout of the fms on the facades of the research centre building. figure 10. temperature recorded at the fms sensors on the west facade of the research centre building during 6 july 2021 to 9 july 2021. figure 11. daily temperature course recorded by fms sensors on the west facade of the research centre building during 7 july 2021. 336 vol. 38/2022 influence of heated facade air on the results . . . figure 12. temperature recorded at the fms sensors on the east facade of the research centre building during 6 july 2021 to 9 july 2021. figure 13. temperature recorded at the fms sensors on the south facade of the research centre building during 6 july 2021 to 9 july 2021. 20 cm further away from the building facade, is only about 1 to 2 °c lower than for the sensors with the previous type of shield in the previous position (t21, t23, t28), which represents a measurement error of approximately 20 to 21 % compared to meteorological station m37. it is therefore reasonable to assume that the distance of the sensor from the building facade has only a small influence on the resulting sensed temperature. conversely, the type of radiation shield used has a major influence on the measurement result and hence on the resulting temperature data. the temperature course from the east and south facade, shows in figures 12 and 13, showed similar results to the previous west facade. the differences between the sensor temperature with the new type of shield and the air temperature measured by the station on the roof of the adjacent building is up to 1 °c, which is an even smaller difference compared to the west facade. however, on the eastern facade a more significant difference is visible between the sensors with the previous type of shield in the previous (t11, t12) and in the new position (t9-s), i.e., at a greater distance from the facade. this may be due to the gradual heating of the sensor inside the shield during the day, as it is just the eastern facade. it is therefore noticeable how the deviation increases during the day and approaches the values of the sensors with the previous type of shield in the previous position (t11, t12). the temperature course on the east and south facades (figures 14 and 15) are used to show the temperature evolution during the day in more detail. in figure 14, which shows the daily temperature course on the eastern facade, an increasing difference between the temperature on the sensors with the previous type of shield and on the sensor with the previous type of shield but in a more distant position is visible. insufficient airflow in the previous shield type is heating the air inside the shield from the hot air flowing close to the facade. moving the sensor shield away reduces 337 p. kysela, r. ponechal, j. rybárik acta polytechnica ctu proceedings figure 14. daily temperature waveform recorded by fms sensors on the east facade of the research centre building during 7 july 2021. figure 15. daily temperature waveform recorded by fms sensors on the south facade of the research centre building during 7 july 2021. the effect of the hot air, but not nearly enough to achieve similar results as with the new shield type. then, as the sunlight passes onto the south facade, the temperature on the sensors will gradually converge to a roughly similar position. the slight variations between fmss with the same type of shield are due to the specific location within the facade. 5. discussion – suitability of the use of shields in the future and recommendations as it has been shown, the relevance of measured meteorological data is influenced by several factors, which have varying degrees of influence on the accuracy of the measured data. in this paper, we found that the position of the sensor is less important than the quality of the sensor radiation shield for the influence of heated air from the facade. the proper shape of the shield allows constant airflow inside the shield and prevents the sensor from overheating, and thus the relevance of the data is achieved [12]. it is therefore necessary, when designing the location of the sensors, to evaluate what factors may affect the measured data and accordingly select a suitable type of shield that is resistant to the factors that occur at a given location of the measuring equipment. measurements with the new shield and at a greater distance from the building facade fundamentally changed the measured results of heated air from the facade. it can no longer be so strongly argued that the air near the facade prevents passive cooling. especially on the south side, the air temperature measured by the sensor with the new shield was no different from that on the roof of the adjacent building. it is only different in the morning on the east side. in the future, it would be advisable to produce a smaller shield of similar construction that could be placed closer to the facade. it would also be adequate to investigate the behavior of the shield under artificial weather conditions in climate chambers. 338 vol. 38/2022 influence of heated facade air on the results . . . acknowledgements this work was supported by grant system of university of zilina no. 8077/2020. references [1] p. juras, d. jurasova. outdoor climate change analysis in university campus: case study with heat-air-moisture simulation. civil and environmental engineering 16(2):370–378, 2020. https://doi.org/10.2478/cee-2020-0037 [2] d. štaffenová, r. ponechal, p. ďurica, m. cangár. climate data processing for needs of energy analysis. in envibuild 2014, vol. 1041 of advanced materials research, pp. 129–134. 2014. https://doi.org/10. 4028/www.scientific.net/amr.1041.129 [3] p. juras, p. durica, d. jurasova, et al. building of research centre as the experimental laboratory for civil engineering. iop conference series: materials science and engineering 415(1):012032, 2018. https://doi.org/10.1088/1757-899x/415/1/012032 [4] p. durica, a. iringová, r. ponechal, et al. energetické a enviromentálne navrhovanie a hodnotenie budov. edis-vydavateľské centrum žu, žilina, 2017. isbn 978-80-554-1296-2. [5] x. lin, k. g. hubbard, g. e. meyer. airflow characteristics of commonly used temperature radiation shields. journal of atmospheric and oceanic technology 18(3):329–339, 2001. https://doi.org/10.1175/15200426(2001)018<0329:acocut>2.0.co;2 [6] r. ponechal, p. kysela, j. rybárik. vplyv režimu vetrania na výsledky simulácie tepelnej pohody v lete v menších rodinných domoch. in simulace budov a techniky prostředí 2020, 11. konference ibpsa-cz. 2020. 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[2021-10-24]. https://www.baranidesign.com/ [10] j. yang, q. q. liu. a temperature error correction method for a thermometer screen. acta physica polonica a 132(4):1301–1305, 2017. https://doi.org/10.12693/aphyspola.132.1301 [11] p. juras, d. jurasova. influence analysis of climate data time-step on the accuracy of ham simulation. matec web of conferences 196:02029, 2018. https://doi.org/10.1051/matecconf/201819602029 [12] x. lin, k. g. hubbard, e. a. walter-shea, et al. some perspectives on recent in situ air temperature observations: modeling the microclimate inside the radiation shields. journal of atmospheric and oceanic technology 18(9):1470–1484, 2001. https://doi.org/10.1175/15200426(2001)018<1470:sporis>2.0.co;2 339 https://doi.org/10.2478/cee-2020-0037 https://doi.org/10.4028/www.scientific.net/amr.1041.129 https://doi.org/10.4028/www.scientific.net/amr.1041.129 https://doi.org/10.1088/1757-899x/415/1/012032 https://doi.org/10.1175/1520-0426(2001)018<0329:acocut>2.0.co;2 https://doi.org/10.1175/1520-0426(2001)018<0329:acocut>2.0.co;2 https://doi.org/10.4028/www.scientific.net/amm.887.411 https://doi.org/10.4028/www.scientific.net/amm.887.411 https://doi.org/10.1051/matecconf/201710700083 https://www.baranidesign.com/ https://doi.org/10.12693/aphyspola.132.1301 https://doi.org/10.1051/matecconf/201819602029 https://doi.org/10.1175/1520-0426(2001)018<1470:sporis>2.0.co;2 https://doi.org/10.1175/1520-0426(2001)018<1470:sporis>2.0.co;2 acta polytechnica ctu proceedings 38:332–339, 2022 1 introduction – measuring climate data 2 methods of measurement – the importance of radiation shields 2.1 facade meteorological station (fms) 2.2 radiation shield 3 installation and positioning on the facade, selection of representative days 3.1 installation of new radiation shields 3.2 location of fms and used shields 3.3 meteorological control station 4 evaluation of measured results 5 discussion – suitability of the use of shields in the future and recommendations acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.38.0630 acta polytechnica ctu proceedings 38:630–634, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague rescue of a stone bridge with respect to current condition and standards petr řeřichaa,∗, petr fajmana, lucy davisb a czech technical university in prague, faculty of civil engineering, department of mechanics, thákurova 22, 166 29 praha, czech republic b mcgill university, faculty of engineering, 45 sherbrooke street west, montreal, canada ∗ corresponding author: petr.rericha@fsv.cvut.cz abstract. a three span stone masonry bridge dating back to the middle of the 19th century, still in the roadway network, is assessed to prevent its demolition. its industrial heritage value and ecological concerns were the principal reasons for the assessment. the carbon footprint of the stone arch replacement would be approximately 200 t co2 emission owing to 430 m3 reinforced concrete in the new structure. besides the cultural monument, considerable energy, co2 emissions and natural resources can be saved in accordance with the sustainable development goals. standards, guides and commercial software do not provide an adequate support for the assessment of masonry arch bridges, therefore an innovative two phase application of a commercial linear sttructural analysis code and an original in-house code was developed for the purpose. thousands of stone ach bridges still in service worlwide give the case study more general importance. keywords: natural resources preservation, sandstone, bridge, arch, stress, displacement. 1. introduction the three-span sandstone arch bridge was built in 1857 and remains an important part of the second-class roadway network, see figure 1. it was considered for a reconstruction with total demolition of the arches. the owner planned to remove the arches and top parts of the piers and replace them with a reinforced concrete structure that would be a reminder of the arches of the stone bridge. in order to prevent an irrecoverable loss of the industrial heritage, an extensive load capacity assessment was conducted for the bridge with the replacement of the pavement. standard assessments determine load rating based on the limit states analysis. the load models used for the purpose differ in various countries substantially, for instance either the ultimate (in europe) or serviceability limit states (in the us) are preferred. instead, the check of the available building standard’s criteria is used herein. the conditions of the en 1990 part 2 traffic loads on bridges have been checked to testify that the bridge with its valuable stone arches can remain a part of the roadway network without limitations. a number of clauses of the en 1996-1-1 could be applied even though the standard is not designed for bridges. “there is a multitude of assessment methods available for masonry arch bridges, yet there is no widely accepted framework for their application”, this quotation from [1] is a sad fact. tens of papers report on sophisticated methods based on advanced material and structural models. none of them are acceptable for the design practice for two principal reasons, figure 1. front view of the sandstone bridge at ponikla. (a) the application requires specific high competencies of the designer and specialized software (b) the input material and structure data are not available for the models. the present approach is simple: the assessment method can use only the data available in a common design environment and with reasonable cost. this admits relatively simple material models. the finite element technology is preferred since it can model the components of the bridge and their interactions in a way that other methods and codes (modified mexe, plasticity theorems like ring [2], castiligliano theorem like ctap code [3] and others) cannot. at the same time it is widely available. the absence of design standards or other “widely accepted framework” gives the load rating of an important bridge the character 630 https://doi.org/10.14311/app.2022.38.0630 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 38/2022 rescue of a stone bridge with respect to current condition and standards figure 2. new design of the bridge. of a research task. an assessment method was developed in the course of the solution of the bridge. the objective of the contribution is to outline the principal steps of the method and its application to similar bridges. the method is tailored to masonry bridges management practice and demostrates its application. the sample solution documents the potential of the method for sustainable infrastructure management and ecological impact. 2. the original restoration concept, ecological impact scheduled for reconstruction, the owner planned to remove the arches and top parts of the piers and replace them with a reinforced concrete structure that would remind the arches of the stone bridge, see figure 2. however, considerable economical savings and benefits for the environment can be achieved when the arches are preserved. the construction of the new reinforced concrete structure would require the destruction and deposition of 230 m3 high quality stone masonry built with craftsmanship which would be lost forever. removal and deposition of 400 m3 backfill would cost money and ecological load. the carbon footprint would be left owing to 430 m3 reinforced concrete with approximately 200 t of co2 emission. the most important offense against sustainable building would probably be the life span of the new reinforced concrete structure. the stone arches are 170 years old and still in service in spite of the neglected maintenance in the last decades. with adequate protection against water leaks they can serve for 150 more years. the service life of the proposed reinforced concrete structure willl certainly be substantially shorter, as in particular the struts above the piers with notch hinges are susceptible to early deterioration. the ecological aspects of the reconstruction speak for the preservation of the stone arches. 3. load capacity of stone arch bridges 3.1. material models the dominant material failures and limit states in the stone arch bridges are (a) the tension cracking in the bed joints, (b) the compression failure in the bed joints. the former occurs already at the service load levels and may result in gradual development of virtual hinges and stability loss of the arch with the growing live load. it must be represented in the material model in order to assess the redistribution of the internal forces. the trouble with this limit state is that there are no generally accepted quantitative criteria worldwide. recently the czech guide tp199 of the ministry of transport of the czech republic introduced an admissible relative depth of the cracks in the bed joints. the other trouble is that the predestined orientation of the cracks makes the no-tension models in general purpose codes almost unusable. this failure mode can effectively be included only in beam elements of the 2d models or shell elements of the 3d models where the kinematic assumptions and dominant cross-section normal stresses facilitate the specific no-tension model. the thick shell and beam finite elements with the no-tension property are derived in [4]. the latter limit state is a kind of a brittle failure, partially contained in case of the arch bridge with fill. the failure criterion for this limit state is the compression strength of the masonry. this property is the only one that can be determined in a diagnostic survey with acceptable cost and reliability. en 1996-1-1 provides support for determining the masonry compression strength from the compression tests of the core bored specimens of the stones and by nondestructive boring tests of the mortar (pzz 01 device). the tests were performed for the ponikla bridge masonry arches and the compression design strength was determined fd = 2 mpa. the value is rather conservative since the sandstone ashlars of the vault are in good condition. the backfill and pavement are linear elastic. in order to eliminate extensive backfill failures, the drucker/prager criterion is checked in the fill elements and indicated by asterisks in the graphic output. the elastic properties used in the fem models are summarized in table 1. they are deliberately conservative for the backfill. a review of the backfill properties was conducted in the published literature [5] and these are the values at the low end. during the method development, a parametric study showed that softer fill always implies worse stresses in the vault drawing 631 p. řeřicha, p. fajman, l. davis acta polytechnica ctu proceedings figure 3. the minimum stresses [kpa] and crack depths in the ponikla central span arch. the displacements are 1000 times scaled up. e [gpa] g [gpa] ν ρ [t/m3] arch 10 5 0.25 2.0 backfill 0.1 0.043 0.15 1.8 pavement 31 12.917 0.2 2.5 spandrel 0.1 0.04 0.25 2.2 piers/abutments 10 4 0.25 1.8 table 1. elastic properties considered in this case study. an important conclusion. the fill material constants from the table are on the safe side and can be used in bridge assessment when nothing is known about the fill quality. the value 0.4 gpa is recommended when at least a compacted cohesionless material can be assumed. selection of the young moduli of the arch parts is facilitated by a convenient property of the fem model, see the next section [6]. the short summary of the material models and properties exposes the uncertainties involved which inevitably imply low reliability of the analytical models of the masonry arch bridges. on the other hand, most of them were built in the 19th century according to empiric rules with ample reserve of load capacity. decades of their service testify to their robustness and reliability. these aspects compensate the low reliability of the analytical models, asserts boothby in [7]. 3.2. finite element bridge models it is difficult to account in a single complex model for the interaction of the spans and piers, the transverse asymmetry of the load and structure and non-linear arch behavior. the assessment method therefore uses different fem models for the tasks. the checks required by the standards are performed in a non-linear 2d (plane strain condition) model of a single span featuring the no-tension material specified above. the simplest timoshenko beam elements [4] are used for the stone arch so that the normal stresses in the cross-section planes are used in the above failure conditions. the backfill is meshed by standard cst triangles and the pavement by linear timoshenko beam elements. the meshing is shown in figure 3. clamped arch springings are assumed. the results are slightly non-conservative owing to the plane strain 2d idealization which neglects all transverse effects. correction factors are used to make up for the error, see the next paragraph. the linear elastic no-tension material model secures robust convergence of the equilibrium iterations so that unskilled users can use the model and code. the model also implies that stresses and crack depths do not change when the young moduli of all materials are multiplied by a common factor. a non-commercial code was developed for the purpose and will be available as an octave/matlab script when a minimum user interface is completed. the other two models are a 3d linear model of one span with the same boundary conditions as in the non-linear model and a 3d model of the whole bridge including piers and abutments. they are primarily used to assess the differences between the 2d and 3d models and the interaction of the arches, piers and abutments.the differences between the 2d and 3d models are expressed in terms of correction factors for the 2d solutions. the factors are then used to correct the results of the non-linear model. this is not quite consistent but the bed joints cracking is localized to relatively small volumes of the vault and the imperfection is thus accepted. a front view of the bridge and its 3d fem model are shown in figures 1 and 4. the spans are all approximately 11.4 m and the full width of the bridge is 7.6 m. the single span 3d model is an extraction of the full bridge separated by adjacent pier’s symmetry planes. on the planes, the symmetry boundary conditions are applied (zero normal and free in-plane 632 vol. 38/2022 rescue of a stone bridge with respect to current condition and standards figure 4. full 3d bridge model with two tandem axle forces. displacements). the elements are based on isoparametric bricks/quadrilaterals enriched by rotational degrees of freedom at all nodes. the commercial package rfem from dlubal software [8] was used in the sample solution. any other package can be used with an adequate modeler. the model’s build itself is rather easy since details of the sructure can be omitted. extraction and processing of the relevant stresses and other data may be more demanding and depends largely on the available software. the other purpose of the full 3d model is to asses the interaction of the spans, piers and abutments. the dominant effect of the interaction is the widening of the arch subject to the local live load, see section 3.3 for the live load model. the vertical displacements and rotations of the arch springings owing to the interaction are neglected in the method. the arch widening is obtained as the difference of the averaged horizontal displacements of the right and left springings. it is then used in the non-linear 2d model as an imposed displacement (kinematic load). 3.3. load model the load model lm1 of the en 1991-1-1 is used. it is known to be critical for bridges with these space parameters. the same position of the two axle tandems is considered in all calculations, as shown in figure 1. the position above the quarter span is generally acknowledged to be the most adverse for the arch. the uniform continuous live loads of lm1 are not shown in figure 4 but applied on the loaded half of the span in the calculations. it is worth mentioning that lm4 class 1800 kn of the en standard gives approximately the same total load on the half span but is almost uniformly distributed. the dead load is included via mass densities of the bridge components materials. the load factors of the ultimate limit state are used according to the en and national application document. loading in the 2d model is taken as the average over the bridge width of the 3d model loads. 3.4. results for the design load of the ponikla central span the solution of the ponikla bridge for the design loads of the en 1991-2 is shown in figure 3 with axle tandems of the lm1 allocated at a quarter span of the arch. the dead load of the arch, fill and pavement is accounted for. the third load component is the imposed horizontal displacement 0.13 mm of the right arch springing, fill boundary and pavement end. it equals the arch widening obtained in the full 3d linear model, see the last paragraph of section 3.2. the absolute values of the minimum stresses in the arch elements are inserted at the arch face where they occur (the maximum stresses are always nonnegative). the same quantities are inserted at the lower and upper faces of the pavement although they are irrelevant with respect to the arch assessment. the 0.2 m thick reinforced concrete pavement is considered linear elastic in the model. the bending “waves” characteristic for beams on an elastic layer can be observed in the figure, note the alternating faces of the minimum stresses in the pavement elements. the crack depths in the arch elements are indicated by the inserted line segments. the crack mouth widths in the bed joints can be computed from the crack depths, the voussoir lengths and the minimum strains in the elements when the timoshenko kinematic assumption is employed. the arch model is a 1d continuous beam, where the arch elements lengths depend on the chosen mesh density. the voussoirs are approximately 0.4 m long whereas the elements just about 0.33 m. the load intensities and material properties used in the non-linear plane strain 2d model are averages 633 p. řeřicha, p. fajman, l. davis acta polytechnica ctu proceedings cross-section locations 0 0.25 0.5 extrados stress correction factor 1.18 1.08 1.1 intrados stress correction factor 1.05 0.94 table 2. the 3d/2d correction factors for the compressive stresses at three cross-section locations. across the vault width 7.6 m. however, the load model lm1 for two lanes is not symmetric with respect to the longitudinal symmetry plane of the bridge. the asymmetry effect is evaluated through comparison of the 3d model solutions for the true lm1 load distribution with the solution for the average uniform distribution in the transverse direction. the ratios of the two stress’ sets are used as 3d/2d correction factors and are summarized in table 2 for the four cross-section locations along the arch span. only compressive stresses are considered. the stress values from figure 3 are multiplied by the correction factors from table 2 in the last step of the method. the assessment of the bridge using the method determined its safety in the ultimate limit state as defined in en 1991-1. 4. conclusion a method for the assessment and load rating of the stone masonry arch bridges is presented. it is designed for application in bridge management. a single material property, the vault masonry compression strength, is necessary. users need no special skills beyond adequate experience with the application of the fem codes. the safety assessment of the ponikla bridge using available standards together with activities of other institutions and individuals in the cultural heritage protection helped to avert the demolition of the bridge’s arches. the simple material model and the dedicated finite element can be used to check the complex structure against available standards conditions. an extension of the concept to 3d shells is currently under way and then the parallel linear 3d model will not be necessary for the application of the method. preservation of the ponikla bridge represents a contribution to the responsible consumption and production goal of the sustainable development policy. acknowledgements the authors acknowledge the support of the project naki dg20p02ovv001 (tools for the preservation of historical values and functions of arch and vaulted bridges) provided by the ministry of culture of the czech republic. references [1] n. gibbons, p. j. fanning. ten stone masonry arch bridges and five different assessment approaches. in arch’10 6th international conference on arch bridges, pp. 482–489. fuzhou, 2010. [2] m. gilbert. ring: a 2d rigid block analysis program for masonry arch bridges. in proceedings of the arch01: 3rd international arch bridges conference, pp. 459–464. paris, 2001. [3] r. j. bridle, t. g. hughes. ctap theory. journal inst highways transp 36:21–22, 1989. [4] p. řeřicha. a mindlin shell finite element for stone masonry bridges with backfill. acta polytechnica 62(2):293–302, 2022. https://doi.org/10.14311/ap.2022.62.0293 [5] l. j. davis. load rating of a masonry arch bridge: comparison of 2d and 3d finite element models. master’s thesis, czech technical university in prague, faculty of civil engineering, 2021. [6] p.-o. persson, g. strang. a simple mesh generator in matlab. siam review 46(2):329–345, 2004. https://doi.org/10.1137/s0036144503429121 [7] t. e. boothby. empirical design of masonry arch bridges. journal of architectural engineering 26(1):02519002, 2020. https: //doi.org/10.1061/(asce)ae.1943-5568.0000388 [8] rfem 6, fem structural analysis software. [2021-0416]. https://www.dlubal.com/en/products/rfem-feasoftware/what-is-rfem 634 https://doi.org/10.14311/ap.2022.62.0293 https://doi.org/10.1137/s0036144503429121 https://doi.org/10.1061/(asce)ae.1943-5568.0000388 https://doi.org/10.1061/(asce)ae.1943-5568.0000388 https://www.dlubal.com/en/products/rfem-fea-software/what-is-rfem https://www.dlubal.com/en/products/rfem-fea-software/what-is-rfem acta polytechnica ctu proceedings 38:630–634, 2022 1 introduction 2 the original restoration concept, ecological impact 3 load capacity of stone arch bridges 3.1 material models 3.2 finite element bridge models 3.3 load model 3.4 results for the design load of the ponikla central span 4 conclusion acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2023.43.0117 acta polytechnica ctu proceedings 43:117–125, 2023 © 2023 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague cooperation of active steering control of wheelsets with subsystems of an autonomous urban rail vehicle jan vrba czech technical university in prague, faculty of mechanical engineering, department of automotive, combustion engines and railway engineering, technická 4, 160 07 prague, czech republic correspondence: vrba@fs.cvut.cz abstract. the paper presents the motivations for the application of actively controlled elements in the running gear of a rail vehicle and the challenges that need to be addressed before deploying vehicles with actively controlled elements in real operation. furthermore, the paper presents the design of an active element concept and its control for steering the axles of an urban rail vehicle to improve the vehicle’s passage through small radius curves. the design takes into account the fundamental issues of cost and safety of the application of active elements, which are addressed in the presented control design by using data from autonomous driving subsystems such as the environmental database or the anti-collision system. keywords: active element, actuator, rolling stock, autonomous driving, tram, running gear. 1. introduction routes for urban rolling stock, especially trams, usually run through city centers, where directional curves with very small radii (less than 25 meters) are often used for historical reasons. curves with even smaller radii (around 15 meters) can often be found in depots, see example in figure 1. when a vehicle passes through such a small curve, the lateral clearance of the wheelsets in the track is exhausted and large slip forces are generated at the wheel-rail contact. these longitudinal and transverse forces lead to undesirable phenomena in the form of high wheel and rail wear and noise and vibration emissions. the wear of wheels and rails results in high costs for both the infrastructure manager and the vehicle operator, who has to arrange for machining of the worn wheel profile and subsequent replacement of the wheel tyre. wheel reprofiling, which has to be carried out regularly due to the wear of the wheel profile, and the subsequent replacement of the wheel tyre are the largest maintenance costs for tram vehicles and thus contribute substantially to the service component of the lcc (life cycle cost). improving the passage of trams through small radius directional curves has been the focus of much attention from researchers and vehicle designers around the world for many decades. improvement of smallradius curve passage of tram vehicles can be achieved by only mechanical solutions in the form of optimisation of the rolling stock running gear, for example longitudinal stiffness and primary suspension design, use of pivot bogies, lubrication of the flanges or the articulated tram vehicle concept itself. another possible purely mechanical solution to improve the passage through the curve is the use of wheelset with indepenfigure 1. demonstration of high wear of rails and turned material from vehicle wheels in tram depot [1]. dently rotating wheels (irws), whereby the torsional link between the individual wheels of the wheelset is unbound compared to conventional wheelsets, see figure 2. this solution allows the wheels to rotate at different angular speeds and deals to minimisation of longitudinal slip forces. the use of independently rotating wheels (irws) is only widespread in tram vehicles, because of the ability to pass through curves of very small radii and also because of the possibility to achieve a fully low-floor vehicle without ramps or steps over the bogies. the use of irws has a great advantage in the design of low-floor vehicles passing small radius curves, but on the other hand, vehicles 117 https://doi.org/10.14311/app.2023.43.0117 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en jan vrba acta polytechnica ctu proceedings figure 2. tram wheelset with independently rotating wheels [3]. with irws have poorer running characteristics on the straight line [2] and at higher speeds. the lack of torsional coupling between the wheels prevents the irws wheelsets from performing the sinusoidal motion that conventional wheelsets perform, which ensures more even wear on the wheel surface. due to the long development in the mechanical parts of rolling stock running gear, a major advance in the improvement of sharp curve negotiating can no longer be expected. for this reason, a great deal of interest and research capacity has been devoted in recent decades to the use of actively controlled elements in rolling stock running gear. the first active elements in locomotive running gear were already used in the 1950s to achieve a mechanical optimum. however, the rapidly developing fields of electrical engineering, sensorics or controllers in the last decades allow the use of active elements on a much wider scale and thus support the quest for faster, more comfortable, more economical and ultimately safer rolling stock (hereafter referred to as rs). however, despite several decades of research and development in the field of active elements applied to rolling stock running gear, it is not possible to claim that their use in new rolling stock running gear is more widespread. the second significant development trend in rail transport, and again especially in urban rs, is the development of autonomous vehicles and their gradual deployment in operation. currently, fully autonomous metro vehicles are already routinely deployed. moreover, partial systems leading to future fully autonomous driving for trams are developed and tested in operation. an example of such a subsystem for autonomous tram driving is the deployment of an anti-collision system (acs) on current tram vehicles to improve operational safety. a number of synergies can be found between these two development trends, both in terms of the interconnection of the vehicle control itself and therefore the active elements, and in terms of the sensors that provide the input variables for the control algorithm of the active element in the running gear and the vehicle as a whole. the aim of the article is to briefly introduce the figure 3. electro-hydraulic actuator. reader to the possibilities of using active elements in the rs running gear. the paper also describes the proposed concept of an active element in the tram vehicle running gear for the purpose of turning the wheelset into a curve and its control. the proposed concept emphasizes on trying to solve some basic issues of the application of active elements in the rs running gear, such as cost or safety, by linking the control of the active element with the sub-systems of the autonomous control of the tram vehicle. this linking can lead to a reduction in the cost of the active element application due to the use of sensors that are primarily placed on the vehicle for other purposes, and to an increase the safety of the control by verifying the input variables by two independent sources. 2. active elements in rs running gear the active element (actuator) is a force element used in the construction of the rolling stock, with the help of which we can actively influence the driving and force effects of the rolling stock in interaction with the track. an example of an electro-hydraulic actuator is shown in figure 3. 2.1. the collision scenario: the frontal impact of two identical trams the classification of the use of active elements can best be made according to their location in the running gear of the rolling stock into active elements used in secondary suspension and in primary suspension. as a special subcategory of elements in secondary suspension, it is possible to classify active elements used to tilt the carbody or the concept of placing active elements between the carbodies of a multiple unit train or articulated tram in the vertical or lateral direction. 2.1.1. classification by purpose of use a different approach to the classification of active elements is presented by stichel et al. [4] according to the purpose of their use. thus, active elements can be divided into four categories: • active elements isolating the vehicle body from track irregularities (increasing ride comfort), 118 vol. 43/2023 cooperation of active steering control of wheelsets • active elements controlling the bogie kinematics (ensuring the driving stability), • active elements steering and guiding the wheelset (radial positioning of the wheelset), • active elements for other special purposes (tilting of the carbody, more even loading of the bogies, leveling of the floor height in relation to the platform, ...). 2.1.2. classification by source of force actuators can also be divided into pneumatic, hydraulic, electro-hydraulic or electric actuators in terms of the initial source of force they exert. information regarding their characteristics or technical maturity can be advantageously drawn from the field of civil aviation, where the use of actuators is much more widespread and where the use of hydraulic and electrohydraulic actuators is still predominant. however, the electro-mechanical actuator is increasingly being used because of its good characteristics [5]. hydraulic actuators are very often used in aviation due to their technical maturity, high power density or good safety in case of failure. usually, actuators are powered from one central pump producing a constant oil pressure and controlling the oil flow by means of servo valves (the so-called “fly by wire” concept) [5]. another frequently used actuators are electrohydraulic actuators, which operate according to the “power by wire” concept, where each actuator has its own servo motor controlling a pump that produces pressure and fluid flow according to the current need. this solution brings weight reduction as it eliminates the need for manifolds, oil reservoirs or valves [5]. the electro-mechanical actuator is the least technically proven, but very promising for future use. this type of actuator is the best controllable, lightest, least demanding for maintenance, but it can cause mechanical jamming if the ball screw malfunctions. this type of actuator is used, for example, in electrical multiple units for carbody tilting [5]. the actuators presented are applied as linear actuators and thus influence the behaviour of the vehicle on the track by means of their extension. a different approach is the torque control of traction motors that drive independently rotating wheels. each traction motor is thus individually controlled by its own traction converter and when passing through small radius curves, the angular velocity of the outer wheels is increased in the case of irws wheelsets. moreover, the wheels may be also braked on the inside of the curve. this has the effect of ensuring the necessary difference in rolling distance between the inner and outer wheels and thus improving the passage of the rolling stock through the curve. in this case, the traction motor can therefore be considered as the active element. 2.1.3. classification according to the input variable of the active element control in the past years, many control algorithms have been developed and described in the literature, which control individual actuators in the rolling stock running gear and thus control the steering of the wheelset into a curve, or control individual traction motors driving the irws. in particular, two categories of control method can be defined in terms of the variables entering the control algorithm. the first method of active element controlling is the feedback control based on the knowledge of the lateral position of the wheelset in the track. this method was introduced in the works of, for example, lu et al. [6], oh et al. [7] or ahn et al. [8]. however, this control method is very problematic for practical application on real rolling stock, since the measurement of the lateral deflection of a wheelset is very difficult to perform in an operationally demanding environment such as a rs running gear. hyundai rottem applied this method of active torque control of the irws to a test tram vehicle, and a reduction of up to 30 % in lateral wheel-rail contact forces was achieved when the vehicle was driven on a test track [9]. the method of controlling the active steering of the wheelset based on the radius of the currently negotiating curve seems to be much more applicable. the radius of the curve can be sensed by sensors on the vehicle (automatic rail curvature detection from lidar or camera sensed data), or the information can be retrieved from a track database on the vehicle based on the current position of the vehicle on the track. this input variable has been used to control actuators by authors such as hur et al. [10], stichel et al. [11], and for active control of irws by čapek [2]. a commercial application of active wheelset steering designed in this way was presented by crrc on cetrovo metro vehicles. the steering of the wheelsets is performed by electro-hydraulic servo actuators. first tests in operation show a significant reduction of the steering forces in wheel-rail contact and also a reduction of noise emissions [4, 12]. 2.2. active element application into rs running gear the positive effect of properly selected and controlled active elements in the running gear of a rolling stock on the reduction of wear of wheels and rails, noise and vibration emissions when passing small radius curves, increase in driving comfort or increase in maximum vehicle speed can be considered sufficiently proven. nevertheless, a wider expansion of active elements into the running gear of rolling stock has not yet taken place. exceptions are, for example, some pendolino electric multiple units with active tilting of the carbody in order to reduce the lateral forces acting on the passenger when passing through a curve, or an active element placed instead of a yaw damper in the 119 jan vrba acta polytechnica ctu proceedings secondary suspension of vectron locomotives in order to actively steer the bogie when passing through a curve [13]. the author of the paper discusse in detail the research on the use of active elements in rs running gear in [14]. from the conducted research it is evident that the most attention is paid to active elements improving the vehicle’s passage through the curve and in particular to active elements located within the primary sus-pension and actively controlled irws. the wider spread of active elements in the rs running gear is hindered by unresolved issues, including in particular the question of safety of their use in normal operation, cost or maintenance requirements. another relatively unresolved issue may be the relative movements of the wheelset and the drive or brakes. unresolved issues for actively controlled independently rotating wheels are primarily the magnitude of the needed steering torque when passing a small radius curve [2], the cost and number of traction converters, or finding the right steering algorithm to avoid wheel climbing on the flange during track irregularities or at curve transition. 3. autonomous driving systems in general, rail vehicles have a significant advantage over road vehicles in terms of autonomous driving in terms of directional guidance in the track and thus have one less degree of freedom. as mentioned in the introduction, autonomous driving, particularly of metro vehicles or similar transport systems, is a commonly deployed technology. autonomously controlled trains are also being tested and the implementation of etcs on european railways can also be seen as a major step towards autonomy. certain uniqueness of trams compared to road vehicles, but also compared to other types of rolling stocks, leads to the need for specific development of some autonomous driving subsystems. these specific systems are mainly the anti-collision system (acs) and the track map information database. due to the operation of trams in urban areas, the issue of sufficiently accurate vehicle localisation on the track must also be taken into account. for the other sub-systems of autonomous driving, it can be assumed that it will be possible to adopt them for autonomous tram vehicles from other rs. acs is mainly used on tram vehicles on the german market, but is gradually becoming a part of domestic vehicles as well. the system’s task is to detect an obstacle in the vehicle’s passage profile (by using a camera, radar, lidar, etc.), to alert the driver with a light or sound signal and, if necessary, to directly intervene in the driving of the vehicle. anti-collision systems developed at the same time often work on inputs from lidar and cameras, supplemented by an imu unit that records vehicle movements. lidar data is usually used for obstacle detection and camera figure 4. hd map of the railway track [17]. data is used for obstacle type recognition by image processing [15]. the environment database contains a static map database and thus provides the vehicle information about, among other things, the vertical and horizontal profile of the track, the track surroundings or the infrastructure along the track. the second part of the vehicle environment database can be described as dynamic, as it is modified in real time based on data from other vehicles in the system, dispatching, etc. today, localization is often solved by rfid and odometry technology for tram vehicles. however, for the use of autonomous vehicle localization, the technology of combining gnss with odometry and imu unit seems to be more suitable. by combining these signals, a relatively very accurate vehicle location can be achieved even in built-up urban areas within centimetres [16]. another recently much discussed solution is the use of hd map technology, where the surroundings of the tram line are first scanned and stored in the vehicle environment database. the vehicle then scans its surroundings while driving and determines its location based on matching with the same objects in the database. this method is particularly suitable for urban environments, which are diverse in terms of objects and also have the risk of poorer gnss signal reception. this method can thus refine positioning to gnss [17]. this method of localization can be advantageously used for vehicles with an anti-collision system based on lidar data, as the same sensors can be used for obstacle detection and localization. a sample hd map of the railway track is shown in figure 4. 4. conceptual design of active wheelset steering control as already mentioned in section 2.2, the use of active elements is hindered by the issues of price, reliability and, last but not least, the safety of their application. these key factors have been taken into account in the design of the control concept for active elements in tramway running gear for the purpose of steering individual wheelsets into a curve. when travelling on a straight track, the actuator in its nominal position 120 vol. 43/2023 cooperation of active steering control of wheelsets will be used to transfer the traction and braking forces from the wheelsets to the bogie frame. the design is based on the assumption that the active elements will be applied to the running gear of a tram vehicle equipped with an anti-collision system operating on the basis of object detection in the vehicle’s running profile. for obstacle detection, it is assumed to use one or more lidars mounted on the front of the vehicle in combination with an imu unit mounted on the same part of the vehicle. in addition to obstacle detection, the lidar data will also be processed to make the position of the vehicle on the track from gnss more accurate by automatically searching for matches between the imaged surroundings and the surroundings stored in the hd map in the track database. based on the found matches, the distance of the vehicle from objects, buildings, etc. will be evaluated and thus the position of the vehicle on the tram line will be determined unambiguously and accurately enough. 4.1. location and type of active element for the design of the active element control and its application, an active element is considered on each side of the wheelset located within the primary suspension with the function of steering the wheelset into a curve and transferring the driving and braking forces. in parallel with the active element, a passive primary longitudinal suspension is considered. this solution increases the requirement for the force exerted by the actuator and thus increases its weight and dimensions, which makes it more difficult to incorporate the active element into the running gear, but the main advantage is to ensure safe driving of the vehicle in the event of a failure or deliberate removal of the active element from service. in the event of a failure of the active element or its control, it is thus considered that the vehicle will be able to reach the currently serviced line at a reduced speed and then leave for a service operation in the depot. fu and bruni [5] discussed in detail the comparison of dif-ferent concepts of placement and types of active elements in the primary suspension of the rs from the safety point of view. their suggestion of the most suitable solution is shown in figure 5 on the right and represents a variant where each actuator is duplicated by a different type of actuator in case of failure. hydraulic and electro-hydraulic actuators are used. although this solution appears to be the safest, it is doubtful whether it can be applied to the running gear of conventional rs, let alone trams, where the problems of incorporating new elements are compounded by the low floor level. apart from the large installation dimensions, the high cost of the application and the complicated and demanding servicing of two different types and twice as many actuators can be expected. the parameters of the selected actuator type will also enter the active element control algorithm. based (a). (b). figure 5. the considered concept of placing active elements in the tram bogie (a), the concept evaluated as the safest according to [5] (b). figure 6. solution of electro-hydraulic actuator with fail-safe hydraulic circuit [18]. on the actuator characteristics described in section 2.1.2, an electro-hydraulic actuator can be tentatively considered. this appears to be a compromise between simplicity of control, installation dimensions, maintenance requirements and the ability to ensure safe operation of the actuator. although the electromechanical actuator has the above mentioned characteristics better, it may cause mechanical jamming if the ball screw malfunctions, which will make further operation of the vehicle impossible. this is unacceptable in a tramway system, as the inability to operate one vehicle can block the entire transport system. on the other hand, a safety circuit can be added to the elec-tro-hydraulic actuator to ensure that the actuator is taken out of operation if necessary. the design of an electro-hydraulic actuator with a fail-safe function has been addressed by umehara et al. [18], see figure 6. 4.2. active element control 4.2.1. input variables into active element control in terms of the cost of the active element application, a large part of the cost can be attributed to the sensors that need to be fitted to the vehicle to obtain the input variables needed for the active element control algorithm. similarly, the issue of safety is largely linked to the correct sensing of input quantities. algorithms based on the measurement of the lateral 121 jan vrba acta polytechnica ctu proceedings position of the wheelset in the track are difficult to apply in practice, due to the near impossibility of reliably measuring this quantity in such a demanding urban tram operation environment. for this reason, the control design of the active element is performed only with input quantities that can be obtained from the track environment database, i.e. the current radius of the passing curve, or with quantities that are already sensed on the current conventional vehicles – the vehicle speed. vehicle speed is a quantity that is normally sensed with sufficient accuracy already on current vehicles and thus there is no need to install additional sensors. the radius of curve will be obtained from the track environment database based on the vehicle position, which will be obtained using hd mapping and gnss. thus, the same sensors located at the front of the vehicle for the acs are used to locate the vehicle and there is no need to use additional sensors to obtain input variables for controlling the active element. the design also assumes that other parameters of the track, obtained from the environmental database, could be input to the control algorithm of the active element. in particular, the track lateral clearance, track gauge (in small radius curves these parameters are often extended), superelevation or track quality (influence of vertical and lateral irregularities). based on this information, the steering of the wheelsets will be controlled and the vehicle speed will be regulated. in order to obtain such detailed information about the infrastructure, it is necessary to find a tool that is able to map the track and its surroundings in sufficient detail. 4.2.2. basic active element control relations as input variables to the control of the active element, the easily obtainable variables of the instantaneous vehicle speed and the radius of the directional curve r will be input. in the next phase of development of the control algorithm, the plan is to include information about the gauge g or clearance in the track σ in the control of the active element. the optimal displacement of the actuator ∆l can then be expressed as: ∆l = a · b r + f(g, σ) , (1) where 2b is the wheelbase of the bogie and 2a is the lateral distance of the actuators steering the wheelsets, see figure 7 [5]. based on the optimal actuator extension, the required actuator force can be expressed as: f = ( kx · ∆l + ft_dv 2 ) · k . (2) the actuator must be dimensioned based on the requirement for the actuator displacement ∆l, the longitudinal stiffness of the primary suspension kx and figure 7. bogie with actively steering wheelsets passaging through a curve [5]. then the transfer of the maximum traction force of the vehicle to each driven wheelset ft_dv, which will be transferred from the wheelset to the bogie frame via the actuators. the coefficient k can be seen as a safety coefficient, the value of which will be further investigated to guarantee safe steering of the wheelset in any operating situation (empty/fully loaded vehicle, high/low coefficient of adhesion, etc.), but at the same time not over-dimensioning the actuator. oversizing the actuator leads to an increase in its size and weight, making it even more difficult to place the actuator into the vehicle running gear. due to the positioning of the acs sensor on the front of the vehicle, the individual wheelsets will need to be steered with a delay depending on the vehicle speed and vehicle dimensions (vehicle front length, bogie axle and wheelbase distances, length of individual articles). the extension speed and other actuator parameters will also have an influence and will be included in the control algorithm. at the current design stage, the control is intended to be forward, which appears to be a suitable solution for this task. it has the advantage of being able to better adapt to different boundary conditions that may occur during operation [11]. the detailed design of the active element control will be developed in the next phase of the research. 4.3. safety issues of active element application and its control for the evaluation of system fault tolerance, inspiration can be drawn from the aerospace industry, which is significantly further along in this field. comparisons in terms of risk priority number and based on the proposed methodology [5] of how to assess fault tolerance 122 vol. 43/2023 cooperation of active steering control of wheelsets can be used. the risk priority number is determined based on two main criteria, the severity of the failure in terms of economic loss or personal injury and the probability of failure. a third additional criterion may be the ability to detect a failure using a monitoring system. from the point of view of the safety of vehicle operation, the following factors and operating situations appear to be particularly problematic: • correctness of control input information (sensors), • actuator malfunction status: ▷ inactive actuator, ▷ actuator stuck in utmost position. • control algorithm error: ▷ maximum extension of both wheelset ac-tuators in the opposite direction, ▷ extending only one wheelset actuator in the opposite direction. all these states need to be considered in the design of the active element, its control and the whole vehicle and then the system needs to be evaluated using, for example, the methodology mentioned above. as in aviation, in the case of the design of the control of an active element, it would be advisable to obtain the key variables entering its control from two sources. the correctness of the main input information of the control algorithm, i.e. the curve radius, will thus be verified by comparing the value stored in the vehicle environment database based on the knowledge of the vehicle position with the measured and automatically evaluated value using sensors placed on the front of the vehicle primarily for the purpose of obstacle detection. this mechanism should prevent the vehicle from derailing due to a bad steering input variable. if these two values differ, the active element will be set to the nominal position and the vehicle will drive through the curve at a reduced speed. the leading of the wheelset shall be still provided by a active element, which, however, will not steer the wheelset. in the event of a failure of the active element or its control, the actuator is disabled using the fail safe function. the leading of the wheelset is shall be provided only by a passive primary suspension designed with a longitudinal stiffness that allows the vehicle to pass safely through the curve with the smallest radius at a low speed defined by the derailment safety calculation. the longitudinal stiffness of the passive primary suspension shall be designed sufficiently large to allow a vehicle with a disabled active element to complete its service on the line (albeit at reduced speed and with accumulated delay) and to depart to the depot. at the same time, however, it must be taken into account that the higher the longitudinal stiffness of the primary suspension, the greater the force the actuator must exert in normal operation to overcome this stiffness when turning the wheelset into a curve. an increase in the required actuator force leads to an increase in actuator mass and volume and is therefore undesirable. thus, the longitudinal stiffness of the primary sus-pension must be very carefully tuned to compromise the two described conflicting requirements. 5. conclusion in this paper, the design of the concept of controlling the active element in the tram vehicle running gear in order to actively steer the wheelset into a curve is presented. the aim of this application is to reduce the longitudinal and lateral slip forces at the wheel-rail contact when passing small radius curves, thereby reducing wheel and rail wear and noise and vibration emissions. the proposal attempts to reflect the problems that have so far discouraged the wider application of active elements in rolling stock running gear and outlines possible solutions that will be further addressed by the author. for the control of active elements, it is proposed to use the subsystems of autonomous vehicles that are currently already installed on vehicles or can be expected to be installed on vehicles in the near future. these systems are in particular an anti-collision system, vehicle position detection using hd maps or the use of an environmental database. by exploiting these synergies, the cost of controlling the active elements is reduced, as there is no need for expensive sensors to measure the input variables, and the security of their use is also increased, as the input information is verified from two different sources. the aim is also to further supplement the forward control of the active element with additional track parameters that are stored in a detailed environmental data-base and have an impact on the vehicle’s passage through the curve. the article deals with the active steering of wheelsets using actuators located in the primary suspension of double-axle bogies of the classic concept of modern articulated trams. however, it can be assumed that future advances, in particular the reduction of the weight of the electric equipment and the mechanical parts of the vehicle or the autonomous driving will change the trend in the production of tram vehicles and will allow the creation of a light double-axle autonomous tram, where the actuator will have the function of steering the single-axle bogies when passing through a curve, and on the contrary, stabilizing the bogies when driving on a straight line. an example of such a vehicle concept is shown in figure 8. in such a case, the design of the positioning of the element, which would probably be placed between the bottom of the vehicle carbody and the bolster or frame of the single-axle bogie, and the control algorithm would change, but the main ideas outlined in the paper remain valid. inspiration for the design of a similar running gear can be drawn from the solution applied to the cobra trams, see figure 9. in further research, the author wants to focus on creating mbs models of articulated trams of different 123 jan vrba acta polytechnica ctu proceedings figure 8. possible solution for a two-axle autonomous tram presented by the company škoda [19]. figure 9. running gear of cobra tram [20]. concepts in the environment of suitable mbs software. the next step will be to create a detailed control algorithm for the active elements in matlab simulink and then link these two models. by cosimulation, it is possible to link the control of the active element to the vehicle model and to demonstrate the advantages of such a solution by simulating the driving of a vehicle with a controlled active element, to compare the lateral forces and wear during the passaging the reference track of vehicle with passive and active suspension, or to simulate failure conditions and evaluate their criticality. in the next phase, the failure states of the actuator and its control will be simulated by means of cosimulation and the safety of the presented design will be assessed. the final goal of the research is to test the active wheelset steering design on an experimental roller rig. list of symbols a half of the bogie wheelbase [mm] b half the lateral distance of the actuators [mm] g track gauge [mm] f actuator force required [n] ft_dv traction force transmitted by one wheelset [n] kx longitudinal stiffness of the prim. suspension [n·m−1] ∆l required actuator extension [m] acknowledgements the research was supported by sgs21/112/ohk2/2t/12. references [1] f. fritz. spurführung in engen gleisbögen. der nahverkehr 3(2):52–61, 1985. [2] j. čapek. optimalizace jízdních vlastností nízkopodlažních tramvají. dissertation thesis. fme ctu in prague, 2013. [3] ghh – bonatrans. tram wheelset with independent wheel axle. [2022-04-06], https://www.ghh-bonatrans.com/en/products-andservices/wheelset/. 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[2022-02-17]. arxiv:2111.15043 [18] y. umehara, s. kamoshita, k. ishiguri, y. yamanaga. development of electro-hydraulic actuator with fail-safe function for steering system. quarterly report of rtri 55(3):131–137, 2014. https://doi.org/10.2219/rtriqr.55.131 [19] skoda.cz. technologie a design. [2022-03-30], https: //www.skoda.cz/stranka/technologie-a-design. [20] m. kummer, s. krause. “cobra” – ein neues konzept für niederflur-straßenbahnfahrzeuge. schweiz eisenbahn revue 4:174–183, 1993. 125 https://doi.org/10.1109/tits.2008.2011712 https://arxiv.org/abs/2111.15043 https://doi.org/10.2219/rtriqr.55.131 https://www.skoda.cz/stranka/technologie-a-design https://www.skoda.cz/stranka/technologie-a-design acta polytechnica ctu proceedings 43:117–125, 2023 1 introduction 2 active elements in rs running gear 2.1 the collision scenario: the frontal impact of two identical trams 2.1.1 classification by purpose of use 2.1.2 classification by source of force 2.1.3 classification according to the input variable of the active element control 2.2 active element application into rs running gear 3 autonomous driving systems 4 conceptual design of active wheelset steering control 4.1 location and type of active element 4.2 active element control 4.2.1 input variables into active element control 4.2.2 basic active element control relations 4.3 safety issues of active element application and its control 5 conclusion list of symbols acknowledgements references https://doi.org/10.14311/app.2022.33.0444 acta polytechnica ctu proceedings 33:444–451, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague engineering properties of geopolymer concrete: a review eliana parcesepea,∗, carmine limab, giuseppe maddalonia, maria rosaria peccea a university of sannio, department of engineering, piazza roma, 21, benevento 82100, italy b items srl, piazza guerrazzi, 1, benevento 82100, italy ∗ corresponding author: parcesepe@unisannio.it abstract. geopolymer concrete (gpc) could be a solution that uses a cementless binder and recycled materials for producing concrete, while reducing the carbon dioxide emission and the demand for raw materials. in addition to the environmental aspect, previous studies on gpc showed that it can achieve mechanical characteristics higher than those of ordinary portland concrete (opc) such as greater strength a few days after casting, and it can be suitable for structural applications. in this paper, the state-of-the-art review of gpc is presented through an extensive literature analysis to determine the most recent information regarding the engineering properties of geopolymer concrete and the critical issues that prevent its widespread use and to put forward suggestions for future research. in particular, the physical properties in both fresh and hardened states and the mechanical characteristics are investigated; the structural performance of geopolymer concrete elements is also outlined. keywords: fresh and hardened properties, geopolymeric concrete, recycled materials, structural properties. 1. introduction the issue of global warming and, more generally, environmental pollution concerns all areas of science, technology and industry, including construction. in particular, the construction field is characterized on the one hand by an enormous demand for energy and on the other hand by the significant production of raw materials. from this point of view, the use of more eco-friendly building materials, such as geopolymer concrete (gpc), can be a valid alternative to ordinary portland concrete (opc). the environmental and economic concerns associated with conventional cement-based building materials have led the scientific and technical community to explore possibilities in the use of alternative materials. currently, the concrete industry faces challenges due to the growing demand for portland cement, such as the increasingly limited limestone reserves and the increase in carbon taxes due to greenhouse gas emissions resulting from the production of opc [1]. these issues require the development of alternative binders, such as alkali-activated cement, with the aim of reducing the environmental impact of buildings, the use of a greater percentage of pozzolan waste and improving concrete performance [2]. from this perspective, geopolymers are alternative binders that attract considerable attention due to their early compressive strength, low permeability, good chemical resistance and excellent fire-resistant behaviour. moreover, geopolymers are ecological materials whose production reduces co2 emissions with respect to opcs [3]. despite the positive results available in the scientific literature, the use of gpcs in practice is still limited mainly due to production costs, the wide variety of properties that can be obtained with different mixes of gpc, the lack of more extensive studies both on material and structural elements and the absence of specific design rules and codes. in this paper, a comprehensive literature review is summarized to assess the critical issues of geopolymer concrete and identify the basic aspects for a research project aimed at an experimental program. the main features of the material are summarized to briefly describing its composition, production methods and current applications. then, the physical and mechanical characteristics of the material are discussed, and the structural performance of the reinforced elements made of geopolymer concrete is also analysed. 2. an overview on geopolymers 2.1. composition the term "geopolymer" is generically used to describe an amorphous alkali-aluminosilicate, which is also commonly used to represent "inorganic polymers", "cements activated with alkalis", "geo-elements", "related ceramics with alkali ", etc. despite the variety of nomenclatures, these terms describe all the materials synthesized using the same chemical process [5, 6]. geopolymers consist of basic and additional components: the former is aluminium silicate (binder), sand, alkali (mortar activator solution) and aggregates (to produce concrete); additional components 444 https://doi.org/10.14311/app.2022.33.0444 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 33/2022 engineering properties of geopolymer concrete figure 1. classification of geopolymer concrete. figure 2. geopolymer concrete road pavement [4]. can be extra water, plasticizers and fibres. specifically, geopolymer concrete can be produced by polymerizing aluminosilicates such as fly ash (fa), metakaolin (mk), slag (sg), rice husk ash (rha) and wood ash with high calcium content (hcwa) by activation with alkaline solution. figure 1 shows the composition of the raw materials at the base of geopolymer concretes (approximately 80% of the composition) and the activators used (approximately 20% of the composition). various alumina silicate resources have been adopted for producing geopolymers, but from the analysis, fa is the most commonly used and widely tested resource used for geopolymer concrete, showing a high reactivity and more durability than metakaolin-based geopolymers. however, slagbased geopolymers are considered to have high early strength and greater acid resistance than metakaolin and fly ash-based systems [3, 7]. these aspects show that the microstructures and chemical properties of geopolymers vary widely, although many physical properties of geopolymers prepared from various aluminosilicate sources may appear to be similar. it would be important to identify more standardized mixtures, while considering economic aspects, to be able to define the mechanical characteristics with reliable formulae and provide design procedures, also achieving classification. 2.2. research and applications scientific interest in the field of geopolymers has considerably increased since 2016 [8]; however, geopolymer concrete has not yet obtained international acfigure 3. prefabricated geopolymer concrete beam [4]. ceptance as a building material mainly because the production cost of geopolymer concrete is not competitive. furthermore, reliable data are needed on the practicality of using geopolymer concrete as a structural reinforcement element to develop design procedures. the economic aspect is certainly very influential on the diffusion of the material due to the transport system, which is not as effective as that of cement. mathew et al. [9] estimated that the cost of gpc based on coal ash and granulated blast furnace slag (ggbs) can be more than twice that of opcbased concrete if the difference in the rate of transport is considered; otherwise, normalizing this effect, the cost difference between the two types of concrete is equal to 7%. however, the advantages in terms of ambient temperature processing, low carbon dioxide emissions, environmental friendliness, carbon dioxide reduction targets and reutilization of waste must be considered; therefore, a procedure to assess the social cost of this material needs to be developed. the geopolymer industry is becoming established, and an increasing number of geopolymer supplier companies are becoming active based on research activities. several applications regard the transportation sector in the usa due to the short setting time of geopolymer cement, which makes it an ideal solution for repairing highways and airport runways (figure 2). furthermore, gpc is now used in australia both for prefabricated elements and in situ casting (figure 3) [4]. 445 e. parcesepe, c. lima, g. maddaloni, m. r. pecce acta polytechnica ctu proceedings research density csa stsb fsc ymd poisson ratio activator/ binder ratio curing temperature and time [kg/m3] [mpa] [mpa] [mpa] [gpa] [−] [−] [−] [10] 2330-2430 30-80 3.8-6 5-1 23-31 0.12-0.16 0.35-0.4 60-80 ◦c for 24 h [11] 1876-2555 65-77.9 2.8-5.1 nre 11.2-41.2 0.15-0.19 0.4-0.65 60 ◦c for 24 h [12] 2074-2185 24.9-82.5 1.2-4.3 3.4-5.4 8.2-22.7 nre 0.82-0.92 nre [13] 2400 46.3-57.2 3.1-4.5 nre 17.1-30.8 0.16-0.21 0.4-0.6 60-80-120 ◦c for 4-6-8-10 h [14] 2400 16.2-52.6 2.9-8.4 nre nre 0.13-0.18 0.4 22 ◦c till testing and 50 ◦c for 48 h [15] nre 29-43.5 nre 6.86 10.7-18.4 nre 0.4-0.55 85 ◦c for 20 h [16] nre 17.7-37.9 3.2-5.4 2.9-4.1 nre nre nre 60 ◦c for 48 h [17] 2147-2408 47-56.5 2.8-4.1 4.9-6.2 23-39 0.23-0.26 0.45-0.59 23 ◦c till testing a cs: compression strength. b sts: splitting tensile strength. c fs: flexural strength. d ym: young modulus. e nr: not reported. table 1. properties of geopolymer concrete. 3. mechanical properties of geopolymer concrete 3.1. fresh and hardened properties extensive studies have been performed to assess the performance of geopolymer concrete, showing that the behaviour is affected by the following parameters: • chemical composition and mineralogy of the precursor materials. • quantity, shape (solid, liquid) and type of alkaline activator (optimal na/al ratio: 1-1.5). • si/al ratio of the precursor materials (optimal: 2.5-4) and quantity of the available calcium source, such as portland cement, blast furnace slag and lime. • total water/solids ratio (precursors + alkaline salts). • time and temperature of curing. regarding the precursor materials, mixtures with fine particles activated with alkali exhibit greater workability and require less water due to the reduction in porosity and to the increase in the surface of the finer particles [18, 19]. the molarity of the activator, the superplasticizer and the water content strongly influence the workability of geopolymer concrete [20, 21]. in particular, the addition of naphthalene and polycarboxylate superplasticizing additives increases the workability [22, 23], but the presence of superplasticizers in contents greater than 2% is responsible for a slight degradation of the compressive strength [24]. therefore, the chemical process, called "geopolymerization", depends on the environmental conditions that occur during the process. the existence of an optimum curing temperature (60◦c) that allows the achievement of the best physical and mechanical properties was observed. the stress-strain behaviour of concrete provides insight into its ability to ensure an adequate degree of safety and serviceability in structural applications. different mixes of geopolymer concrete (gpc) were analysed by various authors, and a summary of the engineering properties is reported in table 1. it is noted that conditioned curing at a high temperature for at least 24 h is generally necessary to obtain mean values of compression strength greater 446 vol. 33/2022 engineering properties of geopolymer concrete figure 4. comparison between eurocode 2 and experimental formulae: (a) splitting tensile strength, and (b) elastic modulus. than 50 mpa; therefore, only prefabricated elements can be made that require a large space with a conditioned environment. therefore, it is important to focus research on mixtures and curing techniques but also on the modulus of elasticity that has a scattered correlation with compression strength. 3.2. experimental relationships and standards to establish correlations within the mechanical properties of geopolymer concrete, several attempts have been made from experimental results by various authors. sofi et al. [24] found that the splitting tensile strength and flexural strength of gpc were comparable to those models presented by the australian standard (as 3600) for opc. regarding the stiffness, the modulus of elasticity of gpc was found to be lower than the values predicted by the standards, particularly by aci318 for opc concrete [25]. ryu et al. [6] compared the formulae proposed by aci 363r-92 and model code 1990 with the experimental results. additionally, in this case, the splitting tensile strength (fct) with respect to the cylindric compressive strength (fct) is lower than that provided by the formulae proposed by the standards; therefore, the following equation (equation 1) is proposed: fct = 0.17 (fc)3/4 (1) other formulations were suggested by lee-lee [25] for both the splitting tensile strength fct (equation 2) and elastic modulus ec (equation 3) compared to the compressive cylindric strength fc: fct = 0.45 (fc)1/2 (2) ec = 5300 (fc)1/3 (3) comparing the proposed formulations for the gpc with those reported in eurocode 2 for strength classes of concrete from c12/15 to c50/60 (figure 4), it is noted that also in this case, the european standards overestimate the properties. in particular, the considerable difference concerning the elastic modulus can lead to problems in the serviceability state. therefore, it is necessary to study the possibility of increasing the ratio between the elastic modulus and resistance in mixtures by analysing which components have the greatest influence on the elastic modulus in detail. in the case of tensile strength, it is clear that performance worsens considerably with the increase in compressive strength; therefore, attention must be focused on the analysis of the microstructure and the inert-matrix interface for higher strengths. 4. behaviour of the structural elements 4.1. reinforced concrete beams and columns the research on geopolymer concrete has been extended to structural elements such as beams and columns. studies on fa-based geopolymer concrete have shown that the behaviour of gpc beams is similar to that of ordinary reinforced concrete beams, but the predictions from the standards have led to conservative designs [29]. even for gpc columns, insignificant differences were found with respect to ordinary columns. the failure modes observed in the compression tests show the opening of the cracks and the crushing of the concrete in the central section and the buckling of the reinforcement bar in the compression zone [28]. it is well known that crisis due to bending generally occurs with yielded reinforcement and therefore is not substantially affected by the constitutive bond of the concrete. therefore, tests should be carried out above all on beams that fail in shear because the shear mechanisms are substantially influenced by the tensile strength of the concrete and the interlock characteristics of the cracks, i.e., also by the inert/matrix behaviour. furthermore, the more relevant differences in the flexural behaviour of rc beams occur under serviceability conditions that are 447 e. parcesepe, c. lima, g. maddaloni, m. r. pecce acta polytechnica ctu proceedings research variable remarks beam [26–28] reinforcement ratio (ρ) flexural strength and ductility influenced by ρ, similar behaviour to opc beams also in shear, applicability of the calculation methods of reinforced opc beams [29] quantity of recycled aggregate greater number and width of cracks, but greater deformation and ductility [30] reinforcement configuration higher shear strength of gpc beams than equivalent opc beams [31] steel fibre volume (ω) increase in the cracking load and the ultimate strength as ω increases column [32] compression strength of concrete higher ultimate strength, ductility and stiffness of gpc columns than equivalent opc columns [33] steel fibre volume the addition of steel fibres increases the bearing capacity up to 56% [30] compression strength of concrete, reinforcement ratio crushing breakage of a fragile type similar to reinforced opc columns [34] effect of the confinement increase in the strength by approximately 30%, increase in ductility [35] eccentricity of the load, slenderness ratio no significant difference between the columns in gpc and opc table 2. summary of the structural performance of gpc elements. influenced by cracking and the elastic modulus; therefore, experimental tests could also be developed on ties to better understand crack propagation, spacing and width. to increase the performance of geopolymer concrete, the effects of additional steel fibres were investigated in beams and columns [31, 33]. furthermore, to increase the load carrying capacity and ductility of gpc columns, confinement can be applied using double layer gfrp wrapping [34]. table 2 shows the summary of previous literature on fa-based geopolymer concrete elements. 4.2. bond between reinforcing steel bars and geopolymer concrete the mechanism of the bond influences the embedded length of the reinforcing bar and consequently the load-bearing capacity of the structural elements, the crack opening and spacing, and the anchorage length. the bond-slip mechanism of the steel-concrete interface of rebar is especially influenced by the concrete strength in tension, which has been commented in par. 3.2. investigations on the gpc bond behaviour are rather limited; however, research was started by sofi et al. [36], who observed a lower influence of the type of fly ash on the bond strength. interface behaviour was investigated in further studies through pull-out tests summarized in table 3. in general, the tests showed that the bond strength between fly ash-based gpc and steel bars is on average higher than that of opc [37]. in some cases, the link between steel and gpc is so strong that the break involved steel bars, while in the case of traditional concrete, pull-out breakage occurred [15]. further tests are necessary to develop formulations for reliable anchorage lengths in gpc. 5. conclusions the paper presents a comprehensive review on the performance of gpc, demonstrating the benefit of using gpc but also the lack of results in many important aspects that influence the structural behaviour and the reliability of the properties, which does not allow us to assess reliable formulations for design. observations that can address further research are summarized as follows: • the development of the mix proportion of gpc is more difficult than that of opc due to a range of parameters being involved in the matrix of 448 vol. 33/2022 engineering properties of geopolymer concrete research bond strength [mpa] variables [36] 10.5 − 14.7 amount and type of fly ash, bar diameter [38] 14.5 − 35.6 bar diameter, concrete resistance [37] 24.1 − 31.9 curing time [39] 12.7 − 16.6 bar diameter, embedded length, steel fibre volume [40] 3.1 − 6.7 smooth bars: bar diameter, cover/bar diameter ratio, embedded length table 3. summary of bond strength research. geopolymer concrete; therefore, more accuracy in the choice of components is needed. most of the research work is limited to heat curing conditions; however, geopolymer products can spread, especially if they can suitably be developed under ambient curing conditions. therefore, the variation in the properties of ambient temperature cured materials and geopolymer concrete prepared under field conditions needs to be investigated to achieve classifiable products with reliable standard properties. • gpc has considerable potential for use as a construction material, especially due to its very high strength. however, the mechanical properties (elastic modulus and tensile strength) increase less than those of opc with the same compression strength; therefore, the serviceability performance could be the key issue of the design. furthermore, research is lacking on long-term mechanical properties (creep and shrinkage), permeability and overall durability issues that have to be analysed by microstructural analysis. • experimental studies on structural elements show gaps in tests on serviceability conditions, especially in crack propagation. therefore, further research in the fields of multiaxial stress states (efficiency of confinement) and stiffness degradation is needed. • the economic issue is certainly a challenge to face. a solution to reduce the cost may be performed with raw materials regarding the manufacturing process of sodium hydroxide or replacing the fine aggregates with alternative materials such as crusher dust; however, the impact of using such materials on the strength of concrete has to be studied. • the currently available standards are not exhaustive both for the material and for the structural elements; therefore, the unavailability of adequate manufacturing and design provisions represents the main challenge for the mass use and diffusion of geopolymer concrete. in conclusion, the literature analysis revealed that the mechanical properties of geopolymer concrete are affected by curing conditions, and its manufacturing process requires a proper mixed design. therefore, there is an urgent need to develop a user-friendly geopolymer concrete design procedure to carry out a more efficient analysis of the experimental results considering that the material used is comparable for all tests and theoretical models. this is the first step to assess design formulations with adequate safety factors for design procedures. references [1] s. b. singh, g. ishwarya, m. gupta, et al. geopolymer concrete: a review of some recent developments. construction and building materials 85:78-90, 2015. https: //doi.org/10.1016/j.conbuildmat.2015.03.036. 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[40] e. s. dewi, j. j. ekaputri. the influence of plain bar on bond strength of geopolymer concrete. aip conference proceedings 1855:030017, 2017. https://doi.org/10.1063/1.4985487. 451 https://doi.org/10.1155/2014/312139 https://doi.org/10.1016/j.conbuildmat.2017.03.014 https://doi.org/10.1007/s10853-006-0534-5 https://doi.org/10.1016/j.cemconres.2015.02.016 https://doi.org/10.4028/www.scientific.net/amm.578-579.441 https://doi.org/10.1680/macr.14.00125 https://doi.org/10.1063/1.4985487 my title https://doi.org/10.14311/app.2022.36.0231 acta polytechnica ctu proceedings 36:231–236, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague probabilistic safety assessment of koeberg spent fuel pool sibongiseni thabethe university of pretoria, national nuclear regulator, centre for nuclear safety and security, engineering building 2, lynwood road, po box 395, pretoria 0002, south africa correspondence: sthabethe@nnr.co.za abstract. the effective management of spent fuel pool (sfp) safety has been raised as one of the emerging issues to further enhance nuclear installation safety after the fukushima accident on march 11, 2011. sfp safety-related issues have been mainly focused on (a) controlling the configuration of the fuel assemblies in the pool with no loss of pool coolants, and (b) ensuring adequate pool storage space to prevent fuel criticality owing to chain reactions of the fission products and the ability for neutron absorption to keep the fuel cool. in support of regulatory functions, the centre for nuclear safety and security (cnss) seeks to perform confirmatory analysis for all potential accident scenarios that may occur in the koeberg nuclear power plant sfp. probabilistic safety assessment (psa) was done using the systems analysis program for hands-on integrated reliability evaluations (saphire) computer code. we present preliminary psa results of initiating events that lead to boiling and cause fuel uncovering, resulting in possible fuel damage in the koeberg nuclear power plant sfp. keywords: computer code, fuel assemblies, probabilistic safety assessment, spent fuel pool. 1. introduction highly radioactive spent fuel assemblies that are unloaded from the nuclear reactor core are typically stored for a certain period in cooling water pools called spent fuel pools (sfp). the safe storage of these spent fuel assemblies in the sfp is very vital since many radioactive fission products could potentially be released into the environment if a severe accident occurred in the sfp [1]. the fukushima daiichi nuclear accident has proven that it is crucial to investigate potential severe accidents and corresponding mitigation measures for the sfp of a nuclear power plant (npp) [2, 3]. the necessary mitigation measures, the effects of recovering the sfp cooling system and makeup water in sfp as the accident progresses have also been investigated respectively based on the events of pool water boiling and spent fuel uncovery [3–5]. to use risk insights in the decision-making processes in an adequate manner, it is very important to establish a systematic approach that integrates in a sound, transparent and justifiable manner all the elements required as stated in iaea tecdoc 1436 [6]. requirement 21 paragraph 4.71 of the iaea general safety requirements part 4; states that the regulatory body should carry out a separate independent verification to satisfy itself that the safety assessment is acceptable and to determine whether it provides an adequate demonstration of whether the legal and regulatory requirements are being met [7]. this work looks at all internal possible initiating events that may cause severe accidents in the koeberg nuclear power plant sfp. however, external events such as seismic activity and aircraft crash were not considered for this study. a list of abbreviations is provided in appendix a. 2. probabistic analysis the systems analysis programs for integrated reliability evaluations (saphire) code was used as the modelling tool. this work was carried out by (1) identifying initiating events and scenarios, (2) accident sequence modelling, (3) quantification of top events and (4) analysis of results. the main modelling assumptions for the sfp psa model are listed below: • the sfp water is at initial level of 19.65m. • the initial pool water temperature is at 50◦c. the sfp cooling loop is designed to provide a 50◦c bulk sfp temperature with the maximum component cooling system (rri) temperature and a maximum heat load of 2.88 mw during normal and outage operation. • the model is done for the normal operating mode which is 94.2 % of an 18-month fuel cycle. • no gate seal failure during sfp boiling. • make up from the reactor cavity and sfp cooling system is not considered because of limited capacity. • the demineralised water distribution system (sed), fire fighting water supply (jpp) and the mobile fire fighting system (jps) are the considered make up systems. make up from the reactor cavity and sfp cooling system (ptr) is not considered because of limited capacity 231 https://doi.org/10.14311/app.2022.36.0231 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en sibongiseni thabethe acta polytechnica ctu proceedings figure 1. loss of sfp cooling event tree. figure 2. event tree model of the loss of inventory due to pipe rupture. 3. results and discussions event tree models were developed for the identified initiating events; this was done to better understand accident progression in the sfp resulting from the initiating events. the loss of sfp cooling, loss of sfp inventory due to pipe rupture, loss of sfp inventory due to flow diversion, loss of offsite power and station blackout are the initiating events that were modelled in this study. these initiating events either lead to a loss of sfp cooling, a loss of sfp inventory or both. the success criteria for a loss of cooling or inventory is generally to recover ptr cooling before boiling or before a loss of ptr suction. it also entails making up inventory before ptr suction uncovery and making up inventory prior to fuel uncovery. 3.1. event sequence modelling the loss of sfp cooling initiating event results from a failure of the reactor cavity and spent fuel pool cooling system (ptr) whereby all trains are unavailable for sfp cooling. the loss of sfp cooling event tree is shown in figure 1. following a loss of sfp cooling it will be ideal to recover cooling before boiling occurs (ptrcrecbbnmom). although boiling occurs when the pool heats up to a temperature of 100◦c, it is crucial to maintain the pool level at a safe level, in this case 15,5m. the pumps take suction from the fuel pool at an inlet located below the pool water level (∼ 15.5 m), transfer the pool water through a heat exchanger and return it back into the pool through an outlet typically located below the cooling system inlet and some large distance from it. recovering ptr cooling before the loss of suction at the 17 m level would be ideal in this case (ptrrecbs). if make-up cannot be established and ptr cooling is not restored the sfp level will drop from the 17 m level to the point where it is assumed that fuel damage occurs i.e. at the 9,85m level. the success criterion will be to establish sed make up before fuel damage (sed-mu-bd). in the event where sed is unavailable for make up to the sfp, an operator will be instructed to use the jpp system (jpp-mu-bd). as a last resort, if make up cannot be established through the sed and jpp systems, an operator will have to start the mobile fire pump (jps-mu-bd). this end state indicates that fuel damage has not occurred, the pool is boiling as illustrated by the boil in the event trees. the ok end state indicates that the accident has been successfully mitigated. this state comprises of all states where spent fuel damage has not occurred and either cooling has been re-established or make up to the sfp has been established. the loss of sfp inventory due to a pipe rupture, this initiating event considers the loss of sfp inventory due to pipe breaks in the ptr system. after a pipe rupture event the operator action required is to isolate the break before the loss of the ptr pump suction (ptr-iso-rp). a similar action is required in the case of loss of inventory due to flow diversion, with the operator being required to isolate the ptr flow diversion before the loss of suction (ptrfdiso). the loss of sfp inventory due to a flow diversion, this event is defined as a loss of sfp inventory due to diversion through an interfacing system valve failing 232 vol. 36/2022 koeberg spent fuel pool safety figure 3. event tree model of the loss of inventory due to flow diversion. figure 4. event tree model of losp. figure 5. event tree model of sbo. open or having been left open during ptr operation. the event tree models for the loss of inventory due to pipe rupture and flow diversion are shown in figure 2 and 3 respectively. at the 15.5 m level, ptr pump suction is lost. this means ptr cannot be recovered and make-up via sed, jpp or jps is the only remaining options. this event considers maintaining the sfp water level so that ptr repairs can still be undertaken even after boiling has started. success for this function event is defined as the establishment of sed make-up to the sfp before sfp boiling (sed-mu-bb). should the sed be unavailable for make up to the sfp, an operator will be instructed to use the jpp system (jpp-mu-bb). as a last resort, if make up cannot be established through the sed and jpp systems, an operator will have to start the mobile fire pump (jps-mu-bb). the loss of offsite power is defined as the failure of off-site power supplies. both units are tripped and the emergency diesel generators (edgs) and acacia power station supply are started. off-site power restoration is attempted as soon as possible. edgs will automatically actuate and supply power to the emergency ac buses (lha-b_e). if the edgs start and run, and the operator starts a ptr pump, sfp cooling can be re-established (ptr_clbb19nmom). the ptr pumps must not be restarted with a level of less than 17.0 m in the sfp. the suction of the ptr pumps located at the 15.5 m level in the sfp. below this level ptr cooling cannot be restarted (ptr_c_bs17nmom), therefore make-up is the only remaining option (sed-mu-bd, jpp-mu-bd and jps-mu-bd). the sbo initiating event is the failure of the offsite power supplies, including emergency off-site power from acacia. both units are tripped, the edgs fail to start or run for 24 hours and offsite sources are not recovered. following a sbo, it is desirable to recover power before sfp boiling. this can be done in one of two ways: either the recovery of the grid or by aligning 233 sibongiseni thabethe acta polytechnica ctu proceedings initiating event frequencies initiator time factor conditional initiating frequency (iec) initiating event frequency (ief) loss of sfp cooling (nm) 9,42e-01 9,52e-04 8,97e-04 loss of sfp inventory due to flow diversion (nm) 9,42e-01 3,57e-02 3,36e-02 loss of sfp inventory due to ptr pipe rupture 9,42e-01 1,37e-08 1,22e-08 loss of offsite power (nm) 9,42e-01 4,50e-01 4,24e-01 table 1. summary of initiating event frequencies. figure 6. failure model of the sed system. figure 7. failure model of the jpp system. figure 8. failure model of the jps system. the fifth diesel to either lha or lhb (9lhs_sbo). it is also to recover the grid before sfp boiling 234 vol. 36/2022 koeberg spent fuel pool safety (grid_bb_nm_om) and to start ptr cooling before boiling (ptr_sbo_nm_om). the loss of inventory due to boiling causes the sfp level to drop from the 19.3m level to the 17m level in 37.5 hours in normal mode. success for this function event is defined as the recovery of the grid before the 17 m level is reached (grid_bs_nm_om). as already mentioned, the ptr pumps must not be restarted at a level less than 17m, ptr cooling must be started before loss of suction (ptr_c_sbo_bs_nm_om). it is also desirable to recover the grid before fuel damage (grid_bd_nm_om). the available make up systems before fuel damage occurs are the sed, jpp and jps systems (sed-mu-bd, jpp-mu-bd and jps-mu-bd). the worst-case scenario is a drop in the pool water level to an extent where the fuel assemblies will be uncovered, which is assumed to be at the 9.85m level. this will lead to the fuel being damaged as indicated by the end state fd in the event trees. fuel damage will occur when all possible mitigating actions fail to be implemented. several thermal hydraulic studies on the initiating events have also been performed. these studies are often used to support psa because they reveal crucial information on sfp behaviour [8, 9]. 3.2. accident sequence quantification to quantify accident sequences, fault tree models were used to quantify the function events in the event trees. accident sequence quantification involves firstly the quantification of the initiating event frequencies. the conditional (iec) and initiating event frequencies (ief) were calculated for normal mode of operation (17 months) i.e., a time factor of 94.2 % of an 18-month fuel cycle was applied to the iec to calculate the ief. the iec and the calculated ief were obtained from koeberg nuclear power plant documentation and are shown in table 1. the failure models of the make-up systems take into account human failure events, those function events which require human action were quantified following step by step standardized plant analysis risk-human reliability analysis (spar-h) methodology [10] for quantifying human failure events (hfe). a number of performance shaping factors (psf) are taken in to consideration. these factors include available time, stress/stressor, complexity, experience/training, etc. the failure model of the sed is modelled on the basis that the operator fails to start or align sed make up or the sed isolation valves fail or there is an overall loss of the sed. the loss of the sed results from two events, if the tank leaks externally or the pumps fail to start and run. the failure of the pumps was not modelled in detail and were treated as undeveloped transfers, in this case they are treated as basic events. spar models are generally not as detailed as those contained in licensee pras. the failure probabilities for valves, pumps and tanks were obtained from koeberg internal documentation. the failure model of the jpp system modelled jpp system fails to start and run from standby or operator fails to start or align jpp make-up. the failure model of jps because of the failure of the mobile fire pump from standby or mobile fire pumps suction or discharge valves fail to open or operator ails to establish jps make up. the failure models of the jpp and jps are shown in figure 7 and 8 respectively. the other function events in the losp and the sbo event trees are operator actions such as "operator starts ptr cooling after losp before sfp boiling at 19.3m" (ptr_clbb19nmom) are operator actions and are currently being re-quantified using saphire hra calculator. 4. current and future work the purpose of this work is to do an independent verification of the current koeberg nuclear power plant spent fuel pool psa model. fuel damage is the event consequence state of interest and is considered for quantification. system and failure probabilities were inputted into the model for quantification. since the failure models of the make-up systems involve human or operator action, the function events which require human, or operator action will be re-quantified following a step-by-step standardized plant analysis risk-human reliability analysis (spar-h) methodology for quantifying human failure events (hfe). 5. conclusions this study entails the independent verification of the current koeberg nuclear power plant sfp psa model. initiating events that lead to boiling and potential fuel uncovery were reviewed and investigated. the event tree models for the loss of sfp cooling, loss of sfp inventory, loss of offsite power, station blackout were developed. the accident sequence modelling highlighted the importance of recovering sfp cooling and making up sfp inventory to prevent fuel uncovery. operator or human action is a key component in always maintaining critical pool levels using the available make up systems. through appropriate mitigation measures the increase in the fuel temperature could be stopped, limiting the release of fission products to the environment and preventing severe accident that could result in significant damage to the spent fuel assemblies in sfp. references [1] f. niehaus. use of probabilistic safety assessment (psa) for nuclear installations. safety science 40(1-4):153-76, 2002. https://doi.org/10.1016/s0925-7535(01)00047-9. [2] g. mignot, s. paranjape, d. paladino, et al. large scale experiments simulating hydrogen distribution in a spent fuel pool building during a hypothetical fuel uncovery accident scenario. nuclear engineering and technology 48(4):881-92, 2016. https://doi.org/10.1016/j.net.2016.06.005. 235 https://doi.org/10.1016/s0925-7535(01)00047-9 https://doi.org/10.1016/j.net.2016.06.005 sibongiseni thabethe acta polytechnica ctu proceedings [3] x. wu, w. li, y. zhang, et al. analysis of the loss of pool cooling accident in a pwr spent fuel pool with maap5. annals of nuclear energy 72:198-213, 2014. https://doi.org/10.1016/j.anucene.2014.05.030. [4] s. carlos, f. sanchez-saez, s. martorell. use of trace best estimate code to analyze spent fuel storage pools safety. progress in nuclear energy 77:224-38, 2014. https://doi.org/10.1016/j.pnucene.2014.07.008. [5] x. wu, w. li, y. zhang, et al. analysis of accidental loss of pool coolant due to leakage in a pwr sfp. annals of nuclear energy 77:65-73, 2015. https://doi.org/10.1016/j.anucene.2014.11.010. [6] international atomic energy agency, risk informed regulation of nuclear facilities: overview of the current status, tecdoc series (iaea-tecdoc-1436), 2005. https://www-pub.iaea. org/mtcd/publications/pdf/te_1436_web.pdf. [7] international atomic energy agency, safety assessment for facilities and activities, general safety requirements, no. gsr part 4 (rev. 1), 2016. https://www-pub.iaea.org/mtcd/publications/pdf/ pub1714web-7976998.pdf. [8] k.-i. ahn, j.-u. shin, w.-t. kim. severe accident analysis of plant-specific spent fuel pool to support a sfp risk and accident management. annals of nuclear energy 89:70-83, 2016. https://doi.org/10.1016/j.anucene.2015.11.024. [9] j. c. de la rosa blul, p. mcminn, a. grah. analysis of the inherent response of nuclear spent fuel pools. annals of nuclear energy 124:295-326, 2019. https://doi.org/10.1016/j.anucene.2018.10.014. [10] w. j. galyean, a. m. whaley, d. l. kelly, et al. spar-h step by step guidance, technical report inl/ext-10-18533, rev. 2, idaho national lab. (inl), idaho falls us, 2011. bautechnik 98(7):475-81, 2020. https://doi.org/10.2172/1027888. 236 https://doi.org/10.1016/j.anucene.2014.05.030 https://doi.org/10.1016/j.pnucene.2014.07.008 https://doi.org/10.1016/j.anucene.2014.11.010 https://www-pub.iaea.org/mtcd/publications/pdf/te_1436_web.pdf https://www-pub.iaea.org/mtcd/publications/pdf/te_1436_web.pdf https://www-pub.iaea.org/mtcd/publications/pdf/pub1714web-7976998.pdf https://www-pub.iaea.org/mtcd/publications/pdf/pub1714web-7976998.pdf https://doi.org/10.1016/j.anucene.2015.11.024 https://doi.org/10.1016/j.anucene.2018.10.014 https://doi.org/10.2172/1027888 acta polytechnica ctu proceedings 36:231–236, 2022 1 introduction 2 probabistic analysis 3 results and discussions 3.1 event sequence modelling 3.2 accident sequence quantification 4 current and future work 5 conclusions references https://doi.org/10.14311/app.2022.33.0200 acta polytechnica ctu proceedings 33:200–206, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague technical effectiveness of cement-based mortar for high-reflective building envelope through building energy simulations: preliminary results alessandra gugliandoloa,∗, enrico genovaa, francesco baldia, claudia caponeb, maria anna segretoa a enea, energy efficiency unit department (duee), via martiri di monte sole 4, 40129 bologna, italy b italcementi spa, via stezzano 87, 24126 bergamo, italy ∗ corresponding author: alessandra.gugliandolo@enea.it abstract. in areas with high levels of solar radiation, decreasing the amount of solar energy absorbed by the building envelope is useful to reduce the need of air conditioning and "heat island" effects. most high-reflective products, however, suffer from low durability. the cool-it project is developing an innovative high-reflective cement based mortar for precast products to be used as outer layer in buildings for both vertical and horizontal surfaces, or for road pavement. the mix design is aimed at increasing the durability of this cement-based component while retaining high reflectance to solar radiation. this paper presents the preliminary results of the project, based on the simulation of the energy demand of a residential building, intended as a support to optimize the proposed mixes. the model is analysed in three different locations in italy, for one year of operation. this allows evaluating the trade-off of the energy demand between the winter increase and the summer reduction. keywords: building energy demand, building envelope, cool material, energy efficiency, high reflectance. 1. introduction urban areas are characterized by high density of built surfaces, which generally are low reflective and low permeable. this modification of land surface results in higher air temperature compared to the surrounding rural areas, with differences up to 3◦c. this phenomenon, normally referred to as urban heat island (uhi), increases the energy needs and consumption for summer cooling, thus contributing to global warming. the uhi can be countered through the implementation of so-called "cool" technologies. a "cool material" is characterized by high solar reflectance (thereafter denoted also as "albedo") and high infrared emissivity, where the first is the ability to reflect incident solar radiation, and the second allows to return most of the fraction to the atmosphere solar absorbed by thermal radiation. several technical solutions have been developed for the construction sector, especially in outdoor pavements but also in the building envelope [1–3]. also in the latter, the most common application field of cool materials is in horizontal surfaces, where the solar radiation peak is reached in the summer season. 2. state of art the large use of cement-based pavement products highlights that increasing their solar reflectance may result in a significant mitigation of the heat island effect and in the improvement of urban comfort. furthermore, the diffusion of cement-based products in building roofs and walls, namely tiles and concrete façades, suggests that the solar reflectance of cementbased components can play a relevant role in reducing the energy demand for building cooling. mainly related to pavements, researches in this field characterize the albedo of concretes and their constituents or focus on developing methods and technologies to obtain high reflective cement-based products. solar reflectance values of 0.35 − 0.40 and 0.25 − 0.30 are considered typical for new and weathered concretes respectively, if conventional mixes are used [4]. levinson and akbari [5] studied the effects that composition and exposure of portland cement concrete have on its solar reflectance. they noticed that concrete albedo grows during the cement hydration reaction but stabilizes after six weeks from casting. the concretes were on average more reflective if produced by means of white than grey cement. the authors also observed a correlation between the composition of concrete and its solar reflectance, which was influenced by cement albedo, fine aggregate albedo and, after abrasion, also by the solar reflectance of coarse aggregates. marceau and vangeem [6] widened the base of available data by measuring the solar reflectance of concrete constituent materials (portland and slag cements, fly ash, fine aggregates) and concrete specimens comparable to those commonly used in exterior flatwork. designed with combinations expected to result in low albedo, the concrete specimens showed a minimum solar reflectance of 0.34 and a maximum 200 https://doi.org/10.14311/app.2022.33.0200 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 33/2022 re-asr behaviour of recycled concrete of 0.64 up to 0.69, with an average value of 0.47. the authors highlight that the influence on concrete albedo is more relevant for cement than for the other constituents and confirm that the higher the solar reflectance of cement, the higher that of concrete. since albedo is related to the product surface, a way to increase this property in cement-based components consists in surface treatments of conventional mix designs, generally by the addition of pigments or the application of coatings. several solutions have been developed for the building sector, generally related to roof tiles and shingles [7, 8]. in the refurbishment of existing constructions, acting on the finishing is appropriate to reduce costs and technical barriers to the boost of albedo. however, the enhancement of solar reflectance can be irreversibly decreased by aging processes, especially in case of abrasion and surface damages. the effects of ageing on the solar reflectance of building envelope materials are object of investigation [9–11]. on the other hand, different studies aim at increasing the solar reflectance of concrete by acting on its composition. for this purpose, the partial substitution of the binder, the use of white cement and the incorporation of lightly coloured aggregates are mainly investigated. it has been observed that fly ash reduces albedo with respect to conventional mixes, without a consistent trend [4]. on the contrary, the solar reflectance of concrete increases consistently with the content of ground-granulated blast furnace slag, which appears to be even more effective than the use of white sand and latex [4]. besides the ratio of binder replacement, the type of aggregates and the surface finish, also curing conditions influence the solar reflectance of hardened concretes. recent research has observed that solar reflectance increases if the moist level of curing conditions is higher, with more marked difference if the water-to-cement ratio increases [12]. 3. objective the research herewith discussed investigates the use of building and urban simulations as a support tool to design innovative high-reflective cementitious mortars to be used on the building envelope. more in detail, the aim is optimizing the use of "cool materials" as constituents of cementitious mixtures. different from surface treatments and curing treatments, this solution affects the building product, namely a precast tile or panel, in all its thickness. consequently, the solar reflectance, which is a surface property, is kept in the long term. optimization is necessary because not only the "cool material" but also the other constituents of the mix influence the albedo. furthermore, combination with the effects of mix design strategies described in the literature should be explored, in order to amplify the enhancement of solar reflectance. the focus of the preliminary investigation presented in this paper, based on building simulations, is on assessing benefits and criticalities of the developed products whose albedo varies in a range in terms of building energy demand for heating and cooling, depending on geographic location, climatic conditions and urban density. 4. materials and method 4.1. design and characterization of cementitious "cool" mortars innovative precast tiles (40 × 40 or 40 × 60 cm, 2 or 3 cm thick), suitable for both horizontal and vertical application on the building envelope, were produced using high-reflective cementitious mortars. the mixes included "cool" binders or "cool" additions, both based on commercial static cool materials (cm). these cms increase the attitude of the mortars to reflect radiations in the total solar spectrum, with high efficacy in near-infrared region (nir, with wavelengths between 700 and 2500nm), where about 50 % of solar radiation falls. the compatibility of these inorganic cms with the alkaline ph of cementitious matrix was verified. white and coloured traditional mortar tiles surfaces were prepared by means of a high-fluidity matrix, made of white cement cem i 52.5r, calcareous sands 0 − 2 mm, superplasticizer, shrinking-reducing and water-proofing agents. for the cool cementitious tiles manufacture, a cool binder, made up of selected cement and cool materials proportions, is premixed, before the addition to the other recipe ingredients. after mixing in a planetary−type mixer, the cementitious mortar is cast in vertical moulds. after 24h−demoulding, the tile is kept in a conditioned room at 20◦c and 55 % rh. at fresh state, rheological behaviour of different preparations was mainly monitored through the fluidity characterization over time (en 7044 standard), e.g. higher than 280 mm up to 30 min. at hardened state, compressive (> 54 mpa at 28 days) and flexural strengths (> 9 mpa at 28 days) were characterized according to en 196-1. hydraulic shrinkage resulted in − 500 µm/m after 28 days (en 12617 standard). preliminary promising durability tests were performed in compliance with en 12380-8, en 13295, en 11164, en 13687-1 standards. solar reflectance was measured according to the standards astm e903-12 and astm g173-03, by means of a uv-vis-nir spectrophotometer with 150 mm integrating sphere. improved optical properties were assessed calculating the increase in solar reflectance of cool cementitious tiles compared with white/coloured control samples, ranging from 0.10 − 0.40(grey surfaces) to 0.60 − 0.81 (white surfaces). by way of demonstration, figure 1 shows typical solar reflectance spectrum of experimental cool mortars compared with relative white and grey ref201 a. gugliandolo, e. genova, f. baldi et al. acta polytechnica ctu proceedings reflectance cool surface neighbouring buildings 0.30 − 0.80 walls no 0.30 − 0.80 walls yes 0.30 − 0.80 roof no 0.30 − 0.80 roof yes 0.30 − 0.80 walls + roof no 0.30 − 0.80 walls + roof yes 0.40 walls + roof yes 0.50 walls + roof yes 0.72 walls + roof yes 0.76 walls + roof yes table 1. list of building simulations for each location. erences w/wo cm. it is worth noting that the cementitious solutions with cool materials enlarge the current possibilities of application for coloured/grey surfaces more significantly than for the white ones: the former improve the solar reflectance of the relevant reference in the entire wavelength range, with high benefits in nir region; the latter show more limited enhancement margins, despite the cm addition, being themselves high-reflective surfaces. accelerated and natural aging tests are in progress. )ljxuh� ��� h[shulphqwdo�zklwh�dqg�frorxuh kljk�uhiohfwlyh�pruwduv�frpsduhg� zlwk� wkh� uhohydqw�pdwul[�zlwkrx frro�pdwhuldov� ��� ��� ��� ��� ��� ��� ��� ��� ��� ���� ���� ���� ���� ���� 6r od u�5 hi oh fw dq fh ȝ��qp� :klwh�frqwuro ([s�zklwh pruwdu�� ([s�zklwh pruwdu�� *uh\�frqwuro ([s��juh\ pruwdu�� ([s��juh\ pruwdu�� figure 1. solar reflectance of experimental white and coloured high-reflective mortars compared with the relevant matrix without cool materials. 4.2. methodological approach the analysis of the effects that the use of the proposed cementitious products has on the energy performance of residential buildings is based on the numerical modelling and simulation of a reference building. the use of a numerical approach allowed performing an indepth study of the different performance indicators of interest for the subject, and applying it to different locations, and to different material properties. the performance of the material is assessed based on the following performance indicators: • increase in winter energy demand; • decrease in summer energy demand; • total energy balance, after conversion to tonnes of oil equivalent (toe); • difference in operational costs, which also takes into account the different tariffs for electricity (main energy vector for summer cooling systems) and natural gas (energy vector for winter heating systems in the reference building). the present study is focused on the impact of the technology at building level, hence excluding the impact of the proposed material on the local climate. as the shading effect generated by neighbouring buildings can have a relevant size [9], the difference between the cases with, and without, neighbouring buildings is considered. the following procedure was designed: first, the performance of the building was simulated with generic, increasing values for the surface reflectance of the building surface material, allowing to get a better understanding of the effects of surface reflectance (r) on the building’s energy performance. to this scope, different simulations were performed to consider the effects of applying the material on the roof only, on vertical opaque surfaces only, or on both. then, the analysis focused on two main scenarios: the use of a relatively dark material (r = 0.4), and of a white material (r = 0.72); in both scenarios, the performance of the "standard" material was compared to that of the proposed one, characterized by a higher surface reflectance (r = 0.5 for the dark material, r = 0.76 for the white one). these values were provided by the material manufacturer, based on internal laboratory tests. the full list of simulations performed for each location considered in this study is provided in table 1. the study was performed using energyplus [13] together with designbuilder [14] as graphical user interface (figure 2). the description of the building and of the inputs to the model is provided in the following section. the main parameter that varied across the different simulations is the "solar absorbance", which controls the share of solar radiation that is absorbed by the material surface. as the material under study is opaque, it was assumed that r + a = 1. 202 vol. 33/2022 re-asr behaviour of recycled concrete 4.3. reference building morphology, size and construction features of the reference residential building were based on the 2011 italian general census on population and housing [15]. from this data, it emerges that the most representative period of the national residential stock is the decade 1971 − 1980. in the top ten italian towns for population, the census shows that residential buildings are generally made of at least four storeys and sixteen dwellings. to define the building model, therefore, it was assumed that: • the building has 6 floors and is equipped with a single staircase, serving three building units per floor; • the dimensions of the building are 30 m × 12 m; • each housing unit has a usable area of 90 m2, while the stairwell has an area of 30 m2; including walls, the gross floor area of each storey is 360 m2; the internal height is 2.70 m; • the roof is flat. it was assumed that the two minor façades of the building face north and south respectively; the stairwell faces east and is considered to be an "unconditioned" thermal zone (z1). for each floor, every apartment is considered as a single thermal zone (z2, z3, z4). the external walls don’t have overhangs and shielding. according to the uni 10339 standard, evaluations were conducted to ensure appropriate thermohygrometric conditions, air changes and comfortable occupational conditions. two density values of different occupations were assigned between the apartments and the stairwell. from the point of view of the model construction, the building is composed of 6 overlapping blocks (30 m × 12 m). each block is divided into four parts, representing the three floor housing units and the stairwell. the building model is referred to the building techniques commonly used in italy in the 1971 − 1980 decade (uni/tr 11552:2014). it was assumed that the building structure is a reinforced concrete frame, that is, the most widespread solution in italian residential construction after the second world war. the external walls of the reference building consist of a single layer of perforated bricks (medium perforation), plastered on both sides, with a total thickness of 35 cm. the floor slabs, made of concrete beams and clay blocks, are 30 cm thick. the windows have an aluminum frame without thermal break and a 4−mm double glass with 12−mm air gap. roller shutters (with a box emerging from the inner side of the wall) are used as shielding system. for each building unit, the window area is equal to 1/8 of the floor area, which is the minimum allowed by the italian building regulation. the energy systems of the reference building chosen for this study were defined with reference to a "standard" building built in the 1970s. the systems in the building are, therefore, the following: • centralized heating system (natural gas boiler) with radiators; • centralized hot water generation system (dhw, natural gas boiler); • decentralized systems for summer air conditioning, based on a compression refrigeration cycle. the energy performance of the reference building was simulated in dynamic conditions in three italian cities, located in the northern (bologna), central (rome) and southern (palermo) part of the country. the energy demand of the building was simulated over the time of one year. the weather data (in the form of an .epw file) used in the simulation was a representative average of climatic data gathered in the 2003 − 2017 period. figure 2. building model and neighbouring buildings in the graphical user interface. orientation is specified on the lower left. 5. results and discussion the results of the parametric analysis carried out in this research, collected in table, show the influence that increasing the solar reflectance of different envelope components has on the overall building energy demand for heating and cooling. from this energy perspective, only the application to the case of palermo appears to be beneficial, as it is the only one where the benefits during the cooling season outweigh the losses during the heating season. the reason behind this effect can be better understood looking at figure 3, where the yearly energy demand for winter heating and summer cooling is shown for the three locations: both in bologna and in rome, the gas demand for heating is much higher than the electricity demand for cooling in summer, while difference shrinks in the case of palermo. the tendency for the costs is similar to that for energy demand, as expected; as a general trend, the cost per toe is lower for electricity than for natural gas (840 eur/toe and 1030 eur/toe, respectively). this works against the use of high-reflectance surface materials, as it increases the weight of the natural gas 203 a. gugliandolo, e. genova, f. baldi et al. acta polytechnica ctu proceedings reflectance ng (winter) el (summer) total [-] [kwh/y] [toe/y] [€/y] [kwh/y] [toe/y] [€/y] [toe/y] [€/y] bologna 0.40 173208 12.58 12962 5823 1.46 1223 14.04 14185 0.50 175466 12.75 13131 5270 1.32 1107 14.06 14238 0.72 180548 13.11 13512 4043 1.01 849 14.13 14361 0.76 181505 13.18 13583 3832 0.96 805 14.14 14388 roma 0.40 114342 8.31 8557 5155 1.29 1083 9.59 9640 0.50 116872 8.49 8746 4528 1.13 951 9.62 9697 0.72 122660 8.91 9179 3182 0.80 668 9.71 9848 0.76 123747 8.99 9261 2949 0.74 619 9.73 9880 palermo 0.40 44164 3.21 3305 6994 1.75 1469 4.96 4774 0.50 45569 3.31 3410 6102 1.53 1281 4.84 4692 0.72 48858 3.55 3656 4230 1.06 888 4.61 4545 0.76 49485 3.59 3703 3906 0.98 820 4.57 4524 table 2. results of simulation: primary energy demand (ng = natural gas; el = electricity). figure 3. annual energy demand for winter heating and summer cooling of the reference building, without "cool" cementitious tiles, for the three simulated locations. demand, which increases when using these materials. consequently, the results of the simulations carried out on the reference building highlight that the use of high-reflective products, such as those developed in this research, might be more successfully implemented in hot-summer climate conditions. figure 4 shows the effect of the presence of neighbouring buildings in the case of palermo. as expected, differences are negligible if the "cool" product is used as roof covering, while the shades casted by neighbouring buildings reduce the benefits of the high-reflective wall, thus making it less effective. furthermore, in case the "cool" product is used on the vertical envelope, the overall energy demand increases in presence of neighbouring buildings. for the reference building, they appear to enhance the criticalities of high reflectance during the heating period than the benefits during the cooling season. the results for the specific application to the four "realistic" scenarios chosen for this study are shown in figure 5, which confirms the same tendency: increasing the surface reflectance is only convenient for the case of � figure 4. influence of neighbouring buildings on the effects that the "cool" cementitious tiles would have on the annual energy demand of the reference building in palermo. palermo, where the natural gas demand for winter heating is low (close to one fourth of the demand in bologna). moving from a "standard" dark cover to a "high-reflective" one, the electricity demand for cooling is reduced by approximately 13 %, while the total energy demand (expressed in toe) is reduced by 2.5 %. 6. conclusions and further developments in this work, dynamic energy simulations of a building model were used to develop a support tool, with the aim of optimizing the design and testing of precast tiles for the building envelope based on new highreflective (i.e. with enhanced nir-reflective characteristics) cementitious constituents. the effects of different tiles on the energy demand of a reference building were evaluated in three italian locations. the preliminary results showed that the use of high204 vol. 33/2022 re-asr behaviour of recycled concrete � figure 5. annual energy demand of the reference building and related cost savings according to location (bo = bologna, pa = palermo, rm = rome), treated envelope components (w if only walls, r if only roof, w+r for both) and solar reflectance of the "cool" product. in grey the range between the albedo of the high-reflective dark material (r = 0.5) and that of the white one (r = 0.76). reflective cementitious products might be more successfully implemented in hot-summer climate conditions, where natural gas demand in winter season does not offset the electrical energy saving in building air conditioning expenditures. in the simulations performed for this work, this corresponded to the case of palermo. future analysis needs to be broadened in terms of location and building features. on the one side, locations representative of a wider variety of climate conditions (within italy and in other mediterranean countries) will be considered. on the other side, building orientation will be included as a parameter for simulations. furthermore, different building types must be investigated, since the building use affects the morphology of the envelope (especially the ratio between roof and wall surfaces) and the energy demand for heating and cooling. improving the building model could support the cost-benefit analysis of cooling materials in order to design more performing cementitious mortars. not carried out in this early stage of this research, the economic assessment should take into account both the investment cost of the "cool" material to be used in the mix and the variation in the operational costs for the building use. from this perspective, the parametric curves based on the building model will help in integrating the economic evaluation of the developed products with the effects of long-term aging on their solar reflectance. acknowledgements this paper is related to the industrial research and the experimental development performed in the framework of the italian project cool-it, funded by the italian ministry of economic development (ricerca di sistema elettrico nazionale). the authors thank eng. a. cacciatore from italcementi for the valuable contribution to the research and application activities carried out in the innovation laboratories. references [1] m. santamouris. using cool pavements as a mitigation strategy to fight urban heat island a review of the actual developments. renewable and sustainable energy reviews 26:224-40, 2013. https://doi.org/10.1016/j.rser.2013.05.047. [2] a. pisello, f. rossi, f. cotana. summer and winter effect of innovative cool roof tiles on the dynamic thermal behavior of buildings. energies 7(4):2343-61, 2014. https://doi.org/10.3390/en7042343. [3] i. hernández-pérez, g. álvarez, j. xamán, et al. thermal performance of reflective materials applied to exterior building components a review. energy and buildings 80:81-105, 2014. https://doi.org/10.1016/j.enbuild.2014.05.008. [4] k. boriboonsomsin, f. reza. mix design and benefit evaluation of high solar reflectance concrete for pavements. transportation research record: journal of the transportation research board 2011(1):11-20, 2007. https://doi.org/10.3141/2011-02. [5] r. levinson, h. akbari. effects of composition and exposure on the solar reflectance of portland cement concrete. cement and concrete research 32(11):1679-98, 2002. https://doi.org/10.1016/s0008-8846(02)00835-9. 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[9] r. paolini, m. zinzi, t. poli, et al. effect of ageing on solar spectral reflectance of roofing membranes: natural exposure in roma and milano and the impact on the energy needs of commercial buildings. energy and buildings 84:333-43, 2014. https://doi.org/10.1016/j.enbuild.2014.08.008. [10] r. paolini, g. terraneo, c. ferrari, et al. effects of soiling and weathering on the albedo of building envelope materials: lessons learned from natural exposure in two european cities and tuning of a laboratory simulation practice. solar energy materials and solar cells 205, 2020. https://doi.org/10.1016/j.solmat.2019.110264. [11] n. xie, h. li, w. zhao, et al. optical and durability performance of near-infrared reflective coatings for cool pavement: laboratorial investigation. building and environment 163, 2019. https://doi.org/10.1016/j.buildenv.2019.106334. [12] y. qin, y. zhao, x. chen, et al. moist curing increases the solar reflectance of concrete. construction and building materials 215:114-8, 2019. https: //doi.org/10.1016/j.conbuildmat.2019.04.164. [13] d. b. crawley, l. k. lawrie, f. c. winkelmann, et al. energyplus: creating a new-generation building energy simulation program. energy and buildings 33(4):319-31, 2001. https://doi.org/10.1016/s0378-7788(00)00114-6. [14] designbuilder. designbuilder software v 4.7.0.027, 2021. www.designbuilder.co.uk. [15] censimentopopolazione. http://dati-censimentopopolazione.istat.it/. 206 https://doi.org/10.1016/j.solmat.2009.12.012 https://doi.org/10.1016/j.enbuild.2014.08.008 https://doi.org/10.1016/j.solmat.2019.110264 https://doi.org/10.1016/j.buildenv.2019.106334 https://doi.org/10.1016/j.conbuildmat.2019.04.164 https://doi.org/10.1016/s0378-7788(00)00114-6 www.designbuilder.co.uk http://dati-censimentopopolazione.istat.it/ https://doi.org/10.14311/app.2022.33.0290 acta polytechnica ctu proceedings 33:290–294, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague expansion of sustainability in the aci 318 building code shana kelley kpff consulting engineers, 1601 5th avenue, ste 1600, seattle, wa 98101, usa correspondence: shana.kelley@kpff.com abstract. the american concrete institute (aci) has long recognized the growing importance of accounting for environmental sustainability in the design of concrete structures. as the next version of the aci 318 building code is being developed (aci 318-25), a new sustainability subcommittee, subcommittee n, has been formed and tasked with expanding the provisions related to sustainability in the code. this paper reviews the current provisions in the aci 318 code that are related to sustainability and explores the possible future expansion of code and commentary language to provide better guidance on sustainable design. areas of focus for expansion of code language, including definitions of sustainability metrics, as well as the defined baseline for comparison, are discussed. while sustainable design is increasingly being specified in concrete projects, the sustainable properties of concrete are often interrelated with other aspects of design, including resiliency and intended lifespan. approaches to acknowledging the impacts of life cycle considerations on the sustainability of concrete construction are also explored. finally, the anticipated timeline for the development of aci 318 sustainability provisions is described, as well as potential interactions with other code writing organizing that are currently developing sustainability provisions. keywords: aci 318, code, sustainability. 1. introduction historically, concrete building codes have focused mainly on provisions related to strength, serviceability, stability, and durability of the structure. however, as the priorities of policy makers, owners, and designers have increasingly included environmental sustainability, the question of the role of the concrete building code in setting standards related to sustainability has arisen. this paper includes a summary of how the current aci 318 building code addresses sustainability, an exploration of the current practices in the design and specification of sustainable concrete, and finally a description of the areas of expansion that are currently being worked on by the new aci 318-n sustainability subcommittee. 2. concrete sustainability at aci and in the aci building code as the focus on environmental sustainability of building materials has increased in recent years, the american concrete institute (aci) has expanded the committees, standards, and publications related to concrete sustainability. in particular, aci technical committee 130 sustainability of concrete, has been actively producing technical publications on approaches to the design, specification, and performance of sustainable concrete. there are also numerous committees studying concrete topics related to sustainable concrete including supplementary cementitious materials, alternative cements, and recycled aggregate. as the practice of incorporating sustainability into buildings has become more common, the need to incorporate provisions related to sustainability within the concrete building code has increased. provisions directly referencing sustainability were first incorporated in section 4.9 of the 2014 version of the american concrete institute building code, aci 318-14 [1]. these provisions, which remain essentially unchanged in the newly released aci 31819, acknowledge that design professionals are permitted to specify sustainability requirements, but do not define how sustainability of concrete could be specified or considered in the design of a concrete structure [2]. while the code language with direct references to sustainability remained unchanged in the aci 31819 code, there were several additions and expansions within that code edition that are associated with materials that are increasingly being used in projects targeting sustainability goals. these include new provisions regarding the use of alternative cements and recycled aggregates. the current code cycle for aci 318 is working toward further expanding the concrete sustainability provisions with the establishment of a new sustainability subcommittee, aci 318-n. this subcommittee is now working toward developing new provisions related to sustainability within the building code. 290 https://doi.org/10.14311/app.2022.33.0290 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 33/2022 aci 318 building code sustainability expansion 3. how sustainability is being considered in design before addressing how the approach to defining sustainability can be incorporated into a concrete building code, it is important to understand how the current market is regulating environmental sustainability in concrete. 3.1. environmental sustainability in relation to cement content since the cement in concrete contributes the majority of the emissions in the production of concrete, the majority of environmental sustainable efforts have been associated with cement reduction. until recently, most design professionals and owners utilized either maximum amounts of cement or minimum amounts of cement replacement with supplementary cementitious materials to define sustainability. however, as information related to the environmental impacts of materials has become more readily available, the ways that the sustainability of concrete is being defined are changing. 3.2. a new focus on embodied carbon and environmental product declaration much of the historical focus on the sustainability of buildings has been on the reduction of the carbon utilized for the operations of buildings. thus, energy codes have increasingly required better insulated structures and more efficient mechanical systems. however, as these systems have improved and the operational carbon footprint of structures has reduced, the focus is now increasingly shifting to reductions of embodied carbon in buildings. embodied carbon is the global warming potential (gwp) associated with the manufacture of a product. in order to quantify the gwp of a building, design professionals are increasingly utilizing environmental product declarations (epds) to calculate the impacts of elements of a building. epds are reports, typically abiding by the requirements of the international organization for standardization (iso) 21930 for construction products [3], that quantify various environmental impacts of the production of a product. the declaration of environmental indicators includes environmental impacts, resource use, energy use, and waste flows. as shown in figure 1; gwp, ozone depletion, and freshwater use are amongst the impact categories that are typically included in the summary of impacts for a product. many industries have created industry-wide epds that reflect the average impacts of a product across an industry, but individual manufacturers are increasingly creating epds for their specific products. it should be noted that epds typically do not account for the full life cycle impacts of the use of a product, as their scope usually only includes the "cradle to gate" portion of the cycle, where the "gate" is figure 1. example of the summary of environmental impacts from an epd for a concrete mixture [4]. the gate of the manufacturing facility. so while epds can help quantify the impacts of the production of a product, the impacts from transportation to site, construction, use, deconstruction, and end-of-life are not included. this increased availability of information regarding the environmental impacts of concrete is starting to change the way that design professionals, building owners, and building officials are requiring and specifying the sustainability of concrete. 3.3. collection of epds into searchable databases as the number of epds has grown, numerous databases have been developed to aggregate the information in one place to better compare products. one of these online directories, the embodied carbon in construction calculator (ec3) has an extensive and growing collection of epds that it utilizes to calculate the gwp of building projects. as can be seen in figure 2, concrete represents by far the largest number of epds collected [5], likely due to the fact that each concrete mixture at a plant has a separate epd and because the concrete industry has rapidly adopted the use of epds. 291 shana kelley acta polytechnica ctu proceedings figure 2. graph showing number of epds for materials in ec3 carbon calculator software [5]. 3.4. life cycle assessment and resiliency in addition to the trend of examining the relative environmental impacts amongst a group of products within a certain industry, designers are increasingly examining the overall impacts of building construction and use over the entire life of the building. this process, called life cycle assessment (lca), is used to calculate the environmental impacts associated with all the life states of a product. often when lca is undertaken, the performance of a structure is compared to a baseline structure to calculate the degree to which the building has reduced its impacts. many sustainable certification systems for building construction require that an lca for a structure be performed, sometimes with a requirement for reductions of impacts or performance below a set threshold. while lca studies do assign a lifespan to certain products and adjust the overall impact of those products based on the selected lifespan of the structure, the resiliency of the structure is not necessarily accounted for with lca. with concrete structures, one of the benefits often cited is the resiliency of the structure in the event of natural disasters, extreme exposures, an extended lifespan, or other conditions. the question of how those qualities are to be accounted for is one that should be addressed when examining the life cycle impacts. 3.5. mandating of sustainability in local building codes with the increased focus on embodied carbon in building structures, local jurisdictions are beginning to mandate or consider mandating some characteristics of concrete related to sustainability. these ordinances and legislation vary in their scope and requirements, but many set limits on gwp and/or cement content. the advent of these laws lends an importance to ensuring that the concrete building code, aci 318, adequately addresses the items that must be considered when accounting for the sustainability of concrete structures. 4. aci 318-n sustainability subcommittee to address the increasing trend of defining limits on the environmental impacts of concrete in the design of buildings, the aci 318 building code has established a new sustainability subcommittee. this new subcommittee, aci 318 subcommittee n, is tasked with expanding the provisions related to sustainability in the concrete building code. the members on this subcommittee include practicing engineers, researchers, and concrete producers. many have extensive experience specifying and producing concrete to meet sustainability goals. 4.1. timeline for code development the aci 318-n subcommittee had its first official meeting at the october 2019 aci fall convention in cincinnati, ohio. the last published edition of the aci 318 building code was aci 318-19. it is anticipated that the next version released will be aci 318-25 and any provisions developed by the 318-n subcommittee that are approved by the main aci 318 committee will be included in the next edition. 292 vol. 33/2022 aci 318 building code sustainability expansion 4.2. possible areas of sustainability expansion in aci 318 while the manner and details surrounding the incorporation of sustainable concrete have not yet been established, the subcommittee is currently considering creating an appendix with sustainability provisions and commentary that will be part of the code. since the inception of the aci 318-n subcommittee, a number of task groups have been established to begin the process of writing code provisions related to sustainability. these task groups include the following: • scope section task group this group is addressing both the existing sustainability section of the code and its reference to the proposed sustainability appendix. • sustainable design of structural concrete task groups there are two separate task groups associated with this topic: ⊲ sustainable design this task group is focused on how the design of structures should account for concrete when sustainability is an objective. in particular this group will be looking at approaches such as life cycle assessment and how aspects of concrete should be accounted for. ⊲ resiliency this task group is examining how resiliency of concrete structures relates to the consideration of the sustainability of a structure and how it should be considered. this group is also examining resiliency in general and how it is accounted for in the code. • concrete mixture design information task group this group is working to develop provisions that outline how concrete mixtures can be specified to require sustainability metrics. the group is currently examining several approaches and is debating what methods to include. the interface between this section and the sustainable design section is still under consideration. • recycled aggregate task group this group is looking at the new provisions in aci 318-19 related to the use of crushed hydraulic-cement concrete and recycled aggregate. in particular, the new provisions require testing and quality control programs for these types of aggregate [2] and the task group is examining whether additional guidance could be provided in the commentary of this section regarding those programs. this code section falls within the overview of a separate aci 318 subcommittee, thus any changes to this section will require coordination across subcommittees. • cements and alternative cements task group chapter 26 of aci 318-19 contains a list of acceptable astm specified cementitious materials along with a new section regarding alternative cements [2]. this task group is examining whether additional cementitious materials with astm specifications should be added and whether any additional clarifications are needed for the alternative cement section. as with the recycled aggregate topic above, any changes to the provisions related to cement will need to be coordinated across aci 318 subcommittees. since the aci 318 code has both code language and commentary, these groups will be writing proposed sections for both. the commentary will likely include discussions of what approaches could be taken to achieve the code standards. as this subcommittee’s work progresses, the relationships between the work of the different task groups will emerge and there will be more clarity regarding the form and content of the proposed provisions. 4.3. interactions with other sustainable concrete code writing organizations as the aci 318-n subcommittee is beginning its work to expand the code language related to sustainability in the aci 318 building code, the subcommittee is reviewing the work of other concrete code writing bodies and their approach to the incorporation of sustainability into their codes. in particular, aci 318-n has studied the sustainability sections of the fib model code 2010 and sought insight into the active developments in fib commission 7: sustainability (com7). in order to further foster coordinating and information sharing, aci 318-n will have a joint meeting with com7 at the fib international conference on concrete sustainability that will occur in prague in september 2020. 5. conclusion this paper has addressed how the current aci 318 building code addresses sustainability, the changing landscape of sustainable design and specification, and the possible areas of expansion within aci 318. the efforts related to this expansion are in their infancy, but with the wide use of aci 318 the possible impacts are great. the implementation of sustainable regulations on concrete are growing and will continue to grow regardless of the incorporation of sustainability in this building code. however, if efforts are made to make new provisions pertinent and easily implementable, the potential to make specifying sustainable concrete more consistent and logical are significant. acknowledgements the author gratefully acknowledges the travel support for this conference provided by the american concrete institute technical cooperative representative program. the opinions expressed in this paper are the authors own and do not necessarily represent the opinions of aci. 293 shana kelley acta polytechnica ctu proceedings references [1] american concrete institute. building code requirements for structural concrete (aci 318-14) commentary on building code requirements for structural concrete (aci 318r-14) ed aci committee 318, farmington hills, mi: american concrete institute, 2014. http: //aghababaie.usc.ac.ir/files/1506505203365.pdf. [2] american concrete institute. building code requirements for structural concrete (aci 318-19) commentary on building code requirements for structural concrete (aci 318r-19) ed aci committee 318, farmington hills, mi: american concrete institute, 2019. https://www.usb.ac.ir/filestaff/ 5526_2020-1-25-11-12-7.pdf. [3] iso 21930:2107. sustainability in buildings and civil engineering works core rules for environmental product declarations of construction products and services, geneva, international organization for standardization, 2017. [4] astm international. central concrete environmental product declaration mix 3daz75z2 south san francisco (wet) plant, 2020. https://www.astm.org/c ertification/docs/epd_central_concrete.html. [5] building transparency. embodied carbon in construction calculator, 2020. https://www.buildingtransparency.org. 294 http://aghababaie.usc.ac.ir/files/1506505203365.pdf https://www.usb.ac.ir/filestaff/5526_2020-1-25-11-12-7.pdf https://www.astm.org/certification/docs/epd_central_concrete.html https://www.buildingtransparency.org acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.39.0023 acta polytechnica ctu proceedings 39:23–31, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague brain data and driver’s attention during simulated drive roman mouček∗, petr brůha, pavel šnejdar university of west bohemia, faculty of applied sciences, department of computer science and engineering, technická 8, 306 14 plzeň, czech republic ∗ corresponding author: moucek@kiv.zcu.cz abstract. the attention of drivers is a serious issue and one of the critical factors of road safety. the question is whether the electrical activity of the human brain can be correctly measured/collected and utilized to monitor and interpret the driver’s attention during simulated driving. this article summarizes four experiments that have been designed, performed, and evaluated in the neuroinformatics laboratory at the university of west bohemia. simulated driving under various conditions in a car simulator was organized, and electrophysiology, mainly electroencephalography, data were collected from participants/drivers. the results include experience with the design of such experiments and the suitability of methods based on the collection and interpretation of electroencephalography data for driver attention detection. keywords: attention of drivers, electroencephalography, event-related potentials, simulated driving, p300 component. 1. introduction the attention of drivers is a serious issue and one of the critical factors of road safety. inattentive drivers are dangerous not only to themselves but mainly to their surroundings and cause many accidents. however, the decline of attention, especially during long rides, is natural, and it is necessary to take this phenomenon into account. this paper summarizes some of the experiments that have been designed, performed, and evaluated in the neuroinformatics laboratory at the university of west bohemia. simulated driving under various conditions in a car simulator was organized, and electrophysiology (mainly electroencephalography, event-related potential) data were collected from participants/drivers. electroencephalography (eeg) is a method and technique that records and evaluates the electrical activity of the (human) brain. a set of electrodes is placed at various (often predefined) locations on the scalp. these electrodes capture voltages (resulting from the firing of large neural circuits) that must be amplified and recorded. event-related potentials (erps) are changes in the electrical brain activity that are time-locked to particular events (stimuli). erps are extracted from the underlying eeg data. the main research objective of the presented set of four experiments was to find and verify if the electrical activity of the human brain (eeg/erp data) could be correctly measured/collected and utilized to monitor and interpret the driver’s attention during simulated driving. the paper is organized as follows. the materials and methods section deals with the fundamentals of eeg/erp methods and their benefits and drawbacks in investigating driver’s attention. it further introduces the neuroinformatics lab at the university of west bohemia and summarizes four performed experiments. the results section provides the results and conclusions from individual experiments. the conclusion section summarizes experience from these experiments and gives several cautious prospects for the future. 2. materials and methods this section provides a fundamental insight into the advantages and disadvantages of collecting, monitoring and analyzing the electrical activity of the human brain using the methods and techniques of electroencephalography and event-related potentials when interpreting the driver’s attention during simulated driving. then the neuroinformatics lab at the university of west bohemia is introduced, and the description of four experiments dealing with driver’s attention and using the eeg/erp methods is provided. 2.1. electroencephalography and event-related potentials electroencephalography has many advantages: affordability, non-invasiveness, routine examination protocols, and the opportunity to measure spontaneous activity. however, it also has a significant disadvantage, evident in scientific experiments. the resulting picture of brain activity (the eeg signal) is rough since it represents many sources of neuronal activity. thus it is challenging to derive individual neurocognitive processes from the measured brain activity. another disadvantage is the relatively difficult collection of a quality eeg signal outside the laboratory since only observable and clean data can be reasonably analyzed 23 https://doi.org/10.14311/app.2022.39.0023 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en r. mouček, p. brůha, p. šnejdar acta polytechnica ctu proceedings to interpret the driver’s attention during simulated driving. erps have two advantages compared to behavioral methods. they help determine which stage or stages of processing are influenced by a given experimental manipulation; a detailed set of examples can be seen, e.g. in [1]. the second advantage of erps is that they can provide an online measure of stimuli processing, even when there is no behavioral response [2]. erp components are obtained from the eeg signal by averaging the epochs around the events. the p300 (also labelled as p3) component depends entirely on the task performed by the subject and is not directly influenced by the physical properties of the stimulus [1]. the p300 component is sensitive to the probability of the target stimulus. p300 amplitude increases when the probability of the target stimulus class decreases. the amplitude of the p300 component becomes larger when a greater number of non-target stimuli precedes it. p300 amplitude is also larger when the subject pays more attention to a task. on the other hand, p300 amplitude is smaller if the subject does not know whether a given stimulus is/is not a target. it means that more complex tasks can increase p300 amplitude because the subject pays more attention to these tasks and simultaneously decrease p300 amplitude because the subject is not certain of the stimulus category [1]. p300 latency is associated with stimulus categorization; it increases when stimulus categorization is postponed. since p300 latency does not depend on consequent processes (e.g. response selection, in our case, the driver’s reaction), it can be used to determine if a performed experiment influences the processes of stimulus categorization or other processes related to a response (driver’s reaction). the experimental design of eeg/erp experiments is a challenging and often critical step that influences other technical issues related to eeg/erp research. an apparent and detailed description of erp experimental design as a set of rules and strategies (e.g. focusing on specific, large, and easily isolated components or comparing only erps elicited by the same physical stimuli) is also provided in [1]. although these rules and strategies constrain the experimental design, they are not a standard part of the software tools used to design erp experiments, and they are often violated in many published studies. consequently, it was not easy to get inspired by the published studies when we designed the first experiments related to drivers’ attention. 2.2. related studies while most studies examining the attention of drivers use behavioral methods (i.e. the behavior of the driver under certain conditions is investigated – e.g., how often he/she leaves his/her lane), not many experiments also use eeg and erp methods and techniques. the suitability of eeg-based techniques for recording drivers’ activity during simulated driving was investigated in [3] where oscillations of brain electrical activity (frequency bands) were analyzed. as a result, an increase in alpha activity was interpreted as less attentional activity and a decrease in alpha activity as more attentional activity. significant differences between drivers were observed. the erp method (p300 amplitude) is utilized in [4] where the impact of the secondary task performance (an oddball auditory task) on a primary driving task (lane keeping) was investigated. the study showed that when performing a simple secondary task during driving, the performance of the driving task and this secondary task were unaffected. however, analysis of brain activity showed reduced cortical processing of irrelevant and potentially distracting stimuli from the secondary task during driving. the use of eeg data for the evaluation of driver fatigue was provided in [5]. energy parameters were computed, and finally, the evaluation model for driver fatigue was created based on the eeg data from the electrodes fp1 and o1. muscle artifacts were minimized by the experimental protocol design, and all trials showing artifacts linked to eye movements or blinks were removed before averaging erps. the model accuracy was about 92.3 %. the impact of a surrogate forward collision warning system and its reliability according to the driver’s attentional state was introduced in [6]. both behavioral and electrophysiological (amplitudes and latencies of several erp components) data were recorded and evaluated. these results showed electrophysiological data as a valuable tool to complement behavioral data allowing a better understanding of how these systems impact the driver. using the erp technique, it was found in [7] that the brain activity associated with processing the information necessary for the safe operation of a motor vehicle was suppressed when drivers were talking on a cell phone. the p300 amplitude was reduced, and the p300 latency was delayed when participants were engaged in phone conversations and reacted to intermittent lead vehicle deceleration. the effect of regular and prior sleep restricted to five hours during simulated driving was studied in [8]. the study was carried out on 20 younger and 19 older healthy men. after a short sleep, younger drivers showed significantly more sleep-related deviations and greater 4 to 11 hz eeg power, indicative of sleepiness [8]. a systematic framework for measuring and understanding cognitive distraction in three experiment settings (laboratory control, driving simulator, and instrumented vehicle) was presented in [9]. participants completed eight tasks commonly performed by the driver; primary, secondary, subjective, and physiological measures (the p300 component) were collected and integrated into a cognitive distraction scale. they 24 vol. 39/2022 brain data and driver’s attention during simulated drive concluded that impairments in driving were directly related to the cognitive workload of these activities. simultaneous recording of eeg and eye-tracking to investigate situation awareness and working memory load in distracted driving was introduced in [10]. the driver-vehicle-environment state-space related to drivers’ conditions and environmental factors around the vehicle were described using (also) an ontology. the amplitude of the p300 component reflecting individual differences in navigation performance in a driving task was investigated in [11]. two groups of navigators with good and poor navigation performance participated in a driving task; the p300 amplitude was measured while two types of triggers were presented (intersections and street signs). in general, poor navigators showed a larger p300 amplitude than good navigators. to identify eeg oscillatory parameters for cognitive mechanisms involved in high and low controllability tasks, time spent on a task, task load, and cognitive controllability (using either re-active or pro-active driving task) in simulated driving scenarios were investigated in [12]. the results on thirty healthy participants demonstrated that the controllability of a driving situation had a similar effect on oscillatory eeg activity like time on task and task load. the drivers’ fatigue was measured in [13] where eeg and forehead eog were fused. the percentage of eye closure was calculated using eye movement data (recorded by eye tracking glasses as the indicator of drivers’ fatigue level). the prediction correlation coefficient and root mean square error between the estimated and real fatigue levels were used to evaluate the performance of a single modality and fusion modality. the experimental results on twenty-one healthy subjects showed that this fusion could improve the performance of driving fatigue detection. a real-time driving fatigue detection system based on a wireless dry eeg acquisition system was presented in [14]. the prediction of fatigue in ten healthy subjects was consistent with the observation of reaction time recorded during simulated driving. the study in [15] concluded that the eeg signal from the prefrontal brain region (forehead area) could be used to detect fatigue of drivers, although the signal classification accuracy (the svm algorithm used for classification) was not high. a survey on eeg-based driver state detection systems and their corresponding analysis algorithms over the last three decades was provided in [16]. the authors concluded that the current eeg-based driver state monitoring algorithms were promising for safety applications, but many improvements were still required in eeg artefact reduction, real-time processing, and between-subject classification accuracy. a two-level learning hierarchy radial basis function model was used to detect eeg-based driving fatigue in [17]. the authors concluded that the proposed model achieved a better classification performance compared to other used artificial neural networks. multi-channel eeg recordings during a sustainedattention driving task were presented in [18]. they included 62 sessions of 32-channel eeg data for 27 subjects driving on a four-lane highway. a recurrent residual network used to analyze eeg data captured during simulated sustained attention driving tasks was presented in [19]. the authors demonstrated the competitive results achieved by comparing this network with other benchmark models. an attention-based multiscale convolutional neural network with a dynamical graph convolutional network proposed to detect driving fatigue was introduced in [20]. the authors concluded that their model outperformed six widely used competitive eeg models with a high accuracy of 95.65 %. an eeg-based spatio–temporal convolutional neural network for driver fatigue evaluation was introduced in [21]. fatigue driving experiments were conducted to collect eeg signals from eight subjects alert and in fatigue states. the results indicated that the network fulfilled a better classification accuracy of 97.37 % than the eight methods it was compared with. the study [22] investigated whether measures from low-cost devices monitoring peripheral physiological states (eye-tracker, heart rate monitor, and a highfidelity 32-channel quick-gel eeg system) were comparable to standard eeg measures in predicting lapses in attention to system failures. twenty-five participants were engaged in a fully autonomous lane-changing driving task. the results showed that current lowfidelity technologies were not sensitive enough to reliably model reaction time to critical signals. to summarize the related studies section, we can conclude that eeg and erp methods and techniques are promising for detecting driver fatigue and the impact of various disturbing events and tasks on driver’s attention. although the studies usually work with fatigue, vigilance or cognitive performance, we will use the term attention to align with the fundamental explanation of eeg/erp methods. we were surprised that many studies did not pay too much attention to design issues of such experiments since only a wellprepared design can lead to relatively clean data and reasonable interpretation. 2.3. neuroinformatics laboratory equipment the neuroinformatics laboratory at the university of west bohemia is equipped with all necessary hardware infrastructure for driving simulations and recording eeg/erp signals and additional biosignals. the experimental car simulator (a front part of a real škoda octavia car, figure 1) is equipped with the logitech g27 wheel, accelerator, and brake. these are connected to the control computer via the usb port. the tracks are prepared mainly using the world racing 2 game produced by the synetic company [23]. 25 r. mouček, p. brůha, p. šnejdar acta polytechnica ctu proceedings figure 1. car simulator. the track is projected on the wall in front of the car simulator. the stimuli were presented either using a custom programmable hardware stimulator (for visual stimulation with leds) placed on the car windshield or the presentation software tool (for auditory stimulation) produced by neurobehavioral systems, inc. [24]. v-amp produced by the brain products company was used as an eeg amplifier. it also served as an input of the sensors (also produced by the brain products company) capturing other biosignals. the brainvision recorder [25] was used for recording and local storing eeg/erp and other biosignal data (e.g. respiration rate or skin conductance). matlab, eeglab, erplab, brainvision analyzer software tools, and python tools (especially the mne library) were used to process and analyze experimental data. standard eeg caps (the 10-20 system defining locations of scalp electrodes) were used depending on the size of the participants’ heads. the reference electrode was placed approximately 0.5-1 cm above the nose, and the ground electrode was placed on the ear. the biosignal sensors and the eeg cap were connected to the v-amp amplifier. three computers were usually used: the first for the presentation of auditory stimuli (if they were presented), the second for storing recorded data, and the third for the track presentation. the diagram of the standard experimental setup is depicted in figure 2. eeg/erp data were recorded with the sampling frequency 1 khz; no filters were used during data recording. the resulting signal was stored in three files: .eeg file containing raw data; .vhdr file containing metadata that describe raw data in .eeg file, and .avg file containing the averaged signal around the used stimuli. iir filter was applied to data from the fz, cz and pz electrodes. these three electrodes were also selected for further processing. 2.4. methodology – experiments four already published eeg/erp-based experiments investigating driver’s attention during simulated driving are presented to show the evolution of our experimental work in eeg-based simulated driving. although we could theoretically include dozens of mostly unpublished experiments in this work, various driving scenarios and overall driving conditions would confuse the article. an appropriate experiment design (including the choice of the event-related component, type of stimuli, driven track, or entire data collected) has to be set and evaluated to achieve research objectives. when observable and clean data are collected, it is reasonable to interpret the driver’s attention during simulated driving. the event-related component p300 was selected based on its nature, use in related studies, and the outcomes of our preliminary unpublished experiments. in the first presented experiment, we proposed and tested the constraints related to the complexity of the experimental design. the experiment design was improved in the second presented paper. the third paper introduced more tiresome conditions for participants; not only a monotonous track but various daytime conditions and sleep deprivation were part of the experiment protocol. the last experiment used two signals collected in parallel to detect the driver’s attention during a simulated drive – brain electrical activity and respiration. eeg/erp data were recorded with the sampling frequency of 1 khz; no filters were used during data recording. the resulting signal was stored in three files: .eeg file containing raw data, .vhdr file containing metadata that describe raw data in the .eeg file, and .avg file containing the averaged signal around the used stimuli. various iir filters were applied to data from the fz, cz and pz electrodes in the processing phase, typically in the frequency range 0.01-20 hz. the fz, cz and pz electrodes were also selected for further processing. the segmented data containing artifacts were first automatically and then manually rejected. 2.4.1. experiment 1 the main objective of this work [26] was to determine if eeg/erp methods and techniques and the event-related p300 component itself can be successfully utilized to investigate how the mental load of drivers affects their attention during a simulated drive. the second objective was to test the constraints of the experimental protocol complexity. a relatively challenging virtual track containing sharp turns and allowing for potential collisions with oncoming cars (target stimuli) was designed and tested on two groups (nine volunteers) of drivers (affected/unaffected by alcohol). the occurrence of the p300 component was investigated. then, an alternative protocol relying on simpler target stimuli (flashing 26 vol. 39/2022 brain data and driver’s attention during simulated drive figure 2. experimental setup for simulated driving. diodes and simple sounds) was proposed, implemented, and tested. 2.4.2. experiment 2 this paper’s [27] objective was to detect changes in a driver’s attention during a monotonous simulated drive. it was done using eeg/erp techniques, performing auditory stimulation, and investigating properties of the p300 component related to attention. the erp experiment was designed to investigate the driver’s attention changes during monotonous 40 minutes long simulated drive. the experiment was performed on 14 participants who drove a car simulator on a monotonous track (a motorway without disturbing obstacles). the participants were audibly stimulated (simple beeps of various duration and the sound of a crying child). the data were preprocessed (filtering, artefact rejection, epoch extraction, and baseline correction) and investigated for the p300 component’s occurrence and peak latency in subsequent eight-minute intervals. 2.4.3. experiment 3 the objective of the experiment [28] was to introduce more tiresome conditions for participants. not only a monotonous track but also various daytime and sleep deprivation were part of the experiment protocol to detect changes in the driver’s attention during a monotonous simulated drive. eeg and erp techniques and auditory stimulation were utilized; changes in the peak latency of the p300 component over time were investigated. the same hypothesis as in the previous experiments (the peak latency of the p300 component increases over time as the driver becomes more tired) was considered, but the experiment design differed and evolved in time. a monotonous track was constructed in this experiment, and a simple auditory experiment was designed (three stimuli – simple target and non-target stimuli of 500 ms duration time and different frequencies, a rare stimulus of 1000 ms duration time). each participant (11 participants in total) underwent four twenty-minute drives held over two days; each day, morning and afternoon sessions were held; on the 27 r. mouček, p. brůha, p. šnejdar acta polytechnica ctu proceedings first day, the participant usually slept, and on the second day, he/she suffered from sleep deprivation. each drive was divided into time intervals of the same length (the stimulation was provided only in some time intervals to ensure relaxation time and prevent participants from familiarity with the presented stimuli). the latencies of the averaged p300 components were compared depending on the daytime and sleep deprivation. 2.4.4. experiment 4 the paper’s [29] objective was to consider two biosignals collected in parallel to detect the driver’s attention during a simulated drive; these bio-signals were the electrical activity of the human brain (as in the previous experiments) and respiration. moreover, the collected data were validated using a stacked autoencoder to guarantee their quality and annotated to make them publicly available and reusable. the experiment was built on the previous experiments, considering the basic assumptions related to the p300 component and driving a car simulator on a monotonous track. the experiment (simulated drive) was prolonged to 60 minutes and divided into three sessions. auditory stimulation was performed (target and non-target stimuli of the same length), and the background sound of drizzling imitated the natural environment. the respiration rate was captured parallel with the brain’s electric activity using the same hardware amplifier. the data from a group of 15 participants were annotated, stored, preprocessed, validated, and partly analyzed (classified using a stacked autoencoder). 3. results the first experiment [26] brought significant results – the proposed track appeared too complex to provide experimenters with the observable occurrence of the p300 component, but its simplified alternative version was more promising. the p300 component was not observed in the grand average waveforms when the more complex target stimuli (sharp turns and possible collisions) were part of the virtual track. a similar trend of the grand average waveforms for both groups of subjects (affected/unaffected by alcohol) was observed. when target stimuli were elicited by external hardware devices (flashing diodes or sound generators), the p300 component was easily observable. it was shown that the experiment design had to be relatively simple; otherwise, the event-related p300 component could not be identified. it is necessary to be sure if and when the p300 component is elicited. the subsequent experiments thus followed a more simple design. the experiment design was improved in the second driving experiment [27]. as a result, the p300 component was easily observable in the grand average waveforms. however, both variants of the experiment design (using a different location of the reference electrode) did not show gradually increasing peak latency concerning the length of the simulated drive. there were 30 % of artifacts in the epochs related to target stimuli. the target stimulus (a 600 ms long sound of a crying child) was too long and finally made the p300 component less analyzable. selecting a long target stimulus (a crying child) was probably not a good design solution since it stretched the p300 component length. stretched components are less analyzable; their absolute coordinates are distorted. there were also not many target stimuli, and because of the removal of the artifacts, only 70 % of them entered the averaging process. the experiment’s prolongation and introducing more tiresome conditions (e.g., exposing participants to heat, absence of sleep, or placing them in a darkened room) were further considered. the p300 component was clearly identified in all experimental sessions of the third experiment [28]. figure 3 shows the grand averages on the electrode fz (frontal central electrode) for all experimental sessions. on the other hand, the prolongation of the peak latency of the p300 component overtime was not observable when the grand average measure of each participant was investigated. however, the prolongation of the peak latency of the p300 component was observable when the techniques of peak latency and fractional 50 % area latency were applied to compute the grand average for each experimental session. the latency of the p300 component was not influenced by the time of day. the latency of the p300 component increased with sleep deprivation. the experiment showed that the p300 component’s prolongation was identifiable when the techniques of peak latency and fractional 50 % area latency were applied. we concluded that for the following experiments, it would be beneficial to have a larger number of target stimuli (i.e., the simulated drive has to be prolonged) and more participants. in the fourth experiment [29], the p300 component was identified in most participants during all driving sessions. a high amplitude of the n2 component, when compared to the amplitude of the p300 component, was detected in some cases. prolongation of the peak latency of the p300 component was evident in most participants when their peak latencies were averaged. however, prolongation of peak latency in time was not clearly observed when grand averages for all participants were investigated. average respiration rate and respiration rates for most participants showed a decreasing trend during the experiment. in this experiment, the trend of the increasing latency of the p300 component and decreasing respiratory rate was relatively clearly visible (note: the data were not statistically evaluated). since the experimental results were naturally affected (e.g., by different brain reactions of participated drivers, the sensibility of captured data to the environmental noise, 28 vol. 39/2022 brain data and driver’s attention during simulated drive −100 0 100 200 300 400 500 600 700 800 900 −15 −10 −5 0 5 10 15 time [ms] vo lta ge [µ v ] morning + usual sleep afternoon + usual sleep morning + sleep deprivation afternoon + sleep deprivation figure 3. grand average on the electrode fz – experimental sessions differ in the daytime and duration of sleep [28]. or participants’ overall mental conditions), there was a significant effort to eliminate these circumstances by experiment design, setting of experimental conditions, and appropriate use of data preprocessing and processing methods. moreover, the data were finally validated using the stacked autoencoder. 4. conclusions various experiment protocols were designed (dozens of experimental protocols were tested, and four published ones were presented in this paper) to set and verify constraints significant for the successful investigation of drivers’ attention utilizing the human brain’s electrical activity and the eeg/erp methods and techniques. the initial experiments were over-specified; the experiment protocols were too complicated, naive, and unrealistic, given the method used. over time, the experiment design was improved; the event-related p300 component was finally easily identifiable. the interesting results regarding the latency of the p300 component were observed, e.g., its increase with overall fatigue or sleep deprivation of drivers was detected. on the other hand, we had to cope with the troubles related to participants’ willingness to wear an eeg cap for a longer time, handle a longer drive in the simulator, or reduce unnecessary movements. the number of produced artifacts finally put time constraints on the designed experiments. all the work related to the collection, long-term storage, processing, and interpretation of eeg/erp data/metadata and publication of results also revealed the troubles with the data/metadata descriptions, procedures, and reproducibility of the findings. even reproducing a single experiment by the same experimenter proved practically impossible in a few months. the need for standardized data annotations and formats, procedures, and workflows turned out to be enormous to share the data, procedures, and outcomes with the community and understand data, procedures, and conclusions after a particular time. the investigation of driver’s attention has finally become interesting for automotive in-car assistance systems and self-driving car development. the assistance systems that prevent drivers from causing accidents have been improved and can monitor drivers’ behavior and vital signs (mainly related to attention) such as movements, eye blinks, and heartbeat. moreover, self-driving cars are being developed to spare or even prevent people from driving. however, the data collection methods presented here are still complicated and impractical, and the results are not so reliable to be used in cars during actual driving. on the other hand, these methods can be used for verification purposes in laboratory conditions. list of symbols eeg electroencephalography erp event-related potentials p300 p300 component acknowledgements this work was supported by the university specific research project sgs-2019-018 processing of heterogeneous data and its specialized applications (project sgs-2019018). 29 r. mouček, p. brůha, p. šnejdar acta polytechnica ctu proceedings references [1] s. luck, g. woodman, e. vogel. event-related potential studies of attention. trends in cognitive sciences 4(11):432–440, 2000. https://doi.org/10.1016/s1364-6613(00)01545-x [2] s. luck. an introduction to the event-related potential technique. an introduction to the event-related potential technique. mit press, 2005. 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2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague comparison of selected parameters for evaluation of rail surface damage intensity jiří šlapák∗, tomáš michálek university of pardubice, faculty of transport engineering, department of transport means and dignostics, studentská 95, 532 10 pardubice, czech republic ∗ corresponding author: jiri.slapak@upce.cz abstract. this paper deals with the issue of evaluation of a rail surface damage (rsd) intensity. some ways of calculating parameters that represent the rsd are described. in this context, a multi-body model of a railway vehicle was created and several simulations of this model on a curved track were performed. furthermore, these simulations were evaluated and the rsd parameters were compared. keywords: wheel/rail interaction, rail surface damage, wear number, multi-body simulation. 1. introduction in recent years, some railway infrastructure managers have started to use vehicle ratings based on damaging effects of vehicles on tracks. these damaging effects are directly related to maintenance requirements of tracks. one of the damaging effect is the rail surface damage (rsd), which occurs when wheels roll on rails. rsd primarily represents a wear of rails by abrasion and secondarily a relationship between the wear and a rolling contact fatigue (rcf). this method of evaluation may motivate vehicle operators to use and purchase track-friendly vehicles. this paper is focused on several methods for evaluating rsd intensity and these methods are compared. these evaluation methods are described. the parameters that represent rsd intensity are compared depending on selected vehicle parameters. by means of multi-body simulations of vehicle running, a quantification of the parameters representing rsd intensity and damaging effects can be performed. 2. rail surface damage rolling of wheels on rails is possible due to normal forces acting in wheel/rail contact areas and the existence of adhesion in these contact areas. in general, when wheels rolls on rails, creepages and tangential (creepage) forces occur in the wheel/rail contact areas. the creepages occur in the longitudinal and lateral directions and also as a spin (rotation around the vertical axis). the longitudinal creepage is primarily related to traction and braking forces and also to the conditions of the wheelset/track interaction (specifically on the delta-r function) in curves. the increase in the lateral creepage is caused by an increase in the angle of attack of the wheelset. the spin is related to the inclination of the wheel/rail contact area. creepages and spin result in tangential (creepage) forces and spin moment that cause loading of the rail surface. due to this loading, the rail surfaces are damaged. in more detail, the issue of creepage forces is described in [1]. the first damaging effect is the wear of the rails and wheels by abrasion. the amount of wear depends on some design parameters of a vehicle, some wheel/rail contact conditions (coefficient of friction/adhesion, materials) and some track parameters. the rolling contact fatigue (rcf) is another damaging effect caused by wheels rolling on rails. rcf causes cracks on the rail surface. under certain conditions, crack initiations can be removed by wear on the rail surface. thus, the wear can be beneficial in terms of the rolling contact fatigue. 2.1. wear number the parameter called the wear number tγ [nm/m] is the first option for evaluating and comparing the damaging effects of vehicles that result in rsd. this parameter is based on the physical assumption that the wear of the rails and wheels is caused by friction work performed in the wheel/rail contact. according this assumption, the wear number is defined as: tγ = |txγx| + |tyγy| , (1) where t [n] is the tangential (creepage) force and γ [−] is the creepage in the wheel/rail contact. the letters x and y describe the longitudinal and lateral direction of these quantities. equation 1 applies when spin and spin moment are neglected. since these quantities (specifically creepages and lateral creepage force) cannot be measured on a real vehicle, it is necessary to determine the value of the wear number using multi-body simulations of vehicle running. the wear number presented in equation 1 corresponds to the specific friction work performed in the wheel/rail contact. furthermore, the wear number in this form is used in the methodologies of some railway infrastructure managers for setting track acess charges (e.g. methodology [2]). 42 https://doi.org/10.14311/app.2022.35.0042 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 35/2022 comparison of selected parameters 2.1.1. rcf prediction method based on tγ the wear number tγ is also used in the rcf prediction method. the non-linear dependence of the wear number and the so-called rcf damage index is shown in figure 1 and described in [3]. this index indicates whether the rails are damaged due to wear, rcf or a combination of both, which is more common option. according to figure 1: • rcf damage index increases from 0 to 1 · 10−5 as the wear number values incease from 15 n to 65 n. when the wear number value is 65 n, the probability of rcf crack initiation is greatest. • then the index value decreases to 0 as the wear number increases to 175 n. in this part, a wear begins to predominate over rcf. • for the wear number values greater than 175 n, the rcf damage index has negative value. this mean that only wear damage occurs. figure 1. dependence of the wear number tγ and rcf damage index. [3] figure 1 applies to r260 steel. for other steels, the position of the characteristic points differs. 2.2. rail surface damage parameter according to en 14363 in the standard en 14363 [4], the current evaluation of rail load in lateral direction is performed using the quatistatic lateral guiding force ya,qst. this force is also used as indirect evaluation parameter for the rail surface damage intensity especially the wear of rails, but sometimes it shows a very weak connection with rsd. another parameter for the evaluating of the rail surface damage intensity is presented in annex k of standard en 14363 [4]. the standard proposes the parameter tqst which is a combined quantity of lateral yqst, longitudinal tx,qst and vertical qqst forces acting in the wheel/rail contact and represents the rail surface damage intensity. the parameter tqst is defined as: tqst = qqst 10000 · ( 330 · f2 − 62 · f + 4 ) , (2) where f = yqst qqst + 0, 62 · |tx,qst| qqst . (3) the constants in these equations are derived as regression parameters from the dependence of tqst and tγ . the parameter f (equation 3) is dimensionless then the unit of the parameter tqst (equation 2) is newton [n]. this parameter has been defined in order to be able to measure the values of input parameters (forces) on a real vehicle without multi-body simulation. compared to the guiding force ya,qst, the parameter tqst better includes the influence of friction conditions in the wheel/rail contact area. the parameter tqst has only been defined for the guiding wheel of a vehicle. 3. multi-body simulations the values of previously defined quantities were obtained from multi-body simulations of running vehicle. the model and multi-body simulations of railway vehicle running were performed using simpack simulation software. 3.1. model of railway vehicle for the purposes of this study, the multi-body model of a conventional passenger car of an electric unit was used. the fundamental parameters of this model are listed in table 1. parameter value unit nominal mass of the carbody 40000 kg mass of the bogie frame 5200 kg mass of the wheelset 2100 kg bogie distance 19 m nominal bogie wheelbase 2.4 m vertical stiffness of the primary spring (per wheel) 1.6 kn/mm vertical stiffness of the secondary spring (per side of bogie) 0.7 kn/mm table 1. fundamental parameters of the multi-body model of the railway vehicle. setting the wheel/rail contact conditions is a very important part of the simulations. the wheel profile ore s1002 and the rail profile 60e1 were used. the rail inclination of 1:40 was considered. the nominal friction coefficient value 0.4 was chosen. the fastsim algorithm was chosen to calculate the tangential (creepage) forces. figure 2. the vehicle model created in simpack. 43 jiří šlapák, tomáš michálek acta polytechnica ctu proceedings figure 3. basic comparison of the mentioned parameters used to evaluate rsd intensity depending on the curve radius r. only for the guiding wheel on vehicle. 3.2. track conditions the simulations were performed on several curved tracks with a radius from 250 m to 1200 m. the length of the curve was set to 600 m. the cant d and cant deficiency i values are constant for all simulations. • d = 150 mm • i = 130 mm according to these conditions, the vehicle speed was set in the range of values from 77 km/h to 169 km/h. in the order to consider a real model of the track, reference track irregularities were used and a model of elastic track foundation was created. 4. simulation results time records of the quantities acting in the wheel/rail contact (lateral, longitudinal and vertical forces) and the parameter wear number tγ were monitored and exported from the multi-body simulations. from these records, the mean values of the quantities in the fully curved part of the tracks were calculated. the exported data from simulations were processed in matlab. because the parameters tqst and ya,qst are defined for the guiding wheel of a vehicle, all comparisons and evaluations have been processed for the guiding wheel only. 4.1. comparison of evaluation parameters for the first look at the comparison of the mentioned rsd intensity parameters (tγ , tqst, ya,qst), the parameter values depending on the curve radius are plotted in figure 3. this figure only shows the situation on the guiding wheel of vehicle for the nominal setting of the multi-body model and the simulation. according to figure 3, all rsd intensity parameters depending on the curve radius have the same trend and the shapes of the graph curves are similar. the values of the parameters progressively increase as the curve radius decreases. the figure further shows that the parameter tqst values are smaller than the wear number tγ values in curve radii greater than 700 m. then for smaller values of the curve radii, the parameter tqst values are greater then the values of the wear number tγ . however, this only applies to the guiding wheel of the vehicle. figure 3 further shows that the curve of the wear number tγ has a convex character in the whole range of curve radii. this also applies to the parameter tqst, except for the situation of the very small curve radius where the curve begins to be concave. 4.2. influence of selected parameters according to the standard en 14363 [4], the formula for the parameter tqst (equation 2) was defined for a wide range of selected operating conditons of a vehicle. therefore, vehicle running simulations were performed with a focus on the influence of these parameters: • friction coefficient in the wheel/rail contact µ, • weight of vehicle body m, • bogie wheelbase 2a, • longitudinal stiffness of wheelset guiding kx per one wheel. for all these parameters, the dependences of tqst and tγ on the curve radius r are plotted in graphs. to compare of rsd intensity parameters, the difference between tqst and tγ is defined and calculated as: tγ − tqst [n ]. (4) furthermore, the relative difference is defined and calculated as: (tγ − tqst) /tγ · 100 [%]. (5) 4.2.1. influence of friction coefficient in the first part, the influence of the friction coefficient values in the wheel/rail contact was investigated. friction coefficient values of 0.2, 0.4 and 0.6 were selected. figure 4 shows comparison of the wear number tγ and the parameter tqst (calculated according to [4]) on the guiding wheel of the vehicle. for different 44 vol. 35/2022 comparison of selected parameters values of the friction coefficient µ in the wheel/rail contact area, the individual curves are plotted as a function of the curve radius r. for all investigated values of the friction coefficient, this figure shows that the values of the parameter tqst are very close in the range of the curve radius from 700 m to 1200 m. this behavior of the parametr tqst is different from the wear number tγ . further, as the curve radius value decreases, the values of the parameter tqst as well as the parameter tγ values increase. figure 4. dependence of the tqst and tγ on the curve radius r in defined range of the friction coefficient values µ. figure 4 shows that the friction coefficient value in the wheel/rail contact area has a great influence on the values of wear number tγ and also shows that the parameter tqst follows this trend. figure 5. dependence of the ya,qst on the curve radius r in defined range of the friction coefficient values µ. in figure 5, the influence of the friction coefficient µ on lateral guiding force ya,qst is shown. as the friction coefficient values increase, the slope of the plotted curves increases. this behavior is the same as for the parameters in figure 4. but the curves are shifted, which causes a large mismatch between the force ya,qst and the parameters tqst and tγ . the next figure 6 shows the difference and the relative difference between the values of the wear number tγ and the parameter tqst in defined range of friction coefficient values. in general, the best match of these parameters occurs for the smallest value of the friction coefficient. but for very small curve radii, the value of the relative difference for the friction coefficient of 0.6 is the smallest. this mean that in very small curve radii, tqst corresponds better to the wear number tγ with increasing value of the friction coefficient. on the other hand, for large curve radii, the parameter tqst best corresponds to the wear number tγ under the conditions of the smallest value of the friction coefficient. for a radius curve of 1200 m and a friction coefficient of 0.6, the value of the relative difference is 78%. figure 6. the difference tγ − tqst depending on the curve radius r (above) and the relative difference (tγ − tqst) /tγ depending on the curve radius r (below) in the defined range of friction coefficient µ values. 4.2.2. influence of vehicle weight in the next part, the influence of the vehicle weight was investigated. for this evaluation, the vehicle body mass m was set to 35, 40 and 45 tons. figure 7 shows the values of the parameter tqst and the wear number tγ as a function of the curve radius r for the defined values of the vehicle body weight. according to this figure, the weight of vehicle has an effect on the wear number tγ values. this effect is most evident for the small curve radii. on the other hand, the values of the parameter tqst do not change when the vehicle weight changes. this applies in whole range of the curve radii expect the very small curve radii. 45 jiří šlapák, tomáš michálek acta polytechnica ctu proceedings figure 7. dependence of the tqst and tγ on the curve radius r for defined values of the vehicle body mass m. figure 8. dependence of the ya,qst on the curve radius r for defined values of the vehicle body mass m. figure 8 shows that the weight of the vehicle body has an effect on the guiding force ya,qst. the wear number tγ (in figure 7) has a similar effect. furthermore, the dependence of the force ya,qst on the curve radius is concave in very small curve radii, as well as the dependence of the parameter tqst on the curve radius. in figure 9, the difference and the relative difference of the parameters tqst and tγ are shown in relation to figure 7. in very small curve radii, the best match of the investigated parameters occurs at higher values of the vehicle weight. then in the large curve radii, the best match of the parameters occurs for the lightest vehicle but the differences disappear in very large curve radii. in the value of the curve radius of 1200 m, the relative difference value is 72% and this value does not depend on the vehicle weight. figure 9. the difference tγ − tqst depending on the curve radius r (above) and the relative difference (tγ − tqst) /tγ depending on the curve radius r (below) for defined values of the vehicle body mass m. 4.2.3. influence of bogie wheelbase in this part the influence of the bogie wheelbase 2a on the parameters tqst and tγ is analyzed. the value of the bogie wheelbase was set to 2.0, 2.4 and 2.8 meters. figure 10. dependence of the tqst and tγ on the curve radius r for defined values of the bogie wheelbase 2a. in figure 10, the influence of the bogie wheelbase values is evident for both parameters of rsd intensity over the whole range of the curve radii. again, the values of tqst are smaller than the values of tγ for large curve radii. then, for small curve radii, the values of tqst increase over the values of tγ . 46 vol. 35/2022 comparison of selected parameters figure 11. dependence of the ya,qst on the curve radius r for defined values of the bogie wheelbase 2a. according to figure 11, the influence of the bogie wheelbase 2a on the guiding force ya,qst values and on the parameter tqst is similar. for large and very small curve radii, it is difficult to predict and describe the values of the force ya,qst . the dependence of the tγ values on the curve radius has the same trend for different values of bogie wheelbase. this does not apply to the parameter tqst whose trends are changing in the very small curve radii. this is also shown in figure 12 where the both differences increase (in terms of absolute values) in the very small curve radii under the condition of the bogie wheelbase of 2.0 m. but for the other values of the bogie wheelbase in the very small curve radii, the differences decrease with decreasing the curve radius. figure 12. the difference tγ − tqst depending on the curve radius r (above) and the relative difference (tγ − tqst) /tγ depending on the curve radius r (below) for defined values of the bogie wheelbase 2a. in general, figure 12 shows that the best match between the tqst and tγ parameters occurs for the vehicle with the longest bogie wheelbase. according to the first graph in figure 12 and for the curve radius of 250 m, the value of the difference is 150 n for the vehicle with bogie wheelbase of 2.0 m. it is the highest value of the absolute diference of the rsd intensity parameters. but from the point of view of the relative difference of these parameters, the value corresponds to the relative difference of 37%. a much larger relative difference occurs in curve radius of 950 m where the value is about 74% and it is the highest value of the relative difference of the parameters for this analysis. 4.2.4. influence of longitudinal stiffness of wheelset guiding the influence of the longitudinal stiffness of the wheelset guiding kx per one wheel on the parameter tqst and the wear number tγ is the last analyzed part. the values of the longitudinal stiffness of the wheelset guiding per one wheel were set to 2.0 · 107, 3.5 · 107 and 5.0 · 107 n/m. figure 13. depencence of the tqst and tγ on the curve radius r in defined range of the longitudinal stiffness of wheelset guiding kx per one wheel. figure 14. depencence of the ya,qst on the curve radius r in defined range of the longitudinal stiffness of wheelset guiding kx per one wheel. figure 13 shows that the longitudinal stiffness of the wheelset guiding in defined range of values has 47 jiří šlapák, tomáš michálek acta polytechnica ctu proceedings a small effect (in comparison with other investigated parameters) on the values of the parameter tqst and the wear number tγ . from this point of view, the behavior of these parameters is the same. thus, in general, changing the stiffness value causes the same reaction in the parameter tqst as in the wear number tγ . according to figure 14, the behavior of the dependence of the guiding force ya,qst on the curve radius is the same as the dependences on figure 13. this means that the effect of the longitudinal stiffness of wheelset guiding kx in a defined range of values on the guiding force ya,qst values is small. figure 15. the difference tγ − tqst depending on the curve radius r (above) and the relative difference (tγ − tqst) /tγ depending on the curve radius r (below) in the defined range of the longitudinal stiffness of wheelset guiding kx per one wheel. this fact is confirmed in figure 15. this figure shows that the difference and the relative difference values are almost idential for the defined range of the longitudinal stiffness values. some effect of the longitudinal stiffness to the difference and the relative differrence of rsd intensity parameters occurs in very small curve radii. 5. conclusions it can be assumed that the wear number tγ represents the wheel and rail abrasion wear and indirectly also represents the rolling contact fatigue effects. it is used by railway infrastructure managers as an indicator of damage and maintenance requirements of a curved track. unfortunately, the wear number must be obtained from multi-body simulations and cannot be measured on a real vehicle. this fact means a risk that the results of the multi-body simulations can be affected by the model settings. the correctness of this settings cannot be experimentally verified. the lateral guiding force ya,qst is a parameter that is used to evaluate the lateral load of the rails. this force exhibits similar behavior and trends as the wear number tγ depending on the curve radius, but its values are different. for the reason, an alternative quantity, the parameter tqst is defined in the standard en 14363. based on multi-body simulations of running a conventional passenger four-axle railway vehicle, it was found that the parameter tqst shows the same trends as the wear number tγ on the outer guiding wheel of a vehicle. however, there are differences between the values of these parameters, which vary depending on the curve radius. the parameter tqst values are smaller than the wear number values in the large curve radii. conversely, the parameter tqst values are higher than the wear number in the very small curve radii. the best match of these parameters occurs in curve radii from 400 to 600 meters. the worst match occurs in the very large curve radii where the relative difference between the parameter tqst and the wear number tγ has a value higher than 70 percent. this mean that this approximation of the parameter tqst is bad for very large curve radii. the friction coefficient value in the wheel/rail contact area has the greates influence on the rsd intensity parameters. further, the vehicle body weight affects the wear number values, but has almost no effect on the parameter tqst value. the values of the longitudinal stiffness of wheelset guiding have the same effect on both rsd intensity parameter. the defined parameter tqst for evaluation of rsd intensity follows the trend of the wear number. but between these parameters, there is bad match in the large curve radii and the parametr tqst does not respond to the vehicle weight change. the parameter tqst can represent the wear number, but the accuracy of this parameter could be improved. acknowledgements this work was supported by the scientific research project of the university of pardubice no. sgs_2021_010. references [1] voltr, p.: calculation of locomotive traction force in transient rolling contact. in: applied and computational mechanics, 11, 69-80, 2017. [2] feredal office of transport. base price wear in the train-path pricing system 2017 – instruction for determining vehicle prices. bern, 2017. [3] iwnicki, d. s.: the effect of profiles on wheel and rail damage. in: international journal of vehicle structures & systems,1(4), 99-104, 2009. [4] en 14363:2016+a1:2018. railway applications – testing and simulation for the acceptance of running characteristics of railway vehicles – running behaviour and stationary tests. brusel: european committee for standardization, 2018. 48 acta polytechnica ctu proceedings 35:42–48, 2022 1 introduction 2 rail surface damage 2.1 wear number 2.1.1 rcf prediction method based on t 2.2 rail surface damage parameter according to en 14363 3 multi-body simulations 3.1 model of railway vehicle 3.2 track conditions 4 simulation results 4.1 comparison of evaluation parameters 4.2 influence of selected parameters 4.2.1 influence of friction coefficient 4.2.2 influence of vehicle weight 4.2.3 influence of bogie wheelbase 4.2.4 influence of longitudinal stiffness of wheelset guiding 5 conclusions acknowledgements references https://doi.org/10.14311/app.2022.33.0245 acta polytechnica ctu proceedings 33:245–249, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague durability of high belite cement as new technical solution for concrete sara iricoa,∗, sabine mutkea, federica bertolab, daniela gastaldib, livio capellib, fulvio canonicoa,b a wilhelm dyckerhoff institut for building materials technology, dyckerhoff gmbh, dyckerhoffstraße 7, 65203 wiesbaden, germany b buzzi unicem innovation lab and technology (built), via restano 3, 13100, vercelli, italy ∗ corresponding author: sara.irico@dyckerhoff.com abstract. the study presents an investigation on the durability performance of concretes produced with an industrial scale high belite cement. this binder, thanks to the high amount of c-s-h gel and the lower c3s/c2s ratio, is characterized by high durability properties. good resistance to freeze/thaw cycles, very low chloride migration and excellent resistance to sulfate attack were demonstrated. the accelerated carbonation test revealed that the belite cement, due to the high c2s phase amount, is able to increase its strength over time when exposed to high co2 concentration. durability and strength results were compared with those of cem i, cem ii, and cem iii. in addition, a comparison of the performance of a self-compacting concrete produced with high belite cement and cem i (as reference) is also presented. the results show that the self-compacting concrete with the high belite cement is able to reach sufficient strengths already after 1 day of hydration, with a low heat development and improved durability properties, compared to cem i. keywords: belitic cement, low heat cement, low heat concrete. 1. introduction new technical solutions aimed at building sustainable and durable structures need a synergy between the research on innovative binders, with reduced co2 emissions, and the development of the advanced concrete technology. today, the use of supplementary cementitious materials (scms) is the main strategy to lower the environmental impact of the cement and concrete production. however, the long-term availability of fly-ash and slag will drop in the next future and has to be considered. while, limestone is the most used scm being economic and widely available, but its use in concrete is an issue concerning durability requirements [1]. in this study, a high belite cement (hbc) is proposed as technical solution for concretes requiring low heat development and long service life. this type of binder is characterized by a reduction of co2 footprint around 8-10% when compared to conventional portland cement, thanks toa reduced limestone demand as raw material and a lower burning temperature needed for clinker production.. the larger amount of c-s-h gel developed by the belite phase than alite is the basis to expect an improvement in durability performance, a possible driving force to move from portland clinker to belite clinker or future blends with scms. the hbc (4540 cm2/g) used in this study was produced at industrial scale by buzzi unicem s.p.a. the c2s content is higher than 54 wt.-% and the heat of hydration is lower than 227 j/g during 7 days. performance and durability of the hbc were compared with those of cem i 52.5n (3600 cm2/g), , cem ii/b-ll 32.5r (5690 cm2/g), and cem iii/a 52.5n lh (4150 cm2/g), as references. 2. results 2.1. compressive strengths related to the heat of hydration an undoubted advantage in the use of hbc is represented by its very low heat of hydration; in fact, as shown in figure 1 it exhibits the lowest value between the different cements, also considerably lower than cem iii. this fact makes hbc highly interesting for mass concrete applications and for possible blends with portland clinker and scms when low heat cement is required. figure 1 shows also the comparison of the cements in terms of compressive strength, mortars were produced according to the en 196-1 standard. after 1 day hbc develops a slightly lower compressive strength than cem ii, due to the slower hydration rate of the c2s phase,and responsible for the low heat development. at 7 days the compressive strength of hbc and cem ii are almost comparable, while from 28 days on hbc exhibits higher results. 2.2. high belite cement for concrete at exposure classes xd, xs and xf the concretes were produced according to the boundary conditions dictated in the german institute for 245 https://doi.org/10.14311/app.2022.33.0245 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en s. irico, s. mutke, f. bertola et al. acta polytechnica ctu proceedings figure 1. comparison of the mortar compressive strength development correlated with the heat of hydration referred to the mass of cement. figure 2. compressive strength of the c1 concretes after 28 days of curing. construction technology (dibt) approval process for cements. two concrete compositions were considered in this study. the resistance to chloride penetration, according to the baw code of practice (similar to nt built 192), was performed on concrete c1 produced by using rhine gravel and sand with a16/b16 grading curve, 320 kg/m3 of cement, and a w/c ratio of 0.5. no plasticizers were used in order to avoid their influence. figure 2 shows the compressive strengths of concrete c1 produced with the different cements used at 28 days of curing; hbc shows exhibits a higher strength compared to cem ii, but lower than the other two cements. the resistance to the freeze/thaw cycles immersed in 3% sodium chloride solution, according to the cdf method (din en 12390-9), was instead performed on concrete c2 produced with: a16/b16 grading curve, 320 kg/m3 of cement , a w/c ratio of 0.5, and an air figure 3. chloride migration coefficients of c1 concretes. the solid square symbols show the compressive strength values at 28 days of hydration of mortars. void content of 4.5 ± 0.5 vol%. as shown in figure 3, hbc shows a higher resistance against chloride migration compared to cem i and cem ii, but lower than cem iii. the migration coefficient of hbc is low enough for hydraulic engineering applications (≤ 10 × 10−12 m2/s for xs1-2, xd1-2 or ≤ 5 × 10−12 m2/s for xs3 and xd3, in accordance with [2]). few information are reported in literature about the resistance of hbc to freeze/thaw cycles. the results, shown in the graph in figure 4, reveal that hbc behaviour is comparable to that of cem i; both cements are suitable for concrete applications with xf4 exposure class, presenting a scaling after 28 cycles lower than the threshold limit. on the contrary, both cem ii and cem iii show a weak resistance against 246 vol. 33/2022 durability of high belite cement figure 4. resistance to freeze/thaw attack with deicing salt (cdf method). figure 5. expansion due to sulfate attack at 20 ◦c and 5 ◦c. the stress induced by the 28 freeze/thaw cycles going beyond the threshold limit. these data confirm that the freeze/thaw resistance is a "weakness" property of limestone and slag cements, as well known. the resistance to freeze/thaw cycles is often an obstacle to the implementation of sustainable binders with lower clinker factor. 2.3. sulfate resistance the sulfate resistance of the cements was tested according to the sva flat prism (4×1×16 cm) method. the expansion of the prisms cured in a saturated ca(oh)2 solution was used as reference in order to evaluate the influence of sulfate attack at 20 ◦c and 5 ◦c. hbc has an excellent resistance against the aggression of sulfate, as confirmed by sui et al. [3], and by the experimental results in figure 5 where hbc exhibits the lowest expansion compared to the other cements. the resistance of portland cement exposed to sulfate solution is related, in general, to the c3a content, by the way a high amount of portlandite development can make concrete more vulnerable to sulfate attack, with the consequence formation of gypsum. the positive influence of the low c3s/c2s ratio (figure 5), often not considered in the interpretation of the results, is a significant factor in the choice of a sulfate resistant cement [4]. the very high expansion of the cem ii at 5 ◦c is probably due to the formation of thaumasite [5]. 2.4. accelerated carbonation in order to compare the depth of carbonation in the different cements considered, mortar samples were prepared in accordance with en 196-1, stored in water up to 7 and 28 days, and then exposed to accelerated carbonation (chamber with 4% of co2 at 20 ◦c, and 70% rh) for 28, 56, 90, 140 days. the carbonation depth was determined at each due date with 247 s. irico, s. mutke, f. bertola et al. acta polytechnica ctu proceedings figure 6. accelerated carbonation (4% co2 at 20 ◦c, and 70% rh) and influence on the strength development. the phenolphthalein test in accordance with rilem recommendation. in addition, prior to determine the co2 penetration, on the same samples, compressive strengths were also evaluated to understand the effect of the belite content and the influence of carbonation curing on the physicochemical properties (figure 6). the carbonation-cured hbc has a significant growth in the compressive strength during the exposure time. jang at al. [6] demonstrated that the co2 uptake capacity is proportional to the belite content and most of the belite phase is consumed by the carbonation reactions during the curing, resulting in the production of calcite. they observed that the carbonation curing of cement with high belite content leads to a more dense microstructure (with lower connected porosity) than those with high alite content. the hypothesis states that the carbonation reactions occurring at the inner layer of an unreacted belite particle are likely more rapid than that of the hydration reaction, resulting in the densification of the microstructure between the inner layer of c-s-h and the outer layer of unreacted belite particles, which significantly influences the strengths. the depth of carbonation of hbc is proved to be strongly dependent on its grade of hydration, in fact when cured just for 7 days it is considerably higher than when it is cured for 28 days. 2.5. high performance concrete with hbc a challenge in the application of hbc is represented by the slower strength development compared to conventional cements. research on cement science is addressing efforts in order to overcome this barrier to the implementation. on the other hand, it must be said that in case of new eco-binders or innovative cements, not included in the standard en 197-1, the evaluation of the performance considering standard mortars with a w/c of 0.5 (en 196-1) could be not sufficient to explore their potential. on this purpose, progresses in concrete technology play a relevant role in order to match the needs of implementing ecobinders characterized by lower environmental impact with requirements on compressive strength, rheology and durability. the behavior of a self-compacting concrete (scc) with 420 kg/m3 of hbc, 100 kg/m3 of fly-ash, and a w/c ratio of 0.36 was compared with a scc made with cem i 52.5r, as reference. the compressive strength of the hbc-scc after 1 day reaches 20 mpa, and after 7 days is comparable to that of cem i-scc. hbc has also the great advantage of a very low heat of hydration. the durability of the two sccs was evaluated in terms of freeze/thaw resistance and chloride penetration. the scaling due 248 vol. 33/2022 durability of high belite cement to freeze/thaw cycles (according to the cdf test) is in both cases very low: 450 g/m2 for cem i-scc, and just 192 g/m2 for hbc-scc. the chloride migration of hbc-scc is lower than that of cem i-scc, 2.2 × 10−12 m2/s and 4.7 × 10−12 m2/s, respectively. 3. conclusions the durability of concretes made with an industrially produced hbc was explored. the high resistance against chloride migration and freeze/thaw scaling of hbc, in comparison with cem i, cem ii, and cem iii (as reference), were evaluated on concretes produced according to the boundary conditions dictated in the german institute for construction technology (dibt) approval process for cements. hbc revealed to be suitable for exposure classes xs3, xd3 and xf4, thanks to its good resistance against freeze/thaw cycles, and its extraordinary ability to counteract the chloride penetration. almost no expansion was measured on hbc samples due to sulfate attack. the accelerated carbonation showed that hbc, thanks to the high amount of belite phase (higher than 54wt%), increases its compressive strengths during the co2 exposure. this interesting phenomenon needs to be deeply investigated at mineralogical and microstructural level, as well as the high durability properties of this cement related to the c-s-h gel formation. the durability of hbc is probably the main driving force for the future applications of this binder, together with its reduced environmental impact, and its low of heat of hydration. the study also demonstrated that when formulated as self-compacting concrete, with low w/c, hbc can reach already after 1 day enough compressive strength (higher than 20 mpa) to work efficiently. the potentialities of this binder need to be further explored also in blends with scms in order to develop sustainable and more durable future cementitious binders. references [1] w. neufert, i. reuken, g. weber g, et al. erforschung des dreistoffgemisches klinker, hüttensand und kalksteinmehl mit dem ziel der absenkung des klinkeranteils im zement zur verminderung der co2 emissionen in der zementproduktion, abschlussbericht reduzierung der umweltwirkung der betonbauweise, 2016. https://www.dbu.de/opac/ab/dbu-abschlussbe richt-az-28790-02.pdf. [2] bundesanstalt für wasserbau (hg.). bawmerkblatt chlorideindringwiderstand von beton (mcl), karlsruhe: bundesanstalt für wasserbau (baw-merkblätter, -empfehlungen und -richtlinien), 2012. https://henry.baw.de/bitstream/handle/20. 500.11970/102465/bawmerkblatt_chlorideindringwi derstand_mcl_2012_k.pdf?sequence=1&isallowed=y. [3] s. tongbo, f. lei, w. zhaijun, et al. properties of belite-rich portland cement and concrete in china. journal of civil engineering and architecture 9(4):384-392, 2015. https://doi.org/10.17265/1934-7359/2015.04.002. [4] a. neville. the confused world of sulfate attack on concrete. cement and concrete research 34(8):1275-96, 2004. https: //doi.org/10.1016/j.cemconres.2004.04.004. [5] m. collepardi. a state-of-the-art review on delayed ettringite attack on concrete. cement and concrete composites 25(4-5):401-7, 2003. https://doi.org/10.1016/s0958-9465(02)00080-x. [6] j. g. jang, h. k. lee. microstructural densification and co2 uptake promoted by the carbonation curing of belite-rich portland cement. cement and concrete research 82:50-7, 2016. https: //doi.org/10.1016/j.cemconres.2016.01.001. 249 https://www.dbu.de/opac/ab/dbu-abschlussbericht-az-28790-02.pdf https://henry.baw.de/bitstream/handle/20.500.11970/102465/bawmerkblatt_chlorideindringwiderstand_mcl_2012_k.pdf?sequence=1&isallowed=y https://doi.org/10.17265/1934-7359/2015.04.002 https://doi.org/10.1016/j.cemconres.2004.04.004 https://doi.org/10.1016/s0958-9465(02)00080-x https://doi.org/10.1016/j.cemconres.2016.01.001 https://doi.org/10.14311/app.2022.33.0092 acta polytechnica ctu proceedings 33:92–97, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague strength and durability characteristics of self compacting concrete (scc) with recycled aggregate and manufactured sand sashidhar chundupallea,∗, nirmala seeria, sudarsana reddy veerab, venkata ramana nelluric a jawaharlal nehru technological university anantapur, department of civil engineering, 28-3-322-2, sarada nagar ii cross, 515002 anantapur, india b priyadarshini engineering college, department of civil engineering, lakshmi nagar, ramachandrapuram-pudi rd, ramachandrapuram, 517561, tirupati, india c university b.d.t college of engineering, department of civil engineering, university bdt college of engineering, 577004 davanagere, india ∗ corresponding author: sashidhar.civil@jntua.ac.in abstract. the investigations on self compacting concrete (scc) with recycled coarse aggregate and manufactured sand (m-sand) were performed in this current study. the scc mixtures were produced with the recycled coarse aggregate of proportions of 0 to 100% with step increment of 25% with a ratio of water binder at 0.36. in addition, the mixtures were also prepared by mixing it with m-sand. the feasibility of utilizing the recycled coarse aggregate in scc was evaluated through strength and durability studies. the obtained results demonstrated that the scc produced from these materials can be effectively recommended for their usages in concrete industries. keywords: m-sand, mechanical properties, recycled coarse aggregate, self compacting concrete. 1. introduction self compacting concrete (scc) show superior rheological properties as compared to conventional concrete. these properties in scc are generally achieved due to higher ratio of cement paste to aggregate. accordingly, the scc was developed to overcome the problems associated with the labor quality and the noise and dust problems as well. the mechanical characteristics such as tensile and compressive strength of concrete evaluated with recycled aggregate will be lower than the typical concrete. however, this can be overcome by using superplasticizer [1, 2]. the studies conducted on the fresh and hardened scc using recycled coarse and fine aggregate confirmed that both the coarse as well as fine recycled aggregate can be effectively used for the production of scc [3, 4]. accordingly, in this study, an equal consistency was observed in scc mixtures when it is prepared with the recycled coarse aggregate with 0, 50 and 100% proportions as a substitute for the natural coarse aggregate [5]. regarding the fine aggregate, the manufacturing sand (commonly known as m-sand) can be a suitable alternative for river sand in concrete. the incorporation of m-sand contributes to increase in paste volume, which eventually helps for scc for its improved properties. it means that as compared to the river sand, the m-sand produces relatively high paste content, which is beneficial for scc [6]. the utilization of recycled coarse aggregate of 0 to 40% proportions (with a step increment by 10%) in scc as a substitute for the natural coarse aggregate showed relatively decreased mechanical strength as compared to those made with natural aggregate [7, 8]. on the other hand, it was reported that the utilization of fine and recycled coarse aggregate enhances the rheological and fresh properties of scc [9]. few authors [10, 11] have used the crushed red bricks and crushed ceramics materials as a replacement of coarse aggregate. however, the scc made by m-sand and fly ash contributed to raise of the strength characteristics of scc [12]. from the above discussion, it is observed that only a very few studies were carried out with amalgamation of coarse and fine aggregate as suitable replacements in scc. hence, the current experimental study is focused on enhancing the strength characteristics and durability of scc through the substitution of both coarse and fine aggregate, where these aggregate were substituted with recycled coarse aggregate and m-sand, respectively at the proportions of 0, 25, 50, 75 and 100%. 2. experiments and methods 2.1. materials in this current study, the conventional portland cement grade 53 with specific gravity (sg) of 3.05 and fineness value of 310 m2/kg, was used. similarly, the class f’ fly ash was found to be as per the astm c 618 standard with a relative density of 2.13 and fineness value of 360 m2/kg. the chemical ingredients of 92 https://doi.org/10.14311/app.2022.33.0092 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 33/2022 analysis of concrete and cement epd figure 1. photographic image of the experimental setup for rapid chloride permeability test. the cementitious materials are tabulated in table 1. natural granite was used as a coarse aggregate with a max. size of 1.2 cm with sg around 2.66 and the water absorption value of 0.53. however, the local demolished wastes were also used as the recycled aggregate, which was with a sg around 2.52 and water absorption value of 5.06. the local river sand with sg around 2.58 and water absorption value of 0.95 was employed as the fine aggregate components. the sg and water absorption value of m-sand used in the study are 2.46 and 1.20, respectively. a chemical based superplasticizer named polycarbolic ether was also used to achieve a good workability. chemical present cement fly ash value (%) sio2 19.79 65.6 al2o3 5.67 28.1 fe2o3 4.68 2.9 cao 61.81 0.95 mgo 0.84 1.55 so3 2.48 0.21 chloride 0.003 lime saturation factor 0.92 table 1. chemical compositions of scc. 2.2. mix proportions scc mixtures at proportions of 0 to 100% replacements (with step increment by 25%) with the recycled aggregate and m-sand were prepared, while keeping the ration of water binder (w b) constantly at 0.36. throughout this work, cement and fly ash was taken to be 351 and 150 kg/m3, respectively. 2.3. test methods in this current work, the scc prepared using the aggregate of recycled coarse and m-sand was tested as per the efnarc and is guidelines [13]. 2.3.1. split tensile and compressive strength test the split tensile and compressive strength characteristics of various mixtures were evaluated on the test setup with capacity of 2000kn compression testing machine with a least count of 1kn. 2.3.2. rapid chloride permeability test (rcpt) the rcp tests were conducted on the different concrete mixtures in order to investigate the developed concrete’ ability to resist the penetration of chloride ions. for this purpose, the samples were cut into cylindrical specimens and subjected to impressed voltage of 60v between the anode and cathode using a dc power source as shown in fig 1. the response current was monitored without any interruption for the period of 6 h at 30 min of interval time. 2.3.3. acid resistance test the resistance against the acid attack on scc specimens was conducted in accordance with the astm c 267 01. after the process of moist curing for 28 days, the weight of each specimen has been taken and then the specimens were immersed in 3% hydrochloric (hcl) acid solution. during this test, the changes in weight as well as the compressive strength characteristics of specimens were evaluated after 28 days and 56 days of immersion of the specimens in 3% hcl acid solution. 3. results and discussion 3.1. compressive strength analysis 3.1.1. compressive characteristic strength of rca in nca (natural coarse aggregate) concrete mix with 0% m-sand the compressive characteristic strength results of the developed various concrete mixtures are displayed in 93 s. chundupalle, n. seeri, s. r. veera, v. r. nelluri acta polytechnica ctu proceedings s. no mix type average strength (mpa) m0/days m25/days m50/days m75/days m100/days 7 28 7 28 7 28 7 28 7 28 1 r0 29.77 42.66 28.88 41.77 28.1 40.44 27.42 40.00 27.07 39.11 2 r25 28.27 38.88 27.11 38.66 26.66 38.00 26.22 37.55 25.86 36.6 3 r50 26.76 36.24 25.55 35.92 25.33 35.57 24.88 35.14 24.44 34.52 4 r75 25.00 33.77 23.88 33.33 23.53 32.88 23.35 32.44 23.00 32.00 5 r100 23.77 31.58 22.66 31.11 22.00 30.44 21.55 30.11 21.33 29.77 r0 first letter stands for rca and the suffix number indicates the % used m0 first letter stands for m-sand and the suffix number indicates the % used table 2. estimated compressive strength of various concrete mixtures. table 2. from the obtained results, it can clearly be noticed that the increasing concentration of recycled aggregate concrete considerably decreases the effective compressive strength continuously, which is also well agreed with the literature reports. further, it can also be noted that the compressive strength of 100% nca is maximum and the compressive strength decreases with increasing recycled coarse aggregate at both 7 and 28 days. this might be due to the poor quality of the recycled aggregate as compared to the natural aggregate. the percentage decrease in the compressive strength for 7 days sample with reference to the basic mixture is found to be 7.45, 13.43, 19.38 and 22.37% for 25, 50, 75 and 100% replacement of recycled coarse, respectively. similarly, the percentage decrease in the compressive strength for 28 days sample with reference to basic mix is estimated to be 8.86, 14.58, 20.83 and 23.62% for 25, 50, 75 and 100% replacement of recycled coarse, respectively. 3.1.2. compressive strength of m-sand substitution in fine aggregate with 0% substitution of recycled coarse the compressive investigation results of replacing the m-sand in fine aggregate are given in table 2. in this work, as aforementioned, the fine aggregate has been replaced by m-sand at different percentages such as 0, 25, 50, 75 and 100% and the percentage amount of recycled aggregate was taken to be 0%. from the test results of the obtained compressive strength, it is noticed that the increasing substitution of m-sand leads to a continuous decrement in the eventual compressive strength of the concrete mix. accordingly, the strength property of 28 days immersed 100% msand scc is estimated to be 29.77 n/mm2 and on the other hand, it can be seen that the compressive strength of cubes is decreasing with the increasing m-sand substitution in fine aggregate. this may be because of the reason that the crushing process of m-sand fundamentally affects its shape characteristic, where the grading of m-sand and the proportion of micro fines may not be the same as fine aggregate. the percentage decrease in the compressive strength of 28 days immersed samples with respect to basic mix is estimated to be 2.08, 5.20, 8.86, 6.23 and 8.32% for 25, 50, 75 and 100% replacement of recycled coarse, respectively. notably, this current study was essentially carried out to devise the possibility of replacing the 100% sand by m-sand. using m-sand, the water demand may increase due to its high fines; however, the fines in m-sand play role to increase in paste volume, which will be useful in the development of scc with improved properties. 3.1.3. compressive strength of recycled coarse aggregate concrete and substitution of m-sand in fine aggregate the compressive test results of the recycled coarse aggregate replaced in natural coarse aggregate and replacing m-sand in fine aggregate are shown in table 2. from the obtained results on compressive strength, it can be noticed that with increasing replacement of recycled aggregate and m-sand, the effectual compressive strength is decreasing continuously in 7 and 28 days immersed samples. however, it is worth to mention that the researchers were widely used different waste materials other than the m-sand in fine aggregate. the percentage decrease in the compressive strength for 7 days immersed samples with respect to reference mix was found to be 8.93, 14.91, 16.42 and 28.35% corresponding to 25, 50, 75 and 100% replacement of recycled coarse aggregate and m-sand in fine aggregate. similarly, the percentage decrease in the effective compressive strength for 28 days immersed samples with respect to reference mix was estimated to be 9.37, 15.61, 19.78 and 30.21% corresponding to 25, 50, 75 and 100% replacement of recycled coarse aggregate and m-sand in fine aggregate. however, for the mixture having 100% of recycled aggregate and 100% of m-sand, the compressive strength value was estimated to be 29.77 mpa, which is almost equal to m 25 grade concrete. therefore, it is suitable and can possibly be recommended to utilize 100% recycled aggregate and m-sand in producing the scc with improved properties. 94 vol. 33/2022 analysis of concrete and cement epd s. no mix type average strength (mpa) m0 m25 m50 m75 m100 1 r0 3.65 3.61 3.56 3.44 3.4 2 r25 3.49 3.45 3.42 3.34 3.25 3 r50 3.38 3.36 3.33 3.27 3.15 4 r75 3.25 3.22 3.19 3.16 3.05 5 r100 3.19 3.16 3.13 3.05 2.94 table 3. estimated split tensile strength of the various concrete mixtures. mix m0 m25 m50 m75 m100 remarks astm c1202 charge passed (coulomb) chloride penetrability r0 1155.00 1197.00 1246.00 1274.00 1316.00 > 4000 high r25 1217.00 1259.00 1300.00 1318.31 1357.00 2000 − 4000 moderate r50 1273.00 1310.00 1350.12 1368.24 1404.00 1000 − 2000 low r75 1326.45 1370.00 1400.00 1419.54 1446.00 100 − 1000 very low r100 1384.00 1420.00 1449.41 1470.90 1491.00 < 100 negligible table 4. estimated rcpt value of scc mixtures after 56 days. 3.2. split tensile strength 3.2.1. split tensile strength of rca in nca concrete mix with 0% m-sand from test results of split tensile strength, it can clearly be noticed that the increasing replacement of recycled aggregate continuously leads to the decrement in the split tensile strength as listed in table 3. as per the literature reports, researchers typically observed a decrement in the strength when the % of recycled aggregate is increased. however, m. seethapathi et al [12] encountered a kind of reverse trend and reported that the split tensile strength were increased with increasing percentage of the recycled aggregate. but there were no specific reasons mentioned for the observed reverse trend in their studies. in this present study, the mixture r0m0 was taken as a reference mix for all the mixtures. the split tensile strength value of the 28 days immersed 100% recycled coarse aggregate was found to be 3.19 n/mm2. it was observed that the split tensile strength was noted to be decreasing with increasing recycled aggregate replacements in 28 days immersed samples. this could be ascribed to the low quality of recycled aggregate as compared to the natural coarse aggregate. the decrement % in the split tensile strength for 28 days immersed samples with respect to reference mix was estimated to be 4.58, 7.98, 12.30 and 14.42% for 25, 50, 75 and 100% replacement of recycled coarse, respectively. 3.2.2. split tensile strength of m-sand in fine aggregate with 0% recycled coarse aggregate the split tensile strength of m-sand replaced in fine aggregate samples is displayed in table 3. it can be noticed that the increasing replacing percentage of m-sand, the split tensile strength is continuously decreasing. the split tensile potency of 28 days immersed 100% m-sand in fine aggregate was estimated to be 2.81 n/mm2. it can clearly be noticed that the split tensile is decreasing with increasing rca replacement ratios in 28 days immersed samples. this could be due to the relative improved quality of msand that varies with river sand. the percentage decrease in split tensile for 28 days immersed materials with respect to the reference mixture r0m0 is found to be 1.10, 2.52, 6.10 and 12.30 corresponding to 25, 50, 75 and 100% replacement of m-sand in fine aggregate. 3.2.3. split tensile strength of the recycled coarse concrete and replacement of m-sand in nca and fine aggregate the split tensile strength results of the recycled coarse aggregate replaced in the natural coarse and m-sand replaced fine aggregate are presented table 3. it can be noticed that the increasing replacing % of recycled coarse aggregate and fine aggregate by msand continuously decreases the split tensile strength in 28 days immersed samples. notably, no literature reports are available studying the replacement of rca in nca and m-sand in fine aggregate. the percentage decrease in split tensile for 28 days immersed samples with respect to the reference mixture r0m0 is found to be 5.79, 9.60, 15.50 and 31.76% corresponding to 25, 50, 75 and 100% replacement of recycled coarse aggregate and m-sand in fine aggregate samples. however, it is noteworthy that the mixture having 100% recycled aggregate and 100% m-sand, the split tensile value was estimated to be 2.77 mpa. therefore, it can possibly be recommended to utilize the 50% recycled aggregate and m-sand in producing the scc towards having the improved split tensile 95 s. chundupalle, n. seeri, s. r. veera, v. r. nelluri acta polytechnica ctu proceedings mix m0 m25 m50 m75 m100 before after before after before after before after before after r0 29.77 26.40 28.88 25.46 26.22 23.06 25.33 22.09 24.00 20.90 r25 27.74 24.17 26.94 23.46 24.94 21.65 24.00 20.80 22.67 19.62 r50 25.77 22.38 24.94 21.60 23.21 20.04 22.54 19.41 21.34 18.37 r75 24.00 20.75 23.21 20.04 21.67 18.70 20.93 18.02 19.94 17.10 r100 22.27 19.18 21.40 18.42 20.07 17.25 19.53 16.75 18.74 16.05 table 5. compressive strength of scc mixtures after 56 days of acid immersion. strength. 3.3. rapid chloride permeability test (rcpt) table 4 displays the rapid chloride (cl) permeability characteristics of scc mixtures after 56 days cured samples. from the table 4, it can be noticed that the increasing percentage of recycled aggregate leads to the increment in the rcpt values of scc mixtures. the permeability value of scc mixture without any recycled coarse aggregate replacement was found to be 1155 and with 100% replacement, the value was increased to 1384. the decrease in permeability values could be attributed to the insufficient properties of the recycled coarse s as compared to the natural coarse aggregate. it could also be observed that with increasing replacements of m-sand in fine aggregate, the permeability is also increasing in the samples. accordingly, the permeability value of the mixture with 100% replacements of fine aggregate with msand was estimated to be 1316. it is also observed that permeability is found to be increasing when the percentage of recycled coarse and m-sand is replaced at a time in natural coarse aggregate and fine aggregate. the permeability value with 100% replacement of recycled coarse aggregate and m-sand was found to be 1491. 4. conclusions from this current study and the obtained results, the following conclusions could be arrived; • from the obtained results of the present investigation, it can be recommended that the recycled coarse aggregate and m-sand can be effectively used in developing scc with relatively improved properties. • the compressive strength of scc is decreasing with increasing % replacements of recycled coarse aggregate in natural coarse aggregate and m-sand in fine aggregate. the compressive strength of the mixture having 100% recycled aggregate and 100% m-sand was estimated to be 29.77 mpa, which is almost equal to m 25 grade concrete’s strength • the split tensile strength of scc is decreasing with increasing percentage replacement of recycled coarse aggregate in natural coarse aggregate and m-sand in fine aggregate. the split tensile strength of the mixture having 100% recycled aggregate and 100% m-sand was estimated to be 2.77 mpa. • permeability values are found to be increasing as the replacement of recycled coarse aggregate and m-sand in natural coarse aggregate and fine aggregate are increasing. • when immersed in hcl, the compressive properties of scc were decreasing when the replacements of recycled coarse aggregate and m-sand in natural coarse aggregate and fine aggregate are increasing. • from the strength properties observed in this study, the scc can be effectively produced with recycled coarse aggregate replacement as well as m-sand replacement. references [1] j. m. v. gómez-soberón. porosity of recycled concrete with substitution of recycled concrete aggregate. cement and concrete research 32(8):1301-11, 2002. https://doi.org/10.1016/s0008-8846(02)00795-0. [2] l. evangelista, j. de brito. mechanical behaviour of concrete made with fine recycled concrete aggregates. cement and concrete composites 29(5):397-401, 2007. https: //doi.org/10.1016/j.cemconcomp.2006.12.004. [3] h. okamura, m. ouchi. self-compacting concrete. journal of advanced concrete technology 1(1):5-15, 2003. https://doi.org/10.3151/jact.1.5. [4] s. c. kou, c. s. poon. properties of self-compacting concrete prepared with coarse and fine recycled concrete aggregates. cement and concrete composites 31(9):622-7, 2009. https: //doi.org/10.1016/j.cemconcomp.2009.06.005. [5] z. j. grdic, g. a. toplicic-curcic, i. m. despotovic, et al. properties of self-compacting concrete prepared with coarse recycled concrete aggregate. construction and building materials 24(7):1129-33, 2010. https: //doi.org/10.1016/j.conbuildmat.2009.12.029. [6] p. nanthagopalan, m. santhanam. fresh and hardened properties of self-compacting concrete produced with manufactured sand. cement and concrete composites 33(3):353-8, 2011. https: //doi.org/10.1016/j.cemconcomp.2010.11.005. [7] k. c. panda, p. k. bal. properties of self compacting concrete using recycled coarse 96 https://doi.org/10.1016/s0008-8846(02)00795-0 https://doi.org/10.1016/j.cemconcomp.2006.12.004 https://doi.org/10.3151/jact.1.5 https://doi.org/10.1016/j.cemconcomp.2009.06.005 https://doi.org/10.1016/j.conbuildmat.2009.12.029 https://doi.org/10.1016/j.cemconcomp.2010.11.005 vol. 33/2022 analysis of concrete and cement epd aggregate. procedia engineering 51:159-64, 2013. https://doi.org/10.1016/j.proeng.2013.01.023. [8] s. santos, p. da silva, j. de brito. mechanical performance evaluation of self-compacting concrete with fine and coarse recycled aggregates from the precast industry. materials 10(8), 2017. https://doi.org/10.3390/ma10080904. [9] e. gueyisi, m. gesoglu, z. algin, et al. rheological and fresh properties of self compacting concrete containing coarse and fine recycled concrete aggregate, cement and concrete composites 113:622-630, 2016. [10] m. m. kamal, m. a. safan, z. a. etman, et al. behavior of recycled self-compacting concrete. erj engineering research journal 36(3):317-27, 2013. https://doi.org/10.21608/erjm.2013.67433 . [11] m. k. mounir, a. s. mohamed, a. e. zeinaband, et al. effects of polyprophelene fibers on development of fresh and hardened properties of recycled self compacting concrete, international journal of engineering and advanced technology 2(5):86-94, 2013. [12] a. tamilarasan, s. sankaran. experimental investigation of mechanical and durability properties of self compacting concrete with fly ash and msand, national conference on recent advances in coomunication, computation, electrical science and structures (ncraccess 2015), p. 11-16, 2015. [13] m. seethapathi, s. r. r. senthilkumar, k. chinnaraju. experimental study on high performance self compacting concrete using recycled aggregate, journal of theoretical and applied information technology 67(1):84-90, 2014. 97 https://doi.org/10.1016/j.proeng.2013.01.023 https://doi.org/10.3390/ma10080904 https://doi.org/10.21608/erjm.2013.67433 acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.38.0678 acta polytechnica ctu proceedings 38:678–686, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague circular design strategies through additive manufacturing: modom, a “circular building” housing model francesca giglio, massimo lauria, sara sansotta∗ university mediterranean of reggio calabria, darte department architecture and territory, via dell’università, 25, 89124 reggio calabria, italy ∗ corresponding author: sara.sansotta@unirc.it abstract. the doubling of the global use of raw materials in the last century is an established environmental emergency due to an all too common “take – make – dispose” linear development model. strategic plans within the green deal, such as the circular economy action plan, highlight the key role of building design as an enabling driver for process cyclicality. against this backdrop, the paper describes the results of an experimental research project aimed at the technological design of a “circular” housing model. the integration of circular economy principles into the design process was pursued through the technology transfer of additive manufacturing principles as an enabling technology of industry 4.0. the research is based on the first phase of critical analysis of two types of case studies: one referring to circular buildings, the other referring to 3d printed buildings, through a common reading method based on “circular” indicators extrapolated from the current literature. the evaluation of the results obtained determined the prerequisites for designing a replicable circular building model by 3d printing using a lignin-based biopolymer. the adoption and combination of these seemingly opposing themes was a key strength and asset to the project in terms of benefits such as energy savings, lead time, and cost savings at all life cycle stages. keywords: additive manufacturing, circular building, bio-composite. 1. introduction the doubling of the global use of raw materials in the last century is now an acknowledged environmental emergency due to a linear “take – make – dispose” development model that is still too widespread [1]. strategic plans within the green deal, such as the circular economy action plan, highlight the key role of building design as an enabling driver for process cyclicality. several factors, including the consequences of the construction sector’s responsibility for co2 emissions, have prompted the european community to propose a number of resolution strategies through guidelines and directives, such as agenda 2030 and the paris agreement, to raise awareness of sustainable development. in line with the goals set by systemiq and the ellen macarthur foundation [2], including the design and production of circular buildings and the closing of construction cycles, there is an urgent need to address the total impact of emissions, resource consumption and waste generation in the construction sector. in this sense, the strategies of the circular economy and the action plan focus on responsible design as a key tool for the optimal use of resources [3]. the change of course towards circular processes is implemented by taking the linear model and changing the final step of waste into a resource, thinking in phases of reuse/recycling of products, components, elements and materials [4]. emphasising the relationship between the circular economy and sustainability concepts, the influence of digital technologies through industry 4.0 and life cycle assessment breaks through [5]. the transition to a circular economy model can be driven by the process of industrial digitisation that enhances the connection of products and factories to achieve a more sustainable production cycle, both economically and environmentally. these principles are translated thanks to the unbreakable link between the dictates coming from the circular economy and the innovations coming from the technological era in which we find ourselves. the current historical context is influenced by innovations from the fourth industrial revolution, focusing on digitalisation/automation processes. the relationship between the circular economy and kets undoubtedly also involves the construction sector, posing a significant challenge for the latter’s future, given its considerable impact on the environment [6]. the link with the circular economy can be made according to specific common goals, such as reducing waste, rethinking production chain processes and using technologies derived from industry 4.0 such as iot, big data, 3d printing, additive manufacturing techniques etc. broadly speaking, additive manufacturing is “a process of joining materials to make objects from 3d model data, usually layer upon layer, as opposed to subtractive manufacturing methodologies” [7]. in de678 https://doi.org/10.14311/app.2022.38.0678 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 38/2022 circular design strategies through additive manufacturing . . . sign, architect frank duffy first proposed the concept of building “in layers” in the 1970s. stuart brand developed the concept in the 1990s, and today it is more widely known, though not yet a widely adopted method, for carrying out a circular-compliant building design [8]. brand’s model considers the building to be made up of six separate and interconnected layers, each with its characteristics and life span: site, structure, skin, services, space and things [9]. in this sense, the layer-by-layer fabrication capability of 3d printing translates stuart brandt’s concept well. it presents unique advantages in the circular construction approach, such as rationalisation of connections, reduced construction time, efficient use of energy, and reduction of production waste. the relationship between circular economy and additive manufacturing is emerging as a new systemic logic to define new opportunities for circular sustainability. in this scenario, the paper presents the results of experimental university research whose objective concerns the technological design of a “circular” housing model, applying the principles of additive manufacturing and relating them to the concept of circular building. starting from the results of the systematic work of retrieval, classification and interpretation of the themes and case studies, the design of a housing micro-architecture to be 3d printed through the use of bio-based material, a lignin-based biopolymer, was initiated. in response to the increasingly common need for temporary solutions that incorporate the possibility of being assembled/disassembled easily and in a short time. the research became a moment to reflect on the social, cultural and environmental effects of designing, building and managing sustainable technological solutions through an innovative concept of low-cost housing with expandable units or with local materials. finally, the research became an opportunity to reflect in applicative terms on the low-cost approach in a broader vision, considering the entire life cycle of buildings and focusing on the circular economy theme pursued through additive manufacturing. 2. the scientific context: circular economy enabled by additive manufacturing from the scenario of profound environmental emergency described above, the need for a transition to a circular model is driven to a large extent by the lack of finding resources [10], so it becomes necessary to start thinking about processes whose aim is to reuse/reduce and finally recycle elements, products and materials already placed in the production cycle. this means that all resources extracted from the ecosystem must be exploited within no-waste processes, avoiding the removal of additional assets, thus maximising reuse practices at all scales. a definition of a circular building is provided in the report “a framework for circular buildings: indicators for possible inclusion in breeam” [11] – in line with the transition agenda for circular buildings in the netherlands. thus, a circular building is defined as “a building that is developed, used and reused without unnecessary depletion of resources, [. . . ]. it is constructed in an economically responsible way and contributes to the well-being of people and the biosphere [. . . ]. the technical elements can be dismantled and reused, [. . . ]”. for these purposes, brandt’s conceptual model mentioned above can provide designers with a useful means of shaping their decisions to conform to circularity. a package of specific requirements can be associated with the function that each appropriately defined layer performs within the building. this allows a very effective selection of the characteristics of the constituent materials, focusing exclusively on their relevant purposes [12]. by identifying the different layers, very specific strategies can be adopted for each of them. this helps a lot in eliminating redundancy both in the design of the building and in the production material. in addition, building elements with more specific characteristics increase their residual value, making their reuse opportunities easier to determine and recycling cheaper to perform. building in layers leads to focusing on each element and its specific characteristics when conceiving and designing a building, thus shaping the assembly components separately – making them easier to remove individually, even at different times, depending on their different lifespans. finally, layered construction has implications with respect to reducing production costs [13]. in this sense, additive manufacturing processes have the characteristics to support circular economy initiatives. additive manufacturing reduces the waste generated during production and saves input materials [14]. furthermore, it supports reuse and recycling processes by using recycled materials as inputs for the production process. additive manufacturing also encourages repair and refurbishment processes and extends the product life cycle by printing on-site the necessary parts for broken/unfunctional products. this transition reconfigures the supply chain to achieve the ideal of a circular economy together with improved resource efficiency [15]. ultimately, the concept of layering, transferred to the field of additive manufacturing, can be further synthesised and made “circular” by the monomatericity of the layers that make up the 3d printed construction solution. advances in materials science and technology offer designers a wide range of more sustainable materials for a multitude of applications. material innovations with respect to 3d printing extrusion processes are moving towards supporting the use of biomaterials for production, which, through the printing methodology, are able to play both structural and non-structural roles. interest in using biomaterials for 3d printing is ex679 f. giglio, m. lauria, s. sansotta acta polytechnica ctu proceedings figure 1. methodological flow-chart. panding as several studies have explored that the use of biomaterials requires less material usage and with zero waste, further increasing the environmental benefit [16]. the focus on the design and production of 3d-printed circular buildings using bio-based materials, therefore, provides an interesting scenario for the sustainable production and recycling or recycling of waste materials such as wood flour, rice husks or miscanthus fibre etc. the decomposition process of the biocomposite can be understood as the reverse of the production process, just as with cultivated wood, and can also be disposed of by burning without producing additional co2. the latter is in line with the ellen mac arthur foundation’s definition of circularity, as the preservation of the value of buildings and their components is guaranteed by optimising the cycles of use and reuse with minimum use of virgin resources. furthermore, the importance of both technical and biological cycles is emphasised [11]. according to the biological metaphor, they “melt without a trace”, as what disappears reappears embodied in a new element without any waste throughout this process. in this scenario, the goal coming from the circular economy of initiating intelligent production based on the reduction of resource consumption and waste production through the optimisation of industrial processes is translated. 3. the modom, a “circular building” housing model the research was developed by adopting a deductive schematic scalar methodological procedure, moving from the definitions of the characteristics/qualities that identify a circular building to the construction of the variables that can be implemented through 3d printing processes. in this phase, a theoretical reference model was developed for the purposes of low-cost temporary housing solutions. specifically, the study is structured in three phases: (i) the adoption of research criteria through circular qualities application and case studies; (ii) the experimentation; (iii) the evaluation of the potential impact of circular building on cost scenarios (figure 1). 3.1. a research criteria: circular qualities application and case studies in the first instance, the research aimed to identify the relationships between circular economic and industry 4.0 enabling technologies within the construction sector, as described in the paragraph “circular economy enabled by additive manufacturing”. the need to implement criteria and indicators to assess the circular potential of technical options on the one hand, and 680 vol. 38/2022 circular design strategies through additive manufacturing . . . their efficiency on the other, to optimise resource use and reduce waste was highlighted. the different declinations of circular design have been analysed about design strategies, theories, approaches and principles that may respond to those of the circular economy, even if not directly derived from them. starting with experiments strongly related to circular design and moving on to experiments from additive manufacturing, the research identified some case studies, subsequently analysed using common reading indicators, deduced from the current literature in the sector, the circular design qualities1. computerised sheets have been defined against sixteen circularity criteria. on the one hand, these aim to measure the circularity of a building product, and on the other hand, to demonstrate the added value in the revision framework from the beginning of the design process to the decommissioning phase. these criteria are recurrent when strongly related to design for reuse, design for longevity and design for disassembly. ultimately, any building project that meets these requirements can be defined as circular. in a perspective that aims to outline the experiences and principles related to sustainable building concepts, it is necessary to critically describe, through some of the circular design qualities prepared by vub, the experimentation of the first prototype of “circular building”, carried out by arup associates, during the london design festival in 2016. the experimentation aims to test the maturity of circular economic thinking within the construction industry. the circular building was constructed using rented rather than purchased materials and products. every part of the building can be removed with minimal damage, reused, regenerated or recycled at the end of its life. the use of mechanical and pressure connections is in line with the construction solutions adopted, allowing it to meet the criteria of “independent” and “reversible” and thus facilitating the deconstruction process. in this sense “circular building” becomes a promoter of and responds to indispensable qualities for a circular model, such as the possibility of being easily disassembled, thanks to the non-use of resins and the use of so-called reversible elements. the evaluation criteria highlighted how the variables chosen in the design are closely related to determining the levels of circularity, such as the proportion of materials used, reversibility strategies and the circularity of resources. within the category of purely “circular” experiments is the example of the “people’s pavilion” designed by the bureau sla and overtreders w team 1the bbsm research was conducted by vub (vrije universiteit brussel) architectural engineering and was financed by european regional development fund (erdf) and the brussels-capital region. project partners: uclouvain, rotor, belgian building research institute. source: https: //www.vub.be/arch/page/circulardesign. although less relevant for our purposes, the additional circular design qualities of the belgian study identified are: safe and healthy, durable, manageable, accessible, independent, compatible, multi-purpose, varied. using 100 % materials borrowed from traditional suppliers, manufacturers, and local inhabitants. the temporary use of the pavilion meant that the disassembled elements were returned intact to the suppliers, providing for a future phase of assembly/use of the parts and designing, ex-ante, the “reused” criterion. this last phase is in line with the ellen mac arthur foundation’s definition of circularity, as the preservation of the value of buildings and their components is guaranteed by optimising the cycles of use and reuse with minimum use of virgin resources [17]. the “u build – box house” project was analysed to conclude the cycle of circular experiments. the system is based exclusively on a flat-pack kit made up of wooden parts and stems from studio bark’s desire to make construction “truly accessible” to the public. the components can be quickly fitted together to assemble the structure of a building and easily disassembled, recycled or reused at the end of the building’s life. a further feature of the project is producing the box with local materials while also paying attention to the principle of resourcing [18]. in this first systemic tranche, the methodology of reading has shown how circular buildings respond to the circular indicators “reversible” and “durable”. concepts that are widely addressed in all circular design strategies. the circular design qualities have been applied as reading indicators to experiments coming from additive manufacturing. this method of reading aims to show that even a project realised with construction strategies different from the traditional ones, such as those coming from additive manufacturing, can be read through circularity indicators, underlining the link between circular economy and 3d printing. specifically, the following experiments were analysed: “office the future”, “urban cabin”, ““passivedom”, and finally “amie”. this paragraph aims not to describe the design choices but rather to offer a critical analysis of the projects mentioned, focusing on the circular qualities that can be found in projects from additive manufacturing. suppose in the case of circular buildings, the recurring circular design qualities lie in the “reversible” and “durable” of the material resources used, in the case of 3d printed buildings, the systematic quality is “100 % pure”, thanks to the monomatericity that distinguishes the experiments. another fundamental requirement for additive manufacturing construction strategies is to minimise the number of connecting elements, which are sometimes moulded and made dry-assemblable, thus reducing construction waste. the analysis of the case studies concerning qualitative indicators and the study of research topics related to the principles of the circular economy and innovations from industry 4.0 led to a design reflection. “reversible”, “durable”, and “100 % pure” requirements were identified as key indicators for a possible temporary housing solution. 681 https://www.vub.be/arch/page/circulardesign https://www.vub.be/arch/page/circulardesign f. giglio, m. lauria, s. sansotta acta polytechnica ctu proceedings figure 2. composition diagram, modom circular housing model. 3.2. the experimentation the experimentation is an opportunity for research activity, still in progress by the scientific heads of the study. the first results concern developing a housing system, a micro-architecture designed as an aggregable and temporary housing system. starting from the archetypal home, modom is a 3d printed housing model using a lignin-based biopolymer whose properties are similar to those of wood, but whose workability can be likened to that of a 3d printable plastic material. lignin is, in fact, a rigid timber component discarded in the pulp and paper production process. the process combines recovered lignin with natural fibres – such as flax, hemp or other fibre plants – to create a composite processed at high temperatures in the same way as synthetic thermoplastics. the circular building consists of 6 stackable modules in 2 transport kits with three types of dimensions. the width of each module is 1 500 mm, the height of which varies according to the kind of module a, b and c, 5 200, 4 600 and 4 000 mm, respectively (figure 2). the composition-design scheme varies according to the functionality of each module. the kits will be prepared with a special module, including the off-grid systems for the toilet. for greater space flexibility, these kits can be enriched with two additional “extra” modules corresponding to two shaded loggias to filter the perception of inside outside (figure 3). modom is assembled without the use of connecting elements. in fact, during the printing phase, a malefemale connection system is installed directly on the printed module. in addition, the technological solution chosen avoids the installation of foundations on which to erect the architecture. in fact, each module is provided with grooves in the lower part, which allows the module to be raised and prevents humidity from rising (figure 4). stuart brand’s “layer model” was translated through a careful stratigraphy, including the skin of the circular building to the cavity prepared for the systems (figure 5). the printing methodology, based on shell&infill, allows to configure through a single material its technological-structural properties not only through the percentage of “infill” filling of the material (0–100 %) but also through the type of “pattern” filling that will be used (honeycomb, linear, triangular, grid, etc.) [19]. this makes it possible to define whether the moulded element can perform structural functions or act, for example, as a container for implants. 3.3. potential impact of circular building on cost scenarios adopting the design choices mentioned in the previous paragraph has highlighted a further key aspect of the experimentation regarding the impacts on the entire life cycle. we refer to the construction technologies 682 vol. 38/2022 circular design strategies through additive manufacturing . . . figure 3. axonometric projection of modom modules. figure 4. cross-section and detail of modom. 683 f. giglio, m. lauria, s. sansotta acta polytechnica ctu proceedings figure 5. stuart brand’s six-layer model applied to the ’shell&infill’ principle of 3d printing. through the adoption of 3d printing and concerning the modularity of the designed system. according to a mckinsey report [20], modular construction can speed up construction projects by up to 50 % and potentially reduce project costs by 20 %. if in a first phase, the trial shows a high production cost, due to the machines that do the printing, and a longer design time, time and costs are halved compared to the cost items on “foundations”, “off-site process” and “maintenance”. modom does not require foundations, as the modules are designed to be lightweight for transport. the off-site production process is much faster than the equivalent on-site construction process. the moulded modules, including connection systems, will be installed on-site, allowing for ease of assembly and reducing assembly/disassembly time. in this sense, the potential of additive manufacturing significantly reduces the need for resources by exploiting the capabilities of materials that could not be obtained through conventional processes. in experimentation, the concepts of the flexibility of spaces and reversibility of the construction process substantiate the project as part of an ecosystem; their objective is to establish a circular economy process. finally, the maintenance aspect has a significant impact on the percentage of costs that would be incurred in a conventional building. in the 3d printing process, the elements that need to be replaced or repaired are not already placed on the market but are themselves printed for the specific case. this means that parts and resources that would otherwise be taken from the ecosystem are not introduced into the production cycle. 4. conclusion the lines of research that revolve around the themes dealt with can represent a fertile field of experimentation of the knowledge of methodologies and operational tools necessary to develop models of housing innovation integrated by a different approach to the management of the project process. at the end of the experimentation, the project was subjected to the same evaluation of the circularity used to read the case studies. this resulted in its validation through a feedback process, through the circular design qualities (figure 6). more specifically, it should be noted that modom responds to the circular indicators of: • “reversible”, through the design of male-female connections without the use of adhesives or sealants, allowing this to be completely disassembled; • “100 % pure”, the entire structure is moulded through the lignin-based biopolymer with structural stiffness capabilities; • “renewed”, the choice of material has a significant influence on the circularity characterisation, the 684 vol. 38/2022 circular design strategies through additive manufacturing . . . figure 6. critical analysis of the “modom” through circular design qualities (vub). initial costs furniture operating costs maintenance and repair end of life total traditional construction 550.00 €/m2 110.00 €/m2 450.00 €/m2 700.00 €/m2 190.00 €/m2 2 000.00 €/m2 modom 830.00 €/m2 90.00 €/m2 280.00 €/m2 200.00 €/m2 100.00 €/m2 1 500.00 €/m2 table 1. a quantity of life cycle cost. lignin, in fact, comes from a recycling process of production waste; • “durable” through the ability of the material to maintain the characteristics for which it was designed over time. a further aspect to be considered for the design of a circular building is that the construction solution using 3d printing can significantly impact the cost of the entire life cycle. in anticipation of the production of its full-scale prototype, the evaluation carried out on the project shows how modom can guarantee savings of up to 25 % of the total cost compared to construction using traditional technologies. based on data from technical literature (regarding the costs of traditional construction) and empirical data (regarding the costs of buildings realised with the additive manufacturing technique) [21], the following parametric cost quantities per m2 in the life cycle were defined (table 1). the increase in initial costs is amortised during the extended life cycle phase. specifically, the costs are amortised during the use phase, both in management and repair, thanks to the connection techniques and systems that have a significant impact, together with the material used. finally, the estimation accounts for the 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[2022-01-18]. https://www.mckinsey.com/businessfunctions/operations/our-insights/modularconstruction-from-projects-to-products [21] l. di angelo, p. di stefano, e. guardiani. search for the optimal build direction in additive manufacturing technologies: a review. journal of manufacturing and materials processing 4(3):71, 2020. https://doi.org/10.3390/jmmp4030071 686 https://eur-lex.europa.eu/legal-content/en/txt/?uri=com%3a2019%3a640%3afin https://eur-lex.europa.eu/legal-content/en/txt/?uri=com%3a2019%3a640%3afin https://doi.org/10.1007/978-3-030-55969-4 https://doi.org/10.1016/j.spc.2020.10.005 https://doi.org/10.3390/su12187659 https://assets.website-files.com/5d26d80e8836af2d12ed1269/5dea6b3713854714c4a8b755_a-framework-for-circular-buildings-breeam-report-20181007-1.pdf https://assets.website-files.com/5d26d80e8836af2d12ed1269/5dea6b3713854714c4a8b755_a-framework-for-circular-buildings-breeam-report-20181007-1.pdf https://assets.website-files.com/5d26d80e8836af2d12ed1269/5dea6b3713854714c4a8b755_a-framework-for-circular-buildings-breeam-report-20181007-1.pdf https://assets.website-files.com/5d26d80e8836af2d12ed1269/5dea6b3713854714c4a8b755_a-framework-for-circular-buildings-breeam-report-20181007-1.pdf https://assets.website-files.com/5d26d80e8836af2d12ed1269/5dea6b3713854714c4a8b755_a-framework-for-circular-buildings-breeam-report-20181007-1.pdf https://assets.website-files.com/5d26d80e8836af2d12ed1269/5dea730af4e45c41063c7e21_cop-construction-report-20190116.pdf https://assets.website-files.com/5d26d80e8836af2d12ed1269/5dea730af4e45c41063c7e21_cop-construction-report-20190116.pdf https://assets.website-files.com/5d26d80e8836af2d12ed1269/5dea730af4e45c41063c7e21_cop-construction-report-20190116.pdf https://assets.website-files.com/5d26d80e8836af2d12ed1269/5dea730af4e45c41063c7e21_cop-construction-report-20190116.pdf https://doi.org/10.1016/s0010-4485(97)00083-3 https://doi.org/10.3390/su13010234 https://doi.org/10.3390/buildings12010053 https://doi.org/10.1016/j.promfg.2018.02.101 https://doi.org/10.1109/tvcg.2017.2655523 https://www.mckinsey.com/business-functions/operations/our-insights/modular-construction-from-projects-to-products https://www.mckinsey.com/business-functions/operations/our-insights/modular-construction-from-projects-to-products https://www.mckinsey.com/business-functions/operations/our-insights/modular-construction-from-projects-to-products https://doi.org/10.3390/jmmp4030071 acta polytechnica ctu proceedings 38:678–686, 2022 1 introduction 2 the scientific context: circular economy enabled by additive manufacturing 3 the modom, a ``circular building'' housing model 3.1 a research criteria: circular qualities application and case studies 3.2 the experimentation 3.3 potential impact of circular building on cost scenarios 4 conclusion references https://doi.org/10.14311/app.2022.33.0518 acta polytechnica ctu proceedings 33:518–524, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague life cycle assessment of carbon concrete composites: a circular economy path beyond climate mitigation? christoph scopea,∗, edeltraud guentherb, torsten mieleckec, julia schuetzc, eric muendecked, konstantin schultzed a technische universität dresden, chair of sustainability management and environmental accounting, 01187 dresden, germany b unu-flores, unu institute for integrated management of material fluxes and of resources, 01067 dresden, germany c lcee life cycle engineeering experts gmbh, 64295 darmstadt, germany d ingenierbüro grassl gmbh, 13158 berlin, germany ∗ corresponding author: christoph.scope@tu-dresden.de abstract. sustainable construction and materials play an ever-important role to stay within our planetary boundaries. in support, innovative carbon concrete composites (ccc) promise significant raw material savings by integral design. we aim to illustrate current environmental hotspots and a feasible recycling scenario of ccc that meets circularity requirements. we modelled a cradle-tograve life cycle assessment for two potential building structural applications (sandwich wall, ceiling reinforcement) made of ccc. we based our recycling scenario on previously conducted large-scale experiments. results show a relative larger energy intensity and abiotic depletion of fossil fuels for variants of ccc but lower global warming. yet, recycling is, second to embodied emissions of basic materials, the driving force of total environmental impacts. the presented recycling path (demolition, pyrolysis for carbon fabric, reuse in fiber fleece) offers less "green credentials" than steel. keywords: carbon concrete composite, life cycle assessment, recycling. 1. introduction the european construction industry including buildings contributes to 9% in eu’s gdp, but is also held accountable for roughly 90% of used mineral resources, 40% of energy consumption, and 35% carbon emissions [1]. to minimize negative effects on our planetary boundaries, a dedicated pathway to sustainable construction is key. an ever-increasing regulatory attention is paid to climate change and circular economy that could eventually improve sustainable development goals #9 to #13. circular economy, as synthesized by potting et al. [1], allows to align environmental strategies along nine "rs". decisionmakers are asking to what extend new building materials can help achieving these goals. they may offer options of considerable raw material savings ("r2 reduce") by integral design ("r5 refurbish"), remedial maintenance ("r0 refuse") or closing material loops ("r8 recycle"). in this study, carbon concrete composites (cccs) are examined as type of textile reinforced concrete (trc). ccc is understood as cement-based carbon fiber reinforced composites using a fine-grained concrete [2] and mesh-like textile reinforcements or carbon fiber rods as basic materials. in comparison to steel reinforced concrete (src), the use of highperformance carbon fibers offers a much wider "range of application specific properties" [3]. cccs still need to showcase whether its application is environmentally, socially and financially viable [4] though seemingly functionally superior including beneficial raw material savings. despite of over 20 years of intensive studies on ccc by two german collaborative research centers in dresden and aachen [5] and a succeeding joint research center "c3 carbon concrete composite" little is known about sustainability of ccc. an initial scoping review revealed that pertinent literature on ccc is dominated by structural and mechanical tests. there is only a few published work on sustainability assessments. many cases refer to carbon fiber reinforced plastics (cfrps) in other product contexts, e.g. automotive or railway. for example, das [10] applied lca for cfrp components in cars quantifying primary energy consumption (pe) and greenhouse gas emissions to show lightweight advantages. cfrp has also been studied for reinforcements of bridges and façades. cadenazzi et al. [15] assessed five life cycle impact assessment (lcia) indicators whereas hájek et al. [8] calculated four, e.g. global warming (gw) and acidification (ap): cfrp performed better than src in both cases. for ccc, williams portal et al. [9] examined reinforcement technologies, e.g. src and ccc, with cradle-to-gate lca. ccc scored the lowest total environmental impact. laiblová et al. [7] compared 518 https://doi.org/10.14311/app.2022.33.0518 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 33/2022 lca of carbon concrete composites materials value* unit life cycle examined product source carbon concrete composites (ccc) 34-109 [kg co2eq/m2] a1-c3** façade sandwich panels [6] 21.83 [kg co2eq/m2] a1-c3 façade element [7] 0.35*** [kg co2eq/mpa] a1-a3 façade element [8] carbon fiber reinforced plastics (cfrp) 295.93 [kg co2eq/m3] a1-a3 slab, case "ca1" [9] 345 [kg co2eq/kg] a1-a3 car component, "pan smc" [10] 30-52 [kg co2eq/kg] a1-a3 car component, various manuf. [11] cfrp rebar/strip 19.70 [kg co2eq/kg] a1-a3 rebar [12] carbon fiber 17.43-34.39 [kg co2eq/kg] a1-a3 pan carbon fiber [13] 24.20-31 [kg co2eq/kg] a1-a3 lignin and pan carbon fiber [10] 11.40 [kg co2eq/kg] a1-a3 pan carbon fiber [14] epoxy resin 5.25-8.25 [kg co2eq/kg] a1-a3 gabi database, for de/europe steel reinforced concrete (src) 129 [kg co2eq/m2] a1-c3 façade sandwich panel [6] 26.33 [kg co2eq/m2] a1-c3 façade element [7] concrete c25/30 197 [kg co2eq/m3] a1-a3 german mix, cement 11.1% epd 0.09 [kg co2eq/kg] a1-a3 ∼ epd reinforcing steel 0.31 [kg co2eq/kg] a1-a3 girders for concrete slabs epd 0.68 [kg co2eq/kg] a1-a3 reinforcement steel wire epd * all indicator values are based on dimensions given in each study. ** "life cycle" refers to examined life cycle stages using nomenclature of en 15804:2014. *** normalized to tensile strength in bending table 1. reported values for global warming within past studies (sources given upon request). façade elements made of src, ccc, and trc with basalt or glass fibers by cradle-to-grave lca. they identified no clear winner, though all trc variants did better in 4 out of 6 lcia indicators as compared to src. hülsmeier et al. [6] showed in a cradle-to-grave lca that ccc performed better than src on gw (-13 kg co2eq/m2). a multifunctional, vacuum-insulated façade element made of ccc even outperformed that standard panel of ccc (-74 kg co2eq/m2). recently, stoiber et al. [12] used epd data to analyze bridge designs made of ccc, src, and steel. again, ccc showed lower gw (-34 to -48%), but higher ap (-40 to 48%) scores. those studies served as benchmark, but were partly limited in data availability, e.g. for carbon fiber fabric or end-of-life (eol). to address these research gaps, we compared variants of a prefabricated sandwich wall system and strengthening schemes for an existing concrete ceiling made of either ccc or src. our research aims to identify hotspots for carbon concrete from an environmental perspective and is guided by following: 1. what environmental impacts are characterizing compared structural applications? 2. what level of energy and resource depletion is embodied in each material life cycle? 3. is substitution of src with carbon concrete an eligible measure to foster circular economy? we improved the analysis by collecting nondisclosed company data, updated materials for ccc, and integrated data from joint research projects, e.g. recycling processes. for practitioners, the results signal current hotspots along the value chain of carbon concrete to proactively start redesign strategies. civil engineers will be enabled to discuss material choices within the early design process to improve environmental sustainability. for city planners, we illustrate indicators allowing to measure policy impact and scale. for scholars, we will add insights to existing research dialogues in the area of ccc and environmental sustainability. the method and data used in this article is described in section 2. results and discussion are presented in section 3. section 4 concludes upon our research objectives. 2. materials and methods we began with a scoping systematic review to maintain evidence-based research practices and identify comparable datasets. table 1 illustrates a current 519 ch. scope, e. guenther, t. mielecke et al. acta polytechnica ctu proceedings sandwich wall system ceiling slab reinforcement variant a1 (src, c25/30) variant b1 (ccc, prefab.) variant c (src, c25/30) variant d (ccc) unit inner / outer shell / height 0.19 / 0.08 0.14 / 0.03 0.047 0.006 m xps insulation 0.035 na m n.x (outer shell) 15 na kn load capacity na 5 kn/m2 u-value 0.27 na w/(m·k) sound insulation 77 71 na db concrete 25/30 (% in-situ) 601.7 (40%) na 109.8 (100%) na kg/m2 special concrete for ccc na 406.05 na 14.1 kg/m2 steel reinforcement b500 46.31 na 2.99 or 0 na kg/m2 stainless steel b500 ng 1.04 na 0 or 2.99 na kg/m2 carbon fiber fabric na 2.16 na 0.65 kg/m2 polystyrene 2.7 na kg/m2 net weight 651.75 410.91 112.99 14.75 kg/m2 table 2. functional unit for two structural applications and life cycle inventory (own illustration). lack of coherent knowledge: the range of reported values for just gw varies a lot. we see the importance of well-defined functional units: differences on level of basic materials, e.g. carbon fiber versus steel girders (up to +9, 118%), are substantially greater than on level of entire structures, e.g. ccc versus src used in façade panels (-15 to 73%). 2.1. functional unit and life cycle inventory our reference functional unit was defined as 1 m2 of: a) a prefabricated sandwich wall made of src for a 3-storied building or b) a ceiling slab reinforcement of an existing src slab to increase load capacity from 1.5 to 5 kn/m2. each functional unit is not solely mass-based, but fulfils rated structural and mechanical requirements (see table 2). the wall systems were designed as non-stiffening sandwich outer walls with total dimensions 6 × 2.8 meters. we did not consider connections or cut-outs for existing openings, facades or corners. for the wall systems, we modelled four variants: two variants (a1-a2) of src with different concrete compressive strengths (c25/30 and c50/60) both using in part in-situ concrete. the two variants made of ccc (b1b2) were dimensioned to meet the same requirements. variant b1 was modelled as entirely prefabricated, the other b2 required partly in-situ concrete. the existing ceiling slab was 23.5 cm thick using c25/30. for the ceiling reinforcement, one src and ccc variant each (c and d) was calculated while assessing only the reinforcement materials. calculated material input data in table 2 reveals: the special concrete for ccc amounts to 67.4%, respectively 12.8% of concrete c25/30 in variant a1 and c. weight of steel reinforcement amounts to 4602,192% of carbon fiber fabric. total net weight differs by 240.84 (wall) or 98.05 kg/m2 (ceiling). 2.2. system boundaries and cut-offs our cradle-to-grave lcas encompassed all life cycle stages except the use stage. we followed the nomenclature of en 15804 for life cycle stages. service life was assumed 50 years as suggested by german regulatory bodies. no extended service life scenario was applied. petroleum-based polyacrylonitrile (pan) was used as precursor for ccc. for use stage, no conversion scenarios, replacement cycles or maintenance activities were expected. component layers subject to wear, e.g. paint coats, were not considered, too. the base scenario for eol: the wall is demolished, the building materials are separated and transported to dedicated plants for further processing. based on findings of joint research center "c3 carbon concrete composite", we assumed that 95 percent of initial carbon fiber fabric was reclaimed for material reuse after processing by pyrolysis. material reuse was modelled as producing glass fleece with recycled carbon fibers. avoided production of primary glass fleece was credited. we also modelled "r8 recycle" paths for concrete fraction, (stainless) steel, and insulation materials by road gravel, recycled steel, and thermal recovery. for cut-offs, we neglected plants, machines, and infrastructure necessary for manufacturing and construction. the lcas were mainly set-up for reference area germany. for base scenario, transport distances for the materials ranged between 50 and 500 km. we used gabi software version 9.2.0.58 sp 38 and cml2015 as lcia method. the dataset for carbon fiber rovings and a special fine-grained concrete mix "c3b2-hf-2-145-5" similar to c80/95 was based on another research subproject [16] within joint research 520 vol. 33/2022 lca of carbon concrete composites center "c3 carbon concrete composite". we used non-disclosed data of a manufacturer of carbon fiber fabrics and concrete plants otherwise generic datasets of gabi. ���� �� ��� ��� ��� ��� ���� &uhglw�iru�qdwxudo�jdv��khdw� :dwhu 6xoskxulf�dflg (sr[\�uhvlq 7udqvsruw 1lwurjhq 3urfhvv�vwhdp��qdwxudo�jdv� :dvwh�lq�pdqxidfwxulqj 3rzhu�pl[ 3$1�ilehuv figure 1. hotspot analysis cradle-to-gate for carbon fiber fabric (own illustration). 3. results and discussion 3.1. hotspot analysis for single life cycle stages first, we discuss hotspots identified within manufacturing and upstream activities (cradle-to-gate). figure 1 shows results for the carbon fiber fabric. the manufacturing of precursor material and succeeding production processes (summarized as "pan fibers") are the dominating influencing factors. only odp (ozone depletion) and partly adpel (abiotic depletion, non-fossil resources) are also strongly influenced by electricity consumed and its underlying power mix. manufacturing companies, e.g. toray, hexcel, sgl carbon or teijin, should thus concentrate on corporate efficiency strategies within production and identify suitable green alternatives to natural gas. all other value chain members should increase the share of renewables within their power mix [11] to refer to circular economy strategy "r2 reduce" [1]. moreover, that industry cluster should foster research on alternatives to petroleum-based pan like lignin (see section 3.3) assorted to "r0 refuse" [1]. for concrete, hotspot analyses are already published [17] and not displayed here. ���� ���� ���� ���� �� ��� ��� ��� ��� ���� '��&uhglw�iru�jodvv�iohhfh &���3\uro\vlv &�����'hprolwlrq�dqg�wudqvsruw figure 2. hotspot analysis of the end-of-life stage for carbon concrete (own illustration). second, we look at the eol stage in our base scenario. this includes demolition, transportation to pyrolysis plant, pyrolysis [18], manufacturing of a recycled carbon fiber fleece (a case of downcycling). figure 2 shows a dissatisfying picture: pyrolysis is the dominating process due to its energy intensity. except for indicator ap, none of the seven other environmental impact categories can be offset. the crediting along that "r8 recycle" [1] pathway for the glass fleece production is not enough. thus, current recycling technologies for carbon fiber fabric in ccc offers a true circular economy path, but has a negative net environmental effect, e.g. it requires more energy input than credited. although better than facing difficulties like wind turbines [19], this is far from ideal. 3.2. cradle-to-grave comparison results for the full life cycle illustrate a mixed picture. exemplary, we show adpfos (abiotic depletion, fossil resources), pe, and gw scores for each considered life cycle stage (see figure 3-5). for the wall system, each square meter of variant b1 made of ccc leads to lower carbon emissions (gw: -6.1 kg co2eq/m2) but is characterized by higher energy intensity (pe: +238 mj/m2) and fossil resource depletion (adpfos: +263 mj/m2). same is true for ceiling reinforcement: -2.6 kg co2eq/m2, +94 and +74 mj/m2. apparently, the value chain of carbon fiber and its fabric uses low-carbon power generation as compared to steel. base scenario shows another 521 ch. scope, e. guenther, t. mielecke et al. acta polytechnica ctu proceedings � ���� � ��� ��� ��� ��� ����� ����� 0 -� p ð 9duldqw�$���65&� 9duldqw�%���&&&� figure 3. comparing adpfos along all life cycle stages for wall systems (own illustration). comparative advantage for src with respect to recycling (pe: -108 mj/m2) and resulting credits (-36 mj/m2). about 85% of used steel derives from recycled and remelted material in german construction industry. this is an ongoing success story of a circular "r8 recycling" [1] path already paying back for src. for indicators not shown here: ccc variant b1 keeps an advantage on ap and eutrophication (ep); odp is roughly equal; src variant a1 has lower damage in photochemical ozone creation (pocp). figure 5 illustrates examined effects of four scenarios (violet bars): positive values imply an environmental advantage of variant b1 (ccc); negative values vice versa a disadvantage. for base scenario, variant b1 has lower gw scores (up to +11.9 kg co2eq/m2) than all other design variants (blue bars). scenario 1 assumed a 20% share of steel imported from china within the src value chain. this increased all lcia indicators negatively for variant a1 (gw: +16.9). scenario 2 modelled real location sites of carbon fiber and fabric manufacturers. the advantage of low-carbon water power generation in the us and scotland is entirely offset by environmental impacts of the resulting long-distance oversea transportation (gw: -1.1). scenario 3 ignored the eol stage (gw: +14.8) and scenario 4 assumed a future reuse of recycled carbon fibers for the same purpose, namely carbon fiber fabric (gw: +16.4). this is a clear signal to foster that true recycling path. sensitivity analyses tested for data uncertainty (orange bars): a dataset for steel reinforcement with -50% lower and a dataset for carbon fibers with +50% higher carbon emissions is chosen. the advantage for gw turns negative (gw: -7.8 / -6.8). ��� � �� �� ��� ��� ��� 0 -� p ð 9duldqw�&��65&� 9duldqw�'��&&&� figure 4. comparing pe scores for ceiling slab reinforcements (own illustration). 3.3. limitations as with any lca, (model) choices were made with effects on overall results. first, we did not quantify the use of lignin-based pan, though there is promising research that illustrates lignin as future, more sustainable sourcing. other alternatives to high purity pan, e.g. based on algae or waste cotton linter are rather at an experimental stage. second, we did not study natural fiber materials, e.g. flax, jute, or sisal. they still lack a good balance of impact strength, stiffness, and toughness within the desired flexural and tensile properties. both strategies would support "r0 refuse" [1] and diminish the volume of petroleum-based pan and geopolitical risks. third, we did not explicitly model strategies towards green concrete. we used an optimized special fine-grained concrete mix that was developed and tested for ccc [16]. the benchmarking src was modelled with german average mix of c25/30 and c50/60 by the german cement works association. fourth, we did not account for recycled aggregate concrete or other types with recycled materials. as this is a feasible option for both, src and ccc, so we did not add this complexity to "r8 recycling" [1]. currently, crushed concrete was credited for road gravel. fifth, future analyses could evaluate eco-designs related to modularity and removable joints. this path would follow circular economy strategy "r3 reuse" [1]. sixth, we do not include extended service life scenarios, yet. 4. conclusions our study aimed to earmark comparative environmental advantages for the innovative building material carbon concrete and existing circular economy potentials. we applied cradle-to-grave lca compar522 vol. 33/2022 lca of carbon concrete composites ��� �� � � �� �� �� )xoo�olih�f\foh nj�&2�ht�pð ¨�9duldqw�$��%���zdoo� ¨�9duldqw�$��%� ¨�9duldqw�%��%� ¨�6fhqdulr����$��%�� ¨�6fhqdulr����$��%�� ¨�6fhqdulr����$��%�� ¨�6fhqdulr����$��%�� ¨�6hqvlwlylw\�6whho�uhlqi���$��%�� ¨�6hqvlwlylw\�&duerq�ilehu��$��%�� figure 5. difference in gw scores between wall system variants a (src) and b (ccc) for full life cycle of design variants, scenarios, and sensitivity analyses (own illustration). ing src and ccc variants. scope et al. [20] may meet expectations of the scholarly community framing lca as main pillar of a life cycle sustainability assessment. first, we identified hotspots of environmental impacts located in manufacturing stage of carbon fiber fabric and concrete as well as during recycling processes of ccc. second, we quantified abiotic depletion and primary energy consumption for each material life cycle for a functional unit of 1 m2. during manufacturing and construction stages, the examined wall system variant a1 made of src is less energyintensive (adpfos: -181 mj/m2; pe: -102 mj/m2), but emits roughly +14% more greenhouse gases. for the full life cycle, both materials get closer on gw (+6%), but drift apart on adpfos and pe (-24%/19%) in base scenario. analogies are observed for the ceiling slab reinforcement: src keeps an advantage on adpfos and pe (-53/-54%), but emits +2.7 kg co2eq/m2. net weight of ccc structures is significantly reduced by 59% (wall system) and 665% (ceiling). third, we have discussed a couple of realized or potential circular economy paths. currently, tradeoffs exists: using variant b1 for a whole 3-storied building could amount to increased primary energy of +107,957 mj, but achieved savings of 109 tons materials and 2.8 tons greenhouse gases emissions. city planners then need additional sustainability criteria. overall, our assessment indicates that ccc provides a circular economy material loop, less material weight, and resulting lower carbon emissions. carbon concrete may improve the climate mitigation potential of buildings, but industry and building professionals are encouraged to emphasize discussed technology pathways to improve energy intensity and specific circular economy strategies. we extensively discussed our study’s limitations with room for further research in many fields. practitioners shall not "overlook" [9] carbon concrete but study the new material and start piloting projects to empirically test estimated savings. acknowledgements the authors thank the german federal ministry of education and research for funding the program "twenty20 partnership for innovation" and the entailed project "c3 carbon concrete composite" (grant no. 03zz0320a). references [1] abergel t, dean b, dulac j, hamilton i, wheeler t 2018 2018 global status report: towards a zero-emission, efficient, and resilient buildings and construction sector (iea and unep). 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[20] c. scope, e. guenther, j. schütz, et al. aiming for life cycle sustainability assessment of cement-based omposites: a trend study for wall systems of carbon concrete: dresden nexus conference 2020 session 4-circular economy for building with secondary construction materials to minimise resource use and land use. civil engineering design 2(5-6):143-58, 2020. https://doi.org/10.1002/cend.202000024. 524 https://doi.org/10.4028/www.scientific.net/amr.1106.69 https://doi.org/10.1061/(asce)mt.1943-5533.0001160 https://doi.org/10.1007/s11367-011-0264-z https://doi.org/10.1007/s11740-018-0802-7 https://doi.org/10.1016/j.jclepro.2020.124300 https://doi.org/10.1520/acem20180113 https://doi.org/10.1002/best.201700058 https://doi.org/10.1016/j.jclepro.2020.123558 https://doi.org/10.1021/acssuschemeng.8b01026 https://doi.org/10.1016/j.scitotenv.2019.135054 https://doi.org/10.1002/cend.202000024 https://doi.org/10.14311/app.2022.33.0078 acta polytechnica ctu proceedings 33:78–84, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague predicting leakage of the vercors mock-up and concrete containment buildings a digital twin approach laurent charpina,∗, jessica haelewynb, anass cherki el idrissib, julien niepceronc, benoît massonc, charles toulemondea, guillaume boulantd, jean-philippe mathieua, françois hamonb, sylvie michel-ponnelleb, jean-marie hénaulte, frédéric tailladee, jean-luc adiaa, florian escoffierb a edf r&d mmc, les renardières, avenue des renardières, 77250 ecuelles, france b edf r&d ermes, saclay, 7 boulevard gaspard monge, 91120 palaiseau, france c edf dt, 19 rue pierre bourdeix, 69000 lyon, france d edf r&d pericles, saclay, 7 boulevard gaspard monge, 91120 palaiseau, france e edf r&d prisme, chatou, 6 quai watier, 78400 chatou, france ∗ corresponding author: laurent.charpin@edf.fr abstract. edf operates a nuclear power generation fleet made up of 56 reactors. this fleet contains 24 reactors designed as double-walled concrete containment building. the inner concrete containment vessel has no metallic liner and is a prestressed reinforced concrete building. the inner concrete containment vessel is designed to withstand a severe accident, in terms of mechanical and sealing behaviour. the tightness of the containment is tested every 10 years, by carrying out a pressurization test and by measuring the leak rate. the leak rate is required to be below a regulatory threshold to continue operation of the concrete containment building for the next ten years. ageing of concrete due to drying, creep and shrinkage leads to increase prestress loss and then leak rate with time. for some containment buildings, the leak rate gets closer to the regulatory threshold with time, so important coating programs are planned to mitigate and limit the leak rate under the regulatory threshold. therefore, it is very important for edf to have a concrete containment building leak rate prediction tool. to address this issue, an important research program around a 1/3 scale concrete containment building mock-up called "vercors" have been launched at edf. the mock-up is heavily instrumented, and its materials (concrete, prestressing cables) have been widely characterized and studied. an important numerical effort has also been made to implement structural computations of the mock-up and to capitalize these computations as well as their post-processing (so as to compare automatically with the monitoring data) in what can be called a digital twin of the mock-up. this digital twin is now used to predict the leakage of vercors mock-up before yearly pressure test, and also to optimize the repair programs on the real containments. keywords: air leakage, concrete containment building, creep, drying, shrinkage. 1. introduction 1.1. industrial context edf is the main electricity utility in france and operates a 56 reactors fleet of nuclear power plants (npps). pwr safety relies on three confinement barriers: fuel cladding, primary circuitry integrity, and reactor containment building. concrete containment buildings (ccbs) leak-tightness is checked every 10ăyears during the integrated leakage rate test (ilrt). this test must be performed and validated in order to continue the plant operation [1]. double-wall containment buildings leak-tightness is achieved thanks to the inner prestressed reinforced ccb (without a metallic liner) and dynamic air filtering between the two containment buildings. in case of an accident, the concrete remains in compression. the design of the containment building is calibrated to this aim. in france, the choice has been made to use grouted tendons, therefore increasing posttension is not an option (which is singular compared to ccbs in other countries). edf and its industrial partners has acquired a vast experience of repair solutions that are thoroughly tested and qualified in the lab and have shown good efficiency in the field. however, their operational implementation remains difficult due to accessibility and schedule considerations, and therefore needs to be optimized. to perform this optimization and ensure a high level of safety of the containments, knowledge about concrete behaviour and capability to predict concrete delayed strains as 78 https://doi.org/10.14311/app.2022.33.0078 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 33/2022 predicting leakage of the vercors mock-up and ccbs well as leak-tightness evolution are required. the vercors programme is the current main effort at edf to bridge the gap between material scale knowledge of concrete behaviour and leak-tightness prediction. 1.2. vercors mock-up, a unique experiment a 1/3 scale mock-up of a double-wall ccb has been built by edf in the south of paris (edf lab les renardières). it is called vercors (vérification réaliste du confinement des réacteurs / realistic assessment of reactors containment, figure 1). for more details on the project and the first international simulation benchmark (organized in 2015 on vercors early-age behaviour), the reader can refer to [2]. the second vercors benchmark about long-term mechanical behaviour and leak-tightness has been reported in [3]. a third benchmark has started in january 2021. in addition to cost, the huge advantage of the scale reduction is the acceleration of drying: since the mock-up is 1/3 compared to real containment buildings, it is expected to dry 9 times faster. in addition, the 1/3 scale makes it possible to reduce all featured of the mock-up (tendons, rebars, etc.), therefore representativity to real containment buildings is very satisfactory. aggregate size and sensors have not been reduced by the same ratio for practical reasons. vercors was built starting in 2013 in less than 1 year. prestressing started after completing concrete casting, at least 28 days after casting of the dome. the first irlt has been performed in the end of year 2015. in march 2016 the air conditioning system, ensuring representative temperature and humidity conditions both in the inner containment and between the two containments, has been started. then, irlts have been performed every year (except during the covid lock-out period), to represent the tests performed every 10 years on real ccbs. instrumentation has been the focus of large efforts, with both the classical instrumentation found on ccbs but in much larger numbers (more than 300 vibrating wire strain sensors, 200 pt100 temperature sensors, pendula and invar wires) and also innovative sensors (2 km of optic fibers, 20 tdr moisture sensors, 80 strain gages on rebars). vercors concrete has been thoroughly characterized in edf and national and european partner labs to achieve a consistent knowledge of elastic properties, strength, drying properties, delayed strains at different ages and different temperatures. more information can be found in [4]. 1.3. leak-tightness model strategy the strategy to use vercors to be able to predict the ccb behaviour during irlts and to optimize coating programs relies on the construction of a ccb digital twin. it is based on physical ideas described in [5]. the present paper is the focus of the two first steps, as described on figure 2: • validation of the physical models (for drying, shrinkage and creep at moderate temperature) using laboratory data obtained in edf laboratories and in macena pia project and a comparison of the staggered thermo-hygro-mechanical analysis to vercors results. • calibration of a global analytical leakage model on existing vercors ilrt results, which is then used to predict the leakage during next irlts. 1.4. objective of the present paper in this paper, the procedure presented in figure 2 to the vercors mock-up is followed. the main assumptions of the structural computation will be presented as well as the results obtained. then, the global leakage model and its results will be presented. 2. vercors structural computation the objective of this t-h-m computation is twofold: validating the models involved by comparison to the monitoring data on the vercors mock-up, and providing inputs to the leak-tightness model. 2.1. model assumptions most assumptions of the vercors modelling have been presented elsewhere [2], so only the main features as well as references will be provided in the present paper. all computations are performed with code_aster (www.code-aster.org) in the framework of the vercors digital twin developed at edf. 2.1.1. constitutive modelling the thermal model is a linear conductivity model. the thermal model parameters (heat capacity, conduction coefficient) have been measured on vercors concrete. the drying model is that developed in granger’s works from mensi’s earlier developments [6]. it is a non-linear diffusion model using water concentration as an unknown (hence, boundary conditions needs to be expressed in water concentration which requires the knowledge of the desorption isotherm, since the ambient moisture is known from air relative humidity). the drying model and calibration issues are presented in detail in [6], and the parameters used in this study are available in [7]. the mechanical model is based on benboudjema and sellier’s works as well as internal edf works [8–10]. it is an additive model taking into account elasticity, drying shrinkage, basic creep and drying creep. the calibration of the model is described in [7] as well. this calibration phase involving a large number of drying, shrinkage and creep experiments at 20 ◦c and 79 www.code-aster.org l. charpin, j. haelewyn, a. cherki el idrissi et al. acta polytechnica ctu proceedings ������$�wklug�ehqfkpdun�zloo�vwduw�rq�-dqxdu\����������� � figure 1. outer containment wall of the vercors mock-up and sectional schematic view figure 2. model validation strategy proposed in the present paper, corresponding to points 1 and 2 of the above strategy. 40 ◦c yields a set of parameters which is used in the present study. concerning the properties of prestress cables, they were communicated by fressinet who did the posttensioning. the relaxation of prestressing cables is not accounted for. passive reinforcements are not taken into account in the model. the mechanical model is based on benboudjema and sellier’s works as well as internal edf works [6, 7, 11]. it is an additive model taking into account elasticity, drying shrinkage, basic creep and drying creep. the calibration of the model is described in [5] as well. this calibration phase involving a large number of drying, shrinkage and creep experiments at 20 ◦c and 40 ◦c yields a set of parameters which is used in the present study. concerning the properties of prestress cables, they were communicated by fressinet who did the posttensioning. the relaxation of prestressing cables is not accounted for. passive reinforcements are not taken into account in the model. 2.1.2. fe model the numerical model of the vercors mock-up is focused on the inner pre-stressed concrete containment building. the mesh used for thermal and hydric computations is composed of 331962 nodes and 304928 hexahedral linear elements, while the mesh used for the mechanics computation is composed of 211776 nodes and 40656 hexahedral quadratic elements (figure 3). 80 vol. 33/2022 predicting leakage of the vercors mock-up and ccbs� figure 3. vercors mesh. the pre-stress cables are individually modelled using bar elements. deviations around the access hatch and other passages through the walls are represented. the metal elements closing the through-wall passages (pipes, hatches) are also modelled in 3d. the mesh is refined at the surface of concrete to correctly capture the drying gradients. in the cylindrical part, the wall thickness is discretized using 12 elements which size vary between 2 cm and 6 cm. 2.1.3. boundary conditions and loadings the thermal and hydric boundary conditions are very important since drying, creep and shrinkage are highly influenced by temperature and humidity. the ambient temperatures and humidity are the results of: • the external weather (especially in the early months of the experiment during which the external containment was not completed). • the behaviour of the air-conditioning system which is designed to impose temperature and humidity similar to field conditions (both inside the containment and in the space between internal and external containment), but needs to be stopped for about 3-4 weeks for ilrts. • the supply of humidity due to the flooding of the basement raft during ilrts (which is related to the procedure also used on real ccbs to measure the leakage through the containment wall only), which brings moisture close to 100 % rh during about 2 weeks. in this work, the boundary conditions used are a schematic representation of the available measurements. for the temperature, the temperature sensors installed on the wall surface are used (due to significant boundary layer effects, the wall temperature is quite different to ambient temperature, which has also been modelled directly in other works, but is not presented here). for moisture, the rh measurements performed on both sides of the containment walls are used. the mechanical boundary conditions are as follows: displacements at the bottom surface of the basement raft are assumed to be zero. the post-tension of the cables is performed in 8 phases instead of the 35 real phases. it was shown to be equivalent in terms of post-tensioning global and local mechanical behaviour. finally, and most importantly, the computation starts at the beginning of drying of concrete (shortly before the beginning of prestress). hence, early-age effects are not represented. then, the mechanical fields must be understood as relative to the mechanical state at the end of the early-age period. 2.1.4. vercors digital twin the data associated to vercors (monitoring, materials), the parameters identification procedures, the structural computation of vercors and related post-processing functions have been capitalized in what can be described as the digital-twin factory of vercors.) the aim of such a tool is to improve quality and traceability of the studies, and also to save time when changes are performed to a part of the study. the different tools needed for building such a digital twin are described in a more detailed manner in [4]. 2.2. results of the staggered t-h-m analysis modelling in this section, some comparison to monitoring results are presented, and some conclusions are drawn on the validation of the models. 81 l. charpin, j. haelewyn, a. cherki el idrissi et al. acta polytechnica ctu proceedings figure 4. simulated and measured moisture (bottom of the wall, see arrow) 2.2.1. hydric behaviour first, the results of the hydric computation are compared to moisture sensor results available on the vercors mock-up. these results have been obtained using tdr sensors [6, 12, 13]. the comparison is shown for one of the sensors on figure 4. the set of parameters studied here is the set called "2018" on figure 4. it has been calibrated on drying experiments performed on well-hydrated samples at rh close to 50 %, see [7]. the simulation assumes an initial saturation ratio of 1 which is consistent with the fact that the vercors concrete has been artificially kept wet during construction. the "2016" set of parameter performs poorly and is considered as non-representative of the field concrete since it was calibrated on a drying test performed at 24h (nonmature concrete). the 2018 model does not perfectly match the experiment: a fast drying is predicted before the heating system is turned on (600 days or march 2016), whereas for the tdr post-processing, it has been assumed that the saturation is still equal to 1 when heating is turned on. 2.2.2. vercors digital twin this inability to reproduce the moisture field with the "2018" identification lead to different investigations: • as explained in [6], the calibration of granger model on mass-loss data is an ill-posed problem. the set of parameters might be inadequate for this reason. the calibration is now being improved by using other sources of data: mass-loss at different rhs, moisture profile measurements in large samples in vercors mock-up. • the model itself might be inadequate for the extrapolation from small laboratory samples to large samples. therefore, a new model is currently tested on vercors mock-up based onărichardfick’s law [14]. figure 5. simulated and measured strain (middle part of the wall, see arrow) • the calibration procedure for the tdr sensor, linking permittivity to saturation, is also a complicated process [13], a precise temperature dependence correction is required. as a comparison, another set of parameters called "2020" has also been used. the identification of the thermal activation coefficients of the model has been refined, which enables predicting more accurately the slow drying before heating is turned on. 2.2.3. mechanical behaviour the strain computed in the mechanical computation are then compared with measured strains. it is chosen here to use the vibrating wire sensors as a reference as they are available on ccbs. as an example, the comparison is performed on one sensor (at mid-height) on figure 5. the total strain is represented (the strain is corrected from effects of temperature on the sensor, but not from thermal dilation effects of the structure). the comparison of simulation and measurements for the "2018" set of parameters is quite satisfactory. many conclusions car be drawn: • the low strain occurring before the start of the ac/heating system in march 2016 is well reproduced. • the acceleration of the strain kinetics after march 2016 is also well reproduced, showing that the thermal activation is well represented (at least globally) in the model. • the swelling related to water uptake during the pressure tests (due to lower temperature and higher moisture) is underestimated. on the contrary, the previous set of parameters "2016" performs much less well. indeed, this set of parameters was only partly calibrated on vercors concrete. hence, we see that despite an unsatisfactory representation of the moisture in the concrete wall, the mechanical prediction using the "2018" set of 82 vol. 33/2022 predicting leakage of the vercors mock-up and ccbs � figure 6. local leakage prediction method. parameters are consistent with measurements. this shows that the calibration procedure of mechanical behaviour can "compensate" an unsatisfactory hydric calibration to a certain extent. to improve our understanding of this phenomenon, new studies will be started to quantify the uncertainty of the moisture and strain predictions depending on the constitutive models calibration methodology. 3. vercors leak prediction 3.1. model assumptions the leakage model is based on the assumption that leaks through the wall can be classified into two categories: • diffuse leaks that flow mostly through the concrete porosity. hence, these leaks are insensitive to the crack opening, but mostly related to the sound concrete permeability and its evolution, which is itself driven by concrete saturation. • localized leaks (which are measured during the ilrt using leak boxes which collect and measure the flux of the air flowing through localized defects). hence, these leaks are sensitive to the mechanical state of the containment since cracks can open and close due the pressure applied during the ilrt. this effect is considered to be dependant to the evolution of the prestress. 3.1.1. porosity leaks the input for the computation of the diffuse leaks is a moisture profile computed with the t-h-m model. then, a second computation is performed to reproduce the air-flux through the wall at a given saturation profile. the model used requires the knowledge of the vercors van genuchten isotherm parameters to compute the relative permeability according to mualem’s law. the model intrinsic permeability is calibrated in order to reproduce the correct porosity leak flow in "vd2" test of 2018. the model is then used to predict the porosity leakage in "vd3" test in figure 7. leakage measurements and vd3 prediction (o = measurements, x = prediction). 2019, using the same parameters but changing the saturation profile according the t-h-m computation results. the saturation profile used corresponds to the computation labelled "2016" in the previous figures, since at the time when the study was performed, it was thought to best reproduce the tdr measurements (which have since then been corrected). 3.1.2. local leaks the evolution of local leaks is assumed to be proportional to the prestress decrease (average on the full mock-up and the 2 in-plane directions in this approach). an affine relation is found between the prestress loss and the local leakage, which is used to extrapolate the value of the local leakage to the next vd by using the predicted prestress loss, as shown on figure 6. 3.2. leak-tightness prediction results using this methodology, a prediction for the leakage drying the "vd3" pressure test is obtained, only based on information which were already available before the pressure test. the results are shown on figure 7 as well as the measured result. the leakage predictions are very close to the measurements (for local and diffuse leaks). therefore, the global leakage model is considered very satisfactory. it is believed that despite being a quite simple model, it correctly captures the leakage behaviour of the mock-up. 4. perspectives in 2020 many changes will be brought to the vercors modelling: • new efforts on the drying law (validation and calibration) will be made. • our ability to do the same predictions using a reduced set of monitoring data for calibration (as they are available on ccbs) will be tested. 83 l. charpin, j. haelewyn, a. cherki el idrissi et al. acta polytechnica ctu proceedings • works a localized version of the leakage model will be continued, in order to be able to predict better where on the mock-up leakage occurs, assess the effect of local prestress loss and take refurbishment into account in a more efficient manner. moreover, a third benchmark has started in 2021. references [1] a. simon, a. courtois. structural monitoring of prestressed concrete containments of nuclear power plants for ageing management, structural mechanical in reactor technology 21:6-11, 2011. [2] m. corbin, m. garcia. international benchmark vercors 2015 overview, synthesis and lessons learned, edf, septen, 2015. [3] l. charpin, j. niepceron, m. corbin, et al. ageing and air leakage assessment of a nuclear reactor containment mock-up: vercors 2nd benchmark. nuclear engineering and design 377, 2021. https: //doi.org/10.1016/j.nucengdes.2021.111136. [4] j.-p. mathieu, l. charpin, p. sémété, et al. temperature and humidity-driven ageing of the vercors mock-up, euro-c, austria, 2018. [5] l. charpin, c. toulemonde, j. l. adia, et al. double wall containment buildings leak-tightness prediction: strategy and application, in international rilem conference on early-age and long-term cracking in rc structures, 2021. [6] l. charpin, a. courtois, f. taillade, et al. calibration of mensi/granger constitutive law: evidences of ill-posedness and practical application to vercors concrete, tince 2018, paris-saclay, france, 2018. [7] l. charpin, j. haelewyn, j.-p. mathieu. identitication of drying, creep and shrinkage constitutive laws for concrete at 20c and 40c, application to vercors mock-up, in smss, rovinj, croatia, 2019. [8] f. benboudjema. modélisation des déformations différées du béton sous sollicitations biaxiales. application aux enceintes de confinement de bâtiments réacteurs des centrales nucléaires, doctoral dissertation, université de marne la vallée, 2002. https://doi.org/10.1051/978-2-7598-1846-4.c009. [9] a. sellier, l. buffo-lacarrière. vers une modélisation simple et unifiée du fluage propre, du retrait et du fluage en dessiccation du béton. european journal of environmental and civil engineering 13(10):1161-82, 2011. https://doi.org/10.1080/19648189.2009.9693184. [10] a. foucault, s. michel-ponnelle, e. galenne. a new creep model for npp containment behaviour prediction, sscs 2012, aix en provence, france, 2012. [11] l. granger. comportement différé du béton dans les enceintes de centrales nucléaires : analyse et modélisation, ecole nationale des ponts et chaussées, ph.d. thesis, 1996. [12] d. vautrin, f. taillade, a. courtois, et al. adaptation of a tdr probe design for the estimation of water content in concrete, tince, paris, 2016. [13] v. guihard, j. haelewyn, a. courtois, et al. relevance and means for measuring water content in concrete structures. an illustration with pwr concrete containment, tince, paris, 2018. [14] j. carette, f. soleilhet, f. benboudjema, et al. identifying the mechanisms of concrete drying: an experimental-numerical approach. construction and building materials 230, 2020. https: //doi.org/10.1016/j.conbuildmat.2019.117001. 84 https://doi.org/10.1016/j.nucengdes.2021.111136 https://doi.org/10.1051/978-2-7598-1846-4.c009 https://doi.org/10.1080/19648189.2009.9693184 https://doi.org/10.1016/j.conbuildmat.2019.117001 https://doi.org/10.14311/app.2022.33.0039 acta polytechnica ctu proceedings 33:39–44, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague conformity assessment model for mechanical coupling devices and anchors for reinforcing steel for use in concrete structure lee brankley, ayhan turgul, lyn morgan, ladin camci∗ cares, pembroke house, 21 pembroke road, sevenoaks, kent tn13 1xr, united kingdom ∗ corresponding author: ladincamci@ukcares.com abstract. the use of mechanical coupling devices and anchors for reinforcing steel for use in concrete structures can simplify the design and construction of reinforced concrete and reduce the amount of reinforcement required. technical approvals of these products by suitably qualified organisations are acceptable under building regulations and by major specifiers as the appropriate way of assessing non-standard or new and innovative products. specifiers, manufacturers, traders, and customers increasingly demand certainty in the quality management of mechanical coupling devices and anchors entering the supply chain. the supply chain is extremely complex. the weak traceability systems and pressure on the supply chain to reduce cost have created the environment where risks increase. this unique and comprehensive model describes the technical and assessment requirements for the conformity of mechanical coupling devices and anchors. it covers all stages in the supply chain from the production, assembly, product performance and evaluation of mechanical properties including tensile strength, ductility, permanent elongation (slip), cyclic loading performance and fatigue performance. the essential elements of the quality management system combined with additional requirements on process control, product testing and robust installation methodology are essential and agreed by all relevant sectors of industry. keywords: certification, mechanical rebar anchors, mechanical rebar couplers, mechanical rebar splicing systems, product conformity. 1. introduction the cares technical approval scheme for couplers and anchors provides a comprehensive assessment process, ensuring complete product traceability to the construction site. a key part of assessment process is the development of a technical approval report which not only indicates the performance characteristics and conformity of the product, but also ensures that correct assembly to the rebar is undertaken with robust installation procedures contained within each report. the cares technical approval scheme predominantly covers couplers for use with bs 4449 [1] reinforcing steel. cares with its global presence have recognised the need for more global approvals and have recently introduced a scheme which not only supports cares approved rebar to bs 4449 [1], but also rebar to en 10080 [2], astm a615/a615m [3], astm a706/a706m [3], iso 6935-2 [4] and ss 560 [5]. the cares technical approval certification scheme for the supply and installation of couplers and anchors has been developed in conjunction with the cares technical committee, made up of various interested parties including leading structural engineers, specifiers, and coupler producers. the schemes are based on the demonstration of the continuing operation of the quality management system consistent with iso 9001 [6], the appropriate quality and operations assessment schedule [7] and the ability to manufacture, supply and / or install material or product complying with bs 8597 [8], sellafield engineering standard [9], highways england manual [10] or more recently, iso 15835 [11]. at the initial approval stage, all quality affecting manufacturing processes are audited whether produced internally or outsourced via approved subcontract suppliers. once approved, all cares approved coupler / anchor manufacturers must ensure any design, material specification, supply chain or installation instruction changes are vetted by cares to ensure any necessary testing is further carried out to ensure product conformity is maintained prior to the change. the cares coupler technical approval scheme is further supported both downstream and upstream by the cares steel certification scheme where reinforcing bar fabricators must also be cares approved for installation of couplers on the reinforcing steel (cares src08 scheme). 2. objective the objective of the technical approval scheme is to give confidence to the purchaser that the construction 39 https://doi.org/10.14311/app.2022.33.0039 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en l. brankley, a. turgul, l. morgan, l. camci acta polytechnica ctu proceedings schedules no. description ta1-a technical approval of couplers for reinforcing steel for use in structures and structural elements designed in accordance with the fatigue requirements of structural eurocodes ta1-b technical approval of couplers for reinforcing steel and reinforcement anchors for bs 8110 [12] and en 1992-1-1 [13] applications for static loading in tension or tension and compression. ta1-c technical approval of tension couplers for reinforcing steel for sellafield engineering standard [9] applications ta1-f technical approval of couplers for high cycle fatigue and low cycle loading and static loading applications in tension. src08 quality and operations assessment schedule [7] for the application of mechanical couplers to reinforcing steel table 1. quality and operations assessment schedules [7]. product complies with the appropriate specification without the need to undertake separate verification. additionally, it provides confidence in both traceability and correct installation methodology. this involves the application of quality assurance principles to assess the supplier’s and supply chain quality systems and, as appropriate, product testing to ensure conformity with the requirements contained in each scheme at each part of the supply chain. 3. assessment and certification system 3.1. principles the assessment and certification system employed in the scheme has been developed around the following common principles: • the scheme is concerned with ensuring that the construction product complies with the relevant product standards and / or client specifications. • the responsibility for complying with the standards rests absolutely with the approved firm. • the means of providing assurance of consistent compliance with the standards or specifications shall be the formal management system for quality, including process control, which the approved firm is required to have documented and to be operating to the satisfaction of cares. this system is subject to ongoing audits at periodic intervals. • a key method of verifying compliance of the product is by the independent assessment and surveillance of the approved firms management system for quality, including witness testing of the product as appropriate • additionally, independent inspection and testing of the product is undertaken by testing laboratories approved by cares. • long term quality level reviews and quality complaints are a key feature of the ongoing surveillance programme. 3.2. quality and operations assessment (technical assessment) schedules the schemes quality and operations assessment schedules [7] for couplers and anchors are shown in table 1. 3.3. assessment the assessment consists of: • full assessment in accordance with the relevant quality and operations assessment schedule [7]. • the manufacturing process route integrity, including records, resources, training, contract review, product requirements, manufacturing controls and interfaces. • review of installation methodology and product dimensional conformance. • product conformity assessment by sampling in accordance with the relevant quality and operations assessment schedule [7]. • preparation and issue of the cares technical approval report for the couplers / anchors assessed. 3.4. applicant the applicant is required to have and be able to demonstrate: • the ability and resources required to produce the coupler / anchor to the relevant standard(s) and / or specifications consistently. • a quality management system which complies with the requirements of iso 9001 [6], and the relevant quality and operations assessment schedule [7]. the system shall be described formally in the supplier’s quality manual. • adequate control of approved suppliers and incoming product. • the ability of the coupler manufacturer to control its supplier is essential in the case of approval of cementitious grouted couplers, where the grout has 40 vol. 33/2022 model assessment of reinforcing steel in concrete structure tight performance requirements as part of the latest coupler technical approval scheme ta1-f. • robust installation methodology which is readily transferrable to downstream fabricators or site installers. 3.5. certification a recommendation for approval is produced when the manufacturing procedures and product conformity have been assessed and the scheme criteria have been satisfied and all reported deficiencies have been addressed to the satisfaction of cares. a key part of this recommendation process is the preparation and subsequent approval of the coupler / anchor technical approval report. copies of all the issued technical approval reports are freely and readily available to download from the cares website (www.ukcares.com). the certificate of approval is valid for one year with renewal subject to continuing satisfactory performance of the construction product. firms granted certificates of approval are required to reproduce the cares logo, and the certificate of approval number on all relevant documentation. 3.6. ongoing surveillance the assessment and certification system employed in the scheme has been developed around the following common principles: firms granted certificates of approval are required to undergo surveillance audits by cares twice per annum, to ensure continued compliance with the required scheme and product standard. samples of the construction product may be witnessed tested at the manufacturer in addition to the requirement to test "mirror" samples at an independent laboratory approved by cares. all technical approval schemes require that key variables of the original tested component remain unchanged. the following variables as applicable to the type of approved coupler shall not be changed without the prior agreement and subsequent assessment by cares: a)coupler component material or grade or process route of the raw material used to manufacture the coupler b)cross-sectional area of the splice sleeve c) bar engagement length d)reinforcing bar profile (rib pattern), grade or specification e) bar locking device f) thread geometry g)torque h)swaging pressure i) die geometry j) production process or production location k)approved grout supplier and / or change in grout specification or properties (in the case of cementitious grouted coupler) l) installation instructions a key condition of the cares technical approval schemes are that any subsequent changes to the above must be reviewed by cares prior to the implementation of the change, thus ensuring that any extra testing is undertaken to ensure continued product conformity. failure to do so will usually lead to the removal of the technical approval and the cares approval certificate. new suppliers deemed critical for the processed part will be assessed by cares before production can commence. all sub-suppliers are visited at least once every three years as part of the ongoing surveillance installation instructions are validated at each surveillance visit, by means of witness sampling of the coupler and application to the reinforcing bar. 3.7. key attributes of the approved coupler / anchor the approved coupler / anchor attributes include a)tensile properties, ensuring the connected reinforcing steel properties are not compromised b)slip < 0.10 mm (permanent elongation after unloading from 60% / 65% of the characteristic yield strength of the reinforcing steel). this is key requirement in all the cares coupler schemes not only at the initial assessment but also ongoing factory production control surveillances. control on slip is essential to ensure adequate concrete crack control. c) mode of failure, within or outside the coupler assembly. d)performance in low cycle / high cycle fatigue according to each cares scheme appropriate to the coupler final application, whether highway bridges, critical installations such as nuclear or areas of potential seismic activity. e) designed to allow consistent and robust installation methods readily repeatable whether at fabricators or on site 3.8. methodology of conformity assessment and ongoing surveillance for the coupler / anchor approval key requirements of the cares assessment and surveillance methodologies are: a)use of assessors who have a background in the steel production and / or construction industry is critical. these assessors will also have a thorough knowledge of principals of iso9001, with clear 41 www.ukcares.com l. brankley, a. turgul, l. morgan, l. camci acta polytechnica ctu proceedings attention to factory production control (fpc), maintenance of traceability, construction product properties and installation methodology. b)the manufacturing site visits, including visiting of any key sub-contract operations are agreed with the client in advance and an audit itinerary issued to ensure full understanding of the requirements. c) a key part of each audit of the client and any subcontractors is to establish confidence in their fpc and associated procedures, purchase control and specifications, incoming inspection of goods and final product inspection and release. d)management of change is facilitated using schedules of operations which are required to be produced by each approved client and required to be updated regularly to detail any changes to the agreed manufacturing process route, critical subcontractors, purchasing specifications and product drawings. also included are the issue status of operational procedures for any rebar end preparation / threading machines leased to cares approved fabricators. these schedules are reviewed during audits to ensure there an adequate management of change record, thus ensuring confidence in the maintenance of the product properties. e) sampling of the assembled product is a key feature at each audit whether at the manufacturer or fabricator following the agreed installation procedure. these are subsequently tested at independent laboratories which are approved by cares f) continued approval and certification is only allowed following successful auditing and product testing. any areas for improvement are followed up the issue of non-conformances which require the client to undertake a root cause analysis, immediate remediation, and corrective actions. 4. identification and traceability system cares certification provides full traceability from the molten steel to the construction site by addressing all stages of production, delivery, and receipt. once installed and cast into concrete within the structure, construction components cannot be easily removed even if there is a product failure or a manufacturing deficiency. there is no ’product recall’ when things go wrong with steel in the structure of a prized project. its performance must be assured from the outset. cares have embraced the digitalisation era by developing and promoting its cares cloud. this cares digital platform plans to cover all stages in the supply chain from receipt of raw materials, the manufacture and processing of steel products through to delivery to the end user. it delivers product test results and sustainability credentials that are traceable across the whole supply chain ensuring that rebar is effectively tested, marketed, and traced. critically, in the latter stages of this development it could also confirm where these products are installed in the structure. 5. construction product supply chain cares approach the supply chain is designed to give confidence to the purchasers and the technical approval schemes are supported by the steel schemes as shown in figure 1. for a product to be cares approved, all the supply chain must be approved, whether it is the steelmaker, roller, steel trader or fabricator, there shall be no break in this link. coupler installation is rigorously reviewed during surveillance at approved fabricators as part of the cares steel scheme src08, ensuring agreed installation procedures are maintained in line with those detailed in the appropriate technical approval report. witnessed coupled rebar is taken for subsequent independent testing at an agreed rate over the surveillance period. 6. assessment models and specification • cares ta1-a quality and operations schedule for the technical approval of couplers for reinforcing steel for use in structures and structural elements designed in accordance with the fatigue requirements of structural eurocodes: this scheme is based on the requirements of the highways england [10] specification for highways structures, with continual product assessment by independent testing and ltql. testing includes both tensile and high cycle fatigue testing. • cares ta1-b quality and operations schedule for the technical approval of couplers for reinforcing steel and reinforcement anchors for static loading in tension or tension and compression: this scheme is based on the requirements of bs 8597 [8], with continual product assessment by independent testing and ltql. testing includes both tensile and optional compression testing. • cares ta1-c quality and operations schedule for the technical approval of tension or tensioncompression couplers for reinforcing steel and reinforcement anchors for nuclear applications based on sellafield engineering standard [9]: this scheme is based on the requirements of the sellafield engineering standard 42 vol. 33/2022 model assessment of reinforcing steel in concrete structure figure 1. concept of ’extended’ product responsibility applied to reinforcing steel. [9], with continual product assessment by independent testing. testing includes both tensile, including cold soak tensile testing, low cycle, and high cycle fatigue testing. • cares ta1-f quality and operations schedule for the technical approval of couplers for high cycle fatigue and low cycle loading and static loading applications in tension: this scheme is based on the requirements of iso 15835 [11] with enhancements. continual product assessment in line with the standard by independent testing. testing includes tensile, optional low cycle and high cycle fatigue testing. this is the first cares scheme to include a cementitious grouted coupler, hence evidence of conformance and product performance of the specified grout is a key part of this scheme. this built on cares experience of using grout on site for bonded tendons in post tensioning installations. 7. conclusion cares is an independent, not-for-profit certification body. the certification scheme for both the steel supply chain and installation of couplers in its requirements, providing for good management control, consistent product and technical conformity, industry developed disciplines, rules and standards and auditing by industry professionals with many yearsexperiences in the appropriate industry. cares reduce the risk of using construction products which do not meet the specification requirements, and to enable feedback and corrective action in case problems arise. major construction clients, designers, and contractors worldwide, recognise and even specify cares and its certification scheme to gain assurance of consistent product quality and technical competence. cares maintain its construction products development by attendance at the european organisation of technical approval (eota) meetings and actively participate in various iso, en and bs standard meetings. cares schemes are prepared in collaboration with the relevant technical committees of interested parties which include senior structural engineers, specifiers, and producers. references [1] bs 4449:2005+a3:2016: steel for the reinforcement of concrete bar, coil and decoiled product, european standards, 2016. [2] en 10080:2005 steel for the reinforcement of concrete weldable reinforcing steel general, 2005. [3] astm a615/a615m: 2018 standard specification for deformed and plain carbon steel-bars for concrete reinforcement & astm a706/a706m:2016 standard specification for deformed and plain low-alloy steel bars for concrete reinforcement, 2018. [4] iso 6935-2:2019 steel for the reinforcement of concrete part 2: ribbed bars, 2019. [5] ss 560:2016 (singapore standard) specification for steel for the reinforcement of concrete weldable reinforcing steel bar, coil and decoiled product, 2016. [6] iso 9001:2015 quality management systems requirements, 2015. [7] cares quality and operations assessment schedules. www.ukcares.com. [8] bs 8597:2015 steels for the reinforcement of concrete reinforcement couplers requirements and test methods, 2015. 43 www.ukcares.com l. brankley, a. turgul, l. morgan, l. camci acta polytechnica ctu proceedings [9] sellafield engineering standard es_0_3110_2 issue-4 mechanical splices to reinforcement for concrete. part 2 manufacturing, installation and construction requirements. https://www.ukcares.com/tapdf/5067.pdf. [10] highways england mhwa amendment 2014 manual of contract documents for highways works volume 1, series 1700, 2014. [11] iso 15835-1:2018 steel for the reinforcement of concrete reinforcement couplers for mechanical splices of bars requirements, iso 15835-2:2018 steel for the reinforcement of concrete reinforcement couplers for mechanical splices of bars test methods & iso 15835-3:2018 steel for the reinforcement of concrete reinforcement couplers for mechanical splices of bars conformity assessment scheme, 2018. [12] bs 8110 structural use of concrete (replaced by eurocode 2), british standard. [13] en 1992-1-1:2004, eurocode 2: design of concrete structures part 1-1: general rules and rules for buildings, 1992. 44 https://www.ukcares.com/tapdf/5067.pdf https://doi.org/10.14311/app.2022.33.0467 acta polytechnica ctu proceedings 33:467–472, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague forced carbonation of recycled concrete aggregates marc-patrick pfleger∗, markus vill university of applied sciences vienna, favoritenstraße 226 a-1100 vienna, austria ∗ corresponding author: marc-patrick.pfleger@fh-campuswien.ac.at abstract. the production of concrete, especially the contained cement clinker, causes a high proportion of the manmade gaseous air pollution. studies show that the decarbonation of limestone effects roughly 8% of worldwide co2 emissions, which underlines the need for action and research. due to the present building and infrastructure stocks and their obsolescence, or rather replacement, the amount of demolition material is permanently increasing. in many cases the demolition material consists of high-quality concrete, which could be used again as recycled aggregate in the production process to conserve sources of raw materials. most of the concrete waste is not carbonated which means that the potential as co2 sink can be exploited in the course of the recycling process. since the recycled and carbonated material is used as a replacement for primary raw material, the state of carbonation itself is irrelevant to any steel corrosion or associated problems. the main objective of this work was to investigate the co2 absorption of concrete waste, to investigate and prove its use as a possible carbon sink. at this point, the carbonation velocity had to be optimised by the use of special equipment. first test series show that the ecological footprint of concrete can be significantly reduced if concrete waste is used for carbon storage and admixed as recycled aggregate during the production process. the aim of this study was to point out further possibilities for the ecological optimisation of concrete besides alternative binders, which are the subject of research in various projects worldwide. keywords: carbonation, concrete, recycled aggregates. 1. introduction the protection of the environment and the global climate is currently a major field of action. not only changing climate conditions are perceptible, but also rising raw-material demand, land consumption and the pollution of important habitats occur as main problems. since every industrial sector has more or less impact on the environment, the question arises where significant entry points for ecological optimisations can be found. often a close connection can be found between the amount of materials used by a particular business branch and its environmental impact. the construction industry and related trades are associated with an enormous need for raw materials. further processing uses high amounts of energy, which means that there is a tangible potential for greenhouse gas reduction. one of the most commonly used materials for civil engineering structures and buildings at present is concrete or rather reinforced concrete. especially the contained cement clinker is a highly climate relevant substance. according to the current state-ofthe-art, structural concrete cannot be adequately replaced, neither technically nor economically. its robustness, stability and formability, in combination with low material and manufacturing costs, are the main reasons for the continuously rising demand. air-pollution is increasing analogously to the growing amounts of produced binder or rather the contained cement clinker. the fact that nowadays about 8% of the worldwide co2 emissions (by population) result from producing cement has to be highlighted as absolutely critical. the mentioned emissions result only partly from combustion processes to provide the needed heat to process clinker out of lime stone. if modern technology is used, over 50% of the overall co2 emissions concerning the cementitious binder production result from the lime stone decarbonation, as seen in table 1. emissions due to the raw material decarbonation are process-related and cannot be substituted, if the known characteristics of cement shall not be modified. especially the corrosion protection of the reinforcement depends on the alkalinity of the hardened concrete. this paper brings the described problems with greenhouse gas in context with rising amounts of demolition materials, especially concrete recycling products. the majority of concrete waste is not carbonated, in particular the inner sections of structural elements of many civil engineering structures, like bridges with large thicknesses of concrete members. 2. background for ecological optimisation of concrete as described before, concrete is an indispensable construction material, because of its technical charac467 https://doi.org/10.14311/app.2022.33.0467 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en marc-patrick pfleger, markus vill acta polytechnica ctu proceedings raw material decarbonation combustion of fossil fuels combustion of biogenic fuels total 2016 524 260 72 856 2017 517 256 71 844 2018 515 254 75 844 table 1. specific co2 emissions in [kg] per [to] clinker in austria [1]. teristics. therefore, optimisations concerning the ecological footprint of concrete can actually only be made if the occurring of actual disadvantages can be consistently avoided. in general, improvement potential can be seen in alternative binder recipes or using high-grade recycling processes for an ideal reuse of concrete waste. the minimum share of cement clinker in the most commonly used standardised cement mixtures (cem ii) is about 65%. the rest consists, depending on the exact cement type, of fly ashes, blast furnace slag or lime dust. the currently available amount of fly ash and slag is already completely used up, so that no more substitution can be expected. furthermore, due to the focus of future energy policy, the available quantity of additives, based on fossil combustion processes, will decline. therefore an extensive amount of research activity is done worldwide to develop alternative binders for concrete applications, like [2], [3]. commonly the main objective is to reduce or avoid the share of cement clinker in the binder mixture. problems with durability may occur as a consequence and are not investigated enough in long term tests. apart from binder optimisation the rising stream of demolition material has to be involved in high quality recycling. particularly in urban regions new buildings can only be produced if a site is previously cleared from its original building stock. it can be observed worldwide that a building’s life cycle is decreasing, which means that the building stock becomes a more important source of raw material in the sense of "urban mining". generally demolished concrete structures are processed by one or more crushers to receive a useable concrete granulate. depending on the valid standards, this concrete recycling aggregate can be used with a certain percentage as replacement for natural aggregates. concerning the national documents of austria, additional differentiation is made for admixing, depending on the grain size of the recycling product [4]. usually those recycled aggregates have a grain size distribution from 0 to 30 mm or smaller. the maximum grain size in general depends on the desired configuration of the jaw crusher. 3. carbonation of concrete 3.1. carbonation as concrete damage the carbonation of concrete is usually known as a classic concrete damage and can be described as the reaction of co2 from the surrounding atmosphere with calcium hydroxide, as a product of the hydration of cement clinker. the calcium hydroxide is contained in the pore water of a concrete matrix and its concentration may vary due to the particular binder mixture and the surrounding conditions during hydration. the specific concentration of lye determines the alkalinity of the pore water and thus the capability to prevent the reinforcement from corrosion. under the absorption of co2 this potential declines over time, starting from the concrete surface. therefore the tolerable carbonation depth is generally limited to the minimum concrete cover of the reinforcement. concrete technological measures are taken to prevent fast carbonation. main ambient parameters for the carbonation velocity can be: • carbon dioxide level of the surrounding atmosphere, • ambient air and concrete temperature, • concrete moisture, • pressure. additional influencing factors result mostly from the material itself such as content of cement, porosity or (micro) cracks which provide an increased surface area for carbon dioxide absorption and reaction [4]. 3.2. carbonation of concrete as carbon sink the inner cross-section parts, or rather the area between the outer reinforcement layers, are usually not carbonated until the demolition of a structure. emitted co2 caused by the calcination of the binder remains in the atmosphere and has a negative impact on the materials ecological footprint. the question arises as to which surrounding conditions and related time span are required until the carbonation process is completed after a concrete structure is recycled into aggregates. as described before, the carbonation of concrete is a natural process, though natural conditions only lead to a slow reaction process with the actual co2 consumption not defined. nevertheless, there is carbon absorption at any time where the essential conditions, such as concrete moisture, are given. the chemical conversion of calcium hydroxide into calcium carbonate effects that a solution transforms into solid [5]. as a result, the gas permeability of the cement matrix decreases and 468 vol. 33/2022 forced carbonation of recycled concrete aggregates figure 1. not-carbonated share of a cross-section. the carbonation velocity asymptotically approaches zero within the grain [6]. based on this knowledge, an artificial method shall be preferred in comparison to carbonation under natural conditions. controllable parameters and the possibility of measurements during the carbonation ensure a consistent recycling product [7]. 4. experimental setup and methods the test setup was designed to investigate forced carbonation of recycled concrete aggregates, which means that no correlation with natural carbonation could be expected. the first step of the experimental carbonation test series was to determine the exact amount of carbon dioxide which can be bound by a concrete matrix. for this purpose, special equipment was necessary, since standard carbonation chambers do not allow to vary and monitor the reaction conditions in real time as desired [8]. especially the used experimental setup allowed to monitor and control the exact co2 consumption of the sample material in real time. further it was planned to find the optimal parameters to fully carbonate a particular sample as fast as possible. 4.1. reaction parameters as basis for the equipment design, the previously defined external parameters were taken into consideration. thus temperature, humidity as well as the amount of co2 in the sample atmosphere should be varied. increasing the ambient pressure was planned for future sample series. the range of reaction parameters was defined according to feasible values in large-scale techniques or rather concerning industrial facilities as shown in table 2. from to co2 0,04% 30% humidity 0% 100% temperature 20◦c 65◦c table 2. range of reaction conditions. 4.2. equipment design the equipment consists of a carbonation chamber that can be top-loaded with the sample material. a removable grate is filled with the sample aggregates up to 25cm layer thickness. the container sits hermetically on a catch, so that a forced flow of the ambient atmosphere through the material can be ensured by the integrated fans. the perfusion of the sample container happens from bottom to top. as shown in figure 2, all fittings and connections are located at the cover of the carbonation chamber. so only the tempered and humidified granulates are inside the reaction tank which consists of stainless steel. the acryl glass lid is prepared for an attachable steel frame to ensure sufficient stiffness if tests with overpressure will be carried out in the future. a main requirement was that the co2 absorption of the samples should be measurable at any time of the carbonation process. a standard carbonation chamber does not allow a real-time monitoring of the actual co2 consumption as desired. further, any implementation of additional custom equipment is easier, dealing with a setup from scratch. it is planned to generate a completely automated process by using artificial intelligence to set the ideal reaction conditions and vary them over the test’s duration. due to the moistened material and the increased humidity inside the chamber it is not possible to obtain reasonable results by measuring the sample’s mass during the experiment. the material moisture varies during the test. additionally, condensation will distort 469 marc-patrick pfleger, markus vill acta polytechnica ctu proceedings figure 2. simplified scheme of the test set-up. any weighing process inside the chamber. co2 reacts with the calcium hydroxide contained in any concrete waste to caco3, so that a direct conclusion can be drawn about the absorbed carbon. therefore the increasing weight of the aggregates shall be proved by other boundary parameters such as the decreasing gas bottle mass, the opening time of the valve and the flow volume of co2. all these three parameters where monitored and compared to validate the accuracy of the data. the measured values are documented and analysed in real time, which allows statements about the actual co2 consumption according to current reaction conditions. at the end of the experiment dry mass of the sample can be weighed and compared with the initial mass in order to verify the results derived from the experiment. data-logging and the real-time analysis is physically located in an extra housing where also the co2 probe was placed. gas from the reaction chamber is extracted, analysed and pumped back. since only carbon consumption was expected here, no precautions were taken to reduce the carbon dioxide share of the test atmosphere. due to the more or less constant reaction of co2 the valve is only opened when necessary to keep the concentration just as desired. ideally the reaction conditions are varied during the carbonation process to achieve more efficiency. furthermore, initial tests show that varying the reaction conditions according to different cement types gives better results. 4.3. samples granulated concrete with accurately known recipe was found to be a suitable sample material for the first test series, which also should serve for data verification. several concrete test cubes with 15cm edge length were produced and stored in the air for 60 days under standardised conditions for 60 days exposed to air. 5. test series 5.1. sample concrete aggregates all experiments carried out with crushed concrete aggregates were done by using the same concrete mixture to achieve ideal comparability. the determined samples were made from c25/30 containing cem ii b with a maximum grain size with 22mm in the matrix. the binder portion was defined as 15% of the wet concrete mass. the water-cement ratio was kept at 0.5. 5.2. preparation of the sample material the first step of each test series was the determination of the samples dry mass. to extract residual moisture the grains were put in a dry kiln until the dry weight was proven by mass stability. as basis for all test series the aggregate mixture was separated into grain fractions 0 to < 8mm and 8 to 22mm. the fraction <8mm had 26.5% of the total mass, thus 73.5% were weighed with 8-22mm. depending on the particular experiment the samples were moistened with a defined amount of water to exploit the influence of material moisture to the carbonation velocity. in fact, the ideal material moisture cannot be expressed with a simple percentage, since the porosity, in particular the pore diameter, is a key factor for the amount of water needed to moisten the pore walls [5]. 470 vol. 33/2022 forced carbonation of recycled concrete aggregates figure 3. carbonation results for c25/30, cem iib, grain size 0 − 8mm. 5.3. structure of the grain fractions after separating the grain fractions visual inspection alone showed that most of the cementitious parts of the crushed down concrete find themselves mainly in small grains. this means that also most of the potential carbon sink was initially expected in small grain fractions. 5.4. influence of the layer thickness several carbonation tests were done with mainly small grain sizes from 0 to 8mm. as expected, the permeability of a sample layer consisting of only small grain fractions is poor. the effect is seen when checking the carbonation of grains from the centre of the bulk. when testing coarse aggregates, a more consistent carbonation through the whole layer depth was verified, whereas aggregate mixtures do not ensure enough permeability as well as recycling sands. the issue with poor permeability can be obviated by using a non-static process. 6. results 6.1. concrete aggregates the following results show the total amount of bound carbon dioxide in the samples over the test duration. ideally the mass increasing shall not be compared directly with the total mass at the experiment’s start, because of the contained natural aggregates in the concrete matrix, which are not reactive. when the ecological improvement is calculated, only the binder or cement clinker share shall be considered. 6.1.1. grain sizes under 8mm figure 3 shows exemplarily how carbon absorption proceeds at grain fractions below 8mm. the experiments duration was 24 hours with 20% co2 in the carbonation chamber, heated to 40◦c. the carbon dioxide concentration as well as the temperature was constant during the whole test. the sample’s dry mass was determined to 11.289g at the beginning. it can be seen that the co2 consumption is very high at the beginning, which can be explained by the rapid carbonation of the surface areas of the crushed figure 4. carbonation results for c25/30, cem iib, grain size 8 − 22mm. down concrete and the high share of cement of the recycling sands. the further course of the graph varies visibly over time. in this case this phenomenon cannot be explained by the saturation of the aggregate. when the test was completed it was seen that poor carbonation results were achieved in the centre of the bulk. this can be explained by too little gaseous exchange due to the high packing of the small grains. the test ends with 238g co2 bound in the sample, resulting in 21g co2 per 1kg recycled aggregate (grain size < 8mm). 6.1.2. grain sizes over 8mm figure 4 shows exemplarily how carbon absorption proceeds at grain fractions over 8mm. the outer parameters of the experiment were exact the same as mentioned in 6.1.1., which means: 24 hours of carbonation with 20% co2 at 40řc in the test setup. both the carbon dioxide concentration and the temperature were stable during the whole test. the sample’s dry mass was determined to 11.385g at the start. again it can be seen that co2 consumption is high at the beginning, whereas the gradient is less prominent when smaller fractions are analysed. the graph shows a clearly more flat carbonation curve at the start due to the smaller surface to volume ratio. furthermore, the reaction velocity decreases permanently over time and approaches zero. after the removal of the sample material it was seen that constant carbonation took place in all layers of the bulk. the test ends with 284g co2 bound in the sample, resulting in 25g co2 per 1kg recycled aggregate (grain size 8-22mm). the test series show that co2 can be absorbed with 2.1% to 2.5% of the sample mass in a static process. that mass increase is accompanied by the formation of lime in the concrete matrix which means that simultaneously the porosity of the samples must decrease. this phenomenon was proved by an additional saturation experiment before and after the carbonation process. 471 marc-patrick pfleger, markus vill acta polytechnica ctu proceedings 6.2. exemplary presentation of the ecological benefit based on the results from 6.1. the ecological advantage of recycling concrete with forced carbonated can be pointed out. due to current austrian national standards, the admixture of up to 50% recycled which is explained by unpredictable increased water needs [4]. the following example states, that 50% of the natural aggregates are substituted by a forced carbonated recycling product. it is expected that a concrete strength of c25/30 with cem ii shall be achieved. it can be assumed that 900kg of carbonated aggregates are admixed while concrete production. thereby 22.5kg carbon dioxide can be credited per m3 fresh concrete if the ecological footprint of the material is calculated. in comparison to the carbon emissions due to the binder production with approximately 170kg per m3, an improvement of about 13% can be achieved. 7. conclusion the exemplary stated test series show the potential of forced carbonation of recycled concrete aggregates in principle. an absorption of 2.1% to 2.5% of the sample’s mass was achieved by the test setup. as the cement clinker production increases permanently worldwide and thereby the carbon dioxide emissions rise, ways for ecological optimisation must be found. the experiments show that forced carbonation is suitable to utilise existing carbon sink potentials of demolished concrete. if the carbonation of recycled concrete aggregates can be implemented in current processes with high carbon dioxide emissions, such as cement plants, the ecological footprint of concrete actually can be reduced. currently, the life-cycle assessment of concrete does not take recarbonation into account. in practice carbonation of structural concrete elements is generally present. however, only the surface areas can be practically considered during a buildings period of use. at the end of a structure’s life-cycle, a demolished concrete is stored in piles and may be used as recycled aggregates for mixing concrete or technical backfill material. as the test showed an increased natural carbonation cannot be expected, because of insufficient air flow or rather gas exchange through the stored material. a forced carbonation process can efficiently use the crushed down material as carbon sink in a much more predictable period of time. for this reason an uninterrupted recycling process leading to the final product is possible. the further purpose is to show if the expected differences in the carbonation behaviour of different cement types will occur during the tests. if decisive differences are seen, the findings can simplify a possible automation for the adjustment of ideal carbonation parameters. references [1] g. mauschitz. emmissionen aus anlagen der österreichischen zementindustrie, berichtsjahr 2018. technical university of vienna, 2019. https://www.zement.at/downloads/downloads_2019/ emissionen_2018.pdf. [2] t. higuchi, m. morioka, i. yoshioka, et al. development of a new ecological concrete with co2 emissions below zero. construction and building materials 67:338-43, 2014. https: //doi.org/10.1016/j.conbuildmat.2014.01.029. [3] j. juhart, c. baldermann, g-a. david, et al. erescon neue betone für den infrastruktur-bereich, energieund ressourcenoptimiert. eigen, 2015. https://www2.ffg.at/verkehr/studien.php?id= 1112&lang=de&browse=program. [4] önorm b 4710 1:2017-06-15. austrian standards institute, vienna, 2017. [5] h. grube, j. krell. zur bestimmung der carbonatisierungstiefe von mörtel und beton. beton 36(3), 1986. [6] j. gajda, f. m. miller. concrete as a sink for atmospheric carbon dioxide: a literature review and estimation of co2 absorption, portland cement concrete: portland cement association, 2000. . [7] c. gehlen. probabilistische lebensdauerbemessung von stahlbetonbauwerken. dafstb (= dafstb-heft. band 510). beuth, 2000. [8] m. seidemann, a. müller a, h. ludwig. weiterentwicklung der karbonatisierung von rezyklierten zuschlägen aus altbeton (2. phase: prozessoptimierung im labormaßstab und technologieentwurf), 2015. https://www.dbu.de/opa c/ab/dbu-abschlussbericht-az-23689_02.pdf. 472 https://www.zement.at/downloads/downloads_2019/emissionen_2018.pdf https://doi.org/10.1016/j.conbuildmat.2014.01.029 https://www2.ffg.at/verkehr/studien.php?id=1112&lang=de&browse=program https://www.dbu.de/opac/ab/dbu-abschlussbericht-az-23689_02.pdf acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.34.0020 acta polytechnica ctu proceedings 34:20–25, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague heat transfer model verification for thermal monitoring system of integral bridges martin jonáš∗, jan zatloukal czech technical university in prague, faculty of civil engineering, experimental centre, thákurova 7, 166 29 prague 6, czech republic ∗ corresponding author: martin.jonas@fsv.cvut.cz abstract. thermal actions considered on abutments of integral bridges by the european standard (eurocode 1) might be in variety of certain situations underestimated. based on this assumption, thermal monitoring system has been designed and installed into structure of bridge no. 27-117. temperature profiles in five certain spots are being measured to provide sufficient basis for evaluation of real thermal actions on abutments of integral bridges. for purpose of measurement verification, elementary heat transfer models for finite element method solver were created. these models are being loaded by simplified temperature profiles reached in real time in-situ. evaluated models provide verification data to compare with measured temperature profiles in structure and might provide information about temperature profiles probably reached during extreme weather situations. keywords: thermal actions, integral bridge, thermal monitoring, heat transfer. 1. introduction while integral bridges do not allow free temperature dilatation of bridge deck, shifts are transferred into abutments and subsequently into surrounding soil. therefore, proper investigation of interaction between soil and structure is necessary. soil-structure interaction problem of integral bridges is widely described in literature. more than sufficient background is available for thermal actions on bridge decks and effects bound with them. in case of temperature loading of abutments is the situation weaker. overall influence of lack of more accurate approach to temperature loading of abutments might not be very significant. however, in some cases it might be suitable to consider distribution of temperature in the structure more precisely. thermal actions considered on integral bridges being designed in the czech republic are given by european standard en 1991-1-5 [1], its national annex čsn en 1991-1-5[2] and in case of road bridges also by technical conditions tp 261[3] issued by ministry of transport. these documents provide wide and specific approach to consideration of thermal actions on bridge structures. however, in case of bridge piers and abutments, none of these documents can provide specific background sufficient enough for proper analysis of specific temperature states of these parts of structures. on the other side, it is not expected, that neglecting of more precise thermal action analysis of structure parts might lead to more or less serious damage of structure. rather, it could lead to the long-term negligible degradation of materials used in structure, which could result in reduced service life of structure. to cover these potential excess of standard background, stationary measurement system was designed and installed into bridge no. 27-117. system is created to monitor temperatures in multiple profiles deployed across the whole structure. last, but not least, heat transfer models of measured spots were created for purpose of measurement verification. their accessory use is for preliminary modelling of potential temperature states reached in structure during the whole year, while the full operation of system is not launched. 2. thermal actions on abutments of integral bridges background given by standards and conditions [1][2][3] for thermal loading of piers and abutments is very simple. except of uniform temperature component, linear temperature component is given by recommendation [1]. national annex, nor technical conditions does not specify more precisely horizontal/vertical temperature components, as they do in case of bridge deck or beams, for example. therefore, linear horizontal temperature component is used according to recommendation of values for concrete piers and walls. linear temperature difference with value of 5 °c between outer edges of concrete piers is used and linear temperature difference with value of 15 °c between inner and outer edge of wall is used. 3. monitoring system on bridge no. 27-017 for purpose of proper analysis of thermal actions on integral bridges, stationary measurement system was designed and installed into the bridge no. 27-017 in železná ruda alžbětín. this bridge was chosen for its suitable location in the šumava mountains, where more extreme weather conditions can occur (during 20 https://doi.org/10.14311/app.2022.34.0020 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 34/2022 heat transfer model verification for thermal monitoring system of integral bridges the winter 2020-21 temperatures reached about -20 °c). the main function of monitoring system is measurement of temperatures in multiple spots deployed across the bridge structure. its secondary function is measurement of strain and humidity in the bridge deck. whole system will be fully automatic and will send measured data to the server on its own. figure 1. deployment of all the measured spots in the ground plan of the bridge structure. temperature profiles, which are being measured are chosen according to expectations and purpose of measurement. two spots are deployed in bridge deck, where, on each spot, temperature of steel beam and concrete is measured. for bridge deck, standards give more possibilities of vertical temperature difference consideration. therefore, it is not expected, that measured temperature profiles in bridge deck would exceed values given by standard. other three spots are located in abutment shaft, each spot is implemented by 6 thermometers. measurements in the abutment shaft are divided across the width of abutment, so that possibility of sunlit thermal influence is covered. while the standard[1] gives temperature difference of 5 °c, it is highly probable, this value will be exceeded in measured abutment. pt 1000 thermometers were used due to their robustness, accuracy and simplicity of operation. thermometers dedicated for concrete temperature measurement are all specifically prepared for quick, accurate and safe installation into bridge structure. this preparation consists of thermometer encapsulation into "shells" made of high-performance concrete (using msk-2 substance[4]) and further preparation of prefabricated casing beams, where are thermometers precisely deployed in accurate positions. concrete casing beam is further installed into bridge structure figure 2. example of the thermometer deployment into the bridge deck with usage of the prefabricated casing beam. only by inserting into grid of reinforcement bars and fixating its position in proper place. thermometers measuring temperature of steel are attached to the filler steel beam by hot glue from the outer side, so that the contact of thermometer and steel is secured. figure 3. installed casing beam with thermometers in abutment shaft. cables are led along reinforcement rods for protection before damage during concreting of abutment shaft. measurement is processed by the switchboard solely designed for purpose of inexpensive, reliable and simple monitoring systems suitable for structures, where monitoring of specific values is required or suitable. switchboard is designed for significant reduction of costs, while usually used loggers are expensive and difficult to obtain in reduced time. thus, price is reduced from few hundred thousand to several tens of thousands czk. core of switchboard is tiny raspberry pi computer, which processes measured data. temperature measurement itself is processed by lucid io reading units, which are designed and widely used for resistance sensors. power supply might be secured by connection into power grid, or by usage of small solar panel. backup power supply is provided by common 12v batteries. switchboard is connected via modem to the internet and sends measured data to the server regularly. temperature measurement period is basically set to 30 minutes. 21 martin jonáš, jan zatloukal acta polytechnica ctu proceedings figure 4. installed switchboard on-site. 4. thermal models while the commissioning of measurement system was delayed, computer simulations seemed as a suitable replacement of first data evaluation. 2d heat transfer models of measured spots in structure were created to simulate first data of temperature division along the thickness of abutment shaft or bridge deck during season extremes. however, their main purpose is verification of measured data in following months, when measurement system will fully start its operation. the thermal model is created for oofem solver software [5] and solves heat transport. material properties, such as density, thermal capacity and conductivity are defined. the linear 2d elements are used in these models and geometry with mesh were created in salome processor. 4.1. abutment model abutments model describes cross-section of measured spot in the abutment shaft. model consists of three layers, 2 concrete layers and one layer of soil. two layers of concrete are used to enable finer element mesh in the contact of concrete with air. width of model is 3 m, height is 1,8 m, from which layer of concrete takes 0,6 m, rest height of 1,2 m represents soil. concrete is split into two layers, first layer with height of 0,1 m, second of 0,5 m. quadrangle mesh was used, always 10 elements per edge of face, resulting in 100 elements per one face. size of mesh is chosen so that positions of sensors in real structure are in mesh nodes. initial and boundary conditions are set for this model very simple. initial temperature is set for all nodes to equal initial value. boundary condition is set for both edges. dirichlet’s condition is used on bottom edge (immutable set temperature). top edge has two boundary conditions, newton’s (contact with ambient air) and stefan-boltzmann’s (heat radiation). load is performed by temperature profile typical for extreme situations. figure 5. mesh for abutment shaft model with visible layers. 4.2. bridge deck model two different models in bridge deck are created, according to deployment of sensors in real structure. first model replaces spot under traffic lane, second one represents spot under granite pavement. model of bridge deck under traffic lane consist of cut out from cross-section of bridge deck. model contains all materials of bearing structure, insulation and layers of roadway. bearing structure is a composition of steel filler beams, cetris plates and concrete, which fills whole remaining area. on the top of concrete, insulation made of asphalt strips (naip) is made. three layers of different asphalt concretes are laid on the insulation. these three layers have parameters set identical, while the thermal parameters are similar and it would be necessary to obtain them experimentally. filler beams are of heb240 profile with narrowed top flange. on the bottom flanges, cetris plate of 20 mm thickness is put as lost formwork. concrete layer of 280 mm thickness is concreted. insulation layer (naip) is 10 mm thick. three layers of asphalt concrete are embedded into one, 135 mm thick. whole model is 1200 mm wide. triangular mesh with elements of minimal size with 5 mm edge is used. maximal element size with 100 mm edge is set. initial condition represents initial temperature of all nodes varying to situation being solved. three boundary conditions are used on the top edge of model, newton’s (contact with ambient air), stefan-boltzmann’s (heat radiation) and neumann’s (sun glare). on the bottom edge is neumann’s boundary condition omitted. temperature and glare functions are set according to available information. second one, model of cut-out from the bridge deck under pavement contains all materials of bearing structure and layers of pavement. bearing structure is a composition of steel filler beams, cetris plates and concrete, which fills whole remaining area. pavement consist of two layers, bed layer of concrete and granite pavement. filler beams are of hea300 profile with narrowed top flange. on the bottom flanges, cetris plate of 20 mm thickness is put as lost formwork. concrete 22 vol. 34/2022 heat transfer model verification for thermal monitoring system of integral bridges figure 6. mesh for bridge deck model under traffic lane with visible layers. layer of 440 mm thickness is concreted. concrete bed layer of pavement is 100 mm thick with 200 mm thick layer of granite pavement on it. whole model is 1200 mm wide. mesh, elements, elementary and boundary conditions are identical with the model of bridge deck under traffic lane. 4.3. material properties material parameters for each modelled material are defined in the model. heat capacity c, volumetric mass density γ, thermal conductivity λ and emmisivity ε have to be specified. most of the values are taken from engineeringtoolbox [6]. parameters for concrete were measured at the laboratory and their values are for concrete used in abutments c = 806 jkg−1k−1, λ = 2.638 wm−1k−1, γ = 2327 kgm−3 and for concrete used in the bridge deck c = 770 jkg−1k−1, λ = 2.238 wm−1k−1, γ = 2253 kgm−3. 5. model situations each model was loaded by three extreme model situations, representing coldest days in winter, most warm days in summer and days with maximal temperature difference during 12 hours. these situations were considered to simulate maximal temperature difference across the thickness of bridge deck or abutment shaft. 5.1. minimal temperature according to data from chmi[7], lowest reached air temperature was -22 °c, what was considered into design of thermal loading function. time period for computation was set to 200 hours (8 days and 8 hours) and model was loaded with following temperature function: t(t) = 6 sin [( 2π 86400 ) ∗ (t− 9 ∗ 3600) ] − 16 (1) this function represents cold winter day with clear sky, when minimal temperature of -22 °c is reached before sunrise and highest day temperature rises to -10 °c. initial temperature was set to 0 °c for all nodes. results confirmed expectation about standard values exceeding in abutment shafts, reaching the difference of about 14 °c, see fig 7. in bridge deck, reached values did not exceed standard values. figure 7. horizontal temperature difference in abutment shaft. 5.2. maximal temperature maximal temperature profile expects warm summer day with maximal temperature of 35 °c (maximal reached temperature was 34,8 °c) and minimal temperature of 21 °c in the night. amplitude of temperature sin function was set to 7 °c, temperature shift to 28 °c, period of time was 200 hours and initial temperature 20 °c. bridge deck models were loaded also by sun glare. solar irradiance is represented by time-dependent function of heat flux q(t). to ensure exclusion of negative part of sine wave, heaviside step function was used. amplitude of heat flux q(t) was set to 800 wm−2. results have shown again, that in case of abutment shaft, reached horizontal temperature differences are significantly higher than standard considers. however, the difference was lower than in minimal temperature situation. in case of bridge deck, values did not exceed standard again. figure 8. heating of both surfaces during the day in the bridge deck, in spot under pavement. 5.3. maximal temperature fluctuation maximal temperature fluctuation might be represented by a day with high afternoon temperatures, which falls deep in the night before dawn. this period occurs in the early autumn months. representative 23 martin jonáš, jan zatloukal acta polytechnica ctu proceedings day with extreme temperature fluctuation was modelled. maximal temperature is 26 °c and minimal during the night falls to 0 °c (ground frosts). initial temperature was set to 10 °c and period of time to 200 hours. bridge deck models were loaded by sun glare again. amplitude of heat flux q(t) was set to 650 w−2. results have confirmed the expectations again, in abutment shaft, temperature difference reached almost 10 °c. the difference is therefore lower than in minimal temperature and comparable with maximal temperature simulation. temperature fluctuation in the abutment shaft is shown in fig 9 in case of bridge deck, values did not exceed standard again. figure 9. temperature development in sensor spots, surface is represented by dark red line (600 ⇒ 600 mm from contact soil-concrete), contact with soil by blue line. 6. first control measurement together with switchboard installation, first control measurement was taken. data from all the sensors were taken and stored. due to weather, measured temperature profiles were with minimal temperature difference between outer edges. weather few days around installation dramatically changed more times and during the installation, temperature was almost constant during whole day. therefore, computation of verification model was practically disabled, while the model needs few-day settled weather. however, it was possible to evaluate at least temperature change in measured spots, what can be seen for abutment shaft in fig 10. 7. further progress together with full operation of the monitoring system, models will be finally used for their main purpose, verification of measured data, what was not possible until today. during the verification process, model parameters should be fitted. special attention should be dedicated to adjustment of loading functions, where some parameters are at least uncertain, especially solar irradiance, which may be highly influenced by many impacts. 8. conclusion automatic measurement system should provide longterm data for completion of thermal actions standard figure 10. measured temperature profiles in three spots in the abutment shaft (1 ⇒ 1 m from the edge of abutment; 2 ⇒ 4 m from the edge; 3 ⇒ in the middle of the abutment). background needed for proper complex analysis of integral bridges. according to models, evaluated before full system operation, thermal actions on abutments reach significantly higher effects than literature considers. therefore, detailed evaluation of long-term measurement will be important to implement into analysis sources and consider in future designed bridges. price and simplicity of used measurement system is very strong argument for installation of similar systems into newly constructed structures, where available sources for analysis might be insufficient, or consider irrationally high load effects. e.g., almost identical systems are now being installed into three tunnels under construction in czech republic. acknowledgements this work has been supported by the technology agency of the czech republic under the project th04010038 system for permanent monitoring of material degradation in civil structures. references [1] en 1991-1-5: eurocode 1: actions on structures part 1-5: general actions thermal actions. tech. rep., the european union per regulation, 2003. [2] čsn en 1991-1-5. eurokód 1: zatížení konstrukcí část 1-5: obecná zatížení zatížení teplotou. tech. rep., úřad pro technickou normalizaci, metrologii a státní zkušebnictví, 2005. [3] m. drahorád, m. foglar, b. polák, v. hrdoušek. integrované mosty, tp261. tech. rep., ministerstvo dopravy, 2017. [4] z. bažantová, k. kolář, p. konvalinka, et al. multifunkční silikátový kompozit programovatelných vlastností nejen pro rychlé opravy cementobetonových konstrukcí. materiály pro stavbu 22(9):34–37, 2016. [5] b. patzák. oofem project home page. http://www.oofem.org, ctu in prague, faculty of civil engineering, prague, since 2000. 24 vol. 34/2022 heat transfer model verification for thermal monitoring system of integral bridges [6] engineeringtoolbox. material properties. https://www.engineeringtoolbox.com/materialproperties-t_24.html. [7] čhmú. měsíční a roční data dle zákona 123/1998 sb. http://portal.chmi.cz/historicka-data/pocasi/mesicnidata/mesicni-data-dle-z.-123-1998-sb. 25 acta polytechnica ctu proceedings 34:20–25, 2022 1 introduction 2 thermal actions on abutments of integral bridges 3 monitoring system on bridge no. 27-017 4 thermal models 4.1 abutment model 4.2 bridge deck model 4.3 material properties 5 model situations 5.1 minimal temperature 5.2 maximal temperature 5.3 maximal temperature fluctuation 6 first control measurement 7 further progress 8 conclusion acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.35.0049 acta polytechnica ctu proceedings 35:49–53, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague the long-pitch corrugation development in small radii curves jan valehrach∗, tomáš říha, jiří vendel, otto plášek brno university of technology, faculty of civil engineering, institute of railway structures and constructions, veveří 331/95, 602 00 brno, czech republic ∗ corresponding author: valehrach.j@fce.vutbr.cz abstract. the paper deals with the phenomenon of long-pitch corrugation in curves of the small radii and also the effects that accelerate or slow down its development. the paper presents data from sections of railway track measured for new rails, rails before and after grinding or replacement. the effect of train speeds running through the curves and the associated cant deficiency or excess were studied. the tendencies of long-pitch corrugation development are described, which can be used to estimate when maintenance action or rail replacement will be necessary. keywords: cant defeciency, cant excess, long-pitch corrugation, speed, track, train. 1. introduction various technologies to remove rail defects such as the long-pitch corrugation are present. the spectrum starts with the preventive grinding and spans all the way to a complete replacement of the rails. to understand which technology is for the track section or even for the specific curve at the time the most durable, a broad microgeometry data collection is necessary. based on the data history it is then possible to predict the further defect development and to lay down the maintenance guidelines in order to improve the possessions plan and to minimize negative effects on the railway operation. although there are procedures and methods to measure a microgeometry of rails, in most cases these applications are simplified for practical use in the track record, for compiling substantial amounts of data, or for the "rail reprofiling reception" [1]. the evaluation for the purposes of the development of the long-pitch corrugation then happens only based on line manager requests. for the purposes of measuring and monitoring the defects are categorized according to čsn en 13231 3 [1] into four wavelength ranges: • 10 – 30 mm, • 30 – 100 mm, • 100 – 300 mm, • 300 – 1000 mm. according to [2], wavelengths in the range 100–300 mm are considered as long-pith corrugation (corrugation primarily on the surface of inner rails in curves). the categorization mentioned above is not very suitable for the long-pitch corrugation monitoring, hence the length of the waves could be even shorter than 100 mm [3, 4], especially in the curves of small radii. following [5], based on the long-pitch corrugation creation theory, own measurements, and data comparison, it could be concluded, that for the long-pitch corrugation monitoring the range 30-100 mm and the range 100-300 mm may be merged, since wave amplitudes in both ranges usually reach similar or even same values. therefore, according to the above stated, for the rail long-pitch corrugation in particular the ranges 30-100 mm a 100-300 mm are to be monitored. 2. the long-pitch corrugation development monitoring, evaluation, and data quantification of the measured data were conducted using the rms and p2p calculations. the assessment originates from the 2006 standard čsn 13231-3:2006 [6], and not from the actual 2012 standard čsn 13231-3:2012 [1] due to the exclusion of the rms method. to be able to use the data measured before the acceptance of the 2012 standard, older standard was used. all the measurements were conducted on the railway line no. 326 (brno – česká třebová) between traffic point brno – maloměřice st. 6 and railway station adamov. own measurements took place in sections called hady (ha1.1 and ha1.2 to ha1.4), bílovice (bv1.1 and bv1.2) and babice (bs1.2), the track recording car data were used for the section adamov (ad1.1). once the calculated values rms and p2p are inserted into the graphs, the development speed gradient of the monitored magnitude could be obtained. examples of the graphs for measuring places in section hady are shown in figure 2 and figure 3. the line-up of the measuring places according to the gradient is shown in table 1. the steeper gradients are positioned lower. the consecutive order is also affected by the wavelength range. when the weighted cant deficiency (or cant excess 49 https://doi.org/10.14311/app.2022.35.0049 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en j. valehrach, t. říha, j. vendel, o. plášek acta polytechnica ctu proceedings figure 1. placement of the sections on the railway line no. 326 (brno – česká třebová). figure 2. rms progression in time for section hady. respectively) is calculated for each measuring place (curve) as shown in table 2, the rank of the values in comparison with the tab. 1 matches the wavelength range 100-300 mm. figure 3. p2p progression in time for section hady. 30-100 mm 100-300 mm bs1.2 bs1.2 bv1.2 ha1.1 bv1.1 bv1.2 ha1.1 ha1.2-4 ha1.2-4 bv1.1 table 1. measuring places line-up according to gradient. curve lweighted [mm] bs1.2 21.0 ha1.1 46.1 bv1.2 67.6 ha1.2-4 73.8 bv1.1 87.0 table 2. weighted cant deficiencies in curves for all recorded trains (not rounded). 3. development prediction 3.1. prediction based on the previous defect progression sections hady and bílovice using the gradients, the timeframes required for reaching the rms and p2p values were calculated for the purposes of the prediction of the future defect development. the timeframes respect the replacement works, or the rail grinding according to the particular place or section. the timeframe size differs significantly according to 50 vol. 35/2022 the long-pitch corrugation development in small radii curves section hady bílovice measuring place 1.1 1.2 1.3 1.4 1.2 number of years 0 – 24 40 – 63 0 – 24 40 – 63 0 – 24 40 – 63 0 – 24 40 – 63 from timeframe 30-100 rms 5.1 6.9 4.1 5.1 4.9 5.7 3.6 4.5 5.0 p2p 4.7 6.3 4.0 4.2 4.7 5.6 3.4 5.3 5.1 100-300 rms 22.0 5.6 28.3 37.0 21.0 17.4 49.7 14.2 4.7 p2p 23.0 4.9 26.1 30.3 23.3 14.9 47.7 13.7 3.9 table 3. weighted cant deficiencies in curves for all recorded trains (not rounded). figure 4. number of years needed to reach the wave defect progression of the same level as before the rails replacement. the wavelength range. whilst the wavelength range of 30-100 mm indicates the timeframe in years, the wavelength range of 100-300 mm reaches tens of years in timeframe estimation. the interval of 17 to 19 years could be associated with the essential modernization of the railway line at the end of the 1990s, where the sections are placed. on the other hand, to assume that the defect development lasts for two decades would not be right, since major maintenance efforts such as rail grinding took place a number of times. lastly, the considerably longer timeframe for the defect progression for the measuring place ha1.4 is not the result of the slow development of the defects, but the reason probably is the significantly worse state of the rails before their replacement in 2015. when calculating the timeframe required for reaching the defect progression of the same level as before rail grinding, the results are shorter. the reason is given by the lesser absolute values to be reached on one hand, but also by the initial state of rails on the other, hence the state of rails after grinding does not necessarily have to be equal to new rails. unfortunately, for the measuring places bílovice 1.1, babice 1.1, and 1.2 the initial state before the replacement of the rails in 2016 was not documented. section adamov the same principles of the development prediction were applied to the data from the track recording car in the section adamov. figure 5. number of years needed to reach the wave defect progression of the same level as before the rail grinding. the p2p from the track recording car is simplified just to values in 0.25 m steps, where all 4 values representing one meter of the rail are given the same value. these values are for the purposes of this paper signed as p2p modified (p2pm). furthermore, when all the values for the measuring place are summarized, the p2p modified sum is obtained (p2pms). this simplification regrettably brings the loss of information about the development progress, on the other hand, it helps to reduce local inaccuracies. when the p2pms values are drawn into the chart, the gradient could be assessed. the drawn values are referred to the initial state of the new rails, i.e. after the rail replacement. together with the known p2pms values before the rail replacement, it is then possible (by extrapolating the gradients) to search for the time, when the p2pms reaches the same value as it was before the rail replacement. this search is practically possible to be conducted on all the four wavelengths, however, the soundest ranges to be examined are the 30-100 mm and the 100-300 mm. to clarify: the wavelength range 1030 mm, which is embodying the non-periodical rail defects, could show locally randomized development progress. contrariwise, the wavelength range 3001000 mm represents more the defects of the track or even the defects of the whole substructure and do not reflect the maintenance done on the superstructure. the referential value of the developed defect for 51 j. valehrach, t. říha, j. vendel, o. plášek acta polytechnica ctu proceedings section hady measuring place 1.1 1.2 1.3 1.4 number of years 0 – 24 40 – 63 0 – 24 40 – 63 0 – 24 40 – 63 0 – 24 40 – 63 from timeframe 30-100 rms 2.2 3.0 2.3 2.8 2.4 2.8 2.4 3.0 p2p 2.2 3.0 2.2 2.4 2.4 2.9 2.4 3.8 100-300 rms 3.8 1.0 3.9 5.1 2.7 2.2 5.0 1.4 p2p 3.8 0.8 3.3 3.8 2.8 1.8 4.4 1.3 table 4. number of years needed to reach the wave defect progression of the same level as before the rails were ground. figure 6. wave defects development prediction based on the previous measurements done by the track recording car. the measuring place adamov 1.1 was taken from the measurement in 2016. maintenance works were carried out in 2018 (the values for the gradient obtention were from 2018, 2019, and 2020). the type of the regression curve (linear, exponential, . . . ) is based on the coefficient of determination so that its value is as close to 1.0 as possible. the p2pms value corresponding with the initial state before the maintenance works was for the measuring place adamov 1.1 found in 2035, which represents the progression of the defect for 17 years. it is interesting that this figure is comparable with the progression between 1999 (the newly modernized railway line put into service [7]) and the maintenance works in 2018. the development prediction, however, could not reflect some minor maintenance works, and also due to not very rich set of measured values, it is burdened with some uncertainty. moreover, the maintenance works in 2018 were of much lesser scale in comparison with the complex modernization in 1999. that is why the future timeframe still remains to be misty (see figure 6). 3.2. prediction based on the given limits the most suitable method for prediction is to use the gradients for calculation of rms and p2p into the values table 17 (čsn 13231-3:2006 [6]). figure 7. timeframe needed for the defect progression according to table 17 (čsn 13231-3:2006 [6]). the same applies to the standard sždc s2/4 [8] for the level 0.1 mm. figure 8. timeframe needed for the defect progression according to standard sždc s2/4 [8] for the level 0.1 mm. the longest timeframe for the defect progression or development appears to be in babice, although there were some negative gradients for the wavelength range 30-100 mm excluded from the assessment, and also the rms value for the wavelength range 100-300 mm is cropped. 52 vol. 35/2022 the long-pitch corrugation development in small radii curves 4. conclusions based on the measurements of the actual state there is the possibility to predict the speed of the further development of the long-pitch corrugation in curves of small radii. simultaneously, there is a possibility by monitoring weighted cant deficiencies (respectively cant excesses) to estimate in advance in which curves the progression of the defect will be faster. currently, a spectrum of measures exists to suppress the development of the long-pitch corrugation: starting with the methods aiming to prevent or slow down the defect progression and concluding with the measures designed to remove already developed defects. it is thus possible to use elastically enhanced superstructure or to grind off the defects extensively. that is why infrastructure managers face the difficult decision of which of the methods would be most suitable for their respective cases. once they had some appropriate tool to predict the defects progression, they can easily, based on the previously conducted measurements, decide for the actions generating lesser expenses or minimize the track possessions. performed measurements have also shown, that long-pitch corrugation monitoring via the p2p values is the most suitable way. assessments have additionally proven, that such monitoring should not be conducted without observation of other factors and parameters as well, hence the long-pitch corrugation is the result of the processes happening in an interconnected system infrastructure – vehicle – traffic. some parameters change over time very dynamically (rail wear, composition of trains), others are consistent (such as curve radii). long-term monitoring shows, that even in sections, appearing for the first sight as homogeneous, where unified construction and parameters take place, still some anomalies are present with different properties. these could be not only of the construction type (like the superstructure composition), but also even historical (previous actions not just to the infrastructure itself, but to the entire system infrastructure – vehicle – track). that amplifies the need for the selection of the appropriate parameters for the monitoring. regarding the current state, where the tracks are regularly diagnosed by the track recording car for the superstructure, there is no lack of high-quality data. in the case of the national line corridors, this means consistently three times during one year to obtain such data. naturally therefore there should be the drive to assess the data not only for the backward analysis but also for the defect development prediction. in the modern era, where powerful hardware and neural networks are present, data analysis is even easier. there is also the need for defect progression prediction on the designer side. the current state of the general knowledge of track designers is not overly broad: the usual conclusion is that the usage of curves of small radii is wrong, and it will eventually lead to long-pitch corrugation development. this is the space, where quality information about the parameters affecting the progression of long-pitch corrugation could be used most. track designers like to use guidelines and therefore some kind of a handbook, where the significant parameters, superstructure choices, and other crucial aspects would be summarised, would eventually lead to a significant decrease in the construction and maintenance costs and also in the number of possessions. as a result, society would obtain superior, safer, more dependable, and a neighbourly conscious railway track. references [1] čsn en 13231-3. železniční aplikace – kolej – přejímka prací – část 3: přejímka reprofilace kolejnic v koleji. praha: úřad pro technickou normalizaci, metrologii a státní zkušebnictví, třídící znak: 736374. 2012. [2] s. scholle. možnosti měřících vozů tudc železničního svršku pro detekci kolejnicových vad. příspěvek na workshopu: výhybky a výhybkové konstrukce. kolejnicové vady. brno, 2015. [3] c. esveld. modern railway track. 2nd ed. zaltbommel: mrt-productions, 2001. isbn 90-800-3243-3. [4] h. funke. brooušení kolejnic. praha: elka press, 1992. 122 s. [5] j. valehrach, p. guziur, t. říha, o. plášek. assessment of rail long-pitch corrugation. iop conference series: material science and engineering [online], 2017, roč. 236, č. 1, 8 s. issn: 1757-8981, https://doi.org/10.1088/1757-899x/236/1/012048. [6] čsn en 13231-3. železniční aplikace – kolej – přejímka prací – část 3: přejímka reprofilace kolejnic v koleji. praha: úřad pro technickou normalizaci, metrologii a státní zkušebnictví, třídící znak: 736374. 2006. [7] k. černý, r. jeschke, m. navrátil, et al. železniční trať brno – česká třebová na starých pohlednicích. polnička: tváře, 2019. isbn 9788090739476. [8] sždc s2/4. předpis pro zajišťování diagnostiky železničního svršku měřícími prostředky s kontinuálním záznamem. účinnost od 1. ledna 2020. správa železniční dopravní cesty, státní organizace. praha, 2020. 53 https://doi.org/10.1088/1757-899x/236/1/012048 acta polytechnica ctu proceedings 35:49–53, 2022 1 introduction 2 the long-pitch corrugation development 3 development prediction 3.1 prediction based on the previous defect progression 3.2 prediction based on the given limits 4 conclusions references https://doi.org/10.14311/app.2022.33.0238 acta polytechnica ctu proceedings 33:238–244, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague stability of concrete containing blast-furnace slag following exposure to cyclic elevated temperature ahmad iravania,∗, volkert feldrappeb, andreas ehrenbergb, steffen andersa a university of wuppertal, institute of structural engineering, department construction materials, pauluskirch str. 11, 42285 wuppertal, germany b fehs building material institute, bliersheimer str. 62, 47229 duisburg, germany ∗ corresponding author: a.iravani@uni-wuppertal.de abstract. concrete is widely used in constructions such as industrial floors or airducts in steeland casting industry where it is often exposed to long-term or cyclic elevated temperatures. for these applications, thermal stability of concrete is of vital importance. the strength reduction due to elevated temperatures depends on the temperature level and concrete composition. in this study, the effects of blast-furnace slag cement (cem iii/a) and basaltic aggregates were investigated at temperatures 250◦c to 700 ◦c in comparison to conventional portland cement (cem i) containing quarzitic aggregates. the concretes were cyclically exposed to high temperatures. special attention was paid to mass loss, residual compressive and residual flexural strength depending on type of cement and aggregate as well as the number of thermal cycles. mass loss and strength loss increased with increasing maximum temperature level, as expected. it was generally observed that concretes containing cem iii/a displayed significantly higher residual mechanical properties for almost all temperature levels. concretes containing a combination of cem iii/awith basaltic aggregates showed significantly higher stability at elevated temperatures compared to other concrete mixtures. it is further shown that apart from the maximum temperature the number of thermal cycles is important for the residual mechanical properties. keywords: blast furnace slag, concrete, cyclic elevated temperature, residual compressive strength, residual flexural strength. 1. introduction durability is one of the most important characteristics of concrete constructions, but it can become decisive for elevated temperature environments including cyclic heating and cooling. applications are for instance concrete floors or airducts in steel and casting industry. the use of concretes with higher long-term thermal stability when exposed to continuous cycles of heating and can reduce the cost and down time for repair or replacement. the behaviour of concrete exposed to single hightemperature events such as fire has been extensively studied [1]. however, the stability of concrete subjected to long-term or cyclic exposure to elevated temperatures is still a research topic [2–4]. physical and chemical changes in concrete such as microcracking caused by different thermal extension of aggregates and cement stone or dehydration of calciumsilicate-hydrate phases in cement stone during are irreversible changes. the hot strength of concrete was compared to its residual strength within the course of cooling process and after reaching ambient temperature. results demonstrated significant differences in strength of the hot, cooling and cooled down concrete [5, 6]. frangi et al. [3] reported additional 10% reduction in the compressive strength comparing residual compressive strength and hot compressive strength, as long as temperature remains below 600 ◦c [3]. lucio-matin et al. [7] report an increasing number of micro-cracks up to 25 cycles when concrete subjected to 1, 25 and 75 cycles between 290 ◦c and 550 ◦c. the cracks initiated in the first heating cycle, with the number of cracks increasing up to the 25th cycle, but remained unchanged henceforth. these findings demonstrate a kind of thermal fatigue in concrete. due to the comparatively low compressive strength of the concrete used here, explosive spalling is not dealt with in this study. it is commonly known that the type of cement, the aggregates as well as the interaction of cement stone and aggregates affect the behaviour of concrete at high temperatures. concretes containing basaltic aggregate or blast-furnace slag as aggregate have shown better thermal stability compared to conventional quarzitic concrete [8–10]. generally, the lower thermal expansion of e.g. basalt or slags reduce thermal stresses and micro-cracking in the concrete structure. as cement paste shrinks during heating, lower thermal expansion of the aggregates is sup238 https://doi.org/10.14311/app.2022.33.0238 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 33/2022 blast-furnace slag concrete sustainability mix 1 mix 2 mix 3 mix 4 water 286 286 286 286 [kg/m3] cem i 42.5r 550 550 − − [kg/m3] cem iii/a 42.5n − − 550 550 [kg/m3] quartz 0/4 mm 1354 − 1354 − [kg/m3] basalt 0/4 mm − 1534 − 1534 [kg/m3] stabilizer (1% cem.) 5.5 − 5.5 − [kg/m3] w/c-ratio 0.52 0.52 0.52 0.52 [−] initial compressive strength at the age of 46 days 58.4 73.9 65.0 75.4 [mpa] initial flexural strength at the age of 46 days 8.7 11.7 7.0 10.8 [mpa] table 1. overview of concrete mixtures. concrete age climat remarks 1 day − demoulding after casting 1 28 days submerged in water optimal condition for hydration 28 35 days 20 ◦c / 65 % r.h. drying 35 38 days 50 ◦c in dry box reducing water content 38 46 days 20 ◦c / 65 % r.h. hygric homogenization 46 days − initial compressive strength as reference 47 days cyclic temperature exposure starting high temperature exposure temperatures: 250 ◦c, 500 ◦c, 700 ◦c cycles: 1, 5 and 20 table 2. overview of conditioning process. posed to reduce micro-cracking. according to another studies, the residual compressive strength of concrete containing blast-furnace cement (cem iii) is significantly higher compared to concretes with portland cement (cem i), after single temperature exposure. higher residual strengths are reported for concretes containing combinations of cem iii and basalt as aggregate following exposure to cyclic heating [2, 4]. furthermore, alkali-activated binders using ground granulated blast-furnace slags have shown to improve resistance to high temperatures between 500 − 800◦c. this can be attributed to the ability of alkali-activated slags to chemically react forming calcium-silicate-hydrate phases (chs) [10]. therefore, the residual strength of concrete after exposure to cyclic heating/cooling provides essential questions for further investigations concerning the effect of blast-furnace slag in cyclic high-temperature applications. it the aim of this study to quantitatively investigate increases in residual compressive strengths if temperature resistant aggregates, namely basalt, as well optimized binders containing blast furnace slag are used in concrete subjected to thermal cycles at elevated temperatures. as reference concrete conventional concrete made of quarzitic aggregates and ordinary portland cement are tested. apart from the residual compressive strength, residual flexural strength and mass loss are considered. 2. experimental program 2.1. materials and curing of concrete mixes the experimental program is designed to show the effects of blast-furnace slag cement and basaltic aggregates on the residual strengths and mass loss of concrete. as reference concrete made of quarzitic aggregates and ordinary portland cement are used. the resulting matrix of concrete mixtures was fully varied (table 1). in order to be able to compare the results w/cratio and lime content must kept constant. mixture 1 is the reference containing ordinary portland cement (cem i) and quarzitic aggregate. mixture 2 is a combination of cem i and basalt. mixtures 3 and 4 have been developed to investigate the effect of cement type, replacing cem i by blast-furnace slag cement (cem iii/a). specimens were cast in standard prisms (40×40×160mm3). the maximum grain size was limited to 4 mm. both cements were supplied by the same manufacturer representing market-based products. thus, the clinker in both cements is the same. 239 a. iravani, v. feldrappe, a. ehrenberg, s. anders acta polytechnica ctu proceedings figure 1. sketch of the heating program, each complete cycle having a duration of 24 h. figure 2. residual compressive strength after 1 heating cycle (a), 5 heating cycles (b) and 20 heating cycles (c). crosses (x) denote untestable mixtures due to complete degradation. in order to reach comparable moisture contents in all mixtures and to reduce the water content to a value usually reached after about 90 days (d) of curing in air, pre-storage was scheduled (table 2). after demoulding, specimens were stored in water for 28 d, followed by 7 d in standard climate (20 ◦c, 60 % relative humidity). afterwards concretes were dried at 50 ◦c for three days in order to reduce the physically bounded moisture. this pre-drying was implemented to reduce the impact of temperature-induced steam pressure build up on the strength of the specimens. finally, the specimens were stored again in standard climate for 7 d to ensure a homogenous moisture distribution. at the time of the first cycle of thermal exposure, specimens had an age of 45 − 48 d. mass loss, residual flexural strength and residual compressive strength were measured after exposure to 1 and 5 heating cycles with the maximum temperature of 250 ◦c, as well as 1, 5 and 20 heating cycles at maximum temperatures of 500 ◦c and 700 ◦c. 2.2. heating programme / temperature regime heating started at ambient temperature and increased with a heating rate of 5 k/min until the desired maximum temperature 250 ◦c, 500 ◦c or 700 ◦c was reached in a conventional tempering furnace. temperature was kept constant for 4 h to ensure the homoňgeneity of the temperature in the specimen and to allow chemical reactions to take place. the cooling process was free but slow cooling for about 16 h until the temperature reached about 50−60 ◦c. measurements and strength tests were always carried out after cooling down in ambient condition. figure 2 shows a sketch of the cyclic heating regime. regardless the temperature level, one cycle took 24 h. therefore, specimens which were heated up to 250 ◦c, were stored longer at ambient temperature before the next cycle compared to specimens heated to 700 ◦c. the specimens were examined in terms of water release (mass loss) immediately before testing the residual flexural and residual compressive strength. 3. results and discussion 3.1. residual compressive strength generally, residual compressive strength depends on the maximum temperature during heating. it is demonstrated that the type of cement and aggregate significantly affect the residual compressive strength. furthermore, the number of cycles affect the residual strengths significantly. the residual strengths after heating is expressed relative to the initial strength of concrete prior to thermal treatment at the concrete age of 45 − 48 d, as described in section 2.1. 3.1.1. effect of maximum temperature and number of heating cycles figure 2a (left) displays the residual compressive strength for all mixtures after a single cycle with maximum temperatures of 250 ◦c, 500 ◦c and 700 ◦c. the known increasing reduction of residual strength [1, 5] with an increasing temperature level is confirmed. as expected, basaltic aggregates increase the residual strength compared to quartz at temperature levels of 500 ◦c and higher. 240 vol. 33/2022 blast-furnace slag concrete sustainability figure 3. residual compressive strength for concrete containing cem i and basalt (a) and quartz (b). figure 4. combined effects of type of cement and aggregate on the residual compressive strength after cyclic heating with maximum temperatures of 500 ◦c and 700 ◦c. figure 2b and 2c display the residual compressive strength after 5 and 20 thermal cycles respectively. an ongoing reduction of residual compressive strength can be seen after 5 heating cycles for all tested mixtures (figure 2b). similar to previous findings, the reduction of strength is significant for maximum temperatures of 500 ◦c and 700 ◦c. in addition to the dehydration and degradation of the cement paste, the increase in thermal expansion of aggregates, especially quartz, leads to the formation of micro-cracks resulting in higher degradation. crack formation is further increased by alternating expansion and shrinkage while subjected to thermal cycles. accordingly, stability of concrete significantly depends on the maximum exposed temperature as well as the number of exposed heating/cooling cycles. similar results have been obtained within the current work, which are in accordance with findings e.g. of alonso et al [5]. in figures 3b and 3c it is furthermore obvious that none of the mixtures containing cem i was testable after 20ăthermal cycles due complete degradation to powder. mixtures containing cem iii/a maintained considerable residual compressive strengths even after 20 thermal cycles. 3.1.2. effect of blast-furnace cement and basaltic aggregates the effect of basaltic and quarzitic aggregates on residual compressive strength are depicted in figure 3a and 3.b depending on the maximum temperature. a comparison shows the higher thermal stability of the basalt-containing concretes subjected to 1 and 5 thermal cycles at both temperatures of 500 and 700 ◦c. the residual strength of concrete containing cem i and basalt is about 55% and 30% after exposure to 5 heating cycles at 500 and 700 ◦c, respectively; whereas in the concrete containing quarzitic aggregate, the residual strength is lower than 20% in both cases. this better performance of the basaltic aggregates is attributed to the lower thermal expansion of basalt. nevertheless, both mixtures exhibited complete degradation before 20 thermal cycles were reached. in order to characterize the effects of different types of cements, the residual compressive strengths are compared of mixtures with cem i and cem iii/a are displayed in figure 4. a pronounced decrease in compressive strength is observed for all mixtures before 5 thermal cycles. generally, mixtures containing cem iii/a show higher residual strengths compared to cem i. the stability of cem iii/a containing mixtures is particularly visible after 5 thermal cycles, where cem iii/a obviously reach a maximum 241 a. iravani, v. feldrappe, a. ehrenberg, s. anders acta polytechnica ctu proceedings figure 5. residual flexural strength of the concretes after 1 cycle (a), 5 cycles (b) and 20 cycles (c). figure 6. mass loss induced by cyclic heating for all tested concretes. degradation. it has reached a stable structure without further degradation. the authors currently have no a comprehensive explanation at hand. one explanation is that the powdered blast-furnace slag, which is itself stable against temperature as basalt, changes development of cracks and micro-cracks or the thermally induced shrinkage of the cement paste. another approach is a possible re-hydration tendency of cem iii/a due to the blast-furnace slag and moisture uptake in the air between temperature cycles. this re-hydration could result in a chemical closure of micro-cracks [11, 12]. 3.2. residual flexural strength apart from the residual compressive strength, the residual flexural strength was measured for all mixtures, temperature levels and numbers of cycles. figure 6 displays the results after exposure to 1 (figure 6.a), 5 (figure 6.b) and 20 (figure 6.c) thermal cycles. a significant reduction of the residual flexural strength occurs after any thermal treatment. the reduction is determined both by the temperature level as well as the number of thermal cycles. the loss of residual flexural strengths is generally higher compared to the reduction of the compressive strength. figure 5.b and 5.c show significant stability of concretes containing cem iii/a after 5 and 20 cycles. it is worth mentioning that compared to mixtures with quartz, flexural strengths of mixtures with basalt are higher both, before (see table 1) and after heating to temperatures 500 ◦c and 700 ◦c. this can be explained by to better mechanical properties of basalt. 3.3. mass loss after temperature exposure figure 6 depicts the percentage of mass loss in all concrete mixtures and temperature regimes. the mass loss increases with the maximum temperature. the increase of mass loss is attributed first to the release of 242 vol. 33/2022 blast-furnace slag concrete sustainability physically bound water, followed by beginning of dehydration of csh-phases and the subsequent release of further chemically bound water at higher temperatures. this corresponds to the findings presented in section 3.1.1, where the maximum temperature significantly affected the residual compressive strength for all mixtures. as observed, and described in [12], mass does not change significantly after the first cycle and up to the 5th cycle of heating at temperatures of 500 and 700 ◦c for almost for all mixtures. exposure up to 20 thermal cycles at 500 and 700 ◦c, indicate a slight increase of mass loss for concretes containing cem iii/a. the pronounced mass loss in the first cycle results in shrinkage of the cement stone and subsequent formation of a cracks. this is the reason for high degradation in residual compressive and flexural strengths in the first cycle. this hypothesis is in accordance with findings of lucio-martin et al. [7]. the effect of cement type on the concrete mass loss is evident from figures 6b and 6d, where the higher mass loss for mixtures containing cem iii/a compared to cem i is observed. this might further imply that the bonding of water to the cement paste differs with the type of cement. from the results presented here it could be expected, that water is easier releasable in concrete containing cem iii/a cements. 4. conclusions from the generally known effects of an increasing reduction in residual strengths with increasing temperature, the following conclusions can be drawn: • the residual compressive and residual flexural strength decrease strongest in the first thermal cycle, especially at temperatures of 500 ◦c and higher. the further decrease depends on the type of cement and type of aggregate. the performance gets better if either cem iii/a is chosen as cement or basalt as aggregate. the combination of both shows best results in terms of residual strengths. • at higher temperature levels, concretes containing basaltic aggregates perform better than quarzitic aggregates. • blast-furnace slag cement (cem iii/a) can significantly improve the residual compressive and flexural strengths, particularly after exposure to 20 thermal cycles and in combination with thermally stable aggregates such as basalt. • the mass loss increases with an increasing maximum temperatures, but is not significantly impacted by the number of thermal cycles. it was furthermore observed that the mass loss of cem iii/a is generally about 1 % by mass higher compared to cem i cement. 4.1. residual flexural strength apart from the residual compressive strength, the residual flexural strength was measured for all mixtures, temperature levels and numbers of cycles. figur 5 displays the results after exposure to 1 (figure 5a), 5 (figure 5b) and 20 (figure 5c) thermal cycles. acknowledgements the igf project 20318n of the research association "vdeh-gesellschaft zur förderung der eisenforschung mbh" is funded by the aif within the programs for sponsorship by industrial joint research (igf) of the federal ministry of economic affairs and energy (bmwi) based on an enactment of the german bundestag. we further acknowledge our industrial partners for their project support. references [1] u. schneider. verhalten von beton bei hohen temperaturen, technische universitaät braunschweig, 1982.https://publikationsserver.tu-braunschwei g.de/servlets/mcrfilenodeservlet/dbbs_derivate _00043616/3444-1790.pdf. [2] s. anders. comparison of residual strengths of concretes with quarzitic, limestone and slag sand constituents after cyclic high-temperature exposure. concrete repair, rehabilitation and retrofitting, crc press, p. 223-4, 2015. [3] a. frangi, c. tesar, m. fontana. tragwiderstand von betonbauteilen nach dem brand. bauphysik 28(3):17083, 2006. https://doi.org/10.1002/bapi.200610017. [4] h. y. wang. the effects of elevated temperature on cement paste containing ggbfs. cement and concrete composites 30(10):992-9, 2008. https: //doi.org/10.1016/j.cemconcomp.2007.12.003. [5] k. d. hertz. concrete strength for fire safety design. magazine of concrete research 57(8):445-53, 2005. https://doi.org/10.1680/macr.2005.57.8.445. [6] e. klingsch, a. frangi, m. fontana. experimental analysis of concrete strength at high temperatures and after cooling. acta polytechnica 49(1), 2009. https://doi.org/10.14311/1087. [7] t. lucio-martin t, j. puentes, m. c. alonso. effect of geometry in concrete spalling risk subjected to high temperatures for thermal inertia studies, proceedings of the 6th international workshop on concrete spalling due to fire exposure, p. 71-80, 2019. [8] k. endell. versuche über längen-und gefügeänderung von betonzuschlagstoffen und zementmörteln unter einwirkung von temperaturen bis 1200c: ausgeführt im laboratorium der dozentur für bauwissenschaftliche technologie an der technischen hochschule zu berlin in den jahren 1928/29: ernst; 1929. [9] interner abschlussbericht für die verwendung eines hitzebeständigen betons aus hochofenstückschlacke mit hochofenzement für fundamente eines trockenkühlers internal test report fehs institut für baustoff-forschung (duisburg, germany). 243 https://publikationsserver.tu-braunschweig.de/servlets/mcrfilenodeservlet/dbbs_derivate_00043616/3444-1790.pdf https://doi.org/10.1002/bapi.200610017 https://doi.org/10.1016/j.cemconcomp.2007.12.003 https://doi.org/10.1680/macr.2005.57.8.445 https://doi.org/10.14311/1087 a. iravani, v. feldrappe, a. ehrenberg, s. anders acta polytechnica ctu proceedings [10] interner abschlussbericht für kokerei dünkirchen beton mit hochofenstückschlacke internal test report fehs institut für baustoff-forschung (duisburg, germany). [11] c. alonso, l. fernandez. dehydration and rehydration processes of cement paste exposed to high temperature environments. journal of materials science 39(9):3015-24, 2004. https: //doi.org/10.1023/b:jmsc.0000025827.65956.18. [12] a. iravani and s. anders, effects of cement type, aggregate type and concrete age on the mass loss of concrete exposed to elevated temperature, 5th international workshop on concrete spalling due to fire exposure in, pp.63-72, 2017. 244 https://doi.org/10.1023/b:jmsc.0000025827.65956.18 acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.38.0533 acta polytechnica ctu proceedings 38:533–538, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague a tool for developing a plan for the renovation and remediation of cultural heritage buildings daniel macek∗, renáta schneiderová heralová, eduard hromada, stanislav vitásek, iveta střelcová, lucie brožová, jan pojar czech technical university in prague, faculty of civil engineering, department of construction management and economics., thákurova 7, 166 29 prague, czech republic ∗ corresponding author: daniel.macek@fsv.cvut.cz abstract. cultural heritage monuments are, to a large extent, public goods of collective consumption, and their preservation is in the public interest of the society as a whole. the benefits arising from the owner’s investment and resulting from the existence and the use of a cultural heritage monument are usually not “consumed” solely by the owner, but, to a greater or lesser extent, by the whole society or a particular group. in the case of the renovation and remediation of cultural heritage buildings, the life-cycle costs are determined in the operational phase, before the intended renovation or remediation. they should be used to select an economically sustainable solution, with the maximum potential to preserve the cultural and historical value. the paper presents the application designed for the elaboration of plans for the renovation and remediation of cultural heritage monuments, developed in the form of a web interface. the application processes data at the level of individual structural elements. for faster and more comfortable users’ work, a database of type objects is used, which combines primary data from the level of structural elements. keywords: cultural heritage, remediation, renovation. 1. introduction more than czk 7 billion was spent within the ministry of culture’s subsidy programmes on heritage care in the czech republic in the years 2005–2016. the proposal for the years 2017–2020 amounted to czk 727 million per year. according to the ministry of culture data from 2016, the conservation deficit of cultural monuments in the czech republic exceeded czk 318 billion [1]. these are large amounts of money that should be spent efficiently. and not only in the form of renovation costs, but also in a long-term perspective – on the future costs of the operation, maintenance and further renovation of these buildings, i.e. on their economic sustainability throughout their whole life cycle. the analysis of the life cycle cost performed on the basis of relevant input data on the technical parameters of the building, its structural elements and equipment, on the period of time when related costs originated serves as the tool for the assessment of the economic sustainability of an immovable cultural monument. in this way, the analysis becomes an important background factor for the owner’s, designer’s and future user’s decision-making on the selection of the optimum alternative of the renovation technical solution respecting also the ecological aspects, the cultural and historical value and long-term economic consequences [2]. the life cycle cost (lcc) is the total cost of an asset over its life cycle. when applied to the construction industry, it accounts for the costs associated with the acquisition of building and engineering structures, the costs of their maintenance and the renovation of the structures and equipment, the operating costs and the disposal costs at the end of an asset’s life. in most cases, the evaluation covers the cost of the economic life of the building. when deciding on the choice of potential alternative options, it often happens that only the acquisition costs are incorrectly assessed, but the operating costs as well as the preventative maintenance and renovation costs are often neglected. it is, however, the costs incurred during the building’s use phase that make up a significant portion of the building’s life cycle cost [3]. 2. objectives of the research the aim of the project was to create administration and management tools for the sustainable development of cultural heritage realty. the project provided tools for identification and optimization of technical and economic limits of such properties, with an emphasis on the specifics of cultural heritage. due to their monumental nature, the reconstruction and maintenance costs of historical buildings are often considerably higher than normal. their value is predominantly of a non-utilitarian nature, which further increases the investor’s financial burden. the research team designed specialized software with the purpose of passport, estimation and optimization of maintenance and renewal costs of realty. owners and managers of cultural heritage buildings often lack comprehensive maintenance plans that 533 https://doi.org/10.14311/app.2022.38.0533 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en d. macek, r. schneiderová heralová, e. hromada et al. acta polytechnica ctu proceedings could be part of the money management. the research team prepared procedure of maintenance plan and proposal of conservation principles for cultural heritage structures, which lead to sustainability and maximum longevity of the building and will be in compliance with current trend in cultural heritage protection. the developed tools are pilot-tested in the form of case studies of selected buildings. these studies include economically and technologically sustainable management model, which will also be sustainable in cultural and social terms. the main parts of studies contain a description of the current state and options of possible future operation. for each option, technological solution, costs, possible operational revenues and financial analysis with variant and risk assessment are proposed. 3. life cycle costs of structural elements the lifetime of the construction is limited not only by its technical but also its economic lifetime. with the technical lifetime the emphasis is put on material characteristics of a construction and the lifetime of the construction, which is dependent especially on the provision of building elements with a long-term viability. it concerns those structures of the construction which have, from the viewpoint of the technical lifetime, principal significance because with their damage (loss of performance of their function) the construction is not functional, threatens to collapse and any repairs become technically and economically extremely demanding. from the viewpoint of the cost level for repairs it is more effective to remove the construction and build a new one. in the case of the economic lifetime this concerns the period in which it is appropriate to use the building economically. it is usually shorter than the technical. very often it concerns the loss of economic usefulness which can be connected with the permanent loss of net income with reference to disproportionately high costs and it would seem preferable to remove the building and replace it by a new building and thus re-evaluate the land. the resulting lcc calculation of the relevant inputs which concern the technical parameters of structural elements and the time period for incurring costs related to them should be an important basis for the decision of an investor, a designer and any future user in choosing an optimum variant of a technical solution for a construction and also looking to ecological aspects and long-term economic consequences. costs connected with the implementation, use and disposal of a building are divided into 3 basic groups [4]: • costs directly related with the technical parameters of a construction – investment costs, repair and maintenance of a building costs, reconstruction costs, costs relating to modernization and disposal of a building, • operating costs of a building – energy, cleaning, depreciation etc., • administrative costs related to property management – taxes, insurance, administration of a building etc. 4. life cycle cost of cultural heritage in the case of immovable cultural monuments, the life cycle cost mainly includes the renovation costs. renovation in the sense of the act on state heritage care refers to the maintenance, repair, rehabilitation, restoration or other modifications of a cultural monument. the term “maintenance” can generally be understood as the removal of undesirable changes to a cultural monument that have occurred as a result of its use (e.g., a new coating on the facade of a house). the term “repair” can be defined as the elimination of the consequences of damage to a cultural monument or the effects of its wear (e.g., adding part of the roofing). the term “rehabilitation” can either denote such structural and technological interventions in a cultural monument which have either changed its technical parameters or its function or purpose (e.g., the conversion of an industrial building into an exhibition space), or restored the cultural monument to the previous, historically documented or presumed state, designed on the basis of professionally substantiated assumptions (e.g., the construction of a new roof truss after a fire). “restoration” is understood as a set of specific artistic, craftwork and technical activities respecting the technical and artistic structure of the original – works of fine arts or handicraft. “another modification” is not any modification of a cultural monument, but only the modernization of a converted building with a changed function, as well as the extension and addition to a building. “modernization of a building with an unchanged function” is such a modification of a cultural monument in which its parts are replaced by more modern parts, its amenities are upgraded or its usability is extended, without changing its function (e.g., the installation of central heating instead of local heating by a stove). here, the objective of the renovation is to extend the technical life of buildings and, at the same time, preserve their historical and cultural value [5]. operating costs come up to high amounts. these are the costs incurred throughout the entire technical life of a building, which is very long in the case of immovable cultural heritage. lcc is usually calculated in the pre-investment phase of the life cycle of a construction project, and can thus be used to select effective alternative solutions. the lcc indicator is a cost criterion, its lower 534 vol. 38/2022 a tool for developing a plan for the renovation and remediation . . . value being more beneficial for the investor. the information on the cost development in individual phases and the possibilities and ways of affecting them, as well as the information on the service life of structures and equipment, is the key information relevant for the lcc modelling. in the case of immovable cultural monuments, lcc is calculated in the operational phase, usually before the intended renovation. it should be used to select an economically sustainable solution, with the maximum potential to preserve the heritage and historical value of the monument [6]. the investors engaged in the rehabilitation of immovable cultural monuments funded from public budgets can profit from the optimized construction life cycle costs: they contribute to following the 3e principles (economy, efficiency, effectiveness), which are key to public contracts. in the currently valid wording of the public procurement act, the life cycle cost is listed as one of the options for the application of the basic indicator for the selection of a public procurement contractor, as the “economic efficiency of the tender”. the private investors engaged in the renovation projects of immovable cultural monuments profit from using the optimized construction life cycle costs as well: they contribute to achieving higher value for money (value for money) and a shorter payback period. the evaluation of the life cycle cost of buildings can also be viewed from a broader perspective to include, in addition to the costs related to the rehabilitation, remodelling, renovation and operation of an immovable cultural heritage building (i.e. lcc), also externalities and social (community) benefits and costs arising in connection with construction activities and the building operation in its surroundings. the construction costs defined in this way are denoted as the lifetime costs of a building [7]. 5. evaluation applying life cycle cost the methodology of sustainable acquisition of buildings by public contracting authorities has been designed for newly built, mostly public buildings, but after a modification, it can also be used for the renovation of immovable cultural monuments. the application of the methodology can be split into several steps: • determination of the lcc analysis objective, • determination of the lcc analysis scope, • definition of key parameters, • identification of alternative options for the analysis, • data collection on evaluated alternative options, • economic assessment of the alternatives, including risk and sensitivity analysis, • final report. the methodology of sustainable acquisition of buildings by public contracting authorities is more specific; the costs are structured at individual levels of the lcc analysis respecting the practices of pricing strategies in the construction industry in the czech republic, the recommended methods and background materials for their compilation available for pricing in the czech republic. the methodology contains a list of common risks impacting on the building’s life cycle cost value. it also highlights the specifics of the methodology use by public contracting authorities. the methodology is supplemented by the templates of clear forms that can facilitate its application [8]. the aim is to encompass the complete process of the application of the life cycle cost analysis for immovable cultural monuments. the workflow of the modified methodology application is defined for three levels: • preliminary lcc analysis for the decision on the project implementation, • detailed lcc analysis for designed alternative options of the immovable cultural monument renovation, • detailed lcc analysis for alternative options of key structures, systems and installations. the public sector is aware of the importance of evaluating buildings in terms of their life cycle cost. in the public procurement act currently in force, the life cycle cost is listed as one of the options for applying the basic indicator for selecting a public procurement contractor, as “the economic benefits of a tender”. in this case, the life cycle costs must include the bid price (in terms of lcc, it is part of the acquisition costs, in public construction projects it is equivalent to construction costs) and may include: • costs incurred by the contracting authority or other users during the life cycle of the public contract subject, which may, in particular, refer to ▷ other acquisition costs, ▷ costs related to the use of the public contract subject, ▷ maintenance costs, or ▷ disposal costs at the end of the service life, or • costs caused by the environmental impacts associated with the public contract subject at any time during its life cycle, provided that their monetary value can be quantified; these may, in particular, refer to the cost of emissions of greenhouse gases or other pollutants or other costs of the climate change mitigation. another option for the evaluation of the economic efficiency of projects involving immovable cultural monuments that should be mentioned uses the costbenefit analysis (cba), based on the utility theory. it is an analytical tool used to evaluate investment decisions to assess how they contribute to changing the level of a society’s well-being. the cba method serves 535 d. macek, r. schneiderová heralová, e. hromada et al. acta polytechnica ctu proceedings primarily to evaluate projects from the public sector, comparing the benefits, which express any positive effects (benefits), with costs in the broadest sense (costs), or negative effects of the investment. the method is based on the analysis of the impacts of the investment on the affected entities, the quantification of the identified effects and, finally, the conversion to a common numerical (ideally monetary) unit [9]. 6. pricing of construction works on immovable cultural monuments the pricing of construction works on immovable cultural monuments studies methodologically how to approach the pricing of construction activities related to historic buildings. the uniqueness of historical building elements sets higher demands for the technologies used during their revitalization compared to classic (modern) building structures. therefore, the pricing concept itself is based on the analysis of construction work market prices used for historic buildings in combination with traditional tools and pricing methods [10]. the designed methodology is based on ten pillars: • cs úrs and rts data price systems for buildings. • principles of parametric pricing of buildings and classification of structural elements respecting the needs of immovable cultural monuments. • budgetary principles according to the tskp classifier (classification of building structures and works). • technological handling of rubble. • measurement method of structures. • examples of rehabilitation and repair pricing. • individual calculations of items using a standard calculation formula. • hourly billing rates. • micro budgets referring to defined structural elements. • analysis of market prices of construction works used for historic buildings. the pricing of renovation projects of historic buildings is generally considered problematic due to the type of work and some degree of uncertainty in estimating its amount. the aim of the guide price analysis was to find out whether they actually corresponded to the prices on the construction market. in this context, it must be realized that if the works in the building are regulated by the restoration regime and can only be performed by a conservator with a valid certificate, it is necessary to price these works individually, as guide prices do not take restoration repairs into consideration. furthermore, a relatively large scope of works can be needed during renovations that are not listed in the budgeting programmes and are priced individually in the bids. this mostly refers to carpentry and locksmith structures and artistic elements [11]. 7. monurev the monurev software application, designed as a web-based interface, has been developed for the elaboration of maintenance and renovation plans of cultural monuments. the application processes data at the level of individual structural elements. for faster and more comfortable users’ work, it uses a database of type objects, which combines primary data from the level of structural elements. one of the conditions necessary for a practical and full use of the t-e analysis model is an unambiguously defined form and amount of input data, which will be the subject of the summarization. the obligatory data that must be entered by the analysis maker are automatically complemented by the remaining data necessary for the processing of the analysis before the summarization. the sources of these additional data are two internal databases: • database of characteristic representatives of cultural monuments, • database of typical structural components. by a suitable compilation of the databases, a system has been created which enables the analysis maker who has no professional construction qualifications to obtain the best practically usable outputs. the more elements the database will contain, the lower the distortion of the output information caused by assigning the analysed buildings to the selected representatives of buildings and structural components. a too wide range of selected representatives (reference models), on the other hand, would lead to a complicated and confusing entering process. one database of characteristic representatives of cultural monuments has been physically used in the application programme processing, which, however, practically enables the introduction of different types of classification of buildings running in parallel. this not only involves the introduction of the classification of the same type with a change in the content, but also a type difference in the approach to the generation of the templates of buildings. the only approach used in the application for obtaining the options for the distribution of structural components in the respective type of building is by entering the units of measure, such as the height, width, length of the building. after selecting the reference building and entering its basic size data, individual structural components which the reference building is composed of are unambiguously assigned to this building. this assignment is made through a matrix of conversion formulas compiled for all buildings and all components. each conversion formula contains the characteristic size parameters of the analysed building and an empirically identified conversion coefficient from which the 536 vol. 38/2022 a tool for developing a plan for the renovation and remediation . . . number of structural components in the building is derived. by the summarization, a fictitious building is created differing from the actual analysed building within acceptable tolerance limits. the basic requirement for this database is to define all structural components existing in the building construction production whose service life does not reach the limit service life of the entire building. the criteria for the classification of structural components are the component’s function, its service life and the unit cost of the component’s renovation. the life cycle of an element expresses in what time cycles and at what costs the renovation of the respective structural element will be necessary to maintain its standard use and, at the same time, not to renovate it unnecessarily prematurely when its potential for use has not been exhausted yet. a characteristic feature of the t-e analysis model is the collection and arrangement of all relevant data on the technical and economic condition of the analysed building valid as of the selected date into a uniform input data scheme. the application runs on the server of the faculty of civil engineering, ctu in prague and is available at https://monurev.fsv.cvut.cz/. the users will log into the application with the assigned user password and name. 8. conclusion the monurev application is intended primarily for owners (administrators) of listed buildings. the tool is focused on qualified planning of renovation and maintenance of buildings. the project solution is based on reference databases of objects and structural elements, which will allow users who are not experts in this field to obtain results quickly. on the other hand, the tools allow you to go into details and modify the designed models according to your own specifications. the user chooses which areas to use and how deep the detail he goes into processing the data. the developed methodology and tools for the administration and management of the sustainable development of immovable cultural heritage are pilot-tested in the form of a feasibility study for a selected listed building. the feasibility study includes an economically and construction-technologically sustainable model of further operation of the historic building in the long term. the study includes a description of the current state (building certification), evaluation of the current wear rate of the building and design of variants of further operation of the building (zero variant – only maintenance work, partial reconstruction, complex reconstruction, design of innovative technologies related to environmental protection, etc.). for all variants, the organizational implementation, the implementation schedule, the planned construction-technological solution are described, the investment and operating costs and possible revenues from the operation of the historic building are quantified, and a financial analysis, including economic evaluation, is prepared. all assessed variants and risks. the owner will be able to use the outputs of the feasibility study for further strategic decisions on the operation of the historic building. the life cycle costing and analysis are primarily a tool for informed decision making. as a rule, they involve the evaluation of several investment scenarios in the pre-investment phase, the selection between alternative options in the investment phase, and the choice of alternative structures and equipment with acceptable parameters in the investment phase and in the use phase. the life cycle cost analysis is also a valuable tool to estimate the future costs incurred by the building owner or to evaluate an ex-ante investment decision. the analysis of the life cycle cost of a building should be part of the decision making on the renovation of immovable cultural heritage. the experience from construction practice indicates that the choice of alternative options, structures or equipment of buildings on the basis of the lowest acquisition costs (lowest bid price) only is wrong. the investors should aim at the acquisition of an economically sustainable building, i.e. a building with the lowest life cycle cost. this can be achieved by the incorporation of the life cycle cost analysis into the design of buildings. acknowledgements this contribution was instigated and undertaken at the czech technical university in prague, faculty of civil engineering. it was supported by the program for the support of applied research and experimental development of national and cultural identity for the years 2019 to 2022 (naki ii – dg18p02ovv012) funded by the ministry of culture of the czech republic. references [1] t. krulický, j. vrbka, t. brabenec. a relationship between the value of the whole and separate parts. iop conference series: materials science and engineering 960(4):042094, 2020. https://doi.org/10.1088/1757-899x/960/4/042094 [2] r. r. nadkarni, b. puthuvayi. a comprehensive literature review of multi-criteria decision making methods in heritage buildings. journal of building engineering 32:101814, 2020. https://doi.org/10.1016/j.jobe.2020.101814 [3] j. antuña. construction history: tool to improve maintenance and intervention in built heritage. in s. sementsov, a. leontyev, s. huerta (eds.), reconstruction and restoration of architectural heritage 2021, pp. 3–7. 2021. https://doi.org/10.1201/9781003136804-1 [4] d. m. frangopol, h. furuta. life-cycle cost analysis and design of civil infrastructure systems. american society of civil engineers, 2001. 537 https://monurev.fsv.cvut.cz/ https://doi.org/10.1088/1757-899x/960/4/042094 https://doi.org/10.1016/j.jobe.2020.101814 https://doi.org/10.1201/9781003136804-1 d. macek, r. schneiderová heralová, e. hromada et al. acta polytechnica ctu proceedings [5] h. vochozka. scientific and technological innovation and enterprise sustainable development strategy research. advance in sustainability 1(1):22–27, 2021. https://doi.org/10.26855/as.2021.07.005 [6] s. bertagni, p. boarin, m. zuppiroli. the dialogue between structural interventions and sustainability criteria in rating systems for cultural heritage: the experience of gbc historic building. international journal of architectural heritage 14(1):139–161, 2020. https://doi.org/10.1080/15583058.2018.1511001 [7] j. karásek, j. pavlica. green investment scheme: experience and results in the czech republic. energy policy 90:121–130, 2016. https://doi.org/10.1016/j.enpol.2015.12.020 [8] m. stransky, p. matejka. digital quality management in construction industry within bim projects. in engineering for rural development, pp. 1707–1718. 2019. https://doi.org/10.22616/erdev2019.18.n494 [9] j. zak. shear accumulated equilibrium compliance as permanent deforamtion susceptibility parameter. construction and building materials 271:121510, 2021. https: //doi.org/10.1016/j.conbuildmat.2020.121510 [10] b. nowogońska. consequences of abandoning renovation: case study – neglected industrial heritage building. sustainability 12(16):6441, 2020. https://doi.org/10.3390/su12166441 [11] g. franco. smart cities and historical heritage. techne – journal of technology for architecture and environment (1):158–165, 2018. https://doi.org/10.13128/techne-22734 538 https://doi.org/10.26855/as.2021.07.005 https://doi.org/10.1080/15583058.2018.1511001 https://doi.org/10.1016/j.enpol.2015.12.020 https://doi.org/10.22616/erdev2019.18.n494 https://doi.org/10.1016/j.conbuildmat.2020.121510 https://doi.org/10.1016/j.conbuildmat.2020.121510 https://doi.org/10.3390/su12166441 https://doi.org/10.13128/techne-22734 acta polytechnica ctu proceedings 38:533–538, 2022 1 introduction 2 objectives of the research 3 life cycle costs of structural elements 4 life cycle cost of cultural heritage 5 evaluation applying life cycle cost 6 pricing of construction works on immovable cultural monuments 7 monurev 8 conclusion acknowledgements references https://doi.org/10.14311/app.2022.33.0207 acta polytechnica ctu proceedings 33:207–211, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague equivalent thermal diffusivity of natural and recycled aggregate concrete at temperature up to 350 ◦c izabela hagera,∗, katarzyna mróza, tomasz tracza, václav kočíb, lukáš fialab a chair of building materials engineering, faculty of civil engineering, cracow university of technology, 24 warszawska st. 31-155 cracow, poland b czech technical university in prague, department of materials engineering and chemistry, faculty of civil engineering, thakurova 7/2077, 16629 prague 6, czech republic ∗ corresponding author: izabela.hager@pk.edu.pl abstract. the paper presents the evolution of equivalent thermal diffusivity with temperature of concretes with natural coarse aggregates and recycled concrete aggregates. thermal diffusivity was evaluated based on the temperature measurements made on cylindrical samples using so-called "inverse technique". thermal diffusivity parameter was estimated for concretes manufactured with different replacement ratios of natural by recycled coarse aggregate (0, 50 and 100 %) produced by crushing the worn-out concrete pavement that was dismounted after exploitation period. the diffusivity of concrete was tested up to 350 ◦c. the progressive decrease of thermal diffusivity was observed, and the lowest d values were obtained for higher recycled concrete aggregates content. the results were presented against the background of other authors results and referred to the d values proposed in eurocode 2. keywords: fire, recycled concrete aggregates, temperature, thermal diffusion. 1. introduction by volume,construction and demolition waste (cdw) are one of the largest streams of waste in europe with 850 million tons that are produced per year [1]. the waste separation process enables to obtain good quality aggregates and their use in concrete is economically, environmentally and technically feasible with the respect of all technical standards. the progressive replacement of natural aggregates by recycled concrete aggregates contributes to the implementation of the strategy for sustainable development of the field of construction through the successful implementation of sustainable building materials with a reduced carbon footprint. the concrete industry scepticism based on concern about poor-quality recycled products needs to change with the research projects showing successful application of recycled concrete aggregates rca without compromising on quality and contribute to the decrease environmental impact. the detailed recommendations concerning assessing the properties of recycled aggregates are given in [2], however the imitation concerning their replacement of natural aggregate in the concrete mix is imposed [3]. in the present research the properties of concrete with recycled aggregates were investigated in the view of study their behaviour in fire. the fire behaviour of concrete with natural aggregate is well documented in the literature [4–6]. well known cases of fires in engineering facilities like channel (1996); mont blanc (1999); gotthard (2001) tunnels has drew attention to one of the disadvantages of concrete that is concrete spalling in a fire. the sudden loss of concrete cover and concrete element cross-section reduction may jeopardise the overall load-bearing capacity of the structural element [7]. it has been confirmed that the composition of concrete, as well as boundary conditions, affects the concrete propensity to spall in a fire. one of the main parameters governing concrete spalling is permeability. in denser and less permeable concrete, the spalling risk is higher [8]. concrete permeability is a derivative of the type of aggregate used, transport properties of the cement paste and the permeability of interfacial transition zone itz between the cement paste and the aggregate. the permeability of recycled concrete aggregate differs from natural aggregate, thus transport properties of those concretes may be higher than in traditional concretes [9], thus lowering the risk of spalling behaviour. in view of studying the rca concrete behaviour in the fire the first step that was undertaking was the observation of thermal properties evolution presented in this paper. the aim of this project is to examine the impact of concrete composition, in particular the recycled concrete aggregate amount rca on the thermal properties of concrete. as the heat transfer in concrete plays a crucial role in fire resistance evaluation, the 207 https://doi.org/10.14311/app.2022.33.0207 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en i. hager, k. mróz, t. tracz et al. acta polytechnica ctu proceedings figure 1. thermocouples arrangement in the concrete cylinder, and the furnace heating rate. thermal properties that change with the temperature are essential and need to be provided to the calculation model. thermal diffusivity (d) is a specific property of the material and allows you to determine how quickly a given material reacts to temperature changes. this is a property that measures the rate of heat transfer from the exposed surface of the material to the inner layers. the higher the value of thermal diffusivity, the faster the temperature increase at a certain depth. to predict the behaviour of concrete during a fire and carry out simulations determining the temperature development in a heated element, one need to know the value of thermal diffusivity. thermal diffusivity can be evaluated directly from its definition or its evaluation can be based on the temperature measurements made on cylindrical samples, called an "inverse technique". 2. experimental method the thermal diffusivity (d) of concrete depends on its thermal conductivity (λ), density (ρ) and specific heat of the material (cp). thermal diffusivity can be determined (eq. 1) by directly determining these physical properties by experimental methods or by using an inverse method leading to the determination of d. the unit of thermal diffusivity is [j/kgk], but [mm2/s] or [m2/s] are also used in the literature. d = λ ρ cp ! m2 s " (1) the method determining the equivalent thermal diffusivity of concrete was recently described by felicetti [10, 11] showing good compliance with experimental results from standard fire tests. the main advantage of the "inverse technique" is the determination of thermal diffusivity without the need for cumbersome measurements of thermal conductivity and specific heat values as well as density change evaluation, knowing that all those values vary with temperature increase. the inverse method is quite sensitive to moisture presence in concrete and other physicochemical processes that occur when the material is exposed to high temperature. it consists in heating the cylindrical sample (diameter 100 mm; height 200 mm) in the electric furnace and measuring temperature increase in the centre of the sample (tc centre) and on 2 mm from cylinder lateral surface (te exterior) using k-type thermocouples, figure 1. the temperature differences are measured on the central crosssection where the field of temperature is considered dependant only to distance from the cylinder centre. the temperature increases with the constant heating rate 1 ◦c/min up to 350 ◦c. the test result is the temperature difference te − tc in function of time. during the test. in the determination of thermal diffusivity by the inverse method, the initial assumptions is adopted that the cylinder is axis-symmetric and infinitely long, so the analyse is related to a differential equation for transient heat conduction. to determine the diffusivity that characterises the transient process of heat exchange inside a constant mass we use fourier’s law (eq. 2): ρ cp ∂t ∂t + div (q) = 0 (2) assuming a cylindrical coordinate system (θ, r, z) (eq. 3): ∂t ∂t = d ! d2t dr2 + 1 r ∂t ∂t " (3) and using laplace transformation, we get a formula for the temperature difference between the edge and the centre of the sample (eq. 4): 208 vol. 33/2022 natural and recycled aggregate concrete thermal diffusivity component units oc rac 50% rac 100% cem i 42.5 r kg/m3 414.2 414.2 414.2 water kg/m3 186.4 186.4 186.4 w/c − 0.45 0.45 0.45 river sand 0/4 mm dwudniaki kg/m3 615.1 280.6 569.2 coarse agg. 2/8 mm dwudniaki kg/m3 579.9 280.6 − coarse agg. 8/16 mm dwudniaki kg/m3 562.3 264.1 − recycled concrete agg. 4/16 mm kg/m3 − 825.3 1057.2 plasticizer basf bv 18 % mc 0.9 0.9 0.9 basf glenium sky 591 % mc 1.4 1.4 1.4 cement paste content dm3/m3 320 320 320 mortar content dm3/m3 542 511 540 density kg/m3 2248.5 2236.8 2221.3 water content % 5.2 5.8 6.7 water absorption % 6.9 7.3 7.8 28 days compressive strength mpa 46.1 56.8 54.5 28 days tensile splitting strength mpa 3.3 3.6 3.4 table 1. mix design and properties. t (r, t) = r ! t − a2 − r2 4d " (4) from here we get the formula for d equivalent thermal diffusivity (eq. 5): d = ra2 4∆t (5) where: • r heating rate [◦c/min], • a sample radius [m], • ∆t temperature gradient [◦c/cm], • d -thermal diffusivity [mm2/s]. 3. materials the measurements were carried out for concrete made of cem i 42.5 cement, riverbed coarse aggregate dwudniaki from dunajec river southern poland (grain size 2/8 mm and 8/16 mm) and crushed concrete from the worn-out concrete pavement with grain size 4/16 mm. before being crushed the mechanical properties of concrete from the pavement was determined on drills. the compressive strength of 66.8 mpa, modulus of elasticity 41.2 gpa and apparent density of 2290 kg/m3 characterised the recycled concrete. for all 3 concretes the same water-cement ratio (w/c) and cement paste volume was maintained. additionally, a plasticiser was used to obtain good workability and water content reduction. the mix composition of the concretes is shown in table 1. the concretes were labelled: the reference ordinary concrete (oc) made of natural riverbed aggregates; rac 50% is concrete made of recycled pavement aggregate with 50% of natural coarse aggregate replacement with recycled one; rac 100% recycled aggregate concrete made of recycled concrete coarse aggregates. in all concretes oc, rac 50% and rac 100% riverbed quartz sand was used. the concretes specimens were cast and stored in plastic cubic or cylindrical moulds for the first 24 hours. after preliminary curing, water evaporation was prevented by covering them with plastic lids for 7 days. during the next 28 days, they were stored in natural air-drying conditions at the temperature of 20±5◦c and relative humidity of 50±5 %. the compressive strength and splitting tensile strength of concretes after 28 days were determined on 10 cm3 cubes. all basic physical (density, water content, water absorption) and mechanical (compressive and splitting tensile strength) performances are summarised in table 1. 4. results and observations the temperature evolution in time measured with type k thermocouples was presented in figure 2. the solid line is showing the heating programme. the presented results were used to determine effective thermal diffusivity as a function of temperature, obtained from the eq. 5 and expressed in [mm2/s]. the results are shown in figure 3. due to thermal gradient value close to zero at the beginning of the test, when the furnace starts to heat, the effective thermal diffusivity values are very high. this phenomenon was previously observed by [10], and the correction factor was proposed to reduce this unwanted effect so that the suggestion to compute the instantaneous heating rate was proposed [11]. the equivalent thermal diffusivity decreases with increasing temperature in the range up to 350 ◦c. in heated 209 i. hager, k. mróz, t. tracz et al. acta polytechnica ctu proceedings figure 2. time temperature curves for oc, rac 50% and rac 100%. figure 3. equivalent thermal diffusivity for oc, rac 50% and rac 100%compared with the results of felicetti [11] and ec 2 proposed values. with 1◦c/min cylinders the values of equivalent thermal diffusivity reaches at 100 ◦c 0.76 mm2/s, 0.61 mm2/s and 0.45 mm2/s respectively for oc, rac 50% and rac 100%. the highest value obtained for oc results from the density of this material. with an increasing proportion of recycled concrete aggregates, the d values are reduced. the similar tendencies are observed at 200 ◦c and 300 ◦c. from the graph, we can see the water migration effect occurring in the lower temperature in case of concretes with recycled concrete aggregates. it confirms that those materials enable faster water migration due to their higher permeability. the observed water migration perturbance on figure 3 is linked with specific heat significant peak at a temperature close to 200 ◦c. the comparison with the d range proposed by eurocode 2 (ec2) [12] for both samples with recycled aggregates rac 50% and rac 100% shows thatthe thermal diffusivity values are mostly below both the upper and lower limits given in ec2. the results remain similar to the experimental results of felicetti [10, 11]. 5. conclusions determination of thermal diffusivity is a vital parameter for the correct modelling of heat flow through heated concrete. thanks to the inverse method, we determine diffusivity for much higher temperatures than other stationary and non-stationary methods. non-stationary transient temperature methods, despite their broad application, require the use of specialised and often costly measuring equipment complex in mathematical and numerical terms. in the inverse method, we can use widely available and easyto-use laboratory equipment to perform the tests. although the technique does not allow for specific parameters to established: ρ, cp and λ but their ratio 210 vol. 33/2022 natural and recycled aggregate concrete thermal diffusivity expressed in diffusivity definition d = λ/ρ cpis assessed. acknowledgements this research was supported and funded in part from the emmat project e-mobility and sustainable materials and technologies ppi/apm/2018/1/00027 financed by the polish national agency for academic exchange (nawa). references [1] global recycling. recycled construction waste in europe. https://global-recycling.info/archives/2964. [2] rilem tc 121-drg. specifications for concrete with recycled aggregates. materials and structures 27(173):557-59, 1994. [3] en 12620:2004 + a1:2010. aggregates for concrete, 2004. [4] j.-c. mindeguia, p. pimienta, h. carré, et al. on the influence of aggregate nature on concrete behaviour at high temperature. european journal of environmental and civil engineering 16(2):236-53, 2012. https://doi.org/10.1080/19648189.2012.667682. [5] z. xing, a.-l. beaucour, r. hebert, et al. influence of the nature of aggregates on the behaviour of concrete subjected to elevated temperature. cement and concrete research 41(4):392-402, 2011. https: //doi.org/10.1016/j.cemconres.2011.01.005. [6] i. hager, t. tracz, j. śliwiński, et al. the influence of aggregate type on the physical and mechanical properties of high-performance concrete subjected to high temperature. fire and materials 40(5):668-82, 2016. https://doi.org/10.1002/fam.2318. [7] k. d. hertz. limits of spalling of fire-exposed concrete. fire safety journal 38(2):103-116, 2003. [8] i. hager, k. mróz. role of polypropylene fibres in concrete spalling risk mitigation in fire and test methods of fibres effectiveness evaluation. materials 12(23), 2019. https://doi.org/10.3390/ma12233869. [9] m. choinska, a. khelidj, g. chatzigeorgiou, et al. effects and interactions of temperature and stress-level related damage on permeability of concrete. cement and concrete research 37(1):79-88, 2007. https: //doi.org/10.1016/j.cemconres.2006.09.015. [10] r. felicetti. thermal diffusivity and residual strength of high-performance light-weight concrete exposed to high temperature. proc. 6th int. symposium on the utilisation of high-strength/high performance concrete 2, leipzig (germany) 935-48, 2002. [11] r. felicetti. assessment of the equivalent thermal diffusivity for fire analysis of concrete structures, proc. fib task group 4.3 workshop fire design of concrete structures, coimbra 149-58, 2007. [12] en 1992-1-2. eurocode 2: design of concrete structures part 1-2: general rules structural fire design 2004, 1992. 211 https://global-recycling.info/archives/2964 https://doi.org/10.1080/19648189.2012.667682 https://doi.org/10.1016/j.cemconres.2011.01.005 https://doi.org/10.1002/fam.2318 https://doi.org/10.3390/ma12233869 https://doi.org/10.1016/j.cemconres.2006.09.015 acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.32.0037 acta polytechnica ctu proceedings 32:37–41, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague the importance of a combined 3d model of ground and geotechnical structure ivan vaníček∗, daniel jirásko czech technical university in prague, faculty of civil engineering, thákurova 2077/7, 160 00 prague 6, czech republic ∗ corresponding author: ivan.vanicek@fsv.cvut.cz abstract. the rapid progress of digitalisation in civil engineering is also happening in the branch of geotechnical engineering. bim is a part of this process. the mutual connection of a 3d model of ground with a 3d model of geotechnical structure can help to give a clearer view of the interaction of this structure and ground. at the same time it also makes possible better control for all participants in the construction process, particularly if all conditions of this mutual interaction are fulfilled. there is considered whether the main aim of the construction process, namely that the care devoted to the individual phases of the structure design and execution actually correspond to the risk with which this structure and ground are indeed connected. within this context a closer specification of the eurocode 7 of the second generation will be identified , especially as it relates to different models. keywords: 3d ground model, 3d model of geotechnical structure, bim, phases of geotechnical investigation, principles of sustainable construction. 1. introduction practical problems solved in geotechnical engineering have usually a 2d character. a typical example could be a flood barrier along a river or a long retaining wall. the design and execution of both of these geotechnical structures is based on the characteristic values of geotechnical parameters of the soil with which these structures interact. the problem is that on a relatively long section there may be a place where the geotechnical parameters used in the design are lower than expected and in this local place a failure may occur. in the case of a flood barrier (a dike, for example), a section 10 m long that breaks will in practice devalue the principle of the entire flood barrier, which can be as much as 10 km long. of the various types of retaining walls, the gabion wall has the lowest stiffness in the third dimension, especially in comparison with conventional concrete. therefore, it is easier to cause a local failure at the weakest point. when a 2d ground model is used, at least in the form of geological and geotechnical models of the subsoil, it is easier to identify these potentially dangerous places and thus prevent a loss of functionality. for earth structures of transport infrastructures, where it is a line construction, but where the third dimension is even more important than for a flood barrier (dike) because the width (third dimension) for higher fills or deeper cuts is more important, even though in reality the fault retains its local character, a typical example being the landslide on the motorway d8. practically the same applies to linear underground structures, as in the case of tunnels. from the point of view of the geotechnical construction itself, there is an interesting transition for earth and rock fill dams, for which in the ratio of the length of the dam to its height, it becomes more appropriate to use a 3d computational model. the reason is a more credible assessment of the potential danger of tensile cracking in the sealing of these dams. for foundation structures, given the current more significant use of underground spaces (as for underground floors), the solution of a foundation structure in 3d is almost a necessity. the current requirements for digitalisation in the construction industry, e.g. in the form of the use of bim (building information modelling), are thus met for geotechnical structures. the basic requirement is that the entire structure, i.e. not only the superstructure but also the subsoil and the foundation structures mediating the mutual interaction, be part of one common 3d model. the interconnection thus brings to the fore the possibility of comparing the quality (informative value) of the subsoil model (whether geological or geotechnical) with the complexity of the superstructure ,which can again be geotechnical, as already mentioned in earth structures or underground structures such as tunnels, galleries etc. at the same time, it meets the requirements of čsn en 1997 design of geotechnical structures and the requirements for subsoil (ground) models obtained during different phases of geotechnical investigation. 2. bim one of the most widely used definitions of the bim process is given by the american national committee for the bim standard, which states that bim is a digital description of the physical and operational characteristics of a facility. bim is a shared source of knowledge and information about this device, creating 37 https://doi.org/10.14311/app.2022.32.0037 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en ivan vaníček, daniel jirásko acta polytechnica ctu proceedings a reliable basis for decision-making during its life cycle, defined from the first concept to demolition [1]. the emphasis is on two basic factors: • it is a reliable basis for the decision-making process; • it is a long-term process – covering the entire life of a building – from the concept itself to a final demolition. next, the main attention will be focused on the first factor, which in principle leads not only to a safe but also an effective construction design. all basic participants in the construction process can be part of this decision-making process, starting from the investor, and then on to the implementer of the geotechnical investigation, the designer, the supplier/contractor, and finally to the approval body, which comments on the construction. however, the second factor is no less important, as it mainly allows for time sequence and the continuity of individual steps, thus ensuring gradual development and specification. however, we must not forget the implementation of the principles of sustainable construction, which are currently strongly promoted [2, 3]. this means the possibility of assessing financial efficiency not only in the construction phase, but also in the demolition phase (i.e. through the entire life of the building). therefore, lower demands on energy (plus co2 footprints), lower land consumption as well lower demands on natural materials (by replacing them with non-standard materials) should be evaluated when total price is being determined. the question of a reliable basis for a decision-making process and for the mutual cooperation of the main actors is present from the very beginning. in the first place, there is the investor’s idea of the structure and the initial information about the ground on/in which this structure is to be situated. today, even investors in supermarkets or warehouses understand that one standard, unified project of such a building cannot be put in place, at least without a minimum knowledge of the nature of the subsoil. in the same way, it is not possible to model the upper structure in 3d without relation to the ground/subsoil, respecting its replacement by various springs, as this is contrary to the basic principles of structural design, now clearly expressed in the system of eurocodes for structural design. finally, this is shown by the recommended name of eurocode 0: eurocode – basis of structural and geotechnical design, defining the principles for the design of all building and civil engineering structures, including geotechnical – draft pren1990, september 2020, which will replace en 1990: 2002 [4]. therefore, the importance of interconnected 3d models of the subsoil and the upper structure (whether it is a building or an engineering structure, including a geotechnical structure), is at the centre of current attention. 3. models time sequence the current valid version of eurocode 7 – čsn en 1997 in part 1997-2 [5] in annex b1 presents the following scheme for the stages of geotechnical/ground investigation of the foundation soil in the geotechnical design, the execution of work and the use of the geotechnical structure – see figure 1. it is clear from this scheme that two basic parts are important within the geotechnical investigatory process: • providing a geological model (ground -subsoil, borrow pit), the main task of which is to divide the ground into different lithological layers with approximately identical properties, including different discontinuities, and also including information on the groundwater level. • ensuring the implementation of field or laboratory tests in the field of soil and rock mechanics for individual lithological layers. the geotechnical/ground investigation report (gir) is the main output of this process of geotechnical investigation. note. en 1997 of the second generation uses not only the term geological model, but for this geological model supplemented by the results of field and laboratory tests, also uses the simple term ground model. although the english word ground has many possible meanings in czech, it can probably be expected that it will be translated persistently as a model of the subsoil. although for underground structures (tunnels, metro) it will probably be a more appropriate translation to say the rock environment model, and as well as for the earth structures to say the soil environment model. within the design of the geotechnical structure, two models come to the fore: • the geotechnical model – developed by the designer o the geotechnical structure after the evaluation of geotechnical parameters and coefficients. • the calculation model, when the analytical calculation model and the numerical calculation model come to the fore. after the elaboration of the geotechnical structure project, the selection of the supplier and the actual implementation of all works will follow. note. in en 1997 of the second generation the first model is labelled as "geotechnical design model", which for each lithological layer specifies representative/characteristic values of the geotechnical parameters, and which are subsequently used in the calculation model. from the point of view of time sequence, ground/geotechnical investigation becomes very important in respect to its phasing. most often, and this 38 vol. 32/2022 the importance of a combined 3d model figure 1. stages of ground investigation in geotechnical design, execution of works and exploitation of the structure. is also in the second generation en 1997, for significant structures connected with high risk a distinction is made between: • desk study – a search of all available existing information from the point of view of the requirements of the geological and geotechnical model this phase is connected with the investment plan. in principle, it is based on all available map data – from geological, engineering-geological, hydrogeological to map data of a geo-environmental nature. also it makes use of the database of survey points carried out so far in the area of interest. for significant public civil engineering structures, these documents can be used to evaluate various variants of location, with subsequent use for the eia process. therefore it is important, and this is supported by the cgts cice, that the data obtained so far can be used in such a form that would allow their quick placement in the 3d model of the subsoil for the first evaluation. • preliminary gi – the preliminary geotechnical/ground investigation – the investigation already uses survey methods, although only to a limited extent, which serves primarily to confirm the expectations obtained in the desk study and from the point of view of designing a geotechnical structure, it serves as a preliminary design for documentation for the zoning decision / project plan. • design (detailed) gi – detailed investigation, investigation for the geotechnical structure design – this is the most important phase, as the results and conclusions are directly usable in the design of a geotechnical structure for the documentation phase regarding a building permit. 39 ivan vaníček, daniel jirásko acta polytechnica ctu proceedings • supplementary gi – additional investigation is needed if during the execution phase larger deviations from the expected ones are found, which may affect both the actual technology of execution and any corrective calculation from the point of view of limit states. • confirmatory gi – a confirming investigation as an investigation of the actual state, important for comparing how sufficiently and credibly the investigation was in the first 3 phases and to what extent the designer used in the calculation model values very close to reality, or was too conservative, or too optimistic. this phase is typical for tunnels, also for foundation pits or for cuts of the earth structures of transport infrastructures, where the actual condition can be visually verified with the possibility of confirmation by additional tests. this possibility does not exist for the subsoil of embankments of earth structures. it is natural that for simple structures with low risk, the phasing of the gi is reduced in exceptional cases to a one-stage survey, but the "desk study" phase should always take precedence for the collection of the existing data and the considered single stage investigation should complement or specify these assumptions. 4. mutual interaction between different steps of the ground models with models of the geotechnical structure the importance of the first step, the evaluation of all available information, has already been mentioned, when for example for transport infrastructure structures, it has an influence on the choice of variants, as well as on the environmental impact assessment – eia. however, already at this stage it is possible to apply the principle of sustainable construction. two issues can be mentioned in this regard. the first one is connected with a recommendation whether the principle of reinforcement (for steeper slopes) will or will not be applied for the design of embankments and cuts. the question about required demands on land occupation and purchase become prominent as well the issue concerning volume of excavated and redeposited soil. the second is related to the possibility of using large-volume waste such as construction and demolition waste, excavated soil, and rock excavated during construction of underground structures or underground garages, etc. similarly, this applies to various wastes arising from the extraction of raw materials and their utilisation. this can significantly affect the issue of the balance of cubatures between cuts and embankments, where there is possibility of reducing the consumption of natural materials. the second step – preliminary gi focuses primarily on areas about which there is not so much prior information taken from the first step. the survey points should, in the first instance, provide sufficient information for the assessment of the individual lithological layers, when for these layers the term "geotechnical type" can be used as well. it is not only about layer interface but also about the basic properties of each layer, at least of a descriptive and index nature, when sometimes a set of results of these index properties can be used for a closer separation of individual lithological layers. this naturally also applies to information on filtration properties and groundwater levels, with respect to pore water pressures. however, with regard to the location of survey points and their depth, it is possible to use the recommendations in čsn en 1997-2 annex b.3 [5]. at this stage, however, it can only be on the basis of experience when, for example for the foundation of bridge piers, it has not yet been decided whether they can apply spread or deep foundations (piles). however, the preliminary gi should provide sufficient information for the designer’s decision about foundation engineering. as for this decision the designer can realise the preliminary design according to the limit states based on so-called table values of the mechanical-physical geotechnical parameters utilising ground index properties or different classification systems, e.g. [6]. as field investigation methods have certain advantages in this respect, there is a tendency to prioritise them when their selection for different phases of the survey is specified in annex b.2 in čsn en 1997-2 [5]. similar tables of the mechanical-physical properties of soils can be used for the preliminary design of embankment and cut slopes. for embankments, information is obtained from the places from which this material will be used, as in borrow pits of cuts of transport infrastructures. when non-standard materials are used (such as large volume waste) a previously obtained experience can be applied. preliminary design is used for the planning of the next step of geotechnical/ground investigation, as now the area affected by a proposed structure is more delineated. on the third step, the step of detailed investigation, investigation for the geotechnical structure design is crucial. additional survey points already use the preliminary design of the geotechnical structure, so they can fully follow the recommendations of čsn en 1997 with regard to location and depth. at the same time, however, when planning this phase, it is necessary to divide the individual sections of the earth structure of transport infrastructure according to the risk and the class of consequences, and into different geotechnical categories. this division into geotechnical categories thus specifies the required scope and focus of field and laboratory tests. for sections falling into the 1st geotechnical category, the design can be implemented using previous experience and so descriptive and index properties in sufficient numbers for the individual lithological layer will suffice. simply put, samples taken within the sampling category of class a and 40 vol. 32/2022 the importance of a combined 3d model the quality class of samples 2, in accordance with čsn en 1977-2, will suffice for laboratory tests. naturally, it is also possible to use mechanical-physical properties obtained for neighbouring sections of the earth structure of transport infrastructure, and coming into a higher geotechnical category for the same lithological layer. for sections falling into the 3rd geotechnical category, the gi must be planned so that for each lithological layer it is able to provide a sufficient number of samples for statistical evaluation of the measured mechanical-physical properties for these lithological layers. the number of 5 samples is usually given as a minimum for statistical evaluation. the designer then decides (and is also responsible for) the representative/ characteristic value that he/she assigns to the individual lithological layer and is thus part of the "geotechnical design model". the fourth step has the function of further specification, often coinciding with the last step, and can significantly affect the final price of the structure brought on by a change in design or in the technology of structure execution. the fifth step records the last findings on the properties of the ground/rock and soil environment and its 3d model together with a 3d model of the actual geotechnical structure or the whole structure and is the final output of the bim model and serves for subsequent decisions over time throughout the life of this structure. for transport constructions, this is a decision regarding future interaction with another construction or in necessary decision-making in the case of accidents or disasters, where there is required quick decisions on the choice of remediation method in the event of an incident involving vehicles transporting hazardous substances. 5. conclusion the interconnection of 3d models of ground and geotechnical structures, whether foundation, ground or underground structures, is today referred to as the bim model and is one of the outputs of digitisation in the construction sector. it is used for communication among partners in the gradual specification of individual phases associated with the geotechnical/ground investigation and geotechnical structure design. this eliminates possible errors, inaccuracies, and helps speed up the construction process and also defines the responsibilities of individual partners. attention is paid to the issue also because government resolution no. 82 "concept of the introduction of the bim method in the czech republic" specifies that all above-limit contracts financed from public budgets must be awarded in the bim regime from 2022. this serves to point out the importance of a 3d model of the ground connection with a 3d model of the geotechnical structure, with an emphasis on transport infrastructure. more precisely, the phasing and gradual refinement of the bim model according to the individual phases of geotechnical/ground investigation in connection with the individual phases of the geotechnical structures design has been shown. this is a preference for earth structures such as embankments and cuts and for foundation structures in the designing of transport structures. this is accomplished while respecting the principles of geotechnical structures design according to čsn en 1997 [5], in terms of risk, geotechnical categories, and in terms of requirements for the sampling class or sample quality class. references [1] i. vaníček, j. pruška, d. jirásko. bim model – application in geotechnical engineering. (in czech). in: proc. 47th conference on foundation engineering brno 2019, s. 91-98. [2] i. vaníček. sustainable construction. czech technical university press, praha, 2011. [3] i. vaníček, d. jirásko, m. vaníček. modern earth structures for transpor engineering. engineering and sustainability aspects. crc press, taylor and francis group, london, 2020, 171 p. [4] eurocode. basis of structural and geotechnical design. draft pren1990, september 2020. [5] čsn en 1997. eurocode 7 geotechnical design (in czech navrhování geotechnických konstrukcí. 1997-1, 1997-2, 2006, 2008. [6] a. bond. eurocode 7: half-term report. (invited lecture), 41st conference on foundation engineering. brno, 11th november. 41 acta polytechnica ctu proceedings 32:37–41, 2022 1 introduction 2 bim 3 models time sequence 4 mutual interaction between different steps of the ground models with models of the geotechnical structure 5 conclusion references https://doi.org/10.14311/app.2022.33.0193 acta polytechnica ctu proceedings 33:193–199, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague energy flows along the production and use of secondary materials with a special focus on concrete karin gruhler∗, georg schiller leibniz institute of ecological urban and regional development, weberplatz 1, 01217 dresden, germany ∗ corresponding author: k.gruhler@ioer.de abstract. urban mining in the existing building stock can contribute to securing raw materials and conserving natural resources if the potential of recycling construction waste is consistently exploited. from an ecological point of view, it is on the one hand interesting what amounts of primary materials can potentially be substituted and on the other how much energy need to be invested for this. at present, the recycling of construction waste usually is considered from a material perspective. there is lack of an approach, extending material-oriented considerations by energetic aspects. the aim is to develop a uniform research approach by which energy expenditure during recycling of important construction products can be determined. besides concrete seven further construction products are investigated. recycling paths are described and analysed along waste management processing steps taking into account the quality of the demolition materials and the quality requirements of the possible new application variants in the construction sector. the result is a clear plea for more consistent recycling. the analyses of concrete indicate that "high-quality" recycling only results in energy advantages when "high-grade" demolition material is used. however, so-called "down-cycling" solutions allow resource conservation to be combined with energy savings, even with lower-quality demolition materials. the single-minded focus on "high-quality" recycling according to the general understanding should therefore be questioned. instead, preference should be given to solutions that take resource conservation into account in a more holistic way especially with regard to resource conservation and climate protection. keywords: circular economy, energy consumption, recycling, resource efficiency. 1. introduction the recycling of construction waste can make an important contribution to climate and nature protection, but above all to the conservation of resources. in germany, the german resource efficiency programme formulated the goals of decoupling resource consumption from economic growth, doubling raw material productivity by 2020, reducing the environmental impacts associated with the use of natural resources as far as possible and further developing and expanding the circular economy [1]. in order to enable almost circular economic activity in the construction sector, the corresponding demolition materials must be recyclable and be available with a certain consistency in specific quantities. resource conservation potentials using secondary materials expressed in tonnes of material are known for some selected construction products [2, 3]. however, an assessment of the resource conservation potentials only on the basis of these material figures is too one-sided, as energy aspects are not taken into account. there are individual studies that focus on specific construction products with regard to the energy used in recycling [4]. however, there is no comprehensive overview for all important construction products. there is a lack of a common approach by which important construction products can be presented synoptically and compared to each other in terms of their energy expenditures during recycling. the aim of the present study was to extend the material-oriented studies on resource conservation potentials to include energy considerations and to develop a uniform approach by which important construction products can be presented and compared in a synoptic way with regard to their energy expenditure during recycling [5]. the following questions should be clarified: which energy expenditures are associated with recycling? what is the energy expenditure of recycling compared to the energy expenditure of standard production without recycling? are there energetic differences between "high-quality" application variants (in building construction) and less "high-quality" application variants (in civil engineering and landscaping)? the investigations focused on eight construction products: concrete, bricks, sand-lime bricks, gypsum, flat glass, mineral (stone) wool, pvc profiles, and pvc floor covering. for all of these, we energetically investigated the paths from demolition material to a new application variant in building construction, civil engineering or landscaping. this we did taking into account the quality of the demolition material and the quality requirements of the new application 193 https://doi.org/10.14311/app.2022.33.0193 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en karin gruhler, georg schiller acta polytechnica ctu proceedings process chain demolition material secondary material application variant concrete pc 1 concrete without recycled aggregates coarse adhesions 2/16 type 1 concerete foundation pc 2 recycled aggregates c20/25 broken concrete mixture 2/22 type 2 pc 3 (with brick, limestone, plaster) recycled aggregates ballast base layer 0/32 for road construction for roads table 1. model-like pc for concrete. variant in the construction sector. 2. methodology in order to carry out these investigations, we have developed an analytical approach which generally regulates the methodological procedure, following the idea of a continuous material flow analysis as described in [6]. 2.1. balance framework and terms & definitions starting point of the considerations are demolition materials. demolition materials are quantities that are available in a certain quality after the demolition of a building (e.g. broken concrete without coarse adhesion). these materials come exclusively from building construction. construction site waste is not included in the analysis. in order to be able to supply demolition materials as secondary materials and substitute materials for a new application variant, they must be prepared accordingly. a new application variant is the use of demolition materials for a new purpose and location (e.g. broken concrete as aggregates for foundation concrete). all necessary preparation and further processing steps depend on the quality of the demolition material itself as well as the quality requirements of the new application variant. in order to take these dependencies into account, there is a need to consider the entire process from the demolition material to the new application variant. this is applied in the form of characteristic model-like process chains (pc). due to the large variety of possible process chains (large variety of possible deconstruction qualities, types of processing, further processing methods, application variants) two to three demolition materials and two to three typical application variants were combined into model-like pc for each construction product. the pc are defined in consultation with experts from the relevant construction product associations in germany. table 1 shows the pc for the example concrete. on the way from the demolition material to the new application variant, a secondary material is first created and in a further step a substitute. the terms "secondary material" and "substitute" are described more in detail in the standard din en 15804:201407 [7] (table 2). with regard to din en 15804:2014-07, the selected balance framework starts at the end of the waste treatment step of the preceding product system "building". processes before the end of the waste property stage (such as collection and transport) are part of the waste treatment of the above product system "building". in contrast, processes that are required after the end of waste property stage to allow primary materials to be replaced in another product system are considered to be outside the above system "building" (see [7], para. 6.3.4.5 note 3). these regulations are complied with here. the analyses start with the demolition material at the recycling company and integrate all preparation steps for the production of the secondary material as well as its further processing up to the substitute (preceding demolition technology, collection processes, sorting processes and transports on the site are not considered). 2.2. 3-step analysis approach of process chains the energetic analyses of the model-like pc of the different construction products follow a uniform methodical approach. it is divided into three steps: 1. determination of the energy required to process a demolition material into a secondary material. 2. determination of the energy expenditure for the further processing of the secondary material into a substitute that can replace a primary material in a functionally equivalent way, using energy addons/deductions. 3. comparison of the energy expenditure for the substitute with that for the primary material to be replaced (figure 1). the steps of the methodical procedure are explained more in detail in the following paragraphs using concrete as example (pc 1: from pure crushed concrete to recycled aggregate for foundation concrete c20/25, table 1). 2.2.1. energy expenditure for processing secondary material here the energy required to process the demolition material into secondary material is calculated 194 vol. 33/2022 energy flows of secondary materials term description of important terms secondary material according to din en 15804:2014-07 [7], para 3.29, a secondary material is any form of material recovered from a previous use or from waste and replacing a primary material, recorded at the point at which the secondary material enters the system from another system, recovered from a previous use or from waste from a product system and used as input to another product system. substitute substitutes are "secondary materials having left the system for primary material production [and are declared] in module d if they have functional equivalence to the replaced primary material" ([7], para 6.3.4.5, note 3). table 2. description of the terms "secondary material" and "substitute" according to din en 15804:2014-07. figure 1. methodical procedure. in mj/kg. the following list shows which analysis and calculation steps are required to determine the average energy expenditure. • identification and definition of characteristic main processing steps to produce the secondary material (e.g. pre-screening, crushing, metal deposition etc.). • definition of technical components/machinery for the identified processing steps (e.g. sieve, jaw crasher, magnetic separator etc.). • determination of average energy expenditures for the defined components/machinery (e.g. sieve 0.0021 mj/kg, jaw crasher 0.0022 mj/kg, magnetic separator 0.0147 mj/kg etc.). • calculation of the energy required to produce the secondary material by adding the energy values of the identified process steps and defined machines (e.g. 0.036 mj/kg for all steps). • analysis of the material flows and calculation of the proportions for target product, co-products and waste. for pc 1 of concrete this is 48% target product (recycled aggregate 2/16 type 1), 43% coproducts (crushed concrete 16/22, 22/32, 32/56, 56/x and sand 0/1, 0/2) and 9% waste (pre-screen material 0/8 and impurities/foreign substances). for co-products there is the possibility of further use. they receive an energy allocation. there is no further use for waste and therefore no energy allocation. • calculation of the energy allocation for the coproducts based on the material flow proportions (e.g. 0.013 mj/kg for crushed concrete > 16 mm and sand ≤ 2 mm). • calculation of the energy required to produce the secondary material considering the energy allocations for co-products (e.g. 0.036 mj/kg − 0.013 mj/kg = 0.024 mj/kg) in sum step 1 ends with the energy expenditure for processing the secondary material. 195 karin gruhler, georg schiller acta polytechnica ctu proceedings figure 2. energy expenditures of the selected construction products a comparison. 2.2.2. energy expenditures for further processing up to a substitute here it is analysed which energy expenditure is connected with the further processing of the secondary material to the substitute. methodologically, all necessary measures and processes are analysed and the differences between the production process with recycled material and the standard production process without recycling are worked out. the differences can be associated with both additional and reduced energy expenditure. this means that the previous energy expenditure for the secondary material (0.024 mj/kg) receives an energy add-on or an energy deduction. additional and reduced energy expenditure result mainly from modified recipes and modified process steps1. modified recipes are, for example, different mixing ratios or additional admixtures of auxiliary materials. modified process steps can be additional, shortened or unnecessary process steps. in the case of concrete, it is above all necessary to increase the cement content in the recipe (+ 1%). modified process steps are not necessary (± 0%). this results in a total energy add-on of + 0.052 mj/kg. in total, this results in the energy expenditure for the substitute (0.076 mj/kg). 2.2.3. comparison of the energy expenditure for the substitute with that of the primary material finally, the production of construction products without recycled material is compared in terms of energy to that with recycled material. this is done 1modified transport efforts also lead to additional or reduced energy expenditure. however, they could not be calculated due to the poor availability of data. there is a need for research. at the level of the material to be replaced or the material to be substituted. in this way, the energy expenditure of the substitute is compared with that of the primary material. this comparison allows assessments to be made as to whether recycling makes sense from an energy perspective. the energy characteristics of the primary materials are taken from the source ökobaudat [8] and din en 15804 compliant secondary sources [9], [10], [11]. 3. results following the methodological procedure presented, energy expenditure for all pc of the selected construction products was calculated for the production of secondary materials and further processing up to the substitutes and compared with the energy expenditures of primary materials (to be replaced) (figure 2). in general, the results show that under the given framework there is almost nothing to prevent recycling. the energy expenditure for the secondary material and the substitute is usually lower than the energy expenditure for the primary material. exceptions are mineral wool and one pc of concrete (pc 2) and one of gypsum (pc 1). for mineral materials, recycling is not associated with excessive energy expenditures and makes sense above all with regard to the mass aspect, as recycling reduces the extraction of raw materials and protects the natural environment and landscape. for plastics the energy aspect is more significant. a specially look at concrete shows that the quality of the demolition material as well as the material requirements of the new application variant have a significant influence on the energy expenditures for recycling (table 3). as a rule, the prin196 vol. 33/2022 energy flows of secondary materials construction product concrete pc 1 pc 2 pc 3 demolition material concrete without broken concrete mixture coarse adhesions (with brick, limestone, plaster) ee of all processing steps 0.036 mj/kg 0.031 mj/kg 0.025 mj/kg discharge co-products 43% 34% 10% energy allocation (ea) co-products 0.012 mj/kg 0.009 mj/kg 0.002 mj/kg secondary material recycled recycled ra 0/32 aggregates (ra) aggregates (ra) for road 2/16 type 1 2/22 type 2 construction ee of all processing steps with ea 0.024 mj/kg 0.022 mj/kg 0.023 mj/kg substitute modified recipes + 0.052 mj/kg + 0.092 mj/kg ± 0 mj/kg modified process steps ± 0 mj/kg ± 0 mj/kg ± 0 mj/kg ee with add-ons/deductions 0.076 mj/kg 0.114 mj/kg 0.023 mj/kg for modification (77%) (118%) (14%) primary material aggregate of aggregate of aggregate of gravel (2/32; 80%) gravel (2/32; 80%) gravel round (4/x; and chippings and chippings 50%) and gravel (2/15; 20%) (2/15; 20%) broken (4/x; 50%) ee for processing primary 0.0972 mj/kg 0.0972 mj/kg 0.168 mj/kg material1 (100%) (100%) (100%) application variant concrete crushed stone foundation c20/25 sub-base (roads) 1 source: ökobaudat [8]. 2 using ecoinvent [9] results in a different figure: 0.057 mj/kg. this also influences the figure of the substitute due to the recipe. table 3. energy expenditure (ee) calculations of the different pc of concrete. ciple applies that "high-quality" demolition material (clean/without adhesions) is usually associated with a "high-quality" application variant (building construction) and a "low-quality" (unclean/ with adhesions) one with a "low-quality" application variant (civil engineering/landscaping). however, one and the same demolition material can also be used in different application variants (pc 2 and 3). for example, the demolition material crushed concrete mix can be used both as aggregates for the production of foundation concrete (pc 2) and as a ballast base layer in road construction (pc 3). the first variant has higher quality requirements and is also associated with higher energy expenditure. these result from a different secondary material: recycled aggregates 2/16 type 1 for concrete foundation (pc 2) and recycled aggregates 0/32 for road construction (pc 3). in addition, energy-relevant recipe adjustments (e.g. modified ratios for superplasticizer, water or cement) are required (pc 2) to produce a substitute equivalent to the primary material. a higher cement ratio content in particular has an impact on energy expenditure. compared to the two pc, the crushed stone sub-base for road is thus the energetically better application variant. likewise, two different demolition materials can have one and the same use (pc 1 and 2). in this way, both broken concrete without adhesions (pc 1) and crushed concrete mix (pc 2) can be used for the production of foundation concrete. here, however, the "low-quality" demolition material (crushed concrete mix) is the less efficient variant in terms of energy. it is true that the production of aggregates 2/22 type 2 requires less energy than that of aggregates 2/16 type 1. however, changes in the recipe finally lead to a higher energy expenditure. in comparison of the two process chains, the broken concrete without adhesions is therefore the more energy-efficient starting demolition material. the examples for concrete show that the use of recycling material in building construction can lead to different energy results with different types of demolition material (pure concrete or concrete mix). for example, the energy expenditure for the substitute in pc 1 is approx. 20% lower and in pc 2 approx. 20% higher than the energy expenditure for the primary material to be replaced (aggregates for concrete foundation). in contrast, energy savings of approx. 85% 197 karin gruhler, georg schiller acta polytechnica ctu proceedings are possible in road construction if recycled aggregates are used instead of primary aggregates (pc 3). if these results are taken into account, both "highquality" and "lower-quality" recycling should be given attention in the interests of resource and climate protection. 4. summary and conclusions in line with the objectives we developed a standard uniform balance approach to assess energy expenditure. using this approach we calculated two to three pc for each of the selected construction products und provided a first set of energetic figures for recycling. the analyses show that recycling is worthwhile. compared to the use of primary materials, it is generally not associated with an excessive use of energy. however, there are some exceptions. every building product has its own specific quality requirements and must be considered individually. for mineral materials, recycling makes sense above all with regard to the mass aspect, as recycling reduces the extraction of raw materials and protects the natural environment and landscape. for plastics the energy aspect is more significant. it also becomes clear that the pc of the construction products have to be considered from the demolition material until the new application variant is reached. this is because each new application variant is associated with certain quality requirements for the secondary material up to the substitute, which finally affect the energy expenditure. in summary, the analyses of concrete show that "high-quality" recycling (building construction application) only brings energetic advantages if "highquality" demolition material (pure concrete breakage) is used. "low-quality" recycling (road work application) on the other hand allow resource conservation to be combined with energy savings, even with lowerquality demolition materials. the single-minded focus on "high-quality recycling" according to the general understanding should therefore be questioned. instead, preference should be given to solutions that take resource conservation into account in a more holistic way especially with regard to resource conservation and climate protection. in order to be able to assess this comprehensively, it is always necessary to look at the entire process chain, starting with the construction waste and its quality, the intended recycling product and the intermediate treatment and processing steps. this should not stop at today’s common technologies. rather, it is important to keep an eye on innovative technical developments. for example, the use of geopolymers as cement substitutes can lead to a significant reduction in greenhouse gas emissions [12] and thus have an impact on the advantages of the recycling processes under consideration. acknowledgements the authors would like to thank the federal institute for research on building, urban affairs and spatial development (bbsr) for funding the research as well as the entire project team. in particular, we would like to thank tamara bimesmeier, who made most significant contributions to the development of the results of this publication. references [1] bmub. deutsches ressourceneffizienzprogramm ii: programm zur nachhaltigen nutzung und zum schutz der natürlichen ressourcen, p. 144, 2016. https://www.bmu.de/fileadmin/daten_bmu/pools/ broschueren/progress_ii_broschuere_bf.pdf. [2] g. schiller, c. deilmann, k. gruhler, et al. ermittlung von ressourcenschonungspotenzialen bei der verwertung von bauabföllen und erarbeitung von empfehlungen zu deren nutzung, dessau-roßlau: umweltbundesamt, uba-texte 56/2010, p. 195, 2010. https://www.umweltbundesamt.de/publikationen/ ermittlung-von-ressourcenschonungspotenzialen-bei. [3] c. deilmann, n. krauß, k. gruhler, et al. materialströme im hochbau. potenziale für eine kreislaufwirtschaft (zukunft bauen: forschung für die praxis), p. 88, 2017 https: //www.bbsr.bund.de/bbsr/de/veroeffentlichungen/ zukunftbauenfp/2017/band-06.html?nn=1143814. [4] m. buchert, j. sutter, h. alwas, et al. ökobilanzielle betrachtung des recyclings von gipskartonplatten. endbericht, dessau-roßlau: umweltbundesamt, uba-texte 33/2017, p. 110, 2017. https://www.umweltbundesamt.de/sites/default/ files/medien/1410/publikationen/2017-04-24_ texte_33-2017_gipsrecycling.pdf. [5] t. bimesmeier, k. gruhler, c. deilmann, et al. sekundörstoffe aus dem hochbau forschungsinitiative zukunft bau, band f 3184, p. 252, 2019. https://www.baufachinformation.de/ sekundaerstoffe-aus-dem-hochbau/fb/253180. [6] g. schiller, k. gruhler, r. ortlepp. continuous material flow analysis approach for bulk nonmetallic mineral building materials applied to the german building sector. journal of industrial ecology 21(3):673-88, 2017. https://doi.org/10.1111/jiec.12595. [7] din en 15804:2014-07: nachhaltigkeit von bauwerken umweltproduktdeklarationen grundregeln für die produktkategorie bauprodukt, 2014. https: //www.beuth.de/de/norm/din-en-15804/195229515. [8] ökobaudat. informationsportal nachhaltiges bauen, 2018. http://oekobaudat.de/datenbank/ browser-oekobaudat.html. [9] ecoinvent. the ecoinvent database (zurich), 2019. https://ecoinvent.org/the-ecoinvent-database. [10] ibu. published epds (institut bauen und umwelt), 2018 https://ibu-epd.com/en/published-epds/. [11] sphera. gabi solutions (ef database v2.0.). https://gabi.sphera.com/ce-eu-english/index/. 198 https://www.bmu.de/fileadmin/daten_bmu/pools/broschueren/progress_ii_broschuere_bf.pdf https://www.umweltbundesamt.de/publikationen/ermittlung-von-ressourcenschonungspotenzialen-bei https://www.bbsr.bund.de/bbsr/de/veroeffentlichungen/zukunftbauenfp/2017/band-06.html?nn=1143814 https://www.umweltbundesamt.de/sites/default/files/medien/1410/publikationen/2017-04-24_texte_33-2017_gipsrecycling.pdf https://www.baufachinformation.de/sekundaerstoffe-aus-dem-hochbau/fb/253180 https://doi.org/10.1111/jiec.12595 https://www.beuth.de/de/norm/din-en-15804/195229515 http://oekobaudat.de/datenbank/browser-oekobaudat.html https://ecoinvent.org/the-ecoinvent-database https://ibu-epd.com/en/published-epds/ https://gabi.sphera.com/ce-eu-english/index/ vol. 33/2022 energy flows of secondary materials [12] m. sandanayake, c. gunasekara, d. law, et al. greenhouse gas emissions of different fly ash based geopolymer concretes in building construction. journal of cleaner production 204:399-408, 2018. https://doi.org/10.1016/j.jclepro.2018.08.311. 199 https://doi.org/10.1016/j.jclepro.2018.08.311 https://doi.org/10.14311/app.2022.33.0027 acta polytechnica ctu proceedings 33:27–31, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague environmental impacts of using desalinated water in concrete production in areas affected by freshwater scarcity alessandro arrigonia,∗, tamar ophera, sabrina spatarib, valeria arosioc, heather l. macleana, daman k. panesara, giovanni dotellic a the university of toronto, faculty of engineering, department of civil and mineral engineering, 35 st. george street, toronto, ontario, m5s 1a4, canada b technion israel institute of technology, faculty of civil & environmental engineering, department of structural engineering and construction management, haifa, 32000, israel c politecnico di milano, school of industrial and information engineering, department of chemistry, materials and chemical engineering "g.natta", piazza l. da vinci 32, 20133 milano, italy ∗ corresponding author: a.arrigonim@gmail.com abstract. up to 500 litres of water may be consumed at the batching plant per cubic meter of ready mix concrete, if water for washing mixing trucks and equipment is included. demand for concrete is growing almost everywhere, regardless of local availability of freshwater. the use of freshwater for concrete production exacerbates stress on natural water resources. in water-stressed coastal countries such as israel, desalinated seawater (dsw) is often used in the production of concrete. however, the environmental impacts of this practice have not yet been assessed. in this study the effect of using dsw on the water and carbon footprints of concrete was investigated using life cycle assessment. water footprint results highlight the benefits of using dsw rather than freshwater to produce concrete in israel. in contrast, because desalination is an energy intensive process, using dsw increases the greenhouse gas intensity of concrete. nevertheless, this increase (0.27 kg co2e/m3 concrete) is small, if compared to the life cycle greenhouse gas emissions of concrete. our results show that using untreated seawater in the mix (transported by truck from the coast) in place of dsw, would be beneficial in terms of water and carbon footprints if the batching plant were located less than 13 km from the withdrawal point. however, use of untreated seawater increases steel reinforcement corrosion, resulting in loss of structural integrity of the reinforced concrete composite. sustainability of replacing steel with non-corrosive materials should be explored as a way to reduce both water and carbon footprints of concrete. keywords: carbon footprint, desalinated water, seawater, water footprint. 1. introduction water scarcity affects two-thirds of the global population [1] and climate change, economic development and population growth might worsen this issue in the near future [2]. industrial development plays an important role in the growing demand for water [2], with concrete being a significant contributor. the life cycle of concrete requires large amounts of water, from the production of the raw materials (i.e., cement and aggregates), through mixing, curing, and hydration, to equipment cleaning processes [3]. in 2012, the estimated global water consumption for concrete production was 16.6 km3 and it is expected to grow with the increase in production, by more than 40% by 2050 if no action is taken [4]. aggregate production is the main water-consuming activity in the life cycle of concrete [5], but water consumption at the batching plant is very significant. considering ready-mix concrete, up to 500 litres of water per cubic meter of concrete may be consumed, including water for washing the mixing trucks, on-site equipment and floor [6]. water used at the batching plant is typically sourced from a potable supply network, on-site wells, or harvested stormwater [6]. potable water is used to avoid any adverse effects on the properties of the concrete, such as hydration, strength development and durability performance [7]. however, using available freshwater for concrete production will exacerbate the stress on natural water resources. in water-stressed coastal countries, such as israel, desalinated seawater (dsw) is increasingly used to meet the potable water demand [8]. although reducing the stress on freshwater resources, desalination comes with an environmental cost. in qatar, for instance, 4.66 million tons of co2 were estimated to be emitted by desalination plants in 2014 [9]. one appealing option to reduce the stress on available freshwater resources, and at 27 https://doi.org/10.14311/app.2022.33.0027 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en a. arrigoni, t. opher, s. spatari et al. acta polytechnica ctu proceedings figure 1. flow chart for the product system analyzed. the different paths for the two scenarios are highlighted: desalinated and seawater. the unit processes valid for both processes (e.g., seawater withdrawal, concrete mixing, etc.) are considered unaffected by the type of water used in the mix and therefore excluded from the comparative lca. the same time avoid the impacts associated with desalination, is to replace potable water with untreated seawater in the manufacture of concrete [10]. however, the use of seawater is currently forbidden in reinforced concrete standards because it contributes to the early corrosion of carbon steel reinforcement, which is the present common practice in construction [11]. nevertheless, in recent studies, the use of seawater combined with non-corrosive reinforcing materials, such as glass fibre reinforced polymers, has shown promising results in terms of mechanical properties and durability [12]. the focus of this study is to estimate the consequences of using seawater for concrete production in israel, using a consequential life cycle assessment (lca) methodology. the results are intended to pave the way for further investigation of the feasibility and sustainability of using seawater and marine aggregates for concrete production. 2. goal the objective of this analysis is to assess the potential water and carbon footprint savings for israel if seawater was used in place of municipal grid water in concrete production. the study is aimed to set the stage for further analyses of the feasibility and sustainability of using alternative concrete designs with the ultimate goal of reducing the environmental impacts of the most prevalent construction material in the israeli construction sector. 3. scope and methods the functional unit of the study is the production of 1 m3 of generic non-reinforced concrete (100% ordinary portland cement, 0.85 water-to-cement ratio) in israel. two alternatives differing in the type of water used in the mix are compared in the analysis: municipal grid water (current practice) and untreated seawater. the market demand for concrete is assumed to be constant (18 mm3 per year [13]) and its mix design (i.e., the amounts and types of binders, aggregates and admixtures) and mechanical properties are assumed to be identical in both cases. the production process at the batching plant in this study is the only life cycle phase that differs between the compared scenarios. we use a consequential approach in this lca. unlike attributional lca, which aims to capture all environmental flows associated with the life cycle of the activity, a consequential assessment accounts only for the flows affected by a certain change in the system [14]. according to the principles of consequential lca, we consider the marginal water supply, which is the one that is most likely to react to a change in market demand. in the case of the israeli water market this is the supply of dsw, coming from five major desalination plants along the mediterranean coast. in the case of electricity, the marginal supplier in israel is under most circumstances one of several natural gas power plants [15]. we consider only the variations in impacts resulting from avoiding the desalination 28 vol. 33/2022 concrete made of desalinated water scenario unit process water footprint (l-eq./m3 concrete) carbon footprint (kg co2-eq./m3 concrete) desalinated desalination 18 0.27 distribution 4.8 0.071 mixinga 0.0 excluded total 23 0.34 seawater transportation 1.8 · x 0.016 · x mixinga 0.0 excluded total 1.8 · x 0.016 · x a the energy intensity of the mixing process, and therefore the greenhouse gas emissions and water footprint associated with the process, is assumed to be independent from the water used (dsw or sw) and therefore excluded from the analysis. since the water used for mixing was originally seawater in both scenarios, its water footprint is zero. table 1. water and carbon footprint results for the activities considered in the two scenarios (i.e., activities affected by the change in the system). variable x represents the distance (km) travelled by the truck from the desalination plant to the batching plant. process by replacing dsw with untreated seawater throughout the life cycle of concrete. the ecoinvent consequential database was used for modelling background processes in the system. the lca is limited to two environmental impact categories: climate change and water consumption. inputs and outputs to the system are considered from cradle to grave. 4. product system two scenarios of concrete production in batching plants in israel are modelled. in the base case scenario, representing current practice, water for mixing concrete is sourced from the israeli water grid. dsw is the marginal source of water in israel [16], meaning that a change in potable water demand will affect the extent of its production [17]. therefore, in the base case scenario (named desalinated scenario from now on) seawater is drawn from the sea, desalinated and distributed to the concrete batching plants via the national supply grid. in the alternative (seawater) scenario, once drawn, seawater is transported via trucks to the batching plants. if using untreated seawater proves to be a viable and sustainable solution, pipelines could be installed to deliver seawater to the batching plants. existing desalination plants inlets are assumed to be used as a means for pumping water from the sea into the trucks. the product system analysed is illustrated in figure 1. the mixing process, the delivery of concrete to the construction site, and the properties of the concrete are assumed to be unaffected by the type of water used. in future work the effects on concrete properties and on the service life of the mixing trucks (which might be reduced due to potential corrosivity of the seawater mix) will be included. 5. life cycle inventory both primary and secondary data are used for the analysis: primary data for the inputs and outputs of the israeli desalination plants and the distribution process [18]; secondary data from the ecoinvent 3.6 database [19] for the average amount of water used in the concrete mix (i.e., 170 l/m3), and for the emissions and indirect water consumption of electricity production and road transportation (e.g., water consumed in fuel production). water for washing the trucks is assumed to be reused in the concrete mix itself, as is the current common practice [13]; therefore, no additional benefits are considered for substituting municipal grid water with seawater for this purpose. moreover, additional water required at the batching plant for washing the site and for personnel needs is assumed to be municipal grid water, in both scenarios, and is therefore excluded. a threshold is calculated for the maximum distance from the seawater extraction site for which using seawater shipped by truck to the batching plant would still be beneficial in terms of carbon and water footprint. energy consumed by the desalination plants is supplied through the electricity grid, which uses natural gas combined cycle power plants (60% efficiency) [15]. 6. results and discussion table 1 presents the water and carbon footprints of the activities affected by the change in the system. results are expressed per cubic meter of fresh concrete produced at the batching plant. in the seawater scenario, impacts depend on the distance travelled by the trucks (represented by x in table 1) to transport seawater to the batching plant. for instance, the transport of 170 l of seawater by truck for the production of 1 m3 of concrete is responsible for the consumption of 1.8 l-eq. of water per km. 29 a. arrigoni, t. opher, s. spatari et al. acta polytechnica ctu proceedings the key observations from the results are: • from a consequential perspective, the water footprint of concrete mixing in israel, taking into account direct water use at the batching plant only, is considerably lower than those reported for other regions (e.g., up to 16,000 l-eq./m3 concrete in sicily, italy [6], or 185 l-eq./m3 concrete in sweden [20]). the reason is that the marginal water supply in israel is dsw, which has a water footprint characterization factor of zero since it does not negatively affect the availability of freshwater in the watersheds [21]. therefore, the only freshwater consumed in the process is the water consumed indirectly for electricity generation and fuel production. this applies to both scenarios. • the carbon footprint of desalinating seawater and distributing it to the batching plant is trivial compared with the life cycle greenhouse gas emissions of concrete production. a global average cubic meter of non-reinforced concrete is responsible for approximately 200 kg of co2-eq. [19]. this may be over 400 kg of co2-eq. for high-performance concrete (e.g., 50 mpa [19]). either way, the impact of desalination and water distribution for the concrete mix would be far less than 1% of the total impact. • greenhouse gas emissions for desalination are 1.5 kg co2-eq/m3 desalinated water. this is lower than the values reported for other geographical contexts. in qatar, for instance, 9.5 kg of co2-eq is estimated to be emitted per m3 of desalinated water [9]. there are two potential reasons: (i) only impacts associated with the electricity consumption of the desalination plant (3.5 kwh/m3 dsw) are considered in this study and, (ii) the lower greenhouse gas intensity of electricity used in israeli desalination plants. • the thresholds for reducing water and carbon footprints by using untreated seawater in the mix in place of municipal grid water are 13 and 22 km, respectively. if a batching plant is located less than 13 km away from the desalination plant (x < 13), transporting seawater by truck instead of desalinating it and pumping it through the national pipeline network would reduce both the water and carbon footprints of concrete. if the distance is between 13 and 22 km, desalinating and distributing water would have a lower water footprint but would generate higher greenhouse gas emissions. finally, if the batching plant is located more than 22 km away from the desalination plant, transporting seawater by truck would lead to higher impacts for both water and carbon. 7. conclusions the water and carbon footprint consequences of using seawater instead of municipal grid water for concrete mixing in israeli batching plants are estimated. a net benefit for both impacts would emerge only if the batching plant is located less than 13 km from a desalination plant. however, even in this case, the benefits appear to be negligible compared with the average life cycle impacts of concrete. negligible savings are observed since the marginal water supply in israel is desalinated seawater, which has a water footprint of zero and a low carbon footprint compared to other constituents of concrete, such as cement. nevertheless, if batching plants are located near the coast, given the large amount of concrete produced in israel each year, significant greenhouse gas savings could still be achieved: for every km less than the 22-km threshold, 20 g of co2-eq. could be spared for every m3 of concrete produced. if, theoretically, all israeli concrete was produced in plants that are less than 10 km away from desalination plants, over 3,000 tons of co2-eq. could be saved every year. moreover, by using untreated seawater in the mix, other environmental impacts (e.g., marine ecotoxicity, biodiversity) associated with the desalination process, which are not included in the present assessment, could be avoided. approximately 1.5 m3 of brine are produced for every m3 of desalinated water and concerns are growing over the rising toxic levels from its discharge back into the sea [8]. to confirm the results, a sensitivity analysis on the assumptions and data should be performed. for instance, results might differ if pipes were used instead of trucks to deliver the seawater or if seawater was to replace potable water used for washing the quarried aggregates as well. moreover, even though negligible savings (or larger impacts, when distances from the coast are above the threshold) are observed in israel, contrary conclusions may result in countries where the marginal supply of municipal grid water comes from natural freshwater resources. the impact of seawater on the service life of the machinery used for concrete production and of the concrete structure itself should be further investigated to avoid burden shifting. finally, a comparison of the life cycle costs of the two scenarios would provide valuable information to the concrete industry and the government to evaluate the practicality and economic viability of shifting the concrete industry towards the use of seawater. in a future analysis the challenges of a design practice not yet widespread and the market cost of gfrp should be included. references [1] m. m. mekonnen, a. y. hoekstra. four billion people facing severe water scarcity. science advances 2(2), 2016. https://doi.org/10.1126/sciadv.1500323. 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[14] t. ekvall, b. p. weidema. system boundaries and input data in consequential life cycle inventory analysis. the international journal of life cycle assessment 9(3):161-71, 2004. https://doi.org/10.1007/bf02994190. [15] y. parag. personal communication, 2019. [16] t. opher, e. friedler. comparative lca of decentralized wastewater treatment alternatives for non-potable urban reuse. journal of environmental management 182:464-76, 2016. https://doi.org/10.1016/j.jenvman.2016.07.080. [17] b. p. weidema, n. frees, a.-m. nielsen. marginal production technologies for life cycle inventories. the international journal of life cycle assessment 4(1):4856, 1999. https://doi.org/10.1007/bf02979395. [18] t. opher. a multi-objective lca-based model for sustainability assessment of urban water reuse alternatives at various centralization scales, department of civil and environmental engineering, technion israel institute of technology), 2016. 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[21] a.-m. boulay, j. bare, l. benini, et al. the wulca consensus characterization model for water scarcity footprints: assessing impacts of water consumption based on available water remaining (aware). the international journal of life cycle assessment 23(2):368-78, 2017. https://doi.org/10.1007/s11367-017-1333-8. 31 https://doi.org/10.1016/b978-0-08-102480-5.00009-9 https://doi.org/10.1038/s41893-017-0009-5 https://doi.org/10.1007/s11367-009-0105-5 https://doi.org/10.1088/1755-1315/323/1/012127 https://doi.org/10.1016/j.scitotenv.2018.12.076 https://doi.org/10.1016/j.desal.2018.11.017 https://doi.org/10.1520/acem20180114 https://doi.org/10.1088/1755-1315/296/1/012017 https://doi.org/10.1007/bf02994190 https://doi.org/10.1016/j.jenvman.2016.07.080 https://doi.org/10.1007/bf02979395 https://doi.org/10.1007/s11367-016-1087-8 https://doi.org/10.1007/s11367-017-1333-8 acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.38.0656 acta polytechnica ctu proceedings 38:656–671, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague measuring circularity from buildings to neighbourhoods vanessa gomesa,∗, sara valdiviab, lizzie pulgrossia, maristela gomes da silvac a university of campinas, school of civil engineering, architecture and urban design, department of architecture and construction, av. albert einstein 951, cidade universitária, campinas – sp, 13083-852, brazil b university of campinas, school of technology, r. pedro zaccaria 1300, limeira – sp, 13484-350, brazil c federal university of espírito santo, technology center, department of structures and buildings, av. fernando ferrari 514 – goiabeiras, vitória – es, 29075-910, brazil ∗ corresponding author: vangomes@unicamp.br abstract. the circular economy (ce) aims to eliminate the concept of pollution and waste generation, maintain the integrity of the product over several use cycles, and focus on closing material and energy loops. circularity metrics are relevant for monitoring, reporting and communicating ce implementation progress. applied to buildings, these metrics deliver structured assessments through standardized indicators, which establish a common language among the agents involved, help implement strategies to assess the circular potential of technical options. studies dealing with circularity metrics for buildings are still scarce and somewhat variable within an overall common framework. applications to neighbourhoods are even more incipient. this study applied selected metrics to two building cases with different constructive characteristics, to improve the understanding on how information on circularity is conveyed. the selected metrics highlighted the circularity challenges for the two building designs. however, such metrics disregard the environmental impacts required to induce circular flows and loop closure. it is herein proposed that such metrics are paired with environmental performance profiles produced by e.g., life cycle assessments (lca). the concept of “nested indicators” could be applied to neighbourhood and city scales by referring to the lca concept of functional equivalency as the “relevance” weighting criterion. keywords: circular building, indicator, metric, circularity, circular economy. 1. introduction the circular economy (ce) aims to eliminate the concept of pollution and waste generation so that material circulates in the economy perpetually, that is, it seeks to maintain the integrity of the product over several cycles of use and focus on the closure of material and energy cycles [1]. this helps not only in the security of material supply and sustainable consumption, but also in improving environmental and socioeconomic analyses [2]. this new economic-environmental paradigm has gained relevance in recent years [3]. indeed, our systematic literature review (see appendix a) confirmed that this is a trending research topic, and this will continue or intensify. it also showed that this discussion is notably euro-centred: no article in the final sample focused on e.g., brazil or south america. in 2015, the european commission defined a first action plan that allocated more than ten billion euros, between 2016 and 2020, for the transition from the current linear model to a circular model [1]. in 2019, the “european green deal” was established to address the climate and environmental challenges in europe and to separate economic growth from the use of resources. in 2020, the european commission adopted a new action plan and the proposal for a new regulation of sustainable batteries. finally, in 2021, the global alliance on circular economy and resource efficiency was launched, and several initiatives were adopted in the action plan [4]. all these measures aim to make the economy of europe and the world adjust to protect the environment, enhancing competitiveness and circularity, and thus achieving a green future [4]. despite the debate that has taken place over the last decade, the concept of ce is still being discussed, and remains somewhat confusing, as shown by the over 114 definitions found in a previous review [5]. ce metrics are relevant, as they serve to identify areas in which countries need to focus their efforts to boost their performance in the circular economy [6]. in addition, the ec’s comprehensive performance assessment metrics and methods are critical to defining public policy. therefore, metrics serve to monitor, report and communicate progress towards the implementation of the circular economy [7]. the ec’s concepts apply to companies in all sectors [8], and therefore also to the construction industry [9, 10]. the construction sector plays a critical but 656 https://doi.org/10.14311/app.2022.38.0656 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 38/2022 measuring circularity from buildings to neighbourhoods also a strategic role in achieving global environmental, social and economic goals. the paris agreement provides for the global decarbonization of the construction sector by 2050, intending to avoid the impact of a 2 °c rise in temperature [4]. from the perspective of circularity, the sector is one of the largest consumers of resources and natural capital, with about 40 % of resource consumption and waste generation [11, 12]. furthermore, the end-of-life destination of buildings is usually demolition and, at best, recycling the waste generated. thus, applying ce principles to buildings allows for considerable benefits to be gained by keeping assets in circulation rather than starting a cycle from new resource extraction. for this reason, the european commission has identified “construction and buildings” as one of the seven main product value chains in its action plan for the circular economy [13]. in this context, circularity metrics for buildings are a topic of great interest, as they allow managers to identify and control the application of ce practices in the different phases of construction projects, and with what intensity [14]. furthermore, they implement a common language among all involved actors, which helps to assess the circularity of buildings and monitor the progress of a project according to standardized indicators [15]. indicators and metrics are also needed to implement strategies to assess the circular potential of the technical options that can be adopted and their efficiency [2]. as circular buildings gain in reputation, the valuation model can be used as a benchmark to compare the performance of buildings based on ce [15]. the lack of indicators and metrics has prevented policymakers and stakeholders from setting verifiable recovery targets for new construction and renovation [16]. by providing a meaningful measure of key elements of circular buildings, metrics form the heart of assessment methodologies, and should objectively assess and measure the building’s development towards ce and help to improve the performance of buildings, from the initial stages to the end of their useful life, closing the materials cycle [15]. currently, the most recognized and globally adopted indicator for the built environment is the material circularity indicator (mci), also known as the theoretical indicator of circularity of the product. the mci tool is part of a broader “circular indicators project” developed by the ellen macarthur foundation and ansys granta [17] and evaluates the input type, output type and the technical useful life of the materials [17]. in verberne’s adaptation [18], mci is the basis for subsequent nested calculations at product (pci), system (sci) and building (bci) levels. pci incorporates product disassembly possibilities and can be referred to as the practical “product circularity indicator”. the system circularity indicator (sci) assesses the circularity of products assembled in a system based on their mass contribution, broken down into the brand’s six building layers [19]. lastly, the building circularity indicator (bci) assesses the set of systems considering a factor for the level of importance each of them has [18]. in the past few years, some bci improvements have been proposed. van vliet [20] proposed to omit the building layers in the pci calculation; alba concepts developed a new bci based on product, element (instead of “system”) and building [21]; and van schaik [22] slightly modified it to apply to building foundations. some buildings circularity assessments combine metrics on reversibility and durability [2], the building circularity indicator (bci), the new predictive bci (predictive bci – pbci) [21] and the circular economy key performance indicators of the construction industry (kpis) to determine to what extent a company implements ce in the different construction projects phases [23]. an expanded list of indicators extracted from the systematic literature review (slr) can be found in table 3 (appendix a). despite this, there is still a lack of clear definitions to link what characterizes the circularity of buildings to circular construction technologies and to adequate indicators to measure the circular economy [2] and circularity indicators for the built environment as a whole [3]. hence, two main research questions emerged to be addressed in this paper: • “how clear is the message regarding environmental assessment of the materials flows involved? which kind of information do the studied metrics convey? do they stand-alone or should be combined to other to communicate environmental performance of buildings and the built environment?”, and • “can they be aggregated to address larger scales?”. for this purpose, selected metrics were applied to two case studies of different material and constructive characteristics. 2. method the methodological approach consisted of • a systematic literature review (rsl) to identify existing metrics, as well as their structure, content, components and where they were applied; and • the application of metrics which stood out in the slr to two case studies; followed by an analysis of the information conveyed and its potential combination to other environmental assessment outcomes. material circularity indicator (mci), product circularity indicator (pci), system circularity indicator (sci) and building circularity indicator (bci) were hence selected for application in this study. two residential buildings were selected as case studies. the first case is a six-story wood-framed building, and the second building is a thirteen-story reinforced concrete structure with masonry walls. for our exploratory purposes, only the structure and envelope components were considered in the calculations. the 657 v. gomes, s. valdivia, l. pulgrossi, m. g. da silva acta polytechnica ctu proceedings figure 1. circularity metrics calculation flow. building lifespan was 50 years, and the materials service life followed the brazilian performance standard nbr 15575:2013 [24], and the waste rate was based on brazilian association for recycling of civil construction waste data. 2.1. circularity indicator calculation method the calculations followed the steps in figure 1, once the products and materials used in each case study, their service life, and mass (kg), virgin input (v) and waste (w) involved were determined. calculations begin by the linear flow index (lfi), the utility factor (x) and the utility factor function, f(x). lfi and f(x) are next used to compute the material circularity indicator (mci). the mci and the determining factor of disassembly (ddf) – that is: the amount of material assembled into systems that can be disassembled – feed calculation of the product circularity indicator (pci). the mci is also used to obtain the theoretical value for the system circularity indicator (sci (t)), by considering the products’ contribution to the complete system, in mass. then, the practical value for the sci (sci (p)) is calculated using the pci and the product mass. finally, the factors accounting for system dependency/level of importance (lk_k) and sci (theoretical and practical) are considered, to calculate the theoretical (bci (t)) and the practical (bci (p)) values of the building circularity indicator. to compute “x”, the buildings were decomposed into six systems (layers). table 1 is used to compute the systems lifetime (lsys) and level of importance, a weighting factor between 0 and 1. pci calculations need the determining factor of disassembly (ddf) nof each product. for that we used durmisevic’s [25] classification into seven variables (functional separation; functional dependence, technical life cycle / coordination; geometry of product edge; standardisation of product edge; type of connections; accessibility to fixings), and assigned values from zero (worst) to 1 (best) impact on disassembly. finally, to comsystem lifetime level of [years] importance site 500 0.1 structure 100 0.2 skin 20 0.7 services 15 0.8 space plan 10 0.9 stuff 5 1.0 table 1. lifetime (lsys) in years and level of importance of each system, based on the brand’s six building layers [19]. pute the bci (t) and bci (p), the level of importance (lk_k) of each system is computed (table 1). 3. results and discussion the linear flow index (lfi) offers immediate insight on the potential circularity challenges – i.e., flows with lfi closer to 0, like asphalt or acrylic waterproofing, extruded polystyrene board from external walls or slabs – and accomplishments, i.e., lfi closer to 1, such as steel from window frames and softwood plywood from external walls. the mci shows how linear a material flow is, also from 0 (linear) to 1 (circular). in figure 1 and figure 2, the more the red shape is open, the closer to theoretical circularity is achieved. the wood-framed building has visibly more circularity questions solved than the masonry building, particularly regarding envelope components (figure 3). still, apart from the wood elements themselves, the building’s structure uses several items that stress its circularity performance. the lowest result was presented by the floor mortar cement (0.95), which had the shortest estimated lifetime (13 years) within this group of systems (100 years). materials with longer service life positively affect the utility factor (x) if it suits the system in which the material is inserted. as pointed out by verberne [18], the ideal 658 vol. 38/2022 measuring circularity from buildings to neighbourhoods figure 2. mci and pci results – concrete-framed & masonry building. figure 3. mci and pci results – wood-framed building. figure 4. mass distribution [t] for values the cases studied. figure 5. mass distribution [t] for values the cases studied. 659 v. gomes, s. valdivia, l. pulgrossi, m. g. da silva acta polytechnica ctu proceedings total wood-framed building concrete-framed & masonry building bci (t) 0.992 0.993 bci (p) 0.461 0.127 disassembly potential loss 0,531 0,866 table 2. bci results for the cases studied. balance between service life and reusable materials is still subject for future research. as a product that is going to be recycled does not, by definition, mean that it remains functional and can be safely broken down, the pci highlights those components with the greatest disassembly potential. the masonry building shows lower indicators, but still with some items that can be disaggregated from the system, mainly envelope components (e.g., stone wool board, xps board and granite). the wood-framed building showed improved results for disassembly, both for structure (e.g., steel or wood beams) and envelope (e.g., aluminium or steel elements, rockwool and prefabricated concrete slab) components. the sci aggregates the mci and pci results, normalizing and weighting the results from the mass, to generate a single value that demonstrates a system’s circularity potential. thus, products with the largest mass contribution influence the sci results, according to their circularity potential determined in the previous steps (mci used in the calculation of sci (t); pci used in the calculation of sci (p)). the distance between the theoretical and the practical values indicate the potential “circularity losses” created due to limited disassembly. even in the wood-framed building, concrete use is substantial and responds for about 62 % of the structure mass, followed by the wood beams (floor) summing up to 30 % (figure 4). whilst the theoretical sci value represents a circularity stand close to ideal (mci ∼1), the practical sci value is cut by almost half due to the influence of the concrete’s pci, which is close to zero, despite the good performance of wood elements (pci ∼1) (figure 5). the mass of the envelope elements is concentrated by the gypsum panel (34 %) and wood beams (14 %). in the concrete-framed and masonry building, concrete holds 81 % and 44 % of the structure and envelope mass, respectively (figure 4). concrete blocks account for 16 % of the envelope’s mass. such products have low pci results and therefore push the practical sci towards linearity (figure 5). as the bci (table 2) factors in the systems’ importance, those with shorter lifetimes – in the case, the envelope products – weight and (negatively) influence the most. though both buildings have similar bci (t), the disassembly potential loss (bci (t) – bci (p)) is higher for the concrete-framed building (0.866) and reflects the circularity opportunities designed out for the wood-framed building. 4. conclusions this study’s outcome made clear that – though very intuitive and crucial for societal development peacemaking with its supporting environment – circularity metrics do not convey complete environmental information. by highlighting the renewability, potential re-application and recoverability of materials, they emphasize the use phase, and do not usually consider, for example, the actual impacts to keep materials and energy in the loop. the environmental impacts / costs to reinject potentially returnable material in the industrial loops and transition to circular economy are not accounted for. increased material and energy circularity might even enlarge life cycle impacts until the full circular economy status is reached. so, it is herein proposed that such metrics are paired with environmental performance profiles produced by e.g., life cycle assessments. as to the second research question posed, the concept of “nested indicators” seems to also be smoothly applicable to larger – e.g., neighbourhood and city – scales. a “relevance” weighting criterion is needed and is herein proposed to refer to the life cycle assessment concept of functional equivalency. procedures for applying lca to neighbourhoods have already been investigated in a few studies, including by some of these authors, and can provide the basis for such extended examination. acknowledgements the authors thank the national council of scientific and technological 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[20] m. m. van vliet. disassembling the steps towards building circularity. redeveloping the building disassembly assessment method in the building circularity indicator. master thesis, eindhoven university of technology, 2018. https://pure.tue.nl/ws/portalfiles/portal/ 122509202/vliet_0946226_thesis.pdf [21] d. cottafava, m. ritzen. circularity indicator for residential buildings: addressing the gap between embodied impacts and design aspects. resources, conservation and recycling 164:105120, 2021. https://doi.org/10.1016/j.resconrec.2020.105120 [22] c. w. van schaik. circular building foundations: a structural exploration of the possibilities for making building foundations contribute to a circular economy. master thesis, delft university of technology, 2019. https://repository.tudelft.nl/islandora/object/ uuid:70bad27f-d276-482c-9d54-2f19e4aab7c6 [23] p. núñez-cacho utrilla, j. górecki, j. m. maqueira. simulation-based management of construction companies under the circular economy concept – case study. buildings 10(5):94, 2020. https://doi.org/10.3390/buildings10050094 [24] abnt. nbr 15575-1 – edificações habitacionais – desempenho parte 1: requisitos gerais prefácio. abnt, rio de janeiro, 2013. isbn 978-85-07-04036-1. [25] e. durmisevic. transformable building structures: design for dissassembly as a way to introduce sustainable engineering to building design & construction. doctoral thesis, delft university of technology, 2006. isbn 978-90-9020341-6. https://repository.tudelft.nl/islandora/object/ uuid%3a9d2406e5-0cce-4788-8ee0-c19cbf38ea9a 661 https://docs.wbcsd.org/2020/11/wbcsd_circular_economy_action_plan_2020%e2%80%93summary_for_business.pdf https://docs.wbcsd.org/2020/11/wbcsd_circular_economy_action_plan_2020%e2%80%93summary_for_business.pdf https://docs.wbcsd.org/2020/11/wbcsd_circular_economy_action_plan_2020%e2%80%93summary_for_business.pdf https://doi.org/10.1016/j.resconrec.2017.09.005 https://doi.org/10.3390/su12020618 https://doi.org/10.1080/00207543.2018.1524166 https://doi.org/10.3390/su8010043 https://doi.org/10.1016/j.ijpe.2016.06.008 http://www.jstor.org/stable/24873561 https://doi.org/10.1111/jiec.12586 https://doi.org/10.1080/014461900446867 https://doi.org/10.2779/05068 https://doi.org/10.1016/j.jclepro.2016.10.038 https://doi.org/10.1088/1755-1315/225/1/012058 https://doi.org/10.5334/bc.58 http://www.ellenmacarthurfoundation.org/circularity-indicators/ http://www.ellenmacarthurfoundation.org/circularity-indicators/ https://pure.tue.nl/ws/files/46934924/846733-1.pdf https://pure.tue.nl/ws/files/46934924/846733-1.pdf https://pure.tue.nl/ws/portalfiles/portal/122509202/vliet_0946226_thesis.pdf https://pure.tue.nl/ws/portalfiles/portal/122509202/vliet_0946226_thesis.pdf https://doi.org/10.1016/j.resconrec.2020.105120 https://repository.tudelft.nl/islandora/object/uuid:70bad27f-d276-482c-9d54-2f19e4aab7c6 https://repository.tudelft.nl/islandora/object/uuid:70bad27f-d276-482c-9d54-2f19e4aab7c6 https://doi.org/10.3390/buildings10050094 https://repository.tudelft.nl/islandora/object/uuid%3a9d2406e5-0cce-4788-8ee0-c19cbf38ea9a https://repository.tudelft.nl/islandora/object/uuid%3a9d2406e5-0cce-4788-8ee0-c19cbf38ea9a v. gomes, s. valdivia, l. pulgrossi, m. g. da silva acta polytechnica ctu proceedings a. appendix – table 3 id en ti fie r a ut ho rs t it le y ea r in di ca do r d es cr ip ti on c as e st ud y 1 ve rb er ne , j. j. h . bu ild in g ci rc ul ar ity in di ca to rs an ap pr oa ch fo r m ea su rin g ci rc ul ar ity of a bu ild in g 20 16 m at er ia lc irc ul ar ity in di ca to r (m c i) t he in di ca to r ev al ua te s in pu t ty pe ,o ut pu t ty pe an d te ch ni ca lu se fu ll ife of m at er ia ls. c ur re nt ly, th e m os t re co gn iz ed an d ad op te d in di ca to r gl ob al ly . pr od uc t c irc ul ar ity in di ca to r (p c i) t he in di ca to r in co rp or at es pr od uc t di sa ss em bl y po ss ib ili tie s an d ca n be re fe rr ed to as th e pr ac tic al in di ca to r of pr od uc t ci rc ul ar ity . sy st em c irc ul ar ity in di ca to r (s c i) th e in di ca to r ev al ua te s th e cir cu la rit y of pr od uc ts in a sy st em ba se d on th ei r we ig ht an d m ak in g a se pa ra tio n ba se d on th e six la ye rs of th e sy st em . bu ild in g ci rc ul ar ity in di ca to r (b c i) t he bc ie va lu at io n m od el is co m po se d of th e m at er ia lc irc ul ar ity in di ca to r (m c i) ,p ro du ct c irc ul ar ity in di ca to r (p c i) ,s ys te m c irc ul ar ity in di ca to r( sc i) an d, fin al ly ,t he c on st ru ct io n c irc ul ar ity in di ca to r (b c i) .b c ie va lu at es th e se pa ra te sy st em s as a w ho le w ith a fa ct or of im po rt an ce le ve lo fe ac h sy st em . tw o fic tit io us bu ild in gs (c irc ul ar an d no t ci rc ul ar ). 662 vol. 38/2022 measuring circularity from buildings to neighbourhoods id en ti fie r a ut ho rs t it le y ea r in di ca do r d es cr ip ti on c as e st ud y 2 g er ar d fi nc h, g uy m ar ria ge , a nt on y pe lo si, m or te n g je rd e bu ild in g en ve lo pe sy st em s fo r th e ci rc ul ar ec on om y; ev al ua tio n pa ra m et er s, cu rr en t pe rfo rm an ce an d ke y ch al le ng es 20 21 bu ild in g c irc ul ar ity in di ca to r (b c i) c irc ul ar ec on om y in di ca to r (c ei p) c irc ul ar ec on om y in de x (c ei ) c irc ul ar ity pe rfo rm an ce in di ca to r (c pi ) ec oeffi ci en t va lu e r at io (e v r ) en dof -l ife r ec yc lin g r at es (e ol -r r s) m at er ia lc irc ul ar ity in di ca to r (m c i) pr od uc tle ve lc irc ul ar ity m et ric (p c m ) r eu se po te nt ia li nd ic at or (r pi ) r es ou rc e d ur at io n in di ca to r (r d i) r ec yc lin g in di ce s (r is ) n ew ze al an d c on st ru ct io n. 663 v. gomes, s. valdivia, l. pulgrossi, m. g. da silva acta polytechnica ctu proceedings id en ti fie r a ut ho rs t it le y ea r in di ca do r d es cr ip ti on c as e st ud y 3 fe lix h ei se l, sa bi ne r au o be rh ub e c al cu la tio n an d ev al ua tio n of ci rc ul ar ity in di ca to rs fo r th e bu ilt en vi ro nm en t us in g th e ca se st ud ie s of u m a r an d m ad as te r 20 20 m at er ia lc irc ul ar ity in di ca to rs (m c i) d es cr ib ed ab ov e. m ad as te r c irc ul ar ity in di ca to r t he in di ca to r re gi st er s st oc ks an d m at er ia lfl ow s w ith in a ci rc ul ar bu ilt en vi ro nm en t. le ve l(s ) c irc ul ar ity in di ca to r (c i) t he in di ca to r ev al ua te s ea ch bu ild in g’ s ci rc ul ar ity le ve lb et we en 0 an d 10 0 pe rc en t ba se d on us er s. c ic on st ru ct io n t he in di ca to r re pr es en ts th e ra tio of vi rg in m at er ia ls to re cy cle d, re us ed or ra pi dl y re ne wa bl e m at er ia ls. it is ba la nc ed by effi ci en cy in di ca to rs of th e pr oc es s of re cy cl in g pr io r to th e co ns tr uc tio n ph as e an d th e m as so fw as te ge ne ra te d du rin g th e re cy cli ng pr oc es s. c iu se t he in di ca to r re pr es en ts th e lif e ex pe ct an cy of th e pr od uc ts us ed ,c om pa re d to th e av er ag e lif e of pr od uc ts in st at us qu o. t he ac tu al sc or e is de te rm in ed by ca lc ul at in g th e we ig ht ed av er ag e of al lp ro du ct s of th e va rio us la ye rs of th e sy st em . c ie nd of lif e t he in di ca to r re pr es en ts th e re la tio ns hi p be tw ee n wa st e an d re us ab le an d/ or re cy cl ab le m at er ia ls ge ne ra te d w he n a bu ild in g is re no va te d or de m ol ish ed . u rb an m in in g u ni t an d r ec yc lin g (u m a r ) de sig ne d an d bu ilt by w er ne r so be k w ith d irk e. h eb el an d fe lix h ei se la t n es t (n ex t ev ol ut io n of su st ai na bl e te ch no lo gi es ) of em pa d üb en do rf (f ed er al la bo ra to rie s sw iss m at er ia ls sc ie nc e te ch no lo gy ) in sw itz er la nd . 4 k im be rle e m ar ce llu sza m or a, pa tr ic ia g al la gh er , sa br in a sp at ar i c an pu bl ic c on st ru ct io n an d d em ol iti on d at a d es cr ib e tr en ds in bu ild in g m at er ia l r ec yc lin g? o bs er va tio ns fr om ph ila de lp hi a 20 20 m at er ia lf ra ct io n (m f) in di vi du al fra ct io ns of m at er ia lw as te . m at er ia ld iv er te d (m d ) o ve ra ll de vi at io n ra te of to ta lm at er ia ls. m un ic ip al wa st e m an ag em en t in di ffe re nt ec on om ie s, in cl ud in g th e pa ci fic is la nd s an d a sia n c iti es . 664 vol. 38/2022 measuring circularity from buildings to neighbourhoods id en ti fie r a ut ho rs t it le y ea r in di ca do r d es cr ip ti on c as e st ud y 5 d ar io c ot ta fa va , m ich ie l r itz en c irc ul ar ity in di ca to r fo r re sid en tia l bu ild in gs : a dd re ss in g th e ga p be tw ee n em bo di ed im pa ct s an d de sig n as pe ct s 20 21 m at er ia lc irc ul ar ity in di ca to r (m c i) d es cr ib ed ab ov e. c irc ul ar ity in di ca to rs (c i) sy st em c irc ul ar ity in di ca to r (s c i) d es cr ib ed ab ov e en vi ro nm en ta l pr od uc t pe rfo rm an ce in di ca to rs (e pi ) t he y ai m to in di ca te th e m ac ro ,m es o or m ic ro ch ar ac te ris tic s of a pr od uc t. in di ca do r de po te nc ia ld e r ec ur so s (r pi ) t he in de x m ea su re s th e in tr in sic va lu e of re us in g a m at er ia l, ta ki ng in to ac co un t st at e of th e ar t re cy cl in g te ch no lo gi es . pr od uc t c irc ul ar ity in di ca to r (p c i) li ne ar fl ow in de x (l fi ) in di ca do r de lo ng ev id ad e (l i) t he in de x in di ca te s th e to ta lt im e th e m at er ia li s he ld in a pr od uc t/ se rv ic e sy st em . bu ild in g ci rc ul ar ity in di ca to r (b c i) d es cr ib ed ab ov e. n ew pr ed ic tiv e bc i( pb c i) ba se d on th re e le ve ls, na m el y a pr od uc t c irc ul ar ity in de x (p c i) ,a n el em en t c irc ul ar ity in de x (e c i) an d a bu ild in g c irc ul ar ity in de x (b c i) . a na ly ze d th e co m po ne nt of bu ild in gs in si ng ap or e. st ud y of ca se in 8 pl ac es . 6 ca th er in e d e w ol f, en dr it h ox ha , c or en tin fi ve t c om pa ris on of en vi ro nm en ta l as se ss m en t m et ho ds w he n re us in g bu ild in g co m po ne nt s: a ca se st ud y 20 20 c ut -o ff m et ho d t he in de x is us ed to ev al ua te th e re cy cl in g of co ns tr uc tio n pr od uc ts . r ec yc lin g ra te t he in de x en co ur ag es re cy cl in g at th e en dof -li fe st ag e. a bi lit y to be di sm an tle d or re m ou nt ed t he in de x m ea su re s th e re pa ir an d tr an sfo rm at io n re qu ire d du rin g as se m bl y an d di sa ss em bl y, re us e ca n on ly ha pp en if co m po ne nt s ca n be di sa ss em bl ed an d re as se m bl ed . en dof -l ife (e ol ) c om m er ci al bu ild in g w ith re us ed co m po ne nt s. k op fb au h al le bu ild in g 11 8, w hi ch is de sig ne d w ith el em en ts re co ve re d fro m de m ol iti on sit es . 665 v. gomes, s. valdivia, l. pulgrossi, m. g. da silva acta polytechnica ctu proceedings id en ti fie r a ut ho rs t it le y ea r in di ca do r d es cr ip ti on c as e st ud y 7 m uh am m ad bi la l, k hu rr am iq ba la hm ad k ha n, m uh am m ad ja m al ud di n t ha he em , a bd ur r eh m an n as ir c ur re nt st at e an d ba rr ie rs to th e ci rc ul ar ec on om y in th e bu ild in g se ct or : to wa rd s a m iti ga tio n fra m ew or k 20 20 c om pr eh en si ve ut ili za ti on ra te of in du st ri al so lid w as te r ec yc lin g ra te of re cl ai m ed w as te w at er to ta la m ou nt of so 2 em is si on s to ta la m ou nt of c o d em is si on s r at e of w as te em is si on s to ta la m ou nt of w as te w at er di sc ha rg e w at er co ns um pt io n pe r un it pr od uc t in ke y in du st ri al se ct or s p as si ng ra te of us ed m at er ia ls ba ck in to th e su pp ly ch ai n c om pr eh en si ve di sp os al ra te of da ng er ou s w as te r eu si ng ra te of pr od uc ts /m at er ia ls fr es hw at er co ns um pt io n e ne rg ysa vi ng am ou nt r at e of ca rb on fo ot pr in t av ai la bi lit y of co m pl et e bi ll of m at er ia ls an d su bs ta nc es fo r th e pr od uc t p er ce nt ag e co ns um pt io n of re ne w ab le or cl ea n en er gy o ut pu t of m ai n m in er al re so ur ce e ne rg y co ns um pt io n to ta la m ou nt of in du st ri al so lid w as te di sp os al r ec yc lin g ra te of in du st ri al so lid w as te r el at iv e im po rt an ce in de x (r ii ) e va lu at io n in de x sy st em fo r th e as se ss m en t of c e at th e re gi on al le ve l t he in de x co nt ai ns 16 in di ca to rs cl as si fie d in to 4 gr ou ps . it in cl ud ed in di ca to rs of e c “r ed uc e” an d “r ec yc le ” pr in ci pl es an d no t th e “r eu se ” pr in ci pl e. 666 vol. 38/2022 measuring circularity from buildings to neighbourhoods id en ti fie r a ut ho rs t it le y ea r in di ca do r d es cr ip ti on c as e st ud y 8 c ar m en d ía zló pe z, m an ue l c ar pi o, m ar ía m ar tín m or al es , m on ts er ra t za m or an o d efi ni ng st ra te gi es to ad op t le ve l(s ) fo r br in gi ng bu ild in gs in to th e cir cu la re co no m y. a ca se st ud y of sp ai n 20 21 lo ca lp rio rit y in de x (p j) t he in de x al lo w s kn ow in g an d qu an tif yi ng th e gr ea te r or le ss er we ig ht th at ex pe rt s ha ve gi ve n to re le va nt fa ct or s. c on sis te nc y in de x of th e m at rix (c i) , r an do m c on sis te nc y in de x of th e m at rix (r c i) to ta lp rio rit y in di ce s fr om th e lo ca lp rio rit y in de xe s an d th e de te rm in ed we ig ht in g co effi ci en ts ,t he ov er al lp rio rit y in de x is ca lc ul at ed fo r ea ch of th e re le va nt fa ct or s. le ve l(s ) it is a co m m on eu ro pe an u ni on fra m ew or k of es se nt ia ls us ta in ab ili ty in di ca to rs to m ea su re th e pe rfo rm an ce of bu ild in gs th ro ug ho ut th ei r lif e cy cl e, en ab lin g re du ct io n of em iss io ns an d ci rc ul ar re so ur ce flo w s. t he to ol ai m s to un ite th e en tir e va lu e ch ai n of th e se ct or ar ou nd a co m m on eu ro pe an la ng ua ge to im pr ov e th e pe rfo rm an ce of a bu ild in g. t he te rr ito ry of sp ai n wa s se le ct ed fo r th is st ud y. 667 v. gomes, s. valdivia, l. pulgrossi, m. g. da silva acta polytechnica ctu proceedings id en ti fie r a ut ho rs t it le y ea r in di ca do r d es cr ip ti on c as e st ud y 9 m at an m ay er , m ar tin be ch th ol d 9. d ev el op m en t of po lic y m et ric s fo r ci rc ul ar ity as se ss m en t in bu ild in g as se m bl ie s 20 18 m at er ia l r ec ov er y p ot en ti al in d ex (m r p i) t h e sy st em as se ss es th e re co ve ry p ot en ti al at b ot h m at er ia l an d as se m b ly le ve ls th ro u gh a se ri es of ca te go ri es an d su b ca te go ri es th at is m ai n ly b as ed on q u an ti ta ti ve m at er ia l d at a. u n fa st en in g e ff or t in d ex (u f i) t h e in d ex ev al u at es th e eff or t of re le as in g w id el y u se d fa st en er s. e ff or t is d efi n ed as th e le ve l of co m p le x it y in vo lv ed in ac ce ss in g, u n lo ck in g, re lo ca ti n g an d re m ov in g fa st en er s. t im e (i n se co n d s) an d co st (i n € ) ar e u se d as d is as se m b ly p ot en ti al in d ic at or s t h e in d ex es ti m at es th e p ot en ti al en d -o fli fe d is as se m b ly p ro d u ct d u ri n g ea rl y d es ig n p h as es . t h e d is as se m b ly p ot en ti al is ca lc u la te d fr om a ti m e p er sp ec ti ve an d co st , u si n g li n k -o ri en te d se q u en ce an al y si s. d is as se m b ly e ff or t in d ex (d e i) t h e in d ex q u an ti fi es en d of li fe eff or t an d d is as se m b ly co st . d is as se m b ly eff or t an d co st is m ea su re d th ro u gh ti m e, to ol s, fa st en in g, ac ce ss , in st ru ct io n , d an ge r an d fo rc e co n si d er ed . d is cr et e in d ex sc or es t h e in d ex co n si d er s th re e m ai n li fe st ag es : d is p os al , d is as se m b ly an d re co ve ry , w h ic h in cl u d es re cy cl in g or re m an u fa ct u ri n g. c on n ec ti on in d ex t h e in d ex ta ke s in to ac co u n t se p ar at io n d am ag e, ty p e of to ol re q u ir ed fo r d is as se m b ly an d d is as se m b ly ti m e. a cc es s in d ex t h e in d ex co n si d er s th e co rr el at io n b et w ee n d is as se m b ly se q u en ce an d p ro d u ct li fe ex p ec ta n cy , as w el l as th e p re se n ce of la ye rs th at li m it ac ce ss to ot h er la ye rs w it h sh or te r li fe ex p ec ta n cy . r ec y cl ab il it y in d ex t h e in d ex in cl u d es n ot on ly th e d ev al u at io n of th e v ir gi n to re cy cl ed co n te n t m ar ke t b u t al so th e re la ti on sh ip b et w ee n th e le ve ls of ca rb on d io x id e em it te d d u ri n g p ro d u ct io n an d re co ve ry . d ev al u at io n ra te of p ro d u ct s t h e av ai la b il it y an d eff ec ti ve n es s of re ve rs e su p p ly ch ai n s an d re cy cl in g te ch n ol og ie s ca n b e ga u ge d b y th e ra te of d ev al u at io n of p ro d u ct s. s u rf ac e tr ea tm en t in d ex s u rf ac e tr ea tm en ts ar e ty p ic al ly ap p li ed to b u il d in g co m p on en ts in or d er to m ee t w ea th er or fi re p ro te ct io n re q u ir em en ts . t h os e co at in gs of te n ad h er e to th e co m p on en t u si n g ch em ic al ag en ts th at cr ea te an ir re ve rs ib le b on d , th u s aff ec ti n g th e m at er ia l re co ve ry p ot en ti al of th e co m p on en t. b io d eg ra d ab il it y in d ex b y d efi n it io n , b io d eg ra d ab il it y is th e p ot en ti al fo r m at er ia ls to b e ch em ic al ly co n su m ed b y b ac te ri a or ot h er b io lo gi ca l ag en ts . t h e ab il it y of tr ea tm en ts to su rf ac e to b io d eg ra d e co n tr ib u te s to it s re co ve ry p ot en ti al . b in d er s in d ex t h e in d ex co n si st s of a b io d eg ra d ab il it y sc or e an d a p ri m ar y c o 2 p ro d u ct io n . a ss em b ly le ve l in d ex t h e m on ta ge as se ss m en t is in h er en tl y m or e q u al it at iv e as it fo cu se s p ri m ar il y on sp at ia l d es ig n as p ec ts . c on n ec ti on ty p e in d ex c on n ec ti on s ar e ty p ic al ly m ea su re d b y co m p ar in g tw o va ri ab le s: re ve rs ib il it y an d d is as se m b ly ti m e. s ep ar at io n d am ag e in d ex d am ag e sc al e w it h 5 va lu es . d is as se m b ly ti m e in d ex e st im at in g d ec on st ru ct io n ti m e is n ot as eff ec ti ve as es ti m at in g p ro d u ct d is as se m b ly ti m e. it is b as ed on fi el d ob se rv at io n s, w it h m or e ex p ec te d re fi n em en ts as m or e in fo rm at io n is co ll ec te d d u ri n g fu tu re p h as es of th e p ro je ct . t o ol ty p e in d ex u se d to co n cr et el y re fl ec t th e ra th er ab st ra ct n ot io n of th e le ve l of eff or t. s in ce eff or t is es se n ti al ly a su b je ct iv e co n d it io n , u se rs ca n fi n d ch al le n ge s in d et er m in in g w h at co n st it u te s a h ig h , m ed iu m , or lo w le ve l of eff or t. c om p on en t in te gr at io n in d ex t h e in d ex li n k s th e d eg re e of p re -a ss em b ly in a gi ve n as se m b ly to it s en d -o fli fe m at er ia l re co ve ry p ot en ti al . it sh ou ld b e n ot ed th at th e in te gr at io n in d ex d o es n ot fo cu se s on p ar t co u n t as an in fl u en ci n g fa ct or in m at er ia l re co ve ry p ot en ti al , b u t ra th er on th e re la ti on sh ip b et w ee n on -s it e an d off -s it e as se m b ly . e n d -o fli fe re co ve ry p ot en ti al e n d -o fli fe in d ex a pp lic at io n in wa ll m ou nt s re sid en tia l ou td oo r. 668 vol. 38/2022 measuring circularity from buildings to neighbourhoods id en ti fie r a ut ho rs t it le y ea r in di ca do r d es cr ip ti on c as e st ud y 10 li nd a br aa km an , si lu bh oc hh ib ho ya ,r ob in de g ra af ex pl or in g th e re la tio ns hi p be tw ee n th e le ve lo f ci rc ul ar ity an d th e lif ec yc le co st s of a on efa m ily ho us e 20 21 le ve lo fc irc ul ar ity (l oc ) m at er ia lc irc ul ar ity in di ca to r (m c i) d es cr ib ed ab ov e. d iss as se m bl y d et er m in in g fa ct or s (d ff ) t he y id en tif y po ss ib ili tie s fo r m at er ia l-i nd ep en de nt di sa ss em bl y in pr od uc t de sig n, fo cu sin g on th e in te gr at io n of fu nc tio ns an d co nn ec tio n ty pe s. pr od uc t c irc ul ar ity in di ca to r (p c i) d es cr ib ed ab ov e. sy st em c irc ul ar ity in di ca to r (s c i) d es cr ib ed ab ov e. bu ild in g ci rc ul ar ity in di ca to r (b c i) d es cr ib ed ab ov e. si ng le -fa m ily ho us e in th e n et he rla nd s. 11 sh iy am in i r at na sa ba pat hy ,a li a la sh wa l an d sr in at h pe re ra in ve st ig at io n of wa st e di ve rs io n ra te s in th e co ns tr uc tio n an d de m ol iti on se ct or in a us tr al ia 20 20 w as te d iv er sio n r at e (w d r ) t he in de x ev al ua te s th e lif e cy cl e wa st e ou tp ut un de r co ns id er at io n fo r wa st e m an ag em en t be nc hm ar ki ng . r es id ua ld ev ia tio n ra te (w d r ) is th e pe rc en ta ge of re sid ua ls di ve rt ed fro m th e la nd fil l th ro ug h di ffe re nt in te rv en tio ns su ch as re us e, re cy cl in g, re pa ir, tr ea tm en t an d en er gy re co ve ry . w as te ge ne ra tio n ra te (w g r ) u nd er st an di ng th e vo lu m e, ty pe an d co m po sit io n of c & d w he lp s wa st e m an ag em en t pr of es sio na ls m ak e in fo rm ed de ci sio ns ab ou t su st ai na bi lit y de ve lo pm en t th ro ug h wa st e m in im iz at io n, w hi ch ha s be en w id el y us ed as a k pi in be nc hm ar ki ng pe rfo rm an ce m an ag em en t wa st e. r ec yc lin g r at e ze ro w as te in de x (z w i) t he y ar e us ed to as se ss th e eff ec tiv en es s of wa st e m an ag em en t pe rfo rm an ce an d he lp m ak e in fo rm ed de ci sio ns ab ou t de ve lo pi ng st ra te gi es to im pr ov e wa st e m an ag em en t pr ac tic es . 12 re sid en tia l pr oj ec ts in th e n ew so ut h w al es an d v ic to ria st at es (a us tr al ia ). 669 v. gomes, s. valdivia, l. pulgrossi, m. g. da silva acta polytechnica ctu proceedings id en ti fie r a ut ho rs t it le y ea r in di ca do r d es cr ip ti on c as e st ud y 12 l. br ag an ça , r .m at eu s o bs ta cl es an d ba rr ie rs fo r m ea su rin g bu ild in g’ s ci rc ul ar ity 20 19 m at er ia lc irc ul ar ity in di ca to r (m c i) d es cr ib ed ab ov e. 13 c he ng ka ng g ao a, c he ng bo g ao ,k ai hu i so ng ,k ej in g fa ng pa th wa ys to wa rd s re gi on al ci rc ul ar ec on om y ev al ua te d us in g m at er ia lfl ow an al ys is an d sy st em dy na m ic s 20 20 d ire ct m at er ia li np ut (d m i) to ta lm at er ia li np ut (t m i) h id de n fl ow (h f) d ire ct m at er ia lo ut pu t (d m o ) to ta lm at er ia lo ut pu t (t m o ) g ua ng do ng pr ov in ce . 14 er ne st o a nt on in i, a nd re a bo er i, m as sim o la ur ia an d fr an ce sc a g ig lio r ev er sib ili ty an d d ur ab ili ty as po te nt ia li nd ic at or s fo r c irc ul ar bu ild in g te ch no lo gi es 20 20 r ev er sib ili ty r ev er sib ili ty is th e pr op er ty of a pr oc es s, sy st em or de vi ce to re tu rn it to its or ig in al st at e th ro ug h th e tr an sfo rm at io n or di sa ss em bl y of a bu ild in g or pa rt s of it, pr es er vi ng th e m ax im um in te gr ity of re m ov ed ele m en ts an d en su rin g m in im al da m ag e to th os e ke pt in pl ac e. r ev er sib ili ty is re la te d to tim e an d im pl ie s tr an sie nc e. d ur ab ili ty it is th e ow ne rs hi p of a pr oc es s, sy st em or de vi ce to m ai nt ai n its ab ili ty to pr ov id e th e re so ur ce st ha ti ti s de sig ne d fo r ov er tim e. c lo se ly re la te d to th e tim e, du ra bi lit y in di ca te s pe rm an en ce . lo nd on d es ig n fe st iv al 20 16 (u ni te d k in gd om ). 670 vol. 38/2022 measuring circularity from buildings to neighbourhoods id en ti fie r a ut ho rs t it le y ea r in di ca do r d es cr ip ti on c as e st ud y 15 pe dr o n úñ ez c ac ho u tr ill a, ja ro sł aw g ór ec ki , ju an m an ue l m aq ue ir. si m ul at io nba se d m an ag em en t of c on st ru ct io n c om pa ni es un de r th e c irc ul ar ec on om y c on ce pt – c as e st ud y 20 20 c irc ul ar ec on om y k ey pe rfo rm an ce in di ca to rs (k pi s) th ey al lo w m an ag er st o id en tif y an d co nt ro lw he th er a co ns tr uc tio n co m pa ny is ap pl yi ng c irc ul ar ec on om y pr ac tic es in th e di ffe re nt ph as es of th e wo rk s an d w ith w ha t in te ns ity . t he fin al c e in de x c e in de x (c ei ) t he in de x m ea su re s a de gr ee of pr op en sit y of th e co ns tr uc tio n co m pa ny to im pl em en t th e c e (c al le d sc al e c e) .s ho w s th e m os t lik el y va lu es (s co re ) fo r ea ch in di ca to r, gr ou p of in di ca to rs an d th e in de x so th at th es e va lu es ca n be us ed as cu t off va lu es in th e gr ap hi ca lr ep re se nt at io n in pa ne ls (s ca le c e) . g en er al c on tr ac to r (g c ) c on st ru ct io n. t ab le 3. c irc ul ar ity in di ca to rs an d m et ric s fo r bu ild in gs ,e xt ra ct ed fr om th e sy st em at ic lit er at ur e re vi ew (s lr ). 671 acta polytechnica ctu proceedings 38:656–671, 2022 1 introduction 2 method 2.1 circularity indicator calculation method 3 results and discussion 4 conclusions acknowledgements references a appendix – table 3 acta polytechnica ctu proceedings https://doi.org/10.14311/app.2023.42.0051 acta polytechnica ctu proceedings 42:51–54, 2023 © 2023 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague response of the ultra high performance concrete under dynamic compressive loading nela krčmářováa,∗, jan faltaa, tomáš fílaa, david čítekb a czech technical university in prague, faculty of transportation sciences, department of mechanics and materials, konviktská 20, 110 00 prague 1, czech republic b czech technical university in prague, klokner institute, department of experimental and measurement methods, šolínova 7, 166 08 praha 6, czech republic ∗ corresponding author: krcmarova@fd.cvut.cz abstract. ultra high performance concrete is a modern cementitious material which exhibits excellent mechanical properties such as damage tolerance, fracture toughness and durability. these features make this materials suitable for wide range of applications where is the material subjected to different modes of loading and different loading rates. this paper deals with measurement of the ultra high performance concrete reinforced with steel fibres in quasi-static compression mode of deformation and two elevated strain rates using split hopkinson pressure bar. the results of the measurement show high increase of the mechanical properties with elevated strain rate. keywords: ultra high performance concrete, split hopkinson pressure bar, scanning electron microscopy, quasi-brittle material, dynamic testing. 1. introduction ultra high performance concrete (uhpc) is a cementitous composite material which can be used in wide range of applications, mostly in civil engineering. thanks to excellent mechanical properties such as high damage tolerance, fracture toughness and durability, which uhpc exhibits, it can be used in special applications. these applications include for example marine structures, underground spaces, nuclear waste containers, protection of critical infrastructure, mobile anti-vehicle barrier, and national defence and military facilities [1–3]. uhpc is a fine-grained composite material made of high amount of portland cement, supplementary cementitious materials, reactive powders, limestone or quartz flour, fine sand, high-range water reducers,and small amount of water. the uhpc was first introduced in the mid-1990s by de larrard and sedran [4] as well as richard and cheyrezy [5, 6]. after that uhpc has seen widespread implementation throughout the world. by including fiber reinforcement, post-cracking retention and a level of ductility can be achieved that cannot be observed in conventional concrete [7, 8]. the material is in this special applications frequently subjected to dynamic loading. therefore, it is necessary to test this material under wide range of strain rates from quasi-static to high velocity impacts and different loading modes. this paper deals with testing of uhpc in quasi-static compression and dynamic compression in two strain rates. 2. material and methods 2.1. ultra high performance concrete samples the samples used for testing were prepared from uhpc reinforced by steel fibres (uhpfrc). mixture of the uhpfrc was developed in klokner institute and modified in terms of intended use for explosive tests. all dry components were premixed and placed into bags. this preparation made it possible to speed up the whole mixing process. main characteristics of the mixture are very low water to cement ratio, self compacting character and very high material properties such a compressive strength, flexural strength and also durability. mixture was reinforced by scattered reinforcement – steel fibres in volume of 1.5 %. detailed mixture parameters are shown in table 1. material uhpfrc[ kg m−3] cement 52.5 700 aggregate 0-2 mm 867 microfillers (microsilica + slag) 180 water 175 superplasticizer 40 steel fibres 0.2/13 mm 120 table 1. uhpfrc mixture the samples were drilled out from big block made of the uhpfrc resulting in the shape of cylinders with diameter approximately 20 mm and height 20 mm, see figure 1. samples were prepared from core (middle) of the uhpc block and from the shell (edge) of the block. 51 https://doi.org/10.14311/app.2023.42.0051 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en n. krčmářová, j. falta, t. fíla, d. čítek acta polytechnica ctu proceedings (a). dimensions of the uhpc samples. (b). uhpc specimen. figure 1. uhpc specimen. figure 2. sem image of the uhpc matrix and fibre reinforcement. 2.2. scanning electron microscopy in order to get information about the microstructure and bond between the fibre and matrix, scanning electron microscopy (sem) was performed using jeol jsm-it200 (jeol, japan). in figure 2 is the sem image of the bond between uhpc matrix and steel fibre. the image was obtained using secondary electron detector with magnification of 1000×. 2.3. quasi-static experiments measurements in quasi-static compression mode were performed using instron 3382 (instron, usa) with maximal loading capacity of 100 kn. displacement of the cross-head and applied force were captured at a 50 hz frequency. velocity of the crossbar was set to 1 mm min−1. figure 3 shows the setup for the quasi-static experiments. 2.4. dynamic experiments split hopkinson pressure bar in traditional configuration was used for dynamic testing of the samples, see figure 4. the length of the incident bar was 1600 mm and the length of transmission bar was 2100 mm. the length of the striker bar was 750 mm and the impact velocities were 26 m s−1 and 37 m s−1, respectively. the corresponding gas-gun pressure used for lower figure 3. setup for the quasi-static measurement. strain rate was 4 bar and for higher strain rate was 8 bar, respectively. all of the used bars had diameter of 20 mm and were manufactured from high-strength aluminium alloy (en-aw-7075). incident bar was instrumented with four foil strain gauges (3/120 ly61, hbm, mainz, germany), one pair in the middle of the bar and one pair near the interface between the incident bar and the specimen. one pair of the strain-gauges was mounted at the transmission bar near the interface specimentransmission bar. all of the strain gauges were connected in the wheatstone bridge to capture all deformations appearing during the impact. a pair of high-speed cameras (fastcam sa-z, photron, japan) were used for optical imaging. one high-speed camera was used for the recording of the deformation behaviour of the specimens and the second camera for velocity of the bars measurement. both cameras recorded the scene with frame rate of 300 kfps with the corresponding resolution of the images 256 × 128 pixels. figure 4 shows that in the setup is a mirror. the reason is to protect the high speed camera from the x-ray beam because the setup is designed to be used also for x-ray imaging. the strain pulse was shaped using pulse shaper made from copper in the shape of cylinder with diameter 6 mm and height 1.5 mm to mechanically filter high frequency oscillations caused by wave dispersion effects. actual velocity of the impact at the boundary of the specimen was lower than the striker velocities 26 m s−1 and 37 m s−1, respectively because of the early failure of the specimen during ramp-in period of the strain pulse. actual impact velocities during compression of the specimen were thus lower and corresponded to approximately 5 m s−1 and 10 m s−1, respectively (see figure 5). 52 vol. 42/2023 response of the uhpc under dynamic compressive loading 2500 mm high-speed camera incident bar momentum trap gas gun 2100 mm direction of movement specimenoptical gates momentum trap, dampertransmission bar 800 mm 1400 mm sg1 sg2 300 mm sg3 1600 mm mirrorpulse shaper striker high-speed camera figure 4. results from the quasi-static experiments. (a). typical force and velocity histories during low velocity measurement. (b). typical force and velocity histories during high velocity measurement. figure 5. typical recorded forces and velocities for both strain rates during shpb experiments. (a). results from the quasi-static experiments. (b). results from the low velocity experiments. (c). results from the high velocity experiments. figure 6. results for each deformation mode. 3. results the samples made from uhpfrc were tested in quasi-static compression mode of deformation and in dynamic compression mode of deformation. for each deformation condition were performed 5 measurements. the figure 6 are depicted stress-strain diagrams for quasi-static mode (6a), for low velocity mode (6b) and high velocity mode (6c). from results shown in figure 6 is obvious that there is no significant difference between deformation behaviour in compression of samples from the core and from the shell of the uhpfrc block. scattering of the results can be caused by inner imperfections of the samples. in figure 7 are shown the mean courses and standard deviation of the stress-strain diagram for all sets of samples. for the uhpfrc samples is shown that the material exhibits high change in mechanical properties with increasing strain rate. yield stress σm is varying from σmquasi = 103 ± 17 mpa for quasi-static experiments to σmlow = 135 ± 22 mpa for low velocity experiments, and σmhigh = 158 ± 17 mpa for the high velocity experiments respectively. this corresponds to 31 % and 54 % change in the yield stress with increasing strain rate of the experiment. the identical trend is apparent for young’s mod53 n. krčmářová, j. falta, t. fíla, d. čítek acta polytechnica ctu proceedings figure 7. stress-strain diagram with standard deviation of the results from quasi-static and dynamic experiments. ulus. young’s modulus for the quasi-static experiments was equasi = 4.1 ± 0.7 gpa, for the low velocity experiments young’s modulus was elow = 11.3 ± 4.7 gpa and for the high velocity experiments ehigh = 13.9 ± 5.1 gpa. this is a 174 % and 237 % increase for low velocity and high velocity, respectively. 4. conclusion mechanical properties in compression of the uhpfrc were investigated in quasi static mode and in elevated strain rate for three sets of samples. for each set were used 5 samples drilled from uhpfrc block. the material exhibit high increase of the mechanical properties in the dynamic measurement. the yield stress for the lower impact velocity increased by 31 % and for the higher impact velocity by 54 %. there is even higher increase in the young’s modulus where is the increase of 174 % for lower impact velocity and 237 % for higher impact velocity. there occurred a problem because of the early failure of the specimen during ramp-in period of the strain pulse. actual impact velocities during compression of the specimen were thus lower. promising method how to overcome this phenomena is to arrange this measurement with a gap between the incident bar and the specimen to allow acceleration of the incident bar before the specimen impact. acknowledgements this research was funded by czech science foundation (project junior star no. 22-18033m) and by the grant agency of the czech technical university in prague, grant no. sgs23/134/ohk2/2t/16. references [1] l. ren, x. yu, y. he, et al. numerical investigation of lateral inertia effect in dynamic impact testing of uhpc using a split-hopkinson pressure bar. construction and building materials 246:118483, 2020. https: //doi.org/10.1016/j.conbuildmat.2020.118483 [2] l. a. de béjar. virtual estimation of the griffith’s modulus and cohesive strength of ultra-high performance concrete. engineering fracture mechanics 216:106488, 2019. https: //doi.org/10.1016/j.engfracmech.2019.106488 [3] l. ren, z. fang, k. wang. design and behavior of super-long span cable-stayed bridge with cfrp cables and uhpc members. composites part b: engineering 164:72–81, 2019. https: //doi.org/10.1016/j.compositesb.2018.11.060 [4] f. de larrard, t. sedran. optimization of ultra-highperformance concrete by the use of a packing model. cement and concrete research 24(6):997–1009, 1994. https://doi.org/10.1016/0008-8846(94)90022-1 [5] p. richard, m. cheyrezy. reactive powder concretes with high ductility and 200 800 mpa compressive strength. sp-144: concrete technology: past, present, and future 144:507–518, 1994. https://doi.org/10.14359/4536 [6] p. richard, m. cheyrezy. composition of reactive powder concretes. cement and concrete research 25(7):1501–1511, 1995. https://doi.org/10.1016/0008-8846(95)00144-2 [7] r. thomas, a. d. sorensen. review of strain rate effects for uhpc in tension. construction and building materials 153:846–856, 2017. https: //doi.org/10.1016/j.conbuildmat.2017.07.168 [8] l. leicht, t. fíla, p. máca, m. curbach. dynamic beam-end tests: investigation using split hopkinson bar. international journal of impact engineering 172:104417, 2023. https://doi.org/10.1016/j.ijimpeng.2022.104417 54 https://doi.org/10.1016/j.conbuildmat.2020.118483 https://doi.org/10.1016/j.conbuildmat.2020.118483 https://doi.org/10.1016/j.engfracmech.2019.106488 https://doi.org/10.1016/j.engfracmech.2019.106488 https://doi.org/10.1016/j.compositesb.2018.11.060 https://doi.org/10.1016/j.compositesb.2018.11.060 https://doi.org/10.1016/0008-8846(94)90022-1 https://doi.org/10.14359/4536 https://doi.org/10.1016/0008-8846(95)00144-2 https://doi.org/10.1016/j.conbuildmat.2017.07.168 https://doi.org/10.1016/j.conbuildmat.2017.07.168 https://doi.org/10.1016/j.ijimpeng.2022.104417 acta polytechnica ctu proceedings 42:51–54, 2023 1 introduction 2 material and methods 2.1 ultra high performance concrete samples 2.2 scanning electron microscopy 2.3 quasi-static experiments 2.4 dynamic experiments 3 results 4 conclusion acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.38.0402 acta polytechnica ctu proceedings 38:402–409, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague integral planning – potentials and challenges for the construction of timber buildings. a teaching example martin schachenhofer∗, martin aichholzer, aída santana-sosa university of applied sciences – fh campus wien, department building and design (bauen und gestalten), favoritenstraße 226, 1100 vienna, austria ∗ corresponding author: martin.schachenhofer@fh-campuswien.ac.at abstract. the term “integral planning” describes the creative cooperation of experts from different disciplines with the aim of solving a complex, technical planning task. by involving all the disciplines and stakeholders at an early stage it is possible to achieve better solutions than in a linear planning process in which one designer directs the specifications to the respective specialist planners, who then add the corresponding engineering services. in the course “integral planning”, students from the fields “architecture – green building” and “civil engineering – construction management” are working on a project in an interdisciplinary exchange over two semesters. through the simultaneous interweaving of their specific skills, combined with a very free working method, the students are taught integral planning in a realistic setting with the aim to design a modular timber building system for temporary educational buildings according to the “cradle to cradle” principle. keywords: integral planning, sustainable architecture, teaching architecture, teaching sustainability, modular timber construction, timber building. 1. introduction the word “integral” is borrowed from the latin word “integer” and can be translated as “intact”, “complete” or “healthy”. in the broadest sense, “integral” also means “functional”. in the context of planning, the adjective “integral” can be seen as the understanding that complex systems are only working flawlessly when all sub-components of the system are functioning well. in this way, integral planning is a holistic approach of complex planning tasks [1]. buildings have become extremely complex systems, where sustainability plays a fundamental role. developers demand resilient buildings and the statutory requirements for energy efficiency are also continuing to rise. as a result, planners have to deal with the assessment of the entire life cycle of a building, from construction to deconstruction [2]. this growing complexity requires an interdisciplinary approach, and therefore, a change from a linear planning process to one in which experts and stakeholders communicate with each other on an equal footing. integral planning is a life-cycle-oriented, holistic approach in which all planning participants and stakeholders are involved in an early stage of the project. through the simultaneous involvement of all disciplines such as architecture, structural engineering, technical building equipment, users and facility management, the project can be grasped in its entirety and the necessary arrangements for optimal use can be made (figure 1). decisions taken in an early planning phase, play an essential role for the construction costs and the later life cycle costs [3]. especially in the field of prefabricated timber buildings, where specific expertise is required, rather than for buildings made of brick or concrete, it is fundamental to involve all stakeholders in the planning at an early stage in order to set impulses, simplify processes and optimize the building design. 2. theoretical framework 2.1. effects of integral planning the early involvement of the key players and specialist planners has far-reaching consequences on the life-cycle of a building. buildings are responsible for about 40 % of total energy consumption in the eu and for about 30 % of co2 emissions. furthermore, about 40–50 % of the resource consumption is due to building design and construction. economic considerations and not at least due to the new eu taxonomy regulation, there has to be a change from a linear planning process to a forward-looking approach of a life-cycle-oriented, holistic view on the project. the planning strategies in the early process phases (strategy, initiation, planning) are responsible for the whole life-cycle-oriented performance of a building. the possibility to optimize life cycle costs, consumption of resources and energy and essential parameters to allow a flexible use of the building are remarkably high in these phases. therefore, all necessary and available information and requirements for the project, particularly from the use phase, should be considered at an early stage and at the beginning of the planning in order to optimize the subsequent operation of the property (figure 2) [4, 5]. 402 https://doi.org/10.14311/app.2022.38.0402 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 38/2022 integral planning – potentials and challenges for the construction . . . figure 1. integral planning team [3]. figure 2. cost development vs. change potential over building life cycle [5]. 2.2. integral planning and timber constructions in austria timber construction has experienced a significant upswing in recent years [6] (figure 3), but the collaboration of architects, structural engineers as well as specialist engineers and planners in the manner of conventional, linear planning does not work out properly for buildings with a high degree of prefabrication. it often turns out that the actors of the different disciplines are not sufficiently aware of the interfaces and the scope of services of the other planners in detail. the plan content and presentation depending on the maturity of a project is not uniformly defined across all disciplines. specialist planners are often involved too late in the planning process, the team does not work synchronously and different planning depths prevent targeted cooperation. a lack of standards for digital data processing and data exchange puts an additional strain on cooperation [7]. within traditional planning, details have to be revised frequently. integral planning offers the possibility of dealing with characteristic features of timber construction at a very early stage in order to find superior solutions by permanent exchange of experience and knowledge [8]. it helps all parties involved to achieve greater clarity about the various interfaces in the project. through regular exchange with experts, detailed questions can be solved at an early stage and the standards for digital cooperation can also be established from the beginning. in order to prepare students for the growing trend in timber construction in austria, specific trainings towards integral planning seem to be an adequate approach. in one interdisciplinary and compact course 403 m. schachenhofer, m. aichholzer, a. santana-sosa acta polytechnica ctu proceedings (a). total percentage of timber construction in austria (by floor space). (b). percentege of residential houses made of timber in austria (by floor space). (c). percentage of public buildings made of timber in austria (by floor space). figure 3. percentage of timber constructions in austria [6]. recycling environmental impacts durability building recyclability of individual components lca of buildings life span of buildings (depends on location, use, value) building parts recyclability of individual component layers lca of building parts life span of building parts (exchange, location, quality) building materials recyclability or reusability of building materials lca of building materials (epd) life span of building materials (life span, function, location) table 1. considerations of sustainable use of resources [9]. students learn to work collaboratively with specialist planners for their future careers, while they acquire specific expertise about contemporary timber construction. in addition, students learn to take a holistic view on their projects, a building’s life cycle and life-cycle costs. when considering the life cycle of timber, it becomes essential the fact of keeping the building elements and component in use as long as possible. for this reason, the goal of the course’s project is to develop design strategies for circular economy. 2.3. life-cycle of timber timber constructions have an excellent life cycle assessment (lca) when locally produced. dismantled wooden components can be recycled or even reused and, in this regard, timber is a long-term carbon storage [10]. when approaching holistically a new project, the sustainable use of resources is mandatory, wherein the following three points must be considered (table 1): • an efficient use of resources based on their specific properties and their potential of be reused and recycled. • an in-depth analysis of their environmental impacts throughout the entire life cycle. • an adequate design that maximizes the use of the building throughout time. that requires durability of the structure as well as flexibility in terms of floor plans and room height [9]. conventional models for the cascade use of wood and wood-based materials focus predominantly on the material recycling of waste wood into wood-based materials and thermal recycling. in order to prolong carbon storage and save primary resources, it will be increasingly necessary to follow the higher-value principles of re-use and repair and to apply these increasingly in the construction industry. in the long term, building components should remain in the cycle before they are recycled or thermally recovered (figure 4). in particular, the recyclability of entire components and their layers is mostly an unsolved problem that still need to be assessed. the dismantling capability of a building must be considered in the very early planning phase of the project in order to ease a circular use of its components. within the course “integral planning” the students were given the task of planning a circular timber building. 3. method – design studio “integral planning” the two-semester sequential design studio “integral planning” is a mandatory practical course for the student of the master programs “architecture – green building” and “civil engineering” in the 2nd and 3rd semester. in this course students get in touch with different construction management processes and work out mechanisms for the design of communication 404 vol. 38/2022 integral planning – potentials and challenges for the construction . . . figure 4. timber waste hierarchy [11]. as well as for the distribution of responsibilities in practice. they work cooperatively to find technical and aesthetic solutions to given problems and optimize the interfaces until they jointly arrive at an ideal solution. within the process, lecturers and experts take part as advisors in areas where specific expertise is needed, for example, detailed knowledge of timber construction and manufacturing processes. the first part of the course (integral planning 1) took place in the summer semester of 2020 and the second course (integral planning 2) in the following winter semester. the course was attended by 75 students of the master program “civil engineering – construction management”, who are subsequently referred to as civil engineers, and 22 students of the master program “architecture – green building”, who are referred to as architects. thus, a total of 97 students participated, who were divided into 14 groups with a maximum of 7 members. each group had at least 3 members covering the architectural tasks, where a team-leader was selected to be the contact person for the accompanying lectors in administrative and organizational matters. the following division was proposed for the tasks: • architectural design: architects and designing civil engineers; • structural analysis and construction: civil engineers; • building physics: architects and civil engineers; • construction economics and project management: architects and civil engineers. a total amount of nine lecturers supervised the course, with one of the professors being responsible for the organization and coordination. the other lecturers supervised and advised on the above-mentioned areas. 3.1. planning tasks the students should find an aesthetically and technically elaborated sustainable solution to the following problem: architectural and structural planning as well as economic processing of a modular construction system for temporary educational buildings according to “cradle to cradle” principles. the building structures to be planned had to comply with the vienna building regulations. the key challenge was to plan and construct the educational buildings in a modular timber construction, which can be dismantled or deconstructed into individual parts. the students had the task of developing a modular system from which classrooms and multifunctional rooms as well as sanitary units can be formed. these should be able to be combined to form a temporary educational building, while the different types of use had to be flexible and self-sufficient. the aim of “integral planning 1” was the architectural respectively constructive design. in “integral planning 2”, the project was continued, a submission plan was drawn up, the pre-structural and detailed structural analysis was calculated, and the implementation planning was drawn up for selected areas of the building. at the end, the project results were presented in groups to a high-ranking jury of internal and external experts. the result of one of these groups is presented in the next chapter. 4. case study – group 13 “woodul” members: one female architect, four female and two male civil engineers. the basic module on which the building is based measures 7.50 × 2.50 × 3.63 m. this ensured a room height of 3 m and also the possibility to move the 405 m. schachenhofer, m. aichholzer, a. santana-sosa acta polytechnica ctu proceedings (a). (b). figure 5. woodul – the module that can do everything [12]. figure 6. building types: kindergarten / compulsory school / university (from left to right) [12]. module to the construction site without special transports. the module is a timber frame construction with bending-resistant corners. the modules can be brought to the respective location by crane and a specially planned lifting construction (figure 5). 4.1. material and system the woodul consists of a room module with a loadbearing frame structure made of glulam. through their combination, high levels of flexibility are eased, wherein open areas can be built when joint, or separated environment by inserting different planar elements in their interfaces. in this context, exterior walls element were also proposed as timber panelized system filled with wood fiber insulation materials. for the load bearing structure the group decided to choose glulam made from larch as it is domestic wood with a high density and stable. for the interior and exterior cladding, the student selected silver fir as it is also domestic, light and has very good properties in shrinkage. 4.2. building types the group used three different examples to show how their so-called “woodul” could function in a kindergarten, a compulsory school and a university (figure 6). in the case of kindergartens, for example, the respective applications are geared towards sufficient movement areas and cloakrooms in the group room. in the case of the compulsory school, the group showed how central communal areas enable multifunctional uses, whereas the model for the university offers sufficient space for group work and lecture rooms of different sizes. 4.3. case study compulsory school the group convinced the jury of the flexibility of their system using various building typologies and showed the compulsory school type in detail (figure 7). the building is entered centrally, after a few steps one is in the heart of the building, the entrance hall with the two staircases, which also function as a multifunctional community area. this point is the central hub of the building, connecting all accesses of surrounding classrooms. generous atriums and glass doors at the ends of the corridors bring sufficient daylight into the interior spaces, while flexible interior walls create unique and changeable areas in shared spaces. in this case one classroom consists of at least 4 combined modules. 406 vol. 38/2022 integral planning – potentials and challenges for the construction . . . figure 7. floor plan: compulsory school [12]. figure 8. calculation of summer [12]. 4.4. costs the selected working group calculated the total costs, being 1 354 €/m2 gfa (gross floor area) for their woodul with by assembling costs of 116 €/m2 gfa. they calculated as well the costs for a conventional container made of steel with a prize of 1 020 €/m2 gfa. the higher costs were argued with the higher quality and better lca in their project. one criticism that could be raised at this point is that no statement was made about the life cycle costs. 4.5. energy concept • heating – underfloor heating operated by air-water heat pump. • hot water – decentralized tankless water heating directly at the tapping point. • ventilation – supply and exhaust air through ventilation openings with integrated fans. supply air heating in winter. • cooling – ventilation concept. • power supply – photovoltaic system with monocrystalline modules on the flat roof. 4.6. summer overheating one of the tasks was the calculation of summer overheating according to oib-regulation 6:2019 and simulation according to önorm b 8110-3:2012 (figure 8). key data of the calculation: • classroom consist of 4 modules; • 2× exterior walls; • 2× interior walls; • 1× floor to ground; • 1× intermediate ceiling (warm); • useful area = 68.80 m2; • number of persons = 30; • educational institution; • mechanical ventilation system; • shading by means of vertical lamellas; • night ventilation through windows. 407 m. schachenhofer, m. aichholzer, a. santana-sosa acta polytechnica ctu proceedings 4.7. dismantling a dismantling and reassembling concept of the woodul was developed as an integral part of the whole system. the concept ranges from dismantling individual structural elements to removing the underfloor heating system, describing in detail the assembly and disassembly process of the roof construction. they also calculated that the costs for dismantling will be at 98 €/m2 gfa. 5. conclusion a paradigm shift in the construction sector seems inevitable to archive a sustainable development. the use of renewable and regional materials within circular constructions is seen by the experts as a key aspect to implement when aiming to reduce carbon emissions. in order to be able to fully exploit the potential of natural building materials and their implementation on larger constructions, the current training of civil engineers and architects in europe is still inadequate [13]. extensive knowledge and specific expertise on the subjects of architecture, material properties, structural engineering, fire protection, building physics and prefabrication is required. herein, the design of timber buildings should be performed through an intensive collaborative and interdisciplinary perspective among all design team members including all disciplines, with the aim of developing an optimized holistic solution avoiding collisions, re-design and misunderstanding, what would bring cost and time overruns, dissatisfactions within the team or damages later on. the results of the student projects were of high quality and also the learning factor was rated as very high. students were confronted with the difficulties of working with various disciplines. they played their own role in an on-time, process-oriented organization of planning and learned the importance of each individual team member for the success of the project and therefore for the whole team. furthermore, students also recognized the work of other disciplines as a valuable contribution to the entire process. in the evaluation at the end of the course “integral planning 2”, students gave the course a total rating of 91.8 %. the lowest ratings were received in the question: “were the conditions for a positive completion of the course clear and understandable from the beginning?” (65.5 %) and “was the content of the course structured?” (85 %). the highest approval with 99 % was given to the question: “did the course promoted my interest in the content?” furthermore, the students appreciated with high agreement the very open working atmosphere and the further development of their professional understanding. acknowledgements the authors would like to thank all students of the course “integral planning” for their intensive participation and motivation in trying new methods and approaches. references [1] m. schwaninger. integrale planung: ein innovatives konzept? die unternehmung 42(2):123–136, 1988. [2] b. janssens, g. verbeek. architectural strategies for long-term obedience to increasing building-constructive energy performance requirements. in 8th nordic passive house conference, pp. 1–7. helsinki, finland, 2017. [2022-11-20]. https://www.researchgate.net/publication/ 320474692_architectural_strategies_for_longterm_obedience_to_increasing_buildingconstructive_energy_performance_requirements [3] r. stempkowski, p. dzuban, r. rosenberger. leitfaden für die kostenabschätzung von planungsund projektmanagementleistungen. band 7 integrale planung, wko – wirtschaftkammer österreich / bundesinnung bau, 2014. [2022-11-20]. https://www.bauteilaktivierung.info/wpcontent/uploads/sites/6/2020/09/leitfaden-zurkostenabscha%cc%88tzung-von-planungs-undprojektmanagementleistungen-integraleplanung.pdf [4] ig lebenszyklus bau. der weg zum lebenszyklusorientierten hochbau: die 3 säulen erfolgreicher bauprojekte in einer digitalen wirtschaft, 2016. [2022-11-20]. https://ig-lebenszyklus.at/wpcontent/uploads/2021/05/leitfaden_hochbau.pdf [5] i. kovacic, v. zoller. building life cycle optimization tools for early design phases. energy 92:409–419, 2015. https://doi.org/10.1016/j.energy.2015.03.027 [6] a. teischinger, r. stingl, g. o. praxmarer. holzbauanteil in österreich – statistische erhebung aller holzbauvorhaben in den jahren 1998-2008-2018, 2019. [2022-11-20]. https://www.proholz.at/fileadmin/proholz/media/ presse/pressemeldungen/holzbauanteil/studie_ holzbauanteil_in_oesterreich_1998_bis_2018.pdf [7] f. lattke, m. schlehlein. ‘leanwood – buch 5-teil a das prinzip in der ausführungsund werkstattplanung, 2017. [2022-11-20]. https://www.researchgate.net/publication/ 340377522_leanwood_buch_5_-_die_holzbauplanung [8] a. santana-sosa, a. fadai, m. aichholzer, m. kamenik. systemized design to deliver leaner mid-rise timber housing. in wcte 2020 – world conference on timber engineering 2020, p. 7. 2021. [2022-11-20]. https://www.researchgate.net/ publication/354143822_systemized_design_to_ deliver_leaner_mid-rise_timber_housing [9] a. hafner, k. krause, s. ebert, et al. ressourcennutzung gebäude: entwicklung eines nachweisverfahrens zur bewertung der nachhaltigen nutzung natürlicher ressourcen in bauwerken – abschlussbericht, ruhr universität bochum, 2020. [2022-11-20]. https://www.dbu.de/opac/ab/dbuabschlussbericht-az-34301_01-hauptbericht.pdf [10] a. hafner, s. schäfer. comparative lca study of different timber and mineral buildings and calculation method for substitution factors on building level. journal of cleaner production 167:630–642, 2017. https://doi.org/10.1016/j.jclepro.2017.08.203 408 https://www.researchgate.net/publication/320474692_architectural_strategies_for_long-term_obedience_to_increasing_building-constructive_energy_performance_requirements https://www.researchgate.net/publication/320474692_architectural_strategies_for_long-term_obedience_to_increasing_building-constructive_energy_performance_requirements https://www.researchgate.net/publication/320474692_architectural_strategies_for_long-term_obedience_to_increasing_building-constructive_energy_performance_requirements https://www.researchgate.net/publication/320474692_architectural_strategies_for_long-term_obedience_to_increasing_building-constructive_energy_performance_requirements https://www.bauteilaktivierung.info/wp-content/uploads/sites/6/2020/09/leitfaden-zur-kostenabscha%cc%88tzung-von-planungs-und-projektmanagementleistungen-integrale-planung.pdf https://www.bauteilaktivierung.info/wp-content/uploads/sites/6/2020/09/leitfaden-zur-kostenabscha%cc%88tzung-von-planungs-und-projektmanagementleistungen-integrale-planung.pdf https://www.bauteilaktivierung.info/wp-content/uploads/sites/6/2020/09/leitfaden-zur-kostenabscha%cc%88tzung-von-planungs-und-projektmanagementleistungen-integrale-planung.pdf https://www.bauteilaktivierung.info/wp-content/uploads/sites/6/2020/09/leitfaden-zur-kostenabscha%cc%88tzung-von-planungs-und-projektmanagementleistungen-integrale-planung.pdf https://www.bauteilaktivierung.info/wp-content/uploads/sites/6/2020/09/leitfaden-zur-kostenabscha%cc%88tzung-von-planungs-und-projektmanagementleistungen-integrale-planung.pdf https://ig-lebenszyklus.at/wp-content/uploads/2021/05/leitfaden_hochbau.pdf https://ig-lebenszyklus.at/wp-content/uploads/2021/05/leitfaden_hochbau.pdf https://doi.org/10.1016/j.energy.2015.03.027 https://www.proholz.at/fileadmin/proholz/media/presse/pressemeldungen/holzbauanteil/studie_holzbauanteil_in_oesterreich_1998_bis_2018.pdf https://www.proholz.at/fileadmin/proholz/media/presse/pressemeldungen/holzbauanteil/studie_holzbauanteil_in_oesterreich_1998_bis_2018.pdf https://www.proholz.at/fileadmin/proholz/media/presse/pressemeldungen/holzbauanteil/studie_holzbauanteil_in_oesterreich_1998_bis_2018.pdf https://www.researchgate.net/publication/340377522_leanwood_buch_5_-_die_holzbauplanung https://www.researchgate.net/publication/340377522_leanwood_buch_5_-_die_holzbauplanung https://www.researchgate.net/publication/354143822_systemized_design_to_deliver_leaner_mid-rise_timber_housing https://www.researchgate.net/publication/354143822_systemized_design_to_deliver_leaner_mid-rise_timber_housing https://www.researchgate.net/publication/354143822_systemized_design_to_deliver_leaner_mid-rise_timber_housing https://www.dbu.de/opac/ab/dbu-abschlussbericht-az-34301_01-hauptbericht.pdf https://www.dbu.de/opac/ab/dbu-abschlussbericht-az-34301_01-hauptbericht.pdf https://doi.org/10.1016/j.jclepro.2017.08.203 vol. 38/2022 integral planning – potentials and challenges for the construction . . . [11] a. strohmeyer. ressourcenwende – welche technischen lösungen gibt es bereits und was muss noch getan werden, 2019. [2022-11-20]. https://www.re-source.com/wpcontent/uploads/2020/11/vortrag_strohmeyer.pdf [12] student project woodul, fh campus wien, integrale planung, 2020. [13] j. koppelhuber, k. schafferer. die österreichischen holzbauunternehmen stehen für moderne technologien, innovationen und höchste kompetenz. zuschnitt – zeitschrift über holz als werkstoff und werke in holz 20(78):14–15, 2020. https://www.proholz.at/fileadmin/flippingbooks/ zuschnitt78/zuschnitt_78.pdf 409 https://www.re-source.com/wp-content/uploads/2020/11/vortrag_strohmeyer.pdf https://www.re-source.com/wp-content/uploads/2020/11/vortrag_strohmeyer.pdf https://www.proholz.at/fileadmin/flippingbooks/zuschnitt78/zuschnitt_78.pdf https://www.proholz.at/fileadmin/flippingbooks/zuschnitt78/zuschnitt_78.pdf acta polytechnica ctu proceedings 38:402–409, 2022 1 introduction 2 theoretical framework 2.1 effects of integral planning 2.2 integral planning and timber constructions 2.3 life-cycle of timber 3 method – design studio ``integral planning'' 3.1 planning tasks 4 case study – group 13 ``woodul'' 4.1 material and system 4.2 building types 4.3 case study compulsory school 4.4 costs 4.5 energy concept 4.6 summer overheating 4.7 dismantling 5 conclusion acknowledgements references my title https://doi.org/10.14311/app.2022.36.0119 acta polytechnica ctu proceedings 36:119–126, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague reliability of existing reinforced concrete slabs exposed to punching shear jana marková∗, milan holický, karel jung, miroslav sýkora czech technical university in prague, klokner institute, šolínova 7, 166 08 prague, czech republic ∗ corresponding author: jana.markova@cvut.cz abstract. selected standardised models for the verification of punching shear in reinforced concrete structures are applied for the probabilistic assessment of their reliability level. it appears that the models given in en 1992-1-1 and pren 1992-1-1 lead to more realistic estimates of the reliability level of existing reinforced concrete members with respect to punching shear than the models recommended in some national codes. the controlled perimeter has significant influence on the results and should be harmonized in prescriptive documents. keywords: assessment of existing structures, eurocodes, national codes, partial factors, probabilistic models, punching, reinforced concrete slab, reliability. 1. introduction construction works are designed using provisions specified in national, european or international standards. the eurocodes allow the national selection of nationally determined parameters (ndps) including alternative design approaches, load combinations, partial factors and other safety elements. the actual reliability of a designed structure depends on applied national standards or the ndps recommended in the national annexes to eurocodes. presently, the 2nd generation of eurocodes is nearly finished where some ndps have been removed, some provisions simplified or better clarified and selected theoretical models updated. the national annexes should be newly prepared with guidance on selecting ndps in the cen member countries. the load-bearing capacity, serviceability or durability of a particular structure designed in accordance with original national standards or nationally implemented eurocodes can be expected to be within a broad range. the actual structural resistance depends not only on used theoretical models and selected safety elements (including partial factors), but also on prescriptive rules recommended in applied standards, structural detailing (e.g. requirements on the reinforcement concrete cover, reinforcement ratio) and limiting factors (e.g. deflection, crack width). it was shown that the reliability of reinforced concrete (rc) members designed to the ultimate limit state of punching shear according to prescriptive documents might have a considerable scatter and should be further harmonised [1, 2]. the design of slab-column connection is the most critical for reliability of flat slabs as the localised concentration of shear stresses may lead to the progressive collapse due to punching [3]. this paper is focused on the evaluation of the design procedures for punching shear given in the currently valid eurocodes, newly prepared eurocodes and in original czech national standard. the prescriptive procedures for punching shear are also applied in case studies for the reliability verification of existing rc slabs in residential houses in prague designed according to the original czech codes. furthermore, the appearance of cracking and deformations in partition walls and rc slabs designed to the czech standards precipitated the need to verify the reliability of existing buildings according to the presently valid eurocodes. 2. design procedures for punching shear verification 2.1. introduction various prescriptive documents provide approaches to reliability verifications with respect to punching shear of newly designed or existing concrete structures. there are significant differences in the models as some are empirical while the others are based on physical models. the theoretical models recommended in the eurocodes en 1992-1-1 [4] and pren 1992-1-1 [5] (hereafter abbreviated as "en 1992" and "pren 1992") are analysed and the results compared with those based on the relationship given in the original czech national code csn 73 1201 [6]. such a comparison is particularly useful for the assessment of existing structures as it indicates whether slabs designed according to old standards are to be expected to have insufficient reliability. the punching shear resistance of rc flat slabs without shear reinforcement is mainly influenced by the concrete compressive strength, tensile flexural reinforcement ratio, size and geometry of the column and the slab depth [1, 2]. the influence of the basic variables on obtained reliability levels is further investigated. this provides the background information for surveys of existing structures that should be mainly 119 https://doi.org/10.14311/app.2022.36.0119 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en j. marková, m. holický, k. jung, m. sýkora acta polytechnica ctu proceedings focused on updating information about the variables primarily affecting reliability level. 2.2. en 1992-1-1 eurocode en 1992 [4] provides the following expression for estimating the punching shear resistance of selected cross section (concrete contribution): vrcd = u d (crd,c k (100 ρ fck) + k1 σcp) , (1) where crd,c is 0.18/γc, k1 = 0.1, ρ = (ρx ρy)0.5 with the reinforcement ratios ρx and ρy related to the bonded steel in tension in xand y-directions respectively, u denotes the critical section, and d is the effective depth of the slab. the critical section u = 2d is to be verified. in case where shear reinforcement is required, the punching shear resistance is a sum of concrete and reinforcement contributions [4]: vrd = 0.75vrcd + 1.5d/srasw fywd,ef (1/(u1d)) sin α (2) where asw is the total area of shear reinforcement placed around the column that crosses the critical inclined crack (considering approximately the reinforcement placed between 0.5d and 1.5d from the face of the column), sr is the radial spacing of perimeters of shear reinforcement, fywd,ef is the effective design strength of the punching shear reinforcement, d is the mean of the effective depths in the orthogonal directions, α is the angle between the shear reinforcement and the plane of the slab. adjacent to the column, punching shear resistance is limited to a maximum of [4]: ved = β ved u0 d ≤ vrd = 0.5 vfcd , (3) where u0 is the length of perimeter for an inner column. the control perimeter uout at which shear reinforcement is not required is given as [4]: uout = β ved vrdc d . (4) the outermost perimeter of shear reinforcement should be placed at a distance not greater than kd but within uout. 2.3. pren 1992-1-1 the recently developed version of the eurocode pren 1992 [5] provides the following expression for estimating the punching shear strength of a concrete slab: τrcd = 0.6/γvkpb (100 ρ1 fck ddg/dv)1/3 ≤ 0.6/γv f0.5 ck , (5) where ddg is a size parameter describing the failure zone roughness, which depends on the concrete type and its aggregate properties (newly introduced in the model), kpb is the punching shear gradient enhancement coefficient, dv is the shear-resisting effective depth of the slab, and γv is the partial factor for shear and punching resistance without shear reinforcement. the design shear stress τed can be calculated as: τed = βeved/(b0.5 dv), (6) where ved is the design shear force at the relevant control perimeter, βe is the coefficient accounting for concentrations of the shear forces and b0.5 is the length of the control perimeter. the following reliability condition is to be checked: τecd ≤ τrcd (7) in the case where the reliability condition is not met then the punching shear reinforcement should be provided, and a further control perimeter where shear reinforcement is no longer required shall be checked. where shear reinforcement is required, it should be calculated as: τrcs,d = ηcτrcs,d + ηsρwfywd, (8) where ηc = τrcd/τed, ηs = dv/(150φv) + (15ddg/dv)0.5(ηc kpb)−1.5 and ρw is the shear reinforcement ratio at the investigated control perimeter, ρw = asw/sr st where sr is the radial spacing of shear reinforcement and st is the average tangential spacing of perimeters of shear reinforcement measured at the investigated control perimeter. the punching shear resistance shall be limited to a maximum of τr,d max = ηsys τrc,d where a coefficient ηsys accounts for the performance of punching shear reinforcing systems. the outer control perimeter at which shear reinforcement is not required should be calculated as b0.5, out = b0.5 (dv/dv,out 1/ηc)0.5. 2.4. czech national code csn 73 1201 the czech national standard csn 73 1201 [6] gives the following expression for estimating design value of punching resistance of a concrete section given as: vrcd = 0.5 × 0.42 h κs κh κn γb fctd u, (9) where h is the slab depth; κs is the coefficient of amount of reinforcement (for common slabs κs = 1); κs is the coefficient for the slab depth, for slab depth from 0.15 m to 0.3 m the coefficient is κh = 1.2; κn is the coefficient for normal force n (for n = 0 is κn = 1); γb is the performance factor accounting amount of reinforcement in a cross-section, and effects of axial force, repeated loading, and concrete aging (here considered as γb = 1); and fctd is the design value of the tensile concrete strength. the critical section u = 0.5 h should be verified. where shear reinforcement is required, the following expression is given for internal force given for unity length of the critical perimeter transmitted at the shear reinforcement limit: 120 vol. 36/2022 reinforced concrete slabs exposed to punching shear basic variable distr. units char. µx vx self-weight and permanent actions n mn/m2 gk gk 10 % imposed residential building (50 y.) gum mn/m2 qk 0.6 qk 35 % slab depth n m h h 0.01∗ effective depth n m d d 0.01∗ concrete resistance ln mpa rk rk + 2 σx 15 % steel resistance ln mpa rk rk + 2 σx 8 % load effect model uncertainty ln − θe 1 5 % resistance model uncertainty ln − θr 1 10 % ∗ standard deviation. table 1. probabilistic models of basic variables [7, 8]. qsu = ns λss ass γs rsd, (10) where ns is the number of rows of stirrups, λss is the coefficient of stirrup effectiveness, γs is the partial factor for steel and rsd is the design steel strength. 3. basis of reliability analysis en 1990 [7] allows to use reliability verifications based on the probabilistic methods (see also iso 2394:2015). en 1990 gives provisions primarily for structural design, the reliability basis for existing structures is given in the following documents: • iso 13822 [9] general rules • jrc report [10] general principles of the assessment of existing structures, key background for the new guidance on reliability assessment in eurocodes • fib bulletin 80 [11] guidance for practical applications of the partial factor method for existing concrete structures, background for the new eurocodes • jcss publication [12] theoretical procedures of the probabilistic assessment the basic variables are then considered as random variables with appropriate probabilistic distributions. the structural member or theoretical model may be considered as reliable, if the condition pf < pt (or β > βt) is satisfied; the probability of failure pf is given as: p (g(x) < 0) < pt, (11) here g(x) denotes the limit state function, for which the inequality g(x) < 0 indicates that the limit state is exceeded and failure or unfavourable state occurs. the probability of failure pf may be expressed by the reliability index β = −φ−1(pf ) where φ is the distribution function of standardised normal variable. the probability of failure pt and reliability index βt are the specified (target) values that should not be exceeded during a considered reference period t. the reliability differentiation of structures given in en 1990 (see also iso 13822 for existing structures) is based on three different levels of failure consequences (with respect to three classes cc1 to cc3) for verification of the ultimate limit states. for common structures the target reliability index βt = 3.8 (or equivalently pt = 7.24 × 10−5) is recommended for a 50-year reference period and medium failure consequences (cc2). the reliability analysis of a structural member for the ultimate or serviceability limit states can be determined through the probability pf of the action effects e(x) randomly exceeding the structural resistance r(x) according to the following relationship: pf = p{(θr r(x) − θe e(x)) < 0} (12) where x is the vector of basic variables; θr and θe are the model uncertainties of the resistance and action effects. the knowledge of the reliability level of existing structures designed according to the national standards or nationally implemented eurocodes and also the reliability of prescriptive analytical models in standards can be applied for optimization of design procedures. the structural reliability with respect to the ultimate limit state of punching shear is analysed in the following focusing on an example of rc slabs in typical existing residential buildings built in the czech republic in 2000s. 4. probabilistic analysis case study an important step in any reliability analysis is the specification of probabilistic models for the basic variables. the probabilistic models of resistance parameters and actions are based on the jcss probabilistic model code [13], following here simplifications adopted in [7, 8] to provide generic models. the models are normalised by characteristic values of the basic variables (table 1). the permanent action is described by normal distribution (n), variable actions by gumbel distribution (gum), concrete and steel strengths by lognormal distribution (ln). the model for the permanent actions covers selfweight of the concrete slabs and floor layers. a rather conservative value of coefficient of variation, 10 %, is 121 j. marková, m. holický, k. jung, m. sýkora acta polytechnica ctu proceedings figure 1. cracks in partition walls. figure 2. variation of the reliability index β versus the percentage of required area of reinforcement of a slab considering en 1992-1-1, pren 1992-1-1 and csn 73 1201. considered as the study should be indicative for a population of the concrete slabs. for a particular existing slab, uncertainties in the permanent action can be reduced by measurements of thickness of floor layers and specification of volume densities. similarly, the models for the resistance parameters material and geometrical properties can be updated by in-situ measurements and tests. it is noted that the resistance model uncertainty in table 1 represents a rather generic model, considered to allow for comparisons between various resistance models; for more details see discussion in section 5. the concrete slab depth of 0.25 m was originally designed according to the original czech standards with concrete c25/35 and steel s400. however, after about 15 years considerable cracking appeared in partition walls of the residential buildings (figure 1), leading to the concerns about reliability of the structures. the inspection of load-bearing structures revealed that the actual depth of the existing slabs is 0.22 m only instead of 0.25 m assumed in the design. the shear reinforcement was unsatisfactory, failing to fulfil the prescriptive structural measures. for the verification of reliability of existing structures, the nationally implemented iso 13822 and currently valid standards for structural design (eurocodes) are applied (the original czech standard for the design of rc structures has informative character only after the implementation of the eurocodes in 2010 in the czech republic). selected results of the probabilistic analyses of slab punching shear are illustrated in figure 2 for the existing slab thickness of 0.22 m. for the three punching shear resistance models under consideration (originally applied csn 73 1201 [6] for the design of reinforced slabs, denoted as csn, en 1992 (ec2) and pren 1992 (prec2) in figure 2), the reliability index β increases with increasing shear reinforcement area as (100 % indicates shear reinforcement area needed to achieve 122 vol. 36/2022 reinforced concrete slabs exposed to punching shear figure 3. variation of the reliability index β versus the percentage of required reinforcement area as for two classes of reinforcement (s400 and s500), considering en 1992-1-1 and pren 1992-1-1. β = 3.8 for pren 1992-1-1). when conducting reliability analyses, the partial factors are not applied in the models provided in sections 2.2, 2.3, and 2.4; the characteristic values are replaced by random values of the respective basic variables. it appears that the application of the original czech standard leads to overestimation of the resistance of the concrete slab. application of the present eurocode en 1992 and pren 1992 for the verification of the slab for punching shear leads to a lower reliability level, therefore the shear reinforcement should be sooner added in the structural design. variation of the reliability index β versus the percentage of required reinforcement area as for two classes of reinforcement is illustrated in figure 3. two classes of steel reinforcement are considered here: s400 (as applied in the original design) and s500 (as widely used in the present practice). the actual depth of existing slab 0.22 m is considered. the newly conducted material tests reveal that the actual concrete compressive strength corresponds to c35/45 rather than to c25/30 as assumed in the design; the difference is partly attributed to concrete aging (hardening) and partly to execution where concrete of higher quality might have been supplied to comply with the quality control criteria. considering presently valid en 1992, the reliability level of the slab depth of 0.22 m and concrete strength c35/40 mpa is slightly lower, β = 3.6, in comparison to the target level. however, this may still be considered acceptable for existing residential buildings; see discussion in section 5. in case that pren1992 is applied, the reliability level of existing slab (β = 4) fulfils the target reliability level β = 3.8 recommended in eurocodes and also in iso 13822 for the assessment of existing structures. the latter standard indicates β = 3.8 for medium consequence of failure and the reference period considered as "a minimum standard period for safety (e.g. 50 years)"; both these specifications apply for the structures under investigation. for a higher class of reinforcement (s500 instead of s400), the reliability of slab considerably increases for the reference level of 100 % from 3.6 to 4.5). figure 4. sensitivity factors for basic variables considering the en 1992-1-1 model. sensitivity analyses of the basic variables entering the models under consideration indicate that apart from loads, structural reliability is significantly affected by uncertainties in yield strength of reinforcement, spacing of shear reinforcement, and by resistance model uncertainties; see figure 4 as an example for the en 1992-1-1 model. note that these observations largely depend on a reinforcement ratio and dimensions of the slab the sensitivity factor of concrete compressive strength would increase for lower reinforcement ratios; it would increase for effective depth of thin slabs. the controlled perimeter is deemed to be also an 123 j. marková, m. holický, k. jung, m. sýkora acta polytechnica ctu proceedings important random basic variable. yet as is common practice [14], it is considered here as deterministic since associated uncertainties are deemed to be covered by the resistance model uncertainty. the fact that length of the controlled perimeter has been not harmonized in prescriptive documents indicates large uncertainties in specification of this parameter. 5. discussion 5.1. notes on selected probabilistic models to be representative for a range of the existing slabs under consideration, table 1 provides the generic probabilistic models that should be updated in assessment of a particular slab. it is expected that based on measurements and tests, uncertainties in the following basic variables may be reduced: • coefficient of the permanent action could be decreased to about vg ≈ 5 %. • uncertainty in slab depth, considered here as vd ≈ 4.5 %, could be nearly entirely eliminated; the nearly complete draft of fib model code 2020 [15] assumes vd ≈ 1 % for in-situ measurements of geometry. yet it is emphasized that uncertainties in the measurements and possible variability of effective depth along the controlled perimeter (depending on actual position of reinforcement) should be carefully considered when specifying vd-value. 5.2. resistance model uncertainty resistance model uncertainty, θr, should be specified for each model individually [16–18]. while the statistical information on θr-characteristics for the csn and pren 1992 models is missing, the detailed studies focused on the performance of the en 1992 model [19, 20] indicated slightly conservative bias, µθ r = 1.07 (1.14-1.17), and reasonable scatter, vθ r = 18 % (12 % − 18 %), for slabs with (without) shear reinforcement. the bias µθ r > 1 would slightly increase the reliability levels in figure 2 and figure 3 while considering vθ r = 18 % (in comparison to 10 % given in table 1) would lead to a drop of the reliability level. as a first approximation it is considered here that similar θr-characteristics apply for the pren 1992 model and the reliability levels obtained in the numerical study are thus deemed to be representative for both en 1992 and pren 1992 models. as the pren 1992 model newly accounts for failure zone roughness and punching shear gradient effects, it may indeed provide a higher level of approximation than the presently valid en 1992. csn provides a more empirical-based model with a number of input parameters, depending on case-specific information model uncertainty may have different bias and coefficient of variation in comparison to the en 1992 model. uncertainty in the pren 1992 model and perhaps also in the csn model should be analysed within further research. the database of test results provided in [19] could be utilised in this regard. the sensitivity analysis indicates the importance of resistance model uncertainty. a considerable improvement may be achieved by using a non-linear finite element analysis (nlfea) for which vθ r may be significantly reduced. cervenka et al. [21] derived for validated nlfea models vθ r ≈ 8 % for punching and generally for ’all failure modes’, with biases close to unity. 6. target reliability for assessment iso 13822 assumes the same target reliability level for the design of new structures and upgrading of existing structures. however, for assessment the standard stipulates the possibility to adjust reliability targets by optimizing the total cost to a remaining working life. some national standards (austria, czech republic or germany) explicitly or implicitly assume the same target reliability for new and existing structures, some documents (aashto manual for bridge evaluation, 2011; can/csa-s6-06:2006 canadian highway bridges design code or nen 8700:2011 assessment of existing structures in case of reconstruction and disapproval basic rules) provide less strict criteria for existing structures; for instance nen applicable to buildings gives βt, ex, 15y = 3.3 for medium failure consequences (cc2). the latter is in agreement with the principle that target levels decrease with increasing cost of reliability measures as recognised e.g. in iso 2394:2015. following the iso standard, fib mc 2020 proposes βt, ex, 1y = 3.3. holicky et al. [22] proposed the methodology on how to recalculate the target reliability for various reference periods using the concept of an interdependency interval, k. when annual failure events are nearly statistically independent (such as for steel members exposed dominating wind or snow loads), k is close to 1 y. for situations where uncertainties in time-independent variables are dominating (such as for masonry structures exposed to dominating permanent actions), k becomes close to a reference period. punching of a rc slab exposed to the imposed load is a somehow intermediate situation that might be characterised by k ≈ 10 y. (as considered ’on average’ for rc structures in fib mc 2020). considering this, βt, ex, 50y = 2.8 − 2.9 is estimated. the 50-year β = 3.6 obtained in section 4 seems to be acceptable for the existing slab when considering the recalculated βt, ex, 50y-values. for more details see [23, 24]. 7. conclusions deterministic methods commonly used for the reliability verification of structures fail to provide deeper insights into structural reliability. the probabilistic methods make it possible to quantify and consistently treat uncertainties in the basic variables and to analyse 124 vol. 36/2022 reinforced concrete slabs exposed to punching shear relative importance of a basic variable by sensitivity analysis. reliability of existing reinforced concrete slabs originally designed according to the czech standards is verified considering the presently valid en 1992 and the final draft of pren 1992 using probabilistic methods. it appears that the model given in recently developed pren 1992 leads to more realistic estimates of the reliability level of the slabs with respect to punching than the model previously given in the czech standard. it appears that the use of the en 1992 or pren 1992 models in reliability assessments existing reinforced concrete structures exposed to punching shear improves estimating structural resistance and may help to avoid inadequate structural interventions. sensitivity analysis of the basic variables indicate that the dominating resistance parameter is model uncertainty; uncertainty in the pren 1992 model should be analysed within further research. the controlled perimeter is expected to have also significant influence on the results and should be harmonized in prescriptive documents and their national annexes [4–6, 15]; detailed analyses are however necessary. acknowledgements this study has been supported by the czech science foundation under grant 20-01781s and by the ministry of education, youth and sports of the czech republic under grant ltt18003. references [1] k. jung, j. marková. assessment of structural reliability for punching. in risk, reliability and safety: innovating theory and practice, p. 2421-5. 2016. https://doi.org/10.1201/9781315374987-366. [2] k. jung, j. marková. probabilistic analysis of structural reliability for punching shear. safety, reliability, risk, resilience and sustainability of structures and infrastructure proc. of icossar 2017. technische universität wien, p. 1778-1784, 2017. [3] b. ellingwood, r. smilowitz, d o. dusenberry, et al. best practices for reducing the potential for progressive collapse in buildings, nist interagency/ internal report (nistir). national institute of standards and technology, gaithersburg, md, usa, 2007. https://tsapps.nist.gov/publication/get_ pdf.cfm?pub_id=860696. [4] en 1992-1-1. eurocode 2: design of concrete structures part 1-1: general rules and rules for buildings. cen, brussels, 2006. [5] pren 1992-1-1. eurocode 2: design of concrete structures part 1-1: rules for buildings, bridges and civil engineering structures (complete draft). cen/ tc 250/ sc2, sept 2021. [6] csn 73 1201. design of concrete structures. unmz, prague, 2010 (withdrawn). [7] m. holický. reliability analysis for structural design. sun media, stellenbosch, 2009. [8] m. sýkora, m. holický, j. markova, et al. probabilistic reliability assessment of existing structures (focused on industrial heritage buildings), 1st ed., ctu publishing house, prague, 2016. [9] din en 1990:2010 eurocode basis of structural and geotechnical design, 2010. [10] p. lüchinger, j. fischer, c. chrysostomou, et al. new european technical rules for the assessment and retrofitting of existing structures (jrc science and policy report), report number jrc 94918, 2015. https://doi.org/10.2788/095215. [11] fib com3 tg3.1. partial factor methods for existing structures (fib bulletin 80, recommendation, edited by r. caspeele). fib, 2016. issn 1562-3610, isbn 978-2-88394-120-5. [12] jcss. probabilistic assessment of existing structures (edited by d. diamantidis). joint committee on structural safety, rilem publications s.a.r.l., 2001. [13] jcss. jcss probabilistic model code (periodically updated, online publication). joint committee on structural safety, 2022. www.jcss.byg.dtu.dk. [14] s. alkhatib, a. deifalla. punching shear strength of frp-reinforced concrete slabs without shear reinforcements: a reliability assessment. polymers 14(9):1743, 2022. https://doi.org/10.3390/polym14091743. [15] fib. model code 2020 (nearly complete draft). fib, lausanne, 2021. [16] m. sýkora, m. holický, m. prieto, et al. uncertainties in resistance models for sound and corrosion-damaged rc structures according to en 1992-1-1. materials and structures 48(10):3415-30, 2014. https://doi.org/10.1617/s11527-014-0409-1. [17] m. holický, j. v. retief, m. sýkora. assessment of model uncertainties for structural resistance. probabilistic engineering mechanics 45:188-97, 2016. https: //doi.org/10.1016/j.probengmech.2015.09.008. [18] m. sýkora, j. krejsa, j. mlcoch, et al. uncertainty in shear resistance models of reinforced concrete beams according to fib mc2010. structural concrete 19(1):28495, 2018. https://doi.org/10.1002/suco.201700169. [19] a. marí, a. cladera, e. oller, et al. a punching shear mechanical model for reinforced concrete flat slabs with and without shear reinforcement. engineering structures 166:413-26, 2018. https: //doi.org/10.1016/j.engstruct.2018.03.079. [20] punching shear strength of reinforced concrete slabs without transverse reinforcement. aci structural journal 105(4), 2008. https://doi.org/10.14359/19858. [21] v. červenka, j. červenka, l. kadlec. model uncertainties in numerical simulations of reinforced concrete structures. structural concrete 19(6):2004-16, 2018. https://doi.org/10.1002/suco.201700287. [22] m. holický, d. diamantidis, m. sýkora. reliability levels related to different reference periods and consequence classes. betonund stahlbetonbau 113:22-6, 2018. https://doi.org/10.1002/best.201800039. 125 https://doi.org/10.1201/9781315374987-366 https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=860696 https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=860696 https://doi.org/10.2788/095215 www.jcss.byg.dtu.dk https://doi.org/10.3390/polym14091743 https://doi.org/10.1617/s11527-014-0409-1 https://doi.org/10.1016/j.probengmech.2015.09.008 https://doi.org/10.1016/j.probengmech.2015.09.008 https://doi.org/10.1002/suco.201700169 https://doi.org/10.1016/j.engstruct.2018.03.079 https://doi.org/10.1016/j.engstruct.2018.03.079 https://doi.org/10.14359/19858 https://doi.org/10.1002/suco.201700287 https://doi.org/10.1002/best.201800039 j. marková, m. holický, k. jung, m. sýkora acta polytechnica ctu proceedings [23] r. d. j. m. steenbergen, m. sýkora, d. diamantidis, et al. economic and human safety reliability levels for existing structures. structural concrete 16(3):323-32, 2015. https://doi.org/10.1002/suco.201500022. [24] m. sýkora, d. diamantidis, m. holický, et al. target reliability for existing structures considering economic and societal aspects. structure and infrastructure engineering 13(1):181-94, 2016. https://doi.org/10.1080/15732479.2016.1198394. 126 https://doi.org/10.1002/suco.201500022 https://doi.org/10.1080/15732479.2016.1198394 acta polytechnica ctu proceedings 36(0):119–126, 2022 1 introduction 2 design procedures for punching shear verification 2.1 introduction 2.2 en 1992-1-1 2.3 pren 1992-1-1 2.4 czech national code csn 73 1201 3 basis of reliability analysis 4 probabilistic analysis case study 5 discussion 5.1 notes on selected probabilistic models 5.2 resistance model uncertainty 6 target reliability for assessment 7 conclusions acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.34.0001 acta polytechnica ctu proceedings 34:1–5, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague behavior of predried mature concrete beams subject to partial wetting and drying cycles lenka dohnalová∗, petr havlásek, vít šmilauer czech technical university in prague, faculty of civil engineering, department of mechanics, thákurova 7, 166 29 prague 6, czech republic ∗ corresponding author: lenka.dohnalova@fsv.cvut.cz abstract. the presented research focuses on the behavior of predried concrete beams with 2.5 m span subjected to cycles of nonsymmetric wetting and drying. wetting is induced by partially immersing the specimens in a water basin. the submerged portion of the specimens was relatively low (1/5 to 1/10 of their height) leading to a highly nonuniform and nonsymmetric distribution of eigenstrains due to concrete swelling. unlike conventional experiments on the volume changes of concrete, the measured quantity is not the axial deformation but the vertical displacement instead. this paper presents the experimental data obtained within two wetting and drying cycles, running over 1 year. keywords: concrete, wetting cycles, shrinkage, swelling. 1. introduction despite the relatively high number of studies on concrete creep and shrinkage, both phenomena and their origins have not been comprehensively described and satisfactorily explained. the situation is complicated by numerous factors influencing both creep and shrinkage. it is well known that concrete drying (or moisture changes) has a crucial impact on the evolution of drying shrinkage and drying creep (so-called pickett effect [1]). the absolute majority of experiments keeps the relative humidity constant over the whole test duration [2]. in contrast to this, the real concrete structures experience natural ambient conditions which are subject to annual cycles and daily fluctuations. furthermore, in the case of rain, some concrete members can encounter a direct contact with water. the number of studies on the time-dependent behavior of concrete under cyclic ambient conditions is rather limited, e.g. [3–5] or [6, 7] but the latter deals only with high-strength concretes. a common feature of these experiments is that the entire surface of the specimens is subject to moisture changes and the typically measured quantity is the axial deformation. the only exception is the study of hansen [4] who examined the curvature caused by bending under variable relative humidity which was varied between 50% a 70%. in experiments, typically, moisture changes are imposed by changing the level of relative humidity, only very rarely by immersing the specimens directly into a water. to our best knowledge, so far, not a single study has examined the response of concrete specimens subjected to nonsymmetric wetting and drying cycles in which the specimens experience direct contact with water. indeed, the objective of the present research conducted at ctu in prague is to fill this existing gap. within the experimental part of the research described in the present paper, the predried ordinary-strength concrete beams with 2.5 m span and 3 different heights are subject to partial soaking on their bottom surface followed by uniform drying. with approximately 3.5 months on average, the beams have experienced 2 wetting and drying cycles during the first 14 months of the experiment. 2. materials and methods all specimens in that study were prepared from an ordinary structural concrete c30/37. the composition is listed in tab. 1 and the basic strength characteristics determined at the age of 27 days are summarized in tab. 2. ingredients qtty [kg/m3] cement cem ii/b-s 32.5r 379 water 185 aggregates (fine 822, coarse 969) 1791 superplasticizer sika bv4 1.9 table 1. concrete composition. property unit value compressive strength, fc [mpa] 36.8 ± 1.4 young’s modulus, e [gpa] 30.3 ± 0.3 tensile strength, ft [mpa] 3.38 ± 0.45 fracture energy, gf [n/m] 150 ± 21 table 2. concrete mechanical properties at the age of 27 days. the core of the present experimental program is represented by a set of 5 simply supported concrete beams (see fig. 1) with a constant width of 100 mm and crosssections 100×100 mm (3 specimens), 100×150 mm and 100 × 200 mm (1 specimen each) and a uniform span of 2.5 m subject to non-symmetric wetting and drying 1 https://doi.org/10.14311/app.2022.34.0001 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en l. dohnalová, p. havlásek, v. šmilauer acta polytechnica ctu proceedings figure 1. a complete set of 5 concrete beams with 3 sets of 5 displacement sensors above the supports and at the midspan. external loading at l/5 with a pair of 15-kg weights. cycles. the beams were prepared from a spare backup specimens from a preceding experimental project on shrinkage-induced deformations of concrete beams subjected to nonsymmetric drying, whose details and first results can be found in [8]. the specimens were demolded after 34 days of moist curing. afterwards, the beams were kept approx. 1 year in a large laboratory hall next to the main set of specimens. the ambient conditions were not controlled and thus were susceptible to both annual and daily changes. however, on average the conditions were very close to the standard laboratory environment with relative humidity henv = 50% and t = 20◦c. the specimens were stored horizontally; the orientation of the cross-section was identical to the casting position and perpendicular to the final loading direction. figure 2. companion specimens for supplementary measurement of weight loss/gain during drying and wetting cycles. the original length of the specimens with 100 × 150 mm and 100×200 mm cross-section was 3.0 m. to unify their length with the three specimens 100×100× 2700 mm, these beams were at the age of 386 days trimmed to 2.6 m and soon afterwards (age 403 days) the beams were set to their destined position to minimize the overlap of the short-term creep deformation caused by self-weight with the effect of the prescribed ambient conditions. in addition to this, one specimen with 100 × 100 mm cross-section was loaded by a couple of 15-kg weights placed l/5 = 0.5 m from the supports (see fig. 1). to examine the change in stiffness due to the moisture changes, 14 days after the start of the first wetting cycle, this specimen was unloaded and the external weights were moved to the neighbouring specimen of the same size. in the figures presented in section 3, this is marked by a black dashed line labelled “s”. phase duration age of concrete [days] [days] moist curing 34 0 – 34 pre-drying 375 34 – 408 wetting 1 84 408 – 493 drying 1 132 493 – 625 wetting 2 113 625 – 738 drying 2 112 738 – 850 wetting 3 850 – table 3. phases of the experimental program. another set of 5 companion specimens was prepared from the sawn-off parts. these 200 mm long prisms served to monitor the weight changes due to drying and wetting cycles (see fig. 2). similarly to the beams, there are 3 specimens with h = 100 mm and one of each h = 150 mm and 200 mm. the first wetting cycle started at the age of 409 days, one week after the loading by the beams’ self-weight. the space underneath the beams was filled with water such that the lower part of the beams (20 mm high) became immersed, see fig. 3. the procedure was similar in the case of the companion specimens, only the water level was 5 mm higher, i.e. 25 mm (see fig. 4). the first wetting phase took 84 days, after which the water was drained. the length of the individual phases was not uniform and was not prescheduled in advance. the transition 2 vol. 34/2022 concrete beams subject to wetting cycles figure 3. detail of concrete beams at the left support soon after the onset of wetting. cyan line indicates water level and 20 mm immersed depth. figure 4. 5 companion specimens in a water basin subject to wetting (water level 25 mm). between the phases was chosen with respect to the rate of the moisture mass loss. the other factor was the availability of staff to conduct regular measurements. the duration of the phases is summarized in tab. 3. the beams were equipped with 15 potentiometric linear transducers with an internal spring return (mmr10-12, effective stroke 12.7 mm) installed at the midspan and at the supports to compensate for their potential movements. the sensors were mounted to a steel frame which was anchored to the floor, see figs. 1 and 5. the automatic reading was done every 30 minutes and more frequently, every 2 minutes, at the beginning of the drying/wetting cycle. the weight changes of the companion prismatic specimens were monitored manually using laboratory balance kern & sohn gmbh 572-55 with weighing capacity 20 kg and precision 0.05 g. at the beginning of each drying/wetting cycle, when the rates of weight changes were the highest, the measurement was conducted twice a day. the measuring frequency was decreasing with the rate of mass change until it reached the weekly period. 3. results and discussion the loading by self-weight of the beams was accompanied by their instantaneous deflection, which, owing to the adopted experimental setup, could not have figure 5. detail of the displacement sensors installed at the midspan. been monitored. furthermore, as shown later, the magnitude of this deflection is negligible in contrast to the deformation caused by the changes in ambient conditions. the creep deformation due to self-weight which occurred during the first week and prior to wetting were measured, but, because of their small magnitude (high concrete maturity, predried state, and small loading effect) are not presented here. the deflection presented in the following figures can be completely attributed to the deformations caused by nonuniformly distributed eigenstrains produced by swelling and shrinkage, and partially deflection associated with transient creep (linked to moisture changes) and microcracking. the origin of the time scale corresponds to the beginning of the first wetting. additionally, the individual cycles are marked by vertical dashed lines: blue and labelled “w” stands for wetting, and red, “d”, for drying. the beginning of the first wetting cycle is accompanied by concrete swelling in the immersed region leading to positive curvature and positive (downward) deflection. the behavior during the first two hours after the onset of wetting is presented in fig. 6. the height of the immersed region is the same for all three specimen heights. therefore, the initial deflection should decrease with increasing height and restraining bending stiffness, which nicely agrees with the measurements. the initial evolution of the vertical deflection is very interesting. in all three cases, the deflection rate is almost constant during the first 30 minutes. afterwards, the rate gradually decreases, such as in the case of larger specimens (h = 150 mm in red color and h = 200 mm in blue color) or suddenly stops like in the case of the specimens with h = 100 mm (shown in greyscale). for all three heights, the evolution of deflection has a characteristic plateau which is for h = 100 mm between ≈ 10 and 40 minutes. in the case of larger specimens, the presence of the plateau is not so striking and it is reached later, ≈ 0.5–2.5 hours (h = 150 mm) and ≈ 0.7–6 hours (h = 200 mm), see fig. 7 with time in the log-scale. the corresponding deflection is ≈ 0.45 mm for h = 100 mm and 0.32 mm for the other two cases. 3 l. dohnalová, p. havlásek, v. šmilauer acta polytechnica ctu proceedings 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0 20 40 60 80 100 120 w d efl ec ti on [ m m ] time, t [min] 200 mm 150 mm 100 mm figure 6. vertical deflection at the midspan of the beams during the first 2 hours of the first wetting cycle. the same value of vertical deflection of h = 150 and 200 mm several hours after loading needs to be interpreted with caution. as shown in fig. 8, the prism with h = 150 mm imbibes considerably less water than the other specimens. this material inhomogeneity might be responsible for the smaller swelling strain and thus different structural response. the first few hours after wetting the behavior of the beams is very complex. most probably, it is a result of two competing processes: concrete swelling, which triggers the deflection, and relaxation of the generated stresses caused by a high moisture rate. in addition to this, the region of the swelling concrete is gradually increasing and the distributions of moisture and stresses over the height become less nonlinear. 0 0.5 1 1.5 2 2.5 3 3.5 0.001 0.01 0.1 1 10 100 ds d efl ec ti o n [m m ] time, t [day] 200 mm 150 mm 100 mm figure 7. vertical deflection at the midspan of the beams, time t in log-scale is measured from the onset of wetting. the highest deflection, between 2.5 and 3.0 mm, is reached in h = 100 mm after 2–3 days of wetting. except for a small difference in the deflection, the behavior of beams h = 150 mm and h = 200 mm in the second half of the first wetting cycle is very similar (see fig. 7). the fluctuations in deflection can be attributed to the combination of changing ambient conditions as well as to the water level in the basin. (note also the apparent oscillations of the mass in fig. 8 in this period when the overall equilibrium was disturbed by inaccurate refilling the content of evaporated or imbibed water.) it can be stated that the deflections at the end of the first wetting cycle are very similar except for the beam with h = 200 mm whose deflection is approximately 20% less than in the other cases, see fig. 10. the change in deflection at t = 14 days caused by external weight removal or loading is almost identical (see fig. 7). in this wetting cycle, there are no other consequences of the external loading. the first drying cycle results in an almost identical evolution of moisture loss in all the specimens, see fig. 8. additionally, the development of vertical deflection of all beams is very similar and, at the end of the drying cycle, is around 1 mm (see fig. 10). the exception is the specimen with external loads whose deflection recovers considerably less. at the beginning of the second wetting cycle, in all cases, the total deflection becomes considerably larger than what it was at the end of the first wetting. the highest increase is in the case of the smallest specimens, h = 100 mm, which exhibit similar behavior to the first wetting (sharp increase followed by a decrease but now at slower rates). additionally, in the second wetting cycle, the differences between the beams of different sizes tend to propagate. the highest deflection of all belongs to the specimen with external weights; the difference has prominently arisen already during the first drying cycle. the phenomenon responsible for this behavior might be the drying creep at the bottom surface. figure 8 shows that for h = 150 mm and h = 200 mm, the mass gain in the second wetting cycle is considerably less than in the first wetting. this suggests that at the beginning of this experiment, the concrete specimens were not in equilibrium with the ambient conditions. this might be one of the reasons responsible for the different behavior of the beams. the mass change in the second and at the beginning of the third wetting cycle is very similar, both in terms of evolution as well as the measured values. drying was terminated when the moisture content was still 30 g above the initial value, but it needs to be born in mind that the average ambient humidity (see fig. 9) was higher than during predrying (henv = 0.5). in contrast to this, the differences between the individual beams tend to increase during the second drying and the third wetting, which is now in progress. the decrease in deflection of h = 100 mm beams at the beginning of wetting has vanished in the third cycle. 4. conclusions this paper presents original experimental data on predried concrete beams subject to wetting and dry4 vol. 34/2022 concrete beams subject to wetting cycles 0 20 40 60 80 100 120 0 50 100 150 200 250 300 350 400 450 500 550 d dw w w m as s ch an ge [ g] time, t [day] 200 mm 150 mm 100 mm figure 8. total mass increase of companion specimens subject to wetting and drying cycles. the specimens had uniform length l = 200 mm and breadth b = 100 mm but different height h = 100, 150 and 200 mm. 0.4 0.5 0.6 0.7 0 100 200 300 400 500 15 20 25 30 d dw w r el at iv e hu m id it y [] t em pe ra tu re [ °c ] time, t [day] henv t figure 9. evolution of the ambient conditions during the experiment. ing cycles. in contrast to the experiments in the literature, which are scarce, wetting is nonsymmetric which causes high values of curvature. consistency of the measured data is supported by similarities in the observed trends. however, despite quite similar behavior in the first wetting and drying cycle, further cycling tends to split the response of the beams with different sizes. the behavior in the initial stage of the wetting period is extremely complex and with cycling undergoes changes. in the next part of this research, mathematical modeling will be used to further analyze and better understand the complicated phenomena. the calibration will be adopted from previous research [8] which will enable efficient identification of the behavior related to concrete swelling. acknowledgements the authors gratefully acknowledge financial support from the czech science foundation (ga čr), project 0 1 2 3 4 5 0 50 100 150 200 250 300 350 400 450 500 d dw w ws d efl ec ti on [ m m ] time, t [day] 200 mm 150 mm 100 mm figure 10. development of vertical deflection of the concrete beams with height 100, 150 and 200 mm and span 2.5 m subject to cycles of wetting (w) and drying (d) which are marked by vertical dashed lines. “s” corresponds to the instant when the external weights were switched between two beams with h = 100 mm. number 19-20666s, and from the grant agency of the czech technical university in prague, project number sgs21/037/ohk1/1t/11. references [1] g. pickett. the effect of change in moisture-content on the creep of concrete under a sustained load. journal of the american concrete institute 38:333–355, 1942. [2] v. šmilauer, p. havlásek, l. dohnalová, et al. revamp of creep and shrinkage nu database. in proceedings of the biot-bažant conference. northwestern university, 2021. [3] l. vandewalle. concrete creep and shrinkage at cyclic ambient conditions. cement and concrete composites 22(3):201–208, 2000. https://doi.org/10.1016/s0958-9465(00)00004-4. [4] t. c. hansen. creep of concrete. the influence of variations in the humidity of the ambient atmosphere. proceedings of 6th congress of iabse pp. 57–65, 1960. [5] s. asamoto, a. ohtsuka, y. kuwahara, c. miura. study on effects of solar radiation and rain on shrinkage, shrinkage cracking and creep of concrete. cement and concrete research cem concr res 41:590–601, 2011. https://doi.org/10.1016/j.cemconres.2011.03.003. [6] y. song, q. wu, f. agostini, et al. concrete shrinkage and creep under drying/wetting cycles. cement and concrete research 140, 2021. https://doi.org/10.1016/j.cemconres.2020.106308. [7] h. cagnon, t. vidal, a. sellier, et al. drying creep in cyclic humidity conditions. cement and concrete research 76, 2015. https://doi.org/10.1016/j.cemconres.2015.05.015. [8] p. havlásek, v. šmilauer, l. dohnalová, r. sovják. shrinkage-induced deformations and creep of structural concrete: 1-year measurements and numerical prediction. cement and concrete research 144:106402, 2021. https://doi.org/10.1016/j.cemconres.2021.106402. 5 https://doi.org/10.1016/s0958-9465(00)00004-4 https://doi.org/10.1016/j.cemconres.2011.03.003 https://doi.org/10.1016/j.cemconres.2020.106308 https://doi.org/10.1016/j.cemconres.2015.05.015 https://doi.org/10.1016/j.cemconres.2021.106402 acta polytechnica ctu proceedings 34:1–5, 2022 1 introduction 2 materials and methods 3 results and discussion 4 conclusions acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.38.0382 acta polytechnica ctu proceedings 38:382–388, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague a case study of implementing a renovation concept matthias haase zurich university of applied sciences, institute of facility management, grüental / ra, p.o. box, ch-8820 wädenswil, switzerland correspondence: matthias.haase@zhaw.ch abstract. there is an untapped potential for reducing ghg emissions by district renovation. it needs a detailed energy master planning (emp) of the district and support for the decision-making processes. if applied in a comprehensive manner it can contribute significantly to reducing energy consumption and thus to a long-term sustainable development of districts and climate neutrality in our cities. the emp typically includes combinations of energy supply and consumption, but it is equally important to understand the different solutions for district renovation. the multi-owner structure in many districts requires another set of solution finding that is embedded in potential analysis, stakeholder analysis, participative planning, and multi-actor-management. a district near winterthur, switzerland was used as a case study. site visits and structured interviews with key stakeholders were used to collect data which was then used to model the technical-economic situation and to determine the possibilities for the future. keywords: district renovation, renovation packages, building renovation, energy supply. 1. introduction previous research shows, that renovation strategies on building level need to be derived from the energyefficient renovation of buildings and the use of renewable energy sources to reduce the energy supply of regions or cities. the combination of energy efficiency and renewable energy sources covers both energy supply and demand in the built environment. in this sense, the renovation of buildings is a suitable strategy to reduce demand, while the use of renewable energy aims to reduce carbon dioxide emissions from the energy supply system. however, identifying technical solutions is not enough to implement large-scale renovation strategies and to achieve the predicted decarbonization of the building stock. the renovation rate in europe is clearly below the 3 percent annual target [1, 2]. some of the main barriers to remediation relate to remediation costs and access to finance, as well as complexity, awareness, stakeholder management and supply chain fragmentation [1, 3]. as a result, business models are relevant to implementation and acceleration of renovations. seddon et al. [4] define “business model” as the outline of essential details of a firm’s value proposition for its various stakeholders, and the activity system the firm uses to create and deliver this value proposition [5]. in other words, a business model is the abstraction of a strategy, focused on the system of activities through which a firm creates economic value. opportunities to reduce greenhouse gas emissions through neighborhood renovations are largely untapped. it requires not only in-depth energy planning (emp), but also the support of decision-making processes [4]. this can not only significantly contribute to reducing energy consumption and ensuring the location of energy infrastructure (production, distribution, storage), but also to long-term sustainable development of districts and climate neutrality of our cities. 1.1. energy and emissions to be able to reduce ghg emissions in the built environment it is important to focus on the reduction of co2 emissions from the operation of the buildings [6]. the reduction of energy use should come from the implementation of efficiency measures through energy renovation of the building stock. another possibility is the decarbonisation of the energy supply with on-site renewable energy measures. however, in renovation planning it is often unclear which energy supply options are available and what influence the energy renovation has on co2 emission reductions. 1.2. reduction paths and its elements a two-phase approach is proposed to reduce carbon dioxide emissions: first, reducing energy consumption with efficiency measures in connection with the renovation of the building stock. secondly, co2 emission reductions from on-site renewable energy supply. when it comes to costs and financing, it is crucial to relate different measures with different stakeholders. energy supply is a political (municipal) matter, but the renovation of residential buildings mostly depends on the owners [6]. in order to achieve carbon reduction goals, it is important to find ways to involve 382 https://doi.org/10.14311/app.2022.38.0382 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 38/2022 a case study of implementing a renovation concept figure 1. the settlement seen from above with eight building blocks containing 51 row houses. homeowners in long-term carbon reduction investment strategies [7]. this case study was modelled with different power supply options and the performance was simulated [1]. the performance gives energy consumption over a year and with energy prices the costs were calculated. the cost savings could then be used in part to finance an energy-efficient renovation. in the base case, fossil fuels (oil) are delivered to the district and different power supply options offer energy and cost savings compared to the base case. after that, the energy saving measures are calculated, which lead to energy cost savings for the building owner. finally, local renewable energy sources are integrated into the roofs of the buildings which provide electricity production and income when sold to the utility company. the final investment costs are calculated and presented to the building owners. 2. case study 2.1. situation settlement 51 in dinhard was built in a first stage in 1974 and in a second stage in 1977 as a cooperative settlement. eight blocks contain 51 row houses as shown in figure 1. the row houses have different sizes and different number of rooms (3.5 rooms, 5.5 rooms and 6.5 rooms) resulting in different row house types. some of the houses are already being retrofitted with new windows and additional roof insulation, but so far no district renovation (of all houses) took place. the heating center in front of block f next to the swimming pool (in white in the middle lower part) supplies all row houses with heating and hot water. the oil boiler with 465 kw heat output was replaced in 2019. 2.2. energy consumption an average of 99 370 liters of oil (average for the years 2000 to 2014) was consumed in the district. with an average energy reference area of a row house of 140 m2, an energy figure for heat ew = 140 kwh/(m2a) can be found. this value is somewhat below the swiss average for older, non-refurbished residential buildings (ew avg = 160 kwh/(m2a)) [3]. 2.2.1. energy supply option three different building standards (standard 1, 2 and 3) with different peak power was modelled. for each building standard, the following energy supply options were considered [1]. • oil boiler (status quo), centralized system with distribution system. • oil boiler for heating, hp system for dhw. ▷ oil boiler for heating, centralized system with distribution system. ▷ decentralized heat pump system for dhw. • ground source heat pump, centralized system with distribution system. • pellet boiler, centralized system with distribution system. 2.2.2. energy, costs and emissions the simulations provided energy consumption results for each option. these were taken as input for cost and emission calculations. the following assumptions were considered as shown in table 1 (energy costs and emissions), table 2 (investment costs), table 3 (maintenance costs) and table 4 (energy costs). 3. results the results of the case study are presented in energy use, investment and maintenance costs as well as energy costs, and ghg emissions. the link between costs and emission reduction will then be presented to the investors/housing owners. 383 matthias haase acta polytechnica ctu proceedings energy carrier costs emissions electricity 0.15 chf/kwh 0.25 kg co2eq/kwh pellets 375 chf/t 0.01 kg co2eq/kwh oil 1.05 chf/l 0.28 kg co2eq/kwh table 1. energy costs and emission factors of different energy carriers. unit chf oil boiler central oil boiler gshp pellet boiler+ dec. hp boiler electricity 0 7500 34290 0 pellets 0 0 0 60000 oil 84000 68250 0 0 el. for pumps and motors 2000 1800 2000 2000 total energy 86000 77550 36290 62000 table 2. investment costs of different supply options (in chf). unit chf oil boiler central oil boiler gshp pellet boiler+ dec. hp boiler cleaning, control, fees, services 2500 2000 1000 2500 repair fond 3400 6570 5000 6480 total 5900 8570 6000 8980 table 3. maintenance costs (in chf). unit chf oil boiler central oil boiler gshp pellet boiler+ dec. hp boiler electricity 0 7500 34290 0 pellets 0 0 0 60000 oil 84000 68250 0 0 el. for pumps and motors 2000 1800 2000 2000 total energy 86000 77550 36290 62000 table 4. energy costs (in chf). annual energy use type 1 type 2 type 3 type 4 [kwh] block a 135564 5 1 block b 135564 6 block c 170680 1 5 1 block d 194972 1 7 block e 146388 1 5 1 block f 189878 1 6 1 block g 124643 0 5 block h 97168 0 4 sum 1194858 3 1 43 4 table 5. energy use. unit kwh/(m2a) type 1 type 2 type 3 type 4 heating 118 118 118 118 dhw 22 22 23 24 electricity 24.4 31.6 36.3 39.12 sum 164.4 171.6 177.3 181.12 table 6. energy use in different housing units. 384 vol. 38/2022 a case study of implementing a renovation concept renovation measure description investment costs [chf] roof tiles + wind barrier + 30 cm insulation + vapour barrier 653 158 facade + new windows + insulation panels + doors 705 944 ventilation ventilation unit + heat exchanger + ductwork 324 602 pv 1339 pv modules + fastening structure + inverter + cabling 709 322 pvt 1339 pv modules + inverter + cabling, partly integrated with 354 m2 solar thermal collector modules solar thermal collector (5, 6, 7, 8 m2 for building types 1, 2, 3, 4 respectively) + piping + storage tank 852 422 table 7. renovation measures with investment costs. operational energy energy savings co2 emissions co2 emissions savings [kwh] [kwh] [kg co2] [t co2] ref 1212971 322688,35 reno 1 roof 938563 274409 242573,05 80,1 reno 2 roof+fassade 801354 411617 202069,10 120,6 reno 3 roof + facade + balanced ventilation 664146 548826 161565,16 161,1 reno 4 reno1+pv 821331 391641 224425,58 127,8 reno 5 reno1+pvt 740929 472042 201384,15 150,8 reno 6 reno2+pv 679185 533786 183157,37 169,1 reno 7 reno2+pvt 603721 609251 160880,20 191,3 reno 8 reno3+pv 541977 670995 142653,43 209,6 reno 9 reno3+pvt 466512 746459 120376,25 231,8 table 8. renovation options. 3.1. energy there are a total 8 different blocks with four different types of row houses (type 1, 2, 3, 4). energy use in the eight different building blocks is illustrated in table 5. a total of 1195 mwh is used. table 6 summarizes the energy use per m2 heated floor area for heating, domestic hot water (dhw) and electricity. the different types have different annual energy use between 172 kwh/(m2a) (type 1) and 181 kwh/(m2a). table 7 shows the different renovation measures for the eight buildings. the first measure focuses on the roof and adds a vapour barrier, 30 cm insulation, a wind barrier and new tiles. the investment costs for this measure were calculated to 653 158 chf. the second measure focuses on the facade and includes changing doors and windows as well as insulation panels underneath the windows. the investment costs for this measure were calculated to 705 944 chf. the third measure includes a balanced ventilation system consisting of a ventilation unit per house, heat exchanger and ductwork. the investment costs for this measure were calculated to 324 602 chf. the fourth measure includes a pv system on all roof surfaces, including fastening structure and cabling as well as inverters. the investment costs for this measure were calculated to 709 322 chf. the fifth measure includes a pvt system, adding to the pv system solar thermal collectors (5, 6, 7, 8 m2 for building types 1, 2, 3, 4 respectively), piping and a storage tank. the investment costs for this measure were calculated to 852 422 chf. the five measures were combined to nine different renovation options (reno 1 to reno 9). it follows step by step renovating the roof, the façade, and then integrating the different technologies pv and pvt in different options as listed in table 8. the energy use and savings were calculated for heating, dhw and electricity separately. in the case of pv, a separate calculation included a share of self-consumption (30 %), and the economic implications which arise from different tariffs for purchasing and selling electricity [8]. 385 matthias haase acta polytechnica ctu proceedings figure 2. economic evaluation of the renovation options (pbp and npv). in the two columns on the right the co2 emissions of each renovation option is shown together with the potential savings compared to the reference case. 3.2. economic evaluation figure 2 shows the net present value (npv) and payback period (pbp) of the different renovation options (reno 1 to reno 9) without the different options for energy supply. it can be seen that the pbp varies between 16 (reno 4) and 25 (reno 3) years. with the chosen discount rate of 3.5 % and a lifetime of 25 years two renovation options have positive npv (option 4 and 5) and are economically feasible. however, renovation option (reno 1, reno 7, reno 8 and reno 9) are very close to a positive npv and in combination with a wood pellets boiler they become also economically feasible. the payback periods are lowest for renovation options (reno 4) (15.5 years) and (reno 5) (15.4 years). the highest pbp shows renovation option reno 3 (24,5 years). eight renovation options show a pbp below 20 years (reno 1, reno 4, reno 5, reno 6, reno 7, reno 8 and reno 9). 3.3. ghg emissions table 8 gives the ghg emissions and the savings of the different renovation options. we included investment as well as maintenance and energy costs of the supply options. figure 3 shows investment costs over the saved co2 emissions of the different renovation options for different supply options (see table 3) over the period of 25 years. the lowest costs per t co2 provides the renovation of the roof (reno 1) in combination with a wood pellets boiler (100 chf/t co2). the other renovation options are more expensive with (reno 6) providing the highest costs (489.4 chf/t co2). further, costs are always highest for the oil boiler 386 vol. 38/2022 a case study of implementing a renovation concept figure 3. economic evaluation of the renovation options with different supply options. option, while the two gshp solutions rank second and the wood pellets solution is always the cheapest. the renovation options (reno 2), (reno 4) and (reno 5) (options 4 and 5 have a positive npv) with wood pellets boiler provide the second lowest costs (205, 196 and 214 chf/t co2 respectively). 4. conclusions the results of the case study clearly show the potential to save energy and greenhouse gas emissions. correlation with the necessary investment and maintenance costs and energy costs shows that at least two basic improvement options are advantageous. here, the economic advantage of the energy savings of roof renovation is combined with regenerative energy production on the roof. when presenting the emission reductions and avoided costs, it becomes clear that this effect can be strengthened by additional fees for greenhouse gas emissions. in this case, as the fees increase, depending on the co2 level, more renovation options become financially viable. several renovation options are close to economic profitability, and especially the renovation options (reno 1, reno 7, reno 8 and reno 9) become feasible with a combination of wood pellet boilers. this is due to several reasons. first, the co2 payment savings are greater. this is due to the higher co2 savings of the delivered energy. with the co2 coefficient of wood pellets, 0.036 kg/kwh has been calculated, compared with the co2 coefficient of oil of 0.295 kg/kwh. the economic benefit of each saved kwh consists of energy costs and a higher co2 payment saved. finally, the renovation versions reno 4, reno 6 and reno 8 produce electricity. while the self-consumption rate (30 %) lowers energy costs, the excess of electricity is sold to the grid. this lowers energy costs and increases co2 payments. in the renovation options reno 5, reno 7 and reno 9, additional hot water is produced in pvt collectors. the share of own consumption (43 %) is used directly for hot water in buildings. the remaining part (57 %) is not used. however, it can be used for other purposes (such as heating the swimming pool or heating other neighbours). we want to explore the possibilities of using this extra heat for other purposes in the next phase of research. in this work, each building has a small part of the roof with a solar heat collector (5, 6, 7, 8 m2 for building types 1, 2, 3, 4). if all roof areas were supplied with pvt, the excess heat could be stored in seasonal storage increasing the use of locally produced renewable energy and the district’s self-sufficiency. acknowledgements this work has been supported by the project “ped – positive energy districts” of the zhaw. expert discussions within the iea ebc annex 75 and annex 83 are highly appreciated. references [1] m. haase. implementation strategies for renovation concepts based on participative planning. iop conference series: earth and environmental science 1078(1):012081, 2022. https://doi.org/10.1088/1755-1315/1078/1/012081 [2] i. artola, k. rademaekers, r. williams, j. yearwood. boosting building renovation: what potential and value for europe?, european union, brussels, 2016. 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[2022-06-22]. https://www.bpie.eu/wp-content/ uploads/2016/11/bpie_i24c_deepretrofits.pdf [8] european commission dg energy. financing the energy renovation of buildings with cohesion policy funding, 2014. [2020-04-01]. https://ec.europa.eu/ energy/sites/ener/files/documents/2014_ guidance_energy_renovation_buildings.pdf 388 https://doi.org/10.1007/978-3-030-57764-3_1 https://www.bpie.eu/wp-content/uploads/2016/11/bpie_i24c_deepretrofits.pdf https://www.bpie.eu/wp-content/uploads/2016/11/bpie_i24c_deepretrofits.pdf https://ec.europa.eu/energy/sites/ener/files/documents/2014_guidance_energy_renovation_buildings.pdf https://ec.europa.eu/energy/sites/ener/files/documents/2014_guidance_energy_renovation_buildings.pdf https://ec.europa.eu/energy/sites/ener/files/documents/2014_guidance_energy_renovation_buildings.pdf acta polytechnica ctu proceedings 38:382–388, 2022 1 introduction 1.1 energy and emissions 1.2 reduction paths and its elements 2 case study 2.1 situation 2.2 energy consumption 2.2.1 energy supply option 2.2.2 energy, costs and emissions 3 results 3.1 energy 3.2 economic evaluation 3.3 ghg emissions 4 conclusions acknowledgements references https://doi.org/10.14311/app.2022.33.0085 acta polytechnica ctu proceedings 33:85–91, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague feasibility experiments of seismic concrete block walls without joint mortar ho choia,∗, kiwoong jinb, jaecheon jeongc, bongseok kimc, eonju hwangc a shizuoka institute of science and technology, 2200-2, toyosawa, fukuroi, shizuoka 437-8555, japan b meiji university, 1-1-1, higashimita, tama, kawasaki, kanagawa 214-8571, japan c nara & tech co. ltd, 37-34, nokbaegil, juksan, anseong, kyonggi 17521, korea ∗ corresponding author: choi.ho@sist.ac.jp abstract. the authors developed two types of block systems consisting only of main block and key block without joint mortar in consideration of seismic performance and workability. two types of block systems have different key block shapes: one is the peanuts shape and the other is the dumbbell shape. in this study, the proposed two types of block walls as well as a typical block wall were experimentally investigated to evaluate the seismic performance. in the tests, full-scale, single-story specimens were tested under static cyclic in-plane loading, and failure patterns and cracks were carefully observed. in this paper, the loading bearing capacity, energy dissipation capacity and reuse ratio of block walls are discussed in detail. as a result, the deformability, energy absorption capacity and reuse ratio of the proposed block systems were considerably higher than those of typical block system. keywords: in-plane, key block, main block, out-of-plane, unreinforced masonry. 1. introduction in some regions of asia, europe, and latin america where earthquakes frequently occur, serious earthquake damage is commonly found resulting from catastrophic building collapse. such damaged buildings often have unreinforced masonry (urm) walls, as shown in figure 1(a). since the seismic performance of the urm wall buildings and infill walls are relatively poor compared to the other structural types, these buildings and walls always damaged when an earthquake occurs. furthermore, in japan which is an earthquake-prone country, the use of urm walls is prohibited by the building standards, but concrete block fences with reinforcing bars are often used. however, the damage of the fences always occurred when the earthquake occurs, as shown in figure 1(b). under these backgrounds, the authors developed two types of block systems consisting only of main block and key block without joint mortar in consideration of seismic performance and workability. two types of block systems have different key block shapes: one is the peanuts shape and the other is the dumbbell shape. in this study, the proposed two types of concrete block walls as well as a typical concrete block wall were experimentally investigated to evaluate the seismic performance. in the tests, full-scale, single-story specimens were tested under static cyclic in-plane loading, and failure patterns and cracks were carefully observed. in this paper, the loading bearing capacity, energy dissipation capacity and reuse ratio of block walls are discussed in detail. 2. experimental program 2.1. developed block systems the authors developed two types of concrete blocks to improve the seismic performance of both inand out-of-plane directions and to enhance the workability without the joint mortar [1]. the two types of concrete blocks only consist of main blocks and key blocks, and they have different key block shapes: one is the peanuts shape and the other is the dumbbell shape, as shown in figure 2. as shown figures 2, the gap between the main block and the key block was designed to be 1.5 mm on all sides in consideration of workability and manufacturing accuracy. as can be found in figure 3, the proposed block systems have half-height difference between the main block and the key block. therefore, the seismic performances in the out-of-plane direction as well as the in-plane direction of the proposed block systems are expected to be much higher than that of typical masonry walls. each concrete block material test results are shown in table 1 (the average values of three samples are shown in the table). the typical concrete block and peanut shape concrete block were made by the normal cement-to-sand ratio of 1 : 3.5 used in korea. since the dumbbell shape concrete block was, however, used higher cement ratio, the compressive strength is the highest value among three concrete blocks, as shown in table 1. the compressive strength of the peanut shape concrete block is higher than that of the typical concrete block, because the peanut shape 85 https://doi.org/10.14311/app.2022.33.0085 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en h. choi, k. w. jin, j.c. jeong et al. acta polytechnica ctu proceedings �d��/¶$txlod�hduwktxdnh��,wdo\�������� � �e��1ruwkhuq�2vdnd�hduwktxdnh��-dsdq�������� figure 1. masonry wall damage. figure 2. two types of concrete block systems (unit: mm). compressive strength [n/mm2]1 typical concrete block peanut shape concrete block dumbbell shape concrete block2 block unit 8.8 14.9 22.0 block prism3 5.8 7.7 18.1 1 to whole area 2 with key blocks 3 a layered specimen table 1. mechanical properties of each concrete block. 86 vol. 33/2022 seismic concrete block walls concrete block has the key blocks in hollow parts of the main block. 2.2. test specimens in this study, three full-scale, single-story specimens were designed and fabricated supposing the singlestory storage building, as shown in figure 4(a) typical concrete block wall specimen with joint mortar (specimen cb); (b) seismic block wall with peanuts shape key block without joint mortar (specimen ps); (c) seismic block wall with dumbbell shape key block without joint mortar (specimen ds). the specimen size is 2.0 m by 1.4 m, as shown in figure 4. the typical concrete block wall can resist only friction force to the horizontal load after losing the adhesive force between concrete block and joint mortar. on the other hand, the proposed concrete block wall systems are expected higher deformability due to the shear key mechanism between the main block and the key block. 2.3. test methods a loading system for the static cyclic in-plane tests is shown in figure 5. lateral loads in the positive and negative directions were applied to the left end of the upper beam with hydraulic actuators. as mentioned above, since the reference building of this study is a single-story storage building, the axial load was considered as the weight of the upper beam (13.7 kn, axial stress, σ0 = 0.04n/mm2). figure 6 shows a lateral loading protocol that was controlled by a drift angle r, defined as a lateral drift ∆ at the top-center of specimen divided by the height from the bottom of the specimen, h, as shown in figure 5. the measurement system is shown in figure 7. the relative lateral displacement, the lateral displacement at each layer of wall, the vertical displacement of both ends of specimen, and diagonal deformation of wall were measured, respectively. furthermore, the maximum crack widths at peak loads and residual crack widths at unloaded stages were carefully measured. 3. experimental results 3.1. failure patterns and lateral force-drift angle relationships figure 8 and 9 show the damage patterns after final loading and the lateral force-drift angle relationships of all specimens, respectively. the behavior of each specimen to failure is summarized below. 3.1.1. specimen cb during the first loading drift, r, of 0.05 % rad., the cracks were observed in the entire bed joint (horizontal joint) causing slippage between second and third joint interface. since the shear crack in the bottom of compression side was occurred during r = 0.67 % rad., the test was terminated at r = 1.0 % rad. until the final loading, this specimen showed the rocking behavior between second and third layers. the maximum strength of 14.7 kn was recorded at r = 0.67 % rad. and no remarkable strength deterioration is found until r = 1.0 % rad. 3.1.2. specimen ps the small width crack was occurred on the main block of the right-bottom end at r = 0.25 % rad. this specimen also showed the rocking behavior until final loading because of low axial force according to the single-story storage building. since the crushing and spalling off of the main block where is the rightmost side of the second layer was occurred at r = 5.0 %, the experiment was terminated after r = 5.0 %. the maximum strength of − 17.5 kn was recorded at r = −2.0 % rad. and no remarkable strength deterioration is found until the final loading. 3.2. comparison of in-plane seismic performance of each specimen 3.2.1. lateral force-drift angle relationships figure 10 shows the skeleton curves of each specimen. as shown in the figure, the in-plane seismic performances of specimens ps and ds were much higher than that of specimen cb. in particular, the deformability of the proposed system has improved remarkably due to the shear key mechanism between the main blocks and the key blocks. since all specimens show the rocking behavior, as mentioned above, the lateral loads of each specimen were calculated considering the simple rocking mechanism, as shown in figure 11. as shown in the figure, the axial loads were considered as the sum of the weight of the upper beam (n1 = 13.7 kn) and the self-weight of the specimen (n2 = 6.0 kn, 9.0 kn and 10.8 kn of specimens cb, ,ps, ds, respectively). the calculated lateral loads were shown in figure 9. as can be found in the figure, the calculated lateral loads agreed well with the experimental results. 3.2.2. equivalent viscous damping ratios in order to compare the energy dissipation capacities of all specimens, the equivalent viscous damping ratios were calculated, as shown in figure 12. the results of the proposed systems considerably had higher values than that of specimen cb. furthermore, the remarkable deterioration of the ratios of the proposed systems were not found until the final loading. 3.2.3. reuse ratios of specimens ps and ds the typical concrete block and brick may not commonly reuse because of the using of the joint mortar. on the other hand, the proposed systems consisted of only main and key blocks without joint mortar can reuse. in this study, the reuse ratio is defined as the ratio of no damage main blocks to all main blocks. figure 13 shows the reuse ratio of specimens ps and 87 h. choi, k. w. jin, j.c. jeong et al. acta polytechnica ctu proceedings � .h\�eorfn 0dlq�eorfn �e��'xpeehoo�vkdsh��d��3hdqxwv�vkdsh� figure 3. construction of the proposed block walls. �d��6shflphq�&%� � �e��6shflphq�36� ��� ���� ���� ��� ���� ���� figure 4. details of typical and proposed concrete block wall specimens (unit: mm). � ^śğăƌ�ŭğlj wƌğǀğŷƚŝžŷ�žĩ� žƶƚͳžĩͳɖůăŷğ� ďğśăǀŝžƌ ��� �������� ����� ����� ����� ����� >ăƚğƌăů�śljěƌăƶůŝđ�ăđƚƶăƚžƌ hɖɖğƌ�ďğăŵ ^ɖğđŝŵğŷ , ' figure 5. loading system (unit: mm). 88 vol. 33/2022 seismic concrete block walls � ��� ��� ��� ��� � �� �� �� �� ���� ������������������������� 7d uj hw �g lv so df hp hq w�� p p � 3hdn�guliw�dqjoh��r���� ������ figure 6. lateral loading protocol. � � >5hdu@ /dwhudo�glvs�9huwlfdo glvs� 9huwlfdo glvs� 'ldjrqdo� grirup� /dwhudo glvs� ri �hdfk� od\hu figure 7. measurement system. �d��6shflphq�&%� �e��6shflphq�36� �f��6shflphq�'6� figure 8. final crack patterns (blue: positive dir., red: negative dir., black: initial cracks). 89 h. choi, k. w. jin, j.c. jeong et al. acta polytechnica ctu proceedings � � �� �� �� �� �� �� � � � � � � � ��� ��� ��� ��� �� � � �� �� �� �� �d��6shflphq�&% /d wh ud o�i ru fh ��n 1 � 'uliw�dqjoh���� �&dofxodwlq�uhvxow �� �� �� �� �� �� � � � � � � � ��� ��� ��� ��� �� � � �� �� �� �� /d wh ud o�i ru fh ��n 1 � 'uliw�dqjoh���� �&dofxodwlq�uhvxow�e��6shflphq�36 �� �� �� �� �� �� � � � � � � � ��� ��� ��� ��� �� � � �� �� �� �� /d wh ud o�i ru fh ��n 1 � 'uliw�dqjoh���� �&dofxodwlq�uhvxow�f��6shflphq�'6 figure 9. lateral force-drift angle relationships of each specimen. �� �� �� �� �� �� � � � � � � � ��� ��� ��� ��� �� � � �� �� �� �� /d wh ud o�i ru fh ��n 1 � 'uliw�dqjoh���� �6shflphq�&% �6shflphq�36 �6shflphq�'6 figure 10. skeleton curves of each specimen. �� �� �� �� �� �� � � � � � � � n� �����n1 �8sshu�ehdp� p n� ���������������n1� ri�vhoi�zhljkwv�ri� 6shflphqv�&%��36�� '6��uhvshfwlyho\ ����� �� �� � 5rfnlqj srlqw figure 11. lateral load p due to rocking mechanism. � ��� ��� ��� ��� ��� ��� ��� ��� ��� ���� ���� ���� ���� ���� ���� ���� ���� (t xl yd oh qw �y lv fr xv �g dp sl qj �ud wlr ��h eq 'uliw�dqjoh���� �6shflphq�&% �6shflphq�36 �6shflphq�'6 figure 12. equivalent viscous damping ratios. � � � � � � � �� �� �� �� ��� 5 hx vh �ud wlr 'uliw�dqjoh���� �6shflphq�36 �6shflphq�'6 figure 13. reuse ratios of the main blocks of specimens ps and ds. 90 vol. 33/2022 seismic concrete block walls ds. as can be found in the figure, the proposed seismic block systems can reuse more than 70 % after r = 5.0 % rad. this result imply that the proposed concrete block wall systems are economical and ecofriendly. 4. concluding remarks the current paper presented the experimental tests of the two types of new concrete block walls as well as a typical block wall and investigated the in-plane behaviour, the loading bearing capacity, energy dissipation capacity and reuse ratio. the following major findings were obtained: 1. the in-plane seismic performances of specimens ps and ds were much higher than that of specimen cb. in particular, the deformability of the proposed system has improved remarkably due to the shear key mechanism between the main blocks and the key blocks. 2. the calculated lateral loads based on the simple rocking mechanism agreed well with the experimental results. 3. the energy dissipation capacities of the proposed systems considerably had higher values than that of specimen cb. furthermore, the remarkable deterioration of the ratios of the proposed systems were not found until the final loading. 4. the proposed seismic block systems can reuse more than 70 % after final loading. this result imply that the proposed concrete block wall systems are economical and eco-friendly. the current paper focused only on the in-plane experimental behavior. the experimental data should be investigated from numerical perspectives, and the out-of-plane experimental and numerical behaviours should be carried out, in future studies. acknowledgements the project was funded by the jsps grant-in-aid for scientific research (category (c), grant no. 18k04425, principal investigator: ho choi) and the joint research with nara & tech co. ltd. the authors are thankful to both agency and company for their financial support. references [1] h. choi, k. w. jin, j. c. jeong, et al. feasibility study on seismic block without joint mortar, part1 shape determination using finite element analysis, summaries of technical papers of annual meeting, architectural institute of japan 4:977-978, 2019. 91 https://doi.org/10.14311/app.2022.33.0437 acta polytechnica ctu proceedings 33:437–443, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague influence of steel fibers on the fatigue behavior of high-performance concretes under cyclic loading david ov∗, rolf breitenbücher ruhr university bochum, department of civil and environmental engineering, institute for building materials, universitätsstrasse 150, 44801 bochum, germany ∗ corresponding author: david.ov@ruhr-uni-bochum.de abstract. due to the advancement of high-performance concretes, the development of filigree constructions has been improved in the last decades. however, as the demand to create more filigree designs increases, the vulnerability to fatigue loads of such structures has also become a decisive factor. various construction projects, such as wide-span bridges or wind turbines, are exposed to fatigue loads. especially wind turbines are permanently subjected to wind and wave loads of several hundred million load cycles during their service life. at present, the fatigue behavior of high-performance concretes under cyclic loading is still unknown. in a worst-case scenario, the significantly lower ductility can lead to a sudden failure of the entire structure. in this case, the addition of steel fibers could be advantageous, as they significantly improve the ductility of concretes. however, it is still undetermined how the material fatigue is influenced by steel fibers. hence, systematic investigations on the fatigue behavior of various high-strength concretes with steel fibers were conducted. since the crack-bridging effect of fibers is relevant for tensile stresses, predominantly cyclic bending tests were performed on concrete beams with different steel fiber variations. to accomplish the investigations, a test setup has been developed which allows the simultaneous testing of a total of six specimens. based on the predetermined static concrete strengths, the specimens were subjected to cyclic loads with a defined lower stress level and various upper stress levels. during these cyclic tests, the cycles-to-failure as well as the degradation within the microstructure were detected. keywords: cyclic loading, degradation, fatigue behavior, high strength concrete, steel fibers. 1. introduction with the increasing appearance of filigree designed structures, the necessity to investigate the fatigue behaviour of materials has become more significant in the past decades. since the beginning of the 20th century, railway bridges with large spans have been constructed and the installation of e.g. offshore facilities followed. many of those structures are exposed to fatigue stresses during their service life. figure 1 provides an overview of various structures, which are subdivided in accordance with the numbers of load cycles during their lifetime [1]. particularly wind turbines are subjected to wind loads of up to n = 109 load cycles, which endangers the foundation and tower construction as a result of material fatigue. due to the further development of highperformance concretes, it is possible to produce the tower construction of wind turbines in reinforced and prestressed concrete. however, an essential difference between high-performance concretes and normal concretes is their distinctly more brittle material behavior or significantly lower ductility. considering the foundation of wind turbines, not only compressive forces but also tensile forces occur in the anchoring area. this may result in a sudden failure of the structure under permanent fatigue loading. in this context, the addition of steel fibers could be advantageous, as they significantly improve the ductility and the post-cracking behavior by a crackbridging effect in the concrete. however, the fatigue behavior of steel fiber-reinforced high-performance concrete (sfrhpc) under fatigue stress is still unexplored. in general, the overall test series described in the literature are limited to the fatigue behavior of normal steel fiber reinforced concrete under cyclic stress up to about n = 106 load cycles. for particularly high-performance concretes, the fatigue behavior has only been investigated occasionally so far, see e.g. [2] and [3]. an effective increase in the number of cycles-to-failure could clearly not be determined for sfrhpc, however, the concretes revealed a much more ductile elongation behavior, resulting in a prolonged phase of unstable crack growth [2]. as a result, these concretes showed better failure announcement behavior. in addition to the fiber content, the effectiveness of steel fibers is largely determined by the geometry and length of the fibers [4]. therefore, the aim of this research study is to investigate the extent to which the addition of steel fibers influences the fatigue behavior of high-strength concretes. 437 https://doi.org/10.14311/app.2022.33.0437 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en david ov, rolf breitenbücher acta polytechnica ctu proceedings figure 1. number of load cycles of different cyclically stressed structures [1]. components c80 c120 binder/sand mixture cem i 52.5 r cem i 52.5 r aggregate 2/8 mm quartz gravel quartz gravel aggregate 8/16 mm limestone quartz gravel superplasticizer pce based pce based w/c-ratio (−) 0.47 0.35 table 1. concrete compositions of c80 and c120. steel fiber series shape slength l (mm) diameter d (mm) aspect ratio l/d (−) tensilestrength ft (mpa) sf-a straight 6 0.16 37.5 2,600 sf-b straight 13 0.20 65.0 2,600 sf-c (mix) straight / hooked end 6 / 35 0.16 / 0.55 37.5 / 65.0 2,600 / 1,345 table 2. properties of the steel fiber series. 2. investigation program 2.1. concrete compositions as a basis for the following studies in this paper, particularly two high-performance concretes with compressive strengths of 80 mpa (c80) and 120 mpa (c120) were included in the experimental investigations. in addition to their high-strength properties, both concretes also have self-compacting properties, as they show a self-levelling and self-venting behavior after mixing. in principle, the composition of the two concretes were defined by an industrial partner in the form of a specified binder/sand mixture with a maximum aggregate size of 16 mm. the concrete compositions of c80 and c120 are listed in table 1. 2.2. workability with steel fibers the first part of the experimental investigations was to examine the workability of the fresh concrete due to the addition of steel fibers and to define a suitable maximum fiber dosage. for this purpose and in order to determine the influence of the steel fibers, different fiber variations (fiber geometry, tensile strength and aspect ratio) were specified. in this case, the selected variations are a high-strength micro-steel fiber with 6 mm length (sf-a series), an another high-strength micro-steel fiber with 13 mm length (sf-b series) and a fiber mix of 50 % micro-steel and 50 % macro-steel fibers (sf-c series). the characteristic properties of the different steel fiber series are listed in table 2. in order to assess the change in workability of the high-strength concretes with especially selfcompacting properties, the slump flow and the t500time were determined according to en 12350-8 [5]. during the examination, the mixtures of both concretes were not modified in general, e.g. the cement paste volume was not adjusted due to the addition of steel fibers. only the superplasticizer content was adjusted to control the consistency and processing of 438 vol. 33/2022 fatigue behavior of high-performance concretes figure 2. t500-time (left) and slump flow (right) depending on the fiber variations and content. figure 3. flexural strength of c120 depending on the test age and preliminary storage. the fresh concrete. the results of the t500-time and slump flow tests, determined with the concrete c120 on different fiber dosages, are shown in figure 2. as can be seen from figure 2, an affection of the fresh concrete properties can be detected with higher steel fiber content, since the tests results demonstrates a decreasing slump flow value and thus an increasing t500-time. at a steel fiber content of 1.5 vol.%, occasional agglomerations could be observed in the fresh concrete regardless of the type of steel fibers. by slightly increasing the superplasticizer dosage, it was also possible to improve the slump flow of all mixtures with 1.0 vol.-% fiber dosage to approx. 600630 mm without any signs of segregation. therefore, a maximum steel fiber content of 1.0 vol.-% was defined for the following investigations. 2.3. flexural strength prior to the cyclic bending tests, it is necessary to examine the static strength of the concretes and thereby to be able to define the upcoming stress levels. thus, the flexural strengths of concrete beams (700 × 150 × 150mm3) were determined according to the dafstb german guideline "steel fibre reinforced concrete" [6] in a four-point flexural setup. in order to also take into account the influence of the prestorage conditions, the test series were stored in a preliminary study as follows: on the one hand, the specimens were stored under water according to en 12390-2/a20 [7] until the time of testing and, on the other hand, stored in a dry environment (20 ◦c, 65 % rh) after the 6th day of water. for this purpose, the fibre-free reference concrete (c120) and the steel fibre-modified concrete with sf-a (c120) were initially tested, see figure 3. at first glance, it can be stated that no significant increase in flexural strength due to post-curing was observed as a result of a later test period (56d). by adding the steel fibers (sf − awet), an increase in flexural strength of about 14 % could be achieved compared to the fiber-free samples (refwet). in the case of the dry-stored samples refdry, the flexural strength was measured to be about 46 % lower than refwet due to the resulting shrinkage stresses during dry storage. for refdry compared to sf − awet, however, only a reduction of the flexural strength of roughly 23 % was found, indicating a possible influence of the steel fibers on the shrinkage behavior or on the residual stresses. in order to prevent the shrinkage stresses, the flexural strengths of the two concrete strength classes were subsequently investigated as dry stored specimens. figure 4 shows the average flexural strength values of the concretes c80 (left) and c120 (right), 439 david ov, rolf breitenbücher acta polytechnica ctu proceedings figure 4. average flexural strength of c120 and c80 depending on the different fiber variants. figure 5. test setup for the simultaneous loading of six specimens. subdivided according to the specified fiber variations (ref, sf-a, sf-b and sf-c). the test specimens were also tested at concrete ages of 28d and 56d. in terms of the observed test results, it can be stated that the development of the flexural strength also does not change significantly over time. accordingly, it can be seen in each test series that the flexural strengths between 28d and 56d remain dominantly at the same level. the effect of adding fibers is again very noticeable from the results between ref and with steel fibers (sf-a, sf-b and sf-c). 2.4. cyclic bending test 2.4.1. test setup in order to conduct the cyclic bending tests, an innovative test setup has been developed which enables the cyclic loading of six prismatic specimens (700×150×150mm3) at the same time and therefore substantially shortens the generally long procedure of fatigue tests. in principle, the test setup is hydraulically operated and the frame is designed in a way that allows three specimens to be placed on each side (see figure 5). a main hydraulic cylinder leads the pressure into the system, where it is distributed to each of the six individual test positions. this method ensures a reliable reproducibility, as all six specimens are subjected to the same pressure and force simultaneously. at each individual test positions there are two smaller modified hydraulic cylinders, which can transfer a pre-defined pressure of up to 200 bar into the specimens with a frequency of 5 hz (see figure 6, left). all concrete beams were subjected to the cyclic stresses at a minimum age of 56 days and in a fourpoint-flexural setup, so that the maximum bending tensile stress is located on the upper side of the beam. accordingly, when the concrete reaches the number of cycles-to-failure, the cracking occurs on the upper side of the specimen (see figure 6, right). 2.5. degradation in addition to the determination of the number of cycles-to-failure, the stiffness development of the concretes was also monitored during the fatigue tests. especially by examining the stiffness, a progressive degradation of the test specimens can be visualized 440 vol. 33/2022 fatigue behavior of high-performance concretes figure 6. display of max. bending stress in the test specimen (left) and an example of crack (right). figure 7. schematic test arrangement for us (left) and course of stiffness development (right [8]). [9]. this degradation within the concrete structure caused by cyclic bending stresses was measured after specified numbers of load cycles by means of comparative ultrasonic runtime measurements (longitudinal waves) in the area of maximum bending stresses (figure 7, left). for this purpose, a suitable ultrasonic measuring device was used. based on the ultrasonic runtime measurements, the dynamic modulus of elasticity can be determined. a continuous reduction in the relative dynamic emodulus over the loading periods indicates an increasing number of microcracks as well as other microstructural changes in the concrete structure [9]. if the determined stiffnesses is plotted against the number of load cycles or against the related number of load cycles, an s-shaped curve is formed that can basically be divided into three phases (see figure 7, right). 2.5.1. results for the fatigue tests under cyclic bending stress, preliminary studies on the influence of the pre-storage (between wet and dry) were also required and thereby conducted. in that case, three test specimens without steel fibers (ref) as well as three test specimens with steel fibers (sf-a) were again included and tested with up to n = 107 load cycles at a frequency of 5 hz. the setting of the upper and lower stresses based on the results of the static flexural strength tests and was determined as follows: • upper stress σo = 0.60 · fct, fl • lower stress σu = 0.30 · fct, fl figure 8 shows the courses of the rel. dynamic emodulus for the specimens without steel fibers (ref) and with steel fibers (sf-a) of concrete c120. following the developments of the rel. dyn. emodulus, a significant degradation already occurs in the early load cycles. as expected, the refwet specimens already failed after roughly 300,000 load cycles due to the shrinkage stresses superimposed by cyclic loading. however, a failure of the sf-a specimens did not occur even after 10 million load cycles. but a continuous decrease in the rel. dynamic e-modulus was notable. in a first phase up to about 500,000 load cycles, a relatively steep decrease in the rel. dynamic e-modulus is caused by the initiation of microcracks. this is followed by a flatter decline of the rel. dynamic e-modulus. when comparing the wet and dry stored test series, significant differences can be observed. while the rel. dyn. e-modulus of the wet-stored sf-a test specimens is around 76.5 % after 10 million load cycles, the rel. dyn. e-modulus of the dry-stored sf-a test specimens is around 89.7 %. one of the reasons for the early failure of the refwet test specimens is the drying out and the associated additional shrinkage stresses as these series had been stored in water until the test day. under consideration of the results of the preliminary studies, the following test series are stored solely in a dry environment in order to reduce the dry441 david ov, rolf breitenbücher acta polytechnica ctu proceedings figure 8. decrease of the rel. dyn. e-modulus of ref (left) and sf-a (right). figure 9. decrease of the rel. dyn. e-modulus of sf-a, sf-b and sf-c at various upper stresses. ing effects (shrinkage stresses). subsequently, the concrete c120 was further examined (up to n = 2.5 · 107), taken into account the influence of different fiber variations and additionally various stresses σo = 0.60, 0.70 and σu = 0.30 (figure 9). the results with an upper load of 0.70 are interim results, which are currently recorded up to 12.5 million load cycles. according to the diagrams it can be observed, that the higher the load stress, the greater the drop in the relative dynamic modulus of elasticity becomes. after 100,000 load cycles, a drop to 95.3 % (sf-a), 94.7 % (sf-b) and 97.2 % (sf-c) was observed at a load level of σo/fct, fl = 0.60, whereas a significantly higher drop to approx. 92.4 % (sfa), 91.3 % (sf-b) and 93.7 % (sf-c) was already noticed at σo/fct, fl = 0.70. all test samples have passed through up to 12.5 and 25 million load cycles without failure. regardless of the load level, the loss of stiffness is particularly evident within the first few load cycles. furthermore, it can be seen that the relative dynamic e-modulus of all test specimens decreases further with increasing number of load cycles, as expected. the falling curves show the typical s-shaped course of fatigue tests. 3. conclusion in order to study the influence of steel fibers on the fatigue behavior of high-performance concretes, predominantly cyclic bending tests on concrete beams (700 × 150 × 150mm3) with different steel fiber variations (sf-a, sf-b and sf-c) were conducted. during these cyclic tests (up to n = 2.5 ·107), the cyclesto-failure as well as the degradation within the microstructure were monitored. the presented test results reveal a significant influence of the cyclic bending tests (n = 107) on the effectiveness of the steel fiber (sf-a) when compared with a fiber-free concrete (ref). as the the number of load cycles increased, a steady drop in the dynamic e-modulus could be detected without the steel fiber-modified test specimens failing in contrast to the fiber-free test specimens (refwet failed after 300,000 load cycles). the steel fiber modified test samples were all able to pass through up to 12.5 and 25 million load cycles without any failures. for the following studies, further experimental investigations on series with more fiber variants are planned. moreover, microscopic examinations are planned to examine the microstructure in case of degradation. additionally, a comparison of the fatigue behavior between high-strength and normal-strength steel fiber reinforced concretes is intended. acknowledgements the research presented in this paper is part of the joint project winconfat (project no. 0324016d) funded by the german federal ministry for economic affairs and energy (bmwi). the authors are grateful for their financial support. 442 vol. 33/2022 fatigue behavior of high-performance concretes references [1] j. hegger, t. roggendorf, c. goralski, et al. ermüdungsverhalten von beton unter zyklischer beanspruchung aus dem betrieb von windkraftanlagen. abschlussbericht, fraunhofer irb-verlag stuttgart 3305, 2014. [2] l. lohaus, n. oneschkow, k. elsmeier, et al. ermüdungsverhalten von hochleistungsbetonen in windenergieanlagen. bautechnik 89(8):533-41, 2012. https://doi.org/10.1002/bate.201200024. [3] e. fehling, m. schmidt, t. teichmann, et al. entwicklung, dauerhaftigkeit und berechnung ultrahochfester betone (uhpc).schriftenreihe baustoffe und massivbau structural materials and engineering series 3, kassel university press, 2005. [4] p. b. cachim, j. a. figueiras, p. a. a. pereira. fatigue behavior of fiber-reinforced concrete in compression. cement and concrete composites 24(2):211-7, 2002. https://doi.org/10.1016/s0958-9465(01)00019-1. [5] en 12350-8 2010. testing fresh concrete part 8: self-compacting concrete slump-flow test, 2010. [6] dafstb. dafstb guidline steel fibre reinforced concrete, deutscher ausschuss für stahlbeton e. v., 2012. [7] en 12390-2/a20 2015. testing hardened concrete part 2: making and curing specimens for strength tests; amendment a20, 2015. [8] n. oneschkow. analyse des ermüdungsverhaltens von beton anhand der dehnungsentwicklung, hannover: leibniz university hannover, ph.d. thesis, 2014. [9] r. przondziono, j. j. timothy, m. nguyen, et al. vorschädigungen in beton infolge zyklischer beanspruchungen und deren auswirkung auf transportprozesse im hinblick auf eine schädigende akr. betonund stahlbetonbau 110(1):3-12, 2015. https://doi.org/10.1002/best.201400095. 443 https://doi.org/10.1002/bate.201200024 https://doi.org/10.1016/s0958-9465(01)00019-1 https://doi.org/10.1002/best.201400095 https://doi.org/10.14311/app.2022.33.0188 acta polytechnica ctu proceedings 33:188–192, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague benefits of pervious concrete for urbanized habitats giuseppe giunti∗, daniel miczek zapa beton a. s., vídeňská 495, 142 00 praha 4, czech republic ∗ corresponding author: giuseppe.giunti@zapa.cz abstract. pervious (draining) concrete is a new generation concrete which, for its structure, allows rainwater to enter the subsoil and at the same time is determining the final surface without the need for any additional layer. pervious concrete allows to drain off up to 95 % of precipitation, which represents a significant contribution to ecology and sustainability. therefore, its use is particularly suitable in city parks, bike paths, in the vicinity of apartment buildings, swimming pools and parking lots. rainwater is not purposelessly be drained by the city sewer system, but it is absorbed directly in place and contributing to the environmental improvement of the urban habitat (replenishment of groundwater supplies, water for roots of plants and trees, reduction of the thermal island of the city, etc.). unlike prefabricated paving blocks (interlocking and flat paving), pervious concrete makes it possible to realize structured and complex shapes of finished surfaces without the need for laborious cutting, waste, dust and with generally faster laying. this paper summarizes the current achievements within the buzzi unicem group and the use of pervious concrete in the czech and slovak republic. keywords: environment, pervious, ready mix, water. 1. introduction continuous expansion of impervious materials in construction (as asphalt and concrete) and agricultural cultivation prevent rain water infiltration into the soil. the logical outcome is the decline of the level of underground water. from the natural ecosystem prevails evaporation of water (72 %) over the drain and infiltration, while in the case of urban agglomerations is the opposite, thus draining is prevalent (54 %) upon evaporation and infiltration of water. there is a need to intensify efforts for the purpose of creating systems of water management in the civil engineering. and one of the instruments is pervious concrete, which gives a possibility not to transport water into sewage system, but to soak in a given place and environmentally contributes to the improve the urban environment (refill reserves of underground water, water for the roots of plants and trees, reduce the heat of city islands, etc. 2. pervious concrete pervious concrete has been used in different variants since the 1980s, mainly in the usa, japan and the nordic countries (finland). pervious concrete has so far been successfully applied mainly on less frequented roads, car parks, sidewalks, courtyards, access roads, etc. [1, 2]. in the czech republic, the production of pervious concrete is still in its infancy, including valid legislation, standards and technical regulations. currently, the american test method astm c 1701, which solves and describes the rate of water infiltration into permeable concrete, is used to test the drainage ability of concrete. zapa drop is one of the pervious concrete material in czech republic that can be used not only on the base layers, but mainly on the final surface layers, also in various color variants using mineral pigments, with natural or crushed aggregates. in technical terminology, it is necessary to distinguish between pervious concrete and drainage concrete. drainage concrete is used mainly as a simple base layer part of the structure, pervious concrete instead, must be considered much more important. it is not just one layer, but multilayer structure with high capability to drain water. to reach all these expected performances we need to observe the technological principles of laying or keeping thicknesses of the embankment of the underlying layers. thus, porosity values are therefore essential for the liquid to pass through this pervious concrete. the effective void content for a sufficient water permeability is between 15 30 %. the gaps in the structure of concrete have an average diameter from 1 to 8 mm. the volumetric weight of pervious concrete, which is normally approximately 70 % compared to conventional concrete, is closely connected to its porosity. the water flow through pervious concrete is usually about 0.34 cm/s, which is 200 l/m2/min (but in some cases much more, depending on the maximus diameter of aggregate used). 3. raw materials production of pervious concrete is obtained with the same raw materials used in ordinary concrete, but with the exclusion of fine aggregates. the grading 188 https://doi.org/10.14311/app.2022.33.0188 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 33/2022 benefits of pervious concrete for urbanized habitats of aggregates has a significant effect on the hardened pervious concrete strength results [3]. the grading of coarse aggregates is calculated very precisely in order to improve the physical and mechanical parameters. like in ordinary concrete, portland cement or blended cements as well, can be used as a binder in a pervious concrete composition. accurate dosing of water and cement is used to form a cementitious paste which perfectly envelops the coarse aggregates, creating a system of interconnected cavities which are highly water permeable. normally in pervious concrete, we use retardants and vma stabilizers chemical admixtures. 4. application and use of pervious concrete to design pervious concrete structures for parking lots or roads it is necessary to avoid the use any kind of absorbing and frosty material. pervious concrete zapa drop (which is the commercial name of zapa beton portfolio) can be delivered to construction sites by standard open dumpers or with concrete truck mixers. pervious concrete is produced in s1 consistency (according to en 12350-2 testing of fresh concrete part 2: slump test) and for its laying a finisher machine is used. excessive compaction of pervious concrete results in higher compressive strength but lower porosity (and thus lower permeability) [4]. the base layer must be capable to drain off the infiltrated rain water (in some cases drainage pipes is desirable). a pervious concrete is designed with a low watercement ratio [5], therefore, in order to allow full hydration of the cement, it is necessary to protect its surface covering it with nylon foils or paraffin spray. it is strongly suggested to avoid excessive drying of the structure surface for at least a minimum period of 48 hours after the laying and follow standard procedures of curing. pervious concrete naturally helps water to reach ground water reserve, contributes to the growth of surrounding vegetation and contributes to decrease contaminants such as hydrocarbons which, on the contrary, are present on asphalt surfaces [6]. the porous surface of pervious concrete can significantly reduce the effect of the urban thermal island, which shows noticeably higher summer temperatures than its surroundings. the cooling effect is mainly due to the absorption of huge evaporative heat when moisture evaporates under the porous concrete surface. in addition roads casted with pervious concrete demonstrate their ability to increase surface friction and thereby reduce the risk of aquaplaning and tires noise up to 8 decibels [7]. 5. benefits of using pervious concrete • no limitation of shape, no waste of trimming such as with interlocking pavement; • open possibility to choose any color and size of used aggregate; • significantly higher infiltration ratio than interlocking pavers; • fast drainage of large amounts of water; • less susceptibility to icing due to drainage of surface water; • usage of natural aggregates pleasant for barefoot, ideal for swimming pools, terraces and gardens; • use of crushed aggregates better anti-slip properties, ideal for roads; • minimal shrinkage which eliminates the use of dilatation joints. 6. experimental setting of pervious concrete the aim of the experiment was to verify the properties of pervious concrete, including the its production, transportation and feeding of finisher at the jobsite. as well the time of fresh concrete setting and verifying properties of hardened pervious concrete. during this experiment we tested pervious concrete with different size of aggregates: dmax = 8 mm and dmax = 16 mm. during the casting of zapa drop concrete samples were taken to determine: • density of concrete in fresh and hardened state according to en 12350-6 testing fresh concrete part 6: density and according to en 12390-7 testing hardened concrete part 7: density of hardened concrete; • concrete strength according to en 12390-3 testing hardened concrete part 3: strength of test specimens (test specimens of dimensions 150×150×150 mm; • flexural tensile strength according to čsn en 12390-5 testing hardened concrete part 5: tensile bending strength of test specimens (test specimens of dimensions 100 × 100 × 400 mm); • frost resistance according to čsn 73 1322 determination of frost resistance of concrete (test specimens of dimensions 100 × 100 × 400 mm; • resistance to chemical and de-icing substances according to čsn 731326 -z1, determination of resistance of cement concrete surface to water and chemical de-icing substances (test specimens of dimensions 150 × 150 × 150 mm). 189 giuseppe giunti, daniel miczek acta polytechnica ctu proceedings figure 1. subsoil composition. figure 2. finisher machine. after 24 hours all specimens were demolded and stored in laboratory at 20 ± 5 ◦c and rh 40 to 60 % for 28 days. fresh concrete density with dmax = 8 mm was 2150 kg/m3, while with dmax = 16 mm was 2080 kg/m3. hardened concrete density with dmax = 8 mm was lower than 0.3 % (compare to the fresh density), while with dmax = 16 mm did not differ. compressive strength with dmax = 8 mm reached 38.4 mpa, while with dmax = 16 mm the compressive strength was lower then 30%, due to bigger size of air pores. flexural tensile strength of concrete with dmax = 8 mm was 3.9 mpa, while with dmax = 16 mm was recorded a value lower that 30%, in line what recorded with the compressive strength. test of resistance to chemical de-icing agents appears to be highly problematic due to the porous structure of the concrete surface. so it is necessary to continue the research and define new methodologies to perform such test. the subsoil under pervious concrete was sufficiently compacted by means of a cylinder with subsoil composition, see figure 1. 1. pervious concrete dmax = 8 mm, thickness 150 mm. 2. aggregate crushed dmax = 16 mm, thickness 150 mm. 3. compacted soil. for better stability and water retention it is necessary to have below the pervious concrete a compacted crushed stone layer. in case of greater demand of water retention capacity it is necessary to design a layer thicker than 200 or 300 mm. after compaction, the deformation modulus edef2 of substrate should be more than 50 mpa but to ensure adequate stability deformation modulus edef2 of the substrate must be bigger than 30 mpa. this machine work by pouring material into the socalled basket. at the bottom of the basket are usually 190 vol. 33/2022 benefits of pervious concrete for urbanized habitats figure 3. laying down of pervious concrete. figure 4. detail of pervious concrete surface with dmax = 8 mm. two belts in the shape of a ladder, which convey the material in front of the smoothing bar (iron). with the transverse helix this material reaches the entire span of the screed. then the material is placed between the substrate and the screed, which ensure the primary compaction of the material an even distribution of the material on the substrate. the working span of the machine is always adjustable thanks to the telescopic smoothing bar [8]. pervious concrete placement seems to be very suitable with a finisher machine. the finisher machine smoothly places concrete in plane without using vibration. the final surface was then rolled without the use of trowels in order not to close the porous surface of the pervious concrete. finished concrete surface has been treated with a film-forming cmd set against evaporation to reduce the loss of moisture during the process of maturation. this material forms a thin film against evaporation, which protects the concrete surface from rapid drying due to sun and wind and prevents the formation of micro-cracks due to plastic shrinkage. figure 5. detail of pervious concrete surface with dmax = 16 mm. 7. conclusion all the monitored parameters of concrete in fresh and hardened condition, show the most suitable to use the 4−8 mm aggregate fraction, also due to the fact that the resulting surface had the best walking and "traffic" properties and is therefore suitable for use such as bike paths, sidewalks, park paths, etc. as part of the laying of water-permeable concrete, the use of a paver with subsequent rolling of the concrete surface has proved most successful. this is closely related to the consistency of the concrete mix. the more fluid the mixture is, the better the physical and mechanical parameters, but there is a lower the water permeability of the concrete. therefore, a moist mixture in the consistency s1 with a water cement ratio of 0.2−0.3 seems to be the most suitable. due to the low water content, shrinkage is eliminated and thus the formation of cracks in the cement matrix. further research will monitor the use of plasticizers to reduce the amount of recipe water, as well as the use of air-entrained agents to improve frost resistance and monitor the effects on compressive strength. further 191 giuseppe giunti, daniel miczek acta polytechnica ctu proceedings measurements and research will take place on the laid surface made of water-permeable concrete, followed by tests of the water-permeability of concrete over time and under different climatic conditions. next monitored parameter was the mixing time of fresh concrete in the agitator that for the production of this type of concrete it is suitable to set a longer mixing time for homogeneous mixing of the concrete, due to the small amount of water. a longer mixing time is also necessary for sufficient coating of the aggregate with cement paste, which also affects the resulting physical and mechanical parameters of the hardened concrete. water-permeable concrete with a correctly designed composition of the formation has a positive effect on ecology and the environment only by supplementing the amount of groundwater thanks to its water permeability. furthermore, thanks to its properties, it improves safety for vehicles and pedestrians, especially in the winter months, when there will be no water on the surface, thus eliminating skidding of vehicles or slipping of pedestrians. one undisputed advantage of water-permeable concretes over the use of asphalt should be mentioned, and that is a light gray color that reflects more light from the surface, so there is no need for such strong artificial lighting around water-permeable concrete, which has a double effect on energy savings. the development of water-permeable concrete in the country is still at the beginning. although research is progressing relatively quickly, many problematic issues related to this material still need to be addressed, so that it can be used in different climatic conditions and can successfully face strong competition from traditional concrete or the use of asphalt. references [1] report on pervious concrete. american concrete institute. 2010. isbn 9780870313646. report no. 522r-10. [2] spragues’ ready mix concrete, pervious ready mix concrete, 2015. srmconcrete.com. [3] j. t. kevern, v. r. schaefer, k. wang. mixture proportion development and performance evaluation of pervious concrete for overlay applications. aci materials journal 108(4), 2011. https://doi.org/10.14359/51683117. [4] m. t. suleiman, j. kevern, v. r. schaefer, et al. effect of compaction energy on pervious concrete properties. concrete technology forum-focus on pervious concrete, national ready mix concrete association, 2006. [5] d. desai. pervious concrete effect of material proportions on porosity. civil engineering portal, 2011. https: //www.engineeringcivil.com/pervious-concrete-e ffect-of-material-proportions-on-porosity.html. [6] e. ashley. using pervious concrete to achieve leed points (pdf). national ready mixed concrete association, 2008. https://www.perviouspavement.o rg/downloads/leed_pointsashley.pdf. [7] epa. reducing urban heat islands: compendium of strategies, 2005. https://www.epa.gov/sites/production/files /2014-08/documents/coolpavescompendium_ch5.pdf. [8] finišer, 2017. https://cs.wikipedia.org/wiki/finišer. 192 srmconcrete.com https://doi.org/10.14359/51683117 https://www.engineeringcivil.com/pervious-concrete-effect-of-material-proportions-on-porosity.html https://www.perviouspavement.org/downloads/leed_pointsashley.pdf https://www.epa.gov/sites/production/files/2014-08/documents/coolpavescompendium_ch5.pdf https://doi.org/10.14311/app.2022.33.0463 acta polytechnica ctu proceedings 33:463–466, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague parapet structure from special shaped trc blocks jiří pazderka, petr hájek, michal nývlt∗, hana žáková czech technical university in prague, faculty of civil engineering, thákurova 7 166 29 prague, czech republic ∗ corresponding author: michal.nyvlt@fsv.cvut.cz abstract. the paper describes innovative parapet solution based on special shaped blocks made form textilereinforced concrete trc. the blocks are designed for a placement at the parapet of building with a flat roof. the solution is appropriate for buildings with precast-concrete bearing structure by following the principle of precast technology (short construction time, minimal technological pauses). however, presented parapet trc blocks can be used for energy efficient buildings as well in combination with thick layers of thermal insulation. the technical solution was registered as utility model no. cz27642(u1). keywords: concrete, parapet, prefabrication, roof, trc. 1. introduction nowadays, the emphasis is on energy savings in the field of building construction. the solution of building envelope details is important for contemporary design of buildings. an important part of this issue is the solution of the detail of flat roofs of buildings in the form of a low parapet wall. for buildings whose load-bearing structure consists of precast frame construction system, the parapet wall design is an important part of the overall design. the main advantage of precast technology is the minimization of wet processes on site which resulting to short construction time. in order to minimize the wet processes in precast construction systems, it is desirable that the parapet wall structure should also be prefabricated. however, this assumption is often not fulfilled, because at the present time the construction of the parapet wall in precast frame systems is commonly solved in the form of a masonry wall from various materials. in some cases, the parapet wall is in the form of a specially modified beam which combines the function of supporting the ceiling structure (specially adapted bearing girder) with the function of the parapet wall. however, such prefabricates usually represent a distinct architectural element (not suitable for most buildings) and at the same time almost do not consider the problem of linear thermal bridges (it cannot be used for modern energy efficient buildings). however, the requirement to minimize thermal bridges is one of the key parameters of the design of building envelope structures in today’s building construction. the solution currently used for the construction of a parapet wall on a flat roof in the form of aămasonry wall (mentioned above), presents several disadvantages. this is especially addition of another wet process into site construction which resulting to reduction of efficiency and speed of construction (not exploiting the potential of precast technology). another significant problem is the risk of deformation of the masonry parapet wall (displacement due to shear stress in masonry) over time, which is often associated with aăfailure of the roof waterproofing layer. the parapet made in the form of aăspecially modified beam (specially adapted bearing girder) is a more suitable solution in terms of in situ efficiency but these elements are always part of standardized construction systems with predefined dimensions (the possibilities of using are limited due to architectural design). another problem may be the high weight of the parapet bearing beam and the resulting complications in terms of handling the precast. 2. parapet structure from special shaped trc blocks the above-mentioned disadvantages of existing parapet structures of flat roofs of buildings based on precast frame system (masonry and parapet bearing beams) are largely eliminated using specially shaped parapet blocks made of textile reinforcement concrete (trc) [1]. the essence of the parapet block is that it is made of precast reinforced concrete with an optimized (lightweight) shape solution with aăcrosssection in the shape of the letter "l". the vertical wall of the parapet block is formed by a thin plate (30ămm thick), which is reinforced by reinforcing ribs (50×100 mm) at distances of 0.7 − 0.8 m. the stiffening ribs comprise a bending reinforcement. the low plate thickness between the reinforcing ribs is made possible using high-performance concrete (hpc) in combination with textile reinforcement (trc). the bottom horizontal slab of the block has a variable thickness which increases towards the centre of the block (thickness 30−80 mm). the lower horizontal plate is made of trc and, like the vertical plate, is reinforced with reinforcing ribs (50×100 to 137 mm) spaced 0.7−0.8 463 https://doi.org/10.14311/app.2022.33.0463 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en j. pazderka, p. hájek, m. nývlt, h. žáková acta polytechnica ctu proceedings figure 1. special shaped parapet block (visualization). m apart. these ribs, together with the ribs of the vertical plate, form an "l" -shaped frame structure with reinforcement in the centre of the block. the variable thickness of the horizontal slab is advantageous both in terms of precast technology (concreting will take place from the outer surface of the vertical slab) and in terms of better stability of the shaped piece (lower centre of gravity). the parapet block is equipped with a horizontal cross rib (30×100 mm) at the top, which serves for anchoring of the flashing structure (through the spacer profile). the rib on the top of the parapet block also has a reinforcing function in terms of the horizontal load on the parapet wall (wind pressure, suction, anchoring of other structures). to ensure the fixation of the position, the block is equipped with three holes in the lower horizontal plate, which are used for anchoring the block to the ceiling panel of the frame system. the shape-optimized "thin-walled" design of the parapet block is advantageous both in terms of minimizing the weight of the precast element during transportation and subsequent handling on site. at the same time, it is also advantageous the reducing of overall vertical load on the bearing structures and foundations of the building. the subtle optimized shape of the precast block using trc is also advantageous in terms of eliminating thermal bridges. only a minimal cross-section of a part of the supporting structure penetrates the thermal insulation envelope of the building. in fact, it is not a thermal bridge, because the parapet construction must always be thoroughly insulated around its perimeter. this design predetermines the parapet block for use in energy efficient buildings. for handling by lifting means, the block will be equipped with countersunk threads for screwing in the anchorages. these threads will be located on the upper surface of the horizontal reinforcing rib at the position of the vertical reinforcing ribs. 3. description of the functional principle figure 3 shows the installation of the precast parapet block (no. 1) on the ceiling structure of a precast concrete frame construction system comprising ceiling panels (no. 2) and beams (no. 3). the parapet block (no. 1) is secured (fixed) against horizontal movements by anchoring at the anchor holes (no. 4). the lower horizontal slab of the parapet block (no. 5) is"weighted" by a layer of gravity concrete (no.ă6), which thus acts as a stabilizing layer. at its lower left edge, the parapet block (no. 1) is separated from the concrete layer forming a gradient (no. 7) by means of an inserted thermal insulator strip (no. 8), which eliminates the transfer of horizontal deformations of the concrete layer due to longitudinal thermal expansion. on the upper horizontal reinforcement rib of the parapet block (no. 9) is an anchored construction of the flashing (no. 10). the vertical panel of the parapet block (no. 11) is supplemented by an additional thermal insulating layer (no. 12), which compensates for the higher thermal conductivity coefficient hpc (compared to the adjoining masonry of the external cladding (no. 13)). an additional thermal insulation layer (no. 11) is connected to the thermal insulation system of the facade cladding (no. 14), which is tightened up to the thermal insulation under the flashing (no. 15). from the inside of the roof, a vertical layer of thermal insulator (no. 16) is anchored to the parapet block. this layer follows the thermal insulation of the flat roof (no. 17). the thickness of the vertical thermal insulation (no. 15) is designed to overlap the reinforcing rib (no. 18) of the parapet block. figure 4 shows precast parapet block (no. 1) which forms the supporting structure of the parapet shown in figure 3. the parapet block (no. 1) is characterized in that it is made in the cross-sectional shape of the letter"l", where the thin vertical plate of the block (no. 11) is reinforced with ribs (no. 18). the vertical ribs are reinforced at the point of connection 464 vol. 33/2022 parapet structure from special shaped trc blocks figure 2. installation of special shaped parapet block on the building (simplified visualization). figure 3. example of parapet block installation on the ceiling structure of precast concrete frame construction system. notes: 1 parapet block, 2 reinforced concrete ceiling panel, 3 reinforced concrete beam, 4 anchoring of parapet block, 5 horizontal part of parapet block, 6 monolithic concrete layer forming a gradient and stabilizes the parapet block, 7 monolithic concrete layer forming a gradient, 8 dilatation, elastic strip, 9 the upper part of the parapet block, 10 anchoring of parapet block, 11 vertical part of parapet block, 12 lower thermal insulation layer (compensation for reinforced concrete elements), 13 filling masonry of perimeter wall, 14 main thermal insulation layer (etics), 15 insulation of parapet extruded polystyrene (xps), 16 insulation of parapet block, 17 conventional roofing layers of flat roof, 18 reinforcing rib of parapet block 465 j. pazderka, p. hájek, m. nývlt, h. žáková acta polytechnica ctu proceedings figure 4. precast parapet block. notes: 1 parapet block, 4 anchoring of parapet block, 5 horizontal part of parapet block, 9 the upper part of the parapet block, 11 vertical part of parapet block, 18 vertical reinforcement rib of parapet block, 19 horizontal reinforcement rib of parapet block, 20 rib reinforcement. to the ribs of the horizontal plate (no. 19) by triangular haunches (no. 20) and together with the reinforcing ribs of the lower horizontal plate (no. 19) form"l" frame structure. the solved parapet block (no. 1) is characterized in that it comprises a lower horizontal plate (no. 5) of variable thickness which increases towards the centre of the precast. this plate is reinforced with ribs (no. 19) which have an increasing thickness towards the centre of the prefabricated elements. the lower horizontal plate (no. 5) has three holes (no. 4) for anchoring the precast to the ceiling panel of the framework system. the parapet block (no. 1) is provided with aătransverse rib (no.ă9) at the top of the vertical plate (no. 11), which serves to anchor the parapet block flashing structure and to reinforce it in terms of horizontal loading. 4. conclusion the technical solution of the parapet block for flat roofs described in the paper was registered as a utility model no. cz27642(u1). the block is intended primarily for energy-efficient buildings based on precast frame construction systems. however, it can be used without further modifications for parapet solutions in other building construction systems (e.g. masonry buildings with precast concrete ceiling), where the advantage of the precast technology will be beneficial. the parapet block is the next element used for construction details in buildings, expanding the existing set of thin-walled precast elements, developed at the faculty of civil engineering ctu in prague [2–4]. a potential industrial partner for production is currently being searched. testing of mechanical properties is planned after the production of the first prototype series based on the results, the final dimensions of the elements for serial production will be optimized. acknowledgements this work was supported by the research project sgs19/145/ohk1/3t/11. references [1] j. pazderka, p. hájek.utility model cz27642(u1) parapet structure, especially for building platform roofs, ctu in prague, 2015. [2] j. pazderka, m. nývlt, h. žáková. underground ventilated wall based on trc blocks. iop conference series: materials science and engineering 596(1), 2019. https://doi.org/10.1088/1757-899x/596/1/012037. [3] j. pazderka, p. hájek. two innovative solutions based on fibre concrete blocks designed for building substructure. iop conference series: materials science and engineering 246, 2017. https://doi.org/10.1088/1757-899x/246/1/012046. [4] j. pazderka, e. hájková, m. jiránek. underground air duct to control rising moisture in historic buildings: improved design and its drying efficiency. acta polytechnica 57(5), 2017. https://doi.org/10.14311/ap.2017.57.0331. 466 https://doi.org/10.1088/1757-899x/596/1/012037 https://doi.org/10.1088/1757-899x/246/1/012046 https://doi.org/10.14311/ap.2017.57.0331 acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.38.0340 acta polytechnica ctu proceedings 38:340–346, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague bim based hygrothermal modelling of building constructions balázs fürtön, balázs nagy∗ budapest university of technology and economics, faculty of civil engineering, department of construction materials and technologies, 3. műegyetem rkp, h-1111, budapest, hungary ∗ corresponding author: nagy.balazs@emk.bme.hu abstract. the construction industry is currently one of the least digitised. however, due to recent technological developments, building information modelling and management (bim) systems are increasingly being used to increase the efficiency of constructions and sustainable use of resources and increase the energy efficiency of buildings. in the case of new constructions or building renovations, properly designed hygrothermal behaviour of building materials, elements and structures are unavoidable to achieve the objectives mentioned above. therefore, we can ensure the comfort of our users and healthy living space. bim is the most suitable and widespread method for digital management of building stock and related data and integration. with the possibilities offered by bim, it is possible to integrate building physical modelling right into the architectural design process. however, with standard bim systems, we cannot embed detailed material properties, weather, or the effects of the natural and built environment in the bim model for heat and moisture transfer (ham) modelling for hygrothermal performance evaluation at present. furthermore, the data exchange process is one-way mainly and unregulated between bim and numerical simulation tools. in the presented paper, both a closed bim and an open bim based workflow are presented to bridge the gap between bim and hygrothermal modelling of building components and constructions, that can contribute to the design of sustainable, economic and hygrothermal optimal building constructions and also could be a basis for artificial intelligence-based optimised design solutions. keywords: building information modelling, heat and moisture transfer, numerical simulation. 1. introduction the construction industry is currently one of the least digitised and among the most fragmented. however, due to recent technological developments, building information modelling and management (bim) systems are increasingly being used to increase the efficiency of constructions and sustainable use of resources and increase the energy efficiency of buildings. bim is the most suitable and widespread method for the digital management of building stock. the fragmentation of the aec industry [1] is present in the building physics focused area as well. many disciplines should work together until a hygrothermally sound structure is designed. in the case of new constructions or building renovations, properly designed hygrothermal behaviour of building materials, elements, and structures are unavoidable in order to achieve the aforementioned objectives. therefore, we can ensure the comfort of our users and healthy living space. with bim’s potential, it is possible to integrate building physical modelling right into the architectural design process. though bim has different flavours, national regulations and software developers shape what it means to work with this methodology. using standard bim systems, we usually cannot embed material properties, weather, or the effects of the natural and built environment in the bim model for heat and moisture transfer (ham) modelling for hygrothermal performance evaluation at present. furthermore, the data exchange process is one-way mainly and unregulated between bim and simulation tools. even with today’s available bim systems, performing a ham simulation is usually a struggle because the software developers present no clear workflow. the possibility of creating multidimensional simulations by using commercial software started in 2000 when the fraunhofer institute presented wufi 2d [2]. in 2013, they demonstrated the potential of coupling multidimensional thermal bridge simulation with a whole building simulation model in 3d [3]. the use of comsol to model the hygrothermal behaviour of building constructions is even more modest than 2d or 3d wufi. simplified building constructions demonstrate this software’s capabilities [4, 5]. a few papers deal with simulations of wall and slab connections in 2d [6] or hygrothermal modelling of external wall and window joints [7], or in 3d with different concrete structures to examine the effect of different materials or inhomogeneity [8, 9]. bim can be classified into open bim and closed bim, based on the availability of the file format structure it is using. today, industry foundation classes (ifc) is the commonly accepted semantic standard to describe building-related data. it is generally supported in the aec industry. however, the framework necessary to run hygrothermal simulations does not 340 https://doi.org/10.14311/app.2022.38.0340 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 38/2022 bim based hygrothermal modelling of building constructions figure 1. closed bim workflow based on revit. figure 2. open bim workflow based on archicad. entirely fall into this industry, which means ifc’s support is weak. it is an open, international standard [10]. it promotes vendor-neutral, or agnostic, and functional capabilities across a wide range of hardware devices, software platforms, and interfaces for many different use cases [11], offering the possibility to develop an interconnected, accessible data flow among the built assets entire life cycle. buildings can last for centuries; we do not have enough evidence for the longevity of digital data at this point for a time frame this long. in our research, we created and tested an open bimand a closed bim-based workflow for creating ham simulations to model, evaluate or optimise the hygrothermal performance of building constructions during the integrated design process. then, we made a comparative assessment between the two possible workflows and summarised the possibilities and limitations of the different approaches. 2. methodology we chose comsol multiphysics to run the ham simulations because it is validated according to en 15026:2007 annex a [12] and showed promising results according to other benchmarks, including hamstad [13]. the workflows for closed bim and open bim differ for various reasons. for the closed bim workflow solution, we used revit 2021 for bim and comsol 5.6 for numerical simulation to link the bim model into the multiphysics environment, as the schematic workflow shows in figure 1. a livelink solution for revit made the initial geometry and material setup relatively easy in comsol, especially if we consider some techniques not included in the manuals but will be introduced in the present paper later. for the open bim workflow solution, we used archicad 24 as the bim platform. since there is no standard format between archicad and comsol that is reliable enough to transfer geometry, we used rhino as an interim step to organise and clean the model. unfortunately, the ifc format cannot be imported into comsol directly; this is why we needed this step. we used the geometrygym ifc addon to import the geometry to rhino, as shown in figure 2, respectively. an externally insulated wall corner joint consisting of solid masonry blocks and a mineral wool-based external thermal insulation composite system (etics) is considered for testing the workflows. 3. numerical simulations from closed bim: revit-comsol livelink revit is one of the most widespread in the international bim software landscape. therefore, a transparent workflow to perform ham simulations from revit models is probably needed for professionals. comsol is a multiphysical finite element-based engine that lets users examine coupled phenomena, such as conjugated heat and moisture transfer in structures or building materials. the comsol-revit livelink is a proprietary software solution to bridge the gap between bim and numerical simulation. as such, it is a black box with a bit of hard-to-understand operation. 3.1. geometry import the primary options for sending the bim elements are based on spaces or 3d views. for our purposes, the latter proved to be more useful: most of the time, it is sufficient to have only a portion of the structure modelled to run the simulation; we are focusing on this scenario. 3d views can use view templates, so hiding excessive geometry by type and instance is a semi-automatic workflow. certain geometry types, like planes, are not needed in the numerical model, and cleaning is possible in comsol, but it is worth avoiding the issues. it is essential to set the “parts visibility” in revit to “show parts” only in the 3d view used to synchronise the bim model to comsol to ensure that all set layers are visible in comsol. this way, building materials are transferred to each layer accordingly (see figure 3). 341 balázs fürtön, balázs nagy acta polytechnica ctu proceedings figure 3. the model from revit 2021 (left) imported to comsol 5.6 (right). figure 4. building material properties transferred from revit to comsol. using virtual operations on the imported geometry based on bim is possible in comsol. it could be useful to handle modelling issues on the geometry, especially to form composite faces or domains from separated materials to improve the meshing performance or ignore vertices, faces or domains that are not needed for the ham simulation, e.g. thin layers that can be handled as surface resistances. 3.2. building materials we tested various scenarios of how the livelink can be established: the model needs to be created from native revit elements (walls, slabs, roofs, etc.), imported geometry is not supported as of writing this paper. using these elements allows us to assign building materials to each geometry. building materials can be managed in the material browser. the necessary building physics parameters are listed in the “thermal” tab: thermal conductivity, specific heat, and density. vapour permeability or vapour resistance factor, as well as liquid transport coefficient (or as it is called in comsol “diffusion coefficient”) and moisture storage function (“water content” in comsol) that necessary for moisture transport calculations, cannot be added to the materials in revit. there is an option to sync only the geometry without explicitly syncing material data. the comsol geometry import workflow has the livelink for revit, which can be used instead of manually importing from cad formats (step provides a stable workflow that can be exported from revit natively). parts should be created in revit before synchronising the selected view to access each wall layer and its properties. the livelink process transfers the material properties, and domain selections are created according to parts and materials, which speeds up the preprocessing stage of the simulation work. boundary conditions are not yet stored in the revit model at an element level. the parameters required to run a hygrothermal evaluation are not present in the revit model. they cannot be passed (figure 4); therefore, we extended them from [14] to be able to perform the ham simulation. 342 vol. 38/2022 bim based hygrothermal modelling of building constructions figure 5. meshed element originated from bim. figure 6. temperature distribution (left) and relative humidity distribution (right) from ham simulation based on bim model. 3.3. boundary conditions rooms/zones and analytical spaces can be present in the model, and the comsol livelink sync now works with revit rooms. these are abstract, non-existent elements; bim uses them to specify how space is occupied: hourly schedules often describe the loads, lighting requirements, occupant number, required internal temperature or relative humidity. it could allow the generation of boundary conditions for the internal surfaces of the numerical model. as the model is referenced to real-world coordinates, solar radiation could be added to the external boundary conditions apart from external temperature and relative humidity. small scale models often do not include broader context, but at a façadeor whole building level, numerical simulation shading from neighbouring buildings is an important aspect of the design. during our study, boundary conditions were added according to [12, 14] in comsol as heat flux and moisture flux boundary conditions. 3.4. meshing the tested insulated wall corner joint has meshed in comsol after the import. the model is meshed using a physics-controlled meshing sequence and extra-fine element size in the software that gave us an average element quality of skewness of 0.6627 without errors and warnings (figure 5). the different thicknesses of the layers in the model affect the built-in meshing functions since there are considerably significant differences between the thicknesses of the domains included in the model. the element quality can be improved with manual mesh refinement, solution-based adaptive meshing, or previously mentioned virtual operations. 3.5. physics and results we set the physics in comsol 5.6 to heat and moisture transport in building materials, which models the ham transfer according to [12]. a stationary solver was deployed for the coupled equations, geometric multigrid for the moisture and smoothed aggregation geometric multigrid for heat transfer. the results using the extended material properties and boundary conditions are shown in figure 6. since we put mineral wool thermal insulation on our masonry wall, the relative humidity distribution is high since it lets the vapour through. besides analysing temperature and relative humidity distribution, ham analysis also 343 balázs fürtön, balázs nagy acta polytechnica ctu proceedings figure 7. stages of model conversion from archicad to comsol. allows us to check the temperature factor at internal surface points or evaluate the thermal transmittance and the moisture transmittance of building components. the structure’s heat flux or moisture fluxes can also be visualised for surfaces, domains and sections, respectively. 4. numerical simulation with open bim: archicad-comsol relying on open standards provides accessibility to and control over simulations’ input and output data. finding errors or workflow issues is easier if we can track the path of each piece of information stored initially in the model. in bim, the only widespread open standard for comprehensive communication of stored data is ifc. ifc is an international semantic standard maintained by buildingsmart, with extensive technical specifications covering the materials’ necessary building physics properties. 4.1. geometry as there is no direct support for neither archicad’s proprietary data model nor the openly available ifc format, the open bim workflow is more complicated because we need an intermediary step to convert the geometry into formats that the chosen multiphysics platform (comsol) can handle. for this reason, we decided to choose rhinoceros 7 because the step export from it towards comsol proved to be robust, and it has an extensive ecosystem to extend its capabilities. there are published examples of the rhino to comsol workflow previously used for building envelope analysis [15]. the geometrygym addon lets users import and manipulate the ifc files to rhino, then export it as step towards comsol. the problem is that although material level data can be stored (even extended) in archicad and rhino can display it; we couldn’t find a way to attach non-geometric information to the step geometry apart from layer information (figure 7). unfortunately, building material properties are lost at the export stage from rhino to comsol. in order to get accurate representation, it is advised to export the geometry as non-parametric brep objects, exploding composite structures to their layers in the ifc translation settings to ensure that everything is correct. with parametric export, the pipeline can suffer down the line, as the import capabilities of other applications are externalities from the initial export’s perspective. 4.2. building materials this means that setting up all the properties in archicad for the sake of hygrothermal evaluation is not feasible (for quantity take-offs, mass calculation, custom gdl scripting they are useful to include, as the request command can be used to obtain building physics parameters and do calculations in a custom library part). with recent releases, it is possible to assign custom physical properties to building materials, but they are constant values only, as the default physical properties (figure 8). these values are exported in the ifc file, but we did not take care of it since they cannot be used for the last step of the conversion process. material parameters need to be done from scratch in comsol, which isn’t much work. the problem occurs when we need to assign materials to the objects (this process is automatic with the help of the livelink), it is dependent on the import workflow how selections are stored. colours and layers are the only attributes that can be converted to domain selections. 4.3. meshing, physics and results these processes are exactly the same and should be handled as introduced in sections 3.3, 3.4 and 3.5, respectively. since the geometry, mesh, and boundary conditions are the same as described in the closed bim process, and simulation results are also the same. 5. conclusions in general, it can be stated that there isn’t a satisfying workflow present in the current market. neither closed bim nor open bim solutions can provide the necessary granularity in building physics characteristics that we would require and are still far from the detailed ham modelling possible in comsol [14]. however, we succeeded and performed simple ham simulations based on bim models both from closed bim and open bim processes. the conclusions are summarised in table 1. 344 vol. 38/2022 bim based hygrothermal modelling of building constructions figure 8. building material catalogue in archicad with the default available physical parameters. process closed bim-based ham open bim based ham passing geometry from bim to ham no loss of detail passing building physics parameters from bim to ham limited, depending on the input data. with simpler geometry and parts created, lossless connection no interface is provided for ifc from ham, although it is a step based file format creating domains for faster ham model preprocessing partand material based selections help the work layer or colour based indirect sorting is possible boundary conditions no interface is provided as of now; manual work needed meshing input can be meshed without errors, if sufficiently clean feedback to bim from ham early-stage development for massing – geometry control not implemented table 1. summary of the possibilities with bim to ham workflows. acknowledgements supported by the únkp-21-4 new national excellence program of the ministry for innovation and technology from the source of the national research, development and innovation fund. project fk_128663 has been implemented with the support provided from the national research, development and innovation fund of hungary, financed under the fk_18 funding scheme. references [1] d. hall, a. algiers, t. lehtinen, et al. the role of integrated project delivery elements in adoption of integral innovations. in proceeding of epoc 2014 conferenence. 2014. 20 p. [2022-03-15]. https://epossociety.org/epoc-2014 [2] a. holm, h. m. künzel. two-dimensional transient heat and moisture simulations of rising damp with wufi 2d. in proceedings of the 12th international brick/block masonry conference, vol. 2. 2000. 8 p. [2022-03-15]. https://wufi.de/en/literature/ [3] f. antretter, m. pazold, j. radon, h. künzel. kopplung von dynamischer wärmebrückenberechnung mit hygrothermischer gebäudesimulation. bauphysik 35(3):181–192, 2013. https://doi.org/10.1002/bapi.201310068 [4] a. w. m. van schijndel. multiscale heat, air and moisture modelling and simulation. in g. petrone, g. cammarata (eds.), recent advances in modelling and simulation, chap. 41. 2005. https://doi.org/10.5772/5963 [5] m. janetti, f. ochs, w. feist. 3d simulation of heat and moisture diffusion in constructions, 2011. [2022-03-15]. https://www.comsol.com/paper/3dsimulation-of-heat-and-moisture-diffusion-inconstructions-10921 345 https://epossociety.org/epoc-2014 https://wufi.de/en/literature/ https://doi.org/10.1002/bapi.201310068 https://doi.org/10.5772/5963 https://www.comsol.com/paper/3d-simulation-of-heat-and-moisture-diffusion-in-constructions-10921 https://www.comsol.com/paper/3d-simulation-of-heat-and-moisture-diffusion-in-constructions-10921 https://www.comsol.com/paper/3d-simulation-of-heat-and-moisture-diffusion-in-constructions-10921 balázs fürtön, balázs nagy acta polytechnica ctu proceedings [6] j. berger, w. mazuroski, n. mendes, et al. 2d whole-building hygrothermal simulation analysis based on a pgd reduced order model. energy and buildings 112:49–61, 2016. https://doi.org/10.1016/j.enbuild.2015.11.023 [7] b. nagy, c. tömböly. hygrothermal analysis of window construcion joints. in zbornik radova građevinskog fakulteta 34, pp. 91–100. 2018. https: //doi.org/10.14415/konferencijagfs2018.008 [8] f. colangelo, g. de luca, c. ferone, a. mauro. experimental and numerical analysis of thermal and hygrometric characteristics of building structures employing recycled plastic aggregates and geopolymer concrete. energies 6(11):6077–6101, 2013. https://doi.org/10.3390/en6116077 [9] p. iodice, n. massarotti, a. mauro. effects of inhomogeneities on heat and mass transport phenomena in thermal bridges. energies 9(3):126, 2016. https://doi.org/10.3390/en9030126 [10] international organisation for standardisation. iso 16739:2018. industry foundation classes (ifc) for data sharing in the construction and facility management industries. [11] buildingsmart: industry foundation classes. [2022-03-15]. https://www.buildingsmart.org/standards/bsistandards/industry-foundation-classes/ [12] european committee for standardisation. en 15026:2007. hygrothermal performance of building components and building elements – assessment of moisture transfer by numerical simulation. [13] j. i. knarud, s. geving. implementation and benchmarking of a 3d hygrothermal model in the comsol multiphysics software. energy procedia 78:3440–3445, 2015. https://doi.org/10.1016/j.egypro.2015.12.327 [14] b. nagy, g. stocker. numerical analysis of thermal and moisture bridges in insulation filled masonry walls and corner joints. periodica polytechnica civil engineering 63(2):446–455, 2019. https://doi.org/10.3311/ppci.13593 [15] j. hand. optimising the performance of complex building facades, 2015. in comsol news. [2022-03-15]. https://www.comsol.com/story/download/309771/ newtecnic_cn2015.pdf 346 https://doi.org/10.1016/j.enbuild.2015.11.023 https://doi.org/10.14415/konferencijagfs2018.008 https://doi.org/10.14415/konferencijagfs2018.008 https://doi.org/10.3390/en6116077 https://doi.org/10.3390/en9030126 https://www.buildingsmart.org/standards/bsi-standards/industry-foundation-classes/ https://www.buildingsmart.org/standards/bsi-standards/industry-foundation-classes/ https://doi.org/10.1016/j.egypro.2015.12.327 https://doi.org/10.3311/ppci.13593 https://www.comsol.com/story/download/309771/newtecnic_cn2015.pdf https://www.comsol.com/story/download/309771/newtecnic_cn2015.pdf acta polytechnica ctu proceedings 38:340–346, 2022 1 introduction 2 methodology 3 numerical simulations from closed bim: revit-comsol livelink 3.1 geometry import 3.2 building materials 3.3 boundary conditions 3.4 meshing 3.5 physics and results 4 numerical simulation with open bim: archicad-comsol 4.1 geometry 4.2 building materials 4.3 meshing, physics and results 5 conclusions acknowledgements references https://doi.org/10.14311/app.2022.33.0424 acta polytechnica ctu proceedings 33:424–430, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague comparison between concrete stress diagrams according to the brazilian standard abnt nbr 6118: 2014 victor hugo dalosto de oliveiraa,∗, rodolfo alves carvalhoa, luana ferreira borgesa, paulo chaves de rezende martinsa, rodolfo rabelo nevesb, joão paulo de almeida siqueiraa, gabriel yves da silva oliveirac, cintia adriana azevedo de liz anhaiad a department of civil and environmental engineering, university of brasília, sg-12, campus darcy ribeiro, asa norte, brasília, df, 70910-900, brazil b department of materials engineering and construction, federal university of minas gerais, av. antø̂nio carlos, 6627, pampulha, belo horizonte, mg, 31270-901, brazil c school of exact, architecture and environment, catholic university of brasília, qs 07 lote 01, taguatinga, brasília, df, 71966-700, brazil d department of civil engineering, faculty of pinhais, av. camilo di lellis, 1065, centro, pinhais, pr, 83323-000, brazil ∗ corresponding author: victordalosto@gmail.com abstract. this study aims to compare methods for the determination of concrete properties by means of the stress diagrams present in the brazilian standard abnt nbr 6118: 2014. the area under the stress diagram, the internal reactions, and the application point of the resulting reactions for the parabola-rectangle and rectangular block diagrams are present in order to compare them. deductions and numerical examples were used, and different results were obtained for each formulation. this is due to non-consideration of the relationship between stress and strain in the simplified rectangular block. the rectangular block is applicable only for cases in which the concrete reaches the ultimate strain. these cases are those that concrete crushing determines the section failure in compression with steel yielding in tension (domain 3) or without steel yielding (domain 4). keywords: concrete behavior, parabola-rectangle diagram, rectangular block. 1. introduction exhaustion of the stress capacity of a reinforced concrete element subjected to normal stress can be achieved by crushing the concrete under compression or by an excessive plastic strain of the steel. however, concrete failure is difficult to identify. it is conventional to assume that this failure occurs when the material reaches a maximum compressive strain determined by experimental results [1]. according to american standard aci 318-14 [2], the maximum compressive strain of concrete is around 0.003 to 0.004 under normal conditions, and it may reach 0.008 under special conditions. the brazilian standard abnt nbr 6118: 2014 [3] states a maximum specific strain of 0.0035 for concretes up to class c50 (characteristic compressive strength up to 50 mpa). this brazilian standard adopts an idealized stressstrain diagram to represent concrete behavior, in which the stress distribution takes place according to a parabola-rectangular diagram [3]. this standard allows to replace the parabola-rectangle diagram with a rectangle of equivalent depth, as a simplification of calculation [3]. abnt nbr 6118: 2014 [3] states that the difference in results with both formulations is small, so there is no need to make any adjustment. the rectangular block does not represent the actual stress distribution within the compressed concrete zone, but provides reasonably the same compressive force [4]. however, mendes neto [5] noticed that the design of structures with the rectangular block provides a different and unsafe result compared to the parabolarectangle diagram result. formulations for stress, internal reaction, and the point of application of the resulting reaction are presented in this paper by means of the parabola-rectangle diagram and the rectangular block. solutions are compared for some cases to verify if the use of the rectangular block leads to reasonable and safe results. 424 https://doi.org/10.14311/app.2022.33.0424 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 33/2022 concrete diagrams (abnt nbr 6118: 2014) figure 1. idealized stress-strain diagram. source: adapted from the abnt nbr 6118: 2014 [3]. 2. materials and numerical methods this section presents the concepts and formulations considered for the comparison between parabolarectangle and rectangular block diagrams. abnt nbr 6118: 2014 [3] uses an idealized stress-strain diagram to represent concrete compression at the ultimate limit state, as shown in figure 1, in which εc is the concrete strain; εc2 is the assumed specific compression strain of concrete at the beginning of the plastic level; εcu is the assumed maximum useable compression strain in the concrete; fck is the characteristic strength of the concrete; fcd is the design strength of the concrete; and σc is the concrete stress [3, 6]. this diagram is similar to that presented in section 7.2.3.1.5 of the fib model code 2010 [7] and section 3.1.7 of the eurocode 2 [8]. equation (1) defines this diagram (figure 1): σc = 0.85fcd ! 1 − " 1 − εc εc2 #n$ (1) in which n = 2 for fck ≤ 50 mpa; and n is given by equation 2 for fck > 50 mpa. n = 1.4 + 23.4 % (90 − fck) & 100 '4 (2) for concretes with fck ≤ 50 mpa: εc2 = 0.002; and εcu = 0.0035. equations 3 and 4 give the values of εc2 and εcu for concretes with fck > 50 mpa. εc2 = 0.002 + 0.000085 (fck − 50)0.53 (3) εcu = 0.0026 + 0.035 % (90 − fck) & 100 '4 (4) the brazilian standard [3] states that it is possible to replace this diagram with a rectangle of height y = λx (in which x is the effective depth of the neutral axis), and constant stress αc fcd, without loss of quality in results. for concretes with fck ≤ 50 mpa: λ = 0.8, and αc = 0.85. equations 5 and 6 give values of λ and αc for concretes with fck > 50 mpa. thus, the strains and stresses in a rectangular cross-section at the ultimate limit state can be represented as in figure 2. λ = 0.8 − (fck − 50) & 400 (5) αc = 0.85 % 1 − (fck − 50) & 200 ' (6) the area under the stress diagram can be obtained by integrating stresses in the domain of the specific strains. thus, equation 7 is used when strain values at the most compressed fiber are less than εc2, and equation 8 is used if strain values are larger than εc2. a(ε) = 0.85 fcd ( εc 0 ! 1 − " 1 − ε εc2 #n$ dε (7) a(ε) = 0.85 fcd ( εc2 0 ! 1 − " 1 − ε εc2 #n$ dε + 0.85 fcd ( εc εc2 dε (8) considering the bernoulli hypothesis (plane sections remain plane), the longitudinal strain varies proportionally with the distance to the neutral axis [2]. therefore, if the specific strain is known, the neutral axis is determined. thus, as the resultant of internal reactions equals the volume under the stress diagram, its value becomes known after determining the stress and cross-sectional compressed area. the center of gravity of the compression region is obtained by dividing the stress-strain diagram into infinitesimal areas stress-strain diagram into infinitesimal areas σ dε. the point of application of the resultant of normal concrete stresses can be determined by equation 9, in which the integral of the stress area in the strain domain is determined by equation 7 and 8. ε = ( a xda ( a da = ( ε εσdε ( ε σdε (9) 2.1. strain domains conventionally, failure of a reinforced concrete section occurs when the specific strain of concrete or steel (or both) reaches its ultimate value. the standard abnt nbr 6118: 2014 [3] defines that the ultimate limit state is characterized when the distribution of strains in the section is within one of the specified strain domains. figure 3 illustrates these strain domains, in which εyd is the strain in the tension reinforcement at failure; h is the overall height of a cross-section; d is the distance from the most compressed fiber to the centroid of the longitudinal reinforcement on the tension side of the member; and d′ is the distance from the most compressed fiber to the centroid of the longitudinal compression steel [3, 6]. thus, section failure can occur under excessive plastic strains of the steel characterized by the straight-line 425 v. h. d. d. oliveira, r. a. carvalho, l. f. borges et al. acta polytechnica ctu proceedings figure 2. strain and stress diagrams of concrete. figure 3. strain domains in the ultimate limit state. source: adapted from the abnt nbr 6118: 2014 [3]. a, within domains 1 and 2, or by crushing of the compressed concrete defined by domains 3, 4, 4a, 5, and the straight-line b. the ultimate limit state of domain 2 is characterized by the strain εyd = 1%, and the failure of the cross-section occurs without the concrete reaching its maximum strength. fusco (1981) [1] divides domain 2 into two subdomains: 2a (εc from zero up to εc2) and 2b (from εc2 to εcu). this subdivision aims to identify the value at which the use of compression reinforcement becomes efficient [1]. such consideration becomes important here because it marks the point at which pseudo-plastification of concrete begins. while the section is in subdomain 2a, concrete stresses are less than 0.85 fcd. when concrete strain exceeds εc2, the concrete pseudo-plastification and the constant compression region arise. when the section deformation approaches domain 3, the specific strains become higher, and the plasticized zone expands. therefore, two different equations are required to represent the behavior of compressed concrete, equations 7 and 8. the strain εcu characterizes the ultimate limit state of domains 3, 4 and 4a. in these domains, the failure of the section occurs by crushing the concrete in compression. although the neutral axis goes down in the section and the compressed area increases, the stress diagram proportions remain constant. in these domains, it is possible to find the stresses and reactions in concrete only with equation 8. the strain εcu also characterizes the ultimate limit state of domain 5, with concrete failure in compression. in domain 5, the entire section is under nonuniform compression. the strains converge to a constant value εc2 as the section approaches the line b. thus, the concrete properties in domain 5 are defined by equation 8, changing the lower limit of the first integral. 2.2. calculations with a computer program the matlab software was used for the preparation of scripts in order to make a comparison between the diagrams. the stresses, internal reactions, and center of gravity of the parabola-rectangle diagram were obtained through these scripts, that solve equations 7, 8, and 9. the results were compared with those obtained by the rectangular block. this analysis is called "approach 1". another comparison between the diagrams, with a different approach (approach 2), was performed. values were assigned to the position of the neutral axis, and the internal reactions were obtained with equation 10 for the parabola-rectangle diagram, and with equation 11 for the rectangular block. rc = ( a σ da (10) rc = αc fcd bw λ x (11) 426 vol. 33/2022 concrete diagrams (abnt nbr 6118: 2014) figure 4. concrete strains and stresses in the ultimate limit state. figure 5. representation of calculation solution. 3. results and discussions initially, it should be emphasized that the use of stress diagrams does not apply to line a and domain 1, since there are no compressive stresses in concrete, and its tensile stresses are neglected at the ultimate limit state. 3.1. approach 1 based on the parameters analyzed, the rectangular diagram presents only one way to describe the behavior of the concrete. however, the parabola-rectangle diagram has five different behaviors: 1. while the structure is in domain 2a, there is only a parabolic excerpt (σc < 0.85 fcd); 2. when the pseudo-plastification begins in domain 2b, the excerpt of constant stress 0.85 fcd appears, and there is an expansion of this plastic zone until the section reaches domain 3; 3. for domains 3, 4 and 4a, there is an expansion of the compressed region, in which the stress distribution increases proportionally in the parabolic excerpt and in the constant stress excerpt; 4. for sections in domain 5, there is an overlap of the plastic zone, where the specific strains across the section converge to εc2 and the stresses converge to 0.85 fcd; 5. for line b, the entire section is under uniform compression (σc = 0.85 fcd). figure 4 illustrates the stress distribution and behaviors in the strain domains. results of the parabola-rectangle diagram can be obtained by equations 7 and 8, calculating the area as a function of the neutral axis position (x). for comparison, the solution is represented as proportions of equivalent rectangles of constant stress of 0.85 fcd and nominal height y1 and y2, referring to the portions of constant and parabolic stresses distribution, respectively. figure 5 illustrates the solution, and the results are listed in table 1. domain 4a represents a small region of the domains, and it ends when the section is fully compressed, so its relationships are similar to those of domains 3 and 4. for domain 5 and line b, the neutral axis relation is no longer represented as a function of x/d and becomes a function of x/h, as shown in table 2. thus, the nominal compression of the parabolarectangle stress diagram is similar to the rectangular diagram in domains 3, 4 and 4a, where the height y = 0.8095x is equivalent to the height adopted by simplification (y = 0.80x). however, the point of application of the resulting reaction is different in each type of diagram. in addition, a considerable differ427 v. h. d. d. oliveira, r. a. carvalho, l. f. borges et al. acta polytechnica ctu proceedings (x/d) y/x y1/x y2/x c.g./x domain 2a 0.01 1460 / 29403 ∼= 0.050 1460 / 29403 391 / 1168 ∼= 0.335 0.02 710 / 7203 ∼= 0.099 710 / 7203 191 / 568 ∼= 0.336 0.03 1380 / 9409 ∼= 0.147 1380 / 9409 373 / 1104 ∼= 0.338 0.04 335 / 1728 ∼= 0.194 335 / 1728 91 / 268 ∼= 0.340 0.05 260 / 1083 ∼= 0.240 260 / 1083 71 / 208 ∼= 0.341 0.06 630 / 2209 ∼= 0.285 630 / 2209 173 / 504 ∼= 0.343 0.07 8540 / 25947 ∼= 0.329 8540 / 25947 337 / 976 ∼= 0.345 0.08 590 / 1587 ∼= 0.372 590 / 1587 41 / 118 ∼= 0.347 0.09 3420 / 8281 ∼= 0.413 3420 / 8281 319 / 912 ∼= 0.350 0.10 110 / 243 ∼= 0.453 110 / 243 31 / 88 ∼= 0.352 0.11 11660 / 23763 ∼= 0.491 11660 / 23763 301 / 848 ∼= 0.355 0.12 255 / 484 ∼= 0.527 255 / 484 73 / 204 ∼= 0.358 0.13 12740 / 22707 ∼= 0.561 12740 / 22707 283 / 784 ∼= 0.361 0.14 3290 / 5547 ∼= 0.593 3290 / 5547 137 / 376 ∼= 0.364 0.15 180 / 289 ∼= 0.623 180 / 289 53 / 144 ∼= 0.368 0.16 860 / 1323 ∼= 0.650 860 / 1323 16 / 43 ∼= 0.372 0.166667 2 / 3 ∼= 0.667 2 / 3 3 / 8 ∼= 0.375 domain 2b 0.17 172 / 255 ∼= 0.675 2 / 85 166 / 255 22019 / 58480 ∼= 0.377 0.18 94 / 135 ∼= 0.696 4 / 45 82 / 135 6451 / 16920 ∼= 0.381 0.19 68 / 95 ∼= 0.716 14 / 95 54 / 95 9977 / 25840 ∼= 0.386 0.20 11 / 15 ∼= 0.733 1 / 5 8 / 15 43 / 110 ∼= 0.391 0.21 236 / 315 ∼= 0.749 26 / 105 158 / 315 39211 / 99120 ∼= 0.396 0.22 42 / 55 ∼= 0.764 16 / 55 26 / 55 3697 / 9240 ∼= 0.400 0.23 268 / 345 ∼= 0.777 38 / 115 154 / 345 49859 / 123280 ∼= 0.404 0.24 71 / 90 ∼= 0.789 11 / 30 19 / 45 3481 / 8520 ∼= 0.409 0.25 4 / 5 ∼= 0.800 2 / 5 2 / 5 33 / 80 ∼= 0.412 domains 3 and 4 0.259259 17 / 21 ∼= 0.810 3 / 7 8 / 21 99 / 238 ∼= 0.416∼ 1 table 1. section properties in the ultimate limit state (domains 2, 3 and 4). ence is noted for the nominal compression and the point of application of the resultant reaction within sections in domains 2 and 5, and in line b. 3.2. approach 2 a rectangular section with bw = 1, d = 1, h = 1.2 and fck = 20 mpa was analyzed, in which bw is the section width. regardless of the values adopted for the cross-section, the difference obtained by the diagrams remains proportional. for this reason, dimensionless values were used for bw, h and d. the results for the ultimate limit state are shown in figure 6. as a reference, the limits of the strain domains were marked, where ca-50 steel (steel with 500 mpa yield strength) was adopted. the same fact can be verified for concrete strength group ii (c55 to c90): there is a considerable difference between results with the parabola-rectangle diagram and the rectangular block simplification. a rectangular section with bw = 1, d = 1, h = 1.2 and fck = 80 mpa was analyzed. the relationship between internal reaction and neutral axis position is shown in figure 7. 4. conclusions although the rectangular block simply represents the behavior of the concrete at the imminence of failure, it does not reflect the relation between stresses and strains where concrete is under limit strain of failure. this simplification considers that the material works at its full capacity of strength all over the compressive zone. the nominal compression and point of application of the concrete reaction are almost equal with both diagrams (parabola-rectangle or rectangular block) in the design of linear elements sections submitted predominantly to bending load (domains 3 and 4). however, it can result in unsafe design in sections in domains 2 or 5, typically slabs or columns, respectively. the difference between the diagrams is amplified for high strength concrete elements. since high resistance concretes have lower ductility and an explosive and brittle failure, they have a smaller plastic region and, consequently, values of εc2 close to εcu, with a smaller excerpt of constant stresses. thus, the calculated difference between the stress diagrams becomes larger. 428 vol. 33/2022 concrete diagrams (abnt nbr 6118: 2014) (x/h) y/h y1/h y2/h c.g./h domain 5 1.00 17 / 21 ∼= 0.810 3 / 7 8 / 21 99 / 238 ∼= 0.416 1.05 133349 / 158949 ∼= 0.839 3 / 7 65228 / 158949 805443 / 1866886 ∼= 0.431 1.10 39989 / 46389 ∼= 0.862 3 / 7 20108 / 46389 247923 / 559846 ∼= 0.443 1.15 188621 / 214221 ∼= 0.881 3 / 7 96812 / 214221 1192347 / 2640694 ∼= 0.452 1.20 13709 / 15309 ∼= 0.895 3 / 7 7148 / 15309 87963 / 191926 ∼= 0.458 1.25 10085 / 11109 ∼= 0.908 3 / 7 5324 / 11109 13095 / 28238 ∼= 0.464 1.30 71741 / 78141 ∼= 0.918 3 / 7 38252 / 78141 470187 / 1004374 ∼= 0.468 1.35 323861 / 349461 ∼= 0.927 3 / 7 174092 / 349461 2139027 / 4534054 ∼= 0.472 1.40 5669 / 6069 ∼= 0.934 3 / 7 3068 / 6069 37683 / 79366 ∼= 0.475 1.45 403829 / 429429 ∼= 0.940 3 / 7 219788 / 429429 2698803 / 5653606 ∼= 0.477 1.50 4469 / 4725 ∼= 0.946 3 / 7 2444 / 4725 30003 / 62566 ∼= 0.480 1.75 27725 / 28749 ∼= 0.964 3 / 7 15404 / 28749 37791 / 77630 ∼= 0.487 2.00 2477 / 2541 ∼= 0.975 3 / 7 1388 / 2541 17019 / 34678 ∼= 0.491 3.00 1685 / 1701 ∼= 0.991 3 / 7 956 / 1701 2343 / 4718 ∼= 0.497 4.00 13061 / 13125 ∼= 0.995 3 / 7 7436 / 13125 91107 / 182854 ∼= 0.498 5.00 335 / 336 ∼= 0.997 3 / 7 191 / 336 234 / 469 ∼= 0.499 10.0 94205 / 94269 ∼= 0.999 3 / 7 53804 / 94269 131823 / 263774 ∼= 0.500 50.0 ∼= 1.000 3 / 7 0.571 ∼= 0.500 line b ∞ 1 3 / 7 4 / 7 0.5 table 2. section properties in the ultimate limit state (domain 5 and line b). figure 6. internal reactions for concrete class c20 (fck = 20 mpa). the percentage difference between the two approaches is marked at some representative points, referred to results obtained with the parabola-rectangle diagram. figure 7. internal reaction for concrete class c80 (fck = 80 mpa). the percentage difference between the two approaches is marked at some representative points, referred to results obtained with the parabola-rectangle diagram. 429 v. h. d. d. oliveira, r. a. carvalho, l. f. borges et al. acta polytechnica ctu proceedings acknowledgements this study was financed in part by the coordenação de aperfeiçoamento de pessoal de nível superior brasil (capes) finance code 001. references [1] p. b. fusco. estruturas de concreto: solicitações normais, rio de janeiro: guanabara dois, 1981. [2] american concrete institute aci 318-14. building code requirements for structural concrete (farmington hills: aci), 2014. [3] associação brasileira de normas técnicas. abnt nbr 6118: 2014: projeto de estruturas de concreto procedimento, rio de janeiro: abnt, 2014. [4] a. h. mattock, l. b. kriz, e. hognestad. rectangular concrete stress distribution in ultimate strength design. journal proceedings, 1961. [5] f. n. mendes. bloco retangular versus parábola-retângulo: comparação na flexão composta anais do 4◦ simpósio epusp sobre estruturas de concreto, são paulo: epusp, 2006. [6] j. k. wight. reinforced concrete: mechanics and design, hoboken: pearson, 7th edition, 2016. [7] fib. fédération internationale du béton. model code for concrete structures 2010, lausanne: fib, 2013. https://doi.org/10.1002/9783433604090. [8] european committee for standardization 2004 en 1992-1-1. eurocode2: design of concrete structures part 1-1: general rules and rules for buildings, brussels: cen, 2004. 430 https://doi.org/10.1002/9783433604090 acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.35.0001 acta polytechnica ctu proceedings 35:1–7, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague kalker’s coefficient c11 and its influence on the damping and the retuning of a mechanical drive torsion system of a railway vehicle vojtěch dybala czech technical university in prague, faculty of mechanical engineering, department of automotive, combustion engines and railway engineering, technická 4, 160 00 prague 6, czech republic correspondence: vojtech.dybala@fs.cvut.cz abstract. within the research of electromagnetically excited torsion oscillations in the mechanical part of traction drive systems of modern railway vehicles, which has been realized at the faculty of mechanical engineering at the ctu in prague, there are two separate simulation models in use. the basic calculation model, which is utilized to gain basic characteristics of the torsion system as natural frequencies and natural modes of oscillations. and the complex simulation model, which simulates a drive of the vehicle. this contribution is focused on the basic calculation model, which has been built in matlab. this model in its first version did not apply the contact between wheels and rails. it was necessary to find out, if this simplification is relevant with respect to subsequent simulations within the complex simulation model and its results. therefore, the contact interaction as a traction force in longitudinal direction in the wheel-rail contact was realized via the kalker’s linear theory. this article deals with the comparison between models with and without the implementation of the wheel-rail contact and its influence on the damping within the torsion system and retuning of the torsion system. keywords: kalker, natural frequency, railway vehicle, torsion system, wheel-rail contact. 1. introduction one of the typical features of modern electrical railway vehicles is the individual mechanical drive of the wheelset, e.g. see figure 1. in principal it means that each wheel-set has its own propelling unit. this unit mostly consists of an electrical traction motor, a gearbox and a coupling connecting both of them. for sure there are more concepts of such a mechanical drive unit. figure 1. a partly-suspended drive of a locomotive [1]. for purposes of this research a fully-suspended drive layout has been applied. this type of an individual drive consists of an electrical traction motor, a gearbox and a hollow shaft, which transmits driving and braking power between the gearbox and the wheel-set – figure 2. because generally this research is focused on high-speed and high-power railway vehicles, the fully-suspended type of a drive train was chosen, as it is a typical and an appropriate layout for this type of vehicles. figure 2. a fully-suspended drive of a locomotive [2]. 2. basic mathematical model this mathematical model, built in matlab, is utilized to provide basic characteristics of a torsion system, which schematically represents the design of a mechanical drive train of a railway vehicle. the basic characteristics are: • natural frequencies of oscillations • natural modes of oscillations 1 https://doi.org/10.14311/app.2022.35.0001 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vojtěch dybala acta polytechnica ctu proceedings knowledge of these characteristics is supposed to be important for evaluation of frequency analysis, which will be carried out in subsequent simulations in the above-mentioned complex simulation model, built in matlab simulink. now the complex simulation model applies some simplifications to do simulations effective in terms of simulation time, amount of data and goals of the research. the basic calculation model supposes also some simplifications as the interaction in the wheel-rail contact. because of the frequency analysis it is appropriate to do a review if the simplification of the basic calculation model is reasonable and if there can be a significant impact on the evaluation frequency analysis. 2.1. model with no wheel-rail contact the mathematical model is based on the scheme of the torsion system (figure 3), which respects the layout of the fully-suspended drive presented in figure 2. figure 3. torsion system scheme – fully-suspended drive [3]. equations of motion describing this system were derived via the lagrange method. the matrix notation of these equation of motion is (1). to calculate natural modes and natural frequencies of oscillations these equations are solved as a system without the vector of excitation [m ], see (2). [j ][φ̈] + [k][φ] = [m ] (1) [j ][φ̈] + [k][φ] = [0] (2) the system (2) was solved via the function eig(k, j), which is a pre-programed function in matlab. this function returns: • an eigen vector representing rotation angle φi,j for a rotation mass ji and j natural mode of oscillation • an eigen values vector [λj ] representing j natural angle frequency ωj = √ λj the eigen values are transformed into natural frequencies fj via the formula (3). fj = √ λj 2π (3) 2.2. model with a wheel-rail contact the wheel-rail contact was implemented into the calculation model via longitudinal traction forces t1, which can be seen in the top view of the drive train – figure 4. the wheel-rail contact is simplified so, that lateral forces and spin moment between the wheel and the rail are neglected. practically it means that it represents rolling of a cylindrical wheel on rails, not conical profile wheels and sinus movements of the wheel-set is neglected. also forces t1 on both sides are supposed to be same. figure 4. visualization of wheel-rail forces – top view. the longitudinal traction force t1 is calculated via kalker’s linear theory for the wheel-rail contact (4). t1 = c11 ael bel g sx = c1 sx = k1 sx (4) in this article details about the kalker’s theory will not be presented, but they can be found in [4– 6]. for purposes of this contribution the value of kalker’s coefficient c11 = 4, 984 is taken as a fact. that coefficient itself can varies. because that theory is linear, its validity is limited for small values of slip with respect to adhesion characteristic – phase i in figure 5. for the area of higher slip (phase ii) the direction of the curve k1* decreases. while the value of c11 can be supposed for good conditions in the wheel-rail contact (dry rails), for worst conditions (wet or dirty rails) it can decrease as well. this fact is presented in figure 6 within popovici’s adhesion characteristics. and therefore because of both effects, the calculations were carried out also for variations of c11, specifically for c11/2 and c11/4. sx = rk ωk − vx v (5) due to above mentioned simplifications also only the wheel slip sx in the longitudinal direction is supposed. its deduction is presented in figure 7 and its mathematical representation is formula (5). [j ][φ̈] + [b][φ̇] + [k][φ] = [0] (6) within the torsion system scheme (figure 8) and subsequently the equations of motion in the matrix representation (6) the force t1 creates a damping element in the wheel-rail contact. this damping defines 2 vol. 35/2022 kalker’s coefficient c11 and its influence figure 5. traction characteristic and adhesion characteristic of wheelset [4–6]. figure 6. popovici’s adhesion characteristics [7]. the formula (7). rk is constant, c1 = k1 (figure 5 and figure 6) will varies according to c11 and the vehicle velocity v is a variable as well. the calculations for this system were carried out as an analysis of the influence of c1 and v on the damping factor. bw −r = c1 r2 k v (7) the velocity as a calculation parameter was specified based on the supposed traction characteristics (figure 9) of the vehicle, which is a high-speed locomotive for purposes of the research. the applied values of velocity were from 0 km/h to 200 km/h. the system (6) was solved by the matlab funcfigure 7. wheel slip deduction [2]. figure 8. torsion system scheme – fully-suspended drive. figure 9. vehicle traction characteristic. tion polyeig(k, b, m), which returns eigenvectors and eigenvalues for defined matrices. eigen vectors returns values of angle of rotation as described in 2.1. eigen values are complex numbers in the form according to (8) for this system with damping. λj = −δj ± i ωdmp,j (8) resulting natural frequencies of oscillation are then calculated according to (9). fdmp,j = ωdmp,j 2π (9) 3 vojtěch dybala acta polytechnica ctu proceedings 3. calculation results results of calculations in sections 3.1 and 3.2 below are respective to the theory from sections 2.1 and 2.2 and the subsequent conclusion aims to make an analysis of the velocity influence on the retuning of the torsion system via damping. 3.1. model with no wheel-rail contact the torsion system scheme, which respects the design of the fully-suspended drive in figure 2 is a system with 7 degrees of freedom and seven natural frequencies of oscillation were calculated, see table 1. because the torsion system in this state was considered as a free system the first natural frequency is 0hz and it matches with a free rotation of the system. natural modes of oscillation complying with the natural frequencies are presented in figure 10 to figure 16. such a considered state of the torsion system can be agreed with the situation when a railway vehicle does not generate any force in the wheel-rail contact – standstill of the vehicle or drive without traction or brake force. natural frequencies of torsion system [hz] 1. 2. 3. 4. 5. 6. 7. 0 6 57 337 572 857 2403 table 1. natural frequencies overview [3]. figure 10. first natural mode of torsion oscillations [3]. figure 11. second natural mode of torsion oscillations [3]. table 2 provides a description of these natural modes with respect to dominant oscillation of a specific rotation mass. order respective dominant less of natural oscillations significant natural frequency of a mass oscillations modes [hz] 1. 0 own free – rotation 2. 6 wheel-set towards – hollow shaft 3. 57 wheels of – wheel-set 4. 337 wheel-set pinion towards towards hollow shaft rotor 5. 572 pinion towards – rotor 6. 857 wheel-set towards hollow hollow shaft shaft joints gear wheel towards hollow shaft 7. 2403 pinion towards rotor – pinion towards gear wheel table 2. description of natural modes [3]. figure 12. third natural mode of torsion oscillations [3]. figure 13. fourth natural mode of torsion oscillations [3]. 4 vol. 35/2022 kalker’s coefficient c11 and its influence figure 14. fifth natural mode of torsion oscillations [3]. figure 15. sixth natural mode of torsion oscillations [3]. figure 16. seventh natural mode of torsion oscillations [3]. 3.2. model with a wheel-rail contact in this section the attention will be put on calculated eigenvalues and respective natural frequencies. the first group of calculations was done for the value of the kalker’s coefficient c11 = c1. with respect to the damping figure 17 shows that, the damping reaches very high values in low velocities and strongly decrease with increasing velocity. this fact can be concluded with respect to the third eigenvalue, which relates to torsion oscillations of the wheel-set, see table 2. regarding natural frequencies see figure 18. a small change in the value of the second one can be observed, from 6hz to 4hz. a significant change can be observed regarding the third one when its value decreased from 57hz to 0hz. this means that the damping in the wheel-rail contact is so high, that it suppresses torsion oscillations of the wheel-set. for the second group of calculations, where c1 = c11/2 = k1∗, see figure 5 and figure 6, the situation changed. the damping (figure 19) was very high in low velocities again and suppressed oscillations of the wheel-set on the frequency of 57hz. with increasing velocity, the damping decreases. between 100 km/h and 105 km/h retuning of the system appeared. there figure 17. damping as a function of velocity – c11. figure 18. natural frequencies as a function of velocity – c11. is a visible jump in damping and change in the third natural frequency (figure 20), which started to grow. then the third natural frequency increases with decreasing damping and approximate to the value of 57 hz. also, a small change in the value of the second natural frequency from 6hz to 4hz was observed. figure 19. damping as a function of velocity – c11/2. in the third group of calculations, where c1 = c11/4 = k1∗, see figure 5 and figure 6, the results were equivalent to results from the second group. the difference, which figure 21 and figure 22 presents, is that the point of torsion system retuning occurs in lower velocity, between 50 km/h and 55 km/h. a small 5 vojtěch dybala acta polytechnica ctu proceedings figure 20. natural frequencies as a function of velocity – c11/2. change in the value of the second natural frequency from 6hz to 4hz was observed again. figure 21. damping as a function of velocity – c11/4. figure 22. natural frequencies as a function of velocity – c11/4. the fourth, the fifth, the sixth and the seventh eigen value and respective damping parameters and natural frequencies are not mentioned, because the calculations did not show any influence of the variation of values of the velocity and the kalker’s coefficient. 4. conclusions results of calculations presented in figure 17 to figure 22 proved, that the wheel-rail contact can significantly influence a behavior of a torsion system, which means to change some natural frequencies. specifically, the third natural frequency related to the oscillations of the wheel-set itself in a very strong way and weakly the second natural frequency related to oscillations of the wheel-set towards the hollow shaft. on the other hand, it was presented, that the variability of parameters, which characterize the wheel-rail contact, don’t influence the rest of the torsion system. with respect to a research it can practically mean, that for a research oriented on a wheel-set torsion oscillations and related problematics, the effect of the wheel-rail contact should be considered. on the other hand, within a research of torsion phenomenon regarding the rest of a torsion system, as traction motor, gears and coupling, the wheel-rail contact can be neglected. list of symbols ael main half-axis of contact ellipse [m] bel secondary half-axis of contact ellipse [m] b, bw −r damping parameter [nms.rad−1] c11 kalker’s coefficient [-] f natural frequency [hz] fdmp natural frequency of damped system [hz] j mass of rotation [kg.m2] k torsion stiffness [nm.rad−1] m external torque [nm] t1 tangential force [n] rk wheel radius [m] sx wheel slip [%] φ angle rotation [rad] φ̇ time derivative of angle rotation [rad.s−1] φ̈ second time derivative of angle rotation [rad.s−2] λ eigenvalue [-] ω natural angle frequency [rad.s−1] ωdmp natural angle frequency of damped system [rad.s−1] ωk angular speed of a wheel [rad/s] δ oscillation damping [rad/s] acknowledgements this research has been realized using the support of the technology agency of the czech republic, programme national competence centres, project #tn01000026 josef bozek national center of competence for surface transport vehicles this support is gratefully acknowledged. references [1] t. fridrichovský. studie disertační práce. praha, 2017. [2] v. dybala, m. libenský, b. šulc, c. oswald. slip and adhesion in a railway wheelset simulink model proposed for detection driving conditions via neural networks. in sborník vědeckých prací vysoké školy báňské – technické univerzity ostrava, řada strojní, ostrava, 2018. [3] v. dybala. the electromagnetically excited resonance of the pinion in fully-suspended drive of a locomotive and its sensitivity on the torsion stiffness of the rotor shaft. in sborník abstraktů konference stč, praha, 2021. 6 vol. 35/2022 kalker’s coefficient c11 and its influence [4] j. kolář. teoretické základy konstrukce kolejových vozidel. praha: česká technika – nakladatelství čvut, 2009, p. 276. [5] j. kolář. zborník prednášok ii. – xx. medzinárodná konferencia – súčasné problémy v kolajových vozidlách. in dynamika individuálního pohonu dojkolí s nápravovou převodovkou, žilina, 2011. [6] j. kolář. problémy modelování vlivu svislých nerovností trati do dynamiky pohonu dvojkolí. in železničná doprava a logistika xi, pp. 38–47, 2015. [7] r. popovici. friction in wheel-rail contacts. enschede, the netherlands: university of twente, 2010. 7 acta polytechnica ctu proceedings 35:1–7, 2022 1 introduction 2 basic mathematical model 2.1 model with no wheel-rail contact 2.2 model with a wheel-rail contact 3 calculation results 3.1 model with no wheel-rail contact 3.2 model with a wheel-rail contact 4 cocnlusions list of symbols acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.38.0573 acta polytechnica ctu proceedings 38:573–578, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague land use change impact on urban land surface temperatures: a gis-supported satellite-based case study caroline walder, pelin firat ors, ardeshir mahdavi∗ tu wien, department of building physics and building ecology, karlsplatz 13, 1040 vienna, austria ∗ corresponding author: amahdavi@tuwien.ac.at abstract. this paper illustrates the use of gis techniques and satellite data in order to analyze the impact of land use change on the local urban microclimate. specifically, a case study is presented that concerns the city of vienna. thereby, satellite-based images were used to classify the city of vienna into four zones toward the computation of land surface temperatures in two reference years. the classified maps were then statistically projected into the future, resulting in predicted land surface temperatures. the findings highlight the relationship between urbanization and temperature rise in the urban context. the study used data from landsat 8 satellite in 2013 and 2020. land cover maps were generated with qgis for past and current conditions and future land cover maps were projected and corresponding land surface temperatures were predicted. the analysis of satellite data highlighted land surface temperature increase in the city of vienna. this rise in land surface temperatures correlates with urbanization-driven change in land use and land cover. keywords: urbanization, land surface temperature, gis, satellite data. 1. introduction throughout the ongoing expansion and growth of urban areas and a current percentage of over 50 % of the world’s population living in urban areas, the environment is expected to experience massive changes in the future. due to this rapid increase of urbanization, cities are forced to use and therefore seal surfaces in order to provide enough space for their occupants [1]. numerous cities already show higher air and surface temperatures than their rural surrounding areas, which is commonly known as the urban heat island (uhi) effect. the rise of intra-urban temperatures has a strong effect on the quality of life for humans, animals and on vegetation in general [2]. furthermore, according to the intergovernmental panel on climate change (ipcc), world emissions would have to be cut by 45 % in less than ten years, in order to stay below the 1.5 k global warming threshold and to avoid a climate catastrophe [3]. hence, the investigation of land use and land cover as well as quantifying their changes with regard to land surface temperature is of growing importance in urban development [4]. in this context, by projecting the land surface temperature (lst) in an urban setting from two scenarios into the future, the present contribution may contribute to the efforts toward a better understanding of the relationship between urbanization and urban thermal environment. using the capital city of austria, vienna, as an example, the paper presents a gisbased approach, starting from obtaining data from satellite images to a statistical analysis in order to predict future land surface temperature scenarios. the research thus aims to explore gis techniques to obtain future land cover change projections. moreover, the impacts of these projections on the urban thermal environment are evaluated through analysis of land surface temperatures. 2. methodology 2.1. study site and data for the methodological part of the study, the city of vienna was investigated regarding its climatic and geographical properties. moreover, demographic characteristics of vienna were considered, such as an average increase of the population growth of 0.8 %, which correlates to the city’s rate of urbanization. by 2050, the city of vienna is expected to have about 2.2 million inhabitants, which is an increase of about 16 % compared to 2020 [5]. subsequently, the available satellite data was undergone a feasibility check in earthexplorer, a service tool provided by the u.s. geological survey (usgs) [6, 7]. the landsat 8 data belongs to the dataset “landsat 8 oli/tirs c1 level-1”, which is a part of the “landsat collection 1 level-1” data, courtesy of the u.s. geological survey [7, 8], for the representative years of 2013 and 2020 was evaluated with regard to cloud contamination, clarity issues, and other disturbances that might affect the image accuracy. the most suitable datasets recorded by landsat 8 were acquired in world reference system 2 (wrs-2) 190-026 for vienna, with a maximum cloud coverage of 5 %. in order to mimic similar scenarios for past and present, respective landsat 8 images were downloaded for early september in 2013 and 2020; courtesy of the u.s. geological survey [9]. the data was retrieved with 573 https://doi.org/10.14311/app.2022.38.0573 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en c. walder, p. f. ors, a. mahdavi acta polytechnica ctu proceedings semi-automatic classification plugin (scp) version 7.0.0 in qgis [10, 11], and covers a pixel size of 30 by 30 meters with an actual cloud coverage of below 1 %. 2.2. land use/cover classification within the scp environment in qgis, the retrieved landsat 8 images relevant to the study area were reduced in size and converted to reflectance maps. converting the raw satellite imagery to obtain reflectance requires a metadata file, which was generated by retrieving the data from the usgs database. the metadata file contains the required information for the conversion. secondly, the dark object subtraction 1 (dos1) atmospheric correction was applied to the reflectance images. for the resulting bands, the center wavelengths of landsat 8 were defined manually [12]. for the classification of the images, training data was generated in qgis. the “macro classes” chosen for the training input are water, built-up, vegetation, and bare soil. within the macro classes, a more detailed separation was specified following rivers and lakes, buildings and roads, forests, and grassland, and low vegetated areas [12]. by applying “region growing algorithm”, the “regions of interest” (roi) were selected and assigned to the respective macro classes and classes according to the spectral distance, minimum size and maximum roi width [12]. ultimately, the training input file with 49 evenly distributed training input points was generated and applied to the 2013 and 2020 scenarios. 2.3. land use/cover prediction in order to predict the future land use/cover future scenario, modules for land use change simulations (molusce) was applied in the study [13]. with an artificial neural network (ann), molusce is able to generate classified future land use/cover scenarios from past and present conditions. due to the temporal difference of seven years between the two reference conditions, the simulation process with molusce was additionally executed five times after 2020, in order to reach the target year 2055. the projection was calculated with 5000 iterations and an inaccuracy of 0.1 %. the past and present raster images were analyzed through an overlaying process on the land use/cover data. as a result, molusce creates a changes map, which identifies the changes of the land use/cover classification from past to present scenario. molusce is able to compute a transition matrix of the probability of change between the two given scenarios. the output of this algorithm is a transition potential raster of the model, i.e., certancy map, as well as the simulation results, which conclude the predicted future land use/cover classification [14]. 2.4. land surface temperature calculation calculating lst via the following workflow requires a set of input parameters that can be computed with the prepared satellite imagery. these parameters include emissivity (ε), which was derived through the “normalized difference vegetation index (ndvi) threshold method” as a first step. based on the ndvi threshold method, emissivity can be derived for the 10 to 12 µm range [15, 16] which corresponds to landsat 8, band 10 [15]. the mathematical base of the method is represented in equation (1) [15–19]. ε = εs for ndv i < ndv imin ε = εvpv + εs(1 − pv) + dε for ndv imin ≤ ndv i ≤ ndv imax ε = εvpv + dε for ndv i > ndv imax (1) in equation (1), εs refers to the “soil” whereas εv refers to the “vegetation” emissivities [15–19]. equation (2) gives the “vegetation proportion” based on minimum and maximum ndvi values [15–19]. pv = ( ndv i − ndv imin ndv imax − ndv imin )2 (2) with ndv imin = 0.2 and ndv imax = 0.5 [15–19]. in equation (3) below [15, 16, 19], f refers to “a shape factor” with a mean of 0.55 [19, 20]. dε = (1 − εs)(1 − pv) fεv (3) the calculation of emissivity can be concluded via equation (4) [19]. ε = m pv + n (4) with input parameters as per equation (5) and (6) [19]. m = εv − εs − (1 − εs) fεv (5) n = εs + (1 − εs) fεv (6) based on [19], the emissivity values can be assumed as εv = 0.99, εs = 0.97; and m = 0.004 and n = 0.986, resulting in equation (7) [19]. note that equation (7) was proposed for landsat 5 by [19] for the conditions of ndv imin ≤ ndv i ≤ ndv imax and employed in the present work as a simple approach. ε = 0.004 pv + 0.986 (7) the land surface temperature (lst) calculations were conducted following the workflow proposed by [15] which modified the “mono-window algorithm (mwa)” by [21] and adapted to landsat 8 band 10 [15, 18]. this is given in equation (8) below [15]. lst = [a10(1 − c10 − d10) +(b10(1 − c10 − d10))t10 − d10ta] /c10 (8) 574 vol. 38/2022 land use change impact on urban land surface temperatures . . . where, a10 = −62.7182 and b10 = 0.4339 were employed in the present study [15], whereas c10 and d10 refer to the “internal parameters” (equations (9) and (10)) [15]. c10 = τ10ε10 (9) d10 = (1 − τ10) [1 + (1 − ε10) τ10] , (10) where τ10 is the “atmospheric transmittance” and ε10 refers to the “ground emissivity”. both parameters are stated for landsat 8, band 10 [15]. the “effective mean atmospheric temperature” (ta) can be estimated by the linear relation, based on the “near-surface air temperature” (t0) and considering the atmospheric states of the location [15, 18, 21]. as per [15], t0 can be derived from local meteorological stations and calculated by equation (11) [15, 22]. t0,t = tmin + (tmax − tmin) sin ( π t + tdl/2 − 12 tdl + 2ttmax ) (11) with tmin and tmax as “the daily minimum and maximum near surface air temperatures” [15], obtained from weather stations in and around vienna [23]. t0 was calculated and spatially interpolated for the past and present scenarios. on the other hand, brightness temperature, t10, can be calculated from “band-specific thermal conversion constants” (k1 and k2), “spectral radiance” (lλ), and “radiance rescaling factors” (mi, ai) [12, 24]. these values can be found in the respective metadata file [12, 24]. the mathematical definition for brightness temperature t10 and spectral radiance lλ are provided in equations (12) and (13) [12, 24]. t10 = k2 ln ((k1/lλ) + 1) (12) lλ = miqcal + ai (13) according to [15], atmospheric transmittance (τ10) can be estimated – for the present study – according to “mid-latitude summer” with a water vapor content range of 1.6–4.4 g/cm2. this range was selected by following the equations (14) and (15) [15]. w = w(0)/rw(0) (14) w(0) = (h · e · a)/1000 (15) in equation (14), w(0) refers to the ground-level water vapor content (g · cm−2) of the atmosphere, w refers to the total water vapor content (g · cm−2) of the atmospheric column between the ground and the sensor, and finally, rw(0) corresponds to the ratio between the ground-level and total water vapor content [15]. in equation (15), the term h refers to the relative air humidity (%) observed at the surface, e refers to the saturation mix ratio (g/kg) at a certain air temperature between water vapor and air, whereas for that temperature, a denotes the air density (g/m3) [15]. the input parameters for e and a according to the respective specific air temperatures were interpolated for the weighted air temperature of 26.1 °c and 67 % relative humidity for vienna [25]. this resulted in e = 22.05 and a = 1.178 [15]. rw(0) resulted in 0.6834, for mid-latitude summer [15]. 2.5. land surface temperature projection the results of the lst calculations were statistically projected by using a linear regression line. the dependent variable is lst; the independent variable is time. for each of the approximately 460 000 observed pixels per raster image, two observed lst output values are available: past and present. thus, the observed values were extrapolated for predicting the lst of the future scenario. 3. results and discussion 3.1. land use/land cover the results for the land use/cover (lulc) classification of the past and present scenario showed 3.7 % water area for both scenarios. further, it resulted in 43.6 % and 47.2 % built-up area, 44.3 % and 43.9 % vegetated area, as well as 8.4 % and 5.2 % bare soil, respectively. the predicted scenario of 2055 resulted in a distribution of 3.6 % for water surfaces, 51.4 % built-up area, 40.5 % vegetated area, and 4.5 % bare soil. figure 1 includes the lulc resulting maps. as figure 1 indicates, the majority of the water surfaces are distributed over the danube river and its branches and tributaries. it can be seen that the danube river is separated by several built-up area “stripes”, which can be identified as bridges. for all three conditions, the highest density of built-up area corresponds either to the city center or areas of residential or commercial use as well as traffic areas. areas covered by forest displayed the highest level of vegetation. other green areas, primarily covered with grass, were also identified within the city. the majority of bare soil areas was identified in the city’s north and south parts. this distribution is in line with the agricultural land use in and around vienna. in comparison to 2013, the lulc classification of 2020 shows a slight densification of the built-up area in the city center as well as in the outer districts of the city. it further indicates a drastic decrease of the bare soil areas, especially in the northern part of the city. both reference years showed a small amount of builtup classified spots, evenly distributed over the forest regions in the eastern and western parts of the city. regarding the future scenario, the water surface remained mostly the same and the built-up area increased significantly. the vegetated and bare soil areas experienced a decrease of 7.8 % and 13.1 %, respectively. the molusce algorithm “erased” the sparsely distributed small built-up areas in the eastern and western forest areas of the city and overall densified the vegetated and built-up areas. 575 c. walder, p. f. ors, a. mahdavi acta polytechnica ctu proceedings figure 1. land use/cover classification of vienna. ∆t2020−2013 [k] ∆t2055−2020 [k] min. 0.88 4.35 mean 0.36 1.78 max. 0.11 2.44 table 1. summary of the lst differences between 2020 and 2013 as well as 2055 and 2020 scenarios. 3.2. land surface temperature table 1 summarizes the temperature differences (∆t ) between present and past as well as future and present scenarios for minimum, maximum and mean lsts calculated over the study area. figure 2 further presents the lst maps for the three scenarios pertaining to past, present and future conditions. as indicated in figure 2, the lowest temperatures for the past condition (i.e., 2013) can be obtained in the forest regions in the eastern and western parts of the city, as well as in the water area of the danube river. it can be concluded that, additionally to the city center, the denser regions in the south show higher temperatures compared to their surroundings (figure 2). as already mentioned, the lst for the future scenario (2055) was not computed from the respective classified lulc map but projected based on a statistical (linear) extrapolation. the results indicate that the city center as well as the southern and north/northeastern parts experience significant increase in temperature over the course of the next 35 years. not only the built-up areas show temperature rise, but also the eastern and western forest and grassland regions face higher surface temperatures. 3.3. results evaluation the classification result generated by molusce shows realistic values regarding the growing area of impervious surfaces. it appears plausible that bare soil areas, which in many cases are not only farmland but also unused and barren land, are going to be used for constructing new buildings and roadwork. however, it is possible that some inaccuracies occurred in the course of classifying the lulc of the future condition. for instance, molusce predicted single classified water pixels evenly distributed over the study site, where in reality no water bodies could be identified. it can be thus recommended that in future research efforts more spatial variables are included in the process of the molusce classification for better machine learning performance [14]. although the results indicate a slight shift towards higher temperatures over time, it is nevertheless essential to interpret them with caution: first, as the cloud cover of the selected images were below 1 %, all pixels 576 vol. 38/2022 land use change impact on urban land surface temperatures . . . figure 2. land surface temperatures in vienna. within the images were considered as valid data points and an additional screening for cloudiness and potential quality issues was not performed. also, although the satellite images were collected within a similar period in september of the respective years, the weather conditions might have been different. therefore, it is plausible that these factors might have had an impact on the obtained outcomes. the calculated t0 for the past and present scenario may also include irregularities due to the limited weather station data availability for the past scenario (two stations for 2013, seven stations for 2020). this limited information may account for inaccuracies in the later stages of the lst calculations. moreover, the timing of landsat 8 overpass in vienna (10:45 a.m. cet) is not ideal for evaluating the urban thermal environment, as the thermal bands tend to peak between midday and the early afternoon [12]. in the present study, thermal data from landsat 8 was selected considering its suitable spatial resolution [8]. future studies may also employ readily processed lst products from various satellite sensors. one example may be the aqua modis, which provides data with a coarser spatial resolution [26] but overpasses the study area around 12:30–13:30 p.m. cet. a recent lst product from landsat (landsat level-2 surface temperature science product courtesy of the u.s. geological survey) [27] can also be an alternative that would shorten the workflow and ameliorate the mentioned limitations. 4. conclusion the key objective of this contribution was to explore an approach to predict land use/cover change and lst maps based on satellite imagery [28]. it can be seen from this effort that the utilization of satellite data provides a promising opportunity to predict future developments in the urban settings. the elaborated approach is not limited to the specific study area (vienna), but can be applied to any setting on the urban scale, assuming that the required satellite images are publicly available. further analyses could support the predictive assessment of future climatic conditions. such analyses could include diurnal and seasonal variability in data acquisition. utilizing data from a larger number of weather stations may also improve the lst calculations. these steps can facilitate the generalization of the approach pursued in this paper. references [1] h. ritchie, m. roser. urbanization, 2019. 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https://doi.org/10.3390/s19225049 https://doi.org/10.1016/j.rse.2004.02.003 https://doi.org/10.1080/01431160010006971 https://doi.org/10.1111/j.1365-3040.1995.tb00628.x https://doi.org/10.1111/j.1365-3040.1995.tb00628.x https://kachelmannwetter.com https://www.usgs.gov/core-science-systems/nli/landsat/using-usgs-landsat-level-1-data-product https://www.usgs.gov/core-science-systems/nli/landsat/using-usgs-landsat-level-1-data-product https://www.usgs.gov/core-science-systems/nli/landsat/using-usgs-landsat-level-1-data-product https://www.timeanddate.de/wetter/oesterreich/wien https://www.timeanddate.de/wetter/oesterreich/wien https://modis.gsfc.nasa.gov/data/ https://doi.org/10.3390/rs61111244 https://doi.org/10.34726/hss.2021.80480 acta polytechnica ctu proceedings 38:573–578, 2022 1 introduction 2 methodology 2.1 study site and data 2.2 land use/cover classification 2.3 land use/cover prediction 2.4 land surface temperature calculation 2.5 land surface temperature projection 3 results and discussion 3.1 land use/land cover 3.2 land surface temperature 3.3 results evaluation 4 conclusion references https://doi.org/10.14311/app.2022.33.0008 acta polytechnica ctu proceedings 33:8–14, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague 3d finite element simulation of reinforced concrete column strengthened by partially confined carbon fibre reinforced polymer under concentric loading fariz aswan ahmad zakwana, ruqayyah ismaila,∗, hazrina ahmada, raizal saifulnaz muhammad rashidb a school of civil engineering, college of engineering, universiti teknologi mara, cawangan pulau pinang, kampus permatang pauh, 13500 permatang pauh, pulau pinang, malaysia b department of civil engineering, faculty of engineering, universiti putra malaysia, 43400 serdang, selangor, malaysia ∗ corresponding author: fruqayyah812@uitm.edu.my abstract. in this paper, numerical simulation initiated to investigate the mechanical response of reinforced concrete (rc) column strengthened by partially confined carbon fiber reinforced polymer (cfrp). an experimental investigation of the circular hollow unconfined and confined rc column enhances with a partially cfrp sheet has been performed to investigate its behavior under concentric loading on top of the rc column. from the experimental data, the critical buckling failure of the rc column observed. therefore, the prediction behavior of the rc column strengthened with partially confined cfrp under concentric loading was made with the application of the general-purpose of abaqus finite element simulation. from the outcome of the simulation results, the predicted buckling failure compared and validated against the experimental data. the predicted model discovered to agrees well with the experimental work output. keywords: abaqus, cfrp, finite element, rc column. 1. introduction there is a need to incorporate a finite element analysis program to predict the structural behavior of unconfined and confined rc columns under specific types of loading. additional material added to strengthen the unconfined rc column under those applied loading if often quite costly to implemented [1]. this scenario happened is since the more additional material used to strengthen the column, the more cost implication will involve. in previous research, the experimental program of unconfined and confined rc columns reinforced by partially enclosed cfrp under concentric loading as been completed successfully [2–4]. therefore, the general purpose of the abaqus finite element simulation program introduced to validate the mechanical response output from the experimental results [5]. from then onwards, a different configuration of the cfrp layout onto the rc column can be implemented to investigate its behavior under a finite element approach. besides, implementing finite element software would help in reducing the experimental program cost in buying cfrp to be used for the rc column. the mechanical behavior of the rc column can be predicted due to this various configuration of the cfrp strengthening method approach. 2. methods 2.1. abaqus nonlinear analysis of circular hollow rc column three types of nonlinearities exist in concrete materials, namely material nonlinearity, geometric nonlinearity, and contact nonlinearity [6–11]. for material nonlinearity, the material used to create a concrete model behaves as a nonlinear stress-strain relationship. it is much more reliable to incorporate nonlinear properties of elements instead of just adopting linear features due to its precise computation of stress distributions, which enable us to verify any load-bearing capacity aspects of the model such as plastic collapse. for instance, abaqus provides the constitutive model to predict the nonlinear characteristic of the concrete material. there is three constitutive concrete model available that available to be used during the plastic stage of nonlinear characteristic, namely concrete smeared cracking, concrete damage plasticity (cdp), and cracking model. in this current research, the concrete damage plasticity (cdp) constitutive model adopted in the finite element analysis (fea) simulation due to the complexity of the failure mode of the rc column under axial load. for the second geometric nonlinearity material, large deflection is the most common term to define this type of nonlinearity behavior [7, 8, 10]. the deformation translated into rigid body translations and 8 https://doi.org/10.14311/app.2022.33.0008 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 33/2022 3d fem for reinforced concrete column figure 1. abaqus fea model; (a) concrete model (b) reinforcement model (c) cfrp sheet model. dilation angle eccentricity fbo/fco k viscosity parameter 35 0.1 1.16 0.6667 0 table 1. concrete damage plasticity value. rotations, or substantial strains, or a mixture both. it is very crucial to incorporate significant deformation effect onto the final geometry or position of any component to avoid any over-predicted displacement. also, activating geometric linearity will score a slight increase in computational work when material contact nonlinearity exists in the model. in the third contact linearity material, it refers to boundary conditions between multiple components that exist in the model. in this boundary condition nonlinearity, the stiffness of the assembly my change when two or more parts still connected or split from initial contact. 2.2. finite element model and material properties of circular hollow rc column for the finite element model, solid element, truss element and shell element were selected during the numerical simulation stage for material concrete, reinforcement bars and cfrp sheet respectively. for solid element, an eight-noded linear brick solid element (c3d8) type applied for the rc column. for transverse and longitudinal reinforcement bars, a two-node linear 3-d truss element (t3d2) used for modeling purposes. four-node doubly curved shell element employed for the cfrp sheet. all the models illustrated in figure 1. the constitutive model used for concrete materials is concrete damaged plasticity (cdp). the cdp value shown in table 1. meanwhile, for the cfrp sheet model, hashin damage criteria were applied in the model to create typical snap patterns of composite cfrp material with concrete. 2.3. finite element analysis of circular hollow rc column the circular hollow unconfined rc column measured 2 m long vertically. the details of the dimensions and reinforcement illustrated, as shown in figure 2. there are two experimental programs conducted previously, namely unconfined circular hollow rc column and confined circular hollow rc column strengthen with partially cfrp sheet. both rc column sample was subjected full axial load acting on top of the rc column. figure 3 represents the unconfined circular hollow rc column modeled in abaqus fea. it can appear that axial load was acting on top of the column model while the full fixed boundary condition applied at the bottom surface of the column. 3. results 3.1. experimental results the experimental results of the stress-strain diagram for circular hollow unconfined and confined rc column with cfrp wrapping under full axial load illustrated as in figure 5. a full axial load applied on top of the rc column for both unconfined and confined rc columns, as shown in figure 3 and figure 4, respectively. it can be seen from figure 5 that the rc column strengthens with cfrp can sustain much higher stress as compared to the regular unconfined rc column. the stress-strain value taken at the middle section along the rc column. when the strain value approximately 0.015, the predicted stress value of the confined rc column with the cfrp sheet is performing better than the unconfined rc column with 2 mpa increased to become 20 mpa. the unconfined rc column only manages to achieve 18 mpa. the improved confined circular hollow rc column strengthen with cfrp were able to sustain a higher load concerning higher strain value. 3.2. finite element simulation results the predicted outcome of the stress-strain diagram for both circular hollow unrestrained and restrained rc columns shown in figure 6. from the figure, the confined circular hollow rc column with cfrp exhibits a stress value of 27 mpa, which is higher than the unconfined rc column of 18 mpa. similar to the previous chapter, the predicted stress value taken at the middle section along the rc column. 9 f. a. a. zakwan, r. ismail, h. ahmad, r. s. m. rashid acta polytechnica ctu proceedings � �d�� �e� �f�� figure 2. circular hollow unconfined rc column detail specification; (a) front view (b) a-a section view (c) plan view [2]. � �d�� �e�� �f� figure 3. circular hollow unconfined rc column modeled in abaqus fea; (a) front view (b) bottom plan view showing support boundary condition (c) upper plan view showing full axial load imposed on top of the column. � �d�� �f�� �e� 3duwldoo\� &)53�vkhhw� �ghqrwhg� zlwk�uhg� frorxu�� �g�� figure 4. circular hollow confined rc column strengthen with cfrp sheet experimental ser-up and modeled in abaqus fea; (a) front view from experimental set-up (b) front view (c) upper plan view showing full axial load imposed on top of the column (d) bottom plan view showing support boundary condition. 10 vol. 33/2022 3d fem for reinforced concrete column � ϭ͕ϭ ϱ͕ϭ ϭϭ͕ϭ ϭϱ͕ϭ ϯϭ͕ϭ ϯϱ͕ϭ ϯϭ͕ϭ ϭ͕ϭϭϭ ϭ͕ϭϭϱ ϭ͕ϭϭϭ ϭ͕ϭϭϱ ϭ͕ϭϯϭ ϭ͕ϭϯϱ ^ƚ ƌğ ɛɛ ͕�ʍ ;d wă ϳ ^ƚƌăŝŷ͕�ɸ �džɖğƌŝŵğŷƚăů�;ƶŷđžŷĩŝŷğěϳ �džɖğƌŝŵğŷƚăů�;đžŷĩŝŷğě�ǁŝƚś��&zwϳ figure 5. stress-strain relationship of circular hollow unconfined and confined rc column with cfrp under full axial load. � ϭ ϱ ϭϭ ϭϱ ϯϭ ϯϱ ϯϭ ϭ͕ϭϭϭ ϭ͕ϭϭϱ ϭ͕ϭϭϭ ϭ͕ϭϭϱ ϭ͕ϭϯϭ ϭ͕ϭϯϱ ^ƚ ƌğ ɛɛ ͕�ʍ ;d wă ϳ ^ƚƌăŝŷ͕�ɸ eƶŵğƌŝđăů�;ƶŷđžŷĩŝŷğěϳ eƶŵğƌŝđăů�;đžŷĩŝŷğě�ǁŝƚś��&zwϳ figure 6. predicted stress-strain relationship of circular hollow unconfined and confined rc column with cfrp under full axial load. � �d� �e� figure 7. predicted von mises stress of circular hollow unconfined rc column (at the end of simulation analysis); (a) front view (b) upper plan view showing failure behavior on top of the column. 11 f. a. a. zakwan, r. ismail, h. ahmad, r. s. m. rashid acta polytechnica ctu proceedings � �d� �e� figure 8. predicted von mises stress of circular hollow confined rc column strengthen with partial cfrp sheet (at the end of simulation analysis); (a) front view (b) upper plan view showing failure behavior on top of the column. � ϭ͕ϭ ϱ͕ϭ ϭϭ͕ϭ ϭϱ͕ϭ ϯϭ͕ϭ ϯϱ͕ϭ ϯϭ͕ϭ ϭ͕ϭϭϭ ϭ͕ϭϭϱ ϭ͕ϭϭϭ ϭ͕ϭϭϱ ϭ͕ϭϯϭ ϭ͕ϭϯϱ ^ƚ ƌğ ɛɛ ͕�ʍ ;d wă ϳ ^ƚƌăŝŷ͕�ɸ �džɖğƌŝŵğŷƚăů�;ƶŷđžŷĩŝŷğěϳ eƶŵğƌŝđăů�;ƶŷđžŷĩŝŷğěϳ �džɖğƌŝŵğŷƚăů�;đžŷĩŝŷğě�ǁŝƚś��&zwϳ eƶŵğƌŝđăů�;đžŷĩŝŷğě�ǁŝƚś��&zwϳ figure 9. validation process stress-strain relationship of circular hollow unconfined and confined rc column with cfrp under full axial load. 12 vol. 33/2022 3d fem for reinforced concrete column 4. discussions from the experimental and numerical results shown in the previous chapter, the validation process shown in figure 9. for the circular hollow unconfined rc column, the stress-strain behavior agrees well between both experimental and numerical approaches. however, numerical results outcome of circular hollow confined rc column with cfrp sheet exhibit over predicted stress-strain value up to strain value of 0.02. it can be seen that abaqus fea can consistently able to perform and verify the real experimental results, but there are some results display variances between the values calculated by the numerical analysis and experimental output. the reason may be due to: • finite element simulation is the best tool to predict any structural behavior due to axial load. however, there are several constitutive concrete models available that may correspond with the rc column model. the different constitutive models may contribute to different failure behavior to the rc column. therefore, the most appropriate constitutive concrete model needs to be selected to achieve the most reliable failure mode of the rc column at the end of numerical simulation. • apart from that, the connection and bonding between the cfrp sheet and the rc column model also need to be taken into account in the numerical simulation. the relationship between both material also may contribute to the failure mode of the rc column, as shown in the previous validation process. • other important factors also may affect the structural behavior of the rc column, namely convergence of finite element simulation, the global meshing size of the rc column model, parameter values inserted in material properties of finite element simulation, and loading conditions applied onto the rc column. 5. conclusions this research paper represents the verification process between the actual experimental results and predicted numerical analysis. applying abaqus fea was able to verify the actual experimental results of the unconfined circular hollow rc column. however, when adopting additional material of the cfrp sheet onto the rc column, the predicted behavior was over anticipated as compared to experimental results. therefore, abaqus fea proves to be the most reliable method in predicting the structural behavior of the rc column by selecting the most appropriate constitutive concrete model. acknowledgements this work fully sponsored by the universiti putra malaysia (upm) research grant under the putra grant initiative (grant reference no: 9439300). the authors would like to express their most profound appreciation to upm for providing all facilities and human resources during the laboratory work phase. full gratitude dedicated to universiti teknologi mara, cawangan pulau pinang, kampus permatang pauh, pulau pinang. references [1] l. n. k. sai. madupu, k. s. sai ram. strengthening of axially loaded reinforced concrete rectangular columns with gfrp strips. materials today: proceedings 18:3758-65, 2019. 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https://doi.org/10.1016/j.finel.2019.07.004 my title https://doi.org/10.14311/app.2022.36.0280 acta polytechnica ctu proceedings 36:280–285, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague effect of decision parameter efficiency on target reliability andrew way∗, nico de koker, celeste viljoen stellenbosch university, faculty of civil engineering, hammanshand road, stellenbosch, 7600, south africa ∗ corresponding author: acway@sun.ac.za abstract. target reliability forms the basis of most modern design standards and is intended to represent an optimal balance of safety and risk of failure. previous research noted discrepancies between target reliability obtained using generic and case-specific cost optimization for sls cases. the source of the discrepancies was identified as the efficiency of the decision parameter at increasing reliability. this research extends the investigation to cases of uls failures and found similar discrepancies. generic cost optimization assumes a linear dependence between the decision parameter and the structural resistance. where the dependence of resistance on the decision parameter is superlinear, the generic was found to under-predict target reliability by up to 15%. a factor is proposed to adjust generically-obtained uls target reliability to be more appropriate to specific uls cases. the factor accounts for the efficiency of the decision parameter, the case-specific cost of safety and parameter variation. the adjustment factor represents a first step towards mapping generic to case-specific target reliability in the ideal of promoting safer and more cost-effective structures. keywords: cost optimization, structural reliability, target reliability. 1. introduction target reliability in structures is determined through the optimization of costs, considering the consequences of failure [1]. where human safety is at risk in the case of failure, lower limits on reliability are imposed based on societal risk criterion [2]. the costs of increasing safety and the consequences of failure play an important role in the determination of target (optimum) reliability and have been considered extensively in various research contributions [2–6]. the effect that the decision parameter efficiency has on the reliability limit state, however, has not been given as much consideration. in the cost optimization process, a function describing the structural cost is developed, in conjunction with the reliability limit state that governs the design of a structure. the parameter with the greatest influence on increasing the reliability in the limit state is identified as the decision parameter. the optimal reliability is found at the decision parameter value that minimizes the structural cost function. previous research [7–9] has shown that in rc structures governed by sls design, the efficiency of the decision parameter at increasing safety in the limit state equation also plays a role in the determination of the optimum reliability. generic target reliability through cost optimization is typically based on research by rackwitz [4]. in this generic assessment, the structural resistance is linearly dependent on the decision parameter. however, when the decision parameter in the governing limit state takes on a form that is not linear, the target reliability will differ from that obtained through the generic cost optimization. this research aims to investigate various uls limit states to determine the extent to which the decision parameter efficiency affects target reliability and how deviations from the generic target reliability may be predicted and accounted for. 2. target reliability from cost optimization 2.1. generic cost optimization a summary of the generic cost-optimization framework to determine target reliability is presented here, from [4]. the total cost of the structure normalized by the initial structural cost, z(d), is shown in equation 1 as a function of a decision parameter, d. z(d) = cp c0 + u c0 · pfsls γ + ( cp c0 + a c0 ) · ω γ + ( cp c0 + h c0 ) · pfuls γ (1) with cp c0 + c1 · d c0 initial cost of structure independent of d c1 initial costs of structure dependent on d γ age-averaged discount rate ω obsolescence rate u, a, h costs related to sls failure, obsolescence and uls failure pf probability of failure costs attributed to inspection and maintenance are assumed to be included in cp; costs related to fatigue and ageing failure are not considered. the reliability 280 https://doi.org/10.14311/app.2022.36.0280 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 36/2022 effect of decision parameter efficiency on target reliability limit states and probabilities of failure for uls and sls that are linked to the cost function in eq. 1 are given as: guls = ruls(d) − s (2) pf,uls = p (guls < 0) (3) gsls = rsls(d) − s (4) pf,sls = p (gsls < 0) (5) the decision parameter that which most efficiently increases reliability at a specific limit state. in a generic case, it typically affects only the structural resistance, which is modelled lognormally, with mean and coefficient of variation (vi) as ruls ∼ ln(d, 0.3) and rsls ∼ ln(d/1.5, 0.3) for uls and sls, respectively [4]. in effect, this definition reflects the fact that the loading at uls is typically ≈ 1.5 times that at sls and that the decision parameter required at sls is assumed to be affected in a similar fashion. the load is therefore modelled as s ∼ ln(1, 0.3) for both uls and sls. target reliability from cost optimization is defined by the value of decision parameter which minimizes the total cost of the structure. this corresponds to the point d : ∂z(d)/∂d = 0. from eq. 6, the optimal value of d is dependent on various cost ratios, as well as γ and ω, all of which have been investigated in previous research [3, 10–13]. ∂z(d) ∂d = 0 = ( 1 + h c0 + c1d c0 ) ∂pf,uls ∂d + c1 c0 ( ω + 1 γ + 1 ) pf,uls + u c0γ ∂pf,sls ∂d (6) little consideration has been given, however, to the derivative of the probability of failure with respect to the decision parameter at uls and sls (∂pf,uls/∂d and ∂pf,sls/∂d, respectively). this "efficiency parameter" describes how the probability of failure is affected by changes in d and is a measure of how efficient d is at increasing the reliability of the limit state. 2.2. efficiency parameter from eq. 2 to 5, the efficiency of the decision parameter at increasing safety is dependent on the limit state equations for uls and sls. previous research by van nierop et al. [7–9] investigated target reliability in reinforced concrete (rc) structures where the design is governed by sls considerations. this work indicated that discrepancies exist between the target reliability derived using the generic cost optimization and that derived from a case-specific cost optimization. this discrepancy was attributed to the difference in efficiency of the decision parameter in the generic sls formulation compared to that of the specific case, i.e. generic︷ ︸︸ ︷ ∂pf,sls ∂d ̸= specific︷ ︸︸ ︷ ∂pf,sls ∂d and proposed a parameter, ν, that could be used to map the specific case to the generic, of the general form: generic︷ ︸︸ ︷ ∂pf,sls ∂d = ν · specific︷ ︸︸ ︷ ∂pf,sls ∂d . in the context of target reliability, however, it is more convenient to consider it in terms of the reliability index, as the conversion from probability of failure to reliability index, β, is case-independent: generic︷ ︸︸ ︷ ∂βgen ∂d = ν · specific︷ ︸︸ ︷ ∂βspec ∂d . (7) van nierop et al.[8] investigated this discrepancy in target reliability for rc structures governed by sls design. in the current research, the efficiency parameter is investigated in a broader structural context to determine the extent to which it affects target reliability in general. 3. effect of decision parameter on target reliability 3.1. form of decision parameter in limit state equations to investigate the effect of the decision parameter efficiency on target reliability, the governing limit state equation, g (eq. 2 5), is considered in more detail. in the generic cost optimization, a linear dependence is assumed between the decision parameter and the resistance in the generic limit state equation. that is, linear increases in the decision parameter linearly increase the mean of the resistance, and thereby increase the reliability. a typical example of this is the bending resistance of a simply supported rc beam, given by the simplified limit state in eq. 8, where as, fy, x, w and l are the area of tension reinforcement, reinforcement yield stress, moment lever arm, distributed load and length, respectively. the decision parameter is as, which clearly has a linear effect on the resistance and therefore the limit state. in cases like this, the assumption of linearity, and by implication the target reliability derived from it, will also be appropriate. g1(as) = fy · as · x − w · l2 8 (8) in cases where the decision parameter form in the limit state equation is not linear, whether sub-linear or super-linear, the generic assumption of linearity is no longer appropriate. consider the tension resistance of a structural steel rod with radius r, subjected to a tensile load, tu, as shown in the limit state in eq. 9. 281 a. way, n. de koker, c. viljoen acta polytechnica ctu proceedings comparing eq. 8 to eq. 9, it is clear that increasing r is more efficient at increasing the resistance than increasing as, and thus that ∂βg1/∂as < ∂βg2/∂r. in the latter case, a higher target reliability will result. g2(r) = πr2fy − tu (9) this suggests that it is the form of the dependence of the governing limit state equation on the decision parameter that governs the efficiency of the decision parameter and thereby the target reliability. decision parameter efficiency will apply to any structure and not just rc structures governed by sls requirements. cases of uls limit states are investigated below. 3.2. uls test cases the target reliability of a number of specific uls limit states are determined and compared with those obtained from generic cost optimization. varying degrees of decision parameter efficiency are implied by the chosen limit state equations, to determine the effect that the decision parameter form has on the target reliability. a more generalized structural cost equation is used to determine the target reliability as shown in eq. 10, where cf are all costs associated with failure of the limit states under consideration. the normalized failure cost is varied, with particular points of interest at cf /c0 = 1, 3.5 and 7.5, which are representative of small, medium and large consequences of failure (cof), respectively [11]. similarly, the normalized costs of increasing safety (cos) are varied, using c1/c0 = 5×10−2, 5×10−3, and 5×10−4 for high, moderate and low costs of increasing safety, respectively [3, 14]. furthermore, dn in eq. 10 represents a normalized decision parameter, as discussed below. z(d) =1 + c1dn c0 + ( 1 + c1dn c0 + a c0 ) ω γ + ( cf c0 ) pf γ (10) for the intents of evaluating the effect of decision parameter efficiency on target reliability, the rest of the cost optimization parameters are taken directly from [4], with the exception of the case of lower variation, and are shown in table 1. the various uls failure mechanisms in table 1 are chosen to illustrate the effect of decision parameter efficiency on target reliability, in comparison to that obtained from the generic cost optimization. the following assumptions/simplifications are made, and are discussed below: • all action effects (s, m, t, n) are considered with both high and low vs and are modelled as ln(µs = 1, vs = 0.3) and ln(1, 0.15), in turn; • all resistance effects are modelled with a mean value stemming from the resistance term from table 1 and vr = 0.3 and 0.1, in turn. e.g. for case 2: r ∼ ln(fy as z, 0.3) and ln(fy as z, 0.15), in turn; • all decision parameters are normalized with respect to the value of decision parameter, d0, that gives β = 0, i.e. dn = d/d0; • model factors are included as part of vi. the above-mentioned assumptions are made so that meaningful comparisons can be drawn between decision parameter efficiencies, in terms of target reliability. in reality, the statistical distribution and variation of each resistance and load parameter in the various limit states will vary; they are kept consistent here, for the sake of comparison with the generic case. similarly, the model factors assumed to be included in vi will vary for each case. the normalization of the decision parameter is performed so that the normalized costs of increasing safety (c1/c0) can be used for all the cases in table 1. this avoids the subjectivity of considering costs specifically for each case of decision parameter, and is similar in principle to that performed for target reliability in existing structures from [15]. all reliability analyses are performed using the first order reliability method. 4. results and discussion the results of the optimization for higher variation are shown in 1 and 2. red vertical lines indicate small, medium and large cof. the magnitudes of the target reliability in figures 1 and 2 is lower than typical due to the high variation, which is intended to cater to a wide array of applications. from figures 1 and 2, the case of concrete column buckling (decision parameter of a 4th order) has the highest target reliability. uls case limit state equation decision parameter form (order) 1 generic r(d) − s generic decision parameter d linear (1st) 2 rc beam bending fy · as · z − m area of tension reinforcing as linear (1st) 3 steel tension rod πr2fy − t steel rod radius r quadratic (2nd) 4 rc column buckling ecπ3d4/64l2 − n concrete column diameter d quartic (4th) cost optimization parameters γ = 0.035 ω = 0.02 a/c0 = 0.2 cf /c0 : 0.1 − 7.5 c1/c0 : 5 × 10−2; 5 × 10−3; 5 × 10−4 table 1. uls test case limit state equations and cost optimization parameters. 282 vol. 36/2022 effect of decision parameter efficiency on target reliability 0 1 2 3 4 5 6 7 cf/c0 10 6 10 5 10 4 10 3 p f ,1 vr, vs = 0.3, 0.3 generic rc bending tie rod column buckle 0 1 2 3 4 5 6 7 cf/c0 3.00 3.25 3.50 3.75 4.00 4.25 4.50 4.75 5.00 t, 1 vr, vs = 0.3, 0.3 moderate cos generic rc bending tie rod column buckle figure 1. annual probability of failure (left) and annual target reliability (right) for the considered uls limit states for moderate cos (c1/c0 = 5 × 10−3) with high variation. the difference in target reliability with respect to the generic case is ∆βt,1 ≈ 0.6 for all three cases of cos. the generic case and that of bending in an rc beam (both linear decision parameters) have the lowest target reliability and are practically identical. the case of a steel tension rod (quadratic decision parameter) falls in between the generic and concrete buckling cases, with ∆βt,1 ≈ 0.3. these results confirm that target reliability increases with increasing decision parameter efficiency. the process is repeated for lower values of vr = 0.1 and vs = 0.15 to determine whether or not similar trends apply to limit states with variation more representative of typical applications [2, 16], where parameters and/or model factors exhibit lower variation. for the sake of brevity, only the results for moderate cos are shown. from figure 3, similar trends can be observed for lower variation, with higher values of target reliability. similar trends were observed for cases of low and high cos (not shown here). differences in magnitude of target reliability between the generic and column buckling cases are slightly lower for lower variation, at ∆βt,1 ≈0.5. in the consideration of target reliability, higher values are justified when the cof are large, when parameter and model uncertainty is low or when the costs of increasing safety are low. in this case, the latter is true for uls cases 3 and 4. as a result of the heightened decision parameter efficiency, the cost of increasing safety is effectively lowered. the generic cost optimization under-predicts the target reliability for these cases due to the assumed linear form of the relationship between the decision parameter and the resistance term in the limit state equation. figure 4 illustrates this visually by comparing the target reliability obtained from the generic with that from the specific, for moderate cos, high variation and a range of cof of 0.1 ≤ cf /c0 ≤ 7.5. similar results were found for other cos and values of variation. thus, for any specific limit state with a decision parameter form other than linear, the target reliability obtained from the generic cost optimization will need to be adjusted using a factor that considers the decision parameter efficiency. 0 1 2 3 4 5 6 7 cf/c0 3.75 4.00 4.25 4.50 4.75 5.00 5.25 t, 1 vr, vs = 0.3, 0.3 low cos generic rc bending tie rod column buckle 0 1 2 3 4 5 6 7 cf/c0 2.25 2.50 2.75 3.00 3.25 3.50 3.75 4.00 4.25 4.50 t, 1 vr, vs = 0.3, 0.3 high cos generic rc bending tie rod column buckle figure 2. annual target reliability for the considered uls limit states for low and high cos (c1/c0 = 5 × 10−4 and 5 × 10−2, from left to right, respectively) with high variation. 283 a. way, n. de koker, c. viljoen acta polytechnica ctu proceedings cost of safety vi low moderate high form of decision parameter in governing limit state linear quadratic quartic linear quadratic quartic linear quadratic quartic high 0.0 0.3 0.6 0.0 0.3 0.6 0.0 0.4 0.7 low 0.0 0.2 0.4 0.0 0.2 0.4 0.0 0.4 0.7 table 2. target reliability adjustment factor (ν) values for various forms of decision parameter, with reference to eq. 11 for all considered consequence of failure classes (0.1 ≤ cf /c0 ≤ 7.5). 0 1 2 3 4 5 6 7 cf/c0 3.6 3.8 4.0 4.2 4.4 4.6 4.8 5.0 5.2 t, 1 vr, vs = 0.1, 0.15 moderate cos generic rc bending tie rod column buckle figure 3. annual target reliability for the considered uls limit states for moderate cos (c1/c0 = 5×10−3) with low variation. from the results, an additive factor of the form in eq. 11 captures the deviation from the generic target reliability more effectively than the multiplicative form proposed in [7]. generic︷ ︸︸ ︷ βt,1,gen + ν = specific︷ ︸︸ ︷ β1,t,spec (11) values for the target reliability adjustment factor, ν, are shown in table 2 for first, second and fourth order decision parameter forms. these values are applicable for low, medium and high cof and are given for high (vr, vs = 0.3, 0.3) and low (vr, vs = 0.1, 0.15) variation. it should be noted however, that these are indicative first estimates and that further research is required, especially for cases where the cos of a specific decision parameter varies substantially from the considered ratios. additionally, the determination of similar values for sls cases should be considered in future research. 5. conclusions target reliability determined by generic cost optimization forms the basis of design of most modern design codes. to maintain acceptable levels of reliability and simultaneously design cost effective structures, target reliability should be as reflective of the specific structure under consideration as possible. previous 3.25 3.50 3.75 4.00 4.25 4.50 4.75 gen 3.25 3.50 3.75 4.00 4.25 4.50 4.75 sp ec vr, vs = 0.3, 0.3 moderate cos generic rc bending tie rod column buckle figure 4. comparison of annual target reliability for generic vs specific cases for moderate cos, high variation and 0.1 ≤ cf /c0 ≤ 7.5 research reported discrepancies between sls target reliability obtained using generic cost optimization and that obtained using structure-specific cost optimization. the efficiency of the decision parameter at increasing safety was identified as the cause of the discrepancies. this research investigated cases of uls failures and also identified discrepancies in target reliability of up to ∆β = 0.7 between generic and specific optimization. for forms of decision parameter in the limit state equation other than linear, the target reliability differs from that obtained through generic cost optimization and needs to be adjusted to be representative of the specific case. a factor is sproposed to appropriately adjust the target reliability, based on decision parameter efficiency, parameter variation and cost of safety. future research aims to determine similar values for sls, as well as a more comprehensive consideration of decision parameter forms in governing limit states. acknowledgements this research was supported by post-doctoral funding from research subcommittee b of stellenbosch university. references [1] iso. iso 2394:2015 general principles on reliability for structures. international organization for standardization (iso), geneva, switzerland, 2015. 284 vol. 36/2022 effect of decision parameter efficiency on target reliability [2] r. caspeele, r. steenbergen, m. sykora. partial factor methods for existing concrete structures. tech. rep., fib bulletin 80, 2016. [3] k. fischer, c. viljoen, j. köhler, m. faber. optimal and acceptable reliabilities for structural design. struct safe 76:149–161, 2019. https://doi.org/10.1016/j.strusafe.2018.09.002. [4] r. rackwitz. optimization the basis of codemaking and reliability verification. struct safe 22:27–60, 2000. https://doi.org/10.1016/s0167-4730(99)00037-5. [5] o. ditlevsen, h. madsen. structural reliability methods, vol. 178. new york: wiley., 1996. [6] m. holický, d. diamantidis, m. sýkora. determination of target safety for structures. in 12th international conference on applications of statistics and probability, vancouver. 2015. https://doi.org/10.14288/1.0076258. [7] s. van nierop. target reliability of concrete structures governed by serviceability limit state design. master’s thesis, stellenbosch university, stellenbosch, south africa, 2017. [8] s. van nierop, c. viljoen, r. lenner. target reliability of concrete structures governed by serviceability limit state design. 15th international probabilistic workshop, dresden, germany 2017. [9] r. lenner, c. viljoen, s. van nierop. a comparative study of target reliability index derivation for reinforced concrete structures governed by serviceability limit state. struct conc 20(2):670–677, 2019. https://doi.org/10.1002/suco.201800202. [10] d. diamantidis, m. sýkora, e. bertacca. obsolescence rate: framework, analysis and influence on risk acceptance criteria. in life cycle analysis and assessment in civil engineering: towards an integrated vision: proceedings of the sixth international symposium on life-cycle civil engineering (ialcce 2018), ghent, belgium. crc press., vol. 5, p. 379. 2018. [11] joint committee on structural safety (jcss). probabilistic model code, 2001. [12] r. rackwitz. optimization with lqi acceptance criterion. munich, germany: jcss basic documents on risk assessment in engineering, 2008. [13] r. van coile, d. hopkin, l. bisby, r. caspeele. the meaning of beta: background and applicability of the target reliability index for normal conditions to structural fire engineering. in 6th international workshop on performance, protection & strengthening of structures under extreme loading, protect2017, guangzhou (canton), china. 2017. https://doi.org/10.1016/j.proeng.2017.11.110. [14] k. fischer, b.-v. c., f. m.h. deriving target reliabilities from the lqi. in lqi symposium, kongens lyngby, denmark. 2012. [15] m. sýkora, d. diamantidis, m. holický, k. jung. target reliability for existing structures considering economic and societal aspects. structure and infrastructure engineering 13(1):181–194, 2017. https://doi.org/10.1080/15732479.2016.1198394. [16] m. holický. reliability analysis for structural design. sun press, sun media, stellenbosch, south africa., 2009. https://doi.org/10.18820/9781920689346. 285 https://doi.org/10.1016/j.strusafe.2018.09.002 https://doi.org/10.1016/s0167-4730(99)00037-5 https://doi.org/10.14288/1.0076258 https://doi.org/10.1002/suco.201800202 https://doi.org/10.1016/j.proeng.2017.11.110 https://doi.org/10.1080/15732479.2016.1198394 https://doi.org/10.18820/9781920689346 acta polytechnica ctu proceedings 36:280–285, 2022 1 introduction 2 target reliability from cost optimization 2.1 generic cost optimization 2.2 efficiency parameter 3 effect of decision parameter on target reliability 3.1 form of decision parameter in limit state equations 3.2 uls test cases 4 results and discussion 5 conclusions acknowledgements references https://doi.org/10.14311/app.2022.33.0370 acta polytechnica ctu proceedings 33:370–376, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague experimental investigation on the mechanical behaviour of aac blocks for sustainable concrete masonry elena michelini∗, daniele ferretti, alice sirico, roberto cerioni university of parma, department of engineering and architecture, parco area delle scienze, 181/a, 43124 parma, italy ∗ corresponding author: elena.michelini@unipr.it abstract. to satisfy the increasing demand of energy efficient buildings, aac manufacturers are nowadays encouraged to produce blocks with ever lower densities. however, a compromise between energy-saving requirements and mechanical performances is needed to ensure structural safety, as well as an adequate structural durability. this paper reports a comprehensive experimental study on aac mechanical properties (compressive and tensile strengths, as well as fracture energy), and on their dependency from material density and moisture content. the collected data are compared with some well-known analytical relations taken from the literature, which are often used for the calibration of mechanical parameters required for mathematical and/or finite element modelling of aac load-bearing masonry, as well as of aac masonry-infilled framed structures. these comparisons highlight some critical issues in the formulation of analytical relations having a general applicability; however, it was found that rilem suggestions are appropriate for the considered aac productions, at least for densities greater than 400 kg/m3. keywords: autoclaved aerated concrete, mechanical properties, sustainable concrete masonry. 1. introduction the increasing interest in energy efficient buildings has promoted a growing use of innovative masonry products in the construction market. this tendency to search for new and non-standard masonry techniques is related to the widespread awareness that a large proportion of thermal losses in buildings is due to masonry walls. therefore, one of the most efficient strategies for the reduction of greenhouse gases produced by space heating is the adoption of products with increased thermal insulation performances [1–3]. among them, autoclaved aerated concrete (aac) is becoming a quite common solution for the realization of sustainable concrete masonry units (cmus), to be used in the construction of new buildings, as well as in the retrofitting of existing ones [4–10]. aac is a porous lightweight concrete whose cellular structure is generally obtained through a gas-producing chemical reaction of sand, lime, gypsum, and cement slurry, with the addition of an expanding agent (usually aluminium powder). other waste materials, such as fly ash, bottom ash, air-cooled slag, slate waste, glass or perlite waste, agriculture and industrial waste, etc. can be added to increase the sustainability of the production process [11–14]. thanks to the large amount of entrapped air ranging from 60% to 85% by volume the volume of the final product is up to five times that of the raw materials used in the production process. adequate strength and dimensional stability are obtained through autoclaving at high temperature and pressure, with the formation of a crystalline binder, called tobermorite [15]. thanks to the presence of small air voids (in the range of 0.1 − 1 mm) uniformly distributed in the cement paste, aac masonry blocks have many advantages in comparison with conventional concrete: lighter weight (typically from one-sixth to one-third of conventional concrete), lower transportation and building costs, excellent workability and easiness of laying, reduced construction times, remarkable acoustic and thermal insulation. besides its low thermal conductivity (which can reach 0.08 w/(mk), or even less for lower density values, see [16]), aac is characterized by an outstanding air-tightness, which further enhances building thermal performances, creating a comfortable living environment. furthermore, the use of aac, with its simple construction details, reduces energy losses due to thermal bridges at junctions [6]. the flip side of the coin is that the reduction of aac density, which improves thermal properties and reduces structural masses (and consequently the selfweight and the seismic forces acting on the building), exerts an unfavourable effect on material mechanical strengths. this could represent a limitation when aac blocks are used for the realization of load-bearing masonry walls, since in this case it is mandatory to find a compromise between satisfactory insulation requirements and minimum compressive strength values, according to standard codes [17]. for example, regardless of the type of adopted ma370 https://doi.org/10.14311/app.2022.33.0370 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 33/2022 mechanical behaviour of aac blocks figure 1. sampling scheme for compression and flexural tests. sonry units, en 1998-1 [18] requires a minimum normalised compressive strength of 5 mpa, so to ensure adequate stability and robustness of masonry walls in seismic areas. even if not explicitly required by design standards, minimum tensile strength values (and consequently minimum compressive strengths, since these two properties are strictly related to each other) should be also provided for cladding and infill panels in framed structures, to limit crack formation under static and seismic loads. it is indeed well known that one of the most common drawbacks in using aac for the realization of cladding and infills is related to its limited fracture toughness and to its brittleness, which can reduce in some cases the durability of masonry panels, causing the appearance of premature and excessive damages [19, 20]. aim of this work is to provide a further insight on the relation between density and mechanical properties of aac. despite the large number of research works on this topic, a well-established relation between compressive strength and density has not yet been found, since available experimental data are referred to different raw materials in the admixture and to different autoclaving conditions (e.g., [3, 21–24]). on this point, another critical aspect is that temperature, pressure and curing time values are often not specified, despite their marked influence on the formation process of a stable form of tobermorite. a further complication relies on the fact that compressive strength is also dependent from the moisture content at time of testing, as well as from the possible treatments undergone by the specimen to smooth its surfaces (i.e. use of sandpaper, or of a water grinding machine). to better clarify these aspects, an experimental campaign is presented on aac specimens characterized by four different densities and four different moisture contents. the main mechanical properties, i.e. compressive and flexural tensile strengths, and the fracture energy, were determined according to relevant standards [25–27]. the best-known relationships between these properties and the material density, as taken from the literature [23, 24, 28], were then checked against the collected data. 2. experimental program 2.1. preparation of aac specimens all the specimens were cut from commercially available aac masonry blocks with nominal dimensions equal to 600 × 250 × 300 mm. the blocks were manufactured at the production plant starting from the following raw materials: sand with high silica content, cement, lime, water, gypsum, and aluminium powder as expanding agent. the blocks were autoclaved for 11 hours, at a temperature of 180 ◦ c and a pressure of 12 bars. four different types of commercial blocks were investigated, with nominal density equal to 300, 350, 480 and 580 kg/m3 (respectively indicated with symbols a, en, e, and s in table 1). the sampling scheme followed for compression and flexural tests is depicted in figure 1. as can be seen, 6 cubes with 100 mm side and three 50 × 50 × 200 mm prisms were cut from the bottom, middle and top part of each block (with respect to the direction of rising), by using a diamond blade cutter and a hacksaw for aac. three of these six cubes were used 371 e. michelini, d. ferretti, a. sirico, r. cerioni acta polytechnica ctu proceedings figure 2. test setup for the determination of: (a) compressive strength; (b) flexural tensile strength; (c) fracture energy. for the determination of the oven-dry density (db, dm and dt), while the remaining three were tested in compression (cb, cm, ct), according to [25, 29]. prismatic samples were instead tested in flexure (fb, fm, ft) according to [26]. for each examined density, the sampling was repeated on six blocks (so having a total of 144 cubes and 72 prisms). according to the adopted standards, before the tests, specimens were cured at laboratory conditions until the attainment of a moisture content equal to (6 ± 2) %. in order to investigate the effect of moisture content on mechanical strengths, additional samples (108 cubes and 54 prisms) were realized from 18 aac blocks with nominal density of 350 kg/m3. these specimens were subjected to different curing conditions so to achieve a moisture content respectively equal to (0 ± 2) %, (15 ± 2) % and an "upper limit" equal to the moisture content of the samples at their delivery to the laboratory (approximately ranging from 20 to 30%). before conducting the tests, flatness and parallelism requirements were checked; in some cases, it was necessary to smooth sample surfaces with sandpaper. regarding fracture energy determination, the only available standard for aac is represented by rilem recommendations [28], which suggests to carry out wedge-splitting tests. these tests are quite complex to be performed and require a completely different setup with respect to flexural tests. on the contrary, for standard concrete, fracture energy is usually determined by means of three-point bending tests on notched specimens. previous works (i.e. [22]) proved that this test method can be extended also to aac, providing results in accordance with those of wedgesplitting tests. for this reason, in this work fracture energy tests were carried out on notched prismatic samples having the same geometry as those tested in flexure, following the japanese code [27]. it was decided to extract only one prismatic specimen from the middle of each block, together with two cubes for the determination of the corresponding oven-dry density and compressive strength. three blocks were tested for each considered density and moisture content (also in this case, the latter was varied only for those blocks with a nominal density of 350 kg/m3). 2.2. determination of oven-dry density as already stated, three twin cubes extracted from each block (db, dm and dt according to figure 1) were used for the determination of the oven-dry density. to this aim, the specimens were dried in a ventilated oven at the temperature of (105 ± 5) ◦ c until a constant mass was reached. the oven-dry bulk density was then calculated as the oven-dried mass divided by the volume. the moisture content was determined as the ratio between the loss of mass during drying and the corresponding oven-dry mass. 2.3. compression tests according to [25], compression strength was calculated as the average of the results obtained on three standard cubes with an edge length of 100 mm, cut from the top, middle and bottom part of each block. compression tests were performed by using an instron 5882 press working under loading control (figure 2a), with a loading rate of 0.05 mpa/s, as suggested for masonry elements with an expected compressive strength lower than 10 mpa. loading was applied perpendicular to the direction of rise. 2.4. flexural tests aac modulus of rupture (tensile strength in bending, so-called mor) was obtained from prismatic samples subjected to three-point bending over a net span of 150 mm. an instron 8862 press was used to the scope, with a loading rate of 10 n/s (figure 2b). flexural tensile strength was determined as [26]: fct = 1.5 fl bfr h2 fr (1) where f is the failure load, l is the net span, bfr and hfr are the specimen dimensions in correspondence of the cracked section. 372 vol. 33/2022 mechanical behaviour of aac blocks figure 3. evaluation of the average fracture energy for specimens with nominal density of 350 kg/m3 and moisture content ws of (0 ± 2) % by considering: a) the load p -deflection δ curve; b) the load p-cmod curve, according to [27]. density class thermal conductivity nominal density oven-dry density compressive strength flexural tensile strength (w/(mk)) (kg/m3) (kg/m3) (mpa) (mpa) a 0.07 300 ± 50 295 (0.007) 1.93 (0.04) 0.65 (0.13) en 0.08 350 ± 50 346 (0.008) 2.56 (0.03) 0.77 (0.08) e 0.11 480 ± 50 506 (0.002) 3.94 (0.08) 1.13 (0.05) s 0.13 580 ± 50 588 (0.005) 5.49 (0.08) 1.44 (0.05) table 1. properties of aac specimens for different density classes, with (6 ± 2) % moisture content. 2.5. fracture energy tests the test setup for the determination of fracture energy was similar to that adopted for flexural tests (figure 2c). however, in this case, the specimens were notched at their mid-length (with a notch depth equal to 0.3 times the beam depth) and the tests were performed under crack mouth opening displacement (cmod) control, with a loading rate of 0.01 mm/min. the specimens were also instrumented with a linear variable displacement transducer (lvdt) for the measurement of midspan deflection. fracture energy gf was calculated as [27]: gf = 0.75 w0 + w1 alig (2) w0 being the area below load-cmod curve up to specimen failure, w1 the work done by the specimen deadweight and by that of the loading jig, and alig the area of the broken ligament. the applicability of this relation (which was originally developed for standard concrete) to aac was preliminary checked by comparing the so obtained fracture energy with that calculated as the total work of fracture given by the area under the complete load-midspan deflection curve, divided by the ligament area [30]. the deadweight of the specimen and of the loading arrangement was taken into account also in this case. as an example, fracture energy deduced from the load-displacement curve (where the midspan deflection was that measured by the lvdt) and that calculated from the load-cmod curve according to the japanese standard [27] is reported in figure 3 for three prismatic samples with nominal density of 350 kg/m3 (class en), and a moisture content of (0 ± 2) %. 3. results and discussion 3.1. oven-dry density for the four density classes considered in this study, the average oven-dry density was found to be equal to 295, 346, 506 and 588 kg/m3, with a coefficient of variation (cov) reported in brackets in the table below 0.01. these densities were almost coincident with the nominal values provided by the manufacturer, as can be seen in table 1. the declared nominal values of thermal conductivity are also reported in the table for each density class. 4. compressive strength the average cube compressive strength is summarized in table 1 for all the considered density classes, for a predefined moisture content equal to (6 ± 2) %. as can be seen from figure 4a, compressive strength increased with density due to the corresponding reduction in material porosity, but the relation between the two variables was found to be non-linear. the experimental relation between compressive strength and density was checked against some wellknown analytical expressions available in the literature. as can be seen, experimental data laid within the range suggested by rilem recommended practice [28], except for the lower density values. in this case, experimental compressive strengths were higher 373 e. michelini, d. ferretti, a. sirico, r. cerioni acta polytechnica ctu proceedings figure 4. comparison between experimental data and analytical relations available in the literature: a) cube compressive strength fcc vs. density ρ; b) dimensionless cube compressive strength fcc/fcc (6 %) vs. moisture content ws. figure 5. (a) comparison between experimental data and analytical relations available in the literature in terms of flexural tensile strength fct vs. cube compressive strength fcc; (b) experimental relation between fracture energy gf and density ρ, for a moisture content of (6 ± 2) %.. than the predicted ones. in the same graph, the relations proposed by argudo [24] and by chen et al. [23] were also plotted. in these relations, which were obtained from the best-fitting of experimental data collected by the authors, the compressive strength is expressed as a function of the bulk density corresponding to a moisture content of 10% (ρ10 %), by assuming that the ratio ρ10 %/ρdry is equal to 1.1. the comparison between the experimental data collected in this study and the two considered analytical relations shows that a clear relation between compressive strength and density has not yet been found for aac. figure 4b highlights the influence of moisture content at time of testing on the dimensionless compressive strength. it can be seen that the maximum strength was found at the end of the drying process. even if standard codes allow to perform compressive tests on dried specimens (by introducing a correction factor on the so obtained strength), the actual moisture content normally reached by aac blocks in one or two years in external constructions is about 4−6 % [30]; for this reason, it seems more reasonable to provide the compressive strength value corresponding to that condition. in addition, the compressive strength of specimens tested in dry conditions may be influenced by the required thermal treatment. as can be seen in figure 4b, experimental normalized strength at dried condition was found to be significantly lower than that suggested by rilem recommended practice [28], while experimental values for other moisture contents fell inside the proposed range. 4.1. flexural tensile strength and fracture energy experimental values of flexural tensile strength coming from three-point bending tests are summarized in table 1. as for ordinary concrete, flexural tensile strength is generally expressed as a function of concrete compressive strength, instead of density. as can be seen in figure 5a, the relation suggested by 374 vol. 33/2022 mechanical behaviour of aac blocks rilem [28] was found to well fit the experimental data collected in the performed experimental campaign, while the semi-empirical equations suggested more recently by argudo [24] and by chen et al. [23] had a significantly lower precision, especially for higher compressive strengths. this is a further evidence of the limits of semi-empirical relations available in the literature, which suffer the dependency of aac mechanical properties from the raw materials adopted in the production process, the autoclaving conditions and the moisture content of the specimens at time of testing, and consequently they could hardly be generalized. a possible strategy to overcome these limitations could be the formulation of different relationships associated to distinct production processes, and to fix a predefined value of moisture to be reached before testing. even if tensile strength is the main parameter governing crack formation, it can be useful also to know the fracture energy, since it affects crack propagation within the material. when masonry behaviour is analysed through finite element analyses, the knowledge of fracture energy is necessary for the calibration of a proper block cohesive law. however, the dependency of fracture energy with material density has been little studied so far. a possible relation based on the data collected in this experimental program is shown in figure 5b. this relation well fit also the average of the data collected by ferretti et al. [22] in a previous experimental work, on a different aac production with a density of about 550 kg/m3. however, a further validation with other experimental programs is required. 5. conclusions to reduce thermal losses in buildings, an increasing attention is being paid to the reduction of cmus thermal conductivity and, consequently, of their density. however, minimum mechanical properties should be guaranteed also for lower density blocks, so to ensure adequate structural performances and the compliance with durability requirements (i.e. limiting crack formation in infills and claddings). this work focuses on the mechanical characterization of aac blocks belonging to four density classes. based on the obtained experimental results and their comparison with some semi-empirical relations available in the literature (calibrated on other aac productions), the following conclusions can be drawn: • an almost linear relation can be inferred between thermal conductivity and density, as well as between fracture properties (fracture energy) and density, while the relation between compressive strength and density seems to have an exponential form; • the relation between mor and compressive strength is almost linear; • significant variations may be obtained in compressive strength for a given density value, mainly due to different production processes and material curing; • eco-mechanical indexes, which are often used as synthetic indicators of environmental and mechanical performances for concrete (see, e.g. [4]), are hardly applicable in the considered case, unless modifying the definition of the reference environmental data (which are typically related to the production process, i.e., global warming potential and embodied energy, and are consequently quite similar for the four density classes). on the contrary, the major advantages of low density blocks are related to energy savings during the use phase of the building life cycle, which should be somehow taken into account. acknowledgements this work is part of a research founded by ekoru s.r.l., within pon national project n. f/090017/00/x36, cup b18i17000450008. authors gratefully thank dr. f. talento, arch. a. riva and dr. l. bergamonti for their valuable help. references [1] uygunoğlu t and kec̨ebas̨. lcc analysis for energy-saving in residential buildings with different types of construction masonry blocks. energy and buildings 43(9):2077-2085, 2011.https: //doi.org/10.1016/j.enbuild.2011.04.011. 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[12] w. wongkeo, p. thongsanitgarn, k. pimraksa, et al. compressive strength, flexural strength and thermal conductivity of autoclaved concrete block made using bottom ash as cement replacement materials. materials & design 35:434-9, 2012. https://doi.org/10.1016/j.matdes.2011.08.046. [13] p. walczak, j. malolepszy, m. reben, et al. utilization of waste glass in autoclaved aerated concrete. procedia engineering 122:302-9, 2015. https://doi.org/10.1016/j.proeng.2015.10.040. [14] h. el-didamony, a. a. amer, m. s. mohammed, et al. fabrication and properties of autoclaved aerated concrete containing agriculture and industrial solid wastes. journal of building engineering 22:528-38, 2019. https://doi.org/10.1016/j.jobe.2019.01.023. [15] n. narayanan, k. ramamurth. structure and properties of aerated concrete: a review. cement & concrete composites 22:321-9, 2000. https://citeseerx.ist.psu.edu/viewdoc/download ?doi=10.1.1.474.5811&rep=rep1&type=pdf. 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[20] r. cerioni, n. pongiluppi, d. ferretti, et al. damage assessment of autoclaved aerated concrete buildings: some italian case studies. international journal of masonry research and innovation 5(1), 2020. https://doi.org/10.1504/ijmri.2020.10024371. [21] y. song, b. li, e.-h. yang, et al. feasibility study on utilization of municipal solid waste incineration bottom ash as aerating agent for the production of autoclaved aerated concrete. cement and concrete composites 56:51-8, 2015. https: //doi.org/10.1016/j.cemconcomp.2014.11.006. [22] d. ferretti, e. michelini, g. rosati. cracking in autoclaved aerated concrete: experimental investigation and xfem modeling. cement and concrete research 67:156-67, 2015. https: //doi.org/10.1016/j.cemconres.2014.09.005. [23] y. chen, m. peng, y. zhang, et al. mechanical properties of autoclaved aerated concrete with different densities. advances in civil engineering materials 2(1):441-52, 2013. https://doi.org/10.1520/acem20130063. [24] j. argudo. evaluation and syntesis of experimental data for autoclaved aerated concrete ph.d. thesis, university of texas at austin, 2003. [25] en 772-1:2015. methods of test for masonry units part 1: determination of compressive strength, 2015. [26] en 1351:1997. determination of flexural strength of autoclaved aerated concrete, 1997. [27] jci-s-001-2003. method of test for fracture energy of concrete by use of notched beam, 2003. [28] f. h. wittman. autoclaved aerated concreteproperties testing and design (rilem recommended practice). rilem technical committees, 1993. [29] en 772-13:2000. methods of test for masonry units determination of net and gross dry density of masonry units (except for natural stone), 2000. [30] r. g. mathey, w. j. jr. rossiter. review of autoclaved aerated concrete products. nbsir 87-3670. national bureau of standards, 1988. 376 https://doi.org/10.1088/1757-899x/640/1/012016 https://doi.org/10.31031/rdms.2017.01.000518 https://doi.org/10.1016/j.conbuildmat.2015.02.077 https://doi.org/10.1016/j.matdes.2011.08.046 https://doi.org/10.1016/j.proeng.2015.10.040 https://doi.org/10.1016/j.jobe.2019.01.023 https://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.474.5811&rep=rep1&type=pdf http://cementwapnobeton.pl/pdf/2011/2011_special/straube.pdf https://doi.org/10.1016/j.conbuildmat.2015.02.057 https://doi.org/10.1007/s10518-018-0353-5 https://doi.org/10.1504/ijmri.2020.10024371 https://doi.org/10.1016/j.cemconcomp.2014.11.006 https://doi.org/10.1016/j.cemconres.2014.09.005 https://doi.org/10.1520/acem20130063 acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.38.0316 acta polytechnica ctu proceedings 38:316–322, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague excellence in building science education: experiences with a central european experiment ardeshir mahdavia,∗, bob martensb, ulrich ponta, matthias schussa, helene teufla, christiane bergerc a tu wien, department of building physics and building ecology, karlsplatz 13, 1040 vienna, austria b tu wien, institute of architecture and design, karlsplatz 13, 1040 vienna, austria c aalborg university, department of architecture, design and media technology, rendsburggade 14, level 3, 9000 aalborg, denmark ∗ corresponding author: amahdavi@tuwien.ac.at abstract. the spectrum of educational programs in building science is wide and diverse. whereas the academic landscapes across the world vary significantly, general discourse postulates certain rather broad characterizations. for instance, some schools, especially in the anglo-saxon context, are suggested to target a selective, private, and high-tuition approach. others, particularly in the european context, have a more broad, public, and affordable nature. even though highly simplistic and perhaps even misleading, this distinction has been frequently accompanied by the implicit assumption that true excellence (the so-called world-class) education in general and building science education in particular is possible only in the former settings. in this paper, we report on a specific academic degree program, namely the master in building science and technology (bst) program, which may be argued to cast doubt on this assumption. initiated at tu wien, austria, bst was offered over a period of almost two decades. in this paper, we briefly present the genesis of this program, its features, its accomplishments, and its termination. thereby, our primary objective is to inform similar and future initiatives, particularly across central europe, where many public universities exist, the building industry has a substantial presence, and the education of a future generation of technically competent, environmentally conscious, and socially responsible building planners and engineers is of essence. keywords: building science, building technology, curriculum development, environmental education, multidisciplinary teaching. 1. introduction the construction and operation of buildings have major implications. these include the use of resources (energy, materials), the impact on environment (emissions), and the influence on human health, comfort, and satisfaction (indoor-environmental quality) [1–6]. this observation underlines the importance of the education and preparation of professionals responsible for buildings’ design and operation [7, 8]. due to various reasons, including the inherent complexity and multi-disciplinary nature of the building delivery process [9], this education cannot be limited to a single trade and a single format (such as the one offered by conventional architecture schools). this explains the increasing appearance of multiple building-related educational concentrations at different levels. one such concentration pertains to a specific domain that is variously referred to as building science, building physics, or building performance. professionals working in this domain are expected to display in-depth understanding and skill in matters related to buildings’ technical quality. as such, they must address, among other things, buildings’ thermal, visual, acoustic, air quality, fire safety, as well as energy and environmental performance [3, 6, 10–12]. in this context, the present contribution discusses the attributes of and experiences with an educational graduate-level program in building science and technology (bst) area, implemented and operated over a period of 15 years at tu wien in austria. thereby, the primary objective is to inform similar and future initiatives, particularly across central europe, where many pertinent academic institutions exist, the building industry has a substantial presence, and the education of a future generation of technically competent, environmentally conscious, and socially responsible building planners and engineers is of essence. 2. background the conception and initiation of the bst program was motivated by a number of observations and experiences. whereas the academic landscapes across the world vary significantly, general discourse postulates certain rather broad characterizations. for instance, some schools, especially in the anglo-saxon context, are suggested to target a selective, private, ranking oriented and high-tuition approach, others, particularly in the european context, have a more broad, public, and affordable nature. even though highly simplistic and perhaps even misleading, this distinction has 316 https://doi.org/10.14311/app.2022.38.0316 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 38/2022 excellence in building science education . . . been frequently accompanied by the implicit assumption that true excellence (the so-called world-class) education in general and building science education in particular is possible only in the former settings. the initiators of the bst program considered the potential for synthesis of these elements. the idea was that an educational program with high conceptual leaning toward fundamental science foundations can be implemented in a public technical university geared toward industry-driven professional education. likewise, the idea was that the merit-based admission to the program and continued performance monitoring can be also implemented in a low-tuition public educational setting. specifically, one of the initiators had experienced in-depth, both the conventional architecture and engineering curricula in german speaking countries and the so-called elite university programs in north america. the idea was to conceive a master program in bst that would appeal to candidates with a diverse background in their undergraduate education, would be open to acquire a deeper theoretical foundation, and engage in intensive learning process concerning digital tools and computational modelling. the selection of english as the program’s language facilitated the emergence of an international body of students and made it possible to implement a formal admission procedure for the incoming students. 3. a brief history the bst program could not be initially started as a “default” master program at tu wien. this was due to internal circumstances in the pertinent college (faculty of architecture and planning). there were concerns on the side of the senior staff that resources would be diverted from the architecture program to a new program whose nature and necessity they may have insufficiently realized. on the other side, some representatives of the student body were opposed to introduction of admission conditions and processes intended by the initiators of bst. note that the majority of the disciplines at tu wien do not require any admission requirements other than high school degree. however, a program offered exclusively in english (as opposed to german) could be exempted from this practice, a circumstance that the bst initiators intended to use so as to define necessary conditions for admission to the program. as the result of these oppositions, the program had to be initially installed as a post-graduate master program within the tu’s continued education framework, which is not tuition-free and is mainly meant for professionals seeking a flexible educational opportunity while remaining active in their jobs. during this phase, the program included three foci, namely building physics, structural design, and design computing. in the framework of two delivered cohorts this program educated a number of well-qualified graduates. however, the circumstances were not seen as ideal in view of the program initiators. on the one hand, the more desirable access rules (i.e., no or a very small tuition) available to tu’s regular programs was not available for bst students. on the other hand, the operation of the program with three parallel foci (building physics, design computing, structural design) was not found to be the most efficient solution, implying the need for further topical concentration. benefiting from a window of administrative opportunity, it was possible to move the bst to the regular educational program. subsequently, the curriculum was also modified to provide a more robust focus on building physics. from this time on, the bst was offered as a regular 2-year master program until its discontinuation in the year 2021. the reasons for program’s termination were not inherent to the academic nature of the program, which has been shown to be very successful. rather, top-down decisions at the higher level of tu governance ultimately led to circumstances that made the termination of the program inevitable (see section 5 for further details on the circumstances responsible for this development). 4. the structure of the curriculum as indicated before, the bst curriculum experienced a larger revision (augmented concentration on building physics content accompanied by computational and diagnostics tools and methods) and a few smaller adjustments. the result of these development was a fairly stable, robust, and efficient curriculum. the syntax and content of this curriculum are presented in figure 1 in a compact form. in terms of the course load, the curriculum has the bologna-conform 4-semester 120 ects format [13]. following a university-mandated overall curricular framework, the entailed courses in the programme (see figure 1 for details) are structured in terms of thematically coherent groups referred to as modules. the general sequential logic of the program corresponds to the envisioned students’ progression in knowledge and skills acquisition. focus on foundational material in the first semester, intensification of disciplinary material in the second semester, the multi-disciplinary, collaborative and praxis-oriented project course in the third semester, and the individual research work in term of the final semester’s master thesis was intended to offer the right superposition of program syntax and the program content. 5. experiences with the operation the long-term multi-year experience with the bst program involved on the one hand highly positive aspects, but also a number of challenges. to start with the positive, a few observations are listed below outlining the key contributing factors to the program’s success: • the aforementioned decision pertaining to the introduction of admission requirements for bst pro317 a. mahdavi, b. martens, u. pont et al. acta polytechnica ctu proceedings figure 1. structure of the bst curriculum. gram made it possible to maintain, in comparison to other (bachelor and master) programs of the college, a smaller student to teacher ratio. • english as the language of the bst program may have encouraged a more diverse (internationally oriented) body of students, which is arguably less common in central european programs. • the disciplinary diversity of the backgrounds of the students (architecture, engineering, computing) supported the emergence of an inquisitive and openminded attitude within the student body. this sharpened also the understanding and appreciation of the interdisciplinary and collaborative aspects of the building design and delivery process. • the foundational courses in mathematics, physics, and computing made it possible to bring up the students with background in architecture to the necessary prerequisite levels of students with backgrounds in engineering and informatics. on the other hand, courses regarding tools and media, as well as a specifically tailored building construction course introduced students with engineering and informatics background to the building technology skills and knowledge. • early focus on courses that dealt with the scientific method supported the development of a better grasp of the nature of scientific research. this contributed also to students’ preparations for their master thesis. • a pair of bst attributes, one regarding the orientation and the other regarding the approach may have contributed to its success and reputation. the former attribute pertains to a parallel emphasis on both theory (scientific foundations) and praxis (applied skills). the latter attribute is related to the concurrent application of both computational and experimental tools and methods. • a major course in the third semester of the program, namely the “project course” was found to be especially effective. this course was not focused on frontal transfer of knowledge. rather, it regularly selected practically relevant research and/or planning agenda regarding the quality and performance of the built environment as its subject. students worked collectively and in teams on the task at hand, thereby assuming various roles (e.g., building design, system configuration, performance modeling). • a few courses emerged in a kind of evolutionary step based on experiences gained in the early years of the program. one course, labeled “master thesis seminar” accompanied the students in the preparation of their master thesis proposal. earlier experiences had shown that completing a high-quality master thesis in the final semester of the studies was not a trivial matter. the seminar facilitated, in the 318 vol. 38/2022 excellence in building science education . . . preceding semester, the initial consideration of possible topics, their merits, and the feasibility of their completion. thereby, the students could select the topic of their thesis themselves, or request the supervisor for suggestions. another course, the “master progress seminar”, accompanied the students in the actual developmental work for their master thesis. thereby, the students could attend the progress presentations of their colleagues, receive feed back from the supervisors, and discuss their work with the other students. • the general sequential logic of the program fulfilled the expectations with regard to the students’ progression in knowledge and skills acquisition. focus on foundational material in the first semester, intensification of disciplinary material in the second semester, the multi-disciplinary, collaborative and praxis-oriented project course in the third semester, and the individual research work in term of the final semester’s master thesis proved to offer the right superposition of program syntax and the program content. • the generally research-oriented master thesis in the final semester of the program did serve multiple purposes. aside from acting as final exercise in independent structured query, the master thesis enabled the students to pursue, in an in-depth manner, subjects that they considered to be essential in view of their future plans and aspirations. frequently, the research conducted in the framework of the master thesis provided the students material for scientific publications in conference proceedings and journals, hence improving their career chances for those targeting future academic employment opportunities. • upon completion of their master studies, a considerable number of graduates decided to pursue doctoral studies both at tu’s department of building physics and building ecology and in other schools. as alluded to before, the program faced also a number of challenges with regard to both academic performance and administrative boundary conditions. instances of such challenges are briefly addressed in the following: • given the students’ non-homogeneous backgrounds and differences in their skill levels, the knowledge transfer process could not be maintained in a consistently optimal manner. hence, in a few occasions, the learning material and the knowledge transfer pace was turned out to be too advanced for some students and insufficiently challenging for others. • the bst master degree was conceived as a fulltime educational program. nonetheless, due to the absence of stipends and despite the low tuition level, a fraction of (self-supported) students needed to work on the side. this resulted, in certain cases, in scheduling frictions, reduced level of concentration, and ultimately a lengthened duration of studies. • the role, value, and accomplishments of the bst program were perhaps insufficiently visible at the college and university level. the lack of precedence for such a pioneering internationally oriented building science program offered exclusively in english language may have contributed to this circumstance. as perceived by the program initiators and staff, the most critical challenge faced by the program, was the insufficient level of engagement (structural and institutional support) from the college and university instances. consequently, the program could be offered over the course of multiple years only due to the sheer dedication and ultimately self-exploitation of the involved faculty. every aspect of the program had to be taken care of exclusively by the immediate bst staff (predominantly members of the building physics and building ecology department), who had also many other responsibilities (teaching in bachelor and master programs in architecture, supervision of doctoral studies, academic research). moreover, processes regarding the promotion of the program, the students’ admission process, the curriculum design, the scheduling and offering of the courses, the evaluation of students’ performance, and the dissemination of students’ research did not benefit from college or university support. • the absence of a promotional budget made it difficult to make the programme broadly known. this probably reduced the pool of potential applicants. moreover, university’s position in international ranking schemes (a factor amongst those influencing candidates’ choices of schools), the absence of stipend offers, and the impression that studying would require the knowledge of german language, might have led some very talented students to seek admission in other universities. 6. performance and output the objective presentation and evaluation of the standing and performance of an academic program is not a trivial matter. nonetheless, some factual data and statistics related to the bst program’s educational and academic output may offer a measure of performance accessible to some degree of objective scrutiny. since the inception of bst in the year 2004, roughly 160 students (57 % female and 43 % male) graduated from the program with a master in bst (as of september 2021). figure 2 illustrates the distribution of the topical foci of the master thesis of the graduated students. the median duration of the studies amounted to 6 semesters. this is longer than the 4-semester duration intended by the program initiators. however, as mentioned before, a number of students worked part-time, hence requiring more time to complete their studies. 319 a. mahdavi, b. martens, u. pont et al. acta polytechnica ctu proceedings figure 2. topical distribution of the bst graduates’ master theses. figure 3. survey results concerning past bst students’ professional foci after their graduations. aside from the master theses themselves, related publications represent another measure of the academic output of the bst program. altogether, 91 conference papers and 29 journal papers were co-authored by the bst students. the distribution of the thematic foci of these publications corresponds roughly to that of the completed master theses (see figure 2). the performance and ultimately the real-world success of the bst program can be also approached based on the carriers pursued by the graduates. upon completion of their master studies, 18 students pursed doctoral studies at the department of building physics and building ecology or, in a few cases, other units of tu wien. due to certain constraints (specifically with regard to data privacy issues), it is not possible to put together a comprehensive record of the activities and carriers of the bst students after their graduation. however, a recently conducted survey among the bst’s program’s graduates resulted in 61 responses. the processing of the data obtained via this survey yields an overview regarding the professional careers the bst students pursued subsequent to their graduation (see figure 3). in the course of the survey, impressions, opinions, observations, and ideas were solicited as well. the responses were positive in general. a few instances of this feedback (statements in the survey) are included in the following: • “the master program was first of all a professional boost in my career and a socially fulfilling experience – i have enjoyed having an international team of lecturers as well as group colleagues not only from various cultural, but also professional backgrounds.” • “the multicultural and interdisciplinary environment of the program was very interesting and widened my horizons both intellectually and professionally and for that i’d be always grateful.” • “i found very necessary in the job market the digital tools i learned at the tu vienna. i would recommend more advanced . . . courses . . . during master studies. energy certifications are also very demanded in the market, emphasis should be given into that too.” • “i personally found the pace of studies a bit slower than i expected . . . for example . . . , the cfd analysis also could have lasted a whole semester.” • “[the] master study . . . had a very good quality of education and opened several career doors to its graduates!” 7. lessons learnt the complex background of the bst program and the rich set of experiences with its operation cannot be summarized in terms of a few observations. however, some qualitative reflections, structured in terms of a kind of brief swot analysis (strengths, weaknesses, opportunities, and threats), may provide some insights that could be of use for similar initiatives in the future. • strengths: as alluded to before, the major strengths of the program lie in its combination of scientific foundations, digital media, diagnostics skills, balance between theoretical models and practical applications, balance between mandatory and elective courses, and last but not least, the vibrant multicultural composition of the student body. 320 vol. 38/2022 excellence in building science education . . . • weaknesses: bst was intentionally open to applicants from different educational backgrounds. this had advantages (for instance, in the operation of the project course and its real-world collaborative agenda). however, this diversity represented also a challenge in view of the optimal depth and sophistication of material coverage. but the most critical challenge the program faced was perhaps not an endogenous one. rather, it was mainly due to the boundary conditions. for an academic program to be sustained over the long haul, the institutional understanding and support is essential. of course, an inspired educational initiative requires, as the necessary condition, the “bottom-up” engagement and enthusiasm of the involved faculty. this is, however, not sufficient in the long run, as the bst experience clearly demonstrates. institutional support is essential, both in administrative terms (promotion, admission, service, tuition and stipend options) and the academic sense (faculty positions and time). • opportunities: as bst no longer exists, the discussion of opportunities is relevant only to future efforts. many central european technical universities have an excellent background in engineering education. extending their repertoire with novel computationally driven and interdisciplinary programs does not negate this excellence. rather, it facilitates the transition to creative academic offerings of the future. moreover, adaptation of certain strategies in the graduate programs of the so-called top universities does not mean that one must also adopt their so-called elitist features (highly restrictive admission procedures and exorbitant tuitions). • threats: again, the termination of the bst program makes the discussion of the threats relevant only in view of possible future initiatives. a general threat, not only related to bst, but to most academic programs, concerns the rapid developments in the so-called information society. recent trends with home-officing and online-teaching have been used as arguments to question the future feasibility and viability of the “traditional” academic programs. there is no reason why new forms of digitally supported and hybrid methods and platforms of knowledge transfer and life-long learning should not be adopted. but the related processes need to be careful not to throw the baby of empathic learning environments with the bath water of conventional frontal classroom teaching. specifically, the design and operation of built environment is an inherently collaborative and contact-intensive process. care should be thus taken that future developments in academic formats would not neglect this essentially communal attribute of the building delivery process. acknowledgements recent and past faculty staff and external lecturers contributing to the programme included, amongst others, christiane berger, peter ferschin, kristina kiesel, ardeshir mahdavi, nikolaus maly, bob martens, kristina orehounig, ulrich pont, claus pröglhöf, peter schober, matthias schuss, balasz somogvary, christian steineder, georg suter, sigrun swoboda, farhang tahmasebi, helene teufl, milena vuckovic, dawid wolosiuk, gabriel wurzer, robert zach. whereas these colleagues contributed to the bst programme in different positions and at different times, they are not responsible for the content of the present contribution. the authors acknowledge the dedication of the bst students to the objectives of the programme and their contributions to a vibrant educational environment. references [1] global alliance for buildings and construction, international energy agency, united nations environment programme. global status report for buildings and construction 2019: towards a zero-emission, efficient and resilient buildings and construction sector, 2019. [2021-09-29]. https://www.iea.org/reports/global-statusreport-for-buildings-and-construction-2019 [2] international energy agency. transition to sustainable buildings: strategies and opportunities to 2050, 2013. [2021-09-29]. https://iea.blob.core. windows.net/assets/1e300ab6-44de-41dc-8714ee12a4800943/building2013_free.pdf [3] ansi/ashrae standard 55. thermal environmental conditions for human occupancy, 2017. 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utilities) will face new challenges in their traditional way of doing business. therefore, adapting their business models to remain competitive is seen as an important step. we chose to characterize these business models by content, structure and governance of transactions for creating value by exploiting business opportunities. in the energy sector the following characteristics to the business models for energy supply have been identified. the most predominant archetypes of business models for the energy supply are presented and discussed. further, we propose to set up (or use existing) innovation clusters, based on these promising bm to ensure that innovative business environments (innovation clusters) will grow that have the potential for upscaling and replication of district decarbonization solutions. there are no specific business models for energy supply applied to renovation of districts. uncertainties in the supportive measures for the application of der makes it difficult to develop new business models for the utilities. keywords: business models, energy supply, innovation cluster. 1. introduction renovation strategies on building level need to be derived as a combination of energy efficiency upgrades for buildings and the use of renewable energy to decarbonise the energy supply, on district or city scale. to this end, the annex 75 of the iea ebc programme sets off to define a methodology to identify which strategies are most energy saving and cost-effective when applying both energy efficiency and renewable energy measures [1]. by combining energy efficiency and renewable energy sources, the approach addresses both energy supply and demand in the built environment. in this sense, building retrofitting is an appropriate strategy to reduce demand, while the use of renewable energy aims at decarbonizing the energy supply system. nevertheless, to apply the large-scale renovation strategies and achieve the projected building stock decarbonisation, identifying the technical solutions is not enough. the renovation rate in europe remains well below the targeted annual 3 % [2, 3]. some of the main barriers to renovation have to do with the renovation cost and access to finance, as well as complexity, awareness, stakeholders’ management and fragmentation of the supply chain [4]. as a result, business models are relevant to implementation and acceleration of renovations. seddon et al. [5] defines “business model” as the outline of essential details of a firm’s value proposition for its various stakeholders, and the activity system the firm uses to create and deliver this value proposition. in other words, a business model is the abstraction of a strategy, focused on the system of activities through which a firm creates economic value. 1.1. energy supply grid renovation strategies on district level comprise of energy efficiency measures on building level as well as energy supply measures on the district level. energy supply for the buildings stands in for the supply of both electric and thermal energy. most of the energy use of buildings is related to space heating (and cooling), therefore the needed thermal energy can be extracted directly from a dh(c)-network or provided by a heat pump (hp) or even directly converted using electric heaters/coolers. energy supply companies are responsible for supplying buildings and district with their needs in terms of electric and thermal energy, however still most of the provided energy 417 https://doi.org/10.14311/app.2022.38.0417 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en matthias haase, emilia motoasca acta polytechnica ctu proceedings figure 1. illustration of energy supply system options (blue: gas, green: renewable: dotted: new connections). is mostly electric energy. electricity supply companies today face a strong decrease in the number of customers. this has to do with new market actors which are operating with new business models centered on renewable energy and energy efficiency technologies. to understand the situation better, it is helpful to acknowledge that electricity supply companies act in an organized electricity market based on different marketplaces. the physical electric energy system encloses the infrastructure (generation, transport, distribution and use together with their components). thus, the market consists mainly of the following actors [6]: • electricity generator: who generates electricity and sell it to the energy suppliers. • electricity suppliers who purchase the electricity from the generators and sell it to consumers. • consumers who use electricity and pay monthly fees to suppliers. • transmission system operators (tso), who are responsible for transporting electricity for long distance and ensuring grid stability and reliability by real time dispatch. • distribution network operators (dso), who are responsible for delivering electricity to the consumers and measuring the consumption. • regulators, who set the market rules and oversee the functioning of the market. 1.2. energy supply value stream in the eu countries some energy supply companies operate transnationally, however still a large part of the worlds’ electric energy supply is either based on fossil fuels like coal, gas, and oil or nuclear energy. the production, transmission and distribution of electricity accounts for the largest share of the world’s anthropogenic greenhouse-gas emissions, while the use of emission-free nuclear energy comprises serious security risks and unsolved problems of hazardous waste, therefore the role of renewable energies as the most important instrument to mitigate climate change and reduce negative effects of energy production is increasing. despite the fact that utilities (with national or transnational activities) still have a dominant position they are confronted with disruptions of their current way of doing business and face the challenge to develop new business models for electricity generation from distributed and highly intermittent renewable sources. in this sense, the energy supply company changes into a service provider based on the idea of an innovation cluster which can be understood in two ways. the traditional view is that it is a collection of companies situated within some level of proximity, allowing for more collaboration, interaction, development of stronger ties and a natural growth of a collaborative strengths within the cluster. in our work, these new energy service providers or innovation clusters acts as ecosystems that create an active flow of information and resources for ideas to transform into reality. through these ecosystems, a process is started which creates expertise in new areas (here buildings and energy), helps to diversify the economy, and allows businesses to meet their customers where they are. additionally, an innovation ecosystem provides the means to create economic stability and resource sharing [7]. the value of an innovation ecosystem lies in the access to resources and the flow of information for the ecosystem’s stakeholders. this information flow 418 vol. 38/2022 characteristics of business models for innovation clusters . . . exists already in some energy supply companies and creates more investment opportunities for the right institutions to connect with the right ideas for their businesses and portfolios, at the right time, for the right reasons. for energy supply companies which want to activate value streams in the building sector it is important to understand the different business models. 1.3. research question the transformation of today’s electric power sector to a more sustainable energy production based on renewable energies will change the structure of the industry [8]. in this transformation towards a smart energy system interaction between sectors and technologies the main stakeholders (energy service providers; utilities) will face new challenges in their traditional way of doing business. therefore, adapting their business models to remain competitive is seen as an important step. when looking at the business model literature reveals that there exist basically two possibilities: (1.) ownership of renewable energy assets [9]. (2.) utilities need to develop from commodity providers to energy service providers [10, 11]. according to this idea utilities should evolve to comprehensive energy-solutions providers for residential and commercial customers to create new sources of revenues. but which characteristics of these new value creation streams exist and which could be extended and developed further? 2. methodology new business models are needed that are able to work with energy demand reduction and supply. literature review was conducted in the scopus and science direct database based on the method iterative method proposed by bocken et al. [12] starting with a cascading keyword search (business model, energy supply, districts, characteristics) resulting in 844 publications. of these were 69 review articles of which 50 were reviewed to identify prominent schemes. of the 710 research articles, 241 were doublets and disregarded resulting in a list of 468 research articles. a total of 523 articles were reviewed to identify examples and case studies. a total of 34 in-depth interviews with different stakeholders were conducted between november 2020 and september 2021 with energy experts from the netherlands, switzerland, span, austria and germany. this formed the basis for compiling a number of main characteristics of business models for energy supply. we chose to characterize these business models by content, structure and governance of transactions for creating value by exploiting business opportunities. “content” refers to the performed activities (transactions/exchanges of goods and information), together with the required capabilities and resources. “structure” describes the links among the activities (transactions) and the participant parties, while “governance” provides information about who performs the activities and where, thus it refers to legal organizational form, incentivizing of participants and the way information, goods and resources flow. the analysis and compilation of the most common business models for energy supply can help to develop their own bm following the bmc (business models canvas) principle. 3. results in the energy sector the following characteristics to the business models for energy supply have been identified that can be characterized by servitization (energyas-a-service), financing and ownership, public interest, customer’s role, decentralization, energy tariff’s structure, infrastructure, flexibility, and management and control. 3.1. servitization (energy-as-a-service) the bm have a more service-oriented character, thus the companies are selling the functionality of the product rather than the product energy. the product is thus replaced by a combination of products and services and the notion of the value moves from exchange to application. in the energy transition context, servitization is correlated with energy services and energy efficiency. this means the perception of having a certain percentage cost savings due to the reduction of end-user’s energy consumption. the variations of energy services range from additional services such as aggregated information and provision of data analysis to advanced services, such as performance and activity control, depending on servitization intensity. energyas-a-service activities include energy planning, project design, implementation, maintenance, management, operation evaluation and equipment supply. in this bm, performance is directly linked to the energy savings achieved where its remuneration includes savings guarantees. 3.2. financing and ownership for the energy supply (mainly of the renewable energy sources (res)) three main ownership models are predominant: consumer’s ownership, collective community ownership and service-based with company ownership [13]. since res capacity may have an influence on the grid stability and energy supply security, the value stream is based on the notion that owners may influence grid balance. one emerging bm is the remuneration of these services to increasing or decreasing grid stability. in some bm there is a clear distinction between the customer or/and third-party ownership, where the prosumers or aggregators can choose to contribute (or not) to grid balance activities. this relates to the utility ownership, i.e. the energy supply company has the full authority to manage and maintain the grid and control the renewables production. 419 matthias haase, emilia motoasca acta polytechnica ctu proceedings collective community ownership is often considered as a bm where the source of income can be controlled locally. therefore, financing these kinds of investments are more likely to be accepted within the community, because it helps developing local res ownership and avoids that value leaks out of the local economy. financing of res technologies is a crucial factor for both micro-generation (where the costs are barrier because of the long-term investment in the infrastructure assets and the success or failure of investment depends on the institutional support) or for large-scale res technologies (where the large upfront cost is often described as a “barrier that prevents customers from financing res and hence outsourcing financing to a third-party in order to remove this barrier”) [14]. alternative financing sources for re investments are the newly emerging energy cooperatives, where the financial risk can be mitigated by local community investment or collectively fundraising for re through crowdfunding platforms. 3.3. public interest the public sector may wish to steer a district energy project towards a variety of specific local objectives [15], including: cheaper local energy for public, private and/or residential customers (e.g., the alleviation of fuel poverty); local job creation; local wealth retention; low-carbon power generation; and/or local air pollution reduction. economic modelling helps quantifying these objectives and thus creating additional value by extending standard financial modelling. 3.4. customer’s role understanding the different roles of the customer is centrally important to reduce the cost in the energy supply and specifically in the demand side management (dsm) for compensating consumers for their participation in demand response programs [14]. the relationship with customers has been modified since some years, by intensifying customer engagement or by delivering new services, by providing consumption and production related information real-time or by installing enhanced two-way communication channels. the knowledge about consumer’s behavior, including attitudes, tastes and needs are critical success factors for the bm operating in decentralized systems. the reason is that multiple roles for the consumers are possible. first, there are active producers and consumers who produce and self-consume green electricity and/or heat simultaneously and alternating (e.g. day/night). thus, customers act as financial investors in res and demand user services as light, heat, etc. instead of an energy commodity. this results in interactions with local beneficiaries, project supporters/protestors/activists, and not least with technology hosts. 3.5. decentralization energy systems can be designed on different levels of decentralization. nowadays, due to the distributed nature of res and their smaller production capacity a new decentralized energy market has been established. it requires different revenue models than the classical ones of centralized energy production. the associated bms should provide distributed solutions (e.g. for each consumer separately), thus implying higher costs compared to previously dominating one-size-fitsall solutions. this provokes a strategy shift from big to small, from wholesales to a customer-oriented strategy, from commodity to service, and from long-term energy planning to a more flexible energy planning including further services/infrastructure like e.g. energy storage. in res dominated energy production, the ownership of the res and the vicinity between production and consumption sites plays a determining role in the choice of bm. 3.6. energy tariffs structure today, energy company’s revenues are typically based on a flexible fee and fixed price per energy. this means that the more energy is consumed the better it is for the utility. this is the reason why the current revenue model creates a incumbrance for utilities to offer incentives for energy efficiency, because thereby energy delivery decreases and revenues reduces. it is often argued that the current revenue model is the greatest obstacle between the current utility structure and a modernized energy delivery system or thirdparty owned decentralized power generation based on renewable sources [16]. here, a decoupling of the relationship between sales volume and revenues is needed, a dynamic pricing (see flexibility below) should orientate on the wholesale prices for energy (time-of-use tariffs, off/peak prices, etc.), flatrate tariffs should be coupled to incentives to reduce energy consumption, new const structures would need to be more customer oriented. 3.7. infrastructure the infrastructure includes key resources to create a value proposition. especially renewable energy assets follow different strategies than traditional assets (high investment, low operational costs) and depend on key activities of the energy provider (capacity), key partnership within a network of suppliers and partners to make the bm work. even co-operations or joint ventures between utilities and independent developers are needed to build up portfolios and inhouse competencies to extend the revenue model. in this respect, utility-side renewable energy business models are more attractive to utilities in terms of risk and return expectations than customer-side renewable energy business models. thus, when considering risk return expectations and transactions costs, energy companies will primarily focus on large scale projects. 420 vol. 38/2022 characteristics of business models for innovation clusters . . . archetype type novelty oriented energy bms going green bm building energy communities bm lock-in oriented business models bm that offer energy functionalities complementarities-oriented energy supply business models optimizing grid operations combining value propositions acting locally efficiency-oriented energy business models scaling-up running platforms table 1. summary of business models. hence, customer-side renewable energy business models are not expected to advance in scale in the near future [8]. 3.8. flexibility flexibility is the ability of energy supply systems to manage net load variation (and generation outage) with their existent resources. this spans over various time horizons when intermittent res, such as solar and wind energy are present in the system. flexibility can be stimulated either from consumption or from generation side by coupling them with timing service. the decentralized electricity and/or heat generation is thus not just developing sources of renewable energy but also finding ways of locally based balancing of production and consumption. trading of flexibility services are important for reliable energy supply systems, due to their main functions (integration of intermittent resources, congestion management and portfolio optimization) which affect different users. 3.9. management and control mc indicates the actors that take the responsibility of maintaining and keeping the energy supply hardware in optimal operating conditions. many factors affect management (consisting of operation, control and governance), among other the proximity of the technology to the consumption’s site, the contract, the partnership and the legal form. the management activities are actually optimization activities that improve electricity grid balance and trading service, maintain the co-owned infrastructure and lever the fluctuation of renewable energy production and grid balance. table 1 summarizes the types of business models that these innovation clusters can follow. there were basically four different business model archetypes identified which can be split in several types and even sub-types. however, there are other objectives that are to be fulfilled as well. buildings are more commonly seen as micro-energy hubs with energy generated, stored, used and saved in buildings and districts as indicated by bpie (2016), aiming at [17]: • maximising energy efficiency of the buildings. • increasing on-site or nearby res production and self-consumption. • encouraging energy storage capacities in buildings (or nearby). • incorporating demand-response capacity in the building. • decarbonizing the heating and cooling energy for buildings. • empowering end users via smart meters and controls. • making dynamic price signals available for all consumers. • fostering business models aggregating micro-energy hubs. • (re)building smart and interconnected districts (renovate/retrofit). • building up infrastructure for further market uptake of electric vehicles. the most predominant archetypes of business models for the energy supply are presented in table 1. • novelty-centered bms are the ones where new ways of performing the economic transactions have been adopted. novelty is rooted in one or several activity system design elements and present in different innovation forms. accounting for the content element, the fossil fuel energy is replaced in these bms with res, thus they are mostly technology driven bms. nowadays these bm provide a strong predominance in the solar pv businesses, resulting in a pattern category named “going green”. “building energy communities” is the second pattern category where new organizations based on co-participation are addressed in the structure element, while the governance element is based on shared resources and governance. • lock-in centered bms refer to the ability of the company to attract, maintain and improve customer and partner association with the bm. • complementarities-centered bms refer to the bm bundling goods and services together instead of providing each separately and finally 421 matthias haase, emilia motoasca acta polytechnica ctu proceedings • efficiency-centered bm are the ones where measures are taken to achieve increased transaction efficiencies. 4. conclusions the energy sector is undergoing a continuous process of transformation where, together with decentralization and digitalization, a ultimate shift of energy supply towards renewable, co2 neutral energies is taking place. the classical structure of the electrical energy industry that emerged after the liberalization of the electricity and gas markets in europe including established business models, is subject to disruptive and substantial changes. business models in the electric power sector have to be seen as interlinked with the regulatory and policy frameworks that characterize the sector. up to now, the energy system is based on a distribution system which stateor national government-appointed regulatory commissions regulate. this influences also the revenues of electricity distribution companies and thus the viability of distributed renewable energy businesses (der) in distribution networks are exposed in part to these regulatory frameworks. correspondingly, market rules are established by a central authority, independent system operators (isos) or regional transmission operators (rtos), all entities monitored and regulated by an energy regulatory commission (ferc) in wholesale electricity markets. in addition, the electric power sector is subject of significant national and eu policy support, taking the form of subsidies or favorable rules for a variety of technologies, like e.g. solar-based energy (electricity and heat) generation. on the other hand, new der business models selling services in wholesale electricity markets must conform to the market rules and regulations established by these authorities. one way out would be to change these market rules and regulations together with authorities and policy makers. better understanding these regulatory and policy interdependencies is decisive to ensure the sustainable development of these businesses. • four different archetypes of bm were identified that ensure a maximum of impact. • there are no specific business models for energy supply applied to renovation of districts. • uncertainties in the supportive measures for the application of der makes it difficult to develop new business models for the utilities. acknowledgements this work has been developed 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[2022-10-12]. https: //www.bpie.eu/publication/buildings-as-microenergy-hubs-delivering-climate-solutions/ 423 https://www.techstreet.com/ashrae/standards/or-20-020-energy-master-planning-identifying-framing-constraints-that-scope-your-technology-options?product_id=2113219 https://www.techstreet.com/ashrae/standards/or-20-020-energy-master-planning-identifying-framing-constraints-that-scope-your-technology-options?product_id=2113219 https://www.techstreet.com/ashrae/standards/or-20-020-energy-master-planning-identifying-framing-constraints-that-scope-your-technology-options?product_id=2113219 https://www.techstreet.com/ashrae/standards/or-20-020-energy-master-planning-identifying-framing-constraints-that-scope-your-technology-options?product_id=2113219 https://www.renewableenergyworld.com/baseload/renewable-energy-and-the-utility-the-next-20-years/#gref https://www.renewableenergyworld.com/baseload/renewable-energy-and-the-utility-the-next-20-years/#gref https://www.renewableenergyworld.com/baseload/renewable-energy-and-the-utility-the-next-20-years/#gref https://www.bpie.eu/publication/buildings-as-micro-energy-hubs-delivering-climate-solutions/ https://www.bpie.eu/publication/buildings-as-micro-energy-hubs-delivering-climate-solutions/ https://www.bpie.eu/publication/buildings-as-micro-energy-hubs-delivering-climate-solutions/ acta polytechnica ctu proceedings 38:417–423, 2022 1 introduction 1.1 energy supply grid 1.2 energy supply value stream 1.3 research question 2 methodology 3 results 3.1 servitization (energy-as-a-service) 3.2 financing and ownership 3.3 public interest 3.4 customer's role 3.5 decentralization 3.6 energy tariffs structure 3.7 infrastructure 3.8 flexibility 3.9 management and control 4 conclusions acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2023.43.0111 acta polytechnica ctu proceedings 43:111–116, 2023 © 2023 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague recycled materials for sublayers richard svoboda∗, dana hubáčková, jan valehrach brno university of technology, faculty of civil engineering, veveří 331/95, 602 00 brno, czech republic ∗ corresponding author: svoboda.r@fce.vutbr.cz abstract. the article focuses on the recycled materials used in sleeper substructure. in the introduction, it summarizes the requirements for bearing layers of railway substructure and recycling of waste construction materials. the article presents the materials on which laboratory tests have been carried out and discusses in detail the most perspective – recycled concrete. for concrete recyclate, the article describes the results of laboratory tests – sieve analysis, determination of optimum moisture, and determination of bearing capacity using california bearing ratio, immediate bearing index and static deformation modulus. keywords: recycled construction materials, concrete recyclate, sieve analysis, optimum moisture, california bearing ratio, static plate load test. 1. introduction the reuse of construction waste in the construction industry is becoming a topical issue, due to the ecology and the increasing pressure to recycle all waste materials, but also due to the increasing scarcity of building materials generating a rapid increase in their price. we are responding to this global trend by researching the applicability of recycled materials in the sleeper substructure of railway tracks. the paper focuses on recycled construction waste, its possible use in railway substructure and laboratory tests carried out on recycled construction waste with regard to its possible use in sublayers of railway substructure. 2. design of the railway substructure the design of the substructure for the czech republic is prescribed by the instruction sž s4 (valid from 1st january 2021) [1]. the layers that distribute the loads from the ballast are (from top to bottom) the sub ballast layers and the capping layers, see figure 1. structural layers are designed according to annex 6, table 3 of the instruction sž s4. this table specifies, according to the highest speed limit and the expected operational load, what grain size and thickness of the gravel layer must be used, see the table 1. the capping layers are designed according to a special calculation that determines the equivalent static deformation modulus on each layer, the calculation is based on the dornii method. 3. requirements for the capping layers instruction sž s4 prescribes very high requirements for the material of sub ballast layers, therefore it is rather considered to use them in the capping layers. the following requirements are imposed on the materials of unbonded layers according to chapter 6 of the instruction tkp [2]: • grain size; • non-frosting and permeability; • heterogeneity; • content of fine-grained and foreign particles; • resistance to weathering and mechanical wear. according to instruction sž s4 [1], suitable materials for the capping layers are: • gravel; • crushed aggregate; • stabilization and improved soil; • geosynthetics; • asphalt concrete; • other materials, with the approval of sž gř 013 1, to ensure the required load-bearing capacity throughout the service life of the structure. 4. recycling of construction waste construction and demolition waste comes from the demolition, reconstruction and construction of building facility, not only buildings but also linear structures. in 2020, nearly 16.5 million tonnes of waste were produced in the czech construction sector, which is 42.9 % of the total waste production of 38.5 million tonnes [3]. the construction sector thus holds the first place among the different sectors (see figure 2). the largest share of waste is tailings, which are not recycled and should be reused. most other construction waste is recyclable. their effective recycling 1railway infrastructure administration, department of track management 111 https://doi.org/10.14311/app.2023.43.0111 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en r. svoboda, d. hubáčková, j. valehrach acta polytechnica ctu proceedings figure 1. construction of the railway substructure according to the instruction sž s4. figure 2. waste production in 2020. expected highest operational track class speed limit loads [millions throughout composition of trackbed layers [kph] gross whole tons/year] lifetime min. 200/šd 0/32 kv (min. 150 with the) <2 a – d agreement of infrastructure manager) ≤80 2-8 a – d min. 250/šd 0/32 kv >8 a – d min. 300/šd <2 a – d min. 250/šd 0/32 kv 81-120 2-8 a – d min. 300/šd 0/32 kv >8 a – d min. 300/šd <2 a – d min. 300/šd 0/32 kv var. i: min. 400/šd 0/32 kv 121-160 2-8 a – d var. ii: min. 250/šd 0/63 kv var. i: min. 400/šd 0/32 kv >8 a – d var. ii: min. 250/šd 0/63 kv 161-200 for all var. i: min. 400/šd 0/63 kv (incl.) operational a – d var. ii: min. 100/asphalt concrete + 250/šd loads 0/63 kv table 1. design of sub ballast layers according to the instruction sž s4. 112 vol. 43/2023 recycled materials for sublayers figure 3. samples of waste material from ballast cleaning. figure 4. recycled concrete. should be preceded by selective demolition, which is both economically demanding and places high demands on the organisation and timing of demolition work [4]. the recycling of construction waste itself usually consists of a simple modification of its physical characteristics. in the case of concrete, asphalt or brick waste, recycling consists of crushing and sorting the material into the required fractions. during the process, the recyclate is further cleaned of unwanted particles, for example by removing floating particles or by magnetic separation [5]. 5. considered waste for use in the railway substructure in the first phase of the project, we were searching for possible wastes that could be used as capping layers. for bonded capping layers, which are mixed with soil in varying proportions, we considered the following materials [6, 7]: • high temperature fly ash; • fluid fly ash; • bottom ash; • coal slag; • cement kiln dust. furthermore, we searched for suitable waste materials for unbonded capping layers, as possible materials figure 5. recycled asphalt. we considered [6, 7]: • waste material from ballast cleaning; • recycled concrete; • recycled asphalt; • recycled brick; • waste from washing concrete mixers; • waste from washing concrete mixers; • waste from washing concrete mixer trucks; • foam glass. 6. available recyclates for capping layers the following were subsequently selected as suitable and available recyclates for the base layers of the railway substructure: • waste material from ballast cleaning; • recycled concrete; • recycled asphalt. waste material from ballast cleaning (see figure 3) was collected at several different locations. its investigation has shown that its physical and mechanical properties are highly dependent on the content and origin of fine-grained particles. sample 2, which was significantly darker than the other two samples, probably contained a large amount of coal dust, which significantly degraded its properties. concrete recyclate from company dufonev r.c., a.s. was selected as another suitable material (see figure 4). recycled concrete has the best physical and mechanical properties in the current state of research and is described in detail below. the possibility of using asphalt recyclate (see figure 5) is also currently being investigated. its use alone in base layers seems rather inappropriate and 113 r. svoboda, d. hubáčková, j. valehrach acta polytechnica ctu proceedings figure 6. sieve analysis for concrete recyclate. figure 7. apparatus for the proctor compaction test. it is being tested to increase its load-bearing capacity by mixing it with other materials. 7. recycled concrete the following tests were carried out on concrete recyclate from dufonev r.c., a.s.: • grain size determination (according to czech state standard čsn en 933-1 [8]); • determination of moisture content and maximum bulk density (according to czech state standard čsn en 13286-2 [9]); • determination of immediate bearing index (ibi) and california bearing ratio (cbr) (according to czech state standard čsn en 13286-47 [10]); figure 8. modified proctor compaction test. • plate load test (according to instruction sž s4 železniční spodek [1]). 7.1. sieve analysis the sieve analysis confirmed the declared fraction of 0/32 concrete recyclate, showing that it has a continuous grain line and a content of fine particles only 3.2 %, see figure 6. 7.2. determination of moisture content and maximum bulk density considering the grain size of the samples and the subsequent cbr (ibi) test, the modified proctor compaction test was chosen as a suitable option for determining the optimum moisture. specifically, a test type with a 4.5 kg hammer and a 150 mm wide proctor mould was chosen (see figure 7 and figure 8). the optimum moisture content, i.e. the moisture content at which the mixture of concrete recyclate and water reaches its highest bulk density, was determined to be 13 % (see figure 9). this relatively high value is due to the natural water absorption of the concrete. 7.3. determination of immediate bearing index (ibi) and california bearing ratio (cbr) subsequently, the ibi and cbr tests were performed: the mixture of concrete recyclate and water was mixed to the optimum moisture content, the mixture was 114 vol. 43/2023 recycled materials for sublayers figure 9. determination of optimum moisture (recycled concrete). figure 10. the ibi test. compacted in a proctor mould using the same procedure used to determine the optimum moisture content, and then the ibi test was performed (see figure 10). after the ibi test, the compacted body was turned bottom side up and placed under water for 96 hours to mature. the cbr test was then performed (see figure 11). in the ibi and cbr tests, a cylinder of prescribed dimensions is pressed into the test body. the dependence of force and penetration of the cylinder is recorded. the force required for 2.5 mm and 5 mm penetration is compared to the standard force. in addition, if the force-displacement curve is concave in the beginning, the value of the initial penetration against which the standard force is compared is corrected (see figure 11. the cbr test. figure 12. cbr test for concrete recyclate. figure 12 and table 2). the cbr (ibi) value is defined as the higher of the two penetration values, rounded to the nearest 5 % for values greater than 29 %. by the procedure described above, the immediate bearing index (ibi) was determined, which was 105 %, and the california bearing ratio (cbr) after the mature period, which was even 205 %. these high values confirm that recycled concrete is a suitable alternative in terms of load-bearing capacity for use in substructure. 7.4. static plate load test in a test stand the static plate load test in the test stand was carried out in the laboratories of the faculty of civil engineering, department of railway structures, czech 115 r. svoboda, d. hubáčková, j. valehrach acta polytechnica ctu proceedings revised standard penetration penetration force force cbr [mm] [mm] [kn] [kn] [%] 2.5 3.1 23.62 13.2 180 5.0 5.6 40.76 20.0 205 table 2. determination of the cbr value. figure 13. static plate load test in the test stand (author ctu in prague). technical university in prague. the static plate load test was carried out in a small box with internal dimensions 790×900×460 mm, 3 compacted layers of recycled concrete were placed in the test stand. the test was made according to sž s4 railway substructure with a load plate with a diameter of 0.3 m, see figure 13. the test results are clearly shown in the table 3. the high values of the static deformation modulus confirm suitable properties for use in the layers of the substructure in terms of load-bearing capacity. 8. conclusion tests carried out on waste from ballast cleaning show a large variance of results according to the location and origin of the contamination of the ballast bed. for this reason the use of this material appears to be rather problematic. the most perspective material at present is recycled concrete, which shows very high load-bearing capacity values in laboratory conditions. to verify other properties, we prepare test sections with recycled concrete in the capping or sub ballast layer. acknowledgements this article was written with the support of the ministry of industry and trade within the trio programme, project advanced technologies for installation and restoration of settlement in 1. loading cycle y1 [mm] 1.23 static deformation modulus of 1. loading cycle e1 [mpa] 36.5 settlement in 2. loading cycle y2 [mm] 0.46 static deformation modulus of 2. loading cycle e2 [mpa] 97.1 ratio of static deformation modulus e2/e1 2.66 table 3. results of the static plate load test in the test stand. the protective layers of railway substructure with the efficient use of secondary raw materials, number fv40081. references [1] správa železnic, s.o. instruction sž s4: železniční spodek. prague, 2021. [2] správa železnic, s.o. instruction tkp: technické kvalitativní podmínky staveb státních drah: kapitola 6, konstrukční vrstvy tělesa železničního spodku. prague, 2008. [3] czech statistical office. waste generation by nace sections. [2022-06-01], https://www.czso.cz/ documents/10180/143330119/2800202101.pdf. [4] třídění odpadu cz. stavební a demoliční odpady. [2022-06-02], https://www.trideniodpadu.cz/stavebni-odpad. [5] asociace pro rozvoj recyklace stavebních materiálů v české republice. podstata recyklace stavebních odpadů. [2022-05-29], https://www.trideniodpadu.cz/stavebni-odpad. [6] m. luňáček. pokročilé technologie zřízení a obnovy konstrukčních vrstev tělesa železničního spodku s efektivním využitím materiálů z druhotných surovin. roční zpráva o řešení projektu v programu trio v roce 2021: evidenční číslo projektu fv40081, brno, 2021. [7] m. luňáček. pokročilé technologie zřízení a obnovy konstrukčních vrstev tělesa železničního spodku s efektivním využitím materiálů z druhotných surovin. roční zpráva o řešení projektu v programu trio v roce 2020: evidenční číslo projektu fv40081, brno, 2020. [8] úřad pro technickou normalizaci, metrologii a státní zkušebnictví. čsn en 933-1. zkoušení geometrických vlastností kameniva – část 1: stanovení zrnitosti – sítový rozbor. prague, 2012. [9] úřad pro technickou normalizaci, metrologii a státní zkušebnictví. čsn en 13286-2. nestmelené směsi a směsi stmelené hydraulickými pojivy – část 2: zkušební metody pro stanovení laboratorní srovnávací hmotnosti a vlhkosti – proctorova zkouška. prague, 2011. [10] úřad pro technickou normalizaci, metrologii a státní zkušebnictví. čsn en 13286-47. nestmelené směsi a směsi stmelené hydraulickými pojivy – část 47: zkušební metoda pro stanovení kalifornského poměru únosnosti, okamžitého indexu únosnosti a lineárního bobtnání. prague, 2012. 116 https://www.czso.cz/documents/10180/143330119/2800202101.pdf https://www.czso.cz/documents/10180/143330119/2800202101.pdf https://www.trideniodpadu.cz/stavebni-odpad https://www.trideniodpadu.cz/stavebni-odpad acta polytechnica ctu proceedings 43:111–116, 2023 1 introduction 2 design of the railway substructure 3 requirements for the capping layers 4 recycling of construction waste 5 considered waste for use in the railway substructure 6 available recyclates for capping layers 7 recycled concrete 7.1 sieve analysis 7.2 determination of moisture content and maximum bulk density 7.3 determination of immediate bearing index (ibi) and california bearing ratio (cbr) 7.4 static plate load test in a test stand 8 conclusion acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.38.0470 acta polytechnica ctu proceedings 38:470–477, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague use of energy profile indicators to determine the expected range of heating energy consumption tobias loga∗, britta stein, guillaume behem iwu – institut wohnen und umwelt, institute for housing and environment, rheinstraße 65, de-64295 darmstadt, germany ∗ corresponding author: t.loga@iwu.de abstract. a small set of query variables designed to collect information about the energy-related features of residential buildings is presented. these “energy profile indicators” include information about those visible characteristics of a building which have a notable impact on its energy performance and are simple to assess. the queries are an interesting source for a rough energy performance calculation for single buildings as well as for housing portfolios or housing stocks. a method has been developed to transform the energy profile indicators into input data for a physical calculation model. it consists of procedures to estimate the envelope area, u-values, and efficiency values of the heat supply system. to all model input variables an uncertainty is assigned. if information from a query is not available, the model input is set to a state representing the average building stock and the uncertainty of this quantity is adjusted to a value reflecting the variance in the stock. the resulting uncertainty of the calculated energy use is determined. examples of the application of the method are given to show the influence of different unknown quantities including occupant behaviour. experiences on the coherence with metered consumption are reported. keywords: monitoring indicators, building stock, surveys, statistical evaluation, realistic physical model, uncertainties, target/actual comparison. 1. introduction various activities in the building sector aim at gathering building data, executing energy performance calculations and assessing the metered energy consumption, often in the context of energy performance certificates (epcs). however, huge challenges are faced when cross-section and longitudinal analyses are needed to get an overall view of the refurbishment progress and energy consumption of housing stocks. this is not only due to data gaps but also to the used calculation methods and software applications focussing on official proof of requirements and rating. the detailedness and complexity of input data, recurrent revisions of the regulatory framework, but also the lack of software applications to store, calculate and analyse multi-building data are barriers for aggregation and statistical evaluation. the idea of the concept presented in this paper is to tackle these challenges by introducing an additional level of energy assessment addressing evaluation purposes. the overall objectives are: • facilitate the collection and maintenance of information about energy-related features of housing stocks, allowing for an assessment of the overall state of refurbishment, annual refurbishment rates and for a comparison with milestones on the refurbishment path. • provide reliable estimates of the energy consumption of single buildings and of housing stocks for different levels of insulation and various heat supply system types to be used for prognoses in the context of refurbishment scenarios. • enable a target/actual comparison of the consumption after implementation of refurbishment measures for single buildings and also for housing stocks. in order to reach these aims, a methodical framework has been developed within the research project mobasy, consisting of two parts [1]: (1.) a set of monitoring indicators representing those visible characteristics of buildings that have the most important effect on its energy performance, suitable for use in questionnaires and quick on-site inspections. (2.) an energy performance calculation using the mentioned monitoring indicators as input data and providing a realistic bandwidth of the expected energy consumption, reflecting the uncertainty of the data acquisition (uncertain or unavailable information) and of the model input data (thermal properties of materials and components, occupant behaviour, climate) depending on the specific field of application (assessment of past consumption or prognosis). this paper gives an overview of the monitoring indicators (“energy profile indicators”), and of the principles of the uncertainty assessment attached to the realistic energy performance calculation (“simple combined physics-probability model”). the influence of 470 https://doi.org/10.14311/app.2022.38.0470 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 38/2022 use of energy profile indicators to determine the expected range . . . characteristics of indicators (query variables) building size and form living space, number of attached buildings, number of full storeys, heating situation in attic and basement, number of dwellings thermal properties of the opaque envelope (categories: roof, ceiling, wall, floor) building: year of construction; per envelope category: existent insulation upgrade, year of insulation upgrade, insulation thickness + covered fraction thermal properties of windows (two types) per window type: number of panes, low-e coating existent, type of window frame, year of installation, fraction of the second window type, if applicable heat supply system (heating / dhw) heat generators (type, specification, year of installation), thermal solar system, heat storages (type, location inside or outside of thermal envelope), heat distribution (type, insulation level, location inside or outside of thermal envelope) further systems ventilation system (with / without heat recovery), pv system (with / without electrical storage) table 1. overview of energy profile indicators for residential buildings. the origin and detailedness of data on the uncertainty of the estimated energy consumption is illustrated for a singleand a multi-family house. furthermore, results are presented from the application of the method to a housing stock sample, providing statistical information on the energy-related state, on the estimated and actual consumption. 2. energy profile indicators a set of “energy profile indicators” has been developed in [2], then continuously further refined and applied in several projects for portfolio assessment and quality assurance (examples: [3, 4]). the indicators have also been used in house owner surveys in 2009 [5] and 2016 [6] on the basis of random sampling1 to get an image of the refurbishment state and progress of the german residential building stock. table 1 gives an overview of the query variables. these indicators represent the physical characteristics of a building that have the biggest impact on its energy use for heating and domestic hot water (dhw). the variables are designed to gather information by asking (technically informed) building owners and by executing on-site inspections of residential buildings. for these purposes, a two-page questionnaire [7] and a structured data table [8] have been developed. energy profile indicators provide information that can be used to estimate the energy consumption of a building. for this purpose, a transformation to a physical model including an uncertainty assessment has been developed. 3. simple combined physics-probability model there are different approaches to quantify the uncertainties of the building energy assessment [9]. the 1the 2016 evaluation is based on about 17 000 evaluable questionnaires (18 % of ca. 92 000 queries sent to a random sample of german house owners) [6]. concept presented here uses forward uncertainty, that tries to quantify the uncertainty in the calculation outputs propagated from uncertain input variables through mathematical models. due to the stationary, quasi-linear energy calculation model, a very simple, non-sampling probabilistic method can be employed, assuming normal distributions and applying the gaussian error propagation law. 3.1. principle the principle of the indicator-based energy performance calculation is shown in figure 1. the first step is a transformation of energy profile indicators to input variables of the physical model. as a result, a “calculation value” is provided for each quantity, directly used by the energy performance calculation (“model input variables”), and an “uncertainty range” is attributed, which represents the span of values that might include the actual (unknown) real value. preferably, the parameters of the transformation should be derived from empirical data of large building stock samples. however, for certain quantities extended surveys or large measurement campaigns are not available. in these cases, the calculation value and uncertainty range can only be estimated. a pragmatic way to do this is to ask an expert: which is the highest, which is the lowest value that might be present and still be seen as “not unusual”? the average of both is the calculation value, half the difference between both represents an indicator of the uncertainty. as an important means for transparency and inducement for continuous improvement of the method, the derivation of calculation values and uncertainties must be documented for each variable. the calculation values are used as input for the physical model to determine the theoretical energy consumption. in a separate calculation, the uncertainty of the calculated energy consumption is determined by combining the effect of the uncertainties of all input 471 t. loga, b. stein, g. behem acta polytechnica ctu proceedings figure 1. principle of the combined physics-probability model based on energy profile indicators. values. gaussian error propagation law (square root of the added-up squares of the uncertainties caused by each single variable) is used for this purpose. since most of the probability density distributions of input variables are assumed to be symmetrical and the energy balance procedure used (seasonal method) is mainly linear, the systematic deviations produced by this simplification are assumed to be small. the result of this combined probability-physics model is an expectation value and an expectation range of the energy use for heating and dhw. 3.2. systematic uncertainty assessment the uncertainty of the energy performance calculation is in principle depending on the type of data source and on the completeness of information (see figure 2). type of data source: information provided by building owners is a useful data source, however inspections can provide additional information and thus reduce the uncertainty. more trustworthy are design data from refurbishment planning (including issued epcs), at best if the implementation is secured by quality assurance (qa). completeness of information: information can be missing from all mentioned data sources (even in a quality assured planning process). when typical or average values from the building stock are used instead the uncertainty of the calculation increases. this global principle has been translated into an algorithm: for each model input variable five uncertainty classes a to e are defined with rules for selection and uncertainty values to be used. as an example, table 2 shows the uncertainty assessment of the insulation thickness, applied separately for all opaque envelope categories. analogue definitions are available for the other input variables2. 2these are: envelope area, u-values of original constructions, insulated fractions by envelope type, thermal conductivities figure 2. influences on the uncertainty of an energy performance calculation. the method presented in this paper is focussing on data from building owners and from on-site inspections, associated with uncertainty classes c and d. these data sources facilitate the energy assessment of a large number of buildings – as a start from files and records – updates can be provided in the course of insulation by envelope type, u-value of windows, thermal bridging surcharge, heating degree days, annual solar radiation, effective passive solar aperture, internal heat load, efficiency of ventilation heat recovery, dhw heat need, efficiency of heating system, efficiency of dhw system. tabled values and explanations in english language are displayed at the end of the supplemental document [10]. a detailed description of the algorithms for a selection of variables is given in annex d of [1] (german language). 472 vol. 38/2022 use of energy profile indicators to determine the expected range . . . uncertainty description uncertainty of class insulation thickness∗ relative absolute a insulation thickness from design data, quality assured 5 % 0.5 cm b insulation thickness from design data 10 % 1 cm c insulation thickness determined by inspection on-site / record or statement of building owner 25 % 2 cm d insulation available; insulation thickness unknown; use of average values from the building stock differentiated by implementation period 40 % 5 cm e insulation thickness and year of implementation unknown; use of average values from the building stock (all buildings) 50 % 8 cm ∗ relevant for the calculation is the minimum in cm, derived from the relative and absolute uncertainty. table 2. example of uncertainty classification for an input variable (insulation thickness). of on-site investigations. even if input is missing for some indicators (class e), the estimation of the actual energy use of the sample or portfolio can be commenced – with corresponding larger expectation ranges. if available, design data with or without quality assurance (classes a or b) can also be integrated. the improvement achieved by gradually adding reliable data from inspections and from quality assured refurbishments will result in narrowed ranges of the expected energy use. 3.3. mechanisms of the energy performance calculation based on energy profile indicators following the above-described principles, a coherent method for an energy performance calculation by use of energy profile indicators has been developed and implemented in form of an excel-workbook and an r script (description of the method in [1]). core aspects are the estimation of the thermal envelope area [11] and of the u-values [12]. the energy performance calculation is based on the tabula seasonal method [13], using climate data differentiated by postcode: in the first instance, the physical model is calculated by use of the local long-term average climate (of the past 20 years). for performing target/actual comparisons for specific years, calculated heat losses and gains are calibrated by the actual temperatures and solar radiation of the metering period. 4. showcase: influence of different parameters on the uncertainty of the estimated consumption for a single building, the described method allows a prognosis of the energy consumption, providing an estimation value (result of the physical model) and an estimation range (result of the uncertainty assessment). the actual consumption is expected to be found within the estimation range, however in rarer cases it can also go beyond that. table 3 shows examples of expectation ranges for a single-family and a multi-family house (sfh, mfh), depending on the state of refurbishment, on the data acquisition type and on the utilisation of the buildings. these are results from the combined physicsprobability model with uncertainty assessment as described above. the examples are intended to illustrate the uncertainty of the consumption estimate based on energy profile indicators (house owner statement) as opposed to those based on design data, considering particularly the influence of unknown occupant behaviour. for the variants 1.1 to 1.3 the calculation is assumed to be based on energy profile indicators, with input from house owners. for the single-family house, the relative uncertainty of the calculated energy use for heating and dhw is ±35 % before (var. 1.1) and ±72 % after a deep refurbishment with passive house components (var. 1.2). the relative increase is mainly caused by the influence of occupant behaviour. however, the state of the building is much better defined after refurbishment, so the absolute uncertainty caused by the envelope is only ±25 kwh/(m2a) compared to ±52 kwh/(m2a) before refurbishment. if the real climate conditions of the climate zone are used, the uncertainty of the energy demand caused by the climate is reduced from ±26 % (var. 1.2) to ±19 %, (var. 1.3), having only a small effect on the total uncertainty. in variant 1.4, design data from the planning of the refurbishment (detailed calculation of envelope area, of u-values, and of supply system performance) and long-term climate data are used. this is the typical pattern of energy performance certificates, issued in the context of modernisation. it reduces the total uncertainty from about ±70 to ±60 %. the effect is rather small due to the predominating large uncertainties of utilisation and climate. variant 1.5 shows a situation where the design data are quality-assured (update of information about actual implementation 473 t. loga, b. stein, g. behem acta polytechnica ctu proceedings e xe m pl ar y bu ild in g sf h (s in gl efa m ily ho us e) [1 95 8– 19 68 ] m f h (m ul ti -f am ily ho us e) [1 95 8– 19 68 ] st at e of re fu rb is hm en t u nr ef ur bi sh ed r ef ur bi sh ed (p as siv e ho us e co m po ne nt s) u nr ef ur bi sh ed r ef ur bi sh ed (p as siv e ho us e co m po ne nt s) t yp e of da ta so ur ce ex po st (h ou se ow ne r st at em en t) d es ig n da ta d es ig n da ta + q a ex po st (h ou se ow ne r st at em en t) d es ig n da ta d es ig n da ta + q a c lim at e lo ng -t er m av er ag e sp ec ifi c ye ar lo ng te rm av er ag e sp ec ifi c ye ar lo ng -t er m av er ag e sp ec ifi c ye ar lo ng te rm av er ag e sp ec ifi c ye ar o cc up an t be ha vi ou r un kn ow n de ta ile d m ea su re m en ts ∗ un kn ow n de ta ile d m ea su re m en ts ∗ v ar ia nt 1. 1 1. 2 1. 3 1. 4 1. 5 2. 1 2. 2 2. 3 2. 4 2. 5 calculatedenergy use(naturalgas) [kwh/(m2a)] e ne rg y us e∗∗ • h ea ti ng 20 3 50 50 50 50 15 6 31 31 31 31 • d h w 25 9 9 9 9 26 15 15 15 15 su m 22 8 59 59 59 59 18 3 45 45 45 45 u nc er ta in ty ∗∗ ∗ • e nv el op e ± 23 % ± 42 % ± 42 % ± 15 % ± 7% ± 17 % ± 29 % ± 29 % ± 12 % ± 5% • sy st em ± 9% ± 10 % ± 10 % ± 7% ± 5% ± 9% ± 12 % ± 12 % ± 10 % ± 6% • u ti lis at io n ± 19 % ± 52 % ± 52 % ± 52 % ± 13 % ± 4% ± 14 % ± 14 % ± 14 % ± 4% • c lim at e∗∗ ∗∗ ± 16 % ± 26 % ± 19 % ± 26 % ± 13 % ± 16 % ± 33 % ± 24 % ± 30 % ± 10 % to ta l ± 35 % ± 72 % ± 70 % ± 60 % ± 20 % ± 26 % ± 47 % ± 42 % ± 37 % ± 14 % ∗ in do or te m pe ra tu re s an d w in do w s op en in g ha ve be en m ea su re d, e. g. in a re se ar ch pr oj ec t (a ss um pt io n fo r th e sh ow n ca se : bu ild in g ha s ut ili sa ti on cl os e to th e av er ag e of al lb ui ld in gs ); ho w ev er ,i nt er na lh ea t so ur ce s an d ex te rn al sh ad in g ha ve no t be en m ea su re d or m od el le d in de ta il. ∗∗ n at ur al ga s, in kw h/ (m 2 a) ;c al cu la ti on w it h si m pl e pr ob ab ili ty -p hy si cs m od el . ∗∗ ∗ r el at iv e un ce rt ai nt y, re la te d to en er gy us e fo r he at in g an d d h w . ∗∗ ∗∗ in cl ud in g un ce rt ai nt y of ex te rn al sh ad in g (a ss um pt io n fo r al lv ar ia nt s: n o de ta ile d as se ss m en t of th e re du ct io n of pa ss iv e so la r ga in s ca us ed by su rr ou nd in g tr ee s an d bu ild in gs or by us e of sh ut te rs et c. ). table 3. estimation of the actual energy consumption before and after deep refurbishment for two example buildings and assigned uncertainty, depending on the type of data source and measurements (a pdf demonstrating the calculation flow for the 10 building variants can be viewed in the supplemental document mentioned above [10]). 474 vol. 38/2022 use of energy profile indicators to determine the expected range . . . during refurbishment) and where the utilisation and the climate are measured in detail, for example in the framework of a research project. in this case, the uncertainty of the consumption estimate can be reduced to ±20 %3. for the multi-family house with 24 flats (var. 2.1 to 2.5), the effect of the utilisation uncertainty is significantly smaller because of stochastic effects (compensation between flats with high and low intensity of usage). it may be noted that a very deep refurbishment has been assumed here, resulting in very low energy consumption and causing large relative uncertainties. however, the absolute uncertainty of about ±40 kwh/(m2a) for sfh (var. 1.3) and of ±20 kwh/(m2a) for mfh (var. 2.3) is not changing much for less ambitious refurbishments, resulting in smaller relative uncertainties (see results for the building sample presented in the following chapter). if design data are not available for such a deep refurbishment, the thermal envelope’s uncertainty is predominated by the unknown effects of thermal bridging. if a characterisation is possible (for example as a result of on-site inspection: no relevant constructive weaknesses), the uncertainty of the energy use caused by the envelope can be reduced from ±29 % to ±16 % (mfh, var. 2.3), the total uncertainty reduced from ±42 % to ±34 %. these uncertainties of consumption estimates are valid with view at a single building. however, if large building samples or stocks are considered, the uncertainty of the predicted total consumption (and thus the uncertainty of the average) is reduced to 1/10 of the shown values for 100 buildings and to 1/100 for 10 000 buildings. in summary, the expectation ranges of the energy consumption determined by quantifying input uncertainties seem rather large, especially for single-family houses. this is not only true for the calculation based on energy profile indicators but also for the epc calculation based on design data. for both, the predominating factors determining the uncertainty of the prognosis of an annual consumption are unknown occupant behaviour and climate conditions. 5. application on a sample of apartment blocks from housing companies in the following, the application of the combined probability-physics model on a housing sample from three housing companies is reported. the energy profile indicators and the metered energy consumption values were collected for 155 building entities (mostly 3the type of energy calculation procedure has only a very small effect (for example the seasonal method has only an uncertainty of ±2 % compared to the monthly method [14]). thus, nearly the same uncertainties would result if a highresolution simulation programme was used. large building blocks) with 3329 apartments4. the energy characteristics of the building sample comprise a wide span from unrefurbished to deep renovations [12], which also includes the use of passive house components and installation of ventilation systems with heat recovery. in two of the three companies, a large effort was required to collect the information from individual building-related files. in the third company, however, energy profile indicators were already introduced in 2008 for the complete portfolio and since then maintained and updated [3]. these data could therefore be used directly. for some of the buildings, the indicators were collected during a quick on-site inspection – the typical effort was half an hour per building. table 4 shows the results for a subset where metered consumption is available for both, heating and dhw. as a parameter for the theoretical heat loss the “thermal conductance” is defined, which is the heat loss of the building per kelvin temperature difference, related to the reference floor area. it can be stated that the average actual consumption by conductance class is very close to the calculation model averages (the ratio average actual to average theoretical energy use varies from 95 % to 115 % per class). the standard deviations of the actual energy consumption and the average uncertainty of calculation are in similar ranges. in summary, the method seems quite suitable for modelling the consumption of housing stocks and for evaluating measures of different depths with respect to their actual effect. one conclusion that can be drawn from this is, for example, that the average energy consumption for heating and dhw of the class with best insulation level is about one third of that with the worst. 6. conclusions a method has been developed that is based on a set of monitoring indicators collectable by a questionnaire or a quick on-site examination. these “energy profile indicators” are used as input for an energy performance calculation to determine an expected range of actual energy consumption. examples show how the uncertainty of consumption prognosis depends on information about data source and data completeness. moreover, the methodology was applied to a sample of multi-family buildings involving statistical evaluations. for different levels of insulation, the estimated as well as the actually measured consumption were determined, with the actual consumption by conductance class being very close to the calculation model averages. future work should focus on the expansion of the database to include also single-family houses and different heat supply systems, especially heat pumps and solar systems. 4the datasets of the sample can be viewed at the excel workbook mentioned above [8]. 475 t. loga, b. stein, g. behem acta polytechnica ctu proceedings thermal conductance class∗ range w/(m2k) 0.01–0.80 0.81–1.00 1.01–1.20 1.21–1.50 1.51–2.00 2.01– equivalent insulation thickness of opaque elements∗∗ average cm 31.6 21.5 12.1 13.0 2.4 2.7 u-value of windows average w/(m2k) 0.80 1.29 1.52 2.62 2.68 2.68 availability of mechanical ventilation with heat recovery percentage 100 % 0 % 0 % 0 % 0 % 0 % calculated energy use for heating and dhw (comparison value)∗∗∗ average kwh/(m2a) 50 73 96 103 143 163 std. dev. kwh/(m2a) ±7 ±5 ±10 ±11 ±14 ±20 av. uncert. kwh/(m2a) ±16 ±20 ±22 ±27 ±39 ±43 actual energy use for heating and dhw (metering) average kwh/(m2a) 50 78 110 110 135 148 std. dev. kwh/(m2a) ±9 ±21 ±15 ±21 ±16 ±23 ratio actual / calculated energy use 0.99 1.07 1.15 1.07 0.95 0.91 frequency number of building n = 15 n = 10 n = 8 n = 23 n = 5 n = 11datasets number of annual n = 28 n = 25 n = 21 n = 46 n = 10 n = 22consumption values ∗ the “thermal conductance” is the heat transfer coefficient by transmission and ventilation, related to the reference floor area of the building. ∗∗ thermal resistance of opaque envelope, expressed in cm insulation with 0.035 w/(m · k) thermal conductivity. ∗∗∗ energy use, calculated by the combined probability-physics model, balance scope and climate consistent with metering (example: if heat is metered in the apartments the heat losses of the central heat distribution system are disregarded in this comparison). table 4. comparison of theoretical and actual consumption for a building sample (heating + dhw). averages, uncertainties and standard deviations differentiated by thermal conductance class. (status of database analysis: 2022-02-17). in practice, the simplification of data input, the ability to adequately consider data gaps and uncertainties and the output of estimation ranges may provide benefits for different purposes: • individual houses (occupants, owners, energy consultants, . . . ): ▷ the small effort to provide the indicators for an existing house makes it a useful method in energy advice campaigns. the current energy consumption can be interpreted and assessed. achievable consumption bandwidths after refurbishment can be provided for the investigated house. ▷ for refurbished buildings as well as for new build, the method provides a target range of the energy consumption. the occupant or owner can compare the actual consumption against this individual benchmark. if the consumption is above the expectation range, the implemented measures, the operating conditions, and the occupant behaviour can be checked and improved, if necessary. besides, measurements of the actual utilisation conditions and of the local climate during the consumption period can help to significantly narrow down the expectation range. ▷ energy profile indicators can be attached to epc issuing (e.g. “statistic forms”, xml-files). this enables a plausibility control of the energy performance calculated for the epc, for issuers as well as for inspectors on quality assurance. ▷ the fact that visually provable indicators are used is also a benefit for application cases in the context of pricing rules, dependent on energy performance (building value, financing, sales, rents, . . . ) and for grant applications. • housing portfolios (housing companies, energy service and billing companies, . . . ): ▷ the collection and regular update of energy performance indicators provides a basis to determine the current level of insulation and the current fraction of efficient/renewable systems for the entire stock as well as the respective annual improvement rates. these achievements can e.g. be compared with the climate protection targets for the portfolio or for the national stock. ▷ the average estimated and, if available, actual consumption values and ranges per conductance class are benchmarks that can be useful in the preparation of refurbishments, especially in the communication with tenants, to strengthen the confidence in the actual effect of the measures. ▷ by matching consumption values of single buildings against the benchmarks, cases with suspiciously high consumption can be identified. an uptake of low-cost measures can improve the situation, similar to the target/actual comparison described for individual buildings. ▷ the calculation for all buildings of a portfolio may also be used as building stock model for developing refurbishment strategies. • national housing stock (energy experts, authorities, energy agencies, . . . ): ▷ by using the energy profile indicators for a random-sample-based house owner survey, the achievements regarding levels of insulation and fractions of efficient/renewable systems can be determined for the national housing stock, similar to the evaluation of the energy profile database of a housing portfolio. 476 vol. 38/2022 use of energy profile indicators to determine the expected range . . . ▷ in addition, the random sample can be used in national building stock models in form of a microsimulation model or by aggregation to average buildings to forecast the development of energy use by energy carrier and calculate different alternative scenarios. ▷ in the context of national information campaigns, the publication of benchmarks consisting of average consumption values and typical spreads for different insulation categories can provide useful information for occupants and owners that inspires confidence in different types of measures. acknowledgements the work described in this paper is part of the research project mobasy, supported by the german government (federal ministry for economic affairs and climate action (reg. n°: 03sbe0004a); project duration: 11/2017–10/2022; more information at: https: //www.iwu.de/forschung/energie/2017/mobasy/). the analysis of the housing sample has been implemented in collaboration with three german housing companies: bauverein ag darmstadt, wohnbau gießen and nassauische heimstätte wohnstadt frankfurt/main. we very much appreciate the support and cooperation. references [1] t. loga, m. großklos, a. müller, et al. realbilanzierung für den verbrauch-bedarf-vergleich (mobasy teilbericht). tech. rep., darmstadt, 2021. isbn 978-3-941140-67-7. https://doi.org/10.13140/rg.2.2.22472.24328/1 [2] t. loga, n. diefenbach, j. knissel, r. born. entwicklung eines vereinfachten, statistisch abgesicherten verfahrens zur erhebung von gebäudedaten für die erstellung des energieprofils von gebäuden („kurzverfahren energieprofil“). tech. rep., darmstadt, 2005. isbn 3-932074-76-9. https://doi.org/10.13140/rg.2.2.20794.52160 [3] t. loga, m. repp, s. swiderek. verbrauchsbenchmarks – ein soll-ist-vergleich für wohnungsunternehmen. in 12. effizienztagung bauen + modernisieren 2020. 2020. [2022-03-02]. https://www.researchgate.net/ publication/350710240_verbrauchsbenchmarks__ein_soll-ist-vergleich_fur_wohnungsunternehmen [4] t. loga, k. müller. evaluation of the tabula database – comparison of typical buildings and heat supply systems from 12 european countries, iwu, darmstadt, 2012. tabula work report, [2022-03-02]. http://www.buildingtypology.eu/downloads/public/docs/report/ tabula_workreport_evaluationdatabase.pdf [5] n. diefenbach, h. cischinsky, m. rodenfels, k.-d. clausnitzer. datenbasis gebäudebestand. datenerhebung zur energetischen qualität und zu den modernisierungstrends im deutschen wohngebäudebestand, iwu, darmstadt, 2010. [2022-10-06]. https://www.iwu.de/fileadmin/ publikationen/gebaeudebestand/datenbasis/ endbericht_datenbasis.pdf [6] h. cischinsky, n. diefenbach. datenerhebung wohngebäudebestand 2016, iwu, darmstadt, 2018. [2022-10-06]. https://www.iwu.de/fileadmin/ publikationen/gebaeudebestand/2018_iwu_ cischinskyetdiefenbach_datenerhebungwohngeb%c3%a4udebestand-2016.pdf [7] t. loga. energy profile. questionnaires thermal envelope and heat supply system, iwu, darmstadt, 2021. [2022-03-02]. completed with example data: https: //www.iwu.de/fileadmin/tools/energyprofile/ energyprofile_forms_buildingsystem_eng.pdf template with active form fields: https://www.iwu.de/ fileadmin/tools/energyprofile/energyprofile_ formbuildingsystem_datacapture.pdf [8] t. loga. energy profile indicators. data table, showcase, iwu, darmstadt, 2021. [2022-03-02]. https://www.iwu.de/fileadmin/tools/ energyprofile/energyprofileindicators_ datatable_showcase_eng.xlsx [9] w. tian, y. heo, p. de wilde, et al. a review of uncertainty analysis in building energy assessment. renewable and sustainable energy reviews 93:285–301, 2018. https://doi.org/10.1016/j.rser.2018.05.029 [10] t. loga, b. stein, g. behem. cesb22 – annex of the paper: use of energy profile indicators to determine the expected range of heating energy consumption, cesb22 prague, documentation of the example building calculations, 2022. [2022-03-02]. https://www.iwu.de/fileadmin/publikationen/ energie/mobasy/cesb22_energyprofileindicators_ annex_showcaseexamples_calculationflow.pdf [11] t. loga, n. diefenbach, j. knissel. entwicklung eines flächenschätzverfahrens zur abschätzung von bauteilflächen als grundlage für die energetische bewertung von wohngebäuden. bauphysik 27(2):102– 116, 2005. https://doi.org/10.1002/bapi.200590017 [12] t. loga, g. behem. target/actual comparison and benchmarking used to safeguard low energy consumption in refurbished housing stocks. in proceedings of the eceee summer study 2021, pp. 461–471. 2021. [2022-03-02]. https://www.researchgate.net/ publication/355124720_targetactual_comparison_ and_benchmarking_used_to_safeguard_low_energy_ consumption_in_refurbished_housing_stocks [13] t. loga, n. diefenbach. tabula calculation method – energy use for heating and domestic hot water. reference calculation and adaptation to the typical level of measured consumption, iwu, darmstadt, 2013. tabula documentation, [2022-03-02]. https://episcope.eu/fileadmin/tabula/public/ docs/report/tabula_commoncalculationmethod.pdf [14] t. loga. heizgrenztemperaturen für gebäude unterschiedlicher energetischer standards. abschätzung der heizgrenztemperaturen und bilanzzeiten für das heizperiodenbilanzverfahren nach en 832/din v 4108-6. bauphysik 25(1):2–14, 2003. https://doi.org/10.1002/bapi.200300020 477 https://www.iwu.de/forschung/energie/2017/mobasy/ https://www.iwu.de/forschung/energie/2017/mobasy/ https://doi.org/10.13140/rg.2.2.22472.24328/1 https://doi.org/10.13140/rg.2.2.20794.52160 https://www.researchgate.net/publication/350710240_verbrauchsbenchmarks_-_ein_soll-ist-vergleich_fur_wohnungsunternehmen https://www.researchgate.net/publication/350710240_verbrauchsbenchmarks_-_ein_soll-ist-vergleich_fur_wohnungsunternehmen https://www.researchgate.net/publication/350710240_verbrauchsbenchmarks_-_ein_soll-ist-vergleich_fur_wohnungsunternehmen http://www.building-typology.eu/downloads/public/docs/report/tabula_workreport_evaluationdatabase.pdf http://www.building-typology.eu/downloads/public/docs/report/tabula_workreport_evaluationdatabase.pdf http://www.building-typology.eu/downloads/public/docs/report/tabula_workreport_evaluationdatabase.pdf https://www.iwu.de/fileadmin/publikationen/gebaeudebestand/datenbasis/endbericht_datenbasis.pdf https://www.iwu.de/fileadmin/publikationen/gebaeudebestand/datenbasis/endbericht_datenbasis.pdf https://www.iwu.de/fileadmin/publikationen/gebaeudebestand/datenbasis/endbericht_datenbasis.pdf https://www.iwu.de/fileadmin/publikationen/gebaeudebestand/2018_iwu_cischinskyetdiefenbach_datenerhebung-wohngeb%c3%a4udebestand-2016.pdf https://www.iwu.de/fileadmin/publikationen/gebaeudebestand/2018_iwu_cischinskyetdiefenbach_datenerhebung-wohngeb%c3%a4udebestand-2016.pdf https://www.iwu.de/fileadmin/publikationen/gebaeudebestand/2018_iwu_cischinskyetdiefenbach_datenerhebung-wohngeb%c3%a4udebestand-2016.pdf https://www.iwu.de/fileadmin/publikationen/gebaeudebestand/2018_iwu_cischinskyetdiefenbach_datenerhebung-wohngeb%c3%a4udebestand-2016.pdf https://www.iwu.de/fileadmin/tools/energyprofile/energyprofile_forms_buildingsystem_eng.pdf https://www.iwu.de/fileadmin/tools/energyprofile/energyprofile_forms_buildingsystem_eng.pdf https://www.iwu.de/fileadmin/tools/energyprofile/energyprofile_forms_buildingsystem_eng.pdf https://www.iwu.de/fileadmin/tools/energyprofile/energyprofile_formbuildingsystem_datacapture.pdf 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https://www.researchgate.net/publication/355124720_targetactual_comparison_and_benchmarking_used_to_safeguard_low_energy_consumption_in_refurbished_housing_stocks https://www.researchgate.net/publication/355124720_targetactual_comparison_and_benchmarking_used_to_safeguard_low_energy_consumption_in_refurbished_housing_stocks https://www.researchgate.net/publication/355124720_targetactual_comparison_and_benchmarking_used_to_safeguard_low_energy_consumption_in_refurbished_housing_stocks https://www.researchgate.net/publication/355124720_targetactual_comparison_and_benchmarking_used_to_safeguard_low_energy_consumption_in_refurbished_housing_stocks https://episcope.eu/fileadmin/tabula/public/docs/report/tabula_commoncalculationmethod.pdf https://episcope.eu/fileadmin/tabula/public/docs/report/tabula_commoncalculationmethod.pdf https://doi.org/10.1002/bapi.200300020 acta polytechnica ctu proceedings 38:470–477, 2022 1 introduction 2 energy profile indicators 3 simple combined physics-probability model 3.1 principle 3.2 systematic uncertainty assessment 3.3 mechanisms of the energy performance calculation based on energy profile indicators 4 showcase: influence of different parameters on the uncertainty of the estimated consumption 5 application on a sample of apartment blocks from housing companies 6 conclusions acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.38.0176 acta polytechnica ctu proceedings 38:176–182, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague proof of concept – using prospective hybrid mfa-lca to evaluate the environmental implications of circular economy using a case study of wood nikolaos emmanouila, sarah cecilie andersena,b,∗, morten birkveda a the university of southern denmark, sdu life cycle engineering, department of green technology (igt), campusvej 55, dk-5230 odense m, denmark b danish technological institute, buildings & environment, gregersensvej, dk-2630 taastrup, denmark ∗ corresponding author: sara@dti.dk abstract. consistent evaluations of impacts induced by implementation of circular economy (ce) design processes and solutions within the built environment, necessitates decision-support tool development/advancement, as ce does not allow for business-as-usual assessments only. a preliminary test of concept that seeks to quantify the environmental implications of ce on a case study of wood, is presented here. the core methodology is based on coupling of the shared socioeconomic pathways (ssps) with material flow analysis (mfa) and consequential life cycle assessment (clca). applying this novel approach, a prospective consequential hybrid mfa-lca analysis was initiated, to evaluate the mitigation challenges of ce design processes, through different formalized and generally accepted (i.e. consensus) scenarios. the case study is based on danish reference buildings, meeting the current building regulations, designed to replace conventional building materials with wood. this “wood-approach” for test of concept case study, is chosen due to the increasing interest in wood construction. the development and calibration of a prospective model for different building material consumptions will further illuminate the connection between the to-be generated emissions and the marginal productions of the materials in question, under specific sets of societal development scenarios. keywords: hybrid mfa-lca, consequential lca, circular economy, shared socioeconomic pathways (ssps). 1. introduction realistic future scenarios are needed to create robust policies for the future, and when investigating ce benefits and implications. ce research is mainly focused on obtainable benefits for the biosphere and technosphere, neglecting equally important societal aspects. new methods are required, to address different possible future socioeconomic trajectories. the process of addressing the operating space for humanity needs to be based on concrete assumptions on how different future scenarios are developed, corresponding to different mitigation and adaptation challenges. to forecast different pathway variations under certain socioeconomic paradigms and include these assumptions in the evaluation of the environmental impacts of ce solutions, a methodology is proposed, coupling various existing methods. the scenario/assessment framework is based on the most recent development within climate modelling, the shared socioeconomic pathways (ssps) framework, extended with dynamic material flow analysis (mfa) and dynamic consequential life cycle assessment (clca). this combination, allows for prospective consequential hybrid mfa-lca, addressing societal challenges and changes along different future pathways, by quantifying material stocks and evaluating the consequences of such changes, by calculating the environmental implications of ce implementation processes. see a schematic overview in figure 1. while the individual methods used in this paper are well known and applied in their respective established line of research, the coupling of them, is only found in isolated parts of the literature, e.g. [1], but mostly only as partial couplings. the ssps are gaining traction, considering prospective aspects of scenario development, possibly as they have also been used in the ipcc 6th assessment report [2]. they have, however, also been used in the literature to assess e.g. the co2 emissions of global cement production [3], tokyo’s building sector emissions [4] and investigating the impact of material efficiency on climate mitigation [5], all under a socioeconomic perspective. in this study, we investigate how the ssp framework can be used to express different future pathways, presented through a case study on woody biomass, and thus contribute to the increasing pool of evidence and perspectives, for further prospective environmental implications quantification, when subjected to competing supply demands. the increasing focus on wood for energy and construction purposes allows to create 176 https://doi.org/10.14311/app.2022.38.0176 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 38/2022 proof of concept – using prospective hybrid mfa-lca . . . figure 1. schematic overview of applied methods. 1) global ssp narratives are downscaled to national storyline elements, using national trends and scenarios, 2) shared policy and technology assumptions (spas, stas) are created based on literature. together with model assumptions, the inventory data for the hybrid mfa-lca is created, 3) calculations of the model estimates possible future impacts of different environmental indicators. a case study with multiple potential pathways. the case study applied is thus used to illustrate a way to construct future scenarios for decision making, rather than to showcase a result for decision support. 1.1. case study denmark has set a target of lowering ghg national emissions by 70 % compared to 1990 levels [6], and aims to phase out coal by 2030 [7], leaving biomass as the largest renewable energy source [8]. additionally, there is a strong focus on shifting from conventional to wooden structures in denmark. these competing wood applications are based on different political agendas, as well as future societal developments, e.g., the fact that wood has the ability to be reused/recycled more under different cascading options, implying that wood can potentially contribute to resource efficiency for both the energy and construction industry, using it according to the waste hierarchy. 2. theory and proceeding methods 2.1. theory scientists use climate models to understand how greenhouse gases (ghgs) and aerosols interact with earth’s climate system, and forecast possible changes [9]. by implementing socio-economic, technological, energy and land-use changes as input variables, climate models can be used to create socio-economic dependent emission scenarios and allow for further examination of future climate impacts and mitigation strategies under various societal paradigms [10]. in the need for a common and consistent set of updated and more detailed future scenarios, investigating the impact of different climate policies, that take into account basic elements of demographic and economic drivers, as well as developments in energy demand, land-use and air pollutants, based on different narratives for the future [11], the so-called ssp framework was created. the ssp framework consists of five different narratives, one for each ssp, and every narrative expresses a different pathway based on future socioeconomic trends. the five narratives are expressed as: a world of sustainability (ssp1), business-as-usual (ssp2), a nationalist oriented world (ssp3), a world where societies suffer from increasing inequality (ssp4) and a future of increased economic growth and intensive energy use (ssp5) [12]. all scenarios include assumptions regarding climate policies and future technological developments, socalled shared policy assumptions (spas) and shared technology assumptions (stas). spas are designed by considering mitigation challenges and expected impact of the mitigation policy of each narrative, along with flexibility given to the modelers to implement their own policy interventions [12]. 2.2. scenario modelling this section presents the general methodology and logic behind each step, and then exemplifies application, onto the case study. the exemplification addresses the ssp2 (business-as-usual) narrative, the ssp1 pathway (as a more sustainable future), and ssp5 (taking the highway) as an energy intensive future. this should illustrate the effect of increased wood consumption under different futures. 2.2.1. national ssp narratives and scenario set-up the global ssp narratives, as found in riahi et al. [12], are adapted into national narratives. quantitative documentation of the ssp elements such as population, urbanization and energy projections, is available in the international institute for applied systems analysis (iiasa) ssp database [13], at global and national scale. the qualitative assumptions for all ssp scenarios can be seen in table 1. the population projections in ssp2 and ssp1 are in line with the global narratives, however ssp5 population projections diverge, due to high-tech and highliving standards, resulting in a higher growth rate. this deviation is based on the expectation on rich organization for economic co-operation and development (oecd) countries [14]. education is a key element that can influence people’s environmental awareness, leading to a change of consumption patterns and lifestyle [14], e.g., live in smaller houses in order to decrease material and energy consumption. environmental awareness in ssp1 and ssp5 is 177 n. emmanouil, s. c. andersen, morten birkved acta polytechnica ctu proceedings population environmental energy fossil fuel renewable’s recycling growth awareness demand development development rates ssp1 moderate very high low low high high ssp2 moderate moderate moderate moderate moderate moderate ssp5 high high high high moderate high table 1. qualitative assumptions for national storylines, based on global narratives and national policies. spas/stas ssp1 ssp2 ssp5 biomass energy phased out phased out moderate timber construction skills very high moderate high building components’ service life moderate no progress moderate wood cascading high no progress moderate-high waste treatment of building materials (eol) high moderate high table 2. shared policy assumptions (spas) and shared technology assumptions (stas) included in the model based on national policies. considered high, however the ssp2 storyline follows historical patterns. denmark’s future energy system is in line with the global political narratives. thus, the ssp1 pathway is characterized by a relatively low energy demand, and very restrictive national environmental policies with high taxes on fossil fuels, to keep the mitigation challenges low. these policies create constrains on fossil fuel use, which leads to a rapid improvement of conversion technologies on biomass and other renewable sources. oppositely, ssp5 represents an energy intensive and fossil fuel dependent future, with low fossil fuel prices and energy taxes. ssp2 represents an intermediate world. lastly, in line with core paradigms of the individual pathways, we assume that both ssp1 and ssp5 scenarios will have higher recycling rates and more sustainable material use of construction materials, than ssp2 scenarios. 2.2.2. national shared policy assumptions and shared technology assumptions the ssp framework allows modelers to implement their own assumptions regarding climate change policies, as well as future sectoral trends and technological developments, while the spas consider e.g., national legislation, the stas are considering future trends. the spas and stas included in the case study model can be seen in table 2. the case study is focused on the increased wood consumption in two main sectors: construction and energy, thus the spas and stas concerns questions regarding timber construction skills, cascading etc. to ensure that woody biomass is used in sustainable manner for the production of electricity and heat, danish political parties came to an agreement to set legal requirements on biomass applications [15]. this political decision was used for the spas in the case study. in ssp1 and ssp2, wood used as biomass for energy is phased out by the end of the century. in a sustainable scenario (i.e., ssp1) this policy is assumed to become very strict, leading to rapid change on how woody biomass is used. for ssp2 scenarios, we assume this policy agreement leads to a decrease of wood for energy at a slower rate than for ssp1, until wood is finally phased out. for ssp5, we assumed that the policy agreement is not strict enough, hence biomass follows the trend of the projected global ssp narrative. there are no current policies in denmark to specify how wood should / or shouldn’t be used in construction. stas implemented in the case study are targeted future trends in the building sector; ways that wood could help decarbonize the built environment, alternative wood cascading options, as well as optimal end-of-life (eol) treatment options of construction materials. the assumptions are distinguished in civil engineering skills, increased expertise of wood construction and increased future service life (durability) of building components. 2.3. coupling forecasting with mfa-lca the demanded wood stock for construction and energy is quantified by coupling the above forecasted storylines with time dependent mfa-determined inventory, from 2020 until 2100, and then with clca. the case study can be examined for every different ssp scenario, to express different future mitigation and adaptation challenges of ce of wood. the inputs are construction materials like concrete, bricks, steel, and wood which enter the system (inputs), and based on the scenario specific narratives, and the spas and stas, one can calculate the forecasted co2 emissions and waste streams (output). every ssp runs a certain set of parameters, concluding on different results, that help understand the potential under the ce concept. the sections below are purely illustrated through the case study. 178 vol. 38/2022 proof of concept – using prospective hybrid mfa-lca . . . variable assumption average persons per dwelling (av.per.dw.) ssp1av.per.dw. > ssp5av.per.dw. > ssp2av.per.dw. share of wood structures (wood.str.) ssp1wood.str. > ssp5wood.str. > ssp2wood.str. share of conventional structures (conv.str.) ssp2conv.str. > ssp5conv.str. > ssp1conv.str. service life of building components ssp1life (+20 %) > ssp5life (+10 %) > ssp2life recovered wood, class a+ ssp1a+ (25 %) > ssp5a+ (15 %) > ssp2a+ recovered wood, class a ssp1a (25 %) > ssp5a (15 %) > ssp2a recovered wood, class b ssp1b (25 %) > ssp5b (15 %) > ssp2b recovered wood, class c ssp1c (25 %) > ssp5c (15 %) > ssp2c recovered wood, class d ssp2d > ssp5d >ssp1d table 3. variables included in the model and the associated assumptions of the mfa model. 2.3.1. building stock modeling to estimate future building stock, the quantity of materials needed for construction purposes in denmark, until 2100, must be identified. the assumptions are shown, qualitatively, in table 3. the reference buildings used in our study are based on competitive conventional and timber structure design, to illustrate the difference in material consumption [16]. the bill of materials (bom) used, provide material quantities of the different archetypes, which are aggregated into material groupings and then projected to national level. the included archetypes are distinguished in detached (single-family) houses, semidetached houses and multi-dwelling houses. in denmark, the proportion of constructed buildings from 2009 to 2019 was mainly one to five floors, representing about 82 % of the total gross area [17]. assuming this trend will remain constant for the period 2020 to 2100, the archetypes account for 82 % of the future building stock. by using national housing statistics, the future building stock is extrapolated. the variables include the projections on number of people as indicated in the ssp database [13], and the country’s “share of residents per dwelling” statistics [18], which is assumed to remain constant in the future. the number of “average persons per dwelling” is estimated for each ssp in order to calculate the “total amount of buildings”. to calculate the total amount of wood used to construct the “total amount of buildings”, the share of wood and conventional structures (concrete/steel/bricks) is estimated. the wood needed to renovate all new constructions, is based on the amount of people under each ssp, extrapolated consistently until 2100. the second variable used to model renovation of the future building stock is “service life of building components”. in ssp2 the building service life is assumed business-as-usual, in ssp1 to last longer and in ssp5 assumed longer than ssp2 but still less than ssp1. by assuming different service lives per building component, the total amount of wood needed to renovate the new construction can be estimated, by creating renovation steps based on the service life of each wood building component. wood needed for renovation, is based on the existing building stock, documented by national statistics [19]. the same way as used in renovating new construction, the building components and their current service life are modelled, and the demand for wood is calculated by creating renovation steps based on the service life of each building component made of wood, excluding structural components. the renovation of the existing building stock does not relate to any applied assumptions, and it is thus modelled the same way for all ssps, since wood supply is assumed to satisfy the demand for renovation of existing building stock first. eol is modelled for both wood products and the other construction materials. to account for substitution potential of using wood instead of other building materials, the potential applications, and mass flows, of wood recovered from construction and demolition waste (c&dw) were examined. wood is classified into different cascading classes (a+, a, b, c, d) based on the size and the condition of what is recovered from c&dw, as presented in a finish study [20]. ssp1 and ssp5 take advantage of these potential flows, however ssp2 is modelled based on current share. shares for reuse of concrete and bricks, and recycling of remaining building materials [21, 22] are assumed for each ssp. 2.3.2. energy consumption projections energy consumption is projected by extrapolating current energy sector until 2030, when coal is predicted to be phased out. in 2030, it is expected that biomass energy still plays an important part in the danish energy system [23]. three different scenarios were created to model biomass energy. for ssp1 it is assumed that biomass will follow the same decreasing trend as coal, after 2030, and eventually be phased out. in ssp2, biomass energy after 2030 follows coal’s current annual decrease rate. ssp5 scenarios are modelled by implementing the annual energy demand growth as 179 n. emmanouil, s. c. andersen, morten birkved acta polytechnica ctu proceedings exemplary key findings, across projected future development pathways ssp1, baseline ssp2, baseline ssp5, baseline total energy related emissions (million metric tons co2eq) 2.574 3.393 8.448 tot. construc. related emissions (million metric tons co2eq) 4.665 6.755 15.748 per capita annual construction related emissions in period 2020–2100, for dk (kg co2eq/capita/yr) 6.05 9.32 11.1 wood biomass for energy and construc. (million metric tons) 86.43 126.09 291.02 potential avoided emissions from increased wood in construction (decrease in co2eq emissions) -17.50 % -10.78 % -15.88 % table 4. exemplary findings from applying the different future narratives, to the case study. the case study is executed on country level. addressed in the ssp database, resulting in biomass energy still existing. all remaining energy sources (oil, wind, solar, natural gas, hydro, geothermal, biogas, heat) have a reference year 2030, and follow the annual growth rates, as obtained from the ssp database. biomass energy production includes many different sources, but we focus only on woody biomass, thus the sources included in our model are wood chips, wood pellets and firewood. waste wood is excluded from the model because it is assumed that this fraction includes the waste wood in c&dw, which will be calculated in the eol, while wood waste from other sources, such as households is assumed very low, thus excluded. it is assumed that the share of each stream used to produce biomass energy is the same in the future as it is in 2019. parameters relating to production and imports of biomass energy, and the respective calorific values are obtained from national energy statistics [24]. since emissions from trading wood occur to the country of origin [25], import quantities are excluded from the model, to account only for the fraction of woody biomass produced in denmark. 2.3.3. life cycle assessment the mfa is based on a variety of parameters and assumptions that investigate the consequences of a change in demand of wood for energy and construction purposes, under different futures. thus, by choosing the clca approach, the model inherently has the ability to account for when marginal suppliers change their production capacity in response to an increased wood demand. the functional unit is defined as: “the accumulative amount of kilos of building materials per square meter of floor area (kg/m2) for denmark in 2100”. the model excludes the operational phase, but considers “benefits and loads beyond system boundary”, calculating credits and drawbacks from ce implementations. the model is dependent on the future scenarios for denmark, under the ssp framework. the modelling was made by using openlca 1.10.3 [26], the database used is ecoinvent 3.7 [27] and the impact assessment used is recipe midpoint (h) v1.13 [28]. 2.3.4. substitution potential the quantified wood stock assumes a trend in the future for more wooden structures. by running a baseline scenario, assuming that there will be no increase on wooden structures for all ssp scenarios, and by comparing the results with those obtained from the scenarios derived though the above mfa calculation, we can identify differences in material quantities used. class d quantities from c&dw, are used for energy recovery. the energy from wood combustion of a given year, will substitute the marginal fossil fuel energy source (oil or gas) in the same year, for all ssp scenarios. 3. results and discussion coupling the ssp framework with mfa and lca, provides the opportunity to model prospective wood chain in denmark. selected scenarios represent different narratives of extreme changes towards sustainability (ssp1), intensive energy dependency (ssp5), and development based in historical developments (ssp2). in the absence of strong policies, ssp2 is the future that society will most likely move towards. to enhance the comparison between the different narratives, we demonstrate some findings of the case study pathways in table 4, which highlight how the presented approach can evaluate the complex interconnected systems that ce concept touches upon. the outcome of cases assessed with the method can provide insight for political decisions associated. in context of the case study, it could further help 180 vol. 38/2022 proof of concept – using prospective hybrid mfa-lca . . . establish an overview for the future framework and development of forest-sector based industries and wood trade for denmark. exploring ways to meet modelled demands, underlines that wood is a limited resource. hence, forest plantation strategies could include measures for forest management, to optimize balance between afforestation/deforestation and wood harvesting, and set regulatory agendas, e.g., restricting the amount of primary wood used for fuel. 4. conclusion in this study, we investigate how the ssp framework can be used to express different future pathways, and thus set the foundation for further prospective sustainability performance quantification, when subjected to e.g. competing supply demands. the case study applied is used to illustrate a way to construct future scenarios for decision making at policy level, rather than to showcase a result for decision support. the suggested coupling methodology is an investigative approach to assess sustainability potential under different future pathways, for the applied case study it is wood used in denmark, achieved by coupling climate modelling with environmental engineering methodologies. by applying this concept, upstream supply chain activities and hotspots with considerable environmental implications are identified (by forecasting the substitution potential of the building materials), but also crucial societal parameters that will contribute to set future policies to keep the world temperature from rising. the proof of concept and method presented, carries limitations and implications. it requires careful parameterization and supporting peer-reviewed literature to limit uncertainties and excess opinionated model parameterization, when downscaling to national storylines. also, by adding more socioeconomic elements in the model, e.g., gdp or urbanization, the results may diverge. future development should be on establishing more robust scenario projections. acknowledgements the second author is funded by the danish technological institute, innovation fund denmark 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[26] greendelta. openlca. ed. berlin, [2021-01-29], https://www.openlca.org/. [27] g. wernet, c. bauer, b. steubing, et al. the ecoinvent database version 3 (part i): overview and methodology. the international journal of life cycle assessment 21(9):1218–1230, 2016. https://doi.org/10.1007/s11367-016-1087-8. [28] m. goedkoop, r. heijungs, m. huijbregts, et al. recipe 2008: a life cycle impact assessment method which comprises harmonised category indicators at the midpoint and the endpoint level. report 1: characterisation. amsterdam, 2009. 182 https://traeibyggeriet.dk/media/va5icbto/resume_juni2020_opdateret.pdf https://traeibyggeriet.dk/media/va5icbto/resume_juni2020_opdateret.pdf https://bit.ly/3aanerk https://bit.ly/3ahunvm https://bit.ly/3j3ficv https://doi.org/10.1080/20426445.2017.1389835 https://doi.org/10.1016/j.buildenv.2019.106633 https://doi.org/10.5334/bc.55 https://ens.dk/sites/ens.dk/files/basisfremskrivning/deco_2020_27082020.pdf https://ens.dk/sites/ens.dk/files/basisfremskrivning/deco_2020_27082020.pdf https://doi.org/10.1787/63937a2b-en https://www.openlca.org/ https://doi.org/10.1007/s11367-016-1087-8 acta polytechnica ctu proceedings 38:176–182, 2022 1 introduction 1.1 case study 2 theory and proceeding methods 2.1 theory 2.2 scenario modelling 2.2.1 national ssp narratives and scenario set-up 2.2.2 national shared policy assumptions and shared technology assumptions 2.3 coupling forecasting with mfa-lca 2.3.1 building stock modeling 2.3.2 energy consumption projections 2.3.3 life cycle assessment 2.3.4 substitution potential 3 results and discussion 4 conclusion acknowledgements supplementary materials references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.38.0131 acta polytechnica ctu proceedings 38:131–137, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague european environmental databases. opendap, spanish context. sheila oteroa,∗, soledad montillaa, josé antonio tenorioa, guillermo sotorrioa, teresa garnicab, belén abadc, marta condeb a instituto de ciencias de la construcción eduardo torroja, csic grupo de investigación construcción avanzada y sostenible, dep. construcción, calle serrano galvache 4, 28033 madrid, españa b universidad de córdoba, grupo de investigación ersaf, escuela técnica superior de ingeniería agronómica y de montes, avenida medina azahara 5, 14071 córdoba, españa c inditecma, s.l. calle del carmen, 1 – 3°, 28660 boadilla del monte (madrid), españa ∗ corresponding author: sheila@ietcc.csic.es abstract. with the aim of facilitating sustainability assessment through life cycle assessment, the eduardo torroja institute of construction sciences. csic is developing a public database of environmental information: opendap. one of the objectives of this database is to be compatible with other european databases but to be fully open, public and free of charge. with the participation in the project “wood for sustainable construction task force”, whose coordinator was the university of cordoba, 10 epds have been developed for different wood products and species, with a detailed study of each stage, using a spreadsheet and field studies for data collection. in this article we aim to demonstrate the need for environmental quantification of services and products in the construction sector, due to the importance of the global impact of this sector. with common procedures that allow us to obtain comparable and useful results for the final client who selects them. both for generic and specific products, being stable and exportable. allowing us to compare, reduce and improve our sustainability. keywords: lca, databases, epd, opendap, environment. 1. introduction life cycle analysis (lca) involves the study of a product or service from the moment of its conception (eco-design) to its ultimate end (cradle to grave). it should include phases beyond the system boundaries, tending towards the circular economy as a model (favouring reuse and recycling), from cradle to cradle. sustainability assessment through lca should be at environmental, social and economic levels. gerta, j. & traverso, m. [1] conducted a study on the analysis of sustainability through lca in the construction sector. in 171 cases reviewed between the years 2010 and 2021, only 11 % of the cases conducted a complete study on sustainability assessment with a life cycle approach. the rest are based analyses, which do not include the three pillars of sustainability, with the economic and especially the social pillars being the least evaluated. the calculation of impacts is also oriented towards the interests of the owner, which gives a partial and incorrect view. the construction sector is responsible for 33 % of global energy consumption, 40 % of raw material consumption and generates 40 % of solid waste [2, 3]. life cycle sustainability assessment at product and building level at an early stage (from design or choice of materials) can be a great responsibility and brings great benefits for the improvement of environmental, social and economic sustainability. through the european technical committee cen/tc 350, sustainability in buildings [4], standards such as iso 14025 [5] are defined, which defines common requirements, criteria and life cycle structure for environmental product declarations (epd). epds are an environmental communication tool that presents the life cycle performance of a product or service in a standardised and verifiable form. its calculations are based on lca, which consists of four phases. in the first phase, the objectives and scope are defined, where issues such as system boundaries, allocations, data quality, etc. are set, which can substantially vary the results according to the selected criteria. studies such as the one carried out by kellenberger et al. [6], estimate that a simplification in which transport, construction and end-of-life stages are eliminated, in energy terms, represents a 27–42 % reduction in the results obtained with respect to the consideration of all the stages. the calculation of stages a1–3 is the most common analysis due to the possibility of knowing the data reliably and closely (raw material extraction (a1), raw material transport (a2) and manufacturing (a3) stages). the second phase is data collection, one of the most extensive and complex phases. it can be based on literature, field research or specialised databases such as ecoinvent, gabi, elcd, etc. software such as gabi, simapro or openlca is commonly used for its 131 https://doi.org/10.14311/app.2022.38.0131 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en s. otero, s. montilla, j. a. tenorio et al. acta polytechnica ctu proceedings calculation (phase three of the lca). the evaluation phase of the data obtained is based on generally preestablished methodologies. these are classified and characterised according to iso standards 14040 and 14044 [7, 8]. the most popular methods are the ilcd, ecoindicator99, recipe, cml, etc. environmental product declarations (epd) for construction products, standardised by en 15804 [9], can have different scopes depending on the stages considered within the product life cycle. product category rules (pcr) are the documents that define the epd requirements for a product category or product family. they are currently being developed by the standardisation committees. until now, these pcrs have been developed by programme managers, with their own, non-comparable criteria. these programme managers display in stable and verified databases the lists of the epds they carry out, such as global epds, environdec, epd norge, etc. there are also platforms that similarly classify and display these epds with different quality and traceability options. examples of these platforms are ecoplatform, ókobaudat or opendap. in this article we highlight the problems presented by the development of a life cycle analysis and its presentation by means of an epd in the construction sector at the metadata, methodologies and results levels. this research stems from the work being carried out by the eduardo torroja institute of construction sciences csic (ietcc) in spain, whose ultimate goal is to develop the national database of environmental information on construction products: opendap [10]. it is an official, public and free accessible database of environmental information on construction products, both generic and specific. we will show how we work in the field of lca, the factors that have the major influence on the results and the shortcomings found. at the same time, we will also present some of the environmental information calculated and the decisions we have been taking in order to move forward. in the study by gerta, j. & traverso, m. [1], the four most influential factors in this field of work, among the 19 case studies analysed, are: system boundaries, the software used, the database and the life cycle inventory analysis (lcia) methodology. it is during the development of our national database (ndb) that we are encountering major challenges. that generates great doubts as to whether the “routes” for the calculation of the lca are marked and agreed with full transparency, traceability and, therefore, whether results can be compared. and, above all, whether this information can be used as one of the criteria for the selection of materials, as well as for the overall environmental accounting in construction: the building. 2. methodology responding to the need to provide stable and free environmental data, the project to develop a national database under the name of opendap was launched in 2013. this project is carried out at the ietcc, which is a centre attached to the state agency of the spanish national research council (csic), the main implementing agent of the spanish science, technology and innovation system. our research group, advanced and sustainable construction, has been trying for years to envision the construction of the future as a way to adapt to and mitigate climate change. 2.1. description of the structure of the opendap db as an example since 2013, this project has had to adapt to regulatory changes and updates. from a methodological point of view, the new une-en 15804:2012+a2:2020 greatly modifies many pre-established concepts in the development of the epd (largely due to its adaptation and convergence with another methodology that is evolving in parallel, such as the product or organisational environmental footprint, pef or oef): • new impact categories are added. • cradle-to-gate reporting (a1–a3), end-of-life modules (c1–c4) and benefits and burdens beyond system boundaries (d) are mandatory, except in exceptional cases. • biogenic carbon is required as an input for certain products. • the concepts of reference useful life (rul) and endof-life (eol) equations are defined and improved. • guidelines are provided for the assessment of generic data. regarding this last point, the pne-pren 15941 standard [11] is currently being revised, which reconsiders the quality, content and traceability of these products, as well as their classification. the classification and filtering system is the socalled ilcd, developed by the joint research centre (jrc), which is the european commission’s science and knowledge service. the ilcd format establishes the fields that each environmental data item displayed in the database has to contain, and also defines its content. the fields can be optional or mandatory. the aim is to generate databases that contain the same structure and allow interoperability between them. opendap is part of the international indata working group [12], whose main objective is to generate a european lca data structure for construction products based on current standards. it follows the same format, but extends its content with additional fields whose data remain from the epds. the format is called elcd+epd. within indata there are 11 countries represented (opendap is the representative of spain) that maintain a common database where their own databases converge. 132 vol. 38/2022 european environmental databases. opendap, spanish context. figure 1. spreadsheet development of one of the processes for 1 m3 of laricio pine sawn timber [13]. the software for the structure of the database is soda 4lca, also developed by the jrc. this format allows the data to be viewed, sorted, imported, exported and searched in ilcd format. these data can be downloaded in xml or simply visualised, so that they can be exported and worked on in any external calculation tool. 2.2. participation in the sustainable timber construction operational project the project consisted of developing an intelligent tool for the appropriate selection and prescription of woodbased products for construction to be incorporated in sustainable buildings, providing technical and environmental contents to these products. the ietcc was a member of this project, which ended in september 2020. together with the university of cordoba, they led the environmental development tasks [13]. one of the tasks of this project was to carry out 10 wtps of different wood products and species, belonging to the first and second transformation industries, performing their lca from “cradle to grave”. the flows of production, installation, end of life and reuse or recycling were established. for this purpose, questionnaires in sawmills and in industries of second processing of timber and also specific referenced literature were mainly used, always in the spanish geographical area. une-en 16485:2014 [14], as pcr for wood products, was followed. the calculation was performed without using any software or specialised database, exposing our limitations and simplifications. the data collection, the inventory of data and the representativeness of each sector involved in the manufacture of each product in spain were promoted. the processes basically consisted of machinery in the forest, or sawmill or second processing of timber (stages a1–a3), transport machinery (a2-a4-c2), the installation stage which combined lifting and fastening machinery, the dismantling stage (c1) (to simplify, this stage was consider the reverse of the installation), the waste treatment stage (c3) and dumping (c4), where it is considered that 100 % is reused or recycle, with the benefits being passed on to module d. as a result, the input flows considered are basically the inputs of diesel, oil (lubricant), petrol and a small amount of steel for the fastenings of the installation stage. with the exception of the manufacturing stage, whose main input flows are electricity and oil consumption. currently, the consultation of these daps, as well as the complete tool (search engine and intelligent selector of wood products) can be consulted on the maderia website [13]. this work was carried out using a spreadsheet. each process (e.g. consumption of a chainsaw in the forest) was detailed and accounted in the corresponding life cycle stage. subsequently, the inputs necessary to carry out that process were disaggregated (example: oil and petrol consumption of the chainsaw in the bush, figure 1) and after disaggregating the components of each substance (according to referenced and recognised sources), they were multiplied by the characterisation factors, the sums of which gave as a result the impacts for that process. this was replicated for each process and each stage. the limits of the system were set by the cpr pcr and the capacity to obtain such information. the study assumes a lack of data on waste, packaging, etc., but has very good quality input data taken directly from the manufacturer. 2.3. use of free environmental software and databases the work carried out within the sustainable timber construction (go madera) task force the need to use a calculation tool. openlca [15] is a free and open source tool for sustainability and life cycle assessment. it is compliant with current regulations, provides a data quality system, allows importing and working with different free and commercial databases, and allows importing calculation methods compatible with them, as well as exporting results to ilcd editors. it has a download platform for free databases and paid options, as well as methods and examples. following the work done in go madera, we wanted to replicate the results with the same input data. the database was another handicap of the previous project. agribalyse was the database selected. it is a french life cycle inventory (lci) database for the agricultural and food sector, published in 2020 and free of charge. the database contains certain field machinery, which, while not matching those of the timber industry, maintains certain approximations. as a compatible calculation method, the openlca 2.1.1 method package was used, including the “epd 2018” method, which contains calculations in accordance with une-en 15804:2012+a1:2014. this standard is in force and in coexistence with the new version until the end of october 2022. it was impossible to find a method that had the parameters for the up133 s. otero, s. montilla, j. a. tenorio et al. acta polytechnica ctu proceedings figure 2. screenshot of stage a1 in the openlca programme. figure 3. screenshot of opendap in ilcd+epd format. dated version of the standard (15804+a2), and was compatible with agribalyse or another free database. compatibility between the database and the calculation method is another important point to take into account. if this is not the case, there will be elementary flows that are not recognised by the methods and will not be taken into account for their computation. with this work, we have come to calculate the 7 indicators of the 15804+a1 and the water scarcity indicator. figure 2 shows the a1 step in the openlca software. the quantities entered coincide with those in figure 1. when using the agribalyse database, when we enter the entry “gasoline or lubricating oil”, each of them has a series of elementary flows associated with it that make up these entries. this makes the calculation more complete. 2.4. publishing the opendap database the ultimate aim of this work is to develop an official, public and freely accessible database with national character and international links. the opendap website [10] can currently be consulted and its contents and criteria can be studied in depth. in the “search” section there is a direct link to the database itself where the epds generated in the go madera project (currently under construction) can be consulted in ilcd+epd format as shown in figure 3. fully compatible and interoperable with the rest of the european environmental databases that have followed the criteria set by the european commission. 3. results and discussion as a first result, table 1 shows the comparative environmental impacts obtained for the functional unit of 1 m3 of laricio pine structural sawn timber for use as a structural element (beam or pillar), in dry environment and indoor use. the middle column shows the results obtained by means of a spreadsheet and a detailed field study; and the left column shows the results obtained by using openlca. the inputs and quantities of matter and energy were in both cases the same. and the right column shows the ratio between the two. in general, the results are quite different. the impacts with very small ratios are because the openlca results are larger, due to the simplifications assumed in the spreadsheet. the odp impact has very marked 134 vol. 38/2022 european environmental databases. opendap, spanish context. spreadsheet analysis openlca ratio (cradle to grave) (cradle to grave) (except module d) (except module d) spreadsheetanalysis openlca contribution to the global warming, gwp [kg co2eq] 2.19e+02 8.21e+02 0.27 destruction of the stratospheric ozone layer, odp [kg cfc-11eq] 2.13e-03 1.40e-04 15.21 acidification of land and wáter, ap [kg so2eq] 3.17e-01 2.86e+00 0.11 eutrophication, ep [kg po3− 4 eq] 5.89e-02 5.60e-01 0.11 formation of ground level ozone, pocp [kg ethylene eq] 1.93e-02 abiotic resourse depletion (fossil), adp fossil [mj] 6.85e+02 1.02e+04 0.07 abiotic resourse depletion (elements), adp elements [kg sb eq] 1.11e-03 photochemical oxidation [kg nmvoc] 2.51e+00 table 1. comparison of the environmental impacts obtained according to the calculation made in a spreadsheet and in the openlca software for the same product. spreadsheet ratio openlca ratio epd ratio epd ratio epd ratio (a) (a) (b) (b) (a) epd(1∗) (1*) (a) epd(2∗) (2*) (a) epd(3∗) (3*) epd(2∗) epd(3∗)a1–a3 a1–a3 a1–a3 a1–a3 a1–a3 gwp 1.57e+02 2.12 3.33e+02 -0.44 -7.55e+02 -0.48 -6.95e+02 2.41 1.38e+02 -5.03 odp 9.22e-04 0.06 5.47e-05 120503.26 4.54e-10 2.69 2.03e-05 ap 1.17e-01 11.47 1.34e+00 5.33 2.52e-01 3.56 3.77e-01 6.42 2.09e-01 1.80 ep 2.57e-02 11.03 2.84e-01 5.54 5.12e-02 5.73 4.95e-02 5.65 5.02e-02 0.99 pocp 8.82e-03 8.24e-02 6.19e-02 1.80e-02 3.44 adp fossil 3.78e+02 10.09 3.81e+03 11.88 3.21e+02 3.92 9.72e+02 8.45 4.51e+02 2.15 adp elements 2.33e-04 16.77 1.39e-05 0.21 1.12e-03 2.01 1.16e-04 9.66 table 2. comparison of the results of the sum of stages a1-2-3 according to the spreadsheet of the go madera project, in the openlca software and with three epds declaring similar products and their corresponding ratios. differences. reviewing openlca, the most influential process is diesel production and within this, there is a single stream, methane, possibly not considered in the spreadsheet. as a second result in this article, and to reinforce the environmental impact data obtained in the go madera project, in table 2 we make a comparison of this first epd obtained in openlca and locate three similar published epds. the stages selected were the sum of a1–3 because they are those declared in these three epds. • the epd 1* has as its functional unit 1 m3 egger sawn timber planed. realised under the programme ibu – institut bauen und umwelt e.v. the owner of the epd is fritz egger gmbh & co. og. it uses the software and db gabi 6 from 2013. • epd 2* has as functional unit 1 m3 radiata pine sawn board. it is carried out under the international epd® system operated by epd international ab. the owner of the epd is baskegur, the basque timber association. it uses the simapro 8.1 software and the ecoinvent 3.1 database. • epd 3* has as functional unit 1 m3 of swedish sawn dried timber of spruce or pine with a moisture content of 16 %. carried out under the international epd® system operated by epd international ab. the owner of the epd is swedish wood. uses simapro 8.5.2.0 software and db ecoinvent 3.4. the analysis of the results was calculated using the ratio between the results obtained in openlca and the rest. the most similar ratios are located in the gwp. analysing the openlca results, the impact results are influenced to a greater extent by the diesel production processes and secondly by the production of low voltage electricity. simplification in the go wood project would explain these divergent results. regarding the other epds compared, we see that their ratios also show discrepancies, always with higher values compared to the openlca results. this comparison with other epds is not very regulatory. on the one hand, they are of similar products and with different system boundaries and allocations. their 135 s. otero, s. montilla, j. a. tenorio et al. acta polytechnica ctu proceedings cprs are different because they are epds based on the rules of different programme managers. but, even epd 2* and 3* which share the same administrator and have used the same type of db and calculation program (in different versions), have large discrepancies in all impacts except eutrophication, as their ratio shows. this analysis only shows that the epds are a document, as of today declarative. they are not useful for comparison, they cannot be analysed and they are not useful for decision-making. the new updates of the regulations try to generate convergences, such as the development of common pcrs. even so, there is a notable lack of a common, transparent and free route. until now, programme administrators have provided quality assurance to their epds through their own regulations and external verifications. the jrc is creating this common route by providing a reporting format (ilcd) and software for database development (soda4lca). it promotes the generation of shared inventory data, but these do not have the required traceability. the indata initiative tries to make up for the lack of nearby data by converging formats from other european databases. but it is necessary to develop a national database that characterises the technologies of each process, which are as up to date as possible and specific to each geographical area: a national database. a software, with calculation methods according to current regulations (15804+a2), and an inventory database is necessary for the complete development of any epd. the work we have done with a spreadsheet has allowed us to master and understand each process. but the large number of components that influence each process makes it almost impossible to execute at that level with quality. 4. conclusion climate change is one of the greatest challenges of our time and its adverse effects undermine the ability of all countries to achieve sustainable development. the sustainable development goals (sdgs) and their translation into the 2030 agenda were agreed to mitigate climate change. these commitments must turn current policies on their head. environmental assessment and quantification at the product and building level must achieve even stronger, clearer and more binding guidelines. at product level, environmental quantification is being driven by initiatives at european level with support for both epd and pef development. at the building level, methodologies such as level seek to unify a common calculation method. it is necessary to quantify and qualify and, with this, to be able to decide. we must build with environmental knowledge. there must be a regulatory framework that establishes strong criteria setting limit values, as is the case in other areas of construction (acoustic, structural or thermal conditions). now the field of construction is complex and is constantly expanding. the interaction between the agents that make up this community, it is essential and powerful for its continuity and effectiveness. in short, lca studies involve a very complex analysis and calculation, that at the level of building is exponential. the multitude of factors that are involved in each stage of their life cycle, and amplitude of each branch, would the infinite formulation. therefore, we act with simplifications and modelling that allow advance testing, development lines and national data set. the environmental analysis of a building involves an assessment and prior knowledge of the user, the designer and manufacturer, which should direct their business models. with this drive shaft opendap emerge: offer information. references [1] j. g. backes, m. traverso. application of life cycle sustainability assessment in the construction sector: a systematic literature review. processes 9(7):1248, 2021. https://doi.org/10.3390/pr9071248 [2] g. akhanova, a. nadeem, j. r. kim, s. azhar. a multi-criteria decision-making framework for building sustainability assessment in kazakhstan. sustainable cities and society 52:101842, 2020. https://doi.org/10.1016/j.scs.2019.101842 [3] j.-h. choi. strategy for reducing carbon dioxide emissions from maintenance and rehabilitation of highway pavement. journal of cleaner production 209:88–100, 2019. https://doi.org/10.1016/j.jclepro.2018.10.226 [4] 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[2019-05-06]. https://revista.une.org/11/centc350-sostenibilidad-en-la-edificacion.html [5] une-en iso 14025:2010. etiquetas y declaraciones ambientales. declaraciones ambientales tipo iii. principios y procedimientos. [6] d. kellenberger, h.-j. althaus. relevance of simplifications in lca of building components. building and environment 44(4):818–825, 2009. https://doi.org/10.1016/j.buildenv.2008.06.002 [7] une-en iso 14040:2006/a1:2021. gestión ambiental. análisis del ciclo de vida. principios y marco de referencia. modificación 1. [8] une-en iso 14044:2006/a2:2021. gestión ambiental. análisis del ciclo de vida. requisitos y directrices. modificación 2. [9] une-en 15804:2012+a2:2020. sostenibilidad en la construcción. declaraciones ambientales de producto. reglas de categoría de producto básicas para productos de construcción. [10] opendap. base de datos de información ambiental de productos de la construcción. 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[2021-09-20]. https://www.indata.network/data/ [13] proyecto de investigación grupo operativo “madera construcción sostenible”. financiación en el marco del programa nacional de desarrollo rural 2014-2020, para el año 2018. bdns (identif.): 405133. línea de ayuda feader 05046001 16 421 00 cooperación (artículo 35), 2020. [2020-06-10]. https://www.maderia.es/sites/ default/files/media/file/diptico-go-madera.pdf [14] une-en 16485:2014. madera aserrada y madera en rollo. declaraciones ambientales de producto. reglas de categoría de productos de madera y derivados de la madera para su utilización en construcción. [15] openlca. the open source life cycle and sustainability assessment software. [2021-04-14]. https://www.openlca.org/ 137 https://www.indata.network/data/ https://www.maderia.es/sites/default/files/media/file/diptico-go-madera.pdf https://www.maderia.es/sites/default/files/media/file/diptico-go-madera.pdf https://www.openlca.org/ acta polytechnica ctu proceedings 38:131–137, 2022 1 introduction 2 methodology 2.1 description of the structure of the opendap db as an example 2.2 participation in the sustainable timber construction operational project 2.3 use of free environmental software and databases 2.4 publishing the opendap database 3 results and discussion 4 conclusion references my title https://doi.org/10.14311/app.2022.36.0253 acta polytechnica ctu proceedings 36:253–260, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague simplified modelling of the performance of concrete tunnels during fire and post-fire damage classification ruben van coilea,∗, balša jovanovića, ranjit kumar chaudharya, xavier deckersb, andrea lucherinia a ghent university, department of structural engineering and building materials, technologiepark-zwijnaarde 60, 9052 gent, belgium b fire engineered solutions ghent a jensen hughes company, oudenaardsesteenweg 32 g, 9000 ghent, belgium ∗ corresponding author: ruben.vancoile@ugent.be abstract. the performance of concrete tunnel structures during and after fire is not well understood. this is an obstacle to the adoption of risk-based approaches for fire safety design of tunnel structures. upon the request of the belgian fire safety consultancy fesg, a simplified assessment of the collapse probability and post-fire damages for a reference tunnel structure has been made. the structural system is modelled through 2d beam finite elements, where spalling rates have been assumed based on available literature data. structural stability is verified for both the heating and cooling phases of the fire. in those cases where the structure survives up to burnout, the residual deformations and thermal damage to the tunnel structure are assessed. keywords: damage, failure probability, fire, spalling, tunnel. 1. introduction the structural fire design of concrete road tunnels is commonly done with reference to prescriptive guidance and regulations. the actual performance achieved in case of a fire is not well understood, both in terms of the probability of failure (collapse during fire), as well as the probability density function of the damage post-fire (permanent damage and residual deflections, repair costs). this hampers the adoption of risk-based approaches to the fire safety design of road tunnels, and is an obstacle for risk-based codecalibration. the belgian fire engineering consultancy fesg has been developing a simplified risk assessment tool for tunnel structures. this risk assessment requires an evaluation of the probability of structural failure during fire and of the extent of the post-fire damage and downtime. this will allow for the comparison of design alternatives, e.g. with respect to active and passive fire safety measures. for further details on the project and a conceptual framework for decision making for tunnel fire safety, see [1]. in cooperation with fesg, a simplified assessment of the performance of a reference concrete tunnel structure during and after fire has been made. a 2d beam finite element model is adopted, taking into account the effect of concrete spalling through nominal spalling rates. the fire is modelled through a heating phase according to the nominal rws curve, followed by a decay phase based on limited experimental data. details of the approach and key outcomes are provided in the following. 2. modelling approach 2.1. reference tunnel section the reference tunnel cross section is visualized in figure 1, with tunnel lining cross-sectional characteristics (per unit length) listed in table 1. the cross-section is inspired by an existing design consisting of two one-directional tubes and a central connecting tube for evacuation purposes and maintenance. for simplicity (calculation time), all walls are identical and no reinforcement curtailment has been modelled. soil resistance to outwards movement of the tunnel is modelled through springs. a structural mesh of 0.5 m is applied. a fine thermal mesh is applied (5 to 10 mm element width perpendicular to the heat transfer direction). the structural modelling is done using safir [2]. other input parameters for the analysis (material and mechanical properties of concrete and reinforcement) are listed in table 2. the adopted concrete model is an extension to the en 1992-1-2:2004 [3] model, taking into account explicit modelling of transient creep [4]. concrete tensile strength is neglected. the concrete compressive strength and reinforcement yield strength are considered in the model with their expected values. this is the preferred approach for implementation in the larger risk-based framework. furthermore, due to the complex structural behaviour of the tunnel system, it is not a priori clear if a larger or smaller value of the material strength and stiffness is "more conservative". the concrete poisson coefficient is an artefact from model development. a lower temperature-dependent poisson coefficient can 253 https://doi.org/10.14311/app.2022.36.0253 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en r. v. coile, b. jovanović, r. k. chaudhary et al. acta polytechnica ctu proceedings figure 1. section of the reference tunnel and dimensions (lining axis positions). the blue lines are springs modelling the soil interaction. parameter ceiling and floor walls thickness [mm] 600 410 concrete cover [mm] 70 100 reinforcement axis distance to concrete surface [mm] 75 105 reinforcement area (single layer) [mm2] 5300 �25 − 100 2010 �16 − 100 table 1. tunnel cross-sectional characteristics (per unit length). be recommended, in accordance with [5]. parameter value concrete compressive strength [mpa] 42 concrete tensile strength [mpa] 0 concrete poisson coefficient [−] 0.3 reinforcement yield strength [mpa] 560 reinforcement initial modulus of elasticity [gpa] 210 reinforcement poisson coefficient [−] 0.3 table 2. material parameters. upon cooling, concrete loses an additional 10% of strength in accordance with en 1994-1-2:2005 recommendations. the reinforcement fully recovers upon cooling. the latter is a simplification that overestimates reversibility for reinforcement which has exceeded 600◦c [6]. this simplification is in line with the state-of-the-art, but means that cooling results for reinforcement that has exceeded 600◦c need to be interpreted with care. fundamental research is needed to develop a model for the material behaviour during the cooling phase. the tunnel is loaded with 1 m of soil (weight 17.5 kn/m3) and 2 kn/m2 permanent load at ground level. the lateral earth pressure coefficient is set at 0.4. the tunnel is considered to be above the ground water table (no upwards water pressure). these values were chosen to represent a city tunnel for through traffic. a characteristic imposed load of 30 kn/m2 is considered. the mean imposed load is assessed as 0.2 times the characteristic value, in analogy with building design [7]. improved live load modelling for geotechnical structures, and the consideration of the local geotechnical parameters are recommended. 2.2. fire exposure as the tunnel is symmetrical, fire is considered in one of the road tubes only (see figure 1). the likelihood independent simultaneous occurrence of fire in both tubes is considered negligible. within the larger risk assessment tool, a detailed assessment of fire severity is made, taking into account fire spread and the interaction with active fire safety measures. for the structural performance evaluation, these fire severities are mapped onto an equivalent heating duration for the nominal rws fire curve [8]. the following nominal durations are considered: 15/30/60/120/180 min. the fire exposure is applied to the walls and ceiling, considering a convective heat transfer coefficient of 50 w/(m2k) and emissivity of 0.7, based on en 19921-2:2004 [3]. the decay phase is modelled through equation 1, with θc, θref and θmax respectively the fire curve temperature in the cooling phase, the reference ambient temperature (20◦c) and the maximum rws fire temperature during the heating phase. the time in minutes since the nominal start of the fire and the time of heating phase duration are denoted by t and tmax, while the speed of the decay is governed by the decay parameter b. to evaluate the decay parameter b in equation 1, the decay phases of 19 test measurements were extracted from [9]. an average value of 0.025 min−1 was estimated and adopted (mean value preferred for cost-benefit evaluations in a later stage). for concrete structures, the cooling phase behaviour is of great importance, both due to the delayed heating of the cross-section, as well as 254 vol. 36/2022 modelling of concrete tunnels damage by fire figure 2. example fire curves. rws heating phase and exponential decay phase. temperatures to be interpreted as adiabatic surface temperatures. due to the fire-induced permanent deformations resulting in a possible force redistribution. the obtained temperature-time curves are visualized in figure 2. θc − θref = (θmax − θref ) e−b(t−tmax) (1) 2.3. concrete spalling spalling is modelled considering nominal spalling rates, as recommended in [10] and applied by [11]. specifically, the start of spalling is fixed at 1 min after the nominal start of the exposure, followed by a constant spalling rate of, for example, 3 mm per minute. spalling is stopped as soon as either (i) the reinforcement layer is reached, or (ii) when the fire enters the decay phase. the spalling is implemented in safir through an automated procedure, as described in [11]. considering the current state-of-knowledge, it is recommendable to consider all unprotected concrete to be prone to spalling. earlier definitions of spalling-free concrete mixes have later been shown susceptible to spalling after all, see e.g. [12]. especially in the case of severe fire exposures as adopted for tunnels, (limited) spalling of unprotected concrete can be assumed. the addition of polypropylene fibres is considered to reduce the concrete propensity to spall [13]. most literature sources ascribe this to an increased porosity (and thus the reduction in pore pressures) during fire. concrete with polypropylene fibres is assigned a spalling probability of 40%. this number is based on a study of spalling occurrence in an unpublished test campaign and should be considered as engineering judgement. for protected concrete, spalling is considered not to occur. this does not imply that local spalling is deemed impossible in case of protected concrete. the occurrence of spalling across the entire exposed surface is however deemed very unlikely. hua et al. [11] report spalling rates for tunnels of up to 5 mm/min. in the current study nominal spalling rates of 1/2/3/3.75/5 mm/min are considered, as well as a no-spalling case. the spalling severity figure 3. spalling rate data listed by hua et al. (2021) and fitted gamma distribution. spalling rate [mm/min] probability 1.00 0.091 2.00 0.313 3.00 0.292 3.75 0.139 5.00 0.165 table 3. assessment of conditional probabilities for the spalling severity. probabilities (conditional probability given spalling) are derived from the data listed by [11]. a theoretical distribution describing the spalling rate probability density is fitted on the data whereby each literature data point is treated equally. applying the akaike information criterion (aic) [14], a gamma distribution (equation 2, with γ the gamma function, and x the spalling rate) with shape factor k = 3.6 and scale factor θ =0.7 is chosen as the best fit (see obtained fit in figure 3). based on this distribution, probabilities for each spalling rate used in this study are calculated and presented in table 3. these probabilities have been calculated considering the nominal spalling rates as upper bounds for their respective interval (with the exception of the highest nominal spalling rate of 5 mm/min). f(x) = 1 γ(k) · θk xk−1e(x/θ) (2) 2.4. post-fire damage assessment reinforced concrete structures exhibit permanent damage following fire [6]. concrete exhibits a permanent loss of strength and stiffness following heating to elevated temperatures and subsequent cooling, and also reinforcement exhibits a permanent loss of 255 r. v. coile, b. jovanović, r. k. chaudhary et al. acta polytechnica ctu proceedings damage states thermal damage repairability ds0 d300 ∼= 0 no repair required ds1 0 < d300 < c/10 clean surface and replace damaged concrete ds2 c/10 ≤ d300 < c clean surface and replace damaged concrete ds3 c ≤ d300 < d/4 clean surface and replace damaged concrete ds4 d/4 ≤ d300 < d/2 demolish and reconstruct table 4. thermal damage states, with c the concrete cover and d the thickness of the cross-section. damage states structural damage ceiling repairability structural damage wall repairability ds0 ∆ s/l < 1/240 no repair required ∆ c/h < 0.5% no repair required ds1 1/240 ≤ ∆ s/l < 1/120 no repair required ∆ c/h ≥ 0.5% demolish and reconstruct ds2 1/120 ≤ ∆ s/l < 1/60 demolish and reconstruct −ds3 1/60 ≤ ∆ s/l demolish and reconstruct table 5. structural damage states, with ∆ s the mid-span residual vertical displacement, l the ceiling span, ∆ c the mid-height residual horizontal displacement, and h half of the wall height. figure 4. axial force (top; compression blue) and bending moment (bottom) before the fire. strength following exposure to temperatures in excess of 600◦c [6]. both these damage effects are on a material level. the fire exposure also results in fire-induced forces (restraining effects), permanent thermal strains, and load redistribution due to changes in stiffness. these effects result in residual deformations post-fire. these residual deformations may jeopardize the serviceability of the tunnel, and also result in second order load effects. these latter damage effects are on a system (structural) level. the concept to pre-evaluate the repair costs of concrete structures following fire exposure is relatively new. a recent in-depth investigation for concrete structures has been presented in [15]. the evaluation of concrete fire damage through the 300◦c isotherm (d300) and residual deformations is recommended. these damage parameters are then used to determine the damage class and repair feasibility. the study by ni and gernay also elaborates an approach for repair cost of fire-damaged structures based on the damage states. specifically, rsmeans data is used to inform the cost evaluations. a similar approach as proposed by ni and gernay is adopted in the following to evaluate the damage class and repair cost for fire-damaged tunnel structures. the same limit states as proposed by [15] are used here to determine the damage of the structure. table 4 and table 5 below gives the damage states and repairability limit states for thermal and mechanical damages for concrete structures following a fire event. 256 vol. 36/2022 modelling of concrete tunnels damage by fire figure 5. axial force (top; compressive blue) and bending moment (bottom) after approx.. 60 min of fire exposure (beginning of the cooling phase). figure 6. axial force (top; compression blue) and bending moment (bottom) after 12 hours of fire exposure (end of the fire event simulation. for the damage states of the tunnel ceiling, the damage classification for slabs is adopted, while column limits are adopted for the walls. this is based on the consideration that the tunnel cross-section can be viewed as a 2d frame. 3. results 3.1. example case analysis 60 min rws heating phase, no spalling in this case study, the walls and ceiling in the left tube are exposed to fire from the interior only. these linings can be considered thermally thick as the cooling conditions on the unexposed side (e.g., adiabatic, or cooling to ambient) do not influence the results. upon the start of the cooling phase, the hot exposed surface quickly starts to cool as well. the heat wave however 257 r. v. coile, b. jovanović, r. k. chaudhary et al. acta polytechnica ctu proceedings figure 7. thermal damage state (ceiling and wall) in function of rws heating phase duration and nominal rate of concrete spalling. figure 8. structural damage state (ceiling and wall) in function of rws heating phase duration and nominal rate of concrete spalling. continues to penetrate into the interior of the lining. at 130 minutes the ceiling bottom reinforcement reaches its maximum temperature of approximately 270◦c. this maximum temperature is within the range where the reinforcement can be considered to fully recover [6]. similar results are obtained for the fire-exposed walls. as the concrete cover is larger for the walls, the maximum reinforcement temperature at the exposed side is lower (189◦c, attained at approximately 168 minutes). the depth of the 300◦c isotherm reaches a maximum around 113 minutes with a depth of 60-65 mm. this result applies to both the ceiling and wall. the thermal damage classification is ds2 both for the ceiling and wall. the tunnel section withstands the modelled exposure without structural failure. however, the fire exposure does result in permanent deformations and load redistributions. importantly, to obtain a full view on the structural response, the calculation needs to be continued for beyond the end of the heating phase. in figure 4, the axial force (top) and bending moment (bottom) in the tunnel section are visualized at the start of the exposure (compression blue, tension yellow colour; bending moment drawn at the tension side). figure 5 visualizes the same at approximately 60 min, and figure 6 at 12 hours since the start of the nominal fire exposure. in these figures, displacements are visualized with a factor 10. the maximum ceiling mid-span deformation is obtained at approximately 115 min since the start of the exposure (approximately 55 mm). the recovery of the deformation takes multiple hours. the residual total mid-span deformation is 36.4 mm. the mechanical damage classification for the ceiling is ds0. the same applies to the wall. 3.2. damage classification summary none of the simulated tunnels fail during the fire exposure. the possibility of failure has however been confirmed through simulations with characteristic loading. in the current evaluation, expected values for the load conditions have been considered as indicated in section 2.1. figure 7 and figure 8 show the matrix plots for the thermal and mechanical damage for different rws fire duration and nominal spalling rates. the plots also include damage classifications for fire protected structures (i.e. tunnels cladded with passive fire protection boards with a nominal rating of 1h or 2h protection). combining the results of figure 7 and figure 8 with the spalling probabilities listed in section 2.3 above, damage class probabilities are obtained in function of the equivalent fire severity (figure 9 for the ceiling damage classes for unprotected tunnel linings). figure 9 highlights how also limited fire exposure results in a damage class of at least ds2, and a high proba258 vol. 36/2022 modelling of concrete tunnels damage by fire figure 9. structural damage state probabilities (probability of damage class dsx or higher) for unprotected tunnel ceiling in function of rws heating phase duration. note that the damage class is evaluated for the heating durations 15/30/60/120/180 minutes only. bility of class ds3. probabilities of ds4 are however observed only from 120 minutes heating duration onwards (note that only a limited number of discrete heating durations are modelled here). the results of figure 9 are subsequently combined with the rws heating phase duration probabilities provided by the fesg fire severity model, thus resulting in damage class probabilities for a given tunnel with specific traffic intensity and active/passive fire safety measures. within the larger risk assessment tool, these damage class probabilities are subsequently translated into assessments of the downtime and repair costs. 4. conclusions the structural fire performance of a reference concrete tunnel has been evaluated, taking into account different rws heating durations and nominal spalling rates. the total simulated time is 12 hours to explicitly evaluate the cooling of the structure. the importance of an explicit evaluation of the cooling phase behaviour has been demonstrated through an example, and the probabilities of achieving set thermal and structural damage states post-fire have been assessed. the results will be incorporated in a simplified risk assessment tool, allowing to compare the risk profile of design alternatives for concrete road tunnels. acknowledgements this study has been made possible through funding from fesg, supported by vlaio (flanders innovation & entrepreneurship). 5. bibliography [1] r. van coile, m. schepers, m. pachera, m., et al. risk acceptance framework for specifying fire safety measures in tunnels. 13th international conference on structural safety and reliability (icossar 2021), 20-24/06 shanghai, china, 2022. [2] j.-m. franssen, t. gernay. modeling structures in fire with safir®: theoretical background and capabilities. journal of structural fire engineering 8(3):300-23, 2017. https://doi.org/10.1108/jsfe-07-2016-0010. [3] en 1992-1-2:2004. eurocode 2: design of concrete structures part 1-2: general rules structural fire design. european standard, 2004. https://www.phd.eng.br/wp-content/uploads/2015/ 12/en.1992.1.2.2004.pdf. [4] t. gernay, j. m. franssen. a formulation of the eurocode 2 concrete model at elevated temperature that includes an explicit term for transient creep. fire safety journal 51:1-9, 2012. https://doi.org/10.1016/j.firesaf.2012.02.001. [5] t. gernay, a. millard, j.-m. franssen. a multiaxial constitutive model for concrete in the fire situation: theoretical formulation. international journal of solids and structures 50(22-23):3659-73, 2013. https://doi.org/10.1016/j.ijsolstr.2013.07.013. [6] fib. fire design of concrete structures structural behaviour and assessment. fib bulletin 46. the international federation for structural concrete. lausanne, switzerland, 2008. [7] b. jovanović, r. van coile, d. hopkin, et al. review of current practice in probabilistic structural fire engineering: permanent and live load modelling. fire technology 57(1):1-30, 2020. https://doi.org/10.1007/s10694-020-01005-w. [8] a. j. breunese, c. both, g. m. wolsink, g.m. fire testing procedure for concrete tunnel linings. tno-rapport 2008-efectis-r0695, bleiswijk, the netherlands, 2008. [9] h. ingason, s. gustavsson, m. dahlberg. heat release rate measurements in tunnel fires. brandforsk project 723-924 (sp report 1994:08). swedish national testing and research institute, boras, sweden, 1994. [10] fib. performance-based design of concrete structures (committee draft 2021-06-24). the international federation for structural concrete. lausanne, switzerland, 202. [11] n. hua, a. tessari, n. elhami khorasani. characterizing damage to a concrete liner during a tunnel fire. tunnelling and underground space technology 109, 2021. https://doi.org/10.1016/j.tust.2020.103761. [12] t.g. van der waart van gulik, a. j. breunese, r. jansson, r., et al. spalling behaviour of a non-spalling qualified concrete. 4th international workshop on concrete spalling due to fire exposure, 08-09/10 leipzig, germany, 2015. [13] c. maluk, l. bisby, g. p. terrasi. effects of polypropylene fibre type and dose on the propensity for heat-induced concrete spalling. engineering structures 141:584-95, 2017. https: //doi.org/10.1016/j.engstruct.2017.03.058. [14] h. akaike. a new look at the statistical model identification. ieee transactions on automatic control 19(6):716-23, 1974. https://doi.org/10.1109/tac.1974.1100705. 259 https://doi.org/10.1108/jsfe-07-2016-0010 https://www.phd.eng.br/wp-content/uploads/2015/12/en.1992.1.2.2004.pdf https://www.phd.eng.br/wp-content/uploads/2015/12/en.1992.1.2.2004.pdf https://doi.org/10.1016/j.firesaf.2012.02.001 https://doi.org/10.1016/j.ijsolstr.2013.07.013 https://doi.org/10.1007/s10694-020-01005-w https://doi.org/10.1016/j.tust.2020.103761 https://doi.org/10.1016/j.engstruct.2017.03.058 https://doi.org/10.1016/j.engstruct.2017.03.058 https://doi.org/10.1109/tac.1974.1100705 r. v. coile, b. jovanović, r. k. chaudhary et al. acta polytechnica ctu proceedings [15] s. ni, t. gernay. a framework for probabilistic fire loss estimation in concrete building structures. structural safety 88, 2021. https://doi.org/10.1016/j.strusafe.2020.102029. 260 https://doi.org/10.1016/j.strusafe.2020.102029 acta polytechnica ctu proceedings 36:253–260, 2022 1 introduction 2 modelling approach 2.1 reference tunnel section 2.2 fire exposure 2.3 concrete spalling 2.4 post-fire damage assessment 3 results 3.1 example case analysis 60 min rws heating phase, no spalling 3.2 damage classification summary 4 conclusions acknowledgements 5 bibliography bibliography acta polytechnica ctu proceedings https://doi.org/10.14311/app.2025.53.0037 acta polytechnica ctu proceedings 53:37–41, 2025 © 2025 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague rheology properties of metakaolin activated by lithium water glass vendula kellnerová, tomáš david, veronika blovská, pavel reiterman∗ czech technical university in prague, faculty of civil engineering, thákurova 7, 166 29 prague 6, czech republic ∗ corresponding author: pavel.reiterman@fsv.cvut.cz abstract. alkali-activated materials play an important role in the current industry, because they offer alternative way towards low carbon technologies. however, their crucial utilization still lays in the field of composites resistant to high temperatures, especially in the heavy machinery and chemical industry. the current paper documents rheological properties of set of pastes on the basis of czech metakaolin as precursor, which was activated by lithium water glass exhibiting various silicate modulus. the rheological properties were studied in terms of standard flow test and optimized composition exhibiting similar workability were subsequently investigated by using rotary rheometer. the rheology was monitored in time to document performed changes during the geopolymerization. the attained flow curves were fitted by herschel-bulkley model and basic rheological characteristics of optimized pastes were derived. the research confirmed prolonged workability of the studied paste activated by lithium water glass and better. the pastes activated by lithium water glass of lower silicate modulus exhibited lower values of yield stress. keywords: lithium water glass, metakaolin, rheology, flow curves. 1. introduction last decades the scientific activity is focused on the development and improvement of alternative binding systems exhibiting lower carbon dioxide emissions in comparison with traditional systems suchlime and portland cement (pc). the activation of aluminosilicates is a promising way offer, so called geopolymers [1]. these materials are well known for more then a century. the first systematic research was published by kuhl in 1930, however the patent by the similar author was filled at 1908. these initial works were focused on the utilization of blast furnace slag as a precursor and sodium hydroxide as an activator [2]. with increasing interest in alkali-activated materials (aam) in last decades were intensively studied the other possibilities of these binders with the aim to replace pc in a production of buildings and structures. aam could be formulated on the basis of different precursors [3, 4]. promising results were attained with blast furnace slag, however in relation to gradual reduction of heavy metallurgy in regions such a west and middle europe. hence, the attention is currently focused on calcined clays, which seem to perspective materials for such an application, because of their wide availability [5, 6]. besides number of successful utilizations of aam in building structures, the main importance lies in the field of high temperature application, because of superior resistance of such formulated composites [7–9]. the final properties are logically influenced by the mineral composition of used precursor. thus, the properties of hardened composite could be highly different. in addition, the microstructure and entire performance of aam based composite is also determined by the used activator. aluminosilicate could be successfully activated by the water glass, usually potassium or sodium), or by sodium hydroxide or a combination of both [10]. the relatively fast process of polymerization leads to the gradual loss of the workability, what limits standard industrial application of amm. in technical praxis is preferable applied potassium-based activator, due to prolonged workability time [11]. incorporation of lithium ions is relatively rare, because of higher price of such materials, however intensive research was performed in the field of utilization of lithium slag, however the lower reactivity was monitored [12]. the present paper is focused on the experimental study dealing with monitoring of rheology properties of pastes on the basis of metakaolin activated by the lithium water glass, which was selected due to its better performance in terms workability of aams. 2. experimental program the experimental program was focused on the systematic description of rheology properties of the set of alkali-activated pastes. czech metakaolin was used as a precursor and lithium water glass of different modulus served as an activator. the used precursor is metakaolin mefisto k05 (čluz a.s., nové strašecí, czech republic) originating by calcination of kaolinitic clay at approximately 750 °c. its chemical composition is introduced in table 1. the previous 37 https://doi.org/10.14311/app.2025.53.0037 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en v. kellnerová, t. david, v. blovská, p. reiterman acta polytechnica ctu proceedings sio2 al2o3 fe2o3 cao mgo k2o na2o tio2 so3 49.1 47.3 0.9 0.2 0.1 0.5 0.0 1.6 0.1 table 1. chemical composition of used metakaolin. water glass li 2.7–5.0 li 3.0–3.5 li 3.7–4.2 li 3.7–5.0 middle value 3.85 3.25 3.95 4.35 table 2. used lithium water glass. research confirmed, that amorphous content is just about 74.5 % [13] and poccolanic activity in terms of chapelle test 1 967 mg/ca(oh)2 per a gram of pozzolan [14]. the used lithium water glass, produced by vodní sklo, ltd., exhibited different silicate modulus, which are introduced in table 2. the pastes of aam were prepared by mixing of metakaolin and the activator by 1:1 for 2 minutes in a laboratory mixer. subsequently was determined spreading in accordance with čsn en 1015-3 using traditional hägermann flow table, figure 1a. the resulting spreading represents diameter of the pastes after vibration, figure 1b. the consistency was monitored after mixing, 15, 60 and 150 minutes to assess the stability in time. the second phase of the laboratory campaign was focused on the optimization of the dosage of an activator to achieve similar consistency. the targeted spreading was approximately 310 mm. the pastes of optimized composition were investigated using rotary rheometer to attain basic characteristic describing properties of this type of fluid. the instrument produced by anton paar with ball apparatus was used for this propose, figure 2. the flow characteristics were measured using dynamic procedure 5 and 60 minutes after mixing. the acquired flow curves were then plotted by herschel-bulkley model (1), τ = τ0 + kγn, (1) where τ is a shear stress (pa), τ0 is an yield stress (pa), k is a consistency index (-), γ is shear rate (s−1) and n is the flow index (-). this methodology was successfully applied in previous research [15]. 3. results and discussion the performed experimental program was focused on the detailed description of rheology properties of aam on the basis of metakaolin activated by lithium water glass. the importance of rheology properties has relation to technical praxis, where the sudden loss of workability limit further utilization of amm in the industry. the results of the first phase of the project are shown in figure 3. it is evident, that changing silicate modulus of used lithium water glass influenced the final flow of studied pastes. specific feature of fresh aam is sudden loss of workability, when sodium or potassium activator is applied [10, 11]. (a). (b). figure 1. (a) flow table (b) final spreading. however, thanks to used lithium water glass slight gradual improvement of the workability was monitored within 150 minutes. 38 vol. 53/2025 rheology properties of metakaolin activated by lithium water glass figure 2. used configuration of used rotary rheometer. figure 3. flow of aam pastes in time. the second phase of the experimental program was dealt with the optimization of the dosage of the activator. with respect to the results of initial testing, different amount of activator is necessary to achieve given value of flow. hence, the further set of paste was prepared in order to reach flow approximately 310 mm. the detailed results in table 3 document reduction of the activator in such designed experiment. the derived flow characteristics from equation 1 on the basis of rheology measurement are introduced in table 4. the derived values of yield stress (pa) well document differences between single studied pastes and used activators. the value of this property describes the minimal stress needed to flow. from the plotted curves illustrated in figure 4a and figure 4b is evident, that the paste with the highest applied silicate modulus exhibited the highest value of the yield stress. that was monitored for both time intervals. it can be assumed, that remaining mixtures exhibited negligible differences. from this point of view lower silicate modulus seems to more suitable for application requiring better workability of the mixture. similar findings were reported by vance et al. [16], who described the mechanism of improved workability in case of aam pastes with low values of silicate modulus. it was concluded, that changing charge (a). (b). figure 4. (a) flow curve of pastes after 5 minutes (b) flow curve of pastes after 60 minutes. on the surface of used precursor and ionic species in the activator. similar mechanism was reported by rovnaník et al. [17], who studied rheology properties of aam pastes on the basis of metakaolin activated by sodium water glass. they mentioned higher reactivity of metakaolin due to its mainly amorphous character, thus immediate gelation accompanied by the increase of yield stress in time. however, the increase of yield stress in this research program studying effect lithium water glass has not been monitored. liang et al. [18] studied combination of metakaolin and lithium slag as precursors activated by sodium water glass and sodium hydroxide. the yield stress and workability ware increasing with increased incorporation of lithium slag, however the setting time was decreased. this behavior is caused by the increase of si/al ratio of used precursor, which proportionally accelerate setting of the paste. the initial better workability in mixtures incorporating lithium slag was influenced by its lower water adsorption. 39 v. kellnerová, t. david, v. blovská, p. reiterman acta polytechnica ctu proceedings designation used activator metakaolin [g] water glass [g] flow [mm] dosage change [%] li 3.25 li 3.0–3.5 500 500 330 0 li 3.85 li 2.7–5.0 500 615 305 23 li 3.95 li 3.7–4.2 500 600 310 20 li 4.35 li 4.5–5.0 500 500 300 0 table 3. flow of aam pastes with optimized dosage of the activator. 5 minutes 60 minutes k n τ0 r2 k n τ0 r2 [-] [-] [pa] [-] [-] [pa] li 3.25 1.74 1.34 7.98 0.98 3.98 1.13 6.01 0.97 li 3.85 2.78 1.18 4.69 0.98 0.19 2.00 10.25 0.97 li 3.95 0.26 1.88 6.59 0.97 0.68 1.52 6.90 0.97 li 4.35 0.74 1.65 19.23 0.98 3.39 1.09 15.40 0.98 table 4. flow characteristics of studied pastes. 4. conclusions the performed experimental program was focused on the rheology properties of aam paste activated by lithium water glass, which introduce interesting possibility for enhancement of suitable technological properties. the study was focused on the determination of basic rheological properties such as flow and yield stress, of which values were plotted using herschel-bulkley model. the attained results declared increased workability of studied pastes in time, thus the sudden loss of workability has not been reported within 150 minutes. this feature differs from other research works using traditional ways of metakaolin activation like sodium/potassium water glass. from the rheology characteristics point of view, the lower silicate modulus led to the more promising values of yield stress, however the differences between single mixtures were negligible. the lower silicate modulus seems to more suitable also due to expected higher mechanical properties of hardened composites. acknowledgements this research was supported by the financial support of technology agency of czech republic under project fw10010109 and ctu in prague under the project sgs24/051/ohk1/1t/11. references [1] j. davidovits. geopolymer: chemistry and application. institut géopolymère, saint-quentin, france, 2008. 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https://doi.org/10.1007/s00397-014-0793-z [17] p. rovnaník, p. rovnaníková, m. vyšvařil, et al. rheological properties and microstructure of binary waste red brick powder/metakaolin geopolymer. construction and building materials 188:924–933, 2018. https://doi.org/10.1016/j.conbuildmat.2018.08.150 [18] g. liang, w. yao, a. she. rheology and microstructure of lithium slag/metakaolin geopolymer pastes: insights from particle packing and water dynamic evolution. journal of building engineering 95:110261, 2024. https://doi.org/10.1016/j.jobe.2024.110261 41 https://doi.org/10.1007/978-1-4614-3445-0_20 https://doi.org/10.1016/j.conbuildmat.2024.137696 https://doi.org/10.1016/j.conbuildmat.2017.05.032 https://doi.org/10.3390/ma12060996 https://doi.org/10.1063/5.0137817 https://doi.org/10.1007/s00397-014-0793-z https://doi.org/10.1016/j.conbuildmat.2018.08.150 https://doi.org/10.1016/j.jobe.2024.110261 acta polytechnica ctu proceedings 53:37–41, 2025 1 introduction 2 experimental program 3 results and discussion 4 conclusions acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.38.0269 acta polytechnica ctu proceedings 38:269–274, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague measurement and cfd analysis of a local radiant cooling solution shiva najaf khosravi∗, helene teufl, ardeshir mahdavi tu wien, department of building physics and building ecology, karlsplatz 13, 1040 vienna, austria ∗ corresponding author: shiva.nkhosravi@gmail.com abstract. this paper entails an empirical and computational assessment of the air flow field in the close proximity of a vertically positioned radiant cooling panel. this radiant cooling solution differs from the conventional large-area radiant cooling systems (e.g., ceiling panels). it involves rather small-sized vertical panels positioned close to occupants. moreover, the panels are designed so as to manage potential surface condensation of water vapor via integrated drainage elements. hence, the panels can be operated with relatively low surface temperatures. the low panel surface temperature and its proximity to the occupants are intended to compensate for the potential lower cooling power due to the relatively small panel size. in this paper, we specifically explore the air flow field close to the local radiant cooling panel via laboratory measurements and cfd (computational fluid dynamics). thus, possible issues regarding discomfort due to draft and turbulence risk close to the radiant panel can be examined. to this end, a prototypical local radiant cooling panel was installed in a mock-up office room of a laboratory. during the experiments, the air flow speed was measured and simulated at several heights (between 10 and 110 cm from the floor) and distances (ranging from 1 to 50 cm from the radiant panel). the results allow for the evaluation of the draft discomfort risk as well as the reliability of cfd in reproduction of the measurement results. a further step involved the numeric analysis of the effect of the human model on the air flow patter. keywords: radiant cooling panel, thermal comfort, air flow, cfd simulation. 1. introduction and background it has been suggested that climate change due to greenhouse gas (ghg) emissions is one of the most significant challenges facing humanity [1]. specifically, the global reliance on non-renewable energy sources leads to an increasing concentration of greenhouse gases in the atmosphere and thus contributes to climate change. in this respect, particular attention should be given to the household sector as a major consumer of energy [2]. the move from uniformly conditioned indoor environments to smaller, individually controlled zones, can be advantageous in view of thermal comfort and energy efficiency [3]. limitations of traditional cooling systems, such as noise, draft, vertical thermal gradients, and high energy demand necessitate the investigation of alternative cooling systems. in this context, the application of water in chilled radiant panels can be of interest [4]. these panels can be broadly classified based on their installation location. the most common form is the ceiling-mounted cooling panel. radiant cooling ceiling systems were investigated, among others, in the laboratories in european countries in the early 1990s [5]. they have been shown to have the potential to provide adequate thermal conditions [6]. in some systems, the pipes are inserted in the surface layer of a wall, ceiling, or floor [7]. lately, the integration of radiant cooling into furniture (e.g., in desks) has received attention [8]. thereby, the chilled water could circulate through the table top to remove the sensible heat, while latent heat is removed by an all-air cooling system [6]. as compared to conventional cooling systems, radiant cooling panels have the potential to offer smaller vertical temperature gradients, lower air movement velocity, and reduced local discomfort [9]. whereas conventional air-conditioning systems are mainly based on convection, radiant panels utilize a combination of radiation and convection [10]. the thermal performance of the cooling panels has been investigated via both experimental and computational methods. the thermal comfort of five human subjects in a laboratory test room equipped with cooling ceilings was analysed by nagano and mochida [11]. in their experiment, the temperature difference between the ceiling and the room air was less than 5 k. according to their findings, the mean radiant temperature for a supine human body should be applied in the design of ceiling radiant cooling. another study compared the radiant cooling systems with airconditioning systems in terms of thermal comfort, energy consumption, and cost. the results suggested that the radiant ceiling panel system is capable of creating a smaller vertical variation of air temperature and a more comfortable environment [12]. experimental studies are of course highly instructive, but also rather expensive and time-consuming. if carefully applied, cfd-based numeric simulation can provide alternative or at least complementary means of investigation. computational fluid dynamics has 269 https://doi.org/10.14311/app.2022.38.0269 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en s. najaf khosravi, h. teufl, a. mahdavi acta polytechnica ctu proceedings become an important tool in the prediction of thermal comfort in occupied spaces [13]. compared to the experimental techniques, cfd simulations can provide detailed values regarding the flow distribution and concentration fields in the whole domain, rather than just targeted points for data collection [14–16]. nevertheless, proper cfd application necessitates sufficient grid resolution, accurate choice of numerical models, estimation of numerical errors, and control over other numerical parameters for precise verification and validation [17–19]. despite the dramatic progress in its application, cfd has not replaced experimental and theoretical analyses, but represents a highly useful complementary tool [20–22]. as such, experimental studies have been conducted to probe the precision of numerical simulation of the indoor thermal environment [23]. myhren et al. [24] applied cfd simulations for two office rooms equipped with separate heating and ventilation systems, to investigate potential cold draught problems, the differences in vertical temperature gradients, and air speed levels. another study compared the energy efficiency of the cooling panel with the all-air cooling system via cfd simulation and experimental method. thereby, the cooling panel was found to be more energy efficient [25]. predicting thermal comfort by applying virtual human manikins in the cfd simulation has been reported by numerous studies [13]. according to our observation, there are no relevant standards in cfd simulations for the sizes, shapes, and postures of human model, and their application mostly depends on the purposes of the research works [26–29]. because of the excessive simulation time, the majority of cfd studies have applied simple (e.g., rectangular) shapes. in some cases, the nature of the study necessitated a complex human model [30]. within the framework of a recently completed research study, the potential of vertical radiant panels in the proximity of users was investigated and related technological requirements regarding envelope tightness and ventilation systems as well as water vapor condensation risk were evaluated [27]. as mentioned before, the personal radiant cooling panels are designed so as to manage potential surface condensation of water vapor via specific elements. one option is to remove the condensed water via a drainage system. however, in our experience, a much simpler option would be sufficient in most case, namely the placement of an easy to clean metal container underneath the vertical cooling panel. in the present contribution, we explore the potential of cfd simulations to analyse the airflow patterns and the velocity fields inside a test room with a vertical radiant panel positioned in the close proximity of a workstation. simulated results were compared with previously obtained measurements. the objective of the numeric analysis was to capture the air flow patterns and local discomfort risk. the study included also the air velocity field around a simplified human model. figure 1. test room equipped with a cooling panel. 2. approach predicting the thermal performance of a cooling panel is not a trivial task. in general, this system represents a mixed convection heat-transfer mode, including human body buoyancy-driven thermal plumes, and radiation through the panel and wall surface. to this end, a prototypical local radiant cooling panel was installed in a mock-up office room of a laboratory (see figure 1). to deal with potential surface condensation, a metal container was positioned below the local cooling elements. during the laboratory experiments, the ambient air temperature and relative humidity in the room were kept at 30 ± 0.5 °c and 40 ± 3 %, respectively. the target panel surface temperature was 10 °c. for these indoor air and panel surface temperatures, the cooling power of the radiant element was estimated to be roughly 100 w per square meter panel area [27]. the air flow speed was measured and simulated at several heights (10, 35, 60, 85, and 110 cm above the floor) and distances (1, 1.5, 2, 3, 4, 5, 10, 15, 20, 30, 40, and 50 cm from the radiant panel) (see figure 2) [31]. in this study, the potential discomfort risk due to draft was explored and was not found to be significant [31]: at a distance of 10 cm and more from the panel, the air flow velocity was never found to be above 10 cm s−1. the cfd model’s boundary conditions were adapted from the thermal conditions of the laboratory. note that during the experiment the surface temperature of the panel’s frame was higher than the chilled part of the panel. within the cfd model, the cooling element was simplified and no distinctions were made between the surface temperature of the frame and the chilled part of the panel. as a result, for the cfd simulations the whole panel temperature 270 vol. 38/2022 measurement and cfd analysis of a local radiant cooling solution figure 2. floor plan and sensors locations at the test room. was considered to be 11 °c. cfd model was implemented in the finite volume code ansys fluent 19.0 [32]. the model involves the vertical cooling panel mounted in the test space and a simplified human model for some simulation cases. ansys design modeler and ansys meshing were applied as a pre-processor to create the geometry, mesh, and the computational domain. in this model, a mesh with hexahedral element was generated. buoyancy due to density change was applied in the energy equation. the discrete ordinates (do) model was applied as the radiation model. the pressure-based solver was applied. the simple segregated solver was used for pressure-velocity coupling. various studies have assessed the performance of turbulence models. for rans-based models, selecting a proper turbulence model is essential for heat transfer analysis. the answers to the question of the best-performing turbulence models are not always consistent. for instance, the standard k-ε or the rng k-ε turbulence models are widely implemented for cfd simulations of cooling panel, while sst-k ω model has been applied in other studies. in addition, some studies applied a laminar flow model. the results of the literature review revealed that the k-ε family of turbulence models presented similar results consistent with laboratory data [33–35]. the skw and sst turbulence models showed a lesser performance level in predicting air velocity [36, 37]. this disparity can be due to the physical features of the case studies, as well as different computational parameters and settings applied in different studies. hence, for each specific simulation study, the selection of the turbulence model may have to be reassessed. in the present study, cfd simulations were carried out using the same geometry but with different turbulence models (rng k-ε and sst-k ω). to check the reliability of the turbulence model, the simulation results were compared with experimental. note that we used the discretization scheme employed for the energy, momentum, turbulent kinetic energy, and specific dissipation rate was the second order upwind scheme. pressure staggering option (presto) was used for the pressure discretization scheme. 3. results and discussion the air velocity could be an influential parameter for the thermal comfort: increased airspeed can aid the evaporation of sweat thus leading to a cooling effect, particularly if loose clothing is worn. nevertheless, too high air velocity may cause discomfort and a draughtiness sensation. average air speed is typically recommended not to exceed 0.15 m s−1 in the occupied zone to prevent discomfort [38]. the cfd method can complement the experimental results by allowing the detailed examination of the velocity field at different spots. given the complexity of the physical processes involved, a candidate simulation model should ideally incorporate complete three-dimensional processes, suitable turbulence models, realistic boundary conditions, and temperature-dependent material properties. in this study, to gain confidence concerning the model’s reliability and the employed turbulence model, we first compared the computational results with the laboratory measurements. subsequently, the study covered the human model analysis. 271 s. najaf khosravi, h. teufl, a. mahdavi acta polytechnica ctu proceedings figure 3. velocity as a function of distance from the panel for two different turbulence models. figure 4. simulated velocities plotted against corresponding measurements for two different turbulence models. 3.1. turbulence model two different turbulence models (k-ε and k-ω) were applied for the cfd simulation. during the experiments, air velocity was measured at 75 locations. figures 3 and 4 illustrates the velocity distribution profile (on the sensors location) obtained from the cfd simulation for two different turbulence models, together with the measured velocity values in the test room. the effect of turbulence models on the air velocity distribution can be seen in this figure. as such, the k-ε and the k-ω family of turbulence models overestimated the values by up to 0.05 and 0.1 m s−1 respectively. in this study, the cfd simulation results obtained while using the k-ε family of turbulence models were found to more closely correspond to the experimental results. the air velocity values were below the limits of the standards at the assumed seating position of the occupants [38]. as mentioned earlier, in this study we considered a highly simplified human model (body temperature 35) in three distances from the cooling panel (20 cm, 35 cm and 55 cm). the cooling was provided by radiant panels and the manikin acted as the source of heat generation in the room. the flow in the room is dominated by buoyancy. the solver settings were identical to those used for the aforementioned comparison with the experimental results. the main objective of this query was to explore the potential influence of the human presence on the air flow pattern around the radiant cooling panel. figure 5 illustrates the simulated velocity contours for the aforementioned simulation scenarios. the results indicate that the velocity in the vicinity of manikin lies in the range of 0.10 to 0.28 m s−1. the velocity range in the zone can be suggested to be 272 vol. 38/2022 measurement and cfd analysis of a local radiant cooling solution figure 5. simulated velocity contours around a human model at three distances from the radiant panel, namely distance of human model from panel, namely 20 cm (a), 35 cm (b), and 55 cm (c). within the recommended maximum velocity range to avoid draft [39]. with increasing distance from the cooling panel, its effect on the velocity field clearly diminishes. 4. conclusion using local radiant cooling panels has been suggested to be a potentially promising alternative to 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https://doi.org/10.1016/j.buildenv.2017.03.040 https://doi.org/10.1177/1420326x13480056 https://doi.org/10.1088/1742-6596/2069/1/012122 https://doi.org/10.1007/bf01061452 https://doi.org/10.1016/0045-7930(94)00032-t https://doi.org/10.1016/0017-9310(72)90076-2 https://doi.org/10.2514/3.12149 https://doi.org/10.1016/s0378-7788(98)00020-6 acta polytechnica ctu proceedings 38:269–274, 2022 1 introduction and background 2 approach 3 results and discussion 3.1 turbulence model 4 conclusion references https://doi.org/10.14311/app.2022.33.0032 acta polytechnica ctu proceedings 33:32–38, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague stainless steel-frcm system for strengthening of rc beams: towards a sustainable strengthening technique francesco bencardino∗, mattia nisticò department of civil engineering, university of calabria, via p. bucci cubo 39b, 87036 rende, cosenza, italy ∗ corresponding author: francesco.bencardino@unical.it abstract. fabric-reinforced cementitious matrix (frcm) systems for strengthening concrete structures are an alternative to traditional techniques. the frcm system is a composite material consisting of two or more layers of cement-based matrix reinforced with dry fibres in the form of open mesh or fabric. when adhered to concrete structural members the frcm system acts as supplemental external reinforcement. many existing reinforced concrete (rc) members exhibit degradation due to the carbonation of concrete and/or corrosion of internal reinforcing steel bars. these rc members can be strengthened using stainless steel, in strip format (unidirectional fibres), embedded in a cementitious based matrix. the system, named stainless steel-frcm, can be applied according to the externally bonded (eb) technique. in order to reduce times and costs of intervention, number of used materials, as well as the amount of chemical compounds, a novel inhibiting-repairing-strengthening (irs) technique is proposed and experimentally tested. using a suitable matrix (thixotropic mortar with passivation properties) the main three operations of steel bars corrosion inhibition/protection, restoring of deteriorated concrete, and installation of the external strengthening can be carried out in one-step. for evaluating the effectiveness of both new strengthening system and installation technique an extensive experimental investigation was planned and developed. a part of the experimental research includes two groups of three rc beams (3.00 m and 4.80 m long): one strengthened with irs technique, one strengthened with eb technique and one control beam. these beams were tested under monotonic loading. further two beams, one beam for each group, strengthened according to irs technique, were also tested under cyclic loading. the experimental results show the validity of the proposed solution in terms of structural performance and environmental sustainability. keywords: innovative materials, repair, strengthening. 1. introduction the repair and rehabilitation process of reinforced concrete (rc) structures and infrastructures is become a fundamental target in last years. several approaches for strengthening concrete members by composites have been developed and increased significantly as an alternative to traditional methods. combining fibres of different origin with inorganic cementitious matrices new class of composite materials were defined under the acronym fabric-reinforced cementitious matrix (frcm) or similar names as textilereinforced systems. lightweight and high strength of frcm systems fit perfectly in the rehabilitation process. the improvement in terms of performance of strengthened local members with frcm composites was experimentally confirmed by several researches [1–6]. in this context, steel-fabric reinforced cementitious matrix (s-frcm) strengthening system has proven to be one of most effective innovative reinforcement used in the civil engineering field [7]. as well as the structural aspect, the use of environmentally sustainable repair and reinforcement techniques and materials is becoming increasingly important. so eco-friendly geopolymeric matrices act as corrosion inhibitor and together to the repair of the cover concrete with inside the steel strengthening fibre represent a new effective application [8–10]. this latter technique, alternative to the traditional externally bonded (eb) application, is proposed with the name of irs (inhibiting/repairing/strengthening) because of it includes these three operations in one. the effectiveness of this methodology of strengthening has been experimentally evaluated on the basis of obtained test results. the irs strengthening procedure is a novel ecofriendly intervention and can be representing the best choice when considering the structural effectiveness as well as sustainable development. in fact, the use of this technique does not require passivating materials of the internal reinforcements (applying geopolymeric matrices), shorter execution times of the intervention (strengthening and restoration/repair of the surfaces in one step), reduced intervention times and consequently less use of skilled manpower. the economic 32 https://doi.org/10.14311/app.2022.33.0032 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 33/2022 stainless steel-frcm system for rc beams � � � � ����������������� � ���������������� � d�� e� )ljxuh����3uhsdudwlrq�ri�wkh�vxuidfhv�dqg�furvv�vhfwlrq�diwhu�wkh�vwuhqjwkhqlqj�surfhvv��d��(%�)5&0� figure 1. preparation of the surfaces and cross-section after the strengthening process: a) eb-frcm system and b) irs-frcm system. advantage therefore represents a key aspect in the study and development of this irs innovative technique. in order to compare the flexural capacity of beams with the two applications of reinforcement techniques (eb and irs), an experimental flexural test was carried out. the specimens are formed by two groups of beams (a and b) with a length of 3000 and 4500 mm, width of 150 mm for both, 250 and 400 mm of height. the beams were supported by simple supports and a total of eight beams have been tested under four-points bending, subjected to monotonic and cyclic loading. 2. application techniques the preparation of the surfaces and the technique of applying the composite material represents a fundamental phase of the strengthening process. according to the traditional strengthening technique (eb) after the restoration of the cover concrete, if necessary, two layers (of about 5 mm each one) of cementitious matrix are applied. a new innovative application (irs) was experimentally tested with effective results. it consists to restore the deteriorated concrete and to apply external strengthening in one step with installation of the stainless-steel strip in the cover concrete with reduction of time and cost of the intervention. the final layer of mortar shows more consistent thicknesses (30 ÷ 35 mm). details of the two techniques eb-frcm and irs-frcm are outlined in figure 5. the beams were prepared with the eb technique through the cleaning of the surfaces from any type of material (dust and other substances) and subjected to light mechanical scarification and small mechanical carvings. with reference to the irs technique, the specimens were cast in the form work without the cover concrete. the idea is to simulate the existing rc elements with damaged cover concrete, which in the strengthening phase is totally removed. the surfaces were wet to avoid absorption of the mixing water of the matrix. in figure 5 the state of the surfaces before and after the strengthening process is reported. 3. experimental programme 3.1. test specimens starting from previous research [1], eight rc beams subjected to four-points bending were tested in the laboratory of the department of civil engineering at university of calabria. the beams were designed to fail in flexure before in shear, using appropriate internal shear reinforcement (stirrups). two types of concrete beams with different amounts of internal steel reinforcement were used. two 16 mm and 33 francesco bencardino, mattia nisticò acta polytechnica ctu proceedings � d�� e�� f�� figure 2. geometry of control and strengthened beams: a) rc control beam; b) eb technique and c) irs technique. group l b h l/h a a/d as a ρs b a′ s c stirrups [mm] [mm] [mm] [-] [mm] [-] [-] [%] [-] [mm] m-a 3000 150 250 12 900 4.1 2!12 0.6 2!8 !8/150 mmc-a 2!14 0.9 m-b 4800 400 1500 2!16 0.7 c-b 2!14 0.5 aas : tension steel reinforcement. bρs = as/ (bd) : internal tension steel ratio. ca′ s : compression steel reinforcement. table 1. geometrical properties and internal steel reinforcement of tested beams. 12 mm steel bars were used as bottom reinforcement in the monotonic tested beam (group a and b, respectively) and 14 mm in the cyclic tested specimens. the shear reinforcement consisted of 8 mm steel stirrups placed at intervals of 150 mm and two 8 mm steel bars as top reinforcement to hold the stirrups for all beams. the aim is to investigate the structural performance of the eb-s/frcm and irs-s/frcm systems considering two large-scale rc beam groups (3000 mm group a and 4800 mm group b). some geometrical dimensions of the two groups have been changed in order to evaluate potential scale effect. beams referred to as m were tested under static monotonic loading; the character c indicates the beams subjected to static cyclic load. to complete the label of the specimens, the abbreviations eb and irs indicate the reinforcement technique used. the internal and external reinforcement ratios and geometry of the beams are presented in figure 2 and table 1. the complete labels of the beams were reported in table 5 (see later). the width (bf ) of s-frcm fabric was equal to 100 mm and 150 mm for the beams of the groups a and b respectively, in order to keep equal the strengthening ratio ρf = (bf tf )/ (bd). where tf is the thickness of the external strip of steel reinforcement. 3.2. material properties and characterization in order to evaluate the mechanical properties of the concrete, compressive and tensile tests were carried out on cylindrical samples (150 mm of diameter × 300 mm of height). the average compressive strength (fcm) at 28 days was 16.8 mpa for group m and 33.5 34 vol. 33/2022 stainless steel-frcm system for rc beams mechanical property m beams c beams compressive strength, fcm [mpa] 16.8 33.5 tensile strength, fctm [mpa] 1.7 3.7 table 2. mechanical property of concrete. mechanical property m beams c beams diameter ![mm] !8 !12 !16 !8 !14 yield strength, fym [mpa] 543.8 367.1 492.0 510.6 501.2 table 3. mechanical and geometrical property of internal steel reinforcement. stainless-steel fibre geopolymeric matrix nominal thickness, tf [mm] 0.24 compressive strength, fcmm [mpa] ≥ 45 tensile strength, ff [mpa] 1470 flexural tensile strength, ftfm [mpa] ≥ 6 ultimate strain, εfu [%] 2.0 elastic modulus, ef [gpa] 190 table 4. mechanical properties of s-frcm system. mpa for group c; whereas 1.7 mpa and 3.7 mpa the average tensile strength (fctm), respectively. the average yield strength of steel (fvm) of bars !8 (group m), !8 (group c), !12, !14, and !16 mm was 543.8, 510.6, 367.1, 501.2, and 492.0 mpa, respectively. moreover, a summary of used concrete and internal steel properties are given in tables 2 and 3. s-frcm composite material system was used for strengthening the beams. it consists of stainless-steel strips embedded in an inorganic geopolymeric matrix. the unidirectional reinforcing strip is made of stainless-steel cords, particularly resistant to corrosion, suitable for interventions on substrates subject to rising damp and/or exposure to aggressive environments. the matrix is a polymers-based inorganic mineral with the addition of synthetic fibres, suitable for structural repairs of deteriorated cover concrete being able to be applied with thicknesses between 2 and 40 mm. the geopolymeric matrix also act as inhibitor of corrosion and represents an eco-friendly material for green technology. table 4 presents the mechanical properties of stainless-steel strips and matrix provided in the technical data sheets of the manufacturer/supplier [11, 12]. 3.3. cyclic loading procedure specimens subjected to cyclic loading were tested according to 12 monotonous cycles, as shown in figure 3. the periodic amplitude was selected on the basis of the theoretical yielding loads (fy) of the group c, using four levels of load corresponding around to 25% (fy/4), 33.33% (fy/3), 50% (fy/2) and 75% (3fy/4) of them. from the theoretical calculations, the yield loads of c-a-irs and c-b-irs beams are almost equal. with reference to the theoretical average yielding load of the two strengthened beams and taking into account the applied load before test (self-weight of the beams and the heavy equipment) the values of the load cycles according to the percentages above described were defined. the values of the load cycles correspond to 12, 18, 30, and 42 kn, respectively. the load history was repeated three times for each step and later the beams were monotonically loaded until failure. 3.4. test set-up each beam was subjected to two equal loads applied symmetrically to the mid-span as shown in figure 2. the shear span (a) was 900 mm and 1500 mm for group a and b, respectively. the distance from support to point load was chosen to have the a/d ratio equal to 4.1 in all tested specimens. the load was monotonically applied to the beams of group m by means of a hydraulic actuator and recorded manually using a 300 kn load cell. the specimens of group c were loaded by using hydraulic unit that controls a 140 kn jack and 160 kn load cell, with a frequency acquisition of 2 hz. this equipment allows to apply a load and keep it constant. the load equipment was connected to a rigid steel frame for the application of the load on the elements. the vertical displacements were recorded using three linear variable displacement transducers (lvdts). two were placed at mid-span (both in the front and back side) and one at quarter span. finally, the strain values of all materials (concrete, internal tensile steel reinforcement and external strengthening) were measured by means electrical strain gauges 35 francesco bencardino, mattia nisticò acta polytechnica ctu proceedings figure 3. loading history for the c group beams. � � d�� e�� f� )ljxuh����/rdg�glvsodfhphqw�uhodwlrqvklsv�� figure 4. load-displacement relationships. 36 vol. 33/2022 stainless steel-frcm system for rc beams specimens failure mode fu [kn] εc [‰] εs [‰] εf [‰] ∆fu µδ m-a-cb concrete crushing 37.22 3.67 13.38 8.64 m-a-eb end debonding 43.22 1.50 3.75 1.87 1.16 2.16 m-a-irs intermediate debonding 47.39 2.63 9.81 11.62 1.27 3.73 m-b-cb concrete crushing 76.33 3.68 17.54 4.38 m-b-eb end debonding 89.52 3.53 12.03 5.99 1.17 1.83 m-b-irs concrete crushing 98.96 4.65 15.49 10.73 1.30 3.05 c-a-irs concrete crushing 83.60 1.89 15.31 10.43 c-b-irs intermediate debonding 90.00 1.25 6.24 7.26 table 5. experimental results. � � d�� e� f�� � � g�� h� i�� )ljxuh����([shulphqwdo�idloxuh�prghv�ri�wkh�ehdpv��d��0�$�(%��e��0�$�,56��f��0�%�(%��g��0�%�figure 5. experimental failure modes of the beams: a) m-a-eb, b) m-a-irs, c) m-b-eb, d) m-b-irs, e) c-a-irs and f) c-b-irs. in the middle section of the beams. in the specimens monotonically tested the load was periodically paused in order to identify and mark crack formations and growth while in the cyclic tests it was performed at the end of each step. 4. experimental results and discussion static and cyclic flexural response at failure was experimentally studied. the ultimate load and overall behaviour of all tested beams are analysed and discussed through the comparison of load-deflection curves to measure the influence of the two strengthening methods (eb and irs) on the flexural capacity. the load-deflection curves are shown, for all tested beams, in figures 4a, 4b and 4c. the deflection reported represents the average value measured by lvdts located in the centre of beam span. the classical vertical flexural cracks were observed during the loading and at failure. table 5 summarises the failure modes of all specimens, the maximum load (fu), the strain measured on the top surface of the compressed concrete (εc), on the bottom tension steel bars (εs) and on the stainless-steel strips (εf ). ∆fu indicates the ratio between the ultimate load of strengthened beam and that of control beam; furthermore, for the beams monotonically tested, an evaluation of the ductility in terms of displacement is given as ratio between the deflection in the mid-span section at failure and at yielding of the internal tensile steel (µδ). from a first analysis, both strengthening systems of group m show an increase of the failure load and a decrease of the ultimate deflection and ductility compared to the control beams (m-a-cb and m-bcb). furthermore, the structural behaviour of the beams m-a-irs and m-b-irs was better compared to the corresponding rc beams strengthened with 37 francesco bencardino, mattia nisticò acta polytechnica ctu proceedings traditional eb technique (m-a-eb, m-b-eb). from the analysis of deformations and failure modes the irs solution provided a greater utilization of all the materials, compared to the traditional eb technique. the failure modes of the strengthened beams with s-frcm strips are shown in figures 5a-f. on the other hand, the beams of group c (c-airs and c-b-irs) shown an excellent behaviour under cyclic loads without premature debonding of the reinforcement system until failure. these first results of group c is a part of a wider experimental programme aimed to investigate the performance of fullscale beams strengthened with the traditional (eb) and innovative (irs) methods in structures affected to variable loads during the service life (such as deteriorated rc deck bridges). 5. conclusions this experimental investigation demonstrated the effectiveness of the irs technique in strengthening of flexural rc members and in reduction of debonding process, showing also an excellent behaviour both under cyclic and monotonic loads. the test results also exhibited that the innovative irs technique had superior flexural performance to the traditional eb strengthened technique, particularly when considering the economic, eco-friendly, and of environmental sustainability viewpoint. in detail, the results showed that with eb and irs technique, the average flexural strength of the beams is improved by around 15% and 30% respectively, compared to control beam under static loading; and great behaviour under cyclic loading for the beams strengthened by means the irs innovative solution. finally, the results obtained from this first phase of the research programme suggest a more interest towards the 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[12] betonfix monolite n st6-0319, 2019. https://www.kimia.it/sites/default/files/docs/ st/it-betonfix-monolite-n.pdf. 38 https://doi.org/10.1016/j.conbuildmat.2016.05.053 https://doi.org/10.1061/(asce)cc.1943-5614.0000960 https://doi.org/10.1016/j.compositesb.2016.03.022 https://doi.org/10.1061/(asce)cc.1943-5614.0000218 https://doi.org/10.1061/(asce)cc.1943-5614.0000882 https://doi.org/10.1016/j.compositesb.2018.04.051 https://doi.org/10.3390/fib8020008 https://doi.org/10.1016/j.engstruct.2019.109348 https://doi.org/10.1016/j.istruc.2019.02.005 https://doi.org/10.1016/j.compositesb.2012.09.019 https://www.kimia.it/sites/default/files/docs/st/it-kimisteel-inox.pdf https://www.kimia.it/sites/default/files/docs/st/it-betonfix-monolite-n.pdf https://doi.org/10.14311/app.2022.33.0020 acta polytechnica ctu proceedings 33:20–26, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague using an analysis of concrete and cement epd: verification, selection, assessment, benchmarking and target setting jane anderson∗, alice moncaster open university, walton hall, milton keynes, keynes mk7 6aa, uk ∗ corresponding author: jane.anderson@open.ac.uk abstract. the carbon embodied in buildings is an important proportion of our emissions and needs to be radically reduced in order to support climate change mitigation. the highest proportion of embodied carbon is usually emitted during the product stage, and within the structural elements. therefore, reducing the carbon embodied in the structural materials is likely to have a major impact. in most buildings, the majority of embodied carbon comes from steel and concrete. but although there are now hundreds of registered environmental product declarations (epd) for cements and concretes, there has been very limited independent published information comparing the embodied carbon of different concrete mixes and raw materials. this lack of comparative data limits the potential to make appropriate decisions at early design stages leading to low carbon buildings. the authors have recently conducted a review of verified epd for concrete mixes and for concrete’s key constituents, including cement, identifying the range of carbon coefficients. this paper provides guidance on making use of the coefficient ranges provided in that research: to support the verification of epd for concrete and its raw materials; in material selection; in assessing building level embodied carbon; in benchmarking; and in the setting of reduction targets. keywords: benchmarking, embodied carbon, epd, lca, methodology. 1. introduction and theoretical background the carbon embodied in buildings, from the use of construction materials which produce carbon dioxide and other greenhouse gases during their life cycle, is an important proportion of our carbon emissions. international energy agency (iea) estimate that embodied carbon accounted for 11% of global co2 emissions in 2017, most of this from cement and steel manufacture [1]. iea and the world business council for sustainable development cement sustainability inititive (wbcsd-csi) estimate the greenhouse gases emissions from cement manufacture are already 7% of our total global emissions [2]. iea also note that cement demand has nearly doubled between 2000 and 2016, mainly due to increased demand in china, and latterly, india [1]. with almost all cement used in concrete, it is clear that concrete has a major influence on global greenhouse gas emissions. environmental product declarations (epd) are standardised presentations of the environmental impact of products. iso 21930:2007 [3] was widely used by many of the epd programmes across europe, but en 15804 [4] has been developed following a mandate from the european commission to harmonise the provision of epd for construction products across europe, ensuring that epd to en 15804 have a common methodological approach to lca and provision of environmental impacts. iso 21930 was updated in 2017 to closely align with en 15804 [5]. constructionlca’s infographic [6] shows that the numbers of epd using en 15804 have risen rapidly with over 7000 epd available globally at the start of 2020. as a significant construction material, epd have been produced for both cement and concrete products, including both ready-mix concrete and precast concrete. as detailed in their paper [7], anderson and moncaster made a systematic review and analysis of all the published epd globally for ready-mix concrete and its constituent materials (cement, aggregates and admixtures) at the end of 2019. they found 252 epd for ready-mix concrete reporting data for over 2000 individual products, 108 epd for cement and cementitious materials covering 118 products, 88 epd for aggregates covering 117 products and 9 epd for specialist aggregates, and 16 epd for admixtures coming from 25 epd programmes in total. these epd all represent products available in the market including both national and regional industry average data provided by trade associations and manufacturer specific data for both average and specific products, and many of these epd, especially those for ready-mix concrete, provided individual data for more than one product, in some cases providing data for over 100 individual products in one epd. these epd were reviewed in terms of the data on "cradle to gate" embodied carbon coefficients (the global warming potential impact indicator for mod20 https://doi.org/10.14311/app.2022.33.0020 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 33/2022 analysis of concrete and cement epd ules a1-a3 within epd) and their embodied energy (the primary energy and secondary energy resource indicators for modules a1-a3 within epd), together with other data such as clinker and cementitious content for cements and 28-day strength for concretes, where they are provided. this paper describes how the information reported in the systematic review and analysis can be used in epd verification, materials selection, embodied carbon assessment at building level, in benchmarking and in setting improvement targets. 2. previous studies hammond and jones provided an overview of the embodied energy and carbon coefficients reported in academic and industry literature in their inventory of carbon and energy database (ice database) [8]. they found 112 datapoints for ready-mix concrete, 92 datapoints for cement and 36 datapoints for aggregate, though in relation to embodied carbon they note that "there is often an absence of such data". damineli et al. reviewed 156 randomly selected papers from 1988-2009, 59 from brazil (covering 604 concretes) and 97 international (covering 981 concretes), considering the embodied carbon of different concretes together with binder content, and 28 day compressive strength [9], and this work was updated by scrivener et al. including examples of recent developments, including data from concretes formulated with up to 70% replacement of binder by filler [10]. the ice database was updated in 2011 [11] using only 3 new datapoints for cement, aggregates and ready-mix concrete. the ice database was updated again in 2019 [12] using embodied carbon data from published epd. this version used 22 datapoints for ready-mix concrete (16 ready-mix sources listed), 14 datapoints for cement (4 sources listed) and 164 datapoints for aggregate (22 sources listed). pomponi and moncaster reviewed carbon coefficients for cement and concrete in academic literature, identifying 58 coefficients across the two products [13]. van den heede and de belie similarly identified 12 datapoints for cement [14], salas et al., reviewed the literature on lca for cement and identified 16 embodied carbon datapoints [15], ganassali et al. identified 32 epd for cement [16], kurda, silvestre and de brito identified 17 cement datapoints and 20 aggregate datapoints [17] and braga, silvestre and de brito identified 16 concrete datapoints [18]. passer et al. have written about how epd are being used in the european market [19] and jelse and peerens about how they can be used for green public procurement [20]. ganassali et al. discuss how they produced benchmark values for cement using 32 epd, using the median value of the range for the reference value, with limit and target values set using the boundaries of the upper and lower quartiles of the range distribution [16]. jones has developed embodied carbon benchmarks using the arithmetic mean of datapoints from selected epd and others sources for each product [12]. 3. description of the systematic review and analysis of epd and summary of its results 3.1. methodology the review was conducted at the end of 2019. during the data collection phase, epd were downloaded from all the known epd programmes, using ecoplatform [21] and the north american pcr catalogue [22] and a general literature search. in the categorisation phase, epd were categorised into cement, different concrete mixes, aggregates, and dividing the epd which report multiples into a set of individual results. in the third phase, the data was extrapolated from the epd cataloguing and tabulating like with like. the data included the embodied carbon coefficients (gwp for a1-a3) together with other relevant data for all the separate products contained within each product group. in the fourth phase, visualisations were developed and in the fifth phase, the results were checked and verified. 3.2. cement embodied carbon coefficients figure 1, taken from anderson and moncaster [7] which explains some of the reasons for variation, shows the embodied carbon coefficients for all the identified cement epd plotted on a graph against clinker content. the types of cement (e.g., cem i, cem ii etc) are differentiated using colour. 3.3. ready-mix concrete embodied carbon coefficients figure 2 from the same source [7], shows the ranges for embodied carbon coefficients for all the identified products within ready-mix concrete epd plotted against the 28-day strength where it has been provided. epd ranges are identified based on the country of concrete production, with average epd produced by national trade associations highlighted as a shaded range, labelled, e.g., "french generic". this clearly shows that the ranges of impact vary significantly between countries, with french and german industry generic epd having much lower impacts for the same 28-day strength than the corresponding us and canadian; and all epd from mexico and saudi arabia having much higher impacts than those epd from australia and the u.a.e. for example. 4. using the embodied carbon coefficient ranges for cement and concrete in the process of verifying the results, the authors found in several cases datasets which were significant 21 jane anderson, alice moncaster acta polytechnica ctu proceedings outliers, and in these instances contacted the epd programmes to check if there were possible errors in the data provided. in these cases, several errors were identified and epd have been reissued. this led the authors to consider how the data collected in the systematic review and analysis could be used. 4.1. in epd verification the above example clearly illustrates how the systematic review and analysis can be used to check epd for cement, concrete and aggregates to identify if the results are reasonable. for cement epd, figure 1 can be used to check the clinker content and embodied carbon are within the expected region of the graph, especially for cem i, ii, iii and iv cements. the systematic review analysis also provided a box and whisker graph showing the range of embodied carbon for epd by country of production and by type of cement (cem i, cem ii etc), not included here, allowing verifiers to check the plausibility of embodied carbon results. additional graphs are provided in the systematic review [7], but not included here, for cem i and cem ii cement epd showing the embodied carbon broken down by co2 from calcination and from fuel use, and the primary and secondary fuels use, broken down by renewable and non-renewable sources, which again can be used to consider the plausibility of data provided in the epd for verification. for concrete epd for verification, we recommend checking them against figure 3 which provides the embodied carbon ranges (mean and upper and lower quartile) for different countries and 28-day strengths to see that the embodied carbon is in the expected region of the graph. this graph also shows how many epd results were included in each grouping. 4.2. in material selection cement selection must be considered alongside the functionality of the concrete that is required. figure 3 shows that that there is great variation in impact reported for any given strength, but that generally, there is an increase in embodied carbon per m3 as the 28-day strength of the concrete is increased. however purnell cautions against the selection of low strength concretes over high strength concretes on the basis of their reduced embodied carbon, highlighting that significantly more of a low strength concrete may be required to fulfil a particular function [23]. a similar conclusion was drawn by damineli et al. who showed that assessing co2 per m3 and per unit of structural performance (kgco2e/m3/mpa) suggested that c50 concrete was the optimal choice ide greater savings in embodied carbon (20-35%) than those achieved by replacing cement with pulverised fuel ash (10-25%) for example [23]. carbon intensities for ready-mix concretes with 28-day compressive strength over 10 mpa (from [7]) are shown in figure 4. many manufacturers are now able to provide epd for the range of concretes that they product, for example using national tools such as the french betie epd tool [24] or industry association tools such as the csi epd tool developed for wbcsd csi [25] or that developed for the norwegian ready mixed concrete association, fabeko. there is also the epd tool developed by basf for manufacturers to assess concretes [26] and other manufacturer specific epd tools such as that for tarmac [27]. therefore, when selecting a concrete, we recommend that specifiers do the following: (1) consider the range of embodied carbon impacts shown in figure 3; (2) ask local concrete producers if they are able to provide information on the embodied carbon (carbon footprint or epd) for their concretes; (3) look for a producer able to provide a concrete at the lower end of the embodied carbon range for its strength; and (4) make sure that any impacts from extended transport distances do not outweigh the benefits of reduced embodied carbon in production provided in module a4 of epd. 4.3. in building lca during early design stages, we recommend using regional generic data for concrete where available, as at this stage in the design this is the type of data recommended by en 15978 [28]. if generic data is not available for a particular region, then we recommend identifying a region with similar technology for cement and concrete production and using figure 2 to pick an appropriate embodied carbon value (from an industry generic epd if available, or from the median of the range otherwise). ganassali recommends the median as it is not sensitive to the outliers in a sample composed of a small number of data [16]. at later stages of the design, it is recommended to use specific data, e.g., from manufacturer specific epd based on the products you have chosen to use in the building (see section on material selection). 4.4. in benchmarking benchmarks can be provided for a product generally (e.g., cement or concrete) or for a specific product, such as cem i cement or c30 concrete. specifying benchmarks at the more specific level will ensure that the products meeting the benchmark are not just lower carbon products, but products which have lower carbon impacts than other products with similar functionality. for concretes, it is important that the functionality is considered in defining the benchmark. concretes with lower embodied carbon per m3 may have lower compressive strength, and this may lead to a requirement to use more concrete which could have an adverse environmental impact. for situations where compressive strength is relevant, we recommend using carbon intensity (co2eq/m3.mpa) as shown in figure 4 to set benchmarks. 22 vol. 33/2022 analysis of concrete and cement epd figure 1. graph showing relationship of embodied carbon for cements by clinker content, source [7]. figure 2. embodied carbon per m3 concrete by compressive strength, shown for each country, source [7]. 23 jane anderson, alice moncaster acta polytechnica ctu proceedings figure 3. box and whisker graph showing mean, upper and lower quartiles of embodied carbon for ready-mix concretes from concrete epd, by 28-day strength, source [7]. figure 4. box and whisker graph showing mean, upper and lower quartiles for carbon intensity ready-mix concretes, source [7]. 24 vol. 33/2022 analysis of concrete and cement epd in setting benchmarks, the geographical and related technological situation should also be considered ideally a regional benchmark should ensure that at least some of the production achieves the benchmark, whilst also stretching producers adopting "business as usual". 4.5. in setting reduction targets the lower quartile ranges in figure 3 and figure 4 show the best performance shown by 25% and 50% of products with existing epd, and therefore what should be achievable in setting long-term reduction targets for the majority of the market. this is also important information for manufacturers who wish to stay competitive in the carbon-aware market. 5. conclusion this paper has demonstrated how the analysis of existing epd for cement and concrete can provide multiple useful information for verifiers, specifiers, designers, building assessors and manufacturers, including for epd verification, material selection, building lca, benchmarking and target setting. a similar detailed analysis of many other products groups from the construction sector, particularly other materials and products with major impacts, would be useful for the construction industry as a whole. acknowledgements the authors are very grateful to the whitbybird foundation for funding j. a.’s initial work on the original paper [7]; and to the open-oxford-cambridge arts and humanities research council (ahrc grant number ah/r012709/1) doctoral training partnership for funding the continued project. funding for a. m. came from the school of engineering and innovation at the open university references [1] international energy agency (iea). global status report, 2018. https://webstore.iea.org/download/direct/2408? filename=2018_global_status_report.pdf. 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[28] cen/tc 350. en 15978:2011 sustainability of construction works assessment of environmental performance of buildings calculation method. brussels, 2011. 26 https://doi.org/10.1007/s11367-015-0926-3 https://doi.org/10.1007/978-3-319-66981-6_55 https://www.eco-platform.org/who-is-participating.html https://docs.google.com/spreadsheets/d/1ls7ukmug1cawnmghkiqivgcgehqoeiciih5t95inzy8/pubhtml https://doi.org/10.1680/adcr.13.00013 https://www.fedbeton.be/docs/technique/651_12_fr.pdf https://quantis-intl.com/csi-epd-tool-cement/ https://www.basf.com/us/en/media/news-releases/2014/01/p-13-478.html http://sustainability-report.tarmac.com/wp-content/uploads/2016/08/declaring-more-product-information.pdf acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.38.0110 acta polytechnica ctu proceedings 38:110–115, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague thoughts on the selection of the appropriate simulation models in building performance assessment christiane bergera,∗, ardeshir mahdavib a aalborg university, department of architecture, design and media technology, rendsburggade 14, 9000 aalborg, denmark b tu wien, department of building physics and building ecology, karlsplatz 13, 1040 vienna, austria ∗ corresponding author: chbe@create.aau.dk abstract. building performance simulation serves the derivation of the relevant building performance indicators (e.g., energy use, indoor-environmental conditions) given the assumptions of certain model input parameters (i.e., description of the building, boundary conditions, occupants’ presence and behaviour). simulation can be employed for multiple purposes, including but not limited to building design support, building systems configuration, and code compliance demonstration. it has been suggested that the level of detail and resolution of simulation models must match their deployment purpose. however, there is arguably a lack of definitive guidelines for the purpose-dependent selection of appropriate simulation models. to address this challenge, the present contribution suggests that the attributes of a simulation model in general, and the type of the adopted occupant model in particular, must correspond to the specifics of the building performance indicator under investigation. to make progress in this area, a typological classification of building performance indicators is proposed along three salient dimensions, namely the indicators’ topical domain (e.g., energy use, thermal comfort, noise control), their spatial attributes, and their temporal attributes. following a detailed analysis, the paper presents a high-level approach to derive the basic requirements concerning occupant models as a categorical function of the simulation purpose. keywords: building performance, simulation purpose, model selection. 1. about building performance simulation computational models can assist the explanation and prediction of diverse entities and processes in various areas. as such, a carefully constructed and validated computational model of an entity or a process can be used as its virtual (digital) twin. at their core, computational models embody algorithms that use provided input data to derive the values of designated output variables. in case of building performance simulations, model input parameters typically include information on buildings’ physical properties (construction, fabric, systems) as well as external boundary conditions (climate, weather) and internal processes (e.g., occupants’ presence and activities). the output of the simulation process includes the values of the variables that capture buildings’ performance. these variables, or performance indicators, may be related to different domains. they may address, for instance, buildings’ energy use, or their indoor-environmental performance (thermal, visual, acoustic, air quality). note that a number of such performance indicators pertain either directly to occupants’ needs or can be affected by occupants’ actions [1, 2]. hence, building performance simulation frequently requires reliable models of occupants’ presence in buildings and their interactions with buildings’ control systems (e.g., heating, cooling, ventilation, lighting). as described above, the building performance simulation process generated data relevant for assessment, evaluation, and comparison (e.g., through ranking) purposes. the generated data typically constitute values of the relevant building performance indicators (bpis). in practice, these values are compared with target values that may be specified as per requirements in codes and standards or otherwise included in relevant contractual documents. building designers, operators, and other pertinent stakeholders can use the simulation process and tools to pursue various queries. two common forms of queries are as follows. the first form, which we may label as direct, looks for the values of performance indicators for a given set of input assumptions. the second, indirect, form looks for those constellations of input variable values that would yield a specific – desired – output. options to address this latter kind of queries include, for instance, iterative simulation runs, parametric studies, and optimization tools. 2. application purposes of building performance simulation the discussion hitherto suggests to view building performance simulation as a process by which values of 110 https://doi.org/10.14311/app.2022.38.0110 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 38/2022 thoughts on the selection of the appropriate simulation models . . . attribute description topical the relevant field of performance query (e.g., energy performance, daylight availability, thermal comfort, noise control) spatial the physical extent of the simulated object (e.g., workstation, single office, meeting room, whole building, urban district) temporal specific time stamp(s) or time interval(s) for which the value of the bpi is computed (e.g., temperature at a specific time, monthly or annual energy use) table 1. schematic presentation of the key attributes of bpis (see also [3]). bpis can be derived based on available or assumed input information on buildings’ geometry, construction, systems, occupants, and context. the main concern of the present paper is the role of occupants in building performance simulation. however, performance simulation studies may be conducted for very different reasons or purposes [4–6]. common instances of these purposes are as follows: • analysis of the behavior of building elements/components (e.g., numeric thermal bridge simulation). • whole building design support (e.g., examination of the performance implications of decisions regarding building geometry, fabric, envelope, etc.). • design and configuration support of building systems for indoor-environmental control (e.g., heating, cooling, ventilation, illumination). • support of optimal building operation (for instance, via model-predictive control methods). • performance assessment at the urban level (concerning, for instance, the patterns of air flow and migration of pollutants). • compliance assessment with building standards and codes, as well as benchmarking vis-à-vis building quality certification systems. • generation of content (e.g., for project competitions, promotion of new building projects, educational material). given the diversity of performance-related queries, it has been reasonably argued that the choice of a proper simulation model must consider the purpose of the inquiry. as mentioned before, the simulation process typically results in bpi values. hence, we can simply suggest that the selection of a simulation model (as well as the embedded occupant model) should take the specifics of the relevant bpi into account. in other words, the simulation model must fit the purpose of simulation, and the embedded occupant model must fit the simulation model. to put it in more succinct terms, the simulation query implies an appropriate bpi, the bpi an appropriate simulation model, and the simulation model an appropriate occupant model. 3. toward a building performance indicator typology the above observations provide a conceptual basis for the discussion of the selection process of a suitable simulation model for a given purpose. however, we must attempt to explore the practical implications of these observations. to this end, it may be useful to arrive at a basic typology or classification of bpis. detailed ontologies of building performance data and bpis have been proposed and discussed in previous publications [7–11]. for the purposes of the present discourse, we need to consider only three bpi dimensions or attributes (see table 1). 4. about the resolution level of building performance indicator values given the previous discussion, we can now turn our attention to the question of the appropriate level of resolution as applied to bpis. given vast computational resources, one would be tempted to suggest that we should obtain bpi values at a very high level of resolution level regarding time and space. after all, values of a bpi such as energy use that are derived on a minute-by-minute and on a room-by-room basis could be easily aggregated to hourly, daily, monthly, or annual values for individual rooms, different floors, or whole buildings. note that the reverse process is rather non-trivial: if initial bpi values are computed for longer time intervals or at the whole building level, they cannot be disaggregated in a straight-forward manner into values of higher temporal or spatial resolution. these observations must be interpreted carefully, lest a misunderstanding ensues. the problem of fitting simulation models to specific queries cannot be solved by conducting simulations always at the highest possible level of spatial and temporal resolution. the premise of such an approach is that highly detailed simulation results could be adapted, via averaging and aggregation operations, to fit the needed resolution level of target bpis. but there are both practical and conceptual problems with this approach. the reliable application of high-resolution simulation models is computationally expensive and requires considerable expertise. there is also the much-discussed 111 christiane berger, ardeshir mahdavi acta polytechnica ctu proceedings problem of scarcity of detailed and exact input information particularly at the early stages of building design process. hence, in an early design stage scenario, a high-resolution simulation model would require large amounts of detailed yet uncertain data. in other words, it can be argued that the added value of higher resolution is questionable, if it comes with higher uncertainty. this implies that high-resolution simulation models are not necessarily more suitable for all tasks, neither do they automatically yield more accurate results. specific tasks require simulation models with the appropriate resolution, which does not always mean the highest possible resolution. to approach this question in more concrete terms, consider the relationship between the temporal structure of the simulation algorithm on the one side, and the type of the modelled physical process on the other side. for instance, when modelling thermal processes in buildings, we need to properly consider phenomena pertaining to thermal inertia, thermal lag, and thermal storage. the nature of these phenomena mandates transient simulation, which in turn requires minimum temporal resolution levels of the applied numeric solutions. this requirement is perhaps of lesser relevance in case of visual and acoustic simulation. these observations explain why it has not been possible to come up with fast and easy rules for matching simulation models to the nature of performance queries. they also underline the essential role of users’ knowledge, expertise, and experience in conducting reliable simulation studies. nonetheless, our discussion so far does suggest the possibility of sketching certain general approaches toward the systematic and appropriate identification of the adequate simulation models for a given task, including the consideration of information requirements regarding the deployed occupant models. to this end, however, we must first reflect upon the available options with regard to the representation of the patterns of occupants’ presence and behaviour in buildings. 5. about models of occupants occupant models and their integration in simulation models can be discussed from different viewpoints [4, 12, 13]. for the purposes of the present contribution, we can direct our attention to the thermal performance simulation domain as a case in point. consider the kinds of information that must be embedded in occupant models. generally speaking, occupant models suitable for thermal simulation are expected to entail information on occupants’ attributes including their location in the building, their activity, and the thermal resistance of their clothing. occupant models must also capture occupants’ passive and active effects on buildings [1]. passive effects denote occupant-induced introduction of sensible and latent heat as well as various emissions (co2, water vapour, etc.). active effects concern the interactions of occupants with buildings’ various indoor-environmental control devices and systems, including, for example, windows, blinds, luminaires, and fans. it is conceivable that we could classify the presence and behaviour models of occupants in buildings in a manner that corresponds to the dimensions of bpis (domain, temporal attribute, spatial attribute). as such, we can assign the passive and active effects of occupants to specific performance domains. occupants’ activity rates, for instance, are relevant to the thermal simulation domain. their clothing’s sound absorption is, on the other hand, relevant to the performance domain of room acoustics. the spatial attribute is likewise relevant to occupant models. for instance, we can represent occupants in terms of a group (e.g., all occupants in a building, in a floor, in a room). occupants can be also individually assigned to specific locations such as workstation or a single-occupancy office. as far as the temporal attribute of occupants’ presence in buildings is concerned, intervals of various duration may be selected. similarly, occupants’ control-related actions may be assumed to ensue within such intervals. alternatively, if the simulation runs are event-driven, occupants’ actions can be mapped to specific instances in time and marked with a time stamp. a further relevant aspect in constructing occupant models is the approach chosen to represent occupants’ locations and their actions. these can be expressed either in terms of recurrent (fix) patterns, or in a probabilistic manner. as such, stochastic instances of occupant models may be, in certain cases, more adequate than conventional schedules and diversity profiles. we can thus conclude that occupant models can be broadly classified with regard to their location in a conceptual multi-dimensional space (relevant domain, spatial attribute, temporal attribute). consider, for instance, the thermal simulation domain. at one end of the spectrum, a simple simulation model may be single-zone spatially and annual (or monthly) temporally. in this case, occupants are representationally reduced, through fix daily schedules, to their contribution to the internal gains (which is typically added to gains associated with equipment and lighting) and the fresh air volume flow they require. at the other end of the spectrum, simulation tools and processes that involve agent-based modelling, would represent each occupant individually. in this case, the occupants’ influence on the resulting high-resolution values of the relevant bpis (energy use, indoor environment) is modelled probabilistically and dynamically. 6. high-level directions for the selection of occupant models in building performance simulation it may be rather difficult to arrive at strict rules for the query-driven selection of simulation models (and entailed occupant models). however, the previous discussion provides the basis for the formulation of 112 vol. 38/2022 thoughts on the selection of the appropriate simulation models . . . some general directions in the matter. to this end, consider three topical areas, namely (1.) the code compliance use case, (2.) the temporal dimension of the bpis, and (3.) the potential of probabilistic modelling. 6.1. code compliance the relevant bpis are commonly predefined when the use case is the examination of building projects in view of their compliance with applicable codes and standards. frequently, even the computational tools that can be applied are specified a priori. furthermore, in the code compliance use case, the obtained values of bpis are generally expected to be, at least in principle, reproducible by independent parties. what does this mean for the selection of the model used to represent the occupants? for one thing, the occupant model’s resolution should preferably match the deployed computational model. for instance, in case the pertinent standard requires the values of an aggregate bpi, the application of neither a high-resolution simulation model nor a high-resolution occupant model would be critical. examples of aggregate bpis would be monthly values of buildings’ estimated heating or cooling loads. it may be useful here to reflect on a “historical” circumstance. in a number of instances, simplified methods for code-based assessment of building performance were thought as substitutes of previously applied prescriptive codes. a case in point is standardbased documentation of the thermal quality of buildings’ construction (fabric, envelope). such procedures did not per se consider occupants and their role in the buildings’ performance. in the course of a paradigmatic shift from prescriptive codes to performancebased ones, the computational prediction of the energy demand of buildings was suggested to supplement – if not replace – prescriptive mandates regarding, for instance, the maximum thermal transmittance values of the components of the buildings’ envelope such as windows and walls. in other words, the inclusion of information on occupants was not necessarily meant to address the sensitivity of the buildings’ thermal performance in view of occupant behavior. as such, it could be argued that the inclusion of normalized occupant-related information in calculations was part of a procedure to document the influence of other factors (especially the physical quality of the building fabric) on the thermal performance of buildings. the application of simplified occupant models in thermal performance assessment procedures can be of course questioned. however, the reasoning behind the initial adaptation of simplified occupant representations in code-based calculations must be understood, lest the criticism would miss the point entirely. 6.2. the temporal dimension when deciding what simulation and occupant models to choose, one must pay attention to the temporal dimension of the pertinent bpis. we mentioned already the relative slow characteristics of the thermal behavior of buildings (particularly with regard to thermal conduction) and its distinction from visual and acoustic processes. thermal performance simulation must specifically consider the thermal inertia and lag effects. hence, the selected simulation models and respective numeric methods must be capable of representing transient phenomena. whereas hourly thermal simulations appear to represent the standard in basic simulations of the energy use of (and the thermal conditions in) buildings, finer resolutions levels (e.g., sub-hourly intervals, event-driven simulation procedures) could be preferable, especially in those cases that involve regular human interaction with control devices such as windows, blinds, and luminaires. 6.3. probabilistic modeling it has been suggested, falsely, that occupant models should be stochastic, because occupants “behave stochastically”. what would be more reasonable to suggest is that the overall patterns of the presence and behavior of occupant populations in buildings can be realistically reproduced using probabilistic formalisms. consequently, it could be appropriate to use, in specific use cases, probabilistic modeling methods [1, 5]. however, these methods obviously do not generate single values for bpis, but rather result in distributions. this has been suggested to be useful: stochastic models can theoretically deal with the inherent uncertainties of processes and events attributable to building occupants. but one needs to be careful, as unvalidated or insufficiently validated probabilistic models could perhaps generate patterns that morphologically resemble actual occupant behavior processes, but would be entirely different than actually monitored behavioral data. the empirical foundation of a stochastic model needs to be relevant to and representative of the specific object of the simulation (a rarely feasible option in the building design phase), otherwise the resulting simulation data would not be reliable. if carefully conceived and deployed, there is still considerable potential in the application of stochastic modeling techniques in building performance simulation. however, in typical building design use cases, it could be more useful to address the matter of uncertainty in performance simulation via sensitivity analysis [14, 15]. thus, one could explore the implications of variance in model input data without suggesting that the simulation results more accurately predict the future performance of a design. 6.4. a summarized view the previous discussion can be broadly summarized in terms of the following matrix (see table 2). thereby, desirable features of occupant models are listed as 113 christiane berger, ardeshir mahdavi acta polytechnica ctu proceedings simulation purpose spatial temporal probabilistic resolution resolution modeling compliance with codes l/m l/m na building design/retrofit support m/h m/h l building systems design support m/h h m building operation support h h h table 2. fitting spatial and temporal resolution levels of occupant models as well as their suitability for inclusion of probabilistic features as a function of the simulation application use case (l: low; m: medium; h: high). a function of the simulation use case (i.e., demonstration of compliance with codes, building design and retrofit support, building systems design support, and building operation support). the desirable features of occupant models are specified for spatial and temporal resolution levels, as well as the models’ suitability for inclusion of probabilistic features [3]. here, the categories of spatial resolution are labelled low (applicable, for instance, in case of building floors and whole buildings), medium (applicable to rooms), and high (relevant, for instance, to individual workstations). similarly, the categories of temporal resolution are labelled as low (for instance, monthly or annual), medium (hourly intervals), and high (sub-hourly intervals, event-driven simulation). the suitability of adopting stochastic occupant models in the building performance simulation process is characterized, qualitatively, in terms of the attributes non-applicable (na), low, medium, and high. 7. concluding remark the discussion in the previous chapter in general, and the entailed summary matrix in particular (table 2) are of course not suggested to represent a clear-cut recipe for the purpose-dependent selection of occupant models in building performance simulation applications. composing such a recipe would not be a trivial matter: building design and operation processes are highly complex and dynamic. they depend on a host of typological constraints, local circumstances, climatic conditions, economic parameters, and cultural factors. the objective of this contribution was rather to provide a bird-eye view of the relevant considerations with regard to identification of the simulation approaches and tools that would be fitting to the nature of specific performance queries. we are arguably not in a position to automate this matching process between models and queries. but obtaining a better understanding of the nature of this process could support the responsible experts in their choice of proper simulation models in general and occupant models in particular. references [1] a. mahdavi. the human dimension of building performance simulation (keynote). in v. soebarto, h. bennetts, p. bannister, et al. (eds.), proceedings of the 12th conference of the international building performance simulation association, sydney, australia, pp. k16–k33. 2011. isbn 978-0-646-56510-1. [2] d. yan, t. hong, b. dong, et al. iea ebc annex 66: definition and simulation of occupant behavior in buildings. energy and buildings 156:258–270, 2017. https://doi.org/10.1016/j.enbuild.2017.09.084 [3] i. gaetani, a. mahdavi, c. berger, et al. occupant-centric simulation-aided building design, chap. fit-for-purpose occupant modelling: choosing the right approach. 2023. book chapter submitted for publication in iea ebc annex 79 book. [4] a. mahdavi, f. tahmasebi. building performance simulation for design and operation, chap. people in building performance simulation, pp. 117–145. 2nd ed. routledge, london, 2019. https://doi.org/10.1201/9780429402296-4 [5] a. mahdavi, f. tahmasebi. the deployment-dependence of occupancy-related models in building performance simulation. energy and buildings 117:313–320, 2016. https://doi.org/10.1016/j.enbuild.2015.09.065 [6] a. j. m. lindner, s. park, m. mitterhofer. determination of requirements on occupant behavior models for the use in building performance simulations. building simulation 10(6):861–874, 2017. https://doi.org/10.1007/s12273-017-0394-8 [7] a. mahdavi, d. wolosiuk. integration of operational data in building information modelling: from ontology to application. in c. lungu, et al. (eds.), proceedings of the 13th rehva hvac world congress clima2019, bucarest, romania. 2019. paper id 101. [8] a. mahdavi, m. taheri. ework and ebusiness in architecture, engineering and construction, chap. a building performance indicator ontology, pp. 385–390. 1st ed. crc press., london, 2018. https://doi.org/10.1201/9780429506215-48 [9] a. mahdavi, m. taheri. an ontology for building monitoring. journal of building performance simulation 10(5-6):499–508, 2017. https://doi.org/10.1080/19401493.2016.1243730 [10] t. hong, s. d’oca, w. j. n. turner, s. c. taylorlange. an ontology to represent energy-related occupant behavior in buildings. part i: introduction to the dnas framework. building and environment 92:764–777, 2015. https://doi.org/10.1016/j.buildenv.2015.02.019 [11] z. deme belafi, t. hong, a. reith. a library of building occupant behaviour models represented in a standardised schema. energy efficiency 12(3):637–651, 2019. https://doi.org/10.1007/s12053-018-9658-0 114 https://doi.org/10.1016/j.enbuild.2017.09.084 https://doi.org/10.1201/9780429402296-4 https://doi.org/10.1016/j.enbuild.2015.09.065 https://doi.org/10.1007/s12273-017-0394-8 https://doi.org/10.1201/9780429506215-48 https://doi.org/10.1080/19401493.2016.1243730 https://doi.org/10.1016/j.buildenv.2015.02.019 https://doi.org/10.1007/s12053-018-9658-0 vol. 38/2022 thoughts on the selection of the appropriate simulation models . . . [12] i. gaetani, p.-j. hoes, j. l. m. hensen. a stepwise approach for assessing the appropriate occupant behaviour modelling in building performance simulation. journal of building performance simulation 13(3):362–377, 2020. https://doi.org/10.1080/19401493.2020.1734660 [13] i. gaetani, p.-j. hoes, j. l. m. hensen. occupant behavior in building energy simulation: towards a fit-for-purpose modeling strategy. energy and buildings 121:188–204, 2016. https://doi.org/10.1016/j.enbuild.2016.03.038 [14] c. j. hopfe, j. l. m. hensen. uncertainty analysis in building performance simulation for design support. energy and buildings 43(10):2798–2805, 2011. https://doi.org/10.1016/j.enbuild.2011.06.034 [15] r. rezaee, j. brown, g. augenbroe, j. kim. assessment of uncertainty and confidence in building design exploration. artificial intelligence for engineering design, analysis and manufacturing 29(4):429–441, 2015. https://doi.org/10.1017/s0890060415000426 115 https://doi.org/10.1080/19401493.2020.1734660 https://doi.org/10.1016/j.enbuild.2016.03.038 https://doi.org/10.1016/j.enbuild.2011.06.034 https://doi.org/10.1017/s0890060415000426 acta polytechnica ctu proceedings 38:110–115, 2022 1 about building performance simulation 2 application purposes of building performance simulation 3 toward a building performance indicator typology 4 about the resolution level of building performance indicator values 5 about models of occupants 6 high-level directions for the selection of occupant models in building performance simulation 6.1 code compliance 6.2 the temporal dimension 6.3 probabilistic modeling 6.4 a summarized view 7 concluding remark references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2024.47.0117 acta polytechnica ctu proceedings 47:117–120, 2024 © 2024 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague the carbonation resistance of concrete on the basis of blended binders containing milled limestone pavel reiterman∗, vendula davidová, tomáš david czech technical university in prague, faculty of civil engineering, thákurova 7, 166 29 prague, czech republic ∗ corresponding author: pavel.reiterman@fsv.cvut.cz abstract. the current building industry is essentially changing due to implementation of new approach, which is predominantly motivated by the need to reduce its negative environmental impact. the paper is focused on the experimental evaluation of carbonation resistance in terms of accelerated tests performed on the concrete on the basis of blended binding system incorporating milled limestone. the used commercially produced cement was additionally modified by the limestone powder. the obtained results confirmed, that currently used design standards are convenient to real carbonation process, however further clinker replacement by the milled limestone causes significant loss of the durability of the hardened concrete. keywords: concrete carbonation, carbonation front, milled limestone, cement replacement. 1. introduction the effort to find out new low-carbon technologies is well visible in all branches of human activities, especially in the industry. it is well visible on the field of new design procedures and predominantly on the field of material engineering. besides increasing utilization of waste materials to protect non-renewable sources, mostly in europe is emphasize the implementation of technologies related to lower carbon dioxide emissions. hence, the environmental trend has been projected to the building industry as well. concrete is globally the most used building material, which production is due to content of portland cement highly energy consuming and related also to enormous emissions of carbon dioxide. it is caused by very high autogenous carbon dioxide emissions during limestone calcination, which is primary raw material for portland cement production. approximately 980 kg of co2 per a tune of clinker origin during clinker production. the reduction of such an immense co2 emissions due to cement utilization is currently solved by two different fundamental ways. the first is based on the replacement of concrete by the other materials or significant reduction of its volume in the given structure. it is often reached by the application of concrete with increased mechanical and durability performance. the preservation or rather increase of the durability is a crucial aspect of such an approach. the high-performance concrete (hpc) and its material variations were intensively studied during the last decades. the fundamental aspect of hpc is the high-quality binder matrix, which ensures the low permeability of and thus long service-life of the structure. the super resistance of hpc to severe conditions was confirmed by the number of experimental studies [1, 2] and also verified by case studies [3]. the matrix of hpc is based on portland clinker and other mineral additives, but due to application of efficient plasticizers it is possible to formulate concrete mixtures with very low water-to-cement ratio ensuring low porosity [4]. the improved properties of used binder suitably allow utilization of aggregate of low quality, although the final durability of the concrete is still higher [5]. the second approach is based on the systematic reduction of portland clinker amount in the cement and the formulation of new cement types [6], that could be used in case of less demanding application, because traditional portland cement of often overused, thus is used in cases, where different, low carbon binder, could be successfully applied. this reduction of clinker portion is performed by the replacement of the other cementing material such as fly ash, silica fume, blast furnace slag, etc. unfortunately, there is enormous lack of cementing mineral additives exhibiting poccolanic properties, hence the milled limestone is preferably used. cem ii/a-ll or cem ii/b-m are massively applied instead of formerly preferred cem i or cem ii/b-s during ready mix concrete production. the mechanical properties of these cements are primarily prescribed by the standard [6], however the long-term durability of the concrete incorporating these types of cement incorporating limestone powder is highly questionable. the essential weakness of this mineral additive is its negligible contribution to the cement hydration from the chemical point of view, where it is responsible for the formation of a small amount of monocarbonates [7]. the effect of limestone powder is primarily physical in hydrating paste, where the increased inner surface stimulates formation of new precipitates. also, standards for concrete production assume limestone powder as an inert additive. thus, such a formulated binder exhibits higher degree of hydration at the time. although positive impact 117 https://doi.org/10.14311/app.2024.47.0117 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en p. reiterman, v. davidová, t. david acta polytechnica ctu proceedings on the paste and concrete rheology, the presence of milled limestone is often responsible for the reduced freeze-thaw resistance and increased permeability [8]. this mineral additive will be widely used in future due to its wide availability and low price, however it is more advantageous to combine milled limestone with active mineral additives such as fly ash, blast furnace slag or calcined clay [9, 10]. unfortunately, poccolanic additives are not especially in european union readily available, because the heavy machinery and similar producers of these by products have been removed to other regions during past decades. 2. experimental program the performed experimental program was focused on the evaluation of carbonation resistance of conventional concrete on the basis of blended portland cement. with respect to the preferable utilization of milled limestone as cement replacement and the technological measure for achievement of good workability of fresh concrete, was part of the above-mentioned cement replaced. 3. materials the conventional mixture of ready mix concrete c30/37 exhibiting consistency s5 has been designed on the basis of local sources. cem ii/a-ll 42.5 r was selected as main binder material. the aggregate consisted of fine siliceous sand and natural siliceous gravel. the polycarbocylate-based water reducer was complemented with the stabilizing admixture to prevent bleeding. the detailed composition of studied mixtures is shown in table 1. components lcr lc15 cem ii/a-ll 42.5 r 320 272 milled limestone 100 148 sand 0–4 mm 980 980 gravel 4–8 mm 260 260 gravel 8–16 mm 380 380 plasticizer 2.5 2.5 stabilizer 1.5 1.5 water 175 175 w/b 0.42 0.42 table 1. composition of studied mixtures. 4. procedures the mechanical properties of designed concrete mixtures were monitored in time in terms of compressive strength test using cubic specimens of edge 150 mm [11]. the measurement was performed in selected time interval up to 365 days. the carbonation resistance of the concrete was measured using accelerated carbonation test in accordance with čsn 13295 [12]. the cubic specimens were after 90 days of curing under humid conditions inserted figure 1. evolution of compressive strength in time. into the test chamber with 1 % co2 and rh about 80 %. after 56 days are specimens extracted and development of carbonation front is determined using phenolphthalein test, which indicates ph just about 9.5. the carbonation depth was calculated as an average of six measurement on each cube. the process of carbonation in time could be described according equation 1, dcarb,field = kfield · √ t · γ (1) where dcarb,field is carbonation depth (mm), γ is safe factor 1.1, kfield coefficient of carbonation (mm·year0.5) and t is time (years). the coefficient of carbonation could be derived on the basis of accelerated test, which is conducted under specific conditions, by using equation 2, kacc kfield = cacc cfield (2) where kacc is coefficient of carbonation obtained during accelerated test with co2 concentration cacc (%), that is often higher then conventional aerial concentration of carbon dioxide cfield (%); 0.03 % has been applied. present calculation was successfully applied in previous research [8, 13]. 5. results and discussion the experimental program was focused on the longterm properties of conventional concrete on the basis of blended binder with addition of milled limestone. the complemented part of the campaign was the determination of compressive strength in time during 365 days. the results illustrated in figure 1 document that after 90 days of curing both studied mixtures fulfilled the requirements of c30/37. on the other hand, using logarithmic scale it is obvious, that long-term gains of compressive strength are after 90 days negligible for these binding systems. that is significant difference in comparison with the strength development is case of binder incorporating active or pozzolanic additives [14, 15]. the calculation of direct environmental savings is usually performed on the basis of reduced carbon dioxide emissions, however it is necessary to take into the consideration also the duration of service-life of 118 vol. 47/2024 the carbonation resistance of concrete figure 2. estimation of the progress of carbonation front. such a structure. hence, it is crucial to evaluate new technologies in terms of the attained durability. in this experimental program was the process of carbonation applied. obtained data confirm, that reference mixture, which was modified by milled limestone exhibited slightly lower resistance to the carbonation with respect to used cement, figure 2. according to applied estimation the carbonation front would pass the concrete cover after approximately 90 years. the second mixture with 15 wt. % replacement by milled limestone exhibited significant decay of the carbonation resistance. the concrete cover would be passed after 55 years, that performs loss of durability performance about 40 %. from the other point of view, it is necessary to design thickness of concrete cover 50 mm for lcr and 60 mm for lc15, respectively, for the service life 100 years. 6. conclusions the performed experiments confirmed, that mechanical properties of such designed concrete mixtures are nearly similar from this point of view. with respect to the level of cement replacement the differences between single mixtures are negligible. the scope of the program was the evaluation of the resistance of carbonation. the obtained results showed crucially reduced resistance to the carbonation due to additional cement replacement by milled limestone. just 15 % of limestone reduced this durability factor by 40 %, thus for similar service life concrete cover would be increased from 50 to 60 mm, what is not feasible with respect to the increased amount of concrete. it is crucial to note, that reference concrete mixture was based on the cem ii/a-ll cement, where could be the clinker replaced up to 20 %. following cement grade “b”, thus cem ii/b-ll, could exhibit clinker replacement up to 35 %. performed replacement of cement by milled limestone, could be assumed as artificial formulation of cem ii/b-ll in case of the mixture lc15, in addition the extreme level of clinker replacement. besides negligible differences in mechanical properties, the final service life would be due to 15 wt. % cement replacement reduced from 90 to 55 years. that is why is crucial to take into the consideration the durability of final concrete mixture, not only direct reduction carbon dioxide emission related to the used cement type. acknowledgements this work has been supported by the project no. fw06010592. references [1] j. li, z. wu, c. shi, et al. durability of ultra-high performance concrete – a review. construction and building materials 255:119296, 2020. https: 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[7] a.-m. poppe, g. de schutter. cement hydration in the presence of high filler contents. cement and concrete research 35(12):2290–2299, 2005. https://doi.org/10.1016/j.cemconres.2005.03.008 [8] p. reiterman, r. jaskulski, w. kubissa, et al. assessment of rational design of self-compacting concrete incorporating fly ash and limestone powder in terms of long-term durability. materials 13(12):2863, 2020. https://doi.org/10.3390/ma13122863 [9] k. de weerdt, k. kjellsen, e. sellevold, h. justnes. synergy between fly ash and limestone powder in ternary cements. cement and concrete composites 33(1):30–38, 2011. https: //doi.org/10.1016/j.cemconcomp.2010.09.006 [10] i. bekrine, b. hilloulin, a. loukili. multiscale investigation of cement pastes with low and high-grade calcined clays and slag at early and advanced ages. journal of building engineering 78:107570, 2023. https://doi.org/10.1016/j.jobe.2023.107570 119 https://doi.org/10.1016/j.conbuildmat.2020.119296 https://doi.org/10.1016/j.conbuildmat.2020.119296 https://doi.org/10.1016/j.conbuildmat.2022.129029 https://doi.org/10.1016/j.conbuildmat.2022.129029 https://doi.org/10.5390/susb.2011.2.1.063 https://doi.org/10.1016/j.conbuildmat.2023.131495 https://doi.org/10.1016/j.conbuildmat.2023.131495 https://doi.org/10.1002/suco.202200734 https://doi.org/10.1016/j.cemconres.2005.03.008 https://doi.org/10.3390/ma13122863 https://doi.org/10.1016/j.cemconcomp.2010.09.006 https://doi.org/10.1016/j.cemconcomp.2010.09.006 https://doi.org/10.1016/j.jobe.2023.107570 p. reiterman, v. davidová, t. david acta polytechnica ctu proceedings [11] zkoušení ztvrdlého betonu – část 3: pevnost v tlaku zkušebních těles [testing hardened concrete – part 3: compressive strength of test specimens]. standard, czech standard institute, prague, 2009. [12] výrobky a systémy pro ochranu a opravy betonových konstrukcí – zkušební metody – stanovení odolnosti proti karbonataci [products and systems for the protection and repair of concrete structures – test methods – determination of resistance to carbonation]. standard, czech standard institute, prague, 2004. [13] i. sáez del bosque, p. van den heede, n. de belie, et al. carbonation of concrete with construction and demolition waste based recycled aggregates and cement with recycled content. construction and building materials 234:117336, 2020. https: //doi.org/10.1016/j.conbuildmat.2019.117336 [14] n. singh, p. kumar, p. goyal. reviewing the behaviour of high volume fly ash based self compacting concrete. journal of building engineering 26:100882, 2019. https://doi.org/10.1016/j.jobe.2019.100882 [15] p. reiterman, o. holčapek, o. zobal, m. keppert. freeze-thaw resistance of cement screed with various supplementary cementitious materials. reviews on advanced materials science 58(1):66–74, 2019. https://doi.org/10.1515/rams-2019-0006 120 https://doi.org/10.1016/j.conbuildmat.2019.117336 https://doi.org/10.1016/j.conbuildmat.2019.117336 https://doi.org/10.1016/j.jobe.2019.100882 https://doi.org/10.1515/rams-2019-0006 acta polytechnica ctu proceedings 47:117–120, 2024 1 introduction 2 experimental program 3 materials 4 procedures 5 results and discussion 6 conclusions acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.38.0495 acta polytechnica ctu proceedings 38:495–501, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague calibration of a summer building simulation model based on monitoring of user behaviour dóra szagri∗, ainur kairlapova, balázs nagy, zsuzsa szalay budapest university of technology and economics, faculty of civil engineering, department of construction materials and technologies, műegyetem rkp. 3. k.i.85., 1111, budapest, hungary ∗ corresponding author: szagri.dora@emk.bme.hu abstract. occupant behaviour is a field, that has always been of great interest to researchers. it could significantly modify the operation of the building and the user’s energy needs, and it is also difficult to model it according to reality. evaluation of measurements is a crucial step to calibrate dynamic simulations. our goal was to analyse the indoor comfort conditions according to measurements, particularly in summertime, and find what solution closes the performance gap between the measured and simulated results. in this research, we investigated an apartment building that underwent an energy efficiency renovation. we have installed a weather station and monitoring sensors in selected apartments, with which we monitored the temperature, relative humidity, and co2 values of certain rooms, the presence of the inhabitants and the window opening and the operation of shading. in this paper, we focus on the monitoring and simulation results of the topmost apartment. the results can help us better understand how buildings work and how to implement user behaviour in dynamic simulations, how to calibrate the model according to measurements and make suggestions to increase the comfort of the residents. keywords: user behaviour, summer overheating, indoor comfort, in-situ measurement. 1. introduction the warming climate has an impact in many areas, not only in the way we design and develop our buildings but also in the way we operate them and adapt our habits to this changing climate. measurements show that there is an increased risk of overheating in buildings [1, 2] and it can be predicted using external temperatures and characteristics of the housing and its surroundings [3]. these increased internal temperatures have an impact on human health, especially in the case of the elderly [4, 5] so we must use the right tools and building management [6–8] to reduce the internal temperature and adapt to the increased external temperature. another qualitative factor is the co2 value, which mainly represents the indoor air quality, but it can also be used to predict and calculate the air change rates in the building [9–11]. thermal behaviour and energy consumption of a building can be assessed with dynamic simulation. however, simulation without calibration can lead to substantial error in the calculated energy consumption. if properly calibrated, the building energy simulations can estimate the real energy consumption 90 % of the time [12]. in modelling, various uncertainties (specification, numerical, scenario, heuristic), inter-model variability, or even post-design changes can be important, increasing the performance gap. according to the literature, the most common causes are specification uncertainty in building modelling, occupant behaviour, and poor practice in operation, with substantial impacts (10–80 %) on energy use [13]. in the calibration process of a naturally ventilated building, the window openings are key factors, as well as the opening of the doors [14]. calibration can be differently accurate for insulated and uninsulated buildings, which is why the two often need to be treated differently [15]. in this research, we examine a renovated, naturally ventilated apartment building located in budapest and focus on the summer thermal comfort. three apartments in the building were equipped with different sensors to determine the user habits and the indoor air conditions (temperature, relative humidity, and co2 concentration) inside the building. we monitored presence, window opening and shading operation in selected rooms. in addition to the measurements, dynamic whole-building simulations were carried out. the monitoring system and the measurements gave a better view about the user habits and also helped the evaluation of the overheating in the summer period. in this paper, we focus on the topmost apartment of the building, which is expected to have the highest risk of overheating. the monitoring measurement results were used to calibrate the model and evaluate the efficiency of using real-user behaviour data in simulation model. 2. materials and methods the analysed building is located in budapest (hungary) on the gellért hill (figure 1). on the rectangular plot there are two two-storey high flat-roofed apartment buildings with basement (building “a” and “b”). in this study, we focus on building b, which 495 https://doi.org/10.14311/app.2022.38.0495 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en d. szagri, a. kairlapova, b. nagy, z. szalay acta polytechnica ctu proceedings figure 1. monitored building’s location (building “b” is the subject of this study) [16]. figure 2. monitoring sensors in the apartment (window opening, shading, motion detection). was awarded a grant to carry out the posterior thermal insulation and window replacement. during the renovation, 11 cm of external facade insulation (graphite eps) and plaster was applied, 10 cm of plinth wall xps thermal insulation were also installed. the flat roof was renovated with 20 cm of eps 100 thermal insulation, and the waterproofing was made with epdm membrane. the windows and doors were replaced with modern pvc windows and doors (uw = 1.15 w/m2k). the new windows and doors are insulated structures of the same size as the original. shutters have been installed to increase summer heat protection. the building envelope improvements can result in significant reduction in energy consumption, so the renovation is partly about expected savings and partly about increasing the resident’s comfort 2.1. in-situ measurements the following parameters were measured in selected rooms of the apartments: temperature and relative humidity (sensirion sht85), co2 (sensirion scd30), presence detection (hc-sr501 pir sensor), window opening and shading operation (fm-106 wh reed relays). a custom wi-fi capable monitoring system and online user interface have been developed for the buildings by bcd kft. (figure 2). temperature, relative humidity, co2 values were recorded at every minute for more than a year from 2020 summer until 2021 autumn. the temperature measurement tolerance was ±0.1 °c, the relative humidity tolerance was ±1.5 %, and the co2 tolerance was ±30 ppm + 3 %. during the processing of the data, the data series had to be cleaned and organised, and hourly averages were made for easier handling; this also helped its application in dynamic simulations. in addition to the internal measurements, a weather station installed on one of the balconies provided the external weather data. the weather station’s real-time data, which can be monitored online, includes temperature, relative humidity, wind direction and speed, rainfall data and solar radiation. in this case, data cleaning and averaging was also done to be able to apply it in the simulation. 2.2. building energy modelling the modelling and energy analysis of the building was carried out in design builder v6.1.5.002 [17] software, modelling the individual apartments and rooms, which are separated into thermal zones. during the assessment of overheating of dwellings, single-zone simulation can predict the overheating well, but it doesn’t account for the thermal dynamics of the building or cross-ventilation. a multi-zone model was used, as this can give more realistic results [18]. the mod496 vol. 38/2022 calibration of a summer building simulation model . . . (a). analysed apartment rooms. (b). whole building’s model with surrounding buildings and trees. figure 3. 3d model of the analysed building. structure thermal transmittance (u-value) [w/m2k] external wall 0.24 flat roof 0.16 basement slab 0.79 windows and doors 1.1; 1.4; 1.6; 1.8; 3 internal walls 10 cm 2.24 internal walls 38 cm 1.25 load-bearing wall (basement) 0.96 table 1. u-values of the analysed building. elling also included balconies, surrounding buildings and trees, as these affect the building’s exposure to environmental impacts (figure 3); the transmissivity of the plants was adjusted according to their type, form and how much light they can transmit. there was no artificial cooling in the building and energy efficient lighting with controls was assumed. the ventilation in the model (v1, v2) was set with the “calculated” method in designbuilder, where a control mode defines how the exterior openings are opened and closed. wind and buoyancy pressure causes flow through the openings and cracks in the building and determines the rate of ventilation. window opening is controlled by a timer and by ventilation setpoint temperatures. in our base case, this control mode was based on the monitoring data and windows were opened and closed according to the measured data. we assumed the crack template as “good” since the building underwent a renovation, improving the airtightness. in the more advanced method, the ventilation rate through each opening and crack is calculated based on the pressure difference: q = c · (dp ) · n, (1) where q volumetric flow through the opening, dp pressure difference across the opening/crack, n flow exponent, varying between 0.5–1.0 depending on the flow type (turbulent or laminar flow), c flow coefficient, related to the size of the opening/crack. the thermal transmittances of the building are in table 1; the u-values of the retrofitted structures meet the requirements of the hungarian regulation [19]. for the boundary conditions, the outdoor weather values were applied based on the measurement results. the estimated occupancy in the whole building was 0.04 people/m2, with a built-in residential occupancy schedule [20], led lighting with 2 w/m2 power density and lighting control. in the selected apartment, the occupancy schedule from the monitoring was applied. during the period under study, the technical building system was not particularly important, as only natural ventilation was used. the external blinds were operated according to the monitored data in the selected apartment and with a setpoint of 250 w/m2 in the rest of the building. the unknown parameters were determined iteratively during the runs. the simulation was used to determine how much the measured user habits help the modelling and how they affect the overheating of the building during the summer week under study. in comparison, we tried different simulation alternatives. in the v1 model, we used the measured values for the analysed flat, and all the other parts of the building got default values, as described earlier. in comparison, two additional modelling approaches were considered: in the v2 model, the analysed flat was assigned with the same default 497 d. szagri, a. kairlapova, b. nagy, z. szalay acta polytechnica ctu proceedings figure 4. measured values in the living room (temperature and relative humidity). figure 5. measured values in the bedroom (co2 and window opening). values (occupancy, shading etc.) as the whole building, thus excluding the measured user habits. in the v3 modelling approach, we used simplified ventilation modelling (scheduled natural ventilation) with 3 ach value in the building, with 2 °c “delta t” limit to control it. the latter one means, that the ventilation stops if the external temperature can potentially heat the internal space; when the external air temperature is less than 2 °c cooler than the internal one, the ventilation turns off. the assumed ventilation rate is an arbitrary value, but is in line with the values found in the literature [21] and in the regulation [19, 22]. 3. results and discussion 3.1. monitoring measurements due to the limited scope of the study, only a few values are highlighted. figure 4 shows the temperature and relative humidity values in the living room in the analysed period (august 3 – august 8, 2020). the temperature inside the room was 27.4 °c with a 29.1 °c maximum value. it can be seen that the indoor temperature in the living room was above 26 °c in more than 92 % of the time studied, which is not comfortable in summer. the temperature in the bedroom was also above 26 °c all the time. the relative humidity was 53 % in average in the living room and 51 % in the bedroom; co2 concentration inside the living room was not significant, 439 ppm in average and there was no major co2 enrichment during the week; so, in figure 5 we have highlighted the bedroom’s co2 values, which are a bit more interesting. in the bedroom the average co2 was 472 ppm, but during the nights, there are some enrichments (maximum: 791 ppm). window positions were recorded in the monitoring system, when the residents changed their position, where “0” means a closed window, “1” is tilted and “2” is a fully open position. it can be observed that the measured co2 decreases after the night when residents opening the bedroom window. the results show that during the second half of the week, the occupants left the dwelling; no significant change in co2 is detectable, and the indoor temperature and relative humidity values are more uniform, showing no large variations, the effect of the ambient temperature is more significant. although it is not the subject of this article, it is worth mentioning that the 2nd floor’s living room in building “a” – which has not been renovated – had much higher values. during the week under review, the average temperature was more than 31.3 °c with a maximum 498 vol. 38/2022 calibration of a summer building simulation model . . . figure 6. measured and simulated temperature values in the living room. (a). comparison of the living room’s temperature (measured versus simulated values). (b). simulation’s deviation from the measured values. figure 7. results of the dynamic simulation. value of 33.5 °c. in addition, the variability of the curves is much higher, with less damping of the effect of the outside temperature since that building is not thermally insulated. 3.2. dynamic simulations in the modelling, the number of occupants in the rooms was adjusted to reflect reality. furthermore, the presence, shading and window use of the occupants formed the basis for the simulation input data. figure 6 shows the measured and simulated temperature values in the living room. in the analysed period, the temperature varied between 25.3 °c and 29.1 °c in the living room. the chart shows all three versions tested. it can be seen that the average deviation from the real-life data is 0.91 °c for the model calibrated with measurement, 1.13 °c for the model with default values, and 1.70 °c for the simplified ventilation case. the v2 version appears to follow the measured calibrated values quite well, but in many cases, it underestimates the internal temperature; the simplified model (v3) shows more significant differences. one reason for the differences in the second half of the week is presumably that the residents were not in the apartment during this period, which the v1 model was able to take into account, while in the other two cases this was not possible with the predefined schedules. the two charts below (figure 7) illustrate the differences within the models more clearly. figure 7a shows the relation of the simulated temperature values in the living room compared to the monitoring data. in the graph, parallelly to the ideal line (where the measured and simulated values are equal), two additional dashed lines show ±1 °c difference from that. it is almost impossible to achieve a perfect match during calibration compared to the measured temperatures due to the several unknown factors and effects, but our goal for the simulation is to be able to reach a match 499 d. szagri, a. kairlapova, b. nagy, z. szalay acta polytechnica ctu proceedings within ±1 °c difference in most cases. a dynamic simulation that returns the monitoring measurement results within a degree difference can be considered sufficiently accurate. the model calibrated with monitoring data (v1) appears to fall much closer to these theoretical limits than the other two approaches (v2 and v3). if we observe the results, 57.1 % of the v1 model is within the ±1 °c limit, 45.2 % for the v2 model and only 29.2 % of the simulated values for the v3. figure 7b shows the deviation from the measured values over time for the different models. the difference between the versions is even more apparent here, with the version calibrated with measurements (v1) showing an average deviation of only 0.91 °c among the different modelling options. the outlier for all models was on measurement day 2, where the calibrated model also underestimated the monitoring measurement values by about 1.5 °c. however, in the rest of the measurement, the calibrated model has visibly lower differences than the default calculated (v2), or default scheduled (v3) dynamic simulations. latter performed the worst against the monitoring measurement, achieving up to 3 °c difference even on those days, where the calibrated model’s difference was just half of that. from these figures, it is also visible that the calibrated model usually performed on average up to around 1.5 times better than the default calculated v2 model and up to about 2.5 times more precision than the default scheduled v3 model. figure 7 shows that using the proposed calibration procedure, the dynamic simulations can achieve better accuracy compared to default simulation techniques. 4. conclusion measurements provided accurate and real data on the operation and behaviour of the buildings. furthermore, this study also offered the opportunity to analyse the period without occupants in a renovated and thermally insulated building. with the measured user habits, the thermal comfort within the apartment was not achieved; further studies are needed to see if this can be improved by other ventilation and shading strategies or if mechanical cooling is necessary. one of the lessons of the research is, when creating and calibrating models, it is important to take into account user habits and also the software’s limitations. based on the results of the study, it is worthwhile to assess and record user habits and implement them in the simulation, as this will provide more accurate results than other simplified modelling options. schedules based on the monitoring measurement helped to calibrate the model and led to smaller deviations from the measured temperatures. although the use of simplified ventilation may seem simpler and more efficient (faster run time), calculated ventilation rates lead to more accurate results. in the further part of the research, the user habits of the other apartments will be implemented, and a detailed comparison of the insulated and uninsulated buildings will be carried out. another interesting aspect of the question is how the inclusion of coupled heat and moisture transport would affect the results, and the calculation time in this case, how much more accurate the values would be compared to the measurement. acknowledgements “optimisation of buildings and building elements from life cycle and building physics perspective based on complex numeric modelling” (project fk 128663) has been implemented with the support provided from the national research, development and innovation fund of hungary, financed under the fk_18 funding scheme. references [1] a. sakka, m. santamouris, i. livada, et al. on the thermal performance of low income housing during heat waves. energy and buildings 49:69–77, 2012. https://doi.org/10.1016/j.enbuild.2012.01.023 [2] a. quinn, j. d. tamerius, m. perzanowski, et al. predicting indoor heat exposure risk during extreme heat events. science of the total environment 490:686–693, 2014. https://doi.org/10.1016/j.scitotenv.2014.05.039 [3] j. l. white-newsome, b. n. sánchez, o. jolliet, et al. climate change and health: indoor heat exposure in vulnerable populations. environmental research 112:20–27, 2012. https://doi.org/10.1016/j.envres.2011.10.008 [4] a. a. williams, j. d. spengler, p. catalano, et al. building vulnerability in a changing climate: indoor temperature exposures and health outcomes in older adults living in public housing during an extreme heat event in cambridge, ma. international journal of environmental research and public health 16(13):2373, 2019. https://doi.org/10.3390/ijerph16132373 [5] j. a. f. van loenhout, a. le grand, f. duijm, et al. the effect of high indoor temperatures on self-perceived health of elderly persons. environmental research 146:27–34, 2016. https://doi.org/10.1016/j.envres.2015.12.012 [6] m. hendel, k. azos-diaz, b. tremeac. behavioral adaptation to heat-related health risks in cities. energy and buildings 152:823–829, 2017. https://doi.org/10.1016/j.enbuild.2016.11.063 [7] j. taylor, p. wilkinson, r. picetti, et al. comparison of built environment adaptations to heat exposure and mortality during hot weather, west midlands region, uk. environment international 111:287–294, 2018. https://doi.org/10.1016/j.envint.2017.11.005 [8] s. porritt, l. shao, p. cropper, c. goodier. adapting dwellings for heat waves. sustainable cities and society 1(2):81–90, 2011. https://doi.org/10.1016/j.scs.2011.02.004 [9] y. yan, b. zhipeng, j. chunrong, et al. measuring air exchanges rates using continuous co2 sensors. in air waste management association symposium on air quality measurement methods and technology 2007, pp. 101–108. 2007. vol. 165 cp. 500 https://doi.org/10.1016/j.enbuild.2012.01.023 https://doi.org/10.1016/j.scitotenv.2014.05.039 https://doi.org/10.1016/j.envres.2011.10.008 https://doi.org/10.3390/ijerph16132373 https://doi.org/10.1016/j.envres.2015.12.012 https://doi.org/10.1016/j.enbuild.2016.11.063 https://doi.org/10.1016/j.envint.2017.11.005 https://doi.org/10.1016/j.scs.2011.02.004 vol. 38/2022 calibration of a summer building simulation model . . . 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2022 1 introduction 2 materials and methods 2.1 in-situ measurements 2.2 building energy modelling 3 results and discussion 3.1 monitoring measurements 3.2 dynamic simulations 4 conclusion acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.38.0145 acta polytechnica ctu proceedings 38:145–150, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague supporting sustainable policies through an urban energy-environmental model and a multi-criteria analysis: a case study in an italian province lorenzo belussi∗, benedetta barozzi, alice bellazzi, ludovico danza, anna devitofrancesco, matteo ghellere, francesco salamone itc-cnr, via lombardia 49, 20098 san giuliano milanese, italy ∗ corresponding author: belussi@itc.cnr.it abstract. public authorities (pas) need to define cross-cutting strategies for urban planning including policies for sustainable and energy-efficient buildings and innovative urban solutions. the article presents a decision support tool that combines an urban energy environmental model (ueem) and a multi-criteria analysis (mca) to support the development of sustainable local policies. the ueem, developed with a bottom-up approach incorporating energy and environmental items, provides a representation of the performance of local urban areas and quantifies the impact of new interventions in expansion areas. the ueem is based on the definition of virtual archetypes built on the characteristics of the area under consideration. 92 building archetypes and 40 urban archetypes are developed. the energy performance of each building archetype is calculated with a dynamic simulation tool. the environmental performance of urban areas (overheating risk and outdoor thermal comfort) is analysed through a grasshopper-based parametric model. in addition, soil permeability is calculated. the ueem results are aggregated into a single index using the mca, providing a municipal rating index (mri). the weights of the mca are estimated through the analytical hierarchy process (ahp) based on a survey submitted to local stakeholders (municipalities, environmental associations, experts). the model is applied to the province of monza and brianza in northern italy. keywords: urban planning, urban energy environmental model, multi-criteria analysis, analytical hierarchical process. 1. introduction city planning has raised several environmental, social and economic issues that public authorities (pas) have often tried to address with inadequate tools, with the result that urban areas are often a collection of environmental, social and economic problems [1]. to make them sustainable, it is necessary to develop and to adopt new planning approaches that include a multi-sectoral scope [2]. this requires a comprehensive knowledge of the energy and material exchanges that characterize the urban metabolism of cities or urban areas [3] and the need of adequate tools to quantify and analyse these flows for the formulation of sustainable planning and policy recommendations [4]. multidimensional approaches and specific methods that can assess and summarise multiple aspects of urban planning can help to address these issues. among the several assessment methods applied [5], the multicriteria analysis (mca) has been widely used [6]. in [7], the authors describe a decision-making system based on multicriteria decision analysis (mcda) to support policy makers in defining resilient territorial systems: the result is a single index that combines seven indicators to identify the resilience class of municipalities in portugal. the mcda to support the sustainable development of cities, described in [8], is based on a composite index that aggregates 35 energy and environmental indicators. the results of these studies have practical implications for the decisionmaking process providing stakeholders with a tool for defining policies developed starting from the real needs of cities and/or urban areas. such studies are typically based on aggregated data for the representation of urban performance. this article combines an urban energy and environmental model (ueem) developed in a previous research [9, 10] with an mca, developing an integrated decision support system (dss) for representing the performance of a local urban area. the proposed dss quantifies the impact of new interventions in expansion areas too. considering that ueem is based on a bottom-up approach moving from the definition of specific virtual archetypes (figure 1) stated on information available to pas and statistical studies, the model provides an estimation of the current energy performance of the building stock, in terms of primary energy consumption and co2 emissions, as well as the environmental characteristics of the urban building fabric in terms of potential overheating. the method is used to evaluate the impact of new neighbourhoods in a northern italian province. 145 https://doi.org/10.14311/app.2022.38.0145 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en l. belussi, b. barozzi, a. bellazzi et al. acta polytechnica ctu proceedings figure 1. example of 3d model built from shapefile. 2. materials and method this study combines a ueem and the mca to support decision makers in the sustainable development of new neighbourhoods. the ueem method was developed using a bottom-up approach, from single components (buildings and neighbourhoods) to the overall urban area. the twofold analysis is performed by considering the available information from opensource database, mostly from the regional territorial information system, for the geometrical, morphological and functional characterization of the building stock and the surrounding urban areas. the ueem is applied for the representation of the energy and environmental performance of urban areas. the outcomes of the ueem are managed by mca for the definition of an aggregated index called municipal rating index (mri) for the identification areas suitable for new sustainable urban development. the following section presents the overall model and the case study. 2.1. case study the case study is the province of monza and brianza, with an area of 405 km2, located to the north-east of the city of milan in the lombardy region, divided into 55 municipalities with an anthropized soil ratio of 41 %, due to the intensive residential and industrial urbanization that has been going on since the 70s. the analysis uses open vector and raster data retrieved from the geographic portal of lombardy region [11]. the available vector data refer to building properties, presence of vegetation and soil use through the corine land cover classification [12] implemented by the regional map “destinazione d’uso dei suoli agricoli e forestali – agricultural and forest land use” (dusaf) [13] for the year 2018 (version 6.0). raster data provide the buildings footprint at different time periods (regional technical map – ctr) [14]. analyses are also performed at sub-municipal level. in this case, the boundaries of the territorial census sections (tcs) provided by istat (national institute of statistics) are used [15]. the management and processing of the georeferenced data is performed with the grassgis software [16]. the mca evaluation of the building expansion policies of the municipalities is focused on the energy and environmental performances and is based on the intervention scenarios defined for the transformation areas identified by each municipality. the following aspects related to the transformation are considered: • all design hypotheses comply with the predictions and requirements of the municipal development plans concerning urban parameters (building typology, height, distances, coverage ratio, etc.). • the energy performance of the planned buildings complies with the requirements of the current legislation for near zero energy buildings (nzebs). • for each planned building use, the shape of planned buildings corresponds to the one most commonly used after the last ctr recognition (2003 to date). • building footprint and vegetation soil coverage fraction equals the territorial census section-(tcs) mean values of the municipalities which have the same planned building use. 2.2. energy and environmental models the building stock is classified according to the following features: • building typologies: residential, office, school, commercial, supermarket and industry. • construction period: before 1976, 1976–1990, 1991– 2005, after 2005, considered as milestones in the italian legislation having affected the thermal and energy performance of buildings over the years. • urban context: dense (distance between buildings ≤ 10 m), open (distance between buildings ≤ 20 m) or isolated (distance between buildings > 20 m) based on available local georeferenced data. the granularity of buildings with residential use, offices and schools is further enhanced, due to a more accurate characterization of the building stock representative of the areas considered in the case study. residential buildings are classified into four categories (single family, detached house, multifamily building and apartment block) with respect to predefined shape 146 vol. 38/2022 supporting sustainable policies . . . intensity of importance definition explanation 1 equal importance two attributes contribute equally to the objective 3 moderate importance experience and judgment slightly favour one over another 5 strong importance experience and judgment strongly favour one over another 7 very strong importance an attribute is strongly favoured 9 extreme importance the evidence favouring one attribute over another is of the highest possible order of affirmation 2, 4, 6, 8 intermediate values when compromise is needed table 1. judgement scale proposed by saaty [17]. ratio (s/v) threshold values: 0.38, 0.57 and 0.72. offices are classified depending on the number of floors: ≤ 2 floors for “low-rise”, > 2 floors for “medium-rise”. finally schools are identified based on their location: “compact” in a dense area or large and “isolated” in the suburbs. by combining all the features, a matrix of 92 archetypes is defined. each reference building is then characterized by the performance of the envelope, hvac, energy carriers and occupancy profile found in standard references or eu projects. the primary energy consumption [kwh] and co2 emissions [kg co2] of the archetypes, expressed per square meter of building area, are calculated using the energyplus simulation tool and its interface designbuilder®, including heating, cooling, domestic hot water production, lighting and ventilation energy services. finally, the archetypes are applied to the existing buildings and aggregated at different spatial scales: building, tcs, municipality and aggregation of municipalities scale. the aggregations provide a comprehensive and structured knowledge of the performance of the area. starting from the building level, the aggregation of data is done with the weighted average of kpis (key performance indicators) within a given area. the environmental model assesses the overheating risk and the permeability of the area. the urban area is classified according to the land use typology defined by dusaf. the parameters used to characterize the areas are defined in accordance with the urban weather generator (uwg) engine [18], which considers both building morphology and the land cover characteristics. in relation to the overheating assessment performed with the uwg tool analysis, the urban areas warmer than 1.80 °c compared to the reference rural areas are considered “overheated”. the permeability is assessed starting from the vegetation coverage ratio (vcr), assigning an average runoff coefficient to each census area defined by dusaf. as a reference, runoff coefficients are defined by the american society of civil engineer and the water environment federation, calculated for return periods of 10 years or less and corresponding to the main characteristics of the area [19]. for each rural typology area, excluding water bodies, the value 0.15 has been taken as a reference. for anthropized areas, runoff index is determined by a linear interpolation between 0.15 and 1.00 in proportion to the mean vcr value of the class (from 100 % to 0 %). finally, the maps of overheated areas and the runoff index are rasterized at high resolution (0.75 m/px) to allow the calculation of the environmental kpis at the municipality level: the ratio between non-overheated areas and municipality area (overheating risk) and the mean runoff index (permeability). 2.3. multi-criteria analysis mca is used to identify and compare different policy options and assess their effects and impacts. in this study, the mca is applied to identify the most suitable areas for new urban development. combining the scores of different indicators into a single municipal index, moreover, the index is used to evaluate the impact of new urban settlements with respect to the baseline scenario. the analytic hierarchy process (ahp) is the method used to develop the mcda system [20]. this method consists of decomposing a problem into a hierarchy and comparing the attributes, two at a time, with respect to their effect on the other attributes. seven criteria concerning energy and environmental performance of each municipality were considered in the analysis: (1.) building energy consumption; (2.) anthropogenic co2 emissions (buildings and transport); (3.) urban overheating; (4.) soil imperviousness; (5.) soil consumption; (6.) areas of environmental interest; (7.) areas of agricultural interest. the criteria are compared to measure their relative importance with respect to the main goal based on the following question: “how important is attribute a with respect to attribute b?”. the comparison is made using the nine-points judgment scale reported in table 1. each criterion is scaled on a five-point 147 l. belussi, b. barozzi, a. bellazzi et al. acta polytechnica ctu proceedings figure 2. example of a kpi assessment for energy and environmental analysis. in ex-ante and ex-post analysis, the more colourful the municipality, the worse the performance. scale with a min-max procedure. intermediate values are not considered. a number of stakeholders participate together in the decision-making process and in this case four groups were involved: • province’s employees (p). • mayors and technicians from involved municipalities (m). • scientists involved in energy and environmental research (s). • professional associations (a). a questionnaire is submitted to each stakeholder group reporting the pairwise comparison of the indicators. the analysis of the completed questionnaires led to the calculation of the weight of each group followed by the definition of the overall weight of each criterion for the calculation of the mri. the individual judgments have to be aggregated in order to fulfil the process. two main approaches can be applied for this task: aggregating individual judgments (aij) when individuals in a group act to merge their own preferences to reach a synthetic judgment, and aggregating individual priorities (aip) when the individuals act for themselves [21]. in the present work the aij approach is applied and the geometric mean is used for the aggregation process. 3. results and discussion the kpis are calculated for each municipality using the bottom-up approach to point out the mca of the current state of the territory (ex-ante scenario) and of the intervention scenario (ex-post scenario). figure 2 explains the calculation process for the kpis of energy (e.g. energy consumption) and environmental (e.g. overheating risk) analyses. the kpis for the new scenarios are represented as the variation (percentage or absolute value) with respect to the baseline scenario. as shown by the energy assessment (figure 2c), most of the municipalities have on average a high energy index (195–205 kwh/m2y) and, as a consequence, show a high co2 consumption. 148 vol. 38/2022 supporting sustainable policies . . . figure 3. mri map. n criteria weight weight weight weight mean (s) (p) (a) (m) weight 1 building energy consumption 15.0 % 4.2 % 24.5 % 15.4 % 13.2 % 2 anthropogenic co2 emissions 26.5 % 7.5 % 15.6 % 13.1 % 15.1 % 3 urban overheating 10.6 % 4.3 % 8.3 % 9.0 % 8.1 % 4 soil imperviousness 12.9 % 13.8 % 9.1 % 14.1 % 13.1 % 5 soil consumption 9.0 % 17.0 % 16.6 % 14.7 % 14.8 % 6 areas of environmental interest 14.6 % 26.5 % 15.0 % 19.9 % 20.0 % 7 areas of agricultural interest 11.4 % 26.8 % 11.0 % 13.8 % 15.7 % table 2. weighting factors. this is due to the old municipal building stock plus in addition to the high extension of non-residential buildings with a high electrical consumption due to lighting and hvac systems. thanks to the compliance of new buildings with zeb requirements, the transformation scenario ensures a slight decrement in the municipal energy index (figure 2e). the ex-ante environmental assessment (figure 2d) points out a medium-high overheating risk (deep redcoloured municipalities) due to the high urbanization rates of the past. the assessment of the transformation scenarios (figure 2f) points out differences in overheating risk management between municipalities. permeability performances follow the same trend of the overheating risk. the analysis of the questionnaires completed by the stakeholders led to the calculation of the weights for each criterion, as shown in table 2. the analysis shows that areas of environmental interest account for 1/5 of the overall weights and they represent the highest priority area. following and very close to each other are areas of agricultural interest, anthropogenic co2 emissions, soil consumption, building energy consumption and soil imperviousness. urban overheating ranks last and far below the others. the mean weight is calculated as the geometric mean among (of) the weights of each stakeholder. mri is calculated as the sum of calculated and weighted indicators in order to assess each municipality (figure 3). 4. conclusions the paper develops a simplified model for analysing the current energy and environmental performance of urban areas supporting pas in evaluating the impact of new expansion areas and driving future urban policies. this ueem is applied to the case study of the monza and brianza province but it can be replicated in different contexts. it uses information on buildings and urban areas provided by pas, as well as indicators and calculation methods that are widely accepted and used. the kpis can be modified to meet the needs of the pas. the model can support planning, monitoring and visualization of activities. in the planning phase it supports decisions to maximize benefits while reducing negative impacts of new interventions at building and urban scale (energy and co2 emissions, overheating risk reduction, improvement of green solutions 149 l. belussi, b. barozzi, a. bellazzi et al. acta polytechnica ctu proceedings applications, optimization of run-off coefficient and urban comfort). the model allows monitoring of the variation of energy and environmental indicators over time and the impact of the different scenarios at urban scale. the breakthrough of this study consists in the detailed evaluation of the performance of new urban areas in order to identify development strategies to achieve the energy and environmental targets of 2030 and 2050. future developments will provide the evaluation of the customized uwg model to better reflect the italian building stock against field data, the integration of new indices for a better description of the performance of urban areas including social and economic issues, as well as the definition of a municipal rating index used for the purposes of aggregating energy, environmental, economic and social data. references [1] 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[21] e. forman, k. peniwati. aggregating individual judgments and priorities with the analytic hierarchy process. european journal of operational research 108(1):165–169, 1998. https://doi.org/10.1016/s0377-2217(97)00244-0. 150 https://doi.org/10.3390/su13179858 https://doi.org/10.1186/s40410-016-0052-y https://doi.org/10.3389/frsc.2019.00001 https://doi.org/10.1016/j.eiar.2012.06.007 https://doi.org/10.1016/j.landurbplan.2021.104129 https://doi.org/10.1016/j.landurbplan.2021.104129 https://doi.org/10.3390/buildings9120244 https://doi.org/10.1016/j.scitotenv.2020.143806 https://doi.org/10.1016/j.scitotenv.2020.143806 https://doi.org/10.1016/j.jenvman.2018.07.075 https://doi.org/10.1088/1757-899x/609/7/072018 https://doi.org/10.26868/25222708.2019.210290 https://www.geoportale.regione.lombardia.it/en-gb/home https://www.geoportale.regione.lombardia.it/en-gb/home https://www.regione.lombardia.it/wps/portal/istituzionale/hp/dettaglioservizio/servizi-e-informazioni/enti-e-operatori/territorio/sistema-informativo-territoriale-sit/uso-suolo-dusaf/uso-suolo-dusaf https://www.regione.lombardia.it/wps/portal/istituzionale/hp/dettaglioservizio/servizi-e-informazioni/enti-e-operatori/territorio/sistema-informativo-territoriale-sit/uso-suolo-dusaf/uso-suolo-dusaf https://www.regione.lombardia.it/wps/portal/istituzionale/hp/dettaglioservizio/servizi-e-informazioni/enti-e-operatori/territorio/sistema-informativo-territoriale-sit/uso-suolo-dusaf/uso-suolo-dusaf https://www.regione.lombardia.it/wps/portal/istituzionale/hp/dettaglioservizio/servizi-e-informazioni/enti-e-operatori/territorio/sistema-informativo-territoriale-sit/uso-suolo-dusaf/uso-suolo-dusaf https://www.regione.lombardia.it/wps/portal/istituzionale/hp/dettaglioservizio/servizi-e-informazioni/enti-e-operatori/territorio/sistema-informativo-territoriale-sit/uso-suolo-dusaf/uso-suolo-dusaf https://www.geoportale.regione.lombardia.it/en-gb/download-ctr https://www.geoportale.regione.lombardia.it/en-gb/download-ctr https://www.istat.it/it/archivio/104317 https://doi.org/10.1016/j.envsoft.2011.11.014 https://doi.org/10.1016/0022-2496(77)90033-5 https://doi.org/10.1080/19401493.2012.718797 https://swmm5.org/2017/10/12/runoff-coefficients-for-infoswmm-infoswmm-sa-icm-and-xpswmm/ https://swmm5.org/2017/10/12/runoff-coefficients-for-infoswmm-infoswmm-sa-icm-and-xpswmm/ https://swmm5.org/2017/10/12/runoff-coefficients-for-infoswmm-infoswmm-sa-icm-and-xpswmm/ https://doi.org/10.1007/978-3-642-83555-1_5 https://doi.org/10.1016/s0377-2217(97)00244-0 acta polytechnica ctu proceedings 38:145–150, 2022 1 introduction 2 materials and method 2.1 case study 2.2 energy and environmental models 2.3 multi-criteria analysis 3 results and discussion 4 conclusions references https://doi.org/10.14311/app.2022.33.0001 acta polytechnica ctu proceedings 33:1–7, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague comparative flexural performance of steel fibre reinforced self-compacting concrete (scfrc) ribbed slab with different fibre provision area hazrina ahmada,∗, mohd hisbany mohd hashimb, afidah abu bakarb, fariz aswan ahmad zakwana, ruqayyah ismaila a faculty of civil engineering, universiti teknologi mara, cawangan pulau pinang, kampus permatang pauh, 13500 permatang pauh, pulau pinang, malaysia b faculty of civil engineering, universiti teknologi mara, 40450 shah alam, selangor, malaysia ∗ corresponding author: hazrina180@uitm.edu.my abstract. the flexural performance of three scfrc ribbed slabs with different fibre provision area were investigated in this paper; in both ribs and flange (sfws), ribs only with additional welded mesh in flange (sft) and in ribs only (sfr). short hooked end fibres of 35 mm length of 1% volume fraction was blended with the flowable self-compacting concrete (scc) were used as the material for the slabs. slab samples of 2.8 × 1.2 × 0.2 m were constructed and loaded until failure under fourpoint bending. investigation was carried out in view of the load bearing capacity, deflection, energy absorption capacity as well as the failure modes. the influence of the steel fibre provision on the strain distribution was also examined. keywords: ribbed slab, self-compacting concrete, steel fibres. 1. introduction steel fibre reinforced concrete (sfrc) can be applied in structural slabs that usually cater uniformly distributed low and moderate loads owing to the ability of the steel fibres (sf) to reduce brittleness and enhance its mechanical properties. in consideration of the workability and fibre distribution of the fibre concrete used, this study focuses on the utilization of self-compacting fibre reinforced concrete (scfrc) that is able to be cast in place without any vibration. the flow ability of the scfrc offer advantage to the construction process that further leads to a reduction of manpower cost and time [1]. at present, the inclusion of steel fibres is now extended as the main reinforcing material in slab applications to partially or totally replace conventional reinforcements. a number of studies had been performed utilizing scfrc material in flat slab [2–7]. however, limited studies were found on literatures concerning a ribbed profiled structure. to date, studies on the application of steel fibres in slab structures with rib profile were still limited to its application in normal steel fibre reinforced concrete with low volume fraction (0.5%) [8–10]. the objective of this research is to experimentally determine the ultimate flexural strength, loaddeflection curves and the energy absorption capacity of scfrc ribbed slabs varying in the provision area of the steel fibres (fully and partially steel fibre reinforced). 2. experimental program nine (9) numbers of scfrc ribbed slabs were cast with 2800 mm length, 1200 mm width and overall thickness of 200 mm. two ends of the slabs were simply supported on rollers providing clear effective span length of 2600 mm. two-line loads were applied on the slabs under displacement control of 0.1 mm/sec. sensitive linear voltage differential transducers (lvdt) were used to measure the displacement at the slab centre (rib and flange soffit). grade c30/37 plain self-compacting concrete (pscc) mix was used for the fabrication of the ribbed slab samples with the mix proportions shown in table 1. the volume fraction of the hooked end steel fibres was 1% which corresponds to 80 kg/m3. detail properties of the steel fibres is shown in table 2. the mechanical properties of the pscc and scfrc mix used in this research is discussed elsewhere [11]. similar scc mix was used for the scfrc mix in combination with the steel fibres. the scfrc mix was classified as sf1 [12], thus no vibration was applied during casting of the samples. details on the variation of the slab samples are listed in table 3 while figure 1 presents the steel fibre provision in the slab samples applied in this research. the scfrc ribbed slab are divided into three categories; i.e. full steel fibre reinforced (sfws), partially steel fibre reinforced with welded mesh (sft) and partially steel fibre reinforced without welded mesh (sfr). each type of samples also varies in terms of its topping flange thicknesses (80, 100 and 120 mm). 1 https://doi.org/10.14311/app.2022.33.0001 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en h. ahmad, m. h. mohd hashim, a. abu bakar et al. acta polytechnica ctu proceedings cement cem i 42.5r pulverized fly ash 10 mm coarse aggregate fine aggregate water w/c steel fibre content 315 kg/m3 105 kg/m3 830 kg/m3 865 kg/m3 185 kg/m3 0.44 80 kg/m3 table 1. scc mix composition. fibre type tensile strength (n/mm2) length l (mm) diameter df (mm) fibre aspect ratio l/df hooked end 1250 35 0.55 65 table 2. steel fibre properties. reinforcement designation topping flange thickness (mm) fully steel fibre reinforced sfws sfws80 80 sfws100 100 sfws120 120 partially steel fibre reinforced with brc sft sft80 80 sft100 100 sft120 120 partially steel fibre reinforced without brc sfr sft80 80 sft100 100 sft120 120 table 3. scfrc sample details. � 6)7��3duwldo�6)�zlwk�zhoghg�phvk� 6)5 �3duwldo�6)� 6):6 �)xoo�6)� figure 1. steel fibre provision area. 2 vol. 33/2022 steel fibre reinforced self-compacting concrete sample first crack load pcr (kn) ultimate load pult (kn) displacement at ultimate load δult (mm) energy absorption at pult (knmm) total energy absorption (knmm) sfws80 35.78 57.83 8.75 403.15 1298.91 sfws100 47.67 60.00 7.8 367.56 1379.22 sfws120 46.04 74.71 12.47 716.73 1540.65 sft80 42.89 50.66 3.17 104.23 823.49 sft100 49.14 51.64 2.97 99.96 1019.92 sft120 39.53 49.25 7.95 320.01 794.38 sfr80 47.89 55.65 3.17 117.11 482.52 sfr100 44.69 49.3 2.1 66.72 375.83 sfr120 46.26 54.79 2.61 87.46 264.72 table 4. flexural test results. 3. experimental results and discussion 3.1. first crack and ultimate strength of ribbed slabs the presence of the steel fibres significantly influences the first crack occurrence of the scfrc ribbed slab. the experimental results of the flexural test are presented in table 4. the first crack load of all sfws, sft and sfr samples were found to be approximately double the first crack load of conventional ribbed slab sample without any steel fibres. the first crack values used for comparison are 17.59, 21.39 and 21.99 kn for 80, 100 and 120 mm flange thickness, respectively. these results are reported elsewhere [13]. additionally, the first crack load of all samples was observed to be more than 60% of the ultimate load achieved, proving the ability of the steel fibres to transfer stresses across the cracks within the concrete matrix. furthermore, the provision of steel fibres also significantly influences the ultimate strength of the ribbed slab that corresponds to the major crack initiation in the ribbed slab samples. the sfws samples that were fully reinforced with steel fibres achieved the highest ultimate load even with the absence of any conventional reinforcements. partially reinforced samples (sft and sfr) however, achieved lower ultimate loads. this might be due to the faster occurrence of the major crack in the sample since the steel fibres were only provided in the ribs causing the stress transfer within the matrix to be more rapid. the presence of the welded mesh in the topping flange also showed no significant contribution to the ultimate strength of the partially steel fibre reinforced samples as it is not in the tension zone. 3.2. load-displacement the load-displacement curves for all scfrc ribbed slab samples are shown in figure 2. before the first crack occurrence, the elastic region was steeper for the partially sf reinforced samples (sft and sfr) in comparison to the sfws samples that showed a more gradual increment. beyond the first crack occurrence, only the sfws samples exhibited displacement-hardening response towards the ultimate load displaying the efficiency of the sf to transfer the stresses within the cracked samples. this hardening behaviour is in relation to the existence of the sf in the topping flange that played an important role in resisting the formation of the cracks as it propagates from the ribs to the flange section. at this point, some of the steel fibres will be subjected to pull-outs as the loading continues. this displacement-hardening response was less significant in the partially sf reinforced samples (sft and sfr) even with presence of the welded mesh reinforcement in the topping flange. beyond the ultimate load, all samples experienced displacement-softening response where the load carrying capacity gradually decrease as the vertical displacement continues to increase further. gradual softening curves are exhibited by the sfws and sft samples reaching up to 32 mm displacement. in contrast, the sfr samples underwent more abrupt softening resulting lower displacement reflecting to a more brittle behaviour of the structure. in view of the topping flange thickness for each type of samples, the increase in the topping flange thickness significantly affects the softening curves. steeper curves were observed for 120 mm topping flange thicknesses of all ribbed slab samples exhibiting a more brittle post-cracking behaviour as the topping thickness increases beyond half of the total slab depth (100 mm). this brittleness might be as the result of the topping thickness that enters the tensile zone of the sample reducing the function of the rib section. this finding also agrees well with the previous findings where higher slab thickness resulted in higher brittleness of the section [3, 14]. 3 h. ahmad, m. h. mohd hashim, a. abu bakar et al. acta polytechnica ctu proceedings � � �� �� �� �� � � �� �� �� �� �� �� / rd g� �n 1 � 'lvsodfhphqw��pp� 6):6�� 6):6��� 6):6��� 6)7�� 6)7��� 6)7��� 6)5�� 6)5��� 6)5��� figure 2. load-displacement curves. figure 3. energy absorption capacity. 3.3. vertical displacement at ultimate load, the highest displacement was exhibited by the sfws samples at both ultimate load and at final failure of the samples (table 4). this was followed by the sft and sfr samples. the sfr samples which are more brittle experienced the lowest vertical displacement at ultimate load as well as at final failure exhibiting a lower level of ductility. nevertheless, all samples fulfilled the displacement requirement at service load whereas the values are within the allowable limit (l/500) as stated in the eurocode 2 revealing the capability of the scfrc samples under flexural load. in view of the topping flange thickness, the lowest displacement at ultimate load was observed in the samples with 100 mm topping flange thickness for all types of samples. this can be related to the location of the neutral axis and the transmission of the stresses between the rib and flange section of the ribbed slab. at final failure, however, the 120 mm samples experienced the lowest displacement that is as the result of the brittle behaviour of the scfrc ribbed slab as the topping flange thickness increases. 3.4. energy absorption capacity steel fibre inclusion enhances the energy absorption capacity of the slab as calculated by the area under the load-displacement curve. higher volume of steel fibres in concrete resulted in an increase in the energy absorbed by the slab structure. the sfws samples showed the highest energy absorption capacity at the ultimate load as well as the total energy (see table 4). the energy absorption capacity is in close relation with the total volume of sf in the slab. the sfws samples showed higher energy absorption for higher flange thicknesses. however, a contradict trend were observed in the sft and sfr samples showing the relation towards the provision area of the sf (figure 3). 4 vol. 33/2022 steel fibre reinforced self-compacting concrete � figure 4. strain distribution of sfws and sft samples. � �d��6):6� �e��6)7� �f��6)5� figure 5. crack pattern of the scfrc ribbed slab and steel fibres bridging the cracks. 3.5. strain distribution the neutral axis location of the scfrc ribbed slab samples can be determined based on the strain distribution across the depth of the slab at various stages of loading. for the sfws samples, only for 80 mm topping thickness, the neutral axis falls below the topping flange. the neutral axis falls in flange for all other samples, including all sft and sfr samples. no significant effect of the topping flange on the neutral axis location thickness was observed. the strain distribution for the scfrc ribbed slab samples is presented in figure 4. the view of the bridging steel fibres is also presented in figure 5. from the crack openings, it can be observed that most of the steel fibres were in the longitudinal direction that is perpendicular to the crack direction. the steel fibres had effectively bridged the cracks by holding the matrix together and the hooked end shape had assisted in increasing the bonding to the concrete matrix contributing to the structural strength to resist loads. the crack pattern for the partially reinforced samples (sft and sfr) were different from the sfws samples whereas it only experienced one major crack 5 h. ahmad, m. h. mohd hashim, a. abu bakar et al. acta polytechnica ctu proceedings � figure 6. scfrc ribbed slab at final failure. line with very minimal minor cracks across the sample. the crack started at the rib’s soffit and propagated to the flange soffit. the sft samples underwent ductile failure, gradually failing after the ultimate load was reached. the sfr samples on the other hand experienced a more rapid crack propagation experiencing a more brittle failure. 3.6. crack pattern and failure modes figure 5 showed the crack pattern of the sfws samples. the first crack of all three sfws samples was detected within the central region of the ribbed slab located at the external rib soffit. after the occurrence of the first crack, more cracks developed at the external rib’s soffit on both sides of the sample as well at the middle rib soffit. these cracks then continued to propagate to the sides of the rib sections and further to the soffit of the flange as the loading continued. as the cracks continued to widen at the ribs, it could be visually observed from the slow propagating cracks that the steel fibres had effectively bridged the cracks as the loading continued to increase further. the view of the bridging steel fibres is also presented in figure 5. from the crack openings, it can be observed that most of the steel fibres were in the longitudinal direction that is perpendicular to the crack direction. the steel fibres had effectively bridged the cracks by holding the matrix together and the hooked end shape had assisted in increasing the bonding to the concrete matrix contributing to the structural strength to resist loads. the crack pattern for the partially reinforced samples (sft and sfr) were different from the sfws samples whereas it only experienced one major crack line with very minimal minor cracks across the sample. the crack started at the rib’s soffit and propagated to the flange soffit. the sft samples underwent ductile failure, gradually failing after the ultimate load was reached. the sfr samples on the other hand experienced a more rapid crack propagation experiencing a more brittle failure. in view of the topping thickness variation, the cracks were observed to be more concentrated on the rib soffit of the sfws samples with lower flange thicknesses (80 mm and 100 mm). consequently, more cracks were observed to be distributed over the flange soffit for the 120 mm topping thickness. at the end of the loading test, all sfws samples still remained intact, held by the steel fibres in the ribs and topping flange (figure 6). as for the sft and sfr samples, all samples except for the samples with 120 mm topping thickness remains intact at final failure with the assistance of the welded mesh and bridging of the fibres in the ribs section. samples with 120 mm thickness experienced brittle failure, i.e. breaking into two at the end of the loading process. these conditions might be due to the reduced thickness of the rib section resulting smaller volume of the steel fibres to bridge the cracks. 4. conclusion based on the results, these conclusions can be drawn: 1. all scfrc ribbed slab underwent flexural failure. the fully sf reinforced samples experienced multiple cracks while the partially sf reinforced samples experienced only one major crack, displaying the significant effect of the steel fibre provision under flexural load. 2. the fully sf reinforced samples also achieved the highest ultimate load at higher deflection exhibiting higher level of ductility amongst all samples 3. increase in the flange thickness resulted an increase in the ultimate load for the sfws samples but shows no significant increase in the sft samples. beyond the ultimate load, samples with higher 6 vol. 33/2022 steel fibre reinforced self-compacting concrete flange thickness becomes more brittle resulting in a more rapid strength loss. overall, based on the experimental results, it can be concluded that the fully steel fibre reinforced sample (sfws) with the highest flange thickness displayed a good performance under bending. acknowledgements special thanks to the laboratory staffs of the faculty of civil engineering, universiti teknologi mara for their technical support and the institute of research management and innovation (irmi), uitm for their assistance in managing the research process. this study was funded by the e-science fund (06-01-01-sf0835) from the ministry of science, technology and innovation (mosti) of malaysia. references [1] m. paja̧k, t. ponikiewski. flexural behavior of self-compacting concrete reinforced with different types of steel fibers. construction and building materials 47:397-408, 2013. https: //doi.org/10.1016/j.conbuildmat.2013.05.072. [2] a. blanco, p. pujadas, a. de la fuente, et al. assessment of the fibre orientation factor in sfrc slabs. composites part b: engineering 68:343-54, 2015. https: //doi.org/10.1016/j.compositesb.2014.09.001. [3] j. michels, d. waldmann, s. maas, et al. steel fibers as only reinforcement for flat slab construction experimental investigation and design. construction and building materials 26(1):145-55, 2012. https: //doi.org/10.1016/j.conbuildmat.2011.06.004. [4] steel fiber concrete slabs on ground: a structural matter. aci structural journal 103(4), 2006. https://doi.org/10.14359/16431. [5] h. salehian, j. a. o. barros. assessment of the performance of steel fibre reinforced self-compacting concrete in elevated slabs. cement and concrete composites 55:268-80, 2015. https: //doi.org/10.1016/j.cemconcomp.2014.09.016. [6] k. s. regab, study punching shear of steel fiber reinforced self compacting concrete slabs by nonlinear analysis, international journal of civil, environmental, structural, construction and architectural engineering, 7(9):624-635, 2013. [7] m. d. e. teixeira, j. a. o. barros, v. m. c. f. cunha, et al. numerical simulation of the punching shear behaviour of self-compacting fibre reinforced flat slabs. construction and building materials 74:25-36, 2015. https: //doi.org/10.1016/j.conbuildmat.2014.10.003. [8] a. s. samsudin, structural performance of steel fibre reinforced concrete ribbed slab panel, universiti teknologi mara, 2018. [9] f. a. rahman, a. a. bakar, m. h. m. hashim, et al. flexural performance of steel fiber reinforced concrete (sfrc) ribbed slab with various topping thicknesses. 2017. https://doi.org/10.1063/1.5011493. [10] m. h. mohd hashim, s. h. hamzah, h. ahmad, et al. preliminary investigation on the flexural behaviour of steel fibre reinforced self-compacting concrete ribbed slab. scientific research journal 15(1), 2018. https://doi.org/10.24191/srj.v15i1.4166. [11] a. hazrina, m. h. mohd hisbany, a. b. afidah, et al. effects of steel fibre addition on the mechanical properties of steel fibre reinforced self-compacting concrete (sccfibre). iop conference series: materials science and engineering 431, 2018. https://doi.org/10.1088/1757-899x/431/4/042004. [12] the european guidelines for self-compacting concrete, no. may. european project group, 2005. [13] h. ahmad, m. h. m. hashim, a. a. bakar, et al. flexural performance of full and partially steel fibre reinforced self-compacting concrete (scfrc) ribbed slab. iop conference series: materials science and engineering 615(1), 2019. https://doi.org/10.1088/1757-899x/615/1/012094. [14] l. facconi, f. minelli. verification of structural elements made of frc only: a critical discussion and proposal of a novel analytical method. engineering structures 131:530-41, 2017. https: //doi.org/10.1016/j.engstruct.2016.10.034. 7 https://doi.org/10.1016/j.conbuildmat.2013.05.072 https://doi.org/10.1016/j.compositesb.2014.09.001 https://doi.org/10.1016/j.conbuildmat.2011.06.004 https://doi.org/10.14359/16431 https://doi.org/10.1016/j.cemconcomp.2014.09.016 https://doi.org/10.1016/j.conbuildmat.2014.10.003 https://doi.org/10.1063/1.5011493 https://doi.org/10.24191/srj.v15i1.4166 https://doi.org/10.1088/1757-899x/431/4/042004 https://doi.org/10.1088/1757-899x/615/1/012094 https://doi.org/10.1016/j.engstruct.2016.10.034 acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.38.0072 acta polytechnica ctu proceedings 38:72–76, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague attitude towards lca in hungary and czechia – results of a survey among building design professionals zsuzsa szalaya,∗, antonín lupíšekb a budapest university of technology and economics, faculty of civil engineering, department of construction materials and technologies, 3 műegyetem rkp, h-1111, budapest, hungary b czech technical university in prague, university centre for energy efficient buildings, třinecká 1024, 252 04 buštěhrad, czechia ∗ corresponding author: szalay.zsuzsa@emk.bme.hu abstract. architects and designers have a critical role in promoting life cycle assessment (lca), a scientific methodology for evaluating the environmental impacts of buildings that can help decarbonise the built environment and minimise other negative effects. this paper presents the results of an international survey conducted among design professionals as part of the iea annex 72 project about assessing life cycle related environmental impacts caused by buildings. twenty-three countries were participating in the survey altogether, but in this paper, only the specific situation in two central european countries, hungary and czechia, are presented. the questionnaire explored the designers’ understanding of environmental problems and lca methodology, the drivers and barriers of environmental assessment and the future perspectives. the results show that many architects and designers are concerned about environmental problems and the built environment’s contribution but have a limited understanding of the applicable scientific methods. a full lca is seldomly applied as neither regulations nor clients demand it, and only the operational energy is mandatorily assessed. further improvements in data quality and assessment tools and regulatory and other drivers are needed to increase the use of lca in the construction sector. keywords: survey, life cycle assessment, building design, architecture. 1. introduction decarbonisation of buildings plays a key role in reaching net zero emission levels by 2050, a global challenge to minimize the effects of climate change [1]. it is well known that buildings are responsible for about 35–40 % of energy use and greenhouse gas emissions [2]. this can be divided into the operation of buildings (28 %) and impacts of the construction industry (11 %) [2], which shows the significance of material usage. life cycle assessment (lca) is a scientific methodology for the evaluation of the environmental impacts of products, which is increasingly applied to buildings, both in scientific research and in practice. however, the widespread application is still hindered by several factors, for example a consistent methodology and a lack of incentives. iea ebc annex 72 is an international research project focusing on the assessment of life cycle related environmental impacts of buildings [3]. the project aims at establishing common methodology guidelines for the environmental assessment of buildings. in the framework of this project, a large survey was conducted across the world to explore the attitudes of designers towards the use of lca, as well as the motivations and barriers. this paper presents the results of the survey focusing on two eastern european countries: hungary and czechia. 2. methodology the paper presents the results of a large international survey with a focus on the findings for hungary and czechia, two countries in eastern europe with similar historical and economic background. in these countries, lca is still mostly applied in academic research and there is an emerging number of projects pursuing green certification rating and conducting lca in connection to this. there is no open national database available. the original survey was compiled and carried out in the framework of the iea ebc annex 72, subtask 1 that focuses on lca methodology development and harmonization. the goal of the survey was to get an overview on the attitude, practices and perceived barriers of building designers in lca and also on some related areas, for example building information modelling (bim) applications. the survey was prepared by a coordinating group in annex 72 with the inputs of the whole annex group as a web-based questionnaire and translated into many national languages. the survey was then distributed all over the world by the annex 72 experts in their respective countries in 2019. the full survey with the list of questions and evaluation will be published in an annex 72 publication (in progress). findings of the general survey and the results for germany and for northern europe can 72 https://doi.org/10.14311/app.2022.38.0072 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 38/2022 attitude towards lca in hungary and czechia . . . figure 1. occupation of respondents (based on 42 (hu) + 18 (cz) = 60 responses). be found in [4–6]. in hungary, the survey was distributed via social media and national organizations, such as the hungarian chamber of architects and the hungarian green building council. altogether 83 respondents (61 + 22) filled the questionnaire in hungary and czechia. the respondents had the option to fill in a full or a short version and it was also possible to skip any question or to quit the survey at any time. this resulted in a different number of answers for each question. this paper presents a selection of the data, focusing on the attitude of designers towards lca, motivations for using it, perceived barriers and further development needs. 3. survey results 3.1. background of respondents figure 1 shows the professional occupation of the respondents. in hungary, the majority were architects and civil engineers (86 % in total) and 10 % other (environmental consultant, construction experts), while in czechia also a high share of the reponses came from civil engineers and architects (56 %), but also lca and bim professionals were represented. the respondents are experienced people, with 26 % of more than 20 years and only 5 % of less than 2 years of professional experience (figure 2). regarding the size of the organisation, the majority (64 %) belong to small offices with 1–9 employees (figure 3). figure 4 illustrates the familiarity with environmental lca as declared by the respondents. nearly half of the respondents have some basic knowledge and about a third a good knowledge, while a quarter of them is not familiar with the subject. according to our opinion, these results show that people open to environmental assessment were overrepresented in this survey, as it is probable that this survey was more attractive to people who are interested in environmental issues. hence, the survey is not representative for eastern european designers, and the results are valid only to this pool of respondents. figure 2. work experience of respondents (based on 62 responses). figure 3. number of employees in the organisation of the respondents (based on 61 responses). figure 4. responses to the question: are you familiar with environmental lca of construction products and buildings? (based on 72 responses). 73 zsuzsa szalay, antonín lupíšek acta polytechnica ctu proceedings figure 5. responses to the question: do you consider requirements and assessment results of environmental performance in your design decisions? (based on 82 responses). figure 6. type of clients that demand services with regard to environmental performance (based on 67 responses) . 3.2. motivations for using environmental assessment and lca environmental requirements and assessment results are regularly taken into account by the majority of the respondents, while in some cases only if the client demands it and pays for it (figure 5). the ratio of projects is still low where a sustainability assessment scheme requires this. please note that these questions refer to environmental assessment in general and not specifically to lca. figure 6 illustrates the type of clients demanding services related to environmental performance. private investors and the public sector seem to drive the demand by primarily or partially requiring environmental assessments (67–74 %), while the demand from individual homebuilders is weaker (54 %). figure 7 shows that motivations for using environmental assessment are different in czechia and hungary: in hungary more than half of the responses consider this out of personal conviction or as a core business value while in czechia these drivers are also important but the reasons are more diverse. an important factor is the client in both countries, while cost or competitive advantage were mentioned less often. in these countries, building regulations today only refer to energy efficiency. environmental assessment is not required by the law and its introduction is not expected in the near future, which is mirrored figure 7. motivation for considering environmental performance assessment results in the design decisions (based on 74 respondents, more than one answer could be selected). figure 8. responses to the question: how would you describe your organisation’s (future) use of lca? (based on 54 responses). in the low number of replies in these categories. as seen from figure 8, 39 % of the respondents already use lca in their practice, 30 % plan to use it in the medium term, while 31 % do not plan its application. there was a large difference between the two countries, as 76 % of the czech respondents already use the method, while only 22 % of the hungarians. please note that this question applies specifically to the method of lca, so the scope was narrower than in the previous questions which referred to any type of environmental performance assessment. 3.3. barriers and further development the perceived barriers of environmental performance assessments are illustrated in figure 9. the largest obstacle is the lack of incentives from both clients and regulations. the respondents think that the lack of information/data and time also hinders the spread of these assessments. additionally, the lack of in-house or external expertise and training are important factors. these results are similar to the findings in the nordic countries where the lack of drivers and information were named the two main barriers [6]. one possibility to increase the uptake of environmental assessments is to introduce legal requirements related to environmental performance assessment. most of the respondents of the questionnaire think that life cycle related carbon footprint and de74 vol. 38/2022 attitude towards lca in hungary and czechia . . . figure 9. perceived barriers of environmental perfomance assessments (based on 72 respondents, more than one answer could be selected). figure 10. responses to the question: from your point of view, should life cycle-related requirements in the area of environmental performance be defined/introduced into building codes and laws in future, if not already the case? (based on 61 respondents, more than one answer could be selected). constructability/recyclability requirements should be defined in building codes and laws and many also support resource efficiency-related requirements (figure 10). a high number of respondents voted for the introduction of all three of these requirement types, while only 15 % think that there is no need for regulations in this field. other incentives would also be possible to reward the use of environmental assessments and increase client demand. harmonization of environmental assessment methods and lca would be important for a more widespread use of these methods. also, communication of results should suit the audience. according to figure 11, clients need user-friendly and straightforward communication forms, such as a label, a few important indicators or one single indicator. on the other hand, many designers prefer a comprehensive list of indicators. a label is also accepted by many of them but showing only one single indicator is not sufficient for most of them. designers have an important role to interpret detailed information into more simplified formats. digitalization is a big challenge in the building industry that will transform both the design and construction of buildings. building information modelling (bim) is increasingly used for different applications. the results of the survey show that deriving the bill of quantities is already used or planned by many designers (figure 12). only a small fraction use bim figure 11. preferred communication format of designers (blue – based on 62 respondents, more than one answer could be selected) and clients (red – based on 61 respondents, more than one answer could be selected). figure 12. responses to the question: do you use bim model’s capability to integrate information on the following aspects? (based on 55 responses). for lca and for cost calculations today, but about 40 % plan to integrate these in their workflow in the future. 4. conclusions this paper presented the results of an international survey on the environmental performace assessment of buildings, carried out in the framework of the iea ebc annex 72 project with a focus on hungary and czechia. although the 83 respondents who accessed the survey were not representative for building professionals in these two countries, some general conclusions could be drawn regarding the current use and future potential of environmental assessment methods and lca, as well as about the barriers hindering the widespread application. most of the respondents already had some knowledge in environmental performance methods and lca and many of them regularly apply these methods in their projects, or plan to do it in the future. most important driver today is the personal conviction of designers about the importance of environmental issues, while the client demand is still low and there is a lack of legal incentives. these factors represent the 75 zsuzsa szalay, antonín lupíšek acta polytechnica ctu proceedings largest barriers. the majority of respondents think that requirements on life cycle related carbon footprint and end-of-life should be introduced in building codes. widespread use of environmental assessment could be triggered by further harmonization of methods and bim integration of lca. this would facilitate the use of these methods in early design stages. acknowledgements we gratefully acknowledge the work put into the international survey by maria balouktsi and martin röck in particular and the annex 72 experts in general. elaboration of the hungarian survey results was supported by project fk 128663 that has been implemented with the support provided from the national research, development and innovation fund of hungary, financed under the fk_18 funding scheme. the czech part of the work has been supported by the ministry of education, youth and sports within project interexcellence no. ltt19022. references [1] un environment, international energy agency. towards a zero-emission, efficient, and resilient buildings and construction sector. global status report 2017. 2017. isbn 978-92-807-3686-1. https://www.worldgbc.org/sites/default/files/ unep%20188_gabc_en%20%28web%29.pdf [2] ipcc. climate change 2021: the physical science basis. contribution of working group i to the sixth assessment report of the intergovernmental panel on climate change. cambridge university press, cambridge, united kingdom and new york, ny, usa, 2021. 2391 p. https://www.ipcc.ch/report/ar6/wg1/ downloads/report/ipcc_ar6_wgi_fullreport.pdf [3] r. frischknecht, h. birgisdottir, c.-u. chae, et al. iea ebc annex 72 assessing life cycle related environmental impacts caused by buildings – targets and tasks. iop conference series: earth and environmental science 323(1):012042, 2019. https://doi.org/10.1088/1755-1315/323/1/012042 [4] m. balouktsi, t. lützkendorf, m. röck, et al. survey results on acceptance and use of life cycle assessment among designers in world regions: iea ebc annex 72. iop conference series: earth and environmental science 588(3):032023, 2020. https://doi.org/10.1088/1755-1315/588/3/032023 [5] t. lützkendorf, m. balouktsi. the level of knowledge, use and acceptance of lca among designers in germany: a contribution to iea ebc annex 72. iop conference series: earth and environmental science 588(4):042046, 2020. https://doi.org/10.1088/1755-1315/588/4/042046 [6] f. n. rasmussen, t. malmqvist, h. birgisdóttir. drivers, barriers and development needs for lca in the nordic building sector – a survey among professionals. iop conference series: earth and environmental science 588(3):032022, 2020. https://doi.org/10.1088/1755-1315/588/3/032022 76 https://www.worldgbc.org/sites/default/files/unep%20188_gabc_en%20%28web%29.pdf https://www.worldgbc.org/sites/default/files/unep%20188_gabc_en%20%28web%29.pdf https://www.ipcc.ch/report/ar6/wg1/downloads/report/ipcc_ar6_wgi_fullreport.pdf https://www.ipcc.ch/report/ar6/wg1/downloads/report/ipcc_ar6_wgi_fullreport.pdf https://doi.org/10.1088/1755-1315/323/1/012042 https://doi.org/10.1088/1755-1315/588/3/032023 https://doi.org/10.1088/1755-1315/588/4/042046 https://doi.org/10.1088/1755-1315/588/3/032022 acta polytechnica ctu proceedings 38:72–76, 2022 1 introduction 2 methodology 3 survey results 3.1 background of respondents 3.2 motivations for using environmental assessment and lca 3.3 barriers and further development 4 conclusions acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.38.0592 acta polytechnica ctu proceedings 38:592–598, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague is it all about the windows? residents’ values in residential heritage buildings freya wise∗, alice moncaster, derek jones the open university, faculty of science, technology, engineering and mathematics, school of engineering and innovation, mk7 6aa, milton keynes, uk ∗ corresponding author: freya.wise@open.ac.uk abstract. reducing energy and associated carbon emissions from the built environment is fundamental to meeting our climate goals. retrofit of existing buildings is therefore a key strategy. heritage buildings present particular challenges for retrofitting because of their traditional construction and need to retain historic values. replacing windows is often a critical element of a low energy retrofit, but for heritage buildings this can be problematic. this paper explores the values that residents invest in their windows, and the opportunities and challenges for retrofitting heritage fenestration. qualitative data from 16 case studies of heritage buildings in the county of cumbria, uk, is examined to gain a greater understanding of residents’ views, which are often neglected in policy approaches. findings show that residents, in both designated and undesignated heritage buildings, value their original windows and appreciate the thermal benefits of traditional features such as shutters or curtains. residents generally considered complete window replacement unacceptable but were more positive about options such as secondary glazing or internal shutters. challenges, including costs and finding skilled tradespeople to produce shutters or undertake restoration were identified. the paper concludes with the implications of these findings for retrofitting heritage fenestration for carbon reduction. keywords: heritage buildings, windows, user values, retrofit. 1. introduction it has long been accepted that the built environment is a major source of energy use and resultant carbon emissions, and that these must be urgently reduced to help mitigate climate change [1]. the average rate of building stock replacement across europe is around 1 % per year [2]: retrofitting existing buildings is therefore a key carbon reduction strategy [3]. heritage buildings can be defined by age, planning policy designation, construction type, or specific values [4]. the uk has the oldest housing stock in europe [5] with around 20–30 % of uk buildings likely to have heritage value, although only a small percentage are officially designated as such [6]. the heritage values and traditional, often regionally specific, construction of heritage buildings can present challenges for energy retrofitting, with tension apparent between adaptions for carbon reduction and the retention of heritage [7]. windows are often a focus of retrofit efforts to improve thermal performance and reduce undesirable infiltration in older buildings. however residents value windows for more than just their functionality [8]. the replacement and retrofit of original windows is often a particular source of tension, in both planning policies [9] and to residents [10] and the replacement of original single glazing with modern double glazing is often prohibited in designated heritage buildings [11]. research in this area has tended to focus on planning interpretations of acceptable changes to heritage buildings, such as a study of uk conservation planning professionals’ approach to slim-line double glazing [9], which identified inconsistency both between individual planners and between different uk regions. policy designations are often taken as the arbiter of acceptability for retrofits [12, 13], and the heritage values that residents invest in their buildings tend to be overlooked [14, 15]. this can lead to the assumption that if window replacement is acceptable in planning policy, then it can, and will, be enacted. however, because owner occupied domestic retrofit is predominantly instigated, managed, and funded by homeowners, their views will clearly determine retrofit uptake. residents have been shown to invest heritage values in their buildings, including those that are not designated [16], and these values have significant implications for the retrofits they consider acceptable and will therefore enact [10, 17]. this paper examines the values that residents invest in their windows, the types of fenestration retrofits that they might consider acceptable, and any barriers that they identify to these improvements, through 16 case studies of residential heritage buildings. 2. methodology 16 residential heritage buildings in cumbria, northwest england, were recruited as case studies from previous respondents to a survey of pre-1940 cumbrian buildings [10]. cumbria is a predominantly rural, mountainous, and upland area. it includes the lake district national park cultural landscape 592 https://doi.org/10.14311/app.2022.38.0592 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 38/2022 is it all about the windows? residents’ values . . . figure 1. original windows. world heritage site which has additional planning restrictions. the number of case studies was chosen to be sufficient to provide a diverse range of building types, ages, locations, household compositions and energy usage, following principles of maximum variation selection [18]. the buildings chosen were representative of wider cumbrian heritage buildings but are not representative of the broader uk heritage stock. however, the case studies’ rich data can still provide learning applicable to a wide range of heritage buildings across the uk and europe. occupancy ranged from one to five residents. six buildings were detached, six semi-detached and four terraced, with floor areas varying from 67–373 m2 with a mean size of 144 m2. all buildings were stone built, mainly of limestone and sandstone, but with some of slate, granite, and fieldstone. building ages ranged from pre-1700–1928; two were grade ii and one grade ii* listed (the third and second highest levels of uk heritage protection respectively), six were in protected conservation areas, and seven had no planning designations. for windows, two of the buildings had double glazed upvc windows, two had double glazed timber, one had replica single glazing and ten retained at least some original glazing. the development of the case studies involved a site visit including semi-structured interviews, and building surveys, this paper reports part of a larger study as described in [6]. interviews covered: the building’s history; residents’ attitudes to carbon reduction and heritage retention; their energy behaviours; perceptions of indoor environmental quality; and perceived barriers to carbon reduction. participants also completed a questionnaire on the acceptability of various retrofits, including eight relating to fenestration; providing a commentary on their reasoning and key barriers. participants then led a “tour” of their building, identifying heritage features they valued. the visits were audio recorded, transcribed and analysed, and, with residents’ permission, photographs were taken of various features. the retrofit questionnaire and interview transcripts were checked with residents. most visits took place in early 2020, but as a result of the covid-19 pandemic, visits to case studies (cs) 4, 10, 12 and 16 were conducted virtually in january and february 2021. for these cases, residents were still able to lead a tour of most of their buildings with portable devices, although broadband in some rooms was problematic. ethical approval for the research was obtained. 3. results 3.1. values for windows it was clear that all residents with original windows strongly valued them, identifying them as features they particularly liked about their buildings: • cs3: “i’m unashamedly timber, sash windows and things like that . . . on the front, they are leaded. i really like the leaded windows a lot from both inside and out . . . that reflection of the trees in the leads, i love, i think it’s just absolutely . . . ” (building age: 1928, conservation area (ca)) • cs9: “there are just so many things that make it, not unique but special, you know, the windows, the beautiful window lights and the beautiful windows . . . it’s wonderful, wobbly glass . . . i like the fact that i can look at it from different angles and see a different shape outside.” (1896, ca) • cs14: “i love those windows and i think they are absolutely gorgeous.” (mid-1700s, undesignated (ud)). it was noticeable that the windows that participants valued were all quite different in style and form, and that buildings varied in age and designation, and yet all were appreciated by residents (figure 1). residents with traditional window additions such as original internal shutters and thick, heavy curtains, 593 f. wise, a. moncaster, d. jones acta polytechnica ctu proceedings figure 2. internal shutters and secondary glazing in different cases. figure 3. acceptability of fenestration retrofits. identified that they felt that these provided significant thermal benefits. • cs1: “we’ve got functioning shutters in two of the downstairs rooms which is great . . . they make a massive difference.” (1820, grade ii (gii)) • cs7: “they’re not thermal curtains but they’re heavy and lined . . . the rooms do get warmer quicker [when we close them]” (1789, ud) • cs14: “they [the shutters] haven’t got locking bars on them so they don’t sit flat but they do make a remarkable amount of difference.” (mid-1700s, ud) (figure 2) some residents considered these features so useful that they installed similar modern shutters to additional rooms, with cs6 adding shutters to their bathroom despite having modern double glazing. in addition, some residents also appreciated the thermal comfort offered by shutters in warm weather. • cs6: “it was one thing that we wanted to do, to get them [the shutters] back into operation . . . you’re looking at quite thick timber there, and ok, they don’t meet tightly, but certainly they are effective. we also put shutters in the bathroom.” (c.1700 ca) (figure 2) • cs16: “the house is brilliant actually, so when it was really, really hot in the summer you could close the shutters and yeah . . . you can stay inside and be fairly comfortable.” (c1800 ud) 3.2. acceptability of fenestration retrofits and barriers to retrofitting residents’ combined responses for fenestration retrofit options are shown in figure 3 (note that some cases had multiple window types so the total for some categories is higher than 16). the proportion of the cases for each category that are undesignated (ud) is also shown, suggesting that there is little difference in the acceptability of measures between the nine designated, and seven undesignated, cases. it can be seen that the most acceptable options were those that were additions to existing windows rather than complete window replacement. the option that the most residents already had was thermal curtains (10), followed by modern, timber windows (8, including partial); these were mostly double glazed, although some were single glazed replicas. the majority found upvc and aluminium double glazing unacceptable, something which participants took the opportunity to emphasise in the interviews: • cs2: “it’s the absolute antithesis of what we’ve been talking about. . . i’m dead set against upvc.” (c.1740, grade ii*) • cs5: “we try to maintain that [the character and heritage value] by not putting in horrible plastic windows or whatever it might be! (1897, ud)”. • cs12: “plastic windows everywhere and the wrong proportions simply because that’s what the fitters fit. it just destroys the character of buildings . . . ” (c.1700 ca). 594 vol. 38/2022 is it all about the windows? residents’ values . . . participants mainly viewed internal shutters positively, especially those who already had experience of them. external shutters, however, were universally considered inappropriate for cumbrian heritage buildings, because participants felt that they did not fit the character or architectural style of the area. • cs4: “exterior shutters would be too continental here.” (c1850, ud) • cs11: “exterior shutters wouldn’t look right on this.” (c1760, ud) • cs16: “i don’t think we’d do that, it’s very different in cumbria, you’d be doing for the cold wouldn’t you, like in the south of france, everywhere has shutters . . . ” (c1800, ud). some participants however were unsure about interior shutters, although the reasons varied: cs3 was concerned that they might not match the “spare” style of the house; cs10 was concerned about ease of use; and cs13 felt they would suit the building well but was worried that privacy would be reduced as the current blinds could be angled to provide light and privacy while shutters would have to be fully open to allow daylight in. cs8, meanwhile, felt that interior shutters would be an excellent idea but: • “you have to have built them in, in my opinion, and whether nowadays, for a sensible price you can find a carpenter who can put them in, i have some doubts about that.” (1871, ca). residents considered secondary glazing to have high potential, with only two who would not consider it. even cs15 thought secondary glazing might be an effective improvement to reduce traffic noise through his poor quality upvc double glazing. some concerns were revealed in the interviews, however, cs2 felt that secondary glazing would be “warmer but less aesthetically pleasing” as an addition to their original sash windows. cs3 and cs9 were both concerned about the impact on the character of their original glass, with cs3 concluding against and cs9 tentatively in favour: • cs3: “an architect friend pointed out what i think i’d already intuited anyway, which is that when you look at leaded windows from a distance, or indeed anything that isn’t float glass, it looks different and as soon as you put a piece of glass on it looks like a piece of glass. . . ” (1928, ca) • cs9: “it depends on how it would look across the front window, it might be all right, but we’d need more information to decide” (1896 ca). cs5 and cs8 meanwhile were concerned about the impact of secondary glazing on ventilation. cs5 had installed homemade fixed secondary glazing on one façade but felt that if further secondary glazing was installed it would need to be openable to allow for ventilation which would increase costs, while cs8 already had secondary glazing at the front of the house but were concerned about reducing the air supply for their woodburning stove if it was installed on the rear sitting room windows. both were in favour of secondary glazing but felt it needed some thought. cs14 would not consider secondary glazing, feeling that, because their house suffered from moisture issues, any reduction in infiltration would be detrimental. instead of window replacement or additions cs14 preferred window refurbishment but was concerned about finding appropriately skilled craftspeople. • cs14: “i would get the windows refurbished, they worry me . . . they do need work and i would like it to be done by someone very careful and um, skilled at handling, some of the glass is very old indeed. the problem is, the people who really know their stuff tend to work in the really, really spectacular buildings, so you wouldn’t get them for a near nothing building like this . . . but if we get those windows redone, i want them done really well.” (mid-1700s, ud). residents of listed building meanwhile, identified difficulties or expected difficulties with planning restrictions in considering window alterations. • cs1: “when we moved in, the upstairs [rear] windows were not very nice, not in very good condition, [1970s] transept windows, not in keeping with the house, and the process of getting them through planning, because the house had been listed with those windows in, was actually quite tricky and long winded and expensive . . . they did not like the fact that we wanted to put double glazed sashes in.” (1820, gii). • cs2: “[replacement windows] no, the national park wouldn’t allow it . . . secondary glazing . . . it is a possible option that the [heritage] trust and the national park would accept . . . i know that it would have to go through planning.” (1740, gii*) • cs13: “i think we’d probably try for it [slim line double glazing] i’m not sure we’d get it, but yeah, if we got them done again, particularly the upstairs one which is obviously less visible so we might get away with that . . . it’s just so tall, it looks lovely but (laughter).” (1834, gii). (currently has replica single glazed sashes installed by previous occupants in the 1990s). several participants also highlighted the cost of window retrofits, especially if they were looking for something specialised or unusual. residents generally appeared willing to pay significant amounts for options that were acceptable to their heritage values but some lacked the capital to be able to do so. • cs1: “secondary glazing, yes, expensive, we had a quote to replace everything, it was over £4 000 . . . and that’s for three windows!” (1820, gii) 595 f. wise, a. moncaster, d. jones acta polytechnica ctu proceedings • cs3: “i think it [secondary glazing] was more than a £1 000 a piece, it was quite outrageous . . . ” (1928, ca). • cs9: “it [hardwood replica sash in modern extension] was quite eye wateringly expensive, but, you know! so yes, if we can afford it, yes, we have the right thing done but it’s a question of money really.” (1896, ca). • cs12: “we didn’t want to replace them even though they are only single glazed, so i spent a huge amount of money getting designed and built, the folding glazed shutter in the living room . . . i suppose our windows are triple glazed . . . the glazing bars line up, so that you don’t have glazing bars in front of your windows.” (c1700, ca). (figure 2) 4. discussion this research showed that participants in buildings of varying ages, styles, and levels of heritage designation, including undesignated buildings, clearly value their original windows. residents especially valued the character of traditionally made “wavy” glass, and also appreciated original window features such as shutters which they recognised as having thermal benefits as well as heritage values. these findings agree with research highlighting windows as features of value to residents [16, 17] and is supported by the previous survey work which identified that, out of 147 cumbrian participants, 43 % retained at least some of their original windows, and that 13 % of respondents specifically noted the importance of their original windows (again covering a wide range of types and styles) [10]. residents had clear views on the types of fenestration retrofits they would consider acceptable. complete window replacement with timber double glazing was not acceptable to many residents and modern materials such as upvc and aluminium were almost completely unacceptable; once again this was true in both designated and undesignated buildings. alternative options, such as secondary glazing and traditional window additions such as shutters and thermal curtains, were viewed more positively by the majority of residents, although there were still concerns about the impact of secondary glazing on the character of original glass. interior shutters were considered acceptable or potentially acceptable to many residents. however exterior shutters were universally regarded as inappropriate for cumbrian heritage buildings. this finding highlights the importance of local context when considering potential retrofits, as what may be seen as appropriate in one area may be unacceptable in another. other research has identified the energy savings that traditional window alterations can have, with secondary glazing and interior shutters being shown to reduce the heat loss from sash windows by 63– 73 % [11]. these types of changes have less impact on heritage values, appear more acceptable to residents and may even have the potential to enhance or restore heritage values. they may also have better lifecycle carbon impacts than full window replacement [19]. lifecycle carbon includes both the embodied carbon costs required for raw materials, manufacture, transport, installation and end of life disposal, as well as the operational carbon savings that a retrofit may produce [20]. some retrofit options which reduce operational carbon may nevertheless increase lifecycle carbon [19, 20]; this offers a further benefit for existing windows additions, as they are likely to have lower embodied carbon than window replacement. participants identified several barriers to making fenestration alterations. key themes were: the cost of changes; obtaining planning permission; and finding suitably skilled tradespeople to undertake the work. in many cases these issues were exacerbated when residents tried to apply measures that were more in keeping with their heritage values. residents are likely to require support with these challenges if significant progress is to be made, with both financing and a lack of skilled tradespeople identified as important barriers both from the previous survey [10] and at a european level [15]. in addition to these main barriers, residents also identified individual circumstances that might affect the acceptability and appropriateness of retrofits, such as concern about privacy or maintaining ventilation. these findings suggest that retrofit approaches for heritage buildings should consider window additions rather than replacements, because they are likely to be more acceptable to residents, while also having clear potential for carbon savings. greater awareness of the effect that residents’ values can have on the changes that they will enact is needed and it should not be assumed that window replacement will be acceptable merely because a building is undesignated. barriers such as cost and availability of skilled tradespeople also need to be addressed to achieve high levels of retrofit, both for windows and more broadly. these findings emphasise the need for a holistic approach considering both residents and buildings as interconnected systems [6] and accounting for specific contexts to design appropriate whole house solutions rather than applying individual measures in a vacuum. further research is currently underway to quantitively assess, via modelling and lifecycle assessment, the potential lifecycle carbon impact to the case studies of a range of retrofit measures, including those considered above. 5. conclusions this study has examined the importance of historic windows to the residents of 16 heritage building case studies in northwest england. residents of a wide range of building types and ages were found to invest significant heritage value in their original windows, including residents in undesignated older buildings. 596 vol. 38/2022 is it all about the windows? residents’ values . . . residents generally considered complete window replacement to be unacceptable to their heritage values, especially upvc replacement windows. however, less invasive options such as secondary glazing, thermal curtains and interiors shutters were considered more acceptable. these options may have significant potential to reduce carbon by reducing heat loss from windows in older buildings, especially if lifecycle, rather than just operational, carbon is considered. residents identified barriers to retrofitting such as excessive costs, lack of skilled tradespeople and obtaining planning permission, as well some concerns about the need to maintain ventilation pathways, especially in buildings with moisture challenges. there is a need for support for residents to negotiate these challenges, and these findings highlight the need to consider specific options as part of a holistic approach to building retrofitting that accounts for both the condition of the building and residents’ values. the other key finding is that residents in undesignated buildings also invest heritage values in their historic windows which effect the acceptability of retrofits. it is therefore necessary to consider heritage sensitive measures in a wider range of buildings than just those which are officially designated, if carbon reduction efforts are to succeed at the required scale. acknowledgements we gratefully acknowledge the input of the case study participants in this research, the underlying data cannot be published due to the need to retain participant anonymity. this research is part of a project funded by the uk arts and humanities research council, grant number: ah/l503770/1. references [1] global alliance for buildings and construction, international energy agency, united nations environment program. 2019 global status report for buildings and construction: towards a 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https://doi.org/10.1016/j.buildenv.2011.08.001 https://doi.org/10.1088/1755-1315/329/1/012002 acta polytechnica ctu proceedings 38:592–598, 2022 1 introduction 2 methodology 3 results 3.1 values for windows 3.2 acceptability of fenestration retrofits and barriers to retrofitting 4 discussion 5 conclusions acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.38.0559 acta polytechnica ctu proceedings 38:559–565, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague a methodological approach for life cycle assessment of refurbishment measures – from building to neighbourhood and municipal level simon slabik∗, caya zernicke, michael storck, annette hafner ruhr-university bochum, resource efficient building, department of civil and environmental engineering, universitätsstraße 150, 44801 bochum, germany ∗ corresponding author: simon.slabik@rub.de abstract. life cycle assessment (lca) in the building sector has become a widely used method for quantifying environmental impacts of a building over its entire life cycle. currently, however, no standardized procedure exists for considering refurbishment measures. in addition, the lca, according to the european standard en 15978, is limited to building level only. this paper shows how a methodological lca framework of refurbishment methods can be applied at three levels: single building, neighbourhood, and municipality. initially, the proposed methodological approaches are introduced while the framework rules are defined for each of the three levels. the system boundaries of the lca differ regarding the assessment levels within the given methodology and are adapted accordingly. in addition, the three levels of assessment are defined by the accuracy of measurement results, and data requirements, as well as by the specific value of the calculations and the ownership of the building stock. the assessment levels provide a specific quality and quantity of environmental indicator results. thus, the complex interrelationships of the assessment levels are shown. the developed framework for the environmental assessment of refurbishment measures provides comparability at the building level. at the neighbourhood level, emissions from refurbishment measures are compared with the reductions of emissions through heating energy demand. ultimately, the potential of refurbishment measures at the municipal level can be identified on a large scale and used as a decision-making tool. keywords: life cycle assessment, lca framework, refurbishment, neighbourhood level, municipal level. 1. introduction the german federal government has declared its commitment to achieving greenhouse gas (ghg) neutrality by the year 2045. a gradual reduction of ghg emissions of 65 % by 2030 and 88 % by 2040, compared to the reference year 1990, is mandatory. a corresponding amendment to the national climate protection act has been made. for the building sector, the reduction targets for the next years have been adapted. around 80 % of the buildings that will constitute the building stock in europe in 2050 already exist today [1]. the energetic standard of the buildings varies considerably. about one-third of the final energy demand is assigned to the building sector [2]. decarbonizing the energy stock has become the focus of policy efforts – not only at the national level. in germany, the annual energetic refurbishment rate is around 1 %, although individual measures dominate [3]. however, to achieve the federal government’s goals, it will be essential to increase this quota in the next years. considering the large discrepancy within the energy demand values and the currently increasing renovation activity, energetic refurbishment measures in the building sector will make a significant contribution to climate-relevant effects. given the circumstances, a holistic approach is required to implement sustainable refurbishment measures. lca offers an appropriate instrument; however, there is a current lack of uniform standards and parameters for the assessment of refurbishment measures. 1.1. life cycle assessment of buildings lca quantifies potential environmental impacts of products or services. in a holistic analysis, the input and output flows are evaluated over the entire life cycle. the general assessment regulations for lca are specified in en iso 14040:2020 [4] and en iso 14044:2020 [5]. for buildings specifically, en 15978:2011 [6] sets out additional requirements. one of the main focuses of the lca of buildings is to cover the entire life cycle. starting with production and construction (module a) and continuing through the use phase (module b) to disposal (module c), all life cycle phases according to en 15804:2019 [7] are considered. outside the system boundary, benefits from recycling or reuse are considered in module d. figure 1 shows the life cycle stages and further subdivisions for building lcas. 1.2. life cycle assessment of refurbishment measures in the literature, no consensus exists on the specific application of lcas of refurbishment measures. cabeza 559 https://doi.org/10.14311/app.2022.38.0559 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en s. slabik, c. zernicke, m. storck, a. hafner acta polytechnica ctu proceedings figure 1. life cycle stages for buildings [7]. et al. [8] reported that only few studies have evaluated actual building refurbishments in terms of lca. according to a literature review by vilches et al. [9], the interpretation of the system boundaries is the main difference between existing studies. the inconsistent system boundaries prevent comparative assessments from being made. the updated version din pren 15978:2021 [10], currently still in the draft phase, sets out calculation rules for refurbishment at building level in a normative framework, and which are similar to the rules described by hafner and storck [11]. hence, in this paper, we propose a consistent approach to lca of refurbishment measures and extend the assessment framework from the building to the neighbourhood and to the municipal level. when examining the lca of refurbishment measures, an initial definition is crucial. by refurbishment measures we consider the structural substance of the building envelope, as well as the buildings’ technical installations, to have been modified. thus, structural and material changes are required to be carried out on the facade, the windows or doors, the roof or the top ceiling of the building or the basement ceiling. moreover, replacing the building’s cooling or heating systems is also considered an energetic refurbishment. in accordance with en 15978:2011 [6], the production and transport of all new components, as well as all required construction measures and waste management of the removed components, are also included. when examining the lca of refurbishment measures, an initial definition is crucial. by refurbishment measures we consider the structural substance of the building envelope, as well as the buildings’ technical installations, to have been modified. thus, structural and material changes are required to be carried out on the facade, the windows or doors, the roof or the top ceiling of the building or the basement ceiling. moreover, replacing the building’s cooling or heating systems is also considered an energetic refurbishment. in accordance with en 15978:2011 [6], the production and transport of all new components, as well as all required construction measures and waste management of the removed components, are also included. 2. methodological approach for lca frameworks with consideration of the research gaps and the presented relevance of refurbishment measures, this chapter provides essential boundary conditions and concrete instructions for the implementation of lcas of refurbishment measures. figure 2 shows the overall lca framework approach for all three levels. the following sections will refer to the corresponding illustration in this figure. refurbishment measures are the subject of the investigation, thus the production of all newly introduced building materials (modules a1–3) and the deconstruction (modules c1–4) required is considered. thereby, the implementation point is within the study period, but not necessarily at the beginning. as seen in figure 2, replacement cycles are accounted for the existing building components (modules b2–4 stock) over the assessment period and for the newly installed structures (modules b2–4 new components) after the refurbishment event. the operational use of energy (module b6) is covered similarly. 2.1. building level the refurbishment of buildings is a useful way of saving heating energy as well as extending the life cycle of a building. lcas of refurbishments calculate the potential environmental impact savings through heating energy reduction, as well as the environmental impact of material choice. the system boundaries for lca of refurbishments are similar to the case of a new building. the functional unit is defined as 1 m2 gross external area (gea) of the entire building. within the methodology, it is assumed that the life cycle is extended by 50 years through an energetic refurbishment. to calculate environmental impacts, all components used in the building are determined and connected with environmental product declarations. the refurbishment of a building causes that different building components from different construction periods exist in the building, such as components being removed, new components and remaining components. during the refurbishment, components are removed from the building (e.g. old windows) and are taken into 560 vol. 38/2022 a methodological approach for life cycle assessment . . . figure 2. lca framework for the different level approaches. 561 s. slabik, c. zernicke, m. storck, a. hafner acta polytechnica ctu proceedings account in module c1–4(0) at time zero, shown in figure 2 (grey arrow c0). an exception for this is the carbon content of materials, which is released from the materials in module c. this content should not be accounted for the global warming potential result module of c1–4(0), as it leads to a skewing of the results. all new components added to the building, such as insulation materials or new heating systems, are calculated in module a1–3(0) (blue arrow a0), as in the case of a new building. in addition, there are building components that are not replaced during the refurbishment. the use stage of the building is described by the operational energy use (module b6) as well as the replacement of building components according to their service life (module b2–4), and these are shown as longer arrows in figure 2. for building components that exist in the building and are not replaced during the measure, the replacement cycles are assumed to be based on regular replacement and the year of construction is chosen as the reference value. the newly installed parts are replaced according to regular maintenance cycles, as in the case of new buildings in module b2–4. the reduced operational energy use resulting from the refurbishment measure is calculated further over the life cycle (module b6). at the end of the extended life cycle, it is assumed that all components, from existing components and components installed during the refurbishment, are dismantled, and calculated in module c1–4(50) (grey arrow c50). by calculating lca results of individual buildings, the savings of environmental impacts at the building material level, as well as from an energy perspective, can be presented. different refurbishment options can be compared regarding to the resulting savings, such as environmental or energetic for each building. 2.2. neighbourhood level the neighbourhood has become a crucial perspective for urban planning and the implementation of urban transition projects. benefits of neighbourhood-level implementation range from joint energy supply, for example through district heating systems, to linked mobility concepts and shared facilities. this paper focuses on existing neighbourhoods to assess refurbishment measures holistically using lcas. however, no uniform approach for lca has been established for either new or existing neighbourhoods and all approaches taken have differed considerably [12]. initially, the neighbourhood investigated must be spatially identified and delimited as such. three criteria are decisive for spatial coverage: geographical boundaries, land use and building and settlement structure. the lca system boundary considers all buildings that are subject to refurbishment measures in the defined area. moreover, the assessment period is 50 years, and the functional equivalent is defined as 1 m2 gea. within the methodology for neighbourhood lca, a distinction must be made between 4 four scenarios, as shown in figure 2. scenario 1 represents an untouched building on which no measures are carried out. therefore, it is not part of the assessment framework. in scenario 2, a refurbishment measure is carried out at a time x. the operational energy use before the measurement b6(0−x) decreases subsequently and results in b6(x−50). similarly, the replacement cycles in modules b2–4(0−x) before the measurement and b2–4(x−50) afterwards, are differentiated. the replacement cycles of the building structures of the existing building stock are covered separately and extend over the entire observation period. impacts in module a1–3(x) and module c1–4(x) are determined specific to the time x. in analogy, scenario 3 is to be considered, where demolition and subsequent new construction is implemented at a time x in the neighbourhood. finally, scenario 4 describes a new building that is constructed as a densification at a given time x, and without demolition having to take place beforehand. the assessment period is set to a time frame of 50 years which compensates the life cycle of an individual building and does not have to necessarily end with the demolition of the buildings. thus, modules c1–4(50) and d(50) will not be considered in the lca framework after the 50 years at the neighbourhood level. to model the entire building stock in the neighbourhood, the buildings are clustered based on building typology-specific criteria, such as building age, type of use or typical building constructions. accurate planning documents are necessary for the life cycle inventory, which are frequently not available in sufficient quality, especially in existing buildings. thus, proxy buildings of the clusters are identified with planning documents of sufficient data quality and representative character. different scenarios can be carried out in one cluster, allowing multiple proxy buildings to be designated. a building-specific life cycle inventory (lci) is then conducted for all proxy buildings, followed by an impact assessment combining lci data and environmental impacts. to extrapolate the results to the entire neighbourhood, the building-specific data must first be related to the gea of the proxy building. afterwards, the obtained value is multiplied by the total gea of the cluster. the sum of the clusters presents the environmentally relevant influences of the neighbourhood. 2.3. municipal level municipal self-governance is one of the important elements of the political structure of the federal republic of germany. nowadays, municipal self-governance has a key role in the increasingly diverse interaction of processes. a municipality is authorized to a certain level of autonomy in terms of statutes, finances, planning and execution. issues of municipal policy include, especially recently, the protection of the environment 562 vol. 38/2022 a methodological approach for life cycle assessment . . . figure 3. context of the different level approaches. and natural resources [13, 14]. regarding the building sector, refurbishment can be identified as a municipal climateand environmentally friendly approach to reduce the operational energy consumption. additionally, in public building projects and their tendering, environmental aspects will be given greater prominence. for this, lca of refurbishment measures can provide the data basis for municipal funding [15]. the size of the municipality does not play a role in the methodology, since the lca of refurbishment measures is realized using extrapolations. as a bottom-up approach, lca are carried out for an assembly of building components, which are also the objects of assessment. in this level, the lca system boundary is limited to the exterior envelope components of a building, highlighted colourful in figure 2. the lca is carried out in the functional equivalent in 1 m2 gea. the reference study period will remain 50 years, regardless of the actual date of the refurbishment, see figure 2. different building types were considered, which can differ both in terms of use and year of construction, expressed in the figure 2, by the colour subdivision of the building icons. starting at year 0 of the assessment period, the modules b2–4(0−x) and b6(0−x) of the exterior envelope components of a building are included in the lca framework. as the refurbishment is realized in year x, further life cycle stages will be added: for the exterior envelope components, which will be removed within the measure, the modules c1–4(x) are included in the lca. for the added new components, the modules a1–3(x) are calculated in an analogous manner. independently from the refurbishment measure, the modules b2–4(x−50) of the existing components remained in situ will continue. now, the modules b2–4(x−50) for the newly installed exterior envelope components will be additionally included. the impact of operational energy consumption (b6(x−50)) remains part of the lca after the refurbishment but will be reduced to take account of the consequences of the refurbishment. after the assessment period of 50 years, it is assumed that the investigated municipality will not be demolished, and the existing buildings will remain. therefore, consideration of modules c1– 4(50) and d(50) in the framework is not applied at the municipal level, analogously with the neighbourhood level. for the data basis of the lca framework of refurbishing the exterior envelope of the municipal buildings, a building typology can be used. it can contain data on different types of buildings and years of construction, in addition to information on the structure of the building envelope before and after refurbishment measures. using this information, the life cycle inventory of the exterior envelope can be performed within the lca as described. by using a geographic information system, the calculations can be extrapolated over the functional equivalent of 1 m2 gea for the whole municipality. 3. comparison the correlation of the levels for which lca methodologies are proposed is presented in figure 3. in our framework, the municipality is the largest level of assessment. moreover, various neighbourhoods form an urban structure, which is why this level represents the meso level. finally, the building level is the foundation for the proposed approaches and constitutes the smallest level. as the scale decreases, the level of consideration becomes smaller, and the data requirement increases. assumptions must be made for the municipal analysis, as certain data is no longer available at this level, or it would not be practical to collect missing data for the 563 s. slabik, c. zernicke, m. storck, a. hafner acta polytechnica ctu proceedings framework building neighbourhood municipality object of assessment building proxy building assembly of building components functional equivalent 1 m2 gea 1 m2 gea 1 m2 gea assessment period 50 years 50 years 50 years system boundaries building building exterior building components approximation building specific extrapolation via proxy buildings in clusters extrapolation via building typology life-cycle stages b2–4(0−x) components in situ components in situ b6(0−x) operational energy use before measurement operational energy use before measurement a1–3(x)∗ components newly installed components newly installed components newly installed c1–4(x)∗ components demolished in situ components demolished in situ components demolished in situ b2–4(x−50)∗ components newly installed + retained components in situ components newly installed + retained components in situ components newly installed + retained components in situ b6(x−50)∗ operational energy use operational energy use operational energy use c1–4(50) all building components d(x) informative optional optional ∗ for building level: x = 0 years. table 1. comparative overview of the different lca approaches. entire municipality. in the neighbourhood, buildingspecific data can be collected. collaboration with stakeholders or companies is particularly conducive to data acquisition. highest quality data is required at the building level. due to the different data requirements, the accuracy of the measured variables is affected. for the evaluation of potentials offering refurbishment measures, the municipal level requires less precise data quality. at the neighbourhood and building level, the accuracy of the results increases in line with the increasing availability of data. however, the bottom-up approach ensures that losses in calculation accuracy are limited. the functional equivalent for all approaches is 1 m2 gea and the assessment period is 50 years. both the object of assessment and the system boundaries vary. specific buildings are assessed at the building and neighbourhood level. at the municipal level, in contrast, components of the exterior building envelope are examined. based on these evaluations, and with consideration of special building types, approximations are possible for the neighbourhood and the municipality. upward extrapolation is a central element of the methodologies. at neighbourhood and municipality level, several measurements can be implemented at the same time or with a time lag, which is why the considered life cycle stages must be differentiated from the building specific lca. at the building level, a measurement takes place at the beginning of the assessment period. thus, there is no calculation of the modules b2–4 and b6 before the refurbishment. at neighbourhood and municipality level, however, the operational phases before and after the measurement must be examined differently. moreover, only at the building level, the demolition of the entire building is included in module c1–4(50) after the end of the period of study. table 1 shows the key differences in the frameworks and life cycle stages of the approaches presented. 4. conclusion the proposed methodologies are to contribute holistically to the sustainable transformation of the building stock. through the different levels of assessment, decision-making tools are provided for different stakeholders and authorities, which enables the evaluation 564 vol. 38/2022 a methodological approach for life cycle assessment . . . of the environmental quality of refurbishment measures. therefore, it is essential that energetic refurbishment measures are implemented on a large scale to achieve the climate protection goals set. increasing the refurbishment rate is equally important. the three levels allow for broad applicability based on a coherent methodology. at this stage, the individual methodologies require evaluation through practical application on sample objects. subsequently, data can be determined that will serve for the creation of benchmarks and likewise as a comparison for further studies. in addition, the influence of different materials used during refurbishment measures must be further investigated. as hafner et al. [16] have already shown, renewable materials appear beneficial. particularly at a larger scale in the built environment, carbon storage through enabling technologies in and around the building must be considered. to meet climate protection targets, it is necessary to use existing technologies besides refurbishment measures to reduce ghg emissions and, simultaneously, to store and sequester carbon [17]. acknowledgements this work was funded by the north-rhine westphalian ministry of culture and science within the project “sustainable energy systems in neighbourhoods”, on the basis of the project “holzbau-gis: reduction of greenhouse gas emissions by building and renovating with wood” funded by the federal ministry of food and agriculture (bmel), federal ministry for the environment, nature conservation and nuclear safety, waldklimafonds and on the basis of the project “wood in vertical building extensions – assessment and use of timber in vertical building extensions” funded by bmel. the authors gratefully acknowledge the assistance of all concerned. references [1] the royal academy of engineering. engineering a low carbon built environment: the discipline of building engineering physics. the royal academy of engineering, london, 2010. [2] l. pérez-lombard, j. ortiz, c. pout. a review on buildings energy consumption information. energy and buildings 40(3):394–398, 2008. https://doi.org/10.1016/j.enbuild.2007.03.007 [3] s. rein. datenbasis zum gebäudebestand: zur notwendigkeit eines besseren informationsstandes über die wohnund nichtwohngebäude in deutschland. bundesinstitut für bau-, stadtund raumforschung (bbsr) im bundesamt für bauwesen und raumordnung (bbr), bonn, 2016. [4] din deutsches institut für normung e. v. environmental management – life cycle assessment – principles and framework (en iso 14040:2006 + a1:2020), beuth verlag gmbh, berlin, 2021. https://doi.org/10.31030/3179655 [5] din deutsches institut für normung e. v. environmental management – life cycle assessment – requirements and guidelines (en iso 14044:2006 + a1:2018 + a2:2020), beuth verlag gmbh, berlin, 2020. https://doi.org/10.31030/3179656 [6] din deutsches institut für normung e. v. sustainability of construction works – assessment of environmental performance of buildings – calculation method (en 15978:2011), beuth verlag gmbh, berlin, 2012. https://doi.org/10.31030/1917049 [7] din deutsches institut für normung e. v. sustainability of construction works – environmental product declarations – core rules for the product category of construction (en 15804:2012+a2:2019), beuth verlag gmbh, berlin, 2020. https://doi.org/10.31030/3054172 [8] l. f. cabeza, l. rincón, v. vilariño, et al. life cycle assessment (lca) and life cycle energy analysis (lcea) of buildings and the building sector: a review. renewable and sustainable energy reviews 29:394–416, 2014. https://doi.org/10.1016/j.rser.2013.08.037 [9] a. vilches, a. garcia-martinez, b. sanchez-montañes. life cycle assessment (lca) of building refurbishment: a literature review. energy and buildings 135:286–301, 2017. https://doi.org/10.1016/j.enbuild.2016.11.042 [10] din deutsches institut für normung e. v. sustainability of construction works – methodology for the assessment of performance of buildings – part 1: environmental performance; german and english version (pren 15978-1:2021), 2021. beuth verlag gmbh, berlin. https://doi.org/10.31030/3251250 [11] a. hafner, m. storck. life cycle analysis of vertical building extensions – environmental impacts of different material selection. iop conference series: earth and environmental science 290(1):012046, 2019. https://doi.org/10.1088/1755-1315/290/1/012046 [12] m. lotteau, p. loubet, m. pousse, et al. critical review of life cycle assessment (lca) for the built environment at the neighborhood scale. building and environment 93:165–178, 2015. https://doi.org/10.1016/j.buildenv.2015.06.029 [13] p.-g. albrecht. einführung: die kommunale selbstverwaltung in deutschland. in zivilgesellschaftliche koordination in der kommunalen selbstverwaltung: eine komparative untersuchung administrativ-politischer verfahren und kommunalpolitischer prozesse, pp. 1–22. springer fachmedien wiesbaden, wiesbaden, 2020. https://doi.org/10.1007/978-3-658-31867-3_1 [14] h. büchner. rechtliche grundlagen kommunaler selbstverwaltung. hanns-seidel-stiftung, münchen, 2020. [15] federal ministry of food and agriculture. klima schützen. werte schaffen. ressourcen effizient nutzen: charta für holz 2.0. 4th ed. bundesministerium für ernährung und landwirtschaft, bonn, 2021. [16] a. hafner, s. rüter, s. ebert, et al. greenhouse gas balances for timber buildings – implementation of new requirements for lifecycle-assessments and calculation of empiric substitution factors (ghg – timber buildings), bochum, germany, 2017. [17] m. kuittinen, c. zernicke, s. slabik, a. hafner. how can carbon be stored in the built environment? a review of potential options. architectural science review 0(0):1–17, 2021. https://doi.org/10.1080/00038628.2021.1896471 565 https://doi.org/10.1016/j.enbuild.2007.03.007 https://doi.org/10.31030/3179655 https://doi.org/10.31030/3179656 https://doi.org/10.31030/1917049 https://doi.org/10.31030/3054172 https://doi.org/10.1016/j.rser.2013.08.037 https://doi.org/10.1016/j.enbuild.2016.11.042 https://doi.org/10.31030/3251250 https://doi.org/10.1088/1755-1315/290/1/012046 https://doi.org/10.1016/j.buildenv.2015.06.029 https://doi.org/10.1007/978-3-658-31867-3_1 https://doi.org/10.1080/00038628.2021.1896471 acta polytechnica ctu proceedings 38:559–565, 2022 1 introduction 1.1 life cycle assessment of buildings 1.2 life cycle assessment of refurbishment measures 2 methodological approach for lca frameworks 2.1 building level 2.2 neighbourhood level 2.3 municipal level 3 comparison 4 conclusion acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.38.0429 acta polytechnica ctu proceedings 38:429–435, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague development of a realistic scenario for the thermal energy demand of residential buildings in bavaria till 2050 michael barton∗, franziska pichlmeier, marissa wolter, christian schweigler munich university of applied sciences, cenergie – center for energy-efficient buildings and districts, lothstr. 34, munich, 80335, germany ∗ corresponding author: michael.barton@hm.edu abstract. to achieve the climate protection goals, enormous efforts must be undertaken in all sectors: households, industry, commerce, and transport. following a straightforward approach, the political objectives foresee, that in the building sector a massive reduction of energy use for the heat supply is accomplished. however, previous investigations have shown that this reduction of energy consumption is not feasible. older buildings exhibit especially high energy demand and emissions. yet, due to the low refurbishment rate, no substantial change of the heat demand can be expected within the next decades. by fully renovating the entire residential building stock, approximately 70 % of the final energy demand and related co2 emissions could be saved, still not enough to reach the political goals. therefore, renovation or renewal of buildings and of the use of renewable energy sources have to be implemented jointly for achieving the desired savings. the methods used to estimate the characteristics of bavaria’s residential building stock as well as its heating energy demand and related co2 emissions for the year 2050 are presented. alternative goals are given which base upon the achievable final energy saving for a realistic renovation scenario accompanied by further reduction of co2 emissions by using renewable energies. keywords: refurbishment, building stock, climate neutrality. 1. introduction in order to achieve the european, german and bavarian climate protection goals, enormous efforts must be undertaken to accomplish substantial emissions reduction in all sectors: households, industry, commerce, and transport. following a straightforward approach, the political objectives foresee, that in the building sector this goal is reached by massive reduction of the final energy use for the heat supply of buildings. however, previous investigations have shown that the targeted reduction of final energy consumption in the building sector is not feasible. older buildings exhibit especially high energy demand and high co2 emissions. yet, due to the low refurbishment rate, no substantial change of the heat demand is to be expected in the near future. by fully renovating the entire residential building stock, approximately 70 % of the final energy demand and related co2 emissions could be saved, still not enough to reach the political goals. therefore, goals that achieve reduction of final energy and co2 emissions through a combination of renovation or renewal of buildings and the use of renewable energy sources have to be defined in order to meet reasonable objectives regarding co2 emissions. the proposed pathway could form the starting point for reformulating the political goals for the future energy use, finally targeting climate neutrality of the building heat supply sector. 2. methods to define realistic objectives for an effective transformation of the energy supply to residential houses, a detailed characterization of the heat demand with regard to all relevant criteria is required. there are numerous sources that describe building age classes as well as building types and provide statistical data on flat size, flat layout, specific energy demand and other parameters. however, there is no comprehensive overview available of how the individual criteria influence the heat demand of a specific building. therefore, data regarding the composition of the bavarian residential building stock and its final energy demand has been collected and the impact of different criteria has been shown. as final result a very detailed data set comprising 840 building variants, classified by four criteria, has been obtained from which a large number of specific statements can be derived. as shown in the figures 1 to 4, these data were generated by linking and correlational analysis of individual data sources in order to derive the desired statistical results for the characterization of the building stock and its energy demand for space heating and domestic hot water. at the beginning, the demand of final energy for the heat supply of the bavarian residential building stock has been analysed. in a first step, the perspective of the energy supply has been taken to obtain a reference value for the heat supply data, to be derived from the 429 https://doi.org/10.14311/app.2022.38.0429 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en m. barton, f. pichlmeier, m. wolter, c. schweigler acta polytechnica ctu proceedings figure 1. balance heat supply residential buildings in bavaria 2016. ([1–3]). physical data of the building stock in the following step. (figure 1). for this purpose, it was determined which amounts of the different energy carriers are provided for the heat supply of the residential houses. in the second step, the situation has been investigated from the perspective of the consumers (figure 2). the composition of the bavarian residential building stock has been characterized first, then the final energy demand has been determined, partitioned according to various criteria such as building categories and age. the main part of the physical data comes from the german census [4, 5] and the european wide developed building typology within the tabula project [10, 11]. the energy values of the two approaches – supply and consumption – are congruent with a deviation of less than 5 %. subsequently, a forecast for the development of the bavarian residential building stock and its final energy demand for the period until 2050 has been generated (figure 3). for this purpose, assumptions have been made in a realistic scenario about how the construction of new residential houses, the demolition and the renovation rate for existing buildings and the specific energy demand of the different building types will develop in the future. results for the energy demand and greenhouse gas emissions have been generated with reference to the climate policy objectives of the eu, the federal republic of germany and the federal state of bavaria. these reduction goals were then transferred to the heat supply of the bavarian residential building stock (figure 4), comprising the heat quantities for space heating and domestic hot water. in order to evaluate the reduction potential, it was considered how far the final energy demand of the residential building stock could be reduced until 2050, if all existing buildings were fully refurbished and the new houses had the highest energy standard of the respective year of construction. this shows the theoretical maximum savings potential in final energy and the associated greenhouse gas emissions for the heat supply of the bavarian residential building stock. 3. results and discussion detailed results and analysis on the composition of the bavarian building stock and its energy demand for space heating and domestic hot water preparation have already been published [25, 26]. examples for the use of these data for further analysis supported by appropriate reference building models have been presented in [27, 28]. a rough summary of the results for the heat demand concerning the shares of different building types are shown in table 1. figure 5 and table 2 show the results on the final energy values for different scenarios. political objectives: transferring the global political objectives for the energy use in all sectors to the heat demand (space heating and domestic hot water) of residential houses results in a target value for the final energy of approx. 24 800 gwh. compared to all other scenarios, this represents the politically targeted energy demand of the building sector in 2050. rate of refurbishment constant: a constant renovation rate of 1 % per year was assumed for this “business as usual” case. in this scenario, the energy demand is reduced by 19 % from about 101 300 gwh in 2016 to about 82 000 gwh by 2050. although it is not a pessimistic scenario, the result for the remaining energy demand is more than 57 000 gwh/a above the political goal. rate of refurbishment enhanced: this scenario based on optimistic, yet still realistic assumptions, for the development until 2050 still misses the political objectives significantly. the reduction of the final energy demand by approx. 30 400 gwh to approx. 71 000 gwh is not sufficient. an additional reduction of more than 46 200 gwh to about 24 800 gwh would be necessary to reach the political goal. to reach this goal the effort for renewal and renovation of the building stock would have to be increased by a factor of about 2.5. full refurbishment: the maximum potential for the reduction of the final energy demand can be exploited through the full refurbishment of the residential build430 vol. 38/2022 development of a realistic scenario for the thermal energy . . . figure 2. balance heat demand residential buildings in bavaria 2016. ([4], [5], [6–8], [9–11], [12, 13]). figure 3. balance heat demand residential buildings in bavaria 2050. ([6–10, 14, 15], [9–11]). figure 4. objectives for the heat supply of residential buildings in bavaria until 2050. ([14, 16–20], [2, 21, 22], [23, 24]). 431 m. barton, f. pichlmeier, m. wolter, c. schweigler acta polytechnica ctu proceedings residential building stock bavaria 2050 buildings final energy [pcs] [gwh] residential buildings total 3 501 476 100 % 70 895 100 % buildings unrefurbished 1 307 604 37.3 % 36 706 51.8 % buildings partly refurbished 996 197 28.5 % 23 591 33.3 % buildings fully refurbished 1 197 675 34.2 % 10 598 14.9 % detached houses 2 347 775 67.1 % 46 308 65.3 % semi-detached houses 495 207 14.1 % 7 915 11.2 % terraced houses 515 769 14.7 % 13 246 18.7 % other building types 142 725 4.1 % 3 427 4.8 % buildings with 1 flat 2 358 452 67.4 % 33 100 46.7 % buildings with 2 flats 624 305 17.8 % 14 251 20.1 % buildings with 3–6 flats 334 530 9.6 % 10 352 14.6 % buildings with 7–12 flats 128 601 3.7 % 7 611 10.7 % buildings with 13 or more 55 588 1.6 % 5 582 7.9 % table 1. rough summary of the results for the composition of the bavarian building stock and its energy demand for heating supply. the data for 2050 shows the scenario “rate of refurbishment enhanced”. figure 5. demand and objective values for final energy and for co2 emissions. scenarios 2050 renewable share renewable expansion final energy total final energy final energy resulting co2 reduction based on a heat demand [gwh] [gwh] [gwh] [million t co2] of 101 315 gwh in 2016 (based on 2016) (based on 2050) (based on 2016) rate of refurbishment constant 82 083 (81 %) 64 143 (78 %) +37 396 +140 % 8.27 rate of refurbishment forced 70 895 (70 %) 52 955 (75 %) +26 208 +98 % 5.80 full refurbishment 30 330 (30 %) 7 12 390 (41 %) -14 357 -54 % -3.17 political objective final energy 24 802 (24 %) 6 862 (28 %) -19 885 -74 % -4.40 table 2. requirements to achieve co2 objectives of 3.9 million t co2/year. 432 vol. 38/2022 development of a realistic scenario for the thermal energy . . . ing stock. yet, it is practically impossible to achieve this level of refurbishment. among other aspects, there is not sufficient capacity available, such as materials and workforce, to realize this scenario. and for a variety of reasons, some buildings cannot reach the highest insulation standard. the protection of historic buildings can be given as an example. although not realistic, this scenario provides an answer to the question whether the political objectives are theoretically achievable by refurbishment measures. in this case, the final energy demand of the building stock in 2050 is reduced by approx. 70 % to approx. 30 300 gwh, compared to the situation in 2016. thus, this scenario comes closest to the political objective of 24 800 gwh final energy consumption. the analysis leads to the conclusion that the political objectives for the final energy demand of the bavarian residential building stock cannot be achieved by refurbishment alone, taking into account the current state of building renovation and heating technology. yet, with regard to the current composition of the energy supply in the bavarian building sector, as shown in figure 5 (left section: 2016), it becomes obvious that the political goals have to be re-oriented. currently, approx. 26 750 gwh of the final energy demand for heating is covered by renewable energy sources. this amount exceeds the remaining energy demand, which is targeted by the political goals, exclusively relying on energy saving measures. thus, it has to be concluded that the future pathway for the energetic development of the building sector has to focus rather on the emission of the greenhouse gas co2 than on the building energy demand only. and in return, development scenarios have to incorporate both improvement of the thermal quality of the building structure and variation of the use of fossil and renewable energy sources. for this perspective, a target value for 2050 for the reduction of the co2 emissions related to the heat supply in the bavarian building sector has been calculated, independently from the limit found for the energy demand, as described in figure 4. by applying the politically targeted co2 reduction rate to the bavarian residential building sector, the emissions related to the heat supply have to be limited to 3.97 million tons per year. in order to assess the required contribution of renewable energy for the different refurbishment scenarios described above, it has been assumed that in 2050 fuel oil is fully banned and only natural gas will be used as a fossil energy carrier. the results of this investigation are shown in table 2 and figure 5. for the “business as usual” scenario with constant refurbishment rate, it is found that the use of renewable energy sources has to be increased by 37 400 gwh, equivalent to an increase by 140 % relative to the current value of 26 750 gwh. additional co2 savings of 8.3 million tons would be accomplished in order to reach the co2 emission goal, calculated with the actual emission factor for natural gas. in total, the renewable share would cover 78 % of the final energy demand of the building heat sector in 2050. in case of the “enhanced refurbishment scenario” the contribution of renewables has to be increased by 98 % in comparison to the current status (2016), covering 75 % of the total demand. as stated above, in the case of full refurbishment the objective for final energy would be missed. yet, a rather low absolute amount of renewable energy (12 390 gwh), equivalent to 41 % of the remaining final energy demand, is required to fulfil the objective for co2 emissions. apart from the dramatic reduction of the building energy demand, this scenario could represent a viable transformation path of the energy supply, since the required change of the building stock will in return strongly influence the potential volume for utilization of renewable energy sources. although the absolute amount of renewable energy would decrease by about 54 % compared to 2016, the renewable share would increase from 26 % in 2016 to 41 %. if both objectives shall be fulfilled – final energy, and co2 emissions – 28 % of the remaining demand of final energy are to be covered by renewable sources. from these considerations, it can be concluded that a singular focus on the final energy goal does not provide a viable solution, while concentrating on the objective value for co2 emissions is expedient. as shown in figure 5, by combining efforts in refurbishment of the building stock plus enhanced use of renewable energy sources the goal for co2 emissions can be achieved. to identify a realistic pathway, future development should achieve the maximum possible saving of final energy and accomplish the remaining reduction in greenhouse gas emissions by expanding the use of renewables. this strategy is best represented by the “enhanced rate of refurbishment” scenario. 4. conclusions a method to generate detailed statistical data of the bavarian building stock, its energy demand for heat supply (space heating and domestic hot water) and their future development until 2050 with regard to the political objectives of the eu, the federal republic of germany and the federal state of bavaria have been described. the procedure for creating a realistic scenario to reach the goals until 2050 has been explained. the theoretical maximum potential for saving final energy and reducing the associated greenhouse gas emissions for the heat supply in the bavarian building sector through full refurbishment and construction of new buildings with the highest energy standard until 2050 has been shown. based on the analysis of the possible development of the building structure, it has been concluded that political objective solely related to the building energy demand are unachievable. as an alternative, technically feasible goals have been worked out with focus on reducing co2 emissions as extensively as possible 433 m. barton, f. pichlmeier, m. wolter, c. schweigler acta polytechnica ctu proceedings by renovating residential houses and achieving further savings through the increased use of renewables. a realistic pathway has been presented, comprising a reduction of the final energy demand of the bavarian building sector by 30 % accompanied by a 75 % coverage of the remaining demand by renewable sources. as central result, the co2 emission of the building stock in 2050 is limited to 3.97 million tons per year, representing a reduction by 78 % compared to the situation in 2016. the theoretical and practical implementation of the presented alternative objectives is the subject of further research [29]. in these more intensive considerations, embodied emissions are explicitly taken into account. with overall declining emissions, this part is becoming increasingly important. references [1] bayerisches staatsministerium für wirtschaft, energie und technologie. fortschrittsbericht 2016/2017 zum umbau der energieversorgung bayerns, germany, 2018. 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[25] l. schwan, j. hahn, m. barton, et al. analysis and development of reference buildings for the energy transition in the thermal sector. in proceedings of the 32nd international conference on efficiency, cost, optimization, simulation and environmental impact of energy systems, p. 1741. 2019. isbn 9788361506515. [26] m. barton, j. hahn, l. schwan, c. schweigler. bayerischer wohngebäudebestand und energiebedarf für die wärmeversorgung: aktueller stand und prognose für das jahr 2050. in book of abstracts inuas konferenz 2021, pp. 32–34. 2021. 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[2022-01-20]. https: //www.ar.hm.edu/organisationen/ansprechpartner_ 3/professoren_3/essig/essig_2.de.html 435 https://doi.org/10.1088/1755-1315/588/2/022051 https://www.ar.hm.edu/organisationen/ansprechpartner_3/professoren_3/essig/essig_2.de.html https://www.ar.hm.edu/organisationen/ansprechpartner_3/professoren_3/essig/essig_2.de.html https://www.ar.hm.edu/organisationen/ansprechpartner_3/professoren_3/essig/essig_2.de.html acta polytechnica ctu proceedings 38:429–435, 2022 1 introduction 2 methods 3 results and discussion 4 conclusions references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.38.0546 acta polytechnica ctu proceedings 38:546–552, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague implementation of positive energy district concepts and energy master plans for decarbonization of districts matthias haasea,∗, daniela baerb a zurich university of applied sciences, department of life sciences and facility management, am gruental, 8820 waedenswil, switzerland b sintef, community, hogskoleringen 7b, 7465 trondheim, norway ∗ corresponding author: matthias.haase@zhaw.ch abstract. in order to achieve a holistic approach to community energy planning for neighbourhoods and districts, it is crucial to provide planners, decision makers, and stakeholders with the necessary methods and instruments. however, there is a research gap in terms of planning and implementation strategies and models. to address this gap, our research used literature and document analysis as well as qualitative interviews to identify implementation models and energy supply options for positive energy districts (peds), and to determine which market actors are needed for peds. we also discussed the consequences of scaling up the ped concept. our analysis highlights the importance of integrated energy planning, which is critical for reducing energy consumption, securing the location of energy infrastructure (generation, distribution, storage), and achieving long-term sustainable development and climate neutrality. therefore, understanding the different dimensions of sustainable development in combination with energy supply and consumption is more important than ever for planning and realizing settlements. keywords: positive energy districts (ped), energy master planning (emp), implementation plans. 1. introduction as policy makers set increasingly ambitious energyrelated building and community requirements and standards based on the sustainable development goals of the united nations (un) [1], climate change presents challenges to achieving these goals. the concept of energy master planning (emp) – a roadmap for planning energy efficiency and grid optimization – can facilitate better planning and implementation to meet these goals. stakeholders at all levels, from nations to communities, face the challenge of reducing greenhouse gas emissions to meet the goals of the paris agreement. a promising approach to reducing energy demand, increasing efficiency, and lowering the carbon footprint is through bottom-up approaches to energy planning at the neighborhood level [2]. cities and communities play a crucial role in achieving global sustainability goals as they are the main source of emissions and have the power to implement global goals at the local level while also considering site-specific demands and settings. 1.1. energy master planning (emp) holistic solutions for meeting the heating, cooling, and power needs of energy communities on a district scale can result in significant energy savings, reduced emissions, and increased energy security. while there is considerable literature available on energy master planning (emp) at the neighborhood and district level, including guidance and assessment tools for campuses [3], existing guidance and tools do not fully address the challenges of emp. energy planning involves determining the optimal mix of energy sources to meet a given energy demand, spanning over the lifetime of a neighborhood. emp comprises six steps, as illustrated in figure 1, starting from goal-setting during the early strategic long-term planning phase and continuing through the operational phase characterized by measurement and verification activities. emp focuses on the tactical mid-term planning and implementation phase, encompassing assessment, scenario building, planning, and implementation [4]. the multi-scale aspect (temporal and geographical) of a holistic approach to neighborhood and district energy planning poses major difficulties, as it requires consideration of both quantitative (economic, technical) and qualitative (environmental impact, social criteria) criteria [5]. to apply this approach and provide necessary methods and instruments for master planners, decision makers, and stakeholders, it is crucial to identify and frame the constraints that bound the options towards an optimized energy master planning solution [6]. existing guidance on master planning emphasizes the critical first step of identifying and establishing project goals [7]. sharp et al. (2020) analyzed emp in several countries and identified the importance of a thorough understanding of local and regional goals and constraints in achieving successful emp [6]. as 546 https://doi.org/10.14311/app.2022.38.0546 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 38/2022 implementation of positive energy district concepts . . . figure 1. energy master planning process [4]. more countries aim to improve the efficiency, environmental impact, and resilience of buildings and neighborhoods, early and comprehensive energy master planning on the neighborhood and district level becomes increasingly important [4]. however, various constraints can hinder the full potential of decarbonization. in haase and baer (2021), we identified differences in the implementation of positive energy districts (peds) in two distinct cases of decarbonized district solutions, and noted a research gap in planning and implementation strategies and models [8]. 1.2. positive energy districts and emp the positive energy district (ped) concept is rapidly evolving as an integral part of the strategic energy transition (set) plan for european union member states. while a precise definition is still emerging, the idea of creating energy production plants in built environments that generate more energy than the surrounding neighbourhood consumes is a promising way to advance the clean energy transition and achieve climate neutrality through a holistic approach to reducing energy demand and promoting renewable energy. positive energy districts are based on the fundamental principle of creating an area capable of producing more energy than it consumes over the course of a year, with the flexibility to adapt to changes in the energy market [9]. the joint programme of urban europe envisions positive energy districts as “energy-efficient and energy-flexible urban areas or groups of connected buildings that produce net-zero greenhouse gas emissions and actively manage an annual local or regional surplus production of renewable energy. achieving these goals requires integrating different systems and infrastructures, fostering interaction between buildings, users, and regional energy, mobility, and ict systems, and ensuring the energy supply while promoting social, economic, and environmental sustainability for all stakeholders” [10]. in this sense, the ped provides a natural area of the application of emp concepts. the process manager for emp works with municipal departments and external stakeholders to implement and monitor measures in the plans, evaluate results, and communicate feedback. it is important to note that a local energy planning process should be adapted to suit the context and needs of the local community. the local utility is naturally a major player as it traditionally managed the local energy grids. and the process manager has to understand the different implementation models for local energy supply and how they interact with the ped goal. it lacks a constraints analysis to clearly identify energy supply options for peds. 1.3. issues in energy supply (utility model) in recent years the understanding of the role of utilities has changed dramatically. while in the early 1980s utilities were comparably simple heating and cooling supply schemes in existing or new built building clusters (utility services or energy supply contracting) which included supply components on the supply side, and never considered to refurbish the user side. the supply schemes usually had a highly efficient supply technology such as cooling and heating plants (chp), gas-, oilor coal fired and/or biomass supply boilers for hot water or steam and electricity. each of these efficient technologies was backed up by “normal” heating boilers (hot water or steam). the power was usually fed into the power grid (no direct retail to end users) while the heating was distributed by hot water or steam grids to the end users. end users paid a heating price per kwh consumed and a heating load price covering investment costs (at least partially). the end users have been responsible for the demand side: the house station often equipped with a heat exchanger or a steam or hot water exchanger, a domestic hot-water station and a control system including a heating meter for the measurement of the consumed heating energy. with an increasing number of requirements on the national level such as primary or end energy targets for buildings, regulation to foster renewables, partly and temporarily overloaded grids have added multiple complexities which cannot be handled by the utility model alone. in the eu carbon footprint targets 547 matthias haase, daniela baer acta polytechnica ctu proceedings are in place and primary energy (pe) targets for buildings. the latter pe targets can in most cases only be responded positively by combining demand and supply side measures. this means, that a building which is connected to a supply grid with a biomass boiler (low primary energy factor) can even reduce his efforts on the level of the building insulation to achieve the required pe target. this option however is shortsighted, as these buildings will consume large amount of valuable biomass and has consequences for a larger rollout of the concept which is still not well understood. 2. research question building on the identified research gaps we ask three research questions: (i) what are the characteristics of implementation models towards positive energy districts (peds)? (ii) which energy supply options exist and which are needed for peds? (iii) what are the consequences for a larger rollout of the concept? 3. method we describe the characteristics of models for the energy supply towards positive energy districts (peds) by analysing different recent developments of peds. the work is based on literature and document analysis adapted from the iterative model proposed by bocken et al. [11]. in addition, we conducted 34 interviews with focus groups in four different european countries to understand the role of the different stakeholders. the method was based on structured interviews with different stakeholders from practice, policy and technology provider background. 4. results in today’s resource-constrained environment, and with the emergence of “positive energy districts” concepts, local stakeholders are looking for creative ways to drive additional efficiencies in energy use and reduce associated costs. however, large, coordinated efforts are needed to gain synergy between different energy initiatives and future planned projects to maximize energy use and cost reduction. 4.1. characteristics of new energy supply concepts the transformation of today’s electric power sector to a more sustainable energy production based on renewable energies will change the structure of the industry [12]. in this transformation towards a smart energy system interaction between sectors and technologies the main stakeholders as listed above (energy service providers; utilities) will face new challenges in their traditional way of doing business. therefore, adapting their business models to remain competitive is seen as an important step. when looking at the business model literature reveals that there exist basically two possibilities: (i) ownership of renewable energy assets [13] (ii) utilities need to develop from commodity providers to energy service providers [14, 15]. according to this idea utilities should evolve to comprehensive energy-solutions providers for residential and commercial customers to create new sources of revenues. in the energy sector a few characteristics to implement new energy supply concepts are worth noting (see also [11]): • highly resilient community energy system considering buildings, supply, distribution and storage as basic components which have to match together. • target value approach primary energy, carbon footprint target-oriented system approach for the complete community considering incoming and outgoing energy supply streams • understanding each building as a prosumer each building is understood to consume, produce and store energy: pv on the roof, detached chps in complex grid hub structures may be producing energy. • sectoral coupling power produced and not consumed onsite can be attractive to be stored to operate e-vehicle fleets on campuses or delivery services and likewise to match mobility and energy systems. • consequent use of transfer energy systems the supply will consider supply systems which are able to use multiple fuels in the nearer future. one of the important components is chp or fuel cell which are market ready usually operated with natural gas but will have to be transferred to system immanent synthesized or ptf based fuels such as hydrogen or synth gas. • grid-friendly incentivized business model stabile mid and high-tension systems require gridfriendly local communities which means that the input from the community into the grid and vice versa needs to be controlled to keep loads and frequencies balanced in the regional and national grids. this requires business models which provide direct (subsidies, feed in tariffs) and indirect (legal barriers) incentives for escos and utilities to operate the energy system grid friendly. • no more supply-only-concepts buildings are actively integrated in the energy concept following the idea, that the resilience and dependence of imported fuels (on national but also local level of the community) can only be reduced if building follow challenging building standards even far beyond the national minimum requirements. not touching buildings in an emp business model is obsolete 548 vol. 38/2022 implementation of positive energy district concepts . . . from the macroand microeconomic standpoint and also from the position of preserving the asset values in the future. • innovative instant trade energy distribution systems besides the known power, heating, cooling grids in communities it is necessary to provide open direct marketing structures for locally produced power. vienna utilities are one of the front runners to use block chain technologies to distribute energy packages from house to house and apartment to apartment without involving local grid resources which provides first experience-based input [16]. in the us, the brooklyn blockchain system has been identified as one of leading instant trade systems [17]. • remuneration systems the remuneration system is designed in a way that incentivizes the energy service provider to provide the energy in the simplest way to the end consumer, to focus on the energy target for the single building and the complete community and to integrate and distribute normal supply solutions with detached building based renewable power production in a way which is “grid-friendly” at the time given. also add-ons such as coupling with the mobility sector, by providing quick charging stations for e-mobility are remunerated separately. the utility or esco respectively does not increase the margin by selling more energy to the costumer but by keeping the overall target and the energy mix matching with this demand. • user-centric besides the before-mentioned remuneration systems and missing performance monitoring, a key factor is the user behavior: any kind of energetic measure can be countered by obvious or unknowingly misconduct of the users. socioeconomic analysis is still not very common in energy science but the few interviews with users conducted show that users are not capable to operate their apartments sufficiently. this is not only the case for end users in their private homes but also for skilled staff in hospitals and data centers. the assumption “building automation will fix it” turns out not to work. this sets the requirement for the new implementation models to provide simple to operate, intuitive end users hand holds to operate and also control the consumption directly. a few examples in modern housing complexes exist already providing end users, both private and professionals simple app based smart building tools to control the power and heating/cooling consumption in their buildings. 4.2. emerging models energy supply for the buildings stands for the supply of both electric and thermal energy. most of the energy use of buildings is related to space heating (and cooling), therefore the needed thermal energy can be extracted directly from a district heating (and cooling) network (dh(c)) or provided by a heat pump (hp) or even directly converted using electric heaters/coolers. energy supply companies are responsible for supplying buildings and district with their needs in terms of electric and thermal energy, however still most of the provided energy is mostly electric energy. classical electricity supply utilities are suffering nowadays from what is called the “spiral death”, meaning a strong decrease in the number of energy utility customers. the quitting customers are served by new market actors operating with new business models, centered on renewable energy and energy efficiency technologies. the electric energy system contains the physical infrastructure (generation, transport, distribution and use together with their components) and an organized electricity market based on different marketplaces. the market consists mainly of the following actors [18]: • electricity generator, who generates electricity and sell it to the energy suppliers. • electricity suppliers, who purchase the electricity from the generators and sell it to consumers. • transmission system operators (tso), who are responsible for transporting electricity for long distance and ensuring grid stability and reliability by real time dispatch. • distribution network operators (dso), who are responsible for delivering electricity to the consumers and measuring the consumption. • regulators, who set the market rules and oversee the functioning of the market. in the eu countries some energy supply companies operate transnationally, however still a large part of the worlds’ electric energy supply is either based on fossil fuels like coal, gas, and oil or nuclear energy. the production, transmission and distribution of electricity accounts for the largest share of the world’s anthropogenic greenhouse-gas emissions, while the use of emission-free nuclear energy comprises serious security risks and unsolved problems of hazardous waste, therefore the role of renewable energies as the most important instrument to mitigate climate change and reduce negative effects of energy production is increasing. despite the fact that utilities (with national or transnational activities) still have a dominant position they are confronted with disruptions of their current way of doing business and face the challenge to develop new business models for electricity generation from distributed and highly intermittent renewable sources. electrical energy sector is undergoing a continuous process of transformation where a fundamental shift of energy supply towards renewable, co2 neutral energies is taking place, together with a decentralization and digitalization. the classical structure of the electrical energy industry that emerged after the liberalization of the electricity and gas markets 549 matthias haase, daniela baer acta polytechnica ctu proceedings in europe including established business models, is subject to disruptive and massive changes. 4.3. the role of the public the majority of implementation models for peds involve the public sector to some degree, and in many cases the public sector has partial or full ownership of the project. the degree to which the public sector is involved is determined in part by how much it may wish to steer a district energy project towards a variety of local objectives. ped implementation models that are replicable and scalable both technically and financially at the neighborhood, city and national level are key to the acceleration of district energy production? (1.) the “wholly public” implementation model is the most common globally. the public sector, in its role as local authority or public utility, has full ownership of the system, which allows it to have complete control of the project and makes it possible to deliver broader social objectives, such as environmental outcomes and the alleviation of fuel poverty through tariff control. (2.) implementation strategies that focus on “hybrid public and private” energy supply have a rate of return that will attract the private sector, but the public sector is still willing to invest in the project and retain some control. these models can include: (a) a public and private joint venture where investment is provided by both parties that are creating a district energy company, or where the public and private sector finance different assets in the district energy system (e.g. production of heat/cooling versus transmission and distribution); (b) a concession contract where the public sector is involved in the design and development of a project, which is then developed, financed and operated by the private sector, and the city usually has the option to buy back the project in the future; and (c) a community-owned not-for-profit or cooperative business model where a municipality can establish a district energy system as a mutual, community-owned not-for-profit or cooperative. in this model, the local authority takes on a lot of risk initially in development and if it underwrites any finance to the project. (3.) “private” implementation models are pursued where there is a high rate of return for the private sector and require limited public sector support. they are developed as a wholly privately owned special purpose vehicle but may benefit from guaranteed demand from the public sector or a subsidy or local incentives. few cases are developing “private” models as the majority district energy model, also because there are a number of barriers to be lifted. 5. discussion 5.1. implementation models the electric power sector’s value streams are heavily influenced by the regulatory and policy frameworks that govern it. electricity distribution companies’ revenues are regulated by government-appointed regulatory commissions, which can impact the viability of distributed renewable energy businesses (ders) in distribution networks. wholesale electricity markets are subject to market rules established by central authorities like independent system operators (isos) or regional transmission operators (rtos), which are monitored and regulated by energy regulatory commissions (ferc). der business models looking to sell services in these markets must comply with established market rules and regulations. furthermore, the electric power sector is subject to significant national and eu policy support, including subsidies and favorable rules for technologies like solar-based energy generation. understanding these policy and regulatory interdependencies is critical for sustainable business development. community ownership is often seen as a source of income that can be locally controlled, making these investments more socially acceptable as they help develop local supply ownership and prevent value leakage from the local economy. financing renewable energy technologies is a crucial factor for both microgeneration, where infrastructure investment costs are a barrier, and large-scale renewable energy technologies, where upfront costs are often significant. alternative financing sources, like energy cooperatives or crowdfunding platforms, can help mitigate financial risk. in addition to these models, other promising business models include novelty-centered, lock-in-centered, complementarities-centered, and efficiency-centered models [19]. 5.2. supply options for peds the emp must implement goals and constraints on different levels (national, municipal, neighbourhood) and phases. the measurement and verification depend on a number of criteria: • cost-effectiveness of community projects; • economic decision-making criteria: ▷ life-cycle cost calculation (lcc) for emp, ▷ multiple benefits (bankable lcc on the building level), ▷ bankability and risk mitigation of multiple benefits, ▷ cost effectivenes; • investment costs and capital expenditures; 550 vol. 38/2022 implementation of positive energy district concepts . . . • determination of technical concept and investment costs: ▷ gathering of accurate investment costs, ▷ developing detailed energy demand and supply scenario by simulation, ▷ specific risks in the calculation of investment costs; • optimization of investment cost. 5.3. rollout with increasing complexity of the energy supply in building clusters, the partition of energy service companies (esco) of the total market is steadily increasing. today escos and a few innovative utilities are able to provide highly complex energy services, including generation, distribution, storage, selling, m&v combined with demand side measures like refurbishment of buildings, distribution grids and other demand side activities. the business scheme here usually is the energy supply contracting which delivers demand and supply side measures for a fixed investment cost-based price per kw and a price for the consumed kwh of energy. the ownership of all investments, except those in buildings remains for the duration of the contract (5–20 years) with the esco/utility. in recent years, the energy savings performance contracting has been developed into a business model which is not only able to tap energy efficiency potentials in buildings (hvac and thermal envelope) but also provide complex supply, distribution and storage concepts with chp, pv, biomass and heat pumps. here, the remuneration is based on the energy savings and other life cycle cost savings provided by the energy service company. but also other value streams are important to notice: • energy savings; • avoided maintenance and repair costs; • operation cost reduction; • insurance costs; • building comfort and green neighbourhood value; • risks and de-risking methods and tools; • key risk indicators (kri) in general; • kri in emp for building clusters in particular. 6. conclusions the characteristics of implementation models towards positive energy districts (peds) were collected and which energy supply options exist, and which are needed for peds was analysed. the results show a clear trend towards community models that concentrate and optimize value streams within the community. from the analysis of the results, the conclusions are that integrated energy planning is more important than ever. because this will answer the question which energy supply options exist, and which are needed for peds. the understanding of the advanced emp value stream is providing a different mindset which focuses on the needs of the building owners and the community developer. a few cornerstones of the advanced value stream for peds are high resilient community energy system considering buildings, supply, distribution and storage as basic components which have to match together. the main aspects of energy supply models were identified as important to be able to define value streams as basis for successful implementation of decarbonization projects. publicly owned district heating systems provide the potential for a larger rollout of decarbonization. associated value streams were discussed and point to the need for a shift from energy and ghg emission savings to increased comfort on neighborhood level and risk mitigation. to plan and develop settlements that contribute significantly to reducing energy consumption and secure the location of energy infrastructure (generation, distribution, storage), as well as achieve long-term sustainable development and climate neutrality, it is essential to understand the different dimensions of sustainable development in combination with energy supply and consumption. the implementation of energy master plans (emps) requires a range of mandatory and optional services provided to the public owner of a building cluster. acknowledgements this work is related to iea ebc annex 73, annex 75 and annex 83. the discussion among experts is highly appreciated. references [1] un. transforming our world: the 2030 agenda for sustainable development, united nations, 2015. a/res/70/1, [2022-09-22]. https://sustainabledevelopment.un.org/ [2] m. david, s. schönborn. bottom-up energy transition narratives: linking the global with the local? a comparison of three german renewable co-ops. sustainability 10(4), 2018. https://doi.org/10.3390/su10040924 [3] z. huang, h. yu, z. peng, m. zhao. methods and tools for community energy planning: a review. renewable and sustainable energy reviews 42:1335–1348, 2015. https://doi.org/10.1016/j.rser.2014.11.042 [4] m. haase, d. baer. constraints, stakeholders, and framing goals in energy master planning between neighborhood and district. in a. bisello, d. vettorato, d. ludlow, c. baranzelli (eds.), smart and sustainable planning for cities and regions, pp. 3–13. springer international publishing, cham, 2021. https://doi.org/10.1007/978-3-030-57764-3_1 [5] j. schiefelbein, c. slotterback, j.-p. petersen, et al. implementation of energy strategies in communities – results within the context of iea annex 63. in 30th international conference on efficiency, cost, 551 https://sustainabledevelopment.un.org/ https://doi.org/10.3390/su10040924 https://doi.org/10.1016/j.rser.2014.11.042 https://doi.org/10.1007/978-3-030-57764-3_1 matthias haase, daniela baer acta polytechnica ctu proceedings optimization, simulation and environmental impact of energy systems, ecos 2017, pp. 1–10. san diego, us, 2017. [6] t. sharp, m. haase, a. zhivov, et al. energy master planning: identifying framing constraints that scope your technology options. in ashrae transactions 2020, vol. 126, pp. 789–806. american society of heating, refrigerating and air-conditioning engineers, inc, atlanta, 2020. [2022-09-22]. https://www.techstreet.com/ashrae/standards/or20-020-energy-master-planning-identifyingframing-constraints-that-scope-your-technologyoptions?product_id=2113219 [7] r. jank. annex 51: case studies and guidelines for energy efficient communities. energy and buildings 154:529–537, 2017. https://doi.org/10.1016/j.enbuild.2017.08.074 [8] m. haase, d. baer. towards climate neutrality through integrated energy planning – a cross country comparison and case study analysis of positive energy district concepts between switzerland and norway. in real corp 2021 proceedings, pp. 957–968. 2021. 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[16] vienna energy strategy 2030. [2022-04-19]. https://www.wien.gv.at/stadtentwicklung/ energie/pdf/energierahmenstrategie-2030-en.pdf [17] m. andoni, v. robu, d. flynn, et al. blockchain technology in the energy sector: a systematic review of challenges and opportunities. renewable and sustainable energy reviews 100:143–174, 2019. https://doi.org/10.1016/j.rser.2018.10.014 [18] g. erbach. promotion of renewable energy sources in the eu, european parliamentary research service, 2016. p. 1-20. [19] m. haase, e. motoasca. characteristics of business models for innovation clusters in decarbonation projects. acta polytechnica ctu proceedings 38:417–423, 2022. central europe towards sustainable building 2022 (cesb22). https://doi.org/10.14311/app.2022.38.0417 552 https://www.techstreet.com/ashrae/standards/or-20-020-energy-master-planning-identifying-framing-constraints-that-scope-your-technology-options?product_id=2113219 https://www.techstreet.com/ashrae/standards/or-20-020-energy-master-planning-identifying-framing-constraints-that-scope-your-technology-options?product_id=2113219 https://www.techstreet.com/ashrae/standards/or-20-020-energy-master-planning-identifying-framing-constraints-that-scope-your-technology-options?product_id=2113219 https://www.techstreet.com/ashrae/standards/or-20-020-energy-master-planning-identifying-framing-constraints-that-scope-your-technology-options?product_id=2113219 https://doi.org/10.1016/j.enbuild.2017.08.074 https://doi.org/doi/10.2777/04888 https://jpi-urbaneurope.eu/ped/ https://doi.org/10.1016/j.jclepro.2013.11.039 https://doi.org/10.1016/j.rser.2012.01.072 https://doi.org/10.2172/924651 https://www.wien.gv.at/stadtentwicklung/energie/pdf/energierahmenstrategie-2030-en.pdf https://www.wien.gv.at/stadtentwicklung/energie/pdf/energierahmenstrategie-2030-en.pdf https://doi.org/10.1016/j.rser.2018.10.014 https://doi.org/10.14311/app.2022.38.0417 acta polytechnica ctu proceedings 38:546–552, 2022 1 introduction 1.1 energy master planning (emp) 1.2 positive energy districts and emp 1.3 issues in energy supply (utility model) 2 research question 3 method 4 results 4.1 characteristics of new energy supply concepts 4.2 emerging models 4.3 the role of the public 5 discussion 5.1 implementation models 5.2 supply options for peds 5.3 rollout 6 conclusions acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2023.41.0008 acta polytechnica ctu proceedings 41:8–14, 2023 © 2023 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague level of service of public transport operation – pilot study markéta jirmanová∗, vojtěch novotný, nikol dousková czech technical university in prague, faculty of transportation sciences, department of transportation systems, horská 3, 12803 prague, czech republic ∗ corresponding author: jirmamar@fd.cvut.cz abstract. smart transport planning based on the use of available data while supporting sustainable transport modes is an integral part of the smart city concept. this article presents a critical analysis of the level of service of public transport operation assessment method, which is based on two indicators – the reliability index and the travel speed index. the method is applied to a larger sample of data from several different cities, it makes it possible to evaluate the possible shortcomings of the method, since the method has not yet been applied to such a large number of case studies. keywords: public transport priority, smart city, level of service. 1. introduction the overall increase in urban population mobility has recently created a greater demand for sustainable mobility, which is essential for future urban development and meeting the goals of eu green deal 2050 [1–3]. considering the capacity of city roads to be filled with cars, it is important to focus on urban public transport, which is a suitable and sustainable alternative to individual car transport [2, 4]. from the sustainable development point of view, there is a need to identify an efficient modal-split and to increase the share of other modes of transport, whereas use of private cars must be reduced [5]. considering the options for other modes of transport around the city, public transport must be the backbone of sustainable urban mobility [6, 7]. the role of public transport today is not simple, as high road traffic volumes have a negative impact on its speed and reliability. from the passenger’s point of view, the main criteria required by customers or potential users is the travel time, reliability or the cleanliness of the cars [8]. we can further divide public transport quality indicators into subjective and objective [9]. subjective factors such as the cleanliness and comfort of the indicator are not so easy to assess, and it is necessary to directly ask the opinion of the passengers [10]. objective indicators are those that can be determined from the collected data, these indicators are conclusive, for example travel time, adherence to timetables (reliability), delays and others. these objective indicators have a great influence on the use of public transport, as they have a very high influence when deciding whether a passenger will drive their own car or use public transport. 2. quality of public transport operation and its assessment method in the past, the evaluation of public transport operation was often limited to the evaluation of parameters related to the economics (the so-called circulating or technical speed), or the evaluation of the delay of the connection in reaching the destination. achievement of travel speed and reliability was often expressed in terms of whole lines, not in terms of individual sections, and the achieved values were compared with the timetable data, which by its very nature (timetable for whole minutes, artificially extended travel times in sections with regular congestion, etc.) is not a completely objective evaluation. the development of ict and the real-time positioning of public transport vehicles have brought new data sources and new possibilities for their processing – whether for the purposes of public transport network operation management (for example [12]) or transport planning (for example [13]). although there have been other ways of evaluating public transport operations (for example [14] or [9]), most of them work with a more global view and cannot be used as an analytical tool for evaluating individual sections. in 2017, the quality of public transport operation assessment method was created as a response to the lack of an objective data-based method for evaluating the quality of traffic and identifying problematic locations or sections in terms of public transport operation on urban roads (e.g. trams, buses/trolleybuses) [11]. the aim of the method is to provide transport engineers with an analytical tool to assess the operation of public transport across the network and to identify sections where public transport operations need to be improved through the implementation of priority measures and thereby improving the quality of public transport and which would lead to more people using 8 https://doi.org/10.14311/app.2023.41.0008 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 41/2023 public transport operation service figure 1. quality of public transport operation and its relation to passengers’ perception of the quality of public transport [11]. figure 2. same travel time achieved throughout the day – a section with reliable public transport service [11]. this mode of transportation. the method is generally based on assessment of travel speed and reliability of public transport in intermediate stop sections, which basically corresponds to the passengers’ perception of the quality of public transport operation (see figure 1). the method is based on level-of-service theory and uses objective data-based evaluation of travel speed and reliability [11]. in order to maintain objectivity, both parameters are evaluated only on the basis of the available data on the movement of public transport services in a given inter-stop section, respectively their time positions in the stops. the evaluation of these parameters is then aggregated into an overall level of service for unambiguous evaluation and easy presentability. in this context, the method can form one part of the implementation of the smart city concept [9]. 2.1. reliability evaluation for public transport to be considered reliable, it needs to have the same travel time regardless of the time of day or traffic situation. this means that in the case of a graphical display, the values for each travel time should form a horizontal line as seen in figure 2, on the contrary, in case of unreliable operation, the graph will show high deviations from the average, as seen in figure 3. for interest, the graphs also show the travel time according to the timetable (usually given in whole minutes), which illustrates the inapplicability of evaluating timetable compliance in individual intermediate stop sections. for the purposes of the method, the reliability of the operation is expressed by the reliability index. this is now calculated as the standard deviation divided by the length of the segment (a longer segment will tend to greater deviations, therefore it would not be objective without the length of the segment), the whole multiplied by 1000, for the sake of clarity of the index. 9 m. jirmanová, v. novotný, n. dousková acta polytechnica ctu proceedings figure 3. typical evolution of travel times affected by the traffic situation – a section with unreliable public transport operation [11]. i(rela−b) = σt t s · 1000 = √ 1 n ∑n i=1 [xt t − e (xt t )]2 s · 1000 (1) 2.2. travel speed evaluation the second indicator that the method uses is travel speed. in this case, it is necessary to determine the so-called decisive travel speed, which is determined as a ten percent percentile (10% percentile was determined using small sample of data) (2). this should be the highest most often achievable speed that the vehicle can drive, i.e. the speed that the vehicle should ideally achieve when passing freely through the section. this decisive speed is then used to determine the travel speed index by dividing the average travel speed by the decisive speed (3). v (rela−b;dec) = 0.06 sa−b q10 (tcest;a−b) (2) i (vcest;a−b) = v (cesta−b;avg) v (cesta−b;dec) (3) 2.3. final level of service of public transport operation for both indices, a linguistic value is subsequently determined according to their calculated numerical value. each index has a so-called function for the linguistic values of the quality of public transport. for the reliability index, a linguistic value of 100% “excellent” corresponds to a value of 0.2. furthermore, as the numerical value of the reliability index increases, the linguistic value deteriorates up to the value “unacceptable”, which for 100% corresponds to a reliability index of 1.5 and higher, as can be seen on figure 4. for the travel speed index, the situation is the opposite, as can be seen in figure 5. if the travel speed index is higher than 0.9, its linguistic value is 100% “excellent”, then the lower it is, the worse the linguistic value. if it falls below 0.5, its value is 100% “unacceptable”. from these two linguistic values, the total value of the service level of the given section is then obtained by logical rules. simplistically, the sum of all affiliations of the two indices is made and whichever linguistic value prevails in the sum of affiliations is the overall level of service. further, it is only converted to integer values, where “unacceptable” corresponds to a value of 5 and “excellent” corresponds to a value of 1. 3. testing the method – a pilot study however, the level of service of public transport operation assessment method was originally tested on a small sample of data [11]. the logical next step was to pilot test the method on a larger sample of data and subject it to critical analysis based on the results. pilot testing was carried out on a total of 107 interstop sections of tram or bus services in 4 cities (prague, brno, pilsen and budapest). the sections were selected to form a representative sample – tram and bus sections, sections with and without a dedicated carriageway (e.g. a separate tramway or bus lane), sections with regular congestions and, for example, tram sections in the pedestrian zone. 3.1. obtaining input data due to the different data acquisition systems in the cities, it was necessary to first adjust and unify the input data for the evaluation. this was done using 10 vol. 41/2023 public transport operation service figure 4. reliability index and membership function for linguistic values of the quality of public transport traffic [11]. figure 5. travel speed index and membership function for linguistic values of the quality of public transport traffic [11]. python. in order to apply the data to the method, it is necessary to know mainly the time and date of arrival and departure from both stops of one section. data from cities can have two formats, according to which vehicles report either only arrival or only departure from the stop (arrival – departure, arrival – arrival), therefore the driving time is extended by the time of standing at the stop, or the vehicle reports both, therefore it is possible to get the exact value of the driving time in the section (departure – arrival). a 2-week sample of data was processed from each section. 3.2. using power bi software to evaluate level of service in order to evaluate such a large number of sections, it was necessary to create at least a partially automated evaluation of the level of service. for this purpose, was used software power bi and was created an evaluation sheet for each section separately. this sheet contains basic information about the given section so that it is possible to better imagine the traffic situation on the given section (figure 6). only working days are used to display graphs in the travel time model. thanks to this, it is possible to better recall the traffic situation and clearly see the impact of rush hours on the operation of public transport buses. to identify various extraordinary events in traffic or other inconsistencies in the used data, a table is used that shows the average travel time in hourly sections for each day separately. thanks to the gradual coloring from the darkest red (longest travel time) to white (shortest travel time), it is possible to see different fluctuations during the day and the trend of travel times for each day separately (figure 7). likewise, it is possible to detect data inconsistency using hourly slice charts for each day. it is also possible to find here a comparison of the delay in the section compared to 11 m. jirmanová, v. novotný, n. dousková acta polytechnica ctu proceedings figure 6. travel speed index and membership function for linguistic values of the quality of public transport traffic [11]. figure 7. a table used for identifying various extraordinary events in traffic or other inconsistencies in the used data. the timetable. since the driving time according to the timetable can be artificially adjusted, it is then possible to see whether this value has really been artificially extended or whether it is close to the ideal driving time. 4. results and critical analysis of the pilot study after evaluating the level of service for all of 107 sections, the results were confronted with both expert expectations and the behaviour of the travel speed and reliability indices depending on the type and characteristics of the section. most of the sections that were selected and evaluated correspond to the assumptions of the level of service if there were no traffic abnormalities on the measured days. however, the analysis opened some questions or pointed to the need to calibrate the method: • determination of decisive travel speed is the 10 percent quantile really appropriate? • too much influence of the reliability index on the resulting level of service, especially for short sections. • the need to refine the calibration of the linguistic function of affiliation. 4.1. determination of decisive travel time/speed now the decisive travel time/speed is determined as the ten percent percentile of all measured travel times. 12 vol. 41/2023 public transport operation service figure 8. graph of index dependence on section length and their linear trends. anyway, this value was created as an estimate based on a small sample of data, so it is now necessary to verify that this value is closest to the ideal travel time. one of the main factors that could influence the determination of the decisive travel time/drive is the time span of the data used. if values are used that do not include night connections, such as the time span 8:00–20:00, the value of the decisive travel time on a section loaded with individual car traffic is likely to be significantly higher than if the night lines that go after almost ideal conditions. likewise, the total number of connections per day could have a decisive travel time value, or it is possible that a minimum number of connections per day will need to be determined for which the mathematical evaluation formula will be relevant. 4.2. short sections and excessive influence of the value of the reliability index on the level of service the problem of short sections and too much influence on the overall level of service can be seen in the following graph. for the purposes of this graph, both indices (reliability index and travel time index) were recalculated to the numerical value “level of service”, from which the average of these two values is used to determine the overall level of service of the entire section, these values are displayed on the y-axis, the length of individual sections is then displayed on the x-axis. when displaying trends, ideally, the curve should be horizontal, thus the length of the section would not affect the level of service. as can be seen on the graph (figure 8), the trend of the overall level of service is decreasing, so the longer the section, the better the level of service. at the same time, the level of service trend curve has the same slope as the reliability index trend curve, on the contrary, the travel speed index curve is almost horizontal, from which it can be concluded that the reliability index has a greater influence on the overall level of service of the section compared to the travel speed index. 4.3. linguistic functions of affiliation like decisive travel time, the linguistic affiliation functions were created based on small amount of data. this is not a significant problem when evaluating the method, however, due to the possibility of evaluation on a larger sample of data, it would be good to focus on specific linguistic functions and verify whether these values correspond to reality. 5. conclusion thanks to the evaluation of 107 sections and the implementation of the level of service of public transport operation assessment method on real data in 4 different cities, it was verified that the method could be a suitable evaluation tool for identifying problem sections on the network where the quality of public transport operation is insufficient. and therefore, where it would be appropriate to make preferential measures for public transport, or to optimize the operation of public transport in another way. the calibration and correction of the identified problems in this work is another important step towards the practical use of the level of service of public transport operation assessment method and the improvement of the quality of public transport. references [1] g. beirão, j. s. cabral. enhancing service quality in public transport systems. in wit transactions on the built environment, vol 89. wit press, 2006. https://doi.org/10.2495/ut060811 13 https://doi.org/10.2495/ut060811 m. jirmanová, v. novotný, n. dousková acta polytechnica ctu proceedings [2] eur-lex green deal for europe. brusell, be; 2019. [2022-09-10] https: //eur-lex.europa.eu/legal-content/cs/txt/?qid= 1596443911913&uri=celex:52019dc0640#document2. [3] d. banister. the sustainable mobility paradigm. transport policy 15(2):73–80, 2008. https://doi.org/10.1016/j.tranpol.2007.10.005 [4] n. dousková, v. novotný, p. richter, m. jirmanová. pilot study of using google traffic data to assess conditions for public transport operation on urban roads. in 2022 smart city symposium prague (scsp), pp. 1–5. ieee, 2022. https://doi.org/10.1109/scsp54748.2022.9792550 [5] p. kurfürst. traffic demand management as an environmentally friendly transport policy tool. the centre for transport and energy, prague, czech republic, 2002. [6] g. fernandez-sanchez, a. fernandez-heredia. strategic thinking for sustainability: a review of 10 strategies for sustainable mobility by bus for cities. sustainability 10(11):4282, 2018. https://doi.org/10.3390/su10114282 [7] r. g. mugion, m. toni, h. raharjo, et al. does the service quality of urban public transport enhance sustainable mobility? journal of cleaner production 174:1566–1587, 2018. https://doi.org/10.1016/j.jclepro.2017.11.052 [8] g. beirão, j. s. cabral. understanding attitudes towards public transport and private car: a qualitative study. transport policy 14(6):478–489, 2007. https://doi.org/10.1016/j.tranpol.2007.04.009 [9] o. pribyl, p. pribyl, m. lom, m. svitek. modeling of smart cities based on its architecture. ieee intelligent transportation systems magazine 11(4):28–36, 2019. https://doi.org/10.1109/mits.2018.2876553 [10] a. j. m. seco, j. h. g. gonçalves. the quality of public transport: relative importance of different performance indicators and their potential to explain modal choice. in urban transport xiii: urban transport and the environment in the 21st century. wit press, 2007. https://doi.org/10.2495/ut070301 [11] v. novotný. public transport priority assessment tool. ph.d. thesis, czech technical university in prague, prague, 2017. [12] m. matulin, š. mrvelj, n. jelušić. two-level evaluation of public transport performances. promet – traffic & transportation 23(5):329–339, 2011. https://doi.org/10.7307/ptt.v23i5.151 [13] j. gurjar, p. jain, p. agarwal. comparative performance evaluation of public transport services from city perspective. transportation research procedia 48:2207–2229, 2020. https://doi.org/10.1016/j.trpro.2020.08.277 [14] x. lai, j. teng, l. ling. evaluating public transportation service in a transit hub based on passengers energy cost. in 2020 ieee 23rd international conference on intelligent transportation systems (itsc), pp. 1–7. ieee, 2020. https://doi.org/10.1109/itsc45102.2020.9294662 14 https://eur-lex.europa.eu/legal-content/cs/txt/?qid=1596443911913&uri=celex:52019dc0640#document2 https://eur-lex.europa.eu/legal-content/cs/txt/?qid=1596443911913&uri=celex:52019dc0640#document2 https://eur-lex.europa.eu/legal-content/cs/txt/?qid=1596443911913&uri=celex:52019dc0640#document2 https://doi.org/10.1016/j.tranpol.2007.10.005 https://doi.org/10.1109/scsp54748.2022.9792550 https://doi.org/10.3390/su10114282 https://doi.org/10.1016/j.jclepro.2017.11.052 https://doi.org/10.1016/j.tranpol.2007.04.009 https://doi.org/10.1109/mits.2018.2876553 https://doi.org/10.2495/ut070301 https://doi.org/10.7307/ptt.v23i5.151 https://doi.org/10.1016/j.trpro.2020.08.277 https://doi.org/10.1109/itsc45102.2020.9294662 acta polytechnica ctu proceedings 41:8–14, 2023 1 introduction 2 quality of public transport operation and its assessment method 2.1 reliability evaluation 2.2 travel speed evaluation 2.3 final level of service of public transport operation 3 testing the method – a pilot study 3.1 obtaining input data 3.2 using power bi software to evaluate level of service 4 results and critical analysis of the pilot study 4.1 determination of decisive travel time/speed 4.2 short sections and excessive influence of the value of the reliability index on the level of service 4.3 linguistic functions of affiliation 5 conclusion references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2023.41.0049 acta polytechnica ctu proceedings 41:49–57, 2023 © 2023 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague u smart zone – creating highly realistic virtual environment for vehicle-in-the-loop simulations ondřej piksaa,∗, adam orlickýa, martin schánob a czech technical university in prague, faculty of transportation sciences, department of vehicle technology, horská 3, 128 03 prague 2, czech republic b czech technical university in prague, faculty of transportation sciences, department of forensic experts in transportation, horská 3, 128 03 prague 2, czech republic ∗ corresponding author: piksa@fd.cvut.cz abstract. developing advanced driver assistance systems (adas) and autonomous vehicles (av) based on machine learning is generally an extensive and costly process. due to the increasing complexity of autonomous systems, the need for extensive testing and validation arises. in recent years, computer simulation has been used for these purposes. performing realistic simulations, especially for the purpose of computer vision-based systems, requires a high-quality, almost photorealistic virtual environment. this paper introduces u smart zone, a high-fidelity virtual model of the severní terasa district in ústí nad labem, czech republic comprising of more than 7.5 km of drivable roads with a total area of approximately 1.4 km2 for human-in-the-loop and hardware-in-the-loop (hil) simulations. keywords: virtual environment, human-in-the-loop, hardware-in-the-loop, vehicle simulation. 1. introduction autonomous driving is undoubtedly a technology that will, with its superiority, ultimately revolutionize the way we understand transportation and vehicle ownership. many companies have been trying to develop this innovative technology for more than a decade. however, as the development progresses, it is becoming clear that a long way to fully autonomous vehicles still awaits us. urban traffic is significantly more complicated, primarily due to the number of unpredictable or difficult-to-predict phenomena. the development of sufficiently complex systems for navigating in such a chaotic environment proved to be an issue principally because of the costly and time-consuming nature of machine learning. in addition, regarding security and legislative restrictions, it is often impossible to test experimental vehicle systems in live traffic. the range of learned scenarios is thus limited. as the complexity of autonomous driving increases, so does the need for high-quality and comprehensive testing. autonomous driving systems are tested mainly on closed polygons, in virtual reality, or in a combination thereof, i.e., in augmented reality. while closed polygons offer almost laboratory conditions for testing and the possibility of reconstructing exact situations, virtual reality is significantly less limiting in the amount and scope of tests, which are, nonetheless, confined by preparation time, computing power, and other factors. research in recent years indicates the increasingly efficacious use of virtual simulations for learning or validating of machine-learning-based vehicle systems. the supply of specialized software for “in-the-loop” testing has expanded significantly over the past years. based on our long-term understanding of vehicle simulations, predominantly in the human-in-the-loop domain, and our involvement in the commercial sector in developing advanced driver assistance systems (adas) and autonomous vehicles (av), we have identified one element as yet untapped. that is the photorealism and authenticity of the virtual environment. virtual environments are still relatively simple and idealized. for complex adas and av development and testing, improvements in the simulated environment are necessary to match the complexity of a general urban landscape. in addition, modern vehicle systems built on automatic image recognition systems are becoming progressively routine. and for these systems, it is critical in hil learning to have a virtual landscape closely resembling the actual environment. for this reason, we decided to create an accurate photorealistic model of a sizeable part of the city ústí nad labem in the czech republic for virtual simulations with the possibility of validation via parallel testing. 2. literature review an extensive body of literature exists on developing simulation tools for hil [1, 3]. many of them consider their instruments highly realistic [2–6]. kang et al. [7] presented an overview of a publicly available virtual testing environment. this section introduces some approaches using high-quality, almost photorealistic computer-generated imagery. these computergenerated images work with objects with pre-assigned attributes. that in turn, gives the simulation tool the ability to display a semantic layer alongside the realistic scene, which can then be easily used for artificial intelligence learning or validation. this chapter 49 https://doi.org/10.14311/app.2023.41.0049 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en o. piksa, a. orlický, m. scháno acta polytechnica ctu proceedings figure 1. real environment (left) and its virtual recreation (right) [1]. figure 2. simulation for testing lane keeping and cooperative adaptive cruise control [2]. presents how to create such environment and what constitutes a high-quality environment. the first approach uses a direct reconstruction of the actual environment, for example, based on video footage. this method is utilized, for instance, by gaidon et al. [1]. firstly, they acquire real-world data. consequently, they semi-automatically create a photorealistic dynamic 3d virtual world using off-the-shelf computer graphics engines and graphic assets (realistic 3d models and materials). in their model, main roads are positioned semi-automatically, while secondary roads and background objects (trees, buildings, etc.) are placed manually (the authors indicate these steps could further be automated). the resulting scenario can be seen in figure 1. they then make minor adjustments to these virtual models, yielding a significantly broader range of scenarios for machine learning than they would get by simply annotating the underlying data. alternative approach operates with augmented reality. that is mainly used to validate already learned systems by extending the potential of closed test polygons. authors use a virtual environment to generate road traffic. that includes vehicles, pedestrians, cyclists, or trains. or they use it to add signs, obstacles and other static objects [4, 10, 11]. unfortunately, none of the mentioned-above authors on mixed reality testing showed the visual quality of the virtual elements. a different approach uses completely virtual systems. an example of such a system is a cav hil simulator [2]. this simulator was employed for the development and testing lane keeping and cooperative adaptive cruise control. an illustration of the simulation can be seen in the figure 2. another approach was demonstrated by chen et al. [8]. they verified the effectiveness of their hil simulation platform by path planning and decision-making modules verification, reinforcement learning-based car-following model validation, and autonomous parking algorithm. their scenery was created using openstreetmap, a tool for creating terrain and basic building outlines. various objects like buildings, trees, roads, and other object models were created by 3d modelling software sketchup. the resulting scenario can be seen in figure 3. similarly, li et al. [5] build upon the open-source openstreetmap geographic data to create an elementary road network. using cityengine software, they import the attributes of the virtual road network (including type, width, and the number of lanes) and complete the surrounding environment (see figure 4). introduced by dosovitskiy et al., carla (see fig50 vol. 41/2023 u smart zone figure 3. simulation for testing path planning and decision-making modules, reinforcement learning-based car-following model, and autonomous parking [8]. figure 4. simulation based on osm and cityengine for hil [5]. figure 5. carla is complex hil training and testing software [9]. ure 5) is complex hil training and testing software [9]. “the environment is composed of 3d models of static objects such as buildings, vegetation, traffic signs, and infrastructure, as well as dynamic objects such as vehicles and pedestrians. all models are carefully designed to reconcile visual quality and rendering speed: we use low-weight geometric models and textures but maintain visual realism by carefully crafting the materials and making use of variable level of detail.” [9] they used these assets to create two urban environments, initially by modelling roads and sidewalks, then by manually placing buildings, vegetation, terrain, and other elements of transport infrastructure. the two city models have 2.9 and 1.4 km roads where the test vehicle can move. this system is modular and opensource and is used by many researchers for their hil simulations. gomes et al. [12] took a different approach. in addition to autonomous driving testing, they also focused on the quality of experience of their users. for this reason, they aimed at a higher realism of the simulated environment, especially from the visual side. their method used photogrammetry for all objects in the simulated environment, which you can see in figure 6. continuously more authors use photogrammetry to create photorealistic objects, for instance, shah et al. [3] uses their airsim implemented in the unreal engine (see figure 7 – left) with its access to hundreds of pre-created models. guerra et al. [6] took a similar approach with their flightgoggles. they created their own 3d objects using photogrammetry (see figure 7 – right). both environments are generally used for development and testing of autonomous aerial vehicles. 3. method we decided to advance the current hil simulation environment by creating a large-scale and highly realistic model of the severní terasa district in ústí nad labem, czech republic. that is a nearly 7.5 km of public roads with a total area of approximately 1.4 km2. this model contains a great variety of elements, including directionally divided and directionally undivided sections, a roundabout, a traffic light-controlled intersection, a dense public transport network, and frequent exits for integrated emergency vehicles (see figure 8). thus, it is ideal for running complex simulations. the actual development of the model is divided into four successive steps. these are (i) elevation model, (ii) photogrammetric model, (iii) laser scanning, (iv) manual modelling. 51 o. piksa, a. orlický, m. scháno acta polytechnica ctu proceedings figure 6. scene created only by photogrammetry [12]. figure 7. a snapshot from airsim (left) and flightgoggles (right) [3, 6]. figure 8. extent of the reconstructed section in ústí nad labem, czech republic. figure 9. example of terrain rendering by osm. 52 vol. 41/2023 u smart zone figure 10. simple model crafted on low resolution osm elevation map. 3.1. elevation model the construction of the polygon commenced with an altitude map of the terrain. that defined the core element upon which more specific models were subsequently applied. its visual quality primarily influences the quality of experience of human participants, as it also serves as a visual backdrop providing the model authenticity. at short distances, it is superseded by a more detailed model based on laser scanning or photogrammetry, described below. we compared several different sources of an altitude map. the first was openstreetmap (osm). osm (see figure 9) is preferable in cases with lower standards for an accurate description of the relationship between real and virtual environments, as they have a relatively low resolution of elevation points for the vehicle or pedestrian simulation. beyond that, they offer an additional option, for instance, the automatic generation of buildings. nevertheless, the buildings and terrain generated are highly inaccurate, in particular, terrain lacks the subtle details (unevenness, etc.) found in an actual environment (see figure 10). in our country, a more accurate altimetry called zabaged® (short for “basic base of geographic data”) is available. “digital terrain model of the czech republic of the 5th generation (dmr 5g) represents the image of natural or man-modelled terrain in digital form as heights of discrete points in a triangulated irregular network (tin) with a total mean error of the height 0.18m in an open terrain and 0.3m in a forested terrain.” [13] the model was created using airborne laser scanning acquired data (see figure 11). the availability of such an accurate elevation model makes the surrounding landscape look much figure 11. view of zabaged® elevation with basic texture (up) and without texture (down). figure 12. view of zabaged® elevation without smoothing (up) and with smoothing (down). more authentic and makes it easier to situate objects in it. we further cultivate the visual aspect of the view through low-polygon model smoothing functions (e.g., auto smooth in blender) (see figure 12). 3.2. photogrammetric model the above elevation model delivers realistic and highvisual quality before a close-up. then, it is insufficient from the driver’s perspective. in such cases, photogrammetry can be beneficial as it provides, when used correctly, a relatively accurate point cloud. consequently, accurate surface geometry can be generated, resulting in a high-quality and realistic simulation. and also, this step yields a high-quality texture that further enhances the realism of the overall environ53 o. piksa, a. orlický, m. scháno acta polytechnica ctu proceedings figure 13. the measuring vehicle equipped with cameras for photogrammetry. ment. photogrammetry is a relatively simple and quickly deployable method for creating 3d models, but its use for longer sections still entails significant time effort for data collection. we are trying to solve this partly by placing the measuring equipment on a vehicle, which we then drive through the reconstructed area. the telescopic-modular design, shown in figure 13, allows us to adjust the sensor positions to the nature of the site measured. it is adjustable in height between 1.5–3.0metres above the vehicle in any position, even when exceeding the vehicle outlines. that makes data collection quick and easy. this approach makes it possible to overcome the disadvantages of conventional fixed measuring devices (see figure 14). however, this method brings several challenges for acquiring sufficiently clear images. those range from dynamic shocks to the need for a synchronized shutter speed based on the vehicle speed. we use commercially available gopro hero 9 cameras to maintain this method cost-efficient. the difference between low and high quality images can be seen in figure 15. follow-up experiments showed that it is possible to achieve sufficiently high-quality image while driving. figure 14. the disadvantage of a fixed scanner (in this case, a laser scanner) is the impossibility of collecting data on a site that is not directly visible to the scanner and example of a model reconstructed from the passage of a measuring vehicle (down). figure 15. image of the attainable quality of the photogrammetric model from a detailed environment shot with a static camera (up) and from a driving measuring vehicle (down). 3.3. laser scan the laser scan functions similarly within the framework of the constructed model as photogrammetry. yet, it is a significantly more expensive technology employing, in our case, a high-speed high-performance dual scanner mobile mapping system riegl vmx-2ha (result shown in figure 16). for this reason, we use the low-cost photogrammetry as an alternative. consequently, we compare photogrammetric results with 54 vol. 41/2023 u smart zone figure 16. wired model from high-speed high-performance dual scanner mobile mapping system riegl vmx-2ha. figure 17. combination of multiple textures (plaster + graffiti) before (up) and after (down) application of displacement (focus on graffiti). result obtained via laser scanning. the key difference to be observed will be the quality of the geometric shape of the drivable area to allow the simulation of the realistic behaviour of the vehicle. the comparison will be performed using the professional vehicle physics simulation software carsim. 3.4. manual modelling many objects had to be modelled manually. there are many tools for simplifying complex geometric shapes obtained by photogrammetry or laser scanning (e.g., decimation, the process of discarding points from the data to improve performance) available. however, we have not come across any that would outperform manual modelling regarding object complexity (mesh lod) and the resulting visual quality of the model, if a refined texture is used. in addition, many objects in figure 18. real stop on modelled section (up) and manually modelled stop with texture from real photos (down). our testing area that cannot be reconstructed without manual modelling (see figure 17). we use many other tools to substitute surface details commonly lost using simplified object geometry. to illustrate, we apply the shader effect via vector displacement based on the underlying texture (see figures 17 and 18). this approach achieves simple yet photorealistic objects. for some objects, it is not yet possible to automate the creation of a virtual model with acceptable quality and complexity. an example of such an object can be seen in figure 19. 4. summary and future work in this paper, we reviewed the current work on virtual environments for hil simulation and presented our highly realistic digital model of a complex city environment for hil simulation. the two main contributions of this model are (i) the highly heterogeneous environment for complex simulations, 55 o. piksa, a. orlický, m. scháno acta polytechnica ctu proceedings figure 19. complicated model with no option automatic reconstruction. (ii) the possibility of parallel testing in virtual and actual environments to validate the results. using the real-world environment with all its flaws, including, for example, faulty, worn, or damaged roadway markings or contradictions between traffic signs and road markings, adds depth not previously used in hil simulations. thus, the resulting autonomous system derived from using the real-life environment is better equipped to rectify the randomness inherent to real road traffic. in the future, we intend to extend our model to include additional areas involving adjacent rural roads and highway segments. for modelling the rural roads, we have selected an area of approximately 8.5 km in length involving cramped villages and parts with limited visibility due to vegetation. that will allow simulations of emergencies associated with, for example, collisions with wildlife. also, an approximately 9 km section directly adjacent to the current model area is selected for modelling the highway segments. references [1] a. gaidon, q. wang, y. cabon, e. vig. virtualworlds as proxy for multi-object tracking analysis. in 2016 ieee conference on computer vision and pattern recognition (cvpr), pp. 4340–4349. 2016. https://doi.org/10.1109/cvpr.2016.470 [2] ş. y. gelbal, s. tamilarasan, m. r. cantaş, et al. a connected and autonomous vehicle hardware-in-the-loop simulator for developing automated driving algorithms. in 2017 ieee international conference on systems, man, and cybernetics (smc), pp. 3397–3402. 2017. https://doi.org/10.1109/smc.2017.8123155 [3] s. shah, d. dey, c. lovett, a. kapoor. airsim: high-fidelity visual and physical simulation for autonomous vehicles. in m. hutter, r. siegwart (eds.), field and service robotics, pp. 621–635. springer international publishing, cham, 2018. https://doi.org/10.1007/978-3-319-67361-5_40 [4] y. feng, c. yu, s. xu, et al. an augmented reality environment for connected and automated vehicle testing and evaluation. in 2018 ieee intelligent vehicles symposium (iv), pp. 1549–1554. 2018. https://doi.org/10.1109/ivs.2018.8500545 [5] x. li, y. wang, l. yan, et al. paralleleye-cs: a new dataset of synthetic images for testing the visual intelligence of intelligent vehicles. ieee transactions on vehicular technology 68(10):9619–9631, 2019. https://doi.org/10.1109/tvt.2019.2936227 [6] w. guerra, e. tal, v. murali, et al. flightgoggles: photorealistic sensor simulation for perception-driven robotics using photogrammetry and virtual reality. in 2019 ieee/rsj international conference on intelligent robots and systems (iros), pp. 6941–6948. 2019. https://doi.org/10.1109/iros40897.2019.8968116 [7] y. kang, h. yin, c. berger. test your self-driving algorithm: an overview of publicly available driving datasets and virtual testing environments. ieee transactions on intelligent vehicles 4(2):171–185, 2019. https://doi.org/10.1109/tiv.2018.2886678 [8] y. chen, s. chen, t. zhang, et al. autonomous vehicle testing and validation platform: integrated simulation system with hardware in the loop. in 2018 ieee intelligent vehicles symposium (iv), pp. 949–956. 2018. https://doi.org/10.1109/ivs.2018.8500461 [9] a. dosovitskiy, g. ros, f. codevilla, et al. carla: an open urban driving simulator, 2017. arxiv:1711.03938 [10] y. liu, k. zhang, z. li. application of digital twin and parallel system in automated driving testing. in 2021 ieee 1st international conference on digital twins and parallel intelligence (dtpi), pp. 123–126. 2021. https://doi.org/10.1109/dtpi52967.2021.9540143 [11] x. che, c. li, z. zhang. a test method for self-driving vehicle based on mixed reality. in 2021 ieee international conference on smart internet of things (smartiot), pp. 401–405. 2021. https://doi.org/10.1109/smartiot52359.2021.00075 56 https://doi.org/10.1109/cvpr.2016.470 https://doi.org/10.1109/smc.2017.8123155 https://doi.org/10.1007/978-3-319-67361-5_40 https://doi.org/10.1109/ivs.2018.8500545 https://doi.org/10.1109/tvt.2019.2936227 https://doi.org/10.1109/iros40897.2019.8968116 https://doi.org/10.1109/tiv.2018.2886678 https://doi.org/10.1109/ivs.2018.8500461 http://arxiv.org/abs/1711.03938 https://doi.org/10.1109/dtpi52967.2021.9540143 https://doi.org/10.1109/smartiot52359.2021.00075 vol. 41/2023 u smart zone [12] g. d. gomes, r. flynn, n. murray. a qoe evaluation of an immersive virtual reality autonomous driving experience. in 2020 twelfth international conference on quality of multimedia experience (qomex), pp. 1–4. 2020. https://doi.org/10.1109/qomex48832.2020.9123128 [13] geoportal čúzk – zabaged® – altimetry – introduction. [2023-03-06] https://geoportal.cuzk.cz/ (s(vesl5kbxcrvnynvkwudwtb31))/default.aspx?lng= en&mode=textmeta&text=vyskopiszbg&side= vyskopis&head_tab=sekce-02-gp&menu=30. 57 https://doi.org/10.1109/qomex48832.2020.9123128 https://geoportal.cuzk.cz/(s(vesl5kbxcrvnynvkwudwtb31))/default.aspx?lng=en&mode=textmeta&text=vyskopiszbg&side=vyskopis&head_tab=sekce-02-gp&menu=30 https://geoportal.cuzk.cz/(s(vesl5kbxcrvnynvkwudwtb31))/default.aspx?lng=en&mode=textmeta&text=vyskopiszbg&side=vyskopis&head_tab=sekce-02-gp&menu=30 https://geoportal.cuzk.cz/(s(vesl5kbxcrvnynvkwudwtb31))/default.aspx?lng=en&mode=textmeta&text=vyskopiszbg&side=vyskopis&head_tab=sekce-02-gp&menu=30 https://geoportal.cuzk.cz/(s(vesl5kbxcrvnynvkwudwtb31))/default.aspx?lng=en&mode=textmeta&text=vyskopiszbg&side=vyskopis&head_tab=sekce-02-gp&menu=30 acta polytechnica ctu proceedings 41:49–57, 2023 1 introduction 2 literature review 3 method 3.1 elevation model 3.2 photogrammetric model 3.3 laser scan 3.4 manual modelling 4 summary and future work references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.39.0041 acta polytechnica ctu proceedings 39:41–48, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague dynamic tests of the culvert face within crashtests luboš nouzovský, tomáš kohout∗, pavel vrtal, jakub nováček czech technical university in prague, faculty of transportation sciences, department of forensic experts in transportation, konviktská 20, 110 00 prague 1, czech republic ∗ corresponding author: kohout@fd.cvut.cz abstract. severity of a vehicle impact into solid obstacles alongside the road can be annually illustrated through accident statistics. one example of such solid obstacles is drainage ditch culvert faces. within the research at the faculty of transportation sciences of the czech technical university new equipment has been designed in a form of a modular system of the drainage ditch culvert faces construction. the qualities of this equipment have been tested by means of crash tests with a passenger vehicle škoda fabia making use of criteria of biomechanical endurance of the vehicle passengers. the results have been compared with the test carried out by means of a vehicle crash into concrete culvert faces. keywords: drainage ditch, vehicle crash, occupant safety, deformation block, crash test, acceleration. 1. introduction the objective of this article succeeding the already published primary research “access bridge design measures for safety increase of the road infrastructure” is to prove efficacy of the designed deformable structure of the drainage ditch culvert faces by means of a crash test. the requirements for qualities of a deformable item for the stated application are specific and they thus differ from the so far patented solutions [1–5]. it is first and foremost the requirement for deformation quality in the direction vertical to axes of honeycombs (cells), sufficient capacity in the direction of honeycomb axes and also high durability that will be guaranteed by means of high-early cement mixtures or composites [6]. when crashed, the internal structure of the deformable block will be distorted in a controlled way, thus the vehicle is going to be gradually decelerated. the internal structure of the deformable block should effectively dissipate kinetic energy only by means of a vehicle speed reduction, i.e. the kinetic energy of the vehicle is not primarily used for the deformation of safety zones of the vehicle [7]. at the same time the deformable block has sufficient firmness in the vertical direction to axes of the cells and can shift a load from axletree pressure. 2. crashtests for proving the utility of the deformation structure of drainage ditch culvert faces a crash test of a passenger vehicle under the nominal speed of 50 km·h−1 was carried out, when the vehicle crashed vertically into a drainage ditch culvert face located in the road drainage ditch when the last 5 meters in front of the very crash place (due to the necessity to reiterate the tests and to compare better the results) the tested vehicle was moving at constant speed parallelly with the axis of the ditch. in order to be able to compare the results, 2 crash tests were carried out (figure 1 and 2): • test no. 001 – passenger vehicle m1 × drainage ditch with an integrated deformation zone. • test no. 002 – passenger vehicle m1 × drainage ditch of a classical construction (concrete face). 2.1. parameters of the drainage ditch • outside dimensions (length/width/height): 3.2/2.4/0.95 m, • weight: 5,500 kg, • parameters of the ditch for steering of the vehicle: width 1.9 m, depth 0.75 m. considering that it was a working prototype, and the primary purpose of the test was a dynamic test of the deformation material used, the culvert face was not equipped with a drainage pipe. 2.2. testing vehicles two fabia vehicles of the following parameters were available [8]: • unladen weight: 1,055 kg, • size (length/width/height): 3.99/1.73/1.47 m, • wheel base: 2.47 m, • tread: 1.46 m, • inner height: 0.13 m. the vehicle was for each crash test equipped with dummy h iii 50 % (77 kg) in the place of a driver (seating position 1.) and dummy p6 (22 kg) in the children vehicle restraint system in the back seat right behind the driver (seating position 4.). triaxial accelerometer was placed during each test on the body 41 https://doi.org/10.14311/app.2022.39.0041 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en l. nouzovský, t. kohout, p. vrtal, j. nov’aček acta polytechnica ctu proceedings figure 1. deformation blocks geometry and ditch geometry. figure 2. drainage ditch with an integrated deformation zone – test no. 001 (left) and drainage ditch of classical construction (concrete face) – test no. 002 (right). of the vehicle nearby the airbag control unit (1000 g) and in the head centre of the gravity of both dummies (500 g). in both tests, airbags were taken out of service, primarily because of the scientific approach to the tests. the aim of the tests was to simulate the worst possible scenario in relation to the measured values. the weight of the vehicle in the test including h iii dummy, p6 dummy and the measuring equipment was 1,161 kg. the vehicle was steered into the ditch by means of the guiding system fixed to the pavement and it impacted frontally the culvert face of the drainage ditch, the slope angle being given by parameters of the ditch (see figure 3). the whole course of the crash test was being recorded with a high-speed camera (min. 1000 fr·s−1 at 1280×1024). 2.3. assessing criteria when comparing the two crash tests and quantifying the results, the criteria evaluating the safety of the restraint systems according to en 1317 as well as the biomechanical criteria evaluating head injuries were used [9–11]. 2.3.1. criterion asi (acceleration severity index) acceleration severity index is a criterion used for assessing road restraint systems according to en 1317 [12]. the asi criterion is measured from the measured acceleration on the vehicle body (detector on the tunnel nearby vertical gravity axis projection). it assesses severity range of the pulse affecting the impact severity levels level maximal tolerated values a asi ≤ 1.0 b 1.0≤asi≤1.4 and thiv≤33 km·h−1 c 1.4≤asi≤1.9 table 1. assessment of impact severity by means of the asi criterion. person sitting nearby the measured point of the vehicle. according to en 1317 the value of asi should not exceed the value of 1, or if you like of 1.9 for the given impact severity levels a to c, see table 1. the criterion can be expressed mathematically as: asi(t) = [ (ax/âx)2 (ay/ây)2 (az/âz)2 ]1/2 , (1) where âx, ây and âz are limit values for particular acceleration components and can be interpreted as a value of applied acceleration during which the risk of a grave injury of the passenger is minimized (max. light injuries). for a fastened passenger the limits are generally used âx = 12 g, ây = 9 g, âz = 10 g. the magnitudes ax, ay or if you like az are filtered components of the acceleration. asi is set as maximum of the range asi(t). 42 vol. 39/2022 dynamic tests of the culvert face within crashtests figure 3. testing vehicles and dummies positioning. 2.3.2. criterion thiv (theoretical head impact velocity) the concept of the theoretical head impact velocity (thiv) was developed for assessement of severity index of a vehicle passenger who gets into a collision with the restraint system. the passenger in the vehicle is perceived as a freely moving object (head) that, in compliance with the vehicle changing its speed, when having got into contact with a vehicle restraint system, goes on in its movement until it hits the surface inside of the vehicle. the volume of theoretical head impact velocity is interpreted as a level of crash severity upon the restraint system. thiv = [ ẋ2 b(t) + ẏ2 b (t) ]1/2 , (2) where ẋ2 b(t) or if you like ẏ2 b (t) are the coordinates of the theoretical head velocity in relation to the vehicle coordinate system. the detailed calculation together with the implementation of coordinate systems is a subject to a norm. this criterion is chosen in the case when the vehicle is not peopled by biomechanical dummies. its limit is set by the en 1317 norm to 33 km·h−1. it is rounded up to the whole number. 2.3.3. phd criterion (post-impact head deceleration) the phd criterion used to be an additional criterion of assessing a level of impact severity. since a revision of the norm (en 1317) in 2010 this criterion has not been followed. however, it can be used as an information marker when analyzing performed crash tests: phd = max [ ẍ2 c(t) + ÿ2 c (t) ]1/2 , (3) where ẍ2 c(t) or if you like ÿ2 c (t) is acceleration of the theoretical head after the very first contact with a conceptional structure. the phd should not exceed the value of 20 g. 2.3.4. hic criterion (head injury criterion) this criterion can be perceived as a measurement of the probability of head injury during an impact. it is calculated as a function maximum of the acceleration resultant a(t). hic = [ 1 t2 − t1 ∫ t2 t1 a(t)dt ]2.5 (t2 − t1), (4) where a(t) = final acceleration [g], t1 and t2 = instants of time in the course of the impact determining beginning and the end of the interval the hic volume of which is the highest. the length of the interval (t2˘t1) is 36 ms if it did not come into contact with a solid structure. as for the calculation of a maximal value of the impact the time interval of 15 ms (hic15) is used. the limit value of the criterion hic is 1000 – the limit indicates approx. 50 % probability of severe head injury occurrence [13, 14]. 2.3.5. criterion 3 ms contrary to the previous criterion this one can be applied not only to head injuries. its limit value assesses toleration for occurrence of severe injuries. the limit value for the head is 80 g. the interpretation of the criterion: the acceleration higher than 80 g cannot operate for a longer period than 3 ms [15]. 3. results 3.1. test no. 001 the drainage ditch with an integrated deformation zone, a nominal impact speed of 50 km·h−1 was complied (the real speed was 47 km·h−1). figure 4 demonstrates a course of the test recorded with a high speed camera; the vehicle is due to a deformed structure lifted above the barrier. in the chart of figure 5 accelerations measured in three planted places are being recorded. the red curve corresponds with acceleration measured on the body of the vehicle; the green curve presents acceleration of the head of the driver dummy, the blue one shows acceleration for the child passenger. the blue background stands for a time interval when main material deformation of a structure of an integrated 43 l. nouzovský, t. kohout, p. vrtal, j. nov’aček acta polytechnica ctu proceedings figure 4. a course of the test no. 001 – the vehicle is due to a deformed structure lifted above the barrier. figure 5. test no. 001 of drainage with an integrated deformation zone – acceleration time course. deformation zone was going on (time 0 ms up to approx. 200 ms). from approx. 700 ms the vehicle does not make any forward movement. impact phases: 0 ms – first contact with a drainage ditch culvert face, 0-100 ms – main deformation of the drainage ditch structure and of the vehicle deformation zone, 65 ms – interaction of the drainage ditch structure deformation and vehicle deformation zone – the area of an impact of maximal acceleration upon the body of the vehicle, 100-150 ms – the area with maximum measured acceleration affecting the head of the adult dummy, without any contact with an interior of the vehicle, an apex of forward movement of the dummy, 100-170 (or if you like 250) ms – maximum acceleration affecting the head of the child dummy, without any contact with the vehicle interior, the apex of forward movement of the dummy, 100-230 ms – ongoing deformation of the drainage ditch – the vehicle being lifted up due to a distorting structure, 420-470 ms – backward movement of the driver dummy, contact with the head restraint, approx. from 430 ms – minimum forward movement, vehicle shifts to a final position, figure 6. asi criterion in test no. 001. 480 ms – apex of backward movement of the child dummy and a contact with the head restraint, 530 ms – the vehicle, due to after impact movement, climbed up the distorted structure of the drainage ditch, up to approx. 700 ms. forward movement is noticeable, succeeded by stabilization and “shifting” to a final position in th ditch, 860 ms – the heads of both dummies crashed the door (window glass) – the windows had been due to a better quality of the camera recording removed before the test (usability of the recording for following point trajectory “tracking”). 3.1.1. asi criterion the course of this criterion can be found in the graph on figure 6. the maximal value of asi was measured in the time of 48.5 ms, being it 0.8 (after rounding), the limit value of 1 was not exceeded. the deformable drainage ditch structure thus fulfils requirements for the level a of the impact severity. 3.1.2. hic criterion and the criterion of 3ms hic criterion – the driver and child dummy calculated values of the hic criterion were deep below the set limit value of 1000, see table 2. criterion of 3ms – neither in this case the limit value of 80 g was reached, see table 2. in compliance with the hic criterion it can be said that the probability of a serious head injury is nearly zero. 3.1.3. criteria thiv and phd for test no. 001 any calculation of thiv and phd criteria can be perceived as unnecessary. since during test no. 002 the data depreciation with the driver dummy occurred, these criteria are chosen as referential in such a way so that the results of both tests could be compared. neither the thiv, nor phd criterion exceeded their limit values, see table 3. 44 vol. 39/2022 dynamic tests of the culvert face within crashtests hic36 3ms test no. hic36 t1 t2 [g] t1 t2 001 [ms] [ms] [ms] [ms] driver 17.6 96.6 132.6 12.0 117.4 120.5 child 10.9 135.0 171.0 10.1 145.9 149.1 table 2. overall chart of hic and 3ms criteria, test no. 001. test no. asi thiv phd 001 [-] t1 [ms] [km·h−1] t1 [ms] [g] value 0.8 48.5 28 147.5 5.0 table 3. overall chart of thiv and phd criteria, test no. 001. figure 7. test no. 002 – drainage ditch of a classical construction. figure 8. test no. 002 of drainage ditch of classical construction – acceleration time course. 3.2. test no. 002 drainage ditch of a classical construction, nominal impact speed of 50 km·h−1 was complied (the real speed was 45 km·h−1). during test no. 002 crashing into a concrete drainage ditch face (standard construction) a measuring range of the accelerometer was exceeded with the driver dummy (500 g). that is why a measuring sector of approx. 65 ms to 115 ms could not be assessed for this place. the reason for range exceeding was a contact of the dummy with the steering wheel. note: however, it cannot be stated that within the course of the whole stated time period the acceleration higher than 500 g was affecting the dummy. most probably it was just the impulse higher than 500 g, which at the same time caused a short time measuring fallout with the measuring channel. impact phases: 0 ms – the first contact of the front of the vehicle with a concrete drainage ditch culvert face, 0-22 ms – buffer deformation, 22-70 ms – engine space deformation, 64 ms – the driver dummy – contact of the thorax with the steering wheel, a measuring range of 500 g was exceeded on the accelerometer, 72 ms – the ond of the forward movement – the vehicle bounces from the solid structure, 80 ms – the driver dummy – head contact with the steering wheel, 110 ms – forward movement of both dummies finished, the beginning of the backward movement, approx. 200 ms – crash of both dummies into the head restraint of the seat – the impact of an eccentric character – in between the head restraint and the side structure of the vehicle, 294 ms – crash of the child dummy into the frame of the side window, approx. 400 ms – total stabilization of the movement. 3.2.1. asi criterion the asi reached in the time of 33.6 ms a level of 3.1. for comparison, restraint systems with lowest requirements for safety – impact severity level c – must fulfil the asi value lower than 1.9 (see table 1). 3.2.2. hic criterion and the criterion of 3ms due to a failure of measuring (see above) the criteria hic and 3ms for the driver dummy could not be calculated. in order to be able to compare adequately both crash tests despite this failure the theoretical head impact velocity criterion can be used (thiv) together with an additional phd (post-impact head 45 l. nouzovský, t. kohout, p. vrtal, j. nov’aček acta polytechnica ctu proceedings figure 9. asi criterion in test no. 002. figure 10. deformation of drainage ditch with an integrated deformation zone after test no. 001. deceleration) criterion that can be calculated when the course of vehicle deceleration is known. 3.2.3. criteria thiv and phd as for the thiv criterion a set limit of 33 km·h−1 was exceeded, the value of 51 km·h−1 was measured and calculated. the phd criterion reached value little below the limit of 20 g. 4. discussion due to the deformation of the drainage ditch construction (figure 10) the duration of the crash event is prolonged – transformation of the kinetic energy it spread into a longer time period – which results in a decrease in affecting powers. due to the impact with the drainage ditch culvert face equipped with the deformable block the damage of the left front corner of the car body occurred during test no. 001, together with the deformation of a cooler, a shift in the suspension of the left front wheel in the backward direction (see figure 11). the deformation of the longitudinal beam did not occur, nor tearing of the engine block, the cockpit remained intact, too, the front left door could be opened without any difficulty. due to the impact with the drainage ditch culvert face with concrete face (figure 12) the engine was torn from the right top hinge during test no. 002 together with cabling, the front face was significantly deformed, see figure 13; the cooler, hood, bumper stiffener and mounting (hinge) of the left front fender were deformed. the fender got torn from the hinge, also the setting of the left front fender was deformed figure 11. vehicle damage after test no. 001 – drainage ditch with an integrated deformation zone impact. figure 12. deformation of drainage ditch with an integrated deformation zone after test no. 002. and a manifold destruction of the intake together with a short circuit occurred. based on all this information, a model was created for subsequent simulation and use in forensic engineering [16]. figure 14 shows the mutual comparison of acceleration/deceleration that was measured on the body of the vehicle in the very first 300 ms of the event during both implemented tests. the difference in the course of both events is clearly visible. when carrying out an impact test of the culvert face equipped with deformable blocks, the values of affecting deceleration were five times lower than in the case of the drainage ditch of a standard construction. the significant difference in asi criterion in both tests is clearly visible in figure 15. 5. conclusion the objective of these experiments was to prove that deformable blocks meet the requirements for the restraint function with a calculated deceleration factor and that they cut down a risk of grave and fatal injuries of the vehicle passengers when colliding with the drainage ditch. impact severity assessed by means of the asi criterion was in this case 4 times higher than as for the impact into the deformable block. when assessing a biomechanical force, the hpc criterion for the child dummy head and the thiv criterion can be used. in accordance with the calculated hpc values it can be predicted that the child would – in case of an impact with a drainage ditch with a deformable structure under conditions analogical to implemented tests with a probability of more than 99 % – get by without any injury whereas in the case of an impact into a standard drainage ditch, i.e. unfortunately the most widely spread type of a construction, it would suffer in 40 % from injuries of the highest severity ais 3+ (grave to fatal injury) and only in approx. 2 % it would get by unharmed [17, 18]. according to the thiv 46 vol. 39/2022 dynamic tests of the culvert face within crashtests hic36 3ms test no. hic36 t1 t2 [g] t1 t2 002 [ms] [ms] [ms] [ms] driver – – – – – – child 613.6 71.2 102.8 48.6 86.4 89.6 table 4. overall chart of hic and 3ms criteria, test no. 002. test no. asi thiv phd 001 [-] t1 [ms] [km·h−1] t1 [ms] [g] value 3.1 33.6 51 113.6 19.2 table 5. overall chart of criteria thiv and phd, test no. 002. figure 13. vehicle damage after test no. 002 – drainage ditch with an integrated deformation zone impact. figure 14. vehicle acceleration time course – comparison. criterion, which abstracts from equipping the vehicle with anthropometric devices, the used deformable structure would meet the requirements for road restraint systems according to en 1317. the functionality of deformable drainage ditch culvert faces has been hereby proved experimentally. acknowledgements this work was supported by the project “bezpečnost a obrana pro inovaseed”, cz.1.05/3.1.00/14.0304. references [1] p. barasoain, a. sanchez. shock absorber and manufacturing procedure thereof, 2005. international patent ep 1593872 a1. 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https://www.webofscience.com/wos/woscc/full-record/wos:000576604500245 https://www.webofscience.com/wos/woscc/full-record/wos:000576604500245 https://doi.org/10.4271/973318 https://doi.org/10.7307/ptt.v24i2.282 acta polytechnica ctu proceedings 39:41–48, 2022 1 introduction 2 crashtests 2.1 parameters of the drainage ditch 2.2 testing vehicles 2.3 assessing criteria 2.3.1 criterion asi (acceleration severity index) 2.3.2 criterion thiv (theoretical head impact velocity) 2.3.3 phd criterion (post-impact head deceleration) 2.3.4 hic criterion (head injury criterion) 2.3.5 criterion 3ms 3 results 3.1 test no. 001 3.1.1 asi criterion 3.1.2 hic criterion and the criterion of 3ms 3.1.3 criteria thiv and phd 3.2 test no. 002 3.2.1 asi criterion 3.2.2 hic criterion and the criterion of 3ms 3.2.3 criteria thiv and phd 4 discussion 5 conclusion acknowledgements references my title https://doi.org/10.14311/app.2022.36.0261 acta polytechnica ctu proceedings 36:261–268, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague development of conformity criteria for durability and its influence on service life predictions eline vereecken∗, robby caspeele ghent university, faculty of engineering and architecture, department of structural engineering and building materials, technologiepark-zwijnaarde 60, 9052 ghent, belgium ∗ corresponding author: eivereec.vereecken@ugent.be abstract. to ensure the quality of produced concrete batches, it is often verified whether a specified concrete compressive strength is reached by the application of conformity control. the conformity criteria used for this purpose are related to the probability of accepting a batch, in relation to a specified quality. batches fulfilling the conformity criteria are accepted, whereas others are rejected. in this way, the outgoing quality of the accepted concrete is higher than without the application of conformity control. this is the result of the filtering effect of conformity control and affects the probability of failure of concrete structures. instead of only applying conformity control to the concrete compressive strength, a similar procedure could be applied to durability related variables. one example hereof is the diffusion coefficient of the concrete. when conformity criteria are defined and applied in relation to the diffusion coefficient, the result will be a filtering effect on the distribution of the diffusion coefficient. the outgoing distribution will have a lower mean value and standard deviation compared to the incoming distribution. this will influence the time-dependent reliability of reinforced concrete structures. this paper investigates the effect of conformity criteria for diffusion coefficients on the service life and failure probabilities of reinforced concrete elements. the filtering effect is investigated by a numerical study applying analytical methods and numerical integration. keywords: concrete, conformity control, durability, service life. 1. introduction by requiring quality inspection on concrete properties, producers are stimulated to obtain higher performance. one example of a method for quality inspection is the application of conformity control to the concrete compressive strength [1, 2], based on operating characteristic curves (oc-curves) [2, 3]. these oc-curves relate the probability of accepting a concrete batch to its quality. as such, some concrete batches will be rejected whereas others will be accepted. as a result, a filtering effect is induced on the distribution of the investigated property. in case of concrete compressive strength, this leads to a higher mean value and a lower standard deviation of the outgoing distribution (after conformity control) compared to the incoming concrete offered for conformity control [4, 5]. when the outgoing strength distribution is used in structural reliability calculations, the probability of failure decreases, which illustrates the beneficial effect of quality control [6]. in this article, conformity criteria are applied to the diffusion coefficient, and the effect on the parameters of the probabilistic model is investigated together with the effect on the service life of reinforced concrete structures. the principles of conformity control are summarized in section 2, and the resulting filter effect is explained in section 3, which provides the applied analytical models and methods for numerical integration. the quantification approach is applied to the diffusion coefficient in section 4. in this section, it is also investigated how sampling from a concrete batch influences the probability of accepting the batch. in section 5, the influence of conformity control on the distribution of the initiation period is investigated, together with the influence on the service life. 2. quality control by conformity criteria conformity control verifies whether produced concrete batches comply with specified properties. for this purpose, a sample of limited size is used and from this sample inferences are made with regard to the whole population. as stated in [7], for the diffusion coefficient, the mean value dm, corresponding to the 50%-fractile of the theoretical distribution can be considered as the specified value, or the characteristic value dk corresponding to the 90% fractile [7, 8]. in practice, the fraction above the specified value (corresponding to an undesired situation of a high diffusion coefficient) will be smaller or higher than 50% or 10% respectively. when θ is the fraction of test results higher than dm or dk in the offered distribution, equations (1) and (2) are valid [7] (where x is the diffusion coefficient, considered as a random variable). this value θ will be further denoted as the fraction defectiveness. p [x ≥ dm] = θ1 (1) 261 https://doi.org/10.14311/app.2022.36.0261 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en eline vereecken, robby caspeele acta polytechnica ctu proceedings p [x ≥ dk] = θ2 (2) when a conformity criterion in specified, for each fraction defectiveness θ, the corresponding probability that a concrete batch will be accepted can be calculated, under the assumption of a theoretical distribution of the diffusion coefficient. this probability of acceptance is denoted as pa(θ) and is called the operating characteristic (oc-curve). 3. conformity criteria and their filter effect in [9], an analytical method is provided that can be used to evaluate the filter effect, but is only applicable to some common (basic) conformity criteria. in practice, more complex criteria are used [10]. in such situations, a numerical algorithm can be applied, such as the one developed in [11], which has its basis in bayesian statistics and applies numerical integration. the method applied in this work is also described in [7]. similar as in [7], in the following, the batches offered for conformity control, i.e. the incoming concrete batches, are represented by the index ’i’. the accepted batches, i.e. the outgoing concrete batches, are represented with the index ’o’ the mean and standard deviation of the incoming concrete batches are given by equations (3) and (4) respectively (assuming normal distributions). µi = µm (3) σi = √ σ2 m + σ2 l (4) given the oc-curve of a conformity criterion, the posterior distribution of the diffusion coefficient can be quantified by numerical integration, and the mean and standard deviation can be calculated. the latter can then be compared with the prior (incoming) values to quantify the filter effect of the conformity control. 4. conformity control for the diffusion coefficient of concrete in this section, conformity control is considered for the diffusion coefficient and the filter effect on its distribution is investigated. since there are no standards yet providing conformity criteria for the diffusion coefficient, in this work, some assumptions are made in relation to the format of the conformity criteria, which are similar to those in [7]. 4.1. oc-curves if a value for the diffusion coefficient has to be specified, often the mean value dm (50% fractile) or the characteristic value dk (90% fractile) are used (as also indicated in section 2). to define a conformity criterion, either the sample standard deviation sn could be used, leading to equations (5) and (6) [7], or the criterion could be based on the standard deviation of the incoming batches σi, according to equations (7) and (8) [7]. the latter can be applied when sufficient information is available to have a reasonable estimate of the standard deviation σi. xn ≤ dm + λ sn (5) xn ≤ dk + λ sn (6) xn ≤ dm + λ σi (7) xn ≤ dk + λ σi (8) in these equations, n is the number of samples used for conformity control and xn is the sample mean. the parameter λ corresponding to a certain value of n can be derived based on the aoql (average outgoing quality limit) concept, as described in [3, 12, 13]. the aoq (average outgoing quality) is defined according to equation (9). aoq = θ · pa (9) a boundary for the unsafe region in the θ − pa diagram could be defined, which is represented by the aoql. this aoql equals 50% for conformity criteria (5) and (7), when on average the specified value should be a 50% fractile. similarly, the aoql equals 10% for conformity criteria (6) and (8), where a specified value equal to the 90% percentile is desired [7]. also an uneconomic region in the θ − pa diagram can be defined, for which the boundary is given by equations (10) and (11) according to [14] (for θ1 ≤ 50% or θ2 ≤ 10%). θ1 1 − p 1 a = 50 % (10) θ2 1 − p 2 a = 10 % (11) in these equations, θ1 are the fraction defectives according to equation (1) and p 1 a the corresponding probability of acceptance. accordingly, θ1 and p 2 a are related to equation (2). it should be pointed out that in equations (10) and (11) other values of the aoql can also be assumed, depending on the acceptability requirements. the resulting values of λ to be used for different values of n are summarized in table 1 for the conformity criteria given by equations (5) and (7) (aoql= 50%) and equations (6) and (8) (aoql= 10%), assuming that each time n different samples are taken from the incoming concrete batches. the sample mean xn and sample standard deviation sn are then evaluated based on these n different and independent samples. the corresponding oc-curves for the conformity criterion given by equations (5) and (6) are visualized 262 vol. 36/2022 conformity criteria for durability in service life predictions n λ aoql= 50% aoql= 10% xn ≤ dm + λ sn xn ≤ dm + λ σi xn ≤ dk + λ sn xn ≤ dk + λ σi 3 1.32 0.57 − 1.24 − 0.59 4 1.04 0.51 − 1.07 − 0.60 5 0.90 0.49 − 1.01 − 0.60 6 0.81 0.45 − 0.98 − 0.62 7 0.76 0.44 − 0.96 − 0.62 8 0.70 0.42 − 0.95 − 0.63 9 0.66 0.40 − 0.95 − 0.64 10 0.64 0.39 − 0.96 − 0.64 11 0.61 0.38 − 0.94 − 0.65 12 0.59 0.37 − 0.96 − 0.66 13 0.57 0.37 − 0.96 − 0.66 14 0.55 0.35 − 0.96 − 0.66 15 0.54 0.35 − 0.96 − 0.66 table 1. oc curves for conformity control on the diffusion coefficient. in figure 1. also the boundaries of the uneconomic and unsafe region are indicated. 4.2. influence of sampling on the probability of accepting a concrete batch for derivation of the oc-curves in the previous section, it has been assumed that n different samples are taken from the concrete batch. xn and sn are then evaluated based on these n different samples. however, xn could also represent a moving average. consider for example the case where n = 3 and where six samples are available. in the first situation, conformity control could be applied two times, i.e. once with xn and sn evaluated based on the first three samples, and once with xn and sn evaluated based on the next three samples. if xn is replaced by a moving average, the conformity control can be applied n + 1, i.e. 4 times, i.e. once xn and sn are evaluated based on samples 1 to 3, once samples 2 to 4 are used, once samples 3 to 5 are used, etc. hence, each time a new sample is added to the set and the first sample is thrown out. since in the second situation more tests for conformity control are performed, this will have an influence on the probability of accepting a concrete batch. in this section, this influence will be investigated. the probability of accepting a lot pa will be derived for both situations. each time, the total set of samples is equal to 2n. hence, in the first situation, conformity will be checked 2 times, and in the second situation, with the moving average, conformity will be checked n + 1 times. the influence on the probability of accepting a lot is visualized in figure 2 for the two types of conformity criteria as given by equations (8) and (9) and for n equal to 3, 5 and 15, for the different values of the fraction defectives. here it can be seen that working with the moving average leads to a lower probability of accepting a concrete batch. this because more tests are performed, leading to a more strict criterion. however, moving averages might be sensible to consider if for example from an economic point of view only a limited number of samples can be obtained. 4.3. filter effect of the application of conformity control on the diffusion coefficient in this section, the filter effect on the mean and standard deviation of the diffusion coefficient is quantified. for this purpose, the ratios µo/µi and σo/σi, i.e. the outgoing mean or standard deviation of the diffusion coefficient relative to the incoming values, are evaluated based on the numerical algorithm described in [15]. for aoql= 50% and xn ≤ dm + λ sn, the influence of the filter effect is visualized in figure 3 (i). similarly, for aoql= 10% and xn ≤ dk + λ snthe results are presented in figure 3 (ii) [7]. to derive these results, a set of n samples is assumed together with the values for λ as given in table 1. in figure 3, it can be observed that both the mean value and standard deviation of the diffusion coefficient decrease when conformity control is applied. the magnitude of this reduction depends on the applied conformity criterion, where a lower outgoing mean and standard deviation (i.e. a larger filter effect) are observed for a higher number of samples n. moreover, the filter effect becomes more pronounced if the fraction defectives increases (i.e. lower quality in regard to durability). if the conformity criteria from equations (7) and (8) are applied, the filter effect is influenced by the assumptions on σi. in the following, the filter effect after application of conformity control on the diffusion coefficient is evaluated for equal to 0.1dm. for the ratio σl/σm values of 0.5, 1 and 2 are assumed. the results are summarized in figure 4. all these results are given for the conformity criteria based on aoql = 50 % (xn ≤ dm + λ σi) and aoql = 10% (xn ≤ dk + λ σi). in figure 4, it can be observed that a 263 eline vereecken, robby caspeele acta polytechnica ctu proceedings figure 1. oc curves for conformity control on the diffusion coefficient. higher ratio of the outgoing vs. incoming mean and standard deviation is found for increasing values of σl/σm, and that this effect on the standard deviation is the strongest. if the value of σi would be increase, a stronger reduction in outgoing versus incoming mean would be found. the influence of σi on the ratio of the outgoing vs. incoming standard deviation is rather limited. 264 vol. 36/2022 conformity criteria for durability in service life predictions figure 2. decrease in probability of accepting a concrete batch when going from 2 tests with n different samples to n + 1 tests with a moving average (aoql= 50% and xn ≤ dm + λ sn). figure 3. filter effect on the parameters of the diffusion coefficient. 265 eline vereecken, robby caspeele acta polytechnica ctu proceedings figure 4. filter effect based on xn ≤ dm + λ σi (aoql= 50%) or xn ≤ dk + λ σi (aoql= 10%) with σi = 0.1dm. parameter distr. unit µ σ a b c beta mm 35 5 0 150 cs ln wt.-%/c 0.7 0.21 − − ccr beta wt.-%/c 0.6 0.15 0.2 2 d n mm2/year 50 10 − − ∗a indicates the lower bound of the distribution and b the upper bound table 2. distributions used in the estimation of the service life. 5. influence of conformity control on the diffusion coefficient on the service life of reinforced concrete structures when considering durability of a reinforced concrete structure, corrosion is an important mechanism affecting the durability of the structure. when considering chloride-induced corrosion, the resistance against penetration of the chlorides in the concrete is governed by the diffusion coefficient of the concrete. hence, the initiation period (ti) of corrosion is determined by this diffusion coefficient d, together with the concrete cover c, the surface chloride concentration cs and the critical chloride concentration ccr according to equation (12) [16, 17]. ti = 1 4 dapp,c c2 erf−1 ( 1 − ccr cs ) (12) 266 vol. 36/2022 conformity criteria for durability in service life predictions figure 5. influence of conformity control on the initiation period and on the service life. here, erf−1 is the inverse error function. it has been illustrated in section 4.3 that due to conformity control, the mean value of the diffusion coefficient decreases. when looking at equation (12), a decrease in mean value of the diffusion coefficient will lead to an increase in the mean value of the initiation period, i.e. it takes more time for the chlorides to reach the reinforcement steel, which is beneficial for the durability of the reinforced concrete structure. when assuming the distributions given in table 2 [7, 16], this increase in mean value of the initiation period is illustrated in figure 5 (i). in figure 5 (i), it can be observed that besides the mean value of the initiation period, also its standard deviation increases after application of conformity control on the diffusion coefficient. this effect is caused by the appearance of the diffusion coefficient in the denominator of equation (12). despite this increase in standard deviation of the initiation period, the influence of conformity control on the diffusion coefficient is still beneficial when considering the effect on the service life (tsl). the latter can be evaluated as the time at which there is a 10% probability of exceeding the condition limit state of depassivation [16]. to evaluate the influence of conformity control of the diffusion coefficient on this service life, the latter is estimated both for the incoming and outgoing distribution of the diffusion coefficient (as quantified above). the results are visualized in figure 5 (ii), where a significant increase of the service life is observed after application of conformity control. hence, it can be concluded that an adequate quality control in terms of durability is enabled. 6. conclusions in this work, conformity criteria have been derived for application of conformity control to the diffusion coefficient. it has been assumed that the specified value is characterized either by the mean value (50% fractile) or by the characteristic value (90% fractile) of the distribution. the probability of acceptance has been evaluated for different values of the fraction defectiveness, and based on a limit on the average outgoing quality, conformity criteria have been derived. these investigations are based on analytical methods and numerical integration. moreover, it has also been illustrated that the way samples are taken from the batches influences the probability of accepting a batch. if moving averages are applied, the probability of accepting a batch decreases compared to ordinary averages. the derived conformity criteria have then been applied to quantify the filter effect of conformity control on the distribution of the diffusion coefficient. it has been illustrated that the mean value of the outgoing batches of concrete is lower than the mean value of the 267 eline vereecken, robby caspeele acta polytechnica ctu proceedings batches delivered for conformity control. the same effect has been observed for the standard deviation. these effects will also influence the service life of a reinforced concrete structure. it has been illustrated how the initiation period and the service life both increase due to the beneficial effects of conformity control applied to the diffusion coefficient. as a final note, the authors would like to point out that similar procedures could also be applied to other durability related parameters such as the carbonation rate. for the latter, a lower mean value and standard deviation will be found due to the filtering effect of conformity control, which is again beneficial for the durability of a reinforced concrete structure. when rapid chloride migration tests are applied for the diffusion coefficient, similar procedures can be applied to the obtained drcm values [16] if the relevant statistical information (i.e. the coefficient of variation (cov)) is inserted. list of symbols aoq average outgoing quality aoql average outgoing quality limit oc operating characteristic c concrete cover ccr critical chloride concentration cs surface chloride concentration dm specified mean value of the diffusion coefficient dk specified characteristic value of the diffusion coefficient f probability density function n number of samples used for conformity control pa probability of acceptance sn sample standard deviation ti initiation period tsl service life xn sample mean θ fraction defectiveness µ mean value σ standard deviation ϕ probability density function of a standard normal distribution m variation of the diffusion coefficient of different concrete batches l variation within a concrete batch o outgoing i incoming acknowledgements research foundation flanders (fwo) is gratefully acknowledged for the funding that made this research possible. references [1] s. gavilan, l. c. p. da s. filho. variations of in situ concrete strength by european standard and american code. structural concrete 19(4):1185-94, 2018. https://doi.org/10.1002/suco.201700115. [2] ceb/cib/fip/rilem. recommended principles for the control of quality and the judgement of acceptability of concrete. mater struct 8:387-03, 1974. [3] l. taerwe. a general basis for the selection of compliance criteria. iabse proc. p-102/86, p. 113-27, 1986. [4] r. caspeele, l. taerwe. variance reducing capacity of concrete conformity control in structural reliability analysis under parameter uncertainties. in applications of statistics and probability in civil engineering, p. 2509-16. 2011. https://doi.org/10.1201/b11332-370. [5] r. caspeele, m. sykora, l. taerwe. quantifying the filtering effect of conformity inspection on the reliability of reinforced concrete structures. in: p van gelder lg and dp, editor. proc. 8th int. probabilistic work., szczecin, poland: akademia morska, p. 47-60, 2010. [6] r. caspeele, m. sykora, l. taerwe. influence of quality control of concrete on structural reliability: assessment using a bayesian approach. materials and structures 47(1-2):105-16, 2013. https://doi.org/10.1617/s11527-013-0048-y. [7] e. vereecken, r. caspeele. conformity control of concrete durability parameters and its filtering effect on the design service life. structural concrete 23(2):669-81, 2021. https://doi.org/10.1002/suco.202100459. [8] cen. pren1992-1-1 (draft 7), 2020. [9] r. rackwitz. predictive distribution of strength under control. matériaux et constructions 16(4):259-67, 1983. https://doi.org/10.1007/bf02473691. [10] en 206-1. concrete part 1: specification, performance, production and conformity, 2000. [11] r. caspeele. probabilistic evaluation of conformity control and the use of bayesian updating techniques in the framework of safty analyses of concrete structures. ghent university, 2010. [12] k. govindaraju. single sampling plans for variables indexed by aql and aoql. journal of quality technology 22(4):310-3, 2018. https://doi.org/10.1080/00224065.1990.11979263. [13] ah-s. ang, w. h. tang wh. probability concepts in engineering (second edition). new york, 2007. [14] l. taerwe. aspects of the stochastic nature of concrete strength including compliance control (in dutch). ghent, belgium: ghent university, 1985. [15] r. caspeele, l. taerwe. influence of conformity control on the strength distribution of concrete and the safety level of concrete columns. in safety, reliability and risk of structures, infrastructures and engineering systems, 10th international conference on structural safety and reliability (icossar2009), crc press / balkema, p. 745-51, 2010. [16] fib. fib bulletin 34: model code for service life design. lausanne, 2006. [17] duracrete. be95-1397. modelling of degradation, 1998. 268 https://doi.org/10.1002/suco.201700115 https://doi.org/10.1201/b11332-370 https://doi.org/10.1617/s11527-013-0048-y https://doi.org/10.1002/suco.202100459 https://doi.org/10.1007/bf02473691 https://doi.org/10.1080/00224065.1990.11979263 acta polytechnica ctu proceedings 36:261–268, 2022 1 introduction 2 quality control by conformity criteria 3 conformity criteria and their filter effect 4 conformity control for the diffusion coefficient of concrete 4.1 oc-curves 4.2 influence of sampling on the probability of accepting a concrete batch 4.3 filter effect of the application of conformity control on the diffusion coefficient 5 influence of conformity control on the diffusion coefficient on the service life of reinforced concrete structures 6 conclusions list of symbols acknowledgements references https://doi.org/10.14311/app.2022.33.0232 acta polytechnica ctu proceedings 33:232–237, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague contribution of fly ash to mortar strength development under steam and internal curing ryo insakoa, yuki miyoshia, phat tan huynha,b,c, yuko ogawad,∗, kenji kawaid a hiroshima university, graduate school of engineering, 1-4-1 kagamiyama, higashi-hiroshima, hiroshima 739-8527, japan b ho chi minh city university of technology, faculty of civil engineering, 268 ly thuong kiet street, district 10, ho chi minh city, vietnam c vietnam national university ho chi minh city, 2 linh trung ward, thu duc city, ho chi minh city, vietnam d hiroshima university, graduate school of advanced science and engineering, 1-4-1 kagamiyama, higashi-hiroshima, hiroshima 739-8527, japan ∗ corresponding author: ogaway@hiroshima-u.ac.jp abstract. the purpose of this study is to quantitatively evaluate the effects of steam curing and internal curing on contribution of fly ash to strength development of mortar by using cementing efficiency factor (k-value) that represents strength development performance as a binder of fly ash. in addition, the pozzolanic reaction of fly ash was evaluated from the viewpoint of calcium hydroxide consumption by using thermogravimetry and differential thermal analysis as well as the degree of fly ash reaction by using selective dissolution method. the result indicated that steam curing improved early compressive strength and internal curing improved compressive strength and k-value at all ages. also, a linear relationship between the degree of fly ash reaction and the k-value was shown regardless of the age and the replacement ratio of fly ash. keywords: cementing efficiency factor (k-value), fly ash, internal curing, steam curing, strength development of mortar. 1. introduction fly ash, a by-product of coal-fired power generation, has been commonly utilized as a supplementary cementitious material in concrete to obtain advantages such as long-term strength enhancement and densification of the microstructure. although coal-fired power plant slowly has been shut down in western europe concerning the environmental impact and the fly ash is decreasing, a certain amount of fly ash production is still produced and desired to be utilized. however, there is a concern that the early strength decreases due to replacing a part of cement with fly ash, and most of fly ash is currently used as a cement raw material in japan [1]. steam curing is one of the useful methods for improving the early strength of fly ash concrete, and some properties of steam-cured fly ash concretes have been investigated. yazici et al. reported that when steam curing was performed, the long-term strength of fly ash concrete was low compared with sealed curing at 20 ◦c [2]. meanwhile, pozzolanic reaction of fly ash could continue when the steam-cured fly ash concrete was cured in water after steam curing [3]. on the other hand, there is an internal curing method as a long-term water supply method. internal curing promotes hydration of cement by supplying water from the inside by using internal curing materials with high water absorption. water-absorbing polymer and artificial lightweight aggregate are used as internal curing materials [4, 5]. in addition to these two conventional internal curing materials, roof-tile waste aggregate has been also investigated as an internal curing material. roof-tile is japanese traditional roofing material and sintered clay at high temperature. roof-tile waste aggregate is produced from the roof-tile in its production due to cracking while sintering at high temperature. sekishu kawara, one of the japanese traditional roof tiles, was firstly used as an internal curing material by suzuki et al. [6], and it was reported that the compressive strength of ultra-high strength concrete was improved and the autogenous shrinkage was reduced by partially replacing the coarse aggregate with the roof-tile waste. it was also reported that the strength of steam cured fly ash concrete was improved by using internal curing material [7]. however, the effects of steam curing as well as internal curing on the reaction of fly ash have not been clearly understood. the purpose of this study is to evaluate the effects of steam curing and internal curing on contribution of fly ash to strength development of mortar by using cementing efficiency factor (k-value). furthermore, 232 https://doi.org/10.14311/app.2022.33.0232 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 33/2022 fly ash for mortar strength development cement fly ash cao (%) 65.09 3.59 sio2 (%) 20.43 56.74 al2o3 (%) 4.95 28.06 fe2o3 (%) 2.52 5.75 mgo (%) 1.62 0.92 na2o (%) 0.25 0.79 so3 (%) 2.86 1.07 loss on ignition (%) 0.76 2.08 density (g/cm3) 3.14 2.23 blaine fineness (cm2/g) 4560 3530 table 1. chemical compositions and physical properties of cement and fly ash. crushed sand roof-tile waste fine aggregate density (g/cm3) 2.58 2.28 water absorption (%) 2.05 9.75 table 2. density and water absorption of crushed sand and roof-tile waste fine aggregate. unit content (kg/m3) water cement fly ash crushed sand roof-tile waste fine aggregate f0s0 278 696 0 1290 0 f20s0 269 537 134 1290 0 f40s0 260 389 260 1290 0 f0s20 278 696 0 1032 228 f20s20 269 537 134 1032 228 f40s20 260 389 260 1032 228 table 3. mixture proportion. the pozzolanic reaction of fly ash was assessed, and the relationship between the fly ash reaction and the k-value of fly ash was discussed. 2. experimental method 2.1. materials, mixture proportion and curing methods paste and mortar specimens were prepared in this study. high-early-strength portland cement and type ii fly ash were used as cementitious materials. these cement and fly ash met jis r 5210 and jis a 6201, respectively. the chemical compositions and physical properties of cement and fly ash are shown in table 1. the water to binder ratio was 40% by mass, and fine aggregate to mortar ratio was 50% by volume. roof-tile used in this study was derived from the sekishu kawara, which composed of approximately 70% sio2, and 20% al2o3. the density in saturated and surface-dry condition and the water absorption of crushed sand and roof-tile waste fine aggregate are shown in table 2. the fly ash replacement ratios were 0% (f0), 20% (f20), and 40% (f40) by mass. the roof-tile waste fine aggregate was used for mortar specimens as an internal curing material by replacing crushed sand at 0% (s0) and 20% (s20) by volume. the mixture proportions are shown in table 3. two curing methods were applied in this study. one was sealed curing at 20 ◦c at all ages (20 ◦c sealed curing), while the other was steam curing at a maximum temperature of 50 ◦c for 6 hours after casting. the steam-cured specimens were demolded at the age of 1 day and stored at 20 ◦c and 60% relative humidity. 2.2. compressive strength and k-value mortar specimens, which were cast into cylindrical molds of 50-mm diameter and 100-mm height, were used for the compressive strength test. the compressive strength test was conducted according to jis a 1108 at the ages of 1, 7, and 28 days. the equivalent cement-to-water ratio ((c/w )eq) of the fly-ash mortar was calculated from the compressive strength and the relationship between the cement-to-water ratio (c/w ) and the compressive strength of mortar without fly ash. the k-value was obtained from the (c/w )eq, the c/w and the replacement ratio of fly ash (r) as shown in eq. 1 [8]. k = ! (c/w )eq c/w − 1 " · 1 − r r (1) 233 r. insako, y. miyoshi, p. t. huynh et al. acta polytechnica ctu proceedings 2.3. calcium hydroxide (ch) content and its consumption per unit fly ash content paste specimens cast into 16-ml polypropylene bottles were used for examining the ch content by thermogravimetry and differential thermal analysis. the ch content in the sample was measured at the ages of 1, 7, and 28 days. specimens were cut and samples in the size of 2.5 − 5.0 mm range were obtained. the samples were soaked in acetone to stop hydration and dried in a vacuum desiccator for 24 hours. after that the samples were ground into a powder less than 150 µm before the measurements of the degree of fly ash reaction and the ch content. figure 1. compressive strength of mortar under steam curing. the ch consumption per unit fly ash content can be calculated using the following eq. 2. chconsumption = ch (1 − r) − chf a r (2) • chconsumption: ch consumption per unit fly ash content (%) • ch: ch content in paste without fly ash (%) • chf a: ch content in paste with fly ash (%) 2.4. degree of fly ash reaction the sample used for measuring the degree of fly ash reaction was prepared similarly to that for ch content. the degree of fly ash reaction in paste specimens was measured at 1, 7, and 28 days by using selective dissolution method (sdm). to evaluate the reaction ratio of fly ash, ohsawa et al. proposed a method using 2-m hydrochloric acid (hcl) and 5% sodium carbonate (na2co3) aqueous solution [9]. the procedure of sdm is as follows: a 1-g powder sample was put in a pre-weighed centrifuge tube and weighed precisely, and 30 ml of 2-m hcl solution was added in the tube. then, the tube figure 2. compressive strength of mortar under 20 ◦c-sealed curing. was placed in a water bath at 60 ◦c for 15 min to promote the reaction. the solution was stirred occasionally with a glass rod during this step. the tube was removed from the bath and centrifuged at 4000 rpm (revolutions per minute) for 30 sec to separate solid and liquid phases. the liquid phase was decanted as much as possible without disturbing the solid residue in the bottom. after that, the tube was filled with hot water, centrifuged again and decanted. this operation was repeated 3 times. the tube was then filled with 30 ml of na2co3 solution, placed in a water bath at 80 ◦c for 20 min and stirred continuously. the tube was then centrifuged at 4000 rpm for 1 min. the liquid phase was decanted and the residue was washed with hot water and centrifuged. this washing was repeated 3 times. the tube with residue was dried at 105 ◦c for 24 h and then weighed. the degree of fly ash reaction is calculated using the following eq. 3. α = 1 − x (1 − ig) (1 − ig) k1k2 (3) • α : degree of fly ash reaction • k1 : original fraction of fly ash in ignited base • k2 : residue extracted of a 1-g fly ash • x : residue extracted of the hydrated fly ash in a 1-g hydrated sample • ig : loss on ignition of the hydrated sample • ig′ : loss on ignition of the residue 3. results and discussion 3.1. compressive strength the compressive strengths of mortar specimens under steam curing and 20◦c-sealed curing are presented in 234 vol. 33/2022 fly ash for mortar strength development figure 3. k-value of fly ash with age. figure 4. ch content with age. figures 1 and 2, respectively. at the age of 1 day, the compressive strength of steam-cured mortar was 10% to 35% higher than that of 20 ◦c-sealed cured mortar. the roof-tile waste fine aggregate replacement increased the compressive strength of mortar by 15% to 20% at all ages, regardless of curing methods. at the age of 28 days, the mortar specimens with 20% fly ash and 20% roof-tile waste fine aggregate (f20s20) had the same compressive strength as that with 0% fly ash and 0% roof-tile waste fine aggregate (f0) in the case of 20 ◦c-sealed curing. meanwhile, the compressive strength of f20s20 was lower than that of f0 in the case of steam curing. this could indicate that steam curing mainly promoted the initial hydration of cement and slightly promoted the pozzolanic reaction of fly ash which can occur at later ages, while internal curing by roof-tile waste fine aggregate could enhance the cement and fly ash reaction at early ages as well as the later ages. -5 -4 -3 -2 -1 0 1 2 3 4 0 7 14 21 28 c h c on su m pt io n pe r u ni t f ly a sh co nt en t ( % , g / fly a sh -1 00 g) age(days) f20 20ºc-sealed f20steam f40 20ºc-sealed f40steam figure 5. ch consumption per unit fly ash content with age. 3.2. cementing efficiency factor (k-value) figure 3 shows the k-value of fly ash under steam curing and 20 ◦c-sealed curing with time. the steam curing had a lower k-value than the 20 ◦c-sealed curing for all mixture proportions. this result also suggests that steam curing mainly promoted the hydration of cement. in addition, the k-value in the case of 20% roof-tile waste replacement (s20) was higher than that in the case of no roof-tile waste. it indicated that the internal curing by using the roof-tile waste fine aggregate promoted the fly ash reaction, and the fly ash contributed to the strength development of mortar. the f40s20 in both curing conditions had a significant increase in the k-value from 7 to 28 days when compared with the f40. this shows that the internal curing continuously promoted the pozzolanic reaction of fly ash for a long term when the replacement ratio of fly ash was high. 235 r. insako, y. miyoshi, p. t. huynh et al. acta polytechnica ctu proceedings figure 6. degree of fly ash reaction with age. 3.3. calcium hydroxide (ch) content and its consumption per unit fly ash content figure 4 shows the ch content under 20 ◦c-sealed curing and steam curing. at the age of 1 day, the ch content of steam-cured specimens was higher than that of 20 ◦c-sealed cured specimens. it could imply that steam curing accelerated cement hydration and ch was produced more than that under 20 ◦csealed curing. at the age of 7 days, then, the ch content for each mixture proportion was almost the same, and increased up to the age of 28 days slightly. figure 5 shows ch consumption per unit fly ash content by fly ash reaction under 20 ◦c-sealed curing and steam curing. at the age of 28 days, the ch consumption per unit fly ash content in the case of f20 with 20◦c-sealed curing was greater than that in the case of f20 with steam curing. besides, the unit ch consumption at the ages of 1 day and 7 days was negative. it may indicate that the cement hydration could be promoted by the incorporation of fly ash and more ch was produced in fly ash cement paste compared to the plain cement paste. 3.4. degree of fly ash reaction figure 6 shows the degree of fly ash reaction under steam curing and 20◦c-sealed curing. at all ages, the degree of fly ash reaction in the case of 20 ◦c-sealed curing was higher than that in the case of steam curing. the longer the material age, the greater the difference between steam curing and 20◦c-sealed curing. it could imply that drying after the end of the steam-supply hindered the fly ash reaction. figure 7 shows the relationship between the k-value and the degree of fly ash reaction. the k-value increases as the degree of fly ash reaction increases. the relationship between the degree of fly ash reaction and the k-value can be represented by one straight line regardless of the age and the replacement ratio of fly ash. figure 7. relationship between k-value and degree of fly ash reaction. 4. conclusions the present study investigated the k-value of fly ash regarding compressive strength development, ch consumption as well as the degree of fly ash reaction in order to evaluate the effects of steam curing and internal curing on contribution of fly ash to strength development of mortar. the following conclusions can be drawn within the limits of this study. steam curing improved early compressive strength of mortar regardless of the fly ash or roof-tile replacement. the 20% roof-tile waste fine aggregate replacement increased the compressive strength by 15% to 20% for both 20 ◦c-sealed curing and steam curing. the internal curing could contribute not only to the cement hydration but also to the fly ash reaction. the k-value in the case of 40% fly ash replacement for both steam and 20 ◦c-sealed curing conditions increased from 7 to 28 days by using roof-tile waste fine aggregate. it was suggested that when the replacement ratio of fly ash is high, the effect of internal curing on the pozzolanic reaction may continue for a long time. the k-value in the case of steam curing was lower than that in the case of the 20 ◦c-sealed curing, and it implies that steam curing significantly promoted the cement hydration whereas it slightly promoted the pozzolanic reaction of fly ash. the relationship between the degree of fly ash reaction and the k-value can be represented by one straight line regardless of the age and the replacement ratio of fly ash. references [1] k. ogawa, h. uchikawa, k. takemoto, et al. the mechanism of the hydration in the system c3s-pozzolana. cement and concrete research 10(5):683-96, 1980. https://doi.org/10.1016/0008-8846(80)90032-0. [2] h. yazici, s. aydin, h. yi ‘giter, et al. effect of steam curing on class c high-volume fly ash concrete mixtures. cement and concrete research 35(6):1122-7, 2005. https: //doi.org/10.1016/j.cemconres.2004.08.011. 236 https://doi.org/10.1016/0008-8846(80)90032-0 https://doi.org/10.1016/j.cemconres.2004.08.011 vol. 33/2022 fly ash for mortar strength development [3] j. tomiyama, y. suda, t. saeki, et al. basic study on strength characteristic of fly ash concrete under steam curing. cement science and concrete technology 66(1):359-66, 2012. https://doi.org/10.14250/cement.66.359. [4] k. yokota, s.-i. igarashi. effects of aggregate particles on spatial distribution of superabsorbent polymers for internal curing. cement science and concrete technology 67(1):179-86, 2013. https://doi.org/10.14250/cement.67.179. [5] d. tachibana, k. kimura, k. naitoh. effect of high-performance artificial lightweight aggregate on several properties of lightweight concrete. doboku gakkai ronbunshu 1994(496):89-98, 1994. https://doi.org/10.2208/jscej.1994.496_89. [6] m. suzuki, i. maruyama, t. kawabata. a study on deformation of ultra high strength concrete containing crushed roof tile aggregate. japan concrete institute 29:651-6, 2007.,. [7] y. ogawa, p. t. bui, k. kawai, et al. effects of porous ceramic roof tile waste aggregate on strength development and carbonation resistance of steam-cured fly ash concrete. construction and building materials 236, 2020. https: //doi.org/10.1016/j.conbuildmat.2019.117462. [8] i. a. smith. the design of fly-ash concretes. proceedings of the institution of civil engineers 36(4):769-90, 1967. https://doi.org/10.1680/iicep.1967.8472. [9] s. ohsawa, e. sakai, m. daimon. reaction ratio of fly ash in the hydration of fly ash-cement system. cement science and concrete technology 53:96-101, 1999. 237 https://doi.org/10.14250/cement.66.359 https://doi.org/10.14250/cement.67.179 https://doi.org/10.2208/jscej.1994.496_89 https://doi.org/10.1016/j.conbuildmat.2019.117462 https://doi.org/10.1680/iicep.1967.8472 https://doi.org/10.14311/app.2022.33.0168 acta polytechnica ctu proceedings 33:168–174, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague fire spalling sensitivity of concrete made with recycled concrete aggregates (rca) bruno fernandesa,∗, hélène carréa, jean-christophe mindeguiab, céline perlota, christian la borderiea a université de pau et des pays de l’adour, e2s uppa, siame, allée du parc montaury, 64600 anglet, france b université de bordeaux, laboratoire l2m, cours de la libération, 33 405 talence, france ∗ corresponding author: bruno.fernandes@univ-pau.fr abstract. the fire spalling of concrete is a complex phenomenon, which can affect the integrity of the structures during a fire. this thermal instability is associated with a complex coupled chemo-thermohydro-mechanical mechanism and it can be influenced by many factors, related to material (e.g. permeability, porosity and water content), geometry (e.g. shape and size) and environmental parameters (e.g. mechanical load and heating rate). concrete made with recycled concrete aggregates presents higher porosity, higher water content and different interfaces between aggregates and mortar. all these aspects can lead to a different behaviour under fire exposure, including the spalling risk of these sustainable concretes. the main objective of this paper is to analyse the influence of the use of recycled concrete aggregates on the spalling risk of concrete. in this paper, concrete prisms with different replacement rates of recycled coarse aggregates (0 up to 100%) were exposed to a standard fire curve (iso 834-1) with a constant uniaxial compression load. after heating, samples surfaces were evaluated by means of digital photogrammetry. results showed that concrete with rca is sensitive to explosive spalling. all replacement rates presented higher degree of spalling than concrete made with natural aggregates. keywords: fire, recycled concrete aggregate, spalling. 1. introduction in the last decades, the use of recycled concrete aggregates (rca) has gained popularity due to their potential to make sustainable concrete. as for conventional concretes, the fire behaviour of these concretes, including their spalling sensibility, must be well known to guaranty the safety of the structure in case of a fire. moreover, fire behaviour is fundamental to enhance the resilience of the buildings for these types of extreme events. fire spalling is the detachment of concrete fragments from structures exposed to fire. this phenomenon may affect the integrity of structures, reducing thermal insulation, exposing the steel rebar to fire and reducing the load-bearing capacity [1, 2]. despite the numerous experimental and numerical studies, there is still a lot of uncertainties about the physical origin of the phenomenon [2]. in general, spalling is associated with a coupled thermo-hygrochemo-mechanical mechanism. in general, spalling occurs in different forms, such as aggregate spalling, surface spalling, explosive spalling and corner spalling [3]. previous studies show that many parameters can influence the occurrence of spalling, and it can be divided into material (e.g. permeability, porosity and water content), geometry (e.g. shape and size) and environmental parameters (e.g. heating rate and mechanical load) [3, 4]. the susceptibility to spalling of concrete made with recycled aggregates (rca) is barely known. since rca aggregates are more porous, the use of this kind of aggregates can contribute to vapour migration, reducing spalling risk [5]. at the same time, these concretes present higher water content and different phase interfaces, which also may affect the spalling risk. previous works verified occurrence of this phenomenon. robert et al. [6] observed superficial and localized spalling in full-scale slabs made with rca. authors attributed this behaviour to the high water content of the mix. pliya et al. [5] observed spalling on some cylinder samples exposed to a rapid heating rate. it is noteworthy that in this case, samples are classified as high strength concrete, which is more prone to spalling due to its denser microstructure. the aim of this study is then to evaluate the influence of recycled concrete aggregates (rca) on the spalling behaviour. to this, uniaxial spalling tests were carried out on of ordinary concrete samples containing different rca (coarse aggregates) replacement rates (0%, 10%, 20%, 40%, 70% and 100%). during fire, spalling events are followed and after fire, the residual sample geometry was evaluated by digital photogrammetry, in terms of volume and spalling 168 https://doi.org/10.14311/app.2022.33.0168 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 33/2022 fire spalling sensitivity of rca concrete depth. 2. experimental program 2.1. concrete mixes cement cem ii/a-l 42.5 r ce was used. to achieve workability, limestone filler betocarb hpsc and sika viscocrete tempo-483 superplasticizer were also used in the mixes. sand used in this study had a fineness modulus of 3.10, density of 2650 kg/m3 and water absorption of 0.35 %. two types of coarse aggregates were used: natural aggregates (na) and recycled concrete aggregates (rca). for each type of aggregates, two fractions were used, 4/10 and 10/20. natural aggregates were made from diorite and have a density of 2820 kg/m3 (4/10) and 2840 kg/m3 (10/20), and a water absorption of 0.92 % (4/10) and 0.81 % (10/20). recycled concrete aggregates were crushed and screened by a recycling company, and have a density of 2570 kg/m3 (4/10) and 2590 kg/m3 (10/20), and a water absorption of 5.60 % (4/10) and 4.52 % (10/20). as expected, rca present higher water absorption values, mainly due to the presence of adhered "old" mortar. rca aggregates also present some impurities, such as wood, plastic, brick and bituminous materials. six different ordinary concrete mixes were prepared and mix proportions are given in table 1. for concretes made with rca, na were directly replaced by rca in volume, without any changes in the other components. samples were designed to meet nf en 206/cn:2014 [7] durability requirements for xd3 exposure class. to compensate the higher water absorption of rca and aiming a practical method that can be reproduced in a real concrete plant, all aggregates (sand, na and rca) were pre-wetted for one hour using a soaker hose and then covered with plastic sheet for three hours. all mixes have the same water/cement ratio (0.5) and dosage of superplasticizer (0.9 % c). to avoid errors when determining water content in aggregates, the amount of water in the batch is adjusted to obtain approximatively the same slump (180 − 190 mm). slump, mechanical and physical properties for all studied mixes are reported in table 1. for each mix, mechanical properties at 28 days and at fire test age were measured on 11 × 22 cm and 16 × 32 cm cylindrical specimens. for spalling tests, 20 × 20 × 10 cm (height x length x thickness) prisms were casted. cylindrical samples used to determine 28 days properties were kept submerged in water at 20 řc until test day. samples for fire tests were first kept submerged for seven days and then placed into sealed plastic bags until fire test age (between 88 and 125 days). this curing induces high water content, thus inducing high probability of spalling for all the mixes. at test age, water content (wc), was also measured. as expected, higher the replacement rate, higher the water content in the samples, due to higher porosity of rca. from na to rca-100, a variation of 2.11% is observed. the influence of the water content on the different spalling behaviour between concretes made with na or rca are discussed in section 3. 2.2. spalling evaluation spalling was evaluated by testing concrete prisms subjected to a standard fire (iso 834-1) and uniaxial loading. three samples per concrete mix were tested (with exception of rca-10). before the tests, the sides of the prisms were sealed with aluminium foil tape (figure 1a), which was glued using high temperature glue. this was done to avoid moisture loss through the sides of the concrete blocks and provide unidirectional transport condition of heat and moisture. an intermediate scale furnace, powered by one propane gas burner, was used for the heating process (figure 1b). gas flow was controlled during the test in order to follow the iso 834-1 fire curve. the furnace has an opening of 20 cm x 20 cm and three type k thermocouples were used to measure the heating curve. these thermocouples were placed in the front of the furnace and one centimetre from the heated face of the sample, at three different heights: 4, 10 and 16 cm from the bottom of the furnace opening. more detail can be found in previous works [8, 9]. a hydraulic press was used to apply a uniaxial loading of 5 mpa, in order to be representative of a conventional structure (wall or column for example). the tested specimen was placed between two concrete blocks. the sides of the tested specimen and the support blocks were insulated with rock wool. figures 1c and 1d presents the full test setup. first, the load was applied and after stabilization, the heating process started. the iso 834-1 fire curve was applied for 30 minutes and the three thermocouples allowed to follow the temperature of air close to the sample. during tests, each spalling event has been registered, in terms of occurrence time and intensity (small, medium and big noise). after 30 minutes, heating was turned off and after 5 minutes, the sample was discharged. after heating, post-fire analysis was conducted on all samples with a digital photogrammetry technique. this method allows to construct 3d models from digital photos [10, 11]. the method follows an approach that involves image acquisition and processing, 3d reconstruction, mesh generation and post processing [10]. a digital camera (panasonic dmc-gx80) was used. samples were placed in a rotating table, in order to successfully orientate 3d reconstruction. camera was fixed on a tripod and between each shot, the rotating table was turned around 10ř. a total of 36 images were obtained for each sample. the image processing and 3d reconstruction were done with the meshroom [12] software and later imported 169 b. fernandes, h. carré, j.c. mindeguia et al. acta polytechnica ctu proceedings material / property mix na rca-10 rca-20 rca-40 rca-70 rca-100 cement/filler/sand (kg/m3) 350 / 60 / 804.3 na 4/10 (kg/m3) 331.7 298.5 265.3 199 99.5 na 10/20 (kg/m3) 711.1 640 568.9 426.7 213.33 rca 4/10 (kg/m3) 30.2 60.5 120.9 211.58 302.3 rca 10/20 (kg/m3) 64.9 129.7 259.4 453.96 648.5 water (kg/m3) 175 slump (mm) 194 184 174 176 158 210 fc28 (mpa) 44.8 (±0.2) 39.7 (±0.1) 37.4 (±3.5) 39.7 (±3.0) 34.5 (±2.7) 34.6 (±1.1) j (days) 94 116 88 88 88 125 fc,j (mpa) 46.6 (±1.4) 44.5 (±4.6) 50.8 (±0.5) 49.6 (±0.4) 45.9 (±0.3) 39.1 (±0.3) ec,j (gpa) 34.5 36.8 34.9 39 30.1 30.5 wcj (%) 4.94 (±0.4) 5.02 (±0.5) 5.10 (±0.2) 5.36 (±0.4) 6.41 (±0.5) 7.05 (±0.3) table 1. mix design and properties. figure 1. furnace details and test set-up. 170 vol. 33/2022 fire spalling sensitivity of rca concrete figure 2. developed fire curves. figure 3. first spalling event. in cloudcompare [13], where the model was cleaned and scaled. after this processing, a binary mesh was generated and used in a cast3m [14] algorithm, which post-processed the model to obtain the spalling volume and depths. the error from the methodology applied here is 4.2% (previously determined). 3. results 3.1. fire curves and spalling events fire curves developed during fire tests are presented in figure 2. in grey, the average of the three thermocouples is presented, together with the first registered spalling event. in general, curves follow properly the iso 834-1 fire curve and the perturbations start after the first spalling event. the oven temperature at the time of the first spalling is usually around 550 ◦c to 650 ◦c. the time of first spalling is also presented in figure 3, which presents the time of this event in relation to the replacement rate. except for rca-10 (indicated in figure 2), all samples first spalled between 5 and 10 minutes. the perturbations with rca-10 are related to the fact that the curve in this tests lasted figure 4. spalling events. figure 5. total number of spalling events. more to reach the temperature that may trigger the spalling. besides the first event, each spalling was recorded and categorized in three different qualitative "sound intensity": small, medium and big spalling. in order to have a visualization of these different events along time, figure 4 is proposed. in this figure, results from three representative samples were plotted. in general, all samples started with small spalling events (popup, bottom of the figure 4), then sound intensity increases with the fire duration. the last events usually had big intensity and happened around 10 − 15 minutes. figure 5 presents the total amount of spalling events for each of the tests in relation to the replacement rate. from the figure 5, we can see it a lot of discrepancies in the spalling events and no direct link with the quantity of recycled aggregates can be observed. 3.2. inspection after fire spalling test different analyses were done after the fire spalling tests. first, a visual inspection was done. examples 171 b. fernandes, h. carré, j.c. mindeguia et al. acta polytechnica ctu proceedings figure 6. samples after fire: photos and depth colour map. figure 7. max spalling depth. of samples exposed surface are presented in figure 6. visually it is already possible to observe an influence of the type of aggregates on the spalling severity: na samples present less spalling than mixes made with rca. the impurities in tested samples are also observed. especially in rca 100, the presence of bituminous particles is clearly seen, that burned and melted during the fire test. its noteworthy that the presence of this impurities reduces the strength of concrete made with rca at high temperatures, as observed in laneyrie et al. [15]. figure 6 also presents the colour maps generated in the photogrammetry process. max spalling depth and spalled volume were measured with photogrammetry for all the replacement rates (figures 7 and 8). the first aspect is that concrete made with na presents lower values of spalling depth and volume than any rca replacement rate. these graphs confirms some of the tendencies observed in the visual inspection, i.e. concrete made with na presents lower figure 8. spalled volume. values of spalling volume and depths. another observation is that the evolution of spalling volume and depth with the replacement rate is quite similar. in the first branch, from na to concrete made with 20% of rca, an almost linear increase is observed. in the case of the spalled volume, this increase also appears from rca-20 to rca-40, but the slope of increase is lower. from 40% to 100%, the average values keep quite constant, with no major changes, independently of the quantity of rca. a higher variability of results is also noteworthy for concretes made with rca. first, the use of rca results in a higher water content, as seen in table 1. usually, higher the water content (or the moisture content), higher is the spalling degree [16]. it seems then logical that the rca-100, which had the higher water content at the test age (7.05 %), experimented the more severe spalling (volume and depth). however, the influence 172 vol. 33/2022 fire spalling sensitivity of rca concrete of water content for the other mixes is not that clear. indeed, when comparing na and rca-20, the increase of spalling volume (5.51 %) is much higher than the increase in water content (0.16 %). moreover, it can be seen that spalling volume and depth are quite constant from rca-40 to rca-100, whereas the water content increases significantly (1.5 %). these factors indicate that the water content may influence the spalling of concrete made with rca, but is not the only parameter affecting the spalling sensibility. another influencing parameter for spalling is the compressive strength. usually, the higher the strength, the higher will be the spalling probability [16, 17], due to the lower permeability [16] of the mix. this is especially verified with hpc, where the spalling is a real concern [18]. yet, the influence of compressive strength on spalling is not clearly observed in our study. indeed, the mix with the lower compressive strength at test age (rca100, 39.1 mpa) presents the higher spalled volume. this shows that, in this experimental campaign, the strength was not the main factor affecting spalling sensitivity of concrete made with rca. lastly, internal thermal cracking also may have a contribution to the spalling events. usually, cracks increase the matrix permeability, allowing pressure release and moisture transport, and then a reduction of the spalling risk [1, 19]. these cracks are mainly due to thermal mismatch between aggregates (that expand) and cement matrix (that shrinks after 100 ◦c). the cracking of concrete made with rca under fire exposure is still not clear. on one hand, increasing the replacement rate of rca involves, for a fixed volume of new paste, a higher proportion of old mortar / new paste interfaces, and consequently a lower proportion of new paste / aggregates interfaces. this can probably reduce the density of thermal cracking in concretes made with rca, and then potentially increase the spalling sensibility. on the other hand, the use of rca also may lead to an excess of water (regarding only the one necessary for cement hydration), especially surrounding the aggregates themselves [5]. this extra water may reduce the bond with the new paste, inducing cracks and a local increase of the permeability [5]. this point will be further analysed by means of microscopic observations of heated samples, permeability measurement and thermal expansion measurements. 4. conclusions this paper presented an experimental investigation into the spalling sensitivity of concrete made with rca. based on the test results, the following conclusions are drawn: • visual inspection showed that concrete made with rca presents a spalling more significant than concrete made with na. • the time of the first spalling event can be a good indicator of the spalling occurrence. usually, this time is associated with the temperature reached in the material. in this study, no clear influence of the rca on the time of first spalling was observed, with a high variability of results. • the evolution of the spalling depth and spalling volume with the rca replacement rate both present a peculiar behaviour. first, an increase is observed (from na to rca-20), then values keep almost constant up to rca-100. both indicators showed that concretes made with rca presents a higher spalling sensibility. • different factors may influence the risk of spalling of concrete made with rca: water content, strength and thermal cracking. in this experimental campaign, compressive strength does not affect the spalling behaviour of concrete with rca, since concrete mixes with relatively low strength presented a high spalling sensibility. water content seems to affect the spalling sensibility, since mixes with high water content presented the higher spalling depth and volume. however this is not the main factor, since samples made with rca and without rca, and with similar water content, presented big variations of spalling depth and volume. thermal cracking should be further investigated, along with permeability measurements, microscopic observations and thermal expansion. • lastly, it is important to note that all these experimental conclusions are drawn based on a specific thermal and mechanical state, and for a given geometry of samples. to be more representative of real structures, full-scale fire tests on structural concrete elements made with rca are planned, in order to analyze (i) the spalling sensibility of these concretes in a real situation and (ii) the 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[17] p. pimienta, n. taillefer, p. pimienta, et al. spalling of concrete: a synthesis of experimental tests on slabs. matec web of conferences 6, 2013. https://doi.org/10.1051/matecconf/20130601008. [18] p. pimienta, r. j. mcnamee, j.-c. mindeguia. hysical properties and behaviour of high-performance concrete at high temperature (springer international publishing), 2019. [19] e. w. h. klingsch. explosive spalling of concrete in fire phd thesis (eth zurich), 2014. 174 https://doi.org/10.1002/pse.51 https://doi.org/10.1007/s10694-019-00820-0 https://doi.org/10.1002/suco.201800196 https://doi.org/10.1016/j.jterra.2017.06.001 https://github.com/alicevision/meshroom http://www.cloudcompare.org/ http://www-cast3m.cea.fr/ https://doi.org/10.1016/j.conbuildmat.2016.02.056 https://doi.org/10.1016/j.firesaf.2006.10.006 https://doi.org/10.1051/matecconf/20130601008 https://doi.org/10.14311/app.2022.33.0212 acta polytechnica ctu proceedings 33:212–218, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague tensile behavior of newly developed undercut anchor in cracked and uncracked concrete tomoyuki hatanakaa,∗, naoki funakib, reiji tanakab, shogo fujitac a national institute of technology, yonago college, 4448 hikona-cho, yonago, tottori, 683-8502, japan b tohoku institute of technology, 35-1 yagiyama kasumi-cho, taihaku-ku, sendai, 982-8577, japan c fs technical co., ltd., 1-22-15 takasa, katsushika-ku, tokyo, 125-0054, japan ∗ corresponding author: hatanaka@yonago-k.ac.jp abstract. the authors have been designing a post-installed anchor that fixes itself into concrete material by expanding the anchor tip in an upward direction, and conducted tensile loading tests to confirm its fundamental dynamic characteristics. the test results on three types of test specimens, with anchor morphology as a parameter, indicate that the final failure modes were all anchor bar fractures, and a stable yield strength was confirmed. additionally, the yield strength characteristics of the proposed anchor in cases where cracks are present on the concrete surface, wherein the anchors are fixed, were experimentally confirmed. keywords: fem analysis, hybrid post-installed anchor, self-detecting damaged state function, tensile load test. 1. introduction the concept of "building longevity" plays an important role in the reduction of global greenhouse gas emissions with regards to global environmental problems. there are two primary factors that determine the lifespan of buildings: one is the decline of aseismic performance over time, and the second is the obsolescence of buildings as the functional demands imposed on them change with time. solutions to these factors include seismic repairs and renovations. various postinstalled anchors are used when installing aseismic materials and equipment in existing buildings, and it is predicted that future use of post-installed anchors will continue to increase to achieve longer lifespans for buildings. meanwhile, accidents have occurred owing to the age-related degradation of anchors and cracks in the concrete skeleton structure; hence, an anchor that exhibits stable performance even under harsh conditions is necessary for the wide-range and long-term usage of anchors. with this in mind, the authors have designed multiple types of undercut post-installed anchors with different morphologies. a previous report confirmed the basic dynamic characteristics of the anchors and their performance superiority based on loading test results conducted with these anchors as the test specimens [1–5]. the objectives of the present report are to show the tensile loading test results obtained on post-installed anchors designed by the authors where the anchor tip expands in an upward direction, and to confirm the fundamental yield strength characteristics of the proposed anchor based on these results. 2. experimental investigation 2.1. test specimen details of the anchor investigated in the present report are shown in figure 1 and table 1, and a complete view of the anchor is shown in figure 2. the proposed anchor is composed of a main body, an expanding cap, and an anchor bar. as shown in figure 3, the anchor is placed in a hole with an expanded base. the expanding cap, which is fixed at the anchor bar tip, has four slits; by driving the main body into it, the wedge-shaped tip of the main body cuts into the expanding cap, forcing it to expand radially upwards and fix itself into the concrete slab. the hole into which the anchor is inserted is created by first making a hole using a hammer drill with a straight-drill attachment, and then expanding its base using a specialized hole drill with an expanding bit. in total, three types of anchors with different anchor bar diameters were chosen as the test specimens, and loading tests were conducted on five samples of each type. the concrete slab into which the anchor is applied is a reinforced concrete plate with dimensions of 1,200 mm × 1,800 mm × 400 mm (length × width × thickness), and reinforced steel (d13) was applied in a grid-like manner to prevent fracture formation on the bottom face. the target compression strength of the concrete slab was set as a constant, with a value of 18 n/mm2. the actual measured compression strength of the concrete prior to the loading tests was 19.0 n/mm2. a total of 10 − 12 anchors were applied for a single concrete specimen. sus316 was used as the material for the main body and expanding cap of the anchor, and sus316l was used 212 https://doi.org/10.14311/app.2022.33.0212 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 33/2022 newly developed undercut anchor tensile behavior figure 1. detail of the anchor specimen. figure 2. view of the anchor specimen. for anchor bar. 2.2. test specimen the detail of the test setup used for the loading tests is shown in figure 5, and an overall view of the test setup is shown in figure 6. the loading tests were conducted using an electrically powered hydraulic jack with a maximum capacity of 400kn, where tensile forces were applied to the anchor bar. the tensile load was measured by a center-holetype load cell with a maximum capacity of 200 kn. additionally, a displacement-measuring plate was attached to the anchor bar, as shown in figure 5, to figure 3. detail of borehole. figure 4. scheme of the anchor. measure anchor displacement. the position of the plate attachment was set at approximately 10 mm from the test specimen surface to minimize the effects of elongation on the anchor bar caused by loading. anchor displacement was measured using three displacement transducers attached between the plate and the displacement-measuring jig set up on top of the test specimen. the displacement shown in this report is the average of the values measured by these three displacement transducers. 2.3. test results a representative load-displacement curve obtained by conducting the loading tests are shown in figures 7 (a) 9 (a), and the appearance of the anchor after testing are shown in figures 7 (b) 9 (b). additionally, an overview of the test results are shown in table 2. these results show that the maximum load increases with increased anchor bar diameter and that the failure mode for all test specimens is the anchor bar fracture. additionally, differences in the coef213 tomoyuki hatanaka, n. funaki, r. tanaka, s. fujita acta polytechnica ctu proceedings configuration anchor borehole body expanding cap bar ls ϕ1 ϕ2 lt ϕ3 ϕ4 lh φ umax l la lb le m12 104.0 20.5 12.3 22.0 20.0 12.3 180.0 21.0 28.0 132.0 12.0 14.0 106.0 m16 142.0 26.0 16.3 28.0 26.0 16.3 240.0 27.0 36.0 176.0 15.0 19.0 142.0 m20 177.0 32.0 20.3 38.0 32.0 20.2 300.0 33.0 44.0 220.0 18.0 25.0 175.0 table 1. main parameters of the anchor specimen and borehole. figure 5. test setup. ficient of variation for the maximum yield strength between the anchor bar diameters were small, and a stable tensile yield strength was observed in all the test specimens. as shown in figures 7 (b) 9 (b), there were virtually no cases where the main body of the anchor slipped out of its hole due to loading. 3. tensile loading tests in cracked concrete 3.1. test specimen generally speaking, the yield strength of the anchors significantly decreases when cracks are formed on the anchor-applied concrete surface. with this in mind, we created a testing apparatus like the one shown in figures 10 and 11 to confirm the influence of cracks on the yield strength characteristics of the anchor, and conducted tensile load tests. as shown in the figure, the present testing apparatus is comprised of a pair of guides and moveable blocks into which the concrete block is fixed. the anchors are applied in the upper joined surface sections of the concrete block, and are designed to generate cracks by sliding the moveable block to the left and right. the anchors are connected to air jacks through load cells, and provide strength to the system in the tensile direction. here, we conducted tests where forces corresponding to the longterm loading in cases where the anchor bar yields were applied at a fixed rate while the crack width of the test specimen was gradually increased. the tensile figure 6. overall view of test set.up. load was measured using the load cell connected between the jack and the anchor. additionally, the anchor displacement was measured using the three laser displacement transducers attached to the measuring plate fixed to the anchor bar, and the crack width was measured using six laser displacement transducers set up on the top and either side of the concrete blocks. the anchor displacement and crack width indicated here are the average values between all the measurements made with these displacement transducers. the actual compression strength of the concrete test specimens used in the present test was 19.0 n/mm2. 3.2. test results a representative example of the results obtained from conducting the tests is shown in figure 12, and the appearance of the anchors after loading is shown in figure 13. the figure shows the relationship between the extent to which the anchor slipped out, the ap214 vol. 33/2022 newly developed undercut anchor tensile behavior figure 7. load-bearing behavior during monotonic tension loading and failure mechanism (m12). figure 8. load-bearing behavior during monotonic tension loading and failure mechanism (m16). figure 9. load-bearing behavior during monotonic tension loading and failure mechanism (m20). 215 tomoyuki hatanaka, n. funaki, r. tanaka, s. fujita acta polytechnica ctu proceedings figure 10. test setup. figure 11. view of test setup. figure 12. view of test setup. 216 vol. 33/2022 newly developed undercut anchor tensile behavior tensile load [kn] displacement [mm] failure type∗ max. ave. s.d. c.v. δ∗∗ ave. s.d. c.v. m12-1 65.2 65.0 0.323 0.50 34.1 28.9 3.241 11.2 sf m12-2 64.5 30.8 sf m12-3 65.0 27.5 sf m12-4 65.5 24.6 sf m12-5 65.0 27.7 sf m16-1 118.6 118.8 0.313 0.26 42.6 37.4 2.931 7.83 sf m16-2 119.3 35.0 sf m16-3 118.4 36.2 sf m16-4 119.0 34.7 sf m16-5 118.9 38.6 sf m20-1 173.6 171.7 1.414 0.82 52.1 55.8 4.270 7.65 sf m20-2 170.7 60.2 sf m20-3 169.8 56.6 sf m20-4 171.4 60.3 sf m20-5 173.0 49.7 sf ∗ max: maximum load, ave: average, s.d.: standard deviation, c.v.: coefficient variation [%]. ∗∗ δ: displacement in maximum load ∗∗∗ max: maximum load, ave: average, s.d.: standard deviation, c.v.: coefficient variation [%]. table 2. test results∗∗∗. figure 13. view of test setup. plied load, and the crack width, obtained by gradually increasing the crack width while applying a load that was equivalent to the long-term loading. these results showed that the crack width at the time when the main body of the anchor completely slipped out was 4.7mm in the m12 test specimen and 7.1mm in the m16 test specimen, confirming that the yield strength is maintained even in cases where large cracks form on the concrete surface to which the anchors are applied. 4. conclusion tensile load tests were conducted on an undercut post-installed anchor designed by the authors; fundamental yield strength characteristics of this anchor were analyzed based on the obtained results, and the following findings were obtained: 1. the extent of fixation with the concrete base material increased as a result of making the undercut section of the anchor expand in the upward direction, and the anchor exhibited a stable yield strength. 2. the extent of yield strength decreased even in the presence of relatively large cracks. it was lower in the proposed anchor than in standard metal anchors, and its superior performance was confirmed. acknowledgements the authors wish to thank dr. akira shibuya, technical officials of tohoku institute of technology, for his helpful 217 tomoyuki hatanaka, n. funaki, r. tanaka, s. fujita acta polytechnica ctu proceedings advice. in addition, kind assistance by mr. myunghwan kang and tatsuya kubo, fs technical co. ltd., is deeply appreciated. references [1] n. funaki, a. shibuya, s. fujita, et al. study on the mechanical properties of expansion post installed anchor, part 1 tensile test of expansion post installed anchor equipped with hardware bite. aij summaries of technical papers of annual meeting, 377-378, 2017. [2] m, onuma, a. shibuya, n. funaki, et al. study on the mechanical properties of expansion post installed anchor, part 2 tensile test of hybrid expansion post installed anchor. aij summaries of technical papers of annual meeting, 379-380, 2017. [3] n. funaki, a. shibuya, m. abe, et al. study on the mechanical properties of expansion post installed anchor, part 3 shear test of extension post installed anchor equipped with hardware bite. aij tohoku chapter summaries of technical papers of annual meeting, 61-64, 2018. [4] m. abe, a. shibuya, n. funaki, et al, study on the mechanical properties of expansion post installed anchor, part 4 mechanical property of l shaped anchor. aij tohoku chapter summaries of technical papers of annual meeting, 65-68, 2018. [5] t. hatanaka, n. funaki, r. tanaka, et al. mechanical property evaluation post-installed undercut anchors equipped with hardware bite based on finite element analysis. jci annual convention, vol.41, 1279-1284, 2019. 218 acta polytechnica ctu proceedings https://doi.org/10.14311/app.2023.42.0055 acta polytechnica ctu proceedings 42:55–60, 2023 © 2023 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague experimental validation of motion sensor physilog®5 applied to shoulder joint jan krivošej∗, jana garanová krišťáková, matej daniel, zbyněk šika czech technical university in prague, faculty of mechanical engineering, the department of mechanics, biomechanics and mechatronics, technická 4, 160 00 prague 6, czech republic ∗ corresponding author: jan.krivosej@fs.cvut.cz abstract. the main motivation of this paper is to verify the idea of using the physilog®5 unit for the patients with shoulder movement difficulties. the attached sensor to the patient’s arm then measures motion during which the patient should follow certain paths. finally, if a patient has difficulty with motion requirements, some typical pattern for their problem should emerge. by analysing these patterns, a database of typical problems could be created, which could assist doctors in determining a patient’s diagnosis.the experiment is focused on physilog® concerning the 5th generation. the goal is to experimentally identify and verify the performance of this generation during relatively large motions of the upper limb. for this purpose, an experimental stand representing spherical joint with an accurate absolute position sensing is assembled and calibrated. subsequently, the three physilog®5 sensors are mounted on this stand at different positions. keywords: shoulder joint, motion sensor, wearable, experimental validation. 1. introduction physilog®5 (p5) is well known and its applications are often related to lower limb motion sensing. in this case, the movement of the human lower limbs can be seen as planar. another typical application assumption associated with the lower limb is the measurement of a relatively small angular range. these simplifications often lead to neglecting the acceleration in the perpendicular direction to the considered plane of movement of the limb and as a result the measuring unit often gives very good and relatively accurate outputs. however, during large movements of the shoulder joint this simplification cannot be made. also, the absolute angular position drift that is present due to position acquisition through acceleration integration can be very significant. the article focused on drift reduction for highly dynamic movements is in [1]. fasel et al. [2] also presents functional calibration and strap-down joint drift correction in alpine skiing for computing 3d joint angles. palermo et al. [3] presents paper connected to the calibration of the lower limb sensing. the paper connected to the lower limb motion sensing [4] shows validation of the p5 unit during walking when mounted in a shoe. another validation of physilog® sensor can be found in lefeber at al. [5]. special approach is shown in su et al. [6] where wearable piezoelectric nanogenerators for self-powered body motion sensors are introduced. the p5 unit is not the only option to sense the body/limb motion. hamrs et al. [7] shows the ethos unit, which is an alternative to the physilog®5 and is described as: miniature orientation sensor for wearable human motion analysis. a different approach using a wearable functional arm movement sensing system is presented in nguyen et al. [8], where the measurement is based on the concept of an optical linear encoder. the aim of the paper is to experimentally verify the deployability, applicability and accuracy of the physilog®5 motion measurement unit when used to measure absolute angular position, angular velocity and angular acceleration of the shoulder joint during large movements. 2. experimental stand the considered experimental stand consists of a spherical joint, which is assembled for this purpose as a rotational and an universal joint arranged in a serial configuration. both the rotational and the universal joint are equipped with an accurate absolute position sensing. this approach is a very slight simplification of reality because the centre of the shoulder joint can move a little during motion. further insight into this issue is provided, for example, in wu wt al. [9]. the schematic view of the spherical joint is shown in figure 1, which is hanging on the indicated frame connection. the angle measurement is provided by one renishaw rotational absolute encoder resm20usa150 with line count 16384 per revolution implemented for “z” axis, two renishaw rotational absolute encoders resm20usa052 with line count 8192 per revolution implemented for “y” and “x” axis and renishaw’s signum™ si interface. the spherical motion is realized by simultaneous motion of “z”, “y” and “x” axis (indicated in figure 1). the “z” axis motion is not restricted. motion of “y” and “x” axis 55 https://doi.org/10.14311/app.2023.42.0055 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en j. krivošej, j. g. krišťáková, m. daniel, z. šika acta polytechnica ctu proceedings figure 1. the schematic drawing of experimental stand of spherical joint with considered coordinate system. are restricted to ±90 deg from the hanging position. a physilog®5 unit is subsequently mounted on this assembly on the lower platform, which is together with the entire experimental stand setup shown in figure 2. 3. calibration of the experimental stand generally, calibration is a fundamental process of mechanism to ensure accuracy, reliability, and consistency in measurements. the laser tracker (lt) leica at901 is used with the corner reflector (cr) for the calibration. the lt has accuracy 0.02 mm and the origin of the laser tracker measuring head is set approximately one meter from the cr in the rest position. the calibration setup is schematically shown in figure 3, where the position of the corner reflector and the laser tracker with the reference point located at the origin of the laser tracker measuring head (the laser tracker is standing on the ground and the experimental stand is hanging on the indicated frame connection) are illustrated. the used calibration procedure in the paper is based on modified newton’s method for overdetermined set of equations [10], [11]. the equations are given by constraints between dimensions of mechanism d, measured variables s of three rotational joints and position of the reference point of external measuring v. for the measurement in the j-th position one has f j = f(d, sj , vj) = 0. (1) the real dimensions of mechanism d generally differ from the values d̂ prescribed for manufacturing in design phase. however, one can consider them constant for all measurements. for n measurements one can rewrite equation (1) into compact form f (d, s, v ) = 0, (2) figure 2. the experimental stand with three p5 units mounted on the moving platform (connection to the frame can be seen at the top of figure). figure 3. the schematic description of the calibration setup. where f = [f1, f2, . . . , fn]t , s = [s1, s2, . . . , sn]t and v = [v1, v2, . . . , vn]t . using taylor series expansion yields f (d̂, s, v ) + jd∂d + · · · = 0, (3) 56 vol. 42/2023 validation of physilog®5 where jd is the jacobian matrix of partial derivatives of (2) with respect to d. considering the first term of taylor series only from equation (3) one can express ∂d using pseudoinversion of jacobian matrix jd. using iteration method the corrections can be found as ∂di = −(jt di jdi)−1jt di + f (d̂i, s, v ), (4) and, subsequently, new values are iteratively obtained as di+1 = di + ∂di. (5) considering that the absolute cartesian system is set by lt in the reference point, the whole kinematic model is described using transformation matrices as follows r0cr = t 04r4cr, (6) where r4cr = [x4cr, y4cr, z4cr, 1]t is the radius vector of cr in the coordinate system of platform and t 04 is given as t 04 = t 01t 12(φzm )t 23(φym )t 34(φxm ), (7) where t 01 = t x(x0)t y(y0)t z(z0)t φx (φx0) t φy (φy0)t φz (φz0), (8) describing the position of the experimental demonstrator with respect to the position of the lt, angles φxm ,φym and φzm are measured by the encoders and t 12(φzm ) = t x(x1)t y(y1)t φx (φx1) t φy (φy1)t φz (φzm ), t 23(φym ) = t x(x2)t z(z2)t φx (φx2) t φz (φz2)t φy (φym ), t 34(φxm ) = t y(y3)t z(z3)t φy (φy3) t φz (φz3)t φx (φxm ), (9) describing the transformation from one system to the next one. the arising system has 21 unknown parameters to be calibrated. namely, the vector of parameters to be calibrated is d = [x0, y0, z0, φx0, φy0, φz0, x1, y1, φx1, φy1, x2, z2, φx2, φz2, y3, z3, φy3, φz3, x4cr, y4cr, z4cr]. (10) the constraint equation in the form (1) is given as r0cr − r0crlt = 0, (11) where r0crlt = [x0crlt , y0crlt , z0crlt , 1]t is the radius-vector of the cr position measured by the lt. the calibration process itself is divided into two phases. in the first phase, the data are measured. specifically, the data are synchronously obtained from the lt and from the absolute encoders of the experimental stand. the motion of the stand is performed manually to reflect the motion of the upper limb. in the second phase, the obtained data are used for the offline calibration using iteration method described above ((11), (4) and (5)). 4. experimental results the experimental setup is shown in figure 2, where three p5 units – master (m), slave 1 (s1) and slave 2 (s2) – are mounted on the auxiliary platform (the simulation results are shown only for m and s1 for compactness, s2 gives very similar results in comparison with m and s1). the p5 units using bluetooth for the time synchonized during measurement. the motion of the platform is performed manually to simulate maximally similar conditions relative to the shoulder joint the absolute measured angles are compared with the investigated sensor physilog®5 using its own internal library from “gait up” (this library is locked and cannot be open or edited) and is used for offline data transformation after measurement, where raw data from accelerometer, gyroscope and magnetometer are transformed to quaternions and euler angles in “roll”, “pitch”, “yaw” (rpy) form. this computation is done offline after measurement. the important thing is the time sychnonisation of both measurements, which was done by taps on the platform with sensors before and after the measured motion. the synchronized time window is then framed by dashdotted vertical lines in related graphs. the first experimental results shows the measurements considering the steady state (rest position). the comparison of the obtained data is shown in figure 4. however, even for the steady state position, the m and s1 sensor data show a very significant drift of the “z” axis. the remaining “x” and “y” axis behaviour is on the other hand very good, but still slightly effected (s1) by the drifting “z” axis. note that the measurement range is from −180 to 180 deg and observed vertical line “jumps” represents exceeding of this limit, where measurement “jumps” from −180 to 180 deg and vice versa (this means, for example, that φzp of sensor s1 in figure 4 reaches approx. 2260 deg in 60 s). a complementary view is subsequently shown in figure 5, where the raw data from sensor m accelerometer and gyroscope during measured motion are plotted. the value az obtained from sensor m during the 57 j. krivošej, j. g. krišťáková, m. daniel, z. šika acta polytechnica ctu proceedings figure 4. the obtained data from the sensor – master (top), sensor – slave 1 (middle) and experimental stand (bottom) during the steady state. the synchronized time window is framed by dashdotted vertical lines. figure 5. the raw data from accelerometer (top) and gyroscope (bottom) from the sensor – master during the steady state. the synchronized time window is framed by dashdotted vertical lines. figure 6. the obtained data from the sensor – master (top) and experimental stand (bottom), where the considered steady state is obtained by deflection to another position. the synchronized time window is framed by dashdotted vertical lines. figure 7. the raw data from accelerometer (top) and gyroscope (bottom) from the sensor – master, where the considered steady state is obtained by deflection to another position. the synchronized time window is framed by dashdotted vertical lines. steady state position measurement oscillates around 9.81 m s−2, which represents the gravitational acceleration in “z” axis direction. this acceleration should be removed from the accelerometer data during offline computation using internal library. the second experimental results shows the measurements, where the considered steady state is obtained 58 vol. 42/2023 validation of physilog®5 figure 8. the obtained data from the sensor – master (top), sensor – slave 1 (middle) and experimental stand (bottom) during the pendulum motion around the “y” axis. the synchronized time window is framed by dashdotted vertical lines. by deflection to another position(different from the rest position). the comparison of the obtained data is shown in figure 6. however, sensor m data shows a very significant drift of all three axis (plotted curves for the sensor s1 are very similar). a complementary view is again shown in figure 7, where the raw data from sensor m accelerometer and gyroscope during measured motion are plotted again. it can be seen that the gravitational acceleration vector effects mainly “z” and “x” axes in this case. this acceleration should be again removed from the accelerometer data during offline computation using internal library. the last presented experimental results shows the measurements, where the pendulum motion is performed manually around the “y” axis. the comparison of the obtained data is shown in figure 8. it can be seen that the swing motion pattern in comparison to the experimental stand is present at the sensor s1. however, sensor m suffers from “drift” more than sensor s1, which effect the measurement very badly. 5. conclusion and future work the experimental validation of sensor physilog®5 has been proposed and studied. the experimental stand was designed to meet the kinematic requirement for spherical motion to simulate the kinematics of a shoulder joint. the design has been done as rotational and universal joint arranged in serial configuration. the angle measurement was provided by three renishaw rotational absolute encoders and renishaw’s signum™ si interface. the experimental stand has been calibrated using laser tracker leica at901. the movement of the platform with the sensor was performed manually to simulate maximally similar conditions relative to the shoulder joint. the comparison of the experimental stand and physilog®5 unit shows that the data directly obtained from the internal library during offline computation are not satisfactorily accurate. this fact is mainly seen in the axis, which is most of the time parallel or close to parallel to the vector of gravitational acceleration. this “drift” is very distinctive and has been present during each measurement. its behaviour is very random and very strongly effects the the remaining two axes. it is shown that even the steady position suffers from the mentioned “drift”. future work considers the use of raw data, additional filters, defining areas with minimum drift to obtain sufficiently appropriate data to obtain a reliable measurement for subsequent evaluation of the patient’s shoulder movement difficulties. acknowledgements this research was funded by the czech science foundation project 23-06920s “functionally biomimetic exoskeleton of human 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https://doi.org/10.1016/j.gaitpost.2021.09.203 [5] n. lefeber, m. degelaen, c. truyers, et al. validity and reproducibility of inertial physilog sensors for spatiotemporal gait analysis in patients with stroke. ieee transactions on neural systems and 59 https://doi.org/10.1109/jbhi.2017.2659758 https://doi.org/10.1371/journal.pone.0181446 https://doi.org/10.1016/j.measurement.2014.03.004 https://doi.org/10.1016/j.measurement.2014.03.004 https://doi.org/10.1016/j.gaitpost.2021.09.203 j. krivošej, j. g. krišťáková, m. daniel, z. šika acta polytechnica ctu proceedings rehabilitation engineering 27(9):1865–1874, 2019. https://doi.org/10.1109/tnsre.2019.2930751 [6] c. su, x. huang, l. zhang, et al. robust superhydrophobic wearable piezoelectric nanogenerators for self-powered body motion sensors. nano energy 107:108095, 2023. https://doi.org/10.1016/j.nanoen.2022.108095 [7] h. harms, o. amft, r. winkler, et al. ethos: miniature orientation sensor for wearable human motion analysis. in 2010 ieee sensors, pp. 1037–1042. ieee, 2010. https://doi.org/10.1109/icsens.2010.5690738 [8] k. d. nguyen, i.-m. chen, z. luo, et al. a wearable sensing system for tracking and monitoring of functional arm movement. ieee/asme transactions on mechatronics 16(2):213–220, 2011. https://doi.org/10.1109/tmech.2009.2039222 [9] g. wu, f. c. van der helm, h. d. veeger, et al. isb recommendation on definitions of joint coordinate systems of various joints for the reporting of human joint motion – part ii: shoulder, elbow, wrist and hand. journal of biomechanics 38(5):981–992, 2005. https://doi.org/10.1016/j.jbiomech.2004.05.042 [10] p. beneš, m. valášek, z. šika, et al. experiments with redundant parallel calibration and measuring machine redcam. applied and computational mechanics 1(2):387–392, 2007. [11] p. beneš, m. valášek, j. švéda. measurement and calibration of machine tools in 6 dofs in large workspace. journal of machine engineering 9(3):77–87, 2009. 60 https://doi.org/10.1109/tnsre.2019.2930751 https://doi.org/10.1016/j.nanoen.2022.108095 https://doi.org/10.1109/icsens.2010.5690738 https://doi.org/10.1109/tmech.2009.2039222 https://doi.org/10.1016/j.jbiomech.2004.05.042 acta polytechnica ctu proceedings 42:55–59, 2023 1 introduction 2 experimental stand 3 calibration of the experimental stand 4 experimental results 5 conclusion and future work acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2024.47.0001 acta polytechnica ctu proceedings 47:1–7, 2024 © 2024 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague dowel bars – reinforcing elements of cement concrete pavements stanislav bedřicha,∗, jiří grošekb a czech technical university in prague, faculty of civil engineering, department of road construction, thákurova 7, 166 29 prague, czech republic b transport research center, líšeňská 2657/33a, 636 00 brno, czech republic ∗ corresponding author: stanislav.bedrich@fsv.cvut.cz abstract. our aim is to analyze the function of reinforcement elements (dowel bars) in cement concrete pavement to highlight the influence of these elements on the service life of the pavement as a whole and to find possible alternatives to existing products that are freely available on the czech market. that is, how these elements in the form of dowel bars should work properly so that the function meets the design assumptions of a rigid pavement. we describe the procedure for inserting dowel bars during paving, which is directly related to the possibilities of using different dowel bars. finally, we summarize the possibilities of using alternative materials, shapes, or sizes of dowel bars and the possibilities of their use in the czech republic. keywords: dowel bar, concrete pavement, reinforcing, material, shape, position. 1. introduction as reinforcing elements in cement concrete pavement (ccp), which are placed in transverse shrinkage, expansion and contraction joints, dowel bars in the form of short steel bars of circular cross-section coated with epoxy are used as standard in the czech republic. they are embedded in the concrete pavement and provide a mechanical connection that limits the vertical movement of the slabs while allowing their horizontal movement due to shrinkage and temperature changes in the cement concrete pavement. at the same time, these reinforcing elements ensure a reduction in deflection of the joint and stresses on the approach and departure slabs by increasing the efficiency of load transfer between the slabs. dowel bars were used on czech motorways in the 1990s during the construction of the d5 motorway, which was implemented according to german regulations and experience of that time. the previous motorway constructed, i.e. d1 motorway, was built without reinforcement of transverse and longitudinal joints, which led to sliding of the slabs in all directions and the formation of a step during use. dowel bars are used in jointed unreinforced and jointed reinforced cement concrete pavement figure 1 [1] . in both cases they have the same function. 2. basic requirements for dowel bars and design of their location dowel bars are used in the transverse joints of ccp motorways and airport surfaces. for surfaces that are run in an omnidirectional direction, dowel bars can be used in both transverse and longitudinal joints. on other roads, the interaction of the plates in the figure 1. jointed reinforced concrete pavement [1]. longitudinal joints can be ensured by a so-called tooth (tongue and groove) [2]. dowel bars shall comply with the provisions set out in en 13877-3 [3], en 13877-2 [4] and en 73 6123-1 [2] for the design and construction of the roadway. here it is specified that the minimum diameter of the dowel bars shall be 25 mm and the minimum length 500 mm for motorways and airfields. for other roads a minimum diameter of 16 mm may be designed. in the czech republic, steel bars with a diameter of 25 mm and a length of 500 mm with a coating along the entire length are used as standard. the minimum requirements for the design of the placement of the dowel bars are defined in the standard [4] as follows: • the distance between the dowel bars must not be greater than 12 times their diameter (i.e., in the czech republic 25 × 12 = 300 mm), • the distance between the dowel bars can be doubled for lanes with light traffic loads, e.g., overtaking lanes, 1 https://doi.org/10.14311/app.2024.47.0001 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en stanislav bedřich, jiří grošek acta polytechnica ctu proceedings figure 2. examples of the possible arrangement of dowel bars in transverse joints according to [2], chap. 5.3.7.1. • the distance between the outer dowel bars must not be less than 250 mm. czech technical standard [2] already recommends the arrangement according to figure 2. 3. basic materials of dowel bars and requirements for their properties during the construction of ccp, dowel bars have been manufactured from a variety of materials. nowadays, mainly steel and various types of composite materials (frp – fiber reinforced polymer) are used for the production of dowel bars in the world. in the czech republic, the regulations [3] so far allow us to use only steel bars made of plain coated steel. the steel for the bars is not specified by steel grade, only the tensile strength, which must be equal to or greater than 250 mpa according to en iso 15630-1. the manufacturers [5], [6] state in the documents for this product that the dowel bars are made of s235jr steel. in addition to the material of the bars itself, the coating is very important and has two functions. the first function is to protect the steel bar from corrosion due to moisture and the action of chemical de-icing agents and the second function is to allow easy horizontal movement of the concrete slabs (slip in concrete). according to the regulations [3], the average thickness of the coating should not exceed 1.25 mm. however, this value is very high from our point of view. due to the properties of the standard epoxy coatings used and the amount of deformation, such a high coating thickness can adversely affect the load transfer between the slabs. furthermore, the czech regulations [2] specify a minimum coating thickness of 0.3 mm of plastic, which must be factory applied. for cb ii and cb iii a different type of coating may be used [2]. as mentioned above, dowel bars for ccp can be made from materials other than steel. as an alternative, frp is offered. frp material consists of two parts, a reinforcing component and a binder (matrix). by combining these components, the resulting properties are achieved. the resulting properties depend on the relative proportions and types of the input components. the most commonly used fibres for the production of frp reinforcement are glass fibre reinforcement (gfrp), carbon fibre reinforcement (cfrp), aramid fibre reinforcement (afrp), basalt fibre reinforcement (bfrp) or a combination of these. the binder or matrix provides cohesion and binds the individual fibres together. the matrix transfers the external stresses to the fibres and at the same time has a protective function against environmental influences. basic binders include epoxies, vinylesters and polyesters. according to available evidence from manufacturers, the tensile strength of glass fibre composite reinforcement can be found in a wide range from 550 mpa to 1 300 mpa. carbon fibre composite reinforcement can also reach tensile strengths of around 2 000 mpa. for example, the findings of research [7], which compared the lte (load transfer efficiency) between steel and frp dowel bars, stated that frp dowel bars would result in significantly lower lte than steel specimens of the same cross section. to give an idea, figure 3 compares the load transfer efficiency between different shapes, diameters and materials. this shows that in the case of using frp for ccp in the czech republic, we would have to consider larger dowel bars estimated at around 30 mm. figure 3. load transfer efficiency during load cycles for different materials and diameters of dowel bars [7]. 2 vol. 47/2024 dowel bars – reinforcing elements of cement concrete pavements due to the differences in the properties of the input materials and the associated resulting properties, it is not possible to use this type of material outside of, for example, experimental sections without a technical regulation setting out the minimum requirements. a comparison of the basic material properties of frp and steel is given in table 1. the individual properties of frp materials may vary according to the choice of fibres (combination), matrix and manufacturing. an important parameter for the correct and longterm function of the dowel bars is the shear strength. dowel bars are shear stressed and we do not currently have long-term experience from which all the necessary parameters can be determined. research carried out in the czech republic in earlier years has shown that if frp is used it could lead to failures at ccp joints. the lower resistance of some types of frp to repeated shear stresses has been demonstrated. already after a few thousand cycles of laboratory testing, deformation of the dowel bars occurred [9]. results and comparisons of tested materials (steel/frp) including different dimensions (diameter) are also reported in foreign research, indicating that some properties of frp material can be compensated by increasing the diameter of the dowel bars [10]. in the case of combinations of steel and composite materials, there are experiments with a steel core covered with composite material. however, there is a question as to what the price of the product will be compared to existing available products. given the need to find alternative sources to the standard ones that have been proven for many years, it is imperative that such a regulation be established as soon as possible [11]. 4. shapes of dowel bars the most common shape of dowel bars is a cylinder see figure 4 [6] and will probably remain so. especially for ccp in the czech republic, where two-layer laying is carried out and bars are inserted by machine. in addition, the production of other rolled steel types, e.g. oval diameters, is significantly more expensive and less available or other shapes of dowel bars such as, e.g. diamond see figure 5 [12]. in the case of inserting dowel bars into baskets (or other attachment methods), other shapes can be used that cannot be used in machine insertion to ensure the correct position of the dowel bars and its correct function. the dowel bars on shorter sections are fixed in the baskets before the concrete mix is placed in the desired position so that the optimum function of the bars is fulfilled. the next most common shapes of dowel bars are bars with oval or square or tubular cross-sections. for large areas, diamond-shaped or disc-shaped bars are also used to reduce tensile stresses and allow movement parallel to the joint. if we are thinking about saving materials, a tubular dowel bar filled with, e.g., a composite material is a figure 4. standard dowel bars with a diameter of 25 mm and a length of 500 mm coated with polyolefin powder [6]. figure 5. diamond-shaped dowel bars [12]. suitable alternative. all the proposed shapes of dowel bars have been designed with the intention of reducing the tensile stresses at the joints and preventing cracking or damage at shrinkage or expansion joints, while increasing load transfer. 5. inserting dowel bars the dowel bars are usually inserted halfway through the thickness of the slab (ideally parallel to the road surface) parallel to the direction of travel. • machine placement into fresh mixture – dowel bars are inserted by means of a vibrating device which is part of a concrete paver or one of a set of pavers (usually the first one, designed for the lower concrete). machine placement is used for two-layer paving. the concrete is placed in two layers, with a layer of top concrete on top of the fresh bottom layer, see figure 6. • with the help of support baskets – dowel bars are prepared and fixed in the desired position, usually in steel baskets before machine laying or hand concreting. • additional insertion of reinforcement elements into hardened concrete. in the case of dowel bars, this 3 stanislav bedřich, jiří grošek acta polytechnica ctu proceedings feature material steel gfrp cfrp modulus in longitudinal direction [gpa] 200 35 to 60 90 to 580 modulus in transverse direction [gpa] 200 about 8 to 9 about 10 to 12 tensile strength in fibre direction [mpa] 300 to 600 450 to 1 600 600 to 3 500 compressive strength in fibre direction [mpa] 300 to 600 approx. 1/2 of the tensile strength approx. 1/2 of the tensile strength transverse tensile strength [mpa] 300 to 600 30 to 40 30 to 40 volume weight [kg m−3] 7 850 about 2 100 about 1 700 table 1. comparison of the properties of steel and composite materials [8] is done by drilling or cutting holes of the required size. • by pre-installing the dowel bars (or bars to create holes) in the formwork, see figure 7 [13]. figure 6. finisher set for two-layer cement concrete pavement, construction d1 (strabag). figure 7. mould with bars for subsequent insertion of dowel bars into the joint [13]. the choice of the technology for the ccp is crucial when deciding (from the point of view of implementation) what shape of the bars can be designed. 6. position of dowel bars the basic distribution of dowel bars deviations is illustrated in figure 8 [11]. faulty fit of the dowel bars reduces its efficiency and its ability to provide adequate load transfer. vertical or horizontal tilt (especially vertical skew) can limit movements due to thermal expansion or contraction, which is the most serious problem in relation to ccp life see figure 9. these constraints increase the tensile stresses of the plates and can be the cause of failures (most commonly manifested by the formation of cracks). these constraints can also induce an additional increase in local concrete stresses around the dowel bar, which then leads to failures at the transverse joints. figure 8. basic distribution of deviations from the prescribed dowel bars fit [11]. figure 9. damage to the cement concrete pavement at the transverse joint of the d5 motorway. it is obvious that the friction of the dowel bars in the concrete creates a resistive force that must be overcome both when closing and opening the joint (for this reason, the coating of dowel bars is also tested). it should be noted here that the frictional force is significantly increased by the angular deviation between the axis of the bars and the axis of the road (i.e. horizontal and vertical tilt). already an average angular deviation of 5° can induce a stress of approximately 1 mpa in the cross-section of the plate [6]. since a perfect fit of the dowel bars cannot generally be achieved, it can be assumed that considerable normal forces must be overcome regularly when opening and closing 4 vol. 47/2024 dowel bars – reinforcing elements of cement concrete pavements the joints. on the other hand, the friction of the concrete slab against the base layer no longer plays a significant role at this point. a detailed representation of the temperature curve and the associated joint opening for one day is given in figure 10. the temperature waveform of the concrete slab during the day shows the joint closure/opening. figure 10. daily curve of concrete temperature (red) and joint opening (blue) near zero stress [10]. in the czech republic, since 2014, changes in the accepted tolerances for individual categories of dowel bars misplacement are given in [2] in chapter 5.3.7.2: • the inclined position of the dowel bar relative to the dowel bar length of 500 mm (difference of the dowel barends in the horizontal and vertical direction) may be up to 25 mm, but this value must be met for 75 % of the dowel bars in the joint and the remaining maximum 25 % of the dowel bars in the joint may have an inclination of up to 40 mm; • the deviation of the fit relative to the top surface of the plate must be observed for at least 75 % of the dowel bars in the joint and for the remaining maximum 25 % of the dowel bars in the joint this deviation may be up to 50 mm; • the deviation with respect to the transverse joint (longitudinal displacement) may be up to 75 mm, this value must be observed for a minimum of 75 % of the dowel bars and for the remaining maximum 25 % of the dowel bars in the joint it may be up to 120 mm. i recommend checking the fit of the dowel bars using non-destructive equipment such as ground penetrating radar (gpr) or mit scan. these devices already produce very accurate results in the hands of an experienced surveyor and results processor. to calibrate and verify the results, it is recommended that the results be verified on a bore with a dowel bar made on an untraveled portion of the roadway (e.g. paved shoulder). ferrous minerals in the aggregate, moisture in the concrete or other steel elements in the ccp (e.g. surface reinforcement) may affect the measurement results. detecting the position of slip-rods in fresh concrete by exposing the slip-rods is not recommended due to imperfect subsequent compaction and repair of the cement concrete pavement. failures may subsequently occur at these locations. in addition, on uncured concrete, the propagation speed of the electromagnetic signal varies, leading to inaccuracies in the determination of the layer thickness and therefore the correct position of the dowel bar. a general problem in evaluating the performance of the entire ccp is the evaluation of the performance of the individual parts separately (fresh hardened concrete, base layers, dowel bars, etc.). if “mistakes” are made in the design and then in the implementation, the individual deficiencies are cumulative. the result is a shortened service life of the ccp, even though we have complied with all the prescribed parameters. the same is true for the assessment of the faulty fit of the dowel bars. the evaluation considers individual dowel bars variations at the measured transverse joint, but does not consider the overall effect of the misfit on the roadway behavior, or whether it is a single joint or several transverse joints in a row, or whether the deficiency is in the staging or slow lane, etc. however, these deviations from the correct fit have different effects on the behaviour of individual, interlocking slabs. when checking the dowel bars and their functionality, we must also consider the influence of the dowel bars on the formation of shrinkage cracks (autogenous shrinkage during hydration, etc.) and the subsequent correct functioning of the transverse joints. to give you an idea, if we consider a concrete shrinkage of 0.1–0.25 µm m−1 (depending on the mix) [14] at an approximate stress of 1 mpa, it is likely that deformations due to tensile stresses will occur at the contact between the dowel bars and the concrete already in the first days after concreting, especially for dowel bars placed in an inclined position. this subsequently reduces the efficiency of load transfer between the slabs. it is therefore important not to underestimate the importance of the correct position of this reinforcement element during implementation. 7. errors in the implementation of cement concrete pavement the main cause of errors in the use of dowel bars is the human factor. whether the error is conscious or unconscious, it is often behind the subsequent costly corrective measures. the following are individual errors by technology that occur on construction sites regardless of the contractor. in the case of machine laying, the insertion of the dowel bars is secured mechanically. either the machine automatically inserts the dowel bars into holder with the vibrator, or the bars are inserted manually by an authorized employee and then vibrated into the underlying concrete layer by the machine. the correct positioning of the dowel bars is mainly influenced by the characteristics of the machine (paver, whole assembly), the characteristics of the underlying fresh concrete mixture, etc. 5 stanislav bedřich, jiří grošek acta polytechnica ctu proceedings leaving aside machine error, which we have to take into account, the most common shortcomings are: • insufficiently trained employees (at least they are not aware of the consequences of their omissions); • not adding dowel bars to the machine (dowel bars are not in the transverse joints); • incorrectly set height of vibrating dowel bars; • the fit of the dowel bar to the position of the transverse joint (horizontal displacement) is not checked. other deficiencies that affect the position of the bars, such as the consistency of the fresh concrete, the vibration effect of the paver, the speed of paving; nighttime concreting, or frequent stoppages, must also be technologically addressed by the contractor’s responsible persons. when installing dowel bars using baskets, it is always necessary to: • choose the material and thickness of the wire of the basket so that it is not deformed during laying (covering with dense concrete mix and vibrating equipment); • check the height of the basket, which must correspond to the desired position of the dowel bar, i.e. h/2 – the radius of the dowel bar (check the height of the treasury layer); • fix the steel basket in such a way that it does not shift during concrete pouring; • fix the dowel bars in the basket so that they do not fall out of the basket or shift relative to the position of the planned contraction joint when the concrete mix is included. when preparing for concrete pouring, we must be aware that heavy equipment is used during the execution (various excavators to stretch the concrete mixture), or the concrete mix is dumped from a truck or pumped from a mixer truck. all these procedures are carried out at a certain frequency, where everything has a temporal coincidence. therefore, if any damage occurs, for example due to an imperfectly prepared bin, there may be a delay if the paving workers notice the problem [11, 15]. 8. conslusion cement concrete covers have the advantage of resisting higher loads and therefore have a longer service life compared to asphalt pavements. however, this advantage becomes a disadvantage when you are trying to find information on the implementation of the road after twenty or more years of operation. the documentation is often incomplete and the implementers do not remember the details, are retired, etc. yet one of the most important bases for diagnosis is information that is usually never passed on by the contractor to the client, that there was a problem with the concrete plant, aggregate, cement, air temperature and we could go on. so we can divide the failures caused by the placement of slipforms into two groups: figure 11. non-functioning grout on cement concrete roadway. • failures that occur within a relatively short period of time after commissioning 0–5 years – these failures are caused by e.g. insufficient concrete cover or the sliding spikes are in an inclined vertical position and one of the ends is close to the road surface; • faults that occur after more than 10 years of operation – for these faults, it is often important to have sufficient documentation to enable diagnostics to be properly evaluated and the cause of the faults to be identified [11, 15]. in the field of ccp, reinforcement elements are one area that deserves more attention, in particular [11, 15]: • allow the use of frp composite reinforcement elements or other possible variants by developing a regulation (standard) to allow this; • to consider other possible shape options of dowel bars, e.g. oval or tubular shapes, within the developed regulation; • for major and highly loaded roads, recommend designing larger diameter dowel bars, thereby reducing the magnitude of tensile stresses between the mandrel and the concrete (e.g. for airport carriageways or highly loaded storage areas). a well-maintained and functional joints or rubber seal also contributes to the proper function of the transverse joint. if the condition of the pavement 6 vol. 47/2024 dowel bars – reinforcing elements of cement concrete pavements is not regularly checked and corrected immediately if necessary, the cost of repairing cement concrete pavements will be high regardless of the quality of the work carried out, see figure 11. acknowledgements this article was produced with the financial support of the ministry of transport within the programme of long-term conceptual development of research institutions and from the research results of the student grant programme no. sgs20/046/ohk1/1t/11 references [1] m. navrátil. repair of rwy 12/3, hochtief cz, a.s., asb, 2017. [2023-11-11]. https://www.asb-portal.cz/stavebnictvi/ inzenyrske-stavby/doprava/oprava-rwy-12-30 [2] stavba vozovek cementobetonové kryty – část 1: provádění a kontrola shody [road building – concrete pavements – part 1: construction and conformity assessment]. standard, czech standard institute, prague, 2016. [3] cementobetonové kryty – část 3: specifikace pro kluzné trny [concrete pavements – part 3: specifications for dowels to be used in concrete pavements]. standard, czech standard institute, prague, 2006. [4] cementobetonové kryty – část 2: funkční požadavky [concrete pavements – part 2: functional requirements for concrete pavements]. standard, czech standard institute, prague, 2016. [5] o. brentzel. stahlverarbeitung e.k., hauptstraße 61, d – 67693 fischbach bei kaiserslautern, 2023. [2023-11-11]. https://www.otto-brentzel.com [6] likal, s.r.o. prohlášení o vlastnostech. [2023-11-11]. http://www.likal.cz/prilohyarchiv/c8-2.pdf [7] c. hu, j. ma, j. zhao, et al. experimental study of dowel bar alternatives based on similarity model test. advances in materials science and engineering 2017:3981908, 2017. https://doi.org/10.1155/2017/3981908 [8] f. girgle. analysis of the anchorage area of elements prestressed with non–metallic reinforcement. ph.d. thesis, vysoké učení technické v brně, 2011. [9] j. grošek. research in the field of construction elements and design of concrete pavements. ph.d. thesis, vysoké učení technické v brně, 2016. [10] s. villaret, m. wieland. information systems for continuously assessing the road condition. betonstraßentagung 2019 b 34:306, 2019. isbn 978-3-86446-260-3. [11] s. bedřich, p. pánek, j. veselý, l. vébr. bars – reinforcing elements of cement concrete pavements. in pozemní komunikace 2022, pp. 33–34. czech technical university in prague, 2022. [12] g. fricks. diamond dowel system vs. conventional square or round dowels, 2016. [2023-11-11]. https:// www.linkedin.com/pulse/diamond-dowel-system-vsconventional-square-round-dowels-greg-fricks/ [13] formtech concrete forms, inc. [2023-11-11]. https://catalog.formtechinc.com/products/ pavingflatwork/pavingflatround/surebuilt-dowelsleeve/ [14] z. p. bažant, s. baweja. creep and shrinkage prediction model for analysis and design of concrete structures – model b3. materials and structures 28(6):1– 83, 1995. https://doi.org/10.1007/bf02473152 [15] s. bedřich. reinforcing elements of concrete pavament. silniční obzor 83(6):3–7, 2022. 7 https://www.asb-portal.cz/stavebnictvi/inzenyrske-stavby/doprava/oprava-rwy-12-30 https://www.asb-portal.cz/stavebnictvi/inzenyrske-stavby/doprava/oprava-rwy-12-30 https://www.otto-brentzel.com http://www.likal.cz/prilohyarchiv/c8-2.pdf https://doi.org/10.1155/2017/3981908 https://www.linkedin.com/pulse/diamond-dowel-system-vs-conventional-square-round-dowels-greg-fricks/ https://www.linkedin.com/pulse/diamond-dowel-system-vs-conventional-square-round-dowels-greg-fricks/ https://www.linkedin.com/pulse/diamond-dowel-system-vs-conventional-square-round-dowels-greg-fricks/ https://catalog.formtechinc.com/products/pavingflatwork/pavingflatround/surebuilt-dowel-sleeve/ https://catalog.formtechinc.com/products/pavingflatwork/pavingflatround/surebuilt-dowel-sleeve/ https://catalog.formtechinc.com/products/pavingflatwork/pavingflatround/surebuilt-dowel-sleeve/ https://doi.org/10.1007/bf02473152 acta polytechnica ctu proceedings 47:1–7, 2024 1 introduction 2 basic requirements for dowel bars and design of their location 3 basic materials of dowel bars and requirements for their properties 4 shapes of dowel bars 5 inserting dowel bars 6 position of dowel bars 7 errors in the implementation of cement concrete pavement 8 conslusion acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.38.0281 acta polytechnica ctu proceedings 38:281–287, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague building-integrated photovoltaics (bipv) combined with hydrogen-based electricity storage system at building-scale towards carbon neutrality sergi aguacila,∗, yvan morierb, philippe coutyb,c, jean-philippe bacherb a ecole polytechnique fédérale de lausanne (epfl), building2050 group, fribourg, switzerland b university of applied science of western switzerland (heia-fr, hes-so), energy institute, fribourg, switzerland c tecphy sàrl, renens (vd), switzerland ∗ corresponding author: sergi.aguacil@epfl.ch abstract. electricity storage technologies in buildings are evolving, mainly to reduce their environmental impact and to improve self-sufficiency of buildings that produce their own energy through building-integrated photovoltaics (bipv) installations. to maximize self-consumption – minimizing the import of grid electricity – photovoltaic (pv) systems can be coupled with a hydrogen storage system converting the electricity to hydrogen by electrolysis during the summer season – when the on-site production is higher – and employing it during the winter season with fuel cells. this study focuses on the sizing constraints of solar hydrogen systems at building-scale using an innovative research-centre that will be built in fribourg (switzerland). it presents four stories and a mix-usage (office spaces and research facilities areas) and multi-oriented pv installation in order to produce enough electricity to achieve at least 50 % of electricity self-sufficiency ratio. using the pv production, this study aims to optimise the sizing of a hydrogen storage system allowing to reach the required self-sufficiency ratio with the lowest environmental impact possible. ultimately, the global energy and financial efficiency of the system will be analysed. keywords: building-integrated photovoltaics, building energy analysis, solar hydrogen storage, fuel cell, battery storage system. 1. introduction photovoltaic offers an easy and affordable manner to produce sustainable electricity using solar power. over the last decade, the number of solar panels installation in switzerland has been multiplied by five [1] and the overall electricity efficiency of these panels has kept increasing. solar energy will play a big role in the path to reach carbon neutrality by 2050. however, one issue remains unchanged, solar energy is only available during a short period of time during the day and in most countries, the production varies greatly throughout the year. this means that great selfsufficiency (ss) ratios (equation (1)) can be achieved during the day and summer but energy needs to be stored in order to be reused during the night or winter when the production isn’t sufficient. ssratio = pv production used onsite total electrical consumption (1) daily storage is usually achieved by using battery packs, however, they have a large carbon footprint [2]. nowadays, in most pv installations, very little storage is integrated allowing the building to be self-sufficient for 4 to 6 hours [3, 4]. this paper focuses on the use of seasonal hydrogen (h2) storage at buildingscale. in the summer, excess of production can be used to produce h2 by electrolysis of water and can then be stored in individual tanks. in the winter, when the electrical demand is higher than the on-site production, the h2 can be used in a fuel cell to produce electricity. this process also releases heat which can be recovered for heating or domestic hot water (dhw) converting the fuel cell in a co-generation unit. to simulate the h2 cycle over the year, a ms excel tool has been developed in order to simulate the way the energy is stored and reused. the program allows for two types of storage, a li-ion battery pack and a hydrogen storage system. in order to run a simulation, the program requires the following main input data: • hourly time-step electricity pv production and electrical and thermal demand [kwh]. • h2 storage specification (power and efficiency of the fuel cell and electrolyser, storage capacity). • battery pack specification (capacity and efficiency). • thermal storage specification (hot water tank volume, tank’s insulation, storage temperature). • additional data on components (life cycle, cost, greenhouse gases (ghg) emission). using this information, the program simulates the 281 https://doi.org/10.14311/app.2022.38.0281 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en s. aguacil, y. morier, p. couty, j.-p. bacher acta polytechnica ctu proceedings energy storage during a year and can display output values such as ss ratio, amount of electrical energy imported and exported from or to the grid, amount of thermal energy stored, the overall efficiency of the h2 cycle or the amount of ghg emitted. in a first scenario, the sizing of a h2 storage system allowing to reach 100 % ss was computed. this showed that to achieve such a goal, the amount of h2 stored had to be enormous, around 75 days of autonomy and 1.4 tons of h2. in this paper, the goal is to reach an ss ratio of 80 %. moreover, without using heat recovery on the fuel cell, the overall efficiency of the h2 storage system only reached about 30 %. in a further study [5], the focus was placed on heat recovery on the fuel cell. this time, parts of the simulation were made using polysun software [6]. heat recovery allowed the system to reach 43 % overall efficiency; however, limitations were quickly encountered. using the polysun software, we had very little control of the fuel cell as the program would only consider the thermal demand to switch it on. this is why this article focuses on the writing of an in-house excel tool that allows the user to control every energy flux (electrical and thermal). the global efficiency of the system can be optimized by controlling the electrolyser and the fuel cell in the most efficient way using different control modes that are presented in this paper. an effective sizing of the h2 storage system is then possible and outputs values are available to analyse the project. in order to run the simulation, this paper uses the smart living lab (sll) [7] as a real case study. the building will already be equipped with energy storage in the form of li-ion batteries but will provide enough space for further research, in our case, a h2 storage system with a co-generation fuel cell allowing to produce heat and electricity for the building. 2. methodology the methodology encompasses three phases: (1.) hourly energy demand and production are estimated using the energy modelling and analysis of the building. the thermal and electrical demands are computed using designbuilder [8] with energyplus simulation engine [9]. the pv production is calculated using the polysun software [6]. these data will be used in the further phases in the form of hourly time-step value. (2.) the implementation of a hydrogen-based storage system. in order to simulate the addition of a h2 storage system a dedicated program was written using a combination of ms excel and vba coding. this program includes a li-ion battery pack mainly for daily energy demand and a h2 system for seasonal storage. heat recovery on the fuel cell, hot water storage, and heat distribution are also included in the computation. (3.) the sizing and optimization process. the sizing process is made with the help of design explorer [10]. the optimization process then consists of tweaking the program in order to obtain different improved results such as greatest ss ratio, lowest ghg emission or highest overall efficiency for the h2 system. 2.1. phase 1 – energy modelling and analysis of the building the future sll building in fribourg will be constituted of 4 four floors and a basement for a total surface of about 5 000 m2 of floor area. it will serve as a hybriduse building, with offices and laboratory-type research area covering an energy reference area of 3 859 m2. the complete structure will be utilized as a research platform for conducting studies in real-world settings. the building is fitted with a double-flow mechanical ventilation system with heat recovery, and heat is provided via the air handling unit (ahu) connected to the neighbourhood’s district heating/cooling system using two heat exchangers. a number of radiant panels on the ceiling and a series of clay composite panels in the interior areas add to the building’s thermal inertia. the latter counterbalances the building’s low inertia as, except for the stairwells and basement, which are built of reinforced concrete the rest of the building is made in a light timber frame. domestic hot water (dhw) is created utilizing decentralized electric heaters installed at the consumption end points due to the low consumption of dhw and the need for maximum flexibility. this adaptability will help the structure and its installations evolve, as well as enabling the researchers’ tests to be carried out. an extensive research was done during the project’s design phase in order to employ the best-oriented and productive pv surfaces on the building exterior. the sll building will have a multi-oriented installation of 135 kwp (976 kwh/kwp). figure 1 shows the solar energy model built with rhino [11] and diva for grasshopper [12]. 2.2. phase 2 – design of a hydrogen-based storage system the h2 storage system is composed of an electrolyser, storage tanks and a fuel cell. on the electrical side, the h2 storage system works just like a battery pack, it allows to store electrical energy from the pv panels in the form of hydrogen and reuse when needed. on the thermal side, heat can be recovered from the fuel cell and stored in a hot water tank before being redistributed to the heating system. in parallel to the fuel cell, a small heat pump can cover the additional thermal needs when no hydrogen is available. on the far left of figure 2, an air-water heat exchanger is needed in the case where the fuel cell is working but no thermal demand is present. as it is presented in figure 2, in our case study, the main heating of the building is provided by the district heating, heat recovery from the fuel cell will be used for additional demand that consists of the heating of the basement. 282 vol. 38/2022 building-integrated photovoltaics (bipv) combined . . . figure 1. cumulative annual irradiation of bipv installation. figure 2. schematic design of the fuel cell in the heating system. the choice here was made to recover heat only on the fuel cell (not on the electrolyser) as it’s mainly used in the wintertime when the thermal demand is much higher. the simulation program integrates the following algorithm of energy management: each hour of the year is treated individually, when the pv production is higher than the demand, energy can be stored in the battery, used to create hydrogen or injected into the grid. when the demand is more important than the production electricity can be withdrawn from the battery or from the h2 storage system otherwise it can be imported from the grid. the program lets the user choose all the specifications regarding the h2 storage system, the battery pack and heat recovery on the fuel cell. the program then runs a simulation over a one-year period and presents the following output values that can be used to analyse the results. 2.3. phase 3 – sizing and optimisation process in this phase, the sizing of the hydrogen storage system is made for the sll. to run the simulations, the pv production, electrical consumption and thermal demand for the heating of the basement are added. two parameters can be tweaked in order to modify the simulations. firstly, the components that will be implemented in the building. for example, the volume of the h2 tanks, the capacity of the battery pack, the volume of the hot water tank or the electrical/thermal efficiency of the fuel cell. secondly, the way in which the system is controlled (control mode). that is, in our case, the way in which the fuel cell and electrolyser are activated. to size the system, the base mode has been used, meaning that the electrolyser will be turned on when the production is higher than the demand and the battery pack is fully charged. the fuel cell is turned on when the consumption is higher than the demand and the battery pack is empty. in a second step, using the same system other modes will be tested. four modes were developed specifically to achieve different goals: • maximum self-sufficiency ratio. • lowest injection/withdrawal peaks into and from the grid. • lowest ghg emissions. • maximum overall efficiency of the h2 storage system. in order to run multiple simulations, a vba code has been added to the program. it offers the opportunity to automatically test multiple scenarios by varying the input values and writing the output values in a .csv file. the data will then be analysed with the help of design explorer [10]. it is then possible to choose the best inputs to obtain the targeted results. in our case study, four inputs were chosen to be sized. the four components and the values chosen are as 283 s. aguacil, y. morier, p. couty, j.-p. bacher acta polytechnica ctu proceedings figure 3. sized system from design explorer. follow: • capacity of the h2 storage system (2 000, 4 000, 6 000, 8 000, 10 000, 12 000 [kwh pci]). • capacity of the battery pack (10, 20, 30, 40, 50 [kwh]). • volume of the hot water tank (1 000, 2 000, 3 000, 4 000, 5 000, 6 000, 8 000 [litres]). • efficiencies of the fuel cell: (1.) electrical 50 %, thermal 35 %; (2.) electrical 37 % thermal 43 %. by combining these inputs together, 420 scenarios are computed. to compare all the scenarios, we will look at three output values: the ss ratio, the amount of energy consumed by the additional heat pump and the overall thermal losses of the system. figure 3 shows the 420 solutions presented in design explorer. on the horizontal axis are the 5 input values and 3 outputs. on the vertical axis are the chosen values for the input and the obtained values for the outputs. the blue areas show all the solutions that allow to maximize the ss ratio and lower the heat losses and amount of energy used by the additional heat pump. finally, the green line shows the chosen solution that takes into account the limitations on the outputs and selects the smallest h2 storage, battery pack and hot water tank possible. 3. results in this section the main results are presented. 3.1. optimized sizing of the hydrogen storage system the sizing of the hydrogen storage system was made according to phase 3 of the methodology. using this process, the following values were found: • hydrogen storage system: 10 000 kwh pci (gross calorific power). • li-ion battery: 30 kwh. • hot water tank: 3 000 litres. • fuel cell: 50 % electrical efficiency and 35 % thermal efficiency. a 10 000 kwh pci storage capacity corresponds to 14.3 cubic meters of hydrogen stored at 300 bars or about 300 kg. the li-ion battery was sized to be as small as possible, prioritizing h2 storage. it’s worth mentioning that if the building had to reach the same ss ratio without h2 storage, the li-ion battery would need to be 17 times larger. the hot water tank also serves as an energy storage device. the bigger the tank, the more energy from the fuel cell it can store but as the tank gets bigger, the heat losses increase too. in our case, 3 000 litres appeared to be the ideal size. in this paper, two different fuel cells were considered, one with higher electrical efficiency and the other one with higher thermal efficiency. as it can be seen in figure 3 the only suitable fuel cell to achieve the goals fixed in the previous section is the one with the higher electrical efficiency (50 % electrical efficiency). for all further analysis this sized system will be used. 3.2. output values from the program in this section, the sized system will be used to present different results. all the following solutions use the same system but the control mode used will vary. the control mode is the way in which hydrogen production/consumption is managed. in the previous section, the base mode was used, meaning that the electrolyser and fuel cell are activated whenever it’s possible. this mode allows for the highest ss ratio but also means that the h2 storage is completely empty during a few months in the winter and entirely full during around two months in the summer (figure 4). the maximum ss ratio mode is the one that has been used so far and in our previous researches. the goal for other control configurations is to focus not only on the ss ratio but to show the other advantages of a h2 storage system. the second control mode aims to reduce production and consumption peaks (injection and withdrawal into the grid). one of the main issues with pv panels is the peak production happening around midday when the sun’s intensity is at its highest. the energy that cannot be used or stored is to be injected into the grid. this could soon 284 vol. 38/2022 building-integrated photovoltaics (bipv) combined . . . figure 4. amount of h2 stored over a one-year period time using the max ss ratio mode. figure 5. amount of h2 stored over a one-year period time using the peaks limitation mode. become a problem as more and more pv systems are installed and the grid would have to withstand a huge amount of energy over a short period of time. in this case, the h2 storage system limits the injection peaks by turning on the electrolyser only when the production exceeds a certain value. by comparing figures 4 and 5 we can see how the curve gets much smoother and the h2 storage tanks are almost never empty or full. the lowest ghg emission mode uses an hourly step-time data file containing the co2eq emissions factors for the swiss electricity mix over one year [13]. using this data, the h2 program can choose when to buy/sell electricity to the grid according to its carbon content. by withdrawing electricity from the grid only when the carbon content is low, the ghg emissions of the building related to electricity import can be kept at their lowest. the last mode aims to maximize the overall efficiency of the h2 system. the efficiency of the h2 storage system can be computed using equation (2). ηglobal = ηelectrolyser · qhydrogen qt hermal + qelectrical (2) equation (2) clearly shows that to maximize efficiency, both thermal and electrical energy needs to be used. the aim of this mode is to make sure the fuel cell is turned on only when there is both a thermal and electrical demand. to do so, the program will look at the temperature of the hot water tank and the electricity consumption of the building. if the temperature is not maxed out and there is an electricity demand, then the fuel cell will be activated. the results obtained with these different modes can be seen in table 1. 4. discussion in a previous study [5], a similar system was modelled without heat recovery, the overall efficiency of the hydrogen cycle back then was around 30 %. this value can be computed using equation (1) with qt hermal equalling zero. by recovering heat on the fuel cell, the efficiency can be increased by 65 % percent to reach 49.5 %. this clearly shows that to take full advantage of a hydrogen storage system, thermal recovery must be integrated. looking at the ss ratio, the sized h2 storage system allows to gain around 12 % reaching 80.7 %. however, it has been shown in previous computations that to further increase this ratio the amount of hydrogen needed is impossible to store. it would then become necessary to look at other factors such as the cost and carbon footprint required to build a space dedicated to the h2 storage tanks. regarding the ghg emissions, the chosen sized h2 storage system can help reach the same emissions but can’t improve this value. this is due to two factors. first, the ghg emissions related to the implementation of an electrolyser, storage tanks and fuel cell are still relatively high as these components are not very common. secondly, to the fact that in our computation, when electricity is exported from the building into the grid, the ghg emissions of the pv panels and the grid carbon content are compared. as the carbon content of the electricity from the pv system is always lower than the grid’s, for every kwh exported a negative value is added to the ghg emission balance. when electricity is stored and reused, the model does not take advantage of this benefit. if we were to compute ghg emission balance without this deduction, the h2 storage system would always have a better carbon footprint. this h2 storage system also allows to reduce both injection and consumption peaks. with the dedicated mode, the injection peaks drop from 143 to 99 kw and consumptions from 51.2 285 s. aguacil, y. morier, p. couty, j.-p. bacher acta polytechnica ctu proceedings mode ss ratio ghg emissions overall max withdrawal max injection [%] [kg co2eq] efficiency [%] [kw] [kw] max ss ratio 80.7 9671 34 47.5 140 peaks limitation 72.5 6764 48.5 39 99 max efficiency 77 8071 49.5 45.3 140 ghg limitation 73.6 5658 41 47.5 142.8 no h2 storage 69 5586 0 51.2 142.8 table 1. results from the different program modes. to 39 kw preventing overloads on the network. this could also be very beneficial to a neighbourhood like bluefactory [14] in fribourg, where the sll will be built, as it would be possible to store or distribute energy from/to all the buildings in the area. it should be remembered that all these results are obtained using different control modes and that just one wouldn’t be able to reach all of these values at once. the financial aspect of this h2 storage system still needs be evaluate in further details. it’s is still complicated to estimate prices regarding the components of an h2 storage system. moreover, with today’s electricity prices in switzerland [15] the building would basically have no electrical cost as the amount of energy sold to the grid is much higher than what’s consumed. the constant evolution of electricity prices and the feed-in tariffs should be also taken into account. further research will be necessary to evaluate every aspect of an h2 storage system. financial facets still need be analysed in details to evaluate in which context this installation would be profitable. the h2 program could be optimized by selecting the best mode for every case or by writing other modes allowing to furthermore improve system operation. the h2 storage system could be used not only for the building but in harmony with the grid allowing, when needed, to store energy peaks or to provide great amount of energy. new ideas could also be implemented in this project with the potential of hydrogen vehicle. a quick calculation shows that the sll would have the potential to produce enough h2 for cars to travel about 150 000 km each year. 5. conclusions this paper demonstrates the many interesting aspects of integrating a hydrogen-based electricity storage system at building-scale. this case study shows the potential of h2 storage and the various benefits that it could have. the use of thermal energy released by the fuel cell is necessary to greatly improve the overall efficiency of the h2 cycle. to pursue this project, the ms excel written code will be improved and optimized, further analysis will be conducted to evaluate the financial aspects in more details and new opportunity for the use of h2 will be implemented. the challenges regarding energy storage will become more and more common and this paper clearly demonstrate that h2 will have a role to play in the future of renewable energy. acknowledgements the authors thank the university of applied sciences and arts western switzerland (hes-so) and the ecole polytechnique fédérale de lausanne (epfl) for their support. references [1] t. hostettler, a. hekler. statistik sonnenenergie. referenzjahr 2020, bundesamt für energie bfe, 2021. [2] a. parlikar, c. n. truong, a. jossen, h. hesse. the carbon footprint of island grids with lithium-ion battery systems: an analysis based on levelized emissions of energy supply. renewable and sustainable energy reviews 149:111353, 2021. https://doi.org/10.1016/j.rser.2021.111353 [3] p. couty, d. torregrossa, j.-p. bacher. battery storage systems for building applications. energy in buildings and communities programme 2020. [2021-05-24]. https://arodes.hes-so.ch/record/8139 [4] p. couty, m. j. lalou, p. cuony, et al. positive energy building with pv facade production and electrical storage designed by the swiss team for the u.s. department of energy solar decathlon 2017. energy procedia 122:919–924, 2017. https://doi.org/10.1016/j.egypro.2017.07.410 [5] y. morier, s. aguacil moreno, p. couty. hydrogenbased electricity storage optimization on buildings by coupling thermal and photovoltaic electricity production towards carbon neutrality. journal of physics: conference series 2042(1):012092, 2021. https://doi.org/10.1088/1742-6596/2042/1/012092 [6] vela solaris ag. polysun software, 2018. [2021-05-24]. https://www.velasolaris.com/ [7] smart living lab. smart living lab, 2021. [2021-05-24]. https://www.smartlivinglab.ch [8] designbuilder. designbuilder software v.7, 2021. [2021-01-29]. http://www.designbuilder.co.uk/ [9] u.d. of e. doe, us department of energy, 2018. [2018-04-15]. https://www.energy.gov/ [10] core studio, t. thornton. design explorer v.2 for multi-dimensional parametric studies, 2017. [2018-0509]. http://tt-acm.github.io/designexplorer/ [11] robert mcneel & associates. rhinoceros software, 2019. [2019-02-01]. https://www.rhino3d.com/ 286 https://doi.org/10.1016/j.rser.2021.111353 https://arodes.hes-so.ch/record/8139 https://doi.org/10.1016/j.egypro.2017.07.410 https://doi.org/10.1088/1742-6596/2042/1/012092 https://www.velasolaris.com/ https://www.smartlivinglab.ch http://www.designbuilder.co.uk/ https://www.energy.gov/ http://tt-acm.github.io/designexplorer/ https://www.rhino3d.com/ vol. 38/2022 building-integrated photovoltaics (bipv) combined . . . [12] solemma lcc. diva for rhino. environmental analysis for buildings, 2018. [2018-04-15]. http://diva4rhino.com/ [13] d. vuarnoz, s. aguacil moreno. dataset concerning the hourly conversion factors for the cumulative energy demand and its non-renewable part, and hourly ghg emission factors of the swiss mix during a one-year period (2016 and 2017). data in brief 30:105509, 2020. https://doi.org/10.1016/j.dib.2020.105509 [14] bluefactory. bluefactory – fribourg – freiburg sa, 2014. [2021-05-24]. https://www.bluefactory.ch/ [15] vese. feed-in tariffs for pv solar energy, 2018. [2019-02-01]. http://www.vese.ch/ 287 http://diva4rhino.com/ https://doi.org/10.1016/j.dib.2020.105509 https://www.bluefactory.ch/ http://www.vese.ch/ acta polytechnica ctu proceedings 38:281–287, 2022 1 introduction 2 methodology 2.1 phase 1 – energy modelling and analysis of the building 2.2 phase 2 – design of a hydrogen-based storage system 2.3 phase 3 – sizing and optimisation process 3 results 3.1 optimized sizing of the hydrogen storage system 3.2 output values from the program 4 discussion 5 conclusions acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2025.53.0033 acta polytechnica ctu proceedings 53:33–36, 2025 © 2025 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague modification of properties of fresh mixture for 3d printing using cement-based mortar vendula kellnerová∗, tomáš david, pavel reiterman czech technical university in prague, faculty of civil engineering, thákurova 7, 166 29 prague 6, czech republic ∗ corresponding author: vendula.kellnerova@cvut.cz abstract. portland cement is traditional binding material which is applicable for various modern technologies in these days – the 3d printing is one of them. the development of 3d printers using cement-based materials for the manufacturing of structural elements and moreover entire building is novel concept of structural engineering, which combines advanced material solution and high level of automation. this paper is focused on the study of properties of fresh mixture in order to control its consistency and regime of setting time to achieve continual process of 3d printing. the properties of fresh mortar were modified by the admixture accelerating the process of hardening and stabilizing agent. the performed experimental program showed that both applied admixtures could be used for the control of the consistency of fresh mortar to achieve optimal shape of mortar layer after its extrusion. keywords: accelerating admixture, stabilizing admixture, setting time, flow table, concrete 3d printing. 1. introduction 3d concrete printing is a novel approach in the field of manufacturing of structural elements, alternatively entire buildings, using printing machine and cementitious mixture. there are available several technical concepts for 3d concrete printing [1–5], however the admixture printing seems to be the most frequently used worldwide. the crucial aspect of additive 3d concrete printing is the soft dosage of chemical admixture into the printing nozzle shortly before an extrusion of printing material [1]. such a measure allows to control the process of hardening or modification of the consistency, and other rheological properties of fresh mixture to achieve required plasticity in case of using accelerating and stabilizing admixture [6]. alternatively, could be added mixing water, however that logically leads to the reduction of mechanical properties of hardened mixture, which can be controlled by the use of a plasticizing admixture [7]. the addition of accelerator into the printing nozzle has several advantages. utilization of this kind of chemical admixture causes an increase of strength development, an increase the temperature of fresh mixture, reduces the consistency, what could lead to the defects during printing [5, 8, 9]. the 3d concrete printing is based on the gradual placing of concrete layer by layer. thus, the fresh mixture has to exhibit tougher consistency, in comparison with traditional concrete, to load following layers without immense deformation or loss of the stability. that is why, the stabilizing agents are widely used to modify the consistency of fresh mixture. usually, these admixtures are integral part of the other admixtures used during mixing, however their application into the printing nozzle could increase the applicability of 3d concrete printing, because the stabilizer increases the loadbearing capacity of freshly layered mixture, thus higher number of layers exhibit vertical stability after the printing. the process of 3d printing is highly automatic, where the speed of printing, respectively movement of the nozzle, has to respect the shape and dimensions of the final element. the speed of printing could be basically derived according to equation 1, v ≤ ll · ts n 60 , (1) where v [m s−1] is the linear movement of the printing nozzle, ll [m] is length in one layer, ts [min] is the setting time of extruded mixture, n is the number of following layers exhibiting stability in fresh state. the performed experimental program was focused on the optimization of dosing accelerator and stabilizing agent to maximize the speed of 3d printing. 2. experimental program the experimental program was focused on the modification of mixture used using 3d printing of entire concrete structures, figure 1, which was developed during the collaborating with podzimek a synové and strojirny podzimek ltd. this type of 3d printer is designed for the printing of larger structures under external conditions. the technology of admixture 3d printing allows to control process of hardening and the consistency of fresh mixture to achieve higher homogeneity of the 3d printing. the sense of the continual control is given by the changing external condition. 33 https://doi.org/10.14311/app.2025.53.0033 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en v. kellnerová, t. david, p. reiterman acta polytechnica ctu proceedings figure 1. 3d printer (třešť). 2.1. methods initially, the flow rate of the mixtures was quantified. this procedure contains of positioning a metal cone at the center of the shaking table and subsequently filling it with mixture in two stratified layers. each layer is uniformly compacted using light taps from the choke. excess mixture is then removed, and the surrounding area is meticulously cleaned. the cone is lifted vertically, allowing the mixture to spread across the table surface through 15 consistent impacts at a fixed frequency. the final flow is measured along two orthogonal axes. the setting time was determined using the vicat apparatus. vicat’s ring was filled with a fresh mixture and the setting time was determined using a vicat needle. for the purpose of this test, a non-standard needle with an enlarged area of 2 mm diameter was chosen, as it was not only the paste (a mixture of binder and water) that was tested, but a pre-prepared industrial mixture of a specific composition. it was necessary to use this modified needle for this mixture as the standard needle did not provide meaningful data. the start of setting time was that takes from the addition of water to the mixture until the vicat needle remains suspended 3 to 5 mm above the substrate. 2.2. materials for the purpose of the experiment, an industrially produced mixture from company stachema cz was used. this product is supplied as a dry mixture, which is bagged to protect it from moisture. it is further dosed separately on site and mixed with water to the desired consistency required by the 3d printer. this mixture is composed of portland cement-based binder, fibers, fillers and admixtures. the specific composition is protected by the know-how of the company. for the purpose of regulating the setting time and flow, a liquid accelerator was used, which was manufactured by the same supplier, i.e. stachema cz. the same manufacturer of the dry mixture and the accelerator ensures the compatibility of both components and their efficient use. the accelerator dosage was gradually increased to monitor changes in the behavior of the mixture. the rest of the composition, including the water dose, was kept constant. a stabilizing admixture as a viscosity modifying agent was further tested to control the flow without affecting the setting time. this admixture was again manufactured by the same company stachema cz for the purpose to achieve compatibility of all components. the tested stabilizing admixture was cellulose-based used in a dry, powdered form. for the purpose of the experiment, the stabilizing admixture was dosed separately along with the priming mixture and water. the dosage of water and mixture was constant, the dosage of the stabilizing admixture was varied. 3. results and discussion 3.1. flow rate the use of the accelerator in smaller doses did not have a negative effect on the flow. a reduction in the flow rate was observed only when the dose exceeded 0.6 % of the cement weight. when this limit was exceeded, the final flow rate decreased with increasing accelerator dose, see figure 2. the mixture with a 100 mm flow rate exhibited a very stiff consistency that would be difficult to work properly. similar results were achieved by i. dressler et al. [1]. they monitored the rheological properties using initial 34 vol. 53/2025 modification of properties for 3d printing figure 2. the influence of accelerator on the consistency. yield stress. an increase in accelerator dosage leads to an increase of initial yield stress. the same trend of decreasing consistency with increasing accelerator content was also observed in the research by s. ramakrishnan et al. [10]. in this research, the encapsulated accelerator was used into the mixture. the mixtures containing accelerator in dose of 1.5 and 2.5 % by weight of cement showed lower flow than the reference mixture without accelerator. they noted that it could be also due to the presence of large elongated capsules that reduced the cohesiveness of the mix. reduced flow can cause discontinuity of the printed trace during the 3d printing process, problems with the transport of the mixture through the device and even complete blocking of the print head. therefore, this parameter needs to be monitored. the critical flow value will then depend on the final technical design of the device. 3.2. setting time the acceleration of the setting time of the mixture with increasing accelerator content was expected result. up to a dosage of 0.6 % by weight of the cement showed a slower acceleration. after exceeding this dosage, an increased acceleration effect was observed as can be seen in figure 3. at a high dose of 2.4 % by weight of the mixture, the setting time was accelerated to only 10 min. however, with such a high ramp-up rate, the flow rate was also significantly reduced. these two factors are interdependent and therefore it is always necessary to monitor and achieve the desired properties in both areas. this behavior is also described in work by a. u. rehman et al. [9]. they monitored the setting time using the ultrasonic pulse velocity method (upv). the result shows that dose of 7.5 % of accelerator demonstrated a sharp increase in the upv during the first day. the rapid increase in the upv value indicates a quick transformation of the accelerated figure 3. the influence of accelerator on setting time. mixture from the liquid state to the solid state and enhanced connectivity in its solid particles during the first day. 3.3. stabilizing effect the stabilizing admixture is intended, by its very nature, to limit the flow. this effect was confirmed when this admixture was used in the 3d printing mixture. the final flow rate decreased with increasing percentage of stabilizing admixture, see figure 4. it should be noted that a constant water dose was maintained for all samples in the experiment. figure 4. the influence of stabilizing agent on the consistency. at a dose of stabilizing admixture of 0.02 % by weight of cement, a flow rate of 230 mm was achieved; when the stabilizing admixture was dosed at a high dose of 0.1 % by weight of cement, a flow rate of 160 mm was achieved, which represented a decrease in flow rate of almost 35 %. c. brumaud et al. [11] were also noted that the stress level or rigidity of the system is reduced when cellulose ether is added to the paste. this suggests that cellulose ethers have a priori an effect on the contribution of the nucleation of hydrates between cement grains. according to research c. brumaud et al. [12] the increasing the amount of cellulose ether also increased the viscosity in the distilled water. among others, this research reports the fact that the specific 35 v. kellnerová, t. david, p. reiterman acta polytechnica ctu proceedings composition of the viscosity enhancing agents itself has a major influence on the changes in the rheological properties of the cement mixture. the results show that this type of stabilizing admixture effectively influences the final flow and can effectively control the required final flow with respect to the requirements of the 3d printer. 4. conclusion the experimental program dealt with the possibilities of the consistency control using advanced chemical admixtures in relation to concrete 3d printing. two types of admixtures, which could be added into the printing nozzle were studied. from the results obtained it is evident that the final flow and the setting time can be effectively influenced by the use of the proper admixture. if it is necessary to influence the setting time, accelerator can be effectively used. the importance of using the accelerator is motivated by the printing of smaller elements to ensure load capacity of layered concrete. along with this parameter, however, the mixture flow rate is also influenced, which must be considered and evaluated if the 3d printer is capable of operating with such flow rates. if we only need to affect the flow, thus the workability, the stabilizing admixture has worked effectively. this admixture allowed to reduce the flow rate by up to 35 % with respect to the selected dose. the final dose will depend on the requirements of the 3d printer itself and the requirements for the final consistency of the printed trace. the combination of both chemical admixtures will be studied in detail during further research. acknowledgements this outcome was elaborated with the financial support of technology agency of czech republic under project fw06010592 and ctu in prague under the project sgs24/051/ohk1/1t/11. references [1] i. dressler, n. freund, d. lowke. the effect of accelerator dosage on fresh concrete properties and on interlayer strength in shotcrete 3d printing. materials 13(2):374, 2020. https://doi.org/10.3390/ma13020374 [2] y. weng, b. lu, m. li, et al. empirical models to predict rheological properties of fiber reinforced cementitious composites for 3d printing. construction and building materials 189:676–685, 2018. https://doi.org/10.1016/j.conbuildmat.2018.09.039 [3] y. zhang, y. zhang, g. liu, et al. fresh properties of a novel 3d printing concrete ink. construction and building materials 174:263–271, 2018. https://doi.org/10.1016/j.conbuildmat.2018.04.115 [4] c. zhang, v. n. nerella, a. krishna, et al. mix design concepts for 3d printable concrete: a review. cement and concrete composites 122:104155, 2021. https://doi.org/10.1016/j.cemconcomp.2021.104155 [5] d. marchon, s. kawashima, h. bessaies-bey, et al. hydration and rheology control of concrete for digital fabrication: potential admixtures and cement chemistry. cement and concrete research 112:96–110, 2018. https://doi.org/10.1016/j.cemconres.2018.05.014 [6] m. li, y. weng, z. liu, et al. optimizing of chemical admixtures for 3d printable cementitious materials by central composite design. materials science in additive manufacturing 1(3):16, 2022. https://doi.org/10.18063/msam.v1i3.16 [7] f. j. rubio-hernández, a. adarve-castro, j. f. velázquez-navarro, et al. influence of water/cement ratio, and type and concentration of chemical additives on the static and dynamic yield stresses of portland cement paste. construction and building materials 235:117744, 2020. https://doi.org/10.1016/j.conbuildmat.2019.117744 [8] t. wangler, r. pileggi, s. gürel, r. j. flatt. a chemical process engineering look at digital concrete processes: critical step design, inline mixing, and scaleup. cement and concrete research 155:106782, 2022. https://doi.org/10.1016/j.cemconres.2022.106782 [9] a. u. rehman, b. m. birru, j.-h. kim. set-on-demand 3d concrete printing (3dcp) construction and potential outcome of shotcrete accelerators on its hardened properties. case studies in construction materials 18:e01955, 2023. https://doi.org/10.1016/j.cscm.2023.e01955 [10] s. ramakrishnan, s. kanagasuntharam, j. sanjayan. in-line activation of cementitious materials for 3d concrete printing. cement and concrete composites 131:104598, 2022. https://doi.org/10.1016/j.cemconcomp.2022.104598 [11] c. brumaud, r. baumann, m. schmitz, et al. cellulose ethers and yield stress of cement pastes. cement and concrete research 55:14–21, 2014. https://doi.org/10.1016/j.cemconres.2013.06.013 [12] c. brumaud, h. bessaies-bey, c. mohler, et al. cellulose ethers and water retention. cement and concrete research 53:176–184, 2013. https://doi.org/10.1016/j.cemconres.2013.06.010 36 https://doi.org/10.3390/ma13020374 https://doi.org/10.1016/j.conbuildmat.2018.09.039 https://doi.org/10.1016/j.conbuildmat.2018.04.115 https://doi.org/10.1016/j.cemconcomp.2021.104155 https://doi.org/10.1016/j.cemconres.2018.05.014 https://doi.org/10.18063/msam.v1i3.16 https://doi.org/10.1016/j.conbuildmat.2019.117744 https://doi.org/10.1016/j.cemconres.2022.106782 https://doi.org/10.1016/j.cscm.2023.e01955 https://doi.org/10.1016/j.cemconcomp.2022.104598 https://doi.org/10.1016/j.cemconres.2013.06.013 https://doi.org/10.1016/j.cemconres.2013.06.010 acta polytechnica ctu proceedings 53:33–36, 2025 1 introduction 2 experimental program 2.1 methods 2.2 materials 3 results and discussion 3.1 flow rate 3.2 setting time 3.3 stabilizing effect 4 conclusion acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2025.54.0072 acta polytechnica ctu proceedings 54:72–78, 2025 © 2025 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague characterization of the microstructure of cement paste exposed to gamma radiation by scanning electron microscopy andrea špakováa, jiří němeček1,a,∗, lukáš procházkab, patricie halodováb, jiří němeček2,a a czech technical university in prague, faculty of civil engineering, department of mechanics, thákurova 7, 166 29 prague 6, czech republic b research centre řež, hlavní 130 řež, husinec 250 68, czech republic ∗ corresponding author: jiri.nemecek.1@fsv.cvut.cz abstract. hardened cement paste with a water-cement ratio of 0.4 was examined under five different relative humidity (rh) conditions (11 %, 33 %, 76 %, 96 %, and 100 %). these samples were exposed to gamma radiation with a total dose of 13.82 mgy over 341 days. scanning electron microscopy (sem) was the primary tool used for characterizing the samples’ microstructure at a microscale, aided by image analysis (ia), which showed negligible changes in calcium-silicate-hydrate phases but a decrease in portlandite of 1.5–3.5 % under 33–100 % rh conditions. point elemental analysis was conducted to determine the (ca-s)/si ratio of the inner product, finding a significant increase of up to 20 % at medium rh conditions (33–76 %). keywords: electron microscopy, cement paste, gamma radiation. 1. introduction concrete is a widely used construction material, essential for many different applications, including radiation shielding in light water reactors. these shields protect the power-plants by absorbing and dissipating the harmful irradiation caused by neutron fluxes and gamma-rays emitted from the reactor pressure vessel (rpv). typical concrete members exposed to irradiation are the rpv pedestal and primary shielding wall of a boiling water reactor (bwr), and those of a pressurized water reactor (pwr), as shown in figure 1 [1–3]. currently, more than half of the existing nuclear reactors, including rpvs, are aging and reaching their design lifetimes of 40–60 years, presenting two primary options: shutting down or extending their operating licenses. the approximate lifetime extension is typically 20 years and in many cases, prolonging the current function is preferred over closing operations. this approach eliminates the need to build new powerplant reactors while the old ones still generate the required power. to facilitate the extension safely, it is crucial to closely investigate the impact of gamma and neutron radiation on concrete structures, aiming to find solutions for prolonging the functionality of power-plants worldwide by repairing, maintaining, or replacing the existing structures [1, 3, 4]. the projected doses on concrete after the designed lifetime of 80 years of power-plant operation are 100– 1postdoctoral researcher at czech technical university in prague, orcid: 0000-0002-5635-695x. 2professor at czech technical university in prague, orcid: 0000-0002-3565-8182. figure 1. typical concrete members subjected to irradiation in a) bwr, b) pwr [2]. 200 mgy of gamma irradiation and approximately 6×1019 n cm−2 (e > 0.1 mev) in neutron fluxes. this exposure leads to heat generation and progressive concrete degradation, resulting in a 20 % drop in compressive strength associated with deterioration of properties due to both internal and aggregate damage mechanisms [1, 5]. neutrons (e > 10–100 kev) cause radiation-induced volumetric expansion (rive) through metamictization (amorfization) of minerals in aggregates present in concrete. this process creates lattice structure defects, such as vacancies or interstitials, leading to significant changes in the volume and density of the minerals, which alters properties of concrete, includ72 https://doi.org/10.14311/app.2025.54.0072 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 54/2025 characterization of the microstructure of cement paste exposed . . . cement cao sio2 al2o3 fe2o3 mgo na2o k2o so3 cl − mno cem i 42.5 r (wt. %) 64.78 20.28 4.87 3.55 1.34 0.14 0.76 3.04 0.028 – table 1. percentage representation of cem i 42.5 r components (xrf data provided by manufacturer). ing tensile strength, compressing strength, and elastic modulus [2]. gamma rays can significantly affect concrete, in particular the calcium-silicate-hydrate (c-s-h) gel, through dehydration caused by water radiolysis. water radiolysis is a chemical reaction in which the interstitial liquid of c-s-h gel down chemical bonds, resulting in the formation and accumulation of explosive gases such as hydrogen (h2). during this process, hydrogen gas (h2), and other stable molecules (h2o2, h+, oh−), as well as highly unstable radicals (h·, oh·, e− aq), are formed. hydrogen peroxide (h2o2) then reacts with ch and aft, forming other unstable products, and by further reactions of those, caco3 creation is obtained [4, 6]. based on the testing of gamma-irradiated cementitious samples, the following observations were made [3]: • the formation of the vaterite and aragonite, rather than calcite, occurs during carbonation. vaterite formation in cement pastes was found to fill pores around the c-s-h gel, resulting in increased stiffness and bending strength [4]. • it has been shown, that evaporable water decomposes readily under gamma-ray irradiation in cement pastes and over a long-period exposure of a high dosage of gamma-rays was found, that in 1 % can be decomposed even chemically bound water [7]. • irradiation of c-s-h pellets at a dosage of 189 mgy and 11 % relative humidity (rh) resulted in a decrease in the basal spacing of the c-s-h gel by 0.5–0.7 å. this decrease is associated with a reduction in molecular water content and an increase in hydroxyl groups in the c-s-h gel. nanoindentation, performed with a maximum force of 4 mn, revealed a 15–25 % increase in young’s modulus in irradiated samples [1]. • nanoindentation of samples irradiated at below 11 % rh showed an approximately 25 % increase in nanoindentation young’s modulus due to a combination of water radiolysis and drying, resulting in the densification of the c-s-h gel at a nanoscale level of 100 nm. this densification was accompanied by microcracking, which caused a decrease in the microindentation young’s modulus by ∼17 % at the higher scale of 20 µm [8]. • samples irradiated under common rh conditions (30–60 %) showed minimal differences in nanomechanical properties measured by nanoindentation of cement paste [8] and mortar [9]. • the samples irradiated in water, representing 100 % rh, a significant decrease in nanoindentation young’s modulus by 26 % of the main hydrates, along with a simultaneous loss of portlandite by 9–10 %, was observed [8]. • ordinary cement paste samples exposed to gamma doses of 0–1188 mgy were analyzed using scanning electron microscopy (sem) and x-ray diffraction (xrd). starting at a dose of 130 mgy, a decrease in crystalline phases formed during hydration was observed by xrd, and with increasing dosage, this decrease became more pronounced, accompanied by clinker decomposition. the development of pseudomorphoses, along with the formation of bubbles and cracks due to the separation of chemically bound water, was also observed by sem at a dose of 290 mgy [10]. since gamma radiation can significantly alter the mechanical and chemical properties of cementitious composites, depending on the rh conditions during irradiation. this paper focuses on the characterization of the chemical composition of ordinary cement paste exposed to five different rh conditions using scanning electron microscopy. the results presented in this paper were also reported in the author’s bachelor’s thesis [3]. 2. experiments and methods 2.1. sample preparation a cement paste mixture with a water-cement ratio of 0.4 was prepared using portland cement cem i 42.5 r. the percentage representation of components in the portland cement is shown in table 1. the mixture was poured into cylindrical plastic molds with a 27 mm diameter and 70 mm height and then vibrated. the samples were stored for 24 hours before being demolded. the solidified samples were stored in a 1 % lime water solution for 610 days (1.67 years). afterward, the samples were extracted from the lime water and sliced to a height of approximately 19 mm using a diamond blade (struers secotom-50). the cut samples were then stored at five rh levels (11 %, 33 %, 76 %, 96 %, and 100 %). four samples were kept in containers with salt solutions, while the fifth was submerged in water, representing 100 % rh. the salt solutions did not come in contact with the tested samples. a summary of specific salts and corresponding rh levels is provided in table 2 [3]. 73 a. spaková, j. němeček, l. procházka et al. acta polytechnica ctu proceedings rh (%) 11 33 76 96 100 saturated salt licl mgcl2 nacl kno3 – other condition – – – – water table 2. summary of conditions controlling relative humidity. 2.2. gamma irradiation the samples in containers were left to stabilize at their respective rh conditions for 14 days. half of the samples were then irradiated with co60 for 341 days in the research centrum řež, receiving a total dosage of 13.82 mgy and an average dose rate of 0.675–1.890 kgy/h. the second half of the control samples were kept under the same conditions but were not exposed to gamma radiation [11]. the labeling of the samples was xx–yy, where xx represents the specific rh condition (11/33/76/96/100), and yy denotes whether the sample was irradiated (ir) or control (c) [3]. 2.3. sample preparation before electron microscopy the surfaces of the samples were polished using a struers tegramin 20 machine. the required unscratched and evenly polished surfaces were achieved by applying various sic foils with different grit sizes at varying speeds and pressure forces. the samples were polished without any lubricant while constantly sweeping free particles [3]. the polishing process began with a coarser # 1200 sic foil, followed by # 2000, and finally # 4000. the samples were polished with # 4000 twice to avoid any damage from the accumulation of free particles on the sample. all foils used, along with the applied forces, rotation speeds, and total polishing times for each step, are listed in table 3 [3]. after each step, the samples were placed in a flask filled with isopropyl alcohol and then into an ultrasonic cleaner (usc) for 1 minute to remove any free particles that could scratch the sample during subsequent polishing steps. after each usc process, the samples were dried with compressed air and prepared for the next procedure [3]. 2.4. scanning electron microscopy a phenom xl desktop sem was used to characterize the phase composition of the cement paste samples. sem provides high-resolution images that enable the distinction of different phases in cement pastes. in ordinary cement paste, five main phases are typically easy to recognize with sem: inner and outer hydration products, portlandite, residual clinker, and pores or cracks [3, 12, 13]. 2.4.1. bse imaging and image analysis sem-bse (back-scattered electrons) images at 900× magnification, with a size of 298 × 298 µm2 and a pixel data step 1 step 2 step 3 foil roughness (–) # 1200 # 2000 # 4000 disk rpm (–) 100 120 120 head rpm (–) 70 70 70 time (s) 10 60 150 force (n) 10 10 5 table 3. summary of polishing steps with corresponding rotation speed (rpm), duration, applied force, and sic foil roughness. figure 2. selected image points (red crosses) around the residual clinker were used in eds point analysis in the inner product of the 100–ir sample. resolution of 146 nm, were captured for image analysis [3]. the accelerating voltage was set to 15 kv. a total of ten images were taken for each sample, covering a total area of 0.888 mm2. each of these images was in 8-bit format with grayscale pixel values ranging from 0 to 255 [3]. 2.4.2. point element analysis energy dispersive x-ray spectroscopy (eds) was used to analyze the chemical composition of the inner product formed around residual clinker grains, in point mode, at 3000× magnification. the images had a size of 89.5 × 89.5 µm2. a bsd full detector and an accelerating voltage of 15 kv were used. in total, twenty different points in inner product around at least seven clinker grains were analyzed for each sample, as shown in figure 2. only the inner product phase was selected since phase mixture is less frequent in this phase contrary to outer product [12]. 3. results and discussion a typical sem-bse image for irradiated and control samples at 33 % rh is shown in figure 3. five distinct phases were identified: residual clinker, portlandite, inner product, outer product, and cracks and pores. however, due to the overlapping grayscale threshold between the inner and outer product phases [13], 74 vol. 54/2025 characterization of the microstructure of cement paste exposed . . . figure 3. sem-bse images of samples a) 33–ir, b) 33–c, highlighting five main phases: orange – pores and scratches, red – residual clinker, green – portlandite, cyan – inner product, purple – outer product. figure 4. example of phase separation for the 100–ir sample using image analysis for individual phases: a) pores and scratches, b) inner-outer product, c) portlandite, d) clinker. these phases were grouped together and referred to as the “inner-outer product” during image analysis. an example of this separation process is illustrated for the 100–ir sample in figure 4. the grayscale threshold boundaries for this image were estimated as follows: residual clinker (255 to 185), portlandite (184 to 150), inner-outer product (149 to 41), and pores and scratches (40 to 0) [11]. the mean values and standard deviations for each irradiated and control sample are summarized in figure 5. the results of image analysis show minimal changes between irradiated and control samples across all rh levels. almost negligible changes are observed in the inner-outer product phase. samples at 100 % rh show a slight decrease in residual clinker by ≈1 % and in the pores/cracks phase by ≈2 % compared to samples stored at other rh conditions. this suggests that these changes were primarily due to continuous hydration, not irradiation. a slight decrease in portlandite by 1.5–3.5 % was observed at 33 %, 76 %, 96 %, and 100 % rhs, suggesting decomposition due to irradiation, which is in accordance with other studies [8, 10]. however, the measured changes are within the accuracy of the method, and for a more precise estimation of portlandite, thermogravimetric measurements will be performed in the future. additionally, sem-bse images were examined for crack formation due to gamma radiation. in this case, no cracks were observed in either the irradiated or control samples. this finding does not align with other studies. however, in the study by łowi’nskakluge and piszora [10], crack formation was observed after a gamma dose of 130 mgy, which is ten times higher than the dose used in our study. in the study by němeček et al. [8], crack formation in irradiated samples was only pronounced after prolonged exposure to the sem vacuum chamber [11]. point eds analysis was performed in the inner product phase formed around clinker grains, as marked in figure 2. the typical elemental spectrum of this spot is shown in figure 6 for the 100–ir sample. the elemental atomic concentrations and the stoichiomet75 a. spaková, j. němeček, l. procházka et al. acta polytechnica ctu proceedings 11 % 33 % 76 % 96 % 100 % 0 5 10 15 20 relative humidity v o lu m e re p re se n ta ti on o f cl in ke r (% ) (a) irradiated non-irradiated 11 % 33 % 76 % 96 % 100 % 0 5 10 15 20 relative humidity v o lu m e re p re se n ta ti on o f p o rt la n d it e (% ) (b) irradiated non-irradiated 11 % 33 % 76 % 96 % 100 % 0 10 20 30 40 50 60 70 80 90 100 relative humidity v ol u m e re p re se n ta ti on of in n er -o u te r p ro d u ct (% ) (c) irradiated non-irradiated 11 % 33 % 76 % 96 % 100 % 0 5 10 15 20 relative humidity v ol u m e re p re se n ta ti on of p or es an d cr ac k s (% ) (d) irradiated non-irradiated figure 5. image analysis results of (a) clinker, (b) portlandite, (c) inner-outer product, (d) pores and cracks figure 6. typical sem–eds elemental spectrum of the inner product measured on the 100–ir sample. ric weight concentrations of oxides for the 100–ir and 100–c samples are presented in table 4. additionally, the (ca-s)/si ratios were calculated for all samples and are shown and compared with each other in figure 7. results of the eds point analysis indicated an increase in the (ca-s)/si ratio of irradiated samples. 11 % 33 % 76 % 96 % 100 % 0.0 0.5 1.0 1.5 2.0 2.5 3.0 relative humidity (c as )/ s i ra ti o () irradiated non-irradiated figure 7. the mean value and standard deviation of the (ca-s)/si ratio measured by eds point analysis. a smaller increase is observed at the lowest and highest rh conditions, specifically by 7.3 % at 11 % rh and 12.1 % at 100 % rh. however, a more significant increase in the (ca-s)/si ratio is noted for samples at middle rh levels, with increases of 18.1 % at 33 % rh, 19.6 % at 76 % rh, and 18.9 % at 96 % rh. the 76 vol. 54/2025 characterization of the microstructure of cement paste exposed . . . atomic conc. stoich. wt. conc. element 100–ir 100–c oxide 100–ir 100–c symbol (%) (%) symbol (%) (%) o 53.40 57.9 – – ca 30.81 27.33 cao 57.11 55.88 si 11.62 10.60 sio2 32.97 34.66 mg 1.05 0.78 mgo 2.28 1.92 al 1.06 1.33 al2o3 2.67 3.66 fe 1.33 1.04 fe2o3 2.66 2.16 s 0.84 0.59 so3 2.53 2.10 table 4. percentage representation of elements and their oxides in the inner product around residual clinker, as determined using eds point analysis at typical measured points. minimal changes observed at 11 % rh are consistent with the negligible changes reported for synthetically gamma-irradiated c-s-h pellets at 11 % rh [1, 14]. this was accompanied by an increase in the nanoindentation young’s modulus [1], which is also characteristic of cement paste samples [8]. 4. conclusions this study investigated gamma-irradiated ordinary cement paste (total dose of 13.82 mgy) exposed to five different rh levels (11–100 %) using scanning electron microscopy. based on the observation of bse images and eds point analysis, the following conclusions were drawn: • a negligible change in c-s-h gel volume fraction between irradiated and control samples was observed across all rh levels. • at a dosage of 13.82 mgy, a 1.5–3.5 % decrease in the portlandite phase was observed for rh conditions ranging from 33–100 %, indicating degradation due to gamma irradiation. • observation of sem-bse images showed no visible crack formation in either the irradiated or control samples across all rh levels. • a significant increase in the (ca-s)/si ratio of the inner product was observed for samples irradiated at medium rh, specifically 18.1 % at 33 % rh and 19.6 % at 76 % rh. acknowledgements this work was financially supported by the czech science foundation grant project 23-05435s and the grant agency of the czech technical university in prague (sgs25/083/ohk1/2t/11). the presented results were obtained using the cicrr infrastructure, which is financially supported by the ministry of education and culture – project lm2023041. references [1] a. baral, e. t. rodriguez, w. a. hunnicutt, et al. ultra-high gamma irradiation of calcium silicate hydrates: impact on mechanical properties, nanostructure, and atomic environments. cement and concrete research 158:106855, 2022. https://doi.org/10.1016/j.cemconres.2022.106855 [2] i. maruyama, o. kontani, m. takizawa, et al. development of soundness assessment procedure for concrete members affected by neutron and gamma-ray irradiation. journal of advanced concrete technology 15(9):440–523, 2017. https://doi.org/10.3151/jact.15.440 [3] a. špaková. effect of gamma radiation and relative humidity on cement pastes microstructure studied by scanning electron microscopy. bachelor thesis, czech technical university in prague, 2025. 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composites observed with xrd and sem methods in the range of radiation dose 0-1409 mgy. acta physica polonica a 114(2):399–411, 2008. https://doi.org/10.12693/aphyspola.114.399 [11] j. němeček, a. špaková, j. němeček, p. halodová. micro-scale properties of cement pastes exposed to gamma radiation. acta polytechnica ctu proceedings 53:76–81, 2025. https://doi.org/10.14311/app.2025.53.0076 [12] k. l. scrivener. backscattered electron imaging of cementitious microstructures: understanding and quantification. cement and concrete composites 26(8):935–945, 2004. scanning electron microscopy of cements and concretes. https: //doi.org/10.1016/j.cemconcomp.2004.02.029 [13] j. němeček, j. lukeš, j. němeček. high-speed mechanical mapping of blended cement pastes and its comparison with standard modes of nanoindentation. materials today communications 23:100806, 2020. https://doi.org/10.1016/j.mtcomm.2019.100806 [14] e. e. t. rodriguez, w. a. hunnicutt, p. mondal, y. le pape. examination of gamma-irradiated calcium silicate hydrates. part i: chemical-structural properties. journal of the american ceramic society 103(1):558– 568, 2020. https://doi.org/10.1111/jace.16515 78 https://doi.org/10.12693/aphyspola.114.399 https://doi.org/10.14311/app.2025.53.0076 https://doi.org/10.1016/j.cemconcomp.2004.02.029 https://doi.org/10.1016/j.cemconcomp.2004.02.029 https://doi.org/10.1016/j.mtcomm.2019.100806 https://doi.org/10.1111/jace.16515 acta polytechnica ctu proceedings 54:72–78, 2025 1 introduction 2 experiments and methods 2.1 sample preparation 2.2 gamma irradiation 2.3 sample preparation before electron microscopy 2.4 scanning electron microscopy 2.4.1 bse imaging and image analysis 2.4.2 point element analysis 3 results and discussion 4 conclusions acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2024.47.0008 acta polytechnica ctu proceedings 47:8–14, 2024 © 2024 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague cementitious material development for additive fabrication david číteka,∗, karel hurtiga, vladislav burešb, peter kotešc a czech technical university in prague, klokner institute, šolínova 7, prague 166 08, czech republic b technical university of liberec, department of load-bearing structures, studentská 1402/2, 461 17 liberec 1, czech republic c university of zilina, faculty of civil engineering, univerzitná 8215, 010 26 žilina, slovakia ∗ corresponding author: david.citek@cvut.cz abstract. for the 3d star project and 3d printing purposes, a special fine-grained cement mixture from locally available raw materials was developed. the reason for the development of the custom mixture was the possibility of arbitrary optimization of the developed mixture at any stage of the project and for any type of application. mix design, printing head and the entire system from mixing to extrusion was the subject of research and development for this project. it was therefore necessary to address both issues in parallel and to respond in both sectors to the realities arising from the partial results of the different groups involved in the development. keywords: 3d printing, cementitious material, optimization, testing, material properties. 1. introduction cement binders require a cement hydration process to cure, which under normal conditions takes place at its fastest stage at a significantly slower rate of days and is not fully completed even within a few years. cement hydration is the process of first setting and then hardening of the cement-bonded material [1, 2]. the setting process depends on many factors, but generally this phase takes place within hours. the hardening process is loosely related to the setting process. a fundamental issue, 3d printing of cementitious composites is facing, is designing the mixture in such way that after extrusion and repeated layering, it resists its own weight, and the printed element can be printed/built up vertically. the mixture must be of a suitable consistency to be pumpable and subsequently easy to leave the extruder, but at the same time it must set quickly enough after extrusion or have a rigid consistency to allow the layers to be reapplied on top of each other. this is further related to the very issue of the stability of a freshly printed object that is not sufficiently cured [3, 4]. these mixture requirements ultimately lead to the design of a cementitious composite of a rather complicated composition containing several different additives, including setting accelerators. the resulting mechanical strength is certainly also important to the overall design of the final element, but less important in terms of the extrusion process itself [5, 6]. 2. experimental part 2.1. preliminary tests at the beginning, fine-grained micro concrete was compound (e.g. see table 1). according to requirements on maximum size of grain, sand with grains of 1.25 mm was used. a requirement for the compressive strength of the molding to be at least 60 mpa was required, therefore cem ii 52,5 n cement was used in combination with a superplasticizer. also, polypropylene (pp) fibres with length of 12 mm were added to prevent plastic shrinkage. fine sand 0.10 – 0.63 mm 587 kg m−3 fine sand 0.63 – 1.25 mm 841 cem ii 52.5 n 528 superplasticizer 23 water 200 12 mm pp fibres 1.0 table 1. mix design of preliminary tests. this basic mixture was tested to buildability when the viscosity modifying admixture (vma) is added. the question was, if we are able to provide such material, that can be build by layers. adding vma to the mixture cause increase of material stiffness and thixotropy. these charakteristics allow to print individual layer that withstand the pressure of the layers above it without plastic deformation due to compression, spreading, etc. varienty of vmas were tested. after few trials for each type of vma, printable cement based material was produced (e.g. see figure 1). by using pistol (e.g. see figure 2) filled by fresh mixture, the layers were manually printed out. for each type of vma, the optimum dosage for manually printed material was experimentaly determined. in table 2, type of vma, water content, quantity of admixture, consistecy of fresh mixed material determining by the method specified by standard en 1015-3, bulk density of the material, flexular strength and compressive strength, are listed. bulk density, com8 https://doi.org/10.14311/app.2024.47.0008 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 47/2024 cementitious material development for additive fabrication type of vma vma water flow bulk flexural compressive content content (en 1015-3) density strenght strenght [kg m−3] [kg m−3] [mm] [kg m−3] [mpa] [mpa] modified starch 5.2 215 120 × 120 2 200 14.0 88.0 modified hydroxyethyl methylcellulose 1.0 225 130 × 135 2 170 16.2 83.5 activated bentonite 40.0 225 135 × 135 2 190 17.8 86.0 modified starch + activated bentonite 20.0 / 2.0 235 125 × 130 2 160 15.9 85.0 table 2. mix design of preliminary tests. figure 1. printed layers of cement based material. figure 2. pistol for manually printed cement based material. pressive and flexural strength are determined on beam specimens with dimensions of 40 × 40 × 160 mm at the age of the material of 28 days. as seen on figure 1, the surface of printed material was not optimal. the surface was rough and contained large voids. to make surface smoother, material with fine particles needs to be add. for that purpose ground limestone, silica fume and ground quarts were added. at the same time, 12 mm pp fibres were replace to fibres with length of 5 mm. considering the maximum size of grain, 5 mm pp fibres should be sufficient. the dosage of pp fibres was not changed, so due to its geometry the number of fibres had more than doubled. along with these changes, the amount of cement was reduced by 30 %. the composition of designed mixture is shown in table 3. partical size cumulative distribution is shown in figure 3. this material with adjusted composition was also tested for its ability to print layers manually with pistol. as seen on the figure 4, the surface was much smoother than previous pictured on figure 1, however, if dosage of vma is high or water content is low, the surface could still show defects. as optimal for printing layer the combination of bentonite and modified starch as vma was selected. fine sand 0.10 – 0.63 mm 412 kg m−3 fine sand 0.63 – 1.25 mm 589 cem ii 52.5 n 370 ground quartz 1 188 ground quartz 2 188 ground limestone 41 silica fume 94 5 mm pp fibres 1 superplasticizer 28 water quantity by type of vma vma quantity by type of vma table 3. mix design of preliminary tests. figure 3. partical size cumulative distribution. 2.2. mechanical properties of the material as part of the experimental program, several different types of tests were carried out on the material properties of the mixture. table 4 lists various material parameters at a specific age of the material. 2.3. setting accelerator application to improve the buildability of the printed material and stability of the printed structures the setting accelerator was applicated. usage of powdery setting accelerators was rejected. these ingredients had to be dosed into the dry mixture and the reaction time is thus started as soon as the water is added. based on experience, we concluded that pumping the printing mixture with an accelerator can be very risky in terms of unplanned technological breaks, changes in printing speed and other marginal conditions. 9 d. čítek, k. hurtig, v. bureš , p. koteš acta polytechnica ctu proceedings figure 4. manually printed material with adjusted composition. material age compressive strenght flexural strenght modulus of elasticity layer cohesion shrinkage (en 196-1) (en 196-1) (iso 1920-10) (en 14488-4) (en 12808-4) [day] [mpa] [mpa] [gpa] [mpa] [mm m−1] 1 10.0 3.0 – – – 2 24.0 5.7 20.3 – 0.12 5 37.5 7.8 – – 0.53 7 41.5 8.3 – – 0.84 14 54.0 10.2 29.4 – 1.04 21 60.0 10.8 – – 1.16 28 64.5 11.1 32.1 1.8 1.18 56 73.0 11.9 – – 1.37 90 76.5 12.2 – – 1.49 table 4. mechanical properties of the material. for the purpose of 3d printing, alkali-free setting accelerator used for shotcrete application, was selected as the most optimal. the entire proposed 3d printing system works with a mixture that is mixed, transported, and pumped in a wet state to the printing head where a liquid accelerator is added in the last extrusion stage. its quantity is adjusted according to the printing speed, the amount of material to be extruded and the complexity in terms of the shape of the printed element. this system allows for virtually any printing environment (length of hoses required to pump the mixture, location of the mixing device versus the printing device, and marginal parameters such as temperature, humidity, or glare). the mixture is transported in a liquid or plastic state through the entire system without the risk of the mixture solidifying in the system during a potential downtime or technological break. acceleration occurs only at the very end of the printing process and the reaction time, i.e., the start of the hydration and solidification process, is in the order of minutes. usual range of the accelerator “work” time is about 5–10 minutes. after this time period mixture starts with hydratation of cement components and printed structure became stable. the various setting accelerators were mechanically mixed into the mixture of uniform composition and the initial setting time was measured.the traditional devices for monitoring the setting time of cementitious materials are various types of penetrometers. the measurement of setting time for ordinary cement figure 5. visual evaluation of the effects of the setting accelerator. is carried out with a so-called vicat needle. however, this device and the method itself is not suitable for the expected short times (seconds in extreme cases) that are needed when extruding a mixture for 3d printing. therefore, a sensory evaluation of the material solidification e.g. see figure 5. several variants of the use of the setting accelerating additive were selected from this simple test. these variants were subsequently tested directly during printing using the pump and a printhead. 2.4. mixing equipment the final designed mixture is capable to be mixed in large mixers.. all dry components (cement, sand, 10 vol. 47/2024 cementitious material development for additive fabrication figure 6. the mixing device with maximum capacity of 200 l. fine components) are premixed into 20 kg bags to speed up the preparation and mixing process. large volume mixing plant with a maximum capacity of approximately 200 liters is used for mixing the required volumes for printing test e.g. see figure 6. this mixer is also suitable for mixing material for continuous printing of large structures. the mixer is equipped with a frequency converter for continuous rotation control. the mixing blades are also variable for a different types of mixtures. this is important for mixing materials with different thixotropic properties. 2.5. pumping equipment after initial tests of the base mixture, in which its pumpability was adjusted, the suitable spindle pump for connection to the transport system upstream the printing device e.g. see figure 7 was selected. the spindle pump is composed of a rotor and a stator where, during movement, temporary chambers are created into which the material is drawn in and then pushed out through the discharge nozzle. the pumping equipment has the hopper volume of 150 litres. the pump control is software-integrated into the printing control system. for the initial prints, the mixture was pumped using a inside dn 25 diameter hose system with a hose length of 12 metres. as the mixture was optimized, the consistency of the mixture was gradually changed and the printing speed increased. these changes resulted in increased friction of the mixture in the hoses and significant heating of the mixture, resulting in uncontrollable solidification of the mixture in the hoses and couplings. for this reason, the entire pumping system was redesigned to dn 35 diameter hoses. these hoses were already 3 m long to make them easier to clean. the length of hose system is 12 metres long. the system is fitted with a pressure gauge to monitor the pressure downstream of the pump outlet and all hoses are fitted with quick couplings for easy assembly and disassembly. when the printing is complete, the entire hose system is figure 7. concrete mixture pump with frequency converter. disassembled and transported with the pump to the wash bay where all components are washed, lubricated and ready for the next use. 2.6. extrusion through gradually defining the requirements for the printed cement mixture in the course of the research, we decided to pursue the variant where a liquid setting accelerator is injected in the last stage of the printing process. this variant has several advantages for the complex system under development. one of them is the rapid response to changes in the surrounding climatic conditions, which is not possible when printing the mixture with powder accelerator in the bulk mixture. another advantage is the certainty that the accelerated mixture will not solidify in the conveying hose system when printing is stopped. another indisputable plus is the possibility of accelerating the setting of the mixture, with initial strength coming within 1 to 10 minutes after application, i.e. extrusion. this rate of setting is suitable for printing column elements that have a relatively short footprint in a single layer, and hence the need for the shortest possible setting time. for such fast-rising prints, a setting speed is necessary in order to build on already printed layers. the printing head with nozzle e.g. see figure 8 is the final component before the material is extruded. this is also part, where the setting accelerator is applicated. for perfect mixing of mixture with accelerator and control of the printing speed, a system of specially shaped moving blades mechanically coupled to the print mixture dosing drive was developed and several types of shaping of the accelerator inlet itself were tested. the blades must not only mix the mixture perfectly with the setting accelerator, but they should also restrict the flow of material through the printhead as little as possible and not be prone to clogging the print mixture component during printing. after experimental verification of several basic blade shapes, the variant that best met most of the requirements was selected. this variant underwent further development of the mechanical and material design to better withstand the abrasive environment in the 11 d. čítek, k. hurtig, v. bureš , p. koteš acta polytechnica ctu proceedings figure 8. the printhead with setting accelerator injection and nozzle with quick-release system. print head and shows no significant signs of wear even after several thousand liters of cement mixture flow. all printing nozzles are also developed as parts which are very easy to be printed in small 3d printers so the whole process of parameters changing of the nozzle or simply change of the nozzle can be very smooth and fast. 2.7. mechanical properties of the material with added setting accelerator the setting accelerator has a positive effect on the behaviour of the material in the early stage after the printing, but has a negative effect on the final mechanical properties. the degree of influence on the final mechanical properties is dependent on the accelerator dose used. several test were performed and material characteristics for dosage of 70, 110 and 150 ml min−1 was determined at the age of 28 and 90 days. figure 9 shows the evaluated values of flexural strength and figure 10 shows the evaluated values of compressive strength. the test of flexural and compressive strength was performed on specimens with dimensions of 40 × 40 × 160 mm, where material was pressed into the mold from the printhead. depending of the dose of setting accelerator, the flexural strength is within range of 55 % to 83 % of value of reference material without setting accelerator in the age of material 28 days, the flexural strength is within range of 52 % to 78 % of value of reference material without setting accelerator in the age of material 90 days, the compressive strength is within range of 69 % to 91 % of value of reference material without setting accelerator in the age of material 28 days and the compressive strength is within range of 65 % to 82 % of value of reference material without setting accelerator in the age of material 90 days. figure 9. flefural strength. figure 10. compressive strenght. figure 11. flexural tensile strength test (left) and compressive strength test (right) on 40 × 40 × 160 mm beams. 2.8. mechanical properties of the printed material the possible difference between the laboratory produced specimens (by depositing the material in the moulds) and testing the parameters of the composite after cutting/drilling from the printed object was continuously verified e.g. see figure 11 and 12. an effort was made to achieve as much consistency and homogeneity of the printed mixture as possible before entry and after extrusion. modifications to both the printing technology and the mixture resulted in expected differences in the order of 5–10 %. the deterioration in material parameters is expected due to the different deposition (layering and non-compaction) of the mixture. extrusion increase the air content of the mixture and thus reduce the bulk density. the volumetric mass of the mixture developed is in the 12 vol. 47/2024 cementitious material development for additive fabrication figure 12. drilled cores with diameter of 80 mm. figure 13. printed wall and truss beam. range of 2 080–2 120 kg m−3. finally a tests on drilled cores (figure 12) showed practically indentical results as test on samples from formwork. 3. conclusions the cement-based material suitable for 3d printing technology was designed. the material was designed for pumping and subsequent printing. the buildability of the material was ensured by the increased thixotropy of the material by adding viscosity modifying admixture and by the alkali-free setting accelerator mixed in the printing head. the compressive strength of the material is around 65 mpa and the bending strength is around 11 mpa at the age of 28 days. various structures could be printed with this material e.g. see figure 13. as a final product a set of different construction part was assembled. main parts of the final structure are: double layered thin walls which are partly filled with liquid insulation thermowhite, horizontal system consist of truss beam reinforced by added steel reinforcement and there are also several small parts as a isobeam, which connect horizontal and vertical parts. the main outcome from these project is that all developed and printed structures and construction parts are easy transportable and assemble. several measurement procedures are ongoing. also the “final” construction which represents whole research process and program is now installed outside and exposed to all natural influences. the main subject of ongoing monitoring is the development of cracks, durability and the ability to resist external influences. acknowledgements this work was supported by by the project op vvv cz.02.1.01 / 0.0 / 0.0 / 16_025 / 0007424 3d-star.. references [1] d. čítek, v. bureš, o. melter, et al. experimentální vývoj 3d tisku cementových kompozitů. beton tks (1):7–11, 2022. [2] v. bureš, d. čítek, p. zelený, r. mendřický. návrhy konstrukčních prvků s využitím cementového kompozitu tisknutého 3d technologií. konstrukce (3):24–28, 2023. issn 1213-8762. [3] d. citek, s. rehacek, d. dobias. experimental research on 3d printed construction from cement based composites. in proceedings of 22nd international multidisciplinary scientific geoconference sgem 2022, pp. 233–240. 2022. https://doi.org/10.5593/sgem2022/6.1/s27.30 [4] j. suchomel, v. bureš, j. válek, et al. experimental development of 3d printing using cement composite. p. 220. 2022. in abstract book wmcaus. [5] j.kolísko, d. čítek, k. hurtig, m. kroc. uhpc pro 3d tisk. in 18. konference technologie a provádění (2023). praha 1: česká betonářská společnost čssi, 2023. isbn 978-80-907611-8-6. 13 https://doi.org/10.5593/sgem2022/6.1/s27.30 d. čítek, k. hurtig, v. bureš , p. koteš acta polytechnica ctu proceedings [6] j. suchomel, v. bureš. sequentional on-site printing of building structures. vsb technical university of ostrava, 2022. isbn 978-80-248-4331-5. https://dspace.tul.cz/handle/15240/156782 14 https://dspace.tul.cz/handle/15240/156782 acta polytechnica ctu proceedings 47:8–14, 2024 1 introduction 2 experimental part 2.1 preliminary tests 2.2 mechanical properties of the material 2.3 setting accelerator application 2.4 mixing equipment 2.5 pumping equipment 2.6 extrusion 2.7 mechanical properties of the material with added setting accelerator 2.8 mechanical properties of the printed material 3 conclusions acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2023.40.0008 acta polytechnica ctu proceedings 40:8–14, 2023 © 2023 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague the footbridge jesípek – application of radar interferometry for dynamic response evaluation miroslav čápa,∗, michal poláka, tomáš plachýa, milan talichb, jan havrlantb, lubomír soukupb, filip antošb a czech technical university in prague, faculty of civil engineering, department of mechanics, thákurova 7, 166 29 prague 6, czech republic b czech academy of sciences, institute of information theory and automation, pod vodárenskou věží 4, 182 00 prague 8, czech republic ∗ corresponding author: miroslav.cap.1@fsv.cvut.cz abstract. recent advances in radar systems have led to the development of radar interferometry (ri) methods for contactless vibration monitoring of large-scale structures, i.e. bridges, water tower reservoirs, and factory chimneys. interferometric radars are devices capable to measure with 200 hz sampling frequency and relative movement precision of 0.1 mm up to 0.01 mm. the major part of this paper describes an in-situ footbridge experiment near hradec králové. radar interferometry devices were deployed along with ordinary techniques compounded by accelerometers, wiring, and acquisition station. experiment was focused on the evaluation of basic dynamic structural properties such as natural frequencies and mode shapes. a lot of attention was also given to the result comparison of these measurement methods. test results have confirmed the applicability of ri for bridge vibration monitoring. keywords: bridge dynamics, radar, radar interferometry, accelerometers. 1. introduction radar interferometry is a contactless geodetical measurement method operating with electromagnetic microwaves. the technique is based on a well-known conventional radar principle, i.e. radio detection and ranging. the fundamental characteristic of commonly used radar is its ability to determine the range of the target (i.e. distance) by measuring the time of transmitted and back-reflected echoes. the interferometric radar device enables to characterize slow static displacement and fast variable movement as well. the capability of radar is substantially bigger than the name of this technique implies. typical characteristics such as the velocity, acceleration, displacement, size and shape of the target can be evaluated [1, 2]. the goal of this measurement was to test the radar technique in-situ in real field conditions and to find the capabilities of this method in practice for bridge engineering purposes. the main objective of this project was to find out the advantages and disadvantages of the ri method compared to the common measurement technique using accelerometers. 2. radar interferometry principle the ri method operates with the microwave interferometer device called ibis (image by interferometric survey) developed by the italian company ids (ingegneria dei sistemi). the main characteristic of this radar system is the ability of real-time simultaneous deflection measurement of several points (targets) at the same time. the ibis radar is capable of working in static as well as dynamic modes. the device operates with the following principles: • stepped frequency continuous wave (sf-cw), • differential interferometry. differential interferometry provides target displacement data simply computed from phase shift. the radar emits sinusoidal waves in the direction of the target. the waves are reflected from the target. received echoes carry information about the time and the phase angle. for non-moving mass, there is no phase shift at any time difference. in addition to the echo of moving one, the phase differences are proportional to displacement exhibited in time [1, 3–5]. figure 1. schematic representation of radar interferometry [3]. the principle of differential interferometry is illustrated in figure 1. look at phase angle φ1, φ2 and 8 https://doi.org/10.14311/app.2023.40.0008 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 40/2023 the footbridge jesípek – application of radar interferometry . . . figure 2. projection of measured deflection to vertical movement [3, 6]. figure 3. position of the jesípek footbridge nearby hradec králové city. distance d. distance d is described by the following formula: d = − λ 4π (φ2 − φ1), (1) where λ is wavelength, φ1 is phase angle in time t1, φ2 is phase angle in time t2. all characteristics measured by interferometric radar are determined in the direction of the target, which is not identical to the desired direction. configuration of the radar and object is presented in figure 2. hence, it is necessary to evaluate appropriate displacement by the following formula: dv = dr · r h , (2) where r/h is the projection factor [3, 6]. 3. jesípek footbridge experiment 3.1. structure description this part of the paper describes undertaken experiment near the city of hradec králové. the footbridge for pedestrians and cyclists was observed. the structure links the cycling roadway from hradec králové to vysoká nad labem. the object is situated on the labe river channel called jesípek in the periphery of hk city. see figure 3. the footbridge is designed as the langer beam structure also known as the nielsen-lohse system bridge. the girder with the bridge deck is supported by an arch via connected hangers. the bridge deck figure 4. jesípek footbridge. is composite and compounds of a reinforced concrete slab fixed to the main girder and steel crossbeams through steel joints. the span of the footbridge is 52.4 m. the arch is cambered 11 m [8]. all steel components except slab joints and hangers are made from the steel s355 according to european steel standard grades. for the other parts following materials were used: • slab joints: s235 steel grade. • slab: c30/37-xf3 concrete class. • reinforcement bars: b500b steel grade. • hangers: din en 12385 stainless steel grade. the supporting structure has the following dimensions. the main girder is a welded box shape of 400 × 400 mm, the arc is also a welded profile of 412 × 420 mm. the cross-section of hangers is a circle of 10 mm diameter made of pfeifer pe7. the deck is compounded of i-shape crossbeams with a variable height from 400 mm to 140 mm and a concrete layer 120 mm thick. structure is shown in figure 4 [8]. 9 m. čáp, m. polák, t. plachý et al. acta polytechnica ctu proceedings figure 5. geometric 3d model, crossbeam section numbering [7]. figure 6. measurement devices prepared in position. a detailed view of the accelerometers. 3.2. traffic and weather conditions the examined structure was full-time observed within an ordinary pedestrian and cycling service. traffic on the footbridge was not excluded during the experiment. the measurements were focused to determine the natural frequencies and mode shapes of the structure. an object oscillation was observed. the total length of the continuous measurement was 50 minutes. the experiment was undertaken during the summer season. climatic conditions: • clear and sunny weather, • air temperature between 29.3–30.1 °c, • humidity range was 42–47 % [7]. a pedestrian group with synchronized walking frequency was assembled. a human-induced excitation was used within the experiment as well. 3.3. measurement method the dynamic response was concurrently examined by two techniques: namely, the radar interferometry method and the ordinary method compounded of accelerometers, wiring and acquisition station as well. interferometric radars r1 and r2 two radars ibis made by italian manufacturer ids were used. the properties of utilized interferometric devices are as follows: • radar r1: ▷ ids radar ibis – fs plus. ▷ sn 010-19-000314. ▷ antenna: ibis-ant3-h17v15. ▷ accelerometer (radar vibration only). • radar r2: ▷ ids radar ibis – ru 172. ▷ sn 053. ▷ antenna: ibis-ant3-h17v15. ▷ accelerometer (radar vibration only) [7]. acceleration transducers during the experiment a data acquisition station sirius type 6acc2acc+, and 8 piezoelectric acceleration sensors brüel&kjaer type 8344 with the following characteristics were used: • natural frequency higher than 10 khz, • sensor sensitivity is approximately 2500 mv/g, • transverse sensor sensitivity is less than 5 %, • working frequency range is from 0.2 hz to 3 khz, • operating temperature range is from −50 °c to +100 °c. the lower working frequency range limit of these sensors is low enough to be convenient to use for dynamic experiments realized on bridges [9, 10]. accelerometers were linked via cables to the eightchannel data acquisition station sirius made by the dewesoft company. sensors were attached to steel weights with neodymium magnets. there were four weights, three of which were situated on the right edge of the deck in a position of the crossbeams number 4, 6 and 10. the last weight was situated on the left edge of the deck in the position of crossbeam number 10. see figure 5. sensors were oriented in 3 directions (x, y, z) according to the cartesian coordinate system. the horizontal axis x was aligned with the longitudinal footbridge axis, and the horizontal axis y was perpendicular to the longitudinal axis. the axis z was aligned with the vertical direction. see figure 6. 10 vol. 40/2023 the footbridge jesípek – application of radar interferometry . . . figure 7. radars are based on position. a detailed look at radar r1. both radars r1 and r2 were utilized during the experiment. due to a lack of accessibility on the south bank both radars were placed on the north bank. interferometers were situated beneath the footbridge deck. measured characteristics were evaluated at the bottom of the deck. the acceleration sensors were situated on the top of a concrete slab. measured characteristics were evaluated on the top of the deck. a detailed look at ibis with the direction of aim is illustrated in figure 7. the bottom of the deck has a very diverse surface with a periodic pattern. the entire relevant structure area was fully observed. each crossbeam was incorporated into a different range area (rbin). the resolution was high and the reflection conditions were very good. supporting devices such as reflectors were not necessary to be installed. the radar parameters and settings for this experiment were the following: • ids radar r1: ▷ sampling frequency: 200 hz. ▷ signal reach: 35 m. ▷ radial resolution: 0.75 m. ▷ vertical radar inclination: 13.9 °. • ids radar r2: ▷ sampling frequency: 199.6 hz. ▷ signal reach: 40 m. ▷ radial resolution: 0.75 m. ▷ vertical radar inclination: 7.4 ° [7]. 4. results this section describes the results of performed modal analysis and human-induced vibration response analysis. natural frequencies and mode shapes were evaluated. peak values of displacements and accelerations were determined. dynamic data were converted from the time domain into the frequency domain by using fast fourier transform (fft) analysis. significant frequencies can be seen from the following figures figure 8 and figure 9. the first five natural frequencies determined by the accelerometers and four natural frequencies determined by the radars with related mode shapes are written out in the following table. see table 1. as we can see, radars were able to record the vertical vibration only. the 3rd natural frequency bounded to a lateral oscillation was not determined by the radar interferometers. the direction of aim measurement figure 8. the frequency spectrum of radar data. figure 9. the frequency spectrum of acceleration transducer data. limitation took effect. therefore, additional radar devices should be used for the lateral vibration response evaluation. however, the most interesting part of the experiment was the time interval from 426 s to 516 s in the position of crossbeam number 10 (r1 rbin 22 and r2 rbin 29). this time range represents human-induced vibration by a group of people with a synchronized walking frequency identical to the natural frequency of the structure. excited frequency was bounded to the 2nd vertical bending mode shape. the interferometric radars recorded movement in the direction of the aim. vertical displacement components were computed using equation (2). the acceleration transducer’s time domain data were converted to displacement data by the double numerical integration performance. simpson’s rule formula was used. compared displacement results are shown in figure 10. in the previous chart in figure 10, there can be seen, that positive peak values of displacement determined by interferometric radars are almost identical to the double integration values of the ordinary accelerometer technique. this does not apply to negative peak values 11 m. čáp, m. polák, t. plachý et al. acta polytechnica ctu proceedings radar interferometry accelerometers mode shape f [hz] f [hz] type 1.34 1.34 1st vertical bending mode 1.49 1.53 2nd vertical bending mode 1.62 1st horizontal lateral bending mode 3.18 3.20 3rd vertical bending mode 3.50 3.53 4th vertical bending mode table 1. natural frequencies. figure 10. determined displacements in the crossbeam no. 10 position. where the difference is 2 mm up 3 mm. the maximum amplitude of the oscillation is ±20 mm. acceleration values measured by sensors and values evaluated from radars are shown in figure 11. a double numerical derivation was performed for the radar displacement data evaluation. a strong frequency noise in the radar signal data was observed in the previous chart (figure 11). determined values provided by radar interferometers are abnormally high. the measurement line with transducers brüel&kjaer type 8344 was set up to be able to record accelerations up to a frequency of 120 hz. by further research, other frequencies were detected in the radar data, but the acceleration transducers were unlikely to have not recorded any of them. from the time 2485 s only one radar was working. there was significantly less frequency noise in radar signal data. it was discovered that frequency noise was caused by parallel radar placing. reflected radar r1 and r2 waves apparently have influenced each other. the inconvenient frequency components in higherrange areas were filtered out. a low-pass filter with a cut-off frequency of 35 hz was used. after that, we can see a very good match between the commonly used technique and the ri method. the response evaluated by radar interferometry devices in the time domain 426 s to 485 s reflects very well response determined by accelerometers. peak values of acceleration amplitudes evaluated by both methods are almost identical. amplitude of acceleration was ±2 m×s−2. see figure 12. unlike that, there are still noise frequencies in the time domain from 486 s to 516 s. this part could not be filtered out without any influence on other frequency components occurring in the observed structure. 5. conclusion in this paper, the evaluation of the footbridge experiment realized in the field conditions was shown. the radar interferometry method was tested in practice along with commonly used techniques. after basic data operations and data filtering, there can be stated in this particular test that the radar interferometry (ri) method has reflected a dynamic response with 12 vol. 40/2023 the footbridge jesípek – application of radar interferometry . . . figure 11. comparison of raw data from radars and sensors. figure 12. acceleration values in the crossbeam no. 10 position. a very good match. test results confirmed the applicability of the ibis radar for vibration monitoring in the bridge engineering practice. on the other hand, we should note that inconvenient frequency components contained in the signal can devalue the entire experiment. not all of them can be easily separated and filtered out. also, we should mention that suitable conditions in this particular experiment for the ri method were ensured. interferometric devices had a clear line of view and no additional supporting equipment as reflectors were necessary to be installed. the pattern of the deck bottom surface was convenient as well. all factors should be considered in the planning of further experiments. especially placing and mutual interaction of the interferometric radars should be genuinely considered. acknowledgements the research was supported by the project vi20192022167 of the ministry of the interior of the czech republic and also by sgs22/089/ohk1/2t/11 ctu in prague. references [1] c. gentile. vibration measurement by radar techniques. in proceedings of the 8th international conference on structural dynamics, eurodyn 2011, pp. 92–103. 2011. [2] p. kuras, t. owerko, m. strach. application of an interferometric radar to examination of engineering objects vibration. reports on geodesy 2:209–216, 2009. [3] m. talich. using ground radar interferometry for precise determining of deformation and vertical deflection of structures. iop conference series: earth and environmental science 95:032021, 2017. https://doi.org/10.1088/1755-1315/95/3/032021 [4] c. gentile, g. bernardini. an interferometric radar for non-contact measurement of deflections on civil engineering structures: laboratory and full-scale tests. structure and infrastructure engineering 6(5):521–534, 2010. https://doi.org/10.1080/15732470903068557 13 https://doi.org/10.1088/1755-1315/95/3/032021 https://doi.org/10.1080/15732470903068557 m. čáp, m. polák, t. plachý et al. acta polytechnica ctu proceedings [5] p. v. genderen. multi-waveform sfcw radar. in 2003 33rd european microwave conference, pp. 849–852. 2003. https://doi.org/10.1109/euma.2003.341086 [6] m. talich. přesné monitorování svislých průhybů mostních konstrukcí metodou pozemní radarové interferometrie. in xii. mezinárodní konference geodézie a kartografie v dopravě, pp. 75–88. český svaz geodetů a kartografů, 2014. [7] m. talich, j. havrlant, l. soukup, f. antoš. zpráva o provedeném ověřovacím měření na lávce přes jesípek u hradce králové 2021. [8] transconsult s.r.o. cyklostezka hradec králové pardubice: so201 lávka přes slepé rameno jesípek km 1.368. 2018. [9] t. plachy, m. polák, p. ryjáček, et al. experimental dynamic analysis of the arch road bridge. iop conference series: earth and environmental science 906(1):012061, 2021. https://doi.org/10.1088/1755-1315/906/1/012061 [10] m. polák, t. plachý, a. čítek, et al. experimental dynamic analysis of the existing footbridge in dobřichovice town. matec web of conferences 313:00002, 2020. https://doi.org/10.1051/matecconf/202031300002 14 https://doi.org/10.1109/euma.2003.341086 https://doi.org/10.1088/1755-1315/906/1/012061 https://doi.org/10.1051/matecconf/202031300002 acta polytechnica ctu proceedings 40:8–14, 2023 1 introduction 2 radar interferometry principle 3 jesípek footbridge experiment 3.1 structure description 3.2 traffic and weather conditions 3.3 measurement method 4 results 5 conclusion acknowledgements references my title https://doi.org/10.14311/app.2022.36.0149 acta polytechnica ctu proceedings 36:149–160, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague on the effects of modified lane configuration due to construction works on bridge loading marcel nowak∗, oliver fischer technical university of munich, chair of concrete and masonry structures, theresienstrasse 90/n6, 80333 munich, germany ∗ corresponding author: marcel.nowak@tum.de abstract. construction works are frequently recurring events within highway networks, temporarily altering the characteristics of the traffic flow and hence of the bridge loading process. consequently, highway bridge superstructures usually carrying unidirectional traffic might have to face modified configurations, with possibly altered lane widths and transverse lane positions, or even an increased number of lanes and the occurrence of bidirectional traffic flow. while usually covering only short periods compared to the average service life of a bridge, such scenarios might represent extreme loading situations, potentially dominating the traffic loading process. therefore, their consideration and accurate modeling in the course of traffic load assessment and development of object-specific traffic load models are of utmost importance. the presented study analyzes data from bridge monitoring along federal highway a92 in southern germany. the data covers standard and different modified lane configurations due to construction works. besides analyzing the effects on traffic flow characteristics and resulting occurrence rates of multiple truck presence on the bridge, the consequences for load effects and their extreme value behavior are also evaluated. the findings support verification of related code background works and can serve as base and guidance for future research efforts on bridge load modeling in this context. keywords: bridge loading, modified lane configuration, monitoring, multiple truck presence, transverse distribution factor. 1. introduction throughout the regular service life of a highway bridge, occasional construction works along the road might lead to a temporary modification of traffic lane configuration on the roadway, with possibly altered number of lanes, lane widths, and transverse lane positions. these modifications potentially cause changes in the characteristics of the traffic flow and the loading of the bridge, i.e. the quantity and position of truck vehicles simultaneously present on the bridge. therefore, such scenarios might represent extreme loading situations, potentially dominating the bridge loading process. consequently, the possibility of modified lane configurations on a bridge needs to be considered for code calibration of traffic load models. in the background works for the german national annex to en 1991-2 and the reassessment guideline for existing road bridges (nachrechnungsrichtlinie), this scenario is accounted for [1, 2]. a modified lane configuration with bidirectional traffic on four traffic lanes is considered, with a ratio of 20 % of truck vehicles per direction traveling on the respective fast lane, allowing for the occurrence of truck overtaking events for each driving direction. additional investigations of altered bidirectional four-lane configurations are performed in [3]. based on a (more realistic) reduced width of the respective fast lane, the assumption is made that truck overtaking is not possible due to spatial constraints. the assumed absence of truck vehicles on the fast lane (no overtaking trucks) reduces the level of bridge loading. investigations within the scope of mentioned background works are performed based on numerical simulations and for a limited set of bridge structures. the resulting load levels are evaluated based on extrapolated characteristic values with a return period of 1000 years (even though the occurrence period of modified lane configurations is considerably shorter compared to the standard lane configuration). measurements from an actual structure to compare and verify the effects of modified lane configuration on traffic flow characteristics and bridge loading are unknown to the author. within this context, the presented study analyzes monitoring data from a highway bridge along federal highway a92 in southern germany. the monitoring data covers besides standard two-lane configuration two periods with different modified lane configurations due to constructions works along the considered highway section: a modified unidirectional two-lane configuration and a bidirectional four-lane configuration. direct measurement of the resulting load effects with strain gauges and additional traffic monitoring with a laser scanner for vehicle detection, counting, and classification are performed. the monitoring data allows for an in-depth comparison of the modified lane configurations with the standard case. besides analyzing the effects on traffic flow characteristics and resulting occurrence rates of multiple truck presence 149 https://doi.org/10.14311/app.2022.36.0149 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en marcel nowak, oliver fischer acta polytechnica ctu proceedings on the bridge, consequences for load effects and their extreme value behavior are also evaluated. the findings from this study support verification of related code background works and can serve as base and guidance for future research efforts on bridge load modeling accounting for scenarios of modified lane configurations. 2. methods 2.1. signal processing the raw signal of strain sensors is composed of different parts, mainly differing in their rate of change: • load effects due to temperature and other nonpermanent constraining loads (low rate of change, usually several minutes or longer) • road traffic (static) load effects (medium rate of change, for structure and sensor locations in this work up to a few seconds) • dynamic effects due to excitation by passing vehicles and interaction with structure (high rate of change, fractions of second) • measurement noise (wide range of rates of change, possibly over entire frequency spectrum) signal processing is necessary to extract the relevant parts of the raw signal resulting from road traffic, i.e., passing vehicles’ (static) load. for this purpose, the procedure from [4, 5] is adopted within the scope of this work, performing signal decomposition exploiting the different rates of change of the single signal components. for this purpose, mean filtering (excluding parts of the signal with a low rate of change) and low pass frequency filtering (excluding dynamic effects and most parts of measurement noise) are applied. mean filtering is done based on block-wise determined and linearly interpolated mean values. the choice of block size constitutes a compromise between sufficiently detailed representation of the signal parts with a low rate of change and ‘robustness’ against the influence of the signal part due to traffic loading on the resulting block mean values. for low pass frequency filtering, a butterworth filter of seventh order is applied. the cutoff frequency fcutoff is chosen based on the natural frequencies of the monitored structure. as the focus is on analyzing the load effects resulting from the static load of passing vehicles, the value should be sufficiently low to exclude all significant parts of the signal due to dynamic effects. however, it is also essential to choose the value for fcutoff not too low to avoid significant alteration of relevant parts of the signal. depending on the velocity of a passing vehicle, the signal part due to its static load results in low-frequency contents of the signal that could be affected by the low pass filter [5]. it is acknowledged by the authors that there are more sophisticated and accurate methods for signal processing and decomposition to extract the component representing static traffic load effects. however, the achieved accuracy of the previously described procedure is considered sufficient for the objectives of this work. 2.2. mulitple truck presence according to [6], a bridge loading event (ble) is defined as the presence for a continuous period of time of at least one truck on the influence area of the load effect of interest, meaning two successive events are separated by a time gap with no truck present on the influence area. in other words, a ble consists of a sequence of vehicles consecutively arriving at a bridge (in the same or different traffic lanes and driving directions) with at least one vehicle being present on the structure at each instant of time during the event. in the context of multiple truck presence evaluation, bles can be described by following characteristic values based on the number of contributing truck vehicles: • total number of truck vehicles forming part of the corresponding ble [7]. • number of truck vehicles present on the bridge for a specific instant of time during a ble. • number of truck vehicles contributing to the maximum value of a specific load effect during a ble [8]. while the first two values solely depend on the traffic flow and the length of the considered bridge structure (response independent characterization), the third value is specific to a certain load effect, and hence depending on the structural response (for the same ble, this value can vary for different load effects). bles can be further categorized into following types, based on their transverse location and relative driving direction on the bridge: • in-lane event, with multiple truck presence on the bridge within the same traffic lane • overtaking event, with multiple truck presence on the bridge parallel on adjacent traffic lanes in unidirectional traffic • meeting event, with multiple truck presence on the bridge parallel on adjacent traffic lanes in bidirectional traffic arbitrary combinations of the different event types are also possible. for the scope of this work, multiple truck presence is evaluated based on monitoring data from both structural and traffic monitoring. data from traffic monitoring provides detailed information on the traffic flow over the bridge, with timestamps for every passing vehicle indicating its entrance to and exit from the measurement section. knowing the bridge length, the location of the measurement section along the bridge, and the velocity of each vehicle, timestamps for entrance to and exit from the bridge deck can be obtained. based on this data, detection of bles 150 vol. 36/2022 on the effects of modified lane configuration and their (response independent) characterization is straightforward. data from structural monitoring represents the structural response (e.g. strains) at selected sensor locations due to the traffic flow over the bridge. these load effect time histories contain no direct information on the causative load impact. however, data fusion from multiple sensors also allows for this data evaluation towards multiple truck presence. in a first step, the processed signal (extracted component due to static traffic loads) is analyzed to identify single bles. in approximative accordance with the previous definition, bles are identified as continuous sections of the traffic component of the strain signal exceeding a predefined threshold (continuous presence of vehicles on bridge causing significant structural response). for each identified ble, number and relative position of trucks on the bridge contributing to the maximum load effect εk,max measured at sensor k (response-sensor) at time tεk,max is estimated based on transverse distribution factors (tdfs) ft df,k|i. these factors describe the relative share of structural response εi(t = tεk,max ) for a representative set of n selected sensors (tdf-sensors) at same longitudinal but different transverse positions and indicate the transverse position of passing vehicles: ft df,k|i = εi(t = tεk,max ) n∑ j=1 εj(t = tεk,max ) . (1) this concept is based on the assumption of linear elastic structural behavior under usual traffic loading. in this case, the transverse distribution of structural response depends only on the transverse position of passing vehicles and not on their total weight [9]. the knowledge of this (response dependent) characterization of bles is essential for subsequent extreme value analysis (eva), as different types of bles represent different loading processes to be accounted for separately. 2.3. extreme value analysis for eva, two different methods are applied based on results from previous works [10, 11], the block maxima (bm) method with fit of generalized extreme value (gev) distribution and the level crossing counting (lcc) method with fit of rice’s formula. by applying the bm method, data from load effect time histories is blocked into intervals of a chosen size. the maxima in each interval are determined, generating a series of bm. a gev distribution can then be fitted to this series [12]. by differentiating the load effects according to the underlying ble types, separate bm series can be obtained. a different gev distribution is fitted to each of these series, jointly composing a composite distribution model for the bm series from a mixture of different ble types. each ble type considered for the composite distribution model must occur at least once per block [6]. statistical inference is performed applying maximum likelihood estimation to obtain the parameters of the fitted gev distributions. the estimation is performed by minimizing the respective negative log-likelihood expressions for the bm data. for the lcc method, rice’s formula is fitted to the upper tail of an outcrossing rate histogram. performing lcc, the number of times is counted at which positive values are crossed upwardly in a load effect time history. by normalizing the resulting level crossing histogram to the time history length, the outcrossing rate histogram is obtained, representing for each level the mean rate of its crossing during the reference period. rice’s formula describes the mean rate of upcrossing for a certain level during a reference period and is fitted to the outcrossing rate histogram. rice’s formula is fitted only to the significant tail regions, and the proper choice of the starting point for rice’s formula fit is crucial to the method. it should be as low as possible to ensure sufficient representativeness for statistical extrapolation but not too low to provide still a reasonable approximation of the significant tail region. an optimal starting point can be identified by evaluating the goodness of fit using a modified kolmogorov test. statistical inference is performed applying the least square method to obtain the parameters of the fitted rice’s formula. [13] 3. bridge structure and monitoring 3.1. bridge structure 29/1 bridge structure 29/1 is an overpass of federal highway a92 between the cities of deggendorf (deg) and munich (muc) in the southern part of germany, crossing an outlet ditch in five spans of uniform length of about 16 m. the bridge comprises two separate, identical superstructures, each carrying the roadway for one driving direction with a standard configuration of two traffic lanes plus emergency lane. the crosssection of each superstructure consists of five precast prestressed concrete t-beam girders of 69 cm height with a cast-in-situ slab of 23 cm height on top. the precast girders lie on elastomer bearings and cross each a single span without a rigid connection between them from one span to the next. only the in-situ slab on top is continuous over all five spans. this design reduces the superstructure’s stiffness at the piers, making each span act approximately as an independent single-span system by itself. the total width of one superstructure is 15.35 m, the width of the roadway is 12.0 m (refer also to figures 1 and 2). 3.2. alternating lane configurations federal highway a92 can be considered a minor section within germany’s highway network, with moderate traffic volumes. throughout the year 2019, construction works for renovation of the roadway are carried 151 marcel nowak, oliver fischer acta polytechnica ctu proceedings girder 1 span 1 span 2 span 3 span 4 span 5 dir. muc girder 2 girder 3 girder 4 girder 5 012345 16.00 79.90 16.00 15.9516.0015.95 figure 1. top view of bridge structure 29/1. gi rd er 1 gi rd er 2 gi rd er 3 gi rd er 4 gi rd er 5 1.12 1.12 10 40 10 12.00 15.30 1.65 1.65 1.42 2.86 2.86 2.86 2.86 1.42 14.30 2 3 6 9 figure 2. section of bridge structure 29/1. dir. muc lane 1 dir. muc lane 2 3.75 75 3.753.75 emergency lane dir. muc lane 1 dir. muc lane 2 3.50 6.002.25 25 closed to normal traffic (occasional construc�on traffic) dir. muc lane 1 dir. muc lane 2 dir. deg lane 2 dir. deg lane 1 3.50 3.502.502.25 25 figure 3. lane configurations lcstan (top), lcmod,uni2 (middle), and lcmod,bi4 (bottom). 152 vol. 36/2022 on the effects of modified lane configuration 012345 4-1 4-2 4-3 4-4 4-5 5-2 5-4 3-2 3-4 2-2 2-4 1-2 1-4 4-1t 4-3t 4-28 4-27 4-18 4-17 4-22 4-21 4-12 4-11 4-24 4-23 4-14 4-13 4-26 4-25 4-16 4-15 8.00 8.00 7.988.007.98 7.988.008.007.98 2.80 2.802.40 figure 4. overview of sensor layout of structural monitoring at bridge structure 29/1. 80 1.26 80 1.00 86 1.00 1.00 86 1.00 80 1.26 80 thermo sensorstrain gauge (local sensor)strain gauge (global sensor) 2.86 2.86 2.86 2.86 figure 5. sensor locations within bridge section of span no. 4 (top) and remaining spans (bottom). out in the section of the highway where bridge structure 29/1 is located, leading to alternating lane configurations on the bridge. besides the standard configuration of unidirectional traffic with two traffic lanes plus emergency lane per superstructure (lcstan), a modified unidirectional two-lane configuration with reduced roadway width (lcmod,uni2) and a bidirectional four-lane configuration on a single superstructure with closure of the second superstructure (lcmod,bi4) occur throughout the construction works (figure 3). even though a part of the roadway is closed for regular traffic in configuration lcmod,uni2, construction vehicles pass the bridge in this part for expected multiple truck presence on the bridge, the occurrence of in-lane events is excluded from the scope of this work. due to the short span length of 16 m and the approximate independence of each span from the rest of the structure (refer to section 3.1), the influence line for the considered load effects (strain at bottom of girder at mid-span) is of restricted length. hence, the contribution of an additional vehicle in the same lane is not significant. in this case, the characteristics of the loading process for in-lane events are similar to those of single vehicle events (sves). furthermore, overtaking events for lcmod,uni2 and lcmod,bi4 are likely not to occur due to spatial constraints by the reduced roadway width, making it almost impossible for two truck vehicles passing the bridge parallel in usual velocities. therefore, overtaking events are expected only for lcstan. additionally, multiple truck presence is expected for lcmod,bi4 by meeting events in the bidirectional traffic. besides occurrence due to regular highway traffic, the occasional passing of construction vehicles on the closed part of the roadway in configuration lcmod,uni2 also poses a source of multiple truck presence on the bridge. 3.3. structural monitoring the sensors for structural monitoring are installed on the northern superstructure, which carries the traffic with driving direction towards muc. the monitoring concept comprises 31 sensors (29 strain gauges, two thermal sensors). the sensor layout of the strain sensors consists basically of two components: sensors for global structural response at bottom of webs of t-beams at mid-span, and sensors for local structural response at bottom of flange of t-beams at quarterspan (refer to figures 4 and 5). the layout was designed for the realization of brigde weigh-in-motion (bwim). the bwim application and evaluation of related results are, however, beyond the scope of this work. in the following, the focus is on strain data from three global sensors in span no. 4 (sensors 4-2, 4-3, and 4-4). for the signal processing (section 2.1), the block size for the mean filtering is chosen to 10 min. the cutoff frequency fcutoff for the low pass frequency filtering is chosen based on natural frequencies of the monitored structure. the eigenvalues of the structure are determined using operational modal analysis (oma), and are about 6.7 hz and 7.7 hz for the first two relevant eigenmodes [14]. for further analysis, fcutoff is chosen as 6.0 hz. the tdf threshold values for response dependent characterization of bles identified from structural monitoring data are derived from proof load testing of the bridge. multiple runs with three different proof load vehicles with known weights and dimensions are 153 marcel nowak, oliver fischer acta polytechnica ctu proceedings 0 2 4 6 8 10 12 transverse position on roadway (m) 0.0 0.2 0.4 0.6 0.8 1.0 f t d f ,k |4 2 l c st a n d ir m uc , l an e 1 l c st a n d ir m uc , l an e 2 l c m od ,u n i2 / bi 4 d ir m uc , l an e 1 l c m od ,u n i2 / bi 4 d ir m uc , l an e 2 l c m od ,b i4 d ir de g, la ne 2 l c m od ,b i4 d ir de g, la ne 1 proof load vehicles (fictitious) regular truck vehicles figure 6. tdfs ft df,k|4-2 from proof load testing and evaluation of regular truck vehicles with reference influence line (ril). vehicle axle weight (t) spacing (m) crane 1 12.0/12.0/12.0/12.0 1.65/1.90/1.75 crane 2 11.6/11.5/12.6/12.5 1.65/2.05/1.65 truck 7.9/8.7/9.4/6.1 1.35/3.00/1.80 table 1. parameters of proof load vehicles. performed (refer to table 1). based on the rils derived from the strain data of the proof load events [5], additional fictitious load patterns representing truck vehicles from regular traffic are investigated (refer to table 2). theoretically, tdfs can be evaluated for any set of tdf-sensors at same longitudinal but different transverse positions, according to equation (1). for the scope of this work, the set comprising sensors 4-2 and 4-4 proved to be a proper choice for tdf-sensors able to differentiate clearly between the several transverse position in different traffic lanes for different lane configurations (refer to figure 6 for tdfs ft df,k|4-2, values for ft df,k|4-4 complement to 1). for all considered response-sensors k ∈ [4-2, 4-3, 4-4], instants tεk,max are nearly identical, and therefore ft df,k|i result in equal values. 3.4. traffic monitoring besides the previously described structural monitoring, an additional traffic monitoring unit is installed. this unit consists of a laser scanner for vehicle detection, counting, and detailed classification and a webcam providing image data of the traffic flow on the bridge for comparison and verification purposes. the monitoring unit is placed on top of a telescoping mast right next to the northern superstructure in span no. 4, about 8 m above the top of roadway (see figure 7). due to lateral installation of the monitoring unit, this height is necessary to avoid shadowing id axle weight (t) spacing (m) 11 9.0/11.0 4.5 12 6.8/7.6/5.5 4.3/1.3 1111 10.3/14.9/7.6/7.2 4.9/6.5/5.0 1211 8.4/10.3/6.4/7.8/7.1 4.6/1.3/5.2/4.6 112 12.2/12.4/7.6/7.8 3.7/6.6/1.3 113 8.3/11.2/6.8/6.8/6.8 3.7/5.6/1.3/1.3 14 12.0 (all) 2.6/1.7/1.7/1.7 222 12.0 (all) 1.9/2.9/1.7/2.6/1.7 111 6.0/8.3/10.8 5.9/6.0 11 7.2/12.8 5.9 table 2. parameters of regular truck vehicles (data adopted from [1, 4], id corresponds to sequence of axle groups with number of axles per group). effects and provide the best possible quality of vehicle detection and classification. figure 7. traffic monitoring unit next to bridge. the laser scanner used in the measurement unit is the profiling system tic501 from sick ag. its 2d light detection and ranging (lidar) sensor works with multiple echo technology, making it robust against atmospheric exposure. it detects the travel 154 vol. 36/2022 on the effects of modified lane configuration lane of the vehicles and scans passing vehicles up to 100 times per second, creating a 3d point cloud of the vehicles’ silhouette serving as a base for further classification. the profiling system can cover up to four traffic lanes. furthermore, vehicle dimensions and velocity are recorded, as well as the timestamp of entrance to and exit from the scanning section allowing for determination of inter-vehicle gaps and reconstruction of actual traffic flow. [15] 4. results from monitoring data analysis for the scope of this work, complete daily data sets for the different lane configurations for the following periods are available: • lcstand: 22.11.2019 until 18.12.2019 • lcmod,uni2: 25.10.2019 until 18.11.2019 • lcmod,bi4: 11.10.2019 until 23.10.2019 for further analysis, only data from working days within the mentioned periods is considered. 4.1. multiple truck presence on the bridge as previously stated in section 3.2, the possibility of overtaking events in the same driving direction for lcmod,uni2 and lcmod,bi4 can be excluded due to spatial constraints by the reduced roadway width. analysis of the traffic monitoring data for the different lane configurations confirms this statement (figure 8). -101 0 101 102 103 < 16 m lcstan, dir. muc < 16 m lcmod, uni2, dir. muc -101 0 101 102 103 -101 0 101 102 103 < 16 m lcmod, bi4, dir. muc -101 0 101 102 103 < 16 m lcmod, bi4, dir. deg distance to preceding vehicle (m) di st an ce to fo llo w in g ve hi cle (m ) figure 8. spatial gaps of vehicles on fast lane to adjacent vehicles in slow lane, for different lane configurations. for each truck traveling on the fast lane, the spatial gap to its preceding (distance from front of vehicle in fast lane to end of vehicle in slow lane) and following lcstan lcmod, uni2 lcmod, bi4 0.0 0.2 0.4 0.6 0.8 1.0 f t d f ,k |4 2 muc-l1 mpe-pe muc-l2 0.50 0.25 muc-l1&2 mpe-me(cv) deg-l1&2(cv) 0.65 0.35 muc-l1&2 mpe-me deg-l1&2 0.65 0.35 proof load vehicles (fictitious) regular truck vehicles figure 9. definition of threshold values for mpe identification for different lane configurations. (distance from end of vehicle in fast lane to front of vehicle in slow lane) truck on the slow lane is determined, based on timestamps and velocities from the laser scanner data record. if the vehicle in the fast lane is overtaking another vehicle on the slow lane while crossing the bridge, either of these spatial gaps takes a small value (threshold value of 16 m corresponding to span width of bridge structure). the results in figure 8 show that in general, there are only very few vehicles traveling in the left lane for lcmod,uni2 and lcmod,bi4 being hardly part of an overtaking event. for lcstan, it can be seen, however, that significantly more vehicles travel on the fast lane and that the vast majority of them are overtaking other vehicles. based on the evaluation of tdfs for proof load testings and fictitious regular truck vehicles (see section 3.3), and explanations regarding possible types of multiple presence events (mpes) for the different lane configurations (see section 3.2), appropriate threshold values for ft df,k|4-2 are defined to allow for mpe identification (refer to figure 9). for the different lane configurations, following classifications of bles are specified: • lcstan: sves on lane 1 or lane 2 (muc-l1 and muc-l2 ), mpes as overtaking events (mpe-oe) • lcmod,uni2: sves on direction muc (muc-l1&2 ), sves due to possible construction vehicles on direction deg (deg-l1&2 ), mpes as meeting events with regular traffic on direction muc and possible construction vehicles on direction deg (mpeme(cv)) • lcmod,bi4: sves on direction muc and direction deg (muc-l1&2 and deg-l1&2 ), mpes as meeting events (mpe-me) due to spatial constraints for lcmod,uni2 and lcmod,bi4, and the resulting decreased probability of trucks travelling in the fast lane, no differentiation is made for these lane configurations for vehicles travelling either in lane 1 or lane 2 within the same driving direction. 155 marcel nowak, oliver fischer acta polytechnica ctu proceedings 0 50 100 ε 4 2 ·1 0 6 lcstan threshold for multiple presence events all bridge loading events block maxima events (block size 6h) lcmod, uni2 lcmod, bi4 0 50 100 ε 4 3 ·1 0 6 0.0 0.2 0.4 0.6 0.8 1.0 0 50 100 ε 4 4 ·1 0 6 0.0 0.2 0.4 0.6 0.8 1.0 ftdf, k|4 2 0.0 0.2 0.4 0.6 0.8 1.0 figure 10. tdfs ft df,k|4-2 evaluated for all bles identified from structural monitoring data of working days (bm events with blocks size 6 h highlighted), for different lane configurations and considered response-sensors. muc-l1 mpe-oe muc-l2 0 50 100 muc-l1 mpe-oe muc-l2 traffic monitoring structural monitoring (4-2) structural monitoring (4-3) structural monitoring (4-4) muc-l1&2 mpe-me deg-l1&2 0 50 100 muc-l1&2 mpe-me deg-l1&2 m ea n ra te p er h ou r figure 11. mean hourly occurrence rates of different ble types for lane configurations lcstan and lcmod,bi4, evaluated for working days. the tdf evaluation for the three different lane configurations is shown in figure 10. based on the available data from structural monitoring, relevant bles are identified and classified according to the previously defined criteria for tdfs. the results generally confirm the choice of tdf-thresholds for mpe identification. for all three lane configurations, dense accumulations of data points can be observed beyond the thresholds, representing the respective sves dominating (in the sense of occurrence rates) the bridge loading process in all cases. mpe occurrence in significant numbers is observed for lane configurations lcstan (overtaking events) and lcmod,bi4 (meeting events). for lane configuration lcmod,uni2, the vast majority of bles are sves on driving direction muc. however, due to the occasional passing of construction vehicles on the closed part of the roadway, small numbers of sves on driving direction deg and mpes (meeting events) can be observed. for further quantification of mpe occurrence, the mean hourly occurrence rate of sves and multiple truck presence on the bridge is evaluated and compared for lane configurations lcstan and lcmod,bi4 (figure 11). while slightly differing in absolute numbers, the occurrence rates obtained from traffic monitoring (based on data from laser scanner) and structural monitoring (based on data from strain sensors) show similar tendencies. it can be observed that mpe occurrence rates are significantly higher for bidirectional traffic (meeting events) than for unidirectional traffic (overtaking events), but still relatively small compared to occurrence rates of sves. 156 vol. 36/2022 on the effects of modified lane configuration 0 2 4 k = 4 2 lcstan bm mixed bm for muc-l1 / muc-l1&2 bm for mpe-pe / mpe-me bm for muc-l2 / deg-l1&2 lcmod, uni2 lcmod, bi4 0 2 4 k = 4 3 0 20 40 60 80 100 0 2 4 k = 4 4 0 20 40 60 80 100 εk · 10−6 0 20 40 60 80 100 − lo g( − lo g( c d f )) figure 12. bm data (block size 6 h) of working days for considered sensors and different lane configurations; with and without differentiation of different ble types. 4.2. load effects and extreme value behavior for analysis of the resulting load effects and the impact of multiple truck presence, bm data (block size 6 h) is obtained from the structural monitoring data for the different lane configurations, with and without differentiation of different ble types (refer to figure 12). the bm data reveals only for a few cases actual significance of mpes, e.g. for sensor 4-4 in lcstan. in most cases, the bridge loading process is dominated by sves, primarily by one sve type alone (e.g. muc-l1 for sensor 4-2 in lcmod,stan) or sporadically by a mixture of two sve types (e.g. muc-l1&2 and deg-l1&2 for sensor 4-3 in lcmod,bi4). these observations are supported by illustrations in figure 10 for the highlighted bm data. regarding lane configuration lcmod,uni2 it can be observed that the regular traffic (muc-l1&2) dominates the structural response of sensors 4-2 and 4-3, while data from sensor 4-4 reveals the dominant influence of the occasional construction vehicles passing the bridge on the closed part of the roadway. concerning the absolute load level, data shows only slight variation with changing lane configurations for sensor 4-2. in contrast to that, sensor 4-3 experiences its most severe loading for lcstan, whereas sensor 4-4 faces the heaviest loading for lcmod,bi4. this has mainly to do with the change of transverse position of the traffic lanes on the roadway relative to the single sensor locations, as for all three sensors, the data shows no significant impact of mpe for lane configurations leading to the most adverse loading. regarding the extreme value behavior, extrapolated values with a return period of 1000 years are determined applying the different methods mentioned in section 2.3. due to the limited amount of available method ε4−2 ε4−3 ε4−4 lcstan bm-mix 146.9 144.6 74.3 bm-comp 146.9 154.7 89.1 lcc 158.9 156.1 121.6 lcmod,uni2 (incl. cv) bm-mix 110.8 75.1 342.2 bm-comp − − − lcc 130.3 92.4 114.7 lcmod,uni2 (excl. cv) bm-mix 110.8 75.3 33.1 bm-comp 110.8 75.3 33.1 lcc 130.3 92.5 42.3 lcmod,bi4 bm-mix 112.0 83.4 111.5 bm-comp 111.6 90.3 111.5 lcc 145.2 122.2 149.4 table 3. extreme value estimates with different methods for 1000 years return period. data for the bm method, a choice of a usual block size of 1 d is not feasible. however, a block size chosen too small (e.g. 1 h) might not fulfill the requirement of statistical independence of the different bm events necessary for application of bm method for eva. as a compromise, a block size of 6 h is chosen for the scope of this work. for lane configuration lcmod,uni2, extrapolation is performed for both cases considering and neglecting bles resulting from construction vehicles on the closed part of the roadway. the extrapolation results are shown in table 3. it can be observed that both bm methods with (bm-comp) and without (bm-mix) differentiation of different ble types lead to quite similar results in many cases. the reason for this is that the bridge loading process is dominated by a single ble type in most cases, as previously explained. both bm meth157 marcel nowak, oliver fischer acta polytechnica ctu proceedings ods face problems for lcmod,uni2 when considering bles due to occasional construction vehicles. due to their irregular occurrence on the bridge, the related ble types do not occur in every block, making application of method bm-comp unfeasible. moreover, structural response at sensor 4-4 is dominated by two different ble types in this lane configuration, causing ill-conditioning for the method bm-mix. in general, the most robust results seem to be provided by lcc method, which makes more use of the data by considering each loading event, and not just a few per block. a tendency of slightly higher extrapolated values compared to bm methods can be observed. altogether, the previously identified tendencies for the measured load effects are approximately reflected in the results of eva again. also, the importance of ble due to occasional construction vehicles for sensor 4-4 in lcmod,uni2 is clearly shown. however, in parts significant variation of results between the different methods of eva can be observed. 4.3. discussion of results analysis of traffic flow characteristics and resulting occurrence rates of multiple truck presence on the bridge show that the modified lane configurations have a contrastive impact. on the one hand side, reduced available roadway width leads to a disappearance of overtaking events compared to the standard lane configuration. however, in the case of bidirectional traffic on the structure, additional multiple presence events occur in the form of meeting events. the mean occurrence rates of meeting events in bidirectional traffic are significantly higher than those of overtaking events in unconstrained unidirectional traffic. this can be explained by the different volumes of available truck vehicles potentially involved in the respective mpes. while in unconstrained unidirectional traffic only relatively few truck vehicles travel on the left lane and possibly are part of an overtaking event, a significantly larger number of truck vehicles passes the bridge in the opposite direction in bidirectional traffic that potentially cause a meeting event. the correlation effect of vehicles involved in overtaking events – if a truck travels on the left lane, it is very likely part of an overtaking event instead of crossing the bridge alone (data analysis shows that 65 % to 75 % of trucks traveling in the fast lane are involved in an overtaking event) – usually would lead to a higher occurrence rate. however, this effect is outweighed in this case by the significant increase in the volume of possibly involved truck vehicles in meeting events (even though here the vehicles are entirely uncorrelated). regarding the resulting load effects, it is found that modifications of the lane configuration on the bridge noticeably alter the characteristics of the bridge loading process. however, the related change in occurrence rates of multiple presence events has only for a few cases of considered load effects and lane configurations significant impact on the extreme value behavior. due to the distinct biaxial structural behavior of the considered bridge structure in combination with the limited span width compared to the usual length of regular truck vehicles, it is rather the change of transverse position of the traffic lanes on the roadway playing a more dominant role compared to multiple truck presence. therefore, the loading process for the different lane configurations is dominated by single vehicle events in most scenarios. for bridges with different dimensions and structural systems (uniaxial rather than biaxial structural behavior of superstructure), opposite characteristics are expected, with multiple truck presence playing a more dominant role in the loading process. a particular case is observed in the study for the modified unidirectional two-lane configuration, with occasional bridge crossings of construction vehicles on that part of the roadway closed to regular traffic. even though their occurrence is sparse, they dominate the loading process for specific load effects. concerning the results of extreme value analysis, in parts significant variation of results between different extrapolation methods can be observed. this raises the question of accuracy of the different extrapolation methods and their appropriateness for application in this study, especially in the context of the relatively small extent of available monitoring data and the limited degree of knowledge on the actual load impact leading to the measured structural response (knowledge on underlying loading processes). the lcc method with fit of rice’s formula makes the best use of the available data by considering each loading event and seems to provide the most robust results over the range of investigated load effects and lane configurations. however, the evaluation of the accuracy of the individual extrapolation results is beyond the scope of this study. 5. conclusions the presented study analyzes data from bridge monitoring along federal highway a92 in southern germany. the data covers standard and different modified lane configurations due to construction works. regarding the impact on multiple truck presence on the bridge, the results from the study confirm the basic assumption from code background works that a realistic, reduced width of the fast lane in modified lane configurations effectively prevents truck vehicles from overtaking. analysis of the monitoring data proves an almost complete disappearance of overtaking events for both modified configurations in this study. furthermore, it is found that the mean occurrence rates of meeting events in bidirectional traffic are significantly higher than those of overtaking events in unconstrained unidirectional traffic. this observation can be considered representative for typical highway traffic characterized by small ratios of truck vehicles traveling on the left lane compared to the right lane (for unconstrained unidirectional traffic) and similar 158 vol. 36/2022 on the effects of modified lane configuration truck traffic volumes for both driving directions (for bidirectional traffic). how the change of lane configuration alters the bridge loading process heavily depends on its governing aspects, which are determined by the characteristics of the bridge structure itself. for the bridge in this study (distinct biaxial structural behavior, short span width), single vehicle events play the dominant role in most scenarios. the loading processes for the considered load effects are governed rather by the change of transverse position of the traffic lanes on the roadway than by multiple truck presence on the bridge. therefore, it is crucial for traffic load modeling to accurately represent the single truck vehicles’ parameters (i.e. axle weights and spacings) and their transverse position on the roadway. in the particular case of a lane configuration with partial closure of the roadway, it is either essential to consider possible construction vehicles in this part or to ensure that the closed part of the roadway is, in fact, free of any type of vehicle (e.g. in case of partial roadway closure as a constraining measure to reduce the level of traffic loading on the bridge). extreme value analysis proves to be challenging due to the relatively small extent of available data and limited knowledge on underlying loading processes. application of level crossing counting method with fit of rice’s formula and classification of bridge loading events based on transverse distribution factors towards composite distribution models for block maxima aim to make the best use of the available data. nevertheless, results from different extrapolation methods show considerable variation, and straightforward identification of the most accurate and reliable results is not feasible. therefore, statistical extrapolation is to be performed with great care. further research efforts beyond the scope of this paper are necessary, i.e. towards adopting techniques allowing for comparing the accuracy of the results from the different extrapolation methods based on a given data set of limited extent. further future research efforts should focus on extending investigations on the impact of modified lane configuration on the bridge loading process to a broader range of different types of bridge structures. especially for bridge superstructures with a rather uniaxial than biaxial structural behavior, multiple truck presence is expected to play a more dominant role in the loading process. moreover, a more realistic consideration of scenarios with modified lane configurations could exploit further potential in the scope of object-specific traffic load models. by quantifying the expected occurrence periods of different modified lane configurations throughout regular bridge lifetime, and consistent consideration in bridge load modeling besides the standard configuration based on findings from this study, a more accurate estimate of extreme bridge loading throughout the service life of a bridge structure can be obtained. acknowledgements the research this work is based on is funded by die autobahn gmbh des bundes, branch southern bavaria. references [1] u. freundt, s. böning. teil 1: din-fb 101: einwirkungen auf brücken. in anpassung von din-fachberichten brücken an eurocodes, berichte der bundesanstalt für straßenwesen: brückenund ingenieurbau heft b 77, pp. 13–104. bundesanstalt für straßenwesen, bergisch gladbach, 2011. [2] u. freundt, s. böning. verkehrslastmodelle für die nachrechnung von straßenbrücken im bestand. berichte der bundesanstalt für straßenwesen: brückenund ingenieurbau heft b 82. bundesanstalt für straßenwesen, bergisch gladbach, 2011. [3] u. freundt, s. böning. einfluss der veränderten verkehrsführung bei ertüchtigungsmaßnahmen auf die bauwerksbeanspruchungen. berichte der bundesanstalt für straßenwesen: brückenund ingenieurbau heft b 97. bundesanstalt für straßenwesen, bergisch gladbach, 2013. [4] m. nowak, o. fischer, a. müller. realitätsnahe verkehrslastansätze für die nachrechnung der gänstorbrücke über die donau. betonund stahlbetonbau 115(2):91–105, 2019. https://doi.org/10.1002/best.201900060. [5] m. nowak, o. fischer. estimation of truck weights based on strain measurements from tendons of a post-tensioned concrete bridge. in life-cycle civil engineering: innovation, theory and practice, pp. 1667–1676. crc press, 2021. https://doi.org/10.1201/9780429343292-227. [6] c. c. caprani, e. j. obrien, g. j. mclachlan. characteristic traffic load effects from a mixture of loading events on short to medium span bridges. structural safety 30(5):394–404, 2008. https://doi.org/10.1016/j.strusafe.2006.11.006. [7] m. nowak, f. schmidt, o. fischer. comparison of measured and simulated traffic loading based on bwim data from the millau viaduct. in international probabilistic workshop ipw 2020. 2020. https://doi.org/10.1007/978-3-030-73616-3_16. [8] c. c. caprani. probabilistic analysis of highway bridge traffic loading. ph.d. thesis, national university of ireland, university college dublin, school of architecture, landscape and civil engineering, 2005. [9] a. žnidarič. influence of number and quality of weigh-in-motion data on evaluation of load effects on bridges: doctoral thesis. ph.d. thesis, univerza v ljubljani, fakulteta za gradbeništvo in geodezijo, 2017. [10] m. nowak, d. straub, o. fischer. statistical extrapolation for extreme traffic load effect estimation on bridges. in 14th international probabilistic workshop, pp. 135–153. springer international publishing, 2016. https://doi.org/10.1007/978-3-319-47886-9_10. [11] m. nowak, d. straub, o. fischer. extended extrapolation methods for robust estimates of extreme traffic load effects on bridges. in proc. 6th international symposium on life-cycle civil engineering (ialcce). 2018. https://doi.org/10.1201/9781315228914. 159 https://doi.org/10.1002/best.201900060 https://doi.org/10.1201/9780429343292-227 https://doi.org/10.1016/j.strusafe.2006.11.006 https://doi.org/10.1007/978-3-030-73616-3_16 https://doi.org/10.1007/978-3-319-47886-9_10 https://doi.org/10.1201/9781315228914 marcel nowak, oliver fischer acta polytechnica ctu proceedings [12] s. coles. an introduction to statistical modeling of extreme values. springer london, 2001. https://doi.org/10.1007/978-1-4471-3675-0. [13] c. cremona. optimal extrapolation of traffic load effects. structural safety 23(1):31–46, 2001. https://doi.org/10.1016/s0167-4730(00)00024-2. [14] l. r. casas. evaluation of dynamic bridge excitation bypassing vehicles based on modal contribution analysis. master’s thesis, technische universität münchen, lehrstuhl für massivbau, 2020. [15] sick ag. profiling-systeme tic102, tic501, tic502: zuverlässige fahrzeugklassifizierung über mehrere spuren, 2017. 160 https://doi.org/10.1007/978-1-4471-3675-0 https://doi.org/10.1016/s0167-4730(00)00024-2 acta polytechnica ctu proceedings 36:149–160, 2022 1 introduction 2 methods 2.1 signal processing 2.2 mulitple truck presence 2.3 extreme value analysis 3 bridge structure and monitoring 3.1 bridge structure 29/1 3.2 alternating lane configurations 3.3 structural monitoring 3.4 traffic monitoring 4 results from monitoring data analysis 4.1 multiple truck presence on the bridge 4.2 load effects and extreme value behavior 4.3 discussion of results 5 conclusions acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2023.41.0083 acta polytechnica ctu proceedings 41:83–88, 2023 © 2023 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague design of a network of cycling routes in the kutná hora district lukáš vacka∗, pavel vrtal, tomáš kohout czech technical university in prague, faculty of transportation sciences, konviktská 20, 110 00 prague 1, czech republic ∗ corresponding author: vackaluk@fd.cvut.cz abstract. cycling is a dynamically expanding transport mode of today. at the same time, cycling is an alternative to individual car transport as well as to public transport. the basis of a good cycling infrastructure is a comprehensive network of safe cycling routes linking transport sources and destinations in the area. the partial objectives of the project are to evaluate the management of the existing cycle routes according to the way cyclists are guided and according to the surface of the individual route sections, as well as to determine the basic sources and destinations. the main objectives of the project are to design a network of bicycle routes with a focus on keeping them separate from motorized traffic and linking the identified resources and destinations, as well as determining the difficulty of each newly proposed route. keywords: cyclist transport, path for cyclists and pedestrian, path for cyclists, cycle traffic safety, cycle route management, path for cyclists categorization. 1. introduction cycling is a dynamically expanding transport mode of today. the european union is aware of this trend and annually releases funds for the development of cycling in individual eu countries. cycling is an alternative to individual car transport as well as to public transport. cycling has many benefits. it helps to reduce the risk of various illnesses, such as diabetes, some forms of cancer, cardiovascular diseases, and depression. cycling not only improves physical health, but also has a positive impact on mental health and subjective well-being. cycling takes you from door to door, it offers individuality, flexibility, and freedom (ample opportunity for adapting travel times, speed and routes). if people switch from a car to a bicycle, it saves 150 g of co2 per kilometre and 0.2 g of nox per kilometre and 0.01 g of particulate matter per kilometre [1]. this article discusses the design of a network of cycling routes in the kutná hora district. first, it evaluates the current state of cycling routes, then it maps the significant sources and destinations of transport in the area, then tries to link the identified sources and destinations, and finally determines the difficulty of the individual newly proposed cycle routes. the kutná hora district is located in the south-eastern part of the central bohemia region (nuts ii) [2]. in terms of geomorphology, the district is located between the elbe and sázava rivers. the surface is considered to be rather hilly, due to the occurrence of the hornosázava hills, the čáslav basin and the polabská lowland. the district covers an area of 917 km2. in terms of cycling, it is not a welldeveloped district, as there are only 19 marked cycle routes, which are also largely guided on class ii and iii roads. the potential in the form of a varied and very attractive surface for cyclists is not fulfilled. most of the class iii roads are relatively bicycle-friendly with their low volumes of car traffic. however, due to the latest road traffic rules, which require the driver of a motor vehicle to leave a minimum lateral separation of 1.5m [3] when overtaking a cyclist, the routing of cycle routes no longer appears to be entirely safe even on these roads. figure 1 shows unsuitable sections of cycle routes that are integrated with high volume car traffic. 2. analysis of the current state of cycling transport there are in total 19 marked cycling routes in the kutná hora district. most of them are integrated with car traffic on roads of ii. and iii. class. the territory of the district is crossed by the european eurovelo 4 cycle route, the spinal long-distance class i cycle route no. 1 and the long-distance class ii cycle route no. 19. the other cycle routes are considered to be class iv cycle routes. table 1 lists the individual cycle routes in the territory of the kutná hora district. the first column indicates the number of the cycle route. the second column shows the individual municipalities through which a given cycle route runs through. the third column presents the total length of the cycle route in the kutná hora district [5]. the sections of the individual cycle routes are divided into three categories in terms of the way they are routed: integrated, calmed and protected. an integrated cycle route is a cycle route shared with car traffic where the speed of car traffic is higher than 40 kmh−1. a calmed cycle route is a cycle 83 https://doi.org/10.14311/app.2023.41.0083 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en l. vacka, p. vrtal, t. kohout acta polytechnica ctu proceedings figure 1. sections of class ii roads currently used for cycling routes, and their intensities [4]. cycle cycle route management length [m] route number ev4 hořany, kutná hora, neškaredice, třebešice, čáslav, žáky, 30087 tupadly, potěhy, horky, hostovlice, zehuby 1 solopysky, dobřeň, karlov t., doubrava, tuchotice, nová lhota, 40356 bylany, kutná hora, vrchlice, neškaredice, třebešice, čáslav, žáky, tupadly, potěhy, horky, hostovlice, zehuby 0128 svatá kateřina, nové dvory, ovčáry, jakub, církvice, třebešice 10383 0106 hořany, kutná hora, vrchlice, poličany, malešov, roztěž, albrechtice, 42415 bedřichov, miletice, nepoměřice, žandov, sudějov, janovická lhota, uhlířské janovice, silvánka, smilovice, úžice, talmberk 0117 čáslav, tupadly, schořov, bratčice, hostovlice 15658 0097 chlístovice, týniště, bykáň, krupá, úmonín, lomec, lomeček, 24086 paběnice, újezdec, petrovice i, michalovice, třebětín, tasice 0120 vavřinec, chmeliště, uhlířské janovice, rašovice 11708 0127 úžice, smrk 5635 19 sázava, talmberk, rataje nad sázavou, ježovice, podveky, zalíbená, otryby, 57531 český šternberk, radonice, zdebuzeves, tichonice, kácov, zliv, holšice, chabeřice, zruč nad sázavou, horka ii, březina, vlastějovice, pavlovice 0118 čáslav, filipov, markovice, žleby, zehuby 7082 0098 černíny, opatovice i, červené janovice, zhoř, bludov, tasice 18037 0096 malešov, chlístovice, zdeslavice, švábínov, rápošov, zbraslavice, radvančice, 35090 řeplice, bohdaneč, kotoučov, tasice, bělá, jedlá 0099 chotoměřice t., pančava, krasoňovice, hodkov, 18490 hranice, ostrov, bohdaneč, dvorecko 0121 žandov, pivnisko, kamenná lhota, morány, čestín 7255 0122 podveky, zbizuby, nechyba, vlková, petrovice ii, kasanice, čestín, krasoňovice 15842 0119 mirošovice, opatovice ii, mitrov, kochánov, janovická lhota 9808 0123 zbizuby, vranice, kácov 6323 table 1. management of existing cycle routes. 84 vol. 41/2023 design of a network of cycling routes figure 2. the current state of cycling routes [6]. route guided on less congested roads with speeds of 40 kmh−1 or less. protected cycle route guidance is the guidance of cyclists on dirt, forest or paths for cyclists or on roads with mixed pedestrian and cycling traffic. figure 2 shows the current state of cycle routes in the kutná hora district. the subsections are coloured according to the distribution defined above. integrated sections are red, calmed sections are blue and protected sections are green. the total length of the network of cycling routes in the kutná hora district is 356 km. the length of integrated sections is 298 km, the length of calmed sections is 55 km and the length of protected sections is 3 km. the integrated sections therefore account for almost 84% of the total network, which is a very unsatisfactory and dangerous state. 3. design of cycling roads cycling roads are designed in accordance with čsn 73 6110 (design of local roads), especially according to tp 179 (design of roads for cyclists), which specify the requirements for the design of roads in terms of cycling traffic. cycling roads are designed to be safe and comfortable for bicycle use. the cyclist is a full participant in traffic, so the principle of selfexplanatory and forgiving roads must also be applied to cyclists [7]. cycling measures should logically build on each other and the cyclist should not be confused. in general, each cyclist measure should be continuously maintained for as long a distance as possible. frequent changing of cycling measures reduces the safety of cycling [8, 9]. in recent years, two possible ways of guiding cyclists have come into conflict. guiding cyclists in an integrated way with car traffic and guiding cyclists together with pedestrians on the path for pedestrians and cyclists. modern bicycles and electric bicycles reach speeds of 30–40 kmh−1 on the plain. these speeds are relatively high in shared space with pedestrians and already very dangerous for pedestrians. in integrated lines with car traffic, on the other hand, these speeds are still low and therefore dangerous for cyclists themselves. for this reason, only separate cycle paths should be implemented in the extravilan sections. when designing a network of bicycle routes, it is important to identify key sources and destinations that are attractive to bicycle traffic. the most important sources and destinations are primarily all cities, towns, and villages. the mapping process identifies the largest catchment centres to which linkages should be provided from adjacent smaller communities and between which a basic network of spinal cycle routes should be established. at the district level, cycling does not only represent a traffic function. it is more of a tourist, recreational and sporting function, so the design of the cycle route network should also take into account leisure destinations. leisure destinations include castles, chateaux, churches, fortresses, ruins, water surfaces, rock formations, recreation centres, golf resorts, lookout towers, museums, children’s parks, breweries. the first step of designing the network of cycle routes was to identify the largest catchment villages, among which an aim was to find the most direct spinal routes leading on forest and dirt roads. the czech republic’s advantage over other european countries is undoubtedly the possibility of unlimited use of forest and dirt roads. in foreign countries, these routes are usually private and completely inaccessible. the possibility 85 l. vacka, p. vrtal, t. kohout acta polytechnica ctu proceedings figure 3. cycling routes management in the new concept [6]. of running cycle routes on these roads is therefore out of the question. in our country, if we can negotiate with individual road managers to agree to mark out a cycle route on their road and provide a suitable surface for all types of bicycles, newly proposed protected cycle route corridors can be implemented with a significantly lower cost than would be in the case for completely newly designed roads in the open countryside. of course, the ensuring (in the context of financial investment) of adequate surfacing on these routes would not be provided by the administrator from its own resources but could be helped by subsidies from the european union, that makes substantial sums available each year for the development of cycling. kutná hora, čáslav, zbraslavice, zruč nad sázavou, uhlířské janovice, kácov and rataje nad sázavou were identified as the most important municipalities among which the spinal cycling routes run. for each municipality, the aim was to ensure that the individual routes led to the most important directions. 4. description of the newly proposed cycling routes the new concept of cycling routes in the territory of the kutná hora district counts a total of 40 cycling routes. 7 of them were determined to be spinal. the remaining cycling routes are complementary. the main aim was to find the most direct routes among the largest centres of the district leading on the forest and dirt roads and also to connect the identified sources and destinations. the alignment of the newly proposed cycle routes is represented in table 2. the newly proposed cycle routes can be seen in figure 3. the total length of the newly proposed cycle route network is 593 km. integrated sections amount to 176 km, calmed sections amount to 92 km and protected sections amount to 325 km. it can be seen that there has been an increase in the total length of cycle routes and a significant reduction in integrated sections, which increases the safety and overall attractiveness of the kutná hora district cycle network. almost 71% of the total length is made up of calmed and protected sections. this is an essential fact, because it must not be forgotten that the cyclist is one of the most vulnerable road users [10]. the newly proposed cycle routes were divided into three categories in terms of slope parameters. if the maximum longitudinal slope on a specific cycle route does not exceed 6% or exceeds it only slightly on a negligible number of sections, the cycle route is suitable for all cyclists, including families with children. the second category consists of cycle routes with a maximum longitudinal slope in the range of (6.8〉%. these routes are intended for normal adult cyclists. the last category consists of cycle routes with a maximum longitudinal slope exceeding 8%. these routes are intended for fit, trained cyclists. figure 4 shows all the newly proposed cycle routes specified by colour according to the above classification. cycle routes suitable for all cyclists, including families with children, are coloured green, cycle routes for normal adult cyclists are coloured yellow and cycle routes for fit, trained cyclists are coloured red. 5. conclusion cycling should be considered as a separate mode of transport. managing cycling traffic in an integrated 86 vol. 41/2023 design of a network of cycling routes cycle cycle route management length [m] route number ev4 hořany, kutná hora, neškaredice, třebešice, 32927 čáslav, žleby, ronov nad doubravou 1 solopysky, dobřeň, karlov t., doubrava, tuchotice„ 43307 nová lhota, bylany, kutná hora vrchlice, neškaredice, třebešice, čáslav, žleby, ronov nad doubravou 19 sázava, mrchojedy, ledečko, rataje nad sázavou, malovidy, 59055 český šternberk, soběšín, kácov, zliv, holšice, čížov, chabeřice, zruč nad sázavou, vlastějovice, budčice, chřenovice 0066 čáslav, močovice, krchleby, vodranty, nová lhota, úmonín, bykáň, malešov 16542 0091 kutná hora, přítoky, kutná hora 8790 0096 kutná hora, malešov, hetlín, zbraslavice, chotoměřice t., 34265 pančava, hodkov, želivec, zruč nad sázavou 0097 třebětín, michalovice, újezdec, paběnice, lány, lomeček, lomec 15284 0098 bělá, tasice, červené janovice 12613 0099 hranice, bohdaneč, řeplice 12522 0106 mrchojedy, talmberk, úžice, smilovice, mirošovice, 36294 uhlířské janovice, rašovice, onomyšl, vidice, nová lhota, bylany, kutná hora, červené pečky, nebovidy, kolín 0111 bělá, dvorecko 2373 0117 čáslav, schořov, bratčice, hostovlice, žleby 17372 0119 opatovice ii, malejovice, podmoky 5043 0120 zbraslavice, janovická lhota, uhlířské janovice, chmeliště, vavřinec, církvice 17029 0121 onomyšl, žandov, kamenná lhota 11177 0122 petrovice ii, kasanice, čestín 2587 0123 zbizuby, vranice, kácov 6329 0127 úžice, smrk, skvrňov, církvice 5635 0128 třebešice, církvice, jakub, ovčáry, nové dvory, starý kolín 12844 0199 zruč nad sázavou, řendějov, polipsy, kácov 10302 0200 bylany, miskovice, mezholezy, vysoká, suchdol 6302 0201 miskovice, červené pečky 3483 0299 kácov, račíněves, čestín, chotoměřice t. pančava, zbraslavice, 32932 radvančice, zhoř, chvalov, plhov, paběnice, hraběšín, březí, schořov 0700 rataje nad sázavou, mirošovice, opatovice ii 9391 0701 rataje nad sázavou, útěchvosty, podveky, zbizuby, vestec, 34985 hroznice, vlková, nové nespeřice, stará huť, kamenná lhota, vernýřov, černíny, opatovice i, korotice, úmonín 0702 uhlířské janovice, staré nespeřice, stará huť, 15783 tlučeň, petrovice ii, losiny, kácov 0800 kutná hora, nové dvory, svatý mikuláš, habrkovice, 37109 záboří nad labem, bernardov, horušice, brambory, semtěš, starkoč, zbyslav 0801 čáslav, chotusice, rohozec, svatý mikuláš 11099 0802 kutná hora, hlízov 7999 0803 jakub, rohozec 2581 0804 rohozec, žehušice, horušice 5993 0805 kutná hora, vrchlice, poličany, křesetice, krupá, bykáň 5655 0900 čáslav, vrdy, dolní bučice, zbyslav, zaříčany, brambory 16064 0901 vodranty, hraběšín, damírov, zbýšov, klucké chvalovice 15993 0902 horky, vrdy 4967 0903 třebešice, kluky, olšany, nová lhota 7000 0904 nová lhota, souňov, lomec, korotice 6118 0905 čáslav, žáky, krchleby 5788 0906 michalovice, dědice, dobrovítov, čejkovice, 14773 chlum, klucké chvalovice, přibyslavice zs kutná hora 3483 table 2. management of existing cycle routes. 87 l. vacka, p. vrtal, t. kohout acta polytechnica ctu proceedings figure 4. distribution of the newly proposed cycling routes according to difficulty [5]. way with car traffic in extravilan sections with higher car traffic intensities is always an inappropriate solution. a good quality cycle route network should therefore, for the most part, include protected sections where cyclists are routed away from car traffic. of course, it is not economically desirable to have 100% of cycle routes segregated from car traffic, but leaving cycling integrated with car traffic is only possible on a small number of sections with very low motorised traffic intensities. only in this case will the cycle route network be safe and attractive for potential users. the main aim of this concept was to design a cycle route network that connects all the important sources and destinations in the area while being safe, as most of the sections will be routed away from traffic. approximately 70% of the cycle routes run away from unsafe motorised traffic, the remaining 30% of the cycle routes were maintained on very low volume roads, as in these cases no alternative was found in the form of a comparably direct connection between the identified destinations on forest or dirt roads. however, psychological features highlighting the presence of cyclists (e.g. sdza19, vdzv15 with the pictogram sdza19 or vdzv18) are envisaged on these roads so that drivers of motor vehicles can anticipate a possible collision with this vulnerable road user. the new concept can be implemented in the cycling concept of the central bohemian region for the period 2017–2023 with a view to 2027 and can serve as a basis for subsequent detailed phases of designing cycling routes in the territory of the kutná hora district. the construction of a kilometre of cycle path costs approximately 3–5 million. the exact amount depends on the location of the cycle path (whether intravilan or extravilan) and the type and strength characteristics of the surface used. in this concept, the cycle paths are mainly in the extravilan area and better-quality surfaces are proposed, as heavy agricultural and forestry equipment is expected to be allowed to enter. the economic calculation therefore assumes an amount of 5 million per 1 km of cycle path. the length of the newly proposed sections to be paved is approximately 323 km. the total cost of construction is therefore czk 1 615 000 000. references [1] l. harms, m. kansen. cycling facts, 2018. [2022-08-01], https://www.government.nl/binaries/ government/documenten/reports/2018/04/01/ cycling-facts-2018/cycling+facts+2018.pdf. [2] regiony soudržnosti (nuts 2) a kraje (nuts 3) české republiky. [2022-08-01], https: //www.czso.cz/csu/czso/regiony-soudrznosti-nuts2-a-kraje-nuts-3-ceske-republiky. [3] zákon č. 361/2000 sb., o provozu na pozemních komunikacích, ve znění pozdějších předpisů. [4] openstreetmap. [2022-08-01], https://www.openstreetmap.org/. [5] cyklokoncepce pro období 2017–2023. [2022-08-01], https: //gis.kr-stredocesky.cz/js/dop_cyklokoncepce/. [6] mapy.cz. [2022-08-01], https://mapy.cz/zakladni? [7] centrum dopravního výzkumu. bezpečnostní inspekce pozemních komunikací, 2013. [8] tp 179 navrhování komunikací pro cyklisty, 2017. [9] čsn 73 6110 projektování místních komunikací, 2006. [10] z. schejbalová, t. mičunek, d. schmidt. biomechanical analysis of the dummy responses in case of child pedestrian/cyclist collision with passenger car. neural network world 23(6):609–622, 2013. https://doi.org/10.14311/nnw.2013.23.036 88 https://www.government.nl/binaries/government/documenten/reports/2018/04/01/cycling-facts-2018/cycling+facts+2018.pdf https://www.government.nl/binaries/government/documenten/reports/2018/04/01/cycling-facts-2018/cycling+facts+2018.pdf https://www.government.nl/binaries/government/documenten/reports/2018/04/01/cycling-facts-2018/cycling+facts+2018.pdf https://www.czso.cz/csu/czso/regiony-soudrznosti-nuts-2-a-kraje-nuts-3-ceske-republiky https://www.czso.cz/csu/czso/regiony-soudrznosti-nuts-2-a-kraje-nuts-3-ceske-republiky https://www.czso.cz/csu/czso/regiony-soudrznosti-nuts-2-a-kraje-nuts-3-ceske-republiky https://www.openstreetmap.org/ https://gis.kr-stredocesky.cz/js/dop_cyklokoncepce/ https://gis.kr-stredocesky.cz/js/dop_cyklokoncepce/ https://mapy.cz/zakladni? https://doi.org/10.14311/nnw.2013.23.036 acta polytechnica ctu proceedings 41:83–88, 2023 1 introduction 2 analysis of the current state of cycling transport 3 design of cycling roads 4 description of the newly proposed cycling routes 5 conclusion references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2025.53.0076 acta polytechnica ctu proceedings 53:76–81, 2025 © 2025 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague micro-scale properties of cement pastes exposed to gamma radiation jiří němeček1a,∗, andrea špakováa, jiří němeček2a, patricie halodováb a czech technical university in prague, faculty of civil engineering, department of mechanics, thákurova 7, 166 29 prague 6, czech republic b research centre řež, hlavní 130 řež, 250 68 husinec, czech republic ∗ corresponding author: jiri.nemecek@fsv.cvut.cz abstract. the extension of the lifetime of nuclear power plants is a critical issue today, raising concerns about the microscale mechanisms and mechanical changes in the individual phases of concrete that contribute to its macroscopic deterioration. accurate quantification of these changes is essential for more reliable lifetime predictions. this paper investigates the effects of gamma irradiation on cement pastes exposed to varying relative humidity levels (rh = 11–100 %) using scanning electron microscopy (sem) and nanoindentation. while sem analysis did not reveal significant phase changes through image analysis, nanoindentation highlighted substantial differences in the mechanical properties of samples exposed to either low or high humidity or submerged in water during irradiation. keywords: gamma irradiation, cement paste, humidity, nanoindentation, electron microscopy. 1. introduction concrete, one of the most widely used man-made materials globally, is essential in nuclear power plants (npps) for biological shielding, reactor vessel foundations, spent fuel storage pools and radioactive waste storage containers. however, long-term degradation due to radiation and thermal strains has been extensively documented, resulting in a decline in macroscopic mechanical properties like compressive and tensile strengths, as well as the macroscopic modulus of elasticity, as summarized by [1] and [2]. this degradation ultimately leads to the deterioration of reinforced concrete structures in npps [3] otherwise designed for 80 years of operation [4]. the primary causes of this deterioration are neutron and gamma irradiation, along with heating and drying. during its lifetime, concrete used as a biological shield is exposed to 100–200 mgy of gamma irradiation and neutron fluxes of approximately 6 × 1019 n cm−2 (with e > 0.1 mev) [1]. neutron irradiation mainly impacts the aggregates, altering their crystallographic structure by amorfization of minerals and causing their radiation-induced volumetric expansion (rive) [5], while gamma irradiation affects the cement matrix by causing radiolysis of water and drying of calcium-silicate-hydrates (c-s-h) [4]. water radiolysis is a chemical reaction in which the interstitial liquid within the c-s-h gel breaks down, leading to the formation and accumulation of gases like hydrogen (h2), other stable molecules (h2o2, h+, oh−), and highly unstable radicals (h·, oh·, e− aq). hydrogen peroxide (h2o2) then reacts with calcium hydroxide 1professor at czech technical university in prague, orcid: 0000-0002-3565-8182. 2postdoctoral researcher at czech technical university in prague, orcid: 0000-0002-5635-695x. (ch) and aft phases, forming unstable products. further reactions of these products result in the creation of calcium carbonate (caco3) [6, 7]. however, there have been relatively few studies on gamma-irradiated concrete. microscopic observations reveal that gamma irradiation decomposes evaporable water in cement pastes, and after prolonged exposure at high doses, up to 1 % of chemically bound water can also decompose [4]. during carbonation, vaterite and aragonite formed instead of calcite. vaterite filled the pores around the c-s-h gel, increasing the stiffness and bending strength in cement pastes [7]. nanoindentation at below 11 % relative humidity (rh) resulted in a 25 % increase in nanoindentation young’s modulus due to c-s-h gel densification, accompanied by microcracking that led to 17% reduction in microindentation young’s modulus [8]. samples irradiated at 30–60 % rh showed minimal nanomechanical changes [8, 9], while those exposed to water experienced a 26 % decrease in young’s modulus and 9–10 % loss of portlandite [8]. scanning electron microscopy (sem) and x-ray diffraction (xrd) analyses of samples exposed to doses up to 1 188 mgy showed decreasing crystalline phases, clinker decomposition, and microstructural damage starting at 130 mgy [10]. hunnicutt et al. [11], who performed nanoindentation on synthetic c-s-h pellets exposed to gamma doses up to 0.784 mgy at 11 % rh, found minimal changes in micro-mechanical properties. more recently, [12] reported significant increases in nanoindentation young’s modulus (by 15–25 %) at higher exposure doses of 24 mgy and 189 mgy. khumurovska et al. [9] studied mortar samples exposed to 12–15 mgy under 40–60 % rh for one year, observing negligible changes in young’s modulus and creep compliance. similarly, hilloulin et al. [13] performed 76 https://doi.org/10.14311/app.2025.53.0076 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 53/2025 micro-scale properties of cement pastes exposed to gamma radiation cao sio2 al2o3 fe2o3 mgo na2o k2o so3 cl− mno (wt. %) 64.78 20.28 4.87 3.55 1.34 0.14 0.76 3.04 0.028 – table 1. cement composition of cem i 42.5r (xrf data provided by the manufacturer). rh 11 % 33 % 76 % 96 % 100 % saturated salt solution licl mgcl2 nacl kno3 environment air air air air water table 2. environmental conditions used in sample containers. micro-indentation on samples subjected to gamma doses up to 0.257 mgy, with no changes in young’s modulus but a reported increase in creep modulus, although environmental conditions were not provided. the formation of bubbles and cracks due to the separation of chemically bound water was also observed by sem at a dose of 290 mgy [10]. since it is clear that macroscopic deterioration mechanisms of concrete begin at the micro-scale, conducting experiments at smaller scales is essential. in this context, nanoindentation is a valuable technique for characterizing the micro-mechanical properties of materials [14]. given the limited research and the unclear role of rh, this study aims to clarify the influence of rh during gamma irradiation on the microstruture and micro-mechanical properties of cement paste using sem and nanoindentation. 2. experiments and methods 2.1. samples and sample preparation a cement paste with a water-cement ratio of 0.4 was prepared using portland cement cem i 42.5 r (českomoravský cement, czech republic). the component composition of the portland cement is detailed in table 1. the mixture was poured into cylindrical plastic molds (27 mm diameter, 70 mm height) and vibrated. after 24 hours, the samples were demolded and stored in a 1 % lime water solution for 610 days (approximately 1.67 years). once removed from the lime water, the samples were cut into slices of about 19 mm height using a diamond blade (struers secotom 50). the sliced samples were then used for two types of analyses: sem analysis and nanoindentation under two types of exposition: irradiation and different rh. several different rh levels: 11 %, 33 %, 76 %, 96 %, and 100 % (water) maintained by salt solutions were tested (see table 2). the salt solutions were kept separate from the samples to prevent direct contact. 2.2. gamma irradiation sample slices were placed in closed plastic containers under specific rh conditions provided by salt solutions (or water) and gamma irraditated. the gamma irradiation experiment was conducted at the research center řež in the czech republic using a 60co radiation source. the samples in containers were left to stabilize at their respective rh conditions for 14 days. half of the samples were then irradiated for 341 days, receiving a total dosage of 13.82 mgy and an average dose rate of 0.675–1.890 kgy h−1. the second half of the control (non-irradiated) samples were kept under the same rh conditions but were not exposed to gamma radiation. 2.3. scanning electron microscopy the surfaces of the samples were polished using sic foils with different grit sizes (#2000 to #4000) at varying speeds and pressure forces without any lubricant while constantly sweeping free particles out of the surface. after each polishing step, the samples were placed in isopropanol and an ultrasonic cleaner for 1 minute to remove any free particles. a phenom xl desktop sem was utilized to analyze the phase composition of the cement paste samples. sem imaging offers high-resolution capabilities, allowing for the clear identification of different phases within the cement paste. in typical cement paste, five key phases can be distinguished: inner and outer hydration products, portlandite, residual clinker, and pores or cracks [15, 16]. back-scattered electron (bse) images were captured at 900× magnification, covering an area of 298 × 298 µm2 with a pixel resolution of 146 nm. the sem was operated at an accelerating voltage of 15 kv. for each sample, ten images were taken, covering a total area of 0.888 mm2. these images were then saved into 8-bit grayscale format, with pixel intensity values ranging from 0 to 255, and further analyzed for phase composition using imagej software. 2.4. nanoindentation after irradiation and exposition to different rh, samples were stored in boxes with silica gel (≈ 11 % rh) to prevent further hydration and carbonation until tested. nanoindentation at ambient conditions (temperature ≈ 23 °c, rh ≈ 40–50 %) was carried out using hysitron ti-980 triboindenter equipped with a berkovich diamond tip and using the accelerated property mapping mode (xpm). a grid of 22 × 22 indents, spaced 2.5 µm apart, was applied to each sample. the indentation protocol followed a trapezoidal load function. the load segment involved linear loading for 77 j. němeček, a. špaková, j. němeček, p. halodová acta polytechnica ctu proceedings a) b) c) d) figure 1. example of phase separation for the irradiated sample (rh=76 %) using image analysis for individual phases: a) inner-outer product, b) portlandite, c) clinker, d) pores and cracks. rh 11 % 33 % 76 % 96 % 100 % clinker ir 4.09 ± 0.86 5.11 ± 1.20 4.85 ± 0.88 4.63 ± 1.11 3.50 ± 0.94 clinker nr 4.52 ± 0.75 4.61 ± 0.99 4.34 ± 1.34 4.31 ± 0.80 3.60 ± 0.89 portlandite ir 5.60 ± 0.43 7.53 ± 0.46 4.52 ± 0.36 4.83 ± 0.20 6.64 ± 1.24 portlandite nr 5.02 ± 0.68 8.18 ± 0.65 5.17 ± 0.21 5.75 ± 0.30 7.80 ± 0.34 inner-outer product ir 81.01 ± 0.94 78.90 ± 0.99 81.13 ± 1.00 83.61 ± 1.22 82.61 ± 2.09 inner-outer product nr 81.28 ± 0.78 77.61 ± 1.26 80.93 ± 1.54 81.61 ± 1.13 81.39 ± 0.95 pores and cracks ir 9.30 ± 0.78 8.46 ± 1.20 9.50 ± 0.69 7.42 ± 0.62 7.25 ± 0.80 pores and cracks nr 9.18 ± 0.69 9.60 ± 0.88 9.56 ± 0.58 8.22 ± 0.83 7.20 ± 0.61 table 3. image analysis results: the table shows volumetric ratios (vol. %) of the given phases for the irradiated sample (ir) and the non-irradiated sample (nr). 0.1 seconds up to a maximum load of 1.5 mn, holding the load for 0.1 seconds, and then unloading linearly for 0.1 seconds. the elastic properties were evaluated using the unloading segment of the load-displacement curve, based on the method of oliver and pharr [14]. the reduced modulus, er, and hardness, h, were determined using the equations: er = s √ π 2β √ ac , (1) h = pmax ac , (2) where ac represents the projected contact area of the tip, s is the elastic unloading stiffness, β is the correction factor for the tip geometry (with β = 1.034 for berkovich), and pmax is the maximum load. the reduced modulus er reflects the combined elastic response of both the nanoindentation tip and the material. the young’s modulus, e, of the (isotropic) material can then be determined from the following relationship: 1 er = 1 − ν2 e + 1 − ν2 i ei , (3) where ei and νi are young’s modulus and poisson’s ratio of the nanoindentation tip, respectively. for a diamond tip, these values are typically ei = 1 141 gpa and νi = 0.07. the poisson’s ratio of the sample, ν, was assumed to be 0.2 ([16, 17]). 3. results and discussion 3.1. sem and image analysis for the analysis of sample phase composition, sem images were segmented into five primary phases: residual clinker, portlandite, inner product, outer product, and cracks/pores, using pixel color thresholding. however, due to overlapping grayscale values between the inner and outer product phases [16], these two were grouped and analyzed as a combined “inner-outer product” phase. an example of this segmentation process is shown for the irradiated sample (rh=100 %) in figure 1. the grayscale threshold boundaries used for segmentation were: residual clinker (255–185), portlandite (184–150), inner-outer product (149–41), and pores/cracks (40–0). the average values and standard deviations for each irradiated and non-irradiated sample are provided in table 3. the image analysis results reveal minimal differences between irradiated and non-irradiated samples across all rh levels, with almost negligible changes observed in the inner-outer product phase. at 100 % rh, there is a slight decrease of approximately 1 % in residual clinker and about 2 % in the pores/cracks phase compared to other rh conditions, suggesting these changes are due to ongoing hydration rather than irradiation. a minor reduction in portlandite content (1.5–3.5 %) was noted at 33 %, 76 %, 96 %, and water, indicating potential decomposition from irradiation, consistent with previous studies [8, 10]. however, these changes are within the method’s accuracy, and thermogravimetric analysis is planned to be con78 vol. 53/2025 micro-scale properties of cement pastes exposed to gamma radiation ducted for more precise portlandite estimation in the future. additionally, sem-bse images were analyzed for radiation-induced crack formation, but no cracks were detected in either irradiated or non-irradiated samples. this contrasts with findings from other studies. for example, crack formation was observed by łowińska-kluge and piszora [10] after exposure to a gamma dose of 130 mgy, significantly higher than the dose used in our study. similarly, němeček et al. [8] reported crack formation in irradiated samples, though this was primarily due to prolonged exposure to the sem vacuum chamber and not the gamma radiation exposure. 3.2. nanoindentation a continuous spectrum of e values from 10 gpa to 130 gpa was obtained showing the large differences between individual mechanically dissimilar phases [17, 18]. based on the correspondence with the phase locations observed by sem, typical values of e were found as: residual clinker e = 50–130 gpa, portlandite e = 36–43 gpa, inner product e = 26–38 gpa, outer product e = 19–26 gpa, low stiffness phase e < 15 gpa), values similar to other studies [16– 18]. the typical load-displacement diagrams of these phases separated by post-indentation sem imaging are shown in figure 2. in order to reveal individual phase properties, all indents from a sample were merged together, and frequency density plots were created for each sample. an example of these plots is shown in figure 3. since the spectrum of e is continuous with a single major peak, no distinction was made between individual hydration phases and the hydrates were collectively called ’main hydrates’, meaning responses from inner-outer products and portandite were grouped into a single mechanical phase. the second phase was created by non-hydrated residual clinker. the two mechanical phases were separated with a statistical deconvolution method, which assumed overlapping gaussian distributions of individual phases. more details of the method can be found in [17–19]. based on these results, the difference in micro-mechanical behavior between irradiated and non-irradiated samples was analyzed and summarized in figure 4. the effect of irradiation is evident in figure 4. samples stored at low rh (11 %) showed a 9.5 % increase in e compared to non-irradiated samples and a 19.9 % increase compared to irradiated samples stored at medium rh levels. this increase is likely due to drying shrinkage, leading to the densification and compaction of c–s–h [20, 21]. in irradiated samples, water radiolysis likely depletes water from the interlayer space, further promoting densification. this trend aligns with other studies on well-matured cement pastes exposed to low rhs [8, 12]. studies on younger samples (irradiated at 4 weeks of age) demonstrated an even higher increase in e (+24.8 %) with a smaller radiation dose of 2.88 mgy [8]. these find0 50 100 150 200 250 300 350 400 0.0 0.5 1.0 1.5 2.0 displacement (nm) lo ad (m n ) low stiffness phase clinker inner product portlandite outer product figure 2. an example of typical load-displacement curves of main phases measured for irradiated sample (rh=76 %). 0 20 40 60 80 100 120 0.00 0.02 0.04 0.06 0.08 0.10 0.12 young’s modulus (gpa) fr eq ue nc y de ns ity () irradiated (rh=100%) control (rh=100%) figure 3. a typical example of frequency plots of young’s modulus for irradiated and non-irradiated (control) sample (rh=100 %). 20 40 60 80 100 0 10 20 30 40 relative humidity (%) yo un g’ s m od ul us (g pa ) irradiated control figure 4. summary of young’s moduli of main hydrates for irradiated and non-irradiated (control) samples stored at different rh levels. ings suggest that the hydration degree at the onset of irradiation significantly influences the radiolysis process and the subsequent c–s–h compaction. it is 79 j. němeček, a. špaková, j. němeček, p. halodová acta polytechnica ctu proceedings important to note that the localized increase in e is accompanied by drying microcracking in the samples, which ultimately leads to a reduction in mechanical properties at larger scales [8]. samples stored at medium rhs (33 %, 76 %) exhibited minimal differences between irradiated and non-irradiated states, indicating that water loss due to radiolysis is likely offset by the available moisture in the container. these results are consistent with previous studies on gamma doses ranging from 0.257 mgy to 15 mgy [8, 9, 13]. the samples stored at high rh levels (96 % and water-saturated) showed the most significant deterioration in mechanical properties due to irradiation. the decrease in e was found to be significant (14.6 % at 96 % rh and 15.5 % in water). this deterioration is likely caused by water radiolysis, although the precise mechanism remains unclear. similar findings were reported by němeček et al. [8] for 28-day-old samples irradiated with a dose of 2.88 mgy. future studies by x-ray diffraction, thermogravimetry, and porosity measurements are planned to disclose the mechanism. the residual clinker properties, as for a crystalline phase, were not impacted by gamma radiation. it aligns with the findings reported by rosseel et al. [1]. in their study, it was noted that gamma radiation typically does not significantly affect the crystalline phases of concrete, such as residual clinker, which remains stable under gamma irradiation. 4. conclusions the lifetime extension of nuclear power plants relies on the description of deterioration mechanisms in concrete used for biological shield and containment structures and a precise quantification of mechanical properties of the micro-scale phases. environmental exposure significantly influences the evolution of properties. this study investigated gamma-irradiated ordinary cement paste with total absorbed dose of 13.82 mgy exposed to five different rh levels (11–100 %) using scanning electron microscopy and nanoindentation. based on the observations and measurements the following conclusions were drawn. • insignificant changes in volume fractions of c-sh hydrated phases were observed by sem image analysis across all rh levels. • a 1.5–3.5 % decrease in portlandite phase was observed in medium to high (33–100 %) humidities, indicating its degradation due to gamma irradiation. • no substantial crack formation in either the irradiated or non-irradiated samples was observed by sem and image analysis. • nanoindentation of samples irradiated at low rh (11 %) revealed a 9.3 % increase in young’s modulus for the main hydrates. this increase is likely due to a combination of water radiolysis and drying, which may have resulted in the removal of water from the interlayer space and compaction of the c-s-h gel. • samples irradiated at medium rhs (33 %, 76 %) did not exhibit a significant change in nanomechanical response. • samples irradiated at high rhs exhibited a ≈15 % decrease in young’s modulus, highlighting the significant impact of gamma radiation and water radiolysis on the material’s mechanical properties. acknowledgements this work was financially supported by the project of the czech science foundation grant number 23-05435s. the presented results were 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https://doi.org/10.1016/j.cemconres.2022.106855 https://doi.org/10.1016/j.cemconcomp.2018.09.008 https://doi.org/10.1557/jmr.1992.1564 https://doi.org/10.1016/j.cemconcomp.2004.02.029 https://doi.org/10.1016/j.mtcomm.2019.100806 https://doi.org/10.1016/j.jmps.2006.06.003 https://doi.org/10.1016/s0008-8846(03)00230-8 https://doi.org/10.1016/j.cemconcomp.2012.06.015 https://doi.org/10.1016/j.cemconcomp.2019.03.027 https://doi.org/10.1016/j.cemconres.2020.106036 acta polytechnica ctu proceedings 53:76–81, 2025 1 introduction 2 experiments and methods 2.1 samples and sample preparation 2.2 gamma irradiation 2.3 scanning electron microscopy 2.4 nanoindentation 3 results and discussion 3.1 sem and image analysis 3.2 nanoindentation 4 conclusions acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2023.41.0001 acta polytechnica ctu proceedings 41:1–7, 2023 © 2023 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague stereolithography for manufacturing of advanced porous solids veronika drechslerováa,∗, michaela neuhäuserováa, jan faltaa, jan šleichrta, daniel kytýřb a czech technical university in prague, faculty of transportation sciences, department of mechanics and materials, na florenci 25, 110 00 prague 1, czech republic b czech academy of sciences, institute of theoretical and applied mechanics, prosecká 809/76, 190 00 prague 9, czech republic ∗ corresponding author: drechver@fd.cvut.cz abstract. the aim of this paper is focused on benefits of stereolithography (sla) technology for the fabrication of the lightweight lattice structures with potential for load-bearing function and high absorption of impact energy. sla is an additive manufacturing technology employing the principle of liquid resins curing moderated by radiation of a wavelength from ultra-violet band where resulting material parameters are tunable by setting of the curing process. the batches of samples manufactured using three different resins were subjected to quasi-static uni-axial tensile and compression tests. acquired data were processed to derive deformation behaviour expressed as stress-strain diagrams and fundamental material properties. based on the knowledge obtained from the mechanical tests, the setup of the fabrication parameters, the most suitable resin for manufacturing of the lattice structures and the overall suitability of sla technology for the fabrication of advanced porous materials, were determined. keywords: stereolithography, advanced porous materials, quasi-static testing, resin, mechanical characteristics. 1. introduction in recent decade, additive manufacturing technologies attract the attention of researchers and producers all over the world opening completely new areas of possibilities in structural design and material engineering [1]. it allows the production from metallic lightweight and high strength structures to nano-level bioprinting for tissue engineering purposes [2–4]. stereolithography (sla) is an additive manufacturing technology originally patented (us4575330a) by chuck hull in 1984 [5]. sla works on the principle of curing a liquid photosensitive resin layer-bylayer using a light source – a projector or an ultraviolet laser beam with a typical wavelength range 300–400 nm [6]. the light source in a predefined path hits the surface of the photosensitive liquid and forms a new layer by selective curing. the process is repeated until the entire sample is finished. final products are post-treated by uv light source to reach required and time stable mechanical properties. one of the sla benefits is a good ratio of the resolution compared to the size of the produced part and speed of the printing. this fact is very important for design of complex lattices structures, e.g., auxetics requiring advantageous materials properties such as low density, high energy absorption, resistance to dynamic loading and fatigue, and high toughness [7, 8]. compared to conventional materials, auxetic materials expand laterally during stretching and shrink during compression. this principle of behaviour is known as the auxetic effect and causes materials to exhibit superior mechanical properties. as a result of the given deformation behavior, auxetic materials have a negative poisson’s number, which is defined as the negative ratio between the transverse εy and longitudinal εx deformation of the material under load [7]. the deformation behaviour of auxetic materials is caused by their specific internal structure, which is formed by modifying basic two-dimensional (2d) geometric shapes such as hexagons, triangles or squares [9, 10]. sla is still a relatively new technology with significant effect of resin material and curing parameters to the primary load bearing function and the deformation behaviour of the structure. to quantify this effects and optimization fabrication parameters the experimental campaign including in total of 54 samples was carried out consisting of quasi-stating tensile testing of bulk material and compression testing of the lattice structure. based on acquired data basic mechanical properties of each material used were calculated. from the results obtained, the most suitable material for the production of advanced porous materials and the auxetic 3d structure that showed the highest resistance to compression loading was determined. 1 https://doi.org/10.14311/app.2023.41.0001 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en v. drechslerova, m. neuhäuserová, j. falta et al. acta polytechnica ctu proceedings 2. materials and methods 2.1. sample preparation for the quasi-static tensile loading, dog bone shape specimens were developed according to astm d63814 standard. the standard defines four types of specimens, and this work was based on the type ii specimen (see figure 1), which is suitable for material thicknesses of 7 mm or less. the test specimen was designed with a thickness of 4 mm and its dimensions are shown in table 1. the model was designed using inventor professional 2022 (autodesk inc., usa) and printed at two different angles, see figure 1, to investigate the effect of different layer orientation during printing on its mechanical properties: • the specimen was rotated 90◦ about the x-axis, with the specimen perpendicular to the platform; • the specimen was rotated 60◦ about the x-axis. figure 1. design and orientation of the specimens for tensile test. parameter dim. [mm] overall length l 115 gauge length l0 40 gauge width w 6 thickness t 4 curvature radius r 14 table 1. dimensions of the tensile test specimen. for quasi-static compression loading, samples of 3d auxetic structures composed of elementary 2d re-entrant hexagonal honeycomb cells were created. the structures were designed in four variants differing from each other in the number of cells in the structure and the thickness of the cell struts, and for all designs, a top and bottom contact faces of 0.6 mm thickness was created to ensure a good distribution of the loading force during the specimen measurements. the different designs and dimensions of the structures are listed in table 2 and depicted in figure 2. the auxetic structures were printed in a 30◦ rotation about the x and y-axes, with the structure placed on removable supports. 2.2. materials three different resins compatible with the sonic mighty 4k (phrozen, taiwan) printer were chosen to produce the specimens to demonstrate the dispersion of resulting material properties. aqua blue and abs-like matte gray (phrozen, taiwan) resins were chosen mainly for their high printing accuracy and low toxicity. the samples manufactured from mentioned materials achieve high hardness and toughness. geometry reliability is higher compared to the other resins. this is due to their very low viscosity. the advantage of uv dlp firm grey (photocentric inc., usa) resin is its strength with low yielding under high stress. this property is widely used for the manufacture of components that must not be brittle and where high load capacities are required. printed models exhibit low flexibility but very high toughness, durability and longevity. 2.3. setting production parameters appropriate setting of production parameter values has a crucial effect on the accuracy and overall success of the print. the individual parameters were set using chitubox 1.7.0 proprietary software compatible with the printer. the setting of the individual print parameters is closely linked to the choice of resin, so it was necessary to set the parameters for each of the three resins used separately and test their suitability when printing the calibration matrices. the printing parameters include, e.g., layer height, curing time of the lower and normal layers, lifting speed of the printer’s build plate. the resulting print parameter values for each resin are listed in table 3. in addition to the appropriate choice of values of individual parameters, it is also necessary to place support structures in critical areas of the model (overhangs, complicated shapes) to maintain a smooth printing process without deformation or damage of the printed object. the support structures form the basis for attaching layers of uncured resin in areas where there are no previous cured layers and blank printing would occur. 2.4. samples post-treatment process the sample production process did not end with the completion of printing. the prints already had their form but required post-treatment (cleaning, curing, removal of supports) to achieve the final mechanical properties. the printed samples first had to be suitably separated from the printer platform, using the plastic spatula. thereafter the samples had to be stripped of residual uncured resin. after the water rinsing, an isopropyl alcohol ultrasonic bath was used for clearing of the fine structure. the cleaning time was set to 5 min and the heating temperature to 35 ◦c. the cure luna (phrozen, thaiwan) uv lamp was 2 vol. 41/2023 sla for manufacturing of advanced porous solids design number of cells strut thickness dimensions (w ×d ×h) − − [mm] [mm] 1 4× 3 0.6 15.00× 15.00× 16.20 2 6× 4 0.5 15.00× 15.00× 16.00 3 6× 4 0.3 15.00× 15.00× 16.00 4 8× 6 0.3 15.00× 15.00× 16.00 table 2. design parameters of the samples for compression testing. parameters units resines phrozen aqua phrozen abs-like uv dlp firm layer height [mm] 0.05 0.05 0.05 number of bottom layers [−] 4 4 4 number of transition layers [−] 0 0 0 transition type [−] linear linear linear exposure time [s] 2.5 2 6 exposure time of lower layers [s] 30 30 80 light-off delay [s] 11 11 9 light-off delay for lower layers [s] 11 11 9 platform lift distance [mm] 6 6 5 platform lift distance for lower layers [mm] 8 8 6 lifting speed [mmmin−1] 60 60 80 lifting speed for bottom layers [mmmin−1] 60 60 80 table 3. settings for manufacturing procedure.. figure 2. all types of auxetic structures (design 1-4) according to table 2 for compression testing. used for subsequent curing of the samples. the curing time was set at 120 min to cure the samples throughout. a fully cured sample was plainly recognized by the change in its surface. 2.5. experimental procedure the printed test specimens were subjected to quasistatic tensile and compressive loading for subsequent determination of their mechanical characteristics. three samples from each batch were tested for each resin. the test specimens were weighed before being measured separately and their initial dimensions were recorded for later calculations of mechanical property values. the tensile test was carried out on the instron 3382 (instron inc., usa) universal testing machine (see figure 3). the tested specimen was placed in the clamps and subjected to the uni-axial tension at a loading rate of 0.5 mm min−1 until the failure, with a sampling rate of 100 hz. during the test, the absolute elongation of the test specimen ∆l was recorded depending on the applied axial loading force f . after the loading was completed, the specimen was remeasured to determine its length after the failure. 3 v. drechslerova, m. neuhäuserová, j. falta et al. acta polytechnica ctu proceedings figure 3. tensile test arrangement. for compression testing of the lattices samples an inhouse table-top loading device [11] depicted in figure 4 with nominal capacity of 3 kn and 2µm positioning accuracy with read-out frequency of 200 hz provided by linuxcnc based control system [12]. a load cell with a capacity of 1 kn was used for the structures with a strut thickness of 0.3 mm and a load cell with a capacity 2 kn was used for the other two structures with strut thicknesses of 0.5 mm and 0.6 mm. the measurements were carried out at the loading rate of 2 mm min−1. all the tested samples were compressed up to 50 % deformation. after reaching this strain value, the measurement was stopped. 2.6. data evaluation the evaluation of the acquired data was carried out using matlab (mathworks, inc., usa), where an automatic script was created to calculate the basic mechanical properties and to obtain data for the production of stress-strain diagrams. first, the experimentally obtained data (namely, the load force record f and the absolute elongation of the specimen ∆l) were loaded automatically and other values such as the initial gauge length l0, the length of the specimen after breaking lu and the specimen cross-sectional area parameters ac were manually entered. based on these data, the stress vector σ and strain ε were determined according to the formulas σ = f ac = f t ·w , (1) ε = 4l l0 . (2) subsequently, a temporary stress-strain diagram was created and the data range corresponding to the figure 4. compression test arrangement. linear region of the diagram was defined to determine the young’s modulus of elasticity in tension. the directive of the linear region was further used to construct a parallel to this region in determining the value of the yield stress σy in tension, which was determined as the intersection of the directive with the constructed parallel. furthermore, the tensile strength σu was evaluated according to the relation σu = fmaxt ac , (3) and ductility d = lu − l0 l0 · 100. (4) finally, standard deviations were calculated and stress-strain diagrams were generated using open source project for scientific data visualization gnuplot [13]. during the compression test evaluation, the compressive stress-contraction and relative strain were calculated and subsequently inserted into the stress-strain contract diagrams to better understand the loading behaviour of each auxetic structure, and the porosity υ of the 3d printed structures from each resin was calculated according to the formula υ = vt − vp vt · 100, (5) where vt is the total volume of the structure without pores and vp is the volume of the structure with pores. 3. results 3.1. tensile testing after evaluating the tensile tests of all the tested samples, the obtained values were averaged and resulting 4 vol. 41/2023 sla for manufacturing of advanced porous solids 0 5 10 15 20 25 30 0 0.01 0.02 0.03 0.04 0.05 s tr e s s [ m p a ] strain [−] 90° rotation around x axis 30° rotation around x and y axis figure 5. tensile: stress-strain diagram for specimens made from abs. in values of the basic mechanical characteristics. the results are summarized in table 4. the highest young’s modulus in the tension of all the used resins is for the samples made from the phrozen aqua blue resin (aqb), while the sample with a 90◦ degree rotation around the x-axis is slightly higher. the aqb also has the highest yield strength σy, strength σu, and nominal relative elongation at break εtb, which implies that the linear elastic deformation region is longer than for other resins and a higher loading force is required to induce plastic deformation and failure of the sample. the values obtained were compared with the data reported by the manufacturer and the data were found to be in disagreement with each other. it was also found from the stress-strain characteristic that the specimens made from uv dlp firm resin (dlp) exhibit significant large plastic deformations compared to phrozen abslike resin (abs). there was no significant effect of sample rotation during printing on deformation characteristics in terms of ultimate stress (see figure 5). test samples rotated during printing by 90◦ about the x-axis achieve larger strains than samples that were rotated by 60◦ about the x-axis. 3.2. compressive testing during the evaluating the compression test, it was necessary to calculate the porosities of the 3d auxetic structures to compare the deformation behaviour of the different auxetic structures (shown in table 5). from the stress-strain diagrams it was found that the samples of all auxetic structures made from aqb and abs resisted the compression loading better than the samples made from dlp, i.e. they achieved higher values of ultimate stresses. this phenomena can be observed in figure 6, which shows a stress-strain diagram for a selected structure with the number of cells per face 6× 4 and with the 0.5 mm thickness of the strut. the individual stress increases and decreases in the diagram show the progressive deformation of the layers of the structure. due to the chosen high 0 0.05 0.1 0.15 0.2 0.25 0 0.1 0.2 0.3 0.4 s tr e s s [ m p a ] strain [−] abs matte gray aqua blue uv dlp firm grey figure 6. stress-strain diagram for structures with 6 × 4 cells and thickness of 0.3 mm. figure 7. detail of deformed structures. overall strain of the structure, which was 50 %, a high stress increase can be observed as the reason for the mutual contact of collapsed layers. furthermore, from the developed contraction diagrams it was possible to see the different behaviour of the structures under loading depending on the used resin. while the structures made from phrozen resins clearly show the stress increases and decreases associated with the layer-bylayer deformation of the structure, the dlp resin exhibited load distribution throughout the structure and deformation of the structure as a whole. the different modes of deformation of the structures are shown in figure 7. when comparing the effect of the thickness of the struts on the mechanical properties, it was found that for a structure with the same number of cells and made of the same resin, there is a rapid increase in stress with increasing thickness of the struts. as a result of the different thickness of the struts, the porosity of the structures within the same resin varied by 26 % for aqb, 16.82 % for abs, and 19.65 % for dlp resins. although 4× 3 and 8× 6 structures showed almost identical porosity values, their behaviour under loading was influenced not only by the number of cells but also by the thickness of the struts. the struts with a higher thickness exhibits higher stiffness and the structure collapsed in shear bands amoung the layers. as the thickness of the struts decreased, the struts became more compliant and less brittle behaviour of the structure occurred (see figure 8). different behaviour was observed on the individual structures after unloading. the structures made from dlp showed extensive plastic deformation during load5 v. drechslerova, m. neuhäuserová, j. falta et al. acta polytechnica ctu proceedings specimen position resin mechanical characteristic e σy σu εtb [mpa] [mpa] [mpa] [%] x and y axis rot. 30 ◦ aqb 1246.55± 22.39 30.80± 0.67 43.75± 1.32 3.22± 1.06 x and y axis rot. 30 ◦ abs 809.71± 31.48 17.53± 0.63 26.85± 1.32 2.93± 0.93 x and y axis rot. 30 ◦ dlp 212.94± 16.52 5.20± 0.17 13.05± 0.95 1.46± 0.76 x axis rot. 90 ◦ aqb 1318.02± 23.35 32.80± 1.91 46.66± 1.66 3.56± 0.52 x axis rot. 90 ◦ abs 867.09± 29.55 17.95± 0.52 28.35± 0.64 2.11± 0.49 x axis rot. 90 ◦ dlp 178.64± 16.95 4.50± 0.00 11.51± 0.69 1.47± 0.82 manufacturer data aqb 588 − 24 − manufacturer data abs 116 − 10 − manufacturer data dlp 700 − 26 − table 4. tensile test mechanical characteristic values resin overall porosity [%] 4× 3 0.6 mm 6× 4 0.5 mm 6× 4 0.3 mm 8× 6 0.3 mm aqb 64.69 52.28 78.33 62.68 abs 70.43 65.40 82.22 69.15 dlp 71.98 62.84 82.49 69.17 table 5. porosity of 3d printed structures. 0 0.5 1 1.5 2 2.5 3 3.5 0 0.1 0.2 0.3 0.4 s tr e s s [ m p a ] strain [−] 4 × 3 cells; 0.6 mm strut 6 × 4 cells; 0.3 mm strut 6 × 4 cells; 0.5 mm strut 8 × 6 cells; 0.3 mm strut figure 8. comparison of different types of abs auxetic structures. ing. samples of structures made from aqb and abs showed much better resistance to permanent deformation. for the structures with a strut thickness of 0.3 mm, only minor plastic deformation was observed after unloading and they were almost able to return to their original form. for the other two structures with strut thicknesses of 0.5 mm and 0.6 mm, larger plastic deformations manifested by broken struts were observed, but despite this the structures were able to maintain their integrity. the structures after unloading are shown in figure 9. 4. conclusions a prerequisite for successful sla printing was the appropriate setting of production parameter values, which had a major impact on the accuracy of printfigure 9. damage of samples after loading – 6 × 4 structure with 0.5 mm strut thickness made of dlp (left), 6 × 4 structure with 0.5 mm strut thickness made of aqb (right). ing and its quality. subsequently, test samples were printed and subjected to quasi-static tensile and compression loading. measured data were presented in form of stress-strain diagrams and basic mechanical properties were calculated. based on the performed tests, it was found that the most suitable material for the production of the auxetic structures was aqua blue resin produced by phrozen. its mechanical characteristics reached the highest values and the auxetic structures made from this resin exhibit lower brittleness. among the loaded auxetic structure samples, the structure with 6× 4 cells and 0.5 mm strut thickness showed the highest value of compressive strength and the highest ultimate stress of all tested structures. the structure deformed in layers and showed only minor plastic deformations after unloading. based on the performed experiment, it can be concluded that the stereolithography method for the fabrication of advanced porous structures appears to be suitable but depends on many variables, e.g., printer 6 vol. 41/2023 sla for manufacturing of advanced porous solids setup, resin, the morphology of the specimen etc. the correct choice of the resin and the high printing accuracy of the 3d resin printer, which made it possible to print the designed complex geometry of the samples, led to this conclusion. acknowledgements financial support from the operational programme research, development and education in the project inafym (cz.02.1.01/0.0/0.0/16_019/0000766) and ctu internal project sgs21/131/ohk2/2t/16 are greatly acknowledged. references [1] g. rasiya, a. shukla, k. saran. additive manufacturing – a review. materials today: proceedings 47:6896–6901, 2021. international conference on advances in design, materials and manufacturing, https://doi.org/10.1016/j.matpr.2021.05.181. [2] n. novak, o. al-ketan, m. borovinšek, et al. development of novel hybrid tpms cellular lattices and their mechanical characterisation. journal of materials research and technology 15:1318–1329, 2021. https://doi.org/10.1016/j.jmrt.2021.08.092. [3] n. novak, d. kytýř, v. rada, et al. compression behaviour of tpms-filled stainless steel tubes. materials science and engineering: a 852:143680, 2022. https://doi.org/10.1016/j.msea.2022.143680. [4] g. bouguéon, t. kauss, b. dessane, et al. microand nano-formulations for bioprinting and additive manufacturing. drug discovery today 24(1):163–178, 2019. https://doi.org/10.1016/j.drudis.2018.10.013. [5] l. d. tijing, j. r. c. dizon, i. ibrahim, et al. 3d printing for membrane separation, desalination and water treatment. applied materials today 18:100486, 2020. https://doi.org/10.1016/j.apmt.2019.100486. [6] i. romero-ocaña, s. i. molina. cork photocurable resin composite for stereolithography (sla): influence of cork particle size on mechanical and thermal properties. additive manufacturing 51:102586, 2022. https://doi.org/10.1016/j.addma.2021.102586. [7] n. novak, m. vesenjak, z. ren. auxetic cellular materials – a review. journal of mechanical engineering 62(9):485–493, 2016. https://doi.org/10.5545/sv-jme.2016.3656. [8] m. benedetti, a. du plessis, r. ritchie, et al. architected cellular materials: a review on their mechanical properties towards fatigue-tolerant design and fabrication. materials science and engineering: r: reports 144:100606, 2021. https://doi.org/10.1016/j.mser.2021.100606. [9] k. k. saxena, r. das, e. p. calius. three decades of auxetics research materials with negative poisson’s ratio: a review. advanced engineering materials 18(11):1847–1870, 2016. https://doi.org/10.1002/adem.201600053. [10] r. zhong, x. ren, x. yu zhang, et al. mechanical properties of concrete composites with auxetic single and layered honeycomb structures. construction and building materials 322:126453, 2022. https://doi.org/10.1016/j.conbuildmat.2022.126453. [11] t. fíla, j. šleichrt, d. kytýř, et al. deformation analysis of the spongious sample in simulated physiological conditions based on in-situ compression, 4d computed tomography and fast readout detector. journal of instrumentation 13(11):c11021–c11021, 2018. https://doi.org/10.1088/1748-0221/13/11/c11021. [12] v. rada, t. fíla, p. zlámal, et al. multi-channel control system for in-situ laboratory loading devices. 16th youth symposium on experimental solid mechanics, ysesm 2018 18:15–19, 2018. https://doi.org/10.14311/app.2018.18.0015. [13] gnuplot. [2022-10-05], http://www.gnuplot.info/. 7 https://doi.org/10.1016/j.matpr.2021.05.181 https://doi.org/10.1016/j.jmrt.2021.08.092 https://doi.org/10.1016/j.msea.2022.143680 https://doi.org/10.1016/j.drudis.2018.10.013 https://doi.org/10.1016/j.apmt.2019.100486 https://doi.org/10.1016/j.addma.2021.102586 https://doi.org/10.5545/sv-jme.2016.3656 https://doi.org/10.1016/j.mser.2021.100606 https://doi.org/10.1002/adem.201600053 https://doi.org/10.1016/j.conbuildmat.2022.126453 https://doi.org/10.1088/1748-0221/13/11/c11021 https://doi.org/10.14311/app.2018.18.0015 http://www.gnuplot.info/ acta polytechnica ctu proceedings 41:1–7, 2023 1 introduction 2 materials and methods 2.1 sample preparation 2.2 materials 2.3 setting production parameters 2.4 samples post-treatment process 2.5 experimental procedure 2.6 data evaluation 3 results 3.1 tensile testing 3.2 compressive testing 4 conclusions acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.38.0516 acta polytechnica ctu proceedings 38:516–521, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague research, protection, and possibilities of re-use of post-war industrial heritage in the czech republic lenka popelová∗, tomáš šenberger, tomáš čunderlík, martin lapšanský czech technical university in prague, faculty of civil engineering, thákurova 7/2077, 166 29 praha 6 – dejvice, czech republic ∗ corresponding author: lenka.popelova@fsv.cvut.cz abstract. buildings of an industrial nature dated from the 1950s through ’80s are often quite interesting from the standpoint of typology, architecture and technology, and correspond to the development of technological industrial efforts of that era. still, they sit on the periphery of professional interest, in both the czech republic and abroad. the topic of industrial architecture which existed by the 1950s is already positioned in the professional spheres on the research and protection of industrial heritage, as well as the methodological development of industrial heritage. industrial archaeology usually deals only with the industrialization period, and at the same time is difficult for contemporary researchers to postpone the debate forward. our contemporary research thus plays very important role in evaluation of a typical and atypical examples of industrial buildings of that time. we continually lose many iconic buildings as well as a strata of historical culture as efforts are ongoing to determine both the ways we can protect this portion of industrial heritage and detail the problems that emerge in efforts to reuse it. keywords: industrial heritage, post-war architecture, re-use strategies. 1. introduction the situation in a former “socialist block” is complicated also due to a bias that is unwarranted among the professional public that stems in part from the context so closely connected with an authoritarian socialist period of these buildings, and also even their lack of acceptance of industrial heritage within the commonly evaluated typologies. in 2017 discussion on protection of post-war architecture was started by an independent group of researchers as part of the working group for post-war architecture connected to the icomos czech committee. they published the first systematic inventory, of the prominent architectural works [1], which comprises various sections: cultural monuments, buildings considered for protection, adaptations, devastating adaptations, buildings under demolition. also in 2021 it is still topical and is under revision. it is sorted according to typology and it included approximately 141 industrial cultural monuments, which are mainly connected with mining, hydraulic and railway structures. an output was a text sent to the czech ministry of culture ideas for cultural monuments, containing an inventory of architecturally interesting buildings [2] mainly of ’60s and ’70s. but again with the minimum number of industrial buildings. from the perspective of developing of typological/standardized solutions in general, the topic was dealt at that time also by jan zikmund from the research centre of industrial heritage of the faculty of architecture of the ctu in prague [3]. 1information differs source by source. for a contemporary research there are also important results of research carried out by the national and cultural identity project (naki ii, finished 2020), presentation of values of 1960 and 1970s as part of the national and cultural identity of the czech republic2 [4]. the main result of this research was an implementation of a national heritage/monuments catalogue and so called inventory (an inventory of buildings not protected but of interest for monument conservation). some entries in both databases have been updated with industrial buildings from the 1960s– 1980s [5] – we can find there some exceptional administrative buildings, some telecommunication premises (now at high risk of demolition), bridges, railway stations, and water management structures, power plants (dukovany nuclear power plant), glassworks, rubber factories, heating plants and other technical services premises were also mapped. this inhomogeneous sample was not further analysed but it can serve as a good review for the future research. what is important is that in a published methodology [6] were described architectural qualities and general values of architecture of that time, which could serve also for evaluating industrial heritage. it would follow a general methodology on industrial buildings published by matěj and ryšková [7]. two published monographs [8] on buildings which were for long time under risk of demolition, were part 2it was implemented in 2016–2020 by the department of architecture, faculty of civil engineering, ctu in prague (hereinafter as fce, ctu in prague), and conducted by the national heritage institute. 516 https://doi.org/10.14311/app.2022.38.0516 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 38/2022 research, protection, and possibilities of re-use of post-war . . . figure 1. ostrava-vítkovice railway station. of a mentioned grant naki ii. the first one described ostrava-vítkovice railway station causa (see figure 1). one of the chapters compiled at fsv ctu in prague defines in detail possible approaches regarding thee repurposing of railway buildings which range from minor to major changes, and from single purpose to those with a wider variety of function. in the czech heritage care these possibilities has not been sufficiently formulated yet even in mentioned methodologies. oral history sources were very important within this research. a second book dealt with transgas company’s central dispatching centre of transit pipelines [9], a very notable structure made in the style of new brutalism, which was situated in the historical prague centre. it has been recently demolished despite protests and replaced by a new development (see figure 2). these two examples can be also used to formulate specific research procedures (modern imaging or mentioned oral history methods which would bring a very vivid information on particular buildings). they also helped to formulate more precisely the main values that need to be defined and protected in connection with the post-war industrial heritage (the urban planning context within industrial agglomerations must be taken into an account, but also the transnational overlap of infrastructure, e.g. transgas and the importance of typological design and technical innovations). they have demonstrated that these buildings do not significantly outperform examples from abroad – instead, they are well-informed representatives. as we have already stated, they also defined the potential of new use or re-use, which was completely ignored in the case of the transgas complex, for example (re-purposing towards becoming part of the famous pompidou centre). within the mentioned naki project, lenka popelová published an article on the importance of architectural competitions in the development of individual typologies and forms, and tomáš šenberger published a paper on the construction of post-war czechoslovak industry [10]. some initial findings figure 2. demolition of transgas complex. were published in the publication industrial heritage – recording, research, protection, re-use also [11]. 2. architectural competitions for industrial buildings and development of typology a rather specific issue is an area in which the research deals, which is a promising topic of sorting opinions on typological solutions: competitions. the 1960s were saw a significant increase in competitions, which were methodically launched across most typologies, and published in contemporary professional publications (for example architektura čsr/čssr and československý architect journals, which included a special edition organised by the union of architects) [12, 13]. it was an opportunity for investors to acquire a valued or at least interesting project within the somewhat typically rather homogeneous sphere typical of socialist building planning institutes. but there were rather few competitions available for industrial buildings, and only three were studied in detail, and thus research is still ongoing for this track. 3. architecture mine and havířov railway station: the public and its voice in saving industrial heritage as the research is only academic, it would have a real impact also. thus, it also deals with re-use strategies. conservation is not the sole method possible for preservation. it is rather unambiguous in today’s debate that it quite often fails. examples include issues surrounding a new purpose for vítkovice railway station, and transgas. therefore, the topics for new research must include all possible methods towards protection of key buildings, and should then be utilized in practice. re-use projects involving industrial buildings are an integral part of the effort to preserve the legacy of our recent history. an author of the railway station building in havířov is architect josef hrejsemnou. the building was completed in 1969 and became an important represen517 l. popelová, t. šenberger, t. čunderlík, m. lapšanský acta polytechnica ctu proceedings figure 3. workshop at havířov railway station. tative of the 1960s brussels style architecture. the station’s passenger building, which is decorated by artworks from vladimír kopecký and václav uruba, was integrated into the fabric of the city and became part of the everyday life of its residents. for many years it served its purpose bringing a unique experience to many passengers. the lack of care for the building and its surroundings, together with the high energy consumption of the premises of generous dimensions, resulted in the proposal to rebuild the railway and bus transport hub. the selected design no longer factored in the iconic construction of the passenger hall. underestimation of the values of the existing building, inappropriate design concept, a lack of discussion among the public, as well as architectural competitions contributed to the coalescing of a movement of experts and activists in an attempt to save the passenger building of the station. the representing voice of this initiative is the důl architektury (architecture mine) association, which aims to save havířov railway station by showing and defining the values of brussels style featured at this station. as part of their activities, the initiators have reached out to the professional community and the general public and, through various station-related activities, have opened up a discussion on key topics such as heritage conservation, urban development, and a lack of balance in decision-making on architectural values. the proposal to designate the station’s passenger building as a cultural monument has slowed down a plan for its renovation. the association has put together a number of activities within the structure for a variety of visitor groups. among these are guided tours, discussions, film screenings, concerts, performances, theatre, a minimalist café, artist residencies, and the subsequent presentation of their work at the railway station site (see figure 3). at initiative of důl architektury, students were asked to create projects showing the concepts different from the planned official project. all these activities provided valuable and stimulating experiences for the visitors. the different uses, outlined in the form of simple interventions highlighting the unique features of the site, triggered the interest of the general public. the support by the professional community of conservationists, architects, and historians was combined with a new public demand for the preservation of the station’s passenger building. in 2014, it was finally and ultimately decided that the railway station building in havířov would not be listed as a cultural monument. however, thanks to the change of atmosphere, the railway infrastructure administration and the city administration together decided to abandon the original redevelopment plan. later, the project was reworked and as a result now, the original building is now undergoing a large-scale reconstruction, which will serve both the original and new functions. “the station is alive” motto accurately describes the association activities. an event reaching out to passers-by, a café fulfilling an originally missing function, that is, making an unused space accessible, a projection or a concert outlining new possibilities for its use, to name just a few of simple interventions the association used to engage the professional and general public. through this effort there has been significant support for the preservation of the passenger building. and thus, it has for the time being been preserved, and an opportunity has been created to consider options on how it will evolve into a new function. 518 vol. 38/2022 research, protection, and possibilities of re-use of post-war . . . 4. one selected typology: forgotten buildings for telecommunications built between 1948-1989 and their re-use another topic which was studied in detail were telecommunication buildings. especially in prague, telecommunications buildings, affiliated in most cases with post offices, were built until the first half of the 20th century in a functionalist style. however, with a growing number of telecommunications users, the post office premises were no longer sufficient, calling for the need to build separate telephone exchange buildings, the designs of which were responding to technological developments and innovations, the space requirements of the operations and, in particular, the problem of dissipation of waste operational heat, which used to cause overheating of the buildings. communications buildings offered up completely new, unexplored possibilities for design approaches; they often served as experiments where not everything worked at first. the state project institute of communications (spojprojekt) was tasked with the design of these buildings, which, according to a resolution of the communist party of czechoslovakia, was supposed to build the infrastructure for five million telephone stations by 1980 (in the 1950s the number of users was only 300 000). the priority was the construction of the central telecommunications building, which was to become the main domestic and international telecommunications hub (later designed by the authors of the czech pavilion at expo 58 in brussels, f. cubr, j. hrubý, z. pokorný). the construction of a network of transit and local telephone exchanges in places of need became one of the state interests; the socialist regime saw it as an opportunity to “catch up with the west”. phone line installation was also one of the strategic needs of the military objectives. local telephone exchanges were built in larger communities with a defined need of phone line installation, whereas politics also played a role in the location of actual exchanges (for example, automatic telephone exchange in prague-dejvice was built in this location for a reason – precisely because of the large concentration of communist bosses in this neighbourhood). the actual archetype of the phone exchange was based on a high-tech architecture idea – a housemachine [14–16]. this western trend in architecture impressed the architects from spojprojekt; they were inspired by the works of james stirling, richard rodgers, archigram. etc. however, the development of these buildings is best depicted by the example of the then largest telephone exchange in the world – london’s mondial house, whose designed technology during its eight-year construction had aged so much that it had to be completely replaced before its grand figure 4. telecommunications building in hradec králové. opening. the basic division of exchanges in terms of their importance is constituted by the central telecommunications building followed by transit exchanges, which used to connect small local exchanges with this main central telecommunications building. transit telephone exchanges (tte) can be seen, for example, in hradec králové and ostrava (designed by spojprojekt architects – loos, malátek, eusanreich & aulický) or in ústí nad labem (designed by architects šmolík, vlčková, růžičková). transit telephone exchanges housed, among other things, local exchanges; this is well illustrated by the building of the tte in hradec králové, where the architects visibly differentiated the individual operations in the main cascading mass of technological halls (see figure 4). the building contains three types of internal operations, the first of which is purely administrative and was supposed to ensure smooth operation of the telephone exchanges in terms of human resources. these are offices, entrance areas, an enterprise’s canteen, a meeting room, and a cafeteria. a second operation is technological; it served as a support for the main operations and features equipment and premises such as workshops for maintenance workers, cooling technology, backup generators, sanitary facilities, and storage areas. the largest area was occupied by the remaining type of operations, that is, the actual telephone exchange technology, analogue communications devices. 519 l. popelová, t. šenberger, t. čunderlík, m. lapšanský acta polytechnica ctu proceedings figure 5. telecommunications building in hradec králové in detail. as already mentioned, this technology was very demanding in terms of cooling requirements, and this problem was also reflected in the architecture of the telephone exchanges themselves; in addition to the internal layout, the overheating solution was also reflected in the façade. the classic thermally insulating boletice style façade was preceded by the so-called “radiation envelope”, that is, a double façade with mechanical shutters that could be used to regulate air circulation thus helping to dissipate heat from the building. another characteristic feature of the telephone exchanges designed by the loos, malátek, eisanreich & aulický team is octagonal windows with a distinctive jamb. the shape of windows was to allow the smooth flow of hot air and the overlap of the jamb to allow ventilation of administrative offices without drawing in waste hot air. these elements became typical for the telephone exchanges designed by these four architects from spojprojekt. telecommunications infrastructure was a priority for the socialist state (in its efforts to match the technologies of the west), and so this type of building was not subject to so-called typification. however, the architects loos, malátek, eisanreich & aulický authored a manual for these buildings, which were mainly repeated in smaller towns and included the already mentioned elements of radiation facade, window shape, colour, or connecting bridges of the administrative offices and halls with analogue communications devices in the form of three large-diameter tubes (see figure 5). according to this typification manual, the exchanges buildings in teplice, kralupy nad vltavou, mladá boleslav and česká lípa, for example, were built. telecommunications buildings have served their function for only a few decades, which is an unusually a short period of time in architecture. in particular, their technology was very expensive for the time, and also, great emphasis was placed on the buildings’ aesthetics and functionality. therefore, it is even more unfortunate that the buildings for telecommunications are deteriorating; due to technological advances in telecommunications, the buildings have lost their original purpose and the question that our generation will need to solve is how to deal with this architectural heritage of unique buildings with a specific aesthetic. a majority of these buildings, of which there are more than two dozen in the country, is essentially a large hall without windows, so it is very difficult to adapt these spaces for human use. this was done, for example, in hradec králové, where the transit telephone exchange was given a long horizontal windows on its façade, thanks to which the halls can be used for administration. although the windows has disturbed the original technicist expression, it has preserved the building and we do not have to worry about the building’s fate. it is also one of the few buildings to retain its original purpose – it houses the telephone exchange equipment in the form of a digital switchboard, but its size has been reduced to a technological device the size of a small wardrobe. another successful example of the use of a former telephone exchange is the conversion of the telephone exchange in beroun by ov-a architect studio, where these architects succeeded in supporting the sculptural qualities of the building and making use of its universal skeleton (see figure 4) to build a new administrative centre. in the west, such conversions are more common; we often encounter conversions to the above-listed offices or to a hotel. one such hotel is in the amsterdam city centre, called the w-hotel by bk-architects, with 251 hotel rooms and even a 22-metre roof swimming pool, but has retained its industrial character. a great number of former telephone exchanges now house the headquarters of different companies, a car repair shop, an education club, and a hairdresser and other retail or services. however, these businesses are not permanent, and it is only a matter of time when they are under threat of dilapidation, the presence of asbestos in the structures, or increasing operating costs these businesses will end and the buildings will be condemned to demolition. another aspect is often the land on which the telecommunications infrastructure sits. frequently, they are very lucrative patches of land in city centres, and the owners of these buildings want to sell or develop the land for commercial develop520 vol. 38/2022 research, protection, and possibilities of re-use of post-war . . . ment plans. however, there is no dispute that some representatives of these unique buildings would deserve some form of conservation protection to prevent degradation or demolition. 5. conclusions industrial structures from the second half of the 1900s remain, at least in the czech republic and in regards to typological development, functional, as was shown through the examples mentioned in this text. on these structures the research attempts to systematize information. this current research intends to address the development of particular typologies, as well as a summary of the general history of development of czech industry in the post-war environment [11]. in phd programmes sustainable development and industrial heritage (prof. t. šenberger) and industrial heritage (doc. l. popelová), there are grants helping to develop typologically-oriented research [17], both for the evaluation of individual typologies and general strategies, and thus individual typologies such as glassworks, porcelain plants, the automotive industry, and telecommunication buildings. the inventoried lists are thus continuously being updated, and the detailing of specific values and the options for repurposing of these structures are being refined. regarding the upcoming “industrial revolution 4.0”, premises are at risk of demolishing because of a promoted change. industrial heritage can be seen as a component of cultural heritage. architectural monuments from the latter 20th century no longer face such questions, the time has arrived to consider the evaluation of industrial buildings from this period. based on a rather conceptual approach that considers the need for new production technologies, aspects of non-typical designs, and as well the attempts for quality architectural design and some of the early efforts towards environmental protection, these structures undoubtedly deserve our attention today. references [1] p. vorlík, k. brůhová, j. zikmund, et al. česká poválečná architektura “pod ochranou a bez”. zprávy památkové péče 77(4):456–462, 2017. [2] p. vorlík (ed.), j. zikmund, p. pavel, l. popelová. náměty na kulturní památky – pracovní skupina pro poválečnou architekturu při čnk icomos, 2017. [3] j. zikmund. hledání univer/zality: kontexty průmyslové architektury v československu 1945–1992. české vysoké učení technické v praze, výzkumné centrum průmyslového dědictví fakulty architektury, praha, 2020. isbn 978-80-01-06743-7. [4] npú. analýza a prezentace hodnot moderní architektury 60. a 70. let 20. století jako součásti národní a kulturní identity čr. [2021-10-13]. https://www.npu.cz/cs/npu-a-pamatkova-pece/npujako-instituce/cinnosti/veda-a-vyzkum/9534analyza-a-prezentace-hodnot-moderniarchitektury-60-a-70-let-20-stoleti-jakosoucasti-narodni-a-kulturni-identity-cr [5] npú. památkový katalog. [2021-10-13]. https://www.pamatkovykatalog.cz/soupis/podlerelevance/1/seznam?druh=ts&temaspec=tema&tema= 2&lokalizacezahranici=0 [6] r. vrabelová, n. goryczková, l. popelová, et al. metodika hodnocení a ochrany staveb 2. poloviny 20. století (se zaměřením na architektonické dědictví 60. a 70. let) vzhledem k jejich (možné) památkové ochraně. odborné a metodické publikace. národní památkový ústav, brno, 2020. isbn 978-80-87967-22-5. [7] m. matěj, m. ryšková. metodika hodnocení a ochrany průmyslového dědictví z pohledu památkové péče. odborné a metodické publikace. národní památkový ústav, ostrava, 2018. isbn 978-80-88240-06-8. [8] m. strakoš. nádraží ostrava – vítkovice: historie, architektura, památkový potenciál. národní památkový ústav, praha, 2017. isbn 978-80-87967-14-0. [9] n. goryczková, m. bartová, n. brankov, et al. transgas: areál řídicí ústředny tranzitního plynovodu a budova fmpe v praze: historie, architektura, památkový potenciál. národní památkový ústav, praha, 2019. isbn 978-80-7480-138-9. [10] l. popelová, r. sedláková, t. šenberger. typologické proměny v československé architektuře 60. a 70. let. zprávy památkové péče 78(6):624–638, 2018. [11] l. popelová, t. šenberger (eds.). průmyslové dědictví – evidence, výzkum, ochrana, nové využití; industrial heritage – recording, research, protection, re-use. české vysoké učení technické, praha, 2021. isbn 978-80-01-06657-7. [12] l. popelová. architektonické soutěže 60.let 20. století publikované na stránkách periodik architektura čsr/čssr československý architekt. ph.d. thesis, čvut v praze, 2012. [13] l. popelová. architektonické a architektonickourbanistické soutěže šedesátých let. jejich specifický přínos v dějinách československé architektury. architektúra a urbanizmus xlvii(47):34–51, 2013. [14] automatický telefonní provoz mezi městy čssr. ústřední správa spojů, 1978. [15] o. klika, e. prager, j. hamřík. místní a uzlové telefonní ústředny. nakladatelství dopravy a spojů, praha, 1968. [16] e. prager, j. trnka. elektronické telefonní ústředny. nadas, praha, 1972. [17] l. popelová, t. šenberger. research, protection, and re-use possibilities for post-war industrial heritage in the czech republic – current research and efforts towards protection. architektúra a urbanizmus 55(3-4):184–199, 2021. https: //doi.org/10.31577/archandurb.2021.55.3-4.5 521 https://www.npu.cz/cs/npu-a-pamatkova-pece/npu-jako-instituce/cinnosti/veda-a-vyzkum/9534-analyza-a-prezentace-hodnot-moderni-architektury-60-a-70-let-20-stoleti-jako-soucasti-narodni-a-kulturni-identity-cr https://www.npu.cz/cs/npu-a-pamatkova-pece/npu-jako-instituce/cinnosti/veda-a-vyzkum/9534-analyza-a-prezentace-hodnot-moderni-architektury-60-a-70-let-20-stoleti-jako-soucasti-narodni-a-kulturni-identity-cr https://www.npu.cz/cs/npu-a-pamatkova-pece/npu-jako-instituce/cinnosti/veda-a-vyzkum/9534-analyza-a-prezentace-hodnot-moderni-architektury-60-a-70-let-20-stoleti-jako-soucasti-narodni-a-kulturni-identity-cr https://www.npu.cz/cs/npu-a-pamatkova-pece/npu-jako-instituce/cinnosti/veda-a-vyzkum/9534-analyza-a-prezentace-hodnot-moderni-architektury-60-a-70-let-20-stoleti-jako-soucasti-narodni-a-kulturni-identity-cr https://www.npu.cz/cs/npu-a-pamatkova-pece/npu-jako-instituce/cinnosti/veda-a-vyzkum/9534-analyza-a-prezentace-hodnot-moderni-architektury-60-a-70-let-20-stoleti-jako-soucasti-narodni-a-kulturni-identity-cr https://www.pamatkovykatalog.cz/soupis/podle-relevance/1/seznam?druh=ts&temaspec=tema&tema=2&lokalizacezahranici=0 https://www.pamatkovykatalog.cz/soupis/podle-relevance/1/seznam?druh=ts&temaspec=tema&tema=2&lokalizacezahranici=0 https://www.pamatkovykatalog.cz/soupis/podle-relevance/1/seznam?druh=ts&temaspec=tema&tema=2&lokalizacezahranici=0 https://doi.org/10.31577/archandurb.2021.55.3-4.5 https://doi.org/10.31577/archandurb.2021.55.3-4.5 acta polytechnica ctu proceedings 38:516–521, 2022 1 introduction 2 architectural competitions for industrial buildings and development of typology 3 architecture mine and havířov railway station: the public and its voice in saving industrial heritage 4 one selected typology: forgotten buildings for telecommunications built between 1948-1989 and their re-use 5 conclusions references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.38.0539 acta polytechnica ctu proceedings 38:539–545, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague the potential of unused railway areas jiří kugl czech technical university in prague, faculty of civil engineering, department of urban design, town and regional planning, thákurova 7, 166 29 prague 6, czech republic correspondence: jiri.kugl@fsv.cvut.cz abstract. railway transportation faces many of the issues that are related to standard brownfields – due to changing technologies and industries, more effective systems (of traffic control in this case) and evolving needs many of the areas are actually no longer necessary for proper function of the railway. that is especially prominent in the case of railway stations, where a significant number of the stations use just a portion of their available tracks, buildings and areas (for example for unloading or repair). the remaining areas are sometimes sporadically used, but more effective and conscious management of the station could fairly easily lead up to the release of these areas for another, more useful function for both the city and the station. this paper will explore the differences between “standard” brownfields and unused railway areas like the typical ownership structure, particular location within the city and the effect station has on the city structure, composition and topography of the areas or the fact that railway areas are most often never really fully abandoned and they do continue to serve in some, albeit diminished capacity. paper also aims to map out how much of the railways areas are underused or unused in czech republic (country with highest rail network density in the world). this paper will then recommend the best ways to use and revitalize them and it will show some successful examples of revitalization projects from all around the world. keywords: railway, unused, brownfield, revitalization. 1. potential of unused areas finding solutions of the problem of unused areas, resp. brownfields have long been recognized worldwide as essential for the healthy sustainable development of cities. the use and revitalization of brownfields is one of the answers to the fundamental problem of undesirable and unsustainable growth of cities in the suburbs and the related occupation of land. there is a shift in land perception, when it is now generally considered to be one of the finite resources with all the consequences that follow (there are also opinions that the land is actually one of the most valuable finite resources, as it is a source of livelihood not only for humans, but also for other organisms). the revitalization of unused areas is closely related to current issues of urbanism and spatial planning, namely suburbanization and the so-called urban sprawl (uneconomical, often unconceptual and parasitic development in the suburban landscape). the development of unused railway areas has the potential to dramatically reduce this phenomenon and in addition, it has many advantages over building on the so-called “greenfield” – such as the use of above-standard connections to existing transport and technical infrastructure of the city, healing “wounds” in the urban fabric, suppression of the barrier effect of railway, remediation of contaminated soils and thus the improvement of the environment, etc. the development of unused areas also addresses their negative impact on the surrounding area, whether in the form of a direct impact of architectural and urban degradation of the environment or barrier effect, or in the form of an indirect effect such as the fall of property prices, an overall outflow of funds from the territory potentially leading to an increase in unemployment, an increase in vandalism and crime or a degradation of the aesthetic taste of the population. for not completely abandoned, only not fully utilized railway areas, these indirect consequences are usually less noticeable than for traditional brownfields, but they are certainly not negligible. as a result, the development of unused land has a double basic justification – it not only helps to solve the problem of new development, but also reduces the negative impact of the unused land itself. here it is necessary to emphasize the lesson from practice that many negative processes can be reversed – for example, a gradual outflow of funds from a neglected area, where a quality pilot revitalization project can (and often does) serve as a catalyst for a significant change in the whole area (the best-known example of this approach in the field of railways is the famous high line park in new york). 2. specifics of the unused railway areas first, it is necessary to define what we consider to be unused railway areas and what separates them from the usual term – brownfields. brownfields are defined by czechinvest as: “a property (land plot, building, complex) that is disused and neglected and may even be contaminated. brownfields are remnants of industrial, agricultural, 539 https://doi.org/10.14311/app.2022.38.0539 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en jiří kugl acta polytechnica ctu proceedings residential, military or other activities. a brownfield cannot be used effectively and appropriately without undergoing a process of regeneration.” [1]. calling unused railway areas brownfields would therefore be potentially problematically misleading, as it gives a wrong impression of a narrower focus only on completely disused and abandoned railway areas (because this is how brownfields are generally perceived, at least in the minds of the general public). however, it should be noted that the issue of revitalization of unused railway areas of course shares much in common with the issue of brownfield revitalization. for the purpose of this research the use of the term “unused railway areas” is proposed which gives a needed clearer idea of the character of these areas with no ambiguity. unused railway areas are then defined as follows: “these are railway areas that haven’t been in use for long time or that are used very inefficiently (i.e. although they are sporadically used or railway operates there still to some extent, by more rigorous application of the principles of modern railway operation these areas could easily be released without detriment to the railway function).” compared to standard brownfields, unused railway areas also differ in the following specifics: the role of the railway railway played an important role in the development of the cities through which it passed. it affected both their industry (by connecting it to national and international railway network and thus facilitating the transport of materials and resulting products), as well as the economic situation of the city itself (overwhelmingly positively) and its urban structure. the specific location of the railway and especially the station had a major impact on the direction and manner of subsequent development of the urban structure. the track as a linear (and in the case of larger mainly freight stations also area) barrier often determined the development zones, but also affected the functional structure of the city. railway and significant expansion of industrial areas was often connected, when in some cases special sidings (or even their network) were introduced into individual factories. in many cases, railways and railway stations were built at a considerable distance from the existing city for technical and economic reasons, however, this mostly became a major impulse for the development of the city towards the station, which was soon absorbed by the urban structure. the result was often a de facto shift in the position of the station from the suburbs to the inner parts of the city, when the station sometimes reached the central positions of the newly formed districts. the arrival of the railway to the city was also one of the impulses leading to the demolition of the medieval fortifications. the stations at this time functioned as new symbolic (but also very much actual) gates, entrances to the city. they became the new starting points of the main flow of visitors and passengers in the city. the builders of the station buildings were usually aware of their importance (especially in the golden era of network construction in the second half of the 19th century) and reflected it with a representative architectural design of the station buildings. the structure of the city reacted to this in many respects in a similar way, as if it were a city gate. the protection zone of the track according to the law also plays an important role. the railway is also a significant element in landscape, whether in the form of the track itself or civil engineering works and modifications such as notches, embankments, retaining walls, tunnels, galleries or bridges. and again, it should be mentioned that it effectively forms a linear barrier in the landscape. this effect is more noticeable (to some extent also spatially, but above all functionally) the stronger and faster the operation on a particular track is. single-track local railways thus form a somewhat delicate barrier and are relatively easy to cross (by people and animals), on the other hand, the corridors of interstate trains (not to mention high-speed lines) are practically insurmountable and thus significantly divide the landscape. similarly, the aesthetic effect of the railway in the landscape varies, where some elements such as viaducts can generally be considered as an interesting and distinctive local feature, however, a large freight station is not as a positive element in the landscape from any point of view. great transport infrastructure connections most of these areas, by the logic of their function, have a very good connection to public transport, sufficient parking areas and are very well connected to the road infrastructure. “strategic” interests of the state in some cases, the state may be interested in maintaining the selected line even though the operation has not been carried out on it for a long time and the line is pretty much abandoned. reasons are usually to ensure the state’s defenses. the nature of the area considering the size of the areas, there is a minimum of buildings and objects in general, most of the area consists of tracks. dimensional character usually longitudinal or strongly longitudinal shape. thanks to obvious technical parameters, railway forms an uninterrupted linear barrier. ownership structure with a few exceptions, there is one, maximum two owners, which greatly simplifies the revitalization process. in the czech republic, for example, there are typically two owners, the state-owned české dráhy (especially some railway stations and the land below them) and the national správa železnic (sž) (everything else) – considering those are state companies, it makes the possible different future use of these areas by the cities easier 540 vol. 38/2022 the potential of unused railway areas and more feasible. after the public sale of part of the railway properties, some unused railway areas fell into the hands of private owners (mostly small family houses for railway personal, warehouses and land in the vicinity of the railway lines). an exception is sidings in industrial areas, which have private (and therefore often very confusing) ownership. railway station architecture the station buildings were very often, by their nature, built with an emphasis on high representativeness. in the cases of the most important stations, original architecture was usually applied (to this day with a very high aesthetic value), at other stations, standardized buildings were designed and built. contamination railway operation causes some typical organic and inorganic contamination of the site. the sources are mainly waste from lubricating oils and condenser fluids, the transport of petroleum derivatives, metal ores, fertilizers and various chemicals or the use of herbicides [2]. the most frequently detected contaminants along railway lines are metals, pesticides and components of crude oil or fuels (petroleum products). these chemicals are associated with standard track operation and are likely to be found anywhere along the track [3]. the three most important types of contaminants are polyaromatic hydrocarbons (pahs), heavy metals and polychlorinated biphenyls (pcbs) (to some extent) [4]. terrain the railway is usually located on a very flat terrain without sudden breaks, there is a minimum of vegetation in the track area. 3. extent of unused railway areas in the czech republic research of the railways in the country was carried out which aimed to identify how many unused railway areas are located in the czech republic and what revitalization potential is hidden in these areas. the analyzed sample is based on the sž document “program of reconstruction and revitalization of passenger stations for the years 2018-2022” [5], which was then manually supplemented with some important stations, which were not originally included in the document (because no investment was planned in those buildings in 2018-2022) and several larger freight stations. the final sample was then 712 stations (604 from the original list of sž, 108 added). in the czech republic in 2021 there are a total of 2 612 railway stations and stops. there are 1 082 railway stations and 1 530 stops (typically only a shelter) [6]. the analyzed sample of railway stations therefore makes up about 66 percent of all railway stations in the czech republic. the sample is therefore not exhaustive, however, due to the fact that most of the remaining stations are mainly stations in smaller and very small cities and villages, it is clearly sufficiently representative to identify the overall extent and potential of unused railway areas in the czech republic. the analyzed parameters in the research includes, for example, the position of railway areas within the city, the size of the city, the size of railway areas (number of buildings, tracks, sidings) and especially the revitalization potential of these areas. it is based on the categorization according to the economic profit analysis from the usa that is used today in the czech republic for categorizing brownfields [7]. this categorization was slightly adapted to focus on railway areas and their revitalization potential. the evaluation is qualitative, it is an expert evaluation of many factors, such as the position of the railway area in the city, its connection to other forms of transport, the size of the city itself (and thus its economic strength), composition and location of existing functions in the city and its civic amenities (looking for possible deficit that could be solved by the revitalization of the railway area), the size of the unused railway area (number of the buildings, number of tracks or empty part of the land formerly used for unloading material), its character (building or area) and its location within the station, architectural the quality of the buildings, what the rest of the station looks like and how it is used, how intensive traffic on the track takes place (according to the number of passengers), what the neighboring buildings look like and what is their function or where the development of the city can be expected (because the revitalized railway areas could for example play an important role as a central linking element there). an important monitored factor is also the existence of other unused areas (brownfields) within the residence, such as not fully used castle, representative factory buildings or large farms (in a smaller cities or villages), in short buildings where higher priority of possible revitalization projects can be expected (whether from urban, cultural or aesthetic point of view). it is necessary to mention that some things are not part of the research although they do matter – for example, how many spaces in the station are successfully rented in the long run or where, for example, the station staff lives. those information are too detailed for the purpose of this research which was to get a general idea of the extent of unused railway areas in the country, not to evaluate any particular railway are in great detail. as the figure 1 shows, the research found that in the czech republic, 74 percent of the analyzed stations are located in cities of less than 10 000 inhabitants. the majority (57 percent) of stations are located in central positions within the city and as much as 65 percent, or about two thirds of all stations are located in the inner parts of the cities. this proves the assumption that large number of railway areas do have a very important and prominent position within the city. only 12 percent of the stations are located in an external position, in an area outside of the city. 541 jiří kugl acta polytechnica ctu proceedings figure 1. distributions of railway areas (author’s research). revitalization potential could be identified in as many as 40 percent of stations (288 stations), 87 stations have a high potential a (revitalization of these areas is appropriate and recommended) and 201 stations have a potential b (revitalization of these areas is possible). it is interesting to note that the highest number of stations with a high potential for revitalization (rating a) is located in the smallest cities with a size of less than 1 000 inhabitants. that is partially caused by the uneven character of the analyzed sample and the railway station network in czech republic, where sizable portion (25 percent) of stations is in towns under 1 000 inhabitants. other reason is the fact that the revitalization potential evaluation scales with the size of the city – for example transforming part of the unused train station into a local cafe with small gallery plays a significant role in the life of a small town, but doing the same project in a big city would have negligent effect and therefore the investment would not be warranted. another important thing to note is the fact that for cities larger than 10 001 inhabitants, there is usually a 60–72 percent probability that their railway areas will have a revitalization potential. in comparison, in cities with a size of less than 1 000 inhabitants the probability of a revitalization potential is only around 34 percent. unused railway areas with rating a (great revitalization potential) have an average of about 13 tracks and a median of 9, an average of 10 sidings and a median of 5, an average of 9 buildings, a median of 4, therefore they are rather large stations. rating b railway areas then have an average of about 11 tracks and a median of 7, 7 sidings and a median of 3, an average of 7 buildings, a median of 4, which are medium to large stations. a typical station in the analyzed sample has an average of about 8 tracks and a median of 5, an average of 5 sidings and a median of 3, an average of 5 buildings, a median of 3. this information indicates that stations with revitalization potential are usually large and therefore highly likely to have large unused areas within them, which represents a significant revitalization capital for individual cities. unsurprisingly, the highest number of stations with a high potential for revitalization is in central positions within the cities or at least their districts. by correlation calculations (using pearson correlation coefficient) of the different parameters of the research, it turned out that the most significant factor in evaluating the revitalization potential was the position of the unused railway area within the city (which seems rather logical) and the number of buildings. an important finding is although the fact that no parameter has proven to be a major factor that would dramatically affect the evaluation of revitalization potential. the evaluation thus proved to be really multifactor and it is therefore necessary to monitor all of the set parameters. the research proved that there is a large number of unused railway areas in the czech republic with revitalization potential. these areas – taking into account their characteristics within the cities – represent a huge potential for improving the quality of life of local residents (e.g. as additions to civic amenities or as new public spaces) as well as a large treasury of areas and buildings which are owned by the state and are not used to their full potential. 542 vol. 38/2022 the potential of unused railway areas 4. possibilities of further use for any city, a quality revitalization of unused railway areas is absolutely essential, because as the research analysis has shown, these areas often play an important role in its entire structure. it is highly desirable to find solutions that allow these areas to be used meaningfully and re-integrate them into the fabric and life of the city. in general, it is also appropriate to reduce the barrier effect of the railway as much as possible (by reducing the number of tracks, building footbridges, underpasses, etc.) and try to maximize the permeability of the area (freeing up unused tracks, making them a public space preferably). also, the current assumption in the field of architecture and urbanism also applies that it is always better to use already built-up areas and existing buildings than to build new ones on open green areas (whether in terms of urban context, architectural-historical continuity or in terms of sustainability). it must be emphasized that the physical destruction of railway areas should always be preceded by careful consideration, as such action is against the european union’s transport policy, in particular the marco polo ii program (concerning the transfer of freight from congested roads to railways or waterways). however, many railway areas are decidedly unused and cannot realistically be expected to be re-used efficiently, in those cases it makes great sense to consider another use that will be more beneficial for the city (and, after all, also for the station itself). it is understandable that the size of the city and its budget (or the financial possibilities of sž or other railway carriers) significantly affects the possibilities of revitalization and thus the choice of further use of freed unused railway areas. however, it is still necessary to keep in mind that the use of these areas is in most cases the most financially viable way to supplement or enrich the network of civic amenities, as it is state property after all, where it can be expected (with the support of respective state authorities) there could be a gratuitous or at least very advantageous transfer of real estate into the ownership of the city. one of the possible ways of utilization of unused railway property is also its active lease (targeted at desirable functions), either by the city or sž (in coordination with the city and its vision). this applies, of course, mainly to buildings. an interesting option is also in justified cases the waiving of the lease for a definite period of time (e.g. for state or non-profit organizations or interest groups). in this way, it is possible to support, for example, clubhouses serving local communities, fitness and health groups, groups for children, necessary missing services such as barbershop, surgery, library, but also commercial activities such as start-up offices or social programs such as social apartments for young families or flats for the necessary professions (typically teachers, doctors). this can effectively serve to rather cost-effectively kickstart local community life. many possibilities for further use can also be quite minimalistic – it is understandable that a 3 000 city will not have enough funds to renovate the building into a modern, elegant hamburger bahnhof-style gallery, but sometimes a somewhat simple and industrial, yet functioning gallery (or cultural hall etc.) can be created by just cleaning the building, possible wiring or plumbing done very cost efficiently visibly along the walls and repair of the roof (if necessary). similarly, other creative ways of saving can be used – instead of reconstruction (for example of the former storage hall), the building can ideally drop part of one or more walls (at the very least the parts underneath the windows) and create an unusual generous outdoor shelter which can be used, for example, for outdoor seating or as a roof for holding markets. the costs of such projects are, of course, fundamentally different from a complete reconstruction with repair of the facade, all surfaces, replacement of windows and roofs, and many examples from around the world but also in the czech republic show that such areas have their specific charm, work very well and they usually find their users. from the point of view of costs (and, after all, the time of implementation), this minimalist approach can be applied to practically all possibilities of further use, including the creation of new public spaces. the areas can be done attractive, for example, by adding (as a kind of technical artifact) a parked wagon, which with minor modifications can easily serve as an interesting gazebo, or a lockable storage for a refreshment stand. former loading area doesn’t have to be repaved, it could be just creatively painted over using colors for marking roads, creating visually striking fun patterns, labyrinths or improvised “sport fields”. interesting and attractive in the case of revitalization of unused railway areas does not always mean costly, it could be inspired by the ideas of “tactical urbanism”, have a clear vision and consistent emphasis on low cost and finding ways to achieve it (e.g. looking for possible cooperation with local schools or sponsoring local companies). in general, practically all stations in the czech republic should ideally expand their range of services and shops (for which it is sensible to use those areas and buildings within the station that are not currently used effectively) and follow the global trend aimed at improving the quality of travel for passengers. one has to remember, that even today, for a large number of people, the station is still the gateway to the city, first point of entrance, and as such should be, if not (for financial reasons) representative, then at least lively and interesting. one things that should be noted is that the station buildings were very often, by their nature, built with an emphasis on high representativeness. the station architecture is quite specific and even most laymen are able to identify the station purely by its appearance. for practically all stations it is true that to this day 543 jiří kugl acta polytechnica ctu proceedings figure 2. possible new functions of unused railway areas [8]. they stand with great dignity within their cities and are often (especially in smaller cities) one of the most representative buildings in the area. the vast majority of stations were established during the golden age of the railway and therefore an unusual amount of funds was invested in them. by finding new use for these old buildings, the city can relatively cheaply obtain aesthetical and expensive buildings, far more representative than the city would usually have the funds for. many of the unused railway areas also pose a significant cultural, architectural or historic value. one of the possibilities is the determination of the building or area (or its part) a cultural monument and thus the provision of monument protection. the selection of railway monuments in czech republic was initially somewhat random and was accompanied by many disputes, when the declaration of a monument was cancelled. now, with the growing public interest in industrial heritage, the situation is slowly improving. in 2021, there are 250 railway areas on the central list of cultural monuments, of which about 124 are stations (out of 1 082 functional stations) [9]. when determining the future function of the unused railway areas, one of the major specifics of these areas has to be taken into account – in vast majority of the station, the operation is still running to some extent. revitalization projects therefore should work with this factor and use it to their advantage. the worldwide trend is towards the greatest possible involvement of the station in the daily life of the city, where many of the added or implemented functions improve the quality of life and services of both the neighbourhood and the railway operation itself. for unused railway areas, the following possibilities for further use should be considered (and in some cases they are mutually combinable or consecutive). the examples shown in figure 2 come from countries like usa, germany, france, australia and czech republic. 5. conclusion the research found that in the czech republic, the majority (57 percent) of stations from the analyzed sample (of 712 stations) are located in central positions within the city and about two thirds of all stations are located in the inner parts of the cities. thus proving the assumption that railway areas typically have a very important and prominent position within the city. only 12 percent of the stations are located in an external position, in an area outside of the city. in 40 percent of stations (288 stations) revitalization potential could be identified. from those 87 stations have a high potential a (revitalization of these areas is appropriate and recommended) and 201 stations have a potential b (revitalization of these areas is possible). size of a city of course plays a big role in evaluating the revitalization potential of the railway area, where for example in cities larger than 10 001 544 vol. 38/2022 the potential of unused railway areas inhabitants, there is a 60–72 percent probability of a revitalization potential. successful revitalizations of unused railway areas have the potential to have a very positive effect on their immediate surroundings and so, in addition to their obvious direct benefit (in the form of, for example, the introduction of a new function to the area), they also have an equally important indirect benefit. it is a practice many times proven that successful attractive projects can start a positive development of the whole area. however, even smaller revitalization projects have a significance that should be utilized. due to the fact that the vast majority of unused railway areas are state property (whether čd or sž), the use of these areas is in many cases the most financially viable way, for example, to supplement or enrich the network of civic amenities or public spaces. unused railway areas can be re-integrated into the life of cites, in which they can again play a significant and positive role, which the railways historically always did. references [1] czechinvest. brownfields. [2022-02-08]. https://www.czechinvest.org/en/forinvestors/real-estate-offer/brownfields [2] b. wiłkomirski. railway tracks – habitat conditions, contamination, floristic settlement – a review. environment and natural resources research 2(1):86– 95, 2012. https://doi.org/10.5539/enrr.v2n1p86 [3] department of environmental protection. best management practices for controlling exposure to soil during the development of rail trails. [2016-02-20]. https://www.mass.gov/doc/best-managementpractices-for-controlling-exposure-to-soilduring-the-development-of-rail-0/download [4] united states environmental protection agency. technical approaches to characterizing and cleaning up brownfields sites: railroad yards. [2016-02-20]. https://nepis.epa.gov/exe/zypurl.cgi?dockey= 30004h7u.txt [5] sždc. program rekonstrukce a revitalizace osobních nádraží pro roky 2018-2022, sždc, praha, 2017. [6] správa železnic. základní charakteristika železniční sítě. [2021-06-14]. https://www.spravazeleznic.cz/onas/vse-o-sprave-zeleznic/zeleznicecr/zeleznicni-sit-v-cr [7] b. j. jackson. příručka pro vlastníky brownfieldů, ústí nad labem, 2011. [8] j. kugl. revitalizace železničních ploch. in přestavba a regenerace sídel a krajiny, pp. 34–37. ústav územního rozvoje, brno, 2017. [9] národní památkový ústav. vyhledávání, ústřední seznam kulturních památek. [2021-06-15]. https://pamatkovykatalog.cz/ 545 https://www.czechinvest.org/en/for-investors/real-estate-offer/brownfields https://www.czechinvest.org/en/for-investors/real-estate-offer/brownfields https://doi.org/10.5539/enrr.v2n1p86 https://www.mass.gov/doc/best-management-practices-for-controlling-exposure-to-soil-during-the-development-of-rail-0/download https://www.mass.gov/doc/best-management-practices-for-controlling-exposure-to-soil-during-the-development-of-rail-0/download https://www.mass.gov/doc/best-management-practices-for-controlling-exposure-to-soil-during-the-development-of-rail-0/download https://nepis.epa.gov/exe/zypurl.cgi?dockey=30004h7u.txt https://nepis.epa.gov/exe/zypurl.cgi?dockey=30004h7u.txt https://www.spravazeleznic.cz/o-nas/vse-o-sprave-zeleznic/zeleznice-cr/zeleznicni-sit-v-cr https://www.spravazeleznic.cz/o-nas/vse-o-sprave-zeleznic/zeleznice-cr/zeleznicni-sit-v-cr https://www.spravazeleznic.cz/o-nas/vse-o-sprave-zeleznic/zeleznice-cr/zeleznicni-sit-v-cr https://pamatkovykatalog.cz/ acta polytechnica ctu proceedings 38:539–545, 2022 1 potential of unused areas 2 specifics of the unused railway areas 3 extent of unused railway areas in the czech republic 4 possibilities of further use 5 conclusion references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.38.0124 acta polytechnica ctu proceedings 38:124–130, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague connected design decision networks: multidisciplinary decision support for early building design lca kasimir fortha,∗, patricia schneider-marinb, sebastian theißenc, jannick höperc, nanna dyrup svaned, andré borrmanna a technical university of munich, school of engineering and design, department of civil and environmental engineering, arcisstr. 21, 80333 munich, germany b norwegian university of science and technology, faculty of architecture and design, department of architecture and technology, no-7491 trondheim, norway c university of wuppertal, faculty of architecture and civil engineering, institute of building information modeling, pauluskirchstraße 7, 42285 wuppertal, germany d aalborg university, the faculty of engineering and science, department of the built environment, thomas manns vej 23, 9220 aalborg, denmark ∗ corresponding author: kasimir.forth@tum.de abstract. life cycle assessment (lca) has become the standard method to evaluate environmental impact throughout the life cycle of buildings. however, detailed data about the future building as well as knowledge about the mutual influence of decisions concerning the various disciplines involved are often missing in early design phases, otherwise known to bear the highest potential for emissions savings. hence, a meaningful basis for decision making is lacking. this study suggests a method to digitally represent decisions and their interdependencies in early design phases and visualize their possible consequences for the life cycle of the future building. the method is based on identification of relevant processes and tasks concerning architecture and heating ventilation and air conditioning (hvac). decisions trees of these tasks are used as a point of departure. connecting the decision trees to a multidimensional, connected design decision network (cddn) enables an interdisciplinary design team to pinpoint strategic decision nodes with comparatively more interdependencies with other subsets and high influence on lca results. we believe that a transparent decision making in early design stages can be valuable to both the design team as well as clients and contractors and bear potential for an increased mutual awareness minimizing late and expensive redesigns. keywords: lca, integrated design process, multi-criteria decision making (mcdm), early design phase, interdisciplinary decision network. 1. introduction life cycle assessment (lca) has become the standard method to evaluate environmental impact throughout the life cycle of a building, focusing besides operational energy and carbon also on the embodied energy and carbon of building materials. depending on the type and energy standard of a building, embodied carbon account for up to 80 % over the life cycle, especially for new constructions [1]. the embodied carbon of hvac systems accounts for about 20–30 % of this share, although the material mass share is usually only around 1–2 % of the total building mass [2]. thus, it is essential to consider the interdependencies between architecture and hvac from the lca perspective when optimizing environmental performance. the lca result is highly influenced by decisions made by multiple stakeholders in various disciplines in different stages of the project phase. the content and the point in time of such decisions depend highly on the individual project and the persons connected to the project. a practical example of this interdependent relationship is the choice of insulation material in a construction. in order to maximize the useable area inside the building, it is preferable to use a high performing insulation material that allows for a thinner construction while still fulfilling the energy efficiency requirements. a light construction permits a higher degree of solar access, thereby positively influencing the daylight calculations. however, the high performing insulation material does not carry the same characteristics for fire resistance and sound absorption as standard mineral wool, thereby influencing both the fire strategy as well as the acoustic calculations, not to mention the carbon emissions considered in the lca calculation [3]. in other words, the increasing complexity of buildings combined with tightening of requirements in building codes calls for a high and continuously updated understanding of building and material physics, technology and economy or a completely optimized inter124 https://doi.org/10.14311/app.2022.38.0124 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 38/2022 connected design decision networks: multidisciplinary decision . . . disciplinary design approach. however, the constant efficiency improvement in the industry apportions limited time for each project, which necessitates the use of rules-of-thumb and simplifications, highly dependent on the individual practitioner [4, 5], especially in the early design phases. a study by imam et al. from 2017 [6] investigates 108 modellers’ ability to rank important parameters for building performance simulations. the analysis showed little correlation between which variables were thought to be important by the modellers and which proved to be objectively important. furthermore, they found that qualification level and years of experience do not improve the accuracy. this indicates, that even within own discipline, a full overview and understanding of the importance and consequences of design questions, is a difficult task. besides code requirements, the building designers must satisfy client requirements, often considered as definitive objective requirements. however, many clients do not sufficiently understand the consequence of their prerequisites, which can entail undue difficulties and an unsustainable design process from day one. this calls for a higher transparency and an informed basis upon which clients can set their conditions. the goal of this study is to propose a meta-model of the conventional design decision process as a facilitator for the multi-criteria decisions needed in early design phases. the method offers an easyreadable indication of potential interdependencies across disciplines when making a design decision. we believe that the technique enables a more transparent decision making, showing possible alternative design paths and their connected consequences for the later design stages. 2. literature review / state of the art 2.1. multi-criteria decision analysis and making (mcda & mcdm) multi-criteria decision analysis and making (mcda and mcdm) methods exist to a large extent in literature and are used in many different applications/areas. in general, they aim to find the best option/alternative of complex decisions considering multiple criteria [7]. some of the most commonly used methods are the weighted sum model (wsm), the analytic hierarchy process (ahp), the electre method, the topsis method, among others. although these methods have different individualities, they all correspond to a multidimensional evaluation procedure and aim to decompose the overall objective into sub-objectives. thereby, they are suitable for evaluating many criteria or indicators with different states (quantitative, qualitative). furthermore, they follow a similar process: (i) identification of the problem; (ii) deriving the preferences; (iii) evaluation of alternatives; and (iv) identification of the best alternatives [7]. the architecture, engineering, and construction (aec) industry considering sustainable design decisions shows growing interest in mcdm, but still, it is rarely used. as highlighted by klumbyté et al. [8], within an analysis of 66 articles from 2016–2020 the number of scientific papers using mcdm methods for sustainable construction decision support increased on average around 13 %. however, the total amount of papers is still very low, compared with scientific papers using mcdm in other research areas. another review by tan et al. [9] on combining multi-criteria decision making (mcdm) methods with building information modelling (bim) underlines a growing interest and the fact that mcdm in aec is mostly used in combination with sustainable decision support. while many of the analyzed studies aim to identify the best option of a material choice, including many criteria, e.g., embodied energy and carbon for an lca, they are limited studies considering the influence of multiple disciplines. as an example hereof, sonetti et al. [10] used ahp to identify the best roof design decision out of three design choices considering the following criteria: thermal resistance, construction costs, aesthetics, social usability and environmental impacts. moghtadernejad et al. present advantages and limitations of mcdm methods applied to building facades [11]. one of the main findings is that choosing the best mcdm is challenging, complex and thus not suitable for day-to-day planning. in addition, all considered mcdm methods’ lack in considering multi-domain performance attributes. conversely, this means that interactions between different design disciplines are neglected. thus, there is no decision support across disciplines meaning that the significance and consequences of design decisions on other disciplines are not considered. 2.2. interdisciplinary interactions and dependencies attempts to limit energy use and related emissions in buildings require multidisciplinary efforts between (at the least) architects, building physicists and hvac engineers. the common problem of building-related disciplines “fixing” the “problems” created, often unknowingly, by the architect, can be alleviated by early cooperation with the goal that the building’s construction and hvac systems work as one integrated system. despite the apparent advantages of such an approach, design processes frequently fail in practice, resulting in increased costs and time. with each discipline working with their specific extensive knowledge, there is a lack of understanding of the interactions with other disciplines. if such decisive interactions can be visualized and communicated, designers, consultants and clients profit from a more transparent, smoother process and more integrated results. 125 k. forth, p. schneider-marin, s. theißen et al. acta polytechnica ctu proceedings figure 1. combining discipline-specific decision trees. figure 2. meta-models of conventional and cddn-processes. especially the building’s structure and finishes and the building services interact and overlap in many areas allowing for opportunities to improve the building’s life cycle performance. for instance, insulation levels influence heating demand, and the possibility of low-temperature supply, fenestration and sun shading type and quantity are essential for lighting and cooling demand, and thermal mass is decisive for comfort and heating and cooling demand, whereas building part activation calls for certain choices of surfaces. many such examples can be found for each discipline, leading to the process becoming quickly complex. the proposed method to map such interactions aims to ease the coordination process and identify decisions and alternatives important for life cycle performance. 3. connected design decision network (cddn) approach existing methodologies of mcdm or mcda often take one subset of the building into account. thereby, they do not consider the perspective of several disciplines at the same time and their dependencies between each other as they only examine one specific design question/problem. furthermore, mcdm requires very specific input, such as a choice and definition of the criteria to be applied, an alternative for comparison and a clear objective. therefore, it presupposes that the decisionmaker already has know-how about the criteria, e.g., benchmarks for ghg emissions, and understands the consequences of the mcdm results on other disciplines. however, this is very challenging due to the many complex interrelationships between different design domains and disciplines. in sum, mcdm or mcda are (automatically) optimizing design parameters according to multiple criteria defined by the decision-maker rather than showing interdisciplinary relationships in advance and providing decision support with analysis results (figure 1). due to the described limitations, we suggest a new method, called connected design decision network (cddn). in contrast to the existing methodologies, the cddn enables decision making across multiple disciplines along all planning phases (from early to detailed). based on a baseline cddn of conceptual design decisions, it can be transferred to new projects and easily extended according to project-specific design questions. 3.1. cddn meta-model the meta-model of the cddn extends the conventional design decision process (figure 2). in the conventional process, each subset of the building, e.g., structure, building envelop, heating system, decides several design options in a linear sequence and mainly within the same discipline. design questions with several design options are chosen successively, leading to one decision tree for each subset. the materials are selected in the final step for each option, mainly 126 vol. 38/2022 connected design decision networks: multidisciplinary decision . . . considering costs and functional values fulfilling legal restrictions, such as acoustics or operational energy. in contrast to this conventional process, the proposed cddn process is not limited to decisions within the same discipline or subset, but design options can influence another discipline’s decisions concerning design questions or just specific options. this leads to a combination and network of several decision trees of the conventional process. meaning that the choice of material is not an endpoint, but in fact a decision that influences other design decisions at other and earlier stages of the process. on a very simplified level, decision trees for construction projects increase in their granularity of decisions starting from an early design phase and leading to a detailed design phase. usually, each discipline follows its logic and correlations in these decision trees according to its domain knowledge and norms, as decision trees follow a linear decision-making process. nevertheless, some decisions can be reversible, which is not represented in a conventional tree structure. reversible in this case means that the decisions can be reconsidered at a later stage without decisively influencing or changing the design path at the current stage. in an integrated design process, these design decisions are discussed orally between different domain experts, as each decision might influence the decision of other disciplines. these interdisciplinary correlations can include decisions of different design phases and lead to decision conflicts that can only be resolved by a multi-disciplinary synthesis finding the best compromise. we do not believe that decision trees and oral exchange represent a decision-oriented process well enough. therefore, this study defined a further development based on meta-models and followed by an application example. to formalize this theoretical approach, we firstly developed a meta-model for decision trees of conventional processes. in a second step, we extended the conventional process and developed it further to an integrated approach of a cddn process. generally, each decision tree is a discipline-specific subset and follows different design questions. each of those questions can lead to one or multiple design options that can link in a building information modelling (bim) process to an industry foundation classes (ifc) entity. this design option can be a material, construction technology or logistics and can lead to other design questions and, as a final stage, a material choice and the selections of final options. those options have physical properties and induce costs. the cddn process does not stop at these final material options but also integrates other subset’s decisions. these can occur after a design option but also after a final material selection. in comparison to the conventional process, more semantical information shall be included in the decision-making process, such as data for life cycle assessment. 3.2. case study in this chapter, a possible application example of the proposed cddn approach is described on an early conceptual design level. the use case is a new construction of a non-residential building, for which building structure, building envelope and heating systems are chosen as subsets. in the first stage, we defined a decision tree for each subset. hereafter, we implemented the dependencies between different decisions of subsets (shown with orange highlighted arrows on figure 3). no subset is prioritized, i.e., the decision network can start from every subset and lead to all other ones. in the proposed application example, two different design decision levels for the structural subset are given: the structural system (load bearing wall vs. skeleton system) and the related structural material (concrete vs wood vs masonry/steel). for the building envelope, the two levels considered are (1.) the window-to-wall ratio (high vs low) and the façade system (glazed curtain wall vs solid facade) and (2.) the decision concerning the main exterior wall material (wood vs steel/masonry vs aluminium/ concrete). a dependency between the different subsets, building envelope and structural system, is shown: the decision for a solid façade system calls for a structural design with load-bearing exterior walls whereas a skeleton system favours a curtain wall. in terms of the heating system, four different decision levels are defined. starting with the design question about the available energy sources, further the heat generation, heat distribution medium, and the installation of heat transfer must be chosen. the presented dependency shows the influence of the curtain wall choice on the heat transfer: if a fully glazed curtain wall façade is chosen, heat transfer should not be in front of the windows, i.e., it either needs to be integrated into ceilings or floors or convection heaters. activated ceilings or floors set constraints to the floor construction and material, for example certain surface materials cannot be used (e.g., thick carpet on floors or light dropped ceilings). such decisions might occur much later in the process but are anticipated by an early design choice. additionally, if the window-to-wall ratio is high, special attention to cooling demand is required. in terms of material choices, a skeleton with curtain wall system is typically built either with a concrete, wood or steel structure with the corresponding curtain wall materials or as a hybrid system. each of these materials has a different thermal capacity which, in turn, can influence the choice and size of the heating and cooling systems. 127 k. forth, p. schneider-marin, s. theißen et al. acta polytechnica ctu proceedings figure 3. a case study of a cddn in an early design phase for conceptual decision making. 128 vol. 38/2022 connected design decision networks: multidisciplinary decision . . . figure 4. case study refinement. 3.3. prototypical decision making for validating the suggested framework using a case study, the previously shown application (section 3.2) is considered. a volumetric shape of an office building is the input of the case study, as shown in figure 4. as a starting design decision, the structural system is to be selected. the lca benchmark range for the structural system is derived from a lca benchmark study [12], based on 50 real-world whole building lca results. this decision is shown as a prototype, since the full benchmark calculation is still under development. although the lca results for load-bearing walls seem more promising (figure 5), the skeleton system was decided as the structural system, due to a dependency on the flexibility in the room layout. as the cddn is not a single-criteria optimisation method, but rather giving transparent information about multi-criteria design decision, the lowest lca has not to be necessarily chosen. afterwards, wood was chosen as the main material. as no façade system is yet chosen, the cddn application anticipates the façade system decision, directly leading to curtain walls and the related material decision. at the same step, for the heating systems, the design team is made aware that several systems should be excluded because the heat transfer should be installed in the ceiling or floor due to the previously made decision concerning a curtain wall system. 4. discussion the purpose of the cddn is to formalize and visualize design decisions of different subsets and their dependencies. starting from a predefined general system’s level deciding conceptual design options, every project will develop and enrich their cddn with individual design details. as it is impossible to forecast figure 5. boxplot showing lca result for different structural systems based on 50 real-world buildings. every possible design path, cddn’s are specific for each particular project. therefore, cddn in their conceptual form can be transferred to other projects, while the cddn for a particular project follows the individual design process. cddn identifies critical decisions with a high number of dependencies and a wide range of the results of multiple criteria, and, hence, highlights and communicates strategical decisions. nevertheless, by formalizing design decisions with the help of cddn transparently, decisions leading to sub-optimal solutions or even faulty decisions can be more easily identified in later stages or after finishing the design. as a result, a systematic learning effect occurs, and the cddn can be further developed to avoid similar mistakes in upcoming projects. in the next step, we plan to calculate the lca results for various design paths of the mentioned use case to achieve final values for lca. the calculations for each decision path reveal decisive decisions nodes where interdisciplinary discussion is most beneficial for reduced environmental impacts. 5. conclusion & outlook a connected design decision network (cddn) enables the interdisciplinary team to formalize design decisions and identify strategic decisions due to interdependencies with other disciplines or results. the cddn is based on subset specific decision trees, and considers a hierarchical dependency and can classify reversibility. as a result, a more transparent decision making is possible from early design stages showing the performance of alternative design paths and their connected consequences for detailed design stages. in a next step, prototypical implementation in a tool shall verify the proposed theoretical method and visualise and manage the high number of dependencies and connections. a subset of existing networks will be modified and project-specifically extended in this tool due to upcoming design decisions. furthermore, we plan a link to the bim model to enable a fully integrated, model-based design process. we envision 129 k. forth, p. schneider-marin, s. theißen et al. acta polytechnica ctu proceedings that design decision networks are apparent in the bim model and that a tracking of past decisions enables experience gathering and further development. the cddn must be available for every stakeholder and implemented notifications warn others if changes affect their discipline. this methodology also enables future development, e.g., applying machine learning to the tracked decisions to predict where and when in time decisions are made. potentially, this can also improve the quality of risk management and reduce the number of redesigns in future construction projects. references [1] a. papakosta, s. sturgis. whole life carbon assessment for the built environment, 2017. 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(eds.), values and functions for future cities, pp. 463–477. springer international publishing, cham, 2020. https://doi.org/10.1007/978-3-030-23786-8_26 [11] s. moghtadernejad, l. e. chouinard, m. s. mirza. multi-criteria decision-making methods for preliminary design of sustainable facades. journal of building engineering 19:181–190, 2018. https://doi.org/10.1016/j.jobe.2018.05.006 [12] a. braune, l. ekhvaia, k. quante. benchmarks für treibhausgasemissionen der gebäudekonstruktion: ergebnisse einer studie mit 50 gebäuden, 2021. 130 https://doi.org/10.1051/e3sconf/201911103061 https://doi.org/10.1016/j.enbuild.2017.12.009 https://doi.org/10.1016/j.enbuild.2019.109628 https://doi.org/10.1177/0143624416684641 https://doi.org/10.1007/978-981-33-4745-8 https://doi.org/10.3390/su13052820 https://doi.org/10.1016/j.autcon.2020.103451 https://doi.org/10.1007/978-3-030-23786-8_26 https://doi.org/10.1016/j.jobe.2018.05.006 acta polytechnica ctu proceedings 38:124–130, 2022 1 introduction 2 literature review / state of the art 2.1 multi-criteria decision analysis and making (mcda & mcdm) 2.2 interdisciplinary interactions and dependencies 3 connected design decision network (cddn) approach 3.1 cddn meta-model 3.2 case study 3.3 prototypical decision making 4 discussion 5 conclusion & outlook references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2023.41.0089 acta polytechnica ctu proceedings 41:89–97, 2023 © 2023 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague experimental evaluation of aluminothermic welds lukáš zemana,∗, jaroslav valacha, petr zlámala, nela krčmářováa, veronika koudelkováb, jaroslav zemanc a czech technical university in prague, faculty of transportation sciences, department of mechanics and materials, na florenci 25, 110 00 prague 1, czech republic b czech academy of sciences, institute of theoretical and applied mechanics, prosecká 809/76, 190 00 prague 9, czech republic c pandrol cz, grupe delachaux, jankovcova 938/18a, 170 00 prague 7, czech republic ∗ corresponding author: zemanl16@cvut.cz abstract. the article presents a study of the mechanical processes occurring during the aluminothermic reaction using experimental methods (strain gauges, digital image correlation, thermography, scanning electron microscopy, profilometry). the aluminothermic reaction is a highly efficient welding method due to its exothermic behaviour, however, it places considerable demands not only on the welding technique, but also on the capabilities of the experimental methods used; these limitations are also discussed in the article. the aluminothermic reaction is associated with the formation of a localised heat source with a time evolution dictated by the technological procedure, which manifests itself in heat propagation to the surrounding weld material. the unequal evolution of the temperature field is the fundamental cause of the appearance of the heat affected zone or local deformations or surface curvature, which was the focus of the experimental methods deployed above and the results of which are shown in the article. keywords: aluminothermic reaction, strain gauges, hardness, profilometry, scanning electron microscope. 1. introduction in recent years, the welding process has focused on lowering the human factor and increasing the weld joint quality. the resulting material properties depend on the technological process of welding. production/formation conditions and operating conditions determine the lifetime and the rate of the degradation [1]. for this reason, it is necessary to experimentally observe this process and subsequently evaluate the significant mechanical properties. the welding process is a technology that is widely used in many applications (automotive industry, civil engineering, aerospace, etc.), especially in the case of aerospace, reliability and the associated reduction of the human factor are crucial [2]. the aluminothermic (at) reaction is a promising welding method used for welding rails, joining ground wires and in the construction of guideways for warehouse stackers. the advantage of the at welding technology over the commonly used riveting method is that the resulting joint has mechanical properties similar to those of the original element. moreover, in a riveted connection, a hole is formed in the profile which can initialise defects in the structures and, thus, affect the lifetime of the structure or joint. this supports the idea of applying the at welding technology within the aerospace field in the case of repairing a defective wing spar for example. at welding has its own qualities in terms of the mechanical properties of the resulting weld, including the distribution of the heat field throughout the profile [3]. this makes the at welding method better in its weld quality compared to [4, 5]. moreover, the flash butt welding (fbw) method has a disadvantage in the form of a softer weld compared to basic materials, whereas the metal inert gas/metal active gas (mig/mag) method is strongly dependent on the human factor (the welder’s experience). the at method has a low human factor incidence rate with regards to the appearance of defects in the weld, this fact was now confirmed by the company ssi schäfer, which replaced the mig/mag method with the at method at the beginning of year 2022, thus, reducing the influence of the human factor by more than 50% (non-public study of pandrol cz). to increase the reliability of the welded connection and to understand the welding process itself, it is necessary to use suitable experimental methods for the evaluation of the mechanical properties of the welding structure (weld, affected zone, etc.). here, contact (e.g., strain measurements using strain gauges [6]) and non-contact methods (e.g., optical strain measurements using thermal imaging, laser triangulation inspection [7]) can be used for the observation and evaluation of the welding process [8]. in addition to these macro-level methods, micro-level methods, such as the hardness method or the scanning electron microscopy technique can be performed to evaluate the weld-basic material interface quality or to view changes in the material properties in the affected zone [9, 10]. 89 https://doi.org/10.14311/app.2023.41.0089 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en l. zeman, j. valach, p. zlámal et al. acta polytechnica ctu proceedings 2. aluminothermic reaction the aluminothermic reaction process belongs to the field of powder metallurgy. it involves the composition of a combustible batch of metal oxide (with the target alloy) and aluminium, where aluminium is used as a reducing agent. since the final enthalpy of the product is higher than the enthalpy of the system at the beginning, the overall reaction enthalpy is negative, i.e., a high amount of heat is released during combustion, thus classifying the process as an exothermic reaction. in addition to the heat, the combustion process converts the metal oxide to pure metal, oxygen molecules bind to the aluminium to form alumina, a crystalline compound occurring naturally as a very hard mineral known as corundum. the corundum is lighter than the weld metal, in this case iron or structural steel of two profiles, the heb140 profile of 11.375 steel and the 60e2 profile of the r350 grade, and, therefore, it separates from the weld metal with the slag and remains on the surface. the basic ferritic exothermic reaction is given by equation (1). fe2o3 + 2al→ 2fe+al2o3 (1) the welding technology consists of igniting the mixture in a ceramic crucible to produce the reaction described above. when the reaction takes place, the fusible plug at the bottom of the crucible is melted and the weld metal is poured into the prepared mould (figure 1). the shape of the mould is determined by the dimensions of the profiles to be joined. thus, the slag and corundum can be cast by overflowing outside the mould into the prepared cup, to eliminate the mixture with the weld metal (the process is described more in [11]). in cooperation with the company pandrol ltd. (training centre in kralupy nad vltavou), the welding process of two different types of profiles (heb140 and 60e2) were experimentally investigated. the welding of the joined components was carried out at a neutral temperature (for the czech republic, in the range from 17 to 23 °c and given by ‘novelizace předpisu sždc s3/2 bezstyková kolej’) as this temperature is different for each country, region or area due to the climate of the area [12]. 3. experimental methods to describe the strain and changes in the structure during the at process, several experimental methods were used. these methods were chosen with regard to the possibility of obtaining a comprehensive description of the given issue. 3.1. strain gauge measurement contact strain measurements were performed using ge-lm11 (hbm, germany) foil strain gauges (sgs), sensors with a 6mm active length in the wheatstone quarter bridge configuration. the strain signals from the sgs were recorded using the daq mgcplus solution (hbm, germany) with a 1 s/s sample rate. the strain gauges were bonded to the profile (the position of each sg is depicted in the scheme in figure 2) with an x280 epoxy adhesive (hbm, germany) in locations where the expected temperature was below 175 °c. this temperature was set as a safety value to keep the sgs functional. 3.2. digital image correlation (dic) method in addition to the sg measurements, the strain evaluation was performed using the dic technique [14]. unlike the sgs, this non-contact method enables the measurement of the full-field displacement and strains and/or to define the virtual sgs. an eos 550d (canon inc., japan) digital single-lens reflex (dslr) camera attached with an ef 70–200mm f/2.8l lens (canon inc., japan) to observe the welding process. a black-and-white stochastic pattern was sprayed on the surface of the welded profile (in the hot section from 50 to 150mm from the weld axis) to provide better contrast and to make the dic procedure more precise. an in-house dic software toolkit was used for the displacement evaluation in the series of images (5202× 3467 px image resolution, approx. 28µm in pixel size) captured during the welding procedure with a sample rate of 0.2 fps [15]. the tracking of this dic algorithm consists of two steps. in the first step, the correlation is evaluated at the pixel level using template matching employing the sum of squared differences (ssd) method. in the second step, the sub-pixel precision is obtained according to the interpolation using a third-order bivariate spline over the pixel grid. the resulting best match is found based on the minimisation of the bivariate spline using the limited-memory broyden–fletcher–goldfarb–shanno (lm-bfgs) algorithm [16]. 3.3. thermal imaging the experimental set up was complemented with a flir sc7600 thermal imaging camera to highlight the rate of the heat input and the overall distribution of the temperature field across the profile. the thermal image acquisition was performed at 115 fps and, due to the technical limitations of the camera, it was necessary to cover the hottest area with a foil strip. the resulting experimental set up capturing the welding process can be seen in figure 3. 3.4. profile and hardness measurement after the welding process, it is necessary to evaluate whether the weldment meets the requirements according to the type of application, with an emphasis on the weld geometry and the hardness distribution over the weldment travel surface. a satisfactory geometry is a flatness that is limitingly close to the absolute plane, according to the s3/2 regulation. there is an overhang tolerance of 0.5mm along half a metre distance on each side of the weld axis. profile laser scanning (scancontrol llt 2800-25, micro-epsilon, 90 vol. 41/2023 experimental evaluation of aluminothermic welds figure 1. at process of the pla (préchauffage limité) short pre-heating method using an automatic high flow preheating (hfp) unit and a one-shot crucible [13]. figure 2. time-strain dependency of the individual sgs. some of the sg records are missing or shortened due to damage/failed sensor bonding. figure 3. experimental set up for the strain and thermal measurements of the at welding process. 91 l. zeman, j. valach, p. zlámal et al. acta polytechnica ctu proceedings figure 4. step-shift laser head assembly for measuring the flatness of the running surface. usa) based on the laser triangulation method was performed on both profiles (see figure 4). the preparation of the surface includes polishing the scanning area and a layer of matte white paint to mitigate the frequency of the reflected beam. for the hardness measurements, it is assumed that the hardness value of the weld should be higher than the original material, otherwise deformations in the joint would occur during the process. to inspect the hardness profile of the heat affected zone (haz), a set of indents (poldi 568 01, meopta, czech republic) in accordance with the vickers hardness test method were performed (see figure 5). before the indentation, it was necessary to prepare the measured surface. to achieve a suitable surface roughness, the polishing procedure described in‘monitoring of material surface polishing procedure using confocal microscope’ was applied [17]. this procedure was consistent with the surface preparation procedure specified by the technology provider. 3.5. sem analysis before the scanning procedure, it was necessary to prepare the sample. a block of the sample with dimensions of 20×200×40mm was cut from the profile. the surface of the sample was polished using a diamond abrasive with a grain size of 3µm to achieve the proper surface for etching with 5% nitric acid and alcohol (nital). after the etching process (approx. 2minutes), the nital was cleaned off with alcohol and then dried. the nital etching procedure was necessary to reveal the grain boundaries on the polished surface. scanning electron microscope (sem) analyses (jeol jsm-it200 and tescan mira ii lmu) were employed to reveal the phase changes in the structure, map the thermally affected area in detail, investigate the weld-base metal interface and perform a chemical analysis of the material with an energy-dispersive detector (eds) detector. figure 5. hardness assembly using the meopta vickers apparatus on the heb 140 profile. 4. results and discussion 4.1. strain gauge measurement the time-strain dependencies evaluated based on the values measured by the individual sgs show a certain trend with regards to the symmetrically placed strain gauges. point a indicates the deformation peak at the end of the pre-heating, point b corresponds to the time when the at reaction was completed which means that the liquid metal is in weld form. point c represents the strain values obtained at the time of the maximum heat input (measured using thermal imaging, see figure 8). it is also possible to see the symmetry of the curves between the sg locations along the profile, thus it can be discussed that the magnitude and direction of the curves is affected by the shape and thickness of the profile. some sgs become defective due to the high temperatures at which they were exposed to or from the epoxy peeling off the sample. using a capsule sgs, attached by spot welding, would increase the resistance to high temperatures, but their high cost and greater difficulty in mounting them must be considered. 4.2. optical measurement the captured images, in combination with the dic technique, allow one to determine the displacement/strain field or established set of the ‘virtual’ sgs on the surface of the sample. this strain evaluation technique is non-contact, which is beneficial with regards to the sg failure due to the high temperatures on the surface of profile. in the case of the performed experiments, five sub-areas were selected in the image of the surface of the observed sample. in each sub-area, four correlation points (placed in the corner) were tracked through the image sequence to determine the in-plane displacement vector. the calculated displacement values of all the correlation points in each sub-area were averaged to obtain statistically significant values. the resulting time-vertical/horizontal displacement dependencies evaluated using the dic technique are shown in figures 6 and 7. the individual curves indicate the displacement horizontally up 92 vol. 41/2023 experimental evaluation of aluminothermic welds figure 6. comparison of the average values of the horizontal displacement of the individual correlation sub-areas. figure 7. comparison of the average values of the vertical displacement of the individual correlation sub-areas. and vertically down the weld axis, the thermal dilation can be calculated from the data and the curves matched the trend from the thermographic camera recording. the different heights of the curves are caused by the different amount of heat input as each correlation area is at a different distance from the weld. therefore, the closer the correlation area is to the weld, the values of the horizontal displacement are higher. 4.3. thermographic measurement the at reaction was monitored by a thermal imaging camera to achieve the distribution of the thermal field across the welded profile. due to the technical limitations of the thermal imaging camera (temperature measurability up to 300 °c), the positions of the ‘virtual’ thermal measurement points had to be selected from the thermal field according to this limitation. the positions of the thermal measurement points were selected to be at a distance of 240 to 270mm from the weld axis on both sides of the profile (see the scheme in figure 8). similar to the sg technological limitations for high temperatures, the thermal imaging camera is designed for a maximum temperature of 300 °c. for safe measurement, a maximum temperature of 200 °c was established which corresponds to the mentioned distances, which were calculated from knowledge of the heat propagation in the temperature courses observed in [18]. in the thermographic record, shown in figure 8, the effect from the pre-heating and at reaction (two peaks at approx. 600 seconds) and the subsequent heat input can be observed. the trend of the thermographic curves of the heb140 profile is similar to the trend of the 60e2 profile. as the heb140 profile differs in thickness, the trend is slightly shifted. 4.4. profile measurement the geometry of the plane of the running surface was measured after the metal had cooled. the so-called settlement of the weld after hot grinding has a trend according to the temperature of the material. the higher the temperature, the longer the time is required 93 l. zeman, j. valach, p. zlámal et al. acta polytechnica ctu proceedings figure 8. thermographic record of the temperature profile for the 60e2 profile. the blue zone describes the effect from the pre-heating and at reaction (500–700 seconds). to correct the geometry. this measurement confirms the flatness guarantee provided by the manufacturer when following the technological procedure for the hot grinding of the weld metal. the tolerance for the flatness of a quarter of a metre from the weld axis on each side is ±0.5mm. the plotted lines of the height profile in 2d show differences of up to 0.2mm (see figure 9 and figure 10) and these differences are within the required limit. the same measurement was performed for the heb140 profile, where the results were similar and also satisfy the prescribed tolerance. 4.5. hardness measurement in contrast to the other welding methods, where the weld hardness is lower than the original material, in the case of the at reaction, the hardness of the resulting weld is higher than the hardness of the original material. simultaneously, there is a haz issue that can be mitigated or eliminated by tempering during cooling. due to the higher hardness, there is not an increase in the deformation under the action of operating loads and, thus, there is not any reduction in the service life. figure 11 shows the hardness measurement performed on the running surface of the welded joints. 4.6. sem analysis the sample was etched and analysed using the secondary electron, back-scattered electron (se, bse) and eds detectors. however, only se images are presented due to the significant appearance of the surface (figure 12). to examine the surface, the grain sizes of each metal phase and the elemental composition the se images were sufficient. the investigation indicated lamellar cementite with a perlitic structure of the weld and base material, with different grain sizes corresponding to the introduced temperature. in addition, a softened area (i.e., the haz) with a spherical configuration of lamellar cementite can be observed and all the results were compared with the findings published in [18]. this configuration creates a thermally softened zone, which is a major issue in general welding. 5. conclusion the aim of this study was to evaluate the different experimental methods used to assess the progress of the at reaction. the work provides background knowledge for further improvement of the experimental methods, which pushes the design limits. the used experimental methods allowed us to confirm the existence and the rate of the processes that take place during the at welding. the processes that are known from theory have been demonstrated and their magnitude determined. on the basis of the thermography imaging, it was possible to show the rate of the thermal influence and the maximum temperature at various locations of the profile. this temperature field caused the thermal expansion observed by both strain evaluation methods, i.e., the sgs and the dic method. the welding process affected the structure of the metal not only at the weld location, but also in the surrounding area. this finding was demonstrated both by measuring the hardness as a quantity involving the influence of processing and the “history” of the material and by studying the microstructure revealed by the sem. the fact that at welding, despite its turbulent nature, is a technologically mastered process was also confirmed by laser profilometry, which showed a very precise connection of the welded parts. the results and findings suggest the idea of repairing titanium alloy beams, either main or auxiliary ones, which form the airframe 94 vol. 41/2023 experimental evaluation of aluminothermic welds figure 9. height profile in 3d determined using the laser scanning technique. the colour scale indicates the height level, the edge of the profile has a radius, thus causing the colour transition. however, the plane along the profile is unchanged by the colour scale, this indicates a surface without roughness. figure 10. height profile in 2d determined using laser scanning technique. figure 11. hardness measurement of the running surface of the welded joints. 95 l. zeman, j. valach, p. zlámal et al. acta polytechnica ctu proceedings (a). (b). (c). figure 12. sem analysis: the weld metal includes a larger grain than the base metal with cementite lamellae (top images), haz – spherical configuration of the cementite lamellae compared with the literature [18] (bottom images). 96 vol. 41/2023 experimental evaluation of aluminothermic welds structure, using the at reaction method is possible. this idea is supported by the possibility of the titanium alloy preparation introduced in [19]. moreover, there is another important advantage with regards to the significant reduction in the human factor in the welding process. the application of this method would lead to a relatively fast repair of a deformed aircraft beam by cutting off the damaged part and replacing the new one with a welded joint. another reason to use this method is the disadvantage of the existing rivet joints which is due to the rivet holes that are the initiators of cracks and directly affect the durability and reliability of the structures. references [1] p. mendez, t. eagar. new trends in weldind in the aeronautic industry. in 2nd conference of new manufacturing trends. 2002. 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https://doi.org/10.1088/1361-6501/aac55b https://doi.org/10.1016/j.msea.2022.143680 https://doi.org/10.1007/bf01589116 https://doi.org/10.1016/j.conbuildmat.2021.122249 https://doi.org/10.1016/j.conbuildmat.2021.122249 https://doi.org/10.1016/s0925-8388(02)00999-4 acta polytechnica ctu proceedings 41:89–97, 2023 1 introduction 2 aluminothermic reaction 3 experimental methods 3.1 strain gauge measurement 3.2 digital image correlation (dic) method 3.3 thermal imaging 3.4 profile and hardness measurement 3.5 sem analysis 4 results and discussion 4.1 strain gauge measurement 4.2 optical measurement 4.3 thermographic measurement 4.4 profile measurement 4.5 hardness measurement 4.6 sem analysis 5 conclusion references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.34.0069 acta polytechnica ctu proceedings 34:69–74, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague environmental perspectives of fine grounded concrete powder: lca case study of light-weight concrete block jan peštaa,c,∗, zdeněk prošekb,c a university of chemistry and technology, faculty of environmental technology, technická 5, 166 29 prague 6, czech republic b czech technical university in prague, faculty of civil engineering, thákurova 7, 166 29 prague 6, czech republic c czech technical university in prague, university centre for energy efficient buildings, třinecká 1024, 273 43 buštěhrad, czech republic ∗ corresponding author: jan.pesta@cvut.cz abstract. the construction elements produced with low demand for primary resources are being developed in the building industry. however, not only consumption of primary resources but also environmental impacts of such products should be considered to increase the sustainability of the construction industry. to investigate the environmental impacts of lightweight concrete blocks, life cycle assessment (lca) was used as an analytical tool according to the en 15 804 + a2. firstly, the properties of fine grounded concrete powder (frwc) were measured to declare functionality and potential to use of the considered product. then, the lightweight concrete block containing frwc was designed. the potential environmental impacts of the lightweight concrete block were assessed for the following phases: raw material production, transport of materials, production of concrete block, and end of life of the block. the considered system boundaries includes the production of fine grounded concrete powder, which was produced by recycling demolished concrete structures. the results were related to 1 tun of lightweight concrete block. unsurprisingly, the calculated impact is mainly influenced by cement production. the total impact of the life cycle of 1 t of the lightweight block is 336 kg co2 eq. on the other hand, the lightweight block reached better results in the comparison with the aerated autoclaved concrete block in most of the categories. keywords: light-weight concrete, environmental aspect, lca. 1. introduction during the design phase of a product, the equilibrium among three pillars of sustainability (environmental, economic, and social) is balanced. the environmental perspective is based on the natural limitation of our planet, which can be considered as a system with limited resources. in such a system with limited resources, the production of new products is limited without developing of circularity of materials. the global circularity gap, the difference of materials consumed in the global economy and materials, which are recycled or reused, is more than 90 % [1]. especially, construction and demolition waste represent almost one-third of european waste [2]. moreover, the construction industry consumes more than 30 % of domestic material consumption in the czech republic [3]. to increase circularity, new construction products are being developed containing recycled content. this contribution describes the environmental performance of such a product, which was designed to use recycled concrete. one possible way, how to recycle concrete waste, is the production of fine grounded concrete powder. in comparison with processes, in which concrete is recycled to be used as recycled aggregate, this process due to fine grinding leads to the production of concrete powder with utilization as an admixture in the concrete mixture for lightweight concrete block. to assess environmental perspectives of fine grounded concrete powder (frwc), the case study considering the lightweight concrete block was conducted using life cycle assessment (lca) as a method to analyse environmental benefits and burdens caused by the life cycle of product or service. this method considers not only elementary flows of materials or elements and energies, but it also describes potential impacts, which can be caused by these flows in the environment. the life cycle assessment is standardized by iso 14 040:2006 [4], which describes conditions for the assessment of products and services. the results of such assessment can be used for environmental product declaration (epd) [5]. the epd for construction products is standardized by en 15 804+a2 [6], which also states the method for characterization of environmental impacts. in this contribution, lca was used to assess environmental impacts related to 1 t of the lightweight 69 https://doi.org/10.14311/app.2022.34.0069 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en jan pešta, zdeněk prošek acta polytechnica ctu proceedings concrete block. the assessed system boundaries include recycling of concrete and production of frwc. the environmental performance of considered concrete block was compared with environmental impacts declared in the epd of aerated autoclaved concrete. 2. materials and methods 2.1. lightweight concrete block the lca study was conducted to assess the environmental performance of a lightweight concrete block containing fine recycled waste concrete (frwc), which was used as a fine filler. the density of the block is 800 kg/m3. the proportions of the block are as follows: 500 mm length, 250 mm height, 175 widths. the concrete mixture is described in the following table 1. materials kg per t of blocks superplasticizer (spl) 10 cement i 42.5 r 331 frwc 490 recycled polypropylene fibres 8 water 157 foaming agent 7 table 1. description of materials used for 1 t of blocks. measured energy consumption in the process of preparing concrete mixture (mixing and foam preparation) was 15 kwh for the amount of concrete mixture used for the production of 1 t of blocks. the fire resistance and mechanical properties of the mixture were improved by using 100 % recycled polypropylene fibres. according to their manufacturer (trevos, s. r. o., czech republic), they have 32×10−3 mm in diameter and 4 mm in length. other properties are following: average tensile strength, <3.0 cn/dtex (∼272 mpa); average elongation, ≤50 %; density, 910 kg/m3; young modulus of elasticity, ∼4 gpa. the properties of concrete mixtures were improved by adding polycarboxylates as a superplasticizer (spl). 2.2. production of fine recycled waste concrete frwc was produced by crushing 100-years old concrete to a 0–32 mm fraction on the site of the recycling plant. as a by-side process, the steel scrap was removed from the crushed structure. in the following step, the crushed concrete was transported to the other recycling facility, where frwc was produced by grinding of 0-1 mm fraction of crushed concrete using a high-energy electric mill (sbd 800 from lavaris ltd.). the energy consumption of milling was estimated as 6.25 kw per milled tun of concrete. moreover, steel spins are considered to have a high rate of wear and so, the amount of steel spins used for the production of 1 t is calculated as 0.27 g per 1 t of recycled concrete. during the milling, no losses of recycled concrete are considered. after milling, frwc was transported to the plant, where concrete was prepared by mixing. 2.3. environmental assessment the environmental assessment was performed using the life cycle assessment method [7]. this method is standardized according to iso 14040 [4]. based on this standard, the study was conducted in the four basic steps: goal and scope definition, life cycle inventory, life cycle impact assessment, and life cycle interpretation. these four steps are iterative and the results of each of them can be used for assessment, which will be more appropriate for the goal and purpose of the study. the core category rules for building products are standardized by the european standard en 15 804 + a2 [6], which was applied in this study. in this study, the goal was to assess the environmental impacts of 1 t of concrete blocks containing frwc. the purpose of this study is to discuss the environmental burdens or benefits of using frwc for the production of concrete blocks with similar utilization as aerated autoclaved concrete. 2.3.1. system boundaries in compliance with en 15 804 + a2, the system boundaries include the following phase of the life cycle of the block: mining of raw materials, production of materials (including recycled concrete), transport of materials, preparing of concrete mixture, and end of life of the product (eol). the eol phase includes deconstruction, transport of waste materials, and landfilling of construction waste. during the life cycle, the use phase of the block is not considered. the reference service life is assumed to be higher than 50 years. 2.3.2. life cycle inventory gabi professional was used as software model to describe the boundaries of the system and to assess the functionality environmental impacts [8]. the model was used to describe elementary energies and materials flowing through considered system boundaries. in the following step, the potential environmental impacts caused by these elementary flows were assessed. for the creation of the model, specific data about the production of frwc were used. to model the production of other materials, generic data from the gabi database were used [9]. also, generic data were used for models of unspecific processes such as transport using trucks, czech electricity grid mix production, landfilling of construction waste, and diesel supply. 2.3.3. life cycle impact assessment in this step, elementary flows of described system are classified and characterized. classification of elementary flows represents the assigning of the flow to the impact category in the case, that the flow has the potential to cause impact in that category. one flow can be assigned in several impact categories. in the 70 vol. 34/2022 environmental perspectives of fine grounded concrete powder non renewable energy total raw materials transport production of end of life resources production concrete crude oil (resource) 2.12e+01 1.45e+01 1.31 3.24e-02 5.31 hard coal (resource) 7.06 5.77 8.42e-03 6.39e-01 6.45e-01 lignite (resource) 3.06e+01 2.42e+01 1.03e-02 5.70 6.60e-01 natural gas (resource) 9.94 8.22 1.01e-01 2.54e-01 1.36 peat (resource) 8.08e-03 6.95e-03 1.52e-04 7.88e-05 8.98e-04 uranium (resource) 3.41e-04 2.45e-04 4.97e-07 8.52e-05 1.05e-05 table 2. consumption of non renewable energy resources during life phases of 1 t of lightweight concrete block (consumption in kg). characterization, the potential impact of flow is calculated. the calculation is performed by multiplying of the amount of a flow by its characterization factor for each environmental indicator. characterization factors are estimated by characterization models and in this study, characterization models were used in compliance with the en 15 804 + a2. the calculated results of some environmental indicators represent the potential environmental impact in the category. 3. results results of environmental assessment of lightweight concrete block containing fine recycled concrete powder are described in the following chapter. 3.1. life cycle inventory outputs the life cycle of the lightweight concrete block can be assessed based on the consumption of resources. in table 2, the energy resource consumption is described. regarding consumption of crude oil, the processes related to transport in the life cycle are connected with lower consumption than raw material production. moreover, natural gas consumption is significantly influenced by the end of life processes. however, the processes of raw materials production have the highest consumption regarding all of the considered energy resources. 3.2. potential environmental impacts in this study, environmental indicators according to en 15 804 + a2 were applied to assess potential environmental impacts related to 1 t of the lightweight block. the results of this assessment are described in table 3. the raw materials production phase is responsible for the biggest contribution to impacts in each impact category. however, the eol phase has also a high impact in the category of water use (1.70 m3 world equiv.), which represents almost 21 % of the whole impact in this category. similarly, the production of concrete contributes almost 20 % to the impact in the ionising radiation category. this result is mainly influenced by the production of electricity for processes. nevertheless, the production phase and transport processes cause negligible impact compared to the raw production phase. 4. discussion 4.1. contribution analysis the contribution of each process was analysed to declare the processes with the highest influence on potential impact assessed for the life cycle of 1 t of the lightweight block. in table 4, the relative contribution of processes to impact in a category are declared. categories climate change – fossil, biogenic, and land-use change are not included. in the table, processes contributing more than 10 % of the total impact in each category are stated. results of contribution analyses were conducted only with environmental indicators, which are characterized according to en 15 804 + a2. the concrete recycling process contributes beneficially to several impact categories. unsurprisingly, recycling of concrete decreases the overall impact in the resource use (mineral and metals) category. moreover, the recycling of concrete cause a beneficial impact in the water use category. on the other hand, the process, which is most contributing to several categories, is cement production. the impact of this process represents more than 80 % of impact in the climate change (total) category. also, it is significantly contributing to the impact in categories such as acidification, eutrophication, photochemical ozone creation, resource use (fossil), and water use. the impact in these categories is also influenced by the contribution of landfilling process, which describes environmental impacts caused by construction inert material disposed on the landfill. superplasticizer production is another process, which affects the impacts of considered lightweight concrete blocks. this process, which represents the production of 10 kg of superplasticizers, contributes 96 % to ozone depletion and it significantly increases the results of three other impact indicators (eutrophication freshwater; resource use, mineral and metals; water use). 4.2. comparison of the lightweight block with an aerated autoclaved concrete block the lightweight concrete block is intended to be a commercial product and so its environmental performance was compared to another block, which is available 71 jan pešta, zdeněk prošek acta polytechnica ctu proceedings total raw materials transport production of end of production concrete life climate change total [kg co2 eq.] 3.36e+02 2.99e+02 4.47 9.76 2.31e+01 climate change, fossil [kg co2 eq.] 3.36e+02 2.98e+02 4.44 9.68 2.35e+01 climate change, biogenic [kg co2 eq.] 2.23e-01 6.44e-01 -5.29e-03 6.10e-02 -4.76e-01 climate change, land use and land use change [kg co2 eq.] 3.81e-01 2.19e-01 3.63e-02 1.30e-02 1.13e-01 ozone depletion [kg cfc-11 eq.] 1.79e-06 1.79e-06 8.77e-16 1.19e-13 6.10e-14 acidification [mole of h+ eq.] 7.05e-01 5.13e-01 1.47e-02 2.25e-02 1.55e-01 eutrophication, freshwater [kg p eq.] 3.70e-03 3.61e-03 1.32e-05 2.40e-05 5.05e-05 eutrophication, marine [kg n eq.] 1.94e-01 1.32e-01 6.76e-03 4.45e-03 5.09e-02 eutrophication, terrestrial [mole of n eq.] 2.11 1.43 7.55e-02 4.64e-02 5.62e-01 photochemical ozone formation, human health [kg nmvoc eq.] 5.75e-01 4.19e-01 1.33e-02 1.23e-02 1.30e-01 resource use, mineral and metals [kg sb eq.] 2.35e-04 2.31e-04 3.94e-07 1.45e-06 2.18e-06 resource use, fossils [mj] 2.14e+03 1.62e+03 5.91e+01 1.44e+02 3.13e+02 water use [m3 world equiv.] 9.66 7.78 4.12e-02 1.36e-01 1.70 resource use indicators use of renewable primary energy (pere) [mj] 3.07e+02 2.39e+02 3.40 3.14e+01 3.36e+01 total use of renewable primary energy resources (pert) [mj] 3.07e+02 2.39e+02 3.40 3.14e+01 3.36e+01 use of non-renewable primary energy (penre) [mj] 2.14e+03 1.62e+03 5.93e+01 1.44e+02 3.13e+02 total use of non-renewable primary energy resources (penrt) [mj] 2.14e+03 1.62e+03 5.93e+01 1.44e+02 3.13e+02 use of renewable secondary fuels (rsf) [mj] 1.64e-22 1.64e-22 0.00 0.00 0.00 use of non renewable secondary fuels (nrsf) [mj] 1.93e-21 1.93e-21 0.00 0.00 0.00 use of net fresh water (fw) [m3] 6.34e-01 5.27e-01 3.90e-03 4.64e-02 5.68e-02 output flows and waste categories hazardous waste disposed (hwd) [kg] 8.40e-04 8.40e-04 3.13e-09 3.16e-08 2.73e-08 non-hazardous waste disposed (nhwd) [kg] 1.00e+03 2.78 9.31e-03 5.75e-02 1.00e+03 radioactive waste disposed (rwd) [kg] 6.43e-02 4.37e-02 1.08e-04 1.82e-02 2.28e-03 optional indicators particulate matter [disease incidences] 8.46e-06 6.67e-06 8.51e-08 1.71e-07 1.54e-06 ionising radiation, human health [kbq u235 eq.] 7.52 6.05 1.57e-02 1.21 2.43e-01 ecotoxicity, freshwater [ctue] 1.10e+03 8.09e+02 4.39e+01 5.28e+01 1.97e+02 human toxicity, cancer [ctuh] 7.65e-08 5.60e-08 8.87e-10 1.10e-09 1.85e-08 human toxicity, non-cancer [ctuh] 5.00e-06 2.92e-06 5.24e-08 7.43e-08 1.95e-06 land use [pt] 4.51e02 3.03e+02 2.03e+01 4.74e+01 8.09e+01 table 3. results of environmental indicators for the life cycle of 1 t of the lightweight block according to en 15 804 + a2. 72 vol. 34/2022 environmental perspectives of fine grounded concrete powder concrete cement landfilling of polypropylene steel spl recycling (cem i 42.5) construction fibres parts process production matter production production climate change total -2.83 80.94 4.39 5.31 0.31 3.30 ozone depletion 0.00 0.00 0.00 0.00 3.01 96.04 acidification -1.32 61.45 15.28 3.33 0.76 5.52 eutrophication, freshwater 0.13 3.43 0.69 0.87 11.84 80.51 eutrophication, marine 1.28 55.09 14.41 3.60 0.62 3.74 eutrophication, terrestrial 1.50 55.12 14.53 3.48 0.52 3.54 photochemical ozone formation, human health -0.08 60.15 14.74 4.73 0.52 4.84 resource use, mineral and metals -10.97 7.70 0.61 1.63 9.91 89.69 resource use, fossils -3.35 33.49 9.38 28.63 0.66 11.14 water use -24.03 28.51 16.77 1.50 5.14 58.59 table 4. the relative contribution of processes (only processes with relative contribution higher than 10 % of total impact in a category are stated), %. frwc block aac climate change total [kg co2 eq.] 2.51e+02 2.32e+02 ozone depletion [kg cfc-11 eq.] 1.43e-06 1.90e-03 acidification [mole of h+ eq.] 4.40e-01 6.28e-01 eutrophication, freshwater [kg p eq.] 2.92e-03 2.35e-02 photochemical ozone formation, human health [kg nmvoc eq.] 3.56e-01 1.03e+00 resource use, mineral and metals [kg sb eq.] 1.86e-04 2.69e-03 resource use, fossils [mj] 1.46e+03 1.79e+03 water use [m3 world equiv.] 6.37e+00 1.40e+01 table 5. comparison of the environmental performance of lightweight block with aac block (results of a1-a3 modules. on the market. to investigate products with similar properties and their environmental performance, the environdec database was used as a database of products [10], which were assessed using lca and their environmental performance was declared in environmental product declaration according to en 15 804 + a2. to ensure comparability of products based on their functionality, the aerated autoclaved concrete (aac) blocks were searched. the aac produced by gasbeton was selected as the most appropriate alternative [11]. the selected product is called sysmic idro (density 580 kg/m3). the comparison of their environmental performance related to the 1 m3 od product is presented in table 5. in most of the categories, the lightweight block reached better results than the aac block. on the other hand, the aac block contributes less to the impact in the category climate change. this comparison is limited by system boundaries. the environmental performance of the aac block was calculated only in the cradle-to-gate scope, which includes only a1-a3 phases, so the eol phase is not included in this comparison. the result of the frwc block was calculated based on a measured density of 800 kg/m3. 5. conclusion the lca was performed for the lightweight concrete block. the environmental indicators were used according to en 15 804 + a2 and so the results can be compared with another construction product with 73 jan pešta, zdeněk prošek acta polytechnica ctu proceedings the same function. in the comparison with aac, the lightweight block reached lower environmental impacts in most of the categories. on the other hand, the production of lightweight block caused a higher impact (251 kg co2 eq.) than the production of aac (232 co2 eq.) considering the climate change indicator. unsurprisingly, the cement production influence mainly the result of the climate change (total) indicator. however, the recycling of concrete producing frwc have a beneficial impact not only in this category (-9.41 kg co2 eq.) but also in the categories such as resource use, mineral and metals ( 2.58e-05 kg sb eq.) and water use (-2.32 m3 world eq.). therefore, further research will be conducted to optimize cement reduction and increase the use of frwc. acknowledgements this work was supported by the grant of specific university research – grant no a1_ftop_2021_003. references [1] the circularity gap report 2021. [2] strategy for a sustainable built environment. [3] zpráva o životním prostředí české republiky 2015. tech. rep., 2015. [4] m. finkbeiner, a. inaba, r. tan, et al. the new international standards for life cycle assessment: iso 14040 and iso 14044. the international journal of life cycle assessment 11(2):80–85, 2006. [5] iso. environmental labels and declarations — type iii environmental declarations — principles and procedures(iso 14025:2006), 2006. [6] cen. en 15804:2012+a2:2019 sustainability of construction works environmental product declarations core rules for the product category of construction products. [7] j. guinee. handbook on life cycle assessment operational guide to the iso standards. international journal of life cycle assessment 6(5):255–255, 2001. https://doi.org/10.1007/bf02978784. [8] life cycle assessment lca software: gabi software. [9] t. kupfer, c. m. colodel, m. kokborg, et al. gabi database and modelling principles, 2020. [10] environdec, the international epd system. [11] gasbeton, environmental product declaration. 74 https://doi.org/10.1007/bf02978784 acta polytechnica ctu proceedings 34:69–74, 2022 1 introduction 2 materials and methods 2.1 lightweight concrete block 2.2 production of fine recycled waste concrete 2.3 environmental assessment 2.3.1 system boundaries 2.3.2 life cycle inventory 2.3.3 life cycle impact assessment 3 results 3.1 life cycle inventory outputs 3.2 potential environmental impacts 4 discussion 4.1 contribution analysis 4.2 comparison of the lightweight block with an aerated autoclaved concrete block 5 conclusion acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2023.42.0001 acta polytechnica ctu proceedings 42:1–5, 2023 © 2023 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague effect of aging on mechanical properties of 3d printed samples using stereolitography veronika drechslerováa,∗, jan faltaa, tomáš fílaa, radim dvořáka, daniel kytýřb a czech technical university in prague, faculty of transportation sciences, department of mechanics and materials, na florenci 25, 110 00 prague 1, czech republic b czech academy of sciences, institute of theoretical and applied mechanics, prosecká, 809/76, 190 00 prague 9, czech republic ∗ corresponding author: drechver@fd.cvut.cz abstract. this paper focuses on stereolithography (an additive manufacturing technology working on the principle of curing liquid resins layer by layer using ultraviolet radiation) and the effect of aging on the mechanical properties of the material and printed samples. the aging of the material could be a problem for its subsequent use as the stability of the mechanical properties would not be maintained and unwanted deterioration of the material could occur. as part of the research, sets of samples were printed and subjected to different aging methods and subsequently subjected to quasi-static and dynamic uni-axial load tests. from the data obtained, the basic mechanical properties of the material were calculated and compared with each other. the aim of this paper was to investigate whether aging process causes significant changes in the mechanical properties of the materials used, which could have a consequential impact on their use in different industries. keywords: stereolithography, aging, mechanical properties, quasi-static testing, dynamic testing. 1. introduction the rapid advancement of additive manufacturing technologies has revolutionized the production of complex and customized parts. stereolithography, a popular additive manufacturing technique, utilizes the principle of curing liquid resins layer by layer using ultraviolet radiation [1, 2]. compared to other additive manufacturing methods such as fused filament fabrication, sheet lamination or binder jetting, stereolithography achieves higher print accuracy, the ability to create individual intricate details and the possibility to produce parts with different mechanical properties. while stereolithography has gained significant attention for its versatility and applications, the influence of aging on the mechanical properties of printed material has been relatively limitedly investigated. aging processes (sunlight, x-ray), such as exposure to environmental factors, temperature fluctuations, or long-term usage, can potentially affect the structural integrity of the material and its overall performance [3, 4]. in a previous study, signs of the effect of aging on mechanical properties have already been observed, but it was difficult to find information on this phenomenon because the manufacturer did not provide this type of information and the scientific community has only recently started to focus on this issue. understanding the impact of aging on samples printed by stereolithography is crucial for evaluating the long-term reliability and usability of these materials in various industrial sectors [5]. in this study, we investigate the effect of exposing printed samples by stereolithography to various aging effects and investigate their effect on the mechanical properties of the material and printed samples and on their behavior under loading. for this purpose, the aged samples were tested under quasi-static and dynamic uni-axial loading to evaluate their mechanical response. 2. materials and methods 2.1. materials for manufacturing 3d samples the resin compatible (aque blue from phrozen) with 3d printer sonic mighty 4k (phrozen, taiwan) was used. this resin was chosen based on previous research of drechslerová 2023 [6] in which different resins and their mechanical properties were investigated. among the resins analysed, aque blue resin achieved satisfactory values of mechanical properties and the samples printed from this material showed the required resistance to the tested loading. the chemical composition and more detailed information about the resin are given in the data sheet [7]. 2.2. sample preparation inventor professional 2022 was used to design the test specimen models, and two types of test specimens were designed depending on the type of load test. for tensile loading, dog bone shape were designed. they were modeled based on the astm d638-14 standard. the basic proposed specimen dimensions are shown in figure 1 and listed in table 1. for easier 1 https://doi.org/10.14311/app.2023.42.0001 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en v. drechslerová, j. falta, t. fíla et al. acta polytechnica ctu proceedings figure 1. basic dimensions of tensile sample used from astm d638-14 standard. parameter dim. [mm] thickness t 4 gauge width w 6 overall length l 115 gauge length l0 40 curvature radius r 14 table 1. dimensions of the tensile test specimen. manufacturing and to obtain higher quality of printed samples, the samples were printed rotated 60 ◦ about the x-axis and placed on supports. for quasi-static and dynamic compressive loading, the bulk specimens were created. the design of the test specimens was based on the astm d695-15 standard, and the dimensions of the specimens were adapted to the loading devices. the basic proposed specimen dimensions are given in table 2 and shown in figure 2. to guarantee the quality of the 3d printed specimen, the specimens were rotated 40 ◦ rotation about the x and y-axes during manufacturing and placed on the supports (see figure 3). 2.3. samples post-treatment process test samples printed from resin require a posttreatment process to ensure that the final mechanical properties are achieved. as part of this process, the supports were removed, the samples were mechanically cleaned using spring water and chemically using isopropyl alcohol ultrasonic bath and then cured using a cure luna uv lamp (phrozen, taiwan). the specimens for tensile testing were cured for a total of one hour and the specimens for compressive loading were cured for 40 min. halfway through the time, the cured specimens were inverted to ensure uniform irradiation, including the underside lying on the rotating platform of the uv lamp. cure times were based on the manufacturer’s recommendation of 30 min for the resin. to ensure complete curing to avoid subsequent degradation of mechanical properties, slightly higher times than those given by the manufacturer were chosen. 2.4. aging methods the samples were subjected to two types of aging, sunlight and extreme x-ray irradiation. to avoid exposing the samples to unwanted influences in the intervening figure 2. basic dimensions of the sample for compression. parameter dim. [mm] base diameter w 18 overall height h 16 gauge diameter wc 5 gauge height h 8 table 2. dimensions of the compressive test specimen. figure 3. rotated sample about the x and y-axes. period between the completion of production and the application of the aging method under investigation, the samples were subjected to x-ray/sunlight immediately after removal from the uv lamp. the samples exposed to sunlight were placed between window panes to protect them from the weather conditions. the sets of samples were exposed to sunlight for 7 and 25 days during the month of may in prague to compare the dependence of the effect of a given aging type on the exposure time. using x-ray, the samples were placed one meter away from the radiation source and in the center of the radiation beam and irradiated for 1 and 2 h at 225 kv with radiation intensity of approx. 5000 r h−1. the spread of the samples over a area 2 vol. 42/2023 effect of aging on mechanical properties of 3d printed samples figure 4. printed samples subjected to x-ray. (see figure 4) did not affect the homogeneity of the radiation flux. to demonstrate the effect of aging on the mechanical properties of the material and printed samples, one set of samples was tested directly after production. 2.5. experimental procedure in order to determine the basic mechanical properties and loading behavior, the printed samples were subjected to three types of loading: • quasi-static tensile loading, • quasi-static compressive loading, • dynamic compressive loading. each set of test specimens contained 4 specimens, and a total of 20 tensile spesimens (4 sets – set for each aging effect and set of specimens tested immediately after production) and 40 bulk specimens (quasi-static and dynamic compressive testing) were tested. all specimens were measured and weighted before testing. quasi-static tensile testing was performed using an instron 3382 (instron inc., usa) universal testing machine, in which the test specimens were subjected to the uni-axial tension. the loading rate was set at 3 mm min−1 according to astm d638-14 standard and the test was carried out until the specimen was broken. for the purpose of subsequent test evaluation and to obtain the basic mechanical properties, the dependence of the absolute elongation ∆l of the test specimen on the applied axial loading force f was recorded during testing. after the test, the length of the sample was remeasured. the quasi-static compressive loading was performed on the same loading device as the tensile test. the loading rate was set at 1 mm min−1 and the test was terminated at the time of failure of the middle part of the specimen. during the test, as in the tensile test, the absolute change of the length of the test specimen ∆l was recorded depending on the applied axial loading force f . figure 5. the disintegrating sample in split hopkinson pressure bar. for dynamic compressive loading, the split hopkinson pressure bar (shpb) was used (figure 5) with the striker impact of velocity of 15 m s−1 [8]. using the strain mounted on the bars, the required data was obtained for subsequent evaluation in order to obtain a more detailed description of the behaviour during loading. 2.6. data evaluation all measured data from the load tests were evaluated in matlab (mathworks, inc., usa). to streamline the work, semi-automatic scripts were created and subsequently used to calculate the basic mechanical properties from quasi-static tensile loading and construct the stress-strain diagrams. the curves shown in the diagrams were obtained by averaging the individual specimens tested in a single set. the light broad line along each curve shows the scatter of the experimental values. for subsequent comparison, the obtained curves from each loading test were pasted into one stress-strain diagram. however, the data had to be adjusted (trimmed, smoothed, etc.) before the evaluation and calculation of the mechanical properties was performed. the basic mechanical properties investigated were: • young’s modulus of elasticity e in tension, • yield stress σ0.2 in tension, • tensile strength σu, • ductility d. the tensile strength σu and ductility d were determined from the measured dimensions of the test specimens before (the initial gauge length l0 and the cross-sectional area parameter ac) and after loading (the length of the specimen after breaking lu). these measured values had to be entered manually into the generated semi-automatic scripts. the ductility d and the strain ϵ were calculated on the basis of the following relations d = lu − l0 l0 · 100, (1) ϵ = △l l0 . (2) 3 v. drechslerová, j. falta, t. fíla et al. acta polytechnica ctu proceedings aging mechanical characteristic e σ0.2 σu ϵtb [mpa] [mpa] [mpa] % sunlight (0 days) 1500.66 ± 12.11 26.28 ± 0.85 38.40 ± 1.13 6.59 ± 1.46 sunlight (7 days) 2203.31 ± 32.44 37.60 ± 1.25 57.28 ± 0.47 1.39 ± 0.15 sunlight (25 days) 2108.38 ± 80.75 41.37 ± 1.88 57.08 ± 1.89 1.74 ± 0.25 x-ray (1 h) 1730.42 ± 76.49 32.97 ± 2.40 42.53 ± 2.80 3.43 ± 0.55 x-ray (2 h) 1669.45 ± 53.53 31.00 ± 1.31 41.76 ± 1.09 2.16 ± 0.35 table 3. tensile test mechanical characteristic values. furthermore, the tensile stresses σt and compressive stresses σc were calculated according to the following formulas σt = f ac = f t · w , (3) σc = f ac = f π · (0.5 · wc)2 , (4) where t is the thickness, w is the gauge width and wc is the gauge diameter of the tested specimen. the young’s modulus of elasticity e in tension and the yield stress σ0.2 in tension were obtained from auxiliary diagrams created within the script. the young’s modulus of elasticity e was defined as the ratio of stress to strain on the linear section of the tension curve. 3. results 3.1. quasi-static tensile loading the basic mechanical properties of the material were calculated from the measured tensile test data. the data obtained from the individual measurements were averaged and the average values of the individual mechanical properties are given in table 3. furthermore, average curves of the individual aging effects were generated from the average stress and strain. these average curves were compared with each other and are presented in figure 6. from the results obtained in table 3 it can be seen that highest values of deformation and stress were achieved by samples exposed to aging by sunlight, while the lowest values were achieved by samples tested immediately after manufacturing. samples exposed to x-ray for 1 and 2 h show similarly high values of deformation and stress, while the values of samples irradiated for 2 h lie within the measurement uncertainty range for 1 h of exposure. from a comparison of the curves representing the different effects of aging in the tensile diagram, it can be noticed that the curves corresponding to the samples exposed to solar radiation are almost identical, differing only by a different standard deviation. a similar trend can be observed for the x-ray exposed samples. it can also be observed from the curves that the samples exposed to sunlight show a greater strengthening of the material but an earlier failure compared to, for example, samples tested immediately after manufacture. it can also be seen that the trends of the curves of samples subjected to x-rays are similar, but that those subjected to x-rays for a longer period of time show earlier failure. 0 0.05 0.1 0.15 strain [-] 0 10 20 30 40 50 60 s tr es s [m p a] 0 days 7 days 25 days x-ray (1hr) x-ray (2hr) figure 6. quasi-static tensile: stress-strain diagram. 3.2. quasi-static and dynamic compressive loading the stress-strain diagrams were created from the measured compressive test data. the individual average aging curves were again compared with each other and from the resulting stress-strain diagrams (see figure 7) similar trends to those for tensile loading can be observed. the curves corresponding to the samples exposed to solar radiation have similar trends, showing a steeper increase in the elastic strain region, and higher strengthening. samples tested after 25 days have slightly higher values than samples tested after 7 days. the specimens tested immediately after manufacture show the highest resistance to failure. an important difference with tensile loading is the order of magnitude higher values achieved for compressive loading. when comparing quasi-static and dynamic compressive loading (see figure 8), significant differences in the loading behaviour of the specimens can be observed. the curves show different trends – the spec4 vol. 42/2023 effect of aging on mechanical properties of 3d printed samples 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 strain [-] 0 50 100 150 200 250 300 s tr es s [m p a] 0 days 7 days 25 days x-ray (1hr) x-ray (2hr) figure 7. quasi-static compressive: stress-strain diagram. imens tested by dynamic compressive test show up to twice as much strengthening and failure occurs much earlier (sudden rapid drop in stress in the diagram) than the specimens tested by quasi-static test. in contrast, the area of plastic deformation of these specimens is quite extensive. 4. conclusions using additive manufacturing called stereolithography, two types of 3d printed test specimens were produced – for tensile and compressive testing. subsequently, these samples were exposed to two types of aging – sunlight and extreme x-ray for varying lengths of time. these specimens were subjected to 3 loading tests: quasi-static tensile loading, quasistatic compressive loading, dynamic compressive loading. from the measured data, the basic mechanical parameters were calculated and stress-strain diagrams were plotted. from these, it was possible to observe the acquisition of different values of the basic mechanical parameters and differences in the individual trends of the curves in the diagram depending on the type of aging and the time of exposure. the samples exposed to solar radiation generally reached higher values than the remaining samples, but at the same time they showed earlier failure. other major differences were shown by comparing quasi-static and dynamic compressive loading, where dynamic loading resulted in a significant strengthening of the material during loading. based on the data obtained, it was shown that the aging process causes significant changes in the mechanical properties of the materials used, which could have a consequential impact on their use in different industries. acknowledgements there is very appreciated a kind sponsorship of grants from czech science foundation – junior star (project no. 22-18033m) and ctu internal project sgs22/196/ohk2/3t/16. 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 strain [-] -50 0 50 100 150 200 250 300 s tr es s [m p a] quasi-static dynamic figure 8. comparison of quasi-static and dynamic compressive loading. references [1] c. parulski, o. jennotte, a. lechanteur, b. evrard. challenges of fused deposition modeling 3d printing in pharmaceutical applications: where are we now? advanced drug delivery reviews 175:113810, 2021. https://doi.org/10.1016/j.addr.2021.05.020 [2] z. weng, y. zhou, w. lin, et al. structure-property relationship of nano enhanced stereolithography resin for desktop sla 3d printer. composites part a: applied science and manufacturing 88:234–242, 2016. https://doi.org/10.1016/j.compositesa.2016.05.035 [3] h. agrawaal, j. thompson. additive manufacturing (3d printing) for analytical chemistry. talanta open 3:100036, 2021. https://doi.org/10.1016/j.talo.2021.100036 [4] k. g. topsakal, m. aksoy, g. s. duran. the effect of aging on the mechanical properties of 3-dimensional printed biocompatible resin materials used in dental applications: an in-vitro study. american journal of orthodontics and dentofacial orthopedics 164(3):441–449, 2023. https://doi.org/10.1016/j.ajodo.2023.05.023 [5] n. mite-guzmán, m. lazo, j. triguero, et al. two-dimensional infrared for monitoring the structural variations of uv-aged recycled polypropylene straps used in the ecuadorian banana industry. case studies in chemical and environmental engineering 7:100359, 2023. https://doi.org/10.1016/j.cscee.2023.100359 [6] v. drechslerová, m. neuhäuserová, j. falta, et al. stereolithography for manufacturing of advanced porous solids. acta polytechnica ctu proceedings 41:1–7, 2023. https://doi.org/10.14311/app.2023.41.0001 [7] phrozen. phrozen aqua blue, 2020. [2023-08-04], https://c-3d.niceshops.com/upload/file/ msds_english-phrozen_aqua_blue.pdf. [8] j. šleichrt, t. fíla, p. koudelka, et al. dynamic penetration of cellular solids: experimental investigation using hopkinson bar and computed tomography. materials science and engineering: a 800:140096, 2021. https://doi.org/10.1016/j.msea.2020.140096 5 https://doi.org/10.1016/j.addr.2021.05.020 https://doi.org/10.1016/j.compositesa.2016.05.035 https://doi.org/10.1016/j.talo.2021.100036 https://doi.org/10.1016/j.ajodo.2023.05.023 https://doi.org/10.1016/j.cscee.2023.100359 https://doi.org/10.14311/app.2023.41.0001 https://c-3d.niceshops.com/upload/file/msds_english-phrozen_aqua_blue.pdf https://c-3d.niceshops.com/upload/file/msds_english-phrozen_aqua_blue.pdf https://doi.org/10.1016/j.msea.2020.140096 acta polytechnica ctu proceedings 42:1–5, 2023 1 introduction 2 materials and methods 2.1 materials 2.2 sample preparation 2.3 samples post-treatment process 2.4 aging methods 2.5 experimental procedure 2.6 data evaluation 3 results 3.1 quasi-static tensile loading 3.2 quasi-static and dynamic compressive loading 4 conclusions acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2025.53.0102 acta polytechnica ctu proceedings 53:102–107, 2025 © 2025 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague monitoring the reactivity of pozzolans by thermogravimetric method martin vyšvařil brno university of technology, faculty of civil engineering, veveří 331/95, 602 00 brno, czech republic correspondence: martin.vysvaril@vut.cz abstract. this work focuses on comparing the effect of ten different types of pozzolans on the strength of lime pastes in relation to their pozzolanic activities as determined by the chapelle test and monitoring the reactivity of these pozzolans using the thermogravimetric method. it was found that the chapelle test can in some cases overestimate the reactivity of the pozzolan due to differences between the conditions during the test and in reality. the strengths of lime-pozzolan pastes did not correlate well with the pozzolanic activity values of the pozzolans used. on the contrary, the amount of reacted lime in the pastes, determined by the thermogravimetric method, correlated very well with the compressive strengths of the pastes. keywords: pozzolan, pozzolanic activity, chapelle test, thermogravimetry, compressive strength. 1. introduction air lime-pozzolan mortars have a long tradition of application in the construction industry in many places in the world, especially in volcanic areas. pozzolanic materials are nowadays often ground and sieved into fine particles with a high specific surface area and impart hydraulic properties to air lime mortars. at the same time, air lime-pozzolan mortars retain most of the good characteristics of pure air lime mortars, such as low modulus of elasticity and fast drying. the addition of pozzolans therefore extends the potential range of applications for air lime mortars. the term pozzolanic activity is often used to describe the reactivity of the pozzolan, which includes all the reactions that occur between the active components of the pozzolan, calcium hydroxide and water. pozzolanic activity essentially defines two values, the maximum amount of lime that the pozzolan can react with (bind) and the rate at which it does so. both aspects depend on the type of pozzolan used and, above all, on the quality and quantity of the active ingredients it contains. other influencing parameters are lime/pozzolan ratio in the mix, treatment time, specific surface area, water content in the mix, chemical composition, and temperature [1]. pozzolanic activity can be determined by several direct or indirect methods, where direct methods monitor the presence of ca(oh)2 and subsequent reduction of its amount during the pozzolanic reaction using analytical methods, e.g. frattini test according to en 196-5 or chapelle test [2]. on the other hand, indirect methods are based on measuring mainly the physical properties of the tested samples or the composition of the pozzolan, which is an indicator of the degree of pozzolanic activity. these methods include chemical and mineralogical analysis of pozzolan, determination of reactive sio2 in pozzolan according to en 196-2, electrical conductivity test [3], calorimetric measurements [4], strength characteristics (astm c 593-19), or determination of strength activity index (sai) described in astm c311. the mentioned methods for determining pozzolanic activity differ from each other in terms of complexity, method of evaluation, time-consuming and price of their execution. some methods determine whether and to what extent a given raw material is pozzolanic active, others only characterize a certain property that affects the level of pozzolanic activity. the results of pozzolanic activity obtained by several methods are not always similar or comparable. donatello et al. [5] compared different test methods for determining pozzolanic activity, namely the sai, the frattini test and a similar frattini test, where the cement in the reaction mixture was replaced by lime water. a certain functional dependence was found only when comparing the frattini test and the sai. the results from the test with a saturated solution of ca(oh)2 did not correspond at all. liu et al. compared the differences of six pozzolanic evaluation methods on five pozzolanic materials [6] and concluded that sai is more objective in evaluating the activity based on strength, but there is also the aggregate effect on strength. the electrical conductivity test method is more suitable for evaluating the early pozzolanic reactivity. the evaluation results of the pozzolanicity test are limited by the slow dissolution rate of the active phase, and the dissolution characteristics of ca(oh)2. the high temperature in the modified chapelle test makes the method more suitable for evaluating low-reactivity materials. the evaluation system of the thermogravimetric method (tg) and differential thermal analyses (dta) is reasonable and most similar to the evaluation results of sai. a modified r3 method is proposed to realize a more accurate simulation of the interstitial solution and a more accurate evaluation of pozzolanic reactivity [6]. 102 https://doi.org/10.14311/app.2025.53.0102 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 53/2025 monitoring the reactivity of pozzolans by thermogravimetric method sio2 al2o3 fe2o3 σ zeolite 14.3 2.6 0.1 17.0 pumice 52.6 19.9 2.1 74.6 lava 0.0 3.9 4.7 8.6 spongilite 0.0 0.0 0.0 0.0 chalcedonite 0.0 0.0 0.0 0.0 pozzolanic earth 7.8 0.0 8.1 15.1 trass 4.1 1.0 5.5 10.1 diatomaceous earth 53.4 0.0 0.0 53.4 metakaolin k05 42.8 37.5 0.6 80.9 metakaolin l05 37.0 32.2 0.7 70.0 table 1. content of amorphous hydraulic oxides in pozzolans (wt. %). due to ambiguous results when comparing pozzolanic activity by different methods, the content of this paper focuses on the correlation between the strength of ten different lime-pozzolanic paste systems and the pozzolanic activity of the pozzolan used in these pastes determined by the chapelle test and the thermogravimetric method. 2. materials and experimental procedures commercial hydrated lime cl 90-s (carmeuse czech republic, s.r.o.) was used as a main binder in prepared pastes. ten different pozzolanic materials was used as a partially lime substitute, namely, the natural zeolite (0–125 µm, zeocem, a.s.) z, the pumice (0– 125 µm, vulkalit wr, vulcatec riebensahm gmbh) p, the ground lava sand (0–125 µm, der naturstein garten) l, the spongilite powder (0–125 µm, kalcit s.r.o.) s, the fine chalcedonite powder (0–20 µm, crusil spółka z o.o.) ch, the pozzolanic earth (0–125 µm, kremer pigmente gmbh & co) pe, the trass (0–125 µm, tubag) t, the diatomaceous earth (0–40 µm, lb minerals, s.r.o.) de, and two types of metakaolin (0–10 µm, mefisto k05 and mefisto l05, české lupkové závody, a.s). the particle size distribution of the pozzolans determined by laser particle analyzer, the chemical and mineralogical composition obtained by x-ray fluorescence (xrf) and x-ray diffraction analyses (xrd), as well as some fundamental physical parameters of pozzolans used have been published previously [7–11]. the chemical composition of pozzolans showed a high content of hydraulic oxides (sio2, al2o3, fe2o3) which provided very good conditions for its high pozzolanicity as according to the astm c618, the total pozzolanic content (i.e., the sum of sio2, al2o3, and fe2o3) must be a minimum of 70 % for class n pozzolans. all pozzolans except spongilite and lava fulfilled this condition. the reactivity of pozzolans is significantly influenced by its mineralogical composition. with the exception of chalcedonite and spongilite, the pozzolans contained an amorphous phase, mostly pumice, which was almost completely amorphous. however, the spongilite contained pozzolanically reactive opal and chalcedonite consisted of porous quartz grains with cryptocrystalline sio2 with very small crystal grains of µm size filling the void space of the quartz grains. the content of these small grains was reflected in the distinct specific surface area of the chalcedonite particles. the content of amorphous hydraulic oxides in pozzolans (table 1) was calculated from the total content of the respective hydraulic oxides, the known representation of individual minerals containing hydraulic oxides and the total content of the amorphous phase in particular pozzolans. it should be noted that in the case of chalcedonite, it was not possible to calculate the content of amorphous hydraulic oxides due to the indistinguishability of cryptocrystalline sio2 (chalcedony) and quartz by xrd. and also, there was 14.6 wt. % of opal in spongilite; opal is pozzolanically active but is not a typical amorphous phase (it is detectable by xrd). as can be seen from table 1, pumice and metakaolin had the highest content of amorphous hydraulic oxides. the pozzolanic activity of pozzolans was determined by the modified chapelle test method according to [12] after 1 day and also after 2, 3, 4, and 5 days of reaction with ca(oh)2 in an autoclave at 90 °c (table 2). very high values of pozzolanic activity were achieved by metakaolin, zeolite, chalcedonite, and diatomaceous earth partly due to their porous nature and thus large specific surface area, and also due to the content of reactive sio2 and al2o3. pumice, which had a significantly higher content of amorphous hydraulic oxides than zeolite, had lower pozzolanic activity values after two or more days. it is possible that after the rapid initial pozzolanic reaction, it was further slowed down by the rapid increase of reaction products on the pumice grains and the slowing down of the diffusion of the ca(oh)2 solution to the surface of the pumice grains. the moderate pozzolanic activity of the spongilite confirmed the reactivity of the opal. pozzolanic earth did not reach very high values of pozzolanic activity despite the considerable content of the amorphous phase, due to the fact that more than half of the content of amorphous oxides was fe2o3, which is the least and slowest reactive of them. the higher content 103 martin vyšvařil acta polytechnica ctu proceedings pozzolanic activity [mg ca(oh)2 g−1] 1 day 2 days 3 days 4 days 5 days zeolite 729 1 220 1 263 1 390 1 395 pumice 887 910 958 1 002 1 022 lava 359 370 382 447 515 spongilite 694 803 849 887 910 chalcedonite 755 1 259 1 341 1 349 1 369 pozzolanic earth 513 685 795 870 871 trass 516 618 641 735 762 diatomaceous earth 998 1 147 1 185 1 257 1 292 metakaolin k05 1 065 1 159 1 196 1 283 1 286 metakaolin l05 1 135 1 180 1 224 1 247 1 266 table 2. pozzolanic activity of pozzolans determined by modified chapelle test. of amorphous fe2o3 at the expense of sio2 or al2o3, together with the overall low content of amorphous hydraulic oxides, resulted in relatively low values of the pozzolanic activities of the trass and especially of the lava. if the pozzolans were evaluated according to raverdy’s criterion [13], where the material is considered as pozzolanically active if its chapelle reactivity is 650 mg ca(oh)2 per 1 g of test material and more, it is clear that metakaolin, diatomaceous earth, pumice, zeolite, chalcedonite, and spongilite were pozzolanically active already after 1 day of treatment with ca(oh)2. pozzolanic earth met this condition after 2 days, trass after 4 days, and lava is not pozzolanically active according to this criterion. fine pozzolans were used as a partial replacement of lime in 40 % of the lime weight. the composition of pastes mixes is given in table 3. the amount of water required to achieve the same flow of 45 mm, determined by mini-slump cone test was added [14]. the freshly cast samples (20×20×100 mm) were freely covered by polyethylene foil to avoid their cracking caused by rapid drying. hardened pastes specimens were demolded after 48 h and then cured in a wet chamber at temperature t = (22 ± 3) °c and a relative humidity rh = (95 ± 5) % for 26 days. the reactivity of the pozzolans and the mineral content of the limepozzolan pastes were investigated by tg/dta after 7, 14, 21 and 28 days of aging. three 2 × 2 × 2 cm samples were cut from three different beams of each paste type, crushed and ground in a planetary ball mill for 3 minutes. tg/dta tests were carried out in the temperature range 40–1 000 °c at a heating rate of 10 °c min−1 in a nitrogen atmosphere. the weight of the powder sample analyzed was 300 mg. the compressive strength of pastes was determined after 28 days of curing according to en 1015-11. for the particular lime-pozzolan paste type a set of six samples was evaluated. 3. results and discussion thermogravimetric and differential thermal analysis are commonly used to characterize the composition mixture hydrated lime pozzolan h2o [g] [g] [ml] l-ref 100 0 80 l-z 60 40 80 l-p 60 40 72 l-l 60 40 52 l-s 60 40 70 l-ch 60 40 70 l-pe 60 40 57 l-t 60 40 62 l-de 60 40 120 l-k05 60 40 100 l-l05 60 40 92 table 3. composition of pastes. of lime-pozzolan systems and can also be used to determine the reactivity of pozzolans by monitoring the decreasing ca(oh)2 content over time. the ca(oh)2 content may be underestimated due to carbonation using traditional methods that only measure the mass changes resulting from the decomposition of ca(oh)2 in the temperature range of 400 to 600 °c. the effect of carbonation should be taken into account when calculating the remaining ca(oh)2. the ca(oh)2 content can then be obtained using equation 1 [15]. chr = 4.11(mch) + 1.68(mcc) (1) where chr is the mass of residual ca(oh)2 in the sample, 4.11 is the molar ratio of ca(oh)2/h2o and 1.68 is the molar ratio of ca(oh)2/co2. mch is the mass loss due to dehydroxylation of ca(oh)2 and mcc is the mass loss due to decarbonation of caco3 [15]. the percentage of the consumed lime by pozzolan with respect to the reference sample can be calculated using equation 2 [16]. consumed lime [%] = [ (chr,r×c)−chr,p chr,r×c ] × 100 (2) where chr,r and chr,p is the mass of remaining ca(oh)2 in the reference and lime-pozzolan samples, 104 vol. 53/2025 monitoring the reactivity of pozzolans by thermogravimetric method figure 1. tg/dta curves of lime-pumice paste after 7 days (red), 14 days (green), 21 days (olive), and 28 days (blue). figure 2. content of ca(oh)2 in lime pastes determined by tg/dta. figure 3. amount of ca(oh)2 reacted with pozzolan in lime-pozzolan paste. respectively. c is the proportion of lime in the limepozzolan samples (e.g., for a lime-pozzolan sample with 60 % of lime and 40 % of pozzolan, c is 0.6). in the thermogravimetric determination of ca(oh)2 and caco3 contents in lime pastes, the weight losses of samples from the temperature ranges 400–600 °c (dehydroxylation of ca(oh)2) and 600–900 °c (decarbonation of caco3) were evaluated, as shown in figure 1. when comparing the tg and dta curves of lime-pozzolan pastes after 7-day intervals, an increase in the content of pozzolanic reaction products (hydrated calcium silicates and aluminates – csh and cah), a decrease in ca(oh)2 content and a very slight increase in caco3 content due to carbonation of the pastes were evident (figure 1). the amount of remaining ca(oh)2 in the pastes (figure 2) decreased significantly over time in lime-pozzolan pastes due to the ongoing pozzolanic reaction. lime pastes 105 martin vyšvařil acta polytechnica ctu proceedings with metakaolin, zeolite, pumice and diatomaceous earth, i.e. with pozzolans with high pozzolanic activity, showed a low ca(oh)2 content. the high reactivity of chalcedonite according to the chapelle test was not confirmed by the amount of remaining ca(oh)2 in the paste. the reactivity of the pozzolans was expressed by the amount of ca(oh)2 consumed in lime-pozzolan pastes (figure 3), which increased with time as the pozzolanic reaction progressed. metakaolin and natural zeolite showed a significant and rapid pozzolanic reaction, in agreement with the results of the chapelle test. pumice showed low reactivity initially, but after 14 days it showed a significant increase in ca(oh)2 consumed, similar to diatomaceous earth. very low, but expected, amounts of consumed lime were observed for the lime-lava and lime-spongilite pastes. the low value of reacted lime for the lime-chalcedonite paste was surprising as a much higher value was expected based on the high pozzolanic activities from the chapelle test. from the time evolution of the amount of reacted lime, it is clear that after 21 days almost all amount of the metakaolin k05 is reacted, while for other pozzolans the pozzolanic reaction continues even at 28 days. after 28 days of curing, the compressive strength of the paste samples was determined (figure 4). all limepozzolan pastes outperformed the reference lime paste, although lime-lava and lime-chalcedonite pastes only slightly. this confirms that chalcedonite is not very reactive in lime under standard conditions and that its high pozzolanic activity values from the chapelle test are probably due to different conditions during the test (high temperature, constant stirring). it can therefore be seen that in some cases the chapelle test may overestimate the pozzolanic activity. high strength values were obtained with pastes containing metakaolin, zeolite and pumice, consistent with the amount of lime consumed by these pozzolans, due to the high amount of amorphous hydraulic oxides in these pozzolans. in order to estimate the reactivity of the pozzolans in real conditions, correlations were made between the compressive strengths of the pastes and the values of pozzolanic activity determined by the chapelle test after 24 hours (figure 5a) and after 5 days (figure 5b). as can be seen from the graphs and r2 values, neither of these correlations is very good. therefore, it is not possible to draw any conclusions from the pozzolanic activity values as to the real influence of the pozzolans on the strength of the pastes. there is a much better correlation between the amount of lime consumed by pozzolan (after 28 days) and the compressive strength of the pastes, as can be seen in figure 6 (r2 = 0.9849). the higher the amount of ca(oh)2 consumed, the higher the strength values can be expected. from the amount of reacted lime under real conditions, it is possible to infer the influence of pozzolan on the strength of lime mixtures, and the thermogravimetric method can be used relatively easily to determine the amount of reacted lime. figure 4. compressive strength of lime pastes at 28 days. (a). (b). figure 5. correlation between 28 days compressive strength of pastes and pozzolanic activity of pozzolans determined by modified chapelle test after 1 day of treatment (a); after 5 days of treatment (b). figure 6. correlation between 28 days compressive strength of pastes and amount of reacted ca(oh)2 with pozzolan in lime-pozzolan paste after 28 days. 106 vol. 53/2025 monitoring the reactivity of pozzolans by thermogravimetric method 4. conclusions after comparing the effect of ten different types of pozzolans on the strength of lime pastes in relation to their pozzolanic activities determined by the chapelle test and monitoring the reactivity of these pozzolans using the thermogravimetric method, the following conclusions were made. the true influence of pozzolans on the strength properties of lime pastes can be predicted from the content of amorphous hydraulic oxides and also from the specific surface area of the pozzolan particles. the chapelle test is a fast and suitable method for determining pozzolanic activity for most of the pozzolans, but in some cases it may overestimate the pozzolanic activity due to the elevated temperature and constant mixing of the mixture during the test. determining the amount of lime consumed by pozzolan using thermogravimetry is a time-consuming method for evaluating the effect of pozzolan on the strength of lime composites under real conditions, but it gives better results than the chapelle test, because it is not affected by unrealistic conditions during the determination. in any case, the pozzolanic activity of a pozzolan should be determined by several methods and the subsequent comparison should lead to a critical assessment of the influence of the pozzolan in question on the properties of the prepared mixtures, in order to avoid simplistic conclusions. acknowledgements the results in this work have been achieved with financial support from the but specific research project fast-s24-8512. references [1] p. c. hewlett. lea’s chemistry of cement and concrete. elsevier, oxford, 2004. isbn 9780750662567. 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[14] z. tan, s. a. bernal, j. l. provis. reproducible minislump test procedure for measuring the yield stress of cementitious pastes. materials and structures 50(6):235, 2017. https://doi.org/10.1617/s11527-017-1103-x [15] s. c. l. lima, l. c. b. costa, k. c. s. defáveri, et al. study on assessment of pozzolanic activity: slate cutting waste. aci materials journal 117(3):3–10, 2020. https://doi.org/10.14359/51724611 [16] f. elyasigorji, f. farajiani, m. hajipour manjili, et al. comprehensive review of direct and indirect pozzolanic reactivity testing methods. buildings 13(11):2789, 2023. https://doi.org/10.3390/buildings13112789 107 https://doi.org/10.1016/0008-8846(89)90066-5 https://doi.org/10.1016/j.cemconres.2016.02.015 https://doi.org/10.1016/j.cemconcomp.2009.10.008 https://doi.org/10.1016/j.jobe.2023.107590 https://doi.org/10.1051/matecconf/202440302009 https://doi.org/10.1007/978-3-031-31472-8_32 https://doi.org/10.3390/ma15196881 https://doi.org/10.4028/www.scientific.net/ssp.296.73 https://doi.org/10.4028/www.scientific.net/ssp.296.73 https://doi.org/10.1016/j.conbuildmat.2016.05.062 https://doi.org/10.1617/s11527-017-1103-x https://doi.org/10.14359/51724611 https://doi.org/10.3390/buildings13112789 acta polytechnica ctu proceedings 53:102–107, 2025 1 introduction 2 materials and experimental procedures 3 results and discussion 4 conclusions acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2024.47.0047 acta polytechnica ctu proceedings 47:47–52, 2024 © 2024 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague analysis of concrete taken from a loaded column by determining the modulus of elasticity kristýna hrabová∗, petr cikrle, sabina olejková, dalibor kocáb brno university of technology, faculty of civil engineering, institute of building testing, veveří 331/95, 602 00 brno, czech republic ∗ corresponding author: kristyna.hrabova@vutbr.cz abstract. sustainability places an emphasis on preserving and using existing structures, particularly those that are historically or artistically valuable. the preservation of buildings is closely related to a good assessment of their condition. when making a diagnosis, it is suitable to use non-destructive methods to the greatest extent possible. however, these must also be complemented by core samples, which can be used to determine not only the compressive strength but also other important properties. the paper focuses on the evaluation of a reinforced concrete column that was gradually loaded until failure occurred. during loading, the quality of the concrete throughout the column was monitored using the ultrasonic pulse velocity method and, after loading was finished, the cores were taken from the column. the specimens, obtained from the core samples, were then carefully analysed using both non-destructive and destructive tests. the conclusion describes a possible approach to the assessment of concrete using a core examination. keywords: concrete, column, load, ultrasonic pulse velocity. 1. introduction in september 2015, on the grounds of the general assembly of the united nations, the world community approved the common goals of sustainable development, which should be achieved by 2030. it is a long-term program that should be implemented in all areas of human activity and that establishes 17 sustainable development goals. incorporating sustainability criteria in the design and assessment of structures is necessary, and that is why it became the focus of attention of the research agenda in the last decade [1–3]. requirements for the quality and durability of building materials keep changing – what is currently regarded as totally unsatisfactory may have been very common several decades ago. assessment of existing structures is quite problematic in itself, especially in the case of historically or artistically valuable buildings. assessment of structures related to their expansion, superstructure or simple repair is, from the perspective of sustainability, extremely demanding. to properly propose the procedure for a repair or superstructure of an existing structure, high-quality diagnostic research must be performed, including, among other things, the determination of the properties of the materials used [4]. in reinforced concrete structures, core testing is generally accepted as a reference method for determining the mechanical properties of concrete [5]. with regard to the sustainable development principles and protection of historic buildings, an increased emphasis is placed on their preservation, which leads to more frequent use of non-destructive methods in diagnostics [6]. however, it is not always easy or even possible to determine exactly the most critical parts of a structure without its disruption. although the ultrasonic pulse velocity method, for example, enables the identification of internal defects, the determination of an even distribution of concrete in the structure or the determination of its compressive strength or modulus of elasticity, at least a minimum number of core samples is necessary [5]. if the purpose of concrete structure diagnostics is to learn as much as possible about concrete, only determining compressive strength using specimens obtained by core samples appears to be insufficient. the samples should be examined in more detail. what appears very suitable for this purpose are non-destructive methods such as the ultrasonic pulse velocity method or the resonance method [7]. these methods are commonly used to evaluate the degree of concrete degradation, for example, when testing the resistance to freezing and thawing, see [8], as they can be used to assess the extent of damage to the internal structure of concrete over time. in addition to compressive strength, the static modulus of elasticity can also be determined on the samples, since even partial damage to the concrete during its lifetime will affect its deformations under load. 2. the experiment the concrete used for the column was designed so as to match as much as possible the concrete used in the middle of the 20th century in central europe. the quality of concrete at that time corresponded to the knowledge and possibilities available in the production of cement, the design and the production of concrete. the difference between the construction of concrete structures at that time and today is primarily 47 https://doi.org/10.14311/app.2024.47.0047 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en k. hrabová, p. cikrle, s. olejková, d. kocáb acta polytechnica ctu proceedings figure 1. concrete column under load of various degrees: at 960 kn, the first cracks began to form in the corners of the upper part of the column; at 1 520 kn damage to the upper third of the column is clearly visible (on the left and in the middle) and after cores were taken (on the right). in the requirements for the concrete strength class based on the aggressiveness of the environment, the reinforcement coverage, and the effectiveness of the plasticizingadmixture used. today, the requirements are much stricter and the plasticizing admixure more effective. 3. the material and the specimens the column tested was made from the c16/20 strength class concrete with a minimal amount of plasticizing admixture and with coarse mined aggregate of a maximum size distributionof 16/32 mm. the column had a rectangular cross-section of 200×300 mm and a height of 1.2 m. the corners were reinforced with 4 steel rods b500b with a diameter of 16 mm. stirrups with a diameter of 6 mm were located in an axial distance of approximately 115 mm and, at both ends of the column, the 3 stirrups at the edge were placed closer together (axial distance of 55 mm). the reinforcement coverage was 25 mm. more details on the column design and its reinforcement can be found in [9]. after loading, which is described in the next part of the paper, cores were taken from the column. for the experiment, 12 core samples with the nominal diameter of 75 mm were used; see figure 1. from these cores, 12 specimens were prepared with a diameter of 75 mm and a length of 150 mm on which the quality of the concrete was analysed (or the extent of damage to its internal structure) using both non-destructive and destructive testing. 4. the testing procedure the reinforced concrete column test was based on cyclic compressive loading with gradually increasing stress until the load capacity limit was reached. the column was placed in the test press so that the load acted centrally (figure 2). the load step was set to approximately 1/12 of the assumed load capacity, so the column was loaded in steps of 80 kn. with each increase in the load, the ultrasonic pulse velocity (upv) and deformations were measured over the entire area of the column. after each third increase in load, the column was unloaded to the basic force of 80 kn, which corresponds to a compressive stress of 1.33 n mm−2. the first visually detected concrete failure occurred 48 vol. 47/2024 analysis of concrete by determining the modulus of elasticity figure 2. concrete column under load placed in the test press. at a force of 960 kn, that is, at a stress of 16 n mm−2, specifically at the top of the column – the first cracks appeared in the corners. the last force achieved was 1 520 kn (compressive stress of 25.3 n mm−2) when the concrete in the upper quarter of the column showed substantial damage, see figure 1. the results of continuous measurement by the ultrasonic pulse velocity method discovered the failure of the concrete sooner than the first cracks appeared, see figure 3. before loading was started, it was found, using the upv method, that in the upper part of the column the quality of the concrete is lower (upv locally lower than 3 500 m s−1) than in the lower part of the column (upv locally higher than 4 000 m s−1). therefore, the resulting damage to the upper part of the column was not surprising. in contrast, the ndt results were confirmed by testing. after loading was finished, cores were taken and specimens were prepared on which an analysis of the concrete behaviour was performed depending on its location in the column. first, the dynamic modulus of elasticity were determined using the upv and resonance methods, which is partly described in [10]. upv was determined on each specimen three times along its longitudinal axis using the pundit pl-200 instrument according to the en 12504-4 standard [11], based on which the dynamic modulus of elasticity ecu was calculated according to the čsn 73 1371 standard [12]. in addition, the natural frequency of longitudinal vibration of the specimens was measured using the handyscope hs4 oscilloscope, based on which the dynamic modulus of elasticity ecrl was measured according to the astm c215-19 standard [13]. the static modulus of elasticity ec was then determined on the specimensusing the delta 6-300 press according to the iso 1920-10 standard [14], as well as the compressive strength fc. when ec was tested, a continuous record of the acting force and the deformations was made using a data recorded. 5. the results and their discussion the values of compressive strength and modulus of elasticity determined in the specimens are shown in table 1. the data measured during the ec test were used to calculate the modulus of elasticity of the first loading cycle, which can be considered the initial secant modulus of elasticity ec,0, as defined in the en 12390-13 standard [15]. specimen ecu ecrl ec,0 ec fc [gpa] [gpa] [gpa] [gpa] [n mm−2] 1a 33.64 31.93 25.96 27.54 47.57 2a 36.34 32.33 20.92 22.81 47.07 2b 36.23 35.29 28.12 28.54 48.82 3a 30.99 24.67 18.74 20.68 35.18 3b 33.71 29.86 21.16 24.57 33.82 4a 34.55 28.84 21.90 22.51 35.21 5a 34.23 28.75 18.91 21.07 37.66 5b 32.94 27.68 18.31 21.12 35.75 6a 29.93 26.09 21.15 23.76 32.70 6b 31.38 23.93 20.72 21.39 32.46 7a 26.53 20.16 16.05 18.07 26.70 7b 26.83 21.96 13.99 15.91 27.98 table 1. results obtained for each specimen. it is quite interesting that for the specimens taken from the column subjected to a considerable load, there is almost no dependence on the dynamic and static modulus of elasticity; see figure 4. for all the dependences described below, the simplest regression model, a line, was chosen – the graphs show that more complex models would not achieve significantly better results and, therefore, would not be justifiable. the dynamic modulus of elasticity determined by the upv method is almost completely independent of the static modulus of elasticity – the coefficient of determination r2 only reaches 0.536. for the dynamic modulus of elasticity determined by the resonance method, the dependence is greater (r2 = 0.740) but it is definitely not statistically significant. it is even more interesting that the dynamic modulus of elasticity show a greater dependence on the compressive strength; see figure 5. for the dynamic modulus of elasticity, the coefficient of determination r2 is equal to 0.838, which indicates a statistically significant dependence, although it has been proven that, with concrete, there is no direct dependence of its modulus of elasticity and compressive strength; see, for example [14, 15]. a significant dependence of ec,0 and ec is quite common, since they are two outputs from one test. despite that, the difference in the ratio of ec,0 to ec in the specimens tested is not negligible – it ranges from 0.99 (specimen 2b) up to 0.88 (7b), 0.87 (5b) and 0.86 (3b). the greater the difference between ec,0 and ec, the greater the permanent deformations after the first cycle. this can mean micro cracks and other 49 k. hrabová, p. cikrle, s. olejková, d. kocáb acta polytechnica ctu proceedings figure 3. the distribution of upv across the column area , which was determined under a load of various degrees. figure 4. dependence of the modulus of elasticity ec,0, ecu, and ecrl on the static modulus of elasticity. microdefects in the internal structure of the concrete in the specimen. when testing the static modulus of elasticity, no flexible recovery of the deformations of the specimen takes place after its unloading to the basic level of stress (beginning of the second cycle). therefore, in the following cycles, a smaller difference in the deformations is measured, leading to a higher value of ec. in figure 6, more considerable permanent deformations are apparent in specimen 5b, which leads to a lower ec,0 to ec ratio and to the conclusion that specimen 5b is more damaged than specimen 2b. figures 1 and 3 show how the damage to the configure 5. dependence of the modulus of elasticity ec, ecu, and ecrl on the compressive strength. crete column occurred. approximately the top quarter of the column was so damaged that it was not possible to take solid core samples from this area. the extent of damage to the concrete decreased with the column height and approximately the lower quarter was damaged the least, see figure 3. this is also confirmed by the results obtained for the specimens, when the values determined are related to the location from which the cores were taken with regard to the column height. the values of the properties of the specimens designated by the same figure (i.e., taken from the column at the same height) are represented by the mean value. the data are again fitted with a line re50 vol. 47/2024 analysis of concrete by determining the modulus of elasticity figure 6. development of the deformations of specimens 2b and 5b during the static modulus of elasticity test. gression model. the most significant dependence was found for the static modulus of elasticity (r2 = 0.89), then for the compressive strength (r2 = 0.87) and the dynamic modulus of elasticity determined by the resonance method (r2 = 0.85). on the contrary, almost no dependence related to the location of the specimens in the column was found for the dynamic modulus of elasticity determined by the upv method. figure 7. dependence of the modulus of elasticity and the compressive strength on the height of the location where the specimens were taken from the column. 6. conclusion based on the measurements performed, the following can be stated: • the ultrasonic pulse velocity method can be used to reliably detect a failure of concrete in a column under load. the measurement results obtained by the upv method before loading reliably predicted the area where the greatest damage would occur. • the resonance method produces a very good response to damage to the internal structure of the core concrete, which appears to be more suitable than the upv method when measuring the cores. • the ratio of the initial secant modulus of elasticity to the static modulus of elasticity is connected with the failure of the concrete of the specimen. the more damage caused by microdefects in concrete, the greater the permanent deformations detected and the lower the ratio of ec,0 to ec. • to make a detailed assessment of the quality of concrete, it is necessary to carefully analyse the behaviour of the core specimens. besides the values of permanent deformations, it is useful to analyse the ultrasonic pulse signals and the signal detected by the resonance method. acknowledgements this paper has been worked out under the specific research project no. fast-j-21-7509 at brno university of technology. references [1] fib model code. structural concrete 20(2):860–866, 2019. https://doi.org/10.1002/suco.201970021 [2] k. hrabová, p. lehner, p. ghosh, et al. sustainability levels in comparison with mechanical properties and durability of pumice high-performance concrete. applied sciences 11(11):4964, 2021. https://doi.org/10.3390/app11114964 [3] d. t. doan, a. ghaffarianhoseini, n. naismith, et al. a critical comparison of green building rating systems. building and environment 123:243–260, 2017. https://doi.org/10.1016/j.buildenv.2017.07.007 [4] j. bungey, m. grantham. testing of concrete in structures: fourth edition. crc press, 2006. [5] posuzování pevnosti betonu v tlaku v konstrukcích a v prefabrikovaných betonových dílcích. standard, czech standard institute, prague, 2019. [6] zásady navrhování konstrukcí – hodnocení existujících konstrukcí. standard, czech standard institute, prague, 2001. [7] v. malhotra, n. carino. handbook on nondestructive testing of concrete. crc press, 2004. [8] standard test method for resistance of concrete to rapid freezing and thawing. standard, astm international, usa, 2015. [9] k. hrabová, s. hüblová, p. cikrle, t. vymazal. using the ultrasonic method for the defectoscopy of failures in reinforced concrete columns. in 27th concrete days, vol. 322 of solid state phenomena, pp. 203–208. trans tech publications ltd, 2021. https://doi.org/10.4028/ www.scientific.net/ssp.322.203 51 https://doi.org/10.1002/suco.201970021 https://doi.org/10.3390/app11114964 https://doi.org/10.1016/j.buildenv.2017.07.007 https://doi.org/10.4028/www.scientific.net/ssp.322.203 https://doi.org/10.4028/www.scientific.net/ssp.322.203 k. hrabová, p. cikrle, s. olejková, d. kocáb acta polytechnica ctu proceedings [10] s. hüblová, k. hrabová, j. láník, p. cikrle. assessment of cores using electroacoustic methods. in 27th concrete days, vol. 322 of solid state phenomena, pp. 214–219. trans tech publications ltd, 2021. https://doi.org/10.4028/www.scientific.net/ssp. 322.214 [11] zkoušení betonu – část 4: stanovení rychlosti šíření ultrazvukového impulsu [testing concrete – part 4: determination of ultrasonic pulse velocity]. standard, czech standard institute, prague, 2021. [12] nedestruktivní zkoušení betonu – ultrazvuková impulzová metoda zkoušení betonu [non-destructive testing of concrete – method of ultrasonic pulse testing of concrete]. standard, czech standard institute, prague, 2011. [13] standard test method for fundamental transverse, longitudal, and torsional resonant frequencies of concrete specimens. standard, astm international, usa, 2019. [14] zkoušení betonu – část 10: stanovení statického modulu pružnosti v tlaku [testing of concrete – part 10: determination of static modulus of elasticity in compression]. standard, czech standard institute, prague, 2010. [15] zkoušení ztvrdlého betonu – část 13: stanovení sečnového modulu pružnosti v tlaku [testing hardened concrete – part 13: determination of secant modulus of elasticity in compression]. standard, czech standard institute, prague, 2021. 52 https://doi.org/10.4028/www.scientific.net/ssp.322.214 https://doi.org/10.4028/www.scientific.net/ssp.322.214 acta polytechnica ctu proceedings 47:47–52, 2024 1 introduction 2 the experiment 3 the material and the specimens 4 the testing procedure 5 the results and their discussion 6 conclusion acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2024.49.0055 acta polytechnica ctu proceedings 49:55–59, 2024 © 2024 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague a simplified design of a concrete sandwich structure containing a reinforcing rib jan macháčeka,∗, eliška kafkováa, věra kabíčkováa,b, tomáš vlacha,b a czech technical university in prague, faculty of civil engineering, department of architectural engineering, thákurova 7, 166 29 prague, czech republic b czech technical university in prague, university centre for energy efficient buildings, třinecká 1024, 273 43 buštěhrad, czech republic ∗ corresponding author: jan.machacek@fsv.cvut.cz abstract. this article presents the use of a strut and tie analogy for modelling the behaviour of a concrete sandwich structure, which is formed by thin outer concrete layers made of high-performance concrete and reinforcing ribs. the beams transmitting shear are made of a rigid material with low thermal conductivity (purenit) in combination with a carbon fibre reinforced polymer. the purpose of these ribs is to ensure reliable shear interaction of the outer concrete layers regardless of the thermal insulation of the sandwich structure. a simplified model using the truss analogy in the scia engineer software was used for the design of this structure. furthermore, this design was verified experimentally on a section of the sandwich panel where the feasibility and functionality were tested by a four-point bending test. finally, the theoretical values from the model were compared with the experimental results. this also includes a simple evaluation of whether this simplified modelling of the structure’s behaviour is appropriate. the paper contains a summary of the conditions that could have affected the results. keywords: high performance concrete, precast concrete, sandwich panels, composite reinforcement, shear reinforcement, rigid heat insulation, strut-and-tie model. 1. introduction precast concrete sandwich panels are formed from two thin outer layers made of concrete (nowadays from high performance concrete) which are connected by some kind of shear connectors between which is placed thermal insulation. the connectors where historically from steel [1] but due to high thermal conductivity of steel which resulted in significant heat bridges, which depreciated value of heat transfer coefficient (u-value) of construction, we are trying to replace them. so nowadays they are mostly from some kind of fibre reinforced polymers like for example cfrp, gfrp etc. or is used only rigid thermal insulation placed between concrete layers. the shear connections methods are investigated by many scientists all over the world, for example richard o’hegarty [2, 3], abdelghani benayoune [4], kamil hodicky [5], mathias flansbjer [6] and many others [7, 8]. in this case is the shear connection solved by reinforcing ribs composed from rigid heat insulation (specially from purenit) and braids from cfrp. the problem is that it is very hard to predict behaviour of panels during loading because of panels composition from many components which have different properties. this article deals with the use of strut and tie model for modelling of precast sandwich concrete panel behaviour. the first step was to test mechanical properties of materials which are used in sandwich panel. then was created strut and tie model in scia engineer with values gained from mix content kg m−3 cement i 42.5r 650 technical silica sand 1 200 elkem microsilica 940 u-s 100 technical quartz powder st 6 235 superplasticizer based on pce 18 water 190 total 2 393 table 1. hpc mix design. material tests and was created panel section sample. in the last step the panel section was tested by a fourpoint bending test and the theoretical values from the model were compared with the experimental results. 2. materials used in panel section sample and properties tests 2.1. high performance concrete thin outer layers of pcsp are from hpc in this case specifically from the hpc who’s mix design is listed below. this mixture was developed in department of civil engineering at ctu in prague and later improved in uceeb at buštěhrad. for determining panel’s behaviour during loading test as accurately as possible was necessary to test concrete’s properties. 6 samples were tested in total. 3 cubes with 55 https://doi.org/10.14311/app.2024.49.0055 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en j. macháček, e. kafková, v. kabičková, t. vlach acta polytechnica ctu proceedings figure 1. tensile test of carbon composite reinforcement. figure 2. young’s modulus of carbon composite reinforcement. an edge of 100 mm and 3 beams with the dimensions 40 × 40 × 160 mm. the cubes were tested in compression according to čsn en 12390-3 [9] and the beams were subjected to three-point bending test according to čsn en 12390-5 [10]. the test measured the average strength of concrete in central pressure at cube 97.42 mpa according to čsn en 12390-3 [9] and the average tensile strength of the concrete under bending measured on beams at 15.51 mpa according to čsn en 12390-5 [10]. modulus of elasticity was known from previous tests which was done in uceeb during developing of the mixture and its value is 45.00 gpa. these values were used for calculations. 2.2. carbon fibre reinforced polymer as a shear reinforcement of the panel which is subject of this article is used a composite reinforcement consisting of epoxy resin and carbon fibres tenaxth – e sts40 f13 24k 1600 tex from teijin. before creation of panel were created 8 samples of this reinforcement which were tested by uniaxial simple tension on the test equipment. the maximum force at material failure was monitored (bearing capacity) and the dependence of the relative deformation on stress (approximate young’s modulus of elasticity). the results of test are presented in the figure 1: tensile test of carbon composite reinforcement and figure 2: young’s modulus of carbon composite reinforcement. tensile tests of carbon composite reinforcement revealed that the rovings achieve average tensile strength of 2 968.30 n which corresponds to a stress of 3.28 gpa and that the average young’s modulus is 237.60 gpa. these values were used in the following calculations. 2.3. purenit as part of this work the behaviour of purenit under pressure loading was tested, specifically 3 samples of width 100 mm, thickness 30 mm and height 180 mm. these dimensions were accurately measured by calliper at 3 locations and from the measured values the average value was calculated. after measure the samples were loaded at a speed of 1 mm per minute until failure. from the loading tests was obtained the working diagram of purenit which were used in following calculations. based on the results of loading tests the average breaking strength of purenit is 8.25 mpa as presented 56 vol. 49/2024 a simplified design of a concrete sandwich structure . . . figure 3. young’s modulus of purenit. figure 4. photos from materials tests. in figure 3 and the modulus of elasticity is 226.28 mpa, which approximately corresponds to the data in the technical sheet from manufacturer [11]. after exceeding the load-bearing capacity, the material literally flew to pieces. to the value of 3.50 mpa the material’s behaviour was elastic. 3. strut-and-tie model for the calculation purpose the entire panel was significantly simplified. in calculation model the panel construction is represented by strut-and-tie model which is essentially a rib’s longitudinal section. the strut-and-tie model works as follow: the upper compression bar and lower tension member represents outer concrete layers, so in the calculation model they are from concrete profiles which are 2 cm high, 50 cm wide and have properties which were measured on concrete samples. the diagonals represent shear reinforcement, so their area and properties correspond to area and measured properties of the used reinforcement from cfrp and the shafts represent purenit. figure 5. strut-and-tie model. the model as presented in figure 5 and figure 6 has been loading by 2 forces operating in thirds of the truss beam until the calculated stress in diagonals has the breaking strength value [12]. because strut-and-tie model represent only one rib and experimental panel has two, the forces must be summarized and multiplicated by two. so, the prediction based on the strut-and-tie model is that after force on the press reaches 36.52 kn the shear reinforcement will break and then the panel collapses due to shear failure of the rib from purenit. 57 j. macháček, e. kafková, v. kabičková, t. vlach acta polytechnica ctu proceedings figure 6. the breaking strength. figure 7. schema of four-point bending test. 4. experiment the load test of the panel took place at uceeb and were carried out using a press from which the force was transmitted through a pair of welded i-sections to achieve a four-point bending load. scheme of fourpoint bending test is presented in figure 7 and photo from testing procedure in figure 8. loading was carried out at a rate of 3 mm per minute, and it continued until the structure failed. the first shear reinforcement failed when the force on the press was 30.00 kn, after that panel’s behaviour stopped being linearly elastic and composite reinforcement gradually began to fail until finally the structure collapsed due to the shear failure of the purenit ribs as presented in figure 9. the maximum force on the press was 54.58 kn. 5. comparison as you can see in presented figure 10 below, the strut-and-tie model predicted, that the panel will collapse when the force on the press will be 36.52 kn due to failure of the shear reinforcement. it is very close to the real force 30.00 kn at which the shear reinforcement started to fail but strut-and-tie model didn’t deal with plastic reserve. panel collapsed when the force on the press was 54.58 kn which is much more than was expected. the same we can say about deformation which were smaller than was expected, because purenite with a large cross-sectional area also has large effect. figure 8. photo from four-point bending test. figure 9. detail of the purenit rib shear failure. figure 10. experiment vs. strut-and-tie model. 58 vol. 49/2024 a simplified design of a concrete sandwich structure . . . 6. conclusion the strut-and-tie model provides results which are “on the safe side” so this model could be used for the approximate design of the precast concrete sandwich panels but doesn’t perfectly describe the real behaviour. to achieve more accurate results could be useful to use some software for nonlinear analysis like atena etc. the results could be affected by geometric differences between strut-and-tie model and the real panel. in the real panel the angle of shear reinforcement was 45°, in the model it was 38.65°. the results also could be affected by violation of the shear reinforcement during the creation of the panel which could lead to former shear reinforcement failure than expected. there is also a problem with small number of samples because the only one sample was tested which means that the result isn’t statistically significant. acknowledgements the work on this paper was supported by czech science foundation grant no. 22-14942k entitled “possibilities of using natural fibers for the production of hybrid textile reinforcement in concrete”. the authors would like to acknowledge all financial assistance provided to support this research. references [1] a. einea, d. c. salmon, g. j. fogarasi, et al. state-of-the-art of precast concrete sandwich panels. pci journal 36(6):79–98, 1991. https://doi.org/10.15554/pcij.11011991.78.98 [2] r. o’hegarty, o. kinnane, m. grimes, et al. development of thin precast concrete sandwich panels: challenges and outcomes. construction and building materials 267:120981, 2021. https: //doi.org/10.1016/j.conbuildmat.2020.120981 [3] r. o’hegarty, r. west, a. reilly, o. kinnane. composite behaviour of fibre-reinforced concrete sandwich panels with frp shear connectors. engineering structures 198:109475, 2019. https://doi.org/10.1016/j.engstruct.2019.109475 [4] a. benayoune, a. a. abdul samad, d. n. trikha, et al. flexural behaviour of pre-cast concrete sandwich composite panel – experimental and theoretical investigations. construction and building materials 22(4):580–592, 2008. https: //doi.org/10.1016/j.conbuildmat.2006.11.023 [5] k. hodicky, t. hulin, j. w. schmidt, h. stang. structural performance of new thin-walled concrete sandwich panel system reinforced with bfrp shear connectors. in proceedings of the 4th asia-pacific conference on frp in structures. 2013. https://doi.org/10.25916/sut.26269924.v1 [6] m. flansbjer, n. williams portal, d. vennetti, u. mueller. composite behaviour of textile reinforced reactive powder concrete sandwich façade elements. international journal of concrete structures and materials 12(1):71, 2018. https://doi.org/10.1186/s40069-018-0301-4 [7] a. shams, m. horstmann, j. hegger. experimental investigations on textile-reinforced concrete (trc) sandwich sections. composite structures 118:643–653, 2014. https: //doi.org/10.1016/j.compstruct.2014.07.056 [8] j.-q. huang, j.-g. dai. direct shear tests of glass fiber reinforced polymer connectors for use in precast concrete sandwich panels. composite structures 207:136–147, 2019. https: //doi.org/10.1016/j.compstruct.2018.09.017 [9] czech office for standards, metrology and testing. čsn en 12390-3. zkoušení ztvrdlého betonu – část 3: pevnost v tlaku zkušebních těles [in czech], 2020. [10] czech office for standards, metrology and testing. čsn en 12390-5. zkoušení ztvrdlého betonu – část 5: pevnost v tahu ohybem zkušebních těles [in czech], 2020. [11] izolace-info.cz. purenit – přesvědčivý funkční materiál s rozmanitým využitím [in czech]. https://www.izolace-info.cz/technickeinformace/zateplovanifasady-1/22656-purenitpresvedcivy-funkcni-material-s-rozmanitymvyuzitima.html [12] j. macháček. vliv smykové kompozitní výztuže na únosnost betonových sendvičových panelu [in czech; influence of composite shear reinforcement on bearing capacity of concrete sandwich panels]. master’s thesis, czech techical university in prague, 2022. 59 https://doi.org/10.15554/pcij.11011991.78.98 https://doi.org/10.1016/j.conbuildmat.2020.120981 https://doi.org/10.1016/j.conbuildmat.2020.120981 https://doi.org/10.1016/j.engstruct.2019.109475 https://doi.org/10.1016/j.conbuildmat.2006.11.023 https://doi.org/10.1016/j.conbuildmat.2006.11.023 https://doi.org/10.25916/sut.26269924.v1 https://doi.org/10.1186/s40069-018-0301-4 https://doi.org/10.1016/j.compstruct.2014.07.056 https://doi.org/10.1016/j.compstruct.2014.07.056 https://doi.org/10.1016/j.compstruct.2018.09.017 https://doi.org/10.1016/j.compstruct.2018.09.017 https://www.izolace-info.cz/technicke-informace/zateplovanifasady-1/22656-purenit-presvedcivy-funkcni-material-s-rozmanitym-vyuzitima.html https://www.izolace-info.cz/technicke-informace/zateplovanifasady-1/22656-purenit-presvedcivy-funkcni-material-s-rozmanitym-vyuzitima.html https://www.izolace-info.cz/technicke-informace/zateplovanifasady-1/22656-purenit-presvedcivy-funkcni-material-s-rozmanitym-vyuzitima.html https://www.izolace-info.cz/technicke-informace/zateplovanifasady-1/22656-purenit-presvedcivy-funkcni-material-s-rozmanitym-vyuzitima.html acta polytechnica ctu proceedings 49:55–59, 2024 1 introduction 2 materials used in panel section sample and properties tests 2.1 high performance concrete 2.2 carbon fibre reinforced polymer 2.3 purenit 3 strut-and-tie model 4 experiment 5 comparison 6 conclusion acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2025.52.0136 acta polytechnica ctu proceedings 52:136–143, 2025 © 2025 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague statistical analysis of accident data on d1 motorway jakub woletz brno university of technology, institute of forensic engineering, purkyňova 464/118, 612 00 brno, czech republic correspondence: jakub.woletz@vut.cz abstract. this study analyses incidents on the d1 motorway over the period 2015–2023, with a focus on improving towing operations processes. the analysis includes a breakdown of incidents by type (accidents, technical failures), injuries and deaths, and considers seasonal variations and traffic conditions. the data show that traffic accidents account for most incidents, especially in sections with heavy traffic and difficult weather conditions. at the same time, there is an increase in the number of incidents in 2018 and 2019, while 2020 brings a temporary decrease due to the covid-19 pandemic. this research is part of a long-term project aimed at optimizing and standardizing the processes of extrication and towing activities, with the aim of streamlining interventions, minimizing traffic complications and ensuring rapid traffic recovery on key roads. subsequent steps will focus on designing new standards for towing interventions and improving current protocols. keywords: d1 motorway, incidents, traffic accidents, towing activities, process optimization, technical failures, intervention standardization, injuries, recovery activities, seasonal effects. 1. introduction the d1 motorway connecting prague and brno is the oldest and busiest motorway in the czech republic, as illustrated in figure 1. thanks to its strategic location and importance within the european transport network, it plays a key role in the transport of goods and passengers. the high frequency of traffic, especially trucks and cars, causes d1 to become a frequent site of traffic accidents that affect the flow of traffic [1]. accidents on d1 are caused by various factors, including failure to adapt speed to road conditions, driver fatigue and technical defects in vehicles. in winter months, when road icing is frequent, the risk of accidents increases even more [2]. statistics show that accidents occur most frequently in congested sections of the motorway, especially between kilometers 178 and 210, where traffic complications are common [3]. to ensure a quick response to accidents and to minimize traffic problems, a central dispatch of towing services has been established on d1, which is managed by the directorate of roads and motorways (drd) [1]. this dispatching center ensures the coordination of towing services, which are responsible for the removal of immobile vehicles from the roadway so that traffic can be restored as soon as possible. the towing services cooperating with the rcsd include companies such as pretol, jerex and wuka, which have the necessary equipment to tow both cars and heavy goods vehicles [4]. according to act no. 361/2000 coll. on road traffic, it is the duty of the vehicle owner to remove the immobile vehicle from the road [2]. if the owner is unable or unwilling to do so, the central dispatching center may call a towing service at the owner’s expense. this procedure is common, especially in accidents where the vehicle is an obstruction to traffic and must be removed quickly to avoid further complications [5]. in addition to accidents, towing is also carried out when a vehicle is unable to continue driving due to a technical fault or when it is incorrectly parked, for example in emergency parking lanes. such vehicles can obstruct emergency services or significantly impede the flow of traffic on the motorway [4]. the aim of this paper is to analyze the data on accidents and other incidents on the d1 motorway. this analysis will focus on the number of accidents, the structure of those involved in accidents (cars, trucks, pedestrians) and the distribution of accidents by motorway section. where available, intervention times will also be considered, allowing a better understanding of the nature of accidents and interventions on d1 motorway. the research question is mainly to find out what types of incidents happen most often on d1 motorway, on which sections these incidents happen the most and what is specific to them, what kind of vehicles are involved in these incidents, how frequent are injuries and deaths of people and what is the distribution of incidents in years and months, or whether there are months that are more characteristic of accidents to happen or not. the analysis of accident data on highways is an essential tool for identifying risk factors and improving road safety. such analysis enables the planning of preventive measures and infrastructure optimization to reduce the likelihood of accidents. in the czech republic, the “methodology for managing the consequences of traffic accidents” was developed by the 136 https://doi.org/10.14311/app.2025.52.0136 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 52/2025 statistical analysis of accident data on d1 motorway figure 1. d1 motorway in its analyzed extent [6]. directorate of roads and highways of the czech republic in cooperation with the institute of forensic engineering at brno university of technology. this document provides a detailed guide on procedures for handling accident aftermath on highways and firstclass roads, emphasizing the importance of a quick and coordinated response to minimize impacts on traffic flow and safety for road users [7]. in international literature, statistical methods and spatial analysis are commonly used to gain insights into the causes and effects of traffic accidents. a review of statistical approaches to crash frequency analysis highlights their critical role in modeling risk factors on highways [8]. various approaches to assessing injury severity in accidents offer insight into how different conditions, such as speed and road type, impact accident outcomes [9]. another important factor is unobserved heterogeneity, which means that certain factors influencing the likelihood of an accident may not be directly observable. ignoring these hidden factors can lead to inaccurate conclusions and biased results [10]. temporal instability in accident data also plays a significant role, as patterns of behaviour and traffic conditions may change over time, affecting the accuracy of predictive models [11]. advances in machine learning enable the use of sophisticated algorithms for analyzing and predicting high-risk areas on highways. machine learning has proven beneficial for improving the accuracy and adaptability of these models in various environments [12]. in the european context, factors such as traffic density and infrastructure quality have a crucial impact on road safety, highlighting the need for individualized approaches to enhance safety across different regions of europe [13]. 2. solution methods accident and event data extraction and analysis from large csv files were key steps in understanding the nature of traffic events across the country. working with this data required a combination of several methods and techniques, including the use of excel, vba scripts and analytical techniques to extract relevant information. the data came from the period september 2015 to december 2022 and contained hundreds of thousands of records from across the country, which included various types of traffic events, accidents and road restrictions. the process began with importing this data from csv files into excel, where initial editing and data preparation was required. this data was often in the form of long supplementary text that contained key accident information such as the date and time of the event, road type, road number, accident location and other details that needed to be extracted. visual basic for applications (vba) is a programming language developed by microsoft that allows you to automate tasks in applications such as excel, access, and other microsoft office applications. vba is an extension of visual basic and is integrated directly into these applications, allowing users to create macros and scripts for tasks that would otherwise require manual intervention. one of the main challenges in working with this data was the need to extract specific information from the long text descriptions. for example, it was necessary to isolate the date and time of the accident from this text, which required splitting cells and finding specific formats such as date stamps or time stamps. to do this, we used various methods such as cell splitting using excel functions or vba scripts to automate the extraction of this data. in addition, we needed to 137 jakub woletz acta polytechnica ctu proceedings figure 2. number of interventions on d1 by section. identify the type of road (e.g., highway, class 1 road) and its number, which often required text processing to find keywords and numbers. in the context of data analysis, vba provides efficient tools for manipulating large volumes of data. in this paper, vba was used to process large-scale data on traffic events on the d1 motorway. vba was used to automate various processes such as extracting data from long text descriptions, identifying accident types, splitting data by time stamps, or converting time data into a format usable for analysis in excel. the use of vba not only sped up data processing but also increased the accuracy of the results, which was crucial for a deeper understanding of accident patterns on the d1 motorway. for more advanced analysis, vba scripts written specifically for this purpose were used. these scripts made it possible to automate the identification and counting of the different types of vehicles mentioned in the text, such as cars (“oa”), trucks (“na”), buses (“bus”), motorcycles (“moto”), pedestrians (“chodec”) and other categories such as animals or fixed obstacles, which were classified as other. the scripts were designed to detect different forms and combinations of these categories, including cases where occurrences such as “2× oa” or “oa × na” had to be accounted for, which meant one car and one truck. another challenge was to correctly analyze the duration of the events. for this purpose, the textual duration information, which was given as “2 hours 5 minutes” for example, was converted into a format that excel would recognize and allow for further calculations such as summing or comparing times. these conversions were done using vba scripts that decomposed the time data into hours and minutes and then converted it into a format usable for further work in excel. the whole process therefore involved a combination of different techniques and tools working together to extract, edit and analyze the accident data. the use of vba was key in automating many steps that would otherwise have required manual work and allowed large amounts of data to be processed and analyzed efficiently. this approach not only sped up processing, but also increased the accuracy of the analyses, which was essential for drawing meaningful conclusions from the available information on crashes and road events. 3. results and discussion this analysis is part of a specific research aimed at creating an effective system of evidence for optimizing the processes of recovery and towing activities in the czech republic. the data will be used for a comprehensive progressive analysis of accidents and infrastructure data to improve standards and practices in the field of road obstruction removal. specifically, the analysis focuses on accidents on the d1 motorway in its entirety, from 1 january 2015 to 31 december 2023. this includes data on accidents on all sections of the motorway from prague to the border with poland. this data will be a key source for the creation of a traffic accident database that will be used for further research purposes within the project. due to technical problems three months of data from 2023 are not available, which is likely to have been caused by an error in data collection or transmission. despite this gap, the available data can be used to analyze the trends and distribution of accidents on the d1 motorway. 3.1. analysis of events on d1 by motorway sections the number of incidents on the d1 motorway by kilometer section is illustrated in figure 2, with data broken down by ten-kilometer intervals. some sections show a significantly higher number of incidents than others, a trend influenced by factors such as traffic density, weather conditions, construction works, and the nature of the infrastructure. incidents include not 138 vol. 52/2025 statistical analysis of accident data on d1 motorway figure 3. accident counts along d1 highway with map background. only accidents but also vehicle breakdowns and other events that required the use of a response vehicle. figure 3 shows a map of the d1 motorway with points marking the locations and frequency of accidents. the map was created based on geographical data on key points along the motorway and accident data highlighting areas of risk. each red dot represents a 10 km stretch, with the size of the dot corresponding to the number of incidents. initial coordinates were generated using the python programming language to approximate the location of the incidents and then refined and visualized in qgis to ensure an accurate alignment with the actual route of the d1 motorway. the highest number of incidents occurs in the 0–9 km and 10–19 km sections, which are areas close to prague. these sections are characterized by extreme traffic density, mainly due to the daily commute and the high number of cars and trucks. high traffic volumes and complex traffic situations such as city entrances and exits significantly increase the risk of incidents in this area. the combination of these factors results in a high incident rate at the start of the d1 motorway. on the other hand, the 119–153 km sections (highlands) are also characterized by a significant increase in the number of incidents. this section of the motorway is characterized by its variable terrain and frequent extreme weather conditions such as fog, snow and icing. these conditions are particularly dangerous during the winter months when traffic complications are frequent. in addition, there are regular construction works and closures on this stretch, which can contribute to further incidents. the 190–199 km stretch is an example of an area where construction work is frequent, which together with heavy traffic leads to a high incident rate. closures and changes in traffic arrangements increase the risk of collisions and other complications. on the other hand, sections such as 270–279 km and 280–289 km have very low incident rates. these parts of the motorway are in less busy areas with lower traffic volumes. therefore, there is less likelihood of incidents occurring here. the lower traffic volume in these sections and the absence of critical traffic points can be considered as the main reason for the lower incident rate. also of interest is the 30–39 km section where the number of incidents is elevated. this section is in the mirošovice area, where traffic flow distribution and changes in traffic load occur. it is likely that traffic from the urban area meets the motorway network here, creating more complex traffic conditions and increasing the risk of incidents. conversely, the lower number of incidents in the 230–260 km sections may be due to lower traffic volumes and the absence of complex traffic situations. the lower traffic density in these sections contributes to a relatively lower intervention rate. based on the graph and data analysis, higher incident rates are often associated with heavy traffic, construction and challenging weather conditions, while sections with lower traffic volumes show lower intervention rates. these factors should be considered when designing effective measures to minimize incidents and improve safety on the d1 motorway. 3.2. analysis of the type of events on d1 figure 4 illustrates the ratio of traffic accidents to other incidents on the d1 motorway. the graph highlights that most recorded incidents are traffic accidents, as indicated by the red segment. these incidents frequently necessitate the intervention of towing services due to their severity and the urgent need to clear obstructions from the roadway. a smaller proportion of incidents, shown in blue, represent other incidents, which primarily include technical vehicle breakdowns. although these incidents 139 jakub woletz acta polytechnica ctu proceedings figure 4. ratio of traffic accidents and other incidents. are less frequent than accidents, they still make up a significant proportion of interventions on the motorway. compared to road accidents, these incidents tend to be less serious and can be quicker and easier to resolve, but this does not mean that they should be ignored. 3.3. analysis of events in individual years and months figure 5 shows the number of incidents on the d1 motorway from 2015 to 2022, with the data broken down by month. this representation allows for the analysis of changes in the number of incidents both within each year and across the years. in 2018 and 2019 there is a significant increase in the number of incidents, which reaches a peak. this trend may be due to several factors. one of them is the increasing volume of traffic on d1, which was extremely busy at this time. another reason may be the frequent construction works that were taking place on various sections of the motorway, which led to traffic jams and a higher number of incidents requiring the intervention of towing services. the year 2020, on the other hand, sees a decrease in the number of incidents, which is probably due to the global pandemic covid-19. the restriction of movement and reduction in traffic volume led to fewer incidents, which is clearly visible in the graph. the significantly lower traffic density had a direct impact on the total number of interventions. after 2020, incident numbers gradually stabilized, as shown by the trends in 2021 and 2022. although in some month’s incident numbers did not return to 2019 levels, they were close. this development may be related to the gradual return to normal traffic after the pandemic and the ongoing construction work on the d1 motorway. the chart also shows seasonal fluctuations. for example, winter months such as december, january and february show a higher number of incidents, which is logical given the worsened weather conditions. snow, ice and fog in winter increase the risk of traffic accidents and technical failures. summer months, such as july and august, have a lower number of incidents in some years, probably due to better weather conditions and lower traffic congestion during the holidays. in conclusion, the trend in the number of incidents on d1 over the years clearly indicates seasonal and long-term influences such as construction, pandemics and seasonal conditions. winter months and periods of construction work bring higher numbers of incidents, while summer months and pandemics lead to a decrease. 3.4. analysis of the types of vehicles involved in events figure 6 shows the distribution of incidents on the d1 motorway by type of participants. each segment of the graph represents the share of each participant type in the total number of incidents. key groups identified in the graph include cars, lorries, buses, motorcycles, pedestrians, and other categories such as cyclists, fixed obstacles, and animals. the largest proportion of incidents, shown by the red segment, relate to cars. this is not surprising given that passenger cars make up most of the traffic on d1 motorway. the high volume of passenger traffic increases the likelihood that passenger cars will be frequent participants in accidents and other incidents. the second largest segment, shown in blue, is trucks. lorries make up a significant proportion of traffic on the d1 motorway and, although not as numerous as passenger traffic, their contribution to incidents is significant. this may be due to the greater risks associated with the handling and movement of heavy vehicles, particularly on sections with heavy traffic or poor weather conditions. buses are shown in the light blue segment and account for a smaller proportion of incidents. although buses make up a smaller proportion of traffic, their contribution to incidents is important because of the potential consequences of these incidents, particularly in terms of passenger numbers and vehicle size. motorcycles, which account for a small proportion of the total number of incidents, may in some cases be under-reported in towing service intervention statistics since not all cases require a full tow truck intervention for a motorcycle. in some situations where the motorcycle is not a major road obstruction, it can be removed more quickly and easily without the need for heavy equipment. this factor contributes to the fact that incidents involving motorcycles are not always recorded as full-scale interventions, which may make their proportion of total incidents appear lower. small segments include motorcycles, pedestrians, and other users, which includes bicyclists, fixed obstacles, and animals. incidents involving these participants are less frequent but can have serious consequences. incidents involving pedestrians are particularly risky due to their vulnerability compared to vehicles. 140 vol. 52/2025 statistical analysis of accident data on d1 motorway figure 5. number of interventions on d1 in individual years and months. figure 6. incidents on d1 by participants. overall, the graph shows that cars and lorries make up the vast majority of those involved in incidents on d1 motorway. this is related to their dominance on the motorway and their greater numbers compared to other categories of users. figure 7 shows the number of people injured and killed in incidents on the d1 motorway. it is important to note that the proportion of incidents marked in blue as ’not specified for towing purposes’ include incidents where it was not necessary to specify injuries for towing purposes. this means that in these cases the injury is unlikely to have occurred at all or was not relevant to the towing vehicle intervention. overall, there are over 19 000 cases with injuries and 24 tragic deaths, but this does not necessarily include all cases where towing service intervention was not required. this information is not always relevant to tow truck drivers unless the accident requires specific attention to these aspects. figure 7. number of persons injured and death. 3.5. analysis of participant injuries and deaths 4. conclusions this analysis has provided a comprehensive overview of accidents on the d1 motorway between 2015 and 2023. from the available data we have found that road accidents make up most incidents. vehicle breakdowns and other types of accidents are significantly 141 jakub woletz acta polytechnica ctu proceedings less common but still make up a significant proportion of interventions on the motorway. traffic crashes tend to be concentrated in certain sections of the highway and are influenced by several factors such as traffic volume, weather conditions, and roadway design. from the graphs, we could also infer that there was an increase in the number of incidents in 2018 and 2019, which can be attributed to increasing traffic volumes and frequent maintenance and construction works on various sections of d1 motorway. the year 2020 brought a temporary decrease in the number of incidents due to the covid-19 pandemic, which led to lower traffic volumes. after this hiatus, incidents gradually returned to the levels seen before the pandemic and again concentrated on critical sections of the motorway where traffic is densest or where weather and road conditions are more challenging. seasonal variations in incident rates show that winter months, when weather conditions are less favorable, pose a higher risk to drivers, while summer months have lower incident rates. this suggests that highway conditions play a major role in the incidence of crashes and other events requiring towing service intervention. in the analysis of incident participants, it was found that cars and trucks make up the most of incident participants. motorcycles and buses are represented to a lesser extent, and motorcycles may be statistically under-represented because it is not always necessary to call for tow trucks in their crashes. in such cases it is not necessary to record a full intervention, resulting in a lower proportion of these incidents in the statistics. the analysis of injuries and deaths shows that although most incidents do not involve injuries, there are a significant number of cases with injuries and a few tragic deaths. these incidents have a significant impact on highway operations and require rapid and coordinated intervention to minimize the consequences not only to the participants but also to other drivers and traffic flow. in terms of the ratio of accidents to technical failures, accidents significantly predominate. technical failures, although less frequent, represent an important part of highway interventions and it is essential to ensure effective and rapid procedures for dealing with them to minimize disruption to traffic. in conclusion, this analysis shows that incidents on the d1 motorway are influenced by a wide range of factors, ranging from traffic density to seasonal variations and weather conditions. these factors need to be carefully considered when developing strategies and standardized procedures for towing service interventions. this analysis is an important step in research focusing on optimizing and standardizing the processes of salvage and towing operations. the research will continue toward finding ways to streamline highway interventions to ensure the quickest possible recovery and minimize traffic complications during incidents. the next step will involve developing new standards and protocols for response vehicles, including evaluating current processes and providing suggestions for improvement. optimizing these processes aim to contribute to safer and smoother traffic on czech motorways, not only on d1 motorway but also on other key roads in the country. acknowledgements references [1] directorate of roads and motorways of the czech republic. údržba komunikací – organizace pro údržbu silnic a dálnic [in czech; road maintenance – road and motorway maintenance organisations]. [2024-09-10]. https://rsd.cz/web/guest/silnice-a-dalnice/ udrzba-komunikaci# zalozka-organizace-pro-udrzbu-silnic-a-dalnic [2] ministry of transport of the czech republic. silniční doprava – právní předpisy [in czech; road transport – legislation]. [2024-09-10]. https://md.gov.cz/dokumenty/silnicni-doprava/ legislativa-silnicni-doprava-(1) /silnicni-doprava-pravni-predpisy [3] transport research centre. statistika nehodovosti: dopravní nehody v čr [in czech; accident statistics: traffic accidents in the czech republic]. [2024-09-10]. https://nehody.cdv.cz/statistics.php [4] pretol s.r.o. naše práce – vyprošťování a odtahy vozidel [in czech; our work – recovery and towing of vehicles]. [2014-09-15]. https://www.pretol.cz/nase-prace [5] jerex a.s. zásahová vozidla a dopravní značení [in czech; emergency vehicles and traffic signs]. [2024-09-10]. https://www.jerex.cz/ zasahova-vozidla-a-dopravni-znaceni/ [6] seznam.cz. dálnice d1 – mapa [in czech; d1 motorway – map]. [2024-09-16]. https://mapy.cz/zakladni?q=d1&source=area&id= 31052&ds=2&x=15.4805437&y=49.7460105&z=8 [7] directorate of roads and highways of the czech republic. metodika likvidace následků dopravních nehod [in czech; methodology for dealing with the aftermath of traffic accidents]. [2024-09-20]. https://rsd.cz/documents/38144/56588/metodika_ likvidace_nasledku_dopravnich_nehod.pdf/ 7ee079c5-000f-161a-fb49-f578eb2d6bf9?t= 1642066093042 [8] d. lord, f. mannering. the statistical analysis of crash-frequency data: a review and assessment of methodological alternatives. transportation research part a: policy and practice 44(5):291–305, 2010. https://doi.org/10.1016/j.tra.2010.02.001 [9] p. t. savolainen, f. l. mannering, d. lord, m. a. quddus. the statistical analysis of highway crash-injury severities: a review and assessment of methodological alternatives. accident analysis & prevention 43(5):1666–1676, 2011. https://doi.org/10.1016/j.aap.2011.03.025 [10] f. l. mannering, v. shankar, c. r. bhat. unobserved heterogeneity and the statistical analysis of highway accident data. analytic methods in accident research 11:1–16, 2016. https://doi.org/10.1016/j.amar.2016.04.001 142 https://rsd.cz/web/guest/silnice-a-dalnice/udrzba-komunikaci#zalozka-organizace-pro-udrzbu-silnic-a-dalnic https://rsd.cz/web/guest/silnice-a-dalnice/udrzba-komunikaci#zalozka-organizace-pro-udrzbu-silnic-a-dalnic https://rsd.cz/web/guest/silnice-a-dalnice/udrzba-komunikaci#zalozka-organizace-pro-udrzbu-silnic-a-dalnic https://md.gov.cz/dokumenty/silnicni-doprava/legislativa-silnicni-doprava-(1)/silnicni-doprava-pravni-predpisy https://md.gov.cz/dokumenty/silnicni-doprava/legislativa-silnicni-doprava-(1)/silnicni-doprava-pravni-predpisy https://md.gov.cz/dokumenty/silnicni-doprava/legislativa-silnicni-doprava-(1)/silnicni-doprava-pravni-predpisy https://nehody.cdv.cz/statistics.php https://www.pretol.cz/nase-prace https://www.jerex.cz/zasahova-vozidla-a-dopravni-znaceni/ https://www.jerex.cz/zasahova-vozidla-a-dopravni-znaceni/ https://mapy.cz/zakladni?q=d1&source=area&id=31052&ds=2&x=15.4805437&y=49.7460105&z=8 https://mapy.cz/zakladni?q=d1&source=area&id=31052&ds=2&x=15.4805437&y=49.7460105&z=8 https://rsd.cz/documents/38144/56588/metodika_likvidace_nasledku_dopravnich_nehod.pdf/7ee079c5-000f-161a-fb49-f578eb2d6bf9?t=1642066093042 https://rsd.cz/documents/38144/56588/metodika_likvidace_nasledku_dopravnich_nehod.pdf/7ee079c5-000f-161a-fb49-f578eb2d6bf9?t=1642066093042 https://rsd.cz/documents/38144/56588/metodika_likvidace_nasledku_dopravnich_nehod.pdf/7ee079c5-000f-161a-fb49-f578eb2d6bf9?t=1642066093042 https://rsd.cz/documents/38144/56588/metodika_likvidace_nasledku_dopravnich_nehod.pdf/7ee079c5-000f-161a-fb49-f578eb2d6bf9?t=1642066093042 https://doi.org/10.1016/j.tra.2010.02.001 https://doi.org/10.1016/j.aap.2011.03.025 https://doi.org/10.1016/j.amar.2016.04.001 vol. 52/2025 statistical analysis of accident data on d1 motorway [11] f. mannering. temporal instability and the analysis of highway accident data. analytic methods in accident research 17:1–13, 2018. https://doi.org/10.1016/j.amar.2017.10.002 [12] n. behboudi, s. moosavi, r. ramnath. recent advances in traffic accident analysis and prediction: a comprehensive review of machine learning techniques, 2024. [2024-09-20]. https://doi.org/10.48550/arxiv.2406.13968 [13] d. georgakopoulos, d. nikolaou, j. roussou, g. yannis. the impact of mobility characteristics on public transport and road safety performance in selected european cities. transportation research procedia 72:1547–1551, 2023. tra lisbon 2022 conference proceedings transport research arena (tra lisbon 2022),14th-17th november 2022, lisboa, portugal. https://doi.org/10.1016/j.trpro.2023.11.622 143 https://doi.org/10.1016/j.amar.2017.10.002 https://doi.org/10.48550/arxiv.2406.13968 https://doi.org/10.1016/j.trpro.2023.11.622 acta polytechnica ctu proceedings 52:136–143, 2025 1 introduction 2 solution methods 3 results and discussion 3.1 analysis of events on d1 by motorway sections 3.2 analysis of the type of events on d1 3.3 analysis of events in individual years and months 3.4 analysis of the types of vehicles involved in events 3.5 analysis of participant injuries and deaths 4 conclusions acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2024.47.0066 acta polytechnica ctu proceedings 47:66–70, 2024 © 2024 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague production of concrete pavements using mixed cements jan klobása∗, rudolf hela brno university of technology, faculty of civil engineering, institute of technology of building materials and components, veveří 331/95, 602 00 brno, czech republic ∗ corresponding author: 197260@vutbr.cz abstract. currently, great importance is attached to the reduction of emissions in industrial production, especially in the production of cement, when it is indicated that during production 1 ton of cement produces approximately 0.6 ton of co2. reducing emissions is possible thanks to secondary raw materials, which are produced in large quantities in the czech republic volumes for which a suitable use is sought. some secondary raw materials are suitable for use in concrete mixes, which are used to improve the rheology of the fresh concrete mixture as well as the properties hardened concrete. currently, there is an effort to eliminate the production of cement, to which its partial replacement by admixtures could also contribute. therefore it is necessary determine the optimal admixture that could be used to replace part of the cement in concrete mix. a large consumer of concrete mixes are transport structures, where it is possible for some of the cement used to be replaced by an admixture. keywords: cement, concrete pavemets, admixtures, durability, carbon footprint. 1. introduction concrete is mainly used in buildings and infrastructure around the world, where it is mainly produced using ordinary portland cement (opc) as a binder. in recent years, annual world cement production has increased from 1.0 billion tons to approximately 1.7 billion tons, which is enough to produce 1 m3 of concrete per person. as a result, the cement industry is generally considered to be a high growth industry. however, since the end of the 19th century, industry has been struggling with the need to reduce the burden on the environment, including carbon dioxide (co2) emissions. some estimates suggest that the amount of co2 emissions from global opc production may be as high as 7 % of total global co2 emissions. in addition, the production of opc is associated with serious environmental side effects, such as environmental pollution caused by dust and the enormous energy consumption required by the plasticity temperature above 1 300° c. for these reasons, the cement industry has been challenged in the last 10 years to effectively reduce and control co2 emissions [1–3]. four alternative technologies to reduce co2 emissions in the cement industry were commonly discussed; 1) changing the fuel to a fuel with a lower carbon content, for example from coal to natural gas during the calcination of limestone; 2) adding a chemical absorption process to capture co2 3) changing the clinker production process with efficient grinding and converting from a wet to dry process and 4) adding large volumes of supplementary cementitious materials (scm) such as ground granulated blast furnace slag (ggbs), fly ash (fa) and/or silica fume (sf). of these four technologies, the use of cement mixed with admixtures is the most practical and economical method that can be directly applied in the field of ready-mixed concrete. in addition, the use of ggbs or fa can provide additional environmental benefits, including conservation of natural resources and recycling of industrial by-products. research has shown that the addition of additives is capable of reducing co2 emissions by 22 %. they also pointed out that the greenhouse gas impact calculated from blended cement is reduced by up to approximately 22 % compared to traditional opc [2, 4]. concrete pavements are intended for very high traffic loads and as such are preferably used for the construction of airport runways, expressways or highways. concrete for the production of these structures must meet the requirements for workability, mechanical resistance, resistance to climatic influences, anti-slip properties and must be resistant to the action of chemical de-icing agents, which are used in the maintenance of such surfaces in winter [5]. the steady increase in heavy freight traffic not only puts a strain on highways, but also on other public transport areas. choosing the right technology at the right place is a difficult task for every road construction administration. at the same time, the requirements for durability and sustainability are in the center of attention. therefore, decisions are being made more and more often about the construction of cement-concrete covers on traffic areas. in addition to this, mixing cement with these raw materials can improve the long-term mechanical properties and durability of concrete and thereby extend the life of concrete structures [2–5]. 66 https://doi.org/10.14311/app.2024.47.0066 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 47/2024 production of concrete pavements using mixed cements recipe cem i cem ii/a-s cem iii/a cem ii/c-m cem ii/a-ll cem ii/b-m raw material 42.5 r (sc) 42.5 r 42.5 n (s-ll) 42.5 n 42.5 r (s-ll) 42.5 n [kg m−3] mokrá mokrá hranice hranice radotín radotín cem 360 360 360 360 360 360 dtk 0/4 tovačov 537 537 537 537 537 537 hdk 4/8 rosice 181 181 181 181 181 181 hdk 8/16 rosice 544 544 544 544 544 544 hdk 16/32 rosice 544 544 544 544 544 544 voda 165 165 185 175 170 182 muraplast fk 19 1.8 1.8 1.8 1.8 1.8 1.8 centrament air 202 0.13 0.08 0.07 0.07 0.09 0.08 table 1. composition of concrete recipes. 2. aims the aim of the article is the experimental verification of new types of cements and concrete mixes for cementconcrete road pavements with a significantly reduced carbon footprint and increased durability. the article is aimed at use in the field of transport constructions with a focus on the use of suitable mixed cements, reducing the formation of greenhouse gases to increase the service life of concrete pavements, extend the time for repairs and reduce the total costs for the construction and operation of the highway network in the czech republic. the main goal will be to assess the applicability of mixed cements for the production of cement-concrete covers due to the reduction of environmental impacts on our environment while achieving the required durability, especially the xf4 environment. 3. methodology this work deals with the lower layer of cementconcrete cover. the work was divided into 2 main stages. for the first stage, concretes with mixed cements were designed and produced. in the last stage, the achieved results were processed and compared based on the results of physical and mechanical properties with individual types of cement. 4. used raw materials in the experimental part, 6 concrete mixtures were produced, differing from each other only in the cement used. two cements were used from the mokrá cement plant, portland cement cem i 42.5 r (sc) and portland mixed cem ii/a-s 42.5 r. cements cem ii/a-ll 42.5 r and cem ii/b-m (s-ll) 42.5 n were used from the radotín plant. and cements cem iii/a 42.5 n and cem ii/c-m(s-ll) 42.5 n from the hranice cement plant. four types of aggregates from two areas were used for production. the washed mined aggregate in fraction 0/4 was from the tovačov locality and the coarse crushed aggregate in fractions 4/8, 8/16 and 16/32 from the rosice locality. both the plasticizing additive muraplast fk 19 and the aerating additive centrament air 202 conc. were used from mc bauchemie. the composition of the individual mixes produced is shown in table 1. 5. experiment when designing and mixing individual recipes, the goal was to achieve a consistency of fresh concrete mixture with a cone settlement in the range of 40–60 mm. during the mixing of the concrete mixture, the amount of water and plasticizer was adjusted due to the different compositions of the cements in order to achieve the desired consistency of the concrete. furthermore, the air content was measured on the fresh concrete. a different amount of aeration admixture was added to each mix to achieve an optimum fresh concrete air content of 6.5 %. subsequently, the bulk density of the fresh concrete was determined according to čsn en 123506, which ranged from 2 260 to 2 300 kg m−3. from each recipe, test bodies were produced (cube with an edge of 150 mm, beam 100 × 100 × 400) on which the physical and mechanical properties were determined. at the age of 2, 7, 28 and 90 days, the volumetric weight of the hardened concrete was tested according to čsn en 12 390-7 and the compressive strength according to čsn en 12 390-3 [6–8]. it follows from the measured results, see figure 1 that the replacement of cement with admixtures significantly negatively affects the short-term compressive strengths of concrete. as expected, the highest compressive strength after 2 days of maturation is achieved by road portland cement, which contains the highest percentage of clinker. the higher the replacement of cement with admixtures, the lower the strengths. the lowest compressive strength value after 2 days was evaluated for cem iii/a from hranice. the most significant increase in strength between 28 and 90 days is for the test samples that contain the active ingredient. this difference may be caused by the pozzolanic activity, which becomes apparent after a longer period than 28 days. the highest compressive strength of all concretes was achieved after 60 days of curing. concrete with the used cement cem ii/a-s 42.5 r mokrá achieves the highest compressive strength. as the age of the samples increased, the difference in strength values disappeared. 67 jan klobása, rudolf hela acta polytechnica ctu proceedings recipe weight flexural tensile flexural tensile degree of change strength ref strength frozen frost resistence [%] [mpa] [mpa] [%] cem i 42.5 r (sc) mokrá -0.17 5.9 6.3 106 cem ii/a-s 42.5 r mokrá -0.28 5.4 5.8 108 cem iii/a 42.5 n hranice -0.45 4.9 3.9 80 cem ii/c-m (s-ll) 42.5 n hranice -0.36 5.1 4 78 cem ii/a-ll 42.5 r radotín -0.88 4.8 4.3 89 cem ii/b-m (s-ll ) 42.5 n radotín -0.66 5.3 5.2 96 table 2. evaluation of the frost resistance test. figure 1. strength development over time. 5.1. determination of frost resistance of concrete determination of frost resistance was carried out according to čsn 73 1322. for the frost resistance test, the test specimens are water-saturated beams that are subjected to alternating freezing and thawing. 2 sets of test specimens were prepared for the test. one set is frozen and the other unfrozen serves as a comparison. freezing takes place in freezing cycles, with one cycle consisting of 4 hours of freezing and 2 hours of thawing. once the required number of freezing cycles has been reached, the test is finished. the result is weight loss of frozen bodies expressed in percent, tensile strength after bending and coefficient of frost resistance. the ratio of the arithmetic mean of the strength of frozen bodies and the arithmetic mean of unfrozen beams is called the coefficient of frost resistance. if the frost resistance factor is at least 75 %, we can call the concrete frost-resistant [9]. all concrete formulas passed in terms of frost resistance coefficient, i.e. all formulas have a higher coefficient greater than 75 % (see table 3). some concretes even have a frost resistance coefficient higher than 100 %. this phenomenon can probably be explained by rehydration of unhydrated cement grains. the weight loss for all manufactured recipes was a maximum of 0.88 % of the original weight of the frozen beam. recipe a a300 l [% vol.] [% vol.] [mm] cem i 42.5 r (sc) mokrá 6.6 3.5 0.19 cem ii/a-s 42.5 r mokrá 6.2 4.1 0.21 cem iii/a 42.5 n hranice 7.4 3.7 0.23 cem ii/c-m (s-ll) 42.5 n hranice 7.5 3.1 0.19 cem ii/a-ll 42.5 r radotín 6.9 3.5 0.22 cem ii/b-m (s-ll ) 42.5 n radotín 7.3 4.2 0.18 table 3. characteristics of air in hardened concrete (a – pore volume, a300 – volume of effective pores, l – distribution of air pores). 5.2. air content in hardened concrete the essence of the test to determine the characteristics of the air pores in hardened concrete, their mapping is possible based on an approximation to get an approximate idea of the amount of pores in the aerated test samples.the air pore structure is determined by scanning along a series of measuring straight lines, parallel to the original top surface of the sample. number of air of pores intersected by measuring lines are recorded, as are the individual lengths bowstring. mathematical analysis of the recorded data then enables a description of the system air pores in terms of the required parameters. the measurement is carried out on a cut-out from the test body, which can be seen in figure 2. figure 2. prepared body for determining the air content in hardened concrete. 68 vol. 47/2024 production of concrete pavements using mixed cements the characteristics of air pores in hardened concrete were determined on the test specimens, see table 3, namely the total air content, the content of microscopic pores up to 300 µm and the spatial distribution of air pores. the total air content in the hardened concrete was in the range of 6.2–7.5 %. the effective microscopic air content of a300 ranged from 3.1 to 4.2 %. the spatial distribution of air pores was in the range of 0.19 to 0.23 mm. all these indicators assume that the designed concretes with different cements will resist weather effects, especially frost. 6. discussions from the results of the short-term (2 and 7 day) strengths of the individual formulas, as expected, the formula with road cement from mokré achieved the highest value of compressive strength. the lowest values were achieved by cements with the lowest clinker content in the cement. the most significant increase in strength between 28 and 90 days is for the test samples that contain the active admixture. this difference may be due to the pozzolanic activity, which becomes apparent after a longer period than 28 days. the highest compressive strength of all concretes was achieved after 90 days of curing. as the age of the samples increased, the difference in strength values disappeared. aeration of the concrete mixture also has a significant effect on the magnitude of the determined compressive strength. if the air content is higher in the mixture, the compressive strength of the concrete decreases by up to 5 % for every 1 % of air in the concrete mixture [1, 3, 5]. in terms of concrete’s frost resistance, all formulas achieved frost resistance coefficients above 78 %. for recipes with cements from the wet plant, the coefficient of frost resistance even reached over 100 %. a spacing factor was determined on the hardened concrete in order to compare the results with the values measured in the fresh state. regarding the total air content a, in the fresh concrete the value was in all cases slightly lower than the value obtained from the microscopic determination of the air content in the hardened concrete. the effective microscopic air content of a300 ranged from 3.1 to 4.2 %. the spatial distribution of air pores was in the range of 0.19 to 0.23 mm. all these indicators assume that the designed concretes with different cements will resist weather effects, especially frost [3, 4]. 7. conclusion the study deals with the issue of verifying the use of mixed cements for the production of cement-concrete covers with a focus on reducing the carbon footprint and increasing durability and reducing the overall costs for the construction of transport structures. from the information obtained, it follows that for the production of cement-concrete covers they could also use cements with a lower clinker content and replacement of admixtures. blast furnace slag is among the most suitable admixtures for road concrete that can replace cement. in addition to mechanical properties, other aspects such as ecology and financial complexity are currently being monitored. as the requirement to eliminate the volume of co2, the largest producers of emissions of which are industries, is currently being increased, cement production in particular is energetically demanding, for this reason possibilities are being sought to reduce the amount of cement required without negatively affecting the properties of the proposed concrete structure. in order to eliminate emissions, the use of waste raw materials such as blast furnace slag is suitable. it is possible to replace cement with micro-ground limestone even though it is not a waste material. acknowledgements the paper was written in the framework of the but specific research project fast-j-23-8400: verification of the applicability of mixed cements for the production of concrete pavements. references [1] x. fu, y. li, c. lin, et al. strength, durability and appearance of low-carbon fair-faced concrete containing multiple mineral admixtures. construction and building materials 392:131838, 2023. https: //doi.org/10.1016/j.conbuildmat.2023.131838 [2] a. mohammadi, a. m. ramezanianpour. investigating the environmental and economic impacts of using supplementary cementitious materials (scms) using the life cycle approach. journal of building engineering 79:107934, 2023. https://doi.org/10.1016/j.jobe.2023.107934 [3] i. navarrete, j. valdes, m. lopez, f. vargas. replacement of pozzolanic blended cement by supplementary cementitious materials: mechanical and environmental approach. construction and building materials 394:132263, 2023. https: //doi.org/10.1016/j.conbuildmat.2023.132263 [4] k.-h. yang, y.-b. jung, m.-s. cho, s.-h. tae. effect of supplementary cementitious materials on reduction of co2 emissions from concrete. journal of cleaner production 103:774–783, 2015. carbon emissions reduction: policies, technologies, monitoring, assessment and modeling. https://doi.org/10.1016/j.jclepro.2014.03.018 [5] t. k. gondwal, a. bishnoi, p. mandal. a comprehensive study for physical and chemical properties of road dust to utilize in concrete mix design, collected from diversified locations of delhi ncr. cement 11:100056, 2023. https://doi.org/10.1016/j.cement.2023.100056 [6] zkoušení čerstvého betonu – část 6: objemová hmotnost [testing fresh concrete – part 6: density]. standard, czech standard institute, prague, 2020. [7] zkoušení ztvrdlého betonu – část 7: objemová hmotnost ztvrdlého betonu [testing hardened concrete – part 7: density of hardened concrete]. standard, czech standard institute, prague, 2021. 69 https://doi.org/10.1016/j.conbuildmat.2023.131838 https://doi.org/10.1016/j.conbuildmat.2023.131838 https://doi.org/10.1016/j.jobe.2023.107934 https://doi.org/10.1016/j.conbuildmat.2023.132263 https://doi.org/10.1016/j.conbuildmat.2023.132263 https://doi.org/10.1016/j.jclepro.2014.03.018 https://doi.org/10.1016/j.cement.2023.100056 jan klobása, rudolf hela acta polytechnica ctu proceedings [8] zkoušení ztvrdlého betonu – část 3: pevnost v tlaku zkušebních těles [testing hardened concrete – part 3: compressive strength of test specimens]. standard, czech standard institute, prague, 2020. [9] stanovení mrazuvzdornosti betonu [determination of frost resistance of concrete]. standard, czech standard institute, prague. [10] x. sun, y. li, x. wei, et al. high contents of steel slag in the road concrete: hydration mechanism, mechanical property and durability performance. construction and building materials 400:132703, 2023. https: //doi.org/10.1016/j.conbuildmat.2023.132703 70 https://doi.org/10.1016/j.conbuildmat.2023.132703 https://doi.org/10.1016/j.conbuildmat.2023.132703 acta polytechnica ctu proceedings 47:66–70, 2024 1 introduction 2 aims 3 methodology 4 used raw materials 5 experiment 5.1 determination of frost resistance of concrete 5.2 air content in hardened concrete 6 discussions 7 conclusion acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2023.41.0044 acta polytechnica ctu proceedings 41:44–48, 2023 © 2023 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague analysis of objective eye tracking data used for vehicle assessment jan novotný czech technical university in prague, faculty of transportation sciences, department of vehicles, horská 2040/3, 128 00 prague, czech republic correspondence: jan.novotny@cvut.cz abstract. this paper describes the application of eye tracking technology in the automotive industry in the context of vehicle assessment in customer clinics. the paper presents the mentioned technology, the applied experimental methodology and selected results. in particular, the hypotheses that were expressed during the research of the topic and during the preparation of the experiment are answered. in this way, the innovative technology of eye tracking can be used to reflect the “voice of the customer” in the vehicle design. keywords: eye tracking, customer interest, eye fixation, driver distraction. 1. introduction in the consumer world almost everything is sold to the eyes of the customers. the practicality, functionality and other attributes of a product are obviously important, but the first opinions are formed by the visual, design appearance of the product. therefore, it is crucial for vehicle manufacturers to take the customer interest and opinions into consideration. for this reason, they often organise the so called customer clinics, where they gather their new models, sometimes also their competitors vehicles, and let participants from the public give them their view. this usually happens in a form of an interview or a group discussion, but there never is the real view of the participant. and that opens an opportunity for the eye tracking technology. the eye tracking technology is able to track pupils and therefore detect the exact location where the participant is looking. this study aims to appropriately combine subjective and objective evaluation and develop an innovative methodology about customer qualitative assessments. the subjective evaluation is obtained using a method called “lautes denken”, a form of interview with the participant in the presence of vehicles. for the objective evaluation, the eye tracking technology is used, which consists of putting on special glasses and tracking and measuring the participant’s eye movements. 2. the eye tracking applications the eye tracking technology is frequently used in marketing studies, product or web design assessments. it was confirmed by many studies that the product design strongly influences the buyers decision [1]. another example is a study by lange et al. [2], where a visual demand of in-vehicle touchscreen displays was measured. as is apparent, the eye tracking can be used for distraction levels assessment, which is closely connected with vehicle driving safety. besides the safety and distraction assessment purposes, the eye tracking technology in the field of automotive industry is occasionally used for design assessments. at least there was an effort to do that based on several studies. zhaolin et al. [3] propose a car styling based on the eye tracking technology, where eye movement hotspots maps are used to build an index system. they evaluate the data as a classification model in order to prove that this approach is feasible to objectively evaluate vehicles styling. a study by wang et al. [4] also shows the possibilities to effectively evaluate product design process with the application of the eye tracking technology. because the researchers get the objective data and not only a subjective ranking, score or a narrative from the participants. this opens further possibilities to combine the positive attributes of both types of design assessments. in this paper, a combination of an eye tracking study and a subjective interview design study will be introduced. 3. the eye tracking method the vision is one of the primary senses that tells a lot about person’s behaviour. the input of vision are eyes, more specifically the pupils which process the incoming light. but the perfect image requires a correct angle, therefore the eyes have to move and shift its focus frequently. for the purpose of the eye tracking technology, we recognise three types of movement as shown in figure 1. a gaze is a very brief stopping of the eyes. if there are multiple gazes in close proximity, it can be detected as a fixation, which can already be considered as a focus on certain object and takes between 100–700 milliseconds. and lastly the saccade is a very fast movement between gazes or fixations [5]. the eye tracking technology tobii pro glasses 2 illustrated in figure 2 allows to track and detect all of 44 https://doi.org/10.14311/app.2023.41.0044 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 41/2023 objective eye tracking data for vehicle assessment figure 1. basic eye movements illustration. figure 2. the eye tracker design. the mentioned eye movements and states. based on videooculography, the eye tracker tracks the centre of pupils using reflection of infrared light and camera system in frequency of 50 hz. the designated algorithm i-vt then classifies what type of movement or eye state is happening and where in the coordinate system the person is fixating with regard to the forward view (captured by fullhd camera). the classification is determined by the velocity of movement and can be modified for a given experiment. in this experiment, the velocity threshold parameter between a fixation and a saccade was set to 30◦ s−1 [6]. 4. the experiment demography and description the group of participants in this experiment contained a total of 25 people. complete datasets (subjective and objective data) were obtained for 24 of them. one person had to be excluded from the objective assessment due to a significant eye defect. people with an interest in automobiles who were assumed to be able to talk about the issue at hand were selected. the gender representation was 17 men and 8 women participated in the experiment. participants were divided into three groups according to age. these were those aged 18–29 years (48 %), aged 30–45 years (36 %) and aged 46 years and over (16 %). all participants were brought to a table and asked to provide some personal information. they were also informed about the process of the following measurement, in particular to speak as much and as openly as possible. they were also asked to follow the moderator to the indicated places and to move with their head down. this was so that the participant’s very first glance at a given part of the vehicle was already recorded by the measuring equipment. the participant’s eye tracking glasses were fitted and calibrated and then they were led to a designated spot 2.5 m from the front of the škoda octavia iv style. the first 5 s were reserved for undisturbed static observation, when initial points or areas of interest can be recorded. the moderator then begins questioning, the intention of which is to obtain as many of the participant’s insights and opinions as possible. participant is asked (variant for the front of the car): • “your first impression of the front of the car?” • “how would you characterise it?” • “does anything characterise the quality of the car/evokes the sporty design?” • “what do you like?” • “what do you dislike?” the participant is then led to a marker 2.5 m from the side of the vehicle where the procedure is repeated. the participant is then brought to a mark 2.5 m from the rear of the vehicle. the procedure is then repeated with the octavia iv rs variant. 45 jan novotný acta polytechnica ctu proceedings figure 3. data from the rear of the rs model. 5. evaluation the data from the eye tracking part of the measurement were methodically evaluated using the dedicated tobii pro lab software. areas of interest (aoi) were created to generate the data and to obtain exact information on which parts of the car the participant was looking at during the measurement. a large amount of data was measured and analysed. this paper will highlight the interesting points and the answers to the hypotheses presented below. 5.1. hypotheses 1. do people perceive the spoiler and diffuser as a sporty feature? 2. does the standard deviation of the observation of the electric charging lid take on higher values than the adjacent front door? how do participants perceive electric charging lit? 3. does the term quality associate with the car’s face/joints? (comparison of first view and focus on quality) 4. the acc adaptive cruise control sensor will attract a large amount of fixation (attention). 5. does the average fixation time on the front logo of the style model take higher values than the rs model? 6. the electric charger will be perceived negatively on the rs model. 7. a large number of fixations will be attracted by the additional badges at the rear of the cars. 5.2. evaluation and data demonstration 1. do people perceive the spoiler and diffuser as a sporty feature? the sporty design dominates the rear of the model rs, which is confirmed by the measured data. the exhaust pipes, diffuser and black elements (trims and especially the spoiler) and black badges are the most frequently mentioned and observed as shown in figure 3 proves the spoiler is the most mentioned in terms of sporty design, but the heatmap does not highlight it much – a thin, elongated zone. therefore, both sources of information – objective eye tracking and subjective opinions of participants – must be taken into account to produce a credible result. 2. is the standard deviation of the observation of the electric charging lid higher than the adjacent front door? how do participants perceive electric charging lit? the standard deviation was calculated from the measured individual data. the standard deviation of the electric charging lid is 0.69, the front door is only 0.35. the hypothesis suggesting a larger inconsistency and certain controversy of the electric charging lid is therefore confirmed. subjective opinions also confirm this claim. almost all participants fixated at the front door zone, with an average fixation duration of 0.50 s, there is not much variation between participants. in contrast, the lid was viewed by half of the participants in the first five s, with an average fixation duration of 0.96 s. the value varied frequently, from a minimum of 0.22 s to a maximum of 2.80 s. 3. does the term quality associate with the moulding/joints of the car? (comparison of first view and focus on quality) when looking at a vehicle, one forms an initial opinion in the first few s. he or she forms a first impression, is attracted by various design or practical features, whether positive or negative. a form of conversation, supplementary questions, is then used to distinguish between these positive or negative impressions. therefore, in this study, we mainly measure the first 5 s of a person’s view of a given part of the car, then directly compare it with the interval in which we ask about the perception of quality. differences are observable right away for this 46 vol. 41/2023 objective eye tracking data for vehicle assessment figure 4. heatmaps – first view / question about the quality of the style model. figure 5. fixations division of model style – front view. first view of the front of the style model, both in the heatmaps in figure 4 and in the figure 5. the participants’ first glances are mainly directed at the vehicle logo, the acc sensor cover, the windscreen area and the shape of the bumper. when asked about perceived quality, the fixation of the glances focuses more on the joints around the radiator grille and also more on the trim and bumper materials. a distinct cluster of fixations can be observed on the small cover on the left side of the bumper, this is probably due to the regular shape of the cover where the quality of the moulding can be easily seen. the subjective data also supports the hypothesis, with the moulding receiving the most responses to evoking quality (one third of all recorded responses), followed by the lights quality. 4. the acc adaptive cruise control sensor will attract a large number of fixations (attention). the model style’s acc sensor attracted 7.19 % of all eye fixations, compared to just 2.37 % for the rs model. this is probably because the rs model has a number of other dominant, sporty features such as the rs badge and a variety of aerodynamic elements. however, considering the small size of the sensor (viewing zone), it is still a feature that attracts a great degree of attention. 5. is the average fixation time to the front logo of the style model higher than that of the rs model? the average fixation time on the style’s front logo at the first five-second observation is 0.24 s. the rs model logo recorded an average of 0.46 s. thus, the hypothesis was disproved as the rs model achieved a double the attention. for the style model, attention shifted to the logo only when asked about perceived quality (0.55 s). 6. the rs model will have a negative perception of the electric charger. the side view of the rs model is dominated by the detail of the electric charger lid, which attracted 9.34 % fixation on first glance! the heatmap in figure 6 confirms this fact with a significant cluster of fixations that also had a long duration. if we look at the subjective opinions of the participants, 13 of them directly expressed that they did 47 jan novotný acta polytechnica ctu proceedings figure 6. heatmap – first look at the side of the rs. figure 7. proportion of fixations on rs model details. not like this element, others that they were not used to it. confirming again the hypothesis number 2. 7. a large number of fixations are attracted by the additional badges on the rear of the cars. the accessories, badges and small logos truly draw the attention of the participants for both models. for the style model, the accessories (škoda logo, exhaust pipes, and the signs “octavia” and “g-tech”) attracted 12.01 % of the total fixation duration. as shown in the figure 7, the rs model even has a fixation share of 18.62 % for accessories. in particular, brand and model logos and sports exhausts dominate. 6. conclusion the experiment answered the basic hypotheses concerning the examined models. six hypotheses were confirmed, one hypothesis (number 5) was disproved as the result was the opposite. the results were presented above. the experiment and the paper demonstrates a new measurement methodology combining the benefits of the objective eye tracking technology and the subjective interview technique. this creates a new possibility of applying the eye tracking technology in the assessment of design features of new vehicles. references [1] t. n. reid, e. f. macdonald, p. du. impact of product design representation on customer judgment. journal of mechanical design 135(9):091008, 2013. https://doi.org/10.1115/1.4024724 [2] d. r. large, g. burnett, e. crundall, et al. developing predictive equations to model the visual demand of in-vehicle touchscreen hmis. international journal of human–computer interaction 34(1):1–14, 2017. https://doi.org/10.1080/10447318.2017.1306940 [3] l. zhaolin, l. shengbo, x. shaobing, c. bo. automobile style evaluation based on eye tracking. journal of tsinghua university (science and technology) 55(7):775–781, 2015. [4] z. y. wang, h. s. liu, h. h. shi, h. liu. the research of automobile design evaluation method based on the eye tracking system technology. advanced materials research 230-232:654–658, 2011. https://doi.org/10. 4028/www.scientific.net/amr.230-232.654 [5] eyeware. understanding eye tracking & how it can work for you: definitions, metrics, and applications, 2019.[2022-11-5], https: //eyeware.tech/blog/what-is-eye-tracking/. [6] o. anneli. the tobii i-vt fixation filter 3 / 21. 2012. [2020-10-26], https://www.tobiipro.com/sitea ssets/tobii-pro/learn-and-support/analyze/how -do-we-classify-eye-movements/tobii-pro-i-vt-f ixation-filter.pdf?fbclid=iwar1_em_u-tgzzlsndh jboemnycualvdt9khcbcp1pug3emhxoqbfsdgvj6y. 48 https://doi.org/10.1115/1.4024724 https://doi.org/10.1080/10447318.2017.1306940 https://doi.org/10.4028/www.scientific.net/amr.230-232.654 https://doi.org/10.4028/www.scientific.net/amr.230-232.654 https://eyeware.tech/blog/what-is-eye-tracking/ https://eyeware.tech/blog/what-is-eye-tracking/ https://www.tobiipro.com/siteassets/tobii-pro/learn-and-support/analyze/how-do-we-classify-eye-movements/tobii-pro-i-vt-fixation-filter.pdf?fbclid=iwar1_em_u-tgzzlsndhjboemnycualvdt9khcbcp1pug3emhxoqbfsdgvj6y https://www.tobiipro.com/siteassets/tobii-pro/learn-and-support/analyze/how-do-we-classify-eye-movements/tobii-pro-i-vt-fixation-filter.pdf?fbclid=iwar1_em_u-tgzzlsndhjboemnycualvdt9khcbcp1pug3emhxoqbfsdgvj6y https://www.tobiipro.com/siteassets/tobii-pro/learn-and-support/analyze/how-do-we-classify-eye-movements/tobii-pro-i-vt-fixation-filter.pdf?fbclid=iwar1_em_u-tgzzlsndhjboemnycualvdt9khcbcp1pug3emhxoqbfsdgvj6y https://www.tobiipro.com/siteassets/tobii-pro/learn-and-support/analyze/how-do-we-classify-eye-movements/tobii-pro-i-vt-fixation-filter.pdf?fbclid=iwar1_em_u-tgzzlsndhjboemnycualvdt9khcbcp1pug3emhxoqbfsdgvj6y https://www.tobiipro.com/siteassets/tobii-pro/learn-and-support/analyze/how-do-we-classify-eye-movements/tobii-pro-i-vt-fixation-filter.pdf?fbclid=iwar1_em_u-tgzzlsndhjboemnycualvdt9khcbcp1pug3emhxoqbfsdgvj6y acta polytechnica ctu proceedings 41:44–48, 2023 1 introduction 2 the eye tracking applications 3 the eye tracking method 4 the experiment demography and description 5 evaluation 5.1 hypotheses 5.2 evaluation and data demonstration 6 conclusion references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.38.0361 acta polytechnica ctu proceedings 38:361–367, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague operational energy and embodied impacts of retrofitting the window frames of mixed-mode office buildings rafaela gravia pimenta, leticia de oliveira neves, vanessa gomes∗ university of campinas, school of civil engineering and architecture and urbanism, rua saturnino de brito, n° 224, cidade universitária zeferino vaz. cep: 13083-889 – campinas – são paulo, brazil ∗ corresponding author: vangomes@unicamp.br abstract. design decisions normally consider the building’s operational phase as the main criterion to reduce energy expenses in a building. in less efficient buildings, reducing the operational energy becomes the most important aspect to address in the design, construction and operational phases, for it represents the highest life cycle energy flow. however, energy-efficient solutions often reduce operational energy demand by increasing the building’s embodied energy and greenhouse gas emissions, which have been overlooked in energy performance analyses. this work aims at investigating the operational energy and the consequent embodied impacts resulting from the retrofit of the window frame of mixed-mode office buildings located in a hot climate, with a focus on reducing the cooling energy demand. the method consists of an experimental study based on a case study, in which the energyplus and the simapro software tools are used to evaluate the operational energy and the environmental impacts. results showed that reducing the wwr and increasing the window opening factor conveyed operational energy savings but in some retrofit scenarios tested, these retrofit measures were counterproductive from the ced and gwp perspective. the main scientific contribution of this work is understanding the importance of the building analysis from a life-cycle approach. the results obtained can assist companies and designers to make their decisions from a broader environmental perspective. keywords: energy efficiency, operational energy, embodied energy, life-cycle analysis, office building, envelope, retrofit. 1. retrofit from an operational energy and embodied impacts perspective in brazil, reducing building energy consumption has gained attention in recent years. approximately 47 % from all electricity in the country is consumed by buildings [1]. of this percentage, about 15 % corresponds to the commercial buildings, with lighting and air-conditioning systems being the most representative end uses [1]. according to the international energy agency [2], if energy efficiency strategies in buildings are not addressed currently, energy use for cooling could double by 2040, due to the increased use of air-conditioning. as operational energy represents the largest energy consumption of a building throughout its life cycle, reducing it should be the most important aspect to be addressed by designers, architects and engineers [3]. the design of energy-efficient buildings is, therefore, focused on reducing their operational energy, which can be achieved through the use of passive and active strategies. reducing the operational energy, however, often implies an increase in embodied energy through the use of new materials and technologies [3]. azari [4] shows that, among the publications about energy performance of building envelopes, most are directed to the analysis of operational energy, and few studies investigate the building energy performance from a life cycle perspective. krstic-furundzic et al. [5] point out that building envelope is the main architectural element of a building that impacts thermal comfort and energy performance. in terms of its contribution to reducing energy consumption, the authors show that the analysis of various facade scenarios is crucial and necessary for each specific case and climate, from the design point of view. thibodeau et al. [6], through a comprehensive literature review, indicate that, from a life cycle perspective, it is environmentally advantageous to retrofit a building instead of demolishing and rebuilding it. de angelis et al. [7] identify that in countries that do not have heating demand, carrying out a retrofit focused on reducing energy consumption is preferable than reconstructing the building, since the reconstruction corresponds to 35 % to 40 % of the life cycle impacts. according to tokede et al. [8], there are several studies of building retrofit with a focus on the building’s envelope that prove possible a reduction in the operational energy consumption and, consequently, a reduction in the carbon dioxide emissions to the atmosphere. saade et al. [9] present a literature review correlating operational consumption and environmental impacts, with a focus on the impacts of embodied energy and carbon. the authors found a strong correlation between the gwp (global warming potential) and ced (cumula361 https://doi.org/10.14311/app.2022.38.0361 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en r. g. pimenta, l. o. neves, v. gomes acta polytechnica ctu proceedings tive energy demand) categories, and stated that the advances in buildings operational energy performance led to an increase in their environmental impacts, due to the relative decrease in the operating loads share, and to the resources consumed and emissions generated in materials production. the building’s life cycle analysis depends on the country’s energy matrix. in 2018, the brazilian renewable energy production corresponded to 45.3 % of its total energy production, remaining as one of the highest in the world [10]. therefore, most research studies concerning operational energy and the consequent embodied impacts of buildings do not apply to brazil or to tropical climates, where cooling is one of the buildings’ main end uses. thus, the main objective of this paper is to analyse the ratio between operational and embodied energy resulting from the retrofit of the window frame of mixed-mode office buildings, with a focus on reducing the cooling energy demand. 2. methods 2.1. reference model and scenarios a database containing architectural design and envelope information of 153 mixed-mode office buildings (i.e., operating on natural ventilation and airconditioning modes, alternatively) located in the city of são paulo, brazil, was developed by neves et al. [11] and detailed by pereira [12]. as it contains a representative sample (about 10 %) of mixed-mode office buildings in são paulo, this database was used as a basis to define a reference model, as shown in table 1 and figure 1. table 2 shows the window frame variable parameters used to model the retrofit scenarios, which were chosen based on the literature review [3, 11–15]. the reference model was analysed considering four solar orientations – outdoor facades facing north and east (north-east), east and south (east-south), south and west (south-west) and west and north (westnorth), as shown in figure 1. 2.2. operational energy calculation to quantify the operational energy consumption of the retrofit scenarios, computer simulations were performed in the energyplus software. the geometry of the reference model and scenarios were modelled in the euclid plugin for sketchup, which interfaces with energyplus. the calculation of the office room’s operational energy consumption was performed based on the climate file of the city of são paulo, based on data from the national institute of meteorology (inmet) [16]. the natural ventilation was modelled through the airflownetwork module, which performs pressure, airflow, temperature and humidity calculations in the nodes, and sensitive and latent heat exchange calculations [17]. the energy management system (ems) module was used to simulate the mixedmode system, using as a reference to the indoor operative temperature setpoint the adaptive thermal comfort model from ashrae 55 [18]. the window operation was set as opened and the air-conditioning was turned off when the indoor operative temperature within the thermal comfort range and the room was occupied. otherwise (indoor operative temperature out of range), the window operation was set as closed and the air-conditioning was turned on. when the room was unoccupied, the windows were closed and the air-conditioning system was turned off. 2.3. ced and gwp calculation the life cycle assessment (lca) methodology was used to analyse two categories of environmental impacts: cumulative energy demand (ced) and global warming potential (gwp). the guidelines for lca were based on iso 14040:2006 [23] and iso 14044:2006 [24]. the standards were used to define the objective and scope, to set up the life cycle inventory (icv), to assess the life cycle impacts (lcia) and to analyse the results (table 3). among the life cycle stages indicated by en 15978-2011 [21], only the construction extraction modules were considered, which are: a1–a4 (considering the distance from the region of demolition to the centre of são paulo), b2 (maintenance, considering the repainting of the facades); b5 (operational energy consumption) and c2 (disposal of materials after demolition). modules a5, c1, b1, b3, b4, b6, c3 and c4 were excluded from the analysis. the simapro 8.5 lca platform was used to model the retrofit scenarios. the ecoinvent version 3.4 database was used, enabling changes in the energy matrix of the data sets to bring them closer to national production parameters. the reference period considered was 50 years. the cumulative energy demand (ced) method calculates the total primary energy demand in the life cycle of materials and the cml 2001 baseline method addresses several impact categories, including co2eq emission. the embodied energy analysis was performed based on the results obtained from the ced method (mj/m2 · year) and transformed into kwh/m2 · year to allow a comparison between scenarios. the climate change analysis of the operational and retrofit phases was carried out based on the results of global warming potential (gwp), in kg co2eq/m2 · year. the quantities of materials used in each scenario were calculated in kg or m2, and transportation was calculated in ton ∗ kilometre (tkm), as shown in table 3. the window frames were considered to be made of aluminium and a single pane glazing. 3. results and discussion natural ventilation is known as an assertive passive strategy towards thermal comfort conditions within the humid subtropical climate of the city of são paulo. indeed, among the four solar orientations analysed, natural ventilation was used during 60 % of the room’s occupancy time for both office rooms facing north 362 vol. 38/2022 operational energy and embodied impacts of retrofitting . . . parameters values reference room geometry room area 39.2 m2 average value of database [12] floor 6th intermediate floor of a 12-storey building (average value of the database) [12] floor-to-ceiling height 2.50 m average value of the database [12] window frame window-to-wall ratio (wwr) 25 % average value of the database [12] window opening factor 64 % average value of the database [12] glass solar heat gain coefficient (shgc) colored glass (62 %) most recurrent case on database [12] u-value standard glass (5.8 w/m2 · k) most recurrent case of the database [12] solar shading devices none most recurrent case of the database [12] envelope u-value 2.38 w/m2 · k concrete block and mortar (0.28 m) [19] thermal capacity 258.6 kj/m2 · k solar absorptance 0.5 average value of the database [12] emissivity opaque material (0.9) most recurrent case of the database [12] air conditioning system type split most recurrent system in office buildings [20] coefficient of performance (cop) 3.23 w/w level a procel [21] internal loads occupancy (number of occupants and metabolic rate) 0.14 person/m2 65 w/m2 [22] lights 9.7 w/m2 level a procel [21] equipment 10.7 w/m2 [22] schedule weekdays 8 am to 6 pm [21] natural ventilation strategy cross-ventilated (adjacent facades) most recurrent case of the database [12] table 1. reference model’s input parameters. figure 1. reference model’s office room model and investigated solar orientations. parameters scenarios window frame window-to-wall ratio (wwr) 12.5 %, 25 %∗, 37.5 %, 50 %, 62.5 % window opening factor (% of the window frame that is operable for natural ventilation) 35 %, 64 %∗, 93 % ∗ corresponds to the reference model. table 2. variable parameters. 363 r. g. pimenta, l. o. neves, v. gomes acta polytechnica ctu proceedings figure 2. energy demand for cooling – monthly results. parameters kg m2 tkm window frame wwr aluminium 12.5 % 2.5 6.25 25.0 % 5.0 12.5 37.5 % 7.5 18.75 50.0 % 10.0 25.00 62.5 % 12.5 31.25 glass 12.5 % 80.0 12.50 25.0 % 160.0 24.00 37.5 % 240.0 36.00 50.0 % 320.0 48.00 62.5 % 400.0 60.00 window opening factor aluminium 2 12.50 glass 160 24.00 table 3. window parameters and materials used in the life cycle inventory. (north-east and west-north) and 70 % of the occupancy time for rooms facing south (east-south and south-west). the mixed-mode system was, therefore, responsible for an annual cooling demand reduction of 74 % for the east-south office room and 72 % for the south-west office room, if compared to a fully air-conditioned office room (figure 2). an increase in the cooling energy demand during the hot season (october to march) can be observed in the rooms facing south. as to the rooms facing north, the cooling energy demand varies throughout the year and it is not possible to determine the most critical season. the room with the best thermal and energy performance was the east-south room, since it is the room with less direct solar radiation. the office rooms with openings to north-east and west-north had similar performance. the demand for heating had insignificant results so only the cooling demand values were considered in the analysis. 3.1. wwr and window opening factor scenarios figure 3 presents the cooling energy demand for the wwr and window opening factor variation. the wwr variation had higher impact over the results, being the lowest percentage of wwr (12.5 %) the scenario with best thermal and energy operational demand performance. conversely, the highest value of window opening factor (93 %) resulted in the lowest cooling energy demand values for all solar orientations analysed. low window opening factor values demand more energy for cooling due to its small operable 364 vol. 38/2022 operational energy and embodied impacts of retrofitting . . . figure 3. energy demand for cooling – scenarios for wwr (left) and window opening factor (right). figure 4. embodied energy per m2 · year – wwr scenarios. figure 5. global warming potential impact per m2 · year – wwr scenarios. figure 6. embodied energy per m2 · year – window opening factor scenarios. 365 r. g. pimenta, l. o. neves, v. gomes acta polytechnica ctu proceedings figure 7. global warming potential impact per m2 · year – window opening factor scenarios. area, which impairs natural ventilation. the reduced impact of the window opening factor variation over the results could be due to the fixed wwr of the reference model (25 %), which is relatively low. the office rooms facing north-east and west-north had similar performance for both variable parameters. the cooling energy demand of these rooms, when wwr is 12.5 %, reduced approximately 40 %, if compared to the reference model (wwr = 25 %). if compared to the south-west and east-south rooms, the cooling energy demand was approximately 40 % higher. the reference model scenario (wwr = 25 %) presented cooling energy demand 40 % to 50 % higher for the north facing rooms, if compared to the eastsouth room, which showed the best energy performance for both variable parameters. the electricity showed the highest annual embodied energy and carbon dioxide emissions in all cases (figures 4 to 7). the embodied energy analysis showed the 12.5 % wwr scenario as the best case, even when considering the embodied impacts due to the replacement of the window frames, if the solar office room is facing north (figure 4). when considering a better solar orientation (east-south or south-west), the wwr reduction was not an advantageous option, from the embodied impact perspective. the scenarios with wwr higher than the reference model (37.5 %, 50 % and 62.5 %) showed an increase in the cooling energy demand – and, consequently, in the embodied energy and gwp impacts – for all cases (figures 4 and 5). the 64 % (reference model) and the 93 % window opening factor scenarios showed similar ced and gwp results (figures 6 and 7). in fact, the latter equalled or exceeded the reference model results, showing to be counterproductive, when its embodied impacts were taken into consideration. 4. conclusions this paper aimed at analysing the thermal and energy performance of retrofitting window frames of mixed-mode office buildings located in são paulo, brazil. only single glazed aluminium window frames were herein investigated. though this might limit the information value for temperate and cold climates, such configuration corresponds to typical mixed-mode buildings in brazil and are also ubiquitous in many tropical regions. results showed that reducing the wwr and increasing the window opening factor could indeed convey operational energy savings to the existing buildings. such reduction, however, increases environmental impacts through the addition of materials that demand energy consumption for their production, transport, maintenance and end of life. in some scenarios tested, reducing wwr and increasing the window opening factor outweighed the ced and gwp of the reference model, showing to be counterproductive retrofit measures. the case study here presented demonstrates the importance of analysing the energy performance not only from an operational perspective but also from the embodied impacts perspective. acknowledgements the authors thank the brazilian council for scientific and technological development (cnpq) for the scientific productivity grant [vgs, #306048/2018-3]. references [1] r. lamberts, l. dutra, f. pereira. eficiência energética na arquitetura, 2014. eletrobras / procel programa nacional de conservação de energia elétrica / procel edifica eficiência energética em edificações. 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[13] a. vilches, a. garcia-martinez, b. sanchez-montañes. life cycle assessment (lca) of building refurbishment: a literature review. energy and buildings 135:286–301, 2017. https://doi.org/10.1016/j.enbuild.2016.11.042 [14] p. x. w. zou, x. xu, j. sanjayan, j. wang. review of 10 years research on building energy performance gap: life-cycle and stakeholder perspectives. energy and buildings 178:165–181, 2018. https://doi.org/10.1016/j.enbuild.2018.08.040 [15] v. gomes, m. saade, b. lima, m. silva. exploring lifecycle energy and greenhouse gas emissions of a case study with ambitious energy compensation goals in a cooling-dominated climate. energy and buildings 173:302–314, 2018. https://doi.org/10.1016/j.enbuild.2018.04.063 [16] labeee. [2022-02-16]. https://www.labeee.ufsc. br/downloads/arquivos-climaticos [17] energyplus. engineering reference. united states, u.s. department of energy, 2016. [18] american society of heating, refrigerating and air conditioning. ansi/ashrae 55-2013. thermal environmental conditions for human occupancy. ansi/ashra, atlanta, 2013. [19] associação brasileira de normas técnicas. nbr 15220-2 – desempenho térmico de edificações. parte 2: método de cálculo da transmitância térmica, da capacidade térmica, do atraso térmico e do fator solar de elementos e componentes de edificações. abnt, rio de janeiro, 2005. [20] eletrobras. pesquisa de posse de equipamentos e hábitos de uso – ano base 2005. classe industrial alta tensão, 2008. [2022-02-16]. http://shorturl.at/sxwx9 [21] intituto nacional de metrologia, normalização e qualidade industrial – inmetro. manual para aplicação do regulamento técnico da qualidade para nível de eficiência energética de edificações comerciais, de serviços e públicos. inmetro, rio de janeiro, 2018. [22] associação brasileira de normas técnicas. nbr 16401: instalação de ar condicionado – sistemas centrais e unitários. abnt, rio de janeiro, 2008. [23] international organization for standardization. iso 14040. environmental management – life cycle assessment – principles and framework. iso, geneva, 2006. [24] international organization for standardization. iso 14044. environmental management – life cycle assessment – requirements and guidelines. iso, geneva, 2006. 367 https://doi.org/10.1016/j.rser.2018.12.037 https://doi.org/10.1016/j.enbuild.2018.08.034 https://doi.org/10.1016/j.buildenv.2019.106449 https://www.epe.org.br https://doi.org/10.1016/j.enbuild.2016.11.042 https://doi.org/10.1016/j.enbuild.2018.08.040 https://doi.org/10.1016/j.enbuild.2018.04.063 https://www.labeee.ufsc.br/downloads/arquivos-climaticos https://www.labeee.ufsc.br/downloads/arquivos-climaticos http://shorturl.at/sxwx9 acta polytechnica ctu proceedings 38:361–367, 2022 1 retrofit from an operational energy and embodied impacts perspective 2 methods 2.1 reference model and scenarios 2.2 operational energy calculation 2.3 ced and gwp calculation 3 results and discussion 3.1 wwr and window opening factor scenarios 4 conclusions acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2024.50.0094 acta polytechnica ctu proceedings 50:94–99, 2024 © 2024 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague parametric analysis for interphase shear stress in mos2/pet nanocomposite under thermo-mechanical loading rayka vladovaa,∗, tatyana petrovaa, elisaveta kirilovaa, boyan boyadjieva, apostol apostolova, wilfried beckerb, alexsander moravskic a bulgarian academy of sciences, institute of chemical engineering, acad. g. bonchev str., bl.103, 1113 sofia, bulgaria b technical university – darmstadt, institute of structural mechanics, franziska-braun-str. 7, l5|01 347a, 64287 darmstadt, germany c sofia university st. kliment ohridski, faculty of mathematics and informatics, james bourchier blvd. 5, 1164 sofia, bulgaria ∗ corresponding author: r.vladova@iche.bas.bg abstract. in this study, a parametric analysis on the factors, that influence the value of theoretical interphase shear stress (iss) in nanocomposite mos2/pet, subjected to thermo-mechanical loading has been performed. the theoretical value of iss is calculated based on obtained analytical solutions for the interfacial shear stress. the sensitivity of following parameters on the iss in the considered nanostructure is investigated: the thicknesses of the nanocomposite layers, the length of the nanocomposite, the magnitude of the applied mechanical load, the applied temperature difference. it is found that the interface thickness does not affect the iss value. the magnitude of the applied mechanical load has a strong influence on the magnitude of the iss. the temperature difference (pure thermal loading) also affects the iss value, but to a lesser extent. it should be noted, that in the case of combined loading (thermo-mechanical) the overall effect is additive. the thicknesses of mos2 and pet mostly affect iss, especially the substrate thickness. the obtained results are graphically illustrated and can be used for the fast prediction of iss in micro scale in similar nanocomposite devices or parts thereof such as sensors, nanoand optical electronic devices, energy devices, etc. keywords: molybdenum disulfide (mos2), parametric analysis, interphase shear stress, nanocomposite, thermo-mechanical loading. 1. introduction in recent years, there has been an increased interest in nanocomposite materials, which is indicated by the high publication activity in the global databases web of science and scopus. a large part of the research is focused on the development of new polymer composites and the study of synthesis methods. one of the widespread 2d materials is molybdenum disulfide (mos2) due to its mechanical properties and numerous applications [1]. significantly lower is the share of research regarding analytical methods for stress/strain modeling in nanocomposite structures. considering the increasingly wide application of nanocomposites in industry, it is of particular importance to investigate the specific loads and design of nanocomposite structures. one of the most important characteristics for stress transfer efficiency in nanocomposites, subjected to mechanical or thermo-mechanical loading, is the value of interphase shear stress τ (or strength), which arises at the interphase between nanomaterial and polymer substrate. du et al. 2022 [2] determine the strain field of mos2 and the tangential intermediate shear stress (iss) distribution during the substrate stretching process. the research presents a method for determining the surface properties of graphene/mos2 heterostructures in which the stress field of the upper graphene and the lower mos2 structure is determined. typically, the iss value is obtained based on data from strain measurements using raman and photoluminescence (pl) spectroscopy or with other methods, and then using the relation between iss and strains (from continuum mechanics or shear-lag model). dong et al. [3] use (pl) spectroscopy to study interfacial voltage transfer from a polymer substrate to monoand multilayer molybdenum disulfide (mos2) under stress. the study determined the shear strength limit for various combinations of 2d nanomaterials and polymer substrates by performing stress mapping at various stress levels. it is found that the stress transfer length increases with increasing number of layers and there is no significant change for mono and bilayer mos2. it is essential to investigate the mechanical and physical properties of heterostructures. a study of the mechanical properties of multilayer mos2 and a graphene/mos2 heterostructure were made by [4] by molecular dynamics simulation under uniaxial ten94 https://doi.org/10.14311/app.2024.50.0094 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 50/2024 parametric analysis for interphase shear stress . . . sile stress and normal compression. the study shows that the stiffness of the heterostructure is significantly greater than mos2, which is due to the higher elastic constants of graphene. to bridge the gap between theory and experimental data, the focus of the present work is on the influence of 4 geometrical and 2 loading parameters on interfacial shear stress in nanocomposite structure mos2/interphase/polymer nanocomposite, subjected to thermo-mechanical loading. in this study, a parametric analysis (pa) is performed in order to determine the influence of geometry (thicknesses of nanocomposite layers and length of the structure) and loading (mechanical and thermo-mechanical) on the delamination in the nanocomposite structure mos2/pet. according to our previous works [5, 6], two analytical model solutions for iss are used in pa for the considered nanostructure, as well as the model criteria for delamination. the critical values of the geometric parameters and loading are obtained after which delamination in the structure appears. the proposed method of pa as well as the model solutions and criteria, could be successfully applied to the similar three-layer nanocomposites if the model assumptions are fulfilled (see for details [6]). 2. materials and methods the study used 2d analytical modeling of stress fields in 3-layer nanocomposites subjected to thermomechanical loading (figure 1); the model is validated in our previous works [5–7] for similar structures. the applied method makes it possible to solve an analytical ordinary differential inhomogeneous equation of the 4th order with constant coefficients, with respect to the unknown axial stress function σ1 in the first layer (nanolayer) [6, 7]. all other stresses in the layers, including the interphase shear stress are expressed by σ1 and its derivatives. here, only the most important formulas including two types of derived analytical model solutions for the axial stress σ1 in the nanolayer (eq. (1) and (2)) are presented, with coefficients depending on the geometry of the three-layer nanocomposite, its material properties and external load: σ1(x) = c1 · exp(λ1 · x) + c2 · exp(λ2 · x) + c3 · exp(λ3 · x) + c4 · exp(λ4 · x) −a (1) σ1(x) = exp(−αx) [m1 cos(βx) +m2 sin(βx)] + exp(αx) [m3 cos(βx) +m4 sin(βx)] −a (2) in eq. (1) and (2) the constant a is the solution for non-homogeneous ode and depends on external static load and temperature difference, applied to whole structure and ci and mi are the integration constants in the model solutions, determined from the respective boundary conditions [5]. the value of iss figure 1. scheme of the three layer nanocomposite structure, where: h1 – nanomaterial layer; ha – interphase layer; h2 – substrate layer, p – static tension force [n.m] and σ0 = p/h2. is calculated by σ(a) xy (x) = h1σ ′ 1(x), where in the right hand side eq. (1) or (2) are used, respectively. in this work, both solutions are considered in pa; eq. (2) corresponds to the case of 4 complex roots ±(α ± iβ), while eq. (1) corresponds to the case of 4 real roots λi [7]. for convenience, the following notations will be used in the text below for the type of solution used in the calculation of iss in the pa: case 1 for real roots and case 2 for complex roots. it is worth to note [5], that the type of roots (and solutions for iss, respectively) depends on the chosen geometry of the nanocomposite structure (layers’ thicknesses and its length). for the considered nanostructure mos2/pet the middle layer (figure 1) is an interphase layer between mos2 and pet, with thickness ha. the interphase (adhesive) layer is modelled according to the zhu and narh approach [8]. material properties, geometry, loading and coefficients of thermal expansion (cte) for all layers are given in table 1. the model criterion for the interphase delamination in the considered structure is defined, where uss is the ultimate shear stress of interphase layer: σ(a) xy (x) = h1σ ′ 1 ≥ σ (a) uss (3) graphically, the delamination starts from both ends of the structure and represents the intersection of the iss model curve with the straight horizontal line corresponding to the uss [3]. 3. results of parametric analysis of molybdenum disulfide/interphase/pet nanocomposite the pa of the factors, influencing interphase delamination, was performed and investigated for case 1 and case 2 of combined loaded mos2/interphase/pet nanocomposite (table 1). the factors taken into account in pa are: influence of mechanical load σ0 on iss, layers’ thicknesses h1, ha, h2, layers’ length l and influence of temperature load ∆t . the uss value is 0.26mpa, taken from [3]. all material properties and fixed values for geometry, loading and other data during the pa for case 1 and case 2 of considered nanocomposite are presented in table 1. the values of coefficients of thermal expansion for mos2 and pet 95 r. vladova, t. petrova, e. kirilova et al. acta polytechnica ctu proceedings case studies young’s modulus poisson ratio h1 ha h2 l σ0 [gpa] [–] [nm] [m] [m] [µm] [mpa] for iss mos2 interpet mos2 interpet mos2 interpet all calculation phase phase phase layers case 1 270 11.5 2.3 0.25 0.34 0.43 0.65 3e-08 9e-07 72 15(real roots) case 2 270 3 2.3 0.25 0.34 0.43 0.65 2e-09 5e-05 72 25(complex roots) table 1. material properties, geometry and mechanical load for mos2/interphase/pet nanocomposite. figure 2. influence of the magnitude σ0 of mechanical load on the iss, for considered mos2/interface/pet, for complex roots. are 5e-06 [1/k] and 5.96e-06 [1/k], respectively. in case of thermo-mechanical loading of nanocomposite ∆t varies between 0 (pure mechanical loading) and 500k [7]. all calculations and corresponding parametric analysis were done in mathcad environment. 3.1. pa for case 2 the influence of mechanical load σ0 on the iss, case 2 is depicted on the figure 2. the figure shows that over 25mpa external mechanical load at a fixed geometry for case 2 (table 1) of the considered mos2/interphase/pet nanocomposite structure, delamination is observed. regarding the influence of h1 on iss, case 2, it is depicted on the figure 3. it is seen, that increasing the thickness above that of the mos2 monolayer (0.65 nm) the delamination arises, i.g, the monolayer thickness is a critical value. these results are in agreement with results of dong et al. [3] for mos2/pmma, that “monolayer mos2 is more effective in terms of interfacial stress transfer compared to multilayer one”. figure 4 portrays the influence of h2 on iss, case 2. if pet thickness h2 > 50 µm, the delamination is not appearing. influence of ha is negligible, iss does not depend on ha and it is not presented here. the impact of the length l on the iss, case 2 is seen up to 90 µm; after l > 90 µm – at fixed layers’ thicknesses and load (table 1), the delamination not arises. the maximal figure 3. influence of the thickness h1 on the iss, for mos2/interface/pet for complex roots. figure 4. influence of the thickness h2 on the iss, for mos2/interface/pet for complex roots. value of iss is reached at about l = 15 µm as is clearly seen in figure 5. the effect of thermal loading is depicting on the figure 6. if a sufficient cooling temperature difference (∆t = −100k or more) is applied to the mechanical loading where we have already established delamination, as a result the deboning is no longer observed in the considered structure. this is probably due to the differences in ctes of mos2 and pet and resulting additional compressive strains occurring after an applied thermal load; all this ultimately reduces the iss value below the critical one. in contrast, the opposite situation has place (the positive temperature 96 vol. 50/2024 parametric analysis for interphase shear stress . . . figure 5. influence of length on the iss, for mos2/interface/pet for complex roots. figure 6. influence of temperature load on the iss, for mos2/interface/pet for complex roots. difference increased already observed delamination) in the case of heating, applied to the mechanically loaded nanostructure. 3.2. pa for case 1 before presenting the results from pa for iss case 1, it should be noted, that during the calculations we found that solution for iss case 1 in the considered nanostructure exists at very thin intervals for parameters ha and h2. the used model [5–7] allows more than one solution if the geometry of the structure is appropriately chosen. physically, the case 1 corresponds to thinner nanocomposite, and case 2 to thicker one. the influence of mechanical load σ0 on the iss case 1, is shown in figure 7. at a fixed geometry of the considered mos2/interphase/pet nanocomposite structure, delamination is not observed for applied loads. the influence of h1 on iss case 1, is depicted on the figure 8. the figure shows, that keeping mos2 thickness h1 < 2.66 nm, at fixed other parameters, the delamination is not appearing; over this value it appeared. for comparison, on figure 3 the critical figure 7. influence of the magnitude σ0 of mechanical load on the iss, for considered mos2/interface/pet at real roots. figure 8. influence of the thickness h1 on the iss, for mos2/interface/pet at real roots. value for h1 case 2, after which delamination arises, is 0.65 nm, here, in figure 8 for case 1 it is 2.66 nm, which confirm the fact that thicker layers are more stable against delamination. the influence of ha on iss is negligible and is not represented here. figure 9 portrays the influence of h2 on iss case 1. keeping pet thickness h2 in the range of 0.81 µm ÷ 1.2µm, the delamination is not appearing. it should be noted, that existence of positive roots and, the existence of model solutions for axial stress and iss, respectively, are inextricably bound up with very thin interval for h2. figure 10 presents the influence of length l on the iss: after l >38.25 µm at fixed other layers’ thicknesses, the delamination does not occur. the effect of thermal loading is determined: for higher lengths of structure layers, at 72 µm and more, the influence of ∆t on iss is negligible, as seen in 97 r. vladova, t. petrova, e. kirilova et al. acta polytechnica ctu proceedings figure 9. influence of the thickness h2 on the iss, for mos2/interface/pet for real roots. figure 10. influence of length on the iss, for mos2/interface/pet for real roots. figure 11a. if a sufficient cooling temperature difference (∆t = 500k) is added to the mechanical loading, at fixed length 38.25 µm (i.e. where we have already established onset of delamination), debonding is no longer observed in the considered structure (figure 11b). 4. conclusions in this study, the influence of the mechanical and thermal load, the thicknesses h1, h2, ha, as well as the influence of the structure length on the theoretically predicted values of iss in thermo-mechanically loaded 3layer nanocomposite structure mos2/interphase/pet is shown, by parametric analysis. the model values of iss are used, based on the analytical solutions for iss at two cases with different nanocomposite structure geometry (case 1 and case 2) [6, 7]. as a criterion for influence of these 6 parameters on iss, the model non-linear condition for delamination in the structure is used, i.e., it requires that model iss is greater than uss in the middle interphase layer. received results allow determining the significance of the factors affecting the delamination in the investigated molyb(a). at fixed length 72 µm. (b). at 38.25µm. figure 11. influence of temperature load on the iss, for mos2/interface/pet for real roots. denum disulfide/polymer nanocomposite structure at two distinct cases of its geometry. it is found that the interphase thickness does not affect the iss value for both cases. the magnitude of the applied mechanical load has a strong influence on the magnitude of the iss for both cases. the temperature difference also affects the iss value for case 2, but to a lesser extent and has an additive effect for heating when the delamination already exists as a result of applied mechanical load. in the case of cooling, the effect could be opposite. the thicknesses of mos2 and pet mostly influence model iss for both cases 1 and 2, especially the substrate thickness. the effect of length on iss is more pronounced for both cases for iss at short lengths of the nanostructure. the obtained results are graphically illustrated and can be used for fast prediction of iss in micro scale in similar nanocomposite devices or parts thereof such as sensors, nanoand optical electronic devices, energy devices, etc. acknowledgements the authors gratefully acknowledge the bulgarian national science fund for its financial support via the contract for project kπ-06-h57/3/15.11.2021. 98 vol. 50/2024 parametric analysis for interphase shear stress . . . references [1] s. imani yengejeh, j. liu, s. a. kazemi, et al. effect of structural phases on mechanical properties of molybdenum disulfide. acs omega 5(11):5994–6002, 2020. https://doi.org/10.1021/acsomega.9b04360 [2] h. du, y. kang, c. xu, et al. measurement and characterization of interfacial mechanical properties of graphene/mos2 heterostructure by raman and photoluminescence (pl) spectroscopy. optics and lasers in engineering 149:106825, 2022. https://doi.org/10.1016/j.optlaseng.2021.106825 [3] m. dong, r. j. young, d. j. dunstan, d. g. papageorgiou. interfacial stress transfer in monolayer and few-layer mos2 nanosheets in model nanocomposites. composites science and technology 233:109892, 2023. https: //doi.org/10.1016/j.compscitech.2022.109892 [4] n. ghobadi. a comparative study of the mechanical properties of multilayer mos2 and graphene/mos2 heterostructure: effects of temperature, number of layers and stacking order. current applied physics 17(11):1483–1493, 2017. https://doi.org/10.1016/j.cap.2017.08.018 [5] t. petrova. analytical modeling of stresses and strains in layered nanocomposite structures – opportunities and challenges. bulgarian chemical communications 55(3):349–366, 2023. https://doi.org/10.34049/bcc.55.3.simns05 [6] e. kirilova, t. petrova, w. becker, j. ivanova. mathematical modelling of stresses in graphene polymer nanocomposites under static extension load. in 2019 ieee 14th nanotechnology materials and devices conference (nmdc), pp. 1–4. 2019. https://doi.org/10.1109/nmdc47361.2019.9084003 [7] r. vladova, t. petrova, e. kirilova, et al. comparison of the model axial graphene strain distributions in graphene/epoxy/polymethyl methacrylate (pmma) nanocomposite under mechanical and thermomechanical loading. bulgarian chemical communications 54(4):349– 354, 2022. https://doi.org/10.34049/bcc.54.4.5539 [8] l. zhu, k. narh. numerical simulation of the tensile modulus of nanoclay-filled polymer composites. journal of polymer science part b: polymer physics 42(12):2391– 2406, 2004. https://doi.org/10.1002/polb.20112 99 https://doi.org/10.1021/acsomega.9b04360 https://doi.org/10.1016/j.optlaseng.2021.106825 https://doi.org/10.1016/j.compscitech.2022.109892 https://doi.org/10.1016/j.compscitech.2022.109892 https://doi.org/10.1016/j.cap.2017.08.018 https://doi.org/10.34049/bcc.55.3.simns05 https://doi.org/10.1109/nmdc47361.2019.9084003 https://doi.org/10.34049/bcc.54.4.5539 https://doi.org/10.1002/polb.20112 acta polytechnica ctu proceedings 50:94–99, 2024 1 introduction 2 materials and methods 3 results of parametric analysis of molybdenum disulfide/interphase/pet nanocomposite 3.1 pa for case 2 3.2 pa for case 1 4 conclusions acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2023.41.0079 acta polytechnica ctu proceedings 41:79–82, 2023 © 2023 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague development of advanced driving simulator for driving schools libor topol, michal malý∗ czech technical university in prague, faculty of transportation sciences, department of vehicle technology, konviktská 20, 110 00 prague 1, czech republic ∗ corresponding author: malymic7@fd.cvut.cz abstract. this paper describes development of an innovative advanced driving simulator (ads), that will be implemented into the driver education process as a method to improve education of students and thus their readiness to enter everyday traffic. ads will allow the driving school students to get more practice in general vehicle control, unusual situation solving and use of modern assistant systems, as well as objectively evaluate student’s skills by the instructor. as a conclusion, expected outcomes of innovative simulator design and results validation possibilities are discussed. keywords: simulator, driver, education, safety, evaluation. 1. introduction quality of driver education is a key factor to road safety. this research aims to improve driving school education and thus novice drivers’ readiness to enter everyday traffic. this goal is to be achieved by an advanced driving simulator (ads) development, that will be used in the education process. reduction of beginner’s mistakes that might cause accident is expected. 1.1. accident rate of young and beginner drivers the accident rate of novice drivers is disproportionately higher than for other groups of drivers. as for the accident statistics of young and novice drivers, it can be stated that young drivers under the age of 24 cause around 10,000 accidents each year in the czech republic. despite reduced fatal accidents, young drivers remain the most at-risk group. they cause 9% of accidents and 15% of deaths. according to available data, 84 people were killed due to young drivers under the age of 24, and another 272 were seriously injured in 2020. the highest proportion of persecuted drivers is registered with drivers aged 24 [1]. other statistics show that as a result of accidents caused by young drivers, 35 drivers of passenger cars (44%), 33 passengers in passenger cars (41%), 6 motorcyclists (7%), 3 lorry drivers (4%) and 3 cyclists (4%) died [2]. beginner drivers acquire the skills needed to drive a motor vehicle relatively quickly, but it takes significantly longer to acquire all the perceptual and cognitive skills necessary for safe interaction in the traffic environment. with increasing age and gained experience, there is a decrease in risky and immature behaviour of drivers [3]. 1.2. accident rate of young and beginner drivers the accident rate of novice drivers is disproportionately higher in the long term than for other groups of drivers. the key factors causing increased accident rate among novice drivers are inexperience, underdeveloped personality, and the related willingness to take risks and underestimation of risks. driver training is a factor that clearly affects novice driver’s behaviour in real traffic, both positively and negatively [4]. modern advanced vehicle simulators can help to increase quality of training by enabling training in safe conditions not only for routine tasks, but also for risky situations with a high probability of an accident in case of incorrect or slow reaction. it allows also to fully practice all types of traffic environment (highway, suburbs, city) and driving in different weather conditions (including slippery roads). another significant advantage is the possibility of individualizing the training (a driving school student can focus more on situations in which he struggles), and the possibility of practicing/demonstrating dangerous situations that cannot be simulated during training in a real vehicle, such as reacting to a dangerously overtaking vehicle in the opposite direction [5]. during the training at the driving school, the student will therefore be able to try out various climatic conditions and driving manoeuvres that he would not encounter during the standard training interval and thus be better prepared for real environment [6, 7]. moreover, inappropriate behaviour at railway crossings and dangerous overtaking of cyclists, which also appear in increased accident rates statistics, can be reflected in training scenarios. also, drivers often don’t come across modern driving assistance systems during their training at a driving school and only test them in real traffic, which can be very dangerous both for them and for those around them. advanced driving simulator will provide possibility to learn how to control these systems safely [8]. 79 https://doi.org/10.14311/app.2023.41.0079 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en libor topol, michal malý acta polytechnica ctu proceedings 2. method the car simulator is designed to imitate the driving of a car in a virtual environment as similar to real road conditions as possible. construction of the “light interactive vehicle simulator” (dashboard with pedals, steering wheel, gauges/driver display and visualisation screens) consists of real car parts; the design meets requirements for both hardware robustness and plausibility of scenarios implemented in software [9]. the cabin of the simulator consists of the parts of lower middle-class passenger car lada granta with manual transmission (five forward gears, one reverse), which are firmly attached to steel frame. it comprises following cockpit parts: cockpit dashboard (shortened on the passenger side), central console with controllers, central tunnel including gear lever and handbrake, adjustable driver’s seat with seatbelt, the floor of the vehicle under the driver’s seat, left front door interior trim, three-pedal console, steering wheel with a controlled feedback, functional light controllers (parking, high beam, low beam, fog lights), turn indicators, etc. the functional dashboard instrument panel with speedometer, rev counter, indicator lights and warning notifications is an important part of a cockpit as well. the overall image, sound and background computing processes of the simulation is provided by computational equipment of the car simulator. virtual reality is rendered on powerful graphic workstation hardware with state of art graphic card (gpu nvidia quadro m 4000) ensuring the smooth visualisation. three 31,5” full hd lcd monitors offer panoramic frontal and partially peripheral view from the simulated car. all image information generated by the virtual reality system is shown by this projection system, the horizontal range covers 120° of driver’s view. rear view mirrors are represented by displays with relevant graphics and are placed at the appropriate location of the projection system. the audio system is directly connected to the virtual reality computer and generates sounds that are normally audible by the driver while driving. the sound is provided by a set of six speakers situated around the driver. software equipment of the car simulator reproduces the sensations normally perceived by the driver in a vehicle. if the car simulator is not equipped with a movement platform, movement sensations are excluded. the software of the simulation system consists of mathematical-physical model, virtual environment visualization module and a surround sound generator. the mathematical-physical model periodically reacts to the inputs from the control elements and calculates the values of the forces and moments, from which it determines the upcoming state of the vehicle such as position, speed, etc. taking into account the influence of the surrounding environment. car simulator software behaviour corresponds to four-wheel vehicle with independent axles, front-wheel drive and manual transmission. the virtual reality simulation module is made of few essential components – an image generator that creates an image of the scene based on inputs consisting of vehicle position data, data from the terrain database (virtual environment in which realistically-looking objects surround the road track) projected by display system. to make a driving experience even more realistic, the virtual environment visualisation system is combined with the surround sound generator that gives driver additional information. the vehicle simulator exports operational data into a data file, which contains a complex information about the drive, i.e., position of the car in time, the steering wheel position/turning velocity, pressure levels on individual pedals, position of vehicle control elements and distance from the collision elements. data from individual rides are stored in a matrix, number of columns corresponds with the number of monitored values and number of rows is determined by the distance of measured ride. the visualisation engine unity plays the most important role in the visual quality of the scene. it is a software in which virtual environment is modelled, and it ensures image generation on the screen. unity is a multi-platform engine for creating 2d and 3d scenarios. the subject of the research is to fully adapt this program environment for simulator training on the simulator, including full synchronization with the controllers and communicators built into the simulator, the general issue in sw development is to find a balance between realistic appearance and the complexity of the created scene. the driving simulation in the virtual environment runs in real time and because of that, it is necessary to reach the minimum frame refresh rate of at least 45 frames per second (in ideal case 120) which requires substantial computing power of the vehicle simulator hardware. the visual quality of rendered image could reach the level of current computer games, which is considered as high-end of the real time simulations. that allows for better immersion into simulated driving experience. the growth of computing power also allowed quality enhancement of the parameters evaluated during the ride. the vehicle simulator allows real-time evaluation of: • the offset distance from the centre of the lane, • keeping safe distance from forthcoming vehicle (see figure 1), • respecting maximum allowed speed both in and outside of the city, • stopping position in front of intersection. 3. results the target of this applied research was to develop advanced and innovative driving simulator for education purposes. the goal was achieved by using innovative and advanced technologies. 80 vol. 41/2023 development of advanced driving simulator figure 1. safe distance monitoring. 3.1. innovation 1 implementation of acoustic assistance system to the vehicle simulator. this system monitors all driving parameters and movements in a virtual environment in real time and is able to react to the driver errors or shortcomings during driving. these systems can be used at two levels. at the teaching level, the assisted sound system alerts drivers to upcoming situations and guides them safely in a virtual environment. this category consists of a acoustic overspeeding warning, an explanation of the intersections driving rules or a need for gear change. in the phase of controlled drive training, the driver is no longer guided in a virtual environment, but is alerted about misconducts and dangerous behaviour such as speeding, failing to keep a safe distance or inability to keep the lane. this system can be supplemented with additional warnings depending on the technological equipment of the vehicle simulator [10]. 3.2. innovation 2 eye tracking system is considered as another innovative approach. the system allows to monitor and record the driver’s gaze direction using the video analysis of the eye pupils and head movement while driving (see figure 2). there are various technical solutions available for eye movement tracking, in this particular simulator a distance unit will be used, which offer contactless and thus less distracting measurement. the system uses a series of cameras in front of the driver and specialised software, which is able to monitor the driver and evaluate the direction of view. such systems can even be built into the dashboard and are basically do not affecting the drive. eye tracking data allows to analyse if the driver focused his vision on traffic sign, whether he does not spend excessive time monitoring instrument displays or controls, or the frequency of watching rear-view mirrors. 3.3. innovation 3 one of the latest advanced systems to complement current simulators is the intelligent driver training management system. it is a system that evaluates the results of the drive quality parameters in the vehicle simulator and, based on these results, individualises the training scenarios selection specifically to train the situations with weakest performance that would need to be improved. 4. discussion all the innovative systems mentioned in section 3 can improve the driver training process and may help to safely prepare future drivers for the unexpected situations they may encounter in real conditions. synergic effect of all three innovations is expected when implemented simultaneously. 5. conclusion driver education is a key factor in improvement of the road safety. innovative advanced vehicle simulator can objectively increase the quality of driver training if the teaching process is accordingly adapted. these 81 libor topol, michal malý acta polytechnica ctu proceedings figure 2. eye tracking system. devices have ambition to impact everyday life and indirectly reduce the amount and consequences of vehicle crashes. further detailed testing of the prototype will be conducted. the functional prototype concept is designed to allow further extension and evolution. references [1] t. neřold. meziroční srovnání nehodovosti mladých řidičů do 24 let, 2020. [2022-10-18], https://www.cdv.cz/tisk/mezirocni-srovnaninehodovosti-mladych-ridicu-do-24-let/. [2] dopravní nehody v čr. [2022-10-16], https://nehody.cdv.cz/. [3] m. kulišťák. mladí řidiči. dílčí cíl národní strategie bezpečnosti silničního provozu 2011–2020, 2020. [4] e. chan, a. k. pradhan, a. pollatsek, et al. are driving simulators effective tools for evaluating novice drivers’ hazard anticipation, speed management, and attention maintenance skills? transportation research part f: traffic psychology and behaviour 13(5):343– 353, 2010. https://doi.org/10.1016/j.trf.2010.04.001 [5] j. hoskovec, j. štikar, j. pour. psychologie a technika výcviku řidičů. nadas, 1966. [6] a. malach, v. císař. základy trenažérového výcviku. státní pedagogické nakladatelství, 1984. [7] úz č. 1283, 2018. zákon č. 247/2000 sb. o získávání a zdokonalování odborné způsobilosti k řízení motorových vozidel, 2018. [8] telemax s.r.o. řidičský trenažer osobního automobilu at-217. [2022-10-16], https://www.telmax.eu/cs/product.aspx?r=3zkyvy. [9] p. bouchner, s. novotny. development of advanced driving simulator: steering wheel and brake pedal feedback. in proceedings of the 2nd international conference on circuits, systems, control, signals (cscs’11). 2011. [10] p. bouchner, s. novotný, r. piekník, o. sýkora. assessments of grade crossing warning and signalization devices – driving simulator study. in proceedings of the 2nd international conference on circuits, systems, control, signals, pp. 179–183. 2011. 82 https://www.cdv.cz/tisk/mezirocni-srovnani-nehodovosti-mladych-ridicu-do-24-let/ https://www.cdv.cz/tisk/mezirocni-srovnani-nehodovosti-mladych-ridicu-do-24-let/ https://nehody.cdv.cz/ https://doi.org/10.1016/j.trf.2010.04.001 https://www.telmax.eu/cs/product.aspx?r=3zkyvy acta polytechnica ctu proceedings 41:79–82, 2023 1 introduction 1.1 accident rate of young and beginner drivers 1.2 accident rate of young and beginner drivers 2 method 3 results 3.1 innovation 1 3.2 innovation 2 3.3 innovation 3 4 discussion 5 conclusion references https://doi.org/10.14311/app.2022.33.0539 acta polytechnica ctu proceedings 33:539–545, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague quantitative analysis of the strength generating c-s-h-phase in concrete by ir-spectroscopy frank michael schmidt-döhla,∗, david schulenberga, franziska tralowa, jürgen neubauerb, julian johannes wolfb, dominique ectorsb a hamburg university of technology, tuhh, institute of materials, physics and chemistry of buildings, 21071 hamburg, germany b friedrich-alexander university erlangen-nürnberg, fau, geozentrum nordbayern, schlossgarten 5a, 91054 erlangen, germany ∗ corresponding author: schmidt-doehl@tuhh.de abstract. the c-s-h-phase is the most important strength generating phase in concrete and other cementitious materials. the analysis of c-s-h is therefore an important instrument of innovations in the field of concrete and its durability and sustainability. the quantification and insights in c-s-h are hindered by the predominantly amorphous structure of c-s-h. only the time consuming and expensive solid state nuclear magnetic resonance spectroscopy (nmr) gives a chance to get results, but is restricted to model substances. a new technique, based on the cheap and widespread infrared-spectroscopy (ir) was developed. the quantitative analysis is based on the silicon content in the different structural units of c-s-h and a calibration with natural and synthetic materials with known silicon content in these units. the technique allows to investigate specimens from real structures. specimens with quartz or limestone aggregate can be analyzed directly. in other cases the aggregates must be separated, for example with heavy liquid separation. tests with different mortars showed a good correspondence of measured and expected values of c-s-h concentration. keywords: calcium-silicate-hydrate (c-s-h), quantitative analysis, spectroscopy. 1. introduction undoubtedly the c-s-h-phase is one of the most important chemical substances in the world. it is generated by the hydration of c3s and c2s, puzzolanic reactions and reactions of latent hydraulic components. it is the strength building phase in most cementitious material. if researchers want to improve mineral building materials, and as a consequence improve sustainability of such materials, it is therefore of great importance to quantitatively analyze c-s-h and its variable, actual structure. also in the case of corrosion and durability investigations, where the cs-h is attacked and the concentration is decreased, it will be very helpful to quantitatively analyze c-s-h. still by the extensive amorphous character of c-s-h in x-ray diffraction experiments and other possible amorphous components in the material it is not possible to quantitatively analyze c-s-h at the moment in a general manner. only model substances can be analyzed quantitatively, for example hydrated alite or hydrated white cement with 29si magic angle spinning solid state nuclear magnetic resonance, which is a very expensive and time consuming technique. also special systems can be analyzed with quantitative x-ray diffraction by calculating the c-s-h from the changes of the crystalline species. up to now it is not possible to take a specimen from a real structure and make a quantitative analysis (qxrd). this was the motivation for the development of the method. 2. basic information the method is based on attenuated total reflection (atr) fourier transform (ft) infrared spectroscopy (ir), with a single reflection diamond atr. the generated evanescent wave at the interface is attenuated by the absorption in the specimen, based on the excitation of vibrations in functional groups with a change of the dipole moment. nearly no scattering effects are generated by this technique and the penetration of the evanescent wave into the specimen is less than 5 microns. a quantitative analysis with the atr-technique is based on lambert-beers law, like in transmission. that means the absorbance is directly proportional to the concentration, except very high concentrations [1]. to our experience it is necessary to dilute the calibration substances at least to 50 % to make sure that the calibration curve is in the linear range. the basic structure of c-s-h consists of short chains of sio4-tetrahedrons. the so called q1 structural units are the ends of the chains, the q2 structural units are in the middle. the chain length is connected with the ratio q1/q2. infrared spectroscopy produces direct signals from c-s-h, generated by the 539 https://doi.org/10.14311/app.2022.33.0539 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en f. m. schmidt-döhl, d. schulenberg, f. tralow et al. acta polytechnica ctu proceedings q1 and q2 structural units. they can be used to develop a method for the quantitative analysis of c-s-h. the bands are weak and broad and there are overlapping signals from other phases. and, of course, a quantitative analysis needs a calibration. these difficulties must be overcome. for the development of the method different portland cements, blastfurnace slags, flyashes, silica fume, lime stone powders, quartz powder, hydrated lime, mixtures of these components, and hydrated mortars made of these components were used. the components were characterized by polarized light microscopy, x-ray diffraction, x-ray fluorescence and infrared spectroscopy. the measurements were performed with a perkin elmer spectrum two with universal attenuated total reflectance accessory and the perkin elmer software spectrum. peak fitting was performed with the program peakfit of systat software, erkrath, germany. 3. method for the quantitative analysis of c-s-h in cementitious materials by atr-ir-spectroscopy a detailed inspection instruction is available from the corresponding author (in german). 1. at first a qualitative phase analysis of the specimen is needed. due to the overlapping of peaks, it is important to know the phase assemblage in the sample. 2. the specimen is crushed to a grain size of some millimeters and dried. the drying must be restricted to a minimum of 11 % r.h., because a stronger drying can affect the c-s-h phase. then the specimen has to be ground to a powder. with an atr crystal area of four square millimeters it is necessary to grind to a grain size of about thirty to forty micrometers to get sufficient grain statistics of several thousand grains. dry grinding is possible, but it is necessary to avoid temperatures above 70 ◦c. the powder must kept dry and without carbonation. 3. other minerals than quartz and calcite in the aggregates must be separated from the specimen, because of possible peak overlapping. this separation starts with a careful crushing in step 2, where big aggregates can be separated by hand. the further separation of aggregates can be done with heavy liquid separation with diiodomethane/acetone mixture with a density of 2.4 g/cm3. [2] the separation must be done quantitatively. this technique does not affect the infrared spectra of c-s-h. the very common minerals quartz and calcite need not to be separated. therefore it is possible to produce laboratory concrete specimens, which can be analyzed without a heavy liquid separation. the aggregates from concrete specimens from buildings must be separated normally, because of the many different aggregate types inside. 4. the method needs an internal standard to correct differences between the density of the specimens on the atr-crystal and changes in the intensity of the light source of the spectrometer. the scattering of the c-s-h signals is decreased by working with a relative absorbance. we found potassium ferricyanide, k3[fe(cn)6] to be sufficient as an internal standard for this method. it must be grinded wet in water free ethanol to a grain size of about 20 micrometers without remarkable increase of temperature. any contact with acid, chromium trioxide, oxidizing agents, fluorine, ammonia, nitrates and nitrites must be avoided. because of its solubility in water it is usable only with dry specimens. the standard shows two peaks at 2117 and 2120 cm−1 and a weak band at 500 cm−1. it must be kept dry and guarded from light. 5. the specimen and the internal standard are mixed in a 60 to 40 relation by weight. this mixing must be done wet in a little amount of ethanol, to get a homogenous mixture. the precision of weighting is critical. ethanol must completely evaporate before the measurement. 6. the specimen, mixed with the internal standard is measured with a resolution of 2 cm−1. the atr crystal must be completely covered with powder and the pressure on the powder must be controlled so that the spectrum is not changed with increasing pressure. at least 5 measurements of the material are recommended. 7. the spectrum is converted into absorbance, and an atr correction is performed, assuming ideal contact of powder to the crystal. the atr correction is a mathematical transformation, converting the spectrum into a spectrum comparable with a transmission spectrum. 8. now it is necessary to eliminate interfering signals from other phases, known by step 1. interfering signals can be separated within the necessary peak fit (see the following step). but in the case of components with very broad ir-signals, for example flyash or blastfurnace slag, it is helpful to subtract a typical spectrum of such a component, measured with the same conditions. often it is not possible or recommended to subtract all of the intensities of the interfering substances. the residual intensities will be eliminated by the peak fit, remembering the peaks of the interfering substances. 9. in the next step a peak fit in the area of the c-s-h peaks and of the internal standard is performed. during this peak fit, also the remaining interfering signals from other phases and the background must be considered. we prefer a linear background between 1850 and 2250 cm−1 for the internal standard, and a background with logarithm function 540 vol. 33/2022 quantitative analysis of c-s-h figure 1. calibration curves for the q1 (left) and q2 (right) structural units in c-s-h. in the range of the c-s-h bands, leaned on the minimum points in this range. it is absolutely necessary to restrict the peaks to that, existing in the different phases of the specimen. it is necessary to make this fit with experience on the technique, knowledge of the phases in the specimen and their typical peaks. the absorbance is measured as peak area (voigt-function). 10.then, with the absorbance aq of the q1 and q2 signals of c-s-h and the absorbance of the internal standard ai , a relative absorbance arel = aq/ai of q1 and q2 is calculated. 11.with the help of calibration functions the relative absorbance is changed into a concentration of silicon in the q1 and q2 structural elements of c-s-h in the specimen. 4. calibration with crystalline substances unfortunately there is no c-s-h standard existing, were the concentration of si in the q1 and q2 structural elements is known. therefore we tried to use minerals containing q1 or q2 structural elements as calibration substances to develop the calibration curves for c-s-h. for the calibration of the silicon in the q2 structural units we used the natural minerals cordierite, wollastonite and beryl. for the calibration of the silicon in the q1 structural units the synthesized minerals åkermanite and gehlenite were used. all calibration standards were characterized by x-ray fluorescence, x-ray powder diffraction and ir-spectroscopy. the calibration substances were diluted with fly ash, blastfurnace slag, lime powder, unhydrated portland cement, hydrated lime and silica fume to get specimens with different content of q1 resp. q2 groups. these specimens were measured and the data processed as described in chapter 3. because of the lack of space no more details of this calibration can be presented here. figure 1 shows the calibration curves for silicon in the q1 and the q2 structural units in c-s-h with the data of all calibration substances, diluted by different substances common in cementitious materials, named at the beginning of this chapter, and analyzed by three different people. these calibration curves refer to dry specimens. it can be seen in these diagrams, that although very different calibration substances were used, with different dilution components, and different people performing these measurements, they all fit to the same calibration curves, within an expectable scatter. 5. calibration using in addition fully hydrated activated alite plausibility tests were performed by analyzing fully hydrated activated alite, different hydrated binders, and mortars (with binder enrichment) with wellknown composition. using the calibration described in chapter 4 the tests systematically result in too high total silicon concentrations in the specimens. so the calibration and the results were checked very carefully again. in addition the measured chain lengths are in the range of data reported in the literature, based on 29si mas nmr measurements. this indicates that the measured q1/q2-ratio is correct, assuming that the nmr-data are right. so it seems that the calibration of the q1/q2-ratio by measuring crystalline minerals is possible. but the total si-content must be calibrated with c-s-h specimens with a known sicontent. we used therefore fully hydrated activated alite, described in chapter 6 for the calibration of the total si content in c-s-h. with this technique we got the following calibration functions. csi in q1 = arel, q1 ! (0.163 + 0.122) (1) csi in q2 = arel, q2 ! (1.017 + 0.763) (2) • csi concentration of si in the q1 or q2 structural units of c-s-h respectively • arel relative absorbance of the q1 or q2 peaks respectively, described in chapter 3 541 f. m. schmidt-döhl, d. schulenberg, f. tralow et al. acta polytechnica ctu proceedings age 24 hours 7 days 6 months wt.-% si in q2 in specimen 3.4 ± 0.2 4.0 ± 0.2 3.9 ± 0.2 wt.-% si in q1 in specimen 2.0 ± 0.1 2.1 ± 0.2 1.9 ± 0.2 sum of two lines above 5.4 ± 0.3 6.1 ± 0.3 5.8 ± 0.4 mean chain length 5.4 ± 0.2 5.8 ± 0.4 6.1 ± 0.3 table 1. plausibility tests with fully hydrated activated alite. age 24 hours 7 days 6 months measured weight percent c-s-h (as c1.7sh2.6) 34.8± 1.2 36.4± 0.1 37.4± 1.0 measured weight percent portlandite 20.7± 0.4 20.5± 0.5 20.0± 0.5 table 2. xrd measurements with fully hydrated activated alite. the first numeric value in the equations originates from the calibration with the crystalline substances (see chapter 4). the second numeric value originates from the correction of the total si-content, measured with fully hydrated activated alite. the measurement of the same hydrated cement stone in different laboratories showed, that it will be necessary to make an instrument specific calibration. it is one result of the research that this calibration can be reduced to a one-point-calibration of the second numeric value in equations (1) and (2), together with the proof that without c-s-h in unhydrated cementitious binder the measured absorbance of q1 and q2 signals will be zero. the authors can deliver fully hydrated activated alite to perform such a calibration. the typical quantitation limit of silicon in the q1 or q2 structural unit of 0.3 wt.-% for a number of measurements of 5 and a significance level of 5 % in most cases is sufficient not only for hydrated binders, but also for mortar and concrete. otherwise an enrichment of the binder and/or an increased number of measurements are necessary. 6. plausibility tests with fully hydrated alite activated alite (71.7 wt.-% cao, 25.9 wt.-% sio2, 1.8 wt.-% mgo, 0.6 wt.-% al2o3) was hydrated with a water cement ratio of 1.1 at 23 ◦c in closed containers without moisture loss or uptake from the environment. the total concentration of silicon in this system is very well known. it amounts to 5.72 wt.-%. xrd measurements after 24 h reaction time with water showed no c3s anymore. all of the silicon must therefore be incorporated in the reaction product cs-h. but the detailed structure of the c-s-h is unknown and a function of age. the hydrated system was analyzed by atr ft ir spectroscopy after 24 h, 7 days and 6 months reaction time. differing from the method described in chapter 3 the specimen was not dried until the mixing with the internal standard. additional measurements with thermal analysis proofed, that the moisture loss until mixing with the internal standard was maximum 0.9 wt.-%, increasing the silicon content in the system to maximum 5.76 wt.-%. additional water loss after the mixing with the internal standard do not affect the analysis results. the results are shown in table 1. it is visible, that the mean chain length is slightly increasing with time. the mean chain length is near chain lengths based on nmr-data in the scientific literature [3–6]. the total silicon in c-s-h of course is near the theoretical value of 5.72 wt.-%, because the specimens were used for the calibration of this parameter. in addition the material was analyzed independently by x-ray diffraction with the method of bergold et al. [7] at the same time. assuming a c-s-h composition of c1.7sh2.6, 41.2 wt.-% c-s-h and 20.1 wt.-% portlandite can be expected. the results of the measurements are shown in table 2. it can be seen, that the c-s-h content is lower than the expected one. this was also the result of bergold et al. [7] and is an indication that a part of the c-s-h is totally x-ray amorphous. this may be the dimer molecules. the xrd detectable amount of c-s-h is increasing with time. figure 2 shows the spectrum of the fully hydrated activated alite with age 6 months in the range of the si-o stretching vibrations in c-s-h. it shows 4 peaks resulting from the q2 structural unit at 910, 954, 995 and 1064 cm−1. at the age of 7 and 1 days only the two strongest peaks in the middle can be observed. the q1 structural unit shows two peaks at 793 and 820 cm−1. at 1 day only the strongest peak above 800 cm−1 can be observed. at 7 days the weak splitting is observable. the full spectrum additionally contains the common bands resulting from portlandite, the internal standard, water, carbonate, a peak at 1325 cm−1 perhaps a si-oh bending vibration in cs-h [8], a weak peak at 655 cm−1 and several peaks in the area 450 545 cm−1, perhaps resulting from deformations of the tetrahedrons or the tetrahedron chain. 542 vol. 33/2022 quantitative analysis of c-s-h figure 2. atr ir spectrum of fully hydrated activated alite with age 6 months in the range of the most important bands of c-s-h including the peak fit. background (logarithm function) subtracted before, leaned on the minima at the end of the displayed range. note that the signal of the q1 structural element seems to be more sharp and restricted to lower wave numbers than in the case of a commercial cement stone (see figure 3). cement cem i 42,5 r cem i 42,5 r-sr 0 cem i 52,5 r cem i 52,5 r cem i 52,5 r cement g 680 680 780 325 590 water g 237.82 267.72 232.88 194.7 353.44 silica fume elkem 940 u g − − − − 65.52 water-cement ratio 0.35 0.4 0.3 0.6 0.6 water-binder ratio − − − − 0.54 degree of hydration c3s % 81.3 93.7 73.7 99.4 95.6 degree of hydration c2s % 18.9 35.5 7.8 43.9 35.0 wt.-% si in q2 in specimen 3.6 ± 0.2 3.8 ± 0.2 3.7 ± 0.2 4.6 ± 0.4 5.5 ± 0.4 wt.-% si in q1 in specimen 1.8 ± 0.3 1.3 ± 0.2 1.9 ± 0.2 1.9 ± 0.2 2.4 ± 0.2 sum of two lines above 5.4 ± 0.3 5.1 ± 0.3 5.6 ± 0.2 6.5 ± 0.3 7.9 ± 0.5 expected value (see text) 5.3 6.4 4.9 7.4 10.0 mean chain length (calc.) 6.0 ± 0.6 7.9 ± 1.1 6.0 ± 0.6 6.9 ± 0.6 6.6 ± 0.4 table 3. plausibility tests with hydrated binders. 7. plausibility tests with hydrated binders and mortars using the phase analysis of the cements measured by the rietveld method, the degree of hydration of c2s and c3s measured by the g-factor method [9], an assumed degree of hydration of 100 % for silica fume, the composition of the mixtures, and the drying conditions, the amount of silicon in the q1 and q2 structural elements of c-s-h was calculated in weight percent of the specimens and compared with the experimentally determined value for several hydrated binders. all specimens were stored in closed containers at 23 ± 2 ◦c and above 95 % r.h. until drying. the age before drying was at least 84 days. peaks at about 850 and 900 cm−1 were treated as residual cement phases in this case. table 3 summarizes the tests and their results. it can be seen, that the maximum deviation from the expected total silicon content of the specimens cs-h is about 20 %. positive and negative deviations could be observed. the specimen with the w/c-ratio 0.6 contents more c-s-h as the specimen with the same cement and the w/c-ratio 0.3, as a result of the higher degrees of hydration. the increase of cs-h content by the addition of silica fume is very well visible. the mean chain length of the hydrated binders amounts to 6.0 7.9 and is near chain lengths based on nmr-data in the scientific literature [3–6]. in the same way the amount of silicon in c-s-h of several hydrated mortar binders was analyzed. in this case, the total water content of the specimens was determined by thermal analysis as loss on ignition. after production all mortar specimens were 543 f. m. schmidt-döhl, d. schulenberg, f. tralow et al. acta polytechnica ctu proceedings aggregate type granite, sand basalt, sand limestone, sand quartz sand granite, basalt, limestone g 962.42 962.42 962.42 0 sand, mainly quartz g 641.62 641.62 641.62 1738.07 cement type cem i 52,5 r cem i 52,5 r cem i 52,5 r cem i 52,5 r cement g 535 535 535 580 water-cement ratio 0.5 0.5 0.5 0.55 heavy liquid separation yes yes yes no degree of hydration c3s % 100 100 100 100 degree of hydration c2s % 98.1 98.7 97.2 100 residual phases of aggregate wt.-% quartz 1.8 quartz 1.8 calcite 10.4 quartz 1.3 quartz 12.4 wt.-% si in q2 in specimen 3.3 ± 0.3 3.3 ± 0.3 2.9 ± 0.1 3.8 ± 0.1 wt.-% si in q1 in specimen 2.5 ± 0.3 2.3 ± 0.3 2.0 ± 0.1 2.0 ± 0.3 sum of two lines above 5.8 ± 0.6 5.6 ± 0.4 4.9 ± 0.2 5.8 ± 0.3 expected value (see text) 7.6 7.7 6.9 6.9 mean chain length (calc.) 4.6 ± 0.2 4.9 ± 0.4 4.8 ± 0.1 5.8 ± 0.6 table 4. plausibility tests with binders of hydrated mortars. figure 3. atr-ir spectrum including the peak fit of a mortar with quartz aggregate and enrichment of the binder by grinding and sieving, showing the peaks of the q1 and the q2 structural units in the c-s-h. background (logarithm function) subtracted before, leaned on the minima at the end of the displayed range. stored in closed containers at 23 ± 2 ◦c and above 95 % r.h. until drying. the age before drying was at least 1.5 years (mortar with only quartz: about 0.5 years). the aggregates of the mortar specimens were then removed by two steps: 1) repeated grinding and sieving, removing the coarse grains, 2) heavy liquid separation of the fines with diiodomethane/acetone mixture with a density of 2.4 g/cm3 (only mortars with basalt, granite and limestone). table 4 summarizes the tests and their results. all specimens contain a small amount of residual quartz, probably from the quartz sand. the mortar with only quartz aggregate contains a much higher value, because a heavy liquid separation was not performed in this case. all specimens contain about 0.5 wt.-% calcite, probably by carbonation. the specimen with limestone aggregate contains a much higher value. this indicates that the limestone aggregate cannot be removed well by the heavy liquid separation. but quartz and calcite can be treated during the evaluation of the spectra. no typical minerals from the basalt and the granite could be observed after heavy liquid separation. astonishing is the very high degree of hydration of the c2s. this cannot be an effect of the heavy liquid separation, because also the mortar with quartz and without heavy liquid separation has a degree of hydration of 100 %. figure 3 shows the peak fit of a specimen with quartz sand. because of the high degree of hydration no residual cement phases were considered. in this case all measured si-contents in the specimens c-s-h are lower than the expected values, and the deviation is greater (maximum 29 %). 544 vol. 33/2022 quantitative analysis of c-s-h because of the broader q1-peak used in these peak fits (compare figure 2 and figure 3), the calculated mean chain length is smaller than in the alite and the hydrated binder evaluation. 8. conclusions it can be concluded, that atr-infrared spectroscopy is a very helpful and powerful technique, also in a quantitative manner. infrared spectroscopy is used for the analysis of cementitious materials for a long time, but its potential is not exhausted. also amorphous species are producing signals that can be observed directly. this feature was used to develop a quantitative analysis method for the structurally only weakly ordered c-s-h in cementitious materials. with this technique it is possible to analyze specimens from real life, not only model materials. cementitious materials with quartz or calcite aggregates can be analyzed directly, although an enrichment of the binder will facilitate the analysis. other aggregates must be separated, for example with the help of heavy liquid separation. in addition infrared spectroscopy is a very fast and cheap technique that is common in most laboratories, active in the field of construction chemistry. the described method measures the silicon content in the q1 and q2 structural units in the c-s-h. crystalline minerals containing q1 or q2 structural units were used to calibrate the q1/q2 ratio. but it was not possible to use these crystalline materials for the total silicon content as well. the total silicon content was calibrated with the help of an activated fully hydrated alite with very well known silicon content in the c-s-h phase. plausibility tests on different hydrated binders and the binder in mortars showed that the total silicon content in c-s-h, measured with the method, is in the range of the expected values, but deviations of up to 30 percent could be observed. in particular the measurement of the small q1 absorbance is difficult. in addition also the ratio q1/q2 is measurable and therefore the chain length, which is in the range of literature data based on nmr. in spite of these difficulties researchers and manufacturers have the chance to analyze the concentration and the internal structure of c-s-h as a function of relevant parameters with this method. not only the success of improvements of the time dependent c-s-h quantity and structure, but also the effects of other measures on c-s-h and the results of decomposition of c-s-h by chemical attack can be investigated. in the future it will be helpful to investigate how aluminium and alkalies in c-s-h will affect the irspectra of c-s-h and the method. in addition it is necessary to find an alternative internal standard, that is not soluble in ethanol and water, has a good grindability and chemical stability, is not toxic and does not overlap with the interesting peaks. further activity should pay attention also on the calibration substance and the removal of interfering aggregates. acknowledgements some of the data measurement was performed by m. maaser and s.k. mehra. funding: this work was supported by the deutsche forschungsgemeinschaft [grant number 280067300]. references [1] m. milosevic. internal reflection and atr-spectroscopy, hoboken, wiley, 2012. [2] c. jehn. transportprozesse und chemische reaktionen in silikatischen gesteinskörnern, hamburg: dissertation hamburg-harburg university of technology, 2015. [3] f. battocchio, p. j. m. monteiro, h.-r. wenk. rietveld refinement of the structures of 1.0 c-s-h and 1.5 c-s-h. cement and concrete research 42(11):1534-48, 2012. https: //doi.org/10.1016/j.cemconres.2012.07.005. [4] e. gallucci, x. zhang, k. l. scrivener. effect of temperature on the microstructure of calcium silicate hydrate (c-s-h). cement and concrete research 53:185-95, 2013. https: //doi.org/10.1016/j.cemconres.2013.06.008. [5] i. g. richardson. the calcium silicate hydrates. cement and concrete research 38(2):137-58, 2008. https: //doi.org/10.1016/j.cemconres.2007.11.005. [6] j. skibsted, m. d. andersen, h. j. jakobsen. application of solid state nuclear magnetic resonance (nmr) in studies of portland cement-based materials, zkg int. 60:70-83, 2007. [7] s. t. bergold, f. goetz-neunhoeffer, j. neubauer. mehcanically activated alite: new insights into alite hydration. cement and concrete research 76:202-11, 2015. https: //doi.org/10.1016/j.cemconres.2015.06.005. [8] r. ylmén, u. jäglid, i. panas, et al. monitoring early hydration of cement by ex situ and in situ atr-ftir a comparative study. journal of the american ceramic society 97(11):3669-75, 2014. https://doi.org/10.1111/jace.13186. [9] d. jansen, f. goetz-neunhoeffer, c. stabler, et al. a remastered external standard method applied to the quantification of early opc hydration. cement and concrete research 41(6):602-8, 2011. https: //doi.org/10.1016/j.cemconres.2011.03.004. 545 https://doi.org/10.1016/j.cemconres.2012.07.005 https://doi.org/10.1016/j.cemconres.2013.06.008 https://doi.org/10.1016/j.cemconres.2007.11.005 https://doi.org/10.1016/j.cemconres.2015.06.005 https://doi.org/10.1111/jace.13186 https://doi.org/10.1016/j.cemconres.2011.03.004 acta polytechnica ctu proceedings https://doi.org/10.14311/app.2023.41.0036 acta polytechnica ctu proceedings 41:36–43, 2023 © 2023 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague protection of soft targets – verification of the effectiveness of the anti-terrorist barrier luboš nouzovskýa,∗, zdeněk svatýa, petr hálab a czech technical univesity in prague, faculty of transportation science, department of forensic experts in transportation, horská 2040/3, 128 00 prague, czech republic b czech technical univesity in prague, faculty of civil engineering, experimental centre, thákurova 7, 160 00 prague, czech republic ∗ corresponding author: nouzovsky@fd.cvut.cz abstract. the security of the population and its protection in the context of terrorist attacks is currently one of the major topics of the professional public and security forces. a number of attacks in the last decade have targeted soft targets, which, according to a generally valid definition, can be considered as places with a high concentration of people and a low level of security against violent attacks. in order to ensure maximum effectiveness in protecting soft targets, a number of procedures and methodologies have been developed to determine appropriate measures and their appropriate application. from a technical point of view, these include mechanical barrier systems and elements. it is mechanical barrier elements that are the focus of this article, which uses a selected specific device to present its function, its possible applications, and the issue of testing before use in practice. keywords: mobile barrier, crash test, impact, antiterrorist. 1. introduction in recent times, several terrorist attacks have been perpetrated, which naturally caused a response not only among the general population, but also among the professional public and the security forces. a number of attacks in the last decade have targeted so-called soft targets, which, according to a generally accepted definition, can be considered as places with a high concentration of people and a low level of security against violent attacks. terrorists have exploited their inherent vulnerability because of their open and public location. these include attacks on gatherings during religious festivals, tourist sites, entertainment, or shopping centres. regarding the effort to ensure maximum effectiveness of soft target protection, a relatively large number of methodologies have been developed to select the right measures and their application. measures can be categorized according to several criteria, the most basic of which include a division according to the time of use. these include preventive measures, which operate before an incident and seek to minimise the probability of its occurrence, measures applicable during an incident, which may include, for example, the response of security system staff and their action according to a prepared plan, and measures operating after an incident and mitigating its effects. from a technical point of view, these include organisational and security measures, signalling and monitoring systems and, finally, mechanical barrier systems and elements. mechanical barrier elements are the focus of this article, which presents their function, possibilities of application and the issue of testing before use in practice. 2. barrier elements and systems currently, traffic safety barriers consist mainly of concrete or stone elements, which usually have low containment and low aesthetic value. this issue is summarised in general terms in a document prepared for the european commission “review on vehicle barrier protection guidance” [6]. which explicitly states: “... in the european union detailed documentation on installation methodologies, cost and available products is rather limited ... special care should also be paid on barrier aesthetics and the integration of security by design concepts in the construction of such systems in particular for public spaces, like city centres”. technical devices of this type can be divided into two groups, namely universal (and generally speaking less attractive) and created in accordance with their location. a debated issue, worldwide, is of course the parameter of the level of containment of the proposed product, in addition to its appearance. a basic and generally portable solution that can be seen in many czech and foreign cities is the new jersey concrete crash barrier, in several different designs and sizes. an example is illustrated in figure 1. both experts and city residents for appearance and efficiency sometimes reject these barriers [1]. the efficiency is limited, among other things, largely by the correct placement and use. as a stand-alone element, the functionality is very limited, as is the case if no attachment to other elements or to a solid base is made. it is therefore a barrier that can deter, restrain 36 https://doi.org/10.14311/app.2023.41.0036 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 41/2023 soft targets protection – anti-terrorist barrier figure 1. new jersey crash barrier used in prague [1]. figure 2. concrete blocks used in melbourne [2] and large-volume planters in prague [3]. figure 3. combination of designer flowerpots and benches in cardiff [4] and benches offered as a safety measure in the uk [5]. and, if used correctly, stop a potential attacker, but in most cases will not be effective to the extent required. the aforementioned appearance criterion is also not to be overlooked. stand-alone concrete or similar material cubes, which can be found in various european countries, seem to be less effective and in many designs not very suitable. in addition, to restricting or preventing access to the traffic flow, they can act as part of the urban furniture, e.g. like in figure 2. however, these solutions also have many critics and are therefore not very welcome. the significantly higher efficiency, but very unsuitable appearance for urban use in peacetime, is often mentioned for other types of devices, namely the czech hedgehog [7]. thus, more suitable solutions include, for example, urban furniture that is combined with greenery – the large-volume planters known in prague, but a more interesting and aesthetically acceptable solution than concrete blocks. the figure 3 shows the use of the ’flowerpot’ motif in cardiff, wales. however, the use of even these barriers is bound by many rules and cannot, in principle, be used on their own. in this picture when used with benches. these also can be a safety barrier. however, their risk lies in the purpose of their installation and their very nature – the citizens sitting on them will be the first (only) people at risk in the event of a terrorist act. 37 l. nouzovský, z. svatý, p. hála acta polytechnica ctu proceedings figure 4. urban furniture – mm_at_p_02. figure 5. collision configuration – side views. 3. tested equipment and testing methodology one of the newly developed devices of this type on the territory of the czech republic is an urban furniture, namely a modified flowerpot (see figure 4), which will be referred to as mm_at_p_02 in this article for commercial reasons. it is a planter with a steel frame, an aluminium insert for the substrate and a wooden lining. the dimensions are 1495 × 1495 × 1000mm and the weight of the pot is approximately 490 kg. the weight, including the sand used as a substrate, is 2450 kg. the device in question was subjected to a barrier test by a passenger car (in this case a skoda octavia, 1500 kg) according to pas 68:2013 standard [8]. according to the chosen configuration (see figure 5), the vehicle hit the object at the prescribed speed of 51 km h−1 (thus complying with the specified limits of 48 ± 3 km h−1) perpendicularly, thus verifying its impact properties. the focus is on the behaviour of the vehicle and the obstacle during the impact and in the short interval afterwards. to observe the deformation of the internal parts of the vehicle during the impact, the bumper was removed from the car. one of the test conditions was that the vehicle was not braked. thus, after the impact, the vehicle moved to its final position by inertia and without deceleration produced by the braking system. the entire test was recorded by a set of cameras, including high-speed cameras, and the test was monitored from both the exterior and interior of the vehicle. the deceleration of the car was monitored by several 3-axis accelerometers in the area of the car’s airbag control unit location. the movement in the interior of the vehicle was also monitored, as well as the deceleration acting on the vehicle’s occupants, represented by a hybrid iii 50 percentile male test dummy with 3axis accelerometers in the head, chest and pelvis area. values recorded also include vehicle weight, vehicle speed or displacement and obstacle deformation. all of these data served as auxiliary indicators to evaluate the suitability of the obstacle design used in the test. 4. results of testing – the vehicle and mm_at_p_02 the test was primarily to determine how the tested barrier reacts to being struck by a passenger car. the car was damaged and slowed down by the impact with the barrier in the front part of the bodywork, with the expected unprompted stopping and thus restraint of 38 vol. 41/2023 soft targets protection – anti-terrorist barrier figure 6. selected details of the impact and overall movement (top left: first contact, deformation of the grille and contact with the bumper reinforcement, bottom left: global maximum, maximum penetration of the vehicle into the obstacle). the car by the barrier. some frames from the process are shown in figure 6. the barrier was also damaged, by a displacement of 5.304m in the direction of the x-axis, as well as by the deflection of one wall of the wooden lining, mechanical damage, and deformations to the internal aluminium liner. the test can be described by the classification code pas 68:2013 planter v/1 500(m1)/48/90:3,8/0. after the impact with the barrier at full overlap, there was an area deformation of the bow, which extended up to the headlight mounting point. thus, the external body elements were significantly deformed (breakage of the bonnet including the bonnet and the left and right front wings). the bumper reinforcement was deformed, with the deformation members destroyed, and the reinforcement was fractured. the impact was also transmitted through the brace to the supporting body stringers, which were also deformed. the deformed and fractured brace was further pushed through the radiator walls into the engine compartment where the engine accessories were damaged. the vehicle was immobilized. final position and deformation of the car could be seen in figure 7, the course of acting acceleration in figure 8. from the point of view of the auxiliary criteria for the evaluation of the main purpose of the impact test, it can be stated that some biomechanical criteria used to assess the effects of the impact on the vehicle occupants were slightly exceeded. these include, for example, the thiv criterion, whose limit value is 33 km h−1, in which case a value of 36 km h−1 was reached, the phd criterion was exceeded (by 3.9 g) and the 3ms criterion was exceeded (a maximum of 86.85 g was reached with a limit value of 80 g). in terms of other criteria (asi, hic) the limit values were respected. therefore, it can be considered that the device fulfils the expectations, since the incoming vehicle was arrested and the consequences for the crew would not have been severe. therefore, in the case of an accident, the error forgiving traffic element rule is respected, and in the case of a terrorist attack, exceeding the mentioned limits is not a serious negative. 5. results of testing – anti-terrorist effects for such a used element of urban furniture, it is also necessary to evaluate the consequences of the impact on its surroundings, specifically on the inhabitants in the vicinity. the aim was therefore to evaluate the possible injuries of the people standing behind the tested object in the event of its impact and subsequent removal. the evaluation is based on the performed 39 l. nouzovský, z. svatý, p. hála acta polytechnica ctu proceedings figure 7. view of the final position and deformation of the car. figure 8. course of the resultant acceleration acting on the vehicle. impact test, on the analysis in the simulation environment and on data from the literature. a comparative method was used to evaluate the consequences and severity of possible injuries, where data on similar impact tests and their evaluation were obtained from the literature. due to the uniqueness of the performed test, tests with vehicle types whose front part of the body can be compared to the tested object in terms of its dimensions were used for comparison; specific values were taken from [10–12]. due to the small sample size, small trucks with dimensions at least approximately matching the size of the obstacle were also considered. in addition to these sources, the simulation program pc-crash 12.0, which is used, among other things, in expert practice, was also used and can be used in this sense for a simplified assessment. the person was simulated as a multibody object, which is struck by a solid object at a given velocity. the velocity values were obtained by analysing the measured data in the crash test and are summarized in table 1. in this way, the theoretical impact velocity 40 vol. 41/2023 soft targets protection – anti-terrorist barrier distance from the object [m] impact velocity [kmh−1] 0.5 24.5 1 22.5 2 15.5 3 14 4 10 5 4.5 table 1. impact velocity as a function of the impact distance from the point of impact. distance from the object [m] impact velocity [kmh−1] hic15 [–] acceleration shin [g] acceleration chest [g] 0.5 24.5 247 145 40 1 22.5 227 133 37 2 15.5 156 92 25 3 14 141 83 23 4 10 101 59 16 5 4.5 45 27 7 table 2. biomechanical values as a function of impact velocity. figure 9. dependence of hic15 and possible consequences [9]. was found as a function of the distance of the person standing behind the test object. the data from the individual sources ([10–18]) were compared to see if they were consistent with each other and could be included in the processing. the values were then averaged, as shown in table 2. the reference values were considered to be the acceleration data established by the european enhanced vehicle-safety committee [11] and standardly used e.g. by euro ncap [13]. these are the following limits or dependencies: • the development of a tibial fracture or more severe damage at a load of more than 150 g. • the development of a fracture in the thoracic region or more serious damage at a load of more than 60 g. • head injury according to the graph in figure 9, which shows the relationship between the magnitude of the hic criterion and the probability of injury according to the abbreviated injury scale (ais) [9]. based on a comparison of the highest values of the applied acceleration (at a speed of 24.5 km h−1), the 41 l. nouzovský, z. svatý, p. hála acta polytechnica ctu proceedings body area injury probability of injury fulfilling the limit value head ais2+ (headache, dizziness, minor fractures) 15% −− ais3+ (major fractures, unconsciousness) 5% −− chest fracture the limit is not exceeded 67% lower limbs fracture the limit is not exceeded 97% table 3. possible impact injuries. possible injuries caused by the impact were determined, as summarised in table 3. 6. conclusions the aim of the article was to present the status of the development of protective barriers applicable in urban environments for the protection of soft targets. it also includes the verification of the restraining capabilities and other properties of a specific anti-terrorism measure in the form of mm_at_p_02 flowerpot. the test and its evaluation were carried out according to the british globally used and recognised standard pas 68:2013. thus, the resistance to impact conducted by a moving vehicle at a defined speed was tested. the evaluation included an assessment of the potential consequences to nearby residents, in addition to an indication of the effects on the barrier, the vehicle and its occupants. the evaluation was based on the performed crash test, on analysis in a simulation environment and on data from the literature. based on a comparison of the measured values, literature and simulation data with generally applicable standards and conclusions used in the field of vehicle exterior safety assessment, it was determined that no life-threatening injuries should occur when the test object strikes a person. it can therefore be concluded that the device can fulfil its purpose of restraining a vehicle used as a weapon against soft targets while maintaining the safety of the surrounding area to an acceptable degree. the resulting overall effect in protecting soft targets depends on the specific application and deployment. references [1] redakce euro.cz. rozjetý kamion nezadrží, říká o pražských bezpečnostních zábranách architekt. 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[17] z. schejbalová, t. mičunek, d. schmidt. biomechanical analysis of the dummy responses in case of child pedestrian/cyclist collision with passenger car. neural network world 23(6):609–622, 2013. https://doi.org/10.14311/nnw.2013.23.036 [18] j. yang. review of injury biomechanics in car-pedestrian collisions. international journal of vehicle safety 1(1-3):100–117, 2005. https://doi.org/10.1504/ijvs.2005.007540 43 https://doi.org/10.1016/s0001-4575(03)00007-1 https://doi.org/10.14311/nnw.2013.23.036 https://doi.org/10.1504/ijvs.2005.007540 acta polytechnica ctu proceedings 41:36–43, 2023 1 introduction 2 barrier elements and systems 3 tested equipment and testing methodology 4 results of testing – the vehicle and mm_at_p_02 5 results of testing – anti-terrorist effects 6 conclusions references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2023.40.0039 acta polytechnica ctu proceedings 40:39–47, 2023 © 2023 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague experimental analysis of the railway truss bridge using modern methods petra kálalováa,∗, michal poláka, vojtěch stančíkb a czech technical university in prague, faculty of civil engineering, department of mechanics, thákurova 7, 166 29 prague 6, czech republic b czech technical university in prague, faculty of civil engineering, department of steel and timber structures, thákurova 7, 166 29 prague 6, czech republic ∗ corresponding author: petra.kalalova@fsv.cvut.cz abstract. experimental analyses of civil engineering structures have been carried out for decades. while in the past, mainly static load tests of new structures were performed, a few severe accidents with fatal results caused a lot of emphasis to be given to monitoring existing structures recently. the need for continuous or periodical evaluation of static or dynamic behaviour of structures in operation led to a good progress in the development of modern methods for experimental analysis. the paper deals with one of the modern methods of experimental analysis – the dic method. possible application of the dic method was verified by an experimental analysis of a railway truss bridge in rataje nad sázavou. the experiment was divided into two stages. during the experiment, deflections of the bridge measured by lvdt sensors and the dic method were compared. technological problems of the dic method were found in the first stage. one of the problems was related to stability of the camera tripod. a solution was proposed and verified during the second stage. keywords: experimental analysis, digital image correlation, lvdt sensors, railway bridge. 1. introduction experimental analyses of civil engineering structures have been carried out for decades. while in the past, mainly static load tests of new structures were performed, a lot of emphasis has been given to monitoring existing structures recently. by the monitoring is meant continuous or periodical evaluation of static or dynamic behaviour of structures in operation. this is the consequence of a few severe accidents which had fatal results. for example, it is possible to mention the collapse of morandi bridge (italy) with 43 victims in 2018 [1] or the collapse of nanfang’ao bridge (taiwan) with 6 victims in 2019 [2]. however, collapses of civil engineering structures happen in the czech republic too. almost every citizen has heart about the collapse of the pedestrian bridge over vltava (trója, prague, the czech republic) in 2017. there were no victims but it has stimulated the discussion on other bridges and pedestrian bridges in a state of disrepair in the czech republic. while the reason of collapses of civil engineering structures usually is a combination of many factors, it is indisputable that the actual condition of many bridges and pedestrian bridges in the czech republic is not good. one of the main reasons is neglecting regular maintenance. since there is not enough financial resources and workers for the immediate repair of all the constructions in unsatisfactory condition, it is necessary to control these constructions regularly. some of them may need continuous monitoring of construction condition. after that, it is possible to evaluate which construction is in a state of disrepair, and therefore have to be closed and repaired or demolished immediately, and which is possible to operate with restrictions or after small repairs. because of the reasons above, good progress has been made in the development of modern methods for experimental analysis and monitoring of civil engineering structures. focusing on construction (especially bridges and pedestrian bridges) displacements, it is necessary to mention two methods – terrestrial radar interferometry and digital image correlation. these methods have some advantages compared to classical methods for experimental analysis, such as velocity sensors, acceleration sensors and strain gauges. the main advantage is that the modern methods are non-contact so the experiments can be prepared and held without traffic disruptions which negatively influence its total costs. the next advantage is possibility to monitor parts of constructions which are uneasily accessible. as a counterweight to the benefits, there are disadvantages, such as lower precision, a lack of experience of the experimenters etc. the paper deals with one of the modern methods of experimental analysis – the dic method. possible application of the dic method is verified by the experimental analysis of the railway truss bridge in rataje nad sázavou. at the beginning, the dic method is briefly introduced. it is followed by the description of the experiment which was divided into two stages. technological problems of the dic method were found in the first stage. the proposed solution was verified 39 https://doi.org/10.14311/app.2023.40.0039 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en p. kálalová, m. polák, v. stančík acta polytechnica ctu proceedings during the second stage. the results of both stages are shown in section 4. the idea of the future work, such as mathematical model, is given at the end. 1.1. digital image correlation digital image correlation (dic) is another modern non-contact method for experimental analysis or longterm monitoring of civil engineering constructions. this optic method is based on the correlation between the digital images performed before, during, and after the construction deformation. the dic method was used for a displacement and relative displacement analysis of small laboratory samples in the past. today, with development in technique, it can also be used for monitoring of large civil engineering constructions, such as bridges, building etc. it is hard to appoint the only inventor of the dic method. the initial studies which were closely lined to the dic method can be found mainly in the area of digital photogrammetry. gilbert hobrough dedicated to the digital photogrammetry from the beginning of 1950s. but the american researchers, peters and ranson, are considered to be the pioneers of the dic method. peters and ranson [3] presented a new technology how to measure deformation of a simple object using the image correlation in 1983. the technology was based on computing solution. the technology was further improved in cooperation with sutton [4]. the dic method has gone through great development during the last 40 years. the development was supported by technical advance in the area of cameras and computing systems. the transition from 2d [5] to 3d [6] dic can be considered one of the main milestones. while the only camera can be used for displacement and relative displacement measurement in a plane parallel to the sensor plane (figure 1a), it is possible to determine 3d displacement captured by pairs of cameras (figure 1b). 3d dic is also necessary to use in the case of construction without flat surface. a higher sampling rate of the cameras enables monitoring of dynamic behaviour of the constructions [7, 8], even during extreme situations, such as earthquakes [9]. in addition, a higher camera resolution enables the more precise measurement and the usage of the construction natural surface instead of artificial speckle patterns. thanks to increased computer memory and faster calculations, it is possible to monitor extensive areas of interest. the dic method has the ability to partially substitute visual inspections of the constructions because it is possible to detect concrete spalling [10] or cracks, even the cracks which are not readily seen [11]. the main advantages of the dic method are noncontact measurement and possibility to measure displacements of more points simultaneously. the noncontact measurement is particularly important for constructions or its parts which are uneasily accessible. on the other hand the constructions has to be clearly visible – there cannot be any barriers between 90◦camera planar construction loading direction optical axis computer with computing software (a). 2d dic. optical axis optical axis camera camera ba se lin e stereo angle computer with computing software arbitrary construction (b). 3d dic. figure 1. experimental setup and necessary equipment for the dic method – a camera and a computer with computing software. the cameras and tracked points of the construction. it can be problem for monitoring constructions such as bridges with more spans where piers can hide part of the constructions. in addition, the constructions has to be to be made out of material with speckle pattern, such as concrete. in other cases, the speckle pattern has to be sprayed on the constructions or desks with the speckle pattern has to be installed. the great advantage is that the dic method allows to monitor the whole displacement fields. the field size is limited only by the camera resolution and the computer memory. however, amount of data has to be processed within a reasonable time. the next important competitive advantage, compared mainly to other modern methods such as radar interferometry, is a lower price. the necessary equipment, on a practical level, is only a camera and a computer with computing software. a type of the camera and, therefore, the price depend on required accuracy. high-speed cameras are used most frequently. but it is possible to use macroscopes and microscopes (electronic, atomic) too. during the experiment, the cameras are taking pictures/video which can help to detect undesirable phenomenons affecting the results (e.q. people passing in front of the camera) or cause of the acquired results (e.q. cracks in the construction). installation and set-up of the equipment is fast and easy. b. pan et al. [5] mention a comparison of the dic method with interferometric optical methods in their work. 40 vol. 40/2023 experimental analysis of the railway truss bridge . . . figure 2. the historical railway bridge in rataje nad sázavou. the photo was taken during the second stage of the experiment. 2. description of the bridge a historical steel railway bridge in rataje nad sázavou is a part of railway line called “posázavský pacifik”. the railway line no. 212 runs from čerčany to světlá nad sázavou. on the way to čerčany, there is a tunnel in close proximity to the bridge. on the other side of the bridge, there is a branch line no. 14 running to kolín. the bridge crosses river sázava and a private road. figure 2 shows photo of the bridge taken in 2022. the bridge was build at the beginning of 20th century. the supplier of the steel construction was první českomoravská továrna na stroje in libeň. the abutments and the bridge completion in situ was done by the firm owned by ing. osvald životský. this firm was in charge of the whole railway line. a load test of the bridge was performed on 29th of june 1901 [12]. in the same year, the railway line was put in operation. but the whole railway line was completed as late as 1903. the bridge was restored and renovated twice, in 1961 and 2021. it is a riveted truss bridge with the only span which is 72.0 m long. the clear perpendicular distance between abutments is 70.0 m. the total length of the bridge is 86.2 m. the load-bearing superstructure is composed of two trusses with parabolic upper chords. the roadbed is carried at the bottom of the trusses. the width of the load-bearing structure is 5.23 m. the width of the free space for passing train is 4.12 m. the height of the superstructure is 0.88 m. the clear height of the bridge is 4.12 m. the railway line and the bridge are straight. it was said above that the span of two main trusses is 72.0 m long. the trusses are 4.65 m apart. the height of the trusses is variable because of the parabolic upper chords. each truss consists of upper chord, bottom chord, verticals (struts) and diagonals. the elements are connected in joints. the upper chords are composed of 2 webs, 1–3 flanges and angles riveted together. the bottom chords are composed of 2 webs, 0–5 flanges and angles riveted together. the verticals above the abutments consist of 12 angles a 3 sheet metals riveted together. all the other verticals consist of 4 angles. diagonals consist of 4 sheet metals or 4 angles. the stiffeners between the bottom chord are made of steel sections u200. the stiffeners between the upper chords consist of lattice girders and upper frame stiffeners. the roadbed is on wooden crossbeams which lay on two stringers. the stringers are 1.80 m apart. the stringers are welded from sheet metals to form ’i’ shape. the height of the stringers is 500 mm except the stringers in panel 1–3 and 10 which are higher (580– 630 mm). floor beams are riveted to form ’i’ shape. there are 6 different cross sections of the floor beams. the floor beams height vary 720–724 mm. there are the stiffeners u140 between the stringers. the horizontal load-bearing structure is supported on the abutments by bearings from steel and cast iron elements. the stone masonry abutment are made of blocks of the same size. the wing walls are parallel. the wing walls and adjacent slopes are also made of blocks of the same size. the description of the bridge mentioned above corresponds to the current conditions and therefore to the second stage of the experiment. the first stage was realized before the last renovation so there were some differences in components. the stringers were riveted from web, flange and 4 angles to form asymmetrical ’i’ shape. the height of the stringers was 560 mm. the stiffeners between the stringers consisted of lattice girders. also the stiffeners between the bottom chord differed from the current conditions. it consisted of 2 angles. 41 p. kálalová, m. polák, v. stančík acta polytechnica ctu proceedings figure 3. cross section of the bridge in rataje nad sázavou. deflections of points uzl, uzp were measured by the standard method of experimental analysis – the lvdt sensors. displacements of points sys2, sys3 and sys4 were measured by the modern method of experimental analysis – the dic. construction verification element tracked point lvdt sensor string reference plane string spring figure 4. the scheme of the measuring system – the steel string, the verification element, the core of lvdt sensor. the figure has been taken over with minor changes from [13]. 3. equipment and experimental setup the experiment was divided into two stages. the first stage took place in july 2021. the second stage was planned almost one year later – in june 2022. because of the possible comparison, the aim was the same experimental setup in both stages. but problems in the first stage resulted in some differences which are described below. 3.1. standard method of experimental analysis lvdt sensors were used to measure deflections of the bridge by standard method of experimental analysis. the sensors measured deflections in two points of the bridge. figure 3 shows scheme of the tracked points and how it is marked (uzl,uzp). both points measured by the lvdt sensors were placed in the figure 5. the real photo of the measuring system. same cross section of the bridge (≈ 1/4 of the bridge span). the point uzl was placed on the bottom chord of the left main truss. the point uzp was placed on the bottom chord of the right main truss. the lvdt sensors isdl 50-kd-2405 (inelta sensorsysteme, germany) were used for the experiment. serial numbers of the sensors were 84801 and 84803. steel frames with the fixed lvdt sensors were placed in reference points under the bridge. cores of the sensors were attached to verification elements which were connected to the tracked points by steel string. figure 4 shows scheme of the measuring system and figure 5 shows photo of the real measuring system used in the experiment. the measuring system, the steel string – the verification element – the core of the lvdt sensor, were prestressed by steel spring fixed to the other side of the steel frame. the lvdt sensors were connected to data acquisition system ems pohl. before start of the measurement, adjusted sensitivity 42 vol. 40/2023 experimental analysis of the railway truss bridge . . . figure 6. the desks with artificial speckle pattern were used to the achieve exact location of the tracked points. of the used lvdt sensors was verified by simulating deflection of 10 mm realized by verification element. 3.2. digital image correlation the dic method was used to measure deflection of the bridge by modern method of experimental analysis. three points, which were placed on verticals of the right main truss, were tracked by the dic method in the first stage. figure 3 shows that the tracked point were placed in ≈ 1/4, 1/2 and 3/4 of the bridge span. desks with artificial speckle pattern were used to achieve the exact location of the tracked points. figure 6 shows that the desks were attached to the verticals by clamps. the distance between the tracked points and the cameras was: sys2 ≈ 15 m, sys3 ≈ 30 m, and sys4 ≈ 45 m. three cameras basler aca4096-30um usb 3.0 (8.9 mp; 32 fps) were used for the experiment in the first stage. the cameras were equipped with different cameras lens: sys2 – f = 50 mm; sys3 – f = 75 mm, and sys4 – f = 75 mm. the cameras were attached to two tripods 475 pro geared tripod and connected to a notebook by usb 3.0 cables. figure 7 shows the whole measuring system of the dic method. software vic-snap – correlated solution was used for measuring and synchronization of the data. subsequently, software vid-2d – correlated solution was used for the data evaluation. three floodlights aku parkside served to lighting the desks during reduced visibility, especially at night. there were problems with stability of the tripods, particularly the tripod carrying the camera tracking point sys2, in the first stage of the experiment. because of that problems, five cameras were used in the second stage. two additional cameras were aimed figure 7. the measuring system of the dic method in the first stage of the experiment. figure 8. the measuring system of the dic method in the second stage of the experiment. at two points located on the abutments – sys1 and sys5 (figure 3). it was supposed that these points were fixed and there should not have been any measurable displacements. the cameras were equipped with different cameras lens: sys1 – f = 50 mm; sys2 – f = 50 mm; sys3 – f = 50 mm, sys4 – f = 75 mm, and sys5 – f = 75 mm. all the cameras were attached to only tripod. figure 8 shows the whole measuring system in the second stage. 3.3. additional equipment to consider the influence of temperature on quasistatic deflection of the bridge, it was important to record temperature of the construction and the air. a temperature sensor t1 was placed on the bottom chord of left main truss and it measured temperature of the construction near the point tracked by the lvdt sensor. a temperature sensor stv was placed under the bridge near a table with the notebook and the data acquisition system ems pohl. the sensor 43 p. kálalová, m. polák, v. stančík acta polytechnica ctu proceedings figure 9. influence of temperature changes on deflections of the tracked points during the first stage of the experiment. deflections of points uzl, uzp were measured by the lvdt sensors. deflections of points sys2, sys3, and sys4 were measured by the dic method. temperature t1 responds temperature of the construction near the point tracked by the lvdt sensor. temperature stv responds temperature of the air. stv measured temperature of the air. in the second stage of the experiment both sensors t1 and t2 measured temperature of the air. during both stages of the experiment, next classical and modern methods for experimental analysis were used. high sensitive piezometric accelerometers served to an experimental modal analysis of the bridge. the modern method of radar interferometry measured quasi-static and dynamic response of the bridge. these methods are not described in the paper because the results has not been available until that time. both methods were also used during our last experiment [14]. 4. results 4.1. first stage of the experiment (2021) the first stage of the experiment took place in july 2021. it was mostly sunny during the experiment. the lvtd sensors measured the deflections of two points (uzl, uzp) located on the bottom chord of the main trusses. the measurement performed by the lvdt sensor lasted almost 24 hours – from 28th of july 2021 8:22 to 29th of july 2021 7:57. sampling rate of the sensors was 50 hz. the deflections of three points (sys2, sys3 and sys4) located on the verticals of the right main truss were measured by the dic method. the measurement performed by the dic method lasted from 28th of july 2021 8:22 to 28th of july 2021 21:26. sampling rate of the cameras was 10× lower that the sampling rate of the lvdt sensors, i.e. 5 hz. figure 9 shows influence of temperature changes on deflections of the tracked points. the figure shows a part of the experiment from 28th of july 9:30 to 28th of july 21:00 when both methods were in operation. it can be seen that the temperature changes had an effect on deflections of the lower chords and the verticals of the main trusses. compared to initial time 9:30 (u9:30 = 0), the upward deflection of points uzl, uzp, sys3, and sys4 was dominant. it was most likely caused by different temperatures of the bottom and the upper chords of the main trusses. during a daytime the upper chords are more exposed to the sun than the lower chords that results in higher temperature. the different behaviour of point sys2 and visual observation of the dic equipment during the experiment confirmed concerns about external factor affecting the results. uneven settlement of feet of the tripod carrying the camera tracking point sys2 was observed. it resulted in inclining of the camera and consequently the incorrect results. the solution of this problem was proposed and verified during the second stage. figure 9 also shows dynamic behaviour of the bridge. while the significant deflection of points uzl and uzp measured by lvdt sensors were downward, which responds to passing trains, some significant deflections of points sys2 and sys 3 measure by the dic method were upwards. it can be seen from 16:00 to 17:00 and at 19:37. it is assumed that it was most likely caused by manipulation with the cameras, for example during setting up aperture. this can be another problem of the dic method during long-term measurement. 4.2. second stage of the experiment (2022) the second stage of the experiment took place in june 2022. it was cloudy with occasional heavy rains 44 vol. 40/2023 experimental analysis of the railway truss bridge . . . sys5 sys4 sys3 sys2 sys1 u ca m 5, i u ca m 4, i u ca m 3, i u ca m 2, i u ca m 1, i figure 10. the simplified scheme showing 2d movement of cameras without rotation caused by uneven settlement of tripod feet. the vertical movement (ucam2,i;ucam3,i;ucam4,i) can be calculated and used for the correction of results. and thunderstorms. as in the first stage, the lvtd sensors measured the deflections of two points (uzl, uzp) located on the bottom chord of the main trusses. the measurement performed by the lvdt sensor lasted more than 24 hours – from 28th of june 2022 18:58 to 29th of june 2022 20:28. sampling rate of the sensors was 50 hz. the deflections of three points (sys2, sys3 and sys4) located on the verticals of the right main truss were measured by the dic method. additional two points (sys1 and sys5) located on the abutments were also measured by the dic method. the measurement performed by the dic method lasted from 28th of june 2021 21:20 to 29th of june 2022 15:48. sampling rate of the cameras was lower than in the first stage, i.e. 0.1 hz. the lower sampling rate and the shorter period of measurement was chosen with regard to large amount of data to process. the remaining time of the experiment was used to measure the bridge response to passing trains. for this purpose, the sampling rate of the cameras was higher, i.e. 10 hz. the uneven settlement of the tripod feet was observed during the first stage. it resulted in observable inclining of the camera tracking point sys2 and consequently the incorrect result. the proposed solutions was to measure two additional points (sys1 and sys5) which were located on the abutments, and therefore these points were considered to be fixed. the measured displacement of these points was not largely real and was caused by movement and rotation of the cameras. the displacement could be used for correction of the results. a method of correction is explained by simplified 2d scheme in figure 10 and equations (1) and (2). ucam2,i = ucam1,i + {(ucam5,i − ucam1,i)) ∗ 1 4} ucam3,i = ucam1,i + {(ucam5,i − ucam1,i)) ∗ 2 4} ucam4,i = ucam1,i + {(ucam5,i − ucam1,i)) ∗ 3 4} (1) u2,i,corr = u2,i − ucam2,i u3,i,corr = u3,i − ucam3,i u4,i,corr = u4,i − ucam4,i (2) in equation (1) ucam1,i and ucam5,i are measured displacements of points sys1 and sys5. these points are supposed to be fixed and therefore in the simplified situation (2d movement of the cameras without rotation) the displacements should correspond to vertical movement of cameras caused by uneven settlement of tripod feet. in equation (2) u2,i, u3,i and u4,i are measured deflections of points sys2, sys3 and sys4. u2,i,corr, u3,i,corr and u4,i,corr are corrected results involving the influence of uneven settlement of tripod feet. equation (1) can also be used for more complex situations (3d movement and rotation of the cameras) if all the tracked points are located in one line. during the experiment, it was taken advantage of this fact. figure 11 shows the influence of temperature changes on deflections of the tracked points during the second stage of the experiment. the picture shows the part of the experiment from 28th of june 22:00 to 29th of june 8:00 when both methods were in operation. there are a lot of gaps in measurement by the dic method from 8:00 to 15:48 caused by lack of free disk space, changing weather conditions etc. so the results are not presented here. unfortunately, during the 10-hour period showed in figure 11 there were not significant changes in temperature. what is more important, figure 11 shows a difference between the original (sys2, sys3 and sys4) and the corrected results (sys2_corr, sys3_corr and sys4_corr), or deflections, acquired by the dic method. it can be seen that the difference between the original deflections measured by the dic method and deflections measured by the lvdt sensor is increasing over time. this trend is not observable for corrected results. on the other hand, figure 12 shows that the points which were supposed to be fixed (sys1 and sys5) also vibrated. strong vibration of point sys5 can be seen 45 p. kálalová, m. polák, v. stančík acta polytechnica ctu proceedings figure 11. influence of temperature changes on deflections of the tracked points during the second stage of the experiment. deflections of points uzl, uzp were measured by the lvdt sensors. deflections of points sys2, sys3, and sys4 were measured by the dic method. the corrected results sys1_corr, sys2_corr and sys3_corr involve influence of uneven settlement of the tripod feet. figure 12. measured displacement of points sys1 and sys5 which were supposed to be fixed. the black lines are linear trend lines. strong vibrations of point sys5 can be seen from 9:00. the vibrations were likely caused by multiple reasons – passing trains, diffraction of light, air shimmering etc. from 9:00. the vibrations can be caused by multiple reason – passing trains, diffraction of light, air shimmering etc. this fact stimulates a discussion whether the position and fixation of these points was chosen right. 5. conclusions the experimental analysis of the railway truss bridge in rataje nad sázavou was carried out in 2021 and 2022. apart from classical methods of experimental analysis (lvdt sensors), one of modern methods (the dic method) was also used. during the first stage of the experiment, it was observed by both methods that the temperature changes had an effect on deflections of the lower chords and the verticals of the main trusses. since the dic method has been used for experimental analysis in field only several times, problems were expected. one of the found problems was uneven settlement of the tripod feet. the solution of this problem was proposed. the other problem was necessary manipulation with the cameras (setting up aperture) which caused false upward deflections. during the second stage of the experiment, the proposed solution of uneven settlement of the tripod feet was verified. the results showed that the difference between the original deflections measured by the dic method and the deflections measured by the lvdt sensor had been increasing over time. this trend was not observable for the corrected results. unfortunately, the fixed points (particularly further 46 vol. 40/2023 experimental analysis of the railway truss bridge . . . point sys5) whose measuring was necessary for the correction of results also vibrated. the vibrations were greater during a day while there were changing weather conditions, passing trains over the bridge and other negative effects. this fact stimulates a discussion whether the position and fixation of these points was chosen right. it would be likely better to choose points outside the bridge construction which are closer to the cameras than point sys5. on the other hand, the locations of the points in one line enabled to use simplified situation (2d movement of cameras without rotation) and the related equations. evaluation of the corrections for complex situation (3d movement and rotation of the cameras) would be more difficult. to sum up, there are still problems with long-term monitoring by the dic method which are necessary to deal with. we would like to readjust and verify the solution of uneven settlement of tripod feet during next experiments. the problem of necessary manipulation with the cameras during long-term experiments has also to be solved. we would like to aim our future work at a dynamic response of the bridge. for this purpose, a mathematical model of the bridge will be made. results measured by the dic method, the lvdt sensors and high sensitive piezoelectric accelerometers will be compared to results calculated from the mathematical model. acknowledgements the financial support by ministry of the interior of the czech republic (project no. vi20192022167) and the faculty of civil engineering, czech technical university in prague (sgs project no. sgs20/155/ohk1/3t/11) is gratefully acknowledged. references [1] g. m. calvi, m. moratti, g. j. o’reilly, et al. once upon a time in italy: the tale of the morandi bridge. structural engineering international 29(2):198–217, 2019. https://doi.org/10.1080/10168664.2018.1558033 [2] p. hawryszków. katastrofa mostu nanfang’ao bridge na tajwanie. mosty (6):56–58, 2019. 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[2022-01-11]. http://www.pacifikem.cz [13] m. baťa, v. plachý, m. polák. instrument for relative measurements of displacements of structures. in proceedings of international conference design and assessment of building structures, pp. 33–45. prague; klokner institute, 1996. [14] m. polák, t. plachý, a. čítek, et al. experimental dynamic analysis of the existing footbridge in dobřichovice town. in matec web of conferences, vol. 313, p. 00002. edp sciences, 2020. https://doi.org/10.1051/matecconf/202031300002 47 https://doi.org/10.1080/10168664.2018.1558033 https://doi.org/10.1117/12.7972925 https://doi.org/10.1016/0262-8856(83)90064-1 https://doi.org/10.1088/0957-0233/20/6/062001 https://doi.org/10.1016/j.optlaseng.2013.03.001 https://doi.org/10.1007/s11340-006-6124-2 https://doi.org/10.1016/j.ymssp.2010.08.013 https://doi.org/10.1016/j.engstruct.2020.110551 https://doi.org/10.1117/12.2009647 https://doi.org/10.1680/cien.11.00040 http://www.pacifikem.cz https://doi.org/10.1051/matecconf/202031300002 acta polytechnica ctu proceedings 40:39–47, 2023 1 introduction 1.1 digital image correlation 2 description of the bridge 3 equipment and experimental setup 3.1 standard method of experimental analysis 3.2 digital image correlation 3.3 additional equipment 4 results 4.1 first stage of the experiment (2021) 4.2 second stage of the experiment (2022) 5 conclusions acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.38.0216 acta polytechnica ctu proceedings 38:216–221, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague experiencing circular design: stories of agri-food waste transformed into new materials for architecture silvia tedescoa, elena montacchinia, jacopo andreottib,∗ a politecnico di torino, department of architecture and design (dad), viale p.a. mattioli 39, 10125, turin, italy b roma tre university, department of architecture, via aldo manuzio 68l, 00153, rome, italy ∗ corresponding author: jacopo.andreotti@uniroma3.it abstract. the application of the circular economy model, supported by the european commission, can be the driver of an industrial and architectural conversion and, also, an opportunity to innovate production processes through the elimination of the concept of waste. furthermore, the attention of public and private actors as well as policies and rules are now focused on accelerating the ongoing transition from linear to circular economy. in line with this multi-sectoral shift, the paper intends to illustrate the results of the research activities carried out at the politecnico di torino in cooperation with small and medium enterprises (smes) of the piedmont region (italy). in particular, the article illustrates a double experience developed with two different paths: the cibus’ research project (circular economy in the building sector from agri-food waste) and the innovative master’s degree course design and development the transition to the circular economy. both experiences apply circular economy approaches to grape, hazelnut, and wheat waste to understand whether they could be new “ingredients” for architecture and design. keywords: agri-food waste, circular design, sustainable building products. 1. introduction in recent years, the circular economy (ce), based on the extension of the lifecycle of goods and the elimination of the concept of “waste”, has attracted the attention of public and private actors – governments, academia, businesses – in that it is considered a development model alternative to the current model and on which to invest in. according to the circular economy action plan, the potential of the ce to tackle environmental challenges is considered to be significant in the built environment, due to its high impact [1]. many authors claim that to fulfil the potential that the ce offers, a systemic change is needed in the way products, services, systems, and infrastructures are designed [2, 3]. this change produces a range of new challenges for architects and designers [4], relating to the design process in all its phases. more specifically, circularity criteria may be applied to all the stages of building life cycles, as defined by benachio et al. [5]: project design, manufacturing, construction, operation, end of life. for instance – in the manufacturing and end-of-life stages – one must plan the use of secondary and waste material [6] or at least material that is recycled and recyclable upon end-of-life [7]. in this context, the technology research group of the department of architecture and design at the politecnico di torino, supported by laboratorio sistemi tecnologici innovativi (lastin), has worked for several years together with smes for the development of technical elements and technologically advanced components and materials for architecture and design, and developed circular, short supply chains that may favour the development of local economies. in this regard, we hereby report the research, experimentation, and prototyping work related to the “all you can’t eat” cluster for the valorisation of agri-food waste in the construction industry [8]. growing food consumption and the consequent waste of agri-food products [9] may be considered opportunities, rather than threats, because the majority of such remainders may be potentially employed in numerous sectors, including construction. agrifood waste may be used in thermal insulation [10], masonry [11], plaster [12] and concrete [13]. in this article, we intend to describe an ongoing experience whose goal is the reuse of agricultural waste from grapes, hazelnuts, and wheat as new resources for the construction sector and beyond. the interest in such waste and its potential implementation in different fields was indeed the occasion for the research group to network with three companies – a farm (azienda agricola f.lli durando), a biomedical company (nobil bio ricerche s.r.l.), and a construction material company (sarotto group s.a.s.) – located between the provinces of asti and cuneo and interested in experimenting circular processes with the purpose of – on one hand – triggering industrial symbiosis and – on the other hand – promoting local development. 216 https://doi.org/10.14311/app.2022.38.0216 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 38/2022 experiencing circular design: stories of agri-food waste . . . (a). vine shoots. (b). stalks. (c). pomace. figure 1. three waste types of the grape production. piedmont region grape farmed area [ha] 43.872 quantity picked [t/year] 360.454 waste type vine shoots stalks pomace percentage of end product [%] / 4 15 potential annual availability [t/year] 87.744 14.418 54.068 table 1. synoptic framework of the grape supply chain in piedmont. the experiment involved two closely correlated activities. the first activity – the cibus research project (circular economy in the building sector from agri-food waste), a study performed in the context of the bando talenti della società civile promoted by the foundations giovanni goria and crt – explored the implications of the circular economy model on the design of innovative interdisciplinary agricultural and construction processes. the second experience – the design and development the transition to the circular economy course (master’s degrees in architecture for the sustainability design and in systemic design at the politecnico di torino) – an innovative activity between research and teaching, involved material transformation experiments using waste as “ingredients” and drawing new circularity scenarios to develop design concepts based on waste, recycled, or recyclable materials [14]. the paper focused, in particular, on the reuse of grape processing waste. 2. methods the research activity outlined hereinafter has been performed by the research group within the context of the all you can’t eat cluster, through a consequential method of information analysis and processing. more specifically, the process involved three distinctive stages: analysis of the supply chains; analysis of potential valorisation of waste; identification of circular scenarios and preliminary experimentation. 2.1. analysis of the supply chains: the grape supply chain the analysis of the agricultural waste supply chains was conducted with a methodological approach to understand the common phases of “linear” production – thus not including material recovery and recycling processes: from the field, to the harvest, to processing, to the end product. a crucial step of this phase was the investigation of the waste-generating processes, with an identification of the potentially available types, percentages, and quantities. istat (italian institute of statistics) data, enama (italian body for agricultural automation) data, and data from scientific research were used as a reference to estimate the regional availability of waste based on average production of the end product. upon analysis of oiv (international organisation of vine and wine) data related to the grape supply chain in 2020, it was recorded that italy is the top producer in the world [15] and piedmont contributes 7 % of the national output [16]. nevertheless, upon investigation of the supply chain processes, it was underlined how grape production is distinguished by a considerable quantity of by-products deriving both from the vine maintenance phase and from product processing (table 1). more specifically, three waste types were identified (figure 1): vine shoots, stalks (4 %), and pomace (15 %). in the first case, the by-product is obtained from the annual vine maintenance operations, thus is pruning waste. stalks are the result of crushing, which separates the grapes in bunches, and finally, the pomace is the mixture of the skin and seed residue from racking operations. the data of vine shoots – related to the percentage of residue on the finished product – are not measurable because the by-products are produced by the common annual servicing practices, and they do not directly 217 s. tedesco, e. montacchini, j. andreotti acta polytechnica ctu proceedings (a). brick. (b). paper. figure 2. examples of pomace applications. influence the amount of finished product. nevertheless, there is no generalized recovery and valorisation plan for such by-products. in fact, vine shoots are currently used as biomass for the production of energy, while stalks are used as soil amendments. as for pomace, it is the only by-product destined for recovery processes. according to legge italiana n.238/16 [17], the by-product shall be supplied to distilleries or used as a soil amendment. alternatively, it may be purified to obtain mixtures or molecules. therefore, modern uses of grape by-products contribute to reducing the waste quantity but are not a solution to the issue, because most of the waste is not employed in processes that promote their potential or extend their lifecycle. 2.2. analysis of potential valorisation of waste: vine shoots, stalks, and pomace the investigation of current supply chains has allowed an assessment of the annual waste production, but has also underlined the issue related to its management. in this regard, an exploration of the waste valorisation potential of waste has been conducted, considering the different areas of implementation, including construction and design. the investigation had the aim to identify examples of reuse and recycling that may be classified under two categories: on one hand, commercially available products and the solutions adopted; on the other hand, experimental material resulting from research studies. tools used for such identification included virtual libraries (matrec, material connexion, etc.) and the main scientific publishing platforms (sciencedirect, google scholar, etc.). as for grape waste, pomace is – given its high polyphenol and antioxidant content – the subject of international research. the extraction of such substances may favour the development of new humanuse products in the pharmaceutical, cosmetic, and food sectors [18]. in the fashion sector, interesting experiments were conducted involving pomace for the production of technical biobased fabric [19]. finally, a few examples in the construction field include a research study by the university of parma for the use of pomace to manufacture hollow bricks [20]. the results of the investigation confirmed the limited valorisation of the vine-growing and winemaking supply chain by-products – mainly vine shoots and stalks – as well as the restricted research and experimentation behind recycling in the building sector. 2.3. identification of circular scenarios and preliminary experimentation the study and classification of materials and products were fundamental for the subsequent identification of reuse and experimentation scenarios. based on the exploration of waste valorisation potential, two activities were conducted in parallel. the research was performed throughout the design and development the transition to the circular economy course, and focused on the design of components for architecture and design, functional to the activities performed by partner companies of the project. the mix of architecture and design students from the politecnico di torino has allowed the visualization of new circularity scenarios and the development of design proposals involving waste material. the students presented interesting material concepts for construction and design use. in particular, it has been underlined how pomace may be used to produce: free-standing brick walls with the use of natural binding agents including beeswax; panels; and recycled paper. a few significant examples are outlined as follows (figure 2). for the cibus project, activities focused on the experimentation of new eco-friendly materials for architecture and design. in relation to the winemaking and vine-growing supply chain, the research focused on the development 218 vol. 38/2022 experiencing circular design: stories of agri-food waste . . . density quantity [kg/m3] [g] cl90-s hydrated lime 500 162.24 pomace 202.7 38.42 rice straw 100 2.70 water 1000 178.46 table 2. mix-design prototype 1. density quantity [kg/m3] [g] nhl 3.5 lime 700 227.14 pomace 202.7 38.42 rice straw 100 2.70 water 1000 159 table 3. mix-design prototype 2. of two product types: lightweight plaster and wall panels. the manufacture of lightweight plaster requires lightweight aggregates as identified by uni en 13055:2016 [21], standards, thus dried pomace was analysed to define apparent density (202.7 kg/m3). once the lightweight aggregate classification was defined, a series of prototypes were made using pomace mixed with cl90-s hydrated lime and 3.5 natural hydraulic lime (nhl) as well as rice straw, used as a fibre to prevent crack formation events (tables 2–3 and figures 3–4). as for the grape stalks, medium density fibreboard (mdf) panels including the raw by-product – without the addition of adhesives or binding agents – were experimented with. the prototypes were produced with the application of a constant, 15-metric tonne load for 30 seconds. as done in previous research studies involving pressed panels of agricultural waste [22], the application of such load allowed the material to bind thanks to the lignin in the by-product, and form compact and homogeneous prototypes (figure 5). 3. results and discussion performed within the design and development the transition to the circular economy course, the concept designs by students represent a starting point for experimental activities. the prototypes are based on hypotheses backed by data from scientific literature – for instance, related to the performance features of waste products – and need to be verified and tested in laboratories. experiments and proofs may be the subject of further study in experimental dissertations or research projects and feasibility studies in partnership with companies. within the cibus project, although the experimentation activity is still ongoing, the experiences related to lightweight plasters and mdf panels highlight opportunities and threats in the use of agricultural byfigure 3. prototype 1. figure 4. prototype 2. products for the production of technical elements for building and design. in particular, cohesion issues have arisen in the lightweight plaster samples with pomace at the end of the drying process (28 days), given the features of the by-product employed. in fact, the by-product used for the tests was not a homogeneous material in terms of sizes and components (it included pomace, grapes, and stalks). moreover, the content of organic material – in particular, the alcoholic residue from the vinification process – destabilizes the bond between by-product and binding agent, compromising cohesion. therefore, further testing is required, possibly involving extraction of the waste material at a later stage – directly from distilleries or post-extraction of the molecules/mixtures. as for the pressed grape stalk panels, results are encouraging, and prove that the material may be used in panelling for the architecture and design contexts. despite the prototypes are comparable to mdf wood panels, the introduction of binding agents shall be considered in the performance of new experiments set to achieve better results. the experiments highlight that the theories behind a circular economy in the building sector may be tangible and effective. moreover, they demonstrate: on one hand, the importance of the designers’ knowledge of such topics and their central role in design orientation, choices, and the anticipation of scenarios and solutions; on the other hand, the importance of experimental research in the design process. throughout the course, students had the opportunity to acquire knowledge in the interpretation of complex issues related to the transformation of waste into resources by means of design processes involving them in the first person. furthermore, they developed a critical sense and the ability to design and develop circular economy scenarios, in close contact with real-life challenges requiring both global answers 219 s. tedesco, e. montacchini, j. andreotti acta polytechnica ctu proceedings (a). pressing of stalks. (b). the prototype. figure 5. experimental phase: the sample includes the raw by-product without the addition of adhesives or binding agents. – promoted internationally – and local answers supported by the related tangible actions. the experimental research – conducted in close contact with companies – outlined potential industrial symbiosis scenarios, and involved the design of virtuous processes in which the companies – operating in heterogeneous contexts – may plan to share waste, by-products, services, and know-how. pursuing such corporate policies is functional to obtain significant economic and environmental advantages, and boosts the overall sustainability and competitiveness of production systems, simultaneously reducing industrial process waste and the demand for raw material. 4. conclusion the paper illustrates two experiences – an educational and a research experience, respectively – involving circular economy approaches in the design process and investigating whether agri-food waste can be a new “ingredient” for architecture and design. the circular economy can be a driver of innovation and an opportunity for numerous sectors, including construction. in this perspective, the cibus research projects, as well as the activity performed throughout the design and development the transition to the circular economy course are real examples of experimentation of circularity models, which confirm the growing interest by the scientific community in designing systemic models that foster the exchange of knowledge and material between different sectors. they are also a clear sign of the new challenges that architects and designers will face in the future, such as the inclusion of possible reuse of waste or the environmental impact assessment of new processes and products based on secondary raw materials. they also show the new role that such subjects will need to play as “maestros” of the orchestra among the players, encouraging interdisciplinary design approaches. acknowledgements the following individuals and entities have contributed to the research: prof. roberto giordano, prof. nicolò di prima, azienda agricola f.lli durando, nobil bio ricerche s.r.l., sarotto group s.a.s., with the support of the foundations giovanni goria and crt. this research study involved the students of the design and development the transition to the circular economy interdisciplinary course (ay 2019/20 and ay 2020/21) included in the master’s degrees in architecture for the sustainability design and in systemic design at the politecnico di torino. references [1] european commission. circular economy action plan, 2020. 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[22] c. rojas, m. cea, a. iriarte, et al. thermal insulation materials based on agricultural residual wheat straw and corn husk biomass, for application in sustainable buildings. sustainable materials and technologies 20:e00102, 2019. https://doi.org/10.1016/j.susmat.2019.e00102 221 https://doi.org/10.1016/j.resconrec.2018.10.036 https://doi.org/10.1080/01446193.2018.1435895 https://doi.org/10.1007/978-3-030-36660-5_10 https://doi.org/10.22004/ag.econ.288998 https://doi.org/10.1016/j.egypro.2017.08.147 https://doi.org/10.1016/j.conbuildmat.2011.04.038 https://doi.org/10.1016/j.conbuildmat.2011.04.038 https://doi.org/10.1016/j.conbuildmat.2017.11.140 https://doi.org/10.1016/j.conbuildmat.2017.11.140 https://doi.org/10.1016/j.ijsbe.2016.06.003 https://www.oiv.int/public/medias/7545/it-produzione-di-vino-2020-prime-stime-oiv.pdf https://www.oiv.int/public/medias/7545/it-produzione-di-vino-2020-prime-stime-oiv.pdf https://www.regione.piemonte.it/web/pinforma/notizie/anteprima-vendemmia-2019-dati-dellannata-piemonte https://www.regione.piemonte.it/web/pinforma/notizie/anteprima-vendemmia-2019-dati-dellannata-piemonte https://www.regione.piemonte.it/web/pinforma/notizie/anteprima-vendemmia-2019-dati-dellannata-piemonte https://www.politicheagricole.it/flex/cm/pages/serveblob.php/l/it/idpagina/12012 https://www.politicheagricole.it/flex/cm/pages/serveblob.php/l/it/idpagina/12012 https://doi.org/10.5772/intechopen.76319 https://www.vegeacompany.com https://doi.org/10.1016/j.jobe.2019.100892 https://doi.org/10.1016/j.susmat.2019.e00102 acta polytechnica ctu proceedings 38:216–221, 2022 1 introduction 2 methods 2.1 analysis of the supply chains: the grape supply chain 2.2 analysis of potential valorisation of waste: vine shoots, stalks, and pomace 2.3 identification of circular scenarios and preliminary experimentation 3 results and discussion 4 conclusion acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2024.49.0077 acta polytechnica ctu proceedings 49:77–84, 2024 © 2024 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague image-based random fields in numerical modeling of heterogeneous materials david šilhánek, jan sýkora∗ czech technical university in prague, faculty of civil engineering, department of mechanics, thákurova 7, 160 00 prague, czech republic ∗ corresponding author: jan.sykora.1@fsv.cvut.cz abstract. modeling of spatial variability using random fields is nowadays standard in computational modeling of heterogeneous materials. the only difficulty remains in determining the input parameters of the random field in order to represent the modeled material morphology as accurately as possible. the most frequent method of constructing a random field relies on various covariance kernels that primarily use as input the values of the correlation lengths, which are typically estimated ad-hoc. however, another possibility is to extract input parameters from the modeled material morphology. moreover, this approach enables to calculate the covariance kernel itself. the performance of the image-based procedure is compared with standard methods of random field construction on the response of the two-phase elastic material. keywords: karhunen-loève expansion, random field, monte carlo, two-phase elastic material. 1. introduction the stochastic finite element method is an extension of the deterministic finite element method that introduces randomness into the computational scheme, see [1, 2]. mostly, the input parameters are cast in a probabilistic setting, and random fields (see [3–5]) play a crucial role in this because they allow us to incorporate randomness and spatial variability. so, our goal is to investigate the efficiency of constructing a random field using several different strategies on the material response, which is here represented by the elastic problem of the two-phase medium. 2. estimation of the rve cell the first step in our work is estimating the dimensions of the representative volume element (rve), which means an area with statistically homogeneous properties or at least with properties insignificantly affected by a particular choice. let us consider a two-dimensional heterogeneous composite material with two phases represented by an integer matrix containing only 0 (black color) and 1 (white color), see figure 1 depicting several cut-outs of the original medium. each matrix member corresponds to one pixel in the image and simultaneously to one rectangular element in the finite element mesh. the domain under the study contains different-sized circular white-labeled inclusions with the following prescribed grain-size distribution curve: diameter 2 px (10 %), 4 px (10 %), 8 px (15 %), 16 px (20 %), 24 px (20 %), and 32 px (25 %). its overall dimensions are 10 000 × 10 000 px, and the volume fraction of inclusions is c1 = 0.5. here, the determination of the rve size stems from an engineer’s approach a priori estimating the dimen(a). 150 × 150 px (b). 100 × 100 px (c). 50 × 50 px figure 1. samples of the windows with predefined dimensions taken randomly from the original domain. sions based on easily observable parameters, namely the volume fraction of the white phase and the optimal correlation lengths. authors are aware of a more correct approach to identifying rve size from the statistical analysis of model responses. however, this does not have to be always possible and brings ob77 https://doi.org/10.14311/app.2024.49.0077 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en david šilhánek, jan sýkora acta polytechnica ctu proceedings figure 2. the convergence graph computed for the volume fraction of the inclusions as a function of the window size. the solid line denotes the mean and the light-hatched area represents the ± standard deviation. stacles, especially for the time-demanding numerical simulations. the first study addressing the optimal rve size is built upon the parameter of volume fraction. a window of predefined dimensions is randomly placed 1 000 times in the original medium. consequently, the difference in the studied properties and their statistical moments are then evaluated on the obtained dataset. the entire procedure is repeated for various dimensions of the placed window. the plotted graphs serve for the visual determination of optimal rve dimensions. the results obtained from the first study are shown in figure 2. the mean of the volume fraction is almost constant for all predefined window sizes, contrary to the standard deviation, which steeply converges with increasing dimensions, and the values around the window size of 100 × 100 px reach the final plateau. so, optimal rve sizes in the view of the volume fraction are the dimensions above 100×100 px. the second study focuses on estimating the rve size based on the optimal correlation lengths (lx, ly). the algorithmic procedure is similar to the previous case, except that an additional step of identifying the correlation lengths for every placed window with predefined dimensions is added to the calculation. this step does not significantly increase the computational costs, since the relatively small window sizes enter the optimization process of correlation lengths. the built-in matlab function is used as an optimizer for minimizing the value of error εlopt between the correlation functions cg and ct p p : εlopt = 1 nx · ny nx∑ i=1 ny∑ j=1 |ct p p,ij −cg,ij(lx, ly)|, (1) where nx denotes the window dimension in x-direction, ny denotes the window dimension in y-direction, ct p p is the image-based correlation kernel stored as a matrix and enumerated according to equation (4), cg is the gaussian correlation kernel based on equation (3). the optimal correlation lengths are plotted in figure 3. as we can notice, the values of optimal correlation length become constant for window dimensions greater than 70 × 70 px. moreover, the mean values of correlation length for the same threshold also correspond to the optimal values computed for the entire original domain (10 000×10 000 px), as one can ensure from figure 4, where the absolute values of the error are plotted. the optimal values of correlation length are primarily affected by the spatial distribution of inclusions, the sizes of the inclusions, and the value of the volume fraction. so, the observed evolution of error curves coincides with the study findings examining the effect of volume fraction on the window size. consequently, the influence of window size on optimal correlation lengths is negligible for dimensions above 100 × 100 px and almost disappears for the window sizes above 150 × 150 px. usually, the s2 function is computed using the discrete fast-fourier transform algorithms (dft) [5] to save an enormous amount of computational time. however, this procedure requires the periodic medium as an input, which is not our case of windows randomly placed into the original medium, thus the effect of the non-periodic domain on the final s2 dft-based values needs to be carefully investigated. for this reason, the algorithm computing s2 values for the non-periodic medium has been implemented as a comparison tool with dft-based values. the computational testing protocol is the same as in previous studies and the observed values of error are obtained from the following equation as: εs2,p,s2,np = 1 nx · ny nx∑ i=1 ny∑ j=1 |speriodic 2,ij − snonperiodic 2,ij |, (2) where nx denotes the window dimension in x-direction, 78 vol. 49/2024 image-based rf in numerical modeling of heterogeneous materials figure 3. the convergence of optimal correlation lengths as a function of the window size. the solid line denotes the mean and the light-hatched area represents the ± standard deviation. figure 4. the difference between the optimal correlation length obtained from the entire original domain and mean values of the correlation length computed for the various dimensions of the placed window. ny denotes the window dimension in y-direction, speriodic 2,ij is the i, j-th element of two-point probability matrix computed with the assumption of periodicity according to equation (5), and i, j-th element of two-point probability matrix obtained for the nonperiodic medium with the help of equation (7). as expected, the curves in figure 5 show good convergence of values of error with higher window sizes. therefore, the effect of the periodic assumption on the optimized values of correlation lengths is negligible for the window sizes of our interest, and the fast evaluation of the two-point probability based on the dft algorithm can be performed in our computational scheme. considering the outcomes mentioned above, we decided to proceed further with an rve size of 100 × 100 px. it is a reasonable compromise between the obtained results – the variance of the volume fraction and the optimized correlation length – and computational cost. another enlarging of the window area would not bring a huge benefit since the values of mean computed for the correlation length and volume fraction are further invariable, and their standard deviations reach almost the plateau. 3. numerical example this numerical example is devoted to the assessment of several proposed techniques of random field constructions in stochastic material modeling. since the efficiency of the random fields is demonstrated mostly on the model inputs, the emphasis is here mainly on checking the quality of the prediction of material behavior. to begin with, the ideal elastic model simulating the mechanical behavior is employed for both material phases. their material properties – e young’s modulus and ν poisson’s ratio – differing by an order of magnitude, are summarized in table 1. the geometrical domain with applied boundary conditions is depicted in figure 6. the dirichlet boundary conditions are imposed on the left side of the domain as ux = 0, on the right side as incrementally increasing up to ux = 1.5 · 10−6 m, and 79 david šilhánek, jan sýkora acta polytechnica ctu proceedings figure 5. the comparison of the periodic and non-periodic assumptions on the values of s2 function. phase e [mpa] ν [-] black phase, 0 1 325 0.43 white phase, 1 25 000 0.20 table 1. elastic properties of black and white phase representing the material constants of polypropylene composite materials [6]. figure 6. a scheme of the tensile st problem. finally uy = 0 at bottom left corner. the entire domain with dimensions of 100 × 100 px is discretized into 10 000 finite rectangular elements. the observed responses are displacements ux, uy, and average stresses σx. 3.1. random field the spatial variability of material properties is modeled here by the random fields, which are essentially dependent on the auto-covariance function. since the correct determination of the auto-covariance function plays a crucial role, several methods of constructing this key function are tested within this study. the classical approach, published in literature [4, 5, 7], is based on the approximation of the covariance function by the gaussian correlation kernel, which is a function of a material variance σ2, correlation lengths lx, ly, and the vector of relative position r = (x−x0, y −y0), where (x0, y0) are coordinates of an arbitrarily chosen point (pixel in our case). the gaussian kernel is then expressed as: cg(r) = σ2 · exp ( − (x − x0)2 2 · l2 x − (y − y0)2 2 · l2 y ) . (3) the input correlation lengths are chosen based on the expert’s guess or their values can be estimated from the optimization process minimizing the absolute difference between the gaussian covariance kernel (see equation (3)) and image-based covariance kernel, see [5], derived from a two-point probability function. the latter-mentioned approach represents relatively new concepts of extracting the spatial covariance from images according to the following formula: ct p p (r) = (κ0 − κ1)2 · (s1 2(r) − (c1)2), (4) where κf is the selected material property (e.g. poisson’s ratio) of given phase f , c1 denotes the volume fraction of the white phase, s1 2 is the two-point probability function computed by the fast fourier transformation, see [7], as: sf 2 (r) = idft { dft{χf (r)}dft{χf (r)} } nx · ny , (5) where dft is the discrete fourier transformation, idft is the inverse discrete fourier transformation, · · · stands for the complex conjugate, nx · ny is the image area, f is the given phase, χf is the characteristic function defined as: χf (x) = { 1 if the point x lies in the phase f, 0 otherwise. (6) 80 vol. 49/2024 image-based rf in numerical modeling of heterogeneous materials if the non-periodic medium is considered, the formula used for the evaluation of a two-point probability function arises directly from its definition: sf 2 (x, x′) = ∑(2·nx−dx−1) x=0 ∑(2·ny−dy−1) y=0 χf (x)χf (x′) (2nx − dx) · (2ny − dy) , (7) where x = (x, y) are the coordinates of the starting point, x′ = (x+dx, y+dy) are the coordinates of the ending point. finally, the auto-covariance matrix entering the random field construction is obtained as a resulting covariance computed from equations (3) or (4), which is further reassembled into the row vector. then, this reshaped product determines the first row in a symmetric square toeplitz matrix representing the autocovariance matrix such that ci,j = ci+1,j+1. another possibility of computing the auto-covariance matrix is directly from the definition itself. based on the assumption that our original domain is sufficiently large enough, many independent samples are collected via a window of predefined dimensions randomly placed in the original structure. each of these cut-outs can be then rearranged into a row vector, and subsequently, the auto-covariance is calculated as follows: cs(x, x ′) = e[(x − e(x))(x ′ − e(x ′))]. (8) 3.1.1. karhunen-loève expansion the karhunen-loève expansion is an extremely useful tool representing the stochastic process as an infinite linear combination of orthogonal functions. based on the spectral decomposition of the discretized form of the auto-covariance function c(x, x ′), and the orthogonality of eigenfunctions ϕi, the gaussian random field of parameter κ(ω) can be written as: κ(ω) ≈ κµ + s nϕ∑ i=1 √ φiϕiξi(ω), (9) where ξi(ω) is the standard independent and identically distributed random variable, κµ is the mean of material parameters, s is the standard deviation of material parameters, φi represents a corresponding eigenvalue, nϕ is the number of eigenmodes. smaller values of nϕ lead to more significant dimensionality reduction, and oppositely, higher values give better descriptions of the resulting random field. the log-normal formulation of the random field κln(ω) stemming from the gaussian expression is given by: κln(ω) ≈ exp ( κµ,g + sg ∑nϕ i=1 √ φiϕiξi(ω)√∑nϕ i=1(√φiϕi)2 ) , (10) tag label cov. correlation trimmed kernel length #1 gauss (3) lopt no #2 gauss t (3) lopt yes #3 tpp (4) − no #4 tpp t (4) − yes #5 s (8) − no #6 s t (8) − yes #7 gauss + (3) 2 · lopt no #8 gauss + t (3) 2 · lopt yes #9 gauss ++ (3) 10 · lopt no #10 gauss ++ t (3) 10 · lopt yes #11 gauss − (3) 1 2 · lopt no #12 gauss − t (3) 1 2 · lopt yes #13 gauss −− (3) 1 10 · lopt no #14 gauss −− t (3) 1 10 · lopt yes table 2. description of all tested random field constructions. the first six versions #1–#6 characterized the main approaches of constructing random fields. the rest of them #7–#14 served for simulating the wrong initial guess of experts about correlation lengths. where κµ,g and sg are the gaussian mean and standard deviation, respectively, computed from their lognormal counterparts (κµ,ln, sln) according to the following equations, see [2]: κµ,g = ln ( κµ,ln ) − 1 2s2 ln, (11) sg = √ ln (( sln κµ,g )2 + 1 ) . (12) 3.1.2. description of tested random field all tested versions of random field constructions are summarized in table 2. overall, 14 different versions have been examined with the following specifications: the labels – gauss, tpp, s – refer to the equations employed for assembling the covariance kernels and auto-covariance. since the karhunen-loève expansion delivers the real-valued random fields, the additional label – t – is introduced as another option referring to value filtering. it means that realvalued random fields are trimmed back to the binary values preserving the volume fraction. finally, the correlation lengths indicate the values entering the covariance kernels. the versions with the labels – plus and minus – serve for simulating the wrong initial guess of experts, i.e. the underestimations and overestimations of correlation lengths, respectively. 3.2. homogenization bounds since the initial guess of experts about the inputs entering the covariance kernels is missing here, the theoretical homogenization bounds are exploited for the comparison of our proposed techniques with easily 81 david šilhánek, jan sýkora acta polytechnica ctu proceedings figure 7. the evolution of average stresses as a function of loading steps, nϕ = 50. figure 8. the values of error computed for the average stress as a function of the number of eigenmodes. computed rules. the first homogenization bound is the rule of mixtures, which is given by: κhom = κ1 · c1 + κ1 · (1 − c1), (13) and the inverse rule of mixtures is defined as: κhom = ( c1 κ1 + 1 − c1 κ0 )−1 . (14) κhom is the homogenized material property, and the results stemming from these two equations are labeled as rom and irom, respectively. 4. results for the comparison, it is necessary to introduce the metrics of error. they represent the absolute values calculated as a difference between the reference set of material responses, and the response set having the random fields as an input. the equations evaluating the values of error for the average stress σx and its variance var(σx) are expressed as: εσx = ∣∣∣∣σx,real − σx,rf σx,real ∣∣∣∣, (15) and: εvar(σx) = ∣∣∣∣var(σx,real) − var(σx,rf) var(σx,real) ∣∣∣∣, (16) where the subscript real represents the reference set assembled from 1 000 monte carlo simulations with a domain size of 100×100 px, which is randomly taken from the original structure. then, the subscript rf refers to set collected for the 1 000 monte carlo simulations utilizing the random fields as an input. the first graph (figure 7) illustrates the evolution of average stresses as a function of loading steps for different constructions of random fields. the results are plotted for the fixed number of eigenmodes, which is equal to 50. the true values of the average stresses computed from the reference set are shown by the solid blue line and the homogenization bounds are depicted as dashed blue lines. the first six versions #1–#6 of random field construction are only compared for the sake of clarity. from the presented graph we can see that the trimmed versions show good correspondence with the reference values, in contrast to values carried out for real-valued random fields, which shift to the upper homogenization bound. a similar conclusion is observed from figure 8 depicting the values of error computed for the average stress depending on the number of eigenmodes. as we can see from this graph, including more eigenmodes in the karhunen-loève expansion does not tend to have smaller values of error; however, it has a significant impact on values of error 82 vol. 49/2024 image-based rf in numerical modeling of heterogeneous materials figure 9. the variance error in dependence on the number of eigenvectors included in kle. figure 10. the ratio of the variances computed for the average stresses of the reference set and set assembled from the random field-based simulations #1–#6, nϕ = 250. figure 11. the average stress for rf with arbitrarily chosen correlation lengths nϕ = 50. computed for the stress variance illustrated in figure 9. all trimmed versions achieve the minimum values for nϕ = 200, while the real-valued versions converge with the higher number of involved eigenmodes. the last comparison is finally depicted in figure 10 showing the ratio between the variances of average stresses determined from the reference set and set assembled from the random field-based simulations, respectively. other interesting findings arise from the study of the wrong-determined input correlation lengths caused by the expert guess. it can be seen from figures 11 and 12 that the random fields with the incorrect correlation lengths as an input can have fatal consequences on the material response and can bring no benefit to the probabilistic analysis of the studied problem. from the predicted lines, the splitting into the realvalued and trimmed groups is more observed than the clustering based on the different covariance kernels. 83 david šilhánek, jan sýkora acta polytechnica ctu proceedings figure 12. the ratio of the variances computed for the average stresses of the reference set and set assembled from the random field-based simulations #7-#14, nϕ = 250. taken together, the values of error for the real-valued random fields are large in comparison to other methods, and therefore they do not appear to be adequate approaches for simulating such type of material. 5. conclusion within this contribution, the efficiency of the imagebased procedure of random field construction was studied with classical methods on the elastic problem of the two-phase material. good results have been obtained for trimmed versions of random fields. here, our proposed techniques for identifying properties from images, e.g. correlation lengths or imagebased covariance, are beneficial and can be applied to such problems. unfortunately, these random field constructions can no longer exploit the potential of the karhunen-loève expansion, and their contribution to the stochastic finite method in terms of acceleration is not significant. acknowledgements the authors are thankful for financial support from the student grant competition of ctu, project no. sgs23/152/ohk1/3t/11 and the czech science foundation, project no. 22-35755k. references [1] r. g. ghanem, p. d. spanos. stochastic finite elements: a spectral approach. springer new york, usa, 1991. https://doi.org/10.1007/978-1-4612-3094-6 [2] b. rosić, h. g. matthies. computational approaches to inelastic media with uncertain parameters. journal of the serbian society for computational mechanics 2(1):28–43, 2008. [3] r. j. adler, j. e. taylor. random fields and geometry. springer-verlag new york, usa, 2007. https://doi.org/10.1007/978-0-387-48116-6 [4] j. havelka, a. kučerová, j. sýkora. dimensionality reduction in thermal tomography. computers & mathematics with applications 78(9):3077–3089, 2019. https://doi.org/10.1016/j.camwa.2019.04.019 [5] m. lombardo, j. zeman, m. sejnoha, g. falsone. stochastic modeling of chaotic masonry via mesostructural characterization. international journal for multiscale computational engineering 7(2):171–185, 2009. https: //doi.org/10.1615/intjmultcompeng.v7.i2.70 [6] j.-h. yun, y.-j. jeon, m.-s. kang. analysis of elastic properties of polypropylene composite materials with ultra-high molecular weight polyethylene spherical reinforcement. materials 15(16):5602, 2022. https://doi.org/10.3390/ma15165602 [7] j. havelka, a. kučerová, j. sýkora. compression and reconstruction of random microstructures using accelerated lineal path function. computational materials science 122:102–117, 2016. https://doi.org/10.1016/j.commatsci.2016.04.044 84 https://doi.org/10.1007/978-1-4612-3094-6 https://doi.org/10.1007/978-0-387-48116-6 https://doi.org/10.1016/j.camwa.2019.04.019 https://doi.org/10.1615/intjmultcompeng.v7.i2.70 https://doi.org/10.1615/intjmultcompeng.v7.i2.70 https://doi.org/10.3390/ma15165602 https://doi.org/10.1016/j.commatsci.2016.04.044 acta polytechnica ctu proceedings 49:77–84, 2024 1 introduction 2 estimation of the rve cell 3 numerical example 3.1 random field 3.1.1 karhunen-loève expansion 3.1.2 description of tested random field 3.2 homogenization bounds 4 results 5 conclusion acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2024.51.0121 acta polytechnica ctu proceedings 51:121–124, 2024 © 2024 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague use and access possibilities in the field of training future drivers using a vehicle simulator libor topol czech technical university in prague, faculty of transportation sciences, konviktská 20, 110 00 prague 1, czech republic correspondence: topolli1@fd.cvut.cz abstract. novice drivers between the ages of 18 and 24 have a higher risk of accidents, which results from long-term published statistical data. the goal of training at driving schools should be the comprehensive preparation of future drivers for all traffic situations under various weather conditions. using vehicle simulators allows this practice and training of future drivers, which can be fulfilled and practiced in all weather conditions. moreover, the vehicle simulator practice can be subsequently (machine) evaluated. based on the analysis of such driving, complex erudition and readiness of the future driver behavior in real road traffic can be achieved. this article discusses the capabilities of simulator software that can be applied to the education and training of future drivers. the article aims to present possibilities of evaluation software and related practical use in driver education in driving schools. at the end of the article, the expected benefits in the design of the evaluation software for the vehicle simulator are discussed, emphasizing road safety in the education of future drivers. keywords: evaluation software, novice driver, driver training, driving school, driving simulator, simulator scenario, accident rate, road safety. 1. introduction vehicle simulators are becoming increasingly popular in driver training and help educate young and novice drivers. there are several reasons why simulators can be applied in driving schools. simulator training has been proven to reduce the risk of accidents, allow young drivers to learn from mistakes without consequences, and teach them to drive in a safe and calm environment. moreover, simulator training is cost-effective. 1.1. accidents of young and novice drivers according to statistics from the czech statistical office (čsú), 94 794 traffic accidents were recorded in the czech republic in 2020 [1], with the number of accidents caused by drivers aged 18–24 being 8 743. this means there drivers caused roughly 9.22 % of all traffic accidents in the czech republlc in 2020. this statistic shows that of the total number of 518 people killed in the traffic accidents in 2020, 66 were caused by drivers aged 18–24. these drivers also caused 239 serious injuries and 3 298 minor injuries [2]. it is important to remember that young and novice drivers have a higher risk of being involved in traffic accidents than experienced drivers. therefore, these drivers must gain as much experience and skills as possible through practice and training. 2. vehicle simulators in driving school due to their general characteristics, vehicle simulators contribute to the education of future drivers. this involves training in vehicle control and gaining experience from traffic situations in a simulated environment, which provides safer conditions compared to real traffic. the second significant advantage of vehicle simulators is the reproducibility of individual situations [3]. drivers can be presented with any critical situations they may encounter only once in their lives, but overall safety depends on their correct decision-making. because of these features, the modern interactive vehicle driving simulators [4] are popular in the field of driving education and particularly contribute to acquiring the necessary experience for future drivers [5]. however, nowadays, vehicle simulators only provide driving simulations, during which the future driver gains the correct driving habits and experience to solve complex situations. nevertheless, it is only up to the subjective assessment of the trainer whether the driver has already gained enough driving experience on the simulator and is ready to drive in real traffic. based on the study [6], the trainer’s assessment should be supported by data analyses of the drives. however, with the development in computer technology and the several-fold increase in the computing power of today’s computers, simulators can provide an accurate diving experience (thanks to their welldeveloped mathematical models [7] and relevant hap121 https://doi.org/10.14311/app.2024.51.0121 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en libor topol acta polytechnica ctu proceedings figure 1. a proposed structure of the evaluation protocol. figure 2. diagram of the driver’s lane stability indicator information. tic [8] and motion responses – e.g. [9]) but they are also capable to analyze the course of driving itself and evaluate the correct behavior during driving. with this idea, a newly developed vehicle simulator was created at the faculty of transportation sciences, czech technical university in prague, on which evaluation software for analyzing drivers’ driving was developed [10]. 3. evaluation software of future drivers the developed simulator for teaching in driving schools is a standard design of a single-seater vehicle simulator with 180° projection using 3 32” monitors. all of the vehicle’s controls are obtained using a logitech gaming set modified as part of this development to receive all information from all inputs of the steering wheel module. simulator software was developed in the unity game engine, which has high-quality visual output and is simple to use when creating scenarios [10]. the module itself, which evaluates drivers’ driving, is built on this engine. the driving evaluation software obtains parameters in real time about the environment and the vehicle, the values of the input parameters, the current vehicle position in the virtual scene, the vehicle’s speed, and others. these parameters are recorded and evaluated to provide meaningful information about the quality of the drive based on objective indicators. the indicators are divided into five groups according to the assessed areas. the software’s output provides an evaluation report with driving results from individual areas (see figure 1). in the area of lane-keeping evaluation, the software provides information regarding the stability of the drivers following their lane. the aoo – area of out indicator is evaluated for this information, calculating the proportion of the vehicle’s exit area and the distance traveled. the exit area ts is defined as the sum of the individual areas of the vehicle outside its lane (so). the unit of this indicator is m2 km−1. other indicators are the time spent outside one’s lane, the number of departures, and the maximum deviation from one’s lane as shown in the picture – figure 2. the second evaluated area is the driver’s speed compliance. as part of developing scenarios for a new type of simulator, areas are set up in the virtual environment based on their permitted speed, for example, village areas with a speed of 50 km h−1 and a residential zone with a speed of 20 km h−1. this set of scenarios makes it possible to evaluate whether the driver complies with the speed limits. as part of the evaluation protocol, information is provided from this area regarding the number of speed limit violations, a list of areas in which speed was exceeded, and by what amount the speed was exceeded. information is also provided regarding the time for which the speed 122 vol. 51/2024 use and access possibilities in the field of training future drivers . . . figure 3. the proposed model of the evaluation protocol. has been exceeded and the maximum value of the speed limit. the third area is the evaluation of driving in relation to the surrounding traffic; the time safety distances compared to other vehicles are primarily evaluated. the safety time gap for this evaluation software was set at 2 s. the indicators provided in the evaluation sheet are how long drivers did not keep the safety distance, how many times they failed to maintain the safe time gap and the minimum time gap from the vehicle. if a cyclist appears on the scene, the evaluation software also provides information regarding the lateral distance during overtaking. regarding traffic regulations, the following indicators are evaluated: the number of vehicle crossings at red lights, the number of violations of traffic rules based on vertical road signs, and the list of violated rules. speeding is not assessed in this area. the last group of indicators is focused on the correct control of the vehicle concerning safety and its wear. from the safety point of view, the accurate and timely use of turn signals is primarily evaluated. for each intersection, areas are created where turn signals should be switched on. in these areas, it is possible to assess whether drivers can timely signal their intention of the driving direction and, if not, calculate the delay. the second area assesses parameters for the correct vehicle control that does not lead to vehicle malfunction. the proper vehicle shifting, engine revving, and low-speed driving parameters are evaluated. in this area, the assessed indicators are, for instance, the number of engine shutdowns, the ratio of driving at low to high revolutions, and the number of engine revolutions exceeded. all the evaluated areas are subsequently generated into an evaluation sheet, which contains general information in the header, such as the name of the driver, the date of the simulation, and the name of the simulated scene. currently, the evaluation report is provided through a pdf file generated after the completed drive (see figure 3). 4. conclusion the article further explores the utilization and potential of unity software, which can be enhanced and currently adapted to meet the demands of simulator training for young and novice drivers through further development. with the advancement of computing technologies, there is now the possibility of a multiple-fold increase 123 libor topol acta polytechnica ctu proceedings in the computational power of today’s computers. this allows vehicle simulators to analyze the course of simulated driving and subsequently evaluate both correct and incorrect driver behaviors during the drive. in the individual driving simulation scenarios, the following factors are evaluated and categorized into these areas: evaluation of driving based on lanekeeping, the second area focuses on the driver’s adherence to speed limits, the third area evaluates driving behavior concerning surrounding traffic, such as maintaining safe distances from other vehicles. the fourth area involves evaluating the vehicle’s adherence to traffic regulations. this includes assessing the number of times the vehicle crosses a red light and violations based on vertical traffic signs, as well as ensuring proper vehicle control with a focus on safety and wear, particularly evaluating the correct and timely use of turn signals. further steps in software development could involve providing information through an online database solution and utilizing artificial intelligence (ai) to create individual scenarios. this would enable future drivers to have access to information online, where both the driver and instructor could monitor progress in training. problematic areas for the driver should also be visible. the instructor could then focus on specific areas where the future driver is having difficulties. utilizing this data could also benefit examiners, who could use it during the final exam to systematically test whether the driver safely controls problematic areas. current computer software technologies allow for implementation into vehicle simulators at acceptable prices, even for driving schools. the presented software environment, utilizing virtual reality in scenarios, opens up further possibilities for modernizing and improving software intended for driver training in this field. acknowledgements this work was supported by the student grant competition of ctu – sgs22/124/ohk2/2t/16 “development of the interactive vehicle drive-simulator software with use of state-of-the-art technologies and training methods”. references [1] český statistický úřad. nehody v dopravě – časové řady [in czech; traffic accidents – time series]. [2024-10-11]. https://csu.gov.cz/produkty/nehody_ v_doprave_casove_rady [2] besip. mladí řidiči – statistické analýzy [in czech; young drivers – statistical analysis]. [2024-10-11]. https://test.besip.gov.cz/statistiky/ statistiky-nehodovosti-v-ceskerepublice/statisticke-analyzy/mladi-ridici [3] r. ezzati amini, c. al haddad, d. batabyal, et al. driver distraction and in-vehicle interventions: a driving simulator study on visual attention and driving performance. accident analysis & prevention 191:107195, 2023. https://doi.org/10.1016/j.aap.2023.107195 [4] p. bouchner. interactive driving simulators – history, design and their utilization in area of hmi research. international journal of systems applications, engineering & development 10:179–188, 2016. [5] j. genschow, d. sturzbecher, g. e. willmes-lenz. novice driver preparation – an international comparison. carl schünemann verlag gmbh, bremen, 2013. report on the research project fe 82.0325/2007. isbn 978-3-95606-096-4. https://bast.opus.hbznrw.de/opus45-bast/frontdoor/deliver/index/ docid/741/file/m234b_englisch_barrierefrei.pdf [6] f. alonso, m. faus, j. v. riera, et al. effectiveness of driving simulators for drivers’ training: a systematic review. applied sciences 13(9):5266, 2023. https://doi.org/10.3390/app13095266 [7] p. bouchner, s. novotný. development of advanced driving simulator: steering wheel and brake pedal feedback. in proceedings of 2nd conference on circuits, systems, control, signals, pp. 170–174. 2011. isbn 978-1-61804-035-0. [8] p. bouchner, s. novotný. car dynamics model – design for interactive driving simulation use. in recent researches in applied informatics, pp. 285–289. 2011. isbn 978-1-61804-034-3. [9] s. el hamdani, p. bouchner, t. kunclova, et al. fidelity assessment of motion platform cueing: comparison of driving behavior under various motion levels. sensors 23(12):5428, 2023. https://doi.org/10.3390/s23125428 [10] a. orlický, p. toman, j. svoboda, et al. automatic evaluation system for unity, czech technical university in prague – faculty of transportation sciences, 2019. [software]. 124 https://csu.gov.cz/produkty/nehody_v_doprave_casove_rady https://csu.gov.cz/produkty/nehody_v_doprave_casove_rady https://test.besip.gov.cz/statistiky/statistiky-nehodovosti-v-ceske-republice/statisticke-analyzy/mladi-ridici https://test.besip.gov.cz/statistiky/statistiky-nehodovosti-v-ceske-republice/statisticke-analyzy/mladi-ridici https://test.besip.gov.cz/statistiky/statistiky-nehodovosti-v-ceske-republice/statisticke-analyzy/mladi-ridici https://doi.org/10.1016/j.aap.2023.107195 https://bast.opus.hbz-nrw.de/opus45-bast/frontdoor/deliver/index/docid/741/file/m234b_englisch_barrierefrei.pdf https://bast.opus.hbz-nrw.de/opus45-bast/frontdoor/deliver/index/docid/741/file/m234b_englisch_barrierefrei.pdf https://bast.opus.hbz-nrw.de/opus45-bast/frontdoor/deliver/index/docid/741/file/m234b_englisch_barrierefrei.pdf https://doi.org/10.3390/app13095266 https://doi.org/10.3390/s23125428 acta polytechnica ctu proceedings 51:121–124, 2024 1 introduction 1.1 accidents of young and novice drivers 2 vehicle simulators in driving school 3 evaluation software of future drivers 4 conclusion acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2023.40.0001 acta polytechnica ctu proceedings 40:1–7, 2023 © 2023 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague the experimental evaluation of tensile forces in stays of a cable-stayed footbridge magdaléna boháčová czech technical university in prague, faculty of civil engineering, department of mechanics, thákurova 7, 166 29 prague 6, czech republic correspondence: magdalena.bohacova@fsv.cvut.cz abstract. the topic of this paper is an experimental analysis of the dynamic behaviour of stays of an existing footbridge focusing on determination of the tensile forces in stays. the examined structure was the footbridge across the vltava river in lužec nad vltavou. it is a cable-stayed footbridge, the horizontal load-bearing structure consists of uhpc components, which are prestressed by a pair of external cables. the horizontal load-bearing structure is suspended on a 39.85 m high a-shaped steel pylon by means of a system of 17 pairs of stays. as a part of the work the tensile forces in stays were determined using the vibration frequency method. the method is based on the knowledge of natural frequencies of the stays, which were investigated by the experimental analysis. keywords: vibration, footbridge, cable-stays, vibration frequency method, tensile force. 1. introduction the reason of choosing the footbridge in lužec nad vltavou town as examined structure was the frequent appearance of a problem with oscillation of its longest stays caused by wind. the dynamic experiment focused on the oscillation of stays was carried out in april 2021. natural frequencies of each stay were evaluated using fast fourier transform (fft). the aim was to determine the tensile forces in stays using two simple models within the vibration frequency method (a string model and a simply supported beam), compare the obtained results and specify, which results are more accurate. since the stays are structural elements with a relatively long free vibrating length and with a relatively low bending stiffness, the using of the vibration frequency method was suitable here [1]. there was also an experimental modal analysis and dynamic load test carried out on the structure in october 2021 focusing on the dynamic behaviour of the horizontal load-bearing structure. also, a theoretical analysis of the dynamic behaviour of the structure was performed. the results of the experimental and theoretical dynamic analysis are described in [2]. 2. description of the footbridge the footbridge is located in lužec nad vltavou town nothwards from prague. it crosses vltava river and it creates a pedestrian and cycling connection between lužec nad vltavou town and bukol town. it was put into operation in 2020. it is a cable-stayed footbridge with two asymmetric spans, which are 99.18 m and 31.90 m long. the horizontal load-bearing structure is supported by pair of longitudinally sliding bearings on the o1 abutment and it is frame-connected with the o3 abutment. the height of the frame-connection varies from 500 mm to 1000 mm and its shape is parabolic. the width of the horizontal load-bearing structure is 4.50 m and its total length is 131.58 m. the width of free space for walking and cycling is 3.00 m. the longitudinal slope of the footbridge deck varies along the length from +8.00 % to −8.00 %. the horizontal load-bearing structure consists of uhpc components. the components are made of concrete c 110/130. the length of typical component is 3.998 m and the length of atypical component is 1.998 m. the footbridge deck consists of 31 typical and 2 atypical components. the minimum width of the girders, which are a part of the component, is 400 mm and it varies along the height up to 500 mm in the direction of longitudinal axis of the component. there are linear rises between the girders and the components desk, which thickness is 60 mm. the width of cross girders, which are in 1⁄4 and 3⁄4 of the length of the component (in 1⁄2 of the length of the atypical component), varies symmetrically along the height from 200 mm to 400 mm and there are linear rises between the cross girders and the components desk. the longitudinal slope and transversal slope of the upper surface of the components is ±0.00 % and the longitudinal slope of the footbridge is created by the suitable composition of the components. the height of the cross girder above the abutment o1 varies from 750 mm to 500 mm. the uhpc components are prestressed by a pair of external cables. the diameter of the cables is 15.7 mm and the cables consist of 19 strands. the type of strands is st 1640/1800 mpa by vsl. the horizontal load-bearing structure is suspended on a 39.85 m high a-shaped steel pylon by means of a system of 17 pairs of stays, 5 pair of stays are reverse (see figure 1). the stays are anchored into the massive 1 https://doi.org/10.14311/app.2023.40.0001 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en magdaléna boháčová acta polytechnica ctu proceedings figure 1. the footbridge in lužec nad vltavou town. head of the pylon and into certain components of the horizontal load-bearing structure or into the anchor block on the o3 abutment. the type of stays is “full locked coil strands” by redaelli. the diameter of stays varies from 32 mm to 64 mm. the vertical load-bearing structure is made of pair of pillar struts. the pillar struts are rectangularshaped and the heel dimensions are 1000 × 600 mm, the head dimensions are 600 × 600 mm. the material of the pillar struts is steel s355j2+n and sheet metal thickness is 20 mm. the axial distance of the pillar struts in the heel is 7060 mm, the struts converge into the massive head. there are 12 stays anchoring levels towards lužec nad vltavou and 5 stays anchoring levels towards bukol. the pylon is filled with c 30/37 – xc2 – scc – dmax 16 concrete up to the level of the footbridge deck. 3. the experimental analysis as was said above, one of the aims of the experimental analysis was to find out the current natural frequencies of vibration of the stays of the footbridge, to be able to determine the tensile forces. the experiment was carried out in april 2021. 3.1. measurement line the vibration of the stays was measured by piezoelectric acceleration transducers brüel&kjær type 8344. the working range of these sensors is from 0.2 hz to 3 khz, their sensitivity is very high such as approximately 2500 mv/g and their natural frequency is 10 khz. during the measurement, the sensors were magnetically attached directly to each stay and via cables connected to the 8-channel data acquisition station sirius type 6acc-2acc+ made by dewesoft. all channels have their own unique 2 × 24-bit a/d converter (so called dualcore) that is able to measure with dynamic range up to 160 db. 3.2. measurement system at first, the system of marking the stays was introduced. the level with longest stays, which are the closest to lužec nad vltavou, was signed as “y01”, where “y” means “l” for the left side of the footbridge deck and “r” for the right side (see figure 2). then the sign of the pairs of the stays gradually grows from “y01” to “y17”, where “y17” signs the pair of stays closest to bukol. twenty-six records of vibration of stays in time were acquired. the minimum number of records of vibration of each stay was two. the acceleration transducers were placed in the plane of the stays perpendicularly to the longitudinal axis of the stay (see figure 3). during some records, the transducers were placed also perpendicularly to the plane of the stays. however, these records were then not used for evaluation of natural frequencies of the stays, because the properties of the anchoring of stays (the boundary conditions) in the perpendicular direction are not clear enough. the wind velocity varied between 0 km/h and 20 km/h and was measured by anemometer. 2 vol. 40/2023 the experimental evaluation of tensile forces figure 2. marking the stays – cross section. sign of f1 f2 f3 f4 f5 f6 f7 f8 the stay [hz] l01 1.09 2.17 3.24 4.32 5.40 6.47 7.56 8.62 r01 1.12 2.22 3.32 4.43 5.53 6.63 7.75 8.95 l02 1.14 2.25 3.37 4.49 5.58 6.73 7.85 8.95 r02 1.13 2.22 3.33 4.43 5.53 6.64 7.76 8.88 l03 1.34 2.66 4.00 5.34 6.62 7.98 9.31 10.63 r03 1.32 2.63 3.94 5.25 6.53 7.86 9.18 10.45 l04 1.41 2.81 4.21 5.61 7.00 8.41 9.80 11.18 r04 1.44 2.84 4.27 5.69 7.12 8.53 9.94 11.35 l12 2.37 4.74 7.12 9.43 11.49 13.16 15.19 17.56 r12 2.41 4.85 7.25 9.61 11.69 13.34 15.44 17.88 l15 1.99 3.97 5.96 7.96 9.93 11.83 13.59 15.13 r15 2.04 4.09 6.13 8.16 10.18 12.15 13.87 15.47 l16 1.83 3.65 5.46 7.29 9.12 10.94 12.67 14.33 r16 1.85 3.68 5.52 7.35 9.20 11.03 12.79 14.46 l17 2.97 5.92 8.86 11.75 14.62 17.24 19.40 21.67 r17 2.95 5.89 8.83 11.75 14.59 17.22 19.27 21.47 table 1. the comparison of the values of the tensile forces of the selected stays obtained from the calculation according to the various models. figure 3. acceleration transducers brüel&kjær type 8344 attached directly to a stay by neodym magnet, the silver tape is used as a security tool only. 3.3. evaluation of natural frequencies of the stays at first, the measured data from all channels were transformed using fft from the time domain to the frequency domain (see figure 4 and figure 5). the peaks in a grid of integer multiples were considered as natural frequencies of the investigated stays because this phenomenon is typical for a string behaviour (see figure 4 and figure 5). some of the peaks were relevant to natural frequencies of the footbridge deck (marked by grey colour in figure 4 and figure 5), as was proved within another experiment carried out on this structure [2], they were ignored for this case. there is a summary of evaluated first eight natural frequencies of selected stays in following table 1. 3 magdaléna boháčová acta polytechnica ctu proceedings figure 4. natural frequencies evaluated using fft – the stay no. r01. figure 5. natural frequencies evaluated using fft – the stay no. l02. 4. the determination of tensile forces in stays there are more approaches how to determine the tensile forces in stays within the vibration frequency method. a rod can be modelled as a string, a simply supported beam, an elastically fixed beam or as a fixed beam [2, 3]. in this case two simplest theoretical models of the stays (the string model and the simply supported beam) were used for evaluation of the results. the first reason for choosing these two simplest models was because of the length of the investigated stays, that was large enough. the second reason were the boundary conditions, which correspond with the simple support. 4.1. string model according to this theory, the stay is modelled as a string with no bending stiffness, perfect flexibility, constant mass per metre along the entire length of the string and very low damping. the tensile force ns (j) in the string can be calculated as [4]: ns (j) = ν (2 · f(j) · l j )2 , (1) where µ mass per metre, the data were taken from the manufacturer of the stays, j the serial number of the natural frequency of the stay, f(j) the j-th natural frequency of the stay, l the length of the stay according to the realization documentation [5]. if the stay was a perfect string, the calculated tensile force would be the same for all used natural frequencies. since that is in fact not possible, the calculation of the standard deviation was performed to be able to compare the calculation according to the various theories. there are the calculated tensile forces, their average value and the standard deviation in table 2. 4 vol. 40/2023 the experimental evaluation of tensile forces sign of the stay mass µ [kg/m] length l [m] serial number of the natural frequency j avg. force [kn] std. dev. [kn]1 2 3 4 5 6 7 8 tensile forces [hz] l01 5.7 96.599 252.8 249.4 248.8 247.9 247.8 247.6 248.5 247.0 248.70 1.69 r01 5.7 96.599 267.2 261.8 261.1 260.4 259.9 259.8 261.0 266.0 262.15 2.66 l02 5.7 88.816 234.3 227.9 226.9 226.4 224.1 226.2 226.4 225.1 227.16 2.91 r02 5.7 88.816 228.3 222.3 221.2 220.3 219.7 220.5 220.8 221.6 221.83 2.55 l03 5.7 81.111 267.7 266.1 267.2 267.3 263.1 265.6 265.6 264.8 265.94 1.43 r03 5.7 81.111 263.1 259.2 258.6 258.4 256.0 257.7 257.7 255.8 258.32 2.13 l04 5.7 73.510 244.9 242.3 242.1 241.9 241.5 242.1 241.5 240.4 242.09 1.22 r04 5.7 73.510 255.5 248.4 249.6 249.3 249.8 249.0 248.4 248.0 249.76 2.24 l12 5.7 28.413 103.1 103.5 103.7 102.2 97.2 88.5 86.7 88.7 96.70 7.06 r12 5.7 28.413 107.2 108.1 107.6 106.3 100.6 91.0 89.6 91.9 100.28 7.66 l15 7.2 36.640 153.1 152.3 152.6 153.0 152.5 150.4 145.7 138.3 149.74 4.90 r15 7.2 36.640 161.4 161.4 161.4 160.9 160.3 158.5 151.8 144.6 157.54 5.77 l16 20.4 42.519 491.3 490.0 488.7 490.0 490.8 490.4 482.9 473.3 487.18 5.80 r16 20.4 42.519 502.2 498.1 498.5 497.4 499.4 498.5 492.1 482.0 496.04 5.93 l17 23.2 48.709 1935.6 1925.8 1918.2 1899.9 1881.2 1816.7 1691.1 1615.8 1835.54 112.21 r17 23.2 48.709 1909.6 1906.3 1905.3 1899.9 1873.4 1812.5 1667.7 1585.8 1820.05 117.31 table 2. the tensile forces calculated according to the string model. sign of the stay mass e [gpa] length i×10−8 [m4] serial number of the natural frequency j avg. force [kn] std. dev. [kn]1 2 3 4 5 6 7 8 tensile forces [hz] l01 164.5 3.69 252.8 249.4 248.7 247.7 247.6 247.4 248.1 246.6 248.54 1.78 r01 164.5 3.69 267.2 261.8 261.0 260.3 259.7 259.6 260.7 265.6 261.99 2.66 l02 164.5 3.69 234.3 227.9 226.8 226.3 223.9 225.9 226.0 224.6 226.97 3.00 r02 164.5 3.69 228.3 222.2 221.1 220.1 219.5 220.2 220.5 221.1 221.64 2.63 l03 164.5 3.69 267.7 266.1 267.1 267.2 262.9 265.3 265.1 264.3 265.71 1.54 r03 164.5 3.69 263.1 259.2 258.5 258.3 255.8 257.4 257.3 255.3 258.09 2.27 l04 164.5 3.69 244.9 242.3 242.0 241.7 241.2 241.7 240.9 239.7 241.80 1.40 r04 164.5 3.69 255.5 248.4 249.5 249.1 249.6 248.6 247.9 247.3 249.48 2.38 l12 164.5 3.69 103.0 103.2 103.0 101.0 95.3 85.9 83.0 83.9 94.81 8.53 r12 164.5 3.69 107.1 107.8 106.9 105.1 98.7 88.3 85.9 87.2 98.39 9.12 l15 164.7 5.91 153.0 152.1 152.0 151.8 150.7 147.8 142.2 133.7 147.92 6.30 r15 164.7 5.91 161.4 161.1 160.8 159.8 158.5 155.9 148.3 140.0 155.71 7.19 l16 165.2 47.85 490.9 488.3 484.8 483.1 480.0 474.9 461.8 445.7 476.18 14.30 r16 165.2 47.85 501.7 496.4 494.7 490.5 488.7 483.0 471.0 454.4 485.03 14.52 l17 164.9 61.90 1935.2 1924.1 1914.4 1893.1 1870.5 1801.4 1670.3 1588.6 1824.71 120.95 r17 164.9 61.90 1909.2 1904.6 1901.4 1893.1 1862.8 1797.2 1646.9 1558.6 1809.22 125.90 table 3. the tensile forces calculated according to the simply supported beam theoretical model. 4.2. simply supported beam in contrast to the string model, in this theoretical model the stay is modelled as a simply supported beam with bending stiffness. the same for both theories is constant mass per metre along the entire length of the stay. the tensile force nb (j) can be calculated as [6]: nb (j) = ν (2 · f(j) · l j )2 − ( j · π l )2 ei, (2) where µ mass per metre, the data were taken from the manufacturer of the stays, j the serial number of the natural frequency of the stay, f(j) the j-th natural frequency of the stay, l the length of the stay according to the realization documentation [5], e young’s modulus, the data were taken from the manufacturer of the stays, i moment of inertia. the moment of inertia was calculated for alternative circular cross section, where the area of the alternative cross section was the same as the area of the real cross section, which states the manufacturer of the stays. since the values of mass per metre µ and length l are the same as in table 2, they are not shown again in table 3. 5 magdaléna boháčová acta polytechnica ctu proceedings sign of mass µ length l n ei the stay [kg/m] [m] [kn] [knm2] l01 5.7 96.599 250.1 −50.5 r01 5.7 96.599 262.0 4.0 l02 5.7 88.816 229.2 −63.8 r02 5.7 88.816 223.2 −41.9 l03 5.7 81.111 266.9 −25.3 r03 5.7 81.111 260.2 −48.0 l04 5.7 73.510 243.2 −23.4 r04 5.7 73.510 251.3 −32.5 l12 5.7 28.413 104.6 −25.2 r12 5.7 28.413 108.8 −27.2 l15 7.2 36.640 155.1 −28.4 r15 7.2 36.640 164.0 −34.3 l16 20.4 42.519 493.0 −41.8 r16 20.4 42.519 502.2 −44.0 l17 23.2 48.709 1967.3 −1241.9 r17 23.2 48.709 1955.2 −1273.9 table 4. results. 4.3. the identification of the cable bending stiffness and the cable tensile force since the value of the bending stiffness ei of the stays was not exactly known, it was possible to perform the identification of the bending stiffness ei and the tensile force n using the equation (2) transformed into (3) [7]. n + ( j · π l )2 · ei = µ (2 · f(j) · l j )2 . (3) the eight lowest evaluated natural frequencies were used for calculating the bending stiffness and the tensile force in the stays. based on the equation (3) [7], it was possible to create eight equations for two unknowns.  1 ( 1·π l )2 ... ... 1 ( j·π l )2 ... ... 1 ( n·π l )2  · ( n ei ) =  µ ( 2·f(1)·l 1 )2 ... µ ( 2·f(j)·l j )2 ... µ ( 2·f(n)·l n )2  . (4) this equation system can be solved using gauss markov theorem [7].  1 ( 1·π l )2 ... ... 1 ( j·π l )2 ... ... 1 ( n·π l )2  t ·  1 ( 1·π l )2 ... ... 1 ( j·π l )2 ... ... 1 ( n·π l )2  · ( n ei ) = =  1 ( 1·π l )2 ... ... 1 ( j·π l )2 ... ... 1 ( n·π l )2  t ·  µ ( 2·f(1)·l 1 )2 ... µ ( 2·f(j)·l j )2 ... µ ( 2·f(n)·l n )2  . results of the identification are shown in following table 4. it is clear from the negative values of the bending stiffness of the stays that this method of determining the bending stiffness and the tensile forces is not suitable for this case. 5. conclusions since the experiment was focused on the vibration of stays, twenty-six records of vibration in time were acquired. natural frequencies of each stay were evaluated using fft. as was said above, the aim was to determine the tensile forces in stays using various theoretical models within the vibration frequency method and compare the obtained results. in this case the simplest models were used, such as the string model and the simply supported beam. also, the identification of the bending stiffness and the tensile forces in stays was performed. as is shown in table 4, the results of the identification were unsatisfactory because of the negative values of 6 vol. 40/2023 the experimental evaluation of tensile forces sign of the stay string model simply supported beam avg. std. dev. coeff. of var. avg. std. dev. coeff. of var. l01 248.7 1.69 0.68 % 248.54 1.78 0.72 % r01 262.1 2.66 1.01 % 261.99 2.66 1.01 % l02 227.2 2.91 1.28 % 226.97 3.00 1.32 % r02 221.8 2.55 1.15 % 221.64 2.63 1.19 % l03 265.9 1.43 0.54 % 265.71 1.54 0.58 % r03 258.3 2.13 0.82 % 258.09 2.27 0.88 % l04 242.1 1.22 0.50 % 241.80 1.40 0.58 % r04 249.8 2.24 0.90 % 249.48 2.38 0.95 % l12 96.7 7.06 7.30 % 94.81 8.53 8.99 % r12 100.3 7.66 7.64 % 98.39 9.12 9.27 % l15 149.7 4.90 3.27 % 147.92 6.30 4.26 % r15 157.5 5.77 3.66 % 155.71 7.19 4.62 % l16 487.2 5.80 1.19 % 476.18 14.30 3.00 % r16 496.0 5.93 1.20 % 485.03 14.52 2.99 % l17 1835.5 112.21 6.11 % 1824.71 120.95 6.63 % r17 1820.1 117.31 6.45 % 1809.22 125.90 6.96 % table 5. the comparison of the values of the tensile forces of the selected stays obtained from the calculation according to the various models. the bending stiffness of the stays, it implies that they are not usable in this case. it was expected that the results obtained from the calculation according to the simply supported beam are going to be more accurate. to be able to compare the results, the values of standard deviations of the tensile forces were calculated. since the values of standard deviation and coefficient of variation are lower for the values of the tensile forces calculated according to the string model, the author considers them more accurate. there is a comparison of the values of the tensile forces of the selected stays obtained from the calculation according to the string model and the simply supported beam in following table 5. acknowledgements the results presented in this paper are outputs of the research project no. sgs22/089/ohk1/2t/11 supported by the czech technical university in prague. references [1] t. klier, t. míčka, m. polák, m. hedbávný. overview of the modified magnetoelastic method applicability. acta imeko 10(3):167–176, 2021. https://doi.org/10.21014/acta_imeko.v10i3.1071 [2] m. boháčová. theoretical and experimental analysis of the dynamic behavior of the existing footbridge, 2022. in print. [3] m. polak, t. plachy. experimental evaluation of tensile forces in short steel rods. applied mechanics and materials 732:333–336, 2015. https://doi.org/10. 4028/www.scientific.net/amm.732.333 [4] b. h. kim, t. park, h. shin, t. yoon. a comparative study of the tension estimation methods for cable supported bridges. international journal of steel structures 7:77–84, 2007. [5] novák & partner s.r.o. realization documentation of the structure (rds): the footbridge across the river vltava in lužec nad vltavou, 2020. realization documentation. [6] z. fu, b. ji, q. wang, y. wang. cable force calculation using vibration frequency methods based on cable geometric parameters. journal of performance of constructed facilities 31(4):04017021, 2017. https: //doi.org/10.1061/(asce)cf.1943-5509.0001002 [7] m. polák, t. plachý. determination of forces in roof cables at administrative center amazon court. procedia engineering 48:578–582, 2012. https://doi.org/10.1016/j.proeng.2012.09.556 7 https://doi.org/10.21014/acta_imeko.v10i3.1071 https://doi.org/10.4028/www.scientific.net/amm.732.333 https://doi.org/10.4028/www.scientific.net/amm.732.333 https://doi.org/10.1061/(asce)cf.1943-5509.0001002 https://doi.org/10.1061/(asce)cf.1943-5509.0001002 https://doi.org/10.1016/j.proeng.2012.09.556 acta polytechnica ctu proceedings 40:1–7, 2023 1 introduction 2 description of the footbridge 3 the experimental analysis 3.1 measurement line 3.2 measurement system 3.3 evaluation of natural frequencies of the stays 4 the determination of tensile forces in stays 4.1 string model 4.2 simply supported beam 4.3 the identification of the cable bending stiffness and the cable tensile force 5 conclusions acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2024.46.0009 acta polytechnica ctu proceedings 46:9–16, 2024 © 2024 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague strategic management of the portfolio of urban real estate: research of the current scientific knowledge lucia dobrucká∗, martin maštálka, vladimíra šilhánková czech technical university in prague, masaryk institute of advanced studies, kolejní 2637/2a, 160 00 prague 6 – dejvice, czech republic ∗ corresponding author: lucia.dobrucka@cvut.cz abstract. municipalities possess a wide range of real estate. however, land, buildings, and premises must not only be managed individually, but also optimized and strategically developed as a comprehensive portfolio linked to the objectives of public administration activities and, of course, to the development plans of a particular municipality. nevertheless, a comprehensive view of the issue of managing and building a real estate portfolio in public administration is missing. this is evidenced by the lack of a methodology that would help local authorities with the task. thus, a quantitative analysis of scientific publications was conducted, dealing with the issue of strategic management of real estate portfolios at the city level, both within the czech republic and throughout the world. this article aims to present a qualitative search of the content of scientific publications, thereby contributing to the creation of a foundation for further systematic research into this issue. keywords: asset management, real estate portfolio, public property management, property management, new public governance. 1. introduction cities own the portfolios of diverse real estate properties that need to be not only managed on a daily basis (facility management, property management) but also strategically optimized and developed. however, czech cities lack methodological guidelines on how to do so. their approaches tend to be intuitive, out-of-date, and fragmented among various municipal departments. furthermore, cities often lack professional knowledge of the field, internationally shared best practices, and a persistent focus on efficiency. therefore, the authors searched the global scientific literature and, in this article, present preliminary results on the strategic management of city-owned real estate portfolios. during the initial research, it became apparent that the topic had been neglected to such an extent that there was not even a clear understanding of the current state of knowledge on this issue, from which future research and related proposals of practical modern solutions could be developed. based on the quantitative analysis, the objective was therefore to create a qualitative search of scientific publications that deal with the issue of strategic management of the portfolio of real estate owned by the public administration at the city level. for this purpose, a qualitative analysis of scientific outputs contained in selected databases identified in the first step will be used. the article is divided into two parts. the first part presents a quantitative overview of the scientific literature, which was conducted in the first half of march 2023. regarding methodology, the authors first searched the catalogues of the czech national library – the complex catalogue and the online catalogue [1, 2] – which contain all publications with an isbn or issn issued and/or available in the czech republic. here, authors searched for various combinations of keywords “nemovitosti” [real estate], “portfolio” [portfolio], “strategie” [strategy], “management” [management], “město/obec” [city/municipality], and “veřejné” [public]. second, scopus’s database [3] was chosen to seek global scientific knowledge on the topic. the two main reasons to prefer scopus over web of science [4] were that scopus collected more texts (e.g., the term “real estate management” yielded 5 366 results in scopus and only 3 494 results in web of science), and that scopus included 12 journals and 96 books concerning real estate that were not indexed in web of science. in scopus, authors searched publications with the words “real estate”, “public administration”, “public sector”, “planning”, and/or “urban/city” in their titles. most of the results were journals, books, and conferences. within these, the authors searched for combinations of keywords such as “real estate”, “asset”, “portfolio”, “strategy”, “management”, “city”, “municipality”, “public”, and “public authority property management”. finally, the authors selected abstracts of texts that implied at least three of the requested words (those were considered the most promising). together, 386 abstracts were read and evaluated based on their relevance to the research topic – the strategic management of real estate portfolios owned by cities. the second part of the article presents the content analysis of 386 abstracts. as a result, only 52 abstracts were 9 https://doi.org/10.14311/app.2024.46.0009 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en l. dobrucká, m. maštálka, v. šilhánková acta polytechnica ctu proceedings connected to the topic and only 24 looked to focus on it directly. attempts were made to download these 24 as full texts for further examination. this search was carried out in spring 2023. finally, the authors selected abstracts of texts that implied at least three of the requested words (those were considered the most promising). together, 386 abstracts were read and evaluated based on their relevance to the research topic – the strategic management of real estate portfolios owned by cities. the second part of the article presents the content analysis of 386 abstracts. as a result, only 52 abstracts were connected to the topic and only 24 looked to focus on it directly. attempts were made to download these 24 as full texts for further examination. this search was carried out in spring 2023. 2. quantitative overview of the scientific literature 2.1. czech national library the authors searched the catalogues of the czech national library, the complex catalogue [2], and the online catalogue [1], which hold all publications with an isbn or issn issued and/or available in the czech republic. here, authors searched for combinations of keywords “nemovitosti” [real estate], “portfolio” [portfolio], “strategie” [strategy], “management” [management], “město/obec” [city/municipality], and “veřejné” [public]. in the complex catalogue (publications issued in the czech republic), the keyword “nemovitosti” [real estate] pointed to 3 251 results (69 % in the czech language). however, the combination of keywords led to much lower results. the combination of “nemovitosti” [real estate] and “management” [management] resulted in 16 texts, and the combination of “nemovitosti” [real estate] and “strategie” [strategy] brought 9 texts. no other combination was found (zero texts) in the first catalogue. in the online catalogue (publications available in the czech republic), authors found the triple-combination of “nemovitosti” [real estate], “management” [management], and “strategie” [strategy], which pointed to 17 texts (all of them in the czech language). however, after reading the abstracts of the texts found, the authors excluded all of them from further reading. none of the texts issued/available in the czech republic focused on the strategic management of real estate portfolios owned by cities. only eight texts deal with the topic of facility management. 2.2. scopus the scopus database [3] was chosen to search for global scientific knowledge on the topic. as already noted, the main reasons to prefer scopus over web of science [4] were that scopus contains more texts and that scopus contained 12 journals and 96 books concerning real estate, which were not indexed in web of science. in scopus, authors searched: (a) publications that included the words “real estate”, “public administration”, “public sector”, “planning”, and/or “urban/city” in their title, and (b) combinations of the keywords “real estate”, “asset”, “portfolio”, “strategy”, “management”, “city”, “municipality”, “public”, and “public authority property management”. the authors selected abstracts of texts that contained at least three of the requested keywords, as those were considered the most promising. the following text presents the key findings. more details about the quantitative search can be found in [5]. altogether, 9 492 texts belong to sources that had “real estate” in their source title (12 journals and 98 books). the oldest text comes from 2003, which might explain why no results were found in web of science. most of the texts originated from the united states (4 008 texts), china (2 089 texts), and asian countries in general. in central europe, most of the texts originate in poland (248 texts). the largest number of texts belonged to the categories of “business, management, and accounting” and “economics, econometrics, and finance”. the “social sciences” covered about a third of the texts. the triple combination of searched keywords (“portfolio”, “strategy/management”, and “city/public”) limited the number of texts to 31 altogether (unique texts after excluding dualities), and none of which were from the czech republic. sources with “public administration” or “public sector” in their source title (47 journals and 353 books) contained a total of 32 411 texts. in this category, most of the articles originated in the united states (10 083 texts), the uk (3 983 texts), and australia (2 225 texts). in central europe, most texts come from lithuania (381); however, most of those texts are written in the national language (and were thus incomprehensible). more than 90 % of the texts belonged to the category of “social sciences”, followed by “business, management, and accounting” and “environmental sciences”. triple combinations (keywords “real estate/assets”, “portfolio” and “strategy/management”) led to only two results, none of which were from the czech republic. therefore, a double combination of words was checked as well. most of the results – 244 576 texts – were found in sources with “planning”, “urban”, or “city” in the title (160 journals and 5 566 books). the primary regional origin was the united states (72 818 texts); nevertheless, the second place belongs to “undefined” (38 113 texts). this fact might mean that a large percentage of texts deal with international (or beyond-national) and/or generally applicable topics. in central europe, most of the texts originate in poland (1 510 texts). approximately 65 % of the texts belonged to “social sciences”. the next categories were “environmental science” and “engineering”. however, similar to the 10 vol. 46/2024 strategic management of the portfolio of urban real estate case above, the triple combinations (keywords “real estate/assets/building”, “strategy/management” and “portfolio”) led to only 20 results, none of which were from the czech republic. due to the large number of texts, the double-combination was checked too. following the three searches described above and comparing the weight of individual words (how often they pointed to relevant abstracts), the more holistic term “public authority property management” seemed to be the most appropriate way of conducting the control search. in scopus, this term brought 649 results, which were then reduced to the categories of “social sciences”, “business, management, and accounting” and “economics, econometrics, and finance”. after this reduction, 299 texts (books, articles, and conference papers) remained. as the last step in this chapter, the results were combined and brought 398 abstracts that could be relevant for further reading. notably, there was almost no overlap between the original three searches and the control search. this suggests two alternatives: the research topic did not have any consistency in keywords (pointing to the wide internal heterogeneity of the topic), or the chosen methodology was incorrect. taking this limitation into consideration, the authors proceeded to content analysis. for the purpose of the study, it was necessary to define the differences between the terms used: • facility management: this integrates multiple disciplines in order to have an influence on the efficiency and productivity of economies of societies, communities, and organizations, as well as the manner in which individuals interact with the built environment. facility management affects the health, well-being, and quality of life of much of the world’s societies and population through the services it manages and delivers [6]. • asset management: asset management is the dayto-day running of a wealth portfolio. it is usually headed by an investment manager. the management of assets involves building a portfolio of investments. this includes assessing risks, finding opportunities, and developing an overarching strategy for reaching a set of financial objectives [7]. • real estate property management: property management is the daily oversight of residential, commercial, or industrial real estate by a third-party contractor. generally, property managers take responsibility for day-to-day repairs and the ongoing maintenance, security, and upkeep of properties. they usually work for the owners of investment properties such as apartment and condominium complexes, private home communities, shopping centres, and industrial parks. their main roles are to manage routine tasks delegated to them by the owners and to preserve the value of the properties that they manage while generating income [8]. 3. analysis of the selected scientific literature 3.1. abstracts as mentioned above, 386 abstracts were considered potentially relevant to the research topic – the strategic management of real estate portfolios owned by cities. when reading these abstracts, the authors focused on the research topic, as well as generally on the spread of topics that could be found in those abstracts (based on the combination of keywords, as explained above). the authors acknowledge that this part is vulnerable to the subjective opinion of the authors. to avoid bias as much as possible, the authors read the abstracts individually, cross-compared their individual evaluations, and resolved any commonly discussed disagreements. the range of topics covered by the abstracts is extensive. they deal with investment strategies on real estate markets [9, 10], systems and methods [11–13], performance measurements and valuations [13–16], the impact of actors and/or participation [17–19], the role of fund managers [20] land ownership and management [21, 22], the relationship between business and finance [23, 24], the control function of state institutions and/or legislation [25–28], and many other aspects. in addition, the abstracts suggest that only a very small number of texts handle municipal real estate assets as one holistic portfolio. researchers tend to prefer individual buildings [29] or sub-portfolios composed of the same type, such as the housing sector [30], office market [9], public utilities [31], heritage [32] etc. no commonalities were discovered that would link abstracts from the perspective of keywords, geographical location, time frame, or any other feature. nevertheless, the authors recognized some hints that framed the relevance of the used keywords and the choices of full texts to be read: • the journals with “real estate” in their source title were about private investors. the very limited number of articles pointed to the public sector. • the word “public” in real estate journals pointed almost exclusively to private companies dealing with publicly traded shares. • the word “property” in journals that focus on public administration primarily refer to authorships and ownership rights. • “asset management” in the public sector often resulted in articles in investment funds, especially pension funds. 3.2. full texts after reading the 386 abstracts, the authors selected 52 abstracts that were related to the topic, while only 24 of them focused directly on the strategic management of real estate portfolios owned by cities. subsequently, the researchers attempted to read those 24 texts and focused on four evaluation questions. 11 l. dobrucká, m. maštálka, v. šilhánková acta polytechnica ctu proceedings (1.) does the text deal with how to manage the portfolio of municipally owned real estate properties? (2.) does the text describe methods and tools for managing such a portfolio? (3.) does the text evaluate methods which were used to manage such a portfolio? (4.) does the text consider what is the “optimal portfolio” of municipally owned real estate properties? the four questions were evaluated on a scale of: 0 (not at all), 1 (there are some notes), 2 (there are significant notes), and 3 (it is the main focus). the table 1 shows if and how the 24 selected fulltexts (which were considered the most relevant according to their abstracts) deal with the four questions. a surprisingly high number of texts (8 texts) were not accessible through authors’ standard scopus accounts. authors could not access three books (deakin [33]; manase [23]; pekdemir [10]) and two chapters in deakin’s book (deakin [34]; dent [14]). also, authors had to exclude 3 book chapters (french and hague [15]; hatch [27]; hatch [35]) as well as one article (byrne [24]) from this article due to their inaccessibility. the article by ignjatovic [36] was written in the croatian language, and the authors did not manage to read it. as for the 13 full texts that were considered the most relevant and accessible, most of them acknowledged some relevance to the real estate portfolio and the methods/tools for how to manage it; nevertheless, their relevance was surprisingly low (scores 1 or 2 in more than one question). only three texts achieved high scores (2 or 3 in more than one question): coram and burnes [38]; phelps [11] and uotila, saari, and junnonen [19]. two texts were somewhat relevant to the four questions: lind and lindqvist [12], and pallis and pallis [41]. one text did not focus on the public domain but was considered inspiring – canelas [21] addressed mixed-functional portfolios owned by landlords in london. three more texts referred to municipal property in terms of one building rather than a complex portfolio. this situation suggests that there is a real gap in the scientific literature on the strategic management of real estate portfolios owned by cities. on the other hand, almost all of the full texts examined referred to new literature which should have satisfied the searched keywords (due to their titles, keywords, and topics, as described in section 2); however, these were not found during the search. this fact points to either incorrect searching methodology or the failure of scopus to present all relevant texts. further details about the research topic can be found below. only a few texts addressed the question of what an optimal portfolio of real estate is from the perspective of cities. phelps [11] provided eight dimensions of the optimized portfolio, which could help to fulfil organizational objectives and increase the quality of life of citizens: • cost (reducing the costs of owning and occupying property releases resources for other purposes), • condition (property in good condition provides a safe environment for users and promotes a positive image), • value (property has value which can be released [through disposal] to support corporate objectives), • income (property can be viewed as a “productive asset” capable of generating income to offset running costs), • sufficiency (property of sufficient capacity is necessary to allow services to be provided at the required standard), • utilization (under-used capacity results in excessive running costs, which deprives services of scarce resources), • functional suitability (a building of the right type in the right location acts as a stimulus to effective service delivery), • and client satisfaction (a building that is liked by occupants and visitors is likely to be well used and will meet its service intentions). phelps [11] did not describe any particular method to set up a real estate portfolio, but explored various approaches adopted by 12 councils in the uk. he examined the link between why uk councils carried out property management or asset management, how they did it, and what the results were. phelps [11] stressed the difference between property management and asset management. understanding that difference and shifting to asset management seemed crucial to applying a long-term perspective on how to deal with real estate portfolios. at the same time, however, this understanding was determined by a more innovative, corporate-like, and entrepreneurial organizational culture allied with strong leadership. based on the 12 case studies, phelps [11] identified key change factors such as strategic focus (vision), organizational commitment (including leadership), organizational culture (based on entrepreneurial approach), and portfolio intelligence (its adequate size and knowledge about it). coram and burnes [38] examined one particular major form of organizational change which continues to have a large impact on the public sector: privatization. the article examines the privatization of the property services agency (psa) to draw lessons on how the public sector can and should manage change. although it is a highly informative and instructive example of criticism of privatization, the article addresses the question of how to perform property management and certainly does not recommend privatization of property management; ultimately, the article does not deal with how the administration of portfolio “management” should take place in order to achieve optimal management or what the optimal portfolio should be. 12 vol. 46/2024 strategic management of the portfolio of urban real estate public sector private sector citation note c ity po rt fo lio m an ag em en t? m et ho ds an d to ol s fo r effi ci en t m an ag em en t? ev al ua te s effi ci en cy of de sc rib ed m et ho ds ? d efi ne s op tim al re al es ta te po rt fo lio ? c ity po rt fo lio m an ag em en t? m et ho ds an d to ol s fo r effi ci en t m an ag em en t? ev al ua te s effi ci en cy of de sc rib ed m et ho ds ? d efi ne s op tim al re al es ta te po rt fo lio ? 1 arkesteijn and volker [37] 0 0 0 0 2 byrne [24] n/a 3 canelas [21] 3 3 1 2 4 coram and burnes [38] 3 3 3 0 1 1 1 0 5 danyliuk and dmytryshyn [29] 0 0 0 0 6 deakin [33] book 7 deakin [34] n/a chapter in deakin 2019a 8 dent [14] n/a chapter in deakin 2019a 9 dmytryshyn [17] 0 0 0 0 10 french and hague [15] n/a chapter 11 gammans [39] 0 0 0 0 12 hatch (2015) [27] n/a chapter 13 hatch and manuel [35] n/a chapter 14 higgins [40] 1 0 0 1 15 ignjatovic [36] n/a croatian 16 ladu [18] n/a 17 lind and lindqvist [12] 2 1 1 1 18 mattaricci and pekdemir [10] n/a book 19 ngwira and manase [23] n/a book 20 pallis and pallis [41] 2 0 1 0 21 phelps [11] 3 3 3 2 22 sayce and connellan [13] 1 1 1 0 23 shaw [42] 1 1 0 1 24 uotila, saari, and junnonen [19] 2 1 0 0 scale: 0 (not at all), 1 (there are some notes), 2 (there are significant notes), 3 (it is the main focus). table 1. overview of the availability of 24 selected texts and their relevance to the topic. 13 l. dobrucká, m. maštálka, v. šilhánková acta polytechnica ctu proceedings the article by uotila, saari, and junnonen [19] describes the case study detailing the participation of citizens and stakeholders in the public-school campus process in southern finland. the process was incorporated into the facility and real estate portfolio management framework. canelas [21] investigated how london landlords managed their real estate portfolios. although landlords are private subjects, their portfolios cover large neighbourhoods and offer commercial and public functions. therefore, the curatorial form of placemaking adopted by many cluster managers – which involves actively “selecting the mix of uses and users, unit placement, and investing in public space to increase footfall, customer circulation, dwell time, and spending” (p. 232) – could be inspiring for municipalities too. more detailed practices included tight control over leases, the establishment of a retail environment on the ground levels (economy), office/residential units on upper floors (social mix), a judicious combination of well-known and unknown brands, and other strategies. public space, its quality, and permeability are considered one of the most crucial aspects for success. from the examined articles, it is still worth mentioning danyliuk and dmytryshyn [29] and the follow-up research of dmytryshyn [17], which did not deal with municipal real-estate portfolios. however, their research could be useful for czech municipalities that have brownfields and other unused buildings/areas in their portfolios. danyliuk and dmytryshyn [29], and dmytryshyn [17] offer a methodology (based on community planning) and a set of 50 indicators from diverse perspectives (interior, exterior, environmental, historical and cultural, and financial) to help municipalities decide how to handle abandoned properties. 4. conclusions as preliminary investigations have shown [5] the issue of the optimal management of real estate portfolios at the city level has been relatively neglected by the scientific community. the aim of this article was therefore to create a qualitative analysis of scientific publications dealing with the issue of the strategic management of the portfolio of real estate owned by public administration at the city level, based on a quantitative analysis, and to create a foundation for further research into this issue and a basis for proposing practical, modern solutions. during the initial research (using the online databases of the national library of the czech republic [1], skc – union catalogue of the czech republic [2], web of science [4], and scopus [3]), it was found that the topic had been largely neglected. no relevant scientific literature was found in the context of the czech republic, and a search of the scopus database showed that this issue was neglected internationally, and with some exceptions [42], it did not find a clear state of current knowledge on this issue. within the quantitative analysis, 398 abstracts were found: publications that had been published in a thematic journal and contained at least three keywords when searched or searched through the term “public authority property management” and reduced focus. of these, only 52 texts (articles, books, and conference papers) were connected to the topic, of which only 24 outputs related directly to the researched issue. none of the findings concern the strategic management of real estate portfolios owned by cities in the czech republic. furthermore, the findings did not demonstrate any common points in terms of keywords, territorial affiliation, or time limits. this topic appears to be neglected, thus making it pertinent for systematic scientific considerations and research. of the 24 selected relevant texts, eight were unavailable, so an in-depth analysis of the texts could only be conducted for 16 of them. among the unavailable sources, there were also three books that dealt directly with the issue of property at the municipal level: deakin [33], ngwira and manase [23], and mattarocci and pekdemir [10]. as part of further investigation, these books must be procured and analyzed in detail. finally, 15 texts were examined in depth (one of 16 was in croatian, so the team was unable to read it). only three texts – coram and burnes [38]; phelps [11], and uotila, saari, and junnonen [19] – were evaluated with high relevance to the monitored topic. two texts were somewhat relevant to the proposed four research questions: lind and lindqvist [12] and pallis and pallis [41]. one text did not focus on the public domain but was deemed inspiring – canelas [21], which addressed mixed functional portfolios owned by landlords in london. three more texts referred to municipal property in terms of one building rather than a complex portfolio. given the overall low number of relevant and available texts, this indicates a lack of anchoring of the examined issue. despite the doubts expressed in the methodological part, this situation suggests that there is a real gap in the scientific literature dealing with the strategic management of city-owned real estate portfolios. based on the analyses carried out, it can be asserted that no common point in the form of key themes, territorial, or time limits is evident in the examined texts. the only exceptions are book publications from the beginning of the second decade of the 20th century in the usa, which are inaccessible to researchers or cannot be obtained through the usual scientific sources. it can therefore be said that, on a global scale, there are very few publications on the issue of strategic management of the portfolio of public real estate at the city level and that this issue is not comprehensive. however, as the analysis showed, there are real estate management strategies and practices in the private sector, such as integrated management, integrated project financing, land development, ppp projects, portfolio strategies, etc. [10, 43–45]. although these are not easily trans14 vol. 46/2024 strategic management of the portfolio of urban real estate ferable to our public sector, given that the context of public administration in the czech 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for designing and optimizing additively manufactured variable-stiffness composite laminates yogesh gandhia,∗, julián noratob, ana pavlovica, giangiacomo minaka a alma mater studiorum – università di bologna, department of industrial engineering, via fontanelle 40, forlì-47121, italy b university of connecticut, department of mechanical engineering, 191 auditorium road, u-3139 storrs, ct 06269, united states ∗ corresponding author: yogesh.gandhi@unibo.it abstract. a method for designing laminates is presented using geometry projection to optimize the layout of additively manufactured variable-stiffness composite laminates. by considering fiberreinforced bars as geometric primitives, the geometry projection methodology is extended to include optimizing regions with intersecting load paths. this is achieved by utilizing a dual representation of bars, which considers the geometric parameters and the element-wise density field representation. the dual representation enables the combining and overlapping of bars, resulting in a localized orthotropic material response at overlapping regions that mitigates the transverse compliant response of fiberreinforced components. the proposed method’s effectiveness is demonstrated through minimizing the compliance of the messerschmitt-bölkow-blohm beam problem, a well-known benchmark problem in topology optimization. keywords: topology optimization, geometry projection, continuous fiber-reinforced polymers, variablestiffness laminates. 1. introduction our study focuses on developing and optimizing variable-stiffness composite laminates (vscls) that can be produced using additive manufacturing. these vscls consist of layers with different orientations of fibers, which are customized to achieve specific mechanical properties, such as maximizing the stiffness-to-weight ratio. designing vscls involves considering various factors, including fiber orientation, stacking order, and layer thickness. unlike constant stiffness composites [1], optimizing vscls, as discussed elsewhere [2], is challenging due to the absence of analytical formulations. therefore, designing vscls necessitate the use of discretization methods, such as the finite element method (fem), to represent variations in material constituents. this leads to increased design variables, making the optimization process computationally intensive. as a result, computational design tools like topology optimization (to) have become crucial for achieving the desired mechanical performance. these tools can also accommodate modifications resulting from advanced manufacturing processes [3], such as continuous fiber-fused filament fabrication (cf4). topology optimization (to) is an iterative process for finding the optimal material distribution in a design domain, minimizing an objective function under constraints. various methods include homogenization, solid isotropic material with penalization (simp) [4], level-set [5], evolutionary structural optimization, phase field, and feature mapping [6]. details on these approaches have been reviewed elsewhere [7, 8]. various techniques have also been advanced to determine the optimal distribution of fiber orientations within the design domain, as reviewed elsewhere [9]. in this paper, a geometry projection (gp) based [10] procedure is formulated to enable the design of vscls. while using fiber-reinforced bars (frbs) as features, we modify the gp methodology in multiple ways. initially, we leverage the dual nature of the geometry projection method to define overlapping frbs in the design domain. these overlapping frbs are then retained and modeled using composite laminate theory to calculate homogenized stiffness matrices, resulting in a local orthotropic material response. additionally, forming overlapping frbs in the design domain reduces the high strain energy density at intersecting load paths, which is further amiable for optimization. the component-wise gp formulation enables the seamless printing of vscls with the clear manifestation of overlapping components. nonetheless, the proposed method to design vscls is limited to a single layer, which we call gp-am. 22 https://doi.org/10.14311/app.2024.48.0022 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 48/2024 gp-am 2. geometry projection formulation for variable-stiffness composite laminates the gp procedure begins by mapping the design variables of the bar to a density field ρb (x; zb), where x represents any point in the design region. the dual geometric parameters/density representation can consider individual bars as high-level geometric objects or field variables in component densities. the design in this work involves the combination of multiple frbs. each bar b ∈ b, where b denotes the set of all bar indices, is represented as a rectangle with semicircular caps whose medial axis is a line segment, which occupies a region ωb ∈ r2. the medial axis is characterized by its two endpoints (x1b,x2b), and the offset distance corresponds to the radius rb of the bar. a membership variable αb ∈ [0, 1] is assigned to each bar and penalized similarly to density-based methods, allowing the optimizer to remove or include it in the design. the design variable vector zb for bar b is therefore expressed as: zb := (x1b,x2b, rb, αb) . (1) the projected density at a location x is determined by the intersection of a ball with a radius r and centered at x with ωb, i.e.: ρb(x; zb) := |br x ∩ ωb (zb)| |br x| . (2) in 2d, under the assumption that r is significantly smaller than the dimensions of the bar, the intersection of br x and ∂ωb can be approximated as a line segment. as a result, the proportion of the projected density can be determined by calculating the proportion of the circular segment with a height of h = r − ϕb, where ϕb represents the signed distance from x to ∂ωb. the projected density for bar b is a uniquely determined function of ϕb, effectively functioning as a regularized heaviside function: ρb(x; zb) := h̃ ( ϕb(x; zb) r ) . (3) the expression for h̃ is given. h̃(x) =  0, if x ≤ −1 1 + 1 π ( x √ 1 − x2 − arccosx ) , if |x| < 1 1, if x ≥ 1 dh̃ dx = { 2 √ 1 − x2/π, if |x| < 1 0, otherwise . a penalized density is determined for each bar which is utilized to compute its elastic stiffness tensor. this is similar to the simp technique employed in densitybased topology optimization [11]. the penalized density is expressed as: ρ̆eff b (x; zb) := (αbρb(x; zb))q , (4) where we recall q is the penalization factor. 2.1. combining components eq. (2) pertains to the projection of a single bar. this study aims to focus on continuous fiber reinforcement, particularly in areas where multiple reinforcements overlap. this requires the material interpolation to accurately represent the stiffness that arises from overlapping bars. a straightforward approach is to compute the element’s elasticity tensor as the sum of all elasticity tensors: ce = cv + nb∑ b=1 ρ̆eff be (cb − cv) . (5) the elasticity tensor interpolation ce given by (5) can be used to interpolate between the solid material (bar material cb) and void material cv. 2.2. elasticity tensor for overlapping components the bar’s material coordinate system (mcs), represented by {ê1b, ê2b, ê3b}, is typically not the same as the laminate coordinate system (lcs), {e1, e2, e3}. we assume that the fiber reinforcement aligns with ê1b, and we define ê2b so that it is orthogonal to ê1b. furthermore, we define e3 = ê1b × ê2b such that it corresponds to the global out-of-plane axis. for plane stress of the laminate, the components of the elasticity tensor cb of bar b in lcs are given by cp b = t ⊤ 1 ĉ p bt 1 and cs b = t ⊤ 2 ĉ s bt 2, (6) with t 1 = [ c2 s2 cs s2 c2 −cs −2cs 2cs c2 − s2 ] , t 2 = [ c −s s c ] , (7) where, c and s denote the cosine and sine of the angle θ respectively, where θ represents the angle between the axes ê1 and x. the 3×3 matrix ĉ p b establishes the relationship between the in-plane strains {εx, εy, γxy} and the stresses {σx, σy, τxy} in lcs. similarly, the 2 × 2 matrix ĉ s b governs the relationship between the out-of-plane shear strains {γxz, γyz} and the stresses {τxz, τyz} in lcs. 2.3. elemental laminate elasticity matrix for a laminate that is symmetrical with respect to the middle plane (z = 0) and consists of 2nb layers, the membrane-bending coupling matrix dmd is equal to zero. we utilize the first shear deformation theory (fsdt) to analyze the mechanical behavior of vscls. in the fsdt, we relax kirchhoff’s theory by considering that the transverse normal no longer remains 23 y. gandhi, j. norato, a. pavlovic, g. minak acta polytechnica ctu proceedings material e1 e2 v12 g12 g13 g23 carbon epoxy as4/3501-6 113.6 gpa 9.650 gpa 0.334 6.0 gpa 6.0 gpa 3.1 gpa table 1. material properties used for all the examples. perpendicular to the mid-plane (z = 0) after deformation. this implies that the elastic displacement field within the laminate is expanded by as: u(x, y, z) = u(x, y) + zψx(x, y) v(x, y, z) = v0(x, y) + zψy(x, y) w(x, y, z) = w0(x, y), (8) where u0(x, y) and v0(x, y) represent in-plane displacements, while w0(x, y) represents the out-of-plane displacement. the subscript (·)0 indicates the displacement field of the reference plane. using four-node, bilinear, quadrilateral, and planestress elements, we construct element stiffness matrices based on the abovementioned assumptions. the stiffness matrix for the eth element can be represented as: dme = nb∑ i=1 [hi+1 − hi] cp ei; dse = nb∑ i=1 [hi+1 − hi]κcs ei; dde = nb∑ i=1 1 3 [ h3 i+1 − h3 i ] cp ei. (9) each bar is defined by the top and bottom planes hi+1 and hi. the shear correction factor is κ = 5/6. additionally, we assume that the layer thickness h is uniform, which means that (hi+1 − hi) = h, and we also assume that the effect of stacking on the bending stiffness is disregarded, with hb = 0, leading to (h3 b+1 − h3 b) = h3 for all bars. 3. optimization problem and sensitivity analysis we address the minimization of compliance for a specified volume fraction limit. we formulate the optimization problem as follows: min{zb} f := log(c+ 1) subject to: v ≤ v̄ ku = f zi ≤ zi ≤ z̄i, i = 1, 2, . . . , nz, (10) where compliance, denoted by c = u⊤f , and u and f represent the global displacement and force vectors, respectively. the value v̄ sets an upper limit on the volume fraction, while k represents the global stiffness matrix. the interval [zi, z̄i] defines the lower and upper bounds on the ith design variable. figure 1. design region, boundary conditions, and initial design for beam in 3−point in-plane bending. the volume fraction, v, can be defined as v(e) := 1∑ e ∣∣ω(e) ∣∣ ∣∣∣ω(e) ∣∣∣ ∑ b ρeff be . (11) finally, for the design-independent loading, the sensitivity of eq. (10) and eq. (11) is as follows: ∂zic = − ∑ e u⊤ ( ∂zik (e) ) u ∂ziv = ∑ e ∣∣ω(e) ∣∣ ∂ziv (e)∑ e ∣∣ω(e) ∣∣ . (12) the stiffness matrices design sensitivities da, where {a = m, d, s} in eq. (9) can be expressed in matrix form as follows: ∂zide =  ∂zidme 03 03×2 03 ∂zidde 03×2 02×3 02×3 ∂zidse  . (13) for the matrix 0n, it’s a zero matrix of size n× n, and for 0m×n, it’s a zero matrix of size m× n. the chain rule is used to compute the design sensitivity of eq. (13). this requires finding the derivative of the transformation in eq. (6) and the penalized effective density. the computation of these derivatives is straightforward but has been left out for brevity. more detailed information can be found elsewhere [10]. 4. examples we consider bars made of cfrp; fiber orientation is continuously aligned to the bar’s axis. table 1 lists unidirectional carbon-epoxy as4/3501-6 material properties used for the bars. the following settings are considered until mentioned otherwise. the method-of-moving-asymptotes (mma) [12] optimizer is used for the optimization routine, with the default parameters described in here [13]. the void material assigned with a young’s modulus of evoid = 0.001 gpa and a poisson’s ratio of vvoid = 0.3. in the initial design (see figure 1), bars have a radius 24 vol. 48/2024 gp-am design combined density 0 1 2 3 table 2. optimal designs and combined density for the mbb beam. the first column corresponds to the compliance value obtained by design, i.e., c = 0.0905 kn · mm. in the second column, transparency is used in the color of the bars for the gp-am designs to help distinguish overlapping components. the color of the bars for the optimal designs in the last column corresponds to their penalized membership variable value αq b, and the bars are plotted with transparency to facilitate the visualization of all bars. 0 100 200 300 10−1 100 iteration f f0 0 100 200 3000.2 0.3 0.4 0.5 iteration v v0 figure 2. objective (left) and volume constraint (right) history for the mbb beam design of tab. 2 corresponding to the gp-am design. equal to the average of their upper and lower bounds during initialization. the sizing variable, α, is set to 0.5, and the move limit, m, is fixed and set to 0.02 throughout the optimization process. the procedure of optimization entails three criteria for stopping. the first criterion is achieved when the 2-norm of the alteration in the vector of design variables is smaller than 0.002. the second criterion is attained when the kkt (karush-kuhn-tucker) optimality condition norm is below 0.002. the third criterion is fulfilled when the modification in the objective function is less than 10−9. the optimization process is stopped if any of these criteria are met. 4.1. beam in 3-point bending the example considers a 150 mm × 50 mm simply supported beam in 3-point bending, as illustrated in figure 1. the maximum volume fraction is 0.5. because the problem is symmetrical, only the right side of the structure is modeled. initially, the entire plate is designed with 27 bar, and the design variables are constrained within the following bounds.{ (0, 0) 2 0 } ≤ { (xb1, xb2) rb αb } ≤ { (150, 50) 5 1 } (14) in this situation, the highest bending stress occurs in the upper left corner where the load is applied, and the normal stresses cause tension on the bottom edge and compression on the top edge, with significant shear stresses near the neutral axis. additionally, the optimal design for a single load application aligns bars’ fiber reinforcement axis with the principal stress at each location [14]. in table 2, it can be observed that there are horizontal bars at the top and bottom edges to represent the normal stress distribution, while inclined bars are used for the shear stress distribution. the overlapping bars contribute to a larger second-moment area of inertia, resulting in higher bending stiffness, which optimally supports the applied load. furthermore, the method proposed in this study yields an optimal solution comparable to the findings of other density-based methods for the minimum compliance problem. figure 2 illustrates that the optimization process follows a usual convergence pattern for compliance; there is a significant decrease in the initial iterations, accompanied by minor adjustments to the design variables in the subsequent optimization iterations. additionally, it displays a standard smooth convergence behavior. 5. conclusions the numerical experiment results demonstrate the effectiveness of the gp-am in generating optimal solutions for the minimum compliance problem. from a manufacturing standpoint, the designs obtained with the proposed methods are amenable to manufacturing techniques for vscl. although not presented here for brevity, the proposed method produces good designs for several design regions, for example, problems: rectangular plate in pure torsion; cantilever beam under inand out-of-plane bending; and square membrane in out-of-plane bending. 25 y. gandhi, j. norato, a. pavlovic, g. minak acta polytechnica ctu proceedings 6. acknowledgement financed by the european union-nextgenerationeu (grant no 2022nw83re_002). the opinions expressed are those of the authors only and should not be considered representative of the european union or the european commission’s official position. neither the european union nor the european commission can be held responsible for them. references [1] h. ghiasi, d. pasini, l. lessard. optimum stacking sequence design of composite materials part i: constant stiffness design. composite structures 90(1):1–11, 2009. https: //doi.org/10.1016/j.compstruct.2009.01.006 [2] h. ghiasi, k. fayazbakhsh, d. pasini, l. lessard. optimum stacking sequence design of composite materials part ii: variable stiffness design. composite structures 93(1):1–13, 2010. https: //doi.org/10.1016/j.compstruct.2010.06.001 [3] j. plocher, a. panesar. review on design and structural optimisation in additive manufacturing: towards next-generation lightweight structures. materials & design 183:108164, 2019. https://doi.org/10.1016/j.matdes.2019.108164 [4] g. i. n. rozvany. a critical review of established methods of structural topology optimization. structural and multidisciplinary optimization 37:217–237, 2009. https://doi.org/10.1007/s00158-007-0217-0 [5] n. p. van dijk, k. maute, m. langelaar, f. van keulen. level-set methods for structural topology optimization: a review. structural and multidisciplinary optimization 48:437–472, 2013. https://doi.org/10.1007/s00158-013-0912-y [6] f. wein, p. d. dunning, j. a. norato. a review on feature-mapping methods for structural optimization. structural and multidisciplinary optimization 62:1597–1638, 2020. https://doi.org/10.1007/s00158-020-02649-6 [7] o. sigmund, k. maute. topology optimization approaches. structural and multidisciplinary optimization 48(6):1031–1055, 2013. https://doi.org/10.1007/s00158-013-0978-6 [8] j. d. deaton, r. v. grandhi. a survey of structural and multidisciplinary continuum topology optimization: post 2000. structural and multidisciplinary optimization 49:1–38, 2014. https://doi.org/10.1007/s00158-013-0956-z [9] y. gandhi, g. minak. a review on topology optimization strategies for additively manufactured continuous fiber-reinforced composite structures. applied sciences 12(21):11211, 2022. https://doi.org/10.3390/app122111211 [10] h. smith, j. a. norato. topology optimization with discrete geometric components made of composite materials. computer methods in applied mechanics and engineering 376:113582, 2021. https://doi.org/10.1016/j.cma.2020.113582 [11] m. p. bendsøe, o. sigmund. material interpolation schemes in topology optimization. archive of applied mechanics 69:635–654, 1999. https://doi.org/10.1007/s004190050248 [12] k. svanberg. the method of moving asymptotes – a new method for structural optimization. international journal for numerical methods in engineering 24(2):359– 373, 1987. https://doi.org/10.1002/nme.1620240207 [13] k. svanberg. mma and gcmma, versions september 2007. optimization and systems theory 104, 2007. [14] p. pedersen. some general optimal design results using anisotropic, power law nonlinear elasticity. structural optimization 15:73–80, 1998. https://doi.org/10.1007/bf01278492 26 https://doi.org/10.1016/j.compstruct.2009.01.006 https://doi.org/10.1016/j.compstruct.2009.01.006 https://doi.org/10.1016/j.compstruct.2010.06.001 https://doi.org/10.1016/j.compstruct.2010.06.001 https://doi.org/10.1016/j.matdes.2019.108164 https://doi.org/10.1007/s00158-007-0217-0 https://doi.org/10.1007/s00158-013-0912-y https://doi.org/10.1007/s00158-020-02649-6 https://doi.org/10.1007/s00158-013-0978-6 https://doi.org/10.1007/s00158-013-0956-z https://doi.org/10.3390/app122111211 https://doi.org/10.1016/j.cma.2020.113582 https://doi.org/10.1007/s004190050248 https://doi.org/10.1002/nme.1620240207 https://doi.org/10.1007/bf01278492 acta polytechnica ctu proceedings 48:22–26, 2024 1 introduction 2 geometry projection formulation for variable-stiffness composite laminates 2.1 combining components 2.2 elasticity tensor for overlapping components 2.3 elemental laminate elasticity matrix 3 optimization problem and sensitivity analysis 4 examples 4.1 beam in 3-point bending 5 conclusions 6 acknowledgement references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2024.47.0104 acta polytechnica ctu proceedings 47:104–107, 2024 © 2024 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague analysis of the effect of hydration heat resulting in the formation of cracks on the experimental block of the spillway at orlik reservoir simona potůčková∗, milan holý, jiří kolísko czech technical university in prague, klokner institute, šolínova 7, 166 08 prague, czech republic ∗ corresponding author: simona.potuckova@cvut.cz abstract. this article concentrates on the numerical analysis of hydration heat resulting in formation of cracks on the experimental block of the spillway at orlik reservoir and its verification with the experimental measurement. in order to eliminate the maximum of critical factors which could lead to appearance of early age cracking and faulty execution of the spillway, an experimental block was concreted. it served as a trial block for all steps of the execution process as well as a validating temperature measurement during cement hydration to confirm with the preliminary numerical analysis. keywords: hydration heat, experimental measurements, numerical analysis, spillway. 1. introduction high hydration heat is a common complication while talking about massive concrete structures. usually, we would try to avoid such complication by concreting in single layers of maximum thickness of 0.3 m, which nowadays might be seen as time consuming. especially now when there is emphasis on fast production in every field, civil engineering meets the same requirements. hence it is needed to study the behaviour of massive structures concreted in one go and to establish general rules to follow in order to avoid any kind of complications. according to czech standards and regulations below, the structure is considered to be massive: • the čsn 73 1208 (the design of waterworks concrete structures) [1] – any structure thicker than 2 m is considered massive. • the čsn en 13670 (execution of concrete structures) [2] – defines as massive any structure thicker than 1 m. • technical quality requirements defined by the ministry of transport, chapter 18, concrete constructions and bridges (řsd tkp 18) [3] – a massive structure is any structure thicker than 0.6 m, which is also nonnegligible regarding hydration heat. the main complication in massive structures is caused by uneven diffusion of hydration heat generated during cement hydration within the cross-section. the difference between the core temperature and temperature on the surface could be defined as an average thermal gradient. it is also recommended to limit the maximum core temperature of the structure because it could affect the final compressive strength of the concrete. another complication due to the high temperature could be formation of secondary ettringite which may also cause cracking. that is why it is recfigure 1. visualisation of the open part of the chute spillway (so 03 on the left) [5]. ommended to not overstep the maximum temperature 70 °c during hydration of cement [4]. 2. the spillway of the orlík dam the experimental measurement and the numerical analysis were carried out at the klokner institute of czech technical university in prague under the supervision of povodí vltavy, state enterprise, which is the investor of the project “orlík dam – securing the orlík dam against the impacts of extreme floods”. the project documentation for the construction of the spillway and experimental block was prepared by the company aquatis, ltd. [5] and then executed by metrostav, ltd. [6]. the geometrical shape of the experimental block is based on one of the chute floor sections of so 03 (figure 1), with a maximum gradient of 40 % (figure 2), plan dimensions of 7.5 m × 12.0 m and a variable construction thickness of 1.5 m – 1.8 m (figure 3), all concreted at the same time [5]. 104 https://doi.org/10.14311/app.2024.47.0104 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 47/2024 effect of hydration heat resulting in the formation of cracks figure 2. floorplan of the chute with marked expansion (green lines) and work joints (red lines) [5]. figure 3. the geometry of the experimental block (dimensions in mm) [7]. extensive temperature measurements were carried out at the construction site for a period of 28 days. encardio type eds-20v-e string strain gauges with a sensitivity of 1 µ strain and a range of 3 000 µ strain and thermometers were used during the measurements (figure 4). metrostav ltd. secured and managed the concreting of the test block on 2nd november 2022 (figure 5). prior to the concreting of the block, the klokner institute, with the consent of the company aquatis, ltd., arranged the installation of a total of 26 strain gauges (figure 6). the observation run for 28 days, and the aim of the study was to control the maximum temperature in the core of the block and the temperature difference between the core and the surface in order to limit the cracking of the surface. 3. results and discussion the results of experimental measurement were then compared with the results of the preliminary numerical analysis. the main requirement for maximum core temperature was to not exceed the limit value specified in the project documentation tmax = 65 °c, while the average thermal gradient also should not exceed the limit value ∆t = 25 °c m−1 [5]. figure 4. the location of strain gauges on the test block (dimensions in mm) [7]. figure 5. finalisation of the surface. the maximum core temperature predicted by atena simulation was 47.5 °c while the maximum core temperature measured on the experimental block was 49.6 °c. based on these results and the graph below (figure 7), the numerical model corresponds closely to the experimental measurement and could be used for further numerical analysis. e.g., the construction is subjected to different boundary conditions (ambient temperature) within its execution throughout the year, hence a parametric study with complementary experimental measurements was also carried out [8]. 105 s. potůčková, m. holý, j. kolísko acta polytechnica ctu proceedings figure 6. installation of strain gauges at the construction site. figure 7. comparison of the evolution of hydration heat during cement hydration – experiment and simulation (marked by red line) [7]. distance between 2 sensors – 0.75 m thermal gradient for distance of 1 m [ °c m−1] position strain temperature temperature average gage difference gradien [n°] [ °c] [ °c] [ °c m−1] b 3 32.47 17.11 22.84 49.58 d (core) 10 31.88 17.26 23.011 49.14 g (core) 25 31.46 17.69 23.627 49.15 table 1. thermal gradient measured within the experiment – position marked “g” is placed in the core of the experimental block and compared to atena simulation [7]. the difference between the core temperature and the temperature on the surface, which could be defined as “average thermal gradient”, causes uneven strains within the cross section, the emergence of compressive stresses in the core of the cross-section, due to the expansion of the concrete, and tension on the surface of the structure. the experimental block was concreted in november 2022, whereas the day temperature was from 15 °c to 19 °c and the night temperature was around 10 °c during the first week after concreting the test block, when the core temperature peaked as well. as it is also visible from the table 1, the weather conditions were very favourable and the maximum average gradient did not exceed the limit value ∆t = 25 °c m−1. it should also be taken in consideration that the average gradient could be the limiting factor for concreting the figure 8. comparison of the cracking observed on the test block (marked with white lines) and the numerical analysis (marked with red lines) [7], [8]. figure 9. microscopic picture of a crack of 0.02 mm width [7]. blocks during summer or winter and corresponding curing of the concrete surface should be designed. according to the picture above, the cracking calculated within the nonlinear analysis corresponds closely to the cracking observed on the test block after 28 days (see figure 8). the size and the location of the cracks is also alike and the normative serviceability limit state requirements for maximal width of crack which cannot be greater then 0.2 mm was also not overstepped (see figure 9) [9]. 4. conclusion the numerical analysis of the impact of hydration heat evolution and thermal gradient was carried out as a part of the project “orlík dam – securing of the orlík dam against the impacts of extreme floods” [7]. the accuracy of the numerical model was confirmed by experimental measurements which were carried out 106 vol. 47/2024 effect of hydration heat resulting in the formation of cracks in co-operation of klokner institute of ctu with the company metrostav, ltd. verified numerical model is further used for a parametric study where the impact of different factors is analysed [8]. the experimental measurement is also going to be repeated throughout the year to be able to monitor the influence of ambient temperature on maturing concrete block and to verify corresponding numerical models. acknowledgements we would like to thank the state enterprise povodí vltavy for the opportunity to publish the results, and the companies metrostav, ltd. and aquatis, ltd. for their cooperation. the article was compiled with the support of the grant rvo 31251 122 1222101j000 1nd. references [1] navrhování betonových konstrukcí vodohospodářských objektů [the design of waterworks concrete structures]. standard, czech standard institute, prague, 2010. [2] provádění betonových konstrukcí [provádění betonových konstrukcí]. standard, czech standard institute, prague, 2010. [3] betonové konstrukce a mosty [concrete constructions and bridges]. technical quality requirements of structures, ministerstvo dopravy české republiky, 2016. [4] beton pro konstrukce [concrete for constructions]. technical quality requirements of structures, správa železnic, 2022. [5] aquatis, ltd. construction documentation: orlík dam – securing of the orlík dam against the impacts of extreme floods, 2023. [6] metrostav ltd. execution documentation: orlík dam – orlík dam – securing of the orlík dam against the impacts of extreme floods, 2023. [7] klokner institute ctu in prague. vd orlík – so 03 open chute spillway – partial report on the experimental measurement and numerical analysis of the impact of hydration heat on the formation of cracks on the experimental block zb4 at orlík reservoir, 2022. [8] klokner institute ctu in prague. vd orlík – so 03 open chute spillway – analysis of the impact of hydration heat on the formation of cracks – parametric study, 2023. [9] červenka, v. et al. atena program documentation, part 1 – theory, 2018. [10] navrhování betonových konstrukcí – část 1-1: obecná pravidla a pravidla pro pozemní stavby [design of concrete structures – part 1–1: general rules and rules for buildings]. standard, czech standard institute, prague, 2004. [11] beton – specifikace, vlastnosti, výroba a shoda [concrete – specification, performance, production and conformity]. standard, czech standard institute, prague, 2021. [12] cement – část 1: složení, specifikace a kritéria shody cementů pro obecné použití [cement – part 1: composition, specifications and conformity criteria for common cements]. standard, czech standard institute, prague, 2012. [13] metody zkoušení cementu – část 9: stanovení hydratačního tepla – semiadiabatická metoda [methods of testing cement – part 9: heat of hydration – semi–adiabatic method]. standard, czech standard institute, prague, 2010. 107 acta polytechnica ctu proceedings 47:104–107, 2024 1 introduction 2 the spillway of the orlík dam 3 results and discussion 4 conclusion acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2024.46.0017 acta polytechnica ctu proceedings 46:17–29, 2024 © 2024 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague cities in transition: krakow‘s social, economic and spatial transformation within the last thirty years (selected aspects) anna karwińska, dorota jopek∗, michał kudłacz, michał wiśniewski cracow university of economics, college of economy and public administration, rakowicka 27, 31-510 kraków, poland ∗ corresponding author: jopekd@uek.krakow.pl abstract. krakow is a city that has undergone an immense transformation in recent years. at the end of the 1980s, the period of communism ended and polish local governments began to operate based on the principles of subsidiarity, decentralisation and independence. in 2004, poland joined the european union, which opened up new prospects for development, including in regional and local terms. the world has become a networked and digitized entity susceptible to the influence of innovation. due to metropolitisation processes, cities, especially those offering above-average earnings, a diversification of economic resources and a relatively high quality of life, began to develop even more dynamically, which had and still has negative consequences. in poland, there are five cities in which development dynamics have a highly specific dimension. apart from the capital city of warsaw, these are krakow, wrocław, poznań and the agglomeration of the so-called tri-city (gdańsk, sopot and gdynia). the aim of this study is to show how krakow, the second-largest city in poland, has evolved over the last three decades at the level of demographics, urban development and economy; how it changed in the historical “milestones” indicated above. therefore, depending on the availability of statistical data, several time periods were selected and compared with each other in the most important areas of krakow’s functioning. the research hypothesis is as follows: krakow’s development policy in key areas optimally uses the opportunities resulting from contextual conditions (social, economic and cultural changes that have taken place in recent decades). the study focuses on the three main problems of the city’s development (the abovementioned key areas of the city’s functioning and development): socio-demographic, spatial and economic changes. the analysis assessed data from the literature, city reports and other available sources, as well as public quantitative data (assessment of data from the national census for the years 1988, 2002, 2011, 2022; local data bank of statistics poland and other repositories and reports). keywords: city, central european city, city development, transformation, krakow. 1. introduction in the last thirty years, polish cities, similarly to other cities in central and eastern europe, have gone through a long process of quantitative and qualitative changes related to the departure from the socialist system, central planning, authoritarian governance, and a non-market, planned economy. this transformation has often been extremely difficult, due to socialist residues in practical operation in different spheres of city functioning, the way municipal policies are implemented, social attitudes and mental orientations of the members of a city’s community. furthermore, it is worth remembering that, in a sense, the very idea of the city as a new type of settlement unit, included the expectation that it would be a “well-functioning” environment, a true place created by people for themselves, right from the very beginning. throughout the period since the first cities were founded, this type of social, economic and spatial organisation has often been criticised. at the same time, attempts have been made again and again to create an “ideal city” that would meet people’s expectations and ambitions. the socialist city, that was to provide equal access to a city’s resources, good living conditions, and an environment for shaping social relationships, among other things, was – in its assumptions – a concept of a “wellfunctioning city”. as it turned out, these intentions remained unrealised, and after three decades since the fall of communism, post-socialist cities continue to be hindered by numerous limitations and the weight of the past, as well as challenges of the future. the transition ushered in a period of urban development that followed free market principles. cultural and economic exchanges have resulted in major changes in both socio-economic and spatial terms, particularly noticeable in large cities where these processes were more intense. it is interesting to note that the proportion of the urban population in poland has been declining for more or less two decades, which is linked to numerous factors [1]. one can point to the increasing proportion of people with tertiary education in professions that can offer their services remotely, which has been made 17 https://doi.org/10.14311/app.2024.46.0017 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en a. karwińska, d. jopek, m. kudłacz, m. wiśniewski acta polytechnica ctu proceedings easier by modern technological advancement. another important factor is the development of individual motorisation and, in the immediate vicinity of major cities, also public transport. also important is the increase in affluence, which grants greater freedom to organise one’s life especially in increasingly democratic conditions, the emergence of new motivations, new value systems, and the spread of cultural patterns from highly developed countries. there are five metropolises that currently observe the fastest changes and development processes in poland. together with warsaw, the capital city of poland, these are krakow, wrocław, poznań, and the so-called tri-city agglomeration (gdańsk, sopot and gdynia) [2]. while the warsaw metropolitan area is populated by ca. 3 million people, the other four cities have populations that range between 1 and 1.5 million. today, krakow is poland’s second-largest city. to a certain extent, it is a genuine case of a post-industrial city of central europe that found new development tools that reflect the conditions of the globalist era [3]. however, the rapid changes accelerate many complex and unwanted processes of the city’s evolution. we see krakow’s development as approaching the idea of a “well-functioning city”. we observe this through many aspects: the way space is organised, the functioning of the city as a social collective, the local economy, the cultural system, the political system (municipal governance and citizen– government relations) and many others. the objective of this study is to indicate where the city of krakow currently stands in terms of its development, particularly concerning the socio-demographic sphere, the city’s economy, the quality of the organisation of space and its forms of use. the choice of aspects for analysis was dictated both by the significance of their impact on krakow’s transformation – of its society, space and economy – and by the availability of data in each thematic scope. the complexity of these subjects forces us to focus on the development of krakow as the only case and not to compare it with the other polish metropolises. the differences between those cities resulted in many ways from the conditions of their development during the post-war period, including the scale of wartime destruction, the scale of urbanisation and industrialisation as well as role of private property in the land use structure. moreover, a comparison of polish cities was already discussed in many studies [4]. seeing such problems as far from the main topic of this paper, we decided to focus on krakow as a representative case of the development processes of polish metropolises. this paper was based on existing statistical data, primarily from the four national censuses (1988, 2002, 2011, 2011), data from the local data bank (bdl) analyses and data contained in the reports on the state of the city of krakow issued yearly by the municipality of krakow, the publications krakow in numbers [5], and various reports prepared by other institutions that operate in the city. the authors are aware that the use of existing data prepared for different purposes always involves matching data to research aims and objectives. this sometimes makes data from such multiple sources hard to compare. however, it should be highlighted that the use of this information carries significant benefits in the form of access to data that has been accumulated over several years. our analysis was supplemented with qualitative data from accessible studies, which we believe enhanced our analysis. 2. overview of the area under study the transformations that take place in the city include the appearance of elements of new structures, new processes, new uses, while others disappear or are modified. these new phenomena are found in all urban subsystems: social, spatial, cultural or economic. the period considered in this article consists of the three decades of transition initiated by the change of the political and economic system in poland after 1989. the focus here is on the transformation of krakow, which is an example of a city that has been developing over the centuries in an evolutionary way. it is one of the oldest and most complex urban systems in poland [6]. even the earliest of the city’s plans show the features of the urban layout that can be observed today. we can find the spatial identity of the city by analysing the structure and manner of spatial development over the course of several centuries of history, and the centre of krakow, the old town, looks identical on contemporary plans as it did in the 19th or early 20th centuries. notably, the beginning of the 20th century saw new developmental impulses related to a deepening of the city’s supra-local uses and the expansion of industry, which contributed to the acceleration of the urban community’s transformation, the emergence of new spatial structure elements, especially the development of manufacturing spaces, as well as economic and cultural change. this transformation intensified after the world war ii due to the concept of developing krakow as a major industrial centre. the excessive growth of manufacturing spaces, the hasty development of housing spaces that favoured “temporary” solutions that connected successive estates to existing transport and utility systems, as well as to social infrastructure, created imbalances in the functioning and development of the city, especially in areas with new development projects. after 1989, krakow’s development goals were redefined, with a particular emphasis on achieving and strengthening its position as one of the main national centres in the political, economic, cultural and academic spheres, but also, ultimately, building its importance as a european-scale metropolis. we intend to identify the trajectories of the transformations that have taken place in the last three decades 18 vol. 46/2024 cities in transition: krakow‘s social, economic and spatial . . . (a). civic budget value in krakow (in millions of pln) [7]. (b). voting turnout for 2016–2021 [7]. figure 1. civic activity in krakow in 2016–2021. in the above-mentioned spheres of urban life, and to analyse their significance for the well-functioning of the city. at present, krakow belongs to a group of supra-regional metropolises and occupies the highest position within it due to its ability to initiate and carry development impulses. it has diverse, rich development resources, both tangible and intangible. it is characterised by a very large surplus of central functions with a market character, which defines its significance as a growth centre (see figure 1). krakow covers an area of 327 km2 and is divided into 18 districts. according to the latest data, 803 282 people lived in krakow as of 31 december 2022 and the population density was 2 457 people per 1 km2. women accounted for 53.3 % of the population (number of women: 428 411) and 8 265 babies were born (in 2021: 8 671). year-to-year, there was a 699 increase in population, a 3.6 % decrease in marriages, an 8.8 % decrease in deaths and a 4.7 % decrease in live births [8]. 3. demographic and social transformation of krakow in the last three decades as mentioned above, the concept of the “wellfunctioning of a city” that we adopted refers to the various areas of collective life (including culture, social relations and economic relations), the material framework in which this collective life plays out, i.e., the way in which urban space is organised and used, the way in which power is exercised, the level of citizenship and self-governance. we are not able to cover all of these areas within the limited volume of this paper, and there is also the limited availability of data that shows different spheres of urban life and the city’s development. when considering the “well-functioning of a city” from a sociologist’s point of view, it should be pointed out that the following features in particular are important here: • the capacity to meet the needs of residents (both as individuals and societal collectives), ranging from essential to self-realisation; • the capacity to create a community; • the ability to sustain development through the identification, maintenance, and enhancement of existing and potential resources (tangible and intangible), and the skilful management of these resources. the possibility of achieving this state of wellfunctioning is, to a significant extent, linked to the characteristics of the city’s collective of residents and users, which is why we intend to explore this issue here. among the most important transformations of krakow as a community, since the beginning of the 1990s, we should point to demographic changes, the recomposition of social structures, including changes in the size of particular groups and strata, and the emergence of new ones. patterns of social inequality, the ways in which social distances are expressed, are modified in relation to these transformations. the last thirty years have seen an increase in the population of krakow. currently, krakow is the second-largest city in poland in terms of population and area (see table 1). distinct demographic processes recorded in the last three decades include a change in the proportion of groups with tertiary, secondary and below-secondary education (see table 2). preliminary results from the 2021 national census indicate that there is a significant increase in educational attainment across all of society, with the most rapid growth in the number of people with tertiary education. significantly, this growth is more pronounced in cities. there is also regional variation: the highest number of people with tertiary education was noted in the masovian voivodeship, followed by the lesser poland voivodeship. it is also important to note the predominance of women in the group with tertiary education [12] (see table 2). in krakow alone, the percentage of residents with tertiary education was higher than in the entire voivodeship. in 1988 – 16.0 %; in 2002 – 20.1 %; in 2011 – 31.6 % [13]. krakow, according to a report prepared by fdi 19 a. karwińska, d. jopek, m. kudłacz, m. wiśniewski acta polytechnica ctu proceedings 1988 2002 2011 2021 2030 2040 2050 [in thousands] projections [in thousands] women 390.9 402.9 404.0 404.0 400.9 388.7 377.1 men 355.7 355.5 355.5 373.9 348.4 339.3 333.4 total 746.6 746.6 757.6 800.6 749.3 728.0 710.5 table 1. changes in krakow’s population in the years 1988–2021 and population projections for krakow in 2030, 2040 and 2050 [9, 10]. year tertiary secondary vocational primary other 1988 7.2 24.4 25.3 38.2 4.8 2002 10.5 32.4 26.0 27.2 3.9 2011 17.2 31.1 22.8 22.1 1.1 table 2. changes in the educational structure of the inhabitants of lesser poland voivodeship in the years 1988–2011 (percentage) [11]. intelligence (foreign direct investment), is a city in which there are good investment conditions due to its business-friendliness, the presence of high-quality human capital, and a general climate conducive to quality of life. the second important demographic process is the change in the proportions between age groups. in the years 2010–2020, the age dependency increased, as for every 100 working-age people in 2010 there were 53 people in retirement age, in 2020 there were 71 [10]. the ageing of the population in krakow will intensify in future decades, which implies the need for changes in social policy, healthcare, and prevention. in addition, there is a growing need to develop certain programmes and institutions to support the representatives of the oldest age groups, so that they can continue to function independently and retain their ability to perform their professional or social roles for as long as possible. it is also worth mentioning here that, according to the population projection for krakow for 2030, the predominance of women will increase, which will necessitate new concepts for solving the problems of the oldest group of residents (among other things, the fact that women tend to have lower pensions) (see table 1). overall, in thinking about the future of the city as a well-functioning residential environment, senior policy will become increasingly important. in krakow, it is coordinated by the representative of the president of krakow for senior citizenship policy. the new 2021–2025 social activity and inclusion programme for seniors, which has been developed, includes an ever-expanding range of measures that foster the maintenance and enhancement of social activity, psycho-physical fitness, and social well-being linked to the satisfaction of higher-order needs. it is also worth emphasising that these programmes are subject to public consultation so that they respond as closely as possible to the real needs of this group of residents. another extremely important demographic process, which is a reflection of global trends, is migration. of course the composition of urban communities has always been changing, even thousands of years ago, and people flowed into certain places in search of freedom, opportunity or change, but they also moved to other cities from their own, often for similar reasons. these multi-directional migrations have always fundamentally influenced the way a city functions and develops, and have had varying degrees of intensity, and depend on many factors. in krakow, multiculturalism is embedded into city’s history and has also, to some extent, been the basis for its development as an important european centre. in modern times, krakow’s diversity began to increase after 1989, as a result of the transformational “opening up to the world”, with this process accelerating after poland’s accession into the european union in 2004. the last two years have seen a significant increase in the influx of forced migrants and refugees from ukraine. according to available data, it was estimated that one in five residents of krakow in 2022 was a foreigner and that the dynamics of the influx of foreigners was increasing significantly [14]. importantly, the gender structure of this population has been changing over the last few years, as the proportion of women has been increasing, although this is a phenomenon that occurs to a higher degree in only certain age groups. the biggest changes in the structure by gender, as indicated by research, can be seen in the age groups between 15 and 29 years of age. in 2022, female dominance emerged. there were almost 127 women per 100 men in the 20–24 age group and 123 women in the 15–19 age group [14]. krakow has considerable potential to become a global city in which multiculturalism plays a positive role by dynamising change and enhancing creativity and innovation, through the possibility of confronting different views and solutions, as well as cultural diffusion processes. factors that facilitate this include, among others, the development of tourism, the significant share of foreign tourists among visitors to the city, the growth of multinational corporations in 20 vol. 46/2024 cities in transition: krakow‘s social, economic and spatial . . . krakow, the international activity of universities, including the presence of foreign students who choose krakow as the place where they want to study. all this fosters a foreigner-friendly “climate” that facilitates adaptation and integration into city life, and in turn, from the point of view of city development, allows better use of the important development assets that newcomers bring [15]. it was mentioned earlier that a well-functioning city is one in which a community of residents and users emerges. of course, this is about community in the cultural sense, one that goes beyond the functional ties that arise from the shared use of space or the use of the same institutions. the actions of various municipal institutions which can be conducive to resident integration, but can also contribute to exclusion or non-acceptance phenomena, play an essential role here. the municipality of krakow has been running the open krakow programme for years, and its importance has currently increased. towards the end of 2020, krakow joined the intercultural cities programme, a joint initiative of the council of europe and the european commission [16]. ensuring a wellfunctioning city requires the development of a network of various actors from various areas of city life. this means that there is a need for new patterns of communication between different actors, new attitudes that enhance cooperation and enabling work on common goals [17]. one of the most important elements in building a community and integrating residents is the correct functioning of the municipal government. although the restoration and development of local selfgovernment has been ongoing since the beginning of the transformation, one would be hard-pressed to claim that we are dealing with a steady and continuous development of how local governance can operate and effectively implement local policy. the legal, organisational or financial environment of local government is not entirely conducive to local development policy and requires numerous amendments [18]. another important element of community building is to give citizens the opportunity to have a say in the affairs of the city, especially the implementation of certain plans that affect the individualised needs of various groups which operate in the city. such an opportunity is provided, first and foremost, by the civic budget, whose significance has been systematically increasing (see figure 1). the foundations of social order and the rules that govern interactions between various entities in social life change as local democracy develops and the scope of the impact that residents have on decision making increases. the results of surveys from a few years ago indicated that the idea of the civic budget had taken root in the social consciousness of krakow’s residents. they are familiar with the concept and recognise the benefits associated with the opportunities created by participating in project formulation and voting. they also drew attention to the integrative aspect of the civic budget, although at the same time they pointed out that the scale of these projects was too small [19]. 4. city planning a spatially well-functioning city should refer to both the way in which space is organised, e.g., in terms of the availability of essential uses, and respect for the city’s existing resources (e.g., the natural environment and cultural heritage). a well-functioning city is a planned city, whose development follows a predefined scenario that considers the protection of the city’s resources and the needs and aspirations of its inhabitants [20, 21]. from an anthropocentric point of view, the most important elements of urban space are those, which respond to the aspirations and needs of inhabitants and city users and thus contribute to their quality of life [22]. the most relevant aspects regarding the city’s spatial transformation were considered to be those related to the development of the real estate market and the accessibility and stock of green spaces. in poland, the post-1989 political transformation led to changes in the planning system. these changes also represented an extreme negation of the previous political system and a negation of the communist principle of the “planned economy” [23]. these changes were significant in the spatial transformation of the city [24]. the lack of local legislation and the possibility to carry out development projects based on planning permits, which often did not comply with the directions of a city’s spatial policy, led to the decay of space in many areas of the city [25, 26]. the development of green spaces, the implementation of housing complexes devoid of transport services, public spaces and access to basic services, was another consequence of this. as a result of dynamic economic development, krakow’s space is subjected to high development pressure related to both the housing market and office spaces and various types of services. areas particularly threatened by uncontrolled and undesirable development include primarily green areas, which is a significant problem in managing krakow’s space. in the 2019 directional document, the scale of development pressure on the landscaped green spaces designated in the spatial development conditions and directions study – a document that defines the city’s spatial development directions but does not constitute local law, hereinafter referred to as the study – was diagnosed. the development pressure is partly linked to the housing market, where the ever-increasing demand for housing is generating an impulse to increase supply. this process is confirmed by data from statistics poland on both the increase in the number of dwellings and their usable floor area. the usable floor area of dwellings increased steadily from 12 647 885 m2 recorded in 1995 (no previous data available), to 15 585 641 m2 in 2002, then to 19 124 334 m2 in 2011, 21 a. karwińska, d. jopek, m. kudłacz, m. wiśniewski acta polytechnica ctu proceedings and reached 25 309 770 m2 in 2021. that is, between 2011 and 2021, the usable floor area of dwellings increased by 6 185 436 m2, which is almost as much as the increase in this value between 1995 and 2011 (6 476 449 m2). a significant proportion of the housing stock is used for longand short-term rental. the high demand associated with the large number of tourists and students temporarily living in krakow generates supply, which is conditioned by the property market being a relatively stable capital investment option. this is confirmed by the data related to the number of flats available on the housing rental market, which shows a very large increase in the last two years. according to the knight frank report, in the years 2017–2019 the stock was 251, in 2020 – 403, in 2021 – 535, in 2022 – 1239, while the forecast for 2023 assumed an increase to 1586 [27]. krakow’s business potential stems from, among other things, the steady supply of skilled labour, the support of the public administration, its favourable location and infrastructure facilities. in the business environment assessment study [28], in the beas 2021 ranking, krakow came third with a total score of 6.7 on a 10-point scale, confirming its strong position on the national and international business market, which has been driven in recent years by the business services sector, the it industry and numerous start-ups. krakow’s investment potential is linked to the development of the real estate market in this area. the total supply of office space in krakow is more than 1.5 million m2 (2021), which means that in terms of office resources, krakow is continuously maintaining its leading position among regional cities. the increased growth rate of the office space market in krakow dates back to 2014 and since then, an average of more than 150 000 m2 of it is being added to the city every year. since 2015, almost 1 million m2 of office space has been developed in krakow, which is more than 50 % of the current stock. experts from colliers in krakow estimate that in 2024 the total office space in the city will approach 2 million m2 [29]. in 2022, krakow was named the second-largest office market in poland, with 1.71 million square metres of office space, behind warsaw (6.27 million square metres) and ahead of wrocław (1.33 million square metres) [30]. the concentration of existing office space is partly a reflection of tenants’ location preferences and the size and type of project. most of the existing floor area (47 %) is located in the northern part of the city, both on the eastern side (prądnik czerwony and czyżyny) and on the western side (prądnik biały and bronowice). the south of the city is also characterised by a division – to the east (the area of zabłocie and bonarka) and to the west (from zakopiańska street, along czerwone maki and bobrzyńskiego streets). the high dynamism of the property market, as mentioned, is often tied with the encroachment of development on environmentally valuable land [31, 32]. the only effective way to prevent this is to put these areas under statutory conservation, which can be instituted by, among other things, the only planning document that is a bylaw, namely the local spatial development plan. the city authorities are progressively strive to cover as much of the city’s area with such plans as possible. as a result, 250 local plans were in force as of 13 april 2023, covering 76.3 % of krakow’s area (krakow city planning office). assignation as a green space in a local plan protects areas from unwanted development and is a necessary measure to preserve the balance between developed and open spaces. in addition to the planning level, the municipal authorities actively work towards increasing the amount of green space in the city and improving access to it for residents throughout the city’s territory. in this respect, we can name new park spaces (reduta park, szymborska park) and micro-interventions involving the creation of pocket parks, which at least minimally improve access to green spaces, which is not evenly distributed across the city. the effectiveness of these measures is attested by statistics poland’s data, according to which the amount of greenery in krakow is gradually increasing. this is indicated by data on both the surface area of parks, greens and neighbourhood green areas, the surface area of communal forests and the indicator of the overall share of green areas within the city’s territory. the area of parks, greens and neighbourhood green spaces, from a level of 1 175 ha in 1998, increased to 1 627.4 ha in 2010. in 2011, there was a decrease in this value to 1 487.9 ha, which nevertheless slowly increased to 1 923.28 ha in 2020. according to 2021 data, the value of this indicator decreased again to 1 910.16 ha (see figure 2a). the area of municipal forests in krakow, after increasing in 1999 by 120.3 ha with respect to 1998, remained relatively stable with slight fluctuations. from 2018 onwards, there has been a gradual increase in the area of municipal forests, from a level of 889.99 ha to 1 060.39 ha in 2021 (see figure 2b). this trend is increasing for forests and recreation and leisure areas, and has been noted in recent studies [34]. this increase was due to the purchase of new forested areas by the city of krakow, i.e., borkowski forest as well as a forest in plaszów. the ratio of green areas to the overall area of krakow, with slight fluctuations, increased from 8.35 % in 2005 to 11.51 % in 2021. it is also worth mentioning that according to research conducted after the covid-19 pandemic, 96 % of krakow’s citizens were found to have access to urban green spaces within 300 m from their homes [35]. according to international world’s greenest cities rankings, krakow is showing a steady improvement in this respect. according to data reported in the husqvarna urban green space index (hugsi), generated from satellite imagery and the percentage of green space in metropolitan areas, green space accounted for 57 % of the area (percentage of urban green space). according to these estimates, around 22 vol. 46/2024 cities in transition: krakow‘s social, economic and spatial . . . (a). area (total) of parks, greens and neighbourhood green areas, on the right. (b). area (total) of municipal forests. figure 2. krakow in the years 1998-2021 [33]. 37 % of krakow’s area is covered by trees and 20 % by grass. the study also showed that krakow had more than 207 m2 of green space per inhabitant. in comparison, prague, with the same 57 % share of green space, had a per capita green space ratio of 183.2 m2 (hugsi). similar results were found by łachowski and łęczek [36] in their study comparing the green areas of poland’s major cities. krakow, with a total share of 57.3 % of green areas, is only 12.3 % less than the greenest koszalin (69.6 %) and 32.1 % more than the least green opole (25.2 %) (warsaw’s share is 53.7 %). 5. infrastructure and transport policy of the city of krakow there are two primary issues related to environmental protection policy that are closely related to transport planning – energy saving and environmental quality, including air quality [37]. in addition to zeroemissivity and space neutrality, urban transport policy should be understood through the prism of efficiency: reliability and capacity. this is mainly achieved by indicators of the quality and volume of deliveries, such as: the estimated total number of kilometres of highcapacity means of transport – metro, suburban rail, trams, and other transport modes unaffected by traffic jams [38]. krakow is a city that has populated rapidly since 1995. the population in question is not people registered in krakow, as this figure grew at a rather moderate pace, but of the city’s users, whose number has grown exponentially: tourists, students, residents of the functional area who use krakow’s resources. typically, also for many cities in poland and europe, the transport infrastructure, especially road infrastructure, has not grown as fast as the demand for it. krakow is an attractive place to work with a supra-regional reach, being, among other things, a centre for global business services (a centre of common services with a global reach). it is also a city with 23 universities including, 10 of which are public. it is a city that received 14 million tourists in the pre-pandemic year. it is a metropolis characterised by high-quality of public and private services including, tertiary services. the transport system has to withstand the “stresses” of the intensity of institutions and processes and, importantly, krakow’s transport system follows assumptions about several important aspects that improve its efficiency. firstly, krakow’s authorities are working to diversify intra-city transport tools. the idea is that users of the city should be able to choose how they want to get around the city. they have at their disposal, among other things, public trams and buses, bicycle paths adapted for electric scooters; dedicated bus lanes used by electric buses and cars, as well as taxis. secondly, it is assumed that krakow’s intra-city transport is based on a public tram and bus transport network, which in many places is independent of traffic (has a dedicated track). although krakow does not have a metro network or a premetro (there is a short underground route in the centre which forms the basis of the 2.8 km long pre-metro), the city’s authorities are developing a proposal of the construction of a premetro with a feasibility study. thirdly, krakow’s urban transport should be clean, and environmentally neutral. krakow has one of the most modern fleets of buses and trams in poland and this part of europe. the vast majority of these are less than five years old, with around 30 % of the bus fleet being electric or hybrid buses. the bus fleet is equipped with the latest catalytic converters and meets the strictest european union standards. the krakow municipal transport company is gradually replacing its bus fleet with electric buses. however, krakow’s transport system has weaknesses and barriers that limit its efficiency. firstly, this relates to the lack of intermodal transport integration. krakow is in the process of building interchange hubs, primarily linked to commuter railway. however, matters are made more difficult by krakow’s historical development, as interchange hubs should be located in central locations, but there is no room for them due to the city’s historical architecture. secondly, there is also a mentality problem, as people largely behave irrationally from an economic viewpoint. oftentimes, despite the fact that it would be faster and cheaper to 23 a. karwińska, d. jopek, m. kudłacz, m. wiśniewski acta polytechnica ctu proceedings figure 3. number of cars in krakow [quantity] [12]. figure 4. load structure of krakow’s transport systems [39]. travel by public transport, krakow’s users choose the car. it is not entirely possible to predict the actual effect that the construction of a tram line may have, as residents may simply continue to prefer travelling by car. it is difficult to project the actual return rate of such a project, even if we monetise the societal effects of building a new tram or metro line. thirdly, many modes of transport in krakow are seasonal. relatively few people use bicycles or electric scooters in winter. it is difficult to predict whether krakow will become a second amsterdam or copenhagen. krakow’s parking system is certainly a problem. on the one hand, many city users would like to drive their car into the city centre, but on the other hand, this entails numerous dysfunctions. meanwhile, the city lacks a coherent policy concept of a direction for the parking system’s development. however, after initial enthusiasm, the idea of building multistorey car parks in the city centre was abandoned. there is still a debate as to whether pavements should be exclusively for pedestrians or should they be taken over by drivers. the issue of architectural constraints related to the valuable urban fabric in the old town is brought up regularly. the future of krakow’s transport system seems to lie underground. the strict city centre has probably already reached its maximum capacity and historical architecture prevents the extension of the transport system, for instance by widening streets for building a new lane. the city’s authorities, for the time being, have opted to extend tram lines and to convert singular car lanes into bus lanes, and choose to remain silent when asked about building a metro. the cost of building a single north–south metro line, which would cut commuting times from distant stations by around 20 minutes in krakow, has been estimated at around €8 billion. however, sceptics reiterate that the metro and its extension would mean long distances between stops, which is not going to work well for short-distance trips. in krakow, there are also those who are in favour of the premetro, but also those who believe that the strict city centre should only be entered on foot. 24 vol. 46/2024 cities in transition: krakow‘s social, economic and spatial . . . 6. cultural heritage and the development of tourism and its impact on krakow’s space after 1989 one of the most important spatial changes that took place in krakow after 1989 was related to the transformation of the historical centre [3]. the city, which was not destroyed by warfare, did not become a site of major conservation or rehabilitation measure after 1945. while major development projects were carried out outside the strict historical centre, the area’s most valuable in terms of cultural heritage were left without protection for several decades. after the world war ii and the establishment of a new political system, conservation that stemmed from everyday efforts by buildings owners ceased to function. it must be stressed, however, that property ownership was not nationalised in krakow, but was instead subject to top-down control monitored by the state. at the same time, the state, which was formally responsible for the state of the material tissue of the historical city, failed to fulfil its obligations. the state of the architectural fabric’s decay quickly worsened the tragic state of the natural environment. towards the end of the 1940s, krakow became the site of one of the polish state’s largest heavy industry projects. the vladimir lenin steelworks, which had been in operation since 1949, was an enormous environmental threat to residents and buildings [40]. the dangerous situation, seemingly hopeless, became one of the main reasons for placing krakow on the unesco world heritage list as early as 1978, along with the first 11 entries. at the time, krakow symbolised the decay of a historical city caused by rapid industrialisation and the resulting environmental disaster. the year 2023 marks the forty-fifth anniversary of this historic event. the development of krakow is in a completely different place. the steelworks’ operation has been limited, heavy industry is no longer a challenge, and has no significant impact on the city’s economy. the process of reducing production began back in the 1980s and accelerated considerably with the post-1989 political changes. at the same time, a reverse phenomenon took place in krakow, which today has a significant impact on the city’s economy and labour market and exerts an enormous influence on the way historical buildings are used. the unesco world heritage listing brought attention to the city, which soon after 1989 became an attractive destination for international tourism as well as was among the cities that found interest in using cultural heritage for its future development [41]. it also necessitated the first large-scale conservation work. in 1985, the polish state established the national fund for the revitalisation of krakow’s historical monuments, whose administrator is the social committee for the revitalisation of krakow’s historical monuments, in operation since 1979. this non-governmental organisation, which was set up while the communist state was still in power, is responsible for distributing pln 30 million each year (ca. 7 million eur). the nearly 40 years of the fund’s operation have had a fundamental impact on structural changes in the state of conservation of krakow’s monuments [42]. today, there are 13 other unesco sites in the vicinity of krakow. which were honoured in this profound manner between 1978 and 2013, via five further entries. this exceptional saturation of the city’s neighbouring areas with cultural values strengthens krakow’s position as a leading tourist destination in poland. shortly after the political transformation, in 1991, krakow was visited by around 1.8 million people a year. at the time, tourism was concentrated in a small area between the castle hill and the main market square. less than a decade later, in 1999, alongside prague and a number of other cities, krakow played the honourable role of a european capital of culture. at that time, the city had already been visited by more than 4.2 million people [43]. there was a marked increase in interest in the city after poland’s accession into the european union and the opening of the polish sky to low-cost airlines. krakow’s john paul ii airport handled around 200 000 passengers in the mid-1990s, and a decade later this number exceeded 2.5 million. shortly before the pandemic, the airport was already handling more than 8 million passengers [44]. the rapid development of the air transport, network primarily with cities in the uk, became an important tool for tourism development after 2004. in 2008, krakow was visited by as much as 7.5 million people. in 2013, this number was 9.25 million, just before the outbreak of the covid-19 pandemic, the number of tourists visiting krakow was over 14 million. the pandemic and the period of movement restrictions have slowed growth. however, data from 2022 show a very clear rebound, with the number of tourists again approaching 8 million [45, 46]. the pandemic has changed the nature of tourism in the city; before 2020, foreign tourists, mostly from western europe, played a very important role. tourists from middle eastern and far eastern countries were an increasingly important group. since 2020, there has been an increase in the share of domestic tourism as well as tourism from neighbouring countries. one of the measures of change in tourism in the city is the number of people who visit its most popular museums. in 2005, the wawel royal castle was visited by ca. 1 million people, the national museum in krakow by 350 000 people, and the krakow museum by around 220 000 people. in 2019, just before the outbreak of the covid-19 pandemic, the wawel royal castle was visited by around 1.4 million people [47], the national museum by around 1.2 million people, and the krakow museum [48] by around 1.3 million people. the total number of krakow’s museum visitors group was bigger than 5.2 million [43]. the inflow 25 a. karwińska, d. jopek, m. kudłacz, m. wiśniewski acta polytechnica ctu proceedings of eu funds and the completion of numerous projects in the culture sector have become an important factor in the development of krakow as a leading polish museum centre. as already pointed out, the covid-19 pandemic had only a short-lived impact on reducing tourism in the city. it has also shown that it does not actually have a critical impact on the city’s economy. tourism’s share in the city’s gross product is around 8 %. however, the pandemic revealed a different phenomenon. after 1989, with the development of tourism in the city, there was an exodus of residents from the city centre. district i, the old town, one of krakow’s eighteenth districts, is an area filled with the most valuable and sensitive tangible cultural heritage assets. as recently as 2000, the district was home to around 56 000 people, in 2010 this figure fell to around 42 000, in 2021 it was already less than 30 000. it is estimated to reach 12 500 by 2050 [49]. while the population of the city as a whole increased from 760 thousand to 800 thousand, the population of the most attractive part of krakow has collapsed. the old town area in particular, the area within the historical city walls, has become a site of drastic change. until around 2000, more than 20 000 people had lived there. today, the number is around 500, the smallest in history. structural changes are also affecting the areas of other districts. after 2000, the former town of kazimierz has become an increasingly visited part of the city. before the second world war it was the symbolic centre of life for the jewish community in krakow. after 2010, the historical district of podgórze, on the other side of the river vistula, became another popular destination. the increased interest in these places quickly translated into strong growth in catering establishments and hotel projects. the outflow of residents was hampered in these parts of the city by the ownership structure. while in the strict centre almost all residential buildings are privately owned, in areas outside this area, especially in the historical town of kazimierz and in the podgórze area, municipal ownership plays an important role. it has ensured that the aforementioned parts of the city have avoided the problem of population loss and drastic transformation into flats for short-term rent. it should be emphasised that former flats converted to short-term rentals play an important role in krakow today alongside large hotels, which are often part of major franchises. this phenomenon began to grow after 2010 and the advent of mobile applications that offer accommodation rental. in 2020, there were more than 32 thousand beds in more than 600 hotels and almost as much beds in more than 1600 short-term rental properties. it is no exaggeration to say that krakow’s historical centre has become an enormous hospitality complex over the past twenty years. this phenomenon has a highly negative impact on the life of the city and conflicts with the interests of residents and with the idea of sustainable development based on a circular economy. the development of the tourism sector was highly appreciated by the local policymakers, making the growing crisis in the city centre invisible [50]. the strategic municipal documents for tourism for the period before the outbreak of the covid-19 pandemic highlighted the need to increase the number of tourists. the only problems recognised at that time were the quality of services offered, the growing number of tourists and the rising interest of tourists in the city’s night life [51]. the moment of covid-19 pandemics outbreak exposed the city centre as empty space distant from the citizens [52]. already in spring 2020 the city authorities asked residents to replace tourists and use the catering services in the city centre more often. the city authorities’ appeal was met with numerous negative or ironic remarks and thus exposed the resentment of krakow’s residents towards the historical centre as a place dedicated to outsiders. the affirmative answer from the local community pushed city authorities to introduce a new sustainable tourism strategy [53]. the negative aspects of tourism, such as gentrification and the hypertrophy of the tourist sector, were named in an official document for the very first time. however, the tools proposed for solving these problems, including adopting more sophisticated software devices, are barely adequate. it needs to be emphasized that more appropriate tools for action need financial support and administrative legal solutions that can be implemented only at the state level. while the problem of overtourism is far from being solved, the city of krakow might be seen as the first in poland to discuss it and to propose some adequate future actions. thus, krakow might be seen as the city that follows the idea to the well-functioning city and that develops policies that could help to overcome globally-rooted crises. 7. conclusions the changes that have taken place in krakow over the last thirty years indicate the development of the city’s intellectual, social and cultural capital. better use of the opportunities offered by the growing cultural diversity of the community of residents and users of the city is an important phenomenon. other opportunities that have been taken advantage of include the presence of outsourcing companies and the related position of krakow as a business location, which in turn affects the city’s reputation as a place to study, but also as a housing environment in general. if the tendencies outlined in the city’s reports, tendencies to strengthen social capital, the growing sense of connection with the city and its affairs, and social integration (including intercultural integration) continue in the coming years, it can be assessed that, from a social point of view, krakow will move towards being a well-functioning city. there is, of course a need to further develop social integration and strengthen civil society. this should include the support of citizens’ agency and 26 vol. 46/2024 cities in transition: krakow‘s social, economic and spatial . . . activity. it requires the inclusion of various groups in the decision-making process [54]. the period after the political transformation of 1989 was a difficult challenge for krakow in terms of spatial planning. the lack of regulations, i.e., spatial development plans, resulted in the proliferation of uncontrolled development in terms of function and form. the successive process of adopting planning documents was too slow compared to the changing dynamics of the real estate market. as a result, many dysfunctional spaces were created, primarily in terms of transport connections and access to necessary uses, which were very often created in areas that play a key role in the city’s environmental system. in the face of unfavourable demographic changes, threats related to climate change and the economic situation, krakow’s authorities are gradually implementing a development strategy based on the principles of a circular urban economy. in short, krakow is a city that is gaining in importance in the regional and supra-regional context. in the analysed period, krakow created new and strengthened some of its pre-existing functions. some of them became dysfunctional – which is not an exception. many cities have succumbed to the pressure of excessive commercialisation, which threatens the loss of local identity and hard-to-reproduce resources (e.g., space). certainly, in the case of krakow, we are dealing with a noticeable and dynamic development of tourism, business – especially in the global business services industry – and an increase in the number of city users (development of the city’s functional area). the development and growth of kraków’s economic significance is accompanied by a slightly too short-sighted spatial policy, which results in the impairment of transport infrastructure, the dysfunctional development of residential and office buildings, which results in, among others, the building up of city cross-ventilation corridors. this is also due to the rapid commercialisation of the city, which many cities, including krakow, tolerate, because it is the “price” of rapid development, wage growth, the impact on the city budget, maintaining an appropriate level of interest from investors and tourists. therefore, it seems that, in the future, krakow will always have to analyse the environmental impacts of its development policies. there are many concepts that describe the need to rationalize development actions, with the above-mentioned circular urban economy being notable. taking into account the evolution of krakow in the years under study, the city-forming nature of krakow’s development policy should be clearly indicated. however, it can be stated that the research hypothesis was verified negatively. on the one hand, there has been clear progress in krakow in terms of city budget revenue, average wage levels and the number of companies with foreign capital classified as modern. krakow has developed dynamically in terms of infrastructure. however, this development has come at a price. krakow has become a city with chaotic residential block layouts, and has been excessively commercialized to meet tourist needs. krakow did not use its resources optimally, but in a specific way, fostering development in the hotel, catering and global business services industries. at the same time, positive aspects of krakow’s development are visible: a turn towards innovation, the development of grassroots initiatives (urban activist movements, non-governmental organisations) that began to demand the protection of key city territories. at this stage of the analysis, this study should be considered exploratory and will be further developed in terms of content, research methodology and the type of data used. in subsequent stages, the study will be enhanced with a more detailed set of data from existing databases and supplemented by original research. the final set of indicators and metrics will be used to study other cities to provide comparative material. the study has several limitations. firstly, it was based on pre-existing public datasets, which allowed for their aggregation. secondly, the aim of the study was to explore the scope of available data for krakow, which at this stage precluded comparative analysis with other cities. despite these limitations, the research gap identified is a valuable conclusion. it consists in the lack of studies that analyse and monitor krakow’s development (also in comparison with other cities) and, as a result, model future development trends. references [1] i. e. kotowska (ed.). niska dzietność w polsce w kontekście percepcji polaków: diagnoza społeczna 2013: raport tematyczny. ministerstwo pracy i polityki społecznej, warszawa, 2014. 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http://www.mot.krakow.pl/badanie-ruchu-turystycznego.html https://bip.malopolska.pl/wawel,m,301181,frekwencja.html https://bip.malopolska.pl/wawel,m,301181,frekwencja.html https://stat.gov.pl/obszary-tematyczne/kultura-turystyka-sport/kultura/kultura-w-2019-roku,2,17.html https://stat.gov.pl/obszary-tematyczne/kultura-turystyka-sport/kultura/kultura-w-2019-roku,2,17.html https://stat.gov.pl/obszary-tematyczne/kultura-turystyka-sport/kultura/kultura-w-2019-roku,2,17.html https://www.arcgis.com/apps/dashboards/4a6c48fcd57543af90bd6ab0e6abcd34 https://www.arcgis.com/apps/dashboards/4a6c48fcd57543af90bd6ab0e6abcd34 https://www.bip.krakow.pl/plik.php?zid=127397&wer=0&new=t&mode=shw https://www.bip.krakow.pl/plik.php?zid=127397&wer=0&new=t&mode=shw https://doi.org/10.1057/s41289-021-00175-5 https://www.bip.krakow.pl/zalaczniki/dokumenty/n/327249/karta https://www.bip.krakow.pl/zalaczniki/dokumenty/n/327249/karta https://doi.org/10.1177/02750740122064956 acta polytechnica ctu proceedings 46:17–29, 2024 1 introduction 2 overview of the area under study 3 demographic and social transformation of krakow in the last three decades 4 city planning 5 infrastructure and transport policy of the city of krakow 6 cultural heritage and the development of tourism and its impact on krakow's space after 1989 7 conclusions references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2024.47.0113 acta polytechnica ctu proceedings 47:113–116, 2024 © 2024 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague effect of preparation process on purity of tricalcium aluminate simona ravaszová∗, karel dvořák, andrea jančíková brno university of technology, faculty of civil engineering, veveří 331/95, 602 00 brno, czech republic ∗ corresponding author: ravaszova.s@fce.vutbr.cz abstract. in the realm of cement production, traditional fuels are steadily giving way to alternative sources. while this shift yields economic and ecological advantages, it introduces notable technological challenges. specifically, there is an escalation in certain chemical elements, leading to variability in the mineralogical composition. this variability is subsequently manifested in the characteristics of portland clinker. it becomes imperative, therefore, to synthesize pure clinker minerals. this synthesis is essential for both understanding their behavior in industrially manufactured cement and creating analytical standards for mineral identification through xrd. the primary focus of the research is on the production of pure tricalcium aluminate, a key clinker mineral, within laboratory conditions. a direct laboratory methodology involving solid-phase synthesis is used. the investigation meticulously tracks the impact of homogenizing the raw material mixture on the resultant mineral’s purity. to achieve this, a high-speed pulverisette 6 planetary mill is utilized along with two types of grinding bodies. a corundum grinding capsule with corundum grinding bodies is employed to prevent contamination by foreign ions. simultaneously, a highly efficient steel grinding capsule with steel grinding bodies is utilized. the raw material mixture is then subjected to sintering at six different temperatures ranging from 1 200 to 1 450 °c, and the purity of the produced mineral is quantified using rietveld analysis. laser granulometry results indicate a strikingly similar comminution of the raw material in both grinding cases. iron contamination, as revealed by xrf analysis, is minimal, accounting for only 0.21 %. the purity of 97.6 % c3a, is achieved at 1 450 °c using a steel grinding capsule, with 2.4 % unreacted lime conversely, when using a corundum capsule, a purity of 93.6 % is achieved, accompanied by residues of 2.2 % mayenite and 3.8 % lime. this discrepancy may be attributed to an insufficient degree of raw material comminution in the corundum capsule. intriguingly, a slight iron contamination during the grinding of the raw material mixture exerts a positive influence on purity. in this case, iron acts as a flux, fostering a more favorable reaction of the mineral mayenite. this multifaceted exploration enhances our understanding of clinker mineral synthesis, offering insights into optimizing purity based on grinding methods, sintering temperatures, and the impact of minor contaminants. keywords: tricalcium aluminate, synthesis, contamination, rietveld analysis. 1. introduction tricalcium aluminate c3a is one of the main components of portland clinker. c3a acts as a melt during clinker production and is part of the liquid phase during firing. in the prepared clinker, it becomes part of the interfacial substance. together with c4af, it forms most of the 22 to 28 % of the clinker melt that is essential for the formation of other clinker minerals such as alite c3s and belite c2s, which are the main minerals that bring the cement parameters. ordinary portland clinker contains 8 to 11 % of c3a. in cement, the role of c3a is related to the kinetics of cement hydration. these minerals determine a very high proportion of the final physical properties of the produced clinker. in practice, they are never found in pure form but are contaminated with other chemicals. a better understanding of all the mechanisms involved in c3a formation and hydration requires the ability to prepare pure c3a. [1] there are several different procedures for the preparation of tricalcium aluminate by the solid-state synthesis, but they all involve the following steps: • preparation of raw materials: raw materials such as limestone and corundum are usually first ground into small particles. • mixing the raw materials: the ground materials are then mixed in the correct proportions to ensure that the final product contains the correct amount of calcium and aluminium. • processing of the mixture. • drying and firing: the resulting mass is then dried and fired at high temperature to become a solid tricalcium aluminate [2]. the synthesis of tricalcium aluminate by the solidstate synthesis means that tricalcium aluminate is produced directly from solid materials without the use of liquid solvents. this method is often used to produce large quantities of tricalcium aluminate and to achieve high purity and quality of the final product. solid synthesis is carried out using a mixture of 113 https://doi.org/10.14311/app.2024.47.0113 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en s. ravaszová, k. dvořák, a. jančíková acta polytechnica ctu proceedings calcium oxide (cao) and aluminium oxide (al2o3) in a molar ratio of 3:1, which is then calcined at a temperature of around 1 100–1 200 °c for 2–3 hours. this process leads to the formation of tricalcium aluminate in crystalline form [3, 4]. coprecipitation is a process in which two or more elements or compounds are precipitated as a solid crystal under specific conditions such as temperature and ph. in addition, c3a and cement mixtures containing c3a can be modified by the addition of various additives such as calcium oxide (cao), silicates or carbonates. these additives can alter the properties of the cement, such as hydration (i.e. the reaction of the cement with water), setting, strength and resistance to chemicals. modifications to c3a and cement mixtures can also be made by various processes, such as changing the temperature or pressure during manufacture [5]. this topic is very relevant lately because developments in cement production are increasingly influenced by environmental and economic factors. for this reason, it is very important to know in detail all the mineralogical processes of transformation of each polymorphic modification of each clinker mineral. 2. material and methods to produce cubic tricalcium aluminate, calcium carbonate (p.a. purity, 99.7 %) and aluminium oxide (p.a. purity 99.5 %) were used in an amount corresponding to the molar ratio of c3a or 3 moles of cao and 1 mole of al2o3, see table 1 (chemical composition of the raw material mixture). the homogenisation of the raw materials was carried out by high-speed milling in a laboratory planetary mill frisch pulverisette 6. distilled water was added to the raw material mixture at a powder/water ratio of 0.8. two distinct grinding capsules made of different materials were chosen. first half of the raw material powder was homogenized in a steel-bodied capsule with 25 steel milling bodies for 15 minutes at 500 rpm. the second half of the raw material powder was homogenized in a corundum capsule with 25 corundum milling bodies for 30 minutes at 350 rpm. the reason for the choice of two different materials is possible iron contamination during abrasion of the grinding capsule and the ground material. however, it was also checked if grinding in a corundum capsule achieve the desired degree of comminution as a steel capsule. possible iron contamination will be verified by xrf analysis. the degree of comminution of the raw meal will be verified by laser granulometry by malvern paralytical mastersizer 2000. after milling, the raw material powder was left to dry into nodules 10–15 mm in diameter were formed spontaneously in a laboratory dryer at 105 °c for 24 hours. in the next step, a series of two-phase sintering’s were carried out from both prepared raw material flours. in the first stage of sintering, calcination occurs at 900 °c with soaking time of 60 minutes. in the second sintering stage, the actual solid phase synthesis takes place. the sintering temperatures were chosen in the range in range between 1 200–1 450 °c. the purity of the prepared products was verified by x-ray analysis. caco3 a2o3 h2o c3a cubic 111.124 37.736 120.000 ca/al=1.500 h2o/c3a=0.8 table 1. chemical composition of the raw material mixture in grams and relevant ratios. 3. results and discussion xrf analysis of the raw material powders was carried out due to possible iron contamination during milling in the steel capsule. according to the results, it can be seen that the iron contamination is minimal, specifically 0.21 %. the complete results are shown in table 2. the results of the laser granulometry, which was performed after the xrf evaluation see figure 1, show that the particle size distribution of the powder milled in the steel capsule for 15 minutes and in the corundum capsule for 30 minutes is almost identical. however, the steel capsule ground meal has a higher particle size distribution in the range of 10 to 100 µm. figure 1. particle size distribution (psd) of raw material mixture after milling in pulverisette 6. the evaluation of the samples by xrd analysis, which were milling in a steel and corundum capsule and underwent a two-phase sintering first at 900 °c with a soaking time of 60 min. the samples were then subjected to a second sintering at 1 200, 1 250, 1 300, 1 350, 1 400 and 1 450 °c. from the diffractograms, increasing the firing temperature influences the formation of c3a. as shown in the detail images, with increasing temperature the intensity at 18.1° 2θ angle decreases indicating the decrease of the mineral mayenite ca12al14o33 (m) and the intensity increase of the peak at 17.4° 2θ indicating an increase in the amount of c3a mineral. 114 vol. 47/2024 effect of preparation process on purity of tricalcium aluminate sio2 al2o3 fe2o3 cao mgo na2o3 k2o so3 tio2 mno sum. steel 0.0 36.522 0.21 63.268 0.0 0.0 0.0 0.0 0.0 0.0 100.0 corundum 0.0 37.109 0.0 62.891 0.0 0.0 0.0 0.0 0.0 0.0 100.0 table 2. xrf analysis: chemical composition of raw material mixture after milling. figure 2. xrd analysis of c3a (steel capsule). the x-ray diffraction patterns show that the purest c3a was formed at 1 450 °c, however, the lime cao and the mineral mayenite are still present in the sample; the inability of these components to react is probably due to insufficient comminution of the corundum component of the raw material. the mineral mayenite is more evident on samples milled in a corundum grinding capsule. quantification by the rietveld method was performed in highscore plus version 3.0.5. table 3 and table 4 show the results of the first batch of sintered samples. the rietveld method confirmed that c3a reaches the highest purity at 1 450 °c, however, as already known from figure 2 and figure 3 of the xrd analysis, mayenite and free lime are also present in the samples. these partially reacted minerals are found in greater quantities in the samples that were milled in the corundum capsule. this is likely due to the slight iron contamination proven above by xrf analysis, this iron contamination in this case acting as a melting medium that helped to react the remaining mayenite. mmanaged to demonstrate that the difference in results when grinding in a corundum and steel capsule is minimal, as iron contamination is only 0.21 %. furthermore, it was found that during the preparation of cubic c3a, the purity of this mineral increases with the firing temperature, the highest purity was achieved by the mineral c3a at 1 450 °c in the sample c3a ca12al14o33 cao al2o3 1 200 °c 55.4 24.2 19.3 1.1 1 250 °c 66.5 19.3 13.2 1.0 1 300 °c 78.3 13.9 7.3 0.5 1 350 °c 95.5 1.5 2.6 0.4 1 400 °c 94.4 2.4 3.1 0.0 1 450 °c 97.6 0.0 2.4 0.0 table 3. rietveld analysis: mineral composition (steel capsule). c3a ca12al14o33 cao al2o3 1 200 °c 49.2 31.4 18 1.4 1 250 °c 59.3 26.6 12.6 1.5 1 300 °c 71.3 20.4 7.9 0.4 1 350 °c 94.8 3.6 1.2 0.4 1 400 °c 94.5 3.4 1.6 0.5 1 450 °c 93.6 2.2 3.8 0.4 table 4. rietveld analysis: mineral composition (corundum capsule). that was ground in a steel capsule. here, iron as a flux most likely helped to achieve a c3a purity of 97.6 % according to the rietveld method. 115 s. ravaszová, k. dvořák, a. jančíková acta polytechnica ctu proceedings figure 3. xrd analysis of c3a (corundum capsule). 4. conclusion • laser granulometry results showed similar comminution of the raw material in both cases of grinding. • iron contamination based on xrf analysis was minimal at only 0.21 %. • the highest purity of 97.6 % c3a was achieved at a temperature of 1 450 °c when grinding in a steel capsule, with 2.4 % representing unreacted lime. • grinding in a corundum capsule resulted in a purity of 93.6 %, with a residue of the mineral mayenite of 2.2 % and lime of 3.8 %. this may be attributed to insufficient comminution in the corundum capsule. • slight iron contamination during grinding had a positive effect on purity, as iron acted as a flux, improving the reaction of the mineral mayenite. acknowledgements we acknowledge the financial support to project (fast-s23-8246) study of the formation and stability of various tricalcium aluminate polymorphs. references [1] j. moon, s. yoon, r. m. wentzcovitch, et al. elastic properties of tricalcium aluminate from high-pressure experiments and first-principles calculations. journal of the american ceramic society 95(9):2972–2978, 2012. https: //doi.org/10.1111/j.1551-2916.2012.05301.x [2] j. r. salasin, c. rawn. structure property relationships and cationic doping in [ca24al28o64]4+ framework: a review. crystals 7(5), 2017. https://doi.org/10.3390/cryst7050143 [3] g. shin, f. glasser. interdependence of sodium and potassium substitution in tricalcium aluminate. cement and concrete research 13(1):135–140, 1983. https://doi.org/10.1016/0008-8846(83)90137-0 [4] p. mondal, j. w. jeffery. the crystal structure of tricalcium aluminate, ca3al2o6. acta crystallographica section b 31(3):689–697, 1975. https://doi.org/10.1107/s0567740875003639 [5] f. lee, h. banda, f. glasser. substitution of na, fe and si in tricalcium aluminate and the polymorphism of solid solutions. cement and concrete research 12(2):237–246, 1982. https://doi.org/10.1016/0008-8846(82)90010-2 116 https://doi.org/10.1111/j.1551-2916.2012.05301.x https://doi.org/10.1111/j.1551-2916.2012.05301.x https://doi.org/10.3390/cryst7050143 https://doi.org/10.1016/0008-8846(83)90137-0 https://doi.org/10.1107/s0567740875003639 https://doi.org/10.1016/0008-8846(82)90010-2 acta polytechnica ctu proceedings 47:113–116, 2024 1 introduction 2 material and methods 3 results and discussion 4 conclusion acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2024.48.0052 acta polytechnica ctu proceedings 48:52–55, 2024 © 2024 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague assessment of local mechanical properties of laser powder bed fused aluminium alloy by non-destructive testing based on fimec indentation vittorio villani∗, gennaro salvatore ponticelli, simone venettacci, stefano guarino university niccolò cusano, department of engineering, via don carlo gnocchi 3, 00166 rome, italy ∗ corresponding author: vittorio.villani@unicusano.it abstract. laser powder bed fusion process is a versatile metal additive manufacturing process. although significant progress has been made so far, there is still limited large-scale adoption of this technique by the industry. the main problems are repeatability and lack of proper knowledge. in this work, an innovative and non-destructive testing methodology, based on flat-top cylinder indentation, was used to define the mechanical properties of laser powder bed fused aluminium alloy to highlight any variations induced by the combination of process parameters, for global characterization, and by the building direction, for local characterization. results show similar or improved global mechanical properties of the laser powder bed fused specimens when compared to traditional die-casted ones. indentation tests highlight a local dependence of properties along the building direction in favor of the upper part of the samples. keywords: laser powder bed fusion, non-destructive testing, fimec. 1. introduction additive manufacturing (am) technologies are of growing interest, thanks to their flexibility and ability to reduce time-to-manufacturing, allowing the speedup of industrial process. the cost effectiveness of am solutions depends on the particular application and it is generally proven that for the prototype stage or for smaller productions based on custom designs, am components do have an economical advantage over traditional material processing strategies [1]. am process is generally based on the addition, layer by layer, of thin sheets of materials to generate a physical object from a computer-aided design model, allowing the realization of complex structures [2]. this is recognized to be one of the main advantages in using am, especially for metal additive manufacturing (mam) techniques [3]. mam parts can be realized producing very-little to no waste compared to traditional processes like forging, casting and extrusion [4]. aluminum alloys are known for their wide compatibility with industrial applications, thanks to easy processing, good corrosion resistance, and high specific strength. lpbf can solve some of the main problems found in traditional foundry technologies, generally related to low cooling rate in casting and limited flexibility of preparation and forming processes [5]. main drawbacks of processing pure aluminium with lpbf are related to high reflectivity, high thermal conductivity, and poor flowability of the powders [6]. to overcome these problems, specific alloys are generally used as starting material, e.g., alsi10mg, alsi12, a356, and a357 [7]. however, while significant progress has been made so far, the diffusion of this additive manufacturing technique by the industry is still limited given the lack of standardization of the process itself in terms of repeatability [8] and the high surface roughness of fabricated products [9]. moreover, complexity in the geometry can induce local variations of the resulting properties due to the building direction [10]. therefore, optimization of process parameters and consequently determination of effective final-part mechanical properties on both global and local scales need further investigation. fimec (flat-top cylinder indenter for mechanical characterization) is a non-destructive and noninvasive testing methodology. compared with other other indentation techniques, fimec uses a flat-top indenter tip, whose geometry is optimized to provide results that are not depended on local surface properties such as roughness [11], allowing local and global mechanical characterization of multiple materials [12]. thanks to proven accuracy, ease of implementation, and great flexibility, fimec is of growing interest for several usage scenarios [13], including metal additive manufacturing, for which no known research works are available in the pertinent literature. this study aims to evaluate the local and global mechanical properties of alsi10mg samples fabricated through lpbf by means of the fimec indentation test to highlight any variations induced by the combination of process parameters and building direction. the results are compared with the traditional tensile testing method to verify the suitability of the proposed solution to characterize the samples also for mam processes. 52 https://doi.org/10.14311/app.2024.48.0052 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 48/2024 local mechanical properties of lpbf aluminium alloy by fimec characteristic value chemical composition [wt%] al (78.38) si (18.93) mg (1.36) c (0.94) o (0.39) melting point [◦c] 570 relative density [g · cm−3] 2.63 median diameter [µm] 37 table 1. main characteristics of the metal powder as declared by the manufacturer. 2. materials and methods the starting material is a commercial powder of alsi10mg from m4p materials solutions gmbh whose main characteristics are listed in table 1. fimec tests consisted of penetration of a tungsten carbide tip, 1 mm in diameter and 1.5 mm in height, to a certain depth (i.e., 0.8 mm) at a constant speed (i.e., 0.01 mm min−1), according to the standard astm e2546. the tip was mounted on the mts insight electromechanical testing machine by mts with 50 kn of nominal load that can appreciate a crosshead displacement with a resolution of 0.6 µmm (see figure 1). the tests involved two steps, i.e., (i) loading phase and (ii) unloading to initial position. penetration depth was taken from cross-head displacement. as suggested by the standards, three samples have been indented in five different positions (see figure 2) to evaluate the eventual local variations of the final qualities. figure 1. fimec setup. the main mechanical properties that can be extrapolated from the pressure vs. depth curve obtained with the fimec test are the yield stress σy,f and the elastic modulus ef, using the inflection point py of the loading part of the diagram and the slope s of the unloading portion, as shown in figure 3, and according to the following equations [14]: figure 2. schematization of the indentations along the building direction. σy,f = py 3 , (1) s = 2√ π ea √ a, (2) 1 ea = (1 − ν2 f) ef + (1 − ν2 i ) ei , (3) where ea is the apparent elastic modulus calculated through the slope s, a ≈ 0.785 mm2, ei = 668.35 gpa, and νi = 0.24 are the section, the elastic modulus, and the poisson ratio of the tip [15], while νf = 0.36 is the poisson ratio of the material here investigated [16]. a total of three samples, each of one was tested in five different indentation position were tested during the experimental campaign, as shown in figure 2. finally, the results were compared to traditional tensile tests of both lpbf-ed and die-casted samples carried out in a previous work [9]. 3. results and discussion the characterization of the mechanical properties was carried out at a local level by means of indentation tests through a flat top cylinder tip. the aim was to highlight any dependence of mechanical performance in the direction of the building (see figure 2), which 53 v. villani, g.s ponticelli, s. venettacci, s. guarino acta polytechnica ctu proceedings figure 3. fimec test reference curve. cannot be determined by traditional tensile testing methods. the results of the fimec tests are shown in figure 4 and 5. in the figures, “t” refers to the traditional tensile test carried out for lpbf samples. “h” refers to the traditional tensile test carried out for die-casted samples. figures 4 and 5 show a similar trend for both elastic modulus and yield strength between the two different approaches, when compared to lpbf samples, validating the newly proposed indentation test as a suitable alternative to the conventional method, also being nondestructive since it leaves only a very small imprint on the surface of the component [12]. moreover, fimec diagrams provide additional information about the dependence of mechanical properties on the position in which they are evaluated [17]. this is very important for samples fabricated with lpbf because it is an additive manufacturing technique that involves the deposition of successive layers on top of the previous ones, thus potentially introducing anisotropy. figure 4 suggests that there is a change in the mechanical properties moving from the bottom to the top of the sample. specifically, the bottom part of the specimen, named p1, appears to be characterized by the lowest value of elastic modulus, i.e., around 30.82 ± 2.16 gpa, up to 32.48 ± 2.47 gpa for p5 at the top. this finding can be attributed to the fact that the bottom part of the sample remains at high temperature longer than the upper part during the process, therefore experiencing a heat treatment able to partially relief the residual stresses [18]. a clear variation of the yield stress can be observed in figure 5. its distribution depends on the fabrication process, being the highest values in the bottom part of the samples (i.e., from p1 to p3), which is, as explained for the elastic modulus, the zone closer to the substrate where the cooling rate is higher, generating finer grains and therefore increasing the yield strength [19]. the measurement of the strength of the yield is figure 4. fimec test results: elastic modulus. figure 5. fimec test results: yield stress. probably influenced by microporosity, segregation, and microstructure, but the effect of the cooling rate on grain size can be considered the main influencing factor [13]. comparing the results of the fimec test with “t” and “h” samples, it has to be reported that the elastic modulus of 3d printed samples is lower than die-casted samples due to difference in porosity, while the difference between traditional tensile test and fimec is related to lower speed test of the indentation methodology [11]. 4. conclusions the fimec test recognizes the process history, allowing the quantification of the fabrication effect along the building direction of the sample geometry. moreover, it can be considered as nondestructive since it leaves negligible indentation marks. the comparison with traditional tensile test results demonstrates the suitability of the fimec indentation test to provide local and global information on the mechanical properties in terms of elastic modulus and yield strength. locally, the bottom part of the samples is characterized by the lowest values of the elastic modulus while ensuring high yield strength, because of the 54 vol. 48/2024 local mechanical properties of lpbf aluminium alloy by fimec grain growth induced by the fact that the bottom part remains at high temperature longer. globally, the lpbf process allows to obtain similar characteristics in terms of yield stress, while the elastic modulus drops to half the value of traditional diecasted samples, probably due to the microporosity. references [1] s. singh, r. p. mohanty, s. k. mangla, v. agrawal. critical success factors of additive manufacturing for higher sustainable competitive advantage in supply chains. journal of cleaner production 425:138908, 2023. https://doi.org/10.1016/j.jclepro.2023.138908 [2] s. a. m. tofail, e. p. koumoulos, a. bandyopadhyay, et al. additive manufacturing: scientific and technological challenges, market uptake and opportunities. materials today 21(1):22–37, 2018. https://doi.org/10.1016/j.mattod.2017.07.001 [3] t. debroy, h. l. wei, j. s. zuback, et al. additive manufacturing of metallic components – process, structure and properties. progress in materials science 92:112–224, 2018. 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materials 16(7):2598, 2023. https://doi.org/10.3390/ma16072598 [17] m. di siena, s. genna, p. moretti, et al. study of the laser-material interaction for innovative hybrid structures: thermo-mechanical characterization of polyethylene-based polymers. polymer testing 120:107947, 2023. https: //doi.org/10.1016/j.polymertesting.2023.107947 [18] g. s. ponticelli, l. di salvo, m. giuliani, et al. induced back stress hardening and strengthening effect by repetitive progressive tensile loading of laser-powder bed fused 316l stainless steel. the international journal of advanced manufacturing technology 121(7–8):5125–5138, 2022. https://doi.org/10.1007/s00170-022-09690-3 [19] b. j. mfusi, n. r. mathe, l. c. tshabalala, p. a. popoola. the effect of stress relief on the mechanical and fatigue properties of additively manufactured alsi10mg parts. metals 9(11):1216, 2019. https://doi.org/10.3390/met9111216 55 https://doi.org/10.1016/j.jclepro.2023.138908 https://doi.org/10.1016/j.mattod.2017.07.001 https://doi.org/10.1016/j.pmatsci.2017.10.001 https://doi.org/10.1016/j.procir.2023.03.151 https://doi.org/10.1016/j.jmst.2018.09.004 https://doi.org/10.1016/j.jmatprotec.2016.08.003 https://doi.org/10.1016/j.jmatprotec.2016.08.003 https://doi.org/10.3390/ma13194301 https://doi.org/10.1016/j.matpr.2018.06.119 https://doi.org/10.1016/j.jmapro.2020.08.054 https://doi.org/10.5185/amlett.2016.6171 https://doi.org/10.3390/ma13051154 https://doi.org/10.3390/ma14071742 https://doi.org/10.1016/j.measurement.2006.11.012 https://doi.org/10.1016/j.measurement.2006.11.012 https://doi.org/10.1016/j.msea.2004.04.041 https://doi.org/10.1557/jmr.2004.19.1.3 https://doi.org/10.3390/ma16072598 https://doi.org/10.1016/j.polymertesting.2023.107947 https://doi.org/10.1016/j.polymertesting.2023.107947 https://doi.org/10.1007/s00170-022-09690-3 https://doi.org/10.3390/met9111216 acta polytechnica ctu proceedings 48:52–55, 2024 1 introduction 2 materials and methods 3 results and discussion 4 conclusions references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2024.50.0030 acta polytechnica ctu proceedings 50:30–35, 2024 © 2024 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague nanomechanical properties of neutron-irradiated concrete ondřej liberaa, jiří němeček 1,b, patricie halodováa,∗, zbyněk hlaváča, jiří němeček 2,b a research centre řež, department of material and mechanical properties, hlavní 130, 250 68 husinec-řež, czech republic b czech technical university in prague, faculty of civil engineering, department of mechanics, thákurova 7, 166 29 prague 6, czech republic ∗ corresponding author: patricie.halodova@cvrez.cz abstract. concrete exposed to irradiation from nuclear reactor undergoes atomic-level degradation due to the influence of neutrons, secondary gamma rays, and associated gamma heating. the radiationinduced damage decreases the mechanical properties of the concrete and affects its performance as a construction material. the concrete samples analysed in this work were prepared from blended portland cement cem ii 32.5r mixed with siliceous aggregates with a size of 0–2.5mm. the material underwent irradiation in the lvr-15 research reactor in research centre řež to the target neutron fluence of 1.6–1.8 · 1019 n cm−2 (e>0.1mev) and received the gamma dose of ∼ 500mgy. the temperatures during irradiation were kept within a range of 50–60 °c. the mechanical properties of the individual phases in the cementitious matrix after neutron irradiation were assessed by the nanoindentation. to track the evolution of the nanomechanical properties of individual components under radiation, a comparison was made between the mechanical properties of neutron-irradiated material and pristine concrete. keywords: concrete, nanoindentation, neutron irradiation, mechanical properties. 1. introduction with increasing demands for power generation and nuclear safety all materials used in nuclear reactor are a subject of precise examination regarding degradation caused by long-term radiation exposure [1, 2]. concrete is an essential construction material for various types of nuclear reactor power plants, providing the structural foundation for the reactor pressure vessel and serving as a component of radiation shielding. the irradiation effects influence the behaviour of the concrete structure, leading over time to alterations in its mechanical properties and potential failure [3, 4]. the material degradation caused by gamma and neutron radiation significantly impacts the functional and performance requirements of the shield [5]. microscale analyses of the concrete material are needed to observe the actual changes in the material via irradiation. in this work, microscopic analyses are combined with micromechanical testing to evaluate the effects of neutron irradiation on the microstructure of the concrete. specifically, cement paste of the concrete and its major phases are a matter of examination. the mechanical properties of the cement paste are measured by nanoindentation testing. typically, this can be done by a matrix array of indents over the large area with subsequent statistical deconvolution 1postdoctoral researcher at czech technical university in prague, orcid: 0000-0002-5635-695x. 2professor at czech technical university in prague, orcid: 0000-0002-3565-8182. of the data [5–7]. however, this method may be inadequate for distinguishing the mechanical properties of irradiated materials from those of the reference equivalent. this limitation arises due to the overlapping measured values of the individual phases present in the cement paste, as the anticipated changes in mechanical properties after the irradiation are within the range of a few percent. therefore, we propose an alternative approach in which nanoindentation measurements are manually positioned, and the indented phases are identified using optical microscopy and scanning electron microscopy (sem) coupled with energy dispersive x-ray spectroscopy (eds). the resulting data from measurements of individual phases could serve as a key to interpreting extensive datasets obtained from wide area matrix measurements. 2. materials and methods 2.1. concrete the concrete samples were manufactured using the standard procedure following čsn en 206-1 [8]. the used binder component was blended portland cement cem ii 32.5r (portland cement 62%, limestone 11%, fly ash 21%, gypsum 6% by mass) mixed with siliceous aggregates with a grain size of 0–2.5mm used as a filler. the samples were left for 28 days in closed containers for curing and hardening, subsequently stored for 18 months in laboratory conditions with t = 23 ± 1 °c, rh 45 ± 5% and afterwards cut into slices with ap30 https://doi.org/10.14311/app.2024.50.0030 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 50/2024 nanomechanical properties of neutron-irradiated concrete (a). (b). figure 1. cut neutron irradiated concrete slices (a). mounted concrete sample on a steel block (b). prox. size of 28× 28× 5mm, dried at 60% for 7 days and then placed in the irradiation channel and assembly designed and developed for long-term irradiation campaign in lvr-15 research reactor. the irradiation was carried out in 6 campaigns over a period of 170 days [9]. the reference cement sample was produced using the original recipe, the composition of portland cement cem ii 32.5r slightly differed (portland cement 76%, limestone 18%, gypsum 6% by mass). 2.2. neutron irradiation neutron irradiation was performed in the lvr-15 research reactor with a power of 10 mw in research centre řež to the target neutron fluence of 1.6–1.8 · 1019 n cm−2 (e>0.1mev) with the received gamma dose of ∼ 500mgy. the temperatures during irradiation were kept within a range of 50–60 °c. such fast neutron fluences correspond approximately to 40 years of operation of the biological shielding components at the dukovany nuclear power plant (npp-edu) of vver-440 type. this allows for the simulation of concrete degradation due to neutron flux corresponding to the entire planned lifespan of the npp. after the irradiation, the housing with samples were transported to hot-cells, where cutting, dismounting, and sorting was performed using remote manipulators. samples intended for nanoindentation were transferred to shielded glove-box for further surface preparation. 2.3. sample preparation after the neutron irradiation, the concrete samples were cut in gamma-shielded glove box into smaller pieces in cross-section using gravitational low-speed saw (buehler). the dimensions of cut sample were approx. 1.5× 1.3× 0.5 cm. the reduction of sample dimensions was necessary for the surface preparation process and for the overall reduction of radioactivity of the sample. cut concrete samples are shown in figure 1a. after the cutting, the irradiated concrete sample was glued to cylindrical steel block using hot wax (figure 1b). mounting of the sample was performed for easier manipulation during the hand-polishing process figure 2. light optical microscopy image of cement phase after grinding. and for better handling during nanoindentation and sem analyses. surface preparation of the neutron irradiated concrete sample was performed by dry hand grinding using silicon carbide sandpapers of 400–4 000 grit in gamma-shielded glove box. no water was used during the grinding process to prevent further hydration of the cement paste and to achieve uniform flat surface. adding water during the grinding process would cause higher erosion in the cement paste and therefore uneven surface. for those two reasons, further polishing using suspensions was not used. the surface of the ground irradiated concrete sample was washed with pure ethanol after each grinding step. the whole surface preparation process was identically performed for the reference non-irradiated concrete sample. quality of prepared surface was assessed by light optical microscopy. the cement paste area of interest (light) is shown in figure 2. larger dark areas in the image are siliceous aggregates that are not a subject of examination in this work. the surface in figure 2 has uniform flatness and good visibility of features in the cement paste. visible scratches on the surface are an unavoidable phenomenon due to the applied surface preparation method. however, the unscratched areas between the scratches can be hand selected for nanoindentation measurements in nanoindenter scanning probe microscopy (spm) mode. a 20 nm conductive carbon layer was deposited on the ground surface prior to sem measurements with pvd carbon coater. 2.4. nanoindentation nanoindentation measurements were performed in gamma-shielded hot cell with bruker ti 950 nanoindenter using berkovich diamond indenter tip at room temperature. positions for the measurements were hand selected on the spm scan of the surface performed by the nanoindenter tip. individual selected positions are shown in figure 3. the dimensions of the spm scan area were 70× 70 µm. load-controlled indentation mode with 4mn max. indentation force was selected for nanoindentation 31 o. libera, j. němeček, p. halodová et al. acta polytechnica ctu proceedings figure 3. hand selected indent positions on the spm scan image of the cement paste. measurements. the estimated size of indented area using this load in cement paste is approx. 3µm in diameter with indentation depth in the range of 300– 500 nm. a custom load function with three stages was used: 1 s linear load, 10 s hold, 1 s linear unload. the hold period was used for material relaxation under load and for possible short-term creep behaviour. measured load-displacement curves were analysed by the oliver-pharr method [10]. tip area function was calibrated using fused quartz calibration standard prior to all measurements. 2.5. sem analyses sem analyses were performed on tescan mira gmu electron microscope equipped with eds detector (oxford instruments). surface imaging was performed using se and bse imaging mode. eds chemical mapping were performed over nanoindentation measurement areas for further identification of individual cement paste phases based on chemical composition. 2.6. results and discussion the elemental composition of the scanned cement paste area of the neutron irradiated concrete is shown as eds sum spectrum and individual maps of the major elements in figure 4. majority of the area is composed of ca-si hydrate phase (c-s-h). areas with dominant ca concentration indicate the presence of portlandite ca(oh)2 phase. the al map revealed the presence of larger patches of unreacted porous fly ash particles which are usually round or oval in shape. residual clinker also occurs locally. identified phases are labelled in figure 5a, which is a composition of selected eds maps with se image of the measured area on the background. the same image is complemented with spm image with indent positions overlay in figure 5b. despite the visible scratches caused by dry hand grinding preparation method, the resulting microstructure visibility, and surface flatness of the cement paste area of the concrete were sufficient for precise indent figure 4. distribution of selected elements in the cement paste of neutron irradiated concrete analysed by sem eds. (a). (b). figure 5. selected elemental distribution maps overlay on se image of indented area (a) and the same image combined with spm image with highlighted indent positions (b). positioning in the spm nanoindentation mode. measured nanoindentation hardness (hit) and reduced modulus (er) results relative to the identified phases are shown in figure 6a, 6b. the difference between each phase is clearly visible for both hit and er val32 vol. 50/2024 nanomechanical properties of neutron-irradiated concrete (a). (b). figure 6. comparison of measured reduced modulus (er) and hardness (hit) between identified phases in cement paste area of the neutron irradiated concrete. ues. however, without phase identification knowledge gained with the use of correlation of sem-eds and spm scans, lower measured values of fly ash and portlandite would be easily mixed with c-s-h gel values. the same applies for high fly ash values and clinker. the high scatter of measured nanoindentation values for fly ash is most likely caused by high porosity of the particles. this causes variations in indenter tip contact and uneven deformation area during nanoindentation testing. the variance of measured values of the c-s-h phase is likely caused by multiple factors. first, the surface in the c-s-h area is uneven and to certain extent porous, which means that same nanoindentation effects as for the fly ash phase will apply for c-s-h phase. and second, from previous investigation it is known that c-s-h phase can be divided into two sub-categories: low density (ld) and high density (hd) c-s-h [11]. these two types of c-s-h vary in mechanical properties, thus create the variety of measured results. the c-s-h phases present in cement paste are mixed with other minor phases and are specifically designated as inner (hd) and outer (ld) product [12]. using sem se imaging on the reference concrete (figure 7a, 7b), hd c-s-h areas are visible as lighter areas and ld c-s-h as darker. however, this differentiation is not possible on neutron irradiated concrete, as is shown in figure 7, where both c-s-h products are not distinguishable. therefore, it is to be assumed that it is a mixture of both ld and hd types. (a). (b). figure 7. se image of c-s-h gel distribution on reference non-irradiated concrete (a) and neutron irradiated concrete (b). although it was previously assumed that the effects of neutrons on cement paste would be small or negligible [13, 14], recent research suggests that degradation of cement paste components due to neutron interaction also needs to be considered. the change of surface morphology and chemical bonding of c-s-h globules after irradiation was reported by [15]. other authors have described changes in nano-porosity due to the effects of radiolysis, carbonation [16], formation of secondary vaterite [16], and, on the other hand, also a certain self-healing effect in c-s-h gels [17]. a comparison of er and hit of ld and hd cs-h phases from nanoindentation measurements on reference non-irradiated concrete and mixed c-s-h phase from nanoindentation measurements on neutron and gamma irradiated concrete was made. the results shown in figure 8 indicate that the measured values of the irradiated c-s-h phase are in between the values of the hd and ld c-s-h on reference material and we observe a greater scatter in the measured er and hit values of the neutron-irradiated c-s-h phase. this 33 o. libera, j. němeček, p. halodová et al. acta polytechnica ctu proceedings figure 8. comparison of averaged nanoindentation results of er and hit of low density (ld) and high density (hd) c-s-h phase on reference non-irradiated concrete and mixed c-s-h phase on neutron and gamma irradiated concrete. measurement supports the previous findings that the neutron irradiation has an effect on the mechanical behavior of the c-s-h phase. this effect is to be verified by further investigation. 2.7. conclusions nanomechanical properties of the hydrated cement phases after neutron irradiation were investigated and these conclusions can be drawn: • it was verified that the dry grinding surface preparation method for nanoindentation measurements is suitable for neutron-irradiated concrete. • sem images in combination with sem-eds chemical maps over the indented cement areas provided useful identification of the measured phases. measured mechanical properties were linked to the individual phases creating, separated datasets for each measured phase. • the correlative approach effectively compares overlapping phase data, which is challenging with large array methods without phase identification, and combining both methods yields extensive statistical data with improved phase identification. • the ld and hd types of the c-s-h phase in neutron-irradiated concrete appear indistinguishable in sem images, indicating altered properties due to irradiation. the measured er and hit values of the neutron-irradiated c-s-h phase show greater variability compared to non-irradiated material. list of symbols npp nuclear power plant vver-440 water-water power reactor with electrical output of 440mw sem scanning electron microscope eds energy-dispersive spectroscopy spm scanning probe microscopy se secondary electron image bse backscattered electron image pvd physical vapor deposition c-s-h calcium-silicate hydrate hit hardness [gpa] er reduced modulus [gpa] ld low density c-s-h gel hd high density c-s-h gel acknowledgements this work was financially supported by the project of the czech science foundation grant number 23-05435s. the presented results were obtained using the cicrr infrastructure, which is financially supported by the ministry of education, youth and sports – project lm2023041. references [1] k. l. murty, i. charit. structural materials for gen-iv nuclear reactors: challenges and opportunities. journal of nuclear materials 383(1):189–195, 2008. advances in nuclear materials: processing, performance and phenomena. https://doi.org/10.1016/j.jnucmat.2008.08.044 [2] l. malerba, a. al mazouzi, m. bertolus, et al. materials for sustainable nuclear energy: a european strategic research and innovation agenda for all reactor generations. energies 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https://doi.org/10.3151/jact.15.440 [14] o. kontani, y. ichikawa, a. ishizawa, et al. irradiation effects on concrete structures. in infrastructure systems for nuclear energy, pp. 459–473. 2014. https://doi.org/10.1002/9781118536254.ch27 [15] k. park, c.-w. chung, c. j. lee, t.-h. kwon. investigation of irradiation effects on microstructure of cement paste using small-angle neutron scattering. cement and concrete research 123:105791, 2019. https://doi.org/10.1016/j.cemconres.2019.105791 [16] y. le pape. 3.18 – radiation effects in concrete for nuclear systems. in r. j. m. konings, r. e. stoller (eds.), comprehensive nuclear materials (second edition), pp. 592–614. 2020. https: //doi.org/10.1016/b978-0-12-803581-8.11617-0 [17] a. alex, b. freeman, a. jefferson, e. masoero. carbonation and self-healing in concrete: kinetic monte carlo simulations of mineralization. cement and concrete composites 144:105281, 2023. https: //doi.org/10.1016/j.cemconcomp.2023.105281 35 https://doi.org/10.1016/j.matchar.2007.05.018 https://doi.org/10.1557/jmr.1992.1564 https://doi.org/10.1016/s0008-8846(03)00230-8 https://doi.org/10.1016/j.cscm.2023.e02589 https://doi.org/10.3151/jact.15.440 https://doi.org/10.1002/9781118536254.ch27 https://doi.org/10.1016/j.cemconres.2019.105791 https://doi.org/10.1016/b978-0-12-803581-8.11617-0 https://doi.org/10.1016/b978-0-12-803581-8.11617-0 https://doi.org/10.1016/j.cemconcomp.2023.105281 https://doi.org/10.1016/j.cemconcomp.2023.105281 acta polytechnica ctu proceedings 50:30–35, 2024 1 introduction 2 materials and methods 2.1 concrete 2.2 neutron irradiation 2.3 sample preparation 2.4 nanoindentation 2.5 sem analyses 2.6 results and discussion 2.7 conclusions list of symbols acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2024.47.0079 acta polytechnica ctu proceedings 47:79–83, 2024 © 2024 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague development of hydration temperatures on cement pastes milan meruňka∗, petr šperling, rudolf hela brno university of technology, faculty of civil engineering, institute of technology of building materials and components, veveří 331/95, 602 00 brno, czech republic ∗ corresponding author: 176336@vutbr.cz abstract. this article deals with the possible combination of high temperature fly ash with fluidised bed fly ash. the main objective of the experiment is to verify the effect of the combination of these two energy by-products on simpler systems – cement pastes. in the experimental part, the evolution of hydration temperatures and the time course of volume changes were monitored. the results obtained indicate that the addition of fluid fly ash to the mixture intensifies the hydration reaction, despite the increase in the value of the water to cement ratio of the mixture. at the same time, there is a slight increase in volume changes, which can be considered as a very valuable contribution to technical practice, taking into account possible ecological or economic aspects. keywords: high temperature fly ash, fluidised fly ash, volume changes, cement paste shrinkage. 1. introduction not only concrete mix producers, but also cement producers, are under increasing pressure to reduce harmful emissions into the air. one possible solution is the production and use of portland blended cements, which have a lower content of the basic component – portland clinker – and an increased content of another main component – e.g. silica ash, blast furnace granulated slag or limestone [1]. another way to reduce the amount of cement in the concrete mix is to use so-called active admixtures. these are substances that are involved in the hydration processes in some way and contribute to better physical-mechanical parameters. such substances include, for example, hightemperature fly ash. this is a by-product of the so-called high-temperature combustion of coal. the use of this type of admixture in concrete mixtures can positively influence the rheology [2] or the course of the hydration reaction [3], which is widely used especially in massive structures. however, the gradual transition to a more environmentally friendly – so-called fluidised bed – method of coal combustion is associated with the creation of a completely new and different energy by-product – socalled fluidised bed ash. the grains of this commodity are characterised by an unsuitable chemical composition, which is caused by the more environmentally friendly variant of flue gas desulphurisation. [4] the increased content of reactive cao and sulphates in the cement composite could have a negative effect on the microstructure of the already hardened concrete due to chemical reactions and the possible formation of secondary ettringite or monosulphate. if these reactions took place at a stage when the concrete was still in a plastic state, the formation of ettringite would not be a defect. one possible alternative solution could be at least a partial replacement of high temperature fly ash by fluidised bed fly ash. according to previously published studies, it is assumed that reactive free cao could provide a suitable basis for the initiation and progress of hydration reactions in the cement composite [5], which could positively affect, for example, mechanical parameters [6] and, as a result of the chemical composition of the fluidised fly ash, primary ettringite and monosulphate could be formed, which could at least partially reduce the magnitude of volume changes. 2. experimental part at the beginning of the experiment, the starting materials had to be carefully analysed. specifically, the raw material was cem ii/a-s 42.5 r portland blended cement from the mokrá cement plant (heidelberg – cz). the high-temperature fly ash came from the production of the mělník (cz) lignite thermal power plant (mel) and the fluid fly ash (zl) was produced by heating plant zlín (cz). their specific surface area was determined using an automated blain apparatus and the granulometry of the binder and impurities was determined using a laser granulometer. the results are summarised in table 1. raw material specific gravity specific surface [g cm−3] [cm−2 g−1] cem ii/a-s 42,5 r, mokrá 3.11 4 010 high temperature fly ash 2.21 3 280mělník (mel) fluid ash zlin (zl) 2.82 9 930 table 1. selected physical parameters of input raw materials. in the next phase of the experiment, a total of 7 simpler systems – cement pastes – were mixed. one 79 https://doi.org/10.14311/app.2024.47.0079 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en m. meruňka, p. šperling, r. hela acta polytechnica ctu proceedings (a). (b). (c). figure 1. determination of the consistency of cement paste according to en 1015-3 [7] (a); calmetrix f-cal 8000 test rig for determining the development of hydration temperatures (b) and shrinkage cone test rig for measuring volume changes (c). recipe cem ii/a-s 42,5 r, high temperature fluid ash, water w* cone flow mokrá [g] fly ash, mělník [g] zlín [g] [g] [-] diameter [mm] ref 900 – – 322 0.36 190 25_mel 675 225 – 300 0.33 185 25_mel_zl_80:20 675 180 45 315 0.35 190 25_mel_zl_70:30 675 157.5 67.5 325 0.36 190 25_mel_zl_60:40 675 135 90 335 0.37 185 25_mel_zl_50:50 675 112.5 112.5 345 0.38 190 25_zl 675 – 225 395 0.44 185 * the water to cement ratio value was calculated as the ratio of the weight of water to the total weight of cement and admixtures, regardless of the k-value according to en 206+a2 [8] table 2. composition of individual cement pastes. cement paste reference, without any kind of fly ash. in the other cement paste, 25 % of the binder was replaced by high temperature fly ash (25_mel) or fluid fly ash (25_zl). in the case of the cement pastes with a combination of both types of fly ash, 25 % of the cement was replaced by high temperature fly ash, while in these mixes there was also a 20 %, 30 %, 40 % and 50 % replacement by weight of high temperature fly ash by fluidised fly ash. the marking of individual recipes is based on the relative mixing ratio of the binder components, e.g. 25_mel_zl_60:40 means that in the cement paste the cement has been replaced by high-temperature fly ash of 25 % and the relative mixing ratio of mel and zl fly ash is 60:40 % by weight. the consistency of all the cement pastes was determined according to en 1015-3 [7] with a cone flow diameter value of 190 ± 5 mm as shown in figure 1. this procedure was chosen to directly compare the combination of high temperature fly ash and fluidised fly ash in practical applicability. as can be observed in table 2, along with the increase in the amount of fluidised fly ash in the cement paste, the water to cement ratio value increased simultaneously. in engineering practice, the consistency of the produced concrete or other cement composite could be further modified using superplasticizing additives. however, its use was ruled out in this experiment due to the elimination of another parameter affecting the course of hydration reactions. determination of the hydration temperature history was carried out by the semi-diabatic method according to en 196-9 [9] using the calmetrix f-cal 8 000 test rig shown in figure 2 for 24 hours after mixing. this time interval was in all cases sufficient to reach the maximum hydration temperature. the value of which and the time required to reach it are summarized by table 3. recipe tmax [°c] time tmax [h:m] ref 73.3 11:10 25_mel 58.2 13:55 25_mel_zl_80:20 61.4 12:05 25_mel_zl_70:30 63.3 12:15 25_mel_zl_60:40 65.4 12:10 25_mel_zl_50:50 65.1 12:15 25_zl 73.2 12:20 table 3. maximum temperature reached and time required. 80 vol. 47/2024 development of hydration temperatures on cement pastes figure 2. evolution of hydration temperatures on cement pastes. recipe/time 1 hours 2 hours 3 hours 9 hours 18 hours 24 hours 72 hours [µm] [µm] [µm] [µm] [µm] [µm] [µm] ref -704.1 -675.2 -738.5 -957.5 -948.8 -949.8 -951.1 25_mel -510.5 -592.1 -710.8 -906.7 -903.8 -902.1 -903.9 25_mel_zl_80:20 -569.1 -679.1 -801.1 -1 057.4 -1 051.8 -1 048.3 -1 050.3 25_mel_zl_70:30 -496.3 -615.6 -733.3 -1 039.9 -1 038.6 -1 034.6 -1 037.2 25_mel_zl_60:40 -502.8 -604.9 -710.8 -1 021.1 -1 010.0 -1 001.6 -1 005.5 25_mel_zl_50:50 -509.6 -640.4 -711.4 -1 004.6 -999.3 -991.4 -995.2 25_zl -544.9 -680.5 -800.3 -1 252.1 -1 241.8 -1 226.4 -1 220.8 table 4. overview of volume changes of cement pastes over time. the determination of volume changes was carried out on a shrinkage cone measuring device from schleibinger geräte with a conical measuring vessel, see figure 3. a laser beam was positioned at the centre of the filled measuring vessel and recorded the volume changes of the cement paste with a resolution of 1/10 micron every 5 minutes throughout the measurement period. in this part of the experiment, the volume changes were determined for 72 hours on each cement paste. a significant proportion of the volume changes occur due to chemical and plastic shrinkage in the first 9 hours of curing. thereafter, the values of volume changes remain almost constant. for better clarity, figure 3 presents the volume changes only in the first 18 hours after mixing and table 4 summarizes the values at subsequent time intervals. 3. results and discussion as shown in figure 2, the reference cement paste containing none of the fly ash types shows the fastest evolution of hydration temperature among the cement pastes, as expected. this cement paste reached its highest temperature of 73.3 °c at 11 hours and 10 minutes after mixing. at the same time, it can also be seen from figure 2 that the cement paste with 25 % cement replacement by high temperature fly ash (25_mel) confirmed the characteristic effect of this admixture on the cement composite. during the maturation process, the hydration reaction loosens, the hydration temperature increases more gradually and the maximum temperature reached decreases, which is widely used especially in massive structures. the maximum temperature of 58.2 °c reached for this paste was reached after 13 hours and 55 minutes and the water to cement ratio value of this paste was the lowest of all the formulations. it can also be seen from 81 m. meruňka, p. šperling, r. hela acta polytechnica ctu proceedings figure 3. the course of volume changes on cement pastes. figure 2 that as the amount of fly ash in the cement paste increased, the maximum hydration temperature increased gradually, almost at the same time, despite the increasing value of water to cement ratio, which has a significant effect on the intensity of the hydration reaction. the hydration temperature of the cement paste 25_zl also showed a very interesting evolution, reaching almost the same temperature as the reference mixture, despite the fact that 25 % cement was saved in the formulation and it contained 73 g more water than the reference cement paste. it can be assumed that the reason for the increase in maximum temperatures in cement pastes is the increasing amount of free reactive cao, which leads to the initiation and acceleration of hydration processes and more intense reaction in the cement composite. when determining the volume changes on the cement pastes, it was assumed that the reference cement paste, without any kind of fly ash content, would show the greatest shrinkage. it was assumed that the cement paste containing only high-temperature fly ash would exhibit a lower value of volume changes than the reference mixture due to the loosening of the hydration reaction, and it was also assumed that with increasing amounts of fluid fly ash in the mixture, the formation of new formations in the form of primary ettringite and monosulphate would occur, which could have a positive effect on the volume changes. however, this idea could not be confirmed. as can be seen from figure 3, the cement paste 25_mel showed the lowest value of volume changes. this can most likely be due to the partial replacement of cement with high temperature fly ash, which is associated with the loosening of the hydration reaction, which should have a positive effect on the volume changes of the cement composite in particular. on the other hand, the cement paste 25_zl showed the largest volumetric changes. at the same time, it is also well evident from figure 3 that the magnitude of volumetric changes gradually increased with increasing amount of fluid fly ash. however, it should be remembered that along with the increasing amount of fluid fly ash in the mixture, the value of water to cement ratio also increased, which also has a negative effect on the magnitude of volume changes of cement composites. 4. conclusion the use of suitable energy by-products for concrete production has become commonplace in engineering practice. however, due to the ever increasing costs of concrete and its basic ingredients (cement) and the increasing pressure to reduce the environmental burden, concrete mix manufacturers are looking for other possible alternative solutions to reduce production costs or emissions associated with the production and transportation of raw materials. one possible solution would be to use fluidised fly ash in combination with high temperature fly ash. based on the conducted experiment, it was confirmed that with increasing amount of fly ash in the cement paste, the hydration reaction becomes more 82 vol. 47/2024 development of hydration temperatures on cement pastes intense and the maximum hydration temperature increases despite the increasing value of water to cement ratio, which seems to be closely related to the free cao content. at the same time, it was found that there is a slight increase in the volume changes of the cementitious composite as the amount of fly ash increases, which is most likely due to the increasing amount in the mixture to achieve the same degree of workability. in engineering practice, however, this problem can be addressed by the use of suitable plasticizing admixtures, which should again reduce the amount of water in the concrete formulation. currently, fly ash is mainly used for reclamation or stabilization of soil bodies and its potential as an active admixture is not used at all. the use of this commodity for the production of concrete, at least in partial replacement with high temperature fly ash, could be advantageous not only from an economic point of view, but also from an ecological point of view. acknowledgements fast-j-23-8347 assessment of the effect of portland blended cements in combination of high temperature fly ash and fluidised fly ash on cement composite. references [1] j. gemrich, s. rollová. portlandské směsné cementy – cesta k udržitelnému rozvoji, 2021. [2023-11-20]. https://www.ebeton.cz/clanky/ 2021_1_4_portlandske-smesne-cementy-cesta-kudrzitelnemu-rozvoji-zkouska-fotky-bara/ [2] g. nazari, s. yamanaka, s. date. evaluation of fresh properties and rheology of mortar using carbon-free fly ash and normal fly ash. in materials science and engineering: technological advances and research results, vol. 1005 of materials science forum, pp. 76–81. trans tech publications ltd, 2020. https: //doi.org/10.4028/www.scientific.net/msf.1005.76 [3] l. g. ma, y. s. zhang. study on the effect of fly ash or silica fume to hydration heat of cement. in advanced building materials, vol. 250 of advanced materials research, pp. 4001–4004. trans tech publications ltd, 2011. https://doi.org/10.4028/www.scientific.net/ amr.250-253.4001 [4] m. skřeček. tvorba aft fází při hydrataci směsného cementu s fluidním popílkem. bachelor’s thesis, vysoké učení technické v brně, fakulta stavební, ústav technologie stavebních hmot a dílců, brno, 2014. [5] m. meruňka, m. ťažký, r. hela. effect of the combination of high temperature and fluid fly ash on the compressive strength of concrete and its dynamic modulus of elasticity. in materials science forum, vol. 1070, pp. 211–216. 2022. https://doi.org/10.4028/p-y42l5o [6] m. meruňka, m. ťažký, r. hela. vliv fluidního popílku na objemové změny betonu, 2023. [2023-11-20]. https://stavba.tzb-info.cz/beton-malty-omitky/ 24889-vliv-fluidniho-popilku-na-objemove-zmenybetonu [7] zkušební metody malt pro zdivo – část 3: stanovení konzistence čerstvé malty (s použitím střásacího stolku). standard, czech standard institute, prague, 2007. [8] beton – specifikace, vlastnosti, výroba a shoda. standard, czech standard institute, prague, 2021. [9] metody zkoušení cementu. část 3: stanovení dob tuhnutí a objemové stálosti. standard, czech standard institute, prague, 2017. 83 https://www.ebeton.cz/clanky/2021_1_4_portlandske-smesne-cementy-cesta-k-udrzitelnemu-rozvoji-zkouska-fotky-bara/ https://www.ebeton.cz/clanky/2021_1_4_portlandske-smesne-cementy-cesta-k-udrzitelnemu-rozvoji-zkouska-fotky-bara/ https://www.ebeton.cz/clanky/2021_1_4_portlandske-smesne-cementy-cesta-k-udrzitelnemu-rozvoji-zkouska-fotky-bara/ https://doi.org/10.4028/www.scientific.net/msf.1005.76 https://doi.org/10.4028/www.scientific.net/msf.1005.76 https://doi.org/10.4028/www.scientific.net/amr.250-253.4001 https://doi.org/10.4028/www.scientific.net/amr.250-253.4001 https://doi.org/10.4028/p-y42l5o https://stavba.tzb-info.cz/beton-malty-omitky/24889-vliv-fluidniho-popilku-na-objemove-zmeny-betonu https://stavba.tzb-info.cz/beton-malty-omitky/24889-vliv-fluidniho-popilku-na-objemove-zmeny-betonu https://stavba.tzb-info.cz/beton-malty-omitky/24889-vliv-fluidniho-popilku-na-objemove-zmeny-betonu acta polytechnica ctu proceedings 47:79–83, 2024 1 introduction 2 experimental part 3 results and discussion 4 conclusion acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2025.54.0034 acta polytechnica ctu proceedings 54:34–40, 2025 © 2025 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague optimising truss-based metamaterials for unimodal and pentamodal behaviour nataša jošková∗, marek tyburec, martin doškář czech technical university in prague, faculty of civil engineering, department of mechanics, thákurova 7, 166 29 prague 6, czech republic ∗ corresponding author: joskonat@cvut.cz abstract. our study focuses on the optimisation of the internal structure of unimodal and pentamodal metamaterials, modelled as three-dimensional linear elastic lattice structures. for optimisation, we represent the metamaterials with discrete truss models of their respective periodic unit cells (pucs), whose effective response is determined by the first-order numerical homogenisation. the optimisation is formulated as an inverse homogenisation problem with objective functions comprising a ratio of selected eigenvalues of the effective stiffness matrix, which allows us to dispense with the traditional volume constraint and solve the optimisation problem with a simple gradient method combined with the line search method. we demonstrate the efficacy of the formulation with a design of a unimodal material compliant in a chosen shear deformation mode and we also show that our formulation recovers the traditional pentamodal metafluid. keywords: optimisation, unimodal metamaterial, pentamodal metamaterial, first-order homogenisation, periodic unit cell. 1. introduction metamaterials have physical properties that surpass those commonly found in nature. moreover, these unique properties arise primarily from the microstructure of metamaterials rather than the chemical or physical characteristics of their bulk constituents [1]. in our study, we focus on three-dimensional unimodal and pentamodal mechanical metamaterials [2]. the modality of these materials is derived from the number of free modes of deformation [3], i.e., homogeneous deformations that do not result in an increase in the stored strain energy. in particular, a unimodal metamaterial is pliable in one specific direction but very stiff in the other modes of deformation. conversely, pentamodal metamaterials behave like a fluid (sometimes pentamodal metamaterials are called metafluids), i.e., while they are almost incompressible, they can be easily deformed in a deviatoric way. because of their unique properties, these metamaterials can be applied in various advanced fields. one of the most extensively researched applications is their ability to create acoustic band gaps – specific frequency ranges in which sound waves are unable to propagate through the material due to disrupted transmission [4, 5]. this behaviour makes them highly effective for applications in noise control, vibration isolation, and acoustic filtering, even underwater [6]. another potential application is for cloaking devices that transform waves to make objects invisible or undetectable by bending light or sound, effectively preventing detection or visual observation [7]. the ability to control the propagation of elastic waves also makes them highly effective for seismic isolation [8, 9]. multimodal metamaterials have garnered significant research interest, with pentamodal metamaterials being a primary focus. however, much of the research involves modifying the original pentamodal structure introduced by milton and cherkaev [3] to enhance its overall performance. this includes investigating different shapes [11] and sizes [12] of cross-sectional areas, adding spherical masses to the nodes [13] and incorporating stiffening plates between the metamaterial layers [14], even with different microstructures [10]. in addition, researchers managed both to introduce anisotropy by shifting the central part of the microstructure [15, 16], and, conversely, employ symmetrical designs [17]. in contrast, optimising metamaterial topology through inverse homogenisation, pioneered by the seminal work of sigmund [18], often revolves around enhancing the bulk or shear modulus [19], tailoring poisson’s ratio [20], and designing materials that are both lightweight and stiff [21] or that follow a predefined macroscopic response in the case of nonlinear models [22]. relatively few studies have focused on optimising multimodal metamaterial microstructures. in addition to inverse homogenisation, genetic algorithms have been used to optimise the ratio of eigenfrequencies [23] and components of the homogenised stiffness matrix [24], or to maximise bandwidth [25] to improve the acoustic band gap. in this work, we use the inverse homogenisation with the objective function based on the ratio of effective stiffness’s eigenvalues (or its analogues), distinct from previously discussed approaches. our approach allows for the optimisation of specific deformation modes and enables the analytical expression of the objective function’s gradient. in addition, it naturally avoids 34 https://doi.org/10.14311/app.2025.54.0034 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 54/2025 optimising truss-based metamaterials for unimodal . . . 0 1 y 0.5 0.8 0.6 z 0.4 0 0.2 0 x 0.5 11 0 2 y 1 1.5 1 z 0 0.5 0 0.5 x 1 1.5 22 figure 1. two periodic ground structures considered in the study: smaller ground structure with dimensions of 1 × 1 × 1 (left) and larger ground structure with dimensions of 2 × 2 × 2 (right). the need for the volume constraint. to keep this contribution self-contained, the methodology section briefly covers also the essentials of the first-order homogenisation (section 2.1) in addition to the objective formulation and the adopted optimisation strategy (section 2.3). we demonstrate the efficacy of the proposed objective formulation with two examples (one for the unimodal and one for pentamodal metamaterial) in section 3 and summarise our findings in section 4. 2. methodology 2.1. numerical model assuming the metamaterial as a three-dimensional periodic truss structure allowed us to investigate the response of a periodic unit cell (puc) as its representative volume element. we modelled the puc with a ground structure containing discrete 3d trusses with predefined positions and orientations as shown in figure 1. as the shape of cross sections does not affect the stiffness of the truss structure, for this study, we additionally assumed circular cross sections with area ai for each truss rod i as the primal unknowns for optimisation, collectively stored in a vector of cross-sectional areas a. to determine the mechanical response of the puc, we used a linear truss model, governed by the global stiffness matrix k(a) assembled from the local contributions of individual rods such that formally k(a) = ∑ i lt i tt i kℓ i(ai)tili , (1) where li denotes boolean localisation matrices, ti is a transformation matrix containing directional cosines of the i-th rod, and the element stiffness matrix kℓ i in the local coordinate system reads as kℓ i(ai) = eai li [ 1 −1 −1 1 ] , (2) where e stands for young’s modulus and li refers to the initial length of the i-th rod. 2.2. first-order homogenisation the homogenisation determines the effective response of a heterogeneous puc as if it were an equivalent homogeneous material point subjected to the same uniform strain field e. for the linear problem considered here, the homogenised response is captured by the effective stiffness matrix dhom(a). in our study, we employed the first order numerical homogenisation [26] to obtain the effective properties of the metamaterial. the total displacement field u⃗(x⃗) was thus assumed in the form u⃗(x⃗) = u⃗ e(x⃗) + u⃗ ∗(x⃗) , (3) where u⃗ e denotes the macroscopic part of the displacement field and u⃗∗ is the fluctuation caused by the heterogeneity of the metamaterial [27]. for the discrete system, degrees of freedom (dofs) related to the macroscopic displacement of the j-th node are coupled to the vectorial representation of the symmetric second-order tensor e via matrix qj containing coordinates of node j, ue j = xj 0 0 0 1 2 zj 1 2 yj 0 yj 0 1 2 zj 0 1 2 xj 0 0 zj 1 2 yj 1 2 xj 0  e = qj e . (4) the displacement degrees of freedom u, including both the macroscopic and fluctuation part, then follow as u = [ q i ] [ e u∗ ] = q̂ û , (5) where q contains vertically concatenated contributions qj , i stands for the identity matrix, and u∗ denotes the fluctuation dofs. due to the assumed periodic boundary conditions, not all dofs in u∗ are independent. accounting for the periodic sourceimage pairs and preventing rigid body motions by setting zero fluctuation at one selected node, the fluctuation dofs can be expressed in terms of a subset of unknowns υ, û = [ e u∗ ] = [ i 0 0 p ] [ e υ ] = p̂ υ̂ , (6) 35 n. jošková, m. tyburec, m. doškář acta polytechnica ctu proceedings with p being a boolean matrix facilitating the discussed boundary conditions. combining the discrete first-order displacement decomposition (5) and periodicity enforcement (6) leads to the strain energy formula of a puc e(e, υ) = 1 2 υ̂tp̂tq̂tk(a) q̂ p̂ υ̂ = 1 2 [ e υ ]t [ qtk(a)q qtk(a)p ptk(a)q ptk(a)p ] [ e υ ] = 1 2 [ e υ ]t [ k̂ee(a) k̂eυ(a) k̂υe(a) k̂υυ(a) ] [ e υ ] . (7) from the homogenisation perspective, υ can be treated as internal unknowns, which can be uniquely solved for a prescribed e, leaving us with the condensed form ẽ(e) = 1 2 et ( k̂ee(a) − k̂eυ(a)k̂−1 υυ (a)k̂υe(a) ) e = 1 2 etkeff(a)e . (8) comparing the last row of (8) with an expression for the strain energy stored in a homogeneous material of volume v subjected to e yields the final formula for the effective stiffness matrix dhom(a) = 1 v keff(a) . (9) for a detailed exposition and discussion of the adopted homogenisation scheme, we kindly refer the reader to our previous work [28]. 2.3. topology optimisation to optimise the structure of multimodal metamaterials, we utilised the inverse homogenisation [18] with an objective function o comprising a ratio between a projected stiffness α, defined via a unit modal deformation p as α(a) = ptdhom(a)p , (10) and k-th largest eigenvalue λk of the residual stiffness matrix dres obtained by subtracting the contribution of the projected stiffness α, i.e. dres(a) = dhom(a) − α(a)ppt . (11) in particular, the objective function for a unimodal metamaterial contains the fifth largest eigenvalue (out of six) of dres, as its goal is to maximise the gap between the compliant mode pertinent to α and the second smallest stiffness eigenvalue (recall that the smallest eigenvalue of dres is zero by definition (11)), oi(a) = λ5(a) αi(a) , (12) while the objective function for a pentamodal material includes the largest eigenvalue, ov(a) = αv(a) λ1(a) , (13) because all other stiffness components shall vanish compared to the stiffness αv in a predefined mode p. this objective function is volume independent as both α and λ scale linearly with the linear scaling of a, and thus no additional volume constraint is needed. formally, our optimisation problem reads max a o•(a) (14a) a ≤ ai ≤ a , (14b) where we set the upper bound a such that the diameter of the largest circular cross-section is at maximum one tenth of the shortest truss rod in the ground structure, and the lower bound a = 10−4 · a is posed to avoid an ill-conditioned system in homogenisation (8). note that • stands either for i or for v such that (14) covers both (12) and (13). despite a seemingly complex structure of the objective, a gradient with respect to the design variables a is readily available via the chain rule, i.e. ∂ ∂ai oi(a) = ∂λ5(a) ∂ai αi(a) − ∂αi(a) ∂ai λ5(a) αi2(a) (15) and ∂ ∂ai ov(a) = ∂αv(a) ∂ai λ1(a) − ∂λ1(a) ∂ai αv(a) λ2 1(a) , (16) respectively. the sensitivity of the projected stiffness α with a given p follows directly from the definition (10) ∂α(a) ∂ai = pt ∂dhom(a) ∂ai p . (17) a similar expression also holds for the sensitivity of the k-th largest eigenvalue, that is ∂λk(a) ∂ai = vt k ∂dres(a) ∂ai vk , (18) where vk is the eigenvector pertinent to the k-th largest eigenvalue and ∂dres(a) ∂ai = ∂dhom(a) ∂ai − ∂α(a) ∂ai p pt . (19) the last missing link in the chain rule is the sensitivity of the effective stiffness matrix ∂dhom(a) ∂ai = 1 v ∂keff(a) ∂ai = 1 v ∂ ∂ai ( k̂ee(a) − k̂eυ(a) [ k̂υυ(a) ]−1 k̂υe(a) ) . (20) dropping the explicit dependence on a and using the terse notation k̂••,ai for the partial derivative with respect to ai, we obtain ∂keff ∂ai = k̂ee,ai − k̂eυ,ai k̂−1 υυ k̂υe + k̂eυk̂−1 υυ k̂υυ,ai k̂−1 υυ k̂υe − k̂eυk̂−1 υυ k̂υe,ai = k̂ee,ai − k̂eυ,ai θ + θtk̂υυ,ai θ − θtk̂υe,ai , (21) 36 vol. 54/2025 optimising truss-based metamaterials for unimodal . . . where θ is the solution to an adjoin problem k̂υυθ = k̂υe . (22) the sensitivities of the four sub-blocks k̂ee, k̂eυ, k̂υe, and k̂υυ follow the same pattern as they are (potentially asymmetric) projections of k(a), recall (7), and thus they all require only ∂k(a) ∂ai = lt i tt i e li [ 1 −1 −1 1 ] tili . (23) being able to express the gradient of the objective function analytically led us to the adoption of a steepest ascent method. to compensate for the fact that we did not compute a hessian, we supplemented the steepest ascent method with the line search method equipped with the armijo rule [29]. we started by randomly initialising feasible design variables a that satisfy the box constraints (14). in each iteration, we first calculated the gradient of the objective function ∇o using the expressions introduced above. next, we defined a modified gradient ∇̂o(a) by projecting the original one on the active box constraints such that ∇̂oi(a) =  0 if ai = a ∧ ∇oi(a) > 0 0 if ai = a ∧ ∇oi(a) < 0 ∇oi(a) otherwise. (24) the modified gradient then constituted the search direction for the inexact one-dimensional maximisation in the current iteration. starting with an initial step length computed as the minimum of candidate lengths ν̃i ensuring that the box constraints are not violated in the new state, i.e., ν̃i =  (a − ai)/∇̂oi(a) if ∇̂oi(a) > 0 (a − ai)/∇̂oi(a) if ∇̂oi(a) < 0 0 otherwise, (25) we halved the step length until a significant objective increase is achieved in terms of armijo rule with coefficient c1 set to 0.1. we iterated until both the relative change in design variables and the relative change in the objective value dropped below 10−6. the structure of the adopted optimisation strategy is summarised in algorithm 1. 3. results we demonstrate the proposed objective formulation with two optimisation problems: unimodal and pentamodal metamaterials. as stated in the previous section, both optimisation problems share the same formulation but differ in the definition of the objective function; recall eqs. (12) and (13). in addition, we show the results for two ground structures depicted in figure 1, with the larger ground structure being a periodic extension of the smaller one. in all cases, we assumed young’s modulus of elasticity e = 1. since the response of the metamaterial model is linear in e, the homogenised stiffness matrices can be understood as normalised with respect to e. algorithm 1 adopted optimisation strategy. randomly initialise a0; i = 0 while ∥ai − ai−1∥ ∧ ∥o(ai) − o(ai−1)∥ > 10−6 compute ∇o(ai) ∇̂o(ai) = modify ∇o(ai) compute ν̃ ν = min(ν̃) repeat anew = a + ν∇̂o(ai) ν = ν/2 until o(ai +ν∇̂o) ≥ o(ai)+c1ν∇̂o(ai)t∇̂o(ai) i = i + 1 ai = anew end 0 2 y 1 1.5 1 z 0 0.5 0 0.5 x 1 1.5 22 figure 2. optimised larger puc of a unimodal metamaterial with minimised shear modulus in the yz direction. 3.1. unimodal metamaterial we start with optimising the microstructure of a unimodal metamaterial with vanishing shear modulus in the yz direction. to this end, we set p = [ 0 0 0 1 0 0 ]t in the objective definition (12). since the whole problem is non-convex, the gradient method renders only local optima. consequently, the optimised microstructure depends on the initial distribution of cross-sectional areas. the best results obtained out of 100 independent starting points a0 are shown in figure 2 for both ground structures, with the line thicknesses indicating the size of the cross-sectional areas. the homogenised matrix dhom(aopt) of both optimised structures took the form dhom =  6.70 1.39 1.39 0.00 0.00 0.00 1.39 5.32 0.00 0.00 0.00 0.00 1.39 0.00 5.32 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 1.39 0.00 0.00 0.00 0.00 0.00 0.00 1.39  × 10−3 , (26) and exhibited almost zero value at the position dhom 4,4 . 37 n. jošková, m. tyburec, m. doškář acta polytechnica ctu proceedings 0 1 y 0.5 0.8 0.6 z 0.4 0 0.2 0 x 0.5 11 (a). 0 1 y 0.5 0.8 0.6 z 0.4 0 0.2 0 x 0.5 11 (b). 0 1 y 0.5 0.8 0.6 z 0.4 0 0.2 0 x 0.5 11 (c). figure 3. optimised pucs of unimodal metamaterials with minimised shear modulus in yz (a), xz (b) and xy (c) directions. the value of the associated objective function oi was 4.79 × 103. the best result obtained for the larger ground structure converged to a geometry that is a periodic extension of the best result obtained for the smaller ground structure, i.e. the larger ground structure comprises eight pucs. furthermore, we subsequently tested the optimisation of unimodal metamaterial for vanishing shear modules in the xz (p = [ 0 0 0 0 1 0 ]t) and xy (p = [ 0 0 0 0 0 1 ]t) planes. the optimised structures for these scenarios are rotated versions of the metamaterial with a minimised shear modulus in the yz direction, as shown in figure 3. the corresponding homogenised matrices are – up to a small numerical difference – only permutations of (26). these observations indicate a strong local optimum for such a design, corroborated further by the fact that the same design was achieved for the majority of independent runs with random initial distribution of truss cross-sectional areas. 3.2. pentamodal metamaterial as the second example, we optimised the same ground structures for a meta-fluid behaviour, using the objective definition (13) with p = [ 1√ 3 1√ 3 1√ 3 0 0 0 ]t . this objective proved to be more challenging than its unimodal counterpart, as there was no strong local optimum. different optimisation runs with different initial states yielded distinct results with suboptimal performance compared to the existing pentamodal design of milton and cherkaev [3], which was attainable with the ground structures we considered. a common remedy to suppress the sensitivity to local minima widely used in the community of structural optimisation is to use a separable local approximation such as the method of moving asymptotes [30]. here, we adopt a different strategy that builds on sequential 1d minimisation problems along individual design variables. this approach fits in the provided algorithm 1 by introducing a gradient modification. in each iteration, we randomly select a single design variable and compute the modified gradient specifically for that variable. if the modified gradient is non-zero, we perform a 1d line search while maintaining the values of the other design variables constant. if the modified gradient equals zero, we proceed to the subsequent iteration, iterating through the randomly selected design variables until the optimised structure is achieved. clearly, such a modification leads to an increase in the number of iterations. starting with all crosssectional areas at the lower bound produced a design that corresponded to the known pentamodal design by milton and cherkaev [3]. admittedly, the results remain highly sensitive to the initial conditions. even with random initialisation of cross-sectional areas from a uniform distribution [a; a 10 ], the majority of optimisation cases ended up in the known pentamodal design. we observed that optimisation with smaller values of initial variables tends to result in a higher objective function value than optimisation using larger ones. the best obtained pucs of the pentamodal material are shown in figure 4. similarly to the unimodal metamaterial, the larger puc converges towards a geometry composed of eight smaller identical pucs. for completeness, we list the homogenised matrix dhom dhom =  7.56 7.56 7.56 0.00 0.00 0.00 7.56 7.56 7.56 0.00 0.00 0.00 7.56 7.56 7.56 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00  × 10−4 , (27) with objective function achieving a value of 4.27 × 103. note that the result for the 2 × 2 × 2 ground structure features two interconnected networks and thus its homogenised response is twice as stiff compared to the stiffness of the classical metafluid structure shown in figure 5. 4. summary this contribution dealt with optimising metamaterials for desired unimodal and pentamodal behaviour. the material pucs were modelled as discrete truss systems, whose effective metamaterial properties were 38 vol. 54/2025 optimising truss-based metamaterials for unimodal . . . 0 1 y 0.5 0.8 0.6 z 0.4 0 0.2 0 x 0.5 11 (a). 0 2 y 1 1.5 1 z 0 0.5 0 0.5 x 1 1.5 22 (b). figure 4. optimised smaller (a) and larger (b) pucs of a pentamodal metamaterial with maximised bulk modulus. the larger puc of the pentamodal metamaterial consists of two independent structures (plotted in black and green). figure 5. examples of existing pentamodal materials: (left) a microstructure proposed by milton and cherkaev [3], and (right) an alternative design by guo and coworkers [10]. determined using the first-order numerical homogenisation, providing us with a link between their microstructure and macroscopic behaviour to be optimised. we formulated the objective function on the basis of a projected stiffness and sorted eigenvalues of the homogenised stiffness matrix. the unimodal metamaterials were designed to be significantly more compliant in a chosen shear direction compared to other modes of deformation. the pentamodal metamaterial targeted maximal bulk modulus while diminishing the remaining eigenvalues of the homogenised stiffness matrix. the optimisation was performed using the steepest ascent method, supplemented with the line search algorithm and constrained by upper and lower bounds for individual cross-sectional areas. to suppress the algorithm’s attraction to local minima, we proposed a simple modification of the steepest ascent, which sequentially performs a series of 1d minimisations along individual design variables. in the case of pentamodal material, this modification significantly improved the optimisation results by reducing the number of independent optimisation runs necessary to render a performant solution. acknowledgements nš and mt acknowledge support by the czech science foundation, project no. ga22-15524s. md’s work was supported by the grant agency of 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https://doi.org/10.1088/1402-4896/acb5cc https://doi.org/10.1016/s0045-7825(98)00227-8 https://doi.org/10.1016/j.compstruc.2016.01.003 https://doi.org/10.14311/app.2024.49.0020 https://doi.org/10.1002/nme.1620240207 acta polytechnica ctu proceedings 54:34–40, 2025 1 introduction 2 methodology 2.1 numerical model 2.2 first-order homogenisation 2.3 topology optimisation 3 results 3.1 unimodal metamaterial 3.2 pentamodal metamaterial 4 summary acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2023.40.0104 acta polytechnica ctu proceedings 40:104–110, 2023 © 2023 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague hygro-mechanical model for concrete pavement with long-term drying analysis jakub veselý∗, vít šmilauer czech technical university in prague, faculty of civil engineering, department of mechanics, thákurova 7, 166 29 prague 6, czech republic ∗ corresponding author: jakub.vesely.2@fsv.cvut.cz abstract. concrete pavements are subjected to the combination of moisture transport, heat transport and traffic loading. a hygro-mechanical 3d finite element model was created in oofem software in order to analyse the stress field and deformed shape from a long-term non-uniform drying. the model uses a staggered approach, solving moisture transfer weakly coupled with mps viscoelastic model for ageing concrete creep and shrinkage. moisture transport and mechanical sub-models are calibrated with lab experiments, long-term monitoring on d1 highway and data from 40 year old highway pavement. the slab geometry is 3.5×5.0×0.29 m, resting on elastic winkler-pasternak foundation. the validation covers autogenous and drying strain on the slab. the models predict drying-induced tensile stress up to 3.3 mpa, inducing additional loading on the slab, uncaptured by current design methods. keywords: concrete pavement, hygro-mechanical analysis, creep, shrinkage. 1. introduction concrete pavements present a proven solution for highways and airports due to its ability to withstand high mechanical loads with long service life compared to bituminous alternatives [1]. effects of wheel load, temperature and moisture generally need to be considered, as they influence stress, strain, deformations and service life [2]. for a detailed pavement analysis, finite element simulations with various factors have been carried out [3–7]. hydration and moisture transport generally induces autogeneous shrinkage strain, drying shrinkage strain with an impact on creep. early microcracks may occur due to internal or external restraints [8]. in addition, moisture gradients generate differential shrinkage between the top and the bottom, leading to warping [9, 10]. stress relaxation plays a role on the warping effect [11]. there are several approaches for creating a thermohygro-mechanical model for concrete. the most simple one uses a superposition principle, solving moisture, thermal, traffic load and asr effects separately [12]. due to a coupled nature of the problem, a staggered solution strategy offers more accurate analysis [13], solving moisture and heat transport and passing the obtained fields to a mechanical model. fully coupled problems with chemical submodels were proposed as well [14–16]. this paper presents a new hygro-mechanical 3d model, implemented in an open-source software oofem [17]. a staggered solution strategy uses solving of the weakly-coupled sub-models in the discretized time steps. the geometry uses 3d representation of a single rectangular concrete slab resting on winkler-pasternak foundation without dowel bars. the aim is to inspect overall performance of the concrete pavement under long-term drying loading scenario. a similar model has been published with limited calibration and interpretation [18]. 2. formulation of a hygro-mechanical model hygro-mechanical model solves transport of moisture and mechanical behavior of the slab. staggered solution strategy is adopted in the multiphysical model, partially separating transport transient problem from the mechanical one and passing transient moisture field to the mechanical problem. such an approach allows to define equivalent time in creep models or to calibrate constitutive laws independently. the models were implemented in oofem, an open-source and object-oriented software for finite element method [17]. 2.1. moisture transport a nonlinear moisture transport model describes concrete as a single-fluid medium with the governing equation: k(h)∂h ∂t = ∇ · [c(h)∇h] , (1) where h is the dimensionless pore relative humidity, k(h) [kg/m3] is the humidity-dependent moisture capacity (k(h) = ∂w ∂h which is derivative of the moisture content w(h) [kg/m3] with respect to the relative humidity), c(h) [kg/m/s] is the moisture permeability in the bažant-najjar’s form: c(h) = c1 α0 + 1 − α0 1 + ( 1−h 1−hc )n  . (2) newton (convection) boundary condition is used in the form qn = β(h− h∞) with the moisture transfer coefficient β. 104 https://doi.org/10.14311/app.2023.40.0104 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 40/2023 hygro-mechanical model for concrete pavement e0 e1 e2 em η1 η2 ηm solidification cs s σ σ s εe εv εf εsh εt ε ksh αt figure 1. rheological scheme of mps model. 2.2. mechanical model the mechanical model consists of three different components. the first one takes a concrete slab with a viscoelastic model for concrete with ageing based on microprestress solidification theory (mps) [19]. the slab is placed on elastic, 2d subsoil, winklerpasternak model. the interaction between the slab and the subsoil is controlled by interface elements. 2.2.1. linear viscoelastic material model for creep and shrinkage a constitutive model for creep and shrinkage originate from b3 model [20] extended with mps formulation [19]. the rheological model in figure 1 shows non-aging asymptotic elastic spring, solidifying kelvin chain, aging dashpot, shrinkage strain, and thermal strain. the viscosity ηf in the flow strain εf depends on the evolution of the microprestress. all these units are connected in series, manifesting total strain decomposition into individual contributions. the governing equation for mps theory reads [21]: η̇f + 1 µst0 ∣∣∣∣ṫ ln h+ ḣ h ∣∣∣∣ (µsηf ) p p−1 = ψs q4 , (3) where µs is a parameter with the dimension of fluidity, p is a dimensionless material parameter influencing the size effect (for p = ∞, the size effect disappears), ψs is a temperature and humidity dependent factor, q4 is a material parameter, t is the current temperature, and t0 is the reference temperature. equation (3) can be simplified into equation (4) under the assumption of constant temperature t = t0 as: η̇f + k3 ∣∣∣∣ ḣh ∣∣∣∣ ηp̃ f = ψs q4 , (4) with parameters: p̃ = p p− 1 , (5) k3 = µ 1 p−1 s . (6) the drying shrinkage is computed as: ˙εsh,d = kshḣ, (7) with shrinkage ratio ksh. the autogenous shrinkage εsh,au is approximated as: εsh,au = ε∞ sh,au [ 1 + (τau/te) w/c 0.38 ]rt,au , (8) where ε∞ sh,au is the ultimate value of autogenous shrinkage strain, w/c is water/cement ratio and parameters τau,rt,au control strain evolution in time te. 2.2.2. winkler-pasternak subsoil model elastic subsoil is treated as a 2d winkler-pasternak model, capturing normal and shear stiffness with c1 and c2 parameters [22–24]. the governing equation reads: f(z) = c1w(z) − c2 ∂2w(z) ∂z2 , (9) where f(z) is surface load and w(z) is displacement. 2.2.3. interface elements the interface elements allow separation between the slab and subsoil, eliminating tension stress. both meshes of slab and subsoil share coinciding nodes at the interface. the traction-separation law takes the form: tn = knδ, kn = k for compression, (10) kn = 0.01k for tension, where δ is displacement between two nodes, positive in separation. shear stiffness is assumed zero. interface elements lead generally to slower converge or even convergence loss hence it is important to use reasonable time step to induce gradual slab deformations. 3. results and discussion the presented numerical model stems from a monitored highway slab. the pilot project started in 2017 as a joint activity among the road and motorway directorate (řsd čr), a contractor skanska, a.s. and the czech technical university in prague [25]. the project involved 8 978 m of concrete pavement built on d1 highway přerov-lipník nad bečvou, the czech republic. the pavement was cast between 07/2018 and 09/2019 with the opening to traffic dec 12, 2019. the binder used a slag-blended, slow hardening binder composed of 75 % cem i 42.5 r(sc) + 25 % ggbfs smš 400, corresponding to cem ii/b-s 42.5 n. 105 jakub veselý, vít šmilauer acta polytechnica ctu proceedings figure 2. vibrating wire gauges under protective covers. detail at one assembled location. 3.1. long-term monitoring system a long-term monitoring system was designed and installed in one concrete pavement slab with dimensions of 3.5 × 5.0 × 0.29 m. the system records temperature and strains at six measuring locations [25]. each location contains three vibrating wire strain gauges located 50 mm from the surfaces and in the mid-height. all the gauges have integrated temperature sensors. in addition, one thermal gauge was placed 150 mm under the pavement in order to deliver the sub-base temperature. ambient air temperature and solar radiation sensors were installed as well. two-lift concrete technology started with a bottom layer 240 mm thick with 370 kg/m3 of the binder, following with the top layer 50 mm thick with 420 kg/m3 of the binder. a two-step installation process was adopted, utilizing protective covers which hid the strain gauges before the casting, see figure 2. after the first finisher had passed, the covers were removed, the gauges put in their positions and the empty space filled back with concrete using hand vibrators, see figure 2. finally, the top layer finalized the slab. almost 55 000 m3 of concrete was placed in both layers, complying to required c30/37 strength class, see figure 3. vibrating strain gauges measure relative head displacements, which can be decomposed to: ε = εve + εt + εas + εds + εf + . . . , (11) where partial strains represent viscoelasticity, temperature effects, autogenous shrinkage, drying shrinkage, fracturing strain, etc. figure 4 shows partial strains on the mid-plane, zeroed at 2 hours after the end of setting for all gauges. autogenous shrinkage plays a dominant role in the first week, reaching −70 µε in the transversal direction (w62). in the longitudinal direction, continuous casting led to prestressing, which adds additional strain. a small average drying shrinkage strain from −30 to −120 µε is apparent after 3 years of drying. the measured strains allowed calculating the curvature of the slab, assuming a planar deformation of the cross-section. figure 5 shows the total curvatures, capturing the temperature variations and demonstrating a slow positive drift due to top drying. 0 10 20 30 40 50 60 0 10 20 30 40 50 60 c u b e m e a n c o m p r. s tr e n g th ( m pa ) concrete age (day) cem 𝖨 42.5 r(sc) mokrá, top layer bottom layer 75% cem 𝖨 42.5 r(sc) + 25% slag smš 400, top bottom 0 1 2 3 4 5 0 10 20 30 40 50 60 m e a n s p lit . te n si le s tr e n g th ( m pa ) concrete age (day) cem 𝖨 42.5 r(sc) mokrá, top layer bottom layer 75% cem 𝖨 42.5 r(sc) + 25% slag smš 400, top bottom figure 3. compressive and tensile strength evolution of top and bottom concretes. -250 -200 -150 -100 -50 0 50 20 18 /0 8/ 01 20 19 /0 8/ 01 20 20 /0 8/ 01 20 21 /0 8/ 01 20 22 /0 8/ 01 pa rt ia l st ra in , εε t ( 1 0 -6 ) location 1 location 2 location 3 (discarded) location 4 location 5 location 6 figure 4. partial strains on 6 locations. -0.5 0.0 0.5 1.0 20 18 /0 8/ 01 20 19 /0 8/ 01 20 20 /0 8/ 01 20 21 /0 8/ 01 20 22 /0 8/ 01 to ta l cu rv a tu re , κ ( km -1 ) location 1 location 2 location 3 location 4 location 5 location 6 figure 5. total curvatures at 6 locations. 106 vol. 40/2023 hygro-mechanical model for concrete pavement parameter value unit parameter value unit m ec ha ni ca lm od el f c 45.0 mpa c1 70.0 mnm−3 cc cement content 370.0 kg/m3 c2 60.0 mnm−1 w/c ratio 0.4 α 10.0 · 10−6 °c−1 a/c ratio 4.91 k 200.0 mnm−1 t0 7 days k3 35 τ1 3 · 10−5 days τm 5000 days ksh 1.5 · 10−3 e 35.0 gpa ε∞ sh,au −87.0 ·10−6 ν 0.2 τau 1.05 days rt,au −4.5 αau 1.166 m oi st ur e tr an sfe r m od el c1 6.0 · 10−4 kg/m·s α0 1.0 k(h) = k1 160.0 kg/m3 βtop 0.05 kg/m2 · d βbottom 0.01 kg/m2 · d table 1. list of material input parameters. 3.2. slab geometry the slab is meshed using 3d, quadratic, 20 node brick elements. interaction between neighboring slabs is neglected. in moisture transfer simulation, the influence of air and soil is represented with newton boundary conditions. the overview of used material parameters for concrete and subsoil is summarized in table 1. parameters ε∞ sh,au,b4, τau, αau and rt,au control the evolution of autogenous shrinkage. the basic creep in the mps theory is influenced by the same four parameters q1–q4 as in the model b3, these are computed from f c, cc, w/c and a/c. the units of solidifying kelvin chain are defined with the lowest τ1 and the highest retardation time τm . time at the onset of drying is t0 = 7 days (for lab experiment t0 = 0.1 day), size effect for drying is neglected setting p = ∞. 3.3. hygro-mechanical simulation of long-term slab drying to simulate the long-term drying, the determination of material properties of concrete and relevant boundary conditions are necessary. material properties used in this paper are derived from drying shrinkage lab experiments. the kinetics of slab drying can be estimated from small lab samples 60 × 100 mm exposed to 50 % rh from the largest side. permeability (and hygric exchange coefficient) was calibrated from moisture transport model. boundary conditions are derived from a 40 year old concrete slab, which was removed during the demolition of d1 highway. the thickness of the old slab is only 240 mm compared to 290 mm used in current simulation. the moisture profile (see figure 6) and the fully saturated state were determined. we assume that the drying process is stabilized after 40 years, which identified ambient relative humidity as 65 % at the top and 80 % at the bottom. moisture transfer coefficients (βtop > βbottom) are set in such a way that the top drying becomes dominant. the linear moisture profile from figure 6 also implies that the permeability needs to stay constant, otherwise non-linear profile is obtained. the explanation for this phenomenon is likely a sorption isotherm and full resaturation with rain water. it was showed on prisms, that rewetting is orders of magnitude faster process than drying, thus occasional rain will lead to linear moisture profile [26]. this leads to setting α0 = 1 and c(h) = c1 according to equation (2). figure 7 shows shrinkage validation of numerical model both for lab experiment and for concrete pavement (strain gauge w52). the strain from gauge w52 was shifted by 40 µε to eliminate longitudinal prestressing. the predictions show that the slab will be drying for approximately 30 years. it should be mentioned that other gauges show smaller mid-plane shrinkage values, attributed likely to other partial strains and restraints. the initial drying takes place predominantly in top and bottom 100 mm of the slab, see figure 6. this leads to stress induction in these areas and subsequent redistribution in time. the early age stress field (first 2 years) shows tensile stress in the top part of the slab (around 3.3 mpa after 60 days and 2.2 mpa after 1 year, see figure 6). this will likely cause cracking in the slab when combined with temperature and traffic load. concrete slab should be able to cope with the resulting fracture process zone since the slab is continuously supported and controlled by displacement to a large extent. similar stresses were found on the analysis of restrained slab with thermo-hygro-mechanical model; exposing 150 mm thick slab to 60 % rh led to tensile stresses up to 3 mpa after 100 days [27]. 107 jakub veselý, vít šmilauer acta polytechnica ctu proceedings 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.5 0.6 0.7 0.8 0.9 1.0 d ep th ( m ) relative humidity (-) after 60 days after 1 year after 5 years after 15 years after 30 years real slab, after 40 years 0.00 0.05 0.10 0.15 0.20 0.25 0.30 -3 -2 -1 0 1 2 3 σx (mpa) figure 6. comparison of relative humidity profile (left) from fe simulation with real slab data after 40 years and σx (right) induced by long term drying. -800 -600 -400 -200 0 100 101 102 103 104 autogeneous shrinkageautogeneous shrinkageautogeneous shrinkage au to ge no us + d ry in g st ra in ( 10 -6 ) time of hydration (days) prism 100×60 mm, t0=0.1 d, rh=50% experiment at but slab 290 mm, t0=7 d, updated monitoring, w52+40 με figure 7. shrinkage validation of concrete slab with monitoring. after 2 years, the compression occurs within the top layer and the tensile stress occurs in the middle of the slab. the tensile stress reaches values up to 1.0 mpa, these are values lower than tensile strength of the concrete. the compression of the top layer of concrete is likely to help close the cracks developed in the earlier phase. the deformed shape induced by long-term drying is known as warping. the simulation predicts that the corners will raise up up to 1.7 mm when subjected to drying and self-weight (see figure 8). 4. conclusions this paper describes behavior of road concrete slab when subjected to the long-term drying. individual material models were calibrated based on lab and field experimental data, the main source being a long-term monitoring system on d1 highway. shrinkage was validated both for lab experiment and for the concrete pavement with moisture transport model. boundary conditions were calibrated from moisture profile of 40 year old pavement. drying induces high tensile stress on top surface, reaching 3.3 mpa after 60 days and 2.2 mpa after 1 year of drying. as the drying front advances, the stress profile changes and the surface becomes slightly compressed. slab warping is predicted as 1.7 mm corner uplift at the maximum. acknowledgements financial support for this work was provided by the czech science foundation under the project gačr 21-03118s and the czech technical university in prague, the grant sgs22/041/ohk1/1t/11. 108 vol. 40/2023 hygro-mechanical model for concrete pavement 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 100 101 102 103 104 co rn er u pl ift ( m m ) time of hydration (days) concrete pavement subjected to drying figure 8. warping induced by long-term drying of concrete. references [1] m. g. lay. handbook of road technology. crc press, 2009. 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[26] l. dohnalová, p. havlásek, v. šmilauer. behavior of predried mature concrete beams subject to partial wetting and drying cycles. acta polytechnica ctu proceedings 34:1–5, 2022. https://doi.org/10.14311/app.2022.34.0001 [27] j. gomes, r. carvalho, c. sousa, et al. 3d numerical simulation of the cracking behaviour of a rc one-way slab under the combined effect of thermal, shrinkage and external loads. engineering structures 212:110493, 2020. https://doi.org/10.1016/j.engstruct.2020.110493 110 http://www.oofem.org https://doi.org/10.3390/ma13235530 https://doi.org/10.1016/j.cemconres.2014.03.008 https://doi.org/10.1007/bf02485965 https://doi.org/10.1016/j.cemconres.2016.04.001 https://doi.org/10.1016/j.trgeo.2021.100696 https://doi.org/10.1007/s11071-019-04977-9 https://doi.org/10.14311/app.2022.34.0001 https://doi.org/10.1016/j.engstruct.2020.110493 acta polytechnica ctu proceedings 40:104–110, 2023 1 introduction 2 formulation of a hygro-mechanical model 2.1 moisture transport 2.2 mechanical model 2.2.1 linear viscoelastic material model for creep and shrinkage 2.2.2 winkler-pasternak subsoil model 2.2.3 interface elements 3 results and discussion 3.1 long-term monitoring system 3.2 slab geometry 3.3 hygro-mechanical simulation of long-term slab drying 4 conclusions acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2024.47.0035 acta polytechnica ctu proceedings 47:35–41, 2024 © 2024 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague possibilities of replacing natural sand in concrete kristina fořtová∗, tereza pavlů, diana mariaková, jakub řepka, tomáš vlach czech technical university in prague, university centre for energy efficient buildings, třinecká 1024, 273 43 buštěhrad, czech republic ∗ corresponding author: kristina.fortova@cvut.cz abstract. this study explores the potential of using fine recycled aggregate (fra) to replace natural sand in concrete production. the research was divided into two parts. in the first part, natural aggregate’s complete fine fraction (0–4 mm) was replaced with fra in conventional concrete for two strength classes. in the second part, individual fine components of high-performance concrete (hpc) were substituted with a similar fraction of fine recycled hpc and concrete aggregate. the physical and mechanical properties of these mixtures with recycled aggregate were evaluated against reference mixtures that contained only natural aggregate, following european standards. results showed that while most of the concrete and hpc mixtures with recycled aggregate showed slight decreases in properties compared to the reference mixes, the most significant reduction in mechanical properties was observed in the flexural tensile strength of the concrete mixture with fra, which decreased by over 50 %. the absorption by immersion increased by up to 85 %. the deterioration in properties must be considered during the design stage and only used in applications where it will not affect the functionality of the structures. keywords: fine recycled aggregate, highperformance concrete, natural sand replacement. 1. introduction concrete is one of the construction industry’s largest consumers of natural aggregate, relying heavily on primary resources such as river sand, gravel, cement, and water. the excessive extraction of natural sand (fna) can result in serious ecological problems, including changes in water direction, coastal erosion, and dead-end diversions. one solution to reduce the negative environmental impact of concrete production is replacing natural aggregate (na) with recycled aggregate (rca). the aggregate accounts for about 70 % of the largest component of concrete [1]. still, from a climate change perspective, it is a minor contributor to emissions, accounting for only about 15 % of the emissions generated by concrete production and transportation [2–7]. the environmental impact of rca can be influenced by its transportation, so it is important to find ways to use all fractions of recycled aggregate, such as using it as a substitute for natural sand in concrete mixtures, at the demolition site. the use of recycled aggregate (ra) in concrete is often associated with a decrease in mechanical properties and durability compared to conventional concrete (nac). to mitigate this reduction, cement or supplementary cementitious material (scm) is added [8]. recycled concrete aggregate (rca) is made of natural aggregate particles combined with adhered cement mortar, with quality depending on the demolition and recycling process, including the method, speed, and a number of crushing and sieving steps [9–13]. the properties of rca are affected by the type and size of the natural aggregate in the parent concrete, as well as the strength of the parent concrete. rca often presents challenges such as an irregular and angular shape and a porous, rough particle structure [14–16]. the use of fine recycled aggregate (fra) in concrete is even more complicated, as the measurement of its water absorption has not been clearly developed, with large variations in evaluation methods, and its absorbability during concrete manufacturing is also unknown. due to these factors, using fra (< 4 mm) in concrete is challenging, leading to standards worldwide that do not allow it as a substitute for natural aggregate. the properties of the mixtures produced and tested in the laboratory are described in each of the two parts of this research. while the first part deals with ordinary concrete, the second part follows up the high-performance concrete. these mixtures have very diverse properties, starting with the granulometry of the aggregate used in the mixture and ending with the values of the mechanical properties. therefore, different properties of the aggregate, which are important in individual cases, must also be taken into account during the design. however, the same properties of the mixture for both cases were investigated (only on different types of samples, corresponding to the character of the mixture). the results are further described in tables and graphs in the results and discussion part. 2. materials and methods this paper deals with the properties of conventional concrete with the replacement of fine natural aggregate (fna) with fine recycled aggregate. there is a 35 https://doi.org/10.14311/app.2024.47.0035 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en k. fořtová, t. pavlů, d. mariaková et al. acta polytechnica ctu proceedings complete replacement of fna with two types of recycled aggregate (ra) – fine recycled concrete aggregate (frca) and fine recycled masonry aggregate (frma) up to a grain size of 4 mm described in the first part of this research. the second part of the research consists of the complete replacement of individual fine-grained components of hpc with the fine recycled aggregate of two types – fine recycled high-performance concrete aggregate (frhpca) laboratory prepared by crushing up to grain size of 1 mm and the same type frca as in the first part of this research. the most critical mechanical-physical properties of both groups of samples, in the first part samples of conventional concrete and in the second part hpc mixtures, were tested and evaluated according to the relevant standards. as has already been stated many times, the main properties of concrete that affect its durability are its porosity and water absorption [17]. water absorption by immersion, which describes the transport behaviour of the material, was obtained on cube samples 100×100×100 mm (for conventional concrete) and beam fragments 100 × 100 × aprox. 80 mm (for hpc), respectively, according to astm c158520. cubes with dimensions of 150 × 150 × 150 mm (for conventional concrete) and 100 × 100 × 100 mm (for hpc), respectively, stored for 28 days during solidification and maturation in a permanent laboratory environment, were used for compressive strength testing according to en 12390-3 (2003). prismatic samples with dimensions of 100 × 100 × 400 mm and 40 × 40 × 160 mm, respectively, cured in water for 28 days, were used to test the flexural tensile strength according to en 12390-5 (2009). the static modulus of elasticity was tested according to en 12390-13 (2014), and the dynamic modulus of elasticity according to en 12504-4 (2005). for both tests, prismatic specimens with dimensions of 100 × 100 × 400 mm (for conventional concrete) and 40 × 40 × 160 mm (for hpc) were used, respectively. 3. complete replacement of fna in conventional concrete mixtures in total, six concrete mixtures were produced and tested to verify the possible replacement of fna with fra. for comparison, the concrete strength class with a compressive strength of 20 mpa, which is used for plain concrete, and the strength class with a compressive strength of 30 mpa, which is often used for structural concrete (reinforced concrete), were chosen. in each strength class, one reference mixture (nac 1, nac 2) is produced, which contains only natural aggregate with a grain size of 0–16 mm. in all other mixtures, natural gravel is used as coarse aggregate (4–16 mm), and fine natural aggregate (0– 4 mm) is replaced by fine recycled aggregate from construction and demolition waste (cdw) of two types concrete aggregate (mixtures frcac 1, frcac 2) and masonry aggregate (mixtures frmac 1, frmac 2). basic physical properties (density, water absorption) and mechanical properties (compressive strength, flexural strength, and modulus of elasticity) were verified and compared. 3.1. fine recycled aggregate 0–4 mm for complete replacement this part of the paper presents the possibility of replacing the complete fine fraction of natural sand (fna) with fine recycled aggregate (fra). one type of coarse na (grain size 4–8 mm and 8–16 mm) and natural mined sand (fna 0–4 mm), one type of frca, and one type of frma (grain size 0–4 mm) were used. a cooperating recycling company prepared both types of ra originating from cdw. the aggregate was sorted, and unwanted components and impurities were separated – the aggregate was also washed during the recycling process. according to valid european standards, the properties of aggregates, which are essential for the design of the mixture, were tested in the laboratory. first, particle size distribution tests were performed according to the en 933-1 standard, and then density and water absorption were determined by a pyknometric test according to the en 1097-6 standard. since the methodology for determining the density and water absorption of the fine fraction still needs to be precisely defined [17] only the results for the coarse aggregate fraction with grain sizes of 4–8 and 8–16 mm are presented. furthermore, the current moisture in the aggregate was measured for the design of the mixture, which is essential in terms of the amount of water added to pre-soak the aggregate during the first phase of mixing the concrete mixture. the properties of na and ra used in this part of the paper are given in table 1. the mixtures are marked according to the type of aggregate used (natural aggregate concrete – nac, fine recycled concrete aggregate concrete – frcac, or fine recycled masonry aggregate concrete – frmac). number 1 indicates the amount of cement for the concrete strength class with a compressive strength of 20 mpa and number 2 with 30 mpa. 3.2. fine recycled aggregate concrete mixtures to verify the properties, six concrete mixtures with aggregates with a grain size of up to 16 mm were produced, in which the content of cement cem i 42.5 r was approx. 260 kg m−3 and water-cement (w/c) ratio 0.65 for mixtures marked 1 and 300 kg m−3 and w/c ratio approx. 0.55 for mixtures marked 2. the composition of individual mixtures, designed according to bolomey’s particle size distribution curve, is shown in table 2. two control mixtures of conventional concrete (nac 1 and nac 2) were produced, which contained only natural mined sand. furthermore, two mixtures corresponding to the amount of cement of strength class 20 mpa (one with replacement 36 vol. 47/2024 possibilities of replacing natural sand in concrete type of aggregate grain size finest particles oven-dried water absorption saturation content particle density capacity wa24 level [mm] f [%] ρrd [kg m−3] [%] [%] natural aggregate (na) 0–4 4–8 2 570 ± 81 1.0 ± 0.0 0.0 4–8 0.3 2 530 ± 12 1.7 ± 0.3 0.0 8–16 0.4 2 540 ± 12 1.9 ± 0.2 0.0 fine recycled concrete aggregate (frca) 0–4 0.6 2 430 ± 60 3.6 ± 0.8 1.6 fine recycled masonry aggregate (frma) 0–4 1.0 2 320 ± 130 6.6 ± 0.8 4.7 table 1. measured properties of na and ra used in this part of the research. concrete mixture cement mixing + w/c natural aggregate recycled aggregate additional water ratio fine 0–4 mm coarse 4–16 mm fine 0–4 mm [kg m−3] [kg m−3] [-] [kg m−3] [kg m−3] [kg m−3] nac 1 260 169 + 0 0.65 709 1 130 0 frcac 1 260 169 + 17 0.71 0 949 843 frmac 1 260 169 + 18 0.72 0 766 971 nac 2 300 165 + 0 0.55 671 1 167 0 frcac 2 300 165 +16 0.60 0 994 800 frmac 2 300 165 + 17 0.61 0 822 920 table 2. composition of concrete mixtures used in this part of the research. frca – frcac 1 and one with replacement frma – frmac 1) and two mixtures corresponding to 30 mpa (again one with replacement frca – frcac 2 and one with replacement frma – frmac 2). due to the high absorbency of the recycled aggregate, a two-phase mixing system was used, wherein in the first phase, the aggregate is mixed with water intended for pre-soaking (it is labelled as additional water in table 2 and it is determined based on the physical properties of the aggregate and the current amount of water contained in the aggregate) and 2/3 of mixing water. after 10 minutes of mixing, all other components are added in the second phase. no plasticiser or other additives were added to the mixtures. replacement ratios and grain size are shown in table 2. 4. replacement of individual fine-grained components of hpc in this part of the research, seven hpc mixtures with complete replacement of individual fine-grained components were prepared and tested in the laboratory. fra from the reference hpc (frhpca) and regular waste concrete from the recycling centre (frca) were used as substitute materials. natural quartz powder (qp) and quartz sand (qs) were replaced by recycled frhpca and frca corresponding to the grain size of individual components, respectively qp 0–0.06 mm was replaced by frhpc (mixture frhpca.hpc.qp) and frca (mixture frca.hpc.qp) of fraction 0–0.063 mm, qs 0.1–0.6 was replaced by frhpca (frhpca.hpc.qs1 mixture) and frca (frca.hpc.qs1 mixture) of fraction 0.125–0.5 – in which the compensation of total amount of qs is 60 %. and finally, qs 0.6–1.20 mm was replaced by frhpca (frhpca.hpc.qs2 mixture) and frca (frca.hpc.qs2 mixture) with a grain size of 0.5– 1.0 mm – in this case, the total amount of compensated qs is 40 %. a reference hpc mixture based on previous investigations by the research team, just with a slightly modified recipe, was designated as na.ref, and was used as a control mixture. therefore, the results do not meet the requirements for uhpc – compressive strength up to 120 mpa according to astm c1856, and tensile strength varies from 6 to 12 mpa according to aci 239r-18. hardened concrete’s basic physical and mechanical properties were verified and compared in this part of the paper. 4.1. fine recycled aggregate 0–1 mm for replacement of individual components of hpc this part of the paper deals with the individual fractions of fine-grained aggregates of the fraction up to 1 mm, i.e. it is a much finer grain size than in the first part of the research. therefore, different aggregate properties important for hpc mixture design were tested. however, it is mainly the chemical composition and specific surface area of the finest component of aggregates with a grain size of 0–0.63 mm. the measured properties of all types of aggregate used in this part of the paper are given in table 3. 4.2. fine recycled aggregate hpc mixtures to verify the properties of hardened hpc, seven concrete mixtures were produced with the same amount of cement cem i 42.5 r 680 kg m−3 and a w/c ratio of 0.25. the mixtures were designed using packing density optimisation. although additional water is usually added due to the higher water absorption of 37 k. fořtová, t. pavlů, d. mariaková et al. acta polytechnica ctu proceedings type of aggregate grain size chemical composition [%] specific surface [mm] sio2 al2o3 fe2o3 cao loi area [cm2 g−1] cement 0–0.063 17.6 5.9 2.1 66.4 8.0 3 640 microsilica 0–0.063 97.0 0.2 0.1 0.3 2.4 quartz powder (qp) 0–0.063 99.7 0.2 0.0 0.0 0.1 3 760 quartz sand (qs) 0–0.063 94.1 2.6 1.0 0.9 1.04 2 820 fine recycled hpc (frhpca) 0–0.063 45.5 3.6 2.9 41.9 6.1 3 390 fine recycled concrete aggregate (frca) 0–0.063 41.1 11.8 4.0 33.4 9.7 4 450 table 3. the basic properties of the finest fractions of aggregate used. hpc mixture cement mixing + w/c super-plasticiser micro-silica natural aggregate recycled aggregate additional ratio qp qs powder sand water 0–0.06 mm 0.1–1.2 mm 0–0.063 mm 0.125–1 mm [kg m−3] [kg m−3] [-] [kg m−3] [kg m−3] [kg m−3] [kg m−3] [kg m−3] [kg m−3] na.hpc 680 168 + 0 0.25 19 75 225 960 frhpca.hpc.qp 680 168 + 0 0.25 19 75 960 225 frhpca.hpc.qs1 680 168 + 0 0.25 19 75 225 384 576 frhpca.hpc.qs2 680 168 + 0 0.25 19 75 225 576 384 frca.hpc.qp 680 168 + 0 0.25 19 75 960 225 frca.hpc.qs1 680 168 + 20 0.25 19 75 225 384 576 frca.hpc.qs2 680 168 + 0 0.25 19 75 225 576 384 table 4. composition of hpc mixtures used in this part of the research. fra (see previous text), in this case, except for one mixture (see table 4), no additional water was added to the mixtures. to improve the workability of the mixtures, a superplasticiser based on polycarbonate was added. the composition, replacement ratios and grain size of all hpc mixtures are shown in table 4. the mixtures are marked according to the type of aggregate used (natural aggregate – na, fine recycled hpc aggregate – frhpca, or fine recycled concrete aggregate – frca), followed by the information that the mixture is hpc. the last part indicates which component of hpc is replaced with the recycled aggregate of the corresponding grain size. 5. results and discussion this part of the paper compares and presents the essential mechanical and physical properties of both types of investigated mixtures, conventional concrete and hpc. all properties were investigated on three samples, the arithmetic averages of the values, including standard deviations, are shown in the tables and graphs. 6. complete replacement of fna in conventional concrete mixtures 6.1. physical properties the table (see table 5) shows dry density and water absorption by immersion of the investigated samples. a slight decrease in density can be observed for all rac mixtures compared to nac, with the highest reduction of 7 % for both mixtures with frma. furthermore, it was found that the water absorption by immersion of concrete with fra is higher than that of mixtures with only na. a maximum increase of 85 % was observed for the frmac 1 mixture. this is caused by the frma aggregate that contains highly porous materials such as bricks, aerated concrete, and mortar. 6.2. mechanical properties compressive strength results, a critical mechanical property of concrete, are shown in the additional graph in figure 1. the chart shows the flexural tensile strength for all evaluated conventional concrete mixtures. the green columns are for the concrete strength class with a compressive strength of 20 mpa and the blue 30 mpa. the columns for the mixtures with complete replacement of the fine fraction of natural aggregate are distinguished by their different texture – see the legend below the graph. the orange marks above the columns indicate the value of the compressive strength of a particular mixture compared to the reference mixture for the given strength class, expressed as a percentage. the effect of using frma in the concrete mixture is similar for both strength classes – it reduces compressive strength by 10 and 15 %, respectively. in contrast, the complete replacement of na with frca has minimal impact on compressive strength. for a concrete strength class of 30 mpa, there is a decrease of only 4 %, and for a lower strength class, even an increase in strength of 3 % is observed compared to the reference mixture. flexural tensile strength results and comparison of all mixtures are shown in 38 vol. 47/2024 possibilities of replacing natural sand in concrete concrete mixture dry density water absorption compressive flexural tensile modulus of elasticity by immersion strength strength static dynamic [kg m−3] [%] [mpa] [mpa] [gpa] [gpa] nac 1 2 241 ± 18 5.89 ± 0.35 33.3 ± 2.5 6.2 ± 0.2 36.7 ± 1.4 38.2 ± 1.8 frcac 1 2 195 ± 11 7.68 ± 0.25 34.4 ± 1.7 5.8 ± 0.3 29.6 ± 0.4 34.5 ± 0.7 frmac 1 2 080 ± 14 10.89 ± 0.38 30.0 ± 2.2 5.5 ± 0.4 22.4 ± 1.0 27.3 ± 1.4 nac 2 2 269 ± 13 5.03 ± 1.11 44.8 ± 0.9 7.6 ± 0.9 35.9 ± 0.5 38.2 ± 0.8 frcac 2 2 206 ± 5 7.70 ± 0.06 42.8 ± 0.8 6.5 ± 0.4 31.4 ± 1.0 35.7 ± 0.6 frmac 2 2 115 ± 12 10.30 ± 0.32 38.0 ± 0.9 6.8 ± 0.6 25.3 ± 0.2 30.0 ± 0.9 table 5. basic mechanical-physical properties of ordinary concrete mixtures with replaced sand. figure 1. compressive strength of all evaluated conventional concrete mixtures. table 5. the flexural tensile strength decreased for all concrete mixes with fra compared to the mixture with na. the most significant reduction – of 13 % is observed in the mixture with na replacement with frca in the strength class with a concrete strength of 30 mpa (frcac 2). the static and dynamic moduli of elasticity show the same trend (see table 5). all mixtures with fra show a decrease in properties compared to mixtures with na, while the decline for mixtures with frma is more significant (39 and 31 %) than for mixtures with frca (19 and 14 %). the dynamic modulus values are slightly higher than the static modulus values, with a maximum difference of 18 % for the frmac 1 mixture. 7. replacement of individual fine-grained components of hpc 7.1. physical properties the use of ra in hpc shows (see table 6) a decrease in dry density of 3–6 %, which is insignificant. in contrast, water absorption by immersion is 3 % lower – 5 % higher (for mixtures with na replacement with frhpca) and 6 % lower – 35 % higher (for mixtures with frca), respectively. from the results, it can be observed that the mixtures with qp replacement have lower water absorption. on the contrary, the mixtures with the most extensive qs replacement have higher water absorption than the hpc control mixture with natural aggregate. this could be related to the high water absorption of frca originating from conventional concrete. 7.2. mechanical properties the graph in figure 2 shows the compressive strength values for all investigated hpc mixes. the red column represents the hpc reference mixture with a natural aggregate only. in the following columns, one entire hpc component (qp, qs1 or qs2) is completely replaced in each case. blue columns represent replacement with fine recycled hpc aggregate, and green columns describe replacement with the fine recycled concrete aggregate of the corresponding grain size. the orange marks above the columns again indicate the value of the compressive strength of the specific mixture compared to the reference mixture, expressed as a percentage. figure 2. compressive strength of all investigated hpc mixtures. the compressive strength of hpc containing replacement na with frca decreases more (11–19 %) than that of hpc containing frhpca (5–9 %). samples with the replacement of qp with frca show the maximum drop in compressive strength. the highest deterioration of properties can be observed in the flexural tensile strength (see table 6). for mixtures with frca, a decrease of 2–32 % was observed, and for mixtures with frhpca, even a 50–55 % reduction. all mixtures show similar values for static and dynamic moduli of elasticity (table 6). a slight decrease compared to the reference mixture can be observed for all mixtures with ra, with the most significant 39 k. fořtová, t. pavlů, d. mariaková et al. acta polytechnica ctu proceedings concrete mixture dry density water absorption compressive flexural tensile modulus of elasticity by immersion strength strength static dynamic [kg m−3] [%] [mpa] [mpa] [gpa] [gpa] na.hpc 2 358 ± 28 3.76 ± 0.20 110.7 ± 7.6 15.3 ± 1.1 45.5 ± 1.1 46.3 ± 1.6 frhpca.hpc.qp 2 271 ± 15 3.72 ± 0.12 105.0 ± 13.7 7.1 ± 0.5 45.0 ± 0.7 43.8 ± 0.5 frhpca.hpc.qs1 2 237 ± 13 3.94 ± 0.09 100.3 ± 11.3 7.0 ± 0.6 41.9 ± 1.0 41.0 ± 0.8 frhpca.hpc.qs2 2 288 ± 15 3.65 ± 0.07 102.4 ± 15.2 7.6 ± 0.9 44.2 ± 1.4 44.0 ± 0.5 frca.hpc.qp 2 277 ± 6 3.52 ± 0.16 89.9 ± 9.3 15.0 ± 0.5 45.3 ± 0.5 43.2 ± 0.4 frca.hpc.qs1 2 215 ± 7 5.08 ± 0.28 96.0 ± 8.1 10.5 ± 2.6 38.6 ± 0.6 39.4 ± 0.4 frca.hpc.qs2 2 268 ± 14 4.19 ± 0.21 98.7 ± 12.6 11.7 ± 0.8 43.6 ± 2.6 42.5 ± 0.5 table 6. basic mechanical-physical properties of hpc mixtures with the partially replaced quartz sand. decline (15 %) again perceptible for the mixture with the highest replacement of qs with frca. 8. conclusions in this study, two cases of possible replacement of natural sand were experimentally verified and discussed. in the first case, the study focuses on completely replacing natural sand with fine recycled aggregate from two types of construction and demolition waste (conventional concrete and masonry) in conventional concrete. the second part of the study describes the complete replacement of quartz sand and quartz powder with fine recycled aggregate from two sources (hpc and conventional concrete) in hpc. the established properties of fra were verified by testing the physical and mechanical properties of concrete and hpc containing fra. the following conclusions can be drawn. • although the density of fra is lower in comparison with natural sand, in both cases, there was only a slight decrease in the density of the resulting mixtures. from this point of view, using ra to replace na in conventional concrete and hpc mixtures should not cause any unexpected complications. • water absorption by immersion of conventional concrete and hpc containing fra increased, which could negatively affect the durability of concrete and hpc, respectively. durability properties should therefore be verified in further tests. • for conventional concrete mixtures, the compressive strength mostly decreases slightly when replacing na with fra. due to the fact that natural sand is completely replaced, it can be stated that during further research of similar mixtures, it is possible to include the decrease in strength properties already at the mixture design stage. in the case of hpc, it turns out that ra’s type and original properties influence the compressive strength of hpc with fra. therefore, the reduction in strength properties must be considered when designing mixtures with aggregates originating from conventional concrete. when using recycled hpc for replacement, the compressive strength drop is insignificant. • for the flexural tensile strength of the hpc mixtures, the most significant decrease was found out of all the investigated properties. however, when using hpc in practice, it is assumed that the material will be reinforced with fibres or other reinforcement. for the future use of hpc with fra, the decrease in flexural tensile strength should not be essential. however, it would be appropriate to directly verify the properties of reinforced hpc with fra in subsequent research. flexural tensile strength results for conventional concretes with the replacement of na with ra show a similar trend to compressive strength results for individual mixtures. • the results of this study align with previous research that observed a decline in the static modulus of elasticity of concrete when fine recycled aggregate is used as a replacement for natural sand, this can be observed for both types of studied concrete types. the study showed that, in addition to being used in conventional concrete, a fine recycled aggregate could partially replace natural aggregate in highperformance concrete. however, a reduction in properties compared to the control conventional concrete and hpc must be expected and verified for both types of mixtures. at the same time, it is necessary to emphasise the quality and properties of the input raw materials used (particularly fine recycled aggregate of lower strength classes), which can significantly influence the properties of the resulting mixtures. indeed, other possibilities of replacement ratios as well as properties of reinforced concrete and hpc with fna replacement, are the subject of further research. acknowledgements this work is a part of project entitled “design of performance-based concrete using sand reclaimed from construction and demolition wastes”, ltain19205, supported by the ministry of education, youth and sports of the czech republic through the program interexcellence references [1] b. estanqueiro, j. d. silvestre, j. brito, m. d. pinheiro. environmental life cycle assessment of coarse natural and recycled aggregates for concrete. european 40 vol. 47/2024 possibilities of replacing natural sand in concrete journal of environmental and civil engineering 22(4):429–449, 2018. https://doi.org/10.1080/19648189.2016.1197161 [2] r. kurda, j. d. silvestre, j. brito. toxicity and environmental and economic performance of fly ash and recycled concrete aggregates use in concrete: a review. heliyon 4(4):e00611, 2018. https://doi.org/10.1016/j.heliyon.2018.e00611 [3] a. braunschweig, s. kytzia, s. bischof. recycled concrete: environmentally beneficial over virgin concrete? 2011. 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https://doi.org/10.1016/j.conbuildmat.2021.125950 https://doi.org/10.1016/j.conbuildmat.2008.04.012 https://doi.org/10.1016/j.conbuildmat.2008.04.012 https://doi.org/10.1016/j.conbuildmat.2016.02.154 https://doi.org/10.1016/j.conbuildmat.2016.02.154 https://doi.org/10.1016/j.cemconres.2013.05.005 https://doi.org/10.1016/j.conbuildmat.2015.03.112 https://doi.org/10.1016/j.conbuildmat.2015.03.112 https://doi.org/10.1016/j.cemconcomp.2006.12.004 https://doi.org/10.1016/j.cemconcomp.2006.12.004 https://doi.org/10.1080/19648189.2013.851038 https://doi.org/10.1016/j.conbuildmat.2017.11.068 https://doi.org/10.1016/j.conbuildmat.2017.11.068 https://doi.org/10.1016/j.conbuildmat.2021.125536 https://doi.org/10.1016/j.conbuildmat.2021.125536 acta polytechnica ctu proceedings 47:35–41, 2024 1 introduction 2 materials and methods 3 complete replacement of fna in conventional concrete mixtures 3.1 fine recycled aggregate 0–4mm for complete replacement 3.2 fine recycled aggregate concrete mixtures 4 replacement of individual fine-grained components of hpc 4.1 fine recycled aggregate 0–1mm for replacement of individual components of hpc 4.2 fine recycled aggregate hpc mixtures 5 results and discussion 6 complete replacement of fna in conventional concrete mixtures 6.1 physical properties 6.2 mechanical properties 7 replacement of individual fine-grained components of hpc 7.1 physical properties 7.2 mechanical properties 8 conclusions acknowledgements references my title https://doi.org/10.14311/app.2022.36.0269 acta polytechnica ctu proceedings 36:269–279, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague adaptive sequential sampling for reliability estimation of binary functions miroslav vořechovský brno university of technology, institute of structural mechanics, faculty of civil engineering, veveří 331/95, 602 00 brno, czech republic correspondence: vorechovsky.m@vut.cz abstract. a novel method for estimation of rare event probability is proposed, which works also for computational models returning categorical information only: success or failure. it combines the robustness of simulation methods (counting failure events) with the strength of approximation methods which refine the boundary between the failure and safe sets. two basic tasks are identified: (i) extension of the experimental design (ed) and (ii) estimation of probabilities. the new extension algorithm adds points for limit state evaluation to the ed by balancing the global exploration and local exploitation, and the estimation uses the pointwise information to build a simple surrogate and perform a novel optimized importance sampling. no connection is presumed between the limit function value at point and its proximity to the failure surface. a new global sensitivity measure of the failure probability to individual variables is proposed and obtained as a by-product of the proposed methods. keywords: exploitation, exploration, reliability. 1. introduction this article promotes a recent work [1] by the author. we revisit the problem of failure probability estimation of an engineering product or a process, which is represented by a computationally expensive mathematical model g(xxx). the model features uncertain or random variables xxx and we assume the joint probability density fxxx(xxx) is known and can be uniquely transformed to the space of independent gaussian random variables of dimension nvar. the model g(xxx) may return categorical output only, that is, failure or success. the problem is to accurately estimate the probability of failure with the smallest possible number of g(xxx) calls. the spectrum of existing methods for failure probability estimation is very rich. their categorization can be done depending on how they treat three groups of input information: (i) the geometry and topology of the input space of input variables indexed by a random vector xxx, (ii) probability density function of xxx, be it the true density or some modified ”sampling” density, and (iii) the limit state function g(xxx). from the point of view of the way that the methods for the estimation of failure probability handle the outputs of the limit state function, two basic groups can be identified. simulation methods such as monte carlo integration, importance sampling [2–4] or asymptotic sampling [5] use the binary information only: they count the failure events out of all nsim function calls and associate these events with some weights to be used in estimates performed via arithmetic or weighted averages. these methods are robust with respect to complicated functions g(xxx), which may be noisy, can have disjoint failure sets, the landscapes may nonsmooth or there can be even regions for which g(xxx) return no answer at all. however, excessively high number function calls needed in simulation methods when dealing with practical problems, in which the true failure probability is very low. a disadvantage of the simulation methods is that they use no or only little information from previous function calls. the other group of methods can be termed approximation methods. they make various assumptions about the shape of g(xxx), and in most cases, the methods work for smooth functions only. they can be seen as methods making use of the gradient optimization in search for the most probable failure regions (form/sorm [6–14], subset simulation [15]) or they adaptively build approximations (surrogate models) of g(xxx) [16]. the surrogate model can be a smooth approximation based of polynomials, using kriging (gaussian process regression) [17, 18], polynomial chaos expansions [19, 20], radial basis functions [21, 22], or classifiers such as artificial neural networks [23–25] or support vector regression surrogates [26–30]. the evaluation of the surrogate must be sufficiently fast to be useful for the robust sampling strategies making the probability estimations. there is also a class of methods which construct approximations of the true distribution function of numerical values of g(xxx) based on the empirical histograms to estimate the cumulative probability distribution at zero value, that is, direct estimation of failure probability. it is clear that the shape of the cumulative distribution function is strongly dependent of the particular formulation of g(xxx). in other words, various reformulations and reparametrizations of g(xxx) which do not alter the failure surface lead to quite different distribution functions of the outcome which complicates this kind of approximation. 269 https://doi.org/10.14311/app.2022.36.0269 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en miroslav vořechovský acta polytechnica ctu proceedings in practical applications of reliability estimation methods, we face a variety of challenges and there are difficulties which can not be overcome by the existing algorithms. suppose, for example, that the state of an existing structure is modelled by an algorithm (e.g., a nonlinear fem simulation). this model can provide highly nonlinear output in the space of input variables. the model can capture the existence of multiple failure modes whose probabilities are not many orders of magnitudes different (a situation with multiple design points in form/sorm), and there can be combinations of inputs for which the algorithm provides no meaningful response. the existence of these scenarios motivates the development of more robust algorithms such as the one presented in this paper. another challenge is connected with the spurious dependence of answers obtained by many existing algorithms on the particular formulation of the limit state function. the same system condition can be expressed via different formulations of the function of input variables leading to dramatically different g(xxx) function ”landscapes” in the input space. the landscape is, however, important for gradient-based algorithms, be it the deterministic ones (design point search) or stochastic gradient descent methods (subset simulation). moreover, the some models can also be formulated in a way that the g(xxx) function value is not reasonably reflecting the condition of the system (differentiating between categories ”safe” or ”very safe”). our argument is that in these cases, it can be better to consider the categorical information only: safe operation or failure. the failure category can actually be represented by a finite number of potentially different failure codes, and there can also be an additional category ”no response”. the algorithm for reliability estimation must be able to work with such a crude information and keep going upon receiving any of these state codes by (i) processing the information and (ii) proposing a new point for evaluation by the model. in this paper, we briefly present a technique consisting of two algorithms. one for the adaptive sequential extension of the experimental design (ed), that is, the table of nsim points each with nvar coordinates, and the other for importance sampling analysis performed in the standard gaussian space. the method is able to work with binary functions, however, if the g(xxx) function returns values usable for building smooth approximation, the estimation task can be performed with an advanced surrogate function with the chance of obtaining more accurate results for small eds. 2. gaussian space with independent marginals the methods presented in this paper use the fact that the rotationally symmetrical gaussian space of dimension nvar can be indexed either by nvar orthogonal coordinates using the cartesian coordinate system, or by (i) nvar −1 independent and identically distributed directions and (ii) one radial distance from the origin a) b) c) d) 2d 3d figure 1. the first seven levels of exploration sets (layers) innvar = 2 andnvar = 3 dimensional standard gaussian space. panels a,c: nested nvar-balls. panels b,d: realizations of random exploration sets. which is independent from the directions. the radial distance from is χ-distributed with nvar degrees of freedom. this allows for direct computations of probabilities associated with nvar-balls of a given radius r, see [1] for mathematical details and the available numerical libraries which facilitate the work in the (hyper)spherical coordinate system. we now define a sampling density for a ring (annulus), which is a rotationally symmetrical region between two nvar-balls defined by two different radii, r < r. the probability density for a point xxx is a scaled standard gaussian density hann (xxx; r,r) = πnvar v=1 φ(xxxv) pann = πnvar v=1 φ(xxxv) pr − pr (1) and zero otherwise, where φ(xxxv) is the univariate standard gaussian density. in this equation, the scaling denominator pann = pr−pr is formed by the difference between two probabilities corresponding to probability content in the two nvar-balls. this density can be rewritten in terms if the radial distance ρ ≡ ∥x∥. 3. extension of the experimental design 3.1. exploration set assume now that the input space is divided into rotationally symmetric layers with predefined probability content. each such layer is obtained as the difference between two nvar-dimensional co-centric balls. the first ball contains, say, 90% of the total probability, the next ball contains 99% so that the ring (an annulus) in between adds 1% to the smaller ball. the next onion-like shell encloses 99.9% probability etc. in this way, the space is divided into layers such that the probability contents are covered in a controlled 270 vol. 36/2022 reliability estimation of binary functions ... figure 2. two-dimensional illustration of the combined set candidates and the selection process (standard gaussian space) for a binary performance function dividing the space into a ”flower-shaped” safe domain and the surrounding failure domain. at nsim = 9 model evaluations, the rare event is hit for the first time. at nsim = 10 an exploitation candidate is selected, while at nsim = 11 the exploration candidate wins using the highest value of the ϕ criterion expressed by the blue color saturation. a complete evolution of the extension of the ed using the ϕ-based selection process for another run of the same problem is available as a video. fashion and the radii of the nvar-balls forming the boundaries are easily obtained using the χ distributed variable discussed above. assume now that the surface of each such ball is uniformly covered by a predefined of points. the complete set of point corresponding to all considered layers is called the exploration set. figure 1 illustrates the nested rings and their coverage by points for nvar = 2 dimensions and 3d. 3.2. exploitation set suppose that the experimental design contains at least one failure point and at least one safe point. from that moment it makes sense to expect that the true boundary between the safe and failure sets passes somewhere between the two points with a different event type. we propose to generate a cloud of ”candidate dots” centered at each failure point and with a gaussian density of standard deviation equal to√ nvar − 1. from all these candidate dots, we only consider those whose two nearest neighbors have two different classifications from g(xxx). the set of all dots fulfilling this requirement collectively forms the exploitation set and they are considered in competition for the best candidate alongside the exploration set. 3.3. ψ criterion for candidate selection suppose the combined exploration-exploitation set of candidates is available. we propose to select the most informative candidate using the criterion which balances the local exploitation with global exploration. each candidate is associated with the criterion value which roughly estimates the probability being resolved by evaluating the g(xxx) function in that candidate ψc = √ fcfs︸ ︷︷ ︸ ave probability (lc,s)nvar︸ ︷︷ ︸ ∝vol. (2) the square root term represents the geometric mean of probability density fc in the candidate ”c”, and the density fs in its nearest neighbor ”s”. this average is multiplied by a term proportional to the volume occupied by the region between the candidate and its nearest neighbor. this volume is estimated as their euclidean distance lc,s raised to the domain dimension. the candidate delivering the highest value of the ψ criterion is selected to extend the current ed. in this way the algorithm automatically balances between the refinement of the failure surface approximation and the expansion of the ed in the input space towards infinity. simply, the candidate yielding the largest (approximate) probability amount is selected. the ψ criterion can be monitored during the extension and its value can support the stopping criterion. figure 2 illustrates the combined exploration and exploitation sets and shows the ranking of candidates using the ψ criterion via the darkness of the blue infill. 4. estimation of probabilities at any stage of the ed extension process, the desired probabilities can be estimated based on point-wise information, i.e., the current ed with known g(xxx) outcomes. we propose to substitute a true computationally expensive model (a simulator) by a computationally cheaper model constructed in a solely non-intrusive way with respect to the original simulator, i.e., in a purely data-driven manner. when the performance function g(xxx) returns continuously distributed, trustworthy, and well-behaved output, traditional smooth surrogate models such as pce, kriging, radial basis functions, etc., can be employed for fast sampling analysis. such a surrogate model has the potential to improve the accuracy of the importance sampling estimation presented in this section because the failure surface may be approximated more accurately. it is guaranteed that the above-described extension of the ed was performed in such a way that the surrogate model was well supported, especially when close to the failure surface. in this paper, however, we focus on cases in which the original simulator is trusted to provide categorical information only, i.e., one of a finite set of classifications. similarly, the surrogate will simply be a finite-state classifier. we propose to use simply a classifier based on the nearest neighbor: at any location in the input space, the event 271 https://vutbr-my.sharepoint.com/:v:/g/personal/vorechovsky_m_vutbr_cz/eawduw0xqyhalhwqtnoww8kbrmgslw4lhi-aejnjk-kt0w?e=zndhae miroslav vořechovský acta polytechnica ctu proceedings −4 −2 0 2 4 −4 −2 0 2 4 x1 x2 nsim = 200 ed + excluded candidates −4 −2 0 2 4 x1 ψ-ranked candidates −4 −2 0 2 4 x1 screening dots −4 −2 0 2 4 x1 r r is integration nodes via hann 0 25 50 75 100 125 150 175 200 number of limit state evaluations (g (xxx) calls), nsim 10−4 10−3 10−2 10−1 pr ob ab ili ty of ev en t t , ψ cr ti te ri on . ψ criterion p(nsim) t (global) p(nsim) t (localized) pt (exact) extension of ed (2-nearest neighbors classification) estimation of probabilities (nearest neighbor surrogate) figure 3. top row: on-the-fly classification of territories. two top left panels: two nearest neighbors for the classification of boundary regions – red and white candidates are censored out, and the retained (blue) candidates are considered for the extension of ed. two top right panels: classification of dots and nodes via the nearest neighbor. bottom: history of the failure probability estimation and the ψ criterion quantifying the approximate ”probability bites” by selected candidates. the complete evolution of all panels is demonstrated in the wavy circle video. type is approximated by the event previously detected in the nearest ed point. suppose we have the classifier at hand in the form of an indicator function 111(nsim) f (xxx) based on nsim evaluated points in which the g(xxx) was evaluated previously. 111(nsim) f (xxx) approximates the true indicator function 111f (xxx). using the importance sampling (is) numerical integration, the probability of an event type t occurring when the indicator function 111t (xxx) signals it is defined as the expectation: pt = e [111t (xxx)] =∫ ·· · ∫ d 111t (xxx) fxxx(xxx) dxxx. let h(xxx) be the is density which is positive wherever event t occurs. the probability of event t can be rewritten as pt = ∫ d 111t (xxx) fxxx(xxx) h(xxx) h(xxx) dxxx (3) = eh [ 111t (xxx) fxxx(xxx) h(xxx) ] , (4) where eh [·] denotes the expectation for xxx being distributed according to h: xxx ∼ h. the is estimation of pt based on the current approximation of 111(nsim) t (xxx) of the true indicator function 111t (xxx) is made with nis integration nodes via the arithmetic average p (nsim) t ≈ 1 nis nis∑ i=1 111(nsim) t (xxxi) fxxx(xxxi) h(xxxi) , xxxi ∼ h. (5) the is estimator is unbiased by construction. we propose to use the following cubature via a global is using the rotationally invariant density hann (xxx; r,r) in an important ring, which was introduced in eq. (1). suppose the screening dots were not localized in a very small region for which the localized importance sampling would be efficient. we propose the use of use a large pool of ”integration nodes” selected from sampling density hann(xxx) which excludes the useless nvarball of the radius r (= the distance of the most central ”screening dot” from the origin; see the magenta points in figure 3). additionally, we propose that the density also excludes the exterior of the nvar-ball with the radius r > r. the outer radius r is obtained using the previous estimation of the rare event p(nsim) t r(p(nsim) t ;nvar) = f−1 ρ ( 1 − p (nsim) t 104 ;nvar ) . (6) based on the information from the screening dots, we presume that the interior of the nvar-ball of radius r does not contain any failure event. the number 104 is selected to guarantee that the ignored space outside the ball of radius r is associated with a probability, which is four orders of magnitude less than the current estimate of the rare event probability. different choices of this threshold are possible. let us now consider the sampling density hann (xxx; r,r) introduced in eq. (1) and substitute it 272 https://vutbr-my.sharepoint.com/:v:/g/personal/vorechovsky_m_vutbr_cz/eawduw0xqyhalhwqtnoww8kbrmgslw4lhi-aejnjk-kt0w?e=zndhae vol. 36/2022 reliability estimation of binary functions into eq. (5) p (nsim) t ≈ 1 nis nis∑ i=1 111(nsim) t (xxxi) fxxx(xxxi) hann(xxxi) = pann nis nis∑ i=1 111(nsim) t (xxxi)︸ ︷︷ ︸ nis,t = pann nis,t nis . (7) in other words, computation of the likelihood ratio (fxxx/h) at each node is not needed as the gaussian densities cancel out. this is because the samples have their density proportional to fxxx . it suffices to simply compute the proportion of is nodes that signaled event t (e.g., failure) and multiply it with pann. the closer the true event domain is to the annulus, the closer hann is to the optimal is density, and so the estimation variance vanishes. with this strategy, the annulus between the two radii r and r is effectively examined. whatever event occurs outside the outer radius r is associated with a negligible probability. the variance of such an is estimator is also simple to obtain varh[p(nsim) t ] ≈ 1 nis {[ nis,t nis p2 ann ] − ( p (nsim) t )2 } = p (nsim) t nis ( pann − p (nsim) t ) , (8) and therefore, by using eq. (7), the coefficient of variation of the estimator associated with the indicator function used is simply covh[p(nsim) t ] ≈ 1 √ nis √ pann p (nsim) t − 1 (9) = 1 √ nis √ nis nis,t − 1 (10) figure 3 presents all the important information about the process of extension of ed (top left) and the estimation of probabilities (top right) which can be performed at any time during the process. the evolution of both is shown in the wavy circle video covering the history from the very first limit state function evaluation up to nsim = 200. the video frames, which can be displayed one by one, correspond to individual stages of the process, thus enabling a detailed inspection of the process. the accuracy of the estimation is excellent already at nsim ≈ 70 when all of the seven ”failure regions” become discovered. the blue line plotted in the bottom diagram shows the amount of the ”probability bite ψ” occupied by the neighborhood of the selected candidate. it can be seen that once about nsim = 30 points have been evaluated, the ψ contributions become smaller than the rare event probability itself. the decrease in ψ with an increasing nsim can be used, along with the stabilization of probability estimations, to decide the profitability of further g(xxx) function evaluations (stopping criterion). 5. global sensitivity measures in [1] we also introduce a new generalized importance measure that makes no use of the performance function’s gradient and considers all (known) points contributing to failure, weighted by the original density. the proposed measure comes as a by-product of the above-proposed technique for reliability estimations. consider that the local contribution to failure probability of any point xxx = {x1, . . . , xv, . . . ,xxxnvar} can be decomposed into individual coordinates in the spirit of the form α-sensitivities α2 v(xxx) = ( xv ρ(xxx) )2 = x2 v∑nvar v=1 x 2 v , (11) so that ∑nvar v=1 α 2 v(xxx) = 1 for any xxx . the denominator ρ2(xxx) is simply the squared euclidean distance between a general point xxx and the origin. these α2 v(xxx) can be viewed as importance measures of the individual dimensions and can be used as additive shares of probability density at any point xxx. this choice is natural as αvs weigh the individual increments to the distance ρ: αv(x) = ∂ρ(xxx) ∂xv . using these shares, the total failure of probability can be written as pf ≡ ∫ · · · ∫ f nvar∑ v=1 α2 v(xxx)fxxx(xxx) dxxx = nvar∑ v=1 ∫ · · · ∫ f α2 v(xxx)fxxx(xxx) dxxx = nvar∑ v=1 pf ,v. (12) in this way, the probability of failure (or analogously any event type) is obtained as a sum of contributions pf ,v of individual variables. each variable v contributes to pf by pf ,v = ∫ · · · ∫ f α2 v(xxx)fxxx(xxx) dxxx (13) = ∫ · · · ∫ f ( xv ρ(xxx) )2 fxxx(xxx)︸ ︷︷ ︸ fv(xxx) dxxx (14) = ∫ · · · ∫ f fv(xxx) dxxx, (15) where we define a part of the standard gaussian density ascribed to a single variable v as (see also the illustrations in figs. 4c and d) fv(xxx) = fxxx(xxx) ( xv ρ(xxx) )2 . (16) this local contribution to pf ,v(xxx) is easy to evaluate as it is dependent only on the coordinates of point xxx and its standard gaussian density. finally, the shares pf ,v can be standardized by pf to form the proposed global importance measures s2 v associated with individual variables s2 v = pf ,v pf , i.e. nvar∑ v=1 s2 v(xxx) = 1. (17) 273 https://vutbr-my.sharepoint.com/:v:/g/personal/vorechovsky_m_vutbr_cz/eawduw0xqyhalhwqtnoww8kbrmgslw4lhi-aejnjk-kt0w?e=zndhae miroslav vořechovský acta polytechnica ctu proceedings true fail. surface figure 4. measures of importance with respect to failure probability. a) illustration of form α-sensitivities constructed using linearization of a failure surface in the most central failure point xxx⋆. b)-c) illustration of the proposed global sensitivity measures s2 v by decomposition of pf into additive contributions by individual variables. an important aspect is that the proposed s2 vs are not based on values of the performance function, as only the binary information indicating an event is needed. this is an important property because the importance measures should not be dependent on the way a performance function is defined if it provides the same failure boundary. a robust importance measure for sensitivity to an event should be invariant under reformulations or reparametrizations of the underlying problem. the evaluation of individual pf ,v (and therefore also s2 v) is very cheap as it can be seen as a byproduct of the sampling analysis employed to deliver an estimation of pf . suppose we have an existing set of points that were sampled proportionally to the standard gaussian density fxxx(xxx). it can be the global importance sample with nsim points that was obtained outside the nvar-ball or in the annuloidal hann(xxx) ∝ fxxx(xxx). from this sample, we only select a vector xxx containing nf points marked as ”failure”. all these points are equally probable, and they each represent the same share of the estimated failure probability pf (xxxi) = pf/nf , i = 1, . . . , nf . this share can be further split into individual directions in the spirit of eq. (11) such that the contribution of the ith point in the vth direction reads pf ,v(xxxi) = pf (xxxi) x2 i,v ρ2(xxxi) = pf nf x2 i,v ρ2(xxxi) , (18) i.e., each point xxxi contributes pf (xxxi) = nvar∑ v=1 pf ,v(xxxi) = pf nf (19) where xi,v is the vth coordinate of point xxxi and ρ2(xxxi) = ∑nvar v=1 xxx 2 v is the squared euclidean distance of point xxxi from the origin. the desired estimation of importance measures s2 v of individual directions can be obtained by summation over nf failing nodes with a fixed direction index v and dividing by the failure probability s2 f ,v ≈ nf∑ i=1 pf ,v(xxxi) pf = 1 nf nf∑ i=1 x2 i,v ρ2(xxxi) . (20) therefore, the proposed sensitivity measure is just a cheap by-product of the proposed method. in cases when the sampling probability is not proportional to the standard gaussian density, it is no longer true that all points have the same contribution of 1/nf , and therefore straightforward re-scaling in a similar manner to importance sampling must be employed s2 f ,v ≈ 1 pf 1 nis nis∑ i=1 111(nsim) f (xxxi) fxxx(xxxi) h(xxxi) x2 i,v ρ2(xxxi) . (21) 6. numerical examples a variety of numerical examples which have been selected to explore different classes of problems posing unique challenges are presented in [1]. for the present short paper, we selected four two-dimensional problems defined in the space of independent standard gaussian random variables; see figure 5 for a quick overview of the selected functions. then, problems in higher dimensions are analyzed. in all definitions of the functions [1], we present the expressions that return smoothly varying output variables. however, the proposed extension algorithm receives categorical information only (such as binary ”failure-success” codes). the same holds for the estimation, which uses only the indicator functions signaling an event. the only exceptions are ”four branch” and ”metaballs” examples for which we also examine the degree of improvement in probability estimation when a smooth interpolation of the point-wise information in the ed via the radial basis function is employed as the classifier. we now briefly comment on numerical results obtained for the four branch example, which is a frequently used problem for demonstration of new techniques and for comparison of results. the following 2-dimensional ”four branch function” [31] is a common benchmark problem in reliability analyses; see, e.g., studies with various parameter settings [4, 17, 32]. the function describes the failure of a series system with four distinct limit state components: two linear and two nonlinear branches of the failure surface. the 274 vol. 36/2022 reliability estimation of binary functions wavy circle wavy line metaballs four branch -4 -2 0 2 4 0 -4 4 -2 0 2 4 6 -4 2 -2 6 -4 0 4 -4 0 4 -2 0 2 4 6 -6 -4 -4 0 4 6 -6 2 -2 -2 0 2 4 6 -6 -4 0 5 0 10 0 25 0 3 figure 5. overview of four two-dimensional examples used for demonstration of the proposed method. design points are visualized using solid black balls. −6 −4 −2 0 2 4 5 −6 −4 −2 0 2 4 5 x1 x2 nsim = 100 ed + excluded candidates −6 −4 −2 0 2 4 5 x1 ψ-ranked candidates −6 −4 −2 0 2 4 5 x1 screening dots −6 −4 −2 0 2 4 5 x1 r r is integration nodes via hann 0 20 40 60 80 100 number of limit state evaluations (g (xxx) calls), nsim 10−4 10−3 10−2 10−1 pr ob ab ili ty of ev en t t , ψ cr ti te ri on . ψ criterion p(nsim) t (global) p(nsim) t (localized) p(nsim) t (rbf) pt (exact) extension of ed (2-nearest neighbors classification) estimation of probabilities (nearest neighbor surrogate) figure 6. the ”four branch” problem. the complete evolution of all panels is available as four branch video. limit state function reads g (x1, x2) = min  3 + 0.1 (x1 − x2)2 − (x1 + x2)/ √ 2 3 + 0.1 (x1 − x2)2 + (x1 + x2)/ √ 2 x1 − x2 + 7/ √ 2 x2 − x1 + 7/ √ 2 (22) this function can be also be viewed in the context of practical ”assessment of existing structures”: there are four distinct part of the failure set which are signalling failure. they can be viewed as different failure modes each of which can bring the structure down. the failure event is defined as g(xxx) ≤ 0. in the present definition, there are two pairs of design points: two points when x1 = x2 = ±3 √ 2/2 at a distance β1,2 = 3 and another two when x1 = −x2 = ±7 √ 2/4 at a distance β1,2 = 3.5. the exact result failure probability is pf ≈ 2.222 · 10−3, and the proposed global sensitivities read s2 f ,1 = s2 f ,2 = 0.5. figure 6 shows the four blue lines forming the failure surface. the associated four branch video demonstrates the behavior of the proposed method by adding points one by one. the extension algorithm refines the boundary proportionally to the probability density featured in the ψ criterion. the consequence of this is that the classification close to the four remote intersections of failure surfaces is not performed correctly for small designs. while this is no problem for the global is probability estimation, which focuses on the high-density regions, it is a source of erroneous estimation for the local is. as can be seen, the magenta dash-dot line in fig. 6 represents wrong and unstable results, which are degraded due to the accentuation of the inaccurately classified regions (heavily covered by the local is integration nodes). the global is applied to the same binary surrogate classifier provides accurate and stable results for eds as small as nsim ≈ 80. the top right panel in fig. 6 shows that the corner regions are not important for the global is. 275 https://vutbr-my.sharepoint.com/:v:/g/personal/vorechovsky_m_vutbr_cz/ewynt1wugfrgrbmdim9ypzkbzetilkdm9l96agrfdjvvnq?e=k6mj8c https://vutbr-my.sharepoint.com/:v:/g/personal/vorechovsky_m_vutbr_cz/ewynt1wugfrgrbmdim9ypzkbzetilkdm9l96agrfdjvvnq?e=k6mj8c miroslav vořechovský acta polytechnica ctu proceedings −4 −2 0 2 4 −4 −2 0 2 4 x1 x2 nsim = 200 ed + excluded candidates −4 −2 0 2 4 x1 ψ-ranked candidates −4 −2 0 2 4 x1 screening dots −4 −2 0 2 4 x1 r r is integration nodes via hann 0 25 50 75 100 125 150 175 200 number of limit state evaluations (g (xxx) calls), nsim 10−8 10−6 10−4 10−2 pr ob ab ili ty of ev en t t , ψ cr ti te ri on . ψ criterion p(nsim) t (global) p(nsim) t (localized) pt (exact) extension of ed (2-nearest neighbors classification) estimation of probabilities (nearest neighbor surrogate) figure 7. the ”wavy line” problem. the complete evolution of all panels is available as wavy line video. −6 −4 −2 0 2 4 5 −6 −4 −2 0 2 4 5 x1 x2 nsim = 150 ed + excluded candidates −6 −4 −2 0 2 4 5 x1 ψ-ranked candidates −6 −4 −2 0 2 4 5 x1 rbf surrogate −6 −4 −2 0 2 4 5 x1 r r is integration nodes via hann 0 20 40 60 80 100 120 140 number of limit state evaluations (g (xxx) calls), nsim 10−7 10−6 10−5 10−4 pr ob ab ili ty of ev en t t , ψ cr ti te ri on . ψ criterion p(nsim) t (global) p(nsim) t (localized) p(nsim) t (rbf) pt (exact) extension of ed (2-nearest neighbors classification) estimation of probabilities (nearest neighbor surrogate) figure 8. the ”metaballs” problem. the complete evolution of all panels is available in the metaballs video. we have examined the simplest gaussian rbfbased classifier to show to what extent the estimation improves compared to the simple nearest neighbor classification. using the global is estimation leads to stable, almost exact results which are already in the range of nsim ∈ (20, 80), see the brown line in fig. 6. the improvement is due to the more accurate classification of the failure surface in the vicinity of the four design points; the ”corners” were not classified correctly neither by rbf nor by the binary surrogate. the efficiency of the rbf classifier is as good as the efficiency of the best methods known in the literature. many existing sampling techniques, such as the sequential importance sampling employed in [4], use 276 https://vutbr-my.sharepoint.com/:v:/g/personal/vorechovsky_m_vutbr_cz/ercz-rgf9y5arclnbencqo0bxytvf53h6fy4aiemspm4fq?e=fgtk1e https://vutbr-my.sharepoint.com/:v:/g/personal/vorechovsky_m_vutbr_cz/ecqnjlpeokzljo1fmbqtxr8bywl62zkxpyrwydut0phb0w?e=kxrag3 vol. 36/2022 reliability estimation of binary functions 0 50 100 150 200 250 300 10−7 10−6 10−5 nvar = 2 0 100 200 300 400 500 600 10−7 10−6 10−5 nvar = 3 0 250 500 750 1000 1250 1500 1750 2000 10−8 10−6 10−4 nvar = 4 0 500 1000 1500 2000 2500 10−8 10−6 10−4 pr ob ab ili ty of fa ilu re ev en t, f ; ψ cr ti te ri on nvar = 5 0 500 1000 1500 2000 2500 3000 3500 4000 4500 10−8 10−6 10−4 10−2 nvar = 10 number of limit state evaluations (g (xxx) calls), nsim 0 50 100 150 200 250 300 0 5 0 100 200 300 400 500 600 0 5 0 250 500 750 1000 1250 1500 1750 2000 0 2 4 6 0 500 1000 1500 2000 2500 0 2 4 6 d is ta nc es of ed po in ts fr om th e pl an ar fa ilu re su rf ac e an d fr om th e or ig in 0 500 1000 1500 2000 2500 3000 3500 4000 4500 0 2 4 6 8 number of limit state evaluations (g (xxx) calls), nsim 0 50 100 2 1 0 100 200 3 1 0 200 400 600 4 1 0 100 200 300 n um be r of fa ilu re po in ts ,n f 5 1 0 50 100 150 200 250 10 1 figure 9. recorded histories of exploration and estimation for the ”linear failure surface” problem in eq. (23) studied for nvar = 2, 3, 4, 5 and 10 (rows). the left column reports histories of the ψ criterion and the estimated p (nsim) f . the right column reports the radial distances of ed points from the origin (empty circles), as well as distances from the planar failure surface (small diamonds). the secondary vertical axis shows the number of failure points. the complete evolution of ed extension in nvar = 2 and 3 dimensions is shown in video 2d and video 3d. too many limit state function calls, and good results are obtained only when building a surrogate model. we repeat again that the proposed algorithm achieves almost the same efficiency; however, it does so using only the categorical information about g(xxx), which makes the proposed method very robust. figs. 7 and 8 show the behavior of the method for other functions. the detailed description can be found in [1], along with more examples posing other challenges to the existing methods, which are elegantly overcome by the proposed method. 6.1. linear failure surface in higher dimensions the previous 2d examples revealed the robustness of the algorithm regarding the complicated failure surface and g(xxx) function values. what remains a question is how the algorithm efficiency scales with dimension. to present a reasonable higher-dimensional example relevant to many practical problems, we use a compromise: a simple linear failure boundary (a line, a plane, or generally a hyperplane). there is no reason to make the linear failure surface rotated in the space of input variables because the proposed framework is rotationally invariant in gaussian space. therefore, it suffices to make the limit state function simply depend on the first dimension only g (x1, x2, . . .) = β − x1. (23) such a problem has a trivial analytical solution: pf = φ(−β). we set β = 4.7534243 to achieve the failure probability pf = 10−6, i.e., a number relevant for ”assessment of existing structures”. fig. 9 presents the results for nvar = 2, 3, 4, 5 and 10 dimensions. the blue line in the left column is the estimated amount of probability resolved by evaluation of the corresponding g(xxx) (ψ criterion), and the red line plots the estimates of the failure probability. it is clear that the purely ”exploratory phase” with the expanding search until the first failure is hit 277 https://vutbr-my.sharepoint.com/:v:/g/personal/vorechovsky_m_vutbr_cz/ebommfgxz-nevawcp1_dsq4bfegzuhbacawxbbi2mamwpq?e=16apuw https://vutbr-my.sharepoint.com/:v:/g/personal/vorechovsky_m_vutbr_cz/eqntikuomepfhdftk4ezx6obcuayl5d6hbsw-bvfvyulcg?e=2eey4q miroslav vořechovský acta polytechnica ctu proceedings consumes increasingly more g(xxx) function evaluations as the space dimension increases; compare the radial distances of points from the origin plotted as empty circles in the right column of fig. 9, which are organized at individual distance levels. for nvar = 10, it takes about 500 g(xxx) function calls to hit the failure event and begin refining the (very large) failure surface. stabilization of the probability estimates necessitates very fine refinement of the failure surface, which also consumes many function calls. the need to spend higher numbers of points in the purely exploratory phase in higher dimensions is an inevitable consequence of the fact that the numerical value of g(xxx) cannot be used to orient the search, e.g., in the direction of the negative gradient, as subset simulation or methods building a smooth surrogate do. the estimated pf for nvar = 10 shows quite a serrated profile, although the coefficient of variation is very small due to the use of a high number nis of integration nodes. the reason is that the boundary approximated via the voronoï cells in the nearest neighbor classification is also very serrated. its extent is large, and the refinement would necessitate very many additional g (xxx) calls. one can also notice that the decrease in ψ criterion is less rapid in high dimensions because the volume of the space simply increases with the space dimension. this is manifested via the increase of the extent of the failure surface part with a high gaussian density. apart from the radial distances plotted via empty circles in the right column of fig. 9, we also plot the distance of points from the planar failure surface as small diamonds (green and red failure points). once the first failure event is hit, the extension algorithm primarily selects the points to refine the failure surface. however, as can be seen, their distance from the origin is considerably greater than the shortest distance of the plane β, which is marked by the horizontal blue line. the complete evolution of the refinement process is captured point-by-point in individual frames of video 2d and video 3d. the non-decreasing, maroon-colored line in the right column of fig. 9 shows that the proposed refinement algorithm tends to the ratio nf/nsim = 1/nvar, which is excellent in nvar = 2 dimensions where almost all limit state function evaluations refine the failure surface from both sides, but less efficient in higher dimensions in which increasingly more points are spent on the exploration of new territories and a smaller share is devoted to boundary refinement. 7. conclusions this paper briefly presents simple yet robust and efficient methods for the sequential extension of experimental design and estimation of failure probabilities for computational models, which can be non-smooth, or returns only a finite number of states or even have blind spots for which there is no result at all. the extension of experimental design balances the gradual exploration of new territories by expanding the covered region and refinement via the exploitation of important regions. the balance is maintained by maximizing the proposed simple ψ criterion which expresses the approximate amount of probability being classified by any proposed candidate for extension. the extension of the experimental design makes no assumptions about the performance function and, therefore, is invariant with respect to its reparametrizations and reformulations, which do not alter the failure domain shape & location, and the method is resistant against noise and jumps. the estimation task can be performed at any time during the extension process by quickly analyzing the point-wise information only. it uses standard importance sampling applied to a surrogate model. the proposed method needs no fine-tuning of parameters depending on the analyst, because there are no such variables. the only freedom is in the density of the initial exploration set and with the option of refining it during the extension process. the method can help in solving hard practical reliability problems for which the existing methods fail due to their strong assumptions about the performance function being well-formulated and well-behaved. simple yet apt global sensitivity measures are proposed, which can be obtained for any rare event type as a by-product of the presented method. the applicability of the methods has been demonstrated for small to medium dimensions; high dimensions (several tens to hundreds of independent input variables) remain a challenge. acknowledgements the author acknowledges the financial support provided by the ministry of education, youth and sports of the czech republic under project no. ltausa19058, and additionally by the czech science foundation under project no. 20-01781s. references [1] m. vořechovský. reliability analysis of discrete-state performance functions via adaptive sequential sampling with detection of failure surfaces. computer methods in applied mechanics and engineering p. under review. 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https://doi.org/10.1115/1.4044040 https://doi.org/10.1115/1.4047636 https://doi.org/10.1016/j.ress.2021.108223 https://doi.org/10.1007/s10483-006-1001-z https://doi.org/10.1016/j.strusafe.2011.06.001 https://doi.org/10.1016/j.ress.2016.01.023 https://doi.org/10.1016/j.strusafe.2017.04.006 https://doi.org/10.1016/j.ress.2021.108260 https://doi.org/10.1016/s0167-4730(97)00017-9 https://doi.org/10.1016/j.strusafe.2004.11.001 acta polytechnica ctu proceedings 36:269–279, 2022 1 introduction 2 gaussian space with independent marginals 3 extension of the experimental design 3.1 exploration set 3.2 exploitation set 3.3 criterion for candidate selection 4 estimation of probabilities 5 global sensitivity measures 6 numerical examples 6.1 linear failure surface in higher dimensions 7 conclusions acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2025.54.0046 acta polytechnica ctu proceedings 54:46–49, 2025 © 2025 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague interface shear strength evaluation between different concrete types eliška kafkováa,∗, jan macháčeka, věra kabíčkováa,b, tomáš vlacha,b a czech technical university in prague, faculty of civil engineering, department of architectural engineering, thákurova 7, 166 29 prague 6, czech republic b czech technical university in prague, university centre for energy efficient buildings, materials and construction of buildings research department, třinecká 1024, 273 43 buštěhrad, czech republic ∗ corresponding author: eliska.kafkova@fsv.cvut.cz abstract. composite concrete elements, combining different types of concrete, are widely used in modern infrastructure projects like bridge deck overlays and the reinforcement of existing structures. a critical property for ensuring effective composite action between concrete materials of different ages or types is shear strength, which must be sufficient to prevent excessive slippage or complete separation. this is a fundamental requirement for achieving composite action. this article presents an experimental study on the shear strength at the interface between two different types of concrete, assessed through a push-off test. this investigation stands directly on earlier research focused on the use of high-performance concrete as lost formwork, providing core protection for concrete with recycled aggregates. the study explored the influence of recycled aggregate concrete with aggregates from bricks (rac) and high-performance concrete (hpc), with particular attention to the treatment of the concrete interface and the impact of interface roughness and surface treatment. the findings validated the hypothesis that shear strength at smooth, untreated interfaces is considerably lower than that at rough, treated interfaces. keywords: shear strength, recycled aggregate concrete (rac), high-performance concrete (hpc). 1. introduction the effective bonding between different types of concrete is crucial for ensuring that composite structures perform as intended. composite action depends on the shear strength at the interface between two materials, and any separation between them can drastically reduce the overall load-carrying capacity [1]. this study evaluates the shear strength at the interface between high-performance concrete (hpc) and recycled aggregate concrete (rac), particularly focusing on the role of surface treatment [2–4]. 2. materials both of the concrete mixtures used in this experiment was evolved at the department of architectural engineering, faculty of civil engineering ctu in prague. the average compressive strength of the hpc mixture measured on cubes of the length of the edge 100 mm after 28 days of hardening was 126 mpa. the average compressive strength of the rac mixture measured on cubes of 100 mm in length of the edge after 28 days of hardening was 23 mpa. the detailed composition of the matrix is described below in tables 1 and 2. the samples were prepared as the whole element 350 mm × 150 mm × 80 mm, interface area was 300 mm × 80 mm. all specific dimensions will be shown in the next chapter. mix content kg m−3 cement i 42.5r 650 technical quartz sand dmax=1.2 mm 1 200 technical quartz powder st 6 235 silica fume (microsilica) 75 superplasticizer based on pce 18 water (at 12 °c) 190 total 2 368 table 1. hpc mix design. mix content kg m−3 cement i 42.5r 274 recycled aggregate 0/4 922 recycled aggregate 4/8 0 recycled aggregate 8/16 733 water (at 12 °c) 253 total 2 200 table 2. rac mix design. 3. experiment the interface between the hpc and rac core was subjected to different surface treatments to evaluate the impact of roughness on shear strength. the four variants were as follows: reference type (smooth interface): in this case, 46 https://doi.org/10.14311/app.2025.54.0046 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 54/2025 interface shear strength evaluation between different concrete types figure 1. horizontal grooves on the left, vertical grooves on the right. no surface modification was applied, leaving the concrete in its natural state, with a smooth interface resulting from the formwork. bubble wrap imprint: a bubble wrap sheet was pressed into the surface of the hpc before casting, leaving an imprint of small, rounded indentations that created a mild roughness on the interface. vertical grooves: vertical grooves were created by inserting small plastic strips into the formwork before casting the hpc. these strips were removed after casting, leaving a series of vertical grooves across the interface, figure 1. horizontal grooves: after the hpc was cast and cured, horizontal grooves were milled into the surface using a mechanical tool. this method created a controlled and uniform roughness pattern at the interface, figure 1. each variant was tested with a set of three samples, and the results were used to calculate the average shear strength at the interface. the shear strength was evaluated using a push-off test, where a compressive load was applied to the upper hpc block while the force required to induce failure at the interface was measured. hpc blocks were on the outer layer and the core was made by rac. the setup of this first test is shown in the figure 2. the second push-off test aimed to examine the effect of concrete shrinkage on the bond between hpc and rac. in this setup, the concrete samples were enclosed in a steel casing (hollow rectangle pipe) that tightly contained the samples during the curing process. this steel casing was used to restrict lateral shrinkage during the hardening phase, which can otherwise lead to microcracks at the interface [5]. shrinkage occurs naturally as concrete hardens, but when confined within the steel casing, the concrete was restricted from expanding outward. this allowed for a more controlled curing process, which is expected to maintain the integrity of the interface and improve the shear strength. setup of the samples were exactly the same with added steel profile outwards [6]. figure 2. setup of the push-off test. vertical section on the left, plan view on the right. 4. results the first were tested unrestrained concrete samples in the four mentioned variants. results of maximal loads and the average shear strength are described in the tables 3– 6. fmax [kn] τ [mpa] øτ [mpa] 26.46 0.551 0.59228.59 0.596 30.14 0.628 table 3. no surface treatment. fmax [kn] τ [mpa] øτ [mpa] 20.79 0.434 0.43921.14 0.443 table 4. foil (imprint of bubble wrap). fmax [kn] τ [mpa] øτ [mpa] 84.67 1.771 1.96198.19 2.048 98.75 2.064 table 5. vertical grooves. fmax [kn] τ [mpa] øτ [mpa] 96.28 1.995 1.94592.91 1.926 91.30 1.914 table 6. horizontal grooves. 47 e. kafková, j. machacek, v. kabičková, t. vlach acta polytechnica ctu proceedings these results indicated that the smooth surface (reference type) exhibited kind of low shear strength, while the textured surfaces (vertical and horizontal grooves) displayed a significant increase in shear strength. the bubble wrap imprint, which created mild surface roughness, showed a lower shear strength than the smooth interface. the smooth interface’s lower strength can be attributed to the lack of mechanical interlocking between the two materials. in contrast, the textured surfaces allowed for greater bonding due to the increased surface area and better mechanical interlocking. the similarity in the performance of the vertical and horizontal grooves suggests that both types of surface treatments created effective roughness that enhanced the shear strength [7]. concrete samples with steel casing were tested subsequently in two surface treatment variants possible for casting in the steel profile – smooth interface and vertical grooves. detailed results are visible in the table 7 and 8 and are presented as a compression tests results due to form of fracture. fmax [kn] fc [mpa] øfc [mpa] 149.7 30.248 26.635128.5 25.672 118.7 23.984 table 7. smooth interface. fmax [kn] fc [mpa] øfc [mpa] 149.7 30.248 26.635128.5 25.672 118.7 23.984 table 8. vertical grooves. during the tests, the load primarily concentrated on the rac segment of the composite sample. the applied force crushed the rac at its upper surface, causing failure in this region, figure 3. this localized crushing at the top of the rac core limited the ability of the test to engage the interface fully in shear or even in distributed compression across the interface area. consequently, the lower part of the interface remained underutilized, further confirming that the test primarily reflected a compressive failure mechanism of the rac rather than the intended shear interaction between hpc and rac. the steel casing successfully mitigated shrinkagerelated cracking, maintaining better compressive integrity. nevertheless, the inability of this test setup to simulate shear stresses limits its applicability for evaluating interfacial bond behavior under composite action. figure 3. steel-cased samples after testing. 5. conclusion this study investigated the interface shear strength between hpc and racb using two distinct push-off test configurations. the findings underscore the critical role of surface treatment in improving interface performance but also highlight the limitations of certain test setups in replicating real-world conditions. the first standard pus-off tests provided a clear assessment of shear strength, demonstrating the effectiveness of grooves in enhancing interfacial bond strength. the results confirmed that roughened surfaces significantly outperformed smooth interfaces, emphasizing the importance of mechanical interlock in resisting slippage. while intended to simulate the effects of shrinkage constraints, the steel-cased tests primarily measured compressive resistance due to the load path bypassing the interface. the crushing of the rac core revealed that this setup was unsuitable for evaluating shear performance and highlighted the impact of material incompatibility on load distribution. the study reaffirms that surface treatment is essential for optimizing interfacial bond strength, particularly in applications requiring composite action under shear. however, the significant strength disparity between hpc and rac underscores the need for careful consideration of material selection and test configurations to ensure meaningful assessments. future research should focus on test setups that accurately replicate shear loading conditions, as well as strategies for mitigating material incompatibility to enhance the overall performance of composite concrete systems. acknowledgements the work on this paper was supported by czech science foundation grant no. 22-14942k entitled “possibilities of using natural fibers for the production of hybrid textile reinforcement in concrete” the authors would like to acknowledge all financial assistance provided to support this research. 48 vol. 54/2025 interface shear strength evaluation between different concrete types references [1] p. m. d. santos, e. n. b. s. júlio. assessment of the shear strength between concrete layers. in proceedings of the 8th fib phd symposium in kgs. lyngby, denmark, pp. 20–23. 2010. [2] f. ceia, j. raposo, m. guerra, et al. shear strength of recycled aggregate concrete to natural aggregate concrete interfaces. construction and building materials 109:139–145, 2016. https: //doi.org/10.1016/j.conbuildmat.2016.02.002 [3] r. c. k. wong, s. k. y. ma, r. h. c. wong, k. t. chau. shear strength components of concrete under direct shearing. cement and concrete research 37(8):1248–1256, 2007. https://doi.org/10.1016/j.cemconres.2007.02.021 [4] h.-s. youm, w.-y. lim, s.-g. hong, c. joh. interface shear strength between ultra-high-performance concrete and normal-strength concrete. aci structural journal 118(2):287–298, 2021. https://doi.org/10.14359/51729350 [5] j. echegaray-oviedo, j. navarro-gregori, e. cuenca, p. serna. modified push-off test for analysing the shear behaviour of concrete cracks. strain 53(6):e12239, 2017. https://doi.org/10.1111/str.12239 [6] z. p. bažant. theory of creep and shrinkage in concrete structures: a precis of recent developments. in mechanics today, vol. 2, pp. 1–93. 1975. https: //doi.org/10.1016/b978-0-08-018113-4.50007-0 [7] s. al-obaidi, y. m. saeed, f. n. rad. flexural strengthening of reinforced concrete beams with nsm-cfrp bars using mechanical interlocking. journal of building engineering 31:101422, 2020. https://doi.org/10.1016/j.jobe.2020.101422 49 https://doi.org/10.1016/j.conbuildmat.2016.02.002 https://doi.org/10.1016/j.conbuildmat.2016.02.002 https://doi.org/10.1016/j.cemconres.2007.02.021 https://doi.org/10.14359/51729350 https://doi.org/10.1111/str.12239 https://doi.org/10.1016/b978-0-08-018113-4.50007-0 https://doi.org/10.1016/b978-0-08-018113-4.50007-0 https://doi.org/10.1016/j.jobe.2020.101422 acta polytechnica ctu proceedings 54:46–49, 2025 1 introduction 2 materials 3 experiment 4 results 5 conclusion acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2025.52.0109 acta polytechnica ctu proceedings 52:109–115, 2025 © 2025 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague public perception of the kamenná district sofie pokorná∗, klára francová, nikola číhalová brno university of technology, institute of forensic engineering, purkyňova 118, brno, czech republic ∗ corresponding author: sofie.pokorna@vut.cz abstract. the article describes the emergence of emergency and workers’ colonies in the city of brno from the beginning of the 20s of the 20th century. it also briefly introduces the kamenná district and its inhabitans from its inception up to the present. here, the social transformation of the locality is highlighted, which is proven mainly by research on education and professional representation in the locality. furthermore, the introduction briefly describes the phenomenon of gentrification and the definition of individual phases by philip l. clay. the primary goal of the work was to find out the public’s current opinion on the locality and thereby help to include the phenomenon of gentrification in one of the phases. the opinion of the public was ascertained using quantitative research by means of questionnaires on the internet and in the field. the results brought the expected conclusions, the public views the kamenná district positively and notices signs of renovation of buildings and streets, a richer cultural life or a change in the social structure of the quarter. using the identified characteristics, gentrification was included in the third phase of its course. keywords: kamenná district, brno, gentrification, phase of gentrification, public opinion about the kamenná district. 1. introduction 1.1. the emergence of worker colonies in brno following the industrial revolution (1760-1840), new businesses were founded in brno, which generated new jobs. it was not possible to fill them with the existing citizens of the city, and therefore many new residents immigrated to brno. the influence of migration and the natural growth of brno required a comprehensive solution to the housing issue – in 1919, a housing crisis occurred, when brno lacked 2 000 apartments. the city responded to this situation by building approximately 10 000 houses during the first republic. these were mainly apartment buildings, which provided about 35 000 new housing units. in this period, functionalist housing estates of a housing estate nature, which were supposed to solve the housing shortage of the weaker economic strata of the population, became a prominent urbanistic element of brno. there were plenty of vacant apartments in the city, but thanks to new construction, even workers’ apartments were relatively expensive [1]. however, families with two incomes could normally afford to live as a rent in such an apartment building. during the 1920s, because of the approaching economic crisis, brno businesses laid off workers or reduced their employment, which pushed many families below the poverty line [2]. the reaction to this was the creation of emergency colonies, which were built to a greater extent from the mid-1920s. to prevent the spontaneous and disorderly creation of these worker colonies, the city municipality approved and financed the construction of colonies in husovice and komárov, but this was not sufficient in capacity. the city also bought disused railway cars and placed them in existing colonies, these cars were to serve as temporary accommodation for homeless families. in 1927, an extreme increase of 187 newly built emergency dwellings is recorded. in 1929, these emergency dwellings already formed several contiguous units, and the number of emergency dwellings was approximately 4.5% of the total residential construction (see table 1). after 1935, the growth of worker colonies began to decline. at the end of the first republic, there were about 20 emergency colonies in brno. in these colonies, over 7 000 inhabitants lived in 1 100 houses, another more than 500 inhabitants lived in shacks, stables, barns or sheds. the size and number of these colonies changed considerably during the 1940s, and some, especially those with more stable settlements, have survived to this day [1]. 1.2. kamenná district 1.2.1. brief history the kamenná district was established on the territory of the municipal gravel pit on červený kopec. thanks to the division of the quarry into floors, the kamenná district was naturally divided into two parts, the socalled “upper rock” and “lower rock”. the lands on which the district was established were both municipal and private but did not belong to the residents themselves. the beginning of construction dates to 1925, when the existing house no. 1 was inhabited, and construction began in the dolní skaly area. this construction was permitted by the city. in the course of 1928, construction also began in horní skala, where, however, 109 https://doi.org/10.14311/app.2025.52.0109 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en s. pokorná, k. francová, n. číhalová acta polytechnica ctu proceedings locality formation of a colony established by number of houses in 1929 notes the demise of the colony number of houses today diviš’s colony after 1925 landowners 154 also, shanghai about 170 kamenná district 1925 residents over 100 about 140 písečník 1919 the city 84 only písečník street (formerly písečník + lozíbky) has been preserved in the original spirit of the colony about 27 podstránská 1928 62 (1933) about 100 v chaloupkách 1928 the city 1946 na hausberku 1928 1946 na rybníčku 1927 residents 60 also, planýrka colony 1940 u kostivárny 1925 residents 42 also, černovičky about 50 colony na novém hřbitově 1925 the city 144 flats the name: in the old cemetery, was also used 1945 polní 1930–1932 residents 34 also, baráková kolonie u polní 1958 hálkova ulice 1750–1780 22 today fibichova about 40 resslova ulice throughout the 30s residents today myslínova about 65 na úhelnách throughout the 30s 26 1999 u olší 1932 the city 45 about 42 vagónová colony between 1933–1939 the city 8 made up of discarded railway wagons 1980s colony za zahradou 1926 the city 1942 table 1. overview of workers’ colonies existing in the 1930s in brno [3]. it was built without a permit. the permit for these buildings was issued retroactively. the construction took place in stages lasting from 7 months to 3 years. residents had to pay the so-called “annual fee”, which was a fee for the use of municipal land within a certain year and with the possibility of appeal even without giving reasons. most of the houses were built in the most necessary formula “kitchen + room” and the material for construction was obtained mainly from demolitions – a frequent source was the demolished na rybníčku colony at the time. in the following years, basic hygiene rooms, utility rooms and additional bedrooms were built to the existing buildings. in 1927, the first water pump was built in the colony, and in the 1930s, sewage was installed in the lower part of the kamenná district. during this period, the private land in the upper part was transferred to another owner, who allowed further construction there. the colony thus grew significantly with the houses of families from the polní colony, which was being destroyed at the time. in the 1940s, 132 emergency houses were registered in the kamenná district. in the 1950s, the city intensively strove for the abolition of the district, which the communists opposed [4–6]. 1.2.2. residents of the kamenná district the first residents were very often workers working in the nearby khona and jílka brickworks with their families. the composition of the population of these mainly poor working men and women was changed only at the end of the 1970s, when part of the original population moved to new brno housing estates. in the natural course of time, the remaining original inhabitants grew old and died out. during this period, the public perceives kamenná district very negatively and its inhabitants as problematic and unadaptable. the inhabitants themselves often perceive their origin as a stigma that worsens their public relations and opportunities in education and employment opportunities. the opposite is the heightened “rock” 110 vol. 52/2025 public perception of the kamenná district patriotism, which appears in a significantly smaller part of the local population [5]. between the 1970s and the 1990s, a new wave of colonization of the kamenná district took place, when young bohemians who were enthusiastic about the genius of the loci moved to the district. from the point of view of gentrification, it is possible to call them pioneers. from a professional point of view, they were often artists, teachers or craftsmen. pioneers bought houses at low prices, renovated their dwellings, started families here and actively participated in the cultural development of the locality. after this transformation, the kamenná district was perceived as a place of artists and bohemians. with the new residents came changes in the form of renovations and additions, which marked the new artistic spirit of the district [7]. today’s residents have changed a lot since the days when the residents were exclusively poor people with blue-collar professions. both the economic situation and the tastes, trends and requirements of people to satisfy housing needs have transformed [8]. the transformation of the population from blue-collar professions, through predominantly artistic and craft professions, has transformed into today’s form, when one could speak of an above-average educated locality, see figure 1 from 2023. as part of the jufos 2023 conference, the results of a quantitative survey in the kamenná district were published, which related to education (see figure 1) and professions (see figure 2) represented among the current residents of the locality. the professions that appeared among the respondents were divided into groups: labor, business, teaching, services, technical, artistic and creative professions, scientific and healthcare. this division took place based on the methodology specified by the czech statistical office [9]. a comparison of education and professional composition helps to classify current residents into a social class according to the internationally used classification, which indicates individual social strata with letters. this division refers not only to the profession and property, but also to the highest level of education achieved. according to this classification, kamenná district is currently composed of representatives of the so-called upper and upper middle class supplemented by the lower middle class. according to the results of the investigation, representatives of the working class or the lower class are no longer present in the kamenná district. in terms of professional representation, 2% were classified in the labor profession, given that this profession was also qualified and conditional on a secondary school education, according to the division used, it would be classified in the lower middle class. even if there are no data on the composition of the population from the years when the kamenná district was established, we can consider it an axiom that it was a working class that had at most a basic figure 1. the result of a questionnaire on the topic of education in the kamenná district from 2023 [7]. education. illiteracy in this district was higher than in ordinary brno districts [10]. however, education has increased over the years. it is important to compare the kamenná district with the rest of the republic or the whole of brno. in this comparison, the current level of education in the kamenná district appears to be extremely high and does not correspond in the least to the original working-class population, see figure 3. the only city with a population of over 100 000 in the south moravian region is brno. 1.3. gentrification gentrification is the socio-cultural-economic transformation of an originally poor or excluded city location into a lucrative and expensive location. as part of this phenomenon, the population of the locality gradually changes, new services and shops are created, properties are repaired and rebuilt, and their value increases during the process. above all, the increase in rents and real estate prices has the effect of displacing the original population and replacing them with people from higher social strata. those affected by gentrification are people with limited economic income. these are mainly the poor, migrants and senior citizens of retirement age [11, 12]. gentrification needs to be understood as a process, not as a final stage. this process unfolds in individual stages that do not have clear boundaries and it is not possible to find the end of the overall phenomenon [13, 14]. in the 1970s, philip l. clay defined the individual phases based on timothy pattsenem’s research in boston, and this division is still used today [11]. 1st phase: the arrival of a small and non-risky group, i.e. pioneers, in a working-class or deprived locality. these pioneers buy real estate here, renovate it for their own needs. the district has vacant properties. the area is perceived as risky and for this reason investors and entrepreneurs do not invest in the district [11]. phase 2: the neighborhood’s reputation improves in the eyes of the public and the first gentrifiers begin to move into the area. their number is higher than the first pioneers, but in their way of thinking they are like the pioneers. newcomers can still find available properties in the locality. real estate agencies are 111 s. pokorná, k. francová, n. číhalová acta polytechnica ctu proceedings figure 2. representation of professions in kamenná district in 2023 [7]. figure 3. comparison of education in the czech republic/south moravian region/cities in the czech republic with over 100 000 inhabitants (only the city of brno)/kamenné district 2021–2023 [7]. starting to get involved in the process and are starting to buy up the remaining vacant properties. following the higher utilization of real estate, rents are starting to rise slowly. at this stage, developers are starting to notice the area. the media are becoming interested in the district, articles are written about it and reports are shot [11]. 3rd phase: in this period, the location is already popular with the public and the public is moving to the district. in general, the district is considered relatively safe for investment, and in this connection, developers are also investing in the area. this can also be seen from the outside and the location is improving physically. real estate prices will start to rise dramatically. the pioneers are slowly becoming a minority group. investors owning apartments in the locality are starting to offer them to the middle class. the process of displacement of old settlers is at its maximum [11]. 4th phase: the location is understood as completely safe, fashionable and a great investment. there is still a large influx of new residents from the middle and upper class to the area. developers and other investors perceive the location as a safe and lucrative investment and are starting to establish specialized shops and services here. to satisfy the demand for housing, non-residential spaces are converted into apartment buildings. the process will also begin to determine the surrounding neighborhoods to satisfy the demand for housing. at the same time, the original inhabitants of the gentrified area are moving into these neighborhoods. at that stage, the displacement of the original inhabitants and pioneers escalates, as the 112 vol. 52/2025 public perception of the kamenná district price of real estate and rents in the area increases drastically [11]. 1.4. research hypothesis as part of previous research, the development in the transformation of the population and the growth of real estate prices in the kamenná district were outlined. given the fact that these are not the only characteristics of the gentrification phenomenon, it was necessary to ask ourselves how the kamenná district as a former emergency colony is perceived by the public today. is it still stigmatizing to live in the kamenná district or is the location already popular and living in it is more of a privilege? if the hypothesis were confirmed, it would also be possible to completely rule out the course of the first phase and include gentrification in one of the following phases of its development. 2. materials and methods as part of this work, the goal was to obtain data from a quantitative survey using the questionnaire survey technique in the city of brno. questionnaires were sent out using e-mails and social networks, pasting qr codes directly referring to the questionnaire, and further personal data collection took place in the field. the key questions of the questionnaire asked if the citizens of brno know the kamenná district, how they perceive it and how they perceive the residents of the kamenná district. 113 respondents took part in the survey, it is an ongoing evaluation, as the survey will continue for several months. 63 women and 50 men were interviewed. most respondents were in the age group of 26 to 40 years. due to the need for an up-to-date perception of the kamenná district, the questionnaire was not age-limited in any way. 68.1% of university-educated people, 28.4% of secondary-educated people and 1.8% of people with completed basic education answered the questionnaire. there was a total of 11 questions in the questionnaire, and after the first three, a set of additional questions was opened only to those who answered that they knew the kamenná district. those who do not know the kamenná district were 15.9%. the questionnaire also contained semi-open research questions. 3. results and discussion as part of the questionnaire survey, in addition to the already mentioned 15.9% of respondents who do not know the kamenná district, it is also worth mentioning those who heard about the former emergency colony but were not there. this group of people is 6.2% and the majority perception of the locality is positive or neutral, which shows that the kamenná district does not currently have a primarily bad reputation. furthermore, the answers of only respondents who visited the kamenná district will be evaluated. figure 4. perception of kamenná district. figure 5. willingness to live in kamenná district. most respondents like kamenná district, a smaller number of respondents perceive it neutrally. it is an exception among those interviewed that someone has a negative view of the kč (see figure 4). specifically, there was one respondent in the 41–55 age group. one of the questions was directed at the development of opinion on the kamenná district in the years. rather, the opinion of the respondents remains the same or improves. the biggest change is observed in the reconstruction of houses and streets, there is also a change in the social occupation of the district and a change in the increasing possibilities for cultural and civic enjoyment. despite the positive perception of the kamenná district, half of the respondents would clearly like to live here (see figure 5). they would like to live in the locality because of the location, the people, the atmosphere, the geniu loci and the cultural events. opponents of living in kamenná district have similar reasons, and these are primarily the location, the atmosphere, the people, but also the too cramped and community environment without privacy. the last set of questions dealt with how the public characterizes the kamenná district and its residents (see figure 6). most respondents perceive the surveyed locality as a village situated in the city or as a bohemian and artistic district. roughly 20% of respondents still perceive the kamenná district as a workers’ or emergency colony. a minority of respondents perceive it as a classic district. the current residents of kamenná district are perceived by 36% of the answers as free thinkers, by 29% as normal citizens and then by 18% of the answers as an educated class with open opinions (see figure 7). today, none of the respondents perceives the residents of kamenná district as working class, but the impres113 s. pokorná, k. francová, n. číhalová acta polytechnica ctu proceedings figure 6. characterization of kamenná district. figure 7. perception of kamenná district residents. sion of an emergency colony and the perception of the residents as criminals remained, even if clearly among the minority group of respondents. here, it was specifically about two respondents who were united in their answers only by the male gender. it was possible to answer this question in several ways, and some respondents used this option, and the answers thus overlapped. 4. conclusions research into the public’s perception of the kamenná district showed that the location is popular and perceived very positively. half of the respondents answered that they would like to live in the kamenná district. whether the locality is still perceived as an emergency colony, which is often fondly confused with a worker’s colony, or whether it is outwardly an artistic district is not clear, but it is clearly evident that this is such a specific area that, due to its location, dense population and cultural life, it acts district as an independent community different from the rest of the city of brno. several respondents answered that they perceive how the kamenná district has improved physically. a few others perceive the development of cultural and civic use in the locality, others a social transformation. all the above-mentioned results of public perception of the kamenná district point to the third or fourth phase of gentrification. previous research on the development of the real estate market or the change in the social structure of the research location also corresponds to this (these are largely visible in the introduction). acknowledgements the contribution was prepared with the support of the specific higher research of the ministry of education and culture, id no. úsi-j-23-8373. references [1] k. kuča. brno: vývoj města, předměstí a připojených vesnic [in czech; brno: development of the city, suburbs and annexed villages]. baset, praha, 2000. isbn 80-862-2311-6. [2] p. svoboda, z. zavadil. brněnský písečník: místo, které nespolykal čas [in czech; brno písečník: a place that time has not swallowed]. národní památkový ústav, metodické centrum moderní architektury v brně, brno, 2017. isbn 978-80-7480-099. [3] s. pokorná. dělnické kolonie města brna [in czech; workers’ colony of brno]. in sborník příspěvků konference junior forensic science brno 2022, pp. 105–114. 2022. isbn 978-80-214-6063-8. [4] m. laichman. brněnské nouzové kolonie za předmnichovské republiky [in czech; brno’s colonies in the pre-monarch republic]. vlastivědný věstník moravský xiv:25–29, 1959. 114 vol. 52/2025 public perception of the kamenná district [5] j. svobodová. brněnské obytné kolonie [in czech; brno residential colonies]. brno v minulosti a dnes xv:381–462, 2000. [6] t. bocholář. způsob života dělnických rodin v brněnské kamenné čtvrti [in czech; life of working-class families in brno’s kamenná district]. masters thesis, ujep, katedra etnologie, brno, 1979. [7] s. pokorná, k. francová. proměna sociální struktury kamenné čtvrti [in czech; the transformation of the social structure of the kamenná quarter]. in sborník příspěvků konference junior forensic science brno 2023, pp. 88–95. 2023. isbn 978-80-214-6165-9. [8] s. pokorná. vliv specifické lokality na cenu rodinného domu: brno – kamenná kolonie [in czech; impact of a specific location on the price of a house: brno – kamenná colony]. vysoké učení technické v brně. ústav soudního inženýrství, brno, 2019. [9] czech statistical office. [2023-03-16]. http://www.czso.cz [10] m. kučera. acta demographica ...: populace české republiky 1918-1991 [in czech; population of the czech republic 1918-1991]. česká demografická společnost, praha, 1994. isbn 80-901-6747-0. [11] j. h. chan, k. iankova, y. zhang, et al. the role of self-gentrification in sustainable tourism: indigenous entrepreneurship at honghe hani rice terraces world heritage site, china. journal of sustainable tourism 24(8-9):1262–1279, 2016. https://doi.org/10.1080/09669582.2016.1189923 [12] s. pokorná. gentrifikace [in czech; gentrification]. in xiii. ročník odborné konference doktorandského studia, pp. 137–146. 2021. isbn 978-80-214-5963-2. [13] l. lees, t. slater, e. wyly. gentrification. routledge, new york, 2007. isbn 9780415950374. [14] d. prokop, p. tebery, m. buchtík, et al. rozděleni svobodou: česká společnost po 30 letech [in czech; divided by freedom: czech society after 30 years]. radioservis, praha, 2019. isbn 978-80-88286-08-0. 115 http://www.czso.cz https://doi.org/10.1080/09669582.2016.1189923 acta polytechnica ctu proceedings 52:109–115, 2025 1 introduction 1.1 the emergence of worker colonies in brno 1.2 kamenná district 1.2.1 brief history 1.2.2 residents of the kamenná district 1.3 gentrification 1.4 research hypothesis 2 materials and methods 3 results and discussion 4 conclusions acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2024.47.0097 acta polytechnica ctu proceedings 47:97–103, 2024 © 2024 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague structural health monitoring of construction jakub niedoba brno university of technology, faculty of civil engineering, veveří 331/95, 602 00 brno, czech republic correspondence: jakub.niedoba@vutbr.cz abstract. the paper is concerned with the possibilities of long-term monitoring of constructions using modern wireless sensors and systems. there are currently many constructions under long-term monitoring performed by modern sensors. the monitoring enables an assessment of the current health of the construction or it can be used to recalculate and adjust the bmi models and subsequently evaluate the static and dynamic resistance of the construction. the sensors and systems installed enable the detection of hidden defects or newly emerging failures and damage. using the results obtained from the monitoring of the construction, it is possible to predict and carry out a timely repair or maintenance of the construction, which will ultimately decrease the financial and time-related costs, ensure greater safety and prolong its lifetime and resistance. keywords: structural health monitoring, wireless sensors, bridge monitoring, diagnostics of engineering structures, box girder bridge. 1. introduction many new structures that are important and help to fulfil the fundamental functions of a state are constructed every day all over the world. from the perspective of building construction, the fundamental functions mainly include provision of infrastructure, ensuring safety, or securing the basic necessities of life. the category of important constructions includes, for example, skyscrapers, dams, bridges, military structures etc. if their operation becomes restricted or an accident happens, there may occur not only threat to life and loss of property but also, for example, a significant and long-term infrastructure deterioration, irreversible environmental damage or lowering of the living standard. structural health monitoring – shm enables continual monitoring of the mechanical-physical properties of structures, subsequent evaluation of their health and determination of damage. a simplified diagram of shm is shown in figure 1 [1]. shm of constructions is defined as continual monitoring and analysis of a selected construction [2]. the monitoring is performed using measuring sensors, which are fitted to the structure or in its close proximity. the sensors monitor the mechanical-physical and geometric properties, and climatic changes in the vicinity focusing on changes over time. the goal of shm is an evaluation of the health of the structure based on the data obtained. subsequently, the data can be used to identify the location, extent, size, and seriousness of the defect or failure. in the past, shm was used as a detection system in astronautics and aviation [1]. it contributed to a significant improvement in safety in these fields and also to a better understanding of the behaviour of materials under both typical and exceptional load conditions. the information obtained could be used figure 1. diagram of shm [1]. to design better structural details, more functional, more economical and safer models etc. identification of damage through shm usually involves the following steps [3]: • detection of damage to the structure, • damage localisation, • damage identification, • examination of the seriousness of the damage, • prediction of the lifetime of the structure. shm involves many different processes, the most important of which are data sensing and transmission, data multiplexing, data processing and interpretation, description of changes occurring in the structure and materials, changes to the virtual model of the structure based on the current state. using the information obtained and bim software, it is possible to devise a plan for maintenance, reassess the static action of the construction and propose a solution to the situation or temporarily restrict the operation of the construction [4]. 97 https://doi.org/10.14311/app.2024.47.0097 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en jakub niedoba acta polytechnica ctu proceedings damage to the structure is detected through the physical phenomena occurring on the structure. the sensor reacts to the physical phenomena by generating electrical, optical or other waves, which are further transmitted to a datalogger and sent to a database. in the database, multiplexing takes place and the data are sorted and selected using specialized software. the software goes through all the data, which is multiplexed and compared, and a new database with the needed results is created. the results can be further processed and evaluated by a subsystem or manually. the processing method is usually selected based on the frame rate of the signal and the assumed amount of the data. the subsystems largely use various algorithms or the deep learning method. in most cases, the data are then converted to graphical output which is located at a web interface, where everyone has their own user access. in the case of extensive or important constructions, the data can be further passed on to various organizations which use them for health and property protection, improvement of the traffic infrastructure capacity, enabling faster arrival of the emergency services etc. 2. wireless systems generally, a system of regular visual inspections is used. the purpose of the visual inspections is an assessment of the health and detection of defects and damage to the construction. the visual inspections are very intuitive and provide direct information about the construction. however, a disadvantage is that they only provide information based on the visual surface condition of the structure. new bridge constructions are often built from modern materials. they are often segmented and robust; therefore, although the inspections are conducted by qualified experts with long experience and valid authorization [5], they may be insufficient for an overall assessment of the health. also, in order for an expert to conduct an inspection, they should be able to get close to the construction being checked. therefore, regular maintenance of the access roads should be carried out. however, especially with old constructions, it is missing or they are not maintained at all. the assessment of the construction health can be affected by a subjective interpretation of the results by the inspector. for the reasons mentioned above, a wrong determination of the current health can be made due to hidden defects which cannot be revealed by a visual inspection [6]. early notification of a failure or defect of the construction is necessary for satisfying the standards and the internal regulations of the organizations ensuring the functionality of the construction, following the professional methodologies for ensuring the safety of the construction, or ensuring the protection of human lives and property. modern monitoring systems can be used as an alternative for inspection of constructions. the standard monitoring systems include sensors, cables and an evaluation system. the cabling is usually used as a communicator between the datalogger and the sensor, and for the supply of electrical power. a disadvantage of the standard systems is the cabling leading to each sensor. in important and large constructions, hundreds of sensors are commonly used. establishing a robust cable network is demanding and timeconsuming. for example, for the tsing ma bridge in honkong [7], 350 sensors were used on one bridge construction. building an extensive cable network is demanding, time-consuming and expensive. the cables also have to be secured against damage caused by the weather such as rain, wind, sun, or even vandalism. furthermore, access must be provided for the staff who will be installing the whole system. in order to make the monitoring easier and to lower the costs, wireless sensors started to be used. these sensors do not require cabling. they are powered directly by their batteries and, in most cases, they contain solar panels, which charge the batteries and, thus, prolong the lifetime of the sensor. the data are transferred from the sensor to the database through wireless communication systems. the advantage of their use is significant time-saving in the entire project, from proposal to implementation. the installation of the sensors is much easier, as it is not necessary to access all the parts of the construction, only the parts where the sensors will be located. moreover, it is not necessary to place on the construction any other additional elements protecting the cabling. the standard lifetime of the sensor batteries is around 2 years, which can be affected by the battery capacity, the number of sensing elements in one sensor and the frame rate of the sensors. with the costs related to the computing and communication technologies decreasing, the costs of wireless sensors decrease as well. this fact has positively affected many areas in which the sensors are commonly used nowadays. application of wireless sensors has been proposed, in many cases, in order to lower the costs and the manual effort needed for their installation. in the construction industry, wireless sensors started to be applied in 1998, when straser and kiremidjian [8] first proposed a solution using radio sensors placed on the construction in order to lower the total costs of establishing a monitoring system. many researchers then started to be interested in the possibilities of using wireless technologies. the researchers‘ efforts resulted in the invention of a lowcost wireless technology, which is now used in the construction industry for monitoring the health of a construction. 3. practical application of the sensors the wireless sensors used for the measurement are shown in figure 2. the sensors contain inclinometer, accelerometer, temperature and humidity sensor. the measurement accuracy of the sensors is shown 98 vol. 47/2024 structural health monitoring of construction in table 1. the data transfer from the sensor to the database is realised through lorawan and nbiot communication. this communication is better for the sensors than the gsm or lte networks, which do not reach as far as the valley at the foot of the dam or the bridge chambers and underground underpasses [9]. the frame rate of the sensors can be set individually; the standard is 2.7–3.0 mhz. for long-term monitoring of a bridge overhead road, the frame rate was set to 2.7 mhz (once in 360 s). the sensors also record data when an unexpected situation occurs. the sensors are powered by the lisocl2 batteries and their lifetime primarily depends on the frame rate. the lifetime of sensors on bridge constructions is around 1 year. then, the battery must be quickly replaced. figure 2. wireless sensor. sensor type accuracy inclinometer 0.001° accelerometer 0.001 g temperature 0.01° humidity 0.01 % table 1. accuracy of the wireless sensor. 4. the construction measured from research and explorations, an overhead road with long-term static problems was selected as the construction to be measured. the road connects the czech republic and slovakia, it is part of the d2 motorway and spans the morava river and the ii/425 road. the overhead road comprises two bridge units (left and right). each bridge unit consists of 16 simple span bridge constructions on the czech side. on the border line, over the morava river, there is a continuous box girder bridge with 3 spans bridging the morava river. the continuous box girder bridge is shown in figure 3. on the slovak side, the overhead road continues with 4 typical simple span constructions. in total, the overhead road comprises 20 simple span units with a length of 35.5 m and a continuous box girder construction with a length of 40.0, 80.0 and 49.1 m (a total of 169.12 m). the ii/425 road passes under the 8th span (counted in the direction of stationing, i.e., away from brno). the total length of the overhead road is 879 m and the width is 13.45 m. the bridge was built in 1980 and it is straight, with the skew angle of 0°. figure 3. view of a box girder bridge against the direction of stationing (view in the direction of brno). the supporting structure of simple span units comprises 9 prefabricated prestressed dps girders with the axis distance of 1 500 mm which are joined in the transverse direction with a reinforced concrete slab with a thickness of 200 mm. the bridge construction spanning the morava river is a single-box bridge with a closed rectangular profile. the width of the chamber is 6 550 mm and its height is variable from 1 300 to 3 500 mm. the thickness of the vertical walls is 200–700 mm; the thickness of the lower plate is 150– 650 mm and that of the top plate is 220 mm. the upper, passable part of the bridge is widened on both sides using 3 300 mm cantilever. the box girder was originally prestressed with 24 tendons pz of 7 mm. in 1991, it was reconstructed and 4 pieces of external tendons were added. all the spans of the supporting structure are placed on pot bearings. the profile grade of the motorway in the overhead road section is directed in the crest vertical curve of r = 22 000 m. the cross slope of the right motorway bridge is one-sided, descending to the right outer side with a gradient of 2.0 %. in the experiment, wireless sensors were fitted to the border overhead road. the sensors were placed on the right border box girder bridge. in total, the construction was fitted with 2×3d accelerometer, 2×3d tilt sensor, 2× temperature sensor and 2× humidity sensor. the sensors were fitted to the walls in the central span of the box girder bridge closer to the czech republic. the location of the sensors is shown in figure 4. as the structure has long-term static problems and failures occur both in the supporting structure and in the substructure, it was decided that an extensive diagnostic of the bridge overhead road would be performed. the diagnostic is focused on the material properties of the structure, detection of failures, checking the condition of the concrete repair 99 jakub niedoba acta polytechnica ctu proceedings with additional concrete cover, and checking the prestressed and load-bearing reinforcement. it is planned to monitor the structure for 5 years and regularly compare the condition of the supporting structure with the previous data and with a model of the structure which will be made based on the data obtained from the extensive diagnostic. figure 4. longitudinal and cross section of the bridge. 5. results of the structural monitoring the structure has been monitored since 26th april, 2022. at the same time, an extensive diagnostic has been performed of the overhead road using both destructive and non-destructive tests. as part of the diagnostic, core bores were taken (by [10]). an evaluation was made of the hardness and strength of the concrete using the schmidt hardness tester (by [10]). carbonation was evaluated (by [11]), the cohesion of the concrete was determined (by [12]), and the condition of the prestressing reinforcement and the position of the reinforcement were checked (by [13]). in the first six month of the monitoring, the functioning of the mechanical connections between the sensors and the structure was checked as well as the functioning of the sensors on the structure. figure 5 shows the temperature and humidity in the chamber. the data show that, even during the summer months, the chamber is excessively humid and a considerable leakage into the structure occurs. figure 6 shows that on 1st june, 26th june and 16th july excessive leakage occurred. this corresponds with the records from the meteorological station located in the adjacent town, see [14]. the meteorological data show that, on 1st june, 25th june and 15th july, there were heavy rains. the most significant change in humidity, recorded on 26th june (from 90.8 to 98.1 %), occurred after all-day rain with the rainfall being 32.7 mm/day. the leakage into the bridge chamber was also confirmed by the preliminary diagnostic results and the main inspection of the structure (see the photo of the bridge chamber in figure 7). the long-term humidity in the bridge chamber in the summer months (june and july), when the average temperature of the surrounding environment is 25.6 °c, is 96.2 %. the tilt of the structure along all the axes is very stable. since the sensing takes place once every 6 minutes, the dynamic traffic load cannot be sensed. vertical lines showing incorrect measurement can be detected in the recorded data. these errors were probably caused by signal drop-outs of the sending device. the data can be used to determine a change in the longitudinal tilt of the box girder bridge caused by the temperature. at a temperature of 15 °c the tilt of the chamber was -0.179°. at the highest temperature recorded, 30 °c, the tilt of the chamber was -0.216°. the difference between the angles can be used to determine a change in the deflexion in the middle of the longest bridge span. the deflexion changed by approximately 25.8 mm. the acceleration of the structure is also stable. a change in the acceleration occurred during local renewal of the abrasive layer carried out at night-time between 1st july and 2nd july. this renewal caused a change in the acceleration along all the axes. the change in the acceleration was recorded by both sensors at the same time. this change was apparent for the entire time of the repair, i.e., over 8 hours. when the road was being repaired, no significant change in the tilt of the structure occurred, probably due to the fact that the extent of the repair was small. 6. conclusion shm of a bridge overhead road on the border crossing confirmed leakage into the structure due to nonfunctional waterproofing. the monitoring also provided preliminary results related to the health of the structure and also the average acceleration and deflexion of the structure. the initial monitoring checked the functioning of the sensors and the initial data obtained were used to determine the limit values of acceleration and tilt, at which irreversible damage to the structure might occur. the limit values will be further compared with a model and then entered in online evaluation software. when the limits are exceeded, the evaluation software will immediately inform the construction manager. this warning system will be used for ensuring the safety of the bridge overhead road, an early warning, and an urgent inspection of the structure. based on the inspection, a decision will be made whether critical damage really occurred and an immediate repair is necessary, or whether the 100 vol. 47/2024 structural health monitoring of construction figure 5. temperature and humidity. figure 6. raw data from tilt sensor. (a). (b). figure 7. on the left – a view of a sensor placed on the wall of a box girder; on the right – a view of the top plate of a box girder, where leakage and urban dripstone occur. 101 jakub niedoba acta polytechnica ctu proceedings figure 8. record of the acceleration during the repair of the road. defect is less serious and its repair can be postponed until the next renovation planned. the preliminary results from shm clearly show that the sensors sense real changes in the health of the construction. based on the results, it was suggested to the construction manager that an immediate replacement was necessary of the full-surface waterproofing of the top plate and of the passable asphalt layers of the construction. based on the recommendation, the manager decided to plan extensive road reconstruction and full-surface waterproofing. during the reconstruction, it will be possible to use shm to monitor changes in the acceleration of the structure, as there will be a considerable change in the actual weight of the structure. it is a full surface reconstruction of the road, where a change in the tilt of the structure is expected. the data from the planned reconstruction will be further compared with the data from the reconstruction that took place on 1st and 2nd july shown in figure 8 and with a model which is being prepared. the project includes a plan for excessive crossing over the box girder bridge. during the crossing, the frame rate of all the sensors will be temporarily changed to 5 hz in order to record in detail the changes in the structure caused by the crossing. it is also planned to fit additional sensors (potentiometers) to the construction for the time of the crossing. based on the data obtained from the sensors, the relationship between the tilt and the deflexion of the structure will be specified. the data from shm can be used to immediately check the structural health and thus ensure safety and sustainability. the advantage is that the data are collected and interpreted continually. therefore, if some damage to the construction occurs, shm will evaluate the condition of the structure and contact the experts responsible for the safety of the structure. based on shm and the decision made by the expert, the risk resulting from the current condition of the construction is assessed and, thus, the safety and usability of the structure is ensured. acknowledgements this paper has been worked out under specific research project no. fast-j-22-7982 at the brno university of technology. references [1] h. wenzel. introduction and motivation. john wiley & sons, ltd, 2009. https://doi.org/10.1002/9780470740170.ch1 [2] d. balageas, c.-p. fritzen, a. güemes (eds.). frontmatter. john wiley & sons, ltd, 2006. https://doi.org/10.1002/9780470612071.fmatter [3] c. neves. structural health monitoring of bridges: model-free damage detection method using machine learning. ph.d. thesis, kth royal institute of technology, 2017. [4] x. li, y. xiao, h. guo, j. zhang. a bim based approach for structural health monitoring of bridges. ksce journal of civil engineering 26(1):155–165, 2022. https://doi.org/10.1007/s12205-021-2040-3 [5] z. i. turksezer, p. f. giordano, m. p. limongelli, c. iacovino. inspection of roadway bridges: a comparison at the european level. in bridge maintenance, safety, management, life-cycle sustainability and innovations, pp. 436–436. 2021. https://doi.org/10.1201/9780429279119-272 [6] j. liu, s. chen, m. bergés, et al. diagnosis algorithms for indirect structural health monitoring of a bridge model via dimensionality reduction. mechanical systems and signal processing 136:106454, 2020. https://doi.org/10.1016/j.ymssp.2019.106454 [7] k.-y. wong, w.-y. k. chan, k.-l. man, et al. structural health monitoring results on tsing ma, kap shui mun, and ting kau bridges. in a. e. aktan, s. r. gosselin, s. r. gosselin (eds.), nondestructive evaluation of highways, utilities, and pipelines iv, vol. 3995, pp. 288–299. international society for optics and photonics, spie, 2000. https://doi.org/10.1117/12.387821 [8] e. g. straser, a. s. kiremidjian, t. h.-y. meng. a modular, wireless damage monitoring system for structures. 1998. [2023-11-11]. https: //api.semanticscholar.org/corpusid:106513216 102 https://doi.org/10.1002/9780470740170.ch1 https://doi.org/10.1002/9780470612071.fmatter https://doi.org/10.1007/s12205-021-2040-3 https://doi.org/10.1201/9780429279119-272 https://doi.org/10.1016/j.ymssp.2019.106454 https://doi.org/10.1117/12.387821 https://api.semanticscholar.org/corpusid:106513216 https://api.semanticscholar.org/corpusid:106513216 vol. 47/2024 structural health monitoring of construction [9] j. balek, p. klokočník. development of low-cost inclination sensor based on mems accelerometers. iop conference series: earth and environmental science 906(1):012057, 2021. https://doi.org/10.1088/1755-1315/906/1/012057 [10] nedestruktivní zkoušení betonu tvrdoměrné metody zkoušení betonu [non-destructive testing of concrete – determination of compressive strength by hardness testing methods]. standard, czech standard institute, prague, 2011. [11] základní postup rozborů silikátů. všeobecná ustanovení. standard, czech standard institute, prague, 2009. [12] výrobky a systémy pro ochranu a opravy betonových konstrukcí – zkušební metody – stanovení soudržnosti odtrhovou zkouškou. standard, czech standard institute, prague, 2000. [13] nedestruktivní zkoušení betonových konstrukcí [non-destructive testing of concrete structures]. standard, czech standard institute, prague, 2012. [14] m. navrátil. repair of rwy 12/3, hochtief cz, a.s., asb, 2017. [2023-11-11]. https://www.asb-portal.cz/stavebnictvi/ inzenyrske-stavby/doprava/oprava-rwy-12-30 103 https://doi.org/10.1088/1755-1315/906/1/012057 https://www.asb-portal.cz/stavebnictvi/inzenyrske-stavby/doprava/oprava-rwy-12-30 https://www.asb-portal.cz/stavebnictvi/inzenyrske-stavby/doprava/oprava-rwy-12-30 acta polytechnica ctu proceedings 47:97–103, 2024 1 introduction 2 wireless systems 3 practical application of the sensors 4 the construction measured 5 results of the structural monitoring 6 conclusion acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2024.47.0121 acta polytechnica ctu proceedings 47:121–127, 2024 © 2024 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague characteristics of fibres based on secondary raw materials and their use in concrete technology martin sedlmajer∗, jan bubeník, jiří zach, vítězslav novák brno university of technology, faculty of civil engineering, institute of technology of building materials and components, veveří 331/95, 602 00 brno, czech republic ∗ corresponding author: sedlmajer.m@fce.vutbr.cz abstract. different types of fibres in cementitious composites, particularly in concrete, are currently used for a number of reasons. fibres are being added to improve mechanical properties (especially steel and glass fibres), to increase the durability – to reduce occurrence of microcracks during the concrete aging (mainly synthetic and cellulose fibres), or to increase the fire resistance (polypropylene fibres). within the study, characterization of different types of alternative fibres (fibres generated during waste recycling that would otherwise end in incinerators or landfills) with possible use in cementitious composites. these were fibres from recycled pet bottles, paper, and mineral wool, whose properties were compared to the traditionally used cellulose and polypropylene fibres. in the experimental part, the thickness, length, shape, and surface of individual fibres were monitored by an optical microscope. furthermore, the amount of heat of combustion was determined by the calorimetric method, and the differential thermal analysis (dta) was carried out for determination of the impact of high temperature on monitored fibres. the microstructure of fibres was monitored using a scanning electron microscope. the focus of the experimental study was on fibres usable in concrete and capable of enduring high temperature stress. keywords: fibre reinforcement, waste fibres, temperature resistance. 1. introduction the ecological impact of individual industrial fields belongs to the key issues of the present time. effort is being made, using constantly evolving modern technologies, to make more use of secondary or alternative raw materials as a substitute for primary raw materials and, at the same time, implement new recycling technologies that will allow transform the waste into the secondary raw materials for other possible industrial usage, including construction. in the given case, these are also renewable raw materials belonging mainly to natural organic raw materials that can be used together with the secondary raw materials in the fibre form even within the concrete technology, and so improve the resulting properties of the composite and possibly influence its behaviour under conditions of extreme stress (e.g., in the case of fire resistance). current requirements for fibres added to concrete are regulated by the en 14889 standards, where the specific requirements for fibres are set regarding their material characteristics. the standard divides the requirements separately for steel and polymer fibres. the requirements for steel fibres are regulated by the en 14889-1 [1] fibres for concrete – part 1: steel fibres. the use of polymer fibres is regulated by the en 148892 fibres for concrete – part 2: polymer fibres. other fibre types are considered suitable providing they meet the requirements of issued technical standards for which a building technical certificate is issued in the czech republic. since the full effectiveness of the en 14889-2 [2] standard, i.e., since 2006, it is allowed to sell only fibres for concrete marked as ce [3] within the whole european union. fibres for concrete are an important ingredient that allows the modification of concrete properties to increase its useful properties based on applying the benefits of a particular type of fibre. especially increasing the tensile, form, and bending strength of the material. at the same time, some kinds of fibres, both natural and synthetic, positively influence concrete behaviour in terms of volume changes of the material, namely during shrinking [4–6]. polypropylene fibres prevent the formation of cracks during the concrete shrinkage [7], both in the solidification stage and also in the hardened concrete. the ability to increase the specific properties of concrete depends especially on the material from which the fibres are made of. fibres are also an important part of concrete where increased resistance to high temperatures is required (fire resistance). the increase in concrete durability at high temperatures using fibres is provided by elimination of so-called explosive spalling. based on several studies [8–11], it has been proven that explosive spalling is caused by increasing pressure of water vapours in a construction, which gradually pass from the heated surface towards a cooler part of the concrete. when the dew point is reached, water vapor condensates into water that gradually fills the porous structure and prevents further passage of water vapor that increases the internal stress of concrete. when internal stress exceeds the value of the tensile strength of con121 https://doi.org/10.14311/app.2024.47.0121 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en m. sedlmajer, j. bubeník, j. zach, v. novák acta polytechnica ctu proceedings crete, an explosive cracking occurs. characteristics of the optimal size of fibres for increasing the resistance against explosive spalling were solved within numerous papers [12–14]. when using any polypropylene microfibres in concrete, the spalling is reduced. however, with decreasing fibre cross-sectional size (below 32 µm) and length (6–12), more positive results in terms of higher resistance to the explosive spalling are obtained. fibres shorter than 6 mm and longer than 12 mm are less effective regarding explosive spalling [12]. as part of the study, the basic characteristics of fibres already used in concrete (polypropylene fibres – pp, cellulose fibres – cel i) were compared to 3 types of fibres produced by recycling of waste or secondary raw materials (cellulose fibres from recycled paper – cel ii, fibres from recycled pet bottles – pet, and fibres from recycled waste mineral wool – mw). a study of microstructure using an optical and scanning electron microscope was carried out for those 5 types of fibres, together with a behavioural study of these fibres at higher temperatures using a thermal analysis and determination of the heat of combustion. the study was carried out to assess possibilities of using alternative types of fibres for concrete with a capability to withstand exposure to high temperatures. 2. types of fibres used as part of the experimental part of the study, characterisation of five types of fibres was carried out. the first two types were fibres which are already industrially produced and used for concrete (pp, cel i). the other three types were developed fibres that are being produced by recycling old materials whose service live has ended and would end in incinerators or landfills (cel ii, pet, mw). namely, it was these types of fibres: • polypropylene fibres – designation pp, • cellulose fibres – cel i and cel ii designations (2 types), • polyethylene terephthalate fibres – pet, • fibres from recycled mineral wool – mw. 2.1. polypropylene fibres – pp polypropylene fibres have a wide scale of usage (sprayed concrete, floors, prefabrication) thanks to their specific properties. related to this are several possible methods of production and geometric properties, which are set out in the standard en 14889-2 for polymer fibres. these fibres are used to reduce the occurrence of shrinkage cracks; they decelerate the concrete sweating, reduce permeability, and prevent ripping out concrete when removing the formwork. hardened concrete with these fibres is more resistant against abrasion, impacts, and aggressive cycles [15, 16]. 2.2. cellulose fibres cellulose fibres have similar usage as the polypropylene ones (sprayed concrete, prefabrication, strength). it is possible to produce cellulose fibres by two methods. the first method is from natural sources (wood, plants, cotton), where special treatments are used to gain cellulose that is subsequently transformed into fibres. the second method for obtaining cellulose fibres is by a chemical or mechanical treatment of old paper and subsequent garneting. cellulose fibres are hygroscopic and are able to absorb humidity up to 85 % of their weight and keep it around a fibre. this results in good anchoring and bonding between the fibres and the matrix. contrary to glass fibres, they do not corrode in the highly acidic environment of concrete. the use of cellulose fibres in concrete increases the resistance to impact, resistance against shrinkage cracks, resistance against freezing cycles, and more [17, 18]. 2.3. polyethylene terephthalate fibres – pet this type of fibres is produced by recycling pet bottles that cannot be used within the original purpose. fibres can be produced in several ways. the most used methods are mechanical separation using cutting tools or melting of recycled pet bottles into a melt, creating an infinite fibre, which is subsequently reduced to a desired length. by using recycled pet fibres for concrete, it is possible to obtain concrete of higher stiffness and impact resistance since occurring cracks do not spread freely through the construction element and the stress is distributed to a larger surface by a fibre web [19]. 2.4. fibres from recycled mineral wool – mw fibres from recycled mineral wool are being developed in order to process the leftover waste wool from construction sites and their subsequent use. mineral wool is mechanically separated into smaller parts which are mechanically spread into individual fibres. fibres for concrete will be added to reduce the occurrence of cracks during concrete aging in the early stages, and thus to increase the durability of the concrete. fibres, on which the experimental study was executed, can be seen in figure 1 below, whereby cellulose fibres (cel i, cel ii) for both variants were processed into pellets that should reduce dustiness and their garneting occurs during their processing in the cementitious composite. 3. methodology and measurement results the determination of important properties for further use of these fibres in concrete demanding to high temperature stress was carried out in prepared fibre samples. these tests were carried out pursuant to the relevant testing standards: 122 vol. 47/2024 characteristics of fibres based on secondary raw materials figure 1. types of fibres used from the left – pp, cel i, cel ii, pet, mw. figure 2. observed fibres under an optical microscope – pp, cel i, cel ii, pet, mw. fibre type / property pp cel i cel ii pet mw thickness [µm] 20–23 24–36 10–22 13–15 8–15 face straight wavy variant straight straight surface smooth smooth rough smooth smooth length [mm]* 12.12 2.53 0.96 5.98 1.97 *the length of fibres was set to on at least 20 randomly chosen fibres from which the average length was derived. table 1. evaluation of observed fibres under an optical microscope. • determination of basic properties (length and thickness) under an optical microscope, • thermogravimetric analysis – tga, to verify the weight loss of fibres during heating, • determination of heat of combustion (en iso 1716), • study of the microstructure using a scanning electron microscope (sem). 3.1. determination of properties on an optical microscope the thickness, length and shape of chosen fibre types were determined by an optical microscope at various magnifications. the record was taken using a digital optical microscope keyence vhx. images from the optical microscope are shown in figure 2 at a magnification of 65. monitored properties are stated in table 1. 3.2. thermogravimetric analysis (tga) within the monitoring of increasing/decreasing weight of chosen types of fibres at increasing temperature, the fibres undergone thermogravimetric analysis (tga). this method is based on the principle of heating a certain amount of material in which the increase or decrease of its mass is monitored subsequently. in terms of monitored fibres, this analysis is important for the determination of a temperature interval when the onset of melting and subsequent burning (evaporation) of the fibres occurs. results are shown in the following 123 m. sedlmajer, j. bubeník, j. zach, v. novák acta polytechnica ctu proceedings figure 3. tga recording of observed fibres. figure 4. dsc recording from pet fibres. figure 3. the x-axis is showing the temperature [°c] while y-axis is showing the sample weight [mg]. the fastest mass loss occurs for pp fibres in the range of 250–420 °c and, at a temperature of approximately 470 °c, complete degradation of the fibre is completed (98.8 % of mass was decomposed). for cellulose fibres cel i and cel ii, similar progress can be seen in the graphic evaluation. approximately around 250–270 °c (cel ii), 290–300 °c (cel i), there is a slight mass loss due to the release of free and physically bonded water; upon further increasing of temperature, there is a rapid mass loss as a result of the degradation of the cellulose fibre. in the case of recycled cellulose fibres (cel ii), a residual mass of about 18 % of the original mass remains even after reaching 750 °c. in the case of pet fibres, the significant mass loss occurs within the range of 350-580 °c. after reaching 580 °c, a complete degradation of pet fibres occurred (mass balance after test: 2 %). for fibre samples from recycled mineral wool (mw), there was only a minimal fluctuation in mass, which eventually dropped only by 5 %. this behaviour was expected due to the differences in material characteristics and primary raw material used for this type of a fibre (particularly its higher melting point). 3.3. differential scanning calorimetry (dsc) from the point of view of determining the effect of high temperature on the heat development during heating, the dsc analysis was carried out on selected types of fibres. this method is based on a constant heating rate of two vessels, one of which is empty, and the second one is carrying the sample. a control unit ensures the constant heating rate of 10 °c/min (within the temperature range of 100–600 °c). the difference in heat fluxes between both vessels is measured. the dsc analysis was carried out on all types of monitored fibres. in terms of applicability in the field of increasing the fire resistance of cementitious composites, the focus was on two polymer fibres – namely pet and pp fibres. the dsc analysis of both fibre types can be seen in figures 4 and 5, where the melting temperatures of both types is clearly stated, which is very important because volume changes of the fibre occur upon its liquefaction, thus preventing explosive spalling of the cementitious composite. the melting temperature (point) tm and the combustion temperature tf for pet and pp fibres were recorded using the dsc analysis. the melting temperature for pp is approximately 74 °c lower than 124 vol. 47/2024 characteristics of fibres based on secondary raw materials figure 5. dsc recording from pp fibres. that of pet; combustion occurred for both types at temperatures above 500 °c. cellulose fibres had a similar development during the dsc, when there was a gradual release of heat. at temperatures around 330–440 °c, a development in heat occurred due to the pyrolysis of cellulose. in mw, there was minimal heat fluctuation; a small amount of heat was released at 400 °c due to the degradation of a resin binder contained on the surface of the fibres. 3.4. determination of the heat of combustion the test was performed pursuant to the en iso 1716 standard. the value determination was performed using a semiautomatic device, ika c 200, under standard laboratory conditions. from the selected fibre samples, the quantity required to determine the 5 combustion heat values of the individual fibre samples was taken in the prescribed manner. the sample taken was pulverized into a fine powder using a friction bowl. to determine the individual combustion heat values, an amount of 0.4–0.8 g was taken from the powder. a precisely weighted amount of the sample was placed into a cup, which was mounted in a holder in a bomb calorimeter. the sample was connected to the combustion circuit of the holder by a cotton filament. the entire sample holder system with the sample was placed into a bomb calorimeter, which was closed and filled with pure oxygen to the inner pressure of 34 bar. the calorimeter was placed in a thermally insulated calorimetric vessel. the vessel was filled with a precisely determined amount of demineralized water. the temperature of the water in the calorimetric vessel was monitored for 3 minutes until reaching a constant value while the water was stirred vigorously. using a combustion circuit, the specimen in the bomb calorimeter was then set on fire and the value of combustion heat was derived from the temperature change in the calorimetric vessel due to the combustion of the specimen. the result is the average of three determinations of the heat of combustion for each sample. results are shown in figure 6 below. figure 6. graphical evaluation of the amount of combustion heat for individual fibres. the recorded data of the combustion heat amount relatively corresponds with the results acquired by the dsc. however, in terms of generated heat, a positive result can be seen since the alternative recycled fibres show significantly lower values than traditionally used fibres. it is a comparison of pp and pet fibres. for they have a similar inner structure, cellulose fibres show almost identical values. 3.5. scanning electron microscopy (sem) three types of recycled fibres (cel ii and pet) were subjected to scanning electron microscopy to approximate the internal structure of the selected fibre types. to take pictures at extreme magnification, the tescan mira 3xmu scanning electron microscope was used. the resulting pictures can be seen in the following figures 7 to 9. in the sem pictures, it can be seen that the cel ii fibres (in the middle) have quite rough surface containing dust impurities and there is also a large variance in the size of individual fibres at the same time. pp and pet fibres show a smooth fibre surface with a full circular cross-section. there is a visible unevenness at the cutting point due to the mechanical separation of fibres to the required length. in both cases, these are 125 m. sedlmajer, j. bubeník, j. zach, v. novák acta polytechnica ctu proceedings figure 7. pp fibre structure at 2 000 magnifications. figure 8. cel ii fibre structure at 50 000 magnification. fibres with a fixed thickness that were produced by the extrusion of microfibre, which is then divided into the required length. pp and pet fibres can be visually compared using pictures since the magnification for both types is 2 000. the pp fibres are significantly thicker compared to the pet and at the same time significantly more damage to the cross-section of the fibre is visible when cutting to the required length. 4. conclusion fibres suitable for production of cementitious composites were chosen from microfibres, which predetermine their suitability in specific applications by their geometry. the emphasis was placed on the selection of fibre from secondary or recycled raw materials since this is the direction that the construction industry will follow in order to maintain sustainability. five types figure 9. pet fibre structure at 2 000 magnification. of fibres were selected to determine key properties for subsequent use as dispersed reinforcement in concrete to increase its resistance to high temperatures. the following sub-conclusions were made: • selected fibres are 3–36 micrometres thick, while the finest fibres are made of mineral wool, pet and cellulose cel ii (8–20 µm). the thickness of pp and cel i is the greatest (20–36 µm). • fibres from recycled paper cel ii are different. it is rather a mixture of cellulose fibres of various thicknesses and lengths with a rougher surface. due to the production technology of the recycled paper, the length variance is large. • in terms of increasing fire resistance, the mw fibres from recycled mineral wool are not suitable since they are resistant to high temperatures, and they will not play their role in the given composite. they can be, however, used as complementary fibres that will improve the cohesion of concrete under thermal stress. • pet fibres show significantly lower development of heat of combustion compared to the pp fibres; cellulose fibres cel i and cel ii have approximately similar values of combustion heat. • the sem images show different fibre sizes and their rough surface in cel ii and smooth surface in pet and pp fibres with circular cross-section. • from the results of thermal analysis point of view, it can be said that cellulose fibres are not suitable for increasing fire resistance because the pyrolysis of fibres occurs at relatively high temperatures when chemically bound water starts being released form the concrete. • pet fibres seem to be an interesting substitution for pp fibres, even though the pet fibres melt at higher temperatures, above 200 °c, (as can be seen from 126 vol. 47/2024 characteristics of fibres based on secondary raw materials the tga and dsc analysis performed) compared to the pp fibres but the heat of combustion for pet fibres is significantly lower than for pp based fibres. from the data recorded for various types of fibres, it can be said that fibres from recycled paper cel ii can be effectively used as an alternative to industrially produced cellulose fibres cel i, however, for concretes under stress of high temperatures, these fibres are not suitable on their own. pet fibres show comparable or even better results in all monitored properties than polypropylene fibres; therefore, there is a possibility of using these fibres as an alternative to pp fibres. first of all, pet fibres are not a primary product, but they are used as a recycled raw material. for an effective use of fibres in terms of improving mechanical and fire-resistant properties of concrete, it would be appropriate to focus on a combination of different fibre types. it can be said that it is possible to find possibilities of use between sources of recycled or secondary raw materials without consuming non-renewable resources. for more detailed comparison of individual fibres, it is necessary to further work with fibres that have been already used in concrete or cement matrix and monitor properties of the entire system of composite material as a whole. acknowledgements the contribution was created within the framework of the project gačr 21-25813s “study of the influence of organic fibres on the properties of cement composites under extreme loading”. references [1] fibres for concrete – part 1: steel fibres – definitions, specifications and conformity. standard, european committee for standardization, 2006. [2] part 2: polymer fibres – definitions, specifications and conformity. standard, european committee for standardization, 2006. [3] council of european union. regulation (eu) no 305/2011 of the european parliament and of the council of 9 march, 2011. [4] b. barr, a. el-baden. shrinkage properties of normal and high strength fibre reinforced concrete. proceedings of the institution of civil engineers – structures and buildings 156(1):15–25, 2003. https://doi.org/10.1680/stbu.2003.156.1.15 [5] k. aghaee, k. h. khayat. effect of shrinkage-mitigating materials on performance of fiber-reinforced concrete – an overview. construction and building materials 305:124586, 2021. https: //doi.org/10.1016/j.conbuildmat.2021.124586 [6] g. ren, b. yao, m. ren, x. gao. utilization of natural sisal fibers to manufacture eco-friendly ultra-high performance concrete with low autogenous shrinkage. journal of cleaner production 332:130105, 2022. https://doi.org/10.1016/j.jclepro.2021.130105 [7] g. m. sadiqul islam, s. d. gupta. evaluating plastic shrinkage and permeability of polypropylene fiber reinforced concrete. international journal of sustainable built environment 5(2):345–354, 2016. https://doi.org/10.1016/j.ijsbe.2016.05.007 [8] n. yermak, p. pliya, a.-l. beaucour, et al. influence of steel and/or polypropylene fibres on the behaviour of concrete at high temperature: spalling, transfer and mechanical properties. construction and building materials 132:240–250, 2017. https: //doi.org/10.1016/j.conbuildmat.2016.11.120 [9] j.-p. won, h.-b. kang, s.-j. lee, et al. thermal characteristics of high-strength polymer–cement composites with lightweight aggregates and polypropylene fiber. construction and building materials 25(10):3810–3819, 2011. https: //doi.org/10.1016/j.conbuildmat.2011.03.023 [10] c. wu, v. c. li. thermal-mechanical behaviors of cfrp-ecc hybrid under elevated temperatures. composites part b: engineering 110:255–266, 2017. https: //doi.org/10.1016/j.compositesb.2016.11.037 [11] t. drzymała, w. jackiewicz-rek, m. tomaszewski, et al. effects of high temperature on the properties of high performance concrete (hpc). procedia engineering 172:256–263, 2017. https://doi.org/10.1016/j.proeng.2017.02.108 [12] c. maluk, l. bisby, g. p. terrasi. effects of polypropylene fibre type and dose on the propensity for heat-induced concrete spalling. engineering structures 141:584–595, 2017. https://doi.org/10.1016/j.engstruct.2017.03.058 [13] j.-c. liu, k. h. tan. fire resistance of strain hardening cementitious composite with hybrid pva and steel fibers. construction and building materials 135:600–611, 2017. https: //doi.org/10.1016/j.conbuildmat.2016.12.204 [14] s. packialakshmi, krishnakumar, s. erskine, b. anuradha. performance evaluation of fire resistant characteristics in polypropylene fiber concrete. materialstoday today 81:1152–1156, 2023. https://doi.org/10.1016/j.matpr.2021.04.445 [15] y. s. h. sanjayan, j. han. effect of fibre type, lenght and numbers of fibres per unit volume on spalling, protection of high strenght concrete. first international workshop on concrete spalling due to fire exposure 2009. [16] a. bentur, s. mindess. fibre reinforced cementitious composites, second edition. modern concrete technology. crc press, london, england, 2nd edn., 2014. [17] h. jamshaid, r. k. mishra, a. raza, et al. natural cellulosic fiber reinforced concrete: influence of fiber type and loading percentage on mechanical and water absorption performance. materials (basel) 15(3):874, 2022. https://doi.org/10.3390/ma15030874 [18] steel fiber reinforced concretes (sfrc): an example of french research. aci materials journal 91(3):273–279, 1994. https://doi.org/10.14359/4333 [19] d. foti. preliminary analysis of concrete reinforced with waste bottles pet fibers. construction and building materials 25(4):1906–1915, 2011. https: //doi.org/10.1016/j.conbuildmat.2010.11.066 127 https://doi.org/10.1680/stbu.2003.156.1.15 https://doi.org/10.1016/j.conbuildmat.2021.124586 https://doi.org/10.1016/j.conbuildmat.2021.124586 https://doi.org/10.1016/j.jclepro.2021.130105 https://doi.org/10.1016/j.ijsbe.2016.05.007 https://doi.org/10.1016/j.conbuildmat.2016.11.120 https://doi.org/10.1016/j.conbuildmat.2016.11.120 https://doi.org/10.1016/j.conbuildmat.2011.03.023 https://doi.org/10.1016/j.conbuildmat.2011.03.023 https://doi.org/10.1016/j.compositesb.2016.11.037 https://doi.org/10.1016/j.compositesb.2016.11.037 https://doi.org/10.1016/j.proeng.2017.02.108 https://doi.org/10.1016/j.engstruct.2017.03.058 https://doi.org/10.1016/j.conbuildmat.2016.12.204 https://doi.org/10.1016/j.conbuildmat.2016.12.204 https://doi.org/10.1016/j.matpr.2021.04.445 https://doi.org/10.3390/ma15030874 https://doi.org/10.14359/4333 https://doi.org/10.1016/j.conbuildmat.2010.11.066 https://doi.org/10.1016/j.conbuildmat.2010.11.066 acta polytechnica ctu proceedings 47:121–127, 2024 1 introduction 2 types of fibres used 2.1 polypropylene fibres – pp 2.2 cellulose fibres 2.3 polyethylene terephthalate fibres – pet 2.4 fibres from recycled mineral wool – mw 3 methodology and measurement results 3.1 determination of properties on an optical microscope 3.2 thermogravimetric analysis (tga) 3.3 differential scanning calorimetry (dsc) 3.4 determination of the heat of combustion 3.5 scanning electron microscopy (sem) 4 conclusion acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2024.47.0108 acta polytechnica ctu proceedings 47:108–112, 2024 © 2024 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague mechanical properties of cement pastes containing plasma-treated silicon-based materials zdeněk prošeka,b,∗, pavel tesárekb, aleš paličkab, oleg babčenkoc a czech technical university in prague, university centre for energy efficient buildings, třinecká 1024, 273 43 buštěhrad, czech republic b czech technical university in prague, faculty of civil engineering, thákurova 7, 166 29 prague 6, czech republic c czech academy of sciences, institute of physics, cukrovarnická 10/112, 162 00 prague 6, czech republic ∗ corresponding author: zdenek.prosek@fsv.cvut.cz abstract. the study delves into the impact of waste silicon-based materials on the mechanical characteristics of the resulting cement pastes. the investigation uses portland cement cem i 42.5r in conjunction with silicon-based waste materials sourced from the packaging industry, specifically from recifa, a.s. variations between the sample groups arise from differences in the concentration of silicon-based materials and the method of surface treatment. the inquiry seeks to assess the effects of two distinct levels of si-based material incorporation, namely 10 wt. % and 20 wt. %, along with plasma treatment involving two different gases: oxygen (o2) and hydrogen (h2). the mechanical attributes under scrutiny encompass compressive and flexural strengths. the evaluation of mechanical properties is carried out on samples aged 28 days, with dimensions measuring 20 × 20 × 100 mm. keywords: cement pastes, plasma treatment, silicon-based waste materials. 1. introduction silicon-based waste materials refer to waste products or by-products that contain a significant amount of silicon (si) or silicon dioxide (sio2) and are generated from various industrial processes. most common types of silicon-based waste materials are silica fume, fly ash, rice husk, glass waste and silicon carbide [1]. silica fume is a by-product of the production of silicon and ferrosilicon alloys in the metallurgical industry [2]. fly ash is a waste product generated from the combustion of coal in power plants [3]. rice husk ash is a waste product from the agricultural industry and contains a high percentage of silica [4]. glass waste materials, such as glass cullet, are rich in silicon dioxide [5]. silicon carbide (sic) waste can be generated from the production or use of silicon carbide abrasives and cutting tools [6]. production of these waste materials depends on several factors, including the type of industry in the area, energy production methods, and the use of raw materials such as silicon and coal. annually, more than 130 million tonnes of si-based waste are produced worldwide. these waste materials are commonly used in cement composites [7]. these materials offer several advantages, including improved durability, reduced environmental impact, and cost effectiveness [8]. silica fume is often used as a supplementary cementitious material in concrete. it improves the mechanical properties and durability of concrete, including increased compressive strength, reduced permeability, and increased resistance to chemical attack [9]. fly ash is used as a supplementary cementitious material in concrete, and its use in concrete can improve workability, reduce hydration heat, and improve long-term strength and durability [3]. rice husk can be used in concrete and other construction materials to improve strength and durability [4]. the use of silicon-based materials in concrete is well established and widely accepted in the construction industry. when used correctly and with proper consideration of these factors, they can improve the performance and durability of concrete and reduce the carbon footprint [2]. alkali-silica reaction (asr) is a significant concern when silicon-based materials are used in concrete. asr is a chemical reaction that occurs between the alkali (sodium and potassium) present in concrete and certain types of reactive silica materials. this reaction can lead to the formation of a gel-like substance, which can expand and crack concrete over time, compromising its structural integrity [5]. plasma surface treatment of silicon-based materials is an innovative approach that has the potential to enhance their compatibility with concrete. plasma treatment can modify the surface properties of these materials to make them more suitable for use in concrete mixes [10]. in the case of potentially reactive silica materials, plasma treatment can help reduce their reactivity and minimize the risk of asr. plasma treatment can also increase adhesion between silicon-based material and cement matrix, ensuring better integration and distribution within concrete [11]. this article deals with plasma treatment of glassbased waste material and its incorporation into cement paste. 108 https://doi.org/10.14311/app.2024.47.0108 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 47/2024 mechanical properties of cement pastes set percentage cem i 42.5r waste glass water ratio type of waste replacement powder w/b materials treatment [wt. %] [g] [g] ref 0 150 0 0.35 without treatment s 10 ref 10 135 15 0.35 without treatment s 10 o 10 135 15 0.35 oxygen plasma s 10 h 10 135 15 0.35 hydrogen plasma s 20 ref 20 120 30 0.35 without treatment s 20 o 20 120 30 0.35 oxygen plasma s 20 h 20 120 30 0.35 hydrogen plasma table 1. composition of tested sets. 0 2 4 6 8 10 12 14 0 10 20 30 40 50 60 70 80 90 100 0.01 0.10 1.00 10.00 100.00 1000.00 v o lu m e [% ] c u m u la ti ve v o lu m e [% ] diameter [μm] glass powder cement figure 1. particle size distribution of tested materials. 2. materials and samples experiments were conducted on cement pastes in which a fraction of the cement was substituted with waste glass powder. the portland cement used was cem i 42.5r radotín, characterized by a notable c3s content of 74.6 %, a lower c3a content of 8.1 %, c2s content of 7.2 %, c4af content of 8.5 %, and a mgo content of 1.6 %. glass powder was created through the recycling of waste generated from glass packaging. as part of the recycling process, the glass shards were crushed and ground. the whole process of recycling was carried out by recifa a.s. the grain size curve of the glass waste powder compared to portland cement is shown in figure 1, with an average particle size (d[4,3]) of 35.9 microns. based on energy-dispersive x-ray spectroscopy (eds) analysis, the glass powder exhibited a substantial composition with 40.0 % oxygen (o), 28.5 % carbon (c), 18.5% silicon (si), and 7.1 % sodium (na). this composition profile suggests a high likelihood of asr. the composition of all the mixtures subjected to testing is detailed in table 1. the reference mixture contained exclusively portland cement. in the remaining mixtures, a portion of the cement was substituted with glass powder at two weight percentages: 10 % and 20 %. the water-to-binder ratio (w/b) was consistently set at 0.35. additionally, the glass powder was subjected to two distinct plasma treatment processes generally used for surface wetting properties modification [12]. the radiofrequency (rf) plasma treatment was carried out at low temperature using a large-area, low-pressure system (ak 400, roth & rau). the treatment lasted for 5 minutes and was performed in either oxygen or hydrogen environments at a pressure of 15 pa, rf power 600 w, substrate stage bias 10–15 v, with temperatures not exceeding 50 °c. this treatment is referred to as “room temperature” treatment and was repeated four times with glass powder mixing for better homogeneity. all tested samples featured nominal dimensions of 20 × 20 × 100 mm and were manufactured using steel triple molds. each mixture resulted in three samples. on the second day after production, the test specimens were demolded and subsequently immersed in a water bath at a controlled temperature of 22 ± 1 °c. after 28 days of curing, the samples were subjected to various tests. 109 z. prošek, p. tesárek, a. palička, o. babčenko acta polytechnica ctu proceedings 1880 1900 1920 1940 1960 1980 2000 2020 2040 2060 2080 2100 ref s_10_ref s_20_ref s_10_o s_20_o s_10_h s_20_h b u lk d en si ty [ kg /m 3 ] sample identification figure 2. bulk density of test materials with standard deviations. 3. experimental methods the mechanical properties of the resulting cementitious compounds were assessed by experimental techniques. the test was carried out on samples aged 28 days. the key mechanical properties investigated included flexural strength and compressive strength. before testing, the samples were subjected to precise measurements and weight determination. these measurements were integral in determining the bulk density of the tested materials, a critical parameter for the comprehensive evaluation of the mechanical characteristics. flexural strength assessment was carried out using a heckert machine (hydraulic press), specifically model fp100. using a three-point test configuration, the methodology combines bending and shear loading. the test was carried out under displacement control at a constant rate of 0.5 mm min−1. the distance between the supports during the three-point bending test was fixed at 90 mm. the formula used for the calculation of flexural strength (ft) is expressed as follows: ft = 3fb,max · ls 2ab2 . (1) compressive strength testing was performed using the heckert hydraulic press, model fp100, with a uniaxial compression test setup. testing was carried out under displacement control at a constant rate of 3 mm min−1. the test was carried out on a suitable section of a fractured beam from a previous threepoint bending test, with effective dimensions of 20 × 20 × 20 mm. compressive strength (fc) of the material, representing the magnitude of stress at the point of failure where the material loses its integrity, was determined based on the maximum force achieved during the test (fc,max) and is calculated as follows: fc = fc,max ab . (2) 4. results and discussion the findings concerning bulk density, as illustrated in figure 2, reveal a discernible trend of diminishing values in relation to the quantity of waste glass powder. this observation aligns with the observations made by other researchers in the field [13–15]. in particular, the reference sample exhibits the highest average bulk density, quantified at 2 062 ± 1.1 kg m−3. furthermore, in all instances of plasma treatment applied to waste glass powder, a marginal increment in bulk density, approximately 3 %, was consistently observed. the outcomes pertaining to flexural strength (figure 3) exhibit a consistent alignment with the bulk density results. an evident reduction in the average bending strength value is observed as the quantity of waste glass powder increases. the reference sample stands out with the highest bending strength, measuring 11.6 ± 0.1 mpa. upon a meticulous comparison of samples featuring surface-treated and untreated waste glass powder, noteworthy distinctions emerge. in most cases, the differences in values are within the range of standard deviation, highlighting the similarity in flexural strength. however, an intriguing exception emerges in the case of 20 wt. % glass powder treated with oxygen plasma, demonstrating a remarkable 13 % increase in flexural strength compared to the reference mixture with 20 wt. % glass powder. the results of compressive strength (figure 4) align with the trends observed in bulk density and flexural strength. as the amount of waste glass powder increases, a corresponding decrease in compressive strength is evident. other authors have also made similar observations in their respective studies [13– 15]. in particular, the reference specimen achieves the highest compressive strength value, measuring 110 vol. 47/2024 mechanical properties of cement pastes 0 2 4 6 8 10 12 14 ref s1_10_ref s1_20_ref s1_10_o s1_20_o s1_10_h s1_20_h fl ex u ra l s tr en gt h [ m p a] sample identification 90figure 3. flexural strength of test materials with standard deviations. 0 10 20 30 40 50 60 70 80 90 ref s1_10_ref s1_20_ref s1_10_o s1_20_o s1_10_h s1_20_h c o m p re ss iv e st re n gt h [ m p a] sample identification figure 4. compressive strength of test materials with standard deviations. 79.9 ± 4.8 mpa. in the context of plasma treatment applied to waste glass powders, no discernible impact on compressive strengths is observed. variations in compressive strengths among mixtures containing the same amount of waste glass powder fall within the range of standard deviation. 5. conclusions this study is focused on investigating the impact of plasma treatment on waste silicon-based materials on the mechanical properties of cement pastes. surface plasma treatment was carried out using two gases, namely hydrogen and oxygen. the cement pastes consist of portland cement cem i 42.5r and glass powder at two different concentrations: 10 wt. % and 20 wt. %. based on the results obtained, the following conclusions can be drawn: • with an increase in the amount of glass powder in the cement composite mixture, there is a corresponding decrease in bulk density, flexural strength, and compressive strength. • both types of plasma treatments applied to waste glass powders had a positive influence on the bulk density of the resulting cement composite, resulting in a 3 % increase. • oxygen plasma treatment exhibited the most substantial positive effect on flexural strength, leading to a 13 % increase in flexural strength for samples containing 20 wt. % of surface-treated glass powder. • both types of plasma treatment did not significantly impact the resulting compressive strength of the cement composite with surface-treated glass powder content. future research endeavors will build upon the findings presented in this article. subsequent investigations will utilize electron microscopy to elucidate 111 z. prošek, p. tesárek, a. palička, o. babčenko acta polytechnica ctu proceedings phase changes within the structure of the cement composite and describe alterations in the cement matrix surrounding both treated and untreated glass powder grains. acknowledgements this research has been supported from the global postdoc fellowship program of the czech technical university in prague and by the czech science foundation project 2305500s effect of surface treatments on the performance of silicon-based secondary materials in cementitious composites. the authors would like to extend their gratitude to recifa a.s., and in particular, to doc. ing. martin lidmila, ph.d., for providing the valuable samples and materials for this study and m. mozerov a j. lorinc for technical assistance in plasma treatment. list of symbols ft the flexural strength [pa] fc the compressive strength [pa] fb,max the maximum force during bending test [n] fc,max the maximum force during compressive test [n] ls distance between the supports during bending test [m] a the width of the specimen [m] b the height of the specimen [m] c3s tricalcium silicate c2s dicalcium silicate c3a tricalcium aluminate references [1] d. tavakoli, m. hashempour, a. heidari, et al. use of waste materials in concrete: a review. pertanika journal of science and technology 26(2):499–522, 2018. [2] r. siddique, n. chahal. use of silicon and ferrosilicon industry by-products (silica fume) in cement paste and mortar. resources, conservation and recycling 55(8):739–744, 2011. https://doi.org/10.1016/j.resconrec.2011.03.004 [3] b. w. langan, k. weng, m. a. ward. effect of silica fume and fly ash on heat of hydration of portland cement. cement and concrete research 32(7):1045–1051, 2002. https://doi.org/10.1016/s0008-8846(02)00742-1 [4] r. madrid, c. nogueira, f. margarido. production and characterisation of amorphous silica from rice husk waste. in wasteeng’2012: proceedings of the 4th international conference on engineering for waste and biomass valorisation, vol. 268, p. 4. 2012. [5] l. m. federico, s. e. chidiac. waste glass as a supplementary cementitious material in concrete – critical review of treatment methods. cement and concrete composites 31(8):606–610, 2009. https: //doi.org/10.1016/j.cemconcomp.2009.02.001 [6] x. li, g. lv, w. ma, et al. review of resource and recycling of silicon powder from diamond-wire sawing silicon waste. journal of hazardous materials 424:127389, 2022. https://doi.org/10.1016/j.jhazmat.2021.127389 [7] f. pahlevani, v. sahajwalla, et al. from waste glass to building materials – an innovative sustainable solution for waste glass. journal of cleaner production 191:192–206, 2018. https://doi.org/10.1016/j.jclepro.2018.04.214 [8] m. amran, s. debbarma, t. ozbakkaloglu. fly ashbased eco-friendly geopolymer concrete: a critical review of the long-term durability properties. construction and building materials 270:121857, 2021. https: //doi.org/10.1016/j.conbuildmat.2020.121857 [9] v. m. malhotra. fly ash, slag, silica fume, and rice husk ash in concrete: a review. concrete international 15(4):23–28, 1993. 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https://doi.org/10.1016/j.jece.2014.03.016 acta polytechnica ctu proceedings 47:108–112, 2024 1 introduction 2 materials and samples 3 experimental methods 4 results and discussion 5 conclusions acknowledgements list of symbols references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2023.40.0048 acta polytechnica ctu proceedings 40:48–53, 2023 © 2023 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague benchmark vercors 2022: blind prediction of time-dependent behavior of concrete containment building with low and high-fidelity models štěpán krátký∗, petr havlásek czech technical university in prague, faculty of civil engineering, department of mechanics, thákurova 7, 166 29 prague 6, czech republic ∗ corresponding author: stepan.kratky@fsv.cvut.cz abstract. the vercors program aims to acquire an extensive experimental dataset collected to provide a solid basis for numerical modeling of concrete containment buildings (ccbs). in order to cover the entire life-span of a real containment, the measurements are done on 3× smaller mock-up which leads to 9-fold acceleration of all processes related to drying. the goal for the participants of the third vercors benchmark was to predict the behaviour of the ccb based on standard laboratory measurements on vercors concrete. the previous paper [1] presented the calibration procedure of material models for moisture transport and time-dependent behavior of concrete and summarized the results obtained with a computationally efficient low-fidelity model (lfm). the present paper compares the responses of the lfm and a high-fidelity model (hfm) with a detailed geometry of the entire containment and presents a comparison with the experimental data collected over the last 8 years on the vercors mockup. keywords: concrete, creep, shrinkage, drying, cyclic temperature, modeling. 1. introduction in order to prolong the lifespan of concrete containment buildings of nuclear reactors by 50 %, detailed knowledge of their time-dependent behaviour is essential. this implies the ability to accurately predict the evolution of various quantities over 60 years. such a task can be accomplished by computational modeling with advanced constitutive models. the main task of the first phase of the vercors 3rd benchmark was to predict the behavior of the concrete containment building (ccb) depicted on figure 1, in particular the evolution of strain, displacement and relative humidity at prescribed locations. for a smoother evaluation of the benchmark and simpler comparison of the results among the participants, the organizers provided a detailed finite element mesh of the ccb. this paper is organized as follows. the main results of the previous paper [1] are summarized first. next, the lowand high-fidelity numerical models (lfm and hfm) are described and their features are presented. finally, the responses of lfm and hfm are compared with the experimental response. 2. previous outcomes 2.1. laboratory experiments the provided dataset constitutes from the results of conventional short-term measurements (mean uniaxial compressive strength fcm = 48.7 mpa, young’s modulus e = 34.3 gpa, uniaxial tensile strength ft = 4.4 mpa) and the following long-term experiments: • basic creep and autogenous shrinkage, • total creep and drying shrinkage at relative humidity of the environment henv = 0.5, • moisture loss, • porosity and aging sorption isotherm. every experiment in this list was conducted under both room (20 °c) and elevated (40 °c) temperature. this data set was used for calibration of constitutive models. 2.2. finite element modeling oofem [3] solver was used to run all numerical simulations. a weakly coupled computational approach was adopted to model the influence of temperature and relative humidity on the structural response. moreover, to further simplify the problem, no crosscoupling between the heat and moisture transport subproblems was assumed in the blind stage of the benchmark. this is partially due to the absence of relevant data. 2.3. calibration of material models the constitutive models for heat and moisture transport (bažant-najjar model [4]) and time-dependent behavior of concrete (modified mps model [5]) were calibrated using axis-symmetric models of laboratory specimens. the experimental data sets were provided by edf (électricité de france) at the beginning of the benchmark. successful calibration of the constitutive models is demonstrated by excellent fits of the data and can be found in the final thesis of the first author [6]. 48 https://doi.org/10.14311/app.2023.40.0048 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 40/2023 benchmark vercors 2022: response to ambient conditions figure 1. section of the ccb vercors mockup [2], dimensions in [mm]. the calibration procedure is briefly outlined in the following sections and the identified values of material parameters for the vercors concrete are listed in tables 1 and 2. 2.3.1. basic creep and autogenous shrinkage under hygrally sealed conditions and at constant room temperature, the mps model reduces to the basic creep compliance function of the b3 [7] model whose parameters q1–q4 were initially estimated from the composition of the concrete mixture. these parameters were slightly adjusted to obtain a more accurate fit of the experimental data. the measured strain in the creep experiment was compensated for the autogenous shrinkage measured on a companion specimen. since the experiment began at the age of 90 days, the recorded (incremental) value of autogenous shrinkage was very small, ≈ 50 × 10−6. parameter value q1 9.0 × 10−6 mpa−1 q2 70.0 × 10−6 mpa−1 q3 25.0 × 10−6 mpa−1 q4 6.0 × 10−6 mpa−1 ksh 1.0 × 10−3 k3 10 kt m 6.5 table 1. identified parameters of the mps material model. parameter value c1 28.2 mm2 · day−1 α0 0.055 hc 0.7 n 10 f 1.08 mm · day−1 table 2. identified parameters of the material model bažant–najjar for moisture diffusion. autogenous shrinkage per se was not considered in the analysis. 2.3.2. drying and drying shrinkage parameters of the bažant-najjar model for moisture diffusion [4] were determined by hand fitting and were calibrated simultaneously with the parameter ksh of the mps model which links the rates of axial drying shrinkage and relative humidity. the response of the models was checked against the measured evolution of drying shrinkage and moisture loss at room temperature. to back-calculate the moisture loss from the relative humidity, the moisture capacity was set to 130 kg/m3 which approximately corresponds to the slope of the measured desorption isotherm. 2.3.3. drying creep parameters for the drying creep, creep at elevated temperature, and transitional thermal creep were determined last. drying creep is controlled chiefly via parameter k3. the experimental data on cylinders allowed to identify parameter kt m which is responsible for the creep rate at elevated temperature. however, under temperature cycles this value would have caused overestimated compliance. unfortunately, the experimental data set did not comprise sufficient information to allow for calibration of the last parameter which damps concrete creep at subsequent thermal cycles, kt c. 2.3.4. heat transfer there were no relevant data for the identification of parameters related to heat conduction; therefore, typical values for concrete were adopted: heat capacity c = 1000 j · kg−1 · k−1, heat conductivity k = 1.7 w·m−1·k−1, and surface factor a = 8 w·m−2·k−1. 49 štěpán krátký, petr havlásek acta polytechnica ctu proceedings figure 2. lfm – structural model. figure 3. lfm – moisture transport model. 2.3.5. damage except for the pressure tests, the stress state in the ccb is chiefly biaxial compression in plane of the cylinder and dome. the out-of-plane stress is far below the strength limit. since the benchmark results do not cover places with significant stress concentrations, the nonlinear behavior does not need to be considered. for this reason, concrete damage (neither compressive nor tensile) was not considered in the first stage of the benchmark. 2.4. low-fidelity model the enormous computational demands of the highfidelity model made it impossible to tune up the boundary conditions and the remaining parameters related to transient thermal creep, which could not be identified from the experimental data. therefore, a computationally efficient numerical model was developed to verify the response to the ambient conditions. the resulting low-fidelity model represents a small segment of the wall in the mid-height of the ccb to eliminate the interaction with more rigid parts of the structure (foundation slab and dome stiffener). the finite element meshes for the individual subproblems – structural analysis, moisture transport, and heat transfer – are shown in figures 2, 3 and 4. the dimensions of the computational models are derived figure 4. lfm – heat transfer model. 0 10 20 30 40 0 400 800 1200 1600 2000 te m p e ra tu re , t [ °c ] time, t [day] int t2 ext t2 int t1 ext t1 figure 5. ambient conditions – temperature prescribed on the inner and outer faces of the concrete wall. from the spacing of the prestressing tendons in the vertical and circumferential directions. 2.5. results a comprehensive description and discussion of the results can be found in the previous paper [1]. the most significant discrepancy in the structural behavior was associated with the response to the (simplified) ambient conditions, in particular temperature (program t2 in figure 5). although a higher creep rate can be anticipated under cyclic temperature, the resulting strains were several times higher than under t1 program with bilinear temperature history. this response was considered as unrealistic. to damp the excessive sensitivity to subsequent temperature cycles, the parameter kt c was introduced and set to kt m/20. 3. high-fidelity model in order to obtain compatible results with the benchmark specifications, a full-lifespan simulation was finally run on the hfm. to detect unintentional mistakes which are likely to be made when modeling large structures with complex boundary conditions, the similarity of the hfm response at the mid-height was subsequently cross-checked with the lfm. 50 vol. 40/2023 benchmark vercors 2022: response to ambient conditions figure 6. hfm – structural model of concrete containment building. figure 7. hfm – structural model of vertical and dome tendons. 3.1. structural model structural hfm (figures 6, 7 and 8) with 635 328 degrees of freedom is composed of the concrete body (quadratic brick elements) and prestressing tendons (linear truss elements); conventional concrete reinforcement is neglected due to its low influence on the global behavior. fully fixed boundary conditions are assigned to the bottom face of the substructure. this simplification was acceptable because of the extreme stiffness of the substructure in comparison with the superstructure. as no specific prestressing schedule was provided, prestressing of all tendons was set to day 278+45 of the simulation. the value of eigenstrain assigned to every tendon is constant over its length and thus does not reflect the direction of prestressing; the prestress losses due to friction are evaluated according to eurocode 2. due to the instantaneous deformation of the concrete structure, even the initial distribution of prestress is not uniform. gradual prestress relaxation is described by the fib model code 2010/eurocode 2 (annex d) approach which uses the concept of equivalent time. the material constants adopted comply with class 2 reinforcement (wire or strands with low relaxation). characteristic strength is defined as 1870 mpa and the young modulus is 190 gpa. the effects of elevated temperature or temfigure 8. hfm – structural model of hoop tendons. 0 20 40 60 80 100 0 400 800 1200 1600 2000 r e la ti v e h u m id it y , r h [ % ] time, t [day] int henv3 ext henv3 int henv2 ext henv2 int henv1 ext henv1 figure 9. ambient conditions – relative humidity prescribed on the inner and outer faces of the concrete wall. perature fluctuations on the relaxation rate are not considered. the periods with the increased value of inner overpressure, which would have led to the development of tensile cracks, are not considered in the analysis as the influence on the long-term behavior is negligible because the cracks completely close once the overpressure vanishes. 3.2. moisture and heat transfer models for moisture and heat transfer with 1 061 450 degrees of freedom represent the concrete body discretized by linear brick elements. the boundary conditions are assigned to the inner and outer face of ccb. similarly to lfm, the real history of ambient conditions provided by edf, which was measured on vercors mock-up, was simplified into several programs with different complexity. unrealistic response of the constitutive model to cyclic temperature was prevented by the parameter kt c which in turn allowed to adopt more complex history of ambient temperature (int/ext t2 in figure 5) and thus to get closer to the actual measurements. ambient relative humidity was defined by henv3 shown in figure 9. 51 štěpán krátký, petr havlásek acta polytechnica ctu proceedings analysis hfm lfm linear linear dof (sm) 635 328 26 166 memory consumption 34 gb cpu i9-11900 rl amd 5 3600 cpu threads used 8 1 time steps 382 382 time consumption (rt) 31.5 hours 0.7 hours table 3. comparison of the characteristics of hfm and lfm. 70 80 90 100 500 1000 1500 2000 2500 3000 r e la ti v e h u m id it y , r h [ % ] time, t [day] ambient hum. ext ambient hum. int measurement hfm lfm figure 10. evolution of relative humidity: lfm, hfm and experiment. 4. results and discussion the original intention of the authors was not only to thoroughly evaluate the behaviour of the ccb but also to use the measurements to improve the behavior of the constitutive model of concrete and possibly also steel relaxation. at the very beginning of the benchmark, the participants were promised that after the initial blind stage of the benchmark, they would be granted full access to the cheops database with detailed information including hundreds of strain, displacement, humidity, and temperature sensors. unfortunately, after months of work of the present authors, the organizers revealed data on a very limited fraction of the structure referred to as pacar area with dimensions 2×2 meters approximately in the middle of the height of the cylinder whose response should be in a good agreement with the lfm model. data of relative humidity were available for a single sensor, which can neither support the credibility of the computational models nor it can provide confidence about the accuracy of the experimental data. comparison of several characteristics of lfm and hfm models is summarized in table 3. all future goals of further studies on nonlinear models, evolution of damage, and the behaviour under increased pressure were thus dropped as there is insufficient experimental evidence. -1200 -1000 -800 -600 -400 -200 0 200 400 600 500 1000 1500 2000 2500 3000 ta n g e n ti a l st ra in , ε t a n [ 1 0 -6 ] time, t [day] measurement hfm-ini. lfm-ini. lfm-t comp. lfm-shr lfm-bc lfm-shr comp. lfm-thermal strain figure 11. evolution of hoop strain close to the inner face of the wall. -800 -700 -600 -500 -400 -300 -200 -100 0 100 200 300 400 500 500 1000 1500 2000 2500 3000 v e rt ic a l st ra in , ε v e r [1 0 -6 ] time, t [day] measurement hfm-ini. lfm-ini. lfm-t comp. lfm-shr lfm-bc lfm-shr comp. lfm-thermal strain figure 12. evolution of vertical strain close to the outer face of the wall. 4.1. relative humidity figure 10 compares the computed and measured evolution of relative humidity in the middle of the concrete wall at the pacar area. at first glance, it is apparent that the computed drying rate by both lfm and hfm is substantially faster than in the experiment. additionally, it is striking that the measured data exhibit significant daily fluctuations which even reach 8 % which is unrealistic given the position of the sensor at the wall middepth. 4.2. evolution of the strain in the pacar area figures 11 and 12 present the evolution of tangential and vertical strain. for clarity, figure 13 shows the first 800 days in more detail. lines “lfm-ini.” and “hfm-ini.” represent the initial and unprocessed fem results. there is a good agreement between hfm and lfm with only subtle differences that might be attributed to the slightly different placement of virtual sensors and/or fem discretization. this concordance allowed to present 52 vol. 40/2023 benchmark vercors 2022: response to ambient conditions -600 -400 -200 0 200 400 300 400 500 600 700 800 ta n g e n ti a l st ra in , ε t a n [ 1 0 -6 ] time, t [day] measurement hfm-ini. lfm-ini. lfm-t comp. lfm-shr lfm-bc lfm-shr comp. lfm-thermal strain figure 13. evolution of hoop strain close to the inner face of the wall detailed view. the results of lfm with several modifications introduced. first, the “lfm-thermal strain” curve was computed, which represents only the strain induced by variations in ambient temperature. subsequently, this strain was subtracted from the original “lfm-ini” curve to produce data series “lfm-t comp” which is in very good agreement with the strains provided in the vercors benchmark. in the vertical direction, the prediction is almost perfect, while in the hoop direction the experimental response is initially slightly underestimated, which can be explained by overestimated prestress losses due to friction. it is of interest that even though the drying rate is overestimated (figure 10), the structural response seems to match. the remaining curves are added to illustrate the influence of the individual factors. line “lfm-shr” represents only the influence of drying shrinkage (without prestressing). basic creep of concrete and steel relaxation only were considered in “lfm-bc”. the significance of drying creep is illustrated in “lfmshr comp” in which both basic and drying creep were considered and the results are compensated for shrinkage (“lfm-shr”). increased pressure during prescribed pressure tests are manifested by the spikes in the hoop strain. in the vertical direction, the spikes have opposite orientation than in the experiment; this is because no equivalent pressure was applied in the vertical direction and the response in figure 12 is merely the consequence of the lateral pressure scaled by the poisson-effect. 5. conclusions the blind prediction submitted by the present authors shows satisfactory agreement with the experimental data even though the simulation completely neglected the construction sequence, the prestressing schedule was unknown, and the calibration of material parameters was based entirely on laboratory experiments. the low-fidelity model which represents a periodic section of the ccb wall can substitute the high-fidelity model which covers the entire ccb if the wall behavior is of interest. the computationally efficient low-fidelity model was essential for the estimation of material parameters which could not be calibrated from the laboratory experiments and subsequently for verifying the correct definition of the high-fidelity model. simulation results for approximately 50 sensors over the structure were submitted to edf for evaluation. unfortunately, the extent of accessible data provided by the benchmark organizers does not allow further research. acknowledgements the authors gratefully acknowledge financial support from the czech science foundation (ga čr), project number 21-03118s, and from the grant agency of the czech technical university in prague, project number sgs22/030/ohk1/1t/11. references [1] š. krátký, p. havlásek. benchmark vercors 2022: mechanical response of the prestressed concrete containment wall to ambient conditions. acta polytechnica ctu proceedings 34:26–31, 2022. https://doi.org/10.14311/app.2022.34.0026 [2] direction production ingeniere, edf. project maquette vercors, plan general, 2012. [3] b. patzák. oofem home page, 2000. [2022-10-13]. http://www.oofem.org [4] z. p. bažant, l. j. najjar. nonlinear water diffusion in nonsaturated concrete. materials and structures 5:3–20, 1972. https://doi.org/10.1007/bf02479073 [5] z. bažant, p. havlásek, m. jirásek. microprestress-solidification theory: modeling of size effect on drying creep. in n. bicanic, h. mang, g. meschke, r. de borst (eds.), computational modelling of concrete structures, pp. 749–758. crc press/balkema, eh leiden, the netherlands, 2014. [6] štěpán krátký. benchmark vercors 2022 – slepá predikce chování železobetonového kontejnmentu [in czech; benchmark vercors 2022 – blind prediction of mechanical response of reinforced concrete containment]. bachelor thesis, czech technical university in prague, 2021. [7] z. p. bažant, s. baweja. creep and shrinkage prediction model for analysis and design of concrete structures: model b3. in adam neville symposium: creep and shrinkage – structural design effects, pp. 85–101. 2000. 53 https://doi.org/10.14311/app.2022.34.0026 http://www.oofem.org https://doi.org/10.1007/bf02479073 acta polytechnica ctu proceedings 40:48–53, 2023 1 introduction 2 previous outcomes 2.1 laboratory experiments 2.2 finite element modeling 2.3 calibration of material models 2.3.1 basic creep and autogenous shrinkage 2.3.2 drying and drying shrinkage 2.3.3 drying creep 2.3.4 heat transfer 2.3.5 damage 2.4 low-fidelity model 2.5 results 3 high-fidelity model 3.1 structural model 3.2 moisture and heat transfer 4 results and discussion 4.1 relative humidity 4.2 evolution of the strain in the pacar area 5 conclusions acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2023.40.0083 acta polytechnica ctu proceedings 40:83–87, 2023 © 2023 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague bar-supported and unsupported dental implants: a qct-fea study in human maxilla luboš řehounek∗, aleš jíra czech technical university in prague, faculty of civil engineering, department of mechanics, thákurova 7, 166 29 prague 6, czech republic ∗ corresponding author: lubos.rehounek@fsv.cvut.cz abstract. two variants of implant-supported overdentures (iods) were investigated by quantitative computed tomography-assisted finite element method (qct-fea) on a patient-specific ctreconstructed model of human maxilla. the analyzed variants are bar-supported (splinted) and unsupported (unsplinted) implant assemblies. the loading was done by a 800 n force slanted by 30° in the buccolingual direction. the results of the analyses show favorable stress distributions for the bar-supported variant. keywords: bar-supported, dental, implant, stress, maxilla. 1. introduction the main purpose of fem (finite element method) is the reduction of a complex problem into a set of algebraic equations to help better understand the problem or predict its behavior. in the biomedical field, use of fem is both particularly useful and hard to do, as the problems are usually very difficult to describe by a simple model and boundary conditions and material properties are often subjects of long studies, complex methods or simply making an educated guess. therefore, use of qct-fea by clinical practitioners can be both beneficial (insight into the patient’s conditions) and virtually impossible (integration into clinical practice) [1]. however, qct-fea is still the only reliable method of estimating the bone’s mechanical properties for patients as no contemporary in vivo method is able to do it. this leaves us with the need to assess its usability in real clinical practice. one of such examples of its use can be the example of bar-supported (or bar-retained) and unsupported dentures. a very popular means of treating edentulism (loss of all teeth) is to prepare the patient’s jaw for placement of 4 load-bearing dental implants that carry an overdenture [2]. these implants can either be unsupported – placed individually with no structural connections, or supported – usually with a soldered metal wire or a solid bar that acts as a stirrup that connects all 4 implants together for better distribution of masticatory forces. the former option is the standard in dental practice as it requires less customization and is less time intensive. however, the bar-supported variant finds its use if the patient’s bone quality is poor, implants are located in the maxilla (upper jaw) or the implants are divergent and additional support is needed to maintain their proper position [3]. moreover, bar-supported (or bar-retained) implantsupported overdentures (iods) have a survival rate of ca. 97 % in a period >7 years post-operation as reported by authors [4–7]. another great benefit of the bar-supported system is anti-rotational stability, as reported by [8–10]. barsupported iods also provide the advantages of both removable and fixed prostheses as they require low maintenance and provide good stability [11]. another benefit of bar-supported iods is the fact that they are not correlated with any significant plaque accumulation, bone loss, peri-implantitis or other adverse conditions [12]. studies also suggest that placing an overdenture in the mandible (lower jaw) provides excellent long-term stability and positive outcomes, but when they are used in the maxilla, outcomes are less favorable [13], hence the need for bar-supported variants. for the mandible, use of two or four splinted implants is a feasible option as bars are not necessary [14]. the qctfea reconstruction is shown on figure 1. 2. materials and methods this work analyzes differences in stress distributions of both variants (unsupported and bar-supported). for that purpose, the qct-fea (quantitative computed tomography-based finite element analysis) method was used. the qct-fea method uses patient-specific ct scans calibrated using a phantom device so that every pixel is assigned a unique value of the hounsfield unit (sometimes referred to as the hounsfield number in ct imaging) for every pixel in every image slice so that the final reconstructed model has specific values for every voxel. the readings correspond to the amount of light reflected back by the material during scanning. the hounsfield unit scale (hu) values are computed according to this equation: hu = 1000 × µtissue − µwater µwater − µair , (1) where the value of water µwater is arbitrarily assigned as 0 hu, µair = −1000 hu and all other values are calculated [15]. 83 https://doi.org/10.14311/app.2023.40.0083 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en luboš řehounek, aleš jíra acta polytechnica ctu proceedings figure 1. render of the patient-specific model that was used for numerical simulations. the maxilla and surrounding bone tissue was extracted from this model, as shown on figure 2. note that ct data often contains a lot of noise or unwanted elements out of the desired threshold range, so preparation of the patientspecific ct data for qct-fea up to this point can take anywhere between minutes and hours based on the size of the specimen. the program used was mechanical finder. it is a qct-fea software that fully utilizes the hounsfield unit conversion into a fully inhomogeneous material model (inhomogeneity on figure 2) for bone based on the readings from the patient-specific ct scan and using a material conversion equation. this equation is provided by keyak [16]: ρ [mgcm−3] = ct value [hu] × a + b, (2) where ρ is material density and ρ ≥ 0. parameters a and b are determined for each voxel by the calibration phantom (a set of rods of known densities). the numerical model consists of the patient-specific 3d bone model of human maxilla and of an imported .stl implant part. the implant assembly was modelled in 2 variants – bar-supported and without bars. the program used to create the models was ansys spaceclaim. position of the implants, their shape and location was modelled according to data provided by an anonymous patient. the analyzed implant variants can be seen on figure 3. the mesh of the model was created with respect to the details of the implants. also, shell elements were created on the surface of the bone material as cortical bone is usually too thin to be represented with a single element and there is a concern for accuracy [17]. figure 2. map of young’s modulus e [gpa] that on the qct-fea model of a human maxilla. image shows an arbitrary transversal section of the maxilla that was obtained by a phantom-calibrated patientspecific ct scan. figure 3. two analyzed variants of the implant assembly.top – the bar-supported variant, bottom – only intraosseous implants. the two variants were analyzed under the same exact conditions in regard to placement of the intraosseous implants, their material properties (ti-6al4v alloy with von-mises yield criterion) and bone properties and geometry. the models with boundary conditions can be seen on figure 4. the analysis was performed as fully non-linear in terms of material and geometry. the analysis was divided into 10 substeps so that every increment adds 80 kn of load. the deformations of the models can be seen in detail in figure 5. both simulations were performed using a personal computer with 64 gb of ram and a 16-thread 4.7 ghz processor. both computations were completed under 1 minute – an important factor for real time use by physicians, as will be discussed. 3. results the results shown on figure 5 show the gradual deformation of both variants in various times. as we 84 vol. 40/2023 bar-supported and unsupported dental implants figure 4. both configurations of the analysis with boundary conditions. geometry of bone material and the position of the implant assembly is identical. the only difference are the supporting bars on the first variant. the model is an extracted part of an anonymized ct scan of a human skull presented on figure 1. the part was extracted and fixed on the upper surface. a 30° slanted 800 n force was applied on the surface of the third implant from the left. figure 5. changes in geometry of the two modelled variants during the simulation. top row – bar-supported variant, bottom row – unsupported variant. the loads at which the deformations were captured are, beginning from left: f = 80 n, f = 400 n, f = 800 n (corresponding steps 1, 5 and 10, respectively). gray color represents the original, undeformed geometry and blue color represents the deformed state. deformations are magnified 50×. can see, the different deformation of the implant is accentuated in the final substep of the analysis. the bar-supported variant shows smaller displacements of the implant that is loaded thanks to the additional stiffness provided by the connections to other implants. the main analyzed quantity was stress risk factor in bone. as we can see from overviews on figure 6, the inclusion of the bar changes the distribution of stress (but mainly its values). specifically, the risk factor is calculated simply as: rf = max. principal stress [mpa] critical stress [mpa] [%], (3) where critical stress is assigned by conversions provided by [16]. 4. conclusions the qct-fea analyses of the interaction of bone and two variants of the implant assembly (bar-supported and unsupported, figure 4) showed a predictable outcome favoring the bar-supported variant. this is illustrated on figure 6 and also on figure 7 for the implant assemblies. however, this does not mean that bar-supported variants are always better in clinical practice as the decision-making process involves many other factors (patient specific conditions like bone quality, necessity of installation of bars, time demands or financial situation of the patient). the results are in agreement with another study on similar topics [18]. another factor worth discussing is the usability of qct-fea simulations for real-time decisions of clinical practitioners on-site. contemporary level of 85 luboš řehounek, aleš jíra acta polytechnica ctu proceedings figure 6. risk factor (rf, equation (3)) of both variants of the assembly at the final computation step (f = 800 n). the first image shows the bar-supported variant, where stresses are better distributed and there are no cracked, crushed or plastic solid elements. the unsupported variants shows much larger pools of concentrations of stress and also some plastic elements (yellow boxes) and cracked elements (white lines). the scale for rf was chosen in the range of 0–30 % to better illustrate the stress distribution in the bone as the scale of 0–100 % would leave the reader oblivious to the small areas. deformations magnified 50×. figure 7. von-mises stress [mpa] displayed on a sagittal section of the implant assemblies. the barsupported variant shows better stress distribution with the help of the bars. deformations magnified 50×. complexity, price availability and the time-consuming process of analysis of most qct-fea software does not allow for on-site patient-specific simulations directly by clinical practitioners. however, it similar tools are refined and practitioners are led to understand inputs and outputs of fea, it might be possible to have some level of involvement in the future. this involvement would mainly include deciding between multiple implant types the practitioner has at their disposal, evaluating the quality of bone via ct scan analyses and deciding on the needs and/or risks of operation. the simulations of presented study took less than 1 minute to complete each – a time short enough to perform on-site. however, the expertise needed to operate qct-fea software, its hard introduction en masse among practitioners and the need to perform phantom-calibrated ct scans beforehand limit its use as a practical everyday tool today. this hinderance begs the question whether one might be better off without the need for a phantom device. there are also other algorithms for prediction of mechanical properties of bone without the need for a phantom device as suggested by prado [19]. their ease of operability as compared to having a phantom device, which might be more difficult to get, but much more easy to work with and more precise once available, seem to be the step in the right direction. their evaluation, however, remains a topic for further research. acknowledgements the financial support provided by the by the faculty of civil engineering, ctu, prague, project n. sgs20/155/ohk1/3t/11 is gratefully acknowledged. 86 vol. 40/2023 bar-supported and unsupported dental implants references [1] k. engelke, b. van rietbergen, p. zysset. fea to measure bone strength: a review. clinical reviews in bone and mineral metabolism 14(1):26–37, 2016. 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[18] ö. kümbüloğlu, b. koyuncu, g. yerlioğlu, et al. stress distribution on various implant-retained bar overdentures. materials 15(9):3248, 2022. https://doi.org/10.3390/ma15093248 [19] m. prado, s. khosla, c. chaput, h. giambini. opportunistic application of phantom-less calibration methods for fracture risk prediction using qct/fea. european radiology 31(12):9428–9435, 2021. https://doi.org/10.1007/s00330-021-08071-w 87 https://doi.org/10.1007/s12018-015-9201-1 https://doi.org/10.7860/jcdr/2014/9648.5020 https://doi.org/10.4047/jap.2015.7.4.338 https://doi.org/10.1111/j.1600-0501.2011.02169.x https://doi.org/10.1111/j.1600-0501.2011.02169.x https://doi.org/10.1177/002203459707601009 https://doi.org/10.1067/mpr.2001.119921 https://doi.org/10.11607/jomi.2161 https://doi.org/10.1111/j.1600-0501.2010.02079.x https://doi.org/10.1111/j.1600-0501.2010.02079.x https://doi.org/10.1016/s0021-9290(97)00123-1 https://doi.org/10.3390/ma15093248 https://doi.org/10.1007/s00330-021-08071-w acta polytechnica ctu proceedings 40:83–87, 2023 1 introduction 2 materials and methods 3 results 4 conclusions acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2023.41.0071 acta polytechnica ctu proceedings 41:71–78, 2023 © 2023 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague research and implementation of motorcycle simulator controls josef svoboda∗, přemysl toman, adam orlický czech technical university in prague, faculty of transportation sciences, horská 3, 128 03 prague 2, czech republic ∗ corresponding author: svoboj88@fd.cvut.cz abstract. the paper is focused on research in the field of motorcycle simulator controls from the rider’s point of view, with a focus on the longitudinal and lateral dynamics of vehicle. the subject of the research is the design and testing of a suitable composition of input signals controlled by the rider and their integration into the virtual scene. the design and testing of the communication interface between the hw simulator and the physical model of the motorcycle is mentioned in the paper as well. furthermore, the work describes the suitable hmi elements of the motorcycle and their integration options into the simulator system. in conclusion of the article, various variants, and approaches for controlling the developed motorcycle simulator are discussed, including the possibilities of validating the achieved results. keywords: motorcycle simulator, motorcycle roll, rider performance, simulator control approaches. 1. introduction powered two wheelers (ptw) riders are an inherent part of transportation system. ptw users are also vulnerable road users. specific group of ptw riders are motorcycle riders. focusing on usage of motorcycle we largely observe transport and recreation. motorcycle is often purchased based on excitement and enjoyment [1]. these factors affect challenges associated with ptw safety. it means other road users, road environment, vehicles and ptw users itself [2]. motorcycle riding popularity grows and together with this trend grows the importance of motorcycle riders’ behaviour research because the human factor plays big role in motorcycle crashes [3]. in the czech republic environment, the yearbook of road accidents in the czech republic 2020 published by police of the czech republic presents that despite the total representation of accidents caused by motorcycles of 2,1% accidents result in 10,2% of total fatalities. data from [4] shows human factor errors in case of accidents containing motorcycle rider. from the point of view of rider, we observe the highest number of identification errors (e.g., incorrect evaluation of the route and thus not adjusting the driving speed). the most endangered group of riders according to absolute number of fatalities are riders 25–44 years, but relatively the group of riders 15–17 years. motorcycle riders are trained in the similar way as personal car drivers. it means that riders start the training in the driving school. specific feature of the rider trainings is differentiation of more than one group of riding licences than e.g., one group up to 3,5 tons in case of driving licence. this fact causes that riders start the training more than once during lifetime – to achieve higher group of riding licence and thus to be eligible to ride more powerful motorcycles. it brings an opportunity to extend and deepen the training. focusing on safety manoeuvres, that trained rider should know and handle, the rider must practise in the way to translate them into longterm as well as procedural memory [5]. in hazard situations we observe that advanced trained riders react faster in hazard situations than only experienced riders [6]. the differences are also seen in motorcycle simulator study. riders performed left and right-hand bends and the novice riders tend to take bad position in lane more than inappropriate speed [7]. the study also shows that riding skills are also significantly documented in laboratory conditions. driving simulators are an inherent part of automotive research [8] as well as part of the training in driving schools [9, 10], moreover according to the driving schools’ teachers see driving simulator as part of intense long-term training for young riders to reduce their hazard behaviour [11]. motorcycle simulators in terms of riders training have also the same potential as car driving simulator. it means training of preventive safety of riders [12]. designing a motorcycle simulator, it is important to define target levels of physical (hardware and physical feedback for rider etc.) and functional (engine, virtual reality, visualisation etc.) fidelity that appear that have major impact on simulator validity. in this way investments into the simulator are better spent on simulator engine and visual databases than into the hardware in terms of physical fidelity [13]. physical fidelity of course could increase riders’ sense of presence and thus increase simulator validity [14] but physical fidelity improvements are accompanied with large cost investments and only small quality of data increases. sufficient functional fidelity leads to good quality of obtained data despite lower levels of physical fidelity [13]. the riding of a ptw vehicle is much different from driving a car. the main dif71 https://doi.org/10.14311/app.2023.41.0071 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en j. svoboda, p. toman, a. orlický acta polytechnica ctu proceedings ference comes from the principle of ptw, which are spontaneously unstable. this property significantly affects longitudinal and lateral dynamics behaviour as well as command and control of the vehicle [15]. the manoeuvrability of the real motorcycle is realized especially by the movements of the rider [16]. these facts are used in all the concepts and must be taken into account when designing the motorcycle simulator. the main difference between control approaches is how inputs from the rider are detected. the most advanced ptw simulators have a system that allows control by human-body feedback. this concept requires a cueing platform with multiple dofs and a complex control system that can provides a realistic response. more common motorcycle simulators have sensors placed on mock-up in such a way that they can detect the movement of the rider indirectly; examples are sensors implemented in footrests, steering sensors, and sensors that allow roll detection. in addition to movement sensors, it is necessary to track, record and evaluate command inputs from the rider side (throttle and brake lever position, clutch, shifted gear, etc.) [17]. based on data acquisition and evaluation, movement of these simulator concepts are controlled by algorithms and filters which aim to recreate riding accelerations and angular velocities by using actuators (servomotors, etc.) [18]. simulators are proved to be the experimental tool for testing of hmi (human machine interface) features. hmi elements could be easily designed and implemented into the virtual scene using vr (virtual reality) glasses [19]. according to the hardware setup we recognize reduced motion, parallel platform, and serial platform motorcycle simulators. even the reduced motion simulator is proved tool for assessing riders training, hazard perception and mental workload. on the other movable platform simulators bring benefits when assessing scenarios including motorcycle counter-steering and transition phase between slow and high speed [17]. in case of motorcycle simulator without motion platforms the challenge is the appropriate setup of handling inputs, visualisation, and engine behaviour to reproduce the optimal riding experience. this is the challenge that the rest of the paper focus on. 2. method in the experimental phase, we focus on research of an appropriate approach for handling inputs of motionreduced simulator. the goal is to find a method to mediate an experience like the rolling performance of the motorcycle, even in the case of a static simulator platform. in this way, we propose specific inputs in several combinations together with hardware adjustment in from rollable handlebars and vr glasses visualisation. the proposed variants are to be tested in a gymkhana scenario and compared with the results from the real test track. figure 1. virtual scene. 2.1. simulator sw and hw our motorcycle simulator is being developed in unity software (see figure 1). this article follows on the research made in [20], where the sw simulator approach and architecture are described, as well as the simulated motorcycle parameters and category. in this paper, we focus on implementation of the lateral part of the physical model. in this way, we implemented three variants of steering possibilities. the simulator sw calculates the final angle of steering based on handlebar rotation, roll, and forces applied on footrests. the input signals enter the lateral part of the physical model, where they are combined and mutually weighted according to the weight curves. the whole virtual reality scene is visualised to the subject rider in the vr (virtual reality) glasses. the simulator hardware consists of the motorcycle and handlebar body that are controlled by the rider wearing vr glasses (see figures 2 and 3). the handlebars are attached to the motorcycle body by a ball joint that allows rolling around the motorcycle longitudinal axis and steering rotation. the rider can steer the motorcycle in virtual environment by the actual roll of the handlebar body (due to inclinometer sensor), handlebar rotation (due to rotation sensor) and by weight transfer that is read on footrests (due to weight sensors). the rotation sensor is integrated into turn servo and thus provides spring and damping resistance of the steering. 2.2. simulator experiment the aim of the article is to examine different control approaches for the developed ptw simulator, while is focused mainly on rider inputs which affect lateral dynamics. longitudinal dynamics in the context of a ptw simulator was investigated within the paper [20]. to objectively assess the approaches, an experiment was carried out with a total of three simulation cornering controls. during each session, the following data output from the virtual scene was collected: time, position (x,y,z), speed, virtual motorcycle roll, virtual steering angle. the data channels regarding rider inputs that were measured and recorded during the experiment presents table 1. measured channels varies for each scenario. 72 vol. 41/2023 research and implementation of motorcycle simulator controls figure 2. simulator hardware block schema. rider inputs sc. 1 sc. 2 sc. 3 throttle position 3 3 3 brake level position 3 3 3 hmd roll 3 3 3 roll of the handlebars 3 3 3 footrests weight sensors 7 3 3 steering angle 7 7 3 table 1. rider inputs scenarios. the experiment took the form of a virtual track test in which the slalom (see figure 4) was used as a representative of the typical manoeuvre to prove and test the handling characteristic of the motorcycle. before the start of each scenario season, the probands were instructed about the control behaviour, and they have enough time for free practice. then during the experimental phase, participants have three attempts to pass the slalom as clean as possible. after the tests performed, we ask riders for their subjective impression of each variant of the control behaviour. in the first scenario, cornering was realized only based on roll of the handlebars around the longitudinal axis. for the second scenario, footrests weight sensors were added. in this scenario the simulation sw used two cornering inputs. in the third scenario we moreover added input from the turn servo with rotation sensor. the third scenario was carried out with fully equipped simulator so there were three rider’s inputs which affect the roll and steering of the virtual model. figure 3. simulator hardware – testing subject while riding. 73 j. svoboda, p. toman, a. orlický acta polytechnica ctu proceedings figure 4. setup of the track test – position of the cones. figure 5. track test experiment layout. 2.3. real test track in order to compare and validate data measured on the simulator sessions, the real track test was performed in the same set-up (see figure 5). the used motorcycle has the same performance and weight characteristics as we implemented in the virtual physical model of the simulator. data were obtained using the developed data acquisition system [21] supported by the commercial measurement unit x-sense mti-g710-2a8g4. 2.4. testing hypotheses to evaluate designed motorcycle control concepts, the following hypotheses was established: • h1: control concepts (scenarios 1–3) do not affect the riding style while riding: . h1a: rider’s movements in terms of handlebar and head roll activity. . h1b: rider’s steering performance. . h1c: rider’s weight transfer. • h2: lateral dynamics data acquired while simulator experiment will not correspond with racetrack rides. 3. results 3.1. simulator scenarios the following subsection focuses on the results across scenarios 1–3. it mainly aims to improve the riding style of the rider during the simulator session. figure 6 presents the average phase shift between the hmd (head mounted display) roll, the footrest weight ratio, and the motorcycle roll. the hmd roll represents the actual roll of the head of the rider. footrests weight ratio acquire values from 0 to 1, when 0.5 means balanced situation – the same force applied both on left and right footrest. phase shifts are compared according to the motorcycle roll that represents the x-axis. the hmd roll phase shift is calculated as the hmd roll peak position minus the motorcycle roll peak position. the footrest weight ratio shift is calculated in the same way. the shift was examined while cornering around cones. in this way, we examined 8 turns. in general, we see a trend that maximum weight applied on the footrest while cornering precedes motorcycle roll peak. the hmd roll, on the other hand, slightly follows the motorcycle roll peak. concerning average values across scenarios 1 to 3 the weight ratio shift equals -2.7m, -3.5m, -2.3m and the hmd roll shift equals 0.8m, 2.0m, -0.1m. we observe differences between the scenarios. in the second the shifts reach the highest values across examined variants. comparing scenarios 1 and 3, we observe similar results. in the case of scenario 3 we see the smallest shifts. the further differences of the first and third scenario examine figure 7. the graphs presented in figure 7 provide a detailed comparison of the motorcycle roll and the steering angle within scenario 1 and 3. all data were plotted as a function of distance from the start gate to the last cones that form the final gate. the distance shift could be observed (the most for participant 2), which could cause the choosing a different trajectory within rides. for participant 1 we observe significantly lower absolute values of the motorcycle roll and the steering angle. the total driving distance suggests about the riding style of this person which was diametrically different from the other probands. 3.2. simulator versus racetrack the results of the roll characteristics during the slalom performed in the simulator and during the track test are provided in table 2. all values were calculated as a mean of the three rides per participant and then the 74 vol. 41/2023 research and implementation of motorcycle simulator controls figure 6. hmd roll and footrests weight ratio shifts from motorcycle roll. figure 7. detailed comparison of the motorcycle roll and the steering angle within scenario 1 and 3. variables racetrack ride subjects: slalom t roll rate peak roll right roll left roll rate time t [s] [° s−1] [°] [°] [° s−1] [s] p01 2.26 68.20 11.10 13.33 28.88 13.15 p02 1.72 96.28 11.90 14.80 34.79 11.43 p03 1.91 69.21 8.41 11.13 25.21 12.17 average 1.96 77.90 10.47 13.09 29.63 12.25 percentage 100 100 100 100 100 100 variables simulator ride subjects: slalom t roll rate peak roll right roll left roll rate time t [s] [° s−1] [°] [°] [° s−1] [s] p01 2.26 58.69 10.39 13.83 30.74 14.79 p02 2.08 101.20 10.49 15.24 37.25 13.11 p03 2.68 51.65 6.17 8.00 15.90 13.46 average 2.34 70.51 9.02 12.36 27.96 13.79 percentage 119 91 86 94 94 113 table 2. simulator versus racetrack comparison. 75 j. svoboda, p. toman, a. orlický acta polytechnica ctu proceedings figure 8. racetrack versus simulator rides average values comparison. average value was determined. the track test average values were then used as a basis for the comparison with simulator data. to be able to monitor the change rate of the motorcycle roll, the roll rate value [° s−1] was used. the values in column slalom period defines the mean of the time intervals between the repetition of the slalom wave. figure 8 presents a trade-off of simulator and racetrack rides. it means a comparison of the average values as stated in table 2. in the figure, the racetrack results represent 100%. when comparing racetrack and simulator rides, we observe similar results of roll and roll rate values. regarding time, the simulator rides were generally slower, and the slalom period was longer. 4. discussion in this chapter, we describe the findings based on performed experiment and presented data. the resulting discussion is divided into two sections in compliance with stated hypotheses. 4.1. h1: the control concepts (scenarios 1–3) do not affect the riding style while riding in the first part of results in section 3.1 we focus on rider style from the point of view of head and body movements. in this way figure 6 shows that across tested concepts we observe their influence on synchronization of head and body movements. in the second scenario when the weight sensors as input controller were combined only with actual roll of handlebars, it caused non-negligible shift of forces applied on footrests peak shifts. moreover, it affected the roll of the riders’ head. from this point of view, as well as subject comments, the second scenario handling concept entailed a negative effect on natural riding style. in case of scenarios 1 and 3 we observe similar resulting riding styles. based on the riders’ experience of the testing riders, we evaluated the third scenario as more appropriate. the first scenario was subjectively rated more sensitive. this assumption is assessed in detail in figure 7. analysing motorcycle roll within first scenario ride we observe bumpy curves in areas of cornering. this negative influence on riding style could be caused by an inappropriate perception of the actual roll angle and roll rate. this is also consistent with riders’ subjective assessment. in the third scenario, we see smooth roll curves while cornering across tested subjects – even independently on absolute values belonging personal riding styles of testing riders. in the case of steering, we observe a similar effect on the steering angle as on the rolling angle. the interesting finding is that the riders performed a similar steering angle in scenario 1 knowing that the steering input is not active. riders naturally steer the handlebars, even while focusing only on rolling the handlebar. the shifts between scenarios one and three in figure 7 are caused by minor differences in the final trajectories. in the third scenario, the riders synchronized their movements the most among the concepts tested. 4.2. h2: lateral dynamics data acquired while simulator experiment will not correspond with racetrack rides based on the results of the simulator and racetrack comparison, we observe differences in the duration of the slalom ride and the associated slalom period, where the total time spent is higher in simulator rides. this fact could be related to the different perception of velocity during simulator rides, which was also confirmed by the subjective impressions of participants 1 and 2. although precautions were taken to 76 vol. 41/2023 research and implementation of motorcycle simulator controls ensure better perception of virtual vehicle speed, it will be necessary to improve some parts of the virtual physical model in the future, especially the sound module. furthermore, the results focus mainly on the performance of the rider when cornering. here, it is possible to observe slight nuances between the roll angle on the right and left during both the simulator and the track test rides. this phenomenon is common when riding a motorcycle, and most riders have lower values of roll angle on the right turns. however, the analysis performed showed that this effect is greater while riding a simulator. another finding is that average simulator ride roll rate values reach up 94% of racetrack rider. similar trend is observed from the point of view of roll rate peak values, where simulator ride reaches 91% compared to racetrack ride even though the only movable part of simulator were the movable handlebars allowing rolling and steering. 5. conclusion motorcycle riders are common and moreover vulnerable road users. an important factor in motorcycle road accidents is human error, which strongly affects novice riders. motorcycle training is often multistage, and thus riders applying for higher level of riding license visit training courses multiple times per lifetime. long-term memory and repetition of traffic situations, as well as drilling of rider behaviour, could have a positive effect on road safety in general. in this way motorcycle simulators are one of the tools to achieve this goal. in addition, riding simulators are tools for hmi motorcycle research. in terms of simulator design, the good quality of the data collected is influenced more by functional fidelity than physical fidelity. the reproduction of credible riding experience is very important in the case of the vehicle simulator. in the paper, we analyse the possibilities of reduced motion simulator emendations. in the experiment preparation phased the simulator was equipped with movable handlebar body and footrests weight sensors that are typical rather among movable simulators. the experiment conducted contains an evaluation of three input control scenarios. the results of the simulator study were subsequently compared with the racetrack data. the limitation of the study is the lower number of subjects and the layout of the tracks. data show that combination of handlebar rolls and steering sensor in combination with weight sensors could reproduce similar behaviour in terms of lateral dynamics parameters. moreover, results show more smooth performed riding styles performed within tested scenario 3. this approach allows riders to move and perform weight transferring as they are used to while riding a motorcycle on the road. the proposed controls 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on the race track. 2020 smart city symposium prague (scsp) pp. 1–6, 2020. https://doi.org/10.1109/scsp49987.2020.9133760 78 https://doi.org/10.1093/comjnl/bxp071 https://doi.org/10.1207/s15327590ijhc1002_3 https://doi.org/10.1109/tmech.2009.2035499 https://doi.org/10.1016/s0389-4304(01)00147-3 https://doi.org/10.1177/0954407010396006 https://doi.org/10.1007/s12544-010-0041-2 https://doi.org/10.1109/tvt.2019.2933601 https://doi.org/10.14311/app.2022.39.0055 https://doi.org/10.1109/scsp49987.2020.9133760 acta polytechnica ctu proceedings 41:71–78, 2023 1 introduction 2 method 2.1 simulator sw and hw 2.2 simulator experiment 2.3 real test track 2.4 testing hypotheses 3 results 3.1 simulator scenarios 3.2 simulator versus racetrack 4 discussion 4.1 h1: the control concepts (scenarios 1–3) do not affect the riding style while riding 4.2 h2: lateral dynamics data acquired while simulator experiment will not correspond with racetrack rides 5 conclusion references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2023.43.0045 acta polytechnica ctu proceedings 43:45–50, 2023 © 2023 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague ecological solutions for freight trains martin chýle∗, michal drábek czech technical university in prague, faculty of transportation sciences, deparment of logistics and management of transport, horská 2040/3, 128 00 prague, czech republic ∗ corresponding author: chylemar@cvut.cz abstract. the article presents the potential of freight railway transport in terms of saving of emissions and maximizing the efficiency of operation. attention is paid to the transport of freight trains on electrified lines with higher gradients (up to 25 ‰), alternatively with the possibility of a trip to non-electrified lines and sidings, which generate significant extra costs in the operation of trains and are often a reason for the carrier to deploy a locomotive of independent traction (diesel-electric) even in electrified line sections, even at the cost of increased operating costs and increased emissions. there are many such lines and possibilities of use within the central european region and the czech republic. in the first part of the article, the basic categorisation of dual-mode freight locomotives and their suitability for individual operating programmes is presented. this part also describes current situation on the rolling stock market in general and directions of development taken by various locomotive manufacturers. case studies of deployment of real freight locomotives on chosen czech railway lines are used in the next part to present the possibilities for energy and cost savings when replacing a vehicle with independent traction by an electric or dual-mode locomotive, at least in sections equipped with overhead catenary. the pros and cons of using several possible solutions are described in detail. further considerable savings can be achieved by, among other things, using regenerative braking and returning the generated electricity back to the catenary system when driving downhill. in this respect, however, it is also necessary to cooperate with the infrastructure manager, who must also adapt the railway lines to the operation of these vehicles. finally, general rules for operational deployments on which the replacement of a conventional vehicle by a dual or pure electric vehicle can be considered are formulated. keywords: energetic mix, freight transport, hybrid locomotive, dual locomotive, last mile. 1. introduction since the beginnings of the railway transport, in was common that all railway traction vehicles had only one mode of propulsion, which was available at the time. first of them was steam engine, followed by electric and diesel engine. their development allowed to start the thinking about their possible combination with aim to reduce emissions and energy consumption. current situation in energetic industry and rising energy costs led everyone in the transport sector to the idea of saving energy and changing the energy mix used to power trains. these topics are going to be more and more serious in the future, and every new solution could be beneficial in the effort to make transport more efficient, environmentally friendly and effective. 2. energetic mix and power sources first, it is useful to look at the current situation in european energy industry and present power sources, which are used to power the trains. the energy sources vary widely across european countries. according to the current data from eurostat, the energy mix for the whole european union is very diverse. mountainous countries can benefit from hydroelectric power plants, as well as other renewable energy source; countries with flat landscape use more nuclear energy, or petroleum and fossil fuels. in european union in general, 34.5 % of energy comes from total petroleum products, 23.7 % from natural gas, 17.4 % from renewable energy sources, 12.7 % from nuclear energy, and 11.5 % is still produced from solid fossil fuels. it’s clear that these numbers have a huge variability, depending on local country conditions and political forces [1]. as can be seen from table 1, in czechia is the share of solid fossil fuels much higher than european average – 31.5 %. this is the third worst number in the eu (worse are only poland and estonia) and serious topic to address; according to czech strategic energy plan, this percentage is planned to rapidly decrease in the next few years. on the other hand, the share of nuclear energy is higher than in many european countries (18.6 %), mainly thanks to two big nuclear power plants in dukovany and temelín. by contrast, in germany, for example, the share of nuclear power is just under 6 percent, and petroleum products account for over 35 percent of energy sources. 45 https://doi.org/10.14311/app.2023.43.0045 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en martin chýle, michal drábek acta polytechnica ctu proceedings fossil fuels nuclear energy renewable petroleum country [%] [%] sources [%] products [%] czechia 31.2 18.6 12.7 21.4 germany 17.1 5.8 16.4 35.2 austria 9.7 0.0 32.6 34.5 slovakia 15.4 24.6 13.1 21.9 poland 40.7 0.0 12.5 28.8 table 1. countries and shares of their energy sources. this means that the electricity, used for power supply of rail transport in this country, has higher carbon footprint than in other countries. however, northern parts of germany benefit from renewable sources like solar and wind power plants, which are located on the sea surface. austria has no nuclear power plants; this makes this source here irrelevant. in contrast it has significantly higher share of renewable power sources, thanks to mountainous landscape with wild rivers, where several hydropower plants have been built. the share of petroleum products in austria is also higher than european average, like in germany. one of europe’s pioneers in the field of nuclear energy is slovakia. this relatively small country has two big nuclear power plants (jaslovské bohunice and mochovce), where ¼ of whole energy amount is produced. this number is the third biggest in the eu; better are only france and sweden. however, the share of fossil fuels is still also high here (15.4 %). nevertheless, slovakian officials have repeatedly declared the closure of coal-fired power plants by 2030. poland has long been known for its operation of many coal-fired power plants. they produce more than 40 % of energy, which generates the second worst situation in europe (after estonia). the most important reason for this situation is the abundance of high-quality coal in the country, which can be excavated relatively cheaply and used for combustion in power plants. connected with a lack of nuclear power plants and flat landscape, preventing the expansion of hydroelectric and wind power plants, energetic situation in poland can be considered as the worst from all countries neighbouring with czechia. conditions and status presented in last paragraphs show that the energy mix used to produce electricity can vary considerably across countries. it certainly cannot be said that the energy used for rail transport is equally clean in all countries. especially countries with flatter landscape and low share of nuclear and renewable power sources have yet to take a major step towards reducing its carbon footprint [1]. 3. comparison of hybrid, electric and diesel locomotive to demonstrate the possibility of energy savings, in this chapter the comparison of various locomotive figure 1. siemens vectron ac locomotive, which is equipped with last-mile module (small diesel engine), pulling a freight train outside of the electrified track. types will be done. a dual-source vehicle can generally be considered as a traction vehicle having more than one power sources. usually there is one primary and one secondary power source, which is used only in specific operating situations and under specific conditions. the reasons for using a secondary energy source are various, i.e., operational, technical (absence of overhead contact line, third rail, etc.) or economic (lower power and/or lower unit cost of energy, and thus lower operating costs). these vehicles be designed as power-symmetrical (this means they have the same traction characteristics on electrified and non-electrified lines, but on electrified lines they take advantage of the linear electric power supply), or as power-asymmetrical (on nonelectrified lines the power is lower than on electrified lines, the diesel engine or battery is a supplementary power source). a typical example is siemens vectron locomotive with last-mile diesel module, as shown in figure 1. such a traction vehicle cannot be operated on both energy sources at the same time – it must be time-framed when the vehicle is powered by primary or the secondary energy source. the aim is to compare conventional diesel-electric locomotive with electric locomotive, as well as hybrid and dual locomotive, in the same operational modus – regional freight train on electrified line. in order to properly understand the context and explain the current situation, the following should be mentioned. 46 vol. 43/2023 ecological solutions for freight trains parameter mark value unit calorific value of diesel h 10 kwh·l−1 efficiency of primary traction source νp 97 % efficiency of electric traction in dependent traction νe 85 % efficiency of electric traction in independent traction (battery) νa 74 % table 2. defined efficiency values. diesel-electric locomotives are still deployed on a significant number of trains running on electrified lines throughout the route (or most of it). in 2019, on the czech railway network: • 95.5 % of freight trains were operating on electrified lines, • 86.8 % of transport capacity of freight railway transport were operated by electric locomotives. from the numbers we can dedicate, that diesel locomotives are used almost twice much on electrified lines (8.7 %) than on non-electrified lines (4.5 %) [2]. the aim of this part of the text is, based on comparative calculations, to quantify the energy consumption and the resulting savings when running a train with a two-source locomotive, in different variants (both with auxiliary diesel generator and traction batteries) [3]. in the examples, the comparison of traction energy consumption and costs for four variants is addressed, where a) and b) are comparative variants: a) conventional diesel-electric locomotive with power output 800 kw (called class 742). b) conventional electric locomotive with power output 800 kw, used under an assumption, that all tracks are electrified (called class 210). c) hybrid locomotive with auxiliary diesel engine (called class 218). d) dual locomotive with traction battery (called class 219). the main aim is to calculate the energy consumption. for this purpose, a simple model, working with the methodology of physical resistances of the environment, is used. the most important such components are: • specific traction resistivity. • mean specific curve resistivity. • mean acceleration resistivity. the sum of all these resistivities we can call the mean specific traction resistivity. to determine the values of these resistivities, we need to know a number of physical quantities and values, which depend not only on the specific type and parameters of the vehicle, but also on the parameters of the track and the surrounding environment. all resistivities are treated as dimensionless quantities with the unit n or kn. detailed calculation is based on the model, originally coming from mr. jiří pohl’s model [4]. after the calculation of the mean specific traction resistivity, the traction work can be calculated. this value is the basis for determining the specific work and the resulting specific energy consumption for the given transport performance. the last of the preparatory calculations is the calculation of specific energy consumption. the already calculated specific traction work and the efficiency of the energy source are used to determine it. this is defined by constants in table 2. the result is therefore the specific traction energy consumption according to the formula: qd = wt h [ liter 1000 tkm ] , (1) for diesel traction and: qe = wt ν [ kwh 1000 tkm ] , (2) for electric traction, where wt is specific traction work, h calorific value of diesel and ν efficiency. the value of specific traction energy, which we’ll obtain, can be used in the calculation of the total energy consumption at every power output. 3.1. example as a suitable example for the demonstration of possible use a two-source locomotive was chosen the local freight train from karlovy vary to stráž nad ohří in north-western bohemia. this train runs along the entire route on a double-track electrified line with a 25 kv/50 hz ac power supply system; its locomotive runs off the catenary only when serving the secondary tracks and sidings at the siding stations. currently, the train is pulled by a conventional diesel locomotive class 742 with power output 800 kw. to simplify all the calculations, we used a model of uniform freight wagons. all wagons on the train are uniform: the weight of each empty wagon is 25 tonnes, while the loaded wagon is 75 tonnes heavy. tables 3 and 4 give us an example of typical composition of this train in both directions. total length of the line is 16 km; on the line are significant gradients (in some parts up to 15 ‰) and sharp arcs. the locomotives run on electric power in the intermediate sections (and diesel loco on diesel power); for shunting and operations with wagons, they used their primary power source (742, 210), or a secondary power source (218 – diesel engine, 219 – a battery). 47 martin chýle, michal drábek acta polytechnica ctu proceedings track section composition of the train (wagons) total weight karlovy vary – ostrov nad ohří 6 empty 150 t ostrov nad ohří – stráž nad ohří 3 empty 75 t table 3. train weight in direction karlovy vary – stráž nad ohří. track section composition of the train (wagons) total weight stráž nad ohří – ostrov nad ohří 3 loaded 225 t ostrov nad ohří – karlovy vary 6 loaded 450 t table 4. train weight in direction stráž nad ohří – karlovy vary. track section composition of the train (wagons) total weight jihlava – jihlava město 20 empty + 4 loaded 800 t jihlava město – batelov 18 empty + 2 loaded 600 t batelov – horní cerekev 16 empty 400 t horní cerekev – jihlávka 8 empty 200 t table 5. train weight in direction jihlava – jihlávka. track section composition of the train (wagons) total weight jihlávka – horní cerekev 3 loaded 225 t horní cerekev – batelov 1 empty + 5 loaded 400 t batelov – jihlava město 5 empty + 5 loaded 500 t jihlava město – jihlava 7 empty + 7 loaded 700 t table 6. train weight in direction jihlávka – jihlava. the same model was applied also at line jihlava – jihlávka, which is located in the central part of czechia (table 5 and 6). this line is 37 km long and the maximum gradient is 15 ‰. the height difference between both terminal stations is approximately 160 meters. the assumption of the operating model is the same as in the first example. 3.2. results for the equivalent comparison of the energy consumption of all types of locomotives, the weighted sum method was chosen, where the total traction energy consists of: • for a diesel loco only the diesel consumption, converted by the calorific value of diesel, • for an electric loco only the consumption of electricity, • for a hybrid loco with auxiliary diesel engine the consumption of electricity, added up with a diesel consumption of the auxiliary diesel engine and converted by the calorific value of diesel, • for a dual loco with traction battery the consumption of electricity, added up with a consumption of electricity when running on battery power. total amount of consumed energy q is based on the following formula: q = qd + qe + qa [kwh] , (3) qd, qe and qa are specific traction energy consumption for every traction mode (diesel, electric, battery). energy consumption of the locomotive types are illustrated by tables 7 and 8. as can be seen from the tables, using a locomotive with mainly electric power is saving approximately 75 percent of energy consumption. the significant difference is caused mostly by considerably lower energy consumption when staying in the station, as well as during the running with no traction effort. second reason is using the recuperation, allowing the return of kinetic energy in the form of electrical energy back to the overhead catenary line. it is worth to note that the way in which the locomotive is powered off the overhead contact line (electric, diesel, battery) has no significant effect on the total cost. it is essential that diesel engine is not used on the electrified lines, as the most energy is consumed when running between stations [5]. 4. selected other options for deployment of hybrid and dual locomotives the comparative examples, presented above, can’t cover all aspects of the operation of railway traction 48 vol. 43/2023 ecological solutions for freight trains total (day) total (year) class loco type e [kwh] e [kwh] 742 diesel 2,617 392,619 210 electric 978 146,659 218 hybrid 920 137,981 219 dual 892 133,772 table 7. energy consumption of all types of locomotives on the line karlovy vary – stráž nad ohří. total (day) total (year) class loco type e [kwh] e [kwh] 742 diesel 4,969 1,242,132 210 electric 1,932 483,106 218 hybrid 1,913 478,352 219 dual 1,830 457,555 table 8. energy consumption of all types of locomotives on the line jihlava – jihlávka. vehicles. however, it’s clearly visible, that in freight traffic can electric locomotive, or locomotive with auxiliary power source, bring significant energy savings, connected with appropriate cost savings. hybrid and dual locomotives can be used also in passenger transport. nevertheless, their possible deployment here is much narrower. it is possible to consider their deployment on routes, where part of the train route is on electrified lines and part on nonelectrified lines. typical example is the eurocity train from prague to munich, where the parts prague – plzeň and schwandorf – munich are electrified, while the middle section between plzeň and schwandorf not. the present situation is to change a locomotive twice, but dual locomotive (with a driving wagon) allows to pull the train on whole route and shorten the total travel time. similar examples can be found all around europe [6]. in freight transport, the spectrum of possible deployment is even wider. locomotive with energyclever power can be deployed for operation similar to mentioned above in passenger transport. for example, electric locomotives with auxiliary diesel engine are suitable for deployment on trains where the origin/destination station is not far from the electrified line and the track conditions do not require high power outside the catenary. such a locomotive can also pull a train from a marshalling yard or similar location, for example car factories, coal mines etc., instead of a diesel locomotive. many operators are trying to reduce operating costs on the shunting operation as well. modern shunting locomotives with enough power (i.e., from batteries) can then replace conventional diesel locomotives at shunting yards and local freight trains, like the examples mentioned above. one of practical examples is the locomotive of class eem 923 (built by stadler), which is already being successfully used in switzerland for such applications [7]. the main benefit of using these high-performance dual locos is shortening travel times, as well as reduction of emissions. the travel time is directly proportional to the vehicle’s output (class eem 923: 1 500 kw in electric mode, conventional diesel locomotive: ca. 700–800 kw) [8, 9]. 5. evaluation because of current situation in energetics, as well as in railway industry, the need for environmentally friendly solutions for rail transport is becoming more and more acute. principles and comparisons, mentioned in this paper, are considered as one of the possible solutions to the problem. the main objective is the substitution of conventional diesel locomotives on all spectre of traffic, where the conditions allow to use locomotives with lower emissions (mainly electric or battery powered). electric locomotives with an internal combustion engine could also save operators the costs affected by having a second (diesel) locomotive to cover short sections of the train route (so-called “last mile”). the second important topic is to reflect on the origin of the energy used to power the trains. currently, the situation across european countries is widely variable and the differences between the countries are meaningful. the strategy is to increase the share low-emission sources, like nuclear power, or renewable sources, while the share of fossil fuels and oil products will have to be as less as possible. references [1] eurostst statistics. where does our energy come from? brussels: european commision, 2021, [2022-07-22], https://ec.europa.eu/eurostat/cache/digpub/ energy/2019/bloc-2a.html. [2] správa železnic. statistická ročenka 2020. praha, 2021, [2022-07-22], https: //www.spravazeleznic.cz/documents/50004227/ 94360303/statistick%c3%a1+ro%c4%8denka+2020/ 2286eccb-d554-4b6d-892c-01314d91d9f2. [3] transport measures. cargo calculations. 2020, [2022-07-22], https://www.transportmeasures.org/ en/wiki/manuals/all-modes/1-2-calculations/. [4] j. phol. metodika výpočtu energetické a emisní náročnosti dopravních systémů. praha, 2020. [5] v. janoš, m. drábek, z. michl. quantitative determination of bottlenecks in railway networks with periodic service. in proceedings 20th international conference transport means, pp. 594–598. 2016. [6] r. vávra, v. janoš. long-distance railway passenger transport in prague – dresden/cheb relations. modern traffic engineering in the system approach to the development of traffic networks pp. 257–274, 2020. https://doi.org/10.1007/978-3-030-34069-8_20 [7] stadler rail. zweikraft-lokomotive butler. 2015. [8] v. janoš, m. kríž. infrastructure parameters affecting capacity of railways in ten-t. acta polytechnica ctu proceedings 5:22–25, 2016. https://doi.org/10.14311/app.2016.5.0022 49 https://ec.europa.eu/eurostat/cache/digpub/energy/2019/bloc-2a.html https://ec.europa.eu/eurostat/cache/digpub/energy/2019/bloc-2a.html https://www.spravazeleznic.cz/documents/50004227/94360303/statistick%c3%a1+ro%c4%8denka+2020/2286eccb-d554-4b6d-892c-01314d91d9f2 https://www.spravazeleznic.cz/documents/50004227/94360303/statistick%c3%a1+ro%c4%8denka+2020/2286eccb-d554-4b6d-892c-01314d91d9f2 https://www.spravazeleznic.cz/documents/50004227/94360303/statistick%c3%a1+ro%c4%8denka+2020/2286eccb-d554-4b6d-892c-01314d91d9f2 https://www.spravazeleznic.cz/documents/50004227/94360303/statistick%c3%a1+ro%c4%8denka+2020/2286eccb-d554-4b6d-892c-01314d91d9f2 https://www.transportmeasures.org/en/wiki/manuals/all-modes/1-2-calculations/ https://www.transportmeasures.org/en/wiki/manuals/all-modes/1-2-calculations/ https://doi.org/10.1007/978-3-030-34069-8_20 https://doi.org/10.14311/app.2016.5.0022 martin chýle, michal drábek acta polytechnica ctu proceedings [9] j. gašparík, v. cempírek. railway infrastructure capacity in the open access condition: case studies on sždc and žsr networks. transportation pp. 1–21, 2019. https://doi.org/10.5772/intechopen.88929 50 https://doi.org/10.5772/intechopen.88929 acta polytechnica ctu proceedings 43:45–50, 2023 1 introduction 2 energetic mix and power sources 3 comparison of hybrid, electric and diesel locomotive 3.1 example 3.2 results 4 selected other options for deployment of hybrid and dual locomotives 5 evaluation references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2024.51.0109 acta polytechnica ctu proceedings 51:109–113, 2024 © 2024 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague complex system testing based on api milan sliacky∗, lukáš kacar czech technical university in prague, faculty of transportation sciences, department of transport telematics, konviktská 20, 110 00 prague 1, czech republic ∗ corresponding author: sliacky@fd.cvut.cz abstract. the article describes the practical experience of testing a specific ticketing system for public transport based on blockchain technology. the testing aimed to verify the overall functionality of the complex system as well as the correct execution of the functions of its selected parts, which consist of several separately developed but closely cooperating subsystems: blockchain, clearing, and mobile application subsystem with support of bank payment system. communication between the different parts was implemented through apis developed for this purpose. the availability of api documentation and the implementation on linux os allowed the use of sophisticated tools for testing and performing both manual and automated tests. performed tests made it possible to detect errors in the program code of the respective components and to provide the basis for their elimination. applied approaches and tools can be used to test similar complex information-handling systems. keywords: api, ticketing, public transport, blockchain, clearing, mobile application, functional test, stress test. 1. introduction although the origins of blockchain technology date back to the 90s of the 20th century, it is a relatively new technology whose more massive use, unless we count cryptocurrencies, can only be observed in recent years. this technology is gradually finding applications in various areas of human life [1]. public transport is also becoming one of these areas, specifically in passenger ticketing systems. blockchain has important features that distinguish it from the technologies used in this field. blockchain is distributed, which means the data can be stored in different parts of the public transport system, i.e., on the carrier’s premises and in the means of transport. it is decentralized, which has a positive impact on the security and reliability of the system in the sense of data loss [2]. data is protected against tampering; this is a native feature of blockchain. it contains a distributed state database that is convenient for recording ticketing data (e-wallet status, loyalty program status, ticket purchase transactions, etc.). it is optionally encrypted, which means security in the sense of data protection against theft. however, deploying blockchain in this field also brings some risks, such as the insufficient transaction processing speed caused by technological limits. at the same time, sufficient speed is essential for passenger check-in. 2. solution used there are a variety of blockchain implementations. the so-called industrial blockchain was used in the project, namely hyperledger fabric [3]. maste rnode master node master node micro node micro node maste rnode figure 1. principal blockchain scheme. a common blockchain system consists of several nodes and uses several channels [3]. this project used two types of nodes: master node and micro node. the principal blockchain scheme is shown in figure 1. the master node is fully functional and implemented as a server with a fixed location. the micro node has reduced functionality and is located in the vehicle. each micro node processes transactions from its vehicle and can work online and offline. the blockchain payment system was designed with four channels: electronic wallet, tickets, loyalty system, and whitelist/blacklist (for compatibility with existing systems, where the blacklist contains a list of prohibited transport cards, and the whitelist includes a list of products linked to the passenger identifier). the blockchain subsystem was integrated with two other systems: the clearing 109 https://doi.org/10.14311/app.2024.51.0109 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en milan sliacky, lukáš kacar acta polytechnica ctu proceedings ipts clearing system backup payment method emv payment gateway mobile app backoffice psd2 third party bank master node master node micro node micro node ip ts c oo rd in at orpa ym en ts ys te m s master node mobile application supporting systems view loyalty points statuslegend: internal api api used for testing card reader blockchain blockchain payment system figure 2. blockchain payment system and supporting systems. and mobile application subsystem. there were also supporting systems (see figure 2) for payment. 3. system analysis to propose a suitable testing methodology, an analysis of the blockchain payment system was first carried out to identify key properties for testing. the clearing subsystem and mobile application subsystem were also found to be important. the key features identified are as follows. the physical architecture of the whole system consists of several separate stand-alone subsystems. information exchange (system collaboration) takes place through the apis. all apis are documented. some functions (realized by separate units, like micro node and mobile app) must work correctly offline, without interaction with other subsystems. the primary function of the blockchain payment system is the registration of ticketing data (electronic wallet records, ticket records, loyalty system and whitelist/blacklist). internal communication of the subsystems used was not subject to testing (and documentation was not provided for the testing team). based on the results of the analysis, research was conducted on how the industrial blockchain system works and how functional, security, and stress tests of complex systems connected via api interfaces are performed. security tests are not the subject of this article. 4. methodology first, the reasons and objectives of the functional tests were assessed [4]. functional tests verify the correctness and reliability of all declared functions of the whole system. the stress tests are similar to functional tests but under high load. because we knew a detailed description of how the system and its parts should work, we could have used the so-called white-box testing [5]. a high system load can be obtained by quickly generating transactions at the selected system inputs, generating many offline transactions, and then switching online, generating transactions at a high load ensured in some other way or by a combination of the above methods. functional and stress tests were performed manually or in automated mode. the correctness of the gui design was tested manually. specifically, control logic and control layout were tested. furthermore, the correctness of the response to the specified inputs was verified. the response also depends on the internal states of the system. the result was much like what you would get from an automated testing tool. still, humans have more flexibility than automated tools, so they can follow much more complicated instructions [5]. a well-known disadvantage of manual tests is the limited number of tested combinations at the system inputs and the time-consuming nature of the tests. there is also a higher probability of random error caused by the human factor. automated tests allow the detection of random errors (they occur for certain combinations of inputs and internal states only). they allow testing many combinations of external and internal system states. the identified advantage of the automated testing implementation is the existence of apis and the availability of their documentation. the following apis were used for testing purposes: to card reader (cr api), to clearing (clr api), to blockchain node (blo-api), to blockchain gateway (gw-api), and back office of mobile app (dpapi). all existing inter110 vol. 51/2024 complex system testing based on api dpapi mobile application clr api backoffice ma dpapi gw api clearing cr api card reader clr api gw api dpapi virtual device (postman) blo api blo api blockchain master node clr api blockchain micro node clr api blockchain micro node blo api blo api blockchain master node blo api blockchain master node dpapi web portal psd2 internal api internal api web interface (swagger) clr api figure 3. apis available for functional and stress tests. faces are shown in figure 3. the blockchain native api (blo api) is used for communication between individual nodes and was not used for testing. the following freely available sw tools were used to implement manual and automated tests: postman (mainly for manual api testing), notepad++ (a useful tool for analyzing data records), and swagger (if it has been implemented within the api). within the physical architecture, the micro node was implemented in two versions: industrial pc with debian operating system and raspberry pi 4 microcomputer with raspbian operating system (debian clone). the linux-based operating system enabled specific tools to run automated tests, such as python programming language (scripts with loops), bash scripts, and cron job scheduler for scheduled and/or periodic script execution. 5. implementation of tests the tests were conducted in a laboratory transport ticketing environment according to the existing methodology [6]. to verify the correct execution of the tested functions, it was first necessary to determine the use cases or tasks the system should perform and which should be tested. within each use case, the processes taking place on the system were described in detail. tested tasks are: 1) purchase a ticket in the mobile app, 2a) purchase a ticket via a transport card, 2b) top-up virtual e-wallet in the mobile app, and 3) draw loyalty points. based on the process analysis of the defined use cases, the following functions and features were validated. on the clearing subsystem, it was the correctness of generated/recorded values of transactions on relevant equipment (micro nodes, mobile app). on the blockchain subsystem, they were the loyalty program history, e-wallet transactions history, and transaction record of the ticket purchase. on hmi at all relevant parts of the system under test, it was the correctness of displayed values. quality testing can only be done for automated tests where a sufficient number of retrieved values are available. such tests were successfully carried out for automated ticket purchases on both types of micro nodes, as well as automated e-valet recharges via clearing api. the following parameters were evaluated: transaction success rate (tsr) [7] tsr = total number of successful transactions total number of attempted transactions · 100, (1) and average, minimum, and maximum test execution time, median, and variance. average test execution time is, with some simplification, equal to an average transaction duration or an average latency [8]. each use case was tested separately. the execution of the task means the communication of selected parts of the system through the appropriate apis. for example, purchasing a ticket in the mobile app (ma) means the communication of the mobile application with its back office, psd2, clearing, and blockchain subsystem using the following apis: dpapi, internal api, clr api, gw api, and blo api (see figure 3). for incorrect test results, finding the place of the error in the system (typically an error in the api design) was necessary. 111 milan sliacky, lukáš kacar acta polytechnica ctu proceedings 6. test results the testing was carried out according to the design, and many errors were revealed in most of the tested subsystems (blockchain payment system, clearing, and mobile app subsystem) in the form of a pilot plant. next, we list the main errors found during functional testing. when testing task 1 – purchase a ticket in the mobile app, a timestamp error in bonus program history recorded in the blockchain subsystem was detected. furthermore, missing information was found in the sent records (transport card id and transaction id missing), which made it impossible to trace these transactions in the blockchain database. in task 2a – purchase a ticket via a transport card, a similar timestamp error in bonus program history was detected. furthermore, randomly generated duplicate transactions were detected in both the clearing and blockchain subsystems. these duplicate transactions occurred very rarely – an average of 1 wrong transaction in 1888 correct ones. manual tests probably wouldn’t have caught this bug. in task 2b – recharge the virtual e-wallet in the mobile app, wrong timestamp entries in the e-wallet history recorded in the blockchain subsystem were detected. furthermore, wrong values in the recharge transaction records (balance and previous state of the e-wallet) were detected in the clearing subsystem records. in task 3 – drawing loyalty points, the transaction records in the blockchain loyalty program database had the wrong timestamp – the same as mentioned above. automated tests made it possible to evaluate a qualitative point of view. in task 2a – purchase a ticket via script simulated transport card use, the automated tests were performed for micro node 1 resp. 2 in two stages of system development. the first tests showed the transaction success rate equal to 80.2 % resp. 80.0 % with the average transaction duration (latency) equal to 4.82 s resp. 3.67 s. the final tests indicated the transaction success rate equal to 99,73 % resp. 99.97 % with an average transaction duration of 1.28 s resp. 1.76 s. during the integration work of the system using the results of testing, there was a significant improvement in the system parameters. compared to [8], there is still a possibility of improving efficiency. from the stress tests conducted, it appears that if the blockchain system is overloaded, the transactions on the micro node will go offline, which is a desirable feature. the number of erroneous transactions generated on the side of the micro node and average transaction duration (latency) increased with a higher load on the blockchain system. 7. test results settlement all the findings and shortcomings were incorporated into the corrections and modifications of the system under development, and the system was optimized. the tests demonstrated load resistance, fault tolerance, and practical operational sustainability. the technical and substantive merits of the project were verified in a semi-operational manner, and the operation was flawless within the specified test period. as a result of the project, a system has been built and is ready for deployment in routine operation. it is a system design that will enable mass production of similar systems for use in transport. 8. conclusions testing complex systems consisting of several subsystems connected through interfaces using apis entails the need to use a whole range of different types of testing, especially in the case of functional and stress tests. manual tests are insufficient to verify the system before commissioning, as they do not allow capturing erroneous transactions with a low frequency of occurrence (less than one error transaction per tens to hundreds of correct transactions). automated tests providing a large number of recorded values are very useful. test automation can be done in different ways. it is necessary to use the options provided by the tested system flexibly. since both types of micro nodes ran on a linux os, the advantages of the linux operating system, such as scripting languages or the cron job scheduler, were used in the testing design to generate transactions. another way to automatically generate many specific transactions was using apis. thanks to the documentation provided, this option could be used. various specialized tools allow you to generate transactions directly through the api. in addition to capturing random errors, another advantage of automated tests is obtaining enough data from which it is possible to calculate performance characteristics. finally, we must also mention notepad++, which has proven to be useful when analyzing large logs (with thousands of rows). the described approach is suitable for testing other complex systems based on computer technology, e.g., vehicle simulators, where several independent subsystems cooperate, and communication between them is realized via api. all relevant apis must be documented. an operational system based on linux is an advantage but not a mandatory prerequisite. acknowledgements all the activities described above were carried out as part of the r&d project, id: cz.01.1.02/0.0/0.0/20_321/0024952. the project was supported by the operational program oppik, managed by the ministry of industry and trade of the czech republic. the main research companies of the project were aemis, s.r.o. and business systems a.s. czech technical university in prague, faculty of transportation sciences, through its transport ticketing and information systems laboratory, carried out the testing activity as a subcontract within the implementation of the project. 112 vol. 51/2024 complex system testing based on api references [1] m. del castillo. blockchain 50: billion dollar babies, 2019. [2024-02-11]. https: //www.forbes.com/sites/michaeldelcastillo/2019/ 04/16/blockchain-50-billion-dollar-babies/?sh= 10d4292557cc [2] s. roy, m. ashaduzzaman, m. hassan, a. r. chowdhury. blockchain for iot security and management: current prospects, challenges and future directions. in 2018 5th international conference on networking, systems and security (nsyss), pp. 1–9. 2018. https://doi.org/10.1109/nsyss.2018.8631365 [3] linux foundation. blockchain network. [2024-02-14]. https://hyperledger-fabric.readthedocs.io/en/ release-1.4/network/network.html [4] r. stasonis. how to implement functional test in an automated environment, ddm consulting services inc, usa, 1999. [2024-02-12]. https://www.ddmconsulting. com/design_guides/functest.pdf [5] r. stephens. chapter 13 – testing. in beginning software engineering, pp. 327–358. 2022. isbn 978-1-119-90170-9. [6] m. svítek, j. borka, m. sliacky, et al. the methodology for verifying the interoperability of fare collection and information systems in public transport, ctu in prague, prague, 2015. [2024-02-12]. http://ois.fd.cvut.cz/dokumenty/metodika_ overovani_interoperability_ois.pdf [7] s. mercan, e. erdin, k. akkaya. improving transaction success rate in cryptocurrency payment channel networks. computer communications 166:196–207, 2021. https://doi.org/10.1016/j.comcom.2020.12.009 [8] e. androulaki, a. barger, v. bortnikov, et al. hyperledger fabric: a distributed operating system for permissioned blockchains. in proceedings of the thirteenth eurosys conference, pp. 1–15. association for computing machinery, new york, ny, usa, 2018. https://doi.org/10.1145/3190508.3190538 113 https://www.forbes.com/sites/michaeldelcastillo/2019/04/16/blockchain-50-billion-dollar-babies/?sh=10d4292557cc https://www.forbes.com/sites/michaeldelcastillo/2019/04/16/blockchain-50-billion-dollar-babies/?sh=10d4292557cc https://www.forbes.com/sites/michaeldelcastillo/2019/04/16/blockchain-50-billion-dollar-babies/?sh=10d4292557cc https://www.forbes.com/sites/michaeldelcastillo/2019/04/16/blockchain-50-billion-dollar-babies/?sh=10d4292557cc https://doi.org/10.1109/nsyss.2018.8631365 https://hyperledger-fabric.readthedocs.io/en/release-1.4/network/network.html https://hyperledger-fabric.readthedocs.io/en/release-1.4/network/network.html https://www.ddmconsulting.com/design_guides/functest.pdf https://www.ddmconsulting.com/design_guides/functest.pdf http://ois.fd.cvut.cz/dokumenty/metodika_overovani_interoperability_ois.pdf http://ois.fd.cvut.cz/dokumenty/metodika_overovani_interoperability_ois.pdf https://doi.org/10.1016/j.comcom.2020.12.009 https://doi.org/10.1145/3190508.3190538 acta polytechnica ctu proceedings 51:109–113, 2024 1 introduction 2 solution used 3 system analysis 4 methodology 5 implementation of tests 6 test results 7 test results settlement 8 conclusions acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2023.41.0028 acta polytechnica ctu proceedings 41:28–35, 2023 © 2023 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague survey of user needs and the impact of parking prices on drivers’ motivation to use parking spaces in the capital city of prague kristýna navrátilová∗, david lehet, eva hajčiarová czech technical university in prague, faculty of transportation sciences, department of transport telematics, konviktská 20, 110 00 prague, czech republic ∗ corresponding author: navrakr1@fd.cvut.cz abstract. the paper deals with the possibilities of implementing dynamic tariff regulation of on-street parking spaces in the capital city of prague. achieving optimal results is conditioned by the implementation of advanced technologies, but at the same time, the dynamic tariff regulation must follow the applicable legislation and, above all, be accepted by drivers. for this reason, the paper briefly discusses the technical and legislative possibilities of implementation, and most attention is paid to the conducted survey of user needs. the survey investigates the impact of parking prices on drivers’ motivation to park their vehicles in the area. keywords: parking, on-street parking, dynamic tariff regulation, parking survey. 1. introduction the issue of parking in the urban street network with a focus on appropriate regulation of static traffic is very topical. most cities are seeking to introduce smart parking systems to make more efficient use of existing parking spaces, optimize turnover rates, reduce the number of vehicles whose drivers are looking for a free parking space, introduce more effective monitoring of payment discipline and collect traffic data with subsequent processing. this data provides an important basis for traffic planning and parking control. smart on-street parking in the case of tariff regulation means favoring parking areas where it is desirable to increase the concentration of vehicles. according to modern parking concepts, the main objective of introducing smart systems is to make more efficient use of existing parking areas, without the need to build new areas. at the same time as the introduction of modern technologies, organizational and control processes need to be optimized; monitoring the evolution of parking habits over time is very important for feedback and future planning. the regulation of static traffic needs to be continuously adjusted and changed according to the evolution of the behaviour of vehicle drivers. dynamic regulation is currently referred to as the most advanced method of optimizing parking in urbanized environments and this issue is also discussed in academic literature. geva et al. [1] concluded from simulations that although both distance from the destination and price play a role in finding a free parking space, price was the more important of the two parameters. when the prices for “on-street” parking were comparable to those in parking lots, parking lots were chosen much more quickly. the authors concluded that if parking prices in high demand locations were on par with parking lot prices, drivers would be more likely to choose to park at a greater distance from their destination or to choose parking lots more quickly. based on their results, kurek and macioszek [2] recommend the use of systems to navigate to available parking spaces. in their research, such a system led to a positive subjective evaluation of parking. according to the authors, information from sensors used to detect parking space occupancy can be used to dynamically set parking prices or to create a parking reservation system. aljohani et al. [3] study propose to focus on the selection and design of transport management tools through a market perspective, where transport is viewed as an allocation of resources and services. they believe that in this way it is possible to identify appropriate tools for transport management. it is similar with navigation systems and new technologies such as c2x and sending the necessary data directly to the vehicle for guidance on a free parking space. in the public space of cities, suitable parking spaces are decreasing and to eliminate the search for a free space, the constant burden on road traffic with vehicles looking for a space, navigation and reservation of a free space is an important element of the parking solution together with a defined dynamic tariff during the daily and weekly periods [4–7]. the aim of the paper is to analyze the realistic implementation of dynamic tariff regulation of parking in the capital city of prague suitable for optimizing the distribution of vehicles in the street network. the results will be evaluated especially in terms of deployment of dynamic regulation in real operation as a tool for optimization of vehicle movement and parking in the urban street network. a prerequisite for a successful design is at the same time a survey of user needs, as the human factor is a key factor 28 https://doi.org/10.14311/app.2023.41.0028 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 41/2023 parking prices and spaces in prague zone number of parking spaces share of ppz blue – residents 76 614 64 % purple – mixed 42 325 35 % orange – visitors 1 123 1 % total 120 062 note: data from 2020-08-18. table 1. number of parking spaces and ppzs capacity [8]. owner of vehicle number of owned vehicles tariff zone price 1 year ½ year ¼ year resident 1. vehicle all 1 200 czk 600 czk 300 czk 2. vehicle all 7 000 czk 3 500 czk 1 750 czk 3. and next vehicle 1 36 000 czk 20 000 czk 10 000 czk 2 30 000 czk 17 000 czk 8 500 czk 3 24 000 czk 13 500 czk 7 000 czk table 2. parking prices for residents according to tariff zones [9]. determining the functionality of the solution. tariff regulation must be set in such a way that it provides sufficient incentive, but at the same time the rules are respected, and parking fees are paid. the paper briefly describes the current parking management in prague and evaluates the possibilities of implementing dynamic parking regulations. it also presents the results of a survey of user needs focused mainly on the willingness to pay parking fees. finally, the direction of further research development is outlined. 1.1. current parking management system in prague the regulation of parking in the territory of the capital city of prague is solved by means of paid parking zones (ppzs). the purpose of introducing ppzs is to increase the chance for residents of the busiest areas to park their cars within walking distance from their homes (or properties/businesses). the zones are being established gradually from the historical center to the periphery, the first zone was introduced in 1996, according to the new (currently valid) concept, the ppzs in prague have been introduced since 2016, so far, the last extension was planned for july 2021 [10]. there are 3 basic parking regimes in the zones: • resident mode (blue zone) – residents, subscribers and property owners in the area; • mixed mode (purple zone) – residents, subscribers, property owners in the area and visitors; • visitor mode (orange zone) – short-term parking for anyone. the grantor of the ppzs is the capital city of prague according to the zákon o pozemních komunikacích č. 13/1997 sb. the administrator is technická správa komunikací. in prague, modern technologies are introduced for payment of parking fees directly at the parking meter or via the mpla web application – virtual parking hours. the system enables the use of new payment channels and payment technologies, the aim is to modernize and simplify the issue of parking permits, possible sanctions and increase control. it also has the advantage of devolving the resolution of parking violations and their revenue to the boroughs, leading to higher motivation for staff to deal with violations. table 1 shows a summary of the number of parking spaces in each zone and the percentage of zones by zone type. the areas of the capital city of prague in which ppzs can be established are defined in the regulation no. 10/2020 coll. of the capital city of prague from 1st june 2020. each parking zone has its own operating hours, usually from 8:00 to 20:00, which can be adjusted for each specific location. outside the operating hours, parking in the zones is not regulated. unless the zone operating hours are indicated on vertical signage, the regulation is valid 24 hours a day. the validity of the parking permit is checked on the basis of the vehicle registration number, automatically by means of a monitoring system. for the purposes of determining the fees, ppzs in capital city of prague are divided into 3 tariff zones depending on location. basically, areas in the city center and other busy areas are included in the 1. tariff zone, which is the most expensive, whereas the more peripheral city areas are included in the 3. tariff zone, which is the cheapest. basic parking prices are shown in tables 2 and 3. table 2 shows prices valid for residents – these depend on the number of 29 k. navrátilová, d. lehet, e. hajčiarová acta polytechnica ctu proceedings owner of vehicle price per 1 hour ordinary visitor 1. tariff zone 80 czk, 3.20 ac 2. tariff zone 60 czk, 2.4 ac 3. tariff zone 40 czk, 1.6 ac table 3. parking prices for visitors according to tariff zones [9]. vehicles owned. fee for the first owned vehicle is significantly lower than fee for third and subsequent vehicle. table 3 shows parking fees valid for visitors to ppzs and their differences in relation to different tariff zones. in addition to the existence of the ppzs, another important factor influencing the driver’s choice of parking space is accessibility of locations by public transport and the cost of public transport fees. prague has a very dense network of public transport stops and for a significant number of people this may be a reason to prefer public transport rather than their own vehicle. 2. implementation options the paper deals with the evaluation of the realistic implementation of dynamic tariff regulation in the conditions of the capital city of prague. initially, the technical and legislative possibilities of implementing the proposed system in practice are briefly described. 2.1. technical possibilities long-term parking permits are issued electronically, as are short-term permits. the following two payment channels can be used to pay the fee for a short-term parking session [11]: • flowbird parking meters; • web-based virtual parking meter application. technical parameters of parking meters are very difficult to obtain from openly available sources. the most information can be obtained from the technical conditions of the contracting authority, which defined the parameters and operation for the public contract for the operation of parking meters in paid parking zones [12]. the monitoring center must have the capacity to connect a total of 1500 parking meters. the availability of the parking meter service shall be at least 98% and the functionality shall be at least 95%. in addition, incident groups are defined according to priority. the data connectivity shall use publicly available commercial communication networks and ensure the interconnection of all devices with the monitoring center. precise requirements for payment acceptance, appearance, features, amenities, printed content on the parking ticket, etc. shall be defined. drivers shall enter the registration number of their vehicle into the parking meters. the data link requirements are not adapted to the possibility of frequently uploading new parking prices or even updating them during the day [12]. 2.2. parking legislation the capacity and form of parking facilities are determined by the prague building regulations. parking capacity is dealt with in § 32, § 33 focuses on the form and character of parking. the purpose of the parking regulations is to define minimum parking requirements according to the type of location, building type and connection to public transport, but also to set maximum permissible values. a map of the city’s zones for the purpose of determining the number of parking spaces is available on the website of the institute of planning and development of the city of prague (on the provisions of section 32(2)). ordinance no. 26 on general technical requirements for construction in the capital city of prague, article 10 of 19 october 1999 sets out the dispersal areas and facilities for static traffic. areas intended for a larger number of vehicles are supplemented with greenery, especially trees. five basic criteria are considered to determine the number of parking spaces: • zones 1 to 4; • metro station catchment area; • territorial influence coefficient ku; • coefficient of traffic serviceability of the area kd; • indicators of the basic number of parking spaces depending on the functional use of the building. annex no. 2 to decree no. 11/2014 coll. 11.11 of the city of prague contains a table setting out the basic number of parking spaces. the basic number of parking spaces indicator defines the gross floor area per 1 parking space. the percentage of tied and visitor parking spaces is determined [10]. prague building regulations define parking capacity and the form and nature of parking. parking for buildings is stipulated as the minimum required, and the maximum permitted number of parking stalls. it is calculated as a per cent of the basic number of stalls. dedicated residential parking must usually be located in garages or stacked parking systems. visitor parking must be publicly accessible, but its use may be controlled. surface parking lots are planted with trees, generally one tree must be planted per eight stalls in the parking lot [13]. strategic plan of city of prague formulates a common vision for the city. it determines the principal 30 vol. 41/2023 parking prices and spaces in prague figure 1. feelings while searching for a parking space [1]. development direction in the mediumand long-term and sets out the social and economic objectives for the city. the main points relating to parking are: prioritise environmentally friendly forms of transport, i.e., walking, cycling and public transport; green infrastructure support; make public transport more attractive – the aim is to increase the share of public transport, walking and cycling above 70%; construction of new suburban p+r and b+r parking systems; increase the use of its in traffic control; extension and optimization of ppzs using its including dynamic navigation [14]. the sustainable mobility plan for prague and its suburbs presents an overall view of transport solutions in the capital city. the vision is long-term sustainable mobility, based on a high-quality and accessible integrated public transport network based primarily on rail transport. other preferred modes of transport are walking and cycling. car traffic will be regulated in line with the vision, in the form of urban tolls, parking policy and preference of public transport. the parking policy will go the way of unifying the rules of parking zones, favouring eco-friendly vehicles when parking in the zones and shrinking individual parking zones so that it does not make sense to use the car for journeys within one zone. as part of the revitalisation of the urban space, the number of parking spaces will be reduced in favour of infrastructure for pedestrians, cyclists and public transport. conversely, the plan is to develop the p+r and b+r car park network into a high quality and comprehensive catchment system [15]. 2.3. user acceptance – questionnaire method the basic prerequisite for the functionality of the proposed solution is, in addition to the necessary compliance with applicable parking legislation and technical support, the acceptance of this solution by the users (drivers). the aim of the survey was to determine the willingness to pay parking fees – the effect of parking prices on drivers’ motivation to park their vehicles in a given area or neighborhood. the survey of user needs was carried out in the form of a questionnaire survey. a questionnaire created in ms forms was used and distributed online. the questionnaire was designed to cover all relevant aspects affecting drivers’ parking behavior. the questionnaire focused on collecting information from the following areas: statistical information specifying the respondent’s personal characteristics and his/her general traffic behavior, more detailed information on the respondent’s mobility and daily traffic behavior, and information on the willingness to pay parking fees. as the whole study works with the area of the capital city of prague, the questionnaire focused both on journeys to/from prague and on journeys around prague. the questionnaire was distributed in two rounds. 3. first round of the questionnaire the aim of round 1 was to use open-ended questions to specify the issues so that appropriately worded closed questions could be used in round 2. for participation in round 1, a shortlist of selected respondents was pre-selected to represent groups with potentially different transport behavior. parking issues in the czech republic were also discussed in the paper [1]. respondents were among others asked to rate the statements about their feelings connected to searching for a parking space and parking. the following bar graphs (figure 1) show the perceived issue of finding a parking space and then the parking itself in the czech republic. 31 k. navrátilová, d. lehet, e. hajčiarová acta polytechnica ctu proceedings gender age category occupation permanent residence man 9 up to 29 years 6 employed 12 prague 7 woman 4 30–39 years 6 student 1 other 6 40–49 years 1 table 4. demographic distribution of respondents to round 1 of the survey [11]. gender age category occupation education residency man 236 up to 29 years 216 maternal/ parental leave 4 elementary 2 prague 107 woman 112 30–39 years 64 unemployed 1 high school 163 other 244 do not want to share 3 40–49 years 43 employed 187 university 186 50–64 years 22 student 157 65 years and more 6 housewife/ husband 1 retired 1 table 5. sociodemographic data [11]. 3.1. evaluation of the first round thirteen respondents took part in the first round of the survey and their demographic distribution can be seen in (table 4) below. respondents answered 17 questions relating to their transport behaviour, of which 8 were open-ended. the results, especially the results of the open-ended questions, were used to construct the final (second round) questionnaire. 4. second round of the questionnaire the wider public was invited to participate in round 2. the main round of the survey contained a total of 28 closed questions, which were built on each other and were based on the answers of the respondents. the questions were divided into the following interlinked sections: • statistical information; • mobility and transport behaviour; • willingness to pay for parking and acceptable amounts; • parking preferences. there were 351 respondents. 4.1. evaluation of the second round first, the respondents were asked about several statistical information, the evaluation of these questions is in table 5. the questionnaire was completed by 309 respondents who hold a driving license. these respondents were able to continue to next parts of the questionnaire. the results of the questionnaire showed a correlation between the commuting time from the place of residence to prague and the frequency of commuting. frequent commuting (daily or weekdays) increases the proportion of shorter commuting times. on the other hand, there was no correlation between the possibility of unpaid parking and the use of long-term coupons for travelling by public transport in prague. figure 2 shows the evaluation of the question of car use for trips to and in prague. the numbers of responses are shown. the chart includes the responses of all respondents who answered these questions – i.e., respondents residing in prague and outside of prague. it can be seen that the distribution of car use is very similar for trips to and in prague. the questionnaire was completed by 309 respondents who hold a driving license. these respondents were able to continue to next parts of the questionnaire. the results of the questionnaire showed a correlation between the commuting time from the place of residence to prague and the frequency of commuting. frequent commuting (daily or weekdays) increases the proportion of shorter commuting times. on the other hand, there was no correlation between the possibility of unpaid parking and the use of long-term coupons for travelling by public transport in prague. figure 2 shows the evaluation of the question of car use for trips to and in prague. the numbers of responses are shown. the chart includes the responses of all respondents who answered these questions – i.e., respondents residing in prague and outside of prague. it can be seen that the distribution of car use is very similar for trips to and in prague. as in the case of figure 2 and figure 3 includes the answers of all respondents who answered these 32 vol. 41/2023 parking prices and spaces in prague figure 2. use of the car. figure 3. frequency of car use. figure 4. frequency of car use, respondents residing outside prague. questions – i.e., respondents residing in prague and outside prague. the graph shows that the car is more often used for trips to prague than for trips inside prague. figure 4 uses only responses of respondents living outside prague. for these respondents, the car is used with high frequency for trips to prague. a total of 56% of respondents chose the answer “always” or “mostly”. on the other hand, the car is used rather less frequently for trips inside prague. the answer ’rarely’ or ’very rarely’ was chosen by 53% of respondents. the results of the next part of the questionnaire survey are crucial for the further continuation of the research, as they indicate what price drivers are willing to pay for one hour of regular parking in prague. normal parking is defined as payment for 1 h of parking under normal circumstances, i.e., not an isolated payment that drivers are willing to pay exceptionally or in urgent cases (such as parking near a hospital) and the amount is a one-off. 33 k. navrátilová, d. lehet, e. hajčiarová acta polytechnica ctu proceedings figure 5. maximum accepted parking fee for 1 h of parking. figure 6. use of the car compared to public transport for travelling around prague. the results in the following graph (figure 5) show that the amount that most respondents are willing to pay for an hour of parking ranges from 0–45 czk. the second largest group of respondents was in favour of 50–80 czk per hour. the last part of the questionnaire was focused on the use of p+r parking lots. the questionnaire showed that most respondents do not use p+r parking lots and those who do, do so rather exceptionally. there was also a choice of several reasons why they do not use this type of parking lots and under what circumstances they would start using them. the most frequent reason was another parking option in prague or inconvenient location of these parking lots. the answers showed that the following 3 aspects would motivate the respondents to use these types of parking lots: • suitable location in relation to the destination; • convenient to reach the destination when transferring to public transport; • free parking. the responses indicate that the most common problem that respondents have is the location of parking relative to where they need to go. the purpose of the last question was to determine the frequency of private car use in prague compared to public transport (figure 6). 5. conclusion and future work according to the available literature, dynamic parking is generally perceived as the most advanced method of parking regulation. by appropriately setting the dynamic price and the factors on which it is based, efficient use of existing parking spaces can be achieved. tailoring parking regulation to the area is crucial. pricing should be based on acceptable amounts, and the price can be seen as a tool to optimize the distribution of vehicles. a very suitable solution is the use of a c2x system or the detection of license plate recognition in 34 vol. 41/2023 parking prices and spaces in prague a certain area and the connected virtual reservation of parking space. this results in significantly better vehicle navigation and a reduction in the so-called “wandering vehicle paths” that burden the road network of the area. in addition, the reservation of space and navigation also allows for the effective use of the space equipped with a charging station for electric vehicles. currently, prague has a system of paid parking zones, which is no longer sufficient, and the parking situation is getting worse, especially in housing estates. the introduction of dynamic parking pricing as described in the article is currently hindered, among other things, by the technical parameters of the installed parking meters. the uploading of the price now takes several hours up to a day. current legislation deals with the determining of the number of parking spaces and the parameters of these spaces. the methodology for calculating the price of parking in the relevant zone is defined. the issue of variable pricing is not addressed in the current legislation. parking is also a very politically sensitive issue and often these are the main reasons that prevent a change in the regulatory system. the survey could be summarized as follows: most respondents either do not use vehicle for travelling to and inside prague or use theirs or family vehicle. number of respondents who use car-sharing services is quite low. those respondents who live outside prague more often use vehicle only to get to the city and for their transport around the city, they use public transport. they do not use vehicle inside the city. primary goal of the research was to ascertain what are the acceptable amounts of money to be paid for parking. only few respondents are willing to pay more than 80 crowns per hour. most of the respondents indicated that they would be willing to pay only symbolic amount of 20 crowns per hour. second big group indicated price around 50 crowns per hour. next step of the research should be microsimulation using these data to simulate a behaviour of drivers based on the different prices of parking places. ptv vissim – traffic simulation software will be used for this simulation. the aim is to visualize a simulation of dynamic parking regulation for a selected part of the city. acknowledgements this work was supported by the grant agency of the czech technical university in prague, grant no. sgs22/079/ohk2/1t/16. references [1] s. geva, n. fulman, e. ben-elia. getting the prices right: drivers’ cruising choices in a serious parking game. transportation research part a: policy and practice 165:54–75, 2022. https://doi.org/10.1016/j.tra.2022.08.018 [2] a. kurek, e. macioszek. drivers’ subjective assessment of the ease of finding a vacant parking space in an area equipped with vehicle detection devices. sensors 22(18):6734, 2022. https://doi.org/10.3390/s22186734 [3] m. aljohani, s. olariu, a. alali, s. jain. a survey of parking solutions for smart cities. ieee transactions on intelligent transportation systems 23(8):10012–10029, 2022. https://doi.org/10.1109/tits.2021.3112825 [4] p. toman, j. svoboda, a. orlicky. assessment of the motorcycle rider behaviour – experimental study on the race track. in 2020 smart city symposium prague (scsp), pp. 1–6. ieee, 2020. https://doi.org/10.1109/scsp49987.2020.9133760 [5] t. tichý, d. švorc, m. růžička, z. bělinová. thermal feature detection of vehicle categories in the urban area. sustainability 13(12):6873, 2021. https://doi.org/10.3390/su13126873 [6] j. jíšová, t. tichý, j. filip, et al. the application of the latest territorial components for sustainable mobility in district cities. iop conference series: earth and environmental science 900(1):012012, 2021. https://doi.org/10.1088/1755-1315/900/1/012012 [7] j. brož, t. tichý, v. angelakis, z. bělinová. usage of v2x applications in road tunnels. applied sciences 12(9):4624, 2022. https://doi.org/10.3390/app12094624 [8] auta. pragozor. 2021. [2021-08-29], https://pragozor.cz/cars. [9] výtah z ceníku parkovacích oprávnění. parkuj v klidu. 2021. [2022-10-15], https://www.parkujvklidu.cz/cs/ zakaznici-zon-placeneho-parkovani/ vytah-z-ceniku/. [10] k. navrátilová, e. hajčiarová. detailní analýza dostupných prostředku regulace parkování v uličním prostoru. 2022. [2022-11-20], http: //www.k620.fd.cvut.cz/sgs21-079-ohk2-1t-16. [11] k. navrátilová, d. lehet, e. hajčiarová. možnosti implementace dynamické regulace parkování v podmínkách hl. m. prahy. 2022. [2022-11-20], http: //www.k620.fd.cvut.cz/sgs22-079-ohk2-1t-16. [12] podmínky zadavatele. 2022. [2022-10-20], zps – část 2 technické podmínky zadavatele čistopis-p.pdf (https://tsk-praha.cz). [13] prague building regulations. 2022. [2022-11-20], praugebuilidingregulations_english_1.pdf (https://iprpraha.cz/). [14] strategic plan of city of prague. 2022. [2022-11-20], městské strategie (https://iprpraha.cz/). [15] the sustainable mobility plan for prague and its suburbs. 2019. [2022-11-21], https://poladprahu.cz/wp-content/uploads/2019/ 11/mobility_plan-brochure_en.pdf. 35 https://doi.org/10.1016/j.tra.2022.08.018 https://doi.org/10.3390/s22186734 https://doi.org/10.1109/tits.2021.3112825 https://doi.org/10.1109/scsp49987.2020.9133760 https://doi.org/10.3390/su13126873 https://doi.org/10.1088/1755-1315/900/1/012012 https://doi.org/10.3390/app12094624 https://pragozor.cz/cars https://www.parkujvklidu.cz/cs/zakaznici-zon-placeneho-parkovani/vytah-z-ceniku/ https://www.parkujvklidu.cz/cs/zakaznici-zon-placeneho-parkovani/vytah-z-ceniku/ https://www.parkujvklidu.cz/cs/zakaznici-zon-placeneho-parkovani/vytah-z-ceniku/ http://www.k620.fd.cvut.cz/sgs21-079-ohk2-1t-16 http://www.k620.fd.cvut.cz/sgs21-079-ohk2-1t-16 http://www.k620.fd.cvut.cz/sgs22-079-ohk2-1t-16 http://www.k620.fd.cvut.cz/sgs22-079-ohk2-1t-16 https://tsk-praha.cz https://iprpraha.cz/ https://iprpraha.cz/ https://poladprahu.cz/wp-content/uploads/2019/11/mobility_plan-brochure_en.pdf https://poladprahu.cz/wp-content/uploads/2019/11/mobility_plan-brochure_en.pdf acta polytechnica ctu proceedings 41:28–35, 2023 1 introduction 1.1 current parking management system in prague 2 implementation options 2.1 technical possibilities 2.2 parking legislation 2.3 user acceptance – questionnaire method 3 first round of the questionnaire 3.1 evaluation of the first round 4 second round of the questionnaire 4.1 evaluation of the second round 5 conclusion and future work acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2023.45.0039 acta polytechnica ctu proceedings 45:39–44, 2023 © 2023 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague quality improvement by application of a revised standard – feasibility studies on 2500 soundings håkan garin∗, gunilla franzén president swedish geotechnical society, hällingsjövägen 320, se-434 97 kungsbacka, sweden ∗ corresponding author: hakan.garin@geoverkstan.se abstract. cpt is a geotechnical investigation method that, depending on the geology, has been proved to be an essential tool to establish the geotechnical design model. however, doubt has been raised about the quality of the data. hence, the feasibility to use data from cpt to evaluate reliable parameters for the geotechnical design model also needs investigation. currently, the international standard en iso 22476-1 [1] is being revised, and the revised version will be published in winter 2022. it will include new requirements related to quality parameters such as zero stability, zero-shift, temperature, and pore pressure response. in addition, there will be updated requirements related to the calibration of the cone penetrometer. all this is to improve the quality of the cpt data, thereby increasing the reliability of parameters evaluated from cpt. this paper presents a feasibility study. it compares the zero-shift variation depending on the type of cone penetrometer, the operator, and the geology. more than 2 500 soundings from sweden, norway and finland were the basis for this comparison. the results indicate that the data quality is strongly linked to the type of cone penetrometer, its calibration, and the management of the cone penetrometer in the field. keywords: cpt, quality, zero-shift. 1. introduction cpt (cone penetration test) is a geotechnical investigation method that, depending on the geology, has proved to be an essential tool to establish the geotechnical design model. however, doubt has been raised about the quality of the data and the investigation methods applicability in different types of geological conditions. thus, the feasibility to use data from cpt to evaluate reliable parameters for the geotechnical design model has been questioned. currently, the cpt-standard en iso 22476-1 [2] is under revision and the aim is to ensure high-quality data if the soundings are performed according to the standard. this paper gives an overview of some main revisions in the standard and presents some of the background material for the revision. the analyses in this paper indicate some of the key parameters for the user to consider if reliable data from the cpt is to be obtained. the history of cpt is illustrated in figure 1. it started, as in all sounding methods, as a handheld one that was further developed to include loading and then in the 1970s the first complete electric investigation equipment was developed. today, there are multiple different ground investigation rigs carrying the cpt equipment, from full-scale field labs to multi-purpose hydraulic field rigs. the applicability of the cpt in different ground conditions depends on the maximum resistance of the cone penetrometer utilized. for clay till and firm sand, a cone penetrometer with higher cone resistance should be used, while for clay, silt and fine sand a cone penetrometer with lower maximum cone resistance is preferred. cpt can be used in most ground conditions including organic soils as long as there are no boulders or extremely stiff layers to penetrate. the latter might cause damage to the cone penetrometer. 2. revised version of en iso 22476-1 the first version of en iso 22476-1 geotechnical investigation and testing – field testing – electrical cone and the piezocone penetration test, was published in 2012. a need for clarifications and improvements was identified in relation to several issues including confidence levels, dimensions of the cone penetrometer and calibration. hence, the process of revision was initiated, and the aim is that the revised en iso 22476-1 will be published in late 2022 or early 2023. 2.1. confidence levels in the 2012 version of en iso 22476-1 information on confidence levels with requirements in the field and the information needed for calibration of the cone penetrometer in the lab is mixed together in one table. this causes unnecessary confusion for the user. therefore, the table has been divided into three different tables in the revised version of the cpt standard. the three tables that are used in the revised standard are illustrated in figures 2, 3, 4. depending on the site conditions different test categories will be applied and hence the maximum cone penetrometer resistance will be in one of the three application classes shown in figure 2 – table a1. the 39 https://doi.org/10.14311/app.2023.45.0039 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en håkan garin, gunilla franzén acta polytechnica ctu proceedings figure 1. illustration of the history of cpt. figure 2. new tables in en iso 22476-9 – confidence levels of measurements for the characterisation of geotechnical properties depending on the cone type and test category. confidence level of the data (high, medium, low) in each application class depends on the test category (field) and cone penetrometer class (laboratory calibration). the requirement on the cone penetrometer that shall be shown in laboratory calibration for the different penetrometer classes is given in figure 3 – table 2. this includes measurement uncertainty, temperature stability and bending influence. in figure 4 – table 3 the requirements to achieve specific test categories in the field are given. there are requirements on the cone penetrometer to fulfil a given cone penetrometer class according to the latest calibration documentation. in addition to that, at every sounding, checks should be made that the zero readings (before and after soundings) and the variation in output stability fulfil the given requirements. a cone penetrometer calibrated for cone penetrometer class 1, might not be sufficient to achieve test category b due to its performance in the field. the accuracy of the cone penetrometer is important for the interpretation of the results. it can be shown that uncertainty of the cone resistance of 15 kpa, results in an uncertainty in the measured undrained shear strength of 1 kpa. likewise, the uncertainty of the pore pressure, u2, of 75 kpa, results in an uncertainty in the measured undrained shear strength of 1 kpa. therefore, the pore pressure system must be fully saturated, or errors in pore pressure measurement, u2, up to 500–1 000 kpa may arise, depending on the geological properties. this is an error that may introduce significant uncertainty. 2.2. calibration in the version from the 2012 standard, it was stated simply that the calibration will be performed. however, no detailed requirements on the calibration were included. therefore, it was up to each provider to 40 vol. 45/2023 feasibility studies on soundings figure 3. new tables in en iso 22476-9 – classification of cone penetrometers under laboratory conditions. figure 4. new tables in en iso 22476-9 – test categories of cpt/cptu. 41 håkan garin, gunilla franzén acta polytechnica ctu proceedings figure 5. illustration of the difference between calibration requirements between en iso 22476-1:2012 and iso/fdis 22476-1:202. figure 6. results of achieved test category. perform the calibration based on their best practice. the calibration should be performed every six months. in the revised version the interval has been extended to every twelfth month and there is a requirement that the calibration should be performed in a laboratory that fulfils the requirements of iso/iec 17025. the number of calibration points has been defined as 100 points in three directions of the cone penetrometer. the work needed to perform a complete calibration of the cone penetrometer is significantly extended. figure 5 illustrates the difference in principle between calibration according to the 2012 and 2022 versions. 3. assessment of the 2500 soundings 3.1. background this assessment of the quality of cpt is based on 2500 soundings, collected during 2019 and 2020 in norway and sweden. thirteen different ground investigation contractors were represented and more than 100 individual field engineers. two different manufacturers (geotech and envi) provided 134 cpt-cones. the raw data were collected and, important for a simple database, involved the following information: • cone penetrometer#, • operator, • date, • maximum depth, • zero-readings before, • zero-readings afterwards, • temperature (start/min/max/end if recorded), • qc-max, • fs-max, • u-max. the analyses focus on the following comparisons. 1) achieved field performance test class considering all cone penetrometers and whether only those with a law maximum qc,max are used, 2) differences between different set-ups of the cone penetrometer (manufacturers), 3) the importance of long-term stability, 4) difference in zero-readings. 3.2. achieved test category the achieved test category was determined for all soundings (see figure 6a). if only the cone penetrometers with a maximum qc,max < 2 mpa are included, the results are slightly different (see figure 6b), but this difference is not significant. the results indicate that 98 % of the soundings fulfil the requirement of category b for friction sleeve, 73 % for the cone and 78 % for pore pressure. the friction sleeve, which seldom is used for the evaluation of ground properties, seems easier to obtain with high confidence than cone and pore pressure. it should be recognized that one-fourth of the soundings did not fulfil the requirement of test category b. 42 vol. 45/2023 feasibility studies on soundings figure 7. results of achieved test category for different manufacturers. figure 8. results of long-term stability. 3.3. different set-ups of the cone penetrometer (manufacturers) for manufacturer a 1184 soundings were available and for manufacturer b 519 soundings were. the results presented in figure 7, show that for manufacturer a the test category b is achieved for about 80 % of the soundings, considering cone and pore pressure. for manufacturer b the corresponding values are 44 and 69 % for cone and pore pressure. even though both manufacturers follow the requirement of the standard in relation to dimensions, there are differences in how the cone penetrometer is assembled that give differences in confidence in the results. 3.4. long-term stability two identical cone penetrometers from the same manufacturer that have been calibrated, might give a significantly different confidence in the data. in figure 8 two cone penetrometers are compared. for cone penetrometer #4996 there are 107 soundings available and for cone penetrometer #4315 there are 70 soundings available. for both cone penetrometers, the long-term stability of zero readings for qc is also presented. for cone penetrometer #4996 test category b is only achieved in 55 % of the soundings with regards to cone and pore pressure. in addition, the zero readings indicate very high variation. for cone penetrometer #4315 test category b is achieved in 98 % and 96 % of the soundings for cone and pore pressure. the zero readings show low variation. this check of the field results is essential to determine if the data obtained is reliable to use in the geotechnical ground model. in this case, the results from cone penetrometer #4996 do not have a sufficient confidence. 3.5. concluding remarks the following list indicates some of the items that will influence the achieved test category and confidence level: • each cone penetrometer is an individual one and therefore it is necessary to get information on calibration and field reference readings checks for the individual cone penetrometer with regards to: ▷ long-term stability of the zero readings before ▷ difference in zero readings (before – after) • the result depends on the cone penetrometers’ internal structure/sensors (manufacturer) and model. • performance and handling (human error). • temperature (before vs. during sounding). 4. recommendations the revised standard will give the engineers the tools to ensure that the data obtained from the field have sufficient confidence to be included in the geotechnical 43 håkan garin, gunilla franzén acta polytechnica ctu proceedings design model. however, to obtain quality there has to be teamwork between the field engineer and the engineer evaluating the results. the field engineer needs to be aware of how the performance of the sounding (including treatment of the cone penetrometer) will influence the test category and confidence. any deviation from the normal test procedure or other factors that might influence the results should be documented and forwarded to the engineer evaluating the data. the engineering evaluation of the data needs to know how the test was performed, site conditions and specific information for each sounding. the field reference readings should be evaluated to confirm the quality of the data before the parameters are evaluated. it is the considered opinion of the authors that cpt testing, if applied with required knowledge and within its limitation, will be an essential tool for the engineer to prepare, select and verify data, based upon cpt, for the geotechnical design model. acknowledgements the authors would like to thank the members of the core group iso/tc182/wg7 for collaboration during the preparation of the revised standard. references [1] en iso 22476-1:2012 geotechnical investigation and testing field testing electrical cone and piezocone penetration test. [2] iso/fdis 22476-:2022 geotechnical investigation and testing field testing electrical cone and piezocone penetration test. 44 acta polytechnica ctu proceedings 45:1–6, 2023 1 introduction 2 revised version of en iso 22476-1 2.1 confidence levels 2.2 calibration 3 assessment of the 2500 soundings 3.1 background 3.2 achieved test category 3.3 different set-ups of the cone penetrometer (manufacturers) 3.4 long-term stability 3.5 concluding remarks 4 recommendations acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2024.49.0041 acta polytechnica ctu proceedings 49:41–47, 2024 © 2024 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague numerical modeling of concrete beams subjected to partial wetting and drying cycles štěpán krátký∗, petr havlásek czech technical university in prague, faculty of civil engineering, department of mechanics, thákurova 7, 166 29 prague, czech republic ∗ corresponding author: 1kratkystepan@gmail.com abstract. concrete structures are commonly subjected to cycles of wetting and drying, which can have a significant impact on their behavior. however, experimental data on the effects of wetting and drying on concrete beams are scarce. this study presents the results of a 4-year experimental program on 2.5 m span concrete beams with heights of 100 mm, 150 mm, and 200 mm subjected to one-sided wetting and drying cycles. vertical deflection of the beams was monitored throughout the experiment, and moisture mass balance was measured on companion specimens. the behavior of the specimens was experimentally assessed, and subsequently numerical models using coupled hygromechanical finite element simulations were developed. the purpose of the simulations was to verify the prediction capabilities of the established constitutive models for moisture transport and creep and shrinkage of concrete and, if needed, to propose necessary corrections to the constitutive models. keywords: concrete, creep, shrinkage, drying, cyclic temperature, modeling. 1. introduction although the effects of moisture on the timedependent behavior of concrete have been studied for decades, existing data are typically limited to cases where the concrete element is exposed to constant air humidity uniformly across the whole surface [1–4]. however, in reality, structural members are frequently subjected to cycles of highly variable ambient conditions. data are scarce for cycles of contact with water and subsequent exposure to air, and/or the influence of sunlight [5–7], and focus primarily on axial strain. in 2019, an extensive experimental campaign was initiated at the ctu in prague to study the timedependent behavior of structural concrete exposed to drying conditions [8]. all samples were prepared from a single concrete batch and cured for one month. the key component was a set of simply supported plain concrete beams of various sizes l = 1.75–3.0 m, h = 0.05–0.2 m and sealing setups which influenced not only the drying rate, and the development of vertical deflection, but also the degree of internal restraint and thus the extent of damage. a set of spare specimens was used to investigate the effects of one-sided wetting and drying cycles. within the experiment, deflection of simply supported beams with heights of 100, 150, and 200 mm and a span of 2.5 m was measured. after one year of drying, beams were placed inside a tank and repeatedly partially immersed (≈ 25 mm). the measurement results revealed that the response in the individual cycles varies depending on the number of previous cycles and the moisture content of the specimens. the data suggest that the members may undergo irreversible changes. this paper summarizes the methods and results of the first author’s master’s thesis [9]. the first part describes the computational models, the second part presents the results of the blind prediction and the constitutive models employed, which aimed to understand the modeled problem and assess the predictive capabilities of the material models. the final part shows selected results from the refined simulations, which were designed to replicate the course of the experiment more accurately, and to identify potential causes of the variable behavior of the specimens between individual cycles. 2. numerical modeling a weakly coupled computational approach was used to model the moisture-induced structural response. two sets of computational models were created for simulations: (1.) beams with different heights (as shown in figures 1 and 2), (2.) companion specimens used to measure the moisture content that did not consider the structural response. the oofem finite element solver [10] was used to run all numerical simulations. a finite element mesh of the beams and accompanying specimens represents a symmetric quarter of the real specimen. suitable boundary conditions are defined on the symmetry planes to impose symmetric behavior (zero moisture flux and zero out-of-plane displacement). the mesh is finer near the surfaces, with an edge element thickness of 2 mm, to improve the convergence of computation and to capture the 41 https://doi.org/10.14311/app.2024.49.0041 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en štěpán krátký, petr havlásek acta polytechnica ctu proceedings (i.) longitudinal plane of symmetry, (ii.) mid-span plane of symmetry, (iii.) continuously exposed area to air, (iv.) cyclically submerged area. figure 1. outline of the moisture transport (mt) model. expected strong nonlinear behavior of the moisture profile. for numerical simulations, the primary time step length is chosen to be 0.5 days. to improve convergence at the beginning of the simulations and humidity cycles, the step length is substantially reduced to 10−4 days and then gradually increased with a geometric progression until it reaches the primary step length. the evolution of ambient humidity is shown in figure 3. experimental data indicate that the rate of wetting is much higher than the rate of drying. to realistically capture this phenomenon, during wetting cycles, the surface factor f , which appears in the mixed boundary condition for moisture transfer, is set to a large number to effectively become a dirichlet boundary condition. otherwise, its value is set to 1 mm/day. 3. blind prediction 3.1. constitutive models in the blind prediction, the transport of moisture in concrete is described by a widely accepted model proposed by bažant and najjar [11] that assumes a linear sorption isotherm. in the present case, the moisture capacity is set to 130 kg m−3 (see figure 4). with this assumption, the governing equation for a diffusion of water vapor simplifies to: ∂h ∂t = ∇ · (c(h)∇h) , (1) where ∇· is the divergence operator, ∇h is the gradient of relative humidity, c(h) is the humidity-dependent diffusivity. (i.) longitudinal plane of symmetry, (ii.) mid-span plane of symmetry, (iii.) auxiliary weight, (iv.) pin support, (v.) mid-span symmetry bc, (vi.) longitudinal symmetry bc. figure 2. outline of the mechanical response (sm) model. figure 3. measured (blue) and simplified (red) evolution of ambient relative humidity, wetting cycles assigned to the immersed portion are shown in green. figure 4. experimental data of the sorption isotherm and its linear and bilinear approximation used in the blind prediction and calibrated model, respectively. 42 vol. 49/2024 modeling of concrete beams subject to wetting-drying cycles figure 5. evolution of the average moisture content in the companion specimens (m – measurement in grey scale) and in the blind prediction (s – simulation in colors). the numbers in the legend denote the height of the specimens. the time t corresponds to the material age. the model parameters are prescribed according to the recommended values in the fib model code 2010 [12] from the compressive strength of concrete. the mechanical behavior of concrete is described by a microprestress-solidification (mps) theory [13] modified to diminish the size-effect on drying shrinkage [14]. under hygrally sealed conditions and constant room temperature, this model reduces to the basic creep compliance function of the b3 model [15] whose parameters q1 − q4 were estimated from the composition of the concrete mixture. changes in relative humidity give rise not only to volume changes (shrinkage/swelling) but also to further creep (pickett effect). this additional creep is primarily controlled by the parameter k3 [14]. shrinkage strain and relative humidity are linearly linked via their rates: ε̇sh = kshḣ, (2) where ksh is a material parameter usually treated as a humidityand age-independent constant. in the initial blind prediction, the parameters of the mps model k3 and ksh were calibrated to match the evolution of shrinkage and drying creep given by the (cross-sectional) b3 model. 3.2. evolution of moisture content figure 5 shows the measured and predicted evolution of the average moisture content related to the total volume of the individual specimens. since the initial state is not known exactly, the vertical offset of the experimental measurements is determined by setting the last part of this evolution (which corresponded to a fully submerged condition and almost saturated state) to 130 kg m−3. comparison of the blind prediction to the experimental data demonstrates that figure 6. evolution of vertical mid-span deflection of beams in the experiment (grey scale) and in the blind prediction (colors). the numbers in the legend denote the height of the beams. the time t corresponds to the material age. even though the drying rate is captured quite accurately, the rate of moisture intake during the wetting stages is significantly underestimated. additionally, the moisture balance in the individual drying and wetting cycles is less than the measured values. this implies that the moisture capacity and/or the portion of the specimen that experiences wetting is underestimated. the latter statement is supported by the distribution of relative humidity over the cross section (not presented here), which shows that the fully saturated state is only at the boundary. only 2 mm from the surface, the humidity drops to 97.9 %. 3.3. mechanical response figure 6 presents the evolution of the mid-span deflection of the concrete beams subject to drying and wetting cycles. the readings are zeroed with respect to the instant just before applying the self-weight whose contribution to the overall deflection is negligible compared to the swellingand shrinkageinduced strains. the positive sign corresponds to downward deflection. in the figure, the experimental measurements (series m) on the three beams with different heights of 100, 150, and 200 mm are shown in gray scale while the blind prediction of the fem simulation (series s) is drawn in colors. considering the fact that the presented results are truly the results of a blind prediction, the agreement with the experimental data can be considered promising. during wetting cycles, graphical postprocessing revealed that the evolution of the moisture field in the lower part of the cross section is very similar for all three cross-section heights, which in turn implies that the swelling action is also similar. the computed data series are in correct order and correspond with the experiment; the highest deflection 43 štěpán krátký, petr havlásek acta polytechnica ctu proceedings is observed in the specimen with the smallest height of the cross section (and thus bending stiffness) and vice versa. a stiffer cross section providing stronger internal restraint produces higher magnitudes of selfequilibrated internal stresses. since the moisture intake rate was underestimated in the transport model, the mechanical response is also lagging behind the experiment. however, during drying, the computed deflection tends to return faster to the initial position than what is observed in the experiment. the contribution of creep to the total deflection is rather small, as demonstrated by the computed deflections at the end of the drying cycle. graphical post-processing of the mechanical analysis also revealed that shortly after the onset of wetting or drying, tensile stresses that exceed the tensile strength develop. the origin of these stresses stems from a highly variable field of eigenstrains produced by swelling or shrinkage which need to be balanced by a self-equilibrated normal stresses. the results obtained from an additional analysis in which tensile cracking was introduced using the concept of damage mechanics have shown that cracking leads to an increase in irreversible deformation at the end of the drying cycles. 4. improved modeling approach the results of the blind prediction with bažant-najjar model have shown that this approach does not give a realistic response when modeling drying and wetting cycles in which the concrete is in direct contact with the water. the primary reason for insufficient variation in moisture content is the presence of a linear sorption isotherm, which might be sufficient when modeling monotonous drying. to alleviate this problem, a nonlinear isotherm with a higher moisture capacity needs to be considered when approaching free saturation. however, the combination of bažant-najjar model with nonlinear isotherm would have led to an unrealistic response. for this reason, a more advanced constitutive model proposed by künzel [16] is adopted instead. this model was developed and validated for transport tasks similar to those in the case of the experiment presented. with the assumption of constant temperature and zero moisture sink, the moisture balance equation reads: ∂w ∂h · ∂h ∂t = ∇ · [( dw ∂w ∂h + δppsat ) ∇h ] , (3) where w is the moisture content, dw is the capillary transport coefficient [m2 s−1], δp is the water vapor permeability, psat is the saturation vapor pressure. the expression for the transport coefficient that is dominant at higher values of relative humidity – and so in the present case – reads: dw = 3.8 ( a wf )2 1000 w wf −1 , (4) where a is the water absorption coefficient, with typical values ranging between 0.1–1.0 kg m−2 day−0.5, wf is the moisture content at free saturation. the water vapor permeability of the building material that becomes more significant than the capillary transport at lower levels of relative humidity is defined as: δp = δ µ , (5) where δ is the water vapor permeability in air, µ is the water vapor diffusion resistance factor. 4.1. evolution of moisture content the calibration procedure of the model for moisture transport was complicated by the fact that the evolution moisture content was unknown during the first 408 days of drying when the short companion specimen was cut from the beam, which was then used to measure vertical deflection. for this reason, the calibration can be based only on the moisture balance during the three wetting and drying cycles and the final stage when the companion specimens were immersed in water until their weight stabilized. the best agreement was obtained with the bilinear sorption isotherm shown in figure 4 and parameters a = 0.85 kg m−2 day−0.5 and µ = 250. it should be noted that the measured data were adjusted to compensate for the rather high content of macropores estimated as 10 %. the three sizes of companion specimens were used for calibration, but a higher weight was given to the smallest size 100 mm (figure 7) as it was represented by three samples. compared to the blind prediction, the rate of moisture uptake became significantly accelerated with the künzel model. several snapshots of the graphical postprocessing showing the distribution of relative humidity over the symmetric half of the cross-section with height of 100 mm is shown in figure 8. the state at the beginning and end of the first wetting cycle is shown in the first two pictures, the red color corresponds to the relative humidity 100 % and the blue color to 45 %. initially, the relative humidity profile is extremely non-linear and the progress of wetting is very fast. at the end of the wetting cycle, the bottom half of the 100 mm cross-section is almost saturated. the wetting process is much more dominant than drying, the relative humidity has risen to ≈ 70 % even in the vicinity of the drying surfaces. at the end of the first drying cycle the distribution of relative humidity changes to nicely symmetric with ≈ 65 % in the core. 44 vol. 49/2024 modeling of concrete beams subject to wetting-drying cycles figure 7. evolution of the average moisture content of the accompanying prism with height of 100 mm. response of the calibrated model is shown in red, measurement in grey. (a). 1st day of wetting. (b). 80th day of wetting. (c). 130th day of drying. figure 8. distribution of relative humidity over a the symmetric half of a cross-section (height = 100 mm). 4.2. mechanical response several modifications of the mps model for concrete creep were used to compute the response induced by the changing moisture field and by the sustained loading produced by the self-weight. the objective of this study was to identify the governing mechanisms responsible for the behavior observed in the experiments. a detailed discussion can be found in [9]. graphical postprocessing of the results obtained with the model previously used in the blind prediction shown in figure 9 illustrates the magnitude of normal stresses in the axial direction of the beam that develop as a response to changing moisture field. the black areas indicate surprisingly high compressive stresses exceeding 8 mpa produced by restrained swelling strain shortly after exposure to wetting, while the gray color shows regions where the tensile stress has exceeded the uniaxial tensile strength 3.38 mpa. such stresses are produced only shortly after the onset of drying/wetting cycles when the distribution of eigenstrain is extremely nonlinear and the tensile stresses have not been relieved by relaxation. on the basis of these results, it can be concluded that creep can be treated as linear, but the incorporation of a softening behavior in tension is essential. the evolution of the mid-span deflection obtained with the mps model with tensile damage extension is shown in figure 10. the figure clearly shows that the acceleration in moisture transport has led to a significant acceleration in deflection rate, which is desired for the wetting phase but unintended for all drying phases. in the wetting phases, the computed response for beams with 150 mm and 200 mm height (shown in blue and green lines) is realistic, but the computed deflections reach only approx. 2 3 of the experimental values. on the other hand, the maximum values for the smallest beam with 100 mm cross section (red line in figure 10) are correct, but premature equilibra45 štěpán krátký, petr havlásek acta polytechnica ctu proceedings (a). 1st day of wetting. (b). 80th day of wetting. (c). 130th day of drying. figure 9. distribution of normal stress in the axial direction over a the symmetric half of a cross-section at the midspan (height = 100 mm). the gray areas highlight the region where the tensile strength of concrete has been exceeded. figure 10. evolution of vertical mid-span deflection of beams in the experiment (grey scale) and in the calibrated model (colors). the numbers in the legend denote the height of the beams. tion of the moisture profile has caused an too early decrease in deflection which is not observed in the experiment until the onset of drying. the rate of the structural response to drying is grossly overestimated. in the experiments, the deflection gradually decreases and would have continued if it had not been terminated by another wetting cycle. although it is not apparent from the computed results in figure 10, the contribution to tensile damage is substantial, without it the deflection at the end of the wetting cycles would have been much smaller (negative), which is a consequence of unrealistic creep strains triggered by moisture changes. 5. conclusion this paper investigated the influence of partial wetting and drying cycles on the mechanical response of plain concrete beams. the study has shown that a crucial element is a correct description of a moisture transport and that the constitutive model in the subsequent structural analysis is secondary. the results of the blind predictions demonstrated that the widely used bažant-najjar model for transport cannot accurately capture the evolution of the moisture content when the concrete is in direct contact with water. despite that, the prediction of the structural response which was driven by the (incorrect) moisture field was quite realistic both qualitatively and quantitatively. replacing the constitutive model for moisture transport and incorporating the bilinear sorption isotherm has significantly improved the agreement with the measured moisture balance. despite the effort to calibrate the mps model and introduce several improvements, the response was similar or worse to that of the blind prediction. based on the experimental and numerical results, the following statements can be formulated: (1.) the influence of tensile damage is significant and cannot be ignored in the analysis. (2.) at the onset of wetting, damage develops in the core area of the cross section occurring as a result of the expansion of the surface layer of the cross section. this effect increases with decreasing height of the cross section. (3.) the extent and depth of tensile cracking that develops after sudden exposure to drying increases with cross-sectional height. 46 vol. 49/2024 modeling of concrete beams subject to wetting-drying cycles acknowledgements the authors gratefully acknowledge financial support from the czech science foundation (ga čr), project number 21-03118s, and from the grant agency of the czech technical university in prague, project number sgs23/032/ohk1/1t/11. references [1] p. li, s. he. effects of variable humidity on the creep behavior of concrete and the long-term deflection of rc beams. advances in civil engineering 2018(1):8301971, 2018. https://doi.org/10.1155/2018/8301971 [2] h. cagnon, t. vidal, a. sellier, et al. drying creep in cyclic humidity conditions. cement and concrete research 76:91–97, 2015. https://doi.org/10.1016/j.cemconres.2015.05.015 [3] l. vandewalle. concrete creep and shrinkage at cyclic ambient conditions. cement and concrete composites 22(3):201–208, 2000. https://doi.org/10.1016/s0958-9465(00)00004-4 [4] g. pickett. the effect of change in moisture-content on the creep of concrete under a sustained load. in journal proceedings of the american concrete institute, vol. 38, pp. 333–356. 1942. https://doi.org/10.14359/8607 [5] y. song, q. wu, f. agostini, et al. concrete shrinkage and creep under drying/wetting cycles. cement and concrete research 140:106308, 2021. https://doi.org/10.1016/j.cemconres.2020.106308 [6] s. asamoto, a. ohtsuka, y. kuwahara, c. miura. study on effects of solar radiation and rain on shrinkage, shrinkage cracking and creep of concrete. cement and concrete research 41(6):590–601, 2011. https://doi.org/10.1016/j.cemconres.2011.03.003 [7] r. l’hermite, m. mamillan. nouveaux résultats de recherches sur la déformation et la rupture du betón [in french; new research results on deformation and failure of concrete]. annales de l’institut technique du batiment et des travaux publics 18(207):325–359, 1965. [8] l. dohnalová, p. havlásek, v. šmilauer. behavior of predried mature concrete beams subject to partial wetting and drying cycles. in acta polytechnica ctu proceedings, vol. 34, pp. 1–5. 2022. https://doi.org/10.14311/app.2022.34.0001 [9] štěpán krátký. chování betonových nosníků vystavených cyklům částečného namáčení a vysychání: numerické modelování a experiment [in czech; behavior of concrete beams subject to partial wetting and drying cycles: numerical modeling and experiment]. master’s thesis, czech technical university in prague, 2023. [10] b. patzák. oofem home page, 2000. [2023-01-01]. http://www.oofem.org [11] z. p. bažant, l. j. najjar. nonlinear water diffusion in nonsaturated concrete. materials and structures 5(1):3– 20, 1972. https://doi.org/10.1007/bf02479073 [12] fédération internationale du béton. model code 2010 – final draft, volume 2. no. 66 in fib bulletin. 2012. isbn 978-2-88394-106-9. [13] z. p. bažant, a. b. hauggaard, s. baweja, f.-j. ulm. microprestress-solidification theory for concrete creep. i: aging and drying effects. journal of engineering mechanics 123(11):1188–1194, 1997. https://doi.org/10.1061/(asce)07339399(1997)123:11(1188) [14] p. havlásek. creep and shrinkage of concrete subjected to variable environmental conditions. ph.d. thesis, czech technical university in prague, 2014. [15] z. p. bažant, s. baweja. creep and shrinkage prediction model for analysis and design of concrete structures: model b3. in adam neville symposium: creep and shrinkage – structural design effects. 2000. [16] h. m. künzel. simultaneous heat and moisture transport in building components: oneand two-dimensional calculation using simple parameters. ph.d. thesis, university of stuttgart, 1995. 47 https://doi.org/10.1155/2018/8301971 https://doi.org/10.1016/j.cemconres.2015.05.015 https://doi.org/10.1016/s0958-9465(00)00004-4 https://doi.org/10.14359/8607 https://doi.org/10.1016/j.cemconres.2020.106308 https://doi.org/10.1016/j.cemconres.2011.03.003 https://doi.org/10.14311/app.2022.34.0001 http://www.oofem.org https://doi.org/10.1007/bf02479073 https://doi.org/10.1061/(asce)0733-9399(1997)123:11(1188) https://doi.org/10.1061/(asce)0733-9399(1997)123:11(1188) acta polytechnica ctu proceedings 49:41–47, 2024 1 introduction 2 numerical modeling 3 blind prediction 3.1 constitutive models 3.2 evolution of moisture content 3.3 mechanical response 4 improved modeling approach 4.1 evolution of moisture content 4.2 mechanical response 5 conclusion acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.34.0075 acta polytechnica ctu proceedings 34:75–79, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague sulphate corrosion of cement pastes with a high content of waste marble powder zdeněk prošeka,b,∗, pavel tesáreka a czech technical university in prague, faculty of civil engineering, thákurova 7, 166 29 prague 6, czech republic b czech technical university in prague, university centre for energy efficient buildings, třinecká 1024, 273 43 buštěhrad, czech republic ∗ corresponding author: zdenek.prosek@fsv.cvut.cz abstract. the article focuses on sulphate corrosion of cement pastes with a high content of waste micronized marble powder. micronized marble powder is used as a substitute for portland cement cem i 42.5r. the amount of micronized marble powder used is from 5 to 50 wt. %. the research also tested a reference mixture composed only of portland cement. the used accelerated sulphate corrosion consists of cyclic saturation and drying with 10% na2so4 solution for 56 days. during sulphate corrosion, samples are placed in a controlled environment at 5 ± 1 ◦c and 85 % relative humidity to allow thaumasite to form. during cycling, samples were continuously visually monitored and documented. subsequently, after 56 days, the samples were destructively tested for compressive strength. the research revealed a negative effect of micronized marble powder on the resistance to sulphate corrosion, where in contrast to the reference mixture the decrease in compressive strength is up to twofold. keywords: sulphate corrosion, thaumasite, cement pastes, visual inspection, compressive strength. 1. introduction currently, the most common way to use recycled marble waste is to apply it to cement-based composite materials. cement-based composite materials (in the form of monolithic reinforced concrete, prefabricated parts, etc.) are currently probably the most widespread and most used building material in world. from the point of view of sustainable development, three basic components (cement, sand and water) are needed for the production of cement composite. however, their extraction, treatment and processing have negative effect on nature and the environment [1]. specifically, cement production accounts for 7 % of global co2 production and large amounts of nox. the use of recycled marble in the form of admixtures will significantly reduce the cost of raw material for concrete [2]. for example, the use of waste marble powder in a cement composite reduces co2 emissions by 12 % and reduces the cost of the composite from 40 usd/m3 on 33 usd/m3 [3]. one of the most common applications of marble powder is in the form of admixtures, where it forms a substitute for a binder. when replacing up to 15 wt. % there is no significant decrease of mechanical properties [4–7] and when replacing 50 wt. % cement pastes have twice value the flexural strength [5]. according to the čsn en 206 standard, marble powder is considered as an inert admixture, which does not participate in the hydration of cement. in the case of white and beige marble, it is a mineral filler that contains more than 75 % caco3 [8]. the chemical composition is very similar to limestone, which does not behave completely inertly during the hydration of cement [9]. fine limestone grains act in the cement as nucleation centers for ch crystals and accelerate the hydration of silicate and aluminate phases. as a result, it affects the initial increase in the strength of the cement composite and accelerates the setting and curing time. in addition, even finely ground limestone can participate in hydration processes. it is mainly the hydration of c3s from clinker to form calcium carbonate aluminate hydrate (3cao · al2o3 · 3caco3 · 32h2o) in cements with a higher content of c3a. tricarbonate can transform into a more stable monocarbonate [10] in later stages. the biggest disadvantage of limestone, and thus of sediment formed from caco3, is its sulphate corrosion in combination with silicates in a c-s-h gel. during sulphate corrosion at lower temperatures, crystalline thaumasite is formed (casio3 · caco3 · caso4 · 15h2o), which by volume changes it disrupts the structure, thus deteriorating the mechanical properties and integrity of the cement composite [11]. this article investigates the sulphate corrosion resistance of a cement composite containing waste marble powder. marble (crystalline caco3) has a similar chemical composition to limestone (sedimentary caco3) and thus thaumisite can form during sulphate corrosion, which negatively affects the durability of structures. the sulphate corrosion of a cement composite containing marble powder and limestone powder was deal in an experimental study by m. uysal et al. [12]. the results of the experiments were the 75 https://doi.org/10.14311/app.2022.34.0075 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en zdeněk prošek, pavel tesárek acta polytechnica ctu proceedings set percentage cem i 42,5r micronized marble water ratio replacement [%] [kg] powder [kg] w/b m0 0 3.00 0.00 0.35 m5 5 2.85 0.15 0.32 m10 10 2.70 0.30 0.32 m15 15 2.55 0.45 0.32 m50 50 1.50 1.50 0.32 table 1. composition of tested sets. 0,e+0 5,e+4 1,e+5 2,e+5 2,e+5 3,e+5 3,e+5 4,e+5 4,e+5 5 15 25 35 45 55 65 75 r e la ti ve in te si ty ( co u n ts ) position 2θ [°] pc wmp ● caco3 * quartz + c3s □ c2s * ● ● ● ● ● ●* ● +□ +□ + +□ +□ □ + 4.0 · 105 3.5 · 105 3.0 · 105 2.5 · 105 2.0 · 105 1.5 · 105 1.0 · 105 0.5 · 105 0.0 +□+□ ● marble powder cem 42.5 r figure 1. xrd patterns for portland cement cem 42.5r and micronized marble powder. percentage loss of compressive strength, where the samples with reference cement and samples with limestone and marble powder had the greatest loss of strength, namely 13 %. samples with 30 wt. % marble powder as a substitute for cement, which had better mechanical properties than the reference samples. the loss of strength in this mixture was approximately 9 %. the authors attributed the improvement of the resistance to sulphate corrosion to the reduced porosity and especially to the permeability of the cement composite. 2. materials and samples the experiments were performed on cement pastes, where part of the cement was replaced by waste micronized marble powder. the used portland cement was cem 42.5r radotín. it is a cement with a high c3s content of 74.6 % and a lower c3a content of 8.1 %, c2s 7.2 %, c4af 8.5 % and mgo 1.6 %. the micronized marble powder was created by recycling waste generated from the process of mining and processing of marble stone. the recycling of marble waste was performed using a high-speed mill, which guarantees the separation of individual clumps, reduces the grain size of the material (0–50 microns) and homogenizes the waste material. the research used a high-speed mill from lavaris s.r.o. s1000. according to xrf analysis, micronized marble powder has a high cao content of 62.3 % together with a high loss of ignition of 33.7 %. this indicates a high content of caco3, which can react to thaumasite during sulphate corrosion. xrd analysis was performed for confirmation. figure 1 shows a diffractogram of portland cement and micronized marble powder. the results confirmed the previous assumption. thus, the micronized marble powder is composed of 98 wt. % caco3 and the remaining 2 wt. % are other clay minerals contained in marble. the composition of all tested mixtures is shown in table 1. the first mixture was a reference and contained only portland cement. in other mixtures, part of the cement was replaced by micronized marble powder in various weight percentages, from 5 to 50 %. the ratio of water to the total weight of the dry mixture (water-to-binder ratio w/b) was designed to maintain the same workability of the fresh mixtures in flow table test. the flow varied for individual mixtures up to a maximum size of 5 mm. specifically, the flow was determined to be 180 mm after 15 impacts. all samples tested had nominal dimensions of 40×40×40mm. the samples were produced in steel triple molds whit dimensions of 40×40×160 mm according to čsn en 196-1 [13] and subsequently cut to final dimensions. 6 samples were taken from each set 76 vol. 34/2022 sulphate corrosion of cement pastes m0 the 1st day the 14th day the 28th day the 56th day m5 m10 m15 m50 figure 2. visual determination of sulphate corrosion. (mixture). on the second day after production, the test specimens were demolded and stored loose in a laboratory environment at a temperature of 22 ± 1 ◦c and a relative humidity of 50 ± 2 %. after 28 days, the samples were cut and subjected to sulfate corrosion resistance testing. 77 zdeněk prošek, pavel tesárek acta polytechnica ctu proceedings 0 0.2 0.4 0.6 0.8 1 1.2 m0 m5 m10 m15 m50 w e ig h t lo ss [ % ] figure 3. percentage weight loss due to sulphate corrosion for 56 days, with standard deviations. 0 2 4 6 8 10 12 14 16 18 m0 m5 m10 m15 m50 c o m p re ss iv e s tr en gt h lo ss [ % ] figure 4. percentage decrease in compressive strength due to sulphate corrosion for 56 days, with standard deviations. 3. experimental methods depending on the composition of the recycled material, the resistance to sulphate corrosion was solved. astm c1012-04 [14] has been modified for this purpose. the test samples were stored in a 10% na2so4 solution. due to possible thaumasite formation, the ambient temperature was set to 5 ± 1 ◦c and the relative humidity was 85 ± 5 %. the whole process for accelerated sulfate corrosion was taken from nielsen et al. [15]. the process uses a cycle of alternating saturation and drying of the samples, where one cycle consists of seven days of saturation of the sample with the solution and then another seven days of its natural drying. the samples were visually inspected during cycling and assessed after each completed cycle. subsequently, the samples were weighed and destructively tested for determining compressive strength. the compressive strength was determined using a heckert (fp100 model) hydraulic press. the compressive strength was determined by a uniaxial pressure test. testing was controlled by displacement at a constant rate of 3 mm/min. the test was performed on 6 samples with dimensions of 40×40×40 mm. the compressive strength was determined according to čsn en 196-1 [13]. the compressive strength of a material is the amount of stress achieved at the point of failure when the integrity of the material is lost. the compressive strength was calculated from the maximum force achieved during the test fc,max as: fc = fc,max ab , (1) where: fc is the compressive strength in pa, fc,max is the maximum force in n, a is the width of the sample in m, b is the height of the sample in m. corrosion resistance coefficients were calculated from the obtained compressive strengths of samples in a corrosive environment and samples stored in water, as: kko = fc fref , (2) where: kko is corrosion resistance coefficients, fc is the compressive strength in 10% na2so4 solution [pa], fref is the compressive strength in water [pa]. 4. results and discussion the process of visual examination of the tested mixtures can be observed in figure 2. the monitored samples m50, m15 and m10 showed visible formation of white crystals on their surface after 14 days of accelerated sulphate corrosion in 10% na2so4, . reaction of sodium sulfate with the cement matrix at low temperatures produced thaumasite crystals with a larger volume. the first cracks formed in all examined samples, except for the reference mixture, after 28 days of sulfate corrosion. these were mainly capillary cracks caused by the crystallization of thaumasite. there was no further expansion of cracks after another 28 days (day 56), but leaching of calcium, which subsequently crystallized on the surface of the tested materials. after 56 days, the cement pastes were tested for weight loss due to calcium leaching (figure 3) and compressive strength (figure 4), which are used to determine the corrosion resistance coefficient. all mixtures with marble powder lost weight. the largest weight loss was in the sample m50, namely 0.95 wt. %. this decrease indicates deteriorated sulfate corrosion resistance. the assumption was subsequently confirmed by the compressive strength test, where the sample m50 achieved a 15.5 % lower compressive strength after 56 days in a 10% solution of na2so4. the corrosion resistance coefficient was 78 vol. 34/2022 sulphate corrosion of cement pastes 0.845. the remaining materials with micronized marble powder achieved a corrosion resistance coefficient of 0.9. the results are in agreement with other authors who have dealt with sulphate corrosion in their research [16]. in contrast to accelerated sulphate corrosion, they used the traditional method, which lasted one (whole) year. after a year, samples composed of 20 wt. % caco3 had lower compressive strength by 20 %. 5. conclusions this work is focused on the effect of waste micronized marble powder on the sulphate corrosion resistance of cement pastes. accelerated sulfate corrosion at lower temperatures was used. cement pastes are composed of mixed cement, where micronized marble powder replaces portland clinker. the micronized marble powder was used in 4 different concentrations, namely: 5 wt. %, 10 wt. %, 15 wt. % and 50 wt. %. based on the results, it can be concluded that: • accelerated sulphate corrosion was used, where the results of the reference mixture were comparable to the commonly used sulphate corrosion test. • samples with micronized marble powder showed visible formation of white crystals on their surface after 14 days and the first cracks appear after 28 days. • all samples with micronized marble powder indicate deteriorated sulfate corrosion resistance by losing your weight. • samples with micronized marble powder had lower values of compressive strength from 9 to 15 % after sulphate corrosion. the reference mixture had a compressive strength value only lower about 5 %. future research will build on the results in this article. in the next work, concrete mixtures will be tested. in addition, future research will use an electron microscope to describe the phase changes in the structure of the cement composite. acknowledgements this paper was financially supported by czech technical university in prague under no. sgs project sgs19/148/ohk1/3t/11. the authors also thank lavaris ltd. for the supplied samples and materials. references [1] a. rana, p. kalla, h. k. verma, j. k. mohnot. recycling of dimensional stone waste in concrete: a review. journal of cleaner production 135:312–331, 2016. [2] j. lederová, p. leber. využití průmyslových odpadních materiálů při výrobě stavebních hmot. stavebnictví 8(04), 2008. [3] h. ş. arel. recyclability of waste marble in concrete production. journal of cleaner production 131:179–188, 2016. [4] m. singh, a. srivastava, d. bhunia. an investigation on effect of partial replacement of cement by waste marble slurry. construction and building materials 134:471–488, 2017. https://doi.org/10.1016/j.conbuildmat.2016.12.155. [5] z. prošek, v. nežerka, p. tesárek. enhancing cementitious pastes with waste marble sludge. construction and building materials 255:119372, 2020. [6] m. j. munir, s. m. s. kazmi, y. f. wu. efficiency of waste marble powder in controlling alkali–silica reaction of concrete: a sustainable approach. construction and building materials 154:590–599, 2017. [7] a. khodabakhshian, j. brito, m. ghalehnovi, e. asadi shamsabadi. mechanical, environmental and economic performance of structural concrete containing silica fume and marble industry waste powder. construction and building materials 169:237–251, 2018. https://doi.org/10.1016/j.conbuildmat.2018.02.192. [8] čsn en 206 +a1. beton specifikace, vlastnosti, výroba a shoda. praha: úřad pro technickou normalizaci, metrologii a státní zkušebnictví 2018. [9] r. hela. příměsi do betonu. beton tks 2:4–10, 2015. [10] j. lukáš. současné trendy ve stavebnictví, betony speciálních vlastností. brno, 2007. [11] j. bensted. thaumasite background and nature in deterioration of cements, mortars and concretes. cement and concrete composites 21(2):117–121, 1999. https://doi.org/10.1016/s0958-9465(97)00076-0. [12] m. uysal, m. sumer. performance of self-compacting concrete containing different mineral admixtures. construction and building materials 25(11):4112–4120, 2011. [13] čsn en 196-1. metody zkoušení cementu. část 1: stanovení pevnosti. praha: úřad pro technickou normalizaci, metrologii a státní zkušebnictví 2016. [14] astm c1012 04. standard test method for length change of hydraulic-cement mortars exposed to a sulfate solution. astm standard test method 2004. https://doi.org/10.1520/c1012-04. [15] p. nielsen, s. nicolai, a. darimont, x. kestemont. influence of cement and aggregate type on thaumasite formation in concrete. cement and concrete composites 53:115–126, 2014. https://doi.org/10.1016/j.cemconcomp.2014.06.011. [16] m. vyšvařil, m. rovnaníková. odolnost jemnozrnných betonů vůči síranové korozi v odpadních systémech. tzb-info 2017. dostupné z: https://voda.tzb-info.cz/materialy-vodakanalizace/15420-odolnost-jemnozrnnych-betonu-vucisiranove-korozi-v-odpadnich-systemech. 79 https://doi.org/10.1016/j.conbuildmat.2016.12.155 https://doi.org/10.1016/j.conbuildmat.2018.02.192 https://doi.org/10.1016/s0958-9465(97)00076-0 https://doi.org/10.1520/c1012-04 https://doi.org/10.1016/j.cemconcomp.2014.06.011 acta polytechnica ctu proceedings 34:75–79, 2022 1 introduction 2 materials and samples 3 experimental methods 4 results and discussion 5 conclusions acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.34.0063 acta polytechnica ctu proceedings 34:63–68, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague methylene blue test and adsorption capacity of clays tereza plaček otcovská∗, barbora mužíková, pavel padevět czech technical university in prague, faculty of civil engineering, thákurova 7, 166 29 prague 6, czech republic ∗ corresponding author: tereza.otcovska@fsv.cvut.cz abstract. this article presents results of experimental measurements focused on an ability of clay to adsorb methylene blue dye. this adsorption capacity is determined by a methylene blue test. adsorption capacity could be a specific characteristics of individual types of clay which are basic component of earth. therefore, the methylene blue test could be a good method to identify the composition of an unknown earth. composition of earth is very important for example for final properties of unfired earth structures. the basic common element in all experiments was at least one of the elementary clay minerals (montmorillonite, kaolinite, illite). in the case of adsorption ability measurements, it was determined what is the maximum ability of a given clay to adsorb the methylene blue dye. the highest adsorption capacity was determined for montmorillonite clay, lower for kaolinite clay and the lowest for illite clay. keywords: methylene blue dye, methylene blue test, clay, adsorption capacity, unfired earth. 1. introduction unfired earth is a historic buildings material. unfired earth structures are located all over the earth and there are a lot of famous historic buildings like great wall of china or aït ben-haddou village [1]. unfired earth industry is a minority part of building industry, but unfired earth has a legitimate place among other building materials. unfired earth can be used for load-bearing or non-load-bearing structures with positive effect on the quality of the indoor environment. unfired earth is often used as a architectural element [2–6]. big number of events organized on the topic of unfired clay are a proof of the interest in this building material. the number of published scientific works on the topic of unfired earth construction is growing (fig. 1). there are also a number of companies whose field of interest is unfired clay [7–9]. basic component of earth are clays. montmorillonite, kaolinite and illite are basic clay minerals. adsorption capacity of individual kinds of clays is different. therefore, adsorption capacity could be the characteristic used for analysis of the composition of an unknown earth. this analysis could be very useful for a progress of earth structures, because earth composition has significant influence on final properties of earth structures and the composition of natural earth is very variable [2, 3, 10]. the methylene blue test is a method for measuring of an adsorption capacity. adsorption of methylene blue dye (mb) (c16h18n3scl) on a clay surface is a ground of the methylene blue test. methylene blue dye in a water solution is a cationic dye that can be adsorbed on negatively charged clay surfaces. specific surface area of material, or other characteristics that figure 1. number of publications on the topic of unfired earth in individual yearse. correlates with the specific surface area, is determined by the amount of methylene blue dye adsorbed [11, 12]. specific surface area is a surface area of a solid relative to the unit weight of the solid. the knowledge of the specific surface area is important because there is a correlation between the specific surface area and binder properties, plasticity, shrinkage of unfired earth or adsorption of pesticides and other harmful substances by clays. amount of mb adsorbed depends on a concentration of the methylene blue dye solution, temperature, ph, or contact time between clay and mb [12–14]. 63 https://doi.org/10.14311/app.2022.34.0063 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en t. plaček otcovská, b. mužíková, p. padevět acta polytechnica ctu proceedings ][h!] figure 2. burette with methylene blue solution. 2. description of the methylene blue test the standard čsn en 933-9 [15] describes the methylene blue test, but this procedure was adapted to laboratory equipment and modified on the basis of experience from previous measurements. concentration of dye solution 10 g/kg was used for experimental measurements. earth samples were in the form of a suspension of test material and demineralized water. the basic principle of this experiment is adsorption of mb on the surface of clays. the water in the earth samples serves only as a medium to allow adsorption. dye solution is placed in a burette and then mb is dispensed into earth sample (fig. 2, 3). after this, earth sample with the solution is stirred on a magnetic stirrer for the prescribed stirring time (fig. 4). after the prescribed stirring time has elapsed a drop of the suspension is placed on a filter paper (fig. 5). this drop has a dark blue colour. if a distinct light blue ring does not appear (fig. 6), another dye is added to the earth sample and the earth sample with the solution is stirred again. when the light blue ring appears (fig. 7), earth sample is stirred until the ring disappears or for the prescribed stirring time. methylene blue test is finished when a distinctive light blue ring is still visible after prescribed stirring time has elapsed. amount of adsorbed mb in grams by amount of clay in kilograms is a result of methylene blue test. this value of adsorbed mb by 1 kilogram of clay is figure 3. adding mb solution into the test material suspension. figure 4. controlled stirring of mb solution with the tested material suspension. defined in equation 1. mmm,ads = mmm,s · 1000 msc (1) 2.1. tested material and evaluation of obtained results adsorption capacity was tested for three basic clay minerals. this clay minerals are represented by four 64 vol. 34/2022 methylene blue test and adsorption capacity of clays figure 5. drops of mb solution with the suspension on a filter paper. figure 6. slightly pronounced light blue ring. kinds of clay: montmorillonite (gem), kaolinite (b4), illite-kaolinite (kr) and illite (agl) (fig. 8). chemical composition of clays is given in the table 1. solid component of the earth samples is clay or clay and sand. composition of earth samples and number of experimental measurements are shown in the table 2. minimal number of measurements is 3. outliers in the statistic file were removed by dixon’s test. number of statistical file values after using dixon’s test is 3. average values, standard deviation and relative standard deviation were calculated from obtained results without outliers. furthermore, confidence intervals for all average figure 7. distinctive light blue ring. figure 8. four kinds of tested clay. values were calculated (equation 2). reliability factor of 0.95 was used. l1,2 = x± σ · tk√ n (2) 3. adsorption capacity of individual kinds of clays adsorption capacity significantly depends on individual kinds of clay in the earth sample and the amount of clay in the earth sample. obtained results are shown in the table 3. maximum adsorption capacity is determined for earth sample gem-100 (168.4 ± 13.5 g/kg). this earth sample contains 100 % montmorillonite clay. lowest adsorption capacity is determined for agl25 earth sample (5.7± 0.4 g/kg). this earth sample contains 25 % illite clay and 75 % sand. 65 t. plaček otcovská, b. mužíková, p. padevět acta polytechnica ctu proceedings kind of sio2 al2o3 fe2o3 tio2 cao mgo na2o k2o clay [%] [%] [%] [%] [%] [%] [%] [%] montmorillonite 50.51 31.20 3.37 0.86 0.40 0.42 0.08 1.62 kaolinite 58.13 24.29 4.70 1.21 0.13 0.50 0.50 3.96 illite-kaolinite 59.31 24.71 3.37 1.09 0.19 0.40 0.30 2.82 illite 56.57 18.40 9.72 1.16 1.12 2.54 0.18 2.91 table 1. chemical composition of clays. clay marking clay/ amount amount num. of num. of of the sand of clay of sand measurements statistical file earth ratio [g] [g] values after sample dixon’s test montmorillonite gem-100 100/0 15 0 6 6 gem-75 75/25 11.25 3.75 3 3 gem-50 50/50 7.5 7.5 4 4 gem-25 25/75 3.75 11.25 5 4 kaolinite b4-100 100/0 15 0 5 4 b4-75 75/25 11.25 3.75 3 3 b4-50 50/50 7.5 7.5 5 4 b4-25 25/75 3.75 11.25 3 3 illitekr-100 100/0 15 0 6 6 kaolinite kr-75 75/25 11.25 3.75 4 4 kr-50 50/50 7.5 7.5 4 4 kr-25 25/75 3.75 11.25 5 5 illite agl-100 100/0 15 0 4 4 agl-75 75/25 11.25 3.75 4 4 agl-50 50/50 7.5 7.5 6 6 agl-25 25/75 3.75 11.25 5 5 table 2. tested material overview. clay mark of earth average standard relative standard sample value deviation deviation [g/kg] [g/kg] [%] montmorillonite gem-100 168.4 13.5 8.0 gem-75 130.9 8.5 6.5 gem-50 98.0 12.1 12.3 gem-25 47.2 2.8 5.9 kaolinite b4-100 81.4 6.0 7.4 b4-75 60.3 2.1 3.5 b4-50 43.4 0.6 1.5 b4-25 20.5 0.7 3.2 illitekr-100 49.6 4.9 9.8 kaolinite kr-75 38.0 2.2 5.8 kr-50 25.4 2.2 8.6 kr-25 13.6 1.1 8.1 illite agl-100 19.2 1.4 7.5 agl-75 14.2 0.9 6.0 agl-50 9.6 0.6 5.8 agl-25 5.7 0.4 7.7 table 3. obtained results of the adsorption capacity. 66 vol. 34/2022 methylene blue test and adsorption capacity of clays when comparing the adsorption capacity of individual kinds of clay, it was found that montmorillonite clay gem has the highest adsorption capacity of mb. the adsorption capacity of clays decreases in the following order: kaolinite b4, illito-kaolinite kr, illite agl (fig. 9). figure 9. obtained results of adsorption capacity. when comparing the adsorption capacity of the material in the gem-100, b4-100, kr-100 and agl100 batches, it was calculated that the adsorption capacity of kaolinite clay b4 is 52 % lower (i. e. 48 %) compared to montmorillonite clay gem, illitokaolinite clay kr by 71.5 % lower (i.e. 29 %) and illite clay agl by 89 % lower (i.e. 11 %). as can be seen from the figure 9, the same trend was observed for earth samples with clay content of 75 %, 50 % and 25 %. the confidence interval was calculated for each adsorption capacity average value of individual earth samples. figure 10 shows expected dependence of amount of adsorbed mb on the amount of clay in investigated material for all four clays, indicating area of confidence interval for each clay. methylene blue dye adsorption curve of each clay was created by connecting average values of individual earth sample adsorption capacities (the amount of clay in the earth sample 25 %, 50 %, 75 %, and 100 %). the outside values of the confidence interval range were created by connecting calculated values of the confidence interval limits l1, resp. l2. it can be seen from the graph (fig. 10) that, based on the methylene blue test, the examined clays can be well distinguished from each other. the confidence intervals of the individual clays do not overlap at any point. adsorption capacity is an individual characteristics of each clay. for each clay, the relationship between amount of dye adsorbed and amount of clay in earth sample was investigated. it is evident that in all four cases the dependency approaches a linear function. the graph in the figure 10 shows the linear trend line for individual clays (y = kx + q). in all cases, the regression curves copy the measured values. therefore, it can be assumed that the adsorption of mb is linear depending on the amount of clay in the earth sample. correlation coefficient r2 ∈ 〈0; 1〉 is an indicator of whether the regression curve corresponds to the measured values. when correlation coefficient approaches 1, the regression curve correlates with the measured values. in all investigated cases, r2 ≥ 0.9912. 4. conclusions adsorption capacity of individual kinds of clay is clearly different. therefore, adsorption capacity is a suitable characteristics for analysing the composition of an unknown earth. montmorillonite clay has the highest adsorption capacity. the adsorption capacity decreases for clays in the following order: kaolinite clay, illote-kaolinite clay, illite clay. based on the adsorption capacity itself, it is not possible to determine whether, for example, there is little montmorillonite or a lot of illite in an unknown earth. adsorption capacity is an important guide in the analysis of the composition, however, it is necessary to know other characteristics that differentiate the different types of clays. this other characteristics could be adsorption capacity time and dependence of the amount of adsorbed dye on time. list of symbols mmm,ads amount of adsorbed mb by 1 kg of clay [ g kg ] mmm,s amount of mb in solution which is added to earth sample [g] msc amount of clay in earth sample [g] l1,2 limits of confidence interval tk critical value of student’s t-distribution n number of statistical file values x, y coordinate system values k, q cooefficients of straight of line r2 correlation coefficient acknowledgements the financial support of this experiment by the czech science foundation (gačr project no. 18-10884s) and faculty of civil engineering, czech technical university in prague (sgs project no. sgs19/148/ohk1/3t/11) is gratefully acknowledged. references [1] p. jaquin, c. augarde. earth building: history, science and conservation. [2] g. minke. building with earth: design and technology of a sustainable architecture. ökobuch verlag, staufen. [3] i. žabičková. hliněné stavby, vol. 2002. era 21. [4] h. niroumand, m. f. m. zain, m. jamil. modern rammed earth in earth architecture 457–458:399–402. 67 t. plaček otcovská, b. mužíková, p. padevět acta polytechnica ctu proceedings figure 10. confidence intervals for obtained values of adsorption capacity. [5] j. růžička, j. diviš. the influence of building materials on relative humidity of internal microclimate 290:012–029. [6] a. lajčíková. syndrom nemocných budov. [7] m. navrátil. hliněné omítky picas výroba, vývoj, prodej statistika prodeje. [8] cihly pro obvodové a vnitřní zdivo | heluz. [9] p. stejskal. e-mail consultation with heluz company. [10] j. konta. jílové minerály československa. nakladatelství čsl. akad. věd, 1st edn. [11] j. c. santamarina, k. a. klein, y. h. wang, e. prencke. specific surface: determination and relevance 39(1):233–241. [12] y. yukselen, a. kaya. suitability of the methylene blue test for surface area, cation exchange capacity and swell potential determination of clayey soils 102(1):38–45. [13] k. bellir, m. bencheikh-lehocine, a.-h. meniai. removal of methylene blue from aqueous solutions using an acid activated p. 8. [14] g. blanchard, m. maunaye, g. martin. removal of heavy metals from waters by means of natural zeolites 18(12):1501–1507. [15] e. výbor pro normalizaci. čsn en 933-9, tests for geometrical properties of aggregates – part 9: assessment of fines – methylene blue test . 68 acta polytechnica ctu proceedings 34:63–68, 2022 1 introduction 2 description of the methylene blue test 2.1 tested material and evaluation of obtained results 3 adsorption capacity of individual kinds of clays 4 conclusions list of symbols acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2024.46.0053 acta polytechnica ctu proceedings 46:53–64, 2024 © 2024 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague development and causes of migration in lagging regions in the years 2017–2021: a case study of the slovak republic michal levickýa,∗, viera papcunováa,b, marta urbaníkováa, annamária kozlokováa a constantine the philosopher university in nitra, faculty of natural sciences and informatics, institute of economics and management, tr. a. hlinku 1, 949 01 nitra, slovak republic b masaryk university, faculty of economics and administration, department of regional economics, lipová 41a, 602 00 brno-pisárky, czech republic ∗ corresponding author: mlevicky@ukf.sk abstract. migration represents a process where people leave their homes for various reasons. the aim of the article is to evaluate the level and development of migration in slovakia and to identify its causes in the gemer region, which is among the least economically productive regions of slovakia. we quantified the development and level of migration based on secondary data through selected analytical indicators of migration. we determined the motivations of the residents of the gemer region to migrate using a questionnaire survey. until 2004, slovakia was an emigration country, but in recent years it has turned into a destination country. however, the situation is not the same for all regions of the country. a negative migration balance persists in the gemer region, although restrictions related to the coronavirus pandemic have slowed this trend. we found that up to 21 % of respondents from the region migrate for work. up to 45 % of respondents want to move out of the region, while in 64 % of cases the migration is motivated by economic reasons. the opinion on migration is influenced by the gender of the respondent but is not related to the level of education achieved. keywords: regional differences, migration, lagging regions. 1. introduction migration is a historical social phenomenon that has political, economic and social implications, but it also affects national security by changing the social composition of countries. migration can be a catalyst for conflict but also for national development [1]. migration is most often classified according to whether or not an individual or group moves inside or outside the borders of a country, whether or not a national border is crossed. based on these criteria, a distinction is made between international and internal migration. internal migration takes place at the level of the state, regions, districts and municipalities. on the other hand, international migration includes intercontinental and intracontinental [2]. internal migration can generally be seen as the movement of people from one geographical area to another for the purpose of permanent or temporary residence. the study of this form of migration is therefore important for understanding population distribution, growth and urbanization of territories [3]. most studies suggest that migration is primarily motivated by economic factors. in developing countries, low agricultural incomes and unemployment are considered as the primary factors that lead migrants to move to developed areas with higher employment opportunities. thus, almost all studies agree that most migrants move in search of better economic opportunities [4]. economic migration is triggered by the search for a better livelihood, seeking better conditions, compared to financial opportunities in the country of origin. in all cases, economic migration is voluntary. political migration can be based on armed conflict or a political decision that leads to the deportation, relocation or resettlement of a country’s population this is the case of forced migration. in addition, political migration can also occur when a person, although not in an immediate situation of forced displacement, comes from a territory where human and political rights are violated [5]. migrants are not a homogeneous group. rather, migrant categories are often defined by migrants’ reasons for leaving home. categories of migrants include refugees, economic migrants, migrants fleeing war, environmental migrants, climate migrants, and migrants seeking to reunite with family. further, a migrant may have multiple overlapping reasons for their movement. international refugee law protects some individuals whose flight is perceived as forced and completely excludes others whose flight is perceived as voluntary [6]. the most common reason for migrating from country to country is job opportunities. this is confirmed by several studies. for example, western european countries, which felt the economic devastation caused by the second world war profoundly, were forced to import labour from other countries [7]. from 1955 onwards, 53 https://doi.org/10.14311/app.2024.46.0053 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en m. levický, v. papcunová, m. urbaníková, a. kozloková acta polytechnica ctu proceedings labour migration began from developing countries, especially to the federal republic of germany; and in the 1960s, workers began to be brought mainly from spain, portugal, italy and greece, and then from mediterranean countries such as turkey, morocco, algeria and tunisia [8]. immigrants are still one of the biggest problems of separation in the eu. member states have seen their first tough test in the crisis triggered by immigrants appearing at the eu’s borders in 2015-2016, causing the dublin system to lose its functionality [9]. bell et al. [10] state that an important event in migration policy in the republic of poland, was represented by the introduction of a simplified procedure for the employment of foreigners in the polish labour market in 2006-2007. initially, it was limited to selected types of agricultural and fruit-growing activities and to citizens of countries neighbouring poland, but later it was abandoned to specify the types of economic activity and to cover citizens of armenia, belarus, georgia, moldova, the russian federation and ukraine. changes in poland’s migration situation are intertwined with changes caused by demographic processes: depopulation, rapid increase in the number of people of post-working age, ageing workforce and low birth rate. in practice, of course, the relationship between population density and the net migration rate is not straightforward. in most countries, the residuals from regression analysis suggest that more complex patterns of movement are also taking place. migration is by no means uniformly directed towards the most densely populated regions, as the outlying regions show [11]. migration processes cause a variety of economic and social effects, and the more intense the migration, the more dynamic the processes become. the effects can be both positive (strengthening of the economy, its development, cultural enrichment, development of social integration processes, etc.) and negative (segmentation of the population and increase in ethnic and cultural tensions, overburdening of the social security system, administrative overload, etc.) [12]. in today’s highly mobile world, migration, and in particular internal migration, is becoming an increasingly complex area of governance that is highly interlinked with other key policy areas, including economic and social development, national security, human rights, public health, regional stability, etc. managing migration at national or regional level is a complex and multifaceted task. internal migration includes regular, irregular and forced migration. it has been shown that the increasing scale of migration is in many cases not matched by the availability of infrastructure in urban centres to accommodate migrants [13]. perhaps the most significant aspect of internal migration is that it changes the spatial distribution of the population. internal migration, together with births, deaths and international migration, shapes population changes in a territory. analysing the drivers and dynamics of internal migration is crucial to understanding the progressive shifts in human settlement patterns around the world [14]. however, apart from individuals and households that are mobile both internally and internationally, the “boundary” between internal and international migration can easily become blurred. distance is certainly not the determining criterion. moreover, the nature of international borders may be changing: the european union and its borderless “schengenland” have created a borderless zone for mobility that is, with its regime of passports, visas and borders, more akin to internal migration than to “traditional” international migration. moreover, borders themselves can be mobile; they can appear or disappear (e.g. the dismemberment of the former soviet union or yugoslavia) [15]. on the other hand, according to [16], migration is an inevitable feature of development. migration can improve the distribution of income at origin by providing rural households with opportunities to diversify their income portfolio. also [17] confirms that although filling labour market gaps with migrants may only be a temporary solution, it is nevertheless of positive importance, especially for the further functioning of the market in a particular country. and at the same time, this can often prevent the decline of certain sectors in which the indigenous population refuses to work. according to [17], uncovering the link between internal migration and economic development is difficult. researchers analysing internal migration face two challenges: on the one hand, the inconsistent and difficult conceptualisation and subsequent measurement of internal migration and, on the other hand, its limited popularity within economic research. problems related to defining, measuring and collecting data on domestic flows have hampered research on internal migration. however, there is also an empirical correlation between internal migration and the residential distribution of immigrants, which is the basis of spatial assimilation theory (sat). according to this theory, the early settlement of immigrants from abroad was generally in large urban cities or areas where their national or ethnic groups were more concentrated (ethnic concentration). immigrants then tended to be distributed in a similar way to natives. they leave areas of first arrival and relocate within the host country through internal migration [18, 19]. geographical migration is thus the result of the socio-economic upward mobility of immigrants who, through the process of assimilation, acquire knowledge and become more attached to the host country and the native population. this, in turn, leads to a reduction in ties with the native community, less segregation and a convergence of settlement patterns between immigrants and natives [20]. this is primarily related to the existence of a set of displacement and integration costs, and therefore immigrants tend to first observe the process or racial or ethnic concentration in certain areas. only subsequently does the dispersal of the original community and its integration into the host society occur. 54 vol. 46/2024 development and causes of migration in lagging regions . . . however, the relationship between spatial mobility and assimilation is not always direct. socio-economic improvement and the formation of ties to the host country do not necessarily lead to spatial assimilation. for example, it has been shown that the impact of certain individual characteristics may differ between immigrant groups. similarly, it appears that some individuals, regardless of their socioeconomic status, prefer to reside in ethnic enclaves. the persistence of high moving costs or the existence of discriminatory practices (for example, in the housing market) may also hinder the assimilation process. thus, there is no consensus on the characteristics of immigrants who move within the host society, nor on their possible dispersion from the original concentrations of foreigners [21]. thus, human capital migration theory is not conclusive on whether immigrants should generally be expected to be more mobile than natives. this ambiguity is due to the conflicting forces that affect the internal migration of immigrants. on the one hand, there is a lack of locally specific human capital due to more recent relocation, potentially increasing the propensity to migrate. on the other hand, there is the empirically observed effect of ethnic enclaves where immigrants tend to cluster. these enclaves allow immigrants to benefit from their origin-specific human capital, which would otherwise be worthless in the new host country. migration rates are therefore potentially reduced for immigrants residing in an enclave [22]. the new economics of migration argues that migration is a rational decision made by an individual or family based on a cost-benefit analysis. however, research has increasingly focused on quantitative regression analysis to examine the “causes” and “impacts” of migration largely along the (implicit or explicit) lines of the push-pull model. push factors are factors that, for a variety of reasons, force someone to leave their place of birth and seek new opportunities elsewhere. pull factors, on the other hand, are those factors that attract migrants to a particular place and offer various advantages, benefits and more eastern living conditions [23]. push-pull models dominated much of migration thinking in the mid-twentieth century, until the 1960s, and reflect a paradigm of neoclassical economics based on the principles of utility maximization, rational choice, price differentials between regions and countries, and labour mobility [24]. according to this model, migrants are actors seeking to maximise income or “utility”. neither qualitative nor quantitative approaches have adequately captured the vital role of hard-to-quantify structural factors such as inequality, power and the state in shaping migration processes. they have also not been able to develop a meaningful idea of human agency beyond the voluntaristic assumptions of neoclassical models or the portrayal of migrants as more or less passive victims, as is common in historical-structural theories [25]. internal migration, which involves population movements within a country’s borders for economic, political or social reasons, is considered both a cause and a result of regional imbalances. in this framework, the impact of increasing internal migration on developed and underdeveloped regions may differ due to the influence of different socio-cultural and economic conditions between regions. the imbalance aspect is directly related to the extent to which migration affects parameters such as wages, output, consumption, human capital levels, business migration, unemployment and household income in regions with different levels of development [26]. the benefits of internal migration include the costs associated with job opportunities or other financial benefits. examples of such benefits include higher wages or better employment, better job prospects and cheaper housing. associated costs include the costs of moving a household, the time costs associated with finding new housing and employment, and the loss of local networks or locally specific human capital. in addition to economic considerations, other important costs and benefits relate to local amenities or family issues [27]. karhula et al. [28] add however, that cheap housing in poorer urban areas can be an attractive option for those migrating to study. at least initially, students also have greater opportunities to move to wealthier areas after graduation. over time, however, both processes (the migration of the less well-off as well as the emigration of the better-off) can potentially lead to a concentration of low-paid and low-skilled internal migrants in more deprived neighbourhoods. however, there is also other research on the factors that influence internal migration. research [12] has shown that the fragmentation of rural agricultural land is triggering a process of ruralto-urban migration in india. the higher the extent of subdivision and fragmentation of agricultural land the lower will be the agricultural productivity and the higher will be the extent of rural-to-urban migration. rural industrialization has been found to play a significant role in determining the extent of rural-urban migration in india. the higher the degree of rural industrialization, the higher will be the employment opportunities and hence lower will be the extent of rural-to-urban migration. on the other hand, rural unemployment acts as a driving factor in the process of rural-to-urban migration. in many cases, it has been found that due to unavailability of jobs in the rural sector, people are forced to migrate from the rural to the urban sector. the most important factor which significantly explains the rural-to-urban migration pattern is the urban amenity index. opportunities to lead a better quality of life in urban centres act as a pull factor in this rural-to-urban migration process as well as an impulse factor in determining the extent of rural-to-urban migration. according to [29], rural-to-urban migration has been one of the key drivers of demographic development over the last three decades worldwide. global forces can also affect internal migration in 55 m. levický, v. papcunová, m. urbaníková, a. kozloková acta polytechnica ctu proceedings other ways. as manufacturing in many industrial economies becomes more labour-intensive or less competitive compared to emerging economies, smaller or more peripheral cities may experience population decline [13]. research [30], which examined migration flows from wrocław, shows that there are also permanent migration flows to other rural communities in the voivodeship and other major cities in the country. the migration to rural areas that are not close to the city is significant. against the background of a general decline in the urbanisation index for poland and dolnośląskie voivodeship, this may indicate that anti-urbanisation processes are taking place in the country and the region. moreover, this phenomenon involves not only a redistribution of population between the city and its suburbs, but also an outflow of population from the city to peripheral areas. this is confirmed by [31], who states that permanent migration from urban to rural areas occurs in most cases in peri-urban zones. these migrations are largely a consequence of, but probably also the reason for, the increased amount of housing development in the vicinity of cities. given that the subjects of permanent migration to peri-urban zones are young people just before or just after entering into (in)formal relationships, this value appears to be a good variable, useful in identifying urban functional areas at different levels of the urban hierarchy. permanent internal migration in rural areas is less significant among all directions of migration. they share a common characteristic, that is, they usually take place between places that are not distant, between neighbouring units. however, according to [32], internal migration not only increases the region’s population, but young people’s enthusiasm, self-fulfilment, risk-taking and entrepreneurship will drive innovation and development. at the same time, such migration will bring racial and cultural diversity to cities, fostering integration, while also providing the right conditions for innovation. this is confirmed by [24], who states that population redistribution contributes to the flow of innovation between regions. at the same time, however, chen and rosenthal [33] pointed out that economic benefits seem to be an important driver for young people and those with a university education, but amenities such as climate seem to be more important for older people. one of the other factors influencing internal migration was the amount of information they had about where they wanted to settle and the amount of assistance they could expect to receive there. studies on internal migration provide ample evidence of the existence of various social networks that link urban and rural communities and provide migrants with extensive information and practical assistance. social networks vary in nature: some are based on blood relations, others on professional ties, personal friendships or common geographical origins [34]. networks enable immigrants to accumulate social capital, facilitate the acquisition and distribution of information and the availability of ethnic goods and services, and reduce the costs of migration and the risk of discrimination in labour markets [35]. interestingly, internal migration is largely ignored as a consequence of global, regional and national policymaking, but the topic of urbanization, which is a direct consequence of international and internal movements, is hotly debated [36]. 2. materials and methods the aim of the article is to evaluate the level and development of migration in slovakia and to identify its causes in the gemer region, which is among the least economically productive regions of slovakia. we tried to find answers to the following research questions: (1.) how important are the push and pull factors that motivate people to migrate? (2.) are there significant differences between individual factors? (3.) are there significant differences in opinions in terms of gender, residence and level of education? to evaluate the status and development of migration, we used analytical indicators of migration. for example, gross and net migration, immigration and emigration, the migration balance index, which shows the efficiency of migration, and the gross migration balance. the main source for the calculation of analytical indicators was the statistical office of the slovak republic, specifically the statistical yearbooks of the slovak republic, as well as the regional statistical yearbooks of the slovak republic. we investigated the motivation for migration and its causes based on the push-pull model. we used data from a questionnaire survey in which residents of the gemer region participated. respondents filled out questionnaires in the google forms application. from a methodological point of view, we focused on the psychological barriers of the respondents to fill out contentand technically demanding questionnaires. based on the above, we compiled the most suitable questionnaire with an adequate number of the simplest possible questions. when designing the questionnaire items, we emphasized their mutual symbiosis. pilot testing was carried out on a small sample of respondents in order to find out feedback on the comprehensibility of the questionnaire. after slight adjustments, the result was 6 questions focused on the socio-demographic characteristics of the respondents and 6 questions related to the solved problem. we used random sampling to construct the research sample. there were 316 correctly completed questionnaires. we conducted the research from january to march 2023. table 1 summarizes the personal characteristics of the respondents. the group included 179 women (56.65 %) and 137 men (43.35 %). next, we focused on the characteristics of residence and education. we 56 vol. 46/2024 development and causes of migration in lagging regions . . . variable number of respondents percentage gender female 179 56.65 male 137 43.35 total 316 100.00 residence town 176 55.70 countryside 140 44.30 total 316 100.00 education elementary 8 2.53 secondary 152 48.10 university 156 49.37 total 316 100.00 table 1. personal characteristics of the participants. source: processing by the authors based on their own questionnaire survey: the main causes of migration in the gemer region. 2017 2018 2019 2020 2021 immigrants 7,188 7,253 7,016 6,775 5,733 emigrants 3,466 3,298 3,384 2,428 3,395 total migration 10,654 10,551 10,400 9,203 9,128 migration balance 3,722 3,955 3,632 4,347 2,338 table 2. development of international migration in slovakia and its analytical indicators [37–41]. present these characteristics because they were the subject of our research in our own work. of the total number of respondents, 176 (55.70 %) lived in the city and 140 (44.30 %) lived in the countryside. the structure of the selection set was approximately the same as the structure of the base set for these two characteristics. in terms of education, the research sample consisted of 8 respondents (2.53 %) with completed primary education, 152 respondents (48.10 %) with completed secondary education and 156 respondents (49.37 %) with completed university education. the higher representation of respondents with completed university education is primarily related to their willingness to participate in questionnaire surveys. for the purpose of evaluating the data obtained from the questionnaire survey, we used descriptive and inferential statistics. considering the type of data obtained, we preferred to use non-parametric tests. specifically, we used the friedman test for comparison of multiple dependent samples. the subsequent examination of the two experimental units was carried out using the nemenyi post-hoc test. we will also use the chi-square test of independence to determine the dependence of two nominal variables. all tests were performed at a significance level of 0.05. data analysis for this purpose was created using the real statistics resource pack software (release 7.6). copyright (2013–2021) charles zaiontz [42]. 3. results and discussion 3.1. international and internal migration in slovakia we will evaluate the development and state of migration in slovakia for the period from 2017 to 2021 (table 2). in 2017 and 2018, the number of immigrants showed an upward trend, although the increase was minimal [37, 38]. since 2019, there has been a decrease every year. while only 241 fewer people chose slovakia as a destination country between 2019 and 2020, between 2020 and 2021 the number of immigrants decreased by 1 042 people. this was probably caused by the situation related to the covid19 pandemic at the time. the numbers of emigrants did not change significantly except for one year. in the first monitored year, the number of emigrants was 3 466. subsequently, a decrease in migration values can be observed in 2020. this change was caused by the pandemic described above and its negative consequences. the number of emigrants decreased by 956 between 2019 and 2020. by 2021, however, the situation stabilized again and the values again reached the median and average from previous years. the number of immigrants is always higher than the number of emigrants. between 2017 and 2021, on average, 3 599 more people immigrated than emigrated. this indicates that slovakia is a target destination from an economic, social, demographic and cultural point of view. the volume of migration is the sum 57 m. levický, v. papcunová, m. urbaníková, a. kozloková acta polytechnica ctu proceedings 2017 2018 2019 2020 2021 sr total 95 132 98 414 98 677 87 853 92 768 in the district 42 590 44 222 42 837 39 343 41 772 between districts 27 800 28 809 28 854 25 478 27 019 between regions 24 742 25 383 26 986 23 032 23 977 table 3. development of internal migration in slovakia [37–41]. of the number of immigrated and emigrated persons. it can also be called gross migration. during the monitored period, this volume of migration was the highest in 2017. subsequently, it began to gradually decrease. net migration is the difference between the number of people who moved in and the number of people who left. in the first three years, the value of this indicator was at the level of 3 700, then it increased to 4 347 in 2020. this increase was caused by a greater decrease in the number of emigration than immigration. finally, the migration rate reached its lowest value in 2021. this was due to the fact that in 2021, compared to the previous year, the number of immigrants decreased and the number of emigrants increased [37–41]. internal migration (table 3), which includes population movements within a country’s borders for economic, political or social reasons, is considered both a cause and a consequence of regional imbalance [26]. if we consider the average of migration between 2017 and 2021, we can say that the majority of people – specifically 45 % of internal migration – took place within one district. another 29 % of persons involved in internal migration moved from one district to another within the county. the remaining 26 % moved from one region to another within slovakia. internal migration can also be examined on a regional basis. the largest increase in migration over a long period was recorded in the bratislava region, which is the most economically productive region of slovakia. in 2021, the gross migration rate per 10 000 inhabitants was 56.7 [41]. this value is significantly lower compared to recent years and 2019. the second highest gross rate of migration increase (28.6 persons per 10 000 inhabitants) was in the trnava region, where the gross rate in 2021 increased compared to the previous year. only these two regions in slovakia had a positive population increase during the monitored period. other regions have long-term migration losses, i.e. the number of emigrants exceeded the number of immigrants in all monitored years. the largest migration decreases were recorded in the banská bystrica and prešov regions, in both regions in 2021 there were 16.4 gross migrants per 10 000 inhabitants [37–41]. 3.2. migration in the gemer region the gemer region is located in the south-eastern part of slovakia and had 178 680 inhabitants in 2021 [43]. it includes the districts of rimavská sobota, revúca and rožňava. these districts are not only geographically similar, but also share a common history, similar social structure and culture of life. although they are part of the same region, rimavská sobota and revúca belong to the banská bystrica region, while rožňava belongs to the košice region [23]. the gemer region belongs to the regions of slovakia with the lowest economic performance. due to the availability of data on regional migration, we focused on its evaluation for the period from 2017 to 2020 [44]. in each district of the gemer region, we monitored how the number of immigrants and emigrants changed. we further divided immigrants and emigrants into two groups: immigrated from/to slovakia and immigrated from/to abroad. every year, more people leave the gemer region than move into. this means that the number of emigrants exceeded the number of immigrants every year. noticeably more people moved to other parts of slovakia than abroad. in the case of immigrants to the gemer region, in the monitored years, on average, five times more people came from other parts of slovakia than from abroad. eighteen times more people immigrated to other parts of slovakia than abroad [44]. • median number of people emigrating from slovakia: 882. • median number of persons who immigrated from abroad: 173. • median number of persons migrating to other parts of slovakia: 1 388. • median number of persons migrating abroad: 84. net migration shows a fluctuating development. in 2017 and 2019, net migration increased compared to the previous year. in 2018 and 2020, it was at the level of approximately −370. in the next part, we examine the reasons for migration in the gemer region (table 4), based on the answers to the questionnaire we composed. in addition to socio-demographic questions, we investigated where the respondents currently work and whether they were thinking about moving out of the region. subsequently, we investigated the respondents’ motivations based on the push-pull model. push factors are factors that, for various reasons, force someone to leave the place of their birth and look for new opportunities elsewhere. on the other hand, pull factors are factors that attract migrants to a specific place and offer various advantages, benefits and better living conditions. the combination of these two factors helps 58 vol. 46/2024 development and causes of migration in lagging regions . . . immigrants emigrants net migration from sk from abroad total to sk to abroad total 2017 918 164 1 082 1 427 84 1 511 −429 2018 911 173 1 084 1 388 88 1 476 −392 2019 841 174 1 015 1 460 83 1 543 −528 2020 801 216 1 017 1 315 52 1 367 −350 table 4. development of migration trends in the gemer region [44]. determine the reasons for emigration and immigration of people from one country to another [45]. push and pull factors are usually opposite. for example the push factor of lack of job opportunities in the rural areas is the opposite of the pull factor of abundant vacant jobs in the urban areas [46]. the basic results show that 55 % of respondents would not want to leave the gemer region. a total of 24 % of respondents would like to leave slovakia and move to another country. 21 % of respondents would like to move to another region in slovakia. respondents who would like to leave the region also stated their reasons. in the twelfth question, respondents could choose from three predefined options or enter their own answer. of those who wanted to move, up to 64 % said economic reasons prevailed, 15 % cited demographic reasons and 13 % socio-cultural and political reasons. the remaining 9 % gave their own answer. the respondents most often stated: better work, studies, low living standards, family ties, weak infrastructure, lack of opportunities in the field of culture and sports, or a weak offer of commercial services. based on the established research questions, we further investigated the relationship between the selected socio-demographic characteristics of the respondents and questions related to attitudes towards migration. specifically, we investigated whether there is a difference in the propensity to migrate with respect to gender, level of education of the respondent and with respect to his place of residence (whether the person lives in a city or in the countryside). in all cases, these were nominal variables, and testing was performed using the chi-square test of independence. the results of the calculated p-values are documented in table 5. based on it, it can be concluded that the null hypothesis can be rejected only in the case of the first alternative, as the calculated p-value is smaller than the confidence interval of 95.00 %. we can say that there is a difference in the propensity to migrate between men and women. specifically, 52.55 % of men and 39.66 % of women thought about emigrating from the region. this is an expected result given the traditional model of family functioning, where the man takes responsibility for the material security of the family. this confirms the results of family migration research, according to which couples’ decisions about migration are more often made with regard to the male career than the female career [47]. couples are variable p-value gender – propensity to migrate 0.023 gender – destination of migration 0.160 education – propensity to migrate 0.448 residence – propensity to migrate 0.094 table 5. results of the investigation of the relationship between the respondents’ personal characteristics and their propensity to migrate. source: processing and calculations by the authors based on their own questionnaire survey: the main causes of migration in the gemer region. more likely to move for the male’s job than for the female’s job [48, 49] and social-economic and work characteristics of the man often have a greater influence on the couple’s propensity to migrate than the characteristics of the woman. in other cases, the calculated p-value is higher than 0.05 and therefore we do not reject the null hypothesis. there is no difference in how men and women perceive the goal of migration (domestic or foreign migration). furthermore, the propensity to migrate is not related to the level of education. university-educated people are interested in migrating from the region due to limited employment in the region. people with lower education, whose motivation to migrate includes both limited opportunities for employment and low wages. it is interesting to note that there is no significant difference in the propensity to migrate with respect to the place of residence. we hypothesized that in an economically weak region, the propensity to migrate will be higher among residents who live in the countryside. table 6 shows the respondents’ answers regarding pull factors. on a scale from 1 (the least important factor) to 5 (the most important factor), the respondents expressed the subjective importance of the mentioned pull factors. the arithmetic mean of all items was 3.56, which indicates that pull factors are important for the residents of the region. table 6 contains arithmetic means of each evaluated pull factor, standard deviations and interval estimates of the mean at a confidence interval of 95.00 %. the factors are ranked from most important to least important based on the arithmetic mean. the most important pull factor is better living conditions in the target region, which can be considered an expected result. this confirms the results of studies according to which most migrants 59 m. levický, v. papcunová, m. urbaníková, a. kozloková acta polytechnica ctu proceedings pull factors average st. dev. −95.00 % +95.00 % a. better living conditions 3.965 1.161 3.837 4.094 b. higher wages 3.915 1.225 3.779 4.050 c. better working conditions 3.851 1.234 3.715 3.988 d. career opportunities 3.848 1.251 3.710 3.987 e. better health care 3.475 1.298 3.331 3.618 f. better educational opportunities 3.386 1.274 3.245 3.527 g. family relations 3.358 1.374 3.206 3.510 h. better climatic conditions 2.867 1.307 2.722 3.012 i. political and religious freedom 2.440 1.407 2.284 2.596 scale: 5 = most important, 1 = least important table 6. the importance of pull factors. source: processing and calculations by the authors based on their own questionnaire survey: the main causes of migration in the gemer region. factors p-value factors p-value factors p-value factors p-value a-b 0.987 b-d 1.000 c-g 0.046 e-g 1.000 a-c 0.747 b-e 0.010 c-h 0.000 e-h 0.000 a-d 0.929 b-f 0.000 c-i 0.000 e-i 0.000 a-e 0.000 b-g 0.004 d-e 0.029 f-g 0.943 a-f 0.000 b-h 0.000 d-f 0.000 f-h 0.022 a-g 0.000 b-i 0.000 d-g 0.013 f-i 0.000 a-h 0.000 c-d 1.000 d-h 0.000 g-h 0.000 a-i 0.000 c-e 0.093 d-i 0.000 g-i 0.000 b-c 0.998 c-f 0.000 e-f 0.854 h-i 0.009 table 7. comparison of pairs of pull factors by nemenyi post test. source: processing and calculations by the authors based on their own questionnaire survey: the main causes of migration in the gemer region. move in search of better economic opportunities [4]. other factors in order of importance include higher wages in the target region, better working conditions, the possibility of realizing a career or better health care. it should also be noted that in the case of lagging regions we can talk about labour migration. it is obvious that in this case economic pull factors will reach higher levels of importance [50]. nevertheless, we agree with the opinion [51], according to which non-economic factors also play a very important role in the decision on migration. the least important factors are better climatic conditions or political and religious freedom in the target region. better climatic conditions were not so important for the respondents due to relatively homogeneous climatic conditions in the country. the distribution of the population in terms of religion in slovakia is also relatively homogeneous with majority representation. the situation is similar in the case of religion. in 2021, there were 68.80 % christians in slovakia, 23.80 % of people with no religion and 0.90 % of people with other religion. the order of pull factors shown in table 6 is valid only for a sample set of respondents. if we want to generalize these results (order of pull factors) to the level of the basic set, we need to find out whether the differences in the respondents’ answers are statistically significant. for this purpose, we used the friedman test, through which we verified the validity of the null hypothesis. the null hypothesis assumed that the average level of all pull factors is the same compared to the alternative. the calculated p-value of the friedman test was 0.000, which means that we reject the null hypothesis. there are statistically significant differences between the level of at least two pull factors. to achieve more accurate results, we had to perform nemenyi post test, the results of which are shown in table 7. based on the results of nemenyi post test, it can be concluded that the factors of better living conditions, higher wages, better working conditions and career opportunities are significantly more important than other pull factors (at the 90.00 % confidence level). on the other hand, the factor of political and religious freedom is significantly the least important among the other mentioned pull factors. table 8 captures the responses regarding push factors. push factors are a negative motivation to migrate. these are factors that motivate people to emigrate from their place of residence. in this case as well, the respondents assigned numbers from 1 to 5 to the individual factors. the number 1 corresponded to the lowest importance and the number 5 to the highest importance. the overall average of all responses was 3.251. compared to the value of pull factors, this is a smaller number. this may mean that pull factors 60 vol. 46/2024 development and causes of migration in lagging regions . . . push factor average st. dev. −95.00 % +95.00 % a. lack of jobs 3.943 1.225 3.807 4.079 b. bad economic conditions of the region 3.785 1.231 3.649 3.921 c. high unemployment 3.592 1.341 3.443 3.740 d. few opportunities for career growth 3.560 1.297 3.417 3.704 e. poor health care 3.437 1.307 3.292 3.581 f. poor living conditions in the region 2.851 1.262 2.711 2.991 g. discrimination in the region 2.794 1.321 2.648 2.941 h. lack of political or religious freedom 2.044 1.236 1.907 2.181 scale: 5 = most important, 1 = least important table 8. the importance of push factors. source: processing and calculations by the authors based on their own questionnaire survey: the main causes of migration in the gemer region. factors p-value factors p-value factors p-value factors p-value a-b 0.961 b-c 0.357 c-e 0.981 d-h 0.000 a-c 0.024 b-d 0.135 c-f 0.000 e-f 0.000 a-d 0.005 b-e 0.036 c-g 0.000 e-g 0.000 a-e 0.001 b-f 0.000 c-h 0.000 e-h 0.000 a-f 0.000 b-g 0.000 d-e 1.000 f-g 1.000 a-g 0.000 b-h 0.000 d-f 0.000 f-h 0.000 a-h 0.000 c-d 1.000 d-g 0.000 g-h 0.000 table 9. comparison of pairs of push factors by nemenyi post test. source: processing and calculations by the authors based on their own questionnaire survey: the main causes of migration in the gemer region. represent a generally more important motivation for respondents to migrate than push factors. push factors are arranged in table 8 based on the arithmetic mean from most important to least important. based on the descriptive statistics, it can be concluded that the most important push factor is the lack of jobs, which can be considered an expected result. in order of importance, the poor economic conditions of the region, high unemployment, few opportunities for career growth and poor health care were ranked next. it is interesting that poor health care was a more important factor for the respondents than poor housing conditions, as the majority of the sample was represented by the younger population. the least important push factors included discrimination and lack of political and religious freedom. in order to find out whether the results from table 8 are generally valid for the base set (residents of the gemer region), we will use the friedman test. the calculated p-value was 0.000, which means that we can reject the null hypothesis. therefore, the alternative hypothesis is valid, based on which there is a statistically significant difference in at least one pair of factors (in their arithmetic averages). to obtain more detailed results, we will perform nemenyi post test, the results of which are in table 9. we can say that the lack of jobs is the most important among all other factors (except for bad economic conditions in the region). it can also be concluded that the factor of lack of political and religious freedom is significantly the least important push factor for the inhabitants of the region. 4. conclusion migration is a complex phenomenon and it has many different effects on society. it is important to understand its causes, consequences and risks in order to develop an effective policy in this area. migration can also be defined as the spatial movement of the population and one of the most important components of population development and regional development [52]. we can conclude that slovakia is attracting people in terms of migration in the current situation. more people immigrate to slovakia than emigrate from it. this is mainly due to the fact that business and economic conditions have improved since 2004, which has enabled slovakia to compete with other countries. innovations and reforms increased labour productivity, reduced unemployment and increased the number of foreign migrants [53]. although the current situation is positive, it should not be considered closed. the covid-19 pandemic radically limited migration flows, but already in 2021 there was an intensive increase in migration. during the five monitored years, the average number of internal migrants was 94 569 [37–41]. the majority of these internal migrants, namely 45 %, moved within one district. in other words, they did not leave the district of their residence, but migrated within it. 29 % of internal migrants moved from one district to another within the region. the remaining 26 % moved to another region within the country. the bratislava region was the area within slovakia where the largest increase in migration was recorded in the monitored years. the stated fact confirms the general 61 m. levický, v. papcunová, m. urbaníková, a. kozloková acta polytechnica ctu proceedings empirical research according to which the greatest migration activity takes place in the most developed regions. the level of development of these regions compared to other regions is very high, which is reflected in their attractiveness for migration [54]. the largest migration declines were recorded in the banská bystrica and prešov regions. we also paid attention to the gemer region, which is among the least economically productive regions of slovakia. it is a marginal and disadvantaged region with a lower standard of living, an aging population, a lower number of job opportunities, a lower educational level and lower investments [55]. a negative migration balance with the threat of depopulation is typical for this region. every year, more people leave the gemer region than move into [44]. net migration shows a fluctuating trend. the questionnaire survey showed that 21 % of the inhabitants of the gemer region regularly migrate. based on the results of the questionnaire survey, it follows that the favourite destination countries are hungary, germany, the netherlands, austria, italy, england and the czech republic. within the country, the most common destinations are bratislava, prešov and košice regions. 45 % of respondents would like to move from the gemer region, of which 24 % to abroad and 21 % to another part of slovakia. the gemer region is a region with a long-term negative migration balance, which loses its potential from the labour production factor and thus its chances for convergence decrease [54]. on the other hand, some authors consider migration as a necessary feature of development that can improve income distribution by providing households with the opportunity to diversify their income portfolio [16]. a total of 64 % of respondents cited economic reasons for migration. this result confirmed the empirical assumption about the importance of economic reasons for migration. this is also confirmed by the majority of studies, according to which the economic factors of migration can be considered a key and significant driving force in the decision of migrant workers, and most migrants move in search of better economic opportunities [4, 56]. furthermore, 36 % of respondents to the questionnaire survey stated demographic, sociocultural or other reasons for migration. based on the conducted research, we can conclude that the propensity to migrate is dependent on gender, but it is not dependent on the respondent’s place of residence and his education. on the contrary, based on the results [57], it follows that residents with higher education show the greatest interest in migration. however, this survey was conducted before slovakia joined the eu, when the conditions for migration were more difficult and the educational level of the country’s population was lower. we investigated the motivation to migrate based on the well-known push-pull model [23]. the most important pull factors that motivate individuals to migrate to another place include better living conditions and higher wages. one of the most important push factors that motivates residents to migrate from their place of residence is clearly the lack of jobs. research [56] proved that there is a stronger correlation between the number of migrant employees and the number of job vacancies. discrimination or the level of religious and political freedom do not play an important role in deciding on population migration. acknowledgements this article is a part of the solution of the project vega 1/0466/21 “evaluation of the quality of the business environment in slovakia with an emphasis on entry into business in and the post-pandemic period”, vega 1/0517/22 “mapping of processes and financial impacts at the level of local self-governments in the conditions of the covid-19 pandemic” and part of the solution of the project kega 029ukf-4/2022: preparation of the structure and content of the subject, textbook and e-learning course “entrepreneurial 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619 69 brno, czech republic c academy of sciences of the czech republic, institute of physics, joint laboratory of optics of palacký university and institute of physics as cr, 17. listopadu 50a, 772 07 olomouc, czech republic ∗ corresponding author: jaroslav.kovar@vut.cz abstract. this paper deals with the pile-up phenomenon that can occur during nanoindentation. finite element (fe) simulation of x5crnicunb16-4 steel nanoindentation with pyramidal berkovich indenter was done, and stress and strain beneath the indenter leading to pile-up behavior were analyzed in detail. pile-up also influences the projected contact area, which should be corrected to include the pile-up into the oliver-pharr analysis. accurate calculation of the projected contact area requires knowledge of its boundary. few methods of boundary approximation were used, including approximation by the triangle and the semi-ellipse. for more precise approximation, the expression for parabolical approximation was derived. these methods were compared with the projected contact area calculated by finite element method. the most precise results were obtained using semi-elliptical and parabolical correction, which can be used for the determination of the projected contact area and its boundary. keywords: nanoindentation, berkovich indenter, pile-up, parabolical correction, steel, finite element method. 1. introduction pile-up behavior can occur during the nanoindentation of soft, plastic materials. this leads to accumulation of plastically deformed material beneath the indenter and an increase in the contact area. the pile-up is maximal around the sides of the indenter and minimal under the indenter edges, see figure 1. pile-up can significantly affect the results of oliverpharr analysis (described in [1, 2]), but it is difficult to correct its influence on the projected contact area. this paper is aimed on modelling of x5crniconb164 nanoindentation by finite element method (fem) and the description of the pile-up formation process from the mechanical perspective. additionally, the commonly used corrections of the pile-up are unified and compared with fem results and results calculated by new parabolical correction to get more accurate algorithm for describing pile-up. 2. methods 2.1. fe model to describe the pile-up, fe calculation was done for the nanoindentation of x5crniconb16-4 steel. the fe model similar to one, which was already described in [3], was employed. a blunted pyramidal berkovich indenter with the tip radius of 400 nm was used for calculations. the indentation depth was 500 nm, which was 100 times smaller than the dimensions of specimen and indenter. according to [4], these dimensions should be enough to eliminate the influence of the specimen and indenter boundaries. due to symmetry, only one-sixth of the geometry was modeled (figure 2. the displacement along the z-axis at the bottom of the specimen and displacements perpendicular to the symmetry plane were fixed at them. the specimen was loaded by the displacement of the berkovich indenter. in the first loadstep, a displacement 500 nm was applied on its top, followed by unloading in the second loadstep. the diamond indenter was described by isotropic linear elastic model of material with young’s modulus edia = 1 141gpa and poisson’s ratio νdia = 0.07. the steel was modeled using a multilinear model of material with esteel = 202gpa, νsteel = 0.3 and the yield strength resteel = 500mpa. the stress-strain curve for this model has been defined by linearly interpolated 18 points (described in [3]) and the isotropic hardening was used. the mesh was created from 3d solid 186 elements with the use of the contact conta 174 and targe 170 elements at contact area. to describe the pile-up around the contact area, the mesh was refined here. totally was used 74 439 elements and the finest elements had approximately 2 nm. the contact was solved using the augmented lagrange algorithm with a coefficient of friction 0.1, whose impact has already been determined in [5]. after performing the calculations, the simulated indentation curve has been 18 https://doi.org/10.14311/app.2024.50.0018 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 50/2024 finite element analysis of nanoindentation pile-up . . . (a). projection of the contact area at berkovich indentation of the material with pile-up. (b). cross section of the soft material indent with berkovich indenter. figure 1. visualization of the pile up effect. figure 2. model of geometry of berkovich indentation used for fem calculations [µm] with boundary conditions (at the section planes, the symmetry conditions were used). compared with experimental data (described in [3] in detail). 2.2. corrections of the pile-up to calculate the precise results of young’s modulus and hardness by oliver-pharr analysis, the projected contact area has to be known. few methods of its calculations will be compared in this paper. 2.2.1. oliver-pharr analysis the commonly used oliver-pharr analysis was derived from the sneddon solution, described in [6] which assumed only elastic deformations. as the pile-up is mainly caused by the plasticity, its influence is not included into the oliver-pharr analysis, which is only applicable to sink-in behavior, when the specimen surface bends under the indenter. the projected contact area in oliver-pharr analysis can be determined for the ideal berkovich indenter according to eq. (1). if the indenter is blunted, the projected contact area can be determined as the area of its cross-section at the contact depth hc. to correct the results of oliverpharr analysis for pile-up, the eq. (2) is commonly used, where aop is area obtained by oliver-pharr analysis and ap represents the correction for pile-up. aopideal = 24.56 · h2 c (1) ac = aop +ap (2) 2.2.2. triangular correction the idea of approximation of pile-up region by triangle (figure 3) was proposed by [7] and can be described by eq. (3). for a berkovich indenter, the c can be determined by eq. (4) and with the use of eq. (5) and the assumption of identical pile-ups on each indenter side, the pile-up area bounded by the triangle (apt) can be expressed by eq. (6). apt = 1 2 · tan(π2 − α) · 3∑ i=1 hpi · ci (3) c = 2 · √ 3 · ac (4) 19 j. kovář, v. fuis, r. čtvrtlík, j. tomáštík acta polytechnica ctu proceedings figure 3. approximation of the pile-up by triangles (red line represents the boundary of the pile-up area; blue line represents the boundary of the correction). ac = hc · tanα (5) apt = 3 · √ 3 · tanα tan(π2 − α) · hc · hp ∼= 24.562 · hc · hp (6) 2.2.3. semi-elliptical correction to better describe the boundary of the pile-up, its approximation by semi-ellipse was derived by [8]. with this correction, the approximation of all pile-ups can be expressed by eq. (7). if the indenter deformation is negligible, the eq. (8) can be used, and the pile-up area can be given by eq. (9). although the semielliptical correction provides an accurate description, it can overestimate the pile-up near the indenter’s edge due to the semi-elliptical shape (figure 4). aps = π · c 4 · 3∑ i=1 api (7) ap = hp · tanα (8) aps = 3 · √ 3 · π · tan2 α 2 · hc · hp ∼= 38.582 · hc · hp (9) 2.2.4. parabolical correction parabolical correction was derived to reduce the discrepancy near the indenter edges. this correction, which is described by [9], uses the approximation of the pile-up boundary by a parabola. for simplification, the equations for this correction were derived in the local coordinate system (figure 5). the parabola passes through three points, two on the edges of the indenter (points a and c) and one which is the point of maximum pile-up in its center (point b). the coordinates of these points can be written by eq. (10). by substituting these points into the general equation figure 4. approximation of the pile-up by semiellipses (red line represents the boundary of the pileup area; blue line represents the boundary of the correction). figure 5. approximation of the pile-up by parabola. of parabola described by eq. (11), the equation of the parabolical boundary described by eq. (12) was derived. the projection of the pile-up onto the plane of the specimen surface is bounded by the parabola on one side and the edge of the indent at the other side (figure 5). this boundary can be expressed by eq. (13). assuming of its symmetry, the area of one pile-up can be described by eq. (14). with the substitution of c and ap from previous equations and assumption of the same pile-ups at all three sides of the indent, the total pile-up area can be described by eq. (15). a [ c 2 ; ap ] b [0; 0] c [ − c2 ; ap ] (10) y = a0 · x2 + b0 · x+ c0 (11) yb1 = ap · x2 · ( c 2 )−2 (12) 20 vol. 50/2024 finite element analysis of nanoindentation pile-up . . . yb2 = ap (13) app1/3 = 2 · ∫ c 2 0 ∫ yb2 yb1 1 dy dx = 4 3 · ap · c 2 (14) app = 4 · tan2 α tan 30° · hc · hp ∼= 32.748 · hc · hp (15) 2.2.5. finite element method when the fem is used, the elements in the contact may be selected and the projected contact area calculated as its surface after the projection onto the plane of the specimen surface. this area does not use any approximation of its boundary and if the mesh is enough fine, it can be used for the comparison of the other corrections. the procedure used in this paper is described in chapter 4 in detail. 3. pile-up the fem results confirmed that pile-up occurs during the steel nanoindentation. the location of the equivalent stress maximum was beneath the specimen surface, which was caused by the bluntness of the indenter. as the indentation depth grew, the plastic zone began to form (figure 6). the results of plastic strain in the way of indentation showed a significant zone of compressive strain under the indenter but next this zone, the positive (tensile) values appeared, forming the pile-up region that grows with the indentation depth. the tensile plastic strain is caused by the deformation of the specimen. at the beginning of the indentation, the material is compressed under the indenter, until the creation of plastic zone. plastically deformed material has very limited ability of volumetric compressibility and begins to push surrounding material away, which leads to the creation of pile-up. this can be confirmed by the deformation of the specimen (figure 7), where is shown how is the material in pile-up region pushed to the specimen under the indenter, but behind the side of indenter, the vector of deformation reverses, which describes the pushing the material away and forming pile-up. when the indentation curves were determined by fem, the oliver-pharr analysis was used to calculate the contact depth and its value hc = 472nm was obtatined. as the oliver-pharr analysis does not include pile-up, it was expected that hc should be 500 nm, what was the depth of indentation. this discrepancy was clarified by the plotting the detail of pile-up at indent profile (figure 8). the profile can be divided into the three parts. the first one from the right is the steep line caused by the indenter profile (the graph is scaled to make details better visible). then there is a pile-up region and at the bigger distance from the indent sink-in occurs too. this indicates that the influence of significant plasticity beneath the indenter tip becomes negligible further away from the indent, making sink-in the dominant figure 6. plastic strain in the way of indentation [-] at the indentation depth 8.2 nm [9]. plastic strain was calculated for the blunted berkovich indenter, which bluntness is significant at this indentation depth. figure 7. total deformation [µm] at the indentation depth (displacement of indenter) 0.5µm. figure 8. detail of profile near pile-up. 21 j. kovář, v. fuis, r. čtvrtlík, j. tomáštík acta polytechnica ctu proceedings method fem oliver-pharr triangular semi-elliptical parabolical ac [µm2] 8.0 6.9 7.6 8.1 7.9 ∆ac acfem [%] – −13.6 −4.2 1.2 −1.1 table 1. the calculated projected contact areas and its relative difference to fem results ( ∆ac acfem ) . effect, even in the case of steel nanoindentation. the sink-in influenced calculated contact depth and this is the main reason, why its calculated value is lower than expected value. 4. comparison of the corrections the calculated indentation curves were analyzed using the oliver-pharr method. as the indenter was blunted, the projected contact area for oliver-pharr analysis was determined as the area of indenter cross section at the calculated contact depth (hc = 472 nm). to use corrections described in the chapter 2.2, the height of pile-up (hp) was measured from the end of contact depth to the maximal height of pile-up and then hp = 64.7 nm. note that the height of pile–up is bigger that the location of its maximal point due to the sink-in described in the chapter 3. with these values, the relations for corrections could be used and calculated values were compared in table 1. the boundaries of contact areas obtained for one sixth of the berkovich indent by each approximation were plotted (figure 9). the contact area from fem was determined as the projected area of deformed elements, with only elements having more than half of their nodes in contact. as the mesh was fine at contact area, these results were expected to be accurate enough. the results calculated by oliver-pharr analysis underestimated the projected contact area approximately about 13.6% due to the omission of the pile-up. when the triangular correction was used, the discrepancy dropped to the 4.21%. the triangular correction can be used to correct pile-up but it describes it inaccurately. the semi-elliptical correction overestimated the contact area about 1.16% due to the areas near of the edge of the indenter. the parabolic correction underestimated the projected contact area about 1.07%. both the semi-elliptical and parabolic corrections yielded similar results and are effective for describing the pile-up region. the knowledge of the accurate value of projected contact area is important for the determination of the young’s modulus and hardness. as the hardness is inversely proportional to the contact area and young’s modulus to its square root, the use of good correction of the projected contact area is more important for the hardness. in the same way, the influence of the corrections to young’s modulus and hardness can be determined. figure 9. comparison of the approximations of the indent boundaries with the results of fem. 5. conclusions the fem simulation of x5crniconb16-4 steel showed that the pile-up is caused by huge plastic deformation under the indenter. the plastically deformed material pushes the surrounding material upwards, creating the pile-up under the side of the indenter. far from pileup, the sink-in occurs, which influences the contact depth calculated by the oliver-pharr analysis. the projected contact area calculated from the results of fem calculation was compared with those calculated by oliver-pharr analysis and with the triangular, semi-elliptical and parabolical correction. the results showed that the oliver-pharr analysis underestimates projected contact area when the pile-up occurs. the triangular correction provided a better approximation of it. the most precise results were obtained by the parabolical and semi-elliptical corrections. all formulas for these corrections lead to the same dependencies, which are different by the value of the coefficient. to get more accurate results at experimental nanoindentation, the highest point of the pile-up needs to be determined. determination of this point is difficult due to the small scales of geometry and dimension and could be assessed in future work. acknowledgements this study was realized with the support by the grant fsi-s-23-8186 and with the institutional support rvo: 61388998. jt acknowledges the support by the project op jac cz.02.01.01/00/22_008/0004596 of the ministry of education, youth, and sports of the czech republic and eu. the language was improved by ai software chatgpt. references [1] w. c. oliver, g. m. pharr. measurement of hardness and elastic modulus by instrumented indentation: 22 vol. 50/2024 finite element analysis of nanoindentation pile-up . . . advances in understanding and refinements to methodology. journal of materials research 19(01):3– 20, 2004. https://doi.org/10.1557/jmr.2004.19.1.3 [2] w. c. oliver, g. m. pharr. an improved technique for determining hardness and elastic modulus using load and displacement sensing indentation experiments. journal of materials research 7(06):1564–1583, 1992. https://doi.org/10.1557/jmr.1992.1564 [3] j. kovář, v. fuis, r. čtvrtlík, j. tomáštík. the influence of tilt on berkovich indentation of a steel sample using fem analysis and nanoindentation. powder metallurgy progress 22(1):53–61, 2022. https://doi.org/10.2478/pmp-2022-0007 [4] l. fiala, i. ballo, j. kovář, v. fuis. comparison of finite element simulation of tungsten nanoindentation with berkovich and conical indenter. in engineering mechanics, vol. 29, pp. 63–66. 2023. https://doi.org/10.21495/em2023-63 [5] j. kovář, v. fuis, r. čtvrtlík. influencing of the pile-up and the sink-in by the coefficient of friction in the nanoindentation test. in engineering mechanics, vol. 26, pp. 298–301. 2020. https://doi.org/10.21495/5896-3-298 [6] i. n. sneddon. the relation between load and penetration in the axisymmetric boussinesq problem for a punch of arbitrary profile. international journal of engineering science 3(1):47–57, 1965. https://doi.org/10.1016/0020-7225(65)90019-4 [7] j. hu, y. zhang, w. sun, t. zhang. nanoindentationinduced pile-up in the residual impression of crystalline cu with different grain size. crystals 8(1):9, 2018. https://doi.org/10.3390/cryst8010009 [8] k. o. kese, z. c. li, b. bergman. influence of residual stress on elastic modulus and hardness of soda-lime glass measured by nanoindentation. journal of materials research 19(10):3109–3119, 2004. https://doi.org/10.1557/jmr.2004.0404 [9] j. kovář. computational modelling of nanoindentation. ph.d. thesis, brno university of technology, 2023. 23 https://doi.org/10.1557/jmr.2004.19.1.3 https://doi.org/10.1557/jmr.1992.1564 https://doi.org/10.2478/pmp-2022-0007 https://doi.org/10.21495/em2023-63 https://doi.org/10.21495/5896-3-298 https://doi.org/10.1016/0020-7225(65)90019-4 https://doi.org/10.3390/cryst8010009 https://doi.org/10.1557/jmr.2004.0404 acta polytechnica ctu proceedings 50:18–23, 2024 1 introduction 2 methods 2.1 fe model 2.2 corrections of the pile-up 2.2.1 oliver-pharr analysis 2.2.2 triangular correction 2.2.3 semi-elliptical correction 2.2.4 parabolical correction 2.2.5 finite element method 3 pile-up 4 comparison of the corrections 5 conclusions acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2024.49.0048 acta polytechnica ctu proceedings 49:48–54, 2024 © 2024 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague the concept of fractured rock-mass modeling using dfn-based statistical volume elements martin lebeda∗, petr kabele czech technical university in prague, faculty of civil engineering, department of mechanics, thákurova 7, 166 29 prague, czech republic ∗ corresponding author: martin.lebeda@fsv.cvut.cz abstract. the overall mechanical response of a fractured rock mass is, to a large extent, affected by naturally occurring fractures that exhibit sizes from millimeters to kilometers. thus, in analysis of underground structures, such as tunnels, it is required that the fractures’ influence on the stress and deformation state in the vicinity of the structure is taken into account. in the present work, we examine the applicability of the statistical volume element (sve) approach to determining the apparent stiffness tensor of an equivalent continuum representation of the fractured rock, which is then used in the framework of the finite element method. the equivalent continuum properties are determined by volume-averaging the effect of individual fractures that intersect the sve, while the fractures are represented using the “parallel plate model”. stochastically generated discrete fracture networks are used to represent the fractures’ geometry. presently, we solve the problem linearly for an incremental change of the stress state. an application of the concept is demonstrated on simulation of tunnel excavation. keywords: fractured rock mass, discrete fracture network (dfn), finite element method (fem), statistical volume element (sve). 1. introduction rock masses generally exhibit material heterogeneity at multiple scales. at the level of microscale, the heterogeneity can be associated with variable properties of mineral grains and microstructural defects. on the other hand, on the macroscale the behavior of rock mass is dominated by the stiffness of the rock matrix, presence of different types of discontinuities (generally called fractures) and the filling material or fluids inside the fractures. considering the typical dimensions of underground structures, such as tunnels, their analysis and design generally correspond with the macroscale. hence, when these structures are excavated in fractured rock, the effect of fractures must be taken into account [2]. the finite element method (fem) is often used for the analyses. then, one efficient way is to include only a few largest fractures as discrete discontinuities in the model, while the rest of the fractures is substituted by an equivalent continuum with effective or apparent properties of the rock mass. these overall parameters are evaluated by averaging or homogenization procedure [3]. in this paper, we focus on the equivalent continuum modeling and follow up on our previous work [4], where stochastically generated discrete fracture network (dfn) models [5, 6] provide a geometrical description of the fracture system in the rock mass. note that this approach makes it possible to utilize data from geological survey to statistically describe the fractured rock mass. the equivalent continuum methods are commonly used in conjunction with the concept of the representative volume element (rve) [7–9]. in principle, the rve is the smallest volume, which encompasses a sufficient sample of the substructures (e.g., fractures), so as to provide effective constitutive properties, which are nearly invariable with respect to the rve placement. on the other hand, on the scale of the whole analyzed problem (e.g., tunnel), rve must be small enough with respect to the spatial variation of macroscopic stress to represent a material point. our previous research [4] shows that, for dfns with power-law fracture size distribution (with minimum fracture size on the order of decimeters), the rve size should be on the order of tens of meters. however, in the case of tunnel analysis, this rve size does not meet the second criterion, as the stress shows high gradients in the close proximity of the excavation (units of meters). in this regard, la pointe et al. [10] questioned the applicability of the continuum simulations on the scale of 10 to 100 meters, while fracture sizes occurring in the rock mass may range from millimeters to kilometers. furthermore, ostoja-starzewski [11] pointed out the spatial randomness of the rock material and incorporated the concept of statistical volume element (sve) on the level of mesoscale, separating the microscale of heterogeneities from the macroscale that is usually connected with the rve approach. the procedure of determination of overall properties using volume elements smaller than representative is in agreement with widely accepted study presented by kanit et al. [12], which suggests the evaluation of the effective properties as mean values of apparent properties of rather small volumes considering sufficient number of realizations. against this background, 48 https://doi.org/10.14311/app.2024.49.0048 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 49/2024 the concept of fractured rock-mass modeling using . . . fracture set 1 2 3 4 5 6 7 8 9 fisher distribution mean dip [°] 85.365 36.871 3.360 87.507 62.308 88.629 78.236 88.629 83.253 mean strike [°] 222.949 49.597 313.665 316.780 133.438 346.277 1.040 269.482 85.957 κ [-] 35.056 2.704 28.633 25.351 2.913 18.730 3.815 17.300 3.962 power law distribution amin [m] 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 amax [m] 1 000 1 000 1 000 1 000 1 000 1 000 1 000 1 000 1 000 α [-] 3.328 3.062 3.800 3.789 3.382 3.042 3.000 3.607 3.296 p30 [m−3] 0.269 0.220 0.061 0.246 0.427 0.232 0.146 0.581 0.380 table 1. parameters of dfns [1]. we adopt the sve approach to simulations of tunnel excavation, while taking into account the spatial, orientational, and dimensional randomness of rock fractures. in this study, we present ongoing work on development of the 3d fe stress and displacement analysis that incorporates parameters of equivalent continuum obtained by averaging procedure in conjunction with the 3d stochastic dfn. we use the averaging method based on the approach proposed by oda et al. [9] that treats individual fractures by the “parallel plate model”, which means that the deformation response of a fracture to normal and shear stress is represented by two dimensionless stiffness parameters. at present stage, we have implemented the fe analysis linearly with constant fractures’ stiffness for an incremental change of stress state. although the linear solution is admissible only for small stress changes, the applicability of the sve concept can be demonstrated. 2. dfn model of fractures’ geometry the description of the rock-mass inherent fracturing state represents the crucial part of underground structures’ analysis. the stochastically generated dfn approach provides an adequate balance between the computational efficiency and the accuracy [5, 6]. the quantitative input information about simulated fracture network is often obtained by geological survey that usually consists of structural-geological mapping of fracture traces on rock outcrops or tunnel walls and of inspection of boreholes (observation windows). as the survey provides only a limited information about fractures deep in the rock mass volume, the data are used in the framework of so-called stochastic dfn modeling. in this approach, individual fractures are idealized as polygons, whose size, orientation, and spatial placement are assumed to follow certain probabilistic distributions. the distributions’ parameters are identified so as to match (in a statistical sense) the traces’ sizes, directions, and density within the observation windows. based on the calibrated probabilistic distributions any desired number of dfn realizations can be generated [13]. in the following parts of this study we use fracture network models generated by in-house software dfn_toolbox [14]. parameters of these dfns were calibrated based on data collected in the past by the czech geological survey at the underground research facility bukov, which is located at a depth of 550 metres below the earth’s surface. detailed description of the used measurement procedures and the acquired data are presented in report [1]. positioning of fractures’ centers in the model volume is ruled by poisson random generation process. the abundance of fractures is defined by volumetric density p30 (number of fractures per unit volume). fractures are modeled as squares, whose sizes a (radii of circumscribed circles) are controlled by the power law distribution with parameters amin, which is the minimum fracture size (location parameter), and α, which is the law’s exponent (shape parameter). for practical reasons, only fractures smaller than chosen value amax are generated. the orientation of fractures is determined by fisher distribution, with parameters mean dip and mean strike (from which the mean unit normal vector µ can be determined), and κ, which is the concentration parameter. the whole network consists of nine fracture sets, where each of them is defined by individual density, orientation, and size parameters. all of the parameters are listed in table 1. note that steeply dipping fractures prevail (namely sets 1, 4, 6, 8, 9) while subhorizontal (set 3) and mildly dipping fractures are less abundant. 3. averaging procedure the overall effective properties of fractured rock mass are calculated by averaging procedure that was originally proposed by oda et al. [9]. the main equation (1) of the procedure represents the relation between macroscopic stress σkl and the corresponding strain εij that are connected by the fourth order compliance tensor cijkl: 49 martin lebeda, petr kabele acta polytechnica ctu proceedings εij = 1 2g ( 1 2(δikδjl + δilδjk) − ν 1 + ν δijδkl ) σkl + 1 e [( 1 kn − 1 ks ) fijkl + 1 4ks (δikfjl + δjkfil + δilfjk + δjlfik) ] σkl = cijklσkl, (1) where g is the shear modulus, e is the young’s modulus, ν is the poisson’s ratio of the intact rock, kn is the nondimensional parameter related to the fracture’s normal, ks is the nondimensional parameter related to the tangent stiffness, δij is kronecker’s delta, fij and fijkl are so-called second and fourth rank crack tensors, respectively: fij = 1 v m∑ k=1 s(k)l(k)n (k) i n (k) j , (2) fijlm = 1 v m∑ k=1 s(k)l(k)n (k) i n (k) j n (k) l n(k) m , (3) where v is the volume of the domain over which averaging is performed, s(k) is the area of k-th fracture inside the sampling volume, l(k) is a typical size of the fracture (equal to the diameter of a circle with the same area), n (k) i are the components of a unit vector normal to the fracture. note that, the typical fracture size l is evaluated using the whole area of the fracture and not of the part inside the sampling volume because it is a property of the fracture independent on the size of the sampling volume. the dimensionless stiffness parameters kn and ks are, in this study, set while considering fractures’ mechanical response in the depth of hundreds of meters below the ground level, which corresponds, for example, to the potential location of a deep underground radioactive waste repository. it is assumed that the rock mass is geostatically compressed to the point that all fractures are closed, and their normal stiffness is nearly infinite. on the contrary, fractures are assumed to be vulnerable to slip in the case of changing stress state due to, for example, tunnel excavation, meaning that the shear stiffness is almost nil. in that sense, the parameters are set as kn = 10 000, ks = 1.0. further figure 1. illustration example of three sves corresponding to finite elements. in each sve, only the intersecting portions of the underlying dfn fractures are shown. investigation of this assumption is needed, however the present study deals with the applicability of the sve concept, which is feasible regardless of stiffnesses choice. 3.1. statistical volume element the concept of using the sve to obtain averaged mechanical properties of fractured and heterogenous materials has already been studied by other authors at the level of microscale, for example, [15, 16]. the most frequently used shapes of sve are square and circle in the case of 2d models and cube or sphere for 3d models. the size of the statistical sampling element should be chosen regarding coefficient of variation of the monitored variable [17]. in this study, we use either cubical sves with a fixed side size (in the concept verification stage, section 4) or sves, whose shape, size, and position correspond with those of the finite elements (for the fe simulations, section 5) to obtain the apparent stiffness tensor of the fractured rock mass. to this end, each sve’s compliance tensor is first calculated using equations (1) to (3), s(k), l(k), n (k) i in which are determined from all fractures of the underlaying dfn, which intersect the sve (figure 1). subsequently, the stiffness tensor is obtained by inverting the compliance. note that, in general, this procedure yields fully anisotropic tensors. finally, the stiffness tensors are assigned to the corresponding finite elements, which generally means that each finite element is composed of a unique material (figure 2). the whole process has been implemented by means of python api in the open-source fe code oofem [18]. 4. the sve concept verification first, we need to verify that the sve concept is applicable to analyses, such as, fe simulation of a tunnel excavation in a fractured rock modelled by dfn with parameters given in table 1. the verification is inspired by the study presented by kanit et al. [12]. in particular, we investigate, whether, when averaging 50 vol. 49/2024 the concept of fractured rock-mass modeling using . . . (a). distribution of the apparent modulus ex obtained by sves. for comparison, the value of ex calculated by averaging over large av on the same underlaying dfn is 27 100 mpa. (b). section view of the underlaying dfn (right). figure 2. distribution of the apparent modulus ex obtained by sves and section view of the underlaying dfn. is performed on volumes smaller than rve, similar apparent moduli are obtained when samples are taken from multiple locations of one realization of the dfn as well as by sampling one location in multiple dfn realizations. to this end, we use 10 different statistical realizations of the dfn model with parameters described in the section 2. the fractures are generated with centers in a cubical domain with the side size of 50 m. as a reference, we firstly calculate the effective moduli for all dfn realizations by volume-averaging over a sub-domain of 30 × 30 × 30 m3. the sub-domain is placed at the center of the larger dfn to reduce the boundary effect. based on a previous study [19], this sub-domain, denoted as large averaging volume (av) should be large enough to serve as rve. secondly, moduli are calculated from 10 sves with dimensions of 2 × 2 × 2 m3, which are placed randomly far apart but all in the central portion of one realization of the dfn. thirdly, moduli are obtained for one sve with dimensions of 2 × 2 × 2 m3, which is placed at the same location near the center in all 10 realizations of the dfn. the sve size was chosen so as to roughly correspond with the size of typical finite elements used in the subsequent simulations. although the complete stiffness tensors are calculated, we use young’s moduli ex, ey, ez, which correspond to the tensor elements d1111, d2222, d3333, respectively, to illustrate the results. the data shown in figures 3–5 confirm the expected difference between values obtained by using the sves and by using the large averaging volume. when moduli are evaluated over the large av, the results from 10 realizations show coefficient of variation (cov) of 5.2 % or less. in the case of the sve calculations, the results are more dispersed with cov ranging between 15.8 and figure 3. comparison of stiffness ex. the boxes indicate the quartiles, the whiskers indicate the minimum and maximum and the cross indicates the mean. figure 4. comparison of stiffness ey. the boxes indicate the quartiles, the whiskers indicate the minimum and maximum and the cross indicates the mean. 51 martin lebeda, petr kabele acta polytechnica ctu proceedings figure 5. comparison of stiffness ez. the boxes indicate the quartiles, the whiskers indicate the minimum and maximum and the cross indicates the mean. 19.7 % and the mean values of apparent moduli are higher. furthermore, both sves approaches (spatial sampling and multiple realizations) are in a fair agreement. especially, in the case of the ey and ez, the mean values differ by less than 2 %. in the case of the difference is approximately 7 %. 5. numerical simulation of tunnel excavation to demonstrate the use of the sve modeling approach, 3d simulation of tunnel excavation is performed. the modelled rock domain consists of 30×30×10 m3 prism, where the short dimension is parallel to the tunnel axis. the tunnel passes through the center of the domain and has a diameter of 3 m. the displacement of the rock body is constrained by statically determinate supports. the excavation problem is solved in two calculation steps, see figure 6. in the first step the initial stress state is induced and in the second step the excavation of the tunnel is simulated by removal of elements. to impose the initial stress state, the fe model is loaded by normal stresses that are applied to the outer surface of the rock body. the initial loading stress tensor is based on field measurements from rožná mine [20], which is a part of the underground research facility bukov. the data from vertical boreholes showed that the directions of the principal stresses are vertical and horizontal. the vertical principal stress is calculated as: sv = −ρ · g · h, (4) where ρ is volumetric weight, g is gravity acceleration, h is depth beneath the ground level. the parameters for calculation of the vertical principal stress (table 2) are adopted from [21]. as the major (a). state prior to excavation. (b). state after excavation. figure 6. the finite element mesh. parameter value major horizontal principal stress sh [mpa] −22.8 major horizontal principal stress sh [mpa] −14.6 volumetric weight ρ [kg m−3] 28 000 depth of the mine beneath the gnd level [m] 550 vertical principal stress sv [mpa] −15.4 table 2. the initial loading stress parameters. and minor horizontal principal stresses, sh and sh, respectively, we use the average of the values measured on different levels of the mine. the values of the initial stress are listed in table 2. orientation of the model is set so, that the tunnel axis is parallel with the minor horizontal principal stress, which means that the direction of sh corresponds to the global axis y. for the purpose of comparison, simulations are calculated, firstly, using the uniform effective stiffness tensor, obtained by averaging over the large volume, in all finite elements. secondly, the concept of sve is applied with the same underlaying dfns. in the latter case, the stiffness tensor is calculated by averag52 vol. 49/2024 the concept of fractured rock-mass modeling using . . . ing over sves that coincide with the finite elements, which yields nonuniform stiffness over the analyzed domain, as illustrated in figure 2. specifically, the values of modulus ex are plotted over individual elements along the front cross-section perpendicular to the tunnel axis located at y = −3.75 m. the figure also shows the section of the underlaying dfn. it is noted that the case of uniform stiffness, the stress state prior to the tunnel excavation is also uniform within the modelled domain. on contrary, the calculations using the sve concept exhibit nonuniform initial stress state, which is caused by different material stiffness in each finite element (see figure 2). 5.1. results of the study the calculations were performed for 10 realizations of the stochastic dfn. the main monitored result values are horizontal and vertical convergences of the tunnel walls measured on the cross sections passing through the tunnel center, see figure 7 and figure 8. the results show that the absolute values of tunnel walls convergences are lower in the case of the sve approach, which is in agreement with section 4, where higher stiffnesses were obtained while using sves. however, the variances of the results are comparable regardless of the size of the averaging volume. further investigation of this phenomenon is needed since it may be caused by various effects, e.g., low number of statistical realizations or the specific spatial, orientational, and dimensional characteristics of the dfn fracture sets. 6. conclusion the applicability of the sve concept incorporated in the 3d fem simulation of tunnel excavation using overall rock-mass parameters obtained by averaging procedure based on dfn has been examined. the results of the study lead to the following conclusions: • the variance of the overall moduli obtained by averaging over sves (cov = 15.8–19.7 %) is higher compared to averaging over large volume (cov = 5.2 %), which is an expected outcome. • the agreement of the two different sve evaluation methods (spatial sampling and multiple realizations) is fairly good with the highest difference of 7 %. • lower convergences of tunnel walls appear while using the sve approach compared to large averaging volume, which is in agreement with the higher stiffnesses obtained in case of sves. • on the other hand, the variance of convergences of tunnel walls are comparable regardless of the size of the averaging volume. this phenomenon requires further investigation since it is not an expected outcome. figure 7. horizontal convergence of tunnel walls. the boxes indicate the quartiles, the whiskers indicate the minimum and maximum and the cross indicates the mean for 10 realizations of stochastic dfn. figure 8. vertical convergence of tunnel walls. the boxes indicate the quartiles, the whiskers indicate the minimum and maximum and the cross indicates the mean for 10 realizations of stochastic dfn. acknowledgements this paper was financially supported by czech technical university in prague under sgs project no. sgs23/032/ohk1/1t/11. references [1] l. gvoždík, p. kabele, j. říha, et al. transport of radionuclides from deep geological repository/testing of conceptual and numeric models. tech. rep., progeo s. r. o., 2020. 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[2023-08-22]. http://www.oofem.org [19] m. lebeda, p. kabele. the effect of sampling volume size on the apparent stiffness of jointed rock mass. acta poytechnica ctu proceedings 34:38–42, 2022. https://doi.org/10.14311/app.2022.34.0038 [20] z. bukovská, et al. súrao tz 464/2020. získání dat z hlubokých horizontů dolu rožná [in czech; data acquisition from the deep horizons of the rožná mine]. tech. rep., súrao – správa úložišť radioaktivních odpadů, 2020. [21] k. souček, m. vavro, l. staš, et al. súrao zz 221/2018. komplexní geologická charakterizace prostorů pvp bukov – část ii geotechnická charakterizace [in czech; complex geological characterization of the bukov urf – part ii geotechnical characterization]. tech. rep., súrao – správa úložišť radioaktivních odpadů, 2018. 54 https://doi.org/10.14311/app.2023.40.0061 https://doi.org/10.1029/wr020i001p00079 http://www.iaea.org/inis/collection/nclcollectionstore/_public/38/013/38013872.pdf http://www.iaea.org/inis/collection/nclcollectionstore/_public/38/013/38013872.pdf https://doi.org/10.1061/jmcea3.0002642 https://doi.org/10.1016/0167-6636(92)90004-w https://doi.org/10.1007/bf01023618 https://doi.org/10.1016/j.probengmech.2005.07.007 https://doi.org/10.1016/j.probengmech.2005.07.007 https://doi.org/10.1016/s0020-7683(03)00143-4 https://gitlab.com/pkabele/dfn_toolbox https://doi.org/10.1016/j.compgeo.2019.103229 https://doi.org/10.1016/j.cma.2022.114772 https://doi.org/10.1016/j.actamat.2006.03.042 http://www.oofem.org https://doi.org/10.14311/app.2022.34.0038 acta polytechnica ctu proceedings 49:48–54, 2024 1 introduction 2 dfn model of fractures' geometry 3 averaging procedure 3.1 statistical volume element 4 the sve concept verification 5 numerical simulation of tunnel excavation 5.1 results of the study 6 conclusion acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2023.41.0066 acta polytechnica ctu proceedings 41:66–70, 2023 © 2023 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague safety analysis of the method of implementation of crash cushions tereza šimková∗, michal frydrýn czech technical university in prague, faculty of transportation sciences, konviktská 20, 110 00 prague 1, czech republic ∗ corresponding author: simkote1@cvut.cz abstract. the topic of the article are vehicle restraint systems. specifically, the paper focuses on the safety analysis of the way of implementation of crash cushions. within the safety analysis carried out on the territory of the ústí region of the czech republic, the most frequently occurring deficiencies in crash cushions and their possible impacts on road safety were identified. the output is the creation of catalogue sheets of crash cushions, which contain the most frequently installed crash cushions on the road network and a summary of methodological requirements for their adequate implementation. the work is intended to serve as an effective tool and a quality source of information for road managers and road safety auditors during their subsequent inspection. keywords: crash cushion, passive safety, traffic accident, road safety systems, safety analysis. 1. introduction road transport is one of the world’s most used modes of transport, and the road network is being intensively developed (reconstructed and built) over time. an integral part of this is the increasing degree of automobile use and the higher density of vehicles on the roads. the development of road transport brings a number of positives in the form of more efficient accessibility between territorial units, but also negatives. a typical example is the high number of traffic accidents. traffic accidents cause both health and economic losses. with the development of modern technology, both active vehicle safety, such as various assistance systems and technical devices aimed at preventing accidents, and passive safety, aimed at mitigating the consequences of accidents, are improving. however, the number of road accidents and accidents with serious consequences is still high and it is necessary to focus on developing [1–3] a safe transport system with socially acceptable levels of safety. in the event of an accident, the aim of passive safety is to mitigate the consequences as much as possible. among the elements which primarily help to increase passive safety on roads are road restraint systems, such as crash barriers or crash cushions to which this article is devoted. crash cushions are mainly used to protect isolated obstacles and dangerous places in the vicinity of roads where it is not possible to install crash barriers or other elements [4, 5]. the purpose of the crash cushion is to absorb kinetic energy and safely restrain the vehicle or redirect it back into the lane. the correct effectiveness of a crash cushion occurs provided that it is correctly implemented. otherwise, the consequences of road accidents could be aggravated or, if not correctly implemented, they can also cause an accident. 2. crash cushions crash cushions are classified as road restraint systems and are placed mainly where for spatial reasons it was not possible to install a crash barrier and the placement of a crash cushion seems to be a more suitable solution. one of the reasons for this may be, that one of the parameters of the barriers, such as the minimum length of the barrier in front of a fixed obstacle, cannot be respected and the solution is to install a crash cushion. crash cushions are divided into two basic types according to their function, which are gating and non-gating crash cushions. unlike non-gating crash cushions, gating crash cushions are also tested for side impacts. their purpose is not only to restrain the vehicle but also to redirect it back into the lane. in the case of side impacts, they perform a similar function as crash barriers. non-gating crash cushions are placed in places where side impacts are not expected and are designed only for frontal impacts. the resulting choice of crash cushion depends on the type of road where the crash cushion is installed and the type of obstacle or danger point which is supposed to protect. crash cushions shall comply with the requirements set out in the technical literature specifying the spatial arrangement requirements, their containment level or requirements defining the connection to other road restraint systems. failure to comply with the requirements defined in the technical literature could result in malfunctioning and possible loss of the ability to restrain the vehicle or negatively affect the accident and its consequences [6]. 2.1. containment level the basic parameter of crash cushions is the containment level, which is the amount of impact that the crash cushion is able to resist in order to effectively 66 https://doi.org/10.14311/app.2023.41.0066 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 41/2023 safety analysis of crash cushions speed limit minimal containment level >110 kmh−1 110 >90 and≤110 kmh−1 100 >70 and≤90 kmh−1 80, 80/1 ≤70 kmh−1 50 table 1. the containment level of crash cushions on roads [7]. restrain or redirect the vehicle. in general, the resulting containment level depends primarily on the speed limit at the location where the crash cushion is installed. the containment levels are divided into several classes, which correspond numerically to the speeds of the vehicles in the performed impact tests [6]. the different containment levels are classified according to the impact tests they must meet for that level. in total, there are 18 types of impact tests, which are defined by several parameters that must be met for a specific impact test. these parameters include the impact track to the crash cushion, the total vehicle mass and the impact speed. in total, five containment levels are defined and the choice of the containment level on the road in a specific location depends on the maximum permitted speed in a specific section [7]. table 1 shows the individual containment levels for individual maximum permitted speeds on the road. crash cushions are only tested by passenger vehicles. for other motor vehicles, such as trucks, crash cushions are not adapted, as it would be difficult to construct such a crash cushion in terms of space and economic requirements. these crash cushions would have to have a large number of shock absorbing segments, which would significantly affect its size and make it virtually impossible to install on the road network. at the same time, its design would be unsuitable for the impact of passenger vehicles due to the required stiffness it would have to have to restrain trucks, which could exacerbate the consequences of such accidents in the case of an impact. 3. safety analysis of crush cushions in order to verify the correctness of the implementation of the crash cushions, a local traffic safety investigation was carried out on 18 crash cushions. specifically, these are crash cushions located on the road network in the ústí nad labem region. as part of the inspection, a detailed check of the technical condition of the crash cushions and a check of the level of road traffic safety in the given locations were carried out. at the same time, it was verified whether the crash cushions were made in accordance with the relevant technical regulations such as en 1317-2 [8], en 1317-3 [7], tp 114 [9], tp 158 [6]. additionally, the potential deficiencies in the construction of the crash cushions were identified. 3.1. classification of crash cushions defects several types of deficiencies that may have an impact on road safety have been identified as part of the safety analysis of selected crash cushions. the most frequently occurring deficiency identified in the monitored crash cushions is the inappropriate placement of the crash cushion in the chevron marking. according to tp 158 [6], it is possible to place a crash cushion in the chevron marking, provided that the specified minimum distances of the crash cushion from the nearest outer edge of the chevron marking are observed. however, from the point of view of road safety it is more appropriate to place the crash cushions outside the chevron marking on the unpaved part of the road, if the spatial conditions allow it. this approach generally preserves more space for possible emergency manoeuvres in the area of the chevron marking, which has the positive consequence of increasing safety on the roads. in the event of a loss of control of the vehicle, the chevron marking serves as an area where the vehicle can be returned to the lane and the manoeuvring space is significantly reduced by placing the crash cushion in this area. another category of deficiencies that were identified during the analysis is the unjustified placement of the crash cushion, or crash barriers that are connected to the crash cushion. in general, road safety systems are placed, where there is an increased danger in the vicinity of the road and in places where there is an identified risk of collision with other road users. generally, it is more pleasant for road users that, in the event of loss of control, the vehicle moves into a free space and gradually decelerates, rather than hitting an unjustifiably installed restraint system [10]. in several cases, it was found that crash cushions are installed in places where there are no obstacles or dangerous places that would represent traffic safety risks. on the contrary, their presence can increase the probability of a traffic accident. if the vehicle goes off the road, it may hit the crash cushion and worsen the severity of the accident than if the vehicle were to go into a free space, where it would slow down gradually and thus no collision would occur. based on the findings, it is recommended to install crash cushions only in situations that pose a traffic safety risk and the installation of a crash cushion will improve the current level of safety. the same is applicable for the crash barriers, which are connected to the crash cushions. the necessity of installation crash barriers behind the crash cushion depends on the type of fixed obstacle or place of danger to be protected. a common shortcoming is placement of crash barriers behind the impact attenuator even if it only protects a separate fixed obstacle. more appropriate solution is to place the impact attenuator immediately in front of the fixed obstacle and deflect the impact attenuator at such an angle that the fixed obstacle is protected on both sides. in this case, the crash barriers do not have to continue behind the crash cushion, because 67 tereza šimková, michal frydrýn acta polytechnica ctu proceedings they would not have any protective function at all. as already mentioned, the main parameter of crash cushions is their containment level. very often it has been found during local road safety inspections that the crash cushion has a higher containment level than is necessary for a specific speed. however, it should be noted that a crash cushion with a containment level than is required at a certain location does not have a negative effect on the level of traffic safety. the detected negative effect on traffic safety is only from the point of view of the spatial arrangement in the vicinity of the crash cushion. crash cushions with a higher containment level generally have larger dimensions and their part may in some cases restrict the space for possible emergency manoeuvres. it is therefore appropriate in these cases to place the crash cushion at a containment level that corresponds to the local maximum permissible speed and does not interfere with the chevron marking due to its size [6, 7]. 4. catalogue sheets of crash cushions during the safety analysis of the crash cushions it was found that only five of the eighteen locations monitored could be considered to have a socially acceptable design of crash cushions in terms of road safety. these conclusions show that although these are passive safety elements that should contribute to increasing road safety, the placement of crash cushions is not always implemented in accordance with road safety. based on the resulting knowledge, catalogue sheets of crash cushions have been created, which contain the most frequently occurring types of crash cushions on the road network in the czech republic. the catalogue sheets contain an overview of the containment levels of crash cushions, which are their main parameter and are decisive for choosing the type of crash cushion in a specific location [11, 12]. for the correct implementation of crash cushions and maintaining a socially acceptable level of safety, the catalogue sheets also include a summary of methodological requirements according to which it is desirable to implement crash cushions. furthermore, the catalogue sheets contain the principles of placement of crash cushions, the rules of their spatial arrangement, as well as the design requirements of transitions of crash cushions to crash barriers. in total, the datasheets contain 65 specific types of crash cushions from six companies. (snoline spa, hiasa grupo gonvarri, alpina sicherheitssysteme gmbh, sps – schutzplanken gmbh, luco grande lavori stradali, saferoad holland b.v.). this includes both gating crash cushions, which redirect the vehicle in the event of side impacts, and nongating crash cushions, which are designed for frontal impacts only. the catalogue sheets divide the crash cushions into groups according to the manufacturing company and similar design features. the crash cushions then differ mainly in their dimensions, and with this their containment level varies, as it is generally the case that the longer the crash cushions are, the higher their containment level increases. each sheet contains a group of similar types of crash cushions from the same manufacturer and several parameters are given for each specific variant of crash cushion. the first parameter is the containment level of the crash cushion and then the speed for which the crash cushion is adapted. at the same time, the containment level of the crash cushion should correspond to the maximum permissible speed in the section where this safety element is installed. other parameters are the basic dimensional values of the crash cushions. the length and width of the crash cushion, or for wedge-shaped types of crash cushions, the width at the front of the crash cushion (front width) and the width at the rear of the crash cushion (rear width) are given. the technical drawings of each type of impact cushion with basic dimensions are also an integral part of the resulting catalogue to help visually identify the type of impact cushion. an example of a catalogue sheet for a crash cushion is shown in figure 1. 5. conclusion the aim of passive safety systems, which in this case are specifically crash cushions, is to minimise the consequences of road accidents. in order to ensure an adequate level of road safety, it is therefore desirable that all safety systems are implemented not only in accordance with the conditions set out in the technical literature, but also with regard to the specific location, and that the situation is assessed comprehensively in order to maintain a socially acceptable level of safety. for this purpose, for example, a road safety audit or other similar tools may be used, into which the assessment will be integrated. as part of the safety analysis of crash cushions, it was found that crash cushions are often not implemented in accordance with traffic safety assumptions and in some cases not even in accordance with the requirements in the technical literature [6, 7]. in order to increase the safety of traffic, the above-mentioned catalogue sheets of the most frequently installed crash cushions have been created, which also contain a summary of the methodological requirements for implementation of the crash cushions. the catalogue sheets of crash cushions can also be used by road construction designers and road safety auditors, who will be able to distinguish the type of crash cushion according to the sheets and verify the correctness of its implementation. it will also be possible for the professional public to determine the parameters that belong to a specific crash cushion. it will also make it easier to select the type of crash cushion for specific locations according to the parameters that are optimal for the situation in terms of safety. this will provide a broader range of knowledge in the field of road safety systems, which can be used effectively, for example, in the preparation of safety inspections or safety audits and thus contribute to improvement road safety. 68 vol. 41/2023 safety analysis of crash cushions figure 1. catalogue sheet of crash cushion for tau medium. 69 tereza šimková, michal frydrýn acta polytechnica ctu proceedings references [1] h. i̇brahim yumrutaş, z. othman ali. experimental performance evaluation of an innovative hybrid barrier system filled with waste materials. construction and building materials 316:125231, 2022. https://doi.org/10.1016/j.conbuildmat.2021.125231 [2] t. silva, j. neves. ascendi’s safety barriers upgrading program. in a. akhnoukh, k. kaloush, m. elabyad, et al. 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[11] j. nováček, t. kohout, p. vrtal, et al. application of rsa principles to improve the bim in the road design process with focus on the road restraint systems. the international archives of the photogrammetry, remote sensing and spatial information sciences xlvi-5/w1-2022:159–164, 2022. https://doi.org/ 10.5194/isprs-archives-xlvi-5-w1-2022-159-2022 [12] z. svatý, k. kocián, t. mičunek. integration of safety assessment in bim for transportation infrastructure. the international archives of the photogrammetry, remote sensing and spatial information sciences xlii-5/w3:143–148, 2019. https://doi.org/10.5194/ isprs-archives-xlii-5-w3-143-2019 70 https://doi.org/10.1016/j.conbuildmat.2021.125231 https://doi.org/10.1007/978-3-030-79801-7_23 https://doi.org/10.7307/ptt.v25i6.436 https://doi.org/10.3390/ma15051712 https://doi.org/10.1016/j.trpro.2021.07.076 https://doi.org/10.5194/isprs-archives-xlvi-5-w1-2022-159-2022 https://doi.org/10.5194/isprs-archives-xlvi-5-w1-2022-159-2022 https://doi.org/10.5194/isprs-archives-xlii-5-w3-143-2019 https://doi.org/10.5194/isprs-archives-xlii-5-w3-143-2019 acta polytechnica ctu proceedings 41:66–70, 2023 1 introduction 2 crash cushions 2.1 containment level 3 safety analysis of crush cushions 3.1 classification of crash cushions defects 4 catalogue sheets of crash cushions 5 conclusion references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2024.49.0036 acta polytechnica ctu proceedings 49:36–40, 2024 © 2024 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague measurement of tensile properties of selected rovings věra kabíčkováa,b,∗, jakub hájeka,b, jan macháčeka,b, eliška kafkováa, tomáš vlacha,b a czech technical university in prague, faculty of civil engineering, department of architectural engineering, thákurova 7, 166 29 prague, czech republic b czech technical university in prague, university centre for energy efficient buildings, třinecká 1024, 273 43 buštěhrad, czech republic ∗ corresponding author: vera.kabickova@fsv.cvut.cz abstract. this article is focused on measuring the tensile properties of chosen fibre materials, such as flax, alkali resistant glass or viscose. all selected materials were tested both pure and impregnated using epoxy resin. in this article, the tensile strength and the young’s modulus were observed. young’s modulus was measured using digital image correlation. the suitability of the location of small speckle pattern targets, which are used for measurements of deformations, was first verified on the viscose fiber samples. targets were applied directly to the roving and further to the sleeves made of epoxy resin used to fix samples in the test equipment. after the evaluation, the selected location was then applied to the selected types of rovings. finally, the measured results were compared. keywords: roving, tensile strength, modulus of elasticity, digital image correlation. 1. introduction civil engineering in general and especially cement production remains as one of the highest co2 emitters in the industrial sector. significant efforts need to be made on promoting material efficiency and innovation in order to limit its negative impact on the environment [1]. this goes hand in hand with the development of alternative reinforcements in the form of composite and various fibers. fiber-reinforced polymer (frp) rebars as an alternative composite reinforcement have been considered as a non-corrosive alternative, but they are also associated with specific disadvantages [2]. synthetic fibers such as carbon, basalt or alkali-resistant glass reach high mechanical properties, frp rebars as a combination of epoxy resin with these fibers are logically less stiff and significantly less ductile when compared to steel rebars [3]. for the frp reinforcements, there are several rovings (yarn) together in one rebar. finer reinforcement from individual roving woven into technical textiles is also used as an alternative reinforcement for subtle concrete elements or in combination with traditional steel reinforcement for constructions. this type of reinforced concrete is called textile reinforced concrete (trc). technical textiles are usually applied without any impregnation, only with surface alkali-resistant coating, or impregnated using epoxy or polyester resin. the basic material parameter of this reinforcement in addition to the linear density of the yarn, interaction conditions and other basic information is the tensile strength of the reinforcement (single roving) and a young’s modulus of elasticity in tension. this article deals with these two basic material parameters. for this article, which is focused on the basic tensile parameters, the procedure according to aci 440.3r-04 about the test methods for fibre-reinforced polymers (frp) for reinforcing or strengthening concrete structures was chosen and adapted, but due to the smaller cross-sectional area, it was more inspired by [4]. this method was developed for multi-filament yarns without any impregnation, but the same testing procedure was applied to the tensile testing of single roving with epoxy polymer matrix. to secure the samples to the testing machine epoxy sleeves on both sides of roving were used. comparison of different methods for the determination of the modulus of elasticity of composite reinforcement produced from roving was also described in [5], the tensile strength and static modulus of elasticity were measured and calculated in [6]. 2. materials and methods 2.1. materials and samples for the experiment, three different fiber materials were chosen – alkali resistant glass, flax and viscose (figure 1). alkali resistant glass is already commonly used as a reinforcement in concrete structures, but research shows that natural fibers can also be considered as an alternative concrete reinforcement [7]. flax has the best mechanical parameters of natural fibers. on the other hand, viscose, which is produced from natural material and is industrially compostable, has a great ductility. these materials were mutually compared. the alkali resistant glass fibres roving cemfil® 5325 had the length weight of 2 400 tex 36 https://doi.org/10.14311/app.2024.49.0036 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 49/2024 measurement of tensile properties of selected rovings (a). alkali resistant glass, flax and viscose fibers. (b). preparation of samples. figure 1. rovings used. fiber type specific gravity [g/cm3] t [tex] tensile strength [mpa] elastic modulus [gpa] ar glass (cem-fil® 5325) 2.68 2 400 >1 000 72.0 flax 1.40–1.50 1 680 343–2 000 27.6–103.0 viscose 1.51 1 840 210–530 3.0–4.5 table 1. material characteristics of used roving [8–13]. (=2 400 g km−1), the specific gravity of 2.68 g cm−3, the tensile strength of more than 1 000 mpa and the modulus of elasticity of 72 gpa. the theoretical maximum tensile force before breaking of all fibers at one time calculated from the technical data sheet is more than 896 n [8]. another material used is flax. the chosen roving had the length weight of 840 tex, so two rovings were laid next to each other to reach a value of approximately 1 600 tex for better comparison [9] the primary use of natural flax roving is in the food industry, which means that there are not many mechanical properties available. in addition, its mechanical properties vary depending on many conditions, such as the production process (growth conditions, harvesting, etc.) or relative humidity. in literature, it can be found that the specific gravity is around 1.5 g cm−3, the tensile strength is between 343–2 000 mpa and the modulus of elasticity varies between 27.6–103 gpa [10, 11]. the last selected material was viscose. the viscose fibres roving viscord® had the length weight of 1 840 tex. viscose fibers have the specific gravity of 1.51 g cm−3 and usually reach the modulus of elasticity of 34 mpa. there is a certain scatter of tensile strength, which varies between 210–530 mpa [12, 13]. the characteristics of the materials can also be seen in table 1. some rovings were impregnated with 2-part epoxy resin sikafloor 150. the mixing ratio of part a and part b was 74:26 by weight [14]. to ensure fixation of the samples to the testing machine, both ends were fitted with the epoxy prisms 15 × 15 × 100 mm, as seen in figure 1. the distance figure 2. the location of the small speckle pattern targets. between these prisms was 300 mm. this method of preparation followed a novel tensile test device for effective testing of high-modulus multi-filament yarns by r. rypl et al. [4]. 2.2. experiment for the experiment, galdabini quasar 100 hydraulic testing machine was used. the samples are fitted with small speckle pattern targets, which allows the area to be tracked, and the deformations are then calculated. for this experiment, two locations of the targets were chosen – directly to the roving (figure 2, blue arrows) and further to the epoxy prisms (figure 2, black arrows). the suitability of the location of these targets was first verified on the viscose fiber samples and the selected location was then applied to the other types of rovings. especially in the case of samples without epoxy resin (pure roving), the small speckle pattern targets may rotate during the experi37 v. kabìčková, j. hájek, j. macháček et al. acta polytechnica ctu proceedings figure 3. comparison of chosen viscose data (dic_obj – the targets on the prisms, dic – the targets on the roving). fiber type σmax [mpa] edic_roving [gpa] edic_prisms [gpa] deviation [%] cell epox_2 610 6.93 7.67 10.8 cell epox_3 621 7.50 8.68 15.7 cell epox_4 612 7.21 8.18 13.4 cell epox_5 569 6.38 7.28 14.1 cell epox_6 521 5.42 6.80 25.3 average 587 6.69 7.72 15.5 table 2. data of tensile strength and young’s modulus of elasticity measured on the viscose samples using dic. ment. this may cause deviations in results, or even interruption of measurement. speckle pattern targets on the edge of epoxy prisms do not rotate and are easily applied, but apparently will be influenced by a change of the stiffness. the samples were tested in tension with a constant loading speed of 1 mm min−1 and the force and displacement were recorded. another observed value was the young’s modulus. for the measurement of this value, dic (digital image correlation) was used. photos taken during the experiment are then evaluated by the istra4d software. the photos for the dic analysis were taken in an interval of 0.5 seconds. as mentioned in the introduction, the test method was inspired by standard of american concrete institute aci 440.3r-04 and adapted due to the smaller crosssectional area. this adjustment was mostly inspired by [4] for multi-filament yarns without any impregnation and used also for the homogenized ones. 3. results and discussion first, the effect of the location of small speckle pattern targets was measured. in previous measurements an extensometer was used for the alkali resistant glass roving and results were similar [15]. using extensometer is more reliable, because it does not matter whether the roving rotates during the loading process. but also it is more demanding and can’t be used at pure rovings because of the damage of fibers during the installation. in contrast with extensometer, using dic is easier, but the setup of the experiment must be more thorough. one the other hand, it is possible to measure a larger number of samples in a short time. but there is another problem associated with dic and that is the location of the targets – when the targets are on the roving, as previously mentioned, they tend to rotate during the experiment (as seen in figure 2 on the upper target) and there is a risk that the dic camera would stop measuring due to the loss of the targets. placing the targets on the epoxy prisms would solve this problem ever since it cannot rotate, on the other hand final results can be affected by many other influences (such as the effect of the supports). in this part of the article, the impact of targets’ location was first verified on the viscose fibers rovings. according to the technical data sheet, the viscose has the highest elongation and seems to be the best for this verification. figure 3 shows that the courses of the curves are similar, but the slopes of the curves measured from prisms are steeper. in table 2, the results of measurements are presented. we can see a comparison of the young’s modulus and the deviation from the values measured from rovings despite the fact that the targets were placed on the edge of the epoxy prism. the values measured from prisms are higher than from roving and the average deviation is 15.5 %. this deviation is caused by higher stiffness of the prisms. it can also be seen table 2 that the deviation fluctuates 38 vol. 49/2024 measurement of tensile properties of selected rovings figure 4. stress-strain chart of chosen samples of pure and impregnated rovings. fiber type σmax [mpa] e [gpa] flax epox 522 25.93 flax pure 126 8.10 glass epox 1 534 75.03 glass pure 390 52.05 viscose epox 587 6.69 viscose pure 389 5.86 table 3. average data of tensile strength and young’s modulus of elasticity. significantly. placing the targets on the epoxy prisms is therefore not very reliable. afterwards, all types of rovings, both pure and impregnated, were measured using dic and, according to the results above, the targets were placed directly to the rovings. the results in the figure 4 and table 3 shows that the pure rovings reach lower strength and stiffness than the impregnated rovings. it is caused by the fact, that the stress cannot be transmitted through the whole cross-section of roving as in the case of the impregnated roving. all fibrils, thanks to the epoxy resin homogenization, then collapse at once. it was also found that values of impregnated alkali resistant glass rovings are closer to those in the literature compared to the pure ones (measured young’s modulus of pure glass roving is 52 gpa, measured young’s modulus of impregnated glass roving is 75 gpa and the value of the young’s modulus in the literature is 72 gpa). however, this conclusion cannot be applied to all types of fibers. since mechanical properties of natural fibers depend on many conditions (in addition, in the case of viscose fibers, many mechanical properties are not available), most of the measured values fit within the range of values in the literature. the highest young’s modulus reach glass fibers with the value of 75 gpa, the lowest reach viscose fibers with the value of 6.7 gpa. this seems to be significantly lower, on the other hand, viscose fibers show by far the highest ductility, which may be useful in terms of reinforced concrete. 4. conclusion as expected, pure rovings without epoxy resin have significantly worse results, because there are gradual violations of individual fibrils. similar behavior like presented results happens inside of the concrete matrix with textile reinforcement produced from rovings. in the textile reinforcement without impregnation, there is no full saturation of roving by cementitious particles and the fibrils collapse gradually. this leads to a reduction in load-bearing capacity, but to an increase and better behavior in ductility. the suitability of the use of pure, partially, or fully homogenized roving is determined by the specific applications and requirements. presented comparison of the selected materials was also more or less expected. the best potential as a reinforcement of concrete has alkali resistant glass roving, which 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introduction 2 materials and methods 2.1 materials and samples 2.2 experiment 3 results and discussion 4 conclusion acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2025.52.0050 acta polytechnica ctu proceedings 52:50–56, 2025 © 2025 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague commonly used methods to monitor driver’s drowsiness and fatigue with a focus on the interior camera filip kotas∗, jan leistner czech technical university in prague, faculty of transportation sciences, konviktská 20, 110 00 prague 1, czech republic ∗ corresponding author: kotasfil@cvut.cz abstract. some of the main causes of road accidents nowadays are fatigue, drowsiness and driver inattention. in many cases, these accidents are fatal and can cause significant financial loss. most of these accidents happen between midnight and 6am. one way to reduce the number of accidents caused by these causes is to use an interior camera. this camera can detect fatigue or drowsiness and alert the driver in time, and in the worst case, take over the car and stop safely at the side of the road. the aim of this article is to provide a comprehensive overview of the methods used to detect driver fatigue, drowsiness and inattention. a further objective is then to develop and process a survey that focuses on public awareness and opinions on the integration of interior cameras in vehicles across europe. keywords: adas, driver drowsiness, driver fatigue, driver attention system, vehicle interior camera. 1. introduction in the last few years, traffic safety has become an increasingly popular topic in the automotive industry. the main goal of this research is to reduce the number of accidents related to driver fatigue and distraction. according to [1], from 2022, the number of fatal accidents on the roads in the united states, was 693, which is 1.6% of all fatal accidents in that year. in europe, the number of accidents caused by driver fatigue is around 20% [2]. this situation increases the pressure to develop technologies to prevent those kinds of accidents from happening. this pressure is evidenced by the european parliament regulations that require car manufacturers in europe to incorporate these systems in their vehicles to pass the homologation. one of the latest approaches in accident prevention is the integration of interior cameras that have the ability to monitor driver behaviour and detect early signs of fatigue, drowsiness and inattention. there are several approaches used for this detection: facial and eye movement analysis, detection of the position of the head and the rest of the body or monitoring physiological signs of fatigue such as falling heart and respiratory rates. however, it has not yet been possible to develop a system that is 100% reliable under all operating conditions. the introduction of interior cameras brings a lot of benefits on one hand, but they can raise questions about privacy and ethics on the other. public awareness will play an important role in the successful implementation of these systems. in order to get a better idea of public opinion towards these technologies, a survey has been created with a focus on public awareness and views on the deployment of interior cameras in vehicles in europe. the aim of this article is to provide an overview of methods for detecting driver fatigue, drowsiness and inattention using an interior camera. it also focuses on the analysis of the results of a survey that maps public awareness and opinions towards these technologies. this research aims to contribute to a better understanding of how the public perceives these new technologies and to offer recommendations for the effective implementation of these technologies within the european union. 2. methods of monitoring the driver’s state 2.1. classification of methods to detect driver fatigue and drowsiness methods to detect driver fatigue and drowsiness can be divided into two categories: invasive and non-invasive. invasive methods monitor physiological parameters and often require sensors to be in contact with the body. examples of such methods are the eeg (electroencephalogram), which monitors brain activity, or the ecg (electrocardiogram), which monitors the electrical activity of the heart. these methods often provide more accurate information but are difficult to use and can cause discomfort to the driver. noninvasive methods are preferred for their convenience and less intervention for the user. they can, for example, analyse facial expressions, eye movements or detect changes in driving. methods are based on sensing using external technology such as an interior camera or sensors in the vehicle. another division that can be used is based on the observed parameters: behavioural, psychological and vehicle parameters. behavioural parameters include 50 https://doi.org/10.14311/app.2025.52.0050 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 52/2025 driver drowsiness and fatigue with interior camera monitoring changes in driver behaviour, changes in driving style or increased blinking frequency. psychological parameters include tests of attention and concentration that attempt to detect cognitive decline. vehicular parameters monitor technical aspects of the vehicle, such as distance from lanes, changes in speed or sudden course corrections. 2.2. common methods for monitoring driver health there are many methods used nowadays to detect driver state. in this paper, authors will focus on the most used methods and their accuracy in use. 2.2.1. perclos perclos (percentage of eyelid closure) is a method based on tracking the movement of eyes, specifically their closure time over a period of time. the main idea is that as fatigue increases, the frequency and total time of eye closure increases. the method analyzes how long during a certain time interval the eyelids are closed above a certain threshold. most often, an eye is considered closed when the eyelid covers about 70–80% of the eye. the measurement is most often made using camera systems that monitor a person’s face and eyes. the following formula 1 is used to calculated perclos [3]. perclos = 100× ( number of eye closure events total number of frames ) (1) a higher percentage of perclos means that the eye was closed a greater proportion of the time, indicating greater fatigue. typically a threshold must be set which, if exceeded, indicates driver fatigue. the threshold value is usually between 20 and 40%. 2.2.2. ear ear (eye aspect ratio) is an algorithm based on geometric analysis of the eyes (figure 1). typically, several points are tracked around the eye to determine its shape. figure 1. facial points (p1–p6) while the eye is a) open and b) closed [4]. points p1 and p4 determine the width of the eye, the remaining four points determine the vertical dimensions of the eye. the ear ratio is then calculated using the following formula 2. ear = |p2 − p6|+ |p3 − p5| 2|p1 − p4| (2) if the eye is open, the ear reaches higher values because there is a greater distance between points p2, p3 and p5, p6. the threshold is often set to 0.2, lower values, for example for more than 0.5 seconds, indicate a closed eye. the mar (mouth aspect ratio) method works on the same principle, which monitors the driver’s mouth and focuses on yawning analysis, one of the typical signs of fatigue. 2.2.3. machine learning machine learning is another frequently used option for driver state recognition. the principle is a model that is trained on a set of historical datasets to determine desired states based on more complex patterns of behaviour. general procedure for use: (1.) data collection: machine learning models are dependent on quality and large amounts of data. this data can be obtained, for example, from a camera that captures a face and registers visual signs of drowsiness. it can also be various sensors that monitor heart rate, head movements or skin resistance. in general, the driver’s behaviour, reaction time, accuracy and errors in performing tasks are then monitored. (2.) data processing: the data collected often contains noise and unnecessary information. the data therefore needs to be filtered and normalised. then, key signs are extracted from the data, which may be, for example, head drops, eye closure, etc. this data can then be used to train the model and to test the accuracy of prediction and detection. (3.) model training: the model is trained on the previously mentioned processed data, where the desired states (“sleepy”, “tired”, . . . ) have already been labelled. it then uses various algorithms to partition the data, such as classifiers (support vector machine or k-nearest neighbors) or deep learning (neural networks). (4.) prediction and detection: once the model is trained, it can be used to monitor the actual data in real time. again, already processed data (different from the data on which the model was trained) is then used to determine the accuracy of the model, where the model results are compared to the data processing results. 2.2.4. other methods there are many other methods that are used to detect driver fatigue and distraction. however, they often cause discomfort (invasive methods such as eeg) or are not accurate enough compared to the methods already mentioned in this article. still, it is a good idea to use these methods to obtain additional data that may indicate, driver fatigue. different correlations between the signals of the different methods support the recognition of fatigue and attention. these methods can focus on heart or breathing rate, as well as depth of breathing. as driver fatigue increases, the 51 filip kotas, jan leistner acta polytechnica ctu proceedings heart rate typically decreases, breathing becomes shallow, and the breathing rate decreases. fatigue can also be determined by monitoring facial temperature with an infrared camera, where a drop in temperature, particularly around the eyes, can indicate drowsiness and fatigue. all of these signals can be measured and may indicate fatigue or drowsiness, but their accuracy is usually not as high as it would need to be. it is therefore a good idea to use them in combination with more accurate methods and then look for correlations between the results of each method. 2.3. accuracy of commonly used methods in a 2012 paper [5], the authors compared several methods to determine fatigue. the experiment was conducted over several days, with test subjects being tested every two hours. testing consisted of completing a 10-minute pvt (psychomotor vigilance task) test, during which eeg signals, rr interval (time between consecutive heartbeats) and fatigue were measured simultaneously using the perclos method. at the same time, subjects completed a questionnaire that determined their subjective feeling of fatigue on the kss (karolina sleepiness scale). subjective feeling of fatigue graded around 4 to 10 a.m. in terms of accuracy of the methods, the perclos method showed the best results, achieving an accuracy of around 90%. the rr interval achieved an accuracy of around 86% and the accuracy of the delta power eeg signal was determined to be around 82%. to get a better idea of the extent to which fatigue and drowsiness affect driver reaction time, in [6] the authors measured reaction time in two age groups. the first group was around 20 years old and the second around 50 years old. the difference in reaction time between the first and second groups was on average 0.04 seconds. verbal ratings of levels of drowsiness and fatigue are shown in the table 1. dimension level status of the driver level 0 adequate rest, short driving time, no fatigue fatigue level 1 driving for more than 2.5 hours, obvious fatigue level 2 driving for more than 4 hours, very tired level 0 enough sleep, good energy, very awake sleepiness level 1 feel sleepy and have a certain desire to sleep and rest level 2 feel very sleepy, especially eager to sleep and rest table 1. a verbal evaluation of different levels of fatigue and sleepiness. the experiment was conducted by having the system generate a random interval, after this interval a simple command (e.g., turn left, stop, or turn right) was announced to the driver, and then the reaction time was measured. in the figures 2 and 3, a reaction time comparison of different levels of fatigue and sleepiness can be seen. figure 2. how sleepiness affects the driver’s reaction time. based on data published by [6]. figure 3. how fatigue affects the driver’s reaction time. based on data published by [6]. these results ideally demonstrate the effect of fatigue and drowsiness on driver reaction time. an interesting approach to measuring fatigue is proposed in [7], where the authors detect fatigue by tracking chest movement with a camera. the first phase consisted of laboratory testing to find the optimal position for camera placement. in the second phase, they already measured the data in a driving simulator, in order to train the model to recognize fatigue. the method achieved 90% correlation of results with measurements from a plethysmography band (used to measure changes in chest volume). during the testing the expected premises were confirmed. the loose clothing and poor lighting brought down the measurement accuracy. the authors in [8] use ear, mar and hand gesture recognition to determine driver fatigue. the authors encountered a problem where the system reported a false positive when the driver put his hand to his head but did not cover his mouth (a typical reaction when yawning). they solved the problem by additionally training the model for this particular situation and thus achieved an overall yawning recognition accuracy of 92.75%. in [9], the authors propose a modification of the perclos method, to which they added a method 52 vol. 52/2025 driver drowsiness and fatigue with interior camera of image analysis, edge detection, in which the image is converted to a greal-scale image and then edge pixels are detected. by setting the tresholding of the perclos method to 80%, the proposed modification achieved more accurate results, and in addition, compared to the rgb image, it needs 1/3 memory. in the following table 2 an overview of three states can be seen. eyes closed eyes open eyes open < 50% > 50% glasses 84.29% 72.66% 92.67% no glasses 86.35% 77.84% 94.36% table 2. performance of perclos with and without glasses for all three stages. the paper [10] presents the cascaded convnet framework, which achieves a success rate of 98.1% in detecting eye conditions to determine driver fatigue, with varying image quality. 84 898 images were used to train this model. the model makes decisions based on: blink rate, maximum time for eye closure, perclos, ear, yawning and head pose. it uses an infrared camera as hardware. the authors in [11], focus on the effect of contrast and lighting conditions on the accuracy of eye state determination using perclos method. their results achieved an accuracy of over 95%. in [12], the authors developed a method based on deep learning of face video from which they recognize perclos, fom (frequency of open mouth) and em (eye-mouth state). based on the recognized parameters, the model then detected fatigue with 98.3% accuracy. a wearable bracelet to measure fatigue that converts the results to the kss is presented in [13]. the bracelet measures gsr (galvanic skin response) signal, drowsy level, heart rate, pulse rate variability and respiration rate. it uses a trained svm model as a classifier to determine the fatigue level, which achieves an accuracy of 98.3%. driver behavior/attention recognition is measured in a different way. often a so-called two-stream, or a shot of the driver and the road in front of the vehicle at the same time, is used. in the driver shot, eyetracking tracks where the driver is looking and then looks for correlations in what is happening on the road. an example is the work of [14], in which the authors use a neural network that recognizes vehicles, cyclists, etc. in front of a vehicle and then compares this data with data from eyetracker. the model achieved an accuracy of 99.68%. the authors in [15] used a similar approach. they tried to find a correlation between the data from the eyetracker and the driving of the vehicle, for example, when looking to the left or overtaking, they monitor whether the driver is looking in the appropriate direction. the accuracy of the results (87.2%) is reduced by the impaired camera view from the vehicle, where the side view is not properly visible. 3. public awareness questionnaire the questionnaire is aimed at mapping public awareness of the deployment of interior cameras in passenger cars within the eu. the main objective is to find out the public’s awareness of the mandatory introduction of interior cameras, their opinion on this technology and whether they would like to have it in their vehicle. the questionnaire asks the respondent for basic information such as age, driving experience or whether they plan to buy a new vehicle in the near future in which a camera may already be implemented. the anonymous questionnaire aimed at gauging the public’s opinion on the enactment of interior cameras in passenger cars produced in the eu. the main objective was to find out what respondent’s awareness and opinion on the deployment of interior cameras is. 4. results 4.1. summary of methods there are methods to determine fatigue and drowsiness. there is a number of symptoms that can be measured, some of which are common and quite obvious and some of which are less well known and harder to detect. for example, those that can be detected by image recognition, such as yawning, blinking, closing the eyes or head position, can be described as common. the less easily detectable ones include a decreasing rr interval, reduced respiratory rate, reduced skin conductance or even a slightly lower body temperature. for attention recognition, correlations are most often sought between images from cameras on the driver and on the road in front of the vehicle. deep learning models (neural networks) are often used to detect vehicles and road events in general. these models can be very reliable, but good model training is required. in the measurement process, the influence of elements that may have a negative impact on the accuracy of the measurement results should be minimized or, better still, eliminated altogether. examples include the use of hardware with sufficient image quality or the preference for infrared over rgb cameras when measuring at night. from the research carried out, it appears that methods based on multiple data sources are generally more accurate and, more importantly, more robust. the table 3 lists and ranks the methods found according to their accuracy. some methods achieve high accuracy, but not all of these methods directly detect fatigue levels, but focus on the correlation between subjective and objective results, which is usually simpler and less complex. another factor that has a large impact on accuracy is the influence of the environment. there is a huge difference between measuring in laboratory “ideal” conditions versus measuring in real-world conditions. 53 filip kotas, jan leistner acta polytechnica ctu proceedings method accuracy measurement target source eyetracking, neural network 99.68% two-stream, finding correlations between the driver and the road [14] deep learning – perclos, fom, em 98.3% recognising fatigue with deep learning [12] svm 98.3% measuring fatigue on the kss scale using a wearable bracelet [13] cascaded network framework 98.1% eye condition detection with different image quality [10] perclos 95% effect of contrast and lighting conditions on eye condition detection [11] perclos, edge detection 72.66–94.36% recognition of three eye conditions for people with and without glasses [9] ear, mar, hand gestures 92.75% recognition of yawning even if the hand is in front of the face [8] perclos 90% correlation with the pvt and the subjective fatigue questionnaire [5] monitoring of respiration rate 90% camera tracking of chest movement – correlation with plethysmography band results [7] rr interval 86% correlation with the pvt and the subjective fatigue questionnaire [5] eyetracking, monitoring of vehicle parameters 87.2% finding the correlation between eyetracking and driving (e.g. turning) [15] response time monitoring – monitoring the effect of fatigue and drowsiness on driver reaction time [6] table 3. methods and their accuracies expressed as a percentage in measuring fatigue. 4.2. results of the questionnaire the questionnaire got answers from 108 respondents, with a distribution of 46 women and 62 men. figure 4 shows the gender division of respondents. 53% of respondents belong into the age group 18–27, the second most populated group is 28–37 years old. figure 5 shows the representation of age groups. figure 4. gender division of participants. figure 5. the strongest represented age group of respondents is 18–27. important experience indicator in drivers is their annual kilometres driven. contrary to expectations that come form the age group division (figure 5), most respondents replied, that their annual mileage is more than 10 000 kilometres. annual kilometres driven overview can be found in the figure 6. figure 6. majority of respondents drive more than 10 000 km annually. when looking at figure 7, one can see that 55% of the respondents knew about interior camera prior to the questionnaire. when asked about the source of the information, most of them stated that they got their information on the internet (figure 8). figure 7. more than half of the respondents had some prior knowledge about the interior camera. the vehicle age could indicate whether the respondent is planning on buying a new vehicle, 51% indicated that their vehicle is older than 8 years (figure 9). the new vehicle, will be most probably equipped with the interior camera. in relation to this question, respondents were asked whether the presence of the 54 vol. 52/2025 driver drowsiness and fatigue with interior camera figure 8. source of information about the interior camera. figure 9. almost a half of respondents have a car that is 8 years or older. interior camera would affect their choice. as figure 10 depicts, only 8% of respondents said, that they would have not any problem with the presence of interior camera. the rest of the respondents said they would rather buy an old car instead or the presence of interior camera would not be a factor when choosing a new vehicle. figure 10. the presence of interior camera would make a significant factor for 46% of respondents when picking a new car. along with the increased safety on the road while using interior camera, the question of one’s privacy comes to mind. the respondents were asked to rate on a scale from one to ten, whether they felt their privacy would be invaded. figure 11 shows respondents’ rating of privacy risks when interior camera is installed. figure 11. number 1 on the scale suggests the respondents find interior camera as an invasion of privacy while number 10 suggests the opposite. respondents were also asked to evaluate on scale from 1 to 10 the increase of their safety they fell would interior camera deliver. figure 12 depicts the respondents’ answers to this question. figure 12. number 1 on the scale suggests the respondents believe there would be no increase in their safety when interior camera is installed, number 10 suggests otherwise. 5. conclusion the results of the research suggest that more data sources are needed to be used in moving towards the development of methods for the recognition of fatigue, sleepiness and attention. the method then becomes more accurate and robust when applied. when selecting data sources, the compactness of each measurement method needs to be considered so that the correlations sought between them are as unambiguous as possible. the questionnaire received responses from 108 respondents, with a mix of 42.6% female and 57.4% male across all age groups. when asked about their driving experience, almost half of the respondents reported driving more than 10 000 kilometres per year. 55 filip kotas, jan leistner acta polytechnica ctu proceedings this pool of respondents, with a majority of experienced drivers, provided a unique insight into the opinion on an interior camera. when asked about drivers’ privacy and security concerns, the average privacy risk rating was 5.13 out of 10, where 10 means respondents were not at all concerned about privacy intrusion. regarding the increase of security of passengers while using the interior camera the average rating was 6.1 out of 10 where 10 means the respondents felt that interior camera would greatly increase their safety while driving. the final part of this questionnaire was aimed at those who were likely to buy a new car soon. the main question was whether the presence of an interior camera would influence respondents’ choice of vehicle. most respondents said that the presence of an interior camera would not be a priority when choosing a new car, while 38% said that the presence of the camera would bother them and that they would rather choose an older car without this feature. only 8% of respondents said they would not mind having an interior camera in their new car. although the majority part of respondents knew about the interior camera prior to this questionnaire, the overall level of information seems to be only scratching the surface. in order to prevent some misinformation about the data privacy, which seems to be the biggest issue with the respondents, the car manufacturers ought to spread the information among customers with advertising campaigns or through social media. the lack of this information can lead to a negative view on the technology. the next step in this topic could be to develop a method to recognize desired states and test it in both laboratory and real-world conditions. regarding public awareness, the next approach could be to create another questionnaire for the public, but this time focusing on the interest and opinion about the different features of the system that will be implemented in the vehicles. references [1] drowsy driving, 2022. [2024-10-03]. https: //www.nhtsa.gov/risky-driving/drowsy-driving [2] t. akerstedt, c. bassetti, f. cirignotta, et al. sleeping at the wheel. cloître imprim, 2013. [2024-10-03]. https://esrs.eu/wp-content/uploads/ 2018/09/livre_blanc_va_v4.pdf [3] m. madrigal. feeling drowsy? an in-depth look at perclos for driver drowsiness detection, 2023. [2024-10-03]. https://www.ridgerun.ai/post/ perclos-for-driver-drowsiness-detection [4] c. dewi, r.-c. chen, c.-w. chang, et al. eye aspect ratio for real-time drowsiness detection to improve driver safety. electronics 11(19):3183, 2022. https://doi.org/10.3390/electronics11193183 [5] e. c.-p. chua, w.-q. tan, s.-c. yeo, et al. heart rate variability can be used to estimate sleepiness-related decrements in psychomotor vigilance during total sleep deprivation. sleep 35(3):325–334, 2012. https://doi.org/10.5665/sleep.1688 [6] z. zhang. study on the influence of driving fatigue on drivers’ effective reaction time. in 2022 10th international conference on traffic and logistic engineering (ictle), pp. 89–94. ieee, 2022. https://doi.org/10.1109/ictle55577.2022.9902106 [7] j. solaz, j. laparra-hernández, d. bande, et al. drowsiness detection based on the analysis of breathing rate obtained from real-time image recognition. transportation research procedia 14:3867–3876, 2016. https://doi.org/10.1016/j.trpro.2016.05.472 [8] a. s. agarkar, r. gandhiraj, m. k. panda. driver drowsiness detection and warning using facial features and hand gestures. in 2023 2nd international conference on vision towards emerging trends in communication and networking technologies (vitecon), pp. 1–6. ieee, 2023. https: //doi.org/10.1109/vitecon58111.2023.10157233 [9] j.-j. yan, h.-h. kuo, y.-f. lin, t.-l. liao. real-time driver drowsiness detection system based on perclos and grayscale image processing. pp. 243–246. ieee, 2016. https://doi.org/10.1109/is3c.2016.72 [10] n. t. singh, saurav, n. pathak, et al. real-time driver drowsiness detection system using cascaded convnet framework. in 2023 international conference on sustainable computing and smart systems (icscss), pp. 828–833. ieee, 2023. https: //doi.org/10.1109/icscss57650.2023.10169434 [11] t. p. nguyen, m. t. chew, s. demidenko. eye tracking system to detect driver drowsiness. in 2015 6th international conference on automation, robotics and applications (icara), pp. 472–477. ieee, 2015. https://doi.org/10.1109/icara.2015.7081194 [12] z. cui, h.-m. sun, r.-n. yin, et al. real-time detection method of driver fatigue state based on deep learning of face video. multimedia tools and applications 80(17):25495–25515, 2021. https://doi.org/10.1007/s11042-021-10930-z [13] l. b. leng, l. b. giin, w.-y. chung. wearable driver drowsiness detection system based on biomedical and motion sensors. in 2015 ieee sensors, pp. 1–4. ieee, 2015. https://doi.org/10.1109/icsens.2015.7370355 [14] t. huang, r. fu. driver distraction detection based on the true driver’s focus of attention. ieee transactions on intelligent transportation systems 23(10):19374–19386, 2022. https://doi.org/10.1109/tits.2022.3166208 [15] y. zou, s. du, h. han, et al. two-stream (2+1)d cnn based on frame difference attention for driver behavior recognition. in 2023 10th international conference on dependable systems and their applications (dsa), pp. 782–788. ieee, 2023. https://doi.org/10.1109/dsa59317.2023.00110 56 https://www.nhtsa.gov/risky-driving/drowsy-driving https://www.nhtsa.gov/risky-driving/drowsy-driving https://esrs.eu/wp-content/uploads/2018/09/livre_blanc_va_v4.pdf https://esrs.eu/wp-content/uploads/2018/09/livre_blanc_va_v4.pdf https://www.ridgerun.ai/post/perclos-for-driver-drowsiness-detection https://www.ridgerun.ai/post/perclos-for-driver-drowsiness-detection https://doi.org/10.3390/electronics11193183 https://doi.org/10.5665/sleep.1688 https://doi.org/10.1109/ictle55577.2022.9902106 https://doi.org/10.1016/j.trpro.2016.05.472 https://doi.org/10.1109/vitecon58111.2023.10157233 https://doi.org/10.1109/vitecon58111.2023.10157233 https://doi.org/10.1109/is3c.2016.72 https://doi.org/10.1109/icscss57650.2023.10169434 https://doi.org/10.1109/icscss57650.2023.10169434 https://doi.org/10.1109/icara.2015.7081194 https://doi.org/10.1007/s11042-021-10930-z https://doi.org/10.1109/icsens.2015.7370355 https://doi.org/10.1109/tits.2022.3166208 https://doi.org/10.1109/dsa59317.2023.00110 acta polytechnica ctu proceedings 52:50–56, 2025 1 introduction 2 methods of monitoring the driver's state 2.1 classification of methods to detect driver fatigue and drowsiness 2.2 common methods for monitoring driver health 2.2.1 perclos 2.2.2 ear 2.2.3 machine learning 2.2.4 other methods 2.3 accuracy of commonly used methods 3 public awareness questionnaire 4 results 4.1 summary of methods 4.2 results of the questionnaire 5 conclusion references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2024.50.0044 acta polytechnica ctu proceedings 50:44–48, 2024 © 2024 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague estimation of mechanical properties of neutron-irradiated 08ch18n10t steel from hardness testing aleš maternaa,∗, petr haušilda, petra klatovskáb a czech technical university in prague, faculty of nuclear sciences and physical engineering, trojanova 13, 120 00 prague 2, czech republic b ujv řež, a.s., hlavní 130, řež, 250 68 husinec, czech republic ∗ corresponding author: ales.materna@fjfi.cvut.cz abstract. through-thickness vickers hardness of a neutron-irradiated small segment of the core barrel made from 08ch18n10t austenitic stainless steel was characterized experimentally and results were compared to ones simulated numerically with assumption of ludwik’s form of hardening equation and neutron dose dependent material parameters according to work of sorokin. simulated values of hardness trace the lower bound of scattered experimental data. based on the same material input data and on the correlation with measured hardness profile, increase of the material yield strength of the core barrel due to reactor operation was estimated. keywords: 08ch18n10t austenitic stainless steel, neutron embrittlement, hardness, yield strength, fem, wwer-440 reactor internals. 1. introduction radiation embrittlement of key parts of pressurized water reactors is a limiting factor for safe operation of nuclear power plants. 08ch18n10t (chemically similar to aisi 321) is a titanium stabilized austenitic stainless steel used as the main structural material for reactor internals (namely the core barrel, the core basket and the block of guide tubes) of the wwer440 type nuclear reactor. results presented in this paper are a part of a larger project with aim to monitor the degree of degradation of irradiated core internals of the wwer-440 reactor and to find the proper temperature regime to recover their mechanical properties. the effectiveness of such regenerating annealing is monitored by hardness and fracture toughness measurements and by the tests to assess stress corrosion cracking susceptibility. the size of the experimental program is highly limited by the capacity of the specialized testing facility and by the amount of disposable material. therefore, an attempt was made to numerically simulate some tests to get additional material characteristics, to crosscheck measured values against the results of other tests and against the literature data. the paper will summarize the results of vickers hardness measurement of the core barrel irradiated during the reactor operation and, particularly, the effect of radial distance of the indent from the reactor active zone on local hardness. simultaneously, a finite element model of the indentation process was used to discuss the change in tensile properties of irradiated 08ch18n10t steel, mainly the change of the yield strength. 2. mechanical properties of the neutron-irradiated core barrel 2.1. material a small ring segment for material properties testing was cut from the core barrel by plasma arc. the core barrel was a part of reactor core internals in the nuclear power plant greifswald, unit 1 (npp greifswald 1), which was operated for 15 cycles, i.e. 4204 full power days. approximate irradiation damage of examined segment is 2.4 dpa (displacements per atom). the core barrel was made from a titanium stabilized chromium-nickel austenitic stainless steel 08ch18n10t with chemical composition (see table 1) similar to aisi 321. 2.2. sample preparation hardness measurement was performed on surfaces of miniaturized compact tension (mct) specimens, which were used primarily for fracture toughness measurements. all mct specimens were fabricated from the ring segment using electro-spark wire-electrode cutting, drilling and grinding in the controlled environment of hot chamber using remote-handling systems. 2.3. hardness measurement hardness hv5 was measured at room temperature in semi-hot chamber by the zhv30 hardness tester from zwick/roell indentec company. the thickness of 4mm of mct specimens allows to monitor change of hardness in 5 positions through the wall of cylindrical core barrel. at least 8 indentations were performed for each position. 44 https://doi.org/10.14311/app.2024.50.0044 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 50/2024 mechanical properties of neutron-irradiated 08ch18n10t in wt % c mn s p ni cr ti 08ch18n10t ≤ 0.08 1.0–2.0 ≤ 0.02 ≤ 0.035 9.0–11.0 17.0–19.0 ≥ 5c ≤ 0.6 table 1. chemical composition of 08ch18n10t austenitic stainless steel (in wt %). 2.4. numerical simulation of indentation simultaneously, the indentation process was simulated numerically by means of nonlinear finite element program marc 2015 within the large strain framework. change of hardness during the neutron irradiation was simulated by the change of input material work hardening curve. 2.4.1. model tensile properties the effect of neutron irradiation on tensile properties of 08ch18n10t steel was studied experimentally in [1, 2]. although the mechanical properties for the same material from different producers and for different mechanical treatments can vary, due to lack of other more relevant data the work of [1] was taken as a basis for modelling of mechanical response of material used for fabrication of the core barrel in npp greifswald 1. in present study, tests at room temperature only and on specimens irradiated with doses of about 2.4 dpa were performed. therefore, original more general dependencies with the description of stress-strain curves for different test temperatures, irradiation temperatures and doses, were simplified to test temperature 20 °c and doses between 0–7 dpa. standard tensile test diagrams for 0 and 7 dpa are plotted in figure 1. neutron irradiation led to increase of yield and tensile strengths and to decrease of ductility. it is supposed, that elastic-plastic behavior of 08ch18n10t steel follows the power equation in the form σeq = σy(d) +a(d) · (εp eq )n, (1) where σeq and εp eq are equivalent values of the true stress and the accumulated true plastic strain, respectively, and dose d dependent material properties yield strength σy and coefficient a are expressed as follows: σy(d) = 280 + ∆σy(d) mpa, (2) a(d) = 943 − 0.158 · ∆σy(d) mpa, (3) where ∆σy(d) = 612 √ 1 − exp (−0.143 ·d) mpa, (4) is the yield strength increase due to neutron irradiation. strain hardening exponent n is 0.55 for studied irradiation up to 7 dpa. model power law hardening curves in true stress – true (logarithmic) plastic strain coordinates for 0 and 7 dpa are plotted in figure 2 together with tensile curves from figure 1 converted from engineering (eng) to true values using following relations: σtrue = σeng(1 + εeng), (5) figure 1. tensile diagrams at 20 °c for 08ch18n10t steel in original and neutron-irradiated state (7 dpa) [1]. figure 2. comparison of model work-hardening curves (full lines) with tests (dotted lines) at 20 °c for 08ch18n10t steel in original and neutron-irradiated state (7 dpa). εp,true = ln (1 + εeng) − σtrue e . (6) elastic properties are characterized by the young’s modulus e = 196 000 mpa and poisson’s ratio ν = 0.26. 2.4.2. fe mesh and boundary conditions due to symmetry of vicker’s indenter, only one fourth of the indenter and material were modelled. the rigid indenter was pushed against the deformable material by the maximum force of 12.2575n, which corresponded to one fourth of 5 kgf during hv5 measurement. coulomb type friction with friction coefficient 0.16 was supposed according to general recommendation in [3] for contact with diamond indenter. fine mapped mesh under the indenter (see figure 3 and figure 4) consists of 8-noded hexahedral elements with size of 2 µm in material surface direction. nodes 45 a. materna, p. haušild, p. klatovská acta polytechnica ctu proceedings figure 3. finite element mesh under the indenter before the indentation. figure 4. finite element mesh under the indenter after the indentation. in both symmetry planes were constrained in normal directions, nodes in bottom surface of the material specimen were fixed in all directions. the size of the specimen was more than 100× higher than expected impressions depths. as in the real measurement, numerical hardness was calculated from the length of the diagonal d left by the indenter after unloading according to formula hv5 = 1.8544 · 5 d2 [ kgf mm2 ] . (7) the length of the diagonal was evaluated from the coordinate after unloading of the last node in contact with indenter. 3. results and discussion 3.1. computed hardness increase of vickers hardness during the neutron irradiation up to 7 dpa in 0.2 dpa steps were simulated numerically and plotted in figure 5. as can be seen, the numerical results form a stepwise curve rather than a smooth one due to the discrete nature of the contact used in the finite element method. the length of the diagonal of the impression can increase only figure 5. computed dependence of vickers hardness on the dose. figure 6. computed indent profile along its diagonal at 2.4 dpa. by the length of one element which is 2µm. this step in length leads to step of about 5 and 12 units of hv5 for 0 and 7 dpa respectively. therefore, the numerically determined values of hardness were fitted by the formula similar to the one for yield strength in eq. 2. smooth dependence of the hardness on neutron dose has the final form hv5 (d) = 160 + 174.3 √ 1 − exp (−0.143 ·d). (8) alternatively, as a rough estimate for studied steel, the length of the diagonal can be determined also as the seven times the size of the maximum penetration depth, which comes from the shape of the vickers indenter. this approach is valid for indentations, for which the elastic recovery is negligible and the surface around the indenter remains flat. as can be seen in figure 6, both conditions are almost fully satisfied – the unloading led to only small displacement of the indenter tip and the material surface exhibits only mild pile-up. slightly shorter diagonals determined from penetration depths led to slight overestimation of hardness but the dependence on dose is now smooth and justifies the used fit. 3.2. measured hardness measured vickers hardness in 5 various distances from the inner surface of the core barrel, together with error bars indicating one standard deviation, are plotted in figure 7. both the hardness and hardness scatter decrease with increasing distance from the inner wall as the neutron fluence decreases across the thickness. 46 vol. 50/2024 mechanical properties of neutron-irradiated 08ch18n10t figure 7. measured vickers hardness in the wall of the core barrel. if we suppose the dose of the measured segment of the core barrel 2.4 dpa then from eq. 8 follows hv5 (d = 2.4 dpa) = 254. this value is lower than average measured values across the thickness of the core barrel but is within the scatter of measured data at the lower boundary given by the standard deviation. here are a few ideas why the simulation underestimated measured average hardness 277 by 9%: • 18cr–10ni–ti steel used in the study [1] has slightly different mechanical properties than the steel of rings used for the fabrication of the core barrel in npp greifswald 1. • deformation response of material under the indenter did not follow the power law used in eq. 1 in the whole range of plastic strains. • in contrast to the stiff but deformable real indenter, the rigid indenter in the simulation leads to a slightly lower prediction of the impression area and thus to a slight overestimation of the calculated hardness. • hypothetically, the segment of the core barrel was exposed to higher neutron fluence which led to higher hardening. according to eq. 8, necessary dose to achieve hv5 277 should be 4.6 dpa in average. there are many empirical relationships (eg. [4] for irradiated austenitic steels) between hardness and yield strength. in this study, yield strength profile across the wall of the core barrel was estimated assuming that hardening of neutron-irradiated 08ch18n10t steel follows eqs 1–4 and the hardness comes from eq. 8. all unknown material parameters in these equations are related to neutron dose and, therefore, solution is unique. estimated yield strength in figure 8 decreases from 700mpa close to inner surface of the barrel to 660mpa 20–25mm under the inner surface. coefficient a varies only slightly between 877 and 883mpa and exponent n is 0.55. figure 8. estimated yield strength through the wall of the core barrel. 4. conclusions the paper deals with the characterization of the tensile material properties of the neutron-irradiated segment of the core barrel from the reactor internals of npp greifswald 1, which was shut down after almost 16 years of its operation. due to small amount of material, values of measured hardness together with finite element simulation of the indentation was used instead of the standard tensile test. the results can be summarized as follows: • ludwik’s form of hardening equation eq. 1 and neutron dose dependent material parameters in eqs 2–4 according to work of sorokin [1] can be used for the description of the tensile properties of neutron-irradiated 08ch18n10t austenitic stainless steel used for fabrication of the core barrel. although these material properties underestimate the measured hardness by 9%, computed hardness lies within the scatter of experimental data. • numerically determined dependence of the hardness on the neutron dose is expressed by the equation hv5 (d) = 160 + 174.3 √ 1 − exp (−0.143 ·d). • tensile properties of the examined segment of the core barrel vary across the wall thickness. yield strength reaches 700mpa close to the inner surface and then gradually decreases to 660mpa. strength coefficient is about 880mpa and strain hardening exponent is 0.55. references [1] a. a. sorokin, b. z. margolin, i. p. kursevich, et al. effect of neutron irradiation on tensile properties of materials for pressure vessel internals of wwer type reactors. journal of nuclear materials 444(1-3):373–384, 2014. https://doi.org/10.1016/j.jnucmat.2013.10.016 [2] m. ruščák, m. žamboch, o. erben. assessment of wwer reactor pressure vessel internals and program of lifetime management. in proceedings of the ninth international symposium on environmental degradation of materials in nuclear power systems – water 47 https://doi.org/10.1016/j.jnucmat.2013.10.016 a. materna, p. haušild, p. klatovská acta polytechnica ctu proceedings reactors, pp. 997–1003. 1999. https://doi.org/10.1002/9781118787618.ch104 [3] j. m. antunes, l. f. menezes, j. v. fernandes. three-dimensional numerical simulation of vickers indentation tests. international journal of solids and structures 43(3-4):784–806, 2006. https://doi.org/10.1016/j.ijsolstr.2005.02.048 [4] j. t. busby, m. c. hash, g. s. was. the relationship between hardness and yield stress in irradiated austenitic and ferritic steels. journal of nuclear materials 336(2-3):267–278, 2005. https://doi.org/10.1016/j.jnucmat.2004.09.024 48 https://doi.org/10.1002/9781118787618.ch104 https://doi.org/10.1016/j.ijsolstr.2005.02.048 https://doi.org/10.1016/j.jnucmat.2004.09.024 acta polytechnica ctu proceedings 50:44–48, 2024 1 introduction 2 mechanical properties of the neutron-irradiated core barrel 2.1 material 2.2 sample preparation 2.3 hardness measurement 2.4 numerical simulation of indentation 2.4.1 model tensile properties 2.4.2 fe mesh and boundary conditions 3 results and discussion 3.1 computed hardness 3.2 measured hardness 4 conclusions references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2024.49.0008 acta polytechnica ctu proceedings 49:8–12, 2024 © 2024 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague dynamic behavior of a cable-stayed footbridge depending on the calculation accuracy kristian d‘amico∗, jiří máca czech technical university in prague, faculty of civil engineering, department of mechanics, thákurova 7, 166 29 prague, czech republic ∗ corresponding author: kristian.d.amico@fsv.cvut.cz abstract. footbridges are analyzed in terms of dynamic response for pedestrian comfort. this problem is solved mainly on light and at the same time rigid constructions, which are easily oscillated by pedestrian load. it is often solved on steel footbridges, but with the technological development of uhpc, we are capable nowadays to build relatively light and long span concrete footbridges. behaviour of a thin concrete cross-section with its geometry is closer to steel structures, but at the same time we have to deal with rheological effects such as creep and shrinkage. there are many influences on the structure that can affect the dynamic behavior of the structure, including the non-linear behavior of the cable system. this paper presents an initial entry into the computational issues of more complex constructions that have more input influences. keywords: dynamic characteristics, natural frequencies, footbridge, cable-stayed bridge, finite element method, uhpc, stiffness matrix, mass matrix. 1. introduction dynamic analysis precedes the dynamic experiment insitu, unfortunately construction companies are mostly forced to buy bridge dampers in advance, so calculation inaccuracy may lead to an uneconomical design and therefore an accurate model is required. critical freqency range in the vertical direction is between 1.25–2.3 hz and for the horizontal natural frequences between 0.5–1.2 hz. footbridges with frequencies for vertical or longitudinal vibrations in range 2.5–4.5 hz might be excited by resonanse by the 2nd harmonic of pedestrian load. in that case critical frequences range for vertical and longitudinal vibrations expands to 1.25–4.5 hz. the frequencies are further analyzed more precisely, and the amount of oscillating mass in bridge engineering is also tracked. dampers are designed into the locations, where the acceleration reaches critical values where the natural damping of the structure is insufficient. in practice, if there is an inaccuracy in the calculation, a damper can be designed in a place where it will not be needed [1]. 2. dynamic characteristics the main dynamic characteristics that enter into the dynamic calculation of natural frequencies are stiffness matrix [k] and mass matrix [m ]. a two-dimensional model using the finite element method was chosen for monitoring changes in the behavior of the structure [2, 3]. 2.1. local stiffness matrix the full stiffnes matrix of an element: ke = e l  a 0 0 −a 0 0 0 12i l2 6i l 0 12i l2 6i l 0 6i l 4i 0 − 6i l 2i −a 0 0 a 0 0 0 − 12i l2 − 6i l 0 12i l2 − 6i l 0 6i l 2i 0 − 6i l 4i  , (1) where e is young’s modulus of the material [kpa], i is the moment of inertia for the y-axis in base of the cross-section [m4], a is the element area [m2], l is the lenght of the element [m]. this matrix belongs to the deck and the pilons, but the cables behaviour is different. the cables are non-linerar elements that can only be tensed and have zero bending stiffness. in this analysis, the effect of rope sag is neglected for simplification reasons. to achieve the correct behavior of a cable-stayed structure, the geometric stiffness matrix of the element also needs to be included: kg = p 30l  0 0 0 0 0 0 36 3l 0 −36 3l 0 3l 4l2 0 −3l −l2 0 0 0 0 0 0 0 −36 −3l 0 36 −3l 0 3l −l2 0 −3l 4l2  , (2) where p is the stressing/tension force in the element [kn]. 8 https://doi.org/10.14311/app.2024.49.0008 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 49/2024 dynamic behavior of a cable-stayed footbridge . . . the axial force in the member is denoted by p , with the positive axial force in this case representing pressure. combining the element stiffness and geometrical stiffness we recive the local matrix of stiffness: kl = ke + kg. (3) 2.2. local mass matrix normally, cables are considered intangible, because they oscillate at low frequencies and thus introduce a large amount of low frequencies into the calculation, which usually unnecessarily interfere the global assessment. for the academic purposes, the cables in the numeric model are considered to have weight for realistical results. the weight of the structure is expressed often using the diagonal mass matrix, which expresses that the mass is equaly divided to end nodes: md = ηl 2  1 0 0 0 0 0 0 1 0 0 0 0 0 0 1 0 0 0 0 0 0 1 0 0 0 0 0 0 1 0 0 0 0 0 0 1  , (4) where η is weight per unit length [t m−1]. a more accurate option is to evenly distribute the mass along the element by consistent mass matrix: mk = ηl 420  140 0 0 70 0 0 0 156 22l 0 54 −13l 0 22l 4l2 0 13l −3l2 70 0 0 140 0 0 0 54 13l 0 156 −22l 0 −13l −3l2 0 −22l 4l2  . (5) in the past, it has been proven that the difference in the calculation is in the order of a few percent, so the diagonal weight matrix is considered an acceptable input, the error is negligible. commercial softwars often use diagonal form, difference with the consistent one will also be compared in this analysis. increment to the weight matrices is a mass matrix with the influence of the bending moment: mi = ρi 30l  0 0 0 0 0 0 0 36 3l 0 −36 3l 0 3l 4l2 0 −3l −l2 0 0 0 0 0 0 0 −36 −3l 0 36 −3l 0 3l −l2 0 −3l 4l2  , (6) where i is material moment of inertia. the mass moment of inertia is the inertia in rotation around the center of the cross-section. in the vast majority of cases, this has minimal effect. this must be taken into account in the case of very high beams or extremely fast loads. inertia in the stiffness matrix corresponding to the rotation of the cross-section. but it is related both to the inertia resulting from the transverse movement and to the inertia resulting from the rotation of the cross-sections. inertia, related to the member tip rotation parameter, which will come out in a consistent cross-section matrix. if a consistent inertia matrix is considered, then this term generally cannot be neglected. combining the element mass and the mass with the influence of the rotational bending moment we recive the local mass matrix: ml = md/k + mi. (7) 2.3. transformation to global matrices using the transformation matrix [a] the local matrix is transformed into the global coordinate system, and we get the global matrix of stiffness: a =  c s 0 0 0 0 s c 0 0 0 0 0 0 1 0 0 0 0 0 0 c s 0 0 0 0 −s c 0 0 0 0 0 0 1  , (8) where c is cosine of the angle the element subtends from the global system, s is sine of the angle the element subtends from the global system. kg = at kla, mg = at mla, (9) where kg is the global stiffness matrix, mg is the global mass matrix. 3. fem model to verify the effects of the imputs, an already tested structure was needed, as such cable-stayed footbridge in čelakovice was chosen. the two-dimensional finite element model was parametrized in matlab by the project documentation, which was avaible including the static evaluation [4]. 3.1. geometry this is the footbridge with field spans of 21.5 m + 156 m+21.5 m, pylons are 36 m high each with 14 pairs of cables (figure 1). the bridge deck is supported at the crossing points with pylons and bridge abutments on the shores of the river labe. the deck is made of uhpc with 9 kristian d‘amico, jiří máca acta polytechnica ctu proceedings figure 1. sectional view. figure 2. cross section. figure 3. pylon. a strength c110/130 (figure 2), the cables and pylons are made of steel (figure 3). the mathematical model is parameterized by dividing the construction into elements, where the end nodes at the beginning and end are assigned code numbers. the proposed 2d structure was designed in autocad (figure 4). 3.2. calculation elements were further divided so that material and physical characteristics could be assigned to them (table 1). each pair of cables was also assigned a prestressing force according to the static calculation (table 2). at the connection point of the cables between the pylon and the bridge plate, joints were defined. where the bearings were supported, the bonds were loosened in the matrices so that the structure could move freely in the permitted directions. numerically, this meant that at the support locations the rows and columns in the global matrices were zeroed/cleared because there is no unknown displacement but zero displacement/rotation. for each finite element, the above-mentioned matrices were calculated in the local coordinate system, which were further transformed into the global coordinate system and global matrices k and m were formed. the natural shapes and natural frequences were calculated by direct solution by searching for eigenvalues on the diagonal, the correctness of the results was also verified by the method of inverse iterations. another verification of the results was the modeling of the structure in the scia engineer, which confirmed the correctness of the procedure (figure 5). since the script is in matlab (figure 6), for final results the following command served the necessary outputs: [v, d] = eig(k, m), (10) where v are the eigenform vectors, d are eigenvalues – natural frequencies. 10 vol. 49/2024 dynamic behavior of a cable-stayed footbridge . . . figure 4. 2d drawing to determine code numbers on the lements. element type e [mpa] a [m2] i [m4] η [t m−1] ρ [kn m−3] bridge deck 45 000 0.51368 0.012369 1.3356 26 pylon 210 000 0.12478 0.0098726 0.9795 78.5 cables 164464 − 165067 0.0053 − 0.00163 1.12612 × 10−8 − 1.0571 × 10−7 0.0044 − 0.0136 82.53 − 83.70 table 1. material and geometric characteristics of the elements. cable number area [mm2] area of doubled cable [mm2] pre-stressing force [kn] 1 (101, 401) 266 532 75.6 2 (102, 402) 266 532 82.1 3 (103, 403) 266 532 87.1 4 (104 ,404) 521 1 042 133.6 5 (105, 405) 815 1 630 287.8 6 (106, 406) 815 1 630 287.8 7 (107, 407) 815 1 630 282.1 8 (201, 301) 266 532 90.7 9 (202, 302) 266 532 90.5 10 (203, 303) 416 832 117.8 11 (204, 304) 521 1 042 155.7 12 (205, 305) 639 1 278 185.1 13 (206, 306) 815 1 630 205.9 14 (207, 307) 815 1 630 276.8 table 2. cable parametres. figure 5. numerical model verification in scia engineer. figure 6. the shape of the structure ploted in matlab. 11 kristian d‘amico, jiří máca acta polytechnica ctu proceedings figure 7. comparison of calculation combinations. 4. results various combinations of the stiffness and mass matrix were used in the dynamic calculation of the eigenshapes and eigenfrequencies. a comparison of individual combinations can be seen on the graph (figure 7). the biggest effect is noticeable at higher frequencies, where the curves move away from each other. as expected, the addition of the mi global mass matrix has the least influence on the calculation, because it is a bridge deck with a low structural height. in the area around 1–2 hz, which is observed in bridge structures, the difference between the results is up to a two-digit percentage number. furthermore, the results of this analysis were compared with the results of others who also calculated vertical frequencies on the same construction. the green curve is the actual measured frequencies on the structure (figure 8), ad1 and ad2 are provided values from other mathematical models [5, 6], ad3 are the results from the most accurate form of this analysis, which is a combination of ke +kg +mk +mi. the vertical frequencies are similar, but even if the results are based on the same project documentation, a difference of 5–10 % can be seen [5, 6]. 5. conclusion the analysis helps for better understanding of the behaviour of cable-stayed bridges in terms of dynamics. in contrast with the commercial software, which are often black boxes, the hand-writed numerical code of a finite element model structure is much clearer in terms of changes in the influence on dynamic behavior. by testing different combinations of accuracy, it was proven that for such a complex construction, accuracy has a large effect on frequency results. it is important to continue to collect data from new uhpc constructions, for possible calibration of models, in order to achieve a correct prediction of the structure’s figure 8. comparison of vertical frequencies with other results on the same construction [5, 6]. behavior and avoid unnecessary design of dampers. the results confirm the sensitive behavior of such a complex non-linear construction, and with today’s computer performance, where there is no need to save on computing memory, it is better to choose a more accurate form of calculation. acknowledgements the authors gratefully acknowledge support from the czech technical university in prague, project sgs23/032/ohk1/1t/11 development and application of numerical algorithms for analysis and modelling in mechanics of structures and materials. references [1] european commission, joint research centre, a. goldack, et al. design of lightweight footbridges for human induced vibrations – background document in support to the implementation, harmonization and further development of the eurocodes. publications office, 2009. https://doi.org/doi/10.2788/33846 [2] z. bittnar, p. řeřicha. metoda konečných prvků v dynamice konstrukcí [in czech]. sntl, praha, 1981. [3] m. baťa, v. plachý, f. trávníček. dynamika stavebních konstrukcí [in czech]. sntl, praha, 1987. [4] m. kalný, j. komanec, v. kvasnička, et al. lávka přes labe v čelákovicích – první nosná konstrukce z uhpc v čr [in czech; footbridge over the elbe river in čelakovice – the first uhpc superstructure in the czech republic]. beton (4):10–18, 2014. [2023-09-01]. https://www.ebeton.cz/wp-content/uploads/20144-10_0.pdf [5] j. štěpánek. dynamická analýza lávky pro pěší [in czech; dynamic analysis of the footbridge]. bachelor’s thesis, czech technical university in prague, faculty of civil engineering, 2016. [6] v. šáňa. vibration of footbridges human-structure interaction. ph.d. thesis, czech technical university in prague, faculty of civil engineering, 2016. 12 https://doi.org/doi/10.2788/33846 https://www.ebeton.cz/wp-content/uploads/2014-4-10_0.pdf https://www.ebeton.cz/wp-content/uploads/2014-4-10_0.pdf acta polytechnica ctu proceedings 49:8–12, 2024 1 introduction 2 dynamic characteristics 2.1 local stiffness matrix 2.2 local mass matrix 2.3 transformation to global matrices 3 fem model 3.1 geometry 3.2 calculation 4 results 5 conclusion acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2023.43.0018 acta polytechnica ctu proceedings 43:18–27, 2023 © 2023 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague determining 3d coordinates based on the track geometry description adam hlubuček czech technical university in prague, faculty of transportation sciences, department of transport telematics, konviktská 20, 110 00 prague 1, czech republic correspondence: hlubuada@fd.cvut.cz abstract. this paper is focused on the data description of the railway infrastructure. its aim is to present the possibilities of constructing of linear elements representing tracks in 2d and their subsequent transformation into 3d using the parametric description of horizontal and vertical curves. the transformation is based on determining the spatial coordinates of the newly emerging 3d linear elements. this process is supposed to be implemented in such a graphical editor environment that allows the relevant data to be transferred to the the database based on the multipurpose railway infrastructure model. with the use of this data, it is possible to perform, among other things, the visualization of the railway infrastructure. keywords: railway infrastructure, track geometry, data description, multipurpose railway infrastructure model, railtopomodel. 1. introduction in recent years, the data description of railway infrastructure has become more important, as many intelligent transport systems are being developed, for which it is an essential input. it is desirable to address the question of how to describe the infrastructure in such a way that the description can be used for the widest possible range of target applications [1]. this is what also the multipurpose railway infrastructure model aims for. the multipurpose railway infrastructure model is a data model reflecting some principles of the uic railtopomodel [2, 3] with data stored in the form of a relational database. it is gradually being developed in the railway laboratory at ctu in prague, faculty of transport sciences [4], whereas its latest version, referenced in this paper, is the version 12.2. one of the fundamental aspects of the railway infrastructure description in an expression of the track geometry are the spatial characteristics of the railway line which are the basis for the location of many other infrastructure facilities. when expressing the track geometry, there are several different ways to do this. the multipurpose railway infrastructure model allows us to express the individual points of the centre line of the track described by coordinates as well as to describe its geometric parameters analytically, using the appropriate attributes [5, 6]. both these approaches have their specific advantages and cases in which it is appropriate to apply them. therefore, in order to create a consistent data description, it is necessary to design software tools that allow both of these approaches to be used so that the outputs provided are in accordance with each other. 2. consistent filling the model with data for the purpose of filling the multipurpose railway infrastructure model database with data, a specialized editing script working in the environment of a computer-aided design software is being developed. in the following text, this script together with the computer-aided design software will be referred to as the graphical editor. the graphical editor allows individual instances of the model classes to be visualized as graphical objects arranged into corresponding layers. these graphical objects can also be described by relevant data. each graphical object is described by several items corresponding to the attributes of the respective class. within a specific graphical object, each item is expressed using a record of the structure [table][attribute][attributev alue], where table expresses the name of the table, in which the value of the respective attribute is to be stored, attribute expresses the name of the attribute to be stored and attributev alue expresses its value. on the basis of such a description of all graphical object belonging to layers corresponding to the multipurpose railway infrastructure model classes, the data from the computer-aided design software can be uploaded to the relational database, based on its structure. the mutual consistency of individual data items, which cannot be verified at the database level, is supposed to be ensured on the basis of functionalities of the graphical editor. among other things, the graphical editor provides the creation of graphical object 18 https://doi.org/10.14311/app.2023.43.0018 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 43/2023 determining 3d coordinates based on the track geometry description representing individual data objects of the multipurpose railway infrastructure model and describing them with respective data items. some of the data is entered by the graphical editor user, other can be obtained based on the spatial aspects of these objects in the environment of the computer-aided design software. the way the data objects are graphically visualized usually depends on the class which they belong to. in addition, the model makes it possible to assign some data objects to individual network levels. according to the railtopomodel, a network level is a data object expressed by an instance of the levelnetwork class. it can be described by the descriptionlevel attribute expressing the respective level of detail [2, 3]. the multipurpose railway infrastructure model introduces the dimension and representation attributes, additionally. these attributes make it possible to distinguish between the description of the network using different spatial dimensions and whether the network is described only schematically or realistically. of course, the way of visualization of individual data objects also depends on the attribute description of the respective network level which these data objects are assigned to. when interested in creating the track geometry data description, it is therefore necessary to pay attention to the development of such software tools of the graphical editor that allow coherent transformation across the different description methods and levels. providing these transformations is one of the other functionalities of the editing script. since the track geometry data provide considerably detailed information, it is appropriate to express them at a detailed level corresponding to the value micro of the descriptionlevel attribute of the levelnetwork class. graphical representation of the multipurpose railway infrastructure model data is suitable either in 2d or in 3d which can match the values xy and xyz of the dimension attribute. in order to enable the derivation of attribute values based on the spatial aspects of the visualized data objects, it is desirable to work with such network levels described by the value realistic of the representation attribute. 3. linear elements and associated positions in accordance with the railtopomodel, the basic units of the topological network description are the so-called net elements which can be classified into non-linear and linear elements [2, 3]. according to the multipurpose railway infrastructure model, each linear element is also additionally described by its length in meters, expressed by the length attribute. the interest in describing linear elements by its length was originally promoted by the railml community in order to allow the position within a linear element to be expressed in a more practical way than by specifying the relative value of the intrinsiccoord attribute [7]. in the latest railtopomodel version 1.4, the length attribute attribute was also added to the netelement class [8]. since the information of the geometrical track description has mainly linear features and because the linear elements make it possible to better express the information about the admissible routing within the network, the recommended way of expressing the network structure for the needs of describing track geometry is to use linear elements. in that case, the line elements represent individual track sections (routes) between nodes. after all, such an approach implies the micro description level of the network. each linear element is represented by an instance of the linearelement class, within railtopomodel version 1.4 renamed to linearnetelement. individual net elements can be connected by positioned relations which are the instances of the positionedrelation class. each positioned relation connects exactly two net elements and in the case of a linear element, it can be bound either at its beginning or at its end. each positioned relation is also described by the navigability attribute, expressing whether it is passable and, if so, in which direction [2, 3, 8]. the multipurpose railway infrastructure model allows us to define any number of associated positions bound to a particular linear element. each associated position, represented by an instance of the associatedposition class, is described by its intrinsicreference and deltaposition attributes. the intrinsicreference attribute expresses the intrinsic coordinate within the relevant net element. in the basic concept, for a linear element, it can either take the value of 0 (at the beginning of the element) or the value of 1 (at the end of the element). the deltaposition attribute expresses the difference in position measured along the net element from the position expressed by the intrinsicreference attribute to the resulting position in meters. this attribute can also take the values of negative numbers. therefore, the resulting position parameter p of each associated position can be calculated as p = c · l + ∆p, (1) where c expresses the value of the intrinsicreference attribute, ∆p expresses the value of the deltaposition and l expresses the value of the length attribute of the linear element to which the respective associated position is bound. whereas net elements can appear in different dimensions, the values of the resulting positions calculated from the attributes of corresponding associated positions bound to the linear elements of different dimensions matching each other are generally slightly different. this fact can be demonstrated on the case of two network levels, which will be referred to as the 2d network level and the 3d network level according to the respective dimension. 19 adam hlubuček acta polytechnica ctu proceedings let the linear elements expressed at the 2d network level are visualized as the perpendicular projection of the centre line of the real objects that they represent into the xy horizontal plane and the respective instance of the levelnetwork class is described by the attribute values as follows: • descriptionlevel← micro, • dimension← xy, • representation← realistic. let the linear elements expressed at the 3d network level are visualized basically accurately representing the centre line of the real object in three-dimensional space xyz and the respective instance of the levelnetwork class is described by the attribute values as follows: • descriptionlevel← micro, • dimension← xyz, • representation← realistic. when we compare the linear element expressed at the 2d network level with the linear element expressed at the 3d network level, they have different lengths (except for cases where they express a horizontal track section). this difference is also reflected in the calculation of the corresponding resulting positions on the linear elements and caused by the gradient profile, which is not taken into account when determining resulting positions at the 2d network level. the described ways of graphic visualization can also be reasonably applied to objects of other classes assigned to the stated network levels. the direct description of linear elements does not say anything about their shape. they can nevertheless be visualized based on the geometric entities that are localized to them. these are instances of the terminal classes of the geometryentity module, which is one of the specific multipurpose railway infrastructure model extensions used by many projects [5, 6]. the geometryentity module was designed using some aspects of the railml® 3.1 data format [9], which is a exchange format based on the railtopomodel, so that portability can be ensured in the future. nevertheless, the module has some specific features of its own. its structure can be seen in the figure 1. 4. horizontal curves when filling in the data description of the multipurpose railway infrastructure model using the graphical editor, it is advantageously feasible to construct the linear elements of the micro level first in 2d using horizontal curves. horizontal curves are instances of all the terminal classes which are specializations of the horizontalcurve class contained in the geometryentity module. together with the horizontalcurve abstract class, the module includes also the verticalcurves and the superelevation abstract classes, which all are specializations of the geometryentity abstract class, the top class of the geometryentity module, derived from the netentity abstract class. generally, net entities, as introduced by the railtopomodel, are those objects representing the facilities and properties of the railway infrastructure. they can be localized to individual net elements. the railtopomodel is so general that it does not define specific classes of net entities [2, 3, 8]. although the railml® specifications do so, the railml® 3.1 data format implements the horizontalcurve class as a common class for which instances are horizontal curves of all types [9]. nevertheless, the multipurpose railway infrastructure model introduces a separate class for each curve type, allowing instances of these classes to be described with more specific attributes. the currently used version of the geometryentity module includes the following terminal classes of horizontal curves: • straighthc – describing straight horizontal curves, • circulararchc – describing horizontal curves of the shape of a circular arc, • cubicparabolahc – describing horizontal transition curves of the shape of a cubic parabola, • clothoidhc – describing horizontal transition curves of the shape of a clothoid. each of these specialized classes of horizontal curves has several attributes defined. the only attribute common to all multipurpose railway infrastructure model classes except association classes is id inherited from the baseobject class. this attribute has the meaning of a unique identifier across all objects of these classes. based on its value, records of all relational database tables belonging to one data object are joined. attributes that are common to a significant number of named object classes representing the formalized and user-defined naming of the relevant objects are the name and longname attributes inherited from the namedresource class. the attributes azimuth0 and deltaazimuth are the attributes that are common to all specialized classes inherited from the horizontalcurve class. the azimuth0 attribute determines the azimuth at the starting point of the respective horizontal curve expressed in degrees and it can take values from 0 to 360. the deltaazimuth attribute expresses the difference between the azimuth at the end and the azimuth at the beginning of the respective horizontal curve. it take the value of a positive number for right-turning curves and the value of a negative number for left-turning curves. for each instance of the straighthc class it takes the value of 0. in order to express the azimuth also at the horizontal curve end point, we can define the azimuth1 parameter. for each horizontal curve, this parameter can be calculated as follows: 20 vol. 43/2023 determining 3d coordinates based on the track geometry description geometry geometryentity id char(36) indexes verticalcurve id char(36) horizontallength double elevation0 double deltaelevation double indexes horizontalcurve id char(36) azimuth0 double deltaazimuth double indexes straighthc id char(36) horizontallength double indexes circulararchc id char(36) radius double indexes cubicparabolahc id char(36) radius0 double radius1 double indexes clothoidhc id char(36) radius0 double radius1 double indexes straightvc id char(36) indexes parabolavc id char(36) parabolavertexradius double indexes superelevation id char(36) anchoredaxisreference int indexes constantsuperelevation id char(36) superelevation double indexes linearsuperelevationramp id char(36) superelevation0 double superelevation1 double indexes netentity netentity id char(36) indexes network networkresource id char(36) indexes base baseobject id char(36) indexes namedresource id char(36) name varchar(64) longname varchar(128) indexes geometryentity id char(36) indexes verticalcurve id char(36) horizontallength double elevation0 double deltaelevation double indexes horizontalcurve id char(36) azimuth0 double deltaazimuth double indexes straighthc id char(36) horizontallength double indexes circulararchc id char(36) radius double indexes cubicparabolahc id char(36) radius0 double radius1 double indexes clothoidhc id char(36) radius0 double radius1 double indexes straightvc id char(36) indexes parabolavc id char(36) parabolavertexradius double indexes superelevation id char(36) anchoredaxisreference int indexes constantsuperelevation id char(36) superelevation double indexes linearsuperelevationramp id char(36) superelevation0 double superelevation1 double indexes netentity id char(36) indexes networkresource id char(36) indexes baseobject id char(36) indexes namedresource id char(36) name varchar(64) longname varchar(128) indexes figure 1. the geometryentity module as a specific extension of the multipurpose railway infrastructure model. 21 adam hlubuček acta polytechnica ctu proceedings α0 + ∆α < 0 ⇒ α1 = α0 + ∆α+ 360, (2) 0 ≤ α0 + ∆α < 360 ⇒ α1 = α0 + ∆α, (3) 360 ≤ α0 + ∆α ⇒ α1 = α0 + ∆α− 360, (4) where α0 expresses the value of the azimuth0 attribute, ∆α expresses the value of the deltaazimuth and α1 expresses the value of the azimuth1 parameter. in terms of individual specialized classes of horizontal curves, the straighthc class has the horizontallength attribute that expresses the length of the line segment representing the straight horizontal curve in meters, in addition. it takes the value of a positive number. the circulararchc class has the radius attribute expressing the radius in meters, instead. it takes the value of a positive number for right-turning curves and the value of a negative number for leftturning curves. for transition curves, which are instances of the clothoidhc and cubicparabolahc classes, however, two radius values must be expressed, both at the beginning and at the end of the respective curve. this is provided by the radius0 and radius1 attributes. one of these points is often a point with zero curvature. for such a point, the corresponding attribute takes the value of 0, although it does not express the radius. when constructing horizontal curves in the xy plane (where z = 0) using the graphical editor tools, we can express the starting point of each horizontal curve using the x0 and y0 coordinates and its end point using the x1 and y1 coordinates as follows: x1 = x0 + ∆x, (5) y1 = y0 + ∆y. (6) the ∆x and ∆y values can be calculated for each horizontal curve based on knowledge of the class of which it is an instance and the set of values of the following attributes: azimuth0, deltaazimuth and the specific attributes of individual specialized classes of horizontal curves. based on knowledge of the horizontal curve specialized class and the attribute values, the horizontal length of the respective curve can also be calculated. 5. creating linear elements in 2d using horizontal curves if we choose a specialized horizontal curve class and enter the x0 and y0 coordinates (for example by selecting a point of the xy plane) and the values of the relevant attributes in the graphical editor, we are able to plot a curve representing one instance of the given specialization of the horizontalcurve class. after plotting the horizontal curve of specified attribute values in the base position (which can be starting from the beginning of the coordinate system at the azimuth of 90◦, i. e. not yet taking into account the value of the azimuth0 attribute), it is necessary to move it to the determined starting point with the x0 and y0 coordinates and rotate it so that the azimuth at the beginning of it matches the value of the azimuth0 attribute. in that case, the x1 and y1 coordinates can also be calculated by means of the graphical editor. if we declare the x1 and y1 coordinates of the current horizontal curve to be the x0 and y0 coordinates of the consecutive horizontal curve, it is then possible to construct the consecutive horizontal curve in the same way. whereas the value of the azimuth0 attribute of the newly constructed horizontal curve should be equal to the azimuth1 parameter of the already constructed horizontal curve. this procedure can be used to plot the graphical representation of the entire linear element which these horizontal curves belong to. in that case, the mentioned steps must be repeated until all the horizontal curves belonging to the linear element has been constructed. although all the horizontal curves are considered net entities connected to a net element, the graphical representation of the linear element created using the graphical editor at the 2d network level it is based on the spatial aspects of horizontal curves. while net entities of all other classes are localized to an already existing element along with creating their graphical representation, horizontal curves are supposed to be exceptionally inserted (not yet located using associated location) in advance of the linear element itself. the relevant 2d linear element is created by enclosing selected consecutive horizontal curves, then. horizontal curves that can be enclosed into several linear elements can be seen in the figure 2. in order for a linear element to be enclosed, several conditions must be met. if the linear element is to be created from n horizontal curve the (k+1)th horizontal curve must start at the point where the kth curve ends and the azimuth at the beginning of the (k + 1)th curve must be equal to the azimuth at the end of the kth curve, where can k can take the value of a natural number from 1 to n. when plotting subsequent horizontal curves in the above-mentioned manner, these conditions are already ensured. the length attribute of the linear element is also calculated based on the particular horizontal curves used to enclose it. if we denote the horizontal length of the kth horizontal curve from the total number of n horizontal curves forming the linear element as sk, we can express the linear element length l as follows: l = n∑ k=1 sk. (7) these horizontal curves are retrospectively located to the linear element, which was created on their spatial basis, then. each horizontal curve is to be located to the linear element using the line associated location that uses individual associated sections. each associated section is defined by two associated positions 22 vol. 43/2023 determining 3d coordinates based on the track geometry description figure 2. graphically expressed horizontal curves of a station throat area to be enclosed into 2d linear net elements. bound to the same linear element. in order to create associated sections required to create associated locations intended to locate horizontal curves, individual associated positions at the boundaries and interfaces of the individual horizontal curves must be created. for the stated purpose, we need to define n + 1 associated positions. since the associated positions are instances of the associatedposition class, it is necessary to set the values of their attributes intrinsicreference and deltaposition as well. although this can be done in various ways, the following one can be recommended: c1 = 0, (8) ∆p1 = 0, (9) cj = 0, (10) ∆pj = j−1∑ i=1 si, (11) cn+1 = 1, (12) ∆pn+1 = 0, (13) where j takes the value of a natural number from 2 to n, ck expresses the value of the intrinsicreference attribute of the kth created instance of the associatedposition class and ∆pk expresses the value of the deltaposition attribute of the kth created instance of the associatedposition class. within the multipurpose railway infrastructure model, each associated position can be linked to a geo-point. each geo-point can have its coordinates assigned within each defined coordinate system. in the case of geometric or geographical coordinates, this is carried out using the geopointgeocoordinate association class, while the respective coordinates are expressed by the x, y and z attributes. taking into account the coordinate system of the graphic editor, we can fill the x and y attributes related to individual geo-points connected to the associated positions created in order to locate horizontal curves with the x0 and y0 (eventually x1 and y1) values of the respective horizontal curves in meters. since we are describing a linear element in 2d, we are supposed to set the value of the each z attribute to 0. 6. vertical curves after a linear element has been successfully created, other net entities can be located to it. this usually brings with it the creation of new associated positions and associated sections used to define their associated locations. in some cases, existing associated features can also be used. we can also calculate the coordinate values with which the attributes of the geopointgeocoordinate class instances are to be filled, when creating new geo-points based on the new associated positions. the means of the graphical editor can also be used to fulfill this task. in terms of track geometry, the geometryentity extension module of the multipurpose railway infrastructure model allows us to describe selected types of vertical curves and separable superelevation sections. the currently used version of the geometryentity module includes the following terminal classes of vertical curves: • straightvc – describing straight vertical curves, i. e. sections of constant slope, • parabolahc – describing vertical curves in the shape of a parabola. both of these specialized classes of vertical curves has several attributes defined. the common ones of them are the id, name and longname attributes again 23 adam hlubuček acta polytechnica ctu proceedings and in addition the attributes of the verticalcurve class, which are the elevation0, deltaelevation and horizontallength attributes. the elevation0 attribute determines the elevation at the starting point of the respective vertical curve expressed in meters above the reference level (e. g. above sea level) and it can take the value of a real number. the deltaelevation attribute expresses the difference between the elevation at the end and the elevation at the beginning of the respective vertical curve. in order to express the elevation also at the vertical curve end point, we can define the elevation1 parameter. for each vertical curve, this parameter is calculated as follows: z1 = z0 + ∆z, (14) where z0 expresses the value of the elevation0 attribute, ∆z expresses the value of the deltaelevation attribute and z1 expresses the value of the elevation1 parameter. the horizontallength attribute determines the length of the perpendicular projection of the respective vertical curve to the horizontal xy plane in meters. it must have the same value as the difference between resulting positions of the associated positions defining the associated section intended to locate the respective vertical curve to the corresponding 2d linear element. in terms of individual specialized classes of vertical curves, the straightvc class has no additional attributes, while the parabolahc class has the parabolavertexradius additional attribute expressing the radius of the respective parabola in meters defined. it takes the value of a positive number for sag roundings and the value of a negative number for crest roundings. 7. separable superelevation sections the currently used version of the geometryentity module includes the following terminal classes of separable superelevation sections: • constantsuperelevation – describing sections of constant superelevation, • linearsuperelevationramp – describing linear superelevation ramps. both of these specialized classes of separable superelevation sections has several attributes defined. the common ones of them are the id, name and longname attributes again and the attribute of the superelevation class, which is the anchoredaxisreference attribute, in addition. the anchoredaxisreference attribute determines which axis remains at its original height even after the elevation is constructed. if it is the track axis, the attribute takes the value of 0. if it is the left rail axis, the attribute takes the value of −1. if it is the right rail axis, the attribute takes the value of 1. in terms of individual specialized classes of separable superelevation sections, the constantsuperelevation class has the superelevation attribute expressing the height difference between the track rails in millimeters, in addition. it takes the value of a positive number if the left rail is higher, the value of a negative number if the right rail is higher a the value of 0 if both rails are at the same height level. for superelevation ramps, which are instances of the linearsuperelevationramp class, however, two superelevation values must be expressed, both at the beginning and at the end of the respective separable section. this is provided by the superelevation0 and superelevation1 attributes. 8. transformation of 2d linear elements to 3d the 2d linear element that is coherently, completely and unambiguously described by vertical curves can be transformed into the corresponding 3d linear element in the graphical editor. this involves creating a new graphical representation of the newly emerging linear element. the default assumption for doing this is the ability to express the x and y coordinates and corresponding resulting position p at each point of the 2d linear element graphical representation (we can imagine an instance of the associatedposition class to be created in each of these points). since the element is already plotted in the graphical editor which has the tools to obtain these values, this assumption can be considered fulfilled. in order to plot the corresponding 3d linear element, it is necessary to calculate the z coordinate belonging to the individual points of the 2d linear element. it is advisable to proceed according to the associated sections to which the individual vertical curves are located. the determination of the z coordinate is carried out depending on the class of the respective vertical curve and the resulting position at the relevant point. it is based on calculations related to the design of rail transport structures [10, 11]. for straight vertical curves, the z coordinate of each point within the associated location of the respective vertical curve can be calculated based on its resulting position within the source 2d linear element simply by interpolating the stated values of the elevation0 attribute and the elevation1 parameter of the respective vertical curve in the following manner: zp = z0 + (pp − p0) · z1 − z0 p1 − p0 , (15) where pp expresses the resulting position of the given point, p0 represents the resulting position at the beginning of the vertical curve, p1 expresses the resulting position at the end of the vertical curve, z0 expresses the value of the elevation0 attribute, z1 24 vol. 43/2023 determining 3d coordinates based on the track geometry description figure 3. graphically expressed linear elements representing line and station tracks in 2d and in 3d. expresses the value of the elevation1 parameter and zp expresses the z coordinate of the given point. for parabolic vertical curves, the calculation of the z coordinate of each point within the associated location of the respective vertical curve requires knowledge of the parabolavertexradius attribute value, below referred to as rv, in addition. first of all, it is advisable to calculate the values of resulting position pv and the elevation zv at the top of the respective parabola: pv = p1 − (p1 − p0)2 + 2 · (z1 − z0) · rv 2 · (p1 − p0)2 , (16) zv = z0 − (pv − p0)2 2 · rv . (17) with the use of these auxiliary values, the final calculation can already be performed: zp = zv + (pp − pv)2 2 · rv . (18) once this procedure is done for all points of all vertical curves located to the 2d linear element, it is possible to plot the corresponding 3d linear element. how the result of the transformation looks for several linear elements can be seen in the figure 3. the transformation may also include creating 3d associated points bound to the 3d linear element based on 2d points bound to the corresponding 2d element. as individual associated positions may also correspond to individual geo-points with assigned geocoordinates of defined geo-positioning system, this transformation may further include the creation of matching geo-points in 3d. if a suitable geo-coordinate system is used, this basically means to preserve the values of the x and y attributes and to change the z attribute value of the original 2d geo-point from 0 to the calculated value when assigning new geocoordinates to the corresponding geo-points in 3d. 9. visualization the presented method can also be used to visualize the infrastructure data related to track geometry directly based on records in the database of the multipurpose railway infrastructure model. this procedure is nevertheless more complex, as it also includes the construction of the curve representing the 2d linear element. indeed, this can be achieved using tools similar to those used to create the horizontal curves in the graphical editor environment. the construction of the 2d linear element curve assumes that the linear element is coherently, completely and unambiguously described by horizontal curves. in case this data was obtained from the graphical editor where the 2d linear element was created by enclosing of inserted horizontal curves, this condition is already fulfilled. it only must be ensured that the resulting data description was not subsequently improperly tampered with. at the beginning of the 2d curve construction, it is essential to determine the coordinates of the starting point. for this purpose, it is appropriate to find the geo-point which is assigned to the associated position of the values of its attributes intrinsiccoordinate← 0 and deltaposition ← 0 which is bound to the relevant 2d linear element. its coordinates expressed in 25 adam hlubuček acta polytechnica ctu proceedings figure 4. visualization of the multipurpose railway infrastructure model data in software created by martin němec. the appropriate geo-coordinate system determine the starting point. the first horizontal curve, which associated location uses the stated associated position, is supposed to be plotted from the starting point. the subsequent procedure is similar to the one carried out when working in the graphical editor, only there is no need to enter individual attribute values, as these are loaded from the database. an example of visualization of a linear element representing a track based on data from the multipurpose railway infrastructure model database is shown in the figure 4. net entities of other classes can also be used for visualization purposes. e. g., in terms of track geometry, these are the above-mentioned separable superelevation sections. 10. conclusions this paper introduced the possibilities of creating a realistic graphic representation of linear elements representing tracks in 2d and 3d. related procedures and mathematical operations use data compatible with the multipurpose railway infrastructure model structure and the geometryentity module, which is its extension module focused on the track geometry description. their implementation is supposed to be carried out in the graphical editor, which also serves to fill the database of the multipurpose railway infrastructure model with consistent data. the way of data description of horizontal curves, vertical curves and separable superelevation sections using the classes of this the geometryentity module was continuously presented. the 2d linear element formation is based on the insertion of individual horizontal curves intended to be enclosed into it. the 3d linear element creation presupposes the transformation of the 2d linear element into 3d. this is based on the calculation of the z coordinates of the individual points of 2d linear element graphical representation. the method of the z coordinate calculation within a given associated section used to locate any of the vertical curves of the linear element varies based on the the class of the respective vertical curve. the resulting data description stored in the multipurpose railway infrastructure model database can be graphically presented with the use of dedicated visualization tools. further development of the described geometryentity module could include the introduction of other types of transitions curves and superelevation ramps, as well as the possibilities of expressing different values of track gauge and track gauge widening. references [1] a. hlubuček. význam popisu infrastruktury pro inteligentní dopravní systémy na železnici. vědeckotechnický sborník čd 42, 2016. [2020-08-31], https://docplayer.cz/108533917-vyznam-popisuinfrastruktury-pro-inteligentni-dopravnisystemy-na-zeleznici.html. [2] uic. railtopomodel v1.0. 2016, [2022-04-22], http: //www.railtopomodel.org/en/download/irs30100apr16-7594bca1524e14224d0.html?file=files/ download/railtopomodel/180416_uic_irs30100.pdf. 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[2020-08-31], https://www.spravazeleznic.cz/ documents/50004227/87152001/v%c4% 9bdeckotechnick%c3%bd+sborn%c3%adk+spr%c3%a1vy+ %c5%beeleznic+%c4%8d.1-2019/5743ca4b-17ca-4e958f51-cc179180fa64?version=1.0. [5] a. hlubuček. possibilities of high-speed railway turnout data description. acta polytechnica ctu proceedings 31:18–26, 2021. https://doi.org/10.14311/app.2021.31.0018 [6] a. hlubuček. digital track map for the vexa expert system. acta polytechnica ctu proceedings 35:8–13, 2022. https://doi.org/10.14311/app.2022.35.0008 [7] v. p. kolmorgen, c. rahmig. railml 3.1beta dissemination and feedback workshop. berlin, 2018. [8] uic. railtopomodel v1.4. 2022, [2022-04-22], https://www.railtopomodel.org/en/download/ irs30100-apr16-7594bca1524e14224d0.html?file= files/download/railtopomodel/2022-0429_railml_railtopomodel_v1-4.zip. [9] railml. railml schema version 3.1, 2019. [10] m. lindahl. track geometry for high-speed railways. tech. rep., royal institute of technology, stockholm, 2001. [2022-04-22], http://www.europakorridoren.se/spargeometri.pdf. [11] j. procházka. sylabus 10. přednášky z inženýrské geodézie (přechodnice, přechodnicové a výškové oblouky). inženýrská geodézie, 2015, [2022-04-22], https://k154.fsv.cvut.cz/wp-content/uploads/ 2022/01/sylabus_ig_10.pdf. 27 https://www.spravazeleznic.cz/documents/50004227/87152001/v%c4%9bdeckotechnick%c3%bd+sborn%c3%adk+spr%c3%a1vy+%c5%beeleznic+%c4%8d.1-2019/5743ca4b-17ca-4e95-8f51-cc179180fa64?version=1.0 https://www.spravazeleznic.cz/documents/50004227/87152001/v%c4%9bdeckotechnick%c3%bd+sborn%c3%adk+spr%c3%a1vy+%c5%beeleznic+%c4%8d.1-2019/5743ca4b-17ca-4e95-8f51-cc179180fa64?version=1.0 https://www.spravazeleznic.cz/documents/50004227/87152001/v%c4%9bdeckotechnick%c3%bd+sborn%c3%adk+spr%c3%a1vy+%c5%beeleznic+%c4%8d.1-2019/5743ca4b-17ca-4e95-8f51-cc179180fa64?version=1.0 https://www.spravazeleznic.cz/documents/50004227/87152001/v%c4%9bdeckotechnick%c3%bd+sborn%c3%adk+spr%c3%a1vy+%c5%beeleznic+%c4%8d.1-2019/5743ca4b-17ca-4e95-8f51-cc179180fa64?version=1.0 https://www.spravazeleznic.cz/documents/50004227/87152001/v%c4%9bdeckotechnick%c3%bd+sborn%c3%adk+spr%c3%a1vy+%c5%beeleznic+%c4%8d.1-2019/5743ca4b-17ca-4e95-8f51-cc179180fa64?version=1.0 https://doi.org/10.14311/app.2021.31.0018 https://doi.org/10.14311/app.2022.35.0008 https://www.railtopomodel.org/en/download/irs30100-apr16-7594bca1524e14224d0.html?file=files/download/railtopomodel/2022-04-29_railml_railtopomodel_v1-4.zip https://www.railtopomodel.org/en/download/irs30100-apr16-7594bca1524e14224d0.html?file=files/download/railtopomodel/2022-04-29_railml_railtopomodel_v1-4.zip https://www.railtopomodel.org/en/download/irs30100-apr16-7594bca1524e14224d0.html?file=files/download/railtopomodel/2022-04-29_railml_railtopomodel_v1-4.zip https://www.railtopomodel.org/en/download/irs30100-apr16-7594bca1524e14224d0.html?file=files/download/railtopomodel/2022-04-29_railml_railtopomodel_v1-4.zip http://www.europakorridoren.se/spargeometri.pdf https://k154.fsv.cvut.cz/wp-content/uploads/2022/01/sylabus_ig_10.pdf https://k154.fsv.cvut.cz/wp-content/uploads/2022/01/sylabus_ig_10.pdf acta polytechnica ctu proceedings 43:18–27, 2023 1 introduction 2 consistent filling the model with data 3 linear elements and associated positions 4 horizontal curves 5 creating linear elements in 2d using horizontal curves 6 vertical curves 7 separable superelevation sections 8 transformation of 2d linear elements to 3d 9 visualization 10 conclusions references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2024.49.0071 acta polytechnica ctu proceedings 49:71–76, 2024 © 2024 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague qct/fea analyses of palateless splinted and unsplinted maxillary all-on-4 systems luboš řehouneka,∗, mária frolob, aleš jíraa a czech technical university in prague, faculty of civil engineering, department of mechanics, thákurova 7, 166 29 prague, czech republic b charles university, faculty of medicine in pilsen, department of stomatology, alej svobody 1655/76, 323 00 pilsen, czech republic ∗ corresponding author: lubos.rehounek@fsv.cvut.cz abstract. the goal of the presented study was to determine the viability of using splinted and unsplinted maxillary implant-supported overdentures. the main physical quantity used to evaluate their differences was compressive stress in bone and equivalent (von mises) stress in the implant and overdenture components. we took a 3d x-ray scan of the maxilla and imported the data into a qct/fea (quantitative computed tomography-based finite element analysis) software. the loading was done in three load cases. we designed both models of the overdentures in cad, included the polyetheretherketone (peek) overdenture and analyzed the whole model of the reconstructed maxilla with implants. the results show worse (more concentrated) compressive stress distributions for the unsplinted locator variant. the splinted, bar-retained variant shows lower values of stress in all load cases. these findings and their potential for real-time selection of the optimal variant are then further discussed. keywords: qct/fea, implant-supported overdenture, splinted, unsplinted, all-on-4. 1. introduction alongside conventional methods including removable dentures, there has been a growing share of implantbased solutions since the formal acceptance of dental implants for tooth loss rehabilitation in 1978. these implants can either serve in fixed restorations or to stabilize complete removable dentures, as shown by an implant-supported overdenture (iod) [1]. it is vital for this treatment option to have thorough assessment, with its indications initially outlined in the 1990s [2]. this option proves to be advantageous for patients with limited bone quality [3]. edentulousness has been following a downward trend for over 10 years, but there is still a global concern due to demographic shifts towards an aging population. connecting the overdenture to the supporting dental implants can be done by various types of attachments including both unsplinted and splinted designs (bars and clips) [5–7]. to date, consensus has not been reached regarding the superior option [8]. barsupported (splinted) restorations offer better retention and show improved stability for dentures [9]. bars can withstand lateral motion and rotation during function, facilitating a more even distribution of force across fixtures [7, 10]. however, their application may elevate the likelihood of mucosal issues and hyperplasia due to suboptimal oral hygiene [11]. while the unsplinted design may have its advantages in terms of treatment simplicity, duration, hygiene, and cost [12], evidence regarding the impact of splinting versus non-splinting on the load distribution of upper jaw overdentures remains relatively limited and sometimes contradictory [13]. in a literature review with a comparable follow-up duration, survival rates are 88.9 % for unsplinted implants in the upper jaw, whereas splinted attachments showed an implant survival rate exceeding 97 % [14]. the absence of standardized techniques, implant quantities, differing measurements of marginal bone and loading conditions in studies may contribute to the subjective preference for unsplinted or splinted attachments for maxillary overdentures [2]. therefore, a more precise assessment of the biomechanical distinctions between these two alternatives is needed. to simplify complex problems into a system of algebraic equations, we can use the finite element method (fem) that provides insights into the behavior of the analyzed system. fem proves beneficial in solving intricate geometries, achieving numerical solutions for highly complicated stress problems via finite element analysis (fea) [15]. this technique has been effectively applied in examining stress and strain in implant dentistry [16]. with the increasing availability of 3d imaging techniques, such as quantitative computed tomography (qct), accurate reconstruction of the upper jaw bone is now possible, enabling qct/fea for analyses of stress distributions at the bone-implant interface. this study aims to compare stress distribution in the maxillary bone, dental implants, and prosthetic components of upper jaw overdentures in both splinted and unsplinted designs, using qct/fea. the null hypothesis is that the splinted design will demonstrate 71 https://doi.org/10.14311/app.2024.49.0071 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en l. řehounek, m. frolo, a. jíra acta polytechnica ctu proceedings (a). occlusal view. (b). front view. figure 1. the view of the patient’s residual alveolar ridge [4]. (a). the splinted, bar-supported variant. (b). the unsplinted, locator-supported variant. figure 2. analyzed variants of the all-on-4 system. assemblies designed to fit the individual patient. geometries are identical, short of the splints [4]. superior biomechanical performance compared to the unsplinted design. to validate or refute this hypothesis, this study focuses on these two types of iods. usually, other authors’ analyses use a two-phase (cortical and cancellous) linearly elastic, homogeneous model. however, these assumptions do not adequately mirror the intricate nature of bone tissue [17]. the use of 3d imaging and qct/fea can offer more realistic simulations of bone-implant interface behavior [18]. the present study aims to provide more precise results of biomechanical performance of implantsupported upper jaw overdentures by using a more detailed model. 2. materials and methods the anonymous patient who provided his 3d x-ray data for this study was a 76-year-old male with sufficient dexterity. we had obtained the patient’s written consent prior to conducting the study. all extraoral and intraoral criteria for an implantsupported overdenture in the maxilla were met [3]. buccal inclination of the residual ridge (figure 1) was present. the patient was predetermined for an anterior maxillary concept based on insufficient bone quality in distal regions, but sufficient quality in the frontal region. the implants used were manufactured by straumann (sla rn sp roxolid®) and placed in sites with good bone quality as follows: the upper-right first premolar (4.8 mm in diameter, 14 mm in length), upper-right first incisor (3.3 mm in diameter, 12 mm in length), upper-left second incisor (4.1 mm in diameter, 12 mm in length), and upper-left first premolar (4.1 mm in diameter, 12 mm in length). the 3d x-ray scans were acquired with planmeca promax® 3d classic. the approach used to evaluate the patient-specific 3d x-ray scans acquired with planmeca promax® 3d classic using romexis® 3d imaging was done using qct/fea. the 3d x-ray, along with the implant positions, shapes, and locations, were modeled based on data provided by the patient. for a visual representation of the analyzed implant variants, see figure 2. the qct/fea model includes both the cancellous and cortical bone. the design of the complete denture and abutments was done using exocad (exocad gmbh darmstadt, germany) in the dental laboratory. the denture was made without a palate. for visual reference, the denture with the splinted variant is shown in figure 3. the denture was made out of polyetheretherketone (peek). 2.1. methodology the analyses were performed using a specialized qct/fea software for nonlinear analysis of boneimplant interaction mechanical finder v. 12.0 (rccm, 72 vol. 49/2024 qct/fea analyses (a). occlusal view. (b). front view. figure 3. the view of the entire scene. the splinted variant is displayed [4]. tokyo, japan). the software uses sliced image data, like ct or x-ray scans. the qct/fea method allows for analyses using complete material inhomogeneity, where each voxel, and then finite element, possesses individual material properties specific to the individual patient’s bone quality. conventional modeling methods typically represent bone with a two-phase model (cancellous, cortical). for this study, the patient’s maxilla was reconstructed from an x-ray scan, and two different implant geometries were examined. 2.2. boundary conditions the loading was performed in three load cases for both analyzed variants. the first one was a 150 n force with a 35° slant in the buccolingual direction applied in the frontal region (both incisors). the second was a 600 n force with a 5° inclination applied in the distal region on both molars. the third was a 600 n force with the vertical component only, applied in the distal region on the first premolar tooth. to unify the visual appearance of the results, the color bar’s upper and lower limits were normalized. 3. results 3.1. stress distributions in implants the stress distribution in bone is similar for both alternatives in the frontal region. moreover, the equivalent stress levels in implants show almost no difference between the two variants. notably, even in the unsplinted alternative, the load was effectively transfered through multiple implants, thanks to the modeled denture (figure 4a). for the second load case (600 n force with a 5° buccolingual inclination on both molars), the splinted variant showed increased resistance to flexure. in contrast, the unsplinted design showed a significant concentration of load in the outermost implant (figure 4b). the results of the third load case (600 n force with a vertical component only applied on the first premolar) do not show great differences in equivalent stress distribution between the splinted and unsplinted variants. as the load is effectively transferred into the body of the implant directly beneath the first premolar, bending is minimized. consequently, the splinting bar’s contribution is diminished. the denture also helps to distribute the load, reducing disparities between the two variants (figure 4c). 3.2. stress distributions in bone the differences in stress were more significant in bone. the distribution of minimum principal (compressive) stress for the unsplinted design had proven to be inferior to the splinted design as the load concentrated into one of the implants and the bone had a severe stress concentration at one of the implants’ apices (figure 5). this was mainly manifested in the posterior region of the maxilla. conversely, the splinted setup effectively prevented excessive implant displacement with the use of bars. compressive stresses were significantly reduced for all load cases and primarily concentrated in the vicinity of the implants’ axes in the posterior region (figure 5c). 4. conclusion the results of this study show that the splinted variant provides a favorable, more even stress distribution. the values of stress are also lower in the analyzed load cases for the splinted variant, which confirms the null hypothesis. since masticatory forces are transferred through the dental restoration, splinting offers a range of benefits in implant-supported rehabilitations, including improved resistance to lateral loading and prosthetic component fractures [19]. this numerical study concludes the following: • stress distribution in implants loaded in the frontal region (both incisors) shows a comparable pattern for both variants with values of equivalent stress being comparable and the differences negligible. 73 l. řehounek, m. frolo, a. jíra acta polytechnica ctu proceedings (a). first load case. (b). second load case. (c). third load case. figure 4. isolines of equivalent (von mises) stress normalized across all images. left column – the splinted variant, right column – the unsplinted variant. all images display sections of the implant assemblies [4]. 74 vol. 49/2024 qct/fea analyses (a). first load case. (b). second load case. (c). third load case. figure 5. isolines of minimum principal (compressive) stress normalized across all images. left column – the splinted variant, right column – the unsplinted variant. all images display sections of bone in the buccolingual direction [4]. 75 l. řehounek, m. frolo, a. jíra acta polytechnica ctu proceedings • distal load applied on both molars shows the splinted variant distributes the load more evenly compared to the unsplinted variant and shows less bending in implants. on the other hand, load was concentrated to the outermost implant in the unsplinted variant. • force applied in the distal region on the first premolar did not create any significant stress concentrations in implants as the load was transferred solely into the body of the implant directly under the load and the effect of splinting was reduced. • compressive stress in the maxilla was inferior for the unsplinted variant with peaks of stress in the posterior region, namely around the implants’ apices. • the splinting effect played a significant role in preventing excessive implant displacement and bending and as a result, compressive stresses were less concentrated and had lower peaks for the splinted variant. acknowledgements the financial support provided by the by the faculty of civil engineering, ctu, prague, project no. sgs23/152/ohk1/3t/11 is gratefully acknowledged. references [1] g. thalji, m. bryington, i. j. de kok, l. f. cooper. prosthodontic management of implant therapy. dental clinics of north america 58(1):207–225, 2014. https://doi.org/10.1016/j.cden.2013.09.007 [2] s. j. sadowsky. treatment considerations for maxillary implant overdentures: a systematic review. the journal of prosthetic dentistry 97(6):340–348, 2007. https://doi.org/10.1016/s0022-3913(07)60022-5 [3] r. d. mericske-stern, t. d. taylor, u. belser. management of the edentulous patient. clinical oral implants research 11(s1):108–125, 2000. https: //doi.org/10.1034/j.1600-0501.2000.011s1108.x [4] m. frolo, l. řehounek, a. jíra, et al. biomechanical analysis of palateless splinted and unsplinted maxillary implant-supported overdentures: a three-dimensional finite element analysis. materials 16(15):5248, 2023. https://doi.org/10.3390/ma16155248 [5] t. aldhohrah, m. a. mashrah, y. wang. effect of 2-implant mandibular overdenture with different attachments and loading protocols on peri-implant health and prosthetic complications: a systematic review and network meta-analysis. the journal of prosthetic dentistry 127(6):832–844, 2022. https://doi.org/10.1016/j.prosdent.2020.12.016 [6] s. ceraulo, a. leonida, d. lauritano, et al. proposal for a clinical approach to geriatric patients with anchor need on implant for removable denture: new technique. prosthesis 2(3):185–195, 2020. https://doi.org/10.3390/prosthesis2030016 [7] p. amornvit, d. rokaya, s. bajracharya, et al. management of obstructive sleep apnea with implant retained mandibular advancement device. world journal of dentistry 5(3):184–189, 2014. https://doi.org/10.5005/jp-journals-10015-1285 [8] m. d. al amri. crestal bone loss around submerged and nonsubmerged dental implants: a systematic review. the journal of prosthetic dentistry 115(5):564–570, 2016. https://doi.org/10.1016/j.prosdent.2015.11.002 [9] s. m. heckmann, m. g. wichmann, w. winter, et al. overdenture attachment selection and the loading of implant and denture-bearing area. part 2: a methodical study using five types of attachment. clinical oral implants research 12(6):640–647, 2001. https: //doi.org/10.1034/j.1600-0501.2001.120613.x [10] m. a. elsyad, m. m. alokda, a. a. gebreel, et al. effect of two designs of implant-supported overdentures on peri-implant and posterior mandibular bone resorptions: a 5-year prospective radiographic study. clinical oral implants research 28(10):e184–e192, 2017. https://doi.org/10.1111/clr.12984 [11] j. f. martínez-lage-azorín, g. segura-andrés, j. faus-lópez, r. agustín-panadero. rehabilitation with implant-supported overdentures in total edentulous patients: a review. journal of clinical and experimental dentistry 5(5):e267–e272, 2013. https://doi.org/10.4317%2fjced.50817 [12] v. a. r. barão, w. g. assunção, l. f. tabata, et al. effect of different mucosa thickness and resiliency on stress distribution of implant-retained overdentures-2d fea. computer methods and programs in biomedicine 92(2):213–223, 2008. https://doi.org/10.1016/j.cmpb.2008.07.009 [13] m. roccuzzo, f. bonino, l. gaudioso, et al. what is the optimal number of implants for removable reconstructions? a systematic review on implant-supported overdentures. clinical oral implants research 23(s6):229–237, 2012. https: //doi.org/10.1111/j.1600-0501.2012.02544.x [14] g. m. raghoebar, h. j. meijer, w. slot, et al. a systematic review of implant-supported overdentures in the edentulous maxilla, compared to the mandible: how many implants. european journal of oral implantology 7(suppl 2):s191–s201, 2014. [15] v. jagota, a. p. s. sethi, k. kumar. finite element method: an overview. walailak journal of science and technology (wjst) 10(1):1–8, 2013. [16] j.-p. geng, k. b. c. tan, g.-r. liu. application of finite element analysis in implant dentistry: a review of the literature. the journal of prosthetic dentistry 85(6):585–598, 2001. https://doi.org/10.1067/mpr.2001.115251 [17] j. b. brunski. biomechanical aspects of the optimal number of implants to carry a cross-arch full restoration. european journal of oral implantology 7(suppl 2):s111–s131, 2014. [18] m. prado, s. khosla, c. chaput, h. giambini. opportunistic application of phantom-less calibration methods for fracture risk prediction using qct/fea. european radiology 31(12):9428–9435, 2021. https://doi.org/10.1007/s00330-021-08071-w [19] c. e. misch. dental implant prosthetics. elsevier health sciences, 2004. isbn 9780323078962. 76 https://doi.org/10.1016/j.cden.2013.09.007 https://doi.org/10.1016/s0022-3913(07)60022-5 https://doi.org/10.1034/j.1600-0501.2000.011s1108.x https://doi.org/10.1034/j.1600-0501.2000.011s1108.x https://doi.org/10.3390/ma16155248 https://doi.org/10.1016/j.prosdent.2020.12.016 https://doi.org/10.3390/prosthesis2030016 https://doi.org/10.5005/jp-journals-10015-1285 https://doi.org/10.1016/j.prosdent.2015.11.002 https://doi.org/10.1034/j.1600-0501.2001.120613.x https://doi.org/10.1034/j.1600-0501.2001.120613.x https://doi.org/10.1111/clr.12984 https://doi.org/10.4317%2fjced.50817 https://doi.org/10.1016/j.cmpb.2008.07.009 https://doi.org/10.1111/j.1600-0501.2012.02544.x https://doi.org/10.1111/j.1600-0501.2012.02544.x https://doi.org/10.1067/mpr.2001.115251 https://doi.org/10.1007/s00330-021-08071-w acta polytechnica ctu proceedings 49:71–76, 2024 1 introduction 2 materials and methods 2.1 methodology 2.2 boundary conditions 3 results 3.1 stress distributions in implants 3.2 stress distributions in bone 4 conclusion acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2024.50.0074 acta polytechnica ctu proceedings 50:74–80, 2024 © 2024 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague understanding intrinsic stress effects on vibrational response of silicon nanowires sina zare pakzada,b,c, muhammad muzammild, mehdi bostan shirina, basit alia, semih berk cobana, ege nacarkucuka, umut kerimzadea,e, burhanettin erdem alacaa,e,f,∗ a koç university, department of mechanical engineering, 34450 istanbul, turkey b technische universität wien, institute of sensor and actuator systems, gusshausstrasse 27-29, 1040 vienna, austria c technische universität wien, institute of sensor and actuator systems, christian doppler laboratory for piezoelectric silicon mems with enhanced sensitivity and responsivity, a-1040 vienna, austria d koç university, computational sciences and engineering program, 34450 istanbul, turkey e koç university, n2star-koç university nanofabrication and nanocharacterization center for scientific and technological advanced research, 34450 istanbul, turkey f koç university, surface technologies research center (kuytam), 34450 istanbul, turkey ∗ corresponding author: ealaca@ku.edu.tr abstract. silicon nanowires have become integral components in nanoelectromechanical systems and nanoelectronics. this article explores the intricate relationship between intrinsic stress and mechanical behavior in silicon nanowires. utilizing a comprehensive approach involving raman characterization, resonance testing, and thermal processing, the study investigates the induced intrinsic stresses within silicon nanowires. the findings reveal the introduction of 1.4gpa of intrinsic stress and a 1.3mhz frequency shift to silicon nanowire through thermal processing. these results underscore the importance of understanding and utilizing intrinsic stresses in silicon nanowires for the advancement of nextgeneration nanoelectromechanical systems. overall, this article contributes to the ongoing efforts aimed at fully realizing the potential of silicon nanowires in various scientific and technological domains. keywords: silicon, nanowire, resonance testing, raman characterization, intrinsic stress. 1. introduction silicon nanowires (si nws) serve as fundamental components in the integration of microelectromechanical systems (mems) and nanoelectromechanical systems (nems), playing a pivotal role in next-generation electronics, energy applications, and ultra-sensitive sensors [1–3]. with their unique properties and the inherent piezoresistivity, si nws exhibit remarkable potential across various domains, bridging fundamental scientific exploration with potential industrial applications [4, 5]. consequently, a thorough understanding of the fundamentals and properties of si nws is important [6, 7]. the challenges associated with comprehending the mechanical behavior of si nws underscore the need for a reliable testing method applicable to si nws, considering the complexities involved in their fabrication, testing, and characterization [4, 8–12]. resonance testing, owing to its simplicity and successful outcomes, is commonly employed to acquire vibration modes and subsequently obtain mechanical properties [8, 13]. from a modeling perspective, atomistic simulations also offer a reliable approach for validating or examining the intrinsic effects in si nws [6, 14]. exploring the mechanics of si nws offers an inclusive approach to material testing, providing substantial advantages in bridging the gap between theoretical methods and property assessments [6, 7, 15–17]. additionally, by reducing the challenges associated with full-scale processing, it becomes feasible to seamlessly integrate materials characterization and property measurement [11]. in this context, the utilization of high-throughput and automated [7, 18, 19] testing techniques not only facilitates comprehensive data collection but also reveals deformation mechanisms essential for understanding the overall intrinsic properties of si nws [11]. although conducting bending and resonance tests presents complex experimental challenges [6, 15, 20], the findings hold the potential to offer valuable insights into the intricacies of nanoand micro-scale structures, highlighting their importance as subjects of study. given this perspective, investigating vibrational testing, with reference to successful cases in the literature, provides an opportunity to analyze fundamental properties such as modulus of elasticity of si nws [4, 16, 17, 21, 22]. it also facilitates comparisons of external influences, such as surface treatments or thermal processes, involved in the fabrication or assembly of nano-scale structures, particularly si nws within the framework of this study [23]. 74 https://doi.org/10.14311/app.2024.50.0074 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 50/2024 intrinsic stress effects on vibrational response of si nws understanding stress generation in nanoand microscale structures is crucial, not just from a theoretical standpoint, but also because the functionality of future mems and nems relies on comprehending the nature and intensity of stresses induced or generated during various fabrication, assembly, testing, or utilization stages [4, 6, 15]. in this context, raman spectroscopy emerges as a widely employed technique for stress measurement in nws, particularly in studies focusing on si [24]. raman analyses have shown stresses of up to 4gpa and 1.2gpa for highly strained siliconon-insulator [25, 26] and stress-free substrates [6, 14, 15], respectively. this emphasizes the necessity for meticulous examination of any external influences, whether preor post-processing, given the significant impact of observed stress levels. such scrutiny is essential, as these stresses can lead to variations of up to 85gpa in the elastic properties of si nws [6]. while the resonance technique facilitates controlled testing, challenges arise in the fabrication of samples and the operation of the devices under test (duts), leading to a reduction in overall testing throughput. recent studies suggest the potential for integrating fabrication and testing methodologies meticulously, allowing for the acquisition of numerous testing results for si nws fabricated within a single substrate [20]. this offers advantages for statistical analysis and the efficacy of the developed technique [20]. nonetheless, further enhancements to testing and characterization methodologies necessitate a more systematic and interconnected approach, where the introduction of stresses to si nws can be accurately captured. consequently, examining stresses and their subsequent effects on the mechanical properties of si nws becomes feasible from both theoretical and experimental perspectives. the current investigation presents a comprehensive methodology for analyzing stresses induced in si nws through a systematic, interconnected approach involving thermal processes, raman characterization, and resonance testing. the main aim is to establish a linked process for testing and characterizing si nws fabricated on stress-free substrates, while subjecting them to subsequent thermal steps to systematically introduce intrinsic stresses. building upon prior successful studies by the authors, which focused on cofabrication of si nws integrated into micro-scale support pillars [6, 15, 20, 27], characterization of surfaceor fabrication-induced residual stresses [6, 14, 15], and high-throughput resonance testing of si nws, this research seeks to develop a methodology for introducing controlled stress and subsequently capturing it through follow-up testing and measurement assessments. this facilitates the analysis of vibrational modes and intrinsic stresses post-fabrication and after the thermal process, enabling precise induction and subsequent examination of thermal effects on si nws. notably, the co-fabrication process for creating double-clamped si nws within a single substrate reduces sample-to-sample variations, enhancing the statistical robustness of the findings. for detailed insights into the fabrication method, testing procedure, raman characterization, and thermal process, refer to section 2. the outcomes of resonance testing and raman characterization are presented in section 3. 2. materials and methods this section introduces the integrated methodology for raman characterization and resonance testing of si nws fabricated in a double-clamped configuration. section 2.1 provides detailed information on the specific design and fabrication of si nws connected to the support pillars. additionally, section 2.2 provides details on the resonance testing method. section 2.3 discusses raman characterization for quantifying intrinsic stress in si nws, while section 2.4 outlines the thermal process details, presenting the methodology for inducing stress into si nws. 2.1. fabrication the process of fabricating si nws begins with a double-side polished < 100 > si wafer, measuring 525µm in thickness, which is diced into multiple fragments, each sized at 12mm × 12mm. the method involves masking a specific region of these fragments using a photoresist (pr) etch mask, followed by deep si etching. this allows for the creation of suspended si nw and its micro-scale accompanying support structures. to initiate the fabrication, the fragment undergoes a standard rca1 cleaning process, after which a dielectric etch mask is applied via plasma-enhanced chemical vapor deposition. subsequently, photolithography is employed to define the desired feature on the pr. following the patterning of the dielectric etch mask, deep reactive ion etching (drie) is carried out to remove the si beneath the etch mask [27]. this allows for the release of si nws upon etching process. following this, o2 ashing and selective vapor etching using hydrofluoric (hf) acid are performed to remove the protective layers, enabling precise definition of < 110 >-oriented si nws and micro-scale triangular supports within the substrate. 2.2. resonance testing the frequency response of si nws is examined using a laser doppler vibrometer (ldv msa-600, polytec gmbh). the experimental setup entails placing the dut onto a piezoelectric disk, secured with adhesive tape for actuation. vibrational spectra are captured by activating the piezoelectric disk with a sinusoidal chirp signal generated by the ldv system within the specified frequency range. subsequently, the device’s response is analyzed using fast fourier transform (fft) techniques. the ldv is capable of generating an excitation signal ranging from 0 to 25mhz, with 1.25 khz increments, to drive the piezoelectric disk. this induces vibration in the sample positioned on top of the piezoelectric disk. a piezo-ceramic actuator 75 s. z. pakzad, m. muzammil, m. b. shirin et al. acta polytechnica ctu proceedings figure 1. the experimental setup employs an ldv for conducting resonance tests on si nws. dut refers to the si fragment hosting si nws and accommodates specific scanning points assigned to both the si nws and support pillars, employing the msa-ldv setup for resonance testing. disk is utilized to produce high-frequency excitation via a periodic chirped signal. figure 1 illustrates the testing setup and relevant details for conducting resonance tests on the dut hosting multiple si nws. the scan points assigned using the msa-ldv setup undergo resonance testing for comprehensive analysis [23]. 2.3. raman characterization after conducting dimensional analysis of si nws using sca nning electron microscope (sem), raman spectroscopy is employed to quantify the intrinsic stress, denoted as σ, using equations 1 and 2: ∆ω = 1 2ω0 [ [ps12 + q(s11 + s12)] ] σ = (ssc)σ, (1) ∆ω = −1.93 × 10−9 × (σ[pa]), (2) where s11 and s12 are the elastic constants, p and q are the phonon deformation profiles. the stress shift coefficient (ssc) is calculate using s11 = 7.68×10−12 pa−1 and s12 = −2.14 × 10−12 pa−1 for quantifying the stress via raman shift given as ∆ω (ω0 represents unstrained si peak). to accomplish this, line scans are performed along both the si nw and support pillars, with the latter serving as a reference for bulk measurement. the outcome of line scans for the support pillars and si nw is used for the calculation of the raman shift. a lorentz model is applied to fit the raman peaks of nws under tension. further details concerning the determination of stress induced by native oxide formation and comparative computational investigations can be found elsewhere [6, 14, 15]. 2.4. thermal process the thermal processing is conducted utilizing a solaris 150mm rapid thermal process oven. to guarantee figure 2. a array of si nws connected to triangular supports with an etch depth of 5µm (achieved through the fabrication procedure detailed in section 2.1), exhibiting consistent width, thickness, and length. the si nws measure 500 nm in thickness, 1000 nm in height, and 15µm in length. temperature stability, preliminary processes are executed without the sample before the actual processing begins. nitrogen gas is continuously introduced into the chamber throughout the process at a flow rate of 10 sccm. the sample is subjected to heating at 400 °c for 70 minutes, with an initial ramp rate of 150 °c sec−1 until the desired temperature is attained. the stress generated in si nws is anticipated due to the confined size of the nanostructures when subjected to heating in a double-clamped configuration [6, 14]. its existence can be easily misinterpreted as an increase or reduction of resonator modulus of elasticity in the case of tensile or compressive stresses, respectively. 3. results the results of the si nws fabrication process are detailed in section 3.1. furthermore, section 3.2 presents discussions on raman characterization and resonance testing outcomes. 3.1. fabrication of si nws the co-fabrication method employed for si nws enables the creation of multiple samples on a single chip, allowing for subsequent resonance testing following comprehensive structural and intrinsic stress characterizations conducted using sem and raman setups. this fabrication approach facilitates the production of si nws with consistent geometric and lithographic configurations on a single stress-free si substrate, referred to as dut, which in turn allows for detailed testing and characterization. as illustrated in figure 2, si nws with uniform fabrication processes are produced in close proximity using the methodology outlined in section 2.1. here, the use of low-temperature processes and the ability to achieve multiple samples within a single fabrication process, incorporating mems-level structures as support pillars and nemslevel features as si nws, are critical aspects of this methodology. figure 2 showcases an array of si nws 76 vol. 50/2024 intrinsic stress effects on vibrational response of si nws with dimensions of 500 nm in thickness, 1000 nm in height, and 15µm in length. these si nw structures are co-fabricated with an etch depth of 5µm, allowing for sufficient space for subsequent testing procedures. the developed process flow demonstrates notable repeatability, resulting in a high-yield process that facilitates the evaluation of si nws with desired dimensions. the fabrication technique eliminates the need for additional processes, ensuring that si nws can undergo deformations in different directions relative to the modal resonance state. furthermore, each si nw under examination undergoes sem, resonance testing, and raman analysis before the thermal processing step described in section 2.4. subsequently, these analyses are repeated after the thermal process is conducted to induce residual stress into the si nws, allowing for a thorough investigation of changes in raman and resonance frequency shifts, which will be discussed in section 3.2. further details on the fabrication of such si nws can be found elsewhere [27]. 3.2. characterization and testing of si nws following the dimensional analysis of si nws, raman spectroscopy is employed to quantify the intrinsic stress, denoted as σ, utilizing equation 1 and equation 2. line scans are conducted along both the si nw and support pillars, with the latter serving as a reference for bulk measurement. figure 3a illustrates the results of these line scans, indicating raman shifts of 520.24 cm−1 and 519.61 cm−1 for the support pillars and si nw before the thermal process, respectively, resulting in a downshift of 0.62 cm−1. the measurements are repeated on si nws once the thermal process is completed. figure 3b illustrates the results of these line scans, indicating raman shifts of 520.63 cm−1 and 517.14 cm−1 for the support pillars and si nw after the thermal process, respectively, resulting in a downshift of 3.48 cm−1. given the obtained raman shifts for the si nw under investigation before and after thermal processing, figure 4a and figure 4b represent the shifts obtained along the nw longitudinal axis, depicting micropillar supports and si nws, before and after the thermal process, respectively. a lorentz model is employed to fit the raman peaks experiencing tensile stress before and after thermal processing with different magnitudes. the shift towards lower wavenumbers indicates a tensile intrinsic stress along the nw axis, with a tensile stress of 325mpa and 1.8gpa before and after thermal processing, respectively. in this respect, the standard deviation in stresses characterized before and after thermal processing are obtained for three si nws, resulting in 90.7mpa and 265.3mpa, respectively. the resulting stress, induced by a thermal process and measured at 1.4gpa, along with noticeable changes in raman shifts as shown in figure 3 and figure 4, confirms the introduction of thermal effects into the double-clamped si nw configuration. this also (a). (b). figure 3. raman spectra obtained via line scans conducted on suspended si nws and micro-scale supports (a) before and (b) after thermal processing. the averages associated with raman shifts for si nw and supports are given in dashed lines. underscores the accurate calibration of the raman analysis for detecting stresses present in the sinws under examination within this study. furthermore, resonance testing will serve as the subsequent characterization method for a detailed investigation of the residual stress introduced into si nws through thermal processes. further details regarding the determination of stress induced by native oxide formation and comparative computational investigations involving intrinsic stresses utilizing atomistic modeling can be found elsewhere [6, 14, 15]. the stress observed in this study is consistent with previously reported values for si nws, attributed to intrinsic or thermal processes, including thermal expansion mismatch between the si and surface layer [6, 14]. the power calibration of raman spectroscopy is employed to confirm that the stress values are not influenced by phase transitions or measurement artifacts, consistent with prior experimental findings [14, 25, 26]. 77 s. z. pakzad, m. muzammil, m. b. shirin et al. acta polytechnica ctu proceedings (a). (b). figure 4. raman shifts derived from the data in figure 3, displayed along the si nw (a) before and (b) after thermal processing. the blue and violet data points correspond to support and si nw, respectively. to perform resonance testing, the si nw undergoes vibrational analysis following the methodology detailed in section 2.2. the results, illustrating displacement versus frequency for the si nw before and after thermal processing, are presented in figure 5. lorentzian fits are applied to illustrate the peaks observed in the responses, aiding in the identification of the first mode of resonance for si nw. this approach yields resonance frequencies of 14.27mhz and 15.57mhz for the si nw before and after thermal processing, respectively. further calculations of the quality factor (q-factor) [28] for si nw reveal an increase from 37 to 117 for the si nw before and after the thermal process, respectively. the observed shift in frequency response and q-factor for the si nw under examination offers another independent observation of the stresses introduced into si nws through the process outlined in section 2.4. in this context, the introduction of intrinsic stress figure 5. displacement versus resonant frequency, along with lorentzian fits, depicted for si nw before and after thermal process. resonance measurements indicate the occurrence of the first mode (out-of-plane) mechanical resonance at frequencies of 14.27mhz and 15.57mhz for the si nw before and after thermal processing. into the si nw under examination through thermal processing is investigated via raman characterization and resonance testing. both methodologies independently validate the shifts observed in raman data and frequency responses. the results obtained herein offer a dependable methodology for extending the approach developed for examining multiple samples, facilitating detailed statistical analysis and precise investigation of intrinsic stresses and their subsequent impact on the behavior of si nws. 4. conclusions in conclusion, this paper emphasizes the complex interplay between intrinsic stress and mechanical behavior of si nws. through a comprehensive examination utilizing techniques including raman characterization, resonance testing, and thermal processing, the intrinsic stresses induced into si nw under investigation is analyzed. the integration of these methodologies provides valuable insights into the mechanical properties of si nws and facilitates the development of a reliable testing methodology for streamlining the examination of multiple samples. these findings underscore the importance of understanding and controlling intrinsic stresses in si nws for the advancement of next-generation electronics. acknowledgements s.z.p., b.a. and b.e.a. gratefully acknowledge the financial support by tubitak under grant no. 120e347 and 118c155. the authors acknowledge the use of the services and facilities of n2star-koç university nanofabrication and nanocharacterization center for scientific and technological advanced research for the fabrication processes of this work. the authors acknowledge dr. m baris yagci and dr. gülsu şimşek franci from koç university surface technologies research center (kuytam) for the characterization steps of this work. 78 vol. 50/2024 intrinsic stress effects on vibrational response of si nws references [1] s. raman, r. sankar, m. sindhuja. advances in silicon nanowire applications in energy generation, storage, sensing, and electronics: a review. nanotechnology 34(18):182001, 2023. https://doi.org/10.1088/1361-6528/acb320 [2] v. s. vendamani, s. v. s. nageswara rao, a. p. pathak, v. r. soma. silicon nanostructures for molecular sensing: a review. acs applied nano materials 5(4):4550–4582, 2022. https://doi.org/10.1021/acsanm.1c04569 [3] s. a. ahad, t. kennedy, h. geaney. si nanowires: from model system to practical li-ion anode material and beyond. acs energy letters 9(4):1548–1561, 2024. 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nanoelectromechanical germanium nanowire resonators. the journal of physical chemistry c 112(29):10725–10729, 2008. https://doi.org/10.1021/jp8010487 80 https://doi.org/10.1038/ncomms2102 https://doi.org/10.1088/1361-6641/ad0791 https://doi.org/10.1021/jp8010487 acta polytechnica ctu proceedings 50:74–80, 2024 1 introduction 2 materials and methods 2.1 fabrication 2.2 resonance testing 2.3 raman characterization 2.4 thermal process 3 results 3.1 fabrication of si nws 3.2 characterization and testing of si nws 4 conclusions acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.38.0347 acta polytechnica ctu proceedings 38:347–353, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague moisture drying capacity of mineral wool insulated steel-faced sandwich panels by convection siim lomp∗, kristo kalbe, targo kalamees taltech – tallinn university of technology, department of civil engineering and architecture, nearly zero energy buildings research group, ehitajate tee 5, 12618, tallinn, estonia ∗ corresponding author: siim.lomp@taltech.ee, siimlomp@gmail.com abstract. this study analyses moisture dry-out from a steel-faced insulated sandwich panel by forced convection. moisture convection performance was studied by laboratory tests and simulation. two test walls with the lower parts close to the free water level were studied in a laboratory with and without convection. in addition, a real-scale wall was built to south orientate direction. measurements were also used for simulation model calibration. the hygrothermal simulations were performed with the simulation tool delphin in stable climatic conditions to determine the magnitude of the convection moisture dry-out capacity. comparison of the measured and simulated relative humidity showed sufficiently good agreement. the results indicate that convection significantly improves dry-out capacity, especially during summer. however, during autumn and winter their dry-out capacity was smaller. to minimise the wetting of insulation, weather protection during construction and storage is necessary. keywords: drying of structures, sandwich panel, hygrothermal performance, moisture safety. 1. introduction the building envelope of nearly zero energy buildings needs to be well insulated [1]. pihelo [2] showed that the drying-out period is longer in walls with lower thermal transmittance, especially when vapour-tight rigid boards outside the insulation are used [3]. ojanen [4] studied the hygrothermal performance of mineral wool insulation products in highly insulated structures and showed that the vapour-open thermal insulation, like stone wool, allowed significantly faster drying of the inner concrete layer than the more vapour-tight thermal insulations. sekki et al. [5] studied external insulated precast concrete wall panels and showed that, for this structure, the drying period was also shortest with a vapour-open insulation material. ensuring hygrothermal performance is particularly difficult when the surrounding materials (especially external material – wind barrier) are vapour-tight. some examples of such structures are the structural insulation panel (sip) and steel-faced sandwich panel. in sip, the material which surrounds the insulation is usually oriented strand board (osb) or plywood, which is slightly vapour permeable. however, in steel faced sandwich panels, the steel sheets are vapour tight. the structural integrity of sandwich panels could be significantly degraded due to the moisture absorbed after long-term exposure to a humid environment [6]. it is thus important to investigate the moisture dry-out methods of the sandwich panels. when water has leaked into the mineral wool sandwich panel, the speed of evaporation is the limiting factor for the increasing vapour pressure compared to the speed of vapour diffusion [7]. compared to diffusion, the moisture convection allows much larger amounts of moisture to be moved in both the wetting [7] and drying [8] processes. accidental air leaks can limit the intended drying, as the air does not pass through the entire specified area. the steel-faced mineral wool insulated sandwich panel differs from other structures as it has a very clear structure – airtight external surfaces (exterior and interior steel sheet) and air-permeable internal insulation layer. this makes it possible to form a clearly defined area where air must move to dry the structure. as diffusion drying from a steel-faced mineral wool insulated sandwich panel is limited [9], convection drying is a method to increase the efficiency of moisture dry-out. however, there is still relatively limited research on this field. the current study examines convection drying of a steel-faced mineral wool insulated sandwich panel by small-scale and full-scale laboratory experiments and hygrothermal modelling. 2. methods 2.1. laboratory measurements moisture drying measurements in laboratory included: • 3d mock-up test elements to find out convection drying efficiency (figure 1). airflow rate per volume 91 l/(min · m3) (smc pfm 710), tambient +23.0 °c, rh 30 % (hobo ux-100-023); • full-scale drying test between indoor and outdoor environment (average airflow rate per volume 23 l/(min · m3) (smc pfm 725), ti +23.0 to +31 °c, measured outdoors data for te and rhe (humisense a1)) to reveal air pressure distribution and convection drying performance as well to obtain data for the calibration of hygrothermal model. 347 https://doi.org/10.14311/app.2022.38.0347 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en s. lomp, k. kalbe, t. kalamees acta polytechnica ctu proceedings figure 1. 3d mock-up test elements: (a) with indicated t and rh sensor placement (red dots, hobo ux-100-023), airflow sensors (black dots smc pfm 710) and photos of the elements (b). figure 2. full scale test walls and sensor positioning (t/rh humisense a1, airflow smc pfm 725). the upper section of the wall, separated from lower parts, was left to dry by diffusion through two open surfaces (2 × 600 × 100 mm) connected to outside climate conditions while the middle and lower sections were ventilated with air drawn from outside (figure 2). the test wall had a total of 27 temperature and relative humidity sensors. additionally, 5 air pressure difference sensors were installed to determine air pressure distribution inside the wall and rule out any possible air leakages that could have a negative effect on the test. airflow rate was also measured at airflow entry point and airflow outlet. possible air leakages were tightened until the entering airflow rate reached approximately the same value as the outlet airflow rate. once the airtightness of wall had been confirmed, water was added through 10 injection points per upper and per lower section and then left to redistribute over the volume for 10 days. here, 50 ml of water was added through every upper injection point and 100 ml through every lower injection point. after 10 days of redistribution the upper section of the test wall was opened to diffusion and ventilation was started in the lower section. the entire drying period lasted 21 days. 2.2. modelling the numerical simulation tool delphin 5.9 [10] was used for the combined heat, air and moisture (ham) transport modelling. delphin is a well-developed, advanced and validated software application suitable for building sciences. the simulation model takes into account measured indoor/outdoor temperature, rh and measured airflow through the length of a wall. in the calculation model heat and moisture can move in 2 directions and airflow in 1 direction. the water injection points were modelled as a raised moisture content within the mineral wool, equally distributed over the height of wall. this caused the calculation model to 348 vol. 38/2022 moisture drying capacity of mineral wool insulated . . . figure 3. modelled water vapour pressure at sensor positions compared to measured results (figure 2). w conv. is for time range when lower section of the wall was ventilated and wo conv. when not. figure 4. moisture dry-out at stable temperature conditions (figure 1). diffusive drying 2020 refers to a previous research on vapour retarding and vapour permeable tape performance [9]. overestimate moisture distribution equality because in reality there was only one injection point per height of the wall. however, the outputs taken from the simulation model showed sufficiently good conformity with the measured results (figure 3). the calculation model overestimates drying rate near water injection points and underestimates drying rate further away from injection points. more accurate results might have been achieved if the software could have calculated heat/moisture movement in 3 directions and airflow in two directions. the thermal conductivity of mineral wool was determined as 0.04 w/(k · m), bulk density 85 g/m3, porosity 0.97 m3/m3, specific heat capacity 840 j/(kg · k), effective saturation 0.9 m3/m3 and water vapour diffusion resistance factor µ = 1. air permeability of mineral wool is irrelevant in this case because the air flow rate is measured and inserted into the simulation without calculating it by pressure and air permeability. to study the drying rate of a typical steel-faced sandwich panel wall, the simulation models were updated to represent a typical structure where the panels are 6 m long, 8 m high, 0.23 m thick and have a 30 mm vertical connecting joint at both ends, one for air intake and one for air exhaust. the boundary condition on one of the longer sides was simulated using a stable outdoor climate and on the other side as an indoor climate. shorter sides were simulated as adiabatic. for the outdoor climate of these simulations, the finnish moisture reference year, known as jokioinen 2004, was used. jokioinen 2004 (average for winter te -4.4 °c, rh 93 %, ν 3.3 g/m3 and for summer te +14.5 °c, rh 75 %, ν 9.3 g/m3) is a critical outdoor climate if the internal part of the structure is protected. 3. results and discussions 3.1. stable temperature conditions the temperature and relative humidity indoor remained relatively constant throughout the drying period, respectively ≈ +23 °c and ≈ 30 %. mineral wool pore air relative humidity at the beginning of the test was 97–100 %. the test piece (figure 1) which dried by diffusion decreased in mass about 2.5 g/day. another test piece (figure 1) dried by convection showed a significantly faster decrease in mass – about 31.5 g/day (figure 4). the significant impact of airflow gave us the confidence to move forward with the variable temperature conditions test which was carried out on the big test wall (figure 2). 3.2. variable temperature conditions the external temperature throughout the drying period was extremely hot and produced high water vapour pressure levels inside the wall. the drying rate by diffusion depends on the pore air water vapour level in the layer which is near to the open surface compared to the water vapour level in the air which is 349 s. lomp, k. kalbe, t. kalamees acta polytechnica ctu proceedings figure 5. daily running average of measured water vapour pressures in diffusive drying wall section. figure 6. daily running average of measured water vapour pressures in convective drying wall section. in contact with the open surface. further away from the open surface diffusion also takes place inside the mineral wool but the difference in water vapour pressure between mineral wool layers becomes lower when moving further inside the wall, away from the open surface. this limits the moisture movement rate from the centre of wall span to the open surface. however, the water vapour pressure levels remained relatively high in the diffusive drying section (figure 5) compared to the section which was ventilated by airflow (figure 6). water vapour partial pressure in the figures below is a daily running average created with 24 data points. the water vapour levels inside the ventilated test wall section become largely homogeneous at the end of the drying period (figure 6). this indicates that all the moisture inside the wall has moved towards the exhaust of the airflow and exited the wall. the nearer the layer is to the airflow input, the faster it dries because outside air displaces this area faster. the layer near to the airflow exhaust is the last one to dry because all the moisture gets sucked there before exiting the wall. 3.3. drying strategies for summer and winter the updated calculation model simulated drying at various airflow rates. relative humidity of summer outdoor air was reduced to 10 %, which takes into account the assumption that air is processed through an air compressor and excessive moisture is extruded from the air. variable air inflow temperatures were tested to find out the optimal temperature for winter climate conditions (figure 7). it is important to maintain relatively high airflow rates in winter to ensure that the air has not cooled down too much before reaching the outlet, otherwise it would limit the moisture content in air exiting the wall (figure 8). the drying was simulated until air in mineral wool pores reached the water vapour content of outside air. the drying process might be faster if there is a known performance criterion and appropriate maximum humidity content. 3.4. applying drying on a real scale wall sandwich walls in a real-life situation might be more complicated than the test wall built for this research. it is common that sandwich panels have additional holes for fastening secondary constructions which makes the wall more air permeable and increases leakages. a sandwich wall might also have nonperpendicular angles which could make proper tightening even more challenging. proper tightening should make the wall impermeable for air on pressure difference. to apply convective drying on a multi-span wall with many vertical joints it is recommended to place ventilated joints with negative air pressure drop and free joints for compensation air over one so that the air moves from adjacent joints towards the ventilated joint (figure 9 and figure 10). the key to this airflow trajectory is properly tightened possible air leakages (horizontal joints between panels, screw holes, etc.). it is also important to protect the vertical joints against 350 vol. 38/2022 moisture drying capacity of mineral wool insulated . . . figure 7. moisture dry-out duration at stable temperature conditions to outside air humidity (pore air vapour content in summer 9.3/10 = 0.93 (g/m3) and in winter 3.3 (g/m3). blue lines indicate drying in winter at different air inflow temperatures, red line shows drying in summer with reduced moisture content in the air. (a). (b). figure 8. temperature distribution on top view of the wall (vertical length 0.23 m and horizontal length 6 m), winter period, air inflow temperature +40 °c (a) airflow rate 228 l/(min · m3) (b) airflow rate 3 650 l/(min · m3). red area shows temperature over +35 °c and navy blue is for areas under -3 °c. figure 9. scheme on facade for ventilated vertical joints and compensation air joints. 351 s. lomp, k. kalbe, t. kalamees acta polytechnica ctu proceedings figure 10. scheme on top view of facade for ventilated vertical joints and compensation air joints. diagonal rain with a temporary drying strip on the joint. the drying strip on the ventilated joint has to be airtight in order to avoid air leakages near the pressure drop. our research confirmed the proof of concept, but future studies should analyse cost efficiency and feasibility. it is up to discussion whether the method should be used if its cost exceeds the cost of replacing the panels or the energy demand becomes unacceptably high. a method to reduce energy consumption of the drying method in winter would be adding temporary extra thermal insulation on the outer face of the sandwich panel wall. this would rise the temperature of the external face and therefore increase the evaporation of moisture adsorbed on mineral wool fibres near the cold surface. however, arguably the best option would be to implement proper moisture management during both the construction phase and service life. 4. conclusions the following conclusions can be offered for drying a wall of steel-faced mineral wool insulated sandwich panels: • stable temperature conditions test indicated that convective drying has the potential to lower moisture mass dozens of times faster than drying by diffusion. • even very small air leakages can cause uncontrollable airflow, hence taking compensation air from somewhere else reduces drying in areas that are located further away from pressure drop. • the drying of a sandwich wall is modellable in the case of a 2-dimensional heat/moisture transport and 1-dimensional airflow if there are no significant air leakages. • drying in winter requires heating the air that gets sucked into the wall and it is important to maintain relatively high airflow rates in order to avoid air cooling down too much before exiting the wall at the other end. airflow temperature and airflow rate significantly affect drying speed and final moisture content. • air flow rate of the winter drying method which no longer increases the rate of drying significantly is 3 650 l/(min · m3). if the air inflow temperature is +40 °c then it would take 21 days to dry for an 8 m long, 6 m high and 0.23 m thick wall at the air flow rate 3 650 l/(min · m3) to dry to humidity content of outside air (te -4.4 °c, rh 93 %, ν 3.3 g/m3) which resulted total moisture content 21.3 g/m3 per wall volume. • to achieve the lowest moisture content in the wall it is recommended to reduce the air water vapour content by processing it in an air compressor. • air flow rate of the summer drying method which no longer increases the rate of drying significantly is 27 l/(min · m3). it would take 9 days for an 8 m by 6 m and 0.23 m thick wall at air flow rate 27 l/(min · m3) to dry to humidity content of compressed outside air (te +14.5 °c, rh 7.5 %, ν 0.93 g/m3) which resulted total moisture content 9.1 g/m3 per wall volume. • to increase the rate of drying in summer it would be necessary to develop a device between the air compressor and the wall which acts as an hydrophore meant for air and maintains the pressure and flow of dried air into the wall. acknowledgements this work has received funding from the estonian centre of excellence in zero energy and resource efficient smart buildings and districts, zebe (grant no. 20142020.4.01.15-0016) funded by the european regional development fund, by the estonian research council (grant no. prg483), moisture safety of interior insulation, constructional moisture and thermally efficient building envelope, and by the european commission through the h2020 project finest twins (grant no. 856602). ruukki construction oy is acknowledged for the delivery of the studied panels and the valuable feedback for setting up the measurements and discussing the results. references [1] t. kalamees, a. lupíšek, o. c. mørck, et al. what kind of heat loss requirements nzeb and deep renovation sets for building envelope? in central europe towards sustainable building 2016: innovations for sustainable future, cesb 2016, pp. 137–144. 2016. 352 vol. 38/2022 moisture drying capacity of mineral wool insulated . . . 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[7] t. kalamees, j. kurnitski. moisture convection performance of external walls and roofs. journal of building physics 33(3):225–247, 2010. https://doi.org/10.1177/1744259109343502 [8] j. kurnitski. crawl space air change, heat and moisture behaviour. energy and buildings 32(1):19–39, 2000. https://doi.org/10.1016/s0378-7788(99)00021-3 [9] k. kalbe, h. piikov, t. kalamees. moisture dry-out capability of steel-faced mineral wool insulated sandwich panels. sustainability 12(21):9020, 2020. https://doi.org/10.3390/su12219020 [10] j. j. z. a. nicolai, j. grunewald. recent improvements in ham simulation tools. in recent improvements in ham simulation tools: delphin 5 / champs-bes, p. 10. 2007. 353 https://doi.org/10.1016/j.jobe.2016.02.002 https://doi.org/10.3390/app9061222 https://doi.org/10.1016/j.egypro.2017.10.026 https://doi.org/10.1177/1744259120925850 https://doi.org/10.1177/1744259109343502 https://doi.org/10.1016/s0378-7788(99)00021-3 https://doi.org/10.3390/su12219020 acta polytechnica ctu proceedings 38:347–353, 2022 1 introduction 2 methods 2.1 laboratory measurements 2.2 modelling 3 results and discussions 3.1 stable temperature conditions 3.2 variable temperature conditions 3.3 drying strategies for summer and winter 3.4 applying drying on a real scale wall 4 conclusions acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.38.0151 acta polytechnica ctu proceedings 38:151–161, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague a theoretical assessment of transport emissions from institutional buildings in a norwegian municipality oskar fahlstedt∗, rolf andré bohne norwegian university of science and technology, høgskoleringen 7a, 7034, trondheim ∗ corresponding author: oskar.fahlstedt@ntnu.no abstract. municipalities are key actors in reaching the sustainable development goals 9, 11, 12, 13 set by the united nations. the aim of this paper is to develop, on a theoretical level, a sustainable building refurbishment framework for municipalities. the focal point of the framework is reduction of embodied and operational carbon emissions deriving from buildings, including travel-induced emissions. the institutional buildings governed by a norwegian municipality are the case study objects in this study. the framework that is currently is used in the municipality value the building stock based on the quality and cost of refurbishing them. no framework currently includes travel-induced emissions in the evaluation. the intention is to provide a holistic framework that includes this aspect when deciding if a building should be refurbished or demolished. the omission of transport emissions can lead to truncation errors in assessments. the transport to and from the school generates higher emissions in certain scenarios. based on the knowledge in the assessment, a theoretical framework that assesses this is developed. keywords: co2 abatement, travel-induced emissions, location, institutional buildings, municipality. 1. introduction 1.1. energy refurbishments the built environment is responsible for as much as 70 % of human-induced greenhouse gas emissions (ghg) [1, 2], and the united nations (un) predicts that 70 % of the world population will live in cities by 2050 [1, 3]. additionally, predictions are that 85– 95 % of european buildings in operation today will still be in operation by 2050, and most are of inadequate energy standards [4]. thus, they are responsible for a large share of total carbon emissions and sensible actions are refurbishing existing building stocks to lower energy use and indirect carbon emissions. there is no lack of research providing deep-refurbishment methods and frameworks [4, 5]. however, yearly refurbishment rates are at 1 % across europe, and for deep refurbishments (refurbishment intended to lower energy use by a minimum 60 %), rates are between 0–0,2 % [4]. furthermore, an overwhelming body of literature focuses on optimizing on a building scale. thus only emphasizing mitigation of embodied and operational emissions whilst there are reasons to believe that other emission sources are significant when the scope expands from building to urban scale [6]. 1.2. transport emissions and buildings anderson et al. [5] calculate the emissions for three different building types in munich, germany. the authors adopt a process-based (streamlined) lca framework and “expand” it to capture a new impact category they describe as “induced impacts”, which refer to impacts deriving from embodied and use phase emissions from transportation. the buildings are typical for each location in the city which are multi-family houses (city center), row houses (city periphery), and single-family houses (districts / rural areas). the study analyzes the embodied and operational emissions deriving from buildings and user transportation. the scope is not limited to operational transport emissions, as the embodied emissions for transport and infrastructure can represent a large share of total transport emissions. the calculated induced impacts represent 46 to 54 % of total emissions. furthermore, after a sensitivity analysis, it is concluded that moving any of the three building types further away from the city center will increase total emissions [5]. yu et al. [7] argue that building and city scale has been highly researched, while studies on urban precincts (e.g., districts) are not sufficiently covered in the literature. the authors state that assessment on this scale should include [7, p. 2] “1) embodied emissions that are associated with the construction, maintenance and end-of-life treatment of all precinct objects; 2) operational emissions that are generated from the operation of all precinct objects, 3) occupantrelated transport emissions that are generated from daily commuting, business trips and personal travels”. a precinct in adelaide, australia, is the case study, and occupant-related transport and personal travels have the largest share of emissions (40 %), followed by building operational (32 %) and embodied emissions (28 %). anderson et al. [5] and yu et al. [7] argue that it is important to include mobility emissions. fenner et al. [8] share the same opinion and perform a life-cycle 151 https://doi.org/10.14311/app.2022.38.0151 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en oskar fahlstedt, rolf andré bohne acta polytechnica ctu proceedings carbon emission assessment (lcco2a) to calculate embodied, operational, and commuting emissions for a campus building in the usa. however, without considering the embodied impacts of transport as they are perceived as negligible compared to operational emissions. the authors conclude that the emissions deriving from operating the building are the highest. although, not without reservations for a different outcome if a building with the latest energy standards is assessed instead [8]. in a study performed in norway, lausselet et al. [9] evaluated the application of zero-emission neighborhoods (zen). they identify the operational phase and mobility of inhabitants as the most important factors for transforming existing neighborhoods into zen but also highlight other important factors (see [9]). bastos et al. [10] investigate the importance of including mobility when assessing buildings in lisbon and discovered that emissions from transportation can exceed the emissions deriving from the building itself. the study investigates one apartment building in the city center and one semi-detached house in a suburban area. the suburban house has the highest emissions, and relocating it to the city center would reduce emissions significantly. thus, in the investigated scenario, it makes more sense to focus on transportation rather than building emissions [10]. in another study by drouilles et al. [11] is the energy efficiency of five buildings in switzerland’s urban, peripheral, and rural areas is analyzed against national energy targets. the results reveal that energy use in the buildings is twice as high as the national goals. furthermore, including daily mobility of occupants to and from buildings increase emissions by 40 % in the investigated areas. 1.3. the goal of the study no silver bullet exists for reducing emissions in the built environment. instead, it is necessary to implement a wide array of solutions holistically with urban and building research working in synergy to lower carbon emissions from the built environment. therefore, the work of governing bodies such as municipalities is crucial. they withhold a significant mitigation potential that is necessary to utilize for reaching the climate goals. the european union (eu) is striving to be carbon neutral in 2050, and norway is aligning with the goal as well [12–14]. the energy mix is a determinant factor for reaching this goal, and every country adheres to its own set of prerequisites for producing renewable energy [15]. thus, proven methods are not necessarily generic, and instruments to reach carbon neutrality are most likely country or even region specific. the literature has demonstrated the importance of transport emissions in holistic assessments of buildings [5, 7–11, 16]. the objective of this paper is twofold. first, investigate the importance of induced travel emission in a refurbishment strategy with a norwegian municipality as a case study. second, develop a theoretical framework as a suggestion for how to travel-induced emissions could be included together with operational and embodied emissions when finding refurbishment strategies for existing building stock. to guide the research, two questions are asked: (1.) how important is it to include transport emissions in refurbishment and new building assessments in countries that rely on low-carbon electricity? (2.) how can transport emissions be included in a refurbishment framework? 2. materials and methods 2.1. case study – bærum municipality a case study approach was used in a norwegian municipality when documents, building audits, strategies, and cost calculations were scrutinized. important information was also obtained through conversations with municipal officials and consultants managing the building stock. 2.1.1. background bærum municipality is located in the periphery of the norwegian capital oslo. hence, it is an attractive area to live in because of the work opportunities offered and the short commuting distance to the capital. thus, it has seen a steady population growth [17]. the population in the municipality is close to 130 000 [18]. the urban morphology is suburban and rural with detached houses although, some areas are densely populated with multifamily homes (mfh) and commercial buildings [17]. in regards to housing, the composition is 29 % single-family homes (sfm), 13 % semi -detached homes, 17 % row-houses, 3 % other, and 37 % mfh. there are over 100 000 light-duty vehicles registered in the municipality, with primary fuel sources of the cars being 28 % gasoline, 25 % diesel, 23 % electric, and 24 % other (mainly hybrid cars) [18]. as of 2016, 63 % of total emissions derive from light-duty vehicles (ldv), and with the inclusion of heavy-duty vehicles (hdv), it increases to 87 %. this is significant compared to the 3 % from heating the buildings, 4 % from construction machinery, and 6 % other (agriculture, waste management, energy supply, and anaerobic digestion from landfills) that constitute the remaining 13 % of total emissions [19]. the municipal real estate portfolio comprises a vast range of buildings with a combined area of 800 000 m2 to an insured value of 17.6 billion nok excluding the value of infrastructure and land [20]. nevertheless, the population is predicted to grow 11 % by 2030 and increase demand for public buildings. this is a concern for the municipality due to the scarcity of publicly owned land to build on [17, 19–21]. in 2014, the municipality appointed an external consultancy firm to evaluate the condition of the existing building stock, and they concluded that, on average, 24 % of the buildings achieve satisfactory 152 vol. 38/2022 a theoretical assessment of transport emissions . . . figure 1. abc-framework adapted from svein bjørberg in [20]. standards [20, 22]. the buildings were evaluated according to the norwegian standard for “condition survey of construction works – content and execution” (ns3424:2012), and results are reported on a consequence scale from 0–3. a score of 1.0 or less states that there are only minor consequences to be expected considering the standard of the building, but if the receives a score between 2.0–3.0, then there is a risk of medium to severe consequences. they are, for example, related to safety (structural safety, fire safety), health/environment (indoor air quality), aesthetics (surfaces), and economy (renovation, maintenance) [23]. according to the results from the study, average building condition is lower than comparable municipalities, and there is also a discrepancy within the range of buildings not reaching acceptable standards [20, 22]. the municipality is currently categorizing buildings into three levels (a, b, or c) depending on the total score from a quality assessment tool [20]. they are analyzed based on technical quality, adaptability, functionality, plot, and surrounding area. thus, it provides the municipality with an overview of the building stock that makes known where actions are needed to retain and increase the value of their buildings. furthermore, buildings categorized as a-level buildings in the framework are perceived to be worth the investment to reach the adequate standard demanded to align with the sustainable building strategy. the investment to upgrade from c to a is too high. thus, c-level buildings are kept until they pass their functional lifetime. after that, they are liquidated or demolished, and the land can fulfill other purposes [20, 22]. buildings categorized to b-level are under evaluation if they should be upgraded into a-level or downgraded to c-level instead. thus, they are important since categorizing a building into the wrong category can result in a non-sustainable solution [20]. school gfa [m2] hfa [m2] oe [kwh] 1 4639 3935 4,98e+05 2 7662 6170 9,66e+05 3 7215 6390 1,13e+06 4 5933 5084 7,60e+05 table 1. information about the four b-level schools. 2.1.2. institutional b-level buildings four schools previously categorized by the municipality as b-level buildings were assessed. the schools are of varying sizes, with indoor climates deemed inadequate, and none of the buildings have been refurbished in the last 25 years. none of the schools falls below 1.6 in the quality assessment [20, 23]. schools 1, 2, 3, and 4 house 298, 529, 117, and 525 children, respectively. the approximate cost for refurbishing them is 81 million nok and school 1 is the most expensive to refurbish. the children in the municipality are assigned to the schools based on the school district their homes are located in. school 3 is for children with special needs. thus, it does not belong to a specific school district since children are assigned based on their individual needs. information regarding the gross floor area (gfa), heated floor area (hfa), and operational energy (oe) use for 2019 are provided in table 1. 2.2. adapting the abc-framework figure 1 presents the adapted framework, with each line representing a requirement for the buildings. the red line in figure 1 illustrates building degradation over time to different levels followed by refurbishment actions. moreover, ideally, the school building should reach a higher quality level afterward. all school buildings that fall below c-level should be liquidated or demolished at some point as no sustainable investment is viable, and a downtrend arrow 153 oskar fahlstedt, rolf andré bohne acta polytechnica ctu proceedings figure 2. example of assumed distances between homes and schools at 1.5 km radius. illustrates the deterioration. the minimum requirement of the building is to achieve the norwegian technical standard (tek17) [24], and an a-level building should ideally surpass that minimum requirement. the vector for b-level buildings slightly increases in quality over time due to technical requirements becoming more stringent. the dotted line illustrates the minimum requirement from the norwegian technical standard for school buildings. the green dotted line represents the state-of-the-art school building with the best energy efficiency and lowest emissions. thus, as time progresses, the demand for higher functional quality increases since knowledge and technology advance. ideally, the municipality refurbishes a b-level school to a-level that performs better than the requirements in the technical standard. 2.2.1. building emissions the energy use from 2019 for each school was available in the case study. however, no information regarding the school’s embodied emissions was available. thus, allocating the share of emissions from the existing school buildings is difficult. therefore, the impacts from the previous production and construction stages were disregarded in this study. an archetype was used instead. it was based on average data from schools 1, 2, and 4, resulting in a gross floor area of 6078 m2 and a heated floor area of 5063 m2. it is assumed to be a b-level where 524 children attend the school. two improvement scenarios were developed for the school and are presented below: (1.) scenario 1: building a new school building that performs 20 percent better (88 kwh/heated m2a) than the minimum energy requirement for schools in [24]. embodied emissions (a1–b5) are assumed to be 6.9 kg co2eq/m2a. it is assumed that the school demand refurbishment after 30 years and embodied emissions are 65 percent lower (2.42 kg co2eq/m2a) at that point [25, 26]. even more stringent energy requirements are assumed to be in place after 30-years making the school 40 percent more efficient (66 kwh/heated m2a) than the requirement in the exiting energy standard for schools [24]. the end-of-life phase was not included in the assessment. (2.) scenario 2: a refurbishment scenario with the same energy requirements (88 kwh/heated m2a) as in the new building scenario. the embodied emissions are assumed to be lower (2.42 kg co2eq/m2a) since less material will be needed when refurbishing than constructing a new building. the refurbished building demands an additional refurbishment after 30 years with additional embodied emissions (2.42 kg co2eq/m2a). the school must adhere to the more stringent energy requirements (66 kwh/heated m2a) as the scenario with the new building. the electricity mixes used in the study are from the norwegian energy market. the first electricity mix is based on delivered electricity from the norwegian grid without any guarantees of origin (gos), and it has the highest carbon content in this study [27]. the second is a prediction for the average carbon content in norwegian electricity between 2015–2075 when importing from the european continent. third, the same scenario as the previous one, although electricity is only produced in norway [28]. the fourth is a low carbon scenario based on the reported average carbon content in the norwegian electricity mix during 2020 [29]. (1.) 402 g co2eq/kwh [27] (2.) 136 g co2eq/kwh [28] (3.) 18 g co2eq/kwh [28] (4.) 8 g co2eq/kwh [29] 154 vol. 38/2022 a theoretical assessment of transport emissions . . . 2.2.2. transport emissions a highly theoretical approach was used for the transport scenarios when it was assumed that all households were within a 2.5 km radius of the school. no specific routes were analyzed, and for simplicity, trip lengths were divided into 0.5, 1.0, 1.5, 2.0, and 2.5 km. as demonstrated in figure 2, schools 3 and 4 from the case study are located in such a manner that when the uptake radius is 1 km and increasing, they intersect one another. thus this is not a correct approach, but for simplicity, this approach is used as a theoretical exercise. it was assumed that round trips were made, meaning the children were dropped off and picked up each school day. the transport modes were limited to the car (gasoline, diesel, and electric) and bus (diesel and gas). the 2019 values for carbon emissions [g co2eq/pkm] deriving from transport using fossil fuels in norway were extracted from [30]. no specific adjustments to the transportation modes regarding modal share or improved fuel efficiency were made. the electric vehicles were assumed to use the same electricity mix as the buildings. for each distance, a share of students going by car or bus was assumed. these were between 10–50 % for each transport distance. 2.2.3. calculations the calculations was performed as follows: tben = (em1 × gfa) + (oer1 × elm × hfa) + (em2 × gfa) + (oer1 × elm × hfa) (1) tber = (em2 × gfa) + (oer2 × elm × hfa) + (em2 × gfa) + (oer1 × elm × hfa) (2) tte = s × sd × tu × dd × 2 × etm. (3) abbreviations: em1 embodied emissions new building [kg co2eq/m2a] em2 embodied emissions refurbishment [kg co2eq/m2a] gfa gross floor area [m2] oer operational energy requirements [kwh/m2a] elm electricity mix [kg co2eq/kwh] hfa heated floor area [m2] s number of students [p] sd school days a year tu transport utilization [%] dd driving distance [km] etm emissions transport mode [kg co2eq/pkm] the tbe represents “total building emissions”, and the “n” denotes the new building while “r” is the refurbishment scenario. these are the total embodied and operational emissions deriving from the building. for the new building, embodied and operational emissions are divided into two periods (30 years each) since the embodied emissions are after construction and lower after refurbishment. in the refurbishment scenario, the lower embodied emissions are calculated for both periods, while the operational phase differs. this is because the oer is assumed to be stricter once the building needs to be refurbished again after 30 years. tte stands for “total transport emissions” and represents total emissions deriving from the operational phases of the selected transport mode. as mentioned earlier, distances to the school were assumed to be in the range of 0.5–2.5 km and utilization of the transport mode in the range 10–50 %. furthermore, the operational transport emissions were subtracted from the building emissions. to get an easy overview, tables were made (see table 2, 3 and 4) and a negative value marked in red means that operational emissions from transport are higher than embodied and operational from the buildings. 3. results the overall results showed that building emissions are always higher than the ones from transport for the first two electricity mixes (i.e., 402 and 136 g co2eq/kwh). the results changes once the building is supplied with low-carbon electricity mixes (18 or 8 g co2eq/kwh). in figure 3 are total emissions from the school building provided. the results reveal the significance of embodied emissions when the school is supplied with a low-carbon electricity mix. as seen, the new school generates higher emissions over its lifetime due to the initial embodied emissions. the gap between total emissions increases in the scenarios where low-carbon electricity is used compared to the two higher alternatives. distance users 10 % 20 % 30 % 40 % 50 % gasoline car [kg co2eq] in 8 g co2eq/kwh scenario. 2.5 km 4.27e+05 3.06e+05 1.86e+05 6.52e+04 -5.53e+04 2.0 km 4.51e+05 3.54e+05 2.58e+05 1.62e+05 6.52e+04 1.5 km 4.75e+05 4.03e+05 3.30e+05 2.58e+05 1.86e+05 1.0 km 4.99e+05 4.51e+05 4.03e+05 3.54e+05 3.06e+05 0.5 km 5.23e+05 4.99e+05 4.75e+05 4.51e+05 4.27e+05 table 2. in the table above, the results for the refurbishment scenario for the first 30 years presented. the emissions from transportation to and from the school are only higher (see red marking) than the embodied and operational emissions from the building if half of the children go by car over a distance of 2.5 km or greater. 155 oskar fahlstedt, rolf andré bohne acta polytechnica ctu proceedings figure 3. total emissions from the school building for the two scenarios with each of the four electricity mixes analyzed. distance users 10 % 20 % 30 % 40 % 50 % gasoline car [kg co2eq] in 8 g co2eq/kwh scenario. 2.5 km 4.00e+05 2.80e+05 1.59e+05 3.85e+04 -8.20e+04 2.0 km 4.24e+05 3.28e+05 2.31e+05 1.35e+05 3.85e+04 1.5 km 4.48e+05 3.76e+05 3.04e+05 2.31e+05 1.59e+05 1.0 km 4.72e+05 4.24e+05 3.76e+05 3.28e+05 2.80e+05 0.5 km 4.96e+05 4.72e+05 4.48e+05 4.24e+05 4.00e+05 diesel car [kg co2eq] in 8 g co2eq/kwh scenario. 2.5 km 4.13e+05 3.06e+05 1.99e+05 9.17e+04 -1.56e+04 2.0 km 4.35e+05 3.49e+05 2.63e+05 1.77e+05 9.17e+04 1.5 km 4.56e+05 3.92e+05 3.28e+05 2.63e+05 1.99e+05 1.0 km 4.78e+05 4.35e+05 3.92e+05 3.49e+05 3.06e+05 0.5 km 4.99e+05 4.78e+05 4.56e+05 4.35e+05 4.13e+05 table 3. in the table, the results are based on the refurbishment assumptions after 30 years presented. 3.1. first 30 years-low carbon mix the results for the first 30 years are presented in table 2. the values in the tables present the total emissions [kg co2eq] when tte is subtracted from each tbe scenario. the operational emission from transportation is only higher than the emission from the school if at least 50 % of the children travel by car while having a one-way distance of 2.5 km. furthermore, it only applies to the refurbishment scenario since emissions from the new building are always higher. 3.2. remaining 30 years-low carbon mix the embodied and operational emissions are the same for the remaining 30 years for both the building scenarios (see table 3). the emissions from transportation by gasoline cars are the same as in table 2 although higher since more time has passed. furthermore, since the embodied and operational emissions are assumed to be even lower, the emissions from the diesel car exceed the building emission given that 50 % travel 5 km (2.5 km to and from the school) each day. the results for the whole 60-year period are in table 4, and it shows that emissions deriving from the car only exceed emissions from the school if at least distance users 10 % 20 % 30 % 40 % 50 % gasoline car [kg co2eq] in 8 g co2eq/kwh scenario. 2.5 km 8.27e+05 5.86e+05 3.45e+05 3.45e+05 -1.38e+05 2.0 km 8.75e+05 6.82e+05 4.89e+05 4.89e+05 1.04e+05 1.5 km 9.23e+05 7.79e+05 6.34e+05 6.34e+05 3.45e+05 1.0 km 9.71e+05 8.75e+05 7.79e+05 7.79e+05 5.86e+05 0.5 km 1.02e+06 9.71e+05 9.23e+05 9.23e+05 8.27e+05 diesel car [kg co2eq] in 8 g co2eq/kwh scenario. 2.5 km 8.53e+05 6.39e+05 4.24e+05 2.10e+05 -4.41e+03 2.0 km 8.96e+05 7.25e+05 5.53e+05 3.82e+05 2.10e+05 1.5 km 9.39e+05 8.11e+05 6.82e+05 5.53e+05 4.24e+05 1.0 km 9.82e+05 8.96e+05 8.11e+05 7.25e+05 6.39e+05 0.5 km 1.02e+06 9.82e+05 9.39e+05 8.96e+05 8.53e+05 table 4. the table above presents the results for the whole (60 years) period. 50 % go by car over a distance greater than 5.0 km (2.5 km to and back from the school). 3.3. sensitivity analysis the results revealed that transport emissions only exceed building emissions under specific circumstances. the impacts from the operational phase of the school are already low, making the embodied emission decisive. thus, lower embodied emissions were assumed instead. in the original scenario, embodied emissions from refurbishments are 65 percent lower than the reference value of 6,9 kg co2eq/m2a obtained from literature [26]. the case study revealed that the energy use of school 1 is just above the requirements in tek17 [24]. only minor refurbishments were assumed instead, and embodied emissions were set to be 80 percent lower than the reference value. a low-carbon electricity mix (8 g co2eq/kwh) scenario was tested with gasoline and diesel cars. the results for the whole period (i.e., 60 years) are presented in figure 4. the figure shows the results for the first 30 years (left side), after another 30 years (middle), and the total emission on the right-hand side. the share of car users in the diagram (30–50 percent) with distances between 1.5–2.5 km is when transport emissions can be 156 vol. 38/2022 a theoretical assessment of transport emissions . . . figure 4. in the figure are the result for the first 30 years (left), the remaining 30 years (middle), and total emissions (right-hand side) when embodied emissions are reduced 88 % compared to the reference value for the first period presented. higher. since the role of embodied emissions was important, another analysis was performed. now minor refurbishment (80 percent lower embodied emissions) was assumed for the first period. however, for the second period, it is refurbished to align with the energy requirements (66 kwh/heated m2a) more material is needed. thus, embodied emissions were assumed to be 4.36 kg co2eq/m2a which follows guidelines for the use of “climate friendly” materials [25]. the results (see figure 5) reveal that car use emissions exceed the building emissions under specific circumstances during the first 30 years. once the building is refurbished after 30 years, the transport emissions never exceed the ones from the building. even if the electricity mix with the lowest carbon content is supplied to the building. 4. discussion 4.1. expanding the scope from building to urban scale the situation in the assessed municipality is similar to the rest of europe. it manages a large building stock, and refurbishments are needed [4]. with the scarcity of land and a growing population in the municipality, the argument for refurbishment is even stronger [19]. the literature reported that emissions from user transport should be included in assessments since they make a significant share of the total emissions [5, 7, 9, 10]. however, emissions deriving from user transport to and from the schools were only higher when the embodied and operational emissions were assumed to be very low, and half of the children are driven to school each day over a distance greater than 2.5 km. thus, the transport was less significant than predicted when considering previous studies [5, 7, 9, 10]. fenner et al. [8] reported that operational emissions were higher than transport emissions. however, the authors highlighted that their results could be different with a high-performing building scenario. this study investigated a high-performance building scenario, and results showed that building emissions were higher except under certain conditions. the school had to be supplied with a low-carbon electricity mix while having low embodied emissions. the aspect of embodied transport emissions was excluded in this assessment as well as in the one by fenner et al. [8]. however, anderson et al. [5] included them and showed that they are significant and thus ought to be considered in future assessments. the theoretical approach taken in this study means that it has limitations. for example, the archetype school was based on average values, while transport routes and modes were based on assumptions. future studies should aim towards finding applicable refurbishment packages and new building scenarios for one of the existing b-level schools. the case study discovered that schools were divided into districts so that transport routes could be analyzed in greater detail. thus, indicating the benefit of assessing schools. this is because households are confined within a specific area which means that it is possible to calculate accurate transport distances to their assigned school. this shows the strength of this approach since it enables holistic perspectives with solutions that help to reduce the total emissions on a municipal scale. combining the framework the municipality in the case study currently uses with the approach taken in this study avoids sub-optimization. the abc-framework identifies and sets the goals for the exiting stock whilst expanding the scope to include transport avoids carbon lock-ins. moreover, using the school districts makes it possible to optimize the location of a school within a district and evaluate if it is better to refurbish or build a new building at another location. 157 oskar fahlstedt, rolf andré bohne acta polytechnica ctu proceedings figure 5. in the figure are the result for the first 30 years (left), the remaining 30 years (middle), and total emissions (right-hand side) when embodied emissions are assumed to increase due to more extensive refurbishments needed after 30 years. 4.2. potential barriers for implementation the assessment in this study aimed to develop a theoretical framework, and more work is required. higher accuracy is achieved using bottom-up methods with on-site measurements [8, 31]. nevertheless, bærum municipality manages a large stock [20], and considering the urgency to reduce carbon emissions [12–14], it is not an economically sustainable strategy to address every building individually. therefore, it is necessary to decide a break-even point where the strategy should move towards using the top-down method based on statistical data to create archetypes [5] or potentially achieve more accurate results through clustering techniques [32]. murray et al. [32] claimed high accuracy of using clustering techniques to find archetypes of the swiss building stock. however, the techniques demand knowledge, and if it is not accessible among municipality officials, external support is needed, which can be costly. this potential barrier of how to transfer knowledge from the scientific community to decision-makers (dms) and city planners was not covered in the literature. thus, it should be investigated in future studies. there is some ambiguity about whether the current abc framework demands deep user knowledge. if the dms consider it to go beyond their skill level, a sophisticated black-box model is potentially beneficial as it theoretically can achieve high accuracy without demanding extensive technical knowledge about building physics. the municipality could then decide upon kpis, with data about building characteristics and location generating potential emissions based on the input. another option is to extrapolate data from a small sample or to use statistical data to create archetypes for their stock and then run energy simulations to find suitable refurbishment packages, which are methods already present in literature [11, 31]. however, complex machine learning techniques are often used to manage the computational power, which in turn demands extensive theoretical knowledge of the users [16]. 5. conclusion in section 1 two research questions were asked, and this section intends to answer them. the built environment is heterogeneous and varies depending on region, municipality, city, neighborhood, and building type, which is also the situation in bærum municipality. therefore, implementing holistic life-cycle frameworks covering both building and urban scales is recommended. it will aid dm in finding emission abatement strategies with less risk for carbon lock-in due to sub-optimization at the building scale. the results from this study reveal that the induced emissions from transportation can exceed embodied and operational building emissions. however, this was a theoretical exercise with a fictional school building, and further research with better data is needed. a sensitivity analysis demonstrated the importance of embodied emissions once a building uses low-carbon electricity. moreover, in the assessment, the hypothesis that transport emissions would be higher than building emissions was not the case. however, the work indicates the potential for higher reductions of ghg-emission on an urban scale when the method is applied. finally, the social aspect was not considered, but it should be in the future. for example, the economic situation determines the modal choice, and what the user perceives as valuable will set other requirements that ultimately influence emissions. the literature findings and information obtained from the case study have led to the development of a theoretical framework (see figure 6). it consists of 158 vol. 38/2022 a theoretical assessment of transport emissions . . . figure 6. theoretical framework based for holistic assessment of refurbishment strategies for existing buildings. three main emission drivers, the location of the building, the building itself (embodied and operational energy), and purchasing power (i.e., “social” for now) of inhabitants within each of the catchment areas of the schools. it is a suggestion for future detailed assessments. the social aspect was added impromptu due to the idea that household income will be deterministic for the accessibility of certain modes of transport. however, this aspect needs further development before being implemented hence the dotted line. the aforementioned emission drivers then determine the necessary inventory data for the calculation of environmental impacts. furthermore, emissions deriving from transportation are measured against emissions from refurbishing the building. a sensitivity analysis should be performed in situations when transport emissions are higher than the ones from the investigated building(s). this since greater reduction potential might be achieved if the services of the building (institutional in this case) are established at another location. for the school buildings, it insinuates that the school should be moved to a more suitable location. to move a building is drastic. it foremost refers to moving the service provided to location optimization strategies to reduce emissions that are not limited to the building scale but include transport from building users. references [1] un-habitat. the strategic plan 2020-2023, 2019. 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[30] statistisk sentralbyrå. co2-utslipp per person-/passasjerkilometer fordelt på kjøretøy og drivstoff, g co2/pkm. 2010-2019, 2021. [2022-02-22], https://www.ssb.no/transport-og-reiseliv/ artikler-og-publikasjoner/mindre-utslipp-fraveitrafikk-fly-og-tog?tabell=439538. [31] m. hu. life-cycle environmental assessment of energy-retrofit strategies on a campus scale. building research & information 48(6):659–680, 2020. https://doi.org/10.1080/09613218.2019.1691486. [32] p. murray, j. marquant, m. niffeler, et al. optimal transformation strategies for buildings, neighbourhoods and districts to reach co2 emission reduction targets. energy and buildings 207:109569, 2020. https://doi.org/10.1016/j.enbuild.2019.109569. 161 https://www.ssb.no/transport-og-reiseliv/artikler-og-publikasjoner/mindre-utslipp-fra-veitrafikk-fly-og-tog?tabell=439538 https://www.ssb.no/transport-og-reiseliv/artikler-og-publikasjoner/mindre-utslipp-fra-veitrafikk-fly-og-tog?tabell=439538 https://www.ssb.no/transport-og-reiseliv/artikler-og-publikasjoner/mindre-utslipp-fra-veitrafikk-fly-og-tog?tabell=439538 https://doi.org/10.1080/09613218.2019.1691486 https://doi.org/10.1016/j.enbuild.2019.109569 acta polytechnica ctu proceedings 38:151–161, 2022 1 introduction 1.1 energy refurbishments 1.2 transport emissions and buildings 1.3 the goal of the study 2 materials and methods 2.1 case study – bærum municipality 2.1.1 background 2.1.2 institutional b-level buildings 2.2 adapting the abc-framework 2.2.1 building emissions 2.2.2 transport emissions 2.2.3 calculations 3 results 3.1 first 30 years-low carbon mix 3.2 remaining 30 years-low carbon mix 3.3 sensitivity analysis 4 discussion 4.1 expanding the scope from building to urban scale 4.2 potential barriers for implementation 5 conclusion references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2025.54.0001 acta polytechnica ctu proceedings 54:1–7, 2025 © 2025 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague numerical verification of fracture toughness values for very thin 3d printed polyamide samples petr bočan∗, aleš jíra czech technical university in prague, faculty of civil engineering, department of mechanics, thákurova 7, 166 29 prague 6, czech republic ∗ corresponding author: petr.bocan@fsv.cvut.cz abstract. this researche presents an experimental and numerical analysis of the mechanical properties of very thin polyamide (pa12) samples fabricated by 3d printing with selective laser sintering (sls). the research methodology focuses on testing mechanical properties such as fracture toughness and simple tensile on pa12 samples ranging in size from 0.50 mm to 2.00 mm, which were then subjected to numerical analysis replicating the experiment. significant differences between the experimental data and the numerical analysis were found, mainly due to the choice of material model and the selection of inappropriate material parameters. with the numerical analysis, these parameters were appropriately set up and the results of the numerical analysis began to agree more closely with the experimental data. keywords: 3d printing, polyamide pa12, fracture toughness experiment, simple tensile experiment, numerical analysis. 1. introduction currently, 3d printing technology is one of the most advanced methods in additive manufacturing, with widespread use across various industries. this technology makes it possible to create more complex structures, which leads to fostering numerous innovations, making it a highly researched area. in my previous project [1], i investigated the fracture toughness of very thin polyamide (pa12) samples produced using sls (selective laser sintering) 3d printing. the experiment was designed following a relevant standard, which determined the geometry of the samples, the experimental procedure, and the final calculation of fracture toughness. the experiment were carried out on samples with widths ranging from 0.50 mm to 2.00 mm. to account for the ortotrophy of 3d printed samples, the print orientation was varied, with layers printed either perpendicular or parallel to the anticipated crack propagation. a more detailed description of the experiment and its findings can be found in section 3.1. another study [2] also demonstrated the influence of thickness in a simple tensile test, which is more described in section 3.2. this project was further supplemented by a numerical analysis conducted using preprocessor gid and then calculated in atena software [3]. the geometry of the numerical model copied that of the experimental samples, and a basic material model was established. the load was defined by deformation in form of displacement. the pa12 datasheet [4] provided only fundamental mechanical properties, such as young’s modulus, tensile strength, and elongation at break. therefore, additional mechanical properties, such as compressive strength and fracture energy from other experiments, were incorporated into the numerical analysis. unfortunately, as shown in figure 1, signif0.0 0.5 1.0 1.5 distance [mm] 0 10 20 30 40 fo rc e [n ] sample 2 0.50 mm 0.0 0.5 1.0 1.5 2.0 2.5 distance [mm] 0 100 200 300 400 fo rc e [n ] sample 2 2.00 mm figure 1. comparison of numerical analysis and experimental data, where the blue curve is the experimental data, orange curve is the linear numerical model and green curve is the quadratic numerical model [1]. icant differences emerged between the experimental and numerical analysis results. these significant deviations led to a more detailed 1 https://doi.org/10.14311/app.2025.54.0001 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en petr bočan, aleš jíra acta polytechnica ctu proceedings investigation into the numerical analysis to identify possible sources of error. the diagrams reveal no match between the linear part of the experimental data and the numerical model, suggesting that the main problem may stem from the chosen material model. in the case of thin samples, the mechanical properties given in the data sheet are insufficient and the values may vary. as a result, a simple tensile experiment was performed on thin pa12 samples to describe their behavior in the linear region. this experiment enabled the determination of young’s modulus and tensile strength for thin samples, which were then applied to the numerical model under simple tensile loading. 2. material and method of manufacture pa12 (nylon 12) material was used in my experiment produced by sls (selective laser sintering) 3d printing technology. nylons are part of the thermoplastic polymer family and also they are referred to as polyamides (pa) because of their repeating units connected by amide bonds. there are numerous subtypes with varying crystal structures and material properties, including nylon 6, nylon 12, and nylon 66, among others [5]. one of the popular additive manufacturing techniques is powder bed fusion (pbf), also known as sls. this method is widely used for printing metals and polyamide components by fusing powder particles layer by layer using a high-energy heat source. today, 3d printing technologies are extensively used for additive manufacturing in various industries. however, sls technology presents certain challenges, especially when printing thin samples. one of the main weaknesses is the inadequate bonding of powder particles, and another issue is the inconsistent sample width. poorly bonded powder particles tend to fall off during handling, which reduces the sample width and weakens its load capacity, ultimately affecting fundamental mechanical properties like the modulus of elasticity. these irregularities can significantly reduce specimen width, influencing the experimental results and subsequently influencing the appropriate choice of material properties in numerical calculation. 3. experimental part two types of experiments were designed to verify the results of the numerical analysis. first, i started with an experiment dealing with fracture toughness [1], and then i supplemented the research with a simple tensile experiment. both experiments were conducted on specimens with widths ranging from 0.50 mm to 2.00 mm. the geometries of the specimens were designed according to the relevant standard. a mark 10 load press was used for loading in both experiments. the fracture toughness experiment was supplemented figure 2. the geometries of specimens for fracture toughness experiment according to en iso 12737 [6]. dimensions are given in millimetres. with 3d macro dic to monitor crack propagation and crack length readings. 3.1. fracture toughness experiment the fracture toughness experiment was carried out according to en iso 1273 [6]. with minor modifications, the geometry of the specimens, the experiment procedure, and the final calculation of the fracture toughness value were taken from the standard. in the last research [1], i investigated the fracture toughness of two specimen geometries as shown in the figure 2. the calculation of the fracture toughness value kq according to the standard en iso 1273 [6] is given in the equation 1. kq = fq b √ w ∗ f(a/w ), (1) where fq is the force determine from the experiment data, b is the sample thickness, w is the length of a ligament. an essential parameter in the calculation is the geometric factor f(a/w ) shown in equation 2, which is a dimensionless function of a/w , where a is the length of the crack. 2 vol. 54/2025 numerical verification of fracture toughness values . . . gseducationalversion pr in t d ir ec tio n pr in t d ir ec tio n pr in t d ir ec tio n pr in t d ir ec tio n h v vertical printed sample horizontal printed sample vertical printed sample horizontal printed sample p ri nt o ri en ta ti on p ri nt o ri en ta ti on horizontal printed sample vertical printed sample figure 3. demonstration of the use of orthotropy using different printing direction [1]. f(a/w ) = (2 + a/w ) ∗ [0, 886 + 4, 64(a/w ) − 13, 32(a/w )2 + 14, 72(a/w )3 − 5, 6(a/w )4]/(1 − a/w ) 2 3 . (2) the orthotropy of the material was accounted for by different printing directions (hereafter referred to as h and v). horizontally (h) printed specimens had individual layers printed perpendicular to the expected crack propagation, while vertically (v) printed specimens had layers parallel to the expected crack. orthotropy is a material property, where the material has different properties in perpendicular directions. this property is typical for composite materials. figure 3 shows a schematic of the proposed printing direction. to refine the adequacy of these results, the fracture toughness values of the pa12 material were determined in other researches. j. schneider and s. kumar [7] investigated the influence of ligament length (10, 15, and 20 mm) on fracture behavior. their study used test samples of 1 mm thick subjected to three-point bending tests. the results demonstrated a clear dependence of fracture toughness kic on ligament length, with measured values ranging from 3.7–4.5 mpa √ m. a. salazar [8] further examined the effect of three distinct temperature conditions on fracture behavior, reporting fracture toughness values in the range of 2.7–3.2 mpa √ m. d. i. stoia et al. [9] focused on the influence of different printing orientations and processing energy levels. their experiments resulted in fracture toughness values ranging from 0.8–2.2 mpa √ m. in summarizing the results of other experiments, the fracture toughness value for pa12 material was around the values 0.8–4.5 mpa √ m [7–9]. figure 4 showing the average fracture toughness kq value for each sample thickness. for the horizontally printed samples with widths of 0.50 mm and 0.75 mm, significant variations in the values were observed, leading to unstable values in results. however, from a certain sample thickness onward, the values became more consistent and began to increase with specimen thickness. the values of the horizontally printed samples ranged from 1.4–2.6 mpa √ m. in contrast, for the vertically printed specimens, the fracture toughness values around 1 mpa √ m did not increase 0.50 0.75 1.00 1.25 1.50 2.00 sample width b [mm] 1.0 1.5 2.0 2.5 3.0 fr ac tu re to ug hn es s k q [m pa *m 1/ 2 ] horizontal printed samples sample 2 sample 3 0.50 0.75 1.00 1.25 1.50 2.00 sample width b [mm] 0.5 1.0 1.5 fr ac tu re to ug hn es s k q [m pa *m 1/ 2 ] vertical printed samples sample 2 sample 3 figure 4. final fracture toughness values for different print thicknesses and orientations [1]. with thickness, remaining relatively constant throughout the tests. 3.2. tensile experiment a tensile experiment was conducted by en iso 527-1,2 standards [10, 11]. the primary objective of this experiment was to determine the ultimate tensile strength of very thin specimens. additionally, the experiment aimed to closely examine the linear behavior of thin samples under simple tensile loading. the geometry of the specimens, as specified by the standard, is depicted in figure 5. the orthotropy of the material was considered by varying the print orientation, as was done in the fracture toughness experiment. vertically printed specimens had layers oriented parallel to the tensile load, while horizontally printed specimens had layers perpendicular to the load. the thickness of each printed layer was set to 0.100 mm. figure 6 provides a schematic representation of the sample printing process for both print orientations. the small number of samples in the case of 0.50 mm thick samples is due to the heavy handling during the experiment and sample cleaning, and therefore the results for this thickness cannot be considered relevant. the same applies to the 1.25 mm thick samples where only three samples were printed. however, for other thicknesses, an adequate number of samples 3 petr bočan, aleš jíra acta polytechnica ctu proceedings figure 5. geometry of the specimen used for the simple tensile experiment with variable widths from 0.50 mm to 2.00 mm. gseducationalversion pr in t d ir ec tio n pr in t d ir ec tio n pr in t d ir ec tio n pr in t d ir ec tio n h v vertical printed sample horizontal printed sample vertical printed sample horizontal printed sample p ri nt o ri en ta ti on p ri nt o ri en ta ti on horizontal printed sample vertical printed sample figure 6. samples print direction for horizontal and vertical samples. were tested, allowing their results to be considered valid. in the technical data of pa12 [4], the tensile strength is given as 41 mpa, where it is not specified at which thicknesses or at which print orientation it was found. therefore, it was necessary to obtain the tensile strength values from other experiments, where the print orientation and thinner samples were my area of interest to search. the studies showed that the orientation of the print and the thickness of the sample are the main factors in the tensile strength values. the slager study [12] examined different print orientations and thicknesses of the sample, where horizontally printed samples of 0.8 mm ranging around the tensile strength value of 20.5 mpa, for vertically printed samples it was around 27.8 mpa. sindinger [13] showed a large increase in the value with increasing thickness, where the values for horizontally printed samples were around 25–42 mpa and the values for vertically printed samples were around 26–45 mpa. other studies have shown that tensile strength is not as much affected by the orientation of the print with increasing thickness [12–15]. the experiment revealed notable differences between vertically and horizontally printed specimens, particularly in the maximum load at which the specsamples widths tensile strength [mm] [mpa] 0.50 25.30 ± 0.06 0.75 22.61 ± 1.55 1.00 28.26 ± 1.87 1.25 32.51 ± 1.19 1.50 32.48 ± 0.97 2.00 34.24 ± 2.88 table 1. tensile strength for the vertically print samples [2]. samples widths tensile strength [mm] [mpa] 0.50 – 0.75 11.76 ± 1.04 1.00 21.29 ± 0.95 1.25 – 1.50 21.16 ± 2.62 2.00 20.07 ± 1.45 table 2. tensile strength for the horizontally print samples [2]. imens fractured, as shown in figure 7. from these measurement data was calculated the ultimate tensile strength of each specimen width. the ultimate tensile strength for each specimens were calculated as: ft = fmax a0 , (3) where ft is the tensile strength, fmax is the maximal force from experiment data and a0 is the cross-section area measured for each samples. tables 1 and 2 present the tensile strength results for each specimen width in both print directions. values are not provided for horizontally printed specimens with widths of 0.50 mm and 1.25 mm due to issues with handling during the experiment and the insufficient number of specimens tested. as expected, the tensile strength values for vertically printed samples were higher compared to those for horizontally printed samples. this is because vertically printed specimens have layers oriented parallel to the applied load. conversely, horizontally printed specimens have layers perpendicular to the load, making them more susceptible to breaking with less stress. anyway, the pa12 technical datasheets [4] specify a tensile strength of 41 mpa, which significantly exceeds the maximum value observed in this experiment. this discrepancy highlights that the manufacturer’s certified values may not be applicable for very thin specimens, emphasizing the need for careful consideration of material properties in such cases. 4 vol. 54/2025 numerical verification of fracture toughness values . . . 0 1 2 3 distance [mm] 0 20 40 60 lo ad [n ] thickness 0.50 mm 0 1 2 3 4 5 distance [mm] 0 30 60 90 120 150 lo ad [n ] thickness 1.00 mm 0 1 2 3 4 distance [mm] 0 50 100 150 200 250 lo ad [n ] thickness 1.50 mm 0 1 2 3 4 5 distance [mm] 0 50 100 150 200 250 300 350 lo ad [n ] thickness 2.00 mm figure 7. diagrams of load – distance values for specimens with widths 0.50, 1.00, 1.50, 2.00 mm. blue curves indicate samples printed in the horizontal direction, red curves indicate samples printed in the vertical direction [2]. 4. numerical analysis the numerical model was developed using gid 16.0.6 software, which serves as a preprocessor for numerical analysis. finite element analysis (fea) was performed with atena software [3], typically used for the nonlinear analysis of concrete structures but adaptable for other materials when the appropriate material model is selected. this software excels in numerical simulation experiments, providing detailed insights into crack progression and maximum load capacity before failure. as mentioned in the introduction, the manufacturer provides limited mechanical parameter information for pa12. the values given in the datasheets include young’s modulus e = 1.47 gpa, ultimate tensile strength ft,u = 41 mpa and elongation at break ϵu = 0.13 [4]. consequently, the fracture energy gf and poisson’s ration µ had to be determine through additional experiments. the material properties used in the fea for the previous project [1] are given in table 3. material properties pa12 material young’s modulus e [gpa] 1.47 poisson’s ratio µ [-] 0.4 density ρ [kg m−3] 1010 thermal expansion α [k−1] 10−12 tension strength ft [mpa] 41 fracture energy gf [kn m−1] 7 table 3. material properties used in the calculation for previous research [1]. these material properties showed large differences in the fea results compared to experimental data. it may happen because of the wrong selection of the material model and young’s modulus. thus, a large parametric study of the simple tensile calculation on the value of effective young’s modulus was performed. the calculation showed the value of effective young’s modulus around 250 mpa, which is 6 times lower than stated by the manufacturer. also, the material model was set with elasto-plastic behavior using cementitious2 − user. 5. conclusions previous research [1] investigated the fracture toughness of very thin pa12 samples, involving both experimental and numerical parts. the experimental phase included extensive testing on sample thicknesses ranging from 0.50 mm to 2.00 mm and incorporated orthotropy by varying the printing directions. numerical analysis was then conducted using atena software [3] to simulate the experiments, but significant discrepancies were observed between the numerical predictions and experimental data. these differences were likely due to an inadequate material model and the selection of inappropriate 5 petr bočan, aleš jíra acta polytechnica ctu proceedings material parameters. the material model used had a quasi-brittle behavior, which was not suitable for pa12, as this material exhibits elasto-plastic behavior. it is well established that for thin samples produced via sls powder technology, manufacturer specified material properties are not directly applicable, and effective material properties for these thin samples must be determined experimentally. the manufacturer’s datasheet provide limited information on the mechanical properties of pa12, such as young’s modulus and tensile strength, which can be determined by a simple tensile test. therefore, this research focused on performing a simple tensile experiment to analyze the linear behavior of the samples under load, which allowed the determination of effective young’s modulus and ultimate tensile strength for these thin samples. again, orthotropy with different printing directions was considered. as with the fracture toughness tests, it was found that the printing direction affects the material’s response to load. specifically, samples with layers printed parallel to the load direction showed better performance. the experiments demonstrated increasing values with specimen thickness and highlighted substantial differences between the manufacturer’s provided material properties and those obtained through experimentation, as summarized in table 4. this underscores the fact that standardized parameters cannot be taken into account for very thin samples. material data-sheets experiment properties young’s modulus 1.47 0.20–0.26 e [gpa] tension strength 41 22–35 ft [mpa] table 4. the comparison of material properties provided by the manufacturer’s datasheet with the experimental results for vertically printed samples. the results were verified through finite element analysis (fea) using atena software [3]. initially, a study was conducted to determine the effective young’s modulus, which was found to be eeff = 250 mpa. this value aligned with young’s modulus obtained from experimental data. subsequently, the material model was modified to an elasto-plastic model using the cementitious2 − user option. these adjustments to the material properties and model proved effective, as the numerical analysis results began to closely match the experimental values. figure 8 shows the result for thicknesses 0.50, 1.00, 1.50 and 2.00 mm, which can be seen there is pretty close agreement between experimental data and fea results. the small deviations between the experimental data and fea are due to the choice of one typical value of effective young’s modulus. 0 1 2 3 4 5 distance [mm] 0 10 20 30 40 50 60 70 lo ad [n ] thickness 0.50 mm experimental data fea calculation 0 1 2 3 4 5 distance [mm] 0 50 100 150 lo ad [n ] thickness 1.00 mm experimental data fea calculation 0 1 2 3 4 5 distance [mm] 0 50 100 150 200 250 lo ad [n ] thickness 1.50 mm experimental data fea calculation 0 1 2 3 4 5 distance [mm] 0 50 100 150 200 250 300 350 lo ad [n ] thickness 2.00 mm experimental data fea calculation figure 8. comparison of experimental data of simple tensile and fea calculation using elasto-plastic material material model and the effective young’s modulus eeff = 250 mpa in the calculation. 6 vol. 54/2025 numerical verification of fracture toughness values . . . acknowledgements the financial support from czech science foundation (project no. 23-04971s) and czech technical university in prague (sgs project no. sgs23/152/ohk1/3t/11) is gratefully acknowledged. references [1] p. bočan. experimental verification of the influence of thickness on 3d printed samples on fracture toughness parameters. master’s thesis, czech technical university in prague, department of mechanics, prague, 2024. [2] p. bočan, a. jíra. determination of mechanical properties of very thin 3d-printed specimens for numerical analysis of tensile strength and fracture toughness. acta polytechnica 65(3):263–275, 2025. https://doi.org/10.14311/ap.2025.65.0263 [3] červenka, j., l. jendele, v. červenka. atena program documentation, 2014. cervenka consulting. [2024-08-20]. https://www.cervenka.cz/ [4] sinterit sp. z o.o. pa12 smooth – material’s technical data sheet, 2022. [5] k. s. randhawa, p. ashwin. a review on tribo-mechanical properties of micro-and nanoparticulate-filled nylon composites. journal of polymer engineering 41(5):339–355, 2021. https://doi.org/10.1515/polyeng-2020-0302 [6] en iso 12737. metallic materials – determination of plane-strain fracture toughness, 2011. [7] j. schneider, s. kumar. multiscale characterization and constitutive parameters identification of polyamide (pa12) processed via selective laser sintering. polymer testing 86:106357, 2020. https: //doi.org/10.1016/j.polymertesting.2020.106357 [8] a. salazar, a. rico, j. rodriguez, et al. monotonic loading and fatigue response of a bio-based polyamide pa11 and a petrol-based polyamide pa12 manufactured by selective laser sintering. european polymer journal 59:36–45, 2014. https://doi.org/10.1016/j.eurpolymj.2014.07.016 [9] e. linul, l. marsavina, d. i. stoia. mode i and ii fracture toughness investigation of laser-sintered polyamide. theoretical and applied fracture mechanics 106:102497, 2020. https://doi.org/10.1016/j.tafmec.2020.102497 [10] en iso 527-1. plastics – determination of tensile properties – part 1: general principles, 2020. [11] en iso 527-2. plastics – determination of tensile properties – part 2: test conditions for moulding and extrusion plastics, 2012. [12] j. j. slager, b. c. earp, a. m. ibrahim. influence of build orientation and part thickness on tensile properties of polyamide 12 parts manufactured by selective laser sintering. polymers 16(16):2241, 2024. https://doi.org/10.3390/polym16162241 [13] s.-l. sindinger, c. kralovec, d. tasch, m. schagerl. thickness dependent anisotropy of mechanical properties and inhomogeneous porosity characteristics in laser-sintered polyamide 12 specimens. additive manufacturing 33:101141, 2020. https://doi.org/10.1016/j.addma.2020.101141 [14] n. lammers, m. kersemans, i. de baere, w. van paepegem. on the visco-elasto-plastic response of additively manufactured polyamide-12 (pa-12) through selective laser sintering. polymer testing 57:149–155, 2017. https: //doi.org/10.1016/j.polymertesting.2016.11.032 [15] a. g. rodríguez, e. e. mora, m. a. velasco, c. a. n. tovar. mechanical properties of polyamide 12 manufactured by means of sls: influence of wall thickness and build direction. materials research express 10(10):105304, 2023. https://doi.org/10.1088/2053-1591/acf6f7 7 https://doi.org/10.14311/ap.2025.65.0263 https://www.cervenka.cz/ https://doi.org/10.1515/polyeng-2020-0302 https://doi.org/10.1016/j.polymertesting.2020.106357 https://doi.org/10.1016/j.polymertesting.2020.106357 https://doi.org/10.1016/j.eurpolymj.2014.07.016 https://doi.org/10.1016/j.tafmec.2020.102497 https://doi.org/10.3390/polym16162241 https://doi.org/10.1016/j.addma.2020.101141 https://doi.org/10.1016/j.polymertesting.2016.11.032 https://doi.org/10.1016/j.polymertesting.2016.11.032 https://doi.org/10.1088/2053-1591/acf6f7 acta polytechnica ctu proceedings 54:1–7, 2025 1 introduction 2 material and method of manufacture 3 experimental part 3.1 fracture toughness experiment 3.2 tensile experiment 4 numerical analysis 5 conclusions acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2024.51.0022 acta polytechnica ctu proceedings 51:22–28, 2024 © 2024 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague green mobility in courier services – aspects of using electric vehicles alica kalašová∗, kristián čulík university of žilina, faculty of operation of transport and communications, department of road and urban transport, univerzitná 1, 010 26 žilina, slovakia ∗ corresponding author: alica.kalasova@fpedas.uniza.sk abstract. countries in the eu, as part of the paris agreement, committed to achieving eu climate neutrality by 2050. to meet this goal, the eu aims to reduce its net greenhouse gas emissions across the entire economy by at least 55% by 2030 compared to 1990 levels, continuing the reduction of emissions until 2050. the transport sector, responsible for a quarter of eu greenhouse gas emissions, must transform, requiring a 90% reduction in greenhouse gas emissions by 2050. it is crucial to enhance the efficiency of both personal and freight transport in the eu and reduce dependence on fossil fuels. our paper presents the cost calculation results of using electric vehicles with a total weight of up to 3.5 tons in the courier services sector, compared to conventional vehicles. based on our analysis, electric vehicle usage is effective and environmentally beneficial. however, bevs (battery electric vehicles) have higher acquisition costs than icevs (internal combustion engine vehicles). this paper analyses the vehicle fleet of a significant courier company. thanks to their data, it was possible to provide a closer look at the economic efficiency of bev in courier services. keywords: electromobility, alternative drive, electric vehicle, environmental impacts, calculation. 1. introduction transportation accounts for one-quarter of greenhouse gas emissions in the eu, and progress in decarbonizing the transport sector has been slower than in other economic industries. while emissions have significantly decreased in other sectors, emissions from transportation have risen in recent years. enhancing the efficiency of both personal and freight transport in the eu and reducing its dependence on fossil fuels are essential. in recent years, a growing demand for products purchased online from the comfort of one’s home has led to direct delivery to the customer’s doorstep. the outbreak of the covid-19 pandemic has further deepened this trend, and at certain times, online shopping has become the only alternative to accessing specific products. delivery through courier services is gaining momentum, resulting in increased air pollution as more vehicles from various courier companies travel the roads. therefore, it is essential to address the possibility of alternative drives for these types of vehicles. some companies operating in courier services have already taken steps towards carbon neutrality. worldwide, there are many examples of using electric vehicles for delivering packages to end consumers, which do not produce harmful air emissions and contribute to environmental protection. typical examples include electric scooters or e-bicycles used for deliveries within the central areas of large cities, significantly speeding up the delivery process. this topic has also been addressed by the eu, and as part of the paris agreement, member states committed to achieving the eu’s climate neutrality by 2050. to meet this goal, the eu aims to reduce its net greenhouse gas emissions across the entire economy by at least 55% by 2030 compared to 1990 levels, and it will continue gradually decreasing emissions until 2050. in march 2023, the council adopted new rules for further reducing co2 emissions from new passenger cars and vans, amending the eu regulation from 2019. these revised rules establish gradual emission reduction targets. from 2030 to 2034, emissions must be reduced by 55% for new cars and 50% for new vans compared to the 2021 targets. all new passenger cars and vans must have zero emissions from 2035. the proposal aims to reduce co2 emissions in the road transport sector in line with eu climate targets. it involves increasing emission reduction targets for 2030 and introducing new targets for 2035 and 2040. the proposed rules expand the scope of the regulation to cover almost all new heavy-duty vehicles with certified co2 emissions, including smaller freight vehicles, city buses, coaches, and trailers, subjecting them to emission reduction targets. according to the updated rules, all new city buses will have zero emissions from 2035. long-distance buses and coaches would continue to be subject to overall targets [1]. in 2021, there was a significant increase in interest in electric vehicles (evs) in the eu (figure 1), both in the m1 (passenger cars) and n1 (light commercial vehicles) categories. the adoption rate rose from 10.7% to 17.8% within one year. additionally, the use of electric vans experienced growth from 2.1% to 3.1% [3]. globally, electric vehicles are gaining 22 https://doi.org/10.14311/app.2024.51.0022 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 51/2024 green mobility in courier services . . . figure 1. new registrations of electric cars, eu-27 [2]. popularity, with countries such as china and the usa witnessing a year-on-year increase in their numbers [4]. thanks to the collaboration with the department of road and urban transport and a significant courier company which operates electric vehicles in slovakia, it was possible to obtain the necessary dataset. this article describes an economic calculation based on these data, providing a closer insight into the economic efficiency of battery electric vehicles (bev) in courier services. this article serves as a response to the following research questions: (1.) is the use of electric vehicles economically viable? (2.) do they represent a green future in delivery services? (3.) is the utilization of these vehicles profitable? (4.) do low operational costs outweigh the high acquisition price of the vehicle? 2. materials and methods 2.1. analysis of the current state in slovakia in slovakia, the trend of electric vehicles has a slower pace (figure 2); however, some companies operating in this sector are starting to adopt alternative propulsion systems for delivering parcels. examples include dpd, sps, and gls slovakia. other companies are also making efforts with their pilot projects in electromobility. when it comes to electric vehicles, several questions arise. bevs have higher acquisition costs than icevs (internal combustion engine vehicles). an essential factor in decision-making are operational costs connected with a type of the vehicle drive. the electricity needed for charging is generally less expensive than the fluctuating fuel prices, which are currently higher than in previous years. however, indeed, electricity tariffs for businesses in slovakia have also increased. another crucial factor is the cost of service and maintenance for electric vehicles and insurance costs. the insurance prices are related to the vehicle’s value, which tends to be higher for bevs. 2.2. characteristics of the selected company the analysed company is among the leading providers of standard and express parcel services, serving thousands of customers daily across slovakia [6]. in slovakia, the analysed company became the first to utilize voltia env200 electric vehicles with zero co2 emissions for parcel delivery, making it the pioneer in deploying these special vehicles for urban delivery. by 2025, the company aims to deploy 129 vehicles in slovakia [7]. the voltia vehicle, based on the nissan env200, is from a slovak manufacturer. despite its compact size, it offers up to twice the cargo space compared to other electric vehicles in its category, with 8m3 of cargo volume, a payload capacity of 580 kg, and a range of up to 200 km on a single battery charge, producing no co2 emissions [8]. table 1 shows an overview of the mileage of electric vehicles (bev) from the analysed company and the number of parcels delivered by these vehicles. table 2 provides the number of vehicles that the company has in the capital city, bratislava, and other locations across slovakia. 2.3. cost calculation 2.3.1. fuels, energy to perform the given calculation, the authors had access to the company’s internal data, which they provided for our analysis. it was possible to compare the usage of three electric and diesel vehicles, specifically 23 alica kalašová, kristián čulík acta polytechnica ctu proceedings figure 2. number of registered bevs and phevs in slovakia in 2021 [5]. the number of kilometres travelled the number of parcels delivered by electric by electric vans in 2022 vehicles in 2022 929 623 1,5 million table 1. basic data on analysed vehicles [9]. type vehicle in outside bratislava bratislava bevs stellantis (citroen, peugeot, opel) 24 18 mercedes-benz e-sprinter – 6 nissan e-nv2000 20 27∑ 44 51 total 95 icevs (diesel) peugeot boxer 570citroen jumper fiat ducato table 2. number of vehicles of analysed company [10]. nissan env200 and fiat ducato, for parcel delivery in bratislava, nové zámky, and košice. the analysis involved analysing the kilometres driven for each month in 2022, used electricity in kw100 km−1, the number of delivered parcels, and the cost per kw. based on the number of kilometres driven, we calculated these indicators for average daily, monthly, yearly, and kilometre-based costs. refer to figures 3, 4, and 5, and the summary table 3. for all analysed vehicles, we concluded that unit costs are lower for bevs; however, this applies only to costs related to the most significant differentiating factor, namely the fuel or energy price. 2.3.2. acquisition costs the acquisition cost of an electric vehicle is more expensive than that of a combustion engine vehicle, primarily due to battery costs, which constitute the majority of the vehicle’s total. see table 4. 2.3.3. repairs and maintenance bevs have less expensive repairs and maintenance compared to icevs. the main reason is that they have fewer moving parts. electric motors are less prone to wear because they do not have components like pistons, clutch, or gearbox. there is no need for oil changes. in a vehicle with conventional fuel, there are many moving and functional parts that are susceptible to malfunctions and possible part replacements. also, thanks to regenerative braking, the lifespan of the vehicle’s brake pads and discs is extended. research shows that the repairs and maintenance of a bev are up to 30% less expensive than a conventional vehicle [11]. similar conclusions were reached by the authors in [12]. the authors established the average costs for repairs and maintenance for both bevs and icevs through market research and price quotations. the findings are presented in table 5. 24 vol. 51/2024 green mobility in courier services . . . figure 3. comparison of the vehicle costs – vehicles allocated in bratislava [10]. figure 4. comparison of the vehicle costs – vehicles allocated in nové zámky [10]. figure 5. comparison of the vehicle costs – vehicles allocated in košice [10]. city/vehicle nissan e-nv200 (bev) fiat ducato (icev) bratislava 0,1043 € km−1 0,2031 € km−1 nové zámky 0,0587 € km−1 0,2047 € km−1 košice 0,1109 € km−1 0,1984 € km−1 table 3. comparison of unit fuel/energy costs [10]. 25 alica kalašová, kristián čulík acta polytechnica ctu proceedings type vehicle price range bev nissan e-nv200 29 200–34 125 € without vat bev citroen e-jumpy 50 kwh 35 140–36 350 € without vat bev citroen e-jumpy 75 kwh 40 350–41 560 € without vat icev fiat ducato 20 850–29 550 € without vat table 4. vehicle acquisition costs of bevs and icevs in 2022. year/distance bev icev 2/40 000 km 178.00 eur 234.42 eur 4/80 000 km 514.36 eur 359.52 eur 6/120 000 km 397.69 eur 822.31 eur 8/160 000 km 692.36 eur 422.58 eur average price 222.80 eur 229.85 eurper year table 5. service and maintenance costs for vehicles. city nissan env200 fiat ducato total costs [€] total costs [€] bratislava 1 724.91 3 509.48 nové zámky 1 605.02 5 873.37 košice 955.47 1 940.41 table 6. total vehicle costs and conversion to unit costs. 2.3.4. insurance costs based on market research, we analysed insurance providers and the prices for their services. various insurance programs differ in the scope of services offered, including coverage for insurance events. nevertheless, the average insurance price for electric vehicles is €217.62 per year, for diesel vehicle (icev) is €262.41 per year. insurance costs were taken from source [13] for the vehicles included in this study. 2.3.5. government subsidies currently, there is no government subsidy for the acquisition of electric vehicles in slovakia; however, in the past, the state provided subsidies [14]. 2.3.6. vehicle tax vehicle taxes are subject to vehicles of categories: l1e to l7e, m1, m2 and m3, n1 to n3, o1 to o4. the subject of the tax is a vehicle used for business. vehicles of categories l, m and n, whose only energy source is electricity, have a zero-tax rate in slovakia [15]. 2.3.7. summary of costs based on the conducted calculations related to the vehicle propulsion costs, i.e., electricity or diesel, and defining expenses such as service and maintenance, insurance, and motor vehicle tax, it is possible to calculate total operating costs per vehicle and year. the summary and comparison are in table 6. 3. results several conclusions could be formulated from the analysis of the operating costs of an electric vehicle compared to a vehicle with a conventional engine. they are described in this chapter. 3.1. advantages and disadvantages of bev the benefits of bevs in our territory are as follows: + fuel costs: energy for charging is less expensive than fuel. + repair and maintenance costs: repairs and maintenance for a bev is less expensive (compared to icev). + insurance costs: prices are comparable with minimal differences. + vehicle tax: exempt from (motor) vehicle tax (in slovakia). + government subsidy: a possibility to cover the difference between the price of bev and icev. currently, it is not available in slovakia. + environmental benefits: contribution to environmental protection, zero emissions. + convenience: smooth, quiet operation, automatic transmission, and more. the disadvantages of bevs in our conditions are as follows: high acquisition costs: the value of bev depends on innovative technologies and expensive batteries. low operation range – however, it is adequate for urban delivery. vehicle lifespan – 8 years (estimation), recommended battery replacement period. of course, for an economic comparison of bevs and icevs, it is necessary to study all the related costs. authors all over the world are trying to quantify tco (total costs of ownership). for example, the tco assessment in [16] shows that bevs are currently more expensive than icevs. 3.1.1. costs the main factor in the operation of electric vehicles is the lower cost of electricity compared to fuel (diesel). it is the primary motivation for businesses to acquire 26 vol. 51/2024 green mobility in courier services . . . city bev icev total energy costs unit costs per km total fuel costs unit costs per km bratislava 1 284.49 0.104 2 706.22 0.203 nové zámky 1 164.60 0.059 5 070.11 0.205 košice 515.05 0.112 1 137.15 0.198 total 2 964.14 8 913.48 table 7. comparison of total fuel and energy costs. and operate such vehicles, regardless of the environmental impact and companies’ efforts to become carbon neutral. based on the obtained data and calculations, this article confirmed that operating an electric vehicle can save fuel costs. charging an electric vehicle is less expensive than refuelling a diesel delivery vehicle. table 7 summarizes the costs of analysed vehicles in three slovak cities. even though, in all cases, the diesel vehicle covered more kilometres than the electric one, the operation of the electric vehicle is less expensive. 3.2. benefits for company the idea of expanding the fleet of vehicles with an alternative fuel vehicle has ecological and economic significance [17]. based on our findings after calculating the ownership costs of an electric vehicle compared to a combustion engine vehicle, we can assert that electric vehicles represent the future in the courier services sector. an electric vehicle has its ecological justification but also ensures a certain comfort for the employee with its quiet and smooth ride or automatic transmission; these factors can influence the driver’s performance, whose working environment will improve. despite the high acquisition cost of the vehicle, low operating costs can ensure economic efficiency. several companies should consider incorporating such vehicles into their fleets because they can save costs and contribute to environmental protection. 4. conclusions based on the provided data, it is clear that electric vehicles are efficient primarily due to their low operating costs. the energy costs required for charging bevs are significantly lower than the fuel costs for icevs. it is the most significant differentiating factor in the utilization of electric vehicles. similarly, the cost of repairs and maintenance is lower, the ev is exempt from motor vehicle tax (in slovakia), and the insurance cost is comparable to that of combustion engine vehicles. the drawback is that the acquisition cost of the vehicle is higher. however, in most cases, despite this, it can save companies a considerable financial resource, thanks to its low operating costs. one form of support is state subsidies. currently, there is no project in slovakia supporting the development of electromobility. based on past projects, however, we can consider subsidies as a successful tool for promoting and developing electromobility in slovakia. a subsidy of several thousand euros can cover the price difference between an electric and a diesel vehicle. it serves as motivation for potential owners to purchase electric vehicles. the subsidy also applied to n1 category vehicles, which could be appealing to businesses and courier service companies. it is essential to consider the lifespan of the vehicle, approximately 8 years, during which the battery degrades, and its replacement is theoretically necessary with prices ranging from €10 000 upwards. nevertheless, this article proves that bevs represent the future of urban delivery services. it is also important to note that the significance of battery electric vehicles (bevs) in the courier service sector is substantial because courier vehicles cover only short distances daily, with a significant portion of stop time. battery electric vehicles (bevs) currently face several drawbacks hindering their broader adoption. their limited driving range poses a significant challenge, particularly in long-distance freight transport. additionally, heavy batteries reduce the vehicle’s effective weight, impacting its performance. moreover, the weight of the cargo also affects the electric car’s energy consumption. electric vehicles represent the green future in the courier service sector because online shopping is still more popular, leading to increased demand for courier companies. some customers are also interested in a company’s ecological practices, so they consider the company’s behaviour toward the environment [18]. acknowledgements this paper has been developed under support of the project: mšvvš sr vega no. 1/0178/22 kalašová, a.: basic research of the sharing economy as a tool for reducing negative externalities. references [1] european council. clean and sustainable mobility. 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https://e-mobility.sk/wp-content/uploads/2022/01/elektromobilita-v-%c4%8d%c3%adslach-q42021-2.pdf https://e-mobility.sk/wp-content/uploads/2022/01/elektromobilita-v-%c4%8d%c3%adslach-q42021-2.pdf https://doi.org/10.26552/pte.c.2021.2.6 https://www.dpd.com/sk/sk/news/dpd-roz-iruje-flotilu-elektrick-ch-dod-vok-naslovensku-ich-m-u-sto/ https://www.dpd.com/sk/sk/news/dpd-roz-iruje-flotilu-elektrick-ch-dod-vok-naslovensku-ich-m-u-sto/ https://www.dpd.com/sk/sk/news/dpd-roz-iruje-flotilu-elektrick-ch-dod-vok-naslovensku-ich-m-u-sto/ https://www.dpd.com/sk/sk/news/elektricke-dodavky-voltia-sa-vslovenskych-mestach-osvedcili-spolocnost-dpd-ich-pocet-navysi-trojnasobne/ https://www.dpd.com/sk/sk/news/elektricke-dodavky-voltia-sa-vslovenskych-mestach-osvedcili-spolocnost-dpd-ich-pocet-navysi-trojnasobne/ https://www.dpd.com/sk/sk/news/elektricke-dodavky-voltia-sa-vslovenskych-mestach-osvedcili-spolocnost-dpd-ich-pocet-navysi-trojnasobne/ 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https://www.financnasprava.sk/sk/podnikatelia/dane/dan-zmotorovych-vozidiel/info-dzmv https://doi.org/10.3390/wevj12010021 https://doi.org/10.2478/ttj-2013-0027 acta polytechnica ctu proceedings 51:22–28, 2024 1 introduction 2 materials and methods 2.1 analysis of the current state in slovakia 2.2 characteristics of the selected company 2.3 cost calculation 2.3.1 fuels, energy 2.3.2 acquisition costs 2.3.3 repairs and maintenance 2.3.4 insurance costs 2.3.5 government subsidies 2.3.6 vehicle tax 2.3.7 summary of costs 3 results 3.1 advantages and disadvantages of bev 3.1.1 costs 3.2 benefits for company 4 conclusions acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2023.42.0077 acta polytechnica ctu proceedings 42:77–82, 2023 © 2023 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague high-speed x-ray radiography for experiments in impact dynamics using high-power x-ray tube, cesium iodine scintillator and laboratory optical camera jan šleichrt∗, jan falta, tomáš fíla czech technical university in prague, department of mechanics and materials, faculty of transportation sciences, konviktská 20, 110 00 prague 1, czech republic ∗ corresponding author: sleichrt@fd.cvut.cz abstract. x-ray radiography and computed tomography have become well-established methods for investigation of internal structure of objects and for defectoscopy. recently, the methods have even been used for in-situ analysis of materials under mechanical loading. although the techniques would be very suitable for analysis during dynamic events, their application is constrained by typical achievable frame rates. therefore, fast imaging is usually limited to facilities providing sufficient flux like particle accelerators. in this paper, we test imaging performance of a laboratory-based setup with a high-power x-ray tube, a scintillation panel, and an optical camera. fast-rotating object and typical specimens for impact testing are irradiated with different power settings and quality of captured images is evaluated and analyzed. it is found out that the system can be successfully used for imaging at several hundred frames per second allowing for inspection of slow impact dynamics experiments. keywords: high-speed x-ray radiography, scintillator, high-speed camera, impact dynamics. 1. introduction x-ray imaging is an experimental technique allowing for visualization of the internal structure of the irradiated objects. in engineering applications, the technique of planar x-ray radiography is well-established for defectoscopy, identification of internal imperfections, or control of manufacturing processes. typically, an x-ray detector working on a flat panel or direct conversion principle is currently used to capture the planar x-ray images, or even volumetric computed tomography reconstruction of stationary objects in very high resolution (voxel size corresponding to units of micrometers) [1]. in-situ investigation of slowly moving objects (typically stop-and-go inspection on a conveyor during manufacturing process) or objects subjected to quasi-static mechanical loading is also ordinarily possible [2, 3]. although the method would be perfectly suitable even for in-situ analysis of the internal structure of materials subjected to dynamic processes, like impact or vibration, such applications are still extremely rare in laboratory or field environments as the frame rate of conventional x-ray detectors is not sufficient. the problem can be overcome using particle accelerators or flash x-ray systems providing sufficient photon flux for better imaging performance of scintillation panels in combination with high-speed cameras [4–6]. however, this comes with disadvantages regarding the costs, system complexity, accessibility, flexibility, constraints in image resolution, statistics, and size of the object. recently, state-ofthe-art detectors using direct deposition technology allowing for high frame rates have been introduced, but still exhibit high costs, low scalability, and limited size [7]. in this paper, we introduce a system for high-speed x-ray radiography of transient events consisting of a conventional high-power x-ray tube, large area scintillation panel, and a variety of optical cameras (including costly high-speed cameras as well as budget scientific or standard cameras). the imaging performance of the system is tested exhibiting superiority in achievable frame rates over standard radiation imaging detectors and allowing for successful imaging of transient dynamic processes. the advantages and disadvantages of the system are discussed as well. 2. materials and methods 2.1. imaging principle an observed object is irradiated by a beam of an x-ray emitting source. after penetration through the object, the x-ray beam hits the scintillation panel, which is in principle a thin luminescent layer producing visible light when excited by ionizing radiation. the visible light image produced by the scintillation panel is then captured using a laboratory optical camera. the measurement principle is shown in figure 1. this arrangement requires aligning the camera with the axis of the x-ray beam. ionizing radiation hitting the cmos chip elements of the camera causes randomly distributed high-saturated “bright” pixels, so-called zingers. this unsolicited noise has a significant effect on image quality and should be prevented, minimized, or filtered [8]. in our case, signal intensity (and thus maximum imaging speed) is prioritized to image quality. therefore, zingers are widely present in the images and are filtered during post-processing by a simple median filter at the cost of reduction in 77 https://doi.org/10.14311/app.2023.42.0077 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en j. šleichrt, j. falta, t. fíla acta polytechnica ctu proceedings area of exposured object camera scintillator conversion x-ray to visible light high-power visible spectrum x-ray exposured object x-ray tube figure 1. x-ray imaging principle. detail detectability. 2.2. x-ray tube irradiation was performed by an mxr-225hp/11 x-ray tube (comet ag, switzerland). this highpower x-ray tube has a tungsten target and dual focal spot capability providing spot size of 1 mm at maximum power of 1 800 w in the large spot mode and 0.4 mm at maximum power of 800 w in the small spot mode. the generated beam is approximately conical with angular span of 30–40◦. acceleration voltage can be set in range of 50–225 kv providing sufficient penetration through objects with medium size and density (penetrated thickness corresponding to several centimetres of light metals and alloys). 2.3. scintillation panel the conversion of emitted photons from the x-ray tube to the visible-light spectrum was performed by the cesium iodine (csi) scintillation panel – gpxs j10666 (hamamatsu photonics, japan) based on the luminescence effect. physical dimensions of the panel are 500 × 500 mm2 with effective active area of 450 × 450 mm2. this large-format panel combines the advantages of a low after-glow effect and high relative light output with its maximum intensity peak lying in wavelength range of 400–600 nm. 2.4. laboratory optical camera a standard laboratory camera kiralux cs135mun (thorlabs, usa) with a monochromatic 10-bit cmos chip was used together with a standard c-mount camera lens mvl6wa (thorlabs, usa) with a fixed focal length of 6 mm, minimum focal distance of 200 mm and maximum aperture of f/1.4. the camera was operated using generic software provided by the manufacturer with the images grabbed directly to a pc hard drive using its usb3.0 interface. the chip has a full frame resolution of 1280 × 1024 px with pixel size of 4.8 × 4.8µm2 and maximum quantum efficiency of 60 % at 600 nm matching with the typical wavelength of light emitted by the scintillation panel. the camera can operate at 165.5 fps with full frame images and supports chip windowing, binning and amplification while its maximum imaging speed is 6 712 fps at resolution of 16 × 2 px. besides the kiralux camera, two other cameras were used: sony cyberhot rx100 iv (sony corporation, japan) representing a non-scientific budget equipment and photron sa-z ultra high-speed camera (photron, japan). typical performance of these two cameras is briefly discussed in section 4 for illustrative purposes only. although the photron sa-z high-speed camera is available to the authors and can provide significantly better performance, it was intentionally used only for demonstration and indepth analysis of the ultra high-speed camera results is considered beyond scope of this study. more importantly, long-term exposure to high-intensity radiation can potentially damage extremely costly equipment. therefore, performance that can be reached using an affordable camera is demonstrated instead. 2.5. experiment the experimental program was divided into two main parts: i) investigation of full-beam intensities and image quality of typical specimens in stationary position with a variety of x-ray tube and imaging camera settings, and ii) imaging of fast rotating object. the experimental setup was composed of the high-power x-ray tube, a sample holder – tripod, the scintillation panel, and the laboratory optical camera. the individual components were axis-aligned (see figure 1). typically, the x-ray tube was operated with an output power of 1 500 w, although its maximum output power rating is 1 800 w. the tube has, however, a limited lifetime at the maximum output power and therefore most of the imaging was performed at the maximum continuous power settings. several objects representing typical specimens for impact dynamic experiments were irradiated, namely: closed-cell aluminium foam (cylinder with approximate dimensions of 60 mm in diameter and 30 mm in height, or larger integral blocks), additively manufactured stainless steel auxetic lattice filled with a polymeric filling (cube with approximate dimensions of 12 × 12 × 12 mm3), additively manufactured aluminium alloy auxetic lattice (cube with approximate dimensions of 14 × 14 × 14 mm3) [9, 10]. a cpu cooling fan including its aluminum cooler chassis was used as a fast-rotating object while a single blade of the fan was covered by lead to increase imaging contrast. the fan was rotated at a constant angular velocity of approx. 17 rps with peripheral speed of approximately 3.5 m s−1 at the end of fan’s blade. the x-ray images were captured using different settings of the camera and the x-ray tube to optimize for image quality and speed. various settings of frame rate, exposure time, binning, gain, x-ray voltage and power were used during the experiments to improve contrast and image statistics at a given frame rate. the experiments were performed in an x-ray bunker shielded with high density concrete blocks doped with 78 vol. 42/2023 high-speed x-ray radiography for experiments in impact dynamics b) c) csi scintillator positioning system thorlabs camera sony camera cpu fan comet x-ray tube a) aluminium foam sony camera thorlabs camera comet x-ray tube csi scintillator figure 2. experimental setup: (a) an entire experimental setup with an aluminium foam in front of the x-ray tube; (b) cpu fan with aluminium cooler in front of the x-ray tube; and (c) cameras behind the scintillator. 0 10 20 30 40 50 60 70 100 120 140 160 180 200 220 c h ip s a tu ra ti o n [ % ] voltage [kv] scintillator chip saturation, 165 fps, 6 ms no binning, 1500 w, no gain no binning, 1500 w, gain +5.3 db binning 2x2, 1800 w, gain +5.3 db figure 3. average flat field intensities expressed as a fraction of chip’s saturation recorded at 165 fps using an exposure time of 6 ms. the distance between the x-ray tube and the scintillator was 1 060 mm and between the scintillator and the camera 150 mm. iron ore. lights in the bunker were turned off during the imaging process to measure output of the scintillator unaffected by ambient light conditions. resolution of the system was limited primarily by the focal spot size of the x-ray tube (1 mm for maximum power of 1 800 w, 0.4 mm for power of up to 800 w) and by thickness of the active layer of the scintillator of 0.4 mm. in general, details smaller than focal spot size or length of tangential projection through the scintillation layer can be only hardly detected. on the other hand, resolution of the camera images was much higher even for settings with binning and windowing as it was around 0.2 mm in the worst case. the experimental setup showing arrangement with a stationary object as well as with the cpu fan is shown in figure 2. 3. results 3.1. full beam intensity full beam images (i.e., images of the irradiated scintillation panel when no object is placed in the x-ray beam) were taken for different settings of the x-ray tube and the camera. image noise and dark image values were evaluated and, in the worst case, both were approximately as high as 3 % of chip’s saturation range (10-bit). based on a common practice requiring a minimal signal-to-noise ratio (snr) of 10 db for the darkest part of the object (i.e., approx. 8 % of the chip’s range above dark values) and at least the same range of 10 db between the darkest part of the image and bright full beam background (i.e., approx. 25 % above dark values), it was estimated that full beam intensity has to reach at least approximately 30 % of the chip’s range to provide minimal required image quality. recorded average full beam intensities at 165 fps (exposure time of 6 ms) with a distance of the scintillator from the x-ray focal spot 1060 mm are shown in figure 3. the distance of the camera from the scintillator was kept at approx. 150 mm for all the tests. this distance is lower than the minimum object distance of the lens, therefore an extension element was used to reduce it. depending on the x-ray tube voltage and camera settings (pixel binning and signal amplification), the imaging performance can be investigated revealing that the required intensity is not reached without amplification or binning. however, as the intensity is linearly dependent on the exposure time, it can be expected that, for certain settings (gain of +5.3 db, binning of at least 2 × 2), proportionally higher frame rates can be reached while still fulfilling the minimal required image quality. 79 j. šleichrt, j. falta, t. fíla acta polytechnica ctu proceedings 0 5 10 15 20 25 30 35 40 45 100 120 140 160 180 200 220 c h ip s a tu ra ti o n [ % ] voltage [kv] scintillator chip saturation no gain, 1060 mm gain +5.3 db, 1060 mm no gain, 1610 mm gain +5.3 db, 1610 mm no gain, distance correction gain +5.3 db, distance correction figure 4. average intensities of full beam images for two distances of the scintillator from the x-ray tube. dashed lines represent an extrapolation of the measured values from 1 060 mm to a distance of 1 610 mm fulfilling the inverse-square law. 225 kv, 1500 w, 165 fps, 6 ms, gain +5.3 db, no binning steel sample aluminium sample aluminium foam aluminium sample 500 fps window a) 0 20 40 60 80 100 0 200 400 600 800 1000 1200 c hi p sa tu ra tio n [% ] 225 kv, 1500 w, 165 fps, 6 ms, no gain, no binning line profile via 3d-printed samples line profile via aluminium foam 165 fps b) -20 0 20 40 60 80 100 0 100 200 300 400 500 600 c hi p sa tu ra tio n [% ] 225 kv, 1500 w, 165 fps, 6 ms, no gain, binning 2x2 line profile via 3d-printed samples line profile via aluminium foam 165 fps d) 0 20 40 60 80 100 0 100 200 300 400 500 600 c hi p sa tu ra tio n [% ] 225 kv, 1500 w, 165 fps, 3 ms, gain +5.3db, binning 2x2 line profile via 3d-printed samples line profile via aluminium foam 165 fps e) (shutter 3 ms, equivalent to 330 fps) 0 20 40 60 80 100 120 140 0 100 200 300 400 500 600 c hi p sa tu ra tio n [% ] 225 kv, 1500 w, 500 fps, 2 ms, binning 2x2 no gain no gain gain +5.3db gain +5.3db 500 fps f) 0 20 40 60 80 100 0 200 400 600 800 1000 1200 c hi p sa tu ra tio n [% ] 225 kv, 1500 w, 165 fps, 6 ms, gain +5.3db, no binning line profile via 3d-printed samples line profile via aluminium foam 165 fps c) figure 5. intensity line profiles through typical specimens at different camera settings; (a) x-ray image; (b)(d) 165 fps; (e) shutter speed for 330 fps; (f) 500 fps. 3.2. intensity with distance the expected behavior of intensity according to the inverse-square law (quadratic decrease of intensity with distance) was confirmed by placing the scintillation panel at two distances from the x-ray focal spot. the first distance was 1 060 mm and the second distance was 1 610 mm. average intensities of full beam images for both distances at different settings of the x-ray tube are shown in figure 4. dashed lines represent the values from 1 060 mm extrapolated to a distance of 1 610 mm using the inverse-square law showing good correlation with the measured values. note that the recorded intensities for 1 060 mm do not perfectly match the values in figure 3. because of the open installation of the scintillator, its maximum light output deteriorated over time during long term exposure to excessive radiation. effects of long term after-glow, ghosting, and non-stable scintillator output (caused by dust accumulation etc.) were present. in general, the lifetime of scintillator is usually extremely sensitive to radiation exposure and the environmental conditions. however, the effects observed during this study were temporary and the scintillator restored approximately its initial performance after some time (couple of hours to days). 3.3. typical specimens intensity line profiles were used as a tool to investigate image quality. representative x-ray image of typical specimens including the closed-cell metal foam stacked together with the additively manufactured lattices made of stainless steel and aluminum alloy as well as line profiles corresponding to different camera settings are shown in figure 5. recorded intensities in most of the cases fulfilled the requirements summarized in the previous section 3.2 while images with frame rates of up to 500 fps with reasonable quality were captured. with application of binning, intensities close to saturation of the chip were reached in figure 5d (165 fps, no gain, binning 2 × 2, exposure time 6 ms). figure 5e represents an image where a 80 vol. 42/2023 high-speed x-ray radiography for experiments in impact dynamics 165 fps, gain +5.3 db, no binning 165 fps, gain +5.3 db, no binning 165 fps, no gain, binning 2x2 500 fps, gain +5.3 db, binning 2x2 figure 6. x-ray images of the typical specimens taken at 225 kv and 1 500 w with different camera settings including their histograms normalized to full range of the chip. the images from left to right correspond to the line profiles (b), (c), (d) and (f) in figure 5. 165 fps, raw, gain +5.3 db, no binning 165 fps, cor., gain +5.3 db, no binning 365 fps, raw, gain +5.3 db, binning 2x2 365 fps, cor., gain +5.3 db, binning 2x2 figure 7. cpu fan rotating at approx. 17 rps recorded at two different frame rates. raw images and images after corrections are shown including their histograms normalized to the full range of the chip. gain of +5.3 db was added to the settings of figure 5d. shutter time was reduced to 3 ms to prevent overexposure. the resulting image is therefore equivalent to frame rate of 330 fps. as it is not possible to reach this frame rate without windowing, the full-frame image with binning was taken at 165 fps to allow for direct comparison with the image in figure 5d. increase in image statistics and contrast at a cost of higher noise, lower resolution and smaller active area is apparent in figure 5f. corresponding x-ray images of the typical specimens taken with different settings are shown in figure 6 including histograms normalized to full range of the chip. 3.4. rotating cpu fan typical images of the cpu fan rotating at constant angular velocity of approx. 17 rps captured at frame rate of 165 fps and 365 fps are shown in figure 7. raw uncorrected images and images after dark field, flat field correction (normalization using full beam images), and removal of zingers using a median filter are compared including histograms. the image quality at both frame rates allows for tracking of the fan and motor pole’s position and is considered suitable for subsequent image processing and analysis. motion blur effect caused by peripheral velocity of the fan of approx. 3.5 m s−1 is apparent in the images at both frame rates, requiring post-processing techniques of blur removal to perform more complex analysis of the moving fan itself. based on the x-ray tube focal spot size of 1 mm, it is estimated that the velocity limit not requiring post-processing of blur effects is approx. 0.16–0.50 m s−1 which is, together with the achieved frame rates, suitable for dynamic testing at low and intermediate strain rates. 4. discussion 4.1. advantages and disadvantages based on the performed experimental work and the acquired results, the advantages and disadvantages of the used system for high-speed x-ray imaging in impact dynamics can be summarized. the main advantages are formulated in the following points: relative affordability of the individual components, versatile laboratory-based system, very high frame rates in comparison with standard radiation imaging detectors, the flexibility of the system that can be application optimized using different optics, cameras, geometry, field of views, etc. with low costs of the upgrade. on the other hand, the limitations lay in relatively low image resolution, small pixels of the optical cameras reducing imaging efficiency, the complexity of the system composed of many elements, radiation harm to cameras, obligatory use of windowing, binning, and amplification for increasing frame rates, and low energy resolution. performance of the system can be significantly improved by the high-speed camera (photron sa-z) as the results of similar image quality were possible at frame rates of 1–2 kfps (see an illustrative image in figure 8a) because of the significantly higher sensitivity of the chip. on the other hand, still, acceptable results could be achieved even with the budget sony cybershot rx100 iv at 240 fps (illustrative image in figure 8b). 4.2. application potential the achieved results indicate that the system is capable of high-speed x-ray imaging of impact dynamics experiments performed at low and intermediate strain rates and can provide acceptable frame rate, resolution, and image detail. the system will be 81 j. šleichrt, j. falta, t. fíla acta polytechnica ctu proceedings photron, 1000 fps, 225 kv, 900 w sony, 240 fps, 225 kv, 1500 w, color image c)b)a) figure 8. a) hollow aluminum profile with cross-section of 180 × 90 mm2 captured using photron sa-z high-speed camera at 1 000 fps (225 kv, 900 w). the image was taken using a mirror behind the scintillator to prevent direct irradiation of the photron camera. the frame of the mirror is visible around the x-ray projection highlighted by red rectangle; (b) rotating cpu fan captured using sony cybershot rx100 iv at 240 fps (225 kv, 1 500 w); (c) dynamic testing device with linear motors installed in the x-ray imaging setup. used together with an in-house developed dynamic testing device with linear motors allowing for precisely controlled and time-synchronized experiments with impact velocities of 0–8 m s−1 and typical impact forces up to 5 kn. the potential research fields include crack propagation, fracture and failure, lattice collapse, internal damage, or strain rate-sensitive filling materials. the dynamic testing device with linear motors installed within the x-ray imaging setup is shown in figure 8c. 5. conclusions the performance of high-speed x-ray imaging system using large area csi scintillation panel and the affordable scientific optical camera was investigated through series of experiments analyzing its application potential for continuous x-ray radiography in impact dynamics. it was concluded that the system could be used for imaging of dynamic impact events conducted at low and intermediate strain rates with acceptable image quality. acknowledgements the research was supported by the czech science foundation (project junior star no. 22-18033m). references [1] j. dudak. high-resolution x-ray imaging applications of hybrid-pixel photon counting detectors timepix. radiation measurements 137:106409, 2020. https://doi.org/10.1016/j.radmeas.2020.106409 [2] p. koudelka, t. fila, v. rada, et al. in-situ x-ray differential micro-tomography for investigation of water-weakening in quasi-brittle materials subjected to four-point bending. materials 13(6):1405, 2020. https://doi.org/10.3390/ma13061405 [3] d. vavrik, p. benes, t. fila, et al. local fracture toughness testing of sandstone based on x-ray tomographic reconstruction. international journal of rock mechanics and mining sciences 138:104578, 2021. https://doi.org/10.1016/j.ijrmms.2020.104578 [4] x. zhai, z. guo, j. gao, et al. high-speed x-ray visualization of dynamic crack initiation and propagation in bone. acta biomaterialia 90:278–286, 2019. https://doi.org/10.1016/j.actbio.2019.03.045 [5] e. strassburger, s. bauer, s. weber, h. gedon. flash x-ray cinematography analysis of dwell and penetration of small caliber projectiles with three types of sic ceramics. defence technology 12(3):277–283, 2016. https://doi.org/10.1016/j.dt.2016.01.011 [6] s. moser, m. wickert, s. nau. high-speed x-ray imaging and 3d analysis of impact-formed fragments. in m. versluis, e. stride (eds.), 32nd international congress on high-speed imaging and photonics, vol. 11051, p. 110510o. spie, 2019. https://doi.org/10.1117/12.2525447 [7] d. vavrik, j. dudak, i. kumpova, j. zemlicka. application of 2 × 5 mpx3 camera with monolithic sensor for phase contrast imaging and computed tomography. journal of instrumentation 17(03):c03020, 2022. https://doi.org/10.1088/1748-0221/17/03/c03020 [8] j. mertens, j. williams, n. chawla. a method for zinger artifact reduction in high-energy x-ray computed tomography. nuclear instruments and methods in physics research section a: accelerators, spectrometers, detectors and associated equipment 800:82–92, 2015. https://doi.org/10.1016/j.nima.2015.08.012 [9] t. fila, p. koudelka, j. falta, et al. impact behavior of additively manufactured stainless steel auxetic structures at elevated and reduced temperatures. advanced engineering materials 23(1):2000669, 2020. https://doi.org/10.1002/adem.202000669 [10] j. sleichrt, t. fila, p. koudelka, et al. dynamic penetration of cellular solids: experimental investigation using hopkinson bar and computed tomography. materials science and engineering: a 800:140096, 2021. https://doi.org/10.1016/j.msea.2020.140096 82 https://doi.org/10.1016/j.radmeas.2020.106409 https://doi.org/10.3390/ma13061405 https://doi.org/10.1016/j.ijrmms.2020.104578 https://doi.org/10.1016/j.actbio.2019.03.045 https://doi.org/10.1016/j.dt.2016.01.011 https://doi.org/10.1117/12.2525447 https://doi.org/10.1088/1748-0221/17/03/c03020 https://doi.org/10.1016/j.nima.2015.08.012 https://doi.org/10.1002/adem.202000669 https://doi.org/10.1016/j.msea.2020.140096 acta polytechnica ctu proceedings 42:1–6, 2023 1 introduction 2 materials and methods 2.1 imaging principle 2.2 x-ray tube 2.3 scintillation panel 2.4 laboratory optical camera 2.5 experiment 3 results 3.1 full beam intensity 3.2 intensity with distance 3.3 typical specimens 3.4 rotating cpu fan 4 discussion 4.1 advantages and disadvantages 4.2 application potential 5 conclusions acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2023.45.0033 acta polytechnica ctu proceedings 45:33–38, 2023 © 2023 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague directing classical geotechnical engineering to future challenges gunilla franzén∗, håkan garin president swedish geotechnical society, hällingsjövägen 320, se-434 97 kungsbacka, sweden ∗ corresponding author: gunilla.franzen@geoverkstan.se abstract. geotechnical engineers have traditionally been focused on soil mechanics, rock mechanics and engineering geology. thus it has been sufficient to master these topics to be recognized as a key participant in building and civil engineering projects. however, gradually concerns within both projects themselves and societies more generally are shifting towards new questions and challenges. therefore, ensuring that geotechnical engineers remain central to all relevant projects, there is a need to master increasingly different and new ways of working together, obtaining new skills and increasing professionally pertinent knowledge and experience. this paper will give an overview of the issues geotechnical engineers have to address and master, if they are to take a lead in projects that seek to meet future challenges. this is really about working in a multi-disciplinary environment, using relevant technical methods without losing specifically engineering judgement, and then using open-minded communication skills to question previous practices that can proceed to find solutions to those future challenges identified. how is this to be done successfully within the profession? this paper offers some ideas. all professionals, and geotechnical engineers cannot be an exception, want to influence and contribute to solving current and future challenges, and thus we need to expand our knowledge awareness. the list of topics where we need to develop our knowledge is rather lengthy and include environmental considerations, contracts, law, conceptualisations of sustainability, robustness, monitoring, digitalisation, standardisation, and climate change. this paper will highlight two examples where geotechnical engineers with increased knowledge sensitivity and skills could make a significant contribution. first, there is an example from göteborg about limiting a project impact within its zone of influence. designing the geotechnical structure itself is only the first step, for in addition there is a need to control the building process, have sufficient monitoring, handling the expectations of society while ensuring recognition and use of a particular professional scope of operation. the other example concerns the re-use of material in our embankments to limit transportation. which criteria should be used to determine the suitability of materials for re-use? how can we cope with societal reactions? finally, the paper will discuss how geotechnical engineers, by combining geotechnical knowledge with new skills, can contribute with a sustainability foundation involving both relevant concepts and principles looking forward. keywords: future challenges, re-use, sustainable, zone-of-influence. 1. introduction geotechnical engineers tend to be recognised for knowledge of soil mechanics, rock mechanics and engineering geology. for a long time, it was sufficient to master these topics to realise their key role in building and civil engineering projects. but gradually projects and societies shifted focus towards new questions and challenges. in order to ensure that geotechnical engineers continue as key experts in such projects, we need to master new ways of working together, develop new skills and expand our knowledge. the question raised is how geotechnical engineers with their specialised understanding of the behaviour of soil, rock and groundwater can contribute to solving readily identifiable societal challenges. what additional skills and knowledge are needed? how do we come to terms with the new working environment that is steadily developing around us? this paper will give an overview of the issues that geotechnical engineers have to grasp, if they are to take a lead in future projects that challenge us all. it relates to working in a multi-disciplinary environment, using standard operating techniques while enhancing our engineering judgement, facilitating open-minded communication, and questioning traditional practices to discover new solutions to emerging problems. this paper does not pretend to give a solution to all the issues, but presents some ideas for the geotechnical community to discuss and develop further. 2. facing new demands and working methods in a historical context the main demand made from society on engineers was that they should build houses, schools and transportation systems that were functional and safe. these basic requirements are today often taken for granted and the focus in current projects 33 https://doi.org/10.14311/app.2023.45.0033 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en gunilla franzén, håkan garin acta polytechnica ctu proceedings is shifting to examining possible futures. this includes durability during service life (and beyond), a robust or resilient capacity if the unexpected happens and of course now there is sustainable production with sustainable materials. it is not only the demands on the final product that are changing, but also the very framework for geotechnical design. it is one thing to design and build the foundation of a house if the construction site is in a newly developing area with no previous buildings involving conflicting activities. it is another to recognise that as a result of urbanisation, buildings are constructed in confined areas within an already built environment, causing conflicts with nearby structures and utilities, and managing issues with transportation of building material and excavation material as they arise, becoming aware of a perhaps potentially hazardous working environment due to contaminated soils and maybe the negative reaction of a neighbourhood disturbed by the working process. digitalisation during the last decades has rapidly changed the way we work. the pen and paper has in most cases been replaced by a computer with databases, 3d models, an endless list of documents and multiple platforms for communication. this, in combination with globalisation, make it possible to have colleagues from all over the world contributing to the project. as a result, many geotechnical engineers find that they spend more and more time in multi-disciplinary meetings and discussing topics that first seem to have very little to do with their classical training in geotechnical engineering. however, with the right mindset, the geotechnical engineer has the potential to contribute with a sustainable foundation for the future. 3. a sustainable foundation for the future sustainable foundation is a rather vague expression, with perhaps as many interpretations as readers. for the agricultural industry it is about using suitable crops that give food to an increasing population without destroying the potential to use the ground for coming generations. in forestry it is about finding the best way to produce timber so it can be used as an environmentally friendly building material, with limited negative impact on the eco-system and preserving the ground conditions for new woods. the geotechnical engineer contributes to a sustainable foundation of everything that is built, by combining geotechnical engineering judgement with new skills and knowledge. two main aspects of a sustainable foundation are the selection of appropriate execution methods and building materials. but a sustainable foundation is also about dealing with negative impacts within a zone of influence, and thus avoiding restriction on a neighbours’ possibilities to use their own site for further development. in the following sections there are given two examples of how the geotechnical engineer can contribute to a sustainable foundation. 3.1. appropriate execution methods – limiting the impact within the zone of influence göteborg in sweden is situated in an area with a soft soil condition. a piled foundation is the solution for many of the new buildings in the city. this case concerns the installation of piles on a site that is next to the ongoing construction of what will be the highest building in the nordic countries, namely karlatornet. for the building riksbyggen has planned to install about 270 displacement piles (concrete, length 60 m). normally this is a standard execution, but in this case the foundation of karlatornet put restrictions on the execution. the requirement is given that the impact from the installation of the displacement piles at the riksbyggen site on the foundation at karltornet is to be less than a 2 mm deformation. this turns out to be the main challenge of the project. figure 1 gives an illustration of the building site. using numerical analyses a prognosis was developed using the available information of the ground properties and site geometry. this shows that it is not enough to use pre-drilling in each pile installation point, but that additionally it is necessary to remove clay with augurs to thirty meters depth in a trench along the length of the riksbyggen site facing karlatornet. an extensive monitoring program was performed to help convince the owner of karlatornet that the installation would cause no harm to their foundation. measuring points and inclinometers were installed in four lines. manual measurements of the inclinometers were performed three times a week during the installation of the displacement piles. in total this meant 14 weeks of measurements. analyses, presentation, and discussion of the results were conducted at least once weekly during the execution of the pile installation. after three weeks the results indicated large deformation and the work had to be stopped. the analyses showed that the reason for the large deformation were that the contractor had thought that they could install the larger share of the piles before they went on with pre-drilling and removal of clay. the measurements showed clearly that this was a mistake, and thus additional clay removal was performed before continuation of installation. in the following weeks removal of clay was performed in connection with the installation of the displacement piles. as a result, the measured deformation corresponded fairly well with the prognosis. the prognosis for deformation is shown in figure 2 in combination with measurements of actual deformations after completion of the pile installation. based on the measured results in line 4 it was possible to predict the deformation at the interface of the largediameters displacement piles under karlatornet. the 34 vol. 45/2023 geotechnical future challenges figure 1. illustration of the building site, lindholmen, göteborg [red dots is the position of the inclinometers, blue circles the surface measurement points, the red line section used in figure 2 and brown bold dashed line is the position of the trench]. the distance between riksbyggen and karlatornet is about 100 meters. figure 2. prognosis of deformation and measured results presented along the dotted line (red) in figure 1. deformations were slightly larger than 2 mm, but the owner of karltornet was persuaded that the pile installation had not caused any damage on the foundation of karlatornet. for riksbyggen the possibility to cause damage within the zone of influence was avoided and hence also the risk of huge claims against them. this example shows the work situation that develops for many geotechnical engineers. they start with the geotechnical issue but find themselves then presented with endless meetings discussing measuring techniques, evaluating monitoring results, giving results, and offering their engineering judgement to convince owners and clients that there are indeed no geotechnical risks. in addition to the skills of classical geotechnical engineering, they also need to master measurement techniques, deal in uncertainties, undertake risk management, consider legal (national) regulation and furthermore be excellent in their communication. the question that is raised is of course if it is relevant to have this kind of requirement. it can even be questioned if it is possible to measure a 2 mm deformation with confidence. however, in this case riksbyggen did not want to enter into a time-consuming discussion about the value of the criteria. it might have led to a more reasonable value, but in any case there would have been a need to perform monitoring in order to prove that the value in fact was not exceeded. the work at the building site would thus be postponed until there was a new agreement on this value between karlatornet and riksbyggen. that could take weeks or even months and cost a lot of 35 gunilla franzén, håkan garin acta polytechnica ctu proceedings money. therefore, it was judged to be more efficient to perform the measuring program and try to prove that there was no influence on karlatornet due to the installation of displacement piles at riksbyggen. both the removal of clay and the monitoring program were expensive, but less expensive than waiting or ending up with a claim from karlatornet. 3.2. appropriate building material – reuse of material excavated material used to be something that was removed from the building site without much discussion. now there are numerous questions raised regarding dealing with excavated material. is it contaminated? can it be re-used on-site or at a close-by site? how can we limit the transportation of material? to a great extent these questions are addressed by environment experts. however, if we start discussing reuse of the material, the geotechnical engineer with knowledge of classical soil mechanics could make a huge difference. looking at the material as a resource as indeed it is, then by adding binders, removing water, compacting it, or combining it with a geotextile, it becomes possible to make it into a suitable building material for many applications. similar suggestions have been presented previously for example by vaníček et al. [1] and that the consumption of natural aggregates could be decreased by improving the properties of the natural aggregates or applying non-standard aggregates. in the same reference several possibilities for including sustainability as an aspect in design is presented. it is unfortunate that the application of the concept of sustainable design is still not common practice. in quite a few cases it turns out not to be actual technical reasons that restrict the possibility for reuse of material, but references to common practice or public opinion. we have quite a few rules of thumb and common practice conventions from past periods and when we also had plenty of good quality building material easily accessible. it might be necessary for us to question some of these rules and grasp the opportunity to test less high-quality material. this would help to see if by compaction, using geotextiles or binders we can get as good a final product as possible. the life-cycle assessment of that product might prove to be much more sustainable. it is not only other technical experts that might question the reuse of material. people seem to applaud reuse of material, as long as it is not close to their own backyards with maybe a risk that there is contamination in the material used. as technical experts we may know that there is no risk for the spreading of contamination, but we cannot ignore the concerns of the public. a lot of time needs to be spent on showing the benefits of re-use and that all risks are being considered. as geotechnical engineers we have a possibility to contribute to a sustainable foundation by reusing material preferably close to the excavation site or by adding non-standard material and methods. the key is to start looking at the excavated material as a resource and have the courage to question some of our conventional truths. 3.3. the contribution from geotechnical engineers these two examples show that the geotechnical engineer can play an important role in multidisciplinary projects. however, that requires that we take our knowledge about soil mechanics, rock mechanics, geology, and groundwater, and add other skills. in figure 3 examples of the skills we need are illustrated. figure 3. illustration of some of the skills that are essential to a geotechnical engineer in addition to their engineering skills. communication is one of the most important skills, and this means the ability to communicate with different audiences. communication with society, authorities and clients requires the ability to present a huge amount of information in an easily accessible format for a non-engineering audience. this needs to be done without oversimplifying or excluding the uncertainty involved in the conclusions. if the audience instead is fellow engineers from other technical fields, the data can be presented but the challenge is then not to be misunderstood due to having a slightly different vocabulary. chapter 4 describes how the eurocode can be a tool for facilitating communication among engineers. quality control has always been a key aspect for engineers, but with multi-disciplinary and even multi-cultural projects, it becomes even more important. the key is to take the time to ask ourselves the question – is the proposed design reasonable? does it correspond to notions of engineering judgement and their working conventions? we should also bear in mind that in all projects and especially in a multicultural project it might not be obvious for everyone 36 vol. 45/2023 geotechnical future challenges to question the design or ask a principal for a second opinion. principals do have a huge responsibility to create a working environment where the knowledge of others is used and everyone feels free to improve the design. uncertainty we strive to present precise and easily accessible information. our computer codes today allows us to present calculations in colourful graphs to several decimal precision. this might impress the client, but if we have to make sure that the design is safe and robust, we should add our interpretation of the uncertainty intrinsic to the results. no analysis is better than the quality of the input data and the precision allowed by the calculation model. geotechnical engineers know this well, and thus as a rule we should always present openly the uncertainty discerned in the result and not just the result itself. risk management is not a new topic for geotechnical engineers as basically that is what we always have been working with. what is new is a need to learn to use the terminology and tools that other parts of the project are deploying, so that we can communicate the geotechnical risks in the project and contribute to control of those other risks involved. knowledge in related or cognate fields. chemistry, measurement techniques, computer science, economics and law for example. the list can be extended with disciplines where either we need to develop our own understanding or find people for consultation and collaboration. the latter requires that we have an open-minded approach and are willing to listen to other disciplinary perspectives and knowledge and then be prepared to revise our own internal verities. 4. geotechnical engineering using a multidisciplinary toolbox 4.1. the cube illustrates the societal requirements on matters of geotechnical structure the requirements of society on geotechnical structure may be illustrated by a cube – figure 4. on the first axis is safety, ensuring the structure shall have an appropriate probability that it will not fail during service life due to anticipated variations in the load and design situation. on the second axis we have serviceability, as the structure should be built and keep its function during the service life without causing a negative impact within the zone-of-influence on other houses, utilities and infrastructure. deformation, groundwater, noise, and pollution are some of the impacts to consider. on the third axis robustness, durability, and sustainability, give some additional aspects to ensure that the geotechnical structure meets the societal requirements. eurocode, and especially en 1997, gives the geotechnical engineer the tools to verify that the geotechnical structure is within the cube, and hence reduce the likelihood that the structure will fail, that functionality figure 4. the cube illustrates the requirements that a society imposes on the geotechnical structure, that it be safe, functional, durable, robust and sustainable. will be reduced, degraded, or bring unreasonable damage or even that sustainability is neglected. for an ultimate limit state (safety) and serviceability limit state (function) the code is mature and developed. however, for the aspects on the third axis only the basics are presented and additional guidance is needed on a national level. in many cases this guidance needs to be developed together with the other disciplines. 4.2. eurocode as a common tool for all civil engineering eurocode has been developed to facilitate communication and cooperation within the building process. therefore, all civil engineers have the same basis of design, en 1990. in the second generation of eurocode that is soon to be developed there is a minor change to the title of en 1990, the specific words – and geotechnical – has been added. the question arises whether this is an editorial change or poses a major challenge. in a paper presented by van seters et al. 2021 [2], four main contrasts between structural engineers and geotechnical engineers are identified. the four contrasts can be summarised as follows. contrast 1: structural engineers know the specification for design including the material properties. geotechnical engineers also have the specification for design in mind but must assume also ground properties based on a limited amount of data. contrast 2: structural engineers can select the appropriate material and dimensions to be delivered to the building site. geotechnical engineers have to work with the available ground and minimize the uncertainty in the properties and handle any remaining uncertainty in the design. contrast 3: structural engineers calculate and offer calculation models that have been developed and proved for different materials and design situations. the geotechnical engineer calculates a models rele37 gunilla franzén, håkan garin acta polytechnica ctu proceedings contrast clarifications and additions in en 1997 know vs. assume ground model, nominal value select vs. minimize uncertainty geotechnical reliability, geotechnical complexity class, geotechnical category calculation vs. engineering judgement validation of calculation model, four verification methods (calculation, prescriptive rules, observational method, testing) build vs. adapt inspection, monitoring and maintenance plan, impact within the zone of influence table 1. summary of additions to en1997 due to the contrasts between structural and geotechnical engineers. vance and its applicability has to be proven for each design. engineering judgement is in this case key for the geotechnical engineer to select relevant calculation models or other tools for verification of the structure. contrast 4: the structural engineer will prepare an execution specification that will be used on site to build the structure. supervision is performed to ensure that the specification is followed. for the geotechnical engineer the execution is the first time the hidden secret of the ground is revealed. in addition to the execution specification, there is a need for inspection and monitoring, including a plan for adapting to what is actually found at the building site. a more extensive description of these contrasts is found in [2]. the conclusion is that it is a challenge to provide the same base for structural and geotechnical engineers. therefore, clarifications and additions has been made in en1997, to facilitate the applicability of eurocode for geotechnical engineering. this is done without contradicting en 1990 and hence keeps the common base between structural and geotechnical engineers. in table 1 a summary of some of the clarifications and additions are presented. 4.3. eurocode as a common basis for communication as geotechnical engineers our focus may be soil, rock, groundwater, or engineering geology. all of us are supposed to use eurocode 7 as a base for the design of a geotechnical structure. that is a challenge but also an opportunity. in the same way there are large differences in geology and national practice between countries in north, south, east, and west europe. that is also a challenge but also an opportunity for development if we share our experience and identify differences during our collaboration. in a more transnational working environment eurocode creates a common basis for communication, enhancing shared understandings and vocabulary. however, in order to avoid potential conflicting views in projects, it is important to remember that the implementation and understanding of eurocode might still involve some subtle differences across countries. 5. conclusions the conclusion of this paper can be summarized in two statements. engineering skills must ensure that the geotechnical structure is inside the red cube. eurocode will assist with the tools to prove compliance with basic requirements on safety, serviceability, durability, robustness, and sustainability. it is necessary to be always ready to think outside the box or beyond orthodoxies so that that there is no inhibition or limit to a professional capacity to contribute to a sustainable future and come to terms with, and master a new working environment! acknowledgements the authors would like to thank their colleagues in tc250/sc7 and the corresponding national mirror groups, for their extensive contributions and efforts to ensure that the 2nd generation of eurocode 7 will be a useful tool for the practising geotechnical engineer. adrian van seters is especially recognised for his work as leader for this extensive work. chapter four of this paper is an extension of a previous discussion paper presented by van seters and franzén. references [1] i. vaníček, d. jirásko, m. vaníček. modern earth structures for transport engineering. crc press, 2020. https://doi.org/10.1201/9780429263668 [2] a. van seters, g. franzén. eurocode 7 a toolbox for geotechnical engineering. in: proc. 20th international conference on soil mechanic and geotechnical engineering, sydney, australia, p. 4753–4758, 2020. isbn 978-0-9946261-4-1. 38 https://doi.org/10.1201/9780429263668 acta polytechnica ctu proceedings 45:1–6, 2023 1 introduction 2 facing new demands and working methods 3 a sustainable foundation for the future 3.1 appropriate execution methods – limiting the impact within the zone of influence 3.2 appropriate building material – reuse of material 3.3 the contribution from geotechnical engineers 4 geotechnical engineering using a multidisciplinary toolbox 4.1 the cube illustrates the societal requirements on matters of geotechnical structure 4.2 eurocode as a common tool for all civil engineering 4.3 eurocode as a common basis for communication 5 conclusions acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2025.52.0094 acta polytechnica ctu proceedings 52:94–99, 2025 © 2025 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague traffic accident analysis using edr data: a case study of a multicollision event daniel mikyska brno university of technology, institute of forensic engineering, purkyňova 464/118, 612 00 brno, czech republic correspondence: daniel.mikyska@vut.cz abstract. this paper focusses on the importance of using event data recorder (edr) data in the reconstruction of traffic accidents. it presents a case study of a highway accident in which aggressive overtaking led to a series of collisions between three vehicles. although the speed recorded at certain moments was not accurate due to vehicle skidding, the edr data allowed confirmation of key manoeuvres and movements of the vehicles prior to the collision. the paper highlights the importance of edrs in accident analysis, where they helped reconstruct the drivers’ manoeuvres and decisions, contributing to the elucidation of the causes of the accident. the mandatory introduction of edrs in new vehicles from 2024 is an important step towards better crash analysis and improved road safety. keywords: edr data, crash data, analysis of traffic accidents. 1. introduction an event data recorder (edr), often referred to as a vehicle black box, is a device that records key data on the operation of a vehicle at the time of an accident. these data include, for example, vehicle speed, brake system status, accelerator position, airbag activation, and seat belt status. edr is an increasingly important tool for analysing the causes of road accidents and plays a key role in reconstructing the events that led to the accident [1–5]. edrs were originally developed to improve vehicle safety and collect data for the automotive industry, but over time, their use has expanded into areas such as forensics and litigation. in the united states, for example, the national highway traffic safety administration (nhtsa) has established standards for edrs and, since 2012, edrs have been a mandatory part of new vehicles [1, 4–7]. in the european union, this obligation is effective from 7 july 2024 [8]. this article focusses on a case study that shows a case where the recorded speed was not valid at a certain moment, but the data helped in reconstructing the accident itself and confirm the behaviour of the accident participants. 2. traffic accident introduction a traffic accident that occurred on a highway was used as a case study [9]. it occurred when a black vehicle aggressively overtakes a red vehicle from the right. as a result, the driver of the red vehicle attempted to avoid the collision and carried out an evasive manoeuvre. in the course of the manoeuvre, he drove onto the grass strip near the bollards that divide the road directionally. on reversing, the vehicle skidded, resulting in a first collision with the silver vehicle, which was currently travelling in the right-hand lane. the red vehicle continued to skid and struck the van standing in the breakdown lane. the edr data played a very important role in the reconstruction of this accident and may have confirmed the facts and movements of the red vehicle. figure 1 shows the accident process. the data from the logger was read using bosch equipment (crash data revival 23.0.) and also by the techstream system from the vehicle manufacturers toyota, lexus and scion. the data read from both recorders correspond to each other and capture the same traffic accident. 3. cdr data five events were recorded throughout the accident (figure 2). the first two incidents are associated with a collision between a red and a silver vehicle, and the remaining three are then associated with a collision between a red vehicle and a van. these can be determined from the recorded event lines. events are identified in the cdr log by the abbreviation “trg” (trigger) and the corresponding numerical designation. the event with the highest numerical designation is the last recorded event [9]. 3.1. trg 1 the first event was recorded as a frontal/rear/side collision, but the frontal impact was recorded as the triggering factor. the data are recorded in the time interval −4.7 to 0.0 seconds. this means that a precollision movement of the red vehicle of 4.7 seconds can be imported before the actual collision. in figure 3 you can see the speed of the vehicle. at a time of 4.7 seconds before the collision, the vehicle was travelling at 165 kmh−1 and then accelerated to 171 kmh−1 by 3.2 seconds before the collision, marked in red. during this time interval, the accelerator pedal 94 https://doi.org/10.14311/app.2025.52.0094 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 52/2025 traffic accident analysis using edr data figure 1. the accident process. figure 2. summary of recorded events in the cdr log. figure 3. table of precollision data 1 (trg 1). was fully depressed (column “accelerator pedal”), indicated in green. subsequently, the brake pedal was depressed, and the vehicle was braked from 2.2 seconds before the collision (column “service brake”), shown in blue. the braking is related to the manoeuvre mentioned above, which the driver of the red vehicle had to perform in an attempt to avoid a collision with the black vehicle overtaking him from the right. this manoeuvre could also be read and confirmed from the recorded steering wheel rotation and vehicle rotation values, figure 4, columns “steering input”, marked in green, and “yaw rate”, marked in blue. before the vehicle driver started to brake, the steering wheel was slightly turned to the right, which was consistent with the course of the road itself. at the moment of braking, the steering wheel was turned to the left and then sharply back to the right 1.7 seconds before the collision. this caused the vehicle to skid. the fact that the vehicle entered the grass strip during the manoeuvre also contributed to the skid. the driver then tried to compensate for the skid by turning the steering wheel to its maximum angle. the steering wheel rotation values correspond with the vehicle’s rotation values, where the vehicle was first turned slightly to the right and then turned to the left. then, from a time of 1.2 seconds, the vehicle was on a skid and turned significantly to the right corresponding to values greater than 30° s−1. as a consequence, from the moment of braking the vehicle, the recorded speed of the vehicle in the previous figure 3 cannot be considered valid, or rather the recorded speed corresponds to the speed of the wheels turning, but not to the speed of the vehicle as a whole. 95 daniel mikyska acta polytechnica ctu proceedings figure 4. table of precollision data 2 (trg 1). 3.2. trg 2 the second event related to the impact with the silver vehicle was recorded as a side collision and occurred 58ms after the previous event. this time would correspond to the damage locations on the left side of the silver vehicle, see figure 5, where the red color indicates the likely location of the first recording, i.e., trg 1, and the yellow color indicates the stronger impact, i.e., the likely location of recording trg 2. in terms of the nature of the overall damage to the left side, it is clear that the red vehicle was not moving directly against the side of the silver vehicle but was moving at a small angle relative to its longitudinal axis. the collision therefore took place with a significant slip, but only to the point of greatest deformation, i.e. the point of the second entry marked in yellow in the figure. the intensity of the first impact was 16 kmh−1 (delta v). in the second event, only the lateral acceleration values were recorded using sensors located on the b-pillar, c-pillar, and floor of the vehicle. the graph of lateral acceleration recorded by the floor sensor is best illustrated at figure 6. this is because it shows the overall pattern of acceleration changes over the recorded time. positive values represent acceleration from left to right, negative values from right to left. initially, a slight acceleration from right to left can be seen, which at time −20ms switched to acceleration from left to right. when the vehicle hits the other, the sensor accelerates to the left, corresponding to the impact of the right front corner so intensity of the second impact was with lateral acceleration −27.6ms−2. subsequently, this impact caused the red vehicle to turn. this then corresponds to the next graph progression, i.e. the acceleration of the right rotation of the vehicle. this has gradually decreased as a result of the vehicle being in a sled. figure 7 shows the probable corresponding collision position of the vehicles that triggered the recording of the second event (trg 2). 3.3. trg 3 and trg 4 the third and fourth incidents were related to a red vehicle that crashed into a van in the breakdown lane. figure 5. silver vehicle damage. figure 6. graph of lateral acceleration of the vehicle (sensor on the vehicle floor). the time difference between these events was one millisecond, so it can be said that it is one impact. 1.442 seconds elapsed between this impact and the previous impact. the red vehicle was hit by the left side, specifically the part at the level of the left rearview mirror. it hit the left rear corner of the van, see figure 8, which also corresponds to the damage on the left a-pillar of the red vehicle, which can be seen marked in green in figure 8. figure 9 shows the propable corresponding collision position of the vehicles that triggered recording of trg 3 and trg 4 the trg 3 event was recorded as a side impact. as far as the data itself is concerned, only the lateral acceleration of the vehicle was recorded, similar to the previous event. all the already mentioned lateral acceleration sensors recorded a very strong lateral 96 vol. 52/2025 traffic accident analysis using edr data figure 7. vehicle collision position trg 2. (a). damage to the left rear corner of the van. (b). damage to the red vehicle on the left rearview mirror. figure 8. damage of vehicles (trg 3 and trg 4). figure 9. vehicle collision position trg 3 and trg 4. impact, i.e. acceleration from left to right. this acceleration reached peak values of almost 280m s−2. the maximum value of lateral delta v recorded in trg 4 was 56.4 kmh−1. event trg 4 was recorded in its entirety, similar to event trg 1. data were recorded up to 4.6 seconds before the left side of the red vehicle struck the van. the fourth event is also related to the first recorded event in that an overlap of the recorded data can be observed. this means that the data recorded in event trg 4 at 4.6 seconds before the impact is identical to the data recorded in event trg 1 at 3.2 seconds before the impact with the silver vehicle in the righthand lane, or before the first recorded impact. it is important to re-emphasise that the recorded speeds are not valid as the vehicle was in a skid. furthermore, the impact speed of 20 kmh−1, that is, the speed at time 0.0 s, is not valid as it does not correspond to the lateral acceleration of the vehicle that was recorded in the event trg 3. 3.4. trg 5 the last recorded event occurred 172.5ms after the previous one and 1.6 seconds after the first one. it was recorded as a side impact that occurred as a result of the red vehicle rotating after striking the left side of the van. the corresponding vehicle damage that caused the recording can be seen in figure 11. in terms of the data itself, only the lateral acceleration values were recorded again. the values of all sensors showed an increasing and fluctuating lateral acceleration acting from left to right. floor sensor for the lateral acceleration captured maximum value 36,8m s−2. therefore, these were secondary impacts of the red vehicle on the van due to vehicle rotation. 4. summary in the context of this accident, the accident data from the edr recorder may have played an important role. the presence of the black vehicle was only mentioned in the statements of the accident participants. since there was no cctv footage available to confirm this 97 daniel mikyska acta polytechnica ctu proceedings figure 10. table of precollision data 1 (trg 4). (a). damage to the left side of the van. (b). damage to the left rear of the red vehicle. figure 11. damage of vehicle (trg 5). fact, accident data was the only corroboration possibility. data confirmed that the driver of the red vehicle had made an evasive manoeuvre consistent with avoiding a vehicle overtaking him from the right. furthermore, the data confirmed the occurrence of a skid and therefore the rotation of the vehicle. all data could be considered valid, except for the speed of the vehicle. this was valid only up to the moment of skidding and subsequent vehicle rotation. even so, the vehicle speed values before the collision, which could be considered valid, could have helped the expert as input data for the accident simulation. if the edr had been implemented in the silver vehicle travelling in the right lane, it could have helped more with the analysis of the accident event and possibly with the generation of the accident event simulation. 5. conclusions electronic data recorders (edrs) are a key tool for detailed accident analysis, and their use is becoming an essential standard in the automotive industry. the study showed the important role that edr data played in the investigation of a specific accident, where the recorded data could be used to accurately reconstruct the vehicle’s movement and analyse the driver’s reactions to the situation before the collision. for example, the data obtained confirmed an evasive manoeuvre that led the vehicle into the grass strip and subsequently skidded, which in turn led to a series of collisions. the mandatory implementation of edr in new vehicles opens new possibilities for even more accurate and comprehensive accident analysis. this technology not only facilitates investigations but can also serve as an important tool to improve road safety. in the future, the greater deployment of these recorders in combination with other advanced technologies such as autonomous driving systems or advanced driver assistance systems could significantly reduce the number of accidents and improve overall road safety. therefore, edr is becoming an invaluable source of data not only for forensic analysis, but also for research and development of traffic safety measures. however, it must 98 vol. 52/2025 traffic accident analysis using edr data be added that the validity of the data is not always 100% and it is necessary to find the right methods to proceed with the data reading and the interpretation of the data itself, so that the data are not used without considering all the facts that occurred during the accident process and that may have influenced the data recording itself. acknowledgements references [1] nhtsa. event data recorder. [2024-09-17]. https://www.nhtsa.gov/research-data/event-datarecorder [2] crash data group. edr explained. [2024-09-17]. https://crashdatagroup.com/pages/edr-explained [3] p. mitchell. black box 101: understanding event data recorders (edr). [2024-09-17]. https://www.pushormitchell.com/2020/11/blackbox-101-understanding-event-datarecorders-edr/ [4] exchange aaa. event data recorder. [2024-09-17]. https://exchange.aaa.com/automotive/automotivetrends/event-data-recorder/ [5] european parliament. technical development and implementation of event data recording in the road safety policy. [2024-09-17]. https: //www.europarl.europa.eu/regdata/etudes/stud/ 2014/529071/ipol_stu%282014%29529071_en.pdf [6] crash data group. the bosch cdr tool: key applications for insurance and law enforcement crash investigators. [2024-09-17]. https: //crashdatagroup.com/pages/the-bosch-cdr-tool [7] black box recovery. a brief history of event data recorders – in trains, planes, vessels & cars. [2024-09-17]. https://blackboxrecovery.com/blog/2018/1/24/abrief-history-ofevent-data-recorders-in-trainsplanes-vessels-cars [8] european commission. nařízení komise v přenesené pravomoci kterým se doplňuje nařízení evropského parlamentu a rady (eu) 2019/2144 [in czech; commission delegated regulation supplementing regulation (eu) 2019/2144 of the european parliament and of council], 2022. [2024-09-17]. https://ec.europa.eu/info/law/betterregulation/have-your-say/initiatives/12989bezpecnost-vozidel-technicke-pozadavky-azkusebni-postupy-pro-evropske-schvalenitypuzapisovacu-udaju-o-udalostech-edr-_cs [9] brno university of technology. archive institute of forensic engineering. 99 https://www.nhtsa.gov/research-data/event-data-recorder https://www.nhtsa.gov/research-data/event-data-recorder https://crashdatagroup.com/pages/edr-explained https://www.pushormitchell.com/2020/11/black-box-101-understanding-event-datarecorders-edr/ https://www.pushormitchell.com/2020/11/black-box-101-understanding-event-datarecorders-edr/ https://exchange.aaa.com/automotive/automotive-trends/event-data-recorder/ https://exchange.aaa.com/automotive/automotive-trends/event-data-recorder/ https://www.europarl.europa.eu/regdata/etudes/stud/2014/529071/ipol_stu%282014%29529071_en.pdf https://www.europarl.europa.eu/regdata/etudes/stud/2014/529071/ipol_stu%282014%29529071_en.pdf https://www.europarl.europa.eu/regdata/etudes/stud/2014/529071/ipol_stu%282014%29529071_en.pdf https://crashdatagroup.com/pages/the-bosch-cdr-tool https://crashdatagroup.com/pages/the-bosch-cdr-tool https://blackboxrecovery.com/blog/2018/1/24/a-brief-history-of event-data-recorders-in-trains-planes-vessels-cars https://blackboxrecovery.com/blog/2018/1/24/a-brief-history-of event-data-recorders-in-trains-planes-vessels-cars https://blackboxrecovery.com/blog/2018/1/24/a-brief-history-of event-data-recorders-in-trains-planes-vessels-cars https://ec.europa.eu/info/law/better-regulation/have-your-say/initiatives/12989bezpecnost-vozidel-technicke-pozadavky-a-zkusebni-postupy-pro-evropske-schvaleni-typuzapisovacu-udaju-o-udalostech-edr-_cs https://ec.europa.eu/info/law/better-regulation/have-your-say/initiatives/12989bezpecnost-vozidel-technicke-pozadavky-a-zkusebni-postupy-pro-evropske-schvaleni-typuzapisovacu-udaju-o-udalostech-edr-_cs https://ec.europa.eu/info/law/better-regulation/have-your-say/initiatives/12989bezpecnost-vozidel-technicke-pozadavky-a-zkusebni-postupy-pro-evropske-schvaleni-typuzapisovacu-udaju-o-udalostech-edr-_cs https://ec.europa.eu/info/law/better-regulation/have-your-say/initiatives/12989bezpecnost-vozidel-technicke-pozadavky-a-zkusebni-postupy-pro-evropske-schvaleni-typuzapisovacu-udaju-o-udalostech-edr-_cs https://ec.europa.eu/info/law/better-regulation/have-your-say/initiatives/12989bezpecnost-vozidel-technicke-pozadavky-a-zkusebni-postupy-pro-evropske-schvaleni-typuzapisovacu-udaju-o-udalostech-edr-_cs acta polytechnica ctu proceedings 52:94–99, 2025 1 introduction 2 traffic accident introduction 3 cdr data 3.1 trg 1 3.2 trg 2 3.3 trg 3 and trg 4 3.4 trg 5 4 summary 5 conclusions acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2025.52.0129 acta polytechnica ctu proceedings 52:129–135, 2025 © 2025 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague burnout in agile software development zdeněk vafek brno university of technology, institute of forensic engineering, purkyňova 464/118, 612 00 brno, czech republic correspondence: zdenek.vafek@vut.cz abstract. this article examines the relationship between personality traits measured by the hexaco model and the prevalence of burnout syndrome as measured by the maslach burnout inventory (mbi). the research was conducted on a sample of nine developers working in the it sector, who were divided into two groups according to project management methods – the agile scrum methodology and the traditional waterfall approach. the aim of the study was to determine whether the implementation of the scrum method can contribute to reducing the risk of burnout and how individual personality traits correlate with selected dimensions of burnout syndrome. the results show that individuals with higher extraversion and conscientiousness show lower levels of emotional exhaustion and depersonalization, while honesty-humility and agreeableness have less influence on burnout syndrome. at the same time, the introduction of the scrum methodology contributed to better work stress management and work organization in most cases. the results are limited by the small sample size, but they can provide valuable insights for further research as a pilot study. keywords: scrum, burnout syndrome, it, stress, hexaco, maslach burnout inventory, agile. 1. introduction in the current times, it is necessary to be able to adapt quickly to changes at work. at the same time, the work demands are elevating, as well as the overall pressure on the individual, which also increases the level of stress that the individual is exposed to. the increased stress level also affects the environment of the individual, thus the stress is transferred to the individual’s surroundings. as a result, the stress level has increased among working individuals and the general population [1]. recent psychological researches show that prolonged stress stimulation without recovery phases can lead to overload of the body. this disturbance in self-regulation has an impact on overall mental well-being, where chronic stress poses risks that are associated with physical, mental and psychosomatic disorders [2, 3]. this research focuses on two main questions: (1.) can the implementation of scrum reduce the risk of burnout in developers? (2.) how do personality traits according to the hexaco model correlate with the different dimensions of burnout syndrome? the aforementioned chronic stress can be a cause of burnout syndrome, whose underlying dimensions are exhaustion, cynicism and poor professional performance. individuals who suffer from burnout tend to be cynical about their work and are unable to perform adequately [4]. these aspects of burnout can further result in the long-term disengagement of an individual from work, reducing the capacity of the work team to do the job, which also results in financial losses to society [3]. the average financial loss due to stress in the us is approximately $300 billion per year [5]. the maslach burnout inventory (mbi) is the most used professional method for detecting burnout [6]. it consists of a total of 22 statements that fall into three categories (emotional exhaustion, depersonalization, personal satisfaction). the respondent answers questions from all three categories on a 0–6 scale. as a result, the individual is assigned to a category that rates the degree of burnout in each category as low, moderate, or high [7]. • emotional exhaustion – a feeling of emotional exhaustion and loss of energy due to work. • depersonalization – a cynical and insensitive attitude towards others that manifests itself in alienation and distance. • personal success – reduced perception of one’s own competence and success at work, leading to negative self-evaluation and feelings of ineffectiveness. due to the constant demands for change in the final product, an agile approach to project management has been adopted in recent years, which responds flexibly to client requirements. the most widely used agile method is the scrum method, which is used by approximately 63 % of agile teams [8]. the advantages of the scrum method are its focus on team collaboration and its iterative approach to project management [9]. the scrum method is built on three basic pillars called scrum teams, scrum events and scrum artifacts [10]. scrum artifacts provide workflow information for all team members and stakeholders. scrum artifacts include: 129 https://doi.org/10.14311/app.2025.52.0129 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en zdeněk vafek acta polytechnica ctu proceedings • product backlog – a list of all features, requirements, enhancements and fixes that must be completed to create the final product. this list is managed by the product owner. • sprint backlog the part of the product backlog that the development team chooses to implement during one iteration (sprint). these tasks are selected to meet the sprint goal. • increment the final product of each sprint that is functional and ready for deployment, while meeting the definition of done criteria. in addition to the artefacts, ceremonies are used here, primarily to ensure the rhythm of the work and to regularly assess progress. each ceremony has a given purpose, which aids planning, process improvement and overall team synchronization. • sprint planning – used to plan activities for the upcoming sprint. the entire team, including the product owner, selects tasks from the product backlog and creates a plan to complete each item. • daily scrum – a short meeting that takes place once per day. here, each member of the development team answers questions about what they have completed since the last daily scrum, what they are doing now, what they will continue with next, and if they have any problems. • backlog grooming – takes place approximately once a week and is used to improve backlog quality, prioritize tasks, estimate workload (assigning story points value) and split large tasks into smaller ones. • sprint review – used to present the increment to the stakeholder team. this step includes discussion on what has been completed, and feedback on the increment is obtained. • sprint retrospective – a meeting at the end of the sprint where the team evaluates their work and the whole team tries to identify what worked badly during the sprint, what needs to be improved and what worked well. a scrum team typically has up to 10 members and consists of: • the product owner (one person) – takes care of the product backlog, tries to maximize the value of the product, communicates with all stakeholders (stakeholders), and ensures that their feedback is discussed with the development team and also reflected in the priority of individual tasks [11]. • scrum master (one person) – ensures that the scrum method in the team is used correctly, removes obstacles that may hinder the achievement of the goal and coaches the team to help implement the requirements as efficiently as possible at the same time. scrum master is also in charge of organizing and moderating scrum events. • the development team – responsible for the implementation of the sprint backlog, for which they select the most appropriate tools and techniques at their discretion. the development team works together to ensure that the resulting increment meets the definition of done. one of the most important responsibilities is also to update the progress during the sprint. the basic unit for expressing task complexity here are story points, which take into account multiple factors such as task complexity, uncertainty or risk, amount of work, complexity and task scope. the fibonacci sequence is the most commonly used to determine the value of story points [12]. these story points are then used to plan the amount of work for one sprint. a common value for the number of story points for one developer is approximately 13 story points per sprint [11, 13]. the hexaco is a six-factor framework of human personality designed to measure six major dimensions of personality based on respondent self-assessment. the model examines personality in six dimensions that include social and emotional behavior as well as ethical and moral aspects, providing a comprehensive picture of personality in everyday situations. each dimension contains of several specific statements to which the respondent answers on a scale of 1 to 5 (strongly disagree to strongly agree). hexaco dimension [14]: • honesty-confidence – in the hexaco model it has been shown to be key to understanding our behavior towards money, power and relationships. those who score high on this scale avoid manipulation for personal gain and do not seek wealth or high social status. they are truthful and humble in relationships. on the other hand, those scoring low may be willing to break rules or use sycophancy to achieve material gain. • emotionality – emotionality is characterized by traits such as anxiety, sentimentality, or fearlessness. those scoring high on the emotionality dimension of the hexaco model seek emotional support and are empathic, often due to life stress and fear of danger. on the other hand, those with low scores are less sensitive to pain and stress, do not seek sharing their feelings, and tend to be emotionally distant. • extraversion – extraverts are sociable, but this trait is not the only one that describes this pole of the dimension. in addition, individuals who score high on this scale describe themselves as confident, active, talkative, energetic, cheerful, and optimistic. extraverts enjoy other people (company), like to be part of groups and various social gatherings, enjoy excitement, and keep a cheerful mind. • agreeableness – is characterized by content such as forgiveness, tolerance and equanimity versus irritability, anger and recklessness. on the agreeableness scale in the hexaco model, individuals with 130 vol. 52/2025 burnout in agile software development high scores are those who are willing to forgive and are careful in their assessment of others, which allows them to work well together. on the other hand, those with lower scores tend to confront people who have caused them harm, criticize the shortcomings of others, and hold fast to their opinions. • conscientiousness – those scoring high on this scale describe themselves as focused, ambitious, diligent, persistent, systematic, strong-willed, disciplined, reliable, punctual and orderly. these socially desirable characteristics are related to academic and occupational performance, but they also have a negative side when they exceed a tolerable level and manifest themselves as excessive orderliness, pedantry or workaholic behavior. low scorers describe themselves as careless, indifferent, inconsistent, pursuing their goals with little commitment. • openness to experience – the scale captures interests and level of interest in new experiences and impressions. individuals scoring high on this scale report a rich imagination and are more receptive to experiencing positive and negative emotions more than closed individuals. they take new ideas and unconventional values into consideration. they describe themselves as scientific, intellectual, gifted with imagination, willing to experiment, and interested in the arts. they are ready to critically reformulate existing norms and adopt new social, ethical and political values. they often behave unconventionally, try new ways of doing things and prefer change. the main parts of the hexaco that can be used in conjunction with the maslach burnout inventory are dimensions called emotionality and conscientiousness. at the same time, individuals with lower emotional stability and low conscientiousness might be expected to have a higher risk of emotional exhaustion. low agreeableness may be related to higher levels of depersonalization, and low extroversion may indicate less resilience to social stressors, thus a higher risk of burnout [15, 16]. 2. materials and methods the data collection for the research on the relationship between burnout syndrome and personality of the individual was conducted on a group of nine developers (eight male and one female) aged 24 to 35 years, who were selected from an it company in the usa. although the sample size is small (9 developers), it was chosen to allow for the possibility of conducting pilot research in the company. the timeframe for the implementation of scrum was approximately 3 months, which was sufficient for the first observation of changes. in the future, it would be useful to control for confounding variables such as developers’ previous experience with different project methodologies. seven of them had completed the maslach burnout inventory questionnaire before implementing the agile scrum methodology in their team, while the remaining two developers (developer_7 and developer_8) remained in the original waterfall approach to project management. all nine developers work remotely at the same company. the hexaco-60 questionnaire was used to measure personality traits and has high internal consistency with cronbach’s alpha > 0.80. to measure the level of burnout syndrome, the maslach burnout inventory was used, which has cronbach’s alpha > 0.74, indicating high internal consistency. the surveys were conducted using online questionnaires that were sent to the respondents. personal information about respondents was collected with the consent of the respondents, for the purpose of linking the resulting data; however, as soon as the responses were linked, the personal information was removed, and the data anonymized. evaluation of the questionnaire was done using excel software and subsequent data visualization was done using python library matplotlib.pyplot. pearson’s correlation coefficient (1) was used to calculate the correlation between variables, which is used for quantitative variables where a linear relationship is expected: r = n ( ∑ xy) − ( ∑ x) ( ∑ y)√[ n ∑ x2 − ( ∑ x)2 ] [ n ∑ y2 − ( ∑ y)2 ] , (1) where r is a correlation coefficient between −1 and 1 (−1 negative linear correlation, 0 no linear correlation, 1 positive linear correlation), n is the number of pairs of values,∑ xy is the sum of the products of the corresponding values of the variables x and y,∑ x and ∑ y are the sums of the values of the variables x and y,∑ x2 and ∑ y2 are the sums of the squares of the individual variables. a python script was used to calculate the pearson correlation coefficient using the pandas library. 3. results and discussion a total of nine developers participated in the questionnaire surveys, all of whom are from the same company where they work remotely. all of the developers, except developer_7 and developer_8 have recently implemented the scrum method to manage the project they are working on. the remaining developers have stuck with the waterfall method for project management. 3.1. hexaco for the survey using the hexaco questionnaire, one containing 60 questions were selected. the hexaco 131 zdeněk vafek acta polytechnica ctu proceedings honestyemotionality extraversion agreeconscienopenness to -humility ableness tiousness experience developer_1 1.9 4 4 3 2.2 2.2 developer_2 1.6 4 3.6 3.2 1.7 2.7 developer_3 2 3.1 2.3 1.9 1.2 1.4 developer_4 2.4 3 2.5 2.9 3 2.5 developer_5 1.5 3.8 3.2 2.2 1.9 3.1 developer_6 2.2 3.8 2.2 2.3 2.5 2.3 developer_7 1.9 1.9 4 3 1.4 2.7 developer_8 1.8 3.8 3.4 3.4 1.4 2.4 developer_9 1.6 3.1 2.3 1.9 1.8 1.7 table 1. hexaco results. questionnaire results are summarized in table 1. the distribution of scores is visualized in figure 1. honesty-humility. most developers tend to have lower scores, indicating a tendency to be honest, humble and fair. the highest scores appear for developers 4 and 6, but consistently this is lower than the average, therefore suggesting that the group has low tendencies towards manipulation and self-promotion. emotionality. this shows a higher range of scores ranging from 1.9 to 4. developers 1, 2, and 5 show high emotionality, indicating a higher susceptibility to stress, anxiety, or emotional sensitivity. developers 7 and 9, on the other hand, will be more resilient to stress and manage their emotions better. extraversion. scores range from 2.2 to 4. developers 1, 7 and 8 have higher scores, indicating their sociability and preference for a live environment. developers like 6 and 9, who have lower scores, may be more introverted. agreeableness. most scores are around 2–3, indicating a medium level of tolerance, kindness and cooperation. developer 3 has the lowest score, indicating that they may be more competitive or less willing to compromise in conflict situations. conscientiousness. developers are in the range of 1.2–2.5. developer 4 may exhibit more organization, reliability, and a strong work ethic compared to others. openness to experience. the scores here are mostly between 1.4 and 3.1. developer 5 has the highest score, reflecting creativity and a desire for new experiences, while developer 3 has the lowest score, indicating a preference for routine and traditional thinking. 3.2. maslach burnout inventory a questionnaire survey for burnout syndrome was conducted twice. the first time was on 16.08.2024, when the scrum method for managing the development team started to be used for all respondents except developers 7 and 8. developer_1 noted a significant improvement in emotional exhaustion and depersonalisation, which may be related to the introduction of scrum, which may have contributed to reduced stress and better organization of work. for developer_2, there was a slight improvement in emotional exhaustion, but depersonalization remains high. scrum may have contributed to stabilization here, although not to a significant reduction in stress. developer_3 remains stable, with no significant changes. scrum at this developer is unlikely to have had a significant impact on reducing burnout or depersonalization. in developer_4, there was a slight improvement in all areas, which could be related to better organization of work thanks to scrum. for developer_5, there was a slight increase in emotional exhaustion and depersonalization, suggesting that scrum did not have as positive an effect here as would be expected. despite the increase in depersonalization and emotional exhaustion, developer_6 remains in the lower levels, which may indicate that scrum has helped maintain better control over the workload. for developer_7, which does not use scrum, there was no improvement. on the contrary, the level of depersonalization increased and personal achievements decreased. the absence of scrum may be contributing to remaining at a high level of exhaustion. developer_8 also did not experience a significant improvement; on the contrary, there was an increase in emotional exhaustion. not using scrum could play a role here in maintaining higher stress levels. developer_9 shows stable results with a slight increase in emotional exhaustion, but the improvement in personal accomplishments suggests that scrum can contribute to better work efficiency. developers who started using scrum (all except developer_7 and developer_8) tended to see improvements in emotional exhaustion and depersonalization, or their results held steady. scrum probably helped to structure the work better and reduce workload. developer_7 and developer_8, who did not implement scrum, reported higher or consistently high levels of emotional exhaustion and depersonalization, suggesting that the absence of scrum may have contributed 132 vol. 52/2025 burnout in agile software development figure 1. hexaco results in standard distribution. to sustaining higher levels of stress. the results are consistent with the assumptions of the effect of scrum on psychological well-being, but the limitations of the small sample size need to be considered. future research should include a larger number of respondents and a longer time frame to more accurately measure the impact of different methodologies on burnout syndrome. 3.3. correlation of hexaco and maslach burnout inventory correlational analysis shows that extraversion and conscientiousness have a significant effect on changes in aspects of burnout, while honesty-humility and agreeableness play a lesser role in these relationships. although the results suggest a significant correlation between extroversion and lower levels of emotional exhaustion, it is important to interpret these correlations with caution given the small sample size. these findings are preliminary and further research is needed to confirm our hypotheses and compare our findings with previous studies on a similar topic. change in emotional exhaustion. strongly positively correlated with change in depersonalization (0.81). this means that an increase in emotional exhaustion is often related to an increase in depersonalization. it is negatively correlated with extraversion (−0.53) and agreeableness (−0.40). developers with higher extraversion and agreeableness tend to experience smaller increases in emotional exhaustion. a weaker negative correlation is also seen for conscientiousness (−0.24), suggesting that more conscientious people have a smaller increase in emotional exhaustion. depersonalization change (dp_change). again, it correlates strongly with emotional exhaustion (0.81), confirming the correlation between these two burnout factors. it is negatively correlated with extraversion (−0.40), suggesting that people with higher extraversion tend to have lower depersonalization. personal achievement change (pa_change). it is strongly positively correlated with conscientiousness (0.67), suggesting that developers who are more conscientious show a significant improvement in their sense of personal accomplishment. the negative correlation with extraversion (−0.47) and agreeableness (−0.54) suggests that more extroverted and agreeable individuals experience less of an increase in their sense of personal accomplishment. a positive but weaker correlation is seen with honesty-humility (0.31), suggesting that more humble individuals may feel a greater improvement in personal accomplishments. hexaco dimensions and changes in mbi. honesty-humility: slightly positively correlated with change in personal achievement (0.31), indicating that higher honesty may contribute to a better sense of personal achievement. 133 zdeněk vafek acta polytechnica ctu proceedings figure 2. maslach burnout inventory results. figure 3. correlation matrix heatmap. 134 vol. 52/2025 burnout in agile software development extraversion: strongly negatively correlated with emotional exhaustion (−0.53) and depersonalization (−0.40), suggesting that extraverted individuals tend to experience lower levels of these aspects of burnout. conscientiousness: is strongly positively correlated with personal achievement (0.67), suggesting that more conscientious individuals are better able to manage their workload and achieve a sense of accomplishment. 4. conclusions the research shows that personality traits measured by the hexaco model have an impact on the degree of burnout syndrome, with extraversion and conscientiousness being the most significant predictors. the introduction of the agile scrum methodology has proven to be an effective tool for reducing emotional exhaustion and depersonalization, suggesting a positive impact on developers’ overall mental well-being. however, this effect was not observed for all participants, especially for those who had not implemented scrum. thus, the results of the study support the implementation of agile project management methods as a preventive measure against burnout syndrome and highlight the importance of personality characteristics in coping with work stress. this study offers preliminary insights into the impact of scrum on burnout syndrome and the influence of personality traits on burnout. given the limited sample, future research should include a larger group of respondents and more detailed analysis over a longer time frame. acknowledgements references [1] j. siegrist, j. li. work stress and the development of chronic diseases. international journal of environmental 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[9] s. magistretti, d. trabucchi. agile-as-a-tool and agile-as-a-culture: a comprehensive review of agile approaches adopting contingency and configuration theories. review of managerial science 19:223–253, 2025. https://doi.org/10.1007/s11846-024-00745-1 [10] h. lei, f. ganjeizadeh, p. k. jayachandran, p. ozcan. a statistical analysis of the effects of scrum and kanban on software development projects. robotics and computer-integrated manufacturing 43:59–67, 2017. special issue: extended papers selected from faim 2014. https://doi.org/10.1016/j.rcim.2015.12.001 [11] k. schwaber, j. sutherland. the scrum guide, 2020. [2024-10-02]. https://scrumguides.org/docs/ scrumguide/v2020/2020-scrum-guide-us.pdf [12] j. sutherland. scrum: the art of doing twice the work in half the time. crown currency, new york, 2014. [13] d. maximini. the scrum culture. springer, cham, 2018. isbn 978-3-319-73841-3. https://doi.org/10.1007/978-3-319-73842-0 [14] m. c. ashton, k. lee. empirical, theoretical, and practical advantages of the hexaco model of personality structure. personality and social psychology review 11(2):150–166, 2007. https://doi.org/10.1177/1088868306294907 [15] a. s. almeneessier, s. a. azer. exploring the relationship between burnout and emotional intelligence among academics and clinicians at king saud university. bmc medical education 23:673, 2023. https://doi.org/10.1186/s12909-023-04604-7 [16] j. de la fuente, p. sander, a. garzón umerenkova, et al. the big five factors as differential predictors of self-regulation, achievement emotions, coping and health behavior in undergraduate students. bmc psychology 12:267, 2024. https://doi.org/10.1186/s40359-024-01768-9 135 https://doi.org/10.3390/ijerph15030536 https://doi.org/10.3389/fpsyg.2019.00284 https://doi.org/10.1016/j.psym.2012.09.001 https://doi.org/10.1002/wps.20311 https://www.uml.edu/research/cph-new/worker/stress-at-work/financial-costs.aspx https://www.uml.edu/research/cph-new/worker/stress-at-work/financial-costs.aspx https://doi.org/10.1016/s1479-3555(03)03003-8 https://doi.org/10.1002/job.4030020205 https://doi.org/10.1007/s11846-024-00745-1 https://doi.org/10.1016/j.rcim.2015.12.001 https://scrumguides.org/docs/scrumguide/v2020/2020-scrum-guide-us.pdf https://scrumguides.org/docs/scrumguide/v2020/2020-scrum-guide-us.pdf https://doi.org/10.1007/978-3-319-73842-0 https://doi.org/10.1177/1088868306294907 https://doi.org/10.1186/s12909-023-04604-7 https://doi.org/10.1186/s40359-024-01768-9 acta polytechnica ctu proceedings 52:129–135, 2025 1 introduction 2 materials and methods 3 results and discussion 3.1 hexaco 3.2 maslach burnout inventory 3.3 correlation of hexaco and maslach burnout inventory 4 conclusions acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2024.47.0089 acta polytechnica ctu proceedings 47:89–96, 2024 © 2024 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague self-compacting concrete with reduced formwork pressures jiří němečeka,∗, pavel trávníčeka, jan tichýb a czech technical university in prague, faculty of civil engineering, thákurova 7, 166 29 prague 6, czech republic b skanska a.s., křižíkova 682/34a, 186 00 prague 8, czech republic ∗ corresponding author: jiri.nemecek@fsv.cvut.cz abstract. self-compacting concrete (scc) is normally characterized with high pressures exerted on the formwork during casting. the pressures can easily exceed bearing capacity of a regular formwork when casting a high structural member in one step. the contribution shows new ways of reducing the pressures using mineral additives based on calcinated clay minerals and clay nanoparticles whose addition together with interrupted casting process leads to a substantial reduction of the formwork pressures. short interruptions in the casting lead to microstructural changes and flocculation, thixotropy and early strength evolution. the positive effects are studied systematically in microstructural studies performed on modified cement pastes using microscopy and viscosimetry. rheological behavior of standard sccs is improved towards thixotropy of the mixture by using the mineral additives. the newly developed recipes are tested on sccs used on real construction sites. examples of their practical utilization and on-site measurements show on up to 50 % pressure reductions in enriched sccs compared to ordinary sccs. keywords: self-compacting concrete, metakaolin, nanoclay, formwork pressure, thixotropy. 1. introduction the rheology of the fresh concrete has been largely studied primarily to control workability and flowability of the mixtures [1]. with the development of self-compacting concretes (scc) high mixture flowability with no need of external compaction has been achieved [2]. simultaneously, an increase in lateral pressures that act on formwork has been produced by scc. thus, reduction of the formwork pressures is especially important in the case of using scc for tall structures since the pressure that is much higher compared to ordinary concretes grows with formwork height when the casting process is uninterrupted. for an ideal fluid the pressure grows linearly with the height while for concrete it is reduced and depends on many parameters that can be categorized to the three groups: • material properties (such as binder type, water/binder ratio, aggregate type and shape, viscosity and thixotropy of the mixture), • placing characteristics (casting rate, holding periods, consistency, presence of reinforcement), • formwork characteristics (dimension and shape, surface, possible leakage). this paper focuses on the first category and will study the influence of two mixture modifiers. previous studies of the scc rheology showed that the flowability and fresh state strength is controlled mainly by the particle flocculation and thixotropy of the cement paste [3–5]. some additives can increase the floc size due to microscale agglomeration and rapid hydration [6]. such an additive can be reactive calcined clay, the metakaolin [7, 8]. other additives based on clay minerals have been proved to increase thixotropy of the mixture [9]. kawashima et al. [10] showed that clay-modified cement pastes exhibited high recovery rate after shear loading. this paper extends the work and studies microscale and rheological properties of the modified fresh cement pastes and the evolution of lateral pressures monitored on a real construction site. 2. methods at first, the cement pastes modified with selected modifiers were prepared and studied by means of electron microscopy to reveal additive’s shapes and their influence on the microstructure. fresh cement pastes were further studied by means of rotational viscosimetry to find the effect on rheological properties. as the main parameters, the static and dynamic yield strengths and thixotropy index were evaluated. each measurement was repeated three times and the results averaged. at second modified concrete mixtures were prepared and used in real conditions of a construction site. pressures exerted on the formwork by modified mixtures were monitored and compared to reference ones. 2.1. material selection and samples all cement paste samples series were made of blended portland cement cem-ii 42,5 r a-m (čížkovice) in 89 https://doi.org/10.14311/app.2024.47.0089 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en j. němeček, p. trávníček, j. tichý acta polytechnica ctu proceedings scc-o [kg] scc-10 % mk [kg] scc-0.5 % nc [kg] cem ii-42.5r a-m (čížkovice, cz) 340 306 338 water 195±7 195±7 185 agg. 8–16 mm (klecany, cz) 424 417 417 agg. 4–8 mm (zálezlice, cz) 298 292 292 agg. 0–4 mm (černuc, cz) 804 790 790 limestone filler (čertovy schody, cz) 210 210 210 stabilizer 3.8 4.2 3.81 viscosity modifier 1.0 0.3 1.0 plasticizer 2.5 4.5 2.5 metakaolin 34 nanoclay 1.7 mean spread (mm) 74 67 65 table 1. scc mixture components (weight per m3 of concrete) water to binder ratio 0.4. a reference paste (labeled as c-o) was prepared with no addition. in the second series 10 wt. % of cement was replaced by metakaolin (české lupkové závody, czech republic), labeled as c-mk series. chemical composition of the metakaolin was 53.1 % sio2, 41.7 % al2o3, 1.1 % fe2o3 and other minor oxides. in the third series, 0.5 wt. % of cement was replaced by nanoclay (c-nc series). pure magnesium silicate called sepiolite with the chemical formula mg4si6o15(oh)2.6h2o produced by sepiolsa, ltd., spain, was used. it was dispersed in part of the water and ultra-sonicated prior mixing to support its dispersion. fresh properties of the pastes immediately after mixing were evaluated thereafter. scc mixtures were prepared from the same ingredients complemented with silicious aggregate, limestone filler, high range water reducer (hrwr) based on polycarboxylic ethers (basf mastersure 900), viscosity modifier (basf mastermatrix sdc 180) and mixture stabilizer (basf mastersuna 6035) table 1. the mixtures were previously optimized in cooperation with construction company skanska, a.s. (czech republic). the table 1 presents mixture composition, the labeling is as follows: scc-o stands for no additive, scc-10 % mk stands for 10 % cement replacement by metakaolin, scc-0.5 % nc stands for 0.5 % nanoclay replacement. modification of the water and plasticizer content in scc mixtures was done to maintain comparable workability of the mixtures quantified by the inverted cone slump-flow tests (standard abrams cone 30 cm in height, 20/10 cm in diameter was used). the values of the spread are also summarized in table 1. 2.2. viscosimetry on cement pastes the description of the rheological behavior of mixtures was performed by means of viscosimetry. the aim of the viscometry measurements was to find the optimal dosage of ingredients that were selected for their potential to increase the thixotropy of the scc mixture. a study was carried out to measure the static and dynamic viscosities of modified cement pastes. rheological flow curves were measured with a viscotester iq rotary rheometer (haake viscotester iq) using coaxial cylinder geometry (couette geometry, cc38/ti/se). static yield strength, dynamic yield strength and thixotropic index were quantified from the flow curves. dynamic parameters were fitted with the herschley-buckley model (generalized non-newtonian fluid model) from the respective flow curve segments as τdyn = τ0 + kγ̇n (1) where: τ – shear stress, τ0 – dynamic yield strength, γ̇ – shear strain rate, k – consistency parameter, n – flow index. the recipes of all tested pastes were adjusted so that their spread after five taps on the haegerman shaking table was 20±0.5 cm. the mixture was prepared in a volume of about 1 liter and after 5 minutes of stirring a spread test was performed. if the specified spread value is met, the mixture is placed in the measuring cylinders of the viscometer and the first measurement, the static test, took place within 10 minutes. when the static test ended, the dynamic protocol was started immediately. both protocols are described in figure 1 below. measurements on a viscometer were repeated on the same mixture at 30 and 60 minutes. the mixture was mixed by hand before loading into the testing cylinder, a new batch of mixture from the prepared volume was added for each time period. the static protocol has a total of three linear segments, see figure 2a. in the first segment, the intact mixture is gradually loaded, and the static yield strength of the material, τstat (the first local maximum of the shear stress curve vs. the relative deformation) can be deduced from this part of the protocol. when the prescribed maximum speed of the load rotor is reached, the protocol passes to the second segment, in which a constant speed is maintained for 30 seconds. this part of the protocol aims to create a more significant deformation in the mixture, during which the 90 vol. 47/2024 self-compacting concrete with reduced formwork pressures (a). (b). (c). (d). figure 1. rheology protocols. (a). (b). (c). figure 2. sem micrograph of (left) metakaolin particles, (middle) dry nanoclay, (right) nanoclay after dispersion in water. stress drops. the final third segment prescribes deceleration of the rotor. these three segments thus form a closed working loop, the area of which is then interpreted as the thixotropy index, athix, of the mixture (figure 2b). the dynamic protocol consists of a total of eight segments. in the first step, the mixture is stirred at a high constant speed for 30 seconds, followed by stopping the rotor. the rotor is stopped for one second, during which the measured torque stabilizes up to zero. the following up/down segments on a logarithmic scale load the mixture in a total of three cycles (figure 2c). these cycles are then used to evaluate the dynamic yield strength from the last unloading segment, τdyn (figure 1d). 2.3. field tests in cooperation with skanska company, several real construction sites were selected, and measurements of lateral pressures conducted on reinforced concrete walls made from scc mixtures. the walls were instrumented with the lateral and vertical pressure sensors 91 j. němeček, p. trávníček, j. tichý acta polytechnica ctu proceedings figure 3. view on wall formwork (left), instrumentation at the bottom of the wall (middle), schematics of the wall cross section and position of built-in pressure sensors (right). figure 4. particle size distribution of (left) metakaolin particles and (right) nanoclay. placed at the bottom of the formwork. in all cases, the height of the walls was about 3 m, thickness about 25 cm and the length varied from 4–8 m. usual density of ordinary steel reinforcement was used in the walls. for the pressure, the wall height is the governing factor, so the walls were all comparable. the lateral view on the wall is presented in figure 3 – left. the pressure sensors were embedded in the formwork wall (0.17 from the bottom) in a special formwork piece (figure 3 – middle). both lateral and vertical pressures were monitored during the whole casting process (roughly 2 hours) and for the next 2 hours after. the casting process was prescribed so that always a 0.5 m layer of concrete was cast in once from one batch and then 10 mins break with no vibration was held. the process repeated in the cycles up to filling the full wall height (figure 3 – right). the rest time in the casting led to possible development of thixotropy in the mixtures and decreasing of lateral pressures exerted on the formwork. 3. results 3.1. microscopy and particle size it was found that both additives have wide distribution of the particles in the dry (undispersed) state. clearly visible grains of metakaolin with approximate size of 1– 10 µm are shown in figure 2. particle size distribution measured by laser diffraction is depicted in figure 4. the mean particle size is 2.3 µm. nanoclay particles are strongly agglomerated in the dry state as shown in figure 2. water dispersion and ultrasonication leads to disjoining of the fibrous particles as seen in figure 2. clearly, the fibers are characterized by several micrometer length and nanometric diameter. such particles with high aspect ratio can hardly be quantified by laser diffraction. it seems that the graph shown in figure 4 (right) shows mainly the length distribution of the nanoclay fibers. the peak particle size in this distribution is 5.6 µm. in both cases, the additives have smaller average size compared to cement grains. scanning electron microscopy (sem) imaging of the samples did not reveal any significant differences between the samples (figure 5). slightly less portion of portlandite zones is visible in c-mk samples as a possible consequence of portlandite-metakaolin reaction. no further rigorous analysis was performed. 92 vol. 47/2024 self-compacting concrete with reduced formwork pressures figure 5. sem images of the samples (left to right: c-o, c-mk, c-nc). 4. viscosimetry the comparison of the results from static and dynamic tests for the studied mixtures is shown in figures 6 and 7, respectively. apparently, the static yield strength increases dramatically for both modified mixtures compared to a reference. it means, that the modified mixtures have a much higher resistance to static shearing and their strength is higher when the mixtures are in rest. on the other hand, the differences in the dynamic strength are much less pronounced and the reference and modified mixture behave very similarly. the dynamic measurement thus indicates that when the mixtures are in motion (during mixing and flowing in the formwork space), the viscosity appears very similar. however, when they are brought to a rest, they require a significantly different amount of input energy to overcome the static yield strength. the related average break through stress is approximately 90 pa for cement alone, 112 pa for metakaolin and 132 pa for nanoclay modified paste. this difference represents +24.2 % to +46.3 % increase of the yield strength due to the addition of the modifiers. the comparison of the mean values from all tests is presented in table 2. τstat [pa] τdyn [pa] athix [pa] c-o 89.95±12.64 19.19±1.63 306.77±38.20 c-mk 111.68±22.34 21.29±0.67 383.75±92.65 c-nc 131.66±9.91 29.59±3.96 431.20±75.87 table 2. results of viscosimetry. effect of nanoclay or metakaolin on static yield strength at the 60 minutes age of paste is apparent from figure 6. when the mixture is already in motion, the yield stress from the presented mixtures follows rather similar path, which is also observed in the figure 7 where the dynamic yield strength is very similar. the thixotropy index increases with the addition of the modifiers by +25.1 % for metakaolin and +40.6 % for nanoclay addition, see table 2 for exact values. figure 6. examples of viscosimetry flow curves from static test. figure 7. examples of viscosimetry flow curves from dynamic test (last segment highlighted). this again favors using the additives for the reduction of the formwork pressures. 4.1. field tests on standard walls an important test was conducted on real scc mixtures modified by aforementioned additives. it was proved by the study on cement pastes that both metakaolin and nanoclay increase static yield strength and thixotropy and have a high potential for reduction of lateral pressures. results of the field measurements are depicted in figure 8. the increasing saw-like course of the pressure curve corresponds to the casting process done in steps with the break between the 93 j. němeček, p. trávníček, j. tichý acta polytechnica ctu proceedings figure 8. lateral pressures exerted on the formwork in field tests of sccs. cast layers. the stress increases by steps as the wall layers are built. a stress drop can be observed during each cycle and is more pronounced in later stages (cycles 4–6) and for modified mixtures. the evolution of relative differences between the lateral pressures exerted on the formwork by individual mixtures is shown in figure 9. it must be noted that the lateral pressure is highly correlated with the casting method, that is the dropping height of concrete during casting, casted layer thickness, casting speed, duration of breaks between layers, vibrations, and deformation of a formwork. these variables were therefore kept as identical as possible. the wall thickness and reinforcement of the tested walls were also reasonably close and are assumed not to be the factor for a difference in recorded pressures. the main factors influencing the lateral pressures in the formwork were found the introduction of the breaks between casting the layers and also material selection including the mixture spread. a high correlation of the pressure with the mixture spread has been observed. the lower spread the lower lateral pressure can be anticipated. mixtures denoted in the table 1. have been tested on walls of similar height (about 3.2 m) and the recorded pressure is depicted in the figure 8. saw-like pattern of increasing pressure can be observed for all mixtures, but the steps of modified mixtures are gradually diminishing in size up to the point where another increase in load from a new layer isn’t observable. lower average spread lead to a significant lateral pressure reduction for both mixtures by nearly 50 % and 41 % for the metakaolin and nanoclay respectively. because of minor differences in layer height, the pressure after reaching each layer was normalized by the currently achieved height, see figure 9. the lower curve shows a more effective mixture in terms of the lateral pressure reduction (i.e. scc-10 % mk and scc-0.5 % nc). our previously tested standard walls (in section 3) were all about 3.2 m tall and cast in comparable conditions. since the effect of dimensions and casting figure 9. relative differences in lateral pressures. process is of high importance and could distort the efficiency of the additive, another field trial was carried out on a much higher wall of about 8 m height (figure 10). the mixture with the same ingredients specified in table 1 but only with 5 % metakaolin cement replacement was chosen in this specific case and labeled as scc-5 % mk-8 m. the consistency of the mixture was sf2 with the spread 700 to 750 mm. comparison with a standard wall made of the same mixture (scc-5 % mk-3.2 m) was performed on the same site. some differences compared to previous casting of standard walls occurred: • lower layers were cast with significantly higher energy (drop of the mixture from a higher height), • smaller wall layers were laid (0.4–0.5 m), • the waiting periods were shortened to 5–7 min, • the mixture had a higher spread compared to scc10 % mk and scc-0.5 % nc. figure 10. a 8 m high wall made from scc-5 % mk mixture. for the tall wall, the peak lateral pressure was reached later than in a standard wall, after the 7th layer (in 3.2 m). following 11 layers (from 3.2 m to 7.8 m) did not cause an increase of pressure in the bottom layer. a comparison of lateral pressures also with the reference mixture is shown in figure 11. it is evident that the casting process has a tremendous effect on the pressures if we compare the scc-5 % 94 vol. 47/2024 self-compacting concrete with reduced formwork pressures figure 11. lateral pressures exerted on the formwork in field tests of 8 m high wall and comparison to other mixtures. mk-3.2 m and scc-5 % mk-8 m results. maximum pressure on a high wall was moved to the 7th layer and is by 100 % higher compared to the pressure in a standard wall. it might be a consequence of the concrete flow movements that are higher on a higher, layers are done in smaller time intervals and the concrete is poured from a larger height. still the mk additive works well, and the pressures are below the reference pressure exerted by the unmodified mixture on a standard wall (-18 %), figure 11. it could be anticipated that if the casting parameters were the same as in case of standard walls, the pressure reduction would be higher. more tests are needed to prove this. as a side benefit, the wall from scc-5 % mk mixture exhibited high visual quality, was free of defects and met the architect’s requirements (figure 10). 5. conclusions three types of scc mixtures (a reference, with added metakaolin and with nanoclay) were formulated and results of their performance in terms of lateral pressures exerted on the formwork presented. the additives were dosed as 10 % and 0.5 wt. % cement replacement in case of metakaolin and nanoclay, respectively. the dosage was found to be optimal based on previous viscosimetry measurements done on cement pastes. the mixtures were tested in real construction sites conditions and formwork pressures monitored. the binding phase (cement paste) was tested separately in the fresh state by viscosimetry. results show that both metakaolin and nanoclay increase static yield strength. this average increase was quantified as 24.2 % and 46.3 %, respectively. on the other hand, the ability of the pastes to flow under vibration is only slightly influenced by the additives and the dynamic flow properties exhibit similar values for all mixtures. thus, the measurement indirectly proves the thixotropy effects for both additives. also, the thixotropy index evaluated from static tests show that the additives increase the average thixotropy by 25.1 % and 40.6 %, respectively. the casting process on selected reinforced concrete walls (3.2 m in height) was prescribed with 10 mins interlayer breaks (with exception for a tall wall due to time constraints, figure 10.) to allow development of thixotropy effects in the tested mixtures of sccs. while consistency (spread in the cone-flow test) of the mixtures was maintained on a similar level (by addition of superplasticizers) lateral pressures of modified mixtures measured at the bottom of the formwork exhibited much lower values. absolute values of the pressures depend, among other factors, on the structure height and casting schedule. in our case, most of variables were kept as similar as possible. the lateral pressure decrease was compared based on normalized values. in the monitored examples, the metakaolin modified mixture showed 50 % pressure reduction, the nanoclay modified mixture exhibited 41 % reduction. the study proved a large potential of both additives to effectively reduce lateral pressures developed on formwork as follows from both laboratory and field tests. a single test was carried out on a high wall (8 m) with one mixture and different casting process parameters. although, the pressure reduction was not as significant as in case of standard walls mainly due to different casting conditions. still 18 % pressure reduction was achieved compared to a reference mixture. acknowledgements financial support of the technology agency of the czech republic (project trend fw01010521) is gratefully acknowledged. references [1] d. jiao, c. shi, q. yuan, et al. effect of constituents on rheological properties of fresh concrete-a review. cement and concrete composites 83:146–159, 2017. https: //doi.org/10.1016/j.cemconcomp.2017.07.016 [2] m. ouchi. self-compacting concrete. journal of advanced concrete technology 1(1):5–15, 2003. https://doi.org/10.3151/jact.1.5 [3] y. vanhove, c. djelal. prediction of the lateral pressure exerted by self-compacting concrete on formwork. magazine of concrete research 56(1):55–62, 2004. https://doi.org/10.1680/macr.2004.56.1.55 [4] j. wallevik. thixotropic investigation on cement paste: experimental and numerical approach. journal of non-newtonian fluid mechanics 132(1-3):86–99, 2005. https://doi.org/10.1016/j.jnnfm.2005.10.007 [5] n. roussel. a thixotropy model for fresh fluid concretes: theory, validation and applications. cement and concrete research 36(10):1797–1806, 2006. https://doi.org/10.1016/j.cemconres.2006.05.025 [6] k. khayat, j. assaad. use of thixotropy-enhancing agent to reduce formwork pressure exerted by self-consolidating concrete. aci materials journal 105(1):88–96, 2008. https://doi.org/10.14359/19211 95 https://doi.org/10.1016/j.cemconcomp.2017.07.016 https://doi.org/10.1016/j.cemconcomp.2017.07.016 https://doi.org/10.3151/jact.1.5 https://doi.org/10.1680/macr.2004.56.1.55 https://doi.org/10.1016/j.jnnfm.2005.10.007 https://doi.org/10.1016/j.cemconres.2006.05.025 https://doi.org/10.14359/19211 j. němeček, p. trávníček, j. tichý acta polytechnica ctu proceedings [7] j. h. kim, m. beacraft, s. p. shah. effect of mineral admixtures on formwork pressure of self-consolidating concrete. cement and concrete composites 32(9):665–671, 2010. https: //doi.org/10.1016/j.cemconcomp.2010.07.018 [8] s. wild, j. khatib. portlandite consumption in metakaolin cement pastes and mortars. cement and concrete research 27(1):137–146, 1997. https://doi.org/10.1016/s0008-8846(96)00187-1 [9] s. kawashima, j. h. kim, d. j. corr, s. p. shah. study of the mechanisms underlying the fresh-state response of cementitious materials modified with nanoclays. construction and building materials 36:749–757, 2012. https: //doi.org/10.1016/j.conbuildmat.2012.06.057 [10] s. kawashima, m. chaouche, d. j. corr, s. p. shah. rate of thixotropic rebuilding of cement pastes modified with highly purified attapulgite clays. cement and concrete research 53:112–118, 2013. https://doi.org/10.1016/j.cemconres.2013.05.019 96 https://doi.org/10.1016/j.cemconcomp.2010.07.018 https://doi.org/10.1016/j.cemconcomp.2010.07.018 https://doi.org/10.1016/s0008-8846(96)00187-1 https://doi.org/10.1016/j.conbuildmat.2012.06.057 https://doi.org/10.1016/j.conbuildmat.2012.06.057 https://doi.org/10.1016/j.cemconres.2013.05.019 acta polytechnica ctu proceedings 47:89–96, 2024 1 introduction 2 methods 2.1 material selection and samples 2.2 viscosimetry on cement pastes 2.3 field tests 3 results 3.1 microscopy and particle size 4 viscosimetry 4.1 field tests on standard walls 5 conclusions acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2025.53.0013 acta polytechnica ctu proceedings 53:13–16, 2025 © 2025 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague hybrid reinforcement of trc from flax and ar glass jakub hájeka,∗, matthew schwarzkopfb, marica mikuljanb, tomáš vlachc a czech technical university in prague, university centre for energy efficient buildings, třinecká 1024, 273 43 buštěhrad, czech republic b innorenew coe, livade 6, 6310 izola, slovenia c czech technical university in prague, faculty of civil engineering, thákurova 7, 166 29 prague 6, czech republic ∗ corresponding author: jakub.hajek@cvut.cz abstract. this article focusses on the possible use of hybrid reinforcement from inorganic and organic materials in a thin concrete plates, such as textile-reinforced concrete. organic materials, such as flax, jute, or sisal, have high tensile strengths. young’s modulus is, on the other hand, problematic, so the combination with inorganic textiles goes hand in hand. natural fibres are renewable materials, making them an environmentally friendly material, so using them as a partial substitution of inorganic materials would make concrete reinforcement more sustainable. in this article, four-point tests of hybrid reinforced concrete plates with alkali-resistant glass and flax are presented. in addition, a hybrid reinforcement cross-sectional microscopy was done to evaluate the impregnation with epoxy resin. keywords: natural materials, natural fibres, alternative reinforcement, hybrid reinforcement, textilereinforced concrete, flax, flexure test. 1. introduction faced with climate change, we need to make our society more sustainable. carbon dioxide emission needs to be reduced, even if we are talking about concrete reinforcements. alternative reinforcements, such as inorganic textiles (carbon, ar-glass, basalt), helped to invent a textile-reinforced concrete (trc). trc consists of fine-grained concrete and noncorrosive reinforcements (rovings), which enables one to make as thin samples as few centimetres. however, we can push this composite material even further, and inorganic textiles can be substituted with organic textiles such as flax, jute, hemp, or sisal. these materials are renewable, compostable, inexpensive, and can be produced sustainably, so their environmental impact is low [1–3]. on the other hand, these characteristics make them not durable in an alkaline and wet environment, so some kind of protection is needed (impregnation or coating) [4–6]. today, they are already used in polymer composites for applications in the automobile or aviation industry [2]. a lot of research has been done on the mechanical characteristics of natural fibres. they have a low density of around 12 000–15 000 kg m−3, tensile strength of up to 1 000 mpa and even young’s modulus is promising, as high as 70 gpa for flax, which is equal to ar-glass values [7, 8]. on the other hand, when all fibres are combined into a roving, the values of young’s modulus are drastically lower. due to homogenisation with epoxy resin, it is possible to achieve usable values for use in concrete [9]. to evaluate the potential substitution of inorganic reinforcement, five types of concrete plates were designed for the four-point flexural test. hybrid reinforcement was created from ar-glass and flax, and its composition was checked under the microcope. 2. preparation of samples five different samples were prepared for the four-point flexural test of hybrid reinforcement in a concrete. all samples had dimensions 100 × 20 × 40 mm and were reinforced with four rovings. there were five types of reinforcement: ar-glass as reference, combination of ar-glass and one flax, ar-glass with two flaxes, ar-glass with three flaxes and five flaxes. the material characteristics of the yarns are shown in table 1. hobbybeton by baumit was used for the concrete [10]. it has declared strength 25/30 and the largest grain is up to 4 mm, which is convenient for such small samples. for the impregnation and homogenisation of rovings, lamepox 23 epoxy resin was used. it has a tensile strength of 81 mpa and young’s modulus is 3.3 gpa [11]. t ρr ar [tex] [g cm−3] mm2 flax 460 0.46 1.00 ar-glass 2 400 2.54 0.95 table 1. material characteristic of yarns. the reinforcement was made by combining selected yarns together. they were twisted for better manipulation. all rovings were submerged into epoxy resin 13 https://doi.org/10.14311/app.2025.53.0013 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en j. hájek, m. schwarzkopf, m. mikuljan, t. vlach acta polytechnica ctu proceedings and then stretched on a frame. in the end, the rovings were sprinkled with fine-grained silica sand to ensure better bond with the concrete matrix. figure 1. homogenised hybrid rovings from ar glass and flax with epoxy resin and sprinkled with sand (top) and concreted samples in a framework (bottom). hardened rovings were cut into desired lengths to put them in a framework. the reinforcement was pressed with a board at one end and stretched with a rubber band with a crocodile at the other end to ensure that it will stay stretched during the concreting and that it will have the desired cover layer and position (figure 1). after the concreting, the samples were covered with a foil to prevent water evaporation, and the next day they were put in a climatized chamber. after 28 days, the samples were tested by the four-point bending test. with the samples, reference prisms 40×40×160 mm were also concreted to verify the material characteristics of the concrete mixture. 3. experiment and results the experiment was carried out on a zwick/roell z100 hydraulic press. setup and a broken specimen is seen in figure 2. the samples were tested by a four-point bending test with a crossbar speed of 2 mm min−1. force and displacement were recorded. in the table 2, the results of the measurements are presented. it can be seen, that the more flax yarns were added, the more strength decreases. this figure 2. four-point bending test in process (top). broken specimen. the arrow indicates the cuts of the rovings (bottom). is probably because the young modulus of ar-glass is higher than the flax’s. f σmax [n] [mpa] hybrid_x4_1glass 1 929.50 17.08 hybrid_x4_1glass+1flax 1 376.61 13.31 hybrid_x4_1glass+2flax 1 537.11 13.75 hybrid_x4_1glass+3flax 1 272.25 11.22 hybrid_x4_5flax 846.66 7.40 table 2. measured strengths of plates reinforced with four hybrid rovings. f is the maximal measured strength. σmax is the stress on a ideal cross-section without cracks. in figure 3, the chart of the measurements can be seen. after the first peak, which indicates the flexural strength of the concrete, all samples show a multiple crack behaviour with strengthening. this ductile behaviour is desirable because the structure shows it is collapsing, but not at sudden. at first glance, the ’g’ sample with only glass rovings is the strongest. the other samples show smaller drops after the peaks, indicating that the crack width is smaller than at the glass specimens. probably, it is due to the diameter of the reinforcement. thicker reinforcement can withstand higher contact stress. some of the samples have small peaks in the chart. it is probably caused by the epoxy resin, which can 14 vol. 53/2025 hybrid reinforcement of trc from flax and ar glass figure 3. force-displacement chart of representative samples of plates reinforced with hybrid rovings. the number next to ’flax’ and ’glass’ corresponds to the number of yarns used. be too weak to withstand loads, so it started to crack slowly and with that the rovings were slightly pulled out. the tests were accompanied with periodic sounds, which strengthen this conclusion. another feature that could have had an impact was the twisting of the samples. it seems that the rovings were twisted too much because with the twisting, the yarns are shortened and then when the force is applied in tension, they tend to untwist themselves. reference concrete prisms were tested first by a three-point bending test and then in compression. the results were consistent in all batches of specimens. the average compression strength was 44.75 ± 4.44 mpa and the flexural strength was 7.29 ± 0.47 mpa. in addition, some of the reinforcements were placed in epoxy resin and polished to obtain a good image under a vhx-6000 microscope. these images are seen in figure 4 with a close-up on the bottom, where individual fibrils can be seen. it shows us that the reinforcements were properly impregnated and that both materials are perfectly bonded without cavities or spaces. 4. conclusion five different hybrid reinforcements combined from glass and flax were tested in thin plates of concrete. the four-point bending test was performed on plates figure 4. the cross section of 1glass+1flax roving. with dimensions of 100 × 20 × 400 mm. the results showed these conclusions: • twisting of yarns should not be done in an excessive 15 j. hájek, m. schwarzkopf, m. mikuljan, t. vlach acta polytechnica ctu proceedings amount to reduce the phenomenon of ’untwisting’ during the experiment. • the combination of ar-glass and flax leads to lower strengths but also lower crack widths. • the combination and impregnation of different yarns did not show any flaws in the cross section under the microscope. acknowledgements the work on this article was supported by czech science foundation grant no. 22-14942k entitled ’possibilities of using natural fibers for the production of hybrid textile reinforcement in concrete’. the authors would like to acknowledge all financial assistance provided to support this research. references [1] s. joshi, l. drzal, a. mohanty, s. arora. are natural fiber composites environmentally superior to glass fiber reinforced composites? composites part a: applied science and manufacturing 35(3):371–376, 2004. https://doi.org/10.1016/j.compositesa.2003.09.016 [2] r. k. malviya, r. k. singh, r. purohit, r. sinha. natural fibre reinforced composite materials: environmentally better life cycle assessment – a case study. materials today: proceedings 26:3157–3160, 2020. https://doi.org/10.1016/j.matpr.2020.02.651 [3] t. rousakis. natural fibre rebar cementitious composites. in advanced high strength natural fibre composites in construction, p. 215–234. elsevier, 2017. https: //doi.org/10.1016/b978-0-08-100411-1.00009-1 [4] j. a. halip, l. s. hua, z. ashaari, et al. effect of treatment on water absorption behavior of natural fiber–reinforced polymer composites. in mechanical and physical testing of biocomposites, fibre-reinforced composites and hybrid composites, p. 141–156. elsevier, 2019. https: //doi.org/10.1016/b978-0-08-102292-4.00008-4 [5] j. wei, c. meyer. degradation mechanisms of natural fiber in the matrix of cement composites. cement and concrete research 73:1–16, 2015. https://doi.org/10.1016/j.cemconres.2015.02.019 [6] m. torres-arellano, v. renteria-rodríguez, e. franco-urquiza. mechanical properties of naturalfiber-reinforced biobased epoxy resins manufactured by resin infusion process. polymers 12(12):2841, 2020. https://doi.org/10.3390/polym12122841 [7] t. väisänen, a. haapala, r. lappalainen, l. tomppo. utilization of agricultural and forest industry waste and residues in natural fiber-polymer composites: a review. waste management 54:62–73, 2016. https://doi.org/10.1016/j.wasman.2016.04.037 [8] l. yan, b. kasal, l. huang. a review of recent research on the use of cellulosic fibres, their fibre fabric reinforced cementitious, geo-polymer and polymer composites in civil engineering. composites part b: engineering 92:94–132, 2016. https://doi.org/10.1016/j.compositesb.2016.02.002 [9] j. hájek, t. vlach, j. řepka, v. žďára. verification of material characteristic of natural fibers for concrete reinforcement. acta polytechnica ctu proceedings 47:42–46, 2024. https://doi.org/10.14311/app.2024.47.0042 [10] hobby beton. [2024-08-29]. https://www.merkur.si/hobbybeton/ [11] lamepox 23 epoxy laminating resin 1230 g. [2024-08-29]. https://www.samsonkamnik.si/lamepox_ 16 https://doi.org/10.1016/j.compositesa.2003.09.016 https://doi.org/10.1016/j.matpr.2020.02.651 https://doi.org/10.1016/b978-0-08-100411-1.00009-1 https://doi.org/10.1016/b978-0-08-100411-1.00009-1 https://doi.org/10.1016/b978-0-08-102292-4.00008-4 https://doi.org/10.1016/b978-0-08-102292-4.00008-4 https://doi.org/10.1016/j.cemconres.2015.02.019 https://doi.org/10.3390/polym12122841 https://doi.org/10.1016/j.wasman.2016.04.037 https://doi.org/10.1016/j.compositesb.2016.02.002 https://doi.org/10.14311/app.2024.47.0042 https://www.merkur.si/hobbybeton/ https://www.samsonkamnik.si/lamepox_ acta polytechnica ctu proceedings 53:13–16, 2025 1 introduction 2 preparation of samples 3 experiment and results 4 conclusion acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2025.53.0024 acta polytechnica ctu proceedings 53:24–27, 2025 © 2025 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague effect of hydrogen embrittlement on mechanical properties of non-stressed prestressing steel tomáš chobotský∗, martin kubový, andrea milerová, adam čítek czech technical university in prague, klokner institute, šolínova 7, 160 00 praha, czech republic ∗ corresponding author: tomas.chobotsky@cvut.cz abstract. the aim of this paper is to elucidate the effect of hydrogen on the structure of commonly used prestressed concrete steel strand (pc strand) of concrete bridges and other structures in the non-stressed state to determine the general behaviour of the characteristic steel material, which would lead to further investigation of specimens in the prestressed state. for the experimental part of the research, a system was set up for saturating the samples with hydrogen, which diffuses into the steel structure in atomic form. these specimens were then subjected to tensile testing according to en iso 15630-3 to reveal the reduced mechanical properties of the prestressed concrete steel strand specimen caused by the diffusion of hydrogen into the material structure. after the tensile tests, the fracture surfaces were analysed on a scanning electron microscope to verify the effect of hydrogen. keywords: prestressing steel, diffusion, hydrogen embrittlement, corrosion, ductility. 1. introduction the phenomenon of hydrogen embrittlement has been known since the last century and until recently was an often overlooked problem in high strength materials such as prestressed concrete steel strand in concrete structures. also very important is the knowledge that material failure generally occurs with a delay. speaking about prestressed concrete steel strand, should be mentioned that it is a structural element reaching ultimate tensile strength values of around 1 900 mpa (yield strength of around 1 640 mpa), i.e. a very strong steel material, which according to the aforementioned theories, should be very susceptible to hydrogen embrittlement. it is a tensile steel rope twisted from seven wires, which is supposed to bind all the segments and create a prestress in the concrete structure. using this type of bridge structure reinforcement, it is possible to create noticeably larger spans in the reinforced concrete structure that cannot be realized with conventional bundled profiled reinforcement. it should be noted that the motivation behind the hydrogen embrittlement theory on prestressed concrete steel strand is the uncertain explanation of the causes of collapse of several bridge structures in europe and asia. the collapse of a structure is usually a combination of several adverse factors, like a electrochemical corrosion, and it is possible that hydrogen embrittlement may be one of them. it is necessary to discuss the synergistic effect of this corrosion damage mechanism together with the action of supercritical amounts of chloride anions from the spreading salts (localized pitting corrosion damage). 2. experimental part in the experimental part, the susceptibility of commonly used prestressed concrete steel strand to hydrogen embrittlement was investigated. the pc strand samples were exposed in aqueous sodium sulfate solution in the framework of cathodic surface polarization under hydrogen evolution. in the experimental part, the susceptibility of commonly used prestressed concrete steel strand to hydrogen embrittlement was investigated. the pc strand samples were exposed in aqueous sodium sulfate solution in the framework of cathodic surface polarization under hydrogen evolution. by this electrolytic decomposition of water for predetermined intervals and after a subsequent minor technological delay, the samples were subjected to tensile testing. the pc strands were connected as cathode in the system (the anode was a stainless steel mesh of the corresponding surface area), thus hydrogen was generated on the samples, during the time of current passage, which could freely diffuse into the steel structure. the following sections describe the course of the experiment [1, 2]. chemical composition of prestressing steel c mn si p s n 0.83 0.73 0.26 max. 0.008 max. 0.008 max. 0.004 table 1. chemical composition of samples. 3. exposure of samples a system of experimental design was proposed to provide the necessary values of current densities for water decomposition. the diagram shows a block diagram of a direct current power source intended to supply the electrode system used for sample exposure. 24 https://doi.org/10.14311/app.2025.53.0024 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 53/2025 effect of hydrogen embrittlement on mechanical properties figure 1. wiring diagram of the electrode system. the required effective dc value for this electrode system was determined to be 18 a at an effective voltage of 10 v. this corresponds to an impedance value of the electrode system of approximately 0.5 ω. this value assumes the use of an electrolyte in the form of a saturated aqueous sodium sulphate solution. the rectifier circuit is powered from a regulated ac voltage source from 0 to 240 v with a mains frequency of 50 hz. the regulated voltage is fed to the input of a toroidal transformer with a nominal conversion of 240 v/24 v and an output of 400 v a. the output voltage of the transformer is connected to the ac input of a rectifying bridge (graetz bridge) designed to rectify currents up to 40 a. the development of hydrogen on the exposed surface of the reinforcement is ensured by the acceleration of the corrosion kinetics under the reduction of hydrogen from water, see equation 1, due to the cathodic corrosion reaction in the cathodic polarization mode of the causative surface. the actual cathodic polarization takes place in the galvanostatic regime [1, 3]. 2 h2o + 2 e− −−→ h2 + 2 oh− (1) due to cathodic polarization, by wiring the exposed sample of the prestressing steel as the (–) pole in the implementation of the electrolyzer circuit, the corrosion of the steel surface is accelerated under reducing conditions for the development of hydrogen gas with a significant partial pressure of the gas over the free surface of the exposed sample. if there was no polarization of the reinforcement steel surface due to the involvement of a dc current source, the surface of the reinforcement steel would corrode gradually in an aqueous environment under a cathodic corrosion reaction, the reduction of atmospheric oxygen, see equation 2. 2 h2o + 4 e− + o2 −−→ 4 oh− (2) 4. the course of the exposition in the experiment, a box of polymer material of optimal dimensions is used, where holes were drilled in the sides of the centres to accommodate the prestressing reinforcement specimen (exposure of only the central part of the 1.7 m long pc strand). an aisi 316l stainless steel mesh was inserted into the sides of the plastic box up to the walls. this pair of fine meshes served as a sacrificial anode within the electrolyser wiring. the actual surface pattern of the prestressing steel was routed between these meshes. from previously obtained experimental data related to the cathodic protection of steel pipelines placed in soil, it is clear that the supercritical current density, when hydrogen is formed on the steel surface, is approximately 100 ma cm−2 depending on the type of electrolyte. on this basis, a stationary current density of 120 ma cm−2 was chosen for this experiment, which guarantees a sufficiently significant acceleration of the kinetics of atomic hydrogen formation on the surface of the reinforcement steel [3–5]. figure 2. formation of hydrogen on the sample surface. 5. tensile test after the pc strands were finished and removed from the bath, the samples had to be immediately subjected to tensile testing based on the standard en iso 156303 for testing steel for reinforcement and prestressing, i.e. for a duration of at least 30 s. the fact is that hydrogen, which diffuses into the steel structure during water decomposition, tends to escape very quickly, especially at elevated temperatures. at this point, there is a discrepancy among experts as to the rate at which hydrogen escapes from the structure. however, it is safe to say that the faster the tensile test is performed, the more hydrogen remains in the structure. in addition, it is also necessary to discuss the local 25 t. chobotský, m. kubový, a. milerová, a. čítek acta polytechnica ctu proceedings alkalization (equation 1) of the prestressing steel surface on the diffusion of hydrogen gas into the material structure. the samples were divided into two groups according to the length of exposure and the third group was the samples without hydrogen as a reference. group a was exposed for 20 minutes and group b was exposed for 60 minutes. the longer time is unjustified according to the articles of foreign authors, because after a certain time the same amount of hydrogen diffuses into the structure as escapes from the structure, and therefore there is no increase in the amount of hydrogen in the structure. the measured data were processed into a stressstrain diagram and from them the ultimate strenght rm, the contractual yield strength rp0.1 and rp0.2, the ductility agt and the elastic modulus e were derived. the elastic modulus was calculated by interlacing a straight line by the least squares method through the linear region of the stress-strain diagram in the range 300 to 1 350 mpa [6–8]. figure 3. comparison of mean tensile strenght of each group of specimens. figure 4. comparison of mean ductility of each group of specimens. the above figures 3 and 4 show that there is no decrease in ultimate strength and no change in ductility compared to the reference samples, which would confirm the presence and influence of hydrogen on the structure and mechanical properties of the tested samples. table 2 also shows the other observed values that do not show any influence of hydrogen on the structure [8, 9]. group rm rp0.1 rp0.2 a e [mpa] [mpa] [mpa] [%] [gpa] ref 1 913.73 1 723.22 1 757.69 4.38 203.81 a 1 906.13 1 708.29 1 752.91 4.38 209.35 b 1 911.14 1 705.01 1 752.68 4.21 205.91 table 2. mean mechanical properties of each group of specimens. 6. analysis of fracture surfaces after the tensile test, individual wires that were broken during the test were removed from the specimens. typically, there was one, at most two, broken wires out of seven on each specimen. in no specimen were all seven wires in the pc strand broken. these wires were then subjected to fracture surface analysis on an electron microscope. the fractographic analysis of the fracture surfaces on the reference samples and groups a and b did not reveal even minimal indications of characteristic brittle fracture facets initiated by the presence of hydrogen in the structure. in the majority of cases, the fractures were of the ductile fracture pattern characteristic of high-strength steels. shear failure was observed on three samples across the groups, but with the same mechanical properties as other specimens. fractography group sample fracture surface ref 1 ductile with hole 2 ductile 3 shear 4 ductile 5 ductile a 1 ductile 2 ductile with hole 3 ductile 4 shear 5 ductile b 1 ductile 2 ductile 3 ductile 4 ductile 5 shear table 3. type of fracture surface. no areas with indications of brittle fractures were found during the inspection of the areas under the microscope. after evaluation of all images, it can be assumed that in these cases hydrogen had no effect on the fracture behaviour [1, 2, 6, 7]. 26 vol. 53/2025 effect of hydrogen embrittlement on mechanical properties figure 5. fracture surface of sample b3. figure 6. detail of surface crack. 7. conclusion even though this is a high-strength material probably susceptible to hydrogen embrittlement, it cannot be said from the above data that hydrogen would cause significant hydrogen embrittlement of the material. the fundamental finding remains that most of the specimens cracked after tensile testing outside the exposed zone, i.e. outside the middle zone, where preferential failure should occur. this phenomenon leads to the conclusion that there are random defects in the material structure from the manufacturing process, e.g. from melting or subsequent forming, which have a much greater influence on the strength of the structure than hydrogen diffusion and subsequent embrittlement. this assertion cannot be confirmed in any definite way by the above experiment and its results, but it seems to be a plausible explanation for this phenomenon. in the light of previous scientific work, the possible influence of the corrosion products formed (the natural protective passive layer of steel in an alkaline environment) on the majorite-based fe3o4 magnetite, which even in an amorphous very thin layer are able to block very effectively the entry of hydrogen into the steel, can be discussed. the reason for this is the very low diffusion coefficient for atomic hydrogen through the fe3o4 crystal lattice. this fact may have occurred in the case of this experiment as well. research on the effect of hydrogen embrittlement on prestressing steel will continue in the future, as not all possibilities of specimen exposure have yet been tested to explain the phenomena of excentric cracking. acknowledgements this research was supported by the czech science foundation under grant no. 24-12783s. references [1] s. p. lynch. hydrogen embrittlement (he) phenomena and mechanisms. in stress corrosion cracking, p. 90–130. elsevier, 2011. https://doi.org/10.1533/9780857093769.1.90 [2] k. shiozawa, y. morii, s. nishino, l. lu. subsurface crack initiation and propagation mechanism in high-strength steel in a very high cycle fatigue regime. international journal of fatigue 28(11):1521–1532, 2006. https://doi.org/10.1016/j.ijfatigue.2005.08.015 [3] l. m. quej-aké, r. galván-martínez, a. contrerascuevas. electrochemical and tension tests behavior of api 5l x60 pipeline steel in a simulated soil solution. materials science forum 755:153–161, 2013. https: //doi.org/10.4028/www.scientific.net/msf.755.153 [4] b. yanachkov, y. mourdjeva, k. valuiska, et al. effect of hydrogen content on the microstructure, mechanical properties, and fracture mechanism of low-carbon lath martensite steel. metals 14(12):1340, 2024. https://doi.org/10.3390/met14121340 [5] p. pokorný, t. chobotský, n. prodanovic, et al. bond strength and corrosion protection properties of hot-dip galvanized prestressing reinforcement in normalstrength concrete. journal of composites science 8(10):407, 2024. https://doi.org/10.3390/jcs8100407 [6] h. birnbaum, p. sofronis. hydrogen-enhanced localized plasticity – a mechanism for hydrogen-related fracture. materials science and engineering: a 176(1–2):191–202, 1994. https://doi.org/10.1016/0921-5093(94)90975-x [7] a. s. kholtobina, r. pippan, l. romaner, et al. hydrogen trapping in bcc iron. materials 13(10):2288, 2020. https://doi.org/10.3390/ma13102288 [8] a. miyamoto, k. tei, h. nakamura, j. w. bull. behavior of prestressed beam strengthened with external tendons. journal of structural engineering 126(9):1033–1044, 2000. https://doi.org/10.1061/ (asce)0733-9445(2000)126:9(1033) [9] j. a. morales, j. torres, n. rebolledo, j. sánchez. experimental and statistical analysis of the corrosion in tendons in contact with water. frontiers in materials 6, 2019. https://doi.org/10.3389/fmats.2019.00167 27 https://doi.org/10.1533/9780857093769.1.90 https://doi.org/10.1016/j.ijfatigue.2005.08.015 https://doi.org/10.4028/www.scientific.net/msf.755.153 https://doi.org/10.4028/www.scientific.net/msf.755.153 https://doi.org/10.3390/met14121340 https://doi.org/10.3390/jcs8100407 https://doi.org/10.1016/0921-5093(94)90975-x https://doi.org/10.3390/ma13102288 https://doi.org/10.1061/(asce)0733-9445(2000)126:9(1033) https://doi.org/10.1061/(asce)0733-9445(2000)126:9(1033) https://doi.org/10.3389/fmats.2019.00167 acta polytechnica ctu proceedings 53:24–27, 2025 1 introduction 2 experimental part 3 exposure of samples 4 the course of the exposition 5 tensile test 6 analysis of fracture surfaces 7 conclusion acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2025.52.0008 acta polytechnica ctu proceedings 52:8–16, 2025 © 2025 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague implementation of a hydrogen fuel cell in a small urban vehicle michal cenkner∗, přemysl toman, josef svoboda czech technical university in prague, faculty of transportation sciences, department of vehicle technology, konviktská 20, 110 00 prague, czech republic ∗ corresponding author: cenknmic@cvut.cz abstract. the transition to sustainable urban mobility is a key priority for europe as countries seek to reduce greenhouse gas emissions and combat climate change. electric vehicles (evs) have become a popular trend offering a cleaner alternative to traditional combustion engines. however, despite their advantages, evs face challenges related to the difficult extraction of raw materials for battery production, energy storage and long charging times. hydrogen fuel cells (hfcs) are another possible alternative, offering longer range and shorter refueling times while maintaining zero local emissions. this paper discusses the integration of hydrogen fuel cell technology into a small urban vehicle and evaluates its potential in terms of sufficient power and efficiency. the study focuses on the design and implementation of the fuel cell system in a specific prototype vehicle, with the biggest challenge being the placement of the components and definition of sufficient power of fuel cell system integrated as range extender. keywords: hydrogen fuel cell, urban vehicles, modular micro car, package study. 1. introduction in recent decades, europe has focused heavily on ecological transformation to respond to climate change, pollution, and dependence on fossil fuels (european commission, 2021). an important part of this strategy is the green deal for europe, which aims to achieve climate neutrality by 2050. a key element of this process is the transformation of urban mobility, which accounts for a significant share of greenhouse gas emissions. overall, transport accounts for one-fifth of emissions, but three-quarters of this is attributed to road transport [1]. for this reason, small vehicles in cities, such as electric bikes, scooters, and smaller electric vehicles, are growing in importance [2], mostly in the regions that are prominent in ev sales. this brings a reduction in emissions [3] and optimization of transport. with the growth of small vehicles, which are becoming an important tool for reducing emissions and improving transport efficiency in cities, comes the issue of supporting them, for example, by building infrastructure and introducing special zones [4]. these vehicles not only bring lower energy consumption and better use of parking spaces, but also reduce noise pollution and improve the quality of life of residents [5]. electric vehicles are another key part of decarbonizing transport, although they face challenges such as limited range and the need for extensive charging infrastructure. yet, they are developing rapidly in europe, thanks, in particular, to support from governments and the european union. an example is the project to cover the ten-t road network in europe with charging stations [4]. another option is, for example, a method of motivating the driver to efficient driving through gamification of the human-machine interface [6, 7]. however, just the choice of powertrain can also influence driver’s behavior, where a different vehicle behavior can encourage more dynamic driving [8]. in addition to electric vehicles, hydrogen vehicles, which use hydrogen as a fuel and produce only water vapor during operation, are beginning to gain ground. hydrogen technology is promising, but it faces challenges such as insufficient infrastructure and the high costs of hydrogen production from renewable sources [5, 9]. a very interesting comparison [10] of the environmental impacts of using conventional electricity as part of the energy mix shows that hydrogen vehicles are even less environmentally friendly than conventional combustion vehicles, however they can be more ecologic if clean wind or solar electricity is used. an example of a hydrogen vehicle is the toyota mirai, which is an effort to make practical use of hydrogen in passenger transport [11]. this car uses a fuel cell that converts hydrogen into electricity, allowing emission-free operation. another interesting example of the incorporation of a hydrogen fuel cell into an urban vehicle is the rasa from riversimple. this vehicle is aimed at maximum efficiency, lowest possible driving resistance, and very low weight. the rasa utilizes supercapacitors (shown in figure 1) for acceleration and storing recovered energy from braking. as the car slows down, this electricity floods into the supercapacitors and is send back to the motors. hydrogen vehicles can become an important part of urban mobility, especially due to their rapid refueling and possibility of longer range, which is advantageous for, for example, taxis or public transport [3]. how8 https://doi.org/10.14311/app.2025.52.0008 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 52/2025 implementation of a hydrogen fuel cell in a small urban vehicle model hydrogen consumption [kg 100 km−1] toyota mirai 0,79 hyundai nexo 0,95 (data from manufacturer) honda clarity fuel cell 0,84 (data from manufacturer) table 1. hydrogen consumption of mass-produced hydrogen cars. figure 1. package of the rasa vehicle [12]. ever, this effort still requires a lot of research and development, but the results of the push can be seen by comparing two studies from 2020 [11] and 2023 [5]. at the same time, european cities are already experimenting with hydrogen buses and other means of transport that can contribute to climate goals. comparing the hydrogen consumption of several mass-produced hydrogen cars shows different levels of efficiency, with consumption depending on vehicle type, technology, and fuel cell specification. for example, the toyota mirai has a consumption of approximately 0.79 kg of hydrogen per 100 km according to the combined wltp driving cycle (data from manufacturer). however, the record consumption of this vehicle was 0.55 kg per 100 km (data from manufacturer). the hyundai nexo, a hydrogen suv, and the honda clarity fuel cell, a discontinued model, were compared. the results are shown in table 1. these values show that hydrogen cars have similar fuel economy. the biggest differences appear depending on the aerodynamics, weight, and overall design of the vehicle. an advantage of using hydrogen is its high energy density compared to batteries [13]. hydrogen can be integrated into a vehicle’s powertrain in several ways. the simplest is the conventional combustion of hydrogen in a conventional ice [9]. this can be achieved by dispersing low-temperature hydrogen in the manifold, by using liquid hydrogen, which is injected directly into the cylinder after dispersion, or by adding it to gasoline. burning the hydrogen in combustion engine is however comparatively ineffective approach. therefore, the direction that has been chosen for this application, and fits with the idea of integration possibility to bev, is to integrate hydrogen fuel cells as range extender. their function is to convert energy from the electrochemical reaction of hydrogen and oxygen. the two main types of fuel cells are alkaline membrane fuel cells (amfc) and proton exchange membrane fuel cells (pemfc). pemfcs are more suitable for automotive applications because of their faster power rise and lower operating temperature. our main goal is to use an already available fuel cell of a suitable type and test its performance characteristics and suitability for use in a small urban vehicle. the aim is to verify both the feasibility of incorporating the solution into the l7 vehicle platform and the overall potential of using the vehicle in a suitable driving mode. 2. methods 2.1. car platform description in the context of this paper, our focus will be on evaluating the suitability of the implementation of a hydrogen fuel cell into a modular platform for a small urban electric vehicle. for evaluation, a research platform called evgen (figure 2) will be used that allows integration of different types of propulsion for testing purposes. this vehicle was originally designed as a bev (battery electric vehicle) of the l7 category, namely heavy quadricycles, and is therefore limited to a maximum weight of 450 kg and a maximum power output of 11 kw. the electric motor is located at the front with the front wheel drive. 2.2. powertrain design the entire proposed system architecture is shown in figure 3 where the most important blocks (for the purposes of our paper) are marked in green. red arrows indicate dc (ac) current flow. to find a suitable fuel cell power the only one size of the hydrogen tank will be used with capacity for a 1 kg of hydrogen compressed to 350 bar. in order to use the fuel cell as a range extender, a pem (proton exchange membrane) fuel cell type was chosen mainly because of its compact dimensions, suitability for automotive applications and good power-to-weight ratio (see figure 4). another advantage is the possibility of use in a wide range of temperatures (up to 80° c). the limitation of this type of fuel cell is the necessity to humidify the polymer membrane with water and the high requirements for chemical purity of hydrogen and air [14]. another key component in the schema is the dcdc converter, which is used to regulate and manage generated electrical power. the output voltage of a fuel cell is not constant and depends on the load, 9 m. cenkner, p. toman, j. svoboda acta polytechnica ctu proceedings figure 2. modular vehicle called evgen. visualization on the right and vehicle frame on the left. figure 3. powertrain schema. figure 4. assembly of the components of a single pem fuel stack [15]. 10 vol. 52/2025 implementation of a hydrogen fuel cell in a small urban vehicle pemfc units fc25 fc50 fc75 fc90 fc93 fc95 fc100 fc108 simplified models number of cells [-] 25 50 75 90 93 95 100 108 efficiency continuous [-] 0.7 0.7 0.7 0.7 0.7 0.7 0.7 0.7 power continuous raw [kw] 1.5 3 4.5 5.4 5.58 5.7 6 6.48 efficient power [kw] 1.05 2.1 3.15 3.78 3.906 3.99 4.2 4.536 voltage [v] 13-25 25-50 28-75 45-90 47-93 48-95 50-100 54-108 h2 consumption [g h−1] 108 216 324 388.8 401.76 410.4 432 466.56 operating time (1 kg h2) [hour] 9.26 4.63 3.09 2.57 2.49 2.44 2.31 2.14 table 2. pemfc simplified model properties 1/2. pemfc units fc110 fc112 fc116 fc125 fc132 fc150 fc175 fc200 simplified models number of cells [-] 110 112 116 125 132 150 175 200 efficiency continuous [-] 0.7 0.7 0.7 0.7 0.7 0.7 0.7 0.7 power continuous raw [kw] 6.6 6.72 6.96 7.5 7.92 9 10.5 12 efficient power [kw] 4.62 4.704 4.872 5.25 5.544 6.3 7.35 8.4 voltage [v] 55-110 56-112 58-116 63-125 66-132 75-150 88-175 100-200 h2 consumption [g h−1] 475.2 483.84 501.12 540 570.24 648 756 864 operating time (1kg h2) [hour] 2.10 2.07 2.00 1.85 1.75 1.54 1.32 1.16 table 3. pemfc simplified model properties 2/2. high voltage battery pack cells units lifepo4 li-on lipol 3.2v 105ah 3.7v 58ah 3.7v 24ah high voltage battery box rated voltage [v] 96 96.2 96.2 voltage cell max [v] 3.7 4.2 4.2 voltage cell cut-off [v] 2.5 2.75 3 voltage cell rated [v] 3.2 3.7 3.7 continuous cell discharge current [a] 105 58 200 max discharge cell current [a] 210 384 360 cell weight [g] 2000 946 430 capacity/weight ratio [a hr kg−1] 52.5 61.3 55.8 battery box configuration [-] 30s1p 26s1p 26s1p battery box capacity [kwhr] 10.1 5.6 2.3 battery box cells weight [kg] 60 24.596 11.18 table 4. three possible variants of hv battery. temperature, and other factors. a dc-dc converter helps regulate the voltage to provide a voltage required for downstream components, mainly the high voltage (hv) battery and the electric motor [16, 17]. 2.3. powertrain simulation this chapter describes the approach to define a hydrogen fuel cell with sufficient power in combination with minimized battery pack capacity versus the conventional bev solution. for these purposes, we created simplified models of pemfcs and battery packs. the possible fuel cells (fcs) are shown in tables 2 and 3. the examined vehicle is equipped with lifepo4 battery cells, as shown in the table below. one of the goals of the fuel cell integration powertrain simulation assessment is to minimize battery pack. for this purpose, we created 3 possible variants of such a solution compatible with our vehicle platform (see table 4). the simulation model was created in the realis simulation software. the one-dimensional powertrain model consists of bev fitted with pemfc as range extender. the basic parameters of the examined vehicle are listed in the table 5. the powertrain was examined under the wltc class 3 drive cycle. in every simulation scenario, the pemfc is activated when the battery reaches soc 80%. this simulation part takes into account the fact that pemfc need non-negligible amount of time for start. the pemfc is used as a range extender constantly supporting bev battery pack. this control strategy was defined to define the optimal size of the pemfc. using more complex control strategy allow to define wider group of suitable pemfc system sizes. in this article, we operate with simplified pemfc systems variants, and the results aim to sufficient variants definition even with basic control strategy. the 11 m. cenkner, p. toman, j. svoboda acta polytechnica ctu proceedings powertrain model basic parameters units lifepo4 3.2v 105ah soc initial [%] 0.99 frontal area [m2] 2.1 wheel radius [m] 0.3 rolling resistance coef. [-] 0.015 max power [kw] 20 mass (vehicle + rider) [kg] 600 cx [-] 0.4 overall transmission ratio [-] 9 max torque [nm] 50 transmission efficiency [-] 0,9 table 5. basic parameters of the examined vehicle for simulation purposes. figure 5. examined vehicle performance in context of wltc class 3 drive cycle. simulation ends when the battery reaches soc 5%. in scenarios with bev+pemfc model counts with battery cells lipol 3.7v 24ah and bev simulation takes into account all variants listed in table 4. 3. results in this chapter, we present results of series of powertrain simulations of bev and bev equipped with pemfc as range extender. first figure 5 reflects the vehicle performance under wltc class 3 test cycle. we observe that our vehicle reaches maximal velocity 95 kmh−1. the powertrain simulation assessed various pemfc integrations. the following figures 6a–6c present the overall mileage of the vehicle and the mileage when the pemfc systems stop when all the hydrogen is depleted. the maximum mileage in the tested scenarios is 117.6 km. according to the proper usage of hydrogen in tanks figure 7 shows the proper usage towards driving performance. the blue highlighted section of the figure presents pemfc sizes with insufficient power. in this testing section, values represent how much hydrogen was used and the rest of hydrogen stays in the tank. the green highlighted area defines the optimal size of pmefc according to the parameters of the examined vehicle in the context of wltc class 3 drive cycle. in this area, all the hydrogen is used for driving performance and fuel cell sufficiently supports the vehicle battery. the last orange highlighted area of the figure represents testing scenarios with pemfcs with higher than needed power according to our defined basic and simplified pemfc control strategy. the values in this section represent the amount of hydrogen used for driving performance. the rest of hydrogen was consumed into losses. the following figures 8 to 10 presents the output of three integrated pemfc systems – 75, 108 and 200 cells. first example shows the situation when the battery soc reaches 5% earlier than the hydrogen is consumed. the second example presents the situation where all the hydrogen is consumed while driving the vehicle. when the pemfc system stops due to running out of hydrogen, the battery is charged at soc level 80.1%. the third example describes the situation where the soc while driving the vehicle with activated pemfc system reaches 100%. the rest of the power is under defined pemfc control strategy directed into losses. in the end, the powertrain simulations were compared with the performance of the bev variants. the first variant was the same battery size as in the simu12 vol. 52/2025 implementation of a hydrogen fuel cell in a small urban vehicle (a). overall mileage of the whole powertrain system. (b). mileage when the pemfc system stops. (c). vehicle battery pack soc when the pmefc system stops operation. figure 6. configurations with different numbers of cells. figure 7. proper usage of hydrogen and pmefc systems for driving performance. lation with pmefc. the mileage in this variant was 20.5 km. the second variant simulation test was conducted with a midsize battery with lion nmc 58 a.hr cells. in this variant, the mileage reached 51.8 km. in the last test we performed simulation with original lifepo4 105 a.hr cell battery pack with resulting mileage of 97.7 km. 4. discussion in this article, various scenarios of integration pemfc system as range extender into small urban bev were assessed. performed vehicle powertrain simulation reflects wltc class 3 drive cycle as presented in figure 5. the goal of the simulation process was to define a sufficient size of the pmefc stack in the context of a defined pmefc control strategy. all simulations were performed with the same amount of 1 kg hydrogen in the tank. this amount of hydrogen was defined due to compatibility with current hydrogen refill stations that are often constructed with the limitation of minimal 1 kg hydrogen refueling. the method described in this article proved the potential of definition of such sufficient fuel cell system power. figures 6a and 6b show the effect of the pemfc system sizes in the context of vehicle mileage. using this approach, we are able to define sizes of pmefc systems suitable for examined small urban vehicle – sizes fc98, 100 and 108. in more detail in this narrower group of systems in figure 7 we 13 m. cenkner, p. toman, j. svoboda acta polytechnica ctu proceedings figure 8. powertrain simulation output with pemfc – 75 cells. figure 9. powertrain simulation output with pemfc – 108 cells. figure 10. powertrain simulation output with pemfc – 200 cells. highlighted area indicates losses in the system. 14 vol. 52/2025 implementation of a hydrogen fuel cell in a small urban vehicle observe differences of soc when the fuel cell system stops operation (see figure 6c). this fact is strongly affected in this approach with the defined drive cycle – for example fc 132 in figure 6c which indicates that the soc of the battery was significantly lower at the termination of pemfc than in the other – adjacent cases (only 80% soc). this situation is due to the fact that the pemfc termination occurred at a time when the battery was under more load, depending on the position within the driving cycle – here it was the end of a motorway section. figures 6a, 6b and 7 also show trends in context of pemfc power – too low and even too high-power leads to inefficient operation. figures 8 to 10 brings the closer look on the behavior of the battery system while driving in terms of soc. as the whole pemfc system was designed as a range extender, the battery pack itself is able to provide sufficient power for the vehicle. increasing the sizes of the pemfc systems extend the mileage of the entire powertrain system until the suboptimal size is reached. in detail, we observe that the sufficient power of the fuel cell system is able to keep battery soc in constant zone, e.g. between 50% and 90%. if the fuel cell system has higher than sufficient power, it handles the battery soc in a narrower range between e.g. soc 90% and 100%. because while driving under defined drive cycle the is in certain situation low demand for power the rest of the fuel cell system power is directed into losses. this negative effect could be eliminated with a more complex pemfc control strategy. however, the smaller size of the pmefc is sufficient and provides the same overall mileage, because the same amount of hydrogen is used. the main differences that would occur using higher than sufficient pmefc system power would lead to different battery soc when the fuel cell system stops operating because based on control strategy the vehicle would occur in different parts of the drive cycle. comparing the potential of pemfc system integration with described small urban bev concept, we observe the difference in mileage 117.6 km with optimal fuel cell size and 97.7 km with original battery. integrating the fuel cell system, the battery capacity was reduced to 23% from 10.1 kw.hr to 2.3 kw.hr. this reduction represents a weight reduction of 50 kg in the case of battery cells. however, the weight of the fuel cell and all other necessary components of the system must be taken into account, so the resulting weight savings may be insignificant. this is another parameter that should be considered while choosing the right size fuel systems from predefined cells using approach assessed in this article. in terms of package study, the figure 11 presents possibility of fuel cell system integration into current concept considering real dimensions of all subsystems. figure 11. raw 3d package model of the implemented powertrain. 5. conclusion in this paper, the implementation of the hydrogen fuel cell system in a small urban vehicle was analyzed from the point of view of dimensioning the power of the required hydrogen fuel cell, which is used in the powertrain as a range extender. using a 1d multiparametric powertrain simulation within a standardized wltc class 3 cycle, the suitable fuel cell power was found and then compared to a pure battery electric vehicle. based on the results of the simulations, a raw 3d package model was also created, where the simplified geometry of all essential components of the powertrain system is implemented. acknowledgements this research was supported with the czech technical university in prague grant sgs23/131/ohk2/2t/16 development of a modular urban vehicle research platform for testing alternative energy sources and hmi user solutions. references [1] h. ritchie. cars, planes, trains: where do co2 emissions from transport come from? ourworldindataorg 2020. [2024-10-08]. https://ourworldindata.org/ co2-emissions-from-transport [2] s. sagaria, a. moreira, f. margarido, p. baptista. from microcars to heavy-duty vehicles: vehicle performance comparison of battery and fuel cell electric vehicles. vehicles 3(4):691–720, 2021. https://doi.org/10.3390/vehicles3040041 [3] k. lieutenant, a. v. borissova, m. mustafa, et al. comparison of “zero emission” vehicles with petrol and hybrid cars in terms of total co2 release – a case study for romania, poland, norway and germany. energies 15(21):7988, 2022. https://doi.org/10.3390/en15217988 [4] e. m. szumska. electric vehicle charging infrastructure along highways in the eu. energies 16(2):895, 2023. https://doi.org/10.3390/en16020895 15 https://ourworldindata.org/co2-emissions-from-transport https://ourworldindata.org/co2-emissions-from-transport https://doi.org/10.3390/vehicles3040041 https://doi.org/10.3390/en15217988 https://doi.org/10.3390/en16020895 m. cenkner, p. toman, j. svoboda acta polytechnica ctu proceedings [5] m. a. aminudin, s. k. kamarudin, b. h. lim, et al. an overview: current progress on hydrogen fuel cell vehicles. international journal of hydrogen energy 48(11):4371–4388, 2023. https://doi.org/10.1016/j.ijhydene.2022.10.156 [6] m. cenkner, j. svoboda, p. toman, p. bouchner. adaptation of the vehicle simulator on bev and hev, focused on the impact studies of new user interfaces on drivers. in 2023 smart city symposium prague (scsp), pp. 1–7. 2023. https://doi.org/10.1109/scsp58044.2023.10146111 [7] d. rojdestvenskiy, m. cvetković, p. bouchner. real-time driver advisory system for improving energy economy based on advance driver assistant systems interface. in 2018 smart city symposium prague (scsp), pp. 1–6. ieee, czech republic, 2018. https://doi.org/10.1109/scsp.2018.8402655 [8] p. toman, j. svoboda, j. pinkas. study of riding specifics of the electric and conventional powered twowheeler users. in 2021 smart city symposium prague (scsp), pp. 1–6. ieee, prague, czech republic, 2021. https://doi.org/10.1109/scsp52043.2021.9447401 [9] s. e. hosseini, b. butler. an overview of development and challenges in hydrogen powered vehicles. international journal of green energy 17(1):13–37, 2020. https://doi.org/10.1080/15435075.2019.1685999 [10] k. kolbe. mitigating urban heat island effect and carbon dioxide emissions through different mobility concepts: comparison of conventional vehicles with electric vehicles, hydrogen vehicles and public transportation. transport policy 80:1–11, 2019. https://doi.org/10.1016/j.tranpol.2019.05.007 [11] k. turoń. hydrogen-powered vehicles in urban transport systems – current state and development. transportation research procedia 45(2):835–841, 2020. https://doi.org/10.1016/j.trpro.2020.02.086 [12] f. spowers. welcome to riversimple, 2024. [2025-06-05]. https: //www.riversimple.com/welcome-to-riversimple/ [13] e. m. gray, c. j. webb, j. andrews, et al. hydrogen storage for off-grid power supply. international journal of hydrogen energy 36(1):654–663, 2011. https://doi.org/10.1016/j.ijhydene.2010.09.051 [14] x. lü, y. qu, y. wang, et al. a comprehensive review on hybrid power system for pemfc-hev: issues and strategies. energy conversion and management 171(18):1273–1291, 2018. https://doi.org/10.1016/j.enconman.2018.06.065 [15] m. m. tellez-cruz, j. escorihuela, o. solorza-feria, v. compañ. proton exchange membrane fuel cells (pemfcs): advances and challenges. polymers 13(18):3064, 2021. https://doi.org/10.3390/polym13183064 [16] m. gurz, e. baltacioglu, y. hames, k. kaya. the meeting of hydrogen and automotive: a review. international journal of hydrogen energy 42(36):23334–23346, 2017. https://doi.org/10.1016/j.ijhydene.2017.02.124 [17] z. zhang, c. hu. system design and control strategy of the vehicles using hydrogen energy. international journal of hydrogen energy 39(24):12973–12979, 2014. https://doi.org/10.1016/j.ijhydene.2014.06.010 16 https://doi.org/10.1016/j.ijhydene.2022.10.156 https://doi.org/10.1109/scsp58044.2023.10146111 https://doi.org/10.1109/scsp.2018.8402655 https://doi.org/10.1109/scsp52043.2021.9447401 https://doi.org/10.1080/15435075.2019.1685999 https://doi.org/10.1016/j.tranpol.2019.05.007 https://doi.org/10.1016/j.trpro.2020.02.086 https://www.riversimple.com/welcome-to-riversimple/ https://www.riversimple.com/welcome-to-riversimple/ https://doi.org/10.1016/j.ijhydene.2010.09.051 https://doi.org/10.1016/j.enconman.2018.06.065 https://doi.org/10.3390/polym13183064 https://doi.org/10.1016/j.ijhydene.2017.02.124 https://doi.org/10.1016/j.ijhydene.2014.06.010 acta polytechnica ctu proceedings 52:8–16, 2025 1 introduction 2 methods 2.1 car platform description 2.2 powertrain design 2.3 powertrain simulation 3 results 4 discussion 5 conclusion acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2023.43.0039 acta polytechnica ctu proceedings 43:39–44, 2023 © 2023 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague experience with sleeper substructure design for a speed of 200 kph according to the s4 standard dana hubáčková∗, tomáš říha, erik dušek the brno university of technology, faculty of civil engineering, veveří 331/95, 602 00 brno, czech republic ∗ corresponding author: hubackova.d@vutbr.cz abstract. the paper deals with the sleeper substructure design for a speed of 200 kph according to the czech sž s4 standard. the paper includes the design of sub-ballast and capping layers. the second part of the paper is focused on transition area design. the conclusion brings recommendations of alterations which would be appropriate to incorporate into the standard. keywords: sleeper substructure design, sub-ballast layer, capping layer, transition area, deformation resistance, sž s4 standard. 1. introduction this paper is derived from practical experience with the sleeper substructure design for the modernization of the track brno – přerov (for one of the project stages used within the czech republic). this is one of the first practical examples where the design for a track speed of 200 kph is being considered together with the fact that in the vast majority of the section the design uses a new track body with a new track alignment. in the czech republic, there is not much practical experience with such a design and thus it is possible to practically “verify” the design methodology according to the new s4 standard for this speed. particularly, the project was focused on the 2nd and 3rd sections between the railway station blažovice and the railway station nezamyslice. considering the track geometry of the existing railway track, outside municipality borders it is essentially a design of a new railway line only “returning” to the existing stations in their current or near location inside municipality borders. 2. geology the geology of the interest area is not very favourable in terms of the design of the sleeper substructure. this is the area of the outer carpathian foredeep, which was formed at the junction of the bohemian massif and the carpathian system, and is mainly formed by sandy limestone and claystone. these tertiary rocks are overlain by quaternary fine-grained sediments, namely loess, loess clay and clay. coarse-grained sediments (sand and gravel) also occur in the area of the watercourses [1]. 3. geotechnical survey the geotechnical survey carried out along the designed route confirmed the assumption of fine-grained soil occurrence at the future earthworks level. specifically, clay with medium to high plasticity (f6 ci and f8 ch) predominated along the entire length of the route. to a significantly lesser extent, other fine-grained clayey and loamy soils were also represented. locally the bedrock was observed, namely limestone in various degrees of weathering. exploration wells were carried out in the existing stations, which revealed clayey soils of insufficient bearing capacity furthermore, in contrast to the track sections, static plate load tests were also carried out in the existing stations by the established practice to persist their current position. 4. values of static deformation modulus in the track sections, the values determining the deformation resistance of the earthworks level were not directly available, therefore the values of the static deformation moduli for individual soils were taken from the s4 standard [2], appendix 9, table 3 (see figure 1). for the most frequently used soils, i.e. clay with medium to high plasticity, the tabular indicative static deformation modulus values ezp are very low, in the range between 2–4 mpa. in the current station sections, although there has been long-term consolidation since their construction, some static plate load tests came out surprisingly unfavourably: in many places the reduced static deformation modulus was just e0,r = 5 mpa, which is far below today’s requirements for the bearing capacity of the earthworks level. 5. design of sub-ballast layers and capping layers the design is strongly affected by the discrepancy between the very low values of the static deformation modulus of the original soil and the high requirements for the static deformation modulus of the earthworks level. for example, the requirement for the main tracks (speed 200 kph) on the earthworks level is emin,zp = 70 mpa, and on the railway platform emin,p l = 90 mpa. the fulfilment of such requirements, therefore, resulted in the necessity of designing 39 https://doi.org/10.14311/app.2023.43.0039 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en d. hubáčková, t. říha, e. dušek acta polytechnica ctu proceedings figure 1. examples of indicative static deformation modulus values determination [2]. a multiple capping layer system of relatively substantial thickness. in practice, this meant that apart from the sub-ballast layer, the design of at least one or even two capping layers was necessary. the sleeper substructure layers are shown schematically in figure 2. as the basic capping layer, soil enhancement was chosen, with the decision that, in the event of the need to design more base layers, the bottom layer will be made using on-site mixing technology, and the above layers will then be made in the mixing centre of soil excavated from the subsoil. due to the scale of the planned construction, we emphasized economy and ecology during the design, so the choice of soil enhancement turned out to be the best option. in the case of an on-site soil enhancement layer, there is no need to excavate the existing earth mass, there is no need to transport large amounts of other materials to the construction site and also there is no need to use new crushed aggregate, which is already in short supply today. therefore, it is an economic and also ecological option with no need for large volumes of movements. enhanced soil cannot be used in places where adverse effects of water and frost are to be expected. thus, in locations where these adverse circumstances were encountered, capping layers of crushed aggregate were proposed instead of soil enhancement. however, from a design point of view, capping layers of crushed aggregate suffer from a disadvantage in the form of the need for deeper railway ditches, which then increase the volume of excavated soil, and also increase the occupation of the necessary land. in the first phase of the design, the sub-ballast layers were implemented according to table 3 of appendix 6 of the s4 standard (see table 1). 6. transition areas in the next phase of the design, the placement of railway substructure objects (such as bridges and tunnels) located on the railway track route was included. ahead of and behind these objects, a transition area of the railway substructure must be designed to ensure a smooth transition of stiffness in the substructure. the design of transitional areas is described in appendix 24 of the s4 standard [2]. for new objects, a transition area is designed using a transition wedge and backfilling of the transition area. this means that the construction of the sleeper substructure itself remains the same as in the previous section, and the increase in stiffness is obtained using more load-bearing soil of a considerable thickness as it is described in appendix 24 of the s4 standard. for any object embedded in the embankment, this approach is natural, however for objects preceded by a cutting, such as tunnels, the replacement of the soil to increase the stiffness makes no practical sense. despite this fact, appendix 24 of the s4 standard does not specify the arrangement of the transition area ahead of and behind the new tunnel constructions, where due to the deep cutting linked to the tunnel portals excavating the soil under the sleeper substructure to a greater depth does not contribute to the quality of the design. the answer can be found in appendix 24 of the s4 standard, which states that in the case of tunnels, a reinforced sleeper substructure (the standard uses an abbreviation zkpp) is proposed. it would be appropriate to include this information directly in appendix 24, which deals with transition areas in detail. in the case of existing tracks (and newly built tun40 vol. 43/2023 experience with sleeper substructure design figure 2. sleeper substructure layers according to sž s4 [2]. expected highest operational track class speed limit loads [millions throughout composition of trackbed layers [kph] gross whole tons/year] lifetime ≤80 <2 a – d min. 200/šd 0/32 kv (min. 150 with the agreement of infrastructure manager) 2–8 a – d min. 250/šd 0/32 kv >8 a – d min. 300/šd 81–120 <2 a – d min. 250/šd 0/32 kv 2–8 a – d min. 300/šd 0/32 kv >8 a – d min. 300/šd 121–160 <2 a – d min. 300/šd 0/32 kv 2–8 a – d var. i: min. 400/šd 0/32 kv var. ii: min. 250/šd 0/63 kv >8 a – d var. i: min. 400/šd 0/32 kv var. ii: min. 250/šd 0/63 kv for all var. i: min. 400/šd 0/63 kv 161–200 operational a – d var. ii: min. 100/asphalt concrete + 250/šd (incl.) loads 0/63 kv table 1. design of the composition of sub-ballast layers of the sž s4 standard. nels), the transition area is created using a reinforced sleeper substructure. this is done by designing a reinforcing layer (in other words, practically another capping layer) that increases the bearing capacity of the transition area and this layer is included in the calculation and design of the sleeper substructure. the decision-making process for the design of transition areas is demonstrated using a flow diagram shown in figure 3. it is apparent, that the process is excessively complex and that simplification should be proposed shortly to unify the substructure design of the railway tracks. in our specific case for the main tracks the standard requires an increase in the static deformation modulus on the railway platform emin,p l = 100 mpa. to achieve such a high value, we designed a capping layer of crushed aggregate mixed with cement under the sub-ballast layer, while other capping layers were designed of enhanced soil or crushed aggregate. for low-bearing clay, even 3 sub-layers had to be designed in the areas with reinforced sleeper substructures. 7. design of sleeper substructure in stations in the next phase of the design, we dealt with the design of the sleeper substructure for the other tracks in the stations (apart from the main tracks). track speeds ranged from 40 to 130 kph with the entire range of operating loads considered by the regulation. see table 2 for the required deformation modulus for the earthworks level and the railway platform. 41 d. hubáčková, t. říha, e. dušek acta polytechnica ctu proceedings figure 3. flow chart for design of transition area. expected highest operational track class the minimum required speed limit loads [millions throughout deformation modulus [kph] gross tons/year] whole lifetime [mpa] emin,zp emin,p l ≤80 <2 a – d 15 30 >2 a – d 20 40 81–120 <2 a – d 20 40 2–8 a – d 30 50 >8 a – d 30 50 121–160 <2 a – d 30 50 2–8 a – d 40 60 >8 a – d 40 60 161–200 for all operational a – d 70 90(incl.) loads table 2. the minimum required bearing capacity on the earthwork level emin,zp and the railway platform emin,p l. most of the objects of the railway substructure were located in the stations, so transition areas had to be designed for the other tracks in the stations as well. for these, the standard for new objects sets a minimum value of 80 mpa at the level of the railway platform. as it turned out, in some cases it is not possible to solve according to the sž s4 standard. the problem originates in the fact, that the standard does not allow the use of crushed stone fraction 0/63 (šd 0/63 kv – see table 1) for speeds up to 120 kph (inclusive). the fundamental difference in bearing capacity between fractions šd 0/32 kv and šd 0/63 kv is the deformation modulus: material šd 0/32 kv has a deformation modulus of 70 mpa, while šd 0/63 kv has a deformation modulus of 100 mpa. therefore, when a static deformation modulus of 80 mpa is to be achieved, it is practically impossible with the material of the subballast layer of 70 mpa. it is theoretically possible to oversize the capping layer to achieve this value, but this is a very inefficient solution (see figure 4). it is clear from figure 4 that the required value of the static deformation modulus on the railway platform of 80 mpa with the sub-ballast layer šd 0/32 can only be achieved with great difficulty. since the deformation modulus of šd 0/32 kv is lower than the required value of the static deformation modulus, it is not possible to achieve this value by increasing the thickness of the sub-ballast layer. in our model example, we achieved the desired value using the soil enhanced with a hydraulic road binder with a thickness of 1 m as a capping layer (in practice it would 42 vol. 43/2023 experience with sleeper substructure design figure 4. samples of the design for the sub-ballast layer from the šd 0/32 kv and šd 0/63 kv. be 2 capping layers with a thickness of 0.5 m). the uneconomical nature of such a design is demonstrated by the thickness of the capping layer, which is 2.5 times greater with šd 0/32 than with šd 0/63. due to the high count of transition areas in the stations and our effort to maximize homogenization of the sub-ballast layer, we proposed the usage of a sub-ballast layer of šd 0/63 kv in all tracks, even though the standard does not allow this option for lower speeds. another illogicality that we encountered during this step of the design can be found in table 3 of appendix 6 of the s4 standard (see table 1) for a speed of 81– 120 kph and an expected operating load greater than 8 hrt year−1. in this case, the standard allows the use of the fraction 0/63, but at the same time, it prescribes a thickness of 300 mm for both šd 0/32 kv and šd 0/63 kv. however, for the same load and higher speed, the regulation allows the use of a thickness of only 250 mm in the case of šd 0/63 kv. it is therefore logical to use this thickness even for the lower speed. 8. embankments the design of the sleeper substructure in embankments requires information about the static deformation modulus. standard s4 [2] in its current version only determines the suitability of soil for use in an earth body, but it does not include the indicative static deformation modulus. therefore, it would be advisable to include the standard overview of the expected parameters of soil used for embankment construction and values of indicative static deformation modulus at the earthworks level in the same way as it is prescribed for original soil in cuttings. in our case, after agreement with the investor, we assumed the enhancement of the used soil and thus the value of the static deformation modulus at the level of the earthworks level in all embankments is set to ezp = 30 mpa. 9. conclusion the s4 standard is a functional and comprehensive tool for the design of sleeper substructures. its application to the design of the sleeper substructure of a long section of a newly constructed railway track at a speed of 200 kph showed several contexts that were not fine-tuned during the creation of the standard and which would be appropriate to incorporate into the standard in the future, in particular: • include solutions for transition areas of tunnels in appendix 24 (transition areas); • allow the use of a sub-ballast layer of šd 0/63 kv for all track speeds and loads, taking into account the high required static deformation modulus on the railway platform emin,p l at transition areas; • for track speed of 81–120 kph and operational load greater than 8 million hrt year−1 allow a sub-ballast 43 d. hubáčková, t. říha, e. dušek acta polytechnica ctu proceedings layer thickness of 250 mm for šd 0/63 kv; • add the value of the deformation modulus for cement-enhanced soils to table 2 in appendix 6; • add values of the indicative static deformation modulus for soil used in the embankment; • establish a clear design methodology of transition areas – procedure – flow diagram. the experience with the design also shows the necessity of a quality geotechnical survey. in practice, we often encounter the fact that geotechnical surveys are not carried out at all for smaller constructions, while for larger constructions the investor does not carry out a sufficiently detailed geotechnical survey in an attempt to save money or for lack of time. it should be noted that the quality of the design of the sleeper substructure directly depends on the quality of the geotechnical survey. the financial resources spent on a sufficiently detailed and high-quality geotechnical survey will then be returned during the construction itself thanks to the accurate and non-oversized design of the sleeper substructure. references [1] v. tomeček. předběžný geotechnický a hydrogeologický průzkum. souhrnná zpráva: modernizace trati brno – přerov, i. etapa blažovice – nezamyslice. brno, 2009. [2] správa železnic, s.o. standard sž s4. železniční spodek. praha, 2021. 44 acta polytechnica ctu proceedings 43:39–44, 2023 1 introduction 2 geology 3 geotechnical survey 4 values of static deformation modulus 5 design of sub-ballast layers and capping layers 6 transition areas 7 design of sleeper substructure in stations 8 embankments 9 conclusion references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2023.40.0076 acta polytechnica ctu proceedings 40:76–82, 2023 © 2023 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague extracting general knowledge of model parameters for clays out of numerous laboratory tests veronika pavelcováa,∗, tomáš jandab a czech technical university in prague, faculty of civil engineering, department of geotechnics, thákurova 7, 166 29 prague, czech republic b czech technical university in prague, faculty of civil engineering, department of mechanics, thákurova 7, 166 29 prague, czech republic ∗ corresponding author: veronika.pavelcova@fsv.cvut.cz abstract. this paper is concerned with the process of determining the material parameters of advanced constitutive models for clays. provided results are applicable already in the early stages of geotechnical survey when no detailed data from laboratory tests of soils expected at construction site are known. a worldwide database of laboratory tests obtained during four year operation of web calibration application excalibre was used for this purpose. this application allows automatic calibration of three advanced constitutive soil models based on detailed data from triaxial and edometric tests. the paper describes the process of determining the confidence intervals of each material parameter of modified cam-clay and hypoplastic clay models for low (cl) and high (ch) plasticity clay according to unified soil classification system (uscs) and tabulates their typical ranges. the purpose of presented work was to contribute to the use of advanced constitutive models among practical engineers. keywords: hypoplastic clay, modified cam-clay, calibration, material parameters. 1. introduction in engineering practice, it can be observed that majority of geotechnical engineers limit themselves to the use of basic non-linear soil models of the mohrcoulomb or drucker-prager type when describing the interaction of the structure and the subsoil and mostly avoid critical state models when solving standard tasks. the general lack of knowledge of these advanced material models among practical engineers may be one of the reasons. also standard civil engineering studies and the most commercial geotechnical programs primarily promote only basic models. the second reason is that it is significantly more difficult to determine the input parameters of advanced non-linear models in comparison to the basic linear models. while the inputs for the use of basic nonlinear models are shear strength parameters that can be determined directly from the results of standard laboratory or field tests, and their ranges are even tabulated for each soil type, critical state models require prescription of parameters describing the behavior of the given material not only under isotropic compression but also parameters characterizing the current state of the soil. a practical engineer often encounters a situation where, although he knows that there exist models which are capable to describe the behavior of modeled soil significantly more accurately than linear constitutive models, he is no longer able to determine the necessary parameters that govern the model. 2. critical state models both, the hypoplastic clay [1] and modified camclay [1, 2], constitutive models are based on the theory of critical state. the main features of these models are similar but unlike the hypoplastic model the modified cam-clay model distinguishes the elastic and plastic part of strain components and therefore allows the straightforward visualization of permanently strained plastic zones in fem calculations. on the other hand, the model of hypoplastic clay can correctly represent the associated gradual reduction in stiffness upon activation of plastic behavior rather than a sudden drop as predicted by the modified cam-clay model. 2.1. hypoplastic clay the hypoplastic model for clay takes into account the following important properties of soils: • the stiffness depends on the current value of the mean effective stress σeff m and the direction of loading. • the strength depends on the value of the mean effective stress σeff m and current density characterized by the void ratio e. the model distinguishes between material parameters that are constant for all possible states of a particular soil and state variables that evolve during straining. in this sense the hypoplastic model is fundamentally different from the mohr-coulomb model which employs distinct values of its material parameters for the same soil in different density states. in 76 https://doi.org/10.14311/app.2023.40.0076 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 40/2023 extracting knowledge of model parameters out of laboratory tests parameter name unit material parameters n position of the normal consolidation line [-] λ∗ slope of the normal consolidation line [-] κ∗ slope of the swelling line [-] φc friction angle in the critical state [°] ν parameter controlling the value of the shear modulus [-] state parameter e void ratio [-] table 1. material parameters of hypoplastic clay model. figure 1. normal consolidation line and swelling line assumed in hypoplastic model for clay [1]. particular, the hypoplastic model for clay requires five material parameters and one state variable summarized in table 1. 2.1.1. parameters n and λ∗ parameters n and λ∗ define the position and the slope of the isotropic normal compression line (ncl) in the ln(σeff m )×ln(1+e) space as illustrated in figure 1. the normal compression line defines a theoretical response of the soil sample during isotropic compression test. since the model assumes incompressible grains the resulting volumetric strain depends on the decrease of volume of pores only and thus on the void ratio. 2.1.2. parameter κ∗ parameter κ∗ defines the slope of the swelling line in the ln(σeff m ) × ln(1 + e) space as illustrated in figure 1. unlike the ncl which is fixed by parameter n the position of the swelling line depends on the previous compression stress. in addition, unlike simple elastoplastic models, the slope of the unloading-reloading line is non-linear in hypoplasticity, so that the paparameter name unit material parameters e0 position of the normal consolidation line [-] λ slope of the normal consolidation line [-] κ slope of the swelling line [-] mcs slope of the critical state line [-] ν poisson’s ratio [-] state parameter pc the preconsolidation pressure [pa] table 2. material parameters of modified cam-clay model. rameter κ∗ should thus be calibrated by means of simulation of a laboratory test, rather than by directly evaluating its slope. 2.1.3. parameter φc parameter φc is the critical angle of internal friction. the angle of repose can not be measured for finegrained soils and the critical friction angle has to be determined from triaxial shear test. the critical friction angle correspond to the maximal mobilized friction angle for normally consolidated samples where no softening occurs during the triaxial shear test. or the critical friction angle corresponds to the mobilized friction angle after the peak for overconsolidated samples. 2.1.4. parameter ν parameter ν controls the value of the shear modulus. the bulk and shear modulus are linked through the parameter ν in the isotropic normally consolidated state. during shear test the shear modulus evolves non-linearly and its magnitude is being controlled by the parameter ν and by the distance from the failure state. 2.2. modified cam-clay the modified cam-clay takes into account the following important properties of soils: • the stiffness depends on the current value of the mean effective stress σeff m and the direction of loading. • the strength depends on the value of the mean effective stress σeff m a current density characterized by the void ratio e or by the preconsolidation pressure pc. the model introduces five material parameters that are constant for all possible states of a particular soil and one state variable that evolves during straining. all parameters are summarized in table 2. 77 veronika pavelcová, tomáš janda acta polytechnica ctu proceedings figure 2. normal consolidation line and swelling line assumed in modified cam-clay model [2]. 2.2.1. parameters e0 and λ parameters e0 and λ define the position and the slope of the isotropic normal compression line (ncl) in the ln(σeff m ) × e space as illustrated in figure 2. the normal compression line defines a theoretical response of the soil sample during isotropic compression test. since the model assumes incompressible grains the resulting volumetric strain depends on the decrease of volume of pores only and thus on the void ratio. 2.2.2. parameter κ parameter κ defines the slope of the unloading/reloading line in the ln(σeff m )×e space as illustrated in figure 2. unlike the ncl which is fixed vertically by parameter e0 the position of the swelling (unloading/reloading) line depends on the maximal previous compression stress denoted as the preconsolidation pressure pc. 2.2.3. parameter mcs parameter mcs is the slope of the critical state line plotted in the σeff m × j space, see figure 3. the value is computed from the critical state angle φc by: mcs = 2 √ 3 sin φc 3 − sin φc . (1) 2.2.4. parameter ν parameter ν is the poisson ratio known from linear elasticity. it controls the ratio of radial and axial elastic strains under uniaxial loading and consequently the ratio between the stress dependent bulk and shear modulus. 2.2.5. preconsolidation pressure pc the model has one state variable – the preconsolidation pressure pc. for an isotropic compression test the preconsolidation pressure pc corresponds to the maximal mean stress σeff m to which the soil has been subjected. the kinematics of the model is evident from figure 2. soil sample with an initial state a (it has void ratio ea and preconsolidation pressure pa c ) is loaded isotropically up to the compressive mean stress σeff m = pa c . the deformation during this loading is nonlinear but elastic, i.e. if the soil is unloaded from this point it returns along the κ-line back to the state with e = ea and no permanent volumetric strain occurs. if the soil is loaded beyond the point pa c it moves along the λ-line. during this loading both the elastic and the plastic volumetric strain develop. when the soil is unloaded from the point σeff m = pb c it moves along the new κ-line defined by the new value of preconsolidation pressure σeff m = pb c. as evident from the initial and final (in unloaded state) values of the void ratio ea > eb the permanent volumetric strain developed during this loading sequence [1]. 3. automated calibration of constitutive models even though the development of more advanced constitutive soil model theories which take into account bounding surface plasticity, whether those based on the theory of plasticity or hypoplasticity [3, 4], or barodesy [5] continues steadily forward, the development of automated calibration procedures stays behind. the calibration procedures are regarded as the inverse analysis, since the parameters of a constitutive model are not known in advance but the reaction of the system is known. the goal of the calibration is to minimize the objective error function e(u) which is defined as a function of a difference between the observation or experiment uexp and simulation unum. the most common is the least square method. nevertheless, more sophisticated dimensionless formulas can be used [6]. 3.1. excalibre as mentioned in the introduction, the faculty of civil engineering of ctu in cooperation with the faculty of science of charles university developed a excalibre web application [7] which performs automatic deterministic calibration of the hypoplastic model for sands [8], hypoplastic model for clays [9] and modified cam-clay model [10]. from 2018, the application [1] is available free of charge for registered users who agree to provide the uploaded data for research and development purposes. a typical workflow of model calibration in excalibre is the following: (1.) the user downloads a laboratory protocol template which is prepared for the user in the form of an ms excel file. the template is different for clays and sands. (2.) the user fills in the results of laboratory tests. a minimum of one consolidated isotropically undrained (ciup) triaxial test with pore pressure measurement for clay soils or one consolidated isotropically drained (cid) triaxial test for sandy 78 vol. 40/2023 extracting knowledge of model parameters out of laboratory tests figure 3. yield surface of the modified cam-clay model [2]. soils and one standard oedometric (oed) test must be completed for successful calibration. in addition to those already mentioned, it is possible to fill in the classification according to uscs and index characteristics of the soil. these parameters are currently not used in calibration but they are stored for research purposes. (3.) the user uploads the laboratory protocol back to the web application. as a result, the application proposes optimal model parameters and graphs comparing the course of measured and simulated laboratory tests. these graphs serve to visually check the calibration of the model. (4.) it is possible to manually modify individual model parameters to adjust the calibration or to examine the influence of individual parameters on model predictions after the calibration. large pool of available data collected worldwide propose an effective basis for statistical processing, determination of typical ranges of individual parameters and search for correlations between individual parameters presented in this paper. 4. data processing all data work was concentrated in universal scripts written in the python programming language due to the repeatability of laboratory test data analysis. it is a higher-level programming language with dynamic support for data types and support for programming paradigms. the programming language [11] is freely available. the format conversion of laboratory protocols from microsoft excel spreadsheet xlsx excalibre template to comma-separated values csv was the first step of data processing. it was a necessary step due to the complexity of the xlsx file structure. xlsx file is a zip package that contains xml files representing individual sheets of the spreadsheet which are always supported by rels files containing specifications for linking individual xml files together to form a complete file. compared to complex xlsx file, csv is a simple file format that is intended for exchanging tabular data between different systems. the data of the csv file are stored in rows just like in the default table while the individual items in the row are further separated by a predetermined character. the individual sheets of the input xlsx laboratory protocol form separate csv files which are, for organizational reasons, stored in a folder with a name corresponding to the original name of the xlsx file so that one folder represents one complete laboratory protocol. written script cycles through the folders representing individual laboratory protocols with csv files replacing microsoft excel spreadsheet and extracts characteristic information about the laboratory protocol using predefined classes. the extracted data is further inserted into a dictionary-type data structure. a dictionary is a data type sometimes called an associative array. it consists of key-value pairs. each key-value pair maps a key to an associated value. in the algorithm described , the key corresponds to the name of a folder representing one laboratory protocol. key values are assigned using classes so that one line of the dictionary represents the data of one laboratory protocol. the first value of the dictionary corresponds to the relative path to the file, assigned attributes are: uscs classification of soil, basic index characteristics such as specific gravity, atterberg’s limits or angle of repose. it is followed by representation of particle size distribution and loading and quantity of performed oedometric and triaxial tests. the excalibre application contains a public laboratory test database so that these protocols can be used for testing the functionality of the calibration application by a new user. knowing that, filtering of duplicate laboratory protocols was another step of data processing. a simple condition consisting of • uscs classification of soil, • the largest sieve and the corresponding passing in the grain size analysis, • the initial void ratio of the oedometric test of the naturally consolidated soil sample, • the initial void ratio of the oedometric test of the reconstituted soil sample 79 veronika pavelcová, tomáš janda acta polytechnica ctu proceedings hypoplastic clay modified cam-clay λ∗ κ∗ n ν φc λ κ e0 ν mcs count 7 7 7 7 7 7 7 7 7 7 mean value 0.119 0.010 1.396 0.276 25.3 0.297 0.016 2.784 0.307 0.997 standard deviation 0.082 0.003 0.659 0.062 2.0 0.365 0.012 2.521 0.065 0.088 minimum value 0.063 0.006 0.872 0.170 22.8 0.104 0.010 1.237 0.220 0.888 25 % quantile 0.068 0.009 0.960 0.245 23.9 0.107 0.010 1.383 0.250 0.935 50 % quantile 0.094 0.010 1.173 0.280 25.6 0.155 0.010 1.752 0.340 1.008 75 % quantile 0.123 0.010 1.521 0.320 26.2 0.249 0.018 2.734 0.355 1.032 maximum value 0.296 0.014 2.763 0.350 28.8 1.107 0.039 8.263 0.380 1.147 table 3. basic statistical description of calibrated model parameters for ch soil. was used for the determination and removal of duplicate laboratory protocols. the results of this classdriven filtering revealed that only 58 of the total 1 924 laboratory protocols are non-duplicated. the remaining protocols are duplicates of another 161 protocols in various frequencies. in total, the database would therefore contains 219 unique laboratory protocols. considering uscs composition of non-duplicate protocols only low (cl) and high (ch) plasticity clay soils were selected for further processing. all of the laboratory protocols of soils classified as ch or cl have been uploaded to the excalibre web application and material parameters for hypoplastic model for clay soils and modified cam-clay were calibrated and added to the dictionary-like dataframe collecting protocol information. thus the already existing dataframe was extended by 5 columns with material parameters of hypoplastic clay model (see table 1) and another 5 columns for modified cam-clay model material parameters (see table 2). 5. results the database contained only 7 laboratory protocols of soil classified as high-plasticity clay after removing all duplicates. all of them were successfully calibrated for both of constitutive models – hypoplastic clay and modified cam-clay. the summarization of basic statistical description of calibrated model parameters is summed up in table 3. strong correlations between atterberg’s limits (liquid limit plastic limit wl and plastic limit wp ) and slope of the ncl line of both calibrated models have been observed. example of correlation between plastic limit and slope of the ncl assumed in hypoplastic model for clay can be seen in figure 4 and the same trend applies to the position of the ncl. all considerable correlations are summarized in table 4 but limited number of protocols has to be considered. overall 72 laboratory protocols of soil classified as low-plasticity clay contained the database after removing all duplicates of which it was possible to calibrate 56 for hypoplastic clay and 61 for modified cam-clay model. the reason why not all of the protocols were successfully calibrated will be further examined. basic statistical description of calibrated model parameters 25 30 35 40 plastic limit wp [-] 0.10 0.15 0.20 0.25 0.30 sl op e of th e no rm al c on so lid at io n lin e * [ -] figure 4. correlation between plastic limit and slope of the ncl assumed in hypoplastic model for clay. hypoplastic clay mod. cam-clay λ∗ n ν φc λ e0 mcs wl 0.92 0.95 0.41 0.91 0.94 0.40 wp 0.95 0.92 0.52 0.44 0.89 0.88 0.44 table 4. correlations between atterberg’s limits of high-plasticity clay soil and model’s parameters. is also summarized and can be examined in table 5. examples of parameter’s distribution are shown in histogram in figure 5. in comparison to high-plasticity clay, only moderate correlations between atterberg’s limits and model parameters have been observed for low-plasticity clay. in particular, higher soil moisture at atterberg’s limits indicates higher parameter n with correlation coefficients about 0.41. the similar trend also applies to modified cam-clay’s parameter e0 with correlation coefficients about 0.46 for the liquid limit and the 0.41 for plastic limit. correlation 0.40 has been found between moisture at liquid limit and the slope of the ncl line λ. another correlations seem insignificant for practical applications. 6. conclusions this paper briefly explained each individual input parameters of both hypoplastic and modified cam80 vol. 40/2023 extracting knowledge of model parameters out of laboratory tests hypoplastic clay modified cam-clay λ∗ κ∗ n ν φc λ κ e0 ν mcs count 56 56 56 56 56 61 61 61 61 61 mean value 0.067 0.008 0.948 0.217 29.1 0.107 0.010 1.388 0.270 1.120 standard deviation 0.035 0.005 0.283 0.109 6.6 0.062 0.006 0.597 0.112 0.320 minimum value 0.001 0.000 0.507 0.010 2.9 0.002 0.000 0.112 0.010 0.101 25 % quantile 0.042 0.005 0.725 0.155 25.0 0.059 0.008 0.949 0.200 0.984 50 % quantile 0.064 0.007 0.899 0.230 30.0 0.096 0.010 1.246 0.260 1.199 75 % quantile 0.090 0.010 1.170 0.290 33.3 0.149 0.011 1.797 0.390 1.274 maximum value 0.178 0.025 1.667 0.400 43.6 0.284 0.036 3.075 0.400 1.790 table 5. basic statistical description of calibrated model parameters for cl soil. 0.00 0.05 0.10 0.15 slope of the normal consolidation line * [-] 0 2 4 6 8 10 12 14 16 18 fr eq ue nc y (a). hypoplastic clay model. 0.00 0.05 0.10 0.15 0.20 0.25 slope of the normal consolidation line [-] 0 2 4 6 8 10 12 14 16 fr eq ue nc y (b). modified cam-clay model. figure 5. histograms of slopes of the ncl for hypoplastic clay and modified cam-clay model. clay constitutive model and it summarized the issue of automatic parameter calibration. excalibre web calibration application, a practical tool for calibrating these material parameters based on the results of standard laboratory tests, was introduced in section 2 and section 3. then a method of how could existing data be used to obtain general knowledge of the properties of these individual parameters was presented in section 4 and finally, the paper summarized extracted basic statistical data of model parameters with respect to uscs classification in section 5. the main aim of this research is to help with development of a robust automatic calibration procedure which will be applicable at any stage of geotechnical survey to promote the use of advanced models in geotechnical practice. the determination of the confidence intervals along with correlations between material parameters and index soil properties was only the first step of data analysis and other more complex studies will follow. acknowledgements the support provided by the sgs project no. sgs22/030/ohk1/1t/11 and the gačr project no. 22-12178s is gratefully acknowledged. references [1] excalibre – soilmodels automatic calibration. [2022-07-26]. https://soilmodels.com/excalibre-en/ [2] m. šejnoha, j. pruška, t. janda, m. brouček. metoda konečných prvků v geomechanice: teoretické základy a inženýrské aplikace. české vysoké učení technické, 2015. isbn 978–80–01–05743–8. [3] j. jerman, d. mašín. hypoplastic and viscohypoplastic models for soft clays with strength anisotropy. international journal for numerical and analytical methods in geomechanics 44(10):1396–1416, 2020. https://doi.org/10.1002/nag.3068 [4] d. mašín. clay hypoplasticity model including stiffness anisotropy. géotechnique 64(3):232–238, 2014. https://doi.org/10.1680/geot.13.p.065 [5] d. kolymbas. introduction to barodesy. géotechnique 65(1):52–65, 2015. https://doi.org/10.1680/geot.14.p.151 [6] t. kadlíček. parameters identification of advanced constitutive models of soils. ph.d. thesis, czech technical university in prague, 2019. [7] t. kadlíček, t. janda, m. šejnoha, et al. automated calibration of advanced soil constitutive models. part ii: hypoplastic clay and modified cam-clay. acta geotechnica 17:3439–3462, 2022. https://doi.org/10.1007/s11440-021-01435-y [8] p.-a. von wolffersdorff. a hypoplastic relation for granular materials with a predefined limit state surface. mechanics of cohesive-frictional materials: an international journal on experiments, modelling and computation of materials and structures 1(3):251–271, 1996. https://doi.org/10.1002/(sici)10991484(199607)1:3<251::aid-cfm13>3.0.co;2-3 81 https://soilmodels.com/excalibre-en/ https://doi.org/10.1002/nag.3068 https://doi.org/10.1680/geot.13.p.065 https://doi.org/10.1680/geot.14.p.151 https://doi.org/10.1007/s11440-021-01435-y https://doi.org/10.1002/(sici)1099-1484(199607)1:3<251::aid-cfm13>3.0.co;2-3 https://doi.org/10.1002/(sici)1099-1484(199607)1:3<251::aid-cfm13>3.0.co;2-3 veronika pavelcová, tomáš janda acta polytechnica ctu proceedings [9] d. mašín. clay hypoplasticity with explicitly defined asymptotic states. acta geotechnica 8(5):481–496, 2013. https://doi.org/10.1007/s11440-012-0199-y [10] k. roscoe, j. burland. on the generalized stressstrain behaviour of wet clay. in engineering plasticity, pp. 235–609. cambridge university press, 1968. [11] python. [2022-07-26]. https://www.python.org 82 https://doi.org/10.1007/s11440-012-0199-y https://www.python.org acta polytechnica ctu proceedings 40:76–82, 2023 1 introduction 2 critical state models 2.1 hypoplastic clay 2.1.1 parameters n and lambda* 2.1.2 parameter kappa* 2.1.3 parameter phi c 2.1.4 parameter nu 2.2 modified cam-clay 2.2.1 parameters e0 and lambda 2.2.2 parameter kappa 2.2.3 parameter mcs 2.2.4 parameter nu 2.2.5 preconsolidation pressure pc 3 automated calibration of constitutive models 3.1 excalibre 4 data processing 5 results 6 conclusions acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.38.0169 acta polytechnica ctu proceedings 38:169–175, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague exploring machine learning-based archetypes for urban life cycle modeling (ubim) giseli mary colleto, vanessa gomes∗ university of campinas, school of civil engineering and architecture and urbanism, rua saturnino de brito, n° 224, cidade universitária zeferino vaz. cep: 13083-889 – campinas – são paulo, brazil ∗ corresponding author: vangomes@unicamp.br abstract. urban analyses demand simplifications that balance modelling level of detail and scope broadness. thus, classification by archetypes is a promising methodological approach. such an approach is common for energy studies but rarely applied for life cycle assessment (lca) purposes. when archetypes are used in urban lca, they generally result from previous studies for classification and characterization according to parameters that directly affect the operational energy performance of buildings. this paper tackles two research questions: i) is it appropriate to aggregate building stocks based on operational energy (oe) variables when life cycle impacts are investigated? ii) when integrated lca (oe + embodied impacts) is pursued, would variables describing both interests simultaneously result in better representation than using operational energy-based clustering to predict embodied impacts and vice versa? thus, we aim to confirm that, combining variables that govern oe and embodied impacts offers a better result than using oe to predict materials groupings, even if some adherence is lost relatively to single-objective clustering. clustering experiments were carried out for the campus of the university of campinas, brazil. after unsupervised k-medoid (pam) grouping, the data were submitted to a supervised learning (neural networks) classification method. generated confusion matrices demonstrate how adherent the clustering is when considering one interest to predict the other in three situations. results indicate that an operational energy-driven archetype fails to represent buildings from the embodied impacts viewpoint, and that merging operational energy and embodied impact variables would better support integrated life cycle impact predictions. keywords: archetypes, building stock aggregation, clustering, lca, life cycle impacts, urban modelling. 1. introduction considering the growth projected for cities over the next decades, achieving established energy use goals become indispensable to mitigate urban environmental impacts. estimating and understanding environmental impacts, both operational and embodied, are essential to assist in evaluating and monitoring performance and to support strategic planning and the formulation of energy and environmental policies for sustainable development. from the environmental charges induced by the construction sector, studies are increasingly focusing on simultaneously reducing operational energy (oe) and embodied impacts [1, 2] as a decision-making ais. to avoid that only impacts during the operational phase of buildings are considered [3], life cycle assessment (lca) and energy simulations must be integrated [4], to consider environmental burden over a building’s entire life cycle. life cycle assessments at individual building scale have already been extensively covered by literature [5]. however, the need to mitigate local and global impacts to adapt to the limitations of available resources [6] makes environmental assessment at the urban scale increasingly necessary for sustainable planning. assessments and simulations of the urban tissue are even more complex as they face a series of challenges imposed by the scale, complexity and intensity of data needed to adequately describe urban layers and components [7]. however, if we consider the city as a set of individual buildings, the high level of detail in the analysis of segregated structures requires intensive use of financial, temporal, technological and human resources. thus, to carry out these analyses, the complexity at the urban scale allied to the lack of detailed information about the built stock demands strategies to simplify the built stock modelling, to balance level of detail and scope broadness. thus, archetype-based approach, for instance, abstracts existing built stock into a number of reference buildings. construction archetypes are representations (real sample or theoretical average) of groups of buildings aggregated according to similar characteristics, defined according to the scope of the study at hand. for combining simplified modelling and extended scope, the classification by archetypes is commonly used in research on new technologies [8] and energy efficiency policy design [9]. so far, their use has increased yet been focused on operational energy studies at the urban scale, but building archetypes emerge as a promising methodological approach to extend assessment scope 169 https://doi.org/10.14311/app.2022.38.0169 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en giseli mary colleto, vanessa gomes acta polytechnica ctu proceedings beyond energy use. the development of archetypes for energy studies is based on data directly related to the operational energy of buildings. according to the grouping criterion, the representative buildings vary, reflecting the extrapolation of characteristics to the other buildings of the group that he represents and, consequently, the consistency of the simulations for the urban scale. specific studies assessing the environmental impact of built stock at various scales point out lca studies applying the archetype approach use the same variables as energy studies (use typology, construction typology, year/period of construction), either because they aim only at assessing operational impacts, or because they are derived from energy studies, or simply because of data availability [5]. however, an archetype for energy modeling may not be suitable for analyzing the flow of materials through the built stock [10], and representative buildings of energy perspective are not necessarily also representative from the materiality viewpoint. so, for integrated simulations of environmental assessments, the challenge of generating construction archetypes that integrate information on operational and embodied impacts arises. this paper raises two research questions: (i) is it appropriate to aggregate building stocks based on operational energy (oe) variables when life cycle impacts are investigated? (ii) when integrated life cycle (oe + embodied impacts) assessment is pursued, would variables describing both interests simultaneously result in better representation? thus, we aim to confirm that, at the representative building identification stage of the building archetype creation process, combining variables that govern oe and embodied impacts offers a better result than using oe to predict materials groupings and vice versa, even if some adherence is lost relatively to single-objective clustering. 2. materials and methods to achieve the above-mentioned goal, our method comprised two main steps. firstly, based on a systematic literature review on archetype development methods for energy studies [11], the most used clustering parameters were identified according to the relevance of each study (operational energy and lca). then, cluster analyses to define archetypes were carried out for a selected case study, considering three different aggregation criteria: by energy-relevant variables; by materiality-relevant variables; and by combining energy and materiality variables simultaneously. the case study comprised the campus of the state university of campinas (unicamp), located in the municipality of campinas, são paulo, brazil. with 3 893 958 m2 total area and about 50 000 daily visitors before the covid19 pandemic, the study area is comparable to a small town. the campus comprises 598 429 m2 of gross floor area (gfa) for mixed uses, such as: administration, education (classrooms), research (laboratories, workshops), health facilities (hospitals, clinics), libraries, restaurants, cultural facilities, sport facilities, general services, day care center, schools, bank agencies, squares, public spaces and more. for simplification’s sake, buildings smaller than the university standard building (i.e., < 500 m2 gfa) and missing data points were cutoff, so that 226 buildings remained in the final sample. geometric and nongeometric data for detailing the sample was surveyed from several sources: gis, field visits, image analysis, data provided by administrative sectors, and results of previous studies conducted by the research group [12]. from the operational energy standpoint, data collection focused on the most used parameters for eo clustering found in the literature: (i) year of construction; (ii) construction typology (isolated, semi-detached, attached); (iii) use typology (administrative, education, hospital, restaurant, bank, laboratory, staff, library, museum, gymnasium). from the materiality perspective, aggregation focused on the structure (reinforcing concrete, metallic, structural concrete masonry, pre-fabricated concrete, wood) and on the envelope (external wall: solid ceramic brick, structural concrete masonry, ceramic brick, pre-fabricated concrete, wood; and roofing: metal, concrete slab, fiber cement, ceramic, polycarbonate), which govern a substantial portion of building impacts, as demonstrated by life cycle assessment studies [1]. the total area and number of floors are relevant inputs for both perspectives. based on a previous study on the application of different unsupervised machine learning clustering methods to the same neighborhood [7], the k-medoids / partitioning around medoids (pam) algorithm was chosen to generate a preset number of 9 clusters. in kmedoids algorithms, the groups are defined as subsets of data points that are closest to their representative elements – the medoids – which are real objects from the dataset. the identified medoids serve as the basis for developing archetypes to represent each building group. the k-medoids method is more computationally expensive (i.e. has longer processing time) than gravity center calculation, but is suitable if the groups are spherical, each medoid occupies a more central position in the group and can handle any kind of attribute (quantitative and qualitative). three unsupervised learning clustering scenarios were performed: operational energy-based (oe), materiality-based (lca) and integrated clustering (oe+lca) (table 1). after being processed to generate clusters for each scenario and identify respective medoids, the data were submitted to neural net170 vol. 38/2022 exploring machine learning-based archetypes . . . scenario parameters used for clustering method cluster nr. 1 oe-related parameters: year of construction, construction typology, use typology, total area and number of floors 2 materiality-related parameters: structure, external wall, roof tile, total area and number of floors 3 integrated clustering considering simultaneously oe parameters and materiality-related parameters: year of construction, construction typology, use typology, structure, external wall, roof tile, total area and number of floors unsupervised machine learning: k-medoids / partitioning around medoids (pam) 9 table 1. description of the clustering scenarios. (a) . ref: oe | pred: embodied impact. accuracy = 45,8 %. (b) . ref: oe | pred: integrated lc impacts. accuracy = 29.5 % figure 1. confusion matrices relating operational energy (oe) parameters (reference) to predicted embodied (a) and to integrated life cycle impacts (ubim) (b) and respective model accuracies. works, a supervised learning classification method, using the caret package in r language [13], and the nnet method. the statistical procedures were performed using r packages “tidyverse”, for data manipulation [14]; “ggpubr”, for graph elements [15]; “cluster”, for partitioning around medoids (pam) algorithm – k-medoid [16]; “caret”, for neural networks (supervised learning) [17] and “factoextra”, for generating cluster graphs [18]. finally, confusion matrices were generated to demonstrate how adherent the clustering is when considering one interest to predict the other in three situations: (i) considering energy grouping to predict materiality aspects (figure 1a); (ii) considering energy to predict the integrated scenario (oe+lca) (figure 1b), and (iii) considering materiality aspects to predict the integrated scenario (figure 2). figure 2. confusion matrix relating embodied impact parameters (reference) to predicted integrated life cycle impacts (ubim). model accuracy = 58.6 %. 171 giseli mary colleto, vanessa gomes acta polytechnica ctu proceedings figure 3. clustering based on integrated life cycle (oe and embodied) impacts parameters (scenario 3). numbers indicate the buildings ids. such matrices reveal how confused the model is between the predicted classes versus the actual outcomes, and highlights instances in which one class is confused for the other, providing insightful information regarding a model’s accuracy. the matrix columns are the “true” classes (reference), while the rows are “predicted” classes. the main diagonal shows the cases where the model is correct. 3. results and discussion 3.1. literature review and theoretical deepening several efforts are being made to improve the process of creating building archetypes. de jagger et al. [19] summarized the most common variables within 32 studies and colleto and silva [11] synthetized 21 publications that highlighted in the archetype development process. table 2 gathers the main parameters from those two studies, and the relevance of each variable for energy and embodied impact assessment. other parameters have been identified as not directly relevant for lca, despite their high relevance for energy: • building geometry: ▷ building footprint: 4 occurrences in [19], 5 in [11]; ▷ compactness ratio: 1 occurrence in [19], 2 in [11]; ▷ heated volume or floor area: 2 occurrences in [19] 3 in [11]; ▷ total loss area: 2 occurrences in [19], 1 in [11]; ▷ density of internal thermal mass: 2 occurrences in [19]; ▷ ground elevation: 1 case in [11]; ▷ roof type/shape: 1 case in [19], 1 in [11]; ▷ aspect ratio: 1 occurrence in [11]; ▷ envelope shape: 1 occurrence in [11]; ▷ floor loss area / geometry / loss-to-floor area ratio: 1 occurrence in [19] each; ▷ position of internal thermal mass: 1 occurrence in [3]; ▷ total area to net area ratio: 1 occurrence in [11]; • building occupancy: ▷ occupancy: 3 occurrences in [19]; ▷ use of ground floor: 2 occurrences in [19]; • thermal quality: ▷ u-value: 5 occurrences in [19], 3 in [11]; ▷ maintenance state: 3 occurrences in [19]; ▷ air tightness: 2 occurrences in [19]; ▷ construction method: 1 occurrence in [19]; • building installations: ▷ fuel typed used: 6 occurrences in [19], 3 in [11]; ▷ hvac system 6 occurrences in [10], 6 [11]; ▷ dhw cylinder insulation thickness / measured energy demand: 2 occurrences in [11] each; and • building environment: ▷ density of urban area / exposure: 1 occurrence in [19] each. the parameter analysis in table 2 corroborates [10] regarding the unsuitability of an archetype for energy modeling to support materials flow analyses, by reflecting on the relationship of oe variables on materiality data needed for embodied impact assessment. the neural network classification and resulting confusion matrices enabled to quantify such inadequacy for the selected case study. 3.2. sample clustering the three unsupervised clustering scenarios resulted in quite distinct results. for example, in the integrated clustering scenario 3 (figure 3), the 9 clusters (c1 – c9) were identified in spherical groups, containing a number of elements (n): c1 = 27 n, c2 = 29 n, c3 = 10 n, c4 = 34 n, c5 = 18 n, c6 = 21 n, c7 = 39 n, 172 vol. 38/2022 exploring machine learning-based archetypes . . . parameters relevance to embodied impacts relevance to oe bu ild in g ge om et ry construction typology (attached, semi-detached, independent/isolated, continuous) (15) [19] (9) [11] low – influences the building materials proportions, but its relation to material types and masses is limited • influences the quantities of materials (e.g., attached buildings share walls) high – it defines important characteristics for the building’s operational energy performance due to differences in solar incidence, wind, thermal mass etc. façade-area ratio (1) [19] roof-area ratio (1) [19] • proportion of façade materials or roof materials out of the total amount of building materials and their respective impacts building height (3) [19] (5) [11] medium – data support materials quantity surveying • multiplier for material estimation in vertical elements (walls, columns etc) window area (3) [19] (2) [11] • supports window material and – jointly with wwr – wall area estimates window-to-wall ratio (wwr) (4) [19] (3) [11] • relative proportion of materials in windows and walls and their respective impacts floor area (5) [11] • relevant for quantity surveying nr. of dwellings (buildings) / rooms (residences) (2) [11] • multiplier for the bill of materials of a building floor; • materials estimation of internal elements number of floors / stories (5) [19] (7) [11] high – data support materials quantity surveying • multiplier for the bill of materials for a multistorey building total area (8) [19] • multiplier for the total bill of materials high – essential geometric data for operational energy performance calculations and simulations facade materials (1) [11] • essential data to support the bill of quantities calculations high – non-geometric data essential for oe calculations/ simulations bu ild in g oc cu pa nc y use typology / final use (14) [19] (13) [11] low – does not standardize building materials used in construction. can be compensated for by correction factors that represent, for example, important differences in building systems high – equipment and use intensity/schedule are proxies for energy consumption patterns of different end use typologies. bu ild in g ag e construction year/period (20) [19] (19) [11] medium – allows retrieving the probable physical/material configuration from the implemented building regulation. in places without such regulations, it only points out building trends. high – allows retrieving the probable physical/material configuration from building regulations. if absent, utility for inferring building trends for archetypes development is limited. year of building’s last renovation (1) [11] high – enables potential for retrofit/renovation analyses and maintenance, which directly influence a buildings’ materiality. high – enables retrofit potential analysis bu ild in g en vi ro nm en t climatic zone (12) [19] (3) [11] location (3) [19] high – enables appropriate materials selection according to raw material sources/transportation, and energy matrix. high – non-geometric data essential for calculations and simulations table 2. analysis of the main parameters used for building archetype development. numbers within parentheses indicate the occurrences in the reviewed literature. 173 giseli mary colleto, vanessa gomes acta polytechnica ctu proceedings c8 = 48 n and c9 = 1 n. cluster c9 contains only one building (campus hospital), for its exceptional total built area. for each cluster, the medoid (m1 = id1367; m2 = id1076; m3 = id0.12; m4 = id301; m5 = id0.46; m6 = id396; m7 = id99; m8 = id400; m9 = id518) is the representative building to support subsequent archetype development. in confusion matrices (figures 1 and 2), the columns are the “true” classes (reference), while the rows are “predicted” classes (in our case, building clusters). as the main diagonal shows the cases for which the model is correct, the ideal model shows a highly populated main diagonal, which sums up the highest sample percentage (in our case, 226 buildings). all other cells indicate prediction errors, so the “dirtiest” a confusion matrix is, the lowest its accuracy. despite the accuracy achieved, applying oe parameters to predict materiality grouping is the least suitable: due to the lack of data correlations, few clusters had positive results and most of them were not even statistically processed (indicated as “n/a” in figure 1a). contrastingly, the model’s accuracy roughly doubles if life cycle aspects prediction is based on materiality (figure 2) instead of in oe aspects (figure 1b): 133 of the 226 sampled buildings would be correctly grouped, and clusters c8 (41/48 buildings) and c7 (29/39 buildings) were the most accurately predicted. this reinforces our assumption that, regardless of its often use, using energy-based clustering and then performing lca for embodied impacts estimation of built stocks is not recommendable, and variables simultaneously representing both oe and materiality interests would result in better grouping than by using energy clustering for material grouping prediction and vice versa. 4. conclusions balancing the most appropriate parameters for clustering vs. data collection and computational cost is a major challenge for creating representative archetypes to support built stock aggregation modelling. despite improvement opportunities, the exploratory simulations shown support the assumption that an integrated clustering step combining operational and embodied impacts variables offers better outcomes than singleobjective clustering followed by complementary simulation or lca of representative buildings. studies are underway to advance in the archetype development process and data simulation for predicting benchmarks for some use typologies and should be generally reproducible for varied contexts. outcomes are expected to guide existing built stock characterization and future data collection. acknowledgements the authors thank the national council for scientific and technological development (cnpq) for the phd [gmc, process # 140412/2018-2] and scientific productivity [vgs, #306048/2018-3] grants. references [1] h. birgisdottir, a. moncaster, a. h. wiberg, et al. iea ebc annex 57 ‘evaluation of embodied energy and co2eq for building construction’. energy and buildings 154:72–80, 2017. https://doi.org/10.1016/j.enbuild.2017.08.030. 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[13] r core team. r: a language and environment for statistical computing, 2021. [2022-02-16], https://www.r-project.org/. [14] h. wickham, m. averick, j. bryan, et al. welcome to the tidyverse. journal of open source software 4(43):1686, 2019. https://doi.org/10.21105/joss.01686. [15] a. kassambara. ggpubr: ’ggplot2’ based publication ready plots. r package version 0.4.0., 2020. [2022-02-16], https://cran.r-project.org/package=ggpubr. [16] m. maechler, p. rousseeuw, a. struyf, et al. cluster analysis basics and extensions. r package version 2.1.2., 2021. [2022-02-16], https://cran.r-project.org/package=cluster. [17] m. kuhn. classification and regression training. r package version 6.0-90, 2021. [2022-02-16], https://cran.r-project.org/package=caret. [18] a. kassambara, f. mundt. factoextra: extract and visualize the results of multivariate data analyses, 2020. [2022-02-16], https://cran.r-project.org/package=factoextra. [19] i. de jaeger, g. reynders, c. callebaut, d. saelens. a building clustering approach for urban energy simulations. energy and buildings 208:109671, 2020. https://doi.org/10.1016/j.enbuild.2019.109671. 175 https://www.r-project.org/ https://doi.org/10.21105/joss.01686 https://cran.r-project.org/package=ggpubr https://cran.r-project.org/package=cluster https://cran.r-project.org/package=caret https://cran.r-project.org/package=factoextra https://doi.org/10.1016/j.enbuild.2019.109671 acta polytechnica ctu proceedings 38:169–175, 2022 1 introduction 2 materials and methods 3 results and discussion 3.1 literature review and theoretical deepening 3.2 sample clustering 4 conclusions acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2024.47.0042 acta polytechnica ctu proceedings 47:42–46, 2024 © 2024 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague verification of material characteristic of natural fibers for concrete reinforcement jakub hájek∗, tomáš vlach, jakub řepka, vladimír žďára czech technical university in prague, faculty of civil engineering, thákurova 7, 166 29 prague, czech republic ∗ corresponding author: jakub.hajek@cvut.cz abstract. the topic of the article is the verification of the tensile mechanical characteristics of selected natural fibres commonly available on the czech market. due to their high tensile strengths, some natural materials have a certain potential for use in specific engineering applications such as concrete reinforcement. additionally, natural fibres are renewable materials, making them an environmentally friendly material. potentially, its use as a reinforcement in the form of technical textiles for textilereinforced concrete could help create more sustainable reinforced concrete elements. this article deals with tensile tests of chosen available natural fibre materials, such as flax, hemp, jute, and sisal, both pure and impregnated (homogenised) using epoxy resin. in this article, the results and comparisons of tensile strengths and young’s modulus are presented. keywords: natural materials, natural fibres, alternative reinforcement, tensile test, tensile strength. 1. introduction in recent years, there is great interest and pressure to reduce the environmental impact of structures due to the climatic crisis. natural materials, which are renewable, are of great interest in many scientific fields. due to the low price [1] and the relatively good characteristic of the material, natural materials have a great potential in the engineering industry. today, there are already many applications for use in polymer composites for e.g., the automobile or aviation industry [2]. from the point of view of lifecycle assessment (lca) [2], natural materials have a low environmental impact, because they are, for instance, renewable and composable [3]. generally, textile-reinforced concrete (trc) is reinforced by technical textiles from inorganic materials such as carbon, ar-glass, or basalt in a form of orientated rovings. the rovings are used as either pure or homogenised polymer matrix. the use of natural rovings or textiles in concrete could partially substitute the technical textiles, and technically this replacement is simply feasible. there is an option that both types of textiles might be used as hybrid reinforcement [4]. an important parameter for concrete reinforcement is the young’s modulus. in general, reinforcement should have a higher modulus of elasticity than the concrete matrix (around 20 gpa), because it needs to interact with the concrete matrix and must catch the crack development. also, the amount of reinforcement in the cross-sectional area of the element should be less than the amount of concrete. another basic parameter is the tensile strength, which is usually sufficient and thus the principle of composite behaviour of reinforced concrete is fulfilled. according to previous research, as seen in the presented table 1 below, and according to the availability of the czech market, four natural materials with the highest potentially mechanical performance were chosen for the basic testing of tensile mechanical parameters – flax, hemp, jute, and sisal. for reference, commonly used technical textiles were also experimentally verified – carbon, ar-glass and basalt rovings were used for testing and comparison. 2. preparation of samples from the czech company “agritec, výzkum, šlechtění a služby, s.r.o.”, two types of natural fibres were received: flax and hemp. these specimens had to be woven manually by hand, as seen in the left picture of figure 1. on the other hand, the czech company “juta a.s.” provided finished natural roving bulks of flax, hemp, jute, and sisal, so the work was easier as the rovings just needed to be cut into the required lengths. specimens were prepared for the tensile test. for the attachment of the sample to the claws of the hydraulic press, epoxy resin sleeves were required at the ends of the rovings. a silicone mould was created for the preparation of sleeves. there was an approximately 30 cm gap between the sleeves, as seen in the right image of figure 1. dimensions and test setup were prepared according to previous experience [6]. all samples were prepared in groups of 6 samples and in two variants – pure rovings without any treatment and homogenised rovings by epoxy resin. homogenised filaments of roving interact between themselves which leads to better stress distribution throughout the cross-section, and therefore the sample withstands higher loads. the impregnation was done on a prepared frame to ensure that the rovings will be properly stretched. the linear density of the rovings was around 1 600– 2 500 tex depending on the bought available material. 42 https://doi.org/10.14311/app.2024.47.0042 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 47/2024 verification of material characteristic of natural fibers fibre type annual production density tensile elastic elongation strength modulus at break [dry metric tons] [g cm−3] [mpa] [gpa] [%] softwood 1 750 000 000 1.4 100–170 10–50 8.0–14.0 hardwood 1.4 90–180 10–70 11.0–13.0 softwood kraft pulp 26 000 000 1.5 1 000 40 4.4 jute 2 300 000 1.3–1.5 200–770 20–55 2.0–3.0 sisal 378 000 1.5 100–800 9–22 3.0–7.0 kenaf 970 000 1.4–1.5 930 53 1.6 coir 100 000 1.2 180 4–6 30.0 flax 830 000 1.5 350–1 040 28–70 2.0–4.0 hemp 214 000 1.5 690 30–70 1.5–4.0 bamboo 30 000 000 0.6–1.1 140–230 11–17 4.0–7.0 wheat 720 000 000 0.6–0.8 10–200 1–12 2.7 rice husk 120 000 000 0.7–1.0 55 22 2.0–5.0 table 1. overview of properties of selected natural materials [5]. (a). (b). figure 1. hemp and flax fibres arranged in single rovings (a) and poured sleeves with epoxy resin (b). a full overview of the materials used and their characteristic is presented in the table 2. the results were logically calculated in stress so that they can be compared. t ρr ar [tex] [g cm−3] [mm3] flax 1 680 1.5 1.12 flax handmade 1 850 1.5 1.23 hemp 3 720 1.4 2.66 hemp handmade 1 800 1.4 1.29 jute 1 500 1.4 1.07 sisal 2 000 1.5 1.33 carbon 1 600 1.8 0.89 ar-glass 2 400 2.7 0.90 basalt 2 520 2.7 0.94 table 2. material characteristic of measured rovings and yarns. 3. experiment and results the experiment was carried out on a galdabini quasar 100 hydraulic press. the specimens were tested in tension with a constant load speed of 1 mm min−1. the force and displacement of the upper jaw was recorded. young’s modulus was measured by dic (digital image correlation). a large number of photos are taken during the experiment, and afterwards they are analysed in the software istra4d. identified unique areas of pixels are tracked, and deformations are counted. the samples were provided with small spackle pattern targets for a good measurement of the deformations, as seen in figure 2 and figure 3. the photos for the dic analysis were taken in an interval of 0.5 seconds. the impregnated samples were also measured with an extensometer (figure 3) for validation of the data from dic. samples without impregnation could not be measured with the extensometer, because the jaws 43 j. hájek, t. vlach, j. řepka , v. žďára acta polytechnica ctu proceedings figure 2. scheme of the tensile test setup. of the extensometer would damage the pure roving. the handling of the extensometer is also very time consuming. the pure rovings were twisted 5 times every time before the specimen was placed into the press jaws. by this the filaments were prestressed a little so that the distribution of force was better throughout the cross-section of the roving due to the mechanical interaction of single fibrils. then the filaments did not break too much one by one, and some data could be measured. [7] in the table 3, there are presented the results of measurements. there is a comparison of the young’s modulus measured by dic and extensometer on impregnated flax specimens. the values of both methods are very similar. the data measured with the extensometer have slightly lower deformations and therefore higher young’s modulus. but as seen in figure 4, some data, such as flax, have bigger differences. this inaccuracy might be caused by the experiment setup. around loading force of 250 n there was a jump in the data measured by dic. it was caused by the lower jaws of the press, which were slightly raised. the force of 250 n approximately corresponds to the weight of the steel part (15.6 kg) and friction in placement. therefore, the data measured with the extensometer are more precise and reliable. on the other hand, if the setup of the experiment is prepared thoroughly, then the dic method is much easier and faster to handle. (a). (b). figure 3. impregnated jute yarn with extensometer and dic target (left). pure sisal yarn with dic target after failure (right). σmax edic eext [mpa] [mpa] [mpa] flax epox 522 25 933 28 078 flax pure 126 8 104 flax epox handmade 370 40 201 43 418 flax pure handmade 107 11 706 hemp epox 315 18 702 20 849 hemp pure 150 5 390 hemp epox handmade 275 20 042 21 647 hemp pure handmade 115 5 913 jute epox 311 24 794 28 029 jute pure 161 7 188 sisal epox 349 21 188 22 905 sisal pure 231 8 135 table 3. measured data of stress and young’s modulus on natural yarns and technical rovings. the results in the table 3 and figure 5 clearly state that the impregnated rovings have higher strength and stiffness than the pure ones, because the impregnation allows to transfer the stress effectively through 44 vol. 47/2024 verification of material characteristic of natural fibers figure 4. comparison of the flax data measured by dic and extensometer. figure 5. stress-strain chart of representative samples of pure and impregnated samples. the whole cross-section. the filaments of pure rovings break one by one, which makes it very difficult to measure any relevant characteristic data of the material. on the other hand, it shows ductility, which might be potentially useful in the reinforcement of concrete. in addition, only impregnated rovings are evaluated as their values are more relevant to the theme of this paper. in terms of modulus of elasticity, the results are quite similar except for flax. handmade prepared flax roving shows the best average young’s modulus with the value of 43.4 gpa. generally, flax and hemp plants have a better structure of plant cells – microfibrils have a low angle deviation from the direction of growth, which is a good prerequisite for tensile strength and stiffness [8]. the reason behind the large deviation of handmade flax might be a different type of flax plant or also the process of specimen preparation. the filaments of handmade prepared roving were placed parallel one to each other, as seen in figure 1, but the commercially woven yarn is twisted, and the material might be more damaged by the process of making. 4. conclusion generally, it can be said that the theoretical assumptions were correct. flax seems to be the right material to continue the research of concrete reinforcement in the form of a technical textile. the best was hand-made flax with an average young’s modulus of 43.4 gpa. the best average tension strength had commercially bought flax with a value 45 j. hájek, t. vlach, j. řepka , v. žďára acta polytechnica ctu proceedings of 522 mpa. the differences between the rest of the natural materials were small, and it probably largely depends on the origin of the material. two methods of measuring the modulus of elasticity were tested – with dic and with an extensometer. both methods showed similar and solid results. the dic method is faster and easier to deal with, but the setup of the experiment must be well prepared. the extensometer method is more reliable but more demanding. the material characteristics of the chosen natural materials were described in this article. it can be said that measured flax data show good potential as a concrete reinforcement for specific applications, as the young’s modulus of concrete is around 20–40 gpa. acknowledgements the work on this paper was supported by czech science foundation grant no. 22-14942k entitled “possibilities of using natural fibers for the production of hybrid textile reinforcement in concrete” the authors would like to acknowledge all financial assistance provided to support this research. references [1] s. joshi, l. drzal, a. mohanty, s. arora. are natural fiber composites environmentally superior to glass fiber reinforced composites? composites part a: applied science and manufacturing 35(3):371–376, 2004. aiche 2002. https: //doi.org/10.1016/j.compositesa.2003.09.016 [2] r. k. malviya, r. k. singh, r. purohit, r. sinha. natural fibre reinforced 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modulus of elasticity of composite reinforcement produced from roving. in special concrete and composites 2014, vol. 1054 of advanced materials research, pp. 104–109. trans tech publications ltd, 2014. https://doi.org/ 10.4028/www.scientific.net/amr.1054.104 [7] r. chudoba, m. vořechovský, v. eckers, t. gries. effect of twist, fineness, loading rate and length on tensile behavior of multifilament yarns (a multivariate study). textile research journal 77(11):880–891, 2007. https://doi.org/10.1177/0040517507081280 [8] k. lau, p. hung, m.-h. zhu, d. hui. properties of natural fibre composites for structural engineering applications. composites part b: engineering 136:222–233, 2018. https: //doi.org/10.1016/j.compositesb.2017.10.038 46 https://doi.org/10.1016/j.compositesa.2003.09.016 https://doi.org/10.1016/j.compositesa.2003.09.016 https://doi.org/10.1016/j.matpr.2020.02.651 https://doi.org/10.1016/b978-0-08-100411-1.00009-1 https://doi.org/10.1016/b978-0-08-100411-1.00009-1 https://doi.org/10.14416/j.asep.2019.09.002 https://doi.org/10.1016/j.wasman.2016.04.037 https://doi.org/10.4028/www.scientific.net/amr.1054.104 https://doi.org/10.4028/www.scientific.net/amr.1054.104 https://doi.org/10.1177/0040517507081280 https://doi.org/10.1016/j.compositesb.2017.10.038 https://doi.org/10.1016/j.compositesb.2017.10.038 acta polytechnica ctu proceedings 47:42–46, 2024 1 introduction 2 preparation of samples 3 experiment and results 4 conclusion acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2023.42.0037 acta polytechnica ctu proceedings 42:37–45, 2023 © 2023 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague the possibilities of utilising the skiddometer t2go for forensic engineering tomáš kohout∗, pavel vrtal, adam hoffmann, tomáš blodek czech technical university in prague, faculty of transportation sciences, department of forensic experts in transportation, konviktská 20, 110 00 prague, czechia ∗ corresponding author: kohout@fd.cvut.cz abstract. the aim of the paper is to verify the applicability of the t2go skiddometer for measuring the coefficient of friction for forensic practice in accordance with the legislative framework of the czech republic. in the introduction, the article discusses the problem of friction coefficient measurement. the results of the comparison with dynamic measuring devices used in the czech republic and the determination of the mutual compliance rate with these devices, based on data obtained within the national comparative measurement action, are presented. the article concludes with an interpretation and discussion of the results. in cases where no sufficient mutual compliance was found, an analysis and reasoning of these results is provided. last but not least, based on the results, a procedure is proposed for future use of the t2go skiddometer in practice within the legislative conditions of the czech republic. keywords: forensic engineering, friction, walking speed measuring device. 1. introduction the assessment of the safety attributes of the road surfaces, airport surfaces, as well as pedestrian and cycling infrastructure surfaces is one of the most important ways to ensure an adequate level of surface safety, and therefore to reduce the societal losses caused by the inadequate quality of a particular surface. it is generally known that there is a relationship between the risk of an accident and the surface qualities, not only of the road surface, whereby as the skid resistance of the surface decreases, the braking distance increases and thus the risk of an accident increases [1, 2]. currently, one of the most used methods for assessing the quality of surfaces is the measurement by continuous friction measurement equipment (cfme), which determine the coefficient of friction by rolling the measuring wheel on the surface. the disadvantage of most of these devices is the measuring speed, which corresponds to the speed of normal traffic on roads, with the associated complications such as the need to provide road closures to maintain a constant measuring speed, the impossibility of carrying out measurements in difficult conditions due to the need for adequate space to achieve the measuring speed, high consumption of water sprinkling, or the equipment’s high operating costs. these disadvantages are eliminated by cfme devices such as the t2go, with the ability to continuously measure the coefficient of friction of the surface at walking speed, which limits the measurement of this device over long distances, but on the other hand has the ability to measure in confined spaces, at local accident sites, or on sidewalks or pedestrian crossings. the t2go can therefore work well as a complementary measuring device to standard dynamic meters for specific local investigations. this article discusses the experiment of comparative measurement of the t2go skiddometer with standard dynamic measuring devices, the aim of which is mainly to verify the accuracy of the t2go measurement, to determine the rate of mutual compliance of its measurement with standard dynamic measuring devices and also to examine possible ways resulting in the real use of the t2go in practice, in accordance with czech technical standards. 1.1. state of the art for the assessment of road surfaces, a large number of different measuring devices have been developed over time, especially those for the determination of the coefficient of friction by continuous measurement (cfme devices), which are often more suitable and considerably faster for practical use than the previously mentioned conventional stationary devices. at present, there is no uniform approach to the measurement of the coefficient of adhesion, nor to the specific specification of individual measuring devices. therefore, a large number of countries or companies involved in surface quality assessment develop their own measuring devices, which often differ from each other, e.g. in design, measuring speed, type of measuring tyre used, load on the measuring wheel, thickness of the water film required, slip ratio, slip angle, etc [3–5]. several foreign publications deal with the practical use of the t2go device for various purposes. some possible examples include the chang’an university article that discusses the effect of particle pollutants on skid resistance of ac-13 asphalt pavement on wet and dry surface. another research conducted by the shanghai normal university was focused on dynamic variation of the friction coefficient and its deterioration 37 https://doi.org/10.14311/app.2023.42.0037 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en t. kohout, p. vrtal, a. hoffmann, t. blodek acta polytechnica ctu proceedings mechanism if the road surface is covered with snow or ice. however, both of these studies are focused only on the change of the coefficient of friction due to a polluted road surface [6, 7]. the experimental use of the t2go device occurred within the testing of a special road surface, porous rubber pavement, at the university of waterlooo. this special type of surface is not currently widely used in pavements. in canada, its use is only limited to low volume traffic and low driving speed areas like parking lots and driveways. as a pavement material, its performance is still unexplored for the canadian climate. this is precisely the focus of the cited research. in this research, the t2go and british pendulum tester was used to measure the coefficient of friction of this surface and both devices showed similar values. however, it is a very specific surface with diametrically different characteristics than standard road surface types [8]. interconversion of locked-wheel and continuous friction measurement equipment friction measurements was the subject of virginia polytechnic institute and state university research. this research evaluates various methods to convert measurements from the traditional locked-wheel skid testers (lwsts) to cfme, proposing a speed adjustment-based approach (fr60) as the most effective, with comparable accuracy to the full ifi f60 formula but without the need for static reference measurements. the study confirms appropriate repeatability of the three tested devices across different speeds and surfaces. however, these are not devices similar in characteristics to the t2go device [9]. based on the above, it can be concluded that there are scientific publications regarding the use of t2go devices. however, this is only a simple use of the t2go device for various research tasks, not a comparison with other dynamic devices and a search for correlations within different measuring speeds. 2. methods for measuring friction one of the many critical factors for road safety in all categories is friction, i.e. the contact between the vehicle tyre and the road surface, as mentioned earlier. the assessment of the skid resistance of road surfaces can be carried out in several ways. indirectly, surface quality can be determined by analysing the texture of the unpolluted pavement surface, while direct methods of surface analysis are based on measuring the amount of friction between the surface and the measuring device. the most common and simplest method for assessing pavement texture is the so-called gauge (sand) method. the gauge method is used to determine the mean depth of surface texture (mean texture depth – mtd), in other words, it determines the total volume of surface depressions. of the group of methods for direct surface quality assessment by friction measurement, the british pendulum tester is probably the oldest and simplest. a large number of different measuring devices have been developed over time for the assessment of road surfaces, especially those for the determination of the coefficient of friction by continuous measurement (cfme devices), which are often more suitable and considerably faster than the previously mentioned classical stationary devices for practical use. currently, there is no uniform approach to the measurement of the friction coefficient, nor to the exact specification of the individual measuring devices. therefore, a large number of countries or companies involved in surface quality assessment develop their own measuring devices, which often differ from each other, for example, in design, measuring speed, type of measuring tyre used, load on the measuring wheel, thickness of the water film required, slip ratio, slip angle and others [3–5]. 2.1. t2go the t2go depicted in figure 1 is a hand-pushed, portable device capable of continuous measurement of the longitudinal friction coefficient, with a recording step of at least 0.1 m and at a walking speed of approximately 2.5–4.0 km h−1. due to the low operating speed, no space is required to run the device up to the measuring speed and the measurement can be started almost immediately after the device is set in motion. therefore, the t2go is suitable for measurements, in locations with insufficient access or safety level, for standard dynamic devices measuring continuously at travel speeds. the t2go measures the coefficient of longitudinal friction with a fixed slip ratio of 20 % when the measuring wheel is loaded with a force of 70 n and is capable of measuring on dry and wet surfaces and is equipped with a sprinkling system that applies a 0.5 mm water film in front of the measuring wheel on the surface to be measured [10]. slip ratio = slip rate measuring speed · 100 [%]. (1) figure 1. skiddometer t2go. the t2go skiddometer consists of two basic parts, the measuring device body and the operating terminal. the terminal is located on the handle of the device, 38 vol. 42/2023 utilising the skiddometer t2go for forensic engineering which also serves to provide a water supply, by a hose from an 18 l portable tank (carried by the measuring technician on his back), to the nozzle in front of the measuring wheel. the body of the device consists of a load-bearing steel chassis which holds a pair of wheels (with smooth tyres in accordance with astm e1844) in series behind each other and fitted with different sized gears which are connected by a v-belt. in this way, the slip ratio between the front measuring wheel and the rear running wheel is defined. during the measurement, the v-belt between the wheels is tensioned, depending on the roughness of the surface, and the force with which the belt is tensioned is measured by a load cell located in the center of the device above the belt, between the wheels. at the same time, the device is equipped with a sensor for detecting the wheel speed, on the basis of which, in combination with the measured applied force from the belt or the measuring wheel, the longitudinal friction coefficient is determined. the t2go skiddometer is also equipped with sensors for measuring temperature, humidity and a gps module for recording the position of the measuring device at the start and end position of the measured line. the entire measuring device system is powered by a 12 v li-ion battery with a capacity of 6 600 mah. the operating terminal on the handle of the skiddometer is used for basic operation, setting and calibration of the device, starting and stopping the measurement, or regulating the water supply to the device from a portable tank with an integrated pump. all data obtained during the measurement are transmitted on-line via bluetooth port to a tablet, which is part of the t2go skiddometer accessories and is equipped with a dedicated application for managing the measured data, such as storing it, displaying graphs of the measured longitudinal friction coefficient in real time, displaying the speed of the device movement and providing supervision of the appropriate measurement speed or the angle of inclination of the device handle via warning tones. 2.2. comparison the first available comparison (see table 1) is a comparison made by sarsys-asft itself, which establishes a comparison between the t2go and the t-5s skiddometer (at 65 km h−1), also of its own manufacture. this is a comparison of five measurements, over a 100 m length, made on both wet and dry surfaces, consisting of an unspecified mixture. the result of this comparison is a comparison between the two devices, which is 99.52 % for the measurements on the dry surface and 99.52 % on the wet surface [11]. another comparison is the comparison of the measurements (made by the swedish national institute for road and transport research – vti) of the t2go and the vti pft (portable friction tester). in this case it is a comparison of measurements made on ten different samples of wet road markings of different age dry measurements wet measurements t2go t-5s t2go t-5s 0.84 0.83 0.71 0.7 0.83 0.84 0.69 0.69 0.86 0.84 0.72 0.69 0.85 0.85 0.68 0.68 0.85 0.85 0.7 0.68 average average average average 0.846 0.842 0.7 0.688 table 1. comparison of results with t-5s. and quality. the results of this comparison, which can be seen in the table 2, clearly show similar results for the two devices being compared. 3. comparative measurement for the purpose of comparing the measurement results of standard dynamic devices with the t2go, two devices were selected, namely the bv 11 and the national reference device – tatra runway tester (trt). the recording step of both devices is 1.0 m. for this reason, the values obtained by t2go (every 0.1 m) were averaged into a format that corresponds to the data format of the trt and bv 11 device. the characteristics of the individual surfaces correspond to those that are standardly, most frequently, used in the czech republic and cover the entire range of the classification scale evaluating the coefficient of friction of road surfaces. these are mainly asphalt surfaces, cement concrete surfaces, various types of road markings and safety anti-slip surface. 3.1. individual results the input measured data for the following evaluation are of the following structure. for trt and bv 11, one data set corresponding to one measurement was used for each speed. this approach is possible due to the long-term guaranteed repeatability of the measurements of these devices. on the other hand, in the case of the t2go device, the average of the results obtained in three repeatable measurements is used for each comparison section. primarily, this was done to eliminate any unexpected deviation in the measurement. 3.1.1. asphalt surface the surface is characterised by its significant macrotexture, which is mainly determined by the coarseness of the aggregate used in the asphalt mix of the surface cover, which ranges from approximately 4–15 m. the aggregate used also has a relatively pronounced microtexture and protrudes considerably from the surface of the asphalt binder mixture. as can be seen in figure 2, the values of the coefficient of friction measured on this surface are relatively close to each other, across 39 t. kohout, p. vrtal, a. hoffmann, t. blodek acta polytechnica ctu proceedings surface measurement verifictaion deviation precision no. 1 2 device 1 0.74 0.73 0.76 0.02 0.01 2 0.52 0.5 0.66 0.14 0.01 3 0.59 0.6 0.65 0.06 0.01 4 0.66 0.62 0.69 0.03 0.04 5 0.63 0.64 0.65 0.02 0.01 6 0.67 0.72 0.67 0 0.05 7 0.73 0.71 0.67 0.06 0.02 8 0.48 0.5 0.39 0.09 0.02 9 0.67 0.73 0.69 0.02 0.06 10 0.7 0.63 0.66 0.04 0.07 average 0.64 0.64 0.65 0.05 0.03 table 2. comparison of results with vti pft (portable friction tester). figure 2. comparison of t2go × bv 11 × trt measurements on asphalt surface. figure 3. comparison of t2go × bv 11 × trt measurements on a cement concrete surface. all three devices, and approximately match the nature of the surface. 3.1.2. cement concrete surface this surface is formed, in addition to the binder, by aggregate with dimensions of 1–10 mm, while its upper layer is roughened and its projections thus fall primarily into the category of microtexture, with almost minimal macrotexture irregularities. while the friction coefficient values of the bv 11 and the trt are relatively regularly scaled depending on the speed, the friction coefficient values measured by the t2go are in a completely different range of values. the results are shown in figure 3. such significantly different measurement curves between the standard dynamic measuring devices and the t2go are due to the surface characteristics already mentioned and, above all, to the effect of the different measuring speeds of the measuring devices involved. the cement concrete surface has a minimal macrotexture due to the surface 40 vol. 42/2023 utilising the skiddometer t2go for forensic engineering figure 4. comparison of t2go × bv 11 × trt measurements on road markings. figure 5. comparison of t2go × bv 11 × trt measurements on anti-slip surface. modification and is thus evaluated as very slippery when measured with standard dynamic measuring devices, i.e. it has a low resulting coefficient of friction, due to the fact that the microtexture of this surface itself is not able to transfer the necessary amount of frictional forces from the measuring wheel at speeds of 20 km h−1 or more. however, the measuring speed of the t2go, 4 km h−1, is located in the interval where the microtexture has the largest effect on the friction coefficient, which is also reflected in the friction coefficient measured on this surface. 3.1.3. road markings the third and fourth surfaces are road markings without and with ballotine sprinkles, respectively. from figure 4, it is evident that the t2go skid tester measured significantly higher values for the road markings without ballotine sprinkles compared to other dynamic devices, despite the smoother microtexture of the surface, which subjectively feels slippery, aligning with the values obtained by standard dynamic measuring devices. the fourth measured sample is a road marking with ballotine sprinkles, where the surface is covered with abrasive balls up to 1 mm in size to improve the friction coefficient during driving. during t2go skid tester measurements, an opposite trend emerged, and the average friction coefficient for the horizontal marking with ballotine sprinkles was slightly lower compared to the marking without ballotine sprinkles, even though the ballotine sprinkles treatment created macrotexture on the horizontal traffic marking. this reduction in the average friction coefficient is due to the atypical surface of the ballotine sprinkles, which are smooth and lack microtexture compared to the marking without ballotine sprinkles, which has some minimal microtexture. on the other hand, when measuring with standard dynamic devices, the microtexture of the surface is not as crucial due to the higher measuring speed, and the added macrotexture created by ballotine sprinkles enhances its adhesive properties. 3.1.4. anti-slip surface it is a special mixture of binder and crushed bauxite, applied to an existing asphalt surface, with a very distinctive macrotexture and microtexture of individual bauxite grains. the surface texture corresponds to the measured averages of the bv 11 and the trt, which are at very high levels for all three measuring speeds. the average value of the coefficient of friction obtained by the t2go is also very high. the graph clearly shows that all devices show very similar values on this surface. therefore, a successful mutual compliance can be observed on this surface as demonstrated by figure 5. 41 t. kohout, p. vrtal, a. hoffmann, t. blodek acta polytechnica ctu proceedings figure 6. summary evaluation of t2go × bv 11 × trt device. asphalt cement concrete road markings road markings (sprinkle) anti-slip surface [⊘ fp] [⊘ fp] [⊘ fp] [⊘ fp] [⊘ fp] t2go 3 km h−1 0.72 0.91 0.62 0.57 0.84 table 3. average results of t2go. national reference trt measuring device [⊘ fp] [%] [⊘ fp] [%] [⊘ fp] [%] [⊘ fp] [%] [⊘ fp] [%] trt 40 km h−1 0.66 91.60 0.43 47.02 0.19 30.00 0.43 75.21 0.70 83.63 trt 60 km h−1 0.63 88.14 0.30 33.48 0.14 22.49 0.37 65.43 0.67 80.68 trt 80 km h−1 0.60 83.18 0.24 25.93 0.12 18.92 0.36 62.90 0.64 76.61 table 4. percentage agreement of devices t2go and trt. skiddometr bv 11 [⊘ fp] [%] [⊘ fp] [%] [⊘ fp] [%] [⊘ fp] [%] [⊘ fp] [%] bv 11 40 km h−1 0.77 93.15 0.53 58.09 0.23 36.61 0.44 76.63 0.83 99.30 bv 11 60 km h−1 0.69 95.77 0.29 31.42 0.14 22.87 0.36 62.47 0.78 92.86 bv 11 80 km h−1 0.65 90.13 0.19 21.35 0.12 19.17 0.33 58.70 0.73 86.90 table 5. percentage agreement of devices t2go and bv11. 3.2. overall results based on the above analysis of the measured data on these surface types, it is clear that the mutual compliance between the t2go and standard dynamic measuring devices only occurs on specific surface types that indicate a sufficient representation of macrotexture, but also a certain quality of microtexture within the surface structure. a summary of the results of the comparative measurements with trt and bv11 is presented in the following figure 6. this trend can also be observed from the percentage agreement of measurements with t2go, shown in table 3, table 4, and table 5 compared to standard dynamic measuring devices, where the highest level of agreement was found on asphalt surfaces with safety anti-skid treatments, which have the most pronounced macrotexture. some agreement of measured data was also observed on horizontal traffic markings with abrasives, although it was likely negatively influenced by the nature of the microtexture, to which t2go measurements are highly sensitive due to its low measuring speed. figures 7 and 8 below show the resulting comparison of the t2go results compared to the trt and bv 11 devices on each surface. this graphical illustration indicates again a significant dispersion between the 42 vol. 42/2023 utilising the skiddometer t2go for forensic engineering figure 7. comparison of t2go and trt. figure 8. comparison of t2go and bv 11. measured results on the cement concrete surfaces and the two types of road markings. 3.3. the issue of different measuring speeds based on the conducted research, knowledge of technical specifications of the compared devices, and the results of comparative measurements, it is evident that the results of measurements by dynamic devices at higher speeds (trt and bv 11) are significantly influenced by the quality of macrotexture. conversely, the microtexture of the surface does not have an impact on the results due to the high measurement speeds, as the texture of individual parts of the surface layer cannot be taken into account from a physical perspective at these speeds. however, at very low measuring speeds, as used by t2go, the results are almost exclusively affected by the microtexture of the surface. at such low speeds, the t2go skiddometer is capable of considering the micro-textural properties of the surface, but for the same reason, it is unable to assess the qualities of macrotexture. however, it is not entirely necessary to achieve identical values with the t2go skiddometer as with dynamic devices, but it is at least essential to identify a similar trend within the measured data. in this case, it is possible to determine a conversion coefficient between the compared devices. however, this can only be done for two out of the five tested surfaces. from the above-mentioned partial measurement results on individual surfaces, it is evident that there is a discrepancy in the outcomes, which is caused by different measuring speeds in relation to distinct qualitative states of microtexture and macrotexture. on surfaces that show significant or at least sufficient quality in both microtexture and macrotexture, satisfactory results were attained (asphalt and antislip surface). the compared data demonstrate very similar values or reflect a similar trend, indicating a successful possibility of using the t2go skiddometer in practice. at the same time, based on the findings, it cannot be definitively stated that the friction coefficient values determined by the t2go skiddometer, especially on surfaces with unsuccessful mutual compliance, are unequivocally unsatisfactory. they may be considered as values that are suitable for other speed levels, given the characteristic measurement capability of the t2go skidometer, which measures at speeds of 2.5–4 km h−1. as mentioned earlier, due to this measurement speed, the microtexture of the surface is predominantly considered, which is why the friction coefficient values on surfaces with dominant microtexture and less favorable macrotexture are considerably higher compared to measurements 43 t. kohout, p. vrtal, a. hoffmann, t. blodek acta polytechnica ctu proceedings conducted by standard dynamic measuring devices. based on this reasoning, we can say that the t2go skiddometer is potentially applicable for assessing antiskid properties of surfaces in areas where the traffic speed does not exceed 20 km h−1, a speed at which the influence of microtexture on anti-skid properties is most significant. such areas may include pedestrian and cyclist infrastructure, pedestrian crossings, or certain road sections in close proximity to pedestrian crossings, where vehicles decelerate and move at low speeds. 4. discussion all comparative measurements performed with the t2go allow significant clarification of the correlations and deviations in the measurements reported by the manufacturer, which incompletely assess the measurement capabilities of the t2go only on selected types of surfaces. the mutual compliance of the t2go with trt and bv 11 measuring devices on five reference surfaces was assessed. the results of these comparative measurements indicate that the overall mutual compliance of the t2go with standard dynamic measuring devices is too low to consider a successful correlation. when considering the mutual compliance of the t2go with other measuring devices on individual surfaces individually, the results are much more acceptable. in fact, a mutual compliance was found between the trt national reference device and the bv 11 on asphalt surfaces. a sufficient level of mutual compliance has been identified for these two standard dynamic measuring devices even on the antislip surface. for these surfaces and surfaces with similar characteristics, with a dominant macro-texture (and also some micro-texture), the realistic use of the t2go for the assessment of the surface properties of conventional road surfaces can be considered. an insufficient mutual compliance was found in the case of the cement concrete surface and the road markings without balotine sprinkling. thus, the use of the t2go for assessing the slip resistance properties of these 303–304 surfaces and, in general, of surfaces with predominantly microtextured road surfaces in practice cannot be recommended. 5. conclusions at present, it is not technically possible to implement the t2go skiddometer in the czech standards in the usual way, as the standards and technical specifications concerned do not recognise the concept of dynamic measuring devices operating at walking speed. assuming that the european technical specification cen/ts 15901, following the example of astm e3304-22, for the t2go skiddometer is obtained, it is necessary to modify the measuring speed, suitable for dynamic measuring devices operating at walking speed, in the technical specification tp 207, which establishes the conditions of participation and the course of the equipment accuracy experiment. another necessary modification to this technical specification is the addition of an alternative method of verifying the accuracy and correlation of dynamic measuring devices operating at walking speed with the national reference measurement device. a possible solution is to establish a comparison procedure for these devices with the british pendulum tester, which is more suitable for these purposes than standard dynamic measuring devices, even though the british pendulum tester device is only able to measure pavement surface characteristics point by point. another possible solution seems to be the establishment of a separate standard for the assessment of the characteristics of surfaces with low speed of traffic, including pedestrian and cycling infrastructure. this standard would address the assessment of these surfaces by dynamic measuring devices operating at walking speed, with an upper limit of approximately 20 km h−1 for the operating speed of the surfaces assessed under this standard. therefore, the t2go is currently only applicable in the context of forensic expertise for the purpose of indicative determination of the coefficient of friction on asphalt surfaces and generally on surfaces with a dominant surface macrotexture. references [1] c.-g. wallman, h. astrom. friction measurement methods and the correlation between road friction and traffic safety: a literature review. swedish national road and transport research institute, 2001. [2] b. mataei, h. zakeri, m. zahedi, f. m. nejad. pavement friction and skid resistance measurement methods: a literature review. open journal of civil engineering 06(04):537–565, 2016. https://doi.org/10.4236/ojce.2016.64046 [3] t. andriejauskas, v. vorobjovas, v. mielonas. evaluation of skid resistance characteristics and measurement methods. in the 9th international conference on environmental engineering. selected papers, vol. 9, p. 1. 2014. https://doi.org/10.3846/enviro.2014.141 [4] p. iwanowski, k. blacha, m. wesołowski. review of modern methods for continuous friction measurement on airfield pavements. iop conference series: materials science and engineering 356:012002, 2018. https://doi.org/10.1088/1757-899x/356/1/012002 [5] y. xue, p. li, s. jiang, et al. evaluation of skid resistance of asphalt pavement covered with aeolian sand in desert areas. wear 516–517:204620, 2023. https://doi.org/10.1016/j.wear.2023.204620 [6] p. cao, x. yan, x. bai, c. yuan. effects of contaminants on skid resistance of asphalt pavements. in traffic and transportation studies 2010, pp. 1341–1351. american society of civil engineers, 2010. https://doi.org/10.1061/41123(383)128 [7] h. li, c. song, z. guo, n. xu. dynamic variation and deterioration mechanism of the friction coefficient between tire and pavement under the icy and snowy circumstances. international journal of pavement 44 https://doi.org/10.4236/ojce.2016.64046 https://doi.org/10.3846/enviro.2014.141 https://doi.org/10.1088/1757-899x/356/1/012002 https://doi.org/10.1016/j.wear.2023.204620 https://doi.org/10.1061/41123(383)128 vol. 42/2023 utilising the skiddometer t2go for forensic engineering engineering pp. 1–13, 2022. https://doi.org/10.1080/10298436.2022.2148160 [8] t. kabir, a. g. oyeyi, h. al-bayati, et al. porous rubber pavement – in situ performance evaluation of stiffness and friction in canada. in proceedings of the canadian society of civil engineering annual conference 2021, pp. 95–105. springer nature singapore, 2022. https://doi.org/10.1007/978-981-19-1004-3_7 [9] s. b. quiros, g. w. flintsch, e. de león izeppi, k. k. mcghee. interconversion of locked-wheel and continuous friction measurement equipment (cfme) friction measurements. transportation research record: journal of the transportation research board 2672(40):452–462, 2018. https://doi.org/10.1177/0361198118797455 [10] s. fritz. operators handbook t2go. köpingebro, sweden, 2016. [11] c. fors. validering av mobila instrument för funktionsmätning av vägmarkering: intyg. 2018. se-581 95, linköping, sweden, 2018. 45 https://doi.org/10.1080/10298436.2022.2148160 https://doi.org/10.1007/978-981-19-1004-3_7 https://doi.org/10.1177/0361198118797455 acta polytechnica ctu proceedings 42:37–45, 2023 1 introduction 1.1 state of the art 2 methods for measuring friction 2.1 t2go 2.2 comparison 3 comparative measurement 3.1 individual results 3.1.1 asphalt surface 3.1.2 cement concrete surface 3.1.3 road markings 3.1.4 anti-slip surface 3.2 overall results 3.3 the issue of different measuring speeds 4 discussion 5 conclusions references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2024.46.0094 acta polytechnica ctu proceedings 46:94–100, 2024 © 2024 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague methodology for measuring the potential of a tourism destination in relation to sustainable local tourism product development lucie plzákováa,∗, šárka tittelbachováb a czech technical university in prague, masaryk institute of advanced studies, kolejní 2637/2a, 160 00 prague 6 – dejvice, czech republic b metropolitan university in prague, department of tourism, dubečská 900/10, 100 00 praha 10 – strašnice, czech republic ∗ corresponding author: lucie.plzakova@cvut.cz abstract. tourism is very strongly associated with the potential of the area, including the society, in which the tourism activities take place. this potential is often fragmented and it depends on the skills of local actors, stakeholders, and tourism management structures to discover and connect it effectively. and it does not matter whether the potential is tangible or intangible, natural or cultural. the potential of each territory is unique and needs to be summarized, measured, evaluated, and categorized for its effective and sustainable use. this article presents the first part of a methodology for the creation of a local tourism product focusing on the measurement of potential. the methods used in this process are based on decision analysis and process management methods. the methodology is addressed to all tourism actors in each destination, in particular managers of destination management organizations, local authorities, or other initiating entities. the ministry of regional development certified the methodology for the creation of a local tourism product and recommended its use to municipalities and tourism destinations. keywords: tourism, tourism destination, measuring of the potential, local tourism product. 1. introduction to illustrate the importance of the tourism sector, it has long been the third fastest growing economic sector in the world. the world tourism organization (unwto) reports that the number of international arrivals increased 56 times between 1950 and 2018. the number of international tourist arrivals worldwide grew by 4 % in 2019 to reach 1.5 billion. while the year-on-year growth rate declined (against 7 % and 6 % in the previous two years), it was still a growing sector with an impact on the global economy as a share of 10.4 % of global gdp (usd 10 trillion) in 2019. tourism has been affected particularly hard by the covid-19 pandemic, with international arrivals falling by 73 % in 2020 [1]. compared to other crises (e.g., oil crises, the global financial crisis, the gulf war), there has never been such a huge drop in tourism as in 2020. in 2022, the whole world was expecting a recovery of the global economy and also a recovery of tourism to a level comparable to 2019. however, this did not happen. there are several reasons for this, including the long-standing restrictions and border closures in asian countries, inflation, rising energy and gas prices, and last but not least, the war in ukraine, which is having the greatest impact on tourism, especially in europe. the rapid growth in tourism demand prior to 2019 has placed an excessive strain on environmental resources and cultural assets, such as physical attractions or local monuments. this has been compounded by the affordability of travel, due to factors such as the emergence of low-cost airlines, the development of information and communication technologies, the automation and later robotization of processes, the expansion of accommodations based on sharing or collaboration, and the orientation of customers on speed, and experiences and their sharing in real time, leading to the phenomenon of “over-tourism”. tourism destinations have been overloaded, the environment for residents has been affected, and many cities and places around the world have begun to fight back against this phenomenon. examples are the big cities of venice, berlin, and barcelona, as well as destinations of a local nature such as hallstatt. according to pearce [2], not only residents and businesses can be affected by overcrowded places, but ultimately tourists themselves can be too, if the dissatisfaction of residents reaches its limit and escalates into outright aggression. such a situation reveals that the potential of the destination is at its limits or is not being properly exploited. tourism potential is a set of territorial conditions and prerequisites for tourism development, which creates a complex multidisciplinary system. the system includes aspects of the natural environment, cultural and historical heritage values, and the manifestations of current activities of human society. tourism potential is very important because an area must have certain conditions (prerequisites, factors) to become a tourism destination. the world tourism organiza94 https://doi.org/10.14311/app.2024.46.0094 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 46/2024 methodology for measuring the potential of a tourism destination . . . tion defines a tourism destination as a physical space where the tourist spends at least one night [3]. in order to optimally the potential, it is necessary to know how to work with it. this means to know, to be able to measure, to evaluate, and to classify the potential. only then is the potential ready to become a part of a sustainable tourism product. this paper deals with setting up the methodology for measuring the potential of a tourism destination in relation to sustainable local development. 2. materials and methods 2.1. tourism in regional/local development theories resource-based theory states that a firm’s competitive position depends on a unique set of resources and the relationships between them [4, 5]. if we apply this approach to a tourism destination, we can state that the destination’s potential constitutes the competitive advantage of each unique destination through the creation of the key tourism product. in terms of regional development, a neo-endogenous approach (i.e., an approach based precisely on the exploitation of local resources) will be employed. neoendogenous regional development has been related very often to rural areas development. in the case of tourism, even if it is also about local products in cities, the neo-endogenous approach fits well, as the basic characteristic of tourism is that “tourism is tied to its place”, meaning to the potential of a particular territory. it’s more about neo-endogenous thinking in different contexts [6]. the concepts of endogenous and neo-endogenous development are based on the principles of subsidiarity, participation, and the use of the region’s internal potential in a bottom-up approach. neo-endogenous development, based on the balanced use of local resources and/or traditions, generates economic growth, increases regional productivity, and counters the trend towards uniformity by embracing a diversity of cultures, social laws, technologies, and knowledge. within the neo-endogenous concept of regional development, a new strand is built on knowledge, namely the theory of learning regions, where knowledge is the most important asset. according to skokan [7], a learning region is “a more organized collaboration of a broader range of civil society organizations, firms, institutions, and public authorities that are embedded in social and regional structures”. florida [8] makes an interesting point about the relationship between the learning region approach and sharing when he notes the need to connect and share data, information, and actors in the region’s productive infrastructure. this is the only way to achieve a learning process. the local focus underscored in the neo-endogenous approach calls for local knowledge, local resources, and the engagement of local people to be central to development processes [9]. tourism development is built on the unique potential of a place combined with human and social capital. as each individual is part of the socio-cultural potential and is also the one who activates and transforms this potential, the role of the individual in local/regional development is crucial in relation to sustainable tourism development. the consideration of the individual and/or the service sector has generally been historically neglected in regional development theories. a turning point in the perception of tourism as part of regional development occurred in the 1990s and early 2000s, when the importance of tourism to the global economy was demonstrated through the tourism satellite account. conceptualizing tourism within regional development theories is highly complex. the goods offered in the tourism market are non-stackable, they cannot be relocated (tourism is locally bound to natural and socio-cultural potential), and to consume them the consumer (visitor, tourist) must travel from one region to another with the expectation that “something” non-transferable and otherwise non-tradable will become tradable [10]. moreover, consumption cannot be completely separated from that of the local population. luzzi and flückiger [11] state that “tourism is rather a set of cross-cutting activities that are available for consumption by local residents and visitors participate in that consumption”. in essence, tourism offers a set of services with a differentiated product created based on the uniqueness of each destination. the approach of neo-endogenous regional development and also learning regions are reflected in the proposed methodology introduced below. 2.2. destination potential the potential of the destination is the default value of each destination that should know its potential, it should be able to evaluate and use it. when assessing the potential for desalination, three main groups of factors are taken into account: (a) localization factors such as the attractiveness of the local environment and landscapes or local cultural values and remarkableness; (b) realization factors such as transport prerequisites (availability, or time achievability) and material technical prerequisites that express the amenity of the area with accommodation, catering, sports, entertainment, and other facilities; and (c) selective factors or conditions such as the level of destination management, the level of cooperation between individual entities, the perception of the importance of tourism policy, which depends on the level of public support of tourism, and the income situation of potential visitors (objective stimulation factors), as well as psychological and marketing factors such as the position of the evaluated territory against the competition, its image for the desired target groups, and the ability to fulfil their wishes and expectations [12]. 95 lucie plzáková, šárka tittelbachová acta polytechnica ctu proceedings mazur et al. [13] developed a synthetic index of environmental and landscape values for tourism development, noting that their methodology is only one of many possible measures for assessing the current state of tourism development. on the other hand, there are no methodologies to measure the whole potential of a destination, not just one part (spatial, cultural, natural, etc.). such a methodology is missing. 2.3. methods and data decision analysis methods (e.g., saaty’s method for determining weights), management strategies such as brainstorming, process management, and sociological research methods and techniques, namely questionnaires and in-depth interviews, were used to meet the objective of the article. saaty [14] introduced a method for deriving “priority weights” associated with a set of mutually exclusive alternatives. priority weights – or simply priorities – are positive numbers that add up to one, reflecting the importance of alternatives judged by different criteria. this imaginative technique has found various applications in fields such as marketing, health planning, transportation studies, and personnel studies [15]. details of the method are described in saaty’s work [16]. saaty’s method is used in table 3. sociological techniques of quantitative and also qualitative research were used. two questionnaire surveys were conducted in 2021 in six countries: the czech republic, poland, latvia, bulgaria, croatia, and turkey. the first survey focused on destination management organizations (dmo); responses were received from 56 organizations, with the following main findings in relation to measuring and working with territory potential: • dmos consider potential assessment as a key activity; however, this activity is very often outsourced by the organizations responsible for tourism management in the territory. this is due to the lack of competencies, and moreover, external processing is certainly more expensive than an employee equipped with their own competences. • the organizations responsible for tourism management at the regional level lack a methodology for assessing and categorizing potential. based on the second survey of 654 respondents (again from the above-mentioned countries), service providers pointed out that almost one-fifth of them do not use any potential of the destination, while 82 % make use of the potential of the destination for their business, mostly its natural and cultural tangible potential. in-depth interviews were used to determine the extent to which the potential (in the context of a project linked to the czech tradition of fishing and fish farming, which means natural potential) is being managed by stakeholders, such as destination management organizations, service providers, public institutions, and local authorities. twenty-four stakeholders were interviewed in the tourism sector in the czech republic, with the finding that they do not work conceptually with the destination potential. all of these findings led to the need to set up a tool for measuring the potential of the area (whether or not it is a tourist destination) and its subsequent use, so that stakeholders can work with it conceptually. this should bring more control over the use of the destination’s potential, reduce overcrowding in certain parts of the destination, and on the contrary, exploit the potential that has not yet been used. in this paper, we will only discuss the first part of the local tourism development process, namely the part that focuses on summarizing, evaluating, assessing, and classifying the potential. the other steps leading to the complete development of a sustainable local tourism product (e.g., learning groups, conceptualization, indicator setting, auditing, and innovation) are not the subject of this article. 3. results identifying and assessing potential is a key element in the process of creating a sustainable local tourism product. this activity should be undertaken by any tourism destination aiming to become a competitive and sustainable tourism destination in the national and/or international market. summarizing and sorting the potential is the first step in the whole process, and it is built on the basic components of a successful tourism destination. potential is assessed based on six key attributes for creating a tourism product in a destination: • usp and authenticity – the unique selling proposition (usp) represents “what our product or brand has that other do not”. this is the unique characteristic of a product or brand that differentiates it from competitors and is also relevant to the target audience. the usp is the main reason to visit a destination. at the same time, authenticity is linked to this. that is, what we have in our territory that relates both to the territory itself and/or to the traditions, history, culture, and way of life of the local people. • accessibility – the attribute of accessibility tells us whether the potential we have is accessible, in terms of both time (seasonality, opening hours for the public) and place (there is a road leading to it, it is accessible to wheelchairs, access to the property is allowed, etc.). we are therefore assessing the accessibility to visitors and the quality of this access (absence of barriers). • the range of services related to the potential (entities offering basic services) – this includes accommodation, catering, and transport. it is the presence of these providers and the services they offer which, by their existence, make the potential 96 vol. 46/2024 methodology for measuring the potential of a tourism destination . . . potential assessment usp availability scope of services (which are linked to potential) management system (where potential occurs) total max 90 points final valuation (result from table 3) tim e lo ca l ac co m m od at io n bo ar di ng tr an sp or t to th e re gi on ex ist en ce an d qu al ity of de st in at io n m an ag em en t sc op e of ne tw or ki ng m ar ke tin g st ra te gy po te nt ia l n at ur al ta ng ib le a1 a1x1 a1x2 a1x3 a1x4 a1x5 a1x6 a1x7 a1x8 a1x9 ∑ ax a1 value a2 a.. in ta ng ib le a.. an c ul tu ra l ta ng ib le b1 b2 b.. in ta ng ib le b.. bn table 1. identification and assessment of potential. 97 lucie plzáková, šárka tittelbachová acta polytechnica ctu proceedings point range description of assessment ≤ 1.9 meaningless; very low; very inadequate; there is no potential 2–3.9 minor; low(s); inadequate; inoperative; low quality 4–5.9 moderately significant; medium; adequately sufficient; functional; average quality 6–7.9 significant; high; rather sufficient; very functional; very high quality 8–10 very significant; very high; quite sufficient; fully functional; excellent table 2. potential assessment process. each item is worth 10 points. total 90 pts. potential evaluation total points weight final value (from the previous table) (points * weight) usp and authenticity a1x1 0.3 availability (total) a1x2 + a1x3 0.25 scope of services (total) a1x4 + a1x5 + a1x6 0.15 dmo – existence, quality a1x7 0.1 scope of networking a1x8 0.12 marketing strategy a1x9 0.08 total ∑ a1xn a1 value table 3. valuation of potential. exploitable. here we assess the existence of the provider and the quality of the services. • dmos – we assess the very existence and quality (competence) of destination management organizations (dmos) in the territory on which we carry out the summarization and evaluation of the potential for the exploitation of local product creation. • processes or scope of networking – this entails setting up cooperation, communication, and coordination between all public administration entities, business entities, non-profit organizations, and local residents in the territory. • marketing strategy – this sets the framework for the product under consideration (segmentation, demand, product portfolio), including realistic sources of funding: smart objectives, evaluation methods, and tactics. here we evaluate whether there is a marketing strategy for the destination (territory) where we are setting the local tourism product (the destination itself or the wider destination to which our territory belongs). we evaluate the existence of the strategy and its quality (whether it is implemented, monitored, or evaluated). first, the potential of the place/destination/region is summarized and then sorted into two basic categories: natural potential (tangible and intangible) and cultural potential (tangible and intangible). furthermore, tourism services (transport, accommodation, and catering) and the tourism management system and its quality are identified. for this step, we will use table 1, which we will also work with in the following step, namely the assessment of the potential. the assessment takes place in two steps. the first step is to assign each attribute for the selected potential a score value from 0 to 10. the assignment of the points will be done in accordance with table 2. the second step is to rate each potential item and then assign weights to it. the weights are used to differentiate the importance of the above-mentioned key attributes for creating a local product. the weights for each attribute are recorded in table 3, where we already get the final valuation of the potential. for each potential, the valuation must be calculated separately, with the final value entered in the last column of table 1, where we can then sort and compare all of the potentials. based on the assessment of the potential, we gained an overview of the potential available in the area and its value for creating a local product. next comes the step of categorizing it, which means assessing whether the potential is suitable for inclusion in the local product creation process. the categorization of the potential is based on table 4. according to the result of the summarization, sorting, assessment, and valuation, we classify the potential into four categories: • category 1 – unusable potential is not exploitable for local product development. we discard such potential from the process and do not work with it further. • category 2 – sleeping potential. in this category, we classify potential that shows strong barriers to use it for local product development (ownership and related activities not intended for tourism participants, lack of funding to make the attraction operational, etc.). • category 3 – exploitable potential with limitations. in this category, we include the potential usable once the conditions for exploitation are met (the completion of the renovation, a willingness to open 98 vol. 46/2024 methodology for measuring the potential of a tourism destination . . . total points scored categories of potential ≤ 3.1 unusable potential 3.2–5.2 sleeping potential 5.3–8.2 exploitable potential with limitations 8.3–15.5 potential for immediate use * if the value is higher than 11, beware of capacity! table 4. categorization of potential. figure 1. process of measuring the value of the potential for local tourism product development. the facilities, the creation of an offer for visitors, etc.). • category 4 – potential for immediate use. in this category, we classify the potential that can be used immediately in the process of local product development. the whole process of measuring the potential is illustrated in figure 1. furthermore, the potential serves as an input to the creation of a local tourism product with subsequent phases of the process: definition of the local product concept, implementation, audit and evaluation, and innovation, as previously mentioned. throughout the process, a local learning group is established, involving all stakeholders, including local residents. 4. conclusions the methodology for the development of local tourism products, including its initial phase for measuring the value of local potential (which is the subject of this paper) has been piloted in the tourism destination of czech canada and successfully implemented in the destination of the moravian-silesian region in the czech republic, as well as in other 5 countries: poland, bulgaria, latvia, turkey, and croatia. until recently, this region was largely underdeveloped in terms of tourism, with neither foreign nor domestic visitors showing interest. a fundamental change has occurred as a result of the development of a regional and local tourism management system in the region in the form of destination management. the whole community – residents, public regional and local administration, cultural institutions, non-profit organizations, and business entities – has worked for many years on the process of developing, creating, and implementing a tourism product based on industrial heritage. using the proposed methodology for measuring potential, the components of potential were identified on which the highly successful tourism product called technoroute was built. technoroute – the industrial heritage trail – is a key product of the north moravia and silesia tourism region, and it introduces visitors to unique industrial heritage such as mines, open pits, mills, and defunct factories. the key product contains over 30 activities and 28 entities from the commer99 lucie plzáková, šárka tittelbachová acta polytechnica ctu proceedings cial and non-commercial spheres that cooperate on it. however, it is a living mechanism that may have a lower degree of variability depending on visitor demand and available supply. the product was initially launched in 2012 and has been dynamically developed and improved since then. in 2019, 1.7 million tourists visited the route or one of its attractions, with a direct economic benefit of usd 273 million. the above example shows that if the potential of a place is properly harnessed, even a region affected by industry can become a successful key tourism product by telling its stories; conveying pride, knowledge, tastes, and smells; and generating a notable monetary benefit. acknowledgements results from two projects were used to write this article: demina project: project innovation and continuing education in destination management, no. 2020-1cz01-ka202-078343, erasmus+ programme of the european union and cultural tradition of czech fishing in the light of its use in tourism and gastronomy, naki: dg18p02ovv057. references [1] world tourism organization – unwto. world tourism barometer, 2022. 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https://doi.org/10.2478/mgr-2021-0009 https://doi.org/10.1016/0022-2496(77)90033-5 https://doi.org/10.1016/0022-2496(84)90013-0 acta polytechnica ctu proceedings 46:94–100, 2024 1 introduction 2 materials and methods 2.1 tourism in regional/local development theories 2.2 destination potential 2.3 methods and data 3 results 4 conclusions acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2024.47.0058 acta polytechnica ctu proceedings 47:58–62, 2024 © 2024 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague processing and recycling of aggregate residues after ore extraction for stabilization of mined areas andrea jančíková∗, karel dvořák, simona ravaszová brno university of technology, faculty of civil engineering, veveří 331/95, 602 00 brno, czech republic ∗ corresponding author: andrea.jencikova@vut.cz abstract. the article focuses on the treatment and recycling of aggregate. it also addresses the issue of treatments effects on mine tailings, fly ash and acid mine water on the measured parameters of concrete with load-bearing and backfilling paste with non-load-bearing function. four different mixes were developed, and standard tests were performed on them. these mixtures were made with different types of mine tailing’s fillers with varying degrees of treatment. mineralogical and chemical tests were carried out on the treated aggregates, with sieve analysis and their subsequent adjustment for suitable aggregate gradation curve in concrete. strength and volumetric weight tests were carried out on hardened samples at 28, 56 and 90 days. the results of the determined functional specimens showed that it is possible to develop an almost completely waste-free technology using recycled aggregate and fly ash from the ore mining and energy industries and its subsequent processing and use for backfilling of mine deposits with load bearing and non-load bearing function concrete. keywords: mined residues, mine tailings, mechanical properties, filling and static requirements. 1. introduction the mining industry is divided into two major categories, metallic and non-metallic, based on the ore extracted. both industries lead to the generation of large quantities of solid industrial wastes from mine expansions [1]. a large amount of sub waste generated from the processing of metallic ores in the separation of valuable minerals, this waste is called tailings [2, 3]. the generation of large quantities of tailings adversely affects the environment, including air, water, and soil [4, 5]. consequently, a large area of surface land is required for the appropriate storage and management of this waste. an effective solution is the underground disposal of these industrial wastes (tailings, fly ash, mud, etc.) generated from mining and related activities in the form of mine backfill [6–8]. the spaces created after ore extraction in underground mines can be filled with a specific waste material referred to as backfill mix of cement and mined aggregate, fly ash, in a consistency that can be well pumped and transported. backfill material can be divided into two major categories, the first being unconsolidated backfill without static function and the second being backfill with cement and static function [4–6]. unconsolidated backfill, waste is added without any binder. while in cemented backfill, a hydraulic binder is added, usually this binder is portland cemen, which is added to the solid waste material to provide a background for the mechanical properties of the paste or concrete [7, 8]. one form of cemented backfill used in underground mines is a paste with well-defined mechanical properties [3–7]. mine fill mixtures should be homogeneous, mobile, suitable for transport by pumping over distances of hundreds of meters, and workable for a minimum of 120 minutes. after curing, they should ideally reach a strength of at least 4 mpa after 90 days; however, a strength of about half 0.1 mpa may be acceptable in conditions without static function. the aim is to completely minimize the binder component, which should only have the function of ensuring the cohesion of the cured composite. one way to achieve this is use of fluidized power plant fly ash, which is not used at all to produce conventional concretes. in this case, even total replacements of conventional cements as binders are realistic options. fluidized fly ash from “ledvice” was used as a binder for number of samples in this article. backfill mixture for stabilization of old mining sites. the important requirement here is not ultimate strength but low hydration heat development and workability. the residue after separation of heavy and light recoverable components from the “zlaté hory” landfill was used to create the mixture without static function. the residue after separation of heavy and light recoverable components from the “cínovec” deposit was used to form the second mixture with static function. an important requirement is ultimate strength and low hydration heat development. the implementation of the achieved theoretical basis will be carried out according to the developed complex design of the treatment plant technology for the “zlaté hory” underground mine with obtaining the residual material from the treatment in a further usable form. the results are summarized in the trend fw project report 01010061-1-2022. 58 https://doi.org/10.14311/app.2024.47.0058 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 47/2024 processing of aggregate residues for stabilization of mined areas label cem i 42,5 r fluidized fly aggregates water plasticizing additive ash ash “cínovec” [l] [l] 1 “cínovec” c 25/30 300 100 200 1 260 400 10 2 “cínovec” filling 50 33 66 1 720 480 3 table 1. the main concrete and filling recipe in [kg]. label cem i 42,5 r fluidized fly aggregates water plasticizing additive ash ash “cínovec” [l] [l] 3 zh cem 50 33 66 1 720 460 3 table 2. composition of mixtures with “zlaté hory” aggregate without static function in [kg]. label modified fluidized ash aggregates water plasticizing additive (sor) “zlaté hory” [l] [l] 4 zh sor 150 1 720 460 3 table 3. composition of mixtures with modified fluidized ash without static function in [kg]. 2. material and methods based on the results of the laboratory tests, a formulation for a foundation with a static function that uses aggregate from mine “cínovec” was selected for validation in a mining environment. a mixture of fluidized and conventional fly ash supplemented with portland cement was selected as the binder. at the same time, a mixture based on aggregate from mine “zlaté hory” with a filling function without static requirements was also proposed. the main concrete recipe in [kg] is given in table 1 and the recipes of the base mortar cast mixes in tables 2 and 3. for each sample, four samples of 20 liters of mixture were produced. at the same time. control cubes were formed for storage under laboratory conditions. the sample storage conditions were set to a stable temperature of 7 °c ± 1 °c. strength checking and mine water sampling for ph measurements were carried out 90 and 180 days after mixing also with schmidt reflection hardness test. the resulting ph of the sampled mine water, as determined by a digital ph meter, was 4.8. the water was therefore slightly acidic. in the case of structural concrete, the following laboratory tests were carried out: • concrete mix design, • determination of basic technological tests on fresh and hardened mixes (water coefficient, settlement, fresh concrete temperature, bulk density, flexural and compressive tensile strength. in the case of the development of aggregate mixture cement cemented, the following tests were carried out: • design of the gradation curve of the resulting aggregate mixture, • design of the composition of the mixture, • proctor test, • determination of basic technological tests – bulk density, compressive strength. the following laboratory tests were carried out for the aggregates from “cínovec” within this sub-stage: • sieve analysis of aggregate with percentage of fine particles, • design of the composition of cast mixtures, subsequent testing of the resulting recipes, • determination of basic technological tests on fresh and hardened mixtures (water coefficient, spillage, bulk density, flexural and compressive tensile strength) the xrd analysis on samples was performed using a panalytical empyrean diffractometer. the reflection bragg-brentano para focusing geometry device is equipped with a cu anode (λ = 1.54184 å) and programable divergence slits a pixcel3d detector with 255 active channels. the x-ray generator settings were 45 kv and 40 ma. the measured range was 5– 80° with a step size of 0.013° and 38 s per step. each sample was measured four times, and the scans were next simply summed. the total measurement time for each sample was 60 min. the panalytical highscore 3 plus software was used to identify the individual phases. 3. results and discussion there is no relevant standard available for foundation materials, however, the concrete standard čsn en 206 can be used and to verify the parameters of the samples, the following tests were carried out: to perform the strength tests, the recipes of mortar mixes for individual types of fillers and mining wastewater from the extraction of tailings were designed and compared with a reference recipe prepared in proportions corresponding to the standard mortar according to the 59 a. jančíková, k. dvořák, s. ravaszová acta polytechnica ctu proceedings label 1. “cínovec” c 25/30 2. “cínovec” filling 3. zh cem 4. zh sor consistency by spilling [cm] [days] 24.5 25.4 26 26 volumetric mass [kg m−3] 1 830 1 360 11 390 11 390 compressive cubic strength [mpa] 28 18.7 0.3 0.1 0.1 schmidt reflection [-]/ strength [mpa] not determined not determined not determined not determined volumetric mass [kg m−3] 1 800 1 350 1 380 1 380 compressive cubic strength [mpa] 90 21.1 0.4 0.1 0.1 schmidt reflection [-]/ strength [mpa] 22/ 15 ± 4 indefinable indefinable indefinable volumetric mass [kg m−3] 1 780 1 330 1 360 1 360 compressive cubic strength [mpa] 180 21.9 0.7 0.3 0.3 schmidt reflection [-]/ strength [mpa] 26 / 19 ± 4 17 / <10 mpa 16 / <10 mpa 16 / <10 mpa table 4. properties of concrete and mortar cast mixes. label properties unit measured value aggregate “cínovec” 0/2 mm volume weight of aggregate grains [mg m−3] 2.69 volumetric weight of grains after drying in the oven [mg m−3] 2.65 volumetric weight of water-saturated and surface-dried grains [mg m−3] 2.66 absorption after immersion in water for 24 hours wa24 [%] 0.47 table 5. determination of bulk density and water absorption of aggregates “cínovec”. fresh mortar consistency test using a shaking table according to čsn en 1015-3. volumetric weight of fresh mortar according to čsn en 1015-10 (722400), and flexural tensile strength and compressive strength at curing times of 28, 90 and 180 days according to čsn en 1015-11 (722400), determination of flexural and compressive strength of hardened mortars. properties of concrete and mortar cast mixes made of aggregates from “cínovec” and “zlaté hory” are in the table 4. due to the requirement for flowability of the mixes, the consistency of fresh mortar was chosen to have more than 240 mm spillage. to achieve a 240 mm spillage, more water and had to be added to the mixes due to wetting of the separated aggregate and more fine filler to ensure the correct consistency of the mixture. a sieve analysis was carried out on the tin aggregate with results at figure 1. to determine the volumetric weights and absorption rates according to the relevant standard. čsn en 933-1: testing of geometric properties of aggregates – part 1: determination of grain size – sieve analysis. and čsn en 1097-6: testing of mechanical and physical properties of aggregates – part 6: determination of bulk density of grains and water absorption, chapter 9 with results in the table 5. mineralogical analysis shown in figure 2 shows that the proportion of heavy components (pyrite and chalcopyrite) after separation is low. the gravity separation of the heavy component was quite effective. the light component: concentrate separation ratio was determined after processing of 1 000 kg. the deposit was composed of 56 % quartz, 16.5 % muscovite, 14.6 % albite and 12.6 % chamosite. a chemical analysis was also performed on the sample, which can be found in table 6. figure 1. result of sieve analysis of aggregate from mine “cínovec” fraction 0/2 mm. loss on drying (105 °c) % 0.27 sulphides mg/kg dry 23.3 calcium oxide % dry 3.53 magnesium oxide % dry 2.26 potassium oxide % dry 2.13 aluminium oxide % dry 10.5 silicon dioxide % dry 67.8 iron oxide % dry 7.03 sodium oxide % dry 0.937 zinc % dry 0.206 zinc oxide % dry 0.257 table 6. results of the chemical analysis of the “zlaté hory” landfill. despite the low strength in backfill mixes without static function, the amount and type of binder was chosen in both cases to be sufficient to ensure the 60 vol. 47/2024 processing of aggregate residues for stabilization of mined areas figure 2. mineralogical analysis from mine “zlaté hory”. properties zh sor declared property of the backfill mixture “cínovec” c 25/30 declared property of the concrete mix spillage čsn en 1015-3 250/255 mm > 240 mm 240/245 mm > 230 mm bulk mass čsn en 12390-7 28 days 1 390 kg m−3 1 390 ± 100 kg m−3 1 840 kg m−3 1 850 ± 100 kg m−3 60 days 1 380 kg m−3 1 380 ± 100 kg m−3 1 800 kg m−3 1 800 ± 100 kg m−3 90 days 1 360 kg m−3 1 360 ± 100 kg m−3 1 780 kg m−3 1 800 ± 100 kg m−3 compressive strength according to čsn en 12390-3 28 days 0.1 mpa ≥ 0.1 mpa 19.2 mpa ≥ 15 mpa 60 days 0.1 mpa ≥ 0.1 mpa 20.9 mpa ≥ 18 mpa 90 days 0.3 mpa ≥ 0.3 mpa 21.4 mpa ≥ 20 mpa hydration temperature according to čsn en 196-9 max 21.8 ° c at 152 minutes ≤ 25 ° c untill 250 minutes max 22.3 ° c at 44 hours 49 minutes ≤ 25 ° c until 50 hours table 7. results of monitoring, development of the strengths and condition of the samples. compactness of the samples. both binders, due to their chemical and mineralogical nature, should not suffer chemical damage from the acid mine water which was used for the samples. monitoring of the development of the strengths and condition of the samples with test results with comparing to the declared properties of backfilling mixtures with and without static function are shown in the table 7. all tests and trials were carried out according to the following list of technical regulations and standards. • čsn en 206+a2 concrete-specifications, properties, production, and compliance • čsn en 12390-3: testing of hardened concrete – part 3: compressive strength of test bodies • čsn en 12390-7: testing of hardened concrete – part 7: bulk density • čsn en 933-1: testing of geometric properties of aggregates part 1: determination of gradation – sieve analysis • čsn en 1097-6: testing of mechanical and physical properties of aggregates – part 6: determination of bulk density of grains and water absorption, chapter 9 • čsn en 1015-3: test methods for mortars for masonry – part 3: determination of consistency of fresh mortar (using a shaking table) • čsn en 196-9: test methods for cement – part 9: determination of heat of hydration – semi adiabatic method 4. conclusion aggregate samples were taken and analysed from existing deposits and stockpiles at the “cínovec and zlaté hory” locations. samples simulating cleaned mining residues were prepared by gravity separation methods. samples of stockpile mixtures for the foundation of areas with requirements for static and non-static function during active mining were developed, prepared, and tested. in doing so, a nearly waste-free technology was developed to produce stockpile mixes while maximizing the use of existing tailings. simulations of tailings purification and tailings preparation with the constituents of interest removed were performed. it was demonstrated for the windrowed samples of landfill mixtures for the foundation of areas with static 61 a. jančíková, k. dvořák, s. ravaszová acta polytechnica ctu proceedings and non-static function requirements that: • according to the measured results, it can be concluded that it is possible to prepare relatively wellflowing mixtures with very high-water coefficients. • it is possible to achieve good strength results with extremely low dosage of binder based on fly ash mixtures under placement conditions without static function. • modified and recycled aggregates free of heavy pyrite components after ore extraction are very suitable for use in ash fly ash mixtures. acknowledgements we acknowledge the financial support to project (fw01010061) tacr, “comprehensive technology of ore residue processing for the stabilization of mined out areas and for the underlying layers of linear structures” and project (fast-s-23-8246) “study of the formation and stability of various tricalcium aluminate polymorphs”. references [1] s. lamani, m. aruna, h. vardhan. utilisation of mine waste in the construction industry a critical review. international journal of earth sciences and engineering 9:182–195, 2016. issn 0974-5904. [2] h. e. jamieson, s. r. walker, m. b. parsons. mineralogical characterization of mine waste. applied geochemistry 57:85–105, 2015. https://doi.org/10.1016/j.apgeochem.2014.12.014 [3] m. b. j. lindsay, m. c. moncur, j. g. bain, et al. geochemical and mineralogical aspects of sulfide mine tailings. applied geochemistry 57:157–177, 2015. https://doi.org/10.1016/j.apgeochem.2015.01.009 [4] n. sivakugan, k. j. rankine, k. s. rankine. study of drainage through hydraulic fill stopes using method of fragments. geotechnical & geological engineering 24(1):79–89, 2006. https://doi.org/10.1007/s10706-004-1949-x [5] t. belem, m. benzaazoua. design and application of underground mine paste backfill technology. geotechnical & geological engineering 26(2):147–174, 2008. https://doi.org/10.1007/s10706-007-9154-3 [6] b. ercikdi, f. cihangir, a. kesimal, et al. utilization of industrial waste products as pozzolanic material in cemented paste backfill of high sulphide mill tailings. journal of hazardous materials 168(2-3):848–856, 2009. https://doi.org/10.1016/j.jhazmat.2009.02.100 [7] k. pal, h. vardhan, m. aruna. investigation of contaminant transport in groundwater from the tailings pond of uranium mine: a case study. international journal of mining and mineral engineering (ijmme) 2(4):290–309, 2010. https://doi.org/10.1504/ijmme.2010.039039 [8] m. keppert, v. davidová, b. doušová, et al. recycling of fresh concrete slurry waste as supplementary cementing material: characterization, application and leaching of selected elements. construction and building materials 300:124061, 2021. https: //doi.org/10.1016/j.conbuildmat.2021.124061 62 https://doi.org/10.1016/j.apgeochem.2014.12.014 https://doi.org/10.1016/j.apgeochem.2015.01.009 https://doi.org/10.1007/s10706-004-1949-x https://doi.org/10.1007/s10706-007-9154-3 https://doi.org/10.1016/j.jhazmat.2009.02.100 https://doi.org/10.1504/ijmme.2010.039039 https://doi.org/10.1016/j.conbuildmat.2021.124061 https://doi.org/10.1016/j.conbuildmat.2021.124061 acta polytechnica ctu proceedings 47:58–62, 2024 1 introduction 2 material and methods 3 results and discussion 4 conclusion acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2023.40.0117 acta polytechnica ctu proceedings 40:117–122, 2023 © 2023 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague return mapping scheme for the hoek-brown model with tension cut-off tereza žalská∗, michal šejnoha czech technical university in prague, faculty of civil engineering, thákurova 7, 166 29 prague 6, czech republic ∗ corresponding author: tereza.zalska@fsv.cvut.cz abstract. the present paper revisits a stress update procedure for the hoek-brown plasticity model enhanced by a rankine type of tension cut-off failure criterion. limiting tensile stresses not only supports the behavior of weak rock masses with a very low tensile strength but in combination with the original hoek-brown model improves robustness of the stress update procedure, i.e. the stress return mapping algorithm. herein, the primary focus is on the stress return from a space of inadmissible trial stresses for which neither the hoek-brown yield surface nor its derivative can be evaluated. all potential scenarios are thoroughly discussed both in the framework of singleand multi-surface plasticity. the presented procedures were implemented and verified with the help of the geo5 fem software. keywords: hoek-brown criterion, rankine criterion, nonlinear yield function, rock mechanics, finite element method, tension cut-off. 1. introduction in geological engineering, shear strength properties of the rock mass are mostly expressed by the parameters entering the formulation of the hoek-brown (hb) plasticity model. these can be transformed into the pair of basic shear strength parameters, i.e, cohesion c and friction angle φ, exploiting similarity between the hoek-brown and mohr-coulomb (mc) yield functions. then the numerical analysis can be simply governed by the mc model. however, formulation of the hb model is based on the assumption of a nonlinear development of the rock mass strength with increasing loading. on the other hand, equivalent shear strength parameters derived by hoek and brown in [1] are independent of the current stress state. therefore, this approach requires a proper estimation of the stress range for obtaining comparable results, see [2]. in light of this, a direct application of the hb model appears more suitable, which also promoted a recent implementation of the hb model into geo5 fem software [3]. in a numerical solution, violation of the yield function is followed by a correction step bringing the stresses back to the yield surface. formulation of a suitable stress return algorithm typically calls for the determination of the first derivative of a given yield function. point out that the hb yield function is not defined for the maximum trial principal stress exceeding the tensile strength of the rock mass making the standard return mapping inapplicable. to overcome this obstacle, the present paper introduces a stress update algorithm that incorporates the rankine failure criterion in conjunction with the hb model when moving from inadmissible to admissible stress space. within this concept, the rankine failure criterion may also represent a less or more significant reduction of the tensile strength. 2. governing equations application of the generalized hoek-brown criterion covers the whole range of rock masses from an intact one to a highly disturbed weak rock mass. the input parameters of the hb model are [4]: • uniaxial compressive strength σci and hoek-brown constant mi provided by series of triaxial testing, • geological strength index (gsi) used as a classification number of the geological quality of rock mass estimated from in situ geological observation, • disturbance factor d describing the rate of disturbance by prior excavations or blasting, also estimated from in situ geological observation. the material constantsmb, s and a govern the shape of the yield surface and are given by empirical equations as mb = mi exp gsi − 100 28 − 14d , (1) s = exp gsi − 100 9 − 3d , (2) a = 1 2 + 1 6 ( exp −gsi 15 − exp −20 3 ) . (3) 2.1. hoek-brown yield function assuming a standard order of principle stresses σ1 > σ2 > σ3, the generalized yield function is defined as f13(σ1, σ3) = σ1 − σ3 − σci ( s−mb σ1 σci )a , (4) where σ1 and σ3 are the maximal and minimal effective principal stresses, respectively. unlike typical geotechnical notation, the compressive stresses are assumed negative and σ1 has a meaning of confining 117 https://doi.org/10.14311/app.2023.40.0117 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en tereza žalská, michal šejnoha acta polytechnica ctu proceedings pressure. equation (4) represents the main sector of the yield surface in the principle stress space. for specific cases of triaxial compression (lode’s angle θ = 30◦) and triaxial extension (θ = −30◦) two more sectors need to be considered. these are defined as f23(σ2, σ3) = σ2 − σ3 − σci ( s−mb σ2 σci )a , (5) f12(σ1, σ2) = σ1 − σ2 − σci ( s−mb σ1 σci )a . (6) in the principal stress space, the hb model plots as a irregular hexagonal pyramid with curved edges and surfaces, see figure 1. assuming the uniaxial tensile strength be equal to the biaxial one (σ3 = σ1 = σt) yields the tensile strength of the rock mass adopting equation (4) as σt = sσci mb . (7) for the intact rock it holds gsi = 100 which gives a = 0.5. as the minimum value of gsi is equal to 0, the exponent a ranges from 0.5 to approximately 0.666. it is obvious that setting σmax > σt generates singularity in eqautions (4)–(6). the yield condition is not defined beyond the apex and hence it cannot be evaluated. σm σ2 σ3 σ1 figure 1. hoek-brown yield surface plotted in principal stress space. expressing the yield function in terms of stress invariants, i.e., the mean stress σm, the deviatoric stress measure j and the lode’s angle θ gives fhb(σm, j, θ) = (2j cos θ)1/a − sσ 1/a ci + + ( cos θ − sin θ√ 3 ) mbjσ 1/a−1 ci +mbσmσ 1/a−1 ci . (8) note that formulation (8) is not suitable for the complex return mapping procedure, as the derivative of the yield surface along the triaxial compression and triaxial extension edges (lode’s angle θ = ±30◦, respectively) is singular, see [5] with reference to the mc model. 2.2. plastic potential plastic potential functions are formulated on the basis of the mc yield criterion as g13(σ1, σ3) = σ1 1 + sinψ 1 − sinψ − σ3, (9) g23(σ2, σ3) = σ2 1 + sinψ 1 − sinψ − σ3, (10) g12(σ1, σ2) = σ1 1 + sinψ 1 − sinψ − σ2, (11) where ψ is well known dilation angle. constant rate of dilation is assumed in the present formulation. vectors normal to the plastic potential surfaces governing the direction of the plastic yielding are therefore independent of the current stresses and take the form n1 g = { 1 + sinψ 1 − sinψ ; 0; −1 } t, (12) n2 g = { 0; 1 + sinψ 1 − sinψ ; −1 } t, (13) n3 g = { 1 + sinψ 1 − sinψ ; −1; 0 } t. (14) more advanced formulations of the potential functions are also available in literature, see, e.g., carranzatorres and fairhurst [6] who proposed the plastic potential function based on the mobilized value of the dilation angle ψmob depending on current stress state. however, such formulations go beyond the present scope. 2.3. rankine yield function yield surfaces of the rankine type are often used as the additional tension cut-off surfaces. the model assumes the material to be isotropic and unlike the hb model, the stress return mapping is in general driven by an associated flow rule. the rankine yield surfaces are defined as t1(σ1, σt) = σ1 − σt, (15) t2(σ2, σt) = σ2 − σt, (16) t3(σ3, σt) = σ3 − σt, (17) where σt ≤ σt is the prescribed tensile strength. the vectors normal to individual sectors of the yield surface are for the assumed associated flow rule given by n1 t = {1; 0; 0} t, (18) n2 t = {0; 1; 0} t, (19) n3 t = {0; 0; 1} t. (20) note that apart from a physically accepted reduction of a tensile stress σt, recall equation (7), the return to the rankine yield surface will be adopted as an intermediate step when an attempt to return to the hb yield function is made from an inadmissible trial 118 vol. 40/2023 hoek-brown plasticity model with tension cut-off stress space1. to avoid failing the return process we consider an upper limit of the tensile strength σmax t = σt − σ∗ where σ∗ = 1 kpa is assumed in geo5 fem. for the sake of completeness we present the rankine yield function, which plots as a regular triangular pyramid in the principal stress space, also in terms of stress invariants as fr(σm, j, θ) = 2 √ 3j cos ( θ + π 6 ) + + 3σm − 3σt. (21) 3. stress update algorithm in the presented approach, the rock mass is treated as isotropic and no hardening is considered. stress update algorithm respects the standard theory of plasticity. elastic region of the strain is bounded by the yield surface. in case of plastic yielding both the yield and consistency condition must be satisfied. the geo5 fem software assumes the stress-strain relation to be linearly elastic perfectly plastic for the hb model. the total strain increment decomposes into elastic and plastic parts as dε = dεel + dεpl. (22) the increment of stress relates to the elastic part of the total strain and follows the linear relation dσ = del( dε − dεpl). (23) for non-associated plasticity the plastic part of the strain increment is defined as dεpl = ∆λ∂g(σ) ∂σ , (24) with ∆λ being positive for plastic yielding, otherwise it is equal to 0. an associated plastic flow rule assumes g(σ) = f(σ). as already mentioned, a non-associated flow rule is applied when returning to the hb yield surface, while associated plasticity is adopted for the rankine yield surface. derivation of the plastic strain increment follows standard two step procedure employing elastic predictor and plastic corrector steps. the former step gives the trial stresses in the form σtr = σj−1 + del dε. (25) once the yield condition is violated, i.e., f(σtr) > 0, the return mapping is carried out to bring the trial stresses back to the yield surface. depending on the orientation of the elastic guess and the direction of the plastic strain increment, reordering of the principal stresses may occur, i.e. σ1 > σ2 > σ3 may no longer apply. as will be graphically shown in the next section, reordering of principal stresses requires formulation 1the trial stress space is assumed inadmissible whenever condition σmax > σt occurs regardless of whether or not the hb model is defined. of the return mapping algorithm in the framework of multi-surface plasticity. this is because the updated stress, although theoretically located on the yield surface, gives lode’s angle which falls outside the limits −30◦ ≤ θ ≤ 30◦. in such a case, the correct stress lies on the triaxial edge, where two sectors of the yield surface intersect so both corresponding surfaces become active in return mapping. 3.1. single yield surface return strategy implicit return mapping is applied using the newtonraphson method in the form (∆λ)i+1 = (∆λ)i − (f13)i ( df13)i , (26) where df13 is the derivative of the yield function associated with the main sector and ∆λ is the plastic strain increment. the updated stress is provided by σi+1 = σtr − (∆λ)i+1deln1 g, (27) where the direction of the plastic strain increment n1 g remains constant during the return mapping, recall equations (12)–(14). writing the direction of principal plastic corrector as deln1 g = σpl = { σpl 1 , σ pl 2 , σ pl 3 } t (28) gives σ1(∆λ) = σtr 1 − ∆λσpl 1 , (29) σ2(∆λ) = σtr 2 − ∆λσpl 2 , (30) σ3(∆λ) = σtr 3 − ∆λσpl 3 . (31) substituting equations (29)–(31) into equation (4) gives the yield function f13 in the form f13(∆λ) = (σtr 1 − σtr 3 ) − ∆λ(σpl 1 − σpl 3 )− − σci [ s− mb σci ( σtr 1 − ∆λσpl 1 )]a (32) and the corresponding derivative w.r.t. ∆λ as df13(∆λ) = σpl 3 − σpl 1 − − ambσ pl 1 [ s− mb σci ( σtr 1 − ∆λσpl 1 )]a−1 . (33) return to the rankine yield surface t1 follows the same minimization procedure. owing to the linear form of the yield function, the correct increment of the plastic strain is found in one iteration step. therefore ∆λ = − t1 dt1 . (34) the tensile yield surface defined in terms of the plastic multiplier is given by t1 = σtr 1 − ∆λσpl 1 − σt (35) 119 tereza žalská, michal šejnoha acta polytechnica ctu proceedings σ1 σ2 σ3 0 50 100 150 200 0 20 40 60 80 100 f hb f r f hbf r -σm (kpa) j ( k p a) f r f hb -σ1 (kpa) -σ 3 ( k p a) σm figure 2. non-associated return to the hb surface from trial stress violating t1 in two consecutive steps (ψ = 0): first from trial stress (red point) onto the rankine yield surface (orange point), second from rankine surface to hb surface (green point) plotted in the deviatoric plane (left), σm − j plane (middle), σ1 − σ3 plane (right). and the derivative w.r.t. ∆λ is therefore constant dt1 = −σpl 1 . (36) returning from the trial stress space violating the rankine failure criterion considers two scenarios: • correct return to the hb yield surface is expected: this is a two-step return where returning to the rankine yield surface is performed first assuming the direction of plastic corrector given by the hb model, i.e., σpl = deln1 g (non-associated flow rule). the procedure then continues with the second step bringing the stresses back onto the hb yield surface along the same direction. the total plastic strain increment is then the sum of plastic strain increments generated by both steps. see figure 2. • correct return to the rankine yield surface is expected: this is a one-step return along the direction given by σpl = deln1 t (associated flow rule). 3.2. triaxial lines return strategy two active sectors of the hb yield surface apply to both the state of triaxial compression and triaxial extension. considering triaxial stress state in compression for the determination of the multi-surface plasticity return, two yield functions, f13 and f23, are active and the iterative scheme takes the form{ ∆λ1 ∆λ2 }i+1 = { ∆λ1 ∆λ2 }i −(h−1)i { f13 f23 }i , (37) where the jacobian matrix is composed of the partial derivatives of the yield functions with respect to two plastic multipliers ∆λ1 and ∆λ2. therefore h =  ∂f13 ∂(∆λ1) ∂f13 ∂(∆λ2) ∂f23 ∂(∆λ1) ∂f23 ∂(∆λ2)  . (38) the vector of updated principle stresses for the current iterative step is given by σi+1 = σtr − (∆λ1)i+1deln1 g − (∆λ2)i+1deln2 g. (39) vectors providing the orientation of the plastic corrector are defined as σpl 1 = deln1 g = { σpl 11;σpl 12;σpl 13 }t , (40) σpl 2 = deln2 g = { σpl 21;σpl 22;σpl 23 }t (41) and individual components of the stress vector for j = 1 → 3 σj(∆λ1,∆λ2) = σtr j − ∆λ1σ pl 1j − ∆λ2σ pl 2j . (42) expressing the yield function in terms of two plastic multipliers provides the yield function f13 as f13(∆λ1,∆λ2) = = (σtr 1 − σtr 3 ) − ∆λ1(σpl 11 − σpl 13) − ∆λ2(σpl 21 − σpl 23)− − σci [ s− mb σci ( σtr 1 − ∆λ1σ pl 11 − ∆λ2σ pl 21 )]a (43) and its partial derivatives in the form h11 = σpl 13 − σpl 11 − ambσ pl 11[ s− mb σci ( σtr 1 − ∆λ1σ pl 11 − ∆λ2σ pl 21 )]a−1 , (44) h12 = σpl 23 − σpl 21 − ambσ pl 21[ s− mb σci ( σtr 1 − ∆λ1σ pl 11 − ∆λ2σ pl 21 )]a−1 . (45) the other yield function and components of the h matrix would be expressed similarly and the same approach is applied to the state of triaxial extension with sectors f13 and f12 being simultaneously active. attention should be also paid to the triaxial corner of the tensile yield surface. in this case, the proposed approach is adopted for the intersection of the yield functions t1 and t2 and, similarly to a single surface plasticity with one iteration step needed. again, both non-associated and associated plasticity is considered in return mapping scheme depending on whether the stress is expected to be brought back to hb model or directly to the rankine yield surface, recall the last paragraph in the previous section. as an example of the former case, see figure 3. 120 vol. 40/2023 hoek-brown plasticity model with tension cut-off figure 3. demonstration of the return to the triaxial corners of the hb yield surface from trial stress violating tensile yield condition (ψ = 0 is assumed): σtr violates t1 condition (tc corner) σtr′ violates both t1, t2 conditions (tc corner) σtr∗ violates t1 condition (te corner). 4. complex return mapping scheme here, only the return from an inadmissible trial stress space is discussed. as already explained in the previous sections, a direct return of the trial stress for the condition σtr 1 > σt onto the hb yield surface fails as the hb yield function is undefined. the return mapping therefore begins with evaluating the tensile condition (15) employing the reduced tensile strength σt ≤ σt − σ∗. violating this condition calls a non-associated return onto the rankine yield function controlled by the plastic potential formulated for the hb model. if the stress point violates the hb criterion, a non-associated return from the point on the rankine surface onto the hb yield surface follows, see figure 2. otherwise, associated plasticity is adopted for bringing the original trial stress back onto the rankine tensile yield surface, recall the last paragraph in section 3.1. special attention should be paid to the intersection of t1 and f13 surfaces with reference to figure 5. the proposed algorithm first predicts that the return mapping will continue to the hb surface after being preceded by a non-associated return to the tensile yield surface. if no violation of the hb criterion is detected after the first step, the associated return of the trial stress to the tensile surface is called. after this attempt, violation of the hb condition may eventually occur. if it does, the hb criterion is substituted with the linear function fsub, see equation (46), which satisfies t1 condition (σ1 = σt), and the trial stress is shifted to the intersection of two planes using multi-surface plasticity. fsub(σ3) = σt − σ3 − σci ( s−mb σt σci )a (46) it is appropriate to point out that reordering of the principle stresses, and thus detection of the triaxial corner return, may occur during the described steps. in such a case, the above two-step procedure would also be performed in the framework of multi-surface plasticity. a graphical representation is evident in figure 3. the complete return mapping scheme including all potential scenarios is then summarized in figure 4. figure 4. full return mapping scheme including all potential scenarios. 121 tereza žalská, michal šejnoha acta polytechnica ctu proceedings σ1 σ2 σ3 σ1 σ2 σ3 σ1 σ2 σ3 10 0 10 20 0 5 10 15 20 25 10 0 10 20 0 5 10 15 20 25 10 0 10 20 30 0 5 10 15 20 25 f hb f r -σm (kpa) j ( k p a ) j ( k p a) -σm (kpa) -σm (kpa) j ( k p a) delng 1 d eln t 1 figure 5. sequence of the return attempts leading to correct stress lying on the intersection of t1 and f13 (ψ = 0) plotted in the deviatoric planes (top) and σm − j planes (bottom). 5. conclusions a detailed description of the return mapping algorithm for the case of inadmissible trial stresses, here defined as the stress space for which σtr 1 > σt, is presented. to the authors knowledge the number of contributions addressing this issue is limited. in [7] a smooth variant of the hb model is presented and the tension cut-off condition is introduced via an allowable tensile mean stress instead of actual tensile strength. in [8, 9] the authors also deal with the issue of tensile stresses. however, the approach is not explained in all details and its effectivity is not obvious. the present approach exploits simplicity of the rankine yield surface, which is often used as a tension cut-off criterion. the return of inadmissible trial stresses to the main sector or triaxial edges of the hb yield surface assumes two consecutive steps. first, the stress is brought back to the rankine yield surface defined by the reduced tensile strength. second, the implicit iterative procedure for shifting the stress onto the hb surface is applied. both steps assume the same orientation of the plastic corrector. it is shown, that the prediction of all possible situations, which can lead to exceeding the tensile strength and failing the solution, results in the complex algorithm further complicated by the assumption of associated plasticity for tensile yielding. this assumption becomes active once the two-step return procedure onto the hb surface fails and the trial stress is then brought back directly to the rankine yield surface. this algorithm was implemented into geo5 fem software and thoroughly tested. acknowledgements the support provided by the sgs project no. sgs22/030/ohk1/1t/11 and by the gačr project no. 22-12178s is gratefully acknowledged. references [1] e. hoek, c. carranza-torres, b. corkum. hoek-brown failure criterion – 2002 edition. in proceedings of the 5th north american rock mechanics symposium – narms-tac, pp. 267–273. 2002. [2] t. poklopová, m. šejnoha. comparing the hoek-brown and mohr-coulomb failure criteria in fem analysis. in acta polytechnica ctu proceedings, vol. 30, pp. 69–75. 2021. https://doi.org/10.14311/app.2021.30.0069 [3] fine-ltd. geo5 fem, 2022. [2022-09-13]. http://www.fine.cz/ [4] e. hoek, e. t. brown. practical estimates of rock mass strength. international journal of rock mechanics and mining sciences 34(8):1165–1186, 1997. https://doi.org/10.1016/s1365-1609(97)80069-x [5] j. huang, d. v. griffiths. observations on return mapping algorithms for piecewise linear yield criteria. international journal of geomechanics 8(4):253–265, 2008. https: //doi.org/10.1061/(asce)1532-3641(2008)8:4(253) [6] c. carranza-torres, c. fairhurst. the elasto-plastic response of underground excavations in rock masses that satisfy the hoek-brown failure criterion. international journal of rock mechanics and mining sciences 36(6):777–809, 1999. https://doi.org/10.1016/s0148-9062(99)00047-9 [7] r. g. wan. implicit integration algorithm for hoek-brown elastic-plastic model. computers and geotechnics 14(3):149–177, 1992. https://doi.org/10.1016/0266-352x(92)90031-n [8] j. clausen. efficient non-linear finite element implementation of elasto-plasticity for geotechnical problems. ph.d. thesis, aalborg universitet, 2007. [9] j. clausen, l. damkilde. an exact implementation of the hoek-brown criterion for elasto-plastic finite element calculations. international journal of rock mechanics and mining sciences 45(6):831–847, 2008. https://doi.org/10.1016/j.ijrmms.2007.10.004 122 https://doi.org/10.14311/app.2021.30.0069 http://www.fine.cz/ https://doi.org/10.1016/s1365-1609(97)80069-x https://doi.org/10.1061/(asce)1532-3641(2008)8:4(253) https://doi.org/10.1061/(asce)1532-3641(2008)8:4(253) https://doi.org/10.1016/s0148-9062(99)00047-9 https://doi.org/10.1016/0266-352x(92)90031-n https://doi.org/10.1016/j.ijrmms.2007.10.004 acta polytechnica ctu proceedings 40:117–122, 2023 1 introduction 2 governing equations 2.1 hoek-brown yield function 2.2 plastic potential 2.3 rankine yield function 3 stress update algorithm 3.1 single yield surface return strategy 3.2 triaxial lines return strategy 4 complex return mapping scheme 5 conclusions acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2024.49.0020 acta polytechnica ctu proceedings 49:20–25, 2024 © 2024 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague effect of geometry on homogenised properties of selected auxetic metamaterials nataša jošková∗, martin doškář czech technical university in prague, faculty of civil engineering, department of mechanics, thákurova 7, 166 29 prague, czech republic ∗ corresponding author: joskonat@cvut.cz abstract. our study investigates the influence of geometrical parameters of two types of auxetic metamaterials on their effective properties. in particular, we focus on three-dimensional lattice structures, which we represent with discrete beam models of their respective periodic unit cells (pucs). limiting the scope of the study to linear elasticity, we compute the effective response of pucs by plugging the kinematic ansatz of the first-order numerical homogenisation into the strain energy expression arising from the direct stiffness method and minimising the energy with respect to the periodic fluctuation field. the obtained effective stiffness matrices are post-processed to arrive at elastic parameters as poisson’s ratios coefficients, that are reported in different directions with respect to the key geometrical parameters. keywords: auxetic metamaterial, first-order homogenisation, periodic unit cell, effective poisson’s ratio. 1. introduction metamaterials are artificial materials with properties beyond those of materials found in nature. these properties are mainly determined by their microstructure rather than by the chemical or physical parameters of the bulk constituents from which they are made [1]. due to technological advances in recent decades, complex microstructures of these metamaterials can be produced by manufacturing techniques such as 3d and even 4d printing [2], optical lithography [3], or electrospinning. mathematical modelling is then needed for efficient design of metamaterials by circumventing lenghty experimental search for their optimal design. our study of the influence of geometry on the effective poisson’s ratio focuses on two variants of a threedimensional auxetic metamaterial (cubic and hexagonal) proposed by bückmann et al. [3] and shown in figure 1. both designs exhibit a periodic microstructure allowing us to investigate only the response of a periodic unit cell (puc) as their representative volume element (rve). 2. geometry of investigated metamaterial the microstructure of metamaterial is composed of arranged bow-tie structures, the geometry of which is controlled by the angle δ located between the diagonal beam of the central bow-tie structure in the upper left quadrant and the yz plane passing through its initial node, see insets on the right-hand side in figure 2. the 3d metamaterial is created by rotating the central bow-tie structure located in the xz plane around its vertical centre beams, as shown in figure 2. the cubic microstructure is obtained by rotation by 90°, from (a). cubic variant. (b). hexagonal variant. figure 1. two investigated auxetic metamaterials. insets on the right-hand side show a top view of the microstructures. 20 https://doi.org/10.14311/app.2024.49.0020 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 49/2024 effect of geometry on homogenised properties of auxetic metamaterial (a). cubic microstructure. (b). hexagonal prismatic microstructure. figure 2. periodic unit cell of cubic and regular hexagonal prismatic microstructure along with their corresponding side view including angle δ, which controls the geometry of the metamaterial. which the central structure is formed (dark blue). this central structure is complemented by a similar one (cyan) shifted by half a period in yz plane. to create the puc of regular hexagonal prism with equal height and long base diagonal length, we perform 60° and 120° rotations of the bow-tie structure (purple). the height h of both pucs will be considered as a single unit height. here, we model puc with discrete beams; we know the position of each node and the orientation of the beams that connect them. for this study, we assume a circular beam cross-section with diameter d = 0.1h. consequently, cross-sectional characteristics follow as: a = π 4 · d2, i = π 64 · d4, j = 2i = π 32 · d4, (1) where a is the cross-section area, i is the second moment of inertia, and j is the polar moment of inertia. the volume of puc is for the cubic variant vc = h3 and vh = 3 √ 3 8 · h3 for the hexagonal one. 3. direct stiffness method to compute a mechanical response of the puc model, we use a linear discrete beam model that can be described by a linear relation: f = ku, (2) where the vector f contains the forces and moments applied on all nodes, k is the stiffness matrix – assembled from individual submatrices ki for each beam – and u is the displacement vector that successively contains subvectors of displacements ui, vi, wi and rotations φx,i, φy,i, φz,i of individual nodes i. 3.1. local stiffness matrix the local stiffness matrix kl i for the ith beam follows from the bernoulli-euler beam theory [4, 5] and has size 12 × 12, due to the 6 unknowns located at the beginning (index b) and the end (index e) node of the beam. individual parts that contribute to the stiffness matrix can be divided into 4 submatrices pertinent to: (1) membrane behaviour:[ x l b x l e ] = ea li [ 1 −1 −1 1 ] [ ul b ul e ] , (3) (2) bending in xy plane:  y l b m l z,b y l e m l z,e  = 2eiz li  6 l2 i 3 li − 6 l2 i 3 li 3 li 2 − 3 li 1 − 6 l2 i − 3 li 6 l2 i − 3 li 3 li 1 − 3 li 2   vl b φl z,b vl e φl z,e  , (4) (3) bending in xz plane:  zl b m l y,b zl e m l y,e  = 2eiy li  6 l2 i − 3 li − 6 l2 i − 3 li − 3 li 2 3 li 1 − 6 l2 i 3 li 6 l2 i 3 li − 3 li 1 3 li 2   wl b φl y,b wl e φl y,e  , (5) (4) torsion:[ m l x,b m l x,e ] = gj li [ 1 −1 −1 1 ] [ φl x,b φl x,e ] . (6) in the equations above, li refers to the length of a corresponding beam, e denotes young’s modulus, g stands for the shear modulus, and moments of inertia iy, iz are equal to i due to the circular crosssection of the beam. the local stiffness matrix kl i is obtained by combining the submatrices from equations (3)–(6), each contributing to its specific degrees of freedom (dofs). 21 nataša jošková, martin doškář acta polytechnica ctu proceedings 3.2. global stiffness matrix the beams constituting the puc’s microstructure have different orientations. hence, it is necessary to transform local displacements, rotations and end forces from equations (3)–(6) into a global coordinate system. in our study, we took the approach of building a rotation matrix ri for the ith beam with euler angles. the beam’s initial position is established by aligning its local coordinate system with the global one. afterwards, we execute an extrinsic rotation around the global coordinate axes until the desired position is achieved. rotation matrices: rx = 1 0 0 0 cos α − sin α 0 sin α cos α  , (7) ry =  cos β 0 sin β 0 1 0 − sin β 0 cos β  , (8) rz = cos γ − sin γ 0 sin γ cos γ 0 0 0 1  , (9) determine rotation around global x, y, z axes by angles α, β, γ, respectively, while each angle stands for rotation from the latest beam’s position. to obtain the rotation matrix ri, we perform matrix multiplication: ri = rz · ry · rx, (10) where the sequence of the elements is based on order in which the beam is rotated, starting with rotation around the global x axis. the nodal displacements and rotations are transformed on each side of the beam equally, so we can create a transformation matrix ti by arranging the rotation matrix ri on its diagonal: ti =  ri 0 0 0 0 ri 0 0 0 0 ri 0 0 0 0 ri  . (11) the local stiffness matrix is then transformed into a global coordinate system using the relation: kg i = tt i kl iti, (12) from which we obtain the global stiffness matrix kg i for each beam and localise them to the stiffness matrix k using boolean localisation matrices li: k = ∑ i lt i kg i li. (13) 4. homogenisation the homogenisation process substitutes a heterogeneous puc at the microscopic level with a corresponding macroscopic constitutive model, allowing us to study the effective behaviour of the puc when treated as a material considering its microstructure. in our study, we are using the first-order numerical homogenisation to obtain effective metamaterial properties. 4.1. displacement decomposition in the first-order homogenisation, the total displacement field u⃗(x⃗) is assumed in the form: u⃗(x⃗) = u⃗ e(x⃗) + u⃗ ∗(x⃗), (14) where u⃗ e denotes the macroscopic and u⃗∗ is the fluctuation part of the displacement field caused by the heterogeneity of the metamaterial [6]. the macroscopic part u⃗ e of the displacement field corresponds to a situation under which an entire cell composed of homogeneous material would be subjected to a constant macroscopic strain tensor e: e = exx exy exz eyx eyy eyz ezx ezy ezz  , (15) which results in a displacement field ue given as: u⃗ e(xi) = e · xi = u e i v e i w e i  . (16) for the first-order homogenisation, it is further assumed that the volumetric average of the gradient of the entire displacement field u⃗(x⃗) corresponds to the prescribed macroscopic deformation e. by applying the symmetric gradient operator ∇s = 1 2 (∇ + ∇t) and averaging the result over a unit cell ω, we obtain: e = 1 |ω| ∫ ω ∇su⃗(x⃗) dx⃗ = 1 |ω| ∫ ω ∇su⃗ e(x⃗) + ∇su⃗ ∗(x⃗) dx⃗, (17) where ω represents the microscale domain of interest, being the metamaterial’s macroscopic point [7]. the macroscopic part of the deformation u⃗ e(x⃗) in equation (16) is defined such that: e = 1 |ω| ∫ ω ∇su⃗ e(x⃗)dx⃗. (18) consequently, the fluctuation part u⃗ ∗(x⃗) of the displacement field u⃗(x⃗), must have a zero volumetric average gradient, i.e.: 1 |ω| ∫ ω ∇su⃗ ∗(x⃗)dx⃗ = 0. (19) 22 vol. 49/2024 effect of geometry on homogenised properties of auxetic metamaterial for our discrete beam model, equation (16) in a matrix form reads as: ue i =  xi 0 0 0 1 2 zi 1 2 yi 0 yi 0 1 2 zi 0 1 2 xi 0 0 zi 1 2 yi 1 2 xi 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0  e = qe i e, (20) and couples nodal dofs with the macroscopic deformation e, which is the vectorial representation of a symmetric second-order tensor: e = [ ex ey ez γyz γxz γxy ]t . (21) now we can write the original degrees of freedom u depending on the macroscopic deformation e and the fluctuation unknowns u∗. let’s define the extended displacement vector: û = [ e u∗ ] , (22) then we can express degrees of freedom of our discrete beam model as: u = [ qe i ] [ e u∗ ] = q û, (23) where i is the square identity matrix and qe is composed of the blocks qe i corresponding to the expression (20). 4.2. periodic boundary conditions to satisfy the constraint (19), we introduce periodic boundary conditions (pbc), which is a natural model assumption for materials with periodic microstructures. let’s denote π(x) the mapping from the source part γs of the boundary γ onto its periodic image. the fluctuation dofs at a periodic point, denoted as u∗(π(x)), are then equivalent to the corresponding dofs u∗(x) at the boundary of the unit cell γs: u∗(π(x)) = u∗(x) ∀ x ∈ γs. (24) to this end, we established a new vector a, containing unknown periodic fluctuation dofs, which maps to u∗ through the boolean matrix p∗: u∗ = p∗a. (25) this step significantly reduces the number of unknowns in our equations. to prevent the puc from moving as a whole unit during deformation, we prescribe zero fluctuation displacements at the node in the centre of the top edge of the puc, illustrated with a black node in figure 3, while rotations remain free. due to the periodicity, the fluctuation displacements must also vanish at the node in the centre of the bottom face of the puc. figure 3. fixed fluctuation displacements at nodes in the centre of the top and bottom face of the puc, marked down with the black dots. similarly to equation (23), the matrix p̂ connects the macroscopic deformation e and the fluctuation unknowns a, to the extended dofs û: û = [ e u∗ ] = [ i p∗] [ e a ] = p̂ â. (26) by connecting the macroscopic and fluctuation parts of the displacement field from equation (26) and considering equation (23) we get an expression for full-field displacement dofs u based on unknown â: u = qp̂ â. (27) 4.3. energy minimisation for every macroscopic deformation e, there is a certain state into which the cell deforms, because it naturally attempts to reach the state with the lowest energy. the energy e of a discrete beam model, which represents our puc composed of beams and nodes, can be written as: e = 1 2utku. (28) similarly, for a linear elastic material of volume v that is subjected to a uniform deformation e at the macroscopic level, the relation for the energy can be expressed as: em = v 1 2etdhome, (29) where dhom is the material stiffness matrix of the desired homogenised metamaterial and vector e contains the individual macroscopic components of the deformation tensor in the vectorial form introduced in equation (21). plugging the unknowns from equation (27) into equation (28) yields an expression for puc’s energy based on macroscopic deformations e and fluctuation unknowns a: e(e, a) = 1 2 [ e a ]t [ k̂ee k̂ea k̂ae k̂aa ] [ e a ] = 1 2 âtk̂ â, (30) where stiffness matrix k̂, and its four submatrices k̂ij pertinent to e and a, follow from: k̂ = p̂tqtk q p̂. (31) 23 nataša jošková, martin doškář acta polytechnica ctu proceedings since we are interested in the response of the homogenised puc to a prescribed macroscopic deformation e and not in displacement of its individual nodes, we express a with respect to the macroscopic deformation e of the cell. to this end, we keep the macroscopic deformation e fixed and determine the fluctuation displacements a as the solution ã(e) that minimizes the energy e(e, a) for the given deformation e as: ã(e) = argmin a∈rn 1 2(etk̂eee + etk̂eaa + atk̂aee + atk̂aaa), (32) where n represents the number of unknown fluctuation dofs. as a result of the matrix k̂ being both symmetric and positive definite, the quadratic form (30) attains a global minimum at the point of its zero gradient: ∇ae(e, a) ∣∣ a=̃a(e) = kaee + kaaã(e) = 0. (33) this provides us with the expression for the minimizer: ã(e) = −k−1 aa kaee. (34) substituting the expression above into equation (30) yields an energy dependent entirely on the macroscopic deformations e: ẽ(e) = e(e, ã(e)), (35) ẽ(e) = 1 2 et(k̂ee − k̂eak̂−1 aa k̂ae) e = 1 2 etkeffe. (36) comparing the expression (36) with the formula for the energy of a homogeneous material of volume v = |ω| subjected to a constant deformation e in the form in equation (29), we arrive at the relation for the homogenised material stiffness of the auxetic metamaterial as: dhom = 1 v keff. (37) 4.4. effective poisson’s ratio the procedure introduced in the previous sections yields the whole effective stiffness matrix. however, comparing and discussing the entire stiffness matrix is cumbersome. here, we focus on the homogenised poisson’s effect as it is the primal objective of the auxetic metamaterial. because our structures are symmetric in three mutually perpendicular directions, we can expect the overall orthotropic response. consequently, there are three different values of the poisson’s ratio depending on the direction of the prescribed relative deformation. clearly, the poisson’s ratios ν in the xy and xz planes, νxy and νxz, will be the same due to the symmetry of puc. furthermore, νyz and νzy should be equal for the same reason. we will refer to the stress in the metamaterial σ to distinguish the macroscopic level from the microscopic one. the stress-strain relation is linear because we work in the range of hooke’s law, so macroscopic stress can be written as: σ = dhome, (38) which can be broken down into individual components as follows: σx σy σz σyz σxz σxy  =  dxx dxy dxz 0 0 0 dyx dyy dyz 0 0 0 dzx dzy dzz 0 0 0 0 0 0 gyz 0 0 0 0 0 0 gxz 0 0 0 0 0 0 gxy   ex ey ez γyz γxz γxy  . (39) to determine the poisson’s ratio νij , we perform a virtual uniaxial tension/compression experiment in which we prescribe the macroscopic strain in the ith direction and compute the strain in the jth direction for the requirement of zero macroscopic stress in the jth and kth direction. this experiment results in the following relation: ẽei j = dikdjk − dijdkk djjdkk − d2 jk ei. (40) using (40) we can express poisson’s ratio for any direction, following its definition as a negative ratio of the derived and prescribed macroscopic strain. consequently, the effective poisson’s ratio of an auxetic metamaterial is given by: νij = − ẽei j ei = dikdjk − dijdkk d2 jk − djjdkk . (41) 5. results we parameterised the two microstructural geometries (cubic and hexagonal) from section 2, recall figures 1 and 2, with an angle δ ∈ (0°, 45°⟩. this range was chosen to avoid beams’ overlaps. the numerical results comply with our assumptions of equal poisson’s ratio values in following directions: νxy = νxz, νyx = νzx, νyz = νzy, (42) see also overlapping lines in figure 4. we observed auxetic behaviour in the whole range of δ with cubic puc, while the metamaterial with hexagonal puc exhibits pure auxetic properties only for angles δ ∈ ⟨10.56°, 45°⟩. for the lower values of δ, the metamaterial is auxetic only in the xy and xz planes, with the highest poisson’s ratio value of νyz = 0.33, which leads to the lateral contraction during stretching in the yz plane. poisson’s ratios νyx and νzx are the most influenced by the metamaterial’s geometry and attain their minimum, within the investigated range of δ, from which 24 vol. 49/2024 effect of geometry on homogenised properties of auxetic metamaterial 0 5 10 15 20 25 30 35 40 45 -2 -1.8 -1.6 -1.4 -1.2 -1 -0.8 -0.6 -0.4 -0.2 0 xy xz yx zx yz zy (a). cubic microstructure. 0 5 10 15 20 25 30 35 40 45 -1.6 -1.4 -1.2 -1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 xy xz yx zx yz zy (b). hexagonal prismatic microstructure. figure 4. poisson’s ratio as a function of angle δ for cubic and regular hexagonal prismatic microstructure. their value starts increasing and slowly approaching remaining poisson’s ratios. specifically, for the cubic puc, the global minimum νyx = −1.98 is achieved by the geometry of angle δ = 13.78°. hexagonal puc exhibits its minimal poisson’s ratio of value νyx = −1.55 by the angle δ = 15.92°. the remaining pairs of poisson’s ratios, i.e. νxy, νxz and νyz, νzy, on the other hand, tend to decrease with the increasing angle δ across the entire range. additionally, we observe certain values of δ, where some poisson’s ratios coincide. in the cubic puc, νxy and νyz attain the same value of −0.05 for δ = 3.32°. the hexagonal puc exhibits two such values of δ, δ = 16.96° and δ = 36.98°, for which poisson’s ratios are equal to νxy = νyz = −0.26 and −0.63, respectively. in addition, the hexagonal puc have equal poisson’s ratios νxy and νyx of value νxy = −0.82 at the angle δ = 44.71°. 6. conclusion we investigated the influence of the geometry controlled by the angle δ in the bow-tie part of the microstructure on the effective poisson’s ratios of two three-dimensional metamaterials. their pucs were modelled with discrete beam elements, and the effective metamaterial properties were determined using direct stiffness method and the first-order numerical homogenisation, resulting in a connection between their microstructure and macroscopic behaviour. given the symmetries of both investigated microstructural geometries, we obtain three distinct values of poisson’s ratios as functions of angle δ. two of these values are monotonously decreasing with increasing angle δ, while the remaining value (same for νyx and νzx) exhibits a minimum within the studied range of ν. the cubic puc features the minimum value νyx = −1.98 for δ = 13.78°, while the hexagonal puc attains the minimum νyx = −1.55 for δ = 15.92°. in conclusion, only the cubic puc delivers auxetic behaviour in all directions for all investigated values of δ. the hexagonal puc shares the same trait only for δ ≥ 10.56°. acknowledgements this work was supported by the grant agency of the czech technical university in prague, grant no. sgs23/032/ohk1/1t/11. references [1] e. barchiesi, m. spagnuolo, l. placidi. mechanical metamaterials: a state of the art. mathematics and mechanics of solids 24(1):212–234, 2019. https://doi.org/10.1177/1081286517735695 [2] x. zhou, l. ren, z. song, et al. advances in 3d/4d printing of mechanical metamaterials: from manufacturing to applications. composites part b: engineering 254:110585, 2023. https: //doi.org/10.1016/j.compositesb.2023.110585 [3] t. bückmann, n. stenger, m. kadic, et al. tailored 3d mechanical metamaterials made by dip-in direct-laser-writing optical lithography. advanced materials 24(20):2710–2714, 2012. https://doi.org/10.1002/adma.201200584 [4] a. kassimali. matrix analysis of structures. cengage learning, stamford, australia, 2nd edn., 2012. isbn 978-1-111-42620-0. [5] w. mcguire, r. h. gallagher, r. d. ziemian. matrix structural analysis. john wiley, new york, 2nd edn., 2000. isbn 9781507585139. [6] j. c. michel, h. moulinec, p. suquet. effective properties of composite materials with periodic microstructure: a computational approach. computer methods in applied mechanics and engineering 172(1–4):109–143, 1999. https://doi.org/10.1016/s0045-7825(98)00227-8 [7] m. doškář, j. novák. a jigsaw puzzle framework for homogenization of high porosity foams. computers & structures 166:33–41, 2016. https://doi.org/10.1016/j.compstruc.2016.01.003 25 https://doi.org/10.1177/1081286517735695 https://doi.org/10.1016/j.compositesb.2023.110585 https://doi.org/10.1016/j.compositesb.2023.110585 https://doi.org/10.1002/adma.201200584 https://doi.org/10.1016/s0045-7825(98)00227-8 https://doi.org/10.1016/j.compstruc.2016.01.003 acta polytechnica ctu proceedings 49:20–25, 2024 1 introduction 2 geometry of investigated metamaterial 3 direct stiffness method 3.1 local stiffness matrix 3.2 global stiffness matrix 4 homogenisation 4.1 displacement decomposition 4.2 periodic boundary conditions 4.3 energy minimisation 4.4 effective poisson's ratio 5 results 6 conclusion acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.38.0203 acta polytechnica ctu proceedings 38:203–209, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague closing the loop of textile: circular building renovation with novel recycled insulations from wasted clothes andrea augello, olga beatrice carcassi, francesco pittau∗, laura elisabetta malighetti, enrico de angelis politecnico di milano, abc department, via giuseppe ponzio 31, 20133 milano, italy ∗ corresponding author: francesco.pittau@polimi.it abstract. the implementation of new energy policies and standards for nzeb is expected to lead to a significant reduction of ghg emissions from building use in europe in the next decades. on the other side, the growing pressure on insulation materials risks to significantly contribute to the exhaustion of the remaining carbon budget due to the high carbon intensity of conventional insulation for material processing. consequently, storing carbon in construction products and promoting circular economies able to generate up-cycling processes from industrial or post-consumption waste are the key strategies to promote an effective transition toward a carbon-neutral society. fashion & clothing is one of the manufacturing sectors which mostly contributes to waste generation and fossil ghg emission. this paper presents the main outcomes achieved from recydress project, which focuses on the valorisation of wasted textile collected by municipal districts to develop novel thermal insulations for building applications. three alternative conceptual manufacturing processes were defined at lab scale based on different treatment of textile fibres, with produced specimens tested for thermal characterization. finally, the lca results of an etics application for façade renovation were compared considering as functional unit 1 m2 of façade with similar thermal resistance. keywords: textile wastes, circular economy, life cycle assessment, sustainability, thermal conductivity, transient plane source method. 1. introduction to limit the high environmental impacts caused by the construction sector, the european commission has promoted strategies and directives aimed at improving the energy efficiency of existing buildings, largely inefficient from an energy point of view. the european green deal constitutes one of the main issued strategies, which aims at the elimination of greenhouse gas production in europe by 2050, through a gradual transition that will involve various sectors (transport, industry, agriculture, energy, etc.). other instruments, such as the renovation wave, instead foresee an increase in the annual building renovation rate, currently between 0.4 and 1.2 %, over the next ten years, thus decarbonizing about 35 million buildings. to promote the circularity of the economy and reduce the environmental impacts associated with building materials, numerous authors have studied the possibility of integrating wastes of different nature within construction products. among these, textile waste has been shown to have potential as thermal insulators, thanks to thermal conductivities comparable to traditional insulating materials; therefore, the use of these materials within the construction sector could constitute a possible solution to the problem of recycling textile waste, which ends up mainly in landfills (57 %) or incinerated (25 %) at the end of their life. furthermore, the problem of textile waste in europe is expected to increase in the coming years due to the restrictions imposed by the waste framework directive, which imposes the mandatory separate collection of textile waste in all member states by 2025, preventing landfill, incineration, or export of textile wastes to non-european countries. this paper summarizes the main results achieved from the recydress project, aimed at evaluating the development of thermal insulations based on recycled textile wastes. different types of samples were analyzed, evaluating both their thermal and environmental properties, investigating the effect of parameters such as density, treatment, binder, and composition of the insulating material. 1.1. textile-based insulation materials: a short literature review the thermal properties of textile wastes of different compositions have been studied by many authors in the last years. dieckmann et al. [1] tested the thermal properties of insulation panels based on waste chicken feathers mixed with bicomponent fibers: the lowest value of thermal conductivity measured, equal to 0.033 w/mk, was comparable with conventional insulation materials. similar results were obtained by mrajji et al. [2], who investigated the thermal properties of six nonwovens based on different amounts of chicken feathers, cotton, and wool produced with the needling technique: values between 0.0313 and 0.04465 w/mk were measured. the ther203 https://doi.org/10.14311/app.2022.38.0203 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en a. augello, o. b. carcassi, f. pittau et al. acta polytechnica ctu proceedings figure 1. thermal conductivity of textile wastes from literature. mal properties of polyester fabrics were investigated by jordeva [3], analyzing different types of polyester shredded and inserted in casings made of polypropylene: their measured thermal conductivity values ranged from 0.052 to 0.0603 w/mk. good thermal performances of polyester-based materials were obtained by drochytka et al. [4], who tested five samples based on polyester fibers mixed with synthetic bicomponent fibers produced with the airlay technology: values between 0.04319 and 0.05598 w/mk were measured. the same airlay technology was adopted by zach et al. [5], manufacturing and testing five insulation materials based on flax, cotton, and polyester. the measured thermal conductivities ranged from 0.036 to 0.0443 w/mk, depending on density and composition. the use of recycled wool as thermal insulation was studied by rubino et al. [6] on four samples based on merino wool waste bound with chitosan: thermal conductivity values ranging from 0.049 to 0.06 w/mk were measured. these samples were later compared by rubino et al. [7] with other specimens based on merino wool bound with arabic gum: chitosan and arabic gum samples showed similar thermal conductivity values. a few years later, rubino et al. [8] analyzed five samples with different densities based on merino wool bonded with copolyester/polyester bicomponent fibers: lower values of thermal conductivity were obtained, going from 0.044 to 0.057 w/mk. eventually, rubino et al. [9] studied the combination of the previously investigated merino wool/arabic gum samples with organic phase change materials. the measured thermal conductivities (between 0.053 and 0.085 w/mk) increased as function of the temperature and amount of pcms, due to the increased connections among fibers and the reduction of the pore volume. the thermal performances of five samples based on wool and polyester were studied by patnaik et al. [10]: the average values ranged from 0.032 to 0.035 w/mk, confirming that it is possible to manufacture insulation materials with a similar thermal conductivity as that of 100 % waste wool fibers. the thermal behavior of wool and acrylic fibers was investigated by wazna (el) et al. [11], producing four nonwovens based on acrylic and sheep wool wastes with the needling technique, characterized by low values of thermal conductivity (0.0339–0.0355 w/mk). another acrylic nonwoven fabric was investigated by gounni et al. [12], confirming its capability to significantly reduce the heat flux (λ = 0.038 w/mk). hadded et al. [13] investigated the effect of the mechanical treatment on the thermal properties by considering two textile samples obtained through the same process but at different stages: both materials presented low density and high porosity, with similar thermal conductivities (0.033 and 0.039 w/mk). hegyi et al. [14] investigated 13 samples made of different amount of recycled plastics, denim, or sheep wool mixed with bicomponent fibers; thermal conductivities between 0.032 and 0.053 w/mk were measured. the results obtained by the previously mentioned authors are reported in figure 1. 204 vol. 38/2022 closing the loop of textile: circular building renovation . . . figure 2. tested textile samples. dimensions: 10 cm3. 2. materials and methods 2.1. tested materials as shown in figure 2, two different sizes of fibres were selected for thermal characterization: (i) staple textile fibres, and (ii) fabric strips with two alternative options to bind the layers: needle punched and vinyl adhesive. in total, twelve types of wasted textile samples were prepared for testing, distinguished between: (i) loose fibres (denim_1, denim_2, cotton_1, polyester_1, polyester_2), (ii) thermal treatment (cotton/bico), and (iii) cut-out strips (wool_1, wool_2, cotton_2, cotton_3, synthetic_1, synthetic_2). the effect of composition, density, treatment undergone by the materials, or binder on thermal conductivity values was investigated. all textile fibres used in the test campaign were provided by vesti solidale società cooperativa sociale, an ngo operating in northern italy with a business core on collecting and sorting wasted clothes. 2.2. test methodology 2.2.1. thermal characterization: transient plane source method the measurements of the materials’ thermal conductivity were carried out through the transient plane source method, bs en iso 22007-2:2015, which is based on the use of a disk, called hot disk, which produces a thermal impulse on the material under examination and measures its change of thermal resistance; the model adopted by the software then allows to derive the thermal properties such as conductivity, diffusivity, specific heat, etc. for the loose textile wastes, the test methodology involves the use of some small wooden boxes (cube with a side equal to 10 cm) inside which the loose material to be tested is inserted; a hole located about halfway up the side allows the insertion of the disk in the middle of the sample; the dimensions of the boxes were chosen considering the need of guaranteeing an adequate sample thickness to avoid any edge effects that could alter the results. for the composite materials, the disk was simply inserted between two equal samples with a thickness of about 5 cm each. at least six measurements were performed on each sample, to take into account the variability of the results; therefore, the reported thermal conductivities are intended as averages of six or more values. besides, the uncertainties connected to the measurement method were evaluated by calculating the average standard deviations. 2.2.2. life cycle assessment for carbon footprint calculation the life cycle assessment is a methodology used to estimate the environmental burdens associated with the entire life cycle of a product, process, or service, from raw material acquisition through production, use, endof-life treatment (recycling or final disposal). the requirements and guidelines for carbon footprint of products are defined in the standard iso 14067:2018. this methodology is used to evaluate the carbon footprint of the textile-based insulation materials, which will be compared with those of traditional ones, to highlight benefits or disadvantages. two functional units were adopted: 1 kg of insulation material and 1 m2 of panels with a thermal resistance of 2.22 m2k/w. the chosen system boundaries from “cradle to gate” include the phases between the textile wastes collection to the final product manufacturing. among the tested textile samples, only the environmental impacts of cotton waste (cotton_1) were assessed, combining the loose material with different amounts of bicomponent fiber (5 % or 25 % in weight). the analysis was performed on simapro using the ecoinvent 3.8 database as the main source of data by considering the italian energy mix for electricity driven processes. other relevant data about energy consumption of machinery used for the manufacturing process were collected onsite from a italian company operating in the textile industry. 205 a. augello, o. b. carcassi, f. pittau et al. acta polytechnica ctu proceedings figure 3. thermal conductivity of the tested loose (left) and composite (right) materials. 3. results 3.1. thermal characterization the tested samples were stored in the laboratory under a controlled environment before starting the measurements. the moisture content of some textile samples was evaluated using a moisture analyzer (pce-ma100), which measured an average moisture content of 8–9 %. the thermal conductivity values measured on the tested samples are reported in the following scatter plot (figure 3), having the materials’ density as x-axis; the error bars represent the calculated thermal conductivity’s uncertainties. all the loose textile materials showed low values of thermal conductivity, almost always lower than 0.06 w/mk, which makes them suitable as insulating materials; on the contrary, the cotton/bico sample is the only tested composite material that could be applied as insulation material. all the tested parameters, i.e. density, treatment, binder, and composition have a not negligible effect on the samples’ thermal properties. the thermal conductivity increases with the increasing density for four of the five tested loose materials: only the sample “denim_1” has an opposite trend, i.e. a decreased thermal conductivity, going from 30 to 50 kg/m3. the treatment undergone by the materials, investigated by comparing the two types of polyester, seems to have a not negligible effect too: we registered a decrease of 16 % (in average) of the thermal conductivity for polyester_2 on both the tested densities. for what concerns the effects of the binder on the thermal performances, the needled samples do not have excellent thermal performances: wool_1 and synthetic_1 have the best results, with conductivity values lower than 0.075 w/mk; on the contrary, cotton_2 has a thermal conductivity value completely outside the range of the insulating materials (λ = 0.137 w/mk). in the case of adding glue to the samples, there is a further sharp decrement in thermal conductivity: for the cotton_3 sample, an average value of 0.218 w/mk is measured (+ 59 % compared to the needled cotton_2 sample), i.e. an unsuitable conductivity for an insulating material; furthermore, measurement uncertainties are much greater than those calculated for the other tested materials. these problems are probably attributable to the nature of the chosen binder and to the large amount of water contained in it, which keeps the sample to be tested partially moist even for several weeks and which therefore could have had a not negligible effect on the thermal conductivity measurement. for these reasons it was not possible to evaluate the performance of the other two composite materials (synthetic_2 and wool_2) as they were excessively wet and with results not consistent with what was expected as they were probably influenced by the initial materials conditions. this problem could represent the big disadvantage of the recycled textile wastes, since for many building applications it is necessary to guarantee a minimum water-repellent behaviour. 3.2. carbon footprint assessment 3.2.1. production processes the functional unit chosen to calculate the global warming potential due to the production process of the two textile wastes (cotton/bico 95/5 and cotton/bico 75/25) is equal to 1 kg of insulation material. the production process was divided into the following sub-processes: (1.) collection and shredding. it includes: (a) transport of textile wastes to the collection site located in cinisello balsamo (average distance of 15 km) (b) production of low-density polyethylene for packaging (volume of 0.25 m3) (c) transport of textiles to the production site located in recanati (distance of 460 km) (d) electricity consumption for textile shredding of 515 kwh (e) water consumption of 160 l/h (f) lubricant consumption of 10–20 l/h (g) flame retardant consumption of 30–40 l/h (h) allocation of the production of machinery used in this sub-process 206 vol. 38/2022 closing the loop of textile: circular building renovation . . . figure 4. gwp, in kg co2eq per 1 kg of insulation, of the two textile alternatives. material λ [w/mk] thickness [mm] density [kg/m3] rt [m2k/w] cotton/bico 95/5 0.045 100 50 2.22 cotton/bico 75/25 0.045 100 50 2.22 eps 0.035 78 20 2.23 rockwool 0.035 78 60 2.23 glasswool 0.032 71 50 2.22 woodfibre 0.040 89 50 2.22 table 1. properties of the analysed insulation materials. (2.) production of textile-based insulation. it includes: (a) production of bicomponent fibers in south korea (b) transport of bicomponent fibers by ship to the port of genoa (distance of 18 000 km) (c) transport of bicomponent fibers by trucks to the production site located in recanati (d) electricity consumption for textile thermopressing of 293 kwh (e) methane consumption of 35 m3/h (f) allocation of the production of machinery used in this sub-process (g) production of packaging materials the global warming potential of the two insulations is represented in figure 4, expressed as kg co2eq/kg and as percentages respectively. the sample composed of 25 %m of bicomponent fibres presents the highest gwp-value, thus impacting 2.5 times more than the other sample made with 5 % of bicomponent fibres. this difference is mainly caused by the higher amount of bicomponent fibres and thus by their impacting production process, which represents 79 % of the total gwp of the 75/25 sample. 3.2.2. comparison with traditional insulation materials after calculating the impact associated with the production process, a comparison with traditional insulation materials is performed (table 1). the thermal resistance is adopted as functional unit, taking into account the differences in terms of thermal performances: 1 m2 of panels with a thermal resistance of around 2.2 m2k/w. furthermore, the benefits of the recycled and natural insulation materials are highlighted, calculating the amount of co2 stocked by each kg of dry material obtained multiplying the carbon content and finally by converting carbon into co2, through the mass equivalent coefficient 3.67. as already discussed in the previous paragraph, a higher amount of bicomponent fibres negatively affects the carbon footprint of textile-based materials. the quantity of stocked co2 is affected by the percentage of bicomponent fibres too, growing with their decreasing content: thanks to its low amount of bicomponent fibres, the sample cotton/bico 95/5 is characterized by a negative net gwp value. 4. discussion the methodology adopted for the thermal characterization of the insulation materials presents some criticalities and uncertainties related to the test parameters adopted (time, voltage, hypothesis, etc.) and to the measurement method itself. it is known that the thermal conductivity measured by the transient plane source method could be overestimated by about 10 % with respect to real values, as demonstrated by colinart et al. [15]. therefore, a comparison with other measurement methodologies (e.g., heat flowmeter, guarded hot plate etc.) could help for the valida207 a. augello, o. b. carcassi, f. pittau et al. acta polytechnica ctu proceedings figure 5. carbon footprint (gwp) and co2 stored in 1 m2 of textile and conventional insulation materials with a rt = 2.2 m2k/w. tion of the reported results. for what concerns the carbon footprint assessment, as shown in the following figure 5, results are largely sensitive to the share of bico used to bind the fibres though a thermal treatment. nowadays, the eu demand of bicomponent polyester fibres (bico) is almost fully covered by south korea. the long distant transportation and the high-carbon intensity for country-specific energy generation largely affects the gwp of the final product. investigation of possible alternative fibres, resulting from pet recycling process, should be investigated in order to define alternative low-carbon solutions for manufacturing the insulation panels. 5. conclusions this article summarizes the main results obtained in the “recydress project”, which aimed at investigating the properties of textile-based insulations, proposing alternative ways to recycle a material whose end of life is mainly constituted by landfill or incineration. the main findings of this research were: • textile wastes could be applied as insulation materials thanks to their low thermal conductivity values • as verified by many authors, the thermal conductivity increases with the increasing material’s density • the treatment undergone by the textile wastes affects their thermal performance • the chosen binder, i.e. glue, negatively affects the materials’ thermal conductivity, due to the high amount of water contained • the gwp value of textile-based materials is mainly affected by the amount of bicomponent fibers and their production process; besides, the cotton/bico 95/5 sample is characterized by a negative net gwp value, thanks to its low amount of bicomponent fibers. furthermore, future investigations could involve the measurement of the thermal properties of the tested materials with other methodologies, comparing and validating the obtained results. besides, an additional analysis could concern the analysis of other types of textile materials, trying to improve both their thermal and environmental performances. acknowledgements this work was part of the recydress project, funded by duferco engineering s.p.a. particularly, authors wish to thank meng. emilio castelli and meng. ezio palmisani for their constant support during the research activities. authors also acknowledge phd adriana angelotti, phd alessandro dama and phd andrea alongi for their support during the thermal characterization of the samples. references [1] e. dieckmann, r. onsiong, b. nagy, et al. valorization of waste feathers in the production of new thermal insulation materials. waste and biomass valorization 12(2):1119–1131, 2021. https://doi.org/10.1007/s12649-020-01007-3 208 https://doi.org/10.1007/s12649-020-01007-3 vol. 38/2022 closing the loop of textile: circular building renovation . . . 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https://doi.org/10.4028/www.scientific.net/cta.1.287 acta polytechnica ctu proceedings 38:203–209, 2022 1 introduction 1.1 textile-based insulation materials: a short literature review 2 materials and methods 2.1 tested materials 2.2 test methodology 2.2.1 thermal characterization: transient plane source method 2.2.2 life cycle assessment for carbon footprint calculation 3 results 3.1 thermal characterization 3.2 carbon footprint assessment 3.2.1 production processes 3.2.2 comparison with traditional insulation materials 4 discussion 5 conclusions acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2024.47.0028 acta polytechnica ctu proceedings 47:28–34, 2024 © 2024 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague parameters of back filling material prepared from mine tailing, fly ash, and mine water karel dvořák∗, andrea jančíková brno university of technology, faculty of civil engineering, veveří 331/95, 602 00 brno, czech republic ∗ corresponding author: karel.dvorak@vut.cz abstract. the article investigates the influence of mine tailing, fly ash, and mine water on strength parameters as mine paste filling material performing the strength tests on different recipes of mortar mixes with individual types of fillers and wastewater from extraction of mine tailings. recipe is prepared in proportions corresponding to the standard mortar and compared with a reference recipe according to the fresh mortar consistency test using a shaking table according to čsn en 1015-3. resulting measured values on 40 × 40 × 160 mm solids samples for which the evaluation of the volume weight was carried out in the range of 2, 7, 14, 28, 56 and 90 days according to en 1015-10 (722400). the results showed that that it is possible to prepare relatively fluid mixtures with extremely low binder dosage. and not only the water coefficient but also the granulometry of the raw material plays an important role in the long-term strengths of samples. keywords: fly ash, mine water utilization, mine backfill mixture. 1. introduction the mining industry can be divided into two broad categories, namely metallic and non-metallic. precious metals are extracted from mined ores following a well-defined series of mineral processing and metallurgical procedures. this series leads to the generation of large quantities of solid industrial wastes from mine development, such as fragmented waste rock and mined bedrock [1]. another category of solid waste generated from mineral processing (separation of valuable minerals from ores) is called tailings. it follows that mining and mineral processing is associated with the generation of significant quantities of waste. coal ash from thermal power plants during coal mining, tailings from precious metal mining and metallurgical slag from copper mining are significant waste streams generated [2, 3]. the generation of such a huge number of segregated tailings adversely affects the environment, including air, water, and soil [4, 5]. a vast area of surface land is subsequently required to adequately store and manage this waste in the form of segregated tailings. underground disposal of these industrial wastes (tailings, fly ash cinders, red mud etc.) generated from mining and related activities in the form of mine backfill is an effective solution [6–8]. the voids created after ore extraction in underground mines can be refilled with specific waste material referred to as backfill [9]. backfilling of mines helps to prevent subsidence, improves the stability of underground structures, and prevents ore grinding also increases the recoverability of the mined waste and as a result increases production with many other benefits [8–11]. backfilling can be divided into two broad categories, the former being unconsolidated backfill and the latter being cemented reinforced backfill [12]. unconsolidated backfill, waste is added without any binder. while in cemented backfill, a hydraulic binder is added, usually this binder is portland cement which is added to the solid waste material to create background for mechanical properties of the paste [12, 13]. one form of cemented backfill used in underground mines is a paste with well-defined mechanical properties [12–15]. paste backfill is widely applied worldwide because of its techno-economic and its technology success which is evident from the fact of increased demand over the last 40 years [14]. in recent years, several modifications and investigations have been made regarding improvements in the design and environmental friendliness of paste backfill. significant progress in this area has been achieved using alternative binders, the use and modification of backfill materials, investigation of their strengths, rheology of leaching probability, and the economic use of other wastes. this work was devoted to the study of the basic mechanical properties and the effect of different mine waters used for the preparation of tailings backfill mixtures with binder in the form of fluidized fly ash activated by grinding [8–14]. mine tailings, which are deposited after the extraction of minerals, are the largest source of waste in the processing of mining raw materials. worldwide, approximately 5-7 billion tons of waste are generated annually in the form of mined tailings [7, 8]. this waste has traditionally been disposed of in tailings ponds, repositories, which is associated with serious environmental, geotechnical, and economic problems [15– 17]. cement paste backfilling is increasingly used to fill excavated voids in underground spaces [6–8, 10]. backfilling with cement paste recycles the processed tailings. into underground mine cavities, reducing the volume that needs to be deposited on the surface. this process reduces the potential environmental impacts 28 https://doi.org/10.14311/app.2024.47.0028 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 47/2024 parameters of back filling material associated with the issue [8, 18–21]. material recycled in this way can act as a permanent support system or temporary working platform for others [15, 16]. cement pastes treated in this way, in the form of a high-density slurry, consist of mined tailings, a low proportion of cement binder (3-7 %) and processed mine water to meet the design mechanical properties. to minimize the cost of the binder used, other alternative binders and materials are very often used, the most common being high temperature fly ash with pozzolanic activity and silica fume from the metallurgical industry. to achieve better pumpability and lower water coefficient while still maintaining good workability of these mixtures, superplasticizers are also very often used [1–3, 13–16]. the transport of backfill mixtures in underground mines is most often carried out by gravity or mediated by pumping through pipelines or boreholes. for this reason, the prepared slurry should be thickened to achieve a non-sedimentary character for easy pumping into mine cavities created during deep mining operations [11, 12, 19]. hence, the fluidity of the paste backfill is an essential parameter in the design of cement slurry. the rheological properties (i.e., yield strength and viscosity) are related to the socalled amount of energy consumed to ensure fluidity. mine backfill mixtures should be homogeneous, mobile, suitable for transport by pumping over distances of hundreds of meters, and workable for a minimum of 120 min. after curing, the strength should ideally reach at least 4 mpa after 90 days, however, in conditions without static function, a strength of about half, 2-3 mpa may be acceptable [18–22]. several studies have been carried out in recent years to understand the rheological behavior of backfill pastes and their main influencing factors. these studies show that the rheological behavior is influenced by external (e.g., temperature and time) and internal (e.g., cement content, addition of slag, fly ash or superplasticizers, and type of water used) factors. although the effects of these external and internal factors are widely documented in the literature, the reported results are not consistent because the physical, chemical, and mineralogical properties of tailings vary from mine to mine [8–14]. the aim of this research was to absolutely minimize the amount of binder component, which is only supposed to have the function of ensuring the cohesion of the cured composite. the use of fluidized power plant fly ash, which is not used at all to produce conventional concretes, is also interesting. 2. material and methods to create models of non-static landfill mixtures, residues after separation of heavy and light recoverable components from the cínovec, měděnec and zlaté hory landfills were used, for which specific weight tests were carried out, where the measured values of specific weights for the tailings are given in table 1. the cement paste binder has been completely replaced by a binder based on grinding-activated fly ash, specific weight [g cm−3] měděnec 3.56 cínovec 2.74 zlaté hory 2.82 table 1. the resulting measured values of the specific weight of the separated tailings. which is a dry, clinker-free hydraulic binder material commonly used as an active admixture in shotcrete. composition of the fly ash used: • above 50 % sio2 with al2o3 • up to 20 % free lime, cao • up to 8 % sulfur dioxide, so3 the specific weight of the fly ash was measured in the laboratory using a helium pycnometer accupyc 1340 (micromeritics gas pycnometer), which reached 2.78 g cm−3. the specific weight is significantly lower than that of cement (3.1 g cm−3). and its fineness of grinding was also measured similarly to cement according to čsn en 196-6 using the zeb pc-blainestar instrument. specific surface area of the fly ash came out to be 8 200 cm2 g−1. on the contrary the specific surface area of the fly ash is significantly higher than that of cement (3 000 – 4 500 cm2 g−1). results are given in table 2. specific specific weight surface area [g cm−3] [cm2 g−1] fly ash 2.78 8 200 table 2. determination of specific weight and specific surface area. furthermore, the normal consistency and the onset of hardening of the binder based on fly ash activated by grinding were determined similarly to cement according to en 196-3 (722100), determination of setting times and volume stability. the result is given in the following table 3. and table 4. amount resulting consistency [g] [mm] demineralized water 215 3 fly ash 500 table 3. determination of normal consistency. in addition to the independent properties of the binder consisting of fly ash and filler, which were the tailings residues after heavy and light metal separation, the applicability of mine water for the production and subsequent observation of the durability of the tailings mixtures was tested. to study the influence 29 karel dvořák, andrea jančíková acta polytechnica ctu proceedings amount beginning of hardening [g] [min] demineralized water 215 40 fly ash 500 table 4. determination of the beginning of hardening. of mine water, water from the mining area of cínovec and zlaté hory (acidic water and alkaline water) was used. the ph value was monitored with a ad8000 professional multi-parameter ph – bench meter, with the resolution of 0.01 ph and accuracy of ±0.01 seen in table 5. seven mortar pastes based on fly ash binder were created to monitor the process of gaining their strengths. the mixture was always composed of 1 350 g of residues after separation of heavy and light usable components from landfills 225 g of fly ash and either demineralized water (reference) or mine water (water cínovec, water zlaté hory acidic and water zlaté hory alkaline). ph cínovec 8.5 zlaté hory acidic 3.4 zlaté hory alkaline 7.8 table 5. resulting measured ph values for each water type. to perform the strength tests, the recipes of mortar mixes for individual types of fillers and wastewater from the extraction of tailings were designed and compared with a reference recipe prepared in proportions corresponding to the standard mortar according to the fresh mortar consistency test using a shaking table according to čsn en 1015-3. the monitored parameters were: • volumetric weight of fresh mortar according to en 1015-10 (722400), • flexural tensile strength and compressive strength at curing times of 2, 7, 14, 28, 56 and 90 days according to en 1015-11 (722400), determination of flexural and compressive strength of hardened mortars. due to the requirement for flowability of the mixes, the consistency of fresh mortar was chosen using a shaking table and levelling them to a 180 mm spillage. to achieve a 180 mm spillage, more water had to be added to the mixes than for normal consistency due to wetting of the separated aggregate. the proposed formulations of the stockpile mixes and the reference mixes are shown in table 6, table 7 and table 8. 3. results and discussion resulting measured values on 40 × 40 × 160 mm solids samples for which the evaluation of the volume weight amount resulting consistency [g] [mm] cen standard sand en 196-1 1 350 demineralized water 250 spillage 180/180 fly ash 225 table 6. proposed reference recipe for the mortar mixture. cínovec zlaté hory [g] [g] tailings 1 350 1 350 demineralized water 420 580 fly ash 225 225 table 7. proposed recipes for the mortar mixture with demineralized water. was carried out in the range of 2, 7 14 and 28 days according to en 1015-10 (722400), test methods for mortars for masonry – part 10: determination of volume weight of dry hardened mortar. the values obtained for the volume weights are summarized in table 9 and then evaluated in the bar graph in figure 1 for each individual mixture consisting of the residues after separation from the respective mine deposit together with the respective water and a fly ash-based binder activated by grinding. resulting measured values on 40 × 40 × 160 mm solids samples for which the strength tests were evaluated in the time intervals 2, 7 14, 28, 56 and 90 days according to en 1015-11 (722400), test methods for mortars for masonry – part 11: determination of tensile flexural and compressive strength of hardened mortars. the achieved strength test values are summarized in table 10 and table 11 and then evaluated in the bar charts in figure 2 and figure 3 for each individual mix consisting of the residue after separation from the respective mine deposit together with the respective water and a fluidized fly ash-based binder activated by grinding. the results of the fundamental mechanical properties of the backfill mixtures, determined on selected formulations with very high-water coefficient, showed that it is possible to prepare relatively fluid mixtures with extremely low binder dosage. not only the water coefficient but also the granulometry of the raw material plays an important role in the long-term strengths. here, the flotation waste is in principle significantly handicapped, however, at least minimum strengths were achieved here as well. the fluidity was ensured only by the water coefficient without the addition of thickening or plasticizing additives. these agents were deliberately not used here, as they would have made the whole technology significantly more expensive. the effect of the acidic mixing water on the mechanical properties did not show any significant effect and for this reason can be considered suitable 30 vol. 47/2024 parameters of back filling material měděnec [g] cínovec [g] zlaté hory [g] tailings 1 350 1 350 1 350 fly ash 225 225 225 demineralized water 420 (water cínovec) 420 (water cínovec) 620 (zh acidic) 620 (zh alkaline) table 8. proposed recipes for the mortar mixture with the use of post-mining water. after 2 days after 7 days after 14 days after 28 days reference 1 900 1 950 1 970 1 990 cínovec + demi.w. 1 700 1 730 1 800 1 810 zlaté hory + demi.w. 1 620 1 760 1 750 1 770 cínovec 1 780 1 690 1 730 1 750 měděnec 2 040 2 170 2 110 2 150 zlaté hory + acidic w. 1 770 1 760 1 720 1 750 zlaté hory + alkaline w. 1 850 1 850 1 790 1 860 table 9. achieved volumetric weights of given recipes. figure 1. graph showing the volume weights from table 9 of the tested samples at 2, 7, 14 and 28 days. figure 2. graph showing the tensile strength in bending from tables 10 and 11 of the tested samples at 2, 7, 14 and 28, 56 and 90 days. 31 karel dvořák, andrea jančíková acta polytechnica ctu proceedings figure 3. graph showing the compressive strength from tables 10 and 11 of the tested samples at 2, 7, 14 and 28, 56 and 90 days. properties [mpa] reference cínovec + demi.w. zlaté hory + demi w. 2 days tensile strength in bending 0.3 0.2 0.3 compressive strength 0.4 0.2 0.1 7 days tensile strength in bending 0.9 0.4 0.3 compressive strength 3.3 0.9 0.4 14 days tensile strength in bending 2.1 0.9 0.7 compressive strength 7.3 2.8 1.6 28 days tensile strength in bending 3.4 1.2 1.0 compressive strength 8.1 3.2 1.7 56 days tensile strength in bending 3.3 1.4 1.0 compressive strength 8.9 3.7 1.9 90 days tensile strength in bending 3.5 1.5 1.2 compressive strength 8.9 4.1 2.4 table 10. achieved volumetric weights of given recipes. properties [mpa] cínovec měděnec zh + acidic w. zh + alkaline w. 2 days tensile strength in bending 0.1 0.3 0.3 0.3 compressive strength 0.2 0.2 0.2 0.2 7 days tensile strength in bending 0.4 0.6 0.3 0.3 compressive strength 1.3 0.9 0.3 0.4 14 days tensile strength in bending 0.9 1.1 0.6 0.7 compressive strength 2.9 2.9 1.4 1.6 28 days tensile strength in bending 1.2 1.2 1.0 1.0 compressive strength 3.3 3.4 1.5 1.8 56 days tensile strength in bending 1.4 1.5 1.0 1.1 compressive strength 3.6 3.8 1.8 1.9 90 days tensile strength in bending 1.6 1.5 1.2 1.1 compressive strength 4.3 4.2 2.4 2.6 table 11. achieved strength properties of the formulations with mine waters. 32 vol. 47/2024 parameters of back filling material for these backfill mixtures without a static function. 4. conclusion in conclusion, it can be stated that: • according to the measured results, it can be concluded that it is possible to prepare relatively wellflowing mixtures with very high-water coefficients without the addition of plasticizers and with extremely low dosage of binder based on fly ash mixtures under placement conditions without static function. • fbc fly ash appears to be a promising binder for the preparation of backfill mixtures. • the presence of free lime in fly ash can compensate for the significantly lower ph of the mine water. • the lower ph of the water used to produce the tailing mixtures did not have a negative effect neither on volume weights nor strength in bending nor compressive strength of the tested samples in given times. • it will thus be possible to use untreated mine water to produce mixtures. acknowledgements we acknowledge the financial support to project 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prague, czech republic ∗ corresponding author: veronika.pavelcova@fsv.cvut.cz abstract. data validation is a critical step in every data analysis. it is a well-known fact that the results of any data analysis are directly dependent on the input values. this paper works with a worldwide database of laboratory tests collected during four year operation of web calibration application excalibre and elaborates on the data correctness. excalibre application enables automatic calibration of three critical state models based on data from standard laboratory tests. the paper presents the rules and overall methodology for checking the validity of laboratory tests data and summarises the obtained results. keywords: geotechnical laboratory test, data validation, data processing, error, warning, database. 1. introduction general observation of usage non-linear geotechnical material models for soils shows that the majority of practical engineers limit themselves to the use of basic non-linear soil models of the mohr-coulomb or drucker-prager type when solving standard geotechnical tasks. although these models are sufficient when describing approximate interaction of the structure and the subsoil, some complex tasks require more sophisticated solutions. critical state models allow for more accurate behaviour prediction, however, they are not as widely used as they could be. the general lack of knowledge of these advanced material models among practical engineers may be one of the reasons. the second reason is the significantly higher difficulty of determination of advanced non-linear models input parameters in comparison to the basic models. critical state models require prescription of parameters describing the behaviour of the given material under isotropic compression along with parameters characterising the current state of the soil. presented and previous work presented in extracting general knowledge of model parameters for clays out of numerous laboratory tests [1] is motivated by this non-straight-forward determination of input parameters and works forward to simplify the determination process by using knowledge extracted out of all of the data collected during four year operation of web calibration application excalibre [2–4]. this application enables automatic calibration of three advanced constitutive soil models based on detailed data from triaxial and oedometric tests [5]. after finishing all data preparation steps, bayesian inference methods [6] will be used to set confidence intervals for each material parameter that governs critical state models. 2. excalibre as mentioned before, excalibre web application laboratory test database is the main source of data processed in presented research. the application was developed by faculty of civil engineering of ctu in prague in cooperation with the faculty of science of charles university. it runs from 2018 and performs automatic deterministic calibration of the hypoplastic model for sands [7], hypoplastic model for clays [8, 9] and modified cam-clay model [10]. a typical workflow of model calibration using excalibre is the following: (1.) downloading a laboratory protocol template which is prepared in the form of an ms excel file. the template differs for fine-grained and coarsegrained soils. (2.) filling in the results of laboratory tests. a minimum of one isotropically consolidated undrained (ciup) triaxial test with pore pressure measurement for clay soils or one isotropically consolidated drained (cid) triaxial test for sandy soils must be filled in for successful calibration. in addition, one standard oedometric (oed) test is necessary. (3.) uploading the laboratory protocol spreadsheet to the excalibre web application. as a result, the application proposes optimal model parameters and graphs comparing the course of measured and simulated laboratory tests. (4.) manual modification of individual model parameters by user if required. the main advantage of all these critical state models is that they distinguishes between material parameters that are constant for all possible states of a particular soil and state variables that evolve during straining. in this sense critical state models are fundamentally 65 https://doi.org/10.14311/app.2024.49.0065 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en veronika pavelcová, tomáš janda acta polytechnica ctu proceedings parameter data type note uscs string specific gravity float [g cm−3] liquid limit float [%]; only fine-grained soils plastic limit float [%]; only fine-grained soils angle of repose float [°]; only coarse-grained soils soil gradation 2d array size (2, n); sieve size [mm] – passing [%] table 1. data extracted from idx and grad worksheet. parameter data type note initial void ratio float [-] oedometric test data record 2d array dimension (2, n); axial stress [kpa] – axial deformation [-] table 2. data extracted from oed-nat-n or oed-rec-n worksheet. different from the mohr-coulomb model which employs distinct values of its material parameters for the same soil in different density states. 3. protocol structure as mentioned above, laboratory tests uploaded to excalibre application are all in the form of pre-prepared template file in ms excel. this means that all protocols should contain the same type of data and should keep the same structure. although template differs for fine-grained and coarse-grained soils, the majority of data worksheets remains the same. the only difference is the type of prescribed triaxial test. the content of the individual worksheets of the workbook will be described gradually. 3.1. notes worksheet the introductory worksheet of any laboratory protocol is named notes and it contains basic information about sampling: soil type, sampling location, sampling depth, sampling method and possible notes of the laboratory technician. this worksheet is the same for both fine-grained and coarse-grained soils and no data from this worksheet are currently being further processed. 3.2. idx and grad worksheet another type of the laboratory protocol worksheet is named idx and grad. this worksheet contains soil classification, index characteristics and grain size analysis. this worksheet is the same for both finegrained and coarse-grained soils. extracted data to further statistical processing from this worksheet are summarised in table 1. 3.3. oed-nat-n worksheet or oed-rec-n worksheet the laboratory protocol worksheet named oed-nat-n contains a record of an oedometric test of naturally consolidated soil. the laboratory protocol worksheet named oed-rec-n contains a record of an oedometric test of the reconstituted soil sample. the symbol n in the label of the worksheet indicates the order of the oedometric test, in other words, it represents an increasing integer sequence. this worksheet is the same for both fine-grained and coarse-grained soils. extracted data to further statistical processing from this worksheet are summarised in table 2. 3.4. ciup-nat-n worksheet or ciup-rec-n worksheet the first type of triaxial laboratory protocol worksheet is named ciup-nat-n or ciup-rec-n. ciup-nat-n laboratory protocol worksheet contains a record of a consolidated isotropically undrained triaxial test with pore pressure measurement of a naturally consolidated soil. the laboratory protocol worksheet named ciup-rec-n contains a record of a consolidated isotropically undrained triaxial test with the measurement of the pore pressure of the reconstituted soil sample. the symbol n in the label of the worksheet indicates the order of the ciup triaxial test, same as in the case of an oedometric test. this worksheet is used only for fine-grained soils. extracted data to further statistical processing from this worksheet are summarised in table 3. 3.5. cid-rec-n worksheet the other, and last described, type of laboratory protocol worksheet is named cid-rec-n. it contains a record of a consolidated isotropically drained triaxial test of the reconstituted soil sample. the template does not assume the possibility of performing cid triaxial test of naturally consolidated test as soil sampling usually disrupts soil integrity. the symbol n in the label of the worksheet again indicates the order of the triaxial test. this worksheet is used only for coarse-grained soils and extracted data to further statistical processing from this worksheet are summarised in table 4. 66 vol. 49/2024 dealing with error values in database of numerous laboratory tests parameter data type note initial void ratio float [-] initial mean effective stress float [kpa] triaxial test data record 2d array dimension (3, n); axial deformation [-] – mean effective stress [kpa] – axial deviatoric stress [kpa] table 3. data extracted from ciup-nat-n or ciup-rec-n worksheet. parameter data type note initial void ratio float [-] radial effective stress float [kpa] triaxial test data record 2d array dimension (4, n); axial deformation [-] – radial effective stress [kpa] – axial deviatoric stress [kpa] – volumetric deformation [-] table 4. data extracted from cid-nat-n or cid-rec-n worksheet. 4. data validation data validation is a critical step in data preparation for statistical processing as the results of any data analysis are directly dependent on the input values. even though pre-prepared templates for laboratory protocols uploaded to excalibre calibration application help prevent users error inputs, it cannot be assumed that the uploaded data are free of errors. excalibre itself does not implement a data validation at this time. due to this fact and worldwide variability of users it was necessary to validate all data inputs that will be further statistically processed. a set of rules that should catch all types of error data was created. all rules has been consulted with the head of the geotechnical laboratory at the faculty of science of charles university, the lab technician with many years of practical experience. created algorithm works on first-catch principle and distinguishes between two levels of error severity. so called warnings draw attention to values that are physically possible but not probable to occur and therefore require increased attention. on the other hand, errors notify values that should not occur under any circumstances. however, these rules are not strict. they were created based on years of experience not statistical observation. another important note is the fact that the rules described below are created for soils which physical behaviour can be represented using modified cam-clay, hypoplastic clay or hypoplastic sand model for soils. they do not consider, for example, highly organic soils, which are not suitable for excalibre calibration and would not comply with most of the stated rules. all controls are performed cyclically in a predetermined logical order for all uploaded laboratory protocols. then it forms two additional dataframe columns, one with a possible caught warning and the other with a caught error. in the algorithm described, the key of mentioned dataframe corresponds to the name of one laboratory protocol. key values are assigned so that one line of the dictionary represents characteristic data of one laboratory protocol presented in section 3. any automated interventions into protocols are avoided. all necessary data corrections or protocol discarding will be done individually and manually with respect to all laboratory protocol data. 4.1. warning rules overall 19 warning rules were created. individual rules are listed below. the order in which the rules are listed corresponds to the order in which they are implemented in the script. 4.1.1. idx and grad worksheet (1.) specific gravity is out of 2.5–2.9 g cm−3 interval. (2.) liquid limit is greater than 150.0 %. (3.) plastic limit is greater than 60.0 %. (4.) angle of repose is lower than 30.0° or greater than 40.0°. (5.) angle of repose exists but soil consists of more than 10.0 % of fine grains (smaller than 0.063 mm). (6.) number of soil gradation inputs is lower than 5. (7.) the first soil gradation passing is lower than 100 % but greater than 95.0 %. (8.) soil gradation sieve for the first non 100.0 % passing is out of 0.001–63.0 mm interval. 4.1.2. oed-nat-n worksheet or oed-rec-n worksheet (1.) number of oedometric test data inputs is lower than 5. (2.) axial deformation is negative. 4.1.3. ciup-nat-n worksheet or ciup-rec-n worksheet (1.) number of ciup test data inputs is lower than 50. (2.) maximal axial deformation is lower than 1.0 %. 67 veronika pavelcová, tomáš janda acta polytechnica ctu proceedings (3.) initial mean effective stress is out of 20.0– 2 000.0 kpa interval. (4.) initial axial deviatoric stress is greater than 10.0 kpa. 4.1.4. cid-rec-n worksheet (1.) number of cid test data inputs is lower than 50. (2.) maximal axial deformation is lower to 1.0 %. (3.) initial radial effective stress is out of 20.0– 2 000.0 kpa interval. (4.) initial deviatoric stress is greater than 10.0 kpa. (5.) absolute value of maximal volumetric deformation is greater than 0.5 %. 4.2. error rules overall 24 error rules were created. individual rules are listed below. the order in which the rules are listed corresponds to the order in which they are implemented in the script. 4.2.1. idx and grad worksheet (1.) specific gravity is out of 2.0–3.2 g cm−3 interval. (2.) plastic limit is greater than or equal to liquid limit. (3.) liquid limit is greater than 500.0 %. (4.) plastic limit is greater than 80.0 %. (5.) angle of repose is lower than 25.0° or greater than 45.0°. (6.) angle of repose exists but soil consists of more than 20.0 % of fine grains (smaller than 0.063 mm). (7.) number of soil gradation inputs is 0. (8.) the first soil gradation passing is lower than 95.0 %. (9.) soil gradation passing is increasing. (10.) any soil gradation passing is negative. 4.2.2. oed-nat-n worksheet or oed-rec-n worksheet (1.) number of oedometric test data inputs is 0. (2.) initial void ratio is out of 0.2–3.5 interval. (3.) axial stress is out of 1.0–10 000.0 kpa interval. (4.) initial axial deformation is non-zero. 4.2.3. ciup-nat-n worksheet or ciup-rec-n worksheet (1.) number of ciup test data inputs is 0. (2.) initial axial deformation is non-zero. (3.) axial deformation is decreasing (tolerating noise). (4.) axial deviatoric stress is negative. al l u ni qu e co rre ct on ly w ar ni ng on ly e rro r bo th (w + e ) type of protocol 0 50 100 150 200 n u m b e r o f p ro to co ls figure 1. summary of correct and incorrect laboratory protocols. 4.2.4. cid-rec-n worksheet (1.) number of cid test data inputs is 0. (2.) initial axial deformation is non-zero. (3.) axial deformation is decreasing (tolerating noise). (4.) radial effective stress is not constant. (5.) axial deviatoric stress is negative (tolerating noise). (6.) initial volumetric deformation is non-zero. soil index characteristics and granular analysis depend only on the type of soil. therefore, it was possible to determine the limit values for individual parameters. on the other hand, the data record of oedometric, and especially triaxial, tests can vary greatly depending on the tested soil and the chosen test procedure. for this reason, it was usually impossible to determine error limits for the individual parameters that govern these tests. thus, the control rules for these tests were mainly limited to the detection of erroneously entered units of measurement and evident physically impossible results rather than the definition of individual limit values. 5. results out of 214 unique laboratory protocols, only 18 seem to be correct as they passed data validation check without any findings. another 16 protocols have only warnings, no errors and conversely, 44 protocols have only errors and no warnings. finally, there are 136 protocols left that have both warning’s and error’s detection. this observation is clearly summarised in figure 1. however, this figure does not say anything about the amount of errors that each protocol contains. figure 2 shows the total detection of invalid data for each protocol, warnings (figure 2a) and errors (figure 2b) separately. these counts are related to an 68 vol. 49/2024 dealing with error values in database of numerous laboratory tests index of protocol 0 2 4 6 8 n u m b e r o f w a rn in g s (a). warnings. index of protocol 0 2 4 6 8 n u m b e r o f e rr o rs (b). errors. figure 2. number of warnings and errors detected in each laboratory protocol. 0 1 2 3 4 5 6 7 8 number of warnings 0 20 40 60 w a rn in g f re q u e n cy (a). warnings. 0 1 2 3 4 5 6 7 8 9 number of errors 0 20 40 60 e rr o r fr e q u e n cy (b). errors. figure 3. histograms of warnings and errors detected in laboratory protocols. index that is unique to each laboratory protocol and with which the protocol is identified. it can be seen that no more than 8 warnings or 9 errors were found in any of the protocols. majority of protocols reports about 0–2 warnings and 0–4 errors. histograms in figure 3 clearly demonstrates this trends. table 5 and table 6 summarise total number of occurrences of individual warnings and errors across all protocols. it can be observed that the most erroneous data were captured in data characterising sandy soils, namely existence of angle of repose for clay soils (control rule 4.1.1.5 and 4.2.1.6) or invalid record of cid triaxial test (control rules 4.1.4 and 4.2.4). another recurring error are non-zero deformations or unsecured contact of piston and soil sample at the beginning of the deformation tests, most observed during oedometric tests (control rule 4.2.2.3 and 4.2.2.4). it should be mentioned that the absolute counts of invalid data can be distorted by the repetition of individual problems as part of the control rules are interconnected, meaning that if one data is entered incorrectly and multiple rules are created to check its properties, an error will be displayed for multiple rules, although only one data value is missing or incorrect. 6. conclusion the paper introduced the reader to the origin of the researched laboratory protocols, their original use for the calibration of advanced material models in the excalibre web application, and the possibility of their future use in statistical analysis. the importance of checking any input data before statistical processing and the effect of invalid data on the results of statistical analysis was pointed out. the content of the geotechnical laboratory test template for excalibre application was presented by individual sheets of the excel workbook in detail. the paper described the methodology chosen by the authors for data validation and specifically states all control rules applied during the control to individual characteristic of laboratory protocol. finally, the results of the application of the control mechanism to the given data were summarised. the specific purpose of this work was to give an initial estimation of how the real data collected in the laboratory protocols are realistic and physically possible. a manual control by a natural person is the necessary next step in data control. attention will be paid only to individual data detected by the control rules listed in section 4. the correction or removal of 69 veronika pavelcová, tomáš janda acta polytechnica ctu proceedings 4.1.1. idx and grad 4.1.2. oed no. warning 1. 2. 3. 4. 5. 6. 7. 8. 1. 2. frequency 14 1 0 5 56 21 25 4 2 6 4.1.3. ciup 4.1.4. cid no. warning 1. 2. 3. 4. 1. 2. 3. 4. 5. frequency 40 11 4 19 28 35 3 44 38 table 5. frequency of each warning detected in laboratory protocols. 4.1.1. idx and grad 4.1.2. oed no. error 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 1. 2. 3. 4. frequency 0 0 1 0 0 50 18 22 5 0 0 0 77 87 4.1.3. ciup 4.1.4. cid no. error 1. 2. 3. 4. 1. 2. 3. 4. 5. 6. frequency 0 26 2 12 17 43 17 49 13 55 table 6. frequency of each error detected in laboratory protocols. values marked as invalid will always be decided with regard to the overall properties of the tested soil. it is possible that in the course of manual corrections some rules will have to be modified or new rules added. however, the rules presented here are based on years of practical experience in the laboratory and as such they can provide guidance to other researchers dealing with similar issues of validation of standard geotechnical laboratory test records. this paper, together with previously published extracting general knowledge of model parameters for clays out of numerous laboratory tests [1], presents in detail one of the options for dealing with the task of preparation of data originating from an ms excel file for statistical processing using automatic scripts. the further progress of statistical data processing will be presented in the future. acknowledgements the support provided by the sgs project no. sgs23/032/ohk1/1t/11 and the gačr project no. 22-12178s is gratefully acknowledged. references [1] v. pavelcová, t. janda. extracting general knowledge of model parameters for clays out of numerous laboratory tests. in acta polytechnica ctu proceedings, vol. 40, pp. 76–82. 2023. https://doi.org/10.14311/app.2023.40.0076 [2] t. kadlíček, t. janda, m. šejnoha, et al. automated calibration of advanced soil constitutive models. part i: hypoplastic sand. acta geotechnica 17(8):3421–3438, 2022. https://doi.org/10.1007/s11440-021-01441-0 [3] t. kadlíček, t. janda, m. šejnoha, et al. automated calibration of advanced soil constitutive models. part ii: hypoplastic clay and modified cam-clay. acta geotechnica 17(8):3439–3462, 2022. https://doi.org/10.1007/s11440-021-01435-y [4] excalibre. soilmodels automatic calibration. [2023-07-26]. https://soilmodels.com/excalibre-cs/ [5] t. janda, d. mašín. general method for simulating laboratory tests with constitutive models for geomechanics. international journal for numerical and analytical methods in geomechanics 41(2):304–312, 2017. https://doi.org/10.1002/nag.2558 [6] j. kruschke. doing bayesian data analysis: a tutorial with r, jags, and stan. academic press, 2014. isbn 978-0-12-405888-0. [7] p.-a. von wolffersdorff. a hypoplastic relation for granular materials with a predefined limit state surface. mechanics of cohesive-frictional materials 1(3):251–271, 1996. https://doi.org/10.1002/(sici)10991484(199607)1:3<251::aid-cfm13>3.0.co;2-3 [8] d. mašín. a hypoplastic constitutive model for clays. international journal for numerical and analytical methods in geomechanics 29(4):311–336, 2005. https://doi.org/10.1002/nag.416 [9] d. mašín. clay hypoplasticity with explicitly defined asymptotic states. acta geotechnica 8(5):481–496, 2013. https://doi.org/10.1007/s11440-012-0199-y [10] k. h. roscoe, j. b. burland. on the generalized stressstrain behaviour of wet clay. in engineering plasticity, pp. 535–609. cambridge university press, 1968. 70 https://doi.org/10.14311/app.2023.40.0076 https://doi.org/10.1007/s11440-021-01441-0 https://doi.org/10.1007/s11440-021-01435-y https://soilmodels.com/excalibre-cs/ https://doi.org/10.1002/nag.2558 https://doi.org/10.1002/(sici)1099-1484(199607)1:3<251::aid-cfm13>3.0.co;2-3 https://doi.org/10.1002/(sici)1099-1484(199607)1:3<251::aid-cfm13>3.0.co;2-3 https://doi.org/10.1002/nag.416 https://doi.org/10.1007/s11440-012-0199-y acta polytechnica ctu proceedings 49:65–70, 2024 1 introduction 2 excalibre 3 protocol structure 3.1 notes worksheet 3.2 idx and grad worksheet 3.3 oed-nat-n worksheet or oed-rec-n worksheet 3.4 ciup-nat-n worksheet or ciup-rec-n worksheet 3.5 cid-rec-n worksheet 4 data validation 4.1 warning rules 4.1.1 idx and grad worksheet 4.1.2 oed-nat-n worksheet or oed-rec-n worksheet 4.1.3 ciup-nat-n worksheet or ciup-rec-n worksheet 4.1.4 cid-rec-n worksheet 4.2 error rules 4.2.1 idx and grad worksheet 4.2.2 oed-nat-n worksheet or oed-rec-n worksheet 4.2.3 ciup-nat-n worksheet or ciup-rec-n worksheet 4.2.4 cid-rec-n worksheet 5 results 6 conclusion acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2024.48.0015 acta polytechnica ctu proceedings 48:15–21, 2024 © 2024 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague ageing effects on the mechanical properties stability of 3d printed material under compression veronika drechslerová∗, nela krčmářová, jan falta, tomáš fíla czech technical university in prague, faculty of transportation sciences, konviktská 20, 110 00 prague, czech republic ∗ corresponding author: drechver@fd.cvut.cz abstract. the paper deals with the examination of the ageing effects on the mechanical properties stability of 3d printed material via stereolithography under compression when subjected to various conditions, including uv radiation, x-rays, and the effects of time, from the opening of the bottle with the material to the 3d-printing process. the sets of samples under investigation were subjected to quasi-static and dynamic compression loading using an split hopkinson pressure bar. the aim of this paper is to investigate the long-term stability of the samples in terms of their mechanical properties and material behaviour and their degradation pattern. despite the manufacturer’s information, it was found that the mechanical behaviour of the printed samples was significantly affected by the ageing process. keywords: stereolithography, ageing, mechanical properties, compression. 1. introduction the continuous development of additive manufacturing techniques, in particular 3d printing from polymers, for example, using the stereolithography (sla) method, has led to an increase in the popularity and widespread use of these materials. the extensive range of newly available materials has facilitated the design of structural components with specific properties for use in a multitude of industries, including the production of engineering parts, biomedical implants, decorative items, orthodontic appliances, aerospace components, and automotive components, among others [1]. the popularity of sla technology can be attributed to its advantages over other additive technologies. these include faster printing speeds, better resolution, affordable acquisition costs, and a relatively smaller environmental footprint [2]. concurrently, the expansion of applications has led to an increased need to comprehend the behaviour of materials when subjected to diverse external conditions (ageing processes) and under varying loading conditions. the long-term stability and degradation of materials have become a fundamental issue, which is a crucial parameter for the safety of printed parts in practice. the ageing of epoxy resins in the restoration of cultural monuments was investigated in a study [3], where the authors sought to mitigate the effects of photoageing of the resin through the addition of uv stabilisers. the authors of paper [4] examined the impact of uv lamp intensity during printing (known as grayscale printing) on the mechanical properties of biobased epoxy resins, as well as the influence of varying the colour and wavelength of the uv lamp. another study [5] addressed the stability and long-term durability of epoxy-based composites. the authors tested the use of microor nano-sized fillers to strengthen the composites. the aim of this study is to investigate the effect of ageing processes on the mechanical properties of the printed material. in light of the potential for dynamic loading applications, it is imperative that the materials employed undergo testing under dynamic conditions. this study, therefore, focuses on the impact of ageing processes on the mechanical properties of the printed material under compression loading. the dynamic testing of the mechanical properties of the materials in question remains an under-explored area of research. the observed change in the behaviour of the materials under dynamic loading highlights the necessity for this type of testing. additionally, the study examined the potential influence of the interval between the opening of the resin bottle and the printing process. 2. materials and methods 2.1. materials the sla method and a compatible commercially available resin (aqua blue, phrozen, taiwan) were selected for the production of the test samples. the samples were produced on a sonic mighty 4k resin printer (phrozen, taiwan) with a bottom-up architecture and an illumination source in the form of a monochrome lcd display (405 nm paraled matrix 2.0), which ensures high-resolution printing at 52 µm. the printing layer height was set to 0.05 mm, with the exposure time for the bottom layer set at 30 seconds and the other layers set at 2.5 seconds [6]. the selected process and resin were chosen due to their potential for utilising previously measured data from quasi-static tests. 15 https://doi.org/10.14311/app.2024.48.0015 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en v. drechslerová, n. krčmářová, j. falta, t. fíla acta polytechnica ctu proceedings 2.2. sample preparation for compression tests, an axisymmetric specimen was designed in accordance with astm d695-15 [7], with the dimensions of the bulk specimen adapted to align with the technical specifications of the loading apparatus. the bulk specimen with the appropriate dimensions is illustrated in figure 1. in order to achieve the desired level of quality and a smooth printing process, the designed specimens were rotated by the 40° about the x and y axes and placed on supports before printing in the slicing programme [8]. the prepared samples were then printed using the aforementioned resin printer. figure 1. basic dimensions of bulk specimen. subsequent to the printing process, the samples underwent post-processing, which involved cleaning in an ultrasonic bath and uv lamp curing for a period of 40 minutes. figures 2 and 3 depict the printed sample and its surface detail, as observed via scanning electron microscopy (sem) with elemental volume representation according to spectral analysis: carbon (66 ± 2 %) and oxygen (33 ± 2 %). for the sem observations, an accelerating voltage of 10 kv was employed, with a working distance of 10 mm. given that the observed samples were composed of polymer, they were investigated in low vacuum mode with 200× resolution. figure 3 illustrates a detail of each printed layer, with colour highlighting of printing imperfections, including irregularities in the surface of each layer and cured resin particles in an inconvenient location. figure 2. printed bulk specimen. figure 3. detail of the sample surface by sem. 2.3. ageing process a new resin was used to produce the samples to avoid interference due to the time delay between opening the bottle of material and production. the selection of the type of ageing was determined by the availability of laboratory equipment and the potential effects to which the printed parts may be subjected in real-world applications. in consideration of the aforementioned circumstances, the sets of samples were subjected to: • sunlight, • x-rays. a total of 80 samples were subjected to sunlight, with sets of samples tested after one week and less than a month. in the present study, a total of 40 samples were tested, of which 20 were used for evaluation. these samples were divided into four sets of five according to the duration of exposure to sunlight and the type of test. the remaining samples were comprised of erroneous measurement samples and calibration samples. a total of 24 samples were subjected to x-ray radiation for a duration of 2 and 4 hours. subsequently, all the exposed samples were subjected to testing, and 20 representative samples were selected and divided into four sets of five samples each. the employed x-ray tube type was comet mxr 225 hp 11 (comet x-ray, switzerland) was sealed chamber, tungsten target and dual focal spot size option. the x-ray irradiation procedure used acceleration voltage of 50 kv and emission current of 30 ma resulting in nominal output power of 1500 w. the tube has standard tungsten target energy spectrum with a near-conical beam with angle geometry of 30 × 40 degrees. the specimens were placed on a rotating platform with the rotation axis at a distance of 1 m from the focal spot. shielding from x-radiation at this settings and distance was approximately 3.75 · 107 photons mm−2 s−1. the dose rate was approximately (9.25 sv h−1), see bs 4094-2:1971 – recommendation for data on shielding from ionizing radiation. additionally, the impact of the time elapsed between the opening of the resin bottle and the printing process was evaluated, with a time interval of six and 16 vol. 48/2024 ageing effects on the mechanical stability of 3d printed material figure 4. photographs showing the progressive destruction of a specimen during a dynamic compression test. ten months post-opening. in this instance, 12 samples were generated for each time period under examination, and once more, 20 samples were selected from the aforementioned samples, which were then divided into four sets of five samples in accordance with the elapsed time from resin opening to production and the type of test. in order to facilitate a comparison with the measured data and to demonstrate the extent of the effects of ageing, two sets of five samples was produced that were not subjected to any ageing process and tested immediately after the manufacturing process (including post-processing). 2.4. experimental procedure in order to obtain the necessary data to determine the load behaviour and mechanical properties, a total of 14 sets of five specimens in each set (70 bulk specimens) were used. the requisite data for each sample was recorded prior to measurement for subsequent incorporation into the evaluation. two testing methods were employed: • quasi-static compression test, • dynamic compression test (impact test). the quasi-static test was conducted on an instron 3382 universal loading machine, where the specimens were subjected to uniaxial compression. the quasistatic experiments were conducted using loading velocity of 1 mm min−1 resulting in the nominal strain rate of 300 s−1. the test was carried out until a steep force drop was observed, indicating specimen failure. as the experimental campaingn required several months to be completely finished, the experimental setup for dynamic testing could not be kept identical due to other measurements performed in the laboratory. for this reason, two versions of the split hopkinson pressure bar (shpb) were employed: the standard shbp and a modified direct impact hopkinson bar (dihb). in all the experimental setups, the parameters of the experiment were set identical with the average strain rate approximately 1600–1800 s−1, an incident bar of 20 mm diameter and 1600 mm length and a transmission bar of the same diameter and 2100 mm length were used. the striker bars employed were 750 mm in length at 2.0 bar gas-gun pressure and 648 mm in length at 1.8 bar gas-gun pressure [9]. the impact velocities were approximately 15 m s−1. a high-speed camera was installed during the measurements to obtain an image record of the test sample. in all the experiments, a frame rate of the camera was set to 300 kfps. the acquired image recordings during the dynamic test are presented in figure 4. figure 4a illustrates the initial sample location at 0 µs. figure 4b shows the sample at 262 µs from impaction, with a displacement of 1.75 mm. figure 4c illustrates the violation of the observed sample region at 349 µs, with a displacement of 2.88 mm. figure 4d demonstrates the complete deformation of the observed central region of the sample at 396 µs from impaction, with a displacement of 3.25 mm. 2.5. data evaluation the evaluation of the measured data was conducted in matlab (mathworks, inc., usa) in order to obtain the mechanical properties of the material and to generate and compare stress-strain plots. the data obtained from the quasi-static test did not require any pre-processing and could be utilised immediately. dynamic experiments using the shpb apparatus were evaluated using data of the strain-gauges and standard one-dimensional wave propagation theory. the dihb experiments were evaluated similarly to the method described in [10] while only displacement and velocity data were available at the input side. data from all the sensors were time-shifted to the bar’s boundary with the specimen where corresponding force, displacement and velocity quantities were 17 v. drechslerová, n. krčmářová, j. falta, t. fíla acta polytechnica ctu proceedings evaluated. when data from redundant sensors were available they were averaged. wave dispersion effects in the shpb experiment were extensively reduced by a thin soft copper pulse shaper placed at the impact side of the incident bar. overall, very good dynamic equilibrium conditions with almost constant strain rate were achieved in the experiments during as is demonstrated in figure 5. 0 0.5 1 1.5 2 2.5 3 3.5 time [s] 10 -4 -2000 -1000 0 1000 2000 3000 4000 5000 f o rc e [ n ] force specimen input output figure 5. typical dynamic equilibrium of the shpb. as the disintegration of the specimen occured always during the first impact wave in the experiment, the results of the shpb and dihb experiments are comparable. typical strain rate during the experiment plotted against specimen’s displacement is shown in figure 6 and typical dynamic equilibrium of the shpb experiment is presented in figure 5. experiments carried out using the dihb were of single-sided straingauge instrumentation on the output side. the input side was instrumented by the displacement/velocity sensor only. therefore, it was not possible to evaluate force at the input side of some experiments and the equivalent graph for the dihb experiment is not presented here. 0 0.5 1 1.5 2 2.5 3 3.5 4 displacement [mm] 0 200 400 600 800 1000 1200 1400 1600 1800 2000 s tr a in r a te [ 1 /s ] strain rate figure 6. typical strain rate during the experiment. a number of evaluation scripts were employed to calculate the generation of the diagrams and compute basic mechanical properties, including young’s modulus in compression e, compression strength σu, yield stress σy and relative shortening ϵtb. the resulting curves displayed in the stress-strain diagrams represent the mean curves corresponding to each set of specimens. in the comparison diagrams, the curve corresponding to the specimens tested immediately after manufacture is taken as the reference curve and compared with the other average curves of the individual sets of specimens subjected to ageing processes. furthermore, the plots display the standard deviation in transparent coloured areas around each curve, which indicates the variability of the measured data for each test sample in the set. 3. results 3.1. quasi-static compressive loading a comparison of the stress-strain diagrams reveals significant differences between the curves representing samples exposed to sunlight (figure 7a) and open resin (figure 7b) and those not exposed to any ageing effect. the values of yield stress and ultimate strength are notably higher in the former. the average yield stress increased by approximately 76 % for the sets exposed to sunlight and by 37 % for the open resin. the ultimate compression strength decreased by approximately 12 % for the open resin samples in the comparison, while it remained almost identical for the other set (with a difference of 1 %). in the case of x-ray exposure of the samples (figure 7c), the data demonstrate that differences in the engineering stress-strain curves emerge at strain values above 60 %, while at lower strain values the curves are similar. this finding is consistent with the differences in the average compressive strength values, which indicate a reduction in yield stress and ultimate strength of approximately 18 % and 15 %, respectively. 3.2. dynamic compressive loading upon examination of the individual engineering stressstrain diagrams, it becomes evident that the diagrams exhibit greater variability than was observed in the quasi-static tests. in comparison to the average curve of the set of tested specimens immediately after fabrication, the specimens exposed to sunlight for a period of seven days exhibited higher values for both yield stress (by approximately 12 %) and ultimate strength (by 28 %). the specimens that were exposed for 25 days exhibited higher values for yield stress and ultimate strength, with increases of approximately 45 % and 20 %, respectively (figure 8a). the average curves corresponding to the specimens made of open resins (figure 8b) exhibited an increase in the average values of yield stress and ultimate strength of approximately 16 % and 18 %, respectively. additionally, a significant difference in the maximum strain achieved was observed, reaching approximately 40 % for the aforementioned sets. 18 vol. 48/2024 ageing effects on the mechanical stability of 3d printed material (a). (b). (c). figure 7. a comparison of the resulting engineering stress-strain curves for quasi-static compression loading. (a). (b). (c). figure 8. a comparison of the resulting engineering stress-strain curves for dynamic impact loading. in the case of samples that have been exposed to x-rays (figure 8c), a similar behaviour of the samples during the test can be observed as for samples made of open resin, although with a greater difference in the curves depending on the time of exposure to radiation. the yield stress and strength values of the samples tested after an exposure time of two hours were found to be approximately 12 % and 23 % higher, respectively. for the samples tested after an exposure time of four hours, the yield stress and strength values are approximately 21 % and 8 % higher, respectively. 3.3. comparison of quasi-static and dynamic loading the basic mechanical properties were calculated from the measured data, both for quasi-static loading and for dynamic loading, and entered into the table 1 and table 2. concurrently, comparative plots of quasistatics versus dynamics were constructed for each type of aging. the selected plots are presented in figure 9. the achieved values demonstrate that the average yield stress values for quasi-static testing are significantly lower than those for dynamic testing, with a difference of up to 77 % in the framework. with regard to the strengths, it can be stated that in the majority of cases, the relative difference is up to 5 % on average. furthermore, it is evident from table 2 that the calculated mechanical parameters exhibit relatively high standard deviations, which can be attributed to the inherent variability of the individual measured data. 4. conclusions as part of the study, sets of printed samples were subjected to three different ageing effects using stereolithography: sunlight, x-rays, and the effect of the time elapsed between the opening of the resin and its use for production. the long-term stability, degradation, and changes in mechanical properties of the samples were investigated in accordance with the type of ageing. in order to plot stress-strain diagrams and determine the values of the basic mechanical properties, the samples were subjected to destructive loading tests in the form of compression quasi-static and dynamic tests. the results of the study indicated that the long-term stability of the material was affected by the action of ageing processes. the sets of samples exhibited disparate mechanical behaviours under loading, resulting in markedly disparate values for the mechanical properties. for sunlight, it was observed that large changes in mechanical properties occur during the first week of ageing, after which the changes become negligible with increasing exposure time. the study of the effect of opening the resin bottle showed different behaviour for quasi-static and dynamic loading. while for the dynamic compression test, a similar trend could be observed over the course of the test for specimens made from already-opened resin, for the quasi-static test, the data varied depending on the time the resin bottle was opened. for the samples exposed to x-rays, no significant difference was observed as a function of the time the samples were exposed. 19 v. drechslerová, n. krčmářová, j. falta, t. fíla acta polytechnica ctu proceedings mechanical characteristic aging e σy σu ϵtb [mpa] [mpa] [mpa] [%] immediately 1608.50± 20.02 45.60± 1.98 258.63± 11.17 1.55± 0.82 sunlight (7 days) 2678.19± 20.43 80.35± 1.56 257.25± 1.96 6.47± 0.25 sunlight (25 days) 3027.57± 88.02 83.07± 0.58 270.99± 3.46 5.07± 0.48 open resin (6 months) 2095.65 ± 57.62 60.05± 2.79 220.89± 1.92 7.09± 0.37 open resin (10 months) 2257.48± 67.88 63.62± 1.00 235.23± 6.94 6.81± 0.42 x-ray (50 kv,2 hr) 1816.69± 33.38 50.70± 2.77 209.20± 7.08 6.79± 0.64 x-ray (50 kv,4 hr) 1983.25± 32.36 54.87± 1.62 220.28± 6.89 6.80± 0.51 table 1. quasi-static compression test – mechanical characteristic values. mechanical characteristic aging e σy σu [mpa] [mpa] [mpa] immediately 1846.77± 18.29 159.53± 23.77 194.91± 16.81 sunlight (7 days) 1118.87± 71.45 173.49± 11.59 248.53± 0.57 sunlight (25 days) 1175.46± 15.07 218.60± 39.02 232.95± 3.75 open resin (6 months) 1964.58± 140.48 174.48± 16.16 223.11± 1.32 open resin (10 months) 1646.88± 180.23 183.79± 4.99 235.05± 1.34 x-ray (50 kv,2 hr) 1834.27± 69.71 175.63± 5.24 239.26± 12.48 x-ray (50 kv,4 hr) 2384.55± 133.07 187.11± 1.96 209.82± 6.79 table 2. dynamic impact test – mechanical characteristic values. (a). (b). (c). figure 9. a comparison of the resulting engineering stress-strain curves for quasi-static and dynamic loading. acknowledgements there is very appreciated a kind sponsorship of grants from czech science foundation – junior star (project no. 22-18033m). references [1] m. i. farid, w. wu, x. liu, p. wang. additive manufacturing landscape and materials perspective in 4d printing. the international journal of advanced manufacturing technology 115(9–10):2973–2988, 2021. https://doi.org/10.1007/s00170-021-07233-w [2] p. sharma, h. s. mali, a. dixit. mechanical behavior and fracture toughness characterization of high strength fiber reinforced polymer textile composites. iranian polymer journal 30(2):193–233, 2020. https://doi.org/10.1007/s13726-020-00884-8 [3] s. jeong, n. c. cho. a study on aging characteristics of epoxy resins for conservation treatment of cultural heritage by adding uv stabilizer. analytical science and technology 24(5):336–344, 2011. https://doi.org/10.5806/ast.2011.24.5.336 [4] m. bergoglio, e. rossegger, s. schlögl, et al. multi-material 3d printing of biobased epoxy resins. polymers 16(11):1510, 2024. https://doi.org/10.3390/polym16111510 [5] z. wang, x. zhang, c. cheng, et al. 3d printing of architectured epoxy-based composite lattices with exceptional strength and toughness. composites part b: engineering 256:110653, 2023. https: //doi.org/10.1016/j.compositesb.2023.110653 [6] v. drechslerová, m. neuhäuserová, j. falta, et al. stereolithography for manufacturing of advanced porous solids. acta polytechnica ctu proceedings 41:1–7, 2023. https://doi.org/10.14311/app.2023.41.0001 20 https://doi.org/10.1007/s00170-021-07233-w https://doi.org/10.1007/s13726-020-00884-8 https://doi.org/10.5806/ast.2011.24.5.336 https://doi.org/10.3390/polym16111510 https://doi.org/10.1016/j.compositesb.2023.110653 https://doi.org/10.1016/j.compositesb.2023.110653 https://doi.org/10.14311/app.2023.41.0001 vol. 48/2024 ageing effects on the mechanical stability of 3d printed material [7] astm d695-15. standard test method for compressive properties of rigid plastics. https://doi.org/10.1520/d0695-15 [8] v. drechslerová, j. falta, t. fíla, et al. effect of aging on mechanical properties of 3d printed samples using stereolitography. acta polytechnica ctu proceedings 42:1–5, 2023. https://doi.org/10.14311/app.2023.42.0001 [9] m. neuhäuserová, p. koudelka, t. fíla, et al. strain rate-dependent compressive properties of bulk cylindrical 3d-printed samples from 316l stainless steel. materials 15(3):941, 2022. https://doi.org/10.3390/ma15030941 [10] t. fíla, p. koudelka, j. falta, et al. dynamic impact testing of cellular solids and lattice structures: application of two-sided direct impact hopkinson bar. international journal of impact engineering 148:103767, 2021. https://doi.org/10.1016/j.ijimpeng.2020.103767 21 https://doi.org/10.1520/d0695-15 https://doi.org/10.14311/app.2023.42.0001 https://doi.org/10.3390/ma15030941 https://doi.org/10.1016/j.ijimpeng.2020.103767 acta polytechnica ctu proceedings 48:15–21, 2024 1 introduction 2 materials and methods 2.1 materials 2.2 sample preparation 2.3 ageing process 2.4 experimental procedure 2.5 data evaluation 3 results 3.1 quasi-static compressive loading 3.2 dynamic compressive loading 3.3 comparison of quasi-static and dynamic loading 4 conclusions acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2024.51.0029 acta polytechnica ctu proceedings 51:29–35, 2024 © 2024 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague autonomous driving with a focus on hmi for parallel parking maneuvers jan leistner∗, zuzana radová, petr bouchner czech technical university in prague, faculty of transportation sciences, konviktská 20, prague 1, 110 00, czech republic ∗ corresponding author: leistjan@cvut.cz abstract. autonomous vehicles remain a major topic in the automotive industry today, including assistance systems, cooperative communication, and other aspects that lead to full vehicle autonomy. this paper investigates vehicle autonomy with a focus on parking assistance. along with a review of parking assistants, the paper presents a prototype parking assistant with an alternative distance display. the parking assistant prototype, compared to parking without an assistant and conventional parking assistant, was tested in a real-world experiment of a within-subjects study design. the user experience of the proposed distance representation was obtained, and the results indicated that the prototype parking assistant with an alternative distance display helps less experienced drivers get a better idea of the distance from the obstacle. keywords: parking assistance, in-car display, adas. 1. introduction technology is evolving rapidly in the 21st century. the manufacturing, computer, and transport sectors are particularly affected. high-speed rail lines are being built. traffic lights are being automated, and rail safety is being improved. the car industry is also moving forward with the development of autonomous vehicles. an autonomous vehicle can independently assess and respond to traffic scenarios without driver intervention. the society of automotive engineers (sae) has categorized vehicle autonomy into six levels (0–5), with increasing reliance on advanced driver assistance systems (adas) that must be fully reliable and cooperative [1]. the extent to which driver assistance systems are implemented varies depending on the level of autonomy. these consist of driving aids that maintain speed or keep the vehicle within the lane and assistants that aid in parking or ensure safe exit from the vehicle for passengers. parking aids with a numerical distance display are widely available in the aftermarket. therefore, an experiment was designed to test whether the alternative obstacle distance display is a suitable aid for the driver and can be used to park the vehicle closer to the obstacle than using the distance display with a color scale and audio signals. tesla’s numeric obstacle distance display is currently used with both the ultrasonic sensor and vision systems. the vision system replaces the ultrasonic sensors with cameras. the autonomy of vehicles dates back to the early part of the last century [2]. the article says the first autonomous vehicle, controlled by a radio controller, traveled through the streets of new york city in 1925. as mentioned in [3], the subsequent development in this field was the autonomous vehicle built in 1956, guided by electromagnetic circuits embedded into the roadway. however, using it on the streets was too expensive, and the technology was limited to airport roads. an autonomous vehicle that required no modified infrastructure was introduced in 1995 [4, 5]. the test route was from munich to odense in denmark, and the vehicle was in autonomous mode for 95% of the journey. following the introduction of gps navigation systems and digital maps in 2000, there was a significant enhancement in research into vehicle autonomy, with technology giants such as google beginning to develop autonomous vehicles. 2. autonomous vehicles autonomous vehicles utilize sensors and cameras to collect real-time information about their surroundings [6]. the data is then evaluated by an on-board computer, which works alongside map data and interacts with surrounding vehicles and infrastructure. this system enables the vehicle to navigate roads and be conscious of potential obstructions within its immediate environment. the ultimate aim of this technology is to achieve autonomous operation without driver’s intervention. however, attaining this objective demands prolonged effort and extensive research. vehicle autonomy is distinguished into six levels. at present, autonomous vehicle development lies at level 3. google showcased a level 3 autonomous vehicle capable of autonomously driving in specific suburban regions [7]. in the czech republic, cars with level 2 autonomy can be purchased, offering a ’handsoff’ mode in which the vehicle handles steering wheel rotation, acceleration, and braking. this category of autonomy encompasses tesla’s autopilot [8] and cadillac’s super cruise system [9], amongst others. however, it is worth noting that the driver must pay full attention to the road. 29 https://doi.org/10.14311/app.2024.51.0029 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en j. leistner, z. radová, p. bouchner acta polytechnica ctu proceedings figure 1. levels of autonomy [1]. according to sae, the six autonomous driving levels can be divided into two groups (see figure 1). levels 0–2 are classified as driver-support systems, while levels 3–5 are classified as automated driving. 3. parking assistance parking assistance has become a popular topic. as the number of vehicles on the road increases, parking areas reach their limit. drivers often struggle to park their vehicles in designated spaces due to the narrow parking spaces. therefore, vehicle manufacturers are focusing on the development of parking assistance, among other things. since 1933, car manufacturers have enhanced driver convenience in parking [10]. during that year, a car with a special wheel perpendicular to the vehicle’s longitudinal axis was introduced to park effortlessly. another notable innovation was volkswagen’s futura model [11], which enabled all four wheels to pivot at a right angle, resulting in comfortable parking. in 2003, toyota introduced the prius equipped with the intelligent parking assist system [12], shown in figure 2. the car featured sonar sensors and cameras on both of its bumpers. when drivers decided to park the vehicle, they selected the appropriate parking space on the radio display. once a space was selected, the car’s system took control of the vehicle and parked it in the chosen space. initially, there were issues concerning the recognition of pedestrians and figure 2. intelligent parking assist in prius [13]. particular objects such as a pram. toyota was the only manufacturer to offer this system for three years until other manufacturers, including bmw and ford, adopted the system in 2006. 4. ultrasonic sensors the prius’s sonar sensors were replaced by ultrasonic sensors. this newer approach offers more sophistication regarding its integration into the vehicle’s structure and ease of operation. ultrasound is a mechanical wave with a frequency greater than 20 khz [14]. the speed of ultrasound propagation relies on the density of the medium through which the waves travel. the 30 vol. 51/2024 autonomous driving with a focus on hmi for parallel parking maneuvers velocity of ultrasound is the greatest in solids, reaching up to 3 300m s−1. conversely, ultrasonic waves spread the slowest in gases at 345.7m s−1. the propagation speed in liquids approximates 1482m s−1 [15]. the speed of ultrasonic waves is contingent upon the density and bulk modulus of the medium they traverse. this relationship is well established and expressed by equation (1). v = √ k ρ , (1) where k is the bulk modulus and ρ is the density of the medium. ultrasonic sensors work based on calculating the speed at which the signal bounces off an obstacle [16]. the sensor consists of a transmitter and a receiver. the distance is determined by the transmitter sending an audio signal and triggering a time measurement. the transmitted signal propagates through the environment until it encounters an obstacle from which it bounces. the reflected signal then travels back to the sensor, which listens for the “echo”. after the transmitted signal returns to the sensor, a distance calculation is performed. the sound propagates through the air at approximately 340m s−1, which means that a one-centimeter wave travels in 2.94 × 10−5 seconds. equation (2) shows the formula for calculating the distance. d = t · c 2 , (2) where d is the calculated distance, t is the time taken for the ultrasound to return to the sensor, and c is the speed of sound propagation. the variable c depends on the environment in which the sound propagates. the receiver is equipped with a piezoceramic sensor that can change its geometric shape based on the electrical signal, so it can be said that the piezoceramic converts mechanical energy into electrical energy [17]. 4.1. usage of ultrasonic sensors in parking assistance 4.1.1. bosch automated parking system sensors based on ultrasonic waves work in conjunction with a computer that analyses the proximity of objects in the vicinity of the vehicle. bosch states [18] that sensors are mounted on the sides of the vehicle to scan and map possible parking spaces. once the assistant has confirmed that the space it has scanned is suitable for the car to park in, it sends a signal to the driver. upon pressing a button, the assistant calculates the optimal route to the parking space, determining the required steering wheel turns and necessary maneuvers. once the path is determined, the driver can relinquish control and guide the vehicle with gentle pressure on the accelerator and brake pedals. the assistant uses the electronic steering wheel control to guide the vehicle to the parking spot. figure 3. parking scenario’s scheme. the driver remains in full control of the maneuvers and may cancel the operation anytime by touching the steering wheel. in addition, the assistant supervises the exit from the parking space and brings the vehicle to a safe position for the driver to proceed to the traffic. 4.1.2. valeo automatic parking valeo introduced the function park4u in 2007 [19], which was implemented in the volkswagen touran. the parking maneuver is similar to the bosch system in the hands-free mode. valeo states that once ultrasonic sensors have located a fitting parking space, the driver only needs to shift into the reverse gear, and the system takes control of the vehicle. the driver can interrupt the system’s operation by touching the steering wheel. additionally, in 2011, valeo launched the park4u remote mobile application, which allows drivers to leave their vehicles and park them using their phones. this distinguishes it from the bosch parking assistant as it eliminates the need for the driver to remain inside the car to operate the brake and accelerator pedals. 5. experiment an experiment was designed to test the user experience using different hmi elements in parking assistants. parking was done on a remote road with no surrounding traffic. the experiment aimed to determine if and which parking assistance could better help drivers while parking. a suzuki ignis was used for parking without a parking aid and with the prototype, and a skoda yeti was used for parking with a conventional parking assistant. 5.1. parking space for the purpose of the experiment, a parking space was created. the parking space was designed to conform to the dimensions specified in standard csn 73 6056 [20]. the length of the parking space was 5 750mm, and the depth was 2 000mm, as illustrated in figure 3, and figure 4 shows the testing. 5.2. first parking parking without parking assistance. drivers were asked to park the vehicle using only the rear-view mirrors. 31 j. leistner, z. radová, p. bouchner acta polytechnica ctu proceedings figure 4. parking scenario in real testing. figure 5. hmi in skoda yeti vehicle. 5.3. second parking the second method was parking the vehicle with the help of a parking assistant fitted to the car by the manufacturer. a 4-channel ultrasonic sensor system with a display of the distance to the obstacle on the radio display, paired with acoustic signals, was used. human-machine interface, shown in figure 5, informs the driver about the distance from the obstacle. 5.4. third parking parking using the prototype parking assistant. the parking assistant, equipped with an ultrasonic sensor, responds to approaching an obstacle with an acoustic signal and displays the obstacle distance using colored le diodes and an oled display that shows the distance in centimeters. 5.5. implementation of prototype design before building the prototype parking assistant, a simulation of the circuit, shown in figure 6, was performed in a virtual environment. the parking assistant consists of an arduino microcomputer, acting as a control element. arduino processes data from the ultrasonic sensor and calculates a distance from the obstacle. based on the measured distance, particular leds are powered up. the connection of the le diodes requires a combination with a corresponding resistor. the value of the required resistance for each figure 6. virtual simulation of the prototype circuit. diode was determined using ohm’s law expressed by equation (3). the microcomputer supplies the diode with a voltage of up to 5v, and the current required for proper diode operation is 25ma. r = u i = 5 0.025 = 200 ω (3) the circuit also includes a piezoelectric buzzer fitted with a 100 ω resistor to reduce the volume. the leds were wired to connect a selected 200 ω resistor to the cathode. the positive pole of the led is connected to the digital input of the arduino uno board. all eight leds were connected to pins 6 to 12, leaving the other pins available for other components. the piezoelectric buzzer was connected with a 100 ω resistor on the positive pole to the analog input a0 and the negative pole to the gnd pin on the breadboard. the trig and echo pins of the ultrasonic sensor were connected to digital pins 2 and 3, respectively. the positive and negative poles of the component were connected to the breadboard, which was connected to the arduino. in the next stage, an oled display was added to the circuit to show the distance in centimeters measured by the ultrasonic sensor. the oled display has four pins, which were connected as follows: the vdd pin was connected to the positive pole of the arduino, the gnd pin to the negative pole, the scl pin to the analog pin a5, and the sda pin to the analog pin a4 on the arduino board as shown in virtual simulation in figure 6. the source code was created to ensure the correct functionality of all involved components. the distances at which the le diodes light up and the buzzer sounds were defined in the code. the measured distance is also displayed in centimeter (cm) units on the oled display. the first detection distance was set to 150 cm based on data collected from vehicles that had the parking assistant fitted by the manufacturer. the smallest permissible distance, so-called full tone, was set to 30 cm. all the leds light up at the full tone 32 vol. 51/2024 autonomous driving with a focus on hmi for parallel parking maneuvers distance, and the buzzer provides an uninterrupted tone. 6. results in the experiment, the participants were asked to park the vehicle in all three ways. the time needed to park the car and the final distance from the obstacle were measured. the experiment was performed with 11 participants of different age groups and driving experience. participants were also asked whether their vehicle had been equipped with parking assistance of any kind, see table 1. id age km/year parking assistance 1 20 5000 yes 2 20 1500 no 3 20 2500 yes 4 21 600 no 5 20 3500 no 6 19 1500 yes 7 50 9000 no 8 52 9000 no 9 47 38500 yes 10 50 16500 yes 11 53 21000 no table 1. data table. id time distance 1 00:20,1 146 2 00:48,0 115 3 00:43,2 119 4 02:21,2 107 5 00:36,2 150 6 00:51,8 129 7 00:15,0 129 8 00:28,5 141 9 00:48,0 103.5 10 00:40,0 89 11 01:20,0 170 mean values 00:47,2 127.14 table 2. data without parking aid. the results of parking without a parking assistant are shown in table 2, and the results of parking with the conventional parking assistant are demonstrated in table 3, indicating that the mean distance decreased by 33.23 centimeters compared to parking without a parking aid. however, the mean time needed for parking increased by 8.9 seconds. the data collected shows that the mean value of the final distance from the obstacle gradually decreased with the use of parking assistance. in the third part of the experiment, the mean distance decreased even more, as seen in table 4. the mean time needed for parking changed marginally compared to parking id time distance 1 00:19,9 77 2 01:24,7 110 3 00:40,8 76 4 01:18,9 68 5 01:16,4 106 6 01:45,5 113 7 00:18,0 110 8 00:31,6 134 9 01:00,1 54 10 00:40,7 89 11 01:00,1 96 mean values 00:56,1 93.91 table 3. data with conventional parking assistant. id time distance 1 00:22,1 22 2 00:35,8 51 3 00:44,9 91 4 01:21,8 150 5 00:46,7 76 6 01:36,0 145 7 00:14,7 116 8 01:02,0 121 9 00:54,7 57 10 00:24,3 93 11 00:36,8 67 mean values 00:47,3 89.91 table 4. data with prototype parking assistant. with a vehicle without a parking assistant. still, compared to the mean time of parking with a conventional parking assistant, the mean time for parking with the prototype decreased by 8.8 seconds. using the prototype, the participants parked an average of 37.23 centimeters closer to the obstacle than using a car without parking assistance. this might have occurred because the drivers were comfortable with the numerical display of the distance. driver id 6, despite having a parking aid in his car, was able to park faster using only the rear-view mirrors. the reason for this may be that the parking aid can also be stressful for some drivers and can affect the driver’s concentration when parking. however, the mean values of the final distance show that the drivers were comfortable with the alternative distance display, resulting in a close approach. they were also able to park the vehicle more quickly than when using conventional parking assistance. figures 7 and 8 show the measured data during the experiment. in figure 7, a greater parking distance from the obstacle can be observed for most participants when using rear-view mirrors. the distance was measured from the rear bumper of the parked vehicle to the nearest part of the obstacle to which the vehicle 33 j. leistner, z. radová, p. bouchner acta polytechnica ctu proceedings figure 7. comparison of distances from the obstacle. figure 8. comparison of the parking times. was parked (a third-party parked vehicle). figure 8 shows the time it took to park the car. the time was measured from the start of the parking maneuver to its completion when participants applied the parking brake. for participants id 5 and 6, a notable increase in parking time can be observed, which can be explained by trying to park the vehicle as close as possible to the obstacle using the parking assistant. therefore, the measured data show that both conventional and prototype parking assistance improve the driver’s awareness of the proximity of the obstacle but increase the time required to park the vehicle. the parking aid prototype with an alternative distance display is better used on roads with less traffic. 7. conclusion this paper aimed to present the prototype parking assistant and verify whether the alternative distance display could be a suitable aid for drivers. it was explored that the assistant could help park the vehicle closer to the obstacle than when using the conventional parking assistant. the experimental results showed that the parking assistant with the alternative distance display gave drivers a better idea of the distance from the obstacle. therefore, the data mean of the final distance to the obstacle was smaller than that of the other parking aids. the mean parking time was also the shortest when using the alternative distance display. for more experienced drivers, the numerical representation of distance from obstacles made a less significant difference than for those with less experience. the decreased time spent using the parking aids is also more evident for the less experienced and younger participants. the authors believe that having a larger sample of participants would provide a more comprehensive analysis. this should be addressed in the forthcoming experiments. furthermore, future research on parking assistants might also focus on analyzing drivers’ subjective impressions of the user environment, semiautomatic and automatic parking assistants, and the strategy the system uses to park the vehicle in the parking spot. references [1] levels of driving automation, 2021. [2021-08-04]. https://www.sae.org/binaries/content/gallery/ cm/content/news/sae-blog/j3016graphic_2021.png [2] history of the autonomous car, 2012-2021. [2021-0805]. https://www.titlemax.com/resources/historyof-the-autonomous-car/ [3] science: radio auto. [2021-08-06]. http://content.time.com/time/subscriber/ article/0,33009,720720,00.html [4] j. janai, f. güney, a. behl, a. geiger. computer vision for autonomous vehicles. foundations and trends, boston, 1st edn., 2020. https://doi.org/10.1561/0600000079 [5] m. maurer, r. behringer, s. furst, et al. a compact vision system for road vehicle guidance. in proceedings of 13th international conference on pattern recognition, vol. 3, pp. 313–317. ieee, 1996. https://doi.org/10.1109/icpr.1996.546962 [6] the 6 levels of vehicle autonomy explained. [2021-08-05]. https://www.synopsys.com/automotive/autonomousdriving-levels.html [7] waymo. waymo driver, 2023. [2024-03-15]. https://waymo.com/waymo-driver/ [8] tesla. tesla autopilot, 2024. [2024-03-15]. https://www.tesla.com/cs_cz/autopilot [9] cadillac. cadillac’s super cruise, 2021. [2024-03-15]. https://www.cadillac.com/technology/supercruise [10] the ingenious “fifth wheel” parallel parking tool that never hit it big, 2018. [2024-03-15]. https://www.hagerty.com/media/automotivehistory/parallel-parking-made-easy-1930sstyle/ [11] volkswagen futura concept 1992, 2019. [2021-08-06]. https://www.autorevue.cz/getthumbnail.aspx?w= 1600&h=1000&q=60&id_file=266246351 [12] toyota unveils car that parks itself, 2003. [2023-10-27]. http://edition.cnn.com [13] intelligent parking assist system, 2001. [2021-08-05]. https://en.wikipedia.org/wiki/intelligent_ parking_assist_system 34 https://www.sae.org/binaries/content/gallery/cm/content/news/sae-blog/j3016graphic_2021.png https://www.sae.org/binaries/content/gallery/cm/content/news/sae-blog/j3016graphic_2021.png https://www.titlemax.com/resources/history-of-the-autonomous-car/ https://www.titlemax.com/resources/history-of-the-autonomous-car/ http://content.time.com/time/subscriber/article/0,33009,720720,00.html http://content.time.com/time/subscriber/article/0,33009,720720,00.html https://doi.org/10.1561/0600000079 https://doi.org/10.1109/icpr.1996.546962 https://www.synopsys.com/automotive/autonomous-driving-levels.html https://www.synopsys.com/automotive/autonomous-driving-levels.html https://waymo.com/waymo-driver/ https://www.tesla.com/cs_cz/autopilot https://www.cadillac.com/technology/super-cruise https://www.cadillac.com/technology/super-cruise https://www.hagerty.com/media/automotive-history/parallel-parking-made-easy-1930s-style/ https://www.hagerty.com/media/automotive-history/parallel-parking-made-easy-1930s-style/ https://www.hagerty.com/media/automotive-history/parallel-parking-made-easy-1930s-style/ https://www.autorevue.cz/getthumbnail.aspx?w=1600&h=1000&q=60&id_file=266246351 https://www.autorevue.cz/getthumbnail.aspx?w=1600&h=1000&q=60&id_file=266246351 http://edition.cnn.com https://en.wikipedia.org/wiki/intelligent_parking_assist_system https://en.wikipedia.org/wiki/intelligent_parking_assist_system vol. 51/2024 autonomous driving with a focus on hmi for parallel parking maneuvers [14] i. g. main. kmity a vlny ve fyzice [in czech; oscillations and waves in physics]. academia, praha, 1st edn., 1990. [15] ultrazvukové vlnění [in czech; ultrasound waves], 2020. [2024-03-16]. https: //www.wikiskripta.eu/w/ultrazvukové_vlnění [16] z. malá, d. nováková, t. vítů. fyzika i [in czech; physics i]. české vysoké učení technické v praze, 3rd edn., 2009. [17] ultrazvukové senzory přiblížení – funkce, provedení, použití [in czech; ultrasonic proximity sensors], 2017. [2021-08-10]. https: //automatizace.hw.cz/ultrazvukove-senzorypriblizeni-funkce-provedeni-pouziti.html [18] park assist. [2024-03-14]. https://www.bosch-mobility-solutions.com/en/ solutions/parking/parking-assist/ [19] valeo park4u® an automated parking system to park easily, 2021. [2021-08-05]. https: //www.valeo.com/en/park4u-automated-parking/ [20] čsn 73 6056 odstavné a parkovací plochy silničních vozidel [in czech; parking areas for road vehicles], 2011. úřad pro technickou normalizaci, metrologii a státní zkušebnictví. 35 https://www.wikiskripta.eu/w/ultrazvukov�_vln�n� https://www.wikiskripta.eu/w/ultrazvukov�_vln�n� https://automatizace.hw.cz/ultrazvukove-senzory-priblizeni-funkce-provedeni-pouziti.html https://automatizace.hw.cz/ultrazvukove-senzory-priblizeni-funkce-provedeni-pouziti.html https://automatizace.hw.cz/ultrazvukove-senzory-priblizeni-funkce-provedeni-pouziti.html https://www.bosch-mobility-solutions.com/en/solutions/parking/parking-assist/ https://www.bosch-mobility-solutions.com/en/solutions/parking/parking-assist/ https://www.valeo.com/en/park4u-automated-parking/ https://www.valeo.com/en/park4u-automated-parking/ acta polytechnica ctu proceedings 51:29–35, 2024 1 introduction 2 autonomous vehicles 3 parking assistance 4 ultrasonic sensors 4.1 usage of ultrasonic sensors in parking assistance 4.1.1 bosch automated parking system 4.1.2 valeo automatic parking 5 experiment 5.1 parking space 5.2 first parking 5.3 second parking 5.4 third parking 5.5 implementation of prototype design 6 results 7 conclusion references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2024.51.0125 acta polytechnica ctu proceedings 51:125–128, 2024 © 2024 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague optimizing timing for emergency vehicle approaching warnings kajsa weibulla,b,∗, björn lidestama, erik prytzb a the swedish national road and transport research institute, se-581 95 linköping, sweden b linköping university, department of computer and information science, se-581 83 linköping, sweden ∗ corresponding author: kajsa.weibull@vti.se abstract. emergency driving is a demanding task. the its service emergency vehicle approaching provides drivers with an in-car warning that an emergency vehicle is approaching. to ensure that the warning is efficient and reliable it must have a suitable timing. the eva warning must be presented early enough to allow the driver to make a safe move-over maneuver. however, the distribution of eva warnings is based on the most probable path of the emergency vehicle. if an eva warning is distributed too early, it increases the risk of false alarms. in addition, if a warning is given too early the driver might deem it as irrelevant. previous studies have distributed eva warnings based on the distance between the emergency vehicle and surrounding cars. however, if the speed of the vehicle is not accounted for there is a risk of insufficient time for the driver to prepare for the emergency vehicle interaction. to explore suitable eva timing, post-survey data from five driving simulator studies where drivers were exposed to eva warnings were analyzed. the results suggest that 15–20 seconds could be an appropriate eva timing. however, the required time for a move-over maneuver depends on the complexity of the current traffic situation. keywords: emergency vehicle, intelligent transport systems, emergency vehicle approaching, warning. 1. introduction the interaction between emergency vehicle operators and civilian drivers can result in accidents [1]. driving an emergency vehicle often entails high-risk factors such as speeding, multitasking, and interacting with other road users [2]. civilian drivers who encounter an emergency vehicle in traffic can find the situation stressful. due to the soundproofness of new cars and the limitations of emergency lights and sirens, the civilian driver might not have enough time to move over for the approaching emergency vehicle. one way to mitigate the risks associated with emergency vehicle-related accidents is to provide surrounding vehicles with an emergency vehicle approaching (eva)warning [3]. an eva warning is an in-car warning letting the driver know that they soon must prepare for an oncoming emergency vehicle. several previous studies have examined the effects of eva warnings on drivers’ behavior. however, no study has examined what the optimal timing of an eva warning would be. instead, different occasions to distribute the warning, either time or distance-based have been described in previous research. in lenné et al. [4], the civilian driver would receive the warning when they were inside a 300–400 meters radius of the emergency vehicle. in savolainen [5], drivers were notified when the emergency vehicle was 2 000 feet (approx. 610 m) from an intersection that the civilian driver was heading towards. in lidestam et al. [6], eva timings of both 14 and 50 seconds were used. in payre and diels [7], the drivers received the eva warning 30 seconds before the emergency vehicle overtook them. a warning should be given at such a time that the driver has time to prepare for the oncoming situation. however, presenting it too early could also have consequences. firstly, if a warning is presented but too long time passes without a sign of danger, they are probably going to believe that the warning was false and therefore disregard it. secondly, if a warning is presented too early there is a risk that the prognosis the warning was based on will change. for instance, the emergency vehicle decided on a new route. thereby, no longer affecting the drivers who received the warning. it is therefore important to find a timing of eva warning that provides the driver with enough time to move over, but at the same time find a timing that minimizes the risk of either real or perceived false alarms. in the present paper, the optimal time for eva warnings is examined by analyzing survey data from five different driver simulator studies. 2. method short descriptions of the driving simulator studies included are presented below. 2.1. study 1 – highway experiment this study included 110 drivers aged 18–61 years, m = 38.2, sd = 15.2 years. the participants were 125 https://doi.org/10.14311/app.2024.51.0125 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en k. weibull, b. lidestam, e. prytz acta polytechnica ctu proceedings driving on a highway for 30 minutes and received eva warnings three times (11,19 and 27 minutes into the simulation). the drivers received a randomized combination of false and true alarms. in the events with true eva warnings, the drivers were approached by an emergency vehicle 15 seconds after receiving the alert. only participants who received true eva warnings (n = 32) are included in the current study. 2.2. study 2 – highway experiment with eye tracking in the eye-tracking experiment, the same scenario was used as in the highway experiment, but with the warning presented 20 seconds before the emergency vehicle overtook the participant’s vehicle. two interactions with the emergency vehicle occurred, 9 and 15 minutes into the scenario. in study 2, 73 drivers participated with an age range of 19–54 years (m = 38.1, sd = 10.1). only participants from the experimental group (n = 35) are included in the current study, as the control group did not receive any in-car warnings. 2.3. study 3 – offramp the study included 30 drivers, aged 19–82 years, m = 51.5, sd = 14.6. in the scenario, the drivers were instructed to follow the signs on a highway to the swedish city of trosa. however, when approaching the offramp that would take them to trosa, half of the participants received an in-car warning that there had been an accident on the offramp and that they therefore should continue straight ahead. the warning was presented 15 seconds before they would have entered the offramp. only participants from the experimental group (n = 15) are included in the current study, as the control group did not receive any in-car warnings. 2.4. study 4 – crossing scenario in this study, 34 drivers, aged 19–78 years, m = 49.7, sd = 17.4 were included. in the scenario, they were driving in a city environment. after a few minutes, the participant would approach an intersection. half of the participants would receive an in-car warning instructing them to stop before the intersection due to an approaching emergency vehicle. the warning was presented 12 seconds before the driver entered the intersection. only participants from the experimental group (n = 17) are included in the current study, as the control group did not receive any in-car warnings. 2.5. study 5 – augmented emergency lighting study 5 was a replication of lidestam et al. [6] but with experienced instead of inexperienced drivers. in addition, half of the participants were given an ambient in-car light 15 seconds before the eva warning was given. after the eva warning was presented, it took 14 seconds before the emergency vehicle would overtake the participant’s vehicle. in the study, 62 drivers participated, with an age range of 29–80 years (m = 58.3, sd = 12.9). however, in the present paper, only the drivers who received an eva warning and no ambient light or control group are included (n = 20). 3. results in studies 1-4 the participants were asked the same questions about their experience of the in-car warnings in the post-survey. all answers were indicated on a 1–7-degree likert scale (1 = fully disagree, 7 = fully agree). for the purpose of this study, the participants were grouped depending on the warning timing of the study they participated in. the participants in study 4 were placed in group 12 sec. the participants from studies 1 and 3 formed group 15 sec, and participants from study 2 were put in group 20 sec. the potential difference between the groups was examined through between groups anovas followed by a tukey hsd post hoc analysis. the descriptive result of the questionnaire is presented in figure 1. figure 1. post survey questions regarding experience of the eva warning system for 12,15 and 20 sec warning timing (likert scale on y-axis, 1 = strongly disagree, 7 = strongly agree). the participants were asked to what extent they agreed with the statement “the warning improved my driving behavior”. the result indicated a significant difference between the groups, f (2, 80) = 7.59, p < .001, η2 p = .16. the tukey hsd post hoc analysis indicated that drivers who received the eva warning 20 sec before interacting with the emergency vehicle to a larger extent believed that the warning improved their driving compared to the drivers who received the warning 12 (p = .01) or 15 sec (p = .002) before the emergency vehicle interaction. when asked to what extent “the warning was useful”, the anova test suggested a significant difference between the groups f (2, 80) = 3.30, p = 0.042, η2 p = .08. the post hoc analysis suggested that the 20 sec group agreed with the statement to a larger extent compared to the 12 sec group (p = .033). there was no significant difference when comparing the 15 sec group with the 12 (p = .145) and 20 sec (p = .634) groups. there were no indications of significant difference between the groups in terms of how much stress they 126 vol. 51/2024 optimizing timing for emergency vehicle approaching warnings experienced when receiving the warnings (“the warning made me stressed”), f (2, 80) = 1.42, p = .25. when asked to indicate to what degree the agreed with the statement “i would like to receive these warning in my own car in the future”, the anova did not indicate a significant difference between the groups, f (2, 80) = 0.48, p = .62. finally, the participants were asked to what degree they believed “i felt that i could trust the warning”. the anova suggested a significant difference between the groups, f (2, 80) = 8.08, p < .001, η2 p = .17. the post hoc analysis indicated that the 12 sec group experienced a lower degree of trust compared to the 15 (p = .002) and 20 sec (p < .001) groups. however, there was no significant difference between the 15 and 20 sec groups (p = .835). in study 5, the participants were asked about the timing of the eva warning (figure 2). a majority of participants (60 %) believed that the warning had sufficient timing. about a third of participants (35 %) would have liked the warning to come earlier, and five percent would have liked a later warning. figure 2. evaluation of timing suitability (study 5). 4. discussion the results of the questionnaires from study 1-4 indicate that drivers are positive towards eva warnings. independent of the warning timing, the drivers indicated that they would like to have eva warnings in their own vehicles. the indicated level of stress when faced with the warnings was relatively low. however, in some respects, the participants seem to prefer a longer eva timing. this preference was indicated for how much the eva warning improved the participants’ driving behavior and the level of trust associated with the eva warning. furthermore, drivers in the 20 sec group found the eva warning significantly more useful compared to the 12 sec group. the results regarding timing from study 5 suggested that most participants were satisfied with an eva warning presented 14 seconds before the emergency vehicle interaction. however, a third of participants believed that the warning should have been presented earlier. this further supports that a suitable timing of an eva warning could be around 15–20 seconds. the results of the present study suggest that the timing of eva warnings is important. previous studies examining the effectiveness of eva have used a distance-based warning trigger [3, 5]. the distance trigger in lenné et al. [3] would for instance result in 36–48 seconds to plan and perform a safe move-over maneuver when driving 30 kph and only 9–12 seconds when driving 120 kph. to ensure an eva warning that is distributed in time, speed is a factor that must be considered. one limitation of the present paper is the differences between the simulation scenarios used in the different studies. however, the results could still give a first indication of what a suitable eva warning timing could be. the interaction between emergency vehicle operators and civilian drivers is risky, but an eva warning with sufficient timing could mitigate the accident risk. 5. conclusion eva warning can support drivers in interactions with emergency vehicles. drivers are positive about the idea of having eva warnings in their own vehicles. however, the timing of the eva warning could affect the perceived benefit of the system. eva warnings should be distributed with regards to the time it will take for the emergency vehicle to reach the civilian vehicle, rather than based on the distance between the emergency and civilian vehicle. acknowledgements the authors want to thank the students who have helped with the studies included in this paper. namely, tereza kunclová, philip lindblad, emilia sörengård, and petr ondomiši. this work was supported by the swedish transport administration (trv 2020/25755) and nordic way 3 (2018-eu-tm-0026-s). references [1] c. b. custalow, c. s. gravitz. emergency medical vehicle collisions and potential for preventive intervention. prehospital emergency care 8(2):175–184, 2004. https://doi.org/10.1016/s1090-3127(03)00279-x [2] h. hsiao, j. chang, p. simeonov. preventing emergency vehicle crashes: status and challenges of human factors issues. human factors 60(7):1048–1072, 2018. https://doi.org/10.1177/0018720818786132 [3] k. weibull, b. lidestam, e. prytz. potential of cooperative intelligent transport system services to mitigate risk factors associated with emergency vehicle accidents. transportation research record 2677(3):999–1015, 2023. https://doi.org/10.1177/03611981221119459 127 https://doi.org/10.1016/s1090-3127(03)00279-x https://doi.org/10.1177/0018720818786132 https://doi.org/10.1177/03611981221119459 k. weibull, b. lidestam, e. prytz acta polytechnica ctu proceedings [4] m. g. lenné, t. j. triggs, c. m. mulvihill, et al. detection of emergency vehicles: driver responses to advance warning in a driving simulator. human factors 50(1):135–144, 2008. https://doi.org/10.1518/001872008x250557 [5] p. t. savolainen, t. k. datta, i. ghosh, t. j. gates. effects of dynamically activated emergency vehicle warning sign on driver behavior at urban intersections. transportation research record 2149(1):77–83, 2010. https://doi.org/10.3141/2149-09 [6] b. lidestam, b. thorslund, h. selander, et al. in-car warnings of emergency vehicles approaching: effects on car drivers’ propensity to give way. frontiers in sustainable cities 2, 2020. https://doi.org/10.3389/frsc.2020.00019 [7] w. payre, c. diels. i want to brake free: the effect of connected vehicle features on driver behaviour, usability and acceptance. applied ergonomics 82:102932, 2020. https://doi.org/10.1016/j.apergo.2019.102932 128 https://doi.org/10.1518/001872008x250557 https://doi.org/10.3141/2149-09 https://doi.org/10.3389/frsc.2020.00019 https://doi.org/10.1016/j.apergo.2019.102932 acta polytechnica ctu proceedings 51:125–128, 2024 1 introduction 2 method 2.1 study 1 – highway experiment 2.2 study 2 – highway experiment with eye tracking 2.3 study 3 – offramp 2.4 study 4 – crossing scenario 2.5 study 5 – augmented emergency lighting 3 results 4 discussion 5 conclusion acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2025.54.0029 acta polytechnica ctu proceedings 54:29–33, 2025 © 2025 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague experimental analysis of physical properties of lightweight gypsum plasterboard martin hataja,∗, jan jochmana, jan poštaa, mária domonkosb, ondřej zobalc, miroslav nyčd a czech technical university in prague, university centre for energy efficient buildings, materials and construction of buildings research department, třinecká 1024, 273 43 buštěhrad, czech republic b czech technical university in prague, faculty of civil engineering, department of physics, thákurova 7, 166 29 prague 6, czech republic c czech technical university in prague, faculty of civil engineering, department of mechanics, thákurova 7, 166 29 prague 6, czech republic d knauf praha, spol. s r. o., mladoboleslavská 949, 197 00 prague 9, czech republic ∗ corresponding author: martin.hataj@cvut.cz abstract. this article presents an experimental analysis of a newly developed gypsum plasterboard for dry construction. the new board has a density that is 40 kg m−3 lower than existing boards and was created with an emphasis on conserving resources and reducing the energy consumption required for its production. because gypsum is the most energy-intensive raw material, the research team focused on minimizing its content. as a result, the new board uses 12 % of mass less gypsum, which translates into an approximately 6 % reduction in co2 emissions from the annual production process. crucially, this was achieved while meeting the relevant standard requirements for mechanical, thermal-technical, and fire performance, and maintaining the board’s “sustainable” user properties. the article details the experimental analysis conducted on key mechanical and thermal-technical parameters of the board. keywords: gypsum plasterboard, lightweight, mechanical properties, experiments. 1. introduction dry construction in the czech republic has seen significant growth since the turn of the 20th and 21st centuries, when this technology underwent substantial refinement at the national level. today, it is used not only for temporary structures but also for renovations and new buildings. gypsum plasterboard finds application in both vertical and horizontal structures, offering key advantages such as rapid installation, the avoidance of wet processes, and favourable acoustic and fire performance. its main drawback is heightened sensitivity to moisture, where a special type of board must be used [1]. several major producers of plasterboard operate in the czech republic, across europe, and worldwide, including knauf praha, spol. s r.o. all these manufacturers continually strive to improve and innovate their products, with particular attention to principles of the circular economy and energy efficiency. this article introduces a new, lighter plasterboard for sustainable construction (from the current density of 760 kg m−3 down to 720 kg m−3). the manufacturer’s aim is to make the production process more environmentally friendly while maintaining, as far as possible, the same user and technical properties. a research team at the czech technical university (ctu) validated the key parameters of the newly developed board and compared them with the original formulation. the main innovation lies in reducing the board’s gypsum content; in the next phase, there will be a focus on maximizing the use of organic (and therefore renewable) additives. the new board must comply with the mandatory requirements for performance properties under en 520:2004+a1:2009 [2], which include aspects such as strength, acoustics, and thermal performance. dry construction continues to increase its market share, with plasterboard serving as its core component. moreover, the advent of multi-story timber buildings will further boost the role of plasterboard in providing fire protection. 2. weight reduction in general, research background, and practical application reducing the weight of the board, conserving gypsum (a strategic raw material), and thereby lowering co2 emissions during production all address market demands and evolving legislation, while still meeting the requirements of applicable building standards. in the context of plasterboard innovation, the specifications of en 520:2004+a1:2009 [2] are both authoritative and limiting. failure to comply with these standards would necessitate obtaining a standalone european technical approval for the new board a protracted process. for the practical use of new boards, it is essential to describe and quantify their key properties so that 29 https://doi.org/10.14311/app.2025.54.0029 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en m. hataj, j. jochman, j. pošta et al. acta polytechnica ctu proceedings it is clear which structural requirements they fulfil or do not fulfil, as well as the performance range they will achieve. experience from abroad shows that such a board simply represents a new market segment that complements the existing one. similar boards have the longest history in the united states, where they have been in use for several decades and where it first became evident how technologically and financially demanding it is to manufacture them. any “weight reduction” i.e., decreasing the gypsum content must go hand in hand with ensuring usability for future customers. 3. testing methodology testing mechanical and other physical properties is therefore one of the basic ways to verify the quality and required resistance of newly developed plasterboards [3–5] and also how to certify a new product and bring it to the market. it is possible to combine small-format and large-format tests, which can then be used, for example, for validating computational models [6]. the laboratories of the university centre for energy-efficient buildings and the faculty of civil engineering at the czech technical university in prague tested the mechanical and thermal-technical properties of plasterboard. this was a comparative study of standard and lightweight boards, both with a uniform thickness of 12.5 mm. the set labelled a represented the standard board, while set b contained the lightweight board. the company knauf praha, spol. s r.o. supplied samples with standard dimensions to the laboratories: 10 samples from one set for mechanical tests and 8 samples from one set for determining the thermal conductivity coefficient. the samples for mechanical testing were conditioned at a temperature of 20 °c and a relative humidity of 65 %. 3.1. determination of embedment strength at the fastener location the purpose of this test is to determine the embedment strength at the fastener location. during the test, a continuous record was taken of both the applied load and the vertical displacement of the plasterboard. the board was attached by means of a knauf tn 55 drywall screw to two anchoring l-profiles, which form part of the testing apparatus. both l-profiles are made of p8 steel sheet (strength class s235) and are connected to a p8 steel base plate by four m6 bolts of grade 8.8. the tn 55 screw passed through the center of the board’s height and through pre-drilled holes in the anchoring l-profiles. to reduce friction between the steel l-profiles and the board, a polyamide strip was placed on both sides. the load was applied to the board’s centroid via a loading cylinder acting on a p4 s235 distribution plate. testing of the specimens followed the procedure specified in en 383:2007 [7]. the load was controlled according to the standardized load diagram: first, the figure 1. schematic of the embedment strength test at the fastener location. board was loaded to 40 % of the estimated maximum load (fest), then the load was reduced to 10 % of fest, and finally increased until the specimen breaks or reached the limit displacement. the specimen was positioned in the testing apparatus so as to allow vertical deformation of the board and thus embedment at the fastener location. the test setup, including the applied load, is illustrated in figure 1. testing of all specimens was terminated either after 300 ± 120 s or upon reaching the 5 mm deformation limit in accordance with [7]. for all specimens, the applied load and the corresponding deformation of the board were monitored. deformation was measured continuously by inductive lvdt sensors installed on both side edges of the board at the fastener’s height. the average reading from these lvdt sensors was used for data evaluation. 3.2. head pull-through resistance test the purpose of this test was to determine the load capacity of the screw connection under axial loading – i.e., in the same direction that the screw is driven into the material. this test is essential for evaluating the resistance of the board against localized failure caused by the screw head pulling through the surface. in practical applications, such as suspended ceilings, cladding systems, or other constructions where boards are fastened using screws, this type of failure is a critical mode of concern. a high pull-through resistance ensures a secure and long-lasting connection between the board and the supporting structure, particularly under longterm loads or dynamic effects. therefore, the results of this test directly reflect the reliability and performance of the board in real-life installation scenarios. the test was performed in accordance with the standard en 14566:2008+a1:2009 [8], which specifies the requirements and testing procedures for mechanical fasteners used in gypsum plasterboard systems. the test specimen was placed in the apparatus so that the plasterboard was fully supported. a portion of the screw was secured into a polyamide plate, which forms part of the testing setup and is attached to the hydraulic cylinder via steel side plates and an m6 30 vol. 54/2025 experimental analysis of physical properties of lightweight . . . figure 2. schematic of the head pull-through resistance test. threaded rod. a schematic of the test arrangement is shown in figure 2. throughout the experiment, the tensile force was recorded, and its maximum value served as the primary test outcome. the vertical displacement of the loading device was also monitored. 3.3. flexural strength test one of the fundamental mechanical properties specified by en 520:2004+a1:2009 [2] is the minimum flexural strength (flexural breaking load). according to the standard, test specimens measure 400 × 300 mm and are tested in both the longitudinal and transverse directions, with the longitudinal direction being decisive. testing follows a three-point bending method with a 350 mm support span (see figure 3). the load is increased at a rate of 250 ± 50 n min−1, and the total duration from the start of loading until failure must exceed 20 seconds. the test continues until the specimen breaks and is carried out under displacement control, with a verified speed of 2 mm min−1. we can use the optical correlation to evaluate tensile strength in bending of plasterboard [9]. a supplementary test of strength is the shear strength test [10], but this test was not part of this test block. 3.4. thermal conductivity coefficient the thermal conductivity coefficient (λ-value) is a fundamental parameter that describes the ability of a material to conduct heat. its value depends on various factors, such as the chemical composition, moisture content, temperature, density, and porosity of the material. this parameter is typically determined using experimental methods. generally, effective insulation will have a λ-value as low as possible to minimize heat loss. in this study, a commercially available portable heat transfer analyser, the isomet 2104 from applied precision ltd. (figure 4), was used, which comes with interchangeable needle and surface sensors of various ranges supplied by the manufacturer. this device is suitable for measuring heat transfer properties across a wide range of isotropic materials and offers several advantages, including the direct measurement of thermal-technical properties, portability, and a relatively short measurement time. the λ-value measurement is based on the temperature analysis response of the analysed test specimen to heat flow figure 3. static schematic of the flexural tension strength test. figure 4. measurement of thermal properties using the isomet 2104. impulses. for accurate measurements, a surface probe with a 60 mm diameter was placed in proper contact with a flat sample (300 × 300 mm), and measurements were taken at multiple locations on the sample. 4. results comparison in this section, there are present a comparison of the results for the above-described properties of standard and lightweight boards based on the experimental analysis. 4.1. embedment strength at the fastener location all ten specimens were break in the expected manner – by embedment of the board at the fastener location (see figure 5). compared with the standard boards in set a, the lightweight boards in set b exhibited an 11.5 % decrease in the characteristic embedment strength fh,k. with regard to the average foundation modulus ks,mean, there was a 5.1 % reduction for the lightweight boards, and a 5.3 % reduction for the elastic foundation modulus. 31 m. hataj, j. jochman, j. pošta et al. acta polytechnica ctu proceedings figure 5. typical failure mode of an experimental specimen via indentation at the fastener location. 4.2. axial loading of the fastener the primary result of this test was the maximum load reached during loading – that is, the screw’s pull-through resistance. compared with the standard boards in set a, the lightweight boards in set b showed a 15.4 % decrease in their characteristic pull-through capacity. 4.3. flexural strength test according to en 520:2004+a1:2009 [2], 12.5 mm gypsum plasterboards must achieve a minimum flexural breaking load of 210 n in the transverse direction and 550 n in the longitudinal direction. the crucial parameter is the longitudinal bending strength (in the direction of flow of mixture during production). both boards satisfy the longitudinal strength requirement, and the strength values for the standard and lightweight boards are nearly identical. the original (standard) board, made using the existing formulation, exceeds 610 n, while the lightweight boards attain flexural breaking loads only 3.5 % lower on average. the process of the test can be seen in figure 6. 4.4. thermal conductivity measurement the thermal conductivity measurement results for both the standard and lightweight boards are presented in table 1. the average value is calculated based on all eight samples from a single set of test specimens. in the comparison between standard and lightweight gypsum boards, standard boards typically have a higher bulk density, which results in a higher thermal conductivity coefficient. this means they transfer heat more efficiently but provide less insulation. in contrast, lightweight gypsum boards are designed with a lower bulk density, often incorporating air pockets figure 6. flexural strength test. λa [w (mk)−1] λb [w (mk)−1] 1 0.181 0.171 2 0.180 0.172 3 0.185 0.176 4 0.185 0.175 5 0.192 0.176 6 0.194 0.175 7 0.189 0.172 8 0.182 0.171 average 0.1860 0.1735 stdev 0.0048 0.0021 table 1. results of thermal conductivity coefficient measurements for standard and lightweight boards. or lightweight fillers. these modifications reduce their thermal conductivity coefficient. 5. continuity to full-scale construction experiments the initial motivation for this project stemmed from comparative bending tests on full-scale wall assemblies, designed to show how altering the plasterboard type would affect partition wall performance. identical wall sections were fabricated from both board types and tested horizontally to assess their flexural stiffness. however, it became clear that to ensure identical conditions for both sets of specimens – and thus derive valid results a large number of production steps had to be carefully controlled, making the entire process much more time-consuming than anticipated. therefore, as part of this project, methods were developed to enable faster, more efficient, and above all more meaningful comparisons of the boards’ parameters. the results from these bending tests on the same wall panels confirmed that one cannot reliably infer the actual behavior of gypsum boards built into a partition from their similar flexural tensile strength values per en 520:2004+a1:2009 [2]. moreover, preparing completely identical test specimens for partition walls with different boards proved both technically challenging and time intensive. 32 vol. 54/2025 experimental analysis of physical properties of lightweight . . . consequently, the focus shifted to simpler tests on smaller samples – specifically, embedment strength tests at the fastener location and axial loading of the fastener. by applying an appropriate schematic model, these tests can predict how gypsum boards and steel framing interact, thus providing a fairly precise estimate of how partitions made from different boards will ultimately perform. the findings from this project now serve as a valuable foundation for planning, conducting, and evaluating the results of large-scale structural tests. 6. conclusion the use of circular economy principles in material design is increasingly important for advancing sustainability in the building sector. lightweight gypsum plasterboard is an alternative to standard gypsum board, designed to reduce weight and make transportation, use and installation easier. this reduces costs and time by increasing the quantity of gypsum boards transported in a container or truck, thereby enhancing overall efficiency and logistics. it also improves worker safety during handling. it can be manufactured in standard sizes and thicknesses and features an easy score and snap-cutting method, making it a preferred choice for both residential and commercial construction projects and ensuring straightforward and efficient installation. in summary, this experimental analysis demonstrates the potential of lightweight gypsum plasterboard to improve sustainability and resource efficiency in dry construction. by reducing the density by 40 kg m−3 compared to conventional boards, the new product becomes lighter and easier to handle, offering potential savings in transportation and installation costs, as well as generating less waste. moreover, reducing the gypsum content by 12 % of mass results in an approximately 6 % decrease in co2 emissions from annual production. this reduction is a significant step toward decreasing the energy consumption typically required for gypsum production, making the new board more energy-efficient than its traditional counterparts. additionally, the shift towards using organic additives derived from renewable chemical substances, replacing some of the non-renewable inorganic materials, further enhances the environmental sustainability of the product. this change reduces reliance on finite resources and promotes a circular economy within the construction industry, potentially contributing to certifications such as leed by supporting environmentally sustainable practices. this study emphasizes the importance of developing gypsum boards that not only adhere to current standard requirements for production but also maintain sustainable performance characteristics. these include essential mechanical, thermal, and fire-resistant properties. balancing these factors is crucial to achieving both environmental and functional goals. in conclusion, this study demonstrates that lightweight, resource-efficient gypsum boards offer a viable solution for advancing sustainability in dry construction, balancing environmental benefits with functional performance, without compromising established standards or functional performance. list of symbols λ thermal conductivity coefficient [w (mk)−1] acknowledgements this work “development of lighter weight plasterboard for sustainable construction” tq03000239 is co-financed from the state budget by the technology agency of the czech republic under the program for the support of applied research and innovation sigma. references [1] m. nyč. plasterboard. grada, 2005. [2] brussels: european committee for standardization. gypsum plasterboards – definitions, requirements and test methods, 2009. [3] c. petrone, g. magliulo, g. manfredi. mechanical properties of plasterboards: experimental tests and statistical analysis. journal of materials in civil engineering 28(11):04016129, 2016. https: //doi.org/10.1061/(asce)mt.1943-5533.0001630 [4] i. rahmanian. thermal and mechanical properties of gypsum boards and their influences on fire resistance of gypsum board based systems. ph.d. thesis, the university of manchester, 2011. [5] silver spring, md: gypsum association. gypsum board typical mechanical and physical properties (ga-235-2019), 2019. [2024-08-14]. https://www.americangypsum.com/sites/default/ files/2022-01/ga-235_gypsum_board_typical_ mechanical_and_physical_properties.pdf [6] o. zobal, z. bittnar, p. fajman, et al. experimental verification of the bending stiffness of the wall for the program for design of high walls in dry construction. in experimental stress analysis 58th – book of full papers, pp. 566–569. czech society for mechanics, 2020. isbn 978-80-248-4451-0. [7] brussels: european committee for standardization. timber structures – test methods – determination of embedment strength and foundation parameters for dowel-type fasteners, 2007. en 383:2007. [8] brussels: european committee for standardization. mechanical fasteners for gypsum plasterboard systems – definitions, requirements and test methods, 2009. en 14566:2008+a1:2009. [9] o. zobal, j. litoš, j. antoš. analysis of tensile strength in bending of plasterboard with use of optical correlation. in experimental stress analysis 55th, pp. 459–463. technical university of košice, 2017. isbn 978-80-553-3167-6. [10] o. zobal, m. jára, m. hataj. methodology for measuring the shear stress of plasterboards. acta polytechnica ctu proceedings 13:157–161, 2017. https://doi.org/10.1016/s0045-7825(98)00227-8 33 https://doi.org/10.1061/(asce)mt.1943-5533.0001630 https://doi.org/10.1061/(asce)mt.1943-5533.0001630 https://www.americangypsum.com/sites/default/files/2022-01/ga-235_gypsum_board_typical_mechanical_and_physical_properties.pdf https://www.americangypsum.com/sites/default/files/2022-01/ga-235_gypsum_board_typical_mechanical_and_physical_properties.pdf https://www.americangypsum.com/sites/default/files/2022-01/ga-235_gypsum_board_typical_mechanical_and_physical_properties.pdf https://doi.org/10.1016/s0045-7825(98)00227-8 acta polytechnica ctu proceedings 54:29–33, 2025 1 introduction 2 weight reduction in general, research background, and practical application 3 testing methodology 3.1 determination of embedment strength at the fastener location 3.2 head pull-through resistance test 3.3 flexural strength test 3.4 thermal conductivity coefficient 4 results comparison 4.1 embedment strength at the fastener location 4.2 axial loading of the fastener 4.3 flexural strength test 4.4 thermal conductivity measurement 5 continuity to full-scale construction experiments 6 conclusion list of symbols acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2023.43.0057 acta polytechnica ctu proceedings 43:57–61, 2023 © 2023 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague application of ontology/conceptual modelling in the context of risk identification at level crossings tomáš kohout∗, luboš nouzovský, pavel vrtal, karel kocián czech technical university in prague, faculty of mechanical engineering, department of forensic experts in transportation, konviktská 20, 110 00 prague, czech republic ∗ corresponding author: kohout@fd.cvut.cz abstract. this conference paper discusses a new analytical approach to the evaluation of traffic safety at the railway crossings. the new method is based on the use of ontological principles. the ontological tools present a comprehensive decision-making tool, will can be applied to any potential hazardous railway crossing on a road or rail network. the ontology can be considered as a knowledge base that not only stores the data itself, but also defines the relationships between these data, and can therefore be described as a clearly suitable tool for describing risks at level crossings. keywords: level crossing, traffic safety, ontology, data utilisation. 1. introduction safety analysis, or risk analysis, is a topic that resonates across many different disciplines. transport safety analysis is no exception. if we look at a particular area of transport as an interrelated and influencing system, it is necessary to address the question of whether the system is working properly and indicates an adequate level of safety not only for the system as a whole but also in relation to its users. the issue of safety analysis is closely related to the issue of collecting, processing, evaluating and interpreting data that affect the safety level of a particular system. given the considerable amount of measurable data that the modern world makes possible through technology nowadays, it is important to work effectively with this data. the approach of how to store the data is also an important factor. currently, simple tools such as ms excel are used for these purposes. however, it is questionable whether this approach is effective in relation to subsequent analyses, creates a realistic view of the analyzed system and satisfies the needs of subsequent complex analyses over these data. at the same time, it is important to ensure the highest possible standardisation of procedures in these analyses in the context of minimising the potential impact of analysts’ subjective assessments on the risk analysis outputs. the aim of this systems approach to safety analysis is to increase the informative value of the data at all stages of the analysis and risk classification of a particular system. 2. conceptual modelling, ontology conceptual modelling allows you to create a model of a real part of the world. the basis of this modelling is an ontology that guarantees the correct interpretation of all terms. a conceptual model is created by synthesizing a theoretical knowledge base and an appropriate tool. in this context, the theory is represented by an ontology and a graphical modelling language is used as a tool. the ontology, unanimously defines terms and relationships between terms in the specific reality domain described for which the ontology is created. in general, an ontology is a tool for capturing the real world, or parts of it, in the form of standardizing and formalizing the meaning of concepts in the described part of the world. for this reason, ontologies can be beneficially used in modelling, questioning or derivation tasks [1–3]. 2.1. conceptual modelling conceptual modelling can also be defined as conceptual modelling. conceptual modelling allows modelling of real world elements. a conceptual model is realized by combining a theoretical knowledge base and a suitable tool. the theory is represented by an ontology, which uniquely defines terms and relationships between terms from the specific reality domain described for which it is created, and a graphical modelling language is used as the modelling tool. an example of this is the unified modelling language (uml). conceptual modelling is based on modelling the real world domain by using entities and relationships between these parameters. the term entity can be used to imagine a real world object capable of independent existence and is uniquely distinguishable from other entities. the notion of relationship specifies the relations between entities and at the same time describes certain properties. an important term is the notion of attribute, which indicates a property of entities or relationships. the figure 1 shows graphically the relationship between the ontology that gives a clear meaning to the conceptual model, the conceptual model that can be described by the uml modelling language and this specific modelling language. in general, it can be assumed that conceptual modelling represents the initial phase of the analy57 https://doi.org/10.14311/app.2023.43.0057 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en t. kohout, l. nouzovský, p. vrtal, k. kocián acta polytechnica ctu proceedings figure 1. the relation between conceptual model, ontology and uml modelling language. sis. the next phases are the proposal of the logical model and the implementation within the physical model [1, 4–6]. 2.2. unified foundational ontology the foundations of unified foundational ontology (ufo) were specified by giancarlo guizzardi in his phd thesis. ufo is currently being further developed by the nemo research group of which giancarlo guizzardi is a part. it is a universal upper-level ontology that can be used for application to any part of the real world. the ufo ontology is further divided into the following three interrelated units: (1.) ufo-a deals with individual aspects of reality, which are considered to be individual objects, socalled endurants. ufo-a has been used to analyse structural constructs of conceptual modelling, such as object types and taxonomic relations, associations and relations between associations, roles, properties, data types and weak entities, and participation relations between objects. (2.) ufo-b is a part of the ontology, which extends the original ufo-a ontology by dynamic process and events called perdurants, which are varying depending on time. (3.) ufo-c extends the ufo-b ontology and deals with social aspects. by this term we can imagine aspects that are e.g. faith, actions or social roles [1, 3, 7]. 2.2.1. ontouml ontouml is an ontology language for ontological conceptual modeling. ontouml is an extension of the uml language and is based on the ufo ontology. the foundations of ufo and ontouml are based on giancarlo guizzardi’s phd thesis “ontological foundations for structural conceptual models” [1]. in his thesis, guizzardi proposed a new fundamental ontology for conceptual modeling (ufo) and used it to evaluate and redesign the uml for the purpose of conceptual modeling and domain ontology engineering (see the figure 2) [2]. 3. motivation for applying at level crossings the following section discusses the evaluation of the accident rate at level crossings in the czech republic. the input dataset represents accidents that occurred at level crossings between 2010 and 2020. for this period, the railway inspectorate of the czech republic recorded a total of 2 182 accidents in which 1 116 persons were injured and 430 were killed. the figure 3 shows the evolution of the number of accidents and their consequences at level crossings over the 12-year period. considering the evolution of the number of accidents, no decreasing trend can be identified between 2011 and 2021. the number of injured persons also shows a decreasing trend, which can be classified as a rather fluctuating trend over the period analysed. the most important indicator, namely the number of persons killed, does not follow a decreasing trend either, except for the last year analysed, 2021 [8, 9]. the figure 4 shows the evolution of the number of accidents at level crossings by type of signalling system. it can be deducted from the number of incidents that level crossings protected only by a traffic sign have an almost similar absolute accident rate as level crossings equipped with a level crossing signalling system with light signals without barriers. however, taking into account the ratio between the number of level crossings protected only by a traffic sign (3 486 pcs) and the number of level crossings equipped with a signalling system with light signalling without barriers (2 363 pcs), it can be reasonably stated that the relative accident rate is higher and therefore the safety level is lower for level crossings equipped with a signalling system with light signalling without barriers [8, 9]. the mentioned negative statistical evaluation of the level of safety at level crossings equipped with level crossing signalling system with light signals without barriers is clearly confirmed by the statistics summarised in the figure 5. the figure shows the evolution of the number of persons killed in accidents at level crossings according to the type of signalling system. in the period under review, a total of 210 persons were killed at level crossings protected by level crossing signalling system with light signals without barriers, which is three times higher than the number of persons killed at level crossings protected only by traffic sign [8, 9]. to summarise this section, based on statistical data on the number of incidents at level crossings and their consequences, an unsatisfying development trend can be stated. in general, there is no significant yearon-year decreasing trend in the number of accidents or the number of persons injured or killed. also alarming is the negative assessment of the safety level of level crossings with signalling system with traffic lights without barriers. in terms of both relative and 58 vol. 43/2023 application of ontology/conceptual modelling in the context of risk identification figure 2. ontouml metamodel [2]. 59 t. kohout, l. nouzovský, p. vrtal, k. kocián acta polytechnica ctu proceedings figure 3. evolution of the number of accidents at level crossings [9]. figure 4. evolution of the number of accidents at level crossings by type of protection [9]. absolute accident rates, this type of protection poses the greatest risk for road users. the above conclusions of the analysis of the level of safety at level crossings in the czech republic clearly indicate a necessity for a more detailed analysis of the presented issues in order to achieve a socially acceptable level of safety of traffic on roads in accordance with the european transport policy and the national road safety strategy. research by the author of this article focuses on solving this problem. a number of interacting parameters have a direct or indirect influence on the issue of risk analysis at level crossings. for this reason, organizing these parameters by creating an ontology seems to be effective in terms of subsequent safety analyses. at the same time, this method limits possible misinterpretation of individual terms or relationships between terms. 4. practical application a fundamental premise for assessing the risk of level crossings is a comprehensive approach, both in terms of defects of the road and the railway infrastructure. the safety of a level crossing is determined by the probability of occurrence of risks. the potential occurrence of an incident is influenced by the parameters by which the level crossing and its surroundings are described. the main influence on the level of safety is the current qualitative state of the specified parameters. it is essential to reflect that the parameters are interdependent within a single system (level crossing) and influence each other. generally, situations are considered to be at risk when a level crossing is in a figure 5. evolution of the number of persons killed in accidents at level crossings by type of protection [9]. state where a risk condition is identified for several specific parameters. specifically, the following set of parameters in table 1 has been defined that influence the level of risk at level crossings. the following defined parameters describe the level crossing in detail in terms of three specific groups of defined parameters. it is a group of parameters belonging to the road and a group of parameters parametrizing the railway segment of the issues. the last group of parameters contains those parameters that cannot be clearly classified into one of the two above-mentioned categories but are necessary for the completion of the complex parameterisation of the level crossing. if we consider the ontology as a unified knowledge base that not only stores the data itself, but also defines the relationships between these data, it can be described as a clearly suitable tool to describe the risks at level crossings. 5. discussion the next steps in the project are the validation of the model based on ontological principles within a test database of selected level crossings. at the same time, a database of appropriate remediation measures is being created to be addressed to the identified risks. the main potential problem of the research is the limited scope of literature and information sources about the issue. unified foundational ontology is very well described in dr. guizzardi’s dissertation. however, there are few practical examples or examples of work dealing with ontologies ufo-b and ufo-c. 6. conclusion the perspective and effectiveness of this project consists primarily in the development of a unified decisionmaking tool for a comprehensive evaluation of the level of traffic safety at level crossings. the resulting determination of the risk level of individual level crossings will be based on the proposed and validated ontological model. this ontological model will be complemented by the knowledge base of the researcher regarding the risk rating of individual parameters, or a combination of interrelated parameters if they are in an inadequate state. 60 vol. 43/2023 application of ontology/conceptual modelling in the context of risk identification rail road others intensity of rail transport road traffic intensity number of accidents importance of the railway road category railway crossing angle number of tracks speed limit location – urban/rural area line speed layout of the road assessment of obstacles in the clearance zone level of protection vertical curvature of the road layout of the railway technical condition of traffic signs validity of the regulation s 4/3 perceptibility of the level crossing type of railway signalling system variability of the road width technical condition of crossing road possibility of glare distance between level crossing and intersection perceptibility of traffic signs walking route across level crossing table 1. list of defined parameters. in parallel with the identification of traffic safety defects, a database of specific appropriate remedial measures is being created. these measures appropriately address the identified traffic safety risks. in the case of practical implementation of this clearly defined methodological approach to the evaluation of risks related to level crossings and the design of appropriate remediation measures, a socially acceptable level of transport safety at level crossings will be ensured. acknowledgements this work was supported by the grant agency of the czech technical university in prague, grant no. sgs21/137/ohk2/2t/16, no. sgs21/138/ohk2/2t/16 and no. sgs20/199/ohk2/3t/16. references [1] g. guizzardi. ontological foundations for structural conceptual models. ph.d. thesis, university of twente, netherlands, 2005. [2] ontouml community portal. 2017, [2022-04-20], https://ontouml.org/. [3] g. guizzardi, g. wagner. using the unified foundational ontology (ufo) as a foundation for general conceptual modeling languages. theory and applications of ontology: computer applications pp. 175–196, 2010. https://doi.org/10.1007/978-90-481-8847-5_8 [4] j. arlow. uml and the unified application development process. neustadt ila, 2003. [5] m. molhanec. ontology and conceptual modelling (a short introduction). [2022-04-20], https://www.slideserve.com/noura/ontologie-akonceptu-ln-modelov-n-stru-n-vod. [6] m. molhanec. conceptual modelling, formal foundations and ontologies. [2022-04-20], http://progstory.technicalmuseum.cz/images/dokumenty/ programovani-tsw-1975-2014/2006/2006-17.pdf. [7] nemo. [2022-04-22], https://nemo.inf.ufes.br/en/projetos/ufo/. [8] qcm, s.r.o. in 2021, the fewest people died on railways in 19 years. [2022-04-20], http://www.dicr.cz/v-roce-2021-zemrelo-nadrahach-nejmene-osob-za-poslednich-19. [9] railway administration, state organization. level crossings in numbers. [2022-04-20], https:// www.spravazeleznic.cz/o-nas/bezpecna-zeleznice/ bezpecnost-na-prejezdech/prejezdy-v-cislech. 61 https://ontouml.org/ https://doi.org/10.1007/978-90-481-8847-5_8 https://www.slideserve.com/noura/ontologie-a-konceptu-ln-modelov-n-stru-n-vod https://www.slideserve.com/noura/ontologie-a-konceptu-ln-modelov-n-stru-n-vod http://prog-story.technicalmuseum.cz/images/dokumenty/programovani-tsw-1975-2014/2006/2006-17.pdf http://prog-story.technicalmuseum.cz/images/dokumenty/programovani-tsw-1975-2014/2006/2006-17.pdf http://prog-story.technicalmuseum.cz/images/dokumenty/programovani-tsw-1975-2014/2006/2006-17.pdf https://nemo.inf.ufes.br/en/projetos/ufo/ http://www.dicr.cz/v-roce-2021-zemrelo-na-drahach-nejmene-osob-za-poslednich-19 http://www.dicr.cz/v-roce-2021-zemrelo-na-drahach-nejmene-osob-za-poslednich-19 https://www.spravazeleznic.cz/o-nas/bezpecna-zeleznice/bezpecnost-na-prejezdech/prejezdy-v-cislech https://www.spravazeleznic.cz/o-nas/bezpecna-zeleznice/bezpecnost-na-prejezdech/prejezdy-v-cislech https://www.spravazeleznic.cz/o-nas/bezpecna-zeleznice/bezpecnost-na-prejezdech/prejezdy-v-cislech acta polytechnica ctu proceedings 43:57–61, 2023 1 introduction 2 conceptual modelling, ontology 2.1 conceptual modelling 2.2 unified foundational ontology 2.2.1 ontouml 3 motivation for applying at level crossings 4 practical application 5 discussion 6 conclusion acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2023.43.0062 acta polytechnica ctu proceedings 43:62–71, 2023 © 2023 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague modern vehicles for regional railways josef kolář czech technical university in prague, faculty of mechanical engineering, department of automotive, combustion engines and railway engineering, technická 4, 160 07 prague, czech republic correspondence: josef.kolar@fs.cvut.cz abstract. the paper analyses the history of two-axle rail vehicles (rail buses) used for operation on regional lines in czechoslovakia. it also shows trends in the development of modern light autonomous rolling stocks usable for passenger transport on unprofitable regional lines in western europe. the measures adopted in the transport policy of the czech republic for the period 2021–2027 for regional railway vehicles in the czech republic are presented. the final part of the paper presents the results of study of the concepts of a two-axle hybrid and electric rail buses and a conceptual design of a modular solution of a partially low-floor vehicle bemu and hemu for passenger regional rail transport. this paper shows that it is very problematic to create a light rail vehicle (lvr) that will be economical and environmentally friendly for regional transport and at the same time will meet the requirements of standards čsn en 12 663, category p2 and čsn en 15 227, category c i and will be friendly to road users. keywords: modern railways, regional vehicles, rail bus, design vehicles, light rail vehicle, often low-floor rail vehicle, motor rail vehicles. 1. introduction railway companies have been tackling the idea of offering reliable, safe and economically acceptable regional rail transport on non-electrified low-traffic lines since the mid-1920s. the logical development step was the motorization of the vehicle fleet, i.e. the replacement of regional steam passenger trains by light rail buses, derived from road bus constructions, such as the skoda 550. also for example 1 pc of praga m120 and 11 pcs of skoda m 120.1 rail buses, which were supplied in that time for csd, can be stated, see figure 1. after the second world war, the concept of a light railway vehicle, a rail bus for a regional railway, was represented by the m 131.1 motor car, see figure 2, which was manufactured in the years 1948–1956 in the tatra vagonka studenka company in the number of 549 pieces. in operation from 1948 to 1984 and significantly unified the csd fleet used for regional transport. this two-way trafic, two-axle motor car weighting approx. 16.6 tons powered by an air-cooled, twelve-cylinder diesel internal combustion engine from a tatra 111 truck. to prolong the life of the internal combustion engine, its power was reduced to 120 kw for the needs of the m 131.1 and it was supplemented with an oil cooler, as the installation of the internal combustion engine in the car did not allow air-forced cooling. the m 131.1 passenger motor car with the axle arrangement (1a) reached a maximum speed of v = 60 kmh−1 and had a transport capacity of 48 seats. since 1973, the m 131.1 motor cars were gradually replaced by new light two-axle motor cars m 152.0, class designation 810, of which about 680 pieces were figure 1. one-way rail bus skoda m 120.1 when turning using its own turntable [1]. figure 2. two-way motor car m 131.1 – hurvinek [2]. produced for csd. in addition to these motor vehicles, company vagonka studenka also produced approximately 912 pcs blm 24–29 trailer cars, class designation 010 see figure 3. between 2006 and 2012, 210 pcs of these vehicles underwent a partial modernization. the main requirement of of the modernization was to create a light, partly low-floor vehicle for regional transport, class 814. it can be stated that regionova in a two-car (814 + 914), see figure 4, or three-car (814 + 014 + 814) arrangement represented a compro62 https://doi.org/10.14311/app.2023.43.0062 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 43/2023 modern vehicles for regional railways figure 3. motor car m 152 + trailer (810 + 010) [3]. figure 4. two-car unit regionova (814 + 914) [3]. mise solution of partial low-floor and modernization. however, even the two-car version seems to be uneconomical for low-traffic regional lines in the czech republic, as the hydrodynamic power transmission does not allow energy recovery [3]. 2. regional railways – profitability, safety and filling of the goals of the project “vision o” other countries in europe are also facing the problem of profitability of regional lines. in the next part of the paper, i will try to outline the visions of the modern regional transport on low-traffic lines in france, england and germany. as a result of the discussion that took place in france in 2018, it was stated that about 9000 km of regional lines are not competitive and should be closed. régis coat, president of exid concept & developpement, who presented the taxirail project, which aims to secure the future of low-passenger regional routes using small light autonomous rolling stock running seven days a week and 24 hours a day, disagrees. in france, it is possible to operate taxirails on the same lines as freight trains. there is a maximum of two freight trains a day on small regional lines, says coat. “we have taken measures to avoid having taxirail and freight trains on the line at the same time” [3]. taxirail vehicles, see pictures figure 5, should drive fully automated, with a degree of automation (goa) 4, i.e. without a driver. whether the taxirail project will be a realistic and viable solution should be demonstrated by feasibility studies for the caux-seine agglo figure 5. taxirail autonomous vehicle concept [4–6]. agglomeration between le havrea and roun, which was to be launched in september 2021. the standard autonomous light rail vehicle taxirail should be 6m long and 2.9m wide with a maximum speed of 100 kmh−1. it is to be deployed on tens of single-track, non-electrified lines in france with a length of 10 to 80 km. each vehicle will have 16 seats and space for 40 passengers. to reduce the weight of the taxirail to 11 tonnes, they will not be manufactured to the strict crash resistance standards of conventional trains. however, the current national rules for the operation and construction of trains will need to be rewritten for taxirail and stakeholders are being consulted [5, 6]. railbus inc. presents a similar solution for the rail transport with the help of light autonomous vehicles without a driver, which is more suitable for the urban transport. railbus vehicle with a light body mounted on two two-axle bogies, see pictures in figure 6, offers a transport capacity of 22 seated and 18 standing passengers and a maximum speed of 100 kmh−1 [7]. an interesting feature of this transport system is 63 josef kolář acta polytechnica ctu proceedings figure 6. vehicle railbus [7]. the fully solar vehicle power supply system, where the solar panels are located in the track. this solution will probably not be used for the needs of local regional railways, as it would probably require to consistently isolate these sections of the track from the surrounding environment [7]. within the development of light autonomous tram vehicles, see figure 7, graduates of u 12 120, fme ctu, today employees of skoda transportation, are involved as well. an interesting solution of rail transport with the help of light rail vehicles with a driver, which is rather suitable for urban transport, was presented by the british company tdi for the city of coventry, see pictures figure 8. a light construction vehicle with a length of 11m, a width of 2 650m and a height of 3 175m offers a transport capacity of 20 seated and up to 50 standing passengers. minimum traction power 175 kw and maximum vehicle speed up to 70 kmh−1 and weight approx. 11 tons [8]. another inspiring light rail vehicle solution for regional transport is the autonomous battery electric rail bus aachener rail shuttle (ars), developed at the institute for rolling stock and transport systems of the rwth in aachen, see figure 9. the electric bus is 12m long. the running gear with a wheelbase of 8m forms the robust main frame of the vehicle, which is supported on two traction wheelset via the primary suspension. the force effects during an accident are captured only by this main frame, which is dimensioned according to category c-iii of the en 15227 standard. to absorb the impact energy, impact elements (purple colors) are attached to both ends of the vehicle frame [11]. figure 7. possible solution for a two-axle autonomous tram presented by škoda [9, 10]. an over 3m wide lightweight car body is placed on the chassis frame via a pneumatic suspension, which in the 2+2 seating arrangement is equipped with 34 fixed seats and eight folding seats installed in the boarding areas. the rail bus is supposed to carry a maximum of 90 passengers. each axle is driven by an electric motor with a nominal power of approx. 150 kw, which provides a pulling force of up to 40 kn at a maximum acceleration of 1.5m s−2. the mass of the empty vehicle is to be less than 17 t, which would result in a maximum weight of 25 t with a payload of up to 8 t. the maximum wheelset load would thus be only 12.5 t [11]. 64 vol. 43/2023 modern vehicles for regional railways figure 8. concept vehicle coventry lvr [8]. figure 9. concept vehicle ars bus [11]. 65 josef kolář acta polytechnica ctu proceedings figure 10. revolution very light rail [12]. the concept of light rolling stock for regional railways in england is represented by a solution developed by the university of warwick, see figure 10. the twoway diesel-electric revolution vlr is produced by a consortium led by transport design international. the vehicle, the rail-bus is 18.5m long, 2.8m wide and 3.8m high, offers 120 places for passengers, including 56 seats, and a maximum speed of 104 kmh−1 [12]. the hybrid drive is provided by a power pack of a diesel combustion engine with an electric generator located in the chassis and a battery power pack (lithium titanium lto) or lithium manganese cobalt oxide (nmc), which will be recharged by a recuperation or by a trolley wire if the track is electrified. the wheelsets are powered by electric motors, see figure 11. the rail bus is not equipped with a wc module and apparently do not offer space for the transport of bicycles and strollers. the designers calculate a specific weight of 1 ton per 1 meter of car length. therefore the total weight should not exceed 18 tons. due to such a low weight, the cabinets are not sufficiently strong and crash resistant, by standard en 12 663, category p2 and to category c-iii of the en 15227 standard. the units will be used on small regional lines or suburban lines where there is no danger of collisions with freight trains or other larger trains. the high-floor design of the vehicle does not allow its use on regional lines in the czech republic, where the edge of the platform is 600mm above the level of the top of the track. in the next part of the paper, we will see what is the aim the czech republic wants to take in regional transport. figure 11. bogie for revolution very light rail [12]. 3. transport policy of the czech republic for the period 2021–2027 the czech republic is one of the countries with the highest density of railway lines in europe. there are approximately 0.12 km of tracks per 1 km2. in 2021, 9 377 km of state-owned railway lines and over 100 kilometers of regional lines owned by other entities were in operation. a large share of the railway network, approx. 80%, consists of regional lines, on which only approx. 10% of the transport capacity is ensured. nevertheless, it can be stated that even these lines have their influence on ensuring the serviceability of the territory and their importance may increase with the adoption of the measures specified in the transport policy of the czech republic for the period 2021–27. in the material transport policy of the czech republic for the period 2021–2027 with a view to 2050, it is stated on page 38 [13]: “on the railways, a pan-european trend is visible, which will gradually lead to the termination of the diesel traction. battery-rail cars will be gradually introduced in combination with progressive electrification (bimodal cars can also be used temporarily), which will serve non-electrified end sections and which will be continuously dynamically charged at driving along elec66 vol. 43/2023 modern vehicles for regional railways trified sections, or statically charged while standing in electrified stations. therefore battery-rail cars are not a replacement for the line electrification, but a supplement to it.” the renewal of vehicles must also respect the requirements for alternative types of drives, taking into account the gradual process of electrification of other lines. the expansion of these types of drive will most likely be associated with a significant increase in compensation to carriers, paid by the customer. with respect to the proposed measures listed on page 39, the following measures are particularly interesting for regional railway vehicles [13]: • 1.3.1.5 create facilities for the operation of battery vehicles in areas without a line electrification, by building power points both for overnight stay of vehicles (heating) and for charging vehicles at turning stations. • 1.3.1.6 coordinate the plans of the ministry of transport of the czech republic and railway administration state enterprise (sz) for the electrification of other lines with the plans of the carriers for the development of the vehicle fleet, so that investments are not wasted. in practice, this means no longer buying vehicles powered by internal combustion engines and focusing financial resources exclusively on the purchase of electric vehicles (trolley-powered or battery-powered). • 1.3.1.7 coordinate the plans of the ministry of transport of the czech republic and sz for the electrification of other lines with the plans of the ministry of transport of the czech republic and railway administration state enterprise (sz) to equip tracks and vehicles with the unified european train safety system etcs. the goal is that the tracks that have not been electrified yet, will also be electrified at the same time when they are equipped with the unified european train safety system etcs. in this way, an inefficient investment into the equipment of unpromising diesel-powered vehicles with the mobile part of the etcs train security system can be avoided, as these will be discontinued in the foreseeable future. • 1.3.1.8 coordinate the plans of the ministry of transport of the czech republic and railway administration state enterprise (sz) for the electrification of other lines with the plans of the public transport customers to solve line management and requirements for vehicles. do not allow transport provided by diesel-powered vehicles to be ordered on electrified lines and do not allow public transport customers (state and regions) to demand new dieselpowered vehicles from carriers to provide transport. 4. study of rail vehicle for regional transport in the czech republic regional lines in the czech republic quite often (approx. 8 000 times) cross in the form of level crossings with varying degrees of security with local roads. every unprotected crossing increases the risk of an accident between a regional train and a road vehicle. however, to solve this danger by requiring compliance with en 12 663 in the p2 category and en 15 227 in the c i category means that economically and environmentally beneficial light two-axle motor vehicles, referred to as rail buses, will disappear from regional transport. we came to this conclusion based on conceptual studies of a hybrid rail bus and an electric bus, see figure 12. the results of these studies were presented at conferences [3, 4]. wouldn’t it be more appropriate to introduce contractually and clearly defined operating rules on regional lines that would determine when during the day there will be a regular passenger traffic and at what time intervals an irregular freight traffic can be carried out? isn’t this an idea that, with proper use of etcs and it technology, could simply prevent a possible collision between a freight train and a light rail vehicle? is it appropriate to “dope” rail vehicles for passenger regional transport with robust passive safety elements that increase their weight, lengthen their starting and braking distance, increase energy consumption and reduce their competitiveness with respect to the road bus transport? are we not taking these steps against the ideas of our predecessors about an economical regional railway implemented in the passenger transport with light rail vehicles? i would like to point out that meeting the requirements of standards en 12 663 (category p2), en 15 227 (category c i) creates a clear winner in a collision with a passenger car from a rail vehicle. most of these accidents result in significant destruction of the passenger car and serious injuries or death of the car crew members. these accident results go against the ideas of the european “vision 0” project on road and rail. in order to create “friendly” regional rolling stock, it would be more appropriate to demand compliance with the requirements of the en 12 663 standard in the p3 category and the en 15 227 standard in the c ii category, and to equip rail and road vehicles with active safety elements that would significantly eliminate the possibility of their mutual collision. the requirement for the introduction of etcs-level 2 will certainly significantly contribute to a higher level of active safety of rail traffic in regional transport. however, the next, completely logical step should be the initiation of an intensive cooperation between etcs creators and manufacturers of a car navigation and control systems. transferring information about 67 josef kolář acta polytechnica ctu proceedings figure 12. concept of the hybrid rail-bus or the rail electrical-bus [14]. the position of the train to the navigation and control unit of the car, displaying it to the driver and storing it in the recording device of the “black box of the car” can contribute to a significant increase in the responsibility of car drivers for their behaviour at unprotected railway crossings. this cooperation of the mobile part of etcs with road vehicles will be necessary for the development of autonomous (road and rail) vehicles. during 2021, cd company contacted me to try to work out a concept study of the bemu 70 vehicle that could replace the regionova vehicle. the requirement was to design a two-way vehicle with a transport capacity of 74 seats that meets the requirements of csn en 12 663 in the p2 category and csn en 15 227 in the c i category and tsi prm. the result of this study is the design of the four-axle, partially low-floor bemu 74 vehicle, see figures 13 and 14. the height of the edge of the floor in the boarding areas is 550mm, and with the help of inclined ramps it changes to a height of 700mm in the lowfloor barrier-free section. in the interior above the bogies, the height of the floor is 940mm. the basic components of the vehicle’s traction equipment and auxiliary drives are comparable to the parameters used in metro vehicles. a comparison of the basic technical parameters of the vehicles is documented in table 1. in addition to operating on non-electrified regional lines, the bemu 74 dual-power vehicle with a maximum output of 750 kw (640 kw for traction, 110 kw auxiliary drives) should be operable under 3 kv dc, 25 kv/50hz ac traction systems at a maximum speed of up to 120 kmh−1. the vehicle concept is shown in figures 13 and 14. with regard to the requirements of the “green deal” and significant use of recuperation, it is advisable that all wheel-sets will be driven, i.e. use a vehicle with a b0’ b0’ driving arrangement. each wheel-set is driven by an asynchronous traction motor with a nominal power of 160 kw. the proposed electric ac power transmission will be powered by batteries (lto) with a minimum capacity of 130 kwh on the shorter nonelectrified sections of the track (range up to 50 km). for longer non-electrified sections of the track, 68 vol. 43/2023 modern vehicles for regional railways figure 13. concept of the bemu 74 – arrangement of electrical and pneumatic equipment for a two-power, three-system vehicle (ac 25 kv/50hz, dc 3 kv + battery). figure 14. concept of the bemu 74 – arrangement of the interior of the vehicle – space for immobile passengers. bemu 74 regio nove (814 + 914) vehicle length over bumpers [mm] 25 750 28 440 length of body vehicle [mm] 25 270 13 800 + 13 800 = 27 600 width of the vehicle body [mm] 2 950 3 073 number of fixed seats [–] 58 44 + 32 number of folding seats [–] 16 8 total seats [–] 74 84 maximum number of passengers 58 + 110 = 168 76 + 105 = 181(fixed seats + 4 per.m−2) [–] vehicle power [kw] 750 242 in 1 950min−1 transmission of traction power electric drive hydromechanical hyddrodynamic retarder + service brake electrodynamic pneumatic block brakes axle arrangement (a0a0)(a0a0) or b0’ b0’ a’1’ + 1’1’ maximal speed [kmh−1] 120 80 table 1. comparison of basic vehicle parameters bemu 74 and regionova. 69 josef kolář acta polytechnica ctu proceedings figure 15. concept of the hemu 74 – arrangement of electrical and pneumatic equipment. where the energy stored in the battery power-pack (nmc) would not be sufficient, an alternative solution of the hemu 74 vehicle is proposed, see figure 15. its basic drive unit located on the roof of the vehicle consists of an mtu power pack series 1 600 – 736 kw diesel combustion engine with two alternators (for traction and auxiliary drives), see figure 16, which could be modified to burn hydrogen. for the hemu 74 regional vehicle intended for more extensive operation on independent traction lines, it is more appropriate to use the concept of a hybrid vehicle (ice + aku) burning (biogas or hydrogen), as this type of hybrid drive can be better adapted to dynamic operational requirements. the indirect coupling of the ice with the traction wheel allows the ice to work in an optimal mode of operational consumption. the coverage of energy peaks during the moving off will and acceleration of vehicle ensure consumption from the accumulators. a hybrid vehicle with a fuel hydrogens modules (hypm hd 180) and electrical accumulators (li-on) represents greater costs for the fuel modules hypm (platinum design), which needs very clean hydrogen. the fuel hydrogen modules operate as a stationary source, it is poorly regulated (it does not tolerate dynamic changes in the operating load), it cannot recover electrical energy and must be supplemented with a more capacitive accumulator with a supercapacitator. the solution of the interior and the rough structure of the car-body for both variants of the bemu 74 and hemu 74 vehicles is the same. figure 16. mtu power pack series 1600 [15]. the ecological benefits of the proposed solution can be seen in the application of the operating electrodynamic brake (edb) and recuperation, which will reduce not only the energy demand for the driving cycle and therefore the price of fuel, but also ensure the minimal use of friction brakes only for braking and securing the vehicle, thereby significantly reducing dust and vehicle noise. from the performed traction calculations of an idealized driving cycle in a straight horizontal track on a track of 2 470m, it emerged that 35 to 40% of energy consumption can be saved by applying recuperation. acknowledgements this research has been realized using the support of the technology agency of the czech republic, programme national competence centres, project #tn01000026 josef bozek national center of competence for surface transport vehicles this support is gratefully acknowledged. 70 vol. 43/2023 modern vehicles for regional railways furthermore, the paper presents the results of my partial with cd company. references [1] idnes.cz. [2022-05-05], https: //www.idnes.cz/technet/technika/vlaky-dopravakolejove-autobusy-lokalky-motoraky-hurvinek. [2] cdnostalgie.cz. [2022-05-05], https://www.cdnostalgie.cz/vozidla/motorovevozy/-32488/. [3] j. kolář. konstrukční studie částečně nízkopodlažního kolejového autobusu s hybridním pohonem. in prorail 2015 – xxii. medzinárodná konferencia súčasné problémy v koľajových vozidlách – zborník prednášok diel i, pp. 267–274. 2015. žilinská univerzita, strojnická fakulta, žilina. [4] railtech.com. [2021-07-27], https://www.railtech.com/rolling-stock/2021/07/ 27/taxirail-future-for-regional-rail/. [5] railtech.com. [2022-05-05], https://www.railjournal.com/fleet/frenchpartnership-to-develop-light-autonomous-train/. [6] railjournal.com. [2021-07-27], https://www.railjournal.com/fleet/frenchpartnership-to-develop-light-autonomous-train/. [7] raulbus.com. [2022-05-05], https://railbus.com/vehicles/. [8] transportdesigninternational.com. [2022-05-05], https://transportdesigninternational.com/ portfolio/coventry-vlr. [9] skodagroup.com. [2022-05-05], https://www.skodagroup.com/cs/. [10] m. dynybyl. studie proveditelnosti autonomního tramvajového vozidla s dvounápravovým pojezdem. dp 2020, u 12 120, fs čvut, 2020. [11] c. schindler. the aachen rail shuttle ars – autonomous and energy self-sufficient feeder transport. journal of rail transport planning & management 21:100299, 2022. https://doi.org/10.1016/j.jrtpm.2022.100299 [12] transportdesigninternational.com. [2022-05-05], https://www.railway-technology.com/projects/ revolution-vlr-demonstrator-vehicle. [13] portal.cenia.cz. [2022-05-05], https://portal.cenia.cz/eiasea/download/ u0vbx01audi3metfbmf2cmhfmzkzodq4mjaxmtqymdy5njc0ns5wzgy/ mzp270k_navrh.pdf. [14] j. kolář. konstrukční studie částečně nízkopodlažního kolejového elektrobusu. in zborník prednášok i. – xxiv. medzinárodná konferencia – súčasné problémy v koľajových vozidlách, pp. 317–324. vts pri žu, žilina, 2019. [15] flickr.com. [2022-05-05], https: //www.flickr.com/photos/rrpowersystems/29935774845. 71 https://www.idnes.cz/technet/technika/vlaky-doprava-kolejove-autobusy-lokalky-motoraky-hurvinek https://www.idnes.cz/technet/technika/vlaky-doprava-kolejove-autobusy-lokalky-motoraky-hurvinek https://www.idnes.cz/technet/technika/vlaky-doprava-kolejove-autobusy-lokalky-motoraky-hurvinek https://www.cdnostalgie.cz/vozidla/motorove-vozy/-32488/ https://www.cdnostalgie.cz/vozidla/motorove-vozy/-32488/ https://www.railtech.com/rolling-stock/2021/07/27/taxirail-future-for-regional-rail/ https://www.railtech.com/rolling-stock/2021/07/27/taxirail-future-for-regional-rail/ https://www.railjournal.com/fleet/french-partnership-to-develop-light-autonomous-train/ https://www.railjournal.com/fleet/french-partnership-to-develop-light-autonomous-train/ https://www.railjournal.com/fleet/french-partnership-to-develop-light-autonomous-train/ https://www.railjournal.com/fleet/french-partnership-to-develop-light-autonomous-train/ https://railbus.com/vehicles/ https://transportdesigninternational.com/portfolio/coventry-vlr https://transportdesigninternational.com/portfolio/coventry-vlr https://www.skodagroup.com/cs/ https://doi.org/10.1016/j.jrtpm.2022.100299 https://www.railway-technology.com/projects/revolution-vlr-demonstrator-vehicle https://www.railway-technology.com/projects/revolution-vlr-demonstrator-vehicle https://portal.cenia.cz/eiasea/download/u0vbx01audi3metfbmf2cmhfmzkzodq4mjaxmtqymdy5njc0ns5wzgy/ mzp270k_navrh.pdf https://portal.cenia.cz/eiasea/download/u0vbx01audi3metfbmf2cmhfmzkzodq4mjaxmtqymdy5njc0ns5wzgy/ mzp270k_navrh.pdf https://portal.cenia.cz/eiasea/download/u0vbx01audi3metfbmf2cmhfmzkzodq4mjaxmtqymdy5njc0ns5wzgy/ mzp270k_navrh.pdf https://www.flickr.com/photos/rrpowersystems/29935774845 https://www.flickr.com/photos/rrpowersystems/29935774845 acta polytechnica ctu proceedings 43:62–71, 2023 1 introduction 2 regional railways – profitability, safety and filling of the goals of the project ``vision o'' 3 transport policy of the czech republic for the period 2021–2027 4 study of rail vehicle for regional transport in the czech republic acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.38.0255 acta polytechnica ctu proceedings 38:255–261, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague improving the properties of unburned earth to control the interior microclimate michal procházka∗, zbyněk zušťák, pavel svoboda czech technical university in prague, faculty of civil engineering, department of construction technology, thákurova 7/2077, 166 29 prague, czech republic ∗ corresponding author: prochmi2@fsv.cvut.cz abstract. the article concerns the problematics of unburned earth used as material for moulding formwork or clay plaster with lowered contraction, especially about interior unburned earth plaster with the heightened percentage of clay and therefore improved sorption abilities. by adding a plasticizer, it is possible to modify the fluidity of the mixture. with lessening the amount of water it is possible to lower the shrinkage. furthermore, we can increase the ratio of clay to sand and thus we can control the indoor microclimate. this is because the clay provides adsorption properties. unburnt clay has been widely used as a building material in the past around the world. in the last century, this material was rather despised. nowadays, however, this trend is reversing and the unburnt clay is experiencing a renaissance. increasingly people emphasize ecology, a healthy and balanced indoor climate, and demand buildings with low operational and energy intensity. all this can be offered by unburnt clay, because it is energy-efficient in production, easily accessible in almost all parts of the world, harmless, and beneficial to human health, as clay can contribute to the improvement of the internal microclimate to a much greater extent than other building materials, as confirmed by many scientific findings. keywords: earth, clay, microclimate. 1. introduction unburned earth can be defined as building material held together primary by clay. unburned earth is a common material in the building industry around the world. but the european building norms don’t know this material. unburned earth can be defined as soil or a mixture of soils with added ingredients suitable for building purposes which compose of fine-grained fractions of clay particles as the primary binder and dust particles, further composed of sand and gravel fraction used as filler. also other materials can be used in the mixture such as scattered reinforcement formed by a fibrous admixture, pigment and secondary binders and stabilizers, all used in the right amount. the primary source of the binding ability is clay materials. clay is also the main source of the positive features as sorption, especially the regulation of the indoor microclimate. depending on the physicochemical properties, it occurs mainly in the kaolinite group, the montmorillonite group and the illite group. these clay minerals are often forming mixed clay sediments. the structure of clay minerals is layered. it consists of layers of oxygen tetrahedra with a silicon atom in the middle and layers of oxygen octahedra with an aluminum atom in the middle and an interlayer space (see figure 1). water penetrates into this interlayer space, especially in the case of montmorillonibte, which causes swelling. swelling and thus shrinkage during drying is the lowest in kaolinite, higher in illite and highest in montmorillonite. if the water recedes from this intermediate layer, the electrostatic attractive forces, which are the essence of the binding ability of clay minerals, will increase. it is, therefore, not a chemical binder and the process is reversible. water is also important when moving the components of unburned earth, including mineral plates and their arrangement during drying. here the water can be partially replaced. kneading the mixture or vibration helps to ensure a good arrangement. this can reduce the required amount of water needed for the mixture. however, these procedures are mostly associated with the technology used, and they can often not be replaced or used at all, especially for plasters. in terms of soil classification according to čsn en iso 14688-2 [1], the result is most often soils sisa, clsa, grsisa, grclsa, grsasis, grsacls, sagrsis and sagrcls. for practical reasons, they are often mixed from cl, sicl, clsi soils in practice called clay or loess and sa, grsa soils in practice called mortar sand. for some techniques, gr, sagr in practice called gravel or gravel sand is added. we can thus determine the ratio of individual components. by choosing clay minerals and choosing the ratio of individual components, we influence the binding and shrinkage during drying. in terms of soil granularity according to čsn en iso 17892-4 [2], clay represents a fine-grained fraction up to 0.063 mm consisting of clay and dust particles, mortar sand represents a sand fraction from 0.063 to 2 mm consisting of fine, medium and coarse sand particles and gravel fraction. from 2 to 63 mm consisting of fine, medium and coarse gravel particles. granulometry in the fine-grained fraction is not interesting 255 https://doi.org/10.14311/app.2022.38.0255 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en m. procházka, z. zušťák, p. svoboda acta polytechnica ctu proceedings figure 1. structure of clay materials. because it does not say whether and what type of clay particle it is. granulometry together with the shape of the particles is very interesting for the sand and gravel fraction of the filler. its suitable curve can positively affect drying shrinkage and strength. sorption is a general term for absorption, adsorption. absorption is the sorption of one substance into another in its entire volume. adsorption is the binding of a gaseous or liquid substance to the surface of another substance. there are two types of adsorption. physical adsorption arises from van der waals attractive forces. chemisorption is stronger than physical adsorption, it is formed by chemical bonds. in the context of unfired earth or clay particles, we speak of physical adsorption. 1.1. ratios of primary components wo2005092816 (a2) entitled “use of polymer powder compositions which can be redispersed in water for loam construction materials” discloses a clay dose of 10 to 30 % by weight [3]. bricks, blocks, mortars for masonry, plaster or are poured or smoked into formwork are made of unburned earth. the amount of clay typically depends on the type and origin of clay or earth, the content of clay fraction or clay minerals, the type of clay minerals, their binding ability and their effect on shrinkage or swelling in contact with water, granulometry of other particles, but especially on the purpose the use of a clay mixture and the associated dose of mixing water. in the case of the use of unburned earth as clay plasters, the approximate proportion of clay is from 10 to 15 % by weight. the proportion from 15 to 30 % by weight is suitable for clay mixtures with a lower water content for stamping into formwork. the values of the proportion approximately above 30 % are suitable at most for mixtures for the production of bricks, where shrinkage does not carry out such a role, because shrinkage takes place outside the structure. the use of a suitable fibrous admixture such as cuttings, shives, etc. makes it possible to positively increase the doses of clay. the clay dose is determined according to the tests below. the basic binder in unburned earth is clay. pure clay has a high binding capacity. the minimum binding capacity of the mixture is stated by vlastimil havlíček and karel souček in the book: “buildings made of unfired clay” as 250 g/5 cm2 [4]. binding capacity is tested on 8-shape samples with defined moisture (see figure 2). however, the clay shrinks during drying due to the evaporation of water. this can cause cracks in the unburned earth during drying. therefore, it is necessary to add dust and sand or gravel particles to the clay as a filler which reduces the total shrinkage to a tolerable limit of approximately 2 % in length. at higher total shrinkage, for example, cracks begin to form in clay plasters, and separation may also occur from less adhesive substrates. when measuring shrinkage, havlíček and souček [4] start from a poor initial consistency. this corresponds broadly to the consistency for steamed clay. in accordance with the procedures from the book “building with earth” by minke [5], the measurement of shrinkage must be based on the initial consistency for processing according to the technique, and thus on the amount of water in the mixture, which is always given technologically. for clay plasters, gernot minke states a consistency with a pour of 180 mm according to din 1060-3, which it further reduces to 140 mm [5]. in this article, total shrinkage means shrinkage, which is based on determining the consistency of fresh mortar according to čsn en 1015-3 [6] and the prescribed pour value according to čsn en 1015-2 [7] and thus for mixtures over 1200 kg/m3 pour value 175 ± 10 mm. spill values are measured on a shaker table (see figure 3). this value corresponds to measurements performed in practice, where several observations and samplings from several craftsmen were made just before the plaster was applied. the measurement of total shrinkage is performed either according to havlíček and souček [4] in steel or wooden forms measuring 25 × 40 × 220 mm on marks 200 mm distant or according to minke [5] in metal forms of the same dimensions according to din 18952 on marks 200 mm distant or in metal forms size 40 × 40 × 160 mm provided in the faces with pins for measurement according to čsn en 12617-4 [8] or by measuring the resulting length dimension of formed beams from metal forms size 40×40×160 mm 256 vol. 38/2022 improving the properties of unburned earth . . . figure 2. test of binding capacity figure 3. manual flow table. 257 m. procházka, z. zušťák, p. svoboda acta polytechnica ctu proceedings figure 4. test of shrinkage. according to čsn en 1015-11 [9] (see figure 4). the total shrinkage is measured from the above initial consistency of 175 ± 10 mm corresponding to the processing state to the stabilization of the shrinkage at the equilibrium humidity state of the unfired clay. the shape of the form does not affect the result. total shrinkage is expressed in %. the ratio between clay and other particles is thus more or less dependent on the technologically given amount of water in the mixture. another is the dose of water and therefore the consistency for bricks and steamed clay, and it is different for screeds and plasters that need more water for processing and shrinkage causes more problems. the result is that the smallest ratio of clay to other particles is put into the plaster. although this reduces the overall shrinkage, which is essential especially for plasters, it also reduces the binding ability or bonding and reduces other positive effects of the clay. so it is always a compromise between shrinkage and bonding. this compromise can be improved by suitable granulometry. clay in plasters is also a carrier of positive properties such as the adsorption of odours, dusts, allergens, but especially air humidity. moisture is adsorbed by the clay in clay plasters when there is an excess of ambient air and when it is deficient in the ambient air it is evaporated again. thus, clay plasters regulate air humidity in rooms. it is therefore desirable to produce plasters with a higher clay content, which is currently not possible due to shrinkage and process ability. 1.2. ingredients and admixtures further improvements can be achieved by replacing part of the filler with a fibrous admixture. this helps in addition to shrinkage reduction and a better and more even distribution of cracks. at the same time, it increases compressive and bending strengths which is especially desirable for plasters. fibres up to approx. 4 cm are suitable for plasters. higher doses of fibrous admixtures can impair processability. longer fibres and larger quantities can be used especially for lightweight structural or filling clay or for levelling layers. dosing is usually performed by volume. in laboratory conditions dosing is often complicated by the size of the forms. other additives such as secondary or stabilizing binders can be added to unburned earth, which can reduce the reaction to the effects of water or improve the strength characteristics or increase the adhesion to the substrate, as stated in cz document pv 2008663 entitled “unburned earth stabilized with polymers” [10]. bc. kateřina nováková mentions the use of plasticizers in her diploma thesis “research binding capacity of unburnt earth in time” [11] or fluxes that reduce internal friction. however, specific plasticizers are not mentioned, and reference is made in the literature below. the issue of processability of clay as a raw material for the production of fired ceramic goods is well known in the ceramic industry. certain analogies can be found between the ceramic and clay industries, but the nomenclature, composition of the clay and use differ. as gernot minke states on page 39: “in the ceramic industry, fluid thinning mediums are used to attain higher liquidity, thereby allowing less water to be used (in order to reduce shrinkage). typical thinning media are sodium waterglass (na2o · nsio2), soda (na2co3), and humus acid and tannic acid. tests conducted at the brl at the university of kasselshowed that these methods were of very little relevance to earth as a building material. but tests with whey were successful.” [5]. in the field of clay, cheap natural substances are traditionally used to reduce shrinkage, as stated by ivana žabičková in the book “hliněné stavby” on page 31: “je známo též užití koňské moči, která účinně eliminuje trhlinky a zvyšuje odolnost proti erozi.” [12] which means in english: it is also known to use horse urine, which effectively eliminates cracks and increases resistance to erosion. in the book “hliněné stavby novej generacie” [13] on page 83, petr suske mentions the use of recesses to reduce internal friction. 258 vol. 38/2022 improving the properties of unburned earth . . . 1.3. chemical additives for building materials in the concrete industry plasticizers are used to reduce internal friction. the highest effect is achieved with polycarboxylate-based plasticizers. tests of their use for unfired clay have shown their unsuitability. therefore, it is assumed that unfired clay cannot be combined with concrete plasticizers. 2. solutions the above-mentioned disadvantages of unburnt earth are solved by the unburnt earth with controlled shrinkage according to the invention “unfired clay with controlled shrinkage.”. according to it, the mixture contains 95 to 99.99 % by weight. unburnt earth and 0.01 to 5 wt. at least one plasticizer selected from the group: lignosulphonate-based, naphthalene-based and melamine-based [14]. 2.1. unburned earth for casting in the first step, using the addition of a plasticizer, we can increase the plasticity or flow ability of the unfired clay without increasing the shrinkage during drying. this can be used wherever the shape of unfired clay adds formwork. the process for producing unburned controlled shrinkage earth for casting is as follows: first, a dry mixture of unburned, shrinkage-controlled earth containing from 10 % to 30 % by weight is produced. clay by determining the proportion of clay in the unfired clay as a maximum in the range of 10 to 30 % by weight in the first step of the total shrinkage test, while maintaining the total shrinkage of the unburned earth up to 2 %, then in the second step by the fresh mortar consistency test using of the shaking table determines the optimal dose of plasticizer in the range of 0.01 to 5 % by weight, at which the spillage of unburned earth is maximum and in the third step the dose of plasticizer determined in the second step is made up to 100 % by weight. unburned earth with a clay content determined in the first step and into the resulting mixture in an amount ranging from 66.7 to 90 % by weight. water is mixed in an amount in the range of 10 to 33.3 % by weight, determined by a consistency test using a shaker table, so that the amount of water added to the original mixture from the first step corresponds to a pour of 175 mm [14]. 2.2. unburned earth for plasters with reduced shrinkage in the next step, in addition to the addition of the above-mentioned dose of plasticizer, we can reduce the amount of mixing water so that the consistency again corresponds to the plaster. this will create a plaster with reduced shrinkage during drying. the process for the production of unfired controlled shrinkage clay for plasters with reduced shrinkage is as follows: first, a dry mixture of unburned earth with controlled shrinkage containing from 7.5 % to 20 % by weight is produced. clay by determining the proportion of clay in the unburned earth as a maximum in the range of 7.5 to 20 % by weight in the first step of the total shrinkage test, while maintaining the total shrinkage of the unfired clay up to 2 %, then in the second step by the fresh mortar consistency test using a shaking table, the optimum dose of plasticizer is determined in the range of 0.01 to 5 % by weight at which the spillage of unburned earth is maximal and in the third step the dose of plasticizer determined in the second step is made up to 100 % by weight. unburned earth with the proportion of clay determined in the first step and in the resulting mixture in an amount ranging from 66.7 to 90 % by weight. admixed water in an amount in the range from 10 to 33.3 % by weight so that the shake table consistency test shows a spillage of 175 mm [14]. 2.3. unburned earth for plasters with increased sorption in the last step, the addition of the above-mentioned dose of plasticizer and the reduction of mixing water, which would otherwise cause a reduction in shrinkage, can be used to increase the ratio of clay to sand so that shrinkage is maintained. this will cause a plaster with increased sorption. clay is the main carrier of sorption abilities. the plaster thus helps to clean the air and especially to regulate the humidity microclimate. the process for the production of unfired controlled shrinkage clay for plasters with increased sorption is as follows: first, a dry mixture of unburned, shrinkagecontrolled earth containing from 20 % to 50 % by weight is produced. clay by determining the proportion of clay in the unburned earth as a maximum in the range of 19.99 to 49.99 % by weight in the first step of the total shrinkage test, while maintaining the total shrinkage of the unburned earth up to 2 %, then in the second step by the consistency test fresh mortar using a shaker table determines the optimal dose of plasticizer in the range of 0.01 to 5 % by weight, at which the spillage of unfired clay is maximum, then in the third step the total shrinkage test is performed on the mixture of unburned earth according to the first step and plasticizer according to the second step, determines the increased proportion of clay as a maximum in the range of 20 to 50 % by weight, while maintaining the total shrinkage of this mixture up to 2 %, and in the fourth step the dose of plasticizer determined in the second step is made up to 100 % by weight. unburned earth with an increased proportion of clay, determined in the third step and into the resulting mixture in an amount ranging from 66.7 to 90 % by weight admixes water in an amount in the range from 10 to 33.3 % by weight so that the shake table consistency test shows a spillage of 175 mm [14]. 2.4. results of experiments figure 5 shows an approximate behavior curve of a melamine-based plasticizer in a clay mixture. 259 m. procházka, z. zušťák, p. svoboda acta polytechnica ctu proceedings figure 5. evaluation of the effect of melamine-based plasticizer [15]. figure 6. shrinkage of clay mixtures [15]. the k mixture is a control mixture, with excessive shrinkage and therefore with an excessive dose of clay. commonly we would reduce the ratio of clay to sand. but this is not desirable from the point of view of the microclimate. the pl00 mixture has a lower water dose and a melamine-based plasticizer is metered into the other pl mixtures. the reaction of the clay plaster mixture to the gradual addition of a melamine-based plasticizer can be observed on the blue curve. figure 5 shows that the plasticizer had the highest efficacy at a dosage of 0.2 %. after optimizing the amount of mixing water, with a final spill of 184 mm, we can observe a decrease of 15 % [15]. in figure 6, the mixtures are assigned the appropriate shrinkage in percent. the maximum acceptable shrinkage for earthen plasters is 2 %. the results show the possibilities to increase the fluidity of the mixture, further reduce water by 15 % and reduce shrinkage from 4.38 % to 1.88 %. this can be used to increase the clay to sand ratio and thus control the interior microclimate. 3. conclusion unfired clay is widely used in ecological construction. it is suitable for the preparation of unburned earth for casting and plaster. it is then used in the production of external and internal plasters with reduced shrinkage during drying. it will then use the unique properties in the production of internal plaster with increased sorption. this makes it possible to better regulate the cleanliness and humidity of the indoor air and thus have a beneficial effect on the microclimate. the specific resulting effect on the interior microclimate will be the subject of further research. however, it is already certain that the ratio of clay to sand can be increased without increasing shrinkage, which is limiting for clay plasters. acknowledgements we would like to thank the co-author of the patent pavlína znamenáčková, who contributed to the creation of the unfired clay with controlled shrinkage patent with her laboratory work. 260 vol. 38/2022 improving the properties of unburned earth . . . references [1] čsn en iso 14688-2, geotechnický průzkum a zkoušení – pojmenování a zatřiďování zemin – part 2: zásady pro zatřiďování, únmz, praha, 2005. [2] čsn en iso 17892-4, geotechnický průzkum a zkoušení – laboratorní zkoušky zemin part 4: stanovení zrnitosti, únmz, praha, 2005. [3] wacker polymer systems gmbh & co. kg. use of polymer powder compositions that can be redispersed in water for loam construction materials. patent. inventors: h. lutz, a. sommerauer, k. bonin. priorities: 2004-03-18. mpt: c04b28/00, c04b24/26, c04b24/38. application: de200410013468. published as: wo2005092816. german patent and trade mark office. [4] v. havlíček, k. souček. stavby z nepálené hlíny. státní zemědělské nakladatelství, praha, 1958. 98 p. [5] g. minke. building with earth. birkhäuser, berlin, 2006. 208 p. isbn 978-3-7643-7873-8. [6] čsn en 1015-3, zkušební metody malt pro zdivo – part 3: stanovení konzistence čerstvé malty (s použitím střásacího stolku), únmz, praha, 2000. [7] čsn en 1015-2, zkušební metody malt pro zdivo – part 2: odběr základních vzorků malt a příprava zkušebních malt, únmz, praha, 1999. [8] čsn en 12617-4, výrobky a systémy pro ochranu a opravy betonových konstrukcí – zkušební metody – part 4: stanovení smršťování a rozpínání, únmz, praha, 2003. [9] čsn en 1015-11, zkušební metody malt pro zdivo – part 11: stanovení pevnosti zatvrdlých malt v tahu za ohybu a v tlaku, únmz, praha, 2000. [10] m. procházka, p. svoboda. unfired clay stabilized with polymers. patent. inventor: m. procházka. priorities: 2008-10-23. mpt: b28c1/04, c04b14/10, c04b24/24. application: cz2008663a. published as: cz302139b6. industrial property office. [11] k. nováková. research binding capacity of unburnt clay in time. master’s thesis, czech technical university in prague, praha, 2017. [12] i. žabičková. hliněné stavby. era, brno, 2002. 174 p. isbn 8086517217, 9788086517216. [13] p. suske. hlinené domy novej generácie. alfa, bratislava, 1991. 159 p. isbn 8005008945, 978-8005008948. [14] czech technical university in prague. unfired clay with controlled shrinkage. patent. inventors: m. procházka, p. znamenáčková. priorities: 2018-03-22. mpt: c04b14/10, c04b24/18, c04b24/12. application: cz2018143a. published as: cz307890b6. industrial property office. [15] p. znamenáčková. využití plastifikátorů při výrobě hliněných omítek. master’s thesis, czech technical university in prague, praha, 2017. 261 acta polytechnica ctu proceedings 38:255–261, 2022 1 introduction 1.1 ratios of primary components 1.2 ingredients and admixtures 1.3 chemical additives for building materials 2 solutions 2.1 unburned earth for casting 2.2 unburned earth for plasters with reduced shrinkage 2.3 unburned earth for plasters with increased sorption 2.4 results of experiments 3 conclusion acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2025.53.0108 acta polytechnica ctu proceedings 53:108–113, 2025 © 2025 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague rehabilitation of cement concrete pavement structures at bratislava airport andrea zuzulová∗, dominika glasnáková slovak university of technology in bratislava, faculty of civil engineering, department of transportation engineering, radlinského 11, 810 05 bratislava, slovak republic ∗ corresponding author: andrea.zuzulova@stuba.sk abstract. the pavements on the movement areas of the airport are a specific group of pavements, due to their characteristics, different traffic loads and the time constraints for their repairs and maintenance. increased requirements are on the proposed repairs already from the mentioned time point of view, as well as on the lifetime, which makes these repairs more economically demanding. maintaining the optimal and efficient operation of the airport is primarily achieved by appropriate rehabilitation and innovation of the necessary parts of the airport. this was also subject to the replacement of the axial strips of the runway, part of the apron and the junction of the runways. the paper deals with the preparation, technical solution and implementation of these repairs with a description of individual technical parameters. keywords: cement concrete pavements, runway, reconstruction, joint, pavement structure, slab, paving. 1. design of pavements the design of pavement structure is as a set of several activities related to the design of road construction, dimension and model calculations. this includes calculations of load effects, taking into account the properties of the materials, the subgrade conditions, and the climatic conditions. the development of design methods of pavement structure can easily be described as a development from empirical methods to analytic-theoretical methods. this construction was replaced with a two-layer, later threeand multilayer system and with a mathematical solution to calculations of stresses and strains in the layer system was applied. in analytical design methods, individual pavements layer of different materials is characterized by deformation properties, the layer system being laid on the linear-elastic half-space. it is assumed that the layers are of the homogeneous and isotropic material. for the calculation of stresses and strains in the individual layers of the physical model – which are subject to assessment, we apply the theory of the layered half-space. the calculation program laymed is part of the used design method. the analytical design method used is a complex system. input data includes traffic load (calculate an equivalent number of design axles), classification of the water and temperature regime of the subgrade, characteristics of climatic conditions and properties of the material layers – their strength and fatigue. three criteria are used to assess the pavement design: • ratio strength of bounded materials and radial stresses at the bottom of the layer, • subgrade stability as the ration of vertical stresses and compressive strength, • protection against frost penetration. in the moderate climate zone, the ratio of the stress and strength of bounded material in the critical layer will be decisive. pavements with cc cover have properties other than asphalt pavements, behave differently and other procedures and criteria in the calculation of stresses and strains are used [1]. 2. bratislava airport the runway system of the airport are two perpendicular take-off and landing runways and taxiways (figure 1), which enable arrivals and departures for critical aircraft (critical aircraft b 747-400). runway 13-31, which is the main runway, is 3 190 m long and 45 m wide. it is equipped with a system of lighting and navigation devices enabling accurate approach under iii a category conditions. runway 04-22 is 2 900 m long and 60 m wide. it is equipped with lighting and radio-navigation devices for accurate approach under category i conditions. apron, as an integral part of every airport, must have the required functions in order to achieve the safety, smoothness and efficiency of airport operations. these requirements are ensured if all regulations and policies for the relevant type of airport and critical aircraft are fulfilled. all these surfaces have a cement concrete surface. with development of the airport, investment priorities were ranked, while the reconstruction and expansion of the passenger terminal took first place in 2012 in terms of time and capacity. in 2016, the process of implementing operational safety management system continued – safety management system. the gradual 108 https://doi.org/10.14311/app.2025.53.0108 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 53/2025 rehabilitation of concrete pavement structures at bratislava airport figure 1. runway system of bratislava airport. figure 2. cc slabs replacement plan for runway 13-31. implementation of this system is to maintain an efficient and functional environment for the high-quality and safe performance of all activities related to the operation and maintenance of aircraft at bratislava airport. the runways required rehabilitation, which the airport gradually built over five years. in the years 2016–2018, the airport operator performed the repair of runway 13-31, when the axial cement concrete slabs of the cover were replaced [2]. another reconstruction was planned in 2020, in the most complicated place – the junction of the runways, which was finally postponed for a year with the with the beginning of the pandemic. the reason for the postponement was the need for the airport to be operational during the crisis period and its importance in the critical transport infrastructure of the state. in mid-april 2021 with the one-year delay, the airport began the planned reconstruction of the runway junction. the work was realized 24 hours a day. for this reason, the airport was completely closed for all arrivals and departures, only the helipads and the apron remained in operation. 3. replacement of axial cement concrete slabs on runway 13-31 regular maintenance of the runways at the bratislava airport, which is understood as the sum of activities that keep the pavements in operable condition in all weather conditions, is usually carried out before and after the summer season in order to ensure the safety and smoothness of air traffic. as part of the planned spring maintenance of operational areas, maintenance of operational areas, repair of dents and cracks, reconstruction of 13-31 – i. stage, replacement of emergency cc slabs at the junction of tracks and at 109 andrea zuzulová, dominika glasnáková acta polytechnica ctu proceedings the side asphalt strip were included replacing gutters, repairing gutters in concrete gutters, removing weeds and plants from gutters and lights and fixing lights. degradation of runway 13-31 in terms of technical and operational status continued at larger repairs to ensure the required operational safety in the future. as part of these measures, the replacement of the upper layer of the cement-concrete pavement cover was proposed (figure 2). the replacement concerned the two axial strips of the runway, which are the most heavily loaded and are in direct contact with the main landing gear of the aircraft. the length of the repaired section was 604 m in km 1 159 and the total area of the repair was 9 086 m2 [3]. runway 13-31 is divided in the longitudinal direction into eight strips marked from a to h (figure 3). the strips marked a and h have an asphalt cover, and the other strips b to g have a cement concrete cover. the proposed repair concerned the replacement of the cement-concrete pavement cover together with the asphalt intermediate layer in two lanes in the runway axis marked d and e with a total width of 15 m. figure 3. strips of cc slabs on runway 13-31. according to the project from diagnostics and exploratory drilling, the thickness of individual pavement layers on the repaired section of runway 13-31 was assumed. the construction of the pavement along the entire length of the track is not uniform, but consists of different types. runway 13-31 is divided in terms of composition and pavement quality into five homogeneous sections, while the repair concerned part of section 3a. a two slab construction was found, where the lower cc slab was realized in the 1950s and the upper cc slab in the 1980s. a new composition of the pavement structure was proposed. the change is in the top two layers. the other layers of the pavement were left in their original state. the transverse and longitudinal joints of the cement concrete cover were reinforced with dowels at a distance of 250 mm and tie bars of 4 per length of the slab, placed in one plane, parallel to the pavement surface and perpendicular to the joint [4–9]. the work procedure consisted of cutting transverse and longitudinal joints at the junction of slabs c-d and e-f, the existing tooth was removed in the longitudinal joint of slabs c-d, in the longitudinal joint of slabs e-f the existing groove was concreted together with cc slabs. subsequently, the area of cc slabs with an asphalt intermediate layer was demolished. after removing the layers, a different cross-section of the lower cc slabs and the upper cc slabs than was considered in the project was found. for this reason, the project was changed and the number of longitudinal joints increased by two, on each side from the runway´s axis at a distance of 3.0 m. a bond coat spray was applied to the prepared surface and an asphalt intermediate layer with a thickness of 30 mm was laid. subsequently, the concreting of the cementconcrete cover was using a paver in two strips from the end of the section. strip d was implemented first, followed by strip e. transverse and longitudinal joints were reinforced. in the places of the transverse working joints and the longitudinal joint at the junction of slabs d and e, dowels and tie bars were inserted into pre-drilled holes. after paving, transverse and longitudinal joints were continuously cut, pre-sealed and then filled with asphalt sealing (figure 4). after the completion of the construction works, the traffic markings were restored according to the original markings before the repair. figure 4. paving of cement concrete slabs on runway 13-31. 4. reconstruction of the pavement on the apron the apron at the airport were built and expanded gradually according to the demands of air traffic, which results in different types of construction of cement concrete slabs, with variable thickness of individual pavement layers. the handling areas underwent 110 vol. 53/2025 rehabilitation of concrete pavement structures at bratislava airport a significant expansion in the 1980s and later, to a lesser extent, also after 2000. the degradation of the technical and operational condition of the apron, and especially the taxiway, reached a level that required reconstruction. another requirement for the reconstruction was the innovation of the necessary parts of the airport in order to maintain the optimal and efficient operation of the airport, considering the change of the critical aircraft (b 747-400). the apron is divided into two parts from the point of view of operation – the taxiway and stands for check-in and parking of aircraft (figure 5). the mentioned part of the apron fundamentally affects the operation of the airport and the process of aircraft check-in, as they are the busiest stands and at the same time the taxiway allows aircraft taxiing even in low visibility conditions. the reconstruction of part of the apron increased the safety and technical operating conditions. figure 5. apron of bratislava airport. the realized part of the building is bordered on one side by a grass area and on the other side by a monolithic drainage launder. the reconstruction of the building was divided into two parts of construction. the first section is 160.5 m long and the second section is 273.0 m long [10]. based on the results of diagnostics and construction works, the composition of the cc pavement built in the 1980s was verified as follows: • cement concrete 240–260 mm • asphalt concrete 25–40 mm • cement stabilization 50–170 mm • gravel 230–400 mm on the mentioned section, the pavement structure was designed for the new critical (design) b747400 aircraft, the maximum total weight of which is 397 800 kg [4–9]. the aircraft has four double tandem landing gear. the cement concrete pavement cover is made as a single layer. the dimensions of the slabs are designed to be 5.0 m in the longitudinal direction up 4.5 to 5.5 m in the transverse direction (figure 6, figure 7). on the solved traffic surface, all joints are considered as transverse. the reinforcement of the transverse joints is with dowels ø 30 mm and a minimum length of 500 mm at a mutual axial distance of 500 mm. the connection of the new cc cover to the existing cc cover is solved as an expansion joint, reinforced with dowels ø 30 mm and min. 500 mm long at a mutual axial distance of 500 mm, with an assessment of the need for dilation and the need to guide the displacements of part of the surface (strips) due to climatic changes [11]. between the cc cover and the base layer, there is a separation intermediate layer made of geotextile, which serves to prevent the copying of cracks. figure 6. jointed plain on the reconstructed part of the apron. figure 7. realization of cement-concrete pavements on the apron. 5. reconstruction of the junction of runways 04-22 and 13-31 the reconstruction of the junction was 11 days, from april 14 to april 24, 2021 (figure 8). during the closure of the airport in that period, construction work was taking place at the junction of the runways, such as demolition work, removal of the cement concrete 111 andrea zuzulová, dominika glasnáková acta polytechnica ctu proceedings figure 8. junction runway 04-22 and runway 13-31. cover, cutting of slabs, laying of a new cement concrete cover, installation of axis lighting or spraying of daytime markings. the reconstruction of the runway junction was in three parts. in the first week, preparatory work was on runway 04-22, which included the demolition of cement concrete slabs. during this period, the airport was closed for 6 hours at night, but this did not affect regular flights. in the final third part of the reconstruction, joint fillings were completed during night time, which also did not affect air traffic. the system of take-off and landing runways (rw 1331 and rw 04-22) was built gradually with a different type of construction of cement concrete slabs and variable thickness of individual pavements. the rw 13-31 has a two slab construction along its entire length, while the upper slab has a variable thickness of cc slab 200 up to 320 mm, the lower slab has a variable thickness up 220 to 260 mm. the rw 04-22 has a two slab construction along its entire length, while the upper slab has a variable thickness of cc slab up 200 to 260 mm, the lower slab has a variable thickness up 240 to 300 mm. according to the results of diagnostics and exploratory core drilling at the crossing, the pavement construction was determined in the following composition: • cement concrete 200–260 mm • asphalt mixture 25–40 mm • cement concrete 220–260 mm • cement stabilization 50–170 mm • gravel 500 mm the replacement of the upper cover of the cc pavement was realized in two lanes in the runway axis at the junction of both runways (figure 8). the building object on rw 13-31 starts at km 1 893 00 in the direction from threshold 31 and the end of the section is at km 2 199 00 (length 306.68 m, area 4 600.19 m2, 84 pcs of slabs). the main runway was built continuously. the building object on rw 04-22 starts at km 1.288 00 in the direction from threshold 04 and the end of the section is at km 1.592 00. it is divided into two sections (i and ii), the first section is 141.90 m long (area 2 131.08 m2, 42 slabs) and ii. the section is 146.62 m long (area 2 201.79 m2, 44 slabs) [12]. the upper existing cement concrete slab was also demolished with an intermediate layer of asphalt mixture 15 m wide (two axial slabs) with an assumed average thickness of 270 mm [13]. the cutting also removed the existing connections between the slabs. transverse joints at the beginning and end of the reconstructed section were also cut. the demolished material was taken to the landfill. at the end of the demolition work, the entire surface of the bottom slab was cleaned. subsequently, a separation layer of geotextile was laid (area weight 1 000 g m−2) and a new cement concrete cover with a thickness up 225 to 300 mm was laid. the asphalt intermediate layer was replaced by a separation geotextile. the transverse and longitudinal joints of the cement concrete cover are reinforced. the transverse joints are reinforced with ø 30 sliding dowels at a distance of 250 mm from each other, and the longitudinal joints with ø 16 tie bars, 800 mm long, in the number of 4 per length of the slab. the repair of the runway 112 vol. 53/2025 rehabilitation of concrete pavement structures at bratislava airport junction cost the airport 2.27 million euros without vat (figure 9). figure 9. rehabilitation of the junction of the rw 04-22 and rw 13-31. 6. conclusion with this rehabilitation, the airport achieved an improvement in the operational characteristics of runways and the apron. the intention of the airport in the near future is to deal with ensuring the reliability and operability of the runways by their complex reconstruction in their entire extent (over the entire width and the entire length), or by the construction of a new runway. acknowledgements this paper was processed within the project vega 1/0463/24 “implementation of innovative solutions and technologies in a systemic approach to pavement design”. references [1] d. hodakova, a. zuzulova, s. capayova. climate change adaptation in pavement design. iop conference series: materials science and engineering 1252(1):012017, 2022. https://doi.org/10.1088/1757-899x/1252/1/012017 [2] bratislava airport. [2024-2-14]. https://www.bts.aero/ [3] a. zuzulová, d. hodáková, b. puk. obnova letiskových vozoviek na letisku bratislava [in slovak; renewal of airport roads at the airport bratislava]. in letiskové vozovky 2018. kongres studio, 2018. isbn 978-80-89565-34-4. [4] projektování silnic a dálnic [in czech; design of highways and motorways]. standard, czech standard institute, prague, 2018. [5] stavba vozovek – cementobetonové kryty – část 1: provádění a kontrola shody [in czech; road building – concrete pavements – part 1: construction and conformity assessment]. standard, czech standard institute, prague, 2014. [6] cementobetonové kryty – část 3: specifikace pro kluzné trny [in czech; concrete pavements – part 3: specifications for dowels to be used in concrete pavements]. standard, czech standard institute, prague, 2006. [7] navrhovanie cementobetónových vozoviek na cestných komunikáciach [in slovak; designing cement-concrete pavements on roads]. standard, slovenská správa ciest, bratislava, 2015. [8] meranie a hodnotenie drsnosti vozoviek pomocou zariadení skiddometer bv11 a profilograph ge [in slovak; designing measurement and assessment of pavement roughness using skiddometer bv11 and profilograph ge cement concrete pavements on roads]. standard, slovenská správa ciest, bratislava, 2022. [9] cementobetónové kryty vozoviek [in slovak; concrete pavements]. standard, ministerstvo dopravy a výstavby sr, bratislava, 2019. [10] project documentation for building construction (rekonštrukcia – rolovací pás, apron – ii. etapa), 2019. [11] a. zuzulová, d. hodáková, b. puk, m. abík. rekonštrukcia vozovky apronu na letisku m. r. štefánika v bratislave [in slovak; reconstruction of the road apron at the airport m. r. štefánik in bratislava]. in letiskové vozovky 2021. kongres studio, 2021. isbn 978-80-89565-46-7. [12] project documentation for building construction. (rekonštrukcia križovatky vpd 13 – 31 & vpd 04 – 22), 2019. [13] a. zuzulová, d. hodáková, b. puk, m. abík. rekonštrukcia križovatky vzletových a pristávacích dráh 04-22 a 13-31 na letisku m. r. štefánika v bratislave [in slovak; reconstruction of the intersection of runways 04-22 and 13-31 at the airport m. r. štefánik in bratislava]. in letiskové vozovky 2021. kongres studio, 2021. isbn 978-80-89565-46-7. 113 https://doi.org/10.1088/1757-899x/1252/1/012017 https://www.bts.aero/ acta polytechnica ctu proceedings 53:108–113, 2025 1 design of pavements 2 bratislava airport 3 replacement of axial cement concrete slabs on runway 13-31 4 reconstruction of the pavement on the apron 5 reconstruction of the junction of runways 04-22 and 13-31 6 conclusion acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2024.49.0013 acta polytechnica ctu proceedings 49:13–19, 2024 © 2024 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague fractional order models of viscoelastic polymeric solids undergoing large deformations barbora hálkováa,∗, michal benešb a czech technical university in prague, faculty of civil engineering, department of mechanics, thákurova 7, 166 29 prague, czech republic b czech technical university in prague, faculty of civil engineering, department of mathematics, thákurova 7, 166 29 prague, czech republic ∗ corresponding author: barbora.halkova@fsv.cvut.cz abstract. we present a fractional order model for nonlinear visco-hyperelastic solids taking into account large deformations. a three-field form of the hu-washizu principle is introduced to create a stable finite element method in the context of nearly incompressible dynamics. the β-method (a generalized midpoint rule) for time discretization is implemented into a variational finite element framework for efficient computing of numerical approximations to the initial boundary-value problem for hyperbolic equation of motion. finally, a 2-d cantilever beam problem with a step end load is considered in order to demonstrate the algorithm. keywords: fractional viscoelasticity, large deformations, springpot, fractional calculus, finite element, integration algorithm. 1. introduction many polymers exhibit time dependent behavior somewhere between purely elastic and purely viscous materials. as a result, a large number of kelvin-voigt or maxwell elements (and thus a large number of material parameters) are needed to be identified from experimental data to obtain a reasonably accurate description of mechanical response. on the other hand, a fractional calculus, i.e. the theory of derivatives and integrals of non-integer order, seems to be an efficient tool for the theoretical modelling of viscoelastic materials [1]. theoretical models based on the fractional calculus allows us to describe viscoelastic materials with significantly less parameters than the standard approach. for a deeper discussion on this issue we refer the reader to [2]. the fractional viscoelastic model at small strains was introduced e.g. in [3–7]. on the other hand, although rubbery polymers typically exhibit large deformations in engineering applications, much less attention has been given to fractional viscoelasticity in combination with the finite strain theory [8]. the present work provides a computational framework for modelling the fractional viscoelastic behaviour of polymeric solids at finite strains in the context of nearly incompressible dynamics. let the open set ω0 be the reference configuration of a given (compressible or nearly incompressible) body at time t0. here, ω0 is described by a set of continuously distributed points x (particles or material points) which occupy a region within the euclidean space e3. in the absence of displacement discontinuities, a one-to-one deformation map ϕ : ω0 × [0, t ] → e3 describing a motion exists, figure 1. motion of body ω0. such that any displaced position at a current time t ∈ [0, t ], where [0, t ] ⊂ r+ denotes the time of interest, is determined as x = ϕ(x, t) with the difference being the displacement field u(x, t) = x − x. here, the position vector x = (x1, x2, x3) represents the particle x in the reference configuration ω0, x = x1e1 +x2e2 +x3e3, where (e1,e2,e3) defines an orthogonal base system with origin 0. hence, we have x = ϕ(x, t) = x + u(x, t), see figure 1. the deformation gradient f (x, t) is obtained as the gradient of this deformation map f = ∇0ϕ, where ∇0 is the gradient with respect to x, and the jacobian of the deformation map is given by j(x, t) = det f . we also define the right cauchy-green deformation tensors c(x, t) = f t f . let ω0 be the reference configuration of the body of interest with the boundary ∂ω0 and t > 0 be the time horizon. let ∂ωu, ∂ωp be smooth open disjoint subsets of ∂ω0 such that ∂ω0 = ∂ωu ∪ ∂ωp , ∂ωu ̸= ∅ 13 https://doi.org/10.14311/app.2024.49.0013 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en barbora hálková, michal beneš acta polytechnica ctu proceedings and ∂ωu ∩∂ωp = ∅. let f0 be the body force per unit of mass, a given vector field defined on ω0 × (0, t ), u0 and v0 : ω0 → r3 be the prescribed initial position and velocity. further, let t̆0 be the given traction prescribed on ∂ωp × (0, t ) and let the displacement ŭ0 be prescribed on ∂ωu × (0, t ). the local form of the initial boundary value problem for the momentum equation is given by the following system [9]: ρ0 ∂2u ∂t2 = ∇0 · p + f0 in ω0 × (0, t ), (1) u = ŭ0 in ∂ωu × (0, t ), (2) p · n0 = t̆0 in ∂ωp × (0, t ), (3) u(·, 0) = u0 in ω0, (4) ∂u ∂t (·, 0) = v0 in ω0, (5) where n0 is the field normal to ∂ωp , p = p (x,f (x, t)) is the first piola-kirchhoff stress. 2. constitutive relationships 2.1. hyperelastic material in this contribution, we are interested to solve the system (1)–(5), in the context of nearly-incompressible material behavior (rubber like materials), which requires a special numerical treatment – like mixed methods. in particular, the deformation is split into a volumetric part represented by j and an isochoric part of the right cauchy-green deformation tensor ĉ, ĉ = j− 2 3 c. as a consequence, the split permits a different treatment of the incompressible part. the deformation gradient f together with its conjugate first piola-kirchhoff stress measure p , will be retained in order to defined the basic material relationship. the hyperelastic constitutive equation can be generally expressed as: p (f ) = ∂w (f ) ∂f . (6) typically for mixed methods [10, 11], the stored energy function w is additively decomposed into distortional part ŵ and dilatational part u , namely: w (f ) = ŵ (c) + u(j). (7) recall that c = f t f and j = det f . a traditional nearly incompressible neo-hookean potential ŵ (c) = 1 2µ(trĉ − 3) and a simple volumetric function u(j) = 1 2κ(j − 1)2 are used in this work, where µ is the shear modulus and κ denotes the bulk modulus. these assumptions give us the possibility to decompose the stress tensor into pure shear and bulk responses. the calculation is straightforward, we get: p (f ) = 2f ∂ŵ ∂c + 2f ∂u ∂j ∂j ∂c (8) figure 2. the standard viscoelastic model. and ∂j ∂c = √ det c ∂c = 1 2jc−1. (9) to account properly for nearly incompressible material response and to avoid difficulties concerning “locking” of the finite element procedure, we employ a three-field de veubeke-hu-washizu principle [10]. additional variables entering the mixed three-field formulation represent a strain variable θ which is equivalent to j : θ = j, (10) and the hydrostatic pressure: p = ∂u ∂j ∣∣∣∣ j=θ . (11) in the weak formulation of our problem and the subsequent finite element procedure, introduced hereafter, equations (10) and (11) are satisfied in a weak sense. 2.2. fractional viscoelasticity in a viscoelastic model the stress depends not solely on the current strain (elastic model), but it also depends on the entire strain history. the standard viscoelastic model consists ofn maxwell chains coupled in parallel, see figure 2. the present approach is based on the assumption that a viscous response is characterized by a set of rate constitutive equations, namely for a nonequilibrium stress qk (in chain k) as an internal variable, k = 1, 2, . . . , n . the constitutive model can be written as a set of coupled equations [8, 9, 12]: s = 2∂w ∂c − n∑ k=1 qk, p = f s, (12) ∂qk ∂t + 1 τk qk = 1 τk ( 2∂ŵk ∂c ) , qk(0) = 0, (13) where k = 1, 2, . . . , n, w = ∑n k=1 wk, 14 vol. 49/2024 fractional viscoelastic models at large deformations s represents the second (or symmetric) piolakirchhoff stress tensor, wk is the strain energy in chain k, τk is the relaxation time associated with each maxwell chain. classical theory of viscoelasticity employs the models composed of rheological elements such as elastic springs and viscous dampers. meanwhile, the fractional viscoelasticity introduces the springpot element together with the principles of fractional calculus. the fractional derivative of order α of a function u is defined as: dαu(t) = 1 γ(1 − α) d dt ∫ t 0 u(s) (t− s)α ds, (14) where 0 < α < 1, γ is the gamma function. replacing now the integer order derivative in (13) with a fractional order derivative we get: dαk qk + 1 ταk k qk = 1 ταk k ( 2∂ŵk ∂c ) , (15) qk(0) = 0, (16) where k = 1, 2, . . . , n, ταk k can now be interpreted as the most probable relaxation time out of a continuous distribution of relaxation times. the fractional order of differentiation αk then plays the role of a distribution parameter for the corresponding distribution of relaxation times [13]. the fractional maxwell chain is obtained as a parallel connection of n fractional maxwell cells, see the scheme in figure 3. 3. the weak formulation in the rest of the paper, just to simplify and shorten the presentation and avoid unnecessary technicalities, we will consider the case n = 1 (the fractional maxwell cell). the simple model presented here can be straightforwardly extended to a setting with the parallel connection of n cells (in the fractional maxwell chain). let w 1,2(ω0) denote the sobolev space of functions possessing square integrable derivatives and h(ω0) := w 1,2(ω0)3. let us denote by st the displacement solution space at time t ∈ [0, t ] defined as: st = { u(·, t) ∈ h(ω0) ∣∣∣∣ u(·, t) = ŭ0(·, t) on ∂ωu } . finally, we denote by hu(ω0) the linear space of admissible test functions or kinematically admissible figure 3. the fractional maxwell chain. variations, i.e., (virtual) displacements satisfying the homogeneous form of the essential boundary condition (2) as: hu(ω0) = { v ∈ h(ω0) ∣∣∣∣ v = 0 on ∂ωu } . with these notations in hand, the weak form of our problem reads as follows: for sufficiently smooth data f0, u0, v0, t̆0 and ŭ0 find the displacement field u(·, t) ∈ st, pressure p(·, t) ∈ l2(ω0), the volume filed θ(·, t) ∈ l2(ω0) and the internal variable q(·, t) ∈ l2(ω0)3×3, such that u(·, 0) = u0, ∂u ∂t (·, 0) = v0 and q(0) = 0 in ω0 and: ∫ ω0 ρ0 ∂2u ∂t2 · v dω0 + ∫ ω0 ŝ : [f t ∇0v] + pjc−1 : [f t ∇0v] dω0 − ∫ ω0 q : [f t ∇0v] dω0 = ∫ ω0 f0 · v dω0 + ∫ ∂ωp t̆0 · v ds0 (17) for all v ∈ hu(ω0) and all t ∈ [0, t ],∫ ω0 [p− κ(θ − 1)]ψ dω0 = 0 (18) for all ψ ∈ l2(ω0) and all t ∈ [0, t ] and:∫ ω0 (j − θ)q dω0 = 0 (19) for all q ∈ l2(ω0) and all t ∈ [0, t ]. finally: dαq + 1 τα q = 1 τα ŝ, τ > 0, α ∈ (0, 1], (20) holds almost everywhere in ω0 and for all t ∈ [0, t ]. here: ŝ = 2∂ŵ ∂c 15 barbora hálková, michal beneš acta polytechnica ctu proceedings denotes the computational deviatoric second piolakirchhoff stress. the equation (17) denotes the weak form of the equation of motion (1). the second equation (18) yields the constitutive equation for the pressure p, see also equation (11), and the third equation (19) reproduces the constraint condition (10). 4. numerical algorithm here we outline a general numerical solution scheme for the viscoelastic problem (extended to fractional viscoelasticity) within the context of the finite-element method. the point of departure in our developments is the weak formulation introduced in the preceding section. for simplicity, we will assume that the displacement boundary conditions are homogeneous, i.e.: u(·, t) = ŭ0(·, t) ≡ 0 on ∂ωu. we begin by traditional discretization in space by the finite element method. let us define: uh = ψt u ũ, uh ∣∣∣∣ ωe = nnu∑ i=1 ψi u(x)ũi(t) (21) for all uh ∈ hh u ⊂ hu(ω0), θh = ψt θ θ̃, θh ∣∣∣∣ ωe = nnθ∑ i=1 ψi θ(x)θ̃i(t) (22) for all θh ∈ lh ⊂ l2(ω0), ph = ψt p p̃, ph ∣∣∣∣ ωe = nnp∑ i=1 ψi p(x)p̃i(t), (23) for all ph ∈ lh ⊂ l2(ω0) and: qh = ψt qq̃, qh ∣∣∣∣ ωe = nnq∑ i=1 ψi q(x)q̃i(t) (24) for all qh ∈ [lh]3×3 ⊂ l2(ω0)3×3. here we denoted by hh u and lh the finite element subspace of the space hu(ω0) and l2(ω0), respectively. let now 0 = t0 < t1 < · · · < tr = t be an equidistant partitioning of the time interval [0, t ] with the discrete time step ∆t, {tn}r n=0, ∆t = t r . for any function, vector-valued function or tensor ζ, we will use the approximation ζn ≈ ζ(tn) and introduce the notation: ζn+β = (1 − β)ζn + βζn+1, β ∈ [0, 1]. (25) our goal is to develop time discretization schemes for which a discrete form of the problem (17)– (20) can be established. replacing the second order derivative in (17) by the discrete derivative, ∂2u ∂t2 ≈ uh n+1−2uh n+uh n−1 ∆t , and incorporating the generalized midpoint rule (25) into the weak formulation (17)–(19), the final result is the generalized β-scheme of the form: ru = − ∫ ω0 ρ0 ( uh n+1 − 2uh n + uh n−1 ∆t ) · δuh dω0 − β∆t ∫ ω0 ŝn−1+β : [ f t n−1+β(∇0 δu h) ] dω0 − β∆t ∫ ω0 ph n−1+β jn−1+βc−1 n−1+β : [ f t n−1+β(∇0 δu h) ] dω0 − β∆t ∫ ω0 qn−1+β : [ f t n−1+β(∇0 δu h) ] dω0 − (1 − β)∆t ∫ ω0 ŝn+β : [ f t n+β(∇0 δu h) ] dω0 − (1 − β)∆t ∫ ω0 ph n+βjn+βc−1 n+β : [ f t n+β(∇0 δu h) ] dω0 − (1 − β)∆t ∫ ω0 qn+β : [ f t n+β(∇0 δu h) ] dω0 + β∆t ∫ ω0 (f0)n−1+β · δuh dω0 + (1 − β)∆t ∫ ω0 (f0)n+β · δuh dω0 + β∆t ∫ ∂ωp (t̆0)n−1+β · δuh ds0 + (1 − β)∆t ∫ ∂ωp (t̆0)n+β · δuh ds0 = 0 (26) for all δuh ∈ hh u, rθ = − ∆t ∫ ω0 [ κβ ( θh n−1+β − 1 ) + κ(1 − β) ( θh n+β − 1 )] δθh dω0 √ + ∆t ∫ ω0 [ βph n−1+β + (1 − β)ph n+β ] δθh dω0 = 0 (27) for all δθh ∈ lh and: rp = − ∆t ∫ ω0 [βjn−1+β + (1 − β)jn+β ] δph dω0 + ∆t ∫ ω0 [ βθh n−1+β + (1 − β)θh n+β ] δph dω0 = 0 (28) for all δph ∈ lh. note that β serves as a parameter to control the implicitness of the algorithm. for β = 0 or β = 1 we get the explicit method. setting β = 1 2 we obtain the implicit method. next we present the algorithm for the integration of the rate equation (20). first, the discrete fractional derivative of order α at time (n+ 1)∆t can be approximated by [14]: [dαq]n+1 = 1 (∆t)α n∑ j=0 wj(α)qn+1−j , (29) where the weights can be identified and calculated by the recursion relation below: w0(α) = 1, w1(α) = −α, · · · wj(α) = j − 1 − α j wj−1(α), · · · . 16 vol. 49/2024 fractional viscoelastic models at large deformations using (29) in the rate equation (20) we get: [dαq]n+1 + 1 τα qn+1 = 1 τα ŝn. (30) moreover, using (29) we can rewrite the approximation of the fractional derivative as: [dαq]n+1 = 1 (∆t)α qn+1 + 1 (∆t)α n∑ j=1 wj(α)qn+1−j . (31) substituting (31) into (30) it is easy to see that qn+1 can be computed as: qn+1 =(∆t)α[w0(α)τα + (∆t)α]−1ŝn − τα[w0(α)τα + (∆t)α]−1 n∑ j=1 wj(α)[q]n+1−j . (32) the disadvantage of the fractional viscoelasticity is the nonlocal character of fractional derivatives. numerical approximation requires the whole history of the internal variables in the preceding time steps to be saved and included in the calculation of the new time step. letting α = 1 and the sum in equation (32) simply becomes −qn and the classical model follows. it is worth pointing out that the right hand side in (30) is taken from the preceding time step (a semiimplicit integrator). as a result, the discrete rate equation (30) is decoupled from the system at the actual time step n+ 1 and the approximation of the internal variable qn+1 can be computed first (at each discrete time step). then, with qn+1 in hand, the newton-raphson method is applied to solve the nonlinear system (26)–(28). the derivation of a tangent stiffness tensor that is consistent with the integration procedure is essential to ensure a quadratic rate of convergence [15]. a consistent linearization for the set of non-linear equations given in equations (26)–(28), about a configuration u, θ and p, is given by: jg(y(i))∆y(i) = −g(y(i)), (33) y(i+1) = y(i) + ∆y(i), (34) where yt = (ũn+1, θ̃n+1, p̃n+1), gt = (ru,rθ,rp), jg =  cuu 0 ct pu 0 cθθ ct pθ cpu cpθ 0  , c□△ = ∂r□ ∂△i+1 . 5. numerical example to illustrate the performance of the model, we consider a 2-d cantilever beam 1.0×0.4 m with a step end load, see figure 4 and figure 5. the beam is meshed uniformly with 4-node quadrilateral elements and partitioned as shown. the parameters used for this problem are: young’s modulus e = 1.0 × 104 pa, density figure 4. 2-d cantilever beam. figure 5. a step end load. figure 6. vertical displacement of the mid-point a on the free edge. hyperelastic material was considered in this case (neglecting viscous effects, i.e. τ → +∞). ρ0 = 1.0 × 10−4 kg m−3, end load p0 = 20.0 n m−1 distributed over the free edge. the problem was integrated with a time step ∆t = 2.0 × 10−6 s. first, we considered the purely hyperelastic material neglecting viscous effects (τ → +∞). figure 6 shows the motion of point a. note that our results are in very close agreement with the results presented in [16]. in figure 7 the vertical displacement of the midpoint a versus time is displayed for different values of α. as can be observed, the value of α clearly affects the results. vertical displacements seem to decay faster with higher values of α. figures 8 and 9 show the deformed cantilever for parameter α = 0.5. due to the fact that the time step is limited by 17 barbora hálková, michal beneš acta polytechnica ctu proceedings figure 7. vertical displacement of the point a. the influence of different values of α for fixed τ = 10−3 s. figure 8. motion of the beam at time t = 3.0 × 10−4 s. figure 9. motion of the beam at time t = 8.9 × 10−4 s. accuracy requirements rather than stability conditions, the explicit algorithm was used in our simulations (which is significantly faster as no iterations in each time step are needed). 6. conclusion a fractional derivative visco-hyperelastic model for large and nearly incompressible deformations has been formulated based on irreversible thermodynamics with internal variables. the finite element framework is based on a three-field form of the hu-washizu principle to create a stable finite element method. the β-method (the generalized midpoint rule) for time discretization of the equation of motion and a specific semi-implicit approximation of fractional odes governing the evolution of the internal variables enable us to partially decouple the elastic and viscous response to simplify and speed up the numerical algorithm. the consistent linearization of the resulting system of nonlinear equations is also briefly presented. the present work is our first step toward linking the fractional calculus and hyperelasticity. the dynamic response of a 2-d cantilever beam is computed, including both geometrically and materially nonlinear effects. acknowledgements this research was supported by sgs, project number sgs23/001/ohk1/1t/11, and the czech science foundation, the grant no. 22-15553s. references [1] r. c. koeller. applications of fractional calculus to the theory of viscoelasticity. journal of applied mechanics 51(2):299–307, 1984. https://doi.org/10.1115/1.3167616 [2] b. hálková. experimentální a numerické modelování pvb folie [in czech; experimental and numerical modelling of pvb foil]. master’s thesis, czech technical university in prague, 2024. [3] s. w. j. welch, r. a. l. rorrer, j. r. g. duren. application of time-based fractional calculus methods to viscoelastic creep and stress relaxation of materials. mechanics of time-dependent materials 3(3):279–303, 1999. https://doi.org/10.1023/a:1009834317545 [4] j. padovan. computational algorithms for fe formulations involving fractional operators. computational mechanics 2(4):271–287, 1987. https://doi.org/10.1007/bf00296422 [5] m. enelund, l. mähler, k. runesson, b. l. josefson. formulation and integration of the standard linear viscoelastic solid with fractional order rate laws. international journal of solids and structures 36(16):2417–2442, 1999. https://doi.org/10.1016/s0020-7683(98)00111-5 [6] s. müller, m. kästner, j. brummund, v. ulbricht. a nonlinear fractional viscoelastic material model for polymers. computational materials science 50(10):2938–2949, 2011. https://doi.org/10.1016/j.commatsci.2011.05.011 [7] a. schmidt, l. gaul. finite element formulation of viscoelastic constitutive equations using fractional time derivatives. nonlinear dynamics 29(1):37–55, 2002. https://doi.org/10.1023/a:1016552503411 [8] k. adolfsson, m. enelund. fractional derivative viscoelasticity at large deformations. nonlinear dynamics 33(3):301–321, 2003. https://doi.org/10.1023/a:1026003130033 18 https://doi.org/10.1115/1.3167616 https://doi.org/10.1023/a:1009834317545 https://doi.org/10.1007/bf00296422 https://doi.org/10.1016/s0020-7683(98)00111-5 https://doi.org/10.1016/j.commatsci.2011.05.011 https://doi.org/10.1023/a:1016552503411 https://doi.org/10.1023/a:1026003130033 vol. 49/2024 fractional viscoelastic models at large deformations [9] j. c. simo, t. j. r. hughes. computational inelasticity. interdisciplinary applied mathematics volume 7. springer new york, usa, 1998. https://doi.org/10.1007/b98904 [10] j. c. simo, r. l. taylor, k. s. pister. variational and projection methods for the volume constraint in finite deformation elasto-plasticity. computer methods in applied mechanics and engineering 51(1–3):177–208, 1985. https://doi.org/10.1016/0045-7825(85)90033-7 [11] s. n. atluri, e. reissner. on the formulation of variational theorems involving volume constraints. computational mechanics 5(5):337–344, 1989. https://doi.org/10.1007/bf01047050 [12] j. c. simo. on a fully three-dimensional finite-strain viscoelastic damage model: formulation and computational aspects. computer methods in applied mechanics and engineering 60(2):153–173, 1987. https://doi.org/10.1016/0045-7825(87)90107-1 [13] m. enelund, g. a. lesieutre. time domain modeling of damping using anelastic displacement fields and fractional calculus. international journal of solids and structures 36(29):4447–4472, 1999. https://doi.org/10.1016/s0020-7683(98)00194-2 [14] c. lubich. discretized fractional calculus. siam journal on mathematical analysis 17(3):704–719, 1986. https://doi.org/10.1137/0517050 [15] j. c. simo, r. l. taylor. consistent tangent operators for rate-independent elastoplasticity. computer methods in applied mechanics and engineering 48(1):101–118, 1985. https://doi.org/10.1016/0045-7825(85)90070-2 [16] a. prakash, k. d. hjelmstad. a feti-based multi-time-step coupling method for newmark schemes in structural dynamics. international journal for numerical methods in engineering 61(13):2183–2204, 2004. https://doi.org/10.1002/nme.1136 19 https://doi.org/10.1007/b98904 https://doi.org/10.1016/0045-7825(85)90033-7 https://doi.org/10.1007/bf01047050 https://doi.org/10.1016/0045-7825(87)90107-1 https://doi.org/10.1016/s0020-7683(98)00194-2 https://doi.org/10.1137/0517050 https://doi.org/10.1016/0045-7825(85)90070-2 https://doi.org/10.1002/nme.1136 acta polytechnica ctu proceedings 49:13–19, 2024 1 introduction 2 constitutive relationships 2.1 hyperelastic material 2.2 fractional viscoelasticity 3 the weak formulation 4 numerical algorithm 5 numerical example 6 conclusion acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2025.53.0060 acta polytechnica ctu proceedings 53:60–67, 2025 © 2025 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague effective mechanical properties identification of the concrete from the historical bridge structure dominik lisztwan∗, petr nápravník, dalibor kocáb brno university of technology, faculty of civil engineering, institute of building testing, veveří 331/95, 602 00 brno, czech republic ∗ corresponding author: dominik.lisztwan@vut.cz abstract. one of the first and most important tasks in a complex and precise assessment of the current condition and load-bearing capacity of existing structures is a determination of the physical and mechanical parameters of the materials used. this problem is even more complicated when a historical concrete structure is assessed. this study shows the possible way how to perform an effective and comprehensive evaluation of the concrete taken from the historical bridge structure with the use of a wide range of non-destructive and destructive methods. keywords: concrete, bridge structure, diagnostics, core drilling, compressive strength, ultrasonic pulse velocity test, resonance method. 1. introduction nowadays, concrete is the most commonly used material in civil engineering, usually in the form of reinforced, or prestressed concrete structures. the main development of concrete with portland cement and reinforced concrete structures is dated from the second half of the 19th century. consequently, the first standards and regulations for concrete structures were published [1]. at the beginning of the 20th century on the territory of the czech republic, reinforced concrete for structures, especially for bridges, began to be used widely in the construction industry. the quality of the concrete from this time cannot be comparable with modern concrete in general. poor quality of the cement, unsuitable aggregate (inappropriate aggregate grading, alkali-aggregate reaction), and insufficient methods for concrete compacting during the casting are often characteristics of concrete from this era. unknown and non-homogenous quality, deterioration of the concrete, a limited amount of the possible taken samples, etc. are the problems that could negatively affect the resulting characteristics of the tested material. however, if these problems are assumed, there are procedures and methods that can help us with the effective and correct determination of material properties. this paper deals with a complex description of the material properties of concrete samples taken from historical arch bridge structure with the use of a wide range of non-destructive and also destructive methods. the study continues with a complex assessment of the historical reinforced concrete arch bridge. results obtained from this study will be used as inputs for non-linear finite element method analysis of the bridge. the detailed numerical analysis will supplement the assessment of the structure based on the results gained during the full-scale load test described in [2]. 2. materials and methods 2.1. considered structure and sampling the structure determined for evaluation of concrete properties was a historical reinforced concrete arch bridge over the litava stream in the south moravian region in the czech republic (see figure 1 top). the bridge was built after world war i, during the 1920s. for over 100 years the bridge did not undergo any specific maintenance or reconstruction, except repairs of the bridge deck after the damage caused at the end of world war ii. before a sample collection, a detailed visual inspection of the structure was performed. the inspection included the measuring of the main dimensions of the bridge, visual assessment of deterioration and damages of the bridge members, and preliminary localization of the reinforcement bars. subsequently, the locations for samples collecting with the use of a coredrilling rig were determined. the main requirement for the location of the sampling was the consideration, that concrete at this part is not subjected to tension stresses, and the quantity of the reinforcements allows drilling in between reinforcement bars. the additional requirement was that the diameter of the cores must be at least three times greater than the maximum size of the aggregate used in the concrete [3]. due to these limitations, samples of a nominal diameter of 100 mm from support cross girders and from the ends of the main arches (see figure 1 bottom) were collected. five cores from the support cross girders and two samples from the main arches were collected in total. 2.2. classification of samples and preparations of tests due to minimizing damages to the structure caused by core drilling, a limited number of samples were 60 https://doi.org/10.14311/app.2025.53.0060 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 53/2025 mechanical properties of historical bridge concrete figure 1. arch bridge over litava stream subjected to assessment (top), collecting of the concrete samples with the use of the core-drilling method (bottom). obtained. for the need to characterize concrete as precisely as possible, was decided to use two nondestructive methods to rate the samples by their quality. firstly, core samples were labeled, visually evaluated, and the ends of the cores were cut off (see figure 2). for the non-destructive determination of concrete quality combination of the ultrasonic pulse velocity method and resonance method was used. final sorting was done based on the dynamical properties – the dynamic modulus of elasticity of concrete. 2.2.1. ultrasonic pulse velocity method the principle of the ultrasonic pulse velocity method is the transmission of ultrasonic pulses into the tested material and the determination of the transmission time t. subsequently, the ultrasonic pulse velocity (upv) through the material v (see figure 3 top) is determined according to the standard [4]. the dynamic young’s modulus can then be calculated according to standard [5] using the equation: ecu = dv2 (1 + µ)(1 − 2µ) (1 − µ) , (1) where ecu is the dynamic modulus of elasticity in mpa, d is the density of the material in kg m−3, v is the upv in km s−1 and µ is the poisson’s ratio, which was determined using the resonance method. a pundit pl-200 instrument with 150 khz probes was used to determine the dynamic modulus of elasticity. the upv was determined in three longitudinal lines on each test specimen. the average value of upv was used in the calculation of ecu according to equation 1. figure 2. core samples prepared for preliminary non-destructive sorting (top), cores divided into test specimens based on the results of non-destructive testing (bottom). 2.2.2. resonance method any object made of solid material vibrates after a mechanical impulse and this vibration can be different. the natural frequencies of longitudinal, flexural, and torsional vibrations of specimens of regular geometric shapes are used to evaluate the dynamic properties of the material [6]. the dynamic young’s modulus can be calculated in accordance with the standard [7] from the natural frequencies using the equation: ecrl = 4l2f2 ld, (2) where ecrl is the dynamic modulus of elasticity in mpa, l is the length of the test specimen in m, fl is the natural frequency of longitudinal vibration in khz and d is the density of the material in kg m−3. the dynamic young’s modulus can be also calculated from the equation: ecrf = 0.0789c1l4f2 f di−2, (3) where ecrf is the dynamic modulus of elasticity in mpa, c1 is the correction coefficient depending on the i/l ratio, l is the length of the test specimen in m, ff is the natural frequency of the flexural vibration in khz, d is the density of the material in kg m−3 and i is the radius of gyration of the test specimen cross-section in m. each test specimen of concrete was excited by a mechanical impulse using an impact hammer in accordance with figure 3 (bottom). the natural frequencies were determined using a handyscope hs4 oscilloscope 61 d. lisztwan, p. nápravník, d. kocáb acta polytechnica ctu proceedings figure 3. non-destructive evaluation of the concrete samples and specimens. ultrasonic pulse velocity method (top), resonance frequency method (bottom). with an acoustic emission sensor and software (supplied with the oscilloscope) that works based on a fast fourier transform. based on the results of the first assessment, core samples were saw-cut to the test specimens with lengths from 60 to 200 mm. after that, the second non-destructive measure was performed on the adjusted test specimens using the assigned test method for each test specimen. four specimens for modulus of elasticity tests were cut to the length of 200 mm, i.e., length to diameter ratio equals 2.0. the remaining specimens were divided into two sets to determine the compressive strength and uniaxial tensile strength of the concrete. in each group specimens with various dynamic properties were included. specimens subjected to these tests have quality corresponding with the overall quality of the concrete samples obtained from the structure. besides that, three specimens with lengths less than 100 mm were set up for the water absorption of concrete determination. the sorting of the test samples and specimens and the test plan are shown in table 1. 2.3. determination of the physical and mechanical properties after the non-destructive quality identification of the subjected concrete and assorting of the specimens, the determination of the main mechanical, and also physical parameters of the material followed. for the vast majority of analytical, or numerical approaches core sample specimens test method v1 1.1 modulus of elasticity – ec 1.2 uniaxial tensile strength – ft 1.3 compressive strength – fc v2 2.1 uniaxial tensile strength – ft 2.2 water absorption – wa 2.3 modulus of elasticity – ec v3a 3.1 compressive strength – fc 3.2 compressive strength – fc v3b 3.3 compressive strength – fc v4 4.1 compressive strength – fc 4.2 water absorption – wa 4.3 modulus of elasticity – ec v5 5.1 compressive strength – fc 5.2 uniaxial tensile strength – ft 5.3 uniaxial tensile strength – ft v6 6.1 modulus of elasticity – ec 6.2 uniaxial tensile strength – ft v7 7.1 compressive strength – fc 7.2 water absorption – wa 7.3 compressive strength – fc table 1. groups of the specimens and test plan. for the assessment of the load-bearing capacity of the structure, it is important to know as many mechanical material properties of the materials used as possible. in the case of concrete, the basic strength parameters are the compressive and tensile strength of the concrete. apart from the strength parameters, materials are also described in the form of the deformation parameters of the material. usually, the parameter is expressed by young’s modulus – in other words, the modulus of elasticity. in this study, the mentioned mechanical parameters of the concrete were primarily evaluated. in some cases, in addition to these main mechanical parameters, the fracture energy of the concrete can be also determined. this parameter is greatly important in the case of non-linear analysis of the structure. however, in this specific study, the fracture energy of the concrete was not determined due to the limited quantity of samples. also, the basic physical properties like the volume density of all specimens and water absorption were determined. 2.3.1. compressive and uniaxial tensile strength undoubtedly, one of the basic and the most frequently executed tests of hardened concrete is the compressive strength test. in this case, compressive strength was determined on the specimens with a diameter and length of 100 mm (length to diameter ratio equals 1:1). the test follows the instructions in the european standard for hardened concrete testing [8]. assessment of the compressive strength of the concrete is supplemented by characteristic compressive strength value determination and strength grade classification using the approaches given in the [9, 10] and in an invalid standard: designing of concrete structures, used in 62 vol. 53/2025 mechanical properties of historical bridge concrete the era of the 30th years of 20th century [11]. the uniaxial tensile strength test is not often performed type of the test to determine the tensile strength of concrete. in most cases are tensile strength values based on results from splitting or four-point bending tests. these tests are less time-consuming in terms of preparation of the experiment than the uniaxial test. also, in the case of the four-point bending test, there is the possibility to use halves later for the splitting test, hence three values of tensile strength from a single test specimen are obtained. czech standard [12] covers the requirements and process of the uniaxial tensile testing. in this study, the test was supplemented by measurements of the strain during the testing (see figure 4). for this purpose, two longer specimens which were determined to uniaxial strength test were fitted with a pair of strain gauges. values of the strain gained during the test were used for the determination stress-strain curve of tested concrete under the tensile load later on. figure 4. compressive strength test (top), uniaxial tensile strength test with measurement of strain during the test (bottom). 2.3.2. static modulus of elasticity in the case of detailed analysis of the structure, in terms of deformations analysis or nlfea, the modulus of elasticity is a more important parameter than the strength of the material. this constant describes the deformation behaviour of the material during the loading. especially, values of modulus of elasticity for concrete can vary from 10.0 to 35.0 gpa. in the standards and recommendations (for example american [13], or european [14]) formulas for the calculation of modulus of elasticity based on the compressive strength of concrete can be found. this approach can be used only for modern and ordinary concrete. the modulus of elasticity gained from this method has only approximate character and often the real values can differ by tens of percent against reality. for that reason, the best approach to determine the modulus of elasticity is by experiment. in this study, the modulus of elasticity was determined following [15]. during the tests, the strain was measured with two separate methods: commonly used strain gauge extensometer and bonded foil strain gauges (see figure 5). extensometer was attached to the specimens only during the cyclic loading, whereas the bonded strain gauges were used for continuous measurement until the failure of the test specimens. the standard time course of the test consists of cyclic loading between 0.5 mpa and one-third of the compressive strength of concrete. the test contained two preliminary cycles and the third measured cycle. each specimen was then continuously loaded until failure occurred. strain measured with the use of embedded strain gauges was used for the determination of the stress-strain curve for this particular concrete subjected to compression loading. figure 5. test specimen during cyclic loading with strain gauges and extensometer (top), test specimen after failure (bottom). 3. results corresponding to the study, firstly the results of the non-destructive analysis of concrete are presented. figure 6 shows values of the dynamic properties of the whole core samples, and also properties determined on the adjusted test specimens. assessment of the samples and cores was based on the values of dynamic 63 d. lisztwan, p. nápravník, d. kocáb acta polytechnica ctu proceedings figure 6. results of the dynamic modulus of elasticity based on parameters measured with the use of the upv method and resonance method determined on both core samples and adjusted specimens. figure 7. results of the compressive strength tests. modulus of elasticity gained using the upv method – ecu. in the case of the resonance method, the evaluation of the samples and specimens was performed based only on the measurement of longitudinal and flexural frequencies, hence shear modulus of elasticity (from the torsional frequency) was not used. in addition to this, results gained from resonance frequency modulus (ecrl and ecrf ) are presented. it can be seen, that except for samples v3a and v7 results of the dynamic properties are consistent, also the difference between the dynamic modulus of elasticity ecrl and ecrf are negligible. some of the results based on the measurement of saw-cutted specimens show slight deviations in the dynamic properties from the results of the whole test sample. these deviations can be caused for example by large aggregate grains presence, or the local poor quality of the concrete matrix. in the case of sample v3b and specimens with a length of 100 mm, the resonant frequency measurement cannot be performed due to an unsuitable length-to-diameter ratio. figure 7 describes the results of the compressive strength tests performed on the specimens with length to diameter 1:1, thus cube compressive strength values are assessed. specimens that were made from core samples v1 and v7 had slightly higher compressive strength than the rest of the specimens. this effect could be observed due to the shape or size of the aggregate in the specific test specimen. table 2 summarizes the overall compressive strength values, moreover, concrete grade according to [10] is assessed. in this case, tested concrete does not fulfil requirements for the existing lowest concrete grade c 8/10, for this reason, this particular concrete was classified as concrete of non-existing grade (c 6/7.5). this assessment was supplemented by grading concrete using standards for the designing of the concrete structures used in the 1930s [11] when the bridge was 64 vol. 53/2025 mechanical properties of historical bridge concrete figure 8. stress-strain diagrams of tested concrete. built. thus, based on this standard tested concrete meets the requirements for the concrete class named d. the main requirement for this concrete class is that the minimal concrete cube strength must be higher than 12.5 mpa. mean fc [mpa] 16.1 sample standard deviation (ssd) [mpa] 4.1 minimal compressive strength fc,min [mpa] 12.5 characteristic compressive strength fck [mpa] 8.0 concrete grade [en 206+a2] [10] (c 6/7.5) concrete class [čsn 1090:1931] [11] d table 2. compressive strength results. values of the tensile strength are shown in table 3, except for the tensile strength of specimen 1.2 the results are consistent. a significantly lower value of the strength of the mentioned specimens was induced by poor cohesion between the cement matrix and large aggregate grains and the unsuitable shape of the aggregate of this particular test specimen but the observed failure mode was correct. specimen uniaxial tensile strength [mpa] 1.2 0.38 2.1 1.16 5.2 1.19 5.3 1.24 6.2 1.19 mean ft [mpa] 0.95 sample standard deviation [mpa] 0.37 table 3. uniaxial tensile strength results. evaluated data (see table 4) gained during static modulus of elasticity tests shows a mean value of 13.0 gpa, the difference between ec values based on the different measurement techniques of deformations is 4.5 %. these differences are negligible, and they are caused primarily by the different locations of the measurement gauges on the specimens. however, overall values of the static modulus of elasticity are consistent and correspond to the quality and composition of concrete. stress-strain diagrams gained out of the uniaxial tensile and compressive tests are shown in figure 8. diagrams represent a relationship between deformation and stress during the loading of the specimens. as was mentioned before, the strain was measured with the use of foil strain gauges bonded on the specimen’s surface. the slope of the initial linear part of the compression and also the tension part of the diagram corresponds to the values of the modulus of elasticity. this linearity is observed until approximately 1/3 of the ultimate compressive load is reached. the maximal measured strain when the failure occurred was within a range of 1 500 to 2 500 µm m−1 in case of compressive stress and 20 µm m−1 in case of tensile loading. specimen modulus of elasticity ec [gpa] extensometer foil strain gauges 1.1 14.8 12.1 2.3 13.5 13.9 4.3 13.7 14.7 6.1 11.0 10.1 mean ec [gpa] 13.3 12.7 sample standard deviation [gpa] 1.6 2.0 overall mean ec [gpa] 13.0 overall ssd [gpa] 1.7 table 4. modulus of elasticity results. last but not least, the volume density of the concrete was determined. figure 9 represents values of volume density determined on the adjusted test specimens before testing. tested concrete had an average volume density of 2 190 kg m−3 with a sample standard deviation of 41.5 kg m−3. also, water absorption of 6.6 % on samples 2.2, 4.2, and 7.2 was determined. results of similar studies can be found in [16] or [17]. 65 d. lisztwan, p. nápravník, d. kocáb acta polytechnica ctu proceedings figure 9. results of the volume density of concrete determined on the test specimens. 4. conclusion a comprehensive determination of the concrete properties taken from more than 100 years old historical reinforced bridge structures within a detailed assessment of the current condition of the bridge was described. the paper covers the process of effective and detailed determination of material properties of concrete, with the use of the combination of nondestructive and destructive test methods in general. the main mechanical and physical properties of the concrete were determined. in the case of this specific study, the assessment of the concrete was divided into two main steps. firstly, non-destructive measuring was performed. based on the results of the evaluated ultrasonic and resonance measurement, the core samples were divided into groups of test specimens for the particular test method. subsequently, the main destructive tests were executed and evaluated. from the results, it can be seen that preliminary non-destructive evaluation of the test samples is important, especially when a limited amount of test samples is available. the non-destructive methods can significantly help with revealing damages and defects of the internal structure of the concrete before the performing of the destructive test methods. the results obtained from this study will be used as inputs for the numerical model of the bridge for the further assessment of the structure behavior under various types of loading conditions. acknowledgements the contribution was carried out with financial support of the brno university of technology within a project: “actual condition evaluation of the selected bridge structure using load tests” registered under fast-j-22-7995. references [1] l. 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[16] a. ambroziak, e. haustein, m. niedostatkiewicz. chemical, physical, and mechanical properties of 95-year-old concrete built-in arch bridge. materials 14(1):20, 2020. https://doi.org/10.3390/ma14010020 [17] a. hellebois, a. launoy, c. pierre, et al. 100-year-old hennebique concrete, from composition to performance. construction and building materials 44:149–160, 2013. https://doi.org/10.1016/j.conbuildmat.2013.03.017 67 https://doi.org/10.3390/ma14010020 https://doi.org/10.1016/j.conbuildmat.2013.03.017 acta polytechnica ctu proceedings 53:60–67, 2025 1 introduction 2 materials and methods 2.1 considered structure and sampling 2.2 classification of samples and preparations of tests 2.2.1 ultrasonic pulse velocity method 2.2.2 resonance method 2.3 determination of the physical and mechanical properties 2.3.1 compressive and uniaxial tensile strength 2.3.2 static modulus of elasticity 3 results 4 conclusion acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2024.47.0075 acta polytechnica ctu proceedings 47:75–78, 2024 © 2024 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague concrete mixtures with photovoltaic panels glass: a review of the situation in czech republic diana mariaková∗, kristina fořtová, stanislav černý, tereza pavlů czech technical university in prague, university centre for energy efficient buildings, třinecká 1024, 273 43 buštěhrad, czech republic ∗ corresponding author: diana.mariakova@cvut.cz abstract. solar energy production was initiated in the 1990s following by rapid increase of the use of photovoltaic (pv) technology. most of older solar panels in the czech republic were installed between 2000 and 2006 and a global installed capacity of 222 gw was reached by the end of 2015. with the growing global pv market, a large amount of waste is expected to accumulate after 2030 which is associated with the new industry to process this waste, based on reduce, reuse, and recycle. this article summarizes findings and research conclusions regarding local issues collected by experts from the czech republic. previous research showed that waste glass from photovoltaic panels can be used as a pozzolanic material in high-performance concrete, improving its properties. further research on the use of waste from photovoltaic panels has been conducted in the czech republic since 2019 and the conclusions from various research confirm the suitability of using pv panel glass. keywords: phorovoltaic panels glass utilization, pozzolanic properties, concrete production. 1. introduction in recent years, the utilization of photovoltaic (pv) technologies has been growing rapidly, a cumulative global installed capacity of 222 gw was already reached at the end of 2015 [1]. photovoltaics is used for economically advantageous and ecological electricity production, history of pv dates to the 1990. however, it is a technology whose aging is still insufficiently explored, because during the service years only mechanically damaged panels were disposed. currently, the lifespan of the pv panels is defined by the time when the performance drops by 20 %, which is usually declared by the manufacturer after 25 years of use. at the same time, most manufacturers declare a maximum decrease of roughly 10 % after 10 years. nevertheless, the end-of-life period of the first the era of photovoltaic panels is already coming and decommissioning, dismantling and disposal must be figured out. however, this direction is not ideal due to upcoming regulations and stricter requirements for waste management, circular economy, and sustainability [2]. recycling seems to be the ideal way. however, with such a complicated material, a detailed search of options and ways to recycle the panels is required. the launch of solar energy production in the early 1990s was supported by the launch of the grant programs, initially mainly used by germany, but over time they reached the whole world, including the czech republic. this act was the main beginning of the period of massive production of photovoltaic panels and their installation and application on constructions of all types and sizes. despite a few earlier installations, it can be stated that most of the oldest panels in the czech republic were installed between 2000 and 2006. however, the total number of installed panels during this period is very low compared to the following years. in 2006, there was a big turnaround thanks to the introduction of support for the purchase of electricity from renewable sources according to act no. 180/2005 coll. and just as the global pv market is growing, so is the volume of discarded pv panels, with a cumulative global flow of 43 500–250 000 metric tons at the end of 2016, roughly 0.1 % to 0.6 % of the cumulative weight of all installed panels. with these numbers, a large amount of waste is expected, which will continue to grow until a certain time. this growth will subsequently be slightly reduced thanks to the effort to develop panels with a longer service life and less material consumption [3]. total annual global electrical and electronic waste (e-waste) already reached 41.8 million metric tons in 2014, while the annual amount of waste from pv panels was 1 000 times lower in 2014. however, there is an assumption that by 2050 pv waste will exceed this number by up to 10 %. this assumption therefore points to the fact that it is necessary and desirable to use the experience of managing e-waste and turn it into opportunities for the management of waste from pv panels in the future. 2. current trends in pv recycling due to the lifetime of the panels, a greater number of elements for recycling can be expected after 2030. after 2040, approximately 20 000 tons of panels are expected to be recycled annually [4]. the largest share of the weight of crystalline panels is glass (60–70 %) and aluminum frame (around 20 %). for thin-film panels, the proportion of glass and aluminum is over 95 % [3]. both of these materials are normally almost 100 % recycled. other metal materials are valuable 75 https://doi.org/10.14311/app.2024.47.0075 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en d. mariaková, k. fořtová, s. černý, t. pavlů acta polytechnica ctu proceedings raw materials that are worth recovering from waste. plastics can be recycled only partially or not at all. a detailed table of materials with possible recycling methods is enclosed below. currently, one of the frequently used recycling methods for pv panels is thermal recycling. it is an advanced method of recycling panels that are generally heated to a temperature above 500 °c (the design and testing took place at the german company deutsche solar ag). this temperature guarantees the evaporation of the plastic elements (with subsequent combustion in the next chamber). the rest of the materials are manually divided and if the panels are undamaged, it is possible to use almost 85 % of the panel with roughly 70 % less energy consumption. however, this method can only be used for panels made of crystalline cells and therefore does not solve thin-film panels. the second widely used method is mechanicalchemical method, which is mostly applicable for thinfilm panels. although the first step is the dismantling of the aluminum frame, which is done manually, followed by the crushing and sorting of the materials into the specified fractions, this method has a lower proportion of manual labor than the previously mentioned one, but the result is only crushed raw materials. separation methods are used for fluid and wet floats and electrodynamic separation. important metals such as silver and others are obtained chemically and pyrometallurgically and can subsequently be used as raw materials in the metallurgical industry, plastics are disposed of with the possible use of heat from combustion [5]. 2.1. reduce, reuse and recycle the end of the life of pv panels signals the possibility of the emergence of a new industrial sector, which is also a possibility for the emergence of new economic values in accordance with the global path to sustainable development. this industry can go into both the public and private sectors. the basic ideas are reduce, reuse and recycle. the composition of solar panels is expected to require less raw materials as research and development and technological advances continue to mature the industry. currently, two-thirds of globally manufactured pv panels are crystalline silicon (c-si) which includes hazardous materials such as silver, tin, and lead traces. thin-film panels have a higher percentage of non-hazardous materials like glass, polymer, and aluminum. by 2030, current trends in panel efficiency suggest that the raw material inputs for c-si and thin-film technologies could be significantly reduced, decreasing the use of hazardous and rare materials in the production process, and improving the recyclability and resource recovery potential of end-of-life panels. rapid growth in the global solar panel industry is expected to create a strong demand for used or repaired panels and components. early failures in the lifetime of a panel can be repaired and resold at a lower cost, providing an opportunity for countries with limited financial resources to access the solar pv market. this secondary market for used panels and components also presents an opportunity for buyers to purchase partly repaired panels or components. as solar panels reach the end of their lifespan, recycling and material recovery is becoming a preferable option to disposal. the pv recycling industry currently processes end-of-life panels through separate batches within existing recycling plants, allowing for the recovery of major components such as glass, aluminum, and copper at yields greater than 85 % of the total panel mass. in the long term, specialized panel recycling plants can increase treatment capacities, maximize revenues, and recover a greater fraction of the embodied materials. research on pv-specific recycling technologies has been ongoing for the past decade, and it is important to learn from this research to develop cost-efficient and material-efficient recycling plants [1]. however, technical, and regulatory systems need to be established to ensure that pv panel waste streams are sufficient for profitable operation. for the preparation of specific plants for the recycling of entire panels, which according to studies are extremely financially demanding, the czech republic is apparently not currently ready. this assumption is based on the amount of panels to be processed (recycled), which does not lead to the efficient production of the required recycling line until 2040 [3]. although there are efforts not only in the czech republic to reuse the majority of pv panel components, the most likely scenario is when parts of the panels will have to be used in a different way than before. it is necessary to consider the possibility of impurities on existing parts, especially glass ones. in case of increased contamination of glass parts, it is advisable to look for alternative use. in the czech republic, around 2020, leading technical universities and their research teams began to look into the possibility of using glass waste from pv panels in concrete mixtures, including the possibility of subsequent applications from these mixtures. in the following chapters, the current findings on the use of glass parts from pv panels in concrete mixes in the czech republic are briefly summarized. 2.2. concrete mixtures containing pv panels glass our previous research describes a study that investigates the potential of using waste glass from photovoltaic panels in high-performance concrete. the research team founds that the waste glass can be an excellent pozzolanic material, and that it can improve the mechanical and durability properties of the concrete. this research highlights the potential of using the waste glass from photovoltaic panels in other industries [6]. the study was already published in 76 vol. 47/2024 concrete mixtures with photovoltaic panels glass material recycling method glass the advantage of glass is that in most cases the material can be recycled into the original product, but often due to unwanted impurities, glass cannot be used in this way, and it is advisable to look for it. aluminum easily recycled with very low energy consumption. plastic components usually degrade due to the climate conditions and it is not possible to reuse them. photovoltaic cells at the end of their life, crystalline cells are essentially unchanged, and there is already practical experience with recycling whole cells or plates. for thinfilm cells, their recovery is more efficient than production from primary raw materials. heavy metals due to toxicity, it is necessary to separate heavy metals from the environment, even though they represent minority components (weight, price, power consumption). they will probably be replaced by other material in the future (depletion of economically extractable reserves). table 1. list of the materials used in pv panels and current trends in their recycling. 2019 and was among the first of its kind in the czech republic, so it was desirable to follow up with further research. however, in the meantime, the work of other research centers in the czech republic also began, and the results for the period from 2019 to today are summarized below. in our subsequent research, glass from photovoltaic panels was crushed and then sorted into two fractions – fine sand and glass powder (flour). both fractions were used as a substitute for individual components in concrete and the resulting properties and their differences were monitored. at the same time, efforts were still being made to save the amount of primary resources and eliminate the production of carbon dioxide. for this reason, great emphasis was placed in the research on ecotoxicity tests (daphnia acute toxicity test, freshwater algae toxicity test, duckweed growth inhibition test) and subsequently on the effect of the alkali-silica reaction and the assumption of its elimination due to the use of glass powder. in leachates, all tested risk metals were below detection limit. all tested samples were classified as non-toxic [7]. another research presents the conclusions of a study on replacing natural aggregate in cement composites with recycled glass from pv panels. the research came to similar findings as was done in our research. replacement of natural aggregate with recycled glass fraction 0/10 mm is possible and the densities of recycled glass fractions are similar and reach approximately 2.5 mg m−3. the consistency of fresh cement mixture based on recycled glass was within an acceptable range. the flexural and compressive strengths of the cement composites are almost identical and within an acceptable range. with 100 % replacement of natural aggregate with recycled glass, there is a decrease in flexural and compressive strength by 20–30 %. the results of the image analysis confirm the non-disruption of the contact zone between the grains of the recycled glass and the cementing compound. the permeability of cement composites with recycled glass from pv panels shows similar values to conventional cement composites. future research will focus on increasing the flexural strength to a minimum of 6 mpa and testing the alkali-silica reaction of recycled glass grains from solar panels, as well as the surface treatment of the designed cement composites for potential use in interior paving or tiling material. alkali-silica reaction is one of the fundamental problems that can arise, and within the study of concrete mixtures it is advisable to prevent asr if possible. our research was dealing with asr during last year [8]. testing was carried out according to the american standard astm c1260 and the czech technical conditions tp137. after testing according to astm c1260, one reference sample showed cracks of up to 1 cm, while after testing according to tp137, no cracks were seen on the surface of the samples. regarding the expansion, for the concrete mix containing 100 % pv glass as a substitute for silica flour, the expansion after 28 days was higher than 0.1 % (but lower than 0.2 %) and therefore further testing is necessary to clarify the results. the concrete mixture containing 50 % pv glass as a substitute for quartz flour was verified as safe from the point of view of asr due to the fulfillment of the conditions for the exclusion of the occurrence of asr [8]. other major problems include, for example, corrosion. thereby, another experimental study dealt with the biocorrosion of cement composites in beams and crumbling, using photovoltaic glass as a replacement for natural aggregate in four different grain fractions. three species of microscopic algae were used in the experiment as they were found to be part of the biofilm involved in biocorrosion. the results show that the growth of the microscopic algae was small and low in abundance due to the lack of supporting microbiots and the high ph value of the samples. the high level of recycled glass in the samples also affected the growth of the algae as it affected the availability of nutrients for their nutrition [9]. 77 d. mariaková, k. fořtová, s. černý, t. pavlů acta polytechnica ctu proceedings 2.3. concrete masonry units the research of concrete mixtures containing pv glass began to acquire useful information, and other studies began to deal with the design and testing of applications from these materials. another research [10] describes an experiment in which the authors determined the loose bulk density and voids of recycled glass from pv panels according to the standard čsn en 1097. the authors also prepared two concrete recipes, recipe a and recipe b, with recipe a containing both recycled glass and one fraction of aggregate, and recipe b containing only recycled glass. the average density of recipe a was 2 160 kg m−3 and the average density of recipe b was 1 880 kg m−3. the compressive strength of the two recipes was also measured after 28 days and the results were found to be satisfactory. the average compressive strength of recipe a is 21.8 mpa and the average compressive strength of recipe b is 11.7 mpa. the authors also compare the compressive strength and density of their concrete with ytong, a widely used lightweight aerated concrete and concluded that the strengths of concrete units with recycled glass from pv panels are satisfactory for use in building foundations and load-bearing structures of low-rise buildings [10]. the studies states that the results of the experiment indicate that concrete units made with glass from pv panels have satisfactory strength for use in building foundations and load-bearing structures of low-rise buildings. the authors suggest that the addition of polymer fibers could potentially improve the mechanical properties of the material. they also mention that a combination of coarse recycled concrete with fine recycled glass is worth considering when developing new concrete recipes. however, one limitation is the smoothness of the glass surface in relation to cement and the potential reaction between glass and calcium hydroxide, which has been studied in several scientific studies. 3. conclusion the use of waste glass from photovoltaic panels as a replacement material in high-performance concrete has been extensively studied in recent years in the czech republic. results from these studies have shown that crushed and sorted glass can be used as a substitute for individual components in concrete and can improve its mechanical and durability properties. additionally, the use of glass from photovoltaic panels in concrete has been found to have a low impact on the environment, as well as on biocorrosion and the alkali-silica reaction. further research is necessary to increase the flexural strength and clarify the results of the alkali-silica reaction, as well as to test the surface treatment of cement composites for potential use in interior paving or other applications. the overall research has shown promise for the use of waste glass from photovoltaic panels in high-performance concrete. acknowledgements this work was supported by the grant agency of the czech technical university in prague, grant no. sgs21/096/ohk1/2t/11 – influence of chemical properties on the use of waste materials in concrete applications. references [1] s. weckend, a. wade, g. heath. end of life management: solar photovoltaic panels. tech. rep., national renewable energy laboratory (nrel), 2016. https://doi.org/10.2172/1561525 [2] g. ansanelli, g. fiorentino, m. tammaro, a. zucaro. a life cycle assessment of a recovery process from end-of-life photovoltaic panels. applied energy 290:116727, 2021. https://doi.org/10.1016/j.apenergy.2021.116727 [3] b. bechník. recyklace fotovoltaických panelů na konci životnosti, 2011. [4] m. s. chowdhury, k. s. rahman, t. chowdhury, et al. an overview of solar photovoltaic panels’ end-of-life material recycling. energy strategy reviews 27:100431, 2020. https://doi.org/10.1016/j.esr.2019.100431 [5] k. macalova, v. vaclavik, t. dvorsky, et al. recycling of photovoltaic panels a review of the current trends. iop conference series: materials science and engineering 867(1):012029, 2020. https://doi.org/10.1088/1757-899x/867/1/012029 [6] d. mariaková, z. jirkalová, j. řepka, et al. utilization of photovoltaic panels waste glass in high-performance concrete. aip conference proceedings 2322(1):020020, 2021. https://doi.org/10.1063/5.0042427 [7] d. mariaková, k. a. mocová, k. fořtová, et al. waste glass powder reusability in high-performance concrete: leaching behavior and ecotoxicity. materials (basel) 14(16):4476, 2021. https://doi.org/10.3390/ma14164476 [8] d. mariaková, k. a. mocová, k. fořtová, et al. alkali-silica reaction elimination potential of high-performance concrete containing glass powder. materials (basel) 15(19):6574, 2022. https://doi.org/10.3390/ma15196574 [9] k. máčalová, k. kučová, v. václavík, et al. algae as a part of microorganisms involved in biocorrosion of cement composites with total replacement of natural aggregates by photovoltaic glass. iop conference series: earth and environmental science 900(1):012022, 2021. https://doi.org/10.1088/1755-1315/900/1/012022 [10] m. stehlík, j. knapová, v. kostka. possibilities of use of glass recyclate from photovoltaic panels for concrete masonry units. iop conference series: materials science and engineering 549(1):012006, 2019. https://doi.org/10.1088/1757-899x/549/1/012006 78 https://doi.org/10.2172/1561525 https://doi.org/10.1016/j.apenergy.2021.116727 https://doi.org/10.1016/j.esr.2019.100431 https://doi.org/10.1088/1757-899x/867/1/012029 https://doi.org/10.1063/5.0042427 https://doi.org/10.3390/ma14164476 https://doi.org/10.3390/ma15196574 https://doi.org/10.1088/1755-1315/900/1/012022 https://doi.org/10.1088/1757-899x/549/1/012006 acta polytechnica ctu proceedings 47:75–78, 2024 1 introduction 2 current trends in pv recycling 2.1 reduce, reuse and recycle 2.2 concrete mixtures containing pv panels glass 2.3 concrete masonry units 3 conclusion acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2024.50.0069 acta polytechnica ctu proceedings 50:69–73, 2024 © 2024 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague micromechanical characterization of γ-irradiated cement paste exposed to different relative humidity conditions jiří němeček 1,a,∗, lukáš procházkab, patricie halodováb, jiří němeček 2,a a czech technical university in prague, faculty of civil engineering, department of mechanics, thákurova 7, 166 29 prague 6, czech republic b research centre řež, hlavní 130 řež, husinec 250 68, czech republic ∗ corresponding author: jiri.nemecek.1@fsv.cvut.cz abstract. in this study, well-hydrated cement paste samples made from portland cement cem i-42.5r were gamma irradiated with a 200tbq 60co source, resulting in a total absorbed dose of 13.82mgy. during irradiation, the samples were stored under specific relative humidity (rh) conditions of 11%, 33%, 76%, 96%, and water. grid nanoindentation was performed on polished sample surfaces to assess the nanoindentation young’s modulus of both irradiated and non-irradiated control samples. an increase in young’s modulus by 9.3% was observed for irradiated samples at low rh (11%). samples stored at medium rhs (33%, 76%) showed negligible differences in micromechanical properties. however, a significant decrease in young’s modulus, approximately 15%, was measured for samples stored at high rhs (96%, water). keywords: cement paste, c–s–h gel, gamma radiation, nanoindentation, relative humidity. 1. introduction concrete in nuclear power plants, used as biological shielding around the active zone, is exposed to radiation and thermal strains [1]. a decrease in mechanical properties, primarily the compressive strength of concrete, was previously reported in [2, 3]. the deterioration is mainly caused by neutron radiation, gamma radiation, drying, and heating [1]. gamma rays primarily affect the water in calcium-silicate-hydrates (c–s–h) gel due to water radiolysis, causing concrete drying [4]. as most of the studies are conducted on a macro-scale, only a limited amount of research investigates mechanical properties at the micro-scale. micromechanical properties of gamma irradiated mortar with an absorbed dose of 12–15mgy were previously studied by khmurovska et al. [5]. the samples were exposed to standard relative humidities (rhs) of 40%–60%, showing a negligible effect on the young’s modulus of c–s–h phases. similar findings were presented by hilloulin et al. [6], but with a significantly lower total absorbed dose of 0.257mgy. further, nanoindentation of c–s–h pellets irradiated at 11% rh was performed by hunnicutt [7], who irradiated samples up to a dose of 0.784mgy, reporting again negligible changes in micromechanical properties. however, the authors continued irradiating the pellets with significantly higher doses of 24mgy 1postdoctoral researcher at czech technical university in prague, orcid: 0000-0002-5635-695x. 2professor at czech technical university in prague, orcid: 0000-0002-3565-8182. and 189mgy in baral et al. [8]. the higher dose falls within the range of approximately 100–200mgy, which is the total gamma dose to which a biological shield is exposed over an 80-year lifespan [9]. in these cases, a significant increase in the nanoindentation young’s modulus by 15%–25% was reported for the 189mgy total dose. it was also reported by němeček et al. [10] that the rh level in the environment significantly affects the micromechanical properties of 28-day-old cement paste under gamma radiation (absorbed dose 2.88mgy), especially at low (4–11%) and high (100%) rhs. therefore, it is evident that during the gamma irradiation of cementitious materials, not only the total absorbed dose but also the rh level and the samples’ hydration degree affect the final micromechanical properties. thus, this study aims to quantify the effect of rh conditions on mature ordinary cement paste exposed to a higher gamma irradiation dose (13.82mgy). 2. experiments and methods 2.1. material and sample preparation the ordinary cement paste samples were prepared from portland cement cem i-42.r (českomoravský cement, czech republic) with a water-to-cement ratio of 0.4. after mixing, the paste was cast into cylindrical molds with a diameter of 27mm and a height of 70mm, and then vibrated to remove entrapped air. the samples were demolded after 1 day and placed 69 https://doi.org/10.14311/app.2024.50.0069 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en j. němeček, l. procházka, p. halodová, j. němeček acta polytechnica ctu proceedings label 11 33 76 96 w rh (%) 11 33 76 96 100 salt licl mgcl2 nacl kno3 – other – – – – water table 1. labeling and summary of conditions used to control rh. in a 1% limewater solution for 20 months, representing fully hydrated samples with degree of hydration (doh) estimated as 0.88 based on the cemhyd3d calculation [11]. this high doh was chosen to minimize changes due to varying storage conditions during one year of gamma irradiation. subsequently, the samples were cut into smaller cylinders, 20mm thick, and further placed into plastic containers with five different rh (11%, 33%, 76%, 96%, and 100%) conditions. four of these conditions were created using salt-saturated solutions, while the fifth container was filled with limewater, where the samples were originally stored. the details about the rh conditions and the salts used are provided in table 1. 2.2. gamma irradiation the containers with samples were gamma irradiated using a 60co radiation source in the gamma radiation facility of the research centre řež, czech republic. the average dose rate ranged from 0.675 to 1.890 kgy h−1. the samples were irradiated (ir) for 341 days and absorbed a total dose of 13.82mgy. simultaneously, non-irradiated (non-ir) containers were left under the same rh conditions. the plastic containers were replaced every two months to ensure stable rh conditions throughout the irradiation period [10]. 2.3. sample surface polishing after the gamma irradiation experiment, all samples were removed from the containers and dried for 3 days at 50 °c to prepare them for polishing. doh for samples below 76% rh remain 0.88, while those stored at 96% rh and 100% rh showed only a minimal increase of doh, estimated as 0.90. the dried samples were then polished using silicon carbide papers in the following sequence: #1200 grit for 10 s, #2000 grit for 60 s, and #4000 grit for 200 s. between each step, the samples were placed in an ultrasonic cleaner with alcohol for 60 s to remove any free particles [12]. the surface roughness of the samples, characterized by the root-mean-square (rms) parameter, was 41± 7 nm on 55× 55 µm2, which is acceptable for nanoindentation with ∼200 nm deep indents. 2.4. nanoindentation the nanoindentation was performed with a hysitron ti-980 triboindenter equipped with a berkovich diamond tip. on each sample, ten grids of 22× 22 indents were conducted using the accelerated property mapping mode (xpm). the separation between individual indents was set to 2.5µm to provide high spatial resolution and avoid interaction between indents. a numerical study by phani and oliver [13] showed that the safe indentation distance is 10 times the indentation depth. since our indentations reach depths of up to 200 nm, the 2.5µm separation falls within the safe range. a total of 484 indents were performed. the load function was selected as trapezoidal, consisting of linear loading, maximum force holding, and linear unloading, each lasting 0.1 s. the maximum force was set to 1.5mn. the elastic properties were evaluated from the unloading segment of the force-displacement curve using the oliver and pharr theory [14], and the reduced modulus er was determined as er = s √ π 2β √ ac , (1) where ac is the tip projected contact area, s represents the elastic unloading stiffness, and β is the tip geometry correction factor (β = 1.034 for a berkovich tip). the reduced modulus combines the elastic stiffness of both the nanoindentation tip and the material. the young’s modulus, e, of the (isotropic) material can then be calculated using the following equation: 1 er = 1− ν2 e + 1− ν2 i ei , (2) where ei and νi represent the young’s modulus and poisson’s ratio of the nanoindentation tip, respectively. for a diamond tip, the constants are assumed to be ei = 1141gpa and νi = 0.07. ν denotes the poisson’s ratio of the sample, which was assumed to be 0.2 [15]. 3. results and discussion in figure 1, a typical example of a young’s modulus map measured by xpm is shown. a large range of e values, from 10 to 130gpa were measured and based on the mechanical response and chemical composition, five different phases (residual clinker, portlandite, inner product, outer product, low stiffness phase) were recognized [12]. the first phase consists of residual clinker with e values ranging from 50gpa to 130gpa. the second phase composed of portlandite crystals where e are typically measured in the range of 36– 43gpa. additionally, two phases based on c–s–h gel varying mainly in density of hydrates, were identified. the inner product, created primarily around residual clinker (e = 26–38gpa) and outer product, more porous phase connecting the rest of the hydrates (e = 19–26gpa). the last, low stiffness phase was mostly affected by larger pores with e < 15gpa [15, 16]. the typical load–displacement curves of these phases are shown in figure 2. in the next step, all 4840 indents were merged together, and frequency density plots were created for 70 vol. 50/2024 micromechanical characterization of γ-irradiated cement paste young's modulus (gpa) 55 µm residual clinker inner product outer product portlandite low stiffness phase 12 20 30 40 50 60 70 80 figure 1. an example of a young’s modulus map obtained using xpm for the 76-ir sample. 0 50 100 150 200 250 300 350 400 0.0 0.5 1.0 1.5 2.0 displacement (nm) f o rc e (m n ) clinker inner product portlandite outer product low stiffness phase figure 2. an example of typical force-displacement curves of main phases measured for the 76-ir sample. each sample. an example of these plots is shown in figure 3 for samples stored at 33% rh. it is evident that the main peak is predominantly created by the main hydration products, including inner and outer products and portlandite. this behavior was similar for all samples, with only a shift in the main peak being observable. thus, it was decided not to separate the mechanical response into the main five phases but instead into two phases: one created by the main hydrates and the other by residual clinker, using a statistical deconvolution method, which assigns gaussian distribution for each phase, as shown in figure 4 for 76-ir sample. more details of the method can be found in [15, 16]. based on these results, the micro-mechanical behavior between irradiated and non-irradiated samples was analyzed and is summarized in figure 5. the first group consists of samples stored at medium rhs (33%, 76%). the results show negligible differences between irradiated and non-irradiated samples, suggesting that the water loss caused by radiolysis is most likely counterbalanced by the rh conditions in the container. these findings are in accordance with other studies and apply to total gamma doses in the range of 0.257–15mgy [5, 6, 10]. 0 20 40 60 80 100 120 0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10 young’s modulus (gpa) fr eq ue nc y de ns ity () 33% rh irradiated 33% rh non-irradiated figure 3. a typical example of frequency plots of young’s modulus for irradiated and non-irradiated samples stored at 33% rh. 0 20 40 60 80 100 120 0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 young’s modulus (gpa) f re q u en cy d en si ty () main hydrates residual clinker experimental pdf figure 4. an example of statistical deconvolution of young’s modulus for the 76-ir sample. the second group consists of samples stored at low rh (11%), which showed an increase in e by 9.5% for non-irradiated samples and by 19.9% for irradiated samples compared to those stored at medium rhs. this increase is attributed to drying shrinkage leading to the densification and compaction of c–s–h globules [17, 18]. the effect is even more pronounced in irradiated samples, where water radiolysis likely removes water from the interlayer space, further enhancing the densification and compaction effect. the increase of irradiated sample was measured as 9.3%. the increase in irradiated samples aligns with other studies conducted at 11% rh [8, 10]. however, the increase is much smaller compared to our previous study [10], where a 24.8% increase was measured for samples irradiated with a significantly lower dose of 2.88mgy. the main reason for this difference lies in the total hydration times of the samples; in the previous study, the samples were hydrated for only 4 weeks, compared to the well-matured samples that were hydrated for 1.5 years in the current study. this suggests that the degree of hydration at the beginning of irradiation has a significant effect on the 71 j. němeček, l. procházka, p. halodová, j. němeček acta polytechnica ctu proceedings 11 % 33 % 76 % 96 % w(100%) 0 10 20 30 40 50 relative humidity y o u n g’ s m o d u lu s (g p a) irradiated non-irradiated figure 5. summary of young’s modulus results of main hydrates for irradiated and non-irradiated samples stored at different rh levels. overall changes due to irradiation. the increase in irradiated samples aligns with other studies conducted at 11% rh [8, 10]. additionally, it should be noted that the local increase in e is accompanied by micro-cracking of the samples, leading to a decrease in mechanical properties at larger scales [10]. the third group consists of samples stored at high rhs (96%, water–100%). the mechanical properties of this group showed a significant decrease in e by 14.6% (96% rh) and 15.5% (water), indicating the most negative effect and deterioration of the samples due to irradiation. similar finding were also reported by němeček et al. [10] for 28 days old samples irradiated with dose of 2.88mgy. this deterioration is likely due to water radiolysis, although the exact mechanism is still unknown. further analyses, such as x-ray diffraction, thermogravimetry, and porosity measurements by mercury intrusion porosimetry, will be performed to detect changes in structure and chemical composition. the second phase created by residual clinker is not affected by gamma radiation, as it does not impact the crystalline phase [1]. the measured differences in micro-mechanical properties are primarily attributed to the dimensions of the clinker, which are embedded in the surrounding compliant cement matrix. 4. conclusions in this paper, the nanomechanical properties of ordinary cement paste exposed to different rhs and gamma radiation (total dose of 13.82mgy) were investigated. based on the different rh levels, the following conclusions were drawn: • samples irradiated at low rh (11%) showed an increase in the young’s modulus by 9.3%. this increase is attributed to the combination of water radiolysis and drying, possibly due to water removal from the interlayer space and compaction of the c– s–h phases. • samples irradiated at medium rhs (33%, 76%) showed almost identical nanomechanical properties, suggesting that samples in these conditions withstand gamma radiation best. • samples at high rhs showed a decrease of young’s modulus by 14.6% (96% rh) and 15.5% (water), indicating that samples at high rh perform worst during gamma irradiation. future research should complement these findings with additional analyses detecting changes in porosity and chemical composition. acknowledgements this work was financially supported by the project of the czech science foundation grant number 23-05435s. the presented results were obtained using the cicrr infrastructure, which is financially supported by the ministry of education and culture – project lm2023041. references [1] t. rosseel, i. maruyama, y. le pape, et al. review of the current state of knowledge on the effects of radiation on concrete. journal of advanced concrete technology 14(7):368–383, 2016. https://doi.org/10.3151/jact.14.368 [2] h. k. hilsdorf, j. kropp, h. j. koch. the effects of nuclear radiation on the mechanical properties of concrete. special publication 55:223–254, 1978. [3] k. g. field, i. remec, y. l. pape. radiation effects in concrete for nuclear power plants – part i: quantification of radiation exposure and radiation effects. nuclear engineering and design 282:126–143, 2015. https://doi.org/10.1016/j.nucengdes.2014.10.003 [4] p. bouniol, a. aspart. disappearance of oxygen in concrete under irradiation: the role of peroxides in radiolysis. cement and concrete research 28(11):1669–1681, 1998. https://doi.org/10.1016/s0008-8846(98)00138-0 [5] y. khmurovska, p. štemberk, s. sikorin, et al. effects of gamma-ray irradiation on hardened cement mortar. international journal of concrete structures and materials 15(1):1–14, 2021. https://doi.org/10.1186/s40069-020-00452-7 [6] b. hilloulin, m. robira, a. loukili. coupling statistical indentation and microscopy to evaluate micromechanical properties of materials: application to viscoelastic behavior of irradiated mortars. cement and concrete composites 94:153–165, 2018. https: //doi.org/10.1016/j.cemconcomp.2018.09.008 [7] w. hunnicutt, e. t. rodriguez, p. mondal, y. le pape. examination of gamma-irradiated calcium silicate hydrates. part ii: mechanical properties. journal of advanced concrete technology 18(10):558– 570, 2020. https://doi.org/10.3151/jact.18.558 72 https://doi.org/10.3151/jact.14.368 https://doi.org/10.1016/j.nucengdes.2014.10.003 https://doi.org/10.1016/s0008-8846(98)00138-0 https://doi.org/10.1186/s40069-020-00452-7 https://doi.org/10.1016/j.cemconcomp.2018.09.008 https://doi.org/10.1016/j.cemconcomp.2018.09.008 https://doi.org/10.3151/jact.18.558 vol. 50/2024 micromechanical characterization of γ-irradiated cement paste [8] a. baral, e. t. rodriguez, w. a. hunnicutt, et al. ultra-high gamma irradiation of calcium silicate hydrates: impact on mechanical properties, nanostructure, and atomic environments. cement and concrete research 158:106855, 2022. https://doi.org/10.1016/j.cemconres.2022.106855 [9] o. kontani, s. sawada, i. maruyama, et al. evaluation of irradiation effects on concrete structure: gamma-ray irradiation tests on cement paste. in asme power conference, vol. 56062, p. v002t07a002. american society of mechanical engineers, 2013. [10] j. němeček, p. trávníček, m. keppert, et al. nanomechanical analysis of gamma-irradiated cement paste exposed to different humidities. construction and building materials 393:131969, 2023. https: //doi.org/10.1016/j.conbuildmat.2023.131969 [11] d. p. bentz. cemhyd3d: a three-dimensional cement hydration and microstructure development modeling package, version 3.0. [2024-06-26]. https://www.nist.gov/services-resources/ software/cemhyd3d [12] j. němeček, j. lukeš, j. němeček. high-speed mechanical mapping of blended cement pastes and its comparison with standard modes of nanoindentation. materials today communications 23:100806, 2020. https://doi.org/10.1016/j.mtcomm.2019.100806 [13] p. sudharshan phani, w. c. oliver. a critical assessment of the effect of indentation spacing on the measurement of hardness and modulus using instrumented indentation testing. materials & design 164:107563, 2019. https://doi.org/10.1016/j.matdes.2018.107563 [14] w. c. oliver, g. m. pharr. an improved technique for determining hardness and elastic modulus using load and displacement sensing indentation experiments. journal of materials research 7(6):1564–1583, 1992. https://doi.org/10.1557/jmr.1992.1564 [15] g. constantinides, f.-j. ulm. the nanogranular nature of c–s–h. journal of the mechanics and physics of solids 55(1):64–90, 2007. https://doi.org/10.1016/j.jmps.2006.06.003 [16] g. constantinides, f.-j. ulm. the effect of two types of csh on the elasticity of cement-based materials: results from nanoindentation and micromechanical modeling. cement and concrete research 34(1):67–80, 2004. https://doi.org/10.1016/s0008-8846(03)00230-8 [17] j. němeček, j. maňák, j. němeček, t. krejčí. effect of vacuum and focused ion beam generated heat on fracture properties of hydrated cement paste. cement and concrete composites 100:139–149, 2019. https: //doi.org/10.1016/j.cemconcomp.2019.03.027 [18] p. suwanmaneechot, a. aili, i. maruyama. creep behavior of c-s-h under different drying relative humidities: interpretation of microindentation tests and sorption measurements by multi-scale analysis. cement and concrete research 132:106036, 2020. https://doi.org/10.1016/j.cemconres.2020.106036 73 https://doi.org/10.1016/j.cemconres.2022.106855 https://doi.org/10.1016/j.conbuildmat.2023.131969 https://doi.org/10.1016/j.conbuildmat.2023.131969 https://www.nist.gov/services-resources/software/cemhyd3d https://www.nist.gov/services-resources/software/cemhyd3d https://doi.org/10.1016/j.mtcomm.2019.100806 https://doi.org/10.1016/j.matdes.2018.107563 https://doi.org/10.1557/jmr.1992.1564 https://doi.org/10.1016/j.jmps.2006.06.003 https://doi.org/10.1016/s0008-8846(03)00230-8 https://doi.org/10.1016/j.cemconcomp.2019.03.027 https://doi.org/10.1016/j.cemconcomp.2019.03.027 https://doi.org/10.1016/j.cemconres.2020.106036 acta polytechnica ctu proceedings 50:69–73, 2024 1 introduction 2 experiments and methods 2.1 material and sample preparation 2.2 gamma irradiation 2.3 sample surface polishing 2.4 nanoindentation 3 results and discussion 4 conclusions acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2023.45.0059 acta polytechnica ctu proceedings 45:59–65, 2023 © 2023 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague considerations on an embedded pile’s effective length in an analytical calculation according to the commentary on the stn 73 1002 standard jakub stacho∗, monika súľovská slovak university of technology in bratislava, faculty of civil engineering, department of geotechnics, radlinského 11, 810 05 bratislava, slovakia ∗ corresponding author: jakub.stacho@stuba.sk abstract. the analytical model for calculating the bearing capacity of a pile, presented in the commentary to stn 73 1002 – pile foundations, gives a recommendation to reduce the pile’s length in the calculation of the shaft resistance. the recommendation is based on caquot-kérisel’s theory. especially in the case of embedded piles, reducing a pile’s length in the calculation can cause the shaft friction of the embedded pile to be neglected, and the calculated resistance of the pile is thus significantly lower. the results of instrumented static load tests of embedded piles were analysed. the main aim of the study was to verify the validity of reducing the pile’s length in the analytical calculation of the shaft resistance. the results of the static load tests analysed did not show any reduction in the shaft friction on the embedded part of the pile. in the form of a parametric study, the effect of reducing the pile’s length in the calculation of shaft friction was analysed for different dimensions of embedded piles. keywords: design of pile, effective length of pile, length of pile, resistance of pile, shaft resistance. 1. introduction the vertical bearing capacity of the pile can be determined using various analytical calculation models, e.g., [1–3]. the analytical calculation model, which is stated in detail in the commentary on the stn 73 1002 standard [4], is the one most used in slovakia and the czech republic. this calculation model is also customarily deployed in the analytical calculation of the pile’s bearing capacity using the geotechnical software fine geo5. the individual calculation models presented by the authors referenced just above differ from each other in local experience, typical engineering geological conditions or piling technology. the differences between each calculation model lie in the considered mechanism of the failure of the earth environment in the vicinity of the pile, which was stated by, e.g., [5]. the analytical calculation models determining the pile’s bearing capacity have been constantly developed and modified by many authors to consider different geological, geometrical, and technological aspects, see e.g., [6? –17]. these calculation models apply different coefficients, which can consider the geometry of the pile, the adhesion and friction in the pile body-soil interaction, the lateral earth pressure acting on the pile’s shaft, and the piling technology. the analytical model given in the commentary on the stn 73 1002 standard also mentions the matter of the shortening of the pile’s length in calculating the shaft friction according to caquot-kérisél’s theory [4]. they pointed to creating an area of plastic stresses near the base of the pile, which begin to arise when the vertical load reaches the vertical bearing capacity of the pile. in the case of other frequently used calculation models, mostly in other countries, a reduction of the pile’s length in calculating the shaft friction is not used in this form. the decrease of the shaft friction for the loads that reach the pile’s bearing capacity and a relatively large settlement is mainly attributed to the residual stress state in the pile-body soil interface. when the effective length is applied in calculating the pile’s shaft resistance, especially in the case of an embedded (end-bearing) pile, this often means that the shaft resistance of the embedded part of the pile is neglected. the aim of the study presented was to analyse the results of the instrumented static load tests of embedded piles with a focus on the distribution of axial force and the justification of a shortening of the pile’s length in the calculation of its bearing capacity. 2. calculation of the vertical resistance of the pile according to commentary on stn 73 1002 the calculation model and the detailed description of the calculation procedure are given in the commentary to the stn 73 1002 standard [4]. this calculation model is also taken by other authors, e.g., [9, 10]. the static scheme of the calculation model is shown in figure 1. determining the design value of the pile’s vertical resistance and the influence of partial factors of different design approaches was presented by, e.g., [18]. the characteristic resistance of the pile (rk) is given by the sum of the characteristic base resistance (rb,k) and characteristic shaft resistance 59 https://doi.org/10.14311/app.2023.45.0059 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en jakub stacho, monika súľovská acta polytechnica ctu proceedings figure 1. the static scheme of the calculation of the vertical pile resistance (according to [9]). (rs,k) according to the following formula: rc,k = rb,k + rs,k, (1) where rb,k can be calculated as: rb,k = k1 ab rk, (2) rk = c′ d,2 · nc,2 + q′ · nq,2 + 0.7 · γ2 · d 2 · nγ,2, (3) rk =1.2 · c′ d,2 · nc,2 + ( 1 + sin φ′ d,2 ) · q′ · nq,2 + 0.7 · γ2 · d 2 · nγ,2, (4) where nc,2, nq,2, and ng,2 are the ultimate bearing capacity factors; φ′ d,2 and c′ d,2 are drained shear strength properties; γ2 is the unit weight of the soil; and q′ is the effective geostatic stress in the depth of the pile’s base. the characteristic value of the shaft resistance can be determined using the following formula: rs,k = π · d n∑ i=1 hi · fs,i, (5) where d is the diameter of the pile, hi is the length of the pile’s shaft in the i-layer, and fs,i is the friction on the pile’s shaft in the i-layer, determined from the equation 6 as follows: fs,i = k2 · σor,i · tan ( φ′ d γr,1 ) + c′ d γr,2 . (6) the values of coefficients γr,1, γr,2, and k2 are presented by [4]. the parameter σor,i is the geostatic stress in the middle of the i-layer. the calculation model also states that the effective length of the pile, used in the shaft resistance calculation, can be shortened according to caquot-kérisel by the length of lp (figure 1) [4]. this reduction in length can be calculated using the following formula: lp = n2/3 q · d 4 , (7) where nq is the ultimate bearing capacity factor given by the formula (8) as follows: nq = exp (π · tan φ′ d) tan2 ( 45 + φ′ d 2 ) . (8) based on equations (7) and (8), the reduction length lp is controlled by parameters d and φ′. the dependence between these input parameters and the resulting lp value is shown in figure 2. it can be seen that especially for coarse-grained soils, which have a value of φ′ d mostly greater than 30◦, the reduction length lp can be significant. in the case of gravel soil with an angle of shear strength of 40◦ and the pile with a diameter of 1.5 m, the reduction in length can reach up to 6 m. 60 vol. 45/2023 embedded pile’s effective length figure 2. diagram for determining lp length. figure 3. the resistance of the pile determined from a typical load-settlement curve. 3. analysis of the results of the instrumented static load tests a load-settlement curve of the pile can have different shapes. this mainly depends on the engineeringgeological conditions and the pile’s geometry. according to the shape of the load-settlement curve (figure 3), the following resistances can be determined [9]: • ultimate resistance (figure 3 – a); • resistance on the limit of the proportionality (figure 3 – b); • resistance on the limit of the deformation (figure 3 – c); • indicative resistance (figure 3 – d). in the case of end-bearing (embedded) piles, depending on the shape of the load-settlement curve, a settlement of the pile’s head equal to 10 % of the pile diameter should be considered as the limit of the failure according to stn en 1997-1. this corresponds with determining the characteristic resistance of the pile according to figure 3 c. in this case, the loadsettlement curve must be executed up to the pile’s settlement equal to 10 % of its diameter. the loadsettlement curves of different end-bearing piles, which fulfilled the criteria of the required settlement of 10 % of the pile’s diameter, were selected for the analysis presented. the tested piles were instrumented, i.e., the piles were equipped with strain gauges. this allowed for analysis of the load distribution over the pile’s length. subsequently, the results of one tested pile are pre61 jakub stacho, monika súľovská acta polytechnica ctu proceedings figure 4. load-settlement curve of the pile determined by the static load test (left), load-settlement curves of the base and total resistance of the pile used in the calculation (right). figure 5. load transfer over the pile’s length determined by the static load test. sented in detail. the pile tested had a diameter of 410 mm and a length of 17.9 m. the geological conditions at the testing site consisted of fine-grained soils (f) up to a depth of about 13.9 below the surface and coarse-grained soils, sand (s) and gravel (g), below them. the fine-grained soils consisted of organic soil (o) and sandy silt (ms) of soft consistency. a layer of medium-dense silty sand (sm) was located at a depth of 13.9 to 16.5 m below the surface. gravel with fines fraction (gf) was located below the sandy soil. the tested pile was reinforced using a gewi bar. the strain gauges were installed in couples at the depths of 0.5, 13.9, 16.5, and 17.8 m. the load-settlement curve of the pile tested is shown in figure 4 – left. during the test, an unloading and reloading were applied two times. after reaching the maximum loading, i.e., 2000 kn, the final unloading was also applied. the load-settlement curve of the total pile’s resistance and the load-settlement curve of the pile’s base resistance are shown in figure 4 – right. when the maximum load of 2000 kn was applied, the resistance of the base reached about 772 kn. the resistance of the shaft was about 1228 kn. the load distribution over the pile’s length is shown in figure 5. the load distribution is shown for each loading step, i.e., 333, 666, 1000, 1333, 1666, and 2000 kn. in the case of the given static load test, and similarly to the other static load tests analysed, the criterion of settlement of the pile head equal to 10 % of its diameter was met. it is evident that there was no decrease or any reduction in the shaft friction in the embedded part of the pile. the results of three static 62 vol. 45/2023 embedded pile’s effective length pile no. 1 vertical load [kn] 1000 1333 1666 2000 shaft friction – layer no. 1 – low bearing [kpa] 26 35 43 52 shaft friction – layers nos. 2 and 3 – high/end bearing [kpa] 25 30 43 46 stress below a pile’s base [kpa] 3226 4376 5488 6657 pile no. 2 vertical load [kn] 1000 1333 1666 2000 shaft friction – layer no. 1 – low bearing [kpa] 17 20 22 22 shaft friction – layers nos. 2 and 3 – high/end bearing [kpa] 147 220 300 440 stress below a pile’s base [kpa] 1569 1876 2048 2256 pile no. 3 vertical load [kn] 1000 1333 1666 2000 shaft friction – layer no. 1 – low bearing [kpa] 23 27 35 41 shaft friction – layers nos. 2 and 3 – high/end bearing [kpa] 90 149 157 175 stress below a pile’s base [kpa] 1809 2003 3255 4449 note: layer no. 1 (o, mg, cg); layers nos. 2 and 3 (sm and g-f) – see figure 5 table 1. physical properties of holocene organic soils. load tests of the end-bearing piles in similar geological conditions are presented in table 1. the results present the shaft friction in layer no. 1 (fine-grained soil) – low bearing stratum, the shaft friction in layer nos. 2–3 (coarse-grained soils) – high/end bearing stratum, and stress below the pile’s base. in all the cases presented, there is no evident decrease in the shaft friction of the embedded part of the pile. on the contrary, the results clearly show that the piles take over a significant part of the shaft resistance by the friction in the embedded part of the pile – even when the pile’s settlement reaches about 10 % of the pile’s diameter. tomlinson and woodward state that a reduction in the shaft friction can only be expected when the load-settlement curve reaches the ultimate resistance of the pile (figure 3 – a); however, in the case of the end-bearing pile this can occur for a significantly greater load when the settlement of the pile exceeded about 20 % of the pile’s diameter. in the case of the static load test presented it can therefore be assumed that with the increase in the load, the ultimate resistance of the pile can be reached. in that case, a reduction in the shaft friction can be expected. 4. parametric study on the effect of reducing the pile’s length in the calculation a simple parametric study was created to demonstrate how significant the effect of the pile’s length reduction can be in the shaft resistance calculation. the geological profile consisted of two layers, i.e., low-bearing stratum (fine-grained soil) and high-bearing stratum (coarse-grained soil). the fine-grained soil was defined by the following properties: γ = 20 kn m−3, φ′ = 15◦, and c′ = 10 kpa. the coarse-grained soil was defined by the following properties: γ = 20 kn m−3, φ′ = 35◦, and c′ = 0 kpa. the total design resistance rc,d and the design shaft resistance rs,d was calculated for the end-bearing (embedded) pile of a diameter of 0.6, 0.9, 1.2, and 1.5 m; and a length of 10, 15, and 20 m. the embedded part of the pile was equal to 2 m for a 10 m long pile, 3 m for a 15 m long pile, and 4 m for a 20 m long pile. the results are presented in table 2. the total resistance of the pile rc,d and the shaft resistance rs,d,1 were calculated for the case when the reduction length was not considered. in the second case, the shaft resistance rs,d,2 determined for the pile’s length reduced by lp, was computed. a difference ∆rs,d between both shaft resistances was determined. it can be seen that considering the reduced length of the pile in the calculation can cause a significant reduction in the shaft resistance. 5. discussion on the results presented in general, it can be stated that the resistance of the pile and its deformation behaviour is a complex mechanism that is difficult to describe and define with a simple analytical calculation model. when the pile is continuously loaded, shaft friction is first mobilized. subsequently, with a further increase in load, it is possible to reach the limit stress at the pile base. in the case of soils, where a significant difference occurs between the peak and critical/residual shear strength, when the pile is gradually loaded, there is a subsequent decrease in the shaft friction, as shown in figure 6. in this case, using peak shear strength parameters in the calculation can be incorrect. this effect was not observed for the testing piles analysed. this fact is related to the decrease in the shear strength (from the peak to the residual) and is not related to neglecting the shaft friction near the base of the pile. the shaft friction along the pile is usually not constant, e.g., [12]. assuming homogeneous subsoil, the maximum value of the shaft friction is reached in the 63 jakub stacho, monika súľovská acta polytechnica ctu proceedings l [m] d [m] 0.6 d [m] 0.9 d [m] 1.2 d [m] 1.5 lp [m] 1.552 lp [m] 2.329 lp [m] 3.105 lp [m] 3.881 r c ,d [k n ] r s ,d ,1 [k n ] r s ,d ,2 [k n ] ∆ r s ,d [% ] r c ,d [k n ] r s ,d ,1 [k n ] r s ,d ,2 [k n ] ∆ r s ,d [% ] r c ,d [k n ] r s ,d ,1 [k n ] r s ,d ,2 [k n ] ∆ r s ,d [% ] r c ,d [k n ] r s ,d ,1 [k n ] r s ,d ,2 [k n ] ∆ r s ,d [% ] 10 2370 542 354 35 4969 813 432 47 8549 1085 518 52 13140 1356 574 58 15 3849 1127 845 25 7859 1690 1056 38 13300 2254 1058 53 20190 2817 1228 56 20 5537 1920 1545 20 11060 2880 2035 29 18460 3840 2338 39 27770 4800 2453 49 table 2. impact of reducing the pile’s length in the calculation of shaft frictions for different dimensions of the embedded piles. figure 6. load-settlement relationships – mobilization of the shaft friction and resistance at the base of the pile (according to [1]). vicinity above the pile’s base. it subsequently decreases significantly towards the pile’s base (figure 7). according to the commentary to the stn 73 1002 standard, this decrease in the shaft friction is theoretically taken into account in the calculation model precisely by introducing the effective length of the pile. however, only a decreasing in the shaft friction occurs, and therefore it is questionable if complete neglect of the shaft friction in this part of the pile is appropriate. figure 7 also shows the calculation course of the shaft friction when applying the critical depth defined by, e.g., the navfac dm7.02 standard. under certain conditions, applying the critical depth appears to be more appropriate than neglecting the shaft friction near the pile base; however, the number of analysed static load tests did not allow a deeper analysis that could lead to a more detailed conclusion. in the design of the pile, both the ultimate limit state (uls) and the serviceability limit state (sls) must be verified. the sls condition is often stricter and thus decisive for the design of the pile. however, in the practical design of a pile foundation, a paradoxical situation often occurs when the sls condition is fulfilled, the settlement of the piles is a relatively small value (e.g. about 20 ∼ 30 mm), but the calculated resistance of the pile does not meet the given condition. figure 7. transfer of the shaft friction over pile’s length (according to [16]). especially in the case of end-bearing (embedded) piles, a significant resistance of the pile shaft is represented by the shaft friction of the embedded part of the pile, which is a complete neglect in the calculation when the effective length of the pile is considered. the criterion of the maximum settlement of the pile is usually lower than when the pile’s settlement equals 10 % of its diameter. then, if the sls condition is fulfilled, it is unlikely that the pile settlement should increase to such an extent that a significant decrease in shaft friction near the pile base can be experienced. it should be noted that the analytical calculation model stated in the commentary to the stn 73 1002 standard mentions introducing the effective length of the pile into the calculation as a recommendation, not as a required condition. in the end, it is up to the designer how it will be considered in the pile design. 6. conclusions the analytical model for calculating the resistance of a single pile, given in the commentary to the stn 73 1002 standard, introduces the recommendation of applying the pile’s effective length into the pile’s shaft resistance calculation, according to the caquotkérisel theory. theoretically, the formation of an onion-shaped area of plastic stresses near the pile base is assumed when the load approaches the ultimate resistance of the pile. as a result of this effect, 64 vol. 45/2023 embedded pile’s effective length the shaft friction near the pile base should be ignored. especially in the case of end-bearing (embedded) piles, this may mean that the resistance of the embedded part of the pile is not included in the total resistance of the pile, although its contribution to the pile’s total resistance can be significant. the results of the instrumented static load tests were analysed, in which the settlement of the pile head equal to 10 % of their diameter was achieved, and it was assumed that the resistance on the limit deformation (figure 3 – c) was reached. the results of the static load tests showed that even when the pile head settled at 10 % of its diameter, there was no reduction in the shaft friction in the embedded part of the pile; on the contrary, significant shaft friction was also recorded in the embedded part of the pile. based on the theoretical review, it can be assumed that a decrease in shaft friction could only be noted when the ultimate resistance of the pile is reached, which is particularly difficult for end-bearing (embedded) piles of the given geometry installed in similar geological conditions. the application of the effective (reduced) or actual length of the pile in the calculation of the resistance is at the discretion of the designer/statics. in the case of end-bearing (embedded) piles, where the serviceability limit state condition is fulfilled, the pile deformation does not exceed 10 % of its diameter – it seems more appropriate to consider the total length of the pile in the calculation of the resistance. references [1] m. tomlinson, j. woodward. pile design and construction practice. taylor & francis e-library, 5th edn. p. 551, 2007. [2] d. a. brown, s. dapp, w. r. thompson, et al. geotechnical engineering circular no. 8 – design and construction of continuous flight auger (cfa) piles p. 294, 2007. [3] k. fleming, a. weltman, m. randolph, k. elson. piling engineering. taylor & francis e-library, 3rd edn. p. 398, 2008. [4] r. pochman, j. šimek. pile foundations [in czech – pilotové základy]. commentary to čsn 73 1002, 1.vyd. praha, vydavatelství norem. 1989. p. 75. isbn 80-85111-04-7. [5] c. viggiani, a. mandolini, g. russo, et al. piles and pile foundations. taylor & francis, london, uk. p. 278, 2012. [6] s. škrabl. bearing capacity and settlement of vertically-loaded piles. in deep foundations 2002: an international perspective on theory, design, construction, and performance, pp. 53–63. 2002. https://doi.org/10.1061/40601(256)5 [7] j. zhang, l. hanlong, g. yang. a simplified method for calculating ultimate bearing capacity of cast-in-place concrete pipe pile with large diameter in sandy soil. in advances in pile foundations, geosynthetics, geoinvestigations, and foundation failure analysis and repairs, pp. 75–81. 2011. https://doi.org/10.1061/47631(410)9 [8] j. mecsi. geotechnical engineering examples and solutions using the cavity expanding theory hungarian geotechnical society, 2013, p. 232. [9] j. masopust. bored piles [in czech – vrtané piloty]. prague, čenek a ježek s.r.o., 1994, p. 263. [10] j. hulla, p. turček. foundation engineering [in slovak – zakladanie stavieb]. jaga group s.r.o., bratislava, 1998, p. 310. [11] m. randolph, r. dolwin, r. beck. design of driven piles in sand. geotechnique 44(3):427–448, 1994. https://doi.org/10.1680/geot.1994.44.3.427 [12] b. wrana. pile load capacity–calculation methods. studia geotechnica et mechanica 37(4):83–93, 2015. https://doi.org/10.1515/sgem-2015-0048 [13] j. mihalik, f. gago, m. bukova. determination of experimental bearing capacity piles. in juniorstav 2022: proceedings of the 24th international conference of doctoral students. brno: econ publishing, pp. 297–302, 2022. [14] m. drusa, j. vlcek, f. gago, et al. modern methods of designing geotechnical structures. žilina: university of zilina, 2019, p. 277. [15] m. drusa, f. gago, j. vlček. contribution to estimating bearing capacity of pile in clayey soils. civil and environmental engineering 12(2):128–136, 2016. https://doi.org/10.1515/cee-2016-0018 [16] m. f. randolph, j. p. carter, c. p. wroth. driven piles in clay—the effects of installation and subsequent consolidation. géotechnique 29(4):361–393, 1979. https://doi.org/10.1680/geot.1979.29.4.361 [17] j. mihálik, f. gago, j. vlček, m. drusa. evaluation of methods based on cptu testing for prediction of the bearing capacity of cfa piles. applied sciences 13(5):2931, 2023. https://doi.org/10.3390/app13052931 [18] j. stacho. calculation design of bored piles. in juniorstav 2022: proceedings of the 14th international conference of doctoral students, brno: vut, 2012. 65 https://doi.org/10.1061/40601(256)5 https://doi.org/10.1061/47631(410)9 https://doi.org/10.1680/geot.1994.44.3.427 https://doi.org/10.1515/sgem-2015-0048 https://doi.org/10.1515/cee-2016-0018 https://doi.org/10.1680/geot.1979.29.4.361 https://doi.org/10.3390/app13052931 acta polytechnica ctu proceedings 45:1–7, 2023 1 introduction 2 calculation of the vertical resistance of the pile according to commentary on stn 73 1002 3 analysis of the results of the instrumented static load tests 4 parametric study on the effect of reducing the pile's length in the calculation 5 discussion on the results presented 6 conclusions references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2023.43.0013 acta polytechnica ctu proceedings 43:13–17, 2023 © 2023 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague length of transfer time between connecting trains – is there an ideal value? jan froněk, jaroslav chlumecký∗, daniel vymětal university of pardubice, jan perner’s faculty of transport engineering, department of transport technology and control, studentská 95, 532 10 pardubice, czech republic ∗ corresponding author: jaroslav.chlumecky@student.upce.cz abstract. quality public transport is an essential part of global mobility. in general, there is an effort to ensure that as many transport users as possible use public transport. the public transport subsystems also include rail transport. nowadays, traveling by train is not only a matter of overcoming longer distances in national and international transport, but also a matter of daily commuting to work, school or service. suburban rail transport is usually part of integrated transport systems, the principle of which is the possibility to use several types of means of transport in one journey per ticket. an indicator of the proper functioning of these systems is the existence of quality and convenient transfer dates times and reliable transfer links that minimize waiting times and total time spent on transportation. within these systems, there is no network-specific maximum transfer time, which usually depends on the technical solution of the transfer point, the distance that passengers have to cover between vehicles or check-in technology (for example, front door boarding, turnstiles, etc.). the ideal value of the transfer time in terms of the probability that the passenger will not miss the connection and at the same time represents different aspects of the view of this issue. this is on the specific route ostrava – prague, which is the busiest in the czech republic. within it, two reference links will be selected (one for each direction), the mean value of the delay in the stations with connections and other statistical indicators will be determined. furthermore, a simulation of train delays is created using the monte carlo method, from which the probability of a connection is determined and subsequently the degree of dissatisfaction with the route is determined while maintaining the current connections. from which a transfer time is subsequently identified in which the probability of passing is as low as possible, but at the same time in normal situations passengers do not have to wait too long. keywords: public transport, transfer time, train delay, connecting trains, stability of timetable. 1. literature review according to higgins et al. [1], waiting time is a critical aspect for passengers in deciding on the use of public transport and their perception of the quality of the service. during train journeys, there may be delays for certain reasons, which have a significant effect on the quality of public transport, with missing a connection at the planned transfer point being the worst possible option. the study by vanstenwegen et al. [2] solves the railway timetable in belgium and, with the help of linear programming, measures its performance by determining the value of the costs associated with waiting for a connecting link at a transfer station. on the selected railway network, the researchers used this method to find a significant reduction in the costs associated with waiting, with a 30 % reduction in transfer time and the resulting overall delay. however, reducing these costs will increase the number of times a trailer misses (by up to 12 %). according to goverde [3], the term waiting time in rail transport has several meanings, namely the time after the passenger boarding the train before the train starts, the time the train stays at the railway station where the passenger does not leave, the transfer time at the passenger transfer station the train does not miss/miss and then again the time before the start of the train to which the passenger transferred. the total transport time then consists of these individual components of waiting times and have an impact on the quality and competitiveness of public transport. 2. solution in the czech republic within this issue, the authors of the article determine three model situations that are very easily applicable to the environment of suburban and regional railway transport in the czech republic. in principle, there are 3 basic intervals between connections for this form of rail transport. a 30-minute interval in the case of suburban transport to a large agglomeration, a 60minute interval in the case of railway lines connecting to the main suburban lines and an interval of 120 minutes on normal regional lines. this value is crucial for the calculation, as it is the basis for how many minutes the passenger will have to wait if he misses the connection due to the delay. due to the independence of local conditions, no specific route was selected, but a transport time of 60 minutes was set. subsequently, the values for the transfer are determined, namely 5, 10 and 20 minutes, which increase the transport time accordingly. according to the available statistical 13 https://doi.org/10.14311/app.2023.43.0013 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en j. froněk, j. chlumecký, d. vymětal acta polytechnica ctu proceedings figure 1. numbers of delayed trains according to individual time intervals. yearbook of the infrastructure manager in the czech republic [4], specifically the správa železnic, s.o. it is stated that 90.81 % of passenger trains in 2020 were run on time or with a maximum delay of 5 minutes. due to the ease of interpretation, the train connected to the connecting train will be marked first, the connecting train itself will be marked second. the authors assume that the second train at the transfer point will always wait for the arrival of the first train, which is not more than 5 minutes late. this means that the transfer time of 5 minutes will be increased by the time necessary for the transfer of passengers with regard to the amount of delay of the first train. it follows from the above that 9.19 % of trains were operated with a delay of more than 5 minutes in that year. however, this does not mean that in all cases of such a delay the second train will run on time. the authors assume that in the case of a transfer time of 5 minutes the second train will always run if the first train is delayed by more than 5 minutes, in the case of a transfer time of 10 minutes the second train will run if the first train is delayed more than 10 minutes. the connection is not maintained if the first train is delayed for more than 20 minutes. data on individual minute percentages of delays are not available, so it is necessary to determine for the purposes of the calculation what percentage of delayed trains was over 10 minutes and how many over 20 minutes. the percentages of likelihood of delay of trains and likelihood of loss connection you can find in tables 1 and 2. 3. case study in this case study, two connections (ic 548 and ic 549) were observed on the busiest route in the czech republic (prague – ostrava) in the period from 23 march likelihood of delay 5–10 10–20 20–30 <5 min min min min >30 min 90.81 % 3.68 % 2.76 % 1.84 % 0.92 % table 1. likelihood of delay (according to minute intervals). loss of transfer time connection 5 min 10 min 20 min (per. of cases) 9.19 % 5.51 % 2.76 % table 2. likelihood of loosing connection during different transfer times. to 26 july 2022, i.e. 126 connections in the direction of prague and 126 connections in the direction of ostrava. the connections ic 548 and ic 549 are part of the long-distance line ex1 and serve stations (návsí, třinec-centrum, třinec, český těšín, karviná hl.n., bohumín, ostrava hl.n., ostrava-svinov, studénka, suchdol nad odrou, hranice na moravě, olomouc hl.n., zábřeh na moravě, česká třebová, ústí nad orlicí město, choceň, pardubice hl.n., kolín, prahalibeň, praha hl.n.). however, connection ic 548 does not serve the section návsí – bohumín on sundays and connection ic 549 does not serve this section on saturdays. train arrival delays at individual stations are shown in the tables in figure 1. for better clarity, individual delays are categorised. it should be noted that 21 connections of trains ic 548 and ic 549 are listed as cancelled in the návsí – bohumín section, but these are connections that do not run according to the valid timetable. to illustrate the whole situation, the authors de14 vol. 43/2023 length of transfer time between connecting trains figure 2. distribution of train delays within individual stops. figure 3. number of occurences of delayed trains. cided to graphically illustrate the delays on the ic 548 connection in figure 2, where the dark green color shows the arrival of a train to the station with a delay of up to 5 minutes, light green up to 10 minutes, yellow up to 15 minutes, orange up to 20 minutes, red up to 30 minutes, dark red up to 60 minutes and black above 60 minutes, and the trains not running are then shown in white. from the above, it can be seen that train ic 548 performs significantly better than train ic 549, even though it runs part of its route in the morning peak. ic 549, even though it departs at the end of the afternoon peak, managed to depart from its departure station only 43 % of the time with a delay of up to 5 minutes in the period under review. it reached the station návsí with a delay of 5 minutes only in 3 % of cases. on the basis of the obtained frequency of delays on arrivals to the stations, it is possible to determine the individual statistical parameters that will be used to build a monte carlo simulation model. in particular, the mean value of the occurrence of delays λ. it is also known that train delays follow an exponential probability distribution. for simplicity, the authors decided that for train ic 548 the simulation will be performed only at the stations ostrava-svinov, hranice na moravě, olomouc main station, česká třebová, pardubice main station, kolín and prague main station, which are important interchanges. for train ic 549, simulations will take place at the same stations, except for the station praha hl.n., which is the departure station for the train and will be replaced by the station bohumín. within each station, 1 000 000 train delays will be simulated. the simulation was carried out on the basis of a script written in python programming language. the tables at figure 3 show the results of the frequency of delays for trains ic 548 and ic 549 in the simulation. at first glance, it can be seen that there is an increase in the average delay for trains in both directions over the course of the train journey. for a better demonstration of this phenomenon, the figure 4 show the frequency of delays for the ic 548 service at ostrava-svinov and praha hl.n stations and for the ic 549 service at kolín and bohumín stations. it can be seen from the graphs at figure 4 and figure 5 that the average delay is indeed increasing during the train journey, which points to the fact that the timetable of ic 548 and ic 549 is very unstable and is not able to absorb the resulting delays, which clearly has a detrimental effect on connecting links, and therefore it is evident that these will subsequently lead to an increase in dissatisfied passengers, which may escalate into a passenger exodus from public transport and this problem needs to be addressed more. however, it is necessary to add in one breath that during the period under review there were three closures, namely the přerov junction, the ústí nad orlicí – choceň section, kolín – poříčany, which had a clearly negative impact on train travel. the tables in figure 6 show the cumulative probability of train delays on arrival at the monitored stations. this information is very relevant for determining the appropriate changeover time for connecting trains. within the transfer times, the authors decided to establish three categories – stable transfer link (90– 100 % probability of securing a transfer), partially stable transfer link (75–90 %) and unstable transfer link (less than 75 %). the table below then shows the individual transfer links at the stations listed. a connecting train is defined as a connection running a maximum of 30 minutes after the arrival of the first train (at praha hl.n. station this time was set at 45 minutes) and is not a return connection or a 15 j. froněk, j. chlumecký, d. vymětal acta polytechnica ctu proceedings figure 4. numbers of delayed trains of train number 548 according to individual time intervals. figure 5. numbers of delayed trains of train number 549 according to individual time intervals. figure 6. cumulative probability of trains delays. connection in the same category going to the same destination station. the table shows the station, arrival time, number of the connecting train, time until departure of the connecting train, destination station of the connecting train, direction, stability of the connecting link based on the cumulative probability of train delay, number of the next train going in the given direction (without change), time from departure of the first connecting train to departure of the second train, the risk of missing the connection (this is a twodimensional quantity consisting of the complement of the stability of the connection and the time in minutes until the departure of the next train going in that direction) and the proposed transfer time based on the cumulative probability of train delay, so as to ensure a lower bound of the stability of the connection, i.e. 90 %. the tables in figures 7 and 8 show that for train ic 548 all interchanges beyond olomouc hl.n. station are unstable. the transfer coupling at kolín station to train r 1302 in the direction of poděbrady, mělník and ústí nad labem hl.n. is the least stable. the highest transfer risk is then the transfer link at prague main station to train ec 333 in the direction of české budějovice and linz, where the stability of the transfer link is only 38.3 % and the next connection takes 4 hours. for train ic 549 the situation is even more desperate. none of the interchanges are stable, even for the last connections of the day. in practice, these connections are obliged to wait for the arrival of the delayed train even with a longer delay, but the whole situation leads to disillusionment for passengers. 16 vol. 43/2023 length of transfer time between connecting trains figure 7. stability of transfers during the way of train number 549. figure 8. stability of transfers during the way of train number 548. 4. conclusion the analysis of the interchange links and the subsequent simulation shows that the current form of the timetable of trains ic 548 and ic 549 and the timetable of departures of connecting trains is very unstable, where only 5 out of 34 interchange links can be described as stable and 7 as partially stable, the remaining 22 links are unstable. for some links the probability of catching the connection is less than 50 % (9 cases) or even 25 % (3 cases). in many cases, the proposed changeover times are several tens of minutes higher than the current ones, which is definitely not a sustainable solution. another issue may be the reintroduction of a fast train segment on the prague – ostrava route, which could solve the ex1-r8 and ex1-r19 interchanges and reduce the risk of double transfers (ex1-kopřivnice-nový jičín-bílovec-náchodetc.). references [1] a. higgins, e. kozan. modeling train delays in urban networks. transportation science 32(4):346–357, 1998. https://doi.org/10.1287/trsc.32.4.346 [2] p. vansteenwegen, d. v. oudheusden. developing railway timetables which guarantee a better service. european journal of operational research 173(1):337–350, 2006. https://doi.org/https: //doi.org/10.1016/j.ejor.2004.12.013 [3] r. goverde, i. a. hansen. synchronization control of scheduled train services to minimize passenger waiting times. 1999, [2022-04-09], https://citeseerx.ist.psu.edu/viewdoc/download? doi=10.1.1.607.6286&rep=rep1&type=pdf. [4] správa železnic. statistická ročenka 2020. praha, 2021, [2022-04-09], https: //www.spravazeleznic.cz/documents/50004227/ 94360303/statistick%c3%a1+ro%c4%8denka+2020/ 2286eccb-d554-4b6d-892c-01314d91d9f2. 17 https://doi.org/10.1287/trsc.32.4.346 https://doi.org/https://doi.org/10.1016/j.ejor.2004.12.013 https://doi.org/https://doi.org/10.1016/j.ejor.2004.12.013 https://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.607.6286&rep=rep1&type=pdf https://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.607.6286&rep=rep1&type=pdf https://www.spravazeleznic.cz/documents/50004227/94360303/statistick%c3%a1+ro%c4%8denka+2020/2286eccb-d554-4b6d-892c-01314d91d9f2 https://www.spravazeleznic.cz/documents/50004227/94360303/statistick%c3%a1+ro%c4%8denka+2020/2286eccb-d554-4b6d-892c-01314d91d9f2 https://www.spravazeleznic.cz/documents/50004227/94360303/statistick%c3%a1+ro%c4%8denka+2020/2286eccb-d554-4b6d-892c-01314d91d9f2 https://www.spravazeleznic.cz/documents/50004227/94360303/statistick%c3%a1+ro%c4%8denka+2020/2286eccb-d554-4b6d-892c-01314d91d9f2 acta polytechnica ctu proceedings 43:13–17, 2023 1 literature review 2 solution in the czech republic 3 case study 4 conclusion references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2023.42.0072 acta polytechnica ctu proceedings 42:72–76, 2023 © 2023 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague computed tomography system with strict real-time synchronization for in-situ 3d analysis of periodically vibrating objects václav rada∗, tomáš fíla, petr zlámal, petr koudelka, jan šleichrt, michael macháček, daniel vavřík, daniel kytýř czech academy of sciences, institute of theoretical and applied mechanics, prosecká 809/76, 190 00 prague 9, czech republic ∗ corresponding author: rada@itam.cas.cz abstract. in the contribution, we present a laboratory system capable of x-ray computed tomography (xct) scanning of an periodically moving or oscillating object. the system is an in-house developed xct setup with electromagnetic voice coil actuator mounted on top of the rotary stage of the setup. the strict synchronization of the components, the rotary stage, the electromagnetic actuator movement and the detector readout is accomplished with use of the detector hardware trigger and hard real-time linux operating system. cylindrical sample manufactured from epoxy resin with metal particles to enable movement tracking is scanned in a stationary position and during periodical movement induced by the vibration stage. the volumetric data of the scans is compared and the results of this contribution represent an important step towards identification of defects through modal analysis of in-situ harmonically vibrating object. keywords: harmonic motion, real-time synchronization, computed tomography, hardware trigger, subtraction tomography. 1. introduction in-situ x-ray computed tomography (xct) has become commonly used imaging method for the vizualization and analysis of internal structure and defects of materials in the field experimental mechanics. especially materials with complex shapes and internal structure, such as rocks, biomaterials, 3d-printed lattices, other heterogenous materials, etc. subjected to quasi-static mechanical loading and recently, in-situ computed tomography has been the topic of intensive scientific interest. using the xct technique, volumetric investigation of complex phenomena such as crack propagation [1, 2], damage development or failure mechanism identification [3–6] has become possible. furthermore, x-ray imaging techniques have been adopted for state-of-the-art inspection of nonstationary objects and material samples subjected to dynamic loading [7]. x-ray imaging of non-stationary objects or imaging of short-lived deformation phenomenons require precisely timed synchronization of all the components of the xct setup, such as rotary stage position, x-ray detector readout, etc. in this paper, we present an in-house designed xct system for scanning of periodically moving objects with the intention and future goal of modal analysis of oscillating beams using xct or to vizualize and analyze defects inside the specimens induced by dynamic loading, such as fatique crack growth inside the oscillating beams. to be able to precisely and reliably acquire x-ray data, the proposed system employs realtime hardware synchronization of all the necessary components with in-house developed control software including custom software for the x-ray detector image acquisition. the concept and functionality of the system is tested and verified by obtaining the xct of the periodically moving rigid body and comparing it to the reference xct and evaluating the difference between the reference xct volume and the volume of the periodically moving rigid body. 2. materials and methods at the authors’ workplace, an in-house developed x-ray setup is used for xct imaging. the setup consists of numerous positioning axes including rotary stage and can be equipped with various x-ray sources and detectors. in the modal analysis configuration, on top of the rotary stage is placed the shaker, which is the linear voice-coil actuator, which drives the periodic motion of the sample, see figure 1. in this study, the x-ray source used is the sealed type l10321 (hamamatsu photonics, japan) with maximum acceleration voltage of 100 kv, 20 w of maximum target power, and lowest achievable focal spot size of 5 µm. the detector used is the dexela 1512 ndt (varex imaging, usa) cmos detector equipped with csi scintillator and resolution of 1944 × 1536 px and framerate up to 86 hz depending on binning. the shaker is the mgv41 (akribis systems, singapore) with position sensitivity 0.1 µm, position repeatability 1.5 µm, stroke up to 20 mm and frequency range up to 50 hz. 72 https://doi.org/10.14311/app.2023.42.0072 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 42/2023 3d periodically vibrating objects ct analysis figure 1. experimental setup. 2.1. detector synchronization proper x-ray image acquisition of non-stationary or oscillating object requires precise timing and synchronization of the imaging chain and the positioning components, including detector exposure, shaker position or rotary stage position. this is achieved by utilizing the hardware trigger interface of the dexela 1512 ndt detector and integrating it through the hardware trigger interface into the in-house developed control system of the xct setup. the detector exposure takes place while the shaker reaches its peak amplitude (the image exposure was started so that the peak amplitude value was reached exxactly in the middle of the image exposure time window). inherently, throughout the image exposure, the periodically moving rigid body sample is moving, see figure 2. the image exposure occurs at the zone of the peak amplitude due to the intention to use the proposed system for modal analysis of oscillating beams, for instance to 3d visualize eigenshapes of the beams, etc. figure 2. image exposure at the peak amplitude. for detailed diagram of the hardware synchronization logic, see figure 3. as the shaker position (purple line signal in figure 3) reaches the zone of the peak amplitude and the rotary stage is in position and stationary (i.e. not moving – green signal in figure 3 is in the lower position), the image exposure is started (rising edge on the blue line signal in figure 3) with the rising edge of the hardware trigger signal (red line in figure 3). after the image exposure is finished (falling edge on the blue line signal in figure 3), the rotary stage starts moving (rising edge on the green line signal in figure 3) and once the rotary stage reaches desired position to acquire xct projection, whole process is repeated. detector hw trigger image exposition rotary stage moving shaker position figure 3. synchronization of the detector. the control system, which handles the synchronization of the xct experimental setup and the aforementioned signal processing, is done using an in-house developed control software package based on open-source software platform linuxcnc. the control software employs real-time capabilities of the debian gnu/linux 73 v. rada, t. fíla, p. zlámal et al. acta polytechnica ctu proceedings system to assure precise timing and synchronization with the synchronization frequency of 5 khz. 2.2. computed tomography the sample observed by the xct is a cylindrical sample (d ≈ 8 mm; h ≈ 140 mm) manufactured from an epoxy resin with dispersed aluminium particles to increase contrast in the reconstructed 3d images. first, the reference tomography of the sample was acquired (with the shaker in stationary position). the reference tomography volume can be found in figure 4 and parameters of the xct scanning are listed in table 1. the same imaging parameters were used for the acquisition with the shaker periodically moving. the shaker was set up to produce sinusoidal movement with the amplitude of 5 mm and the frequency of 4 hz. with this setup and the shaker movement throughout the image exposure is approx. 245 µm, see figures 2 and 5. in table 1, note the difference in the duration of the tomography scanning. the much higher scanning duration with the shaker periodically moving is caused by the image exposure taking place only when the shaker reaches its peak amplitude. with the harmonic motion frequency 4 hz, the image exposure occurs with 250 ms period, as opposed to the tomography scanning with the shaker stationary, in which case the image exposure takes place in succession without any delay. furthermore, the oscillation of the shaker has negative influence on the rotary stage position control due to moving mass of the shaker on top of the rotary stage. for that reason, a mandatory time delay needed to be added for the rotary stage position error to settle and meet the control system conditions after the rotary stage position increment. only after the positioning condition were met, the exposure of the subsequent image could be started. tomography parameters target power 20 w acceleration voltage 100 kv spot size 30 µm exposure time 25 ms fdd 497.2 mm fod 162.6 mm no. of projections 1 912 frame averaging 10 detector binning 2 × 2 detector pixel size 150 µm magnification ≈ 3.1 voxel size 48 µm scanning duration (shaker stationary) ≈ 12 min scanning duration (shaker oscillating) ≈ 90 min table 1. tomography parameters. figure 4. section through the reference volume showing aluminium particles (white) and voids (black). 0 .0 0 0 0 .0 2 5 0 .0 5 0 0 .0 7 5 0 .1 0 0 0 .1 2 5 0 .1 5 0 time [s] 1000 500 0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 4755 sh a ke r p o si ti o n [ u m ] shaker position, f=4hz, a=5000um sample movement during acquisition, 244.72um image acquisition time window 25ms figure 5. sample movement during image exposure. 3. results the goal of the study is to validate the feasibility and reliability of the tomographical system for the vizualization of periodically moving or oscillating objects. in order to quantify the influence of the periodic motion, the reference tomogaphy volume (with the shaker stationary) was compared to the tomography volume acquired with the shaker performing sinusoidal 74 vol. 42/2023 3d periodically vibrating objects ct analysis figure 6. (a) reference volume (shaker stationary); (b) periodically moving tomography (shaker moving). motion. the overview of the volumes is shown in figure 6. the volumes were binarized with threshold and then subracted in order to vizualize the difference between the volumes using the subtraction tomography method, see figure 7. the difference of the volumes is mostly distributed near the surface of the subtracted volume envelope. the aim of the comparison was to evaluate the thickness of the subtracted volume envelope in radial directions. for that, each horizontal slice of the subtracted envelope was fitted with a circle, the center and the radius of the circle was fitted using the sum of the squared differences, see figure 7c. the circle represents a polar coordinate system which is used for the evaluation. the thickness of the envelope in all directions in the polar coordinate systems of each horizontal slice is shown in figure 8. the results show the envelope thickness ranging from 0.5 up to 2.1 voxels, due to the reconstructed volume of the periodically moving object being blurry compared to the reference volume. 4. conclusions in this study, we present an in-house developed tomography system capable of scanning periodically moving rigid body. the proof of concept was validated with pilot tomography scanning of epoxy resin rigid body. first, the tomography of the rigid body with shaker stationary was acquired and secondly with the shaker performing sinusoidal motion. the resulting volumes of the tomography scanning were binarized and compared and exhibit small difference in the magnitude of less than 2.1 voxels, even though the motion of the rigid body was approximately 5 voxels during the image exposure. the small difference of the resulting figure 7. (a) vertical slice through the subtracted volume; (b) horizontal slice through the subtracted volume; (c) horizontal slice through the subtracted volume with polar coordinate system. 0 45 90 135 180 225 270 315 angle in the polar coordinate system of the fitted circle [deg] 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 av er ag e th ick ne ss o f t he e nv el op e of th e su bt ra ct ed v ol um e [v ox el s] figure 8. thickness of the subtracted volume envelope. volumes show that the system is capable of tomography scanning of periodically moving objects. in this case, the object was moving periodically with the amplitude of 5 000 µm and the maximum difference of 2.1 voxels (voxel size 48 µm is equivalent to 100 µm in the real world). additionaly, the quality of the acquired data can be improved with shorter exposure time of the detector to decrease the motion of the sample during the image exposure, which was not viable due to the limited flux of the x-ray source used in this study. however, in furter studies regarding this topic, the higher flux x-ray source metaljet d2+ will be used, thus allowing for decreasing the exposure time of the detector up to 15 ms which is the limit of the cur75 v. rada, t. fíla, p. zlámal et al. acta polytechnica ctu proceedings rently used detector dexela 1512 ndt (framerate up to 70 hz with binning 2 × 2). with the exposure time 15 ms (instead of 25 ms presented in this study) and the same harmonic motion of the shaker (frequency 4 hz, amplitude 5 000 µm the motion of the sample during the image exposure can be decreased from the 245 µm (5 voxels with the voxel size of 48 µm) up to 89 µm (less than 2 voxels with the voxel size of 48 µm). the ultimate goal of the proposed imaging system is to use it for modal analysis of oscillating beams using xct and for analyzing eigenshapes and eigenfrequencies of oscillating beams. furthermore, the future research will be aimed at in-situ computed tomography of oscillating beams and the analysis of internal defects, such as fatique cracks development, etc. acknowledgements this research was funded by the czech science foundation (project no. 22-1381s). the conference participation was supported by czech national imeko committee. references [1] x. sun, s.-i. takeda, m. wisnom, x. xu. in situ characterization of trans-laminar fracture toughness using x-ray computed tomography. composites communications 21:100408, 2020. https://doi.org/10.1016/j.coco.2020.100408 [2] d. vavrik, p. benes, t. fila, et al. local fracture toughness testing of sandstone based on x-ray tomographic reconstruction. international journal of rock mechanics and mining sciences 138:104578, 2021. https://doi.org/10.1016/j.ijrmms.2020.104578 [3] e. cakmak, m. gussev, t. watkins, et al. in-situ x-ray computed tomography analysis of fracture mechanisms in ultrasonic additively manufactured al-6061 alloy. additive manufacturing 48:102401, 2021. https://doi.org/10.1016/j.addma.2021.102401 [4] y. wang, l. mikkelsen, g. pyka, p. withers. time-lapse helical x-ray computed tomography (ct) study of tensile fatigue damage formation in composites for wind turbine blades. materials 11(11):2340, 2018. https://doi.org/10.3390/ma11112340 [5] p. koudelka, d. kytyr, t. fila, et al. a method for evaluation the fatigue microcrack propagation in human cortical bone using differential x-ray computed tomography. materials 14(6):1370, 2021. https://doi.org/10.3390/ma14061370 [6] h. j. meng, y. wang, j. y. ren. in situ x-ray computed tomography (ct) investigation on localized deformation and crack damage evolution in a bimrock by tracking rock blocks. advances in civil engineering 2020:8851817, 2020. https://doi.org/10.1155/2020/8851817 [7] a. cohen, d. levi-hevroni, p. fridman, et al. in-situ radiography of a split-hopkinson bar dynamically loaded materials. journal of instrumentation 14(06):t06008, 2019. https://doi.org/10.1088/1748-0221/14/06/t06008 76 https://doi.org/10.1016/j.coco.2020.100408 https://doi.org/10.1016/j.ijrmms.2020.104578 https://doi.org/10.1016/j.addma.2021.102401 https://doi.org/10.3390/ma11112340 https://doi.org/10.3390/ma14061370 https://doi.org/10.1155/2020/8851817 https://doi.org/10.1088/1748-0221/14/06/t06008 acta polytechnica ctu proceedings 42:1–5, 2023 1 introduction 2 materials and methods 2.1 detector synchronization 2.2 computed tomography 3 results 4 conclusions acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2023.43.0006 acta polytechnica ctu proceedings 43:6–12, 2023 © 2023 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague the measurement of the influence of the adhesion loss on the dynamic load of the drive reaction rod on the roller rig vojtěch dybala czech technical university in prague, faculty of mechanical engineering, department of automotive, combustion engines and railway engineering, technická 4, 160 07 prague, czech republic correspondence: vojtech.dybala@fs.cvut.cz abstract. the dynamic behaviour and dynamic phenomena in traction drives of railway vehicle has been studied for decades. generally, in these days the motivation for this research is to look for answers related with service problems, which has been risen, e.g. an overloading of mechanical components as wheelsets or an intention to improve control systems of traction drives of railway vehicles. this problematic has also motivated the research at the faculty of mechanical engineering at the czech technical university (ctu) in prague. nowadays it is also usual, that the main research tool has been a simulation based on a mathematical model of a railway vehicle or its specific components, as testing on real vehicles is very demanding regarding costs. however, there is an alternative – a roller rig. at the ctu in prague there has been modernized the second roller rig for purposes of measurement regarding the torsion dynamics research, to support phd students for their theses and possibly to extend the education for students oriented on the railway vehicle’s design. this contribution brings results of measurements just from the problematics of torsion oscillations and dynamics. specifically, further presented measurements are on the loss of the adhesion in the contact of the wheelset and rollers, as the substitution for the railway, its impact on the reaction rod overloading and study of the axle load influence on the transition phenomena. keywords: railway vehicle, torsion, adhesion, roller rig, wheel-rail contact. 1. introduction within the construction of the railway vehicle the traction drive is a big functional system, generally it is an electro-mechanical system. in these terms the electrical part is intended to be from a pantograph to an electromagnetical circuit of a traction motor. the mechanical part then is from a rotor of the traction motor to a contact of a wheelset and a rail. further is dealt with the mechanical part for purposes of this contribution. from the conceptual perspective the mechanical part of traction drive is divided into three groups: • axle-hung drive (figure 1), • partly-suspended drive (figure 2), • fully-suspended drive (figure 3). these three groups differ in the amount of the mass, which is connected to a wheelset (unsuspended mass) and to a bogie frame (primarily suspended mass). the applicability for a specific vehicle is generally derived from the supposed maximal service speed of the vehicle and needs to reduce the dynamic interaction between the wheelset and the track via reduction of mass connected to the wheelset. it can be defined, that the axle hung drive is typical for slower vehicles with the maximal service speed of 140/160 km h−1, the partly-suspended drive for maximal service speeds from up to 190 km h−1 and high-speed vehicles as figure 1. an axle-hung drive [1]. defined by the tsi and fully-suspended drives for medium and high-speed applications as well. from the perspective of this contribution the first two types of drive are in question as they utilize a specific design feature – a reaction rod, see figure 4. the function of the reaction rod is to connect the traction gearbox or the drive to the bogie frame and realize a support for reaction force (s), which comes from traction forces in the wheel-rail contact (t ) and forces from vertical dynamics, given by track irregularities. the reaction rod can be realized as vertical, horizontal or generally inclined (figure 5). the reaction rod is the component, which needs to 6 https://doi.org/10.14311/app.2023.43.0006 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 43/2023 the measurement of the influence of the adhesion loss on the dynamic load figure 2. a partly-suspended drive of a locomotive [2]. figure 3. a fully-suspended drive of a locomotive [2]. be considered within mathematical models for simulations as well. nowadays there are many publications, which describes mathematical models with implemented reaction rod, how to model it and its influences within simulations [4–8]. for this contribution important has been the thesis of martin dub [9], which provides also results of practical measurements on reaction rods of rail vehicles. in both cases, tramcar 14t (figure 6) and metro car (figure 7), the measurements proved the suitability of a reaction rod as a diagnostic feature to investigate bogie assembly and its failure modes during service. if this work proved sufficient sensitivity of a reaction rod for utilization as an auto diagnostic feature it also raises a question if it could be a good part to measure a transition phenomenon of a loss and re-gain of the adhesion in the wheel-rail contact and its influence due to overloading of the reaction rod. 2. reaction rod of roller rig for purposes of measurement of vertical wheel forces and longitudinal tractive and brake forces the second roller rig of ctu in prague has been modernized so, figure 4. a traction gearbox with a vertical reaction rod [3]. figure 5. inclined reaction rod – schematics [3]. that measuring rods and beams has been equipped by strain gauges, see figure 8. the reaction rod on the roller rig is inclined horizontal rod (figure 9). because the drive of the wheelset is an axle-hung type the function between the tangential force in the wheel-roller contact t1 and the force s1, which loads the reaction rod itself, is defined by the equation (1). s1 = t1 rdv n . (1) in principle the equation (1) defines, that any longitudinal force (static, dynamic, transitional) acting in the wheel-roller contact should be measurable via the reaction rod, if the reaction rod has a sufficient sensitivity – flexibility. as the roller rig is disproportional in terms of dimensions, weights and installed power the measuring rods has reduced dimensions of shank with squared cross-section of side length of 5 mm – figure 10. 7 vojtěch dybala acta polytechnica ctu proceedings figure 6. instrumented reaction rod of tramcar 14t [9]. figure 7. instrumented reaction rod of a metro car [9]. 3. measurements the measurements were realized so, that: • both rollers were smeared with a soap, see figure 11, • rollers were accelerated by driven wheel-set, • when the smeared surfaces came into the contact with wheels the excessive slip occurred, • roller rig was stopped. during this procedure the force in the reaction rod was measured. the value of the reaction rod force (s1) was measured with the sampling frequency of 10 khz. this measurement was carried out 20-times for three different vertical loads of the bogie to observe, what is the influence of the vertical load of the wheelset within the transitional phenomenon of the adhesion loss. at first the bogie was not externally loaded, which means that the axle load is approximately 148 kg due to the figure 8. measuring rods and beams [10]. figure 9. inclined reaction rod of roller rig. mass of the driven wheelset, bogie frame etc. for other sets of measurement, the bogie frame was loaded via weights of 100 kg (figure 12) and 200 kg. it means additional axle load of 50 kg and 100 kg respectively. 4. conceptual design of active wheelset steering control an example of a typical course of the measured force s1 in the reaction rod within the measuring procedure is presented in figure 13. the detail of the adhesion loss itself is highlighted by the dashed square and depicted in figure 14. within the measured influence of the adhesion loss four parameters were assessed: (1.) ∆1 – the difference between the average value of the signal before the loss of the adhesion and during the reduced adhesion in the wheel-roller contact, see equation (2). (2.) ∆2 – the difference between the average value of the signal during the reduced adhesion and after the regain of the adhesion in the wheel-roller contact, see equation (3). (3.) ∆1,a – the maximal amplitude of the signal after the loss of the adhesion in the wheel-roller contact. (4.) ∆2,a – the maximal amplitude of the signal after the re-gain of the adhesion in the wheel-roller contact. 8 vol. 43/2023 the measurement of the influence of the adhesion loss on the dynamic load figure 10. reaction rod shank – detail. figure 11. smeared rollers. ∆1 = xb_l − xred_ad = 1 n n∑ i=1 xi,b_l − 1 m m∑ j=1 xj,red_ad . (2) ∆2 = xred_ad − xreg_ad = 1 m m∑ j=1 xj,red_ad − 1 p p∑ k=1 xk,reg_ad . (3) the parameters ∆1 and ∆2 were determined to see how the transition phenomena influence the nominal transferred force. the parameters ∆1,a and ∆2,a were determined to assess dynamic behaviour at the moment of the transition phenomena. figure 15 presents determined values of ∆1 (negative as decreased) and ∆2 (positive as increased). it is clear, that they were not measured same values. ∆2 reaches higher value of its arithmetic average 194 n and standard deviation of 21 n than ∆1 with its arithmetic average of -179 n and standard deviation of 14 n. it is clearly visible, that ∆2 shows bigger scatter of measured values. equivalent behavior with respect to arithmetic averages, when the adhesion is re-gained, can be also observed in case of assessed amplitudes for all three sets of measurements – figure 16, 17, 18. figure 12. loaded bogie frame by weights. the assessment of amplitudes ∆1,a and ∆2,a also proved, that increasing of the axle load by app. 1/3 and 2/3 has influence on the dynamic behaviour of the force s1. specifically, the average values of amplitudes decrease as presented in table 1. standard deviations do not follow this trend exactly in case of the measurement for 100 kg. arithmetic standard load average deviation [kg] ∆1,a ∆2,a ∆1,a ∆2,a 0 120 190 36 50 100 111 170 30 38 200 104 150 29 48 table 1. basic statistic data of measured amplitudes. 5. conclusion within the assessment of static parameters of ∆1 and ∆2 the measured values showed differences in terms of average values and their standard deviation as well. it means that ∆2 reaches higher values and its scatter is also higher, than in the case of values of ∆1. the evaluation of the behaviour of this parameter indicates, that re-gain of adhesion is more dynamic transition phenomenon than the loss of the adhesion. equivalent evaluation can be done also with respect to the parameters of ∆1,a and ∆2,a, where the average value is also bigger in the case of the re-gain of the adhesion. the assessment of the amplitudes indicates the influence of the axle load too. it can be stated, that when the axle load increases the dynamics of the transition phenomena decreases for both, the loss of the adhesion and the re-again of the adhesion. so higher axle load has positive influence on the dynamic overloading of the mechanical part of the traction drive system, which is caused by this kind of transition phenomenon. list of symbols mh shaft torque [n] mm motor torque [n] 9 vojtěch dybala acta polytechnica ctu proceedings figure 13. course of reaction rod s1. figure 14. adhesion loss impact within force s1 – detail. figure 15. change of force s1 – all vertical bogie loads. 10 vol. 43/2023 the measurement of the influence of the adhesion loss on the dynamic load figure 16. amplitudes of force s1 – no vertical bogie load. figure 17. amplitudes of force s1 – 100 kg vertical bogie load. figure 18. amplitudes of force s1 – 200 kg vertical bogie load. 11 vojtěch dybala acta polytechnica ctu proceedings n arm of s1 force [m] rdv orrk wheelset/wheel radius [m] sors1 reaction rod force [n] t ort1 wheelset-roller force [n] xi,b_l value of signal before loss of adhesion [n] xb_l arithmetic average of signal before loss of adhesion [n] xj,red_ad value of signal during reduced adhesion [n] xred_ad arithmetic average of signal during reduced adhesion [n] xk,reg_ad value of signal after regaining of adhesion [n] xreg_ad arithmetic average of signal after regaining of adhesion [n] z horizontal distance [m] α reaction rod angle [°] ∆1 decrease of s1 force [-] ∆2 increase of s1 force [-] ∆1,a max. amplitude of s1 force [-] ∆2,a max. amplitude of s1 force [-] acknowledgements this research has been realized using the support of the czech technical university in prague and related grant sgs20/120/ohk2/2t/12. this support is gratefully acknowledged. references [1] r. lal. selection of suspension arrangement of traction motors : a right approach. [2019-09-07], https://www.yumpu.com/en/document/view/21978467/ selection-of-suspension-arrangement-oftraction-motors-a-rdso. [2] j. kolář. teoretické základy konstrukce kolejových vozidel. česká technika – nakladatelství čvut, 2009. [3] o. suchánek. návrh závěsky u nápravové převodovky příčného částečně odpružného pohonu dvojkolí. bachelor thesis, fme ctu in prague, 2019, [2022-0421], https://dspace.cvut.cz/handle/10467/84788. [4] m. w. winterling. oscillations in rail vehicle. delft university press, mekeleg 4, 2628 cd delft, the netherlands, 1997. [5] r. schneider. torsionsschwingungen von radsatzwellen – systemanalyse teil 1: systemund modellbeschreibung. zevrail 141(11/12):452–461, 2017. [6] r. schneider. torsionsschwingungen von radsatzwellen – systemanalyse teil 2: physikalische untersuchungen und sicherheitsbetrachtung. zevrail 142(01/02):27–36, 2018. [7] j. kolář. dynamika individuálního pohonu dvojkolí s nápravovou převodovkou. in xx. medzinárodná konferencia – súčasné problémy v kolajových vozidlech, žilina, pp. 139–148. 2011. [8] t. fridrichovský. dynamické vlastnosti pohonů moderních kolejových vozidel. doctoral thesis, fme ctu in prague, 2022, [2022-04-21], https://dspace.cvut.cz/handle/10467/99978. [9] m. dub. analýza možností autodiagnostiky pohonů kolejových vozidel. doctoral thesis, ctu in prague, 2016. [10] v. dybala, p. bauer, t. fridrichovský. využití tenzometrie pro měření součinitele adheze na kladkovém stavu čvut. nová železniční technika 2021(1):26–29, 2021. 12 https://www.yumpu.com/en/document/view/21978467/selection-of-suspension-arrangement-of-traction-motors-a-rdso https://www.yumpu.com/en/document/view/21978467/selection-of-suspension-arrangement-of-traction-motors-a-rdso https://www.yumpu.com/en/document/view/21978467/selection-of-suspension-arrangement-of-traction-motors-a-rdso https://dspace.cvut.cz/handle/10467/84788 https://dspace.cvut.cz/handle/10467/99978 acta polytechnica ctu proceedings 43:6–12, 2023 1 introduction 2 reaction rod of roller rig 3 measurements 4 conceptual design of active wheelset steering control 5 conclusion list of symbols acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2025.52.0017 acta polytechnica ctu proceedings 52:17–23, 2025 © 2025 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague calculation of queue formation and queue length at airport check-in counters petr had∗, jiří volt, slobodan stojić czech technical university in prague, faculty of transportation sciences, department of air transport, konviktská 20, 110 00 praha, czech republic ∗ corresponding author: hadpetr1@fd.cvut.cz abstract. this paper focuses on calculation of airport passenger queue formation and its length. the main goal of the paper is to evaluate the appropriateness of the combination of machine learning and determistic algorithm for queue length determination. the study focuses on the departing passenger flows to the check-in counters. in this study the random forest algorithm was applied for passenger number prediction, and followed by application of the deterministic algorithm for passenger queue length estimation. the study was performed using the real data from the václav havel airport prague operations. presented results indicate that the combined approach of machine learning and a deterministic algorithm offers useful insights into passenger queue formation. this approach provides valuable insights into queue dynamics at check-in counters, enabling improvements in planning and management of airport operations during operational peak. keywords: queue prediction, machine learning, passenger flows, airport processor. 1. introduction airports around the world are facing increasing pressure to improve operational efficiency due to rising passenger numbers. one of the key challenges that airports have to face is queuing at individual processors such as check-in counters, security checkpoints and border control. inefficient queue management leads to longer waiting times, which has a negative impact not only on passenger satisfaction but also on the operating costs of both airport operators and airlines. traditional approaches to determining queue lengths rely primarily on historical data and static capacity models, which often fail to respond flexibly to dynamic changes in demand or to specific airport conditions at a given time. with increasing demands for more accurate planning and optimisation of airport processes, it is necessary to look for innovative ways to predict and manage flows of the passengers. the objective of this study is to determine the queue lengths at airport terminal processors through application of the machine learning algorithms for prediction of the number of passengers arriving on individual flights, followed by calculation of the queue length using a deterministic algorithm. this approach enables the prediction of queue lengths in short time intervals for specific check-in counters, allowing better planning and more flexible management of the airport processors. 2. research background the management of queues at the airport terminals has become an increasingly critical issue, as passenger volumes continue to grow. effective queue management is essential for reducing waiting times, improving passenger experience, and maintaining airport operational efficiency. various approaches have been developed to model, analyse, and optimise airport queuing systems, ranging from classical queuing theory to more advanced techniques using machine learning algorithms. 2.1. queuing theory and simulation models queuing theory represents a strong foundation for airport processes performance analysis. thiagaraj and seshaiah [1] present how queuing models are utilised in delay prediction and assessment of airport capacity under varying demand conditions. queuing models provide insights into how to manage the allocation of service counters in order to minimise waiting times during busy time periods. sharma et al. [2] demonstrate the use of simulation models to predict and mitigate queue lengths at airport security checkpoints, reducing the likelihood of queuing during operational peaks. another application of queuing theory is presented by xue et al. [3], who discusses the synergy between queuing models and machine learning for scheduling tasks in high-traffic areas. the combination of these approaches ensures that service level objectives (slos) are met while maximising resource utilisation. this principle can be also applied to airport queues in order to improve efficiency of passenger flows management. al-sultan [4] explores the optimisation of airport check-in processes through a combination of integer programming (ip) and simulation models. while this approach focuses on the optimisation of resource allocation, it emphasises the importance of simulation for 17 https://doi.org/10.14311/app.2025.52.0017 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en f. author, s. author, t. author acta polytechnica ctu proceedings improving passenger throughput and reducing congestion. simulation techniques are often used together with queuing models to provide more realistic and detailed predictions of airport capacity and passenger flow. 2.2. machine learning recent advances in the area of machine learning have enabled more dynamic and accurate predictions of queue lengths. wang et al. [5] use machine learning algorithms, such as decision trees, random forests, and support vector machines (svm), to predict traffic flow patterns. the study highlighted the importance of data-driven approaches for predicting non-linear traffic conditions, which is a concept also applicable to passenger flow at the airports. by incorporating real-time data, machine learning models are able to adjust resource allocation in order to minimise waiting times and prevent development of bottlenecks. moreover, xue et al. [3] introduce a hybrid model combining neural networks and queuing theory for prediction of the traffic intensity. this hybrid approach can be adapted for airport operations, where accurate predictions of passenger arrivals and queue lengths are crucial for optimisation of the terminal resources allocation. machine learning models enhance the queuing model’s ability to predict variations in passenger flow, providing more responsive and adaptive management of the queues at the airport processors. loureiro et al. [6] developed a machine learning approach for prediction of waiting times across different types of queues, including those at the airports. by using supervised learning algorithms, such as decision trees and neural networks, this study shows that machine learning can offer improvements in the accuracy of queue time predictions. such models are particularly valuable in dynamically changing environments, where they can enhance real-time decision-making for resource allocation and improve the overall passenger experience. 2.3. hybrid approaches to queue management the integration of queuing theory with machine learning has shown promise in optimising queue management. chocron et al. [7] demonstrate how combining queuing models with machine learning improves delay prediction in multi class service environments, such as airports. this hybrid approach allows for better precision in resource allocation by accounting for the different needs of various passenger classes, leading to reduced waiting times and improved overall efficiency. additionally, loureiro et al. [6] highlight the potential of machine learning to outperform traditional queuing theory in environments with high variability, such as airport terminals. by continuously learning from real-time data, machine learning models can predict queue lengths more accurately, allowing airports to dynamically adjust staffing and service levels to meet fluctuating passenger demand. based on the limitations of the existing experiments, the presented research focuses attention on the integration of machine learning algorithm with deterministic queue length calculations, specifically applied for passengers arrivals and queueing at the airport terminal check-in counters on a per-flight basis. limiting the approach to the specific process brings the set of the data and available data sources required for the estimations. the selected method combines the machine learning algorithm for passenger arrival predictions with a deterministic algorithm for queue length estimation in 15 minute intervals, providing a flexible and accurate tool for operational planning at airport terminals. this dual approach leverages the strengths of machine learning for forecasting number of arriving passengers, while using deterministic methods to model queue dynamics based on known processing rates and counter availability. it must be noted that proposed estimation approach is a first phase of the wider research intention to evaluate a possibility of more advanced concept of active passengers flow management. therefore, the research question in this phase of the research is targeting the appropriateness of the passengers flow estimation using the combined machine learning and traditional queueing techniques. 3. methodology the approach used in this study to predict queue lengths combines machine learning algorithm for prediction of number of passengers and deterministic algorithm for queue length calculation. the study is limited only to the predictions of the passenger queue length formed at the airport check-in counters. this involves only the departing passenger flows arriving to the airport for the flight. for the purposes of this study the real data from the václav havel airport prague terminal 2 were used. 3.1. available data to develop the prediction model and calculate the length of the queues, historical operational data and passenger arrival information were utilised. the dataset included a general arrival curve for departing passengers, the proportion of passengers using checkin counters, and details on all departing flights from terminal 2 (serving flights to and from the schengen area) during the period from june to august 2024. to maintain confidentiality of the provided information, the presented data have been anonymized. in total, the dataset covered 10 898 flights. the following parameters were available for each flight: • flight number; • airline; • destination; • scheduled time of departure (std); 18 vol. 52/2025 calculation of queue formation and queue length • designation of all check-in counters; • scheduled opening and closing times of all the checkin counters; • number of passengers; • number of checked-in bags. all timestamps used in this research were in the local time zone of prague airport, i.e. europe/prague. 3.2. machine learning for predicting the number of arriving passengers given the available data, the following predictors were chosen to build the prediction models: • airline; • destination; • number of checked-in bags; • low-cost indicator; • weekday; • month; • part of the day (night, morning, afternoon); • max. number of available check-in counters; • handling company; • aircraft type. based on the chosen predictors, three machine learning algorithms were selected to predict the expected number of passengers arriving at the check-in counters. these models were selected for their performance with structured data and their ability to manage complex relationships between the input features and the target variable effectively. • random forest (rf) this model was selected due to its ability to handle large datasets with mixed data types (categorical and numerical) and its resistance to overfitting by averaging the results of multiple decision trees, resulting in more reliable predictions. random forest also provides feature importance scores, allowing to understand influences of the factors to the predictions [8]. • xgboost (extreme gradient boosting) xgboost is a widely-used implementation of gbm (gradient boosting machines). gbm is ensemble learning technique that builds models sequentially, where each new model corrects the errors made by the previous ones. this model was selected due to its high predictive power and efficiency in handling structured data [9]. • support vector machines (svm) svm was selected due to its effectiveness in modeling complex relationships in data with relatively small to medium-sized datasets. svm constructs a hyperplane that best fits the data points in highdimensional space. this model can also effectively handle overfitting through the use of regularization and kernel functions [10]. 3.2.1. model training and testing all models were trained using above mentioned historical data. the dataset was split into training and testing sets, with first 80 % of the data from each month used for training and 20 % for testing. mean absolute error (mae), root mean squared error (rmse) and r-squared (r2) were used to evaluate the performance of each model for prediction of number of passengers. results are provided in table 1. mae [pax] rmse [pax] r2 rf 11.05 15.72 0.85 gbm 11.18 15.73 0.84 svm 11.89 16.98 0.82 table 1. performance metrics for selected models for pax prediction. based on the results provided in table 1, random forest (rf) emerges as the best-performing model for predicting the set target value. the importance of each feature is shown in the figure 1 and ordered from the most impactful feature on the outcome target variable. as shown in the figure, the aircraft type has the highest importance in creating the prediction with an importance score corresponding to 32 %, followed by the total number of baggage with the importance score of 25 %. the remaining parameters considered had the importance levels of less than 10 % each. as the random forest model performs the best in terms of overall error and variance it will be used for creating the input values for the length calculations of the check-in queues. 3.3. queue length calculation using deterministic algorithm this section describes the steps involved in prediction of queue formation and length at the airport terminal check-in counters. the methodology combines deterministic and stochastic elements, including the use of triangular distributions for processing times and deterministic calculations for queue lengths. to prepare the queuing model, the general passenger arrival curve at the airport was used as one of the key inputs. this curve represents the percentage of passengers arriving at the airport in defined 15-minute intervals leading up to the scheduled time of departure of the flight. additionally, the model incorporates the passenger processing times at check-in counters, the opening times of the counters, and the proportion of passengers using the check-in counters. passenger data, including flight details and airlinespecific behaviour (in particular, differences in passenger check-in patterns according to different airlines), were integrated to adjust the number of passengers who really use the airport check-in counters. each 19 f. author, s. author, t. author acta polytechnica ctu proceedings figure 1. feature importance of random forest model. flight’s predicted number of passengers was therefore adjusted based on the proportion of passengers using self and online services. this adjustment is important to accurately estimate the number of passengers arriving at the check-in counters. the number of available check-in counters varies over time. the developed algorithm checks for each time window whether counters are open based on the scheduled opening and closing times and calculate the number of open counters in each time interval. this solution enables the model to dynamically adjust processing capacity based on counter availability. the maximum number of passengers that can be processed in time window j is given by eq. (1): mj = cj × twindow tprocess , (1) where: mj is the maximum number of passengers processed in window j, cj is the number of open counters in window j, twindow is the length of the time window (e.g., 15 minutes), tprocess is the average processing time per passenger. a triangular distribution is used to account for variability in processing time of each passenger. the processing time for each passenger is sampled from this distribution, defined by a minimum, maximum, and mode. the queue length for each time window is calculated based on the number of arriving passengers, the number of passengers already in the queue from previous windows, and the processing capacity of the open counters. the processing capacity is determined by the time window length, the processing times, and the number of open counters. this ensures that the number of passengers processed does not exceed the capacity of the available counters in each interval, and any unprocessed passengers remain in the queue. the formula for queue length calculation is in eq. (2). the queue length at the end of time window j is the number of passengers left unprocessed from the previous time windows plus the new arrivals, minus the passengers processed during window j: qj = max (qj−1 + pj − mj , 0) , (2) where: qj is the queue length at the end of time window j, qj−1 is the queue length from the end of previous time window, pj is the number of passengers arriving in time window j, mj is the maximum number of passengers processed in time window j. the queue length is updated by adding the passengers from the current window, subtracting the passengers processed, and ensuring the queue cannot be negative. 4. results the dataset analyzed for calculating queue lengths consisted of flights from the same data used to test the machine learning algorithm. this was because both the predicted and actual passenger numbers were included in the dataset. these covered the last 5 days in june and the last 6 days in both july and august. the analysis included a total of 2010 flights. the result of the calculation for each analyzed flight was a dataset containing the flight name, scheduled flight date, current arrival curve interval, current 20 vol. 52/2025 calculation of queue formation and queue length figure 2. queue length distribution by time window. queue length in that interval, number of open check-in counters, and number of passengers processed in the given time window. the overall number of records for each flight matched the total number of arrival curve intervals. table 2 provides an overview of queue metrics used across different airline types, highlighting the differences in average, median and standard deviation of queue lengths for legacy and low cost airlines. cumulative metrics for both types of airlines combined are also presented. airline type average [pax] median [pax] standard deviation [pax] legacy 13 2 19 low cost 5 0 11 cumulative 10 0 17 table 2. descriptive statistics of queue lengths by airline type. the average, median and standard deviation of queue lengths at various time windows leading up to scheduled time of departure times are presented in table 3. each row in the table corresponds to 15-minute time interval before std in hh:mm format according to the defined arrival curve. the figure 2 presents distribution of queue lengths over different time windows before std. each of the boxes represents the spread of queue lengths within the time window. this figure provides insights into typical, minimum, maximum queue lengths and shows the overall variability in the queues. based on this initial analysis and purpose of this research, the following two metrics were defined and calculated. 4.1. queue lengths for specific flights across time intervals for each flight, queue length data were collected at defined intervals of the arrival curve, from 4 hours and 20 minutes to 35 minutes before the scheduled time of average [pax] median [pax] standard deviation [pax] 00:35 0 0 0 00:50 0 0 1 01:05 0 0 2 01:20 0 0 3 01:35 2 0 7 01:50 12 4 15 02:05 32 33 23 02:20 32 33 21 02:35 31 28 22 02:50 23 21 16 03:05 15 13 10 03:20 9 8 7 03:35 5 4 3 03:50 2 2 1 04:05 0 0 0 04:20 0 0 0 table 3. descriptive statistics of queue lengths by each time window [hh:mm] before std for both airline types. departure. the results show how queue lengths fluctuated during these intervals, reflecting the variability in passenger arrival patterns and check-in counter operations. for example, for flight abc123 on july 26th, the queue length peaked at 72 passengers, 2 hours and 5 minutes before the scheduled time of departure. afterward, the queue gradually decreased as the checkin counters opened, and all passengers were processed 1 hour and 20 minutes before departure. the progression of the queue for this flight, along with five other similar flights over the following days, is shown in figure 3. 4.2. cumulative queue formation during day a cumulative analysis was also performed to evaluate queue formation during the period of one day. a day 21 f. author, s. author, t. author acta polytechnica ctu proceedings figure 3. queue lengths for flight abc123 in last 6 days of july 2024. figure 4. cumulative queue formation on june 28th across all departing flights from terminal 2. with the highest number of passengers departing from terminal 2 was selected. during this day, queue formation was analyzed across all flights operating within the day, aggregating queue lengths from individual time windows of the passenger arrival curve to provide an overall picture of passenger flow. the result can be seen in figure 4. three operational peaks are visible in figure 4. the busiest period occurred at 9:45 a.m., when the cumulative queue length reached its highest point at 841 passengers across all flights. two smaller peaks were observed during the day, one around 2:30 a.m. with 556 passengers in the queues and another at 04:30 p.m. in the afternoon with 716 passengers waiting to be processed. 5. discussion the combination of machine learning algorithm with a deterministic algorithm for queue length estimation has demonstrated its potential in predicting queue formation at airport check-in counters. reached results replicate the realistic behaviour of the passenger flow at the chosen processor. these were evaluated through the operational peaks of the executed flight schedule. queue lengths for the particular flight within 6 days interval showed the same pattern, reaching the maximum values around two hours before the flight. this indicates similar conditions and absence of any kind of perturbations of the schedule and check-in counter allocation. in terms of cumulative queuing for the chosen day, seasonality presented in three operational waves was correctly indicated. summer charters peak in the night wave, mid-day and evening waves, clearly show the maximum queue capacity demand during the set time frame. based on the results it can be concluded that integration of these two techniques creates the method, which allows for the predictions of passenger numbers and the translation of these predictions into queue dynamics based on specific operational conditions, such as check-in counter throughput and passenger arrival patterns. main limitations of the presented approach are related to its accuracy and potential for further enhancements in this characteristic. the use of dynamic or more granular arrival curves with smaller time intervals or differentiated by flight type (e.g., low-cost, charter, or legacy carriers) could improve the precision of queue length predictions. this can allow the model to capture more details regarding passenger arrival patterns and to better reflect the variability in 22 vol. 52/2025 calculation of queue formation and queue length different types of flights. limitations are also related to the machine learning algorithm. the predictors, such as information regarding significant events at the flight destinations, could be added to improve prediction accuracy. limited time period data are also considered a limiting factor. this study creates a good foundation for the next phases of the research, which will focus on active management of the distribution of arriving passengers. the primary objective is to create a tool, which will actively assign passengers to specific time windows, optimising the flow of passengers through the airport terminal processors and therefore reducing excessive queues on subsequent terminal processors. the queuing curves calculated for the individual flights represents a milestone and foundation for the further development in the research. 6. conclusions this study explored the use of a combined approach to predict the length of passenger queues at airport check-in counters. by utilizing real-world operational data from václav havel airport prague, the random forest model was trained and predicted passenger numbers for individual departing flights from terminal 2. the deterministic model then used these predictions and calculated queue length estimates based on counter availability and passenger processing rates of the check-in counters. these findings provided valuable insights into passenger behavior upon arrival at the first airport processor, as well as the dynamics of queue formation during the check-in process for individual flights. this information enables further research in the field of active management of departing passengers’ arrivals at the airport. while the methodology offers promising results, further research is needed to refine the accuracy of predictions. the findings also provide a foundation for future work, focusing on active passenger distribution management and coordination with other airport processors to reduce excessive queues and improve overall terminal efficiency. acknowledgements the outcomes of the close cooperation between the faculty of transportation sciences, ctu in prague, and prague airport enabled the development of the presented model. all input data was received according to agreement with the prague airport. this work was supported by the internal grant no. sgs24/107/ohk2/2t/16 “development of an optimisation tool to reduce queues in the airport terminal using managed passenger arrivals” of the czech technical university in prague. references [1] h. b. thiagaraj, c. v. seshaiah. a queueing model for airport capacity and delay analysis. applied mathematical sciences 8(72):3561–3575, 2014. https://doi.org/10.12988/ams.2014.43179 [2] d. sharma, d. s. tege, d. mehta. simulation of queuing process of airport passenger at security check-in by using simulink – matlab. international journal of scientific research in computer science, engineering and information technology 9(6):15–20, 2023. https://doi.org/10.32628/cseit2390537 [3] j. xue, f. yan, a. riska, e. smirni. scheduling data analytics work with performance guarantees: queuing and machine learning models in synergy. cluster computing 19(1):849–864, 2016. https://doi.org/10.1007/s10586-016-0563-z [4] a. t. al-sultan. airport check-in optimization by ip and simulation in combination. world academy of science, engineering and technology, international journal of social, behavioral, educational, economic, business and industrial engineering 9(1):403–406, 2015. https://doi.org/10.5281/zenodo.1100262 [5] h. wang, x. wei, j. yao, y. zhang. traffic flow prediction using machine learning methods. in 2021 3rd international conference on machine learning, big data and business intelligence (mlbdbi), pp. 30–35. 2021. https://doi.org/10.1109/mlbdbi54094.2021.00014 [6] c. loureiro, p. pereira, p. cortez, et al. predicting multiple domain queue waiting time via machine learning. in computational science and its applications — iccsa 2023, pp. 404–421. 2023. https://doi.org/10.1007/978-3-031-36805-9_27 [7] e. chocron, i. cohen, p. feigin. delay prediction for managing multiclass service systems: an investigation of queueing theory and machine learning approaches. ieee transactions on engineering management 71:4469–4479, 2024. https://doi.org/10.1109/tem.2022.3222094 [8] m. schonlau, r. y. zou. the random forest algorithm for statistical learning. the stata journal 20(1):3–29, 2020. https://doi.org/10.1177/1536867x20909688 [9] y. zhang, a. haghani. a gradient boosting method to improve travel time prediction. transportation research part c: emerging technologies 58:308–324, 2015. https://doi.org/10.1016/j.trc.2015.02.019 [10] c. d. manning, p. raghavan, h. schütze. support vector machines and machine learning on documents, p. 293–320. cambridge university press, 2008. https://doi.org/10.1017/cbo9780511809071.016 23 https://doi.org/10.12988/ams.2014.43179 https://doi.org/10.32628/cseit2390537 https://doi.org/10.1007/s10586-016-0563-z https://doi.org/10.5281/zenodo.1100262 https://doi.org/10.1109/mlbdbi54094.2021.00014 https://doi.org/10.1007/978-3-031-36805-9_27 https://doi.org/10.1109/tem.2022.3222094 https://doi.org/10.1177/1536867x20909688 https://doi.org/10.1016/j.trc.2015.02.019 https://doi.org/10.1017/cbo9780511809071.016 acta polytechnica ctu proceedings 52:17–23, 2025 1 introduction 2 research background 2.1 queuing theory and simulation models 2.2 machine learning 2.3 hybrid approaches to queue management 3 methodology 3.1 available data 3.2 machine learning for predicting the number of arriving passengers 3.2.1 model training and testing 3.3 queue length calculation using deterministic algorithm 4 results 4.1 queue lengths for specific flights across time intervals 4.2 cumulative queue formation during day 5 discussion 6 conclusions acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2024.48.0001 acta polytechnica ctu proceedings 48:1–8, 2024 © 2024 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague design strategy for the prototyping of 3d-printed continuous fiber-reinforced components for solar-powered vehicle giulia babbi∗, giangiacomo minak university of bologna, department of industrial engineering, via fontanelle 40, 47121, forlì (fc), italy ∗ corresponding author: giulia.babbi4@unibo.it abstract. continuous fiber fused filament fabrication is an advanced 3d printing method that allows designers to produce high-performing lightweight structures, leading toward innovative design philosophies and sustainable production chains. in this work, we proposed a design strategy to manufacture selected components for a solar-powered vehicle, which consists of three stages. firstly, the prototypes’ cad models are developed, considering their geometric and manufacturing requirements. then, a finite element analysis is carried out to ensure optimal fiber reinforcement of prototypes’ critical regions. lastly, the manufacturing stage involves the slicing process and optimization of the time and cost of the printed part employing markforged® technology. adopting the proposed methodology, the experimental campaign investigated the print quality and performance of several prototypes of each component by adjusting printing parameters. keywords: 3d printing, continuous fibers, manufacturing constraints, solar-powered vehicle, markforged®. 1. introduction emilia v is a solar-powered vehicle prototype that looks to move forward the research for sustainable mobility in the automotive field, through the implementation of advanced lightweight high-strength materials. realized in collaboration with the onda solare project and the university of bologna, the prototype faces different challenges starting from the optimization of the materials employed to obtain a performant structure and weight reduction of the assembly [1]. to achieve this goal, emilia v is built with carbon fiber-reinforced composite materials which have been known to have a competent strength and stiffness of the overall structure [2] while maintaining the lightweight property. from one side laminates of long carbon fiber reinforced thermosetting polymers have been employed to realize the main structure (the chassis) of the solar-powered car. through this methodology, the skeleton of the vehicle was produced by means of a lamination process of the carbon fiber plies in a defined sequence followed by a curing period to strengthen and compress the layups. on the other hand, to optimize the manufacturing of load critical components, continuous fiber fused filament fabrication (cf4) has been selected as the technology to study for this application. cf4 is an innovative 3d printing (3dp) technique, that merges the additive manufacturing (am) process with the enhanced mechanical properties of continuous fiber reinforced materials, allowing the designer to realize design-tailored, lightweight, and high-performing components [3]. to increase the strength of the printed parts, during the extrusion process, filaments of continuous fiber, such as carbon fiber (cf), fiberglass (fg), or kevlar, are embedded into the thermoplastic polymeric matrix [4]. the addition of high-strength highresistance continuous fibers leads to an enhancement of the strength-to-weight ratio of the manufactured component, leading to the realization of additive manufactured piece with increased mechanical properties. while generally additive manufacturing techniques are limited in their applications to rapid prototyping and fast tooling mainly [5], this advanced technology allows to experiment new designs and through a focused optimization process, innovative prints that can represent a competitive alternative for replacing metal parts. several companies offer printers with the possibility to reinforce through cf4 manufacturing process, and between them, markforged® provides a series of commercially available machines [6], able to realize short and continuous fiber-reinforced structures while optimizing the printing time and cost. through the printing process it is possible to customize several parameters including the type of fiber reinforcement, its pattern and orientation, and the infill geometry and percentage, which can be tuned to obtain the desired reinforced structure. in addition, the several available materials and fiber fused filaments increase the possible combinations that need to be studied and investigated experimentally, since every different set of printing parameters generate unique mechanical properties of the final part [7]. coupons manufactured through this reinforcement type are being tested and characterized to investigate the limit and the extent of the technology, spacing from tensile [8], compression [9] and shear [10] tests, 1 https://doi.org/10.14311/app.2024.48.0001 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en giulia babbi, giangiacomo minak acta polytechnica ctu proceedings figure 1. rendering of emilia v, by davide grandini. process fused filament fabrication, continuous fiber reinforcement (cfr) build volume 320 × 120 × 154 mm (12.6 × 5.2 × 6.0 in) layer height 100 µm default, 250 µm maximum print bed precision ground composite nozzles 2 (1 for plastics, 1 for fiber) table 1. markforged mark two 3d printer properties (available at https://markforged.com/3d-printers/mark-two. impact damage resistance [11, 12] and so on. however, the experimentations for this technology are still restricted to material characterization and sample testing. this means that there is still a gap in the literature regarding the whole capacities of the cf4 manufacturing process, from the design strategies and parameters manipulation to the experimental campaigns that could be performed. in this work, we investigate the capacity of the cf4 technology as a metal parts replacement methodology. in order to achieve additively manufactured parts with mechanical properties enhanced enough to compete with metal components, an optimized design strategy has to be implemented. while cf4 provides a way to reinforce polymeric structures, a defined and specific workflow is required to adjust the designed models to 3d printing capabilities and limitations, as well as, to efficiently tune the setup parameters to achieve a part able to perform under studied load cases. therefore, in this study, we propose an optimization and design workflow to explore the strategic implementation of cf4 technology to manufacture selected critical components for emilia v shown in figure 1, such as, the brake pedal [13]. 2. materials and set up in this section, the available materials, the experimental set up and the used equipment in this work are shown and expanded in the following subsections. 2.1. set up and equipment’s parameters for the manufacturing part of this project the mark two 3d printer has been employed. this printer allows a print volume of 320 × 120 × 154 mm with a precision of 100–200 µm. to perform fused filament fabrication (fff) and cf4 the printer is equipped with 2 extruders, one which prints the matrix material, and the other which allows the deposition of the fiber filament. its properties are summarized in table 1. the markforged company provides its own slicing software (eiger) in order to convert the designed model specifics into detailed instructions for the printer. to perform with the slicing process, it is required to provide an standard tessellation language (stl) model of the component to be printed. the 3d models have been realized through solidworks® and then the parts files have been converted into the .stl format. once the file is uploaded in the slicing software, it is possible to select the printing parameters that would affect the final properties of the part. it is important to notice that the features that significantly influence the print quality and the prototypes’ mechanical properties can range from machine capabilities, to environmental factors [14], and from chosen materials, to the printing parameters set up [15], which the designer is going to select. however, in this investigation, it has been possible to control the manufacturing materials choice, which is studied in the next subsection, and part of the printing features, such as fiber deposition path, part orientation on the print bed, infill geometry pattern and percentage. from the settings it is possible to select two particular fiber filament deposition paths, already set by the software. these are called “isotropic” and “concentric” deposition path, which are two standard options and cannot be modified. the “isotropic” reinforcement path does not refer to an actually final isotropic property since, even through the application of continuous fibers, it is not possible to produce an isotropic piece 2 https://markforged.com/3d-printers/mark-two vol. 48/2024 prototyping of 3d-printed continuous fiber-reinforced components figure 2. markforged® layer slicing: deposition of a) “isotropic” fiber at 90°, b) “concentric” fiber with hexagonal infill geometry, c) “concentric” fiber with solid infill geometry, d) “concentric” fiber with triangular infill geometry. material tensile modulus [gpa] flexural strength [mpa] carbon fiber 60 540 kevlar 27 240 fiberglass 21 200 onyx 2.4 71 nylon 1.7 50 table 2. mechanical properties of the materials; data provided by the markforged®company, by means of additive manufacturing technique. it is named this way because it allows the designer to fill selected layers with the deposition of continuous fiber filaments at chosen angles. therefore selecting several angles through the layer might lead to a more “isotropic” distribution of the fiber orientations and therefore to a final piece resistant to different load application angles. for example, to obtain a general reinforcement of a piece, fiber filaments are usually deposited at −45°, 0°, 45° and 90°, along at least 4 layers (one layer can only have one fiber orientation angle). moreover, with this option, it is possible to add one or more filaments of fiber that shall follow the boundaries and perimeter of the geometry, generating a reinforcement along the thickness of the part. this is the “concentric” fiber path, which can also be selected alone, providing a fiber reinforcement around the borders of the component section that is going to be filled with the matrix material and pattern. some examples of “isotropic” and “concentric” deposition are shown in figure 2. another important factor that affect the manufacturing process is the part orientation on the printing bed, which has been shown it significantly influence the final part accuracy and in particular its mechanical properties [16, 17]. therefore, it is necessary to study the 3d model of the part in order to efficiently select which would be the growing direction of the part (z axis) and partly control the anisotropy of the final part. through this section, it is also possible to choose the starting face of the print, the one that would represent the 1st layer. then, the designer has to select the infill geometry pattern and its percentage of filling. the slicing software provides several geometries as infill shapes, such as triangular, hexagonal, rectangular, gyroid, solid (100 % fill) [18], some of which are visible in figure 2. although the software recommends to use triangular infill at 37 % filling rate for an optimal print, the hexagonal infill can achieve lighter structures and reduced print time, while the solid pattern leads to heavier components with enhanced strength and stiffness characteristics. it is possible to play with this parameters and obtain several unique combinations of features which each presents different mechanical properties [19]. prior the print launch, the set up requires to carry out a fine calibration of the print bed which must be cleared by any residue or last print, as well as the nozzles from exceed material. a good calibration level is required for a good quality final piece, since if the extruder is too far from the plane, the filaments shall not compact, while if it is too close, the nozzle does not have enough room to extrude material and may even scratch the bed. in addition, it is also necessary to add some sticking washable glue that enhances the adhesion between the printed part and the bed. this is required since the printer chamber is not heated, therefore leading to an increase of detachment issues. 2.2. materials the choice of the manufacturing materials, especially the reinforcement material, is important to define the final part elastic and stiffness features. in table 2 are enlisted the properties of the filaments available by the manufacturer. the available printing materials provided by the markforged® company are divided into two main categories: the matrix (infill) material and the continuous fiber filaments. the printer is compatible only with the materials provided by the company (patent) and other furnished by third parts cannot be used as an alternative. on one side, the continuous fiber filaments provided by markforged® space from carbon fiber (cf), fiberglass (fg) and kevlar (figure 3). 3 giulia babbi, giangiacomo minak acta polytechnica ctu proceedings figure 3. materials mechanical properties; plots provided by the markforged® company. from the table it is highlighted that cf is the most performant under stress and therefore it has been preferred to reinforce critical parts that have to sustain greater loads, while fg has been chosen for parts that required less load response. regarding the thermoplastics, markforged® allows to a large selection of materials, starting from classic pla, nylon and the more complex and performant onyx. onyx is a combination of a nylon base and short (chopped) carbon fibers, randomly distributed, to generally enhance the mechanical properties of the starting component. although it shows improved stiffness and strength with respect to other thermoplastics, this material presents high sensitivity to moisture, which is absorbed even at room temperature conditions, negatively affecting the mechanical properties [16]. therefore, onyx must be stored under vacuum bags or in closed dry boxes designed specifically to maintain low humidity levels. this material is compatible with the long fiber reinforcement technique, and therefore in this experimental investigation onyx has been selected for matrix extrusion. 3. design strategy since cf4 is still limited for industrial use due to the wide range of design parameters available, recent studies recommend adopting a performance-driven design practices, which can support the designer in exploiting the cf4 for general industrial applications. this study aims to develop a design strategy to realize high performance light-weight components for a markforged (mark two) 3d printer in the framework of sustainable mobility (solar-powered vehicle application). the proposed design strategy involves three macro stages. 3.1. step 1: a general workflow to tailor designs based on geometric constraints firstly, a systematic 3d modelling approach allows the designer to develop and tailor the part design, taking into consideration the geometric constraints and performance requirements. starting from an initial draft, the design undergoes a process of optimization, weight reduction, reinforcement addition to adapt the geometry to the am technology [20]. in this stage, the geometry of the component is considered a draft that satisfies the given size requirements and subsequently, shall be modified according to an iterative procedure that takes into account also the load case analysis and the manufacturing requirements. this is required because the initial draft of the part is usually designed according to metal production philosophies, but realizing the same by means of additive manufacturing would lead to a piece not optimized nor efficiently robust to operational load. therefore, a strategy based on the am requirements is applied taking into consideration print orientation, anisotropic behavior and fiber reinforcement constraint (e.g. required minimal thickness for fiber deposition) during the modelling sketch. this optimization process of the design is unique for each realized components thought am, since each part presents different geometries, weight constraints and load conditions. an example of this workflow is shown in figure 4, where the brake pedal model is considered. starting from a sketch that includes the principal dimensions imposed by the emilia v structure, the part has been thickened to its extent in order to generate a more robust piece. however, this leads to a significant increment of the component weight and cost due to quantity of employed material. therefore, as a next step, a weight reduction process has been applied, paying attention to reduce the thickness only on the areas less reactive to the stress application. doing so, the final model is realized accordingly to weight and cost reduction aims as well as structural integrity of the part. 3.2. step 2: load case analysis suited for each component stress condition as a second stage, a load analysis is carried out to examine the stress conditions and highlight the prototypes’ critical regions. the analysis can be performed both on ansys® software which simulates the worst-case scenario where only the matrix material is reacting to the applied loads, or through a simplified mathematical model that leads to a fast approximations of the generated stresses. specific constraints and boundary conditions have to be set with particular attention to contact areas accordingly to the load case scenario investigated. for example, for the brake pedal analysis the chosen load condition regards an emergency brake of 100 kg, constrained at the fulcrum of the rotation through a cylindrical support, where only the onyx material is reacting to the load. in 4 vol. 48/2024 prototyping of 3d-printed continuous fiber-reinforced components figure 4. brake pedal design optimization process. addition, contact boundary conditions have been applied to evaluate the stresses that may arise through contact surfaces. from the analysis outcomes (figure 5), the critical areas of the stress field are highlighted, and are used an input in the definition of the optimal fiber reinforcement path for the structure that allows a strategic deposition of the strengthening material along the stress directions. from the same example, it is shown that some concentrated stress points are present, that would be resolved by the deposition of continuous fiber in those regions. this is possible because the long fiber allows to redistribute the stresses, avoiding to generate load peaks and concentrations in small regions. on the other hand, an approximate analytical evaluation of the stresses is performed through the implementation of a simplified mathematical model. starting with an estimate of the stress the structure generates to react to the bending load applied, the flexural strength σ has been evaluated through its general formulation 1, where m represents the bending momentum expressed in [nmm], and wf is the section modulus indicating the resistance to bending of a selected cross-section. σ = m wf m = load · arm (1) defined by eq. 2, the section modulus is express in its general formula, as the area moment of inertia i divided by yf , which is the maximum distance from the neutral axis of the cross section. however, to calculate the section modulus, it was chosen the formulation which regards the shape of the cross-section investigated in this case, shown in figure 6. therefore, the corresponding modulus calculation for this kind of geometry is indicated by eq. 3. figure 5. fea results of brake pedal loading condition for equivalent stress. wf = i yf (2) wf = b(h3 − h3) 6h (3) the process allows a comparison between the two maximum stress values the component has to sustain and works as a check of the simulation results. for the case of the brake pedal, while the simulation output corresponds to a maximum stress value of 40.4 mpa, from the formulas the obtained result corresponds to 38.5 mpa. in addition, the iterative design procedure expects to adjust the original draft of the component in the case the fea or the mathematical estimation of the loads highlight stress concentration points unsustainable for the structure. 5 giulia babbi, giangiacomo minak acta polytechnica ctu proceedings figure 6. rectangular cross-section. 3.3. step 3: manufacturing process of high-performance advanced light-weight structures lastly, employing the mark two 3d printer, the components have been manufactured as a subsequent step of the slicing process and the optimization of the time and cost of the part. the available instrumentation provides the manipulation of the several setup parameters enlisted in the previous section (fiber deposition path, part orientation on the print bed, infill geometry pattern and percentage, and choice of the reinforcement material). at the end of the slicing process, a new iteration of the design may be required, since from the view layer-by-layer, some defects in the printing can arise. for example, since the printer requires a fixed thickness to deposit the fiber, the design may present features too thin, which the printer is not able to reinforce. therefore, the geometry shall be updated and modified accordingly. in addition, the tolerances of the printer accuracy must be taken into account, in order to avoid obtaining components that cannot work together because of strict or loose plays. through the optimal combination of the tunable manufacturing features, it is possible to obtain a desired final part with a specific enhancement of its mechanical properties and weight reduction. 4. results through the proposed strategy, several prototypes have been manufactured leading to an evaluation of the combinations of chosen design, adjustable printing parameters and material properties. the highperformance advanced light-weight components are obtained through cf4 technology achieving a significant weight reduction while maintaining structural integrity. these critical components include the brake pedal (figure 7a), the suspension joints, the door hinges (figure 7b) and others. all these parts have been assembled on the emilia v chassis and are being tested throughout the duration of the next solar challenge competition at which it participate (sasol solar challenge, south africa, sempt 2024). the obtained component have been compared based on weight, cost, printing time and quality. each of these parts is a result of a different combination of parameters leading to a unique optimization that needs to be tested in the following steps of this work. for the brake pedal carbon fiber has been deposited at −45°, 0°, 45°, 90° in the layers where the thickness changes occur in order to generally strengthen the structure. moreover, three filaments of continuous fibers have been applied at the borders of the cross section along the whole thickness to prevent the formation of stress concentration points. to reduce the weight of the part, the internal matrix had been filled with hexagonal shaped infill. the pedal print had taken around 24 hours to be completed, and the final piece weighted less then 200 grams. similarly to the brake pedal, the suspension joints have been reinforced with cf deposited at the borders and “isotropically” but the infill is solid to increase the stiffness and resistance to the applied load. lastly, the door hinges have been realized through a separated design in order to reinforce the base structure horizontally around the contact joints, while the fulcrum part had been printed transversely to strengthen the main hole. the model has been studied and designed to these requirement, and therefore to fit the two pieces together at the end of the print a dovetail joint has been employed to merge the parts in a whole component. moreover, in this application fg has been selected as the reinforcement material. 5. conclusions a statistical dataset on markforged® is being collected, and subsequently will offer detailed insight into the addressed printing parameters, allowing the generation of a system to catalog the final parts based on their quality, performance, print time, and cost. the limitation and capabilities of the printer are also analysed, leading to a significant know-how of the mark two and therefore, to a finer prints. moreover, the emilia v project is being completed by means of the continuous fiber-reinforced structures to enhance the parts mechanical properties such as tensile strength and flexural strength. this allows to optimize the time and cost of the required components and reduce the solar-powered car’s weight due to the substitution of metal parts with 3d-printed continuous fiber-reinforced prototypes. on the other hand, the planned future work includes tensile, shear, and compression tests of selected samples to investigate the effect of fiber reinforcement patterns and infill geometries on the mechanical properties. furthermore, investigations on the moisture effect on the material mechanical properties is required to be carried out in order to examine the strength and stiffness changes over time during humidity absorbing periods. finally, the proposed methodology will be 6 vol. 48/2024 prototyping of 3d-printed continuous fiber-reinforced components (a). brake pedal: on the left the slicing view of a layer, on the right the final printed component. (b). on the top, door hinge reinforced through fg filaments; on the bottom, suspension joints realized in solid infill with addition of cf reinforcement. figure 7. examples of outcomes of the design strategy and cf4 manufacturing process. updated and optimized during the next studies and therefore implemented as support in other projects, such as the realization of a solar-powered boat that will eventually compete in the monaco energy boat challenge. acknowledgements financed by the european union – nextgenerationeu (national sustainable mobility center cn00000023, italian ministry of university and research decree n. 1033 – 17/06/2022, spoke 11 – innovative materials and lightweighting). the opinions expressed are those of the authors only and should not be considered as representative of the european union or the european commission’s official position. neither the european union nor the european commission can be held responsible for them. references [1] g. minak, m. lukovic, s. maglio, s. kojic. toward a sustainable mobility: a solar vehicle for a new quality of life. iop conference series: materials science and engineering 659(1):012075, 2019. https://doi.org/10.1088/1757-899x/659/1/012075 [2] g. minak, t. m. brugo, c. fragassa, et al. structural design and manufacturing of a cruiser class solar vehicle. jove journal 143:1–15, 2019. https://doi.org/10.3791/58525 [3] s. m. f. kabir, k. mathur, a.-f. m. seyam. a critical review on 3d printed continuous fiber-reinforced composites: history, mechanism, materials and properties. composite structures 232:111476, 2020. https: //doi.org/10.1016/j.compstruct.2019.111476 [4] a. dey, i. n. r. eagle, n. yodo. a review on filament materials for fused filament fabrication. journal of manufacturing and materials processing 5(3):69, 2021. https://doi.org/10.3390/jmmp5030069 [5] j. m. chacón, m. a. caminero, p. j. núñez, et al. additive manufacturing of continuous fibre reinforced thermoplastic composites using fused deposition modelling: effect of process parameters on mechanical properties. composites science and technology 181:107688, 2019. https: //doi.org/10.1016/j.compscitech.2019.107688 [6] markforged®. carbon fiber composite 3d printer: markforged mark two. 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https://doi.org/10.1186/s10033-019-0368-0 acta polytechnica ctu proceedings 48:1–8, 2024 1 introduction 2 materials and set up 2.1 set up and equipment's parameters 2.2 materials 3 design strategy 3.1 step 1: a general workflow to tailor designs based on geometric constraints 3.2 step 2: load case analysis suited for each component stress condition 3.3 step 3: manufacturing process of high-performance advanced light-weight structures 4 results 5 conclusions acknowledgements references https://doi.org/10.14311/app.2024.50.0013 acta polytechnica ctu proceedings 50:13–17, 2024 © 2024 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague influence of laser beam welding parameters on local mechanical properties of duplex stainless steel joints pavel kovačócya,∗, maroš martinkoviča, beáta šimekováa, martin hnilicaa, ingrid kovaříkováb, jiří macháčekb, michal krbaťab a slovak university of technology in bratislava, faculty of materials science and technology in trnava, j. bottu 25, 91724 trnava, slovak republic b alexander dubček university of trenčín, faculty of special technology, ku kyselke 469, 91106 trenčín, slovak republic ∗ corresponding author: pavel.kovacocy@stuba.sk abstract. duplex stainless steels (dss) are dual-phase alloys consisting of austenitic and ferritic phases in the microstructure. dss provide an ideal compromise between mechanical properties and corrosion resistance. welding of dss leads to a change in the ferrite/austenite ratio in the microstructure and to degradation of the properties of the weld joints. therefore, it is very important to control the microstructure of the weld joint directly during the welding process. an innovative method of laser welding applying a dual beam was used. by spreading the energy of the laser beam over two spots, additional heat is introduced into the weld, which ensures a slowdown in the cooling rate of the dss and possibly ensures the desired ratio of ferrite and austenite in the microstructure. the influence of laser welding parameters and energy distribution of the dual beam on the fusion zone (fz) geometry, resulting microstructure and mechanical properties of the weld joints was monitored. the tensile and shear strength was measured. the coefficient of proportionality between shear and tensile strength was calibrated to be as 1.38 ± 0.01. keywords: welding, welding parameters, weld joint, laser, duplex stainless steel, shear test. 1. introduction the duplex stainless steels (dss) have been developed to provide a combination of high strength and resistance to corrosion. these properties are used in a variety of applications, such as chemical, structural, power plant, nuclear, transport and tanker applications, food processing and many others. understanding the complex factors that affect the weldability of these materials is critical to the successful implementation [1–4]. numerous experimental investigations have been performed to the laser beam welding of dss. laser has the potential to accelerate the fabrication of components, however, some metallurgical challenges limit its applications in dss. solidification is fully ferritic and followed by the diffusion-controlled solid-state ferrite to austenite transformation. rapid cooling, when using laser welding, restricts austenite formation and disturbs the optimum phase balance in dss. the ferrite/austenite balance has an influence on mechanical and corrosion properties. optimal properties of dss are obtained at about 50% ratio of austenite and ferrite in the microstructure. consequently, it is expected that the ferrite/austenite balance in fusion zone (fz) is affected by heat input of laser beam. the heat input is mainly affected by welding parameters such as laser beam power, welding speed and focal position [1–11]. other ways that affect the ferrite/austenite ratio at laser beam welding are using of nitrogen as shielding gas and using of laser reheating. both leads to the increasing of austenite contents in fz and heat affected zone (haz). the effect of nitrogen and reheating has been characterized in terms of tensile properties, fractography, hardness and corrosion resistance [1–5]. some investigations have been devoted to using a multi-beam laser welding process. the numerical analysis of the laser welding process using a dual beam have been applied to perform reheating with a second laser beam to achieve appropriate ferrite/austenite ratio [12, 13]. the present study is concerned with dual beam laser welding and its effect on size and microstructure of fz and mechanical properties of dss weld joints. tensile tests of weld joints lead to fractures in base metal, so the strength properties were assessed by applying a shear strength test right in the fz of weld joints [14]. 2. materials and methods the welded materials are a commercial dss en 1.4362 (dss 2304) and en 1.4410 (dss 2507). chemical compositions of materials are shown in table 1. dissimilar sheets of dss 2507–dss 2304 were welded in butt joint configuration. specimen dimensions were 140 × 100mm, thickness 5mm. an indus13 https://doi.org/10.14311/app.2024.50.0013 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en p. kovačócy, m. martinkovič, b. šimeková et al. acta polytechnica ctu proceedings wt.% c mn p s si cr ni mo n cu dss 2304 ≤ 0.03 ≤ 2 ≤ 0.035 ≤ 0.015 ≤ 1.0 22.5 4.5 0.3 0.1 0.3 dss 2507 ≤ 0.03 ≤ 1.2 ≤ 0.025 ≤ 0.015 ≤ 0.8 25 7 4 0.3 0 table 1. chemical composition of materials en 1.4362 (dss 2304), en 1.4410 (dss 2507). figure 1. arrangement of welding. trial fiber laser type ipg yls 5000 with a wavelength of 1.06 µm and a maximum output power of 5 kw was used to weld dissimilar joints. the precitec yw52 laser head was supplemented with a special twin-spot module (figure 1), which enables the splitting of the laser beam. beam power ratios (primary beam: secondary beam) of 50:50, 65:35 and 80:20 was used. the dual beam was applied in a tandem configuration with a distance of 1.7mm between beam axes. constant welding parameters were laser beam power 3400w, welding speed 10mms−1, focal position 0mm. argon was selected as the shielding gas and a flow rate on the surface was 25 lmin−1. beam power ratio were changed on 50:50 (sample 50:50), 65:35 (sample 65:35) and 80:20 (sample 80:20). samples of weld joints were subjected to metallographic analysis and testing of mechanical properties. at metallographic analysis fz geometry (bead shape), resulting microstructure, ferrite/austenite ratio and microhardness hv0.1, were investigated using optical microscope neophot 32, stereomicroscope stemi 2000-c and imagej. testing of mechanical properties such as tensile strength and shear strength were investigated using the universal testing machine tinius olsen 300st. during the tensile strength test, the samples were broken at the base metal. this does not give an overview of the strength of fz of weld joint. the strength properties were therefore assessed by applying a shear strength test equipment (figure 2a). (a). shear testing equipment. (b). scheme of shear test. figure 2. shear strength test. a schematic view of the shear strength test is shown in figure 2b. 3. results and discussion the weld bead shows full penetration for all the weld samples, however, the variation in weld bead shapes is observed due to changes in the dual beam power ratio. the beam power ratio 50:50 leads to the formation of a typical vine glass shaped weld bead. the front side width is 5.59mm and the back side width is 1.10mm (figure 3a). changing the power ratio on 65:35 and 80:20 leads to decreasing of the front side width to 4.04mm and 4.19mm respectively and increasing the back side width to 2.57mm and 2.42mm respectively (figure 3b, 3c). 14 vol. 50/2024 influence of laser beam welding parameters on local . . . (a). sample 50:50. (b). sample 65:35. (c). sample 80:20. figure 3. weld bead geometry and hv0.1. the fz microstructure consists of the networks of austenite at the ferritic grain boundaries. the observed weld beads exhibited different content of austenite in the range from 16% to 26%. the highest content of austenite was found in sample 65:35 using dual beam power ratio 65:35. the laser beam power ratio 50:50 and 80:20 had importantly almost the same influence on ferrite/austenite ratio 84:16% and 83:17% respectively. for further increasing of austenite content, it would by necessary to use above mentioned methods: nitrogen as shielding gas, laser reheating and so on. the microhardness hv0.1 along the weld crosssection (hardness measurement line figure 3a) for varying dual beam power ratio was measured (figure 3). it can be noted that the beam power ratio has no significant influence on hardness values. the highest value was measured at dss 2507 base material (330 to 357). the microhardness of the dss 2304 base material and fz were in the range from 295 to 306 and from 318 to 334 respectively. so, the hardness of fz reaches higher values in comparison with dss 2304 base material and something lower values in comparison with dss 2507 base material. the tensile strength was evaluated for all the weld joints with varying dual beam power ratios (samples 50:50, 65:35 and 80:20). for each sample, three testing specimens were prepared and tested. during the tensile test all the specimens were broken in the dss 2304 base material (figure 4) and the reached values are in the range from 777 to 783mpa. if there is a fracture in the base material, the weld joint is evaluated as satisfactory, but there is no information about the strength of fz and haz. so, the strength properties were assessed by applying the shear strength test right in the fz of the weld joints. for each sample (50:50, 65:35 and 80:20) two testing specimens were prepared and tested. the specimens after shear strength test are shown in figure 5. figure 4. specimens after tensile strength test. figure 5. specimens after shear strength test. typical sheared edge of fz for specimen 65:35 is shown in figure 6. the sheared edge consists of a burnish zone and a fracture zone. in the front side of the weld bead deformed rollover zone can be seen. the shear test was conducted using 0.1mm shearing clearance. the load-displacement diagrams created during shear strength test for the fz (specimens 50:50, 65:35 and 80:20) are shown in figure 7. the diagrams have almost the same course, nevertheless, some differences in the maximum load can be seen (50:50 – 24500n, 65:35 – 23800n and 80:20 – 24100n). this was reflected in the calculated strength values of the tested specimens. the decrease in the load value corresponds to the increase in the austenite content in the fusion zone. the shear strength for fz of samples 50:50 reached values 641 and 649mpa, for sample 65:35 reached 625 and 629mpa and for sample 80:20 reached 630 and 638mpa. to compare and to determine the propor15 p. kovačócy, m. martinkovič, b. šimeková et al. acta polytechnica ctu proceedings figure 6. sheared edge of fz (specimen 65:35). figure 7. shear strength test diagrams. tionality coefficient between tensile strength and shear strength, the shear strength test was also carried out for both dss 2304 and dss 2507 base materials. from the values of tensile strength rm and shear strength rms of the dss 2304 base material, the proportionality coefficient “k” can be calculated. the value of “k” = rm/rms = 780 ± 3mpa / 565 ± 2mpa .= 1.38 ± 0.01. using this coefficient the proportional tensile strength of dss 2507 base material and fz of weld joints were calculated. measured and calculated strength characteristics are given in table 2. tensile test shear test sample strength [mpa] strength [mpa] bm dss 2304 780 ± 3 565 ± 2 bm dss 2507 .= 861 ± 9* 624 ± 2 fz 50:50 .= 890 ± 12* 645 ± 4 fz 65:35 .= 865 ± 9* 627 ± 2 fz 80:20 .= 875 ± 12* 634 ± 4 * calculated tensile strength using “k” .= 1.38 ± 0.01 table 2. measured and calculated strength characteristics. 4. conclusions the use of the dual beam with different values of beam power ratios leads to significant changes in the bead shape, especially the front side width and beck side width were changed. the investigated fz exhibited different content of austenite in the range from 16% to 26%. the highest content of austenite was found in sample 65:35, when the laser beam power ratio was 65:35. this also leads to decreasing of the shear strength of the fz. measurement of hardness did not reveal any significant differences of values for all measured weld joints, so the dual beam power ratio influence on hardness is negligible. the fracture location at the tensile test was in the base material for all specimens, so the weld joint tensile strength is satisfactory but no information was obtained about the strength of fz. therefore, the shear strength test was applied. the shear strength of both base materials and all fz samples was measured. the coefficient of proportionality between shear strength and tensile strength was calibrated to be as 1.38 ± 0.01. the highest value of the shear strength (649mpa) was measured for the fz of sample 50:50. the weld joint prepared according to the abovementioned parameters exhibited the full penetration and defects free fz. the mechanical properties of the fz are satisfactory, however the content of austenite in microstructure is still unbalanced. acknowledgements this work was supported by the slovak research and development agency under the contract no. apvv-210232. references [1] a.-m. el-batahgy, a.-f. khourshid, t. sharef. effect of laser beam welding parameters on microstructure and properties of duplex stainless steel. materials sciences and applications 2(10):1443–1451, 2011. https://doi.org/10.4236/msa.2011.210195 [2] h. s. abdo, a. h. seikh. mechanical properties and microstructural characterization of laser welded s32520 duplex stainless steel. materials 14(19):5532, 2021. https://doi.org/10.3390/ma14195532 [3] g. n. ahmad, n. k. singh, b. n. tripathi, et al. monitoring of thermo-cycles in fibre laser welding of duplex stainless steel 2205 sheets and its correlation with microstructures and mechanical properties. materials research express 10(10):106517, 2023. https://doi.org/10.1088/2053-1591/ad0095 [4] a. baghdadchi, v. a. hosseini, k. hurtig, l. karlsson. promoting austenite formation in laser welding of duplex stainless steel – impact of shielding gas and laser reheating. welding in the world 65:499–511, 2021. https://doi.org/10.1007/s40194-020-01026-7 [5] w. w. zhang, s. cong, s. b. luo, j. h. fang. effects of energy density and shielding medium on performance of laser beam welding (lbw) joints on saf2205 duplex stainless steel. the journal of the minerals, metals & materials society 70:1554–1559, 2018. https://doi.org/10.1007/s11837-018-2872-6 16 https://doi.org/10.4236/msa.2011.210195 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[6] m. bolut, c. y. kong, j. blackburn, et al. yb-fibre laser welding of 6mm duplex stainless steel 2205. physics procedia 83:417–425, 2016. https://doi.org/10.1016/j.phpro.2016.08.043 [7] e. r. fábián, j. dobránszky, j. csizmazia, r. ott. effect of laser beam welding on the microstructure of duplex stainless steels. materials science forum 885:245–250, 2017. https://doi.org/10.4028/www. scientific.net/msf.885.245 [8] a. ghosh, d. misra, s. k. acharyya. experimental and numerical investigation on laser welding of 2205 duplex stainless steel. lasers in manufacturing and materials processing 6:228–246, 2019. https://doi.org/10.1007/s40516-019-00090-2 [9] m. landowski, s. c. simon, c. breznay, et al. effects of preheating on laser beam–welded nssc 2120 lean duplex steel. the international journal of advanced manufacturing technology 130:2009–2021, 2024. https://doi.org/10.1007/s00170-023-12840-w [10] r. sołtysiak, t. giętka, a. sołtysiak. the effect of laser welding power on the properties of the joint made of 1.4462 duplex stainless steel. advances in mechanical engineering 10(1):1687814017751949, 2018. https://doi.org/10.1177/1687814017751949 [11] s. zhao, y. bi. influence of heat input on microstructure and mechanical properties of laser weld metal in 2507 duplex stainless steel by different welding speed and welding power. materials science 30(1):26– 33, 2024. https://doi.org/10.5755/j02.ms.34338 [12] a. fey, s. ulrich, s. jahn, p. schaaf. numerical analysis of temperature distribution during laser deep welding of duplex stainless steel using a two-beam method. welding in the world 64:623–632, 2020. https://doi.org/10.1007/s40194-020-00857-8 [13] s. m. robertson, a. f. h. kaplan. multi-keyhole separation during multi-spot laser welding of duplex steel. optics & laser technology 143:107382, 2021. https://doi.org/10.1016/j.optlastec.2021.107382 [14] m. martinkovič, p. kovačócy. possibilities of analyzing the mechanical properties of welded and soldered joints. journal of physics: conference series 2712(1):012023, 2024. https://doi.org/10.1088/1742-6596/2712/1/012023 17 https://doi.org/10.1016/j.phpro.2016.08.043 https://doi.org/10.4028/www.scientific.net/msf.885.245 https://doi.org/10.4028/www.scientific.net/msf.885.245 https://doi.org/10.1007/s40516-019-00090-2 https://doi.org/10.1007/s00170-023-12840-w https://doi.org/10.1177/1687814017751949 https://doi.org/10.5755/j02.ms.34338 https://doi.org/10.1007/s40194-020-00857-8 https://doi.org/10.1016/j.optlastec.2021.107382 https://doi.org/10.1088/1742-6596/2712/1/012023 acta polytechnica ctu proceedings https://doi.org/10.14311/app.2024.50.0049 acta polytechnica ctu proceedings 50:49–54, 2024 © 2024 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague comparison of berkovich and spherical tip indentation for determining the young’s modulus of polymer thin films encapsulated by a dielectric marina melo de limaa,b,∗, vincent mandrillonb, jennifer hayc, laurent-luc chapelona, christophe poulainb, olivier lebaigued a stmicroelectronics, 850 rue jean monnet, 38920 crolles, france b university grenoble alpes, cea-grenoble, leti, 17 avenue des martyrs, 38054 grenoble cedex 9, france c kla instruments, 105 meco lane, oak ridge, tennessee 37830, usa d university grenoble alpes, cea-grenoble, liten, 17 avenue des martyrs, 38054 grenoble cedex 9, france ∗ corresponding author: marina.melodelima@st.com abstract. pixel integrated micro-lenses for cmos image sensors consist of a stack of polymer acrylate resin films encapsulated by a dielectric layer. due to the mismatch of thermomechanical properties, adhesive or cohesive fractures can occur. this can lead to reliability issues requiring the knowledge of the polymer thermomechanical properties. nanoindentation is a standard method for determining young’s modulus of thin films. however, when performing temperature-dependent nanoindentation studies directly on the polymer film, contamination of the tip can occur near or above the glass transition temperature leading to estimation errors. therefore, the polymer films must be measured including a protective layer, and a multilayer model is used to extract the polymer’s young’s modulus. finite-element simulations of berkovich and spherical indentations on the complete stack were performed, enabling the identification of a contact radius range within which the relative error resulting from using the multilayer model is less than 10%. consequently, room temperature and temperature-dependent tests of the complete stack were performed, enabling the determination of the polymer’s young’s modulus as a function of temperature without tip contamination. keywords: temperature-dependent nanoindentation, berkovich tip, spherical tip, thin films, polymer, acrylate resin, finite-element modeling. 1. introduction in microelectronics, component manufacturing is based on the triptych of yield, reliability, and performance. this study focuses on the reliability of integrated-pixel micro-lenses for cmos image sensors. these micro-lenses are composed of a stack of polymer (acrylate) resin films encapsulated by an inorganic and transparent protective layer (figure 1). the resins that constitute the micro-lenses ensure the convergence of incoming photons onto the underneath photodiodes. the protective layer is a dielectric film added to protect the image sensor during cleaning, packaging, assembly, and from humidity and oxidation throughout the product’s lifespan. one possible limitation of this process is the eventuality of adhesive or cohesive failures due to the mismatch in thermomechanical properties between the protective layer and the polymers. consequently, understanding the thermomechanical properties of the polymers is crucial for both identifying the impact of process deposition parameters and providing predictive failure simulations of the complete lens stack. in this study, a polymer provided by stmicroelectronics is characterized to evaluate the evolution of its young’s modulus as a function of figure 1. sem image and schematic of a micro-lens array (not to scale). temperature using temperature-dependent nanoindentation tests. first, the nanoindentation protocol is described along with the characterized samples. this is followed by the presentation of the analytical multilayer model used to extract the elastic modulus from the experimental data. then, finite-element models of the indentation are presented and utilized to define 49 https://doi.org/10.14311/app.2024.50.0049 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en m. melo de lima, v. mandrillon, j. hay et al. acta polytechnica ctu proceedings the validity range of the aforementioned analytical model. afterward, the resulting elastic moduli from room temperature indentations performed with both berkovich and 50µm radius spherical tips are compared. finally temperature-dependent results ranging from 20 to 250 °c are presented. this temperature range is extended up to 250 °c to identify the glass transition temperature. 2. materials and methods 2.1. experiments nanoindentation using a berkovich tip is the reference method for determining the young’s modulus of thin films. however, when performing temperaturedependent nanoindentation studies directly on a polymer film, contamination of the tip can occur near or above the glass transition temperature leading to estimation errors and tip degradation. therefore, for the purpose of this study, a dielectric thin film, referred to as the top coat, is added atop the polymer to protect the tip. consequently, the characterized samples consist of a 3 µm acrylate polymer film deposited on a standard silicon wafer with [100] orientation, encapsulated by a dielectric layer, either 200 nm or 600 nm thick (figure 2). however, a potential issue is the cracking of the top coat during indentation, which could lead to tip contamination since it has been demonstrated that room-temperature berkovich indentation induces such cracks (figure 2). therefore, this work focuses first on room-temperature experiments to evaluate and validate the use of a spherical tip, which is less likely to create cracks thanks to its round shape. this is achieved by comparing nanoindentation tests conducted using both berkovich and 50µm radius spherical diamond tips. the tested samples include a 3 µm polymer layer deposited on a silicon substrate and two additional samples where the polymer is coated with top coat layers of 200 nm and 600 nm. temperature-dependent nanoindentation tests, ranging from 20 to 250 °c , are then conducted on the complete stack, with a 200 nm thick top coat, using a 50 µm radius sapphire spherical tip. after conducting the test at 250 °c, a subsequent test at room temperature is performed for evaluating if the young’s modulus of the polymer comes back to its initial value. the indentation tests are performed using a mts xp nanoindenter in continuous stiffness measurement (csm) mode. this enables real-time contact stiffness measurements and subsequent calculation of the apparent reduced modulus against the contact indentation depth, using the oliver and pharr method [1]. the reduced modulus e∗ depends on the young’s modulus e and poisson ratio ν (eq. 1). e∗ = e 1 − ν2 (1) for the temperature-dependent tests, two independent lasers are used to heat the tip and the backside figure 2. illustration of the stack evaluated during nanoindentation tests (left). afm image of berkovich indentation cracks (right). of the sample via optical fibers. the sample is fixed to a specifically-designed stage with a graphite glue allowing heat transfer. the setup also includes a water cooling circuit and a copper shield to protect the indentation column from heating during experiments, which are conducted at ambient air. this system provides a ramping rate of 100 °cmin−1 and a precise temperature control (± 0.1 °c), ensuring that the tip and the sample are at thermal equilibrium before contact. thermal drift correction is applied by measuring it at 90% of the unloading segment and assuming it remains constant throughout the test. standard calibration using a fused silica sample is performed before the tests, [1], allowing the use of the tip’s real shape for measurement analysis. 2.2. multilayer model it should be emphasized that the measured reduced elastic modulus is the apparent one of a composite dielectric/resin/substrate modulus. in order to isolate the resin’s response, an appropriate bilayer model must be used. here, the multilayer model of mercier et al. [2], which extends the monolayer model of bec et al. [3], is chosen. the bec model (eq. 2) involves the reduced modulus of the film and the substrate, respectively e∗ f and e∗ s, the film thickness t and the contact radius rc. this monolayer model relies on several hypothesis. first that the system’s equivalent stiffness can be obtained by applying the reciprocal sum of the film and substrate equivalent stiffnesses. secondly, that both exhibit elastic behavior. lastly, that the pressure is applied by a flat cylindrical punch. the relative influences of the film and substrate are weighted by a function derived from finite-element analysis. kz = (1 + 2t πrc )( t πr2 ce ∗ f + 1 2rce∗ s )−1 (2) the mercier’s multilayer model (eq. 3) involves the reduced young’s modulus of the layers and the substrate, respectively e∗ i and e∗ s , the films thicknesses ti, and the calculated contact radius at the layer number i, rci (eq. 4). 50 vol. 50/2024 comparison of berkovich and spherical tip indentation . . . properties top coat polymer silicon substrate thickness [nm] 200 and 600 3 000 / young’s modulus [gpa] 42 and 35 [0.1–10] 165 [4] poisson ratio 0.23 [5] 0.35 [6] 0.22 [4] table 1. material properties used in finite-element simulations. thicknesses as well as the young’s modulus of the films are experimentally determined. poisson ratios and the young’s modulus of the substrate are obtained from the literature. kz = ( n∑ i=0 ti (πr2 ci + 2rciti)e∗ i + 1 2(rcn + 2tn π )e∗ s )−1 (3) with rc(i+1) = rci + 2ti π (4) here, this multilayer model is used for extracting the young’s modulus of the polymer when encapsulated by a top coat, given that the young’s modulus of the top coat is already known. 3. finite-element model the validity of the mercier’s multilayer model depends on several parameters: the mismatch of elastic properties between the films (etopcoat/epolymer) and between the films and the substrate (etopcoat/esubstrate), the contact radius rc and the ratio of the thicknesses of the films (ttopcoat/tpolymer). finite-element simulations of the complete stack are conducted to define, for this specific case, the validity domain of the model. the layers’ thicknesses, their young’s moduli and poisson’s ratios used in the finite-element simulations are detailed in table 1. the poisson’s ratio and the young’s modulus of the substrate, suitable to nanoindentation simulations, are obtained from the literature. the poisson’s ratios of the top coat and the polymer are also sourced from literature. the reduced elastic modulus of the 200 nm and 600 nm thick top coat are experimentally determined to be 42gpa and 35gpa respectively through nanoindentation tests. the tests are performed at room temperature on a sample consisting of the top coat layer deposited on a silicon substrate. indentation is carried out using the csm mode and a standard berkovich tip. the bec model is used to extract the young’s modulus. to establish a reference reduced modulus value for the polymer, nanoindentation tests with both berkovich and 50 µm spherical tip are performed on a 3µm thick polymer film deposited on a silicon substrate. the reduced elastic moduli obtained from performing 16 indentations on each sample are (4.8 ± 0.3)gpa for the berkovich tip indentation and (5.5 ± 0.9)gpa for the 50µm spherical tip indentation. here, berkovich indentation simulations are conducted to study the validity domains of mercier’s multilayer model for room temperature measurements. therefore, berkovich indentation simulations are performed by implementing a young’s modulus of 4gpa, this value being close to the one measured at room temperature, for the polymer. considering that spherical indentation tests are to be performed at elevated temperatures, a consequent reduction in the polymer’s young’s modulus is anticipated. consequently, 50 µm spherical tip indentation simulations are executed across a range of young’s modulus values for the polymer layer, specifically at 0.1, 0.25, 0.5, 1, 2.5, 4, and 10gpa. since the elastic properties of the top coat and the substrate are assumed to change little with temperature, they are kept constant and identical to those used for the berkovich simulations. 3.1. berkovich simulations finite-element simulations of the indentation of the complete stack are performed with a 70° conical indenter [4], and a tip radius of 20 nm, by assuming an elastic behavior. these axisymmetric, non-linear, two-dimensional simulations are performed using the cosmos-brand finite-element solver (no longer commercially available). both the indenter and the sample are modeled with four-node, axisymmetric, plane-strain elements (figure 3). for the sample, the extent of the mesh is 90rc in both the radial and axial directions, where rc, the anticipated contact radius, is calculated as the radius of the indenter at a distance from the apex equal to the specified indentation depth. the radial extent of the fine mesh of the sample is 1.2rc. for the indenter, the radial extent of the mesh is 45rc, and the axial extent is 90rc. the boundary conditions are specified as follows. along the right-hand side and bottom of the sample, all nodes are rigidly fixed. along the axis of symmetry, nodes are constrained to move along the axis of symmetry only (ux = 0). nodes along the top of the indenter are displaced downward by the total displacement prescribed by the user. nodes along the right-hand side of the indenter (far from the contact) are unconstrained. the stiffness of the sample is calculated from the load versus displacement curve by fitting the unloading segment up to 50% of the maximum load. values of young’s modulus are then calculated, taking into account the indenter’s influence [1], and by applying the sneddon equation [7]. subsequently, the mercier’s multilayer model is applied to obtain the corrected young’s modulus value for the polymer. 51 m. melo de lima, v. mandrillon, j. hay et al. acta polytechnica ctu proceedings figure 3. finite element meshes of the tip and films system in the case of a berkovich (left) and 50µm spherical (right) indentation. in this study, simulations are conducted with varying indentation depths to examine the validity of the mercier’s model depending on the contact radius. simulations are conducted for top coat thicknesses of both 200 and 600 nm to enable comparison with experimental results. 3.2. spherical simulations finite-element simulations are performed with a 50 µm radius spherical tip using the comsol software. the tip, the resin thin film and the substrate are modeled with triangular elements, while the top coat is modeled with four-node elements (figure 3). the mesh for the tip and the sample is refined in the contact area up to 6rmax c in both radial and axial directions while the mesh size gradually increases away from the contact region. regarding boundary conditions, all the nodes are rigidly fixed at the bottom of the sample and a downward displacement, prescribed by the user, is applied to the top of the indenter. a convergence study validated the built mesh, with relative errors in the young’s modulus of less than 5% for indentation depths ranging from 2 to 200 nm. the stiffness of the sample is calculated from the load versus displacement curve by fitting the unloading segment up to 50% of the maximum load. subsequently, values of reduced young’s modulus are calculated, accounting for the indenter’s influence [1], and by using the sneddon [7] equation. subsequently, the mercier’s multilayer model is applied to obtain the corrected young’s modulus value for the polymer. 4. results and discussion 4.1. simulations results berkovich and 50µm spherical indentation simulations, performed with a young’s modulus of 4gpa for the polymer, enables the identification of the contact radius range where the mercier’s multilayer model is valid. the model is considered valid when the relative error in the polymer’s young’s modulus is below 10%. this range is used to extract an average young’s modulus value for the polymer from room-temperature nanoindentation tests on the complete stack, in which the polymer is encapsulated by a top coat. simulations with a berkovich tip show that the relative error using the mercier model remains under 10% for a contact radius range comprised between 300 and 1 400 nm for a capping of 200 nm, and between 600 and 1 800 nm for a capping of 600 nm (figure 4). for the 50 µm spherical tip simulations, with a 200 nm capping, the relative error stays below 10% for a contact radius ranging from 300 to 2 750 nm. with a 600 nm capping, the relative error drops below 10% at a contact radius of 1 630 nm and stays below 10% up to the maximum studied value of 5 100 nm. these validity ranges are summarized in table 2 and are used to calculate an average young’s modulus value from the experimental curves, corrected using the mercier’s model. in order to analyze temperature-dependent nanoindentation tests, a second set of indentation simulations is performed with a 50 µm spherical tip and a 200 nm capping, using varying young’s moduli for the polymer (figure 4). the results show that the valid contact radius range is dependent on the young’s modulus of the polymer. therefore, to analyze the temperaturedependent tests, an average young’s modulus is calculated using a common contact radius range. this range corresponds to 1 600–2 500 nm. however, for the specific case of a polymer with a young’s modulus of 0.1gpa, the relative error remains between 12% and 16% and since a young’s modulus value of 0.1gpa is expected only above the glass transition temperature, this error margin is considered reasonable. therefore an average young’s modulus for the polymer will be extracted within this range for all temperature studied, with the understanding that values obtained above the glass transition temperature may be less confident. 4.2. experimental results at room temperature experimental curves obtained at room temperature by indenting the following stack: 200 nm top coat/3µm polymer/silicon substrate, are shown in figure 5. the black and gray lines represent indentation performed with a berkovich tip and a 50µm spherical tip, respectively. the dashed lines indicate the apparent young’s modulus while the solid lines show the corrected young’s modulus using the mercier’s model. an average young’s modulus value is calculated within the contact radius range determined from the simu52 vol. 50/2024 comparison of berkovich and spherical tip indentation . . . figure 4. relative error in the young’s modulus of the polymer, using the mercier’s model, according to finiteelement simulations indentations on the stack: top coat/polymer/silicon. simulations were conducted for varying top coat thicknesses (left graph). in this graph, gray indicates berkovich simulations, while black denotes 50 µm spherical indentations. circles and triangles represent cases of the polymer encapsulated by top coats of 600 nm and 200 nm thickness, respectively. the right graph displays results from 50µm spherical indentation simulations for different young’s moduli of the polymer. stack valid rc and h ranges [nm] berkovich tip 50 µm spherical tip top coat (42gpa 200 nm)/polymer (4gpa 3µm)/silicon 200-400/150-650 300-2750/2-130 top coat (35gpa 600 nm)/polymer (4gpa 3µm)/silicon 600-1800/350-920 500-5100/7-600 top coat (42gpa 200 nm)/polymer (0.1-10gpa 3µm)/silicon / 1 600-2 500/50-110 table 2. valid contact radius (rc) and indenter displacement (h) ranges, extracted from the simulation results, depending on the stack under study and the indentation tip. figure 5. evolution of the young’s modulus as a function of the contact radius (derived from calibration data) for an indented stack consisting of 200 nm top coat/3 µm polymer/silicon substrate. the dashed lines represent the apparent young’s modulus and the solid lines the young’s modulus corrected using the mercier’s model. indentations were performed with berkovich (black curves) and 50µm spherical (gray curves) tips. the vertical lines show the contact radius range used for extracting an average young’s modulus value. lations and using the mercier-corrected curve. this range is delineated by the black and gray vertical lines for the berkovich and 50µm spherical indentations, respectively. these results enable the acquisition of the young’s modulus of the polymer encapsulated with a 200 nm capping (figure 6). the same protocol is used for a capping of 600 nm. coherent values for the young’s modulus of the polymer (figure 6) are obtained, with a minimum of 4.3gpa and a maximum of 5.5gpa. it enables the validation of this protocol at room temperature. 4.3. temperature dependent experimental results the obtained reduced young’s modulus of the polymer on a silicon substrate encapsulated by a 200 nm top coat as a function of temperature is shown in figure 6. here, the protocol used allows obtaining the value of the reduced young’s modulus above the glass transition temperature, without tip contamination. tests performed at room temperature after heating at 250 °c show that the polymer recovers its initial reduced young’s modulus. this demonstrates that the polymer was sufficiently stabilized through thermal annealing before conducting the temperaturedependent nanoindentation test. the glass transition temperature is determined to be around 220 °c by 53 m. melo de lima, v. mandrillon, j. hay et al. acta polytechnica ctu proceedings figure 6. average young’s modulus values determined for different samples, at room temperature, using berkovich and 50µm spherical tips (right graph). average young’s modulus of the polymer as a function of temperature (left graph). the polymer film is deposited on a silicon substrate and encapsulated by a 200 nm top coat. tests were performed with a 50 µm spherical tip. identifying the inflection point on the curve. this value is consistent with previous studies conducted by temperature-dependent wafer curvature tests and presented as the results for ’resin c’ in [8]. 5. conclusions a protocol for temperature-dependent nanoindentation of polymer thin films without tip contamination is described and validated. it involves adding a top coat, here a dielectric capping, and the use of the mercier’s multilayer model to extract the young’s modulus of the polymer. the applicability of this model to the stack studied here is verified by finiteelement simulations of berkovich and 50 µm spherical tip indentations. this enabled the identification of a contact radius range for which the relative error in young’s modulus is less than 10%. consequently, the evolution of the young’s modulus of the polymer as a function of temperature is determined while preventing tip contamination. acknowledgements this work is part of the ipcei microelectronics and connectivity and was supported by the french public authorities with the frame of “france 2030” program. references [1] w. c. oliver, g. m. pharr. an improved technique for determining hardness and elastic modulus using load and displacement sensing indentation experiments. journal of materials research 7(6):1564–1583, 1992. https://doi.org/10.1557/jmr.1992.1564 [2] d. mercier, v. mandrillon, m. verdier, y. brechet. mesure de module d’young d’un film mince à partir de mesures expérimentales de nanoindentation réalisées sur des systèmes multicouches. matériaux & techniques 99(2):169–178, 2011. https://doi.org/10.1051/mattech/2011029 [3] s. bec, a. tonck, j. l. loubet. a simple guide to determine elastic properties of films on substrate from nanoindentation experiments. philosophical magazine 86(33-35):5347–5358, 2006. https://doi.org/10.1080/14786430600660856 [4] m. rusinowicz. mechanical and electrical properties of dielectric thin films: electrical-nanoindentation experiments and numerical simulations. ph.d. thesis, université grenoble alpes, 2023. [5] g. carlotti, n. chérault, n. casanova, et al. elastic constants of low-k and barrier dielectric films measured by brillouin light scattering. thin solid films 493(1-2):175–178, 2005. https://doi.org/10.1016/j.tsf.2005.06.019 [6] j. richeton, s. ahzi, k. vecchio, et al. modeling and validation of the large deformation inelastic response of amorphous polymers over a wide range of temperatures and strain rates. international journal of solids and structures 44(24):7938–7954, 2007. https://doi.org/10.1016/j.ijsolstr.2007.05.018 [7] i. n. sneddon. the relation between load and penetration in the axisymmetric boussinesq problem for a punch of arbitrary profile. international journal of engineering science 3(1):47–57, 1965. https://doi.org/10.1016/0020-7225(65)90019-4 [8] b. vavrille. développement d’une méthode innovante de mesures des propriétés thermomécaniques de films minces. application à un dispositif imageur. ph.d. thesis, université grenoble alpes, 2023. 54 https://doi.org/10.1557/jmr.1992.1564 https://doi.org/10.1051/mattech/2011029 https://doi.org/10.1080/14786430600660856 https://doi.org/10.1016/j.tsf.2005.06.019 https://doi.org/10.1016/j.ijsolstr.2007.05.018 https://doi.org/10.1016/0020-7225(65)90019-4 acta polytechnica ctu proceedings 50:49–54, 2024 1 introduction 2 materials and methods 2.1 experiments 2.2 multilayer model 3 finite-element model 3.1 berkovich simulations 3.2 spherical simulations 4 results and discussion 4.1 simulations results 4.2 experimental results at room temperature 4.3 temperature dependent experimental results 5 conclusions acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2024.49.0097 acta polytechnica ctu proceedings 49:97–103, 2024 © 2024 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague formulation and implementation of nonlinear elasticity in advanced constitutive models in geomechanics tereza žalskáa,∗, michal šejnohab, tomáš jandab, alena zemanováa a czech technical university in prague, faculty of civil engineering, department of geotechnics, thákurova 7, 166 29 prague, czech republic b czech technical university in prague, faculty of civil engineering, department of mechanics, thákurova 7, 166 29 prague, czech republic ∗ corresponding author: tereza.zalska@fsv.cvut.cz abstract. several formulations of a nonlinear elastic behavior within the yield surface of the selected advanced constitutive models are described and compared. since concentrating on geotechnical applications the accompanied numerical simulations are limited to basic laboratory tests such as isotropic compression, oedometer, and drained triaxial compression. the results show a significant influence of the size of the initial load step on final predictions particularly when starting from a very low initial stiffness associated with the assumed zero initial stress. differences in the predicted response arising from different formulations are also discussed. these might be quite significant and the design engineer should be aware of that when choosing a particular computational software. keywords: hardening soil model, generalized cam clay model, soft soil model, nonlinear elasticity, oedometer, isotropic compression, triaxial compression. 1. introduction constructing underground structures in densely populated areas, preventing large scale landslides, as well as being able to accurately simulate a complex response of soils observed experimentally, opens the door to the application of advanced constitutive models [1, 2]. grounding on experimental evidence such models attempt to address a nonlinear response of soils already at initial stages of loading. an illustrative example are the critical state models [1, 3] where the nonlinear stress-strain behavior within the yield surface arises naturally. a large group of formulations introduce such a behavior in the spirit of duncan and chang hyperbolic model [4–7]. while their appearance in available commercial software is common [8–10] (to cite a few), the details on their numerical implementation within a given constitutive model may differ. this in turn may result in predictions, which are considerably influenced by the initial load increment. in some formulations, the load step dependence yields loadingunloading curves which deviate even for loading conditions that do not exceed the yield limit, often resulting in an artificial residual strain. it is therefore crucial to understand such potential drawbacks and the user should be aware of that when creating the computational model for a given geotechnical construction. the present paper revisits this issue in light of three popular constitutive models implemented in the above mentioned commercial codes. in particular, the hardening soil model [5, 6], the generalized cam clay model [10, 11], and the soft soil model [12] are examined in section 2 with emphasis on the implementation of the associated nonlinear elastic constitutive model. the results of numerical simulations together with a thorough discussion are presented in section 3. all calculations are carried out using the geo5 fem software [10]. 2. advanced constitutive models the present section provides details on the formulation of nonlinear elasticity of the selected constitutive models, while the plasticity issues will be presented only briefly with details available in the above mentioned references. 2.1. hardening soil model the hardening soil (hs) model developed in [5] and later extended in [6] to account for small strain yielding is plotted in figure 1. as seen in figure 1a it combines two yield surfaces where the shear part, bounded by a limiting yield surface of the mohr-coulomb type, is derived on the assumption of the hyperbolic stressstrain response in triaxial compression as depicted in figure 1c with ε1 being the major principal strain and q representing an equivalent deviatoric stress measure. in the formulation implemented in geo5 fem, both the shear yield surface and the cap yield surface, which is assumed elliptic and centered at the origin of the meridian plane, plot as a smooth hexagon in the deviatoric plane in the spirit of the matsuoka-nakai (mn) yield surface [13], see figure 1b. the evolution (hardening) of both surfaces is controlled by the current value of the equivalent deviatoric plastic strain γ and preconsolidation pressure pc, respectively. 97 https://doi.org/10.14311/app.2024.49.0097 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en t. žalská, m. šejnoha, t. janda, a. zemanová acta polytechnica ctu proceedings (a). yield surface with zero cohesion in meridian plane. (b). yield surface in deviatoric plane. (c). response in triaxial compression. figure 1. graphical representation of hardening soil model. (a). yield surface in meridian plane. (b). behavior under isotropic compression. (c). elastic and plastic part of total strain increment. figure 2. graphical representation of generalized cam clay model. the nonlinear elastic model due to vermeer [14] writes the elastic modulus eur in the form of a power law as a function of the current value of mean effective stress σm as: eur(σm) = eref ur ( σm − c cot φ σref m − c cot φ )m , (1) where eref ur , σref m , m are model parameters, c, φ are the shear strength parameters, the cohesion and the angle of internal friction, respectively. in particular, eref ur is the reference elastic modulus for a reference stress σref m typically equal to −100 kpa. note that the standard elasticity sign convention is adopted with tension being positive. for simplicity, we shall now consider the volumetric response only and assume a linear variation of the mean stress σm over a given load increment as: σm = ησi m + (1 − η)σi+1 m . (2) setting η = 1 gives fully explicit formulation (forward euler (fe) method, σm = σi m in equation (1)) while η = 0 renders fully implicit (backward euler (be) method, σm = σi+1 m in equation (1)). henceforth, the former formulation will be referred to as the constant elasticity (ce) return while the latter formulation will be termed the variable elasticity (ve) return. further distinction is made by considering either secant formulation in the form: σi+1 m = kur(σm(η))(εi v + ∆εv), (3) or incremental formulation provided by: σi+1 m = σi m + kur(σm(η))∆εv, (4) where kur is the elastic bulk modulus given by: kur = eur 3(1 − 2νur) . (5) in particular, cases with η = 1 (fe), η = 0 (be), and so called mid-point rule (mp), i.e., η = 0.5 → σm(η) = 0.5(σi m+σi+1 m ), will be examined in section 3. 2.2. generalized cam clay model the generalized cam clay (gcc) model has been introduced to reconcile the principal drawback of the modified cam clay (mcc) model associated with overestimating the shear strength and consequently an excessive softening of highly overconsolidated soils. to this end, the supercritical (dilation) part of the mcc model was suitably adjusted, see figure 2a, resulting in the dependence on both the critical friction angle φcs and peak friction angle φ via parameter β given by: β = ( sin φ sin φcs 3 − sin φcs 3 − sin φ )2 . (6) 98 vol. 49/2024 nonlinear elasticity in advanced constitutive models similar to the hs model, the mn yield surface represents the projection into a deviatoric plane. the nonlinear elastic constitutive model derives from the assumed response in isotropic compression shown in figures 2b and 2c. when moving along the unloading-reloading κ-line the rate form of the stress-strain law is provided by: σ̇m = −σm κ∗ ε̇el v , κ∗ = κ 1 + e , (7) where κ is called the swelling modulus, e is the void ratio. integrating over a given time increment gives the evolution of the mean effective stress in the form: σi+1 m = σi m exp [ −∆εel v κ∗ ] . (8) next, writing the stress increment ∆σm as: ∆σm = σi+1 m − σi m = σi m ( exp [ −∆εel v κ∗ ] − 1 ) = ks∆εel v , (9) allows us to express the secant bulk modulus ks as: ks = σi m exp [ −∆εel v κ∗ ] − 1 ∆εel v . (10) the analysis may simplify if leaving the concept of variable elasticity return represented by equation (10) and assume the bulk modulus: ki = −σi m κ∗ , (11) to be constant over the load increment (constant elasticity return), recall equation (7). 2.3. soft soil model in the original mcc model the shear strength depends solely on the slope of the critical state line g which for the triaxial compression reads: g = 2 √ 3 sin φcs 3 − sin φcs . (12) brinkgreve showed in [12] that it is not possible to simulate both the oedometer stress path and full triaxial compression path with good accuracy using the same value of g. this let to the formulation of a soft soil (ss) model combining the cap yield surface in the form of mcc model to predict a correct k0-path and the shear yield surface, here again assumed in the form of mn model, to ensure a correct shear strength. such a yield surface is plotted in figure 3 where mc for triaxial compression is given in terms of the coefficient of lateral earth pressure of normally consolidated ∙ figure 3. yield surface of soft soil model in meridian plane. soils knc 0 [12] whereas ms follows from the solution of nonlinear mn yield function [13]. while the shear yield surface represents a response of an elastic perfectly plastic material, the cap yield surface may undergo isotropic hardening. to avoid potential softening within the cap model, which is not allowed in the present implementation, the mc-value should be larger than the slope of the matsuoka-nakai failure surface ms. considering the nonlinear elastic response, we follow the implementation in [8] and limit attention to the constant elasticity return of equation (11). as suggested in [12] it is possible to relate equation (11) to equation (1) by writing: k(σm) = kref ( σm σref m )m . (13) when setting m = 1, the logarithmic compression law is recovered with: 1 κ∗ = −kref σref m . (14) 3. simulation of simple laboratory tests numerical simulation of three basic laboratory tests is performed in this section to examine and compare individual formulations presented in the previous section. in all cases, the computational model consists of two constant strain triangular elements. while the oedometric test is run in plane strain regime, the axisymmetric state of stress is assumed for isotropic and triaxial compression. the loading and boundary conditions are displayed in figure 4 for individual cases. all simulations assumed zero initial stress. 3.1. hardening soil model the response in isotropic compression and oedometer is simulated to compare the secant and incremental formulation represented by equations (3) and (4). as for equation (2), only the cases with η = 0 (be) and η = 0.5 (mp) are tested. although purely elastic behavior is addressed, the nonzero values of c = 10 kpa and φ = 30° are adopted together with eref ur = 30 mpa 99 t. žalská, m. šejnoha, t. janda, a. zemanová acta polytechnica ctu proceedings (a). oedometer. (b). isotropic compression. (c). drained triaxial compression. figure 4. computational models of simple laboratory tests. 0 0,01 0,02 0,03 0,04 0,05 0 2 4 6 8 10 step 0.0025 step 0.001 step 0.0001 secant approach incremental approach -ε 3 [-] -σ m [m p a] m ea n st re ss vertical strain (a). comparing predictions provided by secant and incremental (be) formulations. 0 0,01 0,02 0,03 0,04 0,05 0 2 4 6 8 10 step 0.005 mp step 0.001 mp step 0.0001 mp step 0.003 be step 0.001 be step 0.0001 be -ε 3 [-] -σ m [m p a] m ea n st re ss vertical strain (b). comparing predictions provided by mp and be implementation of incremental formulation. figure 5. response of hs model in isotropic compression. and σref m = −100 kpa to set the initial stiffness of eur(σm = 0) according to equation (1). the results for isotropic compression appear in figure 5. point out that achieving convergence of the underlying nonlinear equation with σm(η) provided by equations (2)–(4): r = σi+1 m − σm(η), (15) required a relatively small initial load increment. so only a minor difference in the nonlinear elastic response is observed for incremental formulation for the tested load steps. on the other, the secant formulation shows response, see figure 5a, which is step size independent. this conclusion is further supported via results presented in figure 6a. point out that both formulations adopt in the present study the same model parameters in the calculation of eur. however, it is clear from figure 5a that arriving at identical predictions would require adjusting at minimum the power law coefficient m for individual formulations based on the experimental measurements. figure 5b compares the results associated with mp and be implementation of equation (1) in the framework of incremental formulation. it appears that mp approach is less sensitive to the size of the loading step. this approach is further promoted by predicting an identical stress-strain curve in both loading and unloading if keeping the same load increment. note that such an approach is implemented in [8] and currently also in [10]. the above discussion is fully supported by simulating an oedometric test as seen in figure 6. the influence of the load step size, particularly for be approach, is clearly evident. 3.2. comparing generalized cam clay and soft soil models this section demonstrates potential differences between the variable elasticity approach, adopted with gcc model, and the constant elasticity approach, adopted with ss model, in light of incremental formulation. the material data were set such as to arrive at identical predictions in isotropic compression when enforcing the constant elasticity return also with gcc model. the material parameters used in all subsequent simulations are: κ∗ = 0.0094, λ∗ = 0.0678, ν = 0.26 and g = mc = 1.074. the initial value of k0 = 1 κ∗ was considered. before proceeding with our discussion on individual results it is worth mentioning that the geo5 fem 100 vol. 49/2024 nonlinear elasticity in advanced constitutive models 0 0,0025 0,005 0,0075 0,01 0,0125 0,015 0 100 200 300 400 500 step 0.1 step 0.01 step 0.001 secant approach incremental approach -ε 3 [-] -σ 3 [k p a] v er tic al s tr es s vertical strain (a). comparing predictions provided by secant and incremental (be) formulations. 0 0,004 0,008 0,012 0,016 0 100 200 300 400 500 step 0.1 mp step 0.01 mp step 0.001 mp step 0.1 be step 0.01 be step 0.001 be -ε 3 [-] -σ 3 [k p a] v er tic al s tr es s vertical strain (b). comparing predictions provided by mp and be implementation of incremental formulation. figure 6. response of hs model in oedometer. software allows for gradually increasing the initial size of the load step with a number of iterations needed to achieve global equilibrium. in the present study, the elastic simulations essentially adopted two times larger step size with every new load step when this option, denoted here as vs, was exploited. in this case, the parameter step0, see e.g. figure 7, represents just the initial step being gradually increased in the course of analysis. all other calculations considered constant step size throughout the analysis. exposition to the derived results begins with figure 7 to show the importance of a relatively small initial load step size to avoid excessive strains attributed to initially very low stiffness. the influence of stiffness variation over the load increment (ve), equation (10), in comparison to ce return, equation (11), appears for isotropic compression in figure 8. note that the ∗-lines require 10 steps only in comparison to 100 and 1 000 steps associated with red and blue simulations, respectively. promoting ve return over ce return might seem justifiable when leaving purely elastic response and allowing for plasticity as shown in figure 8b where simulations based on variable step size match those with the constant step size in the case of ve rather well. similar conclusions can be drawn from the results in figure 9 derived for oedometer. figure 9a further shows that, unlike the hs model, the assumed stiffness evolution leads to artificial residual strains upon complete unloading even in the case of pure elasticity, which may become significant for larger load steps. on the other hand, this issue will become less important when loading the soil beyond its relatively low elastic limit, because the generated permanent plastic strains will then well exceed their elastic counterparts. the last simulations are concerned with the plastic response predicted by the gcc and ss models for the drained triaxial compression test. as seen in figure 10 the analysis begins with isotropic compression, figure 4b, when the specimen is first loaded to acquire 0 0,025 0,05 0,075 0,1 0,125 0,15 0 2 4 6 8 10 step = 0.1 step = 0.01 step = 0.001 step 0 = 0.001 (vs) k 0 =1/κ∗ pc 0 = 1kpa vs variable step size -ε 3 [-] -σ 3 [m p a] v er tic al s tr es s vertical strain figure 7. nonlinear elastic response of gcc model in isotropic compression: influence of initial step size. an initial pressure of 50 kpa. clearly, no deviatoric stresses are generated during this loading stage. the evolution of deviatoric stresses begins with the second stage of loading corresponding to triaxial compression, figure 4c. note that the deviatoric stress measure j is defined as the square root of the 2nd invariant of deviatoric stresses sij as: j = √ 1 2sijsij . (16) similarly to previous studies the ce predictions linked to variable step size (vs) slightly deviate from those obtained with the constant step size. this is not the case for the approach based on ve return. however, it is fair to mention, that smooth convergence with ve required a slightly larger number of iterations, thus a smaller load step size, in contrast to ce return (star and diamond symbols identify individual load steps). also note that for sufficiently small load steps the difference between the two approaches becomes insignificant. 101 t. žalská, m. šejnoha, t. janda, a. zemanová acta polytechnica ctu proceedings 0 0,005 0,01 0,015 0,02 0,025 -ε 3 [-] 0 2 4 6 8 10 -σ 3 [m p a] step = 0.01 step = 0.001 step 0 = 0.001 (vs) k 0 =1/κ∗ pc 0 = 1kpa gcc model ss model vs variable step sizev er tic al s tr es s vertical strain (a). nonlinear elastic response. 0 0,02 0,04 0,06 0,08 0,1 0 2 4 6 8 10 step = 0.001 step = 0.001 (vs) k 0 =1/κ∗ pc 0 = 1kpa gcc model ss model vs variable step size -ε 3 [-] -σ 3 [m p a] v er tic al s tr es s vertical strain (b). plastic response. figure 8. comparing gcc and ss models in isotropic compression. 0 0,02 0,04 0,06 0,08 0,1 0 100 200 300 400 500 loading (500 steps) unloading (500 steps) loading (vs, 10 steps) unloading (vs, 9 steps) gcc model step in = 0.002 vs variable step size k 0 = 1/κ∗ ss model -ε 3 [-] -σ 3 [k p a] v er tic al s tr es s vertical strain (a). nonlinear elastic response. 0 0,1 0,2 0,3 0,4 0,5 0 100 200 300 400 500 gcc model (500 steps) gcc model (vs, 11 steps) ss model (500 stes) ss model (vs, 9 steps) k 0 = 1/κ∗ vs variable step size) -ε 3 [-] -σ 3 [k p a] v er tic al s tr es s vertical strain (b). plastic response. figure 9. comparing gcc and ss models in oedometer. 0 0,05 0,1 0,15 0,2 0,25 0 15 30 45 60 75 90 s tr es s j, -σ m [k p a] gcc ss gcc (vs) ss (vs) vs variable step size σ m j step in = 0.01 -ε 3 [-]vertical strain figure 10. comparing gcc and ss models in drained triaxial compression. 4. conclusion the paper presented several illustrative examples via simulations of simple laboratory tests to show differences in the implementation of nonlinear elastic constitutive laws arising in the formulation of three particular advanced plasticity models. it has been confirmed that the chosen method of implementation may considerably affect the predicted response. the user of a given commercial software should be aware of that, since using the same model in different softwares does not assure the same results. common to all examined models is their dependence on the mean effective stress which may generate a very low initial stiffness when starting from zero initial stress. this in turn leads to excessive strains providing the initial load steps are not sufficiently small. to avoid a large number of loading steps to reach the prescribed load the program geo5 fem allows for a gradual increase of the initially small step size in dependence on the number of iterations needed to converge for the previous load increment. it has also been observed that with the gcc and ss models irreversible elastic strains occur as the stiffness at the end of the loading and at the beginning of unloading is different. on the other hand, this issue does not seem to play a major role as the influence of irreversible elastic strains become negligible in plastic analyses. in this regard, the approach based on variable elasticity return provides response which 102 vol. 49/2024 nonlinear elasticity in advanced constitutive models seems less dependent on increasing the load step in simulations adopting the variable step size (vs). to conclude we remind that the geo5 fem software, adopts, similar to [8], the incremental formulation with mp approach for the hs model and the concept of constant elasticity return for the ss model. on the other hand, the mcc and gcc models employ the variable elasticity return approach. acknowledgements the support provided by the sgs project no. sgs23/032/ohk1/1t/11 and by the gačr grant no. 22-12178s is gratefully acknowledged. references [1] k. h. roscoe, j. b. burland. on the generalised stress-strain behaviour of ‘wet’ clay. in j. heyman, f. a. lechie (eds.), engineering plasticity, pp. 535–609. cambridge university press, 1968. [2] d. mašín. a hypoplastic constitutitve models for clays. international journal for numerical and anlytical methods in geomechanics 29(4):311–336, 2005. https://doi.org/10.1002/nag.416 [3] d. m. potts, l. zdravkovič. finite element analysis in geotechnical engineering: volume one – theory. thomas telford publishing, uk, 1999. https://doi.org/10.1680/feaiget.27534 [4] j. m. duncan, c.-y. chang. nonlinear analysis of stress and strain in soils. journal of the soil mechanics and foundations division 96(5):1629–1653, 1970. https://doi.org/10.1061/jsfeaq.0001458 [5] t. schanz, p. a. vermeer, p. g. bonnier. the hardening soil model: formulation and verification. in r. b. j. brinkgreve (ed.), beyond 2000 in computational geotechnics, pp. 281–290. 1999. https://doi.org/10.1201/9781315138206-27 [6] t. benz. small-strain stiffness of soils and its numerical consequences. ph.d. thesis, university of stuttgart, 2007. [7] j. veselý. the use of the advanced material models for the numerical modelling of the underground structures in clays. ph.d. thesis, ctu in prague, faculty of civil engineering, 2016. [8] seequent.com. plaxis. [2023-11-03]. http://www.plaxis.nl/ [9] zsoil. [2023-11-03]. http://www.zsoil.com/ [10] fine.cz. geo5 fem. [2023-11-03]. http://www.fine.cz/ [11] t. janda, m. šejnoha. formulation of generalized cam clay model. engineering mechanics 13(5):367–384, 2006. [12] r. b. j. brinkgreve. geomaterial model and numerical analysis of softening. ph.d. thesis, delft university of technology, 1994. [13] j. p. bardet. lode dependences for isotropic pressure-sensitive elastoplastic materials. journal of applied mechanics 57(3):498–506, 1990. https://doi.org/10.1115/1.2897051 [14] p. a. vermerr. formulation and analysis of sand deformation problems. ph.d. thesis, delft university of technology, 1980. 103 https://doi.org/10.1002/nag.416 https://doi.org/10.1680/feaiget.27534 https://doi.org/10.1061/jsfeaq.0001458 https://doi.org/10.1201/9781315138206-27 http://www.plaxis.nl/ http://www.zsoil.com/ http://www.fine.cz/ https://doi.org/10.1115/1.2897051 acta polytechnica ctu proceedings 49:97–103, 2024 1 introduction 2 advanced constitutive models 2.1 hardening soil model 2.2 generalized cam clay model 2.3 soft soil model 3 simulation of simple laboratory tests 3.1 hardening soil model 3.2 comparing generalized cam clay and soft soil models 4 conclusion acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2024.49.0026 acta polytechnica ctu proceedings 49:26–30, 2024 © 2024 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague properties of cement screeds using recycled fine aggregates with respect to cement percentage leoš jouraa, zdeněk prošeka,b,∗ a czech technical university in prague, faculty of civil engineering, department of mechanics, thákurova 7, 166 29 prague, czech republic b czech technical university in prague, university centre for energy efficient buildings, třinecká 1024, 273 43 buštěhrad, czech republic ∗ corresponding author: zdenek.prosek@fsv.cvut.cz abstract. this article deals with the effect of 100 % replacement of natural aggregate by recycled aggregate and at the same time the effect of cement percentage on the observed properties of cement screeds. for these purposes, concrete and brick recycled aggregate is used in combination with cem i 42.5r portland cement, constituting 10 %, 15 %, 20 % and 25 % of the mixtures weight. to determine the properties of the cement screeds, test intervals of 3, 7, 14, 21 and 28 days are set, during which changes in bulk density are monitored. simultaneously, destructive tests are carried out on test beams of 4 × 4 × 16 cm in size. the results found that the use of recycled fine aggregates allows the production of lighter cement screeds, for example, for reconstruction. potentially usable mixes appear to be those containing 15 %, 20 % and 25 % cement compared to recycled aggregate. for the contaminated recyclate fine fraction, the flexural strength has been shown to improve. however, this improvement is not reflected in an increase in compressive strength. keywords: recycled aggregate, cement screed, concrete recyclate, brick recyclate, portland cement. 1. introduction over the last few years, people’s attitudes towards recycling have changed significantly. the same applies to using recycled materials and products that are becoming part of everyday life. unfortunately, in the construction sector, the use of it is problematic. this may be due to a lack of knowledge, insufficient standards, financial disadvantages, deteriorated properties, and people’s attitudes towards building materials made from recycled materials. these barriers need to be overcome as the amount of construction waste is increasing every year. at the same time, landfill space is diminishing, and the storage of recyclable waste should be banned in the czech republic from 2030 [1]. on the other hand, it is very problematic to meet the high demand with the existing quarries and natural aggregate deposits. thus, the pressure to find efficient uses for construction waste is increasing. concrete, brick, and mixed recyclates are the most commonly encountered and can be further divided into coarse (> 4 mm) and fine (0.063–4.0 mm) recycled aggregates (rfa). coarse aggregates are already starting to be used for recycled concrete and the number of researches looking into this application is increasing [2]. however, there is very little research on the fine fraction, even though its representation in the construction waste recycling process is significant. for this reason, this article focuses specifically on the use of the fine recycled fraction for use in cement screeds, where there is some hidden potential. these screeds currently use natural sand as a filler and are used as leveling and spreading layers for floors in hall and apartment buildings. they are applied in small thicknesses, reaching a maximum thickness of 50 mm [2]. they can be distributed as dry bagged mix, wet mix, or as liquid cement screed [2]. many manufacturers modify the properties of these screeds with various additives and admixtures to achieve faster strength increases and better workability [3]. cement screeds are defined by čsn en 13318 [4]. production is governed by en 13892-1 [5] and the most common tests are described in čsn en 13892-2 [6]. in their studies of fresh mixtures using rfa, researchers also look at the following properties: air content, consistency of the mixture, and onset and end of solidification. in the case of hardened mortars, the most commonly monitored properties are: bulk density, flexural strength, compressive strength, and water absorption [7]. for the use of cement screed, a minimum safe flexural strength of 2 mpa is defined [3], and standard čsn en 13813 [8] separates cement screeds from 1 mpa. in the case of compressive strength, the lowest compressive strength is specified by the standard from 5 mpa [8]. for the use of cement screed as a contact leveling layer with the use of an additional floor covering, the minimum compressive strength is set at 12 mpa. for use in residential construction as a floating layer, the strength is set at 20 mpa, and for heavy use up to 30 mpa [9]. existing research in the investigation of the fine recycled fraction uses the term cement screeds and mortars. the mixtures themselves in these investigations tend to be very similar and consist of aggregate, 26 https://doi.org/10.14311/app.2024.49.0026 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 49/2024 properties of cement screeds using recycled fine aggregates . . . figure 1. cumulative passing. cement, and water. for this reason, the properties of the two also agree in many respects on similar conclusions and are presented in this article. in the research carried out by katz [7] focusing on the effect of fra on mortars, it was found that fra is more absorbent and therefore affects the amount of water added and thus workability. furthermore, the quality and purity of fra also affects the final properties, as it may contain clays, glass, and other impurities [2, 7]. more research points to deteriorating tensile and compressive strengths when using rfa [2, 7, 10, 11]. further, some researchers agree on the conflicting results, with some studies saying that fine recycled aggregates affect the resulting mix positively and others negatively. this may be due, for example, to different approaches to incorporating absorbed water into the recycled aggregate [2, 7, 12], production process [2, 7], and the method of crushing [11]. 2. materials the recycled aggregates used in this research were supplied by company moravostav. these are concrete recyclate (fca) and brick recyclate (fba). fca was produced from clean concrete waste that was delivered from a separate demolition site and crushed using a resta jaw crusher. the fba came from crushing brick rubble that was obtained from separate demolition. all these recyclates were supplied with a maximum grain size of 4 mm for production. for the reference mix, natural fine aggregate (nfa) was used, which came from the dobříň gravel pit. for all these aggregates, their grain size curve was known (figure 1) and also the content of pollution by washable particles (< 0.063 mm) (figure 2). portland cement with the designation cem i 42.5 r from the radotín plant was used as a binder. 3. sample production based on the mixture knowledge for cement screeds with nfa, the production ratios for mixtures using recycled fillers are shown in table 1. this is a volume figure 2. content of fine particles. replacement of the filler and the effect of the different percentages of cement in the mix, which was set at 10 %, 15 %, 20 % and 25 % by volume of the filler. the amount of water contained in each mixture was individually adjusted to achieve a similar consistency, which was set at 170 mm ±5 mm. for this purpose, a shaking table was used which complies with the standard čsn en 1015-3. the forms used for the test beams were 40 × 40 × 160 mm in size. a total of 108 beams were produced, so 3 of each mix were allocated for each test. the test dates were set at 3, 7, and 28 days for destructive tests. the actual manufacture was carried out in accordance with the procedures described in the čsn en 13892-1 [5]. after fabrication, the test beams were stored for 7 days in containers of water at 20 °c. they were then left in the air at 21 °c and 65 % humidity to ensure that the curing conditions were suitable for use on site. 4. experimental methods 4.1. bulk density before each non-destructive and destructive test, the test beams were weighed, and their dimensions measured. from this knowledge, the wet and dry bulk 27 leoš joura, zdeněk prošek acta polytechnica ctu proceedings set cement fine natural fine concrete fine brick water [kg m−3] aggregate [kg m−3] aggregate [kg m−3] aggregate [kg m−3] [kg m−3] ref 10 210 1 890 331 ref 15 315 1 785 307 ref 20 420 1 680 284 ref 25 525 1 575 272 c 10 210 1 531 301 c 15 315 1 446 293 c 20 420 1 361 305 c 25 525 1 276 310 b 10 210 1 531 396 b 15 315 1 446 390 b 20 420 1 361 399 b 25 525 1 276 399 table 1. overview of mixtures. weights were calculated according to the scheduled test dates using the formula: d = m v , (1) where d bulk density of saturated sample [kg m−3], m mass of the fully saturated test body [kg], v volume of the test body [m3]. 4.2. flexural strength this test is based on the čsn en 196-1. laboratory hydraulic press from strassentest was used for this test. the beams were placed on 2 supports spaced 140 mm apart and loaded at a speed of 3 mm min−1 using a non-moving part located above the center of the beam until they broke. from the force required to break the beam, flexural strength was calculated according to the formula: ft = 3fb,maxls 2ab2 , (2) where ft flexural strength [mpa], fb,max maximum breaking force [n], ls distance of supports [mm], a beam width [mm], b beam height [mm]. a total of 108 beams were tested in this way, from which 216 halves were created for the next test. 4.3. compressive strength the broken beams from the previous test were further used for the compressive strength test, which was also performed according to čsn en 196-1. for this test, a test press was used, which was supplemented with an insert that defined 2 areas of 40 × 40 mm. the 216 beam halves were inserted between these sections until final failure. after deduction of the maximum achieved force, the compressive strength was calculated according to equation: fc = fc,max ac , (3) where fc compressive strength [mpa], fc,max force at the moment of breakage [n], ac loaded area [mm2]. 5. results and discussion the bulk density results (figure 3) show a gradual decrease in the bulk density of the cement mortar over time. the significant differences between the initial and final measurements are due to the decreasing amount of water in the mortar due to drying and hydration of the cement. linear increases in the bulk weights of the mixtures with increasing cement percentage can be observed. this is due to the higher bulk density of cement, which proportionally replaces the lighter filler. the decreasing bulk density of cement mortar when substituting filler is also confirmed by hubáček [2] in his research at fca and also moraortiz [12], who researched the fba. when comparing ref, c and b mixtures, we can conclude the suitability of mixtures with recycled fine aggregate, as the differences reach values up to 300 kg m−3. this is due to the higher porosity and lower bulk density of the recyclates used compared to the natural aggregate. this could be used, for example, in the reconstruction of old buildings where overloading of the structure is undesirable. from the results on figure 4, there is a linear increase in flexural strength for all mixes with increasing time and percentage of cement, which is the expected result. for short-term strengths, mixes using c and b with 10 and 15 % cement are even more resistant to failure compared to mixes with fna filler. from the 28 vol. 49/2024 properties of cement screeds using recycled fine aggregates . . . figure 3. bulk density. figure 4. flexural strength. point of view of allowing light loads, mixes with b 20, b 25, c 20, and c 25 appear to be suitable after only 3 days. looking at the strengths after 28 days, there is a more significant increase in strength for c 15, c 20, and c 25 compared to b mixes. this may be due to the higher proportion of finer grains in the delivered fba, which is evident from figure 2. similar trends in the use of fca have been achieved by katz [7] and hubáček [2], who also describe decreasing flexural strengths compared to reference mixtures. figure 5 shows the compressive strength results after 3, 7, and 28 days. the results show a linear increase in strength over time for all aggregates used. during the first 3 days, the c 15 and c 20 mixes achieve higher or equal strengths compared to the reference mixes. mixture b achieves noticeably poorer strengths during the first 7 days compared to the others. of note are mixes b 15 and b 25 at 28 days, which have comparable strengths to mixes c 15 and c 25. one possible explanation also according to hubáček [2] is the effect of a higher proportion of washable particles on the filling of the space between the individual grains in the cement mortar. the effect of 100 % replacement of aggregate with fine recyclates on the inferior strengths compared to the reference mixes is also consistent with research conducted by katz [7] and fan [11] for concrete recyclate and moraortiz [12] for brick recyclate. 6. conclusion this article investigated the effect of fine recycled aggregate (concrete and brick) on the performance of cement screeds at 10, 15, 20, and 25 % cement content in the mix with full replacement of natural fine aggregate. cement screeds with recycled fine aggregates have the great advantage of a lower bulk density compared to natural aggregates and can therefore be used in applications requiring low weight. for the flexural strength, the concrete recyclate has worse strengths after 28 days compared to brick with 29 leoš joura, zdeněk prošek acta polytechnica ctu proceedings figure 5. compressive strength. the same cement content. furthermore, mixes with recyclates and at least 20 % cement content appear to be suitable for light loads within 3 days of placement. the flexural strength itself is positively influenced by the higher fraction < 0.063 mm. the compressive strength has a negative effect when using the recycled fines fraction and the resulting strength compared to the reference mixes. mixes containing concrete and brick recyclate and at least 25 % cement and achieving approximately the same strengths, but not the strengths of the reference mixes. mixtures c 15, c 20, c 25, b 15, b 20, and b 25 meet the requirements for use as leveling and base layers for residential construction, as they reach compressive strengths higher than 20 mpa and flexural strength higher than 2 mpa. acknowledgements this paper was financially supported by czech technical university in prague under no. sgs project sgs22/089/ohk1/2t/11 and by technology agency of the czech republic under no. ss03010302. the authors also thank moravostav for the supplied samples and materials. references [1] d. vološinová, r. kořínek, j. kučera. nakládání s komunálním odpadem ve světle nových předpisů [in czech; municipal waste management in light of new regulations]. vodohospodářské technicko-ekonomické informace 63(5):47–53, 2021. https://doi.org/10.46555/vtei.2021.07.004 [2] a. hubáček, l. boehme, i. nováková. possibilities of recycled concrete aggregate in cement-based flowscreed. advanced materials research 897:247–251, 2014. https://doi.org/10.4028/www.scientific.net/amr. 897.247 [3] t. v. oliveira, l. de nazaré pinheiro cordeiro, s. a. l. bessa. experimental study of self-leveling mortars produced with recycled concrete aggregates. case studies in construction materials 17:e01294, 2022. https://doi.org/10.1016/j.cscm.2022.e01294 [4] úřad pro technickou normalizaci, metrologii a státní zkušebnictví. čsn en 13318. potěrové materiály a podlahové potěry – definice [in czech; screed materials and floor screeds – definitions], 2001. [5] úřad pro technickou normalizaci, metrologii a státní zkušebnictví. čsn en 13892-1. zkušební metody potěrových materiálů – část 1: odběr vzorků, zhotovení a ošetřování zkušebních těles [in czech; test methods for screed materials – part 1: sampling, making and curing specimens], 2003. [6] úřad pro technickou normalizaci, metrologii a státní zkušebnictví. čsn en 13892-2. zkušební metody potěrových materiálů – část 2: stanovení pevnosti v tahu za ohybu a pevnosti v tlaku [in czech; test methods for screed materials – part 2: determination of flexural and compressive strength], 2003. [7] a. katz, d. kulisch. performance of mortars containing recycled fine aggregate from construction and demolition waste. materials and structures 50(4):199, 2017. https://doi.org/10.1617/s11527-017-1067-x [8] úřad pro technickou normalizaci, metrologii a státní zkušebnictví. čsn en 13813. potěrové materiály a podlahové potěry – potěrové materiály – vlastnosti a požadavky [in czech; screed materials and floor screeds – screed materials – properties and requirements], 2003. [9] výrobkyprostavbu.cz. podlahové potěry ii: cementový potěr [in czech; floor screeds ii: cement screed], 2014. [2022-05-05]. https://www.vyrobkyprostavbu.cz/podlahovepotery-ii-cementovy-poter/ [10] h.-j. chen, t. yen, k.-h. chen. use of building rubbles as recycled aggregates. cement and concrete research 33(1):125–132, 2003. https://doi.org/10.1016/s0008-8846(02)00938-9 [11] c.-c. fan, r. huang, h. hwang, s.-j. chao. the effects of different fine recycled concrete aggregates on the properties of mortar. materials 8(5):2658–2672, 2015. https://doi.org/10.3390/ma8052658 [12] r. s. mora-ortiz, s. a. díaz, e. del angel-meraz, f. magaña-hernández. recycled fine aggregates from mortar debris and red clay brick to fabricate masonry mortars: mechanical analysis. materials 15(21):7707, 2022. https://doi.org/10.3390/ma15217707 30 https://doi.org/10.46555/vtei.2021.07.004 https://doi.org/10.4028/www.scientific.net/amr.897.247 https://doi.org/10.4028/www.scientific.net/amr.897.247 https://doi.org/10.1016/j.cscm.2022.e01294 https://doi.org/10.1617/s11527-017-1067-x https://www.vyrobkyprostavbu.cz/podlahove-potery-ii-cementovy-poter/ https://www.vyrobkyprostavbu.cz/podlahove-potery-ii-cementovy-poter/ https://doi.org/10.1016/s0008-8846(02)00938-9 https://doi.org/10.3390/ma8052658 https://doi.org/10.3390/ma15217707 acta polytechnica ctu proceedings 49:26–30, 2024 1 introduction 2 materials 3 sample production 4 experimental methods 4.1 bulk density 4.2 flexural strength 4.3 compressive strength 5 results and discussion 6 conclusion acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2024.48.0041 acta polytechnica ctu proceedings 48:41–45, 2024 © 2024 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague fatigue analysis on flax-epoxy composites: insights and implications andrea pagliaro∗, lorenzo cagnetta università degli studi niccolò cusano, roma, via don carlo gnocchi 3, italy ∗ corresponding author: andrea.pagliaro@unicusano.it abstract. this study investigates the performance of flax fiber and epoxy resin composites, focusing on fatigue testing, which is scarcely reported in the literature. fatigue behavior is evaluated under full reverse bending loading conditions: r = −1. apart of the definition of the classic s-n curve, the analysis deeply investigated the evolution of the hysteresis leaf generated during each cycle over the entire fatigue life. this approach allows evaluating interesting parameters such as the evolution of the loss factor and the apparent modulus over life, being these informations fundamental for practical application of natural-fiber composites in engineering applications. keywords: bio-based composite, flax, fatigue, reverse bending, hysteresis. 1. introduction this study investigates the fatigue behavior of composite materials reinforced with natural fibers, addressing a critical yet under-explored aspect of materials science. composite materials, recognized for their versatility and strength, are increasingly being reinforced with natural fibers to enhance their environmental sustainability and mechanical properties. existing literature presents numerous studies on tensile and cyclic tensile testing of natural-fiber composites, offering valuable insights into their initial mechanical performance and failure modes under static and dynamic loads. specifically, the research conducted by charlet et al. [1] focuses on tensile testing of flax composites with an epoxy resin matrix, revealing nonlinear behavior. additionally, the study by mahboob et al. [2] expands on tensile testing by incorporating an analysis under increasing cyclic loads until failure in both tension and compression, elucidating the damage evolution in the material. further contributions by giuliani et al. [3, 4] investigate both the material’s nonlinearity and the influence of strain rate. an additional noteworthy study by giuliani et al. [5] explores the effects of creep and stress relaxation phenomena. the findings emphasize the significance of viscoelastic (ve), viscoplastic, and pseudo-plastic components in the mechanical response of natural fiber-reinforced polymers (nfrps), in contrast to traditional composites (such as glass and carbon fiber-reinforced polymers), whose responses are typically approximated as linear-elastic. nevertheless, a limited number of studies have addressed the evolution of material behavior under fatigue, a critical consideration for long-term structural applications [6, 7]. fatigue behavior is pivotal as it determines the durability and reliability of materials subjected to repeated loading and unloading cycles over extended periods. understanding the behavior of these composites under such conditions is essential for their application in the automotive, aerospace, and civil engineering sectors, where materials are frequently exposed to fluctuating stresses. in addition to conventional analyses, such as stressstrain response and failure modes, this research examines parameters like the apparent modulus and the loss factor throughout the material’s lifespan. the apparent modulus provides insights into the stiffness degradation of the material under cyclic loading, indicating changes in the material’s ability to withstand deformation over time. the investigation of the loss factor is particularly significant due to the superior damping properties exhibited by natural fibers compared to traditional composites. damping properties are crucial for reducing vibrations and enhancing the comfort and safety of structures and vehicles. the superior damping capacity of natural fibers, like flax fibers used in this study, allows composites reinforced with these fibers to absorb and dissipate more energy, resulting in reduced noise and vibration. by exploring these parameters, this study aims to provide a comprehensive understanding of the fatigue behavior of natural fiber-reinforced composites. this understanding will facilitate the prediction of the lifespan and performance of these materials and optimize their design for specific applications where long-term durability and energy absorption are paramount. 2. materials and methods the material under investigation is a natural fiber composite. specifically, pre-preg flax fibers arranged in a fully unidirectional manner and embedded in an epoxy resin matrix were utilized. the laminate has been cured in autoclave stacking 13 layers of unidirectional (ud) pre-preg with 50 % of fiber resin content, resulting in a final cured thickness of 2.75 mm. all the process parameters are reported in table 1. conducting fatigue tests under full reverse loading conditions (r = −1) using axial tension and compres41 https://doi.org/10.14311/app.2024.48.0041 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en andrea pagliaro, lorenzo cagnetta acta polytechnica ctu proceedings temp. pres. time rate 130 °c 5 bar 1 h 2 °c · min−1 table 1. autoclave curing process parameters. sion would have posed substantial challenges due to buckling phenomena occurring during the compression phase. consequently, an alternative approach was necessary, leading to the implementation of bending tests. reverse bending fatigue tests have been conducted on specimens 100 mm long, 10 mm wide, and 2.75 mm thick. such dimensions do not correspond to any standard, as full-reverse-bending tests are not common in the literature due to the non-trivial loading condition. we have been able to perform these tests thanks to dedicated patented jigs allowing to apply pure reverse bending loads using traditional universal testing machine originally meant for axial testing [8] reported in figure 1. all the specimens have been cut out of a single plate through cnc routing utilizing a diamond carbide bit. overall, 11 specimens have been tested under stresscontrolled fatigue loading with a 2 hz sinusoidal load history for varying stress ratios, imposing as run-out value a cycle count of 2 milion cycles. the frequency was carefully chosen to be low enough to avoid self-heating of the material and to prevent undesirable effects from strain rate sensitivity, which this visco-elastoplastic material is particularly prone to. conversely, it was selected to be high enough to reduce the typically long durations associated with fatigue testing. apart the definition of the mere sn-curve, some parameters of interest have been taken into account by analysing in detail the hysteresis cycles obtained at selected load cycles. more in detail, over the entire fatigue life we recorded the hysteresis cycles over the first 200 cycles and then on a logarithmic base acquiring 10 cycles per logarithmic decade, plus the last 200 cycles prior to the final failure. the parameters of interest in this study pertain to the hysteresis loop characteristic of viscous materials subjected to cyclic loading, as depicted in figure 2. the upper image illustrates the loading and unloading phases of a single cycle, for instance, related to a load history with a stress ratio r = 0.1 defined as the ratio σmin to σmax (eq. 7). additionally, the first parameter of interest is here defined as the apparent modulus ea which, at each cycle, is calculated as the ratio ∆σ to ∆ε (eq. 8), being ∆σ the difference between the maximum (σmax) and the minimum (σmin) flexural stress, and ∆ε the difference between the maximum and the minimum strains attained on the outer layer. please note that stress and the strain have been estimated from the force-displacement data acquired by the load cell following eq. 1 and 2, respectively. figure 1. detail of the dedicated jigs utilized for the fatigue tests (top), and schematic diagram of the applied forces and the induced internal moments (bottom). σ(t) = 6f (t)l w · t2 (1) ε(t) = h(t) − h0 h0 ; (2) being f the force read by the load cell, l the distance from the load axis to the neutral axis of the specimen, w the specimen’s width, t the specimen’s thickness, h the deformed outer length and h0 the undeformed length of the neutral axis corresponding to the useful section of the sample between the gripping points. to determine the deformed outer length h(t), data from the testing machine were utilized. specifically, referring to figure 3, the axial displacement of the movable piston in the direction of the force f , defined 42 vol. 48/2024 fatigue on flax-epoxy figure 2. parameters of interest: apparent modulus (top) and energy lost in a single hysteresys cycle (bottom). as z(t), was considered. for symmetry reasons, only half of the sample was analyzed, as shown in eq. 3. by considering the displacement over time and the fixed dimension l, the angle at the specimen’s end can be easily calculated using the relation in eq. 4. subsequently, the radius of curvature with respect to the sample’s neutral axis is calculated using eq. 5, thus providing the curvature of the neutral axis over time. the evolution of the strain on the outer surface is finally computed as eq. 6. z(t) = z(t) 2 (3) α(t) = arcsin z(t) l (4) r(t) = z0 α(t) (5) ε(t) = 1 r(t) · t 2 (6) the second parameter of interest, referring to the lower image in figure 2, is the loss factor η. it is defined by macioce et al. [9] as the ratio of the energy dissipated during the cycle d to the energy stored during the loading phase u , scaled by 2π. additionally, this factor corresponds to twice the damping coefficient ξ, as reported in eq. 9. r = σmin σmax (7) figure 3. neutral position (top), and deformed position which introduce bending moment on the specimen (bottom). ea = ∆σ ∆ε (8) η = d 2πu = 2ξ (9) 3. results the initial results are derived from the standard sn curve for fatigue tests, reported in figure 4. it effectively characterizes the material’s performance up to a maximum of 2 · 106 cycles. to get some more insights apart the sn curve, emphasis is placed on the analysis of the evolution of the hysteresis cycles over the fatigue life. figure 4. experimental results (solid marks) and derived s-n curve (dashed line). two examples of the hysteresis cycles evolution over life are reported in figure 5. the upper graph reports 43 andrea pagliaro, lorenzo cagnetta acta polytechnica ctu proceedings figure 5. evolution of the hysteresis cycles over life for the specimen failed at 14500 cycles (top) and for the infinite life case (bottom). the case of a short lifespan, which associates to a high loading condition. more precisely, it corresponds to the third marker from the left in figure 4, which failed at approximately 14 500 cycles. it is evident that over cycles, the hysteresis leaf progresses towards higher strain values in both tension and compression. such behavior suggests a stiffness decrease over life which, due to the r = −1 condition, should be ascribed to progressive internal damage, as better detailed later on. the lower graph in figure 5 illustrates the evolution of the hysteresis leaf for the longest-lived case examined in this study. this corresponds to the case achieving infinite life (last large marker on the right in figure 4). it is evident that there has been a noticeably limited evolution of strain, albeit showing a slight progression following a trend similar to the previous case. the continuous evolution of the hysteresis cycles over life is reflected in the concurrent evolution of the pre-defined key parameters. to analyze the evolution of the parameters with increasing fatigue cycles, five points were selected from the s-n curve, which are highlighted in figure 4 with larger markers. to each one of these five points, the evolution of the hysteresis cycles have been analyzed over the entire fatigue life, allowing to derive the evolution of the parameters of interest: the apparent modulus ea and the loss factor η, whose evolution are are presented in figure 6. significantly, the trend is notably influenced by the load intensity under full reverse bending conditions. specifically, there is an approximate 10 gpa variation figure 6. evolution of the apparent elastic modulus (top) and damping ratio (bottom) over life for selected specimens at varying fatigue lives. in apparent modulus between the shortest lifespan case, depicted by the lowest curve on the graph, and the infinite lifespan case, represented by the uppermost curve. the highest apparent modulus value, initialing attaining 31 gpa and consistently exceeding 27 gpa over the entire life, is observed under infinite lifespan conditions, while the lower apparent modulus, initially attaining 23 gpa and rapidly dropping below 15 gpa, is attained by the highly-loaded specimen. for all the loading conditions the behavior of the apparent modulus shows a continuous decline until reaching the latter part of its life, where it undergoes a sudden drop just before failure. the large influence of the loading condition on the initial value of the apparent modulus should be ascribed to the highly nonlinear behavior of natural-fiber composites, which shows an almost bi-linear stressstrain curve [5], hence the modulus at the lower loads is much higher than the one at the larger loads. conversely, there is an inverse trend observed with respect to the loss factor. in this instance, all scenarios demonstrate an increase throughout their lifespan, contrasting with the apparent modulus, until showing a sharp increase just before failure. the variation in the loss factor is approximately 1 % across different lifespans. the most favorable outcome in terms of the loss factor occurs for the shortest lifespan, consistently exceeding 4 %. a highly significant result arises from the damping factor values. in all cases, values exceeding 3 % are observed throughout the fatigue life, indicating the persistence or increase of this property with cycle accumulation. 44 vol. 48/2024 fatigue on flax-epoxy 4. conclusions this study conducted a comprehensive analysis of the hysteresis loop to define two critical parameters that facilitate the evaluation of a material’s performance throughout its fatigue life: the apparent modulus (ea) and the damping capacity, closely related to the loss factor (η). the apparent modulus (ea) is crucial for understanding the evolution of a material’s stiffness as it endures over the lifespan. this parameter highlights the relationship between load intensity and evolution of the stiffness over life. as the number of cycles increases, changes in the material’s stiffness are quantified through the apparent modulus. this measure is essential for predicting the material’s performance and lifespan under cyclic loading conditions, providing insights into how stiffness degrades or evolves over time. the second parameter: the damping capacity, is intrinsically linked to the loss factor (η). this parameter evaluates the material’s ability to dissipate energy, a critical aspect of performance under cyclic loads. the study shows that η values never fall below 3 %, indicating consistently good damping capacity. this finding is significant as it demonstrates the material’s reliability in energy dissipation even under extensive cyclic loading. maintaining a minimum loss factor of 3 % suggests robust damping capacity, making the material suitable for applications requiring effective energy dissipation. in conclusion, this study offers significant insights into the fatigue behavior of materials through detailed analysis of the apparent modulus and loss factor. by addressing the evolution of stiffness and damping capacity during fatigue life, the research fills a notable gap in the existing literature. these findings are particularly valuable for the design and optimization of components or structures made of natural fiber composites. understanding the behavior of these materials under cyclic loading conditions can lead to improved performance and longer lifespans for final products. this research advances knowledge in the field and provides practical guidelines for engineers and designers working with natural fiber composites. the comprehensive evaluation of the hysteresis loop and defined parameters serves as a foundation for future studies aiming to enhance material performance in fatigue applications. references [1] k. charlet, s. eve, j. p. jernot, et al. tensile deformation of a flax fiber. procedia engineering 1(1):233–236, 2009. https://doi.org/10.1016/j.proeng.2009.06.055 [2] z. mahboob, i. el sawi, r. zdero, et al. tensile and compressive damaged response in flax fibre reinforced epoxy composites. composites part a: applied science and manufacturing 92:118–133, 2017. https: //doi.org/10.1016/j.compositesa.2016.11.007 [3] p. m. giuliani, o. giannini, r. panciroli. viscoelastic experimental characterization of flax/epoxy composites. procedia structural integrity 12:296–303, 2018. https://doi.org/10.1016/j.prostr.2018.11.086 [4] p. m. giuliani, o. giannini, r. panciroli. characterizing flax fiber reinforced bio-composites under monotonic and cyclic tensile loading. composite structures 280:114803, 2022. https: //doi.org/10.1016/j.compstruct.2021.114803 [5] p. m. giuliani, o. giannini, r. panciroli. creep and stress relaxation of unidirectional flax fiber reinforced laminates. composite structures 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https://doi.org/10.1016/j.compositesa.2016.11.007 https://doi.org/10.1016/j.prostr.2018.11.086 https://doi.org/10.1016/j.compstruct.2021.114803 https://doi.org/10.1016/j.compstruct.2021.114803 https://doi.org/10.1016/j.compstruct.2023.116755 https://doi.org/10.1016/j.compstruct.2023.116755 https://doi.org/10.1016/j.compositesa.2018.03.034 https://doi.org/10.1016/j.compositesa.2018.03.034 https://doi.org/10.1016/j.compositesa.2015.11.003 https://doi.org/10.1016/j.compositesa.2015.11.003 https://doi.org/10.1016/j.cirpj.2022.07.007 acta polytechnica ctu proceedings 48:41–45, 2024 1 introduction 2 materials and methods 3 results 4 conclusions references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2023.41.0058 acta polytechnica ctu proceedings 41:58–65, 2023 © 2023 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague the influence of modern headlamps on the recognition of portable traffic signs and warning triangles martin rak∗, tereza tmejová, tomáš bilík, david jelínek, albert bradáč brno university of technology, institute of forensic engineering, purkyňova 464/118, 612 00 brno – královo pole, czech republic ∗ corresponding author: martin.rak@vut.cz abstract. this article focuses on the issue of portable traffic signs and their visibility using different light sources such as halogen, xenon, or led. the measured values of luminance and retroreflection for each object are used to distinguish the individual results. the results show the differences between the various light sources on the visibility of the measured objects. furthermore, from these results it is possible to evaluate portable traffic signs regarding degradation due to mechanical wear or aging. keywords: retroreflection, portable traffic signs, light sources, luminance, mechanical wear. 1. introduction this paper expands on the authors’ previous publication presented at the jufos conference and uses parts of the text from it [1]. portable traffic signs have a significant impact on road safety in non-standard situations such as road repairs, detours, or traffic accidents. according to accident statistics, approximately 350 accidents in the czech republic are related to traffic signs every year. for this study, the term ‘portable traffic signs’ includes traffic devices and warning triangles since these are used in similar non-standard situations. one of the motives of this study was the existence of several variations of light sources in modern vehicles such as halogen, xenon, led, or laser diode, which are mentioned by [2–4]. the goal was to determine the visibility of portable traffic signs regarding different light sources from different distances, considering factors such as proper placement of the sign or its degradation due to mechanical wear or aging. to achieve this, retroreflection and luminance measurements were carried out on selected samples of portable traffic signs, particularly 20 pairs of portable traffic signs and 5 different types of warning triangles. this project did not address the effects of adaptive technologies of modern headlamps, such as afl or adb, whose effects on visibility have been addressed by [5]. based on the results and experience with the first static measurement, a second static measurement was carried out. the second static measurement compared halogen reflector headlamps and halogen projector headlamps. to reduce measurement inaccuracies, only new traffic signs and devices that are expected to meet the legislative requirements were used and new bulbs were fitted to both types of headlamps. the measurement procedures and evaluation methodologies were identical to the first static measurement. 1.1. current state there is a methodology for the placement of traffic signs in the czech republic set by technical standards [6, 7]. based on these standards, we recognize the following portable traffic signs and traffic devices. portable traffic sign is vertical traffic sign placed on a red and white striped column and active surface of the sign must be retroreflective according to en 12899-1. traffic devices are mainly devices for traffic management. these include traffic cones, direction signs, or guide signs. retroreflective materials should be used for sign faces according to en 12899-1. the white stripes of traffic cones should be retroreflective according to en 13422 [8, 9]. retroreflection is a feature that allows the light cast by a vehicle’s headlights to be reflected back to the driver. this feature allows the driver to see the traffic sign in time and react to it during the day and in the dark [10]. there are three classes of retro-reflectivity – ra1, ra2, and ra3. the higher the class of the sheeting, the better the visibility of the sign or device at night. the methodology of measuring retroreflection, according to the mentioned standards, consists only the laboratory measurement and does not consider the actual visibility by the driver in the real world. 2. method 2.1. methodology for measuring retroreflection retroreflection values were measured on the entire set of portable traffic signs, traffic devices and warning triangles. for this purpose, signs and devices that were commonly used on roads were selected. the aim of the measurement was to verify whether the selected sings and devices meet the requirements of en 12899-1 and en 13422 in the case of traffic cones and ece 27-04 58 https://doi.org/10.14311/app.2023.41.0058 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 41/2023 the influence of modern headlamps figure 1. retroreflection measuring using zehntner zrs 6060 en [1]. in case of the warning triangles [8, 9, 11]. a zehntner zrs 6060.en retroreflectometer was used for the measurements (see figure 1 for example of measuring process). this is a device designed to measure the retroreflection coefficient ra of vertical traffic signs or other similar materials. this device is intended for measuring the retroreflection coefficient in the field. before the actual measurement, it was necessary to clean each sign and to let the measuring device stabilize outside of the transport package for at least 5 minutes before the actual measurement. the measurement of vertical traffic signs (including traffic devices) is carried out at three randomly selected measuring points of the same-colored area. the device itself then averages these values and displays the resulting values of the retroreflection coefficient. these values can be compared with the values specified in the relevant standards en 12899-1 and en 13422. 2.2. methodology for measuring luminance luminance was measured using the lumidisp. this device is luminance distribution analyzer that uses the analysis of luminance ratios with the help of digital photography to photometrically measure luminance and its distribution. the output of this device are luminance maps from which it is possible to determine the luminance value of individual traffic signs at any point in the captured digital image. both portable traffic sign measurements were taken at a location where there was minimal light pollution. the first static measurement took place when the moon was new, and it was clear to partly cloudy. three types of headlights were used for this measurement – halogen reflector, bixenon and led, all on škoda vehicles. the second static measurement took place when the moon was nearly full, and the sky was clear. for this measurement, škoda vehicles equipped with halogen reflector headlamps and halogen projector headlamps were used. the device was placed in front of the vehicle to avoid any disturbance of the windscreens of figure 2. placement of measuring device lumidisp in front of the vehicle [1]. each vehicle, as shown in figure 2. for each type of headlamp, measurements were collected for low beam and high beam settings. all the measurements were at an airstrip with a tarmac surface and a minimum gradient. the measured objects were each positioned 0.5m from the right side of the road and the centerline of the vehicle with the respective headlamps was 1.75m from the right side of the road. the first distance measured was 80m, which is approximately the distance needed for the vehicle to stop from a speed of 9 kmh−1 including the driver’s reaction time of 1.0 s and applying the minimum required braking deceleration of 5.8m s−2 for passenger vehicles. the second distance was 350m, which was the maximum possible distance that could be achieved on the site. averages of the luminance values of individual pixels for the marker area were used to evaluate and quantify the marker luminance. therefore, comparisons can also be made between various traffic signs in different sizes. for the evaluation of the brightness of the triangles, a similar method as for the traffic signs was used. in the case of triangles, only the values of the physical parts of the triangles without background were used. the average luminance of each triangle included the internal non-reflective parts if the triangle had them. 2.3. test measurements to determine the influence of the environment, tilt, or change of position of the measured sign, a test measurement was performed. the test measurement showed that the error of the measured values due to the influence of weather conditions was below 10%. a change of position (up to 10 cm) and a rotation of the sign (up to 5°) did not have any significant effect and were insignificant compared to the influence of the environment. 2.4. headlamp adjustment and intensity measurement prior to the second static measurement, both types of halogen headlamps were adjusted, including replace59 m. rak, t. tmejová, t. bilík et al. acta polytechnica ctu proceedings ment of halogen bulbs and adjustment. h4 and h7 halogen bulbs were used in the measurements for halogen reflector and projector headlamps respectively. the headlamps were then adjusted, and the illuminance measured. these operations were carried out using a tecnolux 12799/lx2/p regloscope. this is a diagnostic device designed to check and adjust all types of headlamps. ’it is an optical-mechanical device working on the principle of direct projection of the image of the light emitted by the headlamp and enabling the inspection and adjustment of headlamps of motor vehicles whose height above the ground is at least 200–1300mm [12]. 2.5. traffic signs selection for the retroreflection measurements, both new signs and pairs of signs that were already in use were measured. the measured values were compared with the respective norms to determine the state of degradation of the signs. the measurements indicated that the retroreflection values can be used to determine to a certain extent the wear of the signs that cannot be determined otherwise (e.g., the absence of a manufacturing label or details of eventual refurbishment). in the case of the second static measurement, only new traffic signs and devices were selected. 3. results 3.1. the first static measurement for the first static measurement, similar škoda vehicles equipped with different types of headlights were chosen. specifically, the vehicles were a škoda octavia iii (2018) with halogen reflector headlamps, a škoda superb iii (2015) with bixenon projector headlamps and a škoda superb iii fl. (2022) with full led matrix headlamps. 3.1.1. distance 80 m low-beam mode measurements with the lumidisp device at 80m in low beam mode resulted in the highest values for halogen headlights for all measured objects. for the remaining types of headlamps, the following patterns were apparent (see figure 3): • objects that were mostly below 150 cm height from the road surface displayed higher luminance values for full led headlamps in comparison to bixenon headlamps, • the remaining objects, which had the bottom edge at a height of 150 cm, displayed higher values for bixenon headlamps in comparison to full led headlamps. high-beam mode at 80m in high beam mode, the highest luminance values were achieved when using the full led headlamps. in all cases, the halogen headlamps performed better than the bixenon headlamps. curiously, for the warning triangles, halogen headlamps came out better in high beam mode, although full led headlamps were better for the cones and other signs. the full led headlamps performed the best with the signs with a border that used the ra3 sheeting, as expected. for the halogen and bixenon headlights, these signs showed similar values to the white signs using ra2 sheeting. figure 4 provides a representation of the average luminance values measured for each portable traffic sign. differences due to degradation can be seen for each pair. figure 5 shows a series of lumidisp outputs in the form of luminance maps demonstrating the differences between the headlamps for the same traffic sign. 3.1.2. distance 350 m low-beam mode for the measurements at 350m, five pairs of traffic signs were selected, which were similar in size and at the same time featured identical colors on different backgrounds, and one warning triangle. again, in the low beam setting, the halogen headlamps gave the best values for all measured objects, while the bixenon headlamps performed the worst. the results for the warning triangle were in the same manner. high-beam mode for measurements at 350m in high beam setting, the full led headlamps performed best, and the bixenon headlamps again performed worst. as was the case at 80m, the halogen headlamps performed best in the high beam setting. figure 6 shows comparison of different types of headlamps and beam modes. 3.2. the second static measurement for the second static measurement, two similar škoda vehicles equipped with halogen headlamps were chosen. specifically, the vehicles were a škoda fabia ii (2004) with halogen reflector headlamps and a škoda roomster (2007) with halogen projector headlamps. only new traffic signs were used for this measurement and new halogen bulbs were fitted in the vehicles to avoid any possible interference with the measurement. when measuring the illuminance, even when new bulbs were used, the projector headlamps of the škoda roomster were found to have a low intensity value, where the right headlamp did not meet the minimum intensity value required for vehicle operation. in order to be able to compare the results, this fact was considered by recalculating the measured luminance ratios using the measured intensity values of the individual headlamps. for the second static measurement at 80m, the position of the sign above the road level was changed. according to tp 65 it is recommended to place the lower edge at a height of at least 60 cm above the road level [6]. in the case of this static measurement, the bottom edge of the sign was placed at a height of 105 cm and 150 cm on the post. the measurements also used traffic devices 60 vol. 41/2023 the influence of modern headlamps figure 3. luminance values (cdm−2) at 80m using a low beam headlamp setting [1]. figure 4. luminance values (cdm−2) at 80m using a high beam headlamp setting [1]. figure 5. comparison of traffic signs in terms of luminance, from left in order halogen, full led, bixenon [1]. 61 m. rak, t. tmejová, t. bilík et al. acta polytechnica ctu proceedings figure 6. comparison of luminance values at 350m using a headlamp setting (low-beam on the left, high-beam on the right) [1]. figure 7. comparison of luminance values at 80m using a low-beam headlamp setting (height 150 cm on the left, height 105 cm on the right). whose design does not allow them to be placed at different heights. for this reason, they were placed at only one height as specified by the design. 3.2.1. distance 80 m low-beam mode measurements with the lumidisp at 80m in lowbeam mode showed the highest values for the halogen reflector headlamps for all measured objects. after correcting the results in relation to the intensity of the headlamps used (“projector +” in figure 7), the halogen headlamps with projectors showed better results in all cases. in the case of a lowering the height of the traffic sign from 150 cm to 105 cm, there was a slight increase in the brightness values for both the halogen reflector headlamps and halogen projector headlamps. as expected, for both headlamps, all the ra1 class signs came out worst. the signs with the larger white and red areas showed the best average luminance values. high-beam mode measurements at the 80m distance in high-beam mode were also made for two different traffic sign heights. the results show that the halogen projector headlamp performed better for the signs placed with the lower edge at a height of 150 cm, even before recalculation to compare the light intensities. this is due to the alignment of the headlamps, where the main beam halogen projector was aligned higher than the main beam halogen reflector. both headlamps were adjusted to the correct tolerance. for markers at 105 cm lower edge height, the halogen headlamp 62 vol. 41/2023 the influence of modern headlamps figure 8. comparison of luminance values at 350m using a different headlamp setting (low-beam on the left, high-beam on the right). reflector came out better, and after conversion to the same luminous intensities, the halogen projector came out better, as it did for the low-beam mode headlamps. 3.2.2. distance 350 m low-beam mode the 350m measurement was conducted with the same traffic signs as the 80m measurement. the lumidisp measurements at 350m in low-beam mode showed the highest values for the halogen reflector headlamp for all measured objects. again, the results were corrected for the intensity of the headlamps used (“projector +” in figure 8). again, in all cases the halogen projector headlamps showed better results. also, at 350m, all traffic signs of class ra1 came out worst for both types of headlamps. the markers with the larger white and red areas showed the best average luminance values. high-beam mode from the luminance measurements at 350m in high-beam mode, the halogen reflector headlamps again performed better. compared to the halogen projector headlamps, the differences were not significant as with the low-beam headlamps. after correcting the results in relation to the intensity of the headlamps used, the halogen headlamps with projectors performed better in all cases. high-beam mode for measurements at 350m in high beam setting, the full led headlamps performed best, and the bixenon headlamps again performed worst. as was the case at 80m, the halogen headlamps performed best in the high beam setting. figure 6 shows comparison of different types of headlamps and beam modes. 3.3. warning triangles within the individual static measurements, interesting observations were made concerning the warning triangles. after comparing the first and second measurements, it was found that there were some similarities between the measurements. of the two measurements, the best results were obtained for the third warning triangle measured and the worst results for the fourth warning triangle. it can therefore be concluded that, in the case of warning triangles, it is not only the type of headlamps used to illuminate the warning triangles that matters but also, and above all, the construction and material properties of the particular triangles. furthermore, it has been found that the reflective properties of individual warning triangles vary quite considerably, and these facts may have an impact on their possible use in real traffic (see figure 9). regarding the possibility of influencing the average luminance values of individual pixels, as mentioned in section 1, also shown in figure 10, which shows a series of lumidisp outputs in the form of luminance maps demonstrating the different warning tringles for the halogen headlight with reflector. 4. discussion the minimum luminance for recognizability of a traffic sign at night out in the rural area ranges from 35 to 340 cdm−2 according to [13]. according to aashto in the range of 20 to 180 cdm−2 [14]. according to elstad et al. [15] in the range of 35 to 70 cdm−2. according to bullough et al. [16] ranging up to 280 cdm−2. according to fletcher et al. [17] in the range of 20 cdm−2. according to freyssinier et al. [18] in the range of 40 cdm−2. all types of headlamps in a high beam setting met these conditions at 80m. in a low beam setting, signs with ra1 63 m. rak, t. tmejová, t. bilík et al. acta polytechnica ctu proceedings figure 9. comparison of luminance values for warning tringles at 80m using a low-beam headlamp setting (the first static measurement on the left, the second static measurement on the right). figure 10. comparison of luminance values for different warning triangles at 80m for halogen headlight with reflector using a low-beam headlamp setting. background, predominantly amber or blue color, did not meet these values. 5. conclusions in this study, average luminance values were measured for selected portable traffic signs as a whole or their sections (e.g., individual colors). these values were determined for the purpose of the first static measurement for different types of headlamps (halogen, full led, bixenon) in low beam and high beam modes at distances of 80 and 350m from the object to be measured. the retroreflection values were compared with the values set by the standards and with the values measured by the lumidisp device. by comparing the measured values from the two devices with each other, it was observed that in 2/3 of the cases the values showed similar patterns, which could be used to determine the loss of retroreflection for degraded signs. the second static measurement was designed to further understand the design characteristics of the halogen headlamps, and therefore measurements were made on the new portable traffic signs and using only new halogen bulbs, with the intention of eliminating measurement deviations. it was found that although halogen reflector headlamps showed higher measured luminance values, after correcting the results in relation to the intensity of the headlamps used, the halogen projector headlamps would perform better in all cases. in the light of the findings, the interpretation of the results of the first static measurement can be revised so that projector headlamps should show better brightness values than reflector headlamps. thus, it can be concluded that the bixenon headlamps used in the first static measurement must have had an overall lower illuminance which was not, however, related to the design characteristics. the aim of the measurements was to determine how the type of headlamp and the different design and material properties affect the visibility of the warning triangles. for the measurements, warning triangles already in use, as well as new triangles purchased for the purpose of the measurements, were used. after evaluating the results from both measurements, it was found that the visibility of the triangles is significantly influenced by their construction and material. in addition, the use of different types of headlamps also affects the results. 64 vol. 41/2023 the influence of modern headlamps all warning triangles met the minimum brightness limit for visibility set by [14]. further measurements on a larger scale would be necessary to achieve more accurate results. these measurements served as a basis for the implementation of measurements in regular traffic, where drivers’ reactions to portable road signs will be verified using eye-tracking. 6. acknowledgements the authors would like to express appreciation for the support of: transport research centre (cdv) (https://www.cdv.cz/), lumidisp (https: //lumidisp.eu/), ctu in prague faculty of transportation sciences department of forensic experts in transportation (https://www.fd.cvut.cz/), and michal křižák, brno university of technology (https: //www.vut.cz/). this article was produced with the use of measurement equipment with the financial support of the ministry of transport within the program of long-term conceptual development of research organizations. this study was part of the project quality internal grants of but (king but); reg. number: cz.02.2.69/0.0/0.0/19_073/0016948. references [1] m. rak, t. tmejová, t. bilík, d. jelínek. vliv moderních světlometů na viditelnost přenosného dopravní značení a výstražných trojúhelníků. in sborník příspěvků konference junior forensic science brno 2022, pp. 47–51. 2022. [2] m. h. rehan. analysis of bmw and audi headlights’ technology: international standards and road-safety in pakistan, 2020. https://doi.org/10.13140/rg.2.2.28133.63208 [3] k. sokanský. světelná technika. české vysoké učení technické v praze, 2011. [4] f. vlk. elektrická zařízení motorových vozidel. f. vlk, 2005. [5] j. d. bullough, n. p. skinner, t. t. plummer, et al. adaptive driving beam headlights: visibility, glare and measurement considerations. tech. rep., rensselaer polytechnic institute. lighting research center, 2016. [6] tp 65. zásady pro dopravní značení na pozemních komunikacích, 2013. [7] tp 66. zásady pro označování pracovních míst na pozemních komunikacích, 2015. [8] čsn en 12899-3. stálé svislé dopravní značení – část 3: směrové sloupky a odrazky, 2008. [9] čsn en 13422. svislé dopravní značení – přenosná deformovatelná varovná zařízení – kužely a válce, 2021. [10] ppk-fol. identifikace a možnosti použití retroreflexní folie pro svislé dopravní značky, dopravní zařízení a signalizační vozíky na dálnicích a silnicích ve správě ředitelství silnic a dálnic, 2015. [11] ece 27-04. uniform provisions concerning the approval of advance-warning triangles, 2014. [12] příloha č. 7 k vyhlášce č. 2011/2018 sb. vyhláška o technických prohlídkách vozidel. základní charakteristiky přístrojů používaných k technickým prohlídkám vozidel, 2018. [13] t. m. allen, f. dyer, g. smith, m. janson. luminance requirements for illuminated signs. highway research record 179:16–37, 1967. [14] a. a. of state highway, t. officials. roadway lighting design guide. american association of state highway and transportation officials, 2005. [15] j. elstad, j. fitzpatrick, h. woltman, et al. requisite luminance characteristics for reflective signs. highway research board bulletin 336:51–60, 1962. [16] j. d. bullough, n. p. skinner. luminance criteria and measurement considerations for light-emitting diode billboards. tech. rep., 2011. [17] k. fletcher, s. sutherland, k. nugent. identification of text and symbols on a liquid crystal display part ii: contrast and luminance settings to optimise legibility. tech. rep., 2009. [2022-09-25], https://apps.dtic.mil/sti/pdfs/ada499459.pdf. [18] j. p. freyssinier, n. narendran, j. d. bullough. luminance requirements for lighted signage. in sixth international conference on solid state lighting, vol. 6337, pp. 357–364. spie, 2006. https://doi.org/10.1117/12.681422 65 https://www.cdv.cz/ https://lumidisp.eu/ https://lumidisp.eu/ https://www.fd.cvut.cz/ https://www.vut.cz/ https://www.vut.cz/ https://doi.org/10.13140/rg.2.2.28133.63208 https://apps.dtic.mil/sti/pdfs/ada499459.pdf https://doi.org/10.1117/12.681422 acta polytechnica ctu proceedings 41:58–65, 2023 1 introduction 1.1 current state 2 method 2.1 methodology for measuring retroreflection 2.2 methodology for measuring luminance 2.3 test measurements 2.4 headlamp adjustment and intensity measurement 2.5 traffic signs selection 3 results 3.1 the first static measurement 3.1.1 distance 80m 3.1.2 distance 350m 3.2 the second static measurement 3.2.1 distance 80m 3.2.2 distance 350m 3.3 warning triangles 4 discussion 5 conclusions 6 acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2024.46.0047 acta polytechnica ctu proceedings 46:47–52, 2024 © 2024 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague general quality of urban greenery as a city indicator in brazil in comparison with central europe jobson larrubia de almeida júniora,∗, michael pondělíčekb a instituto militar de engenharia (ime), chemistry department of postgraduate, praça gen. tibúrcio, 80 – urca, rio de janeiro, brazil b czech technical university in prague, masaryk institute of advanced studies, kolejní 2637/2a, 160 00 prague 6 – dejvice, czech republic ∗ corresponding author: eng.ambiental.jobsonlarrubia@gmail.com abstract. the evidence for climate change and its associated consequences, such as severe droughts, floods, and storms, exposed the need to investigate the ways that countries can cope with them, especially brazil, which is a continental nation. moreover, the covid-19 pandemic contributed to the urgency to consider global uncertainties. to audit it, the pondělíček method, an indicator of the general quality of urban greenery (iokzm), was employed. this approach considers four factors to indicate if one city is sustainable: city altitude [m], average annual temperature [°c], average annual precipitation [mm], and the total greenery area [km2] per inhabitant per year. the outcomes demonstrate that, regardless independently of the brazilian region, geographic localization, estimated altitude range, or precipitation range may not strongly influence the biokzm. the impact measured in the indicator is significantly influenced by the greenery areas in brazil. those estimated areas are much smaller than european union ones. teresina’s fkes demonstrates that if the greenery area is more than 5 (i.e., more than 50 percent), the iokzm will be a positive outcome. the iokzm calculated for brazilian cities is interesting precisely because of their location in the southern hemisphere, as all the evidence of climate change subtly suggests that the climate in the southern hemisphere changes in a different way. hence, this article can be used as an initial tool to assess whether the chosen brazilian cities (belém, são luís, guarulhos, campinas, porto alegre, teresina, and joão pessoa) can be considered “smart cities”. it is a step towards guiding mayors and other public figures in charge to create and maintain smart cities in brazil. in addition, further studies should be conducted to contribute to this collaborative effort. keywords: green cities, climate change, global warming, brazil, smart cities. 1. introduction it is projected that global warming will exceed 1.5 °c during the 21st century, making it harder to limit warming below 2 °c. by 2100, global warming will have led to a temperature increase of 2.8 °c [1]. this global event will generate climate impacts which can threaten the culture, infrastructure, and economy of many countries. according to the united nations office for disaster risk reduction (unisdr) [2], seismic events and deep scarcities are continually projected. however, exponential rises in storms and floods have been registered since 1970. the floods and storms are direct climate consequences of this global variation, as opposed to seismic events and deep droughts. unisdr’s estimation shows that extreme events such as heatwaves, heavy precipitation, droughts, and tropical cyclones were provoked by human influence. according to the intergovernmental panel on climate change (ipcc) [1], human influence has been the main cause of the rise of global sea levels and atmospheric temperatures since at least 1971. this issue is of even more significance due to the global population growing rapidly. it is projected that the population will increase by more than one billion people within the next 15 years, reaching 8.5 billion in 2030, and to rise further to 9.7 billion by 2050 and 11.2 billion by 2100. roughly 10 % of the global population lives in europe (738 million) and 9 % in latin america (634 million). thus, rapid urbanization, climate change, inadequate maintenance of water infrastructures, and poor solid waste management may lead cities to flooding, water scarcity, water pollution, and adverse health effects. the rehabilitation costs may overwhelm the resilience of cities, and the expenditure of inaction is high. these megatrends highlight the urgent challenges facing cities [3]. the localization of the countries around the globe are determinants for the dangerously escalating climate impacts, such as precipitation. thus, the ocean level can rise due to its proportionality, which can impact urban systems and the resilience strategies applied in the cities [2]. approximately 3.3–3.6 billion people live in areas that are highly vulnerable to climate change. and the increasing weather and extreme climate events have exposed millions of people to acute food insecurity, reduced water security, and other impacts [1]. furthermore, the covid-19 pan47 https://doi.org/10.14311/app.2024.46.0047 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en jobson larrubia de almeida júnior, michael pondělíček acta polytechnica ctu proceedings demic has brought into focus the range of challenges faced in responding to a wide range of uncertainties, while also highlighting the need to prioritize local needs in order to facilitate a just and inclusive transition. the pandemic has hit cities around the world and has exposed many vulnerabilities that need to be addressed in order to ensure better resilience to similar future adverse events [4]. the global impacts of both covid-19 and climate change have caused substantial economic damages, particularly when the two are combined. climate change has been detected in climate-exposed and affected sectors such as agriculture, forestry, fishing, energy, and tourism. the rising extreme heat events have driven hundreds of local losses of species. some tropical, coastal, polar, and mountain ecosystems have reached hard adaptation limits. nevertheless, approaches such as urban greening, wetland restoration, and upstream forest conservation have been effective in reducing flood risk and urban heat [1]. the implementation of eco-technologies can upgrade the discussions of management tools such as indicators, standards, budgets, and rating systems, which incentivizes cities to make an effort to achieve their green objectives [5]. that growth of the global population contributes to the importance of smart cities, which involve natural elements. as highlighted by jóźwik and jóźwik [6], sustainable development largely encompasses ecological, physical, and visual or aesthetic issues. the united nations sustainable development goals, particularly goal 11, emphasize the need to take urgent actions in cities to address multiple social, economic, and environmental concerns [4]. according to iso/fdis 37122:2019 [7], smart cities provide social, economic, and environmental sustainability outcomes and respond to challenges such as climate change and rapid population growth. this type of city uses data information and modern technologies to deliver better services and quality of life to the residents, businesses, and visitors of the city without disadvantages for others and natural environmental degradation. smart city is a term that encompasses the following topics: sustainability, technology, urban mobility, transport, planning, digitalization, innovation, resilience, climate change, urbanization, artificial intelligence, big data analytics, public health, remote sensing, social media, information technology, deep learning, internet of things (iot), energy demand, cloud computing, efficiency, renewable energy, energy management, optimization, blockchain, wireless sensor networks, security, and privacy [4]. data sources from different cities can vary depending on them [7]. nowadays, all of these matters remain to integrate public and private services using technological innovation, which typically involves information and communication technologies (ict) in planning, governance, transportation, water treatment, electricity distribution, and public oversight areas. they can draw people with different cultures and qualifications to exchange their abilities and expertise. besides, they can lead the world to a new era of sustainability by reducing the high levels of carbon in the atmosphere, potentializing the mobility and energy sectors, and adapting to climate change [5]. in this context, the global smart cities market is expected to exceed $2 trillion (usd) by 2025 [8]. smart cities are treated as countries’ progress strategies [5], because their structures prevent attitudes to avoid the interruption, incapacitation, or destruction of networks and infrastructure, which can result in serious social problems, waste treatment issues, and economic consequences. they can be applied as a strategy in stage two of the diagnostic and evaluation of cities’ risk, especially approach number 6, which involves analyzing the environment and engaging with local stakeholders [2]. as such, this article proposes to analyze seven brazilian cities using the pondělíček method as a step to offer assistance to mayors and public in creating and maintaining smart cities in brazil. 2. research methods from the 18th of february until april, a literature review was conducted and encountered in google scholar, springerlink, and sciencedirect databases based on this article. likewise, the pondělíček method for calculating the indicator of the general quality of urban greenery (iokzm) the method will be applied in brazilian cities. the iokzm indicator is important and based on an understanding of environmental gradients (biotic and abiotic nature, even their intra and inter actions) for the predominant types of urban greenery, and these are also considered to be predominantly euryvalent species and dated to urban conditions (ecologically less demanding and tolerant) and not stenovalent (ecologically intolerant). therefore, quantities describing the basic factors for generally conceived greenery in cities are part of the construction of the indicator. briefly, the iokzm indicator, according to equation (1), is a constructed indicator that is then a real composite indicator of the level of general quality of greenery in cities in the czech republic and has across-the-board information based on geographic and meteorological data. the indicator mathematically creates an area between four axes, according to figure 1, on which logarithmized values of individual components of the indicator are plotted. the total and target value of the indicator is therefore given by the area of the resulting quadrilateral, formed inside by four right triangles. iokzm = 1 2[ (log fkes · log fprt) + (log fprt · log fprs) + (log fprs · log fnmv) + (log fnmv · log fkes)], (1) 48 vol. 46/2024 general quality of urban greenery as a city indicator . . . figure 1. construction of the okzm indicator. figure 2. localization of selected cities in brazil. where fnmv is the city’s altitude data [m], fprt is the average annual temperature in the city [°c], and fprs is the average annual precipitation in the city [mm]. to apply this indicator in brazilian cities (biokzm), seven cities were chosen strategically to analyze the iokzm influence in different brazilian regions (north, northeast, south, and southeast), as seen in figure 2 and table 1. these have around one million inhabitants, according to the 2022 brazilian geography and statistic institute (ibge) [9] census (table 1), and they were elected because they can represent the variety of influence of the factors according to their regions. moreover, the factor fkes corresponds to the percentage of the total of greenery areas calculated in square km per inhabitant annually. brazilian greenery area data was registered in ibge’s database in square meters (fkes). those were converted into square km and transformed in percentage to be applied in pondělíček’s calculus. in addition, the ibge computes the fkes in different years, according to table 2. all data were taken from the climate data [10], sustainable cities institute [11], https://topographic-map.com [12], and ibge [9, 13] databases, according to table 3, and the iokzm variables were calculated in consonance with them, according to table 3. city brazilian region inhabitants 1 north 1 367 336 2 northeast 1 061 374 3 northeast 868 523 4 northeast 889 618 5 southeast 1 383 272 6 southeast 1 170 247 7 south 1 404 269 table 1. brazilian cities data according to the 2022 census from the brazilian geography and statistic institute (ibge) database. city fkes census year 1 23.1761 2008 2 1 014 837 2015 3 184 359 971 2013 4 83.3303 2012 5 44.6744 2012 6 9.8843 2015 7 47.11 2014 table 2. brazilian greenery areas data registered in ibge’s database in square meters (fkes). 3. results the biokzms were calculated according to the aforementioned databases and the results are shown in table 3. 4. discussion the measured impact of brazilian greenery areas on the indicator was found to be significant, despite the fact that these estimated areas are much smaller than those in the european union. thus, greenery areas which do not represent a high significant number in square km, results in fkes that are close to zero. according to table 3, independently of the brazilian region, these outcomes indicate that geographic localization, estimated altitude range, or precipitation range result in low values in table 4, which make it difficult to compare biokzm with european region iokzm. the brazilian iokzm and european region iokzm are shown in the table 5. it is important to establish which brazilian outcomes exposed a situation based on archaic data for greenery areas. it was not possible to find city censuses on recent greenery areas. if it was possible to calculate the recent brazilian iokzm, these biokzm might possibly decrease too much. the human growth and fast urbanization with insufficient environmental management plan for those cities indicate that a decrease is provoked in the indicator [14, 15], and these issues had happened in brazil. the correlation between fkes and temperature (fprt) is directly evidenced by the iokzm, with deforestation in brazilian cities contributing to higher temperatures [16]. the range of temperature 49 https://topographic-map.com jobson larrubia de almeida júnior, michael pondělíček acta polytechnica ctu proceedings city log 1 × log 2 log 2 × log 4 log 3 × log 4 log 3 × log 1 biokzm 1 1.109 0.684 1.593 2.582 2.984 2 1.54 -2.457 -5.736 3.596 -1.528 3 3.073 1.141 2.497 6.723 6.717 4 1.947 -2.277 -4.854 4.151 -0.516 5 3.751 -1.669 -4.111 9.239 3.605 6 3.747 -2.46 -5.862 8.928 2.176 7 1.829 -1.432 -3.54 4.521 0.689 table 3. variables of biokzm equation. brazil european region city fprs city fprs belém 2,085 prague 480 são luís 2,156 munich 938 teresina 1,447 vienna 675 joão pessoa 1,462 budapest 563 guarulhos 1,580 berlin 535 campinas 1,451 krakow 675 porto alegre 1,019 table 4. comparison of brazilian and european union precipitation range. brazil european region city iokzm city iokzm belém 2.984 prague 7.3776 são luís -1.528 munich 8.9197 teresina 6.717 vienna 7.3308 joão pessoa -0.516 budapest 6.7725 guarulhos 3.605 berlin 6.0678 campinas 2.176 krakow 7.6091 porto alegre 0.689 table 5. comparison of brazilian and the european union iokzm. in teresina is almost like other cities in the european union, according to table 3 and table 6. even with the higher temperature range of brazilian cities, the biokzm of some cities was maintained in negative outcomes. gomes (2022) demonstrated that deforestation disrupts the balance of hydrological systems, especially the flow of rivers in the amazon rainforest and the cerrado. as a consequence, the water flow was reduced because of the decrease in water recharge in the soil [17]. this hydrological disturbance affects the temperature range and the plumiometric range. according to the results in table 3, the pluviometrics can reduce the iokzm. however, comparing these outcomes within table 6, the european union has cities with lower grades in pluviometric terms, and their iokzm are positive. even considering são luis, which has the top fprs, the biokzm are not positive. according to table 3, fprt, fprs, and fmnv combined brazil european region city fprt city fprt belém 26.7 prague 13 são luís 26.8 munich 13 teresina 19.2 vienna 9,5 joão pessoa 21.3 budapest 11.2 guarulhos 19.7 berlin 12 campinas 27.9 krakow 9.2 porto alegre 25.8 table 6. comparison of brazilian and european union temperatures range. were not sufficient to turn the iokzm to a high number. on the other hand, the precipitation indicator that is correlated with negatives biokzm influences this indicator directly, as can be observed in the outcomes from cities number 4 and 6 from table 3. the altitude above sea level slightly influences the iokzm when applied in different countries. this is clear in the pondělíček study, where the cities selected from the european union have altitudes slightly similar (47, 127, 200, 233, 239, 520 m) compared to the brazilian ones (6, 12, 24, 26, 134, 665, 841 m). according to table 3, teresina is the highest city analyzed, and this associated outcome caused a positive biokzm. compared to the pondělíček method for calculating the iokzm of the european union, we can observe that all five european union cities selected for this study exposed higher grades compared to brazilian iokzm, according to table 3. biokzm revealed the influence of the temperature of the cities investigated. the five cities inspected from the iokzm of the european union have temperatures under 13 °c, which is the maximum measured between the european union sample. in contrast, the brazilian minimum and maximum average temperatures analyzed are 19.2 °c and 27 °c, respectively. this article and thesis aims to look at the geographical and climatic conditions of cities differently and to obtain an overview of the quality of selected central european cities in terms of conditions for developing green infrastructure within cities. the selected indicator of the general quality of green cities (okzm indicator) was used to determine the relative potential of selected cities for further support of green infras50 vol. 46/2024 general quality of urban greenery as a city indicator . . . tructure of the city (the indicator works similarly to iq, with the comparison of values among themselves). to measure the potential of the need to care for urban vegetation in europe, the cities closest to prague with populations of more than 1 million in absolute numbers and linked to the central european area were selected: munich, vienna, budapest, krakow, and berlin (berlin and krakow are peripheral cities, but subject to similar development patterns as the others, and their influence on the development of central europe is obvious). similarly, in brazil, selected cities from different zones, from the equator to the temperate zone in the southern hemisphere, were chosen for the biokzm: belém, campinas, guarulhos, joao pessoa, porto alegre, sao luiz, and teresina. in central europe, munich and krakow dominated, taking into account the number of local inhabitants and thus the density of inhabitants in cities. the best in terms of biokzm in brazil were the southern hemisphere cities of campinas, porto alegre, and teresina, which are practically in similar conditions as cities in the northern hemisphere and in europe. based on a comparison of the results of iokzm and biokzm, we conclude that cities in the temperate zone are practically suitable and comparable for assessment thanks to iokzm and its calculation, whereas cities on the equator have extreme conditions and their vegetation within the city cannot be evaluated similarly. in principle, calculations have proven that iokzm works at this level [18–21]. this comparison between iokzm and biokzm from temperate zones signifies a significant advance of international collaboration efforts to minimize global warming and climate change impacts. thus, eu and brazilian temperate zones can be studied with more accuracy in these terms. moreover, this paper gives the scientific community resources to investigate some countries which are in temperate zones as well as to discover if the okzm indicator can be used on them too. countries in the southern hemisphere, such as south africa, the south of australia, new zealand, chile, argentina, and countries in the northern hemisphere, such as the usa, canada, morocco, china, mongolia, north korea, and south korea are in the temperate zones and represent an important part of the global economy. and this fact is related to sustainability. in addition, the aggregation of data from these countries could propose to the scientific community and the governments an okzm network which reflects international cooperation. some secondary data could be created from these preliminary countryspecific data, correlating okzm with specific issues such as the influence on continents, the economy, the relationship with the country’s position in the world (north or south), global demographics, climate change per hemisphere, and governments’ efforts to attain a state of environmental well-being combined with societal cooperation. 5. conclusions iokzm calculated for brazilian cities is interesting precisely because of their location in the southern hemisphere, as all the evidence of climate change subtly suggests that the climate in the southern hemisphere changes in a different way. according to some authors, this is due to the pacific ocean and also antarctica as a land mass with a large glaciation, which cools south america more. the purpose of the biokzm calculation is now mainly to identify trends in the cities, and when comparing data from five years ago to the current data, it can indicate how the geomorphological and climatic conditions are changing. the final use of biokzm is in the field of vegetation care and maintenance in cities. according to biokzm, we can assess the level of care required for vegetation and predict its demands based on the influence of the climate. in particular, financial demands are costs that are important to the care of urban vegetation. high-quality vegetation and functional trees fulfil their role in improving the microclimate of the city better and dampening all extremes. the main role of biokzm is therefore to look for trends in the care of urban vegetation and to help find a trend where it is possible to reduce or increase investments in greenery, depending on the city’s population density. references [1] k. calvin, d. dasgupta, g. krinner, et al. ipcc, 2023: climate change 2023: synthesis report. contribution of working groups i, ii and iii to the sixth assessment report of the intergovernmental panel on climate change. geneva, switzerland, 2023. https://doi.org/10.59327/ipcc/ar6-9789291691647 [2] the united nations office for disaster risk reduction (unisdr). how to build resilient cities: a guide for public local managers., 2012. [2023-12-18]. https://www.unisdr.org/files/26462_ handbookfinalonlineversion.pdf [3] s. h. a. koop, c. j. van leeuwen. the challenges of water, waste and climate change in cities. environment, development and sustainability 19(2):385–418, 2016. https://doi.org/10.1007/s10668-016-9760-4 [4] a. sharifi, p. salehi (eds.). resilient smart cities: theoretical and empirical insights. the urban book series. springer international publishing, 2022. https://doi.org/10.1007/978-3-030-95037-8 [5] v. mendes. climate smart cities? technologies of climate governance in brazil. urban governance 2(2):270–281, 2022. https://doi.org/10.1016/j.ugj.2022.08.002 [6] r. jóźwik, a. jóźwik. influence of environmental factors on urban and architectural design – example of a former paper mill in nanterre. sustainability 14(1):86, 2021. https://doi.org/10.3390/su14010086 [7] international organization for standardization. iso/fdis 37122:2019 sustainable cities and communities: indicators for smart cities, 2019. [2023-1218]. https://www.iso.org/standard/69050.html 51 https://doi.org/10.59327/ipcc/ar6-9789291691647 https://www.unisdr.org/files/26462_handbookfinalonlineversion.pdf https://www.unisdr.org/files/26462_handbookfinalonlineversion.pdf https://doi.org/10.1007/s10668-016-9760-4 https://doi.org/10.1007/978-3-030-95037-8 https://doi.org/10.1016/j.ugj.2022.08.002 https://doi.org/10.3390/su14010086 https://www.iso.org/standard/69050.html jobson larrubia de almeida júnior, michael pondělíček acta polytechnica ctu proceedings [8] e. leite. innovation networks for social impact: an empirical study on multi-actor collaboration in projects for smart cities. journal of business research 139:325–337, 2022. https://doi.org/10.1016/j.jbusres.2021.09.072 [9] database of instituto brasileiro de geografia e estatística (ibge). ibge, 2023. [2023-04-14]. https://www.ibge.gov.br [10] climate data. database climate-data, 2023. [2023-03-10]. https://pt.climate-data.org/ [11] instituto cidades sustentáveis. database of sustainable cities program, 2023. [2023-03-07]. https: //www.cidadessustentaveis.org.br/inicial/home [12] tessadem. database of topographic-map.com, 2023. [2023-03-20]. https://pt-br.topographic-map.com/ [13] instituto brasileiro de geografia e estatística (ibge). preview of population cities based on demographic cense 2022 collected until 25th december of 2022, 2023. [202303-20]. https://www.ibge.gov.br/en/statistics/ social/labor/22836-2022-census-3.html [14] p. de souza nascimento. impactos socioambientais em áreas de expansão urbana de barreiras (ba): análises consolidadas. anais do xvi simpósio nacional de geografia urbana-xvi simpurb 1:3978–3993, 2019. [15] v. c. s. moura. impactos ambientais da urbanização: esforços da pesquisa brasileira e mapeamento e percepção de moradores na cidade de santarém, pará. ph.d. thesis, universidade federal do oeste do pará, 2019. [16] p. dos santos ribeiro, d. j. c. gomes, e. b. de souza, et al. influência do desmatamento na temperatura do ar. revista brasileira de geografia física 16(01):165–176, 2023. [17] d. j. c. gomes, m. m. m. nascimento, f. m. pereira, et al. variabilidade da vazão na bacia hidrográfica do rio araguaia influenciada pela precipitação em anos extremos e desmatamento. brazilian journal of environmental sciences (rbciamb) 57(3):451–466, 2022. [18] copernicus urban atlas. copernicus programme 2018. [2023-02-07]. https://land.copernicus.eu/local/urban-atlas [19] h. dreiseitl, b. wanschura. strenghtening the blue-green infrastructure in our cities, 2016. [2023-02-07]. https://ramboll.com//media/38fc23d12a5d47dcb7b3821716d69270.pdf [20] espon. zelená infrastruktura v urbánních oblastech – teze politik. úúr, 2020. isbn 978-80-87318-99-7. [21] k. gupta, p. kumar, s. pathan, k. sharma. urban neighborhood green index – a measure of green spaces in urban areas. landscape and urban planning 105(3):325–335, 2012. https: //doi.org/10.1016/j.landurbplan.2012.01.003 52 https://doi.org/10.1016/j.jbusres.2021.09.072 https://www.ibge.gov.br https://pt.climate-data.org/ https://www.cidadessustentaveis.org.br/inicial/home https://www.cidadessustentaveis.org.br/inicial/home https://pt-br.topographic-map.com/ https://www.ibge.gov.br/en/statistics/social/labor/22836-2022-census-3.html https://www.ibge.gov.br/en/statistics/social/labor/22836-2022-census-3.html https://land.copernicus.eu/local/urban-atlas https://ramboll.com/-/media/38fc23d12a5d47dcb7b3821716d69270.pdf https://ramboll.com/-/media/38fc23d12a5d47dcb7b3821716d69270.pdf https://doi.org/10.1016/j.landurbplan.2012.01.003 https://doi.org/10.1016/j.landurbplan.2012.01.003 acta polytechnica ctu proceedings 46:47–52, 2024 1 introduction 2 research methods 3 results 4 discussion 5 conclusions references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2023.43.0081 acta polytechnica ctu proceedings 43:81–85, 2023 © 2023 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague simulation assessment of the headway under the etcs l3 petr nachtigall∗, erik tischer university of pardubice, jan perner transport faculty, department of transport technology and control, studentská 95, 532 10 pardubice, czech republic ∗ corresponding author: petr.nachtigall@upce.cz abstract. this paper deals with a simulation assessment of the impact of the introduction of etcs l2 and l3 on the capacity of railway lines. the simulation itself is carried out in the opentrack simulation software, especially in the tool headway calculator. for this research, an infrastructure model has been built, which represents a typical layout of lines and stations in the czech republic. by successive modifications of the model, the significance of the impact of the implementation of atp systems on the capacity of lines can be simulated. in the paper, the influence of the implementation of different application levels of atp systems on the capacity of railway lines is evaluated. keywords: capacity, headway, etcs, simulation. 1. introduction the gradual development of the railway network equipped with the etcs system is moving to the foreground a question about the influence of this system on the throughput of a railway line [1, 2]. in addition to the traditional division of the etcs system into its three basic levels (l1 to l3), based on foreign experience, other implementations of this system are beginning to appear, e.g., with shorter block sections that are about 400 meters long and are largely virtual with stop signs. german research has even found that the effectiveness of these l2 level adjustments can reach nearly the effects of the l3 level. given the considerable technical complexity and uncertainty, this adds to the fact that the l3 level is not used for conventional rail [3–5]. in the conditions of the czech infrastructure, some research [2, 5–7] and has so far been carried out by comparing the methodology of the správa železnic, s.o. and the simulation software opentrack, which showed that the calculation of the subsequent interval using opentrack is possible, and its results are relevant about the methodology used on the czech railways. further research could therefore focus on the application of etcs l2 to this model to determine the performance parameters of the infrastructure under the new conditions. the implementation of etcs l2 with shorter block sections (approx. 400 meters) was chosen as the first logical step [8, 9]. the simulation model and its results for the etcs l3 are the content of this article. 2. methods a model from literature [2] was used for this research. here, the authors demonstrated the possibility of using the model in the opentrack software to simulate the calculation of the headway, which is given by czech regulations. in the next paper, which is currently being prepared, the authors focused on the application of shorter block sections and verification of the impact of the introduction of etcs l2 with shorter block sections on track capacity. for this paper, the simulation model was subsequently modified to test the function of the moving block at the etcs l3 level. in addition to the braking characteristics of the individual trainsets, it was also necessary to set safety limits (safety distance), which will cause the second train to be able to stop safely if the first train loses the connection or stops. there is currently no strict regulation on the size of these safety distances, but the size of this path must depend on: • train speed of the first train, • train speed of the second train, • braking parameters of the first train, • braking parameters of the second train, • slope and radius of curves. however, standard trainsets for high-speed, conventional, and freight trains were used for this research. the slope and the radius of the curves were set to 0. for the simulation of the influence of the atp system on the track capacity, the opentrack software was chosen [10]. it is a specialized software that allows a broader assessment of the railway infrastructure from many points of view. for example, it can be used in the areas of assessment of rail vehicle dynamics, energy consumption, line and station operating intervals, line capacity, transport planning and assessing the effect of changes to rail infrastructure and rolling stock. defining the objective of the model is always crucial for correct model building. microscopic modelling guarantees the most accurate results, but at 81 https://doi.org/10.14311/app.2023.43.0081 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en petr nachtigall, erik tischer acta polytechnica ctu proceedings figure 1. scheme of the model preparation. train category engine vmax length [m] weight [t] nex traxx f 140 ms 120 km h−1 600 1 000 pn čdc 363.5 80 km h−1 600 1 800 os čd 471 140 km h−1 80 155 r čd 661 160 km h−1 132 170 ex čd 680 200 km h−1 185 385 table 1. parameters of used train sets. the same time requires very careful model processing [4]. figure 1 shows the different stages of model preparation. the infrastructure is represented in the simulation model by a network graph. the vertices of the graph are represented by ordered double points that show the kilometric position of each point. in that double point, some characteristics of the track (slope, curve, speed, ...) are changing. the edges connecting them represent the structural and technical characteristics of each part of the track or station area. the parameters of all possible routes need to be inserted into the network and thus prepared. the routes are in the station areas defined according to the table of control, signalling route or situation diagram. from those “routes” are then designed “paths” which are leading from one station to another station. the last step is connecting “paths” to “itineraries” which are leading from the origin station of each train to the terminal station. for this model, a fictitious line with a total length of 18.35 km with three intermediate stations and three stops was designed. the model track forms an infinite loop. the modelled stations are four-track with variant header and header parameters. switches are used on the headers with a variable speed range in the branch direction from 40 to 80 km h−1. the length of each header has a variance of 300–800 metres. an equally important feature of the simulation model in opentrack sw is the characteristics of each engine and trainset. the main features are traction characteristics and vehicle resistance. the basic parameters of each train set are in table 1. it is also necessary to insert a timetable to simulate the considered operational concepts. each train is assigned by a number, category, track occupancy method, timetable, and itinerary with their preferential order in this step. if there isn’t available a constructed timetable, it must be created. this is possible by running the simulation and determining the required journey times and process times. to verify the functionality of the model, the actual verification, validation, and calibration of the model are further performed to prove the functionality of 82 vol. 43/2023 simulation assessment of the headway under the etcs l3 figure 2. model of the track for conventional operation with automatic train control. figure 3. model of the track equipped by etcs l2. figure 4. model of the track equipped by etcs l3. the model for the intended use. the simulation of individual simulation scenarios can then proceed. 3. simulation scenarios to be able to correctly assess the impact of the different application levels of etcs, individual operational scenarios were prepared for each application level of the system separately. for these reasons, modifying the original simulation model for each application level was necessary. 3.1. conventional operation with national train protection system the first simulation scenario was a simulation of conventional operation. in this variant, all stations are equipped with an electronic station interlocking system. the line is protected by a two-way threecharacter automatic block. the distance of the block signals is standardised at 1 000 metres. figure 2 shows a model for conventional operation with automatic train protection. 3.2. operation under the etcs l2 the station interlocking system remains the same as in the first scenario. all main signals in the stations are also retained. the modification is that the point defining the beginning of the speed restriction area given by the signal the main signal is no longer the position of this signal but the position of the limiting infrastructure element. typically, this is the first speed-restricted switch point. the block sections are no longer divided by block signals. they are divided by fictitious signals of etcs (balise groups). these etcs blocks are approximately 400–430 metres long. figure 3 shows the modification of the model for the etcs l2 application level. 3.3. operation under the etcs l3 this application level enables the operation of socalled moving blocks. so that the length of each block depends only on variables from equation (1). this application level isn’t in regular use on any railway line yet. therefore, the authors needed to find the safe security distance between two trains (the upper index in the formula means which train it is) by equation (1): sbezp = l1 eb − l2 sb + l1 inn + l2 inn , (1) where: • sbezp – safe security distance [m], • leb – emergency brake distance [m], • lsb – service brake distance [m], • linn – inaccuracy of the position of a train [m]. in this case, all the station and track interlocking systems relate to the radio block center (rbc). so, all movement authorities are controlled by the rbc. figure 4 shows the model’s modification for the application level etcs l3. 4. results and discussion the authors decided to compare the effect of each application level’s implementation by determining the headway values. the headway for the section is determined as a maximum of the sub-headways for each block in the section. the value of the headway is very important for the calculation of the line capacity and for the construction of a stable timetable. the software opentrack includes the tool headway calculator which was used also in this research. using multiple simulation methods, this tool determines the minimum value of the headway for any pair of trains by the interval halving method. the rules for the simulation are based on směrnice sždc sm 104 – 83 petr nachtigall, erik tischer acta polytechnica ctu proceedings section b-c first train nex pn os r ex arr. dep. arr. dep. arr. dep. arr. dep. arr. dep. se co nd tr ai n nex 84 73 205 106 155 126 85 80 38 38 pn 73 73 106 106 105 76 80 80 38 38 os 73 73 128 113 135 136 80 80 38 38 r 141 73 219 175 192 154 80 149 38 38 ex 200 73 278 153 251 148 172 127 62 38 table 2. results of the headway calculator for one of the sections. comparison of automatic train comparison of automatic train control system and etcs l2 control system and etcs l3 section [s] [%] [s] [%] a-b -15.76 11.39 -18.92 26.18 b-a -20.96 25.44 -22.03 18.15 b-c -22.24 21.86 -22.31 22.70 c-b -26.00 22.97 -26.50 30.00 c-a -20.52 15.82 -21.52 27.91 a-c -13.96 11.19 -14.06 17.54 total -19.91 18.11 -20.89 23.75 table 3. comparison of headways. provozní intervaly a následná mezidobí [11] and směrnice sždc sm 124 – zjišťování kapacity dráhy [12] and regulation sž d1 part one [13]. the simulation is then run in the prepared model to determine the values of the headway in all sections. each pair of trains was simulated separately in all sections. the output for each of the three simulation variants is six tables showing the headway values in seconds. the resulting headway value is then a maximum from each pair of values (one is on the arrival; one is on the departure). an example is bold in table 2. then, the individual simulated variants were compared with each other to determine the effect of the implementation of different etcs application levels. table 3 shows the overall results. 5. conclusion the simulation model carried out by the authors for this paper is loosely related to their previous research in the automatic train protection system simulation. the results obtained confirm the benefits of implementing etcs l2 with benefits. furthermore, the consideration of further track capacity increase using the moving block in the etcs l3 application level was confirmed. the results obtained are of course valid for the proposed research infrastructure model and have no general validity. the simulation results show that the headway of the etcs l2 application level is 19.1 % lower than in the present interlocking system. the headway value for etcs l3 is 23.75 % lower than in the present interlocking system. so that the benefit of the etcs l3 is approximately 4 or 5 %. maybe this is not the way, how to improve the capacity of the czech railway infrastructure. acknowledgements this article is published within the “cooperation in applied research between the university of pardubice and companies in the field of positioning, detection and simulation technology for transport systems (positrans)” project, registration no.: cz.02.1.01/0.0/0.0/17_049/0008394. references [1] j. bulíček. cancellation of delayed trains: passengers’ and capacity points of view. matec web of conferences 235:00022, 2018. https://doi.org/10.1051/matecconf/201823500022 [2] e. tischer, p. nachtigall, j. široký. the use of simulation modelling for determining the capacity of railway lines in the czech conditions. open engineering 10(1):224–231, 2020. https://doi.org/10.1515/eng-2020-0026 [3] z. chen, b. han. simulation study based on opentrack on carrying capacity in district of beijingshanghai high-speed railway. applied mechanics and materials 505-506:567–570, 2014. https://doi.org/ 10.4028/www.scientific.net/amm.505-506.567 [4] d. hansen, m. leuschel, p. krörner, et al. validation and real-life demonstration of etcs hybrid level 3 principles using a formal b model. international journal on software tools for technology transfer 22:315–332, 2020. https://doi.org/10.1007/s10009-020-00551-6 84 https://doi.org/10.1051/matecconf/201823500022 https://doi.org/10.1515/eng-2020-0026 https://doi.org/10.4028/www.scientific.net/amm.505-506.567 https://doi.org/10.4028/www.scientific.net/amm.505-506.567 https://doi.org/10.1007/s10009-020-00551-6 vol. 43/2023 simulation assessment of the headway under the etcs l3 [5] p. nachtigall, j. ouředníček. wider aspects of deceleration supervision in ertms/etcs. matec web of conferences 235:00010, 2018. https://doi.org/10.1051/matecconf/201823500010 [6] p. nachtigall. vliv nastavení mobilní části etcs na brzdnou křivku. perner’s contacts 11(1):131–137, 2016. [7] i. hruban, p. nachtigall, o. štěpán. přínosy zavedení etcs na brzdnou křivku. in vědeckotecnický sborník čd, vol. 40. 2015. [8] p. nachtigall, e. tischer. efficiency of increasing of the track speed using simulation in opentrack. in the 32nd european modeling & simulation symposium, pp. 130–136. 2020. https://doi.org/10.46354/i3m.2020.emss.017 [9] m. ghazel. formalizing a subset of ertms/etcs specifications for verification purposes. transportation research part c: emerging technologies 42:60–75, 2014. https://doi.org/https: //doi.org/10.1016/j.trc.2014.02.002 [10] d. huerlimann, a. b. nash. opentrack: simulation of railway networks version 1.3. eht zurich, zurich, 2019. [11] správa železnic, státní organizace. směrnice sždc 104 – provozní intervaly a následná mezidobí. správa železnic, s.o., praha, 2013. [12] správa železnic, státní organizace. směrnice sždc sm124 – zjišťování kapacity dráhy. správa železnic, s.o., praha, 2018. [13] správa železnic, státní organizace. dopravní a návěstní předpis. d1, změna č. 4, správa železnic, s.o., praha, 2019. 85 https://doi.org/10.1051/matecconf/201823500010 https://doi.org/10.46354/i3m.2020.emss.017 https://doi.org/https://doi.org/10.1016/j.trc.2014.02.002 https://doi.org/https://doi.org/10.1016/j.trc.2014.02.002 acta polytechnica ctu proceedings 43:81–85, 2023 1 introduction 2 methods 3 simulation scenarios 3.1 conventional operation with national train protection system 3.2 operation under the etcs l2 3.3 operation under the etcs l3 4 results and discussion 5 conclusion acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2025.52.0069 acta polytechnica ctu proceedings 52:69–76, 2025 © 2025 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague the influence of road marking visibility on lateral vehicle position and driving speed ivana kučinaa,∗, bernard kosoveca, filip jezidžićb, marija ferkoa,b, darko babića a university of zagreb, faculty of transport and traffic sciences, department of traffic signaling, vukelićeva 4, 10 000 zagreb, republic of croatia b smart view d.o.o., rastočka 8a/2, 10020 zagreb, republic of croatia ∗ corresponding author: ivana.kucina@fpz.unizg.hr abstract. road markings are a vital element of horizontal signalling, forming an integral part of traffic signalling. their role in traffic safety involves providing timely warnings, safe guidance, and necessary information to ensure the safe movement of all road users. under conditions of reduced visibility, often due to weather (night, fog, rain, etc.), road markings are often the only guide for drivers to determine the direction of the road. research conducted using a driving simulator demonstrated the influence of road markings on driver behaviour, specifically on lateral vehicle position and driving speed. the results indicated that drivers behave differently depending on the visibility of road markings. a total of 31 participants took part in the study, with data collected on driving speed and lateral vehicle position for three different visibility levels of road markings on a straight road and on a right and left curve. keywords: road markings, visibility, driving simulator, driving speed, lateral position. 1. introduction road markings are a crucial traffic management component, providing visual guidance for drivers and other road users. together with traffic signs, they are one of the most cost-effective measures for improving road safety [1]. the visibility of road markings depends on several factors, including the type of road marking material, the position of the marking, the number of markings, their age, the type of road, average annual daily traffic (aadt), speed limits, the amount of salt and abrasive used and winter maintenance activities [2]. additionally, the quality and quantity of glass beads play a crucial role, as they directly affect the visibility and retroreflection of the markings [3–6]. the visibility of road markings is crucial for drivers and advanced driver assistance systems (adas) and the development of autonomous vehicles. research from 2017 showed that camera-based detection of road markings improves with increased visibility, specifically with higher retroreflection and contrast ratios [7]. the colour of the road markings also affects their visibility and detection. for example, yellow markings on concrete pavements are more difficult for adas systems to detect due to lower contrast than white markings [8]. the evolution of road markings has prompted numerous studies on selecting materials and maintenance methods and assessing the impact of these markings on driver behaviour under varying conditions. many studies have examined the influence of longitudinal road marking widths and various treatments (transverse lines, chevron markings, dragon teeth, herringbone patterns, optical circles, horizontal warning signs, etc.) on driving speed and the lateral position of vehicles. most of these studies assessed the effect of longitudinal marking widths on speed reduction [9, 10] and the impact of different treatments on the reduction of driving speed [11–17]. some research has also focused on the effect of longitudinal marking widths on a vehicle’s lateral position, contributing to enhanced road safety. wider longitudinal markings have been shown to reduce traffic accidents [18, 19]. optical circles [16] and red median markings [17] significantly affect lateral positioning, while herringbone patterns were found not to influence lateral position in ariën et al. [15] study. however, charlton [20], awan et al. [16], and kazemzadehazad et al. [21] reported a significant impact lateral position. however, plain double yellow centre lines showed no noticeable effect on lateral vehicle position [20]. drivers obtain over 90% of traffic-related information through their sense of sight. since pavement markings and traffic signs are vital sources of this information, their visibility is critical, especially during night-time and in low-visibility conditions. the research primarily focuses on determining the minimum visibility levels of road markings and analysing how their visibility affects driver behaviour. previous research indicates that the minimum retroreflection needed by drivers for safe night driving in dry conditions ranges between 100 and 150mcd lx−1 m−2 [22– 25]. limited research has explored the impact of visibility levels at night on bus traffic safety. the visibility 69 https://doi.org/10.14311/app.2025.52.0069 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en i. kučina, b. kosovec, f. jezidžić et al. acta polytechnica ctu proceedings of pavement markings does not significantly affect the frequency of night-time crashes [26, 27]. some studies have shown that improving visibility or having lower levels of retroreflection can reduce the frequency of collisions in both dry [28–30] and wet conditions [31, 32]. in conditions of reduced visibility, driving speed tends to decrease, whereas in conditions of higher visibility, driving speed increases [33–35]. a recent study by fiolić et al. [35] found no significant differences in the vehicle’s lateral position at varying levels of visibility. the paper aims to determine whether the different visibility of road markings affects the driver’s behaviour, specifically regarding speed and lateral vehicle position. the research is based on data collected from a driving simulator. 2. research methodology the research was carried out using a static driving simulator from carnetsoft b.v. the simulator setup includes a driver section consisting of a seat with pedals, a steering wheel, and a gear shifter, along with three interconnected 30-inch displays. these displays offer a combined resolution of 5760×1080 and operate at a frame rate of 30hz. the screens provide a 210° panoramic view of the environment through six channels: left, middle, and right views, along with three mirror displays. the middle screen also presents the vehicle’s control panel, which includes the speedometer, rev counter, turn signals, lighting, fuel gauge, and other indicators. this setup enhances the realism of the driving experience. the simulation software operates on a computer running windows 10 (64-bit), with 8gb of ram and 4gb of storage, ensuring smooth operation of the driving simulator (figure 1). three driving scenarios were used in the research, each divided into two parts. the total length of the scenario is 12.5 km, with the first part covering 2.24 km and the second part 10.26 km. the average driving time in the simulator ranges from 10 to 15 minutes, depending on speed. the first part, a warmup, lasted approximately 2-3 minutes, depending on driving speed. the second part, where the actual research takes place, consists of three stages featuring identical road elements: a straight section, a right turn, and a left turn. however, these stages differ in the visibility of the road markings, with three levels: poor visibility (v1), medium visibility (v2), and high visibility (v3) (figure 2). the scenarios differ in the sequence of these stages, with varying levels of road marking visibility for each. all scenarios occur on a two-way road outside urban areas, with a total roadway width of 6.50 meters (3.25 meters per driving lane). the simulation replicates night driving conditions, and all participants drove with low beams to ensure consistent conditions. the driving scenario don’t include vehicles from the opposite direction to avoid influencing the participant’s behavior. figure 1. driving simulator setup. figure 2. scenarios: a) v1 – poor visibility, b) v2 – medium visibility, c) v3 – high visibility. the participant begins driving in the first stage, where road marking visibility is at its lowest (v1). this stage starts on a 150-meter straight section, with a posted speed limit of 90 kmh−1 (sign dimensions: 60 cm, height: 1.5m, distance from edge: 1m). afterwards, the participant drives through a 500-meter flat section, where data on speed and vehicle lateral position is collected. this is followed by another 150-meter stretch with traffic signs indicating an upcoming right turn (sign dimensions: 90 cm, height: 2.3m, distance from edge: 1m) and a speed limit of 70 kmh−1 (sign dimensions: 60 cm, height: 1.5m, distance from edge: 1.5m). the right turn begins with a 50-meter transition with a 200-meter radius, leading into a 110-meter turn with a 150-meter radius, and concludes with another 50-meter section with a 200-meter radius. sharp turn signs (dimensions: 60 cm, height: 1.5m, distance from edge: 7.5m) are placed 37, 73.5, and 110 meters before the turn begins. after that, there is a 150-meter straight section with signs indicating 70 vol. 52/2025 the influence of road marking visibility on lateral vehicle. . . an upcoming left turn and a speed limit, followed by a left turn identical to the right turn. the participant then approaches a four-way intersection where they have the right of way, transitioning to the second stage of the scenario. in this stage, road marking visibility is at a medium level (v2). the participant first navigates through the left turn, then a straight section, followed by the right turn, and finally, another intersection where they again have the right of way. in the third and final stage, where road marking visibility is at its highest (v3), the participant drives through a right turn, a left turn, and ends on a straight section, concluding the test. the exact sequence applies to the other two scenarios, with the order of the straight sections and the right and left turns being the only difference. at the start of the study, participants were provided with detailed instructions, after which they signed a consent form. additionally, they completed a questionnaire that collected personal information such as date of birth, gender, date of obtaining a driver’s license, self-assessment of driving ability, driving frequency, estimated kilometres driven per year, and any vision problems. participants were also required to complete a selfassessment of their psychophysical condition before and after the driving simulation. it was explained that mild nausea might occur during the simulation, and the session should immediately stop if they experienced symptoms such as headache, discomfort, or nausea for safety reasons. participants were instructed to drive using only the low-beam headlights. it was emphasized that the study was not assessing their driving quality, and no penalties would be imposed for traffic violations. after the driving session, participants completed another form to assess their general condition post-simulation, aiming to identify any changes in their psychophysical state. the study involved 31 adult participants, all required to have a driver’s license of at least category b. of the 31 participants, 10 were women (32.26%) and 21 were men (67.74%). the average age of participants was 26.68 years (min = 18.89 years, max = 32.69 years, sd = 2.92 years), and the average driving experience was 7.03 years (min = 0.77 years, max = 14.66 years, sd = 3.07 years). participants rated their driving skills on a scale from 1 to 5, with an average score of 4.06 (min = 3, max = 5, sd = 0.62). the estimated annual distance driven by participants averaged 12 354.84 kilometres (min = 1 000 km, max = 50 000 km, sd = 12 149.07 km). 3. results the conducted analysis aimed to determine whether and to what extent different levels of road marking visibility, or the quality of road markings, affect driver behaviour. this study observed two key variables: vehicle speed and lateral position. a one-way anova was conducted to analyse the impact of visibility on these variables across three distinct sections of the road. a comparison was made between the impact of different levels of visibility of road markings on certain road sections on the driving speed and lateral position. 3.1. descriptives the descriptive statistics for speed and lateral position across different visibility levels and road sections provide detailed insights into driver behaviour in the driving simulator study. the descriptive statistics analysis for the left curve section shows that the mean speed for different levels of road marking visibility (poor, medium, high) remains relatively consistent, ranging from 25.574m s−1 to 26.243m s−1. the standard deviation of speed is also stable across visibility conditions, indicating similar variability in driving speed under all three conditions. for the lateral position on the left curve, the means are close to zero for all visibility levels, suggesting that drivers, on average, maintained their position near the centre of the lane. however, the standard deviation of lateral position, which ranges from 0.379m to 0.419m, indicates a noticeable variation in how consistently drivers maintained their position. poor road marking visibility has a slightly higher standard deviation (0.419m) than medium (0.379m) and high (0.393m) visibility conditions, implying greater deviation from the centreline under poor visibility. in the right curve section, speed remained stable, with mean values between 25.632m s−1 and 25.694m s−1 across different levels of road marking visibility. the standard deviation for speed was again similar, indicating no significant change in speed variability based on the visibility of road markings. however, the lateral position shows more variation in standard deviation, especially in poor visibility (0.429m). this suggests that drivers had considerable variability in maintaining their lateral position under poor visibility conditions. in the straight road section, speeds were slightly higher, with the mean ranging from 26.598m s−1 to 27.037 m s−1. the standard deviation for speed remains consistent across visibility levels. for lateral position, the standard deviation under poor visibility was 0.333m, increasing to 0.382m under high visibility. this indicates slightly more variability in lateral position under high visibility conditions, potentially due to drivers feeling more confident and adjusting their position more frequently. the following multiple comparisons were crucial to understanding whether these observed mean differences were statistically significant across different levels of road marking visibility. given that road marking visibility impacts both speed and lateral position, conducting post-hoc tests to identify specific pairs of visibility conditions that differed significantly was important. these comparisons help clarify road 71 i. kučina, b. kosovec, f. jezidžić et al. acta polytechnica ctu proceedings road variable visibility n mean std. deviation std. error minimum maximum section poor 12,126 25.574 5.757 0.052 14.490 37.657 speed medium 11,858 26.168 5.841 0.054 12.764 37.653 [m s−1] high 11,821 26.243 5.621 0.052 16.706 37.660 left total 35,805 25.992 5.748 0.030 12.764 37.660 curve poor 12,126 0.107 0.393 0.004 -1.238 1.434 lateral medium 11,858 0.115 0.379 0.003 -1.433 1.232 position [m] high 11,821 0.090 0.419 0.004 -0.906 1.485 total 35,805 0.104 0.397 0.002 -1.433 1.485 poor 33,139 25.632 5.291 0.029 12.032 37.663 speed medium 33,061 25.694 5.124 0.028 16.070 37.663 [m s−1] high 33,130 25.641 5.243 0.029 15.249 37.661 right total 99,330 25.656 5.220 0.017 12.032 37.663 curve poor 33,139 0.028 0.429 0.002 -2.193 2.834 lateral medium 33,061 0.012 0.400 0.002 -1.201 1.983 position [m] high 33,130 0.019 0.408 0.002 -2.426 2.084 total 99,330 0.019 0.412 0.001 -2.426 2.834 poor 18,619 26.598 4.614 0.034 17.572 37.662 speed medium 18,312 27.037 4.706 0.035 18.543 37.626 [m s−1] high 18,342 27.008 4.722 0.035 16.402 37.627 straight total 55,273 26.880 4.685 0.020 16.402 37.662 road poor 18,620 0.109 0.333 0.002 -1.157 2.091 lateral medium 18,312 0.078 0.326 0.002 -0.884 1.082 position [m] high 18,342 0.092 0.382 0.003 -1.163 1.324 total 55,274 0.093 0.348 0.001 -1.163 2.091 table 1. descriptive statistics. marking quality’s nuanced effects on driving behaviour across various road sections. 3.2. left curve levene’s test of homogeneity of variances for both speed and lateral position showed that the assumption of equal variances is violated for the left curve section, as all the p-values are less than 0.001. because of that, we used welch’s anova and brown-forsythe tests to determine whether there was a significant difference between the observed groups. both the welch and brown-forsythe tests were significant (p < 0.001), indicating a statistically significant difference in speed across the three levels of road marking visibility on the left curve. this section’s road marking visibility levels (low, medium, high) affect vehicle speed significantly. similarly, both tests showed a statistically significant difference in lateral position across the visibility levels (p < 0.001), meaning that road marking quality affects vehicle lateral position significantly on the left curve. next, we proceeded with multiple comparisons using the games-howell post hoc test. the games-howell post hoc test results revealed significant speed and lateral position differences across road marking visibility levels. for speed, both poor and medium visibility resulted in significantly lower speeds compared to high visibility of road markings (md = −0.593ms−1 and −0.669ms−1, respectively, both p < 0.001). however, no significant difference was found between medium and high visibility (p = 0.568). for lateral position, significant differences were observed between poor and high visibility of road markings (md = 0.017m, p = 0.003) and between medium and high visibility (md = 0.025m, p < 0.001). these results indicate that vehicles tend to deviate less from the centre in high visibility conditions compared to poor visibility. no significant differences were found between poor and medium visibility for lateral position (p = 0.303). 3.3. right curve levene’s test of homogeneity of variances for the right curve section revealed different speed and lateral position results. for speed, the test showed significant results (p < 0.001), indicating that the assumption of equal variances is violated across the visibility levels. in contrast, the p-values were greater than 0.05 for the lateral position, suggesting that the assumption of homogeneity of variances is met. based on these results, we used welch’s anova to compare speeds and standard anova to compare lateral positions. the robust tests of equality of means for speed on the right curve yielded the following results: welch statistic = 1.371, with p = 0.254 and brown-forsythe statistic = 1.349, with p = 0.260. both tests indicated no statistically significant difference in speed across 72 vol. 52/2025 the influence of road marking visibility on lateral vehicle. . . dependent visibility (i) visibility (j) mean difference (i–j) std. error sig. variable speed poor medium -0.593* 0.075 < 0.000 [m s−1] high -0.669* 0.074 < 0.000 medium high -0.076 0.074 0.568 lateral poor medium -0.007 0.005 0.303 position [m] high 0.017* 0.005 0.003 medium high 0.025* 0.005 < 0.000 * the mean difference is significant at the 0.05 level. table 2. multiple comparisons (games-howell post hoc tests) for the left curve. dependent visibility (i) visibility (j) mean difference (i–j) std. error sig. variable lateral poor medium 0.016* 0.003 < 0.000 position [m] high 0.009* 0.003 0.013 medium high -0.007 0.003 0.066 * the mean difference is significant at the 0.05 level. table 3. multiple comparisons (tukey hsd post hoc tests) for the right curve. the three visibility levels (poor, medium, and high) since the p-values are greater than 0.05. the results suggest that variations in road marking visibility do not significantly influence vehicle speed on the right curve. since the result was not significant, there is no need for post hoc comparisons for speed on the right curve. the results of the anova for lateral position on the right curve revealed a statistically significant difference across the three visibility levels, with an fstatistic of 12.895 and a p-value of less than 0.001. this indicates that road marking quality impacts how vehicles position themselves laterally on this road section. given the significance of these findings, further post-hoc analysis was conducted to identify which specific visibility levels (poor, medium, high) differ from each other in terms of lateral positioning. the tukey hsd post hoc analysis for lateral position revealed significant differences between the levels of road marking visibility. specifically, vehicles in the poor road marking visibility condition exhibited a significantly greater lateral position compared to those in the medium (md = 0.016m, p < 0.001) and the high road marking visibility condition (md = 0.009m, p = 0.013). conversely, there was no significant difference in lateral position between medium and high road marking visibility (md = −0.007m, p = 0.066). 3.4. straight road levene’s test of homogeneity of variances results for the straight road section indicated significant violations of the assumption for both speed and lateral position, with p-values less than 0.001, suggesting that the variances are unequal across the visibility levels for both variables. consequently, welch’s anova was utilized to compare means and determine the differences in speed and lateral position across the three road marking visibility conditions. the robust tests of equality of means results for the straight road section revealed significant differences in speed and lateral position across the three levels of road marking visibility. specifically, the welch statistic for speed was 51.611 (p < 0.001), and for lateral position, it was 42.709 (p < 0.001). these findings indicate that variations in road marking visibility significantly influence vehicle speed and lateral positioning. consequently, post hoc analyses are necessary to determine the differences between the visibility levels. the games-howell post-hoc analysis for the straight road section revealed significant differences in both speed and lateral position across the visibility levels. for speed, vehicles in the poor road marking visibility condition drove significantly slower than those in the medium (md = −0.438 kmh−1, p < 0.001) and high road marking visibility conditions (md = −0.410 kmh−1, p < 0.001). there was no significant difference in speed between medium and high visibility (p = 0.832). in terms of lateral position, vehicles in the poor road marking visibility condition showed a significantly greater lateral position movement compared to both medium (md = 0.032m, p < 0.001) and high road marking visibility conditions (md = 0.018m, p < 0.001). additionally, vehicles in the medium road marking visibility condition also had a significantly greater lateral position movement than those in the high visibility condition (md = −0.014m, p < 0.001). 73 i. kučina, b. kosovec, f. jezidžić et al. acta polytechnica ctu proceedings dependent visibility (i) visibility (j) mean difference (i–j) std. error sig. variable speed poor medium -0.438* 0.049 < 0.000 [m s−1] high -0.410* 0.049 < 0.000 medium high 0.028 0.049 0.832 lateral poor medium 0.032* 0.003 < 0.000 position [m] high 0.018* 0.004 < 0.000 medium high -0.014* 0.004 < 0.000 * the mean difference is significant at the 0.05 level. table 4. multiple comparisons (games-howell post hoc tests) for the left curve. 4. discussion this study investigated how different levels of road marking visibility impact driver behaviour, mainly focusing on speed and lateral position, across three distinct road sections: left curve, right curve, and straight road. the findings reveal several notable trends in how road marking visibility influences driving performance, contributing to the broader understanding of road safety and driver behaviour in varying visibility conditions. 4.1. speed and road marking visibility across all road sections, the analysis revealed that speed was generally higher in the straight road section compared to the left and right curves, regardless of visibility level. in particular, the straight road saw the highest average speed under medium visibility conditions (27.037m s−1). interestingly, in the left curve section, while differences in speed between poor, medium, and high visibility were statistically significant, these differences were relatively small, ranging from 25.574m s−1 to 26.243m s−1. the gameshowell post hoc test confirmed that speeds in the poor and medium visibility conditions were significantly lower than in the high visibility condition (md = −0.593ms−1 and md = −0.669ms−1, respectively), suggesting that improved road marking visibility encourages drivers to maintain slightly higher speeds on curves. a similar finding regarding reduced driving speeds in low-visibility conditions and increased speeds in better visibility was observed in previous studies [33–35]. the speed remained consistent across all visibility levels on the right curve, with no statistically significant differences. this finding suggests that the geometric challenge of the right-directed curve may substantially influence driving speed more than road marking visibility, leading drivers to adopt more cautious speeds irrespective of visibility. for the straight road, visibility had a more pronounced impact on speed. vehicles in the poor road marking visibility conditions drove significantly slower than those in both the medium (md = −0.438ms−1) and high visibility conditions (md = −0.410ms−1). higher driving speeds in response to better road marking visibility are attributed to the driver’s increased sense of security. with greater detection distances, drivers can follow the road more clearly and prepare for upcoming situations. this is supported by research showing that increasing the retroreflectivity of road markings improves detection distance. for example, road markings with a retroreflection of 100mcd lx−1 m−2 are visible from 91.44 meters away [23, 36]. however, there was no significant difference in speed between medium and high visibility, indicating that further improvements may not significantly alter driving speed once visibility reaches a medium threshold. 4.2. lateral position and road marking visibility lateral position, which indicates how consistently drivers maintained their position within the lane, also showed significant variability based on road marking visibility. in the left curve section, road marking visibility had a significant effect, with vehicles in high visibility conditions deviating less from the centreline than those in poor and medium road marking visibility conditions (md = 0.017m and md = 0.025m, respectively). these results suggest that higher-quality road markings help drivers maintain a more consistent lane position on curves, potentially reducing the risk of lane departures. similarly, lateral position varied significantly on the right curve between visibility conditions. vehicles in poor visibility had a more significant lateral deviation than those in medium and high visibility conditions (md = 0.016m and md = 0.009m, respectively), indicating that poor road markings may cause drivers to struggle to maintain the lane position, particularly in curved sections. on the straight road, the pattern was similar, with significant differences in lateral position between poor, medium, and high visibility conditions. drivers in poor road marking visibility conditions exhibited the greatest lateral deviation, while those in high visibility conditions maintained the most consistent lane position. the medium visibility condition also showed 74 vol. 52/2025 the influence of road marking visibility on lateral vehicle. . . more significant lateral deviation than the high visibility condition, supporting the statement that improved road markings lead to better lateral control, even on straight road sections. previous studies also confirm the fact that the visibility of road markings impacts a vehicle’s lateral position. specifically, reduced visibility of these markings results in a more erratic lateral position, both on straight sections and in curves [37, 38]. however, since different road marking treatments (transverse lines, dragon teeth, herringbone patterns, optical circles, etc.) do affect a vehicle’s lateral position in curves [14–17]. further research is needed to explore how the visibility of various road markings influences a vehicle’s positioning within the traffic lane. 5. conclusions the visibility of road markings is a crucial factor in traffic safety, as over 90% of information is obtained through vision. research has shown that visibility impacts driver behavior. the results of the study can help experts and practitioners in understanding how markings affect drivers in night-time conditions and in making decisions related to the implementation of particular materials for markings, as well as for planning the maintenance activities. therefore, it is necessary to properly maintain markings and signs in order to maintain an adequate level of visibility and thus ensure of increasing road safety. road markings must be properly and consistently maintained to ensure adequate visibility. one limitation of the study is that it was conducted using a driving simulator, which cannot fully replicate the complexity of real-world driving. additionally, the number and age of participants may have influenced the results, presenting another limitation. future research should also consider various weather conditions, such as rain, fog, and snow, during nighttime, and include a larger pool of participants. the findings from the driving simulator should be compared with data from studies conducted under real road conditions, where sections of the road would feature markings with varying levels of visibility (high, medium, and low). acknowledgements references [1] d. babić, m. fiolić, d. babić, t. gates. road markings and their impact 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https://doi.org/10.1016/j.trf.2023.10.025 https://doi.org/10.1155/2020/7843743 https://doi.org/10.1186/s12544-020-00425-7 https://doi.org/10.1177/03611981221097095 acta polytechnica ctu proceedings 52:69–76, 2025 1 introduction 2 research methodology 3 results 3.1 descriptives 3.2 left curve 3.3 right curve 3.4 straight road 4 discussion 4.1 speed and road marking visibility 4.2 lateral position and road marking visibility 5 conclusions acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2024.46.0001 acta polytechnica ctu proceedings 46:1–8, 2024 © 2024 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague sustainable transitions in age-friendly communities in europe: a participatory approach lijun chena,∗, noor marjia, gülbahar emir isika, akshatha ravi kumara, vladimír kočía,b, david tichýa a czech technical university in prague, faculty of architecture, thákurova 9, 160 00 prague 6, czech republic b university of chemistry and technology, prague, technická 5, 160 00 praha 6 – dejvice, czech republic ∗ corresponding author: chenliju@fa.cvut.cz abstract. the ageing european population is driving the demand for age-friendly communities that support older individuals’ health and well-being. as communities navigate this transition, transition design emerges as a promising approach for addressing complex social and environmental challenges. this study explores the potential of transition design to create age-friendly communities in europe. drawing on examples from several european countries, this paper describes the key principles of transition design and how they can be applied to the development of age-friendly communities. this article highlights the importance of participatory processes in involving older people and other stakeholders in the design process, and discusses the role of technology in creating more accessible and inclusive environments. it also considers some of the challenges of implementing a transition design approach in a european context. in conclusion, this study asserts that transition design offers a promising approach for creating sustainable, inclusive, and responsive age-friendly communities. keywords: sustainable transitions, transition design, age-friendly communities, europe, participatory. 1. introduction europeans are currently experiencing longer lifespans than ever before, leading to significant changes in society’s age distribution. the population is rapidly ageing, resulting in a decline in the proportion of working-age individuals within the european union (eu), while the number of older adults continues to grow. this trend is expected to persist in the future as the post-war baby boomer generation reaches retirement age. these demographic shifts are likely to have extensive implications not only for individuals but also for governments, businesses, and civil society. key areas that will be affected include healthcare and social support systems, labour markets, public finances, and pension programs [1]. the elderly population, defined as individuals aged 65 years and above, is projected to experience a significant increase within the eu-27, rising from 90.5 million at the start of 2019 to 129.8 million by 2050. in contrast, the latest projections indicate a 13.5 % decrease in the number of individuals under 55 years old residing in eu-27 by 2050. although the total population of the eu-27 is expected to slightly increase from 446.8 million in early 2019 to a peak of 449.3 million during the period of 2026–2029, it will subsequently decline to 441.9 million by 2050, see figure 1. the phenomenon of population ageing has been observed in europe for several decades and is driven by factors such as historically low fertility rates, rising life expectancy, and, in some cases, migration patterns, particularly in eu member states with a net influx of retirees. demographic projections indicate that the pace of population ageing within the eu will accelerate in the coming decades, with a rapid expansion in the number and proportion of older individuals [1]. the increasing prevalence of population ageing has led to the emergence of the concept of “age-friendly cities”, which was initially introduced by the world health organization (who) in 2005. this term “agefriendly community (afc)” has been adopted in numerous government policy documents, following over ten years of dedicated efforts to promote agefriendly cities and communities. academic and policy researchers have shown significant interest in studying age-friendly communities in the past decade, indicating a growing focus on this field [2]. in europe, the who established the age-friendly environments in europe (afee) project, which aims to create agefriendly cities and communities in europe where people of all ages can reach their full health potential in a sustainable and equitable manner, with a special focus on the three modules european guide for agefriendly environments, tools for monitoring and evaluation, and templates for municipal action plans on age-friendly environments [3]. in recent years, scholars have generally agreed that creating and maintaining an age-friendly environment is a core component of a positive response to the challenges of population ageing [4]. ageing in place is commonly cited as a policy goal (as cited in [4–6]). governments and international organisations now agree that it makes economic and social sense to support older people to continue living in the community for as long as possible. however, enabling older people to age in place is a complex task. this requires comprehensive 1 https://doi.org/10.14311/app.2024.46.0001 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en l. chen, n. marji, g. e. isik et al. acta polytechnica ctu proceedings figure 1. population developments, by age class, eu-27, 2001–2050 (million inhabitants) [1]. planning and the provision of a wide range of support services in the community, as well as the removal of barriers that segregate older people and limit their activities [7]. advances in ageing-in-place policies have highlighted the need for “age-friendly communities” [4]. simultaneously, the emerging importance of environmental gerontology has contributed to the debate on age-friendly communities [8–10]. current research evidence suggests that well-being in later life is also closely related to the physical environment, which is an important factor in moderating the experience and opportunities of ageing [11, 12]. the multifaceted combination of factors has led to a growing urgency for research on the transition to older communities and the creation of sustainable age-friendly communities. in recent years, there has been an increasing number of cases in which the design discipline has been involved in solving transformative social problems, and attention has begun to be drawn to the broader creativity and potential of design beyond the aesthetics of art. transition design is a burgeoning and emerging design discipline that is attempting to shape itself into an integrated discipline capable of solving complex ongoing problems and transforming society with multiple knowledge and skills to act as a facilitator, accelerator and guide for transitions [13]. transition design focuses on solving wicked problems which are multiple, cyclical, and with no apparent relationship between cause and effect in the context of socio-technical change [14, 15], advocating the design of systemic and mediumand long-term strategies to solve complex and interrelated problems found at multiple levels of a system [16]. given the close connection between social and complex issues, the transition design process emphasises the need to design the transition of social behaviour to a sustainable future [17] and to understand the dynamics of complexity based on local and regional lifestyles and stakeholder visions to bring about fundamental changes at the systemic level. therefore, engaging in sustainable transitions in age-friendly communities through a transition design approach and emphasising the importance of stakeholder participation in the design decision-making process will provide a more systematic and sustainable transition strategy and a vision closer to a desired future for active ageing in communities. at the same time, digital technology is recognised as a support for age-friendly environments [18], particularly in terms of the use of friendly and sustainable design, the acceptance of technology, and the need for implementation and caregivers [19–22], will provide more possibilities for the realisation of design actions to accelerate and facilitate inclusive transitions in communities. this paper seeks to address the issue of community transitions in europe and better meet the complex challenges of building age-friendly communities in the future from a design perspective through transition design approaches and principles to systematically address the complex issues associated with population ageing. it adopts an interdisciplinary and participatory approach that integrates gerontology, design, and technology to provide a more comprehensive analytical perspective and solution. the paper will be beneficial in bridging the gap in exploring a more systematic and integrated strategy to address ageing in europe from a design perspective, taking into account demographic trends, design principles, and digital technologies. 2 vol. 46/2024 sustainable transitions in age-friendly communities in europe . . . 2. principles of transition design in age-friendly communities 2.1. reframing new visions vision is driven by the desire for a longer-term, humanist, and more sustainable way of life. transition design argues that, to effectively address complex societal challenges, it is necessary to develop compelling and imaginative visions of the future that can guide and inspire projects today. whether acknowledged or not, every stakeholder affected by a wicked problem has an implicit or explicit vision of the future associated with it [23]. stakeholders directly describe intractable problems and uncover potential connections between issues with the aim of exploring a shared vision of the future, transcending differences and paradigms, and seeking collaboration and consensus [17]. as a result, stakeholders can work together to create a compelling long-term, lifestyle-based vision of the future in which the problems have been solved and the needs of most stakeholders have been met. following on from this, it is possible to backcasting to the present to create a transition pathway to inform current tangible, consensus-based action [24]. design tools and methods can be used to facilitate the creation and refinement of these visions [25]. sandler suggests that vision is more than just painting an ideal picture of the future; it is a process of assessing the current situation, identifying problem areas, reaching a broad consensus on how to overcome existing problems and manage change, and having a shared vision allows communities to focus their diverse energies and avoid conflict between the present and the future (as cited in [26]). vision creates opportunities to critically explore alternative futures and new ways of being. this allows us to suspend disbelief, forget the present, and envision what the future might look like [27], (as cited in [28]). a collective visioning process based on the integration of multiple perspectives will be central to the creation of locally adapted sustainable communities that work together locally, regionally, and globally to meet the real needs of each individual [26]. 2.2. stakeholder participation in the design decision-making process the wicked problems and socio-technical system transitions addressed by transition design pose significant challenges, owing to their intricate social complexities. despite the social roots inherent in many wicked problems, conventional problem-solving methods often fail to consider them [17]. each social problem involves multiple parties that represent different aspects and demands of the society [29]. to effectively address wicked problems and implement transformative changes at a systemic level, it is crucial to acknowledge these social roots and actively involve all stakeholders who are impacted [30–32]. transition design draws upon social science approaches to gain an understanding of the social origins of wicked problems and places stakeholder concerns as well as co-design and collaboration at the core of the problem-solving process [17]. engaging stakeholders’ significance in problem-solving processes is widely recognized, particularly in domains such as policy and governance, environmental issues, backcasting, and conflict resolution [30, 33–36]. however, most traditional design-led approaches have yet to incorporate this important aspect [17]. transition design asserts that stakeholders, due to their deep understanding and involvement in the problem, can both hinder the resolution of wicked problems and play a vital role in developing effective interventions [37]. consequently, it is of utmost importance to actively engage stakeholders throughout the entire design process, considering them as “first point of view” design practitioners who have the power to shape both the design process itself and the future trajectory of design [29, 38, 39]. therefore, the involvement of older people and community-related stakeholders is crucial in the sustainable transition to age-friendly communities, which will largely influence the analysis of this wicked problem and the direction of future design, and help designers and policy makers to more systematically recognize the full picture of a community that is truly in the interest of its residents. 2.3. design intervention – prototyping and testing solutions and observations design intervention is an essential stage when using the transition design approach. according to irwin [17], a myriad of interventions at multiple levels within a larger spatio-temporal context (over a long period of time) to understand the present problem at higher and lower system levels and to look back to understand the root causes and evolution of the problem. knotty problems are linked and addressed through different system levels to understand the divergence and consequences of wicked problems, their causes and development, and where in the system interventions can work most effectively. interventions can take a variety of forms, particularly in policy-related design, and prototyping is a commonly applied research tool that can help policy makers better understand the root causes of public problems and their underlying interdependencies or “problem architectures”, as well as help give form to policies in practice to clarify, guide and illustrate them [40–42]. participatory prototyping, particularly when combined with digital technology, plays a vital role in the practice of transition design. this is a key method used to engage stakeholders, including community members, designers, policymakers, and other relevant actors, in the co-creation and testing of solutions 3 l. chen, n. marji, g. e. isik et al. acta polytechnica ctu proceedings for sustainable and inclusive transitions. design interventions using digital technology aligns with the principles of transition design by emphasizing usercenteredness, collaboration, testing and validation, learning and adaptation, and empowerment. this enables the co-creation of solutions that are contextually relevant, responsive to user needs, and capable of driving sustainable and inclusive transitions in agefriendly communities. transition design principles implemented through prototyping and testing solutions [42] provide the basis for achieving sustainable transitions in age-friendly communities. the active participation of stakeholders, including the elderly, throughout the design process should be emphasized [43]. prototyping plays a crucial role by creating concrete representations and models of proposed interventions, enabling stakeholders to visualize and engage with possible solutions [44]. through iterative prototyping, ideas can be refined and developed based on valuable user feedback and insights from testing [45]. this participatory prototyping process enables age-friendly solutions to be tailored to the specific needs and preferences of older people, promoting inclusiveness and enhancing the overall well-being of the community. by testing these solutions on a small scale, the feasibility, effectiveness, and feasibility of proposed interventions can be evaluated and provide valuable information for the development and implementation of larger age-friendly interventions. meanwhile, working in large, slow-moving systems will involve cycles of activity and intervention, balanced by intervals of observation and reflection to understand how the system responds to perturbations. transition designers must think in terms of “solutions” rather than “design solutions” over long periods of time and at multiple levels [17], and designing for system-level change requires a completely different mindset and posture [28]. therefore, system-level change requires a certain amount of time to observe the transformation and wait until the intervention has had some effect before intervening again to make adjustments. this process contradicts the 21st century expectation of quick, conclusive, profitable and quantifiable results and is therefore challenging [17]. 3. methods in this study, we utilised secondary sources, including peer-reviewed publications, journal articles, reliable sources of information, verifiable media reports, and medical expert opinions related to the ageing population. to achieve the objectives of this study, we conducted data collection and narrative assessment of case studies related to the intersection of transition design principles, digital technologies, and ageing. the following criteria were used to select the case studies: (1.) the case studies dealt with ageing in the population aged 65 years and over. (2.) the case studies were conducted in european countries. (3.) these involve the use of at least one digital technology relevant to the topic. (4.) it is linked to transition design principles and age-friendly living environments. (5.) this research has been conducted within the last ten years and has been documented in english. the selected case studies will be scrutinised against the above criteria to assess how digital-technologyled transition design principles have contributed to and impacted the transition to age-friendly cities and communities. 4. transition to age-friendly communities in europe 4.1. case studies of age-friendly community projects in europe in this section, we will explore case studies of agefriendly community projects in europe that incorporate a digital technology orientation. these projects demonstrate the application of transition design principles in fostering sustainable and inclusive communities for older adults. analysing these case studies provides insight into how digital technology-led transition design principles facilitate the transition to age-friendly communities. the three case studies below highlight innovative initiatives from european countries, and provide authentic citation references for further research and exploration. aal programme: a european funding initiative called the active assisted living (aal) program encourages the creation of cutting-edge information and communication technologies (ict) solutions for senior citizens. many aal program-funded initiatives use participatory prototyping to involve older individuals in the design and evaluation of digital technologies, which might range from social engagement platforms to health monitoring systems [46]. in the design and development process, the aal program uses a variety of prototyping methodologies for user integration, feedback gathering, and evaluation. the walt disney approach promotes creativity by producing new and realistic ideas from many perspectives, whereas brainwriting helps generate ideas from end users or stakeholders. through pairs of participants, co-discovery improves usability testing by fostering natural and vibrant feedback. by replicating system behaviour through a human operator, the wizard of oz technique enables usability testing with prototypes that are not yet completely functional. while paper prototyping provides testing of interface functionality and layout, cognitive walkthroughs offer analytical evaluations from a user’s perspective. the methodical selection of product features is aided by selection lists, and ute analysis identifies the fundamental 4 vol. 46/2024 sustainable transitions in age-friendly communities in europe . . . requirements based on user characteristics, task processes, and environmental factors. storyboards aid in identifying flaws or challenges with acceptance, while self-documentation and shadowing help acquire insights into user wants and behaviours. these methods work together to support iterative changes and usercentred design, which eventually result in better user experiences [47]. smarticipate: smarticipate is an ec-funded project that is spearheaded by the pilot cities of hamburg, rome, and london. smarticipate seeks to increase citizen participation in urban planning processes, especially that of senior citizens. they understand how crucial it is to involve a variety of groups, including older people, in creating age-friendly urban environments. in order to accomplish this, smarticipate uses a participatory prototyping methodology that enables senior citizens to actively contribute their thoughts, observations, and feedback. older folks can actively participate in the design and testing of digital platforms that are tailored to their needs and preferences. this will help communities grow. age-friendly amenities like accessibility, safety, and social inclusion are given top priority in urban development initiatives thanks to this participatory method. smarticipate works to develop dynamic, inclusive cities through citizen, stakeholder, and technological collaboration [48]. urbanage, helsinki: the city of helsinki, finland, has been at the forefront of developing agefriendly communities that embrace technological innovations. the age-friendly community project in helsinki aimed to enhance the quality of life for older adults through the integration of ai technologies. this project has received funding from the european union’s horizon 2020 research and innovation programme [49]. to promote independent living and provide personalized support, the project implemented ai-powered virtual assistants for older adults. these virtual assistants were designed to assist with daily tasks, such as medication reminders, appointment scheduling, and accessing relevant information. the ai algorithms behind the virtual assistants learned from individual preferences and patterns to provide tailored recommendations and reminders. recognizing the importance of accessible healthcare for older adults, the project incorporated telehealth and remote monitoring solutions powered by ai. through wearable devices and smart sensors, older individuals could have their vital signs and health data monitored remotely. the ai algorithms analyzed the data in real-time, detecting anomalies or trends that could indicate potential health issues. healthcare professionals could provide timely interventions and advice, reducing the need for frequent in-person visits. addressing social isolation among older adults, the project implemented ai-driven social engagement platforms. these platforms utilized ai algorithms to match individuals based on common interests, hobbies, and location. older adults could connect with like-minded peers, join virtual interest groups, and participate in online activities and events. the ai technology also facilitated personalized activity recommendations and encouraged social interactions, fostering a sense of community and reducing loneliness. to support aging in place, the project integrated ai-based smart home automation systems. these systems utilized ai algorithms to learn residents’ daily routines and preferences, adjusting lighting, temperature, and other environmental factors accordingly. the technology also incorporated voice commands and wearable devices, enabling older adults to control various aspects of their homes effortlessly. the age-friendly community project in helsinki conducted rigorous evaluations to assess the impact of ai technologies on the well-being of older adults. surveys, interviews, and user feedback were gathered to understand the effectiveness and usability of the implemented solutions. the project reported positive outcomes, including increased independence, improved access to healthcare, enhanced social connections, and a greater sense of security among older adults [50]. 4.2. the role of technology in creating accessible and inclusive environments through the utilization of technological advancements, communities can address the specific requirements of the elderly population and cultivate an environment that embraces diversity [51]. technological innovations have revolutionized the way society caters to the needs of older individuals, providing them with greater autonomy, comfort, and social integration. one significant role of technology in supporting the elderly is improving accessibility through innovative solutions such as automated smart home systems, assistive devices, and digital tools [52]. these advancements enable older individuals to navigate their living spaces with greater independence while ensuring their safety, comfort, and mobility. for example, smart home systems equipped with motion sensors and voice-activated controls can automate tasks like turning on lights or adjusting the thermostat, reducing physical exertion and potential hazards for seniors. in addition to enhancing physical accessibility, technology also fosters social integration among older adults. digital platforms, social media, and communication tools enable virtual connections, reducing social isolation and encouraging meaningful interactions [52]. older individuals can engage with family, friends, and communities online, participate in virtual support groups, and explore shared interests, creating a sense of belonging and social connectedness. furthermore, technology plays a crucial role in enhancing healthcare services for older adults. the advent of telemedicine allows healthcare providers to remotely diagnose, treat, and monitor patients, 5 l. chen, n. marji, g. e. isik et al. acta polytechnica ctu proceedings eliminating the need for frequent visits to medical facilities [52]. this approach proves especially beneficial for older individuals with mobility challenges or limited access to healthcare facilities. remote monitoring technologies and wearable devices enable continuous health tracking, providing real-time data on vital signs, activity levels, and medication adherence. this data empowers older adults to actively manage their health, detect early warning signs, and seek timely medical support. by incorporating technology into the principles of transition design, age-friendly communities can establish environments that prioritize accessibility, inclusivity, and well-being for older adults. transition design emphasizes collaborative efforts between designers, stakeholders, and older individuals themselves to create solutions that address the specific needs and aspirations of the aging population. by leveraging technological advancements, these communities can develop age-friendly infrastructure, services, and social systems that promote independence, social integration, and a high quality of life for older adults. 4.3. challenges we also acknowledge the challenges of funding, policy, and management that arise in the context of the transition design principles applied to age-friendly communities in europe. while these principles provide a foundation for sustainable and inclusive transitions, they are often confronted with various obstacles at institutional and organizational levels. understanding these challenges is crucial for implementing effective strategies and ensuring successful realization of agefriendly communities. funding challenges: • limited funding opportunities for age-friendly community projects. • fragmented funding landscape across sectors and governance levels. • establishing sustainable funding models for longterm support. policy challenges: • policy silos and lack of coordination between aging, health, urban planning, and social welfare policies. • discrepancies and inconsistencies between policies at different governance levels. • implementation gaps in translating policy intentions into practical actions. management challenges: • coordinating multiple stakeholders involved in agefriendly community projects. • capacity building to enhance knowledge and skills among stakeholders. • develop robust monitoring and evaluation mechanisms. the challenges of funding, policy, and management in europe pose some hurdles to the implementation of transition design principles in age-friendly communities. addressing these challenges requires collaborative efforts between designers, policymakers, funders, community leaders, and stakeholders. by acknowledging and actively working to overcome these obstacles, it becomes possible to create supportive funding mechanisms, coherent policy frameworks, and effective management strategies that foster sustainable transitions towards age-friendly communities in europe. 5. conclusions in summary, this study explores the potential of transition design in european engagement in the development of age-friendly communities and proposes transition design principles in age-friendly communities, particularly from a participatory perspective. it highlights the importance of reframing perceptions and visions of ageing environments, involving stakeholders in the design process, and implementing design interventions through prototyping and observation, which is a significant guide for future designers, technicians, community residents, and policy 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cc-by 4.0 licence published by the czech technical university in prague deep learning-based modeling and simulation of heat conduction ondřej šperl, jan sýkora∗ czech technical university in prague, faculty of civil engineering, department of mechanics, thákurova 2077/7, 160 00 prague 6 – dejvice, czech republic ∗ corresponding author: jan.sykora.1@fsv.cvut.cz abstract. the present study focuses on applying deep neural networks (dnns) to surrogate modeling of heat conduction problems. deep learning algorithms, valued for their ability to learn hierarchical data representations through multi-layered networks, excel at identifying complex patterns. in this work, the u-net architecture – widely recognized for its effectiveness in image segmentation – is adapted to model stationary heat transfer, providing a novel approach to a critical challenge in engineering and physics. specifically, we propose a deep learning-based surrogate model to predict stationary temperature fields in 2d rectangular domains representing two-phase heterogeneous materials. keywords: stationary heat transport, convolutional neural network, u-net architecture. 1. introduction surrogate modeling is a popular computational strategy employed to approximate complex, high-fidelity models with simpler, more efficient representations, while aiming to preserve the essential characteristics of the original forward model [1]. this approach is particularly advantageous in scenarios where the original model is computationally intensive, making it impractical for tasks such as optimization, uncertainty quantification, or real-time simulations. the primary objective of surrogate modeling is to achieve a balance between accuracy and computational efficiency, thereby facilitating more rapid and cost-effective analyses. surrogate modeling has found widespread application across various scientific and engineering disciplines, including aerospace [2], civil engineering [3], materials science [4], and chemistry [5]. there are various types of surrogate models, each characterized by distinct advantages and tailored to specific application domains. polynomial response surfaces [6], such as quadratic or cubic models, are simple and efficient but may struggle with highly nonlinear systems. radial basis functions [7] and kriging [8] (or gaussian processes) offer more flexibility in capturing complex behaviors but can be more computationally demanding. artificial neural networks [9] and support vector machines [10] are powerful tools for high-dimensional and nonlinear problems, though they require careful tuning and can be data-intensive. polynomial chaos expansions [11] are particularly useful for uncertainty quantification problems, as they provide a structured way to represent uncertainty in the model outputs. there has been extensive research on constructing surrogate models based on the deep learning approach. these can be generally classified into physics-based, see [12, 13], and data-driven surrogate models [14]. physics-based surrogate models rely on mathematical equations derived from physical laws to describe system behavior, requiring less data and offering greater interpretability but potentially struggling with complex, nonlinear phenomena. in contrast, data-driven surrogate models use statistical and machine learning techniques to learn from large datasets, providing rapid predictions and good generalization. here, we concentrate on the construction of datadriven surrogate models utilizing deep neural networks. each dnn contains multiple layers of interconnected nodes enabling one to learn complex data representations by identifying hierarchical patterns and features, as detailed in [14]. a typical dnn consists of three types of layers: (1.) an input layer that processes input data into the model, (2.) hidden layers that constitute the core of the dnn, applying weights to the inputs and passing them through the activation functions to generate outputs, and (3.) an output layer that computes the final probability scores of the desired outputs. a specialized form of dnn is the convolutional neural network (cnn), designed for image classification and visual data interpretation [15]. cnns operate on the principles of mathematical convolution, extracting salient features from images using learnable filters. they are primarily composed of convolutional layers and max pooling layers, which reduce the number of weights in the neural network. surrogate models founded on the principles of dnn and cnn architectures and constructed for heat transfer modeling represent the main idea of this contribution. our framework extends the work detailed in [16, 17], examining this aspect from various perspectives. the paper is structured as follows: the 79 https://doi.org/10.14311/app.2025.54.0079 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en ondřej šperl, jan sýkora acta polytechnica ctu proceedings methodology section 2 concisely outline the fundamentals of non-stationary heat transport and the subsequent discretization using the finite element method. the effectiveness of the proposed dnn-based surrogate model is evaluated in section 3 through its application to the stationary heat conduction problem with dirichlet boundary conditions. our conclusions will be summarized in the final section 4. 2. methodology initially, we consider a bounded body d ⊂ r3 (reference configuration) with a piecewise smooth boundary γ. specifically, the dirichlet, neumann, and robin boundary conditions are applied to γd ⊂ γ, γn ⊂ γ, and γr ⊂ γ, respectively, such that γ = γd ∪γn ∪γr. to analyze the temporal behavior of d, we consider a time interval [0, ts] ⊂ r+. the temperature evolution in d is given by the function θ : d × [0, ts] → r3, (1) where θ (in degrees celsius) represents the temperature. the heat transport is then governed by the transient heat balance equation with initial and boundary conditions.  cv(x)∂θ ∂t (x, t) − ∇ · (λ(x)∇θ(x, t)) = 0, x ∈ d, t ∈ (0, ts) , θ(x, t) = θd(x, t), x ∈ γd, t ∈ (0, ts) , λ(x) ∂θ ∂n (x, t) = qn (x, t), x ∈ γn , t ∈ (0, ts) , α(θ(x, t) − θ∞(x, t)) = λ(x) ∂θ ∂n (x, t), x ∈ γr, t ∈ (0, ts) , θ(x, 0) = θin(x), x ∈ d. (2) the equations involve several key parameters to define the thermal behavior of the system. the thermal conductivity is represented by λ(x) [w m−1 k−1], while the volumetric heat capacity, cv(x) [j m−3 k−1], is derived as the product of the volumetric mass density ρs(x) [kg m−3] and the specific heat capacity cp(x) [j kg−1 k−1], expressed as cv(x) = ρs(x)cp(x). the final simulation time is denoted by ts [s], and the ambient temperature is given by θ∞(x, t) [◦c]. furthermore, the heat transfer coefficient is α [w m−2 k−1], with θd(x, t) [◦c] and qn [w m−2] representing the prescribed temperature and the heat flux, respectively. we begin by restricting our analysis to the stationary case, which significantly simplifies data management in the neural network training process. this approach allows us to focus on a comprehensive and detailed investigation of the problem. the governing equation for the stationary heat problem is given by:  ∇ · (λ(x)∇θ(x)) = 0, x ∈ d, θ(x) = θd(x), x ∈ γd, λ(x) ∂θ ∂n (x) = qn (x), x ∈ γn , α(θ(x) − θ∞(x)) = λ(x) ∂θ ∂n (x), x ∈ γr. (3) the finite element method (fem) is a powerful numerical technique used to approximate solutions to complex partial differential equations. it works by transforming continuous functions into discrete vectors, making them easier to handle with numerical tools. one of the most common fem approaches is the galerkin method, which we use in this context to discretize the stationary heat conduction equation, see [18]. the method’s core idea is to minimize the error between the exact and approximate solutions by ensuring the residual – the difference between them – is orthogonal to the chosen finite-dimensional space. the galerkin method’s orthogonality condition for the residual guarantees that the approximation error is minimized, yielding a mathematically consistent solution, see [19]. 2.1. dnn-based surrogate models generally, the process of constructing a data-driven surrogate model involves several key steps. initially, a set of design points is sampled – usually from the multidimensional parameter space – often using techniques like latin hypercube sampling or sobol sequences to ensure a representative distribution. this set of design points is then evaluated using the highfidelity fem-based model to generate corresponding output/sampled data. the input-output (labelled) pairs are subsequently used to train the specific type of surrogate model. however, the generation of this synthetic dataset through high-fidelity fem simulations often represents the most computationally expensive bottleneck in the workflow, and the proper size of the dataset needs to be carefully considered. the surrogate models proposed here are based on the cnn architecture, with the u-net architecture chosen as the most suitable option for further development. introduced by ronneberger et al. [17], the u-net builds on the fully convolutional network framework proposed by long et al. [20]. its innovation lies in augmenting the standard contracting network with upsampling layers, replacing pooling operations to progressively increase output resolution. this design allows convolutional layers to produce precise outputs by leveraging contextual information. a key feature is the use of extensive feature channels during upsampling, which ensures effective context propagation to higher-resolution layers. the resulting architecture is nearly symmetrical, forming its characteristic “u” shape. the specific implementation for our numerical example is depicted in figure 1. 80 vol. 54/2025 deep learning-based modeling and simulation of heat conduction figure 1. this u-net diagram illustrates a convolutional network that downsamples the input in the encoder via pooling and convolutions, then upsamples it in the decoder using up-convolutions. skip connections bridge corresponding encoder and decoder layers, preserving spatial information lost during downsampling. 3. example we investigate a 2d rectangular domain representing two-phase heterogeneous materials under dirichlet boundary conditions, with the left and right boundaries held at 20 °c and 5 °c, respectively. the domain is meshed into quadrilateral finite elements, where each element corresponds to a pixel of a specific material phase. the domain includes two material phases with thermal conductivities of λ1 = 1.0 w m−1 k−1 and λ2 = 5.0 w m−1 k−1. heterogeneity is introduced by randomly distributing overlapping λ1 circles within the predominantly λ2 domain, as depicted in figure 2. figure 2. one particular sample from fem-based dataset depicting the morphology of a heterogeneous material. a well-constructed synthetic dataset is vital for training neural networks effectively, ensuring both efficient learning and high performance. we generated a dataset of 20 000 fem simulations with randomly created domain morphologies to capture critical features. given the simplicity of the problem, managing this large number of simulations was straightforward, allowing the network to model complex relationships and adapt to various material fields. since the data preprocessing is almost the most important part of machine learning, the fem-based dataset is further normalized to temperatures varying between 0 °c and 1 °c. this normalization offers several advantages for neural networks: it accelerates learning for gradient-based algorithms, helps mitigate the issue of local minima that can trap neural networks during optimization, enhances model performance by accommodating the scale sensitivity of activation functions, reduces overfitting by promoting better generalization and lower sensitivity to specific input feature values, and ensures numerical stability by preventing computational issues related to extremely large or small numbers. after normalization, the dataset is divided into training, validation, and testing subsets to ensure the development of a robust and generalizable deep learning model. this partitioning is essential for several reasons: it enables an unbiased assessment of model performance on unseen data (testing set), helps prevent overfitting by monitoring training progress, and identifies if the model is memorizing noise or outliers. in addition, the validation set is critical for optimizing hyperparameters – such as learning rate, batch size, or network architecture – and for comparing different models or algorithms. for these reasons, our dataset is split into three distinct parts: 15 000 samples for training, 2 500 for validation, and 2 500 for testing. during the training process, the unknown parameters of the dnn-based surrogate model are determined using an optimization solver based on the training dataset. simultaneously, the loss is evaluated on the validation dataset to monitor overfitting and underfitting issues and to estimate the appropriate number of training epochs. the loss values for both the training and validation datasets in our specific example are illustrated in figure 3. the close alignment between the two curves suggests that the model is neither underfitting nor overfitting, achieving a stable and reliable 81 ondřej šperl, jan sýkora acta polytechnica ctu proceedings figure 3. convergence graph. performance by the end of the training. the presented graph indicates that achieving a relatively well-trained model requires an epoch count exceeding 10. once, dnn-based surrogate model is trained, a qualitative assessment can be conducted using unseen data. prior to this assessment, figure 4 illustrates the temperature fields corresponding to a particular thermal conductivity field from the unseen dataset. figure 4a presents the ground truth temperature field obtained through the fem simulation, whereas figure 4b displays the temperature field predicted by the dnn-based surrogate model. upon visual comparison, the two results appear nearly identical. however, when the error values are visualized in figure 4c, differences ranging from −0.2◦c to 0.3◦c become evident. the evaluation across the entire unseen dataset is presented in figure 5, which displays the mean (figure 5a), standard deviation (figure 5b), and maximum value of errors (figure 5c). these graphs are derived from the dataset comprising the absolute temperature differences between the fembased simulations and the predictions generated by the dnn-based surrogate model. the graph of average absolute error (figure 5a) demonstrates that the proposed dnn-based surrogate model achieves a relatively high degree of accuracy. this observation is supported by the second graph (figure 5b), which illustrates the standard deviation of the absolute error and shows that it remains consistently within similar bounds. 4. conclusion this study demonstrates the efficiency of a dnnbased surrogate model in predicting stationary temperature fields within a heterogeneous 2d rectangular domain subjected to dirichlet boundary conditions. the model was trained using an extensive dataset of 15 000 fem simulations, with careful attention paid (a). fem-based temperature field. (b). temperature field predicted from the dnn-based surrogate model. (c). difference between fem-based and surrogate modelbased temperature fields. figure 4. comparison of temperature fields computed for one specific thermal field. to data preprocessing and normalization to enhance learning and generalization. the evaluation of the model on unseen data revealed a high degree of accuracy, with average absolute errors remaining consistently low and standard deviations within acceptable bounds. the successful application of dnn-based surrogate model highlights its potential for efficient and accurate prediction of complex physical phenomena, offering a promising alternative to computationally intensive fem simulations. 82 vol. 54/2025 deep learning-based modeling and simulation of heat conduction (a). average absolute error of temperature in [°c]. (b). standard deviation of the absolute error in [°c]. (c). maximum value of errors in [°c]. figure 5. qualitative assessment of dnn-based surrogate model conducted using unseen dataset. acknowledgements the authors are thankful for financial support from the student grant competition of ctu, project no. sgs23/152/ohk1/3t/11 and the czech science foundation, project no. 22-35755k. references [1] b. sudret, s. marelli, j. wiart. surrogate models for uncertainty quantification: an overview. in 2017 11th european conference on antennas and propagation (eucap), pp. 793–797. ieee, 2017. https://doi.org/10.23919/eucap.2017.7928679 [2] a. prabhakar, j. fisher, r. bhattacharya. polynomial chaos-based analysis of probabilistic uncertainty in hypersonic flight dynamics. journal of guidance, control, and dynamics 33(1):222–234, 2010. https://doi.org/10.2514/1.41551 [3] a. kučerová, j. sýkora, p. havlásek, et al. efficient probabilistic multi-fidelity calibration of a damageplastic model for confined concrete. computer methods in applied mechanics and engineering 412:116099, 2023. https://doi.org/10.1016/j.cma.2023.116099 [4] y. m. marzouk, h. n. najm. dimensionality reduction and polynomial chaos acceleration of bayesian inference in inverse problems. journal of computational physics 228(6):1862–1902, 2009. https://doi.org/10.1016/j.jcp.2008.11.024 [5] k. mcbride, k. sundmacher. overview of surrogate modeling in chemical process engineering. chemie ingenieur technik 91(3):228–239, 2019. https://doi.org/10.1002/cite.201800091 [6] a. i. khuri, s. mukhopadhyay. response surface methodology. wiley interdisciplinary reviews: computational statistics 2(2):128–149, 2010. https://doi.org/10.1002/wics.73 [7] m. d. buhmann. radial basis functions. acta numerica 9:1–38, 2000. https://doi.org/10.1017/s0962492900000015 [8] m. a. oliver, r. webster. kriging: a method of interpolation for geographical information systems. international journal of geographical information system 4(3):313–332, 1990. https://doi.org/10.1080/02693799008941549 [9] j. zou, y. han, s.-s. so. overview of artificial neural networks. in artificial neural networks: methods and applications, pp. 14–22. humana press, 2008. https://doi.org/10.1007/978-1-60327-101-1_2 [10] m. pecha, d. horák. analyzing l1-loss and l2-loss support vector machines implemented in permon toolbox. in aeta 2018-recent advances in electrical engineering and related sciences: theory and application, pp. 13–23. springer, 2020. https://doi.org/10.1007/978-3-030-14907-9_2 [11] e. janouchová, j. sýkora, a. kučerová. polynomial chaos in evaluating failure probability: a comparative study. applications of mathematics 63(6):713–737, 2018. https://doi.org/10.21136/am.2018.0335-17 [12] g. e. karniadakis, i. g. kevrekidis, l. lu, et al. physics-informed machine learning. nature reviews physics 3(6):422–440, 2021. https://doi.org/10.1038/s42254-021-00314-5 [13] m. raissi, p. perdikaris, g. e. karniadakis. physics-informed neural networks: a deep learning framework for solving forward and inverse problems involving nonlinear partial differential equations. journal of computational physics 378:686–707, 2019. https://doi.org/10.1016/j.jcp.2018.10.045 [14] f. chollet. deep learning with python. simon and schuster, 2021. 83 https://doi.org/10.23919/eucap.2017.7928679 https://doi.org/10.2514/1.41551 https://doi.org/10.1016/j.cma.2023.116099 https://doi.org/10.1016/j.jcp.2008.11.024 https://doi.org/10.1002/cite.201800091 https://doi.org/10.1002/wics.73 https://doi.org/10.1017/s0962492900000015 https://doi.org/10.1080/02693799008941549 https://doi.org/10.1007/978-1-60327-101-1_2 https://doi.org/10.1007/978-3-030-14907-9_2 https://doi.org/10.21136/am.2018.0335-17 https://doi.org/10.1038/s42254-021-00314-5 https://doi.org/10.1016/j.jcp.2018.10.045 ondřej šperl, jan sýkora acta polytechnica ctu proceedings [15] j. gu, z. wang, j. kuen, et al. recent advances in convolutional neural networks. pattern recognition 77:354–377, 2018. https://doi.org/10.1016/j.patcog.2017.10.013 [16] y. wang, j. zhou, q. ren, et al. 3-d steady heat conduction solver via deep learning. ieee journal on multiscale and multiphysics computational techniques 6:100–108, 2021. https://doi.org/10.1109/jmmct.2021.3106539 [17] o. ronneberger, p. fischer, t. brox. u-net: convolutional networks for biomedical image segmentation. in medical image computing and computerassisted intervention–miccai 2015: 18th international conference, munich, germany, october 5-9, 2015, proceedings, part iii 18, pp. 234–241. springer, 2015. https://doi.org/10.1007/978-3-319-24574-4_28 [18] j. havelka, j. sýkora. application of calderón’s inverse problem in civil engineering. applications of mathematics 63(6):687–712, 2018. https://doi.org/10.21136/am.2018.0323-17 [19] o. c. zienkiewicz, r. l. taylor. the finite element method set. elsevier, 2005. [20] e. shelhamer, j. long, t. darrell. fully convolutional networks for semantic segmentation. ieee transactions on pattern analysis and machine intelligence 39(4):640–651, 2016. https://doi.org/10.1109/tpami.2016.2572683 84 https://doi.org/10.1016/j.patcog.2017.10.013 https://doi.org/10.1109/jmmct.2021.3106539 https://doi.org/10.1007/978-3-319-24574-4_28 https://doi.org/10.21136/am.2018.0323-17 https://doi.org/10.1109/tpami.2016.2572683 acta polytechnica ctu proceedings 54:79–84, 2025 1 introduction 2 methodology 2.1 dnn-based surrogate models 3 example 4 conclusion acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2024.48.0046 acta polytechnica ctu proceedings 48:46–51, 2024 © 2024 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague tomographic and mechanical study of 3d printed porous polymer structure under compression gianpaolo pillona,b,∗, radosław grabiecb, jacek tarasiukb, sebastian wronskib, anne sophie bonneta a université de lorraine, laboratoire d’étude lem3 umr cnrs 7239, 7 rue félix savart, 57 070 metz, france b agh university, faculty of physics and condensed matter, władysława reymonta 19, 30-059 kraków, poland ∗ corresponding author: gianpaolo-pillon@orange.fr abstract. this study investigates the influence of open porosity rate and trabeculae diameter on the compressive behavior of structure printed by dlp technology. three types of structures have been studied : two periodic structures (cubic and octet) and one aperiodic structure (voronoi-based). each of these structures are printed with a porosity of 40, 60 and 80 % and also with two different trabeculae diameters: 0.45 mm and 0.66 mm. we investigated porosity by micro-tomograpy with imagej for the data treatment. we determined the curing time by making a compressive test for different lengths of uv cure. the first results of the study show that cubic structure has better young’s modulus and also better mechanical stress than octet and voronoi-based structures. keywords: porous structure, dlp technology, compression test, tomography, porosity, trabeculae. 1. introduction facing the earth’s aging population, there is more and more the need to perform bone to bone transplantation. however, this method has several problems: donor compatibility, infection, disease or in the worse case, transplantation rejection. to face this problem, surgeons have been using artificial bones made of titanium or stainless steel. these structures are presenting a too vigorous young’s modulus that lead to the stress shielding phenomenon around the artificial bone (reduction of the bone density around the prosthesis). porous structures are good candidates because they own a lower young’s modulus and their design allows muscle regrowth inside the prosthesis. the goal of this paper is to investigate the influence of porosity and trabeculae diameter (td) on the mechanical properties of 3d printed porous structures. for this, cubic, octet and voronoi structures are studied under compression, each structure will be studied under 3 porosity rates (40, 60 and 80 %) and each porosity is studied with two different trabeculae diameter (td) (0.45 mm and 0.66 mm). 2. materials and methods 2.1. materials the printer used in this investigation is a digital light processing (dlp) printer photon from the anycubic brand (shenzhen, guangdong, china). dlp technology uses the photo-sensibility of the resin to print the structure and the resin used is the basic resin from anycubic with a wavelength of 405 nm. the uv curing machine is an anycubic wash and cure 3. the volumic image is obtained by using a nanotom (waygate technologies, pennsylvania, usa). the data are treated using the software image j fiji (schindelin, arganda-carreras, cardona, longair, schmid, version 1.48, loci, university of wisconsin) to extract the porosity as well as the trabeculae thickness. the compressive test are carried out using a deben microcompressive machine with a cell capacity of 5 kn and a strain rate of 0.33 · 10−3 s−1 which corresponds to a compressive speed of 0.2 mm min−1. the results presented in this article are an average of 5 compressive tests to have better statistical data. 2.2. structure generation the structures studied in this paper are generated by coding [1]. three types of structures are studied: cubic and octet structures (figure 1) and the voronoi structure (figure 2). figure 1. cubic structure and its original cell [2]. these structures have a different original cell. the angle between the trabeculae is 90° for the cubic structure but only 60° for the octet structure. the third structure studied is a voronoi structure based on the voronoi diagram [1] (figure 3). its particularity is that the structure is randomly generated and each time the distribution is different. three random versions of voronoi structure have been printed 46 https://doi.org/10.14311/app.2024.48.0046 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 48/2024 ct and mechanical study of 3d printed polymer structure under compression figure 2. octet structure and its original cell. figure 3. voronoi diagram and the voronoi network extracted from it (in black and white). and tested to investigate the influence of randomly generated points on the mechanical behavior. 3. preliminary study the preliminary study was necessary in order to find the best printing parameters: the layer thickness (lt: height of the layer cure for one step), the normal exposure time (net, amount of time one layer is exposed to uv light) and internal parameters such as porosity and td. there is also a need to find the best curing time. different structures were printed presenting different internal parameters. porosity and printing defects such as layer non bonding or unwanted photo polymerization were measured by visual or by micro-tomography analysis. the figure 4 presents a few results. the figure 4.1 is a cubic structure printed with a lt of 0.8 mm and a net of 10 s. the problem encountered is that free space is filled with liquid resin even after a series of washing. as the sample is cured, the liquid resin is hardened. hence, the sample presents a porosity of 6 % instead of the 60 % desired. the figure 4.2 is a voronoi structure with the following printing parameters lt: 0.08 mm and a net: 10 s. the phenomenon is the non bonding of the different layers between them. such a problem is either because of a wide lt or a short net. when the printing plate is going up and down, the next cured layer isn’t radiated enough to stick to the previous one. the last defect is as shown on the figure 4.3 and it is the non printing of supports or corners which figure 4. printing defaults. figure 5. cracks and additional parts. ends up by affecting porosity measurements and even the mechanical behavior. the figures 5 are extracted from a tomography scan and two issues are appearing. the first one is the apparition of cracks after the printing and curing process. the cracks could be from the residual stress of the printing process [3]. the amount of residual stress originates from the degree of conversion of the resin and therefore from the amount of uv-light received by the resin. such cracks have an influence on the compressive behavior because they weaken the structure. the other issue observed is the supplementary part from the previous prints 5 (blue arrow). to avoid this, the resin is frequently filtered (at least twice a week). the printing parameters chosen are lt: 0.05 mm, net: 10 s. the internal parameters are 40, 60 and 80 % with a td of 0.45 mm or 0.66 mm. knowing the printing and intern parameters, the right amount of curing time can be determined by making a compressive test for different curing time (respectively 0, 3, 6, 9, 12, 15, 30 and 45 min). the more curing time there is, the more the cross linking density there is and the stronger the mechanical properties are. the chosen approach is to find the shortest curing time with best mechanical properties. figure 6 shows the influence and the evolution of the young’s modulus in function of the curing time. concerning the compressive behavior of the struc47 g. pillon, r. grabiec, j. tarasiuk et al. acta polytechnica ctu proceedings figure 6. influence of curing time on the young’s modulus. ture, there is clearly a limit of the young’s modulus in function of the curing time. it is also evident that all young’s modulus’ structures have a similar behavior. however, for octet and cubic structures, the samples cured within 9 min or more do not follow the “right” behavior. after investigations, the cubic samples and the octet (only 3 min and 6 min of curing time) were printed with a bottle of resin opened in 2020 while the rest was printed with a bottle opened in 2024. therefore there are aging consequences after the bottles are opened and similar results are observed by [4, 5] in her study. these curves are easily modeled by the equation e(t) = a ( 1 + e− t τ ) + b (1) with t the time variable (min) and a, b, τ positive constants values. the results are gathered in the table 1. narrow table a [mpa] b [mpa] τ [min] cubic 117.41 20.96 4.95 octet 90.75 11.80 7.01 voronoi 68.47 21.47 3.23 table 1. values of constants. it is well known that 3τ will give 95 % of the final value (the “final” young’s modulus) while 5τ will be for 99 % of the final value. facing the challenge with the difference of opening time of the resin, the curing time is set at 20 minutes for all structures. to conclude the preliminary study, the printing parameters are: lt of 0.05 mm and net of 10 s. the internal parameters are: 40, 60 and 80 %, with both td, one of 0.45 mm and one of 0.66 mm. the curing time is set at 20 min. 4. results and discussions for a better comprehension of the reader, the structures are named: o_40_66, the letter indicates the type of structure: v for voronoi, o for octet and c for cubic. the first number indicates the porosity and the second number the trabeculae thickness (0.45 mm or 0.66 mm). there is the version but only for voronoi structure (v1, v2 or v3). a result of the generation of voronoi structure, is the difference of distribution of the point and therefore, the difference of the trabeculae thickness among the structure. since the internal structure of voronoi version is not the same, the mechanical properties could be affected. this influence is visible on the result presented in figure 7. the graph is the evolution of the young’s modulus regarding the porosity and also the version of voronoi. figure 7. influence of the voronoi version on the young’s modulus. the young’s modulus are diverging following the version. this result could be explained by the difference of the trabeculae thickness investigated by a tomography scan. the results are show in figure 8. figure 8. trabeculae thickness for voronoi versions v2 and v3. v1 and v2 have a similar histogram of the trabeculae thickness. structures presenting the weakest young’s modulus are the one with the most spread trabeculae thickness (figure 8), v2 in this case and also v1 by extension. the version v3 that has the highest elastic modulus has also the less dispersed and longest trabeculae thickness. this could explained the difference of young’s modulus (figure 8, v_40_66_v3). the young’s modulus of the resin is obtained by doing an average of 5 compressive tests on a bulk cube of 10 mm side cured for 20 min. these results are an average of the voronoi versions. according to figure 9 the evolution of the porosity is similar for all structures. 48 vol. 48/2024 ct and mechanical study of 3d printed polymer structure under compression figure 9. influence of porosity for the 3 different structures. the evolution is pseudo linear. the voronoi sample and the cubic sample presenting both 60 % of porosity have a similar young’s modulus (respectively 112.57 mpa and 112.40 mpa). it is also interesting to see that these samples have the same young’s modulus as the octet sample with 40 % of porosity (129 mpa). the compressive curves of voronoi structure are shown in figure 10. figure 10. compressive curve of voronoi sample with different porosity. it is clear to see that the sample with 40 % of porosity has a better mechanical stress (18 mpa) compared to the other sample with a different porosity (6 mpa and 2 mpa). although these differences, the samples with 40 % and 60 % are experiencing general densification after the stress drop. similar results are observed for the compressive test of octet samples (figure 11). the sample with 40 % of porosity has the best mechanical behavior. however, the sample with 60 % experienced a floor collapsing phenomenon. this effect is well known and it has been observed by [6] on 3d printed triply periodic minimal surface structures. the sample with 80 % of porosity is also experiencing the floor collapsing. it is easy to see the periodic increase and decrease corresponding to each floor collapsing (figure 12). however, compared to the sample o_60_66 the sample c_60_66 has no direct densification after the stress drop. as said in the introduction the structure will be studied with different porosity and also different td. figure 11. compressive curve of octet sample with different porosity. figure 12. compressive curve of cubic sample with different porosity. to ensure that by changing the td, the porosity remains the same, it is necessary to add more trabecule [7]. figure 13 shows the compressive behavior of the voronoi sample with a porosity of 40 % and two td differences. these samples have the same young’s modulus (respectively 138.49 mpa for 0.45 mm and 146.6 mpa for 0.66 mm). therefore, the reduction of the diameter of the trabeculae and the increase of their number lead to similar results. the elastic parts are similar even though the sample with a td of 0.45 mm admits more standard deviation than the other td. there are two plastic stages, the first one starts around a strain of 0.7 and the second one starts at 0.2 of strain for both samples. this equivalence is also occurring for octet samples with 60 % of porosity (figure 14). figure 13. compressive curve of v_40_66_v1 and v_40_45_v1. 49 g. pillon, r. grabiec, j. tarasiuk et al. acta polytechnica ctu proceedings figure 14. compression curve for octet structure presenting a different td. it is obvious to see both octet structures have the same elastic behavior under compression load and the same young’s modulus. the only slight difference is that the sample presenting a td of 0.45 mm has a better mechanical stress than the sample with 0.66 mm (figure 14). while for the voronoi sample the behavior is similar even in the plastic domain, it is quite different for the octet sample presenting 60 % of porosity. indeed, they have a similar young’s modulus but their compressive behavior is totally different. the figure 14 is highlighting this. the elastic and the first plastic stage are similar with both samples presenting a td difference. the sample o_60_66 experiences a floor by floor while the sample o_60_45 has a direct general densification (figure 14). another goal of the study was to compare the different structures (cubic, octet and voronoi). the differences are shown below (figure 15). figure 15. compressive curve the different structures with 60 % of porosity. the structures show the same behavior for the elastic zone, however, the octet structure has a weaker mechanical stress compared to the other structure. then, both voronoi and octet structures experience general densification after their smooth stress drop while the cubic structure has a brutal stress drop and no general densification right away. for samples presenting 80 % of porosity (figure 16), their compressive behavior is totally different. the cubic sample is experiencing as said previously a floor by floor densification like the octet sample but it is less obvious. the voronoi sample has a general densification after the stress drop. figure 16. compressive curve the different structures with 80% of porosity. 5. conclusions this article shed the light on the influence of the porosity on the mechanical behavior of 3d printed porous structure. a preliminary study was necessary to obtain all parameters and the right curing time. the influence of the random distribution for voronoi samples was studied. octet and voronoi structures for specific porosity have the same elastic stage. the main differences are in the plastic response, cubic structure tends to not experience general densification as octet and voronoi do so. then, for the same structure presenting a trabeculae diameter different, the elastic stage is the same but the plastic stage is again totally different and specially for octet structure. acknowledgements this publication was supported by the université de lorraine through the research club orion mat & met. references [1] a. tamayol, k. w. wong, m. bahrami. effects of microstructure on flow properties of fibrous porous media at moderate reynolds number. physical review e 85(2):026318, 2012. https://doi.org/10.1103/physreve.85.026318 [2] r. grabiec, j. tarasiuk, s. wroński. desing of the algorithm, print and analysis of porous structures with modifiable parameters. acta polytechnica ctu proceedings 42:27–31, 2023. https://doi.org/10.14311/app.2023.42.0027 [3] d. xie, f. lv, y. yang, et al. a review on distortion and residual stress in additive manufacturing. chinese journal of mechanical engineering: additive manufacturing frontiers 1(3):100039, 2022. https://doi.org/10.1016/j.cjmeam.2022.100039 [4] v. drechslerová, j. falta, t. fíla, et al. effect of aging on mechanical properties of 3d printed samples using stereolitography. acta polytechnica ctu proceedings 42:1–5, 2023. https://doi.org/10.14311/app.2023.42.0001 [5] v. drechslerová, n. krčmářová, j. falta, t. fíla. ageing effects on the mechanical properties stability of 3d printed material under compression. acta polytechnica ctu proceedings 48:15–21, 2024. https://doi.org/10.14311/app.2024.48.0015 50 https://doi.org/10.1103/physreve.85.026318 https://doi.org/10.14311/app.2023.42.0027 https://doi.org/10.1016/j.cjmeam.2022.100039 https://doi.org/10.14311/app.2023.42.0001 https://doi.org/10.14311/app.2024.48.0015 vol. 48/2024 ct and mechanical study of 3d printed polymer structure under compression [6] m. saleh, s. anwar, a. m. al-ahmari, a. alfaify. compression performance and failure analysis of 3d-printed carbon fiber/pla composite tpms lattice structures. polymers 14(21):4595, 2022. https://doi.org/10.3390/polym14214595 [7] r. grabiec, j. tarasiuk, s. wroński, g. pillon. comparison of elastic properties of periodic and aperiodic porous structures produced by additive methods. acta polytechnica ctu proceedings 48:27–33, 2024. https://doi.org/10.14311/app.2024.48.002 51 https://doi.org/10.3390/polym14214595 https://doi.org/10.14311/app.2024.48.002 acta polytechnica ctu proceedings 48:46–51, 2024 1 introduction 2 materials and methods 2.1 materials 2.2 structure generation 3 preliminary study 4 results and discussions 5 conclusions acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2023.45.0027 acta polytechnica ctu proceedings 45:27–32, 2023 © 2023 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague providing stability to a highway cut in a landslide area with the use of diaphragm wall panels zdeněk fišera, miloslav kopeckýb,∗, martin ondrášikb a skanska a.s., skanska central europe, bohunická 133/50, 619 00 brno, czech republic b university of technology in bratislava, faculty of civil engineering, department of geotechnics, radlinského 11, 810 05 bratislava, slovakia ∗ corresponding author: miloslav.kopecky@stuba.sk abstract. the route of the d1 highway in the lietavská lúčka dubná skala section near žilina is taken forward by a 17 m deep cut in a section about 300 m long. the first proposed solution of the highway route was based on the assumption that it is a stable area. however, during the construction of the highway cut an old landslide area with a depth of shear surfaces of up to 25 m was identified right in the area of the cut. therefore, massive measures had to be designated to assure that the area above the highway and the access to the tunnel portal were not threatened by landslides. when addressing the security of the cut, an unusual solution was used anchored diaphragm wall panels of underground concrete walling oriented towards the slope. currently, this given solution is under construction and is controlled by inclinometers incorporated into selected diaphragm wall panels. keywords: anchors, landslide, concrete diaphragm walls, slope stability. 1. introduction currently in slovakia the most complicated sections of the d1 highway are under construction [1, 2]. route d1 in central slovakia near the regional city žilina is one of them (figure 1). the surface part of the d1 highway route in the section lietavská lúčka dubná skala (outside the višňové tunnel) is approximately 4.8 km long. the route d1 passes here through five landslide areas, and the largest landslide area in terms of surface area and depth of the sliding surface is at 3.795–4.80 km (figure 2). in the section 4.5–4.8 km in front of the western portal of the višňové tunnel, the highway route is led through a right-hand cut with a depth of up to 17 m and the aforementioned complicated landslide area. with regard to the relatively extensive and deep landslide area, even if it is currently inactive, it was reasonable to assume that the 17 m deep cut would endanger the stability of the slope of the cut itself. however, not only the indicated slope of the cut would be endangered, but also a significant part of the area above it. there are at present inactive sliding blocks of claystones and limestone at a distance up to 250 m from the edge of the cut. for the aforementioned reason, this had to be subject to massive measures to secure the slope, so there would be no movement that would progressively reactivate up the slope. this would endanger the part of the highway in front of the portal of the tunnel and thus also access to it. in order to give stability to the cut, anchored diaphragm wall panels oriented towards the slope were used. this is an unusual solution, because underground walls are primarily used as a securing and sealing element in deep excavations and foundations of buildings either for temporary excavation support or as part of the permanent structure [4–7]. currently, the construction of individual diaphragm wall panels is being carried out on the construction site, and the upper part of the wall is being dug out for the purpose of its multi-level anchoring. 2. engineering-geological and hydro-geological conditions the environment of the carpathian flysch generates complicated geological conditions. often, coarse permeable layers (sandstone, conglomerate which are aquifers) with high uniaxial strength alter with weathered soft and plastic claystone, and in some places it is in combination with tectonics [8]. such geological settings present positive conditions for development of slope movements in the form of landslides and block deformations [9, 10]. the landslide area through which the highway route passes has an extent of approximately 600 × 700 m. the development of the landslide area took place in the distant geological past. it was a tectonic uplift of the marginal parts of the basin and a subsidence of the basin itself, where our interest area on such a tectonic edge is located. vertical movements caused the division of the paleogene formation into several blocks. these began to slide towards the valley forced by gravity. subsequently the mesozoic limestone blocks lying in the higher parts of the slope also started to move. in such a way, two block fields have been formed with a ne-sw direction of movement and has created morphological terrace-like elevations on the slope (figure 2 and 3). 27 https://doi.org/10.14311/app.2023.45.0027 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en z. fišer, m. kopecký, m. ondrášik acta polytechnica ctu proceedings figure 1. the route of d1 highway in slovakia with indicated section of interest [3]. figure 2. landslide in front of the tunnel portal. the red line is the location of stabilization measures. figure 3. distinct morphological blocks of rocks under the power line (blue arrows) and a diagram of blocks with shear surfaces (red lines). 28 vol. 45/2023 landslide area highway cut stability figure 4. engineering-geological profile of the landslide model gt-1. figure 5. the design of the original solution from 2008 and its location in relation to the determined shear surfaces. evaluation of the investigation works (drilling and geophysical measurements) revealed that the landslide blocks of claystones can have a thickness of around 20–25 m (figure 4). important information was also provided by inclinometric measurements, which indicated a certain activity on the slopes. the movement of clay blocks before construction can be characterized as follows: • there is a slow movement at a depth of 15–25 m, which, however, does not happen continuously. • the movement vector is not always unambiguous. • the movement has the character of tilting and rotation of blocks along a broader plastic zone. • groundwater plays a significant factor in the landslide area. it acts, both through a pressure effect but mainly by worsening the shear parameters in the disturbed zone. the hydro-geological survey found several groundwater levels in the weathered and tectonically disturbed formation of claystone, siltstones and sandstone at depths of 1.6–18.5 m below the ground. the groundwater has an artesian character, which was manifested by its rise up to 10 m. in places where remediation works included horizontal drainage wells, the global groundwater level was reduced by up to 12 m. in the area outside of the reach of the horizontal drainage wells, a lowering of groundwater level was not recorded. the above, together with a seismic risk in the given area (the city žilina is located in an area where expected macroseismic intensity is up to 8th degree of msk-64 scale), makes it clear that the entire section in question is a relatively complicated area for the construction of the highway. 3. original solution for cut slope stability (period 2008–2019) the very first measures for the stability improvement of the highway cut slope by the retaining wall were from 2008 as from the documentation for the building permit. the retaining wall was designed as a gradual, storey structure consisting of a system of concrete ribs, anchored with rope rock anchors with a length of 21.0 m. at that time, landslides were not considered and the anchors would clearly be insufficient, as can be seen from figure 5. it was only during construction in 2014–2019 that the presence of a landslide area was verified by mapping (figure 2) and inclinometric measurements. in 29 z. fišer, m. kopecký, m. ondrášik acta polytechnica ctu proceedings figure 6. part of the diaphragm retaining wall consisting panel elements in the section of gt1 model. figure 7. excavation for the diaphragm wall panels with width of 2.8m. this period, however, the protection of the slope was not completed. the only conceptual proposal considered was for a possible protection in the form of anchored pile walls, when the piles reached a length of 30–35 m and the anchors about 50 m. it was only after a change of construction contractor in 2021 that permanent stability measures were actually proposed. 4. the new proposed remediation works for the permanent highway cut slope stability (year 2021–2023) during the works on new design of remediation measures of slope stability, various shapes and supporting elements of the entire structure were proposed: as the most optimising design of the structure, segments of diaphragm wall panels of 2800 × 800 mm rotated by 90 degrees were chosen, and where the longer side is perpendicular to the route of the diaphragm wall. the axial distance of these segments is figure 8. exposed panels with visible beams and anchors. 3500 mm. altogether, depending on the terrain configuration and the minimum necessary embedment under the shear surface, there were 81 designed diaphragm wall panels with a length of 25.0–32.0 m (figure 6). perpendicular rotation of the diaphragm wall panels is extremely important for the high rigidity of the structure and the transfer of bending moments. for the design of the structure, 3 characteristic geotechnical models gt1 (figure 4), gt2, gt3 were created. these models were based on long established and refined engineering geological work and monitoring. inclinometers for measuring deformations were built into selected diaphragm wall panels and dynamometers were installed into selected anchors. • piles dn 1200 mm in two rows with 4 rows of anchors, • piles dn 1500 in two rows alternatively anchored with 4 rows of pile, • diaphragm wall panels, etc. construction of the diaphragm wall panels was carried out by a combination of excavation by a cable grab to the level of 7.0 m and afterwards an excavation to the designated depth conducted by a hydrofraise 30 vol. 45/2023 landslide area highway cut stability figure 9. wall cross-section of geotechnical model gt1. figure 10. detail of anchor pass-through in panels and counter-vaults of sprayed concrete and hdw. (figure 7) was carried out. diaphragm wall panels are in the head part interconnected by a head monolithic beam 1500 mm high and 4500 mm wide. according to terrain configuration, 1–2 beams of dimensions 800 × 800 mm were also used, which additionally stabilize the individual diaphragm wall panels against possible rotation (figure 8). however, specifically there is the anchoring of each panel with 1–3 rows of permanent anchors 35.0–54.0 m long, with the number of ropes from 6 to 8 pieces (figure 9). during the installation, steel beams for the anchors were fixed throughout the monolithic diaphragm wall panels. furthermore, holes were drilled through the entire length of each panel 2.8 m and then 31 z. fišer, m. kopecký, m. ondrášik acta polytechnica ctu proceedings the anchor itself was installed (figure 10). steel reinforcement of each individual panel was optimized with an emphasis on maximum saving of steel while maintaining the necessary rigidity and eliminating bending moments. between the individual diaphragm wall panel counter-vaults were constructed from sprayed concrete of min. thickness 15 cm and 2 × reinforced with kari mesh, which are then strongly connected to the panel elements forming a compact element. in order to drain water from behind the countervaults, spatial geo-composite membranes were installed under the sprayed concrete. to permanently lower the groundwater level in the above the structure, 20 horizontal drainage wells will be constructed between the panels, each 100 m long. the second and the third monolithic beams will serve as a basis for placing the visual cladding of the entire wall made of hollow blocks. 5. conclusions the highway cut with a depth of 17 m was located in a landslide area and the depth of the shear surfaces was found only during construction at 25 m. an unusual technical solution was designed and implemented to ensure the stability of the cut. anchored diaphragm wall panels were used, which are mainly used to secure construction pits during the foundation of structures. these diaphragm wall panels, 25–32 m deep, were turned perpendicular to the slope and set securely in place with anchors 35–54.0 m long. the roots of the anchors extended beyond the assumed shear surface. massive measures were proposed due to the fact that the area above the highway and also the access to the tunnel portal were put at risk. inclinometric boreholes were built into the selected diaphragm wall panels and dynamometers were installed in the selected anchor heads. in this way, it will be possible to monitor possible deformations of the diaphragm wall panels and the surrounding environment and thus evaluate the effectiveness and safety of this implemented technical solution. acknowledgements this contribution was undertaken thanks to the support of projects vega 1/0745/21 by the research agency of the ministry of education, research and sport of the slovak republic. references [1] m. kopecký, m. ondrášik, m. brcek, v. jantak. risk analysis of the motorway routes in landslides area. iop conference series: materials science and engineering 471(4):042009, 2019. https://doi.org/10.1088/1757-899x/471/4/042009 [2] m. kopecký, m. brček, j. frankovská, et al. change of a highway route caused by a rockslide. acta polytechnica ctu proceedings 29:11–15, 2021. https://doi.org/10.14311/app.2020.29.0011 [3] wikimedia commons. file:motorway d1 sk map.svg — wikimedia commons, the free media repository, 2023. [11-09-2023], https: //commons.wikimedia.org/w/index.php?title=file: motorway_d1_sk_map.svg&oldid=815228958. [4] m. frydrych, g. kacprzak, p. nowak. hazard reduction in deep excavations execution. sustainability 14(2):868, 2022. https://doi.org/10.3390/su14020868 [5] a. james, b. kurian. diaphragm wall retaining system – a simplified model for design loads. australian journal of civil engineering 20(2):374–388, 2021. https://doi.org/10.1080/14488353.2021.1991549 [6] d. a. mironov, a. i. osokin, e. s. loseva, a. i. kuzhelev. specific features of diaphragm wall construction. iop conference series: materials science and engineering 775(1):012051, 2020. https://doi.org/10.1088/1757-899x/775/1/012051 [7] j. shu, j. sun, d. zhang, h. wei. sequential measurement and analysis of large underground retaining structures by diaphragm wall anchor for the spring area. advances in civil engineering 2019:5291420, 2019. https://doi.org/10.1155/2019/5291420 [8] m. lujan. role of décollement material with different rheological properties in the structure of the aljibe thrust imbricate (flysch trough, gibraltar arc): an analogue modelling approach. journal of structural geology 25(6):867–881, 2003. https://doi.org/10.1016/s0191-8141(02)00087-1 [9] m. súłovská, m. kopecký, j. panuška. stabilization of landslide area for foundations of highway bridge. in sgem international multidisciplinary scientific geoconference expo proceedings, pp. 293–300. stef92 technology, 2016. https://doi.org/10.5593/sgem2016/b13/s02.037 [10] m. kopecký, m. ondrášik, m. súłovská. investigation and monitoring of reactivated landslides in flysch. in proceedings of the 15th european conference on soil mechanics and geotechnical engineering, pp. 1365–1370. ios press, 2011. https://doi.org/10.3233/978-1-60750-801-4-1365 32 https://doi.org/10.1088/1757-899x/471/4/042009 https://doi.org/10.14311/app.2020.29.0011 https://commons.wikimedia.org/w/index.php?title=file:motorway_d1_sk_map.svg&oldid=815228958 https://commons.wikimedia.org/w/index.php?title=file:motorway_d1_sk_map.svg&oldid=815228958 https://commons.wikimedia.org/w/index.php?title=file:motorway_d1_sk_map.svg&oldid=815228958 https://doi.org/10.3390/su14020868 https://doi.org/10.1080/14488353.2021.1991549 https://doi.org/10.1088/1757-899x/775/1/012051 https://doi.org/10.1155/2019/5291420 https://doi.org/10.1016/s0191-8141(02)00087-1 https://doi.org/10.5593/sgem2016/b13/s02.037 https://doi.org/10.3233/978-1-60750-801-4-1365 acta polytechnica ctu proceedings 45:27–32, 2023 1 introduction 2 engineering-geological and hydro-geological conditions 3 original solution for cut slope stability (period 2008–2019) 4 the new proposed remediation works for the permanent highway cut slope stability (year 2021–2023) 5 conclusions acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2025.52.0043 acta polytechnica ctu proceedings 52:43–49, 2025 © 2025 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague commissioning of universal split hopkinson bar for dynamic compression and tension experiments lukáš kocian∗, jan falta, nela krčmářová, tomáš fíla, jan šleichrt czech technical university in prague, faculty of transportational sciences, na florenci 1324/25, 110 00 prague – nové město, czech republic ∗ corresponding author: kocialuk@cvut.cz abstract. split hopkinson bar is an experimental apparatus designed for acquiring material properties in dynamic regime. the apparatus is usually constructed as a single purpose experimental device for specific type of loading, most commonly either compression (split hopkinson pressure bar) or tension (split hopkinson tensile bar). in this paper, a simple and highly modular design capable of testing for multiple types of loading is introduced, which can be a great advantage to obtain material properties under different loading conditions. the device was successfully assembled, including its electrical installation and implementation of a pneumatic launching system and was fitted with multiple sensors. series of calibration procedures was conducted successfuly to analyze correct behaviour of the system. after calibration, the apparatus was subjected to multiple pilot tests with samples manufactured using 3d printing. the results were compared to those obtained using the quasi-static method. keywords: split hopkinson bar, dynamic tension and compression, experimental impact dynamics, additive manufacturing, high strain-rate testing. 1. introduction high strain-rate testing plays a vital role in determining the behavior and mechanical properties of materials subjected to dynamic loads occurring for instance in collisions of moving objects such as cars or trains and represents current trend for material testing under relevant conditions. knowing the material behaviour under dynamic loading has its use in numerical constitutive modeling that helps to predict outcomes of impact events such as traffic accidents or in defense industry. one of the most common testing devices used for dynamic experiments is the split hopkinson bar (shb) which is commonly designed as a single purpose apparatus with a design tailored for a certain application, e.g., compression or tensile testing. the device follows the principles pioneered in the works of kolsky [1], davies [2] and krafft [3]. it uses an impact of a striker accelerated commonly using a pneumatic launching system and the material properties are evaluated by capturing strain data of waves propagating through two bars with a sample placed between them. this classical arrangement of the experimental setup is well established for testing of conventional materials such metals and alloys particularly in uni-axial compression. however, many variants were derived from this classical arrangement over time to serve better in a specific application. specialized setups for testing of materials under specific dynamic conditions, e.g, shear [4], large deformation of sheet metals [5], interlaminar delamination [6], or large deformation of lattice structures [7] have been developed and successfully used. a specific topic is testing under direct impact conditions using instrumented apparatus [8– 10] that is particularly favourable in dynamic analysis of cellular solids [11–13]. while the testing apparatuses are usually designed on purpose for a specific application requiring modifications of the experimental setup, it is beneficial to design an apparatus with an extensive modularity, versatility and flexibility allowing for easy re-assembly and testing under different loading conditions. in this paper, a highly modular modification of standard design with a capability for both uni-axial types of loading (tension and compression), as well as direct impact testing and experiments with multiple bar arrangements is introduced. the device is aptly named universal split hopkinson bar (ushb) and its design, commissioning, calibration and pilot measurements are described and discussed in the paper. 2. materials and methods 2.1. universal split hopkinson bar ushb was designed as a highly modular system allowing for experiments in uni-axial compression and tension while it even supports more complex arrangements, for instance two parallel output bars. no excessive re-assembly is necessary to switch tests from compression to tension and vice versa. bearing supports are constructed in a way to provide easy alignment, installation and removal of the individual bars without affecting rest of the system. to keep the costs low and the design as simple as possible, the bar system has 43 https://doi.org/10.14311/app.2025.52.0043 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en l. kocian, j. falta, n. krčmářová et al. acta polytechnica ctu proceedings figure 1. a full schematic of ushb. two gas-gun barrels (one for tensile testing and other one for compression testing) while the compressed air launching system is mounted at a portable frame and can be easily transported and attached to either the barrels using a flexible high-pressure and highdiameter hydraulic hose. this solution also allows for using of the ushb as a direct impact hopkinson bar. a momentum trap is shared for both tension and compression and consists of another bar mounted to the frame using friction clamps. residual energy is absorbed by hydro-pneumatic damper mounted at the very end of the bar system. impact anvil attached to the input bar is used in case of tensile testing while this arrangement allows for use of small diameter pulse-shapers which is crucial to filter out unwanted high frequency oscillations from the input strain wave. the scheme of the device showing the main modular components and features of the system is shown in figure 1. in standard configuration, the device is made up of three important components: i) a striker which generates the initial strain wave, ii) an incident bar serving as an input of the strain wave into the specimen, and iii) transmitter bar which is used for measurements of output signals behind the specimen. the mechanical properties are then determined using data from strain/velocity sensors mounted at a certain points on the bar. the device is shown in figure 2. 2.2. assembly and installation besides the bar system, the device consists of multiple other sub-systems such as a pneumatic launching system, electro-installation and sensors. most of them are facilitated on the main frame which is 14m long, allowing long pulse (impact) duration and bigger samples to be tested. the bars and the barrels used for load inducing are fastened to the main frame using specific housing. the housing of the bars is specifically designed to only allow axial movement while eliminating oscillations and ensuring proper contact during impact. to absorb the impact, a damping system is used and fastened at the end of the main frame. for tensile testing, a force transmission anvil is used to transform the impact from the striker into figure 2. current state of the ushb. figure 3. the anvil used for tensile loading. tensile strain wave. the anvil can be seen in figure 3. the electro-installation represents a crucial interface and safety system providing control for some of the sensors and the pneumatic system. it is made up of electrical modules which either control the power flow, air flow in the launching system or simply indicate the current state of the device, and a switchboard containing all required circuitry. it was designed with 44 vol. 52/2025 commissioning of universal split hopkinson bar for dynamic . . . figure 4. the pneumatic launch system. multiple layers of safety protection such as key locks to prevent unauthorized use or state-defining lightemitting diodes (led). all of the circuitry necessary for the operation is contained within the switchboard. the pneumatic launching system allows the striker to be fired through air pressure and represents the most common method of striker launching first utilized by krafft [3]. it consists of a compressor, air pressure reservoir and several electronic or manual valves. the system was designed to fulfill requirements of safe and reliable operation needing multiple valves to be in correct positions to allow for a launch. the system can be observed in figure 4. 2.3. sensors in order to record the experimental data during the experiment, the device had to be fitted with multiple sensors. the recording system uses a data acquisition module (tranet epc, elsys ag, switzerland) and signal amplifiers. three types of sensors were used for instrumentation: i) foil strain gauges, ii) linear magnetic encoders and iii) optical gate. strain gauges are common element used to measure wave propagation in the bars. they detect change in resistivity which is directly proportional to strain, a fact used to capture wave shape generated during the striker’s impact [14]. for experiments introduced in this paper, single pair of strain gauges was mounted at both the incident and transmission bars using adhesives. a strain gauge mounted on the bar can be seen in figure 5 linear magnetic encoders were used to measure displacements and velocities of the certain bar’s crosssection. the sensors consist of two components, a thin magnetic strip which is attached to the bar and a readhead which is fastened to the frame using a 3d printed flange. the working principle of the sensor is modfigure 5. strain gauge mounted on the bar’s surface. figure 6. linear magnetic encoder mounted in the vicinity of a strain gauge. ulation of two orthogonal signals (sine and cosine) based on detection of polarity of the magnetic strip in the actual position [15]. single sensor is mounted at each bar in the vicinity of the strain gauges to measure displacement of the bar. the encoder is featured in figure 6. the optical gate is a device used to detect presence of an object in an area. it is made up of two parts: an emitter and a receiver. the emitter supplies a flow of infrared light to the receiver, when it is interrupted the gate generates an electrical signal. it is placed at the barrel of the ushb and works mainly as a trigger for data recording when the beam is interrupted by a striker. the system is used as a trigger for highspeed camera(s) as well. if mounted in two positions with known distance, the sensors can be used to determine approximate striker velocity. the optical gate mounted in front of the barrel is shown in figure 7. data from all the sensors are processed in the matlab software using a set of in-house developed functions for evaluation of hopkinson bar experiments. 45 l. kocian, j. falta, n. krčmářová et al. acta polytechnica ctu proceedings figure 7. the optical gate mounted in front of a gasgun barrel. the measured strain signals and velocities are used for time-domain decomposition of superposed waves to prolong the standard hopkinson bar experimental time window. the decomposition is performed using a method described in [12, 16]. 2.4. calibration two calibration procedures were carried out to evaluate correct operation of the device and acceptable precision of the sensors, namely: force calibration and void testing. as force is calculated using directly measured strain in the bars and known elastic properties of the bars, it is necessary to compare values indicated by strain gauges with the known force values. this is done by so called quasi-static force calibration comparing force indicated by strain gauges with a load-cell mounted in between the bars (as a specimen). the loading is performed by tightening a screw compressing both bars and the load-cell. in this paper, the calibration was done in a way to generate force from 0 to 5 kn by an increment of 500n and then loosening it by the same amount. void test is in principle a split hopkinson bar experiment without a specimen. it can be performed in two configurations – bars together and bars apart. its purpose is to compare wave amplitudes with theoretical values and check that the elastic propagation of waves is correct. void test can reveal bar misalignments, incorrect mounting of strain gauges, excessive friction in supports or contact problems. the device was tested with two striker lengths of 750mm and 1500mm out of which the longer striker represented the maximum length due to the bars’ lengths being 3000mm, twice the length of the striker bar. according to standard wave propagation theory, wavelength of the generated wave is twice as long as the striker length. if the pulse duration was longer, it would mean the input and reflected waves would interfere with each other. both figure 8. the specimen mounted in the experimental setup. strikers were accelerated with two launching pressures of 0.5 bar and 1 bar. six successful void tests were conducted and were also recorded by a high-speed camera. 2.5. sample testing an important part in determining validity of recorded data is repeatability of tests. due to this fact, a suitable reference sample had to be created. it was designed with a circular cross-section with a diameter of 7.5mm and 15mm in length. samples were produced with 3d printing using pla filament (prusament pla, prusa, czech republic) with 0.2mm layer height and 0.4mm nozzle diameter. as an universal testing device, the ushb supports most material types while the most limiting factor constraining the experimental performance is yield strength of the bars. this particular type of specimen was selected due to its shape and production simplicity and expected forces during the impact experiment within range of the force calibration and safely below yield strength of the bars. the sample was at first subjected to a quasi-static test performed using an universal electro-mechanical testing machine. this was done to obtain referential data that could be compared to data obtained during sample testing on ushb. two pilot tests were conducted in dynamic regime. the test conditions were the same with changing striker lengths and launching pressures as in case of the void tests. the tests were also captured using a high-speed camera with a framerate of 210 kfps. the specimen in the experimental setup can be viewed in figure 8. 3. results 3.1. force calibration results the goal of the force calibration was to prove correct function of strain gauges. the results were satisfactory with the first strain gauge almost following curves recorded by the load cell, with the second strain gauge 46 vol. 52/2025 commissioning of universal split hopkinson bar for dynamic . . . figure 9. force calibration results. figure 10. linearity between gauge and load cell data. however, showing some deviation, particularly at the beginning of the test. this was caused by thermal drift which is not inherently compensated in the dynamic experiments. the results could be improved by heating the gauges before calibration or adding an another strain gauge disconnected from the bars. despite this, the amplitude requirements were met with all of the sensors meeting at approx. 5 kn of force with errors for both gauges of 0.5%. the first strain gauge showed almost perfect linear behavior while the second strain gauge exhibited higher error of approximately 5% which is still acceptable for dynamic testing. the results can be viewed in figure 9 and figure 10. 3.2. void tests results void testing was meant to prove correct function of encoders and strain gauges. the main idea was to compare positional data captured by encoders, the result of which can be seen in figure 11, with strain gauge data to analyze wave propagation according to individual striker lengths as seen in figure 12. the result for velocity were as expected with initial amplitude of both bars reaching approximately 6.8 m s−1 with velocity of the output bar raised to approximately 13.5 m s−1 after separation of the bars. this reprefigure 11. measured velocity of both bars during a void test representing almost perfect elastic impact of two bars. figure 12. wave decomposition from sensors of the transmission bar. as the experiment design was not to allow wave superposition of the waves, the decomposed signals should be almost identical with the measured signal. sents almost perfect linear elastic impact of two identical bars. the velocity signals from encoders and strain signals from strain gauges were used for simple wave decomposition using elastic wave propagation theory in slender bars. if done properly with valid signals from sensors, the decomposed forward and backward propagating waves should overlap with the measured signal with respected sign (in compression forward positive and backward negative). the results for wave decomposition are shown in figure 12 and were also up to expectations with overlapping signals and the length of pulses being in accordance with the shorter striker and came out to around 350 µs. 3.3. sample test results the samples were subjected to a quasi-static test with loading velocity of 1mmmin−1 and dynamic tests with striker impact velocity reaching 9.44m s−1 and 6.55m s−1. the comparison of the results can be seen 47 l. kocian, j. falta, n. krčmářová et al. acta polytechnica ctu proceedings figure 13. force displacement diagrams of the first sample test – comparison of quasi-static regime and impact at 9.44 m s−1. figure 14. force displacement diagrams of the second sample test – comparison of quasi-static regime and impact at 6.55 m s−1. in figure 13 and 14. the samples were behaving differently under higher rate of deformation, showing approximately 30 % increase in ultimate strength, an earlier collapse and failure than during quasi-static deformation, which is expected for this type of material and testing. the specimen deforming during dynamic compression is shown in 15. 3.4. tensile testing tensile test is technically different from a compression experiment as the sample must be fastened to both bars and tensile waves have to propagate through it. pilot tensile experiments were conducted, however problems with insufficient strain waves quality were observed. insufficient rigidity of the supports of the tensile anvil was determined as a cause and will require strengthening of this component. due to stress being imposed asymmetrically during the force transfer through the anvil, a certain amount of distortion of the signal is to be expected. this might seem to render the anvil solution obsolete since in sph designs made especially for dynamic tensile testing the use of tubular impactors, as featured, for figure 15. a slideshow showing collapse of the specimen in compression – experiment duration 9ms. figure 16. waves recorded during a tensile void test and results of the wave decomposition. example, in [17], has become a norm due to its capability to stress the incidental bar symmetrically. this setup, however, is technologically far more complex and costly. therefore the anvil solution was used despite its disadvantages. still, the waves were successfully recorded and even the wave decomposition could be calculated from the data revealing parasitic bending of the input bar. the strain waves and results of the decomposition are shown in figure 16. 4. conclusions ushb with modular design allowing for easy rearrangement to different experimental variants was successfully assembled and commissioned. the device was instrumented with strain gauges and linear magnetic encoders for measurement of strain waves, displacements and velocities during the experiments. calibration procedures were successfully conducted in quasi-static regime to check correct mounting, align48 vol. 52/2025 commissioning of universal split hopkinson bar for dynamic . . . ment, sensitivity of strain gauges, and validity of the indicated force. dynamic calibration was performed by series of void tests to check correct function of strain gauges, linear encoders, experiment triggering, and suitability of signals for subsequent wave decomposition. all the tests produced satisfactory results. pilot testing in uni-axial compression were performed with cylindrical specimens produced by 3d printing while the results were compared with quasi-static compression in standard testing device. the evaluated results were comparable with the quasistatic data while positive strain rate sensitivity with significant weakening of the specimen and its rupture at lower strain were observed. although problems with parasitic bending of the impact anvil were present during the tests in tensile arrangement, the strain waves were validly measured and the setup was considered suitable for testing of the specimens in tension after (currently ongoing) strengthening of the anvil support. acknowledgements the research was supported by the czech science foundation (project junior star no. 22-18033m). references [1] h. kolsky. an investigation of the mechanical properties of materials at very high rates of strain. proceedings of the physical society section b 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lukić, et al. split hopkinson tension bar and universal testing machine for high-speed x-ray imaging of materials under tension. instruments 6(3):38, 2022. https://doi.org/10.3390/instruments6030038 [11] s. kim, d. g. kim, m. kim, et al. analyses of impact energy-absorbing performance of openand closed-cell al foams using modified split hopkinson pressure bar. journal of alloys and compounds 965:171349, 2023. https://doi.org/10.1016/j.jallcom.2023.171349 [12] t. fíla, p. koudelka, j. falta, et al. dynamic impact testing of cellular solids and lattice structures: application of two-sided direct impact hopkinson bar. international journal of impact engineering 148:103767, 2021. https://doi.org/10.1016/j.ijimpeng.2020.103767 [13] t. fíla, j. falta, p. koudelka, et al. flash x-ray radiography of internal damage in soft cellular materials during instrumented dynamic penetration. materials letters 354:135372, 2024. https://doi.org/10.1016/j.matlet.2023.135372 [14] k. hoffmann. an introduction to measurements using strain gages. hottinger baldwin messtechnik gmbh, darmstadt, 1989. [15] j. gao. dynamic penetration of cellular solids: experimental investigation using hopkinson bar and computed tomography. materials science and engineering: a 800:140096, 2021. https://doi.org/10.1016/j.msea.2020.140096 [16] l. leicht, t. fíla, p. máca, m. curbach. dynamic beam-end tests: investigation using split hopkinson bar. international journal of impact engineering 172:104417, 2023. https://doi.org/10.1016/j.ijimpeng.2022.104417 [17] r. gerlach, c. kettenbeil, n. petrinic. a new split hopkinson tensile bar design. international journal of impact engineering 50:63–67, 2012. https://doi.org/10.1016/j.ijimpeng.2012.08.004 49 https://doi.org/10.1088/0370-1301/62/11/302 https://doi.org/10.1098/rsta.1948.0001 https://doi.org/10.1098/rspa.1954.0009 https://doi.org/10.1007/978-3-319-71919-1_3 https://doi.org/10.1007/s11340-022-00936-5 https://doi.org/10.1016/j.compositesb.2023.110788 https://doi.org/10.1016/j.compositesb.2023.110788 https://doi.org/10.1016/j.matlet.2023.135823 https://doi.org/10.1007/s40870-015-0042-2 https://doi.org/10.1016/j.matlet.2023.135082 https://doi.org/10.3390/instruments6030038 https://doi.org/10.1016/j.jallcom.2023.171349 https://doi.org/10.1016/j.ijimpeng.2020.103767 https://doi.org/10.1016/j.matlet.2023.135372 https://doi.org/10.1016/j.msea.2020.140096 https://doi.org/10.1016/j.ijimpeng.2022.104417 https://doi.org/10.1016/j.ijimpeng.2012.08.004 acta polytechnica ctu proceedings 52:43–49, 2025 1 introduction 2 materials and methods 2.1 universal split hopkinson bar 2.2 assembly and installation 2.3 sensors 2.4 calibration 2.5 sample testing 3 results 3.1 force calibration results 3.2 void tests results 3.3 sample test results 3.4 tensile testing 4 conclusions acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2025.52.0116 acta polytechnica ctu proceedings 52:116–121, 2025 © 2025 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague the effect of roadside on speed perception martin rak∗, martin bilík brno university of technology, institute of forensic engineering, purkyňova 464/118, 612 00 brno, czech republic ∗ corresponding author: martin.rak@vut.cz abstract. the study examines how various roadside elements, such as flat areas, trees, slopes, and buildings, influence speed perception. data from multiple previous studies were combined and reanalyzed using newly established criteria to assess the proximity and nature of roadside features. the dataset includes 9 776 speed estimates, gathered from a wide range of road types and surroundings, allowing for an analysis of how these elements affect speed estimation accuracy. the findings suggest that flat areas consistently lead to overestimations of speed, likely due to the lack of physical cues that provide visual references. in contrast, trees and forested environments have a stabilizing effect, helping individuals to estimate speed more accurately. built environments, such as industrial zones and urban areas, tend to lead to significant overestimations of speed. these results highlight the importance of roadside environments in influencing speed perception, with natural settings like forests providing clearer visual cues that help drivers and observers better estimate speed. keywords: speed perception, roadside, perception accuracy, speed estimation. 1. introduction the perception of speed is influenced by various roadside elements. those can vary depending on the surrounding area and can include trees, barriers, slopes, buildings, etc. these visual stimuli can provide drivers feedback that affects their perception of how fast they are moving and understanding the role of roadside features in speed perception. dense roadside environments – trees, guardrails, and buildings – create a tunnel effect. this effect occurs when visual stimuli are placed close to the road, causing a sensation of increased speed as drivers pass them quickly. drivers often respond by reducing speed, even if it is not excessive. this reaction has been well documented in urban environments, where buildings or other vertical elements close to the road create a feeling of restriction that prompts slower driving. guardrails, similarly, add to this effect by reinforcing the sense of lane narrowing, which encourages cautious driving [1, 2]. in contrast, open landscapes, such as fields, sparsely vegetated areas, and wide, open spaces, lead to the opposite effect. these environments tend to provide fewer visual cues, which can cause drivers to underestimate their actual speed, leading to unintentional speeding. this effect is most commonly observed on highways and rural roads, with minimal visual feedback. studies show that drivers on roads bordered by wide fields or open landscapes tend to accelerate beyond safe limits [3]. roadside vegetation, such as shrubs and bushes, provides moderate visual feedback that can influence speed perception. while not as dramatic as large trees or buildings, roadside vegetation helps create a continuous visual flow that gently encourages drivers to maintain a slower speed, particularly in suburban or residential areas. research has found that roads lined with trees or vegetation reduce perceived speed and promote safer driving practices [1, 2]. road design plays a critical role in influencing speed perception through elements like slopes and guardrails. guardrails close to the road’s edge act as a visual boundary, narrowing the perceived road width and encouraging slower speeds. additionally, sloping terrain, particularly slopes that rise away from the road, can create an open feel that leads to speed underestimation. conversely, steep roadside slopes can enhance the tunnel effect by making the road feel more confined [1]. recent studies using dynamic measurement techniques reveal that drivers continuously adjust their speed based on the real-time visual information they receive from their surroundings. misjudgments, such as underestimating speed in open areas, can lead to excessive speeds in environments with fewer roadside cues. these studies emphasize the need for road designs that provide more consistent feedback, especially on highways and rural roads [2]. the effect of the roadside environment on speed perception is not purely visual; psychological mechanisms play a significant role. for example, when drivers feel that a road is narrow or enclosed due to roadside elements, they perceive a higher level of risk, leading to reduced speed. on the other hand, open roads with fewer obstacles are perceived as less risky, leading to faster driving speeds, often beyond the posted limits [3]. understanding these psychological responses is essential for designing roadways that naturally encourage safe driving behavior. 116 https://doi.org/10.14311/app.2025.52.0116 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 52/2025 the effect of roadside on speed perception roads that transition from dense, visually stimulating environments (such as urban streets) to open, less stimulating environments (such as rural highways) can cause fluctuations in speed perception. drivers moving from a visually confined area to an open space may suddenly feel that they are moving too slowly and accelerate, sometimes dangerously. this variability in speed perception highlights the need for carefully managed transition zones to maintain consistent speeds across different driving environments [1]. according to hurwitz et al. [4], drivers tend to underestimate their speed, particularly in environments where visual cues, such as horizontal curves and surrounding landscape, play a role. these findings suggest that elements like road curvature, visual landmarks, and the type of environment (e.g., urban vs. rural) can significantly alter drivers’ perception of speed. the study highlights that drivers’ accuracy in estimating their speed decreases in more complex visual environments. this supports the idea that roadside features and the immediate surroundings impact speed perception. this observation aligns well with your focus on evaluating the influence of proximity to roadside elements and the broader environmental context on speed perception. additionally, research on speed perception has consistently shown that environmental factors, such as road geometry and surrounding visual cues, play a crucial role in influencing how drivers perceive their speed [5]. studies, including those focusing on freeway curves, demonstrate that drivers often underestimate their speed when navigating certain road configurations, particularly curves with smaller radii or complex curve combinations. for example, research on freeway curves shows that drivers are most prone to underestimating their speed at a curve’s tangent-to-spiral (ts) point, especially when the curve radius is tight. this highlights the importance of understanding how road characteristics and environmental visual feedback affect speed perception. such findings are directly relevant to this study’s focus on how roadside environments – including proximity to the road and broader landscape features – affect drivers’ speed estimation. by exploring the influence of roadside elements like trees, barriers, or open fields, this research aims to extend the understanding of how visual factors surrounding the road contribute to misestimations of speed, much like how curve geometry has been shown to influence drivers’ perception in freeway settings. previous theses on speed perception have primarily focused on demographic factors, such as the age, gender, and driving experience of respondents, or on the estimated vehicles’ characteristics. these studies aimed to understand how personal attributes or vehicle types influenced speed estimation [6–11]. however, this study diverges by focusing solely on the impact of roadside elements on speed perception, without considering other factors like respondent demographics or vehicle characteristics. data from earlier theses, where still available, were repurposed to create a new dataset for this research. this dataset has been reanalyzed with a specific focus on roadside environments, such as the proximity of trees, barriers, and open spaces, to understand better how these elements influence drivers’ ability to perceive speed accurately. by eliminating the influence of demographic or vehicle-specific variables, this study aims to isolate the effects of roadside features on speed perception. 2. dataset and method this study builds upon data previously collected from several completed master’s theses and one related study, repurposing the existing datasets to create a new, comprehensive dataset. while the original focus of these works was not specifically on the impact of roadside environments on speed perception, this new analysis shifts the attention to how different surroundings might influence drivers’ and witnesses’ ability to estimate vehicle speed accurately. the newly formed dataset incorporates observations and speed estimates from various experiments but applies fresh evaluation criteria centered around the proximity and characteristics of the roadside environment. these criteria include elements such as the type of surrounding landscape (e.g., forest, urban, rural), distance of roadside features from the road (such as trees, barriers, or open fields), and visibility conditions. by reorganizing and analyzing the data through this new lens, the study aims to explore how these environmental factors affect speed perception in ways not considered in the original analyses. this approach provides a novel understanding of roadside environments’ role in speed estimation, contributing to more effective traffic safety measures and road design. 2.1. original datasets the master thesis focuses on the perception of vehicle speed from the pedestrian’s perspective [8], with an experiment designed to gather speed estimation data under various conditions. the measurement setup involved a sample of 13 respondents (7 men and 6 women) who participated in two days of measurements. the experiment was conducted at multiple locations, with vehicles traveling at specified speeds (e.g., 30 km h−1, 50 km h−1, and 90 km h−1). the participants observed these vehicles and provided estimates of their speeds. the total dataset comprised 2 090 speed estimates, with 895 estimates from men and 1 195 from women. these estimates were later analyzed to understand the factors that influenced the accuracy of speed perception, such as vehicle type, gender, and the roadside environment. 117 martin rak, martin bilík acta polytechnica ctu proceedings the experiment in barbora formanová’s thesis [9] took place on a 1.9 km-long road, chosen for its low traffic intensity and suitability for testing speeds at 30 km h−1, 50 km h−1, and 70 km h−1. the road surface was asphalt but showed signs of wear, including cracks and potholes, providing a realistic driving environment. the surroundings of the test road were primarily agricultural fields, offering minimal distractions, which allowed drivers to focus on estimating vehicle speed. 600-speed estimates were collected from 10 drivers across 10 test drives, with drivers estimating speeds in both their vehicles and borrowed ones. this setup allowed for investigating how drivers perceive vehicle speed in real-world conditions. the thesis by faruk pršeš [10] focuses on passengers’ perceptions of oncoming vehicle speed in moving vehicles. the objective was to evaluate how accurately passengers can estimate the speed of an oncoming vehicle under different conditions, considering factors such as age, gender, driving experience, and vehicle type. the experiment was conducted over two days with 23 participants (15 men and 8 women), including drivers and non-drivers. the participants were seated in various vehicles and tasked with estimating the speed of oncoming vehicles under real-world traffic conditions. the study was conducted on a 2 km road with low traffic intensity in a city. the experiment assessed speeds of 50 km h−1, 70 km h−1, and 90 km h−1. the experiment gathered 3 240 speed estimates – 1 620 estimates for the speed of oncoming vehicles and 1 620 estimates for the speed of the vehicle in which the participants were seated. the results were analyzed to assess the accuracy of speed estimations, focusing on how these estimations were influenced by the participant’s age, gender, driving experience, and position in the vehicle. nela budinová’s thesis [11] examines passengers’ perception of vehicle speed, focusing on the influence of driving experience and distractions. the experiment was conducted on a 12 km route, utilizing different road types such as urban, rural, and highway settings. participants with varying levels of driving experience were asked to estimate the vehicle’s speed under two conditions: one where they were fully focused on the road and another where they were distracted by solving sudoku puzzles. the experiment aimed to assess how attention and distraction impact speed perception across different types of vehicles. 2 119 speed estimates were collected, with 2 174 actual speed measurements recorded. a slight discrepancy in the number of estimates was due to signal transmission issues encountered during the experiment. the results provided valuable insights into how distractions and driving experience affect passengers’ ability to accurately estimate vehicle speed in real-world conditions. an additional study [12] examined how accurately individuals can estimate vehicle speeds during nighttime conditions. the study involved 10 respondents (5 men and 5 women) who were tasked with estimating the speed of passing vehicles. the experiment was conducted over two days, with respondents positioned 15 meters from the road. a total of 1 593 speed estimates were collected during the experiment. the results showed that women generally overestimated speeds, while men were likelier to underestimate them. the observer’s position also influenced the accuracy, with estimates being more accurate when respondents observed vehicles from a 30-degree angle compared to a 90-degree angle. other factors, such as visual and auditory abilities, also contributed to variations in speed estimates. overall, the new dataset used in this study consists of 9 776 estimates. 2.2. new criteria this study builds on data from previously conducted theses and one related study, creating a new dataset to explore the impact of roadside environments on speed perception. although the original research was not focused on this specific area, the existing data has been reanalyzed using new criteria designed to assess how different surroundings influence the accuracy of speed estimates. the new criteria are based on the proximity of roadside features, which were divided into several categories. these include the area directly adjacent to the road, defined as within 1 meter of the road’s edge (“surroundings at 1 m”), and the immediate surroundings, which extend up to 5 meters from the road (“surroundings at 5 m”). a broader category considers the wider surroundings beyond 5 meters and approximately up to 15 meters (“surroundings beyond 5 m”). in addition to these proximity-based factors, the study also evaluates the overall location of the landscape, such as whether the road is situated in an urban area, rural environment, or near forested regions (“landscape”). by applying these criteria to the dataset, the study aims to analyze how the presence of nearby objects, open spaces, and various types of development influence drivers’ and witnesses’ ability to estimate vehicle speeds accurately. table 1 shows individual categories that were identified within the defined variables. the mann-whitney u test was used to evaluate the differences between two independent groups. the pvalue (also referred to as significance or simply p) was calculated for each factor, which is a numerical value used in statistical hypothesis testing. a significance level (95 %) was established, after which the p-value was computed and compared with the significance level. if the calculated p-value was less than 0.05, the null hypothesis (that the examined factor affects speed estimates) was considered credible. 118 vol. 52/2025 the effect of roadside on speed perception variable category surroundings at 1 m flat area ditch guardrails uphill slope downhill slope surroundings at 5 m flat area trees fence buildings uphill slope downhill slope surroundings beyond 5 m flat area trees buildings landscape urban area industrial area forest field table 1. variables and their categories. figure 1. deviation distribution by “surroundings at 1 m”. 3. results in the variable “surroundings at 1 m”, significant differences were found between several categories. “flat area” has a positive effect on the deviation, meaning the estimated speed is higher than the actual speed, while “ditch” also has a positive but smaller effect. a significant difference was found between “flat area” and “ditch” (p-value < 0.05), with the flat area causing a larger deviation. similarly, “flat area” increases the deviation compared to “guardrails” (p-value < 0.05), where guardrails have a negative effect on the deviation, meaning the actual speed is closer to the estimate. in the comparison between “flat area” and “uphill slope” (p-value < 0.05), the flat area has a positive effect on the deviation. in contrast, the uphill slope has the opposite effect – reducing the deviation. however, the difference between “flat area” and “downhill slope” (p-value = 0.395) was not statistically significant. additionally, the difference between “ditch” and “guardrails” (p-value = 0.206) was also not significant. figure 1 shows the deviation distribution of each category. in the variable “surroundings at 5 m”, significant differences were found between categories. “flat area” increases the deviation compared to “trees” (p-value < 0.05), which have a stabilizing effect and reduce the deviation. “flat area” also shows a higher deviation compared to “fence” (p-value < 0.05), where the fence has a slightly negative effect and reduces the overestimation of the estimated speed. a significant difference is also found between “flat area” and “buildings” (p-value < 0.05), where both categories figure 2. deviation distribution by “surroundings at 5 m”. increase the deviation, but buildings have a stronger effect. however, some “surroundings at 5 m” comparisons were insignificant. the differences between “flat area” and “downhill slope” (p-value = 0.605), “trees” vs. “downhill slope” (p-value = 0.222), and “trees” vs. “uphill slope” (p-value = 0.187) were all not statistically significant. figure 2 shows the deviation distribution of each category. in the variable “surroundings beyond 5 m”, significant differences were found between categories. “flat area” increases the deviation compared to “trees” (pvalue < 0.05), which have a stabilizing effect and reduce the deviation. buildings increase the deviation more than “trees” (p-value < 0.05), where trees stabilize the difference between actual and estimated speed. however, the comparison between “flat area” and “buildings” (p-value = 0.224) was not statistically significant. figure 3 shows the deviation distribution of each category. in the variable “landscape”, significant differences were found between categories. “industrial zone” greatly increases the deviation compared to “forest” (p-value < 0.05), where the forest has a stabilizing effect and reduces the deviation. field increases the deviation, but less so than the industrial zone, while “forest” again reduces the deviation (p-value < 0.05). in the comparison between “urban area” vs. “forest” (p-value < 0.05), the urban area increases the deviation, but less than the industrial zone. in contrast, the forest has a negative effect and stabilizes the dif119 martin rak, martin bilík acta polytechnica ctu proceedings figure 3. deviation distribution by “surroundings beyond 5m”. figure 4. deviation distribution by “landscape”. ference between actual and estimated speed. figure 4 shows the deviation distribution of each category. 4. discussion analyzing the surroundings at various distances (1 m, 5 m, beyond 5 m) and landscape provides valuable insights into how different environments influence human perception of speed. one clear pattern is that flat areas tend to lead to overestimation of speed. this suggests that in environments characterized by flat terrain, people are likely to think vehicles are moving faster than they actually are. the lack of visual obstacles or cues, such as changes in elevation, might contribute to this overestimation, as it may create a sense of openness where objects appear to move faster. on the other hand, trees consistently show a stabilizing effect on speed perception. in areas with trees, people tend to estimate speed more accurately, possibly due to the visual references trees provide or the environmental factors like shading and wind buffering that help moderate their perceptions. this suggests that in more structured environments where natural elements are present, people have a better sense of the actual speed. in contrast, built environments like buildings and particularly industrial zones significantly increase the overestimation of speed. these areas are likely more complex visually, with large structures and potential obstacles that distort people’s ability to accurately gauge how fast vehicles are moving. the confined spaces and obstructions in these areas could create a false sense of speed, making vehicles seem faster than they are. moreover, forests and urban areas generally reduce the deviation between perceived and actual speeds, with forests showing the most substantial stabilizing effect. this finding indicates that natural settings with consistent environmental features allow for more accurate speed assessments, perhaps because the surroundings provide regular, predictable visual cues that help people judge motion. there were also instances where the differences between certain categories, such as flat area vs downhill slope or flat area vs buildings, were not statistically significant. this indicates that while the environment often affects speed perception, it does not do so uniformly across all situations and settings. 5. conclusions this study shows that the roadside strongly influences human perception of speed. natural settings, such as trees and forests, help people estimate speed more accurately, while flat areas and built environments, like industrial zones and buildings, often lead to overestimations. the lack of visual cues or the complexity of these environments distorts how fast vehicles are perceived. future research should explore why people overestimate speed in these environments, considering visual complexity, environmental familiarity, and weather conditions. all these factors – demographics of the observers, traffic intensity, road surface, lighting conditions, and types of intersections – must be measured simultaneously to capture their combined effect on speed perception. we can only fully understand how they interact and shape perception by studying them together. understanding these factors could lead to better insights into how humans perceive speed, with practical applications in urban planning and road safety. acknowledgements this paper was written within the project of the specific university research entitled “database of vehicle speed estimates in urban areas” (no. úsi-j-23-8652) and was carried out at the institute of forensic engineering of brno university of technology. references [1] y. qin, y. chen, k. lin. quantifying the effects of visual road information on drivers’ speed choices to promote self-explaining roads. international journal of environmental research and public health 17(7):2437, 2020. https://doi.org/10.3390/ijerph17072437 120 https://doi.org/10.3390/ijerph17072437 vol. 52/2025 the effect of roadside on speed perception [2] c. wu, d. yu, a. doherty, et al. an investigation of perceived vehicle speed from a driver’s perspective. plos one 12(10):e0185347, 2017. https://doi.org/10.1371/journal.pone.0185347 [3] d. schaffner, m. van eggermond, n. studer. selfexplaining roads: effects of road design on perception and speed choice. in international conference on traffic and transport psychology icttp. 2022. [2024-10-20]. https://irf.fhnw.ch/handle/11654/34086 [4] d. s. hurwitz, m. a. knodler, d. m. dulaski. speed perception fidelity in a driving simulator environment. in proceedings of the driving simulator conference north america, pp. 343–352. 2005. [5] j. xu, h. li, x. zhang, et al. the influence of freeway curve characteristics on drivers’ speed perception accuracy. plos one 17(5):e0267250, 2022. https://doi.org/10.1371/journal.pone.0267250 [6] l. kamasová. vnímání rychlosti vozidla [in czech; perception of vehicle speed]. master’s thesis, vysoké učení technické v brně, ústav soudního inženýrství, 2018. [2024-10-20]. https: //www.vutbr.cz/studenti/zav-prace/detail/112286 [7] p. hlavinková. vnímání rychlosti vozidla [in czech; perception of vehicle speed]. master’s thesis, vysoké učení technické v brně, ústav soudního inženýrství, 2018. [2024-10-20]. https: //www.vut.cz/studenti/zav-prace?zp_id=109497 [8] m. rak. vnímání rychlosti vozidla chodcem [in czech; perception of vehicle speed by pedestrian]. master’s thesis, vysoké učení technické v brně, ústav soudního inženýrství, 2020. [2024-10-20]. https: //www.vutbr.cz/studenti/zav-prace/detail/120323 [9] b. formanová. vnímání rychlosti vozidla řidičem [in czech; perception of vehicle speed by driver]. master’s thesis, vysoké učení technické v brně, ústav soudního inženýrství, 2021. [2024-10-20]. https: //www.vutbr.cz/studenti/zav-prace/detail/132016 [10] f. pršeš. vnímání rychlosti protijedoucího vozidla [in czech; perception of oncoming vehicle speed]. master’s thesis, vysoké učení technické v brně, ústav soudního inženýrství, 2023. [2024-10-20]. https: //www.vutbr.cz/studenti/zav-prace/detail/135642 [11] n. budinová. vnímání rychlosti vozidla z pohledu rozdílných řidičských zkušeností [in czech; perception of vehicle speed from the perspective of different driving experience]. master’s thesis, vysoké učení technické v brně, ústav soudního inženýrství, 2021. [2024-10-20]. https: //www.vutbr.cz/studenti/zav-prace/detail/127915 [12] m. rak, r. zůvala. analýza nočních odhadů rychlostí vozidel [in czech; analysis of vehicle speed estimates at night]. in sborník příspěvků konference junior forensic science brno 2023, pp. 32–38. vysoké učení technické v brně, ústav soudního inženýrství, brno, 2023. isbn 978-80-214-6165-9. 121 https://doi.org/10.1371/journal.pone.0185347 https://irf.fhnw.ch/handle/11654/34086 https://doi.org/10.1371/journal.pone.0267250 https://www.vutbr.cz/studenti/zav-prace/detail/112286 https://www.vutbr.cz/studenti/zav-prace/detail/112286 https://www.vut.cz/studenti/zav-prace?zp_id=109497 https://www.vut.cz/studenti/zav-prace?zp_id=109497 https://www.vutbr.cz/studenti/zav-prace/detail/120323 https://www.vutbr.cz/studenti/zav-prace/detail/120323 https://www.vutbr.cz/studenti/zav-prace/detail/132016 https://www.vutbr.cz/studenti/zav-prace/detail/132016 https://www.vutbr.cz/studenti/zav-prace/detail/135642 https://www.vutbr.cz/studenti/zav-prace/detail/135642 https://www.vutbr.cz/studenti/zav-prace/detail/127915 https://www.vutbr.cz/studenti/zav-prace/detail/127915 acta polytechnica ctu proceedings 52:116–121, 2025 1 introduction 2 dataset and method 2.1 original datasets 2.2 new criteria 3 results 4 discussion 5 conclusions acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2024.46.0040 acta polytechnica ctu proceedings 46:40–46, 2024 © 2024 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague investigating midr through ai: a case study of the city of most in czech republic akshatha ravi kumar∗, noor marji, gülbahar emir isik, lijun chen czech technical university in prague, faculty of architecture, jugoslávských partyzánů 1580/3, 160 00 prague 6 – dejvice, czech republic ∗ corresponding author: ravikaks@fa.cvut.cz abstract. urban planning, which is inherently multifaceted, requires the development of innovative tools to navigate its complexities. this study introduces a pioneering approach that presents an ai-driven framework tailored for urban data collection and analysis. the impetus for this framework is highlighted through the unique narrative of most city, which is profoundly transformed by mininginduced displacement and resettlement. while most cities serve as a vivid illustration of the challenges cities can face, especially in the wake of industrial imperatives, this study focuses on the potential of ai in addressing such challenges. the proposed framework, while grounded in advanced computational methodologies, is designed with keen emphasis on real-world applications, ensuring its relevance and adaptability. by integrating most city’s detailed account with this ai-centric methodology, this study emphasizes the importance of a data-driven approach in understanding and addressing urban dilemmas. importantly, this study is preparatory, laying the groundwork for the framework’s future application, especially in contexts such as most city. by bridging advanced ai techniques with tangible urban challenges, this research illuminates a path forward, suggesting a future in which urban planning is not only informed by data but also empowered by ai’s analytical process. keywords: mining-induced displacement and resettlement, artificial intelligence, relocation, most city, urban planning. 1. introduction urban planning, a discipline that shapes the fabric of cities and communities, continually faces multifaceted challenges. from demographic shifts to economic transformations, urban planners must anticipate and address myriad issues. one such challenge is mining-induced displacement and resettlement (midr), which is becoming increasingly prevalent in the wake of industrial development. midr not only disrupts the physicality of cities, but also reshapes their socio-economic and cultural landscapes, requiring innovative methodologies for understanding and intervention. mining operations can be found worldwide, with numerous countries relying on mining as a critical component of their economies. australia, canada, china, russia, the united states, and various african and south american countries are all major mining countries. each region’s mineral resources and mining processes are distinct. it spans from artisanal mining on a small scale to large-scale industrial mining. individual miners or local communities are often involved in small-scale mining in developing countries. largescale mining operations by multinational businesses necessitate extensive infrastructure and machinery [1]. mining activities may have a positive or negative impact on urban areas. by understanding the global prevalence of mining activities and their impact, urban planners and policymakers can develop informed strategies to mitigate negative consequences, promote sustainable mining practices, and effectively plan for sustainable post-mining city transitions. on one hand, mining activities contribute significantly to economic development at both local and national levels; they generate employment opportunities and revenue while stimulating related industries, attracting investments, infrastructure development, and economic diversification in cities hosting mining operations. however, mining operations also have substantial environmental consequences, including habitat destruction, deforestation, soil erosion, water pollution, air pollution, and greenhouse gas emissions, which adversely affect the health and well-being of urban and rural populations [2]. furthermore, mining can lead to population influxes, demographic changes, and increased social inequalities. the disruption of traditional livelihoods, cultural practices, and community cohesion caused by mining activities can give rise to social tensions and conflict. additionally, extensive mining activities can strain existing infrastructure and services, resulting in inadequate healthcare, education, water supply, waste management, and transportation systems due to rapid population growth and increased demand. abandoned mines also pose long-term challenges for cities, including safety hazards, environmental contamination, and visual blight. while architectural interventions have addressed the issue of abandoned mines in the past, the topic of expanding mining activities necessitating the relocation of entire cities still needs to be studied 40 https://doi.org/10.14311/app.2024.46.0040 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 46/2024 investigating midr through ai: a case study of the city of most . . . by urban designers and planners. this knowledge gap is of utmost importance as the growing demand for resources leaves cities with little choice but to contemplate the necessity of moving due to the escalating demands and constraints within their existing locations [1]. the term “mining-induced displacement and resettlement” (midr) refers to the forced displacement of communities and individuals as a direct or indirect consequence of mining activities. it occurs when mining operations require the acquisition of land, which results in the displacement of people from their homes and livelihoods. this displacement often necessitates the resettlement of affected communities in new locations [3]. 2. historical context of most city in the ústí nad labem region of the czech republic lies the city of most, an urban area that serves as an intriguing focal point for scholars and practitioners interested in the dynamics of mining-induced displacement and resettlement (midr) [4]. the narrative of this city is intertwined with the themes of obliteration and rebuilding. it was not simply built but was effectively rebuilt following the mass displacement of its original population in the pursuit of brown coal mining [5]. once known as starý most (“old most”), the old city was evacuated, and its inhabitants were resettled in what became nový most, or “new most”. at first glance, the story of most, the town that moved, seems very simple. with communist heavy industrialisation demanding ever-increasing amounts of energy, planners decided in the late 1950s to mine a rich vein of coal under most, gradually constructing a new city to replace the old one. cost analyses determined that the procedure would not only uncover 86 million tons of coal but also net a profit of over two billion crowns, including the expenses of demolition and building new housing and services for up to 20 000 people. although construction and destruction proceeded in fit and started by the mid-1980s, the project was complete. the historic streetscape of old most was gone, except for that of the itinerant church. the efficient surface mine that took its place yielded the expected revenue, fuelling several nearby power plants and bringing promised profits. a new socialist town emerged amidst open coal pits. it was the communist planner’s dream, with mass housing, modern architecture, and rationalised infrastructure [6]. all that remains of old most is a towering late gothic church, conspicuously isolated on a sculpted plateau north of the new city. beyond the church, the pit begins, miles and miles of hollowed-out landscape left for natural regeneration, a vacuous memorial to a vanished city and the coal that lay beneath it. if the juxtaposition of the church and coal pit seems surreal, consider this: the church of the assumption of the virgin mary used to reside 840 metres away, near the centre of most’s liquidated old town. in a triumph of communist engineering in czechoslovakia in 1975, a team of scientists, preservationists and technicians transported the 10 000 ton church on custom-built rails to its new home. although the church stands as a reminder of the lost old town, its miraculous journey has also rendered it a monument to modernity, to the ability of planners and ideologues to reconfigure the natural and human landscape in the name of industrial progress [5]. this deliberate reshaping of a populated area for resource extraction provides an invaluable case study for examining the complexities involved in midr within the context of urban planning and design. the city of most offers a historical and spatial tableau to assess the immediate and long-term consequences of such an endeavour-sociologically, spatially, and psychologically. unique in its history, most presents a golden opportunity to explore how the abrupt changes inflicted by mining activities can morph not only the geography but also the very essence of an urban area [4]. the city’s resettlement and subsequent evolution illuminates the grander themes of spatial perception, social resilience, and the ongoing impact of resource-centred decisions on urban settlements. 2.1. purpose while the historical intricacies of most city provide a rich backdrop, the core thrust of this study diverges from conventional urban studies. it has been approximately 50 years since most was resettled, providing ample data to understand the challenges and consequences of resettled cities. although gathering this large amount of data can be very time consuming and intense, recognizing the burgeoning role of technology in modern research, this paper presents an artificial intelligence (ai) framework specially tailored for data collection in urban scenarios like most. this paper does not aim to present specific data findings from most as it is the initial stages of ongoing research, but to lay the groundwork for how ai can be leveraged to garner insights into significant resettlement situations in figure 1. by bridging advanced computational methods with real-world urban challenges, we seek to illuminate a path forward for more nuanced data-driven urban planning interventions. 3. the role of ai in urban planning 3.1. current applications artificial intelligence (ai), with its computational prowess, is increasingly found in the realm of urban planning. modern cities generate vast amounts of data daily, and ai offers tools to process, analyse, and interpret these data to make informed decisions. from traffic management and infrastructure maintenance to 41 a. r. kumar, n. marji, g. e. isik, l. chen acta polytechnica ctu proceedings figure 1. map showing the resettlement from starý most to nový most [7]. 42 vol. 46/2024 investigating midr through ai: a case study of the city of most . . . the prediction of urban growth patterns, ai-powered algorithms assist in optimizing resource allocation and improving urban living conditions, facilitating more sustainable and informed planning. data analysis: ai has the potential to efficiently analyze vast amounts of data, including socioeconomic data, environmental data, and spatial data [8]. this enables urban planners to gain valuable insights into the complex dynamics of post-mining cities, such as demographic changes, economic trends, environmental impacts, and social challenges. ai algorithms can be applied to diverse datasets, such as satellite imagery, sensor data, social media data, and surveys, to extract valuable information for decision-making, providing urban planners with valuable insights for effective planning and resource allocation [9]. decision-making support: using machine learning algorithms, predictive modelling and simulations, ai can generate scenarios, simulate the impacts of different policy interventions, and optimize resource allocation. this helps stakeholders make evidence-based decisions and assess strategies, potential outcomes, and trade-offs. with regards to midr strategies, ai tools could help identify suitable locations for resettlement, evaluate the economic viability of alternative livelihood options, and optimize the allocation of resources for infrastructure development [9, 10]. community engagement: ai-powered platforms and tools enable residents and stakeholders to participate in decision-making, provide feedback, and express their concerns. for instance, chatbots and virtual assistants can be used for community queries and providing information about the resettlement process in midr contexts. moreover, natural language processing algorithms can analyze social media data and public sentiments, helping urban planners understand community perceptions and concerns [9, 11, 12]. 3.2. potential and limitations the potential of ai to reshape urban planning is immense. by providing real-time insights and predictive modelling, ai empowers urban designers to make decisions that are both reactive and proactive. this forward-thinking approach driven by data can lead to more sustainable, efficient, and liveable urban environments. in addition, with the integration of machine learning, urban systems can continuously adapt and evolve based on incoming data, ensuring that cities remain resilient in the face of changing circumstances. predictive modelling: by analyzing historical data and applying machine learning algorithms, aidriven decision support systems can generate predictive models that help understand the impacts of various decisions on socioeconomic factors, environmental conditions, and community well-being. this assists urban planners in making informed decisions and evaluating the trade-offs associated with different strategies [13]. optimization and resource allocation: algorithms such as genetic algorithms, simulated annealing, and linear programming can optimize decisions related to land use, infrastructure development, and community services. these optimization techniques assist in maximizing efficiency, minimizing costs, and ensuring an equitable distribution of resources [14, 15]. spatial analysis and visualization: by integrating gis (geographic information system) data with ai techniques, urban planners can analyze the spatial distribution of affected communities, identify areas prone to environmental risks, and optimize the spatial layout of resettlement sites. moreover, aidriven visualization tools facilitate effective communication and understanding of complex spatial information [16]. data-driven decision-making: through leveraging historical data, real-time information, and predictive analytics, urban planners can assess the impacts of different development scenarios and make informed decisions regarding land use, infrastructure investments, and service provision [9, 10]. environmental monitoring and management: combined with ai algorithms, remote sensing data can be used to analyze land use changes, vegetation health, and air quality in the city of most. this information can help develop targeted strategies for environmental rehabilitation, identify areas of ecological importance, and implement measures to mitigate future environmental risks [17]. community engagement: natural language processing (nlp) techniques can extract insights, sentiments, and concerns from community interactions and social media data, allowing urban planners to understand community needs and concerns. this fosters participatory decision-making and ensures the resettled community’s voices are heard [9, 11, 12]. infrastructure planning: machine learning models can assist in predicting future demands for housing, transportation, and community facilities through analyzing spatial data and infrastructure requirements. this could potentially help in designing wellintegrated, sustainable infrastructure systems that cater to the evolving needs of the resettled communities [18]. however, the adoption of ai in urban planning remains challenging. ethical implications associated with ai in urban planning strategies include fairness and bias, transparency and explain-ability, privacy and data security, and the need for human oversight and accountability [19]. ensuring fairness requires addressing biases in data collection and algorithmic models to achieve equitable outcomes [20]. transparency and explainability are essential to building trust and understanding the reasoning behind ai-driven decisions. protecting privacy rights and ensuring data security are paramount when handling sensitive information. lastly, maintaining human oversight is crucial to uphold accountability and ensure that ai 43 a. r. kumar, n. marji, g. e. isik, l. chen acta polytechnica ctu proceedings figure 2. policy framework for ai implementation in midr. technologies are supportive tools rather than replacing human judgement in planning [21]. it’s also worth mentioning that along with developing ai models, there are potential risks, biases, and limitations of such technologies. this could include data bias, overreliance on algorithms, limited generalization, and the potential for a technological divide and exclusion. data bias can occur if the training data used for the ai model is biased or incomplete, which could exacerbate existing inequalities. another risk is over-reliance on algorithms without human judgement, which can result in unintended consequences or inaccurate results, emphasizing the need for ai to be seen as a supportive tool rather than a replacement for human expertise. ai models may have limitations in generalizing the complexities of specific post-mining cities like most, requiring cautious interpretation. furthermore, implementing ai in urban planning may create a technological divide, leaving specific communities behind due to unequal access and digital literacy. efforts should be made to address these risks and ensure equitable access to ai-driven tools and their benefits for all communities involved in midr initiatives. in order to properly address these ethical considerations and potential risks, it is vital to establish clear guidelines, regulations, and ethical frameworks for using ai in urban planning. transparency, inclusivity, fairness, and accountability should be at the forefront of ai implementation, ensuring the technology is harnessed to enhance decision-making processes while safeguarding the well-being and rights of the affected communities. 4. proposed ai framework for data collection 4.1. necessity in the realm of urban planning, data are a cornerstone, shaping our understanding of cities and informing our interventions. the intricate challenges that cities face, especially those undergoing significant transformations such as most city, necessitate a robust data-driven approach. data not only capture the current state of affairs but also reveal patterns, allowing planners to anticipate future challenges. however, the sheer volume and complexity of urban data often transcends traditional analytical capacities. herein lies the need for an ai-driven approach capable of delving deep into vast datasets, extracting pertinent insights, and guiding data-centric urban interventions. 4.2. framework design the proposed ai framework is tailored to address the unique challenges and requirements of urban data collection, particularly in contexts such as most city. at its core, the framework employs advanced machine learning algorithms that are trained to process and interpret urban data. these algorithms are designed to handle a diverse range of data sources, from satellite imagery and urban infrastructure data to socioeconomic indicators. the framework also integrates real-time data-collection tools, ensuring that the insights generated are timely and relevant. moreover, given the dynamic nature of cities, the framework is adaptive and evolving in response to new data, ensuring that the analysis remains pertinent as urban landscapes change. in this diagram, see figure 2, the policy framework for ai implementation in midr strategies is 44 vol. 46/2024 investigating midr through ai: a case study of the city of most . . . presented as a comprehensive system composed of five key components: • regulatory frameworks: this component focuses on establishing ethical guidelines, privacy regulations, and transparency requirements to govern the responsible use of ai in midr initiatives. • inclusivity and equity: this component emphasizes conducting community needs assessments, mitigating biases and disparities, and ensuring fair and just resettlement outcomes. • data collection and analysis: this component highlights the importance of collecting high-quality and diverse data relevant to midr, utilizing ai-driven data analysis techniques, and ensuring responsible data handling. • community engagement and participation: this component emphasizes collaborative decisionmaking, transparent communication, and empowering affected communities throughout the midr process. • capacity building and training: this component involves developing ai knowledge and skills among stakeholders, providing training on ethical ai use and implications, and utilizing participatory planning techniques. together, these components form a robust policy framework that promotes responsible and inclusive ai implementation in urban planning strategies for midr. the framework emphasizes ethical considerations, inclusivity, community engagement, data-driven decision-making, and capacity building, ensuring that ai technologies are used to enhance the midr process while addressing the specific needs and challenges of affected communities. 4.3. potential applications although the framework is general in its design, its application to specific scenarios, such as the city of most offers immense potential. given most city’s rich history and the myriad challenges stemming from its resettlement, the framework can be used to collect and analyse data pertaining to their urban infrastructure, socio-economic dynamics, and cultural shifts. by doing so, planners and policymakers can gain a comprehensive understanding of a city’s current state, anticipate future challenges, and design interventions that resonate with its unique context. beyond most, the modular design of the framework allows it to be adapted to other cities facing similar urban challenges, making it a versatile tool in the urban planner’s arsenal. 5. bridging the city of most with the ai framework 5.1. integration the story of most city, with its intricate tapestry of historical, socio-economic, and urban challenges, serves more than just a backdrop; it underscores the need for sophisticated data analysis tools such as the ai framework. although technologically advanced, an ai-centric methodology derives its true value from its application to real-world challenges. most city, with its unique trajectory of resettlement and subsequent urban challenges, exemplify the complex urban scenarios that demand a nuanced, data-driven approach. by integrating the detailed account of most city’s challenges with the proposed ai framework, the research aims to ensure that the technology is not viewed in isolation, but rather as a tool intricately linked with the city’s narrative. this integration not only bolsters the relevance of the ai framework but also enriches our understanding of most city through a data-centric perspective. 5.2. future application it is crucial to emphasize that the current phase of this research is preparatory. although the ai framework is presented in detail, its application to most city remains a future endeavour. however, this preparatory nature does not diminish the significance of this study. this study set the groundwork for imminent data collection and analysis by proposing a robust aidriven methodology tailored for urban settings. the insights garnered from such future applications have the potential to revolutionize our understanding of most city and provide actionable recommendations for its continued urban development. this study, therefore, serves as both a testament to the potential of ai in urban planning and a prologue to its practical application in cities like most. 6. conclusion 6.1. summary urban planning, ever evolving in its complexities, demands innovative tools and methodologies to address multifaceted challenges. this research sought to highlight one such tool: an ai-driven framework tailored for urban data collection and analysis. through the lens of most city’s unique narrative of mining-induced displacement and resettlement, we underscored the pressing need for such advanced analytical tools. the city of most rich history and subsequent urban challenges, serves as a poignant reminder of the intricacies inherent in urban planning. while technologically sophisticated, the proposed ai framework is anchored in addressing real-world challenges, ensuring that its relevance extends beyond theoretical discourse. 6.2. future directions although the current phase of this research lays the foundational groundwork, the journey ahead is both promising and demanding. the next step involves the actual application of the ai framework to most city, embarking on comprehensive data collection and analysis. this hands-on application will not only validate 45 a. r. kumar, n. marji, g. e. isik, l. chen acta polytechnica ctu proceedings the framework’s efficacy, but also provide invaluable insights into the city’s urban dynamics. moreover, as with all technological tools, the ai framework will undergo iterative refinements, adapting to nuances unearthed during its application. beyond most city, the potential of this framework extends to other urban settings with similar challenges, positioning it as a versatile and impactful tool in an urban planner’s toolkit. acknowledgements this research was supported by grant: sgs23/081/ohk1/ 1t/15 by the faculty of architecture, czech technical university in prague. references [1] h. thorsteinsdottir, b. chalmers, c. waldmeier, et al. sustainability reporting in the mining sector: current status and future trends, 2020. united nations environment programme and group of friends of paragraph 47 (gof47). 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[8] s. c. k. tékouabou, j. chenal, r. azmi, et al. identifying and classifying urban data sources for machine learning-based sustainable urban planning and decision support systems development. data 7(12):170, 2022. https://doi.org/10.3390/data7120170 [9] m. bielecka, j. król-korczak. fuzzy decision support system for post-mining regions restoration designing. in artificial intelligence and soft computing. icaisc 2010, pp. 11–18. springer, berlin, heidelberg, 2010. https://doi.org/10.1007/978-3-642-13208-7_2 [10] m. bielecka, j. król-korczak. hybrid expert system aiding design of post-mining regions restoration. ecological engineering 36(10):1232–1241, 2010. https://doi.org/10.1016/j.ecoleng.2010.04.023 [11] l. wang, k. awuah-offei, s. que, w. yang. eliciting drivers of community perceptions of mining projects through effective community engagement. sustainability 8(7):658, 2016. https://doi.org/10.3390/su8070658 [12] d. ali, s. frimpong. artificial intelligence, machine learning and process automation: existing knowledge frontier and way forward for mining sector. artificial intelligence review 53(8):6025–6042, 2020. https://doi.org/10.1007/s10462-020-09841-6 [13] s. c. koumetio tekouabou, e. b. diop, r. azmi, et al. reviewing the application of machine learning methods to model urban form indicators in planning decision support systems: potential, issues and challenges. journal of king saud university – computer and information sciences 34(8):5943–5967, 2022. https://doi.org/10.1016/j.jksuci.2021.08.007 [14] m. wang, c. zheng. ground water management optimization using genetic algorithms and simulated annealing: formulation and comparison. jawra journal of the american water resources association 34(3):519–530, 1998. https: //doi.org/10.1111/j.1752-1688.1998.tb00951.x [15] y. nian, x. li, j. zhou, x. hu. impact of land use change on water resource allocation in the middle reaches of the heihe river basin in northwestern china. journal of arid land 6(3):273–286, 2013. https://doi.org/10.1007/s40333-013-0209-4 [16] m. breunig, p. e. bradley, m. jahn, et al. geospatial data management research: progress and future directions. isprs international journal of geo-information 9(2):95, 2020. https://doi.org/10.3390/ijgi9020095 [17] y. himeur, b. rimal, a. tiwary, a. amira. using artificial intelligence and data fusion for environmental monitoring: a review and future perspectives. information fusion 86–87:44–75, 2022. https://doi.org/10.1016/j.inffus.2022.06.003 [18] l. mcmillan, l. varga. a review of the use of artificial intelligence methods in infrastructure systems. engineering applications of artificial intelligence 116:105472, 2022. https://doi.org/10.1016/j.engappai.2022.105472 [19] a. hagerty, i. rubinov. global ai ethics: a review of the social impacts and ethical implications of artificial intelligence, 2019. [2023-05-12]. arxiv:1907.07892 https://doi.org/10.48550/arxiv.1907.07892 [20] e. ferrara. fairness and bias in artificial intelligence: a brief survey of sources, impacts, and mitigation strategies, 2023. [2023-05-05]. arxiv:2304.07683 https://doi.org/10.48550/arxiv.2304.07683 [21] european commission. ethics guidelines for trustworthy ai, 2019. [2023-05-01]. https://ec.europa.eu/digital-single-market/en/ news/ethics-guidelines-trustworthy-ai 46 https://wedocs.unep.org/bitstream/handle/20.500.11822/33924/srms.pdf?sequence=1&isallowed=y https://wedocs.unep.org/bitstream/handle/20.500.11822/33924/srms.pdf?sequence=1&isallowed=y https://doi.org/10.2861/804163 https://doi.org/10.1016/j.jsm.2019.03.001 https://doi.org/10.1016/j.resourpol.2016.07.011 https://doi.org/10.3197/096734007x243168 https://doi.org/10.1163/156916100746428 https://doi.org/10.3390/data7120170 https://doi.org/10.1007/978-3-642-13208-7_2 https://doi.org/10.1016/j.ecoleng.2010.04.023 https://doi.org/10.3390/su8070658 https://doi.org/10.1007/s10462-020-09841-6 https://doi.org/10.1016/j.jksuci.2021.08.007 https://doi.org/10.1111/j.1752-1688.1998.tb00951.x https://doi.org/10.1111/j.1752-1688.1998.tb00951.x https://doi.org/10.1007/s40333-013-0209-4 https://doi.org/10.3390/ijgi9020095 https://doi.org/10.1016/j.inffus.2022.06.003 https://doi.org/10.1016/j.engappai.2022.105472 http://arxiv.org/abs/1907.07892 https://doi.org/10.48550/arxiv.1907.07892 http://arxiv.org/abs/2304.07683 https://doi.org/10.48550/arxiv.2304.07683 https://ec.europa.eu/digital-single-market/en/news/ethics-guidelines-trustworthy-ai https://ec.europa.eu/digital-single-market/en/news/ethics-guidelines-trustworthy-ai acta polytechnica ctu proceedings 46:40–46, 2024 1 introduction 2 historical context of most city 2.1 purpose 3 the role of ai in urban planning 3.1 current applications 3.2 potential and limitations 4 proposed ai framework for data collection 4.1 necessity 4.2 framework design 4.3 potential applications 5 bridging the city of most with the ai framework 5.1 integration 5.2 future application 6 conclusion 6.1 summary 6.2 future directions acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2023.43.0028 acta polytechnica ctu proceedings 43:28–38, 2023 © 2023 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague integrated cyclic timetable on the railways in the karlovy vary region jakub hora, filip baran∗ university of pardubice, jan perner’s faculty of transport engineering, department of transport technology and control, studentská 95, 532 10 pardubice, czech republic ∗ corresponding author: st51965@upce.cz abstract. the concept of public transport with periodic timetables has been slowly spreading in the czech republic for many years. together with the announcement of tenders for the operation of compulsory transport according to the european parliament’s regulations 1370/2007 and 2338/2016, circular integrations occur in many regions, which usually represent the adjustment of public transport to a concept corresponding to current trends and needs. keywords: integrated cyclic timetable, optimization, railway transport, transfer links transport connections. 1. introduction when the need to prepare tenders for the operation of obligatory transport began to enter public consciousness around 2014, it became clear that the time of integrated transport system (its) development, with all that comes with it, was also coming. the first region to fundamentally change the concept of public transport was the ústí nad labem region, where from 1 january 2015, completely new green buses of the transport of the ústí nad labem region (túlr) system were introduced. although most regions did not require completely new buses (for example, the central bohemian or moravian-silesian region manages to achieve a low average age even without this requirement), the pilsen region was also expected to reduce the average age of the fleet to 0 years in 2020, and the new blue buses in the paint of the integrated transport of the pilsen region (itpr) system were rolled out in the region. from the point of view of the karlovy vary region, both neighbouring regions – ústí and pilsen region – have a well-developed its, elaborate regional tariffs, developed periodic transport, etc. the cross-border neighbours, the german states of bavaria and saxony, also have their own its, and thanks to this there is also an established timetable on cross-border lines. this text shows one of the possibilities for the redevelopment of transport in the karlovy vary region – the possibility of building on the foundations of clockwise transport to the skeleton provided by the fact that all neighbouring regions have periodic transport. this skeleton means an established timetable on lines ex6, r15, r29, and several other regional lines that enter the karlovy vary region from neighbouring areas. at the outset, it is necessary to mention the central bohemian region, which is not directly adjacent to the karlovy vary region, but the integration of rakovnicko ii (in august 2020) and rakovnicko iii (in december 2021) will allow passengers to switch from integrated transport of the karlovy vary region (itkv) to prague integrated transport (pit) connections. here, we will also see a unified its image, user-friendly tariffs, and other elements that result in an influx of passengers to public transport. the aim of the text is to research the possibilities of transport in the karlovy vary region. the methods used will be the implementation of periodic transport, and an attempt to take maximum account of the possibilities of demand-responsive transport. 1.1. a modern concept of public transport why try to introduce cyclic timetable and improve public transport (pt) in general? cyclic transport is a trend that has already proved its worth in many regions of many countries, the city of lindau, the two neighbouring counties and hundreds of other examples of how to make public transport more attractive. at the same time, public transport is no longer just a necessity, it is a service for people, a way of travelling in an environmentally friendly way and, at its very core, a matter of course for people across europe. we live in an age where distance is no longer measured in kilometres, but in minutes. pt will make places more attractive to live that, with poor quality pt, means that residents depend on the use of cars in their lives. for example, commuting from teplá in the absence of reliable and fast interchanges in mariánské lázně means that if a given person travels anywhere outside of mariánské lázně, they are likely to do so by car (which is disadvantageous both economically for people and environmentally for everyone else). this is the case not only in towns with a lack of employment opportunities, but also in towns from which it is necessary to commute to another town for highly specialised (e.g., higher education) jobs. 28 https://doi.org/10.14311/app.2023.43.0028 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 43/2023 integrated cyclic timetable on the railways in the karlovy vary region the karlovy vary region plans to sign new contracts resulting from tenders that will take place during 2022 and 2023. given the high number of lowcapacity buses in the tender documents, it is possible that by the end of 2023 the karlovy vary region will represent a plausible simulation of eastern european public transport, which, although it has potential, gives the impression of an anthill of irregularly running small vehicles. however, it is also possible that the itkv system will adopt the necessary changes and take its place among the leading its in the czech republic – there are many opportunities for inspiration and learning in 2022. figure 1. map of karlovy vary region with railway network [1]. legend of the figure 1: • orange graphic elements – main railway lines, • yellow graphic elements – branch railway lines, • blue graphic elements – stations on railway lines, • brown graphic elements – connecting tracks and sidings, • dashed graphic elements – abandoned lines. 2. materials and methods the methodology is based on the nature of the integrated cyclic timetable (ict). for such a timetable, a so-called periodic (cyclic) node is necessary – a place where the connections of the lines to the node meet and thus form system links between them. the socalled axis of symmetry, which marks the beginning of the period, is used for this purpose. the period can be of different sizes, e.g. most often 1 or 2 hours. the axis is symmetric precisely when, before the end of the period, all the links concerned meet at the periodic node and all leave it again at the beginning of the period. the time during which the line links are in the node is used for transfers, delay compensation time or other technological operations. if there are periodic nodes in the transport network, an integrated timetable can be constructed, which is figure 2. part of railway map with long-distance train lines [1]. an extension of the periodic timetable described above. in order to establish and maintain interchanges at the node, it is necessary to observe the travel times at the individual edges of the transport network carried out by the lines concerned – this is the main focus of the integrated timetable. 2.1. definition of the study area, basic transport links, and introduction of basic cyclic positions it has already been mentioned that the initial idea of the research is to link the cyclic positions in the karlovy vary region to the cyclic positions in neighbouring regions. within the region, the railway backbone network will then be addressed first. this research includes regional railway connections in the karlovy vary region, in addition to connections that go to the cheb junction. the railway network is represented by three long-distance lines of trains of category r (fast train with higher amount of stops than express) and higher (express etc.). train lines ex6 (prague – plzeň – cheb), r15 (prague – ústí n. l. – cheb), and r29 (cheb – germany). the routes of these lines show an important railway junction in cheb. in the karlovy vary region there is another important train line – p1 (plzeň – cheb – karlovy vary) in the section plzeň – cheb forms an hourly interchange to train line ex6, and in the section cheb – karlovy vary it complements passenger trains in the time position of the departure 10 minutes after train line r15 on departure from cheb (or 10 minutes before on arrival from karlovy vary to cheb). on the basis of the above-mentioned transport information, an effort will be made to create integrated interchanges mainly in the vicinity of the sokolov region (north-eastern part of the karlovy vary region) between the main railway lines, where the 4 train lines mentioned in figure 2 are located, and the regional lines, the location of which is shown in figure 1. 29 jakub hora, filip baran acta polytechnica ctu proceedings in the design part of the text, the current valid timetables are used, i.e. timetables from the theoretical railway graph (trg) 2021/2022. in addition, the timetables from this trg are used, which have developed into stable time positions in the region (albeit without the implementation of an integrated timetable). the authors assume a state that is stable but not periodic. the integrated timetable is determined here by the time positions in the surrounding regions, which also determine some time positions in the karlovy vary region. the rest of the time positions are determined in such a way that the one which can be considered as the system optimum is always selected from the available variants. authors are using odd and even specification for time position of trains in further text of article (e – even, o – odd, x – every hour). there are existing restrictions in daily hours of operation for trains which are shown in tables. authors are using these symbols: • x – train is operated on workdays, • 1 – train is operated on 1st day of week (monday), • . . . , • 6 – train is operated on 6th day of week (saturday), • 7 – train is operated on 7th day of week (sunday) and on public holidays. 2.2. railway line 149 the connection of the r15 and ex6 train lines in the east of the region is important for further modifications. this link is railway line 149 karlovy vary – bečov nad teplou – mariánské lázně. the timing of the ex6 and r15 train lines has been stable for years and apart from the cheb junction, they form several junctions along their routes. the timing of train line p1 can also be considered stable, although in the future it may be recommended to try to shorten the journey time between plzeň and cheb to reach the xx:30 node in cheb. for railway line 149, two connection options can be considered in mariánské lázně: (1.) connecting trains to the ex6 train line, or (2.) connecting trains to the p1 train line. connecting to the ex6 train line is clearly more advantageous, why: • a faster turnaround in mariánské lázně will allow the creation of transfer at e:00 in bečov nad teplou junction with a larger margin, • extending railway line of passenger trains to horní nádraží in karlovy vary, will allow a transfer to the r15 train line, • the crossing on the single-track 149 will take place in bečov nad teplou, in the case of the inserted more connections karlovy vary – bečov nad teplou in peak hours for better frequency, the crossing will also take place in karlovy vary dolní nádraží. figure 3. integrated transfer in karlovy vary. proposed measures for the railway line 149: • extension of all trains to karlovy vary station, • binding to trains of train lines ex6 and r15, • two-hourly running throughout the day. resulting situation on the railway line 149: • connection of mariánské lázně (ex6) and karlovy vary (r15), • creation of a junction at e:00 in bečov nad teplou, • together with the optimised bus line, forming an hourly mariánské lázně – teplá route, • improving the connection of railway line 161 to pt in the karlovy vary region. travel times (see the table 1): • mariánské lázně – bečov n. t.: 50 minutes, • bečov n. t. – karlovy vary: 35 minutes. for figure 3 – dashed lines for passenger trains (in czech: os) means that they are not the same train unit. in that case, arrival is in 36th minute of even hour and departure is in 25th minute of odd hour. the proposed draft of the train timetable for railway line no. 149 is in the table 1. 2.3. railway line 161 the transport solutions of railway lines 149 and 161 are directly related. line 161 is a regional line that runs from the central bohemia region to bečov nad teplou in the karlovy vary region. in 2022 the line has very low passenger frequency. line 161 is connected to line 163 in protivec in central bohemia region, i.e., without regular operation of passenger trains. the area around railway line 161 is characterised by the absence of large towns (the largest town is 30 vol. 43/2023 integrated cyclic timetable on the railways in the karlovy vary region train restriction mariánské lázně bečov n. t. karlovy vary 7120 x – 4:41 5:16 7122 x – 6:01 6:36 7100 x, 6, 7 6:11 7:01 7:36 7102 x, 6, 7 7:11 8:01 8:36 7104 x, 6, 7 9:11 10:01 9:36 7106 x, 6, 7 11:11 12:01 12:36 7108 x, 6, 7 13:11 14:01 14:36 7124 x – 15:01 15:36 7110 x, 6, 7 15:11 16:01 16:36 7126 x – 17:01 17:36 7112 x, 6, 7 17:11 18:01 18:36 7114 x, 6, 7 19:11 20:01 20:36 7116 x, 6, 7 21:11 22:01 22:36 7140 x, 6, 7 23:11 0:01 – train restriction karlovy vary bečov n. t. mariánské lázně 7141 x, 6, 7 – 4:00 4:48 7101 x, 6, 7 5:25 6:00 6:48 7103 x, 6, 7 7:25 8:00 8:48 7105 x, 6, 7 9:25 10:00 10:48 7107 x, 6, 7 11:25 12:00 12:48 7109 x, 6, 7 13:25 14:00 14:48 7121 x 14:25 15:00 – 7111 x, 6, 7 15:25 16:00 16:48 7123 x 16:25 17:00 – 7113 x, 6, 7 17:25 18:00 18:48 7125 x 18:25 19:00 – 7115 x, 6, 7 19:25 20:00 20:48 7117 x, 6, 7 21:25 22:00 22:48 7127 x, 6, 7 22:25 23:00 – table 1. simplified draft timetable for railway line 149 [2]. toužim, which has over 3000 inhabitants), there is no district town in the area and no municipality with extended area of competence (mec). primary schools on line 161 in the karlovy vary region can be found in toužim, štědrá, and žlutice. another one is in lubenec. the area has strong tourist potential, but the nature of the area and lack of job opportunities forces residents to commute for work to another towns. in the 2022 timetable, there are interchanges between trains on railway lines 149 and 161, although these are usually interchanges that include a 20-minute wait (for example, the afternoon connection between os 7109 and os 16747). the proposed transfer at e:00 on railway line 149 in bečov nad teplou encourages the introduction of clockwise services on line 161. the almost hourly journey time in the section bečov nad teplou – žlutice determines the o:00 crossing to žlutice. in the event of the termination of the order of passenger trains on line 161 by the central bohemia and ústí nad labem region, the situation can be resolved: • termination of connections in žlutice, with sharp turnarounds (potential for delays), • extension of connections on the territory of the karlovy vary region to chyše town, with approximately half-hour turnarounds. the current frequency through žlutice is minimal on line 161, but given the trains’ stays of up to one hour in žlutice, no other result can be expected. running the trains bečov nad teplou – chyše means the need for a second train unit, while terminating in žlutice, thanks to the stable running times, one train unit would be sufficient. proposed measures for the railway line 161: • modification of the time positions of the connections using the e:00 transfer in bečov nad teplou. resulting situation on the railway line 161: • two-hourly clock in peak hours within the karlovy vary region. for figure 4 – the blue lines show the passenger trains in question. brown lines are trains outside the competence of the karlovy vary region. pink lines are trains of railway line 142. 31 jakub hora, filip baran acta polytechnica ctu proceedings figure 4. schematic timetable for railway lines 149 and 161. 2.4. railway line 145 modification of the timetable and the introduction of cyclic positions according to the existing cyclic positions of another railway line can also be introduced on line 145. train lines p1 and r15 always meet in sokolov around x:00. on railway line 145, it is possible to design a connection position that considers the x:00 junction in sokolov as well as the existing clock positions of cross-border trains to kraslice from germany. line 145 is an important link in the district of sokolov, connecting the district town with the northern part of the district, where the towns of rotava and kraslice are located, which have a combined population of around 10000. the line provides transport services to oloví, which does not have a bus connection (the bus goes to the local part of studenec, 1.5–2 km west of oloví). sokolov, rotava, and kraslice are also connected by bus line 481660, but its route is very different. it does not serve oloví, but in rotava it goes to the city centre, thus providing a better transport service than the train at the 3 km away station. line 145 is a frequently used suburban line, and the current situation in the 2022 trg resembles an hourly cycle in terms of frequency of services. with the introduction of fixed positions from sokolov x:10 to kraslice x:45, and back from kraslice x:11 to sokolov x:46, it is based on a crossing at x:28 in oloví in addition to the connections at sokolov. in the case of rotava, it is desirable, in addition to optimising bus routes in the region, to link the city with the rail network. for the connection with the railway station, the authors propose a bus line on the route rotava railway station – rotava bus station – rotava, zátiší, using elements of demand-response transport, especially at the outskirts of the day. proposed measures for the railway line 145: • introduction of an hourly interval between sokolov and kraslice, • operation in the section kraslice – state border unchanged, figure 5. extension of passenger trains of railway line 144 to chodov [1]. • introduction of a bus line connecting the city of rotava with the rotava railway station. resulting situation on the railway line 145: • maximising the accessibility of sokolov for passengers from kraslice, rotava, and oloví, • providing connections to long-distance transport in sokolov, • establishment of junction at e:00 (peak x:00) in kraslice. travel times: • sokolov – kraslice: 35 minutes, • sokolov – oloví: 18 minutes. the proposed draft of train timetable for railway line no. 145 is in the table 2. 2.5. railway line 144 it is a short branch line from railway line 140 in nové sedlo to loket. the line runs in the direction of the loket – karlovy vary commute, and it is also possible to consider the loket – sokolov commute. the bus journey times are approximately 30 minutes for the loket – karlovy vary route and approximately 20 minutes for the loket – sokolov route. the train has the potential to achieve the same (or even slightly better) times as buses. as an optimal solution, a revision of the train and bus timetables can be considered so that they complement each other in these directions. the positions of passenger trains on the railway line 144 are determined by the fixed positions of longdistance services – same as on railway line 140. the continuity with the connections of the r15 train line that do not stop at nové sedlo u lokte must be solved by the arrival of trains from the line 144 to chodov. this solution can already be observed to some extent and is shown on figure 5 by dashed blue line. 32 vol. 43/2023 integrated cyclic timetable on the railways in the karlovy vary region train restriction sokolov oloví kraslice operational information 17000 x, 6, 7 5:10 5:28 5:45 17002 x 6:10 6:28 6:45 crossing with 17007 in rotava at 6:36 17004 x, 6, 7 7:10 7:28 7:45 17006 x, 6, 7 8:10 8:28 8:45 17008 x, 6, 7 9:10 9:28 9:45 17010 x, 6, 7 11:10 11:28 11:45 17012 x 12:10 12:28 12:45 17014 x, 6, 7 13:10 13:28 13:45 17016 x, 6, 7 14:10 14:28 14:45 17018 x 14:54 15:12 15:29 crossing with 17023 in rotava at 15:20 17020 x, 6, 7 15:10 15:28 15:45 17022 x, 6, 7 16:10 16:28 16:45 17024 x 16:54 17:12 17:29 crossing with 17027 in rotava at 17:20 17026 x, 6, 7 17:10 17:28 17:45 17028 x, 6, 7 18:10 18:28 18:45 17030 x, 6, 7 19:10 19:28 19:45 17032 x, 6, 7 21:10 21:28 21:45 two connected train units 17034 x, 6, 7 23:03 23:20 23:34 train restriction kraslice oloví sokolov operational information 17001 x, 6, 7 4:11 4:28 4:46 17003 – 5:11 5:28 5:46 17005 x, 6, 7 6:11 6:28 6:46 17007 – 6:27 6:44 7:02 crossing with 17002 in rotava at 6:36 17009 x, 6, 7 7:11 7:28 7:46 17011 x, 6, 7 8:11 8:28 8:46 17013 x 9:11 9:28 9:46 17015 x, 6, 7 10:11 10:28 10:46 17017 x, 6, 7 12:11 12:28 12:46 17019 x, 6, 7 13:11 13:28 13:46 17021 x, 6, 7 14:11 14:28 14:46 17023 x, 6, 7 15:11 15:28 15:46 crossing with 17018 in rotava at 15:20 17025 x, 6, 7 16:11 16:28 16:46 two connected train units 17027 x, 6, 7 17:11 17:28 17:46 crossing with 17024 in rotava at 17:20 17029 x, 6, 7 18:11 18:28 18:46 17031 x, 6, 7 19:11 19:28 19:46 17033 x, 6, 7 20:11 20:28 20:46 17035 x, 6, 7 22:11 22:28 22:46 table 2. simplified draft timetable for railway line 145 [2]. the frequency of passenger trains on railway line 144, considering the current situation and the requirement for approximately the same volume of kilometres, corresponds to an hourly interval between connections. two options are evident when proposing a link to the long-distance services of the p1 and r15 train lines. the first option is to connect to the p1 train line at nové sedlo u lokte in the time position e:40/o:16, which, however, implies: • a rather sharp turnover at x:30 in loket to return in time, • the necessity to deviate from the timetable in cases where the loket suburb stop is to be served, • impossibility of connecting to the r15 train line, coupled with the requirement to locate passenger trains on railway line 140. the second possibility is to bind at position e:55/o:05 to the connections of train line r15 in chodov, and around e:00 to bind to passenger trains of railway line 140 in nové sedlo. the requirement for the position of passenger trains on line 140 remains, but the junction at x:30 in loket and other disadvantages are not present. because there is no difference between passenger trains and p1 trains on the railway line 140 in terms of the number of stops, the latter option appears to be preferable. proposed measures for the railway line 144: • hourly interval between nové sedlo u lokte and loket, with a zonal operation nové sedlo u lokte – chodov once every two hours, 33 jakub hora, filip baran acta polytechnica ctu proceedings train restriction nová role chodov nové sedlo u l. loket předm. 27100 x – 5:11 5:14 5:24 27130 x, 6, 7 – – 6:14 6:24 27150 x, 6, 7 6:29 6:41 – – 27102 x, 6, 7 – 7:11 7:14 7:24 27132 x, 6, 7 – 8:14 8:24 27104 x, 6, 7 – 9:11 9:14 9:24 27106 x, 6, 7 – 11:11 11:14 11:24 27108 x, 6, 7 – 13:11 12:14 – 27134 x, 6, 7 – – 13:14 13:24 27152 x, 6, 7 14:29 14:41 14:14 14:24 27110 x, 6, 7 – 15:11 – – 27136 x, 6, 7 – – 15:14 15:24 27112 x, 6, 7 – 17:11 16:14 16:24 27138 x, 6, 7 – – 17:14 17:24 27114 x, 6, 7 – 19:11 18:14 18:24 27140 x, 6, 7 – – 19:14 19:24 27116 x, 6, 7 – 21:11 20:14 20:24 27118 1, 2, 3, 4, 7 – – 21:14 21:24 train restriction loket předm. nové sedlo u l. chodov nová role 27101 x 4:45 4:57 5:00 – 27151 x, 6, 7 – – 5:10 5:23 27131 x 5:34 5:46 – – 27103 x, 6, 7 6:34 6:46 6:49 – 27133 x, 6, 7 7:34 7:46 – – 27105 x, 6, 7 8:34 8:46 8:49 – 27107 x, 6, 7 10:34 10:46 10:49 – 27109 x, 6, 7 12:34 12:46 12:49 – 27153 x, 6, 7 – – 13:10 13:23 27135 x, 6, 7 13:34 13:46 – – 27111 x, 6, 7 14:34 14:46 14:49 – 27137 x, 6, 7 15:34 15:46 – – 27113 x, 6, 7 16:34 16:46 16:49 – 27139 x, 6, 7 17:34 17:46 – – 27115 x, 6, 7 18:34 18:46 18:49 – 27141 x, 6, 7 19:34 19:46 – – 27117 x, 6, 7 20:34 20:46 20:49 – 27119 x, 6, 7 21:34 21:46 – – table 3. simplified draft timetable for railway line 144 [2]. • competitive connection for commuting loket – karlovy vary, loket – sokolov, and further destinations. the proposed draft of train timetable for railway line no. 144 is in table 3. resulting condition on the railway line 144: • the its will complement bus and train connections between karlovy vary, loket, and sokolov, • establishment of a junction at x:30 in loket, • use of the node at o:00 in chodov. travel times: • nové sedlo u lokte – loket: 10 minutes, • chodov – loket: 13 minutes, • karlovy vary – chodov: 10 minutes. 2.6. railway line 142 the suburban line connecting the north of the karlovy vary district with the regional town of karlovy vary crosses nova rola, nejdek, pernink, horní blatná, potůčky and other villages along its route. the journey time along the entire route is approximately 75 minutes, with a slight deviation between trains heading to the krušné hory and trains coming from the krušné hory to the regional town of karlovy vary. the karlovy vary – nejdek line is also served by bus line 421190 (outside nová rola), the karlovy vary – pernink – potůčky line is also served by bus lines 421103 and 421104 (outside nejdek). the (non-)existence of a periodical node at karlovy vary railway station is essential when researching the cyclic positions for railway line 142. the fixed 34 vol. 43/2023 integrated cyclic timetable on the railways in the karlovy vary region positions of the r15 train line at e:45 and o:15 offer two possibilities: the first option means arrival from nejdek shortly before the departure of the train line r15 to cheb at e:45. departure shortly after o:15 – after the arrival of the r15 from cheb. the advantage of this solution is the possibility of running 142 passenger trains on the railway line to the karlovy vary dolní nádraží (hereinafter lower station), arriving at approximately e:51 and departing at approximately o:06. this would link the o:00 junction at the bus terminal adjacent to the lower station, and the transfer links to karlovy vary station. the bus station at karlovy vary dolní nádraží station can be seen in figure 6. the second option means arrival from nejdek before o:15 to karlovy vary, and departure after e:45. the r15 train line would thus be connected to ostrov, kadaň, chomutov, and further to the ústí nad labem region. in this variant, the extension of connections to the lower station would mean departure immediately before the railway line 149 connection, and on return from the lower station as well. in addition, the position of passenger trains on the railway line 149 takes advantage of the possibility of crossing at the lower station at x:30 between the karlovy vary – mariánské lázně and the inserted connections of karlovy vary – bečov nad teplou, which must be taken into account. from the point of view of the junctions in karlovy vary, the first option seems to be more beneficial. from the point of view of other settlements on the route, only nejdek, where there is also a half-kilometre distance between the train and bus stations, has enough lines to form a node. thus, the first option can be recommended, and taking into account the positions of the passenger trains of line 142 when constructing the timetable (not only) of the backbone bus line karlovy vary – nejdek. for railway line 142, considering the existing timetable, a two-hour interval between connections can be recommended, using zonal operation and hourly interval in peak hours in the karlovy vary – nejdek – nové hamry section. the crossing on the single-track railway line 142 takes place in stará rola (x:30) and in nové hamry (e:00). the shortened zonal connections in nové hamry have a sharp turnover at o:00. proposed measures for the railway line 142: • adjustment of the timing of connections and creation of a two-hour interval on the whole railway line with connection to the r15 train line to/from cheb, • extension of trains to the lower station to ensure connection between buses at the o:00 junction at the bus terminal and the connection at the time of figure 6. iveco crossway bus at terminal station at karlovy vary dolní nádraží. the passage of the r15 train line to karlovy vary station, • introduction of zone operation and hourly interval karlovy vary – nové hamry. resulting situation on the railway line 142: • strengthening the role of the railway line 142 in the suburban transport of karlovy vary, • basis for optimisation of the karlovy vary – nejdek and karlovy vary – pernink – potůčky connections. travel times: • nejdek – johanngeorgenstadt: 47 minutes, • karlovy vary – nejdek: 20 minutes. the proposed draft of train timetable for railway line no. 142 is in the table 4. 2.7. railway lines 143 and 141 the former railway line 143 nová role – chodov (today incorporated into railway line 144) and railway line 141 are not backbone lines. they have only single pairs of connections and can only be counted in the its as a supplementary connection – the exact timing should be determined only regarding the overall optimisation of bus lines in the area. the railway line 141 has number of passengers that are not adequate for the cancellation of the line, the analysis of the use of the connections does not show that the lines should remain without service, and the tourist potential of line 141 should be considered. in the case of the railway line 143, the proposed basic positions of the passenger trains of line 142 in nová role at e:20 (to karlovy vary) and o:38 (to potůčky) can be considered. if the number of available vehicles is favourable, the o:00 junction in chodov can be used. in the case of the railway line 141, the location of the arrival and departure of the train line r15 in karlovy vary can be considered, or the timing of the connections can be worked with to improve the offer of connections between the upper and lower karlovy vary railway stations. however, the results 35 jakub hora, filip baran acta polytechnica ctu proceedings kv dolní karlovy stará nové train restriction nádraží vary role hamry johanngeorgenstat 17100 x 5:06 5:23 5:30 6:00 6:35 17130 x, 6, 7 6:06 6:23 6:30 7:00 – 17102 x, 6, 7 8:06 7:23 7:30 8:00 8:35 17104 x, 6, 7 9:06 9:23 9:30 10:00 10:35 17106 x, 6, 7 11:06 11:23 11:30 12:00 12:35 17132 x, 6, 7 12:06 12:23 12:30 13:00 – 17108 x, 6, 7 13:06 13:23 13:30 14:00 14:35 17134 x, 6, 7 14:06 14:23 14:30 15:00 – 17110 x, 6, 7 15:06 15:23 15:30 16:00 16:35 17136 x, 6, 7 16:06 16:23 16:30 17:00 – 17112 x, 6, 7 17:06 17:23 17:30 18:00 18:35 17138 x, 6, 7 18:06 18:23 18:30 19:00 – 17114 x, 6, 7 19:06 19:23 19:30 20:00 20:35 17140 x, 6, 7 20:06 20:23 20:30 21:00 – 17116 x, 6, 7 21:06 21:23 21:30 22:00 22:35 17142 x, 6, 7 22:06 22:23 22:30 23:00 – nové stará karlovy kv dolní train restriction johanngeorgenstat hamry role vary nádraží 17131 x, 6, 7 – 5:00 5:30 5:47 5:51 17101 x, 6, 7 5:26 6:00 6:30 6:47 6:51 17133 x – 7:00 7:30 7:47 7:51 17103 x, 6, 7 7:26 8:00 8:30 8:47 8:51 17105 x, 6, 7 9:26 10:00 10:30 10:47 10:51 17107 x, 6, 7 11:26 12:00 12:30 12:47 12:51 17135 x – 13:00 13:30 13:47 13:51 17109 x, 6, 7 13:26 14:00 14:30 14:47 14:51 17137 x, 6, 7 – 15:00 15:30 15:47 15:51 17111 x, 6, 7 15:26 16:00 16:30 16:47 16:51 17141 x, 6, 7 – 17:00 17:30 17:47 17:51 17113 x, 6, 7 17:26 18:00 18:30 18:47 18:51 17143 x – 19:00 19:30 19:47 19:51 17115 x, 6, 7 19:26 20:00 20:30 20:47 20:51 17145 x – 21:00 21:30 21:47 21:51 17117 x, 6, 7 21:26 22:00 22:30 22:47 22:51 table 4. simplified draft timetable for railway line 142 [2]. of the optimization of bus lines terminating at the bus terminal (adjacent to the lower station) must be considered when adjusting the operation between the karlovy vary stations. travel times: • bečov n. t. – štědrá: 44 minutes, • štědrá – žlutice: 11 minutes. 3. results (graphics of integrated cyclic timetable) the research presented in chapter 3 can be succinctly summarised in a graphic showing the nodes and the temporal positions of the links between them. the proposed solution allows, among others, the following commute times (hh:mm) in table 5. all the above journey times can be reached periodically every one or two hours. compared to the current situation, we can speak of a considerable simplification of timetables, and at the same time of an increase in ordered kilometres of no more than a few percent. the graphic in figure 7 shows a solution that can be considered optimal based on the options based on the locations of long-distance trains, the volume of traffic ordered in the current timetable (2021/2022) and, in particular, the demand on individual transport routes. the possibility of changing trains from mariánské lázně towards žlutice in bečov nad teplou has been maintained, and the proposed solution also includes much shorter travel times both outside the study area and within the study area. it must be said that the colours of figure 5 of passenger train lines do not correspond with the colour layout of the other images. black line is main line where long-distance r15 train line is operated from cheb to ústí and labem and 36 vol. 43/2023 integrated cyclic timetable on the railways in the karlovy vary region connections part of the railway line commute times [hh:mm] plzeň – bečov nad teplou 149 1:51 karlovy vary – kraslice 145 0:59 kraslice – cheb 145 1:19 loket – karlovy vary 144/143 0:37 karlovy vary – toužim 161 0:58 sokolov – nové hamry 142 1:02 table 5. commuting times on selected routes. figure 7. output time positions for regional lines in the karlovy vary and sokolov region. further to prague. purple line is passenger trains outside the competence of the karlovy vary region. 4. discussion the main impulse for the authors to write the article is the vision of the possibility of future use in solving the current dismal state of karlovy vary regional public transport. as the locations determined by the surroundings offered more possibilities for the intracounty routes, the authors explored all the possibilities that opened up and selected the system optimum, it can be said that the locations should serve as a basis for determining the number of vehicles needed and the creation of circulations. the karlovy vary region currently operates modern rail vehicles, and at the same time, the change is so radical that the increase in passengers may be in the tens of percent on some routes (which are the values that both neighbouring regions achieve). only after the implementation of periodic transport will it be possible to determine suitable vehicles for individual lines in the karlovy vary region. however, the current frequencies are a sufficient guide to make an informed estimate of which routes have higher and which have lower potential. at present, both neighbouring regions are dealing with traffic that directly affects their territory, but even passenger demand could increase on these lines due to appropriate measures. for example, if the connections from teplá were linked to the connections to plzeň, it is certain that the number of passengers would increase here as well. neighbouring regions cannot be interested and change the transport on the territory of a foreign region, but it is certain that the proposed measures would bring the karlovy vary region closer to the position of an equal partner in public transport planning for regions that are currently among the best. on the basis of the aforementioned – the article offers ways to improve the quality of rail transport in the karlovy vary region. for even greater materialization of the proposed modifications it seems to be most appropriate to create related vehicle circuits and to determine the type of vehicles and to submit the whole proposal to the authorities responsible for transport in the karlovy vary region, as there is no publicly available information on the performance prices on individual railway lines/train lines. these could show whether the set solution also brings any economic advantage. this article is followed by another article dealing with the integrated timetable in the north-western part of the karlovy vary region: the transport connections cheb – luby u chebu and cheb – hranice v čechách (railway lines without numbers in figure 1), which thus completes the regulation of railway transport in the karlovy vary region. 5. conclusion most of the temporal locations that are presented in the text as research results are largely determined by the locations of the connections to which the investigated connections connect and which are anchored at nodes in the its in neighbouring regions and in germany. it could be argued whether the proposed locations sufficiently respect the local conditions and whether the existing connections will not be disrupted? the answer here is two-fold. firstly, the research and the very essence of periodic transport is to design transport in such a way that (among other things) the number of interchanges is optimized. this criteria is somewhat abstract in terms of measuring if the all possible transfer links are used. and in the end some transfer link isn’t used as transfer because of null transport flows which are not publicly available. second, except for railway lines 143 and 141, these are the backbone lines of the system (or lines that have the potential to become backbone lines), and for such lines, deviations in interval generally occur only at the extremes when travel times and transfer times are 37 jakub hora, filip baran acta polytechnica ctu proceedings shorter, or in the morning peak when surge demand must be accounted for. for railway lines 143 and 141 their nature is reflected and indicated. list of symbols ict integrated cyclic timetable itkv integrated transport of the karlovy vary region itpr integrated transport of the pilsen region its integrated transport system mec municipality with extended area of competence pt public transport trg theoretical railway graph túlr transport of the ústí nad labem region pit prague integrated transport references [1] m. r. bäumerstrasse. openrailwaymap. germany, [2022-07-24], https://www.openrailwaymap.org/. [2] správa železnic. timetable for official use. prague, čd, 2000-2022. 38 https://www.openrailwaymap.org/ acta polytechnica ctu proceedings 43:28–38, 2023 1 introduction 1.1 a modern concept of public transport 2 materials and methods 2.1 definition of the study area, basic transport links, and introduction of basic cyclic positions 2.2 railway line 149 2.3 railway line 161 2.4 railway line 145 2.5 railway line 144 2.6 railway line 142 2.7 railway lines 143 and 141 3 results (graphics of integrated cyclic timetable) 4 discussion 5 conclusion list of symbols references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2025.54.0067 acta polytechnica ctu proceedings 54:67–71, 2025 © 2025 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague prestress load distribution modeling for containment digital twin model matyáš ratislava,∗, bořek patzákb, petr štemberka, jan štěpánc a czech technical university in prague, faculty of civil engineering, department of concrete and masonry structures, thákurova 7, 166 29 prague 6, czech republic b czech technical university in prague, faculty of civil engineering, department of mechanics, thákurova 7, 166 29 prague 6, czech republic c institute of nuclear research řež, a. s. hlavní 130, řež 250 68, husinec, czech republic ∗ corresponding author: matyas.ratislav@fsv.cvut.cz abstract. this paper examines the prestress load distribution in the ducts of the prestressing tendons of a vver-type nuclear reactor containment. it introduces a method for distributing the simplified linear load from the prestressing cable onto the surface area of the duct. furthermore, a procedure for applying this calculation to a detailed numerical 3d model of a containment segment is described. this method can be implemented into computational software, such as oofem, to incorporate this more accurately defined load into a comprehensive stress analysis of the segment. realistic definition of load distribution from prestressing tendons on the surface of the ducts is critical for identifying potential deviations from a uniform prestress distribution within the containment wall. this, in turn, facilitates assessments such as evaluating the extension of the structure’s service life. keywords: unbonded prestressing reinforcement, structural loading, prestressing ducts, digital twin, 3d model. 1. introduction containments represent a crucial component of the protective systems in nuclear power plants using vver nuclear reactors, ensuring safety and integrity in case of emergency situations [1]. these containments are designed as prestressed reinforced concrete structures that utilize prestressing tendons to enhance strength and resilience [2, 3]. the use of a digital twin, which is a virtual model of a physical system, enables analyses and predictions of the behavior of these structures. the aim of this article is to develop a method for determining the surface load from prestressing on the wall of a tendon duct within a containment segment model. calculations performed on this detailed model will be used to refine the digital twin, defined in this paper as a numerical model of the entire containment building. therefore, the precision in defining load distribution from prestress in tendon ducts will directly impact the accuracy of the digital twin. this digital twin will enable improved assessment, prediction, and decision-making in future service-life extension processes for the structure. the structure of the article is as follows: in the first section, the finite element model and its parameters are presented. next, the linear load from the prestressing cables is defined. then, the calculation of the load distribution coefficient is described. following this, the coordinate transformation for applying the calculations discussed in the previous sections to the aforementioned numerical model of the structure is addressed. finally, the results and their potential further applications are discussed. 2. prestress loads in nuclear reactor containment vessels the determination of prestress loads was conducted for the digital twin model of a segment of a vver1000 reactor containment structure. the containment of this reactor is a monolithic, post-tensioned cylindrical reinforced concrete structure, with its walls prestressed by a cable system arranged in cross loops. this prestressing system consists of unbonded tendons arranged in a helical pattern [4, 5]. the modeled segment is a three-dimensional finite element model designed for detailed stress analysis within the containment structure. although this model has been employed in previous studies [6], it can be enhanced using updated data, advanced methodologies (e.g., new optimization techniques for defining boundary conditions or the approaches discussed in this paper), and improved software tools. the model represents a typical cross-section of the containment structure, intentionally excluding the effects of openings, supports, and the containment ring beam on the stress distribution within this segment. due to the containment size, creating such a detailed finite element model for the entire containment is impractical as it would result in extremely demanding computations. the chosen segment has a height of 2.12 meters, a thickness of 1.2 meters, and an arc width of 3 meters at its midplane. the model also includes reinforcing 67 https://doi.org/10.14311/app.2025.54.0067 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en m. ratislav, b. patzák, p. štemberk, j. štěpán acta polytechnica ctu proceedings figure 1. schematics of numerical model for containment segment. steel bars represented by 1d elements. the unbonded post-tensioning tendons are modeled as external loads acting on the inner walls of the posttensioning ducts, which are represented as cavities within the segment mass (see figure 1). the curvature of both the segment and the ducts corresponds to the curvature of the entire containment. the finite element mesh comprises tetrahedral elements, with a higher mesh density in the regions adjacent to the inner walls of the ducts. the method of applying the post-tensioning load is crucial for stress development near the ducts, thus special attention was paid to the analysis of load distribution. 2.1. assessment of line loads for a detailed determination of the load due to prestressing at any point within the duct, it is first necessary to establish the total line load exerted by the cable along its length on the structure. this line load varies along the length of the cable due to prestress losses and has two components: the first acting in the direction of the axis of the containment cylinder, and the other acting along the direction of the cable (frictional force, which reduces the prestress in the cable) [7]. for the digital twin model, the measurement of prestressing force in the anchorages of the prestressing cables fixed to the containment ring beam can be utilized [8, 9]. the measured force (n0) can be reduced by losses due to friction up to the point of interest at a distance s from the prestressing location (see figure 2). from this calculated force in the cable, the line loads can subsequently be determined. the prestressing force n in the cable at a distance s from the point of prestressing can be determined using the following equation n(s) = n0 · exp ( −s · tan(ϕ) r · (1 + tan2(ω)) ) , (1) where n0 is the force at the prestressing location, tan(ϕ) is the friction coefficient, r is the radial distance of the cable from the containment axis, and ω is the vertical cable inclination [7]. figure 2. schematic of line load from prestressing on containment structure. this calculation is based on the parametrization of prestressing cable paths (helix) along the arc. the line load from the prestressing cable (for detailed explanation see [7]) acting in the radial direction to the containment center (along the internal normal) is given by ftot,cent = n(s) r · (1 + tan2(ω)) . (2) the line load acting in the direction of the prestressing cable (tangent to the cable axis) is ftot,fri = ftot,cent · tan(ϕ). (3) for graphical representation of these loads, see figure 2. 2.2. load distribution on duct surface in the previous chapter, the duct was considered as a 1d element defined solely by its centerline. to define the magnitude of the load on the duct surface, the linear load calculated according to the procedure described above must be multiplied by a load distribution coefficient within the duct, denoted as ξ. this coefficient ensures the distribution of the linear load over the surface of the duct. the coefficient ξ depends on the angle µ – the deviation of the normal to the duct surface at the investigated point from the normal to the plane of the structure (i.e., the direction towards the center of the containment), see figure 3. since it is assumed that the load acts only on parts of the inner surface of the duct, the coefficient ξ takes non-zero values only within the interval µ belonging to (−90◦, 90◦). to maintain the calculation for the simplified 1d cable, it must for duct radius rd hold that: rd · ∫ 90◦ −90◦ ξ(µ) dµ = 1. (4) the determination of the load distribution – i.e., the function ξ(µ) – was based on the assumed distribution 68 vol. 54/2025 prestress load distribution modeling figure 3. schematic of µ angle definition. figure 4. ilustrative photograph showing bend of unbounded prestressing cable. of reinforcement within the duct. it is assumed that the non-bonded prestressing reinforcement will be pressed as much as possible against the inner side of the bend (neglecting the influence of the cable self weight on its distribution). the individual wires will be densely packed, and although the prestressing system considered does not involve individual wires being twisted into a complete cable, it is assumed that due to slight twisting of the wire bundle during installation, the wires will tend to stay more packed together. it can thus be considered that the distribution of wires within the duct will resemble the shape of an ellipse or a lens. this shape can be simplified to a lens, with one side formed by a circular segment (representing the wall of the duct) and the other side by a parabola. the parameters of the parabola can be defined from the known width of the wire bundle distribution within the duct and the area enclosed by the shape. the width of the bundle w was estimated based on a visual assessment from a photograph of the bend in cylindrical cables (refer to figure 4). the area of the wire bundle was calculated as the crosssectional area of the wires multiplied by the coefficient η, which accounts for the space between the wires. the coefficient η for the densest possible arrangement of wires is 1.102. this value was determined graphically as the ratio of the total area as + a0 to the area of the circular prestressing wires as in the tightest configuration, see figure 5. due to the shape figure 5. graphical basis for η estimation. figure 6. example of load distribution in tendon duct. of the duct and the twisting of the bundle, the arrangement will be more relaxed, and η will therefore be higher – the coefficient η was estimated graphically as 1.15. the graphical solutions were performed for a tube with a diameter of 225 mm and 450 wires. the parameters a and b of the parabola can be determined from the following equations 2 · ∫ w/2 0 (fp(x) − fd(x)) dx = ap · η, (5) for x = w/2 : fp(x) = fd(x), (6) where the parabola function is fp(x) = ax2 + b, the duct function is fd(x) = rd − √ r2 d − x2, ap is the area of the prestressing reinforcement, and x is as shown in figure 5. neglecting the self-weight of the wires, the wire distribution is symmetrical, and it is therefore possible to integrate over only half of the figure. assuming that the distribution of the function ξ(µ) is similar to the distribution of the prestressing wires, it can be defined for the angle of deviation µ as follows ξ(µ) = fp(x(µ)) − fd(x(µ)) 2rd · ∫ α 0 (fp(x(µ)) − fd(x(µ))) dµ , (7) where x = rd sin(µ) and α = arcsin(w/(2rd)). an example of the ξ distributionin is shown in figure 6. 2.3. coordinate transformation for segment model to define the load from the prestressing cables at point p with coordinates xg = {xg; yg; zg}, which lies on the surface of the ducts in the presented model, 69 m. ratislav, b. patzák, p. štemberk, j. štěpán acta polytechnica ctu proceedings figure 7. schematics of employed coordinate systems. several steps are necessary. this process involves coordinate transformation, the calculation of resulting loads, and their subsequent transformation back to global coordinates. first, the input global coordinates xg must be transformed into the coordinates of the developed model xu, and subsequently transformed into the local coordinates of the duct xl, on whose surface the investigated point is located. these coordinate systems are schematically depicted in figure 7. the transformation from global coordinates xg to coordinates in the unwrapped coordinate system xu is defined by the following relationships xu =  atan ( xg yg ) · r√ x2 g + y2 g zg  . (8) the transformation from coordinates xu to the local coordinates of the duct xl is defined as follows xl = t · xu =  sin(ω) 0 cos(ω) 0 1 0 − cos(ω) 0 sin(ω)  · xu, (9) where ω is the vertical inclination of the ducts. to perform the calculation, it is further necessary to determine the position c on the duct axis where a cut made by a normal plane will intersect the investigated point. for this purpose, it is required to know the global coordinates of any point on this axis. the coordinates of the mentioned point are converted to local coordinates in the same manner as for point p. these coordinates differ from the coordinates of point c only in the local coordinate z, which is equal to zl of point p and is thus already known from the previous calculation. the height of point c in the global coordinate system is obtained as follows zg,c = xl,c · cos(ω) + zl,c · sin(ω). (10) the input for calculating the line load is the distance s (as given in the equations 2 and 3). this distance can be obtained as: s = h − zg,c sin(ω) . (11) if yl,p ≤ yl,c, then for computing the load distribution coefficient ξ, the angle µ is required (else ξ = 0 as discussed earlier), which can be calculated as µ = atan ( xl,p − xl,c yl,p − yl,c ) . (12) after determining the load magnitude from equations 2 and 3) and multiplying it by the coefficient ξ (acquired via equation 7), it is necessary to transform the results back to the global coordinate system. see the following equations fu = t t ·  0 −ftot,cent · ξ ftot,fri · ξ  , (13) fg =  yg,p/yu,p xg,p/yu,p 0 −xg,p/yu,p yg,p/yu,p 0 0 0 1  · fu. (14) 3. conclusion this study provides a detailed analysis of prestress loads in the containment structure of a vver-1000 nuclear reactor for use in a finite element 3d model. the results obtained, which include a reasonably precise determination of the surface loading by prestressing cables at any point on the duct surface based on its global coordinates (see figure 8), contribute to a better understanding of the stress distribution within the structure. the determination was based on engineering estimates informed by observations of actual constructions. the presented computational approach can be effectively implemented in software programs such as oofem, facilitating its practical application in the detailed examination of such structures. subsequent steps will involve incorporating the prestress loads into the presented model, which will also account for additional acting loads. a comprehensive stress analysis of these combined loads will then enable a more accurate identification of potential deviations from a uniform prestress distribution within the containment wall. the results of this analysis will contribute to the precision of the containment digital twin model, which is used to evaluate the extension of the containment lifespan. 70 vol. 54/2025 prestress load distribution modeling figure 8. schematics of load evaluation process and illustrative comparison of simplified and refined load distribution. acknowledgements the presented work has been realized within institutional support by ministry of industry and trade of the czech republic and the ctu in prague project sgs24/038/ohk1/1t/11. references [1] international atomic energy agency. design of the reactor containment and associated systems for nuclear power plants. iaea safety standards series no. ssg-53. iaea, vienna, 2019. [2] z. janda, j. zeman, j. šejnoha. zjednodušené modely interakce volného přepínacího kabelu s konstrukcí [in czech; simplified interaction models of unbounded tendon with structures]. stavební obzor 18(2):44–48, 2009. [3] nuclear containments. tech. rep. 13, fib (international federation for structural concrete), 2001. [2024-07-30]. https://doi.org/10.35789/fib.bull.0013 [4] d. tonarová, p. brabec, z. fencl, et al. hodnocení předpínacích kabelů vyjmutých z kontejnmentu ete [in czech; evaluation of prestressed cables removed from npp temelín containment]. in zvyšování životnosti komponent energetických zařízení v elektrárnách. újv řež, a. s., újv řež, a. s., výzkumný a zkušební ústav plzeň s.r.o., čvut v praze, fakulta strojní, čez, a. s., zču v plzni, fakulta strojní, rti, 2019. [5] p. rericha, j. sejnoha. partial service life assessment of a reactor containment. nuclear engineering and design 235(23):2451–2462, 2005. https://doi.org/10.1016/j.nucengdes.2005.06.010 [6] z. bittnar, j. šejnoha, t. koudelka, et al. je temelín, zs1020 – opatření pro dlouhodobý stabilní a akceptovatelný provoz ochranných obálek je temelín [in czech; npp temelín, zs1020 – measures for the long-term stable and acceptable operation of the containment structures of the temelín npp]. [7] t. krejčí, j. šejnoha. prognóza ztrát předpětí v kabelech válcové části ochranné obálky jete [in czech; prediction of prestress losses in the cables of the cylindrical part of the containment structure of the npp temelín]. tech. rep., české vysoké učení technické v praze, fakulta stavební, katedra mechaniky, praha, 2003. [8] state office for nuclear safety. národní hodnotící zpráva české republiky pro účely topical peer-review „řízené stárnutí“ dle směrnice o jaderné bezpečnosti 2014/87/euratom [in czech; national assessment report of the czech republic for the topical peer review on “ageing management” under the nuclear safety directive 2014/87/euratom], 2017. [9] p. anderson. thirty years of measured prestress at swedish nuclear reactor containments. nuclear engineering and design 235(21):2323–2336, 2005. https://doi.org/10.1016/j.nucengdes.2005.04.002 71 https://doi.org/10.35789/fib.bull.0013 https://doi.org/10.1016/j.nucengdes.2005.06.010 https://doi.org/10.1016/j.nucengdes.2005.04.002 acta polytechnica ctu proceedings 54:67–71, 2025 1 introduction 2 prestress loads in nuclear reactor containment vessels 2.1 assessment of line loads 2.2 load distribution on duct surface 2.3 coordinate transformation for segment model 3 conclusion acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2025.52.0063 acta polytechnica ctu proceedings 52:63–68, 2025 © 2025 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague abrupt change detection in railway noise data jan kruntoráda,∗, tetiana reznychenkob, petr červenkaa a czech technical university in prague, faculty of transportation sciences, department of transport engineering, konviktská 20, 110 00 prague, czech republic b czech technical university in prague, faculty of transportation sciences, department of applied mathematics, konviktská 20, 110 00 prague, czech republic ∗ corresponding author: kruntjan@fd.cvut.cz abstract. current methods for diagnosing the quality of the railway superstructure are mainly based on optical sensors, which are relatively expensive compared to acoustic sensors. as part of the hlukos research project, a pair of microphones is installed near the wheel-rail contact point on a diagnostic vehicle of the railway administration (czech railway infrastructure manager). the research task is to detect when the sound level changes significantly. a likelihood ratio method has been used in this paper to detect abrupt changes, which is a current scientific topic. experiments with different input thresholds are performed on a sample of 250 m of track data. initial experimental results show that this method is meaningfully able to detect locations of abrupt changes with input threshold values h = 4.58 and number of steps from n = 5 to n = 40. keywords: abrupt change, change detection, likelihood ratio, railway diagnostics, railway noise, wheel-rail contact. 1. introduction the last decades have been marked by the development of technological processes and sophisticated sensors, followed by the need for sophisticated information processing systems. addressing these issues is important for a number of reasons, particularly safety, environmental and economic. the large amount of data from a wide range of sensors means that algorithms must be found or developed to analyse the information. the core of the interest is the detection of one or more abrupt changes in some characteristic of the system under consideration. an abrupt change is considered to be any change in system parameters that occurs either immediately or at least very quickly relative to the measurement sampling period used. abrupt changes certainly do not refer to changes of large magnitude; on the contrary, in most applications the main problem is the detection of small changes. moreover, in some specific cases, early warning of small (and not necessarily fast) changes is essential to avoid economic or even fatal consequences, such as small malfunctions in the sensors of aircraft navigation systems or small variations in the geometric position of the track [1]. the quality of track geometry, respect of clearance profile and other parameters necessary for safe railway operation are continuously monitored and evaluated by infrastructure managers. one of the objectives of the hlukos research project is to determine whether abrupt changes in rolling noise recordings at the wheelrail contact point can be detected, and to use these changes to warn of possible local degradation (defects) of the railway superstructure. 2. theoretical background abrupt change detection in noisy signals is a critical area of research that spans various fields, including signal processing, climate science, and power systems. the methods employed for detecting these changes can be broadly categorized into wavelet-based techniques, statistical approaches, and machine learning methods. each of these methodologies offers unique advantages and is suitable for different types of signals and noise characteristics. 2.1. current methods used wavelet transforms have emerged as a powerful tool for detecting abrupt changes in signals, particularly in the presence of noise. the wavelet transform allows for the decomposition of a signal into its constituent components at various scales, making it easier to identify discontinuities or abrupt changes. for instance, ukil and živanović [2] demonstrated the effectiveness of using an adjusted haar wavelet for segmenting power system disturbance signals into event-specific sections, highlighting the method’s robustness in noisy environments. in addition to wavelet methods, statistical techniques such as the kolmogorov-smirnov statistic have been utilized for abrupt change detection. qi et al. [3] proposed a fast framework based on binary search trees and the kolmogorov statistic to identify changes in the statistical properties of signal series, which are indicative of qualitative transitions in the underlying data generation mechanism. this approach is more efficient due to the shortest computation time, the highest hit rate and accuracy than compared conventional methods kolmogorov-smirnov statistic, t-statistic and 63 https://doi.org/10.14311/app.2025.52.0063 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en j. kruntorád, t. reznychenko, p. červenka acta polytechnica ctu proceedings singular-spectrum analyses which was tested on real eeg recordings. another significant method for detecting abrupt changes is the use of bayesian approaches, particularly in online applications such as automatic speech recognition (asr). chowdhury et al. [4] emphasized the importance of developing noise estimation algorithms that continuously track and estimate noise levels, enabling the detection of abrupt changes in the noise spectrum. this adaptability is crucial in real-time systems where the acoustic environment can vary significantly. in the realm of climate science, various detection methods have been developed to identify regime shifts, which can be classified into categories based on the type of change – mean value, variance, frequency, probability density changes, and multivariable analysis [5]. these methods provide a comprehensive framework for analyzing abrupt changes in climate data, emphasizing the need for tailored approaches depending on the nature of the data. the integration of machine learning techniques into abrupt change detection has also shown promise. for instance, sparse identification of nonlinear dynamics has been utilized for rapid model recovery in chaotic systems, allowing for efficient detection of abrupt changes with reduced data requirements [6]. this approach exemplifies the trend towards leveraging advanced computational techniques to enhance detection capabilities in complex systems. in summary, the methods for detecting abrupt changes in noisy signals are diverse and multifaceted, encompassing wavelet transforms, statistical techniques, bayesian approaches, and machine learning. each method has its strengths and is suited to particular types of signals and noise characteristics. in this paper we focus on another technique for detecting abrupt changes, namely the use of likelihood ratio. it is based on the principle of change detection on difference synthetic aperture radar images [7] or pipeline damage detection using torsional guided wave mode [8]. 2.2. problem formulation lets have a dataset which a sequence of signal level observations x = [x1, x2, . . . , xn], where n is a number of observations. the task is to find a sufficient change in the signal level. this is achieved by identifying the intervals at which significant changes in signal strength occur. this allows for the identification of potential issues and an improvement in the accuracy of data analysis. 2.3. likelihood-ratio change detection likelihood-ratio change detection is a statistical method that is used to detect a point in time when the distribution of signal yt changes [9, 10]. the goal of the detection algorithm is to determine s(yt) when the distribution shifts from f(yt, θ0) to f(yt, θ1) where it is assumed that θ0 and θ1 are known. at each time step t, the log-likelihood-ratio is computed as: s(yt) = log f(yt, θ1) f(yt, θ0) (1) this ratio in eq. (1) compares the probability of observing the current value of the signal yt under the assumption that a change has already taken place (with parameter θ1) to the probability of observing the same value under the assumption that no change has yet occurred with θ0. the following algorithm solves the detection problem. it detects the change in the level of the signal under test. it is possible to apply this approach to the fourier transform, thus capturing changes in frequency. initialization. set the values of the signal yt, θ0, and θ1 – means, σ – standard deviation of the data set. signal observation. the signal yt is sampled in k, where k is the sequence number of the large period of sampling. in addition, each large period is divided into n small periods, as indicated in figure 1. figure 1. illustration of the signal divided into k intervals [9]. compute and update statistics. calculate the likelihood ratio for yt on n -th small interval, and update statistics over small intervals using the alpha coefficient in eq. (2). the closer α is to 1, the more weight is given to the value of st−1: st = α · st−1 + (θ1 − θ2) σ2 [ yt − θ1 + θ2 2 ] , (2) where st is the current value of the statistic, yt is the current observed value, and α is the coefficient of forgetting where 0 < α < 1. after obtaining the result on the small interval n , the value of sn 1 (k) big interval is computed, as shown in eq. (3), where sn 1 (k) is the value of the statistic on the k-th interval. sn 1 (k) = (θ1−θ0) σ2 ∑n t=n(k−1)+1 k [ yt − θ1+θ0 2 ] . (3) threshold test. compare sn 1 (k) with a predetermined threshold h. the decision rule is: d = { 0, if sn 1 (k) < h, 1, if sn i (k) > h, (4) 64 vol. 52/2025 abrupt change detection in railway noise data where sn 1 (k) = snk n(k−1)+1 [10]. if sn 1 (k) > h, it is considered that there was a change in the distribution from θ0 to θ1. 3. railway noise data the purpose of the experiment part is to find intervals where noise significantly and rapidly changes using the likelihood ratio method. within the framework of the hlukos research project, data collection is carried out by means of microphones placed symmetrically on the wheelset of the diagnostic vehicle mvžsv2 (owned and operated by czech rail infrastructure manager: technology and diagnostics centre, railway administration, state organisation), in such a reference position (see figure 2), which is closest to the measured wheel and, according to further technical analysis and listening to the sound recording, shows the lowest level of interference by air flow and other ambient phenomena. the key is the choice and correct configuration of the sound analyser, whose function is to process the data recorded by the measuring microphones in real time and convert them into specific acoustic parameters. the problem lies in the combination of the high speed of the diagnostic vehicle (currently max. 160 km h−1, in the near future up to 200 km h−1) and the requirement for very fine sampling of the track parameters. for the solution of this project, a specially designed sensor-acoustic system developed by the project team – ekola group, spol. s r.o., and ctu in prague, faculty of transportation sciences – was used. this system enables the collection of acoustic data with an accuracy comparable to conventional sound measurement technology at a sampling frequency of up to 48 khz, which corresponds to a sampling duration of 0.02 ms [11]. the study was conducted in a station throat section with three single rail switches running against the switch blades at a low speed, within the range of 35.6 to 36.8 km h−1. the data set used of 1 001 observations contained two key variables: id and railway noise values. the id variable is dimensionless and indicates the order of observations, with measurements taken every 0.25 m of track. the id range is from 0 to 1 000, which means that the measured track sample is 250 m long. the next variable is the noise in db. these values reflect the amplitude of the z-network (unweighted) equivalent sound pressure level recorded at corresponding id points. the range of noise values is from 87.5 to 118.4 db. 4. results and discussion a model was defined with the parameters θ0 = 90 and θ1 = 107. these values represent the mean values obtained from the measured signal, which was segmented into two distinct models. the signal was divided into k samples to determine the moment of signal change. figure 2. position of the microphone for measuring the contact noise between the rail wheel and the rail on the mvžsv2 diagnostic vehicle [12]. table 1 presents the number of samples on which sn 1 (k) changes from θ0 to θ1 for different values of n , where n is 5, 10, 20, 40. the threshold h was established at 4.58. n 5 10 20 40 total 15 9 5 3 table 1. demonstration of the relationship between segment length n and detection sensitivity h. figures 3–6 presents a comprehensive chart detailing the detection of changes in railway noise. the top graph displays the raw signal data, the middle graph illustrates the values of the st statistic on the k-th sample, and the bottom graph visualizes the decision-making process for detecting changes in the signal, based on the analysis of sn 1 (k) and h. the coefficient α was applied during calculations over small periods, preserving α = 0.95 when n = 5 and n = 10, and α = 1.0 when n is 20 and 40. for each combination of parameters n and h, the results have shown the number of detected changes were obtained: • at n = 5 and h = 4.58, changes were identified due to the high sensitivity of the threshold, • at n = 10 and h = 4.58, the sensitivity decreased and the number of false alarms decreased, • at n = 20 and h = 4.58, the lowest number of false alarms and the most accurate detection of changes were recorded, • at n= 40 and h = 4.58, three samples exhibited a change in the signal, and no other notable peaks were identified. this suggests that this specific combination of segment length and threshold might limit the detection of additional signal changes. 5. conclusions listening to the sound recording, three clear sound perceptions can be heard in the analysed section of track (at id = 81, 251 and 834). the abrupt change at id = 81 was only detected at n = 5; at ids = 251 and 834 at id = 834 at all n values examined. 65 j. kruntorád, t. reznychenko, p. červenka acta polytechnica ctu proceedings figure 3. abrupt change the visualization of railway noise using likelihood ratio detection: graph of signal detection with the sample length n = 5. figure 4. abrupt change the visualization of railway noise using likelihood ratio detection: graph of signal detection with the sample length n = 10. 66 vol. 52/2025 abrupt change detection in railway noise data figure 5. abrupt change the visualization of railway noise using likelihood ratio detection: graph of signal detection with the sample length n = 20. figure 6. abrupt change the visualization of railway noise using likelihood ratio detection: graph of signal detection with the sample length n = 40. 67 j. kruntorád, t. reznychenko, p. červenka acta polytechnica ctu proceedings the experiment demonstrated that the parameter n is essential for the accurate detection of signal changes. shorter segments (n = 5) enhance sensitivity but increase the likelihood of false alarms. the threshold h must be selected to achieve an optimal balance between sensitivity and accuracy. with a chosen threshold of h = 4.58, a value of n in the range 20–40 seems to be optimal. this research paper advances the knowledge base and capabilities for identifying railway noise, paving the way for more effective monitoring and maintenance strategies. ultimately, this research contributes to the enhancement of railway safety and reliability. acknowledgements this paper has been financed with the state support of the technology agency of the czech republic and the ministry of transport of the czech republic within the project ck03000099 dynamic opto-acoustic method for railway superstructure emission noise assessment within the transport 2020+ programme. references [1] j. kruntorád, l. týfa. vision zero application on czech railways. acta polytechnica ctu proceedings 41:15–19, 2023. https://doi.org/10.14311/app.2023.41.0015 [2] a. ukil, r. živanović. adjusted haar wavelet for application in the power systems disturbance analysis. digital signal processing 18(2):103–115, 2008. https://doi.org/10.1016/j.dsp.2007.04.001 [3] j.-p. qi, j. qi, q. zhang. a fast framework for abrupt change detection based on binary search trees and kolmogorov statistic. computational intelligence and neuroscience 2016(1):8343187, 2016. https://doi.org/10.1155/2016/8343187 [4] m. f. r. chowdhury, s.-a. selouani, d. o’shaughnessy. bayesian on-line spectral change point detection: a soft computing approach for on-line asr. international journal of speech technology 15(1):5–23, 2012. https://doi.org/10.1007/s10772-011-9116-2 [5] q. liu, s. wan, b. gu. a review of the detection methods for climate regime shifts. discrete dynamics in nature and society 2016(1):3536183, 2016. https://doi.org/10.1155/2016/3536183 [6] m. quade, m. abel, j. nathan kutz, s. l. brunton. sparse identification of nonlinear dynamics for rapid model recovery. chaos: an interdisciplinary journal of nonlinear science 28(6):063116, 2018. https://doi.org/10.1063/1.5027470 [7] h. zhuang, z. tan, k. deng, g. yao. adaptive generalized likelihood ratio test for change detection in sar images. ieee geoscience and remote sensing letters 17(3):416–420, 2020. https://doi.org/10.1109/lgrs.2019.2922198 [8] s. mariani, p. cawley. change detection using the generalized likelihood ratio method to improve the sensitivity of guided wave structural health monitoring systems. structural health monitoring 20(6):3201–3226, 2021. https://doi.org/10.1177/1475921720981831 [9] m. basseville, i. v. nikiforov. detection of abrupt changes: theory and application. prentice-hall, new jersey, 1993. isbn 978-0-13-126780-0. [10] k. s. riedel. detection of abrupt changes: theory and application. technometrics 36(3):326–327, 1994. https://doi.org/10.1080/00401706.1994.10485821 [11] j. kruntorád, l. týfa, o. simon. analysis of noise emissions at wheel-rail contact. in o. prentkovskis, i. yatskiv (jackiva), p. skačkauskas, et al. (eds.), transbaltica xv: transportation science and technology, pp. 502–511. springer nature switzerland, cham, 2025. isbn 978-3-031-85390-6. https://doi.org/10.1007/978-3-031-85390-6_47 [12] l. ládyš, o. simon, m. ládyš, et al. ck03000099 dynamická opto-akustická metoda hodnocení emisní hlučnosti železničního svršku. interim report 2023, ekola group, prague, 2024. 68 https://doi.org/10.14311/app.2023.41.0015 https://doi.org/10.1016/j.dsp.2007.04.001 https://doi.org/10.1155/2016/8343187 https://doi.org/10.1007/s10772-011-9116-2 https://doi.org/10.1155/2016/3536183 https://doi.org/10.1063/1.5027470 https://doi.org/10.1109/lgrs.2019.2922198 https://doi.org/10.1177/1475921720981831 https://doi.org/10.1080/00401706.1994.10485821 https://doi.org/10.1007/978-3-031-85390-6_47 acta polytechnica ctu proceedings 52:63–68, 2025 1 introduction 2 theoretical background 2.1 current methods used 2.2 problem formulation 2.3 likelihood-ratio change detection 3 railway noise data 4 results and discussion 5 conclusions acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2024.46.0065 acta polytechnica ctu proceedings 46:65–84, 2024 © 2024 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague municipal waste management performance: a focus on slovakia and its lau-1 districts eleonóra marišová∗, peter fandel slovak university of agriculture in nitra, faculty of european studies and regional development, institute of law, trieda a. hlinku 2, 949 76 nitra, slovakia ∗ corresponding author: eleonora.marisova@uniag.sk abstract. in this paper, we analyse the changes made to the basic eu directive on waste and assess its impact on the waste legislation of eu members. we then examine the slovak waste strategies/programs that have implemented the eu directive on waste, namely the waste prevention program, the waste management program, and the envirostrategy 2030. based on eu waste legislation, the environmental strategy 2030 sets the waste treatment aims for slovakia until 2030. however, it is questionable whether slovakia will achieve the set goals. our research indicates that as of 2021, slovakia’s rate of waste incineration with energy recovery and landfilling rate of municipal waste are below the eu average, while the recycling rate, both for materials and composting and digestion, is higher. in our quantitative analysis, we examine the progress of waste management performance in slovakia from 2017 to 2021, focusing on the lau-1 districts. we estimate composite efficiency indicators using the techniques of data envelopment analysis and malmquist indices. in accordance with the hierarchy of waste treatment methods, the applied models consider desirable waste operations variables (recycling and incineration with energy recovery) and undesirable waste operation variables (landfilling). our results reveal significant variations in efficiency across the lau-1 districts. the average technical efficiency of the 72 districts has improved from 0.714 in 2017 to 0.852 in 2021, indicating that the performance of districts is generally improving and catching up with the best-performing districts. the total performance, as measured by the malmquist index, has improved by 45.5 %. districts with access to waste incineration facilities with energy recovery have exhibited higher efficiency scores, benefitting from this advantage. keywords: waste legislation, strategies, municipal waste management, lau-1 districts, performance, composite indicators, data envelopment analysis, malmquist index. 1. introduction sustainability issues are closely connected to economic growth of all countries in the world and waste generated by developed and less developed countries. slovakia, similarly, to other countries worldwide committed to the fulfilment of 17 sustainable development goals (sdgs) [1]. the philosophy includes environmental, economic, and social pillars of sustainability. we focus mainly on the goal 12 of sdgs: ensure sustainable consumption and production patterns. this goal should be fulfilled by 2030 as it is formulated: substantially reduce waste generation through prevention, reduction, recycling, and reuse. our paper examines how slovak and european waste legislation and waste programs boost municipal waste recycling and advance the circular economy that affect each other. if we talk about municipal waste in connection with circular economy, each european produces about 500 kg of waste per year. less than half of it is 46 % recycled, 27 % is incinerated and 24 % is landfilled [2]. the paper is aimed at analysis of the fundamental eu legislation and programs relating to minimizing the waste in the eu member states and their impact on waste management performance in slovakia. first partial objective is to analyse the changes of the basic eu directive on waste – directive 2008/98/ec of the european parliament and of the council of 19 november 2008 [3] on waste that was amended by directive (eu) 2018/851 of the european parliament and of the council of 30 may 2018 [3] (hereafter waste framework directive). this revised directive brought many stipulations that had to be implemented into the waste legislation of the eu member states to increase waste incineration with energy recovery, use good techniques for its recycling and minimize landfilling. eu member states are to take among others, measures to support the design, production and use of products that use resources efficiently, are durable, repairable, reusable, and updatable. except of this, the measures have to set aims how to reduce food waste as a contribution to the united nations sustainable development goal [4] of reducing global food waste per capita by 50 % by 2030 at retail and consumer level. member states must by january 1, 2025, establish a sorted collection for textiles and hazardous waste from households and ensure that by december 31, 2023, biological waste is either sorted or recycled at the source (e.g., by composting). since the waste framework directive establishes the legal basis in the field of waste man65 https://doi.org/10.14311/app.2024.46.0065 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en eleonóra marišová, peter fandel acta polytechnica ctu proceedings agement in the european union, we deal with the slovak waste legislation and strategies/programs that implemented this eu directive. we analyse the implementation of eu waste legislation into slovak waste prevention program 2018 [5], slovak waste management program, 2021 [6] and environmental strategy 2030 [7]. the second objective of this paper is to analyse how slovak legislation and adopted strategies influence the municipal waste management performance. the analysis is conducted at the level of lau-1 districts. the aim is to assess the trends in waste management performance using waste treatment-specific indicators, as well as composite indicators based on data envelopment analysis and malmquist index methodology. this paper aims to bridge a gap in the current literature by providing a comprehensive analysis of municipal waste management performance at subregional level of lau-1 districts in slovakia, in relation to the environmental goals of eu and slovakia, employing both partial waste treatment-specific indicators and composite indicators the findings will be relevant to waste management practitioners, policymakers, and researchers who are interested in enhancing the efficiency and sustainability of waste management practices and achieving the environmental objectives set by the eu. the paper is structured as follows: section 2 provides the theoretical background of the topic, section 3 describes the data and methodology used, section 4 presents and discusses the results, and section 5 concludes the paper. 2. theoretical background the theoretical background is processed mainly through the interpretation of the basic eu legislation on waste – directive (eu) 2018/851 [3] of the european parliament and of the council of 30 may 2018 amending directive 2008/98/ec on waste [3], which has undergone several serious changes. in july 2018, the eu circular economy package was introduced by the eu commission with particular relevance for management of municipal waste. it is intended to serve the objectives of growth and employment while at the same time ensuring and strengthening environmental protection [8]. rogge et al. [9] state that focus of eu municipal solid waste policy shifted from basic waste handling standards to promoting, recycling, re-use, and energy recovery. an important guiding principle in the designs of eu waste legislation and policy is the so called “waste management hierarchy”. the slovak legislation on waste is represented by waste legislation act no. 79/2015 coll. as amended [10] and slovak programs: waste prevention programme [5] for years 2019-2025, environmental strategy 2030 [7] and waste management programme [6] for years 2021–2025, responding to the eu legislation in the field of waste. we deal in the theory also by amended legislation on waste in the slovak republic – act on waste no. 79/2015 coll. [1] in the valid wording and opinions of foreign and domestic authors who make research in the field of waste. environmental strategy 2030 [7] set the slovakian aims till 2030, that the municipal waste recycling rate, including the preparation for re-use, will be increased to 60 %, and the land-filling rate will be reduced to less than 25 % by 2035. a green procurement will cover at least 70 % of the total number of all public procurements, and the support for green innovation, science and research will be at a comparable level to the eu average. the energy intensity of the slovak industry will be closer to the eu average, and by 2020, the sustainability criteria for all renewable energy production sources will be developed and accepted. the production of electricity and heat from coal will be gradually reduced [7]. cleaning and wasting are quite familiar to us, and once discarded their products have to be dealt with somehow or managed. yet in many ways research on what becomes of all that we discard has only just begun [11]. in charge of the waste management in slovakia are both state and self-government. today, adequate waste services are considered vital to the governance of cities, industries, and refugee camps: a basic human right, an economic opportunity and an ecological imperative [11]. municipal waste is the total amount of used materials coming from households and smaller local businesses, where the collection is ensured by the local government [12]. mura [13] states that municipalities and regions are part of the public administration system, which are most closely connected with the everyday life of citizens. the waste management system consists of the whole set of activities related to handling, treating, disposing or recycling the waste materials [14]. we agree with kahle et al. [15] who states that sustainability is the ability to remain productive indefinitely. but knowledge is still lacking on local waste prevention, especially regarding the methods for monitoring and how local waste management systems can be designed to encourage waste reduction in the households [16]. waste management is one of the major environmental concerns in the world. human activities and changes in lifestyles and consumption patterns have resulted in an increase in solid waste generation rates [14]. the differences in terms of waste handling among the eu members states are immense. slovakia belongs to the lowest quartile of eu states in terms of waste volumes disposed of by landfilling [17]. the current eu legislation increases the requirements on knowledge of food producers, in particular on the packaging of the product, which must fit even more information in a reasonably large font, and on 66 vol. 46/2024 municipal waste management performance: a focus on slovakia . . . the environment, as the amount of waste produced increases with increasing packaging area [18]. we appreciate that beside the eu member states, the republic of serbia as part of the negotiations for eu accession, has begun the process of establishing a waste management system and adapting it to the goals and acquis communautaire. the key document in serbia that aims for environmental awareness is called the waste management program [6] of the republic of serbia and it harmonizes the waste management in the candidate state with eu regulations [19]. there has been a significant body of literature addressing municipal waste management performance. simões and marques [20] conducted a literature review of 107 studies published from 1965 to 2011, providing a comprehensive overview of the topic. in the past decade, numerous studies have been published, focusing on various aspects of waste management performance. the first aspect examined in the literature is the waste management performance of different geopolitical entities, with a particular focus on eu countries or eu nuts2 regions. examples of such studies include chiaotto [21, 22], khan et al. [23], rios [24], and rogge et al. [9, 25], which commonly utilize eurostat datasets. while these studies offer valuable insights, less frequent are subregional analyses that specifically analyse district or municipal-level data. some notable examples of these subregional studies include rogge-jaeger [25], peréz-lópez [26], and struk and boďa [27]. the second aspect explored in the literature pertains to the different methods employed to estimate performance indicators in waste management. while partial waste treatment-specific indicators are frequently used, their ranking ambiguity has led to the adoption of more sophisticated methods that enable the estimation of composite indicators. a mathematical programming approach to model composite indicators can be found in zhou et al. [28]. other approaches such as multi-criteria decision-making methods, employed in castillo [29], have found wide applicability in this field. another commonly used method is data envelopment analysis (dea), utilized in studies by castillo [29, 30], delgado-antequera [31], perézlópez [26], rios [24], and struk and boďa [27]. other dea derivatives, such as the benefit-of-doubt (bod) method used in chiaotto [22] and rogge [9], directional distance functions applied in villavicencio [32] and ye [33], and the free disposal hull (fdh) employed by rios [24], have also been applied. the third topic thoroughly analysed in literature is the selection and classification of variables in waste management performance models. key discussions revolve around the hierarchy and weighting of variables [34], as well as the identification of desirable and non-desirable variables and their controllability [31]. by considering the insights provided by these different strands of literature, a comprehensive understanding of municipal waste management performance can be achieved, resulting in the design of a reliable model, what is the aim of this study. 3. material and methods the theoretical background of the paper is processed mainly through the interpretation of the waste framework direction and above-mentioned strategies of the slovak republic, the amended legislation on waste in the slovak republic – act on waste no. 79/2015 coll. [10] in the valid wording and opinions of foreign and domestic authors who deal with the issue of waste. method of analysis is used to evaluate the slovak strategies in accordance with eu legislation. we examine to what extent the goals set by the slovak government in the above-mentioned strategies have been met in the field of waste in the slovak republic. in the quantitative analysis of the municipal waste management performance at the subregional level of lau-1 districts, we used data of the statistical office of the slovak republic. we have analysed data of 72 districts of the slovak republic, using the database coded zp3802rr [35] and named as “municipal waste and small construction waste from municipalities according to the waste treatment categories per district (in tonnes)”, 2021–2017 period. the data were used to calculate both partial waste treatment-specific indicators and composite performance indicators. following treatment specific indicators were calculated: • total municipal waste [kg per capita], • recycling [kg per capita], recycling rate [%], • landfilling [kg per capita], rate of landfilling [%]. as composite indicators, we used the technical efficiency, technical super-efficiency, and malmquist indices to consider simultaneously multiple metrics of the municipal waste management efficiency, employing the data envelopment analysis (dea) methodology in this study. technical efficiency (te) is used to measure performance (productivity, efficiency) of the waste management of analysed districts in comparison to the best performing districts. in economic terminology, it is defined as the total factor productivity (tfp ) of an evaluated district expressed relative to the highest tfp in the sample of districts. formally, technical efficiency can be expressed by the following equations eq. (1) and eq. (2): t f p = agregate output of the district under evaluation agregate input of the district under evaluation , (1) te = tfp of the district under evaluation maximum tfp in the sample of districts . (2) we estimate technical efficiency using the ccr dea (charnes, cooper, rhodes, [36]). to determine 67 eleonóra marišová, peter fandel acta polytechnica ctu proceedings the ranking of districts based on their efficiency in waste management, we used an adjusted ccr dea model for calculating super-efficiency, as proposed by andersen and petersen [37]. the formal notation of the dea models used to estimate the technical efficiency is presented in the scheme (3): ccr dea model max φ subject to n∑ j=1 yijλj ≥ φyro, r = 1, 2, . . . , s n∑ j=1 xijλj ≤ xio, i = 1, 2, . . . , m λj ≥ 0, j = 1, 2, . . . , n φ – free (3a) super-efficiency ccr dea model max φ subject to n∑ j=1 yijλj ≥ φyro, r = 1, 2, . . . , s n∑ j=1 xijλj ≤ xio, i = 1, 2, . . . , m λj ≥ 0, j = 1, 2, . . . , n; j ̸= o λo = 0 φ – free (3b) where xij ith input of the district j, yrj rth output of the district j, xio ith input of the district under observation o, yro rth output of the district under observation o, λj intensity variable of the jth district, φ technical efficiency (te) measure. the measure is from the interval [1; ∞). however, for the convenience, in the following sections, we report and interpret the inverse value 1/φ, which is from the interval [0; 1]. • if te = 1, then the evaluated district is efficient, that is, it achieves the maximum performance (tfp ) in the sample of evaluated districts and serves as a benchmark for other districts in the sample. • if te < 1, then the evaluated district is inefficient and achieves only (te∗100) % of the performance of the best districts. the level of the best districts can be achieved either by generating [(1 − te) ∗ 100] % higher outputs from the inputs used, or by using only (te ∗ 100) % of the inputs to generate their outputs. • when determining the ranking of districts according to te, efficient districts with te = 1 are ranked at the top and districts with the lowest te value are ranked at the bottom. in a situation where several districts achieve te = 1, it is not possible to unambiguously determine the ranking of efficient units, and in such cases, super-efficiency measures are calculated for efficient districts. super-te measures are from the interval [1; ∞), and the best district is the one with the maximum value of the super-te. the malmquist index (mi) proposed by färe et al. [38] expresses changes in total productivity over time. it is based on estimating shepard distance functions. in our study, we comprehensively evaluate the development of district performance in waste management over time using the output-oriented malmquist index and its components, simultaneously considering several performance indicators. the malmquist index of total productivity can be expressed as follows: mi = (yt+1, xt+1; yt, xt) = dt+1 o (yt+1, xt+1) dt o(yt, xt) [ dt o(yt+1, xt+1) dt+1 o (yt+1, xt+1) × dt o(yt, xt) dt+1 o (yt, xt) ] 1 2 , (4) • if mi > 1, then the tfp of the evaluated district has improved, • if mi = 1, then the tfp of the evaluated district has not changed, • if mi < 1, then the tfp of the evaluated district has worsened. malmquist index can be decomposed into the index of technical efficiency change (tec) and the index of technological change (tc): mi = tec × tc, where tec = dt+1 o (yt+1, xt+1) dt o(yt, xt) , (5) • if tec > 1, then the evaluated district improved its te (catching up with the best districts), • if tec = 1, then the evaluated district did not change its te, • if tec < 1, then the evaluated district worsened its te (lagging behind the best districts). tc = [ dt o(yt+1, xt+1) dt+1 o (yt+1, xt+1) × dt o(yt, xt) dt+1 o (yt, xt) ] 1 2 (6) • if tc > 1, then we observe progress in technology (innovation in technology) in the evaluated district, • if tc = 1, then there has been no change in technology in the evaluated district, • if tc < 1, then we observe regression in technology in the evaluated district. 68 vol. 46/2024 municipal waste management performance: a focus on slovakia . . . figure 1. the scheme of the transformation process in waste management system. we estimated the shepard distance functions using following output-oriented ccr dea models:[ dt o(yt, xt) ]−1 = max φ,λ φ subject to φyot − ytλ ≤ 0 xtλ ≤ xot λ ≥ 0 (7) [ dt+1 o (yt+1, xt+1) ]−1 = max φ,λ φ subject to φyot+1 − yt+1λ ≤ 0 xt+1λ ≤ xot+1 λ ≥ 0 (8) [ dt o(yt+1, xt+1) ]−1 = max φ,λ φ subject to φyot+1 − ytλ ≤ 0 xtλ ≤ xot+1 λ ≥ 0 (9) [ dt+1 o (yt, xt) ]−1 = max φ,λ φ subject to φyot − yt+1λ ≤ 0 xt+1λ ≤ xot λ ≥ 0 (10) in modelling the waste management performance of districts, we assume that the waste management system corresponds to the process of transforming inputs into outputs. in our analysis, the total quantity of municipal waste generated in districts serves as the input, which enters the waste management system. the quantities of municipal waste processed by the alternative treatment methods are regarded as the outputs of the system. the scheme of the transformation process is depicted in figure 1. the inputs and outputs in our analysis are defined in accordance with the above assumption as follows: • input (input variable) (1.) total municipal waste generated in a district per capita per year. we work with the only input variable, which we consider to be a short-term uncontrollable variable. • outputs (output variables) (1.) recycling – material (r02-r13, except r03) (2.) recycling – composting and digestion (r03) (3.) incineration with energy recovery (r01, d10) (4.) landfilling (d01) the four selected outputs variables represent the prevailing quantities of municipal waste in slovakia. to account for population density, all outputs, as well as the input, are expressed in per capita values. consistent with the waste management strategies adopted by the eu and slovakia, the objective is to minimize the disposal of municipal waste in landfills and encourage recycling and environmentally friendly incineration with energy recovery. for this reason, the selected output variables are hierarchically defined into two groups: (a) good outputs: these represent desirable treatment methods of waste, including material recycling, organic recycling, and incineration with energy recovery. for these outputs, we adopt the preference of “the more – the better”. (b) bad outputs: these represent undesirable waste treatment methods, specifically landfilling, with the preference of “the less – the better”. since outputs are generally modelled in dea models as maximization variables, the maximization preference for bad outputs needs to be transformed into minimization. in our study, we adopt the transformation approach proposed by seiford and zhu [39]: firstly, each undesirable (bad) output is multiplied by a coefficient of −1, and then a suitable transformation vector, 69 eleonóra marišová, peter fandel acta polytechnica ctu proceedings denoted as w, is determined to convert all negative outputs into positive ones: yj b = −yb j + w > 0 yj b = −yb j + max(yb j) + 1 (11) where yb j is the bad output and yj b is the transformed bad output. the ccr dea model with bad outputs then has the following form: max f = φ output constraints for good outputs n∑ j=1 yg ijλj ≥ φyg ro, r = 1, 2, . . . , s output constraints for bad outputs n∑ j=1 yij bλj ≥ φyro b, r = 1, 2, . . . , s standard input constraints n∑ j=1 xijλj ≤ xio, i = 1, 2, . . . , m λj ≥ 0, j = 1, 2, . . . , m φ – free, yg j are good outputs. (12) we applied the same adjustment in models for calculating the super-efficiency and the malmquist indices. 4. results 4.1. municipal waste – eu legislation the waste framework directive lays down measures to protect the environment and human health by preventing or reducing the generation of waste, the adverse impacts of the generation and management of waste by reducing overall impacts of resource use and improving the efficiency of such use, which are crucial for the transition to a circular economy and for guaranteeing the union’s long-term competitiveness [3]. this directive stresses the transition to a circular economy in all eu member states and changes of the former waste directive 2008/98/ec by new definition the municipal waste [3]: (a) mixed waste and separately collected waste from households, including paper and cardboard, glass, metals, plastics, bio-waste, wood, textiles, packaging, waste electrical and electronic equipment, waste batteries and accumulators, and bulky waste, including mattresses and furniture; (b) mixed waste and separately collected waste from other sources, where such waste is similar in nature and composition to waste from households. municipal waste does not include waste from production, agriculture, forestry, fishing, septic tanks and sewage network and treatment, including sewage sludge, end-of-life vehicles or construction and demolition waste. we can see that the new stipulation of the waste framework directive includes separately collected waste that is more and more important for the eu. as for the municipal waste, it excludes mainly waste from production, agriculture, forestry, fishing. newly is defined also construction and demolition waste like waste generated by construction and demolition activities, as well as bio-waste that means biodegradable garden and park waste, food and kitchen waste from households, offices, restaurants, wholesale, canteens, caterers and retail premises and comparable waste from food processing plants. the original characteristics of the waste in article 4 waste framework directive [3] was much simpler: any substance or object which the holder discards or intends or is required to discard. as for the waste management, the new directive states that it is not only collection, transport, recovery and disposal of waste, including the supervision of such operations and the after-care of disposal sites, but includes into management also the waste sorting. except of waste sorting, the new directive also stipulates a new tool for producers’ responsibility by adding so called extended producer responsibility scheme that means a set of measures taken by member states to ensure that producers of products bear financial responsibility or financial and organisational responsibility for the management of the waste stage of a product’s life cycle [3]. 4.2. implementation of eu waste legislation into slovak one this amendment was transposed into slovak waste legislation by amending the act on waste no. 79/2015 coll. waste act [10]. as part of the extended responsibility, producers of reserved products basically bear all the costs of dealing with the reserved waste stream, from its sorted collection to the recovery and eventual disposal of unrecoverable residues. exceptions to this rule are cases where the obligation of the producer of a reserved product, which is not registered in the register of producers, is transferred from the waste act to the distributor of this product. pursuant to § 81 par. 4 of the waste act, the manufacturer is obliged to bear the costs of providing the necessary collection containers (e.g., containers or bags). the transfer of costs for separate collection from citizens to producers also creates financial motivation for the improvement of sorting. although the principles supporting extended producer responsibility create room for an increase in the rate of sorting of municipal waste, extended producer responsibility cannot be considered a direct tool for its increase. it is only a means for shifting the costs of separate collection and recycling from local governments and taxpayers directly to producers, which stimulates waste prevention on the part of producers and, on the other hand, creates room for citizens to manage waste more responsibly [40]. 70 vol. 46/2024 municipal waste management performance: a focus on slovakia . . . oecd [41] defines the extended producer responsibility as an environmental policy tool that extends the producer’s responsibility for the product to the postconsumer phase of its life cycle. in other words: the manufacturer is responsible for handling the product even after the product has become waste. “not the consumer, but the producer of the product is the one who creates the demand for packaging and determines the requirements for its composition. therefore, if the customer makes the effort and sorts the packaging, the subsequent costs associated with the waste are transferred to the manufacturer. in the extended responsibility of producers, producers are responsible for sorted waste from packaging”, the analysts of the slovak institute of environmental policy explain in their latest study how to sort out sorted collection [40]. in the legislation of the european community, the idea of extended producer responsibility appeared for the first time when the directive [42] on packaging and packaging waste set member states targets for waste sorting and recycling, as well as requirements for packaging design. however, this directive did not oblige producers to finance the collection and recycling of waste – this resulted only from the legislation of individual states. 4.3. eu prevention of waste according to article 9 of the waste framework directive [3], the member states shall take measures to prevent waste generation. those measures shall, at least: • reduce the generation of waste, in particular waste that is not suitable for preparing for re-use or recycling, • develop and support information campaigns to raise awareness about waste prevention and littering, • target products containing critical raw materials to prevent that those materials become waste, • encourage the re-use of products and the setting up of systems promoting repair and re-use activities, including in particular for electrical and electronic equipment, textiles and furniture, as well as packaging and construction materials and products, • reduce waste generation in processes related to industrial production, extraction of minerals, manufacturing, construction and demolition, taking into account best available techniques, • reduce the generation of food waste in primary production, in processing and manufacturing, in retail and other distribution of food, in restaurants and food services as well as in households as a contribution to the united nations sustainable development goal [4] to reduce by 50 % the per capita global food waste at the retail and consumer levels and to reduce food losses along production and supply chains by 2030; • encourage food donation and other redistribution for human consumption, prioritising human use over animal feed and the reprocessing into non-food products. 4.4. main aims of waste prevention in slovak waste prevention program slovak republic implemented these goals into its waste prevention programme (wpp), 2018 for years 2019–2025 [5]. the main aim is to break the link between economic growth and environmental impacts related to the generation of waste. wpp [5] states that the capacity of the currently operating waste dumps is sufficient, therefore it is not necessary to build new waste dumps. unlike common dumps, however, landfills are more carefully designed to cordon off waste from both society and nature, maintaining their contents in a state of suspended animation. this makes it possible for landfills to one day be recovered as an invented common, a source of new land upon which to build or reclaim for other purposes [43]. the slovak republic is a rural country which is also reflected in the method of sorted collection of biodegradable waste with predominant domestic composting. we analyze the main goals, financing and measures taken in the slovak wpp [5] relating to mixed and biodegradable municipal waste. the main goal of wpp [5] as for mixed municipal waste is to reduce the amount of mixed municipal waste by 50 % by 2025 compared to 2016 mostly by introduction of mandatory mass collection of municipal waste and activities to prevent the creation of mixed municipal waste should be financed from the environmental fund. as for the biodegradable municipal waste, the main goal of wpp [5] is to reduce the amount of biodegradable waste in mixed municipal waste by 60 % by 2025 compared to the situation in 2016. stipulated measures in wpp [5]: • legislative, financial, and informational support for home and community composting, • creation of a unified methodology for monitoring the amount and types of biodegradable waste in mixed municipal waste. illegal handling of biodegradable municipal waste is also a serious problem in slovakia. a large part of it is dumped in illegal landfills or burned in the spring and autumn months on public and private lands. 4.5. waste management program and slovak lau-1 districts competencies another important strategy that implemented the eu legislation, is the slovak waste management programme [6] for years 2021–2025. the new version of the waste framework directive 2008/98/ec established new definitions (municipal waste) [3] related to the concept of waste, as well 71 eleonóra marišová, peter fandel acta polytechnica ctu proceedings as improving existing ones (waste management). it should be noted that the basic definition of waste remains unchanged [44]. slovakia needs to focus strongly on separate collection at source. a low level of separate collection would result in a low recycling rate of municipal waste. the separate collection of municipal waste has increased in the past period, especially for metals, paper, plastics, glass and biodegradable green waste, but there is also a need to focus on the separate collection of other components and types of waste that are currently underestimated (e.g., hazardous waste, textiles). wmp [6] states the tools to improve the separate collection of municipal waste that were included into slovak waste legislation: • act no. 329/2018 coll. on fees for waste disposal – to make landfilling, which is the last in the waste management hierarchy, handicapped and to create an incentive for the sorted collection of municipal waste and to increase the recycling of municipal waste [3], • regulation no. 330/2018 coll. – the new rate for the disposal of mixed municipal and bulky waste, depends on the level of municipal waste sorting in the municipality, • new law on fees is the result of a number of activities at the slovak level and the eu one. as for slovakia, a member state of the eu since 2004, all district offices in the seat of the regions (8) shall be obliged to work out plans of regions based on the objectives and measures set in the slovak waste management program. this way there are together eight regional waste management programs for years set by the slovak program that determines the direction of waste management for a set period and are binding for respective region, based on its particularities with specific goals and measures to support waste prevention. the new act on wastes imposes the duty on the producers that fulfils their duties individually to perform promotional and educational activities in the district, in which they provide for the waste collection, focusing on end users, about management of selected waste streams, separate collection of municipal wastes and waste prevention [6]. 4.6. the strategy of the environmental policy of the slovak republic until 2030 envirostrategy (2030) that was prepared under leadership of slovak ministry of environment in 2019, defines a vision until 2030, which takes into account a possible, probable, and the desired future development, identifies the fundamental systemic problems, sets the objectives until 2030 and proposes a framework for measures to improve the current situation, and it also contains basic result indicators that will enable a verification of achieved results. the basic vision of envirostrategy 2030 is to achieve better environmental quality and sustainable circulation of the economy, which is based on rigorous protection of environmental compartments and using as little non-renewable natural resources and hazardous substances as possible, which will lead to an improvement in health of the population. environmental protection and sustainable consumption will be part of the general awareness of citizens and policy makers [7]. the main aims of envirostrategy 2030 are as follows: • to achieve better environmental quality and sustainable circulation of the economy, • environmental protection and sustainable consumption will be part of the general awareness of citizens and policy makers, • the prevention and adaptation to climate change. the tools to fulfil the main objectives of envirostrategy 2030: • incentive-based municipal waste collection for municipalities, • increase prevention of black dumping and enforcement of the polluter pays, • restaurants and supermarkets will be obliged to make use of the food (charity donation of the food that fulfils food safety requirements), • if they are no longer suitable for consumption, they will be able to compost them or energetically utilize, • renewable energy production will be preferred, which by its nature does not burden the environment and contributes to the long-term sustainable development of the slovak republic. 4.7. results of the quantitative analysis in the opening part of this section, we focus on presenting selected partial indicators that reflect the performance of waste management. through these indicators, we aim to document the progress of municipal waste management in slovakia between 2017 and 2021, while also comparing it to the average performance of the eu-27 countries, slovakia, and the target values set specifically for slovakia. in table 1 we present a comparison of total municipal waste generated. it is evident that slovakia remains below the eu-27 average. however, economically developed districts of western slovakia, such as trnava and galanta, significantly surpass the eu-27 average in waste generation per capita. waste growth index indicates a higher growth rate in slovakia compared to eu-27. the assessment of a waste management system’s performance is primarily contingent on the extent to which recycling methods are utilized. in this regard, there have been significant and positive developments. table 2 illustrates the advancement in recycling, encompassing both material recycling and recycling through composting and digestion. over the 72 vol. 46/2024 municipal waste management performance: a focus on slovakia . . . geopolitical entity 2017 2021 index 2021/2017 eu-27 499 530 106 % slovakia 378 496 131 % best districts sobrance: 126 sobrance: 220 175 % worst districts trnava: 563 galanta: 830 147 % table 1. total municipal waste generated [kg per capita]. geopolitical entity 2017 2021 index 2021/2017 eu-27 231 257 111.3 % slovakia 113 242 214.2 % best districts nitra: 225 galanta: 514 xxx worst districts sobrance: 12 sobrance: 88 xxx table 2. recycling [kg per capita]. geopolitical entity 2017 2021 index 2021/2017 eu-27 46.3 % 48.5 % 104.7 % slovakia 29.9 % 48.8 % 163.2 % best districts žiar nad hronom: 49.6 % žiar nad hronom: 65.3 % 130.0 % worst districts medzilaborce: 6.7 % medzilaborce: 29.8 % 428.6 % slovakia target 2030 min 60 % table 3. recycling rate [%]. geopolitical entity 2017 2021 index 2021/2017 eu-27 127 121 95 % slovakia 229 202 88 % best districts košice i-iv: 16 košice i-iv: 7 44 % worst districts galanta: 429 malacky: 360 xxx table 4. landfilling [kg per capita]. span of five years analyzed, recycling in slovakia has doubled. however, as of 2021, it still falls below the average recycling rate of the eu-27 countries. table 3 provides an overview of the share of recycling in relation to the total municipal waste generated. slovakia has witnessed significant progress in its recycling rate, which has increased from 29.9 % to 48.8 %, surpassing the average rate of the eu-27. this positive trend is also evident within the districts of slovakia. the district with the lowest recycling rate has tripled its rate, showing a remarkable improvement, while the district with the highest rate has seen a 30 % enhancement. although slovakia as a whole has not yet met the recycling rate target set for year 2030, some of the best districts have already achieved this milestone. waste landfilling is considered as a least desirable treatment method. in line with eu landfill directive, member states are required to reduce the amount of municipal waste sent to landfill to 10 % or less of the total amount of municipal waste generated by 2035. in table 4 we provide per capita statistics of slovakia and its districts. landfilling in slovakia remains significantly high, nearly double the average of the eu-27. moreover, the worst-performing districts exhibit even higher values, almost triple the eu-27 average. over the five-year period under analysis, slovakia has managed to decrease its landfill usage by approximately 12 %. the best practicing districts show 56 % decrease. the target rate of landfilling set in slovakia for 2035 is 25 %. however, as of 2021, the current rate of landfilling stands at 40.7 % (table 5). this indicates that there is still a significant gap between the current rate and the desired target, highlighting the need for further efforts and strategies to reduce landfilling and promote more sustainable waste management practices. the district of košice i-iv stands out as the best performer, with a remarkably low landfilling rate of only 1.5 %. this achievement can be attributed to its significant reliance on incineration with energy recovery, a method that effectively reduces the amount of waste sent to landfills. conversely, the worst-performing districts are characterized by low rates of recycling, indicating a need for improvement in their waste management practices. encouraging higher recycling rates 73 eleonóra marišová, peter fandel acta polytechnica ctu proceedings geopolitical entity 2017 2021 index 2021/2017 eu-27 25.5 % 22.8 % 89.7 % slovakia 60.6 % 40.7 % 67.3 % best districts košice i-iv: 3.8 % košice i-iv: 1.5 % 39.5 % worst districts medzilaborce: 93.3 % medzilaborce: 69.7 % 74.7 % slovakia target 2035 max 25 % table 5. rate of landfilling [%]. rank total municipal waste(d) recycling – material(i) recycling – composting and digestion(i) incineration with energy recovery(i) landfilling(d) 1 sobrance (220) kysucké n. mesto (234) galanta (369) košice i-iv (290) košice i-iv (7) 2 trebišov (229) bytča (207) senec (324) bratislava i-v (234) bratislava i-v (47) 3 medzilaborce (309) žiar nad hronom (192) dunajská streda (261) prešov (133) prešov (72) 4 vranov n/topľou (317) zlaté moravce (190) malacky (257) košice – okolie (97) košice – okolie (91) 5 stará ľubovňa (322) senec (174) nitra (233) rožňava (63) sobrance (128) . . . . . . . . . . . . . . . . . . 68 nitra (613) zvolen (60) detva (54) 26 districts with zero values nitra (285) 69 malacky (763) stará ľubovňa (60) rožňava (49) galanta (293) 70 senec (770) poprad (59) gelnica (48) komárno (309) 71 dunajská streda (771) banská štiavnica (51) košice – okolie (36) dunajská streda (338) 72 galanta (830) medzilaborce (37) sobrance (21) malacky (360) (d) decreasing preference, (i) increasing preference. table 6. top 5 and bottom 5 districts in waste management performance by treatment-specific indicators, 2021 (districts sorted by waste in kg per capita). in these districts would contribute to reducing the reliance on landfilling and promoting a more sustainable approach to waste management. the top 5 and bottom 5 ranks of districts based on waste management performance using treatmentspecific indicators in kg per capita are presented in table 6. the corresponding rankings based on the percentage share of quantities treated by selected methods in the total quantity of municipal waste is shown in table 7. as evident from the previous section, partial waste treatment-specific indicators lead to different rankings of the districts, and in some cases, these rankings can be contradictory. to avoid this discrepancy, the next part of this section presents the results of the waste management performance analysis assessed using composite indicators. in the analysis, a dataset comprising 72 districts was utilized, and waste management performance was evaluated by considering one input variable and four output variables simultaneously, employing the dea model (3a). descriptive statistics of the variables used in the analysis are provided in table 8. the technical efficiency composite indicator of the municipal waste performance of the 72 districts of slovakia for the period 2017–2021 is presented in table 9. as evident from table 9, the average technical efficiency of districts in the evaluated period increased from 0.714 in 2017 to 0.852 in 2021. if in 2017 districts, on average, achieved 71.4 % of the performance of the best-performing districts, by 2021, it had already increased to 85.2 %, while simultaneously reducing the variability in district efficiency. this indicates that districts are converging towards the desired state outlined by waste management strategies. unfortunately, it is showing that the pace of convergence is slowing down, as demonstrated in table 10. while in 2018, the average technical efficiency of districts increased by 15 % compared to the previous year, in 2019 it was a growth of 7.2 %. in 2020, there was a decline of 0.1 %, but in 2021, there was an increase of 2.4 %. the average annual growth of 74 vol. 46/2024 municipal waste management performance: a focus on slovakia . . . rank recycling – material rate(i) recycling – composting and digestion rate(i) incineration with energy recovery rate(i) landfilling rate(d) 1 kysucké nové mesto (42) galanta (44) košice i-iv (65) košice i-iv (1) 2 košice – okolie (38) senec (42) bratislava i-v (41) bratislava i-v (8) 3 bytča (38) topoľčany (40) prešov (30) prešov (16) 4 žiar nad hronom (37) nitra (38) košice – okolie (27) košice – okolie (25) 5 gelnica (34) banská štiavnica (38) rožňava (18) senec (31) . . . . . . . . . . . . . . . 68 zvolen (14) rožňava (14) 26 districts with zero values sabinov (58) 69 bratislava i-v (13) detva (14) rimavská sobota (59) 70 poprad (13) revúca (12) kežmarok (61) 71 medzilaborce (12) košice – okolie (10) revúca (62) 72 banská štiavnica (11) sobrance (9) medzilaborce (70) (d) decreasing preference, (i) increasing preference. table 7. top 5 and bottom 5 districts in waste management performance by treatment-specific indicators, 2021 (districts sorted by % rate). statistics 2017 2018 2019 2020 2021 i – total municipal waste mean 364.2 405.6 414.2 444.1 472.4 minimum 126.3 133.4 156.5 159.4 220.2 maximum 563.2 654.2 690.1 1017.9 830.2 standard deviation 99.2 103.7 101.8 132.3 117.2 o1 recycling – material (desirable treatment) mean 52.2 89.5 91.9 85.9 108.9 minimum 4.8 7.1 9.9 10.7 37.3 maximum 151.5 221.4 234.1 168.6 233.7 standard deviation 32.6 42.6 39.5 31.8 38.7 o2 recycling – composting and digestion (desirable treatment) mean 52.2 64.7 77.5 112.4 127.3 minimum 5.0 5.8 11.3 22.3 20.9 maximum 177.5 169.8 174.7 361.1 368.8 standard deviation 31.5 34.0 35.7 62.5 63.7 o3 incineration with energy recovery (desirable treatment) mean 9.4 7.8 7.8 9.3 11.7 minimum 0.0 0.0 0.0 0.0 0.0 maximum 275.0 238.5 258.0 274.2 290.0 standard deviation 46.2 38.2 35.8 44.6 47.2 o4 landfilling (undesirable treatment) mean 250.4 242.4 234.1 229.4 217.1 minimum 15.6 23.4 17.3 6.2 6.6 maximum 429.2 369.3 363.6 371.3 359.7 standard deviation 69.3 65.6 63.1 60.4 58.4 table 8. descriptive statistics of input (i) and output (o) variables, 2017–2021, n = 72 districts [kg per capita]. 75 eleonóra marišová, peter fandel acta polytechnica ctu proceedings descriptive statistics 2017 2018 2019 2020 2021 mean 0.714 0.799 0.842 0.839 0.852 minimum 0.443 0.467 0.499 0.533 0.612 maximum 1.000 1.000 1.000 1.000 1.000 standard deviation 0.159 0.151 0.118 0.118 0.099 no. of inefficient districts (te < 1) 64 61 62 64 62 no. of efficient districts (te = 1) 8 11 10 8 10 table 9. descriptive statistics of technical efficiency of districts of slovakia, 2017–2021, n = 72. descriptive statistics t ec index 2018/17 t ec index 2019/18 t ec index 2020/19 t ec index 2021/20 mean annual t ec index cumul. t ec index mean 1.150 1.072 0.999 1.024 1.051 1.242 minimum 0.710 0.899 0.694 0.751 0.957 0.838 maximum 2.044 1.920 1.169 1.299 1.220 2.220 standard deviation 0.255 0.155 0.077 0.096 0.054 0.270 no. of districts with tec < 1 (regress) 15 21 34 20 11 11 no. of districts with tec = 1 (stagnation) 5 9 5 7 4 3 no. of districts with tec > 1 (progress) 52 42 33 45 57 58 t ec – technical efficiency change table 10. descriptive statistics of technical efficiency change indices of districts of slovakia, 2017–2021, n = 72. technical efficiency over the entire period was 5.1 %. overall, the average technical efficiency of districts increased by 24.2 % throughout the evaluated period. complete results on technical efficiency change are presented in appendix b. table 11 shows the 5 best and the 5 worst districts. the most efficient district in slovakia from 2017 to 2020 was district of sobrance, while in 2021 it was košice i-iv, which held the second position in the previous period. while sobrance are so efficient probably thanks to low waste generation, district of košice i-iv exhibits so high efficiency score due to a high proportion of incineration with energy recovery. the least efficient districts in respective years of the examined period were partizánske, komárno, and revúca. the detailed ranking of districts according to technical efficiency in each year is provided in appendix a. technical efficiency is a relative measure of performance (productivity) of districts, indicating how far the evaluated districts are from the best-performing districts that utilize the most productive waste treatment methods and technologies for municipal waste management. the malmquist index allows for assessing changes in the overall productivity of districts in terms of the transformation of total generated municipal waste into individual components based on waste treatment methods. in table 12 we present malmquist indices for the period 2017–2021. average malmquist index, as a composite indicator of total factor productivity change of districts increased by 16.75 % in 2018 compared to the previous year, in 2019 it was a growth of 6.6 %, in 2020 there was an increase 9.6 % and in 2021 there was an increase of 2.4 %. the average annual growth of tfp measured over the entire period was 9.2 %. overall, the average malmquist index of districts increased by 45.5 % throughout the evaluated period. the district of dunajská streda achieved the highest average annual tfp change index of 1.3, indicating a 30 % improvement in total performance every year during the analyzed period. furthermore, this district has the best cumulative malmquist index of 2.852, suggesting that an initially inefficient district significantly improved its performance at a high pace, resulting in an overall tfp increase of 185.2 %. on the other hand, the district of sobrance exhibited the lowest annual change index of 0.901 and cumulative mi index of 0.657. these values represent a regress of 9.9 % and 34.3 % respectively, likely due to uncompetitive waste management technology. the complete list of districts with malmquist indices of tfp change can be found in appendix c. 5. conclusions the goal of waste strategies in the slovak republic is to reduce the volume of landfill waste by separating its components to be used in the recycling process. by analysing the legislation and strategies of 76 vol. 46/2024 municipal waste management performance: a focus on slovakia . . . rank 2017 2018 2019 2020 2021 1 sobrance sobrance sobrance sobrance košice i-iv 2 košice i-iv košice i-iv košice i-iv košice i-iv sobrance 3 tvrdošín kysucké nové mesto žiar nad hronom košice – okolie košice – okolie 4 žiar nad hronom žiar nad hronom kysucké nové mesto galanta kysucké nové mesto 5 nitra košice – okolie myjava žiar nad hronom prešov . . . . . . . . . . . . . . . . . . 68 komárno bánovce nad bebravou kežmarok medzilaborce hlohovec 69 bánovce nad bebravou považská bystrica považská bystrica považská bystrica považská bystrica 70 galanta rožňava žarnovica rožňava kežmarok 71 dunajská streda medzilaborce rožňava revúca komárno 72 partizánske komárno komárno komárno revúca table 11. top 5 and bottom 5 districts in waste management performance measured by the composite indicator of technical super-efficiency, 2017–2021. descriptive statistics mi 2018/17 mi 2019/18 mi 2020/19 mi 2021/20 mean annual mi cumulative mi mean 1.167 1.066 1.078 1.096 1.092 1.455 minimum 0.673 0.851 0.827 0.774 0.901 0.657 maximum 2.208 1.735 1.413 1.421 1.300 2.852 standard deviation 0.266 0.146 0.113 0.116 0.069 0.388 no. of districts with mi < 1 (regress) 17 26 15 11 6 6 no. of districts with mi = 1 (stagnation) 0 1 0 0 0 0 no. of districts with mi > 1 (progress) 55 45 57 61 66 66 mi – malmquist index of total factor productivity change table 12. descriptive statistics of malmquist indices in districts of slovakia, 2017–2021, n = 72. the slovak republic, as well as the eu legislation, we can formulate the conclusion that a system for waste generators should contribute to the objective of significantly reducing the overall waste generation, in particular as regards halving the amount of residual, non-recycled municipal waste by 2030 and the land-filling rate should be reduced to less than 25 % by 2035. at the same time, the functionality of the extended liability system will be improved. municipal waste management should contribute to the sdgs’ fulfilment, and we need to consider not just environmental and economic but also social factors influencing the waste generation, waste treatment and processing. the quantitative analysis of waste management performance reveals a positive trend in the average technical efficiency of the lau-1 districts in slovakia, increasing from 0.714 in 2017 to 0.852 in 2021, indicating a significant improvement of 19.3 %. this indicates a positive convergence of district productivity towards the performance of the best-performing districts, highlighting progress in waste management practices. we found out that among the best-performing districts belong the ones with access to waste incineration facilities with energy recovery. however, it is important to note that there has been a slight deceleration in the pace of convergence. the initial improvement observed between 2017 and 2018, which amounted to a 15 % increase, decreased to a 2.4 % change between 2020 and 2021. while there is still progress, the rate of improvement has slowed down compared to the earlier period. over the period from 2017 to 2021, the average cumulative municipal waste management performance, measured by malmquist index of total factor productivity change, improved by a significant 45.5 %. this 77 eleonóra marišová, peter fandel acta polytechnica ctu proceedings improvement indicates an equiproportional growth in the quantities of municipal waste treated using desirable treatment methods, accompanied by a decrease in quantities of waste treated using undesirable treatment methods. on average, there was an annual improvement of 9.2 % in the performance of municipal waste management in the lau-1 districts measured by malmquist index of tfp change. if the observed trend from 2017 to 2021 persists, it can be reasonably expected that the targets set for the recycling rate and landfilling rate for the years 2030 and 2035 will be met. however, this expectation is not statistically analysed in this paper. examples of some districts show that the set goals in recycling and landfilling are already being met or even exceeded. on the other hand, there are districts that are far from meeting the goals, and it is not expected that they will achieve them by the specified target years. positive advancements in waste management practices demonstrate the potential for continued progress in achieving sustainable waste treatment goals. acknowledgements this work was supported by the slovak research and development agency under the contract no. apvv20-0076 entitled “waste and construction – modelling the effectiveness of alternative options for cooperation between 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https://datacube.statistics.sk/#!/view/en/vbd_sk_win2/zp3802rr/v_zp3802rr_00_00_00_en https://datacube.statistics.sk/#!/view/en/vbd_sk_win2/zp3802rr/v_zp3802rr_00_00_00_en https://doi.org/10.1016/0377-2217(78)90138-8 https://doi.org/10.1287/mnsc.39.10.1261 https://doi.org/10.1016/s0377-2217(01)00293-4 https://www.minzp.sk/files/iep/sorting_out_waste_sorting.pdf https://www.minzp.sk/files/iep/sorting_out_waste_sorting.pdf https://doi.org/10.1787/9789264189867-en https://eur-lex.europa.eu/legal-content/en/txt/?uri=celex%3a01994l0062-20180704 https://eur-lex.europa.eu/legal-content/en/txt/?uri=celex%3a01994l0062-20180704 https://doi.org/10.9770/jesi.2020.8.2(32) eleonóra marišová, peter fandel acta polytechnica ctu proceedings appendix a. rank of districts of slovakia according to waste management performance measured by the composite indicator of technical super-efficiency, 2017–2021 rank 2017 2018 2019 2020 2021 1 sobrance sobrance sobrance sobrance košice i-iv 2 košice i-iv košice i-iv košice i-iv košice i-iv sobrance 3 tvrdošín kysucké nové mesto žiar nad hronom košice – okolie košice – okolie 4 žiar nad hronom žiar nad hronom kysucké nové mesto galanta kysucké nové mesto 5 nitra košice – okolie myjava žiar nad hronom prešov 6 stará ľubovňa gelnica košice – okolie stropkov žiar nad hronom 7 myjava nitra nitra bratislava i-v galanta 8 brezno myjava svidník zlaté moravce senec 9 bratislava i-v šaľa galanta žilina bratislava i-v 10 ilava banská bystrica gelnica ilava myjava 11 banská bystrica trebišov veľký krtíš tvrdošín tvrdošín 12 šaľa ilava ilava prešov svidník 13 detva tvrdošín šaľa svidník banská štiavnica 14 košice – okolie stará ľubovňa zlaté moravce kysucké nové mesto bytča 15 spišská nová ves skalica bardejov myjava topoľčany 16 zvolen bratislava i-v liptovský mikuláš bardejov stará ľubovňa 17 žilina nové zámky stropkov nové zámky ilava 18 pezinok hlohovec levoča trenčín dolný kubín 19 prešov galanta banská bystrica banská bystrica bardejov 20 snina veľký krtíš trebišov stará ľubovňa humenné 21 zlaté moravce lučenec tvrdošín veľký krtíš krupina 22 bardejov bardejov pezinok senec levoča 23 liptovský mikuláš stropkov senec banská štiavnica skalica 24 turčianske teplice snina bratislava i-v šaľa veľký krtíš 25 bytča prešov stará ľubovňa púchov gelnica 26 veľký krtíš námestovo medzilaborce nitra piešťany 27 gelnica ružomberok hlohovec pezinok nitra 28 skalica senec spišská nová ves gelnica vranov nad topľou 29 lučenec humenné snina dolný kubín snina 30 dolný kubín spišská nová ves ružomberok snina šaľa 31 trenčín vranov nad topľou prešov skalica námestovo 32 humenné liptovský mikuláš žilina námestovo trebišov 33 martin michalovce dolný kubín michalovce lučenec 34 púchov žilina skalica ružomberok trenčín 35 senica pezinok nové zámky lučenec pezinok 36 vranov nad topľou piešťany humenné vranov nad topľou ružomberok 37 ružomberok bytča púchov dunajská streda dunajská streda 38 svidník krupina topoľčany hlohovec turčianske teplice 39 topoľčany topoľčany dunajská streda liptovský mikuláš zlaté moravce 40 banská štiavnica dolný kubín krupina trebišov brezno 41 rimavská sobota trnava levice piešťany bánovce nad bebravou 42 piešťany trenčín michalovce humenné liptovský mikuláš 43 michalovce martin trenčín krupina stropkov 44 nové mesto nad váhom zvolen námestovo levice banská bystrica 45 revúca púchov lučenec levoča zvolen 46 poltár poltár senica turčianske teplice nové mesto nad váhom 47 levice levoča poprad spišská nová ves sabinov 48 nové zámky senica vranov nad topľou topoľčany detva 49 levoča brezno prievidza bánovce nad bebravou poltár 50 námestovo rimavská sobota piešťany malacky michalovce 51 trnava svidník trnava senica malacky 52 žarnovica poprad martin brezno nové zámky 53 poprad malacky turčianske teplice bytča martin 54 krupina levice poltár martin žarnovica 55 medzilaborce kežmarok bánovce nad bebravou zvolen levice 56 hlohovec zlaté moravce banská štiavnica trnava spišská nová ves 57 senec dunajská streda bytča nové mesto nad váhom púchov 58 sabinov prievidza brezno poltár prievidza 59 kysucké nové mesto nové mesto nad váhom zvolen čadca čadca 60 prievidza banská štiavnica malacky detva trnava 61 kežmarok partizánske nové mesto nad váhom prievidza poprad 62 čadca revúca detva sabinov partizánske 63 považská bystrica detva sabinov rimavská sobota žilina 64 malacky sabinov rimavská sobota poprad rimavská sobota 65 stropkov turčianske teplice čadca partizánske rožňava 66 trebišov žarnovica revúca kežmarok senica 67 rožňava čadca partizánske žarnovica medzilaborce 68 komárno bánovce nad bebravou kežmarok medzilaborce hlohovec 69 bánovce nad bebravou považská bystrica považská bystrica považská bystrica považská bystrica 70 galanta rožňava žarnovica rožňava kežmarok 71 dunajská streda medzilaborce rožňava revúca komárno 72 partizánske komárno komárno komárno revúca 80 vol. 46/2024 municipal waste management performance: a focus on slovakia . . . appendix b. technical efficiency change index, 2017–2021 district 2018/ 2019/ 2020/ 2021/ mean cumulative 2017 2018 2019 2020 t ec t ec bratislava i-v 1.028 0.949 1.108 1.000 1.020 1.081 malacky 1.383 1.056 1.079 0.997 1.120 1.571 pezinok 0.986 1.128 0.964 0.971 1.010 1.041 senec 1.489 1.057 0.973 1.127 1.146 1.726 dunajská streda 1.466 1.283 1.008 0.994 1.172 1.885 galanta 2.044 1.086 1.000 1.000 1.220 2.220 hlohovec 1.589 0.967 0.944 0.816 1.043 1.184 piešťany 1.196 0.976 1.063 1.060 1.071 1.315 senica 1.004 1.127 0.986 0.916 1.006 1.022 skalica 1.274 0.914 0.986 1.031 1.043 1.184 trnava 1.256 0.986 0.987 1.000 1.052 1.222 bánovce nad bebravou 1.191 1.392 1.069 1.032 1.163 1.829 ilava 1.071 0.965 1.043 0.932 1.001 1.005 myjava 1.000 1.000 0.957 1.045 1.000 1.000 nové mesto nad váhom 0.989 1.115 1.054 1.062 1.054 1.234 partizánske 1.463 0.999 1.014 1.150 1.143 1.704 považská bystrica 1.015 1.132 1.031 1.110 1.071 1.315 prievidza 1.183 1.208 0.907 1.081 1.088 1.401 púchov 1.048 1.111 1.029 0.880 1.013 1.054 trenčín 1.070 1.049 1.112 0.928 1.037 1.158 komárno 1.006 1.069 1.068 1.197 1.083 1.375 levice 1.064 1.202 0.995 0.938 1.045 1.194 nitra 1.000 1.000 0.880 1.006 0.970 0.885 nové zámky 1.453 0.944 1.077 0.841 1.056 1.242 šaľa 1.130 0.952 0.929 0.993 0.998 0.992 topoľčany 1.136 1.064 0.959 1.185 1.083 1.374 zlaté moravce 0.858 1.405 1.054 0.839 1.016 1.066 bytča 1.045 0.947 1.050 1.223 1.062 1.271 čadca 1.040 1.244 1.100 1.018 1.097 1.449 dolný kubín 1.069 1.109 0.997 1.056 1.057 1.248 kysucké nové mesto 1.795 1.000 0.964 1.037 1.157 1.794 liptovský mikuláš 1.070 1.137 0.896 0.988 1.019 1.077 martin 1.069 0.985 1.003 1.012 1.017 1.069 námestovo 1.396 0.939 1.037 1.009 1.082 1.372 ružomberok 1.229 1.023 0.960 0.994 1.047 1.200 turčianske teplice 0.816 1.212 1.075 1.033 1.024 1.098 tvrdošín 0.988 0.932 1.065 1.006 0.997 0.987 žilina 0.973 1.092 1.130 0.751 0.975 0.902 banská bystrica 1.109 0.930 0.986 0.898 0.978 0.913 banská štiavnica 0.947 1.163 1.169 1.111 1.094 1.430 brezno 0.710 1.059 1.056 1.055 0.957 0.838 detva 0.743 1.134 1.041 1.075 0.985 0.943 krupina 1.323 1.053 0.994 1.076 1.105 1.490 lučenec 1.218 0.899 1.040 1.011 1.036 1.151 poltár 1.130 1.025 1.010 1.051 1.053 1.229 revúca 0.972 1.035 0.846 1.089 0.981 0.927 rimavská sobota 1.040 0.991 0.997 1.064 1.023 1.093 veľký krtíš 1.197 1.084 0.899 1.001 1.039 1.168 zvolen 0.932 0.949 1.036 1.057 0.992 0.969 žarnovica 0.906 1.048 1.071 1.225 1.056 1.246 žiar nad hronom 1.000 1.000 1.000 1.000 1.000 1.000 bardejov 1.156 1.051 0.996 0.973 1.042 1.177 humenné 1.156 1.012 0.969 1.078 1.051 1.222 continued on next page. 81 eleonóra marišová, peter fandel acta polytechnica ctu proceedings district 2018/ 2019/ 2020/ 2021/ mean cumulative 2017 2018 2019 2020 t ec t ec kežmarok 1.262 0.918 1.020 1.034 1.051 1.222 levoča 1.129 1.295 0.885 1.087 1.089 1.406 medzilaborce 0.796 1.920 0.694 1.114 1.042 1.182 poprad 1.139 1.140 0.838 1.139 1.055 1.239 prešov 1.100 1.003 1.107 1.020 1.056 1.246 sabinov 1.120 1.136 0.981 1.156 1.096 1.443 snina 1.120 1.000 0.983 1.006 1.026 1.108 stará ľubovňa 0.976 0.924 0.990 1.059 0.986 0.945 stropkov 1.795 1.040 1.066 0.825 1.132 1.642 svidník 1.001 1.421 0.965 1.022 1.088 1.403 vranov nad topľou 1.194 0.950 1.059 1.041 1.058 1.250 gelnica 1.320 1.000 0.878 1.013 1.041 1.174 košice i-iv 1.000 1.000 1.000 1.000 1.000 1.000 košice – okolie 1.168 1.000 1.000 1.000 1.040 1.168 michalovce 1.238 1.010 1.021 0.936 1.046 1.195 rožňava 1.019 1.242 0.960 1.299 1.121 1.578 sobrance 1.000 1.000 1.000 1.000 1.000 1.000 spišská nová ves 0.996 1.048 0.912 0.954 0.976 0.908 trebišov 2.036 0.926 0.905 1.033 1.152 1.763 82 vol. 46/2024 municipal waste management performance: a focus on slovakia . . . appendix c. malmquist index of tfp change, 2017–2021 district 2018/ 2019/ 2020/ 2021/ mean cumulative 2017 2018 2019 2020 mi mi bratislava i-v 0.973 0.915 1.323 1.037 1.051 1.221 malacky 1.491 1.034 1.152 1.119 1.187 1.987 pezinok 1.027 1.172 1.096 1.088 1.094 1.435 senec 1.539 1.067 1.198 1.247 1.252 2.453 dunajská streda 1.602 1.337 1.204 1.106 1.300 2.852 galanta 2.208 1.075 1.079 1.079 1.289 2.763 hlohovec 1.726 0.957 1.004 0.886 1.101 1.469 piešťany 1.302 0.984 1.174 1.172 1.152 1.763 senica 1.051 1.139 1.057 1.016 1.065 1.286 skalica 1.303 0.957 1.168 1.055 1.113 1.537 trnava 1.376 1.005 1.072 1.12 1.135 1.660 bánovce nad bebravou 1.223 1.394 1.269 1.035 1.223 2.239 ilava 1.11 0.942 1.09 0.988 1.030 1.126 myjava 0.993 1.03 1.133 1.02 1.043 1.182 nové mesto nad váhom 0.982 1.154 1.128 1.182 1.109 1.511 partizánske 1.528 0.985 1.068 1.218 1.183 1.958 považská bystrica 1.063 1.125 1.068 1.184 1.109 1.512 prievidza 1.249 1.195 1.026 1.138 1.149 1.743 púchov 1.086 1.093 0.923 1.09 1.045 1.194 trenčín 1.12 1.1 1.287 1.004 1.123 1.592 komárno 1.049 1.077 1.225 1.312 1.161 1.816 levice 1.166 1.18 0.937 1.151 1.104 1.484 nitra 0.996 1.035 1.099 1.067 1.049 1.209 nové zámky 1.539 0.932 0.993 1.03 1.101 1.467 šaľa 1.083 0.973 1.178 1.046 1.068 1.298 topoľčany 1.182 1.104 1.161 1.226 1.167 1.857 zlaté moravce 0.933 1.386 0.934 1.103 1.074 1.332 bytča 1.098 0.975 1.11 1.421 1.140 1.689 čadca 0.999 1.223 1.111 1.064 1.096 1.444 dolný kubín 1.102 1.15 1.086 1.078 1.104 1.484 kysucké nové mesto 1.795 0.992 0.827 1.399 1.198 2.060 liptovský mikuláš 1.163 1.157 0.979 1.086 1.093 1.431 martin 1.148 1.026 1.126 1.096 1.098 1.454 námestovo 1.37 0.913 1.024 1.086 1.086 1.391 ružomberok 1.282 1.066 1.11 1.092 1.134 1.656 turčianske teplice 0.869 1.25 1.227 1.084 1.096 1.445 tvrdošín 1.024 0.945 1.132 1.066 1.039 1.168 žilina 1.033 1.082 0.971 0.996 1.020 1.081 banská bystrica 1.188 0.969 1.089 0.986 1.054 1.236 banská štiavnica 0.933 1.156 1.413 1.105 1.139 1.684 brezno 0.712 1.098 1.196 1.037 0.992 0.970 detva 0.744 1.096 1.048 1.329 1.032 1.136 krupina 1.334 1.036 1.054 1.204 1.151 1.754 lučenec 1.253 0.879 1.027 1.188 1.077 1.344 poltár 1.086 0.983 1.06 1.142 1.066 1.292 revúca 0.956 1.023 0.844 1.332 1.024 1.099 rimavská sobota 1.015 0.964 1.129 1.042 1.036 1.151 veľký krtíš 1.154 1.045 0.933 1.034 1.038 1.163 zvolen 0.984 1.006 1.174 1.054 1.052 1.225 žarnovica 0.95 1.026 1.097 1.312 1.088 1.403 žiar nad hronom 1.053 1.012 1.044 1.123 1.057 1.249 bardejov 1.099 1.001 1.052 1.053 1.051 1.219 humenné 1.122 1.03 1.027 1.079 1.064 1.281 continued on next page. 83 eleonóra marišová, peter fandel acta polytechnica ctu proceedings district 2018/ 2019/ 2020/ 2021/ mean cumulative 2017 2018 2019 2020 mi mi kežmarok 1.278 0.902 1.042 1.066 1.064 1.280 levoča 1.088 1.272 1.085 1.009 1.109 1.515 medzilaborce 0.673 1.735 0.843 0.929 0.978 0.914 poprad 1.167 1.195 0.985 1.158 1.123 1.591 prešov 1.105 0.977 1.175 1.103 1.088 1.399 sabinov 1.065 1.105 1.05 1.131 1.087 1.398 snina 1.11 0.971 1.07 1.005 1.038 1.159 stará ľubovňa 0.952 0.915 1.084 1.013 0.989 0.957 stropkov 1.648 1.006 1.318 0.774 1.140 1.691 svidník 1.037 1.402 0.965 1.061 1.105 1.489 vranov nad topľou 1.109 0.905 1.111 1.107 1.054 1.234 gelnica 1.235 0.952 0.874 1.167 1.046 1.199 košice i-iv 0.978 1 1.004 0.999 0.995 0.981 košice – okolie 1.131 0.99 1.023 1.065 1.051 1.220 michalovce 1.248 0.977 1.044 0.995 1.061 1.267 rožňava 1.032 1.21 1.03 1.32 1.141 1.698 sobrance 0.865 0.851 1.053 0.848 0.901 0.657 spišská nová ves 1.045 1.029 0.908 0.992 0.992 0.969 trebišov 1.919 0.894 0.98 0.988 1.135 1.661 84 acta polytechnica ctu proceedings 46:65–84, 2024 1 introduction 2 theoretical background 3 material and methods 4 results 4.1 municipal waste – eu legislation 4.2 implementation of eu waste legislation into slovak one 4.3 eu prevention of waste 4.4 main aims of waste prevention in slovak waste prevention program 4.5 waste management program and slovak lau-1 districts competencies 4.6 the strategy of the environmental policy of the slovak republic until 2030 4.7 results of the quantitative analysis 5 conclusions acknowledgements references a rank of districts of slovakia according to waste management performance measured by the composite indicator of technical super-efficiency, 2017–2021 b technical efficiency change index, 2017–2021 c malmquist index of tfp change, 2017–2021 acta polytechnica ctu proceedings https://doi.org/10.14311/app.2025.52.0077 acta polytechnica ctu proceedings 52:77–85, 2025 © 2025 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague applicability of the saf operation – case study of a czech registered business aircraft operator on flights within europe eliška makovcováa, peter olexaa, pavel provinskýb, damir kuchkarova, peter vitteka,∗ a czech technical university in prague, faculty of transportation sciences, department of air transport, horská 3, 143 00 prague, czech republic b czech technical university in prague, faculty of transportation sciences, department of applied mathematics, na florenci 25, 110 00 prague, czech republic ∗ corresponding author: peter.vittek@cvut.cz abstract. the aim of this paper is to develop a fundamental model for assessing the potential of sustainable aviation fuel (saf) utilization in business aviation. it investigates the feasibility and strategic integration of saf within czech registered business aircraft operator on flights within europe, with a focus on its environmental and economic implications. the study evaluates the carriers’ networks by establishing two refueling strategies and identifying key factors for calculating emissions and costs. specifically, the minimum refueling strategy and maximum saf refueling strategy are analyzed. through a thorough analysis, this research aims to offer actionable insights for aviation companies, highlighting the dual benefits of saf in promoting both sustainability and cost efficiency. current eu ets regulations exempt operators emitting less than 10 000 tonnes of co2 annually, a threshold many operators aim to stay below. however, as the eu intensifies its climate targets, smaller operators must anticipate greater regulatory and financial pressures related to emissions. this represents a pivotal move toward a more inclusive and stringent emissions trading system, aligned with sustainability goals. saf adoption emerges as a promising solution for business aviation, as its use would significantly reduce co2 emissions and associated costs for operators. by offering a comprehensive overview and strategic recommendations, this study aims to contribute to the ongoing discussion on sustainable aviation practices and assist stakeholders in making informed decisions regarding the adoption of saf. keywords: sustainable aviation fuel (saf), business aviation, environmental impact, carbon emissions, refueling strategies, economic analysis, czech registered business aircraft operator. 1. introduction the aviation industry, integral to global economic and social connectivity, is simultaneously an important contributor to environmental degradation, primarily due to its substantial carbon dioxide and other greenhouse gases emissions. as global awareness of climate change’s impacts intensifies, there is a compelling need for the aviation sector to reduce its environmental footprint. this urgency is underscored by the growing scrutiny from governments, regulatory bodies, and the public demanding sustainable practices. sustainable aviation fuel (saf) represents a transformative approach to mitigate aviation’s environmental impacts by providing an eco-friendly alternative to conventional jet fuels. according to the u.s. department of energy [1], saf refers to a category of biofuels derived from renewable feedstocks, offering an alternative to conventional fossil-based jet fuels such as jet a-1. these feedstocks include waste oils and fats (such as used cooking oil), agricultural residues, lignocellulosic biomass, and even algae. saf is considered a “drop-in” fuel, meaning it can be blended with conventional jet fuel and used in existing aircraft engines and infrastructure without the need for significant modifications. this research paper delves into the application of saf within the czech business aviation landscape, examining its environmental benefits against economic implications and exploring strategic integration pathways. the role of aviation in global carbon emissions is significant, with the industry accounting for approximately 2 % of global co2 emissions. this figure is projected to rise dramatically if reliance on traditional fossil fuels persists [2]. the international air transport association (iata) has recognized the critical nature of this challenge, setting forth an ambitious agenda to reduce aviation’s emissions to 50 % of 2005 levels by the year 2050, which underscores the pivotal role of saf in achieving these goals [2]. the primary environmental benefit of saf lies in its potential to reduce lifecycle carbon dioxide (co2) emissions by up to 80 % compared to traditional jet fuel, depending on the specific production process employed [1]. this reduction is achieved by replacing fossil carbon with carbon that has been absorbed from the atmosphere during the growth of the biomass feedstock, thus closing 77 https://doi.org/10.14311/app.2025.52.0077 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en e. makovcová, p. olexa, p. provinský et al. acta polytechnica ctu proceedings the carbon cycle. additionally, saf production pathways contribute to lower particulate matter emissions and sulfur oxides. according to the iata study, the saf is expected to achieve the highest co2 emission reductions of all the plans, contributing to 24–70 % (with a median of 53 %) co2 emission reductions in 2050 compared to the corresponding baseline emission levels [3]. considering these challenges, recent studies have underscored the pressing need for the aviation sector to adopt more sustainable practices. klöwer [4] has further quantified aviation’s contribution to global warming, emphasizing this urgent need. one promising solution highlighted by leading industry players is the integration of saf. according to airbus, saf is critical for achieving significant reductions in greenhouse gas emissions, aligning closely with international environmental policies, and thereby facilitating a transition towards a more sustainable aviation industry [5]. additionally, ryanair’s recent agreement [6] with repsol to promote the use of saf across its fleet exemplifies the industry’s growing commitment to sustainable practices. this agreement not only underscores the potential of saf to reduce emissions but also sets a benchmark for other airlines to follow. according to airbus, the integration of sustainable aviation fuel (saf) is a critical step towards achieving the aviation industry’s environmental goals. airbus emphasizes that the deployment of saf can significantly reduce greenhouse gas emissions, thus aligning with the broader objectives of international environmental policies [5]. additionally, the iata’s fly net zero [7] initiative highlights the importance of adopting saf as a key component in the aviation sector’s strategy to reach net-zero emissions by 2050. this initiative underscores the necessity for both regulatory support and technological advancements to facilitate the widespread use of saf in aviation operations, showcasing the industry’s commitment to sustainable practices. additionally, the european union aviation safety agency (easa) has been proactive in its efforts [8] to harmonize fuel and energy planning policies across airlines, as demonstrated by lufthansa’s comprehensive implementation strategy. this strategy includes detailed fuel consumption monitoring and in-flight fuel management policies that are crucial for maximizing saf efficiency. the alignment with icao’s sustainable fuel policies [9] further reinforces the long-term benefits of such strategies. continued support from regulatory frameworks and policy incentives will be crucial in overcoming these challenges [10]. a detailed report by booz allen hamilton [11] underscores the economic impact of business aviation on the european economy. the report highlights that business aviation significantly contributes to job creation, regional development, and economic activity, especially in regions with limited commercial air services. pazourek [12] emphasizes the unique operational characteristics and economic contributions of business aviation in europe. despite the smaller size of business aviation compared to commercial airlines, it plays a vital role in regional connectivity and economic activity. the importance of these considerations is underscored by the resilience of the business aviation sector during the covid-19 pandemic. according to a systematic literature review by pantelaki and papatheodorou [13], this resilience highlighted the sector’s significant contributions to travel efficiency, regional connectivity, and emergency services, which are crucial for both business and societal functions. as such, there is a growing body of research focusing on the sustainability of business aviation, with a particular emphasis on the integration of saf to reduce environmental impact. however, the integration of saf must also consider the broader economic implications within the aviation network. in this context, the role of the market becomes particularly relevant. the travel patterns of the customers are indicating that their preference for private jets is driven by the need for privacy, flexibility, and efficiency. furthermore, a detailed study on business jets [14] reveals that the wealthiest segments of society, who frequently utilize business aviation, have a substantial role to play in the adoption of saf, as it is assumed that this passenger segment has more financial capacity to absorb the higher costs associated with saf but also the potential to lead by example in reducing the aviation sector’s carbon footprint. encouraging this segment to adopt saf could thus play a pivotal role in accelerating the transition towards more sustainable aviation practices industrywide [15]. by leveraging influence, business aviation customers can set a precedent for broader acceptance and utilization of saf across the industry. the integration of saf into existing aviation operations is heavily influenced by the legislative framework and regulatory standards that govern the aviation industry. paper also deals with identifying the regulatory documents and guidelines that dictate the use of aviation fuels, particularly saf, within both international and czech contexts. the study reviews key international regulations set forth by bodies such as the international civil aviation organization (icao) and the european union aviation safety agency (easa), so that the analysis within this study aligns with the relevant regulations. these organizations provide frameworks that dictate everything from fuel standards and safety regulations to environmental compliance. specific attention is paid to icao’s corsia (carbon offsetting and reduction scheme for international aviation), which sets out emission reduction goals and sustainable fuel usage policies. easa’s guidelines on alternative fuels also play a crucial role in defining how saf can be integrated into existing fleets without compromising safety and operational integrity. as part of the eu’s commitment to reduce greenhouse gas emissions, the eu ets (eu emissions 78 vol. 52/2025 applicability of the saf operation – case study . . . trading system) serves as a cornerstone mechanism, allowing for the trading of emission permits among companies and entities. it is a major tool used to regulate emissions from the aviation sector by setting caps on the total amount of greenhouse gases that can be emitted. operators must hold permits equivalent to their emissions, and they can trade these permits to stay within legal limits [16]. in line with eu directives, czech legislation incorporates these eu regulations into its national laws, primarily through act no. 383/2012 on conditions for trading emission allowances. this law ensures that aviation operators within the czech republic are obliged to comply with eu ets regulations and contribute to the national and eu-wide goals of reducing greenhouse gas emissions. additionally, recent policy developments emphasize the need for robust support mechanisms to encourage saf adoption, including incentives and subsidies [10]. eu ets regulation provides an exemption for operators emitting less than 10 000 tonnes of co2 annually, which is nowadays a reason why many operators focus on never exceeding this emissions threshold so that they are not required to pay for emission allowances. however, the proposed changes under the “fit for 55” package suggest that this exemption is likely to be reduced or eliminated in the near future. as the eu intensifies its efforts to meet its climate targets, smaller operators should prepare for increased regulatory and financial obligations related to their emissions. the shift signals a significant step towards a more inclusive and stringent emissions trading system, reflecting the broader goals of environmental sustainability and climate change mitigation. this might be the most significant reason why adaptation of saf could be a solution for business aviation operators as flights on saf will not produce the same amount of co2 as flying on fossil fuels which will lower operator’s emissions costs. on the other hand, current prices of saf are higher than jet a-1 fuel so the pricing of saf will be crucial factor for new fueling strategies of operators. within this framework, the czech republic’s burgeoning business aviation sector represents a key area of focus. this sector, while smaller than commercial airlines, significantly impacts environmental sustainability due to its specific operational characteristics and fuel usage patterns, and is often targeted due to significantly higher co2 emission per passenger/km, than commercial flights. business aviation in the czech republic, characterized by its flexibility and personalized services, faces unique challenges and opportunities in adopting saf. this study addresses these by first outlining the current state of saf technology and its global adoption, highlighting advancements in fuel technology, regulatory incentives, and the economic rationale behind its adoption. the introduction of saf not only promises to reduce the sector’s greenhouse gas emissions but also aligns with global regulatory trends favoring greener alternatives. the objectives of this research are twofold: to provide a detailed analysis of the operational network characteristics of czech registered business aircraft operator, assessing how these can be optimized to facilitate the transition to saf; and to evaluate different refueling strategies, such as minimum fuel and maximum saf refueling strategies, for their environmental and economic efficiency. by offering a comprehensive overview and strategic recommendations, this study aims to contribute to the ongoing discussion on sustainable aviation practices and assist stakeholders in making informed decisions regarding the adoption of saf. further, this paper investigates the specific needs and feasibility of integrating saf into the operator’s network. it examines the logistical considerations, including the availability of saf, the infrastructure required for its distribution, and the potential for retrofitting existing aircraft or acquiring new ones compatible with saf. it also considers the economic implications, analyzing the cost differentials between saf and conventional fuels and evaluating the financial impact of potential regulatory changes, such as carbon pricing or emissions trading schemes that could favor saf adoption. 2. materials and methods this section of the paper delineates the comprehensive methodological framework utilized to examine the integration and implications of saf within czech registered business aircraft operator. to systematically assess the potential of saf, the methodology is organized into several pivotal areas that build upon each other to form a cohesive analysis. the research begins by examining the characteristics of the carrier’s network, which lays the foundational understanding of the current operational context and identifies key areas where saf could be integrated effectively. this initial analysis helps in mapping out the operational scope and identifying specific routes and hubs that could benefit from saf. following the network analysis, the study transitions into defining and applying relevant regulations and standards that govern the use of alternative fuels in aviation. this includes exploring both international guidelines and local regulatory frameworks that influence fuel choices, safety standards, and environmental compliance. these regulations set the parameters for what is feasible within the current legal and operational boundaries and help in shaping the refueling strategies that will be explored. with a clear understanding of the operational context and regulatory constraints, the methodology then delves into the specific refueling strategies. this stage involves a detailed examination of different approaches to integrating saf into the carrier’s fuel supply chain, assessing the logistical, economic, and environmental 79 e. makovcová, p. olexa, p. provinský et al. acta polytechnica ctu proceedings implications of each strategy. it is at this point that the study evaluates the feasibility of two strategies, setting the stage for a deeper investigation into the environmental and economic impacts of each approach. finally, the methodology addresses the attributes for calculating emissions and costs, which are crucial for quantifying the benefits and challenges of adopting saf. this includes detailed calculations of fuel consumption, co2 emissions, and cost analysis, providing the empirical data necessary to support decision-making. these attributes are essential for assessing the viability of saf from a sustainability perspective and its alignment with business objectives. each of these areas is integral to a thorough evaluation of saf’s feasibility and impact on operations, and together they provide a structured approach to understanding how saf can be realistically and beneficially integrated into czech business aviation. the subsequent sub-sections will delve deeper into each of these areas, offering a detailed exploration of the methods and tools used, the data gathered, and the analyses conducted. the analysis begins with calculating the fuel consumption for each flight segment, considering different aircraft types and their specific fuel consumption rates. data is collected according to the average fuel consumption per one hour flight, and adjustments are made based on whether saf or conventional jet fuel is utilized. the efficiency of saf in terms of energy content compared to jet a-1 is also evaluated to determine if there are any significant differences in the amount of fuel required for similar flight operations. 2.1. data collection the initial phase of the methodology focuses on the comprehensive collection of data were provided, containing information on 2 400 flights over the course of one year. the data included departure and arrival airports, flight duration, aircraft type, and specific fuel consumption details for the company’s aircraft. using the provided data, fuel consumption was calculated for individual flights. in the following step of the methodology, as described in section 2.2, saf was theoretically integrated on selected routes according to the refueling strategies detailed in section 2.3. airport information, provided in iata codes, was specifically used to identify airports for theoretical saf integration, while flight duration and aircraft type were employed to calculate fuel consumption. this data was supplemented by interviews with professionals from business aviation and flight operations personnel to gain insights into operational nuances that standard data collection might overlook. the purpose of this rigorous data collection process is to ensure that the study has a robust dataset that accurately reflects the real-world operations of the carrier, which is essential for a valid analysis. 2.2. network analysis: connectivity evaluation using gephi the connectivity analysis is specifically used to select the most frequented airports within the network. these airports are chosen based on their high traffic volumes and strategic importance to the overall network efficiency and service quality. after collecting the necessary data, the study utilizes gephi, an advanced network analysis tool, to evaluate the connectivity of the carrier’s network [3]. this tool allows for the visualization and analysis of the relationships and flows between different nodes within the aviation network, such as airports, flight routes, and aircraft movements. by analyzing the operational data with gephi, the research identifies key nodes (major hubs) and links (frequent routes) with the highest potential for the effective integration of saf. this detailed connectivity analysis is instrumental in developing tailored refueling strategies. by focusing on the most active airports, the study ensures that the strategies developed are relevant and impactful, providing valuable insights into how saf could be realistically integrated into the existing operational framework. based on the connectivity analysis, five of the most frequently serviced airports were selected for theoretical saf integration. this number was set as an illustrative value within which the model’s results will be assessed. this value can be adjusted based on the actual availability of saf at the airports. currently, saf infrastructure is more tied to large airports that are hubs for fleets of legacy carriers such as british airways, lufthansa, klm, joined by nordic region copenhagen and stockholm arlanda airports. 2.3. refueling strategies methodology elaborates on two refined refueling strategies tailored to assess the viability of saf in different operational scenarios based on data provided by one specific czech registered business aircraft operator. these strategies will be described below as model 1 for minimum refueling strategy and model 2 for maximum saf refueling strategy. both models assume the integration of a 40 % saf blend, with 60 % jet a1 at the same five airports, based on the results of the network analysis. this is based on the availability and current regulatory acceptance of such blends for commercial use. this blend ratio is considered to be a practical and realistic option for testing within the existing infrastructure because it does not require significant modifications to aircraft engines or fuel distribution systems. additionally, a 40 % blend is within the range that can substantially reduce carbon emissions while still ensuring optimal engine performance and fuel availability. in both models, the volume of fuel burned is used as an input numerical value, and this value remains consistent between the two models. although model 2 would typically be expected to show a higher fuel burn 80 vol. 52/2025 applicability of the saf operation – case study . . . due to the added weight of fuel associated with fuel tankering. this factor has been excluded in order to focus on illustrating the core aspects of the model. however, these, so called tankering fuel penalties (additional fuel burnt due weight of fuel itself) were not considered in the calculations to maintain the simplicity of a foundational model, as it was deemed outside the primary scope of analysis. industry estimates suggest penalties of around 0,33 % fuel burn penalty per 100 km [17], but the actual penalty can vary depending on aircraft type, payload, flown distance and other variables. the practice of fuel tankering or carrying excess fuel is utilized in the model 2 to maximize the use of saf. this approach intentionally increases saf usage despite its potential drawbacks, reflecting a calculated trade-off to boost the share of sustainable fuel in the fleet’s fuel blend. this decision underscores the commitment to integrating saf more extensively, even if it involves temporary increases in overall fuel consumption, to achieve long-term sustainability goals in business aviation [2]. given the nature of the data, it is not possible to include potential diversions to alternate airports or other events that would lead to the consumption of a certain amount from the fuel reserve in the calculation. the diversions however pose only 0,3 % of cases on average and therefore are not so significant [18]. data regarding diverts or change of routes due to the weather were not provided for this analysis. however, this model does not work with operational specifics such as the lengths of rwy at individual destinations. therefore, possible operational limitations for aircraft weight were not taken into account in creating the model. model 1 for model 1 (minimum refueling strategy), it is specified that the operator only fills the minimal amount of fuel necessary for the next segment at each landing airport. therefore, model 1 operates with the assumption that each landing leaves exactly this amount of fuel of 600 kg, known as “final reserve fuel”, in the tanks. in model 1 the operator fills saf at the five most frequently serviced airports according to the network analysis. at other destinations, jet a-1 is filled. for clarity, a decision tree is created, which describes the refueling variants typical for model 1, see figure 1. model 2 model 2 (maximum saf refueling strategy) works with the version where the operator fills saf at the five most frequently serviced airports to the maximum possible volume. at other destinations, the operator fills jet a-1, but always only adds as much as is necessary to service the next segment. this means that if the operator fills with saf fuel, therefore to the full, and only a short segment follows, no fuel needs to be refilled if there is no saf at this destination and there is a sufficient amount of fuel in the tanks from the previous flight. if saf fuel was figure 1. model 1 – minimum refueling strategy. available at this destination, the operator would again fill to the maximum possible amount. the decision tree for model 2 is evident in figure 2. 2.4. formulas and figures by offering a comprehensive overview and strategic recommendations, this study aims to contribute to the ongoing discussion on sustainable aviation practices and assist stakeholders in making informed decisions regarding the adoption of saf. the two main variables resulting from both models that will help in decision making are fuel costs and total emissions. the use of saf reduces total emissions, however, it increases overall fuel expenses due to the higher production costs associated with saf compared to conventional jet a-1. the fuel costs c when using saf are calculated using the formula: c = [(1 − f40)pjet + f40psaf ] m, (1) where: f40 is the proportion of 40 % saf in the total fuel consumption, pjet is the unit price in eur per 1 kg of jet a1 fuel, psaf is the unit price in euros per 1 kg of 40 % saf, m is the total weight of fuel consumed. the total emissions e when using saf are calculated according to the formula: e = [(1 − f40)ejet + f40esaf ]m, (2) where: f40 is the proportion of 40 % saf in the total fuel consumption, ejet is the unit emissions in kg co2 per kg of burned jet a1 fuel, esaf is the unit emissions in kg co2 per kg of burned 40 % saf blend fuel, m is the total weight of fuel consumed. 81 e. makovcová, p. olexa, p. provinský et al. acta polytechnica ctu proceedings figure 2. model 2 – maximum saf refueling strategy. according to the results of model 1 and model 2 above, all the input parameters to the equations can be determined. based on the results of model 1 and model 2 and by applying the above mentioned formulas, the following values will be presented in the results section: emision reduction [%], share of saf integrated [%], total fuel consumption [kg], total fuel costs with saf integrated [€], fuel cost of jet a-1 [€], increased fuel cost caused by saf integration [€], co2 emissions with jet a-1 only [kg], co2 emissions with saf integrated [kg], co2 reduction achieved through saf integration [kg], and price increase per flight hour [€]. 2.5. attributes for calculating emissions and costs a critical aspect of this study is the calculation of emissions from both saf and conventional fuels. for this calculation, the emissions factor is the most fundamental element. the emission factor of fuel refers to the amount of emissions produced per unit of fuel consumed. it is a key metric used to estimate and compare the environmental impact of different fuels. emission factors are typically expressed in terms of mass of co2 (or co2 equivalent for other ghgs) emitted per unit of energy (e.g., kilograms of co2 per gigajoule) or per unit of fuel (e.g., kilograms of co2 per liter or per kilogram of fuel). in this paper emission factor describe mass of co2 per one kilogram of burned fuel. emissions of carbon dioxide differs (co2) for each type of fuel. the carbon emissions are particularly important, as one of the main advantages of saf is its potential to reduce lifecycle carbon emissions compared to conventional fuels. factors such as the blend ratio of saf with conventional fuel and the specific emission factors associated with each type of fuel are considered. the emission factor for jet a1 is 3.15 kg co2 per kg of fuel burned [19]. this factor reflects the typical carbon emissions associated with the combustion of conventional jet fuel. for 40 % saf blend, the emission factor used is lower, at 2.55 kg co2 per kg of fuel burned. this lower emission factor is due to the reduced carbon content of saf, which is made from renewable sources that can absorb co2 during their growth phase, offsetting some of the emissions produced during fuel combustion. this property of saf contributes to its potential to reduce the overall carbon footprint of aviation fuel when compared to conventional jet fuel. the cost of fueling aircraft with saf versus conventional jet fuel forms a significant part of the economic analysis. this includes the per-liter cost of each type of fuel, potential changes in maintenance costs due to different fuel properties, and the amortization of any additional infrastructure investments required for storing and handling saf. in addition, potential economic incentives such as tax rebates, emission trading credits, or subsidies for using saf are analyzed to determine their impact on the overall cost-effectiveness of saf adoption. jet a-1 fuel price was set at € 0.73 kg−1. this price was quoted for the europe and cis region (commonwealth of independent states) as of april 2023 [20]. a price of a 40 % saf blend, was used at € 2.5 kg−1, which is the price listed for amsterdam schiphol airport (eham) as of april 2023. this reflects the common estimation that saf is typically 2–6 times more expensive than jet a-1. based on an analysis of the current market, it can be concluded that current prices are comparable. 3. results for both refueling strategies, this section outlines the fundamental parameters that shape the operational, economic, and environmental aspects of integrating saf into the fleet operations of a business aviation operator. the results will be presented in the structure defined in section 2.4. 82 vol. 52/2025 applicability of the saf operation – case study . . . emissions reduction 5.5 % share of saf integrated 11,5 % of total fuel total fuel consumption 3 353 469 kg total fuel cost with saf integrated 4 158 806 € fuel cost of jet a-1 2 448 032 € increased fuel cost caused by saf integration 1 710 774 € co2 emissions with jet a-1 only 10 563 427 kg co2 emissions with saf integrated 9 983 503 kg reduction of co2 with saf integrated 579 923 kg price increase per flight hour 146 € table 1. model 1 – minimum refueling strategy – results. 3.1. model 1 – minimum refueling strategy under the minimum refueling strategy incorporates a 40 % saf blend at the five most frequently served destinations, filling the minimum required fuel for the next leg. in this case, the operator will save 5,5 % of emissions. pure saf constitutes 11,5 % of the total fuel which operator consumed per one year. the total fuel consumption is 3 353 469 kg. regarding fuel acquired data, the operator would face a fuel cost of about € 2 448,032 for a jet a-1-only flight. if the 11,529 % of pure saf will be integrated, the fuel cost would rise to € 4 158 806, exceeding the fossil fuel-only cost by € 1 710 774. the economic impact, whether positive or negative, would depend on the individual pricing of flights. the pricing in business aviation is influenced by various factors, such as airport charges, flight length, cross-country charges, and departure times. it cannot be definitively stated whether the model application would have a positive or negative economic impact on the operator. however, in a simplified analysis, the increased costs per flight hour could be budgeted. if the operator would like to compensate the increased costs using this model, it would need to raise the price per flight hour sold by an average of € 146. if all flights were operated on jet a-1 fuel with an emission factor of 3.15 kg co2 kg−1 fuel, the total co2 produced per year would be 10 563 427 kg. this exceeds the 10 000 tonnes co2 limit, necessitating the purchase of emission allowances. it was found that by incorporating saf flights according to the minimum refueling strategy outlined in model 1, the operator would emit 9 983 503 kg of co2. this places them just below the threshold, eliminating the obligation to purchase emission allowances. the model has a positive environmental impact, reducing co2 emissions by 579 923 kg compared to identical flights using jet a-1 fuel alone. summary of results in table 1. 3.2. model 2 – maximum saf refueling strategy in the maximum saf refueling strategy, instead of always refueling the volume required for the next flight, the operator fills the maximum possible amount of 40 % saf blend at the five saf destinations. then, if the operator operates from a destination where saf is not available, they take only the minimum amount of jet a-1 fuel to safely depart for the next leg. applying this strategy, the operator will save 12,7 % of emissions. pure saf constitutes of 26,573 % of the total fuel which operator consumed per one year. the total fuel consumption is 3 353 469 kg. if the entire operation were to operate with jet a-1 fuel only, the fuel cost is € 2 451 583. when using saf, the cost would be € 6 400 570. this strategy would, therefore, increase the fuel cost by € 3 948 987 per year. as mentioned in the analogous section for minimum refueling strategy, only the operator’s pricing strategy will determine whether the increased costs would be offset by the extra flight hours gained. then, if the operator would like to compensate the increased costs using model 2 by increasing the price per hour sold, the price would increase by € 610 per flight hour. if all flights were operated on jet a-1 fuel with an emission factor of 3.15 kg co2 kg−1 fuel, the total co2 produced per year would be 10 563 427 kg. this also exceeds the 10 000 tonnes co2 limit, necessitating the purchase of emission allowances. it was found that by incorporating saf flights as per the maximum saf refueling strategy described by model 2, the operator would emit 9 240 108 kg of co2. the model has a positive environmental impact, reducing co2 emissions by 1 338 640 kg of co2 compared to identical flights using jet a-1 fuel alone. summary of results in table 2. 4. discussion the integration of saf in business aviation involves a complex interplay of economic costs, operational changes, and environmental impacts. the two models developed in the paper encapsulate different strategies for integrating saf, highlighting the practical considerations and potential outcomes from such initiatives. these models serve as a foundation for understanding the broader implications of saf adoption within the sector. model 1 uses a conservative approach by incorporating a 40 % saf blend at the five most frequently 83 e. makovcová, p. olexa, p. provinský et al. acta polytechnica ctu proceedings emissions reduction 12.7 % share of saf integrated 27 % of total fuel total fuel consumption 3 353 469 kg total fuel cost with saf integrated 6 400 570 € fuel cost of jet a-1 2 451 583 € increased fuel cost caused by saf integration 3 948 987 € co2 emissions with jet a-1 only 10 563 427 kg co2 emissions with saf integrated 9 240 112 kg reduction of co2 with saf integrated 1 338 640 kg price increase per flight hour 610 € table 2. model 2 – maximum refueling strategy – results. served destinations, filling the minimum required fuel for the next leg. this strategy aims to optimize operational efficiency while adhering to environmental regulations. financially, this model necessitates an increase in fuel costs due to the higher price of saf compared to conventional jet fuel. despite the increase in fuel expenditure, the implementation of saf under model 1 would lead to a reduction in co2 emissions by 5.5 %. the model suggests a potential reduction in co2 emissions by keeping the total emissions just below the threshold that would require the purchase of additional emission allowances. this aspect alone could provide a cost-saving benefit, offsetting some of the increased fuel costs. furthermore, recent icao findings support that saf can significantly lower lifecycle emissions, making it a viable option for meeting stringent environmental targets [9]. model 2 proposes a more aggressive use of saf, maximizing the amount of saf used at every opportunity where it is available, and only supplementing with jet a-1 where necessary. this model is designed to push the boundaries of how much saf can be integrated into current operations without altering the core functionalities of aircraft or requiring significant changes in infrastructure. the analysis shows that while model 2 leads to higher upfront costs due to the increased consumption of more expensive saf, it also maximizes the environmental benefits. the higher expenditure on saf could potentially be mitigated by the savings from not having to buy emission allowances, as the reduced co2 production keeps the total emissions below critical regulatory thresholds. both models underscore the practical challenges of cost management and operational adjustments required for transitioning to saf. they reflect the ongoing need for strategic planning in business aviation to balance cost, operational feasibility, and environmental responsibility. the introduction of saf, despite its higher cost, presents an opportunity to significantly reduce the environmental footprint of aviation operations, aligning with global sustainability goals. certain limitations were intentionally applied in this study to maintain a fundamental model that is both illustrative and straightforward, enabling future expansions. key simplifications include disregarding the increased fuel consumption associated with tankering and omitting the variability in the sustainable component ratio of saf, which is legislatively mandated to range from 2 % to 6 % in upcoming periods. additionally, while current regulations allow for up to 50 % saf usage, flights with 100 % saf have also been conducted, adding another layer of potential complexity. another limitation is the omission of variability in the emissions factor, which can vary significantly. these constraints were applied solely to ensure the model’s clarity and simplicity, creating a clear baseline that future studies can build upon by incorporating additional variables for a more comprehensive analysis. however, the real-world application of these models would require careful consideration of fuel price volatility, the availability of saf, and the specific operational profiles of different airlines with weather information or data about diversions. as the industry moves towards more sustainable practices, these models provide a framework for understanding the potential costs and benefits of integrating saf but highlight the necessity for adaptive management strategies that can respond to changing economic conditions and regulatory landscapes. the broader adoption of saf is supported by evolving policies [10] and incentives designed to facilitate a smoother transition towards more sustainable aviation. 5. conclusions this study has thoroughly investigated the potential integration of sustainable aviation fuel (saf) in business aviation, emphasizing its economic and environmental implications. the research has demonstrated that while saf presents opportunities for significant reductions in carbon emissions, its integration within business aviation faces various challenges, including higher costs and limited availability. key findings highlight that adopting saf can lead to considerable environmental benefits by reducing the carbon footprint of business aviation operations. however, the economic impact of integrating saf involves increased fuel costs, which are influenced by the current higher prices of saf compared to conventional jet a-1 fuel. despite these costs, the potential savings from reduced carbon emissions and 84 vol. 52/2025 applicability of the saf operation – case study . . . the avoidance of emissions trading costs under schemes like the eu ets could offset some of the financial burdens. the analysis of two refueling strategies – minimum fuel and maximum saf refueling strategies – provided insights into the operational adjustments necessary to accommodate saf. these strategies, though distinct, both align with the global push towards sustainability and comply with emerging environmental regulations. they also illustrate the trade-offs between achieving maximum environmental benefits and managing operational and financial realities. the research underscores the need for further development in saf production and distribution to make it more accessible and economically viable for business aviation. it also calls for continued regulatory support to foster the adoption of saf, including incentives that could mitigate the high costs associated with its use. icao’s ongoing efforts in promoting saf adoption highlight the importance of international cooperation in achieving these sustainability goals. ultimately, this study concludes that while the integration of saf into business aviation is fraught with challenges, it remains a critical component of the industry’s journey towards sustainability. the findings of this study contribute to the broader discourse on sustainable aviation practices, offering valuable insights for operators, policymakers, and researchers aiming to advance the use of sustainable fuels in aviation. acknowledgements references [1] u.s. department of energy. synthetic aviation fuels, office of energy efficiency & renewable energy, washington, dc, 2024. 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[2023-04-03]. https://climate.ec.europa.eu/eu-action/euemissions-trading-system-eu-ets_en [17] d. rutherford, x. s. zheng, b. graver, n. pavlenko. potential tankering under an eu sustainable aviation fuels mandate. icct working paper (19):1–13, 2021. [18] m. špák, p. olexa. enhancement of the diversion airport selection methodology. transportation research procedia 51:232–242, 2020. https://doi.org/10.1016/j.trpro.2020.11.026 [19] greenhouse gas reporting: conversion factors 2021. [2023-07-20]. https://www.gov.uk/government/ publications/greenhouse-gas-reportingconversion-factors-2021 [20] iata. fuel price monitor: fuel price analysis. 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https://www.airbus.com/en/sustainability/environment/climate-change/decarbonisation/sustainable-aviation-fuel https://www.airbus.com/en/sustainability/environment/climate-change/decarbonisation/sustainable-aviation-fuel https://corporate.ryanair.com/news/ryanair-repsol-sign-major-sustainable-fuel-agreement/ https://corporate.ryanair.com/news/ryanair-repsol-sign-major-sustainable-fuel-agreement/ https://www.iata.org/en/programs/environment/flynetzero/ https://www.iata.org/en/programs/environment/flynetzero/ https://www.easa.europa.eu/en/downloads/136795/en https://www.easa.europa.eu/en/downloads/136795/en https://www.icao.int/environmental-protection/pages/saf.aspx https://www.icao.int/environmental-protection/pages/saf.aspx https://www.iata.org/en/programs/environment/sustainable-aviation-fuels/ https://www.iata.org/en/programs/environment/sustainable-aviation-fuels/ https://nbaa.org/wp-content/uploads/aircraft-operations/international/region-v-europe/boozallenhamilton-bizav-impact-on-europe.pdf https://nbaa.org/wp-content/uploads/aircraft-operations/international/region-v-europe/boozallenhamilton-bizav-impact-on-europe.pdf https://nbaa.org/wp-content/uploads/aircraft-operations/international/region-v-europe/boozallenhamilton-bizav-impact-on-europe.pdf https://doi.org/10.1016/j.jairtraman.2022.102299 https://doi.org/10.1016/j.jairtraman.2022.102299 https://www.forbes.com/sites/douggollan/2018/10/10/why-when-and-where-the-super-rich-fly-their-private-jets/ https://www.forbes.com/sites/douggollan/2018/10/10/why-when-and-where-the-super-rich-fly-their-private-jets/ https://www.forbes.com/sites/douggollan/2018/10/10/why-when-and-where-the-super-rich-fly-their-private-jets/ https://www.transportenvironment.org/articles/private-jets-can-the-super-rich-supercharge-zero-emission-aviation? https://www.transportenvironment.org/articles/private-jets-can-the-super-rich-supercharge-zero-emission-aviation? https://www.transportenvironment.org/articles/private-jets-can-the-super-rich-supercharge-zero-emission-aviation? https://climate.ec.europa.eu/eu-action/eu-emissions-trading-system-eu-ets_en https://climate.ec.europa.eu/eu-action/eu-emissions-trading-system-eu-ets_en https://doi.org/10.1016/j.trpro.2020.11.026 https://www.gov.uk/government/publications/greenhouse-gas-reporting-conversion-factors-2021 https://www.gov.uk/government/publications/greenhouse-gas-reporting-conversion-factors-2021 https://www.gov.uk/government/publications/greenhouse-gas-reporting-conversion-factors-2021 https://www.iata.org/en/publications/economics/fuel-monitor https://www.iata.org/en/publications/economics/fuel-monitor acta polytechnica ctu proceedings 52:77–85, 2025 1 introduction 2 materials and methods 2.1 data collection 2.2 network analysis: connectivity evaluation using gephi 2.3 refueling strategies 2.4 formulas and figures 2.5 attributes for calculating emissions and costs 3 results 3.1 model 1 – minimum refueling strategy 3.2 model 2 – maximum saf refueling strategy 4 discussion 5 conclusions acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2024.47.0053 acta polytechnica ctu proceedings 47:53–57, 2024 © 2024 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague preparation of light fraction silicate filer after the mining for the base layers’ road construction andrea jančíková∗, karel dvořák, simona ravaszová brno university of technology, faculty of civil engineering, veveří 331/95, 602 00 brno, czech republic ∗ corresponding author: andrea.jencikova@vut.cz abstract. the article main objective was eco-innovation in the field of utilization and creation of a complex waste-free technology for the separation of components during ore extraction with subsequent processing, recovery of mine tailings in combination with available energy. the article deals with the topic of production and preparation of base layers for road construction in the form of silicate filer after separation of ore mining. the chemical and mineralogical composition on the fine fraction of the separated tailings was carried out by xrd analysis, filer particle size followed up by laser granulometry and other comprehensive measurements and the development of basic parameters by individual standard tests. the results determined on the functional sample demonstrated that it is possible to create an almost completely waste-free technology in the field of separation of components during ore extraction and its subsequent processing and use for the base layers of linear road construction. keywords: mine tailing processing, heavy metals, mineral separation. 1. introduction recently, due to increasing environmental concerns and material price inflation, there has been a growing interest from the scientific community to replace conventional mined fillers with environmentally friendly alternative materials. new concrete technologies for the development of low-cost and environmentally clean silicate materials with significant use of both new mining residues after the extraction of polymetallic ores, where aggregate heaps burden the existing landscape. while in them lies the potential for the replacement of natural aggregates in concrete mixes [1–3]. thus, very ecological option is the use of these tailings for the preparation of base layers for linear road construction. it is also for this reason that, in the current period of extreme construction, there is starting to be a shortage of good quality natural aggregate sources for concrete production [2–6]. silica sand remains one of the most widely used and versatile materials in the world [1–5]. for example, when used as composite fillers, silica materials significantly improve the mechanical, thermal, and rheological properties of polymer matrix composites. fillers are an integral part of modern building materials. when their function is only filling without any bonding value. it can also be described as the finest part of the aggregate that passes through a certain sieve (0.075 mm in the united states and 0.063 mm in europe) and occupies certain weight in mixtures. filler must be free of organic impurities [3–7]. mining tailings are one of the main wastes generated in the mining industry during the concentration phase of the precious metal extraction process. it is the waste rock that remains after ore processing and is eventually disposed of as fine sand after milling [6–10]. the implementation of the achieved theoretical basis will be carried out according to the developed complex design of the treatment plant technology for the “zlaté hory” underground mine with obtaining the residual material from the treatment in a further usable form. the results are summarized in the trend fw project report 01010061-1-2022. the treatment plant will be a crushing unit, the first stage of which will be a semi-mobile crushing unit with a wheel-mounted jaw crusher located in the mine area. input mined material of up to 500 mm fraction will be crushed to an output fraction of up to 200 mm, which will be transported away from the mine to the stockpile. from the homogenization intermediate stockpile, the raw material will be deposited by wheel loader into the hopper of the belt feeder and then by belt conveyor onto a vibratory screening screen with screen openings of 25 and 50 mm. fractions greater than 25 mm will be crushed on a secondary cone crusher with an output fraction below 25 mm. the crushed material will be conveyed by conveyor belts to a vibratory screening plant with 16, 10 and 1 mm screen drops. the fraction above 16 mm will be returned to the previous crushing stage in the cone crusher. the intermediate product, i.e., the fraction from 1 to 16 mm, will be crushed in the next crushing stage on a roller crusher with an output fraction with a mean grain size of 800 µm. all the material will undergo gravity sorting to a precise fraction size for the silicate filer. 2. material and methods the test sample is made from a light fraction of aggregate after gravity separation of heavy minerals from the mine “zlaté hory” in czech republic deposit. a proven separation procedure using a gravity trough was used with basic screening and removal of coarse 53 https://doi.org/10.14311/app.2024.47.0053 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en a. jančíková, k. dvořák, s. ravaszová acta polytechnica ctu proceedings impurities, followed by gravity separation using a gravity float, after which the light fraction was allowed to settle and dried after decantation. the crushing and separation of the coarser fraction was described in the introduction section. this ochre-colored material is primarily intended to produce cement and polymer-based construction materials. as it is being considered to use the residue for separation both in the processing of fresh residue from active mining and old deposits. due to the large volume of samples, it was first necessary to perform a coarse sieving to remove impurities using a techkon tajfun trough sieve. this was followed by the removal of lumps and concretions using a jaw crusher. the samples were then separated in an lsv d140 magnetic separator with magnetic field strength on the surface of a kevlar conveyor belt. the mineralogical composition and granulometry were monitored, especially of the light component, which will be further used to produce road base layers. the xrd analysis on samples was performed using a panalytical empyrean diffractometer. the reflection bragg-brentano para focusing geometry device is equipped with a cu anode (λ = 1.54184 å) and programable divergence slits a pixcel3d detector with 255 active channels. the x-ray generator settings were 45 kv and 40 ma. the measured range was 5–80° with a step size of 0.013° and 38 s per step. each sample was measured four times, and the scans were next simply summed. the total measurement time for each sample was 60 min. the panalytical highscore 3 plus software was used to identify the individual phases. laser granulometry was used to measure the size and distribution of particles in the sample. the measurements range from tens of nanometers to units of millimeters. the laser granulometer can analyze powdered material in a dry air stream or powder dispersed in a liquid medium. to evaluate the particle size distribution, the laser diffraction evaluation software uses two mathematical models. the simpler fraunhofer approximation or the more modern and accurate mie diffraction model. furthermore, the mechanical and physical parameters of the silicate filler thus treated and analyzed were determined according to the individual standards: čsn en 998-2 specification of mortars for masonry – part 2: mortars for masonry [11], čsn en 13139 aggregates for mortar [12], čsn en 1097-3 testing of mechanical and physical properties of aggregates – part 3: determination of bulk density and void ratio of loose aggregates [13], čsn en 1097-4 testing of mechanical and physical properties of aggregates – part 4: determination of the gap properties of dry compacted aggregate [14], čsn en 1097-5 testing of mechanical and physical properties of aggregates – part 5: determination of moisture content by drying in a drying oven [15], čsn en 1097-6 testing of mechanical and physical properties of aggregates [16]. these standard tests examined the following parameters: aggregate fraction/fineness, grinding fineness according to blaine, bulk weight (čsn en 1097-7 [17]), bulk density (čsn en 10973), chloride content, sulphates content, water absorbency (čsn en 1097-6), mass activity index, 226 ra mass activity (bq kg−1 ). 3. results and discussion figures 1 and 2 show the graphical analysis by laser granulometry. figure 1. laser granulometry of silicate filler (light fraction) “zlaté hory” *“lehká frakce” is translated as light fraction in absolute and cumulative distributions. figure 2. particle distribution of the light component of the “zlaté hory” silicate filler *“lehká frakce” is translated as light fraction in absolute and cumulative distributions. these graphs show the result of granulometric analysis of a sample of processed raw material from “zlaté hory”. it can be seen that the mean grain size of the light fraction d(0.5) was 72 micrometers. the minimum particle size d(0.1) was around 24 micrometers and the maximum grain size d(0.9) was 199 micrometers. the granulometric parameters of the “zlaté hory” silicate sand separated in semi-laboratory quantities were essentially identical to those obtained on small and medium-sized samples. 54 vol. 47/2024 preparation of the filer after the mining for the road construction properties requirement according to test result declared filler property čsn en 13139 of the sample aggregate fraction/fineness declaration filler for mortars d(90) 0.1965mm 0 – 0.2 grinding fineness (blaine) – 335 m2 kg−1 <350 m2 kg−1 bulk weight čsn en 1097-7 declared value 2.630 g cm−3 (pycnometry) 2.630 ± 100 g cm−3 (pycnometry) bulk density čsn en 1097-3 declared value 1 600 kg m−3 1600 ± 100 kg m−3 chloride content declared value < 0,1 % does not contain does not contain sulphates fes < 0.1 % < 1 g kg−1 23.3 mg kg−1 (dry substances) < 100 mg kg−1 (dry substances) absorbency declared value 4.8 % < 5 % čsn en 1097-6 < 0.5 0.412 ± 0.025 < 0.5 mass activity index < 100 25.6 ± 5.2 < 100 table 1. the mechanical and physical parameters of the silicate filler determined according to the individual standards. figure 3. result of xrd mineralogical phase analysis of light component sample “zlaté hory” silicate filler, the results shown in the graph are listed in table 2 below the graph. color mineral quantity content blue muscovite 2m1 8,0 % light green pyrite 1,1 % grey chamosite 1miib 5,7 % magenta quartz 75 % light blue albite 14,6 purple chalcopyrite 0,5 % table 2. the resulting measured values of the specific weight of the separated tailings. the result of xrd mineralogical phase analysis after separation for the pyrite-depleted light components is shown in table 2 and figure 3. mineralogical analysis shows that the proportion of heavy components (pyrite and chalcopyrite) after separation is low. the gravity separation of the heavy component was quite effective. the light component: concentrate separation ratio was determined after processing of 1 000 kg of the deposit and was 81.2 % light component and 15.1 % concentrate. a total of 3.7 % were losses. the light fraction obtained was used for the preparation of silicate filler and as a source of aggregate which will be further used to produce road base layers. to verify the filler parameters, tests were carried out within the scope of the requirements of čsn en 13139 aggregates for mortars. the parameters verified and the limits set are given in table 1. hardened mixture for base layers of linear civil engineering structures is considered a functional sample. it used regraded and cleaned fractions of aggregates from the tailings after mining at underground mine “bytíz”. the binder phase included, in addition to mixed portland cement, “opatovice” fly ash and the basic recipe is given in table 3. for aggregate mixtures cemented with cement the čsn en 14227-1 [18] standard applies. these mixtures are usable as a base layer for linear civil engineering structures. the aggregate used as filler is “bytíz” 0/32, which is a residue after separation of heavy minerals from the “bytíz” deposit. due to the origin of the aggregate and to improve the grain size curve, the aggregate “náklo” in the 0/4 fraction was used as a supplementary aggregate. the following 55 a. jančíková, k. dvořák, s. ravaszová acta polytechnica ctu proceedings name of functional sample cem ii/a-m fly ash aggregates aggregates water 42,5r “opatovice” “náklo” 0/4 “bytíz” 0/32 water fw01010061-v2 95 65 890 1 085 145 table 3. overview of parameters of the functional sample containing silicate filler for linear structures. tests were carried out to verify the parameters of the foundation: the compressive strength according to čsn en 13286-41 [19] was 5.9 mpa after 7 days and 10.4 mpa after 28 days, which is higher than the required value of 5 mpa after 7 days and 8 mpa after 28 days. the bulk density according to čsn en 12390-7 [20] exceeded the required minimum value of 2 300 kg m−3. the optimum moisture contentproctor test according to čsn en 13286-2 [21] was 6.8 % which is less than the maximum allowable moisture content of 10 % and the maximum dry mix bulk density according to čsn en 13286-2 was 2 190 kg m−3 which also corresponds to the required standard value of 2 200 ± 100 kg m−3. 4. conclusion the main objective was the use of tailings for the preparation of base layers of linear road structures and the design and development of waste-free technology to produce base mixtures. preliminary research on legislative requirements and proof tests was carried out. samples of structural concretes and mixes for the base layers of linear structures were prepared and tested from samples of “bytíz” aggregates. the resulting functional sample was classified as sc – type 4 – 0/32 – c8/10 based on the positive results obtained. acknowledgements we acknowledge the financial support to project (fw01010061) tacr, “comprehensive technology of ore residue processing for the stabilization of mined out areas and for the underlying layers of linear structures” and project (fast-s-23-8246) “study of the formation and stability of various tricalcium aluminate polymorphs”. references [1] n. sivakugan, k. j. rankine, k. s. rankine. study of drainage through hydraulic fill stopes using method of fragments. geotechnical & geological engineering 24(1):79–89, 2006. https://doi.org/10.1007/s10706-004-1949-x [2] b. ercikdi, f. cihangir, a. kesimal, et al. utilization of industrial waste products as pozzolanic material in cemented paste backfill of high sulphide mill tailings. journal of hazardous materials 168(2-3):848–856, 2009. https://doi.org/10.1016/j.jhazmat.2009.02.100 [3] j. choudhary, b. kumar, a. gupta. performance evaluation of asphalt concrete mixes having copper industry waste as filler. transportation research procedia 48:3656–3667, 2020. https://doi.org/10.1016/j.trpro.2020.08.083 [4] k. pal, h. vardhan, m. aruna. investigation of contaminant transport in groundwater from the tailings pond of uranium mine: a case study. international journal of mining and mineral engineering (ijmme) 2(4):290, 2010. https://doi.org/10.1504/ijmme.2010.039039 [5] e. yilmaz. advances in reducing large volumes of environmentally harmful mine waste rocks and tailings introduction. mineral resources management 27:89–112, 2013. [6] b. dold. submarine tailings disposal (std)—a review. minerals (basel) 4(3):642–666, 2014. https://doi.org/10.3390/min4030642 [7] h. e. jamieson, s. r. walker, m. b. parsons. mineralogical characterization of mine waste. applied geochemistry 57:85–105, 2015. environmental geochemistry of modern mining. https://doi.org/10.1016/j.apgeochem.2014.12.014 [8] m. b. j. lindsay, m. c. moncur, j. g. bain, et al. geochemical and mineralogical aspects of sulfide mine tailings. applied geochemistry 57:157–177, 2015. https://doi.org/10.1016/j.apgeochem.2015.01.009 [9] c. qi, a. fourie. cemented paste backfill for mineral tailings management: review and future perspectives. minerals engineering 144:106025, 2019. https://doi.org/10.1016/j.mineng.2019.106025 [10] m. k. mishra, u. m. r. karanam. geotechnical characterization of fly ash composites for backfilling mine voids. geotechnical & geological engineering 24(6):1749–1765, 2006. https://doi.org/10.1007/s10706-006-6805-8 [11] specifikace malt pro zdivo část 2: malty pro zdění [specification for mortar for masonry part 2: masonry mortar]. standard, czech standard institute, prague, 2003. [12] kamenivo pro malty [aggregates for mortar]. standard, czech standard institute, prague, 2004. [13] zkoušení mechanických a fyzikálních vlastností kameniva část 3: stanovení sypné hmotnosti a mezerovitosti volně sypaného kameniva [tests for mechanical and physical properties of aggregates part 3: determination of loose bulk density and voids]. standard, czech standard institute, prague, 1999. [14] zkoušení mechanických a fyzikálních vlastností kameniva část 4: stanovení mezerovitosti suchého zhutněného fileru [tests for mechanical and physical properties of aggregates part 4: determination of the voids of dry compacted filler]. standard, czech standard institute, prague, 2008. 56 https://doi.org/10.1007/s10706-004-1949-x https://doi.org/10.1016/j.jhazmat.2009.02.100 https://doi.org/10.1016/j.trpro.2020.08.083 https://doi.org/10.1504/ijmme.2010.039039 https://doi.org/10.3390/min4030642 https://doi.org/10.1016/j.apgeochem.2014.12.014 https://doi.org/10.1016/j.apgeochem.2015.01.009 https://doi.org/10.1016/j.mineng.2019.106025 https://doi.org/10.1007/s10706-006-6805-8 vol. 47/2024 preparation of the filer after the mining for the road construction [15] zkoušení mechanických a fyzikálních vlastností kameniva část 5: stanovení vlhkosti sušením v sušárně [tests for mechanical and physical properties of aggregates part 5: determination of the water content by drying in a ventilated oven]. standard, czech standard institute, prague, 2008. [16] zkoušení mechanických a fyzikálních vlastností kameniva část 6: stanovení objemové hmotnosti zrn a nasákavosti [tests for mechanical and physical properties of aggregates part 6: determination of particle density and water absorption]. standard, czech standard institute, prague, 2008. [17] zkoušení mechanických a fyzikálních vlastností kameniva část 7: stanovení měrné hmotnosti fileru pyknometrická zkouška [tests for mechanical and physical properties of aggregates part 7: determination of the particle density of filer pyknometer method]. standard, czech standard institute, prague, 2008. [18] směsi stmelené hydraulickými pojivy specifikace část 1: směsi z kameniva stmelené cementem [hydraulically bound mixtures – specifications – part 1: cement bound granular mixtures]. standard, czech standard institute, prague, 2013. [19] nestmelené směsi a směsi stmelené hydraulickými pojivy část 41: zkušební metoda pro stanovení pevnosti v tlaku směsí stmelených hydraulickými pojivy [unbound and hydraulically bound mixtures part 41: test method for determination of the compressive strength of hydraulically bound mixtures]. standard, czech standard institute, prague, 2004. [20] zkoušení ztvrdlého betonu část 7: objemová hmotnost ztvrdlého betonu [testing hardened concrete part 7: density of hardened concrete]. standard, czech standard institute, prague, 2001. [21] nestmelené směsi a směsi stmelené hydraulickými pojivy část 2: zkušební metody pro stanovení laboratorní srovnávací objemové hmotnosti a vlhkosti proctorova zkouška [unbound and hydraulically bound mixtures part 2: test methods for the determination of the laboratory reference density and water content proctor compaction]. standard, czech standard institute, prague, 2005. 57 acta polytechnica ctu proceedings 47:53–57, 2024 1 introduction 2 material and methods 3 results and discussion 4 conclusion acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2024.50.0036 acta polytechnica ctu proceedings 50:36–43, 2024 © 2024 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague substrate effect in the evaluation of elastic modulus of thin pvd coatings from nanoindentation františek lofaj slovak academy of sciences, institute of materials research, watsonova 47, 040 01 košice, slovakia correspondence: flofaj@saske.sk abstract. the comparison of nanoindentation results on reactive hitus tizrhf+me-ny (me = nb, v, ta and vnbta) and reactive dcms nbmotaw coatings showed that the influence of substrate stiffness is still not fully eliminated in the evaluation of indentation modulus. to obtain comparable results by keeping the systematic error resulting from the existing evaluation procedures the same, nanoindentation should be performed using the same test parameters, a method for depth profile acquisition, a data evaluation procedure, and coatings with similar thicknesses on the same type of substrate. applying these rules to nanoindentation on tizrhf+me-ny coatings showed that the addition of me = nb, v, ta, and vnbta caused some degradation of mechanical properties indicating negligible or even negative “cocktail effect”. keywords: thin pvd films, nanoindentation, substrate effect elimination, multi-element nitride coatings. 1. introduction the nanoindentation testing on thin hard coating/softer substrate (and vice versa) systems always generates “composite” values due to simultaneous loading and response from the coating and substrate. the problem can be solved using several different approaches. possibly the first way to extract coating hardness from the composite values and known as “10% (relative depth) rule”, was proposed by h. bückle in 1959 [1]. it was suggested that the hardness values obtained from the relative depths below 10% of the coating thickness should be used because the contribution from the substrate would still be very limited. unfortunately, this rule fully corresponds to the h. l. mencken quote: “for every complex problem there is an answer which is clear, simple and wrong” [2]. it is only a rule of thumb with limited applicability to hardness measurements and its use for the determination of an indentation modulus is principally not correct [3, 4]. the reason is that the size of the plastic field under the indenter defined by the yield stress can be confined only to the coating within the limited penetration depth range whereas the elastic field would extend into the substrate without any limit. moreover, the stress field is affected by the ratio between elastic moduli of the coating and substrate and even by their interactions [5, 6]. since the “10% rule” is not able to reflect all these interactions, significant efforts were devoted to the refinement of the analysis of nanoindentation data from thin coatings. starting from the early 1990s, various theoretical models have been developed to solve the problem of substrate effect analytically. the common features of these solutions were that they considered indentation as a deformation of coating and substrate in series and that they were fitted to experimentally obtain hardness and indentation modulus (relative) depth profiles. depending on the physical processes obtained in the solutions, three groups of the models can be distinguished: (1.) linear transition models involving the works of jönsson&hogmark [7], burnett&rickerby [8], doerner&nix [9], he&li [10], chicot&lesage [11], puchi-cabrera [12], etc.; (2.) energy models including works of korsunsky&bull [13, 14], (3.) gao’s model & its modifications including studies of gao&chiu [15], menčík [16], song&pharr [17], hay&crawford [5] etc. most of these models, were applicable only to the coating/substrate systems in which the elastic modulus of the coating was less than twice that of the substrate [5]. however, very stiff coatings can provide additional support against the deformation of the substrate. the additional and parallel influence of the coating on the substrate was introduced into the song&pharr’s model [17] by hay&crawford (h&c) [5]. h&c model applies to the systems with ≤ 10-fold ratio between the elastic moduli of coating and substrate and the range of relative indentation depths (plateau width) available for the extrapolation to zero depth was expanded well above 10% relative depth. the disadvantage is that the obtained indentation moduli are highly sensitive to the accuracy of the additional input parameters, especially young’s modulus of the substrate and coating thickness. the results of theoretical works on nanoindentation in coating/substrate systems were reflected in the standardization efforts which were summarized in iso 14577-4:2016 standard [18]. it prescribes that the true hardness and modulus of hard coatings on softer 36 https://doi.org/10.14311/app.2024.50.0036 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 50/2024 substrate effect in the evaluation of elastic modulus . . . substrate young’s modulus poisson’s ratio hit # eit # e [gpa] ν [–] [gpa] [gpa] (0001) sapphire 435 [25] 0.29 [25] 27.57 ± 0.27 458.7 ± 3.3 tempered 100cr6 steel 210 (190–210) [26, 27] 0.285 (0.27–.30) [27] 9.25 ± 1.0 250.0 ± 9.5 (111) si wafer 187 (186.5) [28] 0.223 (0.22–0.28) [28] 13.07 ± 0.13 193.6 ± 1.2 ti6al4v alloy 114 [29] 0.342 [29] 5.20 ± 0.16 133.2 ± 2.2 table 1. mechanical properties of the substrates used in the calculations (e and ν, and measured by nanoindentation in csm mode (hit and eit ). substrates can be determined by the extrapolation of the values from the plateau at the maximum of the corresponding profile to zero penetration depth (load). it should be noted that such a procedure does not explicitly involve any analytical solution and the way the corresponding depth profile was obtained. the simplest way to generate statistically reliable hardness and modulus depth profiles is to perform a sufficiently large number of individual loading/unloading tests until predefined depths in the range from less than 100 nm up to the depths approaching the coating thickness. analogous depth profiles from the reduced number of indents can be obtained when several partial unloading/re-loadings are applied during the loading part of the loading/unloading tests. this mode is usually called “continuous multicycle” (cmc). further advancement of cmc led to the “sinusoidal loading” or “continuous stiffness measurements” (csm) [19–22] mode which involves an overlap of continuous sinusoidal load with small (constant absolute or constant relative) amplitude and conventional continuous loading. the advantage of csm mode is a much higher speed of the generation of the corresponding depth profiles with better accuracy from a smaller number of indents. statistical reliability of csm tests is further improved due to the reduction of thermal drift and low-frequency noise effects [21]. the disadvantage of csm mode is the contribution of “plasticity error” causing deviations at lower frequencies, especially in the materials with high e/h ratio [22]. it is related to small plastic deformation added to the elastic deformation due to a higher total load at the end of the cycle than at its beginning. however, plasticity error can be corrected [21] or significantly reduced using an appropriate combination of amplitude, frequency and loading rate [22]. it can be also eliminated when using the so-called quasicsm (qcsm) mode when sinusoidal oscillations are applied only to repeated, short dwell-time periods during a continuous loading cycle [23]. regardless of the method depth profiles were obtained, iso standard procedure should be used to deduce the corresponding coating properties without the influence of the substrate. sinusoidal modes (csm and csmtf) seem to be easier, faster and more precise than cmc and loading/unloading tests. moreover, the iso procedure is already built in the evaluation software of sinusoidal modes including that on the h&c model and known as “csm for thin films” (csmtf) mode. however, a comprehensive comparison of the evaluation of nanoindentation results considering different methods for depth profile generation, differences between conventional csm and csm treated based on the h&c model with the effect of substrate properties are still missing. therefore, the current work aimed to quantify the differences in the obtained hardness and moduli from the depth profiles obtained using different modes and on the coating/substrate systems with different substrate’s young’s moduli on the statistically sufficient set of coatings. the work is the extension of our previous on 4 different substrates [24] from one up to six types of multi-transition metal-nitride ((tizrhf+me)ny, where me = v, nb, ta and nbmotaw-ny) coatings with different stoichiometry spanning from pure metallic high entropy stabilized alloys (y = 0) up to stoichiometric nitrides (y = 1). 2. materials and methods 2.1. coating deposition the studied coatings were deposited by sputtering on four different substrates from six different composite targets. the substrates involved polished single crystalline (111) si and (0001) sapphire wafers, the polished discs of polycrystalline tempered 100cr6 steel and ti6al4v alloy with different elastic moduli (see table 1). the targets included tizrhf, tizrhfv, tizrhfnb, tizrhfta, tinbvtazrhf and nbmotaw discs with the diameters 76.2mm and thicknesses of around 6.35mm (table 2). the tizrhf-based nitride coatings were produced by reactive high target utilization sputtering (model s500, plasma quest ltd., uk) from the above targets under the sputtering conditions optimized earlier [30] and applying different flows of nitrogen, x (in standard cubic centimeters per minute, sccm), into the sputtering ar atmosphere (120 sccm). the values of x were usually in the range 0–10 sccm when stoichiometric composition was achieved. the structure and composition of these coatings were described earlier [30]. the metallic nbmotaw coating was deposited using balanced dc magnetron sputtering at 300w on a sapphire substrate heated to 500 °c at a floating bias. 37 františek lofaj acta polytechnica ctu proceedings target ti zr hf nb v ta mo w [%] [%] [%] [%] [%] [%] [%] [%] tizrhf (99.6 %)# 33 33 34 – – – – – tizrhfv (>99.5 %)* 32 18 18 – 32 – – – tizrhfnb (>99.5 %)* 33 19 19 29 tizrhfta (>99.5 %)* 33 19 19 – – 29 – – tinbvtazrhf (>99.5 %)* 20 12 12 18 20 18 – – nbmotav (>99.9 %)* – – – 25 – 25 25 25 *testbourne, ltd.; # porexi table 2. the list of the targets and their nominal compositions used for the deposition of the studied nitride coatings. 2.2. nanoindentation nanoindentation tests were performed with freshly calibrated berkovich diamond tip in g 200 (agilent/kla, usa) nanoindenter under load control with the constant strain rate of 0.05 s−1 using four techniques – loading/unloading, cmc, csm and csmtf. in both csm cases, the frequency of 45hz and the amplitude of 2 nm were applied to reduce the contribution of plastic deformation during sinusoidal loading [22]. on each coating, two sets of 16 indents up to 600 (or 800) nm depth were carried out. the depth profiles were treated according to iso 14577-4 with the lower limit of the plateau range of >80 nm determined by tip area calibration. the upper limit depended on the shape of the average depth profile and measurement mode: in the csm case, the upper depth limit was often within 10% of the relative coating thickness whereas when hay&crawford correction [5] was considered, it may exceed that depth due to improved consideration of substrate effect. the thicknesses of the coatings were measured using scanning electron microscopy (sem, models auriga compact, zeiss, germany) on the fractured cross sections of the coatings deposited on si wafers. 3. results and discussion 3.1. depth profiles by different methods to compare the depth profiles generated using four different methods, only one tizrhfvnbta-xn coating (x = 6 sccm n2) with stoichiometric (y = 1) composition and thickness of 1700 nm on the silicon wafer was used. figure 1a shows the depth profile of the indentation modulus obtained using simple loading/unloading tests. each value calculated using oliver&pharr (o&p) method corresponds to the average from 10 indents and 9 different depths were employed to obtain the profile. the data were valid in the range above 80 nm. the extrapolation procedure from the plateau-like region from 80 nm up to 200 nm resulted in the indentation modulus, eit = 295 ± 35gpa whereas the “10% rule” approach gave around 284gpa. the hardness values, hit = 29.5 ± 2gpa were identical in both cases. (a). (b). figure 1. indentation modulus depth profiles obtained on tizrhfvnbta-xn coating (x = 6 sccm n2) on (111) silicon substrate using (a) – 90 simple loading/unloading tests; (b) – 30 tests with 6 partial unloadings in each test. the zone below 80 nm should not be considered due to indenter tip surface area calibration effects. 38 vol. 50/2024 substrate effect in the evaluation of elastic modulus . . . method eit [gpa] eit [gpa] hit [gpa] hit [gpa] iso 10% rule iso 10% rule loading/unloading 295 ± 35 ∼285 29.5 ± 2.0 29.5 cmc ∼300 (∼300) ∼28 ∼28 csm 290 ± 15 ∼270–275 27.2 ± 2.6 27.2 csm+h&c (csmtf) 275 ± 15 ∼250 28.2 ± 2.7 28.2 table 3. summary of the indentation modulus and hardness values calculated using iso and the “10% rule” procedures from the depth profiles by four different methods on 1700 nm thick tizrhfvnbta-xn coating (x = 6 sccm n2) deposited on (111) silicon substrate by reactive hitus. in the case of cmc tests (figure 1b), 10 indents up to 100mn, 200mn and 250mn loads with 6 partial unloading in each test were performed. each unloading was evaluated according to o&p method but the averaging was not performed due to variations in the depths during unloadings within each test. it resulted in relatively large scatter of the data and only rough estimate of the coating properties. the approximate indentation modulus was around 300gpa and hardness was 28gpa. possible differences between extrapolation according to iso standard and the “10% rule” were not observed due to excessive scatter of the data. in the case of csm tests (figure 2a), the average depth profile curve was smooth and large scatter was present only below 100 nm depths. the maximum at 100 nm was followed by a smooth decrease without a clear plateau. the calculation of the corresponding eit depends on subjectively selected depth range limits. in the range from 100 nm to 150 nm, eit = 290 ± 15gpa whereas only around 275gpa were obtained using the “10% rule” approach. the hardness profile exhibited a well-defined plateau up to 250 nm depth. therefore, the same values of hit of 27.2 ± 2.6gpa were obtained when applying iso and the “10% rule” procedure. figure 2b illustrates the effect of the correction on substrate modulus and coating thickness included in h&c correction (csmtf) on the modulus depth profile compared to the standard csm profile. the differences are visible already in the 100–150 nm (“plateau”) depth region and increase with the increase of penetration depth. subsequently, different indentation moduli (full arrows) were obtained from these regions in csm (∼275gpa) and csmtf (∼263gpa). the average values from 9 indents (after removal of the outcasts) based on iso procedure resulted in the average eit = 275 ± 15gpa. at 10% relative depth, the values were ∼270gpa and ∼250gpa, respectively. the hardness profile is not affected by the correction and it was hit = 28.2 ± 2.7gpa. table 3 compares the results of nanoindentation tests using four different methods for depth profiles and evaluated using iso and the “10% rule” procedure. it should be emphasized that the results were obtained on the same and relatively thick (1700 nm) coating on silicon substrate, which eliminated possible effects of variations between coatings, substrates and (a). (b). figure 2. an average (from ≤16 indents) indentation modulus depth profiles on tizrhfvnbta-xn coating (x = 6 sccm n2) on (111) silicon substrate using (a) – continuous stiffness measurements (csm) tests; (b) – the comparison of eit ) depth profiles from 1 indentation test using csm and csm with the correction on substrate properties based on hay&crawford model (csmtf). 39 františek lofaj acta polytechnica ctu proceedings small thickness. the values of eit (and hit ) calculated according to iso standard from the profiles obtained by different methods varied from 275gpa to 300gpa (and 27.2gpa to 29.5gpa), which was within the scatter of each measurement. thus, all four methods are suitable for the determination of hardness and modulus profiles in coating/substrate systems. however, csm-based methods are experimentally easier, faster, more effective and conservative, especially with the correction on substrate properties. the differences between eit values calculated according to iso and the “10% rule” procedures were also not very large and fully overlapped in hit . however, it does not rehabilitate the “10% rule” approach because the coating was relatively thick which could shield the substrate and prevent the collision of the calculation range with the lower limit of validity range. therefore, additional measurements on thinner coatings were performed. 3.2. coating thickness effect analogous nanoindentation tests using csm only were performed on the nbmotaw coating with a thickness of 610 nm deposited using dc magnetron sputtering on (0001) sapphire substrate. figure 3 compares the indentation modulus and hardness depth profiles in csm and corrected csm (csmtf). the h&c correction introduced a reduction of the indentation modulus profile and plateau became more pronounced than in the case of csm. it caused that the modulus could be calculated from the relative depth range from 16% to almost 30% which is well above 10% of the relative depth. the resulting value from csm was higher by 35gpa than that (354 ± 11gpa) obtained from the corrected csm profile. the calculation employing the “10% rule” completely failed: the depth of 61 nm falls into the range where the tip-blunting effects are dominant and the data are invalid. the hardness profile from csm (figure 3b) was not affected by the correction and it was hit = 30.7 ± 1.0gpa. applying the “10% rule” to hardness also failed because of the same reason. thus, figure 3 even more clearly than the data in table 3 demonstrates why the “10% rule” approach should be avoided in the determination of the mechanical properties of thin coatings from nanoindentation. 3.3. the effect of substrate modulus this study involved one stoichiometric tizrhfvnbtaxn coating approximately 1325 nm thick deposited by reactive hitus at x = 6 sccm n2 flow simultaneously on four different substrates listed in table 1. two measurements were performed using independently csm and corrected csm (csmtf) methods on each substrate. the results summarized in figure 4a indicate that eit and hit values from both measurements were within the scatter and they are reproducible. besides that, systematic and substantial differences in the 20–50gpa range between apparent eit values (a). (b). figure 3. depth profiles of (a) – indentation modulus, (b) – hardness, obtained using csm method during nanoindentation of 610 nm thin r-dcms nbmotaw coating. from csm and corrected csm (csmtf) and in the 65–150gpa range the increase of the substrate modulus from ti-alloy to sapphire. thus, the influence of the substrate elastic modulus on the coating modulus evaluation was not fully eliminated both in csm and in the h&c-modified csm and its further refinement is necessary. on the contrary, hardness (figure 4b) varied only from 27gpa to 36gpa with an average value of around 31.5gpa and the differences were mostly within the scatter of individual measurements. 3.4. best practice recommendations the above results suggest that substrate influence in the evaluation of indentation modulus is still not fully eliminated and the current solutions, despite the incorporation of iso 14577-4 procedure, generate systematic differences exceeding measurement scatter. in the case of the presence of systematic errors, the following rules can be recommended to produce 40 vol. 50/2024 substrate effect in the evaluation of elastic modulus . . . (a). (b). figure 4. the effect of substrate modulus on the calculated values of indentation modulus – (a), and hardness – (b), in csm and independent corrected csm (csmtf) models. the data from csmtf were intentionally offset for better visibility [24]. comparable values of indentation modulus in thin coating/substrate systems: • strictly follow iso 24577-4 and principally avoid the “10% rule” despite its simplicity and small bias in hardness determination in sufficiently thick coatings; • use sharp tip and frequent tip area calibration to minimize the depth range in which the results are dominated by tip bluntness [31]; • the coatings with a thickness >1 µm are preferred; reliable results from thinner coatings require very sharp tips and better consideration of substrate effects; • use the same method for obtaining corresponding depth profiles (the csm method with the optimized parameters seems to be suitable); • use the same evaluation method (hay&crawford correction brings some benefits but it is sensitive to substrate modulus and coating thickness. moreover, (a). (b). figure 5. the dependences indentation modulus – (a), and hardness – (b), in tizrhf+me-ny (me = v, nb, ta) on the flow of nitrogen added in the sputtering ar atmosphere (and subsequently, stoichiometry, y) during reactive hitus deposition. the results may not be directly comparable with the earlier data); • use the same substrates with the well-defined elastic modulus; • at least two independent nanoindentation measurements should be performed on each coating to take into consideration coating uniformity as well as measurement accuracy and repeatability. 3.5. application to measurement in tizrhfvnbta-xn coatings the recommendations from section 3.4 were applied in the measurements and evaluation of the effect of v, nb, ta and nitrogen additions on mechanical properties of tizrhf-me-ny coatings. it should be emphasized that although only the sapphire substrates and csmtf method were employed to keep possible systematic errors the same, the resulting values are only relative and should be used only for comparison. the full lines in figures 5a and 5b indicate the de41 františek lofaj acta polytechnica ctu proceedings pendences in the reference tizrhf-ny (me = 0) and tizrhfvnbta-ny (me = vnbta) coatings, respectively, as a function of nitrogen flow. the broken lines correspond to tizrhf+me-ny. the highest values in all systems were above certain critical flow of nitrogen which was around 6 sccm n2 when stoichiometry was achieved [30]. the curves in tizrhf-ny coatings were above all other curves and also the maximum eit = 490gpa and hit = 39gpa were obtained in this system. despite limited reliability of the absolute values, good agreement with the properties reported for similar multi-transition metal nitrides produced by reactive dc magnetron sputtering [32] was obtained. the relative comparison indicates that the additions of me = v, nb, ta and vnbta caused small degradation of hit and eit which became more visible at higher nitrogen flows. the smallest degradation seems to be in the coatings with me = nb whereas it is the most pronounced in tizrhfvnbta-n system. thus, the “cocktail effect” resulting from the additions of additional elements considered in the high entropy alloys, seems to be negligible or might be even negative in tizrhf-me-ny coatings. 4. conclusions the results of the current study suggest that the influence of substrate stiffness is still not fully eliminated in the evaluation of indentation modulus which causes systematic differences exceeding measurement scatter between coatings deposited on sufficiently different substrates (without consideration of substrate influence on coating growth and structure). to keep the systematic error the same and to obtain comparable relative results, nanoindentation should be performed using the same test parameters, method for depth profile acquisition, data evaluation procedure and coatings with similar thicknesses on the same type of substrate. the application of these rules to nanoindentation on tizrhf+me-ny coatings deposited by reactive hitus showed that the addition of me = v, nb, ta and vnbta caused some degradation of mechanical properties indicating negligible or even negative “cocktail effect”. acknowledgements this work was supported by the slovak academy of sciences via the international visegrad fund (project jp39421 of v4-japan joint research program) and slovak research and development agency (project apvv 21-0042). references [1] h. bückle. progress in micro-indentation hardness testing. metallurgical reviews 4(1):49–100, 1959. https://doi.org/10.1179/095066059790421746 [2] h. l. mencken quotes. 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https://doi.org/10.1007/s10853-021-06448-2 https://doi.org/10.2478/pmp-2022-0005 https://valleydesign.com/sappprop/ https://www.matweb.com/search/datasheet.aspx?matguid=d0b0a51bff894778a97f5b72e7317d85&ckck=1 https://www.matweb.com/search/datasheet.aspx?matguid=d0b0a51bff894778a97f5b72e7317d85&ckck=1 https://www.astmsteel.com/product/52100-bearing-steel-aisi/ https://www.astmsteel.com/product/52100-bearing-steel-aisi/ https://www.universitywafer.com/mechanical-properties-monocrystalline-silicon.html https://www.universitywafer.com/mechanical-properties-monocrystalline-silicon.html https://www.universitywafer.com/mechanical-properties-monocrystalline-silicon.html https://blog.truegeometry.com/designs3d/titanium_grade_5_metal20240605.html https://blog.truegeometry.com/designs3d/titanium_grade_5_metal20240605.html https://doi.org/10.3390/ma16020563 https://doi.org/10.1016/j.tsf.2017.09.051 https://doi.org/10.1016/j.surfcoat.2020.125674 acta polytechnica ctu proceedings 50:36–43, 2024 1 introduction 2 materials and methods 2.1 coating deposition 2.2 nanoindentation 3 results and discussion 3.1 depth profiles by different methods 3.2 coating thickness effect 3.3 the effect of substrate modulus 3.4 best practice recommendations 3.5 application to measurement in tizrhfvnbta-xn coatings 4 conclusions acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2024.49.0031 acta polytechnica ctu proceedings 49:31–35, 2024 © 2024 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague laboratory verification of the properties of recycled fine aggregates and the effect of different replacement percentages on concrete properties leoš jouraa, zdeněk prošeka,b,∗, aleš paličkaa a czech technical university in prague, faculty of civil engineering, department of mechanics, thákurova 7, 166 29 prague, czech republic b czech technical university in prague, university centre for energy efficient buildings, třinecká 1024, 273 43 buštěhrad, czech republic ∗ corresponding author: zdenek.prosek@fsv.cvut.cz abstract. this research work deals with the issue of laboratory testing of commonly available recycled fine aggregates, namely concrete recyclate, mixed recyclate and brick recyclate. their geometrical properties: granulometric composition and fine particle content, including physical properties: bulk density and water absorption are investigated. the tests themselves are carried out according to available standards, which are currently mainly standards for testing natural aggregates, and it is necessary to assess the suitability of these tests for recycled aggregates as well. the results found for concrete recyclates confirm the use of these standards to obtain the properties mentioned, but the bulk density and absorption tests are inconclusive in the case of mixed and brick recyclates. subsequently, test concrete specimens were produced in which natural fine aggregate and concrete recyclate were substituted at 33, 66 and 100 %. a series of tests were performed on the hardened concrete: bulk density, absorption, porosity and compressive strength. from the results obtained, the use of concrete recyclate in concretes with the replacement of natural fine aggregate at 33 % is suitable, without a significant reduction in the observed properties. keywords: recycled fine aggregate, concrete recyclate, mixed recyclate, brick recyclate, utility properties. 1. introduction nowadays, the use of recycled materials is becoming more and more popular and recycling itself is taking its place in various areas of our lives. for this reason, it is an ideal time to expand public awareness of recycled materials that can also be used in construction. the construction industry itself is a major producer of raw materials that are generated by recycling construction and demolition waste. the recycling of this waste most often results in recycled aggregates, for which it is now necessary to find a place in modern construction, where we can see, for example, concrete structures for which recycled aggregates could serve as an alternative to natural aggregates. this aggregate can be divided into a fine fraction and a coarse fraction. the coarse fraction is now often used in the production of new concrete and is supported by standards [1]. the use of recycled fine aggregate is also possible in the production of new concrete. according to the available literature, concrete [2], brick [3] or mixed recyclate from brick and concrete [4] can be used as recycled fine aggregate. an important process in this replacement is to define the properties of the recyclate itself, which include its granulometric composition, content of washable particles, bulk density and water absorption, as it is necessary to know the material that enters the concrete production process. for this reason, csn standards that specify test procedures for natural aggregates in concrete are used in this paper and it is necessary to evaluate the suitability of these standards for these recyclates. scientific studies [5, 6] have addressed the effect of concrete recyclates on concrete properties, specifically porosity, water absorption and compressive strength were tested. another study led by jagan sivamani [7] investigated the effect of concrete recyclate on concrete properties. the results of this study indicated deteriorating properties with increasing percentage of replacement. 2. materials and samples the recycled aggregates were supplied by moravostav a.s., which used resta jaw crushers for crushing construction and demolition waste. three basic recyclates are investigated in this paper. concrete recyclate (rec1) came from highly segregated concrete waste, mixed recyclate (rec2) consisted of a mixture of concrete, brick waste combined with soil and brick recyclate (rec3) consisted of pure brick waste. rec1 was selected to replace the natural fine aggregate in 33, 66 and 100 weight percent replacements for the 31 https://doi.org/10.14311/app.2024.49.0031 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en l. joura, z. prošek, a. palička acta polytechnica ctu proceedings set cement fine natural aggregate 0/4 coarse natural aggregate 4/8 coarse natural aggregate 8/16 rec1 water ref 300 700 538 601 0 165 r1_33 300 466 538 601 233 165 r1_66 300 233 538 601 466 165 r1_100 300 0 538 601 700 165 table 1. summary of concrete mix design in kg m−3. impact of the recycled fine aggregate, and reference concrete (ref) was produced at the same time. three test specimens were produced for each test set. the detailed compositions of the concretes are described in table 1. concrete cubes with an edge of 150 mm were used as test specimens. the concrete itself contained portland cement with the designation cem i 42.5 r, which came from the mokrá plant. in addition, natural fine aggregate, defined by the 0/4 fraction, from the dobříň gravel pit, managed by cemex, was used. the coarse aggregate came from the zbraslav quarry with fractions 4/8 and 8/16 used. 3. testing recycled aggregates the dried recyclates were weighed and then soaked in water for 24 hours. after this time, this aggregate was placed on a set of sieves where the smallest sieve was 0.063 mm and with the vibration on, the aggregate was sieved while continuously adding water. the test itself is described by en 933-1[8] and the desired result was the percentage of washable particles. the aggregate was then dried again in an oven at 110 °c, ready for the next stage of the test described in the previously mentioned standard. in this case, this involved the determination of the granulometric composition, which resulted in a grain size curve. furthermore, it was necessary to carry out a test to determine the bulk density and water absorption of the recycled fine aggregates, for which the standard en 1097-6 [9] was used. the standard described the procedure for carrying out the pycnometric method to obtain several types of bulk weights (apparent, surface dried and fully saturated) and percentage of absorption of the recyclates. 4. testing of concrete samples the produced test specimens were unmolded after solidification and stored in containers with water. they were subsequently tested after 7 and 28 days. the first test carried out on the test specimens was the bulk density, which was measured on the fully watersaturated solids according to en 12390-7 [10], and it was necessary to measure the dimensions of the solids themselves to obtain their volume. this was followed by a test to determine the compressive strength of the concrete, which was based on en 12390-3 [11]. the individual specimens were surface dried and placed in a test press. the loading was carried out by a constant figure 1. content of fine particles. increase in force until the resulting failure. the compressive strength was calculated from the maximum force achieved during the test fc,max as: fc = fc,max ab , (1) where fc is the compressive strength [pa], fc,max is the maximum force [n], a is the width of the sample [m], b is the height of the sample [m]. 5. results and discussion the results of the amount of fine particles can be seen in figure 1 and, as expected, the lowest percentage of contamination came out for rec1. in the case of rec2 and rec3, the percentage of contamination already reaches significant values, indicating that these recyclates are very contaminated and their use is all the more complicated. in figure 2 we can observe the resulting grain size curves for the tested recyclates. rec1 shows uniform gradients of the different fractions, but contains the smallest proportion of finer aggregates compared to the other two recyclates. in the case of rec2 and rec3, a more pronounced representation of grain sizes of 0.063–0.25 mm can be seen, which may be due to the higher content of fines. in figure 3, the results of the bulk weights can be observed. in the case of rec1, the resulting values are conclusive and very similar to, for example, the study by cheng-chih fan [2]. for the other two recyclates 32 vol. 49/2024 laboratory verification of the properties of recycled fine aggregates . . . figure 2. particle size distribution. figure 3. bulk density of testing fine recycled aggregate. figure 4. water absorption of testing fine recycled aggregate. rec2 and rec3, the results are considerably high, indicating the unsuitability of this test for these types of aggregates. the problem itself may be due to the difficulty in achieving the surface dried condition of the aggregate, which is the main variable of this test. also related to the results of the bulk weights are the results of the absorption of the recyclates, which are shown in figure 4. in relation to the previously mentioned problem, the results for rec2 and rec3 are completely inconclusive as their absorption rates are very low compared to reality. in the case of rec1, the absorption is assumed and is based on a similar basis as in the study by gómez-soberón [12] and therefore this recyclate can theoretically be tested according to the existing csn standards for aggregates for concrete. the results of the bulk weights of hardened concrete in the saturated state are shown in figure 5. it is possible to observe a significant reduction in the bulk weights of the r1 set concretes compared to ref, and there is a linear relationship between the replacement and the bulk weight reduction itself. the same conclusion was also reached by cheng-chih fan [2]. in figure 6, the results of the compression test after 7 and 28 days can be seen. the resulting strength of ref concrete at 7 days shows lower strengths than the concrete with 33 % replacement. this may have been due to the small number of test specimens or to indiscipline in production. after 7 and 28 days, there is a rule of thumb which indicates that the compressive strength of concrete decreases as a function of replacement size, which is also confirmed by jagan sivamani [7]. 33 l. joura, z. prošek, a. palička acta polytechnica ctu proceedings figure 5. bulk density of concrete specimens. figure 6. compressive strength of concrete specimens. 6. conclusion this work was concerned with the determination of the basic properties of recycled fine aggregates, according to the available standards that apply to natural fine aggregates. at the same time, rec1 was selected, for which test bodies with 33, 66 and 100 % substitutions for natural fine aggregate were produced and the effect of these substitutions on the properties of the concrete itself was investigated. in the case of fine aggregates, it can be said that rec1 (concrete recyclate) can be tested according to current standards for aggregates in concrete. however, rec2 and rec3 are unsuitable for the application of these standards as it is difficult to achieve a surface dried condition of aggregates which affects the determination of their bulk density and water absorption. concretes with a higher proportion of recycled fine aggregates have been shown to reduce the bulk density of concrete, which is also related to the resulting strength of the concrete. concrete using 33 % recycled aggregate replacement is the closest approximation to the properties of the reference concrete. the 66 % and 100 % replacements are already far from the reference concrete in terms of properties. acknowledgements this paper was financially supported by czech technical university in prague under no. sgs project sgs22/089/ohk1/2t/11 and by technology agency of the czech republic (ta čr) research project ss03010302. the authors also thank moravostav for the supplied samples and materials. references [1] úřad pro technickou normalizaci, metrologii a státní zkušebnictví. čsn en 206 +a1. beton – specifikace, vlastnosti, výroba a shoda [in czech; concrete – specification, properties, production, and conformity], 2018. [2] c.-c. fan, r. huang, h. hwang, s.-j. chao. properties of concrete incorporating fine recycled aggregates from crushed concrete wastes. construction and building materials 112:708–715, 2016. https: //doi.org/10.1016/j.conbuildmat.2016.02.154 [3] f. debieb, s. kenai. the use of coarse and fine crushed bricks as aggregate in concrete. construction and building materials 22(5):886–893, 2008. https: //doi.org/10.1016/j.conbuildmat.2006.12.013 [4] h.-j. chen, t. yen, k.-h. chen. use of building rubbles as recycled aggregates. cement and concrete research 33(1):125–132, 2003. https://doi.org/10.1016/s0008-8846(02)00938-9 [5] l. evangelista, j. de brito. mechanical behaviour of concrete made with fine recycled concrete aggregates. cement and concrete composites 29(5):397–401, 2007. https: //doi.org/10.1016/j.cemconcomp.2006.12.004 [6] l. evangelista, j. de brito. durability performance of concrete made with fine recycled concrete aggregates. cement and concrete composites 32(1):9–14, 2010. https: //doi.org/10.1016/j.cemconcomp.2009.09.005 [7] j. sivamani, n. t. renganathan. effect of fine recycled aggregate on the strength and durability properties of concrete modified through two-stage mixing approach. environmental science and pollution research 29:85869–85882, 2022. https://doi.org/10.1007/s11356-021-14420-5 [8] úřad pro technickou normalizaci, metrologii a státní zkušebnictví. čsn en 933-1. zkoušení geometrických vlastností kameniva – část 1: stanovení zrnitosti – sítový rozbor [in czech; tests for geometrical properties of aggregates – part 1: determination of particle size distribution – sieving method], 2012. 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[10] úřad pro technickou normalizaci, metrologii a státní zkušebnictví. čsn en 12390-7. zkoušení ztvrdlého betonu – část 7: objemová hmotnost ztvrdlého betonu [in czech; testing hardened concrete – part 7: density of hardened concrete], 2009. 34 https://doi.org/10.1016/j.conbuildmat.2016.02.154 https://doi.org/10.1016/j.conbuildmat.2016.02.154 https://doi.org/10.1016/j.conbuildmat.2006.12.013 https://doi.org/10.1016/j.conbuildmat.2006.12.013 https://doi.org/10.1016/s0008-8846(02)00938-9 https://doi.org/10.1016/j.cemconcomp.2006.12.004 https://doi.org/10.1016/j.cemconcomp.2006.12.004 https://doi.org/10.1016/j.cemconcomp.2009.09.005 https://doi.org/10.1016/j.cemconcomp.2009.09.005 https://doi.org/10.1007/s11356-021-14420-5 vol. 49/2024 laboratory verification of the properties of recycled fine aggregates . . . [11] úřad pro technickou normalizaci, metrologii a státní zkušebnictví. čsn en 12390-3. zkoušení ztvrdlého betonu – část 3: pevnost v tlaku zkušebních těles [in czech; testing hardened concrete – part 3: compressive strength of test specimens], 2009. [12] j. m. v. gómez soberón. relationship between gas adsorption and the shrinkage and creep of recycled aggregate concrete. cement, concrete & aggregates 25(2):1–7, 2003. https://doi.org/10.1520/cca10442j 35 https://doi.org/10.1520/cca10442j acta polytechnica ctu proceedings 49:31–35, 2024 1 introduction 2 materials and samples 3 testing recycled aggregates 4 testing of concrete samples 5 results and discussion 6 conclusion acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2024.51.0075 acta polytechnica ctu proceedings 51:75–80, 2024 © 2024 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague automatic detection of sleep spindles by neural networks algorithms jan rychlík∗, roman mouček university of west bohemia, faculty of applied sciences, department of computer science, univerzitní 8, 301 00 plzeň, czech republic ∗ corresponding author: rychlikj@kiv.zcu.cz abstract. sleep constitutes an essential aspect of human existence, with the average individual dedicating approximately one-third of their life to this physiological activity. consequently, comprehending and accurately analyzing sleep patterns is of paramount importance. this research aims to introduce, formulate, execute, and assess diverse machine/deep learning methodologies tailored for the processing of eeg signals geared explicitly towards identifying sleep spindles. the learning algorithms underwent training using meticulously annotated data from the montreal archive of sleep studies (mass) data center. the convolutional neural network emerged as the most effective classification model, achieving an accuracy surpassing 67%. keywords: deep learning, eeg data standards, eeg workflows, eeg pipelines, electroencephalography, event-related potentials, human brain, machine learning. 1. introduction sleep, comprising approximately one-third of human life, is crucial for bodily rejuvenation and mental relaxation. however, the prevalence of sleeping disorders in modern society underscores the significance of research in this domain to enhance sleep quality. the brain’s activity undergoes distinct changes during sleep, categorized into rem and non-rem phases; a specific phenomenon within the non-rem phase is known as a sleep spindle. sleep spindles are brief bursts of neural oscillatory activity during non-rem sleep and play a crucial role in memory consolidation. these spindle events are essential indicators of sleep quality and cognitive functions. electroencephalography (eeg), the fundamental method and technique for measuring and collecting electrical activity of the human brain, is also used in sleep data collection; their abundance and complexity necessitate further computer processing. this mitigates human error and promotes more efficient and accurate data analysis. machine and deep learning, particularly neural networks such as convolutional neural networks (cnn), long short-term memory networks (lstm), and dense networks, offer a promising approach to eeg signal and sleep data processing. this paper focuses on identifying sleep spindles. this is accomplished by designing, implementing, and assessing deep learning methods (neural networks). the general aim is to improve understanding and interpretation of eeg data, contribute to advancements in automated sleep analysis, and enhance our knowledge of sleep-related phenomena. the paper is organized in the following way. the state-of-the-art section provides insight into eeg and sleep stages classification, sleep spindle characteristics, and sleep data platforms and archives. it is followed by the sections describing the dataset and neural network architectures used. then, the dataset processing is presented, and the results are provided. the outcomes and future perspectives are summarized in the conclusion section. 2. state of the art the typical use case in eeg signal classification is to compare methods for categorizing preictal, postictal, and interictal classes when epileptic seizures are detected. the model proposed in [1] integrated a twolayer lstm and a four-layer enhanced neural network (nn) deep learning architecture using improved one-dimensional gradient descent activation functions. the study used the pre-processed bonn university database; statistical features were extracted. conventional methods used for the classification included support vector machines (svm) with different kernels, logistic regression, and nns. the results showed that the improved nn algorithm achieved the highest accuracy (nearly 79%), surpassing lstm (71%), nn neural network (61%), and svm (70%), while logistic regression had the lowest accuracy (about 53%) [1]. in collaboration with the department of medical engineering and technomathematics in germany, aachen university of applied sciences used a machine learning approach [2] to score pre-rem sleep stages in mice automatically. using a dataset of polysomnographic recordings from 18 mice over 52 days, the study investigated the impact of dietary variation on sleep. the mice, chronically implanted with eeg and emg electrodes, underwent data pre-processing, 75 https://doi.org/10.14311/app.2024.51.0075 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en jan rychlík, roman mouček acta polytechnica ctu proceedings including low-pass filtering and synthetic data augmentation for a balanced training set. the neural network architecture, consisting of eight convolutional layers and a classifier with two fully connected layers, demonstrated high accuracy in classifying wake, rem, and nrem segments, achieving over 98%, more than 94%, and close to 92% accuracy, respectively. the pre-rem stage was correctly classified in 58% of the segments. the classifier showed robust performance, correctly predicting human-expert assigned stages for segments predicted to be wake, rem, and nrem, demonstrating the effectiveness of the network in the automated classification of sleep stages in mice [2]. a 1d cnn-lstm algorithm for automatic sleep staging (wake, rem, non-rem) on the sleep-edf dataset, achieving 93.47% accuracy with the fpz-cz eeg channel, is described in [3]. the dataset comprises 197 polysomnographic sleep recordings with manually scored hypnograms. with seven layers (four 1d cnn and three lstm layers), the algorithm exhibited robust performance across various physiological signals. evaluations on single-channel eeg and eog classifications, using five test sets for each group, confirmed the model’s reliability. achieving 94.15% accuracy when incorporating fpz-cz eeg and eog signals, the algorithm demonstrates promise for automated sleep staging in future sleep-related research. it offers an efficient alternative to manual expert inspection in large-scale polysomnography (psg) signal analysis, showcasing the effectiveness of deep learning in discerning similar sleep periods [3]. figure 1 displays a characteristic sleep spindle evident in the eeg waveform during non-rapid eye movement (non-rem) sleep. this spindle, observed in frontal and central brain regions, manifests as a brief burst of rhythmic oscillations lasting 0.5 to 2 seconds, with frequencies ranging from 11 to 16hz. the figure provides a visual representation of the spatial and temporal dynamics of the sleep spindle, contributing to the understanding of its electrographic features. recognized for its role in memory consolidation, the accurate identification and analysis of sleep spindles, as illustrated in figure 1, are crucial for investigating sleep disorders and neurological conditions, enhancing comprehension of neural activities during sleep. due to its characteristic properties, it is suitable for testing processing techniques like machine learning and neural networks further to refine the detection and interpretation of sleep spindles. the massive online data annotation (moda) platform is pivotal in generating standardized datasets for training and validating automated detectors of biological signals, including eeg [5]. a canadian study using moda compared the results of expert, researcher, and non-expert scorers with seven spindle detection algorithms, revealing that only two algorithms performed comparably to human experts, showcasing moda’s significance in benchmarking automated sleep analysis methods. figure 1. example of eeg sleep spindles; adopted from [4]. the montreal archive of sleep studies (mass) also serves as an open-access repository, providing psg data for benchmarking automated sleep analysis systems [6]. established as part of the moda project [5], mass is a comprehensive and openly accessible repository of psg recordings with annotated eeg signals, supporting large-scale collaborations in sleep studies [6]. the maas database is structured into cohorts, with subsets categorizing recordings based on specific characteristics [6]. 3. sleep spindles dataset this study used eeg data from the mass archive’s stage ss2 (described in table 1). the data preparation of eeg recordings involves addressing challenges such as the unique occurrence of sleep spindles in specific sleep segments, constituting about two percent of the overall recording. given this imbalance, set balancing is crucial for effective neural network training. the measured voltage values in the eeg signal range from 10−4 to 10−6 volts, necessitating normalization. the eeg records from only the pz-cle channel were further processed, as this channel provides optimal visibility for sleep spindles. the eeg records were subdivided into smaller segments for neural network processing; each segment was further divided into sets using annotations. the dataset’s imbalance, where segments without spindles make up about ninety-eight percent of the dataset, is reduced using a python random number generator for unbiased set balancing, avoiding pattern-based distortions during training. 4. neural network architectures this section briefly overviews the methods (neural network architectures) employed for the automated detection of sleep spindles in the dataset described above. furthermore, these architectures were complemented by their combinations, with the anticipation of preserving the advantages and properties inherent in each architecture. 4.1. dense neural network the dense neural network (dnn) depicted in figure 2, also known as a fully connected neural network, exhibits a structure where each node in one layer connects to every node in the next. this dense connectivity allows comprehensive eeg signal processing 76 vol. 51/2024 sleep spindle detection stage 2 (ss2) memory size 7.26gb total number of measurements 19 total number of spindles 11204 average number of spindles in a measurement 217 time length 151 h maximum number of spindles in a measurement 569 minimum number of spindles in a measurement 86 maximum duration of spindle 2.218605 s minimum duration of spindle 0.335915 s average length of an eeg measurement 7 h 59min table 1. characteristics of stage ss2. figure 2. an example of a standard dense neural network. the network has one input, two hidden, and one output layer, adopted from [7]. and feature extraction, enabling the network to capture intricate data relationships. dnns are widely applied in domains such as image recognition, natural language processing, and signal processing due to their ability to learn complex patterns from extensive datasets. 4.2. cnn the convolutional neural network (cnn) illustrated in figure 3 is a specialized class of artificial neural networks designed for processing structured grid data, particularly effective for tasks involving image analysis and recognition. with distinctive architectural elements, including convolutional layers, pooling layers, and fully connected layers, cnns excel at capturing hierarchical features and spatial hierarchies in input data. convolutional layers use filters to convolve across input data, extracting local features and patterns. pooling layers reduce spatial dimensions, preserving crucial information while minimizing computational complexity. fully connected layers enable high-level abstraction and decision-making. cnns demonstrate notable efficacy in applications like image classification, object detection, and facial recognition, underscoring their significance in computer vision and pattern recognition research. figure 3. an example of an application of filter on an image inside a cnn network, adopted from [8]. 4.3. lstm the long short-term memory (lstm) neural network, a specialized recurrent neural network (rnn) architecture, addresses the challenge of capturing longrange dependencies in sequential data by incorporating memory cells with input, forget, and output gates, making these networks particularly suitable for tasks involving memory and temporal patterns. the gates selectively control information flow, allowing the network to retain relevant context over extended sequences and mitigating the vanishing gradient problem. lstms excel in natural language processing, speech recognition, and time series analysis, effectively capturing temporal dependencies. their role in mitigating the vanishing gradient problem has positioned lstms as a cornerstone in developing advanced deep-learning models for sequential data processing, contributing to diverse areas of artificial intelligence research. the network architecture is depicted in figure 4, showcasing its capability to detect specific eeg signals. 5. dataset processing the current research incorporates three prominent neural network architectures discussed above: dnn, lstm, and cnn. the strategic use of these architectures underscores a sophisticated approach to ad77 jan rychlík, roman mouček acta polytechnica ctu proceedings figure 4. lstm neural network scheme, adopted from [9]. dressing the complexities associated with sleep spindle detection in research studies. the dnn architecture was the primary choice for sleep spindle dataset processing; it provides a basic model for initial exploration and understanding of the dataset. the lstm architecture has been incorporated to process the data further. the inclusion of lstms is motivated by their ability to detect subtle signals that precede the occurrence of sleep spindles. to enhance the data processing capabilities, the cnn was introduced into the architecture. in the context of sleep spindle detection, treating eeg data as an image allows cnns to capture hierarchical features and spatial hierarchies efficiently. the convolutional layers in cnns facilitate extracting local features and patterns, contributing to a comprehensive understanding of the dataset. various combinations of dnn, lstm, and cnn architectures provide a diverse and comprehensive approach to the data processing to improve sleep spindle detection algorithms; we chose the following simple architectures: lstm, dense, cnn and cnn and their combinations of architecture: cnn-lstm implemented in keras python package and cnn-lstm implemented in torch python package. the proposed cnn-lstm implemented in keras package architecture is shown in figure 5. it consists of seven layers, with the initial four layers characterized by convolutional structures, each having 32 neurons. following the non-convolutional layer is a linear layer with 384 neurons, introducing a deeper level of abstraction. subsequently, the penultimate layer is also linear, incorporating 64 neurons. ultimately, the final layer consists of two neurons. 5.1. validation dataset the validation dataset encompasses nearly 200,000 instances, with 51.2% attributed to non-sleep spindles, and the remaining instances dedicated to sleep spindles. this meticulously constructed dataset is a subset derived from the extensive data repository provided by the montreal archive of sleep studies (mass) center. the deliberate balance between the two classes ensures a representative and unbiased sample for thorough evaluation, fostering the reliability and generalizability of the neural network’s performance assessment. the training and testing of the neural network architectures has been performed on a normalized data set. the parameters for the neural networks, such as the size of the input layer and the number of epochs, were determined using genetic algorithms. the number of epochs varies for each architecture. the last method used to process the sleep spindle dataset, the value-based method, is a simple analytic method. its principle is that it sums the measured values on a specific interval and compares the resulting values with each other. for this method, it is crucial to set the border correctly. at the outset of the algorithm, the border was initialized to zero. subsequently, the eeg signal underwent segmentation, with each segment consistently comprising 64 measured values. the absolute sum of all values within the segment was computed and compared against the border. if the computed value exceeded the border threshold, the segment was annotated as a sleep spindle. the algorithm then tallied the number of correctly and incorrectly classified segments, following which the border was incrementally adjusted by a small increment. the optimal threshold value for classification was determined as the one yielding the highest number of correctly identified spindles. 6. results table 2 summarizes the results obtained from testing various classifiers and their combinations. also the figure 6 shows peak accuracies achieved by the used classifiers. network architecture accuracy dense 64.63% lstm 52.81% cnn 60.87% cnn-lstm in keras 52.81% cnn-lstm in torch 67.15% value-based method 64.12% table 2. best results provided by the used classifiers. figures 7 and 8 present the accuracy and loss function values corresponding to the optimal neural network configuration evaluated on the validation dataset. both figures are structured with the number of the validation set epochs delineated on the x-axis, visually representing the network’s performance metrics across multiple epochs. 78 vol. 51/2024 sleep spindle detection figure 5. the cnn-lstm architecture implemented in torch package. the dropout layer prevents getting stuck in the local minimum; flatten links cnn and the linear layer. figure 6. the best classification results (peak accuracies) achieved by the used classifiers. 7. conclusion the paper examines the processing of eeg signals during sleep to identify sleep spindles using mostly various simple and hybrid neural network architectures. the study used data from the recognized mass archive. the used dataset was balanced, divided into training and testing sets, and used to train neural networks. the classification accuracy of lstm and lstm-cnn networks was the lowest at 52.81%. keras’ dense and cnn networks showed slightly better results with an accuracy of 60–65%. the most successful network was the cnn network implemented in the torch package, achieving accuracy exceeding 67%. figure 7. the accuracy on the validation dataset; xaxis = number of epochs over the validation dataset; y -axis = the accuracy value. the achieved classification results are consistent with those reported in the literature, where the best results for eeg sleeep artifact classification reached 70–75%. the value-based method achieved an accuracy of 64.12%, slightly below the state-of-the-art. it may be worth refining the neural network parameters by extending the state area of input parameters to improve performance. also, the potential benefits of the next alternative hybrid deep learning architectures in identifying sleep spindles can be considered. acknowledgements 79 jan rychlík, roman mouček acta polytechnica ctu proceedings figure 8. the loss function on the validation dataset; x-axis = number of epochs over the validation dataset; y -axis = the loss value. this work was supported by the university-specific research project sgs-2022-016 advanced methods of data processing and analysis (project sgs-2022-016). references [1] p. nagabushanam, s. t. george, s. radha. eeg signal classification using lstm and improved neural network algorithms. soft computing 7:9981–10003, 2020. https://doi.org/10.1007/s00500-019-04515-0 [2] n. grieger, j. t. c. schwabedal, s. wendel, et al. automated scoring of pre-rem sleep in mice with deep learning. scientific reports 11:12245, 2021. https://doi.org/10.1038/s41598-021-91286-0 [3] d. zhao, r. jiang, m. feng, et al. a deep learning algorithm based on 1d cnn-lstm for automatic sleep staging. technology and health care 30(2):323–336, 2022. https://doi.org/10.3233/thc-212847 [4] i. b. iotchev, e. kubinyi. shared and unique features of mammalian sleep spindles – insights from new and old animal modelss. biological reviews 96(3):1021–1034, 2021. https://doi.org/10.1111/brv.12688 [5] k. lacourse, b. yetton, s. mednick, s. c. warby. massive online data annotation, crowdsourcing to generate high quality sleep spindle annotations from eeg data. scientific data 7:190, 2020. https://doi.org/10.1038/s41597-020-0533-4 [6] t. paquette. montreal archive of sleep studies (mass), 2015. [2022-01-21]. http://ceams-carsm.ca/mass/ [7] g. ognjanovski. everything you need to know about neural networks and backpropagation — machine learning easy and fun, 2019. [2022-01-04]. https://towardsdatascience.com/everything-youneed-to-know-about-neural-networks-andbackpropagation-machine-learning-made-easy/e5285bc2be3a [8] ibm cloud education. convolutional neural networks, 2020. [2022-05-03]. https://www.ibm.com/cloud/learn/convolutionalneural-networks [9] s. dobilas. lstm recurrent neural networks — how to teach a network to remember the past, 2021. [2022-05-03]. https://towardsdatascience.com/lstmrecurrent-neural-networks-how-to-teach-anetwork-to-remember-the-past-55e54c2ff22e 80 https://doi.org/10.1007/s00500-019-04515-0 https://doi.org/10.1038/s41598-021-91286-0 https://doi.org/10.3233/thc-212847 https://doi.org/10.1111/brv.12688 https://doi.org/10.1038/s41597-020-0533-4 http://ceams-carsm.ca/mass/ https://towardsdatascience.com/everything-you-need-to-know-about-neural-networks-and-backpropagation-machine-learning-made-easy-/e5285bc2be3a https://towardsdatascience.com/everything-you-need-to-know-about-neural-networks-and-backpropagation-machine-learning-made-easy-/e5285bc2be3a https://towardsdatascience.com/everything-you-need-to-know-about-neural-networks-and-backpropagation-machine-learning-made-easy-/e5285bc2be3a https://towardsdatascience.com/everything-you-need-to-know-about-neural-networks-and-backpropagation-machine-learning-made-easy-/e5285bc2be3a https://www.ibm.com/cloud/learn/convolutional-neural-networks https://www.ibm.com/cloud/learn/convolutional-neural-networks https://towardsdatascience.com/lstm-recurrent-neural-networks-how-to-teach-a-network-to-remember-the-past-55e54c2ff22e https://towardsdatascience.com/lstm-recurrent-neural-networks-how-to-teach-a-network-to-remember-the-past-55e54c2ff22e https://towardsdatascience.com/lstm-recurrent-neural-networks-how-to-teach-a-network-to-remember-the-past-55e54c2ff22e acta polytechnica ctu proceedings 51:75–80, 2024 1 introduction 2 state of the art 3 sleep spindles dataset 4 neural network architectures 4.1 dense neural network 4.2 cnn 4.3 lstm 5 dataset processing 5.1 validation dataset 6 results 7 conclusion acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2025.53.0053 acta polytechnica ctu proceedings 53:53–59, 2025 © 2025 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague design of bond strength testing method of concrete layers for creating a material model marek kříž, michaela frantová∗, václav wudi, petr štemberk czech technical university in prague, faculty of civil engineering, department of concrete and masonry structures, thákurova 7, 166 29 prague 6, czech republic ∗ corresponding author: michaela.frantova@fsv.cvut.cz abstract. the need to determine the bond strength of concrete layers in structural modeling is quite frequent and often encounters a lack of relevant experimental data due to the large number of variables which affect the bond strength. this article describes an experimental method that makes it possible to obtain the experimental data necessary for subsequent modeling of multi-layered concrete composite structures. the experiment is designed to be applicable both to laboratory specimens with defined properties and to samples taken from existing structures. the advantage of this method is its simplicity, minimal equipment requirements, and repeatability. the three-layer specimens, representing a sandwich wall, allow the study of the influence of the age and quality of the concrete on the bond strength, and through image analysis, provide valuable information on deformation, interlayer slip, and crack formation. the results of the experiment may contribute to the determination of a bond strength diagram for multi-layered walls, which can be integrated into software tools for comprehensive structural analyses. keywords: bond strength, shear strength, cohesion of concrete layers, shear stress testing methods, image analysis. 1. introduction the majority of reinforced concrete structures cannot be cast all at once, and for various reasons, the concreting process needs to be interrupted during construction. at the point where concreting is interrupted, a construction joint is formed, where concrete from different casting sessions, theoretically of the same quality, but of different ages (and thus in different phases of hydration), meet. when designing and analyzing reinforced concrete structures, we assume that the material properties of the concrete remain constant throughout the structure and also model it as a whole. however, figure 1 shows an example of core drillings through a construction joint in a concrete wall several decades old, where it is clear that the assumption of perfect bond at the construction joint is not correct. the need to understand the bond strength between layers of concrete of different ages arises in the repair of degraded or otherwise damaged structures, in structural strengthening, or in monolithic-precast and sandwich constructions. when strengthening structures or designing monolithic-precast constructions, it is crucial to know the shear strength of the bond at the interface between concrete layers of different ages. the interaction between the individual layers of the structure plays a significant role in the model used for the structural analysis. if we design sandwich structures without connecting elements, the bond strength is always a key factor in the calculation. the bond strength between concrete layers can be measured in various ways. tests differ in terms of figure 1. horizontal construction joint during concreting (the core on the left disintegrated by itself upon removal from the structure). loading type, types of samples attachment, interface angle, etc. each testing type offers specific advantages and disadvantages. a typical problem with shear tests is the inability to create pure shear stress in the sample; in addition to shear stress, the interface is often subjected to torsional moments or compression. a common disadvantage of most tests (see chapter 3) is the need for specific laboratory equipment, the complexity of the tests, or the inability to repeat them. the testing method we propose for measuring bond strength between concrete layers reduces these disadvantages, does not require specific laboratory equipment, and allows for the production of a larger number of test sets. 53 https://doi.org/10.14311/app.2025.53.0053 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en m. kříž, m. frantová, v. wudi, p. štemberk acta polytechnica ctu proceedings figure 2. a) macroscopic view of the transition zone, b) microscopic view of the transition zone [1]. 2. methods of measuring bond strength the bond strength between older and newly placed concrete depends on the adhesion of the fresh concrete mix to the surface of the older concrete, that is, on the ability of the mix to adhere perfectly to the surface. after the fresh concrete mix hardens, an interfacial zone forms at the interface of the layers. the resulting interfacial zone has similar properties to the interfacial transition zone (itz) between aggregate and hydrated cement paste and is weakened due to the so-called wall effect (figure 3), in a similar way to how the itz between aggregate and cement paste is weakened. as a result of the wall effect, there are fewer hydration product particles and more pores at the interface, which are inversely proportional to the adhesion of the fresh concrete and reduce the contact area between the layers. adhesion can be improved in various ways, such as surface treatment, optimizing the composition of the fresh concrete, and using additives and admixtures [2]. figure 3. schematic representation of the wall effect in the aggregate transition zone in concrete. the contact surfaces between the older and newly placed concrete usually have different aggregate-tocement ratios, water-cement ratios, and temperature developments during setting and hardening compared to the newly placed concrete. the failure at the interface between the older and newly placed concrete is similar to the failure in the transition zone at the interface between the aggregate and hardened cement paste [3]. the interfacial zone between the concretes, schematically illustrated in figure 2 a), can be divided microscopically into three layers, as shown schematically in figure 2 b). the first layer can be called the penetration layer, located in the older concrete, and is composed mainly of c-s-h products with a smaller amount of aft (ettringite) and ca(oh)2. the components of the fresh concrete react to a certain extent with the active components in the older concrete. the second layer is the weakest and most porous at the interface, containing ca(oh)2, highly oriented needle-shaped aft crystals, and fewer c-s-h particles. this layer defines the low bond strength of the layers. the third layer is found in the new concrete and has almost the same microstructure as the new concrete, with individual aft crystals oriented towards the interface. factors that influence the bond strength of concrete layers are those that affect the concrete mix during production, and they influence the bond strength at the material level on a molecular scale, such as additives, admixtures, water-cement ratio, optimal aggregate dosing, and the resulting concrete strength and shrinkage. the main admixture that improves the properties of the interface is silica fume (also known as microsilica), which fills the spaces at the interface of the concretes and participates in the hydration process [3]. other admixtures with a positive impact on the concrete interface include blast furnace slag and fly ash; [4]. 3. methods for measuring bond strength to determine the bond strength between the individual concrete layers, which we want to include in the numerical model for the structural analysis, it is necessary to experimentally verify the bond strength. bond strength tests can be divided into predominantly shear and predominantly tensile types. the principle of each type of stress is schematically illustrated in figure 4. to achieve the appropriate type of stress in the experimental determination of bond strength, it is necessary to select the appropriate testing method. various testing methods exist, each of which is influenced differently by factors that define the bond strength, such as the strength of the concrete, surface roughness, etc. the samples should be tested in a manner that simulates how they will predominantly be 54 vol. 53/2025 design of bond strength testing method of concrete layers figure 4. main types of stresses at the interface for testing the bond strength. stressed in the structure. it also depends on whether the test will be conducted on an existing structure or whether test samples will be specifically produced for the experiment [5]. in figure 5 a) and b), tests perfomed in-situ, meaning directly on the structure, are schematically depicted. the torsion test shown in figure 5 b) involves loading the sample with a torque, which subjects the interface only to shear stress. a challenge is expressing the resulting stress, as concrete is a brittle material that is neither entirely elastic nor purely plastic, making it difficult to accurately derive the shear stress from a known force. figure 5. schematic examples of commonly used testing methods. tests in the second row of the table labeled d), e), and f) are tests that induce „pure“ shear stress at the interface, while in case c), pressure is also present at the interface. the last row of the table in figure 5 illustrates tests where the tensile strength at the interface is determined, from which the shear strength is subsequently calculated [6]. the direct tensile strength testing method (g) is easy to perform both in terms of setup and loading. the challenge lies in securing the sample in the tensile device. the main advantage of this test is that it can be performed directly on the structure (in-situ), with the advantage over laboratory testing being that the sample is clamped only on one side for loading. the czech technical standards [7, 8] describe three types of tests for determining bond strength: indirect tensile strength test h), diagonal shear test c), and direct tensile strength test g), only for samples produced under laboratory conditions. however, of the above tests, they best suited our goal of pure shear stress were the bond strength test on z-shaped samples (d, e) [9] and the four-point shear test (f) [10]. both tests are conducted in a press, where the samples are loaded with a vertical force that induces “pure” shear stress at the interface between the concrete layers until the sample fails. the advantage of the z-shaped sample test is the presence of only one interface layer, while the disadvantage is the more complex shape of the samples. another disadvantage of this test is that during loading, the sample can deform, leading to failure through mechanisms other than from pure shear stress. the four-point shear test is essentially the same loading scheme as the four-point flexural test, with the difference being that the loading forces and supports are aligned vertically, with only the interface of the test sample separating them. the disadvantage of this test is that there are two concrete layer interfaces, which may not fail simultaneously. the test is conducted in a press, with the sample being loaded vertically through distribution elements, such as a steel plate, which transfers the load to steel rods with a full square cross-section (figure 6, b)). figure 6. sample loading scheme. when testing bond strength, it is also necessary to evaluate the failure of the sample. samples can fail in different ways, and if they fail in a way other than expected, they must be excluded from the overall analysis. the basic types of failure are schematically illustrated in figure 7. failure can occur at the interface between the layers, in the older concrete layer, or in the newly placed concrete. if failure repeatedly occurs in a place other than the interface, it means that the created interface is not the weakest point of the sample. 55 m. kříž, m. frantová, v. wudi, p. štemberk acta polytechnica ctu proceedings figure 7. possible types of failure in samples. 4. design of a custom experiment the aim of the experimental part was to design and perform a shear bond strength test that could be used both for samples manufactured under laboratory conditions and for samples taken from real multi-layered structures. the test should also be easily repeatable for the planned larger number of samples, which would include concrete of various ages, different strengths, and different surface roughness at the interface. additionally, there was an effort to keep the experiment low-cost, using commonly available tools in a small laboratory and ensuring easy repetition of the test. the evaluation of the experiment consists of the measured load-bearing capacity at the interface of the concrete layers and the type of failure, as well as the possible slip, evaluated by image analysis. based on the evaluation of the advantages and disadvantages of bond strength tests described in the literature and in the previous chapter, a four-point shear test, performed in a standard loading press, was chosen as the starting point. the main advantage of this test is the ability to continuously record the slip at the layer interface under shear stress during the test and the simpler sample shape, which does not require the production of special molds. another advantage is the easier demolding of the samples and the possibility of reusing the molds, unlike samples of more complex shapes. the overall dimensions of the samples were set at 40 × 40 × 160 mm because it was possible to use molds printed on a 3d printer with a silicone insert (figure 8), which do not need to be oiled for easy demolding. due to the smaller dimensions of the samples it was possible to simplify the loading. the loading scheme was simplified by removing local supports and adding continuous loading (figure 9). figure 8. 3d printed mold with silicone insert. figure 9. sample loading scheme. 4.1. test procedure the samples are prepared in two separate concrete pours, which are time-separated. the molds were prepared in such a way as to simulate a sandwich construction, where the inner layer of concrete is cast onto the prefabricated outer layers concreted earlier. this means that the side parts of the samples were cast first. for casting the side parts, plastic blocks, printed on a 3d printer, were used as inserts to fill the centers of the molds. the inserts fit well into the molds and had the same dimensions (with only minor deviations due to 3d printing), as shown in figure 10. figure 10. preparation of molds for the first pouring. the concrete pouring procedure is shown in figure 11, where the left shows the first step of the pouring with the side parts of the sample cast, and the right shows the second step of the pouring with the inner (younger) part of the sample cast. the time 56 vol. 53/2025 design of bond strength testing method of concrete layers figure 11. top: first phase of pouring – older side parts of the sample. bottom: second phase of pouring – complete sample with cast inner (younger) part. gap between the two pouring steps was 10 days. when preparing the samples, the possibility of producing samples with different interface roughness was also tested to determine the influence of interface roughness on bond strength. three samples were produced, each with a shaped interface formed using a textured pad with projections (figure 12 a), which were 0.2 mm high. the pad with different orientations of the projections (0°, 90°, and 45°) was inserted into the mold and attached (figures 12 b, c, d). figure 12. forming the interface shape of the layers. the concrete mixture was manually processed according to the recipe with aggregate fractions, considering the sample size, ranging from 0.6 to 1.2 mm. fresh concrete was vibrated long enough in the molds to ensure that no air voids were present in the shaped interface. the resulting surface of the side parts of the samples is shown in figure 13. for the initial experiment design, it was necessary to make the slip between the layers in the concrete sample as clear as possible. therefore, the samples were loaded 12 hours after casting the inner layer, at a time when the younger concrete had not yet reached its final strength, and the samples were still a bit malleable. the loading process of the sample in a manual hydraulic press is shown in figure 14. before the actual loading, a pattern was drawn on the samples to serve as reference points for image figure 13. side parts of the samples with shaped interface. figure 14. loading in the hydraulic press. analysis. to correctly use the image analysis, the well-lit sample had to be recorded with a slow-motion function (to capture the highest number of frames per second), as close to the sample as possible, perpendicular to the sample, and without moving the camera. it was also necessary to record the press, showing both the sample and the load readings, which would later be converted to force. the load was applied to the sample through a distribution wooden element 79 mm long, to apply pressure only to the center of the sample. figures 15 and figure 16 show selected frames from the video recording (shot at 120 frames per second) during sample loading. the number above the frame represents the frame number, and the white vertical line marks the locations where cracks are forming. in figure 15, the failure occurred simultaneously at both interfaces in the sample, which was an expected and correct failure pattern. in figure 16, the failure was not entirely correct because the crack formed from the bottom surface upwards, and as the crack opened, the interface was improperly stressed, causing failure at only one interface. to evaluate the experiment, a detailed analysis of possible sample failures was carried out, which is described and schematically illustrated in figure 17. 4.2. image analysis image analysis, also known as image processing, is a process that involves analyzing digital images. the goal is to extract specific information from the images, 57 m. kříž, m. frantová, v. wudi, p. štemberk acta polytechnica ctu proceedings figure 15. correct failure of the sample (detailed section of the right side of the sample). figure 16. incorrect failure of the sample (detailed section of the right side of the sample). enhance their quality, and perform graphic modifications for specific purposes using tools available in a programming environment. common tools for image analysis include noise removal, edge enhancement, resizing, contrast adjustment, pattern recognition, face recognition, segmentation (dividing the image into parts), and more. the sample needs to be captured with a camera as close as possible and perpendicular to the sample. to make operations with the recorded images easier, the sample should be properly illuminated and marked with any pattern. the number of frames per second, video resolution, and distance from the sample directly determine the accuracy of the method. during the analysis, an algorithm processes the large number of images automatically. the video recording of the experiment was split into individual frames, which were then converted into a 3d matrix (three matrices for different base colors). this 3d matrix was transformed into a regular matrix representing a grayscale image. each element of the matrix represented one pixel with a value from 0 to 255, depending on the shade of gray. in the next step, the image was thresholded into binary form (black and white). pixels with a value lower than the threshold were considered black, while those with a higher value were considered white. with figure 17. comparison of correct and incorrect sample failures. this modified image, edge detection was performed using a convolution matrix – see figure 18. the change in position of the detected edges across the frames will serve to quantify the deformation occurring at the interface between the sample layers. by capturing the movement of a specific point on the sample relative to the applied load, it will be possible to obtain a shear bond stress-strain curve for the layers. 5. conclusion in everyday construction practice, concrete structures are composed of concretes of different ages and qualities, which are connected, whether it is in monolithicprecast structures, when fresh concrete is added between two fully matured precast elements, or when it is needed to connect concretes of different ages, for example, during strengthening of existing structures. when designing such a structure, it is necessary to consider the interaction between different concrete layers in the model, as perfect bonding between the layers cannot be assumed. for modeling a sandwich structure, for example, it is imperative to clearly understand the degree of interaction and ideally to have sufficient experimental data on shear bond strength. the purpose of this paper was to present an experimental method which can determine the shear bond strength of concrete layers without shear connectors. this experimental method can be used both for samples produced in the laboratory with well-defined boundary conditions and for samples taken from existing structures. the advantage of the proposed experimental method is its low requirements on equipment and easy repeatability. the three-layer samples can represent a simple sandwich wall, where the bond strength can be observed depending on factors such as the age of the concrete, the quality of the concrete in each layer, or the surface texture of the concrete layers. by utilizing image analysis, this experimental method can provide not only the final bond strength but also the deformation progression, slip between layers, and crack formation and propagation. from the proposed shear test at the interface, it is possible to determine the bond strength of the layers in shear and the deformation. based on the 58 vol. 53/2025 design of bond strength testing method of concrete layers figure 18. edge detection from the frames of the test video recording. set of measured strength and deformation values, it is possible to define a stress-strain diagram (or loaddeformation diagram) for multilayer walls, which can be implemented into software for more complex local analyses or global analyses of entire structures. the test performed is appropriate because the sample is mainly subjected to shear stress, and the samples represent structures of a three-layer monolithic-precast wall. acknowledgements this work was financially supported by the czech technical university in prague, project sgs25/039/ohk1/1t/11, which is gratefully acknowledged. references [1] a. d. espeche, j. león. estimation of bond strength envelopes for old-to-new concrete interfaces based on a cylinder splitting test. construction and building materials 25(3):1222–1235, 2011. https://doi.org/10.1016/j.conbuildmat.2010.09.032 [2] x. hui-cai, l. geng-ying, x. guang-jing. microstructure model of the interfacial zone between fresh and old concrete. journal of wuhan university of technology – materials science edition 17(4):64–68, 2002. https://doi.org/10.1007/bf02838421 [3] p. k. mehta, p. j. m. monteiro. concrete: microstructure, properties, and materials. mcgraw-hill education, new york, 4th edn., 2014. [4] r. hela. příměsi do betonu [in czech; admixtures for concrete]. beton tks: technologie a materiály (2):4–10, 2015. https: //www.ebeton.cz/wp-content/uploads/2015-2-04.pdf [5] j. silfwerbrand. shear bond strength in repaired concrete structures. materials and structures 36(6):419– 424, 2003. https://doi.org/10.1007/bf02481068 [6] e. n. b. s. júlio, f. a. b. branco, v. d. silva. concrete-to-concrete bond strength. influence of the roughness of the substrate surface. construction and building materials 18(9):675–681, 2004. https://doi.org/10.1016/j.conbuildmat.2004.04.023 [7] zkoušení ztvrdlého betonu – část 6: pevnost v příčném tahu zkušebních těles [in czech; testing hardened concrete – part 6: tensile splitting strength of test specimens]. standard, czech standard institute, prague, 2001. [8] výrobky a systémy pro ochranu a opravy betonových konstrukcí – zkušební metody – stanovení pevnosti v šikmém smyku [in czech; products and systems for the protection and repair of concrete structures – test methods – determination of slant shear strength]. standard, czech standard institute, prague, 2000. [9] x. wu, x. zhang. investigation of short-term interfacial bond behavior between existing concrete and precast ultra-high performance concrete layer. jianzhu jiegou xuebao/journal of building structures 39:156–163, 2018. https://doi.org/10.14006/j.jzjgxb.2018.10.018 [10] k. m. pufal, g. savaris, s. l. g. garcia. evaluation of direct shear strength of self-consolidating and conventional concretes. revista materia 26(3):1–10, 2021. https://doi.org/10.1590/s1517-707620210003.13036 59 https://doi.org/10.1016/j.conbuildmat.2010.09.032 https://doi.org/10.1007/bf02838421 https://www.ebeton.cz/wp-content/uploads/2015-2-04.pdf https://www.ebeton.cz/wp-content/uploads/2015-2-04.pdf https://doi.org/10.1007/bf02481068 https://doi.org/10.1016/j.conbuildmat.2004.04.023 https://doi.org/10.14006/j.jzjgxb.2018.10.018 https://doi.org/10.1590/s1517-707620210003.13036 acta polytechnica ctu proceedings 53:53–59, 2025 1 introduction 2 methods of measuring bond strength 3 methods for measuring bond strength 4 design of a custom experiment 4.1 test procedure 4.2 image analysis 5 conclusion acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2024.49.0092 acta polytechnica ctu proceedings 49:92–96, 2024 © 2024 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague transforming point cloud data into 3d bim models: a case study on slabs slávek zbirovský∗, václav nežerka czech technical university in prague, faculty of civil engineering, department of physics, thákurova 7, 166 29 prague, czech republic ∗ corresponding author: slavek.zbirovsky@fsv.cvut.cz abstract. we explore the automatized transition of point cloud data into a building information model (bim), an essential task for reconstructions, demolition plans, or auditing. in this study, we use point cloud data generated by 3d laser scanning. the research elaborates on the process of segregating and segmenting floor slabs from point clouds and the developed algorithms categorize point cloud subsets based on different height parameters (z-coordinates). once slabs are identified, a function is applied to create envelopes for each slab level, accommodating any convex or concave shape. the resulting geometry, along with relevant auxiliary data, is exported in the industry foundation classes (ifc) format, a standard for bim data exchange. the exported entities can then be further processed using prevalent software such as revit or archicad. keywords: bim, pointcloud, automatization, ifc. 1. introduction the conversion of point cloud data into 3d building information model (bim) has been predominantly accomplished through manual methods, which have served as a fundamental technique in the industry for the last decades. these manual methods involve precise tracing of the outlines of objects and structures within the point cloud data to create a comprehensive 3d model. while this process allows for greater control over individual elements and ensures the overall integrity of the model, it is time-consuming. moreover, manual methods are often constrained by the expertise of the user, the limitations of the software, and the scale and intricacy of the scanned point cloud. in recent years there has been significant progress in automated methods of transforming point cloud data into 3d models. these automated techniques use sophisticated algorithms and computer software to process data from the point cloud, greatly reducing the need for human intervention in acquiring data for a bim model. by minimizing manual intervention, automated methods offer higher processing speeds. nonetheless, they can also introduce a greater number of errors compared to traditional manual modelling techniques. commercial solutions, such as pointfuse, faramoon, aurivus [1–3], have emerged in the market, providing semi-automatic conversion capabilities for transforming point cloud data into accurate 3d models. also, there was published many solutions which integrate state-of-the-art algorithms such as ransac [4–7]. in this work, we describe a methodology that utilizes all the past developments in the area and enables the extraction of 3d entities along with auxiliary data. the flowchart of this procedure is provided in figure 1 and described in the following sections. 1.1. building information modeling building information modeling (bim) is a process that has a major impact on how construction projects are planned, designed, constructed, managed, or what happens to them after their lifetime. bim integrates various aspects of a construction project into a comprehensive 3d information model, including architectural, structural, and mep (mechanical, electrical, and plumbing) systems. the 3d model provides information that facilitates cooperation between construction participants, the investor, and building management throughout the project’s life cycle. bim enables accurate visualization, analysis, and simulation of building performance, leading to better decision-making, reduced errors, and increased efficiency. with bim, all professions can access real-time information, coordinate designs, reduce collisions, generate detailed documentation, and simulate the construction sequence. suppose there is no bim model available at the end of the life cycle of a building. in that case, it can pose challenges and limitations for effective planning and decision-making during the deconstruction, demolition, or adaptive reuse process. for these reasons, it is good to create at least a partial bim model containing as-built constructions and their materials. conventional methods of creating 3d models from as-built documentation tend to be inaccurate, which is why digital scan technologies have begun to be used in recent decades. 1.2. point clouds point clouds are large data sets that describe an object or space in a coordinate system using individual points. these points usually represent the surfaces of scanned objects and are described by coordinates 92 https://doi.org/10.14311/app.2024.49.0092 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 49/2024 transforming point cloud data into 3d models figure 1. flow chart diagram of the python script used for extraction of floor elements from point clouds and their formatting into the ifc format. 93 slávek zbirovský, václav nežerka acta polytechnica ctu proceedings figure 2. laser-scanned data (point cloud) of a building that was used for testing of the presented algorithms. (x,y,z), rgb values or brightness intensities. figure 2 shows an example of a common point cloud containing a building. the essential parameter of each point cloud is the density, with a higher density value, texture, and finer details can be distinguished on the scanned surface. but this parameter also negatively enables the speed of the scan itself, the requirements for the computing power of the device on which the point cloud will be processed, and the resulting large file. the most common way to obtain the point cloud is lidar (light detection and ranging) technology. it works on the principle of the spatial polar method, which evaluates the position of individual points based on elevation angles and distances. the second method, also commonly used in surveying, is photogrammetry, exploiting images taken from different angles and positions [8, 9]. 1.3. ifc ifc was developed by the international alliance for interoperability (iai), now known as buildingsmart international, with the aim of improving collaboration and communication among different software applications used throughout the life cycle of a building or infrastructure project. ifc defines a common data schema and a standardized set of object classes, properties, and relationships to represent various aspects of a building project, such as building elements, spaces, materials, quantities, schedules, and more. the data model is based on a semantic representation, meaning that it captures not only geometric information but also the meaning and behaviour of the objects and their relationships. 2. algorithms the algorithms for creating and placing slabs within a building can be divided into several parts. in the first step, the point cloud is divided into strips, which are planes parallel to the x-y axes, and those strips with a point density on the histogram higher than a certain percentage (set to 50 % in our study) of the strip with the highest number of points are extracted. figure 3 shows a section of the point cloud and a histogram on which individual horizontal surfaces are visible in places with maximum histogram values. in these strips, the median z-coordinate is taken based on the index and subsequently labelled as the lower or upper surface. then, neighbouring surfaces formed by points are unified, creating the top and bottom surfaces of the slab. the final step involves calling the function create_hull_alphashape. the alphashape function for a hull creation takes a set of 2d points as input and performs a delaunay triangulation. it then removes edges longer than a specified alpha value, resulting in a non-convex polygon. the remaining edges form the boundaries of the hull polygon, and the function returns the set of vertices and edges that define the polygon [10]. an example of the delaunay triangulation and the alpha shape function with α = 5 can be seen in figure 4. 3. ifc file ganeration the ifc files presented in this study were generated according to the ifc4_add2_tc1 structure [11]. the library for python, ifcopenshell 0.7.0, was used to generate an output.ifc file [12]. the structure of an ifc file can be divided into two main parts. the first, which is called a header, contains basic information such as name, author, view definition, implementation level, and file schema. the example of the file header section in step (standard for the exchange of product model data) format can be seen in figure 5. the second part of the file is data section it is the main part of the ifc file and contains the actual data in a structured format. it consists of the ifc entities representing building elements, relationships, and properties. 94 vol. 49/2024 transforming point cloud data into 3d models figure 3. a z-coordinate histogram of point cloud data to detect slab surface candidates. figure 4. alpha shape of a set of 2d points from the delaunay triangulation; α = 5. figure 5. example of the ifc step file header section structure. in addition to the slab geometry, the data part also describes the structure of the project. an essential part of every ifc file is the ifcproject. this instance represents the main project entity. it is a fundamental concept within the ifc schema that serves as a container for various building-related information and acts as a starting point for organizing and representing the entire building or construction project. the tree structure ifcproject > ifcsite > ifcbuilding > ifcstorey is created in the file. this structure makes it possible to place and assign individual elements to different buildings and floors. the generated floor structures can be seen in figure 6. 4. slab ifc geometry by combining the profile definition and the extrusion parameters, the ifcextrudedareasolid entity creates a 3d solid geometry that represents the slab. the profile shape which is described by ifcarbitraryclosedprofiledef entity is extruded along the specified direction to form the solid shape. the resulting solid represents the volume of the slab in the specified dimensions and position. the main ifc entities for the slab are provided in figure 7. 5. conclusion in this study, an algorithm was developed to automate the conversion of point cloud data into a 3d bim model, with a specific focus on slabs. the code was implemented in python and at the moment it 95 slávek zbirovský, václav nežerka acta polytechnica ctu proceedings figure 6. ifc tree structure with assigned slabs. figure 7. ifc entities used to create the slab geometry. is capable of autonomously assessing individual floor levels in a point cloud and identifying floor slabs. subsequently, the information about these slabs is written into an ifc structure, enabling compatibility with commercial software such as archicad, revit, and other user-friendly software tools. this algorithm significantly expedites the process of working with point cloud data during the creation of a 3d bim model for existing buildings. this research showcases the successful development and implementation of a python-based algorithm that automates the conversion of point cloud data into 3d bim models, with a particular emphasis on slabs. the seamless integration of this algorithm with commercial bim software makes it a valuable asset for streamlining the modelling process of existing structures. the significance of this advancement lies in its potential to save time, reduce manual effort, and enhance the overall accuracy of 3d bim models, ultimately contributing to the advancement of the industry. list of symbols bim building information modeling lidar light detection and ranging ifc industry foundation classes iai international aliance for interoperability step standard for the exchange of product model data acknowledgements this work was supported by the faculty of civil engineering at ctu in prague, grant number sgs23/004/ohk1/1t/11 and by the european union’s horizon europe framework programme (call horizoncl4-2021-twin-transition-01-11) under grant agreement no. 101058580, project reconmatic (automated solutions for sustainable and circular construction and demolition waste management). we would like to express our gratitude to cegra and graphisoft for facilitating the provision of a complimentary archicad software license, a crucial tool employed in the processing of point cloud data and the visualization of the automatically generated building information model (bim) in this research. references [1] autodesk.com. pointfuse, 2023. [2023-07-06]. https://pointfuse.com/ [2] faramoon, 2023. [2023-07-06]. https://www.faramoon.io/ [3] aurivus, 2023. [2023-07-06]. https://aurivus.com/ [4] m. a. fischler, r. c. bolles. random sample consensus: a paradigm for model fitting with applications to image analysis and automated cartography. readings in computer vision pp. 726–740, 1987. https: //doi.org/10.1016/b978-0-08-051581-6.50070-2 [5] z. li, j. shan. ransac-based multi primitive building reconstruction from 3d point clouds. isprs journal of photogrammetry and remote sensing 185:247–260, 2022. https://doi.org/10.1016/j.isprsjprs.2021.12.012 [6] t. doougphummet. 3d building internal structural component segmentation from point cloud data using dbscan and modified ransac with normal deviation conditions. master’s thesis, chulalongkorn university, 2022. https://doi.org/10.58837/chula.the.2022.13 [7] w. yue, j. lu, w. zhou, y. miao. a new plane segmentation method of point cloud based on mean shift and ransac. in 2018 chinese control and decision conference (ccdc), pp. 1658–1663. 2018. https://doi.org/10.1109/ccdc.2018.8407394 [8] q. wang, y. tan, z. mei. computational methods of acquisition and processing of 3d point cloud data for construction applications. archives of computational methods in engineering 27(2):479–499, 2020. https://doi.org/10.1007/s11831-019-09320-4 [9] f. remondino. point cloud acquisition & structuring, 2018. [2023-06-19]. https://3d.bk.tudelft.nl/pdfs/ pcp2018/pcp2018_fabioremondino.pdf [10] h. edelsbrunner, d. kirkpatrick, r. seidel. on the shape of a set of points in the plane. ieee transactions on information theory 29(4):551–559, 1983. https://doi.org/10.1109/tit.1983.1056714 [11] buildingsmart. ifc4_add2_tc1 – 4.0.2.1 official, 2022. [2023-05-31]. https://standards.buildingsmart.org/ifc/ release/ifc4/add2_tc1/html [12] d. moult. ifcopenshell 0.7.0 documentation, 2023. [2023-07-06]. https://pypi.org/project/ifcopenshell/ 96 https://pointfuse.com/ https://www.faramoon.io/ https://aurivus.com/ https://doi.org/10.1016/b978-0-08-051581-6.50070-2 https://doi.org/10.1016/b978-0-08-051581-6.50070-2 https://doi.org/10.1016/j.isprsjprs.2021.12.012 https://doi.org/10.58837/chula.the.2022.13 https://doi.org/10.1109/ccdc.2018.8407394 https://doi.org/10.1007/s11831-019-09320-4 https://3d.bk.tudelft.nl/pdfs/pcp2018/pcp2018_fabioremondino.pdf https://3d.bk.tudelft.nl/pdfs/pcp2018/pcp2018_fabioremondino.pdf https://doi.org/10.1109/tit.1983.1056714 https://standards.buildingsmart.org/ifc/release/ifc4/add2_tc1/html https://standards.buildingsmart.org/ifc/release/ifc4/add2_tc1/html https://pypi.org/project/ifcopenshell/ acta polytechnica ctu proceedings 49:92–96, 2024 1 introduction 1.1 building information modeling 1.2 point clouds 1.3 ifc 2 algorithms 3 ifc file ganeration 4 slab ifc geometry 5 conclusion list of symbols acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2023.43.0001 acta polytechnica ctu proceedings 43:1–5, 2023 © 2023 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague integral railway interlocking system and its assessment according to european standards tomáš brandejskýa, vít fáberaa,∗, martin lesob a czech technical university in prague, faculty of transportation sciences, department of applied informatics in transportation, konviktská 20, 110 00 prague 1, czech republic b czech technical university in prague, faculty of transportation sciences, department of transport telematics, konviktská 20, 110 00 prague 1, czech republic ∗ corresponding author: fabera@fd.cvut.cz abstract. this paper analyses the component of the integrated interlocking system which forms the central logical and functional unit implementing all logical and computational functions necessary for railway traffic control in the railway 4.0 concept. the main principle is that this approach centralizes the technology of station interlocking system, track line interlocking systems, level crossing interlocking systems and the functions of train interlocking system – the line part of etcs l2/3 radio block control panel (rbc). the operation control is centralised to the controlling dispatcher centres. the paper discusses the concept of integrated interlocking system including safety issues addressed from the perspective of the requirements of cenelec standards en 50126, en 50128 and en 50129. these requirements are addressed from the perspective of the authors of the paper, who also work as independent assessors of the safety of railway control and command systems. keywords: integrated interlocking system, cloud, european standards, en 50126, en 50128, en 50129, assessment. 1. introduction the current architecture of interlocking systems, including the implementation of etcs, is nowadays mainly solved by locally (decentralised) technologies, which require relatively significant requirements for their construction and operation (figure 1). it is necessary to build a lot of technological buildings, which include backup power sources, air conditioning units, etc. the railway 4.0 concept was introduced in [1]. as it has already been explained in those paper, the majority parts of railway infrastructure, especially outside the ten-t railway corridor with a length of about 7000 km of track lines, is constituting almost 2/3 of the railway network of the czech republic. these tracks often contain a lot of railway stations with a small number of stations track lines and a large number of level crossings on the tracks. these track lines are also single track, which has a major impact on the lack of capacity. the railways 4.0. concept discusses the possibility to combine distributed signalling technology with centralisation of logical and computational parts (integral interlocking system). it makes possible to reduce the number of external technological buildings and replace them with distributed oc object controllers. controlling and commanding of these external technologies is centralised in the integrated signalling system of the izz, which is further interconnected with the central control centres of the cdp. the gsm-r digital radio system, consisting of the msc master controller and the distributed bts figure 1. standard architecture of decentralized railway interlocking system. base stations, dm (digital modul of gsm-r) are also parts of the whole digital railway control system. the whole digital railway system requires high quality high capacity transmission lines consisting of fibre optic cables (figure 2). 2. integral interlocking system the main principle of this approach is to centralize the technology of station interlocking system szz (setting up train and shunting routes in stations), track 1 https://doi.org/10.14311/app.2023.43.0001 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en t. brandejský, v. fábera, m. leso acta polytechnica ctu proceedings figure 2. new architecture of integral railway interlocking system. line interlocking systems tzz (commanding the movement of trains on the line), level crossing interlocking systems pzz (ensuring the safety of level crossings) and the functions of train interlocking system – the line part of etcs l2/3 radio block control panel (rbc). the one way how to make a centralization is to implement the whole system in cloud. this concept has been already produced be several manufacturers and deployed [2–4]. some producers designed own proprietary solution (like ds3 platform from siemens mobility gmbh [5]) others try to use cloud solution based on cots (like tas platform from thales austria gmbh [6]). the architecture of izz realized in the cloud is shown in figure 3. the solution brings many advantages. cloud is a system of tightly knit physical servers, switches and other devices (see section 2.2) that act as one highly performant and highly available computational platform. the cloud allows the parallel execution of a large number of instances of individual applications implementing logical and computational algorithms for railway traffic control. these applications have a similar logic and function to current technologies (pzz, szz, tzz, rbc), but their functions (range of functions, specific parameters and settings) are modified through a defined configuration – they are configured with data from the configuration memory (memory protected against random overwriting) during their initialization. izz with cloud technology also provides communication with remote oc object controllers via hw and sw resources over optical networks via ip protocol with very high level of security and safety. the additional consequence of centralization is cost reduction for building local technologies and their maintainance cost because local technologies are reduced to object controllers (oc). but using the cloud solution defines new problems and requirement to satisfy european standards (architecture, communication with object controllers). figure 3. architecture of railway interlocking system in cloud. 2.1. european standards the solution must be designed to satisfy european standards en 50126, en 50128, en 50129 (figure 4). the standards en 50126 [7, 8] define the life-cycle of the system and gives the instructions to guarantee rams parameters: r – reliability, a – availability, m – maintainability, s – safety. the standard en 50128 [9] defines requirement for software development (software lifecycle). the en 50129 [10] contains requirements for architecture of hardware and guidelines how to evaluate reliability. the standard en 50159 speaks about communication systems. moreover, security must be considered as well, although standard en 50701 for cybersecurity is under preparation process. 2.2. clouds cloud is perceived as system to store personal data, photos or provide hosting for web based applications by public. more specifically, the cloud is a group (cluster) of computers (servers) working in data center. the cloud technology is based on virtualization: the special virtualization software runs on physical computers (host computers) and allows existences of independent virtual computers (virtual machine – vm) with own operating system and applications. the virtualization software that creates, runs and manages virtual machines is called hypervisor. the one of basic architecture of host with virtual systems is in figure 5. there are two types of clouds: public and private clouds. in the case of public cloud (microsoft azure, google cloud) users share hardware (physical computers, storage) through virtual computers with other 2 vol. 43/2023 integral railway interlocking system figure 4. interlocking system and european standards [11]. figure 5. architecture of the host with virtual machines. users. the configuration can change – virtual computers can be added, memory, processor capacity and allocated storage capacity can be increased/decreased to individual users (customers) according to their needs. in the case of private cloud, hardware is dedicated only for one customer, infrastructure is separated and usually communicates under virtual private network. so, computation capacity and other resources are dedicated to one user (customer) and can’t be provided to other users. moreover, other safety precaution are applied like physical separation from other physical computational systems etc. it is clear that only possible variant to implement interlocking system is to use private cloud to ensure rams and security parameters. the most suitable operating system is linux (it is more stable, reliable and more powerfull than ms windows). there are several virtualization software: • vmware – available under linux and windows, in free and commercial version • oracle virtual box – available under linux and windows, in free and commercial version • hyper v – from microsoft, only under ms windows • kvm – kernel-based virtual machine – under linux system kvm is most used under linux today. each virtual machine has its own configuration – it has assigned some amount of physical memory, counts of processor cores, virtual network adapters assigned to physical card, virtual disks. virtual disks can be represented (emulated) by files stored on physical storage of host computers, but it is better to use more sophisticated technologies for disk management like figure 6. architecture of the host with containerization. lvm (logical volume management), zfs (zettabyte file system), ceph, drbd (distributed replicated block device). these virtualization software makes possible full virtualization: several instances of virtualization software allow to run independent instances of operating system and this system can be different from the operating system on the host computer (linux installed in vm which runs under ms windows and vice versa). we can even emulate other computers based on different processor (remember emulators of 8-bit computers like zs spectrum, atari, commodore on pc under ms windows). on the other hand, the second approach is a containerization. it can be said that this approach is a “light” virtualization. the containerization software is represented by open-sources systems like kubernetes, docker. there is only one instance of operation system on host and applications run in a virtual environment – in a “package” called container (figure 6). all virtual instances share one operating system; application must be compiled under the same operating system, i.e. it is not possible to run application for ms windows in the containerization software running under linux. 2.3. virtualization and fulfilment of en 50126, en 50128, en 50129 2.3.1. en 50126 and en 50129 the designer must fulfill a prove rams according to en 50126. thank to the virtual architecture (hypervisors) the cloud system allows a fast and automatic recovery process of hw and sw resources so that a detected failure does not compromise the function and safety of the whole system (ballancing). so, the 3 t. brandejský, v. fábera, m. leso acta polytechnica ctu proceedings figure 7. example of application distribution over cloud. reliability can be increased, but analyses of the reliability of the cloud system can be more difficult because failures with common cause have to be considered (failures of hypervisor for example). to increase the reliability the redundancy of the cloud (hot or cold stand-by) is recommended. due to centralization accessibility and maintainability is higher than in the case of distributed system. one parameter which influences the safety is an architecture of the system. the architecture is recommended in en 50129 like an architecture m of n. as a minimum architecture, 2 of 3 is assumed to allow fail-safe operation for security integrity (sil) at up to level 4. in the architecture 2 of 3, hw and sw must be implemented in independent instances and the results of the computation are compared. so, there must be created such configuration of the hypervisor which ensures that safety related application and comparators run in several (three in minimal) instances in separated virtual machines on separate hardware. thank to high computational capacity of the cloud and relatively low computational demands of railway applications, there can be executed several applications (pzz, tzz, szz, ...) in one virtual machine in parallel. the example how applications can be distributed over the cloud is in the figure 7. two safety applications (sa1, sa2) run in three instances (sa1-i1, sa1-i2, sa1-i3, sa2-i1, sa2-i2, sa2-i3) in separate hw. each instance has own virtual machine. the architecture is 2 of 3, comp1 and comp2 are comparators atteched to sa1 and sa2. non safety applications nsa1, nsa2 and diagnostics modules d1, d2 run each only in one instance (at basic integrity level). because of the high computational capacity, tens intances of safety appplication can run in one hw node in the real integral interlocking system. 2.3.2. en 50128 the fulfilment of en 50128 criteria is not a potential problem. application software running in virtual machines “does not see” its virtual machine, there is no difference if the software is developed for real target platform or virtual computers. so, the lifecycle, techniques, processes can be the same when the software is developed for dedicated hardware. the virtualization can help to fulfill some criteria, for example 7.3.4.9 “where the software consists of components of different safety integrity levels then all of the software components shall be treated as belonging to the highest of these levels unless there is evidence of independence between the higher software safety integrity level components and lower software safety integrity level components. this evidence shall be recorded in the software architecture specification.” if parts of software on different safety integrity level run in separated virtual machines it can be considered to be isolated and independend. when the software is tested it must be focused on load testing, due to sharing physical hardware. 3. conclusions the development of interlocking systems leads to integral interlocking system when all parts (station interlocking system, line interlocking system, level crossing interlocking systems, radio block control panel etc.) and logic is concentrated in one computational node. the approach can decrease cost for infrastructure because components are reduced to object controllers and communication lines. the central computational node can be realized by cloud – some manufacturers have already produced and deployed such solution. the implementation of interlocking system must satisfy cenelec european standard en 50126, en 50128, en 50129 and security require4 vol. 43/2023 integral railway interlocking system ments which are now in proposal. thank to features of the cloud (centralization, stand-by, fail-safe, automatic reconfiguration, . . . ), ram parameters (reliability, accessibility, maintainability) can be improved in comparison with distributed solution. it is recommended to use private cloud than public one. the attention must be given to the configuration to satisfy requirements for hw/sw architecture related to appropriate safety integrity level (for example for 2 of 3 architecture: more instances of safety related application must be executed on separate hw – even in case of reconfiguration (migrate the application to another node) after the failure). references [1] m. leso. the railway 4.0 concept – the vision of a digital railway in the czech republic (cr). acta polytechnica ctu proceedings 35:27–36, 2022. https://doi.org/10.14311/app.2022.35.0027 [2] thales. öbb-infrastruktur and thales work on cloud-interlocking. 2023, [2023-03-16], https://www.thalesgroup.com/en/austria/news/obbinfrastruktur-and-thales-work-cloudinterlocking. [3] siemens. interlocking systems. 2023, [2023-03-16], https: //www.mobility.siemens.com/global/en/portfolio/ rail/automation/interlocking-systems.html. [4] siemens. bane nor and siemens mobility celebrate milestone in digitalization of norway’s rail network. 2023, [2023-03-16], https: //press.siemens.com/global/en/pressrelease/banenor-and-siemens-mobility-celebrate-milestonedigitalization-norways-rail-network. [5] s. steffens, w. valvoda. the development of new ds3 safety platform – from the research to commissioning. signal+draht 113(6):52–59, 2021. [6] r. hametner, p. tummeltshammer, s. resch, w. wernhart. cloud architecture for sil4 railway application. signal+draht 114(3):20–28, 2022. [7] european standard. en50126-1:2017. railway applications – the specification and demonstration of reliability, availability, maintainability and safety (rams) – part 1: generic rams process. standard, cenelec, 2017. [8] european standard. en50126-2:2017. railway applications – the specification and demonstration of reliability, availability, maintainability and safety (rams) – part 2: systems approach to safety. standard, cenelec, 2017. [9] european standard. en50128:2011. railway applications – communication, signalling and processing systems – software for railway control and protection systems. standard, cenelec, 2011. [10] european standard. en50129:2018. railway applications – communication, signalling and processing systems – safety related electronic systems for signalling. standard, cenelec, 2018. [11] d. nenutil. kybernetická bezpečnost pro drážní systémy. vědeckotechnický sborník čd 47:1–23, 2019. 5 https://doi.org/10.14311/app.2022.35.0027 https://www.thalesgroup.com/en/austria/news/obb-infrastruktur-and-thales-work-cloud-interlocking https://www.thalesgroup.com/en/austria/news/obb-infrastruktur-and-thales-work-cloud-interlocking https://www.thalesgroup.com/en/austria/news/obb-infrastruktur-and-thales-work-cloud-interlocking https://www.mobility.siemens.com/global/en/portfolio/rail/automation/interlocking-systems.html https://www.mobility.siemens.com/global/en/portfolio/rail/automation/interlocking-systems.html https://www.mobility.siemens.com/global/en/portfolio/rail/automation/interlocking-systems.html https://press.siemens.com/global/en/pressrelease/bane-nor-and-siemens-mobility-celebrate-milestone-digitalization-norways-rail-network https://press.siemens.com/global/en/pressrelease/bane-nor-and-siemens-mobility-celebrate-milestone-digitalization-norways-rail-network https://press.siemens.com/global/en/pressrelease/bane-nor-and-siemens-mobility-celebrate-milestone-digitalization-norways-rail-network https://press.siemens.com/global/en/pressrelease/bane-nor-and-siemens-mobility-celebrate-milestone-digitalization-norways-rail-network acta polytechnica ctu proceedings 43:1–5, 2023 1 introduction 2 integral interlocking system 2.1 european standards 2.2 clouds 2.3 virtualization and fulfilment of en 50126, en 50128, en 50129 2.3.1 en 50126 and en 50129 2.3.2 en 50128 3 conclusions references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2024.47.0133 acta polytechnica ctu proceedings 47:133–136, 2024 © 2024 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague production of concrete pavements using mixed cements petr šperling∗, rudolf hela brno university of technology, faculty of civil engineering, institute of technology of building materials and components, veveří 331/95, 602 00 brno, czech republic ∗ corresponding author: petr.sperling@vut.cz abstract. this paper discusses the possibility of assessing the k-value of power plant fly ash using durability. compressive strengths, activity indices, pressure water seepage depth and carbonation depth were determined for cement mortars with 10, 20 and 30 % cement replacement by fly ash for ages of 7, 28 and 60 days. the k-values for each cement substitute were determined for each age using the relationship between water/cement ratio and compressive strength. using the results of the depth of pressurized water seepage and depth of carbonation, the determined k-values were assessed and k-values that are safe for use of power plant fly ash in aggressive xc and xd environments were determined. keywords: k-value, activity index, fly ash, concrete admixtures. 1. introduction the use of active admixtures in concrete, such as fly ash, finely ground granulated blast furnace slag, is nowadays desirable mainly due to the reduction of the use of portland clinker, the production of which produces large amounts of co2, which subsequently escapes into the atmosphere [1]. fly ash has an effect on both the properties of fresh concrete and hardened concrete. the initial strengths of concrete with fly ash are lower than those of the reference concrete. the strengths of this concrete reach the strengths of the reference concrete only at later ages (around 90 days) [2]. in terms of durability, fly ash has a slightly negative effect on frost resistance and fly ash has a positive effect when aggressive substances penetrate and corrode the steel reinforcement [3]. the use of active admixtures in concrete in the czech republic is addressed by the so-called k-value concept. the european standard en 206+a2 mentions this concept, but this standard does not provide any possibility to determine the k-value for active admixtures and the concept itself is described in a very general way [4]. the effect of active admixtures on the properties of concrete depends on the nature of the individual materials, the age of the concrete, the external conditions, etc. to take all these influences into account when designing concrete, the k-value concept uses the relationship between the water/cement ratiot and the compressive strength. if the condition of equal compressive strength is satisfied, then relation (1) is valid [5]: w0 = v c + k · p . (1) where w0 is the water/cement ratio of concrete without admixture, v is the water content of concrete with admixture in kg m−3, c is the cement content of concrete with admixture in kg m−3, p is the admixture content in kg m−3 and k is the k-value. the procedure for determining the k-value is given in cen/tr 16639. however, this document is not used in the czech republic. in this methodology, concretes with different water/cement ratios without and with active admixture are mixed. the k-value is determined from a comparison of the dependence of the compressive strength on the water/cement ratio. the result is not a single k-value for a given active admixture, but a set of k-values dependent on the water/cement ratio [6]. however, this method does not take into account the effect of the active admixture on the durability of the concrete. 2. methodology cem i 42.5 r mokrá cement and high temperature opatovice fly ash were used in the experimental work. cement mortars were mixed with 0, 10, 20 and 30 % cement replacement with opatovice fly ash. a constant water/cement ratio of 0.5 was used in the cement mortars and the amount of cement used was the same as in en 196-1: methods of testing cement – part 1: determination of strength. the aggregate was composed of three fractions so that its grain size curve corresponded as closely as possible to that of standardised sand according to en 196-1. the cement mortars were tested at ages 7, 28 and 60 days for compressive strength according to en 196-1, determination of the depth of carbonation according to en 13295: products and systems for the protection and repair of concrete structures – test methods – determination of resistance to carbonation and resistance to pressure water seepage according to en 12390-8: testing of hardened concrete – part 8: depth of pressure water seepage. 133 https://doi.org/10.14311/app.2024.47.0133 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en petr šperling, rudolf hela acta polytechnica ctu proceedings recipes [m−3] ref a10 a20 a30 cement 511 460 409 358 water 254 254 254 254 ash 51 102 153 sand 0.1–0.6 443 443 443 443 sand 0.6–1.2 517 517 517 517 sand 1–4 572 572 572 572 table 1. recipes of cement mortars. using the determined compressive strengths, a k-value was determined based on the relationship between the water/cement ratio and compressive strength. the relationship between water/cement ratio and compressive strength is described by formula (2) [7]. fc = k · ( 1 v/c − a) (2) where fc is the compressive strength in mpa, k je coefficient dependent on the reference cement in mpa, c is the amount of cement in the concrete in kg m−3, v is amount of water in the concrete in kg m−3 and a is the coefficient depends on the age of the concrete. verification of the k-value by durability was carried out using the requirements of the “concept of equivalent concrete performance”. in this concept, limiting values for the change in d (as a % difference from the reference concrete, which leads to the rejection of the use of type ii admixture with a 90 % probability) are given in terms of the durability aspect of the concrete. the change in d properties of cement mortar with fly ash shall not exceed the limit of 30 % deterioration. 3. results this section presents the result of the experimental part. in following figures are shown results of compressive strength, activity index, depth of carbonation, pressure water leakage and k-value for cement mortars with fly ash. in figure 1 we can see the results of compressive strength which are highest for reference mortars a with increasing amount of fly ashe were lower. figure 1. compressive strength of cement mortars. activity indices are corresponding with compressive strengths of cement mortars. highest activity index figure 2. activity index. is for mortar with 10 % of fly ash after 28 days and for mortar with 20 % of fly ash after 60 days. figure 3 shows the depth of pressure water leakage. the highest depth of pressure water leakage is for mortars a20 and a30 after 7 days. this value decreases with increasing age of cement mortars. figure 3. depth of pressure water leakage. in figure 4 are shown result of depth of carbonation. as depth of pressure water leakage is the highest depth of carbonation for cement mortars a20 and a30 after 7 days. figure 4. depth of carbonation. figure 5. k-value for power plant ash. 134 vol. 47/2024 production of concrete pavements using mixed cements age [days] depth of pressure water leakage [mm] difference [mm] change d[%] ref a10 a20 a30 a10 a20 a30 a10 a20 a30 7 20 18.5 30 28 -1 10 8.5 -5.1 51.3 43.6 28 13 14 15 18 1.5 2 5 12.0 16.0 40.0 60 12 12 8.5 9 0 -4 -3.5 0.0 -29.2 -29.2 table 2. durability aspect for pressure water leakage. age [days] depth of carbonation [mm] difference [mm] change d[%] ref a10 a20 a30 a10 a20 a30 a10 a20 a30 7 1.2 2.4 5.6 7.4 1.2 4.4 6.2 100.0 366.7 516.7 28 1 1.3 2.3 3.1 0.3 1.3 2.1 30.0 130.0 210.0 60 1.1 1 1 1.8 -0.1 -0.1 0.7 -9.1 -9.1 63.6 table 3. durability aspect for pressure water leakage. k-values in figure 5 were determined using using the relationship between compressive strength and water/cement ratio of cement mortars. the course of the k-value corresponds to the course of the compressive strengths. 3.1. k-value assessment using durability according to cen/tr 16639 aspects of durability can be assessed by the following durability tests [6]: • carbonation resistance, • resistance to chloride penetration, • resistance to freezing and thawing, • resistance to the effects of seawater, • sulphate resistance. according to the czech supplementary standard čsn p 73 2404 concrete – specification, properties, production and compliance supplementary information, the k-value for fly ash cannot be used for exposure classes xf2-xf4. for this reason, the frost resistance tests were omitted and instead the depth of pressure water penetration was determined [8]. for each type of aggressive environment, limits of change are set from which the concrete must not deviate from the reference concrete. cen/tr 16639 gives the limit values for the change in d as the percentage difference from the reference concrete that leads to the rejection of the use of a type ii admixture with a 90 % probability, in terms of the durability aspect. the change in d properties of the tested concrete with admixture shall not exceed 30 % [6]. durability assessment using pressure water leakage is shown in table 2 and durability assessment using carbonation depth is shown in table 3. 4. discussion in figure 2, we can see that the activity indices are lowest at 7 days of age, as expected, and increase with increasing age. this corresponds to the slower rate of pozzolanic reaction of fly ash compared to the hydration of cement. the slower pozzolanic reaction of fly ash also corresponds to the higher depth of pressure water seepage and carbonation. in figure 3 we can see that the depth of pressure water penetration at 7 days of age is significantly worse (by almost 50 %) than the reference mortar, especially for the formulations with 20 and 30 % fly ash. at the age of 28 days this difference decreases significantly, to 12 % for the recipe with 10 % fly ash, 16 % for the recipe with 20 % fly ash and 40 % for the recipe with 30 % fly ash. at the age of 60 days, the penetration depths of the fly ash formulations are already equal to or lower than the penetration depth of the reference mortar water pressure. as we can see in figure 4, at an age of 7 days, the depth of carbonation is significantly higher for mortars with fly ash (up to hundreds of percent) and the depth of carbonation for these mortars decreases with increasing age. however, in the case of formulation a30 with 30 % fly ash, there is a significant increase in the depth of carbonation even at 60 days of age. the k-values determined using the relationship between compressive strength and water/cement ratio of cement mortars at 7 days of age are 0.43–0.57. already these k-values are higher than the recommended value in en 206+a2, which is equal to 0.4 for fly ash with cem i cement [4]. to verify the durability aspect, the depth of pressure water seepage and the depth of carbonation were determined. in the case of pressure water penetration depth, cement mortar a10 with 10 % fly ash meets the durability condition at all ages, formulation a20 only at ages 28 and 60 days and formulation a30 only at age 60 days. for the depth of carbonation, the durability aspect is met only for formulation a10 at ages 28 and 60 days and for formulation a20 at age 60 days. 135 petr šperling, rudolf hela acta polytechnica ctu proceedings 5. conclusion the aim of this paper was to verify the possibility of determining the k-value of power plant fly ash in the replacement of 10, 20 and 30 % portland cement using the aspect of durability. the compressive strengths obtained were used to determine the activity indices, which were lowest at 7 days of age and highest at 60 days of age. mortar a10 at 28 days of age achieved a higher activity index than 100 % and mortar a20 at 60 days of age achieved an activity index of 100 %. for the depth of carbonation and the depth of seepage by pressure water, the waveforms were similar. in the case of pressure water seepage depth, the seepage depths at 60 days age were equal to or lower than the pressure water seepage depth. the k-values determined from the relationship between water/cement ratio and compressive strength were highest for mortar with 10 % cement replacement with fly ash at 28 days of age. at the age of 60 days for mortar with 20 % cement replacement with fly ash. in the case of pressure water seepage depth, mortar a10 is suitable already at 7 days of age, mortar a20 at 28 days of age and mortar a30 up to 60 days of age. the durability aspect for the depth of carbonation was only satisfied for mortar a10 at ages 28 and 60 days and for mortar a20 at age 60 days. on the basis of the test results obtained and the assessment of the durability aspect, it can be said that for replacements and ages that satisfy the durability aspects, the values set for exposure classes xc and xd can be safely used. however, it should be taken into account that the requirements of en 206+a2 for the exposure classes are given for concrete. it is therefore advisable to verify the k-values on concrete exposed to these aggressive environments. acknowledgements the paper was written in the framework of the project fast-j-23-8236. possibilities of assessment of the k-value of active admixtures with respect to durability. references [1] c. li, x. z. gong, s. p. cui, et al. co2 emissions due to cement manufacture. in energy, environment and biological materials, vol. 685 of materials science forum, pp. 181–187. trans tech publications ltd, 2011. https://doi.org/10.4028/www.scientific.net/msf. 685.181 [2] n. chousidis, e. rakanta, i. ioannou, g. batis. mechanical properties and durability performance of reinforced concrete containing fly ash. construction and building materials 101:810–817, 2015. https: //doi.org/10.1016/j.conbuildmat.2015.10.127 [3] w. j. halstead. use of fly ash in concrete. publication iom. transportation research board, 1986. [4] beton specifikace, vlastnosti, výroba a shoda. standard, czech standard institute, prague, 2021. [5] b. teplý, m. chromá, p. rovnaníková, a. strauss. probabilistic modelling and the k-value concept. in material engineering practice ix, vol. 635 of key engineering materials, pp. 198–203. trans tech publications ltd, 2015. https://doi.org/10.4028/ www.scientific.net/kem.635.198 [6] použitie koncepcie k-hodnoty, koncepcie ekvivalentných vlastností betónu a kombinovanej koncepcie ekvivalentných vlastností betónu. standard, 2014. [7] e. badogiannis, v. papadakis, e. chaniotakis, s. tsivilis. exploitation of poor greek kaolins: strength development of metakaolin concrete and evaluation by means of k-value. cement and concrete research 34(6):1035–1041, 2004. https://doi.org/10.1016/j.cemconres.2003.11.014 [8] beton – specifikace, vlastnosti, výroba a shoda – doplňující informace. standard, czech standard institute, prague, 2021. 136 https://doi.org/10.4028/www.scientific.net/msf.685.181 https://doi.org/10.4028/www.scientific.net/msf.685.181 https://doi.org/10.1016/j.conbuildmat.2015.10.127 https://doi.org/10.1016/j.conbuildmat.2015.10.127 https://doi.org/10.4028/www.scientific.net/kem.635.198 https://doi.org/10.4028/www.scientific.net/kem.635.198 https://doi.org/10.1016/j.cemconres.2003.11.014 acta polytechnica ctu proceedings 47:133–136, 2024 1 introduction 2 methodology 3 results 3.1 k-value assessment using durability 4 discussion 5 conclusion acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2023.42.0087 acta polytechnica ctu proceedings 42:87–93, 2023 © 2023 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague segmentation of pores in carbon fiber reinforced polymers using the u-net convolutional neural network miroslav yosifova,b,∗, patrick weinbergerb, bernhard plankb, bernhard fröhlerb, markus hoeglingerb, johann kastnerb, christoph heinzlc a university of antwerp, imec-visionlab, department of physics, universiteitsplein 1, 2610 antwerpen, belgium b university of applied sciences upper austria, research group x-ray computed tomography, stelzhamerstraße 23, 4600 wels, austria c university of passau, faculty of computer science and mathematics, innstraße 43, 94032 passau, germany ∗ corresponding author: miroslav.yosifov@fh-wels.at abstract. this study demonstrates the utilization of deep learning techniques for binary semantic segmentation of pores in carbon fiber reinforced polymers (cfrp) using x-ray computed tomography (xct) datasets. the proposed workflow is designed to generate efficient segmentation models with reasonable execution time, applicable even for users using consumer-grade gpu systems. first, u-net, a convolutional neural network, is modified to handle the segmentation of xct datasets. in the second step, suitable hyperparameters are determined through a parameter analysis (hyperparameter tuning), and the parameter set with the best result was used for the final training. in the final step, we report on our efforts of implementing the testing stage in open_ia, which allows users to segment datasets with the fully trained model within reasonable time. the model performs well on datasets with both high and low resolution, and even works reasonably for barely visible pores with different shapes and size. in our experiments, we could show that u-net is suitable for pore segmentation. despite being trained on a limited number of datasets, it exhibits a satisfactory level of prediction accuracy. keywords: deep learning, segmentation, u-net, computed tomography, pores, carbon fiber reinforced polymers. 1. introduction fiber reinforced polymers are getting increasingly important in our daily lives. as they are used in safety-critical areas such as aerospace, fast and reliable analysis of these materials is necessary. industrial x-ray computed tomography is a powerful tool to facilitate such an analysis, but it is still a huge effort to analyze, visualize and quantify the data generated in an xct scan [1]. deep learning has been successfully employed in fields like computer vision [2], object recognition [3], medical image analysis [4], face-recognition applications, and material inspection [5–7] over the past few decades. convolutional neural networks (cnn) are one of the highly popular classes of deep learning methods, which has showed a remarkable ability to handle classification and segmentation tasks for image, volume, and video. material science domain experts often need to segment pores in cfrp to characterize materials, especially with regards to pores. however, segmentation of images or volumes remains a challenging problem. despite the availability of various segmentation and classification techniques such as k-means [8, 9], watershed [10] and thresholding techniques, there are still several research efforts to improve segmentation methods [11, 12]. accurate and efficient segmentation plays a important role in ensuring the integrity of safety-critical materials. to achieve a robust and reliable segmentation, we have developed a modified version of the u-net architecture. our contributions in this work are: • the modified 3d u-net designed for efficient pore segmentation of xct scans from cfrp. • a network architecture and testing application in open_ia [13] which allows predictions on consumergrade gpus. • evaluation of the neural network’s performance on unseen data with varying resolutions and sizes. • visual analysis and quantitative comparison of the network’s predictions with the results (labels) generated from the otsu thresholding technique. 2. background and related work segmentation is defined as classification of an image into regions which contain similar characteristics (e.g., similar grey values). several segmentation methods can be applied for segmenting pores in xct scans of fiber-reinforced polymers. here a short overview is given over segmentation methods employed in this 87 https://doi.org/10.14311/app.2023.42.0087 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en m. yosifov, p. weinberger, b. plank et al. acta polytechnica ctu proceedings figure 1. (a) shows 3d rendering of sample 1, sample 2, and sample 3. (b), (c) and (d) respectively display axis-aligned 2d slices of sample 1, 2 and 3. area, the methods that we used in our work and those which we based our work on. 2.1. thresholding-based segmentation global thresholding techniques are used in a wide range of diverse applications. a detailed summary on thresholding techniques has been published by sezgin and sankur [14]. various kinds of threshold methods have been described and compared on carbon fiber polymer samples by rao et. al. [12]. in this paper we have decided to use otsu thresholding [15] to create label images for the training, testing and validation as shown by reh et al. segmenting pores [16]. in addition, it is a non-parametric threshold method. therefore, no additional user-depended parameters have to be defined. 2.2. convolutional neural networks convolutional neural networks now are the state-ofthe-art technique in biomedical image segmentation [17]. one of these cnn architectures is u-net. u-net was initially developed to address the segmentation task of 2d biomedical electron microscopic images and demonstrated remarkable success during the isbi cell tracking challenge in 2015 [17]. it has been successfully applied to various segmentation tasks, including cell, organ segmentation [18] and brain tumour detection [19]. later, u-net was further developed to handle 3d input shapes for segmenting 3d microscopy images [18]. 3. data characteristic in this paper, we have used two different samples of the same material which have been obtained from fiber reinforced composites. 10 × 10 × 2 mm3 carbon fiber reinforced polymer samples were cut out from an 360×510×2 mm3 plate. the plates for sample 1 were manufactured by a wet lay up process with a vacuum bag and for sample 2 a vacuum assisted resin infusion (vari) process was used. both sample plates were manufactured of six layers of an 2×2 twill weave pattern. figure 1 shows the training data (sample 1), the testing data (sample 2) and the testing data with lower resolution 10 µm3 (sample 3). volume graphics was used for registration of different resolution. xct scans were performed on a ge nanotom 180 nf xct-device. using an roi-ct mode, 3.3 µm voxel size can be reached. an additional low-resolution scan with 10 µm voxel size was performed on the vari material (sample 3). for further data processing, following cut-out dimensions of the datasets were used, shown in table 1. resolution size sample 1 3.3 µm 1300 × 900 × 976 sample 2 3.3 µm 1300 × 900 × 976 sample 3 10 µm 366 × 244 × 244 table 1. sample modality 88 vol. 42/2023 cfrp pore segmentation via u-net figure 2. the modified 3d u-net architecture [5]. 4. methods in the following section we describe our modified 3d u-net (section 4.1), the respective data pre-processing steps, as well as the used training process (section 4.2). 4.1. modification of 3d u-net u-net is a convolutional neural network that features high performance with a small amount of training, which can be applied both for 2d and 3d data. we implemented an architecture in python [20] using keras [21] with tensorflow [22] as a backend which was inspired by 3d u-net. the modified 3d u-net network contains two paths respectively called analysis and synthesis path, with a u-shaped architecture containing in total 46 layers (see figure 2). the modified 3d u-net architecture contains the input layer, encoder and decoder sections, a final convolution and transpose output layer. for up-sampling, the decoder subnetworks also contain one more convolution layer. first, encoder sections analyze the whole image, then the decoder sections produce and predict the segmented volume [5]. the modified 3d u-net has two additional convolutional and rectified linear unit (relu) layers in the decoder stages if we compare with original 3d u-net. rather than employing a softmax function, a sigmoid function was utilized in this case to reduce complexity in the final classification. additional layers were integrated into the decoder stages with the aim of enhancing the final prediction accuracy. as demonstrated in our previous study [5], the segmentation results were improved compared to the original u-net architecture. the modified 3d u-net uses an overlap-tile strategy to predict the labels for each test volume. the input volume has dimensions of 132 × 132 × 132, while the prediction size is 122 × 122 × 122. the variation in input and output sizes is due to a 5-voxel overlap in each dimension. this overlapping strategy enhances prediction accuracy, particularly at the borders of the sub-volumes [17]. the neural network model (the weights) was saved in onnx (open neural network exchange) format [23] for use in open_ia [13]. 4.2. data pre-processing, training and testing the preprocessing pipeline involved several steps, including volume normalization, mirror-padding with 5 extra voxels, and division of the ct scan into subvolumes with a 5-voxel overlap in all directions. the labeled image, processed using otsu thresholding, was also split into sub-volumes for training. the primary reason for splitting xct data into sub-volumes with a size of 132 × 132 × 132 is to optimize the training time and augment the training data, thereby reducing the need for scanning multiple specimens. for training, only the sub-volumes from sample 1 were used, which were further divided into three parts: 64 % (358 samples), for training, 20 % (112 samples) for testing, and 16 % (90 samples) for validation. to identify optimal training parameters, a heuristic hyperparameter tuning with talos [24] was employed. in the hyperparameter tuning of our training we tested various combinations of learning rates, optimizers, and epochs to achieve an optimum between accuracy and training time. the hyperparameter tuning process yielded the best outcomes when employing the adam optimizer [25] with a learning rate of 4e-5. the training process, conducted on a workstation powered by an nvidia quadro rtx 6000 with keras 2.3.0 and tensorflow-gpu 2.1.0, was successfully completed in approximately 1 hour, involving 10 epochs and a batch size of 3. to distinguish classes with significant imbalances during training, the dice coefficient [26] 89 m. yosifov, p. weinberger, b. plank et al. acta polytechnica ctu proceedings figure 3. the visualization consists of 2d and 3d representations of a sub-volume extracted from sample 2. (a) displays the superimposed predictions (yellow) on the input volume and the otsu thresholding (red) in a 3d visualization. (b), (c) and (d) respectively display axis-aligned 2d slices of predictions (yellow) overlaid on the input volume and otsu thresholding (red). orange colour represents the agreement between the prediction and otsu thresholding regions. additionally, the detail images provide a closer look at a specific region marked in blue, magnified four times. function was employed as loss function. following the training process, the obtained results were evaluated using the test dataset. in the training process, the input size is fixed at 132 × 132 × 132; however, during the testing stage, data of any size can be applied. in our case, we segmented the entire xct volumes with the trained model, as well as exemplary sub-volumes. for the prediction phase, a different system was employed, featuring a consumer-grade gpu (nvidia geforce gtx 1080). predicting the sample 2 (1300× 900 × 976) using open_ia and the onnx runtime took approximately four minutes. this time includes the normalization of the input, splitting it into subvolumes, performing the prediction, and merging the sub-volumes into the final result. 5. results and discussion in this section, we present some of the results of the predictions in comparison to otsu thresholding. we measure the segmentation accuracy using the dice coefficient function [26]. this function computes the dice similarity coefficient between the prediction and reference segmentations. training and validation accuracy were both determined at appr. 99 %. on the training process, those values were continuously growing, and the final model achieved satisfactory results. the result of the evaluation with the test dataset was a dice coefficient of 0.9822 and an accuracy of 0.9990. the grey values were normalized to be in the range from zero to one for all of the samples before the training and testing process. otsu thresholding segmentations (labels) are binarized to zero or one (white values = 1, black values = 0) for all the samples. for each voxel, the prediction delivers a value between 0 and 1, denoting the probability for this voxel to be a pore. we have used our prediction without any postprocessing to show the probability of the predicted voxels. 5.1. sub-volume prediction and visualization the neural network model in open_ia was utilized to perform segmentation on a sub-volume extracted from sample 2, which had dimensions of 122 × 122 × 122. the obtained segmentation results are presented in figure 3. this subsection of the dataset has been chosen since it contains many different shapes of pores. according to the predictions, all kind of pores with different shapes are segmented both with otsu thresholding and with our modified 3d u-net approach. for bigger pores, the overall results seem quite similar with both methods. in the magnifications, it becomes obvious that the otsu thresholding segmentations are 90 vol. 42/2023 cfrp pore segmentation via u-net figure 4. original input data slice from sample 2 (a), respectively show 2d slices of otsu thresholding (red) (b) and prediction (yellow) (c) overlaid on an input volume slice. figure 5. zoomed version of region 1 and 2: input data slice (a), otsu thresholding (red) segmentation results overlaid on input data slice (b), prediction (yellow) overlaid on input data slice (c). the detail images showcase a closer view of the edge region marked in blue, magnified 12 times and 4 times respectively. under-segmented, especially with long and thin (figure 3c pores and small pores (see figure 3b. the neural network predictions are performing better than otsu thresholding as seen by visual inspection. 3d visualization of the sub-volume and its prediction with overlaid otsu thresholding is shown in figure 3a. differences between otsu thresholding and predictions are clearly visible in the zoomed region in the 3d image. the predicted area covers most of the matching otsu thresholding region. according to the 2d and 3d visualizations of the sub-volume shown in figure 3, both show that the results from otsu thresholding are under-segmented when compared to predictions of the neural network model. 5.2. prediction and visualization of sample 2 this section presents the prediction outcome for sample 2 (see figure 4). the goal is here to show that the model is also able to effectively segment unseen data. train data and test data have an isotropic resolution of 3.3 µm. in figure 4c, yellow colors (prediction) show that long thin pores are segmented much better with the neural network. otsu thresholding is not as effective for this kind of pore segmentation as the neural network. in figure 4b, region 2 clearly shows that otsu thresholding produces under-segmented results. the qualitative result of prediction is similar to the sub-volume segmentation presented in section section 5.1. sample 2 has many different shapes of pores such as rounded pores (big and small), long thin and some big complex pores. segmentation results differ substantially between the otsu technique and the neural network prediction depend on the shapes of the pores. region 1 and 2 of figure 4 are shown in a magnified version in figure 5. the original slice images are transparently overlaid with the otsu thresholding segmentation and the neural network prediction. region 1 contains two pores with different characteristics: one is clearly visible, while the other one is only barely visible. segmentations are almost the same and reasonably good for the clearly visible pore in both methods. for barely visible pores, it is hard to evaluate which method corresponds better with reality by visual inspection. there is not a sharp edge between pore and background transitions and thus it is also hard to make decisions for the network. even so, it is 91 m. yosifov, p. weinberger, b. plank et al. acta polytechnica ctu proceedings figure 6. input data slice from sample 3. (a), and (b), (c) respectively show 2d slices of otsu thresholding and prediction overlaid on an input volume slice from sample 2 (red= otsu thresholding, yellow=predictions). figure 7. zoomed version of region 3: input data slice (a), otsu thresholding overlaid on input data slice (b), prediction overlaid on input data slice (c) (red= otsu thresholding, yellow=predictions). the detailed images display a 2x zoom of the edge region highlighted in blue. also hard to decide which area should be segmented as pore for a human. 5.3. prediction and visualization of sample 3 the model trained on 3.3 µm data is also successfully working on datasets with different resolution. to show that, sample 3 is segmented in open_ia with the neural network and otsu thresholding method, the resulting segmentation outcomes are presented in figure 6. however, there are some limitations with applying the neural network on the different resolution data: the pores (long and thin) are barely visible, much less visible than in the datasets with 3.3 µm resolution, and the transition is more blurred between pore and background. the results show that the prediction of the neural network is also successfully working for the segmentation of different shapes of pores on different resolutions (see figure 6). another result is that the otsu thresholding over-segments pores (see figure 7b). the main challenge in this scenario is the segmentation of long and thin pores, particularly in cases where they are in close proximity to one another (see figure 7). proper segmentation is challenging for the network model and other methods. nevertheless, results are impressive and promising for future improvements. 6. conclusions this paper demonstrates that the modified 3d u-net architecture achieves satisfactory performance, even when trained on a limited training dataset. through visual inspection and a comparative assessment with the findings, the modified 3d u-net performs better in certain areas compared to otsu thresholding. significantly, the generated model with 3 µm resolution also demonstrated a satisfactory performance on the datasets with 10 µm resolution. it is important to note that during training, only the 3 µm resolution (sample 1) was utilized. quality assessment and comparisons in this workflow are conducted through manual visual inspection. for future work, we plan to measure the quality numerically by using xct simulated data on cfrp. however, to measure the quality of estimations and the performance of the deep learning model, the generation of accurate ground truth is a necessity. therefore, we will improve our reference segmentation by incorporating both xct images segmented by human experts and xct simulation techniques. another important point is the uncertainty determination for generation of xct images (radiographs), reconstruction, segmentation and deep learning training. each of these steps can influence the final image segmentation or estimation. to improve segmentation accuracy, uncertainty sources and their levels have to be identified. additionally, we intend to explore the assessment of algorithm reliability through the creation of artificial xct images containing known pore sizes. moreover, we will investigate the impact of introducing noise or image blurring on the algorithm’s sensitivity to various side effects. 92 vol. 42/2023 cfrp pore segmentation via u-net acknowledgements this research was co-financed by the european union h2020-msca-itn-2020 under grant agreement no. 956172 (xcting). references [1] c. heinzl, s. stappen. star: visual computing in materials science. computer graphics forum 36:647– 666, 2017. https://doi.org/10.1111/cgf.13214 [2] d. cireşan, u. meier, j. schmidhuber. multi-column deep neural networks for image classification. proceedings / cvpr, ieee computer society conference on computer vision and pattern recognition ieee computer society conference on computer vision and pattern recognition 2012. 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https://doi.org/https://doi.org/10.1016/j.health.2022.100098 https://keras.io https://tensorflow.org https://onnx.ai http://github.com/autonomio/talos https://doi.org/10.48550/arxiv.1412.6980 acta polytechnica ctu proceedings 42:87–93, 2023 1 introduction 2 background and related work 2.1 thresholding-based segmentation 2.2 convolutional neural networks 3 data characteristic 4 methods 4.1 modification of 3d u-net 4.2 data pre-processing, training and testing 5 results and discussion 5.1 sub-volume prediction and visualization 5.2 prediction and visualization of sample 2 5.3 prediction and visualization of sample 3 6 conclusions acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2024.51.0081 acta polytechnica ctu proceedings 51:81–93, 2024 © 2024 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague an examination of driving speed: the role of perceivable road characteristics in towns martin schánoa,∗, josef novýb, jiří zeisekc a czech technical university in prague, faculty of transportation sciences, department of forensic experts in transportation, pohraniční 1288/1, 405 02 děčín 2, czech republic b czech technical university in prague, faculty of nuclear sciences and physical engineering, department of software engineering, pohraniční 1288/1, 405 02 děčín 2, czech republic c czech technical university in prague, faculty of transportation sciences, děčín department, pohraniční 1288/1, 405 02 děčín 2, czech republic ∗ corresponding author: schano@fd.cvut.cz abstract. understanding the subjective perspectives of drivers is paramount in evaluating the factors affecting driving speed in built-up areas. building on our previous work, where a wide set of infrastructure attributes was analysed, this paper delves deeper into the realm of driver perception. it seeks to answer how drivers’ observations and awareness of their surroundings influence their speed choices. we reconduct our previous analysis, emphasizing the driver’s subjective viewpoint, contemplating the visible and perceivable elements of the road infrastructure rather than their objective presence. this research aims to garner insights into how these subjective elements impact speed choices. this research has uncovered a number of interesting factors that have not been previously used in the calculation of road vehicle speeds. this more nuanced understanding is expected to lead to enhanced safety and efficiency in road traffic management, ensuring the road design aligns seamlessly with driver perception and behaviour. keywords: speed of road vehicles, traffic engineering, road infrastructure. 1. introduction modern traffic engineering has consistently focused on road designs that mitigate the risks of accidents, and in cases where they occur, reduce their aftermath. the intricate role of road infrastructure elements is fundamental, not just in their direct impact but also in their latent effects on driver behaviour. in the constantly evolving discourse surrounding road design parameters and their subsequent influence on drivers, a predominant challenge is the nuanced nature of human perception. each driver, unique in their perception, reacts differently to distinct infrastructure parameters, a complexity further amplified by regional variations in driver populations. while our previous research aimed at determining a comprehensive set of infrastructure parameters and understanding their correlation with driving speeds, there remained a gap in understanding the subjective views of drivers and how their perceptions translate into their chosen speeds. this paper seeks to bridge this gap. instead of only focusing on objective parameters like the sheer presence of infrastructure elements, we delve deeper into how drivers perceive and interpret their surroundings. this research undertakes a closer examination of how the perceptibility of road features influences speed choices. the nuances of driver perception and behaviour are indispensable, as they hold the key to designing roads that align seamlessly with users’ expectations and behaviours, thereby enhancing safety and efficiency. 2. literature review understanding the factors that influence a driver’s choice of speed is crucial for a plethora of reasons. for one, vehicle speed is a fundamental factor in both the frequency and severity of road accidents [1]. a slight increase in speed can disproportionately elevate the risk of severe injuries and fatalities in the event of an accident. hence, by investigating the reasons behind a driver’s speed selection, we can better strategize ways to moderate it, ensuring roads are safer for all users. the relationship between a driver’s choice of speed and the road environment has been a cornerstone of transportation research. such insights are invaluable to public administration, road management agencies, and urban planners. they provide a roadmap for informed decisions that can significantly reduce trafficrelated incidents [2, 3]. moreover, in our era of rapid technological advancements, these findings are not confined to the traditional aspects of road management. they play a critical role in supporting the mathematical modelling of road user behaviour. such models can capture complex interactions between various road users, allowing 81 https://doi.org/10.14311/app.2024.51.0081 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en m. scháno, j. nový, j. zeisek acta polytechnica ctu proceedings figure 1. traffic detector closed; traffic detector open; traffic detector installed and measuring. for simulations that can test the efficiency and safety of road designs, traffic rules, and management strategies before they’re implemented in the real world [4]. while there has been substantial research on roads outside of municipalities [5], largely attributed to their lower environmental complexity [6, 7], the intricate and dynamic nature of municipal areas poses unique challenges. multiple modes of transport, the presence of pedestrians, varying road designs, and frequently changing traffic patterns make urban areas particularly complex. this complexity underscores the need for a deeper understanding of speed choices in these areas. many researchers concur that to truly create safer and more efficient urban transport systems, the issue of driver speed selection within municipal boundaries warrants more extensive exploration [2, 4]. [8] aimed to identify roadway, roadside, and traffic control device variables that may affect the speed of drivers on suburban streets. this includes how speed may vary on different parts of the road, like curves versus straight sections. multiple different groups of variables were investigated, among them were for example curve radius, angle and length, lane width, superelevation, median, bike lines, roadside development, roadside density, access density, traffic control devices. some of the key findings were that posted speed limit, lane width, and road development (like nearby buildings and houses) significantly influenced speed. [9] in their study capture variables like road attributes, traffic characteristics, and environmental factors. they introduced a model termed the ordered variant of the fractional split model. the model results highlight the role of various street characteristics including number of lanes, presence of parking, presence of sidewalks, vertical grade, and bicycle route on vehicle speed proportions. the researchers recognized that even after accounting for many observable factors in their models (for local roads and arterial roads), there’s still some variation in speed distributions that the models can’t capture. they postulated that these “unexplained variations” could be due to site-specific unobserved effects. in other words, each road or site might have its own unique characteristics that influence vehicle speeds. these could be factors like the presence of schools or hospitals nearby, unique local driving cultures, recent incidents affecting driver behaviour, etc. the researchers cautioned that if these unobserved effects were ignored, the resultant models would have biased effects. the uncovering of these site-specific effects is the goal of this paper. 3. methods 3.1. data acquisition the data collection method adopted for this study replicates the approach utilized in our previous research [10]. briefly, an rtb topo.box microwave traffic detector, visible in figure 1, was employed to measure vehicles in both driving directions. this device not only gauges the speed of the vehicles but also discerns between different vehicle categories and measures pass-by noise. designed to be discreet, especially in urban settings, it can be confidently stated that the detector did not influence drivers’ behaviour due to its conspicuousness. the efficacy and accuracy of this detector, as well as its potential limitations in different traffic volumes, have been detailed previously [10]. in this study, similar to our previous work, we did not distinguish between the passage of the integrated rescue system or other vehicles that inherently travel at higher speeds. 3.2. site selection our research incorporated measurements from a total of 30 distinct sites. each driving direction at these locations was evaluated separately, resulting in 60 unique speed measurements. such a granular approach allows for a more nuanced understanding of driver behaviour, taking into account the distinct characteristics of each site. historically, the data collection was initially geared towards identifying and remediating at-risk sites on the road network, examples of which can be seen in figure 2. as such, the locations were ones where there was an existing suspicion of drivers’ non-compliance with the speed limit or other potential road trafficrelated risks. while this may seem limiting, it offers 82 vol. 51/2024 an examination of driving speed figure 2. details of a small selection of the measured locations. (note: the list of sites provided above is illustrative and not exhaustive. for a complete list of sites, please see the appendix complete list of sites at the end of this article.) a unique perspective, shedding light on areas that are potentially more prone to speed-related incidents. however, the distribution of the sites is not incidental and presumably carries a particular unknown bias. the sites are predominantly situated in areas with heightened tourist activity, especially in proximity to the bohemian switzerland national park. this location choice is significant as areas with increased tourist traffic might witness drivers who are unfamiliar with local roads and might not drive “by memory”. therefore, the behaviours recorded might differ from those on roads dominated by local traffic. road infrastructure in the czech republic is categorized into classes based on its purpose and transport importance. in this study, we primarily focused on: • class i roads, which cater mainly to long-distance and interstate transport, • class ii roads, designated for inter-district traffic, • class iii roads, which serve the purpose of linking municipalities with each other or connecting them to other main roads. most of the chosen sites represent thoroughfares of smaller towns and municipalities. these are typically directionally undivided roads with a maximum permissible speed of 50 kph. such roads are especially interesting as they offer a mix of both local and non-local traffic, presenting varied driving patterns. incorporating this larger and more diverse dataset, especially with the application of spearman’s correlation coefficient, enriches our analysis. it provides more robust insights into the correlation between infrastructure elements and the speed choices drivers make. this, in turn, assists policymakers and traffic planners in making informed decisions to enhance road safety. 3.3. set of tested attributes in this study, various attributes of measured locations were tested, which were believed to influence the driver’s subjective perception of safety and consequently their intuitive choice of vehicle speed. attributes were chosen based on the results of our previous research [1], emphasizing the decoding of several ambiguities or intriguing findings. an example from previous results is the negligible influence of bus stops located directly in the driving lane of the measured vehicle, but a significant detected influence of bus stops in a separate bay, i.e., outside the driving lane. on the other hand, an interesting observation from the previous data set is that there was no correlation between lane width and driving speed, contrary to what the literature suggests. new attributes were carefully chosen to provide multiple ways to redefine these intriguing factors. through this approach, we hope to uncover specific patterns and bring deeper understanding as to why certain factors influence or do not influence driver behaviour. in line with this, these attributes were chosen to be observable and perceptible from the driver’s perspective. for instance, if the driver does not see an intersection, a door, or an exit from their view, these elements were not included in the evaluation. 83 m. scháno, j. nový, j. zeisek acta polytechnica ctu proceedings figure 3. speed distribution across the measured sites. the assessed attributes can be categorized into several groups: (1.) pedestrian related attributes • nearest sidewalk to driving lane [m]: indicates how close the nearest sidewalk is from the driving lane. • building and minor structures coverage [%]: the percentage of visible area covered by buildings and other small structures in relation to the total visible area. • urban amenities coverage [%]: percentage coverage of urban amenities like benches, streetlights, and trash bins. • number of view-obstructing objects: objects of sufficient size that could potentially hide a smaller person. • number of pedestrian entry points: places from which the entry of a pedestrian can be reasonably expected (doors, gates, emerging paths, etc.). (2.) vehicle related attributes • passing sight distance [m]: describes the distance to the furthest point of the opposite driving lane that is fully visible from the driver’s perspective. • stopping sight distance [m]: describes the distance to the furthest point of the driver’s own lane that is fully visible from the driver’s perspective. • road width for legal driving [m]: width of the driving lane or strip where a driver can legally operate. • road width for safe driving [m]: the drivable and paved part of the road, regardless of its legal status. • road width for emergency driving [m]: the actual width available to a driver during an emergency. exiting this space could lead to vehicle damage or leaving the roadway irreversibly. • number of vehicle entry points: points from which a vehicle entry can be reasonably expected (gate, driveway, etc.). (note: the list of attributes provided above is illustrative and not exhaustive. for a complete list of attributes and detailed descriptions, please see the appendix complete attribute table at the end of this article.) in total, 36 unique factors were defined, many of them measured at various distances from the measurement site, amounting to 128 evaluated attributes for each measured driving direction. 3.4. data processing to determine the relationships between the attributes of the measured sites and the chosen driving speed (distribution vissible in figure 3), the spearman correlation coefficient was used. this coefficient is standardly employed to measure the strength and direction of the relationship between two continuous variables and is suitable for revealing both linear and nonlinear correlations. the primary reason for its use was the expectation of a nonlinear relationship between the variables. an example can be the sight distance to oncoming vehicles. with increasing sight distance, the driver’s sense of safety might increase, and thus the driving speed. however, this effect will not linearly increase to extreme values. for instance, the difference in driver’s behaviour when changing the sight distance from 50meters to 150meters will be more pronounced than the difference when changing from 1 050meters to 1 150meters. if there is a strong positive correlation between site attributes and the speed of passing vehicles, it can be assumed that an increase in the attribute value will lead to a higher speed of the passing vehicles. conversely, a strong negative correlation would indicate that an increase in the value of a given attribute would result in a reduced driving speed. the spearman coefficient is also robust against extreme values, which can often occur in the set of tested attributes. 84 vol. 51/2024 an examination of driving speed table 1. correlation coefficients for pedestrian related attributes. due to the presumed variations in behaviour of different types of drivers, it was decided to divide the speeds into three categories: (1.) the 65th percentile of the speed of vehicles of all types, representing the regular speed of the majority of vehicles, (2.) the 85th percentile of the speed of vehicles of all types, representing faster driving vehicles that appear more frequently, and (3.) the 98th percentile of the speed of vehicles of all types, representing extreme speeds that are only occasionally observed at a given location. the speed distribution across the measured sites can be seen in the figure below. the sites have a similar character and none of them show extremely different speeds or relationships between individual speed percentiles. 4. results the results are presented in the form of tables divided according to the nature of the attributes into attributes related to pedestrians, vehicles, road, and speed. for increased clarity, a colour scale is used starting at a coefficient of 0 in red colour (i.e., no correlation was found) to 0.6 in green colour (i.e., strong correlation). the interpretation of these coefficients is described in figure 4. positive correlation coefficients represent positive relationships between quantities (i.e., an increase in one correlates with an increase in the other) and negative coefficient values represent negative relationships (i.e., an increase in one correlates with a decrease in the other). first set of results is related to pedestrians (see table 1). nearest sidewalk was not proven as a relevant element in this or previous study. however, it should be noted that the presence of sidewalks is figure 4. colour palette used in correlation evaluation tables. related to road width attributes, which proved to be more relevant (see table 2). likewise, attributes related to pedestrian crossings were not shown to be relevant. slightly higher correlations were identified in attributes related to bus stops. positive correlations were in relation to distance, i.e., the closer, the lower the speed. negative correlations were related to size, i.e., the larger and more equipped the stop, the lower the speed. the same was true for weighted distance. this is an attribute taking into account both size and distance. the value of the attribute increased with the increasing size of the stop and decreased with increasing distance from the stop. however, all correlations related to bus stops were relatively low and therefore less conclusive. this clarified the higher correlation with bus stops (0.347) identified in previous work by the authors, which was not confirmed with this expanded data set and is believed to be inaccuracy due to the smaller data set. the highest correlations of attributes related to pedestrians were identified with the number of viewobstructing objects, number of pedestrian entry points, and building and minor structure coverage. in all cases, a clear correlation was identified in the section approximately 50 meters before the measured site. lower correlations with these attributes at other distances are attributed by the author team to the fact that the placement of the measuring device is not random and its placement in a higher-risk location, i.e., a place with a higher number of view-obstructing 85 m. scháno, j. nový, j. zeisek acta polytechnica ctu proceedings table 2. correlation coefficients for road related attributes. table 3. correlation coefficients for vehicle related attributes. objects and pedestrian entry points, is always preferred. the effect of these attributes is more visible at lower percentiles. this is expected as the faster drivers tend to ignore a lot of potential risks. within vehicle-related attributes (see table 3), lower correlations can be seen with intersection-related attributes and safety barriers attribute. minimal correlations were identified with the presence and quality of road marking. slightly higher correlations can be seen with the number of vehicle entry points, which represents the number of intersections, driveways, garage exits, etc. by far the highest correlations within this category of attributes were identified with stopping sight distance and passing sight distance. of these two, stopping sight distance had a slightly higher influence. for higher percentiles, the correlation value further increases slightly. regarding road-related attributes (see table 2), even in this expanded data set, road width for legal driving (in most cases single line width) was shown to be insignificant. significance gradually increases with road width for safe driving, which is the drivable and paved part of the road, regardless of its legal status. the highest correlation among the road width attributes was identified with road width for emergency driving. this is the actual width available to a driver during an emergency (paved or unpaved). it is usually the width between fixed obstacles, a slope, a guardrail, etc. exiting this space could lead to vehicle damage or leaving the roadway irreversibly. table 4. correlation coefficients for speed related attributes. curvature of the visible part of the road was also identified as a significant attribute. with increasing percentile, the influence of this attribute increases. the most significant attribute in this section was the length of the straight section. however, it is interesting that only the length of the section being entered into has a significantly strong correlation, not the length of the section being exited. this noticeable difference is not caused by any observable trend in the measured locations. from the last part of the attributes related to speed (see table 4), a relatively low effect of changes in the allowed speed can be seen. in this part, it is necessary to say that many measured locations were on the outskirts of towns, as well as many deep inside towns. several unsurprising trends can be observed such as that vehicles drive faster than they would normally through the measured location when the allowed speed is reduced close before the measured location. similarly, for higher percentiles, there is a 86 vol. 51/2024 an examination of driving speed reduction in compliance with traffic signs regarding the maximum allowed speed. from the results presented above, the variance in the importance of individual factors for different groups of drivers represented by different percentiles is evident. for ease of interpretation of these results, we have included below a summary of the most significant factors affecting the various driver groups. 5. discussion this publication has undoubtedly brought new insights into the area of driver’s choice of driving speed, but it has also brought a number of challenges. this work was significantly complicated by the difficulty of defining the correct attributes across a heterogeneous environment. particularly in the czech republic, the road network often contains elements that fundamentally complicate the comparison of various factors affecting driving speed, the comfort or feeling of safety for the driver. examples of such elements include the absence of traffic signs or absence or wear of road markings. in some towns there are extensive sections of roads without proper differentiations between areas for vehicle movement, parking, or pedestrian traffic. sometimes, there are even contradictions between traffic signs and road markings. spaces within municipalities are often not significantly different from the surrounding roads. especially in the high-risk locations used in this study, it is common for some important elements of the urban street network to be missing. this offers the potential to evaluate the absence of individual factors on one hand, but on the other hand, it presents a significant challenge in the proper definition of such factors so that they are measurable by the same rules at all locations. the authors originally planned to subsequently use the data and results of this study in the design of mathematical models for calculating speed characteristics in complex and heterogeneous conditions. this activity will certainly be performed, however, based on the increasing complexity with the growing number of locations and their variability, there arises a requirement for a deeper understanding of the observed phenomena. as part of continuing research activities, the author’s team plans parallel activities using a simulated virtual environment with full control over the characteristics of the street network, including associated risks like pedestrian activity. this will fundamentally increase the number of assessed attributes and, above all, at least partially automate their collection. this will be necessary because the depth of the issue at hand requires a significantly larger amount of data for proper and reliable evaluation of the observed phenomena. 6. conclusions in this article, a number of factors that influence the driver’s decision-making have been uncovered. factor table 5. the highest identified correlation coefficients. that appears as relevant in a multitude of publications, the width of the driving lane, was again manifested as insignificant based on the available data. it seems that other factors associated with the reduction of lane widths, such as narrowing the roadway, directional division of lines, inclusion of a parking lane which adds a number of objects generating pedestrian activity right next to the driving lane and at the same time adding view-obstructing objects, are much more significant than the reduction of the lane width itself. among the most significant identified factors (see table 5) are undoubtedly the number of viewobstructing objects, number of pedestrian entry points, building and minor structures coverage, passing sight distance, stopping sight distance, curvature of the visible part of the road, length of the straight section behind the measured location, but not before. based on these attributes, we believe that strong assumptions can be made about driver’s behaviour in relation to driving speed. however, we recommend adding a number of other factors with lower but still relevant correlation (at least 0.2+) to the evaluation, as well as to any potential mathematical modelling of speed parameters. regarding the individual assessed speed percentiles, it can be said that with higher driver speeds, the relevance of factors such as number of view-obstructing objects, number of pedestrian entry points, and other factors associated with pedestrian activity decreases. in contrast, the importance of attributes such as passing sight distance, stopping sight distance, curvature of the visible part of the road, length of the straight section behind, increases, which represents a higher ability to safely achieve higher speeds (safely in relation to the individual vehicle and its travel, not to the surroundings and other participants). therefore, it can be claimed that although the set of factors affecting speed for these individual types of drivers overlaps considerably, it is not the same. by modifying certain road infrastructure factors, it is possible to target the reduction of predominantly one group with less effect on the other. conversely, there are also factors whose modification may aim to reduce average speeds, may not be so effective at reducing the highest speeds. 87 m. scháno, j. nový, j. zeisek acta polytechnica ctu proceedings acknowledgements this paper was supported by the project “simuluk: simulační prostředí v ústeckém kraji” (cz.02.01.01/00/23_021/0009003) funded by the programme johannes amos comenius of the ministry of education, youth, and sport, supported by eu funds. references [1] j. de oña, l. garach, f. calvo, t. garcía-muñoz. relationship between predicted speed reduction on horizontal curves and safety on two-lane rural roads in spain. journal of transportation engineering 140(3):04013015, 2014. https: //doi.org/10.1061/(asce)te.1943-5436.0000624 [2] v. martinelli, r. ventura, m. bonera, et al. effects of urban road environment on vehicular speed. evidence from brescia (italy). transportation research procedia 60:592–599, 2022. https://doi.org/10.1016/j.trpro.2021.12.076 [3] m. bassani, d. dalmazzo, g. marinelli, c. cirillo. the effects of road geometrics and traffic regulations on driver-preferred speeds in northern italy. an exploratory analysis. transportation research part f: traffic psychology and behaviour 25:10–26, 2014. https://doi.org/10.1016/j.trf.2014.04.019 [4] m. budzynski, a. gobis, l. guminska, et al. assessment of the influence of road infrastructure parameters on the behaviour of drivers and pedestrians in pedestrian crossing areas. energies 14(12):3559, 2021. https://doi.org/10.3390/en14123559 [5] m. abuaddous. operating speed prediction models for tangent segments: a brief review. civil engineering journal 7(12):2150–2164, 2021. https://doi.org/10.28991/cej-2021-03091784 [6] d. d. dinh, h. kubota. profile-speed data-based models to estimate operating speeds for urban residential streets with a 30 km/h speed limit. iatss research 36(2):115–122, 2013. https://doi.org/10.1016/j.iatssr.2012.06.001 [7] a. thiessen, k. el-basyouny, s. gargoum. operating speed models for tangent segments on urban roads. transportation research record 2618(1):91–99, 2017. https://doi.org/10.3141/2618-09 [8] k. fitzpatrick, p. carlson, m. brewer, m. wooldridge. design factors that affect driver speed on suburban streets. transportation research record 1751(1):18–25, 2001. https://doi.org/10.3141/1751-03 [9] n. eluru, v. chakour, m. chamberlain, l. f. miranda-moreno. modeling vehicle operating speed on urban roads in montreal: a panel mixed ordered probit fractional split model. accident analysis & prevention 59:125–134, 2013. https://doi.org/10.1016/j.aap.2013.05.016 [10] m. scháno, j. nový, o. smišek. infrastructure elements influencing the driving speed of road vehicle drivers in towns. in 2023 smart city symposium prague (scsp), pp. 1–7. 2023. https://doi.org/10.1109/scsp58044.2023.10146116 88 https://doi.org/10.1061/(asce)te.1943-5436.0000624 https://doi.org/10.1061/(asce)te.1943-5436.0000624 https://doi.org/10.1016/j.trpro.2021.12.076 https://doi.org/10.1016/j.trf.2014.04.019 https://doi.org/10.3390/en14123559 https://doi.org/10.28991/cej-2021-03091784 https://doi.org/10.1016/j.iatssr.2012.06.001 https://doi.org/10.3141/2618-09 https://doi.org/10.3141/1751-03 https://doi.org/10.1016/j.aap.2013.05.016 https://doi.org/10.1109/scsp58044.2023.10146116 vol. 51/2024 an examination of driving speed a. appendices a.1. complete list of sites 89 m. scháno, j. nový, j. zeisek acta polytechnica ctu proceedings 90 vol. 51/2024 an examination of driving speed a.2. complete attribute table attribute name attribute description nearest sidewalk to driving lane [m] nearest sidewalk to driving lane [m] is the distance between the closest sidewalk and the farthest edge of the driving lane, that is, the width available to the driver between the closest sidewalk. number of view-obstructing objects the number of view-obstructing objects is a metric that quantifies the total count of visible objects along the roadside that are tall or wide enough to obstruct a driver’s direct line of sight, potentially concealing a child from view. this includes, but is not limited to, parked vehicles, large street furniture like advertising boards or bus shelters. number of pedestrian entry points the number of entry points visible from the driver’s perspective on the right side of the road such as doors and gates. building and minor structures coverage [%] measured within the driver’s field of view. the specific value of this attribute corresponds to the ratio of the horizontal plane of view from 0-100 [%] and the weight coefficient. the weight is 2 for larger objects, especially buildings, and 1 for smaller objects, representing a maximum value of 300 [%]. urban amenities coverage [%] coverage by urban amenities elements in the driver’s field of view [%] such as bench, trash can, flower pot. distance from bus stop [m] the distance to the nearest visible bus stop. size of the nearest bus stop [-] the size of the nearest bus stop by score. scoring includes various elements related to the bus stop’s location, design, and visibility features. the higher the value, the bigger and more equipped the bus stop is. 91 m. scháno, j. nový, j. zeisek acta polytechnica ctu proceedings attribute name attribute description weighted distance from bus stop [m] the combination of the size and distance from the nearest bus stop entails that the larger the size, the greater the weighted distance. conversely, the further the stop, the smaller the weighted distance. distance to pedestrian crossing or crossing point [m] the distance to the nearest visible pedestrian crossing or crossing point [m]. size of pedestrian crossing or crossing point [-] the size of the nearest pedestrian crossing or crossing point. scoring includes various elements related to crossing’s design, safety features, and visibility. the more elements the higher the value. weighted distance to pedestrian crossing or crossing point [m] the combination of the size and distance from the nearest pedestrian crossing or crossing point is such that the larger the size, the greater the weighted distance. conversely, the further the crossing, the smaller the weighted distance. passing sight distance [m] the passing sight distance represents the maximum visible length of the road from the driver’s position to opposite line. stopping sight distance [m] the stopping sight distance represents the maximum visible length of the road from the driver’s position to the driver’s line. number of vehicle entry points the number of vehicle entry points quantifies the potential locations along a roadway where vehicles may enter the traffic flow, including driveways, side streets, and other access points.. observable area of the nearest intersection [m2] the observable area of the nearest intersection measures the total visible area from the driver’s perspective at the closest intersection, encompassing the full expanse of the junction where two or more roads meet, including pedestrian crossings and merging lanes. distance to the nearest intersection [m] the distance to the nearest intersection indicates the linear distance from a given point on the road to the closest intersection ahead or behind. weighted distance to the nearest intersection [m] the weighted distance to the nearest intersection adjusts the plain distance measurement by accounting for the size and complexity of the intersection. proportional part of the road covered by safety barriers [%] this attribute describes the percentage of the road’s length that is equipped with safety barriers designed to prevent vehicles from leaving the roadway or colliding with obstacles and also any barriers preventing pedestrians from entering the roadway. vehicle barriers have higher value than pedestrian barriers. the value of the attribute is the sum of both visible pedestrian and visible vehicle barrier length. presence and quality of horizontal road marking [-] the presence and quality of horizontal road marking assesses both the existence and the condition of painted road markings such as lane lines, pedestrian crossings, and other symbols. additional features like reflective surface add to the value of the attribute. road width for legal driving [m] the width of the driving lane or band that a driver can legally use. this is the width between the road marking separating the opposite direction and the road marking or the edge of the paved road. in the absence of a separating lane marking, it refers to the width between the v4 road marking or the edge of the paved road. road width for safe driving [m] the width of the paved and drivable part of the road, regardless of whether the driver can legally move on it. this is space for emergency maneuvers, assuming the driver might ignore traffic rules. 92 vol. 51/2024 an examination of driving speed attribute name attribute description road width for emergency driving [m] the width practically available to a driver in a crisis situation. going beyond this space ends either with vehicle damage or irreversible exit from the road. typically, it includes the unpaved road section, the space between the nearest solid obstacles (including high curbs), slope fills, or cuttings, etc. curvature of the visible part of the road the curvature of the visible part of the road describes the degree and extent of any bends or turns in the segment of the road that is visible to the driver. the value is calculated as the sum of all bends or turns in degrees divided by the total length of visible part of the road. length of the straight section [m] the length of the straight section measures the distance over which the road extends in a straight line without any curves. route descent (-) / ascent (+) this attribute records the gradient of the road in terms of descent or ascent, expressed as a negative or positive value respectively. change in maximum allowed speed [km/h] the change in maximum allowed speed denotes any official alteration in the speed limit, either an increase or a decrease, which drivers must adhere to. the decrease in maximum allowed speed in the vicinity has a negative value of the attribute. the increase has a positive value. distance from the speed limit change before [m] the distance from the speed limit change before measures the length of road prior to reaching the measured site and after the speed limit change. distance from the speed limit change after [m] the distance from the speed limit change after measures the length of road prior to reaching the speed limit change and after the measured site. 93 acta polytechnica ctu proceedings 51:81–93, 2024 1 introduction 2 literature review 3 methods 3.1 data acquisition 3.2 site selection 3.3 set of tested attributes 3.4 data processing 4 results 5 discussion 6 conclusions acknowledgements references a appendices a.1 complete list of sites a.2 complete attribute table acta polytechnica ctu proceedings https://doi.org/10.14311/app.2024.51.0094 acta polytechnica ctu proceedings 51:94–108, 2024 © 2024 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague assessing time and financial costs of bus delays in the ústí nad labem region, czech republic martin schánoa,∗, alexandra dvořáčkováb, ondřej smíšekc a czech technical university in prague, faculty of transportation sciences, department of forensic experts in transportation, horská 3, 128 03 praha 2, czech republic b czech technical university in prague, faculty of transportation sciences, department of logistics and management of transport, horská 3, 128 03 praha 2, czech republic c czech technical university in prague, faculty of transportation sciences, department of vehicle technology, horská 3, 128 03 praha 2, czech republic ∗ corresponding author: schano@fd.cvut.cz abstract. this publication focuses on the development of a regional system for monitoring and evaluating public bus delays in the ústí nad labem region in the czech republic. the case study, through a questionnaire survey, addresses the issue of the value of time loss caused by delays in regional public bus transport in the ústí nad labem region from both a societal and individual passenger perspective. the survey includes, in addition to opportunity costs, the necessity to use earlier buses due to uncertainty and the potential real financial loss of passengers, for instance, due to late arrival to work and p otential employer sanctions. routes for the questionnaire survey were selected based on an analysis of the movement of the bus fleet to represent typical situations applicable to the entire bus transport network. the result is a financial evaluation of the losses caused by bus delays in relation to the number of passengers, the purpose of their journeys, and the value of their time. the findings reveal relatively high costs of time losses, which, when applying a co st benefit approach, allow for the return on larger infrastructure investments aimed at reducing these delays. the research results will contribute to the development of issues evaluating the economic efficiency of investments in transport infrastructure. keywords: value of travel time, bus delay. 1. introduction urban transport is integral to the vitality of cities and regions, catering to the diverse mobility needs of their inhabitants. yet, contemporary cities grapple with an overpowering influx of private vehicles, epitomizing the allure of unfettered mobility for many. however, the surging numbers of cars are straining road infrastructure capacities. the resultant issues encompass reduced traffic speed, prevalent congestion, compromised safety, and escalated environmental concerns [1, 2]. in some cities, the number of cars on the roads affects the quality of traffic to such a great extent that some parts of the city are at certain times completely cut off [3]. often, expanding the existing road networks isn’t feasible due to the constraints imposed by urban structures. as highlighted by [4], excessive reliance on private vehicles poses formidable barriers to fostering sustainable urban mobility. unfortunately, the global trend in the intensity of use of public transport is in decline. [3] present partially positive statistics suggesting stabilization of the use of public transport in europe at around 11 % in western europe and 16 % in eastern europe. based on this, addressing the dwindling public transport patronage isn’t just desirable; it’s essential for a balanced mobility ecosystem. low usage of public transport, especially when compared to private car travel, is often linked to the quality of public transit or the perceived quality of private vehicles in relation to public options. in these instances, the criteria for judging quality aren’t always objective. sometimes, people make decisions based on incomplete or inaccurate information [5]. quality of service in public transport also deeply affects vulnerable populations. low-income groups, the elderly, and persons with disabilities are especially reliant on efficient public transport. their mobility, and by extension their quality of life, hinges on the accessibility and reliability of these systems [6, 7]. by analysing travel time data, it is possible to determine if the system is meeting its scheduled times. improving reliability often has a positive impact on the user experience of passengers [8]. however, certain public transport delays may not necessarily pose a problem for users, in terms of time or financial loss, and might not even be subjectively perceived as an issue that needs addressing. yet, where the threshold of acceptability lies is not clear and likely varies for different population segments in various parts of the world. this study presents responses to these questions from bus transport users within the integrated transport system of the ústí nad labem region, the northern part of the czech republic. 94 https://doi.org/10.14311/app.2024.51.0094 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 51/2024 assessing time and financial costs of bus delays . . . 2. literature review as noted, it is clear there is an obligation to seek what can be done about the unpopularity of public transport. hence, it is essential to understand what affects users’ decision to use public transport. an extensive body of literature identifying key priorities for public transport users was published in recent years. [9] presented study of user loyalty to public transport identified different types of users with different motivations and satisfaction. they also identified the heavy impacts of policies improving service quality, reliability, safety, information, and cleanliness on increasing loyalty among transit users. generally, as customer satisfaction rises, so does customer loyalty, meaning that satisfied customers are more likely to continue using a service and also recommend the service to others. therefore transit agencies need to keep their users satisfied, as these are the users who are likely to positively affect the growth and profitability of the agency. [4] conducted a study to identify factors that influence public transport competitiveness by analyzing current passenger loyalty. they explored the relationships between service quality, passenger satisfaction, switching costs, the appeal of alternative transport, and passenger loyalty and applied their model in a case study in shanghai pt in china. they concluded that pt service quality was a dominant factor in motivating respondents to choose pt as their travel method. comparing the degrees of influence it was determined that service quality has a much higher total effect than car attractiveness and switching costs. their research additionally used seven factors to reflect pt quality. in order of importance to the user, these were: comfort, safety, convenience, timeliness, reliability, economics, and freedom. even decades ago, authors such as [10] and [11] generally recognized the concept that retaining existing clients and working to gain their loyalty is more beneficial than investing in attracting new users. recent authors like [12] or [13] continue to confirm the validity of this perspective in today’s context. numerous other authors pointed out that when the public transport service is unreliable, has a low frequency, or lacks comfort people are likely to switch to using cars as the unsatisfied users not perceiving public transport as a viable alternative [14, 15]. extensive research was also conducted on the satisfaction of public transit users and the factors influencing people’s motivation to use public transport. one of the most important is timeliness [4, 9, 14]. the travel time value is increasingly playing a significant role in decisions about public transport and transportation infrastructure. international studies have focused on this issue since the 1960s when travel time was included as part of the utility function. the widely accepted discrete choice model bases the value of travel time on the willingness to pay for its reduction, expressed as a ratio of sensitivity to travel time changes and costs. this model is further modified by additional parameters, with meta-analyses summarizing european and overseas studies [16, 17] most commonly citing parameters characterizing the specific journey (its purpose, transportation means used, length, quality), the traveler (age and gender, income category, etc.), and the broader economic context (most often gdp per capita, average wage rate, etc.). in the czech republic, the value of travel time is considered when evaluating externalities in the field of transportation infrastructure. among the most significant are the certified methodology for quantification of transport externalities (2013) [18] and the sectoral methodology for evaluating the economic efficiency of transport construction projects (2017) [19]. however, there is no consensus on how much time actually costs and how significant a delay becomes problematic. for this reason, we decided to investigate this question as part of broader activities focused on evaluating the state of the integrated transport system. 3. methods the objective of this paper is to demonstrate the possibilities of using delay data collected by the dúk dispatcher system, utilizing financial valuation of the societal costs of delays. as evident from a review of both foreign and domestic professional publications, this valuation is influenced by various factors, the quantification of which is not entirely trivial. one such factor is the purpose of the trip, which led us to conduct a pilot survey on selected lines and connections within the ústí nad labem region transport (doprava ústeckého kraje, hereafter dúk). the pilot survey aimed to verify the feasibility of obtaining relevant information about the purpose of travel by personally questioning passengers on designated connections, to test the method of their utilization, and to prepare recommendations for a possible subsequent larger-scale survey. the traffic survey was conducted on buses of the integrated transport system of the ústí nad labem region (doprava ústeckého kraje, hereafter dúk). the dúk system has approximately 250–350 buses active during an average weekday, covering the entire area of the ústí nad labem region, which is approximately 5 400 km2. the method for processing this article involved analyzing the positional data of dúk buses and based on it, selecting representative suburban lines for conducting a questionnaire survey. the questionnaire survey was intended to provide a sufficient amount of data about passengers to quantify the cost of delays for specific buses and simultaneously verify the willingness of passengers to share information for further related work. 95 m. scháno, a. dvořáčková, o. smíšek acta polytechnica ctu proceedings attribute description of attribute payload_id a unique identifier of the data payload delivery resulting from the query. payload_time the date and time of the payload receipt. payload_topic the identifier of the task. in this case, /traffic_status. arrivaldt the date and time of arrival at the stop. cislineid long name of the bus line. delay delay generated by dispatching software. finalnode the final stop on the line. gpspositiondt the date and time of the gps position collection. haslowfloor whether the bus has a low-floor. id the identifier of the bus. lastactivitydt the date and time of the last activity of the bus. latitude the bus’s position. lineid the identifier of the bus line. longitude the bus’s position. routeid the identifier of the bus route within the line. stationnode the last stop passed. todeparturedt the date and time of departure. table 1. description of individual attributes of the bus position database. 3.1. bus fleet movement and delay data acquisition and processing the project works with data exports from the dúk’s dispatching software over the period of one month. during this time, about 30 million records of individual bus locations were collected, which were recorded every 15 seconds. the content of the data can be seen in the table 1. from this data, a total of 4.4 million passages through inter-stop segments were reconstructed. the passages through stops are manually recorded by the driver in dúk buses. for this reason, the data on passages were validated through linkage with the passport of bus stops. if the onboard computer reported a stop at a station, the location of that station was compared with the reported location of the bus. if these locations differed significantly, the record of the passage was excluded from subsequent analysis. such cases are mainly due to a driver’s error, for example in setting the onboard computer or in its proper operation. the number of such excluded records fluctuated daily, but generally ranged between 15–20 %. the rest of the data was temporally and spatially consistent with the timetables and was thus used for delay analysis. the onboard computer is connected to the timetables, and information about planned and actual bus arrivals is thus contained in the processed data. this offers the possibility to evaluate the spatiotemporal delays of individual buses. for the purposes of this survey, only delays at the time of arrival at the stop were relevant, as only such delays have a direct impact on passengers. the result was statistically processed to obtain a comprehensive overview of the formation and behavior of bus delays in the heterogeneous environment of the ústí nad labem region. based on the visualization of various delay parameters on the map (see figure 1), representative bus lines were subsequentlyselected to evaluate passengers’ perceptions of different levels of delay. generally, it’s understood that if a passenger chooses to use a dúk bus, they can anticipate arrival to be on time or with a minor delay. the average delay upon arrival at the stop across all bus lines is 3.1 minutes, and as observed in the aforementioned heatmaps, this delay is relatively consistently distributed throughout the day. to identify extreme delays for selecting suitable lines for surveys, the 95th percentile was utilized. this percentile signifies a delay that a daily public transport user might encounter about 1–2 times a month. the 95th percentile value for all dúk bus arrivals’ delays during the surveyed period is 9 minutes. lines 402, 432, and 455 were chosen for the questionnaire survey. these lines intersect the functions of commuting to work, school, and leisure activities. the distribution of delays during weekdays is illustrated in figures 2–7 (average values for the entire network can be seen for comparison in figures 8–9). line 402 has an average daily delay of 3.8 minutes and a 95th percentile of 10 minutes. specifically, during the morning and forenoon hours, when the survey was conducted, these statistics are 2.6 and 7 minutes, respectively. therefore, this line has a medium average delay but high extremes. line 432 has an average daily delay of 1.9 minutes and a 95th percentile of 6 minutes. specifically, in the morning and forenoon hours, when the survey was conducted, these statistics are 1.5 and 7 minutes, respectively. thus, this line has a low average delay and low extremes. line 455 has an average daily delay of 5.0 minutes and a 95th percentile of 9 minutes. specifically, in the morning and forenoon hours, when the survey 96 vol. 51/2024 assessing time and financial costs of bus delays . . . figure 1. delays of individual lines on the map. figure 2. absolute delays on arrival at stops on bus line 402. figure 3. relative delays on arrival at stops on bus line 402. figure 4. absolute delays on arrival at stops on bus line 432. was conducted, these statistics are 2.7 and 9 minutes, respectively. therefore, this line has a high average delay and high extremes. 3.2. bus delay cost evaluation on the selected lines, a total of 15 journeys were chosen at times that covered all required functions of suburban bus transport, i.e., corresponding to regular commutes to work, school, and for occasional trips. on these services, passenger counts for those boarding and alighting were conducted, along with a survey using closed questions. the questionnaire was designed to ensure that sufficient responses could be gathered in short inter-stop sections (typically involving 2–4 minute drives between stops). it was also structured to be fully anonymous, not containing any personal queries about the passenger (such as gender, age, residence, employment, 97 m. scháno, a. dvořáčková, o. smíšek acta polytechnica ctu proceedings figure 5. relative delays on arrival at stops on bus line 432. figure 6. absolute delays on arrival at stops on bus line 455. figure 7. relative delays on arrival at stops on bus line 455. figure 8. absolute delays on arrival at stops throughout the region. figure 9. relative delays on arrival at stops throughout the region. income, etc.). this approach was taken to minimize any resistance from passengers towards completing it. consequently, it was anticipated that the passengers’ responses would be honest, devoid of any inclination to conceal information or provide inaccurate or false answers. the questions were asked by trained personnel directly on specific buses over a period of 3 days from 31.10.2023 to 2.11.2023. overall, mainly considering the length of time taken to fill out the questionnaire, it was possible to obtain answers to all the questions from 191 passengers, which represents approximately 47 % of passengers present on the specific buses. three passengers refused to answer. these outputs were subsequently linked with statistical information from the previous section to obtain a more comprehensive picture of the subjective perception of bus delays from the passengers’ perspective. for the financial valuation of delays, a procedure according to the “sectoral methodology for evaluating the economic efficiency of transport construction projects” (updated version 2023) was chosen. for declared travel purposes to work and school, time values were used as determined by the sectoral methodology for the category bus non-working – short commute, and for other purposes, the value for the category bus other-short distance was used. these time val98 vol. 51/2024 assessing time and financial costs of bus delays . . . question description of the question where are you traveling from? where are you traveling to? for the purposes of determining the relationship of responses with statistical data on delays. purpose of the trip? to identify the anticipated negative effects of possible delays. options were (1) to work, (2) to school, (3) other. do you ride the bus regularly? to determine the weight of the response. severe negative consequences of delay for a passenger who uses the bus once a month carry a different weight than minor consequences for a passenger who uses buses every day. options were (1) every (work) day, (2) several times a week, (3) several times a month, and (4) once a month or less. does the bus delay cause you harm? to determine tolerance for delay and its potential consequences. options were (1) yes, it causes direct or indirect financial damage (fine, need to buy a new ticket, penalty by employer, . . . ), (2) yes, it causes inconvenience (absence or late arrival at school/work), and (3) no. if the delay causes you financial harm, how much? specific value in case of direct financial loss or description of indirect financial loss (e.g., need to compensate for bus delay with longer time at work). if the delay causes you financial harm or inconvenience, how significant is the delay? specific value – for example, 6 minutes for a transfer. if a bus is delayed less than 6 minutes in such a case, no inconvenience occurs. if it’s more than 6 minutes, inconvenience occurs. if buses were to run with absolute reliability on time, could you travel by a later service? a question aimed at the cost of delay variability. a passenger may be so intolerant to delay that even if a bus is late only once a month, potential losses outweigh the daily benefits of a later departure. do you have the option to use a personal car or another alternative mode of transport (bike, . . . ) for this journey? a question aimed at the potential for changing the mode of transport, specifically to distinguish between captive riders (users who are dependent on transit), choice riders (car owners who choose to take transit), and captive-by-choice riders as presented by [9]. table 2. list of survey questions. ues, stated at the 2017 price level, were recalculated to the 2023 price level (considering inflation, gdp growth, and the elasticity of gdp growth for nonworking journeys). for travel to work, this value was increased by 16 % following the recommendation of the “methodology for quantification of transport externalities”. this recommendation is based on a metaanalysis ([17]) that generally agrees on a higher time value for commuting compared to other travel purposes. the following values were used for the financial valuation of delays. the value of travel time (vott) for the observed purposes was determined as follows: • vott work = 198.2 czk/person-hour (approximately 7.7 eur/person-hour), • vott school = 170.87 czk/person-hour (approximately 6.6 eur/person-hour), and • vott other = 143.62 czk/person-hour (approximately 5.6 eur/person-hour). the sectoral methodology procedure respects the necessity to work with the so-called perceived value of time and to value time for individual phases of the journey with the corresponding coefficient, therefore time values were further adjusted by a coefficient of 2.5, which the methodology determines for the duration of delays. the determined values of delay (value of delay) according to the purpose of the journey were thus valued at rates • vod work = 495.50 czk/person-hour (approximately 19.2 eur/person-hour), • vod school = 427.18 czk/person-hour (approximately 16.6 eur/person-hour), and • vod other = 359.05 czk/person-hour (approximately 13.9 eur/person-hour). 4. results the questionnaire survey results reveal the purposes of the journeys. in the early morning hours, travel to work predominated, followed by school trips between 6:32 and 8:21 am, and later hours saw a majority of other journeys, mainly for shopping, medical appointments, etc., as shown in figure 10. figure 11 illustrates the responses regarding the regularity of public transport use. most morning travellers commute daily or several times a week. however, there were also passengers who travel 2–4 times a month or less than once a month. 99 m. scháno, a. dvořáčková, o. smíšek acta polytechnica ctu proceedings figure 10. the purpose of the trip. figure 11. how often respondents use pt. figure 12 presents the responses to whether passengers could use a later bus if they were 100 % certain that the next bus would not be delayed. a considerable number of positive responses to this question were observed, especially in the early morning hours on line 455, which shows statistically the highest delays. high values were also observed on line 455 during the morning hours. figure 12 also reveals answers to whether passengers have the option to use a different mode of transport for their journey. the responses indicate that a significant number of passengers have this option but still choose to use public transport, which is a highly positive outcome. figure 13 displays responses regarding the problems caused by delays, whether financial or non-financial. as the graph indicates, passengers’ tolerance is relatively high, and financial or non-financial consequences occur, except in one case, only after 10 minutes or more. financial consequences arise after 15 minutes of delay. it should be noted, however, that the number of respondents with financial consequences is negligible (only 4 responses) out of the total sample of 191 respondents. from the results of the passenger count in the pilot survey, tables were created for stops where passengers exited the buses. based on the percentage representation of declared travel purposes in the questionnaire survey, the presumed purposes of all passengers travelling in the monitored section of the line were calculated. if no corresponding data from the questionnaire figure 12. the alternative route or means. figure 13. what delay causes problems. survey were available for a given stop or were obviously unrepresentative, then the proportions of travel purposes were determined based on local knowledge (location of schools, companies, shopping centres, etc.). for line 455 český bukov – ústí nad labem, due to low occupancy, the survey was conducted for all three monitored trips twice on two consecutive days, and the average of the determined values was used for each trip. tables 3 and 4 present examples of such processed data for line 432 and trip 113. for each stop with recorded passenger alighting, the cumulative value of delay in person-hours for each presumed travel purpose and in total was determined based on the average value of delay at the stop on weekdays of the last month (for the respective line and trip). the comprehensive results for all 10 trips on the 3 selected lines are presented in table 5. the total value of delays on the monitored trips was 18.97 person-hours, representing societal costs of 8 181.80 czk (eur 336 per day). if this value were considered typical, the annual total for these trips alone would amount to 2 045 450 czk (eur 83 863 per year). 5. discussion while the results are intriguing, it is important to note that the scope of the survey was considerably limited by its 191 respondents. nonetheless, it was possible to discern general trends among passengers. 100 vol. 51/2024 assessing time and financial costs of bus delays . . . table 3. travel purposes for line 432, trip 113. table 4. financial value of delay for line 432, trip 113. table 5. financial value of delay for the observed trips on lines 402, 432, and 455. 101 m. scháno, a. dvořáčková, o. smíšek acta polytechnica ctu proceedings the most significant finding is the discrepancy between perceived losses and actual losses from bus delays. as seen in the differences between figures 2–7 and figure 13, perceptible problems caused by delays do not occur in most real cases. the majority of reported delay problems were observed from 10 minutes onwards. however, there were instances with delays of 21 minutes or even 32 minutes. thus, the overall perception of the timeliness of public transport in the ústí nad labem region is high. nevertheless, losses still occur, albeit unnoticed. the second key finding is that the losses from public transport bus delays are substantial enough to cover investments in infrastructure (e.g., intersection reconstruction or traffic light priority) and still be costeffective. that is, merely from reducing delays, it is possible to cover the entire or a significant part of the costs of measures to reduce delays. this finding adds motivation to create analytical tools for identifying and locating causes of bus delays, as their elimination will be financially efficient. 6. conclusions in conclusion, it can be stated that users of public transport in the ústí nad labem region are more than willing to participate in activities aimed at improving public transport services, even if they are not necessarily daily users. only a very small number of people (∼1 %) refused to respond. the survey showed that approximately 33 % of public transport users have the option of using another mode of transportation for their journey. this is a significant proportion and indicates that bus delays in the region (average of 2–3 minutes) are not perceived as problematic enough for users to widely change their mode of transport. on the other hand, an average of 12 % of respondents stated that if they were 100 % certain that the bus would not be delayed, they could use a later connection for their journey. this points to high time investments forced by potential delays and suggests a quite promising direction for improving the quality of bus services. in total, only about 2 % of public transport users stated that bus delays cause them financial losses. most of them mentioned the obligation to stay longer at work and make up for the delayed time. on the other hand, a total of 25 % of public transport users stated that bus delays cause them non-financial losses or problems. this group of users can significantly influence the perceived quality of service and affect the popularity of public transport. references [1] d. banister. unsustainable transport: city transport in the new century. routledge, london, 2005. isbn 9780203003886. https://doi.org/10.4324/9780203003886 [2] e. nedeliakova, j. sekulova, i. nedeliak, b. abramovic. application of raymond fisk model in research of service quality. communications – scientific letters of the 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[2023-11-19]. https://www.tacr.cz/dokums_raw/ metodiky/tb0100md020_metodika.pdf [19] státní fond dopravní infrastruktury. rezortní metodika pro hodnocení ekonomické efektivnosti projektů dopravních staveb (10/2017, aktualizace 06/2023) [in czech; sectoral methodology for evaluating the economic efficiency of transport infrastructure projects], 2023. [2023-11-19]. https://sfdi.gov.cz/pravidla-ametodiky/rezortni-metodika-pro-hodnoceniekonomicke-efektivnosti-projektu/ 103 https://doi.org/10.1016/j.jue.2014.02.001 https://doi.org/10.1016/j.tranpol.2012.11.005 https://doi.org/10.1016/j.evalprogplan.2009.06.010 https://doi.org/10.1016/j.evalprogplan.2009.06.010 https://doi.org/10.1016/j.tra.2016.08.019 https://www.tacr.cz/dokums_raw/metodiky/tb0100md020_metodika.pdf https://www.tacr.cz/dokums_raw/metodiky/tb0100md020_metodika.pdf https://sfdi.gov.cz/pravidla-a-metodiky/rezortni-metodika-pro-hodnoceni-ekonomicke-efektivnosti-projektu/ https://sfdi.gov.cz/pravidla-a-metodiky/rezortni-metodika-pro-hodnoceni-ekonomicke-efektivnosti-projektu/ https://sfdi.gov.cz/pravidla-a-metodiky/rezortni-metodika-pro-hodnoceni-ekonomicke-efektivnosti-projektu/ m. scháno, a. dvořáčková, o. smíšek acta polytechnica ctu proceedings a. appendices – tables for calculation of cost of delay for each line and route table 6. travel purposes for line 402, trip 107. table 7. financial value of delay for line 402, trip 107. table 8. travel purposes for line 402, trip 109. table 9. financial value of delay for line 402, trip 109. 104 vol. 51/2024 assessing time and financial costs of bus delays . . . table 10. travel purposes for line 402, trip 113. table 11. financial value of delay for line 402, trip 113. table 12. travel purposes for line 432, trip 103. table 13. financial value of delay for line 432, trip 103. 105 m. scháno, a. dvořáčková, o. smíšek acta polytechnica ctu proceedings table 14. travel purposes for line 432, trip 111. table 15. financial value of delay for line 432, trip 111. table 16. travel purposes for line 432, trip 113. 106 vol. 51/2024 assessing time and financial costs of bus delays . . . table 17. financial value of delay for line 432, trip 113. table 18. travel purposes for line 432, trip 123. table 19. financial value of delay for line 432, trip 123. table 20. travel purposes for line 455, trip 102. 107 m. scháno, a. dvořáčková, o. smíšek acta polytechnica ctu proceedings table 21. financial value of delay for line 455, trip 102. table 22. travel purposes for line 455, trip 106. table 23. financial value of delay for line 455, trip 106. table 24. travel purposes for line 455, trip 108. table 25. financial value of delay for line 455, trip 108. 108 acta polytechnica ctu proceedings 51:94–108, 2024 1 introduction 2 literature review 3 methods 3.1 bus fleet movement and delay data acquisition and processing 3.2 bus delay cost evaluation 4 results 5 discussion 6 conclusions references a appendices – tables for calculation of cost of delay for each line and route acta polytechnica ctu proceedings https://doi.org/10.14311/app.2024.47.0023 acta polytechnica ctu proceedings 47:23–27, 2024 © 2024 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague the use of concrete slurry waste to improve the properties of the stabilised soil vendula davidová∗, tomáš david, petr mondschein, pavel reiterman czech technical university in prague, faculty of civil engineering, thákurova 7, 166 29 prague, czech republic ∗ corresponding author: vendula.davidova@fsv.cvut.cz abstract. it is often necessary to use stabilising additives during construction to improve the resulting soil properties to ensure the good quality of civil engineering structures. these additives are often produced at high temperatures and with a high carbon footprint. this paper focuses on soil stabilization with waste concrete slurry, which is produced during the production of fresh concrete. the selected clay soil was stabilized with cement and csw at 2, 4, and 6 % by weight of soil. compressive strength, the resistance to freezing and thawing, initial bearing index (ibi), and california bearing capacity (cbr) were monitored. the research did not confirm better performance of csw binder even at higher dosage compared to cement, which was due to the high water content of the mixtures. positive results were achieved especially in freez-thaw resistance. keywords: concrete slurry waste, soil improvement, compressive strength. 1. introduction concrete slurry waste (csw) is produced as a waste material from the production of fresh concrete, which currently has only limited use. here are some examples of the use of this waste material. hardened csw has been used as recycled aggregate or as a fine aggregate in concrete. the csw has also been used as a lightweight aggregate or as a substitute for lime for liming acidity in agricultural soil, or as substitute for soil stabilisation [1]. to ensure quality construction, it is necessary to ensure good conditions in the building below. however, if there is no good quality soil on the site, additional investment and works are needed to improve the properties. clayey soils often have low strength characteristics, and such soils cannot be used for the construction of civil engineering structures without modification of the properties. for this reason, the inadequate soils can be improved by using of stabilising materials [2]. as a chemical stabilising material, pozzolanic materials are most often used. many researchers, for example z. wang et al. [3] used ordinary portland cement (opc) to stabilize clay soil in china. they noted the degree of hardness decreases with the increase of the clay-water/cement ratio. a. daraei et al. [4] used cement, quicklime, or gypsum to stabilize soil in iraq. the adding quicklime to the soil reduced swelling and permeability. g. barbhuiya et al. [5] used nano-silica to enhance various types of soil. however, these materials need to be manufactured, often at high temperatures and with a high carbon footprint, so it is necessary to look for available materials. y. cheng et al. [6] could use the natural resource of volcanic ash to improve soil properties in kenya. however, such a source is not available everywhere. in the czech republic, csw is far more available, because it is produced at every concrete plant and, moreover, its use is very limited and largely ends up in landfill. by using this material, we reduce the landfilling of waste material, and improve the soils. according to p. reiterman et al. [7] csw contains calcium hydroxide (ch), c-s-h, aluminate hydrates, and non-hydrated particles of opc. thus, this material was supposed to combine the effect of the main practically used soil stabilizers – opc and lime. 2. experimental section 2.1. materials the main material used for this research was a clay soil with medium plasticity, tough consistency, and brown colour. this soil originates from the czech republic – the village of rodov, can be classified as f6 according to čsn 73 6133 [8] and also as cl according to uscs classification [9]. figure 1, shows the grain size distribution of the used soil. a summary of the specific soil properties is shown in table 1. the ordinary portland cement (opc) cem i 42.5 r, and concrete slurry waste (csw) were also added to this soil to improve its properties. csw is a material originating from concrete production. this material is obtained from the maintenance of the plant and from residual concrete that is returned from construction. the aggregate of the large fraction is first separated from this material, then it is stored in a sedimentation pond (figure 2) where the sludge is settled. this sludge was further treated by filtering out excess water and dried at 105 °c. the resulting material was crushed in an om20 laboratory mill from brio hranice. the material was sieved through a 0.5 mm size mesh sieve to achieve the desired properties. 23 https://doi.org/10.14311/app.2024.47.0023 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en v. davidová, t. david, p. mondschein, p. reiterman acta polytechnica ctu proceedings figure 1. grain size distribution – curve. organic content 0.2 % liquid limit 37.6 % plastic limit 19.8 % optimum moisture content (modified compaction according to čsn en 13286-2 [10] 13.0 % maximum dry density 1 930 kg m−3 table 1. specific soil properties. figure 2. csw in sedimentation pond. 24 vol. 47/2024 the use of concrete slurry waste r c3 w2 w4 w6 proportion of soil (f6 cl) 100 % 100 % 100 % 100 % 100 % binder 0 3 % 2 % 4 % 6 % water 13 % 16 % 16 % 19 % 21 % note: c = cement, w = csw table 2. composition of the mixtures. 2.2. composition of the mixtures a reference mix composed of selected soils was first prepared for the purpose of this project. then its properties were improved with use of cement in dose of 3 % by weight of the soil. the cement was further substituted with csw at a dosage of 2 %, 4 %, and 6 % by weight of the soil. water was added based on the results of the optimum moisture content determined by the proctor modified test only for reference soil mixture. the moisture content was then increased by 1 % for every 1 % of added cement to soil, and 1.5 % for every 1 % of added csw binder. before testing began, it was assumed that the csw binder would require a higher proportion of water to achieve the same consistency of the mixture, but this assumption was not confirmed and so the higher water content had a major effect on the final properties. the composition of all mixtures is shown in table 2. 2.3. methods the following tests were performed to compare the effects of the csw binder and opc: initial bearing index (ibi), and california bearing capacity (cbr), compressive strength, resistance to freezing and thawing (f-t). all samples were made according to čsn en 13286-2 [10] modified proctor. with this method, the samples are compacted by a weight falling from a prescribed height and in a prescribed number. then the samples were treated until the day of testing depending on the purpose. the tests according to čsn en 13286-2 [10] were used for the determination of ibi and cbr (cbr was performed after 96 hours of curing in saturated state). these tests are based on the mandrel that is pushed into the test specimens at a certain speed, after the value of the force and the depth of penetration were measured. the ibi and cbr were determined from the measured values. seven samples of each mixture were made for the determination of compressive strength. these samples were sealed in an impermeable bag and left in a humid environment for 7 days (2 samples), 14 days (2 samples), and 28 days (3 samples). the specimens were tested according to the čsn en 13286-41 [11] after the specified time. the resulting compressive strength is determined from the maximum load. a load is applied to the specimen until it breaks in this test. three samples were prepared from each mixture for the f-t resistance. these samples were placed in an impermeable bag and left in a humid environment for 28 days. after this time, the samples were subjected to 13 cycles of freezing and thawing at the temperature of -15 °c. the result is the number of cycles that the test specimens have resisted without signs of destruction. 3. results and discussions the achieved ibi and cbr are shown in table 3 and figure 3. the ibi results of the studied mixtures correspond to the amount of water in the mixtures. mixture r with the lowest water content of 13 % achieved the highest ibi. similarly, mixture w6 with 21 % water achieved the lowest ibi. figure 3. the resulting ibi and cbr. r c3 w2 w4 w6 ibi (2.5 mm) 12.7 9.5 10.5 8.9 4.4 ibi (5.0 mm) 14.8 10.3 11.3 9.1 4.9 the resulting ibi 15.0 10.0 11.0 9.0 5.0 ibi (2.5 mm) 2.3 29.1 9.4 9.6 6.1 ibi (5.0 mm) 2.6 45.8 12.2 12.2 7.0 the resulting ibi 3.0 46.0 12.0 12.0 7.0 table 3. the resulting ibi and cbr [%]. on the other hand, treatment time had a positive effect on the cbr result. the most significant result was observed on c3 mixture with cement as the binder was activated and cbr reached 46 %. the mixtures with csw achieved a higher cbr than the reference soil, but the resulting value was round 10 %. these results correspond with the strength results. again, a longer curing time was needed for better binder activation, and in the short term, the higher dose of csw together with the higher dose of water did not show better properties than the lower dose of csw. 25 v. davidová, t. david, p. mondschein, p. reiterman acta polytechnica ctu proceedings figure 4. evolution of compressive strengths; a) 7 days, b) 14 days and c) 28 days. an overview of the compressive strength results in mpa is summarized in table 4, and figure 4. the reference mixture without the use of stabilising components did not show an increase in strength over time. this result was expected as the soil did not naturally contain any hydraulic components. stabilization with cement was the most effective as it achieved the highest compressive strength. this result is consistent with the research of s. i. haralambos [12], who investigated the effect of cement dosage on different types of soils. he noted that the addition of cement to all soil types increased the compressive strength and stiffness of the soil-cement mixture. curing time r c3 w2 w4 w6 7 days 0.29 0.65 0.34 0.28 0.27 14 days 0.30 0.75 0.34 0.25 0.25 28 days 0.30 0.90 0.35 0.44 0.59 table 4. compressive strengths of the mixtures (mpa). mixtures w2, w4, and w6 initially achieved low strengths due to the higher dose of water. the increase in strength became apparent after a longer curing time (after 14–28 days). after this time, the increase in strength was most significant for the mixture with the highest csw content. the results show that the low dose of csw improved the strength properties insignificantly. higher dose of csw with higher water content was more effective, longer treatment time was required for the soil strength to show. the f-t resistance is shown in figure 5. the reference soil did not resist even 1 of the 13 freezing cycles, therefore it can be considered as non-frostproof. samples with cement and the lowest csw binder dose (and the same water content) resisted the most freezfigure 5. the resistance to freezing and thawing. ing cycles. mixtures with 4 % and 6 % csw binder resisted only 4 freezing cycles. the addition of cement as a stabilizing additive improved the f-t resistance, which corresponds with the y. lu et al. [13] research. they noted that effective content of cement stabilizer was 5 % in their study. the addition of a stabilizing csw improved the resulting frost resistance too, but samples with higher water content in the mixture (w4, w6) resisted a lower number of cycles. overall, it can be noted that the addition of csw improves the resulting f-t resistance, but the water content of the mixture must also be taken into account. 4. conslusion the purpose of this research was to compare the properties of clay soil stabilized with ordinary cement and waste material from fresh concrete production (csw). the research showed that csw improves the compressive strength of the soil, but only after a longer treatment. however, stabilization with cement was more effective. mixtures with cement and low csw content showed comparable f-t resistance, mixtures with higher csw content and higher water dosage showed lower f-t resistance, still better that the refer26 vol. 47/2024 the use of concrete slurry waste ence soil without treatment. increasing water dosage in mixtures with cement and csw binder negatively affected ibi, on the other hand, csw binder positively affected crb and the mixture with cement achieved very good values. overall, even the higher dose of csw binder did not achieve comparable results to cement. this was mainly due to the different dosage of water in the mixture and therefore the comparability of the results is limited. the positive effect of the csw binder was generally more obvious after a longer treatment time than for the samples with cement, which is an important finding for practical use. further research will focus on comparing the properties of soils with the same water content at different percentages of csw binder. acknowledgements this research was funded by czech science foundation under the project 19-11027s. references [1] m. keppert, v. davidová, b. doušová, et al. recycling of fresh concrete slurry waste as supplementary cementing material: characterization, application and leaching of selected elements. construction and building materials 300:124061, 2021. https: //doi.org/10.1016/j.conbuildmat.2021.124061 [2] r. k. sharma, j. hymavathi. effect of fly ash, construction demolition waste and lime on geotechnical characteristics of a clayey soil: a comparative study. environmental earth sciences 75(5):377, 2016. https://doi.org/10.1007/s12665-015-4796-6 [3] z.-f. wang, s.-l. shen, z.-y. yin, y.-s. xu. rapid field evaluation of the strength of cement-stabilized clayey soil. bulletin of engineering geology and the environment 74(3):991–999, 2015. https://doi.org/10.1007/s10064-014-0643-3 [4] a. daraei, a. f. h. sherwani, r. h. faraj, et al. stabilization of problematic soil by utilizing cementitious materials. innovative infrastructure solutions 4(1):33, 2019. https://doi.org/10.1007/s41062-019-0220-5 [5] g. h. barbhuiya, s. d. hasan. effect of nano-silica on physio-mechanical properties and microstructure of soil: a comprehensive review. materialstoday today 44:217–221, 2021. https://doi.org/10.1016/j.matpr.2020.09.115 [6] y. cheng, s. wang, j. li, et al. engineering and mineralogical properties of stabilized expansive soil compositing lime and natural pozzolans. construction and building materials 187:1031–1038, 2018. https: //doi.org/10.1016/j.conbuildmat.2018.08.061 [7] p. reiterman, p. mondschein, b. doušová, et al. utilization of concrete slurry waste for soil stabilization. case studies in construction materials 16:e00900, 2022. https://doi.org/10.1016/j.cscm.2022.e00900 [8] road earthwork. standard, czech standard institute, prague, 2010. [9] standard practice for classification of soils for engineering purposes. standard, astm international, usa, 2018. [10] nestmelené směsi a směsi stmelené hydraulickými pojivy část 2 [unbound and hydraulically bound mixtures – part 2]. standard, czech standard institute, prague, 2014. [11] nestmelené směsi a směsi stmelené hydraulickými pojivy část 41 [unbound and hydraulically bound mixtures – part 2]. standard, czech standard institute, prague, 2004. [12] s. i. haralambos. compressive strength of soil improved with cement. in contemporary topics in ground modification, problem soils, and geo-support, pp. 289–296. american society of civil engineers, reston, va, 2009. https://doi.org/10.1061/41023(337)37 [13] y. lu, s. liu, y. zhang, et al. freeze-thaw performance of a cement-treated expansive soil. cold regions science and technology 170:102926, 2020. https: //doi.org/10.1016/j.coldregions.2019.102926 27 https://doi.org/10.1016/j.conbuildmat.2021.124061 https://doi.org/10.1016/j.conbuildmat.2021.124061 https://doi.org/10.1007/s12665-015-4796-6 https://doi.org/10.1007/s10064-014-0643-3 https://doi.org/10.1007/s41062-019-0220-5 https://doi.org/10.1016/j.matpr.2020.09.115 https://doi.org/10.1016/j.conbuildmat.2018.08.061 https://doi.org/10.1016/j.conbuildmat.2018.08.061 https://doi.org/10.1016/j.cscm.2022.e00900 https://doi.org/10.1061/41023(337)37 https://doi.org/10.1016/j.coldregions.2019.102926 https://doi.org/10.1016/j.coldregions.2019.102926 acta polytechnica ctu proceedings 47:23–27, 2024 1 introduction 2 experimental section 2.1 materials 2.2 composition of the mixtures 2.3 methods 3 results and discussions 4 conslusion acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2025.53.0049 acta polytechnica ctu proceedings 53:49–52, 2025 © 2025 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague solid-state synthesis of pure belite polymorphs michal křištof∗, andrea jančíková, karel dvořák brno university of technology, faculty of civil engineering, veveří 331/95, 602 00 brno, czech republic ∗ corresponding author: michal.kristof1@vut.cz abstract. the solid-state synthesis of pure belite polymorphs is still not described in detail. the study of this issue is important for understanding modification transformations, especially between beta and gamma modifications. in this paper, the two-step firing procedure was used to study belite transformations. the first firing step was designed to produce samples with high content of γ-c2s. these samples were fired again and successfully transformed into β-c2s without any additional chemicals. in addition to firing parameters, the effect of particle size was also monitored. the experiment succeeded in creating a sample containing 64 % gamma belite in total and also a sample containing a total of 90 % of beta belite. keywords: clinker, belite, polymorphism. 1. introduction belite, dicalcium silicate, is one of four main minerals in portland clinker. belite structure is made of sio4− tetrahedrons connected with calcium ions [1]. nowaday, five polymorphs of belite are known, which are referred to as α, α’h, α’l, β and γ. except for gamma modification all of these polymorphs are stable only at high temperatures, but it is possible to stabilize high-temperature polymorphs with suitable cooling method or incorporation of other ions [2]. successful chemical stabilization was studied and described by many authors. for example, kim [3] described that addition of b3+ ions have positive effect to β-c2s stabilization. also, physical stabilization was studied and chan [4] claims that only particles over 10 µm can change its modification in order of α’l → β → γ during cooling. on the other hand, stabilization of different belite polymorphs by solid-state method with no additives is not described in detail. β-c2s exhibits a low rate of hydration in portland cement and its strength is gained mainly after longer hydration time [5]. a more intense increase in strength can be achieved by controlled carbonation during hydration, both for the polymorphic beta modification and also for the gamma modification, which is nonreactive under classic hydration conditions. the effect of incorporating co2 back into the structure of the mineral, in combination with the lower required firing temperature, is an interesting option for reducing the carbon footprint in the cement industry. the production of clinkers with a dominant representation of belite theoretically makes it possible to reduce carbon emissions by 10 % [6]. the degree of carbonation depends mainly on mineralogy, grain size, water factor, co2 concentration and hydration time [7]. 2. material and methods 2.1. preparation of raw material mixture for the preparation of raw material mixture, chemically pure raw materials were used, namely calcium carbonate (caco3, p.a. purity, penta, praha, czech republic) and silicon dioxide (sio2, p.a. purity, penta, praha, czech republic). the mixture for belite was prepared by mixing 125 g of caco3 with 35 g of sio2, which is based on the stoichiometric ratio of calcium and silicon oxides in the c2s mineral. the raw material mixture was wet ground in a fritsch pulverisette 6 planetary mill. grinding took place in an agate grinding capsule with 25 agate grinding balls with a diameter of 1 cm. the length of grinding was set to 10 minutes and the intensity to 350 rpm. the mixture thus prepared was dried at 105 °c for 12 hours. drying was carried out in a binder c 170 laboratory dryer. compact particles of 2–5 cm size were formed from the mixture during drying. 2.2. firing procedure – gamma c2s afterwards, the samples were pour fired in classic 2017 s superkhantal furnace in two steps. first firing of mixtures was set to 1 250, 1 300, 1 350 and 1 400 °c for 120 minutes. in all cases, the temperature was increased 10 °c min−1 and the cooling proceeded slowly until the furnace cooled down on its own. in this step, calcination of caco3, solid phase reactions and gradual transformation of high-temperature forms of belite to gamma modification occurred. due to the results achieved, three additional firing procedures were carried out. specifically, these were firings at 1 450 and 1 500 °c with an isothermal duration of 200 minutes and a firing with a duration of 120 minutes at 1 350 °c and a subsequent duration of 720 minutes at 400 °c during cooling. then, five grams of each sample were milled in mccrone micronising mill for 150 s in 15 ml 49 https://doi.org/10.14311/app.2025.53.0049 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en m. křištof, a. jančíková, k. dvořák acta polytechnica ctu proceedings figure 1. intensities of the diffraction lines of gamma belite at angles 2θ = 23.2°and 23.6°. of isopropanol to study the effect of particle size in the next step. each sample was analyzed immediately after firing and milling by xrd and laser granulometry. 2.3. firing procedure – beta c2s samples prepared by first step firing procedure were immediately fired again. superkhantal furnace and platinum crucibles were used as well as in the first step. the effect of the second firing was observed on ground and unground samples. in this case, the second firing procedure always took place under the same conditions, namely at 950 °c with a duration of 120 minutes and subsequent rapid cooling. during cooling, the samples were gradually taken out of the furnace, poured from the platinum crucibles onto steel sheets and cooled by a stream of compressed air. the only exception is the sample fired at 1 250 °c, which was not fired a second time because it was not sufficiently reacted. the analysis of the samples after the second firing proceeded in the same way as after the first firing. 2.4. sample preparation for xrd analysis after both steps of firing, 5 g of each sample was milled for 150 s with 15 ml of isopropanol in mccrone micronising mill to the required fineness for xrd. the panalytical empyrean diffractometer (cu anode, λ = 1.54184 å) was used for all xrd analysis. to evaluate the measurements records, the software highscore plus in combination with icsd database from 2012 were used. the quantities of mineral phases were determined using the rietveld method. 2.5. particle size characterisation particle size distribution was measured on raw material mixture before firing and then on all samples right after cooling and, in some cases, after addition milling. malvern mastersizer 2000 laser granulometry dry method was used for all analysis. 3. results and discussion after a series of first firings, the minerals β-c2s, γc2s and also ca(oh)2, cao and sio2 were identified in the samples, the presence of which in this case is caused by the low intensity of firing in combination with the selected grain size. the amount of unreacted feedstock ranged from 2 to 16 %, with a higher rate of feedstock reaction occurring as the firing temperature increased. perfect reaction only occurred during additional firings at temperatures of 1 450 and 1 500 °c. it can be confirmed that by firing at higher temperatures and with a longer isothermal hold, a larger amount of γ-c2s modification is formed upon cooling, which can be observed in figure 1. this is due to a higher degree of structural ordering and therefore a higher susceptibility to the β→γ polymorphic transformation, which affects the mineralogy and granulometry of the resulting product. in subsequent second firings, it was confirmed based on xrd analysis that the orthorhombic crystal structure of γ-c2s can be relatively easily transformed into a monoclinic one, just by using a suitable firing mode. that this structural transformation depends on the size of the particles is illustrated by the comparison of the results of the second firing on ground and unground samples. this is shown in table 1, which clearly confirms the fact that the smaller γ-c2s particles convert to β-c2s more perfectly, as purer β-c2s samples were always achieved for the milled samples. however, it should be noted that the parameters of the second firing are not sufficient to complete the reaction of the mixture, and the samples still contained the mineral portlandite, cao and quartz. their amount corresponded with the amount after the first firing. the statement can be illustrated in 50 vol. 53/2025 solid-state synthesis of pure belite polymorphs firing after first firing after second firing temperature unground samples ground samples [ °c] β γ β γ β γ 1 300 90.0 10.0 97.5 2.4 98.5 1.5 1 350 81.8 18.2 97.9 2.1 98.2 1.8 1 350 (400) 81.6 18.4 96.1 3.9 99.2 0.8 1 400 82.5 17.5 94.8 5.2 90.1 9.9 1 450 59.0 41.0 81.2 18.8 88.7 11.3 1 500 35.6 64.5 80.5 19.5 88.6 11.4 table 1. relative representation of beta and gamma belite after firings. figure 2. dependence of the structural transformation of belite on the firing regime. figure 2, where the intense γ-c2s lines are still visible in the untreated samples after the second firing. the average grain size d(50) of the raw material mixture before firing was 12.373 µm. in samples where perfect synthesis did not occur, the particle size decreased to values of 10–11 µm, which can be explained by the calcination of the limestone particles. on the contrary, sufficiently reacted samples, which contain a higher amount of β-c2s, show a d(50) greater than 16 µm. the average particle size of 13.5 µm was measured in additional firings at temperatures of 1 450 and 1 500 °c. milled samples had d(50) in the range of values 6.3–8.1 µm. the grains of all samples were slightly reduced due to the second firing procedure. the only exception is the unground sample (1 300 °c), in which the raw materials reacted further, and the grain size increased slightly during the second firing. 4. conclusion the raw material mixture for belite was prepared at the beginning of the experiment. then, the γ-c2s and β-c2s were produced during two-step firing procedure. particle size distribution and mineralogical composition were determined for all samples. a belite sample with a total γ-c2s content of 64.0 % was prepared by the described preparation of the raw material mixture and the appropriate amount of supplied energy in combination with slow cooling. the conducted experiment further confirmed that the mineral belite in the polymorphic beta modification can be created in high purity even without the use of stabilizing ions. the dependence of this transformation on the particle size was also confirmed, but it was proved that the transformation also occurred in the case of grains smaller than 10 µm. a sample containing a total of 90.0 % β-c2s (ratio β-c2s:γ-c2s = 99.2:0.8) was prepared as follows: • temperature rise 10 °c min−1, maximum temperature 1 350 °c, duration 120 minutes and slow cooling with duration 720 minutes at 400 °c, • grinding in a mccrone laboratory mill in the presence of isopropanol, • temperature rise 10 °c min−1, maximum temperature 950 °c, duration 120 minutes and rapid cooling. acknowledgements we acknowledge the financial support to projects, gačr (czech science foundation) no. 23-05122s and internal grand agency of the brno university of technology, faculty of civil engineering no. fast-s-24-8512. 51 m. křištof, a. jančíková, k. dvořák acta polytechnica ctu proceedings references [1] h. f. w. taylor. cement chemistry, second edition. thomas telford, london, england, 2nd edn., 1997. [2] s. n. ghosh. portland cement phases: polymorphism, solid solution, defect structure and hydraulicity, p. 289–305. elsevier, 1983. https: //doi.org/10.1016/b978-0-08-028670-9.50014-4 [3] y. kim, s. hong. influence of minor ions on the stability and hydration rates of β-dicalcium silicate. journal of the american ceramic society 87(5):900–905, 2004. https://doi.org/10.1111/j.1551-2916.2004.00900.x [4] c. j. chan, w. m. kriven, j. f. young. physical stabilization of the β → γ transformation in dicalcium silicate. journal of the american ceramic society 75(6):1621–1627, 1992. https: //doi.org/10.1111/j.1151-2916.1992.tb04234.x [5] p. hewlett. lea’s chemistry of cement and concrete. butterworth-heinemann, oxford, england, 4th edn., 2003. [6] e. gartner, t. sui. alternative cement clinkers. cement and concrete research 114:27–39, 2018. https://doi.org/10.1016/j.cemconres.2017.02.002 [7] h. sun, f. yu, s. liu, et al. effects of belite incorporation on mechanical and microstructural properties of 7opc mortars cured under carbonation condition. journal of materials research and technology 22:503–518, 2023. https://doi.org/10.1016/j.jmrt.2022.11.168 52 https://doi.org/10.1016/b978-0-08-028670-9.50014-4 https://doi.org/10.1016/b978-0-08-028670-9.50014-4 https://doi.org/10.1111/j.1551-2916.2004.00900.x https://doi.org/10.1111/j.1151-2916.1992.tb04234.x https://doi.org/10.1111/j.1151-2916.1992.tb04234.x https://doi.org/10.1016/j.cemconres.2017.02.002 https://doi.org/10.1016/j.jmrt.2022.11.168 acta polytechnica ctu proceedings 53:49–52, 2025 1 introduction 2 material and methods 2.1 preparation of raw material mixture 2.2 firing procedure – gamma c2s 2.3 firing procedure – beta c2s 2.4 sample preparation for xrd analysis 2.5 particle size characterisation 3 results and discussion 4 conclusion acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2025.53.0097 acta polytechnica ctu proceedings 53:97–101, 2025 © 2025 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague flexural behavior of timber-high-performance concrete experimental slabs tomáš vlach∗, jonáš klimt, eliška kafková, jan macháček, věra kabíčková, jakub hájek czech technical university in prague, faculty of civil engineering, thákurova 7, 166 29 prague 6, czech republic ∗ corresponding author: tomas.vlach@cvut.cz abstract. in general, one of the possible future paths of the building construction industry is the development of robotization and prefabrication of individual building components. presented article continues the development of prefabrication using traditional effective combination of two often used materials in construction – timber and concrete in the form of a slabs. the traditional concept of combining these materials is used for ceiling structures – slabs, panels, floor panels, where timber is most often in the form of beams and a concrete slab is applied over the beams as a material transmitting compressive stress. the key to functionality for this system is the shear connection of both materials. the presented study presents a thin concrete layer made of high-performance concrete, which is connected to a wooden board from glulam using an adhesive bridge. the aim of this presented study is to improve the bending load-bearing capacity and bending stiffness of the glulam slab with a small amount of high-performance concrete and thus achieve a more favorable environmental profile of the ceiling panel. the individual variants differ in thickness, the presence and number of ribs. these variants are compared with a variant of the same thickness of glulam slab without the layer of concrete. keywords: timber-concrete, high-performance concrete, timber-high-performance concrete, composites, timber composites, timber-concrete composites, composite slabs. 1. introduction in the construction sector of civil engineering, it is common to use solid reinforced concrete slabs with steel reinforcement. reinforcement serves to transfer tensile stresses in concrete, which is the weak point of concrete. timber-concrete composites (tcc) offer a sustainable and more environmentally friendly building elements option due to timber’s recyclability, reusability, sustainability and high tensile and bending strength. it works in combination with concrete, which transmits predominantly compressive stresses, where high-performance concrete (hpc) with high compressive strength finds very effective potential of applications [1]. tcc offer many different applications in civil engineering. the first applications were applied more than 50 years ago as a modification of existed floors [2]. the traditional concept of combining these materials is the use of timber in the form of beams. this study presents timber slabs – massive glulam board. the solid plate can be further optimized and lightweighted in different shapes. the advantage of this solution is a larger contact area between the materials and a reduction in contact stress. in the traditional concept, various steel shear connectors [3], composite shear connectors [4, 5], or various locking notches [6] or bonding systems [7–9] are used for connections of tcc. for shear connection in this study is used adhesive bonding with epoxy resin in combination with notch shear connections – system of ribs. the goal of presented study is to enhance the bending strength and stiffness of a glulam slab by incorporating a small amount of hpc, ultimately improving the environmental footprint. the different variants vary in thickness and the presence and quantity of ribs. these variants are compared to a version of the glulam slab with the same thickness but without the concrete layer. 2. materials used for experiment a recipe of hpc was developed by the department of architectural engineering, faculty of civil engineering, czech technical university in prague [10]. hpc mixture design utilizes predominantly local raw materials and is detailed presented in table 1. the hpc mixture is self-compacting and fine-grained. it combines two types of quartz sand and silica flour with silica fume. the maximum grain size is 1.2 mm. dispersed alkali resistant glass fibers were designed into the mixture to eliminate shrinkage microcracks. the water cement ratio was 0.28 and the water binder ratio was 0.24. a reference mixture without any fibers was also mixed. the compressive strength tested on three cubes 100×100×100 mm3 was 131.1 mpa according to the standard csn en 12390-3. the flexural strength tested on six prisms 40 × 40 × 160 mm3 was equal to 16.2 mpa according to standard čsn en 12390-5. the compressive strength was verified on the prism 97 https://doi.org/10.14311/app.2025.53.0097 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en t. vlach, j. klimt, e. kafková et al. acta polytechnica ctu proceedings figure 1. specimen dimension of glulam slabs, scheme of all five types including hpc part and system of longitudinal and transverse ribs. mix content kg m−3 cement i 42.5r 650 technical silica sand 1 200 silica flour (ground quartz) 235 silica fume (microsilica) 100 fibres – ar glass 20 superplasticizers 24 water 180 total 2 409 table 1. recipe of hpc mixture. fragments, and it was 133.8 mpa. a mixture with dispersed ar glass fibers was mixed for the determination of basic mechanical parameters. it was used for tcc presented in this article. the compressive strength tested on three cubes 100×100× 100 mm3 was little bit lower 113.3 mpa according to the standard čsn en 12390-3. the flexural strength tested on six prisms 40 × 40 × 160 mm3 was equal to 17.9 mpa according to standard čsn en 12390-5, little bit higher in comparison with reference mixtures as expected. the compressive strength was verified on the prism fragments, and it was 126.1 mpa. in the presented article only one type of glulam was selected. glue-laminated technology allows different structural products to be manufactured in different sizes and shapes. the manufacture of glue-laminated timber as engineered products for structural uses requires careful procedures for material preparation. for this experiment glulam boards were selected adhesively bonded by birch plywood plates. basic table values of the flexural mechanical parameters of the wood used are presented in the table 2. the adhesiveness in this article to create a bridge in the interface between hpc and glulam was selected epoxy resin sikafloor 150/280 based on the previous successful experience with this epoxy resin in similar research projects. the basic material parameters of paramater glulam bending elasticity modulus em ∥ [n mm2] 6 672 bending elasticity modulus em ⊥ [n mm2] 5 329 bending strength fm ∥ [n mm2] 59.6 bending strength fm ⊥ [n mm2] 50.5 table 2. basic mechanical parameters of used glulam. the epoxy resin are the flexural strength of 15 mpa and the modulus of elasticity of 2.0 gpa. the specific gravity of the resin is 1 100 kg m−3 according to the technical data sheet of the sika company [11, 12]. 3. specimen design and preparation the production process presented is slightly different from the others. if glue is used to transfer shear stress between tcc materials, the wooden and concrete parts are usually manufactured separately and finally glued together. in this experiment, hpc is carried out directly on the glulam part using an adhesive bridge, which facilitates the technological production process. first, a glulam board is prepared for the designed specimen dimensions of 150 × 400 mm in the variants presented in figure 1. in sum three specimens were prepared for each of the five groups. the first is a reference for comparison – a glulam board without any hpc layer with a thickness of 27 mm. the other boards have a thickness of glulam 21 mm, where a layer of hpc was added so that the total thickness is also approximately 27 mm for better comparison. furthermore, a system of longitudinal ribs is designed for the glulam board to lighten the contact area, another variant combines longitudinal and transverse ribs. the last group are glulam boards with only hpc ribs, without a continuous hpc layer. the geometry of all specimens is visible in figure 1. specimen groups are named d.1 to d.5, as visible in the same figure 1. 98 vol. 53/2025 flexural behavior of timber concrete experimental slabs specimen number weight thick width length fcf ,max ϕfcf ,max [kg] [mm] [mm] [mm] [kn] [kn] glulam 27 d.1.1 1.061 26.47 148.8 400.0 21.96 21.98d.1.2 1.045 26.56 148.7 400.0 21.67 d.1.3 1.047 26.68 149.0 402.0 22.31 glulam 21 + hpc 6 d.2.1 2.015 26.65 150.1 400.0 28.09 28.96d.2.2 1.914 26.25 152.3 399.0 28.75 d.2.3 2.013 26.63 152.2 399.0 30.04 glulam 21 + hpc 6 + ribs longitudinally d.3.1 2.028 27.54 152.7 402.0 28.19 28.37d.3.2 1.896 27.34 151.8 401.0 27.06 d.3.3 2.201 28.33 152.2 402.0 29.87 glulam 21 + hpc 6 + ribs longitudinally + transversely d.4.1 2.069 28.12 152.2 402.0 33.97 31.91d.4.2 2.111 27.78 151.0 403.0 30.61 d.4.3 2.078 27.25 151.5 401.0 31.15 glulam 27 + ribs longitudinally d.5.1 1.223 26.57 148.4 402.0 23.99 22.96d.5.2 1.221 26.42 148.7 400.0 22.51 d.5.3 1.286 26.47 148.8 401.0 22.37 table 3. list of samples, individual weights and dimensions, measured maximum force value. the next step, after preparing the glulam boards, was to apply a bonding bridge to transfer shear stress at the interface of both materials. this was done in two stages. the first stage involved applying a coat of epoxy resin, which sealed the wood structure. after it hardened, a second coat was applied, followed by the addition of sand with a grain size of 0.6–1.2 mm. the sand was spread loosely over the surface. after hardening, as specified in the technical sheet, the sample was ready for concreting. the bonding bridge effectively transfers shear loads [8], while the epoxy also protects the wood from the wet concreting process. finally, concreting was carried out. in addition to the specimens, as specified in the plan, the concrete support structures were also cast to determine the basic mechanical properties of the concrete, as listed above. 4. experiment and results the article focuses on the influence of a thin hpc layer on the load-bearing capacity and overall behavior of the slab. loading is carried out using a four-point bending test. the distance between the supports was 300 mm and the distance between the loading supports was 100 mm. the loading process was carried out at a constant loading rate of 0.5 mm min−1 so that the total test length until failure was approximately 15 minutes. the experiment was created on a galbadiny quasar 100 hydraulic press with a maximum cylinder load capacity of 100 kn. numerical values of maximum reached forces the results are given in table 3. the table provides interesting results. the table provides interesting results with a visible positive influence of the hpc layer. these differences between single groups are better visible in figure 2, where the curves from the load tests are presented. the result of the reference group d.1 without a hpc layer shows similar results with group d.5, where only longitudinal thin hpc ribs are presented. this is quite a surprising result, and it may be due to the ribs being properly filled with hpc. the maximum flexural strength of group d.1 was 62.8 mpa. this result corresponds to the value of 59.6 mpa given in table 2 so measurement was correct. however, a significant influence of the hpc thin layer on the load-bearing capacity and a positive influence on the specimen stiffness in groups d.2, d.3 and d.4 are visible. all variants show similar values of strength and stiffness. variant d.2 is the worst of the three mentioned, because it has a smaller interface contact area without any ribs. the concrete surface slipped at the end due to the shear stress. variant d.3 is a little bit stiffer, achieves better results. thanks to the longitudinal ribs the interface contact area is increased, however, there was also shear failure of the interface bond and then failure of the glulam. group d.4 with longitudinal and transverse ribs achieved the best results and there was no slipping in the interface bond area. furthermore, the ribs could be optimized in future work. in figure 3 are presented views on the damaged specimens after the destructive loading tests. 5. conclusions the aim of this presented study was to improve the bending load bearing capacity and bending stiffness of tcc. it is a glulam by incorporating a small amount of hpc. to allow for a better comparison of the results, all samples were made with approximately the same dimensions and were compared to reference samples without hpc. the load bearing capacity of the glulam without hpc corresponds to the data provided in the technical data sheet. the hpc was applied directly to the glulam, with surface treatment involving an adhesive bridge of epoxy resin and silica sand before concreting process. the effect of the single longitudinal hpc ribs on the overall stiffness of the panel was not proven. however, a significant positive effect of the hpc layer in the compression zone was 99 t. vlach, j. klimt, e. kafková et al. acta polytechnica ctu proceedings figure 2. force-displacement diagram for all specimen from groups d.1 to d.5. visible significant influence of the concrete layer on the load-bearing capacity and especially the stiffness of the slab. figure 3. view on the damaged representative specimens after the loading test, one from each group d.1 to d.5. for group d.3 and d.4, the hpc was removed after the testing to see the ribs and the positive influence of the transverse rib. demonstrated, as the bending load bearing capacity increased by approximately 45 %, and the stiffness was also improved. at the same load value, the deformation of the specimen was roughly halved. the ribs showed a positive effect on the shear capacity of interface between both materials. acknowledgements the work on this paper was supported by czech science foundation grant no. 22-14942k entitled “possibilities of using natural fibers for the production of hybrid textile reinforcement in concrete” the authors would like to acknowledge all financial assistance provided to support this research. references [1] a. neville, p.-c. aïtcin. high performance concrete – an overview. materials and structures 31(2):111–117, 1998. https://doi.org/10.1007/bf02486473 [2] e. lukaszewska, h. johnsson, m. fragiacomo. performance of connections for prefabricated timber-concrete composite floors. materials and structures 41(9):1533–1550, 2008. https://doi.org/10.1617/s11527-007-9346-6 [3] m. fragiacomo. experimental behaviour of a full-scale timber-concrete composite floor with mechanical connectors. materials and structures 45(11):1717–1735, 2012. https://doi.org/10.1617/s11527-012-9869-3 [4] s. c. auclair, l. sorelli, a. salenikovich. a new composite connector for timber-concrete composite structures. construction and building materials 112:84–92, 2016. https://doi.org/10.1016/j.conbuildmat.2016.02.025 100 https://doi.org/10.1007/bf02486473 https://doi.org/10.1617/s11527-007-9346-6 https://doi.org/10.1617/s11527-012-9869-3 https://doi.org/10.1016/j.conbuildmat.2016.02.025 vol. 53/2025 flexural behavior of timber concrete experimental slabs [5] l. marchi, l. pozza. timber-concrete composite connections using gfrp notches fastened with self-tapping screws: conceiving, numerical modelling and testing. construction and building materials 294:123579, 2021. https://doi.org/10.1016/j.conbuildmat.2021.123579 [6] k. müller. timber-concrete composite slabs with micro-notches. ph.d. thesis, eth zürich, 2020. https://doi.org/10.3929/ethz-b-000474874 [7] q. fu, l. yan, n. a. thielker, b. kasal. effects of concrete type, concrete surface conditions and wood species on interfacial properties of adhesively-bonded timber – concrete composite joints. international journal of adhesion and adhesives 107:102859, 2021. https://doi.org/10.1016/j.ijadhadh.2021.102859 [8] m. novotná, c. fiala, p. hájek. precast timberconcrete composite floor structures for sustainable buildings – experimental verification. in cesb 13 prague : central europe towards sustainable building 2013, pp. 429–432. 2013. isbn 978-80-247-5015-6. [9] e. appavuravther, b. vandoren, j. henriques. push-out tests on adhesively bonded perfobond shear connectors for timber-concrete composite beams. journal of building engineering 57:104833, 2022. https://doi.org/10.1016/j.jobe.2022.104833 [10] m. kynclova, c. fiala, p. hajek. high performance concrete as a sustainable material. international journal of sustainable building technology and urban development 2(1):63–68, 2011. https://doi.org/10.5390/susb.2011.2.1.063 [11] t. vlach, p. hájek, c. fiala, et al. waffle facade elements from textile reinforced high performance concrete, 2016. [12] t. vlach, j. řepka, j. hájek, et al. cohesion test of a single impregnated ar-glass roving in high-performance concrete. stavební obzor civil engineering journal 29(3):358–369, 2020. https://doi.org/10.14311/cej.2020.03.0032 101 https://doi.org/10.1016/j.conbuildmat.2021.123579 https://doi.org/10.3929/ethz-b-000474874 https://doi.org/10.1016/j.ijadhadh.2021.102859 https://doi.org/10.1016/j.jobe.2022.104833 https://doi.org/10.5390/susb.2011.2.1.063 https://doi.org/10.14311/cej.2020.03.0032 acta polytechnica ctu proceedings 53:97–101, 2025 1 introduction 2 materials used for experiment 3 specimen design and preparation 4 experiment and results 5 conclusions acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2023.42.0027 acta polytechnica ctu proceedings 42:27–31, 2023 © 2023 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague desing of the algorithm, print and analysis of porous structures with modifiable parameters radosław grabieca,∗, jacek tarasiuka,b, sebasatian wrońskia,b a university of cracow, akademia górniczo-hutnicza im. stanisława staszica w krakowie, al. adama mickiewicza 30, 30–059 cracow, poland b university of cracow, faculty of physics and applied computer science, department of condensed matter physics, ul. reymonta 19, 30–059, cracow, poland ∗ corresponding author: rgrabiec@student.agh.edu.pl abstract. the purpose of this paper was to create an algorithm able to creating a porous structure with variable properties, print and analyze them. the basic concepts related to it were introduced and the process of creating an algorithm in rhinoceros software was described. having a suitable test group of porous structures, it was shown that it is possible to modify their porosity. in the next step, the structures were presented for printing and its effect. the obtained physical models were examined by microtomography. the resulting cross-sections were processed in imagej software. having cross-sections of the original bone and printed structures, it was possible to compare their porosity and the average diameter of the trabeculae in the structure. with this procedure, it is possible to deduce whether it is possible to print accurate structures that will serve as porous bone implants. the resulting differential porosity comparison was 2.0–7.5 %, while the thickness was about 18–35 %. keywords: porous, bone, voronoi, rhinoceros, grasshopper, implants, 3d printing, microct. 1. introduction malignant bone tumors, or so-called sarcomas, are quite rare, making medical personnel’s infamiliar with the problem and with treatment techniques. often, even in the early stages of the disease, when it has not managed to significantly occupy the soft tissue, the standard medical procedure is not to get rid of the entire limb, but to cut out areas of exposure, thereby creating cavities in the bones [1]. these defects should be filled with bio-compatible material, which is able to help the bone to reconstruct [2]. the motivation, and the goal of this work, was to design an algorithm to help design such porous structures. aperiodic structures were taken under the closer look, since the literature on this type of model is relatively scarce [3, 4]. a feature of the resulting algorithm should be the easy modifiability of all kinds of parameters, such as the porosity of the structure, or the thickness of the trabeculae from which it is built. our analysis was based on structures with 30 % and 80 % porosity. in addition to the algorithm design, the paper focuses on printing the designed structures and analyzing them. 2. materials and methods 2.1. porous structure porous structures in the materials environment are a frequently addressed topic. periodic structures with uniform distribution are most famous in the literature. however, aperiodic structures are an interesting aspect, as the literature provides much less information on them. the way to create this type of elements that was chosen is voronoi diagrams [5]. figure 1 shows this kind of structure. it is characterized by the fact that each point of the polygon, is closer to its own center (the so-called voronoi center) than any other. figure 1. example of voronoi structure. 2.2. used software several tools were used in this work. the first, used at the beginning of the design process, was rhinoceros (robert mcneel & associates, v.7.3.21039.11201, usa) with the built-in block development environment grasshooper (designed by david rutten, robert mcneel and associates, february 2021, build 1.0.0007, usa). by this softwares a structure was created, with variable parameters. another program that was used is netfabb 2022 27 https://doi.org/10.14311/app.2023.42.0027 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en r. grabiec, j. tarasiuk, s. wroński acta polytechnica ctu proceedings (autodesk, ver. 2023.1, usa). this program was used during cutting the designed structures into a set number of sections for further analysis. in the next step, which was 3d printing, it is necessary to prepare the structure for printing process. for this purpose, the prusa slicer (prusa research, ver. 2.6.0, czech republic) application, dedicated to the printer, was used, which allowed to see the predicted printing parameters such as printing time, layer thickness or filament application accuracy. the last program used in this work was imagej2 fiji (johannes schindelin, ignacio arganda-carreras, albert cardona, mark longair, benjamin schmid, ver. 1.48g, loci, university of wisconsin). this software is used for image analysis. in our case to calculate the parameters of the obtained porous structures. 2.3. structuring algorithm it was necessary, to design an algorithm, used to create aperiodic structures. 2.3.1. algorithm the algorithm was built in grasshooper software by means of blocks, performing specific functions. it consisted of a couple of main sections: creating the shape of the model, parameterizing it, and converting it to a filled structure. the first section, which is visualized on figure 2 opens with the creation of a cube on which the structure will be based. then the distribution of points in the volume of the cube was randomly selected. based on the obtained points, a 3d voronoi diagram was built on them, and then only the edges (trabeculae) were isolated from it. in this way, the target shape of the structure was obtained. figure 2. creation of a voronoi diagram on points, drawn in a cube. in the next section, parameterization follows. the thickness of each trabeculae is selected, as well as the number of points into which the trabeculae is to be divided (a constraint, excluding errors). in addition, it is possible to select the number of voronoi diagram centers, which strongly determine the porosity of the structure. this process were shown on figure 3. once the desired structure is produced, and the parameters are given, it remains to create a solid. to do this, dendro add-on was used. a small volume (resembling a sphere) was built on each point into which the trabeculae were previously divided. these figure 3. parameterization and initial obtaining of the target structure. volumes were then smoothed and combined, resulting in a closed structure ready for export and analysis. we can see connecting and smoothing process on figure 4. figure 4. smoothed, finished structure in the final version. 2.3.2. issues in the way of block programming, there were also unexpected problems, which we can see on figure 5. some of the structures, went beyond their limits. this was caused by the fact that too many points fit into too small space. figure 5. structure escaping beyond its limits. this problem was solved by imposing limits on the number of points and the length of the trabeculae on the model. 2.3.3. structural supervision the finished structures were cut in netfabb software as we can see on the figure 6, and the resulting cross sections were loaded into imagej fiji. having the cross sections in the program it is possible to check the porosity of the structure, including the average thickness of the trabeculae. two functions of the bonej add-on were used for this purpose: 28 vol. 42/2023 desing, print and analysis of modifiable porous structures figure 6. examples of cross sections obtained using the slice function in netfabb software. figure 7. example of a printed model thickness and volume fraction. the average thickness of the trabeculae, we can see in the table 1 and the porosity (the ratio of the amount of voids to the total volume of model) is shown in the table 2. in table 1 we observe twice the output value, because the measured value is the average diameter of the trabeculae, and the value set by the algorithm is the cross-sectional radius. in addition, the average is increased by nodes where multiple trabeculae meet. such an analysis was carried out for a number of structures, in order to see if we can arbitrarily vary the porosity. this kind of controlling the structure is possible. 2.4. printing in order to create physical models of the designed structures, it was decided to use fdm 3d printing, one of the most popular incremental methods of this kind of process. it involves the gradual application of filament to a heated table, using a head heated to about 200 ◦c. the printer used is a czech-made prusa mk3. to the prusa slicer program, structures with porosities of 30 % and 80 %, previously created with the algorithm, were imported. the print layer thickness was set to 150 µm, which guaranteed good print quality. after viewing the predicted structure and making thickness of trabeculae [mm] 0.25 0.5 0.75 1 1.3 num. of pts. tb. th. mean 500 0.5 1.09 1.67 2.27 3.05 450 0.49 1.09 1.66 2.25 3.02 400 0.49 1.08 1.65 2.23 2.99 350 0.49 1.08 1.64 2.21 2.95 300 0.48 1.07 1.63 2.19 2.92 250 0.48 1.07 1.62 2.18 2.89 200 0.47 1.06 1.61 2.15 2.85 150 0.46 1.05 1.59 2.13 2.81 100 0.45 1.04 1.58 2.11 2.78 50 0.42 1.02 1.56 2.08 2.72 table 1. a table that checks whether the average thickness of the trabeculae corresponds to the thickness selected in the algorithm. thickness of trabeculae [mm] 0.25 0.5 0.75 1 1.3 num. of pts. porosity of model 500 0.94 0.77 0.57 0.38 0.19 450 0.94 0.78 0.59 0.41 0.22 400 0.95 0.80 0.62 0.44 0.25 350 0.95 0.81 0.64 0.47 0.29 300 0.96 0.83 0.67 0.51 0.32 250 0.96 0.85 0.70 0.55 0.37 200 0.97 0.87 0.74 0.60 0.43 150 0.97 0.89 0.78 0.66 0.51 100 0.98 0.91 0.82 0.72 0.59 50 0.99 0.94 0.88 0.81 0.72 table 2. table showing variation of porosity against change in number of voronoi centers and thickness of beads. sure that the program did not find possible errors during model creation, printing proceeded. we can see an example of the structure at figure 7. printing time varied between 1 and 3 hours, depending on porosity. it turned out that during printing there were some problems. one of them, was the local sagging of the filament through lack of support. it was not used because, removing the filament from the center of this kind of stucture is impossible (leaving aside the possibility of using a multi-filament head, which the printer used was not equipped with). another mistake was leaving tiny filamentr threads in the pores similar to spiderweb. this is due to a poor choice of head temperature, which pulls the filament too hot. however, they did not significantly affect the quality of the structures made. 29 r. grabiec, j. tarasiuk, s. wroński acta polytechnica ctu proceedings project printed models thickness [mm] porosity [−] number of points bv [mm3] tv [mm3] porosity [−] differential [%] 0.50 0.30 4350 5555 7833 0.29 3.06 0.50 0.80 400 1325 7620 0.83 3.26 0.75 0.30 1480 5376 7915 0.32 6.93 0.75 0.80 130 1413 8084 0.83 3.15 1.00 0.30 680 5639 8168 0.31 3.21 1.00 0.80 57 1561 8187 0.81 1.17 1.25 0.30 365 5474 8023 0.32 5.90 1.25 0.80 30 1818 7922 0.77 3.69 table 3. comparative table of porosity of the designed and printed model. 2.5. microct verified of printed structures the structures made were subjected to analysis, which involved checking the porosity and average bead thickness of the computer-designed models, as well as those printed. for this purpose, a nanotom s microtomograph was used. each of the printed structures, was x-rayed. during the entire process, the x-ray tube operated at 100 kv and a current of 100 µa. the target resolution was set at 15 µm, making a single measurement take about 2 hours. the resulting cross sections, for further analysis, had to be edited. editing, as well as further analysis, was performed in imagej fiji software. the crosssections were cropped as well as rotated appropriately, and then a median filter was performed. figure 8 shows an example of a cross section after the editing process. figure 8. example of an edited structure cross-section. 3. results the first step, was to use the volume fraction function, available in the bonej add-on again. after compiling the results and collecting them in a table 3, it turned out that the discrepancy between the design and the physical model reaches a maximum of 7 %, which is a satisfactory result at this stage of the research. any discrepancies are due to inaccuracies occurring during printing. another goal of the analysis, was to collect data on the average trabeculae thickness of printed structures and their designs by using function thickness. as we see at the table 4 this time the discrepancy reaches the level of a several dozen percent, which is not a very satisfying result. in this case, these discrepancies, are also due to problems during printing. the laid filament, formed into beads, has some kind of defects – small holes, which the filament did not reach during printing. we can see this kind of situation in figure 9. on the other hand, in figure 10, we can see the execution of the same function for the computer project. figure 9. example of visualization of thickness function for printed model. figure 10. example of visualization of thickness function for designed model. due to such a sequence of events, the thickness function sees the place where the defect appeared as two separate trabeculae, which underestimates the 30 vol. 42/2023 desing, print and analysis of modifiable porous structures project printed models thickness [mm] tb.th mean [mm] tb.th std [mm] tb.th mean [mm] tb.th std [mm] differential [%] 0.50 0.85 0.07 0.70 0.28 17.6 0.50 0.72 0.08 0.59 0.15 18.1 0.75 1.20 0.09 0.96 0.35 20.0 0.75 1.05 0.11 0.74 0.24 29.5 1.00 1.55 0.12 1.23 0.38 20.6 1.00 1.38 0.16 1.03 0.30 25.4 1.25 1.89 0.16 1.24 0.31 34.4 1.25 1.72 0.22 1.24 0.40 27.9 table 4. comparative table of the average thickness of the trabeculae of the designed and printed model. figure 11. graph of the average thickness of the trabeculae of the printed model. average result. in the figure 11, we can see the histogram of the distribution of the trabeculae thickness for the printed model, while for the model of the design itself, it appears in the figure 12. for the design, one selected thickness dominates, but there are also larger values, which is the reason for the places where the trabeculae connect (nodes), which by definition must be thicker. for the printed models we see that the distribution is almost symmetrical, and has many results that underestimate the average. this problem can be solved overestimating the thickness of the beam during the execution of the algorithm. with such a procedure, even after underestimating the result of thickness, it will be the target value. 4. conclusions to summarize this work, it is worth noting that it was possible to create an algorithm, capable of creating porous structures with very easily modifiable porosity and average thickness of the beads. this algorithm in the future, after a number of improvements, could serve as an aid for the design of bone implants for each patient individually. in addition, it can be extended to design structures of any shape, not just a cube, which opens its way to many scientific but also business environments. the algorithm and the printing process itself require improvements. the results so figure 12. graph of the average thickness of the trabeculae of the designed model. far, however, and the ease with which they can be obtained, are satisfying. references [1] r. ciaputa, e. brambilla, f. godizzi, et al. first report of primary testicular leiomyosarcoma in two dogs. bmc veterinary research 19(1):101, 2023. https://doi.org/10.1186/s12917-023-03658-5 [2] t. goryń, b. szostakowski, a. pieńkowski. advances in bone reconstructions after sarcoma resection. oncology in clinical practice 14(6):331–340, 2019. https://doi.org/10.5603/ocp.2018.0046 [3] i. e. ramirez-chavez, d. anderson, r. sharma, et al. a classification of aperiodic architected cellular materials. designs 6(4):63, 2022. https://doi.org/10.3390/designs6040063 [4] r. moat, e. muyupa, c. imediegwu, et al. compressive behaviour of cellular structures with aperiodic order. results in materials 15:100293, 2022. https://doi.org/10.1016/j.rinma.2022.100293 [5] g. voronoi. nouvelles applications des paramètres continus à la théorie des formes quadratiques. premier mémoire. sur quelques propriétés des formes quadratiques positives parfaites. journal für die reine und angewandte mathematik (crelles journal) 1908(133):97–102, 1908. https://doi.org/10.1515/crll.1908.133.97 31 https://doi.org/10.1186/s12917-023-03658-5 https://doi.org/10.5603/ocp.2018.0046 https://doi.org/10.3390/designs6040063 https://doi.org/10.1016/j.rinma.2022.100293 https://doi.org/10.1515/crll.1908.133.97 acta polytechnica ctu proceedings 42:27–31, 2023 1 introduction 2 materials and methods 2.1 porous structure 2.2 used software 2.3 structuring algorithm 2.3.1 algorithm 2.3.2 issues 2.3.3 structural supervision 2.4 printing 2.5 microct verified of printed structures 3 results 4 conclusions references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2025.54.0055 acta polytechnica ctu proceedings 54:55–61, 2025 © 2025 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague prestressed textile reinforced concrete with carbon composite reinforcement jan macháčeka,∗, eliška kafkováa, věra kabíčkováa,b, tomáš blažeka, tomáš vlacha,b a czech technical university in prague, faculty of civil engineering, department of architectural engineering, thákurova 7, 166 29 prague 6, czech republic b czech technical university in prague, university centre for energy efficient buildings, materials and construction of buildings research department, třinecká 1024, 273 43 buštěhrad, czech republic ∗ corresponding author: jan.machacek@fsv.cvut.cz abstract. the article deals with the potential of prestressed textile concrete. the prestressing principle, which is nowadays commonly used part of construction, is combined with the developing technology of textile concrete which is concrete reinforced with non-metallic reinforcement. in this case the carbon reinforcement is used because it shows the best mechanical parameters in comparison with glass or basalt reinforcement. the paper presents the advantages of using prestressed textile carbon composite reinforcement of textile reinforced concrete and at the same time deals with problems related to it, such as the correct choice of impregnation using epoxy resin and the technological method of introducing prestress into the sample. the article focuses on comparison between 3 types of epoxy resin, specifically sikafloor 156, lh 300 and eporezit epovill-a. the results are presented on experimental samples tested in four-point bending test. keywords: hpc, textile reinforcement, prestressed concrete, prestressing form, epoxy resin. 1. introduction the principle of prestressed concrete is based on concrete’s relatively hight compressive strength and its very low tensile strength. the main principle is that compressive stress is brought into the concrete before it is loading and due to this the compress reserve is created. it results into better loading capacity of concrete in bend and many other advantages, for example better durability [1]. in commonly used prestressed concrete the prestress is created by steel-based prestressing reinforcement, anyway this paper deals with replacement of this type of reinforcement by cfrp. in case of this article the hpc is used, which in combination with cfrp prestressing reinforcement allows creation of very thin concrete construction elements. cfrp reinforcement displays different properties depending on type of used epoxy resin, so the main purpose of this paper is comparison of three different epoxy resin [2]. concretely sikafloor 156, lh 300 and eporezit epovill-a which were used for homogenization of carbon fibres. a total of 6 sets of samples were made, 2 for each type of epoxy resin, of which there was always one set of samples with non-prestressed reinforcement and one set with pre-stressed reinforcement. this article was written based on the data obtained as part of the diploma thesis “production of prestressed textile reinforced concrete slabs” written by tomáš blažek in 01/2020 [3]. the topic of prestressing of concrete structures by composite reinforcement was investigate by many scientists, for example amr a. abdelrahman, sami h. rizkalla [4], yunxing du, mengmeng zhang, fen zhou, deju zhu [5] and many others [6, 7]. this paper builds on findigs of these researchers and continues in investigation of he given issue. 2. materials and samples 2.1. high performance concrete the samples are created from hpc, specifically from the hpc who’s mix design is listed below in table 1. this mixture was developed in department of civil engineering at ctu in prague and later improved in uceeb at buštěhrad. the cubes of dimensions approximately 100 × 100 × 100 mm and the beams of dimensions approximately 40 × 40 × 160 mm were created to determine the properties of concrete. mix content kg m−3 cement i 42.5r 689 technical silica sand 975 elkem microsilica 940 u-s 177 technical quartz powder st 6 329 superplasticizer based on pce 29.4 water 173 total 2 372.4 table 1. hpc mix design [3]. the cubes were tested in compression according to čsn en 12390-3 [8] and the beams were subjected to three-point bending test according to čsn en 55 https://doi.org/10.14311/app.2025.54.0055 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en j. macháček, e. kafková, v. kabičková et al. acta polytechnica ctu proceedings testing time a b h fcrit σcrit,c σavg,c [days] [mm] [mm] [mm] [n] [mpa] [mpa] kr1.01 3 100 100 99.89 517 231.9 51.78 52.14kr1.02 3 100 100 99.71 522 391.1 52.39 kr1.03 3 100 100 101.2 528 902.4 52.26 kr2.01 10 100 100 100.9 1 264 599.2 125.33 118.15kr2.02 10 100 100 99.7 1 119 892.7 112.33 kr2.03 10 100 100 103.6 1 210 052.9 116.80 kr3.01 28 100 100 101.7 1 400 021.3 137.66 130.40kr3.02 28 100 100 101.9 1 310 562.5 128.61 kr3.03 28 100 100 103.3 1 290 451.6 124.92 table 2. pressure strength [3]. testing time a b h fcrit σcrit,c σavg,c [days] [mm] [mm] [mm] [n] [mpa] [mpa] tr1.01 3 40 39.98 160 1 764.9 4.14 4.60tr1.02 3 40 38.78 160 2 055.1 5.12 tr1.03 3 40 39.49 160 1 889.2 4.54 tr2.01 10 40 40.02 160 3 399.8 7.96 7.93tr2.02 10 40 40.12 160 3 123.9 7.28 tr2.03 10 40 39.97 160 3 638.2 8.54 tr3.01 28 40 40.01 160 6 501.6 15.23 15.83tr3.02 28 40 40.36 160 6 962.0 16.03 tr3.03 28 40 40.21 160 7 001.7 16.24 table 3. tensile strength after bending [3]. 12390-5 [9]. the tests were performed in three times steps, specifically 3 days after concreting, 10 days after concreting and 28 days after concreting. the results of the performed tests are shown in the tables 2 and 3. 2.2. carbon fibre reinforced polymer as a prestressing reinforcement is used a composite reinforcement consisting of epoxy resin and carbon fibres 1 600 tex (which means that 1 km long roving weights 1 600 kg). three types of rovings were tested to demonstrate, why is the epoxy resin so important. the first was roving without any other improvements, the second was roving which was twisted and the third was roving impregnated with epoxy resin. as you can see in the figure 1, the epoxy impregnated rovings were able to transfer more than double the force compared to the unimpregnated rovings. 2.3. epoxy resins 2.3.1. sikafloor 156 two-component epoxy resin, which is most often used for the penetration of concrete substrates and cement screeds. the ratio of filler and hardener is 3:1. in this ratio, the adhesive exhibits the best mechanical parameters. the tensile strength specified by the manufacturer is 1.5 mpa. this information was taken from the manufacturer’s data sheet. 2.3.2. lh 300 + hardener h 287 very high-quality, medium-viscosity, low-molecular resin for lamination high resistance parts (up to 160 °c). this resin has been tested and used in the production of molds and parts for brake system cooling on škoda fabia wrc cars. even though these parts were exposed to temperatures exceeding 250 °c and at the same time to constant impacts, they fully met the extreme requirements for strength and heat resistance. temperature resistance of 160 °c is the temperature at which there are no changes in mechanical parameters and properties. this information was taken from the manufacturer’s data sheet. 2.3.3. eporezit epovill-a epoxy to produce prepregs with increased heat resistance. it is important to work with it at a constant temperature and low air humidity. the main application of the system is in energy (transformers, power transistors), telecommunications industry, capacitors and other forms. the system cures only above 80 °c. the cured system may have an orange-red tint. this information was taken from the manufacturer’s data sheet. 2.4. samples production the design of the prestressing track was based on the design of w. brameshuber and t. brockmann [10, 11]. 56 vol. 54/2025 prestressed textile reinforced concrete with carbon . . . figure 1. graph of force dependence on crossbar displacement – average values [3]. the principle consists in clamping carbon rovings impregnated with epoxy resin into steel jaws and their subsequent displacement, thereby tensioning these rovings to the appropriate value. the problem with this system is how to measure the actual force applied to the roving and how to attach the roving to the jaws. in this case, the actual force transferred to the roving was calculated from the knowledge of the displacement of the cross member, the modulus of elasticity and the cross-sectional area. the calculated values were verified by strain gauges. the method of attaching the rovings to the jaws is problematic mainly because the ends of the rovings are crushed with a strong grip, and on the contrary, with a weak grip, slippage occurs at the attachment point. this problem was solved with chemical mortar sockets. the form was made in such a way that it was possible to concrete an element with dimensions of 20 × 100 × 1200 mm, which will then be divided into 3 samples of length 360 mm. the samples themselves were produced in the following way: in the first step, the rovings were stretched and impregnated with epoxy resin. for streching the mold shown in the figure 2 was used. the design of this mold is shown in the figure 3. subsequently, they were sandblasted to ensure sufficient cohesion with the concrete matrix [12]. heat-cured epoxy resins were cured in a laboratory oven. after the epoxy dried, the rovings were cut from the mold and chemical mortar sockets were made on one side of the rovings. then the rovings were stretched, sockets were also made on the other side of the rovings, and then the final tensioning took place. the reinforcement prestressed in this way was left at rest for 3 days and the stress drop was continuously monitored. after approximately 48 hours, the tension value stabilized and thus concreting was possible on the 3rd day after tensioning of the reinforcement. the introduction of tension was carried out 3 days after concreting, after figure 2. final mold [3]. figure 3. prestressing mold desing [3]. 57 j. macháček, e. kafková, v. kabičková et al. acta polytechnica ctu proceedings figure 4. graph of the dependence of stress on the displacement of the crossbar – the beginning of loading – sikafloor 156 (blue – prestressed, orange – not prestressed) [3]. another 3 days the formwork was removed, and the samples were tested 28 days after concreting [3]. 2.5. experiment to verify the prestressing potential of textile concrete, a 4-point bending test was performed. the loading speed was chosen to be 0.02 mm s−1. all testing took place at the faculty of civil engineering of the czech technical university in prague. you can see a photo from the experiment in the figure 5. figure 5. photo from the experiment [3]. 3. results 3.1. sikafloor 156 from the graph of the dependence of the stress on the displacement of the cross member shown in the figure 4, it follows that the load-bearing capacity of the prestressed elements is increased, and it is evident that after the appearance of the first crack, there is a more massive development of cracks with a significant opening in the non-prestressed elements. this fact clearly proves the positive effect on the formation of the first crack. 3.2. lh 300 the prestress in this case shows the same positive effect that was also noted with the sikafloor 156 epoxy resin. the dependence of the stress on the displacement of the crossbar from the beginning of the loading is shown in the graph in the figure 6. 3.3. eporezit epovill-a even in the case of the last epoxy resin, the positive effect is the same as in the previous cases. as can be seen from the results shown in the figure 7, prestress losses occurred in all variants in time. the causes will be discussed in the following text. there were the lowest losses achieved in samples where lh 300 resin was used. the worst results are achieved when using sikafloor 156 epoxy resin. comparison of all prestressed samples is shown on the graph in the figure 8. prestress losses of individual samples were calculated according to the following formulas: σmf = mf w , (1) ∆σ = (σres + σt) − σmf , (2) where: σres – pressure reserve, σt – tensile stress after bending, σmf – tensile stress from the loading, ∆σ – prestress loss. the prestress losses results are shown in the table 4. 4. conclusion the choice of resins turns out to be successful, mainly due to small stress losses. above all, the lh 300 resin showed minimal stress losses. overall, it can be stated that the prestress is characterized by a shift in origin the first cracks and by eliminating the formation of other massive cracks after their formation. the fundamental effect of the preload is therefore on the 58 vol. 54/2025 prestressed textile reinforced concrete with carbon . . . figure 6. graph of the dependence of stress on the displacement of the crossbar – the beginning of loading – lh 300 (blue – prestressed, orange – not prestressed) [3]. figure 7. graph of the dependence of stress on the displacement of the crossbar – the beginning of loading – eporezit epovill-a (blue – prestressed, orange – not prestressed) [3]. figure 8. graph of the dependence of stress on the displacement of the crossbar – the beginning of loading – all prestressed samples (blue – lh 300, green – eporezit epovill-a, orange – sikafloor 156) [3]. 59 j. macháček, e. kafková, v. kabičková et al. acta polytechnica ctu proceedings r es in sa m pl e b h av g a c f f ir s t l m f e d e p w i p σ r e s σ t σ m f ∆ σ st d ty pe nu m be r [m m ] [m m ] [m m 2 ] [n ] [m m ] [n m ] [m m ] [m m ] [m m 3 ] [m m 4 ] [n ] [m p a] [m p a] [m p a] [m p a] de v si k a sik afl oo r 2. 01 10 0. 0 20 .9 20 90 .0 12 41 10 0. 0 62 .1 10 .5 3. 5 72 80 .2 76 07 7. 7 32 48 3. 12 7. 93 8. 52 2. 52 0. 29 2. 02 10 0. 0 20 .6 20 62 .0 14 02 10 0. 0 70 .1 10 .3 3. 5 70 86 .4 73 06 0. 9 32 48 3. 18 7. 93 9. 89 1. 22 0. 46 2. 03 10 0. 0 20 .8 20 84 .3 12 67 10 0. 0 63 .4 10 .4 3. 5 72 40 .7 75 46 0. 6 32 48 3. 13 7. 93 8. 75 2. 31 0. 17 lh 30 0 3. 01 10 0. 0 26 .7 26 70 .0 23 40 10 0. 0 11 7. 0 13 .4 3. 5 11 88 1. 5 15 86 18 .0 30 67 2. 05 7. 93 9. 85 0. 13 0. 03 3. 02 10 0. 0 25 .1 25 10 .0 21 00 10 0. 0 10 5. 0 12 .6 3. 5 10 50 0. 2 13 17 77 .1 30 67 2. 24 7. 93 10 .0 0 0. 17 0. 01 3. 03 10 0. 0 18 .9 18 90 .0 13 20 10 0. 0 66 .0 9. 5 3. 5 59 53 .5 56 26 0. 6 30 67 3. 43 7. 93 11 .0 9 0. 27 0. 04 le po r ez it ep ov ill -a 5. 01 10 0. 0 21 .7 21 66 .3 12 93 10 0. 0 64 .7 10 .8 3. 5 78 21 .7 84 72 1. 7 29 95 2. 72 7. 93 8. 27 2. 39 0. 35 5. 02 10 0. 0 23 .7 23 73 .7 15 39 10 0. 0 76 .9 11 .9 3. 5 93 90 .5 11 14 49 .5 29 95 2. 38 7. 93 8. 19 2. 11 0. 20 5. 03 10 0. 0 22 .8 22 75 .7 16 64 10 0. 0 83 .2 11 .4 3. 5 86 31 .1 98 20 7. 5 29 95 2. 53 7. 93 9. 64 0. 82 0. 55 table 4. prestress losses [3]. durability of the structure as well as on the design of subtle elements and the maximization of the use of the compressive strength of concrete. fundamental questions that would enable more frequent use of this type of construction is still a large number and dynamic stress, fire resistance are worth mentioning and tests that consider the durability of the structure [3]. acknowledgements the work on this paper was supported by czech science foundation grant no. 22-14942k entitled “possibilities of using natural fibers for the production of hybrid textile reinforcement in concrete” the authors would like to acknowledge all financial assistance provided to support this research. references [1] m. foglar, m. frantová, p. jiříček. betonové konstrukce 3: navrhování betonových konstrukcí na msp, úvod do předpjatého betonu [in czech; concrete structures 3: design of concrete structures for small and medium-sized buildings, introduction to prestressed concrete]. čvut, 2011. [2] l. laiblová, t. vlach, m. ženíšek. využití technických textilií jako alternativní výztuže do subtilních fasádních panelů z vysokohodnotného betonu [in czech; use of technical textiles as alternative reinforcement for subtle facade panels made of high-performance concrete], 2015. [3] t. blažek. výroba desek z předepnutého textilního betonu [in czech; production of prestressed textile reinforced concrete slabs]. master’s thesis, czech technical university in prague, faculty of civil engineering, 2020. [4] a. a. abdelrahnam, s. h. rizkalla. serviceability of concrete beams prestressed by carbon. aci structural journal 94(4):447–454, 1997. [5] y. du, m. zhang, f. zhou, d. zhu. experimental study on basalt textile reinforced concrete under uniaxial tensile loading. construction and building materials 138:88–100, 2017. https: //doi.org/10.1016/j.conbuildmat.2017.01.083 [6] m. zawam, k. soudki, j. s. west. factors affecting the time-dependent behaviour of gfrp prestressed concrete beams. journal of building engineering 24:100715, 2019. https://doi.org/10.1016/j.jobe.2019.02.007 [7] y. du, x. zhang, f. zhou, et al. flexural behavior of basalt textile-reinforced concrete. construction and building materials 183:7–21, 2018. https: //doi.org/10.1016/j.conbuildmat.2018.06.165 [8] únmz. zkoušení ztvrdlého betonu – část 3: pevnost v tlaku zkušebních těles [in czech; testing hardened concrete – part 3: compressive strength of test specimens], 2020. [9] únmz. zkoušení ztvrdlého betonu – část 5: pevnost v tahu ohybem zkušebních těles [in czech; testing hardened concrete – part 5: bending tensile strength of test specimens], 2020. [10] w. brameshuber. report 36: textile reinforced concrete-state-of-the-art, 2005. 60 https://doi.org/10.1016/j.conbuildmat.2017.01.083 https://doi.org/10.1016/j.conbuildmat.2017.01.083 https://doi.org/10.1016/j.jobe.2019.02.007 https://doi.org/10.1016/j.conbuildmat.2018.06.165 https://doi.org/10.1016/j.conbuildmat.2018.06.165 vol. 54/2025 prestressed textile reinforced concrete with carbon . . . [11] w. brameshuber, t. brockmann, b. banholzer. analytical evaluation of the softening behaviour of fine grained concrete. in fracture mechanics of concrete structures (framcos-5), vol. 183, pp. 7–21. 2004. [12] t. vlach. soudržnost uhlíkové a čedičové kompozitní výztuže s uhpc [in czech; cohesion of carbon and basalt composite reinforcement with uhpc], 2014. 61 acta polytechnica ctu proceedings 54:55–61, 2025 1 introduction 2 materials and samples 2.1 high performance concrete 2.2 carbon fibre reinforced polymer 2.3 epoxy resins 2.3.1 sikafloor 156 2.3.2 lh 300 + hardener h 287 2.3.3 eporezit epovill-a 2.4 samples production 2.5 experiment 3 results 3.1 sikafloor 156 3.2 lh 300 3.3 eporezit epovill-a 4 conclusion acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2024.46.0030 acta polytechnica ctu proceedings 46:30–39, 2024 © 2024 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague “smart cities” and their vulnerability oldřich krulíka,∗, jan koloucha, marek pačmagb a ambis university, lindnerova 575/1, 180 00 praha 8 – libeň, czech republic b czech technical university in prague, faculty of biomedical engineering, náměstí sítná 3105, 272 01 kladno 1, czech republic ∗ corresponding author: oldrich.krulik@ambis.cz abstract. in most discussions, “smart cities” are perceived as a largely positive phenomenon that improves the safety but above all the comfort of its inhabitants. the present paper constructively and critically analyses the approach of de facto unregulated development of “smart cities” with emphasis on the risks associated with this phenomenon. examples from recent years, not only in the context of developments in ukraine, show that modern, technological solutions, i.e. e-government tools, can become a target or even a tool of variously motivated attackers (criminal groups, foreign powers). a “smart” city is often potentially more vulnerable than agglomerations managed more traditionally. this paper aims to demonstrate the possible risks through case studies and determine whether there are more comprehensive theoretical approaches to the subject. keywords: smart cities, threats, adaptability, risk management. 1. introduction the presented study aims to interconnect several aspects related to the concepts of “smart cities”, including those that are not immediately obvious. the authors operate with the working hypothesis that positive assumptions dominate about the topic, insofar as certain “pessimistic” expectations are already publicly presented. some examples are related to the significant expansion of information and communication technologies and the potentially rising crime rate in this regard. the authors will therefore try to summarize several lines of concern related to the expansive use of the “smart cities” concept (mentioning, that the agenda needs to be perceived as all inventions in human history – as a “good servant and bad master”). (1.) in the first chapter, the authors try to monitor whether the topic is addressed by international organizations for police cooperation, or whether a certain criminogenic potential has been identified about “smart cities”. (2.) the next part is dedicated to the role of companies that offer technological solutions for “smart cities”. many of these companies come from the people’s republic of china, which may carry certain security connotations. attention is also paid to relevant experience concerning the coronavirus situation. (3.) the third part of the article focuses on the technological and cybersecurity side of things. the focus is primarily on the risks and the possibilities of their elimination in the case of building and maintaining “smart cities”. the concept of a “smart city” during a military conflict is also being specifically pursued. as a follow-up to the previous chapters, observations related to the concept of “smart cities” within ukraine will be telegraphically mentioned. 2. materials and methodology in this article, the authors present an analysis of the four above-mentioned areas (figure 1), linking them and presenting partial conclusions related to the building and use of “smart cities”. the theme of internal security (crime, crime prevention) is thus linked to challenges in the field of industrial competition (and other challenges related to dependence on non-european suppliers of technological goods and services – a process that has been intensified by the coronavirus situation), the effects of armed conflicts (regarding the agglomerations), with special emphasis on the current experience of ukraine. based on an analysis of selected cyber-attacks and risks associated with smart cities, recommendations were made to improve cyber security in this regard. the study also aims to a certain extent to bridge the absence of complete methodological concepts – when the concepts of “smart city” and security are often limited to information, respectively information and communication technologies, the internet of things, and similar aspects, while they usually do not expand much into the area of local public order affairs in the more traditional sense of the word. the authors were also inspired by some existing studies, from which emerge some useful observations. new possibilities traditionally bring with them new challenges, which may not always be entirely positive. the effects and externalities of the processes and concepts of building smart cities urgently require the interest of the scientific community and responsible agents [1]. questions can be asked, for example, 30 https://doi.org/10.14311/app.2024.46.0030 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 46/2024 “smart cities” and their vulnerability figure 1. potential internal and external security-related challenges, which can play a role concerning the “smart cities” agenda. whether general assumptions about the solution uh it’s a longof “smart cities” are based on evidencebased conclusions, or whether they are more or less unfounded guesses. a potential security incident could put such overly optimistic expectations into another perspective [2]. it is necessary to directly assume that any processes and models, including the modernization of cities, will bring with them positive and negative impacts, including the building of new barriers (digital divide, etc.) [3]. cities (and not just “smart” ones) are under increasing pressure to address complex, interrelated challenges (air quality, climate change, carbon footprint reduction, transport, housing affordability, social inequality, biodiversity, and more). at the same time, it can be stated that most developed agglomerations in many countries of the world already contain “smart” elements, such as sensors that monitor air quality, traffic, or street lighting intensity [4]. the promotion of the circular economy (systems in which the waste of one process becomes the fuel of another activity) is not a side of attention [5]. technological optimism or pessimism around new ambitious projects generally concentrates on several themes [6]: (1.) concerns around fairness or inclusiveness (or more generally the legitimacy of democratic governance at the municipal/city level). (2.) concerns about technologies, or specific measures, including the concentration and operationalization of the widest possible spectrum of data, which could expand into the privacy of the widest public. certain types of data are more sensitive or personal than others, and also raise more concerns about possible misuse or “monetization” (for the needs of targeted advertising, or even more negative purposes). (3.) other dimensions of this topic relate to predictive policing or crowd management. however, the data can also represent an essential tool for revealing and correcting some negative trends that may not always be obvious without this data (for example, uneven distribution of services in the territory, which leaves behind various socially excluded locations). (4.) concerns about the sustainability of life in the agglomeration, including the position of marginalized communities. (5.) concerns about insufficient governance transparency. 3. “smart cities”, local public order affairs, and the international organizations for police cooperation examples from recent years show that modern, technological solutions, i.e. e-government instruments, can become a target or even a tool of variously motivated attackers, typically criminal groups or foreign powers. a “smart” city is potentially more vulnerable mainly due to the lack of implementation of security measures or their imperfect interconnection with other components of the information and communications infrastructure. in terms of protecting “smart cities” from criminal activities, interpol and europol do agree on the topic – but the whole agenda is still only in the initial phase. bridging this gap will require a significant effort by several actors, which this study aims to contribute to. in the case of attacks by a foreign power (sovereign state), this issue must be addressed particularly concerning the purpose of the attack. it may still be criminal activities (see e.g. attacks by state-sponsored criminal groups from the russian federation or democratic people’s republic of korea) or attacks like an act of war (see section 6.1). so if we return to the role of international police structures, then along with unmanned aerial vehicles, border security, 5g networks, environmental crime, and others, the topic of “smart cities” was included in the program of the interpol world conference in singapore in 2019. the objective of the conference was to create a global innovation agenda that will present 31 oldřich krulík, jan kolouch, marek pačmag acta polytechnica ctu proceedings the cooperation of the police and private sector [7]. it is also possible to mention the working document of the interpol innovation center, entitled “scanning for the future(s) of policing: first steps towards a new global paradigm” [8], issued in march 2022, where nine basic areas of the global transformation and interconnection were detected (including “smart cities”). it was assumed that in 2050, about 68 percent of the world population will live in cities, which is associated with higher cyber vulnerability in the form of hacking, remote disabling of electricity supplies, etc. within the document, chapter 2 (new citizen relations: community policing) deals with the topic in particular: “in a digital world, it could be increasingly important for police to expand their online community contact and crime reporting platforms. building a strong relationship with communities, such as through dedicated community teams, has long been a key success factor for effective policing. now technological advances are creating new opportunities for positive interactions. multi-channel communication, including app-based crime reporting, can enhance citizen participation and experience while providing real-time awareness to police and enabling them to be more responsive to the needs of their constituents. digital engagement can also be augmented by automated chatbots and ai solutions to provide faster, more tailored services to communities while lessening the stress on limited resources – both online and offline... meanwhile, ai-enhanced analytics could further reduce demand on control rooms and contact centers, for example by indicating if the person calling is a serial caller, particularly vulnerable or wanted in connection with another offence elsewhere.” [8] if we study europol’s document entitled europol programming document 2022-2024, special emphasis is placed on several strategic priorities – but “smart cities” are not mentioned in the entire document at all – so it is possible to conclude, that this agenda is not (yet) a priority for europol [9]. as far as the position of europol in this regard is concerned as ambiguous, since 2020 there was an increase in funds for europol by eur 30 million, for example for the further development of automated biometric identification systems [10]. 4. technologies and “smart cities” – including concerns about the limitation of the standard of human rights protection in the context of the pandemic the massive proliferation of “smart cities” may provide another area for the proliferation of potentially unreliable technologies or applications that may pose serious risks not only to the protection of personal data but also to the cybersecurity of a given city or state. another aspect that might need to be considered is cyber and personal data security. several united states and european agencies marked chinese technologies and applications as a security challenge. some euro-atlantic countries are now pushing internet service providers to discontinue the use of chinese hardware (industrial-grade routers, switches, receivers, etc.) in their networks (or at least in the applications involving critical infrastructure). the covid-19 crisis in the people’s republic of china has greatly accelerated the testing of some technologies and equipment for “smart cities” (drones, robots, surveillance applications, security cameras with artificial intelligence, etc.). the situation has also caused activism in terms of global support, donation, and export of some “smart” technologies with dual-use (health/surveillance) capabilities, the so-called digital silk road. current developments have only revealed more clearly a trend that has been emerging since 2013 (including rhetoric that pits the alleged effectiveness and strength of china’s system against the alleged weaknesses of “western” models). on the other hand, for the euro-atlantic players in general, there is a risk that they will leave the development of security technologies, including the security dimension of “smart cities”, to authoritarian regimes. the people’s republic of china has made the “smart city” concept a part of its national development strategy. this concept is explicitly mentioned in the 13th five-year plan (for 2016 to 2020), adopted in march 2016 [11–13]. since then, the government in beijing has massively supported the development of “smart cities” across the country. in january 2019, it was reported that a total of 500 “smart city” pilot projects were being developed. the government has also encouraged domestic technology companies to become global leaders and to engage in overseas “smart city” projects [14]. the definition of a smart city through the lens of the people’s republic of china was initially understood as a synonym for a “safe city” and the development of monitoring tools for state security agencies. after all, this (although not only) functionality is offered by several chinese companies on a commercial basis (advanced camera systems, including facial recognition technology; collection and analysis of big data; tracking applications, etc.) [13]. the coronavirus has also indirectly helped local governments in the people’s republic of china to further test and tune up the existing applications and solutions. the year 2020 was in this regard planned as the starting point of the full launch of the “national reputation system”1, which sooner or later will probably form a part of the “smart city” model in 1this concept, also known as the social credit system, was developed since 2008/2009 for monitoring natural and legal persons in the people’s republic of china, with an emphasis on various aspects of their economic and social behavior (social credibility) [15]. 32 vol. 46/2024 “smart cities” and their vulnerability this country. in early march 2020, the government in beijing announced stimulus measures to help the economy recover from the coronavirus-related crisis (support for the rapid development of 5g base stations, large data centers, artificial intelligence, the internet of things, etc.) [16]. an approach that combines big data analysis and “social management”, in other words, expects the specific use of data by government bodies for various purposes, including anticipating and preventing protest movements. the registration of an individual’s real name is already mandatory to buy medicine or to use public transport. cloud systems, used by police, are increasingly analyzing individuals’ data, including social media accounts, to identify potentially dangerous trends. the term “predictive police” thus takes on a completely new dimension. this also applies to the counter-terrorism agenda. in particular, xinjiang province is becoming an incubator for high-tech surveillance tools2. companies from the people’s republic of china are increasingly able to provide a “comprehensive smart city package” that connects the various components through partnerships between the nation’s technology companies. data collection efforts are facilitated by the fact that the country is home to internet and e-government industry champions (baidu, alibaba, xiaomi) that have ambitions to expand globally [17]. these companies also quite often offer their services abroad collectively and are increasingly approaching european cities and other public institutions, offering to “improve” their infrastructure (cities, ports, airports, railway stations, etc.). at the same time, the “security” side of the agenda is usually promoted as well [18]. already in december 2019, after all, beijing explicitly mentioned “smart cities” as an area of future cooperation between the people’s republic of china and the european union. many european regional authorities or municipalities may choose such solutions because they do not analyze the geostrategic aspects of such purchases or investments [19]. specific examples of cooperation and transfer of knowledge and technology in europe include: • spain: municipalities such as mazarrón (province of murcía) received temperature-measuring drones in 2020 [20]. • italy: torino received drones for public space monitoring, also in 2020 [21]. • serbia: in april 2019, huawei launched a “smart city pilot project” in the city of niš. this is probably the most significant relevant project of the people’s republic of china on european soil. the concept is trying to include smart lighting and mobility services. this follows the project in belgrade, 2xinjiang (uyghur) autonomous region is the territory where the surveillance system is reinforced with the dense camera system, including facial recognition. it is also associated with a smartphone app that police forces are using to get quick and comprehensive data about individuals. where the city signed a memorandum of understanding with huawei. the serbian capital is already equipped with huawei’s facial recognition surveillance cameras [22]. beijing promotes its concept of “smart cities” through bilateral and regional frameworks, such as the people’s republic of china – association of southeast asian nations (asean) summit, or the forum for cooperation with the post-soviet central asian republic, especially under the banner of the “belt and road initiative”. in addition to donations, beijing promotes its influence through a series of training programs provided to officials and technicians from abroad, especially from developing countries (they are trained in, among other things, how the context can be used for specific social, political, or security purposes) [23]. 5. cybersecurity of “smart cities” all components of “smart cities” can be exposed to new vulnerabilities that can be exploited by cybercriminals or state-sponsored actors. high complexity, high interdependency, and intensive communication lead to unbounded attack surface and cryptography-related issues [24]. the potential consequences of cyber attacks on smart cities are significant, ranging from economic losses and infrastructure damage to threats against public safety and privacy. therefore, it is crucial to implement robust cybersecurity measures. as far as the basic approach is concerned when building “smart cities”, it is imperative that the sd3 (security by design, default, and deployment) rule is respected. this rule assumes that security is an integral part of all processes. at the same time, other regulatory connotations (international and national) should be taken into account. typically, this will include the need to respect the protection of personal data as set out in regulation (eu) 2016/679 of the european parliament and of the council of 27 april 2016 on the protection of natural persons with regard to the processing of personal data and on the free movement of such data (general data protection regulation) or the rules set out in directive (eu) 2022/2555 of the european parliament and of the council of 14 december 2022 on measures for a high common level of cybersecurity across the union (nis 2 directive) to ensure cybersecurity. disruption on a relatively large scale can be thus achieved remotely, without the physical presence of foreign actors on the territory of another state. such a remote attack is then much easier to mask, deny, or deflect as an act of terrorism – instead of foreign power aggression. to enhance the cybersecurity of “smart cities”, several measures should be adopted to mitigate potential risks and protect the digital infrastructure. the authors have to mention several crucial cybersecurity measures that can be implemented: 33 oldřich krulík, jan kolouch, marek pačmag acta polytechnica ctu proceedings (1.) risk assessment and planning. to identify potential vulnerabilities and threats in “smart city” systems, a thorough risk assessment is essential. risk assessment helps in developing comprehensive cyber security plans and strategies for effective risk mitigation. (2.) securing internet of things (iot) devices. “smart cities” heavily rely on internet of things devices, such as sensors, cameras, and connected infrastructure. securing these devices is essential to prevent unauthorized access, data breaches, and potential exploitation. (3.) network security. strong network security measures, including firewalls, intrusion detection and prevention systems, and encryption protocols to safeguard “smart city” networks from unauthorized access and data breaches should be implemented. (4.) secured communication channels. it is essential to secure communication channels between devices, sensors, and systems within the “smart city” infrastructure. this can be achieved through encryption and authentication mechanisms to prevent interception and tampering of data. (5.) regular security updates. all systems but also applications used should be updated daily. updates should include security patches, firmware updates, and software upgrades. known vulnerabilities should also be addressed immediately and patches applied to mitigate potential risks. (6.) access control and authentication. strict access controls and multi-factor authentication mechanisms need to be put in place to ensure that only authorized personnel have access to critical systems and sensitive data. these measures can help prevent unauthorized access by cyber attackers. (7.) data encryption. encryption of data both at rest and in transit must be provided to protect it from unauthorized access. this includes encryption of data stored in databases, transmitted over networks, or processed by various “smart city” applications. (8.) incident response and recovery plans. comprehensive incident response and recovery plans must be in place to deal with cyber-attacks immediately. these plans should include procedures for identifying, mitigating, as well as recovering from security incidents and temporary fallback plans to handle the critical city services without the respective network backgrounds. (9.) vendor security assessments. a key component of building “smart cities” is conducting a thorough security assessment of vendors that provide smart city technologies or services. in particular, it is necessary to identify whether suppliers are adhering to security practices and standards, what their previous reputation is, etc. the supplier and its technology or services may be the target of supply chain attacks, but at the same time, it may be an entity that will carry out one of the previously mentioned attacks. (10.) data protection. strict measures to protect personal data, as well as the accompanying metadata, must be put in place to comply with the relevant regulations and to protect both personal and operational data collected by “smart city” systems. in this respect, it is essential to put in place measures to allow for anonymization or pseudonymization of data. (11.) regular security audits. it is imperative to ensure that regular security audits and penetration tests are carried out to identify vulnerabilities and weaknesses in the infrastructure. these audits and tests can help to identify potential security weaknesses and enable their timely remediation. it’s important to note that the specific cybersecurity measures implemented in a smart city may vary depending on the technologies and systems deployed. cybersecurity in “smart cities” requires a multi-layered approach that includes technology, policies, and active collaboration between stakeholders (system administrators, vendors, etc.). by prioritizing cybersecurity measures, “smart cities” can reap the benefits of advanced technologies while effectively mitigating potential risks and ensuring the safety and privacy of their citizens. a comprehensive cybersecurity strategy should be developed, considering the unique characteristics and requirements of each “smart city” project. 6. military aspects: “smart city” as a battlefield another topic that cannot be ignored here, is the eventuality that the city with “smart infrastructure” would become the scene of military conflict. examples of smart agglomeration are in table 1 [25]. an urbanized world, heavily dependent on modern technologies, contributes to the ever-increasing complexity of future military operations. what is positive in times of peace can be a serious challenge in times of war. let’s imagine the situation: an enemy has compromised the 5g network in a certain big city. traffic lights are not operating and the traffic situation is unmanageable. garbage is not taken out and attracts rodents. disruption of delivery of drinking water or electricity supplies can also occur. if we think of “smart cities” as a battlefield, we need to focus on the infrastructure components that can be attacked in the first row: (1.) physical objects. remotely controlled machines supporting critical services, such as water treatment, can be physically destroyed to affect the comfort of city residents. similarly, electricity supply may be disrupted. 34 vol. 46/2024 “smart cities” and their vulnerability attacker’s position •it is important that the occupied agglomeration does not collapse or disintegrate. •we have to prevent a humanitarian disaster. •it is different if the occupied agglomeration collapses, or decomposes. defender’s position •it is important that the occupied agglomeration does not collapse or fall apart. •in the event of retreat or evacuation of the city, it is expected no intentional damage to its infrastructure. •it is irrelevant, if the agglomeration, occupied by the enemy collapses. •possible problems that arise can be used for international politics or propaganda purposes. table 1. four basic model situations regarding access to “smart agglomeration” in the context of military operations. (2.) information and communication technologies infrastructure. in particular, 5th and nextgeneration internet of things (iot) enabled networks – which can also be the target of some forms of attacks. (3.) sensors. these may be tracking or reconnaissance sensors that are accessed illegitimately, or they may be misused to transmit false data. (4.) computing capacities. data of all kinds can be altered and lead to confusing results. (5.) transportation. physical destruction cannot be ruled out, nor various forms of “remote” communication blocking. (6.) access to cloud services may be temporarily interrupted; data may be manipulated and altered. (7.) unauthorized access to analytical platforms or manipulation of data will make it possible to change decision-making algorithms. (8.) access to mobile or other applications may be denied (including online banking or other functions). without necessarily being “smart cities”, the considerable experience can be drawn from the situation in the middle east, where large cities such as mosul or aleppo have become examples of the collapse that has affected hundreds of thousands of inhabitants [26]. directly in the context of the war against ukraine, it is possible to mention the case of the “remote” takeover of the airport in kyiv in 2016 – which, moreover, was reacted to in the kremlin’s propaganda environment by saying that if the country is unable to defend its infrastructure, it deserves an attack of this kind (the attacker’s guilt, traditionally, was not mentioned at all) [13]. these or other challenges of the security discourse lead to the following statements: • in a city, remotely controlled by the enemy, the quality of life may be reduced considerably. • the progress and habituation of the public to a certain standard are so complex that “returning to offline (non-smart) mode” is no longer an alternative for most cities. • the achilles heel is represented by data centers controlling processes, owned and operated by third parties, which present another set of opportunities for attackers. • the scenario does not even need to be a conflict between states, but also an incident caused by terrorists or violent extremists. 6.1. “smart cities” and war against ukraine the “smart cities” agenda has been monitored within ukraine at least since 2016. the committee for housing and land management received a request from the ministry of regional development, construction and housing and communal services of ukraine to preparea profile of a “smart sustainable city”. voznesensk was chosen as a pilot city (approximately 30 000 inhabitants, mykolayivsk region, in march 2022 the city was heavily damaged). the creation of the related study started in december 2017 and was completed in january 2019. based on the analysis, a list of challenges ukraine faces on the way to “smartization” was created [27]. it was evident, that many cities, despite having some development potential, are trying to use some individual innovations rather than coherent strategies. the problem was also the insufficient integration of activities that were carried out selectively. a positive fact has been the increasing involvement of ict in the city management process. the added value was the systematic improvement of the quality of public services and the increase of social participation in the public decision-making process, which makes it possible to integrate local society and build trust in public institutions [28]. before the outbreak of the current phase of the conflict in ukraine, there was a consensus that, due to the high population density, cities show significant potential for creativity and innovation, energy savings, and environmental friendliness, creating a positive dynamic interaction of these elements for develop35 oldřich krulík, jan kolouch, marek pačmag acta polytechnica ctu proceedings ment [29]. the cities in motion index was chosen for the calculations of relevant indices for ukrainian cities, which takes into account the maximum number of indicators and assesses such socio-economic aspects of the possible development of cities as, for example, human capital, social cohesion, governance, mobility and transport, urban planning and technology [30]. examples of best practices in other countries, especially poland (warsaw, lublin) were also mapped. at the same time, it was found that both in poland and ukraine the concept is only in the initial phase of implementation [31]. in the context of ukraine’s ambitions for accession to the european union, a sustainability project, smart city kharkiv, was launched, initially phased until 2030 [27]. the year 2022, characterized by the escalation of the conflict, continues to be marked by the “smartization” and decentralization of infrastructural processes – in light of the massive destruction and deterioration of living conditions in several large cities of ukraine. several instruments are specifically targeted at rural areas, such as the use of “smart fertilizers”, leading to a less negative impact on the environment, and maintaining biodiversity and food security during war [32]. the key priorities of smart management in the cities of kyiv, poltava, kharkiv, odesa, lviv, and dnipro were determined (multimodal transport systems, cooperation between public and private sector actors, self-managing energy networks, etc.). however, the complex problems that arise from the ongoing war logically complicate the implementation of the concept. at the same time, it is already heard, in the environment of ukraine representatives, that when the fighting ends, the efforts of the authorities will not be to restore the cities to their original state but to “upgrade” them to become pioneers of innovative solutions of all kinds. after about half a year, the war moved into the attrition phase, where the military, human, and economic reserves of the warring countries play a key role [30]. the kremlin perceives the massive destruction of ukraine’s civil infrastructure (electric power grids, heating), ideally across the entire territory, up to the very west, as compensation for unsatisfactory developments on the battlefield. at the turn of 2022 and 2023, there were practically 3 to 4 million people without electricity, running water, and heating, especially in larger agglomerations. it should be noted that kyiv is now, after absorbing the number of internally displaced persons, more populous than berlin, the most populous city in the territory of the european union member states (around 3.6 million people). the most fundamental challenge during the winter of 2022/2023 in kyiv was the water supply – when the electricity goes out, the water in the pipes freezes, and their repair will be extremely expensive. the situation is similar in other cities, especially in odesa. the liberation of kherson can also be seen from this point of view (before the invasion, around 300 000 inhabitants lived here; now it is “roughly 50 000 people” and practically no public infrastructure works there). the same applies to the surroundings of this city, but also to several other regions of ukraine near the front. to this must be added the mining of several localities, which complicates the possible revitalization plans. at the same time, the official speakers of the russian federation did not hide the efforts to encourage emigration from ukraine and strengthen anti-ukrainian sentiments in europe. there are also malicious references to the fact that “power cuts” in ukraine allegedly led to many cases of looting and an outburst of the wide spectrum of crime. the statement of the national police of ukraine on the development of registered crime for november and december 2022, on the contrary, speaks of a decrease in registered crime by 13 to 16 % [33, 34]. police forces, aided by curfews and other restrictive measures, declare an uncompromising stance on looting cases. according to police statistics, this approach is bearing fruit [13, 35]. in summary, it can be stated that ukraine’s ambitions to build “smart cities” have been replaced by ambitions to decentralize energy and other critical infrastructure. the main reason for this approach is to better protect against the cyber attacks that took place at the beginning of the war. in particular, the wiper malware attacks targeted information and communication technologies controlling electricity distribution in ukraine (hermetic wiper, 23 february 2022; isaac wiper, 24 february 2022; caddy wiper, 14 march 2022; industroyer 2 and caddy wiper, 8 april 2022, etc.). in this context, the following preventive or reactive tools that alleviate this situation can be mentioned: • massive investments in digitization, with an emphasis on tools (applications), intended for people who are constantly migrating (nationally and internationally displaced persons), so that these persons also have a channel for communication with public institutions and can prove their electronic identity for various purposes [13]. • building so-called “points of un-breakability”. there are now (february 2023) more than 4 000 such points, and this network is still growing. these are shelters with free heat, electricity, medical supplies, internet, etc. located in schools, medical facilities, private companies, and elsewhere. it can also be a heated tent. this can be seen as a great inspiration for countries whose shelter network is currently very sparse and usually without the mentioned functionalities [13]. • the european commission explicitly mentions ukraine as a pioneer of decentralized energetics (generators, smaller substations, small steam-gas power plants, innovative photovoltaic solutions), as the burden on the environment that this model entails is also important. more than 300 000 power generators can be mentioned, in almost every major 36 vol. 46/2024 “smart cities” and their vulnerability company in the field of production and services, including banks or gas stations [36]. • the e-enemy application, intended for reporting alleged saboteurs, is perceived rather questionably [37– 39]. • cyberspace is becoming an important strategic, tactical, and operational part of the battlefield’s operations. an example might be some ukrainian army operations being canceled due to them being disconnected from starlink [37]. • in addition, it is possible to mention information about the development in transcarpathian ukraine, which is now perceived as a center (cluster) of the development of information and communication technologies. during the first 7 months of the war, around 100 companies and 28 000 experts related to the state-of-art technologies, moved to this region from the rest of the country [1]. 7. results in the current phase, the authors perceive the text as a springboard for other researchers who would focus on topics that can be characterized as security-related meta-aspects of the concept of “smart cities”. given the existing challenges and pressures, it seems possible that the key aspect of the related efforts is not the “city” (or even “big city”), which objectively represents a vulnerable environment, prone to the concentration of a whole range of negatives and weaknesses – but the “smartness”, that can be implemented to any territory (small village, countryside), but also concerning dynamically moving individuals (or groups of individuals, firms, etc.). the issue as a whole also represents a task for progressive public administration, police officers (where the authors are also professionally connected) – but also society as a whole. 8. discussion in the paper, the authors addressed some potentially negative aspects of the concept of “smart cities”. they believe that the sooner these possibilities become the subject of more detailed research, the more likely their possible impacts will not devalue the whole “smart cities” idea. overall, it can be stated that relevant scientific articles, as well as documents, prepared by the international police organizations, perceive the topic only in fragments so far, or about the advantages and opportunities it can bring to law enforcement authorities. large agglomerations are however generally perceived as places where crime tends to be concentrated, as this aspect cannot be perceived (and the countryside cannot be idealized at all). possible investments in surveillance technologies are then an effort to solve the consequences, not the causes of certain negatives. from the point of view of the technological equipment of “smart cities”, the question is to what extent the priorities of free trade are compatible with a certain vigilance towards technologies from outside the european union member states (especially regarding the production of the firms from the people’s republic of china, where the export of technologies can also be seen as part of geostrategic ambitions, the end of which is not easy to see). in this regard, the coronavirus situation represented a multidimensional impulse – many individuals and companies accepted the remotework model, in which it is irrelevant whether a certain person resides in a specific city, near the workplace, or anywhere else. europe has already run out of its “peace dividend”, and for this reason, it seems to be necessary to invest significant financial resources in securing “smart cities” against the robust intervention of a foreign power. this is an additional cost, which was not always expected, but which seems necessary in the current situation. in the event of an armed conflict, large cities become traps that concentrate on several negatives and only a limited spectrum of positives. this statement is directly connected to the experience of the ukraine. this country has involuntarily become a pioneer of solutions, which apparently cannot be perceived as “smart cities”, but as “smart decentralization” or “smart mobility”. any large concentration of people poses a challenge to a functioning infrastructure, which is extremely demanding during the war. priorities of environmental protection or economies of scale have therefore given way in favor of the aforementioned decentralization and mobility. 9. conclusion the concept of “smart cities”, even if we are aware of its positive aspects, poses a significant challenge to modern societies. the concentration of population in cities is not positive in itself. at this point it is necessary to summarise the possible negatives associated with the process of creating “smart cities”: • not all “smart cities” (or their managers respectively) see economic, environmental, and social wellbeing for their current and future residents as their primary objective. • some “smart city” proposals sometimes seem to be motivated by “technology for technology’s sake” (because it is currently “in vogue”, without real need). • large-scale implementation of technologies, without meaningful participation of the interested community, or at least a significant part of it, is also perceived as a step in the wrong direction. • a city can be very “smart” but not necessarily inclusive. it can also be environmentally sustainable, but at the same time, it can be exclusively expensive. • without strict oversight, technology may exacerbate, rather than correct, weaknesses in urban governance. 37 oldřich krulík, jan kolouch, marek pačmag acta polytechnica ctu proceedings the paper presented primarily the risks associated with the use of “smart cities”. through mapping the various vulnerabilities and comparing potential risks, the presented paper may provide a suitable starting point for further research regarding local public order affairs. the whole concept of “smart cities” is also an economic and industrial challenge for europe and the european union, in particular, because many technology companies from the people’s republic of china have already achieved a significant market share in the global market for “smart” technologies and solutions (beating some european or north american competitors). it cannot be ruled out that if euroatlantic companies remain passive, european cities will have to rely on foreign technologies or become uncompetitive compared to cities in other regions that have already incorporated advanced technologies into their daily lives. cybersecurity in “smart cities” requires a multilayered approach that includes technology, policies, and active collaboration between stakeholders (system administrators, vendors, cyber security specialists, etc.). by prioritizing cybersecurity measures, “smart cities” can reap the benefits of advanced technologies while effectively mitigating potential risks and ensuring the safety and privacy of their citizens. a comprehensive cybersecurity strategy should be developed, considering the unique characteristics and requirements of each “smart city” project. “smart cities” will certainly become the environment for future combat operations. as the current conflict in ukraine shows, cyberspace is becoming an important part of modern conflicts and even the battlefields (an example might be some ukrainian army’s operations being canceled due to them being disconnected from starlink). even in the real world, a weak adversary can get a much stronger country into widespread complications. one side may succeed in a fight in the true sense of the word, but its success will be negated by the “winner” losing control of many of “its” agglomerations. examples from recent years, not only in the context of developments in ukraine, represent that modern, technological solutions, i.e. tools of e-government, can become a target or even a tool of variously motivated attackers (criminal groups, foreign powers). at the same time, it can be anticipated that in certain respects the “smart” cities are often potentially more vulnerable than agglomerations managed more traditionally. respective sub-themes and perspectives still exist to a certain extent rather “side by side” and are not interconnected – which the authors to some extent try to bridge in this study. at the same time, the literature related to the mentioned agenda is still relatively limited, not to mention peer-reviewed academic outputs. although these aspects are already on the agenda of international police cooperation organizations, the topic as a whole is still being addressed only in fragments. as a result, it is currently also an assignment for progressive state administration and security forces, which the authors also, to a certain extent, try to represent. acknowledgements the authors would like to thank ambis college, prague, czech republic, for its support. references [1] y. popova, s. popovs. effects and externalities of smart governance. 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https://www.euronews.com/2016/01/18/ukraine-says-cyber-attack-on-kyiv-airport-was-launched-from-russia https://www.euronews.com/2016/01/18/ukraine-says-cyber-attack-on-kyiv-airport-was-launched-from-russia https://www.euronews.com/2016/01/18/ukraine-says-cyber-attack-on-kyiv-airport-was-launched-from-russia https://www.ukrinform.net/rubric-crime/3633228-crime-rate-down-13-in-ukraine-police-chief.html https://www.ukrinform.net/rubric-crime/3633228-crime-rate-down-13-in-ukraine-police-chief.html https://www.ukrinform.net/rubric-crime/3633228-crime-rate-down-13-in-ukraine-police-chief.html https://www.ukrinform.net/rubric-economy/3642827-almost-290000-power-generators-imported-into-ukraine-since-nov.html https://www.ukrinform.net/rubric-economy/3642827-almost-290000-power-generators-imported-into-ukraine-since-nov.html https://www.ukrinform.net/rubric-economy/3642827-almost-290000-power-generators-imported-into-ukraine-since-nov.html https://www.ukrinform.net/rubric-ato/3544619-enemyspotting-chatbot-in-ukraine-boasts-over-344000-reports.html https://www.ukrinform.net/rubric-ato/3544619-enemyspotting-chatbot-in-ukraine-boasts-over-344000-reports.html https://www.ukrinform.net/rubric-ato/3544619-enemyspotting-chatbot-in-ukraine-boasts-over-344000-reports.html https://t.me/evorog_bot https://www.youtube.com/watch?v=vtaxa3omfmw acta polytechnica ctu proceedings 46:30–39, 2024 1 introduction 2 materials and methodology 3 ``smart cities'', local public order affairs, and the international organizations for police cooperation 4 technologies and ``smart cities'' – including concerns about the limitation of the standard of human rights protection in the context of the pandemic 5 cybersecurity of ``smart cities'' 6 military aspects: ``smart city'' as a battlefield 6.1 ``smart cities'' and war against ukraine 7 results 8 discussion 9 conclusion acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2025.53.0028 acta polytechnica ctu proceedings 53:28–32, 2025 © 2025 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague carbonizable calcium silicates by co2 atmosphere in a pressure chamber andrea jančíková∗, michal křištof, karel dvořák brno university of technology, faculty of civil engineering, veveří 331/95, 602 00 brno, czech republic ∗ corresponding author: andrea.jancikova@vut.cz abstract. the paper deals with the possibility of different calcium silicate minerals namely wollastonite (cs), belite (c2s), and rankinite (c3s2) on their curing in co2 atmosphere. it describes the laboratory preparation of these individual minerals from p.a. pure chemical clean materials. for comparison a representative sample of natural mined wollastonite was chosen for carbonation as well. following this, for setting basic parameters analyses determining the properties of the samples by xrd analysis and rietveld evaluation, particle size characterization by laser granulometry and milling fineness by the blaine method. it also addresses the issue of forming samples suitable for carbonation using a hydraulic press and the actual curing and conditions necessary for carbonation of the minerals in pressure chamber. it summarizes the measured results of compressive strengths, weight changes and mineralogical evaluation of the composition of the samples before and after carbonation. in the conclusion, together with a summary of the measured findings, it is pointed out what direction further research in this field should be aimed for. keywords: co2 carbon curing, calcium silicate minerals, belite (c2s), wollastonite (cs), rankinite (c3s2). 1. introduction belite cements, as a low-carbon material for concrete construction and components, are attracting increasing attention of the research world-wide. this is not only because of their relatively low energy consumption, but also because belite clinkers and cements with higher belite content offer higher resistance of the matrix to weather conditions, which can extend the service length. this ensures longer durability for the smaller concrete products such as concrete pavers and tiles compared to traditional portland cement, which is responsible for 7–9 % of global man-made co2 emissions nowadays [1, 2]. unlike conventional portland clinker, which is the most widely used cement worldwide, belitic cement is made up of 40–60 % belite, replacing alite mineral. the industrial production of belitic clinkers requires the same raw materials as portland cement, but in a different proportion, which contributes to sustainability. the ideal sintering temperature for belite clinkers is approximately 1 250–1 350 °c, which is about 100– 200 °c lower than to produce portland clinker. this lower firing temperature leads to a reduction in fuel consumption. the production of belite cement uses on average 15-20 % less energy than the production of portland cement [1–4]. however, the energy savings are slightly compensated by the more difficult milling process, which consumes approximately 10 % more energy than portland clinker. overall, the total energy savings are in the range of 5–10 %. however, this energy saving is insufficient if the fact that belitic cement, without treatment, has very long and slow strength development and develops very low hydration heat [2–5]. this issue is currently being studied extensively and it can be partially avoided by various modifications to the final concrete component. to increase the early strengths of belitic cement, some methods are currently being studied. these are mainly ionic doping of clinker with foreign ions, implementation of nanomaterials into the matrix, curing of the material at higher temperature and humidity and carbonation curing of β and γ-c2s cements in co2 atmosphere. belite appears in several forms under atmospheric pressure: α (hexagonal), αh’, αl’ (orthorombic), β (monoclinic) and γ (orthorombic) [2, 3, 6]. these forms have a very similar arrangement of ca2+ and sio4− ions, except for γ-c2s, which has a different structure. the β form is the most common in industrial clinkers. as mentioned β-belite is very difficult to mill, but when β is transformed into γ-c2s it expands by up to 13 %, leading to a phenomenon called “dusting” where the material spontaneously breaks down. this transformation can be prevented by rapid cooling or the use of stabilisers and is widely used today in portland clinker where gamma γ-c2s is unwanted [4, 7, 8]. on the other hand, it is a very desirable mineral for carbonation curing. among calcium silicates, γdicalcium silicate (γ-c2s) is theoretically relatively easy to sinter. the high-temperature α-c2s can spontaneously turn into γ-c2s powder during cooling, reducing the energy needed for milling. the reactivity of γ-c2s with co2 has been explored as a potential low-carbon building material. research showed that γ-c2s, after carbonation for two hours at 0.2 mpa, can 28 https://doi.org/10.14311/app.2025.53.0028 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 53/2025 carbonizable calcium silicates absorb up to 18.3 % co2 and achieve a compressive strength of 36.4 mpa. carbonation forms caco3 as the main product, along with amorphous silicate structures identified through nuclear magnetic resonance (nmr) [8–10]. for co2 curing of belite and other calcium silicatebased minerals, crucial very specific properties of the curing environment must be achieved. these are mainly the amount of water required to prepare the sample (water coefficient), sample pressing pressure, cement particle size and specific surface area, relative humidity of the curing atmosphere, temperature and co2 concentration [1–4]. according to the carbonation mechanism, moisture acts as a solvent and is critical in carbonation reactions. the ratio of water to solid material (w/s) has a significant effect on the dissolution of ca+ ions and the diffusion of co2 because determines the thickness of the water coating adsorbed on the particle surface. on other hand low w/s ratio can retard ca+ ions dissolution and the diffusion of co2 in due to the very thin water coating on the mineral surface. on the other hand, high w/s ratio may slow down the penetration of ions and slower the rate of co2 diffusion into structure [3–5, 7]. particle size is an internal parameter that significantly influences the rate and efficiency of carbonation. it is generally accepted that finer mineral particle size increases the surface area available for reaction with co2, promotes reaction and space for dissolution of ca+ ions on mineral particles and increases the rate of carbonation. however, ultrafine particles may reduce the speed of carbonation. previous studies have suggested that ultrafine particle size significantly increases the degree of carbonation in the first moment due to the quick formation of carbonate precipitate, which later prevents the particles from encountering water and co2 needed for continuation of the process [5–7]. like the water to solid ratio, relative humidity significantly influences the carbonation efficiency of calcium minerals by affecting water saturation in the pores and capillaries of the materials. the optimum relative humidity content is between 50 to 70 %, when the highest rate of carbonation is achieved [4–8]. all these special conditions need to be investigated individually, along with the question of which of the minerals is relatively easy to produce in the laboratory conditions. then successfully carbonate it to achieve appropriate compressive strengths. this issue of the preparation of individual minerals and their carbonation under the same conditions is dealt with in this article. 2. material and methods 2.1. preparation of raw material powders the mixtures for belite, wollastonite and rankinite samples were prepared from caco3 and sio2. calcium carbonate (caco3, p.a. purity, penta, praha, czech republic), silicon dioxide, (sio2, p.a. purity, penta, praha, czech republic). raw powder materials were prepared according to the required cao: sio2 stoichiometry ratio of 1:1 for wollastonite, 2:1 for belite and repetitively 3:2 for rankinite. due to the requirement of high purity raw material to produce raw powder and proper solid-phase reaction, the loss on ignition (loi) was carried out on sio2. the sio2 was milled at dry first in a fritsch pulverisette 6 planetary mill in an agate 500 ml milling bowl with 25 agate milling balls. mill was set at 350 rpm for 1 min. the firing itself was carried out at a temperature of 1 000 °c for 60 minutes. after firing, loi of sio2 was measured to be 1.27 %. the 160 g of those raw materials in the proper ratios were first dry mixed by hand in a laboratory porcelain bowls. afterwards transferred to a 500 ml agate milling bowl with 25 grinding balls with a diameter of 20 mm. wet milling was carried out in 180 ml of water in a planetary mill (fritsch pulverisette 6). mill was set at 350 rpm for 10 min. after milling, the suspension was poured back into a clean laboratory bowl and placed in a laboratory dryer (binder c 170) at 105 °c for 24 hours. during drying the nodules 10–15 mm in diameter were formed spontaneously. 2.2. firing procedure in the next step, a series of two-step firings were carried out in a superkhantal furnace (classic 2017 s). the samples were placed in platinum crucibles with different volumes ranging from 40 to 120 ml. for belite, the first step firing was set to 900 °c for 60 min with a ramp of 8 °c min−1. during this step calcination of caco3, change in volume and increase in reactivity occurs. in the second step, the actual solid phase reaction between cao and sio2 takes place at 1 450 °c for 60 min. during the production of gamma belite, the sample was left in the furnace and spontaneously cooled to 400 °c. the sample was heated to this temperature for 12 hours to ensure the conversion of beta belite to gamma belite. the samples were then slowly cooled to laboratory temperature. for wollastonite, the first step was the same as for belite and in second step the temperature was set at 1 100 °c for 120 min. samples were then spontaneously cooled in the furnace. for rankinite, the first step was the same as for belite and in second step the temperature was set at 1 450 °c for 600 min. samples were then spontaneously cooled in the furnace. the samples were divided and prepared for the related analyses. stored in labelled (pe) bags. 2.3. sample preparation for xrd analysis samples selected for xrd analysis were milled in a vibratory disc mill (rs 200, retsch, haan, germany) 29 a. jančíková, m. křištof, k. dvořák acta polytechnica ctu proceedings at 1 100 rpm for 20 s. after that, 5 g of powder sample was milled to the required fineness for x-ray analysis in a mill (mccrone micronising mill, glen creston, london, uk) for 150 s in 15 ml of isopropanol. the xrd analysis was performed using a multifunctional diffractometer (xrd, empyrean, panalytical b.v., almelo, the netherlands). the θ-θ reflection bragg–brentano para focusing geometry device is equipped with a cu anode (λ = 1.54184 å), programmable divergence slits, and a pixcel3d detector (empyrean, panalytical b.v., almelo, the netherlands) the purity of the samples was verified via xrd using the rietveld method. the icsd database (released in 2012) was used to qualitatively analyse the diffraction patterns. 2.4. particle size characterisation particle characterization was performed on untreated samples immediately after firing and cooling by a malvern mastersizer 2000 laser granulometry (malvern panalytical ltd., malvern, uk) with a hydro 2000 g fluid dispersion unit, using propan-2-ol as the dispersing agent was used for determining the granulometry. for natural wollastonite, the granulometry was checked both before and after milling of the sample. 2.5. specific weight and specific surface area the same samples as for particle characterization were also tested for specific weight by the accupyc ii 1340 helium pycnometer and specific surface area by the blaine method using the zeb pc blaine star automatic apparatus. 2.6. preparation of carbonizable samples the 80 g of sample and 8.8 g of water were hand-mixed in laboratory bowl and then inserted into a cylindrical mould adapted for hydraulic pressing. the filled cylinder was placed in a hydraulic press and loaded with 10 mpa pressure for 30 seconds. in the first step. in the second step the pressure was interrupted for 30 seconds. in the third step, the sample was again loaded with a pressure of 16 mpa for 120 seconds. the produced solid samples were weighed and placed in the carbonization chamber. 2.7. carbonation and compressive strength test the chamber was set to a pressure of 0.6 mpa, the humidity in the chamber was above 70 % and the co2 concentration was almost 100 %. the samples were left in these conditions for 24 hours. due to the small amount of water in the samples, a wet sponge was also inserted into the chamber to increase the in the chamber. after the carbonization process was completed, the samples were removed from the chamber, weighed and their diameter and height were measured using a calliper. these values were entered into a software that recorded the deformation of the solids during the application of the compressive force. the specimens were placed in a press and loaded at a constant rate until failure. the software automatically converted the maximum loading force to deformation. the fractured specimens after the compressive strength test were ground in a rotary disc mill for xrd and dta analysis. 3. results and discussion to compare the different powder materials, natural mined wollastonite was added to the laboratory created belite, wollastonite and rankinite samples prepared according to previous chapter material and methods. for the carbonation, the aim was to achieve the most similar properties of the samples by milling, both in terms of granulometry and fineness of milling, to be able to clearly compare the different minerals and their ability to carbonate under the same conditions. table 1 summarizes the granulometry measurement of minimum d(0.1), mean d(0.5) and maximum d(0.9) particle size of the suspension. measured by the wet method in propan-2-ol. n label d(0.1) d(0.5) d(0.9) 1 belite 1.771 14.275 68.622 2 lab. wollastonite 1.539 6.618 16.439 3 nat. wollastonite 2.273 11.586 39.512 4 rankinite 3.183 9.333 27.523 table 1. results of particle characterisation by laser granulometry. the fineness of the belite has been considered as a reference because it is determined by the process of converting beta belite to gamma belite during firing and cooling. the other minerals produced, and the natural wollastonite were treated by grinding and their resulting granulometry was slightly finer. the specific weight and milling fineness of the individual samples are given in table 2. from the measured values it can be concluded that the samples had very similar specific weights, which differed between the highest and lowest values only by approx. 6 %. determination of the specific surface area was necessary to perform any additional milling of the samples and to achieve a specific surface area as similar as possible for all samples. n label ρ [g cm−3] [cm−2 g] 1 belite 2.9954 4 600 2 lab. wollastonite 3.0827 5 300 3 nat. wollastonite 3.0451 5 500 4 rankinite 3.0829 4 100 table 2. results of specific weight and milling fineness (specific surface area). 30 vol. 53/2025 carbonizable calcium silicates it can be stated that the achieved specific surface area of the samples, apart from the natural wollastonite, did not need to be modified. the original specific surface of natural wollastonite was 3 500 cm2 g−1. this sample was milled to a finer specific surface of 5 500 cm2 g−1. table 3 shows the mineral content of each sample before the carbonation process, measured by xrd rietveld analysis in software highscore plus 4.1. label 1 2 3 4 γ-c2s 62.3 39.1 β-c2s 35.9 69.8 0.0 25.2 c3s 1.8 wollastonite 25.7 21.0 0.0 pseudo-wollastonite 18.0 cristobalite 1.2 free lime 3.3 calcite 29.5 quartz 9.1 grossular 16.9 diopside 9.9 albite 10.4 clinochlore 1.4 muscovite 1.2 rankinite 14.4 gehlenite 3.3 sum [%] 100.0 100.0 100.0 100.0 table 3. results of xrd analysis according to rietveld refinement before carbonation in %. it is evident that the production of rankinite and wollastonite was not very successful, as the resulting samples contained only 14.4 % and 25.7 % of the required mineral. nevertheless, the samples contained enough minerals that could be carbonated. weight changes after 24 hours of carbonation in the carbonation chamber indicate the formation of carbonates. therefore, a large increase in these carbonated phases is predicted for belite, laboratory wollastonite and rankinite. compressive strength is critical property for carbonizable clicker like powder materials. the results of the increase in weight and compressive strength are given in the table 4. the measured values from table 4 are shown graphically in figure 1. n label ∆m [%] σ [mpa] 1 belite 15.12 56.18 2 lab. wollastonite 15.54 14.41 3 nat. wollastonite 0.42 0.94 4 rankinite 12.06 24.47 table 4. results of changes in weights and resulting compressive strengths. the results of xrd analysis after carbonation show the conversion of minerals to carbonates (mainly calcite, aragonite and vaterite). results are shown in figure 1. graphical representation of the results from the table 4. table 5. below. the analysis showed the formation of carbonation products that were not present in the samples prior to the carbonation process. calcite is the most abundant carbonate, except in the rankinite sample, where mainly aragonite was formed as a carbonation product. the dta confirmed the results of the xrd analysis and the assumptions that natural wollastonite forms a minimal amount of carbonation products. label 1 2 3 4 γ-c2s 13.5 3.2 β-c2s 39.8 25.9 31.1 c3s 2.5 wollastonite 17 13.6 cristobalite 1.0 rankinite calcite 33.5 41.9 24.0 3.0 vaterite 8.9 13.1 12.6 aragonite 1.8 1.1 21.2 quartz 7.4 other minerals 17.0 55.0 28.9 sum [%] 100.0 100.0 100.0 100.0 table 5. results of xrd analysis according to rietveld refinement after carbonation. however, the resulting sample contained the highest amount of calcite, 45.1 %. the higher values for all samples can be explained by the fact that the amorphous phase, which affects the absolute amount, was not calculated. nonetheless, it is evident that γ-c2s carbonates the most of all the minerals of which the samples were prepared. dta and xrd analysis also revealed several newly formed carbonates. these were very similar for the belite, laboratory wollastonite and rankinite. natural wollastonite was confirmed to already contain some carbonates before carbonation process and after carbonation only very small amounts of new carbonates were formed. 31 a. jančíková, m. křištof, k. dvořák acta polytechnica ctu proceedings 4. conclusion in the first step of the experiment, the raw material powders of belite, rankinite and wollastonite were produced, to which sample of natural wollastonite from the bludov mine was added for comparison. initial xrd, dta, milling fineness and particle size characterization by laser granulometry were performed on the samples. in the second step, the solid samples were used for a carbonation process. the results obtained in terms of changes in weights, compressive strengths, xrd analysis and differential thermal analysis show the suitability for carbonation in co2 atmosphere of the individual minerals. from the results of the materials tested, it can be stated that the gamma belite is the most suitable for carbonation. laboratory produced rankinite and wollastonite also show similar values of carbonate formation, but their compressive strengths are lower, which may be a consequence of particle sizes of belite. the complete suitability and comparison of each mineral cannot therefore be accurately determined as the same input conditions were not followed. on the other hand, natural wollastonite can be practically excluded as a mineral suitable for the carbonation process. the formation of new carbonates is minimal, as is its strength. as recommendation for further research from the results obtained are: • focus on carbonation conditions – temperature, moisture, granulometry, porosity. • carbonation process of the same mineral with different water coefficient. • evaluation of porosity on the rate of carbonation. • measurement of crystallography of carbonated minerals. • measurement of the properties of individual carbonates – mainly calcite, vaterite and aragonite. acknowledgements we acknowledge the financial support to projects, (ceitec vut/fast-j-23-8309) mechanical behaviour of calcium phosphate cements wet with liquids of different polarity and, (ga23-05122s) the role combination of fluxes and mineralizers on properties of low-energy clinker. references [1] e. gartner, t. sui. alternative cement clinkers. cement and concrete research 114:27–39, 2018. report of unep sbci working group on low-co2 eco-efficient cement-based materials. https://doi.org/10.1016/j.cemconres.2017.02.002 [2] a. smigelskyte, r. siauciunas, h. hilbig, et al. carbonated rankinite binder: effect of curing parameters on microstructure, strength development and durability performance. scientific reports 10(1):14462, 2020. https://doi.org/10.1038/s41598-020-71270-w [3] s. gupta. carbon sequestration in cementitious matrix containing pyrogenic carbon from waste biomass: a comparison of external and internal carbonation approach. journal of building engineering 43:102910, 2021. https://doi.org/10.1016/j.jobe.2021.102910 [4] m. zajac, j. skocek, m. ben haha, j. deja. co2 mineralization methods in cement and concrete industry. energies 15(10):3597, 2022. https://doi.org/10.3390/en15103597 [5] x. wang, m.-z. guo, t.-c. ling. review on co2 curing of non-hydraulic calcium silicates cements: mechanism, carbonation and performance. cement and concrete composites 133:104641, 2022. https://doi.org/10.1016/j.cemconcomp.2022.104641 [6] e. gartner, h. hirao. a review of alternative approaches to the reduction of co2 emissions associated with the manufacture of the binder phase in concrete. cement and concrete research 78:126–142, 2015. keynote papers from 14th international congress on the chemistry of cement (iccc 2015). https://doi.org/10.1016/j.cemconres.2015.04.012 [7] l. c. lange, c. d. hills, a. b. poole. the influence of mix parameters and binder choice on the carbonation of cement solidified wastes. waste management 16(8):749–756, 1996. https://doi.org/10.1016/s0956-053x(97)00021-4 [8] a. sanna, m. uibu, g. caramanna, et al. a review of mineral carbonation technologies to sequester co2. chemical society reviews 43(23):8049–8080, 2014. https://doi.org/10.1039/c4cs00035h [9] w. ding, h. yang, j. ouyang, h. long. modified wollastonite sequestrating co2 and exploratory application of the carbonation products. rsc advances 6(81):78090–78099, 2016. https://doi.org/10.1039/c6ra13908f [10] w. ashraf. carbonation of cement-based materials: challenges and opportunities. construction and building materials 120:558–570, 2016. https://doi.org/10.1016/j.conbuildmat.2016.05.080 32 https://doi.org/10.1016/j.cemconres.2017.02.002 https://doi.org/10.1038/s41598-020-71270-w https://doi.org/10.1016/j.jobe.2021.102910 https://doi.org/10.3390/en15103597 https://doi.org/10.1016/j.cemconcomp.2022.104641 https://doi.org/10.1016/j.cemconres.2015.04.012 https://doi.org/10.1016/s0956-053x(97)00021-4 https://doi.org/10.1039/c4cs00035h https://doi.org/10.1039/c6ra13908f https://doi.org/10.1016/j.conbuildmat.2016.05.080 acta polytechnica ctu proceedings 53:28–32, 2025 1 introduction 2 material and methods 2.1 preparation of raw material powders 2.2 firing procedure 2.3 sample preparation for xrd analysis 2.4 particle size characterisation 2.5 specific weight and specific surface area 2.6 preparation of carbonizable samples 2.7 carbonation and compressive strength test 3 results and discussion 4 conclusion acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2025.52.0100 acta polytechnica ctu proceedings 52:100–108, 2025 © 2025 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague impact of illuminated advertising devices on road traffic safety luboš nouzovský∗, zdeněk svatý, karel kocián, jonáš maleček czech technical university in prague, faculty of transportation sciences, department of forensic experts in transportation, konviktská 20, 110 00 prague, czech republic ∗ corresponding author: nouzovsky@fd.cvut.cz abstract. this article examines the impact of illuminated advertising devices on road safety in urban environments. by measuring the light parameters of advertising displays and comparing them to public lighting, the study analyzes how the brightness and dynamic elements of these devices affect driver attention, reaction times, and overall ability to perceive traffic situations. the findings indicate that high-brightness, dynamic advertising displays can significantly increase the risk of traffic accidents, especially during nighttime driving. the study also highlights the insufficiency of current legislative regulations, which primarily focus on public lighting systems, and suggests that more comprehensive guidelines should be established to address the specific characteristics of illuminated advertising devices. two measurement methods were employed: horizontal illumination of roadways and a gradient luminance analysis, both of which provided insights into the disruptive influence of these devices on road safety. the article concludes by recommending specific legislative changes, improved regulation of advertising installations, and the development of further research to better understand the long-term effects of these devices on accident rates. by addressing these challenges, this research aims to contribute to safer road environments and minimize the negative impact of illuminated advertisements on traffic flow and driver behavior. keywords: advertising devices, road safety, lighting. 1. introduction the impact of advertising devices on road safety is still the subject of many new studies and publications in the international scientific research community [1]. the solution to this problem is undoubtedly complex and involves not only the examination of the technical characteristics of the advertising devices, such as their dimensions, positioning in relation to the road, lighting intensity or the use of dynamic elements, but also the content of the messages presented. the combination of these aspects can lead to the driver’s attention being diverted from the situation on the road to the advertising devices, which increases the risk of road accidents and has a negative impact on road safety. this may result not only in disruption of traffic flow but also in damage to property or serious health risks to road users [2]. for these reasons, a growing number of expert studies and researches are focusing on analysing this issue, trying to clarify the influence of individual factors and proposing possible measures to regulate them, including adjustments to legislative standards. in the urban environment, advertising has long been an integral part of the public realm. illuminated advertising, especially at night, attracts the attention of drivers and distracts them from the traffic situation. this phenomenon can reduce or interfere with drivers’ attention, increase their reaction time and impair their ability to recognise obstacles or traffic signs in time [3]. studies show that the brightness and contrast of illuminated advertising compared to road lighting can significantly affect visual field perception and contribute to the risk of road accidents [4]. given these risks, it is necessary to further investigate the lighting parameters of these devices, such as their intensity, dynamics and positioning, and to find ways to regulate their impact on road safety [5, 6]. this paper focuses on this issue and presents two types of measurements that can be used to assess the impact of illuminated or light-emitting advertising devices. 2. lighting parameters of advertising devices and their evaluation driver’s thinking and perception is a complex and intricate system influenced by a number of internal and external inputs. the interaction of a person with a vehicle in relation to a specific traffic situation is primarily determined by the driver’s sensory perception, which enables a person to orient themselves in a given situation. visual perception is therefore essential, as it mediates up to 90 % of the perception of the environment. it is all the more important to ensure that this perception is correct, even in low light conditions, when illuminated signs objectively stand out more than other objects [6]. the czech act no. 13/1997 coll. on roads states that it is possible to place and operate advertising devices that “do not dazzle the users of the respective 100 https://doi.org/10.14311/app.2025.52.0100 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 52/2025 impact of illuminated advertising devices on road traffic safety road or otherwise interfere with road traffic” [7]. however, the general nature of these restrictions makes them difficult to enforce in some cases. this may be because there are currently no regulations governing the lighting parameters of advertising devices. the closest thing to this are the technical standards csn en 13201-1 to 5 [8], which define measurement methods, evaluation and glare classes, but apply to public lighting. for the purpose of the measurements processed and evaluated in cooperation with the company sathea vision s.r.o., advertising devices were included in the public lighting system mainly for the following reasons: • advertising devices are usually placed in a similar position to the road as the standard public lighting system. • the light sources used for advertising devices are similar to those used for public lighting systems. • a lighting design methodology has been developed to minimise the negative impact on a driver, which is also applicable to advertisements. • current czech legislation and technical standards, other than those for public lighting, only marginally address the issue of active adverisements. • the impact of illuminated advertisements on the road can be measured using methods designed to measure the quality of public lighting systems. two different approaches were used to assess the effect of the luminance of the selected advertisements, namely horizontal road illuminance measurements and gradient luminance analysis. 2.1. road horizontal illuminance measurement 2.1.1. measurement procedure measurements are taken by an automated data collection platform (figure 1). the platform uses calibrated sensors to measure traffic on three parallel levels in the carriageway. the entire system uses advanced compensation for adverse effects and interference. the sampling and processing speed allows measurements to be made in real-time traffic, and the measured data is interpolated to produce a matrix of equidistant points in accordance with the čsn en 13201 lighting evaluation standard. the measurement is also dependent on road conditions and vehicle manoeuvrability. a class b hand-held luxmeter was used to verify the measurements (figure 2). 2.1.2. locations measurements were carried out at eight locations (i.e. d1 to d8), which are shown in the following maps and visual overview (figures 3–5). all sites are located on two streets in prague, namely rozvadovská spojka and 5. května. the advertising devices have been selected according to their luminosity and variability figure 1. sathea traffic horizontal illuminance measurement vehicle. figure 2. calibration certificate and sonel luxmeter. 101 l. nouzovský, z. svatý, k. kocián, j. maleček acta polytechnica ctu proceedings figure 3. location of 5. května street with marked advertising devices [9]. (i.e. dynamics), whereby the advertising media at locations d1 to d6 have dynamically variable content, while the advertisements at locations d7 and d8 are static. 2.1.3. processing and evaluation measured data was first interpolated and filtered to give the most accurate picture of illuminance along the road. data in the direction of one lane were then selected, and was further filtered using a moving average filter with a window of 7 forward and 7 backward samples (the smoothed data is shown in the graphs below). in each graph, the location of the respective advertising device is always marked with a red bar to assess whether the road illuminance is affected at that location. the following figures show locations d8 and d1 as an example see in figures 6 and 7. the advertisement at the d8 location was the only one found to have a significant effect on the longitudinal uniformity of the lighting due to its position directly above the carriageway. all other advertisements close to the road were found to have a negligible effect on the longitudinal uniformity of the lighting. measured data, including the effect of the advertising device on the illuminance, can also be displayed in a heatmap or greyscale spectrum for better illustration, as shown in figure 8. 2.1.4. summary of horizontal illuminance measurements the data show that the influence of advertising devices in question on the illuminance of the measured sections is minimal and at the level of measurement error. this conclusion was confirmed by manual measurement with a luxmeter, where the change caused by the light emitted by the advertising device was about 10 % of the illuminance on the roadway compared to the state when the advertising was switched off. this is probably due to the orientation of the advertisement, where its normal is approximately parallel to the road. the light rays hit the road at a high angle to the normal of the impact area. illuminance is a function of the angle of incidence, so the effect is negligible. it should be noted, however, that a light source which does not contribute to the illumination of the road and its surroundings can only be considered as a source of glare with a negative effect on the visual performance of the driver. 2.2. gradient luminance analysis the purpose of the measurement is to evaluate the brightness of the light sources in the captured scene. for this measurement, the images were converted to grey scale and then rendered in false colours for subjective evaluation. no further adjustments were necessary when the photos were evaluated numerically. 2.2.1. measurement procedure the measurements were carried out at selected locations in the streets 5. května and rozvadovská spojka. the optical sensor system was placed according to the requirements of čsn en 13201 on a standard observer (i.e. at a height of 1.5 m, at a distance of 60 m from the advertising device and with an inclination of the observation plane of 1°). the composition of the scene was such that the advertising device, roads and public lighting fixtures were recorded at a distance of at least 150 m from the advertising device. the images were taken using a sony ilce-5000 camera with a 19mm fixed focal length lens. all shooting parameters except the shutter speed were kept constant. the sensitivity (iso) was set at 100 and the aperture at f14, and the shutter speed was varied from 1/4000 s to 1 s for each scene to find the point at which the sensor operates in the linear range. the three best images were then selected for further processing – the first for subjective evaluation and the other two for objective (numerical) evaluation. the specifics and limitations of the specific measurements reported in real-time traffic included the following: • high dynamic scene contrast. ▷ high brightness of an advertising device and luminaires. ▷ luminious flux from an advertising device directed at the driver at a more direct angle than public lighting. ▷ restrictive sensor angles against interfering light. • sites do not allow fully static measurements. ▷ dynamic advertisements are switched off at 22:00, a time when there is still a lot of traffic at these locations. 102 vol. 52/2025 impact of illuminated advertising devices on road traffic safety figure 4. location of rozvadovská spojka street with marked advertising devices [9]. figure 5. locations d1 to d8. 103 l. nouzovský, z. svatý, k. kocián, j. maleček acta polytechnica ctu proceedings figure 6. graph of the horizontal illuminance at the d8 location, showing the influence of the advertising device. figure 7. graph of the horizontal illuminance at the d2 location, showing the influence of the advertising device. figure 8. heatmap (left) and grey scale (right) data display. ▷ it is not possible to regulate traffic on roads for fully static measurements. 2.2.2. locations the locations selected for the gradient luminance analysis are identical to those for the horizontal road illuminance measurements. due to the financial, organisational and data requirements of such measurements, the measurements were carried out at locations d1 and d4 to d7. a subjective expert assessment of illuminance and the effect of illuminated advertising on drivers was also carried out for the selected locations. 2.2.3. processing and evaluation objective assessment the only applicable parameter for assessing glare on roads is the threshold increment (ti), which is a measure of the reduction in visibility caused by the restrictive glare of the luminaires in the road lighting system [4]. however, due to the relatively heavy traffic and the time-varying content of the advertisements, it could not be reliably determined. therefore, the ratio of the equivalent veiled luminance of the public lighting and the signage was used as a proxy for the assessment. curtain luminance is defined as: lv = 10erk ϑ2 [cd · m−2] (1) where • erk – illuminance at the point of the observer’s eye in a plane perpendicular to the line of sight and inclined by 2°from the horizontal, • ϑ – is the angle formed by the line of sight and the line connecting the observer’s eye and the source of glare. the most significant fixed source of glare on roads is public lighting. the equivalent veil luminance of the 104 vol. 52/2025 impact of illuminated advertising devices on road traffic safety figure 9. angle comparison between an advertising device and a public lighting luminaire. public lighting system was therefore used as a reference ilustrated in figure 9. if the equivalent luminance did not exceed the reference luminance, the advertisement was considered to be an insignificant source of glare in relation to the public lighting sources already installed and meeting the requirements of čsn en 13201. luminaires between 30 and 300 metres from the monitoring site were included in the assessment. the luminaires are marked l1p (nearest luminaire to the right of the road), l1l (nearest luminaire to the left of the road), l2p (second nearest luminaire to the right of the road), etc. the marking of an advertising device corresponds to the marking of the same advertising device on the map. two locations have been used as examples, namely d5 and d6, whose values are given in detail (see table 1 and 2). the second column indicates the angle in degrees between the plane of observation and the junction of the light source and the camera lens. the solid angle indicates how much of the total field of view is occupied by a given light source. the average illuminance of an object is the calculated value of the luminous intensity of each pixel of the image that falls within the space of a given light source. the relative horizontal illuminance produced by a given source corresponds to the average brightness of the object, angle and solid angle according to the relationship: e = l cos ϑ τ [lx] (2) where • l – average brightness of the object – the value of the luminous intensity of each pixel of the image belonging to the area of the light source, • ϑ – is the angle formed by the plane of observation and the junction of the light source and the camera lens, • τ – solid angle – the size of the object in the field of view. table 3 shows the resulting luminance analysis values. the veiling luminance of public lighting is the sum of all individual luminances. the veiling luminance of an advertisement is, as the name suggests, the veiling luminance induced by the advertisement. the main indicator of glare is the ratio of the veiling luminance of the advertisement to the sum of the veiling luminances induced by the public lighting luminaires. the higher the value, the more significant the glare caused by the advertising device compared to the glare caused by the public lighting system. the results show that one of the advertising devices significantly exceeds the luminance values of the public lighting system. however, such a result does not justify the claim that the advertisements do not comply with the current standard for street lighting. however, it can be stated that the technology used is capable of exceeding the permitted values for public lighting by several times in terms of radiated brightness. subjective assessment this assessment consideres the overall situation at the selected locations and the impact of the advertisements on drivers in relation to other objects and the overall road environment. the aim is to provide a comprehensive picture of the situation at the measured locations in terms of visual presentation. as a result, the sites were classified into three levels according to their impact on drivers, as shown in the situations below: advertisement with a minimal subjective impact on the observer the advertisements at d1 and d4 locations are so unobtrusive within the overall brightness of the scene that their impact on the driver is minimal. subjectively, there is little or no glare to the driver. advertisement with an average subjective impact on the observer for d6 and d7 advertising devices, the effect on the driver depends mainly on the content displayed in terms of the colours used and the intensity of the brightness. advertisement with a high subjective impact on the observer the advertisement at d5 location is a high-brightness device. within the overall brightness of the scene, it stands out significantly above the brightness of other light sources and the brightness of the road. in this comparison, it is clear that the ambient scene values around the billboard, in particular the road surface, are close to zero. such an effect is undesirable because the driver, whose vision is adapted to the high brightness of the billboard, does not have sufficient reserve to distinguish less bright details on the road. this increases the likelihood of missing an obstacle, confusing information from vertical road markings and similar undesirable features. situations are shown in figure 10. 2.2.4. summary of brightness analysis results the advertisements and locations that were most suitable for evaluation in terms of road geometry, luminaires and mast heights were selected to determine the impact of illuminated advertising on drivers. the measurements show that individual advertisements vary in both brightness and position in the field of view. the equivalent veiled luminance of d1 and d4 is less than the equivalent veiled luminance of the public lighting, whereas for d6 and d7 it is close to this value. in the case of sign d5, the measured 105 l. nouzovský, z. svatý, k. kocián, j. maleček acta polytechnica ctu proceedings light source angle ϑ solid average relative horiz. relative veil angle τ brightness illuminance e brightness lv of the object l [°] [sr] [cd m−2] [lx.sr] [cd sr m−2] luminaire l1p 16,328 0,000036 4655,62 0,16301 0,00611 luminaire l1l 14,261 0,000027 4873,28 0,12627 0,00621 luminaire l2p 9,412 0,000011 3286,72 0,03586 0,00405 luminaire l2l 8,944 0,000006 1680,84 0,00992 0,00124 luminaire l3p 6,487 0,000002 719,62 0,00154 0,00037 luminaire l3l 6,708 0,000004 2286,08 0,00976 0,00217 luminaire l4p 5,278 0,000003 2334,78 0,00658 0,00236 luminaire l4l 5,421 0,000002 1486,98 0,00365 0,00124 luminaire l5p 4,615 0,000002 1293,12 0,00297 0,00140 luminaire l5l 4,628 0,000003 839,88 0,00250 0,00117 advertisement d 5 5,850 0,002111 545,22 1,14520 0,33463 table 1. brightness analysis readings for d5 location. light source angle ϑ solid average relative horiz. relative veil angle τ brightness illuminance e brightness lv of the object l [°] [sr] [cd m−2] [lx.sr] [cd sr m−2] luminaire l1p 23,179 0,000025 6662,016 0,15570 0,00290 luminaire l1l 22,815 0,000027 1546,912 0,03857 0,00074 luminaire l2p 10,79 0,000005 5611,392 0,02601 0,00223 luminaire l2l 10,647 0,000018 1968,32 0,03417 0,00301 luminaire l3p 6,994 0,000005 3054,56 0,01399 0,00286 luminaire l3l 6,903 0,000004 713,792 0,00267 0,00056 luminaire l4p 5,135 0,000003 1131,392 0,00349 0,00132 luminaire l4l 5,018 0,000002 282,176 0,00065 0,00026 luminaire l5p 4,056 0,000002 1788,352 0,00411 0,00250 luminaire l5l 4,004 0,000001 269,344 0,00034 0,00021 luminaire l6p 3,315 0,000001 537,344 0,00067 0,00061 luminaire l6l 3,25 0,000002 776,256 0,00126 0,00119 luminaire l7p 2,769 0,000001 328,544 0,00026 0,00034 luminaire l7l 2,704 0,000001 465,792 0,00044 0,00060 luminaire l8p 2,314 0,000001 432 0,00041 0,00076 luminaire l8l 2,288 0,000001 588,448 0,00055 0,00106 advertisement d6 4,251 0,001126 25,831 0,02901 0,01606 table 2. brightness analysis readings for d6 location. location veiled luminance veiled luminance ratio of veil of public lighting of advertisement luminance (ad./pl) location d1 0,039 0,012 0,307 location d4 0,029 0,018 0,620 location d5 0,026 0,335 12,717 location d6 0,021 0,016 0,759 location d7 0,031 0,029 0,935 table 3. results of the brightness analysis of d1, d4, d5, d6 and d7 locations. 106 vol. 52/2025 impact of illuminated advertising devices on road traffic safety (a). d1 (b). d4 (c). d6 and d7 (d). d5 figure 10. brightness analysis in false colour. values were significantly higher than the public lighting illuminance. on the basis of these measurements, with such a result, it cannot be said that some advertisements violate the current standard for street lighting čsn en 13 201. however, it can be stated that, thanks to the progress made in the development of leds, advertisements can achieve luminance values that are not negligible at night, but on the contrary can exceed the luminance of the permitted fixed light sources (i.e. public lighting) many times over. the luminous intensity of some advertisements is significantly higher than that of other light sources. this increases the adaptive brightness of the scene for the driver. compared to the brightness of the road, which is several orders of magnitude lower, this increases the risk of missing an obstacle, reduces the driver’s visual performance and increases reaction time. a particular phenomenon that occurs in the case of an active advertising device with variable content (i.e. a dynamic advertising device) is the change in panel brightness over time. currently, all legislation relating to street lighting only deals with fixed, constant luminous sources. no methodology has been developed to assess lighting systems with time-varying parameters. however, it should be noted that, from a physiological point of view, two undesirable phenomena occur. the first is a sudden change in the brightness of the screen, which can cause blinding glare or at least a significant reduction in visual performance for a short period of time. the second is a reflex associated with peripheral vision. when there is a sudden change in peripheral brightness, a reflexive withdrawal of attention from the stimulus occurs (i.e. the hazard response). these phenomena can pose a greater hazard than glare from fixed sources of constant luminous flux [10]. 3. conclusions the results of the measurements confirm that advertisements, especially those with high brightness and dynamic content, can have a negative impact on road safety. however, this was not clearly demonstrated for all the selected samples. the brightness and contrast of advertisements distract drivers from the traffic situation, which can lead to longer reaction times and an increased risk of accidents. based on the results of this study, it can be concluded that the current legislation in the czech republic, which focuses on roads, traffic or the lighting system, does not sufficiently cover the specifics of illuminated and light-emitting advertising devices. therefore, it would be more than appropriate to adopt more specific legislative measures that take into account not only static but also dynamic advertising devices and set limits on their brightness, dynamics and positioning. advertising can be identified as a source of discomforting glare. no positive effects of advertising devices on road safety or visual comfort of drivers have been 107 l. nouzovský, z. svatý, k. kocián, j. maleček acta polytechnica ctu proceedings identified as part of the classification of advertising devices as part of the public lighting system. the purpose for which such light sources are installed differs from that of public lighting systems. the light sources of the advertising devices measured are not optically modified in any way to improve the visual comfort of the driver (shading, direction of light flow, etc.). however, public lighting luminaires must be adapted in this way. in particular, the absolute value of the luminance of the advertisement and the subsequent calculation of the threshold increment is a parameter that would allow the advertisement to be classified as technically unsatisfactory in the light of current legislation. references [1] j. hinton, o. oviedo-trespalacios, b. watson, n. haworth. beyond the billboard: a review of other external sources of driver distraction. accident analysis & prevention 208:107771, 2024. https://doi.org/10.1016/j.aap.2024.107771 [2] czech republic. ministry of transport. besip strategy 2021–2030, 2020. [2024-08-11]. https://besip.cz/besip/media/besip/data/web/ strategie-besip-2021-2030.pdf [3] k. bucsuházy, m. semela, m. belák, et al. analysis of selected off-road glances during driving in real road traffic. transportation research procedia 45:580–586, 2020. transport infrastructure and systems in a changing world. towards a more sustainable, reliable and smarter mobility. tis roma 2019 conference proceedings. https://doi.org/10.1016/j.trpro.2020.03.054 [4] t. a. dingus, f. guo, s. lee, et al. driver crash risk factors and prevalence evaluation using naturalistic driving data. proceedings of the national academy of sciences 113(10):2636–2641, 2016. https://doi.org/10.1073/pnas.1513271113 [5] t. blodek, p. vrtal, t. kohout, et al. verification of night road safety inspection by luminance analysis. in 2024 24th international scientific conference on electric power engineering (epe), pp. 1–5. 2024. https://doi.org/10.1109/epe61521.2024.10559538 [6] r. brome, m. awad, n. m. moacdieh. roadside digital billboard advertisements: effects of static, transitioning, and animated designs on drivers’ performance and attention. transportation research part f: traffic psychology and behaviour 83:226–237, 2021. https://doi.org/10.1016/j.trf.2021.10.013 [7] parliament of the czech republic. act no. 13/1997 coll. 1997. [8] čsn 13 201-1,2,3,4,5 (1.10.2024), osvětlování pozemních komunikací [in czech; street lighting]. čni praha. [9] google. maps. [2024-08-11]. https://www.google.com/maps [10] l. nouzovský, p. vrtal, t. kohout, z. svatý. using the eye tracking method to determine the risk of advertising devices on drivers’ cognitive perception. applied sciences 12(13):6795, 2022. https://doi.org/10.3390/app12136795 108 https://doi.org/10.1016/j.aap.2024.107771 https://besip.cz/besip/media/besip/data/web/strategie-besip-2021-2030.pdf https://besip.cz/besip/media/besip/data/web/strategie-besip-2021-2030.pdf https://doi.org/10.1016/j.trpro.2020.03.054 https://doi.org/10.1073/pnas.1513271113 https://doi.org/10.1109/epe61521.2024.10559538 https://doi.org/10.1016/j.trf.2021.10.013 https://www.google.com/maps https://doi.org/10.3390/app12136795 acta polytechnica ctu proceedings 52:100–108, 2025 1 introduction 2 lighting parameters of advertising devices and their evaluation 2.1 road horizontal illuminance measurement 2.1.1 measurement procedure 2.1.2 locations 2.1.3 processing and evaluation 2.1.4 summary of horizontal illuminance measurements 2.2 gradient luminance analysis 2.2.1 measurement procedure 2.2.2 locations 2.2.3 processing and evaluation 2.2.4 summary of brightness analysis results 3 conclusions references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2023.40.0027 acta polytechnica ctu proceedings 40:27–32, 2023 © 2023 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague modelling of laminated glass interlayer by fractional viscoelasticity barbora hálková∗, jaroslav schmidt czech technical university in prague, faculty of civil engineering, department of mechanics, thákurova 7, 166 29 prague 6, czech republic ∗ corresponding author: barbora.halkova@fsv.cvut.cz abstract. fractional calculus, i.e. the theory of derivatives and integrals of non-integer order, can be efficiently used for the theoretical modelling of viscoelastic materials. our research is focused on the polyvinyl butyral which is used as an interlayer for the laminated glass. polyvinyl butyral can be classified as a viscoelastic material and the introduction of the fractional viscoelasticity seems to be appropriate tool for its description. this paper briefly introduces the springpot element and its connection into more complex theoretical models. we mainly consider the generalized maxwell model in its standard and fractional form and show their application by fitting the data obtained by experimental analysis. keywords: laminated glass, polymer interlayer, theory of viscoelasticity, fractional viscoelasticity, springpot, generalized maxwell model, fractional calculus, storage modulus. 1. introduction laminated glass, see [1], is a composite material, which consists of a solid glass plates (float, toughened, tempered glass or their combination) and interlayers made of polymers (mostly the polyvinyl butyral, which will be also considered in the following text) or cast resins. the interlayer has primarily the safety function, it improves the post-breakage behaviour of the glass elements. it also ensures interaction between the main load-bearing glass plates, its effect is mostly important in the case of bending, when the interlayer directly affects the stiffness of the cross section and is mostly stressed by shear. the polyvinyl butyral (pvb) type of interlayer can be classified as a viscoelastic material [2]. its behaviour is somewhere between purely elastic and purely viscous. the viscoelastic behaviour is time and load-history dependent, phenomenon such as creep and relaxation are significant. for the accurate description of the viscoelastic material we need to use more complex theoretical models, while the ideal elasticity and viscosity stand for the limit cases, see [3] for broader description. 2. viscoelasticity the standard theory of viscoelasticity commonly uses the theoretical models composed of elastic and viscous elements (springs and dashpots). the fractional viscoelasticity, see [4], applies the principles of fractional calculus and introduces another rheological element, the springpot. however, the fractional calculus is not discussed in detail here because it is out of scope of this article and the interested reader is referred to [5]. e (a). spring. e, τc, α (b). springpot. η (c). dashpot. figure 1. rheological scheme of spring, springpot and dashpot. the linear spring (see figure 1a) is a rheological element, which represents ideally elastic behaviour of the material. the constitutive law, also known as hooke’s law, has the following form: σ(t) = eε(t), (1) where σ stands for the stress, ε for the strain, e for young’s modulus of elasticity. the stress is a linear function of the strain. the viscous damper (dashpot) on the other hand, the viscous damper (dashpot) (see figure 1c) represents the model of ideally viscous fluid. the stress is directly proportional to the strain rate (i.e. to the first derivative of the strain). the constitutive law, also known as newton’s equation of viscosity, has the following form: 27 https://doi.org/10.14311/app.2023.40.0027 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en barbora hálková, jaroslav schmidt acta polytechnica ctu proceedings σ(t) = ηε̇(t), (2) where η stands for the coefficient of viscosity, ε̇ the time derivative of the strain. the springpot (see figure 1b) then represents a transition between the two cases mentioned above. fractional viscoelasticity, see [4], assumes the constitutive law in the following form: σ(t) = ξdαε(t), (3) where ξ and α are parameters of the springpot, dα denotes the α-th time derivative. the point of fractional viscoelasticity is to assume α as non-integer. we have already mentioned that the ideal elasticity and ideal viscosity stands for the limit cases of viscoelastic behaviour, therefore we can define the limit cases for the springpot parameters and consider α only in the interval ⟨0, 1⟩. when we introduce the characteristic time τc in the following form: τc = η e , (4) equation (3) can be rewritten as: σ(t) = eτα c dαε(t). (5) the replacement of ξ by eτα c better illustrates the physical meaning of the constant ξ in the limit cases. for α = 0 we receive: σ(t) = e d0ε(t) dt0 = eε(t), (6) describing purely elastic response, remind equation (1). we see that in this limit the parameter ξ is equal to the modulus of elasticity e and the behaviour of the springpot corresponds to the behaviour of an elastic element. on the other hand, for α = 1 we receive: σ(t) = eτc dε(t) dt = ηε̇(t). (7) in this limit case the behaviour of the springpot corresponds to the behaviour of the viscous element (remind equation (2)), while the constant ξ become the constant of viscosity η. 2.1. relaxation modulus of springpot the relaxation is the phenomenon where the stress gradually decreases over time even if the strain remains constant. on the other hand, the creep can be explained as a phenomenon where the the strain gradually increases over time while the stress remains 0 20 40 60 80 100 time [s] 0.00 0.25 0.50 0.75 1.00 1.25 1.50 1.75 re la xa tio n m od ul us [p a] spring =0,05 =0,2 =0,4 =0,6 =0,8 dashpot figure 2. relaxation modulus of a springpot according to different values of α. constant, for the broader description of these phenomena see [3]. in the following text we focus on the relaxation mode only, because it can be more easily interpreted in connection with the results of our experimental analysis. relaxation modulus of the material is defined as the stress response to the heaviside strain load: ε(t) = h(t), (8) unit-value strain load which is applied instantaneously in time t = 0 and remains constant for times t > 0. in the case of the springpot element we consider the fractional derivative dα as the riemann-liouville type of derivative, see [6]. the resulting relaxation modulus has the following form: r(t) = e γ(1 − α) ( t τc )−α h(t), (9) where γ denotes the gamma function. figure 2 shows the relaxation modulus of a springpot according to different values of α, while the dashed lines represent the limit cases given by elastic and viscous behaviour. 2.2. springpot under harmonic load as a part of our research the experimental analysis of the laminated glass is provided as well. the samples are loaded by a harmonic torque. therefore we are interested in the behaviour of theoretical models under the harmonic load as well. the prescribed strain load and the corresponding stress response have the following forms: ε∗(ω) = ε∗ 0eiωt, σ∗(ω) = σ∗ 0eiωt, (10) where ε∗ 0 and σ∗ 0 are the amplitudes of the harmonic strain and stress in the complex plane, 28 vol. 40/2023 modelling of laminated glass interlayer by fractional viscoelasticity eη (a). maxwell model. ee, τc, α (b). fractional maxwell mode. figure 3. rheological scheme of the maxwell model and the fractional maxwell model. e is the euler constant, i is the imaginary unit. the complex modulus e∗ can now be defined as: e∗(ω) = σ∗ 0 ε∗ 0 . (11) if we separate the real part of the complex modulus, we receive the storage modulus e′ which corresponds to the elastic part of the response. on the other hand, the imaginary part, the loss modulus e′′ represents the viscous part of the response. in the following text we focus on the storage modulus due to better accuracy with which the storage modulus is obtained from experiments. the complex modulus of the springpot element is written as: e∗(ω) = e(iωτc)α. (12) the storage modulus is then obtained by separating the real part of the complex modulus as: e′(ω) = re(e∗) = e(ωτc)α cos ( α π 2 ) . (13) 2.3. theoretical models to improve the description of the viscoelastic behaviour, the rheological elements can be arranged together in series or in parallel to create more accurate models. the overview of some theoretical models can be found for example in [7], while here we limit attention to those, which seem to be appropriate for the description of the pvb interlayer. maxwell cell is a serial connection of an elastic spring and a viscous dashpot. by the replacement of the viscous element with the springpot we obtain the fractional maxwell cell. both rheological schemes with the variable parameters of each element are shown in figure 3. the relaxation moduli of the maxwell cell and the fractional maxwell cell are respectively: r(t) = ee−t/τch(t), (14) r(t) = eeα,1 ( − ( t τc )α) h(t), (15) where e1 η1 e2 η2 en ηn e0 figure 4. rheological scheme of the generalized maxwell model. e1 e2 en e0 e1, α1 τc,1 e2, α2 τc,2 en , αn τc,n figure 5. rheological scheme of the fractional maxwell chain. eα,1 is the mittag-leffler function. for the limit case α = 1 it holds that e1,1(x) = ex, see [8], and therefore equation (14) and equation (15) become identical in this limit case. the storage moduli of the maxwell cell and the fractional maxwell cell are then: e′(ω) = e ω2τ2 c ω2τ2 c + 1 , (16) e′(ω) = e (τcω)2α + (τcω)α cos (α π 2 ) (τcω)2α + 2(τcω)α cos (α π 2 ) + 1 . (17) generalized maxwell model (also known as the maxwell chain) is the parallel connection of a spring and n maxwell cells, see figure 4. the fractional maxwell chain is then obtained as a parallel connection of a spring and n fractional maxwell cells, see the rheological scheme in figure 5. 29 barbora hálková, jaroslav schmidt acta polytechnica ctu proceedings 10 5 10 2 101 104 time [s] 0 20 40 60 80 100 re la xa tio n m od ul us [p a] spring + 1 maxwell cell spring + 3 maxwell cells spring + 5 maxwell cells figure 6. relaxation modulus of the maxwell chain compare to the number of cells. 10 5 10 3 10 1 101 103 angular velocity [rad/s] 106 107 st or ag e m od ul us [p a] fitted maxwell chain experimental data figure 7. fitting experimental data of g′ to 1-cell maxwell chain. the parallel connection of the cells in the generalized model enables us to obtain the relaxation modulus of the maxwell (resp. the fractional maxwell) chain superpositionally. the same applies to the storage modulus. the relaxation modulus and the storage modulus of the generalized models have the following form: r(t) = e0 + n∑ i=1 ri, (18) e′(t) = e0 + n∑ i=1 e′ i , (19) where e0 is the young modulus of elasticity of the spring, ri are the relaxation moduli, e′ i the storage moduli of each of the maxwell (resp. the fractional maxwell) cells. it is obvious that the behaviour of these generalized models depends on the number of the maxwell (resp. the fractional maxwell) cells connected together. this is illustrated in figure 6 which shows the difference in the relaxation modulus of a standard maxwell chain with 1, 3 and 5 connected maxwell cells. 10 5 10 3 10 1 101 103 angular velocity [rad/s] 106 107 st or ag e m od ul us [p a] fitted maxwell chain experimental data figure 8. fitting experimental data of g′ to 3-cell maxwell chain. 10 5 10 3 10 1 101 103 angular velocity [rad/s] 106 107 st or ag e m od ul us [p a] fitted maxwell chain experimental data figure 9. fitting experimental data of g′ to 7-cell maxwell chain. 2.4. validation of theoretical models each of the theoretical models has the finite amount of unknown parameters (moduli of elasticity e, characteristic times τc, viscosities η or the springpot parameters α). these parameters occur in analytical equations prescribing the behaviour of the models, e.g. the relaxation modulus. for our research the data obtained by experiments performed on laminated glass presented in [9] were used. the validation of the theoretical models was provided by fitting the parameters of the storage modulus. the python optimization tool from scipy was used for fitting the parameters, this tool uses the method of non-linear least squares to fit the analytical function to experimental data. the results are displayed in the following figures, where the blue dots illustrate the experimentally obtained data while the red curve is the fitted analytical solution. figure 7 shows the approximation by the 1-cell maxwell chain. however, the approximation is not very accurate. to improve the accuracy we need to add more maxwell cells to the generalized model. the increase of the number of maxwell cells from 1 to 3 is shown in figure 8. the approximation is good on the wider range of frequencies. as is shown in figure 9, the 30 vol. 40/2023 modelling of laminated glass interlayer by fractional viscoelasticity 10 5 10 3 10 1 101 103 angular velocity [rad/s] 106 107 st or ag e m od ul us [p a] fitted frac. m. chain experimental data figure 10. fitting experimental data of g′ to 1-cell fractional maxwell chain. 10 5 10 3 10 1 101 103 angular velocity [rad/s] 106 107 st or ag e m od ul us [p a] fitted frac. m. chain experimental data figure 11. fitting experimental data of g′ to 2-cell fractional maxwell chain. 7-cells maxwell chain approximates the experimental data well in the whole measured frequency range. on the other hand, the increase of the number of the cells from 1 to 7 also significantly increases the number of the unknown parameters that need to be fitted. the approximation by the fractional maxwell chain is shown in figures 10–12. even for the 1-cell fractional maxwell chain (figure 10) the accuracy of the approximation is significantly better compare to standard 1-cell maxwell chain (figure 7). however, the accuracy can also be improved by adding more fractional maxwell cells. figure 11 shows approximation by the 2-cells and figure 12 the 3-cells fractional maxwell chain. in the last mentioned case we can talk about almost perfect fit to the experimental data. the fractional maxwell chain shows one more significant advantage. while the curve of the standard maxwell chain become horizontal on the edges of the measured domain, the fractional maxwell chain still shows the growing trend of the curve. therefore, the fractional maxwell chain might be used for data extrapolation outside the experimentally measured domain. this section clearly shows the results that the frac10 5 10 3 10 1 101 103 angular velocity [rad/s] 106 107 st or ag e m od ul us [p a] fitted frac. m. chain experimental data figure 12. fitting experimental data of g′ to 3-cell fractional maxwell chain. tional maxwell chain can provide better approximations of the experimental data in comparison to standard generalized maxwell model. also it can be used for the data extrapolation for which the standard maxwell chain is not suitable. therefore the use of the fractional models might be beneficial despite their disadvantages which lie in the need of the more difficult mathematical apparatus. 3. conclusion fractional viscoelasticity seems to be an efficient tool to describe the behaviour of a polyvinyl butyral, the material of the interlayer of laminated glass. the principles of fractional calculus are introduced together with the springpot element. this rheological element can be used in connections with springs or dashpots to make more complex theoretical models. for our applications, the generalized maxwell chain models presented in its standard and fractional forms are sufficiently accurate and effective, particularly when considering the approximation of the experimental data. comparing both formulations, the fractional viscoelasticity requires a complex mathematical apparatus to calculate derivatives and integrals of non-integer order. on the other hand, by the use of fractional models the number of the fitted parameters may be reduced in compare to standard viscoelastic models. on top of that, it allows for extrapolation of data outside the experimentally measured domain. therefore, the fractional model can be directly exploited in fitting the experimentally measured master curve very accurately adopting a few parameters only. this smooth approximation can be in turn used in calibrating the maxwell chain model typically adopted in finite element simulations as implementation of the fractional model in the framework of the finite element method is still a subject of an ongoing research. 31 barbora hálková, jaroslav schmidt acta polytechnica ctu proceedings acknowledgements this publication was supported by the czech science foundation, the grant no. 22-15553s and by the grant agency of the czech technical university in prague, grant no. sgs22/031/ohk1/1t/11. references [1] w. laufs, a. luible. introduction on use of glass in modern buildings. tech. rep., epfl, laboratoire de la construction métallique icom, 2003. [2] m. haldimann, a. luible, m. overend. structural use of glass, vol. 10 of structural engineering documents. iabse, zürich, 2008. isbn 978-3-85748-119-2. https://doi.org/10.2749/sed010 [3] m. jirásek, j. zeman. přetváření a porušování materiálů: dotvarování, plasticita, lom a poškození. české vysoké učení technické v praze, prague, 2006. isbn 978-80-01-05064-4. [4] r. c. koeller. applications of fractional calculus to the theory of viscoelasticity. journal of applied mechanics 51(2):299–307, 1984. https://doi.org/10.1115/1.3167616 [5] f. mainardi, r. gorenflo. fractional calculus and special functions, 2013. lecture notes on mathematical physics. [6] r. l. graham, d. e. knuth, o. patashnik, s. liu. concrete mathematics: a foundation for computer science. computers in physics 3(5):106–107, 1989. https://doi.org/10.1063/1.4822863 [7] b. hálková. viscoelastic description of polymer interlayer of laminated glass. bachelor’s thesis, české vysoké učení technické v praze, prague, 2022. [2022-0815]. https://dspace.cvut.cz/handle/10467/102633 [8] r. gorenflo, f. mainardi, s. rogosin. mittag-leffler function: properties and applications. in volume 1 basic theory, pp. 269–296. de gruyter, berlin, boston, 2019. https://doi.org/10.1515/9783110571622-011 [9] t. hána, t. janda, j. schmidt, et al. experimental and numerical study of viscoelastic properties of polymeric interlayers used for laminated glass: determination of material parameters. materials 12(14):2241, 2019. https://doi.org/10.3390/ma12142241 32 https://doi.org/10.2749/sed010 https://doi.org/10.1115/1.3167616 https://doi.org/10.1063/1.4822863 https://dspace.cvut.cz/handle/10467/102633 https://doi.org/10.1515/9783110571622-011 https://doi.org/10.3390/ma12142241 acta polytechnica ctu proceedings 40:27–32, 2023 1 introduction 2 viscoelasticity 2.1 relaxation modulus of springpot 2.2 springpot under harmonic load 2.3 theoretical models 2.4 validation of theoretical models 3 conclusion acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2024.49.0060 acta polytechnica ctu proceedings 49:60–64, 2024 © 2024 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague experimental of verification of alternative hydraulic binders flexural and compressive strength aleš paličkaa,∗, zdeněk prošeka,b, pavel tesáreka a czech technical university in prague, faculty of civil engineering, department of mechanics, thákurova 7, 166 29 prague, czech republic b czech technical university in prague, university centre for energy efficient buildings, třinecká 1024, 273 43 buštěhrad, czech republic ∗ corresponding author: ales.palicka@fsv.cvut.cz abstract. the paper deals with the investigation of alternative binders, namely the two available samples n6 and n7 with comparison from the reference sample cem i 42.5r. both testing binders are provided by company destro. it is a product that contains different ratios of fly ash and slag. by examining the properties, the mix was compared in different proportions and subsequently tested for flexural strength and compressive strength. the results showed the effect of curing time and cement to binder ratio after 28 and 84 days. keywords: alternative binder, fly ash, cement pastes, compressive strength, flexural strength. 1. introduction nowadays, the use of alternative binders is becoming more and more popular in the construction industry as a replacement for cement. the use is more mainstream among the main objectives is to reduce co2 emissions that are produced in the production of cement with the high energy requirement for the creation. individual introduction to alternative binders such as fly ash, which can be characterized as a fine-grained powder with a glassy structure that is produced as a waste material from the combustion of anthracite, lignite, and hard coal, where it is captured from the resulting gases by electrostatic or mechanical separators. due to the numerous production processes using different types of boilers, combustion techniques, separation methods, and types of coal burned, the resulting ash can have quite variable chemical, mineralogical, and granulometric compositions. in the concrete industry, more use is made of fly ash produced from hard coal, as it is not as variable as brown coal fly ash. slag is a coarsegrained substance of black to gray shade that may be of natural origin, i.e., volcanic or a product of the metallurgical industry, where it is produced as a waste product of iron or steel production. previous scientific findings with the implementation of alternative concepts have shown promising results as described by a. mohamed in his scientific work, where he observes the evolution of strength and rapid reactivity of the slag, where the results of the s1f1 (composed of slag and fly ash in a 50/50 ratio) pattern obtained remarkable results [1], a closer scientific evaluation is described by the scientific work of v. shobeiri., where it relies on minimization of gwp (global warming potential) in concrete and replacement of scm (supplied cementitios material) depending on opc (ordinary portland cement) by supplementary materials [2]. unfortunately, experimental methods require extensive experimentation and archiving so that we can meet the paris agreement on co2 reduction, and these experiments are often on a trial-and-error [3]. the experiment deals with selected n6 and n7 binders from different suppliers as replacement cement: (1.) binder n6 – destrocement (fly ash), (2.) binder n7 – destrocement (slag and fly ash). another reason is the cost per mix and the approximation of the same or better properties to portland concrete at a reduced price due to alternative binders that can be recovered from recycled material or incorporated as waste material. subsequently, the use of this waste material significantly reduces the price of the mix. 2. materials and samples the test specimens with dimensions 40 × 40 × 160 mm were manufactured according to en 196-1 [4], using moulds. a total of 5 mixtures were created for testing purposes, and 6 specimens were created for each mixture. the total number of specimens produced was 30. the actual production of the sample consisted of weighing the necessary raw materials in the given ratio of water, cement, and fine aggregate 0/2 and crushed slag in a laboratory balance with an accuracy of ±1 g, which can be seen in the table 1. for all formulations, a water coefficient of 0.718 was established and the workability of the fresh mix was measured, using a cone spill after 15 blows. after the raw materials were weighed, the individual materials were added to the mixing vessel. first cement with an alternative 60 https://doi.org/10.14311/app.2024.49.0060 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 49/2024 experimental of verification of alternative hydraulic binders marking cem i 42.5r destrocement destrocement fine aggregate water [kg] (fly ash) [kg] r [kg] (0/2 mm) [kg] [kg] ref 0.825 2.475 0.592 n6 30 0.578 0.248 2.475 0.592 n6 50 0.413 0.413 2.475 0.592 n7 30 0.578 0.248 2.475 0.592 n7 50 0.413 0.413 2.475 0.592 table 1. overview of mixtures. figure 1. detail of sample after destructive test. binder due to mixing and losses. the third ingredient was water and the last was fine aggregate 0/2 and crushed slag. after all the ingredients were mixed, the material was filled into molds that were greased with a layer of forming oil for better removal of the sample after hardening. immediately after the mixing was completed, the prepared steel triforms were filled about halfway with mortar using a spatula and the inner surface was painted with a thin layer of mineral oil. this first layer of mortar was further compacted. finally, the mortar surface was aligned with the edges of the triforms using a spatula sawing motion and the samples were stored at an ambient temperature of 21 ± 2 °c. after expiration of the time, the samples were and removed from the molds followed by marking the sample and placing it in a water bath at 20 ± 5 °c until testing time. storage was carried out for half of the samples for 28 days and the other half for 84 days. 3. experimental methods the experiment was tested for flexural strength on the basis of en 196-1 [4], on specimens of 40×40×160 mm beams. after measuring and weighing, the specimen was placed on two hydraulic press supports spaced 100 mm apart. using the software, the measurement was started by moving the hydraulic press at a speed of 1 mm min−1 until the specimen was destroyed into two parts, which were further used for compressive strength measurements according to en 196-1 [4]. figure 1 shows the result after destructive testing, the main indicator being the resulting fracture and visible structure of the recycled compound used. this method investigated the introduction of compressive stress up to the time of specimen destruction. the specimen was used from the remaining two parts of the previous flexural strength test using a three-point arrangement. the body of the specimen was placed on the compression of the 40 × 40 mm machine. then the hydraulic press displacement was started by the upper part displacement being constant throughout the test. 4. results and discussion the consistencies of the different mixtures in figure 2 show us that as the cement values decrease, the spillage decreases. in figure 3, we can see the bulk mass of each sample, which shows us the minimum variation with respect to the pore filling. from the measurements of the experiment using the three-point loading method to measure the flexural strength of the test specimens after a period of 28 and 84 days, the values shown in the figure 4. the graph shows the strength range compared to the reference sample. the lower the cement content of the sample, the strength decreases. interestingly, the destructiveness values at 50 % were like those of the reference sample after 84 days. for the n7 mixture, these values are even identical in strength at 30 % and 50 % of the mixture. for fly ash n6 the reaction was confirmed after a longer curing time. samples were tested using the uniaxial test after three points of testing. the highest and lowest compressive strength values were removed 61 a. palička, z. prošek, p. tesárek acta polytechnica ctu proceedings 0 20 40 60 80 100 120 140 160 180 200 ref n6_30 n6_50 n7_30 n7_50 d ia m et er o f sl u m p t es t [m m ] figure 2. spilling fresh mixture. 0 500 1000 1500 2000 2500 ref n6_30 n6_50 n7_30 n7_50 b ul k de ns ity [k g/ m 3 ] 28 days 90 days figure 3. bulk density of testing speciement with plotted standard deviations. 0 1 2 3 4 5 6 7 8 9 ref n6_30 n6_50 n7_30 n7_50 fl ex u ra l s tr en gt h [ m p a] 28 days 90 days figure 4. flexural strength with plotted standard deviations. 62 vol. 49/2024 experimental of verification of alternative hydraulic binders 0 10 20 30 40 50 60 ref n6_30 n6_50 n7_30 n7_50 c o m p re ss iv e st re n gt h [m pa ] 28 days 90 days figure 5. compressive strength with plotted standard deviations. from the obtained values and the arithmetic mean was removed from the remaining values, which can be seen in figure 5. the result shows how the solidification time affects the strength of the samples and after 84 days the strength approaches the reference values. it is remarkable that for specimens n6 50 and n6 30 the values after 84 days are close. similarly, for specimens n7 30 and n7 50 after 84 days, the values are in little difference. historically, although many studies have been conducted to quantify the effect of alternative binders on mechanical properties in concrete, the results were in mass agreement and had similar results. when alternative binders are applied as a partial replacement for cem i, 42.5r materials such as slag and fly ash respond best after a longer curing time and therefore a higher strength increase can be observed after 90 days or more. another factor affecting the individual mixes is the increase in the specific surface area due to the fineness of the material [5–7]. 5. conclusion this work focuses on the effect of waste supplementary cementitious materials on the mechanical properties of cement pastes. cement pastes are composed of mixed cement, where suplementary cementitious materials replaces portland clinker. in our case, we used fly ash and slag. supplementary cementitious materials were used in two different concentrations, namely: 30 wt. % and 50 wt. %. for our testing, the values for specimens n6 50 came out best in terms of replacement and applicability of alternative binders. for better observation and evaluation of the result, the curing time is a priority, which has a great influence on the properties of the different mixtures. a positive indicator is the reduction of hydration heat, which can be positively utilized. however, among the unpredictable factors in the application of an alternative binder, it is important to understand the origin of the material a qestion will be. where the material was used, the method by which it was recycled and shredded, and the storage of the material. the aforementioned factors have implications for unforeseen admixtures that were not included. therefore, it is worthwhile to establish a prescription for each future material to be recycled before use and recycling, so that it is appropriate for approximate laboratory conditions. considering all the results, the material can be used for construction works of a small nature, small weirs, walls, where the load is mainly caused by natural influences and high loads are not expected. the main limiting factor is the failure to guarantee the same properties of the alternative binder. acknowledgements this paper was financially supported by czech technical university in prague under no. sgs project gs24/037/ohk1/1t/11 and by the ga cr research a project under the number ga23-05500s. the authors also thank destro for the supplied samples and materials. references [1] o. a. mohamed, o. najm, e. ahmed. alkali-activated slag & fly ash as sustainable alternatives to opc: sorptivity and strength development characteristics of mortar. cleaner materials 8:100188, 2023. https://doi.org/10.1016/j.clema.2023.100188 [2] v. shobeiri, b. bennett, t. xie, p. visintin. mix design optimization of concrete containing fly ash and slag for global warming potential and cost reduction. case studies in construction materials 18:e01832, 2023. https://doi.org/10.1016/j.cscm.2023.e01832 [3] beyond zero emissions. zero carbon industry plan: rethinking cement. beyond zero emissions, 2017. isbn 978-0-9923580-2-0. 63 https://doi.org/10.1016/j.clema.2023.100188 https://doi.org/10.1016/j.cscm.2023.e01832 a. palička, z. prošek, p. tesárek acta polytechnica ctu proceedings [4] úřad pro technickou normalizaci, metrologii a státní zkušebnictví. čsn en 196-1. metody zkoušení cementu – část 1: stanovení pevnosti [in czech; methods of testing cement – part 1: determination of strength], 2016. [5] j. skibsted, r. snellings. reactivity of supplementary cementitious materials (scms) in cement blends. cement and concrete research 124:105799, 2019. https://doi.org/10.1016/j.cemconres.2019.105799 [6] l. e. burris, m. c. juenger. effect of calcination on the reactivity of natural clinoptilolite zeolites used as supplementary cementitious materials. construction and building materials 258:119988, 2020. https: //doi.org/10.1016/j.conbuildmat.2020.119988 [7] m. m. a. elahi, c. r. shearer, a. naser rashid reza, et al. improving the sulfate attack resistance of concrete by using supplementary cementitious materials (scms): a review. construction and building materials 281:122628, 2021. https: //doi.org/10.1016/j.conbuildmat.2021.122628 64 https://doi.org/10.1016/j.cemconres.2019.105799 https://doi.org/10.1016/j.conbuildmat.2020.119988 https://doi.org/10.1016/j.conbuildmat.2020.119988 https://doi.org/10.1016/j.conbuildmat.2021.122628 https://doi.org/10.1016/j.conbuildmat.2021.122628 acta polytechnica ctu proceedings 49:60–64, 2024 1 introduction 2 materials and samples 3 experimental methods 4 results and discussion 5 conclusion acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2024.46.0101 acta polytechnica ctu proceedings 46:101–107, 2024 © 2024 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague reflections on the crisis management of selected municipal and city authorities during the coronavirus pandemic martin šikýřa,∗, kateřina tomeškováa, radim burešb a czech technical university in prague, masaryk institute of advanced studies, institute of managerial studies, kolejní 2637/2a, 160 00 prague 6 – dejvice, czech republic b czech technical university in prague, masaryk institute of advanced studies, institute of public administration and regional studies, kolejní 2637/2a, 160 00 prague 6 – dejvice, czech republic ∗ corresponding author: martin.sikyr@cvut.cz abstract. the outbreak of the covid-19 pandemic in the spring of 2020 had a significant impact on the operation of all aspects of society, including public administration institutions. this article aims to examine the most challenging issues encountered by selected municipal and city authorities, how they responded to these issues, and how the experience gained has been reflected in crisis plans for managing similar future events. this aim is accomplished through a literature review of public sources and scientific papers on the effects of the covid-19 pandemic on public administration, as well as the results of interviews with representatives of the selected authorities. the results confirmed that the selected authorities were able to rapidly overcome the initial lack of relevant information and unpreparedness to implement anti-coronavirus measures and maintain the standard level of public services provided, albeit in a limited capacity. keywords: covid-19, public administration, anti-coronavirus measures, crisis management. 1. introduction the year 2020 brought the whole world, including the czech republic, an unexpected and threatening challenge in the form of the covid-19 pandemic, in which the rapid spread of the virus caused significant restrictions on most people’s daily lives [1]. the fundamental requirement of most of the responsible state institutions around the world was to prevent the spread of the infection between people, which led to the introduction of anti-coronavirus measures and restrictions on the free movement and gathering of people [2]. most people’s lives moved into the home and the online world [3]. the introduction of anti-coronavirus measures had significant economic and social impacts. many countries experienced a decline in gross domestic product, mainly as a result of household savings and a drop in domestic demand. many businesses were hit by closures of entire operations and workplaces [4]. significant losses were recorded by such businesses as travel agencies, airline companies, accommodation facilities, catering facilities, retail stores, and other services that were repeatedly closed. on the contrary, postal and courier services, information technology, and drugstore, medical, and pharmaceutical goods recorded growth [5]. the key to business success was the ability to flexibly change the business model and start doing business online [6]. the spread of the coronavirus pandemic and the introduction of anti-coronavirus measures resulted in an exceptional situation in healthcare and social services. there was a dramatic increase in the number of patients and deaths, which placed extreme demands on staff [7]. medical and social facilities lacked doctors, nurses, and social workers. this required the involvement of volunteers from other professions [8]. a similarly complicated situation was in education, where it was necessary to deal with the closure of schools and switch to distance learning using computers and the internet. distance learning during the coronavirus pandemic fundamentally changed the educational habits of pupils and students and placed increased demands on their parents as well as teachers [9]. society-wide problems included people’s concerns about the health of themselves and their families, as well as concerns about the uncertain economic and social situation. added to this was the restriction of social contacts and leisure activities, which had negative effects on people’s mental and physical health [10]. dealing with the economic and social impacts of the coronavirus pandemic induced the need for crisis management at all levels of the state, from the national level to the regional level to the level of cities and municipalities. it was important to maintain the standard functioning of public administration offices, even if it was with fewer people or limited office hours [11]. during the lockdowns, there was a surge in working from home, which, in the case of public administration officials, is difficult to implement. most of the officials thus had to go to work even during the lockdowns, while the offices had to routinely deal with changing officials or replacing sick officials. there was also a significant development of electronic communication 101 https://doi.org/10.14311/app.2024.46.0101 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en m. šikýř, k. tomešková, r. bureš acta polytechnica ctu proceedings between public administration offices and citizens [12]. the handling of the coronavirus pandemic within public administration offices was naturally associated with many issues that certainly tested the ability of individual offices to handle similar crises through effective crisis management [13]. the coronavirus pandemic was unprecedented, as it broke out unexpectedly, quickly, and strongly. such an extreme event usually requires a unique ad hoc response according to the current situation, which can hardly be part of any crisis plan [14]. however, every such event is an opportunity to learn something new, and every such experience should or could be incorporated into updated crisis plans for future use [15]. this article seeks to evaluate the extent to which this has been achieved. this article reflects upon the crisis management of selected municipal and city authorities during the coronavirus pandemic. the aim is to examine the most challenging issues that the selected authorities met, how they responded to these issues, and how the experience gained has been incorporated into crisis plans for managing similar events. the assumption is that the selected authorities were able to overcome the initial difficulties resulting from the uniqueness of the situation and maintain the standard level of public services provided. 2. materials and methods this survey of crisis management and the most challenging issues of the selected municipal and city authorities during the coronavirus pandemic is founded on interviews with representatives of selected municipal and city authorities, complete with interviews with representatives of selected state institutions. the interviews were conducted during april and may 2023, with respondents being approached directly based on previous experience and cooperation on other research projects. the decisive criterion was that the respondents were willing to share their knowledge and experiences from the times of the coronavirus pandemic. for many of them, it was not easy to remember and they wished to never experienced anything like that again. representatives of (1.) the ministry of the interior (one respondent: a department director); (2.) the ministry of education, youth, and sports (three respondents: a deputy minister, a methodology specialist, and a department manager); (3.) a city authority in the vysočina region, jihlava district (one respondent: a department manager); (4.) a city authority in the pardubice region, svitavy district (two respondents: a deputy mayor and a department manager); and (5.) five municipal authorities in the pardubice region, chrudim district (five respondents: five council members) were obtained for interviews. a total of twelve respondents from nine state, city, and municipal authorities were interviewed. all respondents were members of crisis teams during the coronavirus pandemic and have experience implementing anti-coronavirus measures. respondents, cities, and municipalities are kept anonymous by agreement. during the interviews, respondents were asked four questions about their experience with crisis management and the introduction of anti-coronavirus measures during the coronavirus pandemic. the first question asked about early memories of the coronavirus pandemic. the second question asked what the most challenging issues were that the selected authorities met. the third question asked how they had responded to these issues. the fourth and last question asked how the experience reflected in crisis plans for managing similar events. some interviews took place in person, others took place online through microsoft teams. each of the interviews lasted approximately sixty minutes, depending on the time available and the interest of the respondents. the aim was to let respondents talk freely and remember as many immediate experiences as possible. the results could be challenged by the limited scope of the survey, including a small set of respondents. however, the surveyed issues relating to the experience of crisis management and the introduction of anti-coronavirus measures are common for most state, regional, city, and municipal institutions dealing with the coronavirus pandemic. this fact is a solid foundation for achieving beneficial results for both managerial theory and practice. the survey brings added value by providing a closer look at the direct experiences of people from the front line – at the level of the state, regions, cities, and municipalities – who had to deal with new challenges related to the spread of the coronavirus pandemic and the introduction of anti-coronavirus measures daily. their knowledge and experience are a good starting point for further research on managing crises in public administration. 3. results in january and february 2020, czech society and responsible institutions monitored the development of the disease in china and other countries worldwide. partial measures were aimed mainly at reducing travel abroad [16]. at the beginning of march 2020, the first patients affected by covid-19 were diagnosed in the czech republic. the number of patients began to increase rapidly, and it was obvious that the worsening situation would require a crisis response. when the world health organization officially designated the spread of covid-19 as a pandemic on march 11, 2020, 102 vol. 46/2024 reflections on the crisis management . . . a state of emergency was declared in the czech republic on march 12, 2020 [17]. the result was, among other things, the closing of state borders, restrictions on the free movement and gathering of people, restrictions on selected business activities, and the closure of schools or other social facilities with the risk of a concentration of a larger number of people [18]. at the national level, the central crisis staff began to function, and similar crisis staffs were also established at the level of regions, cities, and municipalities to take the needed measures, issue appropriate instructions, provide the relevant information, and coordinate the necessary actions [19]. the lack of relevant information was addressed through information hotlines or the websites of relevant governments and other institutions, such as the ministry of health or the ministry of the interior [20]. it was essential to come up with constructive solutions quickly. the first wave of the coronavirus pandemic in the czech republic lasted until mid-may 2020, when the government began to loosen its anti-coronavirus measures and ended the state of emergency on may 17, 2020 [17]. however, during the summer of 2020, the pandemic situation began to gradually worsen, so the mandatory wearing of face masks or respirators in public indoor spaces and public transport was renewed in september 2020. a state of emergency was declared again on october 5, 2020, and lasted until april 11, 2021 [21]. during this second wave of the coronavirus pandemic, there were again restrictions on the free movement and gathering of people, school closures, and restrictions on business activities [18]. local governments had to deal with limited office hours, school closures, or canceled mass events. cities and municipalities also had to deal with purchasing protective devices and distributing them to citizens, carrying out disinfection of public spaces, and assisting the most vulnerable citizens. methodological and coordination support for cities and municipalities was provided by ministries and regions [13]. additional support was provided by regional hygiene stations, the fire brigade, and the police force. these institutions ensured compliance with anti-coronavirus measures, distributed protective devices, performed testing, and traced the infected [17]. the situation during the coronavirus pandemic was not easy for anyone, and most public administration institutions reacted spontaneously to an exceptional and unprecedented event [22]. every decision was made under uncertainty, due to a lack of relevant information and experience. nevertheless, most of the initial and subsequent issues were overcome [23]. this article offers several views from selected state, municipal, and city authorities on what issues they faced during the coronavirus pandemic, how they dealt with them, and how they learned from them. 3.1. view from the ministry of the interior the ministry of the interior is, among other duties, responsible for public administration, and it played an important coordinating, controlling, and methodical role during the coronavirus pandemic. early memories of the interviewed representative of the ministry were associated with surprise at the speed and extent of the spread of the coronavirus pandemic in the spring of 2020. it was a unique situation for which no one was prepared. it was initially expected to be a short-term affair, but it soon became clear that the situation was serious and would last a long time. in the beginning, the effort was to coordinate the activities of local governments “from above”, but this was complicated due to the different approaches of ministries and the inconsistent crisis methodologies or undeveloped crisis plans of cities and municipalities. there are also over 6 000 municipalities in the czech republic, which makes communication and coordination from the center quite difficult. however, cities and municipalities quickly adapted and began to deal with the situation independently at their own discretion. the ministry of the interior endeavored to coordinate the functioning of local governments through methodological recommendations, which were regularly updated and created as comprehensibly as possible. it was important to give cities and municipalities clear information on what to do and how to do it, without the need for complex legal interpretation. to simplify communication, the instructions were given directly to cities and municipalities, additionally using the territorial offices of the ministry of the interior. the operation of offices, office hours, contact with the public, or the gathering of people were dealt with. questions and complaints were also addressed. the feedback was then used to modify the published methodologies. in the future, it would be beneficial to implement the ministry of the interior’s coordinating role with other institutions. the effort is to coordinate the crisis management of municipalities to the level of municipalities with extended powers. however, each crisis is unique and it is always necessary to relearn to adapt to new circumstances, so the creation of crisis plans or crisis scenarios is questionable. a big challenge for the future is also the digitization of public administration, which proved useful during the coronavirus pandemic, but too much dependence on digital means is also not good. therefore, it is necessary to be prepared for various options. 3.2. view from the ministry of education, youth, and sports the ministry of education is responsible for education, and it had to deal with repeated school closures and the transition to distance learning. as stated by the interviewed representatives of the ministry, almost 103 m. šikýř, k. tomešková, r. bureš acta polytechnica ctu proceedings immediately after the outbreak of the coronavirus pandemic, a crisis staff started functioning at the ministry, which communicated with the central crisis staff and lower levels responsible for education. the effort was to maintain the full functioning of the ministry, but it was necessary to solve technical issues (remote access, notebooks, smartphones) or organizational issues related to homework. online communication took place via e-mail and microsoft teams, but also other applications such as whatsapp or social networks such as facebook. over time, a close team of people was formed at the ministry, which worked in a 24/7 arrangement. other people were used as needed. communication with the schools focused mainly on sharing important information, explaining it, and moderating panic. difficulties arose when the media published some information before it was officially announced. this often led to unnecessary panic and chaos. cooperation with the legal department has proven itself in these situations. the information was transmitted to the schools via data boxes. later, a helpdesk designed for schools was launched. to facilitate the work of the helpdesk, separate telephone lines for schools and the public were established. the effort was to make work easier for schools and help them solve their issues, including, for example, conflict situations regarding testing or vaccination. in such situations, schools were advised to refer directly to the ministry. to support schools, various methodologies and instructional videos were created, for example on how to test pupils and students. according to the respondents, the crisis certainly brought about a dramatic shift in the use of information and communication technologies. it taught many people to work in non-standard conditions and to improvise, as well as to show individual dedication. on the other hand, it was necessary to deal with the negative behavior of some people, and it was also necessary to learn to patiently explain and defend the implemented measures. 3.3. view from a city authority in the vysočina region as stated by the interviewed representative of a city authority in the vysočina region, jihlava district, who is responsible for education, like everywhere else, here too, after the outbreak of the coronavirus pandemic, they had to deal with the closure of schools and the transition to distance learning. it turned out that most schools were not technically or organizationally ready to work and teach online. the initial issue of the functioning of the city authority and individual schools was the uncertainty resulting from the lack of relevant information. there was a lot of information coming from superior authorities that was difficult to understand correctly and apply effectively. the situation gradually improved with experience. a crisis team met regularly within the city authority (online and physically) to fulfill a coordinating and methodical role. the functioning of the city authority was maintained. people alternated between working at the office and from home. in the schools themselves, it was necessary to deal with the technical and organizational issues of distance learning. subsequently, it was necessary to coordinate and control the implementation and compliance with anti-coronavirus measures. no new methodologies related to crisis management were developed. only “ten recommendations” were issued for crises. the ability to work online needs to be developed in the future. 3.4. view from a city authority in the pardubice region as stated by the interviewed representatives of the city authority in the pardubice region, svitavy district, responsible for the management of the city authority and education, since the outbreak of the coronavirus pandemic, the effort was to maintain the functioning of the city authority without significant changes. as with everywhere else, they initially had to overcome technical and organizational difficulties. it was also necessary to deal with often chaotic information in the early days and weeks of the coronavirus pandemic. within the management of the city authority, the safety council met regularly every week. the team was supplemented by other people as needed. a crisis telephone line was also set up. the priority was to manage the introduction of anti-coronavirus measures. in the beginning, general solidarity was evident. everyone helped as they could. however, over time various problems had to be overcome, such as the interpretation of some measures or resistance to some measures. it was necessary to handle the technical and organizational issues of distance learning in schools. it was also necessary to deal with the limitation or closure of the operation of services for the public. issues were solved on the fly, without unnecessary paperwork. according to the respondents, based on the experience of dealing with the coronavirus pandemic, it is difficult to create any crisis plans or methodologies. it always depends on the situation. it is important to come up with quick, simple, and clear solutions based on the current need. common sense is also important. do not instill fear or spread panic. communicate and share useful information. learn to improvise. 3.5. view from municipal authorities in the pardubice region the interviewed representatives of five municipal authorities in the pardubice region, chrudim district, were members of the municipal council during the coronavirus pandemic. the first municipality has about 140 residents, the second municipality has about 90 residents, the third municipality has about 120 residents, the fourth municipality has about 100 residents, and the fifth municipality has about 250 residents. in 104 vol. 46/2024 reflections on the crisis management . . . the most challenging issues •the lack of relevant information during the coronavirus pandemic •the unpreparedness for anti-coronavirus measures the response •keep working •do not instill fear or spread panic •communicate useful information •offer quick and simple solutions the reflection •learned to improvise and adapt quickly •built reserve capacities table 1. summary of respondents’ answers. addition to residents, many vacationers in the municipalities come for weekends and holidays. initial memories of respondents were similarly associated with something implausible that seemed impossible. the original hope was that the coronavirus pandemic would not particularly affect the municipalities themselves. later, it became clear that it was necessary to come to terms with reality and submit to the introduced coronavirus measures. the functioning of the municipal administration was preserved online and physically as much as possible. the effort was to convey relevant information and prevent panic. information was sought from all available sources. sometimes it was difficult to understand all the instructions correctly. social life in the municipalities died down. the operation of local social facilities was suspended. within the rules, the operation of local general stores was maintained. everyone handled the situation in their own way. lessons for the future are rare. rather, the hope is that a similar situation will not be repeated. 4. discussion the results of interviews with representatives of selected state, municipal, and city authorities offer several interesting views on what issues the selected authorities faced during the coronavirus pandemic, how they dealt with them, and how they learned from them (see table 1). it was confirmed that the most challenging issue with the outbreak of the coronavirus pandemic was the initial uncertainty resulting from the lack of relevant information [23]. no one had expected such a crisis. no one was ready for it. no crisis plan anticipated such a crisis. there was a lack of experience and resources to handle such a crisis. after overcoming the initial shock, another issue was the technical and organizational unpreparedness to implement anticoronavirus measures [20]. people had to learn along the way to work online and outside their workplaces, usually from their homes. it was further confirmed that selected authorities were able to overcome the initial difficulties resulting from the uniqueness of the situation and maintain the standard level of public services provided [22]. the effort of all was to maintain the functioning of the authorities, moderate panic, and help overcome problems. it was also confirmed that the crisis associated with the coronavirus pandemic has taught people to work online in a shared environment using various information and communication technologies [24]. the crisis also tested people’s ability to express themselves clearly and concisely, as well as to follow instructions and demonstrate both a material and human approach [15]. the issue of updating crisis plans following the experience of the coronavirus pandemic or issuing some “crisis procedures” in case of emergency is debatable [14]. each crisis is unique in its way, and it is difficult to predict what and how it will be necessary to do. rather, it is necessary to develop the ability to learn and adapt quickly. at the same time, it is necessary to build reserve technical and other capacities that could be used in a crisis. in principle, it is necessary to hope for the best but be ready for the worst. however, the tendency to develop and implement obligatory crisis plans is more evident in the state rather than in regional, city, or municipal institutions. the reason is the legitimate effort of state institutions to coordinate the crisis management of lower institutions. such emergency plans should be clear and easy to use, including easy modification according to the current situation. 5. conclusions a survey on a reflection on the crisis management of selected municipal and city authorities during the coronavirus pandemic confirmed the assumption that the selected authorities were able to overcome the initial difficulties resulting from the uniqueness of the situation and maintain the standard level of public services provided. the most challenging issues that the selected authorities met during the coronavirus pandemic included the initial uncertainty resulting from a lack of relevant information and technical and organizational unpreparedness to implement anti-coronavirus measures. the responses to these issues were sought on the fly, according to current possibilities and requirements. the effort was to keep the authorities functioning, allay panic, and help people overcome problems. the issue of projecting the experience of handling the coro105 m. šikýř, k. tomešková, r. bureš acta polytechnica ctu proceedings navirus pandemic into crisis plans turned out to be debatable. the reason is the uniqueness of each crisis and the difficulty of predicting what it will be necessary to do and how to do it. the limitations of the survey include a small set of respondents and the focus on a few selected municipal and city authorities in the czech republic. however, the surveyed issues of crisis management during the coronavirus pandemic are common for most state, region, city, and municipal institutions. this fact is a solid foundation for achieving beneficial results for both managerial theory and practice. the added value of the results could be seen in a closer look at the direct experiences of people from the front line at the level of the state, regions, cities, and municipalities who had to deal with unique challenges related to the spread of the coronavirus pandemic and the introduction of anti-coronavirus measures. their findings are a possible starting point for further research focused on managing crises in public administration. an interesting issue to investigate could be the vertical line of crisis management from the state through regions to cities and municipalities and back, including the enactment of the coordination role of certain state institutions, such as the ministry of the interior. references [1] p. c. terry, r. l. parsons-smith, v. r. terry. mood responses associated with covid-19 restrictions. frontiers in psychology 11, 2020. 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[20] j. slabá. vládní boj proti pandemii: přehled opatření vydaných v souvislosti s pandemií onemocnění covid-19 v česku v letech 2020 a 2021. demografie 64(2):175– 196, 2022. https://doi.org/10.54694/dem.0303 [21] ministry of the interior of the czech republic. 2022 annual report on the state of the public administration of the czech republic for the year 2021, 2022. 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https://doi.org/10.1108/ijpl-06-2020-0047 https://doi.org/10.46585/sp29011320 https://doi.org/10.7441/joc.2022.01.09 acta polytechnica ctu proceedings 46:101–107, 2024 1 introduction 2 materials and methods 3 results 3.1 view from the ministry of the interior 3.2 view from the ministry of education, youth, and sports 3.3 view from a city authority in the vysočina region 3.4 view from a city authority in the pardubice region 3.5 view from municipal authorities in the pardubice region 4 discussion 5 conclusions references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2024.50.0055 acta polytechnica ctu proceedings 50:55–61, 2024 © 2024 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague mechanical properties of dental fillings with nanoparticles magdalena mrózeka,∗, lucie svobodováa, totka bakalovaa, helena gronwaldb, šárka bukovskác, michal krafkaa a technical university of liberec, faculty of mechanical engineering, department of material science, studentská 2, 461 17 liberec, czech republic b pomeranian medical university in szczecin, physical diagnostics and dental physiotherapy, department of propaedeutics, rybacka 1, 70-204 szczecin, poland c technical university of liberec, faculty of mechanical engineering, department of engineering technology, studentská 2, 461 17 liberec, czech republic ∗ corresponding author: magdalena.mrozek@tul.cz abstract. nanoparticles represent an innovative direction in dentistry that offers several possibilities for improving the physical-chemical properties of dental materials. moreover, they may help to reduce tooth sensitivity and prevent the recurrence of tooth caries with their antibacterial properties. particular attention is given to the significant potential of nanoparticles in enhancing the mechanical properties of dental fillings. our aim was to use diamond nanoparticles to modify dental fillings based on glass ionomer cement (gic) and evaluate the effects of nanoparticles on the hardness, strength, wear resistance, and surface texture of the composite. results suggest that modification of gic using diamond nanoparticles can be an effective method for improving mechanical properties and may lead to the development of improved materials for dentistry. keywords: dental fillings, diamond nanoparticles, glass ionomer cement, mechanical properties, tribological properties. 1. introduction the safety of amalgam fillings has been a contentious issue in dentistry. concerns have primarily centred on the risks associated with the handling and use of amalgam materials, including potential exposure to mercury vapours by healthcare professionals, allergic reactions to the metals contained in amalgam, and the challenges related to the safe disposal of amalgam waste. in response to these risks, alternative dental filling materials have been developed and studied, with an emphasis on their safety and ability to replace amalgam fillings [1, 2]. in the latter half of the 20th century, initial mentions of alternative dental filling materials emerged; one is, for example, glass ionomer cement (gic). since its introduction, gic has undergone significant advancements and numerous innovations, establishing itself as a self-curing material predominantly used in restorative dentistry. it is particularly effective for treating deciduous teeth, class iii and v caries, and minor class i and ii caries [3, 4]. gic exhibits reduced moisture sensitivity at the dental filling application site compared to photocomposites. the moist environment within the oral cavity is beneficial for gic, as water acts as a reaction medium in the initial stages and facilitates the gradual hydration of the crosslinked chains in the subsequent stages [5, 6]. gic offers several notable benefits, including the release of fluoride ions, strong adhesion to dentin and enamel, promotion of partial remineralisation of dental tissue, and excellent biocompatibility. however, despite these advantages, gics are currently limited by their inadequate mechanical properties, which constrain their broader use and impact their durability in clinical settings [7]. the mechanical strength of dental fillings is critical, as it determines the material’s ability to withstand the compressive forces generated during mastication. hardness, on the other hand, reflects the material’s resistance to wear and abrasion, which is crucial for the longevity of dental restorations [8]. one additional factor that affects the resulting characteristics of gics is surface roughness. this has a substantial impact on the adhesion between the filling and the dental tissue and influences interactions with microorganisms and salivary components present in the oral cavity [9, 10]. the incorporation of nanoparticles into dental fillings presents a promising way for advancements in dentistry. nanoparticles offer the potential to enhance the mechanical properties, wear resistance, and antibacterial activity of fillings, thereby extending their lifespan and contributing to improved patient outcomes. however, identifying suitable nanoparticles for this purpose requires considerable research and development efforts [11]. this research evaluates the change in performance of a commercially available glass ionomer cement (gic) modified with nanoparticles (dimensions less than 55 https://doi.org/10.14311/app.2024.50.0055 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en m. mrózek, l. svobodová, t. bakalova et al. acta polytechnica ctu proceedings 100 nm). the dimensions, morphology, chemistry, or surface modification of these nanoparticles significantly influence the physicochemical characteristics of the composite material in which they are used. this research aims to assess the mechanical, physical, and tribological properties of the newly developed composite materials suitable for dental purposes. the findings are compared with commercially available gics commonly employed in clinical practice. 2. materials and methods 2.1. glass ionomer cement glass ionomer cement (gic) is a dental filling material composed of a powder and a liquid. the powder consists of finely ground silica glass, calcium, aluminium, and fluoride compounds. the liquid is polyacrylic acid. when these two components are mixed in precise proportions, an acid-base reaction occurs, forming a solid material upon solidification [5, 6]. sdi riva self cure glass ionomer cement (gic), manufactured by sdi dental limited, as a powder/liquid system was used in our research. the powder and liquid components are mixed in a 1:1 ratio, corresponding to one drop of liquid and one plastic scoop of the powder component. the measuring plastic scoop is part of the package. the mixing time of both components should be approximately 30 seconds, while the solidification time of the gic is around 2 minutes. 2.2. nanoparticles nanoparticles of diamond (nd, adámas nanotechnologies, 30 nm) and modified nd (nd-oh, hydroxylated detonation nanodiamonds, adámas nanotechnologies, 30 nm) were used. nanoparticles were selected for their mechanical properties and biocompatibility. the added nanoparticles to the gic were at 0.25wt% and 0.50wt% concentrations (the concentration was calculated for the powdered component). the nanoparticles were integrated into the powder phase of the gic using a mortar and pestle. the resulting mixture was homogenised by mechanical stirring in a digester for several minutes. this procedure was established based on a pre-determined methodological framework. the sample without adding nanoparticles, i.e., according to the manufacturer’s instructions, was a control sample (reference samples). the research was focused on evaluating the effect of nanoparticles on the resulting utility properties of the composite and choosing the appropriate type of nanoparticles. the distribution of nanoparticles in the volume of the composite material and the amount and size of the resulting agglomerates will be the subject of further research. the following tables and graphs will identify the different sample groups by letters; see table 1. 2.3. sample and experimental set-up the gic powder and nanoparticles were thoroughly mixed and enclosed in well-sealed containers. the sample nanoparticles [wt%] a reference sample (no modification) b nd 0.25% c nd 0.50% d nd-oh 0.25% e nd-oh 0.50% table 1. designation of sample types. samples were prepared per the instructions for the commercially used gic, i.e., the powder portion was put on a glass slide using a plastic spoon, and the liquid portion of the gic was measured using a dropper (part of the package). the amount of powder (or powder with nanoparticles) and liquid depended on the size and shape of the sample required; a ratio of 1:1 was always maintained: a) cuboid shape with dimensions 25× 4× 4mm (length×width×height), according to iso 4049:2019 dentistry – polymer-based restorative materials. due to the 3-point bending tests, the size of the samples had to be adjusted (by doubling the height and width of the samples). b) cylinder shape with dimensions ∅ 4mm, height 6mm, according to iso 7489:1986 dental glass polyalkenoate cements. the specific-shaped silicon mould was used to create cuboidal or cylindrical samples (see above; only the lateral sides of the composite came into contact with the mould; a glass square applied pressure to the top and bottom sides of the sample placed at the same time in the mould for a maximum of 3 minutes). once the composite had dried, the sample was easily removed from the mould. without any additional surface treatment, the samples were stored in an eppendorf tube with a few drops of distilled water (as a crack prevention) at room temperature. 2.4. mechanical-physical properties the compressive strength test (figure 1a) was carried out on 5 cylindrical specimens (see section 2.3b) using a testometric machine based on iso 4506. the machine jaw feed speed was 0.05mmmin−1, the preload was 300n (to shorten the test, the preload that the sample should withstand is set to avoid starting from 0n), and the force drop was 50% fmax. upon reaching the maximum load capacity, the material’s structural integrity begins to degrade. the test is concluded when the load capacity diminishes to 50% of the maximum recorded load. specimen failure ended the compressive strength test. the 3-point bending test was performed on the 5 cuboid specimens (see section 2.3a) using the 3-point bending method at bruker nano surfaces division (figure 1b). the feed rate of the machine’s top roller was approximately 0.02mms−1, and the initial position of the top roller was approximately 13mm above 56 vol. 50/2024 mechanical properties of dental fillings with nanoparticles (a). compressive strength test. (b). 3-point bending test. figure 1. conducting experimental activity. the test specimen. the distance between the lower support rollers was 20mm, and the diameter of one roller was 5mm. the same method was used for adhesion tests between the materials (the modified gic to the reference gic). for hardness measurements, the device used struers duramin-40 on block samples; with the vickers method, a load of 0.015 kg and ten indentations were performed on each tested specimen with 1mm spacing between each indentation. the surface roughness of the samples is described in section 3.4, which provides the average sa values for all samples. the tribological test was performed using the anton paar trb3 tribometer (which complies with astm g99 and astm g133 standards) with the linear reversible method. 5 cuboid specimens (see section 2.3a) and a ceramic sphere (al2o3) with a diameter of 6mm as the test counter-body were used; the load was set at 5n, and the ball’s movement speed on the composite’s surface was at 4.50 cm s−1; the total travel distance was 10m. at the end of the test, using a sensofar s neox confocal microscope, the wear based on the width and depth of the wear track was evaluated. as part of the research, the experimental work was aimed at evaluating the amount of wear of composite materials before and after modification with nanoparticles. the aim of the experiment is not to simulate chewing processes in the oral cavity, but the resistance of modified composite materials to the action of external forces during the preservation of moisture of the composite material. 2.5. confocal microscopy the samples’ surface roughness was examined using a sensofar s neox confocal microscope at 20x magnification. the samples were prepared in special silicone moulds that ensured the necessary surface quality. the sample preparation is described in section 2.3, while the surface structure is defined using a silicone mould or a glass square. therefore, there was no need to perform additional treatment of the surface of the samples as part of further measurements. measurements were taken at five locations in the centre of the block sample. according to iso 25178, the height parameter (sa, arithmetical mean height of the figure 2. samples of glass ionomer cement without and with nanoparticles of nanodiamonds (sample b). surface) was evaluated, including the statistical distribution of height values along the z-axis. material surface characterisations were processed using gwyddion (a modular program for visualising and analysing height field data, accessible and open-source software). 2.6. statistical analysis the results for the mechanical properties are tabulated as median and first and third quartiles (q1 and q3), as they provide the most reliable performance when comparing distributions between groups. they were chosen because these tests had more outliers, which skewed the average. the tribological properties and adhesion results are shown as bar graphs of means with a standard deviation (a better expression for the data where standard distribution may be assumed). a one-way anova test for statistical analysis was used. 3. results and discussion 3.1. analysis of mechanical-physical properties we took measurements seven days after sample preparation (example in figure 2). during the first few days, the gic is still “working” (so-called cement maturation); specific reactions occur through which the material matures, and its physical and mechanical properties are constantly improved. after seven days, the material is sufficiently mature and its properties change minimally thereafter. for the reference specimens (group a), compressive strength was measured at 100.1mpa after seven days, and flexural strength was 42.3mpa (table 2). the compressive strength was comparable to the reference specimens for the modified specimens group b and c (both nd), and group d (nd-oh 0.25%) was evaluated with a 12% reduction in values. a slight value increase (of about 14%) was observed only for the samples with higher modified nanodiamond content, 114.9mpa group e (nd-oh 0.50%). the flexural strength decreased for all samples with nanoparticles; only the samples in group e (nd-oh 0.50%) achieved values similar to the reference samples, reaching 40.7mpa (lower by about 4% than group a). nanoparticles can affect gic flexural strength and 57 m. mrózek, l. svobodová, t. bakalova et al. acta polytechnica ctu proceedings compressive strength flexural strength vickers microhardness test [mpa] [mpa] [hv0,015] group median q1 q3 median q1 q3 median q1 q3 a 100.1 89.6 112.8 42.3 40.9 53.4 57.9 54.5 60.0 b 97.7 80.6 101.4 27.6 22.2 34.1 37.6 31.4 40.4 c 95.4 82.3 103.7 19.0 17.1 19.4 47.0 37.6 64.0 d 88.7 64.1 108.6 27.8 22.0 37.1 40.9 36.6 46.7 e 114.9 104.8 115.4 40.7 28.7 52.2 35.1 34.4 38.7 table 2. results of mechanical properties of the samples (designation according to table 1). compressive strength. however, compared to composite resin dental fillings, both parameters measured for modified gics (group e) are twice as low [12, 13]. in the case of microhardness, the added nanoparticles reduce the hardness of the material (table 2). although the samples with higher concentrations of nd nanoparticles showed higher values (47.0hv0.015, group c) than the other samples, they did not exceed the values of the reference samples (57.9hv0.015, group a). 3.2. adhesion of composite materials adhesion of the dental material refers to its ability to adhere effectively to the tooth surface, which is necessary to maintain the stability of the filling and prevent the penetration of bacteria or fluids between the filling and the tissue. the process of preparing adhesion tests is known to be a complex and multifaceted endeavour, involving a range of factors that must be taken into account (e.g., permission to test on a human specimen, tooth preparation for the dental filling, instrument set-up); therefore, we chose an alternative method to assess the adhesion of modified gic specimens to the gic reference material. first, we created a half-block size sample (section 2.3a) from a gic without modification (group a, reference material has been filled to half the length of the mould). after about 2 minutes, when the material had sufficiently solidified but not completely hardened, the second half of the mould was filled with modified gic (nd, nd-oh) or with the reference gic. after seven days of solidification (stored in an eppendorf tube with a few drops of distilled water), samples were tested by the 3-point bend method. the highest adhesion value was reached for the samples with higher nd content (group c) and in both nd-oh concentrations (group d and e). in contrast, we observed (figure 3) poorer results for the samples with nd at lower concentrations (group b). it is important to note that the results from the adhesion tests are only indicative and should be considered along with other relevant information. for example, in practice, the tooth is pretreated before the filling is applied and the surface roughness of the tooth (tissue matter) and the material surface changes. a significant advantage of gic is that there is no need for an ideally dry surface for adhesion of the filling to (a). a bar graph of mean values with standard deviations of the flexural strength of the modified gic to the reference gic. (b). broken sample c (left side unmodified gic, right side modified gic). figure 3. an adhesion test. the tooth, as is the case with photo composites, for example. 3.3. wear resistance various factors can cause the wear rate of dental fillings. abrasion of dental fillings can lead to gradual erosion or flaking and may require repair or replacement. figure 4a shows that the samples containing nd with lower and higher concentrations (group b and c) had lower wear depth compared to the reference samples (group a); the sample with higher concentrations (group c) also had a smaller wear width (figure 4b). research has shown that the presence of nanodiamonds may reduce the wear of the gic composites. for some samples, more sample fragments 58 vol. 50/2024 mechanical properties of dental fillings with nanoparticles (a). (b). figure 4. wear test. a bar graph of mean values with standard deviations for (a) the wear track depth and (b) the wear track width. (quantity or volume) were removed during the tests, as indicated by the higher standard deviations. the results in figure 4 are too limited to conclude that the addition of a small amount of diamond powder improves wear resistance. more detailed observations are necessary. 3.4. composite surface roughness analysis the surface roughness parameter sa was evaluated (figure 5) and the results were processed according to the anova statistical method. the height parameters (table 3) show that the average surface roughness measured for the reference sample was 220 nm. lowest surface roughness was measured for the samples with nd modification at higher concentrations (group c, lower by about 27% than group a) and with nd-oh modification at lower concentrations (group d, lower by about 14%). the highest surface roughness was measured for samples with nd modification at lower concentrations (group b, higher by about 39%) and with nd-oh modification at higher concentrations (group e, higher by about 22%). 3.5. discussion this study (figure 5, composite samples) explored the impact of nanodiamonds (nds) and hydroxylated sa, arithmetical mean height of the surface [nm] group median q1 q3 a 220.2 215.5 242.2 b 306.5 275.5 379.2 c 173.2 154.3 283.1 d 193.5 182.5 201.1 e 269.1 235.0 313.2 table 3. height parameters of samples as a result of confocal microscopy. nanodiamonds (nd-oh) on the mechanical and tribological properties of glass ionomer cements (gics). the incorporation of higher concentrations of nd-oh increased compressive strength and flexural strength, reaching values of 114.9mpa and 40.7mpa, respectively. while nd and modified nd particles influenced the mechanical and tribological properties of gics, the significance level of 5% was not exceeded in any of the measurements. the results from other studies on materials commonly used for dental fillings showed that commercially available dental composites exhibit maximum compressive strengths of approximately 210± 1.8mpa and flexural strengths of 119.6± 2.3mpa. in comparison, amalgam displays compressive strengths of 199.5± 1.8mpa and flexural strengths of 17.7± 1.1mpa [12, 13]. despite the slight improvement in compressive and flexural strength in the modified gic samples (of about 15%), these values remained close to those of the reference samples, indicating that the modifications did not substantially enhance the mechanical properties of gics. the modified gics in our study do not reach the values of other restorative materials. furthermore, all modifications in this study led to a decrease in microhardness relative to the reference samples (commercially available gics), which is consistent with findings in other research [14]. this decrease in microhardness may be attributed to several factors, such as nanoparticle aggregation, insufficient nanoparticle distribution (short processing time of the composite), disruption of the gic matrix, incompatibility of nanoparticles with gic components (low particle adhesion to the matrix), or alterations in chemical reactions (inclusion of a non-native component in the gic). the incorporation of nd at both concentrations resulted in up to a 50% reduction in the wear depth of gics compared to the reference samples, which proved significant at a level of 5%. for future research, it would be advantageous to simulate the oral environment, particularly with respect to humidity levels, as material drying was observed during measurements. in addition, conducting dynamic tests that partially mimic masticatory processes would provide more pertinent insights. it is crucial to acknowledge that all results remain indica59 m. mrózek, l. svobodová, t. bakalova et al. acta polytechnica ctu proceedings figure 5. images of sample surface with 20x magnification (confocal microscopy). the scale is the same for all images: 140 µm. tive, as a complete simulation of the oral environment is inherently challenging. variability in bite force, biting frequency, force composition, and genetic and ethnic predispositions all contribute to the longevity of dental restorations, making it difficult to standardise testing conditions that accurately reflect individual patient experiences. 4. conclusions in this research, two types of diamond nanoparticles were used in two different concentrations, which were added to the base gig composite material. the change in mechanical and tribological properties after the modification of the composite with diamond nanoparticles was mainly monitored. a slight improvement in compressive strength was observed, while the flexural strength and microhardness were significantly lower than the reference gic for all tested modifications (up to 55% and 39%, respectively). adhesion tests revealed that nanoparticles can enhance the composite’s adhesive strength. however, the wear depth was up to half lower for pure nanodiamonds (nd) and up to three times higher on average for nd-oh, indicating that the chemical modification of nanoparticles plays a crucial role within composites. it has been clearly demonstrated that the addition of diamond nanoparticles leads to a significant reduction in the amount of wear of the composite. the application of nanotechnology in dentistry offers significant potential for improving and advancing dental materials; however, a detailed study is still necessary. acknowledgements this publication was written at the tul with the support of the institutional endowment for the long-term conceptual development of research institutes, as provided by the msmt of the czech republic 2024. references [1] m. rathore, a. singh, v. a. pant. the dental amalgam toxicity fear: a myth or actuality. toxicology international 19(2):81–88, 2012. https://doi.org/10.4103/0971-6580.97191 [2] e. rodríguez-farre, e. testai, e. bruzell, et al. the safety of dental amalgam and alternative dental restoration materials for patients and users. regulatory toxicology and pharmacology 79:108–109, 2016. https://doi.org/10.1016/j.yrtph.2015.12.015 [3] l. sun, z. yan, y. duan, et al. improvement of the mechanical, tribological and antibacterial properties of glass ionomer cements by fluorinated graphene. dental materials 34(6):e115–e127, 2018. https://doi.org/10.1016/j.dental.2018.02.006 [4] l. durrant, m. mutahar, a. a. daghrery, et al. clinical performance of glass ionomer cement in load-bearing restorations: a systematic review. medical science monitor 30:e943489, 2024. https://doi.org/10.12659/msm.943489 [5] m. a. fareed, a. stamboulis. nanoclay-reinforced glass-ionomer cements: in vitro wear evaluation and comparison by two wear-test methods. dentistry journal 5(4):28, 2017. https://doi.org/10.3390/dj5040028 [6] n. sikka, m. brizuela. glass ionomer cement. in statpearls. 2023. [2024-05-17]. http://www.ncbi.nlm.nih.gov/books/nbk582145/ [7] k. x. ge, c. y.-k. lung, w. y.-h. lam, et al. a novel glass ionomer cement with silver zeolite for restorative dentistry. journal of dentistry 133:104524, 2023. https://doi.org/10.1016/j.jdent.2023.104524 [8] n. ilie, t. j. hilton, s. d. heintze, et al. academy of dental materials guidance – resin composites: part i – mechanical properties. dental materials 33(8):880–894, 2017. https://doi.org/10.1016/j.dental.2017.04.013 [9] h. rashid. application of confocal laser scanning microscopy in dentistry. journal of advanced microscopy research 9(4):245–252, 2014. https://doi.org/10.1166/jamr.2014.1217 60 https://doi.org/10.4103/0971-6580.97191 https://doi.org/10.1016/j.yrtph.2015.12.015 https://doi.org/10.1016/j.dental.2018.02.006 https://doi.org/10.12659/msm.943489 https://doi.org/10.3390/dj5040028 http://www.ncbi.nlm.nih.gov/books/nbk582145/ https://doi.org/10.1016/j.jdent.2023.104524 https://doi.org/10.1016/j.dental.2017.04.013 https://doi.org/10.1166/jamr.2014.1217 vol. 50/2024 mechanical properties of dental fillings with nanoparticles [10] n. m. magdy, m. z. kola, h. h. alqahtani, et al. evaluation of surface roughness of different direct resinbased composites. journal of international society of preventive and community dentistry 7(3):104–109, 2017. https://doi.org/10.4103/jispcd.jispcd_72_17 [11] i. khan, k. saeed, i. khan. nanoparticles: properties, applications and toxicities. arabian journal of chemistry 12(7):908–931, 2019. https://doi.org/10.1016/j.arabjc.2017.05.011 [12] a. mishra, g. singh, s. singh, et al. comparative evaluation of mechanical properties of cention n with conventionally used restorative materials – an in vitro study. international journal of prosthodontics and restorative dentistry 8(4):120–124, 2018. https://doi.org/10.5005/jp-journals-10019-1219 [13] v. p. chalissery, n. marwah, m. almuhaiza, et al. study of the mechanical properties of the novel zirconia-reinforced glass lonomer cement. the journal of contemporary dental practice 17(5):394–398, 2016. https://doi.org/10.5005/jp-journals-10024-1861 [14] s. wongkhantee, v. patanapiradej, c. maneenut, d. tantbirojn. effect of acidic food and drinks on surface hardness of enamel, dentine, and tooth-coloured filling materials. journal of dentistry 34(3):214–220, 2006. https://doi.org/10.1016/j.jdent.2005.06.003 61 https://doi.org/10.4103/jispcd.jispcd_72_17 https://doi.org/10.1016/j.arabjc.2017.05.011 https://doi.org/10.5005/jp-journals-10019-1219 https://doi.org/10.5005/jp-journals-10024-1861 https://doi.org/10.1016/j.jdent.2005.06.003 acta polytechnica ctu proceedings 50:55–61, 2024 1 introduction 2 materials and methods 2.1 glass ionomer cement 2.2 nanoparticles 2.3 sample and experimental set-up 2.4 mechanical-physical properties 2.5 confocal microscopy 2.6 statistical analysis 3 results and discussion 3.1 analysis of mechanical-physical properties 3.2 adhesion of composite materials 3.3 wear resistance 3.4 composite surface roughness analysis 3.5 discussion 4 conclusions acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2023.45.0020 acta polytechnica ctu proceedings 45:20–26, 2023 © 2023 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague geophysics for underground engineering pavel bláha∗, roman duras, jan gebauer geotest, a. s., šmahova 1244/112, 627 00 brno, czech republic ∗ corresponding author: blaha@email.cz abstract. the growth of underground engineering brought a need for the use of geophysical methods for research of underground constructions rather different in approach compared to that of civil engineering. these changes relate to surface measurement on the one hand and the development of new specialised methods used for underground work on the other. the results of a complete documentation of mining work, the application of the resistivity methods used underground and also the use of 3d electrical resistivity tomography, are all presented here. keywords: geophysics, underground structure, ert. 1. introduction using 2d and 3d methods in geophysical surveys is very enticing, but we must realise that even the latest mathematical procedures and possibilities of electronics have their limitations. in this paper we want to point out some problems in geophysical measurement carried out underground, including interpretation of 2d and 3d geo-electrical measurements. the paper presents the results of measurement in one mine working, between mine workings, and also between mine workings and boreholes. 2. the issue of measurement and interpretation of 2d and 3d ert for a greater objectivity regarding interpretation of the results of geophysical measurements, it is always suitable to process the given measured data in several ways. the same also applies to the measurement of electrical resistivity tomography. figure 1 shows an example of how to interpret ert measurement using the res2dinv and zondres2d programs. the example manifests simple geological conditions in a survey for the construction of motorway d35 at the municipality of sedliště, particularly from profile 41– 41’. the area of interest is formed by sedimentary rocks of the bohemian cretaceous basin typical of claystone, marlstone, etc., covered by soft quaternary sediments typical of loess, loess loam, etc. the results of interpretations from both the programs are not identical, but they are similar. it can be declared without problems that the setup of iterative parameters in both the programs brings about larger resistivity changes than by comparing the results of both the programs mutuall [1]. in both interpretations it is possible to distinguish a near-surface layer of loams without any difficulty. the base of loams is located at a depth of about 7 m according to the res2dinv program, and at a depth of 11 m according to the zondres2d program. based on the result of drilling work, the boundary is defined at a depth of about 12 m, i.e., the interpretation comes out better from the zondres2d program. it is more complicated with the determination of the surface of cretaceous clayey sediments. this geological boundary has not been determined by direct survey work (it was terminated in the quaternary), but only by the results of vertical electrical sounding (ves). based on ves, the boundary is located at a depth of 25 to 35 m. according to the results of res2dinv, it is found at 30 m, and according to zondres2d, at about 35 m. both the programs indicate a horizontal change in resistivity values suggesting a facies change of sediments in the second layer as well as in the third layer. this change is better evident in the uncut resistivity cross sections in the lower part of the figure. the conditions for measuring ert in other figures (the site of pvp/underground research facility/bukov, testing chamber zk-1) were opposite to that in the preceding case. whereas in measurement from the survey for the motorway it can be said that the conditions were ideal (measurement took place on natural ground with normal moisture content, so the contact resistance values on electrodes ranged from about 200 to 1 110 ω), underground measurement was much more complicated. from the monitoring of contact resistance values, it was detected that the contact resistance values between the electrodes and the rock massif were significantly higher. specifically, in the studied profile (lower right, pd) contact resistance values in 2019 changed from 3 000 to 120 000 ω. during processing, the results of such high contact resistance values on these electrodes were eliminated from further processing. when using electrical resistivity tomography underground, we still verified the effect of the origin and version of the interpretation program on the resulting resistivity field. we carried out an extensive analysis on measurements at the beginning of cross-cut bzxiij on its left side just behind the beginning of the research facility, where copper electrodes of 10 mm 20 https://doi.org/10.14311/app.2023.45.0020 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 45/2023 geophysics for underground engineering figure 1. comparison of ert interpretation using the res2dinv and zondres 2d programs. in diameter were used and sunk into the side of the mine working to a depth of 5–10 cm. the resistivity fields detected by different versions of the program are given in figure 2. as in the preceding example from measurement in ideal conditions, the basic geological structure identified by both the programs is the same. but the details of each anomaly are different not only by areal extension, but also by the values of resistivity. in addition to the basic environment designated “a”, five anomalies “b” to “f” were detected using the zondres2d program; the res2dinv program identified six anomalies “b” to “g”. anomalies “b” to “g” have relatively small dimensions and their maximum resistivity values range between four and six thousand ω m in the zondres2d program, whereas in the res2dinv program they range between four and forty thousand ω m. the area of high-resistivity anomalies is approximately the same in both the versions. an exception is the lower limitation of anomalies “b” and “d”. a larger difference is in their maxima. in the “russian” version it is 4 200, 5 700, 5 300, 4 000 and 6 000 ω m and in the “danish” version it is 4 500, 4 200, 5 500, 7 000 and 30 000 ω m. the basic environment “a” in the “russian” version is visually more homogeneous than in the “danish” version. this is due to the fact that the blue colours of the low-resistivity environment of the “russian” version, in a range of 500 to 1 700 ω m, are practically indistinguishable to the human eye. the basic environment has the values of resistivity in the “russian” version 500 to 1 700 ω m and in the “danish” version 400 to 1 500 ω m. 3. measurement and interpretation of ert in 2d and 3d versions underground the first example of the use of electrical resistivity tomography in the 3d version is from measurement in testing chamber zk-1 at the site of pvp bukov. in this chamber, several stages of repeated measurements were carried out, with six profiles being measured: two of them always on the sides of the chamber, one on the footwall of the chamber and one on its top wall. the profiles were measured both separately and on the opposite profiles, i.e., one series of electrodes was, for example, on the lower left profile (ld) and the other on the upper right profile (ph). in all, measurement in testing chamber zk-1 took place five times, in january 2016, then in april 2017, in september 2019, in february 2020 and in may 2021. for those in september 2019 and in february 2020, measurement took place on the lower right profile (pd) always three 21 p. bláha, r. duras, j. gebauer acta polytechnica ctu proceedings figure 2. comparison of the results of ert interpretation using res2dinv and zondres2d programs in zk-1. figure 3. resistivity cross sections through testing chamber zk-1. times immediately one after the other. the results of measurement from 2019 processed by the genie program for 3d interpretation and put together at the technical university of liberec, are depicted in figure 3. the interpreted resistivity values range from 200 to 25000 ω m. the interpreted resistivity values are unusually (even improbably) high. on two subhorizontal cross sections and on one vertical cross section, we can distinguish eight blocks. blocks y and z lie at the margins of the measured space and cannot be considered as reliable. the reason is a very low number of measured points from which it is possible to count resistivity values. these blocks cannot be assigned a geological meaning. the other blocks by their position allow us assign their geological, or more precisely, their geotechnical meaning. 4. measurement and interpretation of ert in 2d and 3d versions underground the first example of the use of electrical resistivity tomography in the 3d version is from measurement in testing chamber zk-1 at the site of pvp bukov. in this chamber, several stages of repeated measurements were carried out, with six profiles being measured: two of them always on the sides of the chamber, one on the footwall of the chamber and one on its top wall. the profiles were measured both separately and on the opposite profiles, i.e., one series of electrodes was, for example, on the lower left profile (ld) and the other on the upper right profile (ph). in all, measurement in testing chamber zk-1 took place five times, in january 2016, then in april 2017, in september 2019, 22 vol. 45/2023 geophysics for underground engineering figure 4. ert cross sections perpendicular to zk-1. in february 2020 and in may 2021. for those in september 2019 and in february 2020, measurement took place on the lower right profile (pd) always three times immediately one after the other. the results of measurement from 2019 processed by the genie program for 3d interpretation, put together at the technical university of liberec, are depicted in figure 3. the interpreted resistivity values range from 200 to 25 000 ω m. the interpreted resistivity values are unusually (even improbably) high. on two subhorizontal cross sections and on one vertical cross section, we can distinguish eight blocks. blocks y and z lie at the margins of the measured space and cannot be considered as reliable. the reason is a very low number of measured points from which it is possible to count resistivity values. these blocks cannot be assigned a geological meaning. the other blocks by their position enable us to assign their geological or more precisely, their geotechnical meaning. block a lies on the right northern wall of the chamber. this block can be considered as a block of rocks in natural state, not being affected in any way by mining work or any other investigation that could have taken place in this space. its resistivity values range between about 10 and 25 thousand ω m. block b can also be characterised similarly, being divided into two sub-blocks b1 and b2. sub-blocks b1 and b2 are probably separated by a discontinuity, or by a conductive zone of fracturing of the rock mass (δ). this zone is also manifested on the right side as a boundary between blocks a and z. the zone of fracturing defined by resistivity probably corresponds to joints of the nnw-sse to n-s strike. according to all three resistivity cross sections, one mega-block can probably be defined, comprising blocks a, b and c and corresponding to the original rock mass. interesting information can be obtained from cross sections drawn perpendicular to the axis of zk-1 (figure 4). the cross sections were constructed only on the basis of ert measurement in september 2019. interpreted resistivity values in the cross sections range from 300 to 15 000 ω m. the cross sections were constructed under the stations of 7.5 and 9.5 m perpendicular to the axis of the working. the most interesting on the perpendicular cross sections are anomalies of low resistivity values above the top wall of the chamber and beneath its footwall. these low-resistivity anomalies indicate the places of the massif loosening after the chamber excavation and the re-distribution of the stress field. we proceed from an assumption that the micro-fissures were formed in the loosened massif and filled by groundwater. the moisture content in the massif may not be at all high. it is sufficient for a decrease in rock resistivity when the fissures create a conductive network. the drop in resistivity values is different above the top wall of the chamber and beneath its footwall. the explanation must be found in the moisture content of the rock above and beneath the working. it is possible to expect full saturation of fissures in the massif by groundwater beneath the footwall of the chamber, whereas above the top wall there remain places in which pores in the rock are not fully saturated. this fact is shown by small anomalies of low resistivity values, whereas beneath the footwall the decrease in resistivity values affects the whole massif concerned. if we proceed from a generally accepted assumption that the fractured massif in the surroundings of the stope has the form of an ellipse in the perpendicular cross section [2], then it is possible to estimate what the ellipse definition of the fractured massif will look like in the vicinity of zk-1. it is interesting that the ellipses do not have their long axes vertical, but that the axis of the ellipse above the top wall is turned to the north. on the cross section at 7.5 m, the ellipse is 23 p. bláha, r. duras, j. gebauer acta polytechnica ctu proceedings figure 5. cross sections from ert between boreholes s1 and s8. turned by about 25 ◦ and on the cross section at 9.5 m by 10 ◦. in relation to the possibilities of the method, the given numbers cannot be taken as absolute numbers, but it can be stated that the ellipse is turned by roughly 15 ◦ to the north. similarly, the stress should also be turned. interesting is the ratio of semi-axes of the ellipse. the resistivity data show that the ratio of semi-axes of the outlined ellipses of fracturing is about 1:2. another testing of possibilities of 3d ert and a new program for 3d interpretation of ert (genieert) took place on the results of measurement between boreholes s1 and s8 at the same site. in this case, all electrodes were placed in the borehole. in relation to the conditions of measurement, it was an ideal situation. the electrodes in the borehole have a minimum contact resistance that changed from 930 to 6 500 ω. the comprehensive interpretation of measurement in boreholes s1 and s8 comprises the processing of electrical resistivity tomography (2019), resistivity logging (carried out by aquatest, a.s.) and the resistivity values obtained from ert. the grey curve depicts the interpreted resistivity values of the first series of ert elements from the sections of one metre size. the outcome of this comparison of all results is shown in figure 5. the interpreted resistivity values range between 300 and 20 000 ω m. on the cross section between boreholes s1 and s8, it is possible to identify four blocks a to d2. the high resistivity values characterise blocks a and b. the other radiographed area is characterised by low resistivity values. blocks a and b are of special significance, differing from the other blocks by their area and the size of the maxima of resistivity values, which reach about 20 000 ω m. the delineation of the blocks is also evident in the results of down-hole logging. these anomalies constitute blocks of rocks which are not practically affected by fracturing. the pictures of distribution of the interpreted resistivity values obtained in 2019 and 2020 bring factually the same results. certain differences can be found in the form of anomalies between 2019 and 2020. the anomalies in 2019 are in area larger and the margins of anomaly a in 2019 are not so rugged. anomalies c and d in 2019 are smaller than was the case in 2020. 5. physical documentation of mine walls below the rqd output in figure 6, graphs of the results of measurement using “speedy” methods are presented one after another, i.e., measurement of temperature, radioactivity and magnetic susceptibility. the results of “speedy” methods provide valuable results, particularly in comparison with other measurements, e.g., resistivity methods. on the curves of dose rate, certain increases of values are evident, located under the stations between about 9 and 18 m, having a significant response in ert measurement (the last graph). the extent of the “radioactive” anomaly is very close to the extent of the resistivity anomaly. an interesting fact appears that a local significant decrease in the values of interpreted resistivity occurred in the inflection point of the resistivity elevation (under the station of about 13.5 m). another interesting fact is a correlation of magnetic susceptibility with ert under the stations of about 3.5 and 17 m, which have a response in resistivity minima on the curves of ert measurement. whereas the geomagnetic anomalies are in. 24 vol. 45/2023 geophysics for underground engineering figure 6. physical documentation of a mine wall. figure 6 presents the results of 2d and 3d geophysical and geotechnical measurements on the wall of chamber zk-1. the first graph illustrates the mechanical state of the rock environment on the left side of the working, represented by the rqd (rock quality designation) coefficient, and also expressed in the form of the frequency of fissures per the unit of distance. both measurements were conducted by geomechanics from the institute of geonics of the czech academy of sciences (ig cas). the rqd coefficient expresses the quality of rock and is usually determined on drill cores. geomechanics of the ig cas developed procedures to determine rqd from measurement on the surface of mine workings. when viewing the shape of curves, it is evident that the quality of the measured rocks is good to excellent. an exception is a section under the stations between about 7.5 and 9.5 m, where the value of rqd diminishes from about 95–100 % to about 55 %, i.e., the quality of rocks is classified by a medium grade. the given fact probably indicates the existence of structural discontinuities. the rqd value 55 accompanies a higher frequency of fissures (joints). relatively narrow, the decrease in apparent resistivity values is more gradual and more extensive in the measured line. it is possible that this concerns the manifestations of local lithological changes (veins penetrating the base rock perhaps?), but for explanation it is again necessary to have more information of a non-geophysical character. the next-to-last graph in figure 6 shows graphs of already mentioned resistivity measurements. specifically, these are profiling curves of spacings a0,5m0,25n0,5b; a1m0,25n1b; a2m0,25n2b. the data were extracted from a cloud of values obtained by the ert method, carried out with a detailed interval (0.25 m) and to a sufficient extent (about 22 m). the curves of resistivity profiling enable the basic quasihomogeneous blocks to be delineated; they are characterised by the extent and value of apparent resistivity. 25 p. bláha, r. duras, j. gebauer acta polytechnica ctu proceedings the resistivity curves are similar by their character to the curves obtained from the measurement of dose rate, where a small difference was recorded only in the initial part of the profile – a local minimum under the station of about 2.5 m on the curve of dose rate has the equivalent shifted to the station of about 3.5 m on the resistivity curves. this difference is probably determined by the oblique course of the plate (vein), which then causes the anomaly. it is necessary to realise that the dose rate characterises the space theoretically to 0.5 of a metre, but in reality even less. the range of geoelectrical measurement can be many times higher. the remaining sections correlate very well. in the last cross section, two resistivity curves are plotted, each of them from a different depth level, indicating the assumed state of the rock environment. the grey curve reflects the resistivity situation at the level to 0.25 m behind the wall of the working and the black curve to 1 m behind the wall of the working. the given values, however, are only indicative because the determining parameters are the size and shape of cells from the 3d model, which are not the same. the displayed data, however, are sufficiently representative for orientation and the basic distribution of the values of resistivity along the wall of the working. 6. conclusions the comparison of programs for 2d interpretation of electrical resistivity tomography (res2dinv and zondres2d) has brought forth a number of interesting findings. we proceeded from our previous knowledge that the resulting resistivity field was not only a picture of distribution of resistivity values in the rock mass, but it could also be affected by the setup of input parameters for iterations. we have obtained for testing the zondres2d program in a functional demo version, and thanks to the cooperation of czech and uzbek specialists, a possibility has arisen to use both the programs to their full extent [3] and [4], during testing work, it appeared that it was possible to change the resulting resistivity field, similarly as in the res2dinv program, by setting up parameters in the zondres2d program as well. when evaluating digital programs of interpretation, we must be aware that we are trying to solve mathematically a not quite correct problem. in this case, we determine a substantially larger number of parameters than the input data we actually have. when comparing the results of both the programs, we began with the interpretation of results in simple geological conditions. in this case, the results were satisfactory because the differences in interpretation using both the programs were comparable with the differences which are obtained when changing the setup of input parameters of iteration. in more complicated surface conditions, the differences in interpretation were higher, but still comparable with changes given by the setup of programs. the largest difference occurred in the application with mine conditions. there, the differences between both the programs even increased, and we even recorded an example where interpretation by the zondres2d program was more reliable and the calculated model approached the reality more than in the case of the output from res2dinv. geophysical measurement using the system of 3d and its 3d evaluation brings quite new possibilities when describing the rock mass in general and this also applies to special conditions underground. in mine workings, the possibility to choose the arrangement of profiles and points of measurement on profiles is limited as compared to surface applications of geophysical measurements. to find a suitable system of points measurement is usually very difficult and requires many compromises. an advantage of 3d processing of geophysical measurements is the spatial information on the geological structure of the area of interest. a disadvantage is the problem of obtaining a sufficient number of high-quality data for a 3d interpretation to bring reliable results. this paper describes several successful examples of the use of 3d geophysical measurement in different lithological types of rocks. acknowledgements geophysical work published in this paper was largely carried out in the underground research facility (pvp) bukov. it was constructed, and is operated by, the radioactive waste repository authority (súrao). to accomplish any activity in the spaces of the deep mine is relatively costly, complicated and requires a coordination of many professions and activities (working facilities, technical support, etc.). thanks to the coordination with súrao, which supports research activities in pvp to the maximum extent possible in the form of technical support and by providing services, it was possible to manage very well the planned measurements and all the other activities. for all this our thanks and appreciation are due to súrao. references [1] p. tábořík, j. gebauer, r. duras, p. bláha. the mystique of ert. exploration geophysics, remote sensing and environment (xxiv.2):29–39, 2017. https://doi.org/10.26345/egrse-028-17-204x [2] g. lanyon. excavation damaged zones assessment. report of fracture systems ltd–nwmo dgr-tr-2011-21, 2011. 95 p. [3] p. bláha. verified technology, development of geotechnical and geophysical methods for obtaining 2d and 3d images of the geological structure. geotest a.s., 2021. 80 p. [4] p. bláha, r. duras, j. gebauer, et al. development of geotechnical and geophysical methods for obtaining 2d and 3d images of the geological structure. final report, geotest a.s., 2021. 53 p. 26 https://doi.org/10.26345/egrse-028-17-204x acta polytechnica ctu proceedings 45:0–0, 2023 1 introduction 2 the issue of measurement and interpretation of 2d and 3d ert 3 measurement and interpretation of ert in 2d and 3d versions underground 4 measurement and interpretation of ert in 2d and 3d versions underground 5 physical documentation of mine walls 6 conclusions acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2025.52.0086 acta polytechnica ctu proceedings 52:86–93, 2025 © 2025 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague comparison of bicycle detection methods in prague traffic pavel mašinda czech technical university in prague, faculty of transportation sciences, department of transport telematics, konviktská 20, 110 00 prague, czech republic correspondence: masinpav@cvut.cz abstract. the paper focuses on the detectors used to detect bicycles. it contains research part which describes various methods of bicycle detection and their possible advantages and disadvantages. the other part focuses on testing the methods under real conditions. testing on 32 different location was conducted. the data from these tests were used to select the appropriate technology for each specific use case. the results show that current detectors are not the best option for bicycle detection and that videodetection and piezoelectric strips are the best way to go. keywords: bicycle detectors, detectors, cyclist movement monitoring, induction loops, microwave radars, piezoelectric detectors, pneumatic detectors, videodetection, magnetometric detectors. 1. introduction the issue of bicycle detection is very topical. detection can be useful for several applications. for example, for dynamically controlled intersections with traffic lights, where bicycles need to be detected, otherwise they might stop at red lights until a car that is detected arrives. this is nowadays mainly solved by induction loops that are able to detect both car and bicycle. a second example of the use of bicycle detectors is counting for analytical and statistical purposes. the number of cyclists passing through strategic areas can be used to determine the main routes used by cyclists, their directionality, an overall overview of cyclist movements in an area, and to determine the utilization of current cycle paths. for example, the latter reason can then be used to justify the need to revitalise or widen a given cycle route or to build new ones where needed. the goal of the research is to define various use cases for bicycle detectors while testing as many detectors currently in use as possible. the result is a recommendation for the use of different detectors for different situations, including how to install them. 1.1. types of bicycle traffic detection this paper looks at the technologies most commonly used, whether intrusive or non-intrusive. there are other options for counting bicycle traffic, such as magnetometric detectors, ultrasonic detectors, infrared detectors or pyroelectric detectors. these technologies are not addressed in the research as they are practically not used for bicycle traffic detection because they are too simple technologies that cannot classify between bicycles and other road users or are very inaccurate. the following bicycle traffic detectors were evaluated: • induction loop, • piezoelectric detector, • microwave radar, • videodetection, • pneumatic detector. 1.1.1. induction loop in its simplest form, an induction loop is a coil of buried wire connected to a source of alternating current that creates a magnetic field. when a vehicle passes over the induction loop, the magnetic field changes and reduces the perceived inductance of the loop [1]. because bicycles do not contain large amounts of conductive materials, the induction loop must be modified to detect them. either by a smaller conductor placement depth or by a specific loop shape. rectangles placed diagonally across the road or diamonds are most commonly used [2]. for actual bicycle detection, the part of the bicycle closest to the road is the most important. this is mainly the rims. nowadays, bicycles made of nonferromagnetic materials such as carbon fibre are increasingly being used, mainly due to their lower weight and improved handling characteristics. thus, this trend may pose a challenge for induction loops in detecting these bicycles [3]. for the purpose of this research, testing was carried out with a carbon fibre bicycle. the bike went back and forth through an induction loop. this data was then paired with data from the loop. the carbon bicycle was detected in only 11 cases out of 63. that is, in less than 18 % of the cases. 1.1.2. piezoelectric detector piezoelectric detectors collect data by converting mechanical energy into electrical energy. when a piezo86 https://doi.org/10.14311/app.2025.52.0086 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 52/2025 comparison of bicycle detection methods in prague traffic electric cable is compressed, a voltage is generated that is linearly proportional to the applied pressure. this enables them to detect bicycles from other road users [4]. piezoelectric detectors are becoming increasingly popular for detecting bicycle traffic. the main reason is their accuracy (manufacturers report an accuracy of around 98 %) and multimodality, i.e. the fact that they can detect more than one mode of traffic. piezoelectric detectors can be placed in places where cars or motorcycles will pass over them in addition to bicycles [4, 5]. 1.1.3. microwave radar microwave radar works on the principle of the doppler effect. the radar transmits a continuous signal of lowenergy microwave radiation to the target area and then analyses the reflected signal. microwave radar for vehicle detection should be positioned at least 1.5 metres above the road. therefore, it must be mounted on a traffic sign or a public lighting pole. the radar should distinguish between a pedestrian and a bicycle, so it can be used in cases where multimodality is desirable. the actual categorisation is primarily done by the speed of the object in most radars. a slow-moving car can thus be classified as a bicycle and cause a false-positive record [6, 7]. 1.1.4. videodetection video detection is a non-destructive detector that works on the principle of extracting information from the image for further software processing. the camera itself is most often placed on the nearest infrastructure element (most often a public lighting pole or traffic light) [8]. when using video detection, one of the most important features is the angle of the image capture. the best camera placement seems to be perpendicular to the road and at a sufficient distance and height. one of the main advantages of using video detection for monitoring bicycle traffic is its multimodality, i.e. the ability to distinguish and monitor the intensity of different traffic modes. when used on a path, the camera can easily distinguish between pedestrians, cyclists and other users. 1.1.5. pneumatic detector pneumatic detectors are rubber tubes that are placed on the road surface. the hose is plugged into the roadside unit at both ends and then placed on the roadway perpendicular to the direction of vehicle travel. detection is made by the change in pressure in the tube that occurs when a vehicle passes over it. this detection method is mainly used to determine the intensity. it can also be used for vehicle classification, occupancy and, if the tubes are placed sufficiently far apart, for speed detection. although there are variants that are specifically made to count bicycles, most pneumatic detectors are more suited to counting cars. in these cases, bicycles are too light or going too slow for detection to occur (some manufacturers quote a minimum speed for identification of 10 km h−1) [9]. pneumatic detectors are not intended to be permanently installed on the roadway, primarily due to limited hose life, battery capacity, and overall vulnerability to intentional damage. the main use of a pneumatic detector is in one-time traffic surveys. part of this paper was testing it for detecting bicycle traffic and it has been shown that most cyclists do not know if they can cross the tube and so avoid it or stop in front of it. so the detection itself proved to be not very suitable. for this reason, the pneumatic detector is not considered in the next part of the paper. 1.2. comparison of different detector types table 1 shows all the considered detectors and some of detector characteristics. one letter is filled in for each detector and each characteristic. the letters are as follows: y – stands for yes which means that the detector meets the given characteristic. p – partly means that the detector only partially satisfies the given condition. for example, with very reduced accuracy. n – no means that the detector does not meet the given characteristic. 2. methodology in order to discover different use cases and to define them, places in prague where bicycle traffic monitoring already takes place were all examined. these locations were then divided into several sets based on various differences. for example, whether the detector is located in an area of car traffic, or whether it is an area inaccessible to cars. other examples of use were selected based on recommendations defined by the suppliers of the cycle counters. some of the selected use cases were later merged together, because of similarities and similar detector recommendations. the following use cases were selected: • traffic with cars on the road. ▷ traffic with cars on the road to the left of parked cars. • cycle lane. ▷ cycle lane to the left of parked cars. • path with possible passage of cars. • a path for cyclists and pedestrians. ▷ a path for cyclists and pedestrians on the footbridge. ▷ a path for cyclists and pedestrians on the pavement. 87 pavel mašinda acta polytechnica ctu proceedings in du ct io n lo op p ne um at ic de te ct or m ic ro w av e ra da r v id eo de te ct io n p ie zo el ec tr ic de te ct or possibility of counting pedestrians n p y y p heading distinction y y y y y resolution of more than two directions p p p y p not susceptible to weather conditions y y p n y detection of non-metallic bikes n y y y y non-intrusive n y y y n not vulnerable to damage from normal traffic p n y y p possibility to count several bicycles passing at the same time p p y y p no need to connect to the power grid p y n n p table 1. comparison of different detector types. use case average accuracy regardless of the detector used traffic with cars on the road 63 % traffic with cars on the road to the left of parked cars 49 % cycle lane 60 % cycle lane to the left of parked cars 41 % path with possible passage of cars 87 % a path for cyclists and pedestrians 77 % a path for cyclists and pedestrians on the footbridge 85 % a path for cyclists and pedestrians on the pavement 69 % table 2. average accuracy on each use case. testing was then carried out at defined locations in prague to determine the accuracy of the technology at that location. the results of this testing are then reflected in the recommendation of the detector selection for the given use case. testing was conducted at 32 locations, but the individual directions at each site were evaluated separately, so the total number is 64 sites. induction loops or microwave radars were installed at most sites. in two cases, video detection was used. during the testing, individual traffic modes were manually counted. testing was conducted at each site for one hour during the morning peak (most often between 7:30 a.m. and 9:30 a.m.) and for one hour during the afternoon peak (between 4:00 p.m. and 6:00 p.m.). the manual data were then paired with the count data. the accuracy of the technology could thus be calculated for each location, direction and time. this was calculated using the following formulae: • if the number of bikes counted on site was lower than the number given by the detector, the following formula was used: accuracy [%] = number of bikes counted on the spot number of bikes counted by the detector × 100 • if the number of bikes counted on site was greater than the number given by the detector, the following formula was used: accuracy [%] = number of bikes counted by the detector number of bikes counted on the spot × 100 3. measured results table 2 shows the tested use cases and the average accuracies that resulted from the testing. it was calculated from all the accuracies counted on each use case regardless of the detector used. 3.1. traffic with cars on the road the first use case of a cycle counter is the counting of bicycles on a road with normal car traffic. in some cases, the lane may be supplemented by a cycle corridor. there may be situations where bicycles need to be counted in traffic with cars on the road, but there is a parking lane between the pavement with 88 vol. 52/2025 comparison of bicycle detection methods in prague traffic figure 1. induction loops on hlubočepská street. the lampposts, where bicycle counters are most often placed, and the lane. in this case, bicycle counting is considerably more difficult. a total of 12 locations falling within this use case were tested. microwave radar was installed at nine locations. at the remainder, an induction loop was installed. the average accuracy of the induction loops in the use case was 89 %. the average accuracy of the microwave radar was 41 %. a major difference between the sites that could explain the level of inaccuracy of the microwave radar is the intensity of car traffic. 2 radars are located on plzeňská street in prague, where around 20 000 vehicles pass on a working day. many vehicles pass through the area at relatively low speeds. the radar, which classifies primarily on the basis of speed, can assess these as bicycles passing [10]. another example can be found on hlubočepská street where the counting is done across the entire width of the street. this can be seen on figure 1. according to the technical parameters, video detection comes out best in this example. its only problem is that most roads with car traffic are too wide for a single camera to cover both directions. and the second-best option is a piezoelectric detector. given its physical properties, it can be said to be suitable for this use case. it can easily distinguish between a car, a motorcycle and a bicycle and because it can distinguish between two signals at least 0.1 seconds apart. the most important thing, however, is to first assess the situation, namely the intensity and average speed of the cars on the site, ideally on a weekday at rush hour. in locations with very low traffic volumes (up to 100 cars per hour), inductive loops (taking into account the fact that they only count metal bikes) or piezoelectric detectors can be used. if a parking lane is present, it is best to avoid the location or find the nearest bottleneck where there is no parking lane [11]. 3.2. cycle lane this example of use is often seen in cities where streets are wide (lane widths greater than 3.5 m). it is a dedicated or protective cycle lane between the car lane figure 2. microwave radar overlooking cycle path on vršovická street. and the pavement. very similar to a dedicated cycle lane is a separated one-way cycle lane, which is used in one-way roads to allow cyclists to go in both directions. as with traffic with cars, there may be situations where bicycles need to be counted in the bike lane over parked vehicles. example of the protected cycle lane can be found on vršovická street near the vršovice train station. in this case the counting is done by microwave radar. this example can be seen on figure 2. neither technology has very high accuracy. again, this is most likely related to car traffic in the vicinity of bicycle traffic. the inaccuracy of the induction loop may be affected by passing cars. when a car passes close to a cycle lane that a cyclist is using at the same time, the cyclist can easily be overshadowed. the average technology accuracies at the test sites falling within this use case are as follows: • microwave radar – 57 % • induction loop – 78 % it should be noted, however, that the radars were tested at two sites and had 88 % accuracy at one site and 25 % at the other. at that location, cyclists often avoided the radar and rode on the adjacent pavement. for the induction loop, again, the installation of the loop matters most. it should not extend into the car lane as it could be disrupted by passing vehicles. the piezoelectric detector also seems to be suitable despite the lack of data to verify it. for video detection, it depends on the installation, but technically it is also suitable. 3.3. path with possible passage of cars this situation arises in places where the intensity of pedestrians and cyclists is significantly higher than that of cars. in city centres, this use case can only be encountered in areas where vehicular access is restricted and free movement of cyclists and pedestrians is allowed. prague’s císařský ostrov is an example. the counting here is done by induction loops. it can be seen on figure 3. 89 pavel mašinda acta polytechnica ctu proceedings figure 3. induction loops on císařský ostrov in prague. there are 6 such locations in the test sample. 2 locations are equipped with induction loops, 2 locations with microwave radars and 2 locations with video detection. the average accuracies of each technology for this use case are as follows: • microwave radar – 91 % • induction loop – 89 % • videodetection – 79 % according to the data, of all the examples of use in motorized traffic, this example is the most suitable for monitoring the movement of cyclists. videodetection achieves the worst results. this is probably due to the placement of the cameras at the test sites, these were installed parallel to the cyclists’ movement and the cameras could have mistaken a cyclist for a pedestrian. according to the data, the induction loop is suitable for this use case. it is essential that the loops are installed across the full width of the roadway. the disadvantages are the passage of a cyclist and a car at the same time and the inability to detect all types of bicycles. the piezoelectric detector is similar to the induction loop but has the advantage of detecting all bikes. observations at sites show that microwave radar is only suitable when the roadway is not too wide for radar coverage. videodetection must be installed appropriately. it has been observed that cameras installed on the side of the observed road achieve better results. 3.4. a path for cyclists and pedestrians a total of 3 use cases were defined on the cycle and pedestrian path. these are merged into one for simplicity and similarity. the cycle and pedestrian path is the most frequent place for bicycle monitoring, at least in prague. the main feature is that motor vehicles cannot enter this location. then there is the fact that pedestrian traffic is often counted at these locations. an example is the path in povltavská street and the a2 cycle path between the troja bridge and the prague zoo. it figure 4. a path for cyclists and pedestrians on povltavská street. is appropriate to count both traffic modes on this section, but currently only cycling traffic is counted using induction loops. image of this situation can be seen on figure 4. a total of 33 sites falling under this use case were tested. two sites had videodetection and the remainder had induction loops, microwave radar or a combination of these. the average accuracies of each technology for this use case are as follows: • microwave radar – 69 % • induction loop – 78 % • videodetection – 94 % video detection performed best in this use case. in these cases, the camera is pointed perpendicular to the trail, which improves its accuracy over a parallel to the trail installation. induction loop is the most commonly used technology on trails. it is installed along the entire length of the trail to avoid going around it. its main disadvantage in this case is the inability to detect all types of bikes. in theory, the piezoelectric detector should perform better, the only disadvantage is the poor quality of the pedestrian count, which can be an important factor in the choice of detector. for microwave radar, it depends on the installation. a suitable installation is primarily for use on a narrower path. at one site, a radar was observed installed next to a tree whose leaves partially obscured its field of view. this location greatly reduces the average accuracy of the radar in this use case. intrusive technologies cannot always be used when used on a footbridge. either because of lack of space or because of interference from the metal structure of the footbridge. it is therefore appropriate to use radar or a camera. however, a camera cannot be installed perpendicular to the footbridge and parallel to the footbridge may not give the best results. for sidewalks, these are locations where cyclists can legally move while being completely separated from 90 vol. 52/2025 comparison of bicycle detection methods in prague traffic the road. this is usually a relatively narrow space, so microwave radar is more suitable than video detection, which needs to be higher and also perpendicular rather than parallel to the pavement. 4. results and discussion the aim of this paper was to find out which technologies are most commonly used nowadays to detect and monitor the movement of bicycles. at the same time, to find out in which situations these bike detectors are used and to propose a suitable technology and installation for each situation. table 3 shows an overview of examples of use and the main detectors mentioned. in the table, the suitability of a given detector for a given use case is indicated by one letter. all the knowledge used to create the table is included in the text above. the selection of suitability was done by professional assessment from the physical characteristics of the detectors and the data from testing and on-site observations. the literature used in the paper was also used when making assessments. the cost of using each detector was not taken into account. test results are limited by the number of detectors tested. only detectors used in prague were evaluated. to gain more insight into the use of bicycle traffic detectors, more investigations should be conducted in different areas. for example, in other cities where other technologies are used. the following letters can be found in the table: • s – suitable, indicates the appropriate detector for the given use case. • p – problematic, indicates a detector that would measure well at a given location, but might have some problems. for example, it requires a specific installation, or it cannot detect all types of wheels (for this reason, the induction loop is never marked as suitable). • ru – rather unsuitable, marks detectors that can only be used in a small number of site-specific cases. the comparison shows that the piezoelectric detector performs best. however, it is judged only on the basis of its technical characteristics. its main disadvantage is the fact that it is intrusive technology, so it must be installed into the road. another suitable detection method for most applications is video detection. this only has a problem with correct installation and with cases where it cannot be installed at a good angle and at a sufficient distance and height. half of the observed videodetection cameras during the testing were installed inappropriately and thus didn’t work as well as they could have. on the other hand, the induction loop comes out of the table the worst, it is not suitable mainly because of the impossibility of counting all the bicycles and is also not always suitable for use in traffic with cars, where there is often interference from passing cars or cars standing in the adjacent parking lane. the next step should be to use the acquired knowledge and data to renew the current technologies and to advance the monitoring of bicycle traffic in prague. it should also be pointed out that cost benefit analysis should further narrow down the choice of the right detector since videodetection and piezoelectric detectors are the most precise but also most expensive of all the considered detectors. to provide better data the testing should be done on all the use cases with all considered detectors, this couldn’t have been done since there are no piezoelectric strips for counting bicycles in prague. the testing should also be done again after the current technologies in use, which are more than 5 years old, will be replaced. that should provide more accurate data about current bicycle detectors. the further use of cyclist detection approaches aligns with the ongoing development of cycling infrastructure in areas undergoing revitalization or modification. this includes existing locations in the city of prague, such as výtoň, where the application of cyclist detection could provide valuable insights for decision-making regarding site revitalization [12]. another potential use for cyclist detection involves the redevelopment of brownfield sites, which focus on repurposing currently unused industrial areas, offering significant potential for enhancing cyclist detection, particularly among young people [13]. in existing urban areas, traffic monitoring is primarily focused on vehicle detection; however, with the integration of new technologies like c-its, cycling traffic can be better incorporated into overall traffic management [14, 15]. moreover, these new detection technologies and data processing methods have the potential for broader application in accurate data collection and future prioritization of cyclists using c-its, similar to trends observed in other western european cities. cyclist detection data should also inform new approaches in design, implementation, maintenance, the adaptation of parking spaces for shared bikes, and other innovative uses of cycling detection technology, as seen in comparable cities. 5. conclusions in this paper 5 commonly used bicycle detectors were evaluated and recommended for specific use cases and installations. bicycle detectors were tested at a total of 32 locations in prague for better overview of their functionalities. these sites have been divided into several use cases. for each use case an average accuracy was calculated based on the conducted tests. these tests served as a basis for recommending the optimal detector. these results were subsequently combined with observations made at the site. such as the fact that the cyclists were using sidewalks for pedestrians instead of cycle paths on the road. each of the detectors currently in use showed some disadvantage. for induction loop it was nonferromagnetic bikes. for microwave radars it was the 91 pavel mašinda acta polytechnica ctu proceedings in du ct io n lo op m ic ro w av e de te ct or v id eo de te ct io n p ie zo el ec tr ic de te ct or traffic with cars on the road ru p s s traffic with cars on the road to the left of parked cars ru ru s p cycle lane p s p s cycle lane to the left of parked cars ru ru s p path with possible passage of cars p s p s a path for cyclists and pedestrians p p s s a path for cyclists and pedestrians on the footbridge ru s p p a path for cyclists and pedestrians on the pavement p s p s table 3. summary of results for each detector and use case. speed of the object or objects moving the same speed as bicycles. for videodetection it was the angle of camera. these disadvantages were also taken into account. the article also highlights the potential use of cyclist detection in new locations to support better decision-making regarding the implementation and development of cycling infrastructure in urban areas. new project proposals and approaches leveraging ai, information modeling, and testing, including data processing, could significantly enhance the accuracy and understanding of cycling traffic usage in cities like prague [16, 17]. acknowledgements references [1] w. guizhu, c. zongfa, x. lingan. induction loop detection of bicycles flow based on pattern recognition (pr) techniques. ifac proceedings volumes 27(12):31–36, 1994. ifac symposium on transportation systems: theory and application of advanced technology, tianjin, prc, 24-26 august. https://doi.org/10.1016/s1474-6670(17)47440-4 [2] partnerství, o.p.s. monitoring návštěvnosti cyklistických a pěších stezek ve městech i v přírodě [in czech; 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smart city symposium prague (scsp), pp. 1–6. 2024. https://doi.org/10.1109/scsp61506.2024.10552692 [17] t. tichý, j. brož, j. růžička, et al. information modelling and smart approaches at the interface of road and rail transport. in 2023 smart city symposium prague (scsp), pp. 1–8. 2023. https://doi.org/10.1109/scsp58044.2023.10146231 93 https://diamondtraffic.com/product/roadtrax-bl https://doi.org/10.1063/5.0170427 https://doi.org/10.3390/su13031232 https://doi.org/10.1109/scsp54748.2022.9792538 https://doi.org/10.3390/app12094624 https://doi.org/10.1109/scsp61506.2024.10552692 https://doi.org/10.1109/scsp58044.2023.10146231 acta polytechnica ctu proceedings 52:86–93, 2025 1 introduction 1.1 types of bicycle traffic detection 1.1.1 induction loop 1.1.2 piezoelectric detector 1.1.3 microwave radar 1.1.4 videodetection 1.1.5 pneumatic detector 1.2 comparison of different detector types 2 methodology 3 measured results 3.1 traffic with cars on the road 3.2 cycle lane 3.3 path with possible passage of cars 3.4 a path for cyclists and pedestrians 4 results and discussion 5 conclusions acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2023.40.0054 acta polytechnica ctu proceedings 40:54–60, 2023 © 2023 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague comparison of behaviour of burnt beams models with effective and actual residual cross-sections in three point bending test lucie keřková kucíková∗, michal šejnoha, tomáš janda, pavel padevět czech technical university in prague, faculty of civil engineering, department of mechanics, thákurova 7, 166 29 prague 6, czech republic ∗ corresponding author: lucie.kucikova@fsv.cvut.cz abstract. glued laminated timber beams exposed to fire of a variable duration were tested in three point bending. three types of model geometries taking into account the effect of burning were created. each model was examined for five material data sets. the maximum deflection obtained from fem simulations was compared with the corresponding deflection wel derived from a three point bending test of burnt beams. the analysis was performed with the help of ansys® software limiting attention to a liner elasticity with the maximum applied force fel selected at the proportionality limit observed experimentally. majority of fem models provided smaller deflections compared to measurements except for the material set adopting young’s moduli from the compression test. the best fit was observed for the material data set obtained from homogenization. no predominant preference among individual geometries was found promoting the computational model based on reduced cross-section as sufficiently accurate. keywords: glt beam, three point bending test, fire test, effective residual cross-section. 1. introduction glued laminated timber (glt) enables wood to be used for more complex constructions with larger spans than used for common wood trusses. however, it also brings larger demands on its reliability. probably, the most pronounced disadvantage of wood as a structural element is its combustibility. it must be taken into account in every design. the main task is a residual load bearing capacity of individual elements. eurocode 5 part 1–2 [1] provides simplified methods to verify mechanical resistance based on a reduced cross-section or reduced properties. the reduced crosssection method introduces an effective cross-section with the same shape as the initial one assuming that the layer close to char line (d0) has zero strength and stiffness, while the properties of the remaining material below this layer are assumed to be unchanged. dimensions of an effective cross-section are the initial ones reduced by the effective charring depth: deff = dchar,n + k0d0, (1) where dchar,n is the notional charring depth which accounts for corner rounding. the coefficient k0 = 1 for unprotected surfaces and time longer than 20 min. the parameter d0 = 7 mm represents zero strength and stiffness layer. point out that d0 could be even larger, in the range of 9.5 to 20.1 mm according to literature [2, 3]. eurocode 5 also enables the application of advanced calculation methods (annex b), usually based on thermal response model. in this paper, we focused our attention on the behaviour of residual cross-section in three point bending. comparison of three beam shapes was performed in finite element software ansys® (section 3), employing the results from three point bending tests (subsection 2.2) and charring depth measurements (subsection 2.1). 2. experimental part the experimental part of research is based on a fire test and supporting mechanical tests. the fire test setup and testing procedure are elaborated in [4, 5] and are briefly outlined here for the sake of completeness. altogether, sixteen glt beams made of norway spruce wood with dimensions 0.10 × 0.32 × 2.38 m (group 1; samples 1–8) and 0.16 × 0.32 × 2.40 m (group 2; samples 9–16) were tested, where all of them contained eight rows of lamellae connected vertically by plain joints and horizontally by finger joints with melamine-urea-formaldehyde glue. prior to the fire test, all beams were subjected to a series of pilodyn 6j indentation tests, briefly mentioned in section 3. a horizontal experimental setup was prepared in a way to simulate the wooden ceiling. each setup (i.e. fire test) contained two beams; one equipped with eleven k type thermocouples in various positions to provide the temperature distribution during the test whereas the second beam was primarily intended for three point bending test. two temperature settings were used; the first one corresponding to čsn en 1363-1 curve (samples 1–4 54 https://doi.org/10.14311/app.2023.40.0054 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 40/2023 comparison of burnt beams models in three point bending (a). four side burning showing measuring points for charring depths. (b). three side burning with depicted lamellae. figure 1. cross-sections with four side burning showing measuring points for charring depths and three side burning with depicted lamellae both rotated anticlockwise. l wel fel h b dmean d0 deff heff beff e1 [mm] [mm] [kn] [mm] [mm] [mm] [mm] [mm] [mm] [mm] [gpa] 1 2280 4.97 6.47 320 100 30.95 7 37.95 244 24 11.99 3 2280 11.96 23.62 320 100 20.90 7 27.90 264 44 11.92 5 2280 9.45 24.07 320 100 17.86 7 24.86 270 50 11.55 7 2280 9.64 37.32 320 100 13.76 7 20.76 278 58 11.79 9 2300 7.28 69.33 320 160 17.42 7 24.42 271 111 11.85 11 2300 9.99 71.01 320 160 23.77 7 30.77 258 98 11.10 13 2300 8.88 92.09 320 160 9.76 7 16.76 286 126 12.03 15 2300 7.59 30.38 320 160 31.13 7 38.13 244 84 11.91 table 1. input data for computational model. and 9–16) and the second one with the same initial stage up to 600 °c which was then set constant (samples 5–8). after the required time (60 min, 40 min, 30 min, 20 min) the test was terminated, beams extinguished with water stream, mechanically cleaned from charcoal layer and let to dry at ambient temperature. 2.1. charring depth charring depth is defined as ”the distance between the outer surface of the original member and the position of the char line”, where ”the position of the char line should be taken as the position of the 300-degree isotherm” [1]. in this paper we assumed the char line to be the line, where the wood colour is changed from its original tone, i.e. visible browning, due to measurement via image analysis. dried and cleaned beams were cut lengthwise into pieces (group 1: 10 pieces, 20 cm – 8 × 25 cm – 18 cm; group 2: 11 pieces, 12 cm – 9 × 24 cm – 12 cm). all sections were documented by a dslr camera canon eos 600d held in hand, with an effort to eliminate image distortion as much as possible. the images were subsequently edited, scaled, and inserted into rectangle with original dimensions. the charring depths were measured along both sides of four innermost lamellae as the distance from sides to first non-white pixel, comprising 66 measuring points (80 mm from top and bottom, 5 mm in between), as depicted in figure 1a. four innermost lamellae were chosen to ensure comparability among all cuts, because both three side (figure 1b) and four side (figure 1a) burning occurred. the whole procedure is described in detail in [4]. the mean values of the measured charring depths for individual beams dmean are listed in table 1. figure 2. load-displacement curves of the three point bending test. 2.2. three point bending test three point bending test belongs to commonly used experiments to determine the mechanical response of a structural element. only odd-numbered dried and cleaned residual beams (i.e. after fire test) were tested in the displacement control regime with loading rate of 0.02 mm · s−1. the load was applied on beam via steel plate or channel section plate, while the displacement of the midsection was continuously measured by a string potentiometer attached to the middle of the beam bottom edge. each beam was supported by rollers on both ends and propped by wood blocks on both sides on each end to prevent tilting. more information could be found in [7]. three quantities were recorded during the test: time, applied force, and maximum displacement in the midspan of the beam. the resulting load-displacement curves are plotted in figure 2. linear sections are 55 l. keřková kucíková, m. šejnoha, t. janda, p. padevět acta polytechnica ctu proceedings ey ez ex gxz gxy gyz νyz νxy νxz er et el glt glr grt νrt νlr νlt [gpa] [gpa] [gpa] [gpa] [gpa] [gpa] [-] [-] [-] iso 1 9.47 0.47 [6] iso 2 e1 b 0.47 ortho 1a 1.30 1.92 9.47 1.70 1.25 0.42 0.28 0.47 0.47 [6] ortho 2 1.30 1.92 e1 b 1.70 1.25 0.42 0.28 0.47 0.47 ortho 3 0.24 0.13 10.08 1.70 1.25 0.42 0.28 0.47 0.47 a mechanical properties of norway spruce obtained by homogenization (mfa=28.24°). b average longitudinal young’s moduli measured by pilodyn device before fire test stored in table 1. table 2. material input data for computational model. marked by crosses. the force-displacement pairs corresponding to the upper cross (wel, fel), i.e. the proportionality limit, are summarized in table 1 together with the beam span l, i.e. the distance between rollers. 3. computational part the main objective of this article is to show whether the simplified approach using the effective crosssection to determine the residual load bearing capacity of wood structural element is sufficient. therefore, this section presents the comparison between finite element models employing the effective cross-section, which is considered rectangular and takes into account the charring depth and the zero strength and stiffness layer, with those reflecting the actual charring depth. besides the influence of the beam shape, five types of material input data were applied to demonstrate the impact of the selected material parameters. all fem simulations were carried out in the student version of the ansys® software. 3.1. material parameters five types of material input parameters were implemented: two considering isotropic behaviour and three assuming material orthotropy, see table 2. all data sets are based on mechanical parameters of norway spruce ortho 1 [6] computed by homogenization [8] for the microfibril angle (mfa) of 28.24°. the iso 1 assumes the longitudinal direction of ortho 1 only. the data sets iso 2 and ortho 2 are the same as iso 1 and ortho 1 except for longitudinal young’s modulus, which is replaced by the mean value of e1 (table 1) provided by the pilodyn measurements prior to a fire test. the moduli e1 [gpa] were computed as: e1 = 19.367 − 0.5641d, (2) where d [mm] is the measured indentation depth [9]. the measurement is elaborately described in [7, 10]. the last data set ortho 3 employs mean young’s moduli in all three directions (er, et, and el) derived from the compression test. this test was performed on small blocks with dimensions of 30 × 30 × (a). block cutting scheme. (b). loading directions. figure 3. sample preparation for compression test. 40 mm cut from glued laminated timber beams not exposed to elevated temperatures (figure 3a). altogether, 17 blocks were loaded in three basic directions (figure 3b) in displacement control regime using the mts aliance 30 kn electromechanical testing machine with loading rates of 2 mm · min−1 (l – longitudinal) and 8 mm · min−1 (r – radial, t – tangential). the moduli were determined from the initial slope of the stress-strain curves obtained from the measured load-displacement curves knowing the sample area and initial extensometer length. 3.2. model definition as was already mentioned, three types of beam geometries were modelled for the sake of comparison, see figure 4, limiting attention to linear elasticity with material parameters summarized in the previous subsection. fifteen such fem models were constructed for each beam, where the length of the modelled beam was set equal to the span between the supports denoted as l in table 1. in the following, the actual residual cross-section of the midspan corresponds to the outline of the residual cross-section determined from the image of the segment cut in the middle of the beam length. the effective cross section employs equation (1), while assuming the mean measured charring depth dmean instead of 56 vol. 40/2023 comparison of burnt beams models in three point bending (a). g1. (b). g2. (c). g3. figure 4. types of beam geometry (sample 1) with depicted meshes. the notional charring depth dchar,n, and coefficient k0 = 1, giving the effective cross-section dimensions as: beff = b − 2deff = b − 2(dmean + d0), (3) heff = h − 2deff = h − 2(dmean + d0), (4) see also figure 5. types of beam geometry: • g1 = effective rectangular cross-section with width beff and height heff , see table 1, applied to the whole beam – constant cross-section (figure 4a). • g2 = actual residual cross-section of the beam midspan applied to the whole beam – constant cross-section (figure 4b). • g3 = actual residual cross-section of the beam midspan applied to the whole beam with pitched ends, both with 0.03 m initial rectangular crosssections (b, h in table 1) and 0.08 m of transition zone lengthwise – variable cross-section (figure 4c). the geometry was modelled as one solid element in spaceclaim 2022 r2 and was imported to ansys workbench 2022 r2 employing the static structural analysis system. the computation was performed considering a default setting, except for the element size and thus the mesh density. limitations for the student license are 128k nodes/elements per model. figure 5. effective cross-section. therefore, the element size had to be adjusted accordingly setting the initial size to 0.02 m. larger elements (0.022 m) had to be adopted in the case of samples 9 and 13 when employing the actual residual crosssection both with and without pitched ends. the element types used in computations were solid186 (a higher order 3d 20-node structural solid element), solid187 (a high-order 3d 10-node tetrahedral structural solid element), and surf154 (a 3d structural surface element). each beam was simply supported, with pinned support (ux = uy = uz = 0) on the left end (x = 0) and roller support (uz = 0) on the right end (x = l), both set as displacement boundary conditions at the bottom edges of beam ends. the force fel (table 1) was applied on the upper beam surface area over the entire width and 0.2 m lengthwise to reduce singularities. note that the curved surface in the case of actual cross-section geometries brought some problems with the definition of the loading area on the upper surface of the beam (g2 and g3) and the bottom edge width (g2) for the introduction of supports, where in both cases borderlines were determined approximately. 4. results the maximum deflection was determined approximately in the middle of the width at the midspan of the beam depending on the position of nodes (i.e. the value in the node closest to the required position was considered). the values closest to the deflection wel in table 1 (i.e. min |wel−wfem|) are highlighted by bold text in tables 3, 4, and 5 for individual geometries and material parameters. the values with absolute difference less than 0.001 m from the measured values (i.e. |wel − wfem| < 0.001) are highlighted as underlined text. according to the highlighted values, there seems to be no significant preference among individual geometries and among material parameters, separately. although less accurate results seem to be predicted for iso 2 material data set considering isotropy with the longitudinal modulus measured by the pilodyn device, while in the combination with g1 geometry it gives relatively good results. probably the best approximation is provided by the data sets based 57 l. keřková kucíková, m. šejnoha, t. janda, p. padevět acta polytechnica ctu proceedings g1 (effective cross-section) beam fire dur. wel wfem [mm] nodes elements[min] [mm] iso 1 iso 2 ortho 1 ortho 2 ortho 3 1 62.32 4.97 6.05 4.78 6.51 5.30 8.46 17323 2964 3 42.15 11.96 9.60 7.62 10.49 8.63 14.58 25355 4788 5 31.40 9.45 8.07 6.61 8.87 7.49 12.58 25355 4788 7 22.90 9.64 9.92 7.96 10.97 9.13 16.01 25355 4788 9 30.82 7.28 10.61 8.48 11.64 9.63 16.44 46461 9744 11 40.70 9.99 14.17 12.09 15.38 13.41 20.75 36888 7540 13 20.55 8.88 10.64 8.38 11.83 9.71 17.64 56968 12180 15 60.95 7.59 8.31 6.61 8.93 7.30 11.51 36888 7540 table 3. deflection w [mm] in the midspan computed by fem model with geometry g1 – effective cross-section (underlined text: |wel − wfem| < 0.001 m; bold text: min(|wel − wfem|) for each beam). g2 (actual cross-section) beam fire dur. wel wfem [mm] nodes elements[min] [mm] iso 1 iso 2 ortho 1 ortho 2 ortho 3 1 62.32 4.97 4.74 3.74 5.04 4.09 5.83 38553 22975 3 42.15 11.96 7.88 6.26 8.61 7.07 11.23 63479 40297 5 31.40 9.45 6.59 5.40 7.24 6.12 9.72 66795 43073 7 22.90 9.64 7.97 6.40 8.80 7.32 11.63 49566 10374 9 30.82 7.28 6.57 5.25 7.39 6.16 10.23 109639 74478 11 40.70 9.99 10.15 8.66 11.20 9.79 14.79 63772 13800 13 20.55 8.88 7.87 6.20 8.88 7.32 12.73 57628 12600 15 60.95 7.59 5.51 4.38 6.10 5.03 8.46 97053 65093 table 4. deflection w [mm] in the midspan computed by fem model with geometry g2 – actual cross-section (underlined text: |wel − wfem| < 0.001 m; bold text: min(|wel − wfem|) for each beam). g3 (actual cross-section with pitched ends) beam fire dur. wel wfem [mm] nodes elements[min] [mm] iso 1 iso 2 ortho 1 ortho 2 ortho 3 1 62.32 4.97 4.73 3.73 4.96 4.00 5.86 43840 26345 3 42.15 11.96 7.84 6.23 8.42 6.88 11.12 68074 43339 5 31.40 9.45 6.56 5.38 7.11 5.99 9.80 70451 45496 7 22.90 9.64 7.97 6.40 8.74 7.26 12.52 79277 52229 9 30.82 7.28 6.61 5.28 7.39 6.16 11.14 111654 75792 11 40.70 9.99 10.13 8.64 11.07 9.67 15.82 118967 80697 13 20.55 8.88 7.87 6.20 8.88 7.32 13.85 118919 81313 15 60.95 7.59 5.46 4.34 5.90 4.84 7.92 103695 69460 table 5. deflection w [mm] in the midspan computed by fem model with geometry g3 – actual cross-section with pitched ends (underlined text: |wel − wfem| < 0.001 m; bold text: min(|wel − wfem|) for each beam). on homogenization iso 1 and ortho 1 in combination with geometries g2 and g3. from highlighted numbers in tables 4 and 5 it could be concluded that the geometries g2 and g3 are more or less comparable and there is no need to model pitched ends. however, the curved bottom ends in geometry g2 brought problems with the introduction of supports and caused non-symmetric vertical reactions. this was eliminated by considering straight edges used in g3 geometry with pitched ends. nonetheless, the simplest geometry g1, considering the rectangular cross-section only, does not deviate from the accuracy point of view when compared to geometries g2 and g3. therefore, it is reasonable to employ the most simple way in estimating the residual load bearing capacity based on the effective cross-section method. by comparing deflections in tables 3, 4, and 5, we observe that the majority of computed values wfem are lower than the measured value wel being numerous in the case of data sets iso 2 and ortho 2 with longitudinal modulus measured by the pilodyn device. this is reasonable, because the fem models are usually stiffer compare to reality approaching the exact 58 vol. 40/2023 comparison of burnt beams models in three point bending solution from below with increasing mesh density. an exception are the results obtained with material parameters ortho 3 data set showing predominantly higher values with some even twice as large as the measured ones, even though some were very close. this model thus appears too compliant. 5. conclusions this paper examined a suitability of the reduced crosssection method (g1) described in eurocode 5 part 1– 2 [1] in the prediction of the behaviour of a beam subjected to fire. to that end, the results obtained experimentally from three-point bending tests were simulated numerically with the help of ansys® finite element software. the results obtained from the application of the reduced cross-section method (g1) were also compared with the response provided by models assuming actual measured residual cross-section of burnt beams (g2 and g3). apart from variable cross-section geometries, five types of material input parameters were implemented, recall table 2. all beams were modelled as simply supported (supports prescribed as displacement boundary conditions on the bottom edge of each end) with the force fel (measured force at the proportionality limit) applied on the upper beam surface area. the linear elasticity was considered only. the maximum deflection was determined in the middle of the cross-section width in the midspan of the beam. the numerically obtained deflections wfem are compared in tables 3, 4, and 5 with the values wel measured experimentally at the proportionality limit. considering the values with the absolute difference less than 0.001 m, we observe no prevailing preference among individual geometries or material parameters, separately. it could also be concluded that geometries g2 and g3 are more or less comparable. however, unlike g3 the curved edges of g2 resulted in nonsymmetric vertical reactions. it is also evident that from the accuracy point of view the most simple geometry g1 is comparable to both more complex geometries g2 and g3 thus promoting the use of the reduced cross-section method in numerical simulations. as for individual material settings, probably the best approximation gives the combination of iso 1 and ortho 1 data sets obtained by homogenization with the geometrical model g3. the effective crosssection based model g1 provided the best fit with iso 2 and ortho 2 data sets. the least accurate results were predicted by combining the g1 model with the ortho 3 data set. remember that the use of ortho 3 resulted in much higher deflections compare to experiments suggesting an excessively large model compliance. nonetheless, there still might be some uncertainties introduced within the model definition such as the way of applying the force, definition of supports, or material input parameters limited to elasticity. from the model definitions point of view we expect a potential improvement by redistributing the force by a steel plate as used in experiments. further advancement should arise from the application of a nonlinear constitutive model to be calibrated on the basis of a nonlinear part of the stress-strain diagram provided by compression tests. we aim to compare both isotropic and orthotropic plasticity models with isotropic hardening in our future studies. list of symbols l span between supports [mm] wel max. displacement at the proportionality limit [mm] fel maximum force at the proportionality limit [kn] b, h initial beam dimensions (width, height) [mm] dmean mean measured charring depth [mm] d0 zero strength and stiffness layer (d0 = 7 mm [1]) [mm] deff = dmean + d0 effective depth [mm] beff , heff effective cross-section dimensions (width, height) [mm] e1 longitudinal young’s modulus determined by pilodyn device [mm] e young’s modulus [gpa] g shear modulus [gpa] ν poisson’s ratio [–] wfem maximum elastic displacement from finite element analysis [mm] r, t, l radial, tangential and longitudinal direction acknowledgements the financial support provided by the czech technical university in prague within sgs project with the application registered under the no. sgs20/155/ohk1/3t/11 is gratefully acknowledged. references [1] european committee for standardization. eurocode 5: design of timber structures – part 1-2: general – structural fire design, 2004. [2] j. schmid, a. just, m. klippel, m. fragiacomo. the reduced cross-section method for evaluation of the fire resistance of timber members: discussion and determination of the zero-strength layer. fire technology 51(6):1285–1309, 2015. https://doi.org/10.1007/s10694-014-0421-6 [3] d. lange, l. boström, j. schmid, j. albrektsson. the influence of parametric fire scenarios on structural timber performance and reliability. tech. rep., sp technical research institute of sweden, 2014. isbn 978-91-87461-78-1. [4] l. kucíková, t. janda, j. sýkora, et al. experimental and numerical investigation of the response of glt beams exposed to fire. construction and building materials 299:123846, 2021. https: //doi.org/10.1016/j.conbuildmat.2021.123846 [5] l. kucíková, t. janda, m. šejnoha, j. sýkora. experimental investigation of fire resistance of glt beams. international journal of computational 59 https://doi.org/10.1007/s10694-014-0421-6 https://doi.org/10.1016/j.conbuildmat.2021.123846 https://doi.org/10.1016/j.conbuildmat.2021.123846 l. keřková kucíková, m. šejnoha, t. janda, p. padevět acta polytechnica ctu proceedings methods and experimental measurements 8(2):99–110, 2020. https://doi.org/10.2495/cmem-v8-n2-99-110 [6] l. kucíková. micromechanical study of spruce wood. master’s thesis, czech technical university in prague, faculty of civil engineering, department of mechanics, 2018. [2022-12-09]. http://hdl.handle.net/10467/74309 [7] l. kucíková, m. šejnoha, t. janda, et al. mechanical properties of spruce wood extracted from glt beams loaded by fire. sustainability 13(10):5494, 2021. https://doi.org/10.3390/su13105494 [8] m. šejnoha, j. sýkora, j. vorel, et al. moisture induced strains in spruce from homogenization and transient moisture transport analysis. computers & structures 220:114–130, 2019. https://doi.org/10.1016/j.compstruc.2019.04.008 [9] m. šejnoha, t. janda, l. melzerová, et al. modeling glulams in linear range with parameters updated using bayesian inference. engineering structures 138:293–307, 2017. https://doi.org/10.1016/j.engstruct.2017.02.021 [10] l. kucíková, m. šejnoha, t. janda, et al. evaluation of impact of elevated temperatures on young’s modulus of glt beams. in acta polytechnica ctu proceedings. nmm 2020 nano & macro mechanics, vol. 30, pp. 41–47. 2021. https://doi.org/10.14311/app.2021.30.0041 60 https://doi.org/10.2495/cmem-v8-n2-99-110 http://hdl.handle.net/10467/74309 https://doi.org/10.3390/su13105494 https://doi.org/10.1016/j.compstruc.2019.04.008 https://doi.org/10.1016/j.engstruct.2017.02.021 https://doi.org/10.14311/app.2021.30.0041 acta polytechnica ctu proceedings 40:54–60, 2023 1 introduction 2 experimental part 2.1 charring depth 2.2 three point bending test 3 computational part 3.1 material parameters 3.2 model definition 4 results 5 conclusions list of symbols acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2023.40.0098 acta polytechnica ctu proceedings 40:98–103, 2023 © 2023 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague the influence of lithium water glass on the durability of concrete klára vašířová∗, josef fládr czech technical university in prague, faculty of civil engineering, department of concrete and mansory structures, thákurova 7, 166 29 prague 6, czech republic ∗ corresponding author: vasirkla@fsv.cvut.cz abstract. the aim of the research is to verify the effect of lithium water glass on the durability and reduction of concrete absorption. for this purpose, the absorption test was made and the influence of the impregnation agent on durability when the test specimens are exposed to cyclic freezing and thawing was verified. finally, the permeability of the test specimens was measured before and after impregnation. on the basis of the tests carried out, the effect of the impregnation agent was determined by comparing impregnated test specimens with reference specimens. keywords: concrete, hydrophobic impregnation, water absorption, durability. 1. introduction nowadays, when the attitude to the environmental protection changes and when more and more attention is paid to the limited reserves of mineral raw materials, there are changes also in the field of construction. for example, for long bridges it is considered that they should safely resist the effects of loading for up to 200 years [1]. it is necessary to adapt the building materials to this. concrete is a very resistant and durable building material. however, its durability can be influenced by the amount and type of pores contained in the concrete. during its lifetime, water and aggressive substances penetrate the porous structure of concrete, which can cause a number of degradation processes, for example corrosion of reinforcement, corrosion of concrete or alkali-silica reaction. as a result of these degradation processes, micro cracks appear and expand, which allows the penetration of a larger amount of water and aggressive substances. over time, the concrete structure may collapse completely. 2. surface protection in order to limit the amount of water penetrated into the porous structure of concrete, it is possible, in addition to changing the composition of the concrete mixture itself, to apply surface protection to the surface of the concrete structure. according to čsn en 1504-2, types of surface protection can be divided into three groups: • hydrophobic impregnation (figure 1) – the surface of the concrete is treated with a hydrophobic agent that has an effect on the size of the wetting angle. there is no reduction in the diameter of pores and capillaries. • impregnation – concrete is provided with a treatment that reduces its surface porosity and strengthens the surface. pores and capillaries are partially or completely filled. • coating – after application of the coating, a continuous protective layer is formed on the concrete surface [2]. 2.1. hydrophobic impregnation when concrete comes into contact with water or another liquid, this liquid soaks into the pores of the building material due to the action of capillary forces. assuming a straight cylindrical capillary, the capillary force fkap acting in this way can be defined by the following relation: fkap = 2πγr cos θ, (1) where γ is the surface tension of the liquid, r is the pore radius, and θ is the wetting angle of the liquid on the surface of the solid. it follows from the equation that the larger the wetting angle θ, the smaller the capillary force [3]. on a concrete surface without surface treatment, water easily soaks into the concrete structure due to the small wetting angle. when a drop of water hits the concrete surface, it spills to the sides. when applying a hydrophobic impregnation, a thin layer is formed on the surface of the concrete structure, which causes an increase in the wetting angle θ (figure 2) that leads to a reduction in the capillary force fkap. in practice, it means that when a drop of water falls on a concrete surface treated in this way, the drop remains wrapped in a ball and runs off the surface. its contact with the concrete surface is minimal which leads to a fundamental reduction in absorbency [4]. 3. the process of impregnation a solution of lithium water glass is used as a hydrophobic impregnation. it is assumed that after its applica98 https://doi.org/10.14311/app.2023.40.0098 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 40/2023 the influence of lithium water glass on the durability of concrete figure 1. hydrophobic impregnation, impregnation, coating (from the left side) [2]. figure 2. increase in wetting angle [4]. tion to the surface of the test specimens, the wetting angle of the surface will increase and thus the water absorption will fundamentally decrease (figure 3). at the same time, there should be no reduction or complete filling of the capillary pores. the influence on the size of the wetting angle can be seen from the following picture, where the top is a non-impregnated test specimen and the bottom is an impregnated one. figure 3. the influence of impregnation on the size of the wetting angle. after its application to the surface of the test specimens, the solution penetrates into the pores and chemically reacts with the calcium ions contained in the hardened cement paste, forming calcium silicate hydrate (c-s-h). it gradually hydrates and creates a crystalline structure, which reduces the absorbency of the surface. for the purpose of the experiment, three series of test specimens were made from hardened cement paste. each series contained 12 blocks measuring 40 × 40 × 160 mm. in the third series, 12 blocks with dimensions of 150 × 150 × 50 mm were also made. a mixture of cement cem i 42.5 (heidelberg cement group, závod mokrá) and water with a water cement ration of 0.28 was used for their production. after their hardening and treatment, the series of test specimens was divided into six reference specimens and six impregnated specimens. in the case of test specimens intended for impregnation, the surface was grinded using 60 grit and then 240 grit sandpaper and cleaned. then the specimens were immersed in a lithium water glass solution for 5 min. this was followed by 24 hours of drying on suitable mats so that all six sides of the surface of the blocks could be dried. 4. testing 4.1. absorption test the aim of the water absorption test is to verify the rate of water absorption of impregnated and nonimpregnated test specimens based on the differences in their weights before soaking and after a certain period of time, and then to compare the results. a prerequisite is slower water absorption for impregnated test specimens, especially in the first minutes. the weight of all test specimens must be considered before soaking. subsequently, the specimens are placed in a container of water on suitable mats so that the water surrounds them from all sides (figure 4). the weights of the individual test specimens are measured after 10 min, 0.5 h, 1 h, 2 h, 3 h, 5 h and 24 h from the moment of their immersion in water. the test specimens are taken out of the water at given time intervals, their surface is wiped with a wet cloth to remove excess water, and their weight is weighed. immediately afterwards, the specimens are put back into the water. for each time interval, the difference in the weight of the test specimen is determined and compared to its initial weight. their percentage is determined from the average of the differences in the weights of the individual impregnated and non-impregnated test specimens compared to their weight before soaking. impregnated test specimens absorb only 55 % of the amount of water during the first test performed in the first interval compared to non-impregnated test specimens. over time, the amount of absorbed water is gradually equalized. impregnation has the greatest 99 klára vašířová, josef fládr acta polytechnica ctu proceedings figure 4. the impregnation of the test specimens. figure 5. the results of the first and second tests of the first series of test specimens. influence in the first hours of the test (figure 5). when the test is repeated with the same test specimens three weeks later, the effect of impregnation is already smaller, but still noticeable at the beginning of the test. between the first and second absorption test of the second series of test specimens (table 1), the specimens were placed in a freezer box, where a test of resistance against the cyclic action of freezing and thawing took place. during this, the surface of the impregnated test specimens was significantly damaged, which had a major impact on the course of the second absorption test, during which the non-impregnated specimens, whose surface was almost undamaged, absorbed less water (figure 6). the third series of test specimens was first placed in a freezer box, where a test of cyclic action of freezing and thawing took place. in this case, there was no significant damage on the surface of the impregnated specimens, as was the case with the previous series. before starting the absorption test, not enough time had passed since the specimens were taken out of the freezer box, and a considerable amount of water series absorption no. 1 freezing and thawing absorption no. 2 porosity 1 x x 2 x x x 3 x x 3–p x table 1. the tests performed on each series. remained in their pores. this is reflected in the test results, where there isn’t any effect of impregnation and the specimens absorbed much less water compared to the other tests (figure 7). the amount of water that bodies can hold is directly proportional to their pore volume. if part of the pore volume is already filled with water, only the remaining volume can be filled during soaking. the test specimens were dried in a laboratory environment and their weight before the start of the second test was higher than before the first test. the resulting weights of the test specimens immersed in water for 24 hours during the second test are the same as their weights at the end of the first absorption 100 vol. 40/2023 the influence of lithium water glass on the durability of concrete figure 6. the results of the first and second tests of the second series of test specimens. figure 7. the results of the test of the third series of test specimens. test. therefore, at the start of the second test, the test specimens were not perfectly dry and could not absorb the same amount of water as they did in the first test. 4.2. test of resistance against the cyclic action of freezing and thawing with this test, it is possible to observe the level of surface degradation of test specimens after their exposure to a certain number of freezing and thawing cycles. it is assumed that the surface of impregnated test specimens will be less damaged after completion of all cycles than the surface of non-impregnated test specimens. the test specimens are placed in the freezer together with the temperature sensors (figure 8). at the beginning, the environment of the freezer box is cooled with cold air to a temperature of −20 °c. after 120 min of maintaining the temperature, the environment is heated by infusing water at a temperature of 20 °c. this temperature is maintained for 180 min. after the water is pumped out, cooling takes place again. in this way, 25 cycles were completed. during the test, the pores of the test specimens are figure 8. test specimens placed in a freezer box. saturated with water. when the temperature drops below the freezing point, the water in the pores turns into ice crystals, increasing its volume up to nine percent. as a result of the increase in the volume of water contained in the pores, pressure arises, which can cause the formation and development of cracks. in the second series of test specimens, which had already passed the absorption test once, there was significant damage to the surface of the impregnated test specimens, while the non-impregnated test specimens remained almost undamaged (figure 9). on the contrary, in the test specimens of the third series, which did not pass any other test before being placed in the freezer box, the surface of the non-impregnated specimens was damaged, while the impregnated test specimens remained without visible damage (figure 10). 4.3. the influence of impregnation on surface porosity the last test performed was the effect of impregnation on surface porosity. the permeability test measures the depth of air penetration into the pores of the test specimens. after determining the specific electrical resistance of the test specimens and the depth of penetration, the quality of their surface can be determined 101 klára vašířová, josef fládr acta polytechnica ctu proceedings figure 9. series 2 after being taken out of the freezer box, impregnated and non-impregnated test specimens from the left. figure 10. series 3 after being taken out of the freezer box, impregnated and non-impregnated test specimens from the left. (figure 11). hydrophobic impregnation should not cause any reduction in the diameter of pores and capillaries. in the case of impregnated test specimens, it is assumed that the degree of permeability will be maintained while the specific electrical resistance increases. for six test specimens, the specific electrical resistance and penetration depth l were measured and the quality coefficient of the covering layer kt was determined. the surface of the specimens was grinded and the values were measured again. this was followed by impregnation and drying for 24 hours, after which the values were measured again. after grinding, there was an increase in electrical resistance. on the contrary, after the impregnation agent was applied, the specific electrical resistance of the test specimens decreased, although the assumption was the opposite. it can be assumed that the electrical resistance decreased due to higher humidity caused by insufficient drying of the test bodies after their impregnation. figure 11. measurement with the torrent device and display of measurement results. the coefficient of quality of the covering layer kt , as well as the penetration depth l, increased after the application of the impregnating agent. the impregnating agent has no effect on the reduction of the capillary pores (table 2). the coefficient of quality of the covering layer can be classified as normal in both cases. 5. conclusion the results of the absorption tests show that the impregnating agent has a significant effect on the absorption of the concrete. in the first few minutes, it reduces the absorption up to 55 % compared to the values of the non-impregnated test specimens. when the test is repeated on the same series of test specimens, the effect of the impregnation is much lower. the performance of the freeze-thaw cycling test also affects the results of the absorption tests. the impregnated surface was damaged on the second series of test pieces. however, when this test was repeated on a third series of test pieces, the effect of impregnation was negligible. for the permeability test, there was no significant improvement or degradation in the surface layer quality and pore size of the test pieces. the values changed only minimally, and this difference can be attributed to the change in humidity after immersion of the test pieces in the impregnation solution. it is demonstrable that hydrophobic impregnation from a lithium hydroxide solution has a positive effect on the durability of concrete structures. for its full use in practice, it is necessary to continue the research, to verify its effects with different composition of concrete mixtures and its effectiveness in the long term. hydrophobic impregnation, impregnation and coatings are undoubtedly a good way to go with the contemporary requirement for long durability of concrete structures. acknowledgements this paper was prepared thanks to the support of the science foundation of the czech republic (gačr), project “advanced lithium silicate sealers: on the way to sustainable building materials” (no. 22-04828s). 102 vol. 40/2023 the influence of lithium water glass on the durability of concrete after grinding after impregnation weight resistance [kωcm] kt l weight resistance [kωcm] kt l [g] i. ii. iii. avg. [mm] [g] i. ii. iii. avg. [mm] 2-1 2320.30 10.0 9.6 10.0 9.97 0.058 16.3 2325.65 9.6 8.9 8.7 9.07 0.127 24.1 2-2 3044.55 6.7 6.5 6.9 6.70 0.086 19.8 3050.09 6.4 6.4 6.5 6.43 0.213 31.1 2-3 1955.98 13.0 13.0 13.0 13.03 0.150 26.1 1960.96 13.0 13.0 12.0 12.53 0.190 29.3 2-4 3015.22 7.0 6.5 7.1 6.87 0.198 30.1 3020.39 6.4 6.3 6.6 6.43 0.356 40.1 2-5 2088.31 12.0 12.0 12.0 12.20 0.181 28.7 2093.88 12.0 11.0 12.0 11.80 0.230 32.2 2-6 2745.00 7.4 7.3 7.8 7.50 0.108 22.2 2749.65 6.7 6.8 6.9 6.80 0.253 33.8 avg. 2528.23 9.38 0.13 23.87 2533.44 8.84 0.23 31.77 table 2. results of the permeability test, test specimens after grinding and after impregnation. references [1] j. margoldová. trvanlivost a životnost betonových konstrukcí, 6/2013. beton tks. [2022-07-20]. https: //www.ebeton.cz/clanky/2013-6-02-uvodnik/ [2] p. tůma. nátěry na beton: typy, požadavky, normové parametry, 3/2014. tzb-info. [2022-07-20]. https://stavba.tzb-info.cz/beton-maltyomitky/10916-natery-na-beton-typy-pozadavkynormove-parametry [3] j. kolísko, d. dobiáš, p. huňka. hydrofobní impregnace betonu – prostředek pro zvýšení odolnosti betonu proti účinkům ch.r.l., 1/2011. beton tks. [2022-07-20]. https://www.ebeton.cz/clanky/2011-172-hydrofobni-impregnace-betonu-prostredek-prozvyseni-odolnosti-betonu-proti-ucinkum-ch-r-l/ [4] a. hubáček. možnosti zvyšování trvalivosti betonů a betonových výrobků pomocí uzavíracích nátěrů, 2/2019. beton tks. [2022-07-20]. https://www.ebeton.cz/clanky/2019-2-20moznosti-zvysovani-trvanlivosti-betonu-abetonovych-vyrobku-pomoci-uzaviracich-nateru/ 103 https://www.ebeton.cz/clanky/2013-6-02-uvodnik/ https://www.ebeton.cz/clanky/2013-6-02-uvodnik/ https://stavba.tzb-info.cz/beton-malty-omitky/10916-natery-na-beton-typy-pozadavky-normove-parametry https://stavba.tzb-info.cz/beton-malty-omitky/10916-natery-na-beton-typy-pozadavky-normove-parametry https://stavba.tzb-info.cz/beton-malty-omitky/10916-natery-na-beton-typy-pozadavky-normove-parametry https://www.ebeton.cz/clanky/2011-1-72-hydrofobni-impregnace-betonu-prostredek-pro-zvyseni-odolnosti-betonu-proti-ucinkum-ch-r-l/ https://www.ebeton.cz/clanky/2011-1-72-hydrofobni-impregnace-betonu-prostredek-pro-zvyseni-odolnosti-betonu-proti-ucinkum-ch-r-l/ https://www.ebeton.cz/clanky/2011-1-72-hydrofobni-impregnace-betonu-prostredek-pro-zvyseni-odolnosti-betonu-proti-ucinkum-ch-r-l/ https://www.ebeton.cz/clanky/2019-2-20-moznosti-zvysovani-trvanlivosti-betonu-a-betonovych-vyrobku-pomoci-uzaviracich-nateru/ https://www.ebeton.cz/clanky/2019-2-20-moznosti-zvysovani-trvanlivosti-betonu-a-betonovych-vyrobku-pomoci-uzaviracich-nateru/ https://www.ebeton.cz/clanky/2019-2-20-moznosti-zvysovani-trvanlivosti-betonu-a-betonovych-vyrobku-pomoci-uzaviracich-nateru/ acta polytechnica ctu proceedings 40:98–103, 2023 1 introduction 2 surface protection 2.1 hydrophobic impregnation 3 the process of impregnation 4 testing 4.1 absorption test 4.2 test of resistance against the cyclic action of freezing and thawing 4.3 the influence of impregnation on surface porosity 5 conclusion acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2023.42.0032 acta polytechnica ctu proceedings 42:32–36, 2023 © 2023 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague ai-assisted study of auxetic structures sergej gredneva,∗, henrik s. steudeb, stefan brondera, oliver niggemannb, anne junga a helmut schmidt university/university of the federal armed forces hamburg, professorship for protective systems, holstenhofweg 85, 22043 hamburg, germany b helmut schmidt university/university of the federal armed forces hamburg, professorship for computer science in mechanical engineering, holstenhofweg 85, 22043 hamburg, germany ∗ corresponding author: sergej.grednev@hsu-hh.de abstract. in this study, the viability of using machine learning models to predict stress-strain curves of auxetic structures based on geometry-describing parameters is explored. given the computational cost and time associated with generating these curves through numerical simulations, a machine learning-based approach promises a more efficient alternative. a range of machine learning models, including artificial neural networks, k-nearest neighbors regression, support vector regression, and xgboost, is implemented and compared regarding the aptitude to predict stress-strain curves under quasi-static compressive loading. training data is generated using validated finite element simulations. the performance of these models is rigorously tested on data not seen during training. the feed-forward artificial neural network emerged as the most proficient model, achieving a mean absolute percentage error of 0.367 ± 0.230. keywords: auxetic structures, regression, machine learning. 1. introduction as a main characteristic auxetic structures posses a negative poisson’s ratio, which makes them a subclass of so called mechanical metamaterials. they show remarkable mechanical properties rendering them particularly useful for applications as crash absorbers. this aptitude stems from their high stiffness and low density combined with a plateau stress over a large deformation regime [1, 2]. the mechanical performance of auxetic structures is mainly governed by their geometry making structural optimization a study topic of paramount importance, which is typically facilitated through finite element simulations. since those can be very time consuming given the structural complexity this work aims at utilizing a machine learning approach to reduce the need for lengthy simulations – a strategy commonly pursued in contemporary engineering sciences [3, 4]. to this end a multitude of machine learning algorithms and techniques are implemented and compared regarding their predictive performance. 2. materials and methods 2.1. auxetic structure and training data the starting point for this study is a specific re-entrant auxetic structure previously studied by bronder et. al. [5]. the structure is shown in figure 1 and is originally defined by five geometry parameters: length, size, waist, strut thickness and angle. as part of a carried out feature engineering process an addition geometry parameter named gap is introduced, which is derived from the original features according to gap = [1 − tan (90◦ − angle)] size − length . (1) figure 1. auxetic unit cell with five defined geometry parameters (red), which represent the initial feature space for the modeling task at hand. an addition parameter was derived during feature engineering (blue). from bronder et. al. [5], modified. for this project a set of 130 stress-strain curves was available, which can be seen in figure 2 . the term stress, in the context of this paper, always refers to the mass-normalized stress in mpa g−1. from all available curves, 15 were reserved for the final performance evaluation at the very end of the project, leaving 115 example curves for model development. the 130 curves were produced by validated finite element simulations from the software abaqus®. alsi10mg structures of 3 × 3 × 3 unit cells were simulated using an explicit solver. 32 https://doi.org/10.14311/app.2023.42.0032 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 42/2023 ai-assisted study of auxetic structures figure 2. all 130 stress-strain curves available for the project. with lightweight applications in mind, stress values were normalized with regard to the structure’s weight. for simplicity reasons compressive strain values are positive. 2.2. error metric the problem at hand is a supervised regression problem where a model predicts numerical values, in this case stresses, while minimizing the prediction error. due to the curves potentially spreading over multiple orders of magnitude, the scale-independent mean absolute percentage error (mape) is chosen as the error metric, which is defined as mape = 1 n n∑ i ∣∣∣∣yi − ŷi yi ∣∣∣∣ , (2) where n is the number of predictions, ŷi the i-th predicted value and yi the corresponding i-th true value. since the mape becomes arbitrarily large for true target values, i.e. the variable values to be predicted, that are zero or close to zero, the metric is evaluated only for strains > 0.01, which corresponds to the initial linear elastic part of the curves. 2.3. problem formulation for the main study two different approaches to the problem itself are studied. in the single point approach (sp) the models take as input the geometry parameters describing the structure as well as a certain strain value and predict the corresponding mass normalized stress value. in contrast, using the whole curve approach (wc) the model predicts a complete sub-sampled curve based on only the geometry parameters. here, a sampling rate of 20 is used, reducing the number of data points per curve from 1 001 to 51. during the exploratory phase of the study different sampling rates were tested with 20 leading to the best results. in particular, better results and faster training times were achieved compared to the original data point density. figure 3. topology of a feed-forward neural network. neurons are represented by circles, while arrow indicate the direction of data transfer. to facilitate a direct and transparent comparison of the results the predicted sub-sampled curves are linearly extrapolated to have the same point density as the original curves. moreover, in an additional approach, the potential of treating the entire curve as a sequence is explored. in this sequential curve approach (sc), the model is tasked with predicting all 1 001 points of the stressstrain curve based on the geometry parameters. 2.4. machine learning models four fundamentally different classes of machine learning models are used to make prediction in order to provide an extensive overview and comparison of methods potentially suitable for the task at hand. representing the class of parametric models, i.e. models that learn by fitting parameters of a mapping function to the data, artificial neural networks (ann) and support vector regression (svr) are implemented. anns consist of a multitude of simple computational units, called neurons, that are arranged in a layered network. in this case, a topology is used where each neuron in one layer is connected to every neuron in the neighboring layers, forming a so called (fully connected) feed-forward neural network. each connection between two neurons provides a degree of freedom for the fitting process [6, 7]. a schematic of the topology of such networks is shown in figure 3. support vector regression (svr) can be viewed – in a simplified manner – as a generalized version of linear regression that is not limited to linear problems [8]. a deeper explanation of svr is omitted at this point, since this model proved to be not suitable for the task at hand. k-nearest-neighbor regression (knnr) and xgboost (xgb) represent the class of non-parametric models. these models do not fit internal parameters to the data. rather, predictions are made based on known data directly. therefore, changing the training data automatically changes the model’s predictions. knnr predicts stresses for a new combination of input parameters, also called features, by identifying the k closest known examples from the input space of the training data. the predicted stress value, then, is the average of these k known stress values [9]. 33 s. grednev, h. s. steude, s. bronder et al. acta polytechnica ctu proceedings figure 4. distribution of mean mass normalized stress values for all 130 available curves. xgboost is an advanced and optimized implementation of decision trees [10]. decision trees consist of a set of “if – then” statements, also called decision rules, that divide the input space into subdomains or partitions. predictions are then made based on the partition to which the sample at hand belongs, typically predicting the average target value of the partition [8]. for the additional sc approach mentioned at the end of section 2.3, temporal convolutional networks (tcns) [11] are employed to generate the sequence from the input features. tcns are a class of neural networks that are particularly well-suited for sequence modeling tasks, as they make use of temporal convolutions to capture dependencies of different lengths in the data. through multiple layers of dilated convolutions, tcns can effectively learn and represent complex patterns across various time scales in the sequence, making them a promising choice for predicting the entire stress-strain curve with high resolution. 2.5. course of action to provide a starting point, first, the four main models introduced in section 2.4 are trained on the original 5 features, namely size, waist, length, angle and strut thickness, using the sp and the wc approach. here, a rudimentary hyperparameter optimization is carried out to obtain parameters that are close to the optimal choice. hyperparameters are parameters describing the architecture and/or functionality of the model. using these trained models, feature engineering is performed, which is a set of techniques aiming at creating data representations that make the learning process easier. for this sake, a selection of feature combinations is evaluated in combination with every model regarding their predictive performance. the feature combinations found to lead to best results – which can be different for different models – are then used to retrain the models and perform a more extensive hyperparameter optimization, resulting in figure 5. performance of each model-approach combination on the final test set. error bars denote the standard deviation with regard to individual curves. the final optimized models. in order to estimate model performance reliably while trying to prevent overfitting, i.e. good performance on known data and bad performance on new data, throughout the model optimization process a cross validation (cv) procedure is implemented. the data set at hand is imbalanced with high-stress curves being highly underrepresented, see figure 4. therefore, the cv procedure is stratified to guarantee that the distribution of mean stress values while splitting data (train, validation and test) stays as close as possible to the distribution in the original data set. for every model, a 4-fold-cv is implemented. the splitting procedure being repeated three times, resulting in 12 different train/validation splits with a size ratio of 75 to 25. at the very end of the project, all optimized models are trained on all 115 curves available for the modeling process and then used to make predictions for 15 neverseen-before curves – and therefore until now unknown geometry describing input parameters – in an attempt to simulate the real application scenario. 3. results and discussion the summarized performances of all trained models can be seen in figure 5. it is apparent that the choice of the approach, meaning sp versus wc, had no significant effect on the predictive performance of the models. the particularly bad performance of knnr (sp) can only be explained with overfitting, even though all possible precautions were taken. here, one particular curve of the final test set, was predicted extremely bad, drastically worsening the average performance of the model. in all prior stages of the model building process its performance was comparable to the other models, especially to knnr (wc). svr (sp) is not shown in the figure because this approach was discarded after the initial hyper parameter opti34 vol. 42/2023 ai-assisted study of auxetic structures mization stage due to extraordinarily bad performance together with unreasonably large training times. in general, the performance of all models, with the exception of knnr (sp), performed even better of the final test set than estimated during the model building process, all models having a mape of at least 0.573 in the case of ann (sp). first of all, this shows that the efforts to provide reliable performance measures through suitable data splitting techniques generally paid off since the expectations from the model building phase could be met and even surpassed in the actual application scenario. the overall improvement in predictive performance can be explained by the larger effective train set. during cross-validation, models were trained on sets consisting of 86 stress-strain curves (75 % of 115), whereas all 115 curves were available to train the final models. this is an increase in data amount of about 28 %. the significant improvement in performance through more data indicates that the amount of training data is at least one of the limiting factors regarding predictive capability. best performance was achieved by feed-forward artificial neural networks using the single point approach with a mape of 0.367 ± 0.230. the network used the original input variables with the exception of angle being replaced with gap, which for this model increased the predictive performance. the final network consist of 16 layers, each containing 18 neurons. figure 6 shows examples of particularly good and bad predictions produced by the model. it is able to predict the general order of magnitude correctly, however, it struggles with predicting the actual shape, which explains the large standard deviation of the mape. this is not surprising since different sets of geometry parameters can lead to stress-strain curves that drastically vary in shape due to stability failure of the structure. in the additional approach, tcns were employed to model the stress-strain curves as sequence data. it is important to note that the tcn faced a more demanding task, as it aimed to predict the full curve consisting of 1 001 data points, which is inherently more complex compared to what the other models were required to accomplish. consequently, the tcn did not prove to be as adept for this specific problem context as was hoped. however, exploring this method served to reinforce the decisions made in the main study, and highlighted the importance of model selection in relation to the complexity of the task. 4. conclusion and future work a model was trained that can predict stress-strain curves for a re-entrant auxetic structure, see figure 1, based only on geometry-describing parameters, achieving a mean absolute percentage error of 0.367±0.230. even though at this point the model cannot replace conventional finite element simulations, it allows to highly reduce the amount of simulations that actually figure 6. particularly good (top) and particularly bad (bottom) predictions produced by the final optimized artificial neural network using the sp approach. need to be carried out by providing a rough estimate of the expected curve within a fraction of a second – compared to up to 48 h for the classical simulation. furthermore, it was shown that feed-forward artificial neural networks are best suited for the task at hand, significantly outperforming all other studied options. additionally, models like support vector regression and temporal convolutional networks were found to be particularly unsuitable for the studied problem. since an increase in training data volume enhanced the predictive performance in this study, further improvements to the models can be expected by incorporating more and more simulated curves into the data set. apart from a simple increase in data volume, additional models are planned to be trained predicting effective quantities like the absorbed energy during the compressive loading. since predicting one quantity per geometry parameter combination instead of a whole curve reduces the complexity of the problem, better results are to be expected. also, since in this case no curve shape needs to be matched, the spread in performance is expected to be reduced. given the expected possible frequency with which predictions can be made, an extension of the framework is conceiv35 s. grednev, h. s. steude, s. bronder et al. acta polytechnica ctu proceedings able where the user chooses a desired specific energy absorption for a particular application and the model provides the corresponding geometry. references [1] a. bezazi, f. scarpa. mechanical behaviour of conventional and negative poisson’s ratio thermoplastic polyurethane foams under compressive cyclic loading. international journal of fatigue 29(5):922–930, 2007. https://doi.org/10.1016/j.ijfatigue.2006.07.015 [2] f. scarpa, p. pastorino, a. garelli, et al. auxetic compliant flexible pu foams: static and dynamic 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[8] m. kuhn, k. johnson. applied predictive modeling. springer, 2013. [9] g. james, d. witten, t. hastie, r. tibshirani. an introduction to statistical learning: with applications in r. springer, 2013. [10] t. chen, c. guestrin. xgboost: a scalable tree boosting system. in proceedings of the 22nd acm sigkdd international conference on knowledge discovery and data mining, pp. 785–794. acm, 2016. https://doi.org/10.1145/2939672.2939785 [11] s. bai, j. zico kolter, v. koltun. an empirical evaluation of generic convolutional and recurrent networks for sequence modeling, [2023-06-30]. https://doi.org/10.48550/arxiv.1803.01271 36 https://doi.org/10.1016/j.ijfatigue.2006.07.015 https://doi.org/10.1002/pssb.200460386 https://doi.org/10.1073/pnas.1911815116 https://doi.org/10.1002/pamm.202000238 https://doi.org/10.1002/adem.202100816 https://doi.org/10.1037/h0042519 https://doi.org/10.1145/2939672.2939785 https://doi.org/10.48550/arxiv.1803.01271 acta polytechnica ctu proceedings 42:32–36, 2023 1 introduction 2 materials and methods 2.1 auxetic structure and training data 2.2 error metric 2.3 problem formulation 2.4 machine learning models 2.5 course of action 3 results and discussion 4 conclusion and future work references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2024.51.0006 acta polytechnica ctu proceedings 51:6–13, 2024 © 2024 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague passive safety of light vehicles – effect of speed on the behavior of the vehicle frame structure michal cenkner∗, josef svoboda, přemysl toman czech technical university in prague, faculty of transportation sciences, konviktská 20, 110 00 prague 1, czech republic ∗ corresponding author: cenknmic@cvut.cz abstract. due to growing concerns about the climate change situation, an increasing number of restrictions are being introduced. one of them is the shift towards smaller and lighter vehicles that are less demanding to produce, operate, and subsequently recycle. these are specifically l6 and l7 vehicles (four-wheelers called quadricycles). in addition, even for these vehicles, there is an attempt to make their operation even more environmentally sustainable by introducing government regulations on the powertrain, which will have to be purely electric from 2035. however, the growing popularity of these vehicles has one major drawback: safety. in the framework of the urban electric vehicle project located at the faculty of transportation sciences, ctu in prague, from the already mentioned category, safety is one of the main topics we want to address. this paper discusses the passive safety of quadricycles and the role of fem simulations throughout the design and development phase of vehicles. the main objective is to validate the main frame of the developed vehicle at an extensive range of impact velocities. in this way, the behavior of the proposed front crumple zone design and their roles in frontal impact can be determined. the metrics to be monitored are, for example, the maximum accelerations on the structure in each part of the frontal crumple zone or the preservation of the survival space. keywords: passive safety, quadricycles, light vehicles, urban mobility, fem, ls dyna, explicit dynamics, crash test, electric vehicle. 1. introduction car transport is very popular. this is evident from the fact that there is an average of 570 cars per 1 000 inhabitants in europe [1]; moreover, this number is still growing. despite the increasing share of alternative drives, cars are not decreasing. even though the number of vehicles in society continues to grow, the roads are getting safer. fatalities decreased by 36 % between 2010 and 2020 [2]. the direct impact is due to the adjustment of legislation towards modern safety technologies, such as the obligation to have advanced emergency braking systems. the general objective is to meet “vision 0”, which aims for zero road fatalities by 2050. a major influence on road safety can also be attributed to the ever-improving design of vehicles, with manufacturers motivated not only by european legislation but also by user tests, most popular in europe being the european new car assessment programme (euro ncap). the specificity of these tests consists of worse conditions for the vehicle being tested, typically higher impact speeds, and different choices of collision partners [3]. vehicles are then scored with a certain number of stars according to how safe they are in the context of passive and active safety. 1.1. definition of quadricycle quadricycles, shown in figure 1 and referred to in legislation as l6 and l7, are a relatively new vehicle category. they are popular in europe as a fourwheeled replacement for motorcycles. they have the advantage of low weight and, therefore, higher fuel efficiency. focusing directly on the definition of the l6 and l7 categories, the main parameters distinguishing them from passenger vehicles (m category) are the maximum weight, which must not exceed 350 kg for the l6 and 450 kg for the l7 [5]. the weight does not include the mass of the batteries. other parameters are a speed limit of 45 km h−1 for the l6 and a power limit of 15 kw for the l7. vehicles in these categories must always have four wheels. the maximum permissible load is 200 kg of passengers or 1 000 kg of cargo. given the strict weight limits and low power, vehicle design aims to keep the body structure as light and, therefore, simple as possible. the vehicle body mostly comprises a combination of tubular frames, carbonbased composite elements, and aluminum profiles, as seen in figure 1. 1.2. passive safety of quadricycles apart from internal strength analyses, quadricycles are not subject to passive safety verification as part of the certification process. euro ncap decided to test the passive safety of selected vehicles from the l6 and l7 categories [5]. according to their standards for passenger car testing, vehicles in these categories have been proven to have a very low level of passive safety [6]. 6 https://doi.org/10.14311/app.2024.51.0006 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 51/2024 passive safety of light vehicles . . . figure 1. ligier vehicle from category l7e [5] on the left and aixam vehicle frame from category l7e [4] on the right. figure 2. comparison of quadricycle – microcar m.go (left) and full-size car – toyota iq (right) in frontal impact [6]. the vehicles were tested at a 50 km h−1 impact speed into a deformable barrier with 100 % overlap [6]. this speed was chosen even though all vehicles tested have a top speed between 80–100 km h−1. the testing company itself has expressed concern over the lack of restraint systems (absence of airbags and seatbelt pretensioners), which would lead to severe or fatal injuries. justification for the said concern is also found in the fact that the selected quadricycles visually look like similarly sized passenger vehicles and will, therefore, be in direct competition with them. to support its argument, euro ncap’s campaign compared different quadricycles and the toyota iq [6], which was classified as a passenger car despite being virtually identical in size and receiving five stars in the rating. in contrast, all the tested quadricycles received only one star. figure 2 shows a comparison of a frontal impact between a quadricycle and a passenger car under the same initial conditions. the same issue was covered in the tlr report [7], where one section compared japanese minicars, known as kei-cars, which are similar in size and weight to quadricycles. the two main conclusions as to why small cars are more dangerous are based on the fact that there is not enough space to absorb energy from a crash due to the small size, and the small weight puts the small car at a disadvantage in a collision figure 3. the probability of serious or fatal injury for drivers of different ages [7]. with a larger vehicle due to momentum transfer. keicars try to compensate for the lack of crumple zones with restraint systems. for example, the seat belt pre-tensioner is set for greater downforce, but this can cause fatal injuries for persons over 65, as shown in figure 3. this brings a whole new perspective to the passive safety of quadricycles. 1.3. state-of-the-art in the field of passive safety of quadricycles the level of passive safety of quadricycles is deficient compared to full-sized passenger cars. this is evident from the euro ncap crash tests carried out [6], de7 m. cenkner, j. svoboda, p. toman acta polytechnica ctu proceedings figure 4. the overview of the tested ev numerical model and initial conditions of the simulation. scribed in more detail in the previous chapter. this situation is not helped by the fact that the higher level of passive safety is not regulated and, therefore, not tested [7]. however, as the passive safety of full-size passenger vehicles increases, safety is also starting to be addressed at the level of manufacturers and research organizations working on new solutions for crumple zones and other elements of passive safety. a paper on the passive safety of urban electric vehicles [8] discusses and summarizes structure testing of bodies made in completely different ways. in the first variant, a plastic variant reinforced with carbon fiber was tested by simulation to prove that the vehicle can be safe even if it is very light. however, the front and rear crumple zones were made of aluminum. in testing, the effect was that the deformation did not hit the survival area at low speeds [9]. the second variant involved using a combination of materials, namely aluminum, magnesium, structural plastic, and multiple-strength steels [10]. for this experiment, more parameters were already monitored; besides deformation, acceleration was also monitored, aiming to not exceed 50 g in a frontal impact at 64 km h−1. subsequently, the restraint systems were also evaluated, simulating the pulse of acceleration at impact. this resulted in a comparable level of passive safety to passenger vehicles. the last variant described in this paper was a frame structure consisting of highstrength steel [11]. this material proved very suitable as it can absorb kinetic energy well, even though it is heavier. for this variant, the validation of the calculations was followed by a real crash test. addressing passive safety makes sense. at lower speeds (approx. 17 km h−1), the use of restraint systems can already be decisive [12]. vehicles are now equipped with a wide range of restraint systems, such as seat belts and seat belt pre-tensioners, airbag systems, deformable steering columns, and even special safety glasses. in addition to restraint systems, passive safety is also affected by the crumple zone system. these are specially tuned for different types of impact. for example, the frontal crumple zone must absorb a large amount of crumple energy, which is converted into plastic deformation. a lot of material is required at higher speeds. for this reason, using alternative materials has been an effort, especially in lightweight vehicles such as quadricycles. 1.4. role of simulations in passive safety the use of finite element simulations (fem) is a standard in the vehicle development cycle [5]. the standard procedure is to perform a crash test of a prototype vehicle under development to obtain the initial pulse of energy at impact, followed by debugging of all vehicle components and all restraining systems made possible by increasing computational power, and then followed again by realistic crashes of the vehicle to verify the fem simulations and debugging before actual production. crash tests, in general, are not only to evaluate the passive safety of the vehicles but also to analyze and test the structure and deformation zone of barriers in the traffic environment [13]. in small-scale production or prototype development of quadricycles, at least the first phase of crash tests is often skipped [6], sometimes even the validation phase. one reason is that simulations using fem can be very accurate and sufficient for simpler vehicles. still, the primary reason is that physical crash tests are not required by legislation [5], as described earlier. examples of the use of fem in the development of quadricycles are given, where further iterative modifications are subsequently made in addition to the verification of the proposed vehicle design [12, 14]. therefore, the question for the future is whether it would be sufficient to verify the passive safety of quadricycles by fem simulations alone. 2. methods the following chapter provides information about the numerical model and the setup of the performed simulations. 2.1. numerical model of the ev the electric vehicle numerical model shown in figure 4 was created in ls prepost software and is based on the original cad 3d model designed. this 3d model was simplified for simulation purposes, meaning that 8 vol. 51/2024 passive safety of light vehicles . . . figure 5. detail of the created mesh and detail of the profile junctions. chassis parts material material ϱ e υ sigy formulation [kg m−3] [gpa] [/] [mpa] frame profiles steel s235jr isotropic, elastic-plastic 7850 210 0.3 220wheel discs bumper car axles tires synthetic rubber isotropic, linear elastic 990 0.15 0.45 – collision wall rigid material rigid material – – – – the column of material formulation describes the selected material model from the ls dyna software library, where ϱ is the density of the material, e is young’s modulus of elasticity, υ is the poison ratio, and sigy is the yield strength of the material. table 1. material characteristics used for parts of the numerical model. some objects with complex geometry or material characteristics were substituted by abstract point mass replacement with the same cog and weight. this substitution was made wherever the original geometry could be neglected, e.g., an electric motor, battery boxes, and seats. the chassis and car body are composed of steel profiles (rectangular and circular cross-sections) modeled as shell elements. specifically, it was section shell type 16 (beathe-dvorkin transverse shear) that provided us with the most accurate solutions and allowed us hourglass control. in a similar way, wheels and pneumatics were modeled where the difference was in chosen thicknesses and the assigned material. the mesh was created using the auto-mesher function and was composed of 0,5 million elements. the characteristic length of the smallest element was 4,72 mm. some elements were manually edited, especially in parts where more profiles meet, as illustrated in figure 5. the material specification for the entire model is in table 1. material erosion has been added to achieve proper behavior. this property is the way to remove elements that have met defined conditions of failure and thus improve the behavior of the model. 2.2. numerical model summary the overview of the whole experiment is shown on the right in figure 4. the numerical model consists of two submodels – the model of an electric vehicle and the barrier, which was modeled as a rigid collision wall. all scenarios were performed as frontal impact with 100 % overlap, and individual simulations differ and depend mainly on the impact speed of the vehicle within the range of 5–100 km h−1. the initial position of the ev front model is unchanged during all simulation runs. to ensure a stable simulation in an acceptable timeframe, the whole numerical model was simplified and optimized. for example, the initial distance between the vehicle and the collision wall was minimalized. to assess the passive safety of the electric vehicle, the body of the vehicle was divided into sections composed of the deformation zones d0-d3 and the survival space d3-d4 (figure 6). the evaluation was carried out according to the values of acceleration and displacement achieved in each zone. 3. results this chapter presents examples of individual scenario results and an overview of data. figure 7 shows the virtual crash test result of the full-frontal impact of the vehicle equipped with bumper reinforcement and deformation block in the front part of the vehicle at a speed of 20 km h−1. the whole vehicle model is simplified for the purposes of the explicit dynamic simulation. 9 m. cenkner, j. svoboda, p. toman acta polytechnica ctu proceedings figure 6. deformation zones and survival space of the assessed vehicle. figure 7. results of the simulation, 20 km h−1. figures 8–10 show the resultant acceleration at initial speeds of 20, 50, and 80 km h−1. the simulations were conducted at an initial speed of 5 to 100 km h−1. each scenario evaluated the impact on the deformation zone and survival space. areas of relevant zones are highlighted in the following figures as well. figure 11 shows the survival space assessment. the vertical axis represents the displacement in [mm] of the survival space and vehicle pillars. the shortening of the survival space is also presented in [%]. survival space displacement was calculated from the displacement of the a and b pillars for each impact speed. figure 12 presents a detailed look at the deceleration peaks occurring in each zone based on impact speed. table 2 then shows overall experiment results containing the vehicle pillar displacements, chassis shortening, and deceleration peak values in each zone. 4. discussion virtual crash tests bring the possibility of more detailed assessment for various impact scenarios. based on deceleration peak values for each impact speed, it’s possible to observe which zones absorbed the kinetic energy of the crash and also which zones were unaffected (table 2). figures 8–10, representing the resultant acceleration on the bottom of the chassis, show the behavior of the deformation elements in the context of initial speeds and actual values observed on the chassis. from this point of view, it is possible to follow in which zones the acceleration starts to infigure 8. resultant acceleration with initial speed 20 km h−1. figure 9. resultant acceleration with initial speed 50 km h−1. figure 10. resultant acceleration with initial speed 80 km h−1. 10 vol. 51/2024 passive safety of light vehicles . . . figure 11. survival space assessment. figure 12. deceleration peaks in zone assessment. survival space shortening chassis deceleration peaks of the chassis initial shortening velocity b pillar a pillar displacement displacement ∆ ∆ d0-d1 d1-d2 d2-d3 d3-d4 [km h−1] [mm] [mm] [mm] [%] [mm] [m s−2] [m s−2] [m s−2] [m s−2] 5 x x x x 36.380 3.610 0.000 0.000 0.000 10 97.4 96 1.4 0.140 100.977 3.620 0.000 0.000 0.000 20 273 267 6 0.600 282.834 1.730 3.560 0.000 0.000 30 537 512 25 2.500 566.725 2.000 12.220 0.000 0.000 40 604 578 26 2.600 631.105 1.820 10.750 15.130 0.000 50 830 795 35 3.500 866.267 1.700 12.780 17.500 0.000 60 973 822 151 15.100 1031.320 1.900 14.650 14.730 16.780 70 1080 827 253 25.300 879.138 1.680 15.400 19.100 19.800 80 1360 852 508 50,800 1490.110 1.710 12.650 21.300 25.400 100 x x x x x 1.800 12.180 23.100 24.100 x – not measurable table 2. overall experiment result values. 11 m. cenkner, j. svoboda, p. toman acta polytechnica ctu proceedings crease rapidly compared to the deformation elements. it means primarily zone d1-d2 in the performed experiment. this zone creates the transition between low deceleration and higher acceleration peak areas while crashing into the barrier. in terms of survival space assessment, it is possible to track the displacement of certain vehicle elements. in our case, the displacement of a and b pillars was analyzed. based on the displacements, it is possible to explore the deformation of the whole survival space. from the point of view of tested construction fitted with a frontal deformation block, we observe survival space defamation from the speed of 50 km h−1 (see figure 11). the displacement of vehicle pillars brings a detailed look at the course of deformation at various impact speeds. thus, comparing different vehicle construction and passive safety system components is feasible. the impact of the frontal crash on the vehicle chassis describes the deceleration peaks measured in defined zones. in compliance with the mutual displacement of a and b vehicle pillars, the higher deceleration peaks occur in the survival space zone from 60 km h−1. moreover, in figure 12, it is possible to observe gradual deformation within the front part of the vehicle. results of the first tested vehicle show the possibility of a detailed virtual testing methodology. based on the data, critical speeds in the context of the survival space and deceleration progress in user-defined zones and peaks in defined locations can be observed. 5. conclusions quadricycles in the l6 and l7 categories represent the innovative automotive trend for urban mobility. this category dynamically grows in recent years. as such vehicles are still relatively new, there are low passive safety legislative regulations. quadricycles are primarily manufactured in small series productions. therefore, the actual crash tests of passive safety systems used usually for mass production series of passenger cars are financially demanding for the quadricycle category. one of the alternatives to the real vehicle assessment is explicit dynamic simulations in the form of virtual crash tests. in this paper, virtual crash tests were conducted at various impact speeds. performed experiments aimed at complex quadricycles’ passive safety methodology, combining virtual dynamic simulations and partial real verification tests to obtain credible results for various crash scenarios. the assessment of survival space shows the potential of critical speed definition for each vehicle. the evaluation of deceleration peak and construction element displacements brings the opportunity to assess the vehicle’s passive safety systems and elements. for the future development of complex methodology based on the results of virtual crash tests, it is essential to perform subsequent simulations of the wider variety of quadricycle constructions. moreover, it is necessary to include more complex testing scenarios regarding vehicle chassis and occupant safety assessment. acknowledgements this paper was supported by the czech technical university in prague grant sgs23/131/ohk2/2t/16 development of a modular urban vehicle research platform for testing alternative energy sources and hmi user solutions. references [1] eurostat. number of cars per inhabitant increased in 2021, 2023. [2024-11-21]. https://ec.europa.eu/eurostat/web/ products-eurostat-news/w/ddn-20230530-1 [2] european parliament. road fatality statistics in the eu (infographic), 2019. [2024-11-21]. https://www.europarl.europa.eu/topics/en/ article/20190410sto36615/ road-fatality-statistics-in-the-eu-infographic [3] m. van ratingen, a. williams, a. lie, et al. the european new car assessment programme: a historical review. chinese journal of traumatology 19(2):63–69, 2016. https://doi.org/10.1016/j.cjtee.2015.11.016 [4] aixam, n°1 for licence-free cars and safety. [2024-11-21]. https://www.aixam.com/en/ french-brand/commitments/safety [5] a. pavlovic, c. fragassa. general considerations on regulations and safety requirements for quadricycles. international journal for quality research 9(4):657–674, 2015. [6] euro ncap. quadricycle ratings explained, 2016. [2024-11-21]. https://www.euroncap.com/en/ car-safety/quadricycle-ratings-explained/ [7] m. edwards, m. seidl, j. carroll, a. nathanson. provision of information and services to perform an initial assessment of additional functional safety and vehicle construction requirements for l7e-a heavy on-road quads, 2014. transport research laboratory. 75 p. [2024-11-21]. https://ec.europa.eu/docsroom/documents/5466/ attachments/1/translations/en/renditions/pdf [8] j. romo, e. canibano, j. c. merino. lightweighting and passive safety for urban electric vehicle. in 2017 electric vehicles international conference (ev), pp. 1–5. ieee, 2017. https://doi.org/10.1109/ev.2017.8242113 [9] european commission. advanced structural light-weight architectures for electric vehicles, 2014. [2024-11-21]. https: //cordis.europa.eu/project/id/265898/reporting [10] european commission. super light architectures for safe and affordable urban electric vehicles, 2018. 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[12] g. i. alexandru, c. g. marius, t. f. daniel, t. d. dragos. dynamics of frontal crash in/without the presence of passive safety systems. iop conference series: materials science and engineering 1220(1):012044, 2022. https://doi.org/10.1088/1757-899x/1220/1/012044 [13] p. vrtal, t. kohout, l. nouzovský, z. svatý. dynamic tests of the protective and security barrier system probar. acta polytechnica ctu proceedings 42:83–86, 2023. https://doi.org/10.14311/app.2023.42.0083 [14] s. kongwat, t. homsnit, c. padungtree, et al. safety assessment and crash compatibility of heavy quadricycle under frontal impact collisions. sustainability 14(20):13458, 2022. https://doi.org/10.3390/su142013458 13 https://doi.org/10.1088/1757-899x/1220/1/012044 https://doi.org/10.14311/app.2023.42.0083 https://doi.org/10.3390/su142013458 acta polytechnica ctu proceedings 51:6–13, 2024 1 introduction 1.1 definition of quadricycle 1.2 passive safety of quadricycles 1.3 state-of-the-art in the field of passive safety of quadricycles 1.4 role of simulations in passive safety 2 methods 2.1 numerical model of the ev 2.2 numerical model summary 3 results 4 discussion 5 conclusions acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2025.54.0017 acta polytechnica ctu proceedings 54:17–22, 2025 © 2025 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague tractable descriptors for digital aggregate generation kaustav das, jan sýkora∗, anna kučerová czech technical university in prague, faculty of civil engineering, department of mechanics, thákurova 2077/7, 160 00 prague 6 – dejvice, czech republic ∗ corresponding author: jan.sykora.1@cvut.cz abstract. this paper introduces a deep learning-based approach for generating 2d aggregate shapes using a small set of tractable and physically meaningful parameters. in contrast to existing methods that often rely on quantities without clear physical interpretations, the approach presented here leverages scale invariant moments to capture essential shape characteristics. additionally, we introduce the idea of using scale-invariant state descriptors, such as area, to control the size of the generated shapes. the neural network is trained to generate shapes corresponding to these parameters, and its ability to learn the relationship between shape constants and state descriptors without explicit data augmentation is demonstrated. the framework thus presented provides a foundation for developing microstructure generators that offer enhanced interpretability by relying on parameters that provide meaningful insights into the description of material morphology composed of non-trivial shapes. keywords: reconstruction, moment invariants, deep learning, concrete. 1. introduction concrete is primarily a matrix inclusion composite consisting of a mortar-based matrix embedded with aggregate-based inclusions. since the casting process results in the introduction of porosity defects and also leads to the formation of interfacial transition zones (itz) in the regions between the aggregates and the surrounding mortar, these are also treated as additional phases during analysis [1, 2]. the heterogeneous structure formed by the spatial arrangement of these constitutive phases is herein termed the concrete microstructure. the role of concrete microstructure on its overall performance is well established with correlation being observed in its mechanical behaviour [3], crack formation [4], hydration mechanisms [5], thermo-mechanical coupling [6] to name a few. in particular, the size, shape and the roughness of aggregates play an important role in the overall strength of concretes [7] and the development of itzs [8], which are accepted as the weakest phase in concretes [9]. however, the grading of aggregates, their property, and their type are also among the controllable parameters in concrete mixtures. therefore, due to the vast number of possible combinations of parameters, development of concrete mixtures for specific applications requires substantial investment of time and resources if performed by exclusively employing experimental methods. alternatively, numerical schemes enable an effective preliminary study of concretes and can greatly reduce experiments by facilitating the inspection of a larger set of parameters in-silico [2]. for materials such as concretes where scale separation is well defined, multi-scale homogenization schemes are used for determining the bulk response by taking into consideration material heterogeneity [10]. however, accuracy of numerical schemes depend upon the level of fidelity used for the definition of the computational domain which in this case, is linked to the distribution of aggregates. generation of complex computational domains or microstructures fall into the purview of microstructure characterization and reconstruction (mcr) techniques where some of the widely adopted methods for modeling heterogeneous material structures either include the use of very large set of parameters or include the use of specialized functions without well defined physical meaning [11]. this is also evident in the modeling of aggregates where spherical harmonics and random fields see wide adoption [12], which by themselves lack clear physical meanings. although it is possible to obtain a limited number of physical descriptors based on polygonal fits such as bounding boxes, circumscribed circles or ellipses etc, they are rarely used for generation of aggregates but are rather used for general evaluation [12]. this paper focuses on the utilization of deeplearning (dl) tool for generation of aggregate shapes in 2d by utilizing a small set of tractable and physically meaningful parameters, which are henceforth termed as geometric descriptors [13] of an aggregate. in this effect, the idea of moment invaraints for image recognition is revisited which was originally proposed in [14] and improved upon in [15]. for more information on image moments, one may refer to [16]. a key highlight in these works is in the use of geometric descriptors that are shape specific and do not change under certain transformations. this idea, as a consequence, leads us to the possibility of further categorizing geometric descriptors 17 https://doi.org/10.14311/app.2025.54.0017 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en k. das, j. sýkora, a. kučerová acta polytechnica ctu proceedings into two distinct classes: first, descriptors that are shape specific and are invariant to certain types of transformations, and second, descriptors that specify the state of the shape, such as size or orientation etc. hence, from hereon, we refer to the first type as shape constant (sc) and the second type as state descriptors (sd). as is demonstrated later, for any shape represented using a set of scs, sd adds a level of tunability for each shape. in the subsequent sections, we focus on moments of binary images that are invariant to both translation and scale, and treat them as shape constants. subsequently, we train a neural network to generate shapes corresponding to these moments. moreover, we condition the network over the area of the shapes such that the network is able to generate similar shapes of varying sizes. this is despite the fact that the data-set is not explicitly augmented to provide any relation between aggregate size and its shape constants. thereby highlighting the ability of the model to learn the mutual independence between the shape invariants and the chosen state descriptor. therefore, in this work, section 2 presents the overall methodology of obtining the shape invariants and details about the neural network architecture that is used. this is followed by the results in section 3 and is finally followed by the conclusion derived from this work in section 4. 2. methodology 2.1. basic theory of image moments computation of image moments relies on defining an image as an intensity distribution function ρ(x, y), which in the case of single-channel binary images is given as i(x, y), where, i(x, y) = 1 if the point (x, y) is in the region of interest or else i(x, y) = 0. as such, the general expression of a generic moment mpq of (p + q)th order is given as: mpq = n p∑ i=0 q∑ j=0 k(x, y)i(x, y), (1) where, n is the normalizing factor and k(x, y) is the moment kernel. for raw-geometric moments, denoted using mpq, n = 1 and k(x, y) = xpyq. the raw moment m00 indicates the area of the region of interest and (x̄, ȳ) gives its centroid such that x̄ = m10/m00, ȳ = m01/m00. since this work particularly focuses on translation and scale invariant moments of aggregates, for any translation invariant moment, also known as central moment, and denoted using µpq, in which case, n = 1 and k(x, y) = (x − x̄)p(y − ȳ)q, the scale and translation invariant moment, also known as normalizedcentral moments are given by: ηpq = 1 µ (1+ p+q 2 ) 00 µpq. (2) s h a p e c o n s t a n t s s t a t e d e s c r i p t o r s encoder head 1 encoder head 2 decoder encoder outputs stack outputdecoder input figure 1. general architecture of the aggregate generation network. consequently, the final expression for scale and translation invariant moments i.e. normalized-central moments is given as: ηpq = 1 µ (1+ p+q 2 ) 00 p∑ i=0 q∑ j=0 (x − x̄)p(y − ȳ)qi(x, y). (3) in the subsequent work, moments derived using equation 3 is used for quantification of aggregate shapes, while m00 is used as the state descriptor for the area. it is to be noted that η00 = 1 and η01 = η10 = 0 and are therefore not used. therefore, the sets of image moments used are given as {m00}, which is the state descriptor (sd) and {η20, η11, · · · , η70, η61, η52, · · · , η07}, which is the set of shape constants (scs). 2.2. neural network for aggregate generation given a set of sd and sc, the objective of this work is to demonstrate the ability of a deep neural network to learn and generate the shape that best fits the description given using and sd and sc. to this effect, a multi-headed autoencoder is implemented that makes use of two encoder-heads to encode the sc and sd respectively. the outputs of each encoder head are combined and used as input for a convolution-based decoder section. the overall architecture of the model is presented in figure 1. each of the encoder heads is composed of a series of fully-connceted layers, each of which apply an affine transform to the incoming vector data. for any incoming vector u, such an affine transform is given by v = uat + b, where v is the output vector and b is a vector of constants, also known as bias. each of the fully-connected layers is followed by a rectified linear unit (relu), which imparts non-linearity to the model. relu is defined as the non-negative part of any argument and as such, relu(x) = x if x ≥ 0 and 0 otherwise. the transformation matrix a and the bias vector b is optimized through training. details about the encoder head 1 is given in table 1 and encoder head 2 is given in table 2. both the encoders are designed to have output vectors of 18 vol. 54/2025 tractable descriptors for digital aggregate generation layer in/out-features activation func. fully connected 1/1 024 relu fully connected 1 024/1 024 relu table 1. architecture of encoder head 1. layer in/out-features activation func. fully connected 33/2 048 relu fully connected 2 048/2 048 relu fully connected 2 048/1 024 relu fully connected 1 024/1 024 relu table 2. architecture of encoder head 2. equal size. each of the output vectors are re-shaped into a matrix and concatenated to obtain a tensor of shape [2, 32, 32], which forms the input for the decoder. the first dimension here is termed as the feature dimension and the remaining dimensions are the y and x directions in the cartesian coordinate system respectively. the decoder comprises of alternating transposedconvolutions and convolution layers [17]. transposed 2d-convolution layers with a stride of 2 and kernels k of size of (4,4) is used to up-sample the images to twice the spatial dimension of the input, while the convolution layers are designed to preserve the size of the output. the depth of the decoder is adjusted such that the input to the decoder is 32 pixels in both dimensions and the output is 512 pixels in both dimensions with a feature dimension of size 1, which in this case necessitates the use of four transposed convolution layers. details about the decoder is presented in table 3. finally, the resulting model is trained to obtain a set of parameters θ. inferencing using this model is subsequently performed by using this fixed set of parameters θ such that given a pair of shape constants and state descriptors, the model is able to generate an approximate 2d aggregate indicated by these quantities. the overall workflow for training the model is presented in figure 2 and the training details is provided in section 2.4. 2.3. aggregate dataset the image dataset consisted of binary images, each containing a single aggregate. aggregates were represented with a pixel value of 1, while 0 indicated the background. the image dimensions were adjusted to the smallest power of 2 that could accommodate the largest aggregate, resulting in each image measuring 512 × 512 pixels. each 1 mm of spatial dimension corresponded to 10.24 pixels. examples of training samples are shown in figure 3. training data comprised (x, y ) pairs, where x represented the input data and y the target. each x consisted of state descriptors (sd) and shape constants (sc). sd corresponded to the area of the aggregates, stop? 𝜽 update parameters target shape predicted shape no yes mse loss d e s c r i p t o r s figure 2. training workflow based on supervised learning scheme. figure 3. example of training dataset containing 8 500 samples. represented by m00, and sc comprised normalized central moments, η20, η11, · · ·, up to the order (p + q) = 7. the target y was the binary image of the aggregate from which x was derived. a total of 8 500 labeled (x, y ) pairs were used for training, while a separate subset of samples was reserved for testing and excluded from training. additionally, no data augmentation was applied to supplement the original dataset. 2.4. training the workflow was implemented using the pytorch framework [18]. the model was trained with the adam optimizer [19] at a learning rate of 0.001, with cosine annealing applied for learning rate scheduling over 96 epochs. the mean squared error was calculated between the model’s output and the target image as the loss function. training was conducted using minibatches of 20 samples. hyperparameters, including the learning rate, batch size, and optimizer settings, were experimentally determined to achieve optimal performance. the entire training process took approximately 2 hours on a workstation equipped with an nvidia rtx 4090 gpu and an amd ryzen9 7950x cpu. inferencing with the trained model typically required about 1 minute for approximately 440 samples. 3. results for the results presented in figure 4, the neural network was applied to reconstruct the aggregates us19 k. das, j. sýkora, a. kučerová acta polytechnica ctu proceedings layer in/out-channels kernel stride padding activation function transposedconv2d 2/512 (4 × 4) 2 1 relu conv2d 512/512 (3 × 3) 1 1 relu transposedconv2d 512/256 (4 × 4) 2 1 relu conv2d 256/256 (3 × 3) 1 1 relu transposedconv2d 256/128 (4 × 4) 2 1 relu conv2d 128/128 (3 × 3) 1 1 relu transposedconv2d 128/64 (4 × 4) 2 1 relu conv2d 64/64 (3 × 3) 1 1 relu conv2d 64/64 (1 × 1) 1 0 relu table 3. neural network architecture of decoder with corresponding convolutional layers. original generated figure 4. reconstruction of original aggregates. original generated 0.25x generated 0.75x generated 1.25x generated 1.75x figure 5. microstructure generator of aggregates based on the scaled area of the original structure. ing geometric descriptors obtained from the original images. it is evident that the model can successfully reconstruct the aggregate shapes using only the 34 quantities. next, to test whether the model could recognize the scale-invariant property of the normalized central moments, the model was applied to the same set of descriptors where only the area, i.e. m00 was scaled by a factor ranging from 0.25 to 2.0 in increments of 0.25. the results are presented in figure 5. it is clear that the model can recognize the invariance between aggregate size and its moments. notably, the dataset was not augmented to account for the scaling invariance of the moments, meaning this property was implicitly learned by the model. the final results presented in figures 4 and 5 were generated by applying the neural network to a set of descriptors that were not previously used for training. due to the requirement of backpropagation during training, the output of the neural network is a grayscale image, with high pixel values indicating aggregate and low pixel values indicating their absence. to obtain the final binarized result, the model outputs were thresholded using otsu’s method, which is readily implemented in the scikit-image library [20]. the quality of the generated aggregates was assessed by comparing the moments of the original aggregates to those of the generated aggregates. these results are shown in figure 6 for the various scaling factors that were used. the scaling error is presented in figure 7, 20 vol. 54/2025 tractable descriptors for digital aggregate generation figure 6. model performance for the various scaling factors evaluated as a mean squared error (mse) of normalized central moments. figure 7. scaling error analysis showing the model’s tendency to generate larger aggregates for scaling factors less than 1 and vice versa. where the area of the generated aggregates is compared to the correct area upon scaling. it is evident that the model tends to generate larger aggregates for scaling factors < 1, while for scaling factors > 1, it produces aggregates with smaller-than-correct areas. this may be attributed to the lack of explicit training on a dataset augmented for scale invariance of the moments. the final study investigates how different area scaling factors affect the overall pixel wise accuracy. the results depicted in figure 8 represent the values of the mean squared error, measuring the deviation of overlap between the predicted and original aggregates. the errors have been adjusted for scaling by dividing the mse-error with the square of the scaling factor. overall, the model demonstrates good fit between the original aggregates and the predicted aggregates for a scaling factor of 1 with considerable deviation from the ideal values for other scaling factors. however, such deviation is observed to be sample specific figure 8. error assessment based on mse exploring the overlap differences between predicted and original aggregates. with the deviation being spread over a wide range of values, with the model performing well in some cases. nevertheless, as has already been highlighted, such a result is expected since the model was not explicitly trained to factor for scaling. 4. conclusion this paper presents a method for generating aggregate shapes in 2d using a deep-learning tool and a small set of tractable and physically meaningful parameters derived from moment invariant analysis. this approach acts as the initial step in developing a microstructure generator of heterogeneous materials that relies solely on the inputs which have clear and interpretable meanings describing material morphology. the method demonstrates the ability of a neural-network model to recognize scale variations in input prompts and adjust output sizes accordingly, despite the absence of explicit training for scale invariance. explicit training for this aspect is expected to enhance the results further. this method is designed with the objective of being integrating with aggregate packing schemes for microstructure generation of concretes. while the current results are focused on 2d structures, the concept can be easily extended to 3d problems, offering broader applicability in the study and design of concrete microstructures. acknowledgements the authors are thankful for financial support from the czech science foundation, project no. 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https://doi.org/10.1016/j.engfracmech.2020.107029 https://doi.org/10.1016/j.jcp.2016.10.070 https://doi.org/10.1016/j.pmatsci.2018.01.005 https://doi.org/10.1016/j.cma.2023.115901 https://doi.org/10.1115/1.4029768 https://doi.org/10.1109/tit.1962.1057692 https://doi.org/10.1016/0031-3203(93)90098-h https://doi.org/10.15579/gcsr.vol1.ch1 https://doi.org/10.1007/978-3-319-10590-1_53 https://doi.org/10.48550/arxiv.1412.6980 https://doi.org/10.7717/peerj.453 acta polytechnica ctu proceedings 54:17–22, 2025 1 introduction 2 methodology 2.1 basic theory of image moments 2.2 neural network for aggregate generation 2.3 aggregate dataset 2.4 training 3 results 4 conclusion acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2024.49.0001 acta polytechnica ctu proceedings 49:1–7, 2024 © 2024 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague experimental dynamic analysis of the footbridge across jizera river in mladá boleslav miroslav čápa,c, vladimír šánab,∗, michal poláka, tomáš plachýa a czech technical university in prague, faculty of civil engineering, department of mechanics, thákurova 7, 160 00 prague, czech republic b czech technical university in prague, faculty of civil engineering, experimental centre, thákurova 7, 160 00 prague, czech republic c pontex consulting engineers, ltd., bezová 1658, 147 14 prague, czech republic ∗ corresponding author: vladimir.sana@fsv.cvut.cz abstract. the text of this submitted paper is devoted to the experimental dynamic analysis of the newly designed footbridge across the jizera river in mladá boleslav. theoretical modal analysis has shown potential risk that some of the natural frequencies of the bridge deck will belong to the range which is typical for pacing frequencies induced by pedestrians. the resonance behaviour of this structure should be reduced by tuned mass dampers (tmd), which would be tuned for separate natural frequencies of this structure. therefore, the experimental dynamic analysis was performed on the footbridge in order to assess the effectiveness of installed tmds. the experiment was divided into two stages, the first one was realized at the footbridge when tmds were not yet installed, and the second one was carried out on the footbridge with installed and activated tmds. moreover, the authors have performed an experimental modal analysis in order to verify the aptness of the computational model and its results. keywords: experimental dynamic analysis, dynamic load test, experimental modal analysis, humaninduced vibration, vandalism. 1. introduction as can be seen from the number of contributions and participants, who regularly presents their papers at the international conferences dealing with the structural dynamics, such as eurodyn, imac, etc., and in technical journals, dynamic behaviour (obtained by either numerical predictions or in-situ experiments) of the footbridges is still at the foreground of international researchers community interest. the newly built structures are designed still more and more subtle thanks to the advanced computational procedures and modern materials. the effort to create more slender and subtle structures is caused in particular by a desire of architects to not break the view of the surrounding landscape. the combination of slenderness, low value of damping, physical properties of used materials, such as mass and stiffness, and static system of the superstructure lead very often to the fact, that some of the natural frequencies are scattered very closely in the region of the pacing frequencies induced by pedestrians. on the other hand, the artistic efforts of the architects create a natural pressure on designers (in the sense of numerical calculations and choice of a correct mathematical model of pedestrian, vandal, etc.), who must properly and with a reasonable measure of the accuracy determine the response of the individual members of the footbridge load-bearing structure. the final maximal level of vibration in the dimension of acceleration is used for structural assessment with respect to pedestrians’ comfort. the maximal values are usually less suitable for a footbridge experimental assessment since their value can be significantly affected by sudden impulse loading, such as stamping in the close vicinity of the used accelerometers, etc. therefore, some guidelines and standards recommend rms (root mean square) values of acceleration for assessing a comfort level of a footbridge structure with respect to pedestrians. in literature, one can find a lot of sources, where authors designed tmd on existing or newly built structures. in [1], authors designed and assessed a tmd on the cable-stayed footbridge. ferreira et al. in [2] proposed a design of a tmd and satmd (semi-active tuned mass damper) on the infinity bridge in stockton, great britain, which is an arch footbridge across river tees. the satmd was used as an efficient tool to prevent the lock-in effect, which is in this case lateral instability of the footbridge induced by a critical number of synchronous pedestrians in the lateral direction. a numerical and experimental study of simple structure with friction tmd was addressed by eliecer et al. in [3] in order to reduce the vibrations of the floor. the bridge deck vibration of the lively footbridge across motlawa river (gdansk, poland) was reduced by high coefficient tmd, see [4]. the suspended footbridge żabia kładka in wrocław and cable-stayed footbridge in poznań, which have been enriched by tmds, were studied in [5]. optimal parameters of tmds, such as their mass, stiffness, and 1 https://doi.org/10.14311/app.2024.49.0001 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en m. čáp, v. šána, m. polák, t. plachý acta polytechnica ctu proceedings figure 1. a view of the investigated footbridge in the direction of stationing. damping properties, can be found e.g. in [6] or [7]. sometimes, the situation requires and permits the usage of tld (tuned liquid damper), where the kinetic energy of the vibrating structure is absorbed by the inertia of moving liquid, see e.g. [8]. 2. description of the investigated structure for the purposes of the presented paper, we have chosen the footbridge, which is located in mladá boleslav (czech republic) and serves to pedestrian and cycling traffic as a part of the cycling route a1. a view of the structure in the direction of stationing is depicted in figure 1 and figure 2. the experimentally investigated footbridge is steel trussed structure with an orthotropic bridge deck, see figure 3, and with two simply-supported spans, which is mounted on two massive abutments and a reinforced-concrete pillar. the length of individual spans are l1 = 68.40 m and l2 = 23.68 m, see figure 4. width of the superstructure of 4.60 m is constant along the entire structure, see figure 2. the footbridge centreline is direct in the horizontal direction and curved in the vertical plane with a radius of r = 811.541 m, see figure 1 and figure 2. the main beams have been constructed from two trussed beams made of s355j2+n and s355j2h category steel. top and bottom box-sectional chords with outer dimensions of 250 × 200 mm are welded to an arc. the thickness of the flanges and webs at the bottom chord is 15 mm. the top chord has a thickness of webs 15 mm and flanges of 15 mm, 20 mm and 25 mm. both truss beams are rigidly connected by horizontal box-sectional bracing with dimensions of 250 × 200 mm. the height of the main beams is variable with respect to the longitudinal axis. the webs of the trussed beams are a combination of rigid tubes, box-sectional rods, providing the shape stability of the top chords, and pair of prestressed rectifiable rods. the bridge deck was designed from a 10 mm plate reinforced with a system of longitudinal and transversal stiffeners with plate profiles and inverted t profiles. the profiles were added to the places with the premised locations of tuned mass dampers (tmds). figure 2. a view of the bridge deck in the direction of stationing. figure 3. a cross-section of the investigated footbridge structure – support [10]. generally, 8 tmds were designed in the theoretical dynamic analysis, see [9]. the distance between separate cross-girders with inverted t profiles is 3000 mm and the mutual spacing of longitudinal plate stiffeners is 360 mm. the distance between inverted t profiles and plate stiffeners at the places, where the inverted t profiles were added, is 270 mm. 3. experiment the in-situ experiment was divided into two phases. while the first stage was focused on the determination of the natural frequencies, relevant mode shapes of the superstructure (experimental modal analysis, informative dynamic test), and the response of the superstructure without installed tmds, the second stage was primarily aimed at verification of the effectiveness of the tuned mass dampers (tmds) after 2 vol. 49/2024 experimental dynamic analysis of the footbridge . . . figure 4. a longitudinal section of the investigated footbridge structure [10]. figure 5. a view at sensor placement. figure 6. a view at sensor placement in a longitudinal section. their installation and activation for a purpose of decreasing a level of vibration of the bridge deck. the dynamic forces were produced by diversely formed groups of pedestrians. the groups of pedestrians used in the second stage corresponded to the groups from the first stage. thus the results from both stages were mutually comparable and we were able to determine the effect of activated tmds. response of the structure was observed in the preselected mesh of points by accelerometers, which were adjusted by steel weights and placed in the correct position. these used accelerometers were piezoelectric seismic sensors of type 8344 (brüel&kjær). the sensors were adjusted to the top chord and on the steel weights directly by neodymium magnets, see figure 5. sensitivity of the accelerometers is 2500 mv g−1 with a frequency range of 0.2 hz–3 khz. eight-channel vibration control stations sirius 6acc – 2acc+ and sirius 8acc have been used to collect seismic sensor data. 3.1. experimental modal analysis experimental modal analysis (dynamic informative test within the meaning of standard čsn 73 6209 [11]) was focused on the determination of natural vibration characteristics of the empty structure. the logarithmic decrement, dominant natural frequencies, and corresponding global mode shapes belong among these characteristics. the structure was excited by regularly jumping person – ambient vibration test (avt) in a predefined spot to excite all desired mode shapes. the dash-and-dot line, depicted in figure 5, represents the approximate spot of the jumping person. the eccentricity of this spot permitted the excitation of potential torsion mode shapes. figure 6 represents the placement of individual measured sections in the longitudinal direction. reference sensors were placed in points 62 and 102 (measurement in y and z directions) throughout the experimental modal analysis. besides this, the next reference sensor was placed in point 104 (measurement in the y direction). during the experimental modal analysis of the second span, the reference sensors were located in point 222 (measurement in the y and the z directions). experimentally obtained data of the time behaviour of the oscillating bridge deck were stored on the hard drive and subsequently processed by fast fourier transform (fft), which transforms the original signal from the time domain to the frequency domain, and natural frequencies are depicted as local peaks. the width of the peak at a specific height is related to the damping connected with this natural frequency. 3 m. čáp, v. šána, m. polák, t. plachý acta polytechnica ctu proceedings figure 7. the example of complex mode indicator function (the black line – evaluated across all measured points in x, y, and z directions, the green curve in the x direction, the red line in the y direction, and the blue curve for the z direction). thanks to the fact, that the force of excitation was produced by a jumping person, frequency response spectra in individual points on the bridge deck have been used for the determination of operating deflection shapes frequency response functions (odsfrf or odshkr(if)). odshkr(if) is generally a complex function consisting of real and imaginary part. since the individual points have been fitted by sensors at different times and therefore the level of excitation force was not constant, the transmissibility functions tkr(if) were determined as well. transmissibility functions permit precise evaluation of the mode shapes. these functions were determined by the equation: tkr = ẅk(if) ẅr(if) , (1) where i means an imaginary unit, ẅk(if) stands for the response (acceleration) of the structure in point k in the frequency domain, ẅr(if) denotes the response (acceleration) of the structure in reference point r in the frequency domain. the natural frequencies and mode shapes were evaluated in the software me’scope ves. the example of evaluated complex mode indicator function with depicted natural frequencies is presented in figure 7. 3.2. dynamic load test according to the standard čsn 73 6209 [11], we have tested such an arrangement of pedestrians, which was in accordance with the requirements of this standard. an ordinary traffic is usually simulated by: • random footbridge crossing with a pedestrian flow density of the same order as the density during standard use of the structure, • excitation of torsional or bending vibration; two synchronized pedestrians stepping on the same foot at the same time; pacing frequency according to natural frequencies of empty structure, • excitation of lateral vibration; two synchronized pedestrians stepping on the same foot at the same time; pacing frequency according to natural frequencies of empty structure. in addition to the ordinary traffic, other form of pedestrian excitation of the footbridge (for example running joggers, swaying and bobbing vandals) were also investigated. the walking pedestrians, running joggers and swaying/bobbing vandals have been synchronized by digital metronome. 3.2.1. the first stage – inactivated tmd on the basis of the previous experimental modal analysis, we have chosen the following pacing frequencies, which were the same as some of the natural frequencies: the first span l1 = 68.40 m • f(2) = 2.15 hz – the first shape of vertical bending vibration; fp = 2.15 hz, fv = 2.15 hz, • f(4) = 2.58 hz – the first shape of torsional vibration; fp = 2.58 hz, • f(5) = 3.44 hz – the third shape of lateral bending vibration (top chords of main beams); fp = 1.73 hz, fp = 3.44 hz fv = 1.72 hz, fv = 3.44 hz, • f(6) = 3.94 hz – the shape of lateral bending vibration; fp = 1.97 hz, fv = 1.97 hz, fv = 1.95 hz, • f(7) = 4.16 hz – the second shape of vertical bending vibration (top chords of main beams); fp = 2.08 hz, fv = 2.08 hz. the following compositions of loading groups have been used: 4 vol. 49/2024 experimental dynamic analysis of the footbridge . . . j 1st span f(j) [hz] 2nd span f(j) [hz] 1 1.16 5.27 2 2.15 5.92 3 2.27 7.31 4 2.58 8.42 5 3.45 11.46 6 3.94 11.92 7 4.15 13.87 8 4.68 15.59 9 6.04 17.21 10 6.11 table 1. natural frequencies of the first and the second spans during the first stage (inactivated tmd). • simulation of an ordinary traffic: 4 pedestrians (random crossing), • synchronized walking/running: 2 pedestrians side by side, • vandalism: 4 qualified vandals. the second span l2 = 23.68 m • f(1) = 5.28 hz – the first shape of vertical bending vibration; fp = 1.76 hz, fp = 2.64 hz, fv = 1.76 hz, fv = 2.64 hz. the following compositions of loading groups have been used: • simulation of ordinary traffic: 5 pedestrians (random crossing), • synchronized walking/running: 2 pedestrians side by side, • vandalism: 4 qualified vandals. in the previous list, f(k) denotes k-th natural frequency, fp stands for exciting frequency produced by pedestrians, and finally, fv means exciting frequency produced by vandals. 3.2.2. the second stage – activated tmd the first span l1 = 68.40 m • f(2) = 2.15 hz – the first shape of vertical bending vibration; fp = 2.15 hz, fv = 2.15 hz, • f(4) = 2.47 hz – the first shape of torsional vibration; fp = 2.47 hz, • f(5) = 3.42 hz – the third shape of lateral bending vibration (top chords of main beams); fp = 1.71 hz, fp = 3.42 hz fv = 1.71 hz, fv = 3.42 hz, • f(6) = 3.94 hz – the shape of lateral bending vibration; fp = 1.95 hz, fp = 1.98 hz, fv = 1.95 hz, fv = 1.98 hz, • f(7) = 4.16 hz – the second shape of vertical bending vibration (top chords of main beams); fp = 2.08 hz, fv = 2.08 hz. the following compositions of loading pedestrian groups have been used: • simulation of ordinary traffic: 8–9 pedestrians (random crossing), • synchronized walking/running: 2 pedestrians side by side, • vandalism: 4 qualified vandals. the second span l2 = 23.68 m • f(1) = 5.27 hz – the first shape of vertical bending vibration; fp = 1.76 hz, fp = 2.64 hz, fv = 1.76 hz, fv = 2.64 hz, • f(2) = 5.92 hz – the first shape of vertical bending vibration; fp = 1.98 hz, fp = 2.97 hz, fv = 1.98 hz, fv = 2.97 hz, • fp = 2.30 hz, fv = 2.30 hz. the following compositions of loading groups have been used: • simulation of ordinary traffic: 4 pedestrians (random crossing), • synchronized walking/running: 2 pedestrians side by side, • vandalism: 3 qualified vandals. 4. results this text summarizes obtained and evaluated results from the in-situ experiment, which was focused on experimental modal analysis and forced vibration (dynamic load test). the table 1 and table 2 present evaluated natural frequencies of the footbridge during the first stage. an example of the second mode shape of the first span is graphically depicted in figure 8. table 3 summarizes the important values of damping. these values were determined for the first vertical bending frequency at the first span and for the first vertical bending frequency at the second span. the following tables (table 4, table 5) present the results of maximal rms values of acceleration at the first span because dominant values of measured dynamic response were observed right there. 5 m. čáp, v. šána, m. polák, t. plachý acta polytechnica ctu proceedings j 1st span 2nd span 1 lateral bending – top chords vertical bending – bridge deck 2 vertical bending – bridge deck lateral bending – top chords 3 lateral bending – top chords torsional – bridge deck 4 torsional – bridge deck torsional – bridge deck 5 lateral bending – top chords lateral bending – top chords 6 lateral bending – bridge deck vertical bending – bridge deck 7 vertical bending – bridge deck lateral bending – top chords 8 lateral bending – top chords vertical bending – bridge deck 9 lateral bending – top chords torsional – bridge deck 10 vertical bending – bridge deck table 2. description of the individual evaluated mode shapes (inactivated tmd). figure 8. the second mode shape f(2) = 2.15 [hz] – the first span (inactivated tmd). span f [hz] ϑ [-] ξ [%] 1st 2.15 0.031 0.49 2nd 5.27 0.061 0.97 table 3. values of logarithmic damping decrement ϑ. 5. conclusion the submitted paper presents the results of the experimental dynamic analysis of the newly built footbridge across the jizera river in mladá boleslav. the calculations, in the design phase, have shown that the structure must be supplemented by tmds to decrease vibrations induced by pedestrians. the experiment should prove whether the installed absorbers are effective or not. as can be seen from table 4 and table 5, the absorbers helped significantly to decrease the level of vibration, which is the most noticeably evident for vandalism at a frequency of 1.95 hz and synchronized running pedestrians at a frequency of 3.44 hz. acknowledgements the financial support has been provided by grant no. sgs22/089/ohk1/2t/11 of the czech technical university in prague, which is gratefully acknowledged. description freq. [hz] spot max [m s−2] ordinary traffic 102-z 0.170 syn. ped. 1.73 102-z 0.089 syn. ped. 1.97 102-z 0.123 syn. ped. 2.08 102-z 0.258 syn. ped. 2.15 102-z 0.245 syn. ped. 2.58 102-z 0.250 syn. ped. 3.44 102-z 0.572 vandals 1.72 102-z 0.117 vandals 1.95 102-z 2.123 vandals 1.97 102-z 1.376 vandals 2.08 102-z 0.340 vandals 2.15 102-z 0.491 vandals 3.44 102-z 0.205 table 4. maximal rms values for individual exciting frequencies – the first stage (inactivated tmd). description freq. [hz] spot max [m s−2] ordinary traffic 102-z 0.111 syn. ped. 1.71 102-z 0.062 syn. ped. 1.79 102-z 0.078 syn. ped. 1.95 102-z 0.086 syn. ped. 1.98 102-z 0.072 syn. ped. 2.08 102-z 0.085 syn. ped. 2.15 102-z 0.111 syn. ped. 2.47 102-z 0.158 syn. ped. 3.42 102-z 0.276 vandals 1.71 102-z 0.141 vandals 1.79 102-z 0.270 vandals 1.95 102-z 0.193 vandals 1.98 102-z 0.153 vandals 2.08 102-z 0.114 vandals 2.15 102-z 0.143 vandals 2.47 102-z 0.211 vandals 3.42 102-z 0.470 table 5. maximal rms values for individual exciting frequencies – the second stage (activated tmd). 6 vol. 49/2024 experimental dynamic analysis of the footbridge . . . references [1] d. makovička, m. studničková, d. makovička, j. korbelář. dynamický tlumič na lávce pro chodce [in czech; dynamic absorber on a footbridge]. stavební obzor 12(5):149–155, 2003. [2] f. ferreira, c. moutinho, á. cunha, e. caetano. use of semi-active tuned mass dampers to control footbridges subjected to synchronous lateral excitation. journal of sound and vibration 446:176–194, 2019. https://doi.org/10.1016/j.jsv.2019.01.026 [3] j. e. c. carmona, s. m. avila, g. doz. proposal of a tuned mass damper with friction damping to control excessive floor vibrations. engineering structures 148:81–100, 2017. https://doi.org/10.1016/j.engstruct.2017.06.022 [4] k. zoltowski, a. banas, m. binczyk, p. kalitowski. control of the bridge span vibration with high coefficient passive damper. theoretical consideration and application. engineering structures 254:113781, 2022. https://doi.org/10.1016/j.engstruct.2021.113781 [5] w. fiebig. reduction of vibrations of pedestrian bridges using tuned mass dampers (tmd). archives of acoustics 35(2):165–174, 2010. https://doi.org/10.2478/v10168-010-0015-3 [6] j. máca. dynamic response of footbridges with tuned mass dampers. iop conference series: materials science and engineering 236:012060, 2017. https://doi.org/10.1088/1757-899x/236/1/012060 [7] h. bachmann, w. j. ammann, f. deischl, et al. vibration problems in structures: practical guidelines. 1. birkhäuser basel, 2011. https://doi.org/10.1007/978-3-0348-9231-5 [8] m. pirner, s. urushadze. liquid damper for suppressing horizontal and vertical motions – parametric study. journal of wind engineering and industrial aerodynamics 95(9–11):1329–1349, 2007. https://doi.org/10.1016/j.jweia.2007.02.010 [9] s. hračov, d. gregor, v. janata, p. nehasil. dynamické posouzení lávky [in czech; dynamic assessment of a footbridge]. excon a.s., 2021. [10] t. lindtner, d. šindler, o. dědek, v. hvízdal. so201 lávka přes jizeru (000 – přehledné výkresy) [in czech; plans (cross sections and lontitudinal sections)]. pontex, ltd., 2022. [11] český normalizační institut. čsn 73 6209. zatěžovací zkoušky mostních objektů [in czech; loading tests on bridges], 2019. 7 https://doi.org/10.1016/j.jsv.2019.01.026 https://doi.org/10.1016/j.engstruct.2017.06.022 https://doi.org/10.1016/j.engstruct.2021.113781 https://doi.org/10.2478/v10168-010-0015-3 https://doi.org/10.1088/1757-899x/236/1/012060 https://doi.org/10.1007/978-3-0348-9231-5 https://doi.org/10.1016/j.jweia.2007.02.010 acta polytechnica ctu proceedings 49:1–7, 2024 1 introduction 2 description of the investigated structure 3 experiment 3.1 experimental modal analysis 3.2 dynamic load test 3.2.1 the first stage – inactivated tmd 3.2.2 the second stage – activated tmd 4 results 5 conclusion acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2025.53.0042 acta polytechnica ctu proceedings 53:42–48, 2025 © 2025 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague parametric study of a reinforced concrete footbridge strengthened by a layer of uhpfrc petr kněž, milan holý∗, jiří kolísko czech technical university in prague, klokner institute, šolínova 7, prague 166 08, czech republic ∗ corresponding author: milan.holy@cvut.cz abstract. this paper deals with the presentation of a parametric study of the strengthening of a reinforced concrete footbridge with a layer of reinforced uhpfrc on its upper surface. in order to increase its load carrying capacity, stiffness and durability, the design of strengthening with 50 mm thick uhpfrc layer was carried out. the geometry of the analyzed structure is based on the real structure. the parametric study solves its strengthening by considering different support options of the structure and its degradation. the design and assessment was carried out in the form of nonlinear numerical analysis using the fem in atena 3d. keywords: uhpfrc, footbridge, reinforced concrete, strengthening, degradation, fea. 1. introduction the parametric study of the effect of strengthening [1–6] the reinforced concrete structure with a uhpfrc [7] layer was based on an existing reinforced concrete footbridge. the existing footbridge is located in the tank of a chemical plant and shows structural damage after years of operation. the footbridge is used to access the platform, which is located in the centre of the chemical tank on 4 circular columns. the footbridge is supported at one end by a central platform and at the other end by the tank wall. both supports are considered to be fixed. for the parametric study, two other methods of supporting the footbridge were considered. the first considered a fixed support at the location of the central platform and an hinged support at the tank wall. the second method of support considered a hinge at each end of the footbridge. the structure is symmetrical, see figure 1. the parametric study was focused on the evaluation of the influence of the 50 mm thick reinforced uhpfrc layer on the upper surface of the reinforced concrete structure. the load bearing capacity as a function of the degradation of the original structure and the boundary support conditions was investigated. the load bearing capacity of the strengthened structure was compared with the load bearing capacity of reinforced concrete structure without degradation. the study was carried out using nonlinear fea [8–10] in atena 3d software. the footbridge has a length of 8.28 m and a rectangular cross-section with a width of 1.2 m and a height of 0.6 m. one end of the footbridge is embedded in a central reinforced concrete platform. the footbridge is reinforced at both surfaces with 6 �22 mm bars, which are supplemented by �8 mm stirrups at a distance of 400 mm. there is a 50 mm layer of concrete on the footbridge, which serves as a footing and protective layer for the structure. this layer will be replaced figure 1. longitudinal sections and structural diagrams of the considered structural variants of the footbridge. by a uhpfrc layer to strengthen the structure. the footbridge is made of c16/20 class concrete and the reinforcing bars are made of 10 216 (e) class steel. the design of the footbridge and its reinforcement is shown in figure 1. the degradation of the reinforced concrete structure includes a reduction of the modulus of elasticity and strength characteristics of the concrete as well as 15 % reduction of the reinforcement cross-sectional area due to corrosion. the new layer made of uhpfrc class c120 fr6 with 2 % steel fibre content is then reinforced with a welded 150/150 mesh made of �6 mm bars in the axis of the slab with a thickness of 50 mm. the coupling to the footbridge slab is carried out by the use of 42 https://doi.org/10.14311/app.2025.53.0042 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 53/2025 parametric study of a reinforced concrete footbridge strengthened by a layer of uhpfrc material e ν fc ft ρ [gpa] [-] [mpa] [mpa] [kg m−3] c16/20 mean 29 0.2 24 1.9 2 300 c16/20 degraded 18 0.2 12 1 2 300 c120 fr6 40 0.2 132 6.0 2 600 table 1. material characteristics for concrete and uhpfrc. material e fy fu ϵlim ρ [gpa] [mpa] [mpa] [-] [kg m−3] b500 200 500 578 0.05 7 850 10216 (e) 200 210 500 0.25 7 850 table 2. material characteristics for concrete and uhpfrc. �8 mm coupling studs in the number of 8 pcs m−2 and alternatively with shear notches. 2. specifications of the parametric study the basic division of the study is into three groups based on the boundary conditions of the footbridge support. the first group consists of the footbridge fixed at both ends, the second group consists of the footbridge hinged at one end and fixed at the other. the last group consists of a footbridge with hinges at both ends. for each support option, several design settings are analysed. the load bearing capacity of the structure in the ideal state without strengthening is verified as a reference state. the ideal condition represents a structure without concrete degradation and without reinforcement corrosion. furthermore, the load bearing capacity of the variously degraded structure without and with strengthening is verified. the degradation of the reinforced concrete footbridge always consists of a reduction in the material properties of the concrete (modulus of elasticity, tensile strength and compressive strength), which is combined with a 15 % reduction of the reinforcement cross-sectional area, which represent reduction due to corrosion. variants considering a degraded structure are subsequently strengthened with a layer of uhpfrc of thickness 50 mm, which replaces the original concrete layer covering the footbridge surface. the uhpfrc layer is considered with or without full shrinkage. full shrinkage is an option for monolithic construction, construction without shrinkage is an option for possible prefabrication. its coupling to the reinforced concrete footbridge is created by using coupling studs and, alternatively, in combination with shear notches. thus, 5 analyses are performed for each variant of boundary conditions. 3. numerical analysis all of the above mentioned variants of the parametric study are analyzed in atena 3d. this is a program that allows nonlinear fea, specialised in concrete structures, where it also allows the monitoring of the possible development of cracks. the geometric, material and contact types of nonlinearities are considered in the calculations. 3.1. materials and material models for the purpose of the analysis, several materiel types are considered using different material models [11]. the material model “cc3d cementitious 2” for normal concrete c16/20 is used and “cc3d cementitious 2 – user” for uhpfrc c120 fr6 with scattered reinforcement is used. the material property values are given in table 1. the second group of materials are steel reinforcing bars and coupling studs. the concrete reinforcement is made of steel class 10216 (e), for the coupling studs and reinforcement of the uhpfrc slab, reinforcement of class b500b is considered. for both steel types the material model “cc3d reinforcement” [11] is used. the values of the material properties are given in table 2. 3.2. numerical models five models were created in software atena 3d for each variation of the boundary conditions of the footbridge supports. for the footbridge with hinged support at both ends(figure 5), the follow-up part of the reinforced concrete platform is not considered in the model. the following models were created: a) the first model consisted of a reinforced concrete footbridge with ideal material properties and no corrosion loss of the reinforcement. the dimensions and reinforcement corresponds to that shown in chapter 1 (figure 2). b) the second model differs from the first only in the material characteristics of the concrete used (degraded concrete) and in the consideration of 15 % reduction of the reinforcement cross-sectional area due to corrosion. 43 p. kněž, m. holý, j. kolísko acta polytechnica ctu proceedings figure 2. numerical model of the reinforced concrete footbridge and part of the platform with reinforcement. figure 3. numerical model with uhpfrc slab, its reinforcement and coupling studs. figure 4. numerical model of reinforced concrete footbridge connected to a uhpfrc slab by shear notches. figure 5. symmetrical half of the hinge-supported version of the footbridge. c) the third model is based on the second model, which has been strenghtened with a uhpfrc slab that is coupled to the reinforced concrete structure by using coupling studs. shrinkage is considered for the uhpfrc slab (figure 3). d) the fourth model is identical to the third model, except that shrinkage of the uhpfrc slab is not considered. e) the fifth model is again a strenghtened degraded reinforced concrete footbridge with a uhpfrc layer, in this case the coupling is created by a combination of shear notches and coupling studs. shrinkage was not considered for this model (figure 4). 3.3. loads in the calculation, the loads were divided into three types: • the first type of load was the dead weight of the structure, which is calculated automatically based on the specified volumetric mass of concrete (2 400 kg m−3), uhpfrc (2 600 kg m−3) and reinforcing bars (7 850 kg m−3). • the second type of load is the remaining dead load. this is only considered for models without a uhpfrc slab where there is a 50 mm layer of concrete on the footbridge – for these models a load of 1.25 kn m−2 is considered. • the third type of load is the service load of the struc44 vol. 53/2025 parametric study of a reinforced concrete footbridge strengthened by a layer of uhpfrc ture. this is applied in increments of 1 kn m−2 until the analysis fails. in addition to the maximum load values before failure, the deformations for a service load of 5 kn m−2, which is the standard load value [12], are also monitored. 4. results of the parametric study the results of the parametric study of the strengthened reinforced concrete footbridge for different boundary support conditions will be presented in the form of tables. the tables show the deflection uq corresponding to the model loaded with the service load. they will also contain the maximum load qmax and the total deflection utot. the chapter will be subdivided into sections according to the boundary conditions of the footbridge support into a section for a footbridge fixed at both ends, a footbridge with one hinge support and a footbridge with a hinge at both ends. the tables will then label the different design options a to e according to the list of numerical models in section 3.2. 4.1. footbridge fixed at both ends from the analysis of the footbridge in its ideal state, we obtained information on the critical cross-section of the structure (figure 6). the results of this design option of the numerical model (a) serve as a reference for the other design options. figure 6. cracks on the footbridge fixed at both ends, the critical section is at the point where the footbridge is embedded in the tank wall (left support). near left support has occured failure for all design options. strenghtening variants using only shear studs did not have a significant effect on the resulting stiffness of the structure and these variants also show a possible effects of shrinkage. the stiffness was increased only for the variant using shear notches for coupling. in the case of the evaluation of the maximum load bearing capacity, for the variants considering a degraded structure, a better load bearing capacity can be achieved by any method of coupling. when comparing the deformations from the applied load, the design option considering shrinkage (c) have higher deformations than the options (d, e) without shrinkage. by strenghtening the footbridge structure fixed at both ends with a layer of uhpfrc on the upper surface, an increase in the load carrying capacity can be achieved for the degraded structure. the effect of strengthening on the stiffness of the structure for normal service loads depends on the method of coupling and shrinkage of the uhpfrc layer. 4.2. footbridge with one hinge from the analysis of the footbridge in the ideal condition with hinge support on one side and fixed support on the other side, we obtained information about the critical cross-section of the structure (figure 7). the results of this design option of the numerical model (a) serve as a reference for the other design options. figure 7. cracks in a single-hinged footbridge, the critical section is at the centre of the footbridge span. for all design options, failure occurred in the middle of the span of the footbridge. the conclusions from the analysis of this variant of structure support are almost identical to those for the footbridge fixed at both ends. however, the resulting load bearing capacity of the degraded structure after strengthening does not reach the load bearing capacity of the reference model (design option a). by strengthening the footbridge with a layer of uhpfrc on the upper surface, an increase in the load-bearing capacity of the degraded structure can be achieved. the effect of strengthening on the stiffness of the structure for normal service loads depends on the method of bonding and shrinkage of the uhpfrc. 4.3. footbridge with two hinges from the analysis of the footbridge in its ideal condition with hinged support at both ends, we obtained information about the critical cross-section of the structure (figure 8). the results of this design option of the numerical model (a) serve as a reference for the other design options. figure 8. cracks in a two-hinged footbridge structure (the symmetrical half of the structure was analysed), the critical section is at the centre of the footbridge span. 45 p. kněž, m. holý, j. kolísko acta polytechnica ctu proceedings design option uq usage qmax usage utot [mm] [%] [kn m−2] [%] [mm] a) footbridge in ideal state 0.63 100 30 100 18.1 b) footbridge in degraded state 1.45 230 26 87 48.1 c) reinforced footbridge. coupled by studs. with shrinkage 1.93 306 30 100 27.6 d) reinforced footbridge. coupled by studs. without shrinkage 1.26 200 30 100 30.3 e) reinforced footbridge. coupled by shear nothes and studs 0.73 116 30 100 14.2 table 3. evaluation of the deformation and maximum load of the footbridge fixed at both ends. design option uq usage qmax usage utot [mm] [%] [kn m−2] [%] [mm] a) footbridge in ideal state 1.2 100 22 100 36.5 b) footbridge in degraded state 4.2 350 17 77 34.9 c) reinforced footbridge. coupled by studs. with shrinkage 5.13 428 19 86 29.9 d) reinforced footbridge. coupled by studs. without shrinkage 3.51 292 19 86 28.1 e) reinforced footbridge. coupled by shear nothes and studs 1.86 155 21 95 23.2 table 4. evaluation of the deformation and maximum load of the single-hinged footbridge. design option uq usage qmax usage utot [mm] [%] [kn m−2] [%] [mm] a) footbridge in ideal state 11.3 100 10 100 58.9 b) footbridge in degraded state 46.8 414 6 60 417 c) reinforced footbridge. coupled by studs. with shrinkage 10.4 92 11.5 115 598 d) reinforced footbridge. coupled by studs. without shrinkage 9.8 87 12.5 125 544 e) reinforced footbridge. coupled by shear nothes and studs 7.5 66 13 130 206 table 5. evaluation of the deformation and maximum load of the two-hinged footbridge. for all design options of the two-hinged footbridge analysis, failure occurred in the middle of the footbridge span. for a service load of 5 kn m−2, a reduction in deformation and therefore an increase in the resulting stiffness of the structure was achieved for the design options considering a degraded structure coupled to a uhpfrc slab. compared to the unstrengthened degraded structure, the resulting values are significantly better. in the case of the maximum load bearing capacity evaluation, for design options of degraded and strenghtened structure a better load bearing capacity can be achieved by any method of coupling than for the unstrengthened structure in the ideal condition. when comparing the degraded design option with the options considering a strengthening plate, the resulting load bearing capacity is significantly higher. 5. evaluation of the parametric study the parametric study was carried out for a total of 15 different design options and offers a range of comparisons. here, a comparison of the load bearing capacity and the stiffness of the different methods of footbridge supports was made first. subsequently, the contribution of the strengthening for each degraded design option was evaluated. a critical point where failure should occur was predicted by analysing the ideal structural condition for each type of footbridge supports. the critical section for the structure fixed at both ends was the section at the tank wall (at the left fixed support). for the single-hinged structure, this was the mid-span of the footbridge. for the footbridge supported by hinges, the critical section is located in the centre of span. the performed analyses confirmed these assumptions. for all design options of each structural system, failure occurred at the predicted location. the comparison of the stiffness and load bearing capacity of the individual variants of the footbridge supports in the ideal condition was carried out for the maximum achieved load bearing capacity qmax and for the deformation (uz) from the applied service load (5 kn m−2). the reference is the load bearing capacity of the footbridge fixed at both ends. the results are given in table 6. structural system qmax usage uz usage [kn m−2] [%] [mm] [%] fixed support (both ends) 30 100 0.63 100 hinged support (one side) 22 73.3 1.2 190.5 hinged support (both ends) 10 33.3 11.3 1 793 table 6. comparison of load bearing capacity and stiffness of the analysed structural systems. 46 vol. 53/2025 parametric study of a reinforced concrete footbridge strengthened by a layer of uhpfrc structural system qmax usage uz usage [kn m−2] [%] [mm] [%] b) footbridge in degraded state 26 100 1.45 100 c) reinforced footbridge. coupled by studs. with shrinkage 30 115.4 1.93 133.1 d) reinforced footbridge. coupled by studs. without shrinkage 30 115.4 1.26 86.9 e) reinforced footbridge. coupled by shear nothes and studs 30 115.4 0.73 50.3 table 7. comparison of load bearing capacity and stiffness of the footbridge fixed at both ends, 15 % reduction of reinforcement cross-sectional area. structural system qmax usage uz usage [kn m−2] [%] [mm] [%] b) footbridge in degraded state 17 100 4.2 100 c) reinforced footbridge. coupled by studs. with shrinkage 19 117.6 5.13 122.1 d) reinforced footbridge. coupled by studs. without shrinkage 19 117.6 3.51 83.6 e) reinforced footbridge. coupled by shear nothes and studs 21 123.5 1.86 44.3 table 8. comparison of load bearing capacity and stiffness of the single-hinged footbridge, 15 % reduction of reinforcement cross-sectional area. structural system qmax usage uz usage [kn m−2] [%] [mm] [%] b) footbridge in degraded state 6 100 46.8 100 c) reinforced footbridge. coupled by studs. with shrinkage 11.5 191.7 10.4 22.2 d) reinforced footbridge. coupled by studs. without shrinkage 12.5 203.3 9.8 20.9 e) reinforced footbridge. coupled by shear nothes and studs 13 216.7 7.47 16.0 table 9. comparison of load bearing capacity and stiffness of the double-hinged footbridge, 15 % reduction of reinforcement cross-sectional area. table 6 shows that, according to theoretical assumptions, the type of support where the footbridge is fixed at both ends achieves the highest load capacity and stiffness. the load bearing capacity of the structure with a hinge on one side is 26.7 % lower and its deformation is 90.5 % higher than for the structure fixed on both sides. the variant with hinges at both ends has clearly the lowest load bearing capacity and stiffness. it has 66.7 % lower load bearing capacity and 1 693 % higher deformation than the footbridge fixed at both ends. another comparison will be made for degraded structures with a 15 % reduction of the reinforcement cross-sectional area due to corrosion. the same parameters will be compared as in the case of the comparison of structural systems, i.e. maximum load bearing capacity and deformation from the applied service load. the evaluation is carried out for the footbridge fixed at both ends in table 7, for the single-hinged footbridge in table 8 and for the double-hinged footbridge in table 9. design options b to e are always compared, where the reference value is the load capacity and stiffness of the unstrengthened structure (option b). tables 7–9 show that in all cases the strengthening of the structure increased its load-bearing capacity and in most cases also reduced the deformation of the structure from the applied service load of 5 kn m−2. in all cases, the best results were achieved by the proposed coupling solution in the form of a combination of shear notches with coupling studs (design option e). for the variants of the footbridge fixed at both ends and the footbridge with one hinge, the increase in load bearing capacity is similar and ranges from 15–24 %. the deformation is higher for the coupled structure than for the un-coupled structure when considering the shrinkage of the uhpfrc slab. therefore, the shrinkage mainly affects the stiffness of the structure, the load bearing capacity is the same for the variant with and without shrinkage. the variant with hinged support at both ends has a load bearing capacity for the unstrengthened and degraded structure that is only slightly higher than the comparative load for determining the deformation. therefore, the unstrengthened variant of the analysis has a significant deflection for this load. the deformation comparison is therefore somewhat biased. in the case that we select the design option in the initial condition for the double-hinged supported structure for comparison (table 6), we find that there is also an increase in the load bearing capacity and stiffness of the structure for all strengthened degraded structures. thus, the strengthening effect is most effective for the hinged-supported structure (simple beam). 47 p. kněž, m. holý, j. kolísko acta polytechnica ctu proceedings 6. conclusion the parametric study was aimed at monitoring the effects of strengthening of existing reinforced concrete footbridge with a 50 mm thick uhpfrc slab on the upper surface. it investigated the effect of different parameters affecting the stiffness and load bearing capacity of the footbridge (different methods of supporting of the footbridge, the effect of concrete degradation and reinforcement corrosion, the method of coupling of reinforced concrete and uhpfrc and the effect of shrinkage of the strengthening slab). an analysis of the ideal state of the structure without strengthening and concrete or reinforcement degradation was carried out for each support option. this was followed by an analysis for structures with degraded concrete (lower modulus of elasticity, tensile and compressive strength) and with a 15 % reduction in reinforcement cross-sectional area due to corrosion. the structures with degradation were then strengthened with uhpfrc slab. the structure was coupled either with coupling studs only or with a combination of studs and shear notches. the results of the parametric study show that the effect of the uhpfrc strengthening layer on the top surface has a different positive effect on the behaviour of the structure for different support types. the most pronounced benefit of the strengthening is for a structure with hinges at both ends, where the strengthening layer is located in the compression region of the crosssection and the excellent compressive properties of the uhpfrc can be exploited to the maximum. when designing the strengthening, it is necessary to take into account the possible effect of shrinkage of the uhpfrc strengthening layer and to select the optimum coupling method. although the use of a combination of shear notches with coupling studs leads to a significantly higher overall stiffness of the final composite structure, it is much more challenging to implement. acknowledgements this article was published with the support of the grant project op tak cz.01.01.01/01/22_002/0000725 strengthening and renovation of industrial buildings using uhpc material. references [1] m. k. ismail, a. a. hassan. structural performance of large-scale concrete beams reinforced with cementitious composite containing different fibers. structures 31:1207–1215, 2021. https://doi.org/10.1016/j.istruc.2021.02.028 [2] m. qiu, j. cao, z. shao, et al. flexural cracking behavior of steel-nc-uhpc composite beam under negative bending moment. engineering structures 322:119071, 2025. https://doi.org/10.1016/j.engstruct.2024.119071 [3] n. bertola, p. schiltz, e. denarié, e. brühwiler. a review of the use of uhpfrc in bridge rehabilitation and new construction in switzerland. frontiers in built environment 7:769686, 2021. https://doi.org/10.3389/fbuil.2021.769686 [4] j. abellán-garcía, j. s. carvajal-muñoz, c. ramírez-munévar. application of ultra-high-performance concrete as bridge pavement overlays: literature review and case studies. construction and building materials 410:134221, 2024. https://doi.org/10.1016/j.conbuildmat.2023.134221 [5] z. haber, i. de la varga, j. f. munoz, b. a. graybeal. uhpc overlays for highway bridge decks: bond behavior, durability, and structural performance. in second international interactive symposium on uhpc. iowa state university digital press, 2019. https://doi.org/10.21838/uhpc.9719 [6] w. nadir, m. m. kadhim, a. jawdhari, et al. experimental investigation on uhpc-nsc composite beams. structures 60:105885, 2024. https://doi.org/10.1016/j.istruc.2024.105885 [7] m. holy, d. citek, p. tej, v. lukáš. flexural strength of thin slabs made of uhpfrc. solid state phenomena 292:224–229, 2019. https: //doi.org/10.4028/www.scientific.net/ssp.292.224 [8] s. he, x. huang, h. zhong, et al. experimental study on bond performance of uhpc-to-nc interfaces: constitutive model and size effect. engineering structures 317:118681, 2024. https://doi.org/10.1016/j.engstruct.2024.118681 [9] s. he, x. huang, j. huang, et al. shear bond performance of uhpc-to-nc interfaces with varying sizes: experimental and numerical evaluations. buildings 14(11):3684, 2024. https://doi.org/10.3390/buildings14113684 [10] a. m. jabbar, a. s. al-zuheriy, q. a. hasan. a numerical investigation of the structural behavior of reinforced concrete beams fully or partially encased with uhpc layers in flexure. structures 70:107706, 2024. https://doi.org/10.1016/j.istruc.2024.107706 [11] v. červenka, l. jendele, j. červenka. atena program documentation. part 1: theory. [2025-01-29]. https: //www.technologismiki.com/uplx/atena_theory.pdf [12] eurocode1 : actions on structures. standard, the national annex, prague, 2005. 48 https://doi.org/10.1016/j.istruc.2021.02.028 https://doi.org/10.1016/j.engstruct.2024.119071 https://doi.org/10.3389/fbuil.2021.769686 https://doi.org/10.1016/j.conbuildmat.2023.134221 https://doi.org/10.21838/uhpc.9719 https://doi.org/10.1016/j.istruc.2024.105885 https://doi.org/10.4028/www.scientific.net/ssp.292.224 https://doi.org/10.4028/www.scientific.net/ssp.292.224 https://doi.org/10.1016/j.engstruct.2024.118681 https://doi.org/10.3390/buildings14113684 https://doi.org/10.1016/j.istruc.2024.107706 https://www.technologismiki.com/uplx/atena_theory.pdf https://www.technologismiki.com/uplx/atena_theory.pdf acta polytechnica ctu proceedings 53:42–48, 2025 1 introduction 2 specifications of the parametric study 3 numerical analysis 3.1 materials and material models 3.2 numerical models 3.3 loads 4 results of the parametric study 4.1 footbridge fixed at both ends 4.2 footbridge with one hinge 4.3 footbridge with two hinges 5 evaluation of the parametric study 6 conclusion acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2023.45.0014 acta polytechnica ctu proceedings 45:14–19, 2023 © 2023 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague an exploratory gallery for luka tunnel on the d3 motorway jiří barták czech technical university in prague, faculty of civil engineering, department of geotechnics, thákurova 7, 166 29 prague, czech republic correspondence: bartakj@fsv.cvut.cz abstract. the luka tunnel is designed for the 0302 jílové – hostěradice section of the d3 motorway. this has a length of 1843 m and will consist of two tunnel tubes of t 9.5/100 category. it is proposed that a 1354 m long exploratory gallery be put in place in the left-hand tunnel tube. it will be used for detailed geotechnical exploration in a rock environment that has been negatively affected by past mining activities. the exploratory gallery will be constructed using a procedure that will limit the possible stability risks during its excavation. the knowledge gained from the excavation of the gallery and from the comprehensive survey operations undertaken will serve as the essential basis for the reliable design and safe construction of the tunnel tubes. the current state of the d3 motorway differs greatly along its individual sections. the more southerly located sections d 305/ii (nová hospoda) to d 309 (úsilné) are in operation. section d 310 úsilné – hodějovice with the české budějovice bypass is under construction, and the construction in the direction of austria has not yet proceeded [1]. the situation is very disappointing particularly in the central bohemian region, even though exploration operations are underway to a limited extent in sections d 301 to d 305/i, as construction has not yet started due to problems with obstructions or objections by activists of various types. the problem is that it is demonstrable that the existing road i/3, which is also part of the main international road route e55, is already completely inadequate both in terms of capacity and in terms of its alignment running through several developed residential complexes. after the completion of the d3 motorway, this road will be usable as an alternative diversion route without major modifications in the event of a required closure of the motorway. keywords: d3 motorway, exploratory gallery, horizontal boreholes, luka tunnel, undermined area. 1. introduction the d3 motorway connects to the existing southern segment of the prague city ring road, continues south to the české budějovice bypass and connects to the austrian road network at the border of the czech republic. the nominal length of the d3 motorway amounts approximately to 172 km and is divided into thirteen basic sections (figure 1), with seven additional sub-sections. the current state of the d3 motorway differs greatly along its individual sections. the more southerly located sections d 305/ii (nová hospoda) to d 309 (úsilné) are in operation. section d 310 úsilné – hodějovice with the české budějovice bypass is under construction, and the construction in the direction of austria has not yet proceeded [1]. the situation is very disappointing particularly in the central bohemian region, even though exploration operations are underway to a limited extent in sections d 301 to d 305/i, as construction has not yet started due to problems with obstructions or objections by activists of various types. the problem is that it is demonstrable that the existing road i/3, which is also part of the main international road route e55, is already completely inadequate both in terms of capacity and in terms of its alignment running through several defigure 1. basic sections of the d3 motorway (modified from [1]). 14 https://doi.org/10.14311/app.2023.45.0014 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 45/2023 d3 motorway luka tunnel figure 2. luka tunnel in jílové u prahy – hostěradice section (modified from [1]). veloped residential complexes. after the completion of the d3 motorway, this road will be usable as an alternative diversion route without major modifications in the event of a required closure of the motorway. eight tunnels are expected to be built along the entire length of the d3 motorway. seven mined tunnels are part of the central bohemian section of the motorway (libeř tunnelland – length of 1517 m, kamenná vrata – length of 1690 m, luka – length of 1843 m, hostěradice – length of 377 m, vršky – length of 177 m, krňany – length of 432 m). the cut-and-cover pohůrka tunnel (999 m long), which is part of the české budějovice motorway bypass, is currently under construction. the luka tunnel is located in section d 302 jílové praha – hostěradice (figure 2). it consists of two unidirectional tunnel tubes t-9.5/4.80 with seven safety cross passages. near the village of horní studené, the motorway route passes through the luka panský vrch hill tunnel, with the highest overburden over 60 m (figure 3), and rises to the surface before the sázava river valley. the route crosses the sázava valley via a bridge with a main span of 250 m, at a height of 100 m above the river. the proposed route of the d3 motorway and the benešov approach road runs outside the developed areas of the individual municipalities and mainly runs across agricultural and forest land in the area affected. the luka tunnel is not only the longest of the tunnels mentioned above, but also it is most likely to be the riskiest in terms of excavation, and so it is necessary to pay special attention to the exploration, design and construction work that it involves. 2. engineering geological and hydrogeological conditions in terms of geology, the area of interest is located in the northernmost part of a belt of volcanic rocks of upper proterozoic age, termed the jílové zone, which is part of one of the oldest formations of the geological unit of the bohemian massif. the beginning of this geological forming of the jílové region falls into the youngest part of the proterozoic period (700 to 540 million years old). in its current form, the jílové zone is made up of basalts, andesites, dacites, rhyolites and their tuffs, through which bodies of sub-volcanic albitic granites penetrate. this complex of mostly very fine-grained, in places also porphyritic rocks, affected by folding and schist-forming processes, is intersected by numerous systems of younger rock veins – lamprophyres, microdiorites and diabases in the jílové belt trending predominantly nne-ssw, and by veins of porphyries and porphyrites of the variscan age, trending mainly transversally e-w. the original character of the rocks of the jílové belt was changed by a schist-forming process and regional recrystallisation associated with the cadomian orogeny at the turn of the proterozoic and the paleozoic eras and by the thermal effects of contact metamorphism associated with the origination of the variscan granitoids of the central bohemian pluton before the end of the paleozoic era (approximately 350 million years). rocks in the vicinity of veins and reticulated veins are metamorphosed by the action of thermal solutions, which gave rise to gold-bearing ore [3]. along the tunnel line, the massif is built by metabasalts, gabbros, rhyodacites and tuffs, through which ore veins penetrate in the transverse direction. the strength of the rocks is 15 jiří barták acta polytechnica ctu proceedings figure 3. schematic longitudinal section through luka tunnel with the exploratory gallery (modified from [2]). generally high to medium, and only in the area of the northern portal does their strength involve decreases. however, the massif is affected by numerous wider fault zones and tectonic lines, the actual occurrence and character of which will adversely affect the excavation of the tunnel. approximately ten of these faults were detected by the geophysical survey. a significant risk from the point of view of tunnel excavation is posed by the existence of past mining activities related to the historic gold mining in the part of the area through which the tunnel passes for a length of approximately 1600 m. in the area of the jílové belt, gold was mined over several periods. the last attempt at gold mining took place in the 1960s at the bohuliby mine. however, gold mining was not very successful or economically profitable, so the bohuliby mine closed its operations in 1968. the planned route of the luka tunnel passes under panský vrch hill, which is unfortunately the area with the least explored remains of historic mining. the occurrence of ancient mining workings in the area of panský vrch hill is evidenced by places of surface depressions (so-called pinges), which are formed on the surface of the area as a result of the collapse of old mine adits [2]. the risk of striking unknown mine spaces during tunnel excavation must be eliminated by excavation of the exploratory gallery and systematic safety drilling ahead of the excavation face. during world war ii, military production took place underground in the given area,(about which there is no detailed information), which increases the risk of the occurrence of unknown underground spaces. the two galleries on the 1st and 2nd mining floors, which were excavated from the bohuliba mine in the 1960s, are an exception to the degree of exploration of this area. they are well described. the gallery on the 2nd floor is at an altitude of 235.26 m above sea level, and in plan it crosses with the luka tunnel at the point where the horizontal alignment of the tunnel is at an altitude of 380.17 m a.s.l. due to the difference between the elevations of approximately 145 m, this gallery will not have any effect on the excavation of the tunnel. the corridor on the 1st floor of the mine is at an altitude of approximately 321.32 m. in ground plan, this gallery crosses the proposed luka tunnel approximately at the point where the horizontal alignment of the tunnel is located at an altitude of 373.2 m above sea level. the difference in elevations amounts approximately to 52 m, so even this gallery should have no major influence on the excavation of the tunnel. hydrogeological conditions are significantly influenced by the undermined area. groundwater is bound to the surface deposits and the zone of surface disintegration of fissures. the direction of flow is roughly conforming to the terrain, and drainage runs through tributaries to watercourses and springs. the locations of the springs are bound to the crossing of significant fissure systems and valleys. a significant volume of groundwater is contained in old mine workings, which are usually filled with stagnant groundwater reserves. controlled drainage into the sázava river is mainly restricted to newer workings from the 19th and 20th centuries. mining workings affect the overall circulation of groundwater, for which they represent preferred ways. the area of the massif under the luka tunnel is permanently drained by old mining workings. 16 vol. 45/2023 d3 motorway luka tunnel figure 4. cross-section with the position of the basic exploratory gallery (modified from [3]). figure 5. cross-section through exploratory gallery in passing bays (modified from [3]). 3. the exploratory gallery 1. exploratory gallery design excavation of the exploratory gallery proposed by the design [3] will be carried out at a length of 1358 m from the southern portal in the left-hand tunnel tube (ltt), without connection with the future primary lining of the tunnel. the uphill driving is advantageous both from the point of view of drainage of the face and the transport of muck to the portal. trackless haulage is assumed. the exploratory gallery will be driven from the construction site utility, located in the area of the future tábor (southern) portal of the tunnel. at the end of the gallery, a shaft is designed in front of the northern portal, which will serve, among other things, for ventilation of the gallery after its completion (figure 3).the cyclic excavation will be carried out using blasting. the cross-section of the basic exploration gallery will be horseshoe-shaped, approximately 3.75 m high, 3.0 m wide, with an excavated area of 13.8 m2; the lining will be without invert (figure 4). the self-supporting capacity of the massif will be increased using radial rockbolts. due to the expected length of the exploratory gallery, passing bays are designed at approximately 175 m intervals. they will be approximately 25 m long and 5.40 m wide. the passing bays will allow for the passing of means of muck transport and the bringing of material and mechanical equipment. the excavated area of the gallery in the passing bay is equal to 25.9 m2 (figure 5). 2. the use of a gallery for exploration in the area of interest due to the detection of an unfavourable combination of engineering geological and hydrogeological conditions within the undermined area, it is clear that the main risks for the excavation of the luka tunnel result primarily from the possibility of encountering unknown mining workings, and which is associated with the risk of inrush of water and water-saturated material. smaller risks follow from encountering extraordinary structural faults and fault zones. these risks need to be checked by 17 jiří barták acta polytechnica ctu proceedings figure 6. arrangement of safety boreholes ahead of the face in cross-section (modified from [3]). figure 7. longitudinal and plan arrangement of safety boreholes ahead of the face (modified from [3]). driving an exploratory gallery. in general, an exploratory gallery is a very effective continuous survey method not only for obtaining conclusive knowledge about the geological structure of the rock massif. it also provides fundamental knowledge about the stability and deformation behaviour of the massif. however, it is necessary to keep in mind the so-called influence of scale, given by the ratio between the dimensions of the excavation face of the exploratory gallery and the dimensions of the partial faces of the sequential excavation of the final working. the stability and deformation behaviour of larger excavation faces (especially in the case of the horizontal excavation sequence) is often worse in comparison with the excavation of the exploratory tunnel; that the tendency to create overbreaks in fractured massifs increases. an important advantage of the construction of the exploratory gallery is the direct detection of the stability of the unsupported excavation around and on the excavation face during excavation, the verification of the tendency of the rock massif to create overbreaks, the testing of the rock breaking technique and excavation support elements, as well as the assessment of the maximum possible excavation advance rate. the hydrogeological behaviour of the massif is verified directly in the excavation by the exploratory gallery. the water-bearing character of the massif negatively affects the mechanical properties of the rock. it reduces the stability of the excavation indirectly proportionally to the increase in inflows into the working. it is desirable during the survey to determine the sections of probable inflows to the future tunnel and to determine their rates and nature. the danger of increased inflows comes in fractured hard rocks and faults. in this sense, the most dangerous are karst formations and undermined areas, in both cases with possible inrushing of water-bearing inconsistent ground. in the event of their possible occurrence, it is necessary to verify the danger zones (or, after their indication in advance, by geophysical measurement) from the exploratory gallery using frontal subhorizontal boreholes into the face-advance core, which allows for a reliable determination of marked inhomogeneities, fault zones, cavities and caverns. it was this procedure that was proposed for the exploratory gallery regarding the luka tunnel [3]. five 24 m long, 65 mm-diameter safety boreholes ahead of the excavation face will be carried out from the gallery during excavation, with the central sub-horizontal borehole being cored and the remaining inclined boreholes non-cored. their orientation and lengths are designed in such a way that they indicate in the advance core of the exploratory gallery excavation the occurrence of old mine workings with a possible danger of an inrush of water or a water-bearing material from the old 18 vol. 45/2023 d3 motorway luka tunnel mine adits, in addition to significant fault zones in the massif. the pre-drilled sections are designed to be 24 m long; the pre-drilled holes at the connection of the sections are designed to overlap 3 m (figures 6, 7). the total length of the pre-drilling section is assumed to be 1 km long (see figure 3), with 235 pieces of pre-drilled holes. 4. conclusions in order to ensure the safety of underground work in the area with the remains of historic mining activities, and especially to prevent any unexpected striking of water-bearing old water-containing mine workings, it is absolutely necessary to carry out boreholes into the area of future mining, in a relatively quite considerable long section (10 500–11 500 km). by fulfilling this legitimate requirement, the risk of an emergency during the course of excavation of the gallery and the future tunnels is significantly reduced. other exploration activities and monitoring will be carried out on an increased scale according to unfavourable local conditions. references [1] ředitelství silnic a dálnic čr. section 0302 jílové – hostěradice. [2017-05-23]. https://www. stredoceskad3.cz/usek-0302-jilove-hosteradice/ [2] d. josefík. d3 motorway luka tunnel. bachelor thesis. faculty of civil engineering, czech technical university in prague. 2017. [3] tubes spol. s r. o. luka tunnel detailed geotechnical survey project, prague, 2021. 19 https://www.stredoceskad3.cz/usek-0302-jilove-hosteradice/ https://www.stredoceskad3.cz/usek-0302-jilove-hosteradice/ acta polytechnica ctu proceedings 45:1–6, 2023 1 introduction 2 engineering geological and hydrogeological conditions 3 the exploratory gallery 4 conclusions references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2024.50.0081 acta polytechnica ctu proceedings 50:81–87, 2024 © 2024 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague about the choice of tungsten carbide indenter to determine mechanical properties of superalloys by using high-temperature microhardness tester bruno passilly∗, amélie kardache université paris saclay, onera, dmas, 29 avenue de la division leclerc, 92320 châtillon, france ∗ corresponding author: bruno.passilly@onera.fr abstract. in the aeronautical field, materials are used in severe environmental conditions (temperature, atmosphere), particularly in engine applications. in order to qualify mechanical properties of new composition ni-based superalloys, onera performs vickers hardness tests from room temperature up to 750 °c close to operating conditions. this method consists in applying a pyramidal tip onto the specimen to characterize hardness and mechanical resistance of the material. this simple method appears to be faster than other methods using classical hot tensile or bending tests. nevertheless, the choice of the indenter tip for high-temperature experiments is crucial. tungsten carbide tip is used for characterizing ni-based superalloys. electron microscopy and x-ray analysis are presented and discussed on new and used tungsten carbide tips. a simple experimental method to control the evolution of the indenter before and after using it in the high-temperature hardness test is explained. evolution of hardness and mechanical resistance versus temperature by using tabor relationship on a nickel-based superalloy sample is compared to evolution of mechanical resistance values determined by classical high temperature tensile tests. a good agreement is found between these two methods with wc indenter. these hardness measurements could be carried out up to 1 000 °c if indenter is still available to characterize layers, coatings, composite materials, additive manufacturing materials or gradient properties materials. keywords: high temperature, hardness, superalloy, mechanical resistance, indenter, ni-based superalloy. 1. introduction the development of the aerospace industry during the last century led to elaborate new materials to increase aircraft performances. studying new nickel-based superalloys demands a better knowledge of mechanical characteristics of each new superalloy composition [1]. conventional high-temperature mechanical tests such as tensile and bending require a large quantity of superalloy. the equipment is heavy and expensive, and a longtime preparation of a lot of specimens as well as long test periods are required to investigate mechanical behavior of only one new superalloy for each tested temperature. in order to increase the quantity of results on the behavior of superalloys in severe thermal conditions, it is essential to test each new composition by developing a high temperature micromechanical test. a lot of nanoindentation means have been developed over the past twenty years for room temperature usage [2] and more recently at high temperature in a limited scale of loading [3]. in order to characterize new superalloys for turbomachinery parts, simple hardness measurements are sufficient to determine hardness and mechanical resistance of a small quantity of the specimen without having to apply a maximum load less than 1n and to measure displacement of the indenter. preparation of specimens is easy and the test period is short. the evolution of hardness and mechanical resistance of ni-based superalloy versus temperature up to 750 °c are presented and discussed by using the wc indenter. 2. high-temperature hardness tester and indenters 2.1. high-temperature hardness tester the high-temperature hardness device shown on figure 1 and 2 is a prototype developed by onera [4]. the maximum load p that could be applied by the indenter on the sample is up to 30n. the maximum temperature that could be reached by the furnaces is about 1 000 °c. working atmosphere is composed of argon with 3% vol of hydrogen under a pressure of working 0.5 bars. this special gas contains less than 0.2 ppm.mole of oxygen. the tip and the specimen are separately heated by cylindrical furnaces to minimize thermal gradient between. the hardness tester has stable and efficient thermal regulations that allow fast heating and little overshoot [5]. a thermocouple is fixed in the ovens near the sample and near the indenter to control and regulate the temperature during tests. the load applied to the sample is measured by load sensor 81 https://doi.org/10.14311/app.2024.50.0081 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en bruno passilly, amélie kardache acta polytechnica ctu proceedings figure 1. diagram of the main components of the hot indentation apparatus. figure 2. high-temperature microhardness prototype developed by onera. (lcm201-100 n, omega) which is located outside the vacuum chamber. the movement of the indenter is controlled by a motorized table which compresses a bellow when the table is moving to indent the sample. a second motorized displacement table supports the optical microscope outside the vacuum chamber to focus indentation zones and to measure the imprints. a cooled porthole is installed between the sample and the indenter to avoid any heating of the lenses. 2.2. indenter choice for high temperature tests 2.2.1. indenter materials principles during an indentation test, the tip must be deformed only elastically and must have a high hardness to be less deformable than the tested material. according to j. m. wheeler and j. michler [6–8], indenter must be 20% harder than material being tested, otherwise the tip will be eroded quickly and will break. to avoid this breakage problem, the tip must have a hardness significantly greater by 100% or more than that of the material (figure 3). for hot indentation, it is then necessary to know the hardness of the indenter at high-temperature test. the choice of the material composing the tip must result necessarily from a compromise between a large number of criteria. indenter material must have good geometric stability at the test temperature. it must have good chemical stability without phase change or chemical reactions with environment or reaction with the tested material. it must keep high hardness and good thermal conductivity at high-temperature. and finally, indenter must be easy to manufacture and to install on the setup. another way to avoid damages on the indenter during indentation tests at high temperature is to minimize time of contact of the tip with the specimen. tiphene shows typical cycle of loading and unloading in less than 1 second [9]. 2.2.2. indenter holders brazing is a permanent heterogeneous assembly process that achieves metallic connection between the two joined parts. to assemble indenter and holder, a filler metal is employed. when the melting temperature of the filler metal is reached, the joint is formed by capillarity. hard brazing is recommended to reach 600 to 1 000 °c. nevertheless, it is recommended to use theses assemblies at a lower temperature than half the melting temperature of the filler metal. ultra-high temperature (uht) vacuum brazing using molybdenum with a melting temperature of around 1 450 °c allows a working temperature of 750 °c. this type of solder was used on a sapphire tip by minnert [10] and had no problem rising to 1 100 °c. furthermore, during the test, if the melting temperature of the filler metal was reached, the tip would move and then it would be oriented incorrectly for the next tests. another way consists in blocking the indenter on a support by using a screw as it can be seen in figure 3. the advantages of this process are that the support is reusable and can resist up to 1 400 °c. however, this way is more interesting if the indenter is in one piece, without brazing. this method requires more material to make an indenter and manufacturers using this 82 vol. 50/2024 about the choice of tungsten carbide indenter to determine . . . figure 3. hardness of different materials versus temperature [6]. figure 4. sample holder with screwing element. technique are very few in the world. to consolidate everything and ensure that the screw will not unscrew, a ceramic cement is used to secure the screw. on the high temperature hardness tester, wc tip is brazed on the holder, and this assembly is screwed on the instrument like in figure 4. 3. results on superalloy 3.1. generalities about superalloy superalloys suited for single crystal casting derived from nickel-based superalloys. the alloys’ designers succeeded in optimizing the mechanical properties of these alloys by introducing large amounts of refractory alloying elements such as w, ta, mo. caron et al. [1] described the evolution of the chemistry of this class of alloys by focusing on advantages and drawbacks resulting from these chemistry changes. 3.2. hardness relationship during microhardness testing, the applied load is measured as a function of the displacement of the indenter in the material. this produces an indentation curve like the one shown in figure 5. in metalworking industry, vickers indenter (a square-based pyramid tip) is preferentially used. figure 5. schematic graph presenting the multiple phases of the loading cycle during indentation tests [2]. moreover, this shape of indenter is used on a commercial vickers hardness tester (wilson vh3100, buehler) in order to compare the hardness value with our device. as there is no displacement sensor on the hot hardness tester, vickers hardness is calculated from the imprint left in the material and not from the loadunload curve. microhardness is then calculated with the following equation (1): hv = 1.854 × fmax d2 (1) in this relationship, fmax represents the maximum applied load during the indentation, and d the average between the two diagonals of the vickers pyramidal imprint. 83 bruno passilly, amélie kardache acta polytechnica ctu proceedings figure 6. hardness of superalloy versus applied load on a commercial tester and hot-hardness tester. 3.3. hardness versus applied load on ni-based superalloy the high-temperature hardness tester is calibrated at room temperature on known metal alloy stainless steel 316l showing a low dispersion of the measured values of hardness which were compared to those obtained on commercial room temperature vickers hardness tester (wilson vh3100, buehler) [4]. figure 6 represents evolution of hardness versus applied load. an average of 5 measures for each applied load is calculated and the standard deviation is represented. 3.4. hardness versus temperature on ni-based superalloy for each chosen temperature, the hardness measurements are collected from hot-hardness test on a nibased superalloy (figure 7). an average of 5 measurements for each temperature is calculated, and standard deviation is presented on figure 8. hardness is decreasing slowly from 20 °c up to 600 °c, and is more important after 600 °c. 3.5. mechanical resistance tabor’s law [11] is used to express the mechanical resistance rm from the hot vickers microhardness result hv, following equation (3): rm ≈ hv 3 (2) it is important to note that the coefficient of proportionality 3 set up by tabor is an average obtained after a study on different metallic materials (steel, aluminum, copper, etc.) [11, 12]. after normalization of the mechanical resistance values calculated from hardness and tensile tests on the same ni-based superalloy, the values obtained experimentally follow the same decrease at the same temperature with a lower percentage value in the case of hardness experimental data (figure 9). this result allows us to affirm that the used indenter is operational up to 750 °c. figure 7. typical imprint of a vickers test on nibased superalloy under a load of 10n. 3.6. scanning electron microscopy on tungsten carbide indenter in order to observe its surface conditions, new and used tungsten carbide tips are analyzed by using electron microscope. the new vickers tungsten carbide tip has a relatively smooth surface, but the presence of porosity is noted on figure 10. figure 11 shows the wc indenter after 20 hours of heating at 750 °c and 2 hours of mechanical stress between the tip and the superalloy. the indenter is slightly blunt and appears to be slightly damaged, with few contaminations of the tip. tungsten carbide tips have good mechanical resistance during indentation tests. a qualitative analysis of the indenter is carried out on an area located in the region of contact between the indenter and the tested material, in the periphery of the indented zone and in the unstressed zone. the presence of aluminum, silicon and nickel is detected only in the region in contact with the material. some elements present in the alloy were stuck in the porosities of the tip. tip cleaning might be necessary to avoid this contamination [13]. 84 vol. 50/2024 about the choice of tungsten carbide indenter to determine . . . figure 8. hardness versus temperature of ni-based superalloy on hot hardness tester. figure 9. normalization of rm as a function of temperature for ni base superalloy. figure 10. sem image of new vickers wc tip at different scales. 85 bruno passilly, amélie kardache acta polytechnica ctu proceedings figure 11. sem image of used tungsten carbide indenter at different scales. figure 12. corrective factor versus time of heating for wc tip at 750 °c. 3.7. life duration of the wc indenter indenter could react chemically with the materials to be tested and could be sensitive to different environmental factors such as temperature and working atmosphere. tip could become fragile, develop cracks, or deteriorate, then the indentation results would be affected. in order to notice a possible degradation of the tip as the tests progress, the evolution of a degradation factor k is measured : k = hvht hvrt (3) hvrt is the hardness measured on a room temperature conventional microhardness tester (wilson vh3100, buehler). after each series of test at 750 °c on the high temperature hardness tester, hardness hvht is measured at room temperature. consequently, tip degradation can easily be represented as a function of the time spent by the tip at high temperature. figure 12 shows the evolution of degradation coefficient k versus time of heating. after 22 hours at 750 °c, variation of k is less than 5%. this result correlated with the sem images demonstrates that the tungsten carbide tip is still usable to provide hot hardness results. the tungsten carbide tip resists mechanically and chemically to high-temperature micro-hardness testing on nickel-based superalloys. the evolution of this degradation factor is a good indicator to determine precisely the start of tip deterioration and therefore the usefulness of replacing it with a new one. tungsten carbide wc appears to react moderately with oxygen and can react slightly with certain transition metals. this indenter is chemically stable under high-temperature conditions and under inert gas atmosphere. 4. conclusions hot-hardness tester prototype can perform a large number of high-temperature tests in just a few hours. in addition, this type of test is non-destructive and requires little quantity of materials and simple preparation. hardness is therefore a very interesting characterization technique since it is useful to determine certain local mechanical properties of materials. the characterization of nickel-based superalloys is performed for a load of 10n and up to a temperature of 750 °c with wc indenter. this test allows us to determine the evolution of the hardness of a ni-based superalloy or mechanical resistance as a function of temperature by using tabor’s law. when these values are normalized, the variation of the percentage of rm versus temperature, obtained in hardness, is very close 86 vol. 50/2024 about the choice of tungsten carbide indenter to determine . . . to the rm obtained in traction and follows the same curve shape with a pronounced decrease after 600 °c. unlike tests at room temperature with the systematic use of diamond, it is necessary to consider an indenter-material pair for each series of tests on material. indeed, the predominance of chemical reactions between the material tested and the indenter does not currently make it possible to find a universal indenter for high-temperature tests. monitoring the health of the indenter is essential to avoid experimental bias leading to erroneous results. for this, developing new high-temperature indentation devices under a scanning electron microscope is promising. indeed, the combination of these two techniques will allow the qualitative observation of the formation of imprints and the interaction between the tip and the sample. this interaction highlighted in this work is certainly harmful for tests carried out at high load, and risks being even more harmful for tests in the nanoindentation range. acknowledgements the work presented in this paper was carried out thanks to the financial support of safran and aid within the framework of the adamant chair (acceleration of the development of alloys and multilayer systems for applications to new turbines). thanks to quentin barrès, nicolas horezan, ariel morel and agnès bachelier for their technical contribution to characterize indenter tips. references [1] p. caron, t. khan. evolution of ni-based superalloys for single crystal gas turbine blade applications. aerospace science and technology 3(8):513–523, 1999. https://doi.org/10.1016/s1270-9638(99)00108-x [2] w. c. oliver, g. m. pharr. an improved technique for determining hardness and elastic modulus using load and displacement sensing indentation experiments. journal of materials research 7(6):1564–1583, 1992. https://doi.org/10.1557/jmr.1992.1564 [3] s. comby-dassonneville, g. tiphéne, a. borroto, et al. real-time high-temperature scanning indentation: probing physical changes in thin-film metallic glasses. applied materials today 24:101126, 2021. https://doi.org/10.1016/j.apmt.2021.101126 [4] b. passilly, a. quelquejeu, a. kardache. mechanical properties of stainless steel by using high-temperature microhardness tester. matériaux & techniques 111:402, 2023. https://doi.org/10.1051/mattech/2023021 [5] hot hardness measurements on materials up to 600 °c during the first hour of using. acta polytechnica ctu proceedings 27:160–163, 2020. https://doi.org/10.14311/app.2020.27.0160 [6] j. m. wheeler, j. michler. indenter materials for high temperature nanoindentation. review of scientific instruments 84(10):101301, 2013. https://doi.org/10.1063/1.4824710 [7] j. m. wheeler, r. a. oliver, t. w. clyne. afm observation of diamond indenters after oxidation at elevated temperatures. diamond and related materials 19(11):1348–1353, 2010. https://doi.org/10.1016/j.diamond.2010.07.004 [8] j. m. wheeler, d. e. j. armstrong, w. heinz, r. schwaiger. high temperature nanoindentation: the state of the art and future challenges. current opinion in solid state and materials science 19(6):354–366, 2015. https://doi.org/10.1016/j.cossms.2015.02.002 [9] g. tiphéne, p. baral, s. comby-dassonneville, et al. high-temperature scanning indentation: a new method to investigate in situ metallurgical evolution along temperature ramps. journal of materials research 36:2383–2396, 2021. https://doi.org/10.1557/s43578-021-00107-7 [10] c. minnert, w. c. oliver, k. durst. new ultra-high temperature nanoindentation system for operating at up to 1100 °c. materials & design 192:108727, 2020. https://doi.org/10.1016/j.matdes.2020.108727 [11] d. tabor. the physical meaning of indentation and scratch hardness. british journal of applied physics 7(5):159–166, 1956. https://doi.org/10.1088/0508-3443/7/5/301 [12] j. t. busby, m. c. hash, g. s. was. the relationship between hardness and yield stress in irradiated austenitic and ferritic steels. journal of nuclear materials 336(2-3):267–278, 2005. https://doi.org/10.1016/j.jnucmat.2004.09.024 [13] x. liu. développement de la caractérisation du comportement local à haute température des alliages métalliques par micro indentation [in french; development of the characterization of the local behavior at high temperature of metallic alloys by microindentation]. ph.d. thesis, l’université de technologie de compiègne, 2017. 87 https://doi.org/10.1016/s1270-9638(99)00108-x https://doi.org/10.1557/jmr.1992.1564 https://doi.org/10.1016/j.apmt.2021.101126 https://doi.org/10.1051/mattech/2023021 https://doi.org/10.14311/app.2020.27.0160 https://doi.org/10.1063/1.4824710 https://doi.org/10.1016/j.diamond.2010.07.004 https://doi.org/10.1016/j.cossms.2015.02.002 https://doi.org/10.1557/s43578-021-00107-7 https://doi.org/10.1016/j.matdes.2020.108727 https://doi.org/10.1088/0508-3443/7/5/301 https://doi.org/10.1016/j.jnucmat.2004.09.024 acta polytechnica ctu proceedings 50:81–87, 2024 1 introduction 2 high-temperature hardness tester and indenters 2.1 high-temperature hardness tester 2.2 indenter choice for high temperature tests 2.2.1 indenter materials principles 2.2.2 indenter holders 3 results on superalloy 3.1 generalities about superalloy 3.2 hardness relationship 3.3 hardness versus applied load on ni-based superalloy 3.4 hardness versus temperature on ni-based superalloy 3.5 mechanical resistance 3.6 scanning electron microscopy on tungsten carbide indenter 3.7 life duration of the wc indenter 4 conclusions acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2023.40.0069 acta polytechnica ctu proceedings 40:69–75, 2023 © 2023 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague effect of electric current on porosity of concrete daniel ňachaja,∗, jiří němeček 1,a, milan kouřilb, martin keppertc, jiří němeček 2,a a czech technical university in prague, faculty of civil engineering, department of mechanics, thákurova 7, 166 29 prague 6, czech republic b university of chemistry and technology, department of metals and corrosion engineering, technická 5, 166 28 prague 6, czech republic c czech technical university in prague, faculty of civil engineering, department of material engineering and chemistry, thákurova 7, 166 29 prague 6, czech republic ∗ corresponding author: nachadan@fsv.cvut.cz abstract. the paper describes the influence of electric current on the porosity of concrete samples after electrically accelerated tests and other electrokinetic treatments. three different concrete mixtures consisting of cem i 42.5r, cem ii / a-m(s-ll) 42.5r, and cem i 42.5r with 10 % replacement of cement by microsilica were examined. direct current (dc) test was performed on fully saturated samples. the samples were loaded with a constant voltage of 20 v for 24 hours in chambers filled with naoh solution. the changes in porosity were examined using mercury intrusion porosimetry (mip), scanning electron microscopy (sem), and open porosity (op). the results of open porosity indicate an increase in porosity for all mixtures measurements of ∼0.6 %–2 % for treated samples. the mip showed increase of volume of pores with radius smaller than 0.01 µm after dc test. according to x-ray diffraction (xrd) and thermogravimetric analysis (tga) there was ∼2 % less calcite after dc test. keywords: electricity, mip, concrete, porosity, image analysis, microstructure. 1. introduction concrete is a composite material composed of fine and coarse aggregate bonded together with a cement matrix. it is the most used building material in the world. its advantage is high compressive strength, which is approximately ten times greater than tensile strength. due to this characteristic, reinforced concrete has been invented. the most common and cheapest rebar is steel reinforcement, which is highly susceptible to corrosion when exposed to aggressive environment. due to this unwanted characteristic, the durability of concretes can be considerably reduced [1, 2]. reinforcement corrosion is influenced by the presence of deicing salts transported through the microstructure and cracks of damaged concrete towards reinforcement. the corrosion of steel reduces the cross-section of the reinforcement, which leads to the loss of its load-bearing capacity. chlorideinduced corrosion of the reinforcement is one of the main causes of degradation of structural concrete and also a contributor to the cost of rehabilitation of concrete structures. in order to prevent reinforcement corrosion and reduce chloride/salt penetration, the concrete cover is usually increased, or the permeability of concrete is decreased [2, 3]. also, non-destructive healing methods based on electromigration have been proposed to eliminate corro1ph.d. at czech technical university in prague, orcid: 0000-0002-5635-695x. 2professor at czech technical university in prague, orcid: 0000-0002-3565-8182. sion [4]. for example, a method of electrochemical chloride extraction (ece) has been successfully used to reduce chloride attacks in concrete bridges [5]. electrochemical remediation techniques are based on the accelerated transport of charged particles or ions inside the pore structure under the influence of an electric field. electrokinetic treatment with nanoparticles has recently been introduced to improve the concrete microstructure and increase durability by blocking concrete pores [6]. in order to improve the properties of concrete mixtures, the permeability of the concrete to ions is often determined as quality control. chloride diffusion measurements require a long time to determine steadystate conditions. therefore, a direct current potential is usually used to accelerate ion migration. one of the standardized electric tests is according to astm c1202 standard [7] also called rapid chloride permeability test (rcpt) which is used to determine concrete chloride permeability [8–10]. electrokinetic techniques are applied in many fields allowing the manipulation of material properties. these techniques are used in the concrete industry, primarily to mitigate corrosion of reinforcement but also to modify the microstructure of cement-based systems, thereby improving their long-term properties. electrokinetic treatment is used to mitigate corrosion and is proposed for microstructural refinement / durability enhancement, crack healing, chemical attack recovery, and reinforcement-concrete bond enhancement [6]. 69 https://doi.org/10.14311/app.2023.40.0069 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en d. ňachaj, j. němeček, m. kouřil et al. acta polytechnica ctu proceedings it was found that the electric current has some side effects and increases the porosity [11, 12]. thus, it is important to investigate the effect of electric current on the microstructure of the cement matrix, as this has not been sufficiently investigated so far. therefore, this paper focuses on the influence of the electric current on the porosity of different types of concrete. 2. experimental methods 2.1. direct current test there are several standardized accelerated tests for chloride penetration to concrete. one of them, the rapid chloride permeability test (rcpt) measures the electric charge transferred through concretes from which it is possible to determine their resistance to chloride ion penetration on various mixtures. the test is carried out by monitoring the amount of electric current that passes through the concrete samples in a specified time. the standard test according to ast c1202 consist of cylindrical 50 mm thick slices with a diameter of 100 m. the concrete samples are exposed to a potential difference of 60 v direct current for 6 hours, where electric current passing through samples is monitored [7]. in the chambers there is the negative pole in 0.52 n nacl solution and positive pole in 0.30 n naoh solution which are in contact with sample’s base. the direct current (dc) test used in this work was inspired by the rcpt test with a modified setting. in the dc test, the voltage was set to 20 v with the duration of 24 hours to better simulate settings used for chloride extraction on real structures. also, the naoh with 0.3 m concentration was present in both chambers. no chlorides were present in the dc test, see figure 1. 2.2. open porosity the open porosity (op) measures the physically bound water in the pores. its weight is needed to determine the total open porosity. the open porosity, ϕop is then calculated according to the equation: ϕop = vw vs = mw − md vs.ρw , (1) where vw is a volume of the water, vs is a volume of the concrete sample, mw is a weight of fully saturated sample, md is a weight of dried sample, ρw is a density of water assumed as 998 kg/m3 at 20 °c. 2.3. mercury intrusion porosimetry several methods are generally used to measure the porous structure of concrete, such as the optical methods, mercury intrusion porosimetry (mip), gas absorption, proton nuclear magnetic resonance (1h-nmr), data logger 20 v power supply + stainless steel cathode 0.3 m naoh reservoir stainless steel anode 0.3 m naoh reservoir concrete sample 50 mm long, 100 mm diameter insulator a figure 1. scheme of dc test. and x-ray microtomography [13]. mip has been used for many years to investigate the porous structure of cement-based materials. mip also provides information about the connectivity of the pores [14]. measurement of porosity by mercury intrusion provides an extensive range of measurable pore sizes from 4 nm to 500 µm. however, it is important to note that mercury intrusion porosimetry only measures the open pores [15]. 2.4. image analysis image analysis (ia) is used for the observation of cement paste phases. the observed parameters are e.g. the size of the phases, their representation, and shape. images are made in a gray range (0–255) of colors, and the colors of pixels vary according to the density of the physical element. the darker colored pixel in the scanning electron microscope (sem)backscattered electrons (bse) image represents elements with a lower proton number, while the lighter colored pixel, on the other hand, represents areas with elements with a higher proton number [16]. during the image analysis, adequate gray-level threshold extraction is necessary. otsu method can perform automatic image thresholding, whose algorithm in its simplest form returns a single intensity threshold that separates pixels into two classes [17]. 2.5. x-ray diffraction x-ray diffraction (xrd) is a technique that provides chemical information for elemental analysis as well as for phase analysis. samples to be analyzed using xrd must be crystalline as concrete samples partly are [18]. xrd analysis is used to show cement microstructure’s phases such as portlandite, calcite, afm, and aft, where the microstructure changes of a cement matrix are expected during current application [19]. 2.6. thermogravimetric analysis thermogravimetric analysis (tga) is a method that captures the change in sample weight as a function 70 vol. 40/2023 effect of electric current on porosity of concrete label cem i 42.5r cem ii 42.5r ms water sand 0–4 mm agg 4–8 mm agg 8–16 mm [kg] [kg] [kg] [kg] [kg] [kg] [kg] ca 261.8 0 0 210.2 1150.0 290.9 581.8 cb 0 261.8 0 210.2 1150.0 290.9 581.8 cc 235.6 0 26.2 210.2 1150.0 290.9 581.8 legend: aggregate (agg), microsilica (ms). table 1. samples labeling with the specific weights of the additives in kg. ca, cb, cc el ca, cb, cc nl storing in 1% lime water (fully saturated) rcpt op mip sem* tga* xrd* time [days] 0 1 361 362 382 383 sample production storing in 1% lime water (fully sat.) drying at 50 °c +weight measurement drying at 50 °c +weight measurement dried samples dried samples pore size distribution phases distribution thermal decomposition amount of crystalline phases figure 2. experimental flow. note: *for ca samples only. of temperature. this method monitors the weight of a sample of the substance under investigation as its temperature changes. tga characterizes and identifies phases from a complex cement matrix [20]. similarly to xrd analysis, changes in the c-s-h phase, portlandite, and calcite can be observed [19]. 3. experimental part 3.1. experimental plan and sample preparation three types of concrete samples were produced, 2 samples of each type, each with a 0.8 water-binder (w/b) ratio and with aggregates of size 0-16 mm as it is shown in table 1. the first sample (ca) was made of cem i 42.5r, the second (cb) of cem ii / am(s-ll) 42.5r, and the third (cc) of cem i 42.5r with 10 % cement replacement by microsilica. the concrete slurry was poured into cylindrical plastic ampules with a base diameter of 100 mm and a cylinder height of 250 mm. the following day, the hardened samples were demoulded, cut into 50 mm thick slices, and stored in 1 % lime water solution. from each type of concrete one sample was loaded with electric current, following the op measurement. later, the pieces necessary for further experiments were taken from all the samples. the mip was performed on every type of the sample. sem, tga, xrd was only performed on the ca samples, because of capacity and time reasons. the overall experimental flow is also shown in figure 2. 3.2. dc test one year saturated samples were exposed to a constant voltage of 20 v for 24 hours. the scheme of the experiment is shown in figure 1. the stainless steel electrodes were placed in two chambers filled with 0.3 m naoh solution. the concrete sample was placed between both chambers to create the conductive path. the electric current is conducted through continuously connected pores filled with pore solution. the samples loaded by the electric current are labeled el, and samples not loaded by the electric current are marked as nl. during the dc test the voltage was set as constant and the current was measured. the charge, q, can then be expressed by formula: q = ∫ i(t)dt, (2) where i is the the current, t is the time. 3.3. open porosity after dc test op was measured. fully saturated samples were placed in an oven at 50 °c, and the sample’s mass was sequentially measured to detect the water loss. samples were considered dried if the daily water loss ratio to the fully saturated sample was less than 0.1 % for at least 2 consecutive days. the period for which the samples have usually dried is approximately 20 days. following the open porosity measurement the dried samples were broken to the pieces for another analysis. 3.4. mip mip was performed on samples weighing ∼1.5 g. the measurements were performed on an autopore iv 9500 instrument for pore sizes ranging from 0.0015 µm to 130 µm. 3.5. image analysis the samples fragment were cast with epoxy resin into cylindrical molds, then cut into 6 m slices and polished according to the procedure described in [21]. prepared samples were observed by sem phenom xl. 71 d. ňachaj, j. němeček, m. kouřil et al. acta polytechnica ctu proceedings 10 bse images were performed on each sample on the spots carefully chosen in the cement matrix. thus, magnification varied from 1350× to 1600× to avoid aggregates. totaly covering area 0.5 mm2, which is sufficient for statistical analysis [21]. 3.6. xrd and tga both xrd and tga experiments were performed only on ca samples for capacity and time reasons. simultaneous thermal analysis (sta), consisting of differential scanning calorimetry (dsc) and thermogravimetry (tg), was carried out using a labsys evo tg/dsc from setaram inc. experiments were done in the temperature range from 30 °c to 1000 °c with a heating rate of 5 °c/min. an argon atmosphere with a flow rate of 40 ml/min was set during the measurements. xrd was performed using the powder diffraction technique using x-ray. it was done by panalytical aeris machine with bragg-brentano arrangement. 4. results and discussion 4.1. results of dc test samples cb and cc show the expected trend of relatively constant electric current. the electric current waveform for sample cc is fluctuating and it is also lower than for sample cb. however, the trend is the same for both samples. the current of sample ca is increasing at the beginning of the test, followed by an constant current progression followed by a current drop after 16 hours of the test. typical curves are shown in figure 3. the results of the calculated total transferred charges are summarized in table 2. the smallest charge was measured for the cc sample that could be assigned to a higher sample resistance due to microsilica supplement. on the other side ca sample without admixture showed the highest charge suggesting the lowest resistance and highest porosity compared to other samples. this could be due to the fact that the ca sample contains more large pores through which more electric current passes, see figure 4. label ca cb cc q [c] q [c] q [c] -el 3967 3285 2996 table 2. total charge measured during dc test. 4.2. results of open porosity the results of open porosity indicate the impact of electric current on the porosity of different concrete mixtures. all three mixtures show an increase in porosity after the dc test, see table 3. the highest increase of open porosity by ∼2 % was found for the ca-el sample. the mixtures with admixtures (cb and cc) exhibited a minor increase in open porosity below 1 %. the larger change for the ca sample 0 4 8 12 16 20 24 32 34 36 38 40 42 44 46 48 time [h] cu rr en t [m a ] ca cb cc 1 figure 3. change of the electric current over time during dc test. could be due to a different admixture and so different microstructure. however, the op can only measure free-water that is evaporated from the sample. label ca cb cc ϕop [%] ϕop [%] ϕop [%] -nl 12.22 12.70 11.47 -el 14.27 13.34 12.30 change +2.05 +0.64 +0.83 table 3. results of open porosity. 4.3. results of mip results of total porosity measured by mip are summarized in table 4. similarly to open porosity, mip did not show significant changes between the samples. the results for ca sample agree with those one obtained from op. however, a decrease in porosity was found for cb and cc samples after the dc test. label ca cb cc [%] [%] [%] -nl 15.69 16.25 16.21 -el 16.62 14.47 15.89 change +0.93 −1.78 −0.32 table 4. results of porosity determined by mip. the pore size distribution curves measured by mip are shown in figure 5 for individual samples loaded and non-loaded by the current. it is possible to observe a shift of the main porosity peak into the smaller values after the dc for all three mixtures. that is in agreement with the study of susanto et al. [11], where a similar shift was observable for mortar samples. 72 vol. 40/2023 effect of electric current on porosity of concrete 0-0.01 0.01-0.1 0.1-1 1-130 0 10 20 30 40 50 60 70 80 pore radius [µm] p or e vo lu m e [% ] ca-nl ca-el cb-nl cb-el cc-nl cc-el 1 figure 4. pores volume changes in the different size ranges of samples loaded and non-loaded by the electric current. 1001010.10.01 0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10 -d v /d lo gr [m l /g ] a) ca ca-dc 1001010.10.01 pore radius [µm] b) cb cb-dc 1001010.10.01 c) cc cc-dc 1 figure 5. pore size distribution curves of samples loaded and non-loaded by the electric current. figure 4 displays the changes in pore distribution in different intervals. there is evident increase of pore volume with a radius smaller than 0.01 µm after the current test. the increase is accompanied by the decrease of pores volume in the range (0.01–0.1) µm. the pores of a radius higher than 0.1 µm do not show any significant changes except for the ca sample where the increase of pores in the interval 1 µm–130 µm by ∼14 % takes place. the change in the distribution of the volume of pores is obvious, as are the trends. this means that after passing through an electric current, the distribution of the pores volume representation according to its radius changes, see figure 4. 4.4. results of image analysis image analysis was applied to sem-bse images of the ca sample. due to similar pixel intensity, only two phases were separated. the first phase was composed primarily of pores, but cracks and scratches were also present, as visible in figure 6. the second phase contains the rest of the phases, mainly hydration products, residual clinker, or small aggregates. the results of ia and phase volume fraction are shown in table 5, indicating a growth of the phase representing pores by 1.78 % for samples loaded with electric current. label pores/cracks other phases [%] [%] ca-nl 2.82 ±0.99 97.18 ±0.99 ca-el 4.60 ±2.18 95.40 ±2.18 change +1.78 ±1.59 −1.78 ±1.59 table 5. volume fraction of phases obtained from image analysis. 4.5. results of tga tg and dsc analysis revealed three distinct thermal decomposition processes. firstly, the physically bound (pb) water and water bound chemically in cs-h hydrates evolved between the ambient temperature and ∼250 °c. portlandite ca(oh)2 decomposes between 450 and 500 °c and finally caco3 polymorphs (calcite, amorphous caco3) between 610 and 800 °c. the tangential method [22] determined the content of portlandite and caco3 polymorphs. the content of portlandite in both samples is equal; the sample ca-nl contains 0.71 % more hydration products as well as 1.9 % more calcite, see table 6. this could mean, that due to the electric current 73 d. ňachaj, j. němeček, m. kouřil et al. acta polytechnica ctu proceedings (a). sem-bse image: ca-nl. (b). ia with phase separation: ca-nl. (c). sem-bse image: ca-el. (d). ia with phase separation: ca-el. figure 6. typical sem-bse image of samples ca-nl, ca-el. example of ia with phase separation: pores/cracks (blue), hydration products, residual clinker (cyan) of samples ca-nl, ca-el. treatment the calcite is leaching out of the sample. label pb water, portlandite∗ calcite∗c-s-h, gypsum [%] [%] [%] ca-nl 6.88 6.52 5.59 ca-el 6.17 6.52 3.69 legend: ∗tangent method [22]. table 6. content of thermally-decomposed components (% by mass) where pb means physically bound. 4.6. results of xrd the phase composition obtained by xrd is presented in table 7. the system is very complicated due to the presence of mixed aggregates containing not only quartz but also many feldspar minerals (anorthite, oligoclase, albite, orthoclase), which are very similar and difficult to distinguish. it must also be kept in mind that concrete contains a lot of c-s-h, which are amorphous and hence not indicated by xrd. moreover, the concrete samples with coarse aggregates pose a risk of homogeneity and representativeness. thus these results should be considered just as an indication. the content of calcite and portlandite seems to be 2 % higher for both of them in ca-nl, which agrees with thermal analysis. phase ca-nl ca-el [%] [%] portlandite 9 7 calcite 6 4 aft 2 1 afm 3 4 table 7. the phase composition in % obtained from xrd. 4.7. mutual comparisons the sample ca exhibits an increase in total porosity for samples loaded by current by all three measurement methods (op +2.05 %, mip +0.93 %, ia +1.78 %). the reason might be given by microstructure changes and dissolution of portlandite, calcite, and aft phases. that was confirmed by xrd/tga analyses and is with the agreement of study of ottosen et al. [19]. the results of the cb and cc samples are a bit different. there was an increase in open porosity, which was not confirmed by mip. the reason might be given by the different pore size measuring ranges between those two techniques where mip did not capture the change in large pores for cb and cc samples, contrary to the ca sample. despite this, the transition to smaller pores is even more pronounced for cb and cc samples as visible in figures 4, 5. overall, the dc test shows that the most of the electric current has passed through the ca sample which is in agreement with the op and mip showing the biggest increase of porosity after the treatment. 5. conclusions this paper describes changes in concrete porosity after the loading by direct current. the porosity was evaluated by three porosity measurement techniques: open porosity, mercury intrusion porosimetry, and image analysis of sem-bse images. the most important conclusions are summarized in the following points: • the influence of the electric current on the porosity of individual samples is different for various concrete mixtures depending on their composition and microstructure. • based on the open porosity results. one day treatment with electric current affects the porosity of concrete mixtures and increases it by 0.64 % to 2.05 %, depending on the mixture. • the increase of porosity for concrete sample made of ordinary portland cement takes place at the expense of other phases like portlandite, calcite, or afm. • based on the mip observation, the pores volume in the range (0.01–0.1) µm is decreased and in the 74 vol. 40/2023 effect of electric current on porosity of concrete range (0–0.01) µm is increased after the dc test. the difference is most pronounced for the sample modified with microsilica. acknowledgements financial support of the czech science foundation (project no. 21-11965s) and the grant agency of the czech technical university in prague (sgs22/088/ohk1/2t/11) are gratefully acknowledged. references [1] c. r. gagg. cement and concrete as an engineering material: an historic appraisal and case study analysis. engineering failure analysis 40:114–140, 2014. https: //doi.org/10.1016/j.engfailanal.2014.02.004 [2] r. loser, b. lothenbach, a. leemann, m. tuchschmid. chloride resistance of concrete and its binding capacity – comparison between experimental results and thermodynamic modeling. cement and concrete composites 32(1):34–42, 2010. https: //doi.org/10.1016/j.cemconcomp.2009.08.001 [3] a. guettala, a. abibsi. corrosion degradation and repair of a concrete bridge. materials and structures/materiaux et constructions 39(288):471–478, 2006. https://doi.org/10.1617/s11527-005-9046-z [4] j. němeček, l. li, y. xi. electrokinetic nanoparticle injection for remediating leaks in oil well cement. construction and building materials 156:63–72, 2017. https: //doi.org/10.1016/j.conbuildmat.2017.08.152 [5] j. němeček, j. kruis, t. koudelka, t. krejčí. simulation of chloride migration in reinforced concrete. applied mathematics and computation 319:575–585, 2018. https://doi.org/10.1016/j.amc.2017.07.029 [6] n. jafariesfad, m. geiker, s. sangesland, et al. electrokinetics application in concrete and well construction. in international conference on offshore mechanics and arctic engineering, vol. volume 11: petroleum technology. 2020. v011t11a031. https://doi.org/10.1115/omae2020-18275 [7] astm. standard test method for electrical indication of concrete’s ability to resist chloride ion penetration, 2012. c1202–18. 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https://doi.org/10.1016/j.micromeso.2020.110125 https://doi.org/10.1016/j.conbuildmat.2018.11.222 https://doi.org/10.1016/j.conbuildmat.2018.11.222 https://doi.org/10.1109/tsmc.1979.4310076 https://doi.org/10.1016/b978-0-08-100117-2.00010-8 https://doi.org/10.1016/b978-0-08-100117-2.00010-8 https://doi.org/10.1080/10934520801973949 https://doi.org/10.1039/an9638800906 https://doi.org/10.1016/j.mtcomm.2019.100806 acta polytechnica ctu proceedings 40:69–75, 2023 1 introduction 2 experimental methods 2.1 direct current test 2.2 open porosity 2.3 mercury intrusion porosimetry 2.4 image analysis 2.5 x-ray diffraction 2.6 thermogravimetric analysis 3 experimental part 3.1 experimental plan and sample preparation 3.2 dc test 3.3 open porosity 3.4 mip 3.5 image analysis 3.6 xrd and tga 4 results and discussion 4.1 results of dc test 4.2 results of open porosity 4.3 results of mip 4.4 results of image analysis 4.5 results of tga 4.6 results of xrd 4.7 mutual comparisons 5 conclusions acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2023.43.0051 acta polytechnica ctu proceedings 43:51–56, 2023 © 2023 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague proposal of operational concept for bulk substrates transport by freight trains and its application within case study of aggregate transport for construction of vlachovice dam martin jacura, lukáš týfa∗ czech technical university in prague, faculty of transportation sciences, department of transportation systems, konviktská 20, 110 00 prague, czech republic ∗ corresponding author: tyfal@fd.cvut.cz abstract. railway freight transport represents an advantageous way for regular transport of bulk substrates. as a result of changes within the industry and energy sectors as well as in the field of consumer behaviour, such business cases decline significantly. nevertheless, transport such of the characteristic have being (and will be) still realized however, in addition, quantities of transported goods will be known in advance – as a part of major construction or environmental remediation projects. in the period of 2020–2021, the department of transporting systems in the ctu in prague, faculty of transportation sciences prepared the study “vlára, vlachovice hydraulic structure, logistic study of material transport for the dam construction” for the state enterprise povodí moravy (the morava river basin). among other things, the study dealt with transport of more than 2.5 million tonnes from two quarries to the construction site of the vlachovice dam by goods trains (the vlachovice dam will be built in moravia – the eastern part of the czech republic). it was necessary to determine an optimal route of the goods trains, their parameters and general operating concept so that the transport costs would be minimised and, concurrently, the transport performance would be as high as possible. this paper sets itself a goal to present a general procedure and principles for proposal of freight trains for a business case characterized by long-term and regular transport of large quantities of goods. for the stated case study, transport technology in the railway station bohuslavice nad vláří is discussed in detail. keywords: railway freight transport, bulk substrate, operating concept, innofreight technology, vlachovice hydraulic structure. 1. railway freight transport and bulk substrates transport railway freight transport represents an advantageous way for regular transport of bulk substrates. as a result of changes within the industry and energy sectors as well as in the field of consumer behaviour, such business cases decline significantly. nevertheless, transport such of the characteristic have being (and will be) still realized however, in addition, quantities of transported goods will be known in advance – as a part of major construction or environmental remediation projects (construction of dams, power stations, transport infrastructure, pipelines; restoration of areas after mineral extraction, removal of soil contaminated by toxic substances). both railway and water transport are together considered as a sustainable way of transportation. sustainability parameters may be various (for example greenhouse gas emissions, dependence on fossil fuels, impacts of transport accidents) and are of different importance. some of them may be considered according to extents of so-called external costs; at the same time, it is necessary to perceive positive benefits of individual kinds of transport (for example economical and social development) [1]. sustainable development of transportation was solved e.g. within the scope of freightvision project [2]. in a specific business case, a rail freight carrier’s revenue depends particularly on the type of rail wagons used, quantity (mass) of goods transported in the train, and chargeable distance (which may not coincide with actual distance travelled) plus any possibly added services. from the viewpoint of costs for freight train running, constant items are: a railway traction vehicle(-s) – its/their depreciation and maintenance costs, costs of energy required for its movement, driver’s personal costs, and costs of covering price of the railway allocation body for allocating and using the railway infrastructure to a specific train. that is why, both carrier and carriage orderer strive to make the freight train carries as much goods as possible. at solving a task of ensuring transport between two points, an optimal train route is sought for. in defiance of the dense railway network in the czech republic, usually no more than two actual options are considered so this preparation phase of transport is not 51 https://doi.org/10.14311/app.2023.43.0051 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en martin jacura, lukáš týfa acta polytechnica ctu proceedings included in detail in this paper. however, determining parameters of train units providing transport and proposing an operating concept for successful management of the given business case represent more demanding task. 2. operational concept of trains carrying known quantity of bulk substrate 2.1. quantitative parameters of goods train before proper proposing an operating concept of goods trains for a specific business case, it is necessary to determine quantitative parameters of the train, in particular, a maximum amount of freight carried in the train and maximum running speed. for most of bulk substrates – in view of their high (powder) volumetric mass – the maximum transported load weight is determinative. the maximum quantity of goods that can be loaded into one rail freight wagon or into one exchangeable transport unit (container, swap body) is based on the comparison of the loading (bulk) apparent density of the goods and the apparent density of the space intended for loading the goods. for solid goods of the bulk material type (unlike liquids), it is important to consider the bulk apparent density that is calculated from the bulk apparent density in the compact state using the swell coefficient. the swell coefficient indicates a multiple of volume increase of bulk material of a given weight that normally takes up in normal bulking without any subsequent compaction compared to the compact state of the material (with minimum air gaps). the larger particle size fraction of the bulk material and the smaller the fraction (min/max grain size), the larger the swell coefficient. if the loading (bulk) apparent density is greater than the apparent density of the space intended for loading the goods, the weight of the goods to be transported is decisive. determining a type of wagons to be assembled into a train or selecting specific logistics technology represent another task. for transports where quantities of goods to be transported (it means duration of a contract) are known in advance, it is usually advantageous to use flat wagons with transport units (containers, swap bodies). advantages of separating the wagon from the transport unit are as follows: • a wagon must meet very stricter (safety) requirements than a transport unit and, therefore, it is a subject to more frequent and demanding inspections and maintenance. • in particular, the wheelsets and bogies of the wagon are subject to more wear and tear than a transport unit and are therefore subject to more frequent and, therefore, it is a subject to more frequent and demanding inspections and maintenance. • from the viewpoint of investment costs (it means depreciation too), a wagon is more expensive than a transport unit. • unification of the carrier’s rolling stock of trailers – so, need for a smaller absolute number of reserve trailers and their significantly simpler maintenance (hence lower operating costs). • at the end of a contract or when at parameters changing, transport units can be easily replaced and any wagon can be used after loading with another type of transport unit for another contract, so it still earns money for its owner (a carrier). • if the wagon must be taken out of service for inspection or maintenance, transport units can be transferred to another wagon and the transport can continue smoothly. after single preparation phase of the transport, the maximum quantity of goods to be transported in one wagon should be known depending on the railway line category (the load carrying capacity of the line) and max. running speed. flat wagon of modern design intended exclusively for haulage of transport units (containers, swap bodies) usually are of the maximum permitted speed of 120 km h−1 when empty and 100 km h−1 when loaded or 100 km h−1 at all times. the maximum load weight of the goods depends on the type of used transport units and their arrangement on a vehicle (load transfer to the individual axles). this kind of wagons usually cannot be included in a train without transport units so they have to be loaded with ballast (dead weight) in such the case. this is due to low dead weight of such wagons which could cause the wagon to derail due to the traction and braking forces in the train during its trip. in the next step of the transport preparation, it is necessary to find out parameters of the railway infrastructure affecting length, speed, and weight of freight trains on their expected route and line capacity. in particular, it includes a number of tracks, line speed, compatibility, maximum permitted train length according to length of station running tracks, decisive longitudinal gradient, and possibility of using dependent electric traction (and the type of railway traction system in a case of its existence). the data must be determined for individual quasi-homogeneous sections of the train route. then, the output represents train composition within individual sections – it means a number and type of traction vehicles, number of wagons, and maximum train speed. this also results in the maximum mass of freight transported in a single train. 2.2. proposal of conception of operation for the proposal of the conception of the operation, a qualified estimation of train running times within individual route sections is essential – for both directions: for loaded rake there and return empty ones. if 52 vol. 43/2023 proposal of operational concept for bulk substrates transport by freight trains freight trains of similar parameters (especially weight, and number and type of traction vehicles) are operated on the line sections in question their journey times can be used with advantage. otherwise, it is appropriate to simulate the train running based on the motion equation of the train of the assumed parameters. for estimating travel times, the authors of the paper recommend to consider stopping the train at all operating posts suitable for overtaking or crossing the given train even though most of the operating posts will be passing in normal operation. it creates a reserve to cover various operational contingencies and, then, the proposed conception of operation will be stable. for this purpose, it is necessary to determine duration of necessary technological operations during the train route (e.g. traction vehicles changing). furthermore, it is necessary to analyse in detail the theoretical diagram of train running valid at the time of the business case realizing and propose train routes within the theoretical diagram of train running and optimum number and parameters of the trains – for both directions of train movement. if it is necessary to shorten journey times in some sections to ensure the operation of other trains it is possible partial using of the journey time reserves (see above) realising the increased probability of timetable instability. if the timetable is not yet known at the time of the business case it is possible to estimate the differences from the current situation within the scope of discussions with the carriers of existing passenger and freight trains and, further, in the case of passenger trains, from the transport service plans of individual regions and the whole country (the czech republic). the operational concept proposal is completed by a proposal of train circuits resulting not only in a number of trains required but also in daily train movements, daily transport volume (in metric tons), and transport capacity (in tonne-kilometres [tkm]). 3. case study of operational concept of aggregate transport for construction of vlachovice dam 3.1. vlachovice hydraulic structure and transport of materials for dam construction the vlachovice hydraulic structure (dam) is situated in the zlín region in the eastern part of the czech republic near the slovakia border and it will be stretched particularly on vlára watercourse. water retention and drawing for following treatment into drinking water represent the main purpose of the dam. the dam is designed to retain 29.1 mil. m3 of water. the structure is designed as a gravity dam with height of up to 40 m. the construction of the dam will require 1 352 mil. m3 of various materials. in the period of 2020–2021, the department of transporting systems in the ctu in prague, faculty of transportation sciences prepared the study “vlára, vlachovice hydraulic structure, logistic study of material transport for the dam construction” [3] for the state enterprise povodí moravy (the morava river basin). according to the above-mentioned study, authors succeeded in proposing transport of 1 024 mil. m3 of aggregate (grain size up to 350 mm) from quarries to the depots situated near the dam by railway transport. so, negative environmental impacts of transport will be significantly less serious than in case of road transport. for this purpose, restored operation on a part of the siding from the bohuslavice nad vláří station to the former vrbětice warehouse [4] and its extension in several variants were proposed within the scope of the aforementioned logistic study. in the study, innofreight technology [5] was chosen as a wagon type or rather logistics technology. within this technology two-piece flat wagons innowaggon of 80 feet in length with xm containers of 24 m3 in volume were chosen for this business case. thus, the innofreight technology allows to take advantage of possibility to separate a railway wagon and transport unit. with regard to requirements for aggregate unloading and loading during day hours, so-called night jump transport was chosen at which two loaded trains one by one leave two quarries after 10 p.m. for the unloading point while four trains work again in the opposite direction for the loading point. regardless of choice of quarries, it is expected that both loaded and empty trains will run to/from bohuslavice nad vláří station from/to staré město u uherského hradiště station. in the viewpoint of train parameters, it can be stated that train length is not a major limiting element. on the contrary, weight standard (maximum load that can be attached behind a traction vehicle) whose lowest value belongs to the bojkovice – slavičín section on the line vlárský průsmyk state border – staré město u uherského hradiště represents the key parameter. this railway line is also restrictive from the viewpoint of safe passage capability of a rail vehicle since its entire length corresponds to line load class c3 (i.e. a maximum weight of 20 t per axle). each train consists of seven loaded wagons, transport weight achieves 1,078 metric tons, i.e. up to 840 metric tons of material transported per train. the proposed operating technology creates the need for eight rakes, i.e. a total of 28 innowaggons. the running time of the wagons (rakes) represents 24 hours and the transport volume is 3 360 metric tons per day. the line section uherské hradiště – bojkovice město is heavily loaded with regional passenger rail transport and the insertion of freight train routes at 05 am–10 pm would be quite problematic. trains routes were inserted into the diagram of train running so that trains pass each other on double-track lines and do not need to cross on any single-track section. trains positions in the diagram of train running are 53 martin jacura, lukáš týfa acta polytechnica ctu proceedings figure 1. bohuslavice nad vláří station track layout (appendix no. 6.3. of [3]) figure 2. handling of rakes at the bohuslavice nad vláří station. set so that the pairs trains arrive to the bohuslavice nad vláří station with a mutual time gap of at least 60 min. 3.2. transport technology at bohuslavice nad vláří station aside from determining the train routes and their positions in the diagram of train running, the study [3] included also the transport technology of operation with rakes at the bohuslavice nad vláří station incorporating also the siding for the deposit of material for construction of the vlachovice dam connecting to the public connected with the railway network. the bohuslavice nad vláří station currently consists of three running tracks and one service track (see diagram in figure 1). to ensure standard train operation in this station, the running track no. 1 must always remain free for the passage of trains and, during the time of train crossings, also the running track no. 2. the running track no. 4 will be used exclusively for entry and exit of rakes to the siding – connection of the siding to this track (via switch no. 5) represents a reason for that. the service track no. 6 will be intended for the staging of rakes waiting to enter the siding or for the arrival of empty rakes from the siding. the transport technology will be simplified by allowing departures to the siding directly from the service track no. 6. at the point of material unloading from the rakes at the end of the siding, this study proposes construction of two dead-end tracks so that two rakes could be unloaded at the same time. handling of rakes at the bohuslavice nad vláří station is indicated on the diagram in figure 2 and will be realized as follows: two trains from the a quarry 54 vol. 43/2023 proposal of operational concept for bulk substrates transport by freight trains (marked a1 and a2) will arrive in succession (so-called on answer-back signal) at the bohuslavice nad vláří station at 02 am on the tracks no. 2 and no. 4. the rake a1 leaves the track no. 4 as soon as possible for the siding. then, the rake a2 will be moved to the track no. 4 and also leaves for the siding when the siding section becomes empty. the station tracks no. 2 and no. 4 will be released by 03 am. two trains from the b quarry (marked b1 and b2) will arrive at the station in succession in the time period 03 am– 04 am on answer-back signal (again on the tracks no. 2 and no. 4). then, the b1 rake will be moved to the service track no. 6 while the b2 rake will remain out of operation on the running track no. 4. both rakes will await for unloading of the rakes a1 and a2 and their return from the siding to the station. simultaneous stay of three rakes in the station around 01 pm represents the most technological demand. the rake b2 will be moved from the track no. 4 to the track no. 1 before entry of the rake a2 from the siding into the station while the rake a2 will be moved to the track no. 2 immediately after its arrival into the station. afterwards, the rake b2 returns to the track no. 4 from the track no. 1 and departs to the unloading point on the siding. when it will reach the unloading point on the siding, the rake a1 will be dispatched from the unloading point with empty wagons. immediately after its arrival at the station on the track no. 4, it will be moved to the track no. 1. subsequently, the rake b1 will depart from the service track no. 6 via the running track no. 4 to the siding. after its departure to the siding, the rake a1 will be moved from the track no. 1 to the track no. 6 and the rake a2 will be moved from the track no. 2 to track no. 4. this manipulation can be carried out during breaks between the passage of other trains through the station. the rakes a1 and a2 will remain out od operation in the station on the tracks no. 4 and no. 6 until 10 pm. around this time, these rakes will be dispatched as trains in the direction to staré město u uherského hradiště. when the station is emptied, the rakes b1 and b2 will be withdrawn from the siding and, after appropriate technological procedures, will be dispatched as trains also in the direction to staré město u uherského hradiště. 4. conclusion in the introduction of this paper, rail freight transport was introduced as a suitable mode of transport for solving specific types of business cases that are not often mentioned in the professional literature – for the transport of bulk substrates with known volume of transported goods. further, input parameters, limiting conditions, and the procedure for planning the conception of the operation of trains that will be applied to such types of presented transport. the transport of aggregate from two quarries for the construction of the dam of the vlachovice hydraulic structure in the east moravia was presented as a specific case of such the business case. the design of the conception of the operation for this business case was prepared for the state enterprise povodí moravy (the morava river basin) by the department of transporting systems in the ctu in prague, faculty of transportation sciences [3]. in the above-mentioned study, it was possible to propose the transport of 1 024 million m3 of aggregate from the quarries to the depots near the dam construction site by the railway transport. it was proposed to resume operation on the siding coming from the bohuslavice nad vláří station. innofreight technology was chosen as the type of wagons and logistics technology in the study. furthermore, the paper describes in detail the optimal transport technology in the bohuslavice nad vláří station proposed in this study. it includes movements of the rakes between individual station tracks and their departures to and arrivals from the siding under the constraining conditions resulting from the preservation of standard railway operation in the station and from the configuration of the track facility station. the transport concept chosen in this study is optimal both in the terms of the required railway vehicles and the transported material. the rakes are interchangeable, the proposed technology does not require any investment in the public railway network and does not restrict the operation of long-distance or regional passenger trains. successful implementation is conditioned by preliminary negotiation with the infrastructure manager (správa železnic, state organisation) so that no closure would be planned on the transport route during this time and loading ban would be declared for the bohuslavice nad vláří station in the period of material transport; there must also be no reduction of the track at the bohuslavice nad vláří station. this study “vlára, vlachovice hydraulic structure – logistic study of material transport” represents also the evidence that railway has an irreplaceable place in the transport of bulk substrates and its use will lead to reduction of negative impacts of transport on the environment. references [1] v. malinovský. comparative analysis of freight transport prognoses results provided by transport system model and neural network. neural network world 31(4):239–259, 2021. https://doi.org/10.14311/nnw.2021.31.013 [2] vision and action plans for european freight transport until 2050. [2022-05-01], https://cordis.europa.eu/project/id/217927. [3] l. týfa, m. jacura, t. javořík, et al. vlára, vodní dílo vlachovice – logistická studie přepravy materiálu. logistic study, fts ctu in prague, 2021. [4] brno university of technology. posouzení použitelnosti železniční vlečky pro stavbu vd 55 https://doi.org/10.14311/nnw.2021.31.013 https://cordis.europa.eu/project/id/217927 martin jacura, lukáš týfa acta polytechnica ctu proceedings vlachovice: f.2 technická zpráva. technical study, fce but in brno, pp. 15 (4 appendices), 2019. [5] m. boháč. patnáct let technologie innofreight v české republice. [2022-05-01], cargovák 12:4, 2020, https://www.cdcargo.cz/documents/10179/3653049/ cargovak_2020_12.pdf/c0ebdba1-1bbb-4515-b8578b3289059b29. 56 https://www.cdcargo.cz/documents/10179/3653049/cargovak_2020_12.pdf/c0ebdba1-1bbb-4515-b857-8b3289059b29 https://www.cdcargo.cz/documents/10179/3653049/cargovak_2020_12.pdf/c0ebdba1-1bbb-4515-b857-8b3289059b29 https://www.cdcargo.cz/documents/10179/3653049/cargovak_2020_12.pdf/c0ebdba1-1bbb-4515-b857-8b3289059b29 acta polytechnica ctu proceedings 43:51–56, 2023 1 railway freight transport and bulk substrates transport 2 operational concept of trains carrying known quantity of bulk substrate 2.1 quantitative parameters of goods train 2.2 proposal of conception of operation 3 case study of operational concept of aggregate transport for construction of vlachovice dam 3.1 vlachovice hydraulic structure and transport of materials for dam construction 3.2 transport technology at bohuslavice nad vláří station 4 conclusion references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2024.51.0114 acta polytechnica ctu proceedings 51:114–120, 2024 © 2024 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague safety aspects of e-cars charging at railway stations roman štěrbaa,∗, jiří cigánekb a czech technical university in prague, faculty of transportation sciences, department of smart cities and regions, konviktská 20, 11000 praha 1, czech republic b všb technical university of ostrava, faculty of electrical engineering and computer science, department of electrical power engineering, 17. listopadu 2172/15, 708 00 ostrava – poruba, czech republic ∗ corresponding author: sterba@fd.cvut.cz abstract. the article focuses on introducing e-cars in the czech republic and the safety aspects of electric cars, particularly when charging at railway stations. safety aspects are comprehensively linked to the charging of electric vehicles, where maintaining the balance of the electrical network is crucial to prevent asymmetric loading. the article’s primary aim is to inform readers about the introduction of railway station charging stations and highlight the proactive measures taken to address possible technical problems that can arise during recharging. fire intervention is a key part of the safety plan, ready to be deployed when an electric vehicle catches fire and poses a risk to the surroundings. keywords: electric car, safety aspect, charging, fire prevention, parking. 1. introduction transportation is essential to our economy and society. mobility is important for the internal market and citizens’ standard of living, enabling them to enjoy the freedom of travel. transport contributes to economic growth and job creation, and it must be sustainable in relation to the new challenges we face [1, 2]. congestions, poor air quality, and, above all, noise pollution affect cities the most. urban transport accounts for roughly one-quarter of co2 emissions from transport [3, 4]. to reduce the emission footprint not only in transportation, the european union (eu) adopted the green deal for europe [5]. the green deal for europe is a set of policy initiatives to set the eu on the path to ecological transformation with the ultimate goal of achieving climate neutrality by 2050. it supports the transformation of the eu into a fair and prosperous society with a modern and competitive economy. this goal can be achieved in transportation by switching from fossil fuels to alternative fuels. alternative sources of propulsion include electric power or hydrogen fuel. in the article, the authors delve into the safety aspects of electric cars. the authors, who work at správa železnic, the czech state rail infrastructure manager, are actively involved in installing charging stations at railway stations across the country. electromobility in road transport originated early in the 19th century. the car, assembled by the dutch professor sibrandus stratinghem in 1835, is considered the first electric car in history. an electric drive powers an electric car. regarding e-mobility, it is necessary to control the fire risks [6]. as described in the article, fighting e-car fires requires three different methods. the czech state rail infrastructure manager (szcz) fire service has already applied all three methods on a training basis in connection to the operation of p+r and parking houses at railway stations. the authors1 participate in the introduction of electromobility at správa železnic. electromobility corresponds with the “policy statement of the government” [7] as well as the corporate strategy of the správa železnic [8]. in cooperation with colleagues, they edited the internal document “infrastructure development plan for electric vehicle charging”, which deals with public charging infrastructure at train stations. project documentation is required before constructing parking places for electric cars at railway stations, and this paper’s authors are currently supervisors. as part of the project documentation, it is always necessary that the “fire safety solution for the building” is being prepared within the scope of § 41 of directive no. 246/2001 coll., as amended, in all contexts following the “methodological guide for design and assessment of a fire safety solution” (edited by the ministry of the interior – general directorate of czech fire service, august 2018) so that details can be incorporated into individual professions. the proposal to ensure the fire safety of buildings is based on the “ methodological recommendation fire safety of buildings – electromobility” (edited by the ministry of the interior – general directorate of the fire service cz, april 2021). 1roman štěrba, head of conception and strategy unit at správa železnic. jiří cigánek, energy expert at strategy department at správa železnic 114 https://doi.org/10.14311/app.2024.51.0114 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 51/2024 safety aspects of e-cars charging at railway stations figure 1. map of charging stations in czechia [9]. 2. public parking with charging stations the “national action plan for clean mobility” [10] envisages the development of transport using alternative fuels and aims to develop electromobility with the support of subsidy programs [10]. in the same way as the laws, decrees, and standards of čsn apply to the design and placement of gas stations (e.g., čsn 73 6060), there are also laws, decrees, and standards that must be followed when placing charging points for electric cars. in particular, from the point of view of fire safety, it is necessary to follow the rules for the placement of charging stations and for parking the electric car while charging. by the end of 2022, 1 364 charging stations and 2 643 charging points had been installed in the czech republic [11]. other charging stations become operational every day. the strategic plan of the “national action plan for clean mobility” is to ensure 19 000– 35 000 public charging points by the end of 2030 [10]. concerning the number of charging stations in operation, the czech republic still lags behind the eu in the funds invested in building charging infrastructure. the current map of charging stations (figure 1) is on the website [9, 12]. the charging station or charging point in the public parking area must meet the parking requirements čsn en 61851-1, čsn en 61851-21, čsn en 61851-23, čsn 332000-4-41, čsn 332000-554, čsn en 621962, čsn en 62196-3 and related standards. when placing charging stations, it is necessary to respect the safe distances from the equipment and the normative distances specified in the standards of the čsn en 7308xx series and related regulations. 2.1. planning public charging infrastructure at train stations when planning public charging infrastructure at train stations managed by správa železnic, it is first necessary to determine whether there is reserved power at that location. if not, a new power line with sufficient power must be built in agreement with the electricity distributor. správa železnic already cooperates with both the electricity supplier and the distributor in the development of charging points within the rail network, e.g., this is the case of the newly designed station prague – dlouhá míle on the newly constructed václav havel airport railway line. the authors are involved in the technical solution of charging points. the following factors need to be taken into account when recharging: (1.) duration of charging (charging speed) determines the performance of the charging point (collection point) and the vehicle’s capability. (a) by the power of the on-board ac charger. if the vehicle can be recharged with 7 kw ac power, it also recharges when connected to a station with a power of 11 or 22 kw. (b) charging management during dc charging. the vehicle battery controls the station’s performance depending on several factors, including weather (today, vehicle batteries can be preheated), battery condition, and device operating temperature. 115 roman štěrba, jiří cigánek acta polytechnica ctu proceedings (2.) duration of charging at a public station calculated as charging the battery from 0 to 100 % is misleading. from the practical experience, it is better to indicate the charging duration from 10 % to 80 %. (a) the average charging volume per charging station is approximately 17 kwh. cooperation with the control and controlled stations is necessary to avoid exceeding the contracted quarter-hour maximums. (b) except for long-distance transit, where there is an interest in charging as much as possible as quickly as possible, people charge part of the battery capacity. still, more powerful charging stations logically provide them with greater comfort (the range increases by 100 km during charging instead of 20 km). (3.) when recharging at railway stations, a combination of stations with different capacities is offered. railway stations represent natural points of interest with good transport accessibility and facilities, thus an opportunity for high-performance stations. every electric car supports ac charging. the onboard device is one of the more expensive components in an electric vehicle, and it limits the number of phases used and the maximum charging. dc charging has not been supported by older electric car models since the beginning of electromobility. while ac charging is constant from the user’s point of view throughout the charging period due to lower powers, dc charging reduces the drop in charging current and significantly slows down charging. charging stations are connected to the distribution system at low or high voltage levels. the connection from the low voltage level is intended for charging powers up to approximately 150 kw. the protection is at 200–250 a at a voltage of 400 v. the connection from the high voltage level is for charging stations with higher output from 150 kw onwards. 2.2. distribution of charging stations currently, the fire service can meet specialized public charging stations for electric cars. the fundamental division of charging stations can be done, for example, according to the following criteria: (a) power: • figure 2 shows slow charging station (regular charging station) with an output of up to 22 kw, • figure 3 shows fast charging station. (b) type of charging: • ac charging, • dc charging. (c) vehicle connection method: • station equipped with a socket (the vehicle is connected with a cable that is an accessory of the vehicle), figure 2. a slow-charging station in czechia [9]. figure 3. a fast-charging station in czechia [13]. • station equipped with an integrated cable. (d) functional equipment: • common charging stations, • stations equipped with “smart” functions. table 1 shows the ranges of electric cars as measured by what car magazine. ultra-fast charging stations with a power of up to 350 kw are already being installed. if there is insufficient reserved power for fast charging stations, a new connection with valid conditions must be built. when installing stations in parking slots or parking garages, the technical conditions of buildings regarding fire safety must be observed. both passive and active fire separation must be observed according to applicable standards. designing a charging station for electric cars needs to be based primarily on decree no. 268/2009 coll., as amended, and the set of standards čsn en 62196 and čsn en 61851. all parking space markings must follow act no. 361/2000 coll., as amended, čsn en 12899-1 and čsn en 1436. for the possible safe implementation of a fire intervention, it is recommended that parking places for charging electric cars shall be implemented at least as wide as parking spaces for physically disabled persons 116 vol. 51/2024 safety aspects of e-cars charging at railway stations car indicated range real range difference tesla model s 75d 490 km 328,3 km 161,7 km hyundai ioniq electric 280 km 188,3 km 91,7 km renault zoe r110 316 km 235,0 km 81,0 km kia e-niro 485 km 407,2 km 77,8 km hyundai kona electric 64 kwh 482 km 416,8 km 65,2 km smart fortwo eq 160 km 95,0 km 65,0 km nissan leaf 270 km 206,0 km 64,0 km smart forfour eq 155 km 91,7 km 63,3 km jaguar i-pace 470 km 407,2 km 62,8 km hyundai kona electric 39 kwh 312 km 254,3 km 57,7 km volkswagen e-golf 231 km 188,3 km 42,7 km bmw i3 94 ah 235 km 194,7km 40,3 km table 1. ranges of electric cars as measured by the magazine what car? [14, 15] on our test track, combining different types of traffic under the same conditions for all vehicles and without the influence of surrounding traffic. (people with reduced mobility), i.e., at least 3.5 m wide (or according to čsn 73 6056, article 6.6.2). 3. fire prevention the fire sections of individual parking places and garages are equipped beyond the scope of building fire safety standards: a) when charging stations are implemented in closed spaces, fire detectors must be installed following the set of čsn en 54 standards connected to alarm security and emergency systems/electronic fire signalization. b) to ensure the safe execution of a fire intervention, a parking space for recharging an electric car is recommended to be at least as wide as a parking space for physically disabled persons, i.e., at least 3.5 m wide (or according to čsn 6056, article 6.6.2). the investor evaluates the possibilities of a fire intervention as part of the processing of the “fire safety solution of the building”. in addition, the investor applies to the manufacturer of the charging station a request for the processing of “fire fighting documentation”, if it is an activity with an increased fire risk for which one there are no normal conditions for intervention2. 4. safety rules when charging electric cars when charging, it is important to remember that it is an electrical device that can be powered by a highvoltage system. when connecting the cable, which is protected by insulation, there is no danger even in the rain, as the charging process starts only after the circuit is closed, when both connectors at both ends are securely connected. the electric current only 2the conditions of § 4 paragraph 2 letter j) of act no. 133/1985 coll., as amended; complex conditions for intervention according to § 18 of decree no. 246/2001 coll., as amended starts flowing into the vehicle after checking both the car and charging infrastructure sides; the connection must be secure. another safety risk is charging an electric car during a storm. the question of what happens if the charging stand or the superior network is struck by lightning arises. during a local thunderstorm, the general rule is to avoid contact with electrical equipment. a lightning strike in or near an electric car can damage the internal electrical installations and equipment of the vehicle and the charging station, even if it is usually protected by surge protection. if the charging cable is disconnected from the vehicle, charging shall stop, and there is no risk of electric shock when touching the body of the electric car. the high-voltage battery system includes precautions against electric shock. 5. preparedness to deal with electric vehicle fires 5.1. detection of increased temperature of electric cars for monitoring the critical temperature of an electric car or battery, it is recommended to choose a system of fixed thermal cameras with direct connection to the existing camera system with recording and connection of the alarm signal from the thermal camera directly to the electronic fire alarm/centralized protection desk system operated by fire service. for monitoring outdoor charging stations, it is recommended to monitor only the critical state, i.e., exceeding the temperature of approximately 90 °c [16]. this system is designed based on more than ten years of czech fire service experience in designing and implementing these systems. also, it considers the newly prepared čsn dealing with the issue of electromobility. 117 roman štěrba, jiří cigánek acta polytechnica ctu proceedings figure 4. a container that can be used in the event of an electric car fire [17]. 5.2. general distribution of electrified vehicles currently, the fire service can meet the following types of e-cars: • micro hybrid: vehicles with start/stop and recuperation, 12 v battery, • mild hybrid: vehicles that are powered by an internal combustion engine supported by an electric motor and lithium-ion batteries 12 v/48 v, • full hybrid (hev): vehicles capable of purely electric driving, nimh battery 288 v/lithium-ion battery 266 v, • plug-in hybrid: (phev) hybrids with the possibility of recharging using a local mains supply or charging station, lithium-ion batteries 300–400 v, • elektromobil (bev): drive only by electric motor, lithium-ion batteries 300–400 v, • rxbev (range-extender): additional internal combustion engine to drive the generator charging the hv battery (extending the driving range, not used to drive the wheels), • fuel cell battery electric vehicle (fcbev): fuel cell vehicle. 5.3. extinguishing methodology and options for intervention in the event of an electric car fire with the boom of e-mobility in the czech republic, the question arises of how to put out a possible electric car fire. in the case of an electric car with traction batteries, the way an electric car burns is different from how a conventional vehicle burns. the traction batteries of electric or hybrid cars are the most considerable risk for firefighters when responding to these vehicles. the battery consists of many cells arranged in segments, and everything is sophisticatedly connected so that the resulting capacity best meets the needs of the given vehicle. everything is then protected against damage by a durable cover, in some cases, formed by, e.g., a titanium shell (tesla). although the packaging protects the battery from damage to a large extent, it represents a major obstacle in a fire or an incipient thermal reaction. the battery is also specific in that when it is mechanically damaged, it may not start burning immediately. however, if a fire occurs, it can re-ignite several times in a row. a chemical reaction occurs inside that can result in a fire within hours or even days. therefore, it is necessary to monitor it, for example, with a thermal camera, and if it is warming up, start to cool it down. lithium-ion batteries, part of today’s electric cars, need a large amount of water to interrupt the chemical processes. certified highpressure equipment (ccs cobra), including special accessories for extinguishing traction batteries, can extinguish traction battery fires. it is thus possible to apply a specially certified fire extinguisher directly into the battery via a high-pressure jet, where the temperature drops and the burning reaction stops. this is achieved by gradually creating holes in the battery’s casing with the help of a high-pressure water jet and an abrasive admixture. after the battery has been extinguished and cooled down, the vehicle continues to be monitored, preferably in a quarantine container that enables transport, monitoring, and safe flooding of the traction battery in the event of another unexpected reaction with regard to ecology. contaminated water must be disposed of ecologically by a specialist company. (1.) using a particular container (figure 4) for extinguishing electric cars appears to be an effective method. it is a container where an electric car is placed and filled with water from a cistern. the electric car is submerged in the container for several days until the traction batteries cool down [17]. (2.) with the development of electromobility, firefighters in the czech republic adopted a unique fire engine specially equipped for extinguishing electric cars. its basis is the toyota hilux pick-up truck, which is structurally modified (figure 5). the fire truck toyota hilux 6x6 is low enough to move even in tight spaces. it fits up to 185 cm in height, allowing it to enter underground garages. in addition to the prescribed equipment for rapid response vehicles, the car is equipped with the ccs 118 vol. 51/2024 safety aspects of e-cars charging at railway stations figure 5. firefighting vehicle in the czech republic for extinguishing electric car fires [18]. cobra high-pressure fire extinguishing and cutting system, which operates with a pressure of up to 300 bar at a flow rate of 28 liters per minute. at the end of the 80-meter hose is a streamer able to regulate the current from a beam to a wide cone, enabling fast and efficient extinguishing of traction accumulators. due to the high efficiency of the fire-fighting medium and the pump, fire brigades can use the vehicle for a wide range of interventions. the equipment also includes holmatro battery rescue tools, positive pressure ventilation, detection technology, and the prescribed equipment for a rapid-response vehicle. a portable and stable thermal imaging monitoring system is also available. (3.) as a third option, special carts (figure 6) that take the electric car to a safe distance from buildings or other vehicles have been developed. then, the firefighting operation can be started [19]. if the battery is mechanically damaged, it may not burn immediately. however, if a fire occurs, it can re-ignite several times in a row. a chemical reaction occurs inside, resulting in a fire within hours or even days. therefore, it is necessary to monitor the battery, for example, with a thermal camera, and if it is warming up, start to cool it down. 6. conclusion with the boom of electric cars, there is a need to create an appropriate charging infrastructure that is safe for users and their surroundings. mobile technology constantly evolves, and a traction battery is being developed for the greatest possible range. správa železnic promotes electromobility and multimodality, employing charging stations at railway stations. it pays attention to the safety risks of charging electric vehicles. therefore, the crucial recommendation is to monitor the temperature of the electric car or battery with a system of fixed thermal cameras with direct connection to the existing camera system with recording and connection of the alarm signal from the thermal camera directly to the electronic fire figure 6. special trolleys for handling an electric car [19]. alarm/centralized protection desk managed by fire service. for monitoring outdoor charging stations, it is recommended to monitor only the critical state, i.e., exceeding the temperature of approximately 70 °c. this system is designed based on more than ten years of experience in designing and implementing these systems. also, it considers the newly prepared čsn dealing with the issue of electromobility. both authors are involved in the charging station project at správa železnic, focusing on charging station design, construction, operation, and fire prevention for both duty and public use. the article described extinguishing methodology, including options for intervention in the event of an electric car fire. acknowledgements references [1] european commission. communication from the commission to the european parliament, the council, the european economic and social committee and the committee of the regions. sustainable and smart mobility strategy – putting european transport on track for the future, 2020. 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[2023-10-01]. https: //www.mdcr.cz/dokumenty/strategie/dopravnipolitika-cr-pro-obdobi-2014-2020-s-vyhled [3] european commission and directorate-general for mobility and transport. white paper on transport – roadmap to a single european transport area – towards a competitive and resource efficient transport system. publications office of the european union, 2011. https://doi.org/doi/10.2832/30955 [4] ministry of transport of the czech republic. koncepce městské aktivní mobility pro období 2021–2030 [in czech; urban and active mobility concept for the period 2021–2030], 2021. 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https://scholar.google.com/scholar_lookup?title=fire%20safety%20of%20lithium-ion%20batteries%20in%20road%20vehicles&publication_year=2019&author=r.%20bisschop&author=o.%20willstrand&author=f.%20amon&author=m.%20rosengren https://scholar.google.com/scholar_lookup?title=fire%20safety%20of%20lithium-ion%20batteries%20in%20road%20vehicles&publication_year=2019&author=r.%20bisschop&author=o.%20willstrand&author=f.%20amon&author=m.%20rosengren https://vlada.gov.cz/en/jednani-vlady/policy-statement/policy-statement-of-the-government-193762/ https://vlada.gov.cz/en/jednani-vlady/policy-statement/policy-statement-of-the-government-193762/ https://vlada.gov.cz/en/jednani-vlady/policy-statement/policy-statement-of-the-government-193762/ https://doi.org/10.1016/j.trpro.2024.01.016 https://www.elektromobilita.cz/en https://www.mpo.cz/cz/prumysl/zpracovatelsky-prumysl/automobilovy-prumysl/aktualizace-narodniho-akcniho-planu-ciste-mobility--254445/ https://www.mpo.cz/cz/prumysl/zpracovatelsky-prumysl/automobilovy-prumysl/aktualizace-narodniho-akcniho-planu-ciste-mobility--254445/ https://www.mpo.cz/cz/prumysl/zpracovatelsky-prumysl/automobilovy-prumysl/aktualizace-narodniho-akcniho-planu-ciste-mobility--254445/ https://www.mpo.cz/cz/prumysl/zpracovatelsky-prumysl/automobilovy-prumysl/aktualizace-narodniho-akcniho-planu-ciste-mobility--254445/ https://www.mpo.cz/cz/energetika/statistika/statistika-a-evidence-cerpacich-a-dobijecich-stanic/seznam-verejnych-dobijecich-stanic-_-stav-k-31--12--2022--271957/ https://www.mpo.cz/cz/energetika/statistika/statistika-a-evidence-cerpacich-a-dobijecich-stanic/seznam-verejnych-dobijecich-stanic-_-stav-k-31--12--2022--271957/ https://www.mpo.cz/cz/energetika/statistika/statistika-a-evidence-cerpacich-a-dobijecich-stanic/seznam-verejnych-dobijecich-stanic-_-stav-k-31--12--2022--271957/ https://www.mpo.cz/cz/energetika/statistika/statistika-a-evidence-cerpacich-a-dobijecich-stanic/seznam-verejnych-dobijecich-stanic-_-stav-k-31--12--2022--271957/ https://www.elektromobilita.cz/en/map-of-charging-stations https://www.elektromobilita.cz/en/map-of-charging-stations https://ekologickaauta.cz/nova-rychlonabijeci-stanice-pro-elektromobily-na-parkovisti-prodejny-lidl-v-hradci-kralove-v-ulici-brnenska/ https://ekologickaauta.cz/nova-rychlonabijeci-stanice-pro-elektromobily-na-parkovisti-prodejny-lidl-v-hradci-kralove-v-ulici-brnenska/ https://ekologickaauta.cz/nova-rychlonabijeci-stanice-pro-elektromobily-na-parkovisti-prodejny-lidl-v-hradci-kralove-v-ulici-brnenska/ https://ekologickaauta.cz/nova-rychlonabijeci-stanice-pro-elektromobily-na-parkovisti-prodejny-lidl-v-hradci-kralove-v-ulici-brnenska/ https://www.autoforum.cz/zajimavosti/britove-zjistili-skutecny-dojezd-soucasnych-elektromobilu-vetsinou-je-to-mizerie/ https://www.autoforum.cz/zajimavosti/britove-zjistili-skutecny-dojezd-soucasnych-elektromobilu-vetsinou-je-to-mizerie/ https://www.autoforum.cz/zajimavosti/britove-zjistili-skutecny-dojezd-soucasnych-elektromobilu-vetsinou-je-to-mizerie/ https://autobible.euro.cz/realny-dojezd-12-elektromobilu-dojede-nejdal-nejvic-lze/ https://autobible.euro.cz/realny-dojezd-12-elektromobilu-dojede-nejdal-nejvic-lze/ https://doi.org/10.1016/j.csite.2023.103889 https://www.garaz.cz/clanek/s-hasici-otevrene-o-elektromobilech-je-to-prusvih-21005165 https://www.garaz.cz/clanek/s-hasici-otevrene-o-elektromobilech-je-to-prusvih-21005165 https://www.auto.cz/galerie/profi/104174/hasici-dostali-specialni-hilux-6x6-pro-boj-s-pozary-elektromobilu?foto=multi https://www.auto.cz/galerie/profi/104174/hasici-dostali-specialni-hilux-6x6-pro-boj-s-pozary-elektromobilu?foto=multi https://www.auto.cz/galerie/profi/104174/hasici-dostali-specialni-hilux-6x6-pro-boj-s-pozary-elektromobilu?foto=multi https://www.autosalon.tv/novinky/ridicuv-chleba/olomoucti-hasici-vymysleli-specialni-voziky-pomoci-kterych-dostanou-horici-elektromobil-z-garaze https://www.autosalon.tv/novinky/ridicuv-chleba/olomoucti-hasici-vymysleli-specialni-voziky-pomoci-kterych-dostanou-horici-elektromobil-z-garaze https://www.autosalon.tv/novinky/ridicuv-chleba/olomoucti-hasici-vymysleli-specialni-voziky-pomoci-kterych-dostanou-horici-elektromobil-z-garaze https://www.autosalon.tv/novinky/ridicuv-chleba/olomoucti-hasici-vymysleli-specialni-voziky-pomoci-kterych-dostanou-horici-elektromobil-z-garaze acta polytechnica ctu proceedings 51:114–120, 2024 1 introduction 2 public parking with charging stations 2.1 planning public charging infrastructure at train stations 2.2 distribution of charging stations 3 fire prevention 4 safety rules when charging electric cars 5 preparedness to deal with electric vehicle fires 5.1 detection of increased temperature of electric cars 5.2 general distribution of electrified vehicles 5.3 extinguishing methodology and options for intervention in the event of an electric car fire 6 conclusion acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2023.40.0111 acta polytechnica ctu proceedings 40:111–116, 2023 © 2023 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague generation of ldpm structure formed by voronoi cells jan vozáb∗, jan vorel czech technical university in prague, faculty of civil engineering, department of mechanics, thákurova 7, 166 29 prague 6, czech republic ∗ corresponding author: jan.vozab@fsv.cvut.cz abstract. a preliminary study of an approach to internal structure generation used in lattice discrete particle models (ldpms) [1]. the presented method used for particle generation and placement is intended to help realistically capture the internal structure of materials. first, a method for structure generation using ldpm is presented. then, the method of particle generation using a voronoi diagram [2] is described. the last part is the optimizations on the algorithm that use apollonius circles to calculate the specific points of the voronoi diagram. keywords: ldpm, voronoi diagram, voronoi cell, internal structure. 1. introduction the use of composite materials such as reactoplasts is increasingly common in today’s industry. the construction industry is no exception. however, other industries such as aerospace or automotive differ in several important ways. this mainly concerns the degree of material curing at which these materials begin to load. this effect has a strong influence on the response of the material under load, especially in combination with viscoelasticity. at the same time, the material properties change over time. one of the structural elements used in construction where these materials are involved involves chemical anchors. particle-filled polymers act as a mortar binder layer between the anchor and the building element to which they are attached. these mortars are considered as homogeneous materials. but at lower scales, they can be characterized as heterogeneous materials [3]. in [4], is described that the modification of conventional continuum models (phase-field and peridynamic) is not possible without discretizing the internal structure. for this reason we decided to use a discrete model. figure 1 shows the real internal structure of such a polymer mortar. the polymer binder that connects the grains is shown in gray and shown in light color. it is clearly shown that the grains are of different sizes. the failure of these materials is mainly around the grains in the mortar [5, 6]. breakage through the grains occurs mainly in high-strength concretes [1]. this allows us to use the lattice discrete particle model (ldpm), which creates the internal structure of the material. it is convenient for the materials we are investigating. the ldpm material model response is dependent on particle distribution, and thus multiple simulations are needed to provide credible results [7]. however, as can be seen in the figure 1, most of the grains are not circular. the ldpm model only allows the generation of circular (2d) or spherical (3d) particles. our goal is to modify the ldpm model to allow the generation of grains of different sizes and shapes. therefore, we decided to modify the particle generation using a voronoi diagram. the computational complexity of this model is also an important factor. this paper focuses on the modification of the algorithm and its subsequent optimization. figure 1. detail of internal structure: hilti hit re 500. 2. ldpm structure first of all, it is important to describe the original method of generating the mesosctucture using the ldpm method [1]. we are given the sample geometry and the prescriptive gradation curve, i.e., the maximum and minimum aggregate size, as input for structure generation. the maximum aggregate size defines the upper limit of the gradation curve. in contrast, the minimum aggregate size defines its lower boundary, i.e., the 111 https://doi.org/10.14311/app.2023.40.0111 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en jan vozáb, jan vorel acta polytechnica ctu proceedings figure 2. delaunay triangulation result given set of particles. figure 3. ldpm structure for circular particles. diameter below which no discretized particles are generated and located. thus, the minimum aggregate size affects the fineness of the discrete network and the computational cost. the process begins with the generation of particles on the exterior of the sample. in our case (2d), points are first generated at the corners and then in the middle of the sides of the square. in the figure 2 they are shown by the blue circular points. this is followed by particle generation on the interior of the sample. the particles are placed from largest to smallest in a random position to ensure that the sample contains as many as possible. in the figure, these particles are represented by circles of different colors. the nearest particles are then detected using delaunyan triangulation, which results in the underlying polytopes in the given space. they represent triangles in the plane and tetrahedrons in space. in the figure 2 they are shown figure 4. structure of circular voronoi particles before composition. with blue lines connecting the individual particles. the following process divides the triangle into parts that belong to the particles that form its vertices. the centres of the sides and the centre of the triangle are calculated, and the part of the triangle is connected to the particle to which it belongs. the individual segments are then combined to form the resulting polyhedral cells (figure 3). when all the polyhedral particles have been formed, their interaction with each other has to be determined through the facets formed by the splitting of the triangles. the displacements and rotations of these neighboring particles form the discrete compatibility equations in terms of rigid-body kinematics. on each facet of the cell, an inter-scale constitutive law is formulated to simulate cohesive fracture, compaction due to pore collapse, frictional slip, and velocity effect. finally, equilibrium equations are formulated for each individual particle. 3. voronoi structure this article studies the potential use of voronoi cell tessellation [2] for the ldpm model. the voronoi cells are particles with a boundary representing half the distance between the particles from which tessellation occurs [8]. boundaries in the fundamental case are represented by lines for points in 2d and planes for points in 3d. for different particles, the voronoi cells are more complex. the simplest planar particles in the plane are circles. when the radii are equal, the boundary is represented by a straight line. when the radii differ, the boundary becomes curved. considering more complex particles, such as ellipses, the boundary complexity becomes even greater. however, it is possible to generate a structure for particles of any shape. in the figure 4 you can see the cell structure created for the same set of particles as for the ldpm struc112 vol. 40/2023 generation of ldpm structure formed by voronoi cells figure 5. structure of circular voronoi particles after composition. ture. the challenging thing about forming a structure using voronoi cells is that if the particles have different radii, or if they are not circular, the boundary between them is formed by a curve. in the figure you can see that in particular, the boundaries of cells formed by particles on the perimeter of a square are curves. furthermore, this effect can be seen for particles that are close together. we deal with this effect by generating more points on the boundary of the particle, thus generating a special cell. once the entire structure is generated, the points belonging to each particle are merged into a complete cell that takes into account the size and shape of the particle. the resulting boundary is a semilinear curve. result is shown in the figure 5. in this case, the particles are represented by 50 points on the particle boundary. for the calculation we used the matlab software [9]. compared to the structure formed by the ldpm method, they are smoother. smaller particles are connected to fewer other particles. at the same time, there are no fragments like those seen in the upper left corner of the picture. for the purple particle, the cell boundary passes through the particle itself. the figure 6 shows a structure made up of elliptical cells. at first glance, it is obvious that the boundaries between the particles are much more curved due to the complexity of the particle shape. therefore, even when the particles are represented by 44 points, small fluctuations in the smoothness of the boundary can be seen. nevertheless, in the figure 7 the boundaries are as we would expect and do not contain any major fragments. 4. optimization of the algorithm such a high number of points representing each particle would greatly affect the speed of the calculation. therefore, we focused on how we could reduce the figure 6. structure of elliptical voronoi particles before composition. figure 7. structure of elliptical voronoi particles after composition. number of points on the particle boundary while maintaining the accuracy of the boundary. therefore, we decided to see what effect replacing random points with specific points on the particle boundary. one of these characteristic points is the point of connection of the three cells. it is also the centre of the circle that touches the three cells. this problem is not trivial and is generally called the apollonius problem or the apollonius circle. f(x, y) = (x − xc)2 + (y − yc)2 = r2 (1) (xs − xci)2 + (ys − yci)2 = (rs − siri)2 (2) equation (1) is general expression of circle, where xc and yc refers to the center of the circle and r is radius. the following equation (2) represents the general solution of the apollonius circle, where si 113 jan vozáb, jan vorel acta polytechnica ctu proceedings figure 8. apollonius circles connecting circular particles. figure 9. optimized voronoi structure of circular particles before composition. is coefficient, that determines whether the resulting circle will be touched by the particle from the interior or the exterior. the result of this system of equations can be seen in the figure 8 for the same set of particles. the linkages between the particles are shown in blue. the red shows the circles between the particles as well as the points that these circles create in contact with particle the boundaries calculated by the unoptimized method are shown in black color. the figure 9 shows the voronoi diagram before composition. at first glance, one can see the much smaller number of cells of which it is composed. the figure 10 shows the resulting structure after composition on the resulting cells. small fragments can be seen where the cells are close together, or at the boundaries of the diagram when the particles have different radii. but the resultfigure 10. optimized voronoi structure of circular particles after composition. ing structure is formed at an average of 8 points per particle. even so, its accuracy is very consistent with the unoptimized method. we have tried a similar approach to calculate points for ellipses. but we encountered the problem here that the general equation of the apollonius circle is noticeably more complicated. with the general equation of the ellipse: f(x, y) =((x − xc) · cos α + (y − yc) · sin α)2 a2 + ((x − xc) · sin α − (y − yc) · cos α)2 b2 = 1, (3) which then has to be converted into the general equation of the conic section: e(x, y) = ax2 + 2bxy + cy2 + 2dx + 2ey + f. (4) then, following the procedure defined in [10], several parameters are calculated to form a determinant, which for a single ellipse is defined as a polynomial of degree 184. the resulting system of equations: ∆1(xs, ys, s) = ∆2(xs, ys, s) = ∆3(xs, ys, s) = 0. (5) the resulting system of equations is a very challenging problem to solve. therefore, we tried to come up with a modification that would make the calculation possible. so we took inspiration from the circle equation, and considered that the ellipses will grow with one parameter until they intersect at one point. from this assumption comes the equation together with the system of equations: fi(x,y, r) = ((x − xci) · cos αi + (y − yci) · sin αi)2 (ai + r)2 + ((x − xci) · sin αi − (y − yci) · cos αi)2 (bi + r)2 − 1 = 0, (6) f1(x, y, r) = f2(x, y, r) = f3(x, y, r) = 0. (7) 114 vol. 40/2023 generation of ldpm structure formed by voronoi cells figure 11. apollonius circles connecting elliptical particles. the result of the system of equations is a polynomial of degree 27. it’s still not a simple solution, but it’s easy to solve the equations numerically. the resulting circles are shown in the figure 11 for the same set of particles. the figure 12 shows the structure and is made up of an average of 6 points per particle. the black line shows the boundaries for particles consisting of 44 points. the structure after composition is shown in red in the figure 13. there are clear fragments and differences from the exact boundary, but the optimized structure is very consistent even with a large saving of points 5. conclusion in this paper, the procedure of structure generation using the ldpm model is presented. it also describes how the structure can be generated using a voronoi diagram. this procedure relies on the representation of particles by points on their boundary for which the voronoi diagram is computed. this is followed by a composition of particles for which the structure already corresponds to their shape. then, a procedure is described on how the algorithm can be optimized using the specific representation. preliminary results have led us to the conclusion that the specifics are more telling in the construction of the resulting voronoi diagram and can be used to save a number of points on the particle boundary. this procedure will be more widely used in the future. it is mainly a matter of trying out additional specific points, then converting the structure to 3d. acknowledgements the financial support provided by the gačr grant no. 21-28525s and by the czech technical university in prague within sgs project with the application registered under the no. sgs20/155/ohk1/3t/11 is gratefully acknowledged. figure 12. optimized voronoi structure of elliptic particles before composition. figure 13. optimized voronoi structure of elliptic particles after composition. references [1] g. cusatis, a. mencarelli, d. pelessone, j. t. baylot. lattice discrete particle model (ldpm) for failure behavior of concrete. ii: calibration and validation. cement & concrete composites 33(9):891–905, 2011. mag id: 2094226000. https: //doi.org/10.1016/j.cemconcomp.2011.02.010 [2] f. aurenhammer. voronoi diagrams – a survey of a fundamental geometric data structure. acm computing surveys 23(3):345–405, 1991. mag id: 1967005434. https://doi.org/10.1145/116873.116880 [3] n. roussel. understanding the rheology of concrete. elsevier, 2011. [4] z. p. bažant, h. t. nguyen, a. abdullah dönmez. critical comparison of phase-field, peridynamics, and crack band model m7 in light of gap test and classical 115 https://doi.org/10.1016/j.cemconcomp.2011.02.010 https://doi.org/10.1016/j.cemconcomp.2011.02.010 https://doi.org/10.1145/116873.116880 jan vozáb, jan vorel acta polytechnica ctu proceedings fracture tests. journal of applied mechanics 89(6):061008, 2022. https://doi.org/10.1115/1.4054221 [5] b. l. karihaloo, a. carpinteri, m. elices. fracture mechanics of cement mortar and plain concrete. advanced cement based materials 1(2):92–105, 1993. https://doi.org/10.1016/1065-7355(93)90014-f [6] k. nagai, y. sato, t. ueda. mesoscopic simulation of failure of mortar and concrete by 3d rbsm. journal of advanced concrete technology 3(3):385–402, 2005. https://doi.org/10.3151/jact.3.385 [7] m. marcon, j. vorel, k. ninčević, r. wan-wendner. modeling adhesive anchors in a discrete element framework. materials 10(8):917, 2017. mag id: 2743109236. https://doi.org/10.3390/ma10080917 [8] q. du, v. faber, m. gunzburger. centroidal voronoi tessellations: applications and algorithms. siam review 41(4):637–676, 1999. mag id: 2051752778. https://doi.org/10.1137/s0036144599352836 [9] matlab. 9.11.0.1837725 (r2021b) update 2. the mathworks inc., 2021. [10] i. z. emiris, g. m. tzoumas. algebraic study of the apollonius circle of three ellipses. in eurocg, pp. 147–150. 2005. 116 https://doi.org/10.1115/1.4054221 https://doi.org/10.1016/1065-7355(93)90014-f https://doi.org/10.3151/jact.3.385 https://doi.org/10.3390/ma10080917 https://doi.org/10.1137/s0036144599352836 acta polytechnica ctu proceedings 40:111–116, 2023 1 introduction 2 ldpm structure 3 voronoi structure 4 optimization of the algorithm 5 conclusion acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2025.52.0057 acta polytechnica ctu proceedings 52:57–62, 2025 © 2025 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague causes of battery fires in railway vehicles and challenges of extinguishing jiří krejčí czech technical university in prague, faculty of transportation sciences, konviktská 20, 110 00 prague, czech republic correspondence: krejcj31@fd.cvut.cz abstract. the aim of this article is to introduce an issue that has not yet been addressed in our region – the fire hazards of bemu units. the methodology of the fire rescue service of the czech republic vehicles was used to establish the basic procedure for extinguishing batteries in railway passenger coaches. keywords: rail transport, environment, alternative drives, bemu units, battery, fire. 1. introduction the railway network of the czech republic can be considered one of the densest in europe. at the end of the year 2023, the total length of public tracks was 9 349 km in our country. it means, that there were 118.5 km of tracks per 1 000 km2 of the territory of our state. the impulse, why the aforementioned network was built in this length, was the industrial revolution and mankind’s effort to simplify and speed up individual jobs. the discovery of the steam engine showed, that the animal power used for decades could be replaced. gradual development and above all the discovery of fossil fuels and the use of their energy caused the gradual abandonment of steam traction. but it is necessary to realize that nothing has only positive sides. human activity was simplified, but the consequence is an environmental damage. in order to prevent damage to our planet in the future, emphasis is placed on so-called sustainable development. this means that human needs are fulfilled, but not at the expense of future generations. although railways can be considered one of the most ecological modes of transport, it also has weaknesses. the biggest problem is represented by independent traction vehicles using a combustion diesel engine for the mentioned fossil fuels (representing primary energy) for their movement. the mentioned fuels are converted into secondary energy by burning, which is then used for the movement itself. during combustion, the carbon contained in fossil fuels is converted into carbon monoxide (in the case of incomplete combustion in internal combustion engines) and carbon dioxide, these gases are subsequently released into the atmosphere. the goal of our society is to replace vehicles with combustion engine on railways. there are two options how to achieve this replacement. the first option is to electrify the line, meaning to build a overhead line traction system. the second approach involves deploying vehicles with alternative power sources, specifically hydrogen-powered vehicles and battery-powered vehicles. given that the czech republic does not have sufficient production and infrastructure for the distribution of hydrogen, it is more advantageous to consider the operation of battery-powered vehicles, specifically bemu (battery electric multiple unit) units, in the czech republic [1]. in addition to the issues related to deploying vehicles into regular operation – such as range, battery capacity, energy recovery options, and others – it is necessary to consider safety. for bemu units, beyond the standard safety measures applicable to other vehicles, special attention must be given to the battery itself. this article focuses on the safety of bemu units on czech railways. since there are currently no guidelines on how to proceed in the event of a bemu fire in our country, key points are highlighted here that should be addressed to prevent complications during emergencies. 2. emergency exits before this article addresses battery fires, it is important to understand the options passengers have to escape from a vehicle in the event of an emergency. since the early days of railway operations, passenger safety has been the highest priority, and significant resources are dedicated to ensuring their protection. some of the first railway safety measures were introduced in response to a railway accident, often referred to as the greatest railway disaster of the 19th century. the accident occurred in 1842 on the line between versailles and paris. the train consisted of three locomotives and several carriages. near meudon, the wheelset of the first locomotive broke. the second locomotive collided with the first (the third locomotive was positioned at the end of the train as a banking locomotive. at the same time, hot coal and steam were released from all engines, which caused the passenger carriages to catch fire. passengers were unable to escape from the burning carriages because, at that time, the doors could only be opened from the outside. 57 https://doi.org/10.14311/app.2025.52.0057 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en jiří krejčí acta polytechnica ctu proceedings as a result, 157 people were killed or went missing out of the 1 530 passengers. this accident led to safety measures being introduced across europe, including in austria. emperor ferdinand i himself ordered a ban on locomotives with two axles and prohibited the use of pushers in all circumstances. the maximum speed for passenger trains was set at 30 km h−1, and for freight trains at 23 km h−1. it was also forbidden to lock carriages that did not allow passengers to escape on their own in case of an accident (this applied to firstand second-class carriages), or measures had to be taken to ensure that passengers could easily unlock the doors if needed [2]. due to various emergency situations, vehicles have been improved and equipped with several safety features to help passengers escape in case of an incident. additionally, there are units within the integrated rescue system that are designated not only for evacuating people but also for handling other potential dangers, such as fires [3]. the simplest way to evacuate passengers is when no new escape routes need to be created, and the prescribed ones can be used. this method is called self-evacuation. in this case, the escape routes must be clearly marked with signs placed along the expected escape path. in a train carriage, a safety hammer is located near the window along with a pictogram that clearly explains the window as an emergency exit. conversely, near the doors, a properly marked handle is placed to unlock the doors, which should then open [3]. however, it is not always possible to use the prescribed escape routes. that is why rescue teams are trained to determine the most suitable, least demanding, and quickest way to create an escape route. for example, they may use doors that have become deformed due to the emergency, making it impossible to enter. in such cases, special equipment is required, which the rescue teams have. this may include hydraulic rescue tools with spreaders, which are now battery-powered to make firefighters’ work easier. additionally, motorized cutting tools or saws can be used. in the event of a derailment or overturning of the carriage, emergency exits can be created through the roof. the roof is one of the easiest areas to create escape routes, as it is made of sheet metal. however, electrical wiring, which is usually routed under the roof, can cause complications [3]. the specific method of evacuation is always determined by the commander of the rescue team, as they are specially trained and prepared for such operations, such as in the case of firefighters. firefighters are trained for common situations they encounter regularly. however, they do not have detailed knowledge of railway vehicles, unlike specialized railway firefighters. 3. fires in e-mobility fires are a part of daily life. however, we have been encountering a new trend: fires involving electric cars, trains, scooters, and other vehicles in recent years. this does not refer to traditional vehicle fires, which firefighters frequently handle, but rather to the growing category of battery fires, most commonly involving lithium-ion batteries. 3.1. what is battery before discussing the issue of fire outbreaks, it’s important to define what a battery is. in czech terminology, a “battery” refers to a cell that can only be used once. in contrast, an “accumulator” (referred to as a battery in english) can be repeatedly charged and discharged during its lifespan. a battery is an electrochemical galvanic cell, where a chemical reaction generates voltage. inside the battery, there are two half-cells, which are electrodes immersed in an electrolyte. these electrodes are separated by a separator to prevent the battery from short-circuiting. during the movement of the vehicle, energy is depleted. this means that one electrode becomes a negative anode, releasing electrons that move towards the positive cathode. during recharging, the cell is connected to an external power source, causing electrolysis, which switches the electrodes’ functions – this is referred to as the electrolytic mode [4]. the battery itself is composed of individual cells. a group of cells is called a module, and these modules are then assembled into packs. in practice, we often encounter various types of batteries, each differing in their chemical composition. some examples include [4]: • li-ion battery – cathode: licoo2, anode: lic6 or c, electrolyte: lipf6, • li-polymer battery – cathode: licoo2, anode: lic6 or c, electrolyte-separator: solid polymer, polymer battery casing • lithium iron phosphate battery (lfp) – cathode: lifepo4, anode: li or lic6 • lithium titanate battery (lto) – cathode: limn2o4 or lifepo4, anode: li4ti5o12 the most commonly used types of batteries are li-ion and li-polymer batteries. 3.2. fires involving lithium-ion batteries as mentioned in the introduction to chapter 3, the most common energy source in electric vehicles is the lithium-ion battery. this technology is crucial for energy storage, not only in transportation but across almost all industries. however, along with these batteries comes a new risk: their potential to catch a fire. when batteries are damaged, unwanted reactions can occur inside the battery itself. these reactions cause a rise in temperature and the release of gases, which can ignite and lead to a fire. it is important to note that during the burning process, it is not the material itself that burns, but the released gases. 58 vol. 52/2025 causes of battery fires in railway vehicles and challenges of extinguishing although battery fires are not very common, they present dangers that are significantly different from other fire hazards in terms of initiation, spread rate, duration, toxicity, and suppression [4, 5]. among the most common types of damage are mechanical stresses. a typical example can be an incident where, for instance, a railway vehicle derailment causes damage to the separators and subsequent short-circuiting of the battery, leading to the release of a large amount of energy. one example, actually not related to transportation, is a house fire where insulation work led to a worker’s walkie talkie falling into a blowing machine. this caused damage to a battery, which subsequently ignited and as a result the fire consumed the entire house. another cause can be electrical stress, such as using an inappropriate type of charging for the battery or resulting from a malfunction. thermal stress is another factor, such as the consequence of exposure to fire. additional causes can include manufacturing defects, intentional damage, and so forth [4, 5]. the technology of lithium-ion batteries is advancing rapidly. this leads to a significant increase in the use of these batteries not only in industry but also in transportation. given the incidents involving batteries, the question of how to ensure safety is becoming increasingly relevant. the most common issue is exceeding the operational temperature limits. this relates to the concept of “thermal runaway”. this is a state that occurs when the temperature of battery reaches a critical value. as a result, the rate of the exothermic reaction increases the temperature, which in turn accelerates the reaction rate further. this temperature rise poses a risk of ignition and related fire hazards. as mentioned earlier, a battery consists of cells or a collection of cells. if one cell is damaged, it can lead to a heat release affecting neighboring cells, resulting in a chain reaction that can impact the entire battery. this can lead to a substantial release of gases, which are difficult to suppress or extinguish [4, 5]. 3.3. issues with batteries and their placement on vehicles nowadays, there are two approaches to dealing with battery fires. the first approach involves developing a safe chemical composition or a secure battery design that prevents thermal runaway or the subsequent spread of fires. it is generally agreed that some chemical cathodes are safer than others. additionally, preventative measures include using so-called shutdown separators that stop the transport of lithium ions once a set temperature is reached. the second approach assumes that thermal runaway will occur and focuses on implementing reliable lightweight elements that concentrate on detecting and mitigating the spread of fires. currently, there is no specific approach to fire suppression, so manufacturers combine various methods for fire detection and mitigation. among the most well-known detection methods are terminal voltage and temperature detection. one of the major issues today is the lack of a globally established regulation for testing and ensuring the safety of batteries. regulations are issued by the relevant governments and are legally enforceable. on the other hand, batteries are tested only under static conditions and not under dynamic movements that are typical for accidents. generally, testing a battery is both physically and financially demanding. from a physical point of view, there is no reliable method to induce thermal runaway in a battery to represent failure modes during operation [5]. although battery-powered railway vehicles have been in operation since the last century, it could be said that the regular operation of these vehicles is still in its early stages. while the vehicles and their individual parameters must comply with fire safety regulations, there has not yet been any major incident involving the ignition of a battery. thus, it can be said that there has not yet been a direct testing of all safety features in action, but only in a laboratory setting. one of the first questions regarding fire safety is the placement of the battery on the vehicle. the current trend is towards low-floor vehicles. this implies that the battery cannot be placed in the lower part of the vehicle (due to maintenance advantages, centre of gravity, etc.). usually manufacturers today place batteries on the roof of the vehicle, where the structure must be reinforced to support the weight, or the battery can be placed in a separate module within the vehicle, where the area for passengers could be located. as mentioned, the first safety measure is prevention; however, if ignition occurs, the primary safety measure for passengers is that the compartment containing the battery should withstand at least 15 minutes before failing. this should be the time required to safely evacuate passengers from the vehicle. but what if there is a derailment or another emergency situation and passengers are trapped inside the vehicle? it must be recognized that there can be a delay between reporting the incident and the arrival of rescue services (which must arrive on-site within 7 minutes of the report). during a fire, it is also necessary to properly direct gases away from passengers. this raises questions about which direction the vent hole should be positioned. if the battery is located on the roof of the vehicle and the vent is directed downward, the heat from the device might affect the roof structure and cause damage. the most common placement is on the side. however, the vehicle could cover the vent and prevent safe gas removal from the fire in the event of a derailment and subsequent side fall. 59 jiří krejčí acta polytechnica ctu proceedings 3.4. methods for extinguishing fires in battery-powered railway vehicles in the czech republic, there is currently no methodological guideline for the localization and extinguishing of fires in battery-powered railway vehicles, specifically for the battery itself. the fire rescue service of the czech republic developed a document on electric mobility insights in the czech republic from 2019 to 2020. this chapter compares whether this document regarding fire extinguishing procedures could be applicable to bemu units as well. the first piece of information for firefighters is to understand what they are dealing with on-site, so they know the appropriate procedure for extinguishing the fire. for example, in modern road vehicles equipped with the ecall system, a message is generated and sent to the operational center. railway vehicles are not yet equipped with this system, so there must be a way to mark or distinguish these vehicles from others. in the case of the first bemu unit operating in the czech republic (bemu regiopanter), the presence of the battery used as an energy source for propulsion is indicated by an orange triangular warning sign with a battery pictogram. apart from the colour coding of different regions in the standard regiopanter electric units (e.g., the prague integrated transport colour), the battery-powered unit is marked with green at the front of the unit, as opposed to the blue colour of the standard electric units with the same commercial designation [4]. in passenger coaches, the battery is most often stored above or close to the rear axle. as described in the previous chapter, due to the requirement for low-floor trainset, the battery is placed on the roof or in a separate vehicle section, known as a so-called powerpack. upon rescue services arrival to the site of intervention, it is necessary to assess whether standard extinguishing agents can be used, considering the risk of electric shock. it is important to recognize that during a fire, a bemu unit may be standing under or even be connected to fixed traction power lines. in such a case, immediate intervention is not possible; the traction power lines must first be shut off and isolated. this differs from electric cars that move freely on roadways. a similar problem arises when electric vehicles are charging, but the difference lies in the power requirement, which is lower compared to trains [4]. after locating and extinguishing the fire, it is essential to continuously cool the battery. this is to prevent the transfer of heat between individual cells and sections of the battery. during the fire, some cells may have ignited but not completely burned out. water, without any additives, is used as a cooling agent and is applied directly to the battery cover for 10 minutes. following this, the battery should be monitored for 5 minutes to check if it continues to heat up or emit smoke. if the battery does, cooling should be repeated for another 10 minutes, and this process should continue in a cycle. to determine if the battery is still overheating, it is advisable to use a thermal camera. after successfully cooling the battery, it is recommended to monitor its condition for an additional 45 minutes. if no self-heating or smoke is observed during this time, the scene can be handed over with post-fire measures established. subsequently, a thorough inspection of the battery is necessary. one effective cooling solution in automotive transport is to submerge the vehicle in a water tank (figure 1). however, this would cause total damage to the vehicle and is not feasible for rail vehicles. therefore, the battery should be designed to allow connection to a fire hose for cooling. it should be noted that this would require the fire truck to be present for several days. additionally, the fire truck’s tank would need to be cleaned afterward, as the water used for cooling will be contaminated with battery fumes [4]. figure 1. submerged electric car in a container of water [6]. 3.5. possibility of using modern technologies for locating batteries with the gradual development of firefighting technology, there are systems that may improve the management of battery fires. one of these is the cobra device (figure 2). this device equipped on some fire trucks, can cut through the battery cover and apply water directly between the battery cells, thus effectively cooling them. however, the fact that the battery cover is breached during the intervention is not in accordance with manufacturers’ recommendations. in cases where this system might be effective, it could lead to damage to other battery components by the water jet, potentially disrupting internal electrical wiring or disabling safety elements designed to prevent electric shock. the water jet may also disturb battery cells that have not been affected by the fire, unnecessarily releasing chemicals into the extinguishing water. additionally, there is a high likelihood of a violent reaction between alkaline metals from damaged cells and water. however, this system might be used, in the case of a derailment where access to the battery 60 vol. 52/2025 causes of battery fires in railway vehicles and challenges of extinguishing figure 2. firefighters using cobra equipment [7]. is needed, as it is generally not recommended unless specified otherwise by the manufacturer [4]. swedish company dafo is now introducing a revolutionary system for extinguishing battery fires. this is achieved using a special substance called forrex, which is a battery component. forrex is designed to prevent oxygen from reaching the battery, thereby stopping the fire. the advantage of forrex is that it is applied immediately after fire detection. as mentioned, the substance quickly extinguishes the fire while also beginning to cool the battery. the lifespan of this substance is approximately 10 years, which corresponds to the typical lifespan of a battery. however, the biggest issue is its acquisition cost. at the required amount of 200 to 300 litters, the cost constitutes about 1 % of the acquisition price of a bemu unit [8]. 3.6. how to prepare for future fires in bemu vehicles as previously mentioned, the regular operation of bemu vehicles is still in its early stages. although manufacturers assume that emergency situations will be rare, it is essential to be prepared for such events. it is important to note that the railway vehicle is not always the cause of the emergency. an example is the incident at a railway crossing in olomouc that occurred in november 2023. at that time, a train collided with a truck. a fire broke out immediately due to the flammable fuel, and passengers had only a few minutes to escape. the first step in ensuring the safety of batterypowered trains is to familiarize emergency responders with the vehicle’s structure. in regions where bemu vehicles will be introduced into regular service, integrated rescue units should be briefed on the specifics of these vehicles. for example, when new passenger cars are launched, some units are provided to firefighters for training purposes, allowing them to practice various rescue techniques and familiarize themselves with the vehicles. however, this approach is not feasible for railway vehicles due to the high costs involved. with the advancement of modern technology, it would be possible to conduct training in virtual reality. there are already applications, such as flaim trainer, that allow firefighters to practice emergency response procedures in a virtual environment. another common practice today is equipping fire trucks with tablets that store structural plans of different vehicles [9]. another solution is the labelling of vehicles. currently, there is no prescribed system to indicate that a vehicle is a bemu unit. as previously mentioned, some vehicles have an orange battery label, but this label was placed by the vehicle manufacturer, not because of the regulation. additionally, it was noted in the previous chapter that bemu units differ in colour from standard emu units. however, the current trend is to standardize the colours of vehicles in integrated transport systems, making it difficult to distinguish between emu and bemu units. therefore, the goal is to establish a clear labelling system for bemu vehicles. in the event of an emergency where the battery catches fire, it would be beneficial for the battery to disconnect from the vehicle itself. this would only be feasible if the battery were located on the roof of the vehicle. firefighters could handle the battery separately from the vehicle. however, a concern arises regarding whether the disconnection process might cause additional damage if not executed properly. this method would require a series of tests on various mechanisms. one clear advantage is that the battery could potentially be submerged in a container of water for cooling. another possible solution that exists today and can be used for extinguishing battery fires is a new innovation from the austrian company rosenbauer, specifically the piercing spike. this device is capable of penetrating the battery casing and spraying water directly from the spike to cool the internal parts of the battery. similar devices are currently used by airport firefighting vehicles to extinguish fires inside airplanes [6]. as previously mentioned, there is currently no established system for extinguishing bemu vehicles. therefore, it is necessary to address this issue to prevent emergencies and potential loss of human lives. 4. conclusion this article was created to outline the issue of fires in bemu units within our country, where there is currently no specific procedure for handling fires in battery-powered trains. first, the article discusses the issue of harmful substances produced on the railway network in the czech republic. this is related to the replacement of internal combustion engine vehicles with those using alternative propulsion systems. second, the article describes the battery and the possibilities related to its ignition. preventive measures for battery fires were then considered, including 61 jiří krejčí acta polytechnica ctu proceedings the placement of the battery on the vehicle and the location of safety vents. in the subsequent sections, the methodological guidelines of the fire rescue service of the czech republic were reviewed and compared to determine if these procedures could be applied to bemu unit fires. finally, the article discusses devices that could be used for fire extinguishing and earlier intervention. acknowledgements references [1] j. krejčí. principle of testing the operational capability of dual-power railway vehicles (trolley/battery electric multiple units) on regional passenger rail transport lines. new railway technique (3):21–22, 2024. 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[2024-09-05]. https://digilib.k.utb.cz/handle/10563/56487 [4] z. malkovský, j. karl, o. suchý, p. thin. aktuální poznatky z elektromobility pro potřeby hzs čr [in czech; current knowledge on electromobility for the needs of the fire rescue service of the czech republic], 2020. ministry of the interior of the czech republic, general directorate of the fire rescue service of the czech republic – technical institute of fire protection, [2024-09-05]. https://www.hasici-vzdelavani.cz/sites/default/ files/download/soubory/201007%20komentovan%c3% bd%20v%c3%bdklad%20k%20mu%20s%20ev-h_0_0.pdf [5] l. bravo diaz, x. he, z. hu, et al. review – meta-review of fire safety of lithium-ion batteries: industry challenges and research contributions. journal of the electrochemical society 167(9):090559, 2020. https://doi.org/10.1149/1945-7111/aba8b9 [6] auto.cz. rakouská společnost představila speciální zařízení pro hašení elektromobilů [in czech; the austrian company has introduced a specialized device for extinguishing electric vehicle fires], 2021. [2024-09-06]. https://www.auto.cz/rakouskaspolecnost-predstavila-specialni-zarizeni-prohaseni-elektromobilu-141654 [7] hasicivo. hasiči se účastní výcviku s hasicím zařízením ccs cobra [in czech; firefighters participate in training with ccs cobra firefighting equipment], 2022. [2024-09-06]. https://www.hasicovo.cz/cs/zpravy/hasici-seucastni-vycviku-s-hasicim-zarizenim-ccs-cobra [8] dafo.cz. forrex, 2024. [2024-09-05]. https://dafo-czech.cz/ [9] flaim trainer, 2024. [2024-09-06]. https://flaimsystems.com/products/trainer 62 https://www.vagony.cz/historie/dvere.html https://digilib.k.utb.cz/handle/10563/56487 https://www.hasici-vzdelavani.cz/sites/default/files/download/soubory/201007%20komentovan%c3%bd%20v%c3%bdklad%20k%20mu%20s%20ev-h_0_0.pdf https://www.hasici-vzdelavani.cz/sites/default/files/download/soubory/201007%20komentovan%c3%bd%20v%c3%bdklad%20k%20mu%20s%20ev-h_0_0.pdf https://www.hasici-vzdelavani.cz/sites/default/files/download/soubory/201007%20komentovan%c3%bd%20v%c3%bdklad%20k%20mu%20s%20ev-h_0_0.pdf https://doi.org/10.1149/1945-7111/aba8b9 https://www.auto.cz/rakouska-spolecnost-predstavila-specialni-zarizeni-pro-haseni-elektromobilu-141654 https://www.auto.cz/rakouska-spolecnost-predstavila-specialni-zarizeni-pro-haseni-elektromobilu-141654 https://www.auto.cz/rakouska-spolecnost-predstavila-specialni-zarizeni-pro-haseni-elektromobilu-141654 https://www.hasicovo.cz/cs/zpravy/hasici-se-ucastni-vycviku-s-hasicim-zarizenim-ccs-cobra https://www.hasicovo.cz/cs/zpravy/hasici-se-ucastni-vycviku-s-hasicim-zarizenim-ccs-cobra https://dafo-czech.cz/ https://flaimsystems.com/products/trainer acta polytechnica ctu proceedings 52:57–62, 2025 1 introduction 2 emergency exits 3 fires in e-mobility 3.1 what is battery 3.2 fires involving lithium-ion batteries 3.3 issues with batteries and their placement on vehicles 3.4 methods for extinguishing fires in battery-powered railway vehicles 3.5 possibility of using modern technologies for locating batteries 3.6 how to prepare for future fires in bemu vehicles 4 conclusion acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2024.51.0014 acta polytechnica ctu proceedings 51:14–21, 2024 © 2024 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague gear ratio optimization of single and two-speed transmission for an electric motorcycle jolana heřmanová czech technical university in prague, faculty of mechanical engineering, department of automotive, combustion engine and railway engineering, technická 4, 166 07 prague 6 – dejvice, czech republic correspondence: jolana.hermanova@fs.cvut.cz abstract. most electric vehicles have a single-speed transmission as the traction characteristic of an electric motor allows operation with sufficient motor torque at low and high speeds. then, the selection of the single-gear ratio is a trade-off between available torque for acceleration (or gradability) at lower speeds and the designed maximum vehicle speed corresponding to the electric motor’s maximum speed limit. nevertheless, the multiple-speed transmission can be used, and the vehicle can benefit from improved acceleration at low speeds and higher maximum speeds. in this paper, the gear ratio of a single-speed transmission for an electric racing motorcycle is optimized to achieve a trade-off between acceleration, lap time, and maximum speed. with the same goal, the gear ratios for two-speed transmission are optimized, and the results are compared. keywords: gear ratio optimization, battery electric vehicle, racing motorcycle. 1. introduction as the traction characteristic of the electric motor is usually sufficient to propel the vehicle, most bevs have a single-speed transmission. there are some exceptions, and the reason for using a two-speed transmission instead of a single-speed transmission has always been the same. it is to achieve sufficient maximum vehicle speed and provide good vehicle acceleration (or gradability). another way, other than using additional gear, to get better acceleration times is to use a more powerful and more expensive motor. figure 1 shows typical electric motor characteristics. the main visible features are these: • high available torque at low motor speed, goes roughly to zero speed with constant torque (the color is hidden for efficiency under 50%), • very low efficiency in high load and low speed (top left corner), under 50%, • area of constant motor power and decreasing torque, approximately (in this specific case) for motor speed above 4000rpm, • the area of best efficiency is located in higher speed and moderate loads, with maximum efficiency of up to 97%, the gear ratio has to be chosen with respect to a motor speed range and vehicle required speed range, and on the other hand, with respect to demanded torque on wheels ensuring sufficient acceleration. the gear ratio then works as a multiplier for torque and a divisor for speed, as shown in figure 2. the figure shows the traction characteristics in a wheel torque equivalent; the first gear (higher gear ratio) of a two-speed transmission multiplies the torque and shortens the speed range, whereas the second gear allows a higher figure 1. electric motor efficiency map example. figure 2. traction characteristics. speed range but lower torque. thus, these conflicting aspects must be considered for the correct gear ratio design. most of the producers in the past used single-gear transmissions mainly due to the complexity of the two-speed transmissions. among these examples in recent decades is the first tesla roadster. it was planned to have two-speed transmission (with gear ratios – gr of 14.3 and 7.4 [1, 2]), but due to higher complexity, the roadster was later produced with a single-speed transmission (gr 8.27) [3]. nowadays, 14 https://doi.org/10.14311/app.2024.51.0014 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 51/2024 gear ratio optimization for an electric motorcycle single-speed transmission two-speed transmission a trade-off between acceleration and maximum speed allows higher maximum vehicle speed, lower complexity allows higher vehicle acceleration lower weight (simpler gearing) electric motor can be used in areas with higher motor efficiency lower efficiency losses in gearing can use a smaller (less powerful) motor, reduced mass and price, and increased efficiency table 1. single-speed and two-speed transmission advantages and disadvantages. another bev that uses the two-speed transmission is a porsche taycan (gr around 15 and 8 at the rear, 8 at the front [4]) and audi rs e-tron gt (gr 15.56 and 8.16 [5]). taycan predominantly uses the second gear. the first gear is used in sport mode and at low speeds. the gearbox weighs approximately 70 kg and uses an electronic limited-slip differential. for transition between the two gear sets, there is a single actuator [6]. the shift to the second speed occurs when reaching roughly 75 kph [7]. another bev using a two-speed transmission is, for example, rimac concept one, which has four motors in total, one for each wheel (located in axle), and with single-speed transmissions at the front and two-speed double-clutch transmission at the rear. for concept-two, they wanted to eliminate the two-speed transmission because of complexity and weight; still, because of demands on better performance (0–100 kph acceleration), they firstly were forced to use the twospeed transmission again. finally, a year later, they presented a car with a single-speed transmission. they achieved it with an electric motor (em) and inverter redesign, which allowed them to use the em at a maximum of 17 700rpm [8, 9]. bmw i8 phev is equipped with a two-speed transmission (developed by gkn) for the electric motor at the front axle [10, 11]. zf also presents its two-speed transmission design with the electric motor [12, 13]. jeep magneto might have a multiple-speed gearbox because of off-road functionalities. also, ev trucks, especially heavy-duty ones, will likely be equipped with multi-speed transmissions because of the high torque at low speeds and efficiency at high speeds. other transmissions are being considered (like cvt) or usage of multiple motors [14]. the main problem with a two-speed transmission is that it must be robust enough to deal with the high torque of the electric motor; there are reliability problems and requirements for more maintenance. it economically makes sense in vehicles with higher battery capacity and in higher performance systems [15]. overall, manual multi-speed motorcycle gearboxes are not very frequent in the market. these are just a few examples. brammo empulse r (production versions available around 2012) was produced with a clutch and six-speed transmission. it benefited over its competitor zero s in a launch and quicker acceleration. otherwise, a manual transmission lowers the comfort and lacks the smoothness of single-speed competitors [16, 17]. a chinese manufacturer, qjmotor, announced a new model, qj7000d, in 2021, featuring manual transmission, but it still has not reached production [18]. it is possible to find limited information about a prototype with a two-stage smeshgear transmission [19]. also, an indian producer, matter, promises a motorcycle with a fourspeed manual transmission (matter aera 5000) [20]. lastly, kawasaki announced in 2020 a motorcycle with a four-speed manual transmission [21]. a different approach was taken by kymco. the announced supernex and revonex promise a simulated six-speed transmission. production-ready variants are still expected [22, 23]. a comparison of fundamental properties of singlespeed and two-speed transmission is shown in table 1. 1.1. two-speed transmission in the research area according to studies [24], using two-speed transmission can bring a 4–5% increase in energy efficiency. in [25], the authors compared a small experimental vehicle with a single-speed transmission and gear ratio of 10 versus the same vehicle with a two-speed transmission with gear ratios of 14 and 10. the results were 3.4% savings in energy consumption (during a specific testing scenario). transmissions usually used in bevs are mostly simplified dcts; another configuration can be based on double-row planetary gears or single planetary gear (with friction brake and shift clutch). amts consist of two gear pairs with synchronizers, shift forks, and actuator motors. the power interruption can also be filled by a motor on the second axle. many other transmission layouts primarily aim to diminish the power interruption during gearshifts and, thus, try to achieve a seamless gear shift. in general, the main problems of two-speed transmissions are gear ratio design and shift schedule definition, then the power interruption, improvement of the shift quality, reduction of the jerk, and reduction of power loss [24– 27]. according to the [24], the multiple-speed transmissions are likely to spread in sports and luxury cars first, then in logistics vehicles, and potentially in ordinary vehicles. 15 jolana heřmanová acta polytechnica ctu proceedings 1.2. the goal of the article this article aims to design, optimize, and compare two transmission variants for an electric racing motorcycle. the 1d simulation models of the motorcycle with single-speed and two-speed transmission were built, the gear ratios were optimized in specific scenarios, and then the results were compared. the scenarios are derived from the requirements of the motostudent competition to reflect the points awarded in the competition as much as possible [28]. then, the vehicle performance was assessed for two different electric motor settings: limited torque variant and maximum torque variant. this reflects the real demands of the motorcycle and its components’ usage. the setting of maximum torque that the electric motor can deliver varies greatly, mainly depending on the performance of the motor controller, motor cooling system, and other systems. the factors, objectives, and other factors had to be defined to perform the optimization. 2. methodology the simulation model and the simulated vehicle are described, followed by the main component parameters, the scenarios, and the description of the optimization process. 2.1. simulation model the simulation model was built in gt-suite 1d simulation software (see figure 3). the model consists of compounds that represent the main components of the vehicle: the vehicle chassis (with its retarding forces), the transmission, the electric motor and battery, the battery controller, and the driver. each component contains its main parameters (vehicle parameters are shown in table 2). the additional mass for the two-speed transmission was not considered because the baseline motorcycle is already equipped with a transmission that transfers the motor power mounted in a rotated position. the transmission consists mainly of two bevel gears, a case, and oil filling. the overall mass of the two-speed transmission was expected to be similar. the model is controlled in a dynamic mode with a pid controller inside the driver compound. it is designed to follow the required speed (the drive cycle is shown in figure 4); in this case, the desired speed is input in the form of a maximum speed limit (distance dependent) that the vehicle could virtually ride with respect to corners. one of the model’s limitations is that it simulates the ride straight. thus, the corners must be processed for the given track in advance. the change in wheel diameter in corners is neglected; this could be solved in future model development. figure 4 shows what the drive cycle looks like. as said, it shows the maximum speed limit depending on the distance driven. as a pid regulator controls the model and takes into account all physical limits, the vehicle accelerates as much as possible in areas of figure 3. simulation model. base vehicle mass 166 kg frontal area 0.407m2 drag coefficient 0.55 tire rolling resistance factor 0.015 tire rolling radius 300.5mm driver mass 80 kg table 2. vehicle parameters. figure 4. aragon drive cycle. high-speed limit, then brakes before the limited speed area and accelerates again. the value of 300 kph was chosen as a speed that is high enough not to be reached by this specific motorcycle to achieve a full throttle demand during all these sections. as the start of the race is straight, the cycle begins with this maximum speed/maximum power demand. the motorcycle’s actual maximum speed and also the simulated speed through the cycle are visible, for example, in figure 5. the gearbox is modeled by a simple compound containing gear ratio, gear efficiency, and moment of inertia. in the case of a two-speed transmission, the gearshift takes 0.2 s. 2.2. electric motor controller setting figure 6 shows the chosen electric motor controller settings implemented into the vehicle model. the maximum torque line represents the maximum torque that the motor is able to deliver according to the manufacturer’s datasheet. it represents the upper limit of what the motorcycle can deliver and what we wish to reach. nevertheless, in reality, it is not easy to reach the datasheet values. this is the reason why we also work with the limited torque variant. it represents what was possible to get from the motorcycle system in real conditions. the goal in future motorcycle development is 16 vol. 51/2024 gear ratio optimization for an electric motorcycle figure 5. results comparison – limited torque. figure 6. motor controller settings. to get as close as possible to the maximum curve. the gear ratio is thus optimized for both variants, with the expectation that the results will give a sufficient and interpolable background (if, in future development, the limited torque curve gets closer to the maximum curve). the work also represents a methodology that can be easily repeated with different inputs, like new motorcycle evolution, new electric motor, different battery cells, etc. 2.3. scenarios before starting the optimization process, the precise goals and specific scenarios have to be defined. the primary purpose of this motorcycle is to race and achieve the best possible results in the competition of many similar motorcycles. it is thus necessary to constantly push the limits further. in this case, the goals can be derived from the parameters of the motostudent competition. table 3 shows the list of disciplines and maximum points awarded. position in the race at aragon is a discipline very similar to the fastest lap (measured in qualification and during the race). thus, the first chosen scenario disciplines awarded by points points race at aragon circuit 150 fastest lap (quali and race) 40 + 30 acceleration test (150m) 80 gymkhana 60 maximum speed 30 table 3. motostudent disciplines awarded by points. is a one-lap race at the aragon circuit as it captures both. the acceleration test and gymkhana have a low speed range in common. the crucial thing in the acceleration test is the available and transferable torque. on the other hand, the gear ratio should be chosen taking into account also the maximum vehicle speed at the finish line at the end of 150m straight. the gymkhana is focused on the motorcycle’s maneuverability and acceleration. it consists of a ride between the cones with many cornering, acceleration, and braking sequences at a very low speed. at this point, the assumption is made that the optimum gear ratio for the acceleration test will also function well in the gymkhana test. also, the torque transferability from the rear wheel to the tarmac has to be taken into account; this adhesion limit makes it purposeless to increase the gear ratio above a certain limit. thus, the second scenario is the 150m acceleration test. the maximum speed can be assessed with respect to the results from one lap of racing at aragon. to some extent, the motorcycle’s maximum speed has to be high enough to achieve a good lap time at the 17 jolana heřmanová acta polytechnica ctu proceedings figure 7. results: single-speed transmission, limited torque, one lap aragon. figure 8. results: single-speed transmission, maximum torque, one lap aragon. aragon circuit; thus, these disciplines are related. the points awarded in the competition have little impact on the overall rating (30 points versus 150+40+30 for race and fastest lap disciplines), so the maximum speed is not a top priority. 2.4. methodology of optimization the single-speed transmission variant is much simpler to assess. a design of experiment (doe) is sufficient to show the dependencies and approximate the optimum. the situation is much more complex for the twospeed transmission. the factors in the optimization case are gear ratio 1, gear ratio 2, the motor speed to shift up to the second gear, and the motor speed to shift down to the first gear. the search algorithm used was genetic algorithms (ga). the scenarios are assessed separately, as they place different demands on the motorcycle’s performance. the maximum torque and limited torque variants are also evaluated. 3. results the results section is divided into several subsections. it starts with results for the single-speed variant – one lap scenario and acceleration scenario. the results for the two-speed transmission variant follow. 3.1. single-speed transmission results the results of the single-speed transmission variant were prepared as a doe with changing the gear ratio in an interval from 1 to 15. the interval is wide enough to contain an area around the optimum and a sufficient number of designs on both sides. limited torque maximum torque gr1 [-] 3.4 3.9 lap time [s] 142.0 136.0 max speed [kph] 170.6 211.0 table 4. results: single-speed transmission. figure 9. results: single-speed transmission, limited torque, maximum speed. figure 10. results: single-speed transmission, maximum torque, maximum speed. 3.1.1. one lap the results for one racing lap for both torque limit variants are shown in figures 7 and 8. it is apparent from the figures that the optimum gear ratio lies in a wider interval. for the variant of limited torque, it is approximately from 2.6 to 4 with a lap time under 145 s, and for the maximum torque variant, it lies in the interval from 3.5 to 5.2 with a lap time under 138 s. the minimum values are 142.0 s for gr of 3.4 for the limited torque variant and 136.0 s for gr of 3.9 for the maximum torque variant (see table 4). the maximum speeds achieved during the one-lap scenario are shown in figures 9 and 10. the vehicle can reach a maximum speed of around 190 kph in the variant of limited torque. in contrast, the variant with maximum torque achieves 211 kph, and the maximum speed is achievable in a wider range of gear ratios (as the maximum motor power does not decrease with the higher motor speed in the case of the maximum torque variant and the motor torque follows thy hyperbola as shown in figure 6). 3.1.2. acceleration the results for the acceleration scenario are shown in figures 11 and 12. the limited torque variant achieves the minimum time of 7.6 s (on 150m acceleration) for a gear ratio of 8.8. still, it achieves a similar 18 vol. 51/2024 gear ratio optimization for an electric motorcycle figure 11. results: single-speed transmission, limited torque, 150m acceleration. figure 12. results: single-speed transmission, maximum torque, 150m acceleration. result with a gear ratio of 7.4 to 9.4. in contrast, the maximum torque variant achieves the lowest values of 6.7 s for gear ratio 7.6, and results are similar for gear ratio from 7.2 to 8. 3.2. two-speed transmission results the results for the two-speed transmission variant were acquired by a genetic algorithm optimization. the algorithm calculated approximately 400 designs for each variant, and the results are firstly shown as interim results in figures as gear ratio to design and then in the table after choosing the best representative design. as was visible from the single-speed transmission results, it is impossible to choose one optimal point. thus, the results should better be interpreted as intervals where the optimum is close. to easily compare, one design was selected from each variant to represent the results. 3.2.1. one lap the interim results for one aragon lap scenario for the maximum torque variant are shown in figure 13 as gear ratio to design number. the figure shows how the algorithm worked through the given space to the optimum values. the acceptable design for each torque variant was chosen from the simulated designs, summarized in table 5. the chosen design for the limited torque variant has a gear ratio of 4.4 for the first gear and 3.0 for the second gear. the gear is shifted when surpassing the motor speed of 4 367rpms; the downshift is done when the speed lowers under 2 360rpms. with these values, the vehicle achieved a lap time of 140 s and, at the longest straight, achieved a maximum speed of 177.7 kph. gr1 and gr2 vs. design maximum torque design 100 200 300 400 g ea r ra tio [] 0 2 4 6 8 10 12 gr1 gr2 figure 13. interim results: two-speed transmission, maximum torque, one aragon lap. limited torque maximum torque gr1 [-] 4.4 5.7 gr2 [-] 3.0 3.0 rpm_up 4 367 7 730 rpm_down 2 360 2 537 lap time [s] 140.0 134.1 max speed [kph] 177.7 211.5 table 5. results: two-speed transmission. the chosen result for the maximum torque variant has a gear ratio of 5.7 for the first gear and 3.0 for the second gear. the upshift and downshift are then at 7 730 and 2 537rpms. the lap time achieved is 134.1 s, and a maximum speed of 211.5 kph. 3.2.2. acceleration as vehicle adhesion is limited, and the gearshift itself represents a short time delay, it is not beneficial to use the two-speed transmission for the acceleration test. thus, the results for the single-speed transmission are applicable. 4. discussion the results for one aragon lap scenario are summed up in table 4 and 5. the first point that should be mentioned is a total difference of 6 s between the limited torque variant and the variant of maximum torque. this difference is roughly the same for single-speed transmission (142 s vs. 136 s) as in the case of using the two-speed transmission (140 s vs. 134.1 s). the maximum torque variant achieves a higher maximum speed, as the motor has more power. the maximum speeds achieved were 190 kph and 211 kph for the limited and maximum torque variants. secondly, using a two-speed transmission instead of a single-speed transmission gives an approximately two-second advantage in the one aragon lap scenario, which is substantial, especially in the racing environment. the comparison of single-speed and two-speed transmission results is shown in figure 5. the figure con19 jolana heřmanová acta polytechnica ctu proceedings sists of (from top) vehicle speed, electric motor speed, electric motor mechanical power (maximum of 42 kw in the maximum torque variant, 36 kw in case of limited torque variant), and gear ratio. it is possible to observe when the gearshifts occur (around rpms of 4 370), and when looking at the electric motor power graph (the third), there are substantial differences in power used. in most cases, the two-speed transmission variant is allowed to use the motor in areas where it can supply more power. thus, the vehicle can be faster in the race. with constant torque in motor torque characteristics shown in figure 6, it is possible to imagine linearly increasing power reaching its maximum as the motor torque starts to decrease. as the limited torque variant reaches its maximum power at this point, it shifts to the second gear shortly after this point is exceeded. the single-speed transmission with shifting impossible is, therefore, a trade-off between higher acceleration and maximum speed. not only the lap time of the single-speed transmission variant is lower, but also the maximum speed achieved. 5. conclusions the 1d simulation model of an electric motorcycle was built. two scenarios were suggested to assess the differences between singleand two-speed transmissions. it was one lap of racing at the aragon circuit and a test of acceleration at the 150m distance. additionally, two variants of motor controller settings were evaluated: the limited torque variant – representing the realistic (rather conservative) setting, and the maximum torque variant – representing the motor datasheet values. the optimum gear ratio for a single-speed transmission was suggested based on the experiment’s design. the gear ratio was changed in intervals from 1 to 15, and the best results in a one-racing-lap scenario were achieved with a gear ratio in intervals from 2.4 to 4 for the limited torque variant and in intervals from 3.5 to 5.2 for the maximum torque variant. the results for the acceleration test are derived in the same manner. the best times were achieved in the intervals from 7.4 to 9.4 for the limited torque variant and 7.2 to 8 for the maximum torque variant. the gear optimization for the two-speed transmission was performed using the genetic algorithms method. in the end, one acceptable design (for each of the two torque variants) was chosen to represent the results. the best time lap was 140 s for the limited torque variant and 134.1 s for the maximum torque variant. the limited torque variant got the best results for gear ratios of 4.4 and 3.0 (for the first and second gear) while upshifting and downshifting at 4 367 and 2 360rpms. the maximum torque variant achieved the best results with gear ratios of 5.7 and 3.0 (first and second gear) while upshifting and downshifting at 7 730 and 2 537 rpms. the maximum speeds achieved in the one race lap scenario were 170.6 and 211.0 kph (limited and maximum torque variant) for single-speed transmission and 177.7 and 211.5 kph (limited and maximum torque variant) for two-speed transmission. to conclude: • the difference between limited and maximum torque variants of motor controller setting is around 6 s, • the possible benefit of a two-speed transmission usage in this vehicle and this scenario is around 2 s per lap, • maximum torque variant: the best lap time achieved was 134.1 s (with two-speed transmission, gr1 = 5.7, gr2 = 3.0), • maximum torque variant: the best acceleration time in the 150 m acceleration test was 6.7 s (with gear ratio of 7.6), • limited torque variant: the best lap time achieved was 140 s (with a two-speed transmission, gr1 = 4.4, gr2 = 3.0), • limited torque variant: the best acceleration time achieved was 7.6 s (gr = 8.8). the benefit of the two-speed transmission over the single-speed transmission is around 2 seconds on a lap, which can be substantial in the racing environment. however, it is not clear if this advantage can balance the higher complexity of the shiftable gearbox system, not only in the higher number of components and added weight, but also in adding the gear shifting mechanism and tuning it to be comfortable for rider use in the stressful and demanding environment that motorcycle racing certainly is. list of symbols bev battery electric vehicle doe design of experiment em electric motor ga genetic algorithms gr gear ratio rp m revolutions per minute soc state of charge acknowledgements this work was realized using the support of: • technological agency, czech republic, programme national competence centres, project 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[2024-04-17]. www.motorcyclesports.net/kymcopatent-reality-or-fiction/ [24] b. gao, d. meng, w. shi, et al. topology optimization and the evolution trends of two-speed transmission of evs. renewable and sustainable energy reviews 161:112390, 2022. https://doi.org/10.1016/j.rser.2022.112390 [25] p. spanoudakis, n. c. tsourveloudis, g. koumartzakis, et al. evaluation of a 2-speed transmission on electric vehicle’s energy consumption. in 2014 ieee international electric vehicle conference (ievc), pp. 1–6. ieee, 2014. https://doi.org/10.1109/ievc.2014.7056116 [26] k. ogawa, t. aihara. development of two-speed dual-clutch transmission for seamless gear shifting in evs. transportation engineering 6:100097, 2021. https://doi.org/10.1016/j.treng.2021.100097 [27] a. rahman, n. hassan, s. i. ihsan. fuzzy logic controlled two speed electromagnetic gearbox for electric vehicle. international journal of technology 13(2):297–309, 2022-04-01. https://doi.org/10.14716/ijtech.v13i2.3913 [28] motostudent, 2023. [2023-10-06]. https://www.motostudent.com/ 21 https://web.archive.org/web/20071020043035/http://www.teslamotors.com/performance/tech_specs.php https://web.archive.org/web/20071020043035/http://www.teslamotors.com/performance/tech_specs.php https://web.archive.org/web/20071020043035/http://www.teslamotors.com/performance/tech_specs.php https://web.archive.org/web/20090130032356/http://www.teslamotors.com/performance/tech_specs.php https://web.archive.org/web/20090130032356/http://www.teslamotors.com/performance/tech_specs.php https://web.archive.org/web/20090130032356/http://www.teslamotors.com/performance/tech_specs.php https://newsroom.porsche.com/en_us/products/taycan/powertrain-18555.html https://newsroom.porsche.com/en_us/products/taycan/powertrain-18555.html https://www.automobile-catalog.com/auta_details1.php https://www.automobile-catalog.com/auta_details1.php https://www.drive.com.au/news/porsche-taycan-a-look-at-its-motors-transmission-and-dynamic-chassis/ https://www.drive.com.au/news/porsche-taycan-a-look-at-its-motors-transmission-and-dynamic-chassis/ https://www.drive.com.au/news/porsche-taycan-a-look-at-its-motors-transmission-and-dynamic-chassis/ https://jalopnik.com/an-extremely-detailed-look-at-the-porsche-taycans-engin-1837802533 https://jalopnik.com/an-extremely-detailed-look-at-the-porsche-taycans-engin-1837802533 https://insideevs.com/features/436049/rimac-two-speed-gearbox-stupid-video/ https://insideevs.com/features/436049/rimac-two-speed-gearbox-stupid-video/ https://insideevs.com/news/343216/check-out-the-new-rimac-c-two-with-nico-rosberg-video/ https://insideevs.com/news/343216/check-out-the-new-rimac-c-two-with-nico-rosberg-video/ https://www.evo.co.uk/bmw/i8/page/0/2 https://www.car-engineer.com/two-speed-eaxle-bmw-i8-plug-hybrid/ https://www.car-engineer.com/two-speed-eaxle-bmw-i8-plug-hybrid/ https://www.car-engineer.com/lightweight-electrified-drive-axle-zf/ https://www.car-engineer.com/lightweight-electrified-drive-axle-zf/ https://jalopnik.com/heres-zfs-new-two-speed-transmission-for-electric-cars-1836428183 https://jalopnik.com/heres-zfs-new-two-speed-transmission-for-electric-cars-1836428183 https://www.recurrentauto.com/research/electric-cars-gears-transmissions https://www.recurrentauto.com/research/electric-cars-gears-transmissions https://www.wired.com/story/electric-car-two-speed-transmission-gearbox/ https://www.wired.com/story/electric-car-two-speed-transmission-gearbox/ https://www.cycleworld.com/2013/10/31/brammo-empulse-r-vs-zero-s-electric-motorcycles-comparison-test-specs-photos/ https://www.cycleworld.com/2013/10/31/brammo-empulse-r-vs-zero-s-electric-motorcycles-comparison-test-specs-photos/ https://www.cycleworld.com/2013/10/31/brammo-empulse-r-vs-zero-s-electric-motorcycles-comparison-test-specs-photos/ https://www.motorcycle.com/manufacturer/2013-brammo-empulse-r-review-91479.html https://www.motorcycle.com/manufacturer/2013-brammo-empulse-r-review-91479.html https://www.webbikeworld.com/qj7000d-patent-designs-for-qj-motors-new-electric-motorcycle-revealed-ahead-of-schedule/ https://www.webbikeworld.com/qj7000d-patent-designs-for-qj-motors-new-electric-motorcycle-revealed-ahead-of-schedule/ https://www.webbikeworld.com/qj7000d-patent-designs-for-qj-motors-new-electric-motorcycle-revealed-ahead-of-schedule/ https://www.smesh.eu/storm-electric-motorcycle/ https://www.matter.in/motorbike https://jalopnik.com/kawasakis-new-ev-motorcycle-has-a-manual-transmission-1843008215 https://jalopnik.com/kawasakis-new-ev-motorcycle-has-a-manual-transmission-1843008215 https://jalopnik.com/kawasakis-new-ev-motorcycle-has-a-manual-transmission-1843008215 https://www.visordown.com/news/new-bikes/kymco-revonex-manual-box-electric-bike-still-happening https://www.visordown.com/news/new-bikes/kymco-revonex-manual-box-electric-bike-still-happening https://www.visordown.com/news/new-bikes/kymco-revonex-manual-box-electric-bike-still-happening www.motorcyclesports.net/kymco-patent-reality-or-fiction/ www.motorcyclesports.net/kymco-patent-reality-or-fiction/ https://doi.org/10.1016/j.rser.2022.112390 https://doi.org/10.1109/ievc.2014.7056116 https://doi.org/10.1016/j.treng.2021.100097 https://doi.org/10.14716/ijtech.v13i2.3913 https://www.motostudent.com/ acta polytechnica ctu proceedings 51:14–21, 2024 1 introduction 1.1 two-speed transmission in the research area 1.2 the goal of the article 2 methodology 2.1 simulation model 2.2 electric motor controller setting 2.3 scenarios 2.4 methodology of optimization 3 results 3.1 single-speed transmission results 3.1.1 one lap 3.1.2 acceleration 3.2 two-speed transmission results 3.2.1 one lap 3.2.2 acceleration 4 discussion 5 conclusions list of symbols acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2023.42.0017 acta polytechnica ctu proceedings 42:17–21, 2023 © 2023 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague mechanical properties of basalt: a study on compressive loading at different strain rates using shpb jan faltaa,∗, nela krčmářováa, tomáš fílaa, martin vavrob, leona vavrob a czech technical university in prague, faculty of transportation sciences, department of mechanics and materials, konviktská 20, 110 00 prague 1, czech republic b czech academy of sciences, institute of geonics, studentská 1768/9, 708 00 ostrava-poruba, czech republic ∗ corresponding author: falta@fd.cvut.cz abstract. this article focuses on the mechanical properties of basalt in compressive loading at different strain-rates. the study employs advanced instrumentation for the evaluation of the results in dynamic conditions, while standard uni-axial loading device is used for evaluation in quasi-static conditions. basalt specimens were subjected to four different loading-rates from 200–600 s−1 on which the stress-strain dependence was evaluated together with dic analysis of crack initiation and disintegration process. understanding the mechanical properties of basalt can provide insights for engineers and designers in creating structures that are durable and able to withstand different loading conditions. the findings of this study can have implications for a wide range of industries, including aerospace, automotive, and construction, among others. keywords: shpb, basalt, compressive loading, dynamic loading. 1. introduction basalt is an igneous rock that is highly versatile in a range of applications. basalt’s high strength and non-corrosive properties make it a suitable material for reinforced concrete structures in specific environments [1]. basalt fibers can be also used as a reinforcing material in composites for various industrial applications [2]. cement-based composites, particularly concrete, are widely used in the construction industry [3, 4]. they play a crucial role in building structures like highway bridges, tunnels, and dams, intended to endure for many generations. however, these structures often exhibit nonlinear behavior under loading, specifically quasi-brittle characteristics. even after deviating from the linear force-displacement diagram, they can still bear loads until reaching a peak point. subsequently, a decline in loading force occurs, ultimately leading to failure a phenomenon referred to as tensile softening [5]. this behavior can be attributed to various factors such as the presence of internal defects (e.g., pores, cracks, transition zones) or material discontinuities (e.g., inclusions for example olivine, labradorite, magnetite, nepheline) that act as obstacles or facilitators for crack propagation. surprisingly, existing standards, such as en 1992-1-1 (2004), completely overlook these stress concentrators, which have the potential to serve as weak elements within the composites. the aim of this work is to reveal the influence of compressive loading at different strain-rates on the mechanical response of the basalt rock and to inspect the failure mechanism of the sample using digital image correlation. 2. materials and methods 2.1. material characterization of the bílčice basalt olivine basalt to nepheline basanite used in the experiment came from the active bílčice quarry (approx. 12 km se from the city of bruntál and near the ride side of the slezská harta dam). the rock deposit is situated in the se edge of the chřibský les stream, which represents the largest of the four known lava streams of the velký roudný stratovolcano. the quarried plio-pleistocene basaltoids are gray to black-gray volcanic rocks with a distinct porphyritic texture. from the structural point of view, two basic types of basaltoids are developed in the deposit: (1) predominant fine-grained, massive, compact, only in places porous basalt with block to tabular jointing, from which the test specimens were prepared; and (2) basalt with a typical orbicular structure with spherical (pea-like) disintegration and well-developed columnar jointing figure 1. rock phenocrysts are predominantly formed by green to green-yellow crystals of olivine (approx. 15–20 % of the rock volume, max. 1–2 % in size), clinopyroxene can occur only subsidiary. rock matrix is composed of clinopyroxene (in particular augite, approx. 37–45 % of the rock volume), calcium-rich plagioclase of labradorite composition (about 20–30 %), magnetite (between 10 and 20 %) and possibly also nepheline (up to 1 %). the proportion of amorphous phase (basaltic glass) is up to 3 % [6]. the bílčice basalt under study exhibit a high bulk density, relatively low water absorption capacity and 17 https://doi.org/10.14311/app.2023.42.0017 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en j. falta, n. krčmářová, t. fíla et al. acta polytechnica ctu proceedings uniaxial compresive strenght fracture toughness ultrasonic wave velocity splitting tensile strength specific density total porosity [mpa] [mpa m1/2] [km s−1] [mpa] [kg m−3] [%] 232 3.09 5.49 12.3 3032 3.44 table 1. basic physical, mechanical and mechanical fracture properties of the bílčice basalt. total porosity, high values of p-wave velocity, and high to very high strength properties table 1 [2]. these properties predetermine the bílčice basalt as a source of high-quality crushed aggregate. figure 1. typical columnar jointing visible in the bílčice quarry wall. 2.2. specimens in this study, samples drilled from a single piece of basalt, which you can see in figure 2, were used. the samples are 20 mm long and 12 mm in diameter. a total of 17 specimens were tested, 5 pc under quasistatic loading (1 mm min−1), 5 pc tested dynamically in contact with an incident bar at a strain-rate of 200 s−1, 5 pc tested dynamically in contact with an incident bar at a strain-rate of 300 s−1, 1 pc by the impact of an incident bar at a strain-rate of 400 s−1, 1 pc by incident bar impact at strain-rate 600 s−1. figure 2. macrophotograph of the massive type of the bílčice basalt with ilustration of one specimen. 2.3. experimental setup the mechanical response of the structures was investigated under dynamic conditions using a split hopkinson pressure bar (shpb) apparatus equipped with strain gauges and a high-speed camera (fastcam sa-z, photron, japan). the camera was set to capture frames at a rate of 300.000 fps with a resolution of 256 × 256 pixels. figure 3 illustrates a diagram and schematic of the loading device. the dynamic loading device comprised two bars: a striker bar housed within a gas-gun barrel and a transmission bar. both bars had a uniform diameter of 20 mm and were constructed from a high-strength aluminum alloy (en-aw-7075-t6). the striker bar was accelerated using a single-stage gas-gun, generating a strain pulse when the striker projectile impacted the front face of the incident bar. to shape the strain pulse, a cylindrical copper pulse shaper was placed on the front face of the incident bar. strain measurements were conducted using foil strain gauges (3/120 ly61, hbm, germany) connected in a wheatstone half-bridge configuration to compensate for any minor deflections in the bars. the data acquisition systems and cameras were triggered by a pair photoelectric sensors, enabling estimation of the impact velocity. 3. results 3.1. quasi-static loading the mechanical response of the investigated structures to quasi-static uni-axial compressive loading was expressed by the average engineering stress-strain diagram from 5 tested specimens with standard deviation was evaluated. the maximum stress value was approx. 270 mpa at strain between 2.5–3.0 %. 3.2. dynamic loading to reveal a possible strain-rate sensitivity of the deformation response, dynamic compression tests using the shpb apparatus were conducted at four different loading-rates with two different loading conditions. 3.2.1. incident bar in contact with specimen in this case, it was the standart placement of the specimen for the shpb assembly, so that the specimen is sandwiched between the incident and transmission bars. for the first low-rate, the speed of the impactor was determined using a pair of short-reaction time through-beam optical gates (fs/fs 10-rl-pse4, sensopart, germany) to approx. 19 m s−1. the generated wave in the incident bar would then result in a strain-rate of 1 000 s−1, but due to the stiffness of the sample and the small total deformation required to failure, the incident bar did not have enough time to accelerate to the required speed before the tested 18 vol. 42/2023 mechanical properties of basalt figure 3. the arrangement of the shpb experimental setup: uni-axial compression with aluminium alloy bars. specimen disintegrated, and the real strain-rate during the deformation was thus around 200 s−1 (4 m s−1). from the trend of the strain-rate curve for the low-rate measurements (green dashed line), an uneven trend can be seen until the breakdown of the sample and the subsequent acceleration up to values of 1 000 s−1. the corresponding average stress-strain curve for low-rate (green solid line) shows similar peak stress values to quasi-static loading and only a steeper stress rise. from the calculated deviation of the stress-strain curve marked as a green shadow around the stressstrain curve, it can be concluded that the behaviour is quite uniform throughout the tested set of five specimens. for the second set of five specimens marked as highrate (yellow color curves), the speed of the impactor was increased to 32 m s−1. as in the previous case, the maximum possible impact velocity was not reached before the sample disintegrated and the deformation rate was roughly 300 s−1 (6 m s−1). no significant effect of strain-rate hardening compared to q-s and low-rate dynamic loading was noted. the process of the deformation recorded by the high-speed camera can be seen in figure 4. the upper slide-show consisting of 8 images with a green border corresponds to a low-rate dynamic loading at 200 s−1, and similarly the following 8 images with a yellow border correspond to 300 s−1. 3.2.2. gap between specimen and incident bar in this case, a 5 mm gap was left in front of the specimen to solve the problem of insufficient time for the incident bar to accelerate. the incident bar thus gained speed before hitting the specimen. due to the insufficient number of specimen, only two were tested with this method, namely at the resulting speeds of 400 s−1 (8 m s−1 purple curves) and at 600 s−1 (12 m s−1, black curves) figure 5. for the specimen tested at a speed of 400 s−1, a similar value of the maximum stress was achieved. the similar response trend according to with the measurement when the sample was in contact with the bars can be seen. a difference in the response trend is only in the gradual build-up of stress, which figure 4. dynamic deformation mechanisms with step of 17µs between images for the green 200 s−1 (4 m s−1), yellow 300 s−1 (6 m s−1), purple 400 s−1 (8 m s−1), black 600 s−1 (12 m s−1). may be caused by inertial effects caused by the impact of the already accelerated incident bar. the second test with the same arrangement only at a higher speed of 600 s−1 shows the same stress gradual build-up but only to a value below 250 mpa. since it is only one specimen, it cannot be concluded that the behaviour of the material has changed, and most likely it is only 19 j. falta, n. krčmářová, t. fíla et al. acta polytechnica ctu proceedings figure 5. mean stress-strain curves with standard deviation (shadows around curves) for q-s (red) and dynamic loading with corresponding strain-rate (dashed lines). green curves – 200 s−1 (4 m s−1), yellow curves – 300 s−1 (6 m s−1), purple curves – 400 s−1 (8 m s−1), black curves – 600 s−1 (12 m s−1). a deviation caused by a different internal structure of the specific tested specimen. the process of the deformation recorded by the high-speed camera can be seen in figure 4. the bottom slide-show consisting of 8 images with a black border corresponds to a dynamic load at 600 s−1, and similarly the above placed 8 images with a purple border correspond to 400 s−1. 3.3. digital image correlation dic is an image processing method that uses tracking and image registration techniques to measure changes in a sequence of images [7]. in this case the modified algorithm of augmented-lagrangian dic providing global kinematic compatibility was used [8]. the camera recordings from dynamic experiments were subjected to a dic analysis to evaluate the captured displacements to describe crack formation and disintegration mechanism. it was found that in experiments where the specimen is in contact with the incident bar, a crack and subsequent disintegration occur primarily on the incident side and always localized. on the other hand, in both specimens tested with an already accelerated incident bar, crack occurred across the entire width of the sample, and the disintegration also appeared earlier on the back side of the specimen, as can be seen in selected images figure 6. 4. conclusions a description of the fundamental mechanical behaviour of the basalt rock specimens was determined from quasi-static uni-axial compression tests. to assess the strain-rate sensitivity of the mechanical response, dynamic compression experiments at four different loading-rates and two different loading modes using the shpb apparatus were conducted. optical measurement methods employing a high-speed camera were applied. dic algorithm was used to evaluate the displacements fields from the camera recordings to investigated failure mechanism under applied loads. based on the results obtained from the experimental investigations, it is possible to conclude: • a strain-rate sensitivity of the deformation behaviour of the investigated basalt rock was not indicated. the decrease in recorded values of stress occurred for last specimen at 600 s−1 was not conclusive and more specimens need to be tested for sufficient statistics. • the starting part of the trend of stress-strain curves was different for two modes of loading (bar in contact and with gap) probably due to inertia effect of bars. this phenomena could be investigated with dic evaluation on both end of the bar and evaluation of their velocities during the test. • loading mode with gap at the front of the specimen seems promising as a method for dynamic testing of tough materials with low deformation around few percent as its creates the possibility to reach higher strain-rates. to clearly describe the effect of such a loading mode compared to the standard placement, it is necessary to perform a larger statistically significant set of measurements and, if possible, include the evaluation of the velocity of both ends of the bars near the specimen using dic. 20 vol. 42/2023 mechanical properties of basalt figure 6. displacement fields for selected specimens taken at the beginning of the disintegration process. acknowledgements this research was funded by czech science foundation (project junior star no. 22-18033m) and by the grant agency of the czech technical university in prague, grant no. sgs23/134/ohk2/2t/16. the conference participation was supported by czech national imeko committee. references [1] m. vyhlídal, i. rozsypalová, h. šimonová, et al. influence of rock inclusion composition on the fracture response of cement-based composite specimens. procedia structural integrity 33:966–981, 2021. https://doi.org/10.1016/j.prostr.2021.10.107 [2] m. vyhlídal, i. rozsypalová, h. šimonová, et al. effect of petrographic composition and chemistry of aggregate on the local and general fracture response of cementitious composites. frattura ed integrità strutturale 16(60):13– 29, 2022. https://doi.org/10.3221/igf-esis.60.02 [3] p.-c. aitcin. high performance concrete. crc press, 1998. [4] a. m. neville, et al. properties of concrete, vol. 4. longman london, 1995. [5] d. lange-kornbak, b. karihaloo. fracture mechanical prediction of transitional failure and strength of singly-reinforced beams. in minimum reinforcement in concrete members, pp. 31–66. elsevier, 1999. https://doi.org/10.1016/s1566-1369(99)80061-2 [6] v. slivka, m. vavro. the significance of textural and structural properties of north-moravian basaltoids for the manufacture of mineral fibres. ceramics silikáty 40(4):149–159, 1996. [7] n. novak, d. kytyr, v. rada, et al. compression behaviour of tpms-filled stainless steel tubes. materials science and engineering: a 852:143680, 2022. https://doi.org/10.1016/j.msea.2022.143680 [8] j. yang, k. bhattacharya. augmented lagrangian digital image correlation. experimental mechanics 59(2):187–205, 2018. https://doi.org/10.1007/s11340-018-00457-0 21 https://doi.org/10.1016/j.prostr.2021.10.107 https://doi.org/10.3221/igf-esis.60.02 https://doi.org/10.1016/s1566-1369(99)80061-2 https://doi.org/10.1016/j.msea.2022.143680 https://doi.org/10.1007/s11340-018-00457-0 acta polytechnica ctu proceedings 42:17–21, 2023 1 introduction 2 materials and methods 2.1 material characterization of the bílčice basalt 2.2 specimens 2.3 experimental setup 3 results 3.1 quasi-static loading 3.2 dynamic loading 3.2.1 incident bar in contact with specimen 3.2.2 gap between specimen and incident bar 3.3 digital image correlation 4 conclusions acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2024.48.0009 acta polytechnica ctu proceedings 48:9–14, 2024 © 2024 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague impact of bolt positioning on the stiffness of angular support brackets dario croccolo, stefano fini, muhammad yasir khan∗ university of bologna, department of industrial engineering (din), viale del risorgimento 2, 40136 bologna, italy ∗ corresponding author: muhammadyasir.khan3@unibo.it abstract. bolts and screws are usually preferred over other joining techniques when assembly and disassembly operations are required. however, they add extra weight to the system, so it is essential to reduce their size and weight by optimizing bolt parameters. additionally, the stiffness of bolted members is very crucial; those with low stiffness may affect the correct functioning of the other connected components. this study focuses on the impact produced by bolt positions, specifically their center of gravity and spacing among them, on the stiffness of angular l-shaped brackets using finite element analysis. bending and torsional loads were applied to the free end of the member and deformations were recorded under these loads. the results revealed that a triangular bolt pattern is recommended for three bolts, whereas for four or more bolts, the pattern should be uniform to avoid stress concentrations. moreover, the center of gravity and bolt spacing have a significant impact on stiffness. by optimizing bolt spacing and the center of gravity, a combination of six bolts can be replaced with four bolts of the same size while maintaining the same stiffness. keywords: stiffness, bolt positioning, angular brackets, optimization, numerical analysis. 1. introduction bolts and screws are widely used in the automotive and aerospace industry due to their easy handling especially during multiple tightening and untightening operations. sometimes, these joints are avoided to keep the system’s weight low. therefore, it is a necessary to avoid the use of extra bolts by optimizing different bolt parameters. moreover, the stiffness of bolted connections plays a crucial role in the uniform distribution of forces. most of the time, it is required to make the structures so stiff that they could not affect the functioning of other connected components. a recently published review article addresses the significant optimization factors i.e. bolt layouts, thread geometry, bolt size, and tightening sequences for bolted joints [1]. there are several ways to improve the stiffness of angular l-shaped brackets by varying bolt parameters. the most important bolt parameters for improving the stiffness are bolt layouts and spacing between bolts [2]. kiamanesh et al. [3] studied the effect of rectangular and circular bolt patterns on extended end plate connections using non-linear 3d finite element analysis. the study revealed that the circular pattern improves the connection strength and enhances the energy dissipation capability. similarly, wang et al. [4] examined the impact of rectangular and staggered bolt patterns on t-stub connections and proposed that the staggered bolt patterns are the best to get the stiffer structures. there are also few studies reported in the literature that used artificial intelligence (ai) techniques to optimize the bolt parameters. as, shen [5] used genetic algorithms to optimize the bolt locations to reduce the accelerations in the electronic board circuit. with optimized bolt positions the acceleration in the circuit reduced 67 percent from 43.9 m s−2 to 14.4 m s−2. similarly, lu [6] used the gray wolf algorithm to optimize the peripheral stresses in the nickel plates using different triangular bolt patterns. in optimized bolt patterns, the stresses reduced by 24 percent. moreover, sari [7] has developed a software to optimize the placement of bolts in bolted connections based on c language. this program takes few values as inputs and suggests the optimized bolt positions for objective function. however, a limited literature is available on the optimization of stiffness for bolted joints by varying bolt parameters. the objective of the following study is to design a bolted connection with higher stiffness using simulation software, and comparing the results obtained with those theoretically. an l-angle piece subjected to variable loads that generate bending and twisting moment was examined. subsequently, an attempt was made to optimize the joints by varying some parameters i.e. bolt layout, center of gravity and bolt spacing and trying to obtain a configuration that guarantees greater stiffness. 2. materials and methods in the study, solidworks has been used for drawings and three-dimensional parametric designs. the numerical analysis was performed using ansys software due to its better solving capabilities, accuracy of strain, stiffness and stress results. 9 https://doi.org/10.14311/app.2024.48.0009 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en d. croccolo, s. fini, m. khan acta polytechnica ctu proceedings the angular l-shaped bracket is subjected to two forces acting on the midpoint of the free corner, one in a vertical downward direction, and other acting horizontally as shown in figure 1. these forces generate bending and twisting moments respectively. the material for the plates and angular bar is structural steel s275 (standard en 10027-1:2016) [8] and the bolts of resistance class 8.8 [9] were chosen for the analysis. the analysis was carried out by verifying the deformation undergone by the angle piece as the load acting on it varies in order to determine its stiffness. moreover, the load was applied from 0 to 30 kn in 16 steps to list the results. in ansys, the bolted joint is represented as an assembly of several solid bodies using the one-sided non-penetration constraint. to achieve this, the resistant section of the bolt is represented by a beam (in ansys command beam) with a circular section with a diameter corresponding to that of the bolt under examination. additionally, the initial preload on the bolts was applied using the bolt preload tool available in ansys. initially, the cad geometry was imported into ansys workbench’s static structural module, while for the materials reference was made to the database present in the software. frictional and bonded contacts were used to model the contacts between the various components. in particular, the frictional contact was used to model the contact at the interface between the plates and in the contact surfaces between the underhead of the screw and plate. bonded contacts were used to connect the different portions of the screw: shank not in socket and threaded portion in socket. the discretization used is different on different components. in particular, a multi-zone approach was used for the bolt components in order to obtain a smooth and sufficiently dense mapped mesh. the elements used for these components are of type solid186 with a size of 1 mm. as far as the plates are concerned, a hex dominant approach has been chosen to carry out a mapped mesh discretization, the elements used are solid187 type 3 mm in size. in all simulations, the load was applied in two different steps. the first step applies the nominal preload to the bolt producing a shortening of the shank. the second step “locks” the preload value achieved through the specific lock function and applies the external loads. the parameters that were analysed to design the bolted connection as the load varies are the following: nominal diameter, number of bolts, the influence of the center of gravity, bolt spacing, and bolt layout. 3. results and discussion 3.1. influence of bolt size on stiffness initially, 3 bolts of sizes m8, m12 and m16 were analysed using a triangular pattern as shown in figure 2. the deformation of angle bar was recorded at the figure 1. l-shaped angular bar subjected to bending and twisting loads. figure 2. triangular bolt pattern. extreme free end in mm, and it varies according to the applied load. the maximum preloads were applied on each bolt following vdi-2230 standard [10] and friction coefficient of 0.2 was taken for the dry conditions. the maximum sliding force that the connection can support is equal to rt = z · µ · fi, (1) where z represents the number of surfaces in contact, µ the coefficient of friction and fi is the applied preload. the same procedure is followed for the rest of the cases. from analysis, it was noted that as the nominal diameter of the bolt increases, the structure shows a more rigid behaviour; however, the increase in stiffness obtained is negligible. figure 3 shows the relationship between applied load and displacement under bending and twisting loads. 10 vol. 48/2024 impact of bolt positioning on the stiffness of angular support brackets figure 3. force vs displacement diagrams (a) bending load (b) twisting load. 3.2. influence of number of bolts on stiffness in this case, three, four, and six bolts of the same size were analysed. the bolts were arranged in such a way that the center of gravity remained the same for all cases. from the results, it was deduced that the stiffness increases as the number of bolts increased as expected; however, this increase is not linear, as greater displacement difference was registered going from 3 to 4 bolts than from 4 to 6 bolts as shown in figure 4. figure 4. force vs displacement for different number of bolts. 3.3. influence of bolt center of gravity on stiffness the center of gravity or centroid is the point where moments act, and it changes when the position of bolts changes. in this analysis, three cases with m12 bolts have been analysed to check the effect of the center of gravity on stiffness as shown in figure 5. the center of gravity in the y direction has been increased and decreased by 30 percent. a similar procedure was also followed for x-coordinates to shift the centroid left and right by 30 percent to check the impact of torsional load. it is evident that the best configuration for bolted joints subjected to bending and twisting moments is achieved by shifting the y-coordinates of the bolts upwards and the x-coordinates towards the negative xdirection, as illustrated in figure 6. shifting the bolts upwards reduces the chance of the l-shaped member separating from the fixed plate. similarly, positioning the x-coordinates of the bolts towards the side of the applied load increases resistance to deformation due to twisting torque. 3.4. influence of bolt spacing on stiffness it was examined from the center of gravity section that bolts should be placed upward and towards the negative x-direction for the given loads. regarding bolt spacing, the previous best bolt locations were chosen with an increase and decrease in center distance by 35 percent. the results indicated that the best stiffness was achieved when the bolts were placed away from the centroid. this is due to the uniform distribution of forces and a reduced chance of the structure rotating. figure 7 shows the effect of bolt spacing for twisting load only. the results explained that the highest stiffness for both twisting and bending moments is given by the increase in the number of bolts i.e. six-m12 bolts; however, it is possible to obtain approximately the same stiffness by optimizing the center of gravity and by increasing the center distance for four-m12 bolts. in particular, it was noted that by increasing the y-coordinate of the center of gravity by 30 percent for four-m12 bolts, decreasing the x-coordinate of the center of gravity by 30 percent, and increasing the center distance of bolts by 35 percent, the stiffness obtained is similar to that obtained by six-m12 bolts. 3.5. influence of bolt layout on stiffness finally, the effect of bolt layouts for four different configurations namely, inline, rectangular, trapezoid and t-shape were analysed as shown in figure 8. the configuration that enjoys greater stiffness was observed to be the rectangular one with an increase in center of gravity by 30 percent. the in-line arrangement is the worst of all, in fact for a load of 30 kn the 11 d. croccolo, s. fini, m. khan acta polytechnica ctu proceedings figure 5. center of gravity (a) xg = 100, y g = 90 (b) increased 30 percent in y (c) decreased 30 percent in y . figure 6. force vs displacement diagrams (a) three cases under bending moment for y (b) three cases under twisting moment for x. recorded deformation was 65 mm. figure 9 illustrates the force and displacement for different layouts. 3.6. stress analysis a brief stress analysis was performed on the optimal configuration, which includes four m12 bolts arranged in a rectangular pattern with y g increased by 30 percent, xg decreased by 30 percent, and the center distance increased by 35 percent. the maximum preload was applied to the bolts, along with the external applied loads. the maximum external load was determined from the maximum sliding force (rt) for each individual bolt. through static analysis, the figure 7. force vs displacement diagram for different bolt spacing subjected to twisting load. safety coefficient for the most stressed bolt was calculated and found to be greater than 2, as shown in figure 10. this clearly indicates that under these loading conditions, the joint is completely safe. 4. conclusions the study presented in this paper demonstrates the influence of bolt parameters on the stiffness of a simple bolted corner connection subjected to bending and twisting moments. in general, the arrangement of the screws plays a fundamental role in the stiffness of the connection. specifically, the parameters that most significantly influence stiffness are the center of gravity and the center distance between bolts. although the nominal diameter of the bolts contributes to increased stiffness, it has relatively little effect on the final stiffness. by appropriately modelling the configuration of the screws, it is possible to achieve considerable stiffness, comparable to using a greater number of screws. a notable example is the configuration with 4-m12 bolts, where the center of gravity is vertically increased and horizontally decreased, and the center distance is increased. in this setup, the displacement of the angle piece due to the load is almost equal to that obtained with 6-m12 bolts. 12 vol. 48/2024 impact of bolt positioning on the stiffness of angular support brackets figure 8. bolt layouts (a) inline (b) rectangular (c) trapezoid (d) t-shape. ] figure 9. force vs displacement for different layouts. figure 10. minimum safety factor calculated through static analysis for the most stressed bolt. 5. future recommendations in the future, more configurations will be investigated using advanced ai optimization techniques, such as evolutionary algorithms, to find the optimal bolt parameters. additionally, the effects of angle piece length, width, and material will be studied. it would also be interesting to include fatigue analysis and examine the effect of bolt preload on stiffness. references [1] d. croccolo, m. de agostinis, s. fini, et al. optimization of bolted joints: a literature review. metals 13(10):1708, 2023. https://doi.org/10.1086/144770 [2] n. r. nelson, n. s. prasad, a. sekhar. structural integrity and sealing behaviour of bolted flange joint: a state of art review. international journal of pressure vessels and piping 204:104975, 2023. https://doi.org/10.1016/j.ijpvp.2023.104975 [3] r. kiamanesh, a. abolmaali, m. razavi. effect of circular bolt pattern on behavior of extended end-plate connection. journal of structural engineering 139(11):1833–1841, 2013. https: //doi.org/10.1061/(asce)st.1943-541x.0000765 [4] p. wang, j. xu, j. li, l. yu. tensile behavior of cruciform stubs with four bolts per row. the structural design of tall and special buildings 32(1):e1988, 2023. https://doi.org/10.1002/tal.1988 [5] b. shen, z. pan, h. lin, w. li. bolt layout optimization design based on genetic algorithm. in iop conference series: materials science and engineering, 13 https://doi.org/10.1086/144770 https://doi.org/10.1016/j.ijpvp.2023.104975 https://doi.org/10.1061/(asce)st.1943-541x.0000765 https://doi.org/10.1061/(asce)st.1943-541x.0000765 https://doi.org/10.1002/tal.1988 d. croccolo, s. fini, m. khan acta polytechnica ctu proceedings vol. 576, p. 012005. iop publishing, 2019. https://doi.org/10.1088/1757-899x/576/1/012005 [6] x. lu, m. zhu, y. liu, et al. triangular position multi-bolt layout structure optimization. applied sciences 13(15):8786, 2023. https://doi.org/10.3390/app13158786 [7] k. sarı, h. dilipak. a design software based on the placement of bolts for bolted joints. artvin çoruh üniversitesi mühendislik ve fen bilimleri dergisi 1(2):104–114, 2023. [8] din. designation systems for steels part 1: steel names; german version en 10027-1:2016. [2017-01-01]. https://www.dinmedia.de/en/standard/ din-en-10027-1/262540217 [9] european committee for standardization. bs en 1993-1-8:2005 eurocode 3: design of steel structures part 1-8: design of joints, 2005. [2005-01-12]. https://www.phd.eng.br/wp-content/uploads/2015/ 12/en.1993.1.8.2005-1.pdf [10] vdi. systematic calculation of highly stressed bolted joints joints with one cylindrical bolt, vdi 2230 part 1, 2014. [2015-11-01]. https://www.vdi.de/en 14 https://doi.org/10.1088/1757-899x/576/1/012005 https://doi.org/10.3390/app13158786 https://www.dinmedia.de/en/standard/din-en-10027-1/262540217 https://www.dinmedia.de/en/standard/din-en-10027-1/262540217 https://www.phd.eng.br/wp-content/uploads/2015/12/en.1993.1.8.2005-1.pdf https://www.phd.eng.br/wp-content/uploads/2015/12/en.1993.1.8.2005-1.pdf https://www.vdi.de/en acta polytechnica ctu proceedings 48:9–14, 2024 1 introduction 2 materials and methods 3 results and discussion 3.1 influence of bolt size on stiffness 3.2 influence of number of bolts on stiffness 3.3 influence of bolt center of gravity on stiffness 3.4 influence of bolt spacing on stiffness 3.5 influence of bolt layout on stiffness 3.6 stress analysis 4 conclusions 5 future recommendations references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2024.47.0019 acta polytechnica ctu proceedings 47:19–22, 2024 © 2024 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague the application of uhpc on the monolithic part of the štvanice footbridge vendula davidová∗, tomáš david, pavel reiterman, vít šmilauer czech technical university in prague, faculty of civil engineering, thákurova 7, 166 29 prague, czech republic ∗ corresponding author: vendula.davidova@fsv.cvut.cz abstract. the use of uhpc with fibers is becoming an increasingly common topic in the world. this material is used for its superior mechanical and physical properties and long service life. however, the production cost is high, hence uhpc is used for special applications or for smaller prefabricated parts. this paper focuses on uhpc application for the construction of the štvanice footbridge, prague, the czech republic. a monolithic part of the structure was made of uhpc from fresh transport concrete, which was produced by skanska transbeton. the special feature is that about 126 m3 of fresh concrete was industrially prepared. this concrete achieved the properties of scc and the final 28 day compressive strength was 120 mpa with tensile strength of 18.6 mpa. due to high hydration heat release rate, an internal water cooling system was designed and sucessfully appled using water from vltava river. keywords: ultra high performance concrete, monolithic construction, production technologies, water cooling. 1. introduction ultra high performance concrete with fibers (uhpc) is an advanced material based on the composition of conventional concrete, with the resulting properties reaching higher values. haber, zachary b. et al. [1] used a definition of this material as a cementitious composite material with water-to-cement ratio less than 0.25, with high percentage of discontinuous internal fiber reinforcement. the compressive strength was greater than 21.7 ksi (150 mpa) and sustained postcracking tensile strength greater than 0.72 ksi (5 mpa). farzad et al. [2] described uhpc as a cement-based material formed with a better gradation of grain components with a water-to-cement ratio less than 0.2, and a significant portion of internal fibers. azmee et al. [3] defined uhpc as a fiber-reinforced, silica fume, cement mixture with the use of superplasticizers with low water-to-cement ratio. it contains very fine quartz sand with size of grain in range from 0.15 to 0.60 mm. a combination of several approaches is used to achieve enhanced properties. a high proportion of cement is used in the composition of uhpc. the dosage per 1 m3 is in the range of 800–1 100 kg, which is about three to four times more than that of conventional concrete. [4] abokifa et al. [5] noted that the most of researches recommended cem i or ii due to a low content of tricalcium aluminate (c3a) that can reduce the required amount of water. in order to achieve low capillary porosity and high density of the cement matrix, the amount of added water is minimized and, as mentioned above, the waterto-cement ratio varies between 0.20 and 0.25. the reduced amount of water in the mixture also causes all the cement grains not to hydrate, which over time causes a self-healing effect of the concrete after cracking. [6] to achieve the highest possible density of the structure, the goal is to create a “perfect” and continuous particle size distribution. aggregate size is thus limited and very fine particles such as micro silica are used for this purpose. these very fine particles fill the voids between the cement and achieve a “filler effect”. [7] to improve the tensile properties of uhpc, a high dose of fine steel reinforcing fibers is normally used. the final dose and type depend on the final strength required. abokifa et al. [8] již použito investigated the effect of dose and fibre type on the final properties of uhpc. the dosage of reinforcing fibers ranged from 0.5 % to 2.5 % by volume depending on the type used and the tensile strength increased with increasing fiber volume fraction. all these improved properties describe concrete of high strength, long service life and improved final properties. thanks to these properties, uhpc structures can be made one-third to one-half the size while maintaining the ability to carry the required loads. this saving has the advantage of producing slender structures and reducing overall costs. reduction of steel reinforcing bars decreases labor costs and provides greater architectural freedom [9], which is why uhpc material was also used for the footbridge. while uhpc production on the lab scale presents no substantial problem, large-scale structures with uhpc transport led to several challenges addressed in the article. 19 https://doi.org/10.14311/app.2024.47.0019 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en v. davidová, t. david, p. reiterman, v. šmilauer acta polytechnica ctu proceedings 2. experimental program 2.1. description of footbridge and associated challenges the footbridge was realized by skanska company while the subsidiary company, skanska transbeton, was responsible for the monolithic part made from uhpc. the main part of the bridge deck is composed from prefabricated uhpc segments. a ramp slopes down on an island, creating a a complicated junction arearelying on a monolithic design. a view of this section is shown in figure 1. figure 1. complicated junction with a framed monolithic part. such a construction required concrete with high compressive and flexural tensile strengths. for this reason, uhpc with fibers was chosen to meet the requirements of the structural design. several challenges arose with the production of uhpc. the high amount of fibre reinforcement had to be uniformly distributed in the concrete matrix. a high dose of cement combined with a high ambient temperature would have lead to overheating of the structure, so it was necessary to reduce the temperature of the concrete using a cooling system using water from the vltava river. finally, the demanding design required high flowability and long workability to guarantee perfect casting of the entire volume of the junction. 2.2. technology of production technology of production the production technology of uhpc is different from that of conventional concrete. uhpc contains a high dose of cement exceeding 500 kg m−3, a high dose of admixtures, a low dose of water with a targeted low water-cement ratio of around 0.25. to achieve a high consistency, which guarantees consistent concrete properties even when realizing complex shapes, a very high dose of plasticizer is usually used, and retarders are often utilized to maintain the required workability. the mix design was firstly developed in the laboratory and further tested in several steps. skanska transbeton faced several challenges in the development of the mix design. the first major challenge was the effective mixing of the high dose steel fibers. the fibers used can be imagined as thicker needles that aglomerate and intertwine with each other. in the laboratory tests, the fibers were easy to dose and disperse because they were hand-dosed, and the clumps were broken before contact with the concrete mix. for efficient mixing in industrial production, a special mixing core, called a planetary mixer, had to be used, which rotates on its axis and the individual paddles rotate in different directions and speeds. a model of this mixer is shown in figure 2. figure 2. planetary mixer core – figure available from https://daswell.com/. another challenge was the high dose of cement and expected casting in the hottest days of the summer. the combination of these factors would generate concrete core temperatures in excess of 100 °c, which is unacceptable. to limit the critical condition, experts from the ctu were involved from the beginning in the mix design. 3. materials in addition to the requirements for mechanical and physical properties, the specific shade of white was prescribed by the architect. for these reasons, special white cement from the most accessible transport location for the czech republic, cem i 52.5 n slovakia – rohožník, was used. to keep the white colour, aggregates up to 5 mm in size were used from the provodín – cz locality. this aggregate is used for glassmaking purposes and is characterized by its high quality and white colour. a special requirement for the sand producer was to dry the material and transport it on covered trucks, as the recipe was very sensitive to the water dosage and wet material could not be used (wet aggregate is normally used for ordinary concrete production). however, these materials did not meet the architect’s requirements for the resulting white colour, and white pigment was also needed to add. the actual composition of the mixture is subject to the company’s know-how, therefore only an approximate composition is given in the table 1 below. 20 https://daswell.com/ vol. 47/2024 the application of uhpc on the monolithic part of the štvanice materials dosage cem i 52.5 n – white over 500 kg m−3 water-to-cement ratio 0.23 super plasticizing admixture 3.8 % wt. to cement white pigment 1.6 % wt. to cement sand fraction 0–5 mm 170 % wt. to cement reinforcing fibers 6 % wt. to total retardant additive 1.25 % wt. to cement table 1. approximate composition of uhpc. due to the mechanical and physical requirements from the structure designers, the use of additional nano-materials and other fine particles was not necessary. 4. cooling system on the basis of preliminary calculations, a number of mock-up tests s were carried out, in order to test the proposed mix design and calibrate simulation model for the prediction of temperature field. as the strength requirements had to be met and it was impossible to change the cement dosage or type, water cooling was proposed as the most effective method, using the water available on site, i.e. water from the vltava river. the cooling system was made of plastic hoses laid in three layers over a total length over 1 km. a view of the cooling system prepared for the uhpc casting is shown in figure 3. figure 3. cooling system before concrete casting. the material of the cooling pipe was chosen with regard to the simplest implementation on site and economic feasability. adek tasri and anita susilawati [10] investigated in their work the effect of the cooling pipe materials on the resulting temperature in the concrete. they concluded that locations further than 100 m from the cooling water inlet showed no differences in the concrete temperatures regardless of steel, pex or pvc pipes. 5. result and discussions 5.1. achieved properties the declining ramp was designed with a longitudinal slope over 8 %, therefore the entire section was cast with a top cover, which limited the concrete placement area, and the entire volume was cast from a limited number of filling holes. these circumstances required a mixture of high flowability with self-compacting properties, which was achieved by a high dosage of plasticizing admixture. figure 4 (left) shows a view of the casting process in the morning, including the top surface covered (concrete is under the worker), figure 4 (right) shows the filling hole during the casting. figure 4. casting process over the morning hours (left), filling hole during the casting (right). the total quantity of fresh concrete produced in the industrial environment of the batching plant was 126 m3. the resulting consistency reached the requirements of the scc mixture, which resulted in a perfect concreting of the junction. the final workability time of the fresh concrete was over 6 hours thanks to the retarder. the average compressive strength tested on 100 × 200 mm cylinders was 120 mpa. the average flexural tensile strength was 18.6 mpa, and the residual strength after a 0.5mm crack opening was 18.3 mpa tested on 150 × 150 × 700 mm beams. these strengths were achieved without the use of special nanomaterials, and a larger aggregate size was used compared to the most of designs. very good results were similarly achieved in the work of a. arora et al. [11]. their binders saved cement and achieved 90-day compressive strengths in the range of 145–165 mpa while they used three different sizes of coarse aggregates with the maximum sizes of 6.25 mm, 4.75 mm, and 2.36 mm. the final date of the uhpc casting was postponed and the actual production of concrete took place during october 2022, which was more than desirable from the point of view of temperatures. in order to minimize the temperature during production and deposition, the production was started at night, therefore the air temperature was around 15 °c. the maximum measured temperature of the concrete was 81 °c at 21 v. davidová, t. david, p. reiterman, v. šmilauer acta polytechnica ctu proceedings 14 hours with internal water cooling. after the maximum temperature of the bridge core was reached, the temperature decrease due to water cooling was monitored. when the temperature of the concrete dropped to approximately 40 °c, the cooling was turned off to avoid overstressing the material due to the temperature difference. the efficiency of the cooling system is also supported by the increase in cooling water temperature by about 20 °c. the resulting temperature history is shown in figure 5. figure 5. temperature evolution and validation of uhpc with internal water cooling. 6. conclusion the production of transport uhpc brings several challenges and risks. by appropriate formulation of the mix design, desired requirements were met. the composition of the concrete is based on a high proportion of cement without the need for additional special nano and fine additives. by limiting the number of input materials, production on an industrial scale is guaranteed and quality production of uhpc in larger quantities was achieved. the proposed formulation met the requirements given by design engineer – in terms of high mechanical and physical properties, as well as the requirements for the required white color given by the architect. acknowledgements this work was supported by the grant of the ctu student grant competition no. sgs23/053/ohk1/1t/11 and sgs24/051/ohk1/1t/11. references [1] z. b. haber, j. f. munoz, b. a. graybeal. field testing of an ultra-high performance concrete overlay. tech. rep., 2017. fhwa-hrt-17-096. [2023-11-30]. https://rosap.ntl.bts.gov/view/dot/37821 [2] m. farzad, m. shafieifar, a. azizinamini. experimental and numerical study on bond strength between conventional concrete and ultra high-performance concrete (uhpc). engineering structures 186:297–305, 2019. https://doi.org/10.1016/j.engstruct.2019.02.030 [3] n. azmee, n. shafiq. ultra-high performance concrete: from fundamental to applications. case studies in construction materials 9:e00197, 2018. https://doi.org/10.1016/j.cscm.2018.e00197 [4] p. richard, m. cheyrezy. composition of reactive powder concretes. cement and concrete research 25(7):1501–1511, 1995. https://doi.org/10.1016/0008-8846(95)00144-2 [5] m. abokifa, m. a. moustafa. mechanical characterization and material variability effects of emerging non-proprietary uhpc mixes for accelerated bridge construction field joints. construction and building materials 308:125064, 2021. https: //doi.org/10.1016/j.conbuildmat.2021.125064 [6] d. wang, c. shi, z. wu, et al. a review on ultra high performance concrete: part ii. hydration, microstructure and properties. construction and building materials 96:368–377, 2015. https: //doi.org/10.1016/j.conbuildmat.2015.08.095 [7] e. ghafari, h. costa, e. júlio, et al. the effect of nanosilica addition on flowability, strength and transport properties of ultra high performance concrete. materials & design 59:1–9, 2014. https://doi.org/10.1016/j.matdes.2014.02.051 [8] m. abokifa, m. a. moustafa. mechanical characterization and material variability effects of emerging non-proprietary uhpc mixes for accelerated bridge construction field joints. construction and building materials 308:125064, 2021. https: //doi.org/10.1016/j.conbuildmat.2021.125064 [9] c. tam, v. w. tam, k. ng. assessing drying shrinkage and water permeability of reactive powder concrete produced in hong kong. construction and building materials 26(1):79–89, 2012. https: //doi.org/10.1016/j.conbuildmat.2011.05.006 [10] a. tasri, a. susilawati. effect of material of post-cooling pipes on temperature and thermal stress in mass concrete. structures 20:204–212, 2019. https://doi.org/10.1016/j.istruc.2019.03.015 [11] a. arora, a. almujaddidi, f. kianmofrad, et al. material design of economical ultra-high performance concrete (uhpc) and evaluation of their properties. cement and concrete composites 104:103346, 2019. https: //doi.org/10.1016/j.cemconcomp.2019.103346 22 https://rosap.ntl.bts.gov/view/dot/37821 https://doi.org/10.1016/j.engstruct.2019.02.030 https://doi.org/10.1016/j.cscm.2018.e00197 https://doi.org/10.1016/0008-8846(95)00144-2 https://doi.org/10.1016/j.conbuildmat.2021.125064 https://doi.org/10.1016/j.conbuildmat.2021.125064 https://doi.org/10.1016/j.conbuildmat.2015.08.095 https://doi.org/10.1016/j.conbuildmat.2015.08.095 https://doi.org/10.1016/j.matdes.2014.02.051 https://doi.org/10.1016/j.conbuildmat.2021.125064 https://doi.org/10.1016/j.conbuildmat.2021.125064 https://doi.org/10.1016/j.conbuildmat.2011.05.006 https://doi.org/10.1016/j.conbuildmat.2011.05.006 https://doi.org/10.1016/j.istruc.2019.03.015 https://doi.org/10.1016/j.cemconcomp.2019.103346 https://doi.org/10.1016/j.cemconcomp.2019.103346 acta polytechnica ctu proceedings 47:19–22, 2024 1 introduction 2 experimental program 2.1 description of footbridge and associated challenges 2.2 technology of production 3 materials 4 cooling system 5 result and discussions 5.1 achieved properties 6 conclusion acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2024.50.0088 acta polytechnica ctu proceedings 50:88–93, 2024 © 2024 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague optimization of fib milling procedure for micromechanical testing of cement pastes jan procházkaa,∗, jiří němeček 1,b, patricie halodováa, jiří němeček 2,b a research centre řež, hlavní 130, řež, 250 68 husinec, czech republic b czech technical university in prague, faculty of civil engineering, thákurova 7, 166 29 prague 6 – dejvice, czech republic ∗ corresponding author: jan.prochazka@cvrez.cz abstract. this study focuses on optimizing the focused ion beam (fib) preparation process for micro-beams made from cement paste prepared from portland cement cem i-42.5r, which includes four phases: inner and outer products, portlandite, and residual clinker. the effects of the primary electron beam, focused ion beam, image drift, imaging issues, and sample moisture cycling were investigated. recommendations on specific fabrication procedures and fib milling parameters, such as accelerating voltage, probe current, and time, were provided. keywords: cement paste, focused ion beam, nanoindentation, micro-beam, c–s–h gel, scanning electron microscopy. 1. introduction ordinary cement paste is a heterogeneous material, primarily composed of calcium-silica-hydrate (c–s– h) gel, portlandite (ch), residual clinker (c), porosity, and other minor phases. the c–s–h gel exists in two variants with different densities. low-density c–s–h gel, along with other minor phases, forms the outer product (op), while high-density c–s–h gel, also with other minor phases, forms the inner product (ip) [1, 2]. these phases exhibit variable chemical compositions and different mechanical properties. nanoindentation can be used to assess the mechanical properties, such as the young’s modulus or hardness of these micrometer-sized phases [3–5]. however, nanoindentation cannot directly measure the compressive and tensile strengths or fracture energy of these phases. these properties can be determined by fabricating micro-pillars or micro-beams using focused ion beam (fib) techniques. subsequently, compressing the micro-pillars [6, 7] or bending the micro-beams [8, 9] with a nanoindenter can be used to derive the elastic and fracture properties of the individual phases. despite the indisputable advantages of the fib milling technique, several potential undesirable effects associated with ion beam nanofabrication, such as temperature-induced phase transformations, gallium implantation, and unintended material redeposition, can significantly influence the measurement results [9]. therefore, when using an ion beam for micro-beam fabrication, it is essential to select appropriate sputter1postdoctoral researcher at czech technical university in prague, orcid: 0000-0002-5635-695x. 2professor at czech technical university in prague, orcid: 0000-0002-3565-8182. ing parameters to minimize the impact of nanofabrication on the mechanical testing results. this study elucidates the crucial steps involved in preparing microbeams using fib techniques and discusses the utilization of optimized parameters to ensure the efficacy and accuracy of both the manufacturing process and subsequent nanomechanical testing. 2. materials and methods 2.1. material the cement paste samples were mixed from portland cement cem i-42.r (českomoravský cement, czech republic) of the chemical composition shown in table 1 without chemical admixtures and the water-tocement ratio of 0.4. a fresh paste was cast in cylindrical molds (27mm in diameter, 70mm in height) and vibrated to avoid excessive air voids in the structure. then, the samples were stored in lime water for 20 months. detailed procedure and sample labeling are described in [10]. 2.2. sample preparation after the curing period in lime water, the samples were dried at 50 °c for 3 days. then, the samples were ground for 1 min on sic paper grit 2 000 and for 2.5 min on sic paper grit 4 000 with no lubrication. the debris was continuously swept away from the papers throughout the grinding procedure. final polishing took place on a soft cloth with alcohol-based diamond suspensions for 15 seconds with 0.25µm grains. the samples were ultrasonically cleaned in an alcohol bath for 1 min after each polishing step [2]. after the polishing, three coarse base-defining marks were engraved in the corners of the analyzed fragment using a diamond tip. 88 https://doi.org/10.14311/app.2024.50.0088 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 50/2024 fib procedure optimization for cement pastes component cao sio2 al2o3 fe2o3 mgo na2o k2o so3 mno cem i (wt. %) 63.77 20.51 4.74 3.30 1.05 0.15 0.95 3.07 0.09 table 1. chemical composition of the used portland cement (xrf data supplied by the manufacturer). process step label deposition trench milling rough undercut fine polishing shortening pattern lines text stairs rectangles polishing rectangle polishing rectangles polishing rectangle dimensions (µm) 175 × 45 × 0.1 25 × 15 × 15 24 × 15 × 15 20 × 3 × 10 3 × 2 × 10 8 × 5 × 10 ib (pa) ∼ 200 ∼ 1 000 ∼ 1 000 ∼ 600 ∼ 600 ts (min) ∼ 240 2× ∼ 120 2× ∼ 90 ∼ 10 θe (°) 55 55 23 23 55 / 23 θb (°) 0 0 32 32 0 / 32 table 2. recommended fib parameters and estimated process times for different steps in micro-beam preparation routine. 2.3. imagining and ion machining polished samples were glued to the sem stub using colloidal silver lacquer (agar scientific), and the same lacquer was applied to the sides of the samples to reduce charging. the location of the four main hydration phases (op, ip, ch, and c) with large enough dimensions suitable for micro-beam fabrication were identified using a scanning electron microscope (sem) phenom xl desktop sem and back-scatter electron (bse) detector. also, the coordinates of locations and basedefining marks were recorded for further aid. the micro-beams and labels for nanoindentation tests were prepared using a dual-beam fib-sem tescan lyra3 gmu, equipped with ga+ cobra fib column tilted 55° to the electron column and gas injection system with platinum, tungsten, and carbon precursors for ion-beam assisted deposition (ibad). the microscope is equipped with external and in-beam detectors for both bse and secondary electrons (se). 5-degree-of-freedom stage allows sample positioning with adjustable tilt around axis perpendicullar to both electon and ion beams and (actual) rotation around axis perpendicullar to the sample surface. the controlling system allows software adjustments of image shift and (apparent) rotation. the image is not deformed by apparent rotation only if the sample is perpedicullar to the beam used. at first, the coarse base-defining marks were made with two platinum alignment tags: one located inside the scratch mark, labeled as coarse, and one on the polished surface, labeled as fine. this set of three base-defining marks allows easier beam positioning using microscopes ranging from optical microscopes to sems. (a) se (b) bse figure 1. sem images of the label, alignment tags, and outline for the beam in residual clinker using (a) se and (b) bse detectors. 2.4. micro-beams after the marking, the regions of interest were identified and labeled using platinum ibad. additional beam outlines and alignment tags were added for easier and more precise ion milling routines, see figure 1. the positioning and orientation of a cantilever must allow enough room for nanoindentation measurement of the selected phase in the proximity of the cantilever root. the root shall be as far as possible from phase interfaces too. the desired dimensions of the final equilateral triangular cantilever for testing are 20 × 8µm. the process of cantilever preparation is divided in four steps: high-current trench milling, high-current rough cantilever undercut, low-current cantilever sidewall and cantilever base polishing, and low-current cantilever shortening. recommended parameters and estimated process times are listed in table 2. 89 j. procházka, j. němeček, p. halodová, j. němeček acta polytechnica ctu proceedings (a) se, perpendicular to the electron beam. (b) bse, perpendicular to the electron beam. (c) in-beam bse, perpendicullar to the electron beam. (d) se, perpendicular to the ion beam. (e) in-beam bse, perpendicular to the ion beam. figure 2. comparison of sem images of residual clinker grain with the sample surface (a–c) perpendicular to electon beam and tilted to the ion beam, and (d–e) tilted to the electron beam and perpendicular to the ion beam, using different detectors (a) se, (b) in-beam bse. 3. results and discussion 3.1. imaging issues as the cement paste is poorly conductive in a vacuum chamber, surface charging is an issue for sem imaging using se because the charged surface has a higher secondary electron emission rate as shown in figures 2(a) and 2(d) due to the negative charge build-up on the scanned surface. bse imaging is less affected by immediate surface charge, see figure 2(b), because the detector collects the signal from electrons interacting elastically with the sample surface. due to their higher energy, bses are less susceptible to deflection by the electric fields caused by surface charging. in-chamber bse detectors have better signal than their in-beam (ib) counterparts that are located inside the sem column, see figures 2(b) and 2(c). on the other hand, in-chamber detectors are prone to mechanical damage and cannot be used with tilted specimens during ion fabrication. in cases where the sample was tilted for fib processes or cross-section documentation, a combination of the se detector, primarily for focusing, and the in-beam bse detector, providing phase and chemical contrast, was used. otherwise, the in-chamber bse detector was used. 3.2. effect of primary electron beam continual sem scanning loads the entire surface of the sample in the view field. in the case of a cementitious sample, the energy deposited by the incident electron beam on the surface cannot dissipate through the material, leading to energy absorption and subsequent thermal effects, such as localized heat damage of the surface. the amount of heat generated depends on the current of the electron beam, and the actual bi de,r (nm) de,d (nm) ie (pa) 1 7.8 13 ∼ 9 2 7.4 12 ∼ 11 3 6.6 11 ∼ 20 4 6.3 11 ∼ 24 5 5.9 10 ∼ 35 6 5.4 9.3 ∼ 50 7 5.0 8.5 ∼ 67 8 4.6 7.8 ∼ 95 9 4.5 7.2 ∼ 130 10 4.6 6.6 ∼ 190 11 5.3 6.1 ∼ 270 12 6.9 5.7 ∼ 330 13 10 5.5 ∼ 450 14 15 5.6 ∼ 550 15 24 6.1 ∼ 750 16 37 7.4 ∼ 880 17 62 11 ∼ 1 000 table 3. electron beam current ib and spot sizes db for resolution-optimised (-r) and focal-depthoptimised (-d) scanning modes, respectively, with acceleration voltage of 15 kev. temperature rise depends on several factors, including exposure time, thermal conductivity, heat capacity, and the geometry of the sample [11]. the microscope used in the study offers two beamfocusing setups optimized for improved resolution or enhanced depth of focus. differences between estimated spot sizes de of both modes and beam current ie for beam intensity (bi ) indices are listed in table 3. figure 3(a) shows the micro-beam prepared under continuous electron imaging with electron beam intensity bi 17 overheating the sample surface. electron 90 vol. 50/2024 fib procedure optimization for cement pastes (a) damaged micro-beam prepared in ip under continuous electron imaging. (b) micro-beam prepared in ip with electron imaging deactivated. figure 3. comparison of micro-beams damaged and undamaged by extensive electron beam heating. imaging using bi 7 and deactivating the continuous acquisition during the fib process minimizes the surface degradation, see figure 3(b). bi 7 with pixel dwell time of 3.2µs and resolution of 512 × 512 px offers an acceptable compromise between low sample heat loading and reasonable preview image quality. to suppress the surface heat loading even further, the continuous image acquisition should be turned off during fib processes. 3.3. effect of ion beam the fib, as well as the electron beam or laser beam, is assumed to be elliptical in the plane perpendicular to the beam with a gaussian-type power profile. as the beam current is defined by masking the defocused beam in the column, the different current settings lead to different standard deviations of the power profile. a higher standard deviation results in blunter edges for higher current ratings. figure 4 shows four identical ion beam etching routines of rectangular object, 1µm wide, 8µm long and deep, in portlandite using four different fib presets defined in table 4. it is clearly seen that with higher beam currents, the edges of the etched objects are less defined and blunter compared to those prepared using lower beam currents. as described in [12], higher current ratings lead to higher temperature rises and broader heated volumes. figure 4. comparison of fib etching routines performed with different ion beam presets. preset da (µm) db (nm) ib (pa) ts (min) 1 500 130.1 ∼ 7 000 0:20 2 200 109.6 ∼ 1 000 2:13 3 150 118.2 ∼ 600 4:09 4 90 121.9 ∼ 200 13:33 table 4. fib preset parameters for ion acceleration voltage of 30 kev and time needed for etching 1µm wide, 8µm long and deep rectangular trench in portlandite. in the case of micro-beams prepared in cement paste phases, both effects contribute to artifacts created on the sides of the micro-beam and the base. the gaussian beam broadening primarily causes blunt edges, while beam-induced heating can lead to geometrical changes in the micro-beam due to possible phase transformations, decomposition, or the release of adsorbed gel water in c-s-h products, resulting in shrinkage and microcracking [9]. figure 5(a) shows preparation of a micro-beam in portlandite interrupted after the second stage of rough undercut using the ib ∼ 7 na because of excessive heat loading induced by the fib. reducing the beam current to ib ∼ 1 na eliminated the excessive heat loading, see figure 5(b). 3.4. effect of image drift because of the low conductivity of cement paste in the vacuum chamber, the accumulation of positive charge due to ga+ ion implantation and secondary electron emission during the ga+ ion beam fabrication can lead to significant charging effects, causing apparent image movement called image drift. this effect can be mitigated by drift correction, where the milling/polishing routine is automatically paused and the reference area is scanned to estimate corrections. the fib/sem used in this study allows for either finer pattern correction, where the fib schedule pattern coordinates are altered for the compensation, or coarser but wider-range image shift correction, where the fib image shift is adjusted for compensation. in the case of cement pastes, the period between correction scans had to be reduced to 30–60 seconds. even though the wider-range correction method of 91 j. procházka, j. němeček, p. halodová, j. němeček acta polytechnica ctu proceedings (a) micro-beam undercut performed using ib ∼ 7 na (b) micro-beam undercut performed using ib ∼ 1 na figure 5. comparison of micro-beams undercut prepared in portlandite using ion beam current of (a) 7 na and (b) 1 na. image shift was used, it could not always compensate for the drift. in such cases, additional polishing steps were added to achieve the desired micro-beam geometry. see figure 6(a). 3.5. effect of moisture cycling hydration and dehydration processes in c–s–h gel are associated with volumetric and mechanical changes in the gel [13]. the changes of water partial pressure in the sample between the storage in the given atmosphere and micro-beam preparation in the sem evacuated chamber induce high artificial stress in the samples, see figure 7. it was found that week-long storage of the samples in the vacuum of sem (p ∼ 10−6 pa) causes significantly fewer cracks in the samples than atmospheric storage overnight between fib procedures. attention must be given to this effect when transporting ready-made micro-beams for nanoindentation testing to another laboratory. we recommend transporting and storing samples in a vacuum to mitigate potential issues. 4. conclusions the preparation of micro-beams in cement paste is a time-consuming process with a significant failure rate. during the preparation of beams for nanoindentation testing using fib techniques, several critical parameters require careful assessment to ensure the (a) micro-beam in op phase recovered by additional polishing. (b) micro-beam prepared in ip phase. figure 6. comparison of micro-beams (a) affected and (b) not affected by excessive sample drift. figure 7. sample surface damaged by moisture cycling. procedure’s success. the general recommendations are given as: • parameters bi 7 of the electron beam equal to electron current ie ∼ 70 pa combined with highergain se and in-beam bse detectors are suitable to use to avoid surface overheating and to ensure reasonable image quality. additionally, continuous image acquisition should be turned off during the fib milling for the same reason. • high currents used during fib milling can lead to excessive heat loading of the sample, resulting in phase transformation, visible bubbles or cracks on the micro-beam, or even geometry changes. safe current values for different milling steps of portlandite are provided in table 2. • to avoid image drift caused by charging due to the accumulation of ga+ ions during fib fabrication, 92 vol. 50/2024 fib procedure optimization for cement pastes it is recommended to perform drift correction by rescanning the measured area at intervals of 30 to 60 seconds. • cyclic water resaturation of the cement paste can lead to the creation of a significant number of microcracks, potentially damaging already fabricated micro-beams. therefore, it is advisable to maintain the sample under vacuum conditions throughout the preparation and testing phases to effectively mitigate these effects. list of symbols bi beam intensity index bse back scattered electron (detector) db ion beam estimated spot size [nm] de electron beam estimated spot size [nm] fib focused ion beam ib ion beam current [a] ie electron beam current [a] ibad ion beam assisted chemical vapour deposition se secondary electron (detector) sem scanning electron microscope ts estimated process time for given beam preparation step [min] θe sample tilt angle respective to the electron beam [°] θb sample tilt angle respective to the ion beam [°] c residual clinker c–s–h calcium – silicate – hydrates ch portlandite ip inner product op outer product acknowledgements this work was financially supported by the project of the czech science foundation grant number 23-05435s. the presented results were obtained using the cicrr infrastructure, which is financially supported by the ministry of education and culture – project lm2023041. references [1] k. l. scrivener. backscattered electron imaging of cementitious microstructures: understanding and quantification. cement and concrete composites 26(8):935–945, 2004. https: //doi.org/10.1016/j.cemconcomp.2004.02.029 [2] j. němeček, j. lukeš, j. němeček. high-speed mechanical mapping of blended cement pastes and its comparison with standard modes of nanoindentation. materials today communications 23:100806, 2020. https://doi.org/10.1016/j.mtcomm.2019.100806 [3] w. c. oliver, g. m. pharr. an improved technique for determining hardness and elastic modulus using load and displacement sensing indentation experiments. journal of materials research 7(6):1564–1583, 1992. https://doi.org/10.1557/jmr.1992.1564 [4] c. hu, z. li. a review on the mechanical properties of cement-based materials measured by nanoindentation. construction and building materials 90:80–90, 2015. https: //doi.org/10.1016/j.conbuildmat.2015.05.008 [5] z. luo, w. li, k. wang, s. p. shah. research progress in advanced nanomechanical characterization of cement-based materials. cement and concrete composites 94:277–295, 2018. https: //doi.org/10.1016/j.cemconcomp.2018.09.016 [6] r. shahrin, c. p. bobko. micropillar compression investigation of size effect on microscale strength and failure mechanism of calcium-silicate-hydrates (c-s-h) in cement paste. cement and concrete research 125:105863, 2019. https://doi.org/10.1016/j.cemconres.2019.105863 [7] c. puttbach, g. s. prinz, c. d. murray. strength and stiffness characterization of ultra-high performance concrete (uhpc) cement paste phases through in-situ micro-mechanical testing. cement and concrete composites 149:105520, 2024. https: //doi.org/10.1016/j.cemconcomp.2024.105520 [8] j. němeček, v. králík, v. šmilauer, et al. tensile strength of hydrated cement paste phases assessed by micro-bending tests and nanoindentation. cement and concrete composites 73:164–173, 2016. https: //doi.org/10.1016/j.cemconcomp.2016.07.010 [9] j. němeček, j. maňák, j. němeček, t. krejčí. effect of vacuum and focused ion beam generated heat on fracture properties of hydrated cement paste. cement and concrete composites 100:139–149, 2019. https: //doi.org/10.1016/j.cemconcomp.2019.03.027 [10] j. němeček, p. trávníček, m. keppert, et al. nanomechanical analysis of gamma-irradiated cement paste exposed to different humidities. construction and building materials 393:131969, 2023. https: //doi.org/10.1016/j.conbuildmat.2023.131969 [11] j. goldstein. scanning electron microscopy and x-ray microanalysis: a text for biologists, materials scientists, and geologists. springer us, 1981. [12] t. ishitani, h. kaga. calculation of local temperature rise in focused-ion-beam sample preparation. journal of electron microscopy 44(5):331–336, 1995. https: //doi.org/10.1093/oxfordjournals.jmicro.a051185 [13] m. b. pinson, e. masoero, p. a. bonnaud, et al. hysteresis from multiscale porosity: modeling water sorption and shrinkage in cement paste. physical review applied 3(6):064009, 2015. https://doi.org/10.1103/physrevapplied.3.064009 93 https://doi.org/10.1016/j.cemconcomp.2004.02.029 https://doi.org/10.1016/j.cemconcomp.2004.02.029 https://doi.org/10.1016/j.mtcomm.2019.100806 https://doi.org/10.1557/jmr.1992.1564 https://doi.org/10.1016/j.conbuildmat.2015.05.008 https://doi.org/10.1016/j.conbuildmat.2015.05.008 https://doi.org/10.1016/j.cemconcomp.2018.09.016 https://doi.org/10.1016/j.cemconcomp.2018.09.016 https://doi.org/10.1016/j.cemconres.2019.105863 https://doi.org/10.1016/j.cemconcomp.2024.105520 https://doi.org/10.1016/j.cemconcomp.2024.105520 https://doi.org/10.1016/j.cemconcomp.2016.07.010 https://doi.org/10.1016/j.cemconcomp.2016.07.010 https://doi.org/10.1016/j.cemconcomp.2019.03.027 https://doi.org/10.1016/j.cemconcomp.2019.03.027 https://doi.org/10.1016/j.conbuildmat.2023.131969 https://doi.org/10.1016/j.conbuildmat.2023.131969 https://doi.org/10.1093/oxfordjournals.jmicro.a051185 https://doi.org/10.1093/oxfordjournals.jmicro.a051185 https://doi.org/10.1103/physrevapplied.3.064009 acta polytechnica ctu proceedings 50:88–93, 2024 1 introduction 2 materials and methods 2.1 material 2.2 sample preparation 2.3 imagining and ion machining 2.4 micro-beams 3 results and discussion 3.1 imaging issues 3.2 effect of primary electron beam 3.3 effect of ion beam 3.4 effect of image drift 3.5 effect of moisture cycling 4 conclusions list of symbols acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2025.54.0023 acta polytechnica ctu proceedings 54:23–28, 2025 © 2025 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague fractional calculus in describing the viscoelastic response of pvb foil barbora hálková∗, michal šejnoha czech technical university in prague, faculty of civil engineering, department of mechanics, thákurova 7, 166 29 praha 6, czech republic ∗ corresponding author: barbora.halkova@fsv.cvut.cz abstract. to address the response of a pvb foil, both traditional and fractional viscoelasticity based formulations are described and compared in this paper. traditional viscoelasticity uses models consisting of elastic springs and viscous dashpots. fractional viscoelasticity is based on the principles of fractional calculus (derivatives and integrals of non-integer order) and introduces another rheological element, the springpot, which behaves as viscoelastic on its own and allows us to construct even more complex models. while limiting attention to the maxwell chain model, both formulations are compared in light of approximation of experimental data provided by rheometer measurements. this is illustrated by plotting the storage modulus derived experimentally as well as computationally, which in turn promotes application of fractional calculus as an efficient tool for smoothing out and storing the experimental data. keywords: viscoelasticity, fractional viscoelasticity, springpot, generalized maxwell model, laminated glass, pvb foil, rheometer experiment. 1. introduction a number of engineering materials, including concrete, bitumen, polymers or even human skin tissue, show viscoelastic properties. the present paper focuses on the behaviour of polyvinyl butyral (pvb) thermoplastic polymer with a particular application in laminated glass panels. laminated glass is a composite material which consists of glass plates and transparent polymer interlayers, both varying in properties and thicknesses. the interlayer ensures an interaction between the individual glass plates and has also an important safety function ensuring that in case of fracture the glass shards remain stuck to the interlayer. the behaviour of viscoelastic materials nonnegligibly depends on time. with reference to concrete structure we are generally concerned with creep describing an increasing deformation over time under constant load. relaxation, on the other hand, represents a continuous decrease of internal stress over time while the strain remains unchanged. this might be a key factor when designing a precast steel reinforcement. to address viscoelastic materials the models combining elastic and viscous elements are used. the presented research endeavour attempts to compare this traditional approach with fractional viscoelasticity. the latter approach is based on fractional calculus which introduces the theory of derivatives and integral of non-integer order. the remainder of the paper is organized as follows. section 2 provides a brief review of theoretical grounds of both traditional and fractional viscoelasticty. the results of experimental measurements together with calibration of corresponding maxwell chain models are presented in section 3. the essential concluding remarks are then summarized in section 4. 2. theory of viscoelasticity a viscoelastic material, as the name suggests, experiences both elastic and viscous properties. the behaviour of such a material is bounded by two limit cases – pure elasticity and pure viscosity. g η (a) (b) figure 1. elastic spring (a) and viscous dashpot (b). purely elastic material is commonly modelled by a linear spring, see the rheological scheme in figure 1(a). the associated mathematical representation corresponds the hooke law which, in the domain of shear response, reads τ = gγ. (1) this law prescribes a linear relationship between the shear stress τ and the shear strain γ. the proportionality is given by the shear modulus of elasticity 23 https://doi.org/10.14311/app.2025.54.0023 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en barbora hálková, michal šejnoha acta polytechnica ctu proceedings g, the material property which we consider constant over time. the response of spring depends on the load only and is independent of time unless the load changes – instantaneous constant stress load causes immediate increase of strain, there is no additional increase or decrease of strain over time. the same applies to an instantaneous loading by a constant strain which manifests by an immediate change of internal stress remaining constant ever since. removing the load also causes the corresponding response to disappear – the spring returns to its original state before the load was applied. for illustration, the elastic response to a constant unit stress load is depicted by the brown line in figure 2 and the response to a constant unit strain load in figure 3. purely viscous material, on the other hand, is modelled by a viscous dashpot, see the scheme in figure 1(b). its behaviour is described by the newton law of viscosity which has the following form τ = ηγ̇. (2) we observe a linear relationship between the shear stress τ and the first time derivative of shear strain, i.e., the shear strain rate γ̇. the proportionality coefficient is represented by the coefficient of viscosity η (considered constant over time). in the case of viscous dashpot, an instantaneous constant stress load leads to an immediate change of strain rate which results in a linear increase of strain over time. therefore, the response of the viscous dashpot is time dependent – the longer the stress load is acting, the larger the deformation of the element is. removing the load causes the strain rate to return back to zero but the deformation does not disappear. it remains constant over time unless the opposite stress load is applied. loading the dashpot by an immediate constant strain would require an infinite strain rate at the onset of its application which in turn would cause an infinite internal stress of the element. passing this moment the strain remains constant and the strain rate and the stress become zero. the viscous response to a constant unit stress and strain load, respectively, is depicted by the pink line in figures 2–3. 2.1. traditional viscoelasticity as was mentioned above, the viscoelastic material behaves somewhere between the two limit cases given by ideal elasticity and viscosity. to describe a viscoelastic material it is standard to use springs and dashpots to compose more complex models by connecting them together in series or in parallel. these models then possess both elastic and viscous properties given by the properties and types of connection between individual elements. there are endless possibilities how to construct a viscoelastic model using springs and dashpots. one of the most simple models is the maxwell model (also referred to as the maxwell cell), which consists of one 0 2 4 6 8 10 time 0 2 4 6 8 10 cr ee p m od ul us spring =0,05 =0,2 =0,4 =0,6 =0,8 =0,95 dashpot figure 2. creep modulus of springpot in dependence on parameter α. 0 20 40 60 80 100 time 0.00 0.25 0.50 0.75 1.00 1.25 1.50 1.75 re la xa tio n m od ul us spring =0,05 =0,2 =0,4 =0,6 =0,8 =0,95 dashpot figure 3. relaxation modulus of springpot in dependence on parameter α. η g figure 4. maxwell model. spring and one dashpot connected together in series, see the scheme in figure 4. loading the model by an instantaneous constant stress would cause an immediate deformation of the spring followed by a linear increase of the deformation of dashpot. upon removing the load the elastic deformation disappears and the viscous deformation of dashpot remains constant. this example shows that by jointly employing the spring and the dashpot provides a model that combines the elastic and viscous response, it therefore behaves as viscoelastic. the properties of the maxwell model are specified by its two parameters – the shear modulus of elasticity g and the coefficient of viscosity η. their ratio is called 24 vol. 54/2025 fractional calculus in describing the viscoelastic response of pvb foil g0 g1 gn η1 ηn figure 5. generalized maxwell chain model. the characteristic time τc = η g , (3) which represents the time when the increasing viscous deformation reaches the same value as the elastic one under the constant stress load. the applicability of several theoretical models for the description of a pvb foil is thoroughly discussed in [1]. it was suggested that one of the most suitable models is the generalized maxwell chain model. this model consists of a single spring connected in parallel with n maxwell cells, see the scheme in figure 5. 2.2. fractional viscoelasticity fractional viscoelasticity is based on the theory of integrals and derivatives of non-integer order. the principle of fractional integration generalizes the cauchy formula for n-times repeated integration inf(x) = 1 (n − 1)! ∫ x 0 (x − y)n−1f(y)dy, (4) where in denotes an integral operator of order n ∈ n. generalizing the factorial function into the gamma function γ allows us to introduce the riemannliouville fractional integral iαf(x) = 1 γ(α) ∫ x 0 (x − y)α−1f(y)dy, (5) where iα denotes an integral operator of non-integer order α ∈ r. the definition of the fractional integral leads us to the two most commonly used definitions of the fractional derivative. the caputo fractional derivative is defined as follows dαf(x) = i⌈α⌉−α [ d⌈α⌉f(x) ] , (6) where ⌈α⌉ denotes the ceiling function. for example, to obtain the 0.8th derivative of function f(x) we take α, τc, g figure 6. springpot. its first derivative and then integrate the result 0.2 times. the riemann-liouville fractional derivative is prescribed as dαf(x) = d⌈α⌉ [ i⌈α⌉−αf(x) ] . (7) both of these definitions lead to the same final results. a broader insight into fractional calculus can be found in [2, 3], to cite a few. the fractional calculus allows us to introduce another rheological element, a springpot, see the scheme in figure 6. the constitutive law for the springpot element has the following form τ(t) = ξdαγ(t), (8) where ξ and α are the model parameters of the springpot. the springpot shows proportionality between the stress and the non-integer α-th derivative of strain. for better understanding of the parameters of springpot, the parameter ξ can be rewritten with the help of characteristic time as ξ = gτα c . (9) substituting this representation into eq. (8) gives τ(t) = gτα c dαγ(t). (10) for the application in the theory of viscoelasticity the limit cases given by pure elasticity and pure viscosity have to be respected. these limit cases are now obvious from eq. (10). for α = 0 we get the following equation τ(t) = gτ0 c d0γ(t) = gγ(t), (11) which is identical to the hooke law describing the behaviour of a purely elastic material, recall eq. (1). for α = 1 we receive τ(t) = gτ1 c d1γ(t) = ηγ̇(t), (12) which corresponds to the newton law of viscosity introduced in eq. (2). 25 barbora hálková, michal šejnoha acta polytechnica ctu proceedings α, τc g figure 7. fractional maxwell model. g0 g1 gn α1, αn, τc1 τcn figure 8. fractional maxwell chain model. the behaviour of springpot is therefore strongly dependent on its parameter α. this dependence is illustrated in the following graphs. figure 2 shows the time evolution of strain due to unit stress load (i.e. the creep modulus). figure 3 then plots the stress response to a unit strain load (i.e. the relaxation modulus). in both graphs the limit cases of pure elastic and pure viscous response are displayed together with the response of a single springpot with varying parameter α. it is evident that reducing the parameter α down to zero brings the response closer a purely elastic material (the brown line in the graph). on the other hand, when the parameter α approaches one, the springpot element becomes to approximate a purely viscous material (the pink line in the graph). the springpot element on its own behaves as viscoelastic. however, a single springpot element is not sufficient to describe most of the viscoelastic materials and it is necessary to introduce more complex rheological models. the standard maxwell model was already presented earlier in this paper. by replacing the viscous dashpot with the viscoelastic springpot the fractional maxwell model (fractional maxwell cell) is obtained, see the rheological scheme in figure 7. connecting more fractional maxwell cells together with a single spring then forms the fractional maxwell chain model, see figure 8. compared to the standard maxwell model the fractional model has more parameters. while for the standard model we need two parameters to specify its behaviour (the shear modulus g and either the viscosity coefficient η or the characteristic time τc), figure 9. dynamic shear rheometer malvern kinexus dsr+. the behaviour of the fractional model yet depends on the springpot parameter α amounting to three parameters to be identified. the standard maxwell chain of n cells has then 2n + 1 free parameters (gi and ηi for each cell and the modulus of the single spring g0). for the fractional maxwell chain we arrive at 3n + 1 free parameters. 3. calibration of theoretical models the maxwell chain in its standard as well as its fractional form was deem suitable for the description of pvb foil. to describe this material properly requires calibration of free model parameters to fit experimental measurements computationally. for this purpose, an extensive experimental program exploiting the dynamic shear rheometer malvern kinexus dsr+ in figure 9 was executed to examine the response of pvb foils with variable thicknesses. by loading the sample in a simple torsion the device determines both elastic and viscous properties of the material, see also [4] for further details. in the rheometer the sample is attached between the two parallel plates as is shown in figure 10. the bottom plate is fixed while the upper plate can move along and rotate around its vertical axis. the sample is loaded by a harmonic (sinusoidal) shear strain while the stress response is monitored by the rheometer. the software then extracts the values of amplitudes of the strain γ0 and the stress τ0 and the phase shift δ between the load and the response. 26 vol. 54/2025 fractional calculus in describing the viscoelastic response of pvb foil figure 10. sample attached to parallel-plate rheometer. these values are converted into the required outputs – the storage modulus g′, the loss modulus g′′, and the complex modulus g∗. these moduli are given by g′(ω) = τ0 γ0 cos δ, g′′(ω) = τ0 γ0 sin δ, g∗(ω) = g′(ω) + ig′′(ω). (13) the rheometer outputs are based on the known diameter of the rheometer plates and the gap between the plates, which corresponds to the thickness of the sample with the assumption of the sample perfectly filling the gap between the plates. the behaviour of pvb is strongly dependent on time and temperature and therefore the measurements were performed for several different frequencies and temperatures and also for two different thicknesses of the material sample, 0.76 and 1.52 mm in particular. we observed some differences in the response of the two types of specimens suggesting the need of their separate calibration. the detailed description of the experiment along with the complete set of results can be found in [5]. here, only the results pertinent to 0.76 mm sample are presented to compare the predictions provided by the standard and fractional viscoelasticity. mathematically, the storage modulus of the standard maxwell chain has the following form g′(ω) = g0 + n∑ i=1 giω 2τ2 ci ω2τ2 ci + 1 , (14) whereas the storage modulus associated with the fractional model reads g′(ω) = g0 + n∑ i=1 gi β2 i + βi cos (αi π 2 ) β2 i + 2βi cos (αi π 2 ) + 1 , (15) where βi = (τciω)αi . it is obvious that the behaviour of the model depends on the number of cells n . in general, the higher the number of cells, the more accurate approximation of the experimental data can be achieved. however, 10 5 10 3 10 1 101 103 105 angular frequency [rad/s] 10 1 100 101 st or ag e m od ul us [m pa ] data 0.76 mm sample 3-cells maxwell chain figure 11. calibration – 3-cells standard maxwell chain vs experimental data. 10 5 10 3 10 1 101 103 105 angular frequency [rad/s] 10 1 100 101 st or ag e m od ul us [m pa ] data 0.76 mm sample 12-cells maxwell chain figure 12. calibration – 12-cells standard maxwell chain vs experimental data. with the increasing number of cells the number of model parameters that need to be calibrated also increases. it is, therefore, necessary to find compromise between accuracy and efficiency. for better illustration see figures 11–12 which show predictions via the calibrated standard maxwell chain model with various number of cells. the black crosses represent the data obtained from the rheometer experiment. the blue line in figure 11 is the approximation of the measured data by the 3-cells standard maxwell chain model (this model has therefore 7 parameters in total). it is obvious that this approximation is not accurate enough and the number of cells need to be higher. a suitable approximation was obtained using 12-cells standard maxwell chain model (37 parameters in total), see the red line in figure 12. table 1 lists the corresponding stiffnesses found for the selected characteristic times assumed to be uniformly distributed as τci = 10j , j = −5, −4, . . . , 6, n = 12. (16) the approximation provided by the fractional model is presented in figure 13 suggesting better fitting capabilities with smaller number of model parameters, see the green line representing the 3-cells fractional 27 barbora hálková, michal šejnoha acta polytechnica ctu proceedings modulus value modulus value g0 0.03733 g7 1.139e-07 g1 2.067e-07 g8 7.309 g2 0.0870 g9 11.9472 g3 0.1149 g10 6.3631 g4 0.1101 g11 3.401 g5 0.02511 g12 1.0174 g6 0.1788 table 1. spring moduli in [mpa] of standard maxwell chain – 0.76 mm sample, fitting to experimental data. 10 5 10 3 10 1 101 103 105 angular frequency [rad/s] 10 1 100 101 st or ag e m od ul us [m pa ] data 0.76 mm sample 3-cells fractional maxwell chain figure 13. calibration – 3-cells fractional maxwell chain vs experimental data. maxwell chain with 10 parameters in total including the characteristic times whose choice, however, is not as straightforward as in the case of standard model. as evident from figure 14, the 3-cells fractional model provides a considerably smoother yet more accurate approximation of experimental data in comparison to the 12-cells standard model. the resulting set of optimal parameters is provided in table 2 for the characteristic times selected as τci = 10j , j = −2, 2, 5, n = 3. (17) 4. conclusion this paper introduces the principles of fractional viscoelasticity and compares the traditional approach with the fractional one to describe the response of a pvb foil. the traditional viscoelasticity employs theoretical models based on elastic springs and viscous dashpots. the fractional viscoelasticity introduces the viscoelastic springpot element. this approach requires more demanding mathematical background including the derivatives and integrals of non-integer order and it operates with more complicated analytical formulas. although analytical operations with the fractional models are more complicated, their application in the approximation of experimental data offer undoubtedly some advantages. the essential one is seen in smoothing the experimental data with strong fitting power even at both tails of the master-curve with a relatively 10 5 10 3 10 1 101 103 105 angular frequency [rad/s] 10 1 100 101 st or ag e m od ul us [m pa ] data 3-cells standard 12-cells standard 3-cells fractional figure 14. calibrated theoretical models vs experimental data. modulus value [mpa] parameter value g0 0.007426 α1 0.7043 g1 25.7932 α2 0.4625 g2 0.05564 α3 0.5241 g3 0.3438 table 2. parameters of fractional maxwell chain – 0.76 mm sample, fitting to experimental data. low number of cells. such a smooth approximation can be then adopted in the calibration of the standard maxwell chain, which still appears more suitable in structural analyses, e.g., in the combination with finite element method. acknowledgements this publication was supported by the czech science foundation, the grant no. 22-15553s and by the grant agency of the czech technical university in prague, grant no. sgs24/038/ohk1/1t/11. references [1] b. hálková. viskoelastic description of polymer interlayer of laminated glass. bachelor’s thesis, czech technical university in prague, 2022. [2] k. oldham, j. spanier. the fractional calculus theory and applications of differentiation and integration to arbitrary order. elsevier, 1974. [3] l. c. becker, i̇. k. purnaras. fractional relaxation equations and a cauchy formula for repeated integration of the resolvent. advances in the theory of nonlinear analysis and its application 2(1):11–32, 2018. https://doi.org/10.31197/atnaa.379282 [4] t. hána, t. janda, j. schmidt, et al. experimental and numerical study of viscoelastic properties of polymeric interlayers used for laminated glass: determination of material parameters. materials 12(14):2241, 2019. https://doi.org/10.3390/ma12142241 [5] b. hálková. experimental and numerical modelling of pvb foil. master’s thesis, czech technical university in prague, 2024. 28 https://doi.org/10.31197/atnaa.379282 https://doi.org/10.3390/ma12142241 acta polytechnica ctu proceedings 54:23–28, 2025 1 introduction 2 theory of viscoelasticity 2.1 traditional viscoelasticity 2.2 fractional viscoelasticity 3 calibration of theoretical models 4 conclusion acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2025.54.0050 acta polytechnica ctu proceedings 54:50–54, 2025 © 2025 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague a study of the behavior of samples of high-performance concrete shells with filled core exposed to compression eliška kafkováa,∗, jan macháčeka, věra kabíčkováa,b, tomáš vlacha,b a czech technical university in prague, faculty of civil engineering, department of architectural engineering, thákurova 7, 166 29 prague 6, czech republic b czech technical university in prague, university centre for energy efficient buildings, materials and construction of buildings research department, třinecká 1024, 273 43 buštěhrad, czech republic ∗ corresponding author: eliska.kafkova@fsv.cvut.cz abstract. the production of concrete has a huge impact on the environment, especially due to its emission of co2. nowadays, it is required to reduce co2 emissions and at the same time reduce the consumption of non-renewable resources. one way to achieve these goals is through the usage of recycled aggregate. however, when these materials are used, there are significant changes in the properties of concrete, particularly a reduction of mechanical strength, modulus of elasticity, and durability. this article presents the possibility of using a thin shell made of high-performance concrete as core protection of a concrete sample with recycled aggregate to improve the abovementioned properties. moreover, using high-performance concrete as the outer layer enables the potential of high-performance concrete to be exploited. the article describes the influence of different shell reinforcements on the failure of the samples under compressive stress. a shell layer without additional reinforcement was used for the reference samples. other sets of samples contained dispersed reinforcement in the form of polypropylene or polyvinyl alcohol fibers. the last reinforcement was a technical textile – a laboratory-prepared composite grid made of carbon rovings. all samples were subjected to a compressive test and the failure mode of the samples was observed in particular. keywords: fiber-reinforced concrete, textile-reinforced concrete, high-performance concrete, concrete with recycled aggregate, fiber reinforcement, textile reinforcement, carbon footprint. 1. introduction carbon footprint, specifically co2 production is a profoundly discussed issue, and the construction industry is not an exception nowadays. hence concrete with recycled aggregate is progressively more desired product and it tends to be more investigated than years ago as its usage is one of the methods how to effectively affect this area. nevertheless, rac (in abbr. recycledaggregate concrete) is disadvantaged by its properties usually deteriorated apart from conventional concrete mixtures, primarily its durability [1]. therefore other improvements are taken into account such as, among other things, outer protection by the hpc (in abbr. high-performance concrete). on the other hand, hpc itself is environmentally inauspicious and there is an effort to lower its consumption in general. due to these facts, hpc used as the only protection layer is incorporating and decreasing its problems also when textile reinforcement is used [2–10]. the experiment described in this article deals with the combination of these two mentioned materials with the view of improved properties unlike using each material separately. samples were made as squared thin shells (protective layer) of hpc and subsequently filled with rac mixture, hollow shells were also made as reference samples. the examination was made as a gradual loading in compressive strength and all variants used were compared. 2. materials both of the concrete mixtures used in this experiment was evolved at the department of architectural engineering, faculty of civil engineering ctu in prague. the average compressive strength of the hpc mixture measured on cubes of the length of the edge 100 mm after 28 days of hardening was 120 mpa. the average compressive strength of the rac mixture measured on cubes of 100 mm in length of the edge after 28 days of hardening was 24.4 mpa. the detailed composition of the matrix is described below in tables 1 and 2. mix content kg m−3 cement i 42.5r 650 technical quartz sand dmax = 1.2 mm 1 200 technical quartz powder st 6 235 silica fume (microsilica) 75 superplasticizer based on pce 18 water (at 12 °c) 190 total 2 368 table 1. hpc mix design. 50 https://doi.org/10.14311/app.2025.54.0050 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 54/2025 a study of the behavior of samples of high-performance concrete . . . mix content kg m−3 cement i 42.5r 274 recycled aggregate 0/4 773 recycled aggregate 4/8 18 recycled aggregate 8/16 845 water (at 12 °c) 290 total 2 200 table 2. rac mix design. figure 1. fiber-reinforced 18 mm thick shell filled with rac (on the left), hpc including carbon grid 18 mm shell filled with rac (on the right). shell preparation included a few following steps. a squared steel section with a separative layer was settled with plastic hoops made by a 3d printer. those hoops were used as a base for longitudinal reinforcement around which the carbon grid was knitted. concrete embedment was made in a horizontal position, samples were first made as 1200 mm long. after concrete hardening, the steel section was pulled out. subsequently, every element was cut into three 400 mm long specimens. for the outer layer made of hpc, three types of reinforcement were used namely carbon textile reinforcement grid, dispersed polyvinyl alcohol fibers, and dispersed polypropylene fibers. within the samples including the carbon textile reinforcement grid, fiberreinforcement rebars (in abbr. frp) were also used as a base for carbon roving knitting for the requested shape of the grid. carbon grid was subsequently carbonated with epoxy resin to hold the shape and the damp surface was covered with fine sand for better cohesion with the concrete mixture. to all the filled samples with rac were also added conventional steel rebars to the inner corners. cross section area of the whole sample was 100 × 100 mm and the length was approximately 400 mm. outer layers were made in two thicknesses – 8 mm and 18 mm. an exemplary scheme is presented in figure 1 and summary of all the specimens is described and counted in table 3. 2.1. experiment a compression test of the specimens was performed at the uceeb laboratory according to the en 12390-3 standard (compressive strength of test specimens) [11], and hydraulic press controls mmc-multitest was used. all samples were gradually loaded until the collapse. 3. results of the compression test the first group of samples tested was hollow shells with 8 mm thickness. plain concrete shells, as expected, were fractured completely due to the fragility of the material as there is no reinforcement. polypropylene fibers had a negative impact toward mechanical properties as they are smooth and thin and caused only the weakening of the shell structure accordingly to the lower compressive strength in comparison with plain concrete shell. the form of fracture was exactly the same as by plain concrete – completely fractured through as much as the concrete without reinforcement. polyvinyl alcohol fiber-reinforced hollow samples reported visible transition apart from previous groups. these sturdy fibers tend to keep the whole element undivided as its integrity is improved although it was already destructed. moreover, compressive strength is not weakened such as by pp fibers whereas is comparable with the plain concrete shell results. furthermore, were textile-reinforced samples tested. above all, compressive strength was significantly increased because of carbon grid reinforcement usage for these elements. the reinforcement grid was not destroyed, conversely, its strength caused concrete spalling around and near the reinforcement carbon grid. a representative sample for every group of reinforcement is shown in figure 2. when testing a group of hollow specimens with a thickness 18 mm of the shell, a form of fracture was equal to the thinner hollow samples. proportions between average compressive strengths were a bit different by dispersed fiber-reinforced elements apparently according to different ratios of fiber length and thickness of the shell layer. the next step was testing 8 mm shells filled with recycled aggregate concrete. the samples including only a plain concrete shell were fractured completely around the core. the outer layer was spalled because of the fragility of the material and also no cohesion or interaction between rac and hpc layers through the construction joint. pp fiber-reinforced specimens reported the same behavior as plain concrete. nevertheless, average compressive strength was a bit improved apart from the first set. pva fibers seemed to be the optimal solution for this outer layer in terms of greater integrity which kept the whole element undivided although many big cracks were visible. the average compressive strength was quite lowered but this was probably caused due to low permeability of the rac mixture which did not reach all space inside so hollow weak spots remained and destruction of the hpc layer was easier because of no interaction with the core. textile-reinforced specimens reported significantly improved behavior apart from all the previous sets. samples were probably over-reinforced because the carbon grid was not broken (as by hollow samples), so except for concrete spalling through the cover layer around the grid, specimens reported greater integrity and also increased average compres51 e. kafková, j. machacek, v. kabičková, t. vlach acta polytechnica ctu proceedings formwork plain concrete masterfiber 012 masterfiber 401 carbon grid 8 mm 3 pcs hollow 3 pcs hollow 3 pcs hollow 3 pcs hollow 3 pcs with rac 3 pcs with rac 3 pcs with rac 3 pcs with rac 18 mm 3 pcs hollow 3 pcs hollow 3 pcs hollow 3 pcs hollow 3 pcs with rac 3 pcs with rac 3 pcs with rac 3 pcs with rac table 3. summary of the tested samples. figure 2. 8 mm hollow shells by materials from left: plain concrete, pp frc, pva frc, trc (carbon grid). figure 3. 8 mm shells filled with rac by materials from left: plain concrete, pp frc, pva frc, trc (carbon grid). sive strength. a representative sample for every group of reinforcement is shown in figure 3. throughout the 18 mm shells filled with rac, forms of fracture were quite similar to the thinner ones. within testing these thicker elements had an important role the construction joint as a weakest part of the element. spalling in this area was frequently visible that is to say cohesion or interaction between rac and hpc layer is near zero. 4. conclusion in this experiment were tested two types of specimens – hollow shells made of hpc in two thicknesses 8 mm and 18 mm and four variants of (non)reinforcement and subsequently these shells filled with recycled aggregate concrete. firstly, hollow sets of specimens will be described. plain concrete shells without reinforcement were always, as was expected, super fragile due to highperformance concrete mixture properties and there were no differences between plain concrete sample variants. polypropylene fibers reported also fragile fracture at all and they also influenced the mechanical properties of the samples. from figure 4 is noticeable that the impact of adding these fibers on compressive strength is a bit unpredictable. some variants were worse than plain concrete although this was not a standard according to other sets. this is probably caused by fiber dosage, orientation, or equal dispersion but these fibers themselves have no effect on compressive 52 vol. 54/2025 a study of the behavior of samples of high-performance concrete . . . figure 4. compression test results in mpa. figure 5. compression test results according to maximal load. strength and they can easily weaken the cross-section area during the loading process. polyvinyl alcohol fibers confirmed the same principle described by pp fiber reinforcement. nevertheless, pva fibers ensure greater integrity thus safer construction fracture. trc samples reported the highest compressive strength values and also the most favorable form of fracture. the remaining sets filled with recycled aggregate concrete reported similar properties when it comes to shell and its materials. nevertheless, it is important to pay heed to construction joint as the weakest area of the sample. nearly all filled samples, with no need to distinguish thicknesses, were broken mostly through the construction joint because of a lack of cohesion or interaction between the inner core and outer layer, which should be improved a lot. all of the average compressive strengths for every type is in figure 4. because of different cross-section area due to filling with rac, results taken as maximal load before collapse are in figure 5 for better comparison between hollow shells and their filled alternatives. in general, significant improvement was caused for each set by just filling with rac and subsequent interaction. when it comes to a complete summary, the best results at all were achieved within pva fibers and textile carbon grid reinforcement. acknowledgements the work on this paper was supported by czech science foundation grant no. 22-14942k entitled “possibilities of using natural fibers for the production of hybrid textile reinforcement in concrete” the authors would like to acknowledge all financial assistance provided to support this research. references [1] c. lima, a. caggiano, c. faella, et al. physical properties and mechanical behaviour of concrete made with recycled aggregates and fly ash. construction and building materials 47:547–559, 2013. https: //doi.org/10.1016/j.conbuildmat.2013.04.051 [2] p. c. aitcin. high-performance concrete. crc press, 1998. [3] p. c. aitcin. the durability characteristics of high-performance concrete: a review. cement and concrete composites 25(4-5):409–420, 2003. https://doi.org/10.1016/s0958-9465(02)00081-1 [4] p. s. song, s. hwang. mechanical properties of highstrength steel fiber-reinforced concrete. construction 53 https://doi.org/10.1016/j.conbuildmat.2013.04.051 https://doi.org/10.1016/j.conbuildmat.2013.04.051 https://doi.org/10.1016/s0958-9465(02)00081-1 e. kafková, j. machacek, v. kabičková, t. vlach acta polytechnica ctu proceedings and building materials 18(9):669–673, 2004. https: //doi.org/10.1016/j.conbuildmat.2004.04.027 [5] t. bittner. experimentální ověřování prvků z textilního betonu. ph.d. thesis, vut, 2019. [6] h. tian, z. zhou, y. zhang, y. wei. axial behavior of reinforced concrete column with ultra-high performance concrete stay-in-place formwork. engineering structures 210:110403, 2020. https://doi.org/10.1016/j.engstruct.2020.110403 [7] e. kafková. influence of different fibre reinforcement on the failure of textile reinforced concrete. bachelor thesis, vut, 2021. [8] r. f. zollo. fiber-reinforced concrete: an overview after 30 years of development. cement and concrete composites 19(2):107–122, 1997. https://doi.org/10.1016/s0958-9465(96)00046-7 [9] w. brameshuber. textile reinforced concrete-state-ofthe-art report of rilem tc 201-trc, rilem, 2006. [10] m. etxeberria, a. r. marí, e. vasquez. recycled aggregate concrete as a structural material, rilem, 2006. [11] european committee for standardization. en 12390-3; testing hardened concrete – part 3: compressive strength of test specimens, 2019. 54 https://doi.org/10.1016/j.conbuildmat.2004.04.027 https://doi.org/10.1016/j.conbuildmat.2004.04.027 https://doi.org/10.1016/j.engstruct.2020.110403 https://doi.org/10.1016/s0958-9465(96)00046-7 acta polytechnica ctu proceedings 54:50–54, 2025 1 introduction 2 materials 2.1 experiment 3 results of the compression test 4 conclusion acknowledgements references my title https://doi.org/10.14311/app.2022.36.0244 acta polytechnica ctu proceedings 36:244–252, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague influence of plastic tension chord deformation on the shear capacity of prestressed beam elements sebastian thoma∗, oliver fischer technical university of munich, chair of concrete and masonry structures, theresienstraße 90, n6, 80333 münchen, germany ∗ corresponding author: sebastian.thoma@tum.de abstract. the shear strength of prestressed beam elements is of great interest with regard to the evaluation of existing concrete bridges. accompanying extensive experimental test series, the adaptation and modification of existing models for the evaluation of the load-bearing capacity also represents an important step. against this background, considerations are made for the isolation of critical influencing factors on the effective concrete compressive strength and quantified on the basis of a numerical framework. subsequently, a sensitivity analysis is performed on this basis to determine the influence of individual parameters and their interaction. taking into account the non-linear relationships at the cross-section level and in the cracked compressive stress field in the web, the shear reinforcement ratio and the strain in the longitudinal reinforcement are of major importance. keywords: plastic tension chord, prestressed concrete, sensitivity analysis, shear strength, stress fields. 1. introduction for the assessment of existing bridge structures and with respect to special questions related to prestressed concrete bridges, theoretical and experimental efforts to evaluate the shear capacity have been increased in the recent past [1–3]. in particular, characteristic bearing mechanisms of prestressed continuous girders with a low amount of shear reinforcement are continuously under discussion. from own experimental investigations, which are briefly presented schematically in section 2, the question for the evaluation of possible influencing factors on the effective concrete compressive strength of the cracked compressive stress field is developed and described by means of a numerical model, cf. section 3. subsequently, the proportion of the variance of the individual input variables in the total variance of the response spectrum is evaluated using a global sensitivity analysis [4] in section 4. from this, dominant parameters and model-inherent interactions can be derived. section 5 discusses main results and some influences of the numerical model are evaluated. existing approaches to consider the phenomenon of compression softening together with the presented results may be viewed from a new angle. this procedure provides a valuable basis for possible model modifications based on it. 2. experimental evidence the aim of the experimental research is the systematic illustration of the influence of a gradually reduced degree of longitudinal reinforcement and the associated effects on internal equilibrium. for this purpose, nine prestressed beam elements are tested using the substructure technique. figure 1 illustrates the concept. this method has already been deployed at the institute to study various aspects in regard to the shear strength of prestressed beam elements [1, 5], while the basic idea for testing detached subsystems has already been used successfully before [6]. including digital image correlation and fibre-optic sensors along longitudinal reinforcement and tendon axis, the experimental results show that the force flow in prestressed beams is subject to a highly statically indeterminate interaction that adopts a static equilibrium depending on communicating damage processes. in this way, the shear capacity is not considered in an exposed manner, but is to be classified with conformity of the stresses and strains under all acting internal forces, whereby in particular the mixed reinforced tension chord of reinforcing steel and tendon controls the cross-sectional strain and dominance of individual load-bearing components. the analysis shows that even with yielding longitudinal reinforcement under high bending moments in field and support regions, the ultimate system capacity is determined by a shear failure as long as the prestressing steel can provide the corresponding increase in strain. the evaluation of the tension chord deformation and the evolution of the cracked stress field in the web prove the great importance of arching actions in the description of the bearing behaviour of prestressed beam elements. a comprehensive analysis and discussion of experimental results will be presented in other publications and are not the subject of this paper. the brief outline of the experiments carried out is intended solely as a background and forms the basis for the considerations set out below, which can assist in evaluating the results and further refining the model concepts. 244 https://doi.org/10.14311/app.2022.36.0244 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 36/2022 influence of plastic chord deformation on shear capacity m v 𝜀sl a) b) c) fce = f(𝜀1) figure 1. experimental setup and focus of presented studies: a) substructure technique for investigating the shear strength of prestressed beam elements, based on the shear field of a continuous beam between the point load in the span and the inner support. reinforcement and tendons are anchored in tension in the cut edges/load plates; b) deformed system and internal forces acting on it. the push cam profiling enables the transmission of shear forces in analogy to bridges in segmental construction. the fans of the cracked compressive stress field are indicated in the field and support areas, a deflection takes place at tendon axis. the effective web compressive strength, controlled by the reduction factor kc, is the focus of the presented investigations. relevant details related to this factor are explained in the following section 3; c) qualitative profile of bending moment and shear force in the tested beam element. the load is successively increased until a shear failure occurs. under increasing load and a considerably reduced degree of longitudinal reinforcement, plastic strains result in the longitudinal reinforcement, which require a corresponding increase in strain in the tendons in order to guarantee the equilibrium of the internal forces. 245 sebastian thoma, oliver fischer acta polytechnica ctu proceedings 3. compression softening approach 3.1. background taking into account the test results, which show that the system load-bearing behaviour of prestressed crosssections is still limited by the shear capacity despite plastic tension chord deformation, and based on assumptions of the plasticity theory for the prediction of the shear strength, the effective concrete compressive strength of the cracked stress field is of crucial importance. as stated in [7], the assumption of a shear force bearing capacity derived from shear reinforcement is always limited by the bearing capacity of the concrete compression struts, which in turn depends on the effective concrete compressive strength and thus on the general state of strain. as a result, in addition to the shear reinforcement ratio, the longitudinal reinforcement ratio and the longitudinal strain coupled by the load distribution and system slenderness may have a major influence on the shear force bearing capacity. the factor kc, see equations 4 and 5, thus decides on the extent to which plasticity-theoretical approaches can be used to describe the shear strength of reinforced concrete beams. for design purposes, the effective concrete compressive strength is usually determined using constant reduction factors, which is a simple, conservative estimate. in the course of one’s own considerations, and especially in the case of plastic chord deformation, it is reasonable to explicitly take into account the actual state of strain. concrete girders, and in particular prestressed systems, develop a varying strain over beam height. the development of arching actions and the deviation of the stress field at the tendon axis cannot be represented in panel tests, which have contributed significantly to the description of various compression softening approaches [8–10]. still, the relations within the membrane element can be transferred to the beam web if an equivalent measure of the longitudinal distortion can be formulated. following the convention in the fib model code 2010 [11] or respectively the modified compression field theory (mcft) [9, 12], the measure of effective longitudinal strain is defined at the mid-depth of the member. however, the member is not defined by the cross-sectional contour, but must be understood as a web (following a classical plasticity-theoretical crosssectional division) between compression and tension chord. the longitudinal strain at half the height of the inner lever arm of the chord forces may serve as a clearer formulation here. 3.2. numerical model for the weighted evaluation of dominant influencing factors on the reduction factor kc and consequently on the effective concrete compressive strength, various assumptions are linked to a numerical model, the solution of which is solved as an optimisation problem, using sequential least squares [13, 14]. by iterating the strain plane for a cross-section under an acting bending moment and normal force, the lever arm of the internal forces is given after integrating the concrete compressive stresses, cf. figure 2. at the level of zm/2, the average longitudinal strain state is obtained, which is further supplemented by components caused by shear, cf. equation 1, adopted from [7]. 𝜎c δ𝜎p 𝜎s 𝜀c zm/2 zm/2 fc ft 𝜀x,mn800 250 figure 2. investigated cross section [mm], strain, stress, inner forces and decisive strain εx,mn due to bending and axial forces 𝜀z 𝜀x 𝜀2 𝜀1 𝜀 𝛾/2 𝜃 kc 𝜀1 a) b) figure 3. core components of the optimisation problem: a) mohr’s circle of strains in pure shear b) reduction factor kc, introduced equation according to [10, 15] since the proportion of longitudinal strain due to shear εx,v is already formulated as a function of the compressive strut angle θ, equation 1 is already part of the constrained nonlinear optimisation. based on equilibrium conditions and an explicit consideration 246 vol. 36/2022 influence of plastic chord deformation on shear capacity of the strains in longitudinal and shear reinforcement, which limit the range of possible strut angles (for a more detailed discussion, please refer to [7, 15]), the correlated values of the compression strut inclination and reduction factor kc, respectively the effective compressive strength fce, can be determined, cf. equations 4 and 5. assuming θ can evolve freely, taking into account its boundary conditions (derived from the yield strength and tensile strength of the reinforcement), the angle can be determined according to equation 2. the iterative routine is obvious in linking equation 2 and figure 3. after convergence the further steps are straightforward, and the principal strain ε1 as well as the reduction factor kc are determined, cf. equations 3 and 4. εx,v = v/2 · cot θ 2 (zs/zm · esas + zp/zm · epap) (1) tan θ = √ ρswfy fce − ρswfy (2) ε1 = εx,mnv + (εx,mnv − εc2) · cot2 θ (3) kc = 1 1.2 + 55 · ε1 ≤ 0.65 (4) fce = kc · ( 30 fc )1/3 · fc (5) the model uses additional, generally accepted simplifications, which are essentially due to the basic outlines of plasticity theory and will not be the focus of further attention here. related to the proposed model structure, some questions with respect to model sensitivity arise: • the principal strain εc2 is assumed to be constant at -0.002. is this, essentially empirically based, assumption [9] valid or do implicitly considered influencing factors falsify a generally valid statement? • what are the consequences of an increased strain level in the longitudinal reinforcement for established models for predicting the shear strength in prestressed beam elements? is εx a sensitive parameter for the response surface y? • are there significant interaction effects between different input parameters of the model? 4. global sensitivity analysis to address the above questions, the numerical model from section 3.2 is subjected to a sensitivity analysis. a global sensitivity analysis (gsa) distils the influence of possible uncertainty of selected n input parameters on the result of a model. possible interactions of the input values (so-called second order effects) can also be determined by variance-based methods. in the following sobol’s method [16, 17] enables a quantified evaluation of these issues. essential information on the characteristics of the gsa approaches used in this research is based on the considerations in [4], unless otherwise referenced. the method does not require an analytical model, but forms its correlation ratio on the basis of a problem spectrum to be defined initially, which contains all input parameters (which may be subject to the gsa), their distribution function and possible bounds. this data set is mapped into n dimensions by means of low-discrepancy sampling and then passed individually to the numerical model. critical aspects and details of the implementation are described below. 4.1. sobol’s sequence aiming for minimized discrepancy, generated sequences of parameter values should avoid holes and clusters in the hypercube. this task is adequately accomplished by sobol’s sequences [16, 17]. in addition, sobol’s sequence ensures order independence for different input parameter axes. without going into further detail, this is enabled by its direction numbers which are derived from distinct primitive polynomials for every required parameter vector [18]. sobol’ sequences are a quadrature rule and become unstable if samples of a size that is not a power of 2 are used, or if the first point is omitted, or if a sequence becomes sparse [19]. this method thus offers a tried and tested approach for evenly covering possible parameter constellations, which essentially eliminates the danger of unfavourable density of a parameter constellation. figure 4 shows an example of uniformly distributed random coordinates and the spread using a sobol’ sequence. the difference in the uniform coverage of the area is obvious. 0 1 0 1 0 1 0 1 0 1 figure 4. random samples and sobol’ sequence in two dimensions (27 = 128 samples) the model study accounts for seven parameters, which are listed in table 1 together with their assumed 247 sebastian thoma, oliver fischer acta polytechnica ctu proceedings uniform distribution. uniform distributions are assumed for the parameter space because no specific configuration with known uncertainty is to be examined, but an overall impression and overview of the model sensitivity is to be given. the limit values of the distributions are based on the test programme referred to at the beginning or assumed as realistic limit values when considering existing reinforced concrete bridges. all parameters are part of the resistance side and are included in the iteration of the initial strain plane and/or subsequent iteration to determine the stress and strain state in the web. in addition to the range of the longitudinal reinforcement ratio, the lower limit of which leads to plastic chord deformations in the crosssection analysis, the shear reinforcement ratio is also chosen relatively low with regard to the conditions in existing prestressed concrete bridges. taking into account the applied variation of the concrete compressive strength, a bandwidth of about 0.7 to 4.0 times the minimum shear reinforcement ratio ρw,min results for the general case according to ρw,min = 0.16ḟctm/fyk [20]. only λ represents a component of the load side to a certain extent, since the ratio of acting moment, which is increased independently of the gsa for all combinations of possible input values, and shear force, which also has a share in εx (see equation 1), is controlled via a variation of the shear slenderness. in addition, the variation of the tendon slope influences the effective shear force (after subtracting the vertical component vp = pmt · sin αp) in the web and ultimately the proportion εx,v. parameter uniform distr. range ρsl [%] [0.8, 2.0] ρsw [%] [0.09, 0.36] λ [-] [2.5 5.0] εc2 [-] [-0.002, -0.001] fc [mpa] [30.0, 50.0] σp [mpa] [550.0, 600.0] αp [deg] [1.0, 7.0] table 1. input parameters for sensitivity analysis. variance-based methods are powerful in quantifying the relative importance of input factors or groups. the main drawback of variance-based methods is the cost of the analysis, which, in the case of computationally intensive models, can become prohibitive even when using the approach described above. with saltellis extension of the sobol’s sequence [17], which is used within this scope, the resulting matrix has n (̇2n + 2) rows, where n is the number of parameters. for a full set of sobol’ indices (s1, s2 & st, cf. section 4.2) a model with 7 factors requires to execute the model at least 16000 times, taking n = 1000. whether the assumption for n was chosen sufficiently large can be judged from the confidence intervals. this results in over 200.000 model evaluations in total due to a relatively finely chosen incremental bending moment rise. in a general case, the validity and robustness of a composite indicator, in this case the reduction factor kc may depend on a number of factors. • the model chosen for estimating the measurement error in the data • the mechanism for including or excluding indicators in the index; the choice of factors fed into a gsa is subjective. particularly in the case of comparatively complex, non-linear numerical models, the model character itself has a considerable influence on the response surface y. theoretically, one could provide the gsa with as many input parameters as possible, but this seems unnecessarily cpu-intensive. not pursued further here, but possible in the first place, seems to be the gradual reduction of the input parameters with subsequent evaluation of the correlation coefficient to the results of a fully packed analysis without static default values. alternatively, qualitative screening using the morris method [21] can be performed as the first step in model survey. • the indicators preliminary treatment; in the investigations presented here, the limiting value 0.65 according to equation 4 is used in the framework of the iteration. the gsa evaluation deliberately excludes this criterion in order to take into account possible influences caused by the asymptotic character of the equation at low principal tensile strains. • the type of normalization scheme applied to the indicators to remove scale effects • the amount of missing data and the imputation algorithm; under high bending moment, it is theoretically possible that the iteration of the strain plane does not reach a stable equilibrium according to the defined stress-strain relationships and limit strains. this leads to a lack of the input value εx,mn in the further iterative consideration of the conditions for the web element, and to a non-response bias depending on the error handling. this scenario was prevented by previous comparative calculations so to avoid inconvergent result data falsifying the gsa. 4.2. sobol’s method after running the numerical model for sobol’s sequence, one gets an array that forms the so-called response surface. sobol’s method evaluates the part of the total variance of the response that can be attributed to input parameter xn. the numerical implementation is based on the salib package [22]. three measures can be obtained for each parameter: • first order index s1: contribution (without interaction) of a parameter to the response variance • second order index s2: interaction of input parameters to the response variance 248 vol. 36/2022 influence of plastic chord deformation on shear capacity • total effect index st: total contribution (including interaction) of a parameter to the response variance a measure of sensitivity is to calculate the variance of the conditional expectation v ar [e(y |xn)] referred to the total variance of y, v ar(y ), cf. equation 6. in other words, the total variance of the response y that can be attributed to input parameter xn defines its weight within the numerical model and, in a further step, enables a better understanding of model-inherent features beyond qualitative screening and, ultimately, opens up initial starting points if a model modification appears desirable. s1n = v ar [e(y |xn = xn)] v ar(y ) (6) any numerical model is some sort of mapping input parameters to output results. the following equation 7 shows the added parts that influence the result y, namely scalar values, the sum of functions evaluated for each parameter (modelling the effect of each individual parameter) and the interaction of parameters. y = f(x) =f0 + np∑ n=1p fn (xn) + ∑ 1≤np fn,n′ (xn, xn′) + . . . + f1,2,...,np ( x1, . . . , xnp ) (7) the total variance of the response y (the total of all y-evaluations) can be decomposed into partial variances, attributing variability of the response y to each input parameter, including interactions. different conditional variances: dn = v ar [e (y |xn)] (8) variance on two conditions: dn,n′ = v ar [e (y |xn, xn′)] − dn − dn′ (9) the first order sobol’ index s1n calculates the impact of the input parameter xn by estimating the partial variance of y explained by this parameter. it estimates by how much the variance of the response is reduced, on average, when the parameter xn is fixed, i.e. it measures the contribution of the parameter xn to the total variance of the response. the total effect index for a parameter xn is defined: stn = sn + ∑ n≤n′ sn,n′ + . . . (10) the total effect index represents the total contribution (including interactions) of a parameter xn to the response variance; it is obtained by summing all first-order and higher-order effects involving the parameter xn. further input variables of the model that are not included in the gsa, but are nevertheless to explicitly assume a variable character, require a recalculation of the sensitivity indices for all sobol’ samples. in the present case, this is the case for the applied bending moment from external load. this procedure is comparatively computationally intensive, but it allows a view on a potentially variable prioritisation of the parameters according to table 1, which should be quite common, especially for non-linear model considerations. 5. results and discussion the gsa shows which of the selected input parameters have an influence on kc and which values would have been sufficiently taken into account within the scope of the investigations with the assumption of a constant value, since no significant contribution to the variance of the result is evident. the major factor is the transverse reinforcement ratio. it is the most important influencing variable over the complete variation of the moment acting on the cross-section. this seems to be plausible with regard to equation 2. described from a phenomenological point of view, the stirrup strain imposes a proportional transverse strain on the concrete compression struts, which reduces the effective concrete compressive strength. for low shear reinforcement levels, this issue is particularly critical because the small cross-sectional area mobilises high strains early on for comparatively small forces after shear cracking. depending on the crack opening and ductility of the reinforcement, rupture of the reinforcement may occur. further questions arise in this regard, which will not be discussed in detail here. a model without parameter interaction leads to∑ st = 1, which is obviously not the case here. under external bending moment of about 0.45 to 0.5 mnm, interesting dependencies between longitudinal and shear reinforcement ratio are revealed, cf. figure 5. the concrete compressive strength also appears to gain in importance in the meantime before st settles back down to the initial level. figure 6 shows also a remarkable interaction between ρsw and εc2 develop in this range. this intermediate effect is due to the limiting conditions of the possible bandwidth of permitted compression strut angles. after exceeding the decompression stresses, small longitudinal strains of the same order of magnitude as εc2 initially enter into the equations. as expected, the influence of the longitudinal strain rises under increasing moment, while the shear reinforcement ratio, which is still important, decreases significantly. a moment of 0.9 mnm for the considered example cross-section (figure 2) results in the longitudinal reinforcement reaching its yield stress. the high longitudinal strain εx,mnv becomes more important for the iterative determination of the compression strut angle and the principal strain ε1 under increasing load. against the background of high utilization rates for bending and shear of many existing 249 sebastian thoma, oliver fischer acta polytechnica ctu proceedings my [mnm] asl asw asl asl asw figure 5. individual parameter importance s1 and total prioritisation st affecting kc concrete bridges, the question arises to what extend the mostly constant factors in standards provided represent a reasonable assumption. directly linked to this is, of course, the question of an adequate formulation of the effective concrete compressive strength or, at a distance, the question of the fundamental capacity of the model conception for shear strength discussed here. the generally constant assumption of the principal strain εc2 even proves to be a definite parameter to be taken into account, largely independent of the acting moment, if one considers modifying the numerical model with respect to kc. the importance of a reasonable assumption or parameterized formulation for εc2 becomes clear when looking at figure 3. selected principal compressive strain and longitudinal strain εx are of the same order of magnitude in absolute terms and have an influence on the compression strut angle according to their difference. 6. conclusions the paper presents a numerical model that allows a closed-loop determination of the effective concrete compressive strength in a beam web under shear loading. of further interest is the reduction factor kc used, which is influenced to different degrees by different parameters. in order to determine the individual influence of the parameters and possible model-determined interaction effects, a gsa was carried out, which was able to identify the shear reinforcement ratio and the assumption of the principal strain εc2 in general, and my [mnm] s2 figure 6. s2 indices: input parameter interaction affecting kc the longitudinal reinforcement ratio and corresponding, ultimately plastic chord deformation under high moment loading in particular as the decisive adjusting screws. in light of the experimental investigations, which achieved high shear capacities despite plastic chord deformation, a few questions arise: • do the empirically derived formulations for determining the reduction factor kc sufficiently approximate the conditions in a prestressed beam web or are the model ideas of plasticity theory described here (especially for cross-sectional segmentation) not applicable? • which possibilities for a more refined estimation of the main strain εc2 seem practicable and reasonable? these questions are part of further considerations, which should be evaluated in the context of the ductility of the reinforcement, which was already briefly addressed in the course of the discussion of results. list of symbols kc reduction factor for concrete compressive strength due to transverse tensile strain [–] εc concrete strain [–] εx,mn axial concrete strain due to normal forces and bending moment at half the height of the inner lever arm [–] εx,v axial concrete strain due to shear [–] ε1 concrete principal tensile strain [–] 250 vol. 36/2022 influence of plastic chord deformation on shear capacity ε2 concrete principal compressive strain [–] θ compression strut angle [deg] fce effective concrete compressive strength [mpa] fc concrete compressive strength, derived from cylinder tests [mpa] ρsl longitudinal reinforcement ratio, defined as ρsl = asl/(bw · d) [–] ρsw geometric shear reinforcement ratio, defined as ρsw = asw/bw [–] λ shear slenderness, defines as λ = m/(v · d) [–] zm lever arm of inner forces [mm] zs lever arm between fc and fs [mm] zp lever arm between fc and fp [mm] σc concrete (compressive) stress [mpa] σs steel stress due to normal forces and bending moment [mpa] σp prestress in tendons [mpa] ∆σp increased tendon stress due to normal forces and bending moment [mpa] αp tendon slope 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[22] j. herman, w. usher. salib: an open-source python library for sensitivity analysis. the journal of open source software 2(9), 2017. https://doi.org/10.21105/joss.00097. 252 https://doi.org/10.1080/00401706.1991.10484804 https://doi.org/10.21105/joss.00097 acta polytechnica ctu proceedings 36:244–252, 2022 1 introduction 2 experimental evidence 3 compression softening approach 3.1 background 3.2 numerical model 4 global sensitivity analysis 4.1 sobol's sequence 4.2 sobol's method 5 results and discussion 6 conclusions list of symbols references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2025.53.0007 acta polytechnica ctu proceedings 53:7–12, 2025 © 2025 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague production of steel-concrete composite columns for blast tests adam číteka,∗, karel hurtiga, martin kryštova, vojtěch šulcb, marek foglarb, david číteka a czech technical university in prague, klokner institute, šolínova 7, prague 166 08, czech republic b czech technical university in prague, faculty of civil engineering, thákurova 7, 166 29 prague 6, czech republic ∗ corresponding author: adam.citek@cvut.cz abstract. the resistance of structures to explosive impacts is a highly relevant issue in modern engineering. during an explosion, structures are subjected to extreme dynamic loads, which necessitate advanced materials and reinforcement strategies. uhpfrc, with its evenly distributed steel fibres, exhibits exceptional mechanical properties that make it well-suited for blast-resistant applications. combining uhpfrc with steel reinforcement is expected to significantly mitigate structural damage under explosive loading. this article focuses on the process of production of testing steel-concrete column specimens under axial compressive load for evaluating their blast resistance. the initial section describes the test elements and the objectives of the experiment. the following section outlines the design and material properties of the concrete mixtures used. subsequently, the production process, including formwork preparation and casting, is detailed. finally, the results achieved from ongoing testing are briefly introduced. keywords: concrete, ultra-high performance fibre-reinforced concrete, blast resistance, steel-concrete composite. 1. introduction explosions are most often associated with industrial accidents or acts of terrorism. in recent years, the increasing threat of terrorist or war attacks, particularly in developed countries, has highlighted the critical need for designing structures capable of withstanding the extreme overpressure loads generated by explosions [1, 2]. while steel-concrete composite structures are widely used globally, primarily for their economic efficiency, their behaviour under blast loading has not been thoroughly investigated. during an explosion, normalstrength concrete is highly susceptible to severe damage and collapse, necessitating reinforcement design to mitigate such risks. explosions generate extreme local loading that is difficult to predict, requiring reinforcement to be designed with a high level of complexity [3]. the use of uhpfrc offers a promising solution, as its dispersed steel fibres provide uniform reinforcement throughout the entire volume of the element [4, 5]. when combined with steel components in a composite structure, this material has the potential to achieve significantly greater blast resistance compared to conventional steel-concrete composites using normal-strength concrete [4, 6, 7]. this article is focused on the process of production of steel-concrete column specimens intended to experimental blast resistance tests. in the end of the article a summary of the findings from the ongoing testing is shown. these tests represent only a part of a complex research project focused on the blast resistance of steel-concrete structures. the experimental design, numerical analysis, and data evaluation were conducted by the faculty of civil engineering at the czech technical university (ctu), which also serves as the lead institution for the entire research project. the optimization of concrete mixtures, material property testing, and production of experimental specimens were performed by the klokner institute (ki). 2. description of specimens a total of ten column specimens were designed and prepared for the experiment, grouped into three fundamental types. type 1 represented a plain steel column with no infill and it was used as a reference. type 2 was a composite steel-concrete column filled with normal-strength concrete (nsc) with a compressive strength of 30+ mpa and type 3 represented composite steel-concrete columns filled with ultra-highperformance concrete (uhpc) with a compressive strength of 120+ mpa. all columns were based on hot-rolled hea 220 steel profiles fabricated from structural steel grade s355. each steel profile was sealed with steel plates at both ends to ensure uniform load distribution and structural integrity. additional side plates were attached to the specimens to assist with handling and positioning 7 https://doi.org/10.14311/app.2025.53.0007 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en a. čítek, k. hurtig, m. kryštov et al. acta polytechnica ctu proceedings figure 1. detailed description of the steel parts of the specimens. during the experiment. these side plates, however, had no structural function and did not influence the test results. to ensure composite action between the steel column and the infill concrete shear connectors in the form of bend reinforcement were welded onto the steel column. to replicate realistic boundary conditions, steel spherical joints were installed at both ends of the columns. these joints simulated ideally pinned connections, allowing for uniform stress distribution of the axial compressive load across the specimen’s crosssection. additionally, the spherical joints facilitated accurate representation of pinned boundary conditions in subsequent numerical analyses. the following table 1. shows description of the specimen. figure 1 illustrates the cross-section and longitudinal section of the test specimen. specimen specimen description 1a, 1b, 1c steel only column 2a, 2b, 2c nsc-filled steel column 3a, 3b, 3c uhpfrc-filled steel column table 1. list of prepared specimens and their description. 3. material properties as previously stated, a key objective of this part of the experimental program was to compare the performance of only steel columns with nsc-filled and uhpfrc-filled columns. the mix designs were adopted from the previous part of the experimental program, which focused on the blast resistance of composite slabs, where concrete mixtures for nsc and uhpfrc had already been developed and optimized by klokner institute [8]. the mixture is characterized by a very low watercement ratio, self-compacting character, and exceptional material properties. reinforcing was provided by dispersed steel fibres in volume of 1.5 % of the total volume. this fibre volume ensured required mechanical properties while maintaining good workability. the key material properties of the uhpfrc mixtures at 28 days are presented in tables 2 to 4. since the casting had to be performed over two days, two sets of results are presented in the table. in case of nsc specimens the same concrete mixture as in the previous part of the experimental program was used [8]. the key material properties of the nsc mixtures at 28 days are presented in tables 5 and 6. since the casting had to be performed over two days, two sets of results are presented in the table. 4. production the prepared steel columns fabricated from steel sections were transported to klokner institute, where the concrete casting for the samples was carried out. samples were casted in horizontal position with flanges upright. this arrangement allowed the column flanges to serve as lost formwork, eliminating the need for additional formwork preparation. the casting process needed to be divided in phases, because only one side of the columns could be casted at the time. between the castings, a two-day technological break was required, after which the samples were rotated to the other side and the second part of the casting was be carried out. in case of nsc, the concrete was compacted using an immersion vibrator (nsc). because of the self8 vol. 53/2025 production of steel-concrete composite columns for blast tests material flow bulk average speed of uz signal [km s−1] modulus of temperature test density beam cylinder elasticity cylinder [°c] [mm] [kg m−3] 150/150/700 mm 150/300 mm 150/300 mm [mpa] 25.0 285×290 2 430 4.64 4.62 44.9 28.3 240×245 2 410 4.61 4.58 44.7 table 2. material properties of uhpfrc. compressive strength [mpa] flexural strength [mpa] beam cube cylinder cylinder beam beam 40/40/160 mm 100 mm 100/200 mm 150/300 mm 40/40/160 mm 150/150/700 mm 176.5 144.3 132.4 129.8 33.3 13.5 152.0 134.3 131.2 122.0 29.1 11.7 table 3. material properties of uhpfrc – compressive and flexural strength. flexural tensile strength residual flexural strength direct tensile strength residual direct tensile strength at crack initiation for deflection of [mpa] at crack initiation ffc,tm,res,i for deflection of [mpa] ffc,tm,fl [mpa] 0.5 mm 3.5 mm 5.0 mm ffc,tm,cr [mpa] 0.5 mm 3.5 mm 11.2 12.6 11.7 9.1 8.5 6.4 4.3 9.6 11.5 10.5 9.1 5.8 5.8 3.8 table 4. material properties of uhpfrc – direct tensile and flexural tensile strength. material slump bulk modulus of average speed of uz signal [km s−1] temperature test density elasticity cylinder beam cylinder [°c] [mm] [kg m−3] 150/300 mm [mpa] 150/150/700 mm 150/300 mm 22.1 100 2 370 33.0 4.57 4.53 22.3 110 2 360 33.1 4.55 4.54 table 5. material properties of nsc. flexural strength beam compressive strength [mpa] 150/150/700 mm cube cube cylinder cylinder [mpa] 100 mm 150 mm 100/200 mm 150/300 mm 4.03 60.7 55.4 52.3 49.5 3.95 59.1 50.2 50.2 47.3 table 6. material properties of nsc – flexural and compressive strength. compacting character of the mixture, the samples from uhpfrc needed not to be compacted. few days after the first part of casting it was necessary to rotate each sample around its longitudinal axis to the opposite position, so it was possible to cast the second side of the section. the casting process was processed in the klokner’s institute laboratories. the uhpfrc specimens were cast in two batches with a total volume of 0.45 m3. similarly, the nsc specimens were cast in two batches with a total volume of 0.5 m3. following casting, the concrete surfaces were treated with an evaporation retardant and covered with plastic foil. the surfaces were treated with water during the first two days after casting. after 28 days, the specimens were transported to the university of pardubice, where the blast tests were performed. in figures 2 and 4 process of casting of the test samples is shown. all the prepared specimens can be seen in figure 3. figure 2. process of casting of the test specimens. 9 a. čítek, k. hurtig, m. kryštov et al. acta polytechnica ctu proceedings figure 3. process of casting of the test specimens. figure 4. process of casting of the test specimens. 5. experimental test as it was mentioned above all specimens were transported to university of pardubice, institute of energetic materials, where blast experiments were performed. for the purposes of these tests on columns subjected to axial loading, it was necessary to design a specialized testing apparatus that would met all the requirements for durability, functionality, and easy handling of the specimens. the axial loading was designed corresponding to a corner-column loading of multiple-story building as 1 300 kn in characteristic values. after multiple iterations, the final arrangement was determined to be a statically indeterminate frame, loaded by prestressing induced by a hydraulic jack. the frame was arranged vertically to avoid parasitic bending. the hydraulic jack was positioned at the bottom of the frame, protected by a steel box to prevent any damage. the testing apparatus is shown in the following figure 5. before the experiment, the elements were positioned vertically in the testing apparatus. the values of the figure 5. testing apparatus. specimen type axial load no. [kn] 1a type 1 (steel only) 1 300 2a type 2 (steel with nsc infill) 1 300 2b 100 3a type 3 (steel with uhpfrc infill) 1 300 3b 100 table 7. material properties of nsc – flexural and compressive strength. axial compressive loads applied to the elements using a hydraulic press are provided in table 7. the outer face of the steel flange in the middle of its height was loaded with a contact explosion of 0.8 kg of semtex 1a moulded to rectangular shape. the blast loading was constant throughout all specimen. during the experiment the time history of displacement of each 10 vol. 53/2025 production of steel-concrete composite columns for blast tests specimen flange was recorded with the use of 4 channels of photonic-doppler velocimetry (pdv). also, the testing frame was equipped with strain gauges on both pillars to check axial load in specimens a and to measure residual load after the blast. after the explosive tests was performed, the specimens were also subjected to visual examination. in this phase, it was decided to test only 5 out of the total 10 specimens. experiments results from the first set were used to further complex and detailed evaluation. this evaluation promising some option to make changes and optimization for the second set of specimens, which should be tested in near future. the design of the testing apparatus and evaluation of the test results was carried out by the faculty of civil engineering at the czech technical university (ctu). 6. results a key finding from the ongoing evaluation of test results is the positive impact of infill concrete on the overall resistance of the steel profile. steel specimens without concrete infill exhibited significant damage. during the explosion, steel fragments from the flange in direct contact with the charge were torn off. these fragments also caused a damage on the flange on the opposite side of the element (figure 6). specimens with concrete infill experienced overall significantly smaller damage. concrete infill prevented fragmentation of the steel flange in contact with explosive. the concrete infill experienced of crushing and spalling on the sides of the specimen in the area closest to the explosion. unlike the steel only specimens without concrete infill, almost no visible damage occurred to the opposite flange. figure 6. side view of specimen no. 1a. tests results also shown positive effect of excellent mechanical properties of uhpfrc. concrete infill made of nsc (figure 7) experienced considerably bigger damage than uhpfrc infill (figure 8). the effect of axial compressive force was not evaluated, as not all unloaded samples were tested in the first set of the experiments. this evaluation will be possible after all the samples have been tested. 7. conclusion this part of research project was focused on behaving steel-concrete columns subjected to contact blast loading. the emphasis of the experimental tests was fofigure 7. side view of specimen no. 2a. figure 8. side view of specimen no. 3a. cused both on comparing performance chosen types of steel-concrete composite columns under axial compressive load. the axial load 1 300 kn simulates a cornercolumn loading of multiple-storey building. to fulfil project goals three types of column specimens were produced. one type was steel only, the other two were steel-concrete specimens with nsc and uhpfrc infill. the emphasis of this article is description of production of the test specimens which was carried out by klokner institute (ki). both uhpfrc and nsc mixtures had been optimized in the previous part of the experimental program. in the end of the article a summary of the findings from the ongoing testing are described. overall, better blast resistance was achieved with specimens with concrete infill. concrete infill prevented fragmentation of steel profile. the positive effect of the excellent mechanical properties of uhpfrc on blast resistance can be observed. specimens with uhpfrc experienced considerably smaller damage than specimens with nsc infill. the results from the first completed set of tests will be used for the further numerical assessment and possible optimization of test set-up for the second part of specimens. test conception, numerical assessment and evaluation of results was carried out by faculty of civil engineering of ctu. acknowledgements the article was written with the support of the grant project of the czech science foundation – project no. 22-33039s. references [1] j. liu, j. wei, j. li, et al. a comprehensive review of ultra-high performance concrete (uhpc) behaviour under blast loads. cement and concrete composites 11 a. čítek, k. hurtig, m. kryštov et al. acta polytechnica ctu proceedings 148:105449, 2024. https://doi.org/10.1016/j.cemconcomp.2024.105449 [2] c. zhao, x. lu, q. wang, et al. experimental and numerical investigation of steel-concrete (sc) slabs under contact blast loading. engineering structures 196:109337, 2019. https://doi.org/10.1016/j.engstruct.2019.109337 [3] h. draganić, g. gazić, d. varevac. experimental investigation of design and retrofit methods for blast load mitigation – a state-of-the-art review. engineering structures 190:189–209, 2019. https://doi.org/10.1016/j.engstruct.2019.03.088 [4] s. abbas, m. nehdi, m. saleem. ultra-high performance concrete: mechanical performance, durability, sustainability and implementation challenges. international journal of concrete structures and materials 10:271–295, 2016. https://doi.org/10.1007/s40069-016-0157-4 [5] m. foglar, r. hajek, m. kovar, j. štoller. blast performance of rc panels with waste steel fibers. construction and building materials 94:536–546, 2015. https://doi.org/10.1016/j.conbuildmat.2015.07.082 [6] m. foglar, r. hajek, j. fladr, et al. full-scale experimental testing of the blast resistance of hpfrc and uhpfrc bridge decks. construction and building materials 145:588–601, 2017. https://doi.org/10.1016/j.conbuildmat.2017.04.054 [7] r. hajek, j. fladr, j. pachman, et al. an experimental evaluation of the blast resistance of heterogeneous concrete-based composite bridge decks. engineering structures 179:204–210, 2019. https://doi.org/10.1016/j.engstruct.2018.10.070 [8] a. citek, m. krystov, k. hurtig, et al. production of steel-concrete composite uhpfrc elements for experimental tests of the blast resistance. key engineering materials 976:113–119, 2024. https://doi.org/10.4028/p-oyi5pk 12 https://doi.org/10.1016/j.cemconcomp.2024.105449 https://doi.org/10.1016/j.engstruct.2019.109337 https://doi.org/10.1016/j.engstruct.2019.03.088 https://doi.org/10.1007/s40069-016-0157-4 https://doi.org/10.1016/j.conbuildmat.2015.07.082 https://doi.org/10.1016/j.conbuildmat.2017.04.054 https://doi.org/10.1016/j.engstruct.2018.10.070 https://doi.org/10.4028/p-oyi5pk acta polytechnica ctu proceedings 53:7–12, 2025 1 introduction 2 description of specimens 3 material properties 4 production 5 experimental test 6 results 7 conclusion acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2023.44.0005 acta polytechnica ctu proceedings 44:5–12, 2023 © 2023 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague thermomechanical calculation within the hera programme tereza kinkorová czech technical university in prague, faculty of nuclear sciences and physical engineering, department of nuclear reactors, v holešovičkách 2, 180 00 prague 8, czech republic correspondence: kinkoter@fjfi.cvut.cz abstract. hera (high burnup experiments in reactivity initiated accident) is one of the programmes which should replace closed halden reactor. as the name implies hera is focused on the tests simulated ria (reactivity initiated accident) and the aim of the programme is to investigate the performance and behaviour of high burnup fuel in ria transients. the first phase of hera programme consists of two steps which are the blind calculation of different ria scenarios (different pulse width, enthalpy increase, hydrogen content and gap size) and in-reactor test with fresh fuel. the set of thermomechanical calculation was performed with the fraptran computer code as part of the first phase of hera. the influence of several parameters on the fuel behaviour was observed, the varying parameters were the pulse width, peak radial average enthalpy increase, hydrogen content and the hydrogen rim size and gap size. in addition, the effect of type and setting of boundary condition was investigated. keywords: ria, nuclear fuel performance, thermomechanical calculations. 1. introduction the simulation, calculation and study of the fuel behaviour during ria (reactivity initiated accident) is very difficult and complex. moreover, deep understanding of the ria processes requires more and more experimental studies and tests of both fresh and irradiated fuel of different types in research reactors and another research facilities. the ria belongs to dbas (design basis accidents), specifically the case of control rod ejection in pwr (pressurized water reactor). after control rod ejection, the fuel rod power increases rapidly but this increase is reduced by doppler effect immediately. thus, ria scenario is well characterized by power peak and amplitude. in general, the ria scenario can be divided into two phases. the first phase, early phase of transient or from the point of view of fuel often called pcmi (pellet-cladding mechanical interaction) phase, describes few first milliseconds of transient. the fuel power increase leads to prompt increase in fuel temperature, which further causes rapid thermal expansion of fuel. it is followed by closure of the gap in the case of fresh fuel or low burnup fuel (there is still gap between fuel and cladding before transient), because in this early phase of transient the temperature of cladding is still unchanged and relatively low (there is no or only small thermal expansion of cladding whereas the thermal expansion of pellets is due to high temperature of fuel faster and larger). on the contrary in the case of high burnup fuel, where the gap is already closed during normal operation, the rapid thermal expansion of the fuel can lead to failure of the “cold” cladding, this is called pcmi-failure. this scenario is schematically shown in figure 1 on the left. for this type of failure the long axial crack is typical, figure 2a. the second phase of the transient is characterized by cladding temperature increase and therefore it is sometimes called high temperature phase. description of fuel behaviour during this phase is similar to fuel bahaviour during loca (loss of coolant accident). due to temperature increase the fgr (fission gas release) increases and this causes the increase of rod inner pressure. in some region the instability criterion can be exceeded which lead to cladding ballooning and the burst can follow. this scenario is shown in the middle in figure 1. for the burst it is typical a short axial crack which can be seen in figure 2b. another failure mode is characterized by radial crack or disruption of the cladding, it is shown in figure 2c. if the energy received by fuel during transient is too high, it can lead to partially melting of fuel and also of cladding. this type of crack is also typical for the post transient failure caused by thermal shock after re-wetting. currently, there is an ongoing programme called hera with the goal of understanding lwr (light water reactor) fuel performance at high burnup under ria condition. another partial goals are to quantify the impact of pulse width on fuel performance, obtain new data on high burnup fuel under pulse conditions, quantify the additional margin provided by modern cladding alloys to pcmi failure limits and offer improved data for modelers using specially designed tests that eliminate key uncertainties in high-burnup fuel tests [2]. the facilities, that house hera programme, are treat (transient reactor test facility) reactor and hot cells in idaho (usa) [3, 4] and nsrr (nuclear 5 https://doi.org/10.14311/app.2023.44.0005 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en tereza kinkorová acta polytechnica ctu proceedings figure 1. schematic drawing of fuel rod processes during ria [1]. figure 2. schematic drawing of three possibilities of failure [1]. safety research reactor) reactor in japan [5]. except of the experimental part, there is also modelling and simulation task included in hera programme. in this part many organisations with many codes are involved. the description of the calculation provided in fraptran thermomechanical code at the ctu (czech technical university) is presented in the following text. 2. modelling and simulation the parameters and the geometry of the calculated cases were given by the hera programme leaders. the simulated geometry copies the geometry and ambient conditions of both experimental facilities (treat and nsrr) as close as possible. then there was built the calculation matrix, which should show the effects of chosen parameters (pulse width, hydrogen content, enthalpy increase and gap size) of the fuel performance during transient conditions and which is described later in this chapter. 2.1. geometry of the design experiments as mentioned above, the geometry in a simulation was dimensionally identical to an experimental one. the specimen parameter treat nsrr fuel fresh uo2 cladding zry-4 (sra) fuel pellet height [mm] 10.160 number of pellets 10 12 total fuel mass [g] 57.9 69.4 cladding inner radius [mm] 4.1785 cladding outer radius [mm] 4.7500 plenum pressure [mpa] 0.1 rod free volume [cm3] 1.23 2.52 water temperature [°c] 20 capsule pressure [mpa] 0.1 table 1. specimen and setup parameters for both treat and nsrr facilities. test specimen was based on a fresh 17 × 17 pwr type fuel design in all cases. the zry-4 cladding and uo2 fuel, typical for lwr were used. during the experiments the fuel was surrounded by stagnant water at room temperature and under normal pressure. in general, there were two types of specimens, because there were two types of experimental facilities (treat and nsrr). the exact parameters and differences between this two experimental setups are provided in table 1 [6]. the figure 3 shows the simplified schema of the capsule. the rodlet is placed in the centre of the capsule, the capsule is partially filled with water and the rest of the capsule volume is filled with argon. the specification of the capsule parameters for both treat and nsrr capsule are in table 2. these parameters represent basic hydraulic information. in general, there are two basic possibilities how the boundary (hydraulic) conditions can be solved in thermomechanical calculations. firstly, the cooling can be solved by system code and results act as a boundary condition in subsequent thermomechanical code. and the second 6 vol. 44/2023 thermomechanical calculation within the hera programme figure 3. schematic drawing of th capsule of experimental setups [6]. capsule parameter treat nsrr water outer radius [mm] 25 120 water volume [cm3] 280 6250 argon free volume [cm3] 500 2350 table 2. the specification of the capsule parameters – treat and nsrr facilities [6]. possibility is to have thermomechanical code which can solve thermohydraulic issues. 2.2. matrix of the calculations the simulations were focused on pcmi and the matrix of the calculation was designed to cover several effects (pulse width, enthalpy increase, hydrogen concentration and gap size). as mentioned before, the cladding can fail due to pcmi and one of the most important aspects of the cladding pcmi failure is the value of hydrogen concentration in cladding material. the higher hydrogen content has a negative effect of cladding failure resistance, especially during pcmi. the cracks start to form in the brittle hydride rim and then propagate through the metallic substrate. it is obvious that the forming of hydride rim and the width of hydride rim grows with the increasing hydrogen content in cladding. temperature of cladding is another very important parameter which influences fuel behaviour during pcmi. thermal expansion and creep rate of cladding increases with temperature and thus the load of the cladding from fuel pellet during pcmi is smaller at higher temperatures. the temperature of the cladding during early phase of ria can be affected by pulse width in a way that wider pulses lead to high temperature of cladding. therefore, the probability of failure during pcmi is higher when the pulse is narrower with the assumption of the same energy. of course, gap size is another parameter affecting figure 4. power distributions for four pulse widths [6]. the probability of pcmi failure. in the case of smaller gap, there is small space where the pellet can expand due to thermal expansion and the loading of the cladding increases. on the other hand, the heat transfer decreases with increase of gap size. last mentioned parameter affecting the pcmi phenomena is enthalpy increase or inserted energy. if the enthalpy increase, the cladding vulnerability to failure during pcmi or whole ria transient gets bigger. the above mentioned phenomena are the most important effects influencing pcmi or early phase of ria. the matrix of calculations considers all these effects and includes 14 sets of parameters. the matrix can be seen in table 3. there are two different gap size values: 18 µm and 48 µm, three hydrogen content values: 400 ppm (to which hydride rim thickness of 80 µm corresponds), 200 ppm (40 µm) and 600 ppm (140 µm), three enthalpy increase values: 650 j/g, 550 j/g and 750 j/g and four pulse width at fwhm (full width half maximum) values: 7.5 ms, 90 ms, 50 ms and 300 ms, where pulse width value of 7.5 ms corresponds to the test facility in nsrr and the other values of pulse width correspond to the facility in treat. the linear heat generation rate (lhgr) versus time for four pulse width variations are shown in figure 4. the pure gaussian temporal profile is assumed for power input. 2.3. fraptran fraptran (the fuel rod analysis program transient) is thermomechanical fortran language computer code. fraptran models and calculations are focused on fuel behaviour in lwr, especially during transients and accidents such as loca and ria. in general, fraptran computes the temperature and deformation dependence on time of the fuel rod. fraptran can be used alone or with the initialization from the frapcon (steady state fuel performance code). the main phenomena simulated by fraptran are heat conduction in fuel rod and heat transfer from cladding to coolant, elastic-plastic deformation of fuel and cladding, fgr, fuel rod gas pressure and cladding oxidation. the input parameters are time-dependent fuel rod power and coolant boundary conditions with several 7 tereza kinkorová acta polytechnica ctu proceedings case facility pulse width at fwhm [ms] peak radial average enthalpy increase [j/g] hydrogen content/rim thickness [ppm/µm] fuel outer radius [mm] 1 nsrr 7.5 650 400/80 4.1605 2 nsrr 7.5 650 200/40 4.1605 3 nsrr 7.5 650 600/140 4.1605 4 nsrr 7.5 550 400/80 4.1605 5 nsrr 7.5 750 400/80 4.1605 13 nsrr 7.5 650 400/80 4.1305 6 treat 90 650 400/80 4.1605 7 treat 90 650 200/40 4.1605 8 treat 90 650 600/140 4.1605 9 treat 90 550 400/80 4.1605 10 treat 90 750 400/80 4.1605 14 treat 90 650 400/80 4.1305 11 treat 50 650 400/80 4.1605 12 treat 300 650 400/80 4.1605 table 3. case matrix ways how to calculate and simulate boundary conditions. the first one is labelled as “coolant” option in input file. if the user chooses this option, the boundary conditions are calculated by fraptran, where nucleate boiling heat transfer, critical heat flux (chf) and post-chf heat transfer correlations are included in thermohydraulic model. in this case the coolant pressure, temperature and mass flux have to be entered by user to the input file and the geometry of the coolant channel has to be specified. user can choose from different nucleate boiling heat transfer, chf and post-chf heat transfer correlations but the default setting is following: dittus-boelter correlation for nucleate boiling heat transfer, epri-1 correlation for chf and modified tong-young and groeneveld 5.9 correlations for transition and film boiling. the second way how to obtain boundary conditions is called “heat” option in the input file. this way is mainly used in the case that the cladding temperature is known, i.e., the temperature is measured by thermocouples during experiment or the temperature history is calculated by specialized thermohydraulic code (mainly system codes) [7]. 3. discussion and results there are several parameters which were observed to compare results of all cases, the most important are failure prediction (from the point of view of all observed effects), cladding outer surface temperature (comparison of used thermohydraulic approach) and stress intensity factor, which was defined by the following relation: sif = σθ · √ πa. (1) where a is the crack length which was assumed to be equal to the thickness of the hydride rim, and σθ is hoop stress. this parameter takes stress field in the cladding and hydride rim into account. there were far more parameters (fuel centreline temperature, hoop and axial strain, elongation, inner rod pressure, . . . ) monitored but only some will be evaluated here. the rod power histories, which represent input parameters, were design in order to the centre of power peak was at the time of 1 second. and the whole simulation was 200 seconds long. 3.1. boundary conditions setting effect as was explained above there are mainly two possibilities how to obtain boundary conditions, which means cladding outer temperature in this case. first way is using boundary conditions calculated by system code relap [8] by the leaders of the hera programme. this data should be relatively accurate because it was calculated by specialized thermohydraulic code. the second way how to calculate boundary condition is using fraptran thermohydraulic model. the comparison of cladding outer temperature calculated with two different approach is shown in figure 5 for the cases 1, 6, 11 a 12. it is shown that the difference between these two calculations is enormous. low cladding temperature and fast decrease of temperature in the calculation by fraptran is caused by inadequate thermohydraulic model for this type of cooling and experimental setup. there is a brief moment when both calculation approaches result in similar temperature values (especially in the cases with narrower power peaks) and it is time interval from the start of calculation (t = 0 seconds) to the time of about 1.02 seconds. in this interval the differences between both approaches are negligible. this observation implies that when the target of interest is pcmi phenomena (these phenomena occur in the early stage of transient) both approaches are inter8 vol. 44/2023 thermomechanical calculation within the hera programme figure 5. cladding outer temperature versus time in cases 1, 6, 11 a 12 and comparison two ways of calculating boundary conditions: full line represents relap temperature results and dash line represents results from fraptran thermohydraulic model. changeable and they should have similar results of fuel behaviour. it is also interesting to compare the results of failure prediction versus boundary condition effect. for example, the case 1, the simulation using relap boundary conditions predicted no failure, whereas the calculation using fraptran thermohydraulic model predicted failure of the rod at the time of 1.006 seconds. another case is shown in the graph in figure 5, the case 12 calculation using fraptran model predicted no failure of rod, on the other hand the simulation using relap cladding temperature predicted rod failure at time of 31 seconds with the initialization of cladding ballooning at the time of 3.4 seconds. another example how failure prediction can be influence by calculation of boundary condition is shown in the graph in figure 6. in the graph the results of the calculation of case 13 (nsrr, 7.5 ms, 650 j/g, 400 ppm, 4.1305 mm of fuel radius) is shown, again it is possible to see the difference in cladding outer temperature (shown in red) between both ways of determining boundary condition. from that difference the different failure prediction follows. the first calculation with fraptran boundary condition predicted no failure, whereas in the second calculation with relap boundary condition there was predicted rod failure due to ballooning and following burst after 23 seconds from the beginning of the calculation. the green curves in the graphs show the strain history and again big difference in both approaches is observed. 3.2. pulse width effect there were four different pulse widths in the case matrix: 7.5 ms, 50 ms, 90 ms and 300 ms. firstly, the difference in temperature history can be seen in the graphs (cladding temperature – figure 5, fuel temperature – figure 7, the results with relap boundary conditions are presented here), the first increase of temperature was in case 12, i.e. with the widest pulse and in the same case the temperature figure 6. case 13: cladding outer temperature and strain with the both types of boundary conditions – calculated by relap and by fraptran thermohydraulic model. figure 7. time dependence of fuel centre temperature for four cases with different pulse width (case 1 – 7.5 ms, case 6 – 90 ms, case 11 – 50 ms and case 12 – 300 ms). reached the highest point. the same observation was in both cladding and fuel temperature. in the graph in figure 7 a little bit different fuel temperature history can be seen, with the comparison with cladding temperature history, the fuel temperature in case 1 (narrower pulse) starts increasing as the last (from investigated cases) but then the increase is very fast and steep, thus the maximum fuel temperature in this case is reached first (from investigated cases). on the other hand, the temperature increase by the case 12 (widest pulse) is not so steep and the maximum temperature is reached later. the effect of pulse width can be seen from the results of cladding hoop stress. in the graph in figure 8 time dependence of hoop stress is shown. the stress peak slightly follows the power peak shape. for example, the stress peak in the case 1 (the narrowest and the highest power peak) is the narrowest and highest. 3.3. enthalpy increase effect the time-dependent enthalpy and the total energy deposited per unit mass of fuel is shown in the graph in figure 9 for cases 6, 9 and 10. the influence of enthalpy increase can be shown also in temperature history in the graph in figure 10. as it was expected, the highest temperature is in the case of the highest 9 tereza kinkorová acta polytechnica ctu proceedings figure 8. time dependence of cladding hoop stress for four cases with different pulse width (case 1 – 7.5 ms, case 6 – 90 ms, case 11 – 50 ms and case 12 – 300 ms). figure 9. time dependence of enthalpy increase and total energy deposited per unit mass of fuel for three cases with different enthalpy increase (case 6 – 650 j/g, case 9 – 550 j/g and case 10 – 750 j/g). enthalpy increase, i.e., case 10 with enthalpy increase of 750 j/g. the difference between these three cases can be seen also in the strain history, for example between cases 6 and 9 (650 and 550 j/g). the hoop strain in the case 6 is about twice as big as the hoop strain in the case 9, the strain history for all three cases can be seen in the graph in figure 11. the strain is then connected with the failure prediction. in the case 10 the rod failure was supposed by code, more precisely at time of 33 seconds and at time of 2.7 second the cladding ballooning initialization was predicted (the time can be seen in the graph in figure 11 it is the sharp change of the green curve, because the ballooning model was shut down thus there is no larger deformation calculated). in the other two cases (6 and 9) no rod failure was predicted. 3.4. hydrogen effect the hydrogen effect is evident, the embrittlement of cladding increases with increasing hydrogen content and following formation of hydrides. but modelling the hydrogen content and its effect on fuel behaviour is more complicated and there are only a few codes which can solve and assume the influence of hydrogen content on fuel performance. in fraptran there is figure 10. time dependence of fuel centreline temperature (full line) and cladding outer temperature (dashed line) for three cases with different enthalpy increase (case 6 – 650 j/g, case 9 – 550 j/g and case 10 – 750 j/g). figure 11. time dependence of cladding hoop strain for three cases with different enthalpy increase (case 6 – 650 j/g, case 9 – 550 j/g and case 10 – 750 j/g). possibility to set hydrogen content, but the influence on results is very limited. the input of hydrogen content is used only in the low-temperature pcmi cladding failure model and it is assumed that hydrogen content is uniformly and homogenously distributed. in other models the hydrogen content is not assumed, so the influence of hydrogen content on other parameters (stress, strain, deformation) is neglected in the code, although in the reality they are affected by hydrogen content. the hydrogen effect can be seen for example in the cases 1-3, the only difference between these cases is in the value of hydrogen content (case 1 – 400 ppm, 2 – 200 ppm and 3 – 600 ppm). the rod failure was predicted only in case 3 and the histories of all parameters were the same until rod failure in this case. due to higher hydrogen concentration in case 3 the failure strain was the lowest one and the hoop strain exceeded it, thus the rod failure at time of 1.003 seconds was predicted by code in case 3. the similar approach is in the calculation of hydride rim, there are only few codes which assume and calculate formation and effect of hydride rim. in the fraptran models the hydride rim is not calculated. in this case the effect of hydride rim can be considered only in the calculations of sif parameter 10 vol. 44/2023 thermomechanical calculation within the hera programme figure 12. time dependence of sif (stress intensity factor) for three cases with different hydrogen content and hydride rim size (case 6 – 400 ppm/80 µm, case 7 – 200 ppm/40 µm and case 8 – 600 ppm/140 µm). (equation (1)) which follow the fraptran stress calculation. sif parameter can be than used for prediction of failure if the sif exceeds some critical value. the influence of width of hydride rim on sif can be seen in graph in figure 12. 3.5. gap size effect the gap size effect of fuel behaviour during early phase of ria, especially the influence of this effect on pcmi phenomena is obvious. if the gap is narrower, there is less space for fuel thermal expansion which leads to bigger hoop stress of cladding. this can be seen from the comparison of calculations of case 1 and case 13 (using fraptran boundary conditions). only difference in these cases is in geometry of gap (fuel pellet diameter, case 1 – 8.321 mm, case 13 – 8.261 mm). during the case 1 calculation the rod failure was predicted by code, whereas in case 13 there was no predicted failure. on the other hand, the vulnerability of cladding to failure in the second high temperature phase of ria is higher if the gap is wider. with the increasing gap size fuel temperature increases and the fuel thermal expansion is bigger, this is caused by decrease of thermal conductivity. the increasing temperature leads to larger fgr which then leads to increase of rod inner pressure and increase of hoop cladding stress and strain. if strain exceed the instability criterion the rod tends to balloon and it can lead to rod burst. an example can be seen from the comparison of case 1 and 13 with use relap boundary condition. in case 13 the rod failure (due to ballooning and burst) was predicted by code, unlike case 1 where no failure was predicted. the gap size and cladding displacement history is shown in figure 13. 4. conclusions to sum up the results, 14 calculations for two types of boundary conditions (total 28) were provided. influence of four effects (pulse width, enthalpy increase, hydrogen content and gap size) on the fuel perforfigure 13. time dependence of gap size and cladding hoop displacement for four different cases (case 1 and 6 – fuel radius of 4.1605 mm and case 13 and 14 – fuel radius of 4.1305 mm). mance was investigated within the hera programme modelling and simulation task. increasing pulse width causes higher maximum cladding and fuel temperatures which lead to larger strains. on the other hand, cladding hoop stress decreases with the increasing pulse width. similar dependence can be seen in enthalpy increase effect, with the increasing energy the cladding and fuel temperature increase and hoop deformation decreases. thus, the fuel rod is more vulnerable to failure during larger enthalpy increases. also, the increasing hydrogen concentration makes the rod more vulnerable to failure and the embrittlement of cladding is bigger. this expected phenomenon can be only limitedly modelled by fraptran, because hydrogen content is considered only in low-temperature pcmi cladding failure model. the creation and size of hydride rim is not assumed in fraptran. then, the gap size plays an important role in the fuel rod performance in both improving or worsening of the fuel performance. if the gap is bigger there is more space for fuel thermal expansion and bigger margin to pcmi failure, on the other hand the bigger gap adversely impacts heat transfer and fuel temperature. and as was shown above, this phenomenon can lead to cladding failure due to ballooning and burst. in addition to hera programme the investigation of influence of boundary condition was provided. two ways how to obtain boundary conditions are described in this paper using specialized thermohydraulic code relap and using fraptran thermohydraulic model. it was shown that the difference is significant with the exception of very early stage of transient, thus the pcmi phenomena can be evaluated by using both of these approaches with similar level of accuracy. the differences can be caused by limitations of thermohydraulic model in fraptran which is intended for the cases with forced circulation in default, so the calculation with mass flux of zero is significantly unstable and so the calculation of stagnant water condition is more complicated. 11 tereza kinkorová acta polytechnica ctu proceedings the more precisely calculation of boundary conditions should be developed in the future work, for example, some analysis using different setting of thermohydraulic model in fraptran should be computed. and then there should be also focus on modelling of phenomena whose simulation was neglected or simplified here, e.g., higher hydrogen concentration in the outer periphery of cladding or implementation of another failure criterion. references [1] t. fuketa. 2.22 – transient response of lwr fuels (ria). in r. j. konings (ed.), comprehensive nuclear materials, pp. 579–593. elsevier, oxford, 2012. https: //doi.org/10.1016/b978-0-08-056033-5.00044-6 [2] high-burnup experiments in reactivity initiated accidents (hera), 2022. [3] c. l. pope, c. b. jensen, d. m. gerstner, j. r. parry. transient reactor test (treat) facility design and experiment capability. nuclear technology 205(10):1378–1386, 2019. https://doi.org/10.1080/00295450.2019.1599615 [4] b. heath, c. jensen. thermal design of the treat facility. nuclear technology 206(9):1436–1448, 2020. https://doi.org/10.1080/00295450.2020.1725370 [5] t. mihara, y. udagawa, m. amaya. experimental capability of nuclear safety research reactor (nsrr), 2017. [6] c. jensen, d. kamerman, c. folsom, s. seo. hera m & s exercise problem description report, 2023. [7] k. geelhood, w. g. luscher, j. m. cuta, i. e. porter. fraptran-2.0: a computer code for the transient analysis of oxide fuel rods. tech. rep., pacific northwest national laboratory, 2016. [8] relap5/mod3 code manual: user’s guide and input requirements, 1995. 12 https://doi.org/10.1016/b978-0-08-056033-5.00044-6 https://doi.org/10.1016/b978-0-08-056033-5.00044-6 https://doi.org/10.1080/00295450.2019.1599615 https://doi.org/10.1080/00295450.2020.1725370 acta polytechnica ctu proceedings 44:5–12, 2023 1 introduction 2 modelling and simulation 2.1 geometry of the design experiments 2.2 matrix of the calculations 2.3 fraptran 3 discussion and results 3.1 boundary conditions setting effect 3.2 pulse width effect 3.3 enthalpy increase effect 3.4 hydrogen effect 3.5 gap size effect 4 conclusions references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2023.40.0093 acta polytechnica ctu proceedings 40:93–97, 2023 © 2023 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague application of cold atmospheric plasma for mold inactivation petra ticháa,∗, mária domonkosa, zuzana rácováb, pavel demoa a czech technical university in prague, faculty of civil engineering, department of physics, thákurova 7, 166 29 prague 6, czech republic b czech technical university in prague, faculty of civil engineering, department of architectural engineering, thákurova 7, 166 29 prague 6, czech republic ∗ corresponding author: petra.ticha@fsv.cvut.cz abstract. mold growth in indoor interiors is an increasing problem that has adverse effects on both occupants and building materials. the aim of this study was to present the new environmentally friendly method (cold atmospheric plasma treatment) for decontamination of indoor environments contaminated by mold spores, with the potential to become an effective replacement for conventional methods, such as ultraviolet-c (uv-c) germicidal irradiation. the spores of aspergillus species on malt extract agar plates were exposed to plasma produced by diffuse coplanar surface barrier discharge. the surface coverage of mold was evaluated using image analysis. cold plasma treatment inactivated spores on the surface of the culture medium in 0.5 min, whereas a weak growth delay was observed after 30 minute exposure to uv-c irradiation. this study demonstrated that cold atmospheric plasma is a more effective method for reducing mold spores on agar compared with germicidal uv-c irradiation. keywords: cold atmospheric plasma, mold, spore, inactivation, mold growth curve. 1. introduction people spend most of their time in buildings. the presence of mold contamination in indoor environments has adverse health effects as a result of inhaling mold spores and microbial volatile organic compounds produced from mold metabolism. the most common health risks related to mold exposure are eye, skin, and respiratory tract irritation and also activation of immune responses through the inhalation of airborne mold spores. their concentrated presence in the indoor environment can cause degradation of the building material and changes in the visual appearance of the surfaces [1]. the major factors affecting indoor mold growth are temperature (between 15 and 30 °c), moisture, and nutrients. cladosporium, aspergillus, penicillium, and alternaria are identified as the predominant mold species in indoor environments [2]. one of the effective ways to manage mold in a building is to eliminate or limit mold spores and their growth. for microbiological decontamination/inactivation, many methods (mechanical, chemical and physical) and technologies (e.g., uvgi technology, hepa filtration technology) are used. they are mostly based on the application of chemical substances that are often toxic [3]. furthermore, some spores are highly resistant to physical and chemical agents. in recent years, cold (non-thermal) plasma has emerged as effective sporicide against a wide spectrum of bacterial and fungal spores [4, 5]. cold atmospheric plasma (cap) is widely used in various applications such as surface modification, inactivation of pathogens, degradation of toxins, water purification, etc. [6, 7]. cold atmospheric plasma is known for its decontamination and disinfection effects against bacteria, viruses and fungi [4]. several studies have reported that cap can effectively inactivate microorganisms in the food industry, medicine (e.g., prevention of food spoilage, foodborne diseases, sterilization of medical devices) [8]. cap also seems to represent a possible alternative method to standard techniques for the inactivation of mold spores, bacteria, viruses, mycotoxins and fungi in the indoor environment or the mold growth on the surfaces of building materials. cap is an ionized gas that contains a wide range of energetic species, such as reactive oxygen and nitrogen species (rons), free electrons, neutral molecules, and ultraviolet radiation. cold atmospheric plasma can be generated from air at atmospheric pressure. atmospheric plasma equipment is significantly cheaper compared to low pressure plasma system because it does not require expensive vacuum chambers and pumps. plasma technology does not use toxic chemicals. these make plasma treatment very fast, economically advantageous and environmentally friendly [8]. the discharge methods for generating cap mainly include, plasma jets, dielectric barrier discharge, floating-electrode dielectric barrier discharge and corona discharge [8]. the effectiveness of cap in inactivating spores/microorganisms is affected by various factors such as plasma reactor type, device setup, gas composition, operating conditions (e.g., input power) and treatment duration. ultraviolet-c (uv-c) light is a short wavelength (200–280 nm) radiation and is primarily used for the 93 https://doi.org/10.14311/app.2023.40.0093 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en p. tichá, m. domonkos, z. rácová, p. demo acta polytechnica ctu proceedings figure 1. photo of plasma treatment of aspergillus brasiliensis on malt extract agar plates using diffuse coplanar surface barrier discharge (dcsbd) in ambient air. disinfection of air and surfaces from microbial contaminants. in this study, the efficacy of mold spore inactivation by uv-c irradiation and cold atmospheric plasma is demonstrated. 2. materials and methods 2.1. mold and growth conditions the mold strain used in this study was aspergillus brasiliensis ccm8222 (atcc 16404). a volume of 0.1 ml of spore suspension (26 micrormycetes/0.1 ml) was inoculated onto one surface of each test specimen using a pipette. the aim of pipetting the suspension on the surface was to inoculate the spore suspension in the center of the petri dishes (diameter 90 mm). in an effort to minimize contamination with spores and dust, the inoculation was conducted in a laminar airflow cabinet and the test specimens were manipulated with gloves. after inoculation, the samples were incubated for 14 days under constant conditions at 23 °c and 86.7 ±1.7 % relative humidity. 2.2. mold inactivation the inoculated samples were treated in a diffuse coplanar surface barrier discharge (dcsbd) developed by robust plasma systems (rps400; roplass, czech republic). the dcsbd consists of a large number of microdischarges and generates a plasma layer with approximately 0.3 mm thickness on the surface of the planar electrode. the area of the planar electrode connected to the hv power supply is 80 × 200 mm. the frequency of supply voltage is usually in the range of 14–18 khz and the corresponding discharge input power ranges from 80 w up to 400 w. the specific property of the dcsbd plasma is the high plasma power density of 100 w · cm−3 and the homogeneity of the dcsbd plasma increasing with the discharge power density. the dcsbd plasma source was applied to the surface of samples with spores for an exposure time ranging from 0.5–10 minutes with a power of 300 w, and to the surface of samples with young mycelium (treated 72 hours after inoculation) for an exposure time of 10 min with a power of 300 w. all figure 2. image analysis of aspergillus brasiliensis on malt extract agar plates using fiji software. samples were treated homogeneously. the plasma treatment of the sample is shown in figure 1. ultraviolet (uv-c) irradiation was performed using a germicidal uv-c lamp (lb 301.2, 2 × 30 w, tubes length 920 mm, wavelength 253.7 nm). the intensity was approximately 450 µw/cm2 across the exposure area. all samples in the uv-c irradiation group were exposed for 30 min and the distance between the lamp and the samples was 30 cm. 2.3. mold growth assessment the mold coverage on each of the plates was determined by image analysis using the open-source fiji software. a digital photograph of the samples was taken using a canon g1x camera. an iso speed (camera sensor’s sensitivity to light), a shutter speed, and an aperture were set to 800, 1/100 s, and f/5.6, respectively. all pictures of the petri dishes were taken under the same lighting conditions to avoid reflections from the petri dishes surfaces. a per-image thresholding method was used to detect the mold growth from its background (growth media) and resulted in a more accurate measurement of mold area coverage (figure 2). imagej is a useful tool for evaluating mold growth on malt extract agar plates. image analysis provides an objective evaluation and improves the comparability of results. the accuracy of the image analysis was increased by using the same lighting conditions. mold growth on the inoculated surface of each test sample was evaluated once a day for 14 days. the percentage of mold coverage c was calculated according to equation (1): c = am a · 100%, (1) where am is the area of the plate surface that was covered by mold, a is the total area of the petri dish. 94 vol. 40/2023 application of cap for mold inactivation figure 3. mold growth of aspergillus brasiliensis on malt extract agar plates. representative pictures were taken 3, 5, 7 and 9 days after inoculation: (a) control sample, (b) uv-c treated sample (exposure time 30 minutes, treated 24 hours after inoculation), (c) plasma treated sample (300 w, exposure time 0.5, 1, 5 or 10 minutes, treated 24 hours after inoculation), (d) plasma treated sample (300 w, exposure time 10 minutes, treated 72 hours after inoculation). 3. results and discussion using aspergillus brasiliensis spores as a target organism, a comparison of low temperature-based decontamination techniques is reported. the inactivation of a. brasiliensis was performed using cap and uv-c irradiation and was determined by image analysis of the mold growth area in petri dishes. there were six treatment groups and one control group (figure 3). in the case of control group, a. brasiliensis grew rapidly after inoculation and the mold completely covered the petri dishes after 12 days. the mold growth was noticeable 2 days after inoculation, the molds grew from the center to the outer edge and formed a circular colony with irregular edges (mold spores were inoculated at the center of the nutrient). the control sample (figure 3) shows the mold growth phases. initial spore inoculation is followed by lag phase (spore adhesion), spore germination, hyphae growth and reproduction. during the lag phase, the spores remain inactive until they have absorbed sufficient moisture and nutrients from the substrate. hyphae are filaments that appear immediately after germination, and as they thicken, they form a mass called mycelium. colonies of a. brasiliensis were initially yellowish and then dark brown to black. in figure 3, morphological differences were observed between the control and cap treated samples, such as color changes and colony growth arrest. cold plasma treatment of the samples (72 hours after inoculation) for 10 min with a power of 300 w completely inhibited spore germination and hyphal growth of a. brasiliensis. as demonstrated in figure 3, 30-minute uv-c treatment, slightly prevented the colonization of a. brasiliensis on samples. in inactivation spore experiments (24 hours after inoculation), plasma treatments were performed for 0.5, 1, 5 and 10 minutes at 300 w. plasma treated samples did not show mold growth during 14 days. the biological effects of plasma treatment have been demonstrated in several studies [4, 8, 9]. several important mechanisms occur in the process of plasma inactivation, such as chemical reactions with atomic oxygen, and uv-induced damage. the microbial inactivation of plasma is mainly attributed to rons. rons include o3 and h2o2, which are powerful oxidants and have similar inactivation mechanisms. these reactive species inactivate molds by progressive oxidative damage to their cell walls and membranes. uv radiation, which is also produced during the formation of cap, is not considered to be a dominant factor in the inactivation process. the results of mold growth image analysis during 14 days of incubation are depicted in figure 4. all data are presented as the mean ± standard deviation. 95 p. tichá, m. domonkos, z. rácová, p. demo acta polytechnica ctu proceedings figure 4. mold growth curves of aspergillus brasiliensis on malt extract agar plates. the mold in the control samples grew according to the typical s-shaped curve (logistic growth). the lag phase lasted two days. thus, the measurements started on the second day and continued until the 14th day. the exponential growth went up to the 7th day (inflection point) and it was possible to observe a color change, to a black tone in the center of colonies. compared to the control group, mold growth was negligibly suppressed by the uv-c germicidal lamp. as shown in figure 4, the coverage area of aspergillus brasiliensis treated with uv-c irradiation was slightly smaller than that of the control group at the same time. spore color can play an important role in inactivation of mold spores by uv-c irradiation. the aspergillus brasiliensis spores are more resistant, in that the pigment of the a. brasiliensis spores absorbs more in the uv-c region and thus protects the spores from uv irradiation [10]. the growth curve results also clearly indicate that the 30-second cap treatment (24 hours after inoculation) was sufficient to completely inactivate the a. brasiliensis spores and prevent mold growth in all samples. the effect of cap treatment on permanent inactivation of a. brasiliensis spores was also evaluated for longer exposure times (1–10 min). based on the above results, plasma treatment is considerably more effective than a conventional uv-c irradiation. cap treatment achieved a high level of inactivation level within a short time. 4. conclusion in this study, it can be concluded that the cold atmospheric pressure plasma has potential sporicidal activity against molds and could be a promising tool for the inactivation of spores in indoor environments or on the surface of building materials. the cold atmospheric plasma treatment was compared with a conventional germicidal uv-c lamp, which is commonly used to reduce mold spores in indoor environments. the results of image analysis of mold growth on malt extract agar plates treated with germicidal uv-c lamp showed a non-significant delay in mold growth dynamics. the appearance of the first mold colonies exposed to uv-c irradiation was delayed by only 24 hours compared to the control samples. furthermore, 0.5 min of cap treatment (24 h after inoculation) was shown to lead to inactivation of a. brasiliensis spores. the inactivation of young mycelium (hyphae) was also tested by cold atmospheric pressure plasma treatment. images of samples with young mycelium exposed to plasma produced by diffuse coplanar surface barrier discharge for 10 minutes at 300 w (72 hours after inoculation) show an obvious change in mold color and an arrest of growth. compared to control samples, it was observed that cold atmospheric pressure plasma treatment caused inactivation of spores and young mycelium growth. the mode of cap action on microorganisms is still the subject of extensive studies, but it is believed that biologically reactive substances generated by plasma (rons, uv radiation, and charged particles) act cooperatively to inactivate microorganisms. however, more studies are needed to understand in detail the mechanism of spore inactivation and young mycelium growth by cold atmospheric plasma. 96 vol. 40/2023 application of cap for mold inactivation acknowledgements this work was supported by the czech science foundation (no. ga22-06621s). references [1] p. ryparová, r. wasserbauer, z. rácová. the cause of occurrence of microorganisms in civil engineering and the dangers associated with their growth. procedia engineering 151:300–305, 2016. https://doi.org/10.1016/j.proeng.2016.07.402 [2] c. du, b. li, w. yu. indoor mould exposure: characteristics, influences and corresponding associations with built environment—a review. journal of building engineering 35:101983, 2021. https://doi.org/10.1016/j.jobe.2020.101983 [3] a. m. elkhatat. recent trends of microbial decontamination for occupational, industrial and domestic applications. bulletin of the national research centre 46(1):75, 2022. https://doi.org/10.1186/s42269-022-00761-9 [4] v. scholtz, j. pazlarová, h. soušková, et al. nonthermal plasma — a tool for decontamination and disinfection. biotechnology advances 33(6):1108–1119, 2015. https: //doi.org/10.1016/j.biotechadv.2015.01.002 [5] b. g. dasan. decontamination of aspergillus flavus and aspergillus parasiticus spores on hazelnuts via atmospheric pressure fluidized bed plasma reactor. international journal of food microbiology 216:50–59, 2022. https: //doi.org/10.1016/j.ijfoodmicro.2015.09.006 [6] m. domonkos, p. tichá. wettability of siox nanofibrous mats and their modification using cold atmospheric plasma treatment. acta physica polonica a 140(2):181–185, 2021. https://doi.org/10.12693/aphyspola.140.181 [7] j. trejbal, v. nežerka, m. somr, et al. deterioration of bonding capacity of plasma-treated polymer fiber reinforcement. cement and concrete composites 89:205–215, 2018. https: //doi.org/10.1016/j.cemconcomp.2018.03.010 [8] m. domonkos, p. tichá, j. trejbal, p. demo. applications of cold atmospheric pressure plasma technology in medicine, agriculture and food industry. applied sciences 11(11):4809, 2021. https://doi.org/10.3390/app11114809 [9] e. lokajová, j. julák, j. khun, et al. inactivation of dermatophytes causing onychomycosis using non-thermal plasma as a prerequisite for therapy. journal of fungi 7(9):715, 2021. https://doi.org/10.3390/jof7090715 [10] v. m. gómez-lópez, p. ragaert, j. debevere, f. devlieghere. pulsed light for food decontamination: a review. cement and concrete composites 18(9):464–473, 2007. https://doi.org/10.1016/j.tifs.2007.03.010 97 https://doi.org/10.1016/j.proeng.2016.07.402 https://doi.org/10.1016/j.jobe.2020.101983 https://doi.org/10.1186/s42269-022-00761-9 https://doi.org/10.1016/j.biotechadv.2015.01.002 https://doi.org/10.1016/j.biotechadv.2015.01.002 https://doi.org/10.1016/j.ijfoodmicro.2015.09.006 https://doi.org/10.1016/j.ijfoodmicro.2015.09.006 https://doi.org/10.12693/aphyspola.140.181 https://doi.org/10.1016/j.cemconcomp.2018.03.010 https://doi.org/10.1016/j.cemconcomp.2018.03.010 https://doi.org/10.3390/app11114809 https://doi.org/10.3390/jof7090715 https://doi.org/10.1016/j.tifs.2007.03.010 acta polytechnica ctu proceedings 40:93–97, 2023 1 introduction 2 materials and methods 2.1 mold and growth conditions 2.2 mold inactivation 2.3 mold growth assessment 3 results and discussion 4 conclusion acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2025.53.0092 acta polytechnica ctu proceedings 53:92–96, 2025 © 2025 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague bonding of timber and high-performance concrete tomáš vlach a,b,∗, nick vanheeswijckc, jan macháčeka, eliška kafkováa, věra kabíčkováa, jakub hájeka a czech technical university in prague, faculty of civil engineering, thákurova 7, 166 29 prague 6, czech republic b czech technical university in prague, university centre for energy efficient buildings, třinecká 1024, 273 43 buštěhrad, czech republic c katholieke universiteit leuven – hasselt university, faculty of civil engineering technology, oude markt 13, 3000 leuven, belgium ∗ corresponding author: tomas.vlach@cvut.cz abstract. timber-concrete composite systems have emerged as a promising building technique, leveraging the strengths of both materials to improve load capacity, stiffness, and overall performance. in the presented study, high-performance concrete with perfect mechanical performance and durability is used in timber-concrete composite systems to further reduce the environmental footprint and optimize structural efficiency. the weakest point in general of these constructions is the interface between the concrete material and the wood. the focus in this study is on shear strength, specifically examining the combination of adhesive bonding with notch shear connections. experimental results, by a push-off test, reveal that the inclusion of shear connectors is essential for effective and secure bonding, with adhesive application methods significantly influencing shear strength. the findings highlight the potential of timber-concrete composite systems high-performance concrete to achieve high shear strength and structural integrity through optimized adhesive and rib configurations. the goal is to make maximum use of the material’s mechanical potential and significantly reduce the primary sources of raw materials. the presented article is based on the diploma thesis of nick vanheeswijck. keywords: high-performance concrete, timber concrete bonding, timber concrete contact, push-of test. 1. introduction the predominant use of concrete as a primary building material has long been established thanks to the variability of shapes, simple technology, and favourable price. in the construction sector, it is common to use solid concrete slabs. however, inherent limitations, such as low tensile capacity, require the incorporation of steel and other reinforcements into the tension zone. tensile cracking in concrete slabs not only compromises structural integrity but also exposes steel reinforcement to moisture, contributing to corrosion and subsequent spalling [1] and contributes significantly to carbon emissions [2]. timber floors offer a sustainable and environmentally friendly building option due to timber’s recyclability, reusability, and sustainability. but also timber floors may face challenges such as excessive vibrations [3], fire resistance, and acoustic problems. in response to these challenges, there has been a growing interest in alternative building solutions that prioritise ecological sustainability in combination with structural efficiency. one such solution is the integration of timber into traditional concrete structures, forming timber-concrete composite (tcc) systems. tcc systems capitalize on the properties of timber and concrete, offering a more efficient structural solution compared to conventional concrete-only or timber-only structures. the composite action of tcc systems distributes forces such that concrete experiences predominantly compression stresses while timber bears tension stresses, optimizing the use of each material’s inherent strengths [4]. in pursuit of further optimizing the ecological footprint of timber-concrete composite (tcc) systems, the integration of high-performance concrete (hpc) presents a compelling avenue [5]. this paper focusses on investigating the behaviour of tcc systems, particularly the efficiency of timber-concrete shear connections. although various types of connectors have been proposed in the past, including mechanical fasteners, notch connections, and adhesives, the emphasis will be on exploring a combination of adhesive bonding with notch shear connections. such a joint aims to achieve high strength against shear forces while minimizing slippage, thereby maximizing the composite effect and structural performance of tcc systems [6, 7]. 2. materials used for experiment a recipe for high-performance concrete commonly used in the laboratory environment was chosen as a reference mixture. it was developed by the faculty of civil engineering, czech technical university in prague utilizing predominantly local raw ma92 https://doi.org/10.14311/app.2025.53.0092 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 53/2025 bonding of timber and high-performance concrete figure 1. specimen dimension, a view of preparing the timber base with selected notches and epoxy resin, a view of the samples after the concreting. terials. hpc mixture design, characterized by its self-compacting fine-grained nature [7], is detailed presented in table 1. the mixture is prepared by initially combining two types of technical quartz sand, silica flour and, with silica fume. the maximum grain size was 1.2 mm. it was without any types of dispersed fibres. the water cement ratio was 0.25 and the water binder ratio was 0.20 for this mixture. the compressive strength tested on cubes with an edge length of 100 mm was equal to 135 mpa according to the standard čsn en 12390-3. the flexural strength tested on prisms 40 × 40 × 160 mm was equal to 11.5 mpa according to standard csn en 12390-5. the same hpc recipe has been also used for several applications and research activities at the ctu like waffle and solid experimental facade elements [8–10]. mix content kg m−3 cement i 42.5r 680 technical silica sand 960 silica flour (ground quartz) 325 silica fume (microsilica) 175 superplasticizers 29 water 171 total 2 230 table 1. recipe of high-performance concrete mixture. in the presented study, two types of commonly used wood were utilized: glulam and softwood (spruce). the required dimensions of the cross-sectional area for the testing were selected 60 × 60 mm, even considering the possibilities of hpc. as the commonly available glulam slabs had a thickness of 30 mm, so two glulam slabs were easily connected using adhesive from epoxy resin to meet the selected dimensions. previous experience confirmed that the bond between the slabs was sufficiently strong to withstand testing without any issues [11, 12]. the solid softwood spruce pieces were used directly in the appropriate dimensions [7]. basic table values of the mechanical parameters of the wood used are presented in the table 2. timber glulam softwood (gl22c) (c22) compressive strength [n mm2] 20 20 bending strength [n mm2] 22 22 shear strength [n mm2] 3.5 3.8 density [kg m−3] 390 410 table 2. basic mechanical parameters of glulam ang softwood. the adhesive used in this research to create a bridge in the interface between concrete and wood materials was selected epoxy resin sikafloor 150/280 based on the previous successful experience with this epoxy resin in similar research projects [7, 8, 13]. the basic material parameters of the epoxy resin are the flexural strength of 15 mpa and the modulus of elasticity of 2.0 gpa. the specific gravity of the resin is 1 100 kg m−3 according to the technical data sheet of the sika company. 3. specimen design and preparation in contrast to the previously mentioned “dry” and “wet” methods [14], this research adopts a third new approach, where concreting is carried out as a last step of specimen preparation. this technology aims at simple use in practice, for example, for the realization of composite elements like load bearing timber concrete sandwich panels. in the presented article two basic types of wood were utilized, glulam and softwood (spruce). the dimensions of the composites were derived from previous similar research created at the department involved push-off tests [11] and the scheme is presented in figure 1. the concrete layers are designed outside and the wooden part inside. the 93 t. vlach, n. vanheeswijck, j. macháček et al. acta polytechnica ctu proceedings variant description code adhesive ribs glulam, no adhesive, no ribs glu 00 – – glulam, adhesive 1x, no ribs glu 10 1x – glulam, adhesive 2x, no ribs glu 20 2x – glulam, adhesive 1x, small ribs 12 mm glu 11 1x small 12 mm glulam, adhesive 1x, large ribs 24 mm glu 12 1x large 24 mm softwood, adhesive 1x, no ribs sof 10 1x – softwood, adhesive 1x, small ribs 12 mm sof 11 1x small 12 mm table 3. all tested variants of glulam and softwood. figure 2. comparison of results presented in the form of a graph, influence of method of resin application (left) and influence of prepared ribs on the shear strength (right). designed cross-section for testing was 60 × 60 mm. as the available glulam had a thickness of 30 mm, two glulam layers were first bonded together using epoxy resin to meet the required dimensions. previous experience confirmed that the bond between the slabs was sufficiently strong to withstand the tests without any issues [11]. the solid softwood pieces were used directly in the appropriate dimensions [7]. in table 3 are presented all tested variants including also different notches to see their influence in effort to achieve maximum mechanical performance. a total of three specimens were always created for each mentioned variant. the first step was the cutting of wood to the required dimensions and creation of selected variants of notches. notches were made of two kinds, always in the transverse direction to create small ribs with width 12 mm and large ribs with width 24 mm. next, an epoxy resin layer was applied to the wood surface, technical quartz sand with a maximum grain size of 1.2 mm was scattered to create a textured surface conducive to concrete bonding and finally fresh concrete was pour after epoxy resin hardening. to prepare these composite specimens, mould was needed to pour the concrete after the wood specimens were modified using the adhesive layer [7]. a view of the specimens in the mold after the concreting is also presented in figure 1. 4. experiment and results the focus of this article is to evaluate the shear strength in a timber-concrete composite with different modifications. the shear strength can be easily calculated by performing a push-off test. during a push-off test, force is applied through the material in the middle and derives shear stress on the interface between the wood and the concrete part. this force gradually increases until failure occurs. the maximum achieved force value is monitored and with known dimensions is later calculated as the shear stress. the loading process was performed using constant speed of loading 1.0 mm min−1. the experiment was created on a hydraulic press galbadiny quasar 100 with a maximum load capacity of cylinder 100 kn. due to the exceeding of maximum force, after first specimens testing was continued on a hydraulic press controls mcc8 and cylinder with maximum load capacity of 600 kn. weak mechanical performance was expected from the glu 00 group without adhesive, which was also confirmed. some samples already experienced delamination during maturation. the measured force was minimal and random. creating a bridge using epoxy resin gave significantly better results, but results were not stable. for some specimens glu 10 it was visible that the bond was not well realized depending on the amount of used resin. therefore, the glu 20 group was created, where the first layer closed the structure of the wood after the hardening and the 94 vol. 53/2025 bonding of timber and high-performance concrete figure 3. comparison of results for the softwood – influence of ribs on the shear strength (left) and view of the sample sof 11 before (middle) and after the loading test (right). second layer served as an adhesive bridge fixing sand. failure occurred only in the wood and the results of shear strength were the best achieved. the maximum shear strength was calculated at 4.92 mpa as visible in figure 2 left. this underscores the importance of the technological process of applying glue. the epoxy resin in general is poorly resistant to the effects of higher temperatures, so the effect of the ribs was also tested. specimen glu 11 featured small ribs and adhesive. with the additional ribs enhancing its shear strength and reducing the spread of results. the failure consistently occurred in the concrete at the height of the ribs. it achieved a maximum shear strength of 1.81 mpa. the alternative variant glu 12 was more effective and demonstrated a significant improvement. it becomes evident that ribs with width 24 mm and depth 10 mm mitigate failure occurred at the height of the ribs (in concrete part). here, the failure was simultaneously in the concrete and in the timber. the testing yielded a maximum shear strength of 3.13 mpa as presented in figure 2 right. softwood has similar mechanical parameters in comparison with glulam, therefore, the number of samples was more limited. only adhesive was used as a shear connector for sof 10, resulting again in very varied results compared to the one with ribs. the maximum force observed was 74 kn with the correct failure in the simultaneously in the wood and concrete, while other specimens reached only 20 kn with collapse in contact area. this results in an average shear strength of 2.06 mpa. the small ribs with the softwood sof 11 produced very strongest results, reaching a shear strength of 4.24 mpa. the failure occurred mostly in the concrete at the height of the ribs. the results in the form of graph and picture of failure specimens with the softwood are presented in figure 3 [7]. 5. conclusions the experimental results reveal critical insights into the bonding performance of various wood-concrete composite specimens. the initial glulam specimen, without adhesive or ribs, showed no bonding. when only adhesive was used, the results varied, highlighting the importance of precise application. the second method of adhesive application showed a significant increase in shear strength by 301 %, demonstrating the highest performance among all specimens. moreover, this method showed reduced variance across test results, suggesting enhanced consistency in the experimental outcomes. enhanced bonding performance was observed with the addition of ribs, where the small cross-section at the height of the ribs was the main weak point. the findings demonstrate softwood’s superior performance over glulam, suggesting a stronger adhesive interaction. when considering small ribs, softwood showed a remarkable 134 % improvement over glulam. the results, which are in line with the literature, indicate that timber composite systems incorporating high-performance concrete can achieve high shear strength and overall structural performance with the use of combination adhesive and ribs. these shear connectors are crucial in maximizing the bonding efficiency and structural integrity of these composites [7]. acknowledgements the article is based on the diploma thesis of one of the co-authors. the work on this paper was supported by czech science foundation grant no. 22-14942k entitled „possibilities of using natural fibers for the production of hybrid textile reinforcement in concrete“ the authors would like to acknowledge all financial assistance provided to support this research. references [1] r. gutkowski, k. brown, a. shigidi, j. natterer. laboratory tests of composite wood–concrete beams. construction and building materials 22(6):1059–1066, 2008. https://doi.org/10.1016/j.conbuildmat.2007.03.013 95 https://doi.org/10.1016/j.conbuildmat.2007.03.013 t. vlach, n. vanheeswijck, j. macháček et al. acta polytechnica ctu proceedings [2] j. estévez-cimadevila, e. martín-gutiérrez, f. suárez-riestra, et al. timber-concrete composite structural flooring system. journal of building engineering 49:104078, 2022. https://doi.org/10.1016/j.jobe.2022.104078 [3] s.-j. pang, k.-s. ahn, s. jeong, et al. prediction of 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[9] c. fiala, j. hejl, v. bilek, et al. construction and static loading tests of experimental subtle frame from high performance concrete for energy efficient buildings. in 23rd concrete days 2016, vol. 259 of solid state phenomena, pp. 275–279. trans tech publications ltd, 2017. https: //doi.org/10.4028/www.scientific.net/ssp.259.275 [10] a. chira, a. kumar, t. vlach, et al. textile-reinforced concrete facade panels with rigid foam core prisms. journal of sandwich structures & materials 18(2):200–214, 2016. https://doi.org/10.1177/1099636215613488 [11] m. novotná, c. fiala, p. hájek. precast timber-concrete composite floor structures for sustainable buildings-experimental verification. in central europe towards sustainable building, pp. 429–432. 2013. [2024-09-09]. https://www.cesb.cz/ cesb13/proceedings/3_materials/cesb13_1432.pdf [12] m. schäfers, w. seim. investigation on bonding between timber and ultra-high performance concrete (uhpc). construction and building materials 25(7):3078–3088, 2011. https://doi.org/10.1016/j.conbuildmat.2010.12.060 [13] t. vlach, j. řepka, j. hájek, et al. cohesion test of a single impregnated ar-glass roving in high-performance concrete. stavební obzor civil engineering journal 29(3):358–369, 2020. https://doi.org/10.14311/cej.2020.03.0032 [14] k. buka-vaivade, d. serdjuks. behavior of timberconcrete composite with defects in adhesive connection. procedia structural integrity 37:563–569, 2022. icsi 2021 the 4th international conference on structural integrity. https://doi.org/10.1016/j.prostr.2022.01.123 96 https://doi.org/10.1016/j.jobe.2022.104078 https://doi.org/10.1016/j.jobe.2021.103900 https://www.umass.edu/bct/research/research-areas/wood-concrete-composite-systems/ https://www.umass.edu/bct/research/research-areas/wood-concrete-composite-systems/ https://doi.org/10.1007/bf02486473 https://doi.org/10.1016/j.conbuildmat.2021.125356 https://doi.org/10.4028/www.scientific.net/ssp.259.275 https://doi.org/10.4028/www.scientific.net/ssp.259.275 https://doi.org/10.1177/1099636215613488 https://www.cesb.cz/cesb13/proceedings/3_materials/cesb13_1432.pdf https://www.cesb.cz/cesb13/proceedings/3_materials/cesb13_1432.pdf https://doi.org/10.1016/j.conbuildmat.2010.12.060 https://doi.org/10.14311/cej.2020.03.0032 https://doi.org/10.1016/j.prostr.2022.01.123 acta polytechnica ctu proceedings 53:92–96, 2025 1 introduction 2 materials used for experiment 3 specimen design and preparation 4 experiment and results 5 conclusions acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2024.47.0084 acta polytechnica ctu proceedings 47:84–88, 2024 © 2024 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague concrete and reliability of existing prestressed bridge structures aleš mezera∗, milan holý, miroslav sýkora czech technical university in prague, klokner institute, šolínova 7, prague 166 08, czech republic ∗ corresponding author: ales.mezera@cvut.cz abstract. a large number of post-tensioned concrete bridges were built in the second half of the last century. they have often been insufficiently maintained during their lifetime (usually around 50 years). nowadays, these structures exhibit significant deterioration, mainly due to leakages and also due to various other deficiencies such as a small concrete cover. their load-bearing capacity needs to be verified. this paper focuses on estimating the load-bearing capacity calculation of existing post-tensioned concrete bridges. in the engineering practice, this is carried out using the partial factor method according to the currently valid standards (czech standards čsn and the eurocodes), which often impose more stringent requirements than the original standards. the partial factor method then often leads to low load-bearing capacities. this study deals with the bridge for which a very low load-bearing capacity has been determined. for this reason, a comparative probabilistic analysis was performed, allowing for a better description of the uncertainties in the resistance and load effect variables. the probabilistic approach appears to be less conservative and yields a higher load-bearing capacity. keywords: concrete, bridge, partial factor method, reliability, load-bearing capacity. 1. introduction in the second half of the last century, the posttensioned bridges in czechoslovakia developed intensively, largely utilising standardized structural members and systems. this was a response to the growing needs of transport and infrastructure, where the main goal was to connect regions as efficiently as possible and to provide for the efficient transport of persons and goods. the choice of this structural system was motivated by the ability to build quickly and use materials efficiently, therefore several systems of posttensioned precast beam bridges were developed and standardised [1]. this study specifically focuses on the ka type posttensioned precast beams. these beams were manufactured off-site under controlled conditions, which guaranteed the high quality of the concrete and prestressing elements. the method was well standardised, which enabled many bridges to be built quickly and economically. the beams were commonly manufactured in several segments to facilitate transportation. in the present territory of the czech republic, a large number of bridges were built from the ka beams, and subsequently, during the operation, the deficiencies of this technology gradually became apparent: • imperfect grouting of cable ducts and poor protection of prestressing reinforcement; • low concrete cover of the reinforcement; • closed hollows that are difficult to survey and can collect water (often the drainage holes of the hollows were not drilled and surveys revealed that in some cases the hollows were full of water); • the beams were assembled on site in several segments and thus contain joints without passingthrough reinforcement, thereby creating weak sections of the structure; • in the transverse direction, joints between the individual beams allow for rotation between the individual beams and this behaviour often results in damage to waterproofing and leaking into the structure, even early after commissioning [2]. it should be pointed out that imperfect grouting is particularly dangerous as it does not protect the prestressing reinforcement against corrosion and, furthermore, creates conditions for moisture condensation that can lead to reinforcement corrosion [3]. 2. bridge under investigation the bridge under investigation was built in 1964 and carries the 2nd class road over a small water stream. the superstructure is made of nine post-tensioned precast beams ka-61/18 (figure 1) and acts as a simply supported beam with span of 19 m. width of the bridge is 9.18 m. at both sides of the cross-section there are 0.75 m-wide precast reinforced concrete cornices with railings. the bridge deck surfacing is made of asphalt and width between the safety barriers is 7.68 m. the one-sided transversal slope of 3 % was created by a variable thickness of concrete deck. reliability analysis of the critical edge beam (figure 2 and figure 3), which exhibits the most severe corrosion weakening, is presented in detail. according to longterm experience gained by surveys of these bridges, 84 https://doi.org/10.14311/app.2024.47.0084 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 47/2024 concrete and reliability of existing prestressed bridge structures figure 1. cross-section at mid-span and longitudinal section of half of beam ka 61/18 with prestressing tendons [1]. figure 2. cross-section of bridge (critical beam marked in red). figure 3. view of bridge. edge beams generally prove to be the most exposed to corrosion weakening of prestressing tendons [4, 5]. the material characteristics of the concrete were determined by a structural survey carried out by the klokner institute of ctu in prague. the characteristic value of the concrete compressive strength was determined by tests on samples as fck = 40 mpa and the mean value of the concrete compressive strength was fcm = 57.4 mpa. for the application of the partial factor method, concrete strength class c 35/45 is considered, which well corresponds to the original documentation [1] (concrete class b500 according to the past classification). the prestressing tendons consist of patented wires of 4.5 mm diameter with a guaranteed ultimate strength fpk = 1 650 mpa and yield strength σ0 .2 = 1 200 mpa. according to [1], yield strength was increased by applying a prestress equal to the yield stress for two minutes. in this way yield strength was increased to σ0 .2 = 1 350 mpa. in reliability analysis, yield strength fp0 .1k = 0 .935σ0 .2 = 1 262 mpa according to [6] was then considered. the wires and tendons approximately correspond to relaxation class 1 according to en 1992-1-1 [7]. the prestress losses in the time of assessment – after 59 years – were determined by tda analysis in scia engineer v. 21.1 [8] with a value of 25.1 % at mid-span. 3. partial factor method reliability analysis of existing structures is commonly based on the principles of iso 1382 [9] and čsn 73 0038 [10]. the partial factor method is widely applied when calculating the load-bearing capacity of existing bridges. čsn 73 6222 [11] provides supplementary guidance in accordance with the eurocodes. in the partial factor method, the basic reliability condition for the determination of the load-bearing capacity can be written using the load combination rule (6.10a, b) according to en 1990 [12] as follows: 85 a. mezera, m. holý, m. sýkora acta polytechnica ctu proceedings γg(gk0 +gk) + γqψ0δnomvn,p f m ≤ r(fpk/γp , fck/γc , fsk/γs , b, ...) (1) γgξj(gk0 +gk) + γqδnomvn,p f m ≤ r(fpk/γp , fck/γc , fsk/γs , b, ...) (2) maximum of the load effects in equation 1 and equation 2 is considered. based on a survey and inspection of the bridge, the actual condition of the structure is taken into account. due to incomplete information and large uncertainties, weakening of the prestressing reinforcement area up to 25 % of the cross-sectional area is conservatively considered, tensile strengthening of the prestressing reinforcement is ignored, and weakening of the shear reinforcement up to 5 % of the cross-sectional area is taken into account. the permanent loads are based on the nominal dimensions of the structure and densities of the materials according to en 1991-1-1 [13]. traffic loads for the determination of the load-bearing capacity are considered according to čsn 73 6222 [11]. the load cases are considered so as the variable loads are always placed in the most unfavourable position as determined from influence line graphs. the capacity in the critical cross-section is estimated considering the ultimate limit state (uls) – bending and shear. considering that the prestressed structure was designed using the theory of allowable stresses, it was also necessary to assess it for the serviceability limit states (sls) – stress and crack width limitations. in the former, the normal stresses are verified for the characteristic, frequent and quasipermanent load combinations and crack widths are verified for the characteristic load combination. table 1 provides load-bearing capacities for shear and bending and the individual load combinations in the ultimate and serviceability limit states. note that in this study, only normal load-bearing capacity is discussed, i.e. the vehicle on the bridge is not restricted in terms of speed and position. load combination vn,pfm [t] uls – shear (6.10a) 41.3 uls – shear (6.10b) 36.8 sls – shear (characteristic combination 76.4– verification of crack width) uls – bending (6.10a) 10.8 uls – bending (6.10b) 16.0 sls – bending (verification of stress limitation) 11.7 table 1. load-bearing capacity for considered load combinations. it follows from table 1 that the critical failure mode is mid-span bending with a very low load-bearing capacity vn,pem = 10.8 t. for example, an unloaded tatra 815 weighs approximately 15 t. this is why a probabilistic approach is also applied to better describe the uncertainties in the resistance of the critical section and the load effects of permanent and traffic loads. 4. probabilistic method the probabilistic approach relies on the determination of the reliability index β or, equivalently, probability of failure pf . for existing structures, according to iso 13822, tab. f.1 [9] the target reliability level is β= 3.8. the limit state function reads: g(x) = θrr(fp, fc, fs, b, . . . )− θe(g0 +g+ δvn,p m ) (3) the symbols and probabilistic models of the basic variables are given in table 2. probability of failure was estimated by the monte carlo method. note that the mean value of the bending resistance of the prestressed section is rm = 1 960 knm and the characteristic value is rk = 1 744 knm (rk/rm = 0.89). the coefficient of variation vr = 0.071 corresponds approximately to the coefficient of variation of the prestressing force taking into account the prestressing losses. the load-bearing capacity vvn,pm = 22 t is estimated iteratively to achieve a reliability index of β= 3.8. the resulting load-bearing capacity is, therefore, approximately doubled in comparison to the value determined by the partial factor method. 5. discussion it is assumed that the difference in load-bearing capacities can be attributed to: • conservative value of the partial factor γg – for an existing bridge it is possible to measure the dimensions of structural members and thicknesses of pavement layers, specify material densities, and therefore to significantly reduce the uncertainties and subsequently update (reduce) the partial factor according to the procedure in čsn 730038 [10], • a conservative value of the partial factor γq – in the determination of the load-bearing capacity, the uncertainties associated with the effects of vehicle loading on the bridge are lower than for the traffic flow considered in the design; therefore, the value of the partial factor 1.35 could be reduced. 6. conclusion post-tensioned prestressed concrete bridges built in the second half of the last century have often been poorly maintained; nowadays their age reaches mostly cca 50 years. these structures exhibit significant degradation (mainly due to adverse long-term effects of leakages and various other deficiencies such as a 86 vol. 47/2024 concrete and reliability of existing prestressed bridge structures symbol variable characteristic xk/µx distribution coefficient source value xk of variation vx b width of beam 0.98 m 1 [1] h height of beam 0.85 m 1 [1] fc concrete compressive strength 43 mpa 0.75 ln0 0.115 fp prestressed force 1 327 kn 0.88 n 0.075 l span length 19 m 1 [1] γc partial factor for concrete 1.50 [12] γg partial factor for permanent actions 1.35 [12] γq partial factor for traffic load 1.35 [12] γs partial factor for reinforcement 1.15 [12] δ dynamic factor 1.20 1 ln0 0.05 g0 self-weight load 493 knm 1 ln0 0.04 g other permanent loads 421 knm 1 ln0 0.10 vn,p m vn (normal load-bearing capacity) 330 knm 1 n 0.05 [14] θe model uncertainty for action effects 1 1 ln0 0.10 [15] θr model uncertainty for resistance 1 0.98 ln0 0.06 [16] table 2. probabilistic models of basic variables. low concrete cover) and their load-bearing capacity needs to be verified. focusing on an example of the representative bridge, the verification by the partial factor method reveals that the mid-span bending failure (with a load-bearing capacity of 10.8 t) is the decisive uls. since this is a very low value, a probabilistic method was also applied to better describe the uncertainties in resistance and load effects. the load-bearing capacity was determined iteratively to achieve the target reliability index β= 3.8. the load-bearing capacity, 22 t, is approximately doubled in comparison to that obtained by the partial factor method. the difference between the load-bearing capacities is attributed to the conservative values of the partial factors for the permanent and variable loads. the probabilistic analysis further shows that the variability of the resistance of the critical section is mainly determined by the variability of the prestressing force including the effect of prestress losses. however, it is important to note that the determination of the corrosion weakening of prestressing reinforcement is a largely uncertain parameter, since surveys reveal conditions of prestressing steel only in a limited number of locations. the obtained results should therefore be considered as indicative only; further research will specifically focus on better characterisation of corrosion effects. acknowledgements this study has been supported by the czech science foundation under grant 23-06222s. references [1] prefabrikované cestné mosty svetlosti 9–12–15–18–21m montované z predpatých nosníkov ka–61 [prefabricated road–bridges of span clearance 9–12–15–18–21 m assembled from post-tensioned beams ka–61]. dopravoprojekt bratislava, 1964. [2] železniční mosty s předpjatou nosnou konstrukcí v síti správy železnic, [railway bridges with prestressed load bearing structure in the network správa železnic]. ctu in prague – faculty of civil engineering, 2021. isbn 978-80-01-06923-3. [3] r. ponechal, p. koteš, d. michálková, et al. effect of water condensate on corrosion of wires in ungrouted ducts. materials 14(24):7765, 2021. https://doi.org/10.3390/ma14247765 [4] m. kryštov, a. čítek, s. řeháček, d. čítek. vyhodnocení stavu diagnostikovaných mostů z dodatečně předpjatých tyčových prefabrikátů ka–73 po 40 letech provozu [assessment of the condition of surveyed bridges post–tensioned ka–73 precast concrete beams after 40 years of service]. 2023. isbn 978-80-908943-0-3. [5] d. čítek, s. řeháček, m. holý, j. kolísko. prefabrikované cestné mosty svetlosti 9–12–15–18–21 m montované z predpatých nosníkov ka–61 [prefabricated road bridges of span clearance 9–12–15–18–21 m assembled from post-tensioned beams ka–61]. spektrum stu, 2022. isbn 978-80-227-5235-0. [6] stanovení zatížitelnosti mostů pk navržených podle norem a předpisů platných před účinností en [assessment of the load-bearing capacity of road bridges designed according to standards and regulations valid before the entry into force of en]. standard, ministerstvo dopravy čr, prague, 2008. [7] european committee for standardization. en 1992-1-1 – eurocode 2: design of concrete structures : part 1-1: general rules and rules for buildings, 2004. [8] advanced expert training, pre-tensioning (precast). https://downloads.scia.net/support/ sciaengineer/manuals/tut%20[eng]%20scia21.1% 20-%20advanced%20expert%20training%20-% 20pre-tensioning%20(precast).pdf, 2021. [2023-11-20]. [9] čsn iso 13822 – zásady navrhování konstrukcí – hodnocení existujících konstrukcí [bases for design of structures – assessment of existing structures]. standard, czech standard institute, prague, 2010. [10] čsn 73 0038 – navrhování a posuzování stavebních konstrukcí při přestavbách [assessment and verification of existing structures – supplementary guidance]. standard, czech standard institute, prague, 2019. 87 https://doi.org/10.3390/ma14247765 https://downloads.scia.net/support/sciaengineer/manuals/tut%20[eng]%20scia21.1%20-%20advanced%20expert%20training%20-%20pre-tensioning%20(precast).pdf https://downloads.scia.net/support/sciaengineer/manuals/tut%20[eng]%20scia21.1%20-%20advanced%20expert%20training%20-%20pre-tensioning%20(precast).pdf https://downloads.scia.net/support/sciaengineer/manuals/tut%20[eng]%20scia21.1%20-%20advanced%20expert%20training%20-%20pre-tensioning%20(precast).pdf https://downloads.scia.net/support/sciaengineer/manuals/tut%20[eng]%20scia21.1%20-%20advanced%20expert%20training%20-%20pre-tensioning%20(precast).pdf a. mezera, m. holý, m. sýkora acta polytechnica ctu proceedings [11] čsn 73 6222 – zatížitelnost mostů pozemních komunikací [load bearing capacity of road bridges]. standard, czech standard institute, prague, 2015. [12] h. gulvanessian. en1990 eurocode – basis of structural design. proceedings of the institution of civil engineers civil engineering 144(6):8–13, 2001. https://doi.org/10.1680/cien.2001.144.6.8 [13] british standards institution and european committee for standardization. en 1991-1-1 eurocode 1: actions on structures – part 1–1: general actions – densities, self-weight, imposed loads for buildings, 2002. [14] j. krejsa, m. sýkora, m. drahorád. applied mechanics and materials. 2016. issn 1662-7482. [15] j. r. casas, d. wisniewski. safety requirements and probabilistic models of resistance in the assessment of existing railway bridges. structure and infrastructure engineering 9(6):529–545, 2013. https://doi.org/10.1080/15732479.2011.581673 [16] r. caspeele, r. steenbergen, m. sýkora. partial factor methods for existing concrete structures. federation internationale du béton (fib), 2016. 88 https://doi.org/10.1680/cien.2001.144.6.8 https://doi.org/10.1080/15732479.2011.581673 acta polytechnica ctu proceedings 47:84–88, 2024 1 introduction 2 bridge under investigation 3 partial factor method 4 probabilistic method 5 discussion 6 conclusion acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2023.45.0053 acta polytechnica ctu proceedings 45:53–58, 2023 © 2023 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague evaluation of deformation and strength parameters of organic soils for the design of geotechnical structures zbigniew lechowicz∗, katarzyna goławska warsaw university of life sciences, institute of civil engineering, department of geotechnical engineering, 166 nowoursynowska str., 02-787 warsaw, poland ∗ corresponding author: zbigniew_lechowicz@sggw.edu.pl abstract. the evaluation of deformation and strength parameters of holocene organic soils obtained from dmt dilatometer tests based on empirical relationships and ann artificial neural networks is presented. the evaluation of undrained shear strength τfu, deformation modulus e0.1% and initial shear modulus g0 of eemian organic soils obtained from dmt dilatometer tests and sdmt seismic dilatometer tests is given, and then its comparison with the values obtained from triaxial tests for the design of underground structures supported by diaphragm walls of the stations of the underground line ii in warsaw, is shown. keywords: deformation modulus, eemian organic soils, holocene organic soils, initial shear modulus, undrained shear strength. 1. introduction earth structures such as dam embankments, levees, dykes, and road and railway embankments are often constructed in difficult geotechnical conditions [1–6]. in general geotechnical practice organic soils are considered as difficult subsoils due to their high compressibility with creep effects, low undrained shear strength and nonlinear variability of material characteristics [7–13]. holocene organic soils are classified as very soft soils with the consistency index ic < 0.5 and undrained shear strength τfu often not exceeding 25 kpa. organic soils formed during the eemian interglacial of the pleistocene period reveal slightly better index properties, and higher stiffness and strength than holocene organic soils. however, due to the composition of the soil skeleton with significant organic matter and calcium carbonate contents, they also show up non-linear mechanical characteristics and a time-dependent response to loading. for the design of embankments on organic subsoil, the crucial problems to be solved include the assessment of embankment stability, and large vertical and horizontal deformations of the subsoil during and after construction [1, 2]. embankment construction on organic subsoil is possible using a staged construction involving ground improvement by consolidation or various ground improvement methods, e.g. inclusions of different rigidity. nowadays, the construction and modernization of facilities and infrastructure in urban agglomerations is often performed in complex geotechnical conditions. subsoil studies and tests with regard to the importance of the structure and complexity of the foundation conditions are based on the combined analysis of field and laboratory test results. in recent practice, among possible in situ tests, the sdmt seismic dilatometer test is increasingly used also for heavily preconsolidated cohesive soils and organic soils. when modelling the behaviour of deep excavation casings by diaphragm walls using a linear elastic-perfectly plastic model, it is necessary to apply deformation parameters determined in the range of small strains. for the design of diaphragm walls in organic soils, it is advisable to use a more advanced soil model, e.g. a non-linear elastic-plastic model with creep [14–18]. practical cases of evaluating deformation and strength parameters of organic soils loaded by embankments are shown in the paper. the evaluation of deformation and strength parameters of holocene organic soils from the antoniny test site located in north-western poland in the noteć river valley and obtained from dmt dilatometer tests is here presented. the constrained modulus moc for the preconsolidated state and the constrained modulus mnc for the normally consolidated state, as well as the recompression index cr and the compression index cc for the normally consolidated state of holocene peat and gyttja, are presented. the undrained shear strength τfu of holocene organic soils evaluated from dmt dilatometer tests based on empirical relationships and artificial neural networks is also shown. the evaluation of deformation and strength parameters of eemian organic soils in the design of underground structures such as deep excavations supported by diaphragm walls for the stations of the underground line ii in warsaw is shown. the values of undrained shear strength τfu, deformation modulus e0.1% at vertical strain ε1 of 0.1 % and initial shear modulus g0 for eemian gyttja evaluated from dmt dilatometer tests and sdmt seismic dilatometer tests are compared with the values obtained from triaxial tests. 53 https://doi.org/10.14311/app.2023.45.0053 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en zbigniew lechowicz, katarzyna goławska acta polytechnica ctu proceedings soil type organic content iom [%] caco3 content [%] water content wn [%] liquid limit wl [%] density unit ρ [t m−3] specific ρs [t m−3] peat 65–75 10–15 310–340 305–450 1.05–1.10 1.45–1.50 gyttja 5–20 65–90 105–140 80–110 1.25–1.40 2.2–2.3 table 1. physical properties of holocene organic soils. soil type organic content iom [%] caco3 content [%] water content wn [%] liquid limit wl [%] density unit ρ [t m−3] specific ρs [t m−3] organic mud 8–12 – 32–34 50–55 1.60–1.65 2.4–2.5 gyttja low-carbonate mineral 7–9 29–38 60–80 75–85 1.60–1.65 2.40–2.45 gyttja high-carbonate mineral-organic 17–25 54–77 80–120 120–130 1.55–1.60 2.30–2.35 table 2. physical properties of eemian organic soils. 2. characteristics of the organic soils 2.1. holocene organic soils the physical properties of typical holocene organic soils: organic mud, high-carbonate gyttja and amorphous peat are shown in table 1. in order to correctly identify organic soils within the basic physical properties, in addition to natural moisture, liquidity limit, bulk density and specific density, it is necessary to determine the organic content iom, by burning the soil at a maximum temperature of +440 ◦c, and to determine the calcium carbonate caco3 content. 2.2. eemian organic soils older organic soils formed during the eemian interglacial of the pleistocene period show slightly better physical and mechanical properties. table 2 shows the physical properties of typical organic soils from the eemian interglacial: organic mud, low-carbonate mineral gyttja and high-carbonate mineral-organic gyttja. 3. deformation and strength parameters 3.1. holocene organic soils experience from organic soils indicates that the value of constrained modulus m is not constant but is an effective stress dependency. for staged construction of embankments on organic soils, when effective vertical stress several times exceeds the initial preconsolidation pressure, for calculation of subsoil settlement not only the constrained modulus moc for the preconsolidated state but also the constrained modulus mnc for the normally consolidated state should be evaluated. analysis of test results for holocene organic soils indicates that based on dmt dilatometer tests the constrained modulus for the preconsolidated state moc and for the normally consolidated state mnc can be calculated using factors roc m and rnc m from the following relations [19]: moc = roc m · ed, (1) mnc = rnc m · ed, (2) for peat: roc m = 0.20 + 1.60 · log kd, (3) rnc m = 0.90 + 0.60 · log id, (4) and for gyttja: roc m = 0.12 + 2.10 · log kd, (5) rnc m = 0.95 + 0.55 · log id, (6) where: ed – dilatometer modulus, kd – horizontal stress index, id – material index. the profiles of the constrained module for the organic subsoil at the antoniny site evaluated from dmt dilatometer tests based on the equations (1)-(6) are shown in figure 1a and compared with the values obtained from the oedometer tests. a comparison of dilatometer data with the results of oedometer tests indicates that the relationship between the compression index cc for normally 54 vol. 45/2023 organic soils for geotechnical structures figure 1. compression moduli and compression indexes for holocene organic subsoil at the antoniny site based on dmt dilatometer tests: a – moduli moc and mnc, b – compression indexes cr and cc. consolidated holocene organic soils and material index id can be expressed in the form [19]: cc = a(id)m. (7) the obtained values of empirical coefficients a and m for peat and gyttja are equal to 1.8, 0.7 and −0.30, −0.20, respectively. analysis of test results indicates that the recompression index cr can be calculated from the relationship between the ratio cr/cc and the lateral stress index kd in the form [19]: cc cr = b(kd)n. (8) for holocene organic soils the values of empirical coefficients are b = 0.27 and n = 1.9. the profiles of the recompression index cr and the compression index cc for the normally consolidated state for organic subsoil at the antoniny site evaluated on the basis of the equation (7) and (8) using the values of empirical coefficients mentioned above are shown in figure 1b and are compared with the values obtained from the oedometer tests. analysis of dmt and triaxial test results carried out by lechowicz indicates that, particularly for peat and gyttja, the relationship between the normalized undrained shear strength and the horizontal stress index kd differs from that proposed by marchetti [20, 21] and can be modified as follows [22, 23]: τfu σ′ v0 = s, (9) where s = (τfu/σ′ v0)nc is the normalized undrained shear strength for normally consolidated soil. the value of parameter s for peat is equal to 0.5, but for calcareous gyttja and calcareous-organic gyttja, it is at 0.40 and 0.45, respectively. a multi-factor relationship was proposed by rabarijoely [25] to evaluate the undrained shear strength τfu of organic soils from dmt dilatometer tests based on the net value of the corrected first pressure reading (p0 − u0), the net value of the corrected second pressure reading (p1 − u0), and the effective vertical stress σ′ v0; and it is as follows: τfu = α0 · (σ′ v0)α 1 · (p0 − u0)α 2 · (p1 − u0)α 3 , (10) where α0, α1, α2, α3 are empirical coefficients, σ′ v0 is the in situ effective vertical stress, p0 and p1 are corrected pressures from dmt tests, and u0 is the in situ pore water pressure. the empirical coefficients in equation (10) for organic soils can be evaluated as functions of the void ratio e and empirical coefficients ci and di shown in table 3 according to the following formula [25]: αi = ci · e + di, (11) where subscript i = 0, 1, 2, 3. 1.1.1 coefficients αi = ci · e + di i = 0, 1, 2, 3 i = 0 i = 1 i = 2 i = 3 ci 0.149 −0.0233 0.0065 0.0114 di 1.003 0.3406 0.1104 0.1847 table 3. values of empirical coefficients ci and di for equation (11). the profiles of the undrained shear strength of holocene organic soils at the antoniny site before 55 zbigniew lechowicz, katarzyna goławska acta polytechnica ctu proceedings figure 2. undrained shear strength of holocene organic soils at the antoniny site based on dmt dilatometer tests. figure 3. architecture of the two-layer artificial neural network 5–4–1 for the evaluation of undrained shear strength of holocene organic soils based on dmt dilatometer tests [24]. embankment construction and after the 3rd embankment stage based on dmt dilatometer tests, using the equations (9) and (10), are shown in figure 2. obtained values of undrained shear strength are compared with the values of corrected undrained shear strength from the field vane tests and the values from ckou triaxial tests. the method of evaluating the undrained shear strength of organic soils from dmt dilatometer tests using the artificial neural network was presented by lechowicz et al. [24] based on the normalized net value of the corrected first pressure reading (p0 − u0)/σ′ v and the normalized net value of the corrected second pressure reading (p1 − u0)/σ′ v from dilatometer tests and organic soil properties such as the organic content iom, void ratio e, and a stress history indicator (oc/nc). the architecture of the two-layer artificial neural network 5–4–1 for the evaluation of undrained shear strength of holocene organic soils based on dmt dilatometer tests is shown in figure 3. 3.2. eemian organic soils for design of deep excavations supported by diaphragm walls, the evaluation of the undrained shear strength τfu, the deformation modulus e0.1% at vertical strain ε1 of 0.1 % and the initial shear modulus g0, are needed. the deformation modulus e0.1% of eemian organic soils from sdmt seismic dilatometer tests using the dilatometer modulus ed and the empirical coefficient re can be evaluated based on equation 12 [26]: e0.1% = re · ed, (12) for eemian organic mud: re = 2.4 + 2.36 · log kd, (13) and for eemian gyttja: re = 2.15 + 2.10 · log kd. (14) the initial shear modulus g0 from sdmt seismic dilatometer tests using soil density ρ and shear wave 56 vol. 45/2023 organic soils for geotechnical structures figure 4. profiles of the deformation modulus e0.1% and shear modulus g0 from sdmt seismic dilatometer tests and the modulus values obtained from triaxial tests cd with measurements of shear wave velocity vs for the eemian gyttja from the płocka underground station. figure 5. undrained shear strength profile from dmt dilatometer tests and τfu values from triaxial tests for the eemian gyttja from the płocka underground station. velocity vs can be evaluated from the following equation: g0 = ρ · v 2 s . (15) figure 4 shows the profiles of the deformation modulus e0.1% and the initial shear modulus g0 from sdmt tests and values obtained from the isotropically consolidated drained cd triaxial tests with shear wave velocity measurements vs for the eemian gyttja from the płocka underground station. figure 5 shows the profile of undrained shear strength from anisotropically consolidated undrained ck0u triaxial tests and from the sdmt test based on the empirical equation (10). empirical coefficients for eemian gytjja are: α0 = 1.25, α1 = 0.30, α2 = 0.12, α3 = 0.30, and for the eemian organic mud are: α0 = 1.12, α1 = 0.13, α2 = 0.10, α3 = 0.44. 4. conclusions the paper presents the problem of evaluating the deformation and strength parameters of holocene and eemian organic soils obtained from dmt and sdmt dilatometer tests. empirical relationships used to evaluate the constrained modulus moc for the preconsolidated state and the constrained modulus mnc for the normally consolidated state, as well as the recompression index cr and the compression index cc of holocene peat and gyttja, are presented. the evaluation of undrained shear strength τfu of holocene peat and gyttja from dmt dilatometer tests based on empirical relationships and the artificial neural network is also shown. the evaluation of undrained shear strength τfu, deformation modulus e0.1% and initial shear modulus g0 of eemian organic soils from dmt dilatometer tests and sdmt seismic 57 zbigniew lechowicz, katarzyna goławska acta polytechnica ctu proceedings dilatometer tests is then presented. a comparison between the evaluated and obtained values from oedometer tests and triaxial tests shows a good agreement. references [1] j. hartlén, w. wolski. embankments on organic soils. elsevier, 1996. 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[20] s. marchetti. in situ tests by flat dilatometer. journal of the geotechnical engineering division 106(3):299–321, 1980. https://doi.org/10.1061/ajgeb6.0000934 [21] s. marchetti, p. monaco, g. totani, m. calabrese. the flat dilatometer test (dmt) in soil investigations a report by the issmge committee tc16. in proceedings of the international conference on in situ measurement of soil properties and case histories, bali, indonesia, p. 41. 2001. [22] z. lechowicz. undrained shear strength of organic soils from dilatometer test. annals of warsaw agricultural university land reclamation 28:85–96, 1997. [23] z. lechowicz, s. rabarijoely. use of dilatometer test in evaluation of organic subsoil strengthening. in proceedings of the conference on recent advances in soft soil engineering, vol. 1, pp. 185–196. 1997. [24] z. lechowicz, m. fukue, s. rabarijoely, m. j. sulewska. evaluation of the undrained shear strength of organic soils from a dilatometer test using artificial neural networks. applied sciences 8(8):1395, 2018. https://doi.org/10.3390/app8081395 [25] s. rabarijoely. the use of dilatometer test for evaluation of organic soil parameters [in polish: wykorzystanie badań dylatometrycznych w wyznaczeniu parametrów gruntów organicznych obciążonych nasypem]. ph.d. thesis, warsaw agricultural university-sggw, land reclamation warszawa, poland, 2000. [26] z. lechowicz, m. bajda, s. rabarijoely, g. wrzesiński. use of sdmt for the evaluation of the geotechnical parameters of organic soils. in: cptu and dmt in soft clays and organic soils by z młynarek & j wierzbicki, poznań, wydawnictwo exemplum pp. 107–118, 2014. 58 https://doi.org/10.1016/j.sandf.2012.01.010 https://doi.org/10.1680/geot.14.p.257 https://doi.org/10.1680/geot.1996.46.1.1 https://doi.org/10.1139/cgj-2013-0002 https://doi.org/10.1680/geot.14.p.144 https://doi.org/10.1007/s11440-017-0545-1 https://doi.org/10.1007/s11440-010-0119-y https://doi.org/10.1680/geot.8.p.027 https://doi.org/10.1016/s0894-9166(11)60023-2 https://doi.org/10.1061/(asce)gm.1943-5622.0000284 https://doi.org/10.1061/(asce)gm.1943-5622.0000284 https://doi.org/10.1016/j.compgeo.2015.04.015 https://doi.org/10.1061/ajgeb6.0000934 https://doi.org/10.3390/app8081395 acta polytechnica ctu proceedings 45:1–6, 2023 1 introduction 2 characteristics of the organic soils 2.1 holocene organic soils 2.2 eemian organic soils 3 deformation and strength parameters 3.1 holocene organic soils 3.2 eemian organic soils 4 conclusions references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2024.47.0063 acta polytechnica ctu proceedings 47:63–65, 2024 © 2024 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague possibilities of reinforcement of subtle translucent concrete panel věra kabíčkováa,∗, jakub hájeka, tomáš vlacha, jakub řepkab a czech technical university in prague, faculty of civil engineering, thákurova 7, 166 29 prague, czech republic b czech technical university in prague, university centre for energy efficient buildings, třinecká 1024, 273 43 buštěhrad, czech republic ∗ corresponding author: vera.kabickova@fsv.cvut.cz abstract. textile-reinforced concrete not only has many advantages that lead to less consumption of primary resources, but also enables the creation of thin and subtle elements. the high performance of such elements allows us to create interesting architectural features, such as translucent concrete. this article focusses on the possibility of reinforcing translucent concrete panels. keywords: concrete, light-transmitting concrete, textile-reinforced concrete, high-performance concrete. 1. introduction concrete is one of the most used building materials, but due to its massive production, concrete has a huge impact on the environment (especially because of the cement and steel used as reinforcement). on the other hand, concrete offers many benefits (such as mechanical properties or the ability to maintain the stability of the internal environment) that make it almost irreplaceable in many cases. this is the reason why it is so important to optimise concrete [1]. there are several ways to improve concrete and reduce its environmental impact – for example, it is possible to partially replace cement using fly ash, silica fume, or slag, reduce the amount of water (that leads to higher strength which allows lower concrete thickness) or replace traditional steel reinforcement with alternative noncorrosive reinforcement that has a lower environmental impact. furthermore, no concrete cover is required to protect the reinforcement, which also leads to a lower thickness of the concrete. reducing the thickness of concrete has an impact on the reduction of used materials, lower load, which means smaller dimensions of the supporting structures, and easier handling [1]. textile reinforced concrete (trc) combines the above improvements. it is reinforced by technical textiles made of carbon, alkali-resistant ar-glass, basalt, or natural materials such as flax or hemp, which can be impregnated using a polymer matrix, e.g., epoxy resin. technical textiles are in the form of orientated rovings and are embedded in a concrete matrix, which is usually high-performance fine-grained concrete [2– 5]. another material that combines the above improvements is fibre-reinforced concrete (frc), where dispersed reinforcing fibres (made of steel, polymers or, for example, glass) instead of traditional reinforcement. these fibres can contribute to higher strength and ductility, but also to enhance fire resistance [5, 6]. overall, there are many ways to improve concrete and thus reduce its environmental impact, but it is also possible to combine multiple functions (e.g., load bearing, acoustic, or lighting) in one material. for example, luminescent or acoustic concrete is being developed and light-transmitting concrete is already available (under the name of litracon, lucem, or licrete). light transmission is ensured by several elements – usually optical fibres, polymers, or glass [7– 9]. based on that, it was decided to explore the possibilities of light-transmitting concrete reinforcement using alternative reinforcement to reduce the thickness of concrete and thus reduce its environmental impact. different types of rovings (carbon, ar-glass, and flax) were tested, and a plexiglass (or polymethyl methacrylate – pmma) elements were chosen to ensure light transmission. 2. preparation of samples translucent samples were made with dimensions of the samples 30×150×400 mm for the flexural strength test. there were 4 groups of trc, 6 samples in total, 4 samples were translucent, and 2 samples were opaque: • u r (2×) carbon roving (tenax sts40 f13 24k 1600tex) + plexiglass, • u r ref (2× – opaque) carbon roving (tenax sts40 f13 24k 1600tex), • l r (1×) flax roving + plexiglass, • arg r (1×) ar glass roving (cem-fil® 5325) + plexiglass. a fine-grained concrete mixture was used for the preparation of samples, which is described in the table 1. 63 https://doi.org/10.14311/app.2024.47.0063 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en v. kabíčková, j. hájek, t. vlach, j. řepka acta polytechnica ctu proceedings reinforcement was prepared as impregnated rovings with epoxy resin. the used rovings are described in the table 2. [kg m−3] cem i 42.5 r 650 silica sand dmax=1.25 mm 1 200 silica fume 75 microsilica 235 superplasticizer 18 water 190 table 1. concrete mixture. fibre type t tensile elastic strength modulus [tex] [mpa] [gpa] carbon (tenax sts40 f13 24k) 1 600 4 000 240 ar glass (cem-fil® 5325) 2 400 >1 000 72 flax 1 680 n/s n/s table 2. material characteristics of used rovings [10, 11]. to ensure a 10 mm distance between the plexiglass elements (figure 1), 10 mm wide wooden bars were placed between them. during the preparation of the translucent trc samples, the rovings were impregnated with epoxy resin, placed in the space where the plexiglass elements are cut and then sprinkled with fine silica sand. after 24 hours, the hardened epoxy resin was able to keep the required distance between the plexiglass elements so the wooden bars could be removed and the same process could be repeated on the other side of the sample (figure 2). the prepared inserts were placed into the mould and weighted down and then the concrete mixture was poured. all the samples were later demoulded and left in water until the day of testing. figure 1. scheme of translucent sample. 3. experiment and results the compressive test according to csn en 12390-3 and the flexural strength test (three-point bending test) according to csn en 12390-5 both at the age of 28 days were performed to measure the mechanical properties of the concrete mixture. the translucent samples were prepared for the flexural strength figure 2. elements with one layer of carbon rovings impregnated with epoxy resin and silica sand (right). figure 3. force-displacement diagram of the translucent samples – rovings (right). test (four-point bending test) with constant loading speed of 1 mm min−1 which was partly according to csn en 12390-5. the average compressive strength on the cubes was 110.69 mpa, and the average flexural strength on the prisms was 17.24 mpa. sample spill flexural average flexural test strength fcf strength fcf ,avg [mm] [mpa] [mpa] u r ref_1 300 18.31 18.00u r ref_2 300 17.70 u r_1 300 18.22 18.79u r_2 300 19.36 arg r 300 8.42 8.42 l r 300 3.35 3.35 table 3. results of the four-point bending test. in figure 3 force-displacement curves of the translucent trc samples are presented. the translucent trc samples achieve high flexural strengths. the highest strength is achieved by the carbon-reinforced 64 vol. 47/2024 possibilities of reinforcement of subtle translucent concrete panel figure 4. number of cracks after four-point bending test: translucent trc sample (left), opaque trc sample (right). samples, the flexural strength of the translucent samples is even higher than the opaque samples with the same reinforcement – 18.79 mpa and 18.00 mpa (see table 3). it might be caused by the microcracks which cause that rovings must bear a certain amount of load from the very beginning of loading and can adapt to it (see figure 4. the flexural strength of glass roving is 2 times lower than carbon roving (18.79 mpa and 8.42 mpa), and the flexural strength of flax roving is even 5.5 times lower than carbon roving (18.79 mpa and 3.35 mpa). after the maximum load, the samples with carbon and flax roving can carry about 4 % of the maximum load, unlike the samples with glass roving, which collapsed immediately. 4. conclusions all translucent samples achieved optimistic results. the highest average flexural strength had samples reinforced with carbon roving (18.79 mpa, which is even slightly more than opaque samples with the same reinforcement – 18.00 mpa), followed by glass roving (8.42 mpa, which is 2 times lower) and flax roving (3.35 mpa, which is 5.5 times lower). the samples with rovings also show ductility behaviour. in summary, all rovings could be used in certain applications of translucent concrete. acknowledgements the work on this paper was supported by czech science foundation grant no. 22-14942k entitled “possibilities of using natural fibers for the production of hybrid textile reinforcement in concrete”. the authors would like to acknowledge all financial assistance provided to support this research. references [1] p. hájek. význam betonu a betonových konstrukcí z hlediska kritérií udržitelné výstavby. časopis stavebnictví 2007. [2022-12-16]. https://www.casopisstavebnictvi.cz/clankyvyznam-betonu-a-betonovych-konstrukcizhlediska-kriterii-udrzitelne-vystavby.html [2] co je to textilní beton (trc). tzb-info [2022-11-13]. https://stavba.tzb-info.cz/beton-malty-omitky/ 18732-co-je-to-textilni-beton-trc [3] t. vlach. interakce textilní výztuže a vysokopevnostní betonové matrice. phd thesis, české vysoké učení technické v praze, 2021. [2022-11-13]. https://dspace.cvut.cz/handle/10467/98215 [4] w. brameshuber. textile reinforced concrete state-of-the-art report of rilem tc 201-trc, 2006. [2022-11-14]. https: //www.rilem.net/publication/publication/100 [5] p.-c. aïtcin. vysokohodnotný beton. 2005. isbn 80-86769-39-9. [6] n. v. chanh. steel fiber reinforced concrete, 2015. [2022-11-13]. https://www.academia.edu/70854322/ steel_fiber_reinforced_concrete [7] litracon. [2022-11-13]. http://www.litracon.hu/en [8] lucem light transmitting concrete – lucem gmbh aachen, germany. [2022-11-13]. https://lucem.com/ [9] licrete® – innovative building material composed of high quality concrete and transparent elements. [2022-11-13]. https://licrete.com/ [10] uhlíkový roving tenax sts40 f13 24k 1600tex. [2022-11-30]. https: //www.havel-composites.com/cs/produkty/uhlikovyroving-tenax-sts40-f13-24k-1600tex-12-6217 [11] cem-fil® 5325 | owens corning composites. [2023-01-02]. https://www.owenscorning.com/en-us/ composites/product/cem-fil-5325 65 https://www.casopisstavebnictvi.cz/clanky-vyznam-betonu-a-betonovych-konstrukci-zhlediska-kriterii-udrzitelne-vystavby.html https://www.casopisstavebnictvi.cz/clanky-vyznam-betonu-a-betonovych-konstrukci-zhlediska-kriterii-udrzitelne-vystavby.html https://www.casopisstavebnictvi.cz/clanky-vyznam-betonu-a-betonovych-konstrukci-zhlediska-kriterii-udrzitelne-vystavby.html https://stavba.tzb-info.cz/beton-malty-omitky/18732-co-je-to-textilni-beton-trc https://stavba.tzb-info.cz/beton-malty-omitky/18732-co-je-to-textilni-beton-trc https://dspace.cvut.cz/handle/10467/98215 https://www.rilem.net/publication/publication/100 https://www.rilem.net/publication/publication/100 https://www.academia.edu/70854322/steel_fiber_reinforced_concrete https://www.academia.edu/70854322/steel_fiber_reinforced_concrete http://www.litracon.hu/en https://lucem.com/ https://licrete.com/ https://www.havel-composites.com/cs/produkty/uhlikovy-roving-tenax-sts40-f13-24k-1600tex-12-6217 https://www.havel-composites.com/cs/produkty/uhlikovy-roving-tenax-sts40-f13-24k-1600tex-12-6217 https://www.havel-composites.com/cs/produkty/uhlikovy-roving-tenax-sts40-f13-24k-1600tex-12-6217 https://www.owenscorning.com/en-us/composites/product/cem-fil-5325 https://www.owenscorning.com/en-us/composites/product/cem-fil-5325 acta polytechnica ctu proceedings 47:63–65, 2024 1 introduction 2 preparation of samples 3 experiment and results 4 conclusions acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2023.40.0033 acta polytechnica ctu proceedings 40:33–38, 2023 © 2023 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague bacterially induced calcite formation at the surface of recycled concrete petr holečeka,∗, hana stiborováb a czech technical university in prague, faculty of civil engineering, department of physics, thákurova 7, 166 29 prague 6, czech republic b university of chemistry and technology prague, department of biochemistry and microbiology, technická 1903/3, 166 28 prague 6, czech republic ∗ corresponding author: petr.holecek@fsv.cvut.cz abstract. the construction industry is one of the main sources of greenhouse gas emissions, and portland cement production is responsible for approximately 8 % of anthropogenic co2 emissions. microbially induced calcium precipitation (micp) has the potential to partially replace cement or modify the properties of materials that would otherwise not find use in construction, for example, in concrete recycling. micp might be an environmentally friendly method to improve the properties of recycled aggregates and form conglomerates from the finest fractions. in this paper, factors influencing micp’s ability to solidify recycled concrete fines are thoroughly investigated. calcium carbonate precipitate crystals produced by the bacterium sporosarcina pasteurii were analyzed using scanning electron microscopy and energy dispersive spectroscopy. keywords: bacteria, micp, recycled fine aggregate, recycled concrete, material properties. 1. introduction concrete is one of the most widely used building materials. it produces significant amounts of greenhouse gas emissions and consumes a lot of natural resources, especially water [1]. and the use of concrete continues to grow. this calls for immediate solutions. one way to make concrete a more environmentally friendly material is to include recycled materials in its production. however, recycled materials must be used carefully in concrete. their use can lead to poorer properties and a decrease in the durability of the concrete. this can devalue environmental benefits [2]. the use of recycled aggregates can affect the properties of the final concrete, such as porosity, density, and durability. these differences are attributed to cement mortar that adheres to natural aggregate particles and forms the aggregate itself [3]. the production of cement is a complex process that consumes a significant amount of raw materials and resources and has a negative impact on the environment [4]. the path to reducing the environmental impact of concrete is challenging. the use of other alternative materials that consume less natural resources, are economical and have the potential to act as a binder, would contribute to a more sustainable and healthy environment [5]. microbially induced calcium carbonate precipitation (micp) offers a possible solution. recycled concrete can be modified with micp to improve its properties for other possible applications, such as aggregate for concrete or biobrick production. 2. use of bacteria in engineering applications 2.1. micp principle microbially induced carbonate precipitation (micp) is the collective name for the formation of calcium carbonate minerals, which is produced by the interaction between various metabolic by-products of bacteria and calcium ions [6]. in this process, calcium carbonate serves as both a binder and a filler [7]. in this way, bacteria produce calcium carbonate in various environments around the world, such as soil, groundwater, etc. micp can be used for engineering applications. therefore, it is necessary to understand the behavior and life of bacteria. the calcium carbonate produced by bacteria is produced by its metabolic processes. each group of bacteria has different metabolic processes. the useful microorganisms in engineering applications use heterotrophic metabolisms and depend on organic carbon sources for their growth [8]. the nitrogen cycle is the most used pathway to achieve calcium carbonate precipitation using bacteria and microorganisms. it involves bacteria using ammonification of amino acids, nitrogen reduction (denitrification), or hydrolysis of urea (ureolysis) [8]. ureolysis is clearly the most widely used metabolic pathway [9]. in this process, bacteria produce the enzyme urease, which then participates in the hydrolysis of urea co(nh2)2 to form ammonia nh3 and carbonic acid h2co3. urea serves as a source of nitrogen [10]. ammonia and carbonic acid further react with water to form positive ammonium ions nh4+ and negative carbonate ions co3 2−. production of ammonium ions 33 https://doi.org/10.14311/app.2023.40.0033 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en petr holeček, hana stiborová acta polytechnica ctu proceedings creates an alkaline environment that promotes precipitation of carbonate. positively charged calcium ions ca2+ bind to the negatively charged bacterial cell wall [8]. as the number of calcium carbonate crystals increases, the cell gradually encapsulates. this causes the bacteria to form a carbonate capsule around itself. most ureolytic bacteria are heterotrophic aerobic bacteria and prefer an alkaline environment for their growth [11]. this is beneficial for use in concrete materials that have alkaline conditions within their structure. the use of bacillus species bacteria [12], which are commonly found in soil [13], has proven to be suitable. however, sporosarcina pasteurii has the maximum ureolytic activity, a higher mineral precipitation rate, and rapid growth and is the bacterium most widely used and studied so far [9]. sporosarcina pasteurii is nonpathogenic, highly stable, has great urease activity, and is cost-effective [12]. the frequency of use of ureolytic bacteria is due to their large distribution. they are found in soils and groundwater all over the world [9]. sporosarcina pasteurii is also used in this work. 2.2. influencing factors for micp the amount of calcium carbonate precipitated depends on the type of bacteria, their activity and metabolism [14]. during ureolysis, bacteria produce the enzyme urease, which accelerates the hydrolysis of urea. therefore, bacteria with high ureolytic activity are selected for micp. these are bacterial strains of the bacillus strain or sporosarcina pasteurii [15]. carbonate precipitation is then carried out by hydrolysis of urea to ammonia and co2 [15]. different types of bacteria can produce different amounts of urease and precipitate calcium carbonate at different speeds [9]. high concentrations of bacteria (106 to 108 cells) increase the amount of caco3 precipitated. the bacterial surface serves as a nucleation site for precipitation of caco3 [9]. however, concentrations of bacteria that are too high lead to a rapid and excessive reaction, followed by blocking the surface pores and poor distribution of the bacterial solution in the material, or even disrupting the material due to internal stresses. therefore, it is always necessary to find the optimum bacterial concentration for each specific application. the concrete structure is a highly alkaline environment with a ph of 12–13. this does not create very suitable conditions for bacteria, and they remain in the form of spores with such a high ph [16]. the optimal ph value for urease and bacteria is around 8. but the ph range for its activity is larger. a ph of 7–10 has been reported [9]. if the concrete is disturbed (microcracks, pores), the ph of the surrounding area will drop. this drop causes bacterial spores to wake up and begin their activity [16]. bacteria also need nutrients for their growth, life, and metabolic activities [12]. a sufficient concentration of nutrients provides bacteria with a source of energy [15]. heterotrophic organisms need a source of carbon and nitrogen. consumption of organic acids affects the precipitation of minerals [15]. ureolysis uses urea as a source of carbon, nitrogen, and energy [9], but peptone, yeast extract, or calcium lactate can be used as an alternative and cheaper carbon source [13, 15]. the microstructural properties, morphology, regularity, and durability of the final material formed by micp are also influenced by the source of free calcium ions in the surrounding area of bacteria [7, 17]. calcium salts, such as cacl2, are suitable calcium sources for the formation of caco3 crystals [12]. the ideal calcium source and optimal concentration are different for each strain of bacteria [9]. there are many factors that influence the course of micp, and they need to be studied very carefully. each bacteria will have different requirements for optimal conditions in different environments. the exact behavior of the bacteria must be known if micp is to become a true engineering method to modify the properties of materials. 3. use of micp the uses of micp in engineering applications are many. bacteria can be added to the concrete to improve its properties. bacterial spores and nutrients can be added directly or encapsulated in capsules. the spores remain inactive due to the high ph in the concrete (ph 12–13). however, when microcracks or pores are formed, the ph drops slightly (a ph value of 11 is sufficient) and moisture penetration occurs. the dissolution of nutrients creates suitable conditions for bacteria. the bacteria then regenerates and can heal microcracks and pores with calcium carbonate [18]. this can reduce the permeability and absorption of water and increase the compressive strength and durability of the structure [15]. bacteria can also restore the mechanical properties of concrete and thus ensure self-healing of concrete [9]. by adding bacteria with the ability to precipitate calcium carbonate to concrete, it can reduce maintenance costs and, together with suitable additives, it can partially replace cement [17]. at the same time, bacteria create an alkaline environment in their surroundings sufficient to protect the reinforcement from corrosion [18]. micp bacteria can also be used to improve soil properties. precipitated calcium carbonate can improve soil erosion resistance or slope stabilization [8]. caco3 acts as a binder that increases soil strength and stiffness and reduces its permeability [14]. it is a gentle, cheap, and sustainable procedure compared to conventional means [9]. bacteria can also be used to remove and immobilize heavy metals and radionuclides from the soil [9, 17]. using micp, soluble heavy metals in the soil are stabilized into insoluble forms. metal ions bind to co3 2− and replace ca2+ cations [17]. lead, copper, cadmium, chromium, arsenic, and many others can be immobilized in the 34 vol. 40/2023 bacterially induced calcite formation at the surface of recycled concrete soil [9]. otherwise, these heavy metals would have to be removed from the soil through complex technological processes. micp can be used to treat heavy metalcontaminated wastewater or water [9]. using the metabolic processes of bacteria, co2 can be sequestered in the soil from the air during the formation of caco3 [17]. like concrete or soil, bacteria can also work in aggregates. micp can improve the properties of aggregates, fill voids and pores, and improve their strength [19]. this effect can be used to improve the properties of recycled aggregate. compared to natural aggregate, recycled crushed concrete has worse properties – strength, absorbency, and porosity. with micp, all these properties of recycled concrete can be improved. properties are modified in the area of the itz zone between the aggregate and the old cement mortar. the surface of the recycled aggregate becomes stronger and can better bond to the new cement matrix [19]. this can be achieved in different ways [20]: • the recycled aggregate can be directly immersed in the bacterial solution. however, this method risks a rapid reaction between bacteria and the nutrient solution, which could block pores and cracks on the surface of the aggregate and prevent bacteria from penetrating deeper layers. it is more suitable for fine-grained aggregates. the process is faster. • the recycled aggregate is first impregnated with a bacterial solution with spores and then a nutrient solution is added. this ensures a more homogeneous distribution of bacteria throughout the aggregate structure. the method is also suitable for larger pieces of aggregate, but the process takes longer. modified recycled aggregate can be used as a bacterial carrier for self-healing concrete [19]. if bacterial spores are added to the concrete mix, their viability may be reduced during the concrete mixing and cement hydration process. encapsulation of bacterial spores in aggregate can increase their chance of survival and they only become activated when the concrete is disturbed. concretes with aggregates treated in this way may have reduced water absorption capacity, increased flexural strength, and resistance to carbonation [21]. microbially precipitated calcium carbonate may not only serve as a filler. it can also be used in bonding materials. using this principle, biobricks can be formed from sand, bacteria, and nutrients [17]. by mixing these ingredients, a strong bond can be formed between the grains of the filler material. the final product is a sandstone-like material [22]. micp offers the production of bricks at low temperatures and low cost [7]. biobricks do not have the strength of conventional bricks (the strength of biobricks is around 1300 kpa [22]), but their production is environmentally friendly, compatible with other materials and has good properties even in contact with water [23]. their water absorption is similar to that of conventional bricks [22]. repeated application of a bacterial solution improves the properties of biobricks, as does the addition of other ingredients [24]. the addition of fibers creates load-bearing bridges between the bonded material and the precipitates, which improve compressive strength, ductility, erosion resistance, and reduce water absorption [25]. 4. calcite precipitates at surface of finely ground recycled concrete to use micp to modify the properties of the material or produce new materials, the course and precipitation process of caco3 must be investigated at the microscale. in this work, the focus is on evaluating the activity of cultured bacteria and their ability to precipitate calcium carbonate in finely ground materials. the resulting products were investigated by sem and eds analysis. for this purpose, materials prepared from recycled concrete drainage trough and finely ground silica sand were used. the recycled drainage trough was prepared from unreinforced precast concrete with a lower amount of cement matrix. portland cement rich in alite was used to produce it, which provided a rapid increase in strength and stiffness. its original strength was c20/25, and its age was 4 years. this concrete was then prepared by crushing and grinding. the ureolytic bacterium sporosarcina pasteurii was used and cultured at 28 °c in a culture medium with 20 g/l urea. the ph value of the solution was 7.3. the cells were then suspended in a physiological solution. the solution was then mixed with the prepared recycled finely ground material. after 8–12 hours of sedimentation, a cementing solution consisting of 4 g/l of nutrient medium, 2.8 g/l cacl2, and 20 g/l urea was added to the sample. the solution had a ph of 6.8 ±0.2. after every 24–36 hours, physiological solution or cementing solution was added, always alternately. after 6 cycles, the samples were left at 28 °c for 7 days and then dried at 65 °c. after this process, partially consolidated samples of uniform material were formed but without significant strength. the material could be crushed between the fingers. in figure 1, the silica sand sample after the micp process can be seen. the sample contains grains of bonded material but is still loose, with no overall cohesion, and with a minor strength of the individual grains. in figure 2 the recycled concrete sample after the micp process can be seen. the sample was joined into individual pieces of material. however, the material did not have significant strength or cohesion, and even after light manipulation, it began to disintegrate into the grains shown in the photograph. 35 petr holeček, hana stiborová acta polytechnica ctu proceedings figure 1. silica sand sample after micp process. figure 2. sample of recycled concrete after the micp process. 5. sem analysis after drying, a portion of the sample was ground again into a fine powder, which was then analyzed using a scanning electron microscope. the powder was deposited on the adhesive surface of the target. sem images of silica sand samples after the micp process can be seen in figure 3a. the densely distributed spherical precipitates of caco3 can be seen. the entire sand grains are encased in crystals and it is almost impossible to find a pure sand grain. the caco3 precipitates reach a size of 35 µm. figure 3b shows a detail of a caco3 precipitate grain with bacterial cavities. this proves the microbiological origin of the crystals. figure 4 shows sem images of recycled concrete samples after the micp process. the spherical precipitates of caco3 are uniformly distributed on the (a). silica sand sample. (b). caco3 precipitate. figure 3. sem images of silica sand sample and caco3 precipitate. sample in addition to the ground concrete grains. the caco3 precipitates reach a size of 25 µm. 6. eds analysis in silica sand, it was verified that the spherical formations on the surface of all grains are composed of caco3. calcium and carbon content, a small amount of chlorine, and other elements. the residual chlorine is from the cacl2 solution; the remaining elements (especially sodium, magnesium, and silicon) are the original aggregates and impurities. at the same time, a small amount of nitrogen was found. nitrogen could 36 vol. 40/2023 bacterially induced calcite formation at the surface of recycled concrete figure 4. sem images of a sample of finely ground recycled concrete and caco3 precipitate. show by-products of bacterial activity. elemental analysis of recycled concrete samples verified that the spherical formations contained calcium, carbon, and trace amounts of chlorine. this shows that it is a caco3 crystal with residual chlorine from the cacl2 solution. however, other elements such as sodium, magnesium, silicon, and iron were detected in the crystals, which were from the original concrete or impurities. the finely ground material around the spherical grains was determined to be the original concrete grains (content of silicon, aluminum, and other elements, especially potassium and magnesium) and the original aggregate grains (high silicon content and lower calcium and aluminum content). samples exposed to the bacteria were compared with samples of the original material. formations containing a combination of the elements calcium, oxygen, and carbon are not present in the original material. in the study by nguyen et al. [13], similar results were obtained when identifying bacterial products in concrete. as a result, it was concluded that the spherical formations in the samples are the product of bacteria and consist of calcium carbonate. 7. conclusions the study investigates the possibility of binding finegrained concrete particles with caco3 precipitates produced by bacteria. the preliminary study shows that silica sand samples allow for a higher precipitation rate of caco3 than those composed of recycled concrete. however, the recycled concrete was bonded to form a more compact material with higher strength. this difference is mainly attributed to the different grain size distribution. if the particles are too fine (sized as bacteria = 1.3–4.0 µm), the bacteria cannot proliferate and do not form enough calcium carbonate crystals. if the particles are too coarse, larger and weaker crystals form. the grain size of the original material has a significant effect on the micp process. at the same time, the recycled concrete underwent a secondary hydration process, making the samples stronger and more cohesive. the micp process for binding recycled concrete has not been explored to date. it is necessary to find optimal conditions and suitable bacteria species that are efficient, economical, use ecological substances, and are safe for humans and the environment. recycled concrete and silica sand were used for performance experiments with finely ground materials. attention was paid to the composition and shape of the micp products and the cohesion of the newly formed material. calcium carbonate crystals in different volumes and sizes were found in the materials. the grain size distribution of the starting material is crucial for the progress of micp and the properties of the resulting material. for recycled concretes, the effect of micp and secondary hydration must be identified and assessed. these results will be used in future experiments and further analyses that will be performed 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https://doi.org/10.3390/su12020696 https://doi.org/10.1016/j.scitotenv.2019.135007 https://doi.org/10.3390/cryst11080887 https://doi.org/10.1016/j.clema.2021.100003 https://doi.org/10.1617/s11527-018-1157-4 https://doi.org/10.3390/su10114079 https://doi.org/10.1016/j.conbuildmat.2020.120751 https://doi.org/10.1016/j.conbuildmat.2020.120751 https://doi.org/10.1016/j.conbuildmat.2020.121573 https://doi.org/10.1016/j.conbuildmat.2020.121573 acta polytechnica ctu proceedings 40:33–38, 2023 1 introduction 2 use of bacteria in engineering applications 2.1 micp principle 2.2 influencing factors for micp 3 use of micp 4 calcite precipitates at surface of finely ground recycled concrete 5 sem analysis 6 eds analysis 7 conclusions acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2024.51.0069 acta polytechnica ctu proceedings 51:69–74, 2024 © 2024 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague comparison of rgb and multispectral cameras for targeted applications in agriculture jaroslav pinkas∗, přemysl toman, josef svoboda czech technical university in prague, faculty of transportation sciences, konviktská 20, prague 1, 110 00 ∗ corresponding author: pinkas.jar@gmail.com abstract. agricultural machines nowadays use advanced satellite guidance systems that allow not only autonomous parallel guidance of machinery on the field but also enable the control of agriculture implements based on the geographical location of the field. by using aerial photogrammetry images, it is possible to identify the spots of land that require chemical protection. this information can be used to create prescription maps for the control of specialised implements, allowing the identification of weed outbreaks that require herbicide for their elimination. using spot-spraying technologies, up to 80% of the active substance can be saved compared to the current common broadcast strategy of applying it to the entire field. this technology automatically controls the sprayer nozzles on the booms only in the spots where it is needed. using an unmanned aerial vehicle (uav) allows us to take a detailed picture of the ground. two main possibilities exist for collecting imagery data with an rgb or multispectral camera. one of the key requirements is the appropriate resolution of the picture, which could be controlled by flying altitude. this paper focuses on comparing rgb and multispectral gathered data toward affected spot identification. keywords: agriculture, qgis, rgb camera, multispectral camera, uav. 1. introduction agriculture has been constantly evolving and adapting to new challenges and needs in recent years. an innovative approach that is gaining increasing popularity is precision farming. this modern way of farming combines traditional knowledge and technology to optimise yields while minimising negative environmental impacts. remote sensing is becoming one of the most widely used methods for obtaining geographic information about the earth’s surface and is also usable in agriculture [1]. image data can provide accurate and detailed images of the earth that are valuable for many research areas, including natural disasters, land use, climate monitoring, and environmental protection. precision farming uses a range of technologies and analytical tools that allow for detailed analysis of fields and the exact needs of plants. through the precise and targeted application of fertilisers or plant protection products, overapplication of these substances can be minimised, which positively impacts the environment while saving costs [2]. the goal of this paper is a comparison of rgb cameras, which are easy to buy for farmers, and more expensive multispectral cameras. 1.1. field monitoring one of the main disadvantages of earth observations (eo) satellite monitoring systems is their orbital interval and the inability to record the earth’s surface under increased cloud cover. for some operations, the required resolution may also be a limitation. an alternative may, therefore, be unmanned aerial vehicles (uav), also known as drones, which are becoming increasingly important in agriculture. these technologies bring several benefits to modern agriculture. drones can be used in the same way as satellite systems, but we can freely choose the sensing term, and the drone sensor offers significantly higher accuracy, around 2 cmpx−1. in addition, drones can help detect diseases or pests at this resolution [3]. a prerequisite for imaging agricultural plots is the selection of a uav with sufficient flight length to carry a suitably selected camera. variants are wing-type uavs with low weight and flight time advantages, but handling and flight path maintenance may not be completely accurate, especially in poor weather conditions. rotorcraft uavs are mainly characterised by their easy manoeuvrability, but the higher purchase price of professional uavs can be a disadvantage. a common equipment requirement should be a gnss sensor with differential rtk (real time kinematic) correction. another requirement should be the ability to fly autonomously along a predefined route [4]. the most important part of the uav is undoubtedly the sensor system, which is subject to high demands for resolution and the ability to sense multiple spectra. in practice, this means that a classical rgb camera can be used for common analyses, such as coverage. still, for deeper analyses, it is necessary to work with the infrared bands normally invisible to the eye using multispectral cameras. the cost of a multispectral camera can exceed the purchase price of the uav itself. cameras are commonly equipped with lenses that detect the green (530–570 nm), red (640– 680 nm), red edge (730–740 nm), and nir (near infrared 770–810 nm) bands [5, 6]. plant stress and 69 https://doi.org/10.14311/app.2024.51.0069 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en j. pinkas, p. toman, j. svoboda acta polytechnica ctu proceedings growth progress can be monitored in these bands by spectral analysis. these recorded data are then used to calculate various indexes. optionally, the uav can be equipped with a so-called sunlight sensor, which records the light conditions during detection [7]. 1.2. photogrammetry aerial photogrammetry is an important technique in image processing and geographic information systems (gis). this method uses aerial imagery to create three-dimensional maps, terrain models, and spatial data. photogrammetry uses the principle of parallax, meaning that a three-dimensional model can be obtained when an object is imaged from different angles. images are usually obtained from aerial vehicles such as aircraft or unmanned aerial vehicles. data recording is done with digital cameras, multispectral sensors, and lidar. during data recording, the exact geographic location is also acquired. the main advantages of this method are the speed of obtaining information with high accuracy and resolution of the data compared to, for example, 3d scanners, and the disadvantages are the high intensity of the data and the need to perform sensor calibrations in the case of longer time measurements [8]. one of the outputs of photogrammetry is a digital terrain model (dtm), which can be created using only a combination of aerial imagery, but more accurate models also use lidar data [9]. this model provides an accurate image of the surface of the earth, which is crucial for use not only in agriculture but also in hydrology or urban planning. another output is an accurate geotagged orthomosaic of the earth’s surface, both in the visible rgb spectrum used, for example, on various map portals and with multispectral data for spectral analysis using vegetation indices. in practice, photogrammetry is then used as a tool for measuring area and volume (e.g., construction), creating 3d models or positional analysis, and monitoring the state of the earth’s surface (water management, forestry, agriculture). this paper focuses on comparing rgb and multispectral gathered data toward affected spot identification. another comparison the article deals with is the method of data collecting using uav and eo. for this purpose, an experiment is designed and performed under actual field conditions. it is necessary to compare real sizes of datasets and the complexity of data postprocessing. the experiments will help to reveal the suitability of the gathered data samples, used recording devices with uav, and data sources in the case of eo. 2. methods 2.1. the experiment the experiment was carried out on the field of zd krasna hora in the czech republic, in land registry počepice, dpb code 3709 (6,23 ha), 3703/1 (3,64 ha), 3804/3 (6,16 ha). field 3709 was chosen as an example for this paper (see the red-marked field in figure 1) because of damage which was, on average, within these three examined fields. the field was sown with rape on 17 august using the strip-till method. the vegetation in the agricultural field was significantly affected by voles and grew irregularly. the local conditions of the crop were suitable for learning the technology of the targeted application of a growth regulator where spraying should be turned off in places where growth is weak or without cover. the aim should, therefore, be that strong grown-up crops would be regulated and growth slowed before winter and that weak or nocoverage sites would not. this fact has an economic impact in terms of spray savings. the dji mavic 3m unmanned aerial vehicle and sentinel 2 satellite were used to collect data. the uav collected images in the usual rgb bands and data outside the visible spectrum, multispectral nir and rededge. an autonomous uav mission at 100m was used to collect rgb data with a resolution of 2.42 cmpx−1 (9mb/frame size) and multispectral data with a resolution of 4.84 cmpx−1 (40mb/frame size). the sentinel 2 satellite acquired data with a resolution of 10mpx−1 (data from october 2, 2023). the satellite images recorded closer to the date of the uav measurement were unfortunately affected by cloud cover. 2.2. data processing the raw data recorded from the uav (multispectral and rgb camera) was pre-processed using pix4d fields using the photogrammetric method. the first step in processing was the cropping of measured data along the field boundaries. for further analysis, the sentinel 2 satellite image of the field was also processed. data measured with the multispectral camera were converted to the ndvi (normalized difference vegetation index) index as presented in formula (1), and the conventional rgb data were converted to the vari index (visible atmospherically resistant index) as presented in formula (2) [10]. ndvi= (nir–red/nir+red), (1) vari= (green-red/green+red-blue), (2) where green = pixel values from the green band, red = pixel values from the red band, blue = pixel values from the blue band, nir = pixel values from the near infrared band. the input format supported by agricultural machinery and its implements is the polygon vector maps, created based on the raster data described above. the tool used for raster data conversion was a mathematical model created in qgis (quantum geographic 70 vol. 51/2024 comparison of rgb and multispectral cameras . . . figure 1. location of the field on which the experiment was carried out. information system). this model optimises the resolution and performs smoothing of the extremes according to the size of the nozzle range. according to the specified breakpoint value, the software selects the pixels where the application should be made and assigns them the desired spray dose. analysis was made using the statistical processing tools of qgis. for these purposes, a raster dataset with the size of the grid the same as the resolution was used, and the calculation was made under polygons overlays. 3. results according to the available sentinel 2 satellite data, the zonal distribution is consistent. still, the data are not appropriate for targeted applications, as the local foci are too small and indistinguishable (see figure 2). on the other hand, uav data is more accurate and thus usable for targeted applications. based on the obtained data, it is visible that the identification of the damaged field areas is well recognisable in both ndvi and vari indices (see figure 3). still, the view of the vegetation cover corresponds better to the ndvi index. for confirmation, the ndvi and vari indices were subtracted from each other, figure 2. comparison of the uav and sentinel 2 ndvi index. and the result was confirmation that for targeted applications such as the identification of damaged patches, both sources are suitable. in general, the multispectral camera data can better identify the crop condition even though it is in a relatively early stage of growth. at the time of data collection (9 weeks after planting), the ndvi may not be sufficiently saturated. however, in our conditions, as presented in figure 3d, the difference between them is not significant; from this point of view, both datasets are sufficient. 71 j. pinkas, p. toman, j. svoboda acta polytechnica ctu proceedings (a). (b). (c). (d). figure 3. comparison of the rgb, ndvi and vari data representation. figure 4. comparison of difference of processed data based on ndvi (left) and vari (right) index. as part of the comparison, both the ndvi and vari data were processed using the computational model created in qgis software. the results of a socalled prescription map are presented in figure 4. spots marked with a light-yellow colour indicate places where the spray should be applied. the difference in the sprayed area between the approaches using ndvi and vari is 3%. this means that both methods for this application (green on brown) are acceptable. the threshold value was 0.60 for both (ndvi and vari) indexes. the results of both sources were acceptable. the differences were only at the borders between weak and strong plants. this finding confirms the better sensitivity of the multispectral camera of plant stages. however, in this usage, in the detection of green on brown, the multispectral advantage is negligible (see figure 5). 4. discussion based on the results, it is evident that the processed data obtained through the satellite system sentinel 2 are not sufficient for the targeted application because the resolution is too low. however, the satellite eo data resolution is sufficient for vra (variable rate applications) because the size of the pixel to change the rate is acceptable [11]. using a uav equipped with an rgb or multispectral camera, it is possible to reach a resolution of up to 2 cmpx−1, which is necessary for data analysis regarding identifying specific spots in the agriculture field. based on multispectral data, we can compare various colour shades even in spots with similar spectral profiles. in this paper, we focus on the problem of detecting brown spots in a green field. for such an application, the rgb and multispectral data transformed into ndvi and vari indexes per pixel were examined and found to be sufficient for target applications. for the qgis model, threshold values need to be identified, which is challenging, especially in detecting green shades on a green background. for example, at this moment, we identify value by creating polygons over areas of interest and calculate their average value. in future releases, it could be interesting to integrate artificial intelligence methods to determine threshold values [12, 13]. as part of the verification process, the prescription maps created in .shp (shapefile) format were 72 vol. 51/2024 comparison of rgb and multispectral cameras . . . figure 5. detailed comparison of rgb and ndvi imagery. figure 6. comparison of the prescription maps – left: target operation, right: broadcast operation. imported into the simulator of the usage implement control unit. specifically, it was the john deere r952i trailed sprayer. this machinery supports the control of individual nozzles, which are spaced 25 cm apart on the sprayer’s shoulder. compared to prescription maps without targeted spray application (see figure 6), there is a 20% savings. 5. conclusions based on the evaluation of data obtained by eo satellite technology (sentinel 2), it was found that the resolution of this technology is not suitable for targeted applications. uav data, along with the use of ndvi and vari indices, provide a more accurate and effective method for targeted agricultural applications. using multispectral images, we can achieve more accurate identification, especially on borders between weak or strong plants in affected areas, with a lower resolution compared to images from rgb cameras when choosing a suitable vegetation index. both ndvi and vari indices are effective in identifying damaged field areas, with the ndvi index providing a better representation of vegetation cover. despite the crop being in a relatively early stage of growth at the time of data collection (9 weeks after planting), the multispectral camera data can identify the crop condition better than the rgb camera. however, rgb data are needed for the initial setup of the multispectral image-based identification model. rgb cameras are cost-effective, user-friendly, and ideal for high-resolution visual tasks like crop monitoring and weed detection, especially for the recognition of green on a brown background. data obtained by an rgb camera is also significantly less data intensive. acknowledgements this paper was supported by the czech technical university in prague grant sgs23/132/ohk2/2t/16 precision farming – research of methods for spot-spraying of chemical products. references [1] d. žížala, v. lukas, j. kumhálová. dálkový průzkum země a precizní zemědělství [in czech; remote sensing and precision agriculture], 2021. [2024-01-12]. https://www.ctpz.cz/media/upload/1646732225_ 17-precizni-zemedelstvi-5-web.pdf [2] a. r. melland, d. m. silburn, a. d. mchugh, et al. spot spraying reduces herbicide concentrations in runoff. 73 https://www.ctpz.cz/media/upload/1646732225_17-precizni-zemedelstvi-5-web.pdf https://www.ctpz.cz/media/upload/1646732225_17-precizni-zemedelstvi-5-web.pdf j. pinkas, p. toman, j. svoboda acta polytechnica ctu proceedings journal of agricultural and food chemistry 64(20):4009–4020, 2016. https://doi.org/10.1021/acs.jafc.5b03688 [3] i. elkhrachy. accuracy assessment of low-cost unmanned aerial vehicle (uav) photogrammetry. alexandria engineering journal 60(6):5579–5590, 2021. https://doi.org/10.1016/j.aej.2021.04.011 [4] j. kim, s. kim, c. ju, h. i. son. unmanned aerial vehicles in agriculture: a review of perspective of platform, control, and applications. ieee access 7:105100–105115, 2019. https://doi.org/10.1109/access.2019.2932119 [5] i. horniacek, v. lukas, r. duffkova, et al. the comparasion of satellite and unmanned multispectral imaging for estimation of plant nutritional status of cereals. in 21st international multidisciplinary scientific geoconference sgem 2021, vol. 21, pp. 615–622. 2021. https://doi.org/10.5593/sgem2021/2.1/s10.74 [6] b. allred, l. martinez, m. k. fessehazion, et al. overall results and key findings on the use of uav visible-color, multispectral, and thermal infrared imagery to map agricultural drainage pipes. agricultural water management 232:106036, 2020. https://doi.org/10.1016/j.agwat.2020.106036 [7] l. kumar, k. schmidt, s. dury, a. skidmore. imaging spectrometry and vegetation science. in f. d. v. d. meer, s. m. d. jong (eds.), imaging spectrometry: basic principles and prospective applications, pp. 111–155. springer netherlands, dordrecht, 2001. https://doi.org/10.1007/978-0-306-47578-8_5 [8] j. l. morgan, s. e. gergel, n. c. coops. aerial photography: a rapidly evolving tool for ecological management. bioscience 60(1):47–59, 2010. https://doi.org/10.1525/bio.2010.60.1.9 [9] s. i. jiménez-jiménez, w. ojeda-bustamante, m. d. j. marcial-pablo, j. enciso. digital terrain models generated with low-cost uav photogrammetry: methodology and accuracy. isprs international journal of geo-information 10(5):285, 2021. https://doi.org/10.3390/ijgi10050285 [10] a. gitelson. hyperspectral vegetation indices. in hyperspectral remote sensing of vegetation, pp. 345–364. 2016. https://doi.org/10.1201/b11222-21 [11] european union agency for the space programme. euspa eo and gnss market report. 2022/issue 1. publications office of the european union, 2022. https://doi.org/doi/10.2878/94903 [12] n. y. murad, t. mahmood, a. r. m. forkan, et al. weed detection using deep learning: a systematic literature review. sensors 23(7):3670, 2023. https://doi.org/10.3390/s23073670 [13] j. elbl, v. lukas, j. mezera, et al. using self-propelled sprayers for the targeted application of herbicides. in proceedings of 23rd international multidisciplinary scientific geoconference sgem 2023, pp. 307–314. 2023. https://doi.org/10.5593/sgem2023/2.1/s10.39 74 https://doi.org/10.1021/acs.jafc.5b03688 https://doi.org/10.1016/j.aej.2021.04.011 https://doi.org/10.1109/access.2019.2932119 https://doi.org/10.5593/sgem2021/2.1/s10.74 https://doi.org/10.1016/j.agwat.2020.106036 https://doi.org/10.1007/978-0-306-47578-8_5 https://doi.org/10.1525/bio.2010.60.1.9 https://doi.org/10.3390/ijgi10050285 https://doi.org/10.1201/b11222-21 https://doi.org/doi/10.2878/94903 https://doi.org/10.3390/s23073670 https://doi.org/10.5593/sgem2023/2.1/s10.39 acta polytechnica ctu proceedings 51:69–74, 2024 1 introduction 1.1 field monitoring 1.2 photogrammetry 2 methods 2.1 the experiment 2.2 data processing 3 results 4 discussion 5 conclusions acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2024.51.0001 acta polytechnica ctu proceedings 51:1–5, 2024 © 2024 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague ways to identify factors contributing to the occurrence of road traffic accidents kairatolla k. abisheva,∗, almagul n. baltabekovaa, assylbek zh. kassenovb, ruslan b. mukanovc, karlygash b. assylovaa a toraighyrov university, faculty of engineering, department of transport engineering and logistics, 64 lomov street, pavlodar, 140008, republic of kazakhstan b toraighyrov university, faculty of engineering, department of mechanical engineering and standardization, 64 lomov street, pavlodar, 140008, republic of kazakhstan c satbayev university, institute of energy and mechanical engineering named after a. burkitbayev, 22a satpayev street, almaty, 050013, republic of kazakhstan ∗ corresponding author: abishev.k@teachers.tou.edu.kz abstract. the article deals with the issues of determining the factors contributing to the occurrence of road accidents. on the basis of statistical studies, the main factors most fully reflecting the process of occurrence of road traffic accidents were identified. for this purpose, an expert-statistical method was used. road safety assessment was carried out on the example of the roads of the city of pavlodar. the results obtained allow, in the future, to build a mathematical model that characterizes the possibility of a traffic accident. keywords: car, accident, driver, road, traffic accident, traffic safety. 1. introduction the movement of cars on the road is considered as a complex interconnected system. the structure of this system includes the following elements: “driver”, “car”, “road” and “environment”. the driver of a motor vehicle is the main link in the “driver-carroad-environment” (dcre) system, the sustainable functioning of which determines the efficiency and safety of road traffic. the driver, driving the car, is in constant tension. in motion, he continuously perceives and comprehends the rapidly changing traffic situation, position, speed and condition of his vehicle, instantly makes decisions and implements them. such an active and continuous course of psychic phenomena in conditions of rapidly changing conditions and danger increases the tension of the nervous system and leads to fatigue, and sometimes to overfatigue of the driver. when driving a car, the driver must receive and adequately analyze a large amount of information (the road, controls, road signs and markings, control devices and the operation of the car’s systems and mechanisms, meteorological traffic conditions, etc.). in addition, he must be able to predict the development of the road situation and choose the safest driving modes. with high dynamism, variable conditions and tight time constraints on decision-making, the driver is constantly under great emotional stress. the complexity of the driver-car-road-environment system lies in the fact that it contains ambiguous relationships and patterns of interconnection between its elements, the system is multicriteria and the processes occurring in it are of a stochastic nature, it is difficult to model the dcre system. the dcre system can be classified as complex due to its multidimensionality, the diversity of the nature of elements, connections, structural heterogeneity, and also due to the fact that the system operates under conditions of significant uncertainty of the environmental impact. its complexity is exacerbated by the random nature of the change in its indicators [1, 2]. 2. materials and methods 2.1. study of the interrelations between the elements of the dcre system in an environment characterized by high traffic intensity, in which huge masses of people and vehicles are involved, road safety activities are multifaceted and diverse. assessing the importance of road safety, it should be noted that this problem has specific features of both a legal and organizational and managerial nature. indicators of the level of road safety are a function of many variables, a number of models for assessing and predicting accidents for interstate comparisons take into account the dynamics of changes in accidents over time, as well as other indicators that affect changes in the level of traffic safety (models of practical actions). such models make it possible to monitor trends in the level of traffic safety in the country and take into account the possibilities of future development of the situation [3]. for motor vehicles, human errors are the cause of 90% of all accidents. at the same time, in 57% of accidents, human error is almost the only factor that could lead to an accident. only 2.4% of accidents can be explained solely by a technical malfunction, 1 https://doi.org/10.14311/app.2024.51.0001 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en k. k. abishev, a. n. baltabekova, a. zh. kassenov et al. acta polytechnica ctu proceedings and an unfavorable environment (that is, phenomena like ice) is the cause of 4.7% of road accidents. the remaining 35.9% of accidents occur due to a complex combination of various factors [4]. considering the human factor in the context of the driver-car-roadenvironment system, the phenomenon is multifaceted. any component of this system has a multifaceted effect on the level of safety of road users. the human factor has a very strong influence on the occurrence of accidents on the roads, despite the fact that recently the level of control automation allows technology to prevent collisions and other accidents. when studying the factors influencing the occurrence of road traffic accidents (rta), the reliability of the system ps depends on the degree of influence of each element, which in general can be expressed as a derivative of the probability of failure-free operation of the car pc, the driver pd and the road pr [5, 6]: ps = pc · pd · pr. (1) the probability of failure-free operation of each element of the system is estimated by the derivative of the probability of failure-free operation of each element pc = π · pci, pd = π · pdi, pr = π · pri. (2) in this system, the “driver” element has been studied in detail as a very complex element. also introduced the concept of “defect” in the work of the driver, which arose as a result of changes in loads, psychophysiological indicators under the influence of road conditions. considering the relationship between the elements of the dcre system from the point of view of assessing the safety of traffic modes, eight significant relationships can be distinguished [7, 8]. but the study of the relationship between the elements of the system is a difficult task, so it is necessary to study the course of the environmental impact on the road and consider the relationship of elements in the “environment-road”, “road-driver”, “road-car” systems. it has been established that safe movement is ensured by the causes of a constant or changing dcre system within the minimum intermediate limits, and this trend can be represented as the following function pred = f {dcre} = const. (3) examination of the system approximation when considering the transport process, as well as road safety, shows that it is difficult to distinguish the main one from the total number of accidents due to a number of reasons. violation of even one indicator of the elements of the dcre system can lead to an accident. therefore, for a comprehensive assessment of the influence of various elements of the system on the accident rate, it is necessary to identify the main causal factors of the dcre system [9–11]. we form a mathematical problem as follows. it is required to find an expert transformation of the cause-and-effect relationships of the occurrence of road accidents, their determining causes, i.e., to find a function of the following type ta = f(x1, x2, x3, . . . , xn), (4) where ta – traffic accident; x1, . . . , xn – mutual set of causes affecting the occurrence of an accident, carried out in two stages. 2.2. study of the interrelations between the elements of the ta system let’s consider the results of the examination of statistical data on the registration of road traffic accidents. the work at the first stage consists of an examination of the primary documentation and statistical data on the accounting of accidents in order to select the main causes that are the causes of the accident from among the causes in the dcre system. since the traffic safety assessment was carried out on the example of the roads of the city of pavlodar, the assessment of the factors of the “road” system was carried out based on the road conditions of the considered road junction. statistical materials of the traffic police department of the city of pavlodar were used as conditional values for the research, the subdivisions of highways in the places of occurrence of accidents were studied. as a result of statistical studies, 19 factors were selected that determine the process of occurrence of an accident in more detail. they are listed below in table 1. professional expertise is required to select a small number of the most informative factors. during the research of the second stage aimed at achieving this goal, an expert statistical method of constructing the objective function was used, based on the methodology described in [12]. each accident is largely characterized by a mutual combination of factors x1,. . . , xn. some factors are much more common in road accidents than others. many of them are not subject to direct quantitative measurement, which makes it difficult for further research to determine the true factor that caused the occurrence of an accident. therefore, in order to obtain maximum information about the factors influencing the occurrence of an accident, it is necessary to determine them essentially in descending order, that is, to obtain an inequality of the following type x1 < x2 < x3 < · · · < xn. (5) according to experts, the factors that make the greatest contribution to the phenomenon under study have the smallest sum of degrees x1, and the factors with the least effect have the largest sum xn. the collection of opinions on the factors that most affect the occurrence of an accident was carried out by conducting a survey among employees of the traffic police department, specialists of the automobile 2 vol. 51/2024 ways to identify factors contributing to the occurrence . . . x factors system x1 driver’s experience “driver” x2 driver’s class x3 driver’s age x4 driver’s gender x5 driver’s condition (drunk, sober) x6 driving time before the accident x7 non-compliance with traffic rules x8 technical condition of the car “car” x9 high traffic intensity “road” x10 insufficient width of the roadway x11 presence of road signs x12 low coefficient of adhesion x13 non-compliance of road conditions with the requirements of the traffic regime x14 road irregularities x15 visibility conditions x16 narrow bridges and railway crossings x17 weather conditions (blizzard, fog, precipitation) “environment”x18 time of day (light, dark) x19 season table 1. factors affecting the accident rate according to statistics. enterprise, road managers, teachers and staff of the department of transport engineering and logistics of toraigyrov university. the number of experts is determined based on the following criteria. the number of experts n� should be 3 times more than the n reasons included in the register n� = 3 · n = 3 · 19 = 57. (6) therefore, 60 expert specialists represented in the survey were involved in the experiment. based on the survey data, a comparative questionnaire or a matrix of degrees has been compiled. a sign of the correctness of filling in the tables is the equality of the amounts received for all columns n∑ j=1 xij = (1 + n) · n 2 , (7) where xij is the degree of the j-th reason of the i-th expert; n is the number of reasons. determining the significance of the influence of the selected factors on the occurrence of emergency situations from the point of view of the interviewed experts is performed only after calculating the sum of the rows and the sum of each row, which should coincide with the amount received by the columns n∑ j=1 xij m∑ i=1 xij = n∑ i=1 xij + m∑ j=1 xij . (8) let’s build a histogram based on the values of the degree matrix and enter into the database of the microsoft excel program the law of distribution of the sum of the degrees of influence of the selected factors on the occurrence of an accident. 2.3. definition of interoperability of examiners one of the important elements of the survey is the degree of mutual compatibility between experts, which is determined using concordance coefficients, which are calculated using the formula below w = 12 · s m2(n2 − n) , (9) s = n∑ j=1 x ( m∑ i=1 xij ∑n j=1 xij ∑m i=1 xij n )2 . (10) with full compatibility between specialists, the concordance coefficient is equal to one, in the absence of this compatibility – zero, i.e., the intervals of changing the value of w lie within 0 ≤ w ≤ 1. the significance of the concordance coefficients is determined using the pearson sign ξ2, which has the number of degrees of freedom n = 1, according to the formula given below ξ2 = m(n− 1)ω = s 1 12mn(n+ 1) . (11) if the calculated value of ξ2 is higher than the table value, then the assumption of randomness in the mutual consistency of expert opinion is refuted. as a result of the experiment conducted to collect expert opinions on formulas (9), (10), the value of the concordance coefficient was calculated w = 12 · 210632 602(193 − 19) − 0.10265. (12) 3 k. k. abishev, a. n. baltabekova, a. zh. kassenov et al. acta polytechnica ctu proceedings y is the degree of significance, x is the factors provoking an accident. figure 1. histogram of the significance of factors affecting the accident rate, according to the results of expert examination. x factors degree of significance x1 driver’s experience 73% x2 driver’s class 53% x3 driver’s condition (drunk, sober) 87% x4 driving time before the accident 45% x5 non-compliance with traffic rules 98% x6 technical condition of the car 85% x7 high traffic intensity 78% x8 low coefficient of adhesion 66% x9 non-compliance of road conditions with the requirements of the traffic regime 60% x10 road irregularities 57% x11 visibility conditions 49% x12 weather conditions (blizzard, fog, precipitation) 79% x13 time of day (light, dark) 48% table 2. important factors affecting the accident rate in pavlodar. also, according to the formula (11), its significance was checked by the pearson sign ξ2 ξ2 = 210632 60·19·20 12 = 110.86. (13) comparing the calculated value of the pearson sign ξ2 for the 5% significance level with the tabular value ξ2 = 26.3, it can be concluded that when assessing the significance of factors affecting the occurrence of an accident with a probability of p = 0.95, the opinion of experts is not accidental. ξ2 cur = ξ2 tab, 110.86 > 26.3. (14) 2.4. factors influencing accident rates analyzing the values of table 1 and the histogram in figure 1, it is possible to determine the most important factors affecting the accident rate. factors with a degree of significance below 40% have also been identified. we exclude them from the table. less significant reasons are: “insufficient width of the roadway” – 31%, “presence of road signs” – 26%, “driver’s age” – 28%, “driver’s gender” – 18%, “narrow bridges and railway crossings” – 16% and “season” – 13%. each of the listed reasons is, to one degree or another, only a factor accompanying the other. the results of the traffic safety assessment on the example of the roads of the city of pavlodar are presented in table 2. as mentioned above, this table does not take into account the factors affecting the accident rate, in which the degree of significance is below 40%. as a result, the number of the most important factors affecting the accident rate in the city of pavlodar is 13. 3. conclusions according to the latest data, there is an increase in the number of cars of all forms of ownership in the republic of kazakhstan. the growth of the country’s car park and the constant increase in traffic intensity have brought to the fore the problem of road safety as the most important state task. in this regard, work aimed at ensuring the reliability and safety of the driver’s activities is of paramount importance. 4 vol. 51/2024 ways to identify factors contributing to the occurrence . . . using the results of the study to determine the factors influencing the accident rate in the conditions of the city of pavlodar, it is possible, in the future, to determine the expert function and build a mathematical model of road accidents. therefore, such research needs to be accelerated. references [1] n. p. patorniti, n. j. stevens, p. m. salmon. a systems approach to city design: exploring the compatibility of sociotechnical systems. habitat international 66:42–48, 2017. https: //doi.org/10.1016/j.habitatint.2017.05.008 [2] m. o. haque, t. h. haque. evaluating the effects of the road safety system approach in brunei. transportation research part a: policy and practice 118:594–607, 2018. https://doi.org/10.1016/j.tra.2018.08.017 [3] v. v. chvanov. metody otsenki i povysheniya bezopasnosti dorozhnogo dvizheniya s uchetom usloviy raboty voditelya [in russian; methods of assessing and improving road safety, taking into account the working conditions of the driver]. infra-m, moscow, 2010. 476 p. isbn 978-5-16-004517-7. 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enhancement of control processes of city buses’ traffic safety. in 2016 smart cities symposium prague (scsp), pp. 1–5. 2016. https://doi.org/10.1109/scsp.2016.7501042 5 https://doi.org/10.1016/j.habitatint.2017.05.008 https://doi.org/10.1016/j.habitatint.2017.05.008 https://doi.org/10.1016/j.tra.2018.08.017 https://doi.org/10.1016/j.procs.2019.01.163 https://doi.org/10.1016/j.jclepro.2018.08.229 https://doi.org/10.1109/scsp.2015.7181573 https://doi.org/10.1016/j.jtte.2017.09.003 https://doi.org/10.1109/scsp.2016.7501042 acta polytechnica ctu proceedings 51:1–5, 2024 1 introduction 2 materials and methods 2.1 study of the interrelations between the elements of the dcre system 2.2 study of the interrelations between the elements of the ta system 2.3 definition of interoperability of examiners 2.4 factors influencing accident rates 3 conclusions references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2023.45.0070 acta polytechnica ctu proceedings 45:70–75, 2023 © 2023 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague possibilities of complex numerical modelling at the czech geotechnical society workplace martin vaníčeka,∗, daniel jiráskob, zdeněk šiškab a geosyntetika, s.r.o., nikoly tesly 3, 160 00 prague 6, czech republic b czech technical university in prague, faculty of civil engineering, geotechnical department, thákurova 2077/7, 160 00 prague 6, czech republic ∗ corresponding author: vanicek@geosyntetika.cz abstract. in the paper, there are examples of complex geotechnical problems in both 2d and 3d that are modelled using plaxis software on the specialist workstation provided by the czech geotechnical society to their members. keywords: numerical modeling, plaxis, embankment, earthquake, reinforced slope, piles. 1. introduction the czech geotechnical society within their activities to support its members purchased workstation based on z6 hp workstation and also software plaxis for solving complex 2d and 3d projects based on finite elements method and also software package autodesk architecture, engineering & construction collection to support bim based 3d design of such projects. this workstation is currently based on geotechnical department of faculty of civil engineering czech technical university in prague with the possibility for remote access by čgts members. at the moment the remote access is tested to determine its quality and stability. hereafter we present selected 2d and 3d complex modelling examples to imagine how complex projects can be solved using this workstation. the dam was designed by the american firm harza engineering company. construction began in 1956 and was completed in the summer of 1961, when the reservoir began to fill. the dam consists of a rockfill stabilization part, a clay core sealing with side slopes of 1:0.3 (v:h) and filters. under the clay core, there are two grout galleries on both the left and right banks. the subsoil of the dam is made up of layers of sedimentary rocks inclined over the water. these are mainly limestone, marlstone, sandstone and conglomerate. on november 12, 2017, a large earthquake of magnitude 7.3 occurred near the iraq-iran border. according to the usgs (united states geological survey), the earthquake was caused by an oblique thrust in the earth’s crust. the epicenter of the earthquake was located near the town of ezgeleh, just 32 km from the dam. the earthquake lasted 30 seconds and is believed to be the most destructive earthquake on the iraq-iran border since 1909. during this earthquake, the crown of the embankment subsided by 0.5 m. at the same time, it led to transverse and longitudinal cracks visible in the embankment crown. based on this event, the competent authority decided to conduct a general inspection of the dam and related facilities [1]. as part of this inspection, there was also a requirement to create a geometric model of the dam and its subsoil and perform numerical calculations to recalculate the stability of the dam body under various relevant design situations. 2. example of a complex 3d geometric and 2d numerical problem the derbendikhan rockfill dam (see figure 1) with a maximum height of 130 m is selected as representative of the complex 2d modeling task. the derbandikhan dam is located in iraq on the diyala–sirvan river approximately 65 km southeast of the city of sulaimaniah and 230 km east of the city of baghdad. figure 1. general overview of the dam. 2.1. geometric 3d model of the dam the construction of the model of the dam body, its surroundings and geology was supported by the materials given by the directorate of the derbandikhan dam. these original drawings of the as built design from 1954 to 1962 – [2] were scanned – see figure 2, which had to be digitized first. 70 https://doi.org/10.14311/app.2023.45.0070 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 45/2023 complex numerical modelling at the cgs workplace a. b. c. d. figure 2. (a.) sample of original drawings intended for digitization –x cross sections of dams. (b.) sample of original drawings intended for digitization – cross-sections through an grout gallery. (c.) sample of original drawings intended for digitization – longitudinal profile of grout gallery. (d.) sample of original drawings intended for digitization – expoloratory boreholes b.a. figure 3. digitized terrain surface – in (b.) also with the foundation joint under the dam body. the 3d model of the embankment was created with the support of the following programs and their complementary modules of these programs: autocad 2022, civil 3d 2022, infraworks see figures 3 to 6 – [3]. digitization of the scanned drawings was done in the raster tools module. based on the contour maps (see the previous chapter), a model of the terrain immediately adjacent to the water body was first created. this part of the surface model was created using the civil 3d application based on the contours digitized using the raster tools module from the map base. subsequently, the terrain model under the dam and outside the dam was extended by a significant part of the surface with the help of the infraworks application and google maps. on the basis of exploratory boreholes drawings, a geotechnical model of the civil 3d application was used to create a model of the geology of the area of interest including 79 core 71 m. vaníček, d. jirásko, z. šiška acta polytechnica ctu proceedings b.a. figure 4. digitized exploratory boreholes of the area of interest and a geological model in the civil 3d. figure 5. model of grout galleries. b.a. figure 6. sealing clay core model, (b.): view of the dam body surface model. wells, both vertical and inclined, or otherwise oriented – see figure 4. original data from boreholes – see figure 2d had to be prepared manually in excel first, including depths and information about individual lithological layers. this database was subsequently imported into the civil 3d geotechnical module. 2.2. numeric 2d model of the dam a numerical 2d model of the dam was made in a cross-section at the station of 330 m, in which the dam reaches its greatest depth. the basic geometry of the model was obtained by sectioning the 3d model of the dam described above. it was further supplemented with various structural elements of the dam body and with the subsoil updated by the latest engineering geological survey. the numerical model was created in the plaxis 2d ultimate 2022 program – see figure 7 using the mohr-coulomb material model. the numerical model consisted of 12 phases simulating the gradual construction of the dam. next, the stages continued to determine the degree of stability of the structure in their different life stages and possible situations. to determine the degree of stability, a gradual reduction of the shear parameters of the materials used was used – the so-called ϕ/c reduction. specifically, the degree of stability of the dam was evaluated at different levels of the water level in the reservoir, including time-dependent situations, such as filling, or different variants of a rapid drop of the water 72 vol. 45/2023 complex numerical modelling at the cgs workplace figure 7. numeric 2d model of the dam. figure 8. shear strain after ϕ/c reduction when the dam is fully filled. figure 9. shear strain after ϕ/c reduction with rapid lowering of the water level in the reservoir. figure 10. shear strain after an earthquake simulation. level (figures 8 and 9). finally, a dynamic analysis was also carried out, where the response of the dam to the loading of the subsoil movement at different frequencies simulating an earthquake was determined (figure 10). the numerical model contained a total of approximately 30 individual phases and made it possible to identify the most critical shear surfaces and the most likely dike failure systems. 3. example of 3d complex project steep reinforced motorway embankment with noise protection barrier founded on piles was selected as representative for complex 3d numerical analysis. high level of complexity is given by interaction between piles and reinforcing geogrids on the plan of existing road. 3.1. geotechnical conditions existing road embankment that will be finally replaced by motorway is founded on highly compressible and low bearing strata of alluvial silts and clays. the existing embankment already consolidated, however the motorway embankment is higher and wider and hence settlement and stability of the whole structure will be highly important. based on geotechnical investigations parameters of both subsoil and existing embankment have been determined. during the construction process the 73 m. vaníček, d. jirásko, z. šiška acta polytechnica ctu proceedings so il p ro pe rt ie s u ni ts a ct iv e zo ne c l– in te rm ed ia te pl as ti ci ty cl ay s c l– so ft hi gh pl as ti ci ty cl ay s g ab io n r oa d co ns tr uc ti on e xi st in g em ba nk m en t fil l su bs oi l ex ch an ge n ew em ba nk m en t fil l g ab io n_ so 17 1 properties colour γunsat [kn m−3] 20.50 20.00 19.50 19.00 20.50 18.50 20.50 19.00 19.00 γsat [kn m−3] 21.00 21.00 20.50 19.50 21.00 19.00 21.00 19.50 19.50 einit [−] 0.33 0.80 0.90 0.30 0.25 0.30 0.25 0.30 0.30 e′ ref [mn m−2] 80.00 3.00 2.00 300.00 300.00 8.00 300.00 15.00 300.00 ν (nu) [−] 0.25 0.40 0.42 0.20 0.20 0.35 0.20 0.35 0.20 c′ ref [kn m−2] 1.00 8.00 2.00 1.00 5.00 8.00 0.10 10.00 10.00 ϕ′(phi) [◦] 35.00 21.00 15.00 45.00 42.00 27.00 39.00 27.00 45.00 ψ (psi) [◦] 7.00 0.00 0.00 20.00 13.00 1.00 10.00 1.00 20.00 k0 [−] 0.4264 0.6416 0.7412 0.2929 0.3309 0.5460 0.3707 0.5460 0.2929 table 1. selected properties of soils used in the analyses. geotechnical model was refined in terms of new embankment fill properties based on its testing. parameters of road structure layers and gabions infill have been determined by competent appraisal according to road layers specifications. most important properties used during the numerical analyses for mohr-coulomb constitutive model are presented in table 1. ground water level was assumed in the level of original ground surface prior to construction of existing road and later it was modelled as depression curve from 1/3 of embankment height in its middle to the level of original ground on the edge of modelled area. properties of materials other than soil (geogrids, piles and noise protection barrier panels) have been assumed as linear elastic without strength limit. the exceedance of strength limit was checked later. 3.2. construction process modelling motorway embankment construction was modelled in 12 main phases and later on followed phases modelling different kinds of loadings (traffic and wind) and stages to checks uls for given loading. main phases of construction: 1. original stress state 2. existing embankment construction 3. ground water level increase to 1/3 of embankment level 4. excavation of existing embankment part needed for motorway construction and so171 5. so171 – gabion height increase and its backfill 6. subsoil exchange below reinforced embankment face 7. 1. layer of reinforced earth structure 8. 2. layer of reinforced earth structure 9. so171 – finalising road construction 10. 3. layer of reinforced earth structure 11. 4. layer of reinforced earth structure 12. piles installation 13. final layer of reinforced earth structure and finalising all other structures modelling of loading as udl followed. the value of characteristic loading from traffic based on czech code 736133 is 10 kn m−2 and characteristic value for wind load of 1.71 kn m−2 was given by the noise protection barrier designer. most critical loading phases: 13. characteristic traffic loading on motorway, 23. uls wind away from motorway + traffic load, 24. φ/c reduction for 23. stability check of the whole structure have been made according to the current legislation based on en 1997-1 design of “geotechnical structures” (eurocode 7). during uls calculations the design approach 3 was followed, when the soil properties are reduced by partial factors (1.25) and loads are increased by partial factors (1.30). 3.3. modelling results graphical output from modelling for the most critical loading (modelling phase 24 after the soil properties reduction by a factor of 1.35) is presented on figures 11 in both 3d and cross-section views. as during the φ/c reduction procedure the reduction factor of the soil properties reached the value of 1.35, which is higher than limiting (safe) value of 1.25 and the structure is still stable, we can conclude that the earth structure comprising reinforced steep slope combined with piled noise protection barrier is and will be perfectly safe. 74 vol. 45/2023 complex numerical modelling at the cgs workplace b.a. d.c. figure 11. (a.) overall horizontal deformatiom perpendicular to motorway axis. (b.) overall deformation in cross-section through motorway. (c.) overall horizontal deformation perpendicular to motway axis in the place of piles for noise protection barrier. (d.) overall horizontal deformation perpendicular to motway axis in the place of piles for noise protection barrier just due to the reduction of soil properties. 4. conclusion the examples described above indicate how complex tasks can be performed at the workstation, not only from the point of view of calculations, but also with regard to the initial modeling of the task using cad together with bim. acknowledgements we would like to thank cgs for creating such a workplace that will help all members solve very complex geotechnical tasks at a very high professional level. references [1] derbendikhan dam in sulaymaniyah governorate, iraq – analysis and safety evaluation of derbendikhan dam, final report, crea hydro & energy, 2022. [2] derbendikhan dam project. harza engineering company, as built design, 1964. [3] p. časnocha. stability analysis of rockfill dam. master’s thesis, faculty of civil engineering, czech technical university in prague, 2022. 75 acta polytechnica ctu proceedings 45:1–6, 2023 1 introduction 2 example of a complex 3d geometric and 2d numerical problem 2.1 geometric 3d model of the dam 2.2 numeric 2d model of the dam 3 example of 3d complex project 3.1 geotechnical conditions 3.2 construction process modelling 3.3 modelling results 4 conclusion acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2024.50.0062 acta polytechnica ctu proceedings 50:62–68, 2024 © 2024 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague local cohesion of splats in hybrid plasma spray coating as observed by in-situ experiment radek mušáleka,∗, tomáš tesařa, jakub minaříka,b, jonáš dudíka,c a czech academy of sciences, institute of plasma physics, u slovanky 2525/1a, 182 00 prague, czech republic b czech technical university in prague, faculty of mechanical engineering, karlovo náměstí 13, 121 35 prague, czech republic c czech technical university in prague, faculty of nuclear sciences and physical engineering, trojanova 13, 120 00 prague, czech republic ∗ corresponding author: musalek@ipp.cas.cz abstract. hybrid plasma spraying combines spraying of dry coarse powders and liquids (suspensions and/or solutions). introduction of secondary miniature phase from the liquid may provide new coating functionality but also may have a significant influence on the local coating cohesion – increasing or decreasing it, depending on the materials combination and deposition conditions. in this study, cohesion of dissimilar splats within the early-stage experimental hybrid thermal barrier coating (tbc) was evaluated by in-situ sem three-point bending (3pb) observation coupled with high-resolution strain mapping. loading was carried out on the annealed sample simulating in-service thermal exposure. observed coating failure confirmed that the coating exhibits a rather high intersplat cohesion lowering its strain-tolerance, which was presumably the reason for the relatively low thermal cycling resistance of the deposit. keywords: plasma spraying, hybrid coating, in-situ sem, strain mapping, cohesion. 1. introduction plasma spraying is a versatile method used for the preparation of various protective coatings [1]. conventional deposition from coarse powders provides high deposition rate and lamellar microstructure, whereas deposition from liquids (suspensions or solutions) provides a new class of coatings microstructures with miniature splats [2]. a cross-over of these two techniques is so called hybrid plasma spraying (figure 1left), where powders and liquids are injected simultaneously into the plasma jet with an intent to combine benefits of both types of feedstocks [3, 4]. generally, powder-originated splats serve here as a matrix, whereas the miniature (secondary) splats are intended to provide an additional coating functionality. hybrid spraying is currently used namely for the development of tribological coatings for wear-intensive applications [5, 6]. in our laboratory, the first experimental hybrid thermal barrier coatings (tbcs) were recently deposited by hybrid water-/argon stabilized plasma torch (wsp-h). yttria-stabilized zirconia (ysz) was used as the matrix and ysz, al2o3, gadolinium zirconate (gzo) and yttrium aluminum garnet (yag) as the secondary phase [7]. deposited coatings showed promising thermal insulation properties and phase stability but their thermal cycling fatigue lifetimes were inferior to the reference conventional coatings, presumably due to their insufficient strain tolerance caused by a rather dense microstructure. straining of hybrid ysz-ysz and ysz-al2o3 coatings was already studied in [8] by in-situ experiment supplemented with strain mapping [8]. general task of strain mapping is to find spatial transformation of the source micrograph into the new “deformed” micrograph (figure 1-right). from the field of displacement vectors, field of strains can be evaluated and transformed into so called strain maps. localized straining in strain maps highlights changes in the material microstructure which are often hardly visible to the naked eye. in the case of relatively brittle thermally sprayed coatings, excessive localized straining usually means activation of coating failure, typically by cracking of splats, their mutual sliding, propagation of magistrate cracks or debonding of the coating from the substrate. strain mapping algorithms usually work with subpixel precision but demand significant computational power especially if sequences of high-resolution strain maps are desirable. older custom-made strain mapping procedure used in our laboratory was based on adapted matlab code using cross correlation method [9]. recently, a new open-source pixel-wise gcpu_opticalflow software using fast pyramidal approach developed by chabib et al. [10] was adapted in our laboratory. aim of this study was to carry out an additional experiment for ysz-al2o3 sample after the heattreatment simulating short exposure to jet-engine relevant temperature and, in particular, to test and validate the updated strain-mapping algorithms enabling the tracking of the coating failure evolution at 62 https://doi.org/10.14311/app.2024.50.0062 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 50/2024 local cohesion of splats in hybrid plasma spray coating . . . figure 1. left) scheme of hybrid spraying combining deposition of ysz from powder and miniature al2o3 from suspension. right) principle of displacement vectors evaluation for strain mapping. higher spatial and temporal resolution. relating the microstructural changes incrementally to the preceding loading stage (instead of to the initial unloaded stage) also helps to relate the failure areas to the local coating microstructure as it travels during the experiment within the observed field of view. 2. materials and methods the coating sample was prepared by hybrid water/argonstabilized plasma torch wsp-h 500 (projectsoft hk, a.s., czechia) using hastelloy-x coupon, nicraly bond-coat and hybrid ysz-al2o3 top-coat. amperit nicraly 413.006, ysz 831.006 (both höganäs, germany) powders, and water-based al2o3 suspension (treibacher industrie ag, austria) with 40 wt.% solid load and mean particle size 1.5 µm were used as feedstocks for the deposition. more detailed information on sample preparation is provided in [7]. in order to simulate thermal exposure of the coating without introducing large-scale coating failure, sample was annealed for 5 hours at 1 150 °c in air (10 °cmin−1 heating ramp, 5 °cmin−1 cooling ramp). heat treated sample was cut into form of “matchstick” type specimens with length ∼28mm. the side of the sample showing coating cross-section was polished up to colloidal silica polishing step (using op-s suspension, struers, denmark) providing planar scratch-free surface for observation. the final cross-section of the sample was 1.70× 1.66mm (width×height). thickness of the substrate, bondcoat, and top-coat were 1.24mm, 0.13mm, and 0.29mm, respectively. sample was then inserted into in-situ loading stage microtest 200 n (deben, uk) which was mounted in the evo ma15 (carl zeiss, germany) sem microscope. loading was carried out using 3-point bending setup with outer support span l = 23mm. diameter of the loading pins was 4mm. after each 50µm of deflection, loading was paused, sample position in the field of view was re-aligned, and sem image was acquired with high resolution (2 048× 1 536 pixels). back-scattered electron imaging mode was used in order to visualize spatial distribution of phases with different mean atomic number. heterogenous microstructure pattern of the coatings could be directly used for the evaluation of local displacement fields using fast gcpu_opticalflow software [10]. from the displacement fields, strain-maps were evaluated for every pixel of the sem images in terms of in-plane εxx (parallel to the coating-substrate interface), outof-plane εxx (spraying direction perpendicular to the coating-substrate interface), and engineering shear strain εxy components. equivalent strain field was computed from the strain field components according to the formula [11]: εeq = √ ε2 xx + ε2 yy + 2ε2 xy. (1) unlike in our previous study [8], strain maps were not related to the initial stage (pristine microstructure before loading), but to the preceding loading step. this approach also allowed to overcome difficulties with the strain evaluation in the areas close to the image edges, which gradually travelled out of the field of view, producing undesirable imaging artifacts. 3. results and discussion microstructure of the as-prepared coating in the area above the central loading pin is shown in figure 2. as intended, the coating consisted of hybrid ysz-al2o3 top-coat, on top of metallic nicraly bond-coat, on top of hastelloy-x substrate. the heat exposure induced distinguishable bond-coat oxidation. within the top-coat, miniature al2o3 splats were well distinguishable among larger splats of ysz matrix. some intermingling of both phases was observed, preferentially on top of the miniature phase. such asymmetric interaction is given by the different capabilities of both coarse and fine splats to remelt underlying material [12]. however, onset of interaction around the whole circumference of al2o3 splats was observed due to the heat exposure. moreover, pores and intersplat cracks may be discerned from the micrographs. some vertical pre-cracks were also observed within the coating microstructure, presumably due to the rather low spraying (“stand-off”) distance resulting in high sample temperature during the deposition. this led to a significant densification of the coating and partial development of the vertical cracks. these are not necessarily detrimental as such opening cracks are intentionally induced in so-called vertically cracked thermal barrier coatings (dvc-tbc) to improve the strain tolerance of the coatings [13]. this strategy is 63 r. mušálek, t. tesař, j. minařík, j. dudík acta polytechnica ctu proceedings figure 2. cross-section of the samples. left) overview. right) detail of the top-coat, bright phase ysz, dark phase al2o3. intermediate phase visible on top of some al2o3 splats (semi-grey phase). figure 3. left) scheme of in-situ 3-point bending experiment. right) load curve obtained during in-situ 3-point bending experiment. an alternative to production of conventional porous tbcs with loose microstructure. the experimental setup and loading-deflection curve obtained during the in-situ experiment are shown in figure 3. the loading was elastic up to approx. 40n after which the plastic loading of the substrate took over. during the sem image acquisition, the load partially dropped showing characteristic periodic steps on the loading record. stiffness of the substrate compared to the coating is too big to see cracking of the coating from the plot. from the sample thickness h (given by the sum of thickness of the substrate and individual sub-layers), support span l, and sample deflection d, macroscopic theoretical flexural strain εtheoretical was evaluated individually for substrate, bond-coat and top-coat surfaces without considering any coating failure according to the classic formula for 3-point bending: εtheoretical = 6dh l2 . (2) it should be noted that as first approximation, it was assumed that the neutral plane of the sample passed through the middle of the sample thickness. the overall straining of the top-coat was significantly higher (1.88%) when compared to the substrate-coating interface (1.40%). at such strains, the coating was obviously already damaged, as observed at the end of the test (figure 4). however, as the stiffness of the substrate was far greater than the stiffness of the coating, evolution of the coating failure did not have any visible influence on the loading curve. loading curves must therefore be coupled with the sem observation or better with the strain mapping, showing directly the coating failure from its very beginning. figure 4 illustrates the final appearance of the sample after the test, showing clearly set of several major through-thickness cracks with the mean spacing of 360 µm in the central area stretching from the coating surface towards the substrate. all cracks propagated fairly vertically with minimum in-plane branching. occasionally, the internal fracture surfaces could be observed directly within the open cracks (figure 5). it is notable to mention that the fracture propagated in the out-of-plane direction without significant interaction with the secondary phase both in the polished plane, and deeper in the bulk of the coating as observed on the fracture surfaces. the observation on the polished cross section may be thus considered 64 vol. 50/2024 local cohesion of splats in hybrid plasma spray coating . . . figure 4. left) overview of the sample after the test. central loading pin is visible at the bottom of the image. right) detail of crack within the top-coat above the central pin. figure 5. detail of cracks from figure 4 propagating through the top-coat. representative for the whole coating. moreover, the fraction of the secondary phase on the fracture surface corresponded to the overall content of the miniature phase on the coating cross-section. this means that the secondary phase splats were not the weakest spots in the coating structure promoting the crack propagation, because should the opposite be true, the content of the secondary phase splats (or even their clusters) on the internal fracture surfaces would be significantly higher than its overall content in the coating microstructure. the selection of images declaring gradual propagation of the cracks above the central loading pin during the in-situ experiment is illustrated in figure 6. for each image of the sequence, strain of each sub-layer was evaluated from the changing distance of two characteristic features preselected on top of substrate and each sub-layer. the original distance of each pair of reference points was approximately 0.9mm, i.e., ∼90% of width of view of the images used for the evaluation (see figure 6). with increasing loading, the distance between the reference points increased due to the straining of the material and evolution of multiple cracks between them. it is obvious that the level of local strain evaluated by such image analysis (ia) was significantly higher than the macroscopic theoretical strain evaluated from the deflection, also better reflecting the ongoing coating failure by changes of the slope of the curves (see figure 7). as expected, straining of the top-coat was the highest due to its greater distance from the specimen neutral axis. nevertheless, such manual strain analysis is not only tedious, but provides only general information about the straining of the layers along one dimension. finally, from the sequence of micrographs, the detailed area of propagation of two major cracks (see figure 4-right) was selected for the 2d computer aided strain mapping (figure 8). displacement and strain fields were evaluated for each pixel of the regions of interest (788× 1051 pixel). strains were related to the preceding loading step using default settings of the gcpu_opticalflow code with size_median_filter = 5 and lmbda parameter = 5.0 × 105. although the option of gpu-accelerated computing with cuda toolkit libraries was not available on the used workstation (ms windows 11 pro, intel i5-13500 processor, 32gb ram), processing of each step took only 27.4± 3.2 seconds, so the whole batch of 20 high resolution images was processed in less than 10 minutes. for comparison, processing of just one loading step using our previous strain mapping code based on matlab cross-correlation routine [9] took 37 minutes, which is ∼80 times longer. another benefit of pyramidal approach is that it does not re65 r. mušálek, t. tesař, j. minařík, j. dudík acta polytechnica ctu proceedings figure 6. evolution of cracking during in-situ test. micrographs with deflection of 0, 200, 400, 600, 800, 1000µm (from top-left to bottom-right) were selected. figure 7. plot of strain on the surface of each sublayer evaluated by image analysis (ia) and from the sample deflection. quire initial guess of maximum displacement value making batch processing significantly easier. moreover, computing time does not increase significantly with increasing maximum displacement detected in the image, unlike in the original matlab-based code. high resolution of the obtained strain maps showed not only the major cracking apparent already to the naked eye, but also minor changing of the coating microstructure as well as directly visualized the activation and de-activation of the crack networks. first crack in the observed area initiated at the topcoat surface, and gradually propagated towards the substrate, later to be accompanied by a second crack initiated in the lateral distance of approx. 130µm. second crack prevailed and its propagation led to the redistribution of the stresses within the coating and closing of the original crack. nevertheless, both cracks propagated by through-splats cracking virtually regardless of the local coating microstructure, in particular presence of secondary phase or splat boundaries. this in other words confirms again that the dissimilar interfaces were not the weak spots of the coating microstructure and that the coating possessed a rather high stiffness, eventually resulting in the limited coating strain tolerance. also after reaching the bond-coat, the cracks propagated perpendicularly to the bond-coat/top-coat interface. in this case, debonding of the splats was typical, especially when the oxides were present at the splat interface. however, intrasplat cracking and plastic necking of the metallic splats was also occasionally observed. 4. conclusions the carried out in-situ sem 3pb experiment confirmed high cohesion of the deposited hybrid yszal2o3 coating as the induced cracks propagated dominantly by through-thickness splat cracking regardless of the local presence of the secondary miniature phase. such behavior leads to the rather low strain tolerance of the coating, which led to relatively poor performance of this coating in the thermal cycling fatigue test when compared to conventional thermal barrier coatings which dissipate the straining energy more efficiently. this finding will be used in the deposition of the next set of hybrid tbcs with the aim of using deposition conditions favoring formation of less compact microstructure. high-resolution strain mapping using pyramidal approach was confirmed to be a fast and strong tool for failure evolution assessment due to the direct visualization of the failure processes within the heterogenous coating microstructure. it is thus a welcome addition to the conventional fractography analysis, which is in the case of plasma sprayed coatings rather complicated as the as-deposited microstructure already contains numerous “defects” such as pores and pre-cracks. 66 vol. 50/2024 local cohesion of splats in hybrid plasma spray coating . . . figure 8. equivalent strain maps showing propagation of the cracks above the central pin for each of the acquired sem micrographs. deflection step 50µm (from top-left to bottom-right). strain maps were related to the previous stage (stepwise evaluation). scale bar: 0.1 = 10% equivalent deformation. acknowledgements financial support through project 22-21478s “highenthalpy deposition of hybrid plasma spray coatings” funded by czech science foundation is gratefully acknowledged. references [1] j. r. davis. handbook of thermal spray technology. asm international, materials park, oh, usa, 2004. [2] p. fauchais, g. montavon, r. s. lima, b. r. marple. engineering a new class of thermal spray nano-based microstructures from agglomerated nanostructured particles, suspensions and solutions: an invited review. journal of physics d: applied physics 44(9):093001, 2011. https://doi.org/10.1088/0022-3727/44/9/093001 [3] s. v. joshi, g. sivakumar, t. raghuveer, r. o. dusane. hybrid plasma-sprayed thermal barrier coatings using powder and solution precursor feedstock. journal of thermal spray technology 23(4):616–624, 2014. https://doi.org/10.1007/s11666-014-0075-4 [4] s. v. joshi, g. sivakumar. hybrid processing with powders and solutions: a novel approach to deposit composite coatings. journal of thermal spray technology 24(7):1166–1186, 2015. https://doi.org/10.1007/s11666-015-0262-y [5] g. bolelli, a. candeli, l. lusvarghi, et al. tribology of nicraly+al2o3 composite coatings by plasma spraying with hybrid feeding of dry powder+suspension. wear 344-345:69–85, 2015. https://doi.org/10.1016/j.wear.2015.10.014 [6] j. w. murray, a. leva, s. joshi, t. hussain. microstructure and wear behaviour of powder and suspension hybrid al2o3-ysz coatings. ceramics international 44(7):8498–8504, 2018. https://doi.org/10.1016/j.ceramint.2018.02.048 [7] r. musalek, j. dudik, t. tesar, et al. microstructure and thermal properties of the early-stage experimental thermal barrier coatings deposited by hybrid plasma spraying. journal of thermal spray technology 33:732–745, 2024. https://doi.org/10.1007/s11666-023-01708-w [8] r. musalek, t. tesar, j. minarik, et al. in-situ sem observation of mechanical failure of hybrid plasma spray coatings. in thermal spray 2024: proceedings from the international thermal spray conference, pp. 266–277. 2024. https://doi.org/10.31399/asm.cp.itsc2024p0266 [9] r. musalek, m. vilemova, j. matejicek, et al. in-situ observation of ongoing microstructural changes in functionally graded thermal spray coating during mechanical loading. surface modification technologies xxviii:571–579, 2014. 67 https://doi.org/10.1088/0022-3727/44/9/093001 https://doi.org/10.1007/s11666-014-0075-4 https://doi.org/10.1007/s11666-015-0262-y https://doi.org/10.1016/j.wear.2015.10.014 https://doi.org/10.1016/j.ceramint.2018.02.048 https://doi.org/10.1007/s11666-023-01708-w https://doi.org/10.31399/asm.cp.itsc2024p0266 r. mušálek, t. tesař, j. minařík, j. dudík acta polytechnica ctu proceedings [10] a. chabib, j.-f. witz, p. gosselet, v. magnier. gcpu_opticalflow: a gpu accelerated python software for strain measurement. softwarex 26:101688, 2024. https://doi.org/10.1016/j.softx.2024.101688 [11] m. g. d. geers, r. de borst, w. a. m. brekelmans. computing strain fields from discrete displacement fields in 2d-solids. international journal of solids and structures 33(29):4293–4307, 1996. https://doi.org/10.1016/0020-7683(95)00240-5 [12] r. musalek, t. tesar, j. dudik, et al. cohesion of dissimilar splats in hybrid plasma sprayed coatings – a case study for al2o3-tio2. journal of thermal spray technology 31(6):1869–1888, 2022. https://doi.org/10.1007/s11666-022-01401-4 [13] a. valarezo, g. dwivedi, s. sampath, et al. elastic and anelastic behavior of tbcs sprayed at high-deposition rates. journal of thermal spray technology 24(1-2):160–167, 2014. https://doi.org/10.1007/s11666-014-0154-6 68 https://doi.org/10.1016/j.softx.2024.101688 https://doi.org/10.1016/0020-7683(95)00240-5 https://doi.org/10.1007/s11666-022-01401-4 https://doi.org/10.1007/s11666-014-0154-6 acta polytechnica ctu proceedings 50:62–68, 2024 1 introduction 2 materials and methods 3 results and discussion 4 conclusions acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2024.49.0085 acta polytechnica ctu proceedings 49:85–91, 2024 © 2024 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague reconstruction of concrete morphology using deep learning ondřej šperl, jan sýkora∗ czech technical university in prague, faculty of civil engineering, department of mechanics, thákurova 7, 160 00 prague, czech republic ∗ corresponding author: jan.sykora.1@fsv.cvut.cz abstract. in this contribution, the concrete morphology is reconstructed with a simple algorithm selecting a pixel value based on the small set of surrounding pixels. a deep neural network (dnn) is used as a classifier, and the authors focus on studying different dnn architectures. the performance of the proposed algorithm is evaluated on several statistical descriptors and the grain size distribution curve. keywords: reconstruction, concrete, deep learning, convolutional neural network. 1. introduction the reconstruction of material morphology has become an essential part of the numerical computational modeling of heterogeneous materials. the first reconstruction algorithms employed optimization routines to minimize selected statistical descriptors, see [1–4]. the drawback of such an approach is the exhaustive computational time caused by sequential pixel-bypixel replacement with subsequent evaluation of statistical descriptors. the second strategy concentrated on material description using random fields with optimized image-based correlation kernels, see [5]. this method dramatically reduces the dimensionality of the problem [6], however, it has the disadvantage of preserving maximal second-order statistical moments. the last approach is texture synthesis, which exploits the relatively simple principle of texture locality and stationarity for material reconstruction, see [7–9]. locality means that a random pixel inside the texture is related only to a small set of surrounding pixels (see [10]), and stationarity can be explained by the following example. assume a movable window placed in different positions in the texture. the regions of texture marked by the movable window seem to be always similar contrary to the situation of the general image, where the observed regions are clearly different. our aim is to reconstruct the image using the latter-mentioned approach, focusing on different dnn architectures, see [11], and the influence of their hyperparameters on the resulting concrete morphology. the authors are aware of more progressive techniques, such as generative adversarial networks or diffusion models, see [12–14], requiring a relatively rich dataset of images used for their training process, that might not always be available. 2. methodology the reconstruction of the concrete cross-section is based on the algorithm shown in figure 1. as an input to the computation, we use a sample of the concrete cross-section characterized by a two-dimensional image, introduced in the algorithm by a matrix of values 0 and 1 representing the black and white colors. the input and output data used for training the model are generated from the original image. the size of the dataset depends on the parameters of the computation; however, it can be estimated that for images ranging from 200×200 to 400×400 px, the size of the dataset is around tens of thousands. the model here is defined as a classification tool determining, based on a given pixel neighborhood, whether a pixel at a given position has a value of 0 or 1. the reconstruction algorithm then uses the trained classifier and the edges of the original image for the prediction of a new concrete cross-section. the quality of the new morphology is evaluated on our selected statistical descriptors. the aim of the reconstruction is to generate a similar sample with a statistically and physically equivalent structure with respect to the studied medium. the pixel neighborhood is an important parameter of the reconstruction algorithm. figure 2 shows the template as a function of the parameters w and h. these two parameters define its dimension, which significantly affects the properties of the entire reconstruction. the pixel neighborhood has two major features: (1.) it specifies the number of input values of pixels needed for the output value. yellow-labeled pixels in the scheme are our input values, and the redlabeled pixel is the output value. since the values of the yellow-labeled pixels have to be known, the reconstruction algorithm must always have the edges of the original image enabling the prediction of the first red pixel. thus, the parameters w and h also characterize the sizes of the boundary regions of the original image, which are needed for the start of the reconstruction algorithm. 85 https://doi.org/10.14311/app.2024.49.0085 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en ondřej šperl, jan sýkora acta polytechnica ctu proceedings figure 1. algorithmic framework. figure 2. template determining set of surrounding pixels. (2.) moreover, it defines a template that moves pixelby-pixel within the reconstructed image and predicts the value of the red-labeled pixel. it can be seen from the scheme that the rectangle of size (2w + 1) · (h + 1) is used as a template, and for this reason, we have to use blue-labeled and red-labeled pixels as input values. these are the values that are not taken from the original image but are randomly and independently generated from a uniform distribution satisfying the volume representation of 0 and 1 values in the original image. once the pixel neighborhood is set and the classifier is trained, the reconstruction algorithm starts the execution with the given template from the upper left corner of the initial image. the initial image is shown in figure 1, with the edges representing the morphology of the original media and the rest being randomly determined pixels so as to preserve the volume representation. the red-labeled pixel is computed with the help of the classifier specifying the probability of 0 and 1 for a given vicinity and the pseudo-random number generator determining the final value. the newly determined red-labeled pixel becomes the yellow-labeled pixel, and then the template is shifted one pixel further to the right. the value of all reconstructed pixels is successively identified in this fashion. the reconstruction algorithm terminates in the lower right corner of the image. subsequently, the metrics of the statistical and physical descriptors are evaluated. 2.1. architecture of dnn deep neural learning is the subclass of machine learning using dnn as a tool for solving complex problems. each dnn has multiple layers of interconnected nodes enabling one to learn complex representations of data by discovering hierarchical patterns and features, see [15]. the regular dnn is composed of three types of layers: (1.) input layer processing input data into the model, (2.) hidden layers representing the key part of dnn and applying weights to the inputs and directing them through an activation function as the outputs, (3.) output layer is the last layer in the neural network calculating the final probability scores of desired outputs. the special case of dnn is the convolutional neural network (cnn) developed for image classification and data visual interpretation, see [16]. it is based on the principles of mathematical convolution extracting the important features from the image with the help of learnable filters. the cnn is primarily composed of 86 vol. 49/2024 reconstruction of concrete morphology using deep learning (a). original segmented image – 1 200 × 1 225 px. (b). the largest possible square image cut from the original segmented image – 816 × 816 px. (c). the final image adjusted for redundant pores – 400 × 400 px. figure 3. input image preparation. convolutional layers and max pooling layers reducing the number of weights in the neural network. our designed classifiers are built upon the standards of dnn and cnn architectures. 3. example the example devoted to the reconstruction of the concrete structure using deep learning techniques is illustrated in this section. the sample of concrete morphology that is being reconstructed is depicted in figure 3a. it is a ct scan of the concrete transformed by a segmentation algorithm into a monochrome image. the white pixels indicate the aggregates and the pores, while the cementitious binder in between is represented by black pixels. the sample image was captured on a 74 mm diameter concrete cylinder. for algorithmic simplicity, the largest possible square image with dimensions of 816×816 px is selected from the original segmented image, see figure 3b. although the training process of our computational model is accelerated on a graphics card (nvidia geforce rtx 2060), it turns out that it is not possible to work with such a huge dataset collected from the largest possible square image. therefore, we reduced the image to a final resolution of 400 × 400 px minimizing the work with large data files. unfortunately, the segmentation algorithm does not allow to identify between the pore and the aggregate and labels both phases identically as white pixels. since the original grain size distribution curve is known, a portion of the small white pixels has been removed so that the grain size distribution curve is as close as possible to the original mixture curve of the concrete sample. the histogram shows the grain size distribution curve of the final image determined by the principal component analysis (pca) and feret’s characteristics (feret), and the real volume distributions of aggregates in the investigated concrete, see figure 4. the imperceptible differences in the histogram are probably mainly caused by the absence of the third grain dimension in the calculafigure 4. grain size distribution curve of final image utilized for the reconstruction process. the original data (blue-labeled) represents the concrete mixture used for the ct scanning. the red-labeled and orangelabeled grain size distribution curves display the results computed for the pca and feret’s algorithm, respectively. tion of the grain size distribution curves. the final 400 × 400 px image, adjusted for redundant pores, is shown in figure 3c. the computational image reconstruction algorithm offers various solutions with different success rates influenced by several variables. the variables affecting the reconstruction process are mainly: (1.) hyper-parameters of the model. these are the parameters controlling the neural network architecture, and some of their aspects are examined here in detail. in particular, the type of neural network layer and its input parameters such as the number of neurons, the composition of the layers in a sequence, or the total number of layers. (2.) the size of the pixel neighborhood is the second essential parameter that significantly affects 87 ondřej šperl, jan sýkora acta polytechnica ctu proceedings original medium #1 #2 #3 #4 #5 #6 #7 #8 #9 #10 figure 5. the original and resulting reconstructed images (#1–#10). model id architecture of dnn #1 15x15_c(16,3x3)_c(16,3x3)_m(2x2)_d(64)_d(64)_d(64) #2 15x15_c(16,3x3)_c(16,3x3)_m(2x2)_d(128)_d(128)_d(128) #3 15x15_c(32,3x3)_c(32,3x3)_m(2x2)_d(64,l1_0.01)_d(64)_d(64) #4 15x15_d(128)_d(128)_d(128) #5 15x15_d(128)_d(128) #6 15x15_d(64)_d(64)_d(64) #7 15x15_c(16,3x3)_c(16,3x3)_m(2x2)_d(128)_d(128)_d(128) #8 15x15_c(16,3x3)_c(16,3x3)_m(2x2)_d(128)_d(128) #9 15x15_c(8,3x3)_c(8,3x3)_m(2x2)_d(128)_d(128)_d(128) #10 15x15_c(8,3x3)_c(8,3x3)_m(2x2)_d(64)_d(64)_d(64) table 1. model id’s and abbreviated names of dnn architecture: for illustration, 15 × 15 stands for the pixel neighborhood, i.e. the dimensions of w and h in pixels, c(16,3x3) is the convolutional layer with 16 filters of spatial size 3x3, m(2x2) is the max pooling layer with 2x2 filter, and d(64) is the dense layer with 64 neurons. the success rate of the entire model. this parameter has also been investigated; however, due to the limited space of the paper, we limit ourselves to claiming that for the problem illustrated here, a pixel neighborhood of 15 × 15 px seems optimal. the hyper-parameters of the model are investigated on the image with dimensions of 300 × 300 px, see figure 5. the reason for reducing the image to acceptable dimensions is again the higher computational effort of the entire study, which is performed on a series of 10 architectures of the neural network with differently composed convolutional, dense, and max pooling layers. the model id’s and abbreviated names of dnn architecture are summarized in table 1. it is obvious that the possibility of designing the dnn architecture is enormous, especially in terms of filter size or number of neurons; thus this study is limited more to the comparison between the cnn and dnn architectures. the resulting reconstructed images for ten different dnn architectures are depicted in figure 5. besides the visual comparison, the following table (table 2) displays the values of errors in the statistical descriptors computed for the original and reconstructed structures. from the wide range of statistical descriptors, three fundamental metrics are selected for our study – the volume fraction (ϕ), the two-point proba88 vol. 49/2024 reconstruction of concrete morphology using deep learning model id ϵwhite ϕ ϵwhite s2 ϵblack s2 ϵwhite l2 ϵblack l2 #1 0.001797 0.000157 0.000158 0.000171 0.000106 #2 0.009209 0.000185 0.000220 0.000205 0.000117 #3 0.010342 0.000238 0.000284 0.000202 0.000148 #4 0.011240 0.000247 0.000303 0.000213 0.000153 #5 0.054454 0.002175 0.002939 0.000796 0.000557 #6 0.055820 0.001978 0.003782 0.000727 0.000627 #7 0.060252 0.002672 0.003554 0.000884 0.000614 #8 0.092767 0.006558 0.007819 0.001403 0.000919 #9 0.094060 0.004631 0.010755 0.001173 0.001081 #10 0.113770 0.010151 0.011235 0.001756 0.001106 table 2. the resulting values of errors calculated for the volume fraction, the two-point probability function, and the lineal path function. bility function (s2), and the lineal path function (l2). these statistical descriptors have been used in reconstruction problems for a significant period and have become an essential part of numerical studies devoted to material morphology. their particular formulations are skipped here, and the interested reader referred to, e.g., [17] or [2] providing basic characteristics, illustrative examples, and implementation strategies. the values of errors are calculated according to the following formulas expressing the differences between values of selected statistical descriptors calculated for the original and reconstructed image: ϵphase ϕ = ϕphase orig − ϕphase rec , (1) where ϕphase orig is the volume fractions of a given phase corresponding to the original image, ϕphase rec is the volume fractions of a given phase corresponding to the reconstructed image. the values of errors for complex statistical descriptors are evaluated as: ϵphase p = r∑ r=1 s∑ s=1 (pphase rs,orig − pphase rs,rec)2 sr , (2) where r and s are the image dimensions, p is the chosen statistical descriptor, here p = {s2, l2}. from the resulting values of errors and comparison of the reconstructed images, two models #1 and #2 have been studied in detail and more runs of reconstruction algorithm have been performed focusing on the statistical moments of error analysis. the most stable classifier is model #2, which is further utilized for reconstructing the larger image of the concrete cross-section in the following paragraph. 3.1. large image evaluation the final reconstruction is carried out on an image of dimensions 400×400 px, the original image is shown in (a). original image – 400 × 400 px. (b). reconstructed image obtained with the model #2 – 400 × 400 px. figure 6. final image reconstruction. figure 6a, and the reconstructed result is in figure 6b. it is apparent that the reconstruction algorithm is quite successful from the visual comparison of these two images. however, some irregularities are observed in the shape of the grains compared to the original image. from the perspective of statistical descriptors, the following values of errors – ϵwhite ϕ = 0.001046, ϵwhite s2 = 0.000171, and ϵwhite l2 = 0.000069 – indicate 89 ondřej šperl, jan sýkora acta polytechnica ctu proceedings figure 7. comparison of grain size distribution curves obtained for the original and reconstructed concrete cross-sections. again good performance of proposed strategy. the last comparison (see figure 7) shows the grain size distribution curves of original and reconstructed images computed for the algorithm using feret diameters (feret) and principal component analysis (pca). in the case of the pca evaluation, the original and reconstructed images differ in the representation of the individual grain size as follows: size range of 0–4 mm – 17.7 %, 4–8 mm – 8.6 %, and 8–16 mm – 9.1 %. in the case of the feret diameters, the grain size distribution differs as follows: size range of 0–4 mm – 8.6 %, 4–8 mm – 3.9 %, and 8–16 mm – 4.7 %. thus, the reconstructed image contains more aggregates of 0–4 mm since both methods result in a higher volume percentage, and conversely, size ranges of 4–8 mm and 8–16 mm are less observed in the reconstructed image. from the perspective of the grain size distribution curve, the reconstruction process of concrete morphology is less effective. 4. conclusion the reconstruction of the concrete cross-section area using deep learning techniques is illustrated in this paper. the proposed model can generate reasonably accurate samples of concrete morphology in terms of the selected statistical descriptors. some limitations are observed in the results of the grain size distribution curve, and the shapes of the aggregates which appear to be more irregular. taking into account that the computational hardware has some limitations and also the fact that we only handled one image, the obtained result can be evaluated positively. acknowledgements the authors are thankful for financial support from the student grant competition of ctu, project no. sgs23/152/ohk1/3t/11 and the czech science foundation, project no. 22-35755k. references [1] c. l. y. yeong, s. torquato. reconstructing random media. physical review e 57(1):495, 1998. https://doi.org/10.1103/physreve.57.495 [2] j. havelka, a. kučerová, j. sýkora. compression and reconstruction of random microstructures using accelerated lineal path function. computational materials science 122:102–117, 2016. https://doi.org/10.1016/j.commatsci.2016.04.044 [3] r. bostanabad, y. zhang, x. li, et al. computational microstructure characterization and reconstruction: review of the state-of-the-art 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[16] z. li, f. liu, w. yang, et al. a survey of convolutional neural networks: analysis, applications, and prospects. ieee transactions on neural networks and learning systems 33(12):6999–7019, 2022. https://doi.org/10.1109/tnnls.2021.3084827 [17] j. zeman. analysis of composite materials with random microstructure. ph.d. thesis, czech technical university, 2003. https://doi.org/10.13140/rg.2.1.2399.0004 91 https://doi.org/10.1016/j.mtcomm.2023.105608 https://doi.org/10.1038/s41598-024-54861-9 https://doi.org/10.1109/tnnls.2021.3084827 https://doi.org/10.13140/rg.2.1.2399.0004 acta polytechnica ctu proceedings 49:85–91, 2024 1 introduction 2 methodology 2.1 architecture of dnn 3 example 3.1 large image evaluation 4 conclusion acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2023.42.0012 acta polytechnica ctu proceedings 42:12–16, 2023 © 2023 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague simulation of a multimaterial model for modified auxetic structures alexander engel∗, anne jung helmut schmidt university/university of the federal armed forces hamburg, protective systems, holstenhofweg 85, 22043 hamburg, germany ∗ corresponding author: a.engel@hsu-hh.de abstract. auxetic structures, which is a term used to describe materials with a negative poisson’s ratio, show beneficial properties like a low density, a high energy absorption capacity and an increased indentation resistance. this enables applications in many fields, such as aerospace and sports industries. given their potential, many studies have already been conducted. previously, the geometry of a selected auxetic re-entrant structure was optimized to maximize its mass-specific energy absorption capacity for ideal usage in lightweight applications. moreover, a homogeneous material was used, whereas the combination of multiple materials could drastically increase the performance of such structures. hence, in this study the use of two different materials combined into a modified re-entrant structure is investigated via finite element simulation. the poisson’s ratio could thus be improved, which leads to a more pronounced and longer lasting auxetic effect. keywords: auxetic metamaterials, multi-material structure, lightweight, energy absorption. 1. introduction auxetic materials are defined by a negative poisson’s ratio ν, which describes the change in volume from the negative ratio between the transversal strain εt and the longitudinal strain εl under uniaxial loading [1]. ν = −εt εl . (1) the deformation behavior is shown schematically in figure 1 for an auxetic and a non-auxetic material under uniaxial tension as well as uniaxial compression. tension compression non-auxetic original shape deformed auxetic figure 1. deformation behavior of auxetic and nonauxetic structures under uniaxial loading. additionally, auxetic materials have several advantages over conventional materials, making them central to many current research projects [2, 3]. these include an increased thermal shock resistance [4], a higher fracture toughness [5], an increased penetration resistance [6, 7], a high volume-specific energy dissipation [8] and a low density, enabling applications in many fields, such as ballistic and explosion protection, as energy absorbers, or in lightweight construca) b) f f wabestructure figure 2. 2d auxetic re-entrant structure in a) nondeformed and b) deformed stage. tion [9, 10]. other applications include piezoelectric composites, medical applications such as stents and bandages, and sports applications through reduced impact forces and friction [10, 11]. several classes of auxetic structures can be distinguished. starting with two-dimensional (2d) structures and the most widespread class, the re-entrant structure [2, 12]. the deformation mechanism of the re-entrant structure is relatively simple to understand and is shown schematically in figure 2. these, as well as other 2d structures, are easily expandable into the third dimension and exhibit unidirectional auxetic behavior [2, 13]. by applying a load, the struts of 12 https://doi.org/10.14311/app.2023.42.0012 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 42/2023 simulation of modified auxetic structures 1 2 43 5 1: height of the pore (hpore) 2: width of the waist (wwaist) 3: width of the half-strut (wstrut) 4: height of the half-strut (hstrut) 5: re-entrant angle (α) figure 3. unit cell of a modified auxetic re-entrant structure. the structures rotate, stretch and bend, resulting in a lateral volume increase and thus auxetic behavior [2, 14, 15]. some of these characteristics could be improved even further by the utilization of a non-homogeneous material for the auxetic structure, since the targeted use of varying stiffnesses for different structural components is expected to have an impact onto the deformation and thus the poisson’s ratio. 2. materials and methods 2.1. geometry the studied structure is a three dimensional re-entrant structure, which has been modified in order to maximze its mass-specific energy absorption capacity for ideal usage in lightweight construction by bronder et al. [9]. through the modification, an additional half strut was introduced to the re-entrant geometry. figure 3 shows the different structural parameters that have been optimized. the optimized values of these geometric parameters are shown in table 1. structural optimized parameter value hpore 7.29 mm wwaist 0.56 mm hstrut 1.66 mm wstrut 0.78 mm α 65.61 å table 1. optimized geometry parameters for the modified auxetic re-entrant structure [9]. for the analysis, a total of 27 (3 × 3 × 3) unit cells were used. this enables one unit cell in the center of the structure to be mostly unaffected by boundary effects, whilst still keeping the necessary time for each simulation manageable. 2.2. materials in order to further improve upon this structure, an effort has been made to move away from a homogeneous material and instead introduce materials with a distinct variation in terms of their stiffness. for that purpose, the horizontal struts where set to be austenitic and the vertical struts were assigned a martensitic material model thus still avoiding major discrepancies in density and composition by sticking to different variants of steel. the martensitic struts are introduced in order to avoid bending of the outer struts which would limit the overall auxetic effect and thus resulting in a lower poisson’s ratio. hence this bending is avoided by utilizing a material with a relatively high stiffness. however, since the austenitic parts have a lower stiffness than the martensitic parts, it is expected, that the horizontal struts can still flex and hinge in order to get auxetic deformation in the first place, hence leading to a more pronounced auxetic effect when compared to a similar structure with a homogeneous material. 2.3. simulations all simulations were done using the finite element software abaqus® (dassault systèmes, vèlizyvillacoublay, france). both materials were assigned an isotropic, elastic-plastic material model. the material data of each bulk material is shown in table 2. parameter value eaustenite 220 mpa emartensite 210 mpa ρaustenite 7.9 g cm−3 ρmartensite 7.8 g cm−3 νaustenite 0.300 νmartensite 0.283 table 2. material data of the bulk materials for austenite and martensite, including the young’s moduli e, the densities ρ and the poisson’s ratios ν. 13 alexander engel, anne jung acta polytechnica ctu proceedings furthermore, for both austenite and martensite, a ductile damage model was introduced with an equivalent plastic strain at the onset of damage of 0.3 and 0.01 respectively. the loading was applied by subjecting a rigid body plate to displacement-controlled compression, which was coupled to the structure via a general contact. a hard contact was defined between the two plates and the auxetic structure in the normal direction, which means that the surfaces of the structure and the two plates cannot be penetrated. in the tangential direction, a friction coefficient of 0.1 was defined [16]. in order to evaluate the poisson’s ratio, the longitudinal and the transversal strain were measured. the longitudinal strain was taken from the load point and the transversal strain was taken from the average strain of the nodes within the corners of the outer unit cells of the middle layer of the structure. the poisson’s ratio was also evaluated in both principal planes perpendicular to the load axis, however, given the symmetric nature of the structure and the uniaxial loading conditions, both lead to the same results. hence, from this point onward the poisson’s ratio will only be shown for one principal plane. the loading conditions as well as the nodes used for evaluation are shown in figure 4. in order to achieve a sufficient accuracy during the simulations, the mesh was found to require at least two elements across the width of one strut. hence, the mesh size was set to be half of the width of a strut. furthermore, modified quadratic tetrahedral elements (c3d10m) were used for the mesh in order to achieve an improved contact properties and good performance over the complete range of deformation [16]. figure 4. loading conditions for the compressive load applied to the modified auxetic re-entrant structure with the highlighted nodes (red) being used to evaluate the poisson’s ratio. for the purpose of studying the effect of introducing multiple materials for the same structure, quasistatic simulations were performed for a homogeneous austenitic structure, a homogeneous martensitic structure as well as the combined multimaterial structure. the unit cells of each structure is shown in figure 5. all austenite all martensite multimaterial figure 5. unit cells of the multimaterial, homogeneous austenitic and homogeneous martensitic structure. 3. results in order to evaluate the auxetic behavior of the different structures, the poisson’s ratio will serve as the decisive parameter. hence, the strain along the compressive load axis will be evaluated as the longitudinal strain εlongitudinal and the strain perpendicular to the load axis as the transversal strain εt ransversal in order to calculate the poisson’s ratios according to equation 1. thus, an increase in the negative amount of the poisson’s ratio correlates to a more pronounced auxetic effect and as such an increased performance. the poisson’s ratios for each of the different structures are shown up until 12 % strain in figure 6. immediately, a difference between the three structures can be noticed, solely based on their material combination. whilst the homogeneous martensitic structure shows barely any auxetic deformation, the other two structures show promising results. the reason for the martensitic structure showing barely any auxetic deformation is based on the very high stiffness of the material. since it is necessary for auxetic structures to flex and bend certain parts, the high stiffness counteracts this deformation. additionally, failure occurs at a very low strain, also resulting in an attenuation of the auxetic effect. hence, the homogeneous austenitic structure shows a stronger auxetic effect because of its increased ductility and 14 vol. 42/2023 simulation of modified auxetic structures all austenite all martensite multimaterial 0 1 2 * * *failure already occured at boundaries 0 1 2 figure 6. poisson’s ratio of the modified auxetic re-entrant structure for the homogeneous austenitic (all austenite), homogeneous martensitic (all martensite) and combined multimaterial (multimaterial) structure. the ability to bend certain parts of the structure due to a lower stiffness. however, the most promising results in terms of the poisson’s ratio are shown by the combined multimaterial structure. the combination of both materials leads to less buckling of the outer struts due to the martensitic parts, whilst still allowing for enough ductile deformation and bending caused by the horizontal austenitic struts. the poisson’s ratios of each structure at different strain points, according to figure 6, are shown in table 3. an increase of roughly 50 % in terms of the negative amount of the average poisson’s ratio can be noticed from the homogeneous austenitic structure to the multimaterial structure. this significant improvement could already be achieved without optimizing the materials used for the model. utilizing carefully selected materials with varying stiffness can certainly help to improve the performance of complex auxetic structures and make them more suitable for application. material setup poisson’s ratio [-] all austenite 1 −0.112 2 −0.109 all martensite 1 −0.007 2 −0.011 multimaterial 1 −0.183 2 −0.157 table 3. poisson’s ratios of the modified auxetic structure for different material setups. 15 alexander engel, anne jung acta polytechnica ctu proceedings 4. conclusion first steps have been made towards the improvement of a selected auxetic structure by introducing multiple materials to an auxetic structure. hence, austenitic and martensitic parts were introduced to a modified auxetic re-entrant structure, whereby martensite has a significantly higher stiffness than austenite thus avoiding buckling of the outer struts. however, the austenitic parts still allow for flexure to get auxetic deformation in the first place. thus, the deformation behavior of these structures has been analyzed and the impact of different geometry parameters on the poisson’s ratio has been studied. it was shown, that the use of selected combinations of materials within the same structure can lead to serious improvements in the overall deformation behavior, and as such the poisson’s ratio, but also the application-oriented performance. in the future, it will be necessary to fabricate samples in order to compare the results from the simulations to actual experiments. references [1] x. hou, v. v. silberschmidt. mechanics of advanced materials, chap. metamaterials with negative poisson’s ratio: a review of mechanical properties and deformation mechanisms, pp. 155–179. springer, 2015. https://doi.org/10.1007/978-3-319-17118-0_7 [2] n. novak, m. vesenjak, z. ren. auxetic cellular materials – a review. strojniški vestnik-journal of mechanical engineering 62(9):485–493, 2016. https://doi.org/10.5545/sv-jme.2016.3656 [3] t. fíla, p. koudelka, j. falta, et al. dynamic impact testing of cellular solids and lattice structures: application of two-sided direct impact hopkinson bar. international journal of impact engineering 148:103767, 2021. https://doi.org/10.1016/j.ijimpeng.2020.103767 [4] t.-c. lim. thermal stresses in auxetic plates and shells. mechanics of advanced materials and structures 22(3):205–212, 2015. https://doi.org/10.1080/15376494.2012.727203 [5] r. s. lakes. design considerations for materials with negative poisson’s ratios. journal of mechanical design 115(4):696–700, 1993. https://doi.org/10.1115/1.2919256 [6] c. qi, a. remennikov, l.-z. pei, et al. impact and closein blast response of auxetic honeycomb-cored sandwich panels: experimental tests and numerical simulations. composite structures 180:161–178, 2017. https: //doi.org/10.1016/j.compstruct.2017.08.020 [7] s. yang, c. qi, d. wang, et al. a comparative study of ballistic resistance of sandwich panels with aluminum foam and auxetic honeycomb cores. advances in mechanical engineering 5:589216, 2013. https://doi.org/10.1155/2013/589216 [8] m. bianchi, f. l. scarpa, c. w. smith. stiffness and energy dissipation in polyurethane auxetic foams. journal of materials science 43(17):5851–5860, 2008. https://doi.org/10.1007/s10853-008-2841-5 [9] s. bronder, f. herter, a. röhrig, et al. design study for multifunctional 3d re-entrant auxetics. advanced engineering materials 24(1):2100816, 2021. https://doi.org/10.1002/adem.202100816 [10] x. yu, j. zhou, h. liang, et al. mechanical metamaterials associated with stiffness, rigidity and compressibility: a brief review. progress in materials science 94:114–173, 2018. https://doi.org/10.1016/j.pmatsci.2017.12.003 [11] z. wang, h. hu. auxetic materials and their potential applications in textiles. textile research journal 84(15):1600–1611, 2014. https://doi.org/10.1177/0040517512449051 [12] f. scarpa, p. panayiotou, g. tomlinson. numerical and experimental uniaxial loading on in-plane auxetic honeycombs. the journal of strain analysis for engineering design 35(5):383–388, 2000. https://doi.org/10.1243/0309324001514152 [13] c. körner, y. liebold-ribeiro. a systematic approach to identify cellular auxetic materials. smart materials and structures 24(2):025013, 2014. https://doi.org/10.1088/0964-1726/24/2/025013 [14] s. bronder, m. adorna, t. fíla, et al. hybrid auxetic structures: structural optimization and mechanical characterization. advanced engineering materials 23(5):2001393, 2021. https://doi.org/10.1002/adem.202001393 [15] x. ren, r. das, p. tran, et al. auxetic metamaterials and structures: a review. smart materials and structures 27(2):023001, 2018. https://doi.org/10.1088/1361-665x/aaa61c [16] m. smith. abaqus/standard user’s manual, version 6.9. dassault systèmes simulia corp, united states, 2009. 16 https://doi.org/10.1007/978-3-319-17118-0_7 https://doi.org/10.5545/sv-jme.2016.3656 https://doi.org/10.1016/j.ijimpeng.2020.103767 https://doi.org/10.1080/15376494.2012.727203 https://doi.org/10.1115/1.2919256 https://doi.org/10.1016/j.compstruct.2017.08.020 https://doi.org/10.1016/j.compstruct.2017.08.020 https://doi.org/10.1155/2013/589216 https://doi.org/10.1007/s10853-008-2841-5 https://doi.org/10.1002/adem.202100816 https://doi.org/10.1016/j.pmatsci.2017.12.003 https://doi.org/10.1177/0040517512449051 https://doi.org/10.1243/0309324001514152 https://doi.org/10.1088/0964-1726/24/2/025013 https://doi.org/10.1002/adem.202001393 https://doi.org/10.1088/1361-665x/aaa61c acta polytechnica ctu proceedings 42:12–16, 2023 1 introduction 2 materials and methods 2.1 geometry 2.2 materials 2.3 simulations 3 results 4 conclusion references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2025.52.0024 acta polytechnica ctu proceedings 52:24–36, 2025 © 2025 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague design of the vertical trajectory for arrival and departure routes at prague airport šimon hrdlička, stanley schmidt, terézia pilmannová∗ czech technical university in prague, faculty of transportation sciences, horská 3, 128 03 prague, czech republic ∗ corresponding author: pilmater@fd.cvut.cz abstract. numerous operational methods and related research focus on the efficient use of controlled airspace and thus on the management of its capacity. during the optimisation processes, various legislative limitations or technological constraints might appear. as the aviation industry continues to evolve, it occasionally happens that one of these pillars lags behind the other. this research focuses on a relatively underexplored aspect of using vertical profiles of air traffic service routes within the tma praha to optimise traffic flow in terms of flight efficiency and environmental impact. virtual points, defining the aircraft’s vertical position limits along the given route, were computed to establish a vertical profile on already published standard departure or arrival routes. these points define the aircraft’s vertical position limits along the given route. along with the proposal of a vertical profile, newly designed arrival and departure routes were implemented into the air traffic control simulation tool escape light and validated simultaneously within x-plane simulator. simulation studies conducted in these environments demonstrated significant fuel savings, averaging 13.2%, with even greater savings observed in the boeing 737-800, exceeding 25%. these results confirm that applying vertical profiles positively impacts fuel consumption and contributes to more environmentally friendly operations. keywords: continuous climb operations, continuous descent operations, standard instrument arrival, standard instrument departure, vertical profile. 1. introduction state-published routes have been fundamental since the early days of air navigation. they allow for the determination of aircraft positions, streamlining traffic flow, and ensuring safe separation from airspaces and other aircraft. with the advent of new technologies, the methods of utilizing these routes have evolved. however, from the early days to the present, routes have predominantly been viewed as horizontal, except for determining minimum safe altitudes. in the vicinity of airports, aircraft often experience frequent changes in their vertical position. if not executed smoothly and simultaneously, the frequency of these changes directly impacts flight efficiency, the environment, and the operational workload of both flight crews and air traffic controllers. the continuous descent operations (cdo) and continuous climb operations (cco) procedures address this issue to some extent [1]. by applying these procedures, fuel consumption can be reduced by up to a few percentage points [2]. a specific and increasingly desirable form of cdo is the continuous descent approach (cda), sometimes referred to as the continuous descent final approach (cdfa), which involves a continuous descent from cruise altitude to the touchdown point. however, to effectively utilize cdo/cco methods, flight execution must also be adapted. for example, when descending from cruise altitude to the destination airport, initiating the descent at a reduced speed is more advantageous than transitioning to level flight to reduce speed during a later approach segment. this operational limitation is less significant than in flights with horizontal phases [3]. however, there might be further restrictions from atc, which may mandate a reduction in forward speed, causing the aircraft to level off, thereby disrupting the entire concept of this method. in areas with higher traffic or complex airspace configurations, their use can be challenging and may not serve the intended purpose of smooth vertical movement [4]. during pre-flight planning, flight crews calculate their optimal descent, which primarily considers published ats procedures and regulations, as well as the needs of the aircraft operator. subsequently, non-published factors such as air traffic control workflow, operational conditions, and traffic composition also play a role, aside from force majeure influences. in order to secure equally fair handling of flights by air traffic service providers, individual needs cannot be the sole consideration. the higher the demand for services and the number of flights, the greater the set of restrictions that must utilized, and vice versa. the scope and degree of measures taken are determined by individual atc units. each air traffic controller (atco) also has an individual style of service delivery. therefore, it cannot be expected that a universal air traffic control method will apply everywhere [5]. 24 https://doi.org/10.14311/app.2025.52.0024 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 52/2025 vertical trajectory for arrival and departure routes at prague airport this research primarily focuses on the design of arrival routes with integrated information about the expected vertical position of the aircraft on standard routes to and from lkpr airport. it also evaluates the impact of these methods on flight efficiency. vertical restrictions are currently applied at many international airports with very high traffic in europe and globally. an example is london heathrow airport, where, given the airspace structure and the volume of traffic utilizing it, there is limited manoeuvring space outside the standard routes [6]. the research aimed to design and validate the vertical profiles of departure and arrival routes at lkpr airport while respecting the existing airspace structure and the sectorisation at the lkpr app control unit. an app unit is a part of the air traffic services facility that includes one or more workstations, each fulfilling specific tasks within a defined scope [7]. prague app’s area of responsibility (aor) is classified as cta 1 prague, a class e airspace, and prague tma, a class c airspace [8]. 2. materials and methods the practical part of the research consisted of four key steps: • analysis of the current state of the art. • identification and calculation of key points along the route, where newly calculated restrictions will be implemented. • determination of values for vertical restrictions. • implementation of the vertical restrictions into the simulation environment and validation. 2.1. analysis of the current state of the art the analysis involves a detailed traffic composition that operates within the airspace. this includes an assessment of aircraft types based on the collection of real data, utilisation of individual flight paths, frequency and representation of various aircraft types, as well as other relevant factors that influence operations in this space. there are two established methods of designing arrival and departure routes. 2d and 3d strategic de-confliction. in aviation, area navigation is a fourdimensional discipline that combines lateral and vertical navigation with speed and time. tma praha has established standard departure and terminal arrival routes (sids and stars) that utilise solely lateral navigation. this reflects the 2d strategic de-confliction of these routes. the only level constraint can be found on turning departures with the first turn defined by an altitude of 1 700 ft amsl. by adding level constraints to these routes, that would correspond to the optimal levels of aircraft on them, full 3d (vertical and lateral) strategic de-confliction would be created [9]. to establish the vertical profile on an already published departure or arrival route, it is essential to define points that will determine the limits of the aircraft’s vertical position on the given route. these limitations must respect cdos and be part of the buffer zone, which physically separates arrival and departure trajectories to continuously ensure vertical separation between aircraft. these trajectories essentially form virtual tunnels from which an aircraft cannot deviate unless explicitly directed otherwise. this allows flight crews to know precisely when and where they can ascend or descend. the currently separate clearances to change level incrementally could be executed based on a single atc clearance aiding in. there are two types of arrival routes, and prague is adapted to both. first one, called open procedure, provides track guidance to a downwind position from which the aircraft is guided by atc using radar vectoring to intercept the final approach track. second route type, closed procedure, continues with track guidance after passing the downwind position up to the final approach point or fix of an instrument approach to the active runway. closed procedure is nowadays used in lkpr by adding an rnav path to the initial approach connecting the downwind positions with the final approach [10]. the standard situation for most traffic to and from lkpr is as follows. ifr departure clearances include a standard departure track and an initial climb altitude of 5 000 ft. the departure track is normally assigned according to the first waypoint in the flight plan. this point should be identical to the last point on the departure track. for climbs above 5 000 ft amsl the clearance is issued by the controller of app praha (the aircraft is located in tma praha – class c area). he also ensures the aircraft is separated from surrounding traffic and from other controllers’ areas of responsibility. the last level assigned by this controller is normally fl140 for propeller aircraft and fl160 for jet aircraft, or their cruise level if this is lower. the aircraft is then instructed to contact acc praha for further climb and other clearances [11]. arriving ifr traffic is transferred to the app praha frequency typically in descent to fl170 (jet aircraft) or fl150 (propeller aircraft) with an already assigned arrival route. taking into account the airspace layout, sectorisation and operational situation, the pilot is cleared to descend progressively down to 4 000 ft amsl – the intermediate approach altitude to one of the lkpr runways [11]. pilots never know in advance what level they can expect at certain points (except at the final approach point). this leads them to plan climbs and descents according to the optimum profile to reduce fuel consumption and maximise efficiency. however, if the aircraft is descending along the optimum descent path, its level at the route intersection may be dangerously close to the level of the aircraft climbing along the 25 š. hrdlička, s. schmidt, t. pilmannová acta polytechnica ctu proceedings optimum path. this shall be prevented by atc by ensuring vertical separation between aircraft. for safety reasons, both aircraft will stabilize at instructed levels and await clearance for further climb/descent. it is this so-called level-off that has a big impact on flight efficiency, fuel consumption, emissions and completely neglects the principles of cdo/cco [2, 12]. therefore, the goal of this paper is to reach a compromise and separate the aircraft vertically at the intersections in advance. this is done by establishing fixes with vertical restrictions already on the arrival and departure routes and their charts. this method will get as close as possible to 3d strategic route de-confliction. 2.2. identification and calculation of key points along the route the second step involves the identification and calculation of key points along the route, where newly calculated restrictions will be implemented. the shortest distance between two points on the surface of a sphere is a great circle, with the centre of the circle located at the centre of the sphere, known as the orthodrome. however, the earth is not a perfect sphere but rather resembles an oblate spheroid, which is important in cartography [8]. aircraft equipped with performance based navigation (pbn) specifications, such as rnav 1 applied on star arrival routes into prague, follow orthodromic segments between the points along the route [13, 14]. to establish a buffer zone that ensures the minimum separation is not violated during continuous altitude changes on the flight route, it is necessary to calculate the coordinates of the points that define this space. this calculation involves several steps and transformations between coordinate systems. initially, it is essential to verify that all segments between which the intersection point is being sought have a length greater than zero. if the length is zero, a solution is not possible [13, 15]. additionally, it is necessary to work with geographic coordinates in radians. an important condition is the conversion of coordinates from the sexagesimal system to the decimal system if they are not already in decimal form [16]. to determine the intersection of two planes containing orthodromes, vectors must be created from the points defining the orthodromic segments. these vectors are then used to calculate the intersection of the planes, which determines the intersection of the orthodromes [16]. the calculation of this intersection is crucial for identifying points where aircraft should cross. the calculation process is as follows: (1.) the coordinates in decimal form are converted to radians according to the eq. 1, where φ and λ represent the latitude and longitude respectively.( φ λ ) = ( lat · π 180 lon · π 180 ) (1) (2.) the next conversion is to the cartesian coordinate system, where two sections of orthodromic lines are expressed by four points: p1, p2, p3 and p4. these points are expressed in longitude and latitude coordinates in radians. the conversion to the cartesian coordinate system is as follows in the eq. 2. p = xy z  = n(φ) cosφ cosλ n(φ) cosφ sinλ b2 a2n(φ) sinφ  (2) (3.) here the radius of curvature in the plane perpendicular to the meridian (transverse radius of curvature) is calculated in the eq. 3. n (φ) = a2√ a2 cos2 φ+ b2 sinφ = a√ 1 − e2 sin2 φ (3) the variables a, b are called the major and minor semi-axes of the meridian ellipse. the constant e is then the (first) eccentricity. the values of a, b, e depend on the geodetic reference system. for international aviation, this is the wgs-84 system according to the icao annex 15 [17] (4). a = 6 378 137 m b = 6 356 752, 31425 m e = 8, 1819190842622 · 10−2 (4) (4.) to determine the intersection, it is necessary to find the intersection of the two planes of which these orthodromes are a part. this intersection will be in the form of a line which, intersecting the surface of the ellipsoid, defines 2 points on the ellipsoid. the general equation of the plane is as depicted in the eq. 5. ax+ by + cz = 0 (5) (5.) from the points p1 and p2 (defining the segment of the first orthodrome) a vector ~v1 is created and from the points p3 and p4 a vector ~v2 is created (6). ~v1 = p1 × p2 = v1x v1y v1z  = y1z2 − y2z1 x2z1 − x1z2 x1y2 − x2y1  (6) (6.) from the vectors ~v1 and ~v2 come unit vectors ~u1 and ~u2 (the individual components of the vector are divided by its length) (7).∣∣∣ ~v1 ∣∣∣ = √ v1x2 + v1y2 + v1z2 ~u1 = u1x u1y u1z  =  v1x | ~v1| v1y | ~v1| v1z | ~v1|  (7) (7.) to calculate the equations to which the orthodromes belong and which determine the intersection we use the following formula (8) ρ1 : u1xx+ u1yy + u1zz = 0 ρ2 : u2xx+ u2yy + u2zz = 0 (8) 26 vol. 52/2025 vertical trajectory for arrival and departure routes at prague airport figure 1. buffer zone around a straight route. the red intersection indicates the position of the first possible point where an altitude change can be made in the expected direction of flight. (8.) the planes ρ1 a ρ2 intersect in one line which has a direction vector ~d (9). ~d = ~u1× ~u2 = dxdy dz  = u1yu2z − u2yu1z u2xu1z − u1xu2z u1xu2y − u2xu1y  (9) (9.) this line already directly corresponds to the searched orthodromic intersections. and from the direction vector, we can directly calculate their geographic coordinates in degrees according to the eq. 10, where the angle measured from the origin of the coordinate system to ~d is the angle θ – geocentric latitude. tan θ = dy dx = ( 1− e2) tanλ λ = tan−1 ( dy dx (1− e2) ) φ = tan−1  dz√ (1− e2)(dx2 + dy 2)  (10) (10.) the resulting point is the point x with coordinates φ and λ after correct conversion back from radians to degrees (11). x = [φ;λ] (11) the whole calculation was then written in the matlab programming language into the function intersection, where the input parameters are the coordinates of the points defining the beginning and the end of the orthodrome in the decimal system. the output parameters are also the coordinates in the decimal system of the intersection of these orthodromes. for safety reasons, an atco cannot issue a clearance that would reduce the minimum vertical or horizontal separation. aircraft must remain vertically separated by 1 000 feet or horizontally by 5nm at all times [18]. therefore, there have been created protection areas to their corresponding routes with a minimum width of 5nm. once the aircraft are at a point 5 or more miles from the intersection, they may change their level, ensuring their trajectories remain non-conflicting. the protection area around the routes determines the new intersection point along the route direction, which is the first possible point where a change in vertical restriction can be defined. however, determining such a point on the routes also involves another limitation: identifying individual points and turns along the routes. to ensure the minimum separation of 5nm around fly-by and flyover fixes, vincenty’s equations were used [19, 20]. an additional buffer must be created in turn areas to allow for further protection area required by turns at fly-by waypoints [21]. these turns are defined by the etp (earliest turning point), which marks the beginning of the turn’s buffer zone. this ensures that it meets all criteria for maintaining the minimum separation, essential for the safe and efficient operation of aircraft along the route [18, 21]. a graphical representation of the buffer zone for a straight segment of the route is shown in figure 1 and for a route with a turn in figure 2. 2.3. determination of values for vertical restrictions in the third step, it was necessary to determine the values for vertical restrictions. these must be defined with sufficient precision, and the actual traffic at lkpr must be taken into consideration. the goal is not just to ensure a safe change in altitude without any probability of infringing on the minimum prescribed separation. the proposed concept must also reflect the performance characteristics of the aircraft operating in this space and consider the airspace organisation, including possible sectorisation. to determine the most commonly used altitudes at the newly established intersection points that define the buffer zone, data from the opensky network were 27 š. hrdlička, s. schmidt, t. pilmannová acta polytechnica ctu proceedings figure 2. buffer zone around a route with a turn. the red intersection indicates the position of the first possible point where an altitude change can be made in the expected direction of flight. figure 3. graphical representation of aircraft vertical position relative to lkpr [22]. utilised [23]. opensky collects data from secondary surveillance radar transponders and automatic dependent surveillance – broadcast (ads-b). data from aircraft arriving and departing from lkpr were selected from days with the highest traffic, as during peak times, aircraft are forced to use the entire length of the arrival route and gradually descend or climb according to available altitudes. the data was filtered to include only aircraft flying under ifr (instrument flight rules) departing from or arriving at lkpr. the graphical representation in figure 3 provides an overview of all the mentioned arrivals and departures. the y-axis represents altitude in feet above the standard pressure setting of 1 013.25 hpa. the x-axis represents the distance of a given arriving aircraft from the touchdown point. for departures, the x-axis shows the distance from takeoff. this illustrates the relationship between the aircraft’s altitude and their distance from takeoff or landing. flights that did not have prague ruzyně as their departure or destination airport were intentionally filtered out from this analysis. the resulting vertical restrictions will be determined by rounding to the nearest available level to ensure that arriving and departing aircraft maintain a minimum separation of 1 000 feet. the restrictions will be plotted on charts at fixes at least 5nm from the intersecting routes. if any fixes are too close to already existing ones, they will be merged, and vertical restrictions will be added to them. the graph in figure 3 shows that some flights reach the altitude of the intermediate approach before the final approach fix. an aircraft performing an ils approach must intersect the glide slope from below rather than catching up with it during descent. for departures, minor fluctuations can be observed, primarily due to acceleration and the conversion of vertical speed into forward speed. when regression is applied to these data, an average level is obtained, 28 vol. 52/2025 vertical trajectory for arrival and departure routes at prague airport figure 4. display of vertical profile regression for arrivals (blue) and departures (red) at lkpr [22]. representing the altitudes at which the aircraft are located. the graphical representation of this curve in figure 4 illustrates the dependence of aircraft altitude on their distance from takeoff/landing, where the descent for arrivals is linear, and the climb for departures follows a second-degree polynomial, representing the ideal altitude for all flights at lkpr. this data determines the optimal altitude at previously identified intersection points. the traffic composition at lkpr is very consistent in terms of aircraft types. therefore, the measured values are very similar and realistically achievable for the vast majority of types without any issues. based on this data, vertical restrictions can be applied to the calculated intersection points of the buffer zone. it is also necessary to ensure that another atc sector is not infringed upon. the aircraft must always remain within the sector of a single controller, except when handed over to the next one. the crew can then adjust the descent profile considering these limiting points and prepare the aircraft for the next phase of flight according to operational needs. these flight phases are also ideal for atc to manage forward speed. before the aircraft reaches the point for the next level change, any intervention in forward speed will have a minimal impact on the descent profile. the earlier the forward speed is managed, the less it will affect the reacquisition of the ideal descent profile. 2.4. implementation of the vertical restrictions into the simulation environment and validation the fourth and final step is to implement the points and calculated vertical restrictions into the escape light simulation environment and validate them. after creating a standalone model of vertical paths and verifying feasibility, actual traffic, including its vertical positions on the paths, was compared with the model in the bada (base of aircraft data) system [24]. buffer zone conditions were then established to ensure that aircraft at no point on the paths fall below the minimum prescribed separation, which is either a horizontal distance of 5nm or a vertical distance of 1 000 feet [18]. thanks to the lateral strategic deconfliction combined with the vertical profile, complete traffic separation is achieved, where each aircraft is pre-informed of the range of flight levels within which it may operate, and when it may leave the limiting altitude. ideally, the crew adjusts the vertical speed so that the aircraft remains in constant vertical motion and reaches the limiting point at an altitude that allows it to continue vertical movement without needing to stop the climb at the published limiting altitude. the air traffic controller can then issue a clearance to climb up to the boundary of their area of responsibility, with the aircraft required to comply with the vertical restrictions duly marked on the chart. an exception is the atco’s clearance, which directly cancels the vertical restriction at specific points, allowing unrestricted climb at those points. the restriction is also lifted in the case of route shortening beyond the points with altitude limitation or providing navigation guidance through vectoring [18]. similarly, an arriving aircraft managed by the arrival control unit may be cleared for descent to the intermediate approach altitude with a single instruction at the beginning of communication. this instruction obliges the crew to adhere to the limiting altitudes at each point unless otherwise specified. in such a case, the aircraft cannot move outside the specified range, and the crew is informed when the limitation expires and can continue changing the vertical position in the expected direction of flight [25]. the entire concept’s feasibility from the atc perspective was validated in a synthetic ats environment using the eurocontrol escape light simulator. this simulator calculates the 4d trajectory of aircraft in real-time, modelling according to the bada dataset, and stores the results 29 š. hrdlička, s. schmidt, t. pilmannová acta polytechnica ctu proceedings at intervals of 5 seconds, corresponding to the refresh rate of the surveillance system. calculations are based on the distance travelled and the individual weight of each aircraft, considering the average amount of fuel on board. the aim is to assess the real-world data against the values obtained from the bada model. if these values do not differ significantly, the proposed values can be considered valid. a significant deviation is considered when values fall outside the overall range of the box plot. the subjects involved in the experiment were individuals with real-world air traffic control experience at the app prague unit or were in the training phase for this unit. each participant had at least 200 hours of experience at the app prague unit, either with real-world operations or in a simulated environment. this level of experience was deemed sufficient for this experiment. the experiment consisted of two simulations: one using the current air traffic control methods and the other using the proposed vertical profiles. the operational conditions, scenario, and traffic composition were identical in both simulations. each subject chose the order in which the simulations were conducted individually. the simulated scenario was designed based on the traffic mix operating at lkpr. according to 2019 data, the most frequently operated aircraft types were the boeing 737 series (38.3% of total traffic) and the airbus a320 series (30.4% of traffic). turboprop aircraft, such as atr 72 and dash-8, accounted for 10.3% of operations. other aircraft types, including long-haul flights and smaller aircraft for individual transport, comprised only a few percent of total traffic. in terms of traffic distribution by wake turbulence categories, the medium category was the most represented in 2019, with a 92.8% share of total traffic. the light and heavy categories were represented equally, at 3.6% each [26, 27]. this statistic indicates that aircraft with similar performance and weight characteristics dominate lkpr. their aerodynamic and physical properties during flight are thus very similar. to verify feasibility from the perspective of pilots and aircraft operations, the prepar3d and x-plane 12 flight simulators were used. simulations were conducted with boeing 737-800 and airbus a320 models, which, including their various variants, represent the majority of movements at lkpr. runway 24 was selected for the design, as it is the most frequently used runway at lkpr, with a maximum capacity of 46 movements per hour [7, 26]. 3. results the final proposal of procedures with the vertical profile is illustrated in figure 5. the procedures include: • intersections determining a five-mile safety zone of the routes (int1 through int4). • new waypoints (lvl1 through lvl8). • vertical restrictions (labelled as fl80). new points were created by establishing a circle with a diameter of 5nm centred on each int and standard routes. these were named lvl1 through lvl8 and determine vertical limitations on the routes. they also respect the boundaries of responsibility areas, which are always 2.5nm from the point whose centre is the standard route. except for lvl5, all these points are derived based on the same principle. lvl5 was defined differently, as it is part of the standard departure route for propeller aircraft, where the first turn is determined by a point corresponding to an altitude of 1 700 feet amsl. however, reaching such a turning point is irrelevant in terms of time and space, as more powerful aircraft will reach it sooner and with less distance travelled from the airport than lower-performing aircraft. moreover, the first fix of this departure route is located at such a distance from the airport that vertical restrictions could not be applied. to create a buffer space for aircraft on the lomki8s arrival route, which intersects with these departures, and to ensure continuous descent and ascent, lvl5 was established as an intersection located 5nm from the line connecting the points forming the arrival route. since the aircraft’s position before reaching the first fix of the departure route can vary, lvl5 was determined to be 11.5nm from vor/dme okl, located at lkpr. this distance is sufficient to ensure that an aircraft in any possible position is at least 5 miles from other traffic. the process of creating all lvl points was designed to avoid creating redundant points that could complicate procedures and make them unclear. an increased risk of errors could also arise in onboard spatial navigation systems, mainly if lvl points are located near fly-by fixes. in cases where such situations occurred, the lvl point was cancelled and merged with the nearest fix from the int point in the flight direction. for each int point, the flight level at which aircraft are located in real operations was determined, which served as the basis for setting vertical restrictions. the procedure is illustrated in figure 6a. the red line represents the median of the data, while the black horizontal lines above and below the box plots indicate the maximum and minimum altitudes. the rectangular outline shows the range of flight levels in which 75% of the traffic is located. based on this information, vertical restrictions would be set at fl90 for departures and fl100 for arrivals after consulting the median with other values, such as the average. values from the bada database were included in the box plots, as shown in figure 6b. according to bada simulations, blue dots represent the ideal flight level for each aircraft type. this approach allowed for the precise design of optimal flight levels, determining vertical restrictions at the given point. based on these values, the optimal level for each lvl point was set, considering the most frequent levels and safety and 30 vol. 52/2025 vertical trajectory for arrival and departure routes at prague airport figure 5. schematic map of lvl point locations including aircraft vertical position restrictions [28]. (a). box plot based on the air traffic at lkpr from the opensky database. (b). box plot based on the air traffic at lkpr compared to bada. figure 6. illustrations of the box plots on the basis of which the level restrictions were determined. 31 š. hrdlička, s. schmidt, t. pilmannová acta polytechnica ctu proceedings figure 7. boxplots of the vertical profile of traffic at lkpr compared to bada. operational factors. this level was included in the standard arrival and departure route maps as a fixed vertical restriction limit. aircraft must adhere to these restrictions unless otherwise directed by atc. this information must also be part of the map. after assessing the actual traffic and the bada database, the information was systematically processed and integrated into analogous box plots. a box plot was created for each int intersection to represent the vertical position of aircraft during departures and arrivals at lkpr. these box plots, illustrated in figure 7, served as the basis for determining vertical restrictions at points referred to as lvl. according to the median (red line) and the average, most departures are near flight level fl100. this level also corresponds to the optimal values that departing aircraft should reach according to the bada database (blue dots). arrivals, on average and according to the median, are approximately at fl130 above the intersection of the routes, while the bada database recommends an optimal level even higher, around fl160. therefore, the appropriate vertical restrictions on the routes were set at flight levels fl130 for arrivals and fl120 for departures. the same approach was used to determine restrictions for other routes. the aircraft’s ability to meet the new route requirements was verified in the escape light and x-plane simulation environments, with the obtained data subsequently processed into graphs shown in figures 8 and 9. in both cases, a reduction in segments with horizontal flight was observed. during descent, the horizontal segment appears mainly at an altitude of 4 000 ft or just before reaching it. this is the part of the flight where the crew prepares the aircraft for the final approach phase. the aircraft must be slowed down, which is related to the deployment of the mechanism and achieving the descent plane of the ils from below. considering the aerodynamic characteristics of the most frequently used aircraft at lkpr, it should be expected that aircraft will not be able to reduce speed and descend simultaneously. some crews, therefore, prefer to reach the intermediate approach altitude earlier so that they can lose speed by inertia before reaching the final approach point. the ils approach was not included in the measured data, as the escape system lacks sufficient accuracy in calculating the vertical trajectory on the final approach route. however, this fact represents only a limitation of the simulation environment considered in the analysis. based on data obtained from actual operations, it can be assumed that aircraft would descend similarly after passing the final approach point, even when implementing the proposal discussed in this article. the vertical trajectory shown in figures 8 and 9 reveals a somewhat irregular descent after passing the final approach point, located approximately 10 miles from runway 24 in prague. since aircraft would continue to approach and follow the same trajectory as in current 32 vol. 52/2025 vertical trajectory for arrival and departure routes at prague airport figure 8. graphical representation of the vertical position of simulated aircraft relative to lkpr before the implementation of new procedures. figure 9. graphical representation of the vertical position of simulated aircraft relative to lkpr after the implementation of new procedures. practice, the values after passing the final approach point are not considered relevant for this research. the same procedure was applied in the x-plane simulator environment on a boeing 737-800 aircraft. the vertical profiles shown in figure 11 also show a significant reduction in horizontal segments before reaching the altitude of 4 000 ft amsl, where the aircraft configuration and speed reduction subsequently occurred to initiate the next approach phase. 4. discussion the analysis of the box plots for departing aircraft suggests that their trajectories in real operations align with the optimal performance profile. this alignment is facilitated by onboard computers that automatically adjust thrust regulation systems based on real-time calculations. these findings align with expectations, as modern aircraft are equipped with advanced flight optimisation systems that automatically manage all phases of flight to maximise safety and efficiency. the resulting trajectories and performance profiles closely match the mathematical bada models, confirming these systems’ high accuracy and reliability in current air traffic operations. however, the analysis of bada models for turboprop aircraft, particularly concerning points int3 and int4, revealed a significant discrepancy in the climb gradient between simulated models and actual operations. specifically, bada models show a lower climb gradient at high aircraft weights than what is observed in real operations. as a result, the bada model lags behind the real-world data box plot. since bada models operate with standardised values, deviations from reality occur, particularly in scenarios requiring a higher climb gradient, such as noise abatement procedures. at lkpr, the operational requirement for 33 š. hrdlička, s. schmidt, t. pilmannová acta polytechnica ctu proceedings a higher climb gradient is set at 10% up to 3,500 feet for departure routes heading north through int3. this requirement can lead to even higher climb gradients in actual operations, reflecting the difference between simulated and real-world data. real operations show that arriving aircraft are at slightly lower altitudes than expected according to optimal trajectories per the bada models. this deviation can be attributed to pilots initiating descent earlier to reach the lowest permitted altitude before starting the initial phase of instrument approach while also decelerating and configuring the aircraft for landing. in practice, aircraft often deviate from the entire planned trajectory of the initial and intermediate approach, with vectoring onto the runway axis for the final approach already occurring when the aircraft is parallel to the runway. when vectoring begins, onboard systems for maintaining the aircraft’s optimal vertical position disconnect, as the aircraft no longer follows the planned route. similarly, the aircraft onboard systems bypass such limitations during trajectory adjustments by vectoring to a point outside the prescribed limitations. after implementing the vertical profile, changes were observed in pilot behaviour during departures and arrivals. simulations indicate that departing traffic ascends at a lower vertical speed. where feasible and desirable in terms of operational procedures, energy is converted into higher forward speed at the expense of faster vertical speed. this change results in smoother operations, fuel savings, and improved passenger comfort. conversely, arriving traffic remains longer at higher flight levels, using the published vertical profile to determine optimal flight levels, which should better align with the aircraft’s performance profile. the realism of simulated data may have been affected by the inherent limitations of the simulation environments. the escape simulation system does not have access to all the parameters that pilots in real cockpits typically use. the calculations performed within the simulations depend on input data corresponding to bada models and any interventions by pseudo-pilots, who simulate the behaviour of aircraft and crews and are responsible for controlling all aircraft and communicating with air traffic controllers. given the large number of tasks and the higher number of aircraft pseudo-pilots must control simultaneously in the simulator, their ability to realistically account for all aspects of the situation in the aircraft is limited. simulators used for measuring on boeing 737 and airbus a320 aircraft also have limitations. the simulations did not include any non-standard operational states, such as system degradation, non-standard flight conditions, weather impact, or special crew requests. such situations could significantly affect the research results, with each change forcing air traffic controllers to revert to traditional control methods. comparing the situation before and after the introduction of vertical restrictions, as illustrated in figures 10 and 11, there was a noticeable decrease in segments where aircraft flew in level flight (with “kinks” in the trajectory) during arrivals. conversely, the segments where aircraft remained at the intermediate approach altitude of 4 000 feet amsl were extended. this change was likely driven by two main factors. first, the relatively high landing weight of the aircraft, around 65 tons, required more time to reduce forward speed before beginning the final approach phase. this could only be achieved in level flight. second, the test pilots’ lack of experience with the new procedure led to a preference for descending before braking, with speed reduction occurring only in the last phase of arrival after reaching the intermediate approach altitude. this descent method contributed to fuel savings, as minimal engine thrust was applied to reduce speed even during level flight. after implementing the new procedures, the average fuel consumption across all simulations decreased by 13.2%. the most significant fuel consumption reduction was observed in the boeing 737-800 simulations in the x-plane environment, where consumption decreased by nearly 25% compared to values before implementing the vertical profile. on the other hand, due to the time-consuming nature of the measurements, a relatively limited data set was obtained. acquiring a more extensive dataset, whether in a simulation environment or real operations, could provide additional valuable insights into the vertical movement of aircraft and their fuel consumption. the simulations almost matched the expected behaviour of air traffic on arrival and departure routes before and after implementing the vertical profile. nevertheless, slightly higher fuel savings were expected in the escape environment, which did not reach the anticipated levels. while simulations and models revealed some deviations and limitations, they also showed that the new methodology offers promising possibilities for further improvement and adaptation in real operations. further research should focus on various scenarios, operational conditions, or the impact of weather on feasibility to obtain even more accurate and comprehensive data. 5. conclusion the proposed system of vertical profiles for arrival and departure routes at tma prague significantly improves air traffic efficiency and safety. this system includes the establishment of points that limit the vertical positions of aircraft, leading to the adaptation of their trajectories, which allows for a smoother flow of arriving and departing aircraft. by setting vertical limitations, the risk of infringing on the minimum required separation between aircraft at crossing routes is minimised. optimisation processes, such as those discussed in this article, contribute to the safe flow of air traffic and the sustainability of aviation. the concept under study is particularly beneficial during 34 vol. 52/2025 vertical trajectory for arrival and departure routes at prague airport figure 10. graphical representation of the vertical position of simulated aircraft relative to lkpr before the implementation of new procedures. figure 11. graphical representation of the vertical position of simulated aircraft relative to lkpr after the implementation of new procedures. peak operational times, where its advantages are most pronounced. simulation studies conducted in the escape and x-plane environments demonstrated substantial fuel savings, averaging 13.2%, with more significant savings of over 25% observed in the boeing 737-800. these results confirm that applying vertical profiles positively impacts fuel consumption and contributes to more environmentally friendly operations. although the specific fuel savings when using the new methods may vary, the results indicate that this system is advantageous in terms of both the workload on flight crews and fuel consumption. while the simulations revealed some deviations and limitations, primarily due to inaccuracies in simulated conditions, the results show a clear positive trend. future research should focus on obtaining a broader spectrum of data and including various operational scenarios that could negatively impact the operational situation. if further research yields positive results, the introduction of 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[2024-08-28]. https://aim. rlp.cz/ais_data/aip/control/aip_obsah_cz.htm 36 https://doi.org/10.2514/6.2010-9115 https://doi.org/10.1109/sii.2017.8279312 https://doi.org/10.2514/1.c033374 https://www.researchgate.net/publication/313755641_continuous_climb_operations_with_minimum_fuel_burn https://www.researchgate.net/publication/313755641_continuous_climb_operations_with_minimum_fuel_burn https://www.researchgate.net/publication/313755641_continuous_climb_operations_with_minimum_fuel_burn https://doi.org/10.1109/dasc.2013.6719602 https://www.aurora.nats.co.uk/htmlaip/publications/2024-03-21-airac/html/index-en-gb.html https://www.aurora.nats.co.uk/htmlaip/publications/2024-03-21-airac/html/index-en-gb.html https://www.aurora.nats.co.uk/htmlaip/publications/2024-03-21-airac/html/index-en-gb.html https://aim.rlp.cz/ais_data/aip/control/aip_obsah_cz.htm https://aim.rlp.cz/ais_data/aip/control/aip_obsah_cz.htm https://doi.org/10.1007/bf00374704 http://hdl.handle.net/10467/111279 https://www.icao.int/environmental-protection/pages/envrep2019.aspx https://www.icao.int/environmental-protection/pages/envrep2019.aspx https://doi.org/10.1007/978-3-662-03714-0_1 https://k154.fsv.cvut.cz/~kremen/p%f8edn%e1%9aka_x_gui2.pdf https://k154.fsv.cvut.cz/~kremen/p%f8edn%e1%9aka_x_gui2.pdf https://en.wikipedia.org/wiki/geographic_coordinate_conversion https://en.wikipedia.org/wiki/geographic_coordinate_conversion https://doi.org/10.1179/sre.1975.23.176.88 https://doi.org/10.5281/zenodo.32999 https://doi.org/10.1109/ipsn.2014.6846743 https://doi.org/10.1109/ipsn.2014.6846743 https://doi.org/10.1002/acs.1176 http://hdl.handle.net/10467/111418 http://hdl.handle.net/10467/103773 https://aim.rlp.cz/ais_data/aip/control/aip_obsah_cz.htm https://aim.rlp.cz/ais_data/aip/control/aip_obsah_cz.htm acta polytechnica ctu proceedings 52:24–36, 2025 1 introduction 2 materials and methods 2.1 analysis of the current state of the art 2.2 identification and calculation of key points along the route 2.3 determination of values for vertical restrictions 2.4 implementation of the vertical restrictions into the simulation environment and validation 3 results 4 discussion 5 conclusion list of symbols references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2023.44.0001 acta polytechnica ctu proceedings 44:1–4, 2023 © 2023 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague effect of heat-treatment on material properties of l-ded printed austenistic alloy 08ch18n10t for nuclear reactor applications štěpán jedlana,∗, martin ševečeka, antonín prantlb, josef hodekb, pavel podanýb, michal brázdab a czech technical university in prague, faculty of nuclear sciences and physical engineering, department of nuclear reactors, v holešovičkách 2, 180 00 prague 8, czech republic b comtes fht a.s., průmyslová 995, 334 41 dobřany, czech republic ∗ corresponding author: jedlaste@fjfi.cvut.cz abstract. this paper deals with the evaluation of material properties of the additively manufactured austenistic alloy 08ch18n10t, which is widely used in the czech republic nuclear power plants temelín and dukovany and other vver reactors around the world. for purposes of utilization of additive manufacturing technologies for nuclear core components fabrication, two sets of samples were prepared from horizontally and vertically l-ded printed blocks from 08ch18n10t material. experiments such as microstructure analysis, porosity and vickers hardness were then performed on l-ded printed and heat-treated 08ch18n10t material, and the obtained material properties were then compared with the properties of l-ded printed 08ch18n10t material without heat-treatment for examination of its effect and also with material properties of conventionally made 08ch18n10t material. keywords: additive manufacturing, 3d printing, laser direct energy deposition, 08ch18n10t, hardness, porosity, microstructure, ebsd, metallography. 1. introduction additive manufacturing (am) provides a faster, cheaper and flexible way to prototype, fabricate, or improve components. so far, am methods have been applied in many industrial applications, but despite the advantages, it is very challenging to implement them in highly regulated and conservative industry field such as nuclear power engineering. this paper deals with the material properties of l-ded fabricated and heat-treated 08ch18n10t material and comparison to conventionally fabricated. the alloy 08ch18n10t has been widely used in many vver reactors around the world and have been studied in detail [1–4]. it has been used for both in-core and out-of core components. with the life extensions of the operating units, it is crucial to find and qualify suitable manufacturing and reparation processes that will allow the utilities to replace or repair components manufactured decades ago. additionally, am provides significant benefits in terms of availability of components and security of supplies. however, am processes are fundamentally different from conventional manufacturing processes, resulting in different material microstructure, mechanical properties or behavior under irradiation [5–7]. therefore, it is crucial to study the am materials in detail before they can be implemented in commercial power plants. figure 1. sample orientation in l-ded deposited block (see green cylinders). 2. materials and methods 2.1. samples preparation the additive manufacturing method laser direct energy deposition (l-ded) was used to fabricate a vertical and horizontal block (see figure 1), using a powdered 08ch18n10t material and 1600 µm laser (see the chemical composition of the conventionally manufactured material in table 1). after fabrication, heat-treatment was applied (3 h – 1150 °c) with air cooling. from each block, six samples were prepared (see figure 1) to further investigate the properties of the as-fabricated material. the samples from the horizontally fabricated block were taken from the side of the deposited block (see figure 1) however, the samples from the vertically deposited block were taken from the top of the fabricated block. 1 https://doi.org/10.14311/app.2023.44.0001 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en š. jedlan, m. ševeček, a. prantl et al. acta polytechnica ctu proceedings c si mn cr ni ti s p ≤ 0.08 ≤ 0.8 ≤ 2 17.0–19.0 9.0–11.0 5 · c–0.7 ≤ 0.02 ≤ 0.04 table 1. chemical composition of conventionally fabricated 08ch18n10t in wt % according to gost 5632:2014 [8]. figure 2. lom analysis image of recrystallized microstructure of l-ded fabricated 08ch18n10t material. 2.2. metallography analysis metallographical analysis of the microstructure was performed using a light optical microscopy (lom) and scanning electron microscopy (sem). for further analysis of grain structures, electron back-scatter diffraction mapping (ebsd) was also used [9]. metallography was performed on heat treated and non-heat treated l-ded fabricated 08ch18n10t to observe differences in microstructure. to reveal the microstructure, the analyzed samples were cut in half, both in the vertical and horizontal plane, and then etched using a v2a etchant (hcl:hno3:h2o – 10:1:10) for ∼40 s. from images of individual samples taken during lom, the porosity was evaluated as a proportion between the area of the individual pores and the area of the whole sample. this was carried out to investigate the differences between samples from horizontally and vertically fabricated blocks. 2.3. vickers hardness vickerss hardness test (hv1) according to en iso 6507 was performed on 12 samples (6× vertical and 6× horizontal) in 9 evenly distributed points, using a struers durascan. in ecos workflow software, the mean value and standard deviation were calculated for each sample. 3. results the results of the lom metallography analysis show complete recrystallization of the microstructure of the l-ded fabricated 08ch18n10t material due to the applied heat treatment (see figure 2). using a lom figure 3. porosity of samples from verticaly (top) and horizontaly (bottom) fabricated blocks. analysis, a greater number of pores was observed in samples from a horizontally fabricated block, as opposed to samples from a vertically fabricated block (see figure 3). this was also confirmed in the porosity evaluation (see table 2), which shows that samples from the vertically fabricated block have ∼2.5× lower porosity than samples from the horizontally fabricated block. sem analysis also showed recrystallization of the microstructure as a result of the applied heat treatment. ebsd mapping results showed columnar grains 2 vol. 44/2023 effect of heat-treatment on material properties . . . fabricated block (samples) porosity [%] vertical 0.040 horizontal 0.108 table 2. mean value of porosity for samples from horizontally and vertically fabricated blocks. fabrication vickers hardness l-ded vertical 132.89 l-ded horizontal 131.80 conventional 127-178 table 3. comparison of vickers hardness mean values for conventionally and l-ded fabricated 08ch18n10t material with heat-treatment. directed from the outside line of individual meltpools toward their center in the non-heat treated material. in the heat treated material, the recrystallized microstructure was observed with large equiaxed grains without specific direction (see figure 4). the results of the vickers hardness test at room temperature (hv1) are similar for both vertically and horizontally deposited blocks, due to the applied heat treatment. heat treatment thus reduces the anisotropy of the am-fabricated materials, which is positive for future applications. the mean values of all samples from vertically and horizontally fabricated blocks were then compared with the values of conventionally fabricated 08ch18n10t material with heat treatment 1020–1100 °c (see table 3) according to gost [10]. 4. conclusions in this study, metallography, porosity and vickers hardness were measured on l-ded fabricated 08ch18n10t material to gain comprehensive information about the effects of applied heat treatment and to compare the material properties with conventionally fabricated material. in conclusion, our comprehensive study of the material properties of l-ded fabricated alloy 08ch18n10t through metallography, porosity analysis, and vickers hardness testing has provided valuable insight into the performance of this material. through metallography, fine microstructure with a homogeneous distribution of large equiaxed grains was observed; however, in the case of samples from vertically deposited block, a lower number of pores were present. this could have been due to better heat transfer during the vertical block deposition process. furthermore, vickers hardness tests revealed that the hardness values of the l-ded fabricated alloy 08ch18n10t in both types of samples (vertical and horizontal) are almost the same, due to the applied heat treatment. the values of the measured hardness are also in close agreement with those typically obfigure 4. ebsd maps of non heat-treated (top) and heat-treated (bottom) material. served in conventionally fabricated materials. this suggests that the 3d printed alloy possesses comparable mechanical strength and wear resistance. in summary, the results of this study affirm the potential of the l-ded fabricated alloy 08ch18n10t as a reliable material with comparable performance to conventionally fabricated; however, further research and testing of its mechanical properties is needed to better understand its capabilities and also the effects of applied heat treatment. it is also foreseen to study the effects of irradiation on am-manufactured materials with optimized heat treatment in the near future. references [1] a. materna, p. haušild, j. ondráček, et al. effects of cold-working and annealing at 700 °c on hardness of 08ch18n10t steel. procedia structural integrity 23:425–430, 2019. https://doi.org/10.1016/j.prostr.2020.01.124 [2] p. pavel, m. petr, n. jan, b. martin. heat treatment of reactor vessel steel 08ch18n10t. in materials science and technology conference and exhibition 2012, ms and t 2012, 2, pp. 1036–1043. 2012. [3] b. fekete, j. kasl, d. jandova, et al. low cycle thermomechanical fatigue of reactor steels: 3 https://doi.org/10.1016/j.prostr.2020.01.124 š. jedlan, m. ševeček, a. prantl et al. acta polytechnica ctu proceedings microstructural and fractographic investigations. materials science and engineering: a 640:357–374, 2015. https://doi.org/10.1016/j.msea.2015.05.093 [4] t. janda, š. jeníček, l. kučerová, et al. influence of higher stabilization temperatures on the microstructure and mechanical properties of austenitic stainless steel 08ch18n10t. metals 13(5):975, 2023. https://doi.org/10.3390/met13050975 [5] n. sridharan, m. n. gussev, k. g. field. performance of a ferritic/martensitic steel for nuclear reactor applications fabricated using additive manufacturing. journal of nuclear materials 521:45–55, 2019. https://doi.org/10.1016/j.jnucmat.2019.04.020 [6] j. simpson, j. haley, c. cramer, et al. considerations for application of additive manufacturing to nuclear reactor core components. tech. rep., oak ridge national lab. (ornl), oak ridge, tn (united states), 2019. [7] š. jedlan, m. ševeček, a. prantl, et al. utilization of additive manufacturing in nuclear power industry. in 2022 8th international youth conference on energy (iyce), pp. 1–6. ieee, 2022. https://doi.org/10.1109/iyce54153.2022.9857541 [8] gost-5632:2014. high-alloy steels and corrosion-resistant, thermally resistant and refractory alloys. standard, gost, 2014. [9] p. gávelová, p. halodová, b. křivská, et al. microstructure of zirconium fuel claddings: tem and ebsd studies of as-received and neutron-irradiated materials. manufacturing technology 20(6):720–727, 2020. https://doi.org/10.21062/mft.2020.088 [10] steel 08x18n10t. auremo, 2023. [2023-09-27]. https://auremo.biz/materials/stal-08h18n10tei914.html 4 https://doi.org/10.1016/j.msea.2015.05.093 https://doi.org/10.3390/met13050975 https://doi.org/10.1016/j.jnucmat.2019.04.020 https://doi.org/10.1109/iyce54153.2022.9857541 https://doi.org/10.21062/mft.2020.088 https://auremo.biz/materials/stal-08h18n10t-ei914.html https://auremo.biz/materials/stal-08h18n10t-ei914.html acta polytechnica ctu proceedings 44:1–4, 2023 1 introduction 2 materials and methods 2.1 samples preparation 2.2 metallography analysis 2.3 vickers hardness 3 results 4 conclusions references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2025.53.0082 acta polytechnica ctu proceedings 53:82–86, 2025 © 2025 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague comparative study of different types of waterproofing screeds with a focus on radon permeability after the freeze-thaw exposure michal nývlt∗, jiří pazderka, martin jiránek, david hes czech technical university in prague, faculty of civil engineering, thákurova 7, prague 166 29, czech republic ∗ corresponding author: michal.nyvlt@cvut.cz abstract. this study aims to verify and independently compare the functionality of waterproofing screeds against the effect of radon with the influence of freezing cycles. the effect of freezing cycles on the sealing ability against radon was investigated on polymer, silicate (mineral), and bituminous screeds. the measured values correspond to commonly used insulation materials, confirming the correctness of the measurement and evaluation methodology. waterproofing screeds are shown to be suitable materials for substructure applications. unlike waterproofing made from strips such as bitumen membrane or pvc membrane, they do not contain joints and can thus offer a more reliable solution. the measured values show minimal differences between the tested waterproofing screeds after freeze-thaw exposure. especially in the case of silicate (mineral) screed and polymer screed, the effect is negligible. the greatest effect of freeze-thaw cycles on the ability to seal against radon was observed for bitumen screeds. keywords: waterproofing screeds, radon, freezing cycles, sealing ability, insulation materials, substructure. 1. introduction radon is a ubiquitous natural radioactive gas. it is formed by the gradual transformation of uranium, which is present in varying amounts in all materials in the earth’s crust. radon itself is converted into other radioactive elements (isotopes of polonium, lead and bismuth), which are trapped in the respiratory tract and irradiated when inhaled [1]. scientific studies have provided clear evidence of a link between radon exposure in building interiors and lung cancer. this risk is already present at the relatively low levels of radon commonly found in residential buildings [1, 2]. waterproof materials provide basic protection for buildings against moisture, groundwater and radon from the soil. these are most commonly polymer and asphalt strips or waterproofing membranes. the loss of their impermeable properties due to premature deterioration may cause a complete loss of protection of the building from radon ingress from the subsoil [3]. degrading agents such as soil bacteria, radon, high temperature and high humidity and possibly combinations of these factors have already been investigated [3] and the results of the experiments published. in the previous research, the effectiveness of the screeds against water has been addressed. since waterproofing materials are designed not only as waterproofing but also as insulation against radon penetration into the interior of a building, the logical next step is to verify their functionality and reliability. in the research, the ability of the screed to insulate against radon and the ability to withstand freezing cycles are analysed and tested. 2. materials and methods a total of 24 supporting base plates were created. cement-bonded lightweight material with product name “fermacel powerpanel h2o” was chosen as a permeable but at the same time strong base material. it is a cement-bonded lightweight concrete panel with a sandwich structure and double-sided reinforcement under cover layers with alkali-resistant fiberglass fabric (5 mm×5 mm). the plate is produced in a thickness of 12.5 mm. the test plates were 325 mm×135 mm. waterproofing screeds, which were used in all previous experiments (polymeric, silicate and bituminous), were applied to the created support plates (figure 1). to perform the calculation, reference specimen was also created without applied waterproofing screed. figure 1. specimen set with applied waterproofing screeds. the prepared specimens were then placed in a freezing chamber. the method of stressing by freezing cycles is identical to that described in chapter 5.1.3 of the thesis (the test specimens were immersed surface with a surface treatment in water at (+20 ± 3 °c) for 82 https://doi.org/10.14311/app.2025.53.0082 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 53/2025 comparative study of waterproofing screeds 6 hours. the specimens were then removed and stored in a freezing chamber at a temperature of (-20 ± 2 °c) for 18 hours). a total of 30 freeze cycles were performed in this way. after 30 cycles, the specimens are left to dry at an air temperature of (+20 ± 3 °c) and a relative humidity of (55 ± 10 %) for at least 14 days. during the cycling time in the freezing chamber, a device was prepared to measure the radon diffusion coefficient d [m2 s−1]. the device used was designed and maintained by the department of architectural engineering of the faculty of civil engineering ctu. it is a measuring device consisting of three receiving chambers above the specimen and one radon source chamber below the specimen (figure 2, figure 3). in the lower chamber (below the specimen) the radon source is common to all 3 specimens. the specimen placed between the two chambers shall be sealed perfectly. a permanently flexible acrylic sealant shall be used for sealing. figure 2. measuring device with one source chamber and three receiving chambers. the radon concentration in the upper and lower chambers is continuously measured by tesla radon detectors. these detectors are located inside each upper chamber (the principle is based on detecting po using a pin photodiode). in total, there are 3 upper chambers in the measuring device and one detector in each chamber. to measure the concentration below the specimen, one detector common to all three specimens is placed in the lower chamber. the data from the detectors are transmitted wirelessly to a control unit located outside the measuring device. the area of a single specimen was 2.93×10−2 m2. the volume of the source chamber was 13.9×10−3 m3, and the volume of the receiver container was 2.7×10−3 m3. the specimens were sequentially loaded into the device and sealed (figure 4, figure 5). after specimen sealing, the increase in radon concentration in the upper and lower chambers is monitored until steady state. for the selected waterproofing screeds, a time interval of 14 days was found to be appropriate. during which time the increase in concentration in the upper chamber was stabilized. crushed concrete made of slag with a high content of radium (1 000–4 000 bq kg−1) and sand that had been (a). used measuring device. (b). used measuring device. figure 3. measuring devices used for experiments. figure 4. measuring device with inserted and sealed specimens. figure 5. the sealed specimens. 83 m. nývlt, j. pazderka, m. jiránek, d. hes acta polytechnica ctu proceedings used as a filter in a water treatment plant were used as a natural radon source. the radon diffusion coefficient d [m2 s−1] was determined according to method a of iso/ts 11665-13 from the known time-dependent curves of the radon concentrations measured on both sides of the tested specimen. a total of 7 sets with three specimens in the measuring device were carried out. • set 1 = 3× reference specimen without waterproofing screed, • set 2 = 3× specimen with polymer waterproofing screed, • set 3 = 3× specimen with bitumen waterproofing screed, • set 4 = 3× specimen with silicate waterproofing screed, • set 5 = 3× specimen with polymer waterproofing screed after 30 freeze thaw cycles, • set 6 = 3× specimen with bitumen waterproofing screed after 30 freeze thaw cycles, • set 7 = 3× specimen with silicate waterproofing screed after 30 freeze thaw cycles. the measurements were taken between the autumn of 2022 and the spring of 2023. after the measurements were taken, the specimens were removed (figure 6) from the measuring device and the thickness of the applied waterproofing screed was measured for each specimen separately (figure 7). figure 6. specimens removed from the measuring device, sealant visible around the perimeter. 3. results and discussion since the measurements were made under nonstationary conditions, the calculation is based on the numerical solution for the non-stationary differential equation of radon diffusion. a very detailed description [4] has been published, where the mathematical procedure and method of calculation is described. (a). (b). (c). figure 7. measuring the thickness of the applied waterproofing screed. in the numerical evaluation of the radon diffusion coefficient using the iterrn software [5], the process relies on time-dependent radon concentration values measured in both the source and receiver containers. the calculation involves iteratively solving equations, starting with an initial estimation of the radon diffusion coefficient range (lower and upper limits) and a defined iteration step (typically 1/10 to 1/20 of the range). simplified description of the procedure: initialization • the estimated range of the radon diffusion coefficient and the iteration step were set. 84 vol. 53/2025 comparative study of waterproofing screeds type of the waterproofing screed layer thickness [mm] average layer number of radon detector thickness s1 s2 s3 [mm] silicate (mineral) screed (reference) 2.73 2.87 3.38 2.99 silicate (mineral) screed (30 freezing cycles) 3.18 2.85 2.96 3.00 polymer screeds (reference) 1.77 1.84 1.28 1.63 polymer screeds (30 freezing cycles) 1.69 1.81 1.73 1.74 bituminous screeds (reference) 1.73 1.87 1.64 1.73 bituminous screeds (30 freezing cycles) 1.71 1.91 1.99 1.87 table 1. measured values for each specimen. type of the screeds radon diffusion coefficient standard deviation [m2 h−1] [m2 h−1] 0 freezing cycle 30 freezing cycle 0 freezing cycle 30 freezing cycle polymer screeds 4.65×10−12 3.73×10−12 3.7×10−13 8.2×10−13 bituminous screeds 4.90×10−12 3.20×10−11 4.0×10−13 5.6×10−12 silicate (mineral) screed 4.15×10−12 4.05×10−12 9.1×10−14 9.2×10−14 table 2. radon diffusion coefficient values and their standard deviations for different types of waterproofing screeds before and after freeze-thaw exposure. • the transient diffusion of radon through the radon barrier is calculated with the initial radon diffusion coefficient set to the lower limit. comparison • the calculated time-dependent radon concentration results in the receiver container are compared with the measured data. • differences between the calculated and measured data sets are recorded. iterative process • the procedure was repeated with a progressively increasing radon diffusion coefficient in each step. • the iteration continued until the upper limit of the radon diffusion coefficient range was reached. optimization • the final radon diffusion coefficient was determined by minimizing variations between calculated and measured data, considering metrics such as average difference or sum of deviations. efficiency • the entire iteration process is designed to be computationally efficient and typically took no more than 30 minutes on a standard contemporary computer. iterrn software (sw.) was developed at ctu for the microsoft windows environment. iterrn shares similarities with transrn software [5]. contains standard ms windows procedures and dialogs for creating, saving, reading and editing data files. the input values derived from the measurements were imported from a text record that was stored in a plain text file with the values separated by periods. measured values for each specimen are shown in table 1. the resulting radon diffusion coefficient d [m2 s−1] values were determined as the arithmetic radon diffusion coefficient. in some measurements, the measuring detector was damaged. this is the reason for the missing measured values (detector labelled s3). unfortunately, this was only discovered during the evaluation of the measured data. 3.1. comparison of all tested waterproofing screeds the mean values of the radon diffusion coefficient d [m2 s−1] are shown in table 2. these values were calculated from three or two specimens. in general, it can be said that the effect of 30 freezing cycles on the resulting values is very low, but visible on the values. the change in the diffusion coefficient was the lowest in the case of silicate (mineral) waterproofing screeds, at the same time with the highest measurement accuracy, see standard deviation. the measured mean value is 4.15×10−12 m2 s−1 in the case of the reference specimen and 4.05×10−12 m2 s−1 after 30 freezing cycles. it is evident from the measurements that the effect of 30 freezing cycles on the radon sealing ability of the silicate screed is negligible. a polymer screeds also achieved very similar results. here we can observe a small change in the radon diffusion coefficient. from the reference value of 4.65×10−12 m2 s−1, the value increased to 3.73×10−12 m2 s−1. in general, the lower the value of the radon diffusion coefficient, the tighter the material. in the case of bitumen screeds, the greatest increase in radon diffusion coefficient d [m2 s−1] can be observed. the value measured for the reference sample is 4.9×10−12 m2 s−1 and after 30 cycles 85 m. nývlt, j. pazderka, m. jiránek, d. hes acta polytechnica ctu proceedings 3.2×10−11 m2 s−1. there was a change of decimal order. the measured values correspond to commonly used insulation materials, which confirm measurements, for example on bitumen membrane (5.93×10−12 m2 s−1) or pvc membrane (1.98×10−11 m2 s−1) carried out on the samples exposed to radon at high level [6]. it can therefore be said that the measurement and evaluation were carried out correctly. waterproofing screeds are thus a suitable material for use in substructures. the optimal value of the radon diffusion coefficient d can be found within the range of 5×10−12 to 1×10−11 m2 s−1. all three screeds meet this condition even after freezing cycles [7]. unlike waterproofing made from strips such as bitumen membrane or pvc membrane, they do not contain joints and can thus be a more reliable solution. the measured values show minimal differences between the tested waterproofing screeds. the results presented here should be considered as preliminary results, more extensive research with a larger number of freezing cycles is necessary to clarify the effect of freezing cycles on the insulating abilities of the tested waterproofing screeds. such research is very time-consuming not only during the preparation of the specimens themselves in the freezing chamber, but also during the actual measurement of the radon diffusion coefficient d [m2 s−1] and the subsequent evaluation of the measured data. 4. conclusion the effect of freezing cycles on the sealing ability against radon was investigated on the aforementioned screeds. • the measured values correspond to commonly used insulation materials, and it can therefore be said that the measurement and evaluation were carried out correctly. • waterproofing screeds are thus a suitable material for use in substructures. unlike waterproofing made from strips such as bitumen membrane or pvc membrane, they do not contain joints and can thus be a more reliable solution. • the measured values show minimal differences between the tested waterproofing screeds. especially in the case of silicate (mineral) screed and polymer screed, the effect is negligible. the greatest effect of antifreezes on the ability to seal against radon was observed for bitumen screeds. 5. acknowledgment the article is based on the diploma thesis of the author [8].this research was supported by project sgs22/138/ohk1/3t/11. references [1] who handbook on indoor radon: a public health perspective. world health organization, geneva, switzerland, 2009. [2] c. cosma, a. cucoş-dinu, b. papp, et al. soil and building material as main sources of indoor radon in băiţa-ştei radon prone area (romania). journal of environmental radioactivity 116:174–178, 2013. https://doi.org/10.1016/j.jenvrad.2012.09.006 [3] k. navrátilová rovenská, m. jiránek, v. kačmaříková. the influence of long-term degradation of waterproof membranes on mechanical properties and on the radon diffusion coefficient – preliminary results. journal of cleaner production 88:369–375, 2015. https://doi.org/10.1016/j.jclepro.2014.06.012 [4] m. jiránek, z. svoboda. transient radon diffusion through radon-proof membranes: a new technique for more precise determination of the radon diffusion coefficient. building and environment 44:1318–1327, 2009. https://doi.org/10.1016/j.buildenv.2008.09.017 [5] z. svoboda. software packages transrn & iterrn, 2008. [6] k. navrátilová rovenská. the effect on the radon diffusion coefficient of long-term exposure of waterproof membranes to various degradation agents. radiation protection dosimetry 160(1-3):92–95, 2014. https://doi.org/10.1093/rpd/ncu096 [7] m. jiránek, j. hůlka. radon diffusion coefficient in radon-proof membranes – determination and applicability for the design of radon barriers. international journal on architectural science 1(4):149–155, 2000. [8] m. nývlt. experimental analysis of the effectiveness of waterproofing screeds in building structures. ph.d. thesis, czech technical university in prague, faculty of civil engineering, prague, 2024. supervisor: jiří pazderka. 86 https://doi.org/10.1016/j.jenvrad.2012.09.006 https://doi.org/10.1016/j.jclepro.2014.06.012 https://doi.org/10.1016/j.buildenv.2008.09.017 https://doi.org/10.1093/rpd/ncu096 acta polytechnica ctu proceedings 53:82–86, 2025 1 introduction 2 materials and methods 3 results and discussion 3.1 comparison of all tested waterproofing screeds 4 conclusion 5 acknowledgment references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2023.40.0022 acta polytechnica ctu proceedings 40:22–26, 2023 © 2023 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague treatment of polypropylene microfibers by atmospheric and low-pressure plasma – application to a reinforced cement composite containing recycled concrete jakub ďurejea,∗, zdeněk prošeka, jan trejbala, štěpán potockýb, radim hlůžeka a czech technical university in prague, faculty of civil engineering, department of mechanics, thákurova 7, 166 29 prague 6, czech republic b czech academy of sciences, institute of physics, cukrovarnická 10, 162 53 prague 6, czech republic ∗ corresponding author: jakub.dureje@fsv.cvut.cz abstract. the effect of atmospheric and low-pressure plasma modification on polypropylene (pp) microfibers was examined. mechanical changes on the microfiber surfaces were observed using scanning electron microscopy (sem). next, wettability was measured using the packed-cell method. the fibers were applied into a cement matrix containing micro-milled recycled concrete. test specimens were made and then the dynamic modulus of elasticity was continuously measured. after 28 days were made in the test specimens central notches to a depth of 14 mm. finally, bending tests were performed. from the results, the fracture energy of the composite material was calculated. it was proven that low-pressure plasma modification as well as atmospheric plasma modification increases the wettability of pp fibers with water. furthermore, it was found that samples containing plasma-modified microfibers have a higher fracture energy compared to the same samples with fibers without plasma modification. conversely, plasma modification had no effect on the dynamic modulus of elasticity. keywords: atmospheric plasma, plasma modification, plasma treatment, polypropylene microfiber, oxygen plasma, wettability, fracture energy, sem. 1. introduction nowadays, proper waste management is very important. old waste concrete can be recycled and subsequently reused. it is most often used as a bottom layer for roads. in this work, recycled concrete was ground using a high-speed mill and then used as a filler for the cement composite material. the advantage of this use is possibility to use all fractions of recycled concrete and possibility to activate some non-hydrated cement in recycled concrete [1, 2]. the cementitious composite material was reinforced using polypropylene (pp) microfiber reinforcement. the properties of the resulting composite material depend, among other things, on the interfacial transition zone (itz). in this zone occurs interaction between the cement matrix and the fibers [3]. to improve the adhesion between the fiber and the matrix, it is possible to modify the surface of the fibers, which will lead to an improvement of the mechanical properties final composite material. using plasma, we can modify the fiber surface both mechanically and chemically [4]. the mechanical effect of plasma modification is caused by ion bombardment. the chemical effect of plasma modification is mainly caused by the many chemical groups that are generated during modification [5, 6]. the effect of plasma modification on microfibers surface depends on many parameters, including time, gas, power, and device. plasma treatments are performed in a low-pressure chamber or at atmospheric pressure [7, 8]. 2. materials and specimens portland cement, micro-milled recycled concrete, plasma treated polypropylene (pp) microfibers and water were used to produce the test samples. the ratio of cement to recycled was 1:1. the water ratio w/c+r was 0.32 for each sample. portland cement cem i 42.5 r radotín (českomoravský cement, czechia) was used. micro-milled recycled concrete (lavaris, czechia) was made from concrete drainage gutters using a high-speed mill. the specific surface of micro-milled recycled concrete is around 36 m2/kg. microfibers fibrofor multi (contec fiber, switzerland) were made of polypropylene. the fibers are made in bundles (type 127), the diameter of individual filament is about 32 µm and the microfiber length is 12 mm ±5 %. the surface of the microfibers was modified using atmospheric or low-pressure plasma. low-pressure plasma treatment was performed by tesla vt214 device using an rf source – gas pressure in the chamber of device was 20 pa. atmospheric pressure modification was performed by roplass rps 400 device using a dielectric barrier discharge. the process parameters of oxygen low-pressure plasma modification were chosen based on previous experiments [9]. 22 https://doi.org/10.14311/app.2023.40.0022 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 40/2023 treatment of polypropylene microfibers . . . set device input power [w] time [s] gas pressure [pa] ref a roplass rps 400 300 480 (4×120) air atmospheric t tesla vt213 100 480 (2x×240) oxygen 20 table 1. process parametrs of plasma modification. set cement [g] recyclate [g] water [g] w/c+r microfibers [g] microfibers [%] ref 1500 1500 960 0.32 28.0 2 a 1500 1500 960 0.32 28.0 2 t 1500 1500 960 0.32 28.0 2 table 2. composition of the samples. the oxygen low-pressure plasma modification of the microfibers was performed for a total of 480 s, while the process was paused after 240 s, the fibers were mixed and then the process was started again. for the same duration, the fibers were modified in atmospheric pressure plasma, where the working was atmospheric air. to achieve a uniform modification of the surface of the fibers, the fibers were mixed during the process after every 120 s. the process parameters of plasma modification are in table 1. a total of three sets specimens were made, each set containing six test specimens. the dimensions of the test specimens were 40 × 40 × 160 mm. samples were unmolded 24 hours after production and were stored 28 days in standard laboratory environment. composition of the samples is shown in the table 2. 3. experimental methods fiber surface was examined by scanning electron microscopy (sem). first, a thin layer of platinum was sputted on the surface of the fibers using a mini sputter coater quorum sc7620 at a pressure of 8 pa. subsequently, the surface of the fibers was examined using a merlin zeiss sem. the surface of the fibers was examined at a magnification of 20,000× (figure 1). the wettability of the fibers was measured by the packed-cell method. the fibers were insert into a container with a perforated bottom, after that this container was immersed into water for 60 seconds. container with the fibers was weighed on the laboratory scale before immersion and 120 seconds after immersion. finally, the percentage weight of water to weight of fibers was calculated (1) [10]: mv = (mm − mn) − (ms − mn) (ms − mn) · 100, (1) where mv the weight of water to the weight of fibers ratio [%], mm the weight of wet fibers and packed-cell measuring set [g], (a). plasma modification in low-pressure by oxygen. (b). plasma modification in atmospheric pressure by air. figure 1. sem image – microfiber surface after modicifation. ms the weight of dry fibers and packed-cellmeasuring set [g], mn the weight of packed-cell measuring set [g]. the dynamic modulus of elasticity of the samples was continuously measured by the resonance method (brüel&kjær, denmark). the modulus of elasticity was measured 7, 14, 21 and 28 days after samples production. the measuring system includes an impulse hammer brüel&kjær type 8206, response sensor brüel&kjær type 4519-003 and measure device brüel&kjær front-end 3560b-120. the dynamic mod23 j. ďureje, z. prošek, j. trejbal et al. acta polytechnica ctu proceedings figure 2. position of the sensor and the impulse hammer on the sample to measure the fundamental frequencies from longitudinal (left), transverse (center), and torsional (right) oscillations; s – sensor, b – impact hammer [11]. ulus of elasticity was determined from longitudinal, transverse and torsional oscillations (figure 2). finally, the dynamic modulus of elasticity was calculated using pulse labshop software version 14.0.1. for measurement of fracture energy was performed a notch in the middle of the length test specimens. notch was performed using an automatic saw with a water-cooled diamond blade (achilli, italy). the depth of the notch was 14 mm, which is approximately one third of the height of the specimen. the notch width was 3 mm. subsequently, a three-point bending test was performed. samples were loaded by electromechanical press (mts, usa). samples were loaded with constant displacement at a speed of 1.5 mm/min. the fracture energy was calculated using the formula (2): gf = af bw , (2) where gf fracture energy [j/m2], af the work of loading force [j], bw the area of the crack ligament [m2]. the work of the loading force was calculated as the integral of the function from the force-displacement graph: af = ∫ smax 0 fds, (3) where s displacement during loading test, f force during loading test. 4. results and discussion in the sem images were observed on the fiber surfaces significant mechanical changes compared to the reference fibers for both types of modification. however, the changes on the fiber surfaces are different for each type of modification. fibers modified by low-pressure oxygen plasma have holes on their surfaces. fibers modified by atmospheric pressure plasma have pimples (drops) on their surfaces. the wettability of the figure 3. weight of water to weight of microfibers. figure 4. dynamic modulus of elasticity from 0 to 28 days. fibers increased after both modifications, the amount of water between the fibers increased in both cases by approximately 20 % compared to the reference values. the chemical effect of plasma modification is approximately the same in both cases (figure 3). the dynamic modulus of elasticity was slightly lower for the samples containing plasma-modified fibers than the reference samples. for fibers modified by atmospheric plasma, modulus of elasticity decreased by approximately 1 %, for fibers modified by low-pressure oxygen plasma decreased by 3.5 % (figure 4). decrease in modulus of elasticity is negligible, most likely it was 24 vol. 40/2023 treatment of polypropylene microfibers . . . figure 5. flexural test – force-displacement graph. caused by slightly worse workability of the cement mixture. that corresponds to a slightly lower density of samples with plasma-modified fibers compared to the reference samples. the fracture energy was higher for the samples with plasma-modified fibers compared to the reference samples. for samples containing fibers modified by atmospheric pressure plasma, it was an increase of approximately 8 %, for samples containing fibers modified by low-pressure oxygen plasma, it was an increase of approximately 25 % (figures 5, 6). 5. conclusion the surface of the fibers was modified both mechanically and chemically. based on the experiment, we can conclude that: • modification by oxygen low pressure plasma caused mechanical changes on the fibers surfaces. there were observed by sem (magnification 20,000×) holes caused by this modification. • modification by atmospheric pressure plasma caused mechanical changes on the fibers surfaces. on the fiber surface were observed by sem (magnification 20,000×) formations that look like drops or pimples caused by this modification. • the wettability increased approximately the same after both modifications. the amount of water between the fibers increased by approximately 20 % in both cases. • modifications had almost no effect on the modulus of elasticity. the modulus of elasticity slightly decreased, for samples containing microfibers modified by atmospheric plasma decreased by 1 %, for samples containing microfibers modified by lowpressure oxygen plasma decreased by 3.5 %. this decrease was probably caused by the slightly worse workability of the fresh cement composite mixture for the samples containing plasma-modified fibers compared to the reference samples. • the fracture energy of samples with plasmamodified fibers increased in both cases of modification. for samples modified by atmospheric pressure plasma, it increased by 8 %, for samples modified by low-pressure oxygen plasma, it increased by 25 %. figure 6. fracture energy of samples. the increase in fracture energy was mainly caused by the mechanical effect of plasma modification. the tested plasma modifications succeeded in increasing the fracture energy of the cement composite material containing micro-milled recycled concrete. both tested modifications were suitable. samples containing fibers modified by low-pressure oxygen plasma had a higher fracture energy than samples containing fibers modified by atmospheric pressure plasma. on the other hand, plasma modification performed in atmospheric pressure is significantly easier to apply in mass production compared to modification performed in low-pressure. the workflow and devices for lowpressure plasma are more complicated compared to atmospheric plasma devices. acknowledgements this work was financially supported by the czech technical university in prague – the project sgs22/089/ohk1/2t/11. the authors also thank the center for nanotechnology in civil engineering at the faculty of civil engineering, czech technical university in prague. references [1] k. mcneil, t. h. k. kang. recycled concrete aggregates: a review. international journal of concrete structures and materials 7(1):61–69, 2013. https://doi.org/10.1007/s40069-013-0032-5 [2] j. topič, z. prošek. properties and microstructure of cement paste including recycled concrete powder. acta polytechnica 57(1):49–57, 2017. https://doi.org/10.14311/ap.2017.57.0049 [3] g. prokopski, j. halbiniak. interfacial transition zone in cementitious materials. cement and concrete research 30(4):579–583, 2000. https://doi.org/10.1016/s0008-8846(00)00210-6 [4] f. d. egitto, l. j. matienzo. plasma modification of polymer surfaces for adhesion improvement. ibm journal of research and development 38(4):423–439, 1994. https://doi.org/10.1147/rd.384.0423 [5] f. awaja, s. zhang, n. james, d. r. mckenzie. free radicals generated by ion bombardment of a semi-crystalline peek surface. plasma processes and 25 https://doi.org/10.1007/s40069-013-0032-5 https://doi.org/10.14311/ap.2017.57.0049 https://doi.org/10.1016/s0008-8846(00)00210-6 https://doi.org/10.1147/rd.384.0423 j. ďureje, z. prošek, j. trejbal et al. acta polytechnica ctu proceedings polymers 9(2):174–179, 2012. https://doi.org/10.1002/ppap.201100056 [6] e. couto, i. h. tan, n. demarquette, et al. oxygen plasma treatment of sisal fibers and polypropylene: effects on mechanical properties of composites. polymer engineering & science 42(4):790–797, 2002. https://doi.org/10.1002/pen.10991 [7] c. mandolfino. polypropylene surface modification by low pressure plasma to increase adhesive bonding: effect of process parameters. surface and coatings technology 366:331–337, 2019. https://doi.org/10.1016/j.surfcoat.2019.03.047 [8] m. shenton, m. lovell-hoare, g. stevens. adhesion enhancement of polymer surfaces by atmospheric plasma treatment. journal of physics d: applied physics 34(18):2754, 2001. https://doi.org/10.1088/0022-3727/34/18/307 [9] j. ďureje, z. prošek, j. trejbal, et al. influence of oxygen and argon plasma treatment on wettability and surface morphology of polypropylene microfibers. acta polytechnica ctu proceedings 34:11–14, 2022. https://doi.org/10.14311/app.2022.34.0011 [10] j. ďureje. plasma modification of microfiber reinforcement to improve adhesion with cement matrix containing recycled concrete. master’s thesis, czech technical university in prague, prague, 2018. [11] čsn 73 1372. nedestruktivní zkoušení betonu – rezonanční metoda zkoušení betonu, 2012. 26 https://doi.org/10.1002/ppap.201100056 https://doi.org/10.1002/pen.10991 https://doi.org/10.1016/j.surfcoat.2019.03.047 https://doi.org/10.1088/0022-3727/34/18/307 https://doi.org/10.14311/app.2022.34.0011 acta polytechnica ctu proceedings 40:22–26, 2023 1 introduction 2 materials and specimens 3 experimental methods 4 results and discussion 5 conclusion acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2023.42.0046 acta polytechnica ctu proceedings 42:46–50, 2023 © 2023 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague volume changes measurement of elastomers using 3d dic barbora kotlánová∗, soňa rusnáková, raphael olabanji ogunleye, jakub javořík tomáš baťa university in zlín, faculty of technology, department of production engineering, vavrečkova 5669, 76001 zlín, czech republic ∗ corresponding author: b_kotlanova@utb.cz abstract. the presented research aims at measuring the volume changes of elastomers using digital image correlation (dic) in the 3d configuration (stereo dic). the deformation measurement method in stereo configuration was applied during the mechanical testing of two types of test specimens (dumbbells, cylinders) in uniaxial tension. in this configuration, two cameras were used. the test specimens were measured up to a specified strain value, and the measured data were used to obtain the strain dependence of poisson’s ratio and the bulk modulus, which are crucial for hyperelastic models. the measurement results reveal that the stereo dic method provides relatively less scattered data in the low-strain regions for dumbbell-shaped test specimens and is suitable for measuring various test specimen shapes. keywords: elastomer, dic, volume change, bulk modulus, poisson’s ratio. 1. introduction elastomeric materials are generally considered volume incompressible in many applications. this deduction is a result of a poisson’s ratio approximately 0.5. rubber-like materials are then characterized by a significant change in shape and a slight change in volume under loading. the properties of elastomers mainly depend on the rubber compound and the production technology. therefore, obtaining data from the literature is limited to a specific material compound, and due to the non-standardized material characteristics of elastomers compared to, e.g., metals, it is necessary to measure the exact data for a different compound individually. the main observed characteristics of elastomers are the bulk modulus and poisson’s ratio indicating the degree of compressibility, which lacks a standardized measurement method. many different experimental methods based on static loading have been developed and applied to detect volume changes in elastomers. in terms of compression dilatometry testing, deformations can be induced by hydrostatic or piston compression [1]. in uniaxial loading conditions, the determination of the poisson’s ratio is often approached by creep and cyclic loading tests [2–4]. changes in the volume of elastomers can be observed as a function of time, temperature, and cyclic loading [5], either compressive or tensile. however, the dependence of the bulk modulus on loading for elastomers still needs to be determined. the development of modern technology of digital image correlation (dic) enhances the possibilities in the field of observing and quantifying deformations and related volume variations [6] not only in elastomeric materials. dic is a non-contact optical method for measuring deformations by comparing reference and deformed images using a speckle pattern applied to the specimen surface [7, 8]. the dic method is generally divided into planar measurements (2d, mono) and spatial measurements (3d, stereo) [9]. the 2d dic method uses only one recording device (camera). the ideal application of 2d dic is limited to planar surfaces where in-plane deformations are expected and the camera must be positioned perpendicular to the measured object [10]. the 2d dic devices can also include a video-extensometer, which can be implemented to a universal testing machine [11]. the 3d dic systems use two or more cameras, and the software is able to record the position of a specific point of the measured object in the spatial area. in addition, 3d dic systems are also more efficient in measuring planar deformations compared to 2d dic systems. planar deformations are accompanied by out-of-plane motions that are caused by circumstances such as deviations from planarity, specimen bending, or even measured poisson’s ratios [12]. this study focuses on the application of the stereo dic method using two types of test specimens to measure the volume changes in elastomeric materials. the aim of the research is to determine the method for measuring the dependence of the bulk modulus on the strain using dic and to verify whether this method is applicable to selected types of test specimens. the presented paper also investigates how the stereo dic configuration differs from the 2d dic method, which the authors have already examined in the previous research [11]. 46 https://doi.org/10.14311/app.2023.42.0046 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 42/2023 volume changes measurement of elastometrs using 3d dic figure 1. experimental setup of the stereo dic measuring device 2. materials and methods the volume changes of elastomers were measured with the dic device (sobriety) with software mercury rt connected to the universal testing machine shimadzu ags – 50knxd during the tensile test. the dic device was set up in the stereo configuration, i.e. using two cameras basler aca2040–90um with kowa lens lm25hc f1.4 f25mm (figure 1). the cameras with a maximum frame rate of 90 fps and resolution of 4 mpx were placed at a distance of 400 mm from the measured test specimen and had an angle of 25◦. the recorded images were 688 × 2048 px, pixel size 5.5 × 5.5 µm with an acquisition rate of 5 fps. two m-tech wlo603 led lamps were used to ensure optimal lighting conditions. the dic setup was calibrated with a grid spacing of 2 mm, and the correlation window size used was 80 × 80 px. two types of test specimens were selected for the experimental measurements, with a sample size of 20 pieces for each type. the dumbbells were cut by press machine with standardized dimensions (figure 2) from 2 mm thick plates which were made from a compound of natural rubber (c5h8, nr) and styrene-butadiene rubber (c12h14, sbr) with a carbon black content of 74 phr (parts of weight per hundred parts of rubber). the cylinders were supplied externally and were extruded from a compound with specified nr with 25 phr of carbon black and the dimensions are shown in figure 2. the test specimens were coated with a speckled pattern using a white spray, and the measurement area was also defined. this preparation of the surface of specimens is important for further software setup of the dic. the ideal speckle pattern contains a consistent size and shape of speckles and is high contrast. the test specimens were clamped in the pneumatic jaws of the universal testing machine, and the crosshead speed was selected: for dumbbells following the standard iso 37200 200 mm min−1 and figure 2. dimensions of test specimens a) dumbbell, b) cylinder for cylinders 50 mm min−1. the line probes in the transverse and longitudinal direction were set for measuring the elongation and transversal contraction, and the quantities in the equations and diagrams were assigned to these probes for the following computational analysis. the measured deformations were between 0 and 100 % for the dumbbells and 0 and 40 % for the cylinders. the range of measured deformations was planned to be the same for both types of test specimens; however, the cylinders were slipping from the pneumatic jaws of the universal testing machine during the measurements, and therefore the deformations were only 40 %. to calculate the poisson’s ratio, which is defined as transverse strain to longitudinal strain in the direction of the applied force, the following equation was used, where ν [-] is the poisson’s ratio, transverse strain εt [-] and longitudinal strain εl [-]. ν = −εt εl (1) the test specimen examined under volume changes exhibits these changes in all directions and, in the case of uniaxial loading, deforms in one longitudinal and two transverse directions, and the relative volume change θ [-] can be defined by the following equation. θ = (1 + εt)2 · (1 + εl) (2) the bulk modulus k [mpa] describing compressibility is defined as the ratio of the stress σ [mpa] and the relative volume change θ [-]. k = σ θ (3) 3. results the bulk modulus and poisson’s ratio values were determined according to equations (1),(2), and (3). the curves of the dependence of the bulk modulus on 47 b. kotlánová, s. rusnáková, r. o. ogunleye, j. javořík acta polytechnica ctu proceedings figure 3. the measuring process divided into 4 steps a) preparation of test specimens, b) test setup, c) measurement, d) data processing and results evaluation strain were obtained from these data, and all measured points were fitted with a cubic polynomial, complexly describing the prediction using the method of least squares. the range of scatter in the measured data for individual test specimens is typical for this type of material. the mechanical properties of elastomeric test specimens from the same batch are particularly affected by the quality of the homogenization of the rubber compound, the degree of vulcanization, and the quality of the technological process. measured data in the low-strain region are partially impacted by the resolution of the measurement method and therefore appear scattered in this region. 3.1. dumbbells the dumbbell-shaped test specimens were measured to 100 % strain with simultaneously recorded stresses using stereo dic device. the resulting dependence of the bulk modulus as a function of strain is shown figure 4. the bulk modulus as a function of strain for dumbbells figure 5. the average values of poisson’s ratio as a function of strain for dumbbells in figure 4. the fitting curve corresponds to the theoretical s-shape characteristic for rubber like materials stress-strains. the beginning of the dependence displays a rise followed by a plateau and the classic s-shape is completed with another rise. the data were evaluated at 25 %, 50 %, 75 %, and 100 % strain values. the average values of poisson’s ratio and the bulk modulus were determined and are presented in table 1. ε [%] ν [-] k [mpa] 25 0.476 ± 0.007 100.0 ± 21.5 50 0.468 ± 0.006 87.3 ± 28.6 75 0.459 ± 0.009 94.8 ± 38.2 100 0.461 ± 0.010 147.4 ± 28.0 table 1. poisson’s ratio and the bulk modulus figure 5 shows the dependence of poisson’s ratio on strain with consideration of average values for defined strain values. the values of bulk modulus versus strain are presented similarly in figure 6. 48 vol. 42/2023 volume changes measurement of elastometrs using 3d dic figure 6. the average values of the bulk modulus as a function of strain for dumbbells figure 7. the bulk modulus as a function of strain for cylinders 3.2. cylinders the data measured during cylinder testing were evaluated to strain values slightly above 40 %. at higher strains, the cylinders were slipping out of the jaws of the universal testing machine; therefore, a higher clamping force had to be applied, and the test specimens were pre-deformed at the edges. despite this, the measured data interleaved with the fitting curve shows the characteristic s-shape of the dependence of the bulk modulus on the strain for elastomeric materials. the apparently wider scatter of the cylinder data is caused by the differently sized scale of x (strain) axis in figures 4 and 7. the data were evaluated in 10 %, 20 %, 30 %, and 40 % strain values. the average values of poisson’s ratio and the bulk modulus were determined for these strain values and are given in table 2 and graphically represented in figures 8 and 9. 4. conclusions the presented paper examines the use of the dic measuring device in the stereo configuration to record volume changes in elastomers. the test specimens of figure 8. the average values of poisson’s ratio as a function of strain for cylinders figure 9. the average values of the bulk modulus as a function of strain for cylinders ε [%] ν [-] k [mpa] 10 0.483 ± 0.003 182.2 ± 35.5 20 0.486 ± 0.003 174.5 ± 32.4 30 0.487 ± 0.003 171.8 ± 34.0 40 0.488 ± 0.003 172.7 ± 35.8 table 2. poisson’s ratio and the bulk modulus two different shapes (dumbbells, cylinders) subjected to uniaxial tension were used. the dic device, which allows the measurement of strain and displacement in multiple directions, was used to record transverse and longitudinal strains for determining poisson’s ratio and the bulk modulus. the measured and calculated data evaluated in the dependences of the bulk modulus and poisson’s ratio on the strain fit the predicted curves. the results show that the observed parameters are non-constant as a function of the strain. the obtained values of the bulk modulus and poisson’s ratio are necessary for commonly used hyperelastic computational models. the measurement method used is suitable for 49 b. kotlánová, s. rusnáková, r. o. ogunleye, j. javořík acta polytechnica ctu proceedings applied mechanical testing. the measurements of the dumbbells were proceeded standardly. however, using a more extended length for measuring the cylinders for proper clamping in the pneumatic jaws would be preferable. another option is using test specimens of different shapes or selecting a different clamping system. measurements with the dic device are generally beneficial due to the post-processing and overall universality. the resolution of the method is dependent on several factors including camera specifications, image quality and interpolation, correlation algorithm, size of correlation window, noise, or speckle pattern. measurements of a similar set of test specimens were conducted in a previous study using the 2d dic. the setup of the 2d dic system is more straightforward in many aspects, as the position of the cameras relative to each other and the measured object is essential in the 3d configuration. the used mercury rt software includes a neck gauge tool for 2d measurements, simplifying the setup of the input measurement quantities. the 3d dic method is characterized by less scattered data in the low-strain region, and the influence of deformations in non-measured planes is reduced due to spatial recording. acknowledgements the research was realized with the support of the project iga/ft/2023/004 tbu in zlín. references [1] j.-b. le cam. a review of volume changes in rubbers: the effect of stretching. rubber chemistry and technology 83(3):247–269, 2010. https://doi.org/10.5254/1.3525684 [2] j. a. s. del moral, j. b. pascual-francisco, o. susarrey-huerta, et al. characterization of viscoelastic poisson’s ratio of engineering elastomers via dic-based creep testing. polymers 14(9):1837, 2022. https://doi.org/10.3390/polym14091837 [3] l. i. farfán-cabrera, j. b. pascual-francisco, o. barragán-pérez, et al. determination of creep compliance, recovery and poisson’s ratio of elastomers by means of digital image correlation (dic). polymer testing 59:245–252, 2017. https: //doi.org/10.1016/j.polymertesting.2017.02.010 [4] l. i. farfan-cabrera, j. b. pascual-francisco. an experimental methodological approach for obtaining viscoelastic poisson’s ratio of elastomers from creep strain dic-based measurements. experimental mechanics 62(2):287–297, 2021. https://doi.org/10.1007/s11340-021-00792-9 [5] j.-b. le cam, e. toussaint. cyclic volume changes in rubber. mechanics of materials 41(7):898–901, 2009. https://doi.org/10.1016/j.mechmat.2009.02.004 [6] j. de crevoisier, g. besnard, y. merckel, et al. volume changes in a filled elastomer studied via digital image correlation. polymer testing 31(5):663–670, 2012. https: //doi.org/10.1016/j.polymertesting.2012.04.003 [7] h. schreier, j.-j. orteu, m. a. sutton. image correlation for shape, motion and deformation measurements. springer us, 2009. https://doi.org/10.1007/978-0-387-78747-3 [8] m. sutton, p. l. reu (eds.). international digital imaging correlation society: proceedings of the first annual conference. springer international publishing, 2017. https://doi.org/10.1007/978-3-319-51439-0 [9] n. candau, c. pradille, j.-l. bouvard, n. billon. on the use of a four-cameras stereovision system to characterize large 3d deformation in elastomers. polymer testing 56:314–320, 2016. https: //doi.org/10.1016/j.polymertesting.2016.10.017 [10] n. novak, d. kytyr, v. rada, et al. compression behaviour of tpms-filled stainless steel tubes. materials science and engineering: a 852:143680, 2022. https://doi.org/10.1016/j.msea.2022.143680 [11] b. kotlánová, j. javořík, m. žaludek, r. o. ogunleye. volume change measurements using 2d dic system. manufacturing technology 23(3):307–312, 2023. https://doi.org/10.21062/mft.2023.044 [12] m. sutton, j. yan, v. tiwari, et al. the effect of out-of-plane motion on 2d and 3d digital image correlation measurements. optics and lasers in engineering 46(10):746–757, 2008. https://doi.org/10.1016/j.optlaseng.2008.05.005 50 https://doi.org/10.5254/1.3525684 https://doi.org/10.3390/polym14091837 https://doi.org/10.1016/j.polymertesting.2017.02.010 https://doi.org/10.1016/j.polymertesting.2017.02.010 https://doi.org/10.1007/s11340-021-00792-9 https://doi.org/10.1016/j.mechmat.2009.02.004 https://doi.org/10.1016/j.polymertesting.2012.04.003 https://doi.org/10.1016/j.polymertesting.2012.04.003 https://doi.org/10.1007/978-0-387-78747-3 https://doi.org/10.1007/978-3-319-51439-0 https://doi.org/10.1016/j.polymertesting.2016.10.017 https://doi.org/10.1016/j.polymertesting.2016.10.017 https://doi.org/10.1016/j.msea.2022.143680 https://doi.org/10.21062/mft.2023.044 https://doi.org/10.1016/j.optlaseng.2008.05.005 acta polytechnica ctu proceedings 42:46–50, 2023 1 introduction 2 materials and methods 3 results 3.1 dumbbells 3.2 cylinders 4 conclusions acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2024.47.0128 acta polytechnica ctu proceedings 47:128–132, 2024 © 2024 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague measurement of asphalt concrete base thickness using ultrasonic pulse echo jaroslav šnédar∗, vojtěch bartoň, petr cikrle brno university of technology, faculty of civil engineering, institute of building testing, veveří 331/95, 602 00 brno, czech republic ∗ corresponding author: 176538@vutbr.cz abstract. this paper deals with the description and use of one of the up-to-date ndt method, the ultrasonic pulse echo, to determine the thickness of an asphalt concrete model. in the article, the authors explain the principle of the method and the limits of its use. furthermore, attention is also paid to the influence of the shape and design of the back wall of the tested elements and the speed of propagation of ultrasonic waves. the research is part of a larger work, defect analysis and implementation of whitetopping technology in the czech republic. keywords: asphalt, thickness, whitetopping, non-destructive testing (ndt), ultrasonic pulse echo. 1. introduction asphalt concrete plays a very important role in the construction industry today. with the development of road transport of goods, services, etc. there was also a need to improve the design and implementation of roadways and parking areas. it is for these reasons that asphalt concrete, i.e. a mixture of aggregate and asphalt binder, began to be used in the past. however, with the increasing intensity of traffic, it is necessary to further adjust and improve these mixtures, especially with regard to their durability and economy. the topic that the authors deal with in this paper is part of a larger research. this research deals with sustainability and the correct implementation of so-called whitetopping. even though the whitetopping technology is relatively old and in some countries such as the usa, sweden, etc. commonly used, its development in the czech republic has only now come to fruition. whitetopping is a method of rehabilitation of roadways, roads, parking areas, etc. the implementation consists of applying a thin layer of high-strength concrete to the existing acp layer, which creates a composite material with new properties. the thickness of the whitetopping layer is from 100 to 200 mm. it is the connection of these two layers and their interaction that is essential for the proper functionality of the new composite. the use of this method is especially suitable in places with a high intensity of heavy freight traffic, such as highways, parking lots next to highways, etc. [1, 2] with the development of whitetopping implementations in the czech republic, there are also requirements for verification of the quality of execution and condition. as already mentioned, for the correct implementation of this remediation method, the connection of the whitetopping layer with the underlying asphalt concrete is essential. in order to be able to assess this, it is first necessary to have an overview of the underlying layer, its thickness, whether the devices can detect and measure it. based on the previous experience of the authors, the ultrasonic pulse echo method was chosen for these measurements, which is very suitable for measuring the thickness of the structure, possibly delaminations and cracks. [3, 4] 2. methodology and principle of upe the pulse echo pundit 250 array (nowadays called pundit pd8050) was evaluated as the most suitable device for these measurements, as it is used in construction practice for a similar issue (measurement of the thickness and properties of cement concrete). the principle of upe consists in sending short beams of damped mechanical oscillation into the material. this oscillation is created in the exciter, which regularly and repeatedly sends electrical impulses, and the sensor then detects them. in the case of upe, only one probe is used, which fulfills both functions and is therefore an exciter and a sensor at the same time. when the probe (piezoelectric) is applied to the surface of the structure, it first functions as an exciter sending a mechanical signal to the structure, then it switches to the sensing mode, when it receives the signal reflected from the opposite surface of the structure or an obstacle and changes it into an electrical image. the image transformed in this way is displayed on the device screen as a so-called “echo”. ultrasonic waves propagate with certain materials and are reflected at the interface of the environment, such as delamination, cracks, plastic or metal objects, etc. air and plastic objects are excellent insulators for ultrasound. when using upe, the signal transit time through the examined element is determined as the time that elapses between the sending of an ultrasonic wave into the environment, the subsequent reflection of the wave from a distant surface (or defects) and the recording of the reflected echo at the point of sending 128 https://doi.org/10.14311/app.2024.47.0128 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 47/2024 measurement of asphalt concrete base thickness (a). b-scan (b). a-scan figure 1. b-scan and a-scan measurements of model thickness. device length [mm] width [mm] height [mm] back of model model no. 1 505 305 102 straight, smooth model no. 2 505 305 110 with compacted macadam table 1. description of asphalt concrete models. the signal. modern devices can display the observed time on their screens as an a-scan graph, which captures the deviations of the impulse as a function of time (figure 1). first, the initial impulse is recorded on the screen, then the impulse of the defect inside the environment (echo of the defect) or the echo of the back wall. [4, 5] the thickness of the structure, or the depth of the detected defect, can be determined from the exact time the ultrasound passes through the examined environment. given the known speed of propagation of ultrasonic waves through the environment under investigation and the time t, which indicates the time between sending and recording the reflected signal from the back wall, the depth of the defect can be determined as d: d = v t 2 . (1) similarly, the thickness of any defects or defects inside the examined structure can be determined [6]. measurements on the models took place in the premises of but fast. an ultrasonic pulsed echo pundit 250 array was used. for the purpose of measurement, 2 asphalt concrete models were created simulating common asphalt concrete base layers. model no. 1 (see figure 2) is made of acp asphalt concrete with an aggregate grain of up to 16 mm. the model is flat on both the top and bottom surfaces, well compacted. the model simulates the underlying layers, on which remediation in the form of whitetopping is normally applied. for description of dimensions see table 1. model no. 2 (see figure 3) is also made of acp asphalt concrete with an aggregate grain of up to 16 mm. in the second model, macadam fraction 63 mm was vibrated on the bottom side, so the surface of the bottom side is not flat. this model is intended to simulate a base when, during the realization of the base layer, asphalt concrete was thickened into a macadam gravel bed. for description of dimensions see table 1. for the sake of systematicity and clarity of measurement, the same measurement grids were drawn on both models. lines in the x-axis direction were marked with numbers a-i, lines marked with roman numerals i-v run in the y-axis direction. the lines are marked with a grid of 50×50 mm. a grid of 50 mm was chosen with regard to the required measurement detail. due to the size of the upe matrix probe (210 mm), the overlap of the individual b-scans was chosen to be 129 j. šnédar, v. bartoň, p. cikrle acta polytechnica ctu proceedings (a). upper surface (b). lower surface (c). side view figure 2. photo of model no. 1. (a). upper surface (b). lower surface (c). side view figure 3. photo of model no. 2. device pulse velocity pulse voltage receiver gain bandwidth [m/s] [v] [db] [khz] model no. 1 2 002 150 50 40 model no. 2 2 001 150 50 40 table 2. default settings of pundit 250 array [5]. 160 mm. measurements were always made as close as 50 mm from the edge of the model to avoid distortion by edge effects. before the actual measurement, the surface speed of the shear wave was determined by measuring at 8 places evenly over the entire area of model no. 1 and no. 2. as already explained, the determination of this speed is very important for the correct calculation of the thickness of the examined element. device settings before measurement on individual models, see table 2. 3. results and comparison of measurements on models with the device’s default settings according to table 2, measurements were made on both models. the models were measured parallel to the x and y axes, i.e. in lines i-iv and a-i. such measurement of the model with a step of 50 mm proved to be sufficiently detailed for the purpose of measuring the thickness of the element. scans of individual lines ii-iv are shown in figure 4, where part (a) is a measurement in line 130 vol. 47/2024 measurement of asphalt concrete base thickness (a). line ii (b). line iii (c). line iv figure 4. scans taken in lines ii-iv on model no. 1. line b c d e f g average deviation deviation [mm] [mm] [mm] [mm] [mm] [mm] [mm] [mm] [%] ii 103 103 104 104 105 102 103.5 1.5 1.4 iii 105 105 105 103 105 104 104.5 2.5 2.4 iv 105 106 105 107 104 105 105.3 3.3 3.2 table 3. results of thickness measurement in selected lines ii-iv and points b – g on model no. 1. (a). line ii (b). line iii (c). line iv figure 5. scans taken in lines ii-iv on model no. 2. ii, part (b) is a measurement in line iii, and part (c) is a measurement in line iv. lines i and v were not measured due to possible edge effects that could distort the measurements. at each point b–g of the given line, the measured thickness of the element was subtracted in the evaluation program. a total of 18 values were therefore measured on model no. 1. subsequently, the average thickness of the measured line was determined and the deviation of the measured and real thickness of the model in the given line was calculated. the measurement results are shown in table 3. measurements were made in the same way on model no. 2. figure 5 again shows scans of lines ii-iv, where part (a) is a measurement in line ii, part (b) is a measurement in line iii and part (c) is a measurement in line iv. the evaluation of the thickness measurement was carried out in the same way as on model no. 1. on model no. 2, however, it was possible to determine only 9 element thickness values, as the backwall echo was not well or completely visible in the rest of the measurement, so it was not possible to determine the thickness. the authors deal with the justification of this phenomenon in the following chapter. the measurement results are shown in table 4. the average asphalt concrete thickness of model no. 1 was determined to be 104.4 mm using the pundit 250 array probe, which is approximately 2.4 mm (+2.34 % deviation) more than the actual thickness of 102.0 mm. the largest difference between the measured and actual thickness was measured in line iv, namely 3.3 mm (deviation +3.2 %). for model no. 2, the average thickness was determined to be 111.2 mm, 131 j. šnédar, v. bartoň, p. cikrle acta polytechnica ctu proceedings line b c d e f g average deviation deviation [mm] [mm] [mm] [mm] [mm] [mm] [mm] [mm] [%] ii 113 113.0 3.0 2.7 iii 109 112 113 109 110.8 0.8 0.7 iv 111 114 111 109 111.3 1.3 1.1 table 4. results of thickness measurement in selected lines ii-iv and points b – g on model no. 2. which is approximately 1.2 mm (+1.09 % deviation) more than the actual thickness of 110.0 mm. the largest difference between the measured and actual thickness was measured in line ii, namely 3.0 mm (deviation +2.7 %). however, this measurement must be taken with a grain of salt, since, as already mentioned, it was not possible to determine the thickness in certain places. 4. conclusion measuring the thickness of the investigated element using an ultrasonic pulse echo is dependent on many factors. in this work, the authors were mainly concerned with the influence of the shape of the back wall. from the previous works of the authors and other researchers dealing with this issue, it was clear that the shape of the back wall will certainly have its importance. to verify this thesis, the authors created 2 models simulating the real performance of the asphalt base layer in situ. when measuring on these models, it was confirmed that the detection of the back wall of the element depends on its shape. if the back wall is straight (model no. 1), the ultrasonic waves are reflected from it, pass through the element back at such an angle that the probe is able to detect them. in the case of uneven, vibrating into the underlying macadam, or an inclined back wall, the slope of which is greater than 40 degrees (inclusive), the ultrasonic waves are either scattered or reflected beyond the reach of the probe. this also corresponds with research conducted by other authors, e.g. the company proceq sa [7]. this is especially important for understanding the principle and limits of ultrasound pulse echo measurements. the aforementioned is also clearly visible from the comparison of measurement results on both models figure 4 and figure 5. in figure 4 are beautifully visible strong echoes of the back wall of model no. 1 at a depth of 102 to 107 mm. while in figure 5 showing the measurement on model no. 2, the echoes of the back wall are clearly visible only on a few scans (places where the underlying macadam did not vibrate). in the rest of the measurements, they are either very weak or completely invisible. table 3 and table 4 also correspond well with these results, in which the thicknesses of the models at individual measurement points are subtracted and the deviations from the reference thicknesses of the models are subsequently determined from them. according to the authors, of these measurements, only the measurements on model no. 1 can be considered completely conclusive. the average deviations found when measuring the thicknesses of the models were set at +2.34 % for model no. 1 and +1.09 % for model no. 2, which are excellent values. it is clear that the ultrasonic pulse echo pundit 250 array is a relatively accurate and suitable device for measuring the thickness of the structure and possible delaminations, in the case of correct device settings and knowledge of the factors influencing the measurement, which was the reason for this research. the authors follow up on these findings in further research into the control and implementation of whitetopping technology. acknowledgements this contribution was created with the support of the junior specific university research project at but no. fast-j-22-8074. references [1] a. mateos, m. millan, j. harvey, et al. mechanisms of asphalt cracking and concrete-asphalt debonding in concrete overlay on asphalt pavements. construction and building materials 301:124086, 2021. https: //doi.org/10.1016/j.conbuildmat.2021.124086 [2] transportation research board and national academies of sciences, engineering, and medicine. thin and ultra-thin whitetopping. the national academies press, washington, dc, 2004. https://doi.org/10.17226/23333 [3] j. šnédar, v. bartoň, p. cikrle. detection of internal defects and failures of concrete structures using non-destructive methods. in rehabilitation and reconstruction of buildings, 23rd, vol. 3 of construction technologies and architecture, pp. 39–44. trans tech publications ltd, 2022. https://doi.org/10.4028/p-4096k1 [4] j. šnédar, v. bartoň, p. cikrle. current possibilities of verifying the filling of cable ducts by using modern devices. in 23rd international conference of doctoral studies, pp. 333–338. juniorstav 2021, 2021. isbn 978-80-86433-75-2. [5] pcte. pundit 250 array – ultrasonic imaging scanner. construction testing equipment, 2019. [2022-11-20]. https://www.pcte.com.au/ pundit-250-array-ultrasonic-imaging-scanner [6] sciencedirect topics. ultrasonic wave – an overview, 2020. [2022-11-20]. https://www.sciencedirect.com/ topics/computer-science/ultrasonic-wave [7] proceq sa. pundit 250 array, 2017. [2022-11-20]. https: //media.screeningeagle.com/asset/downloads/ pundit%20250%20array_application%20booklet.pdf 132 https://doi.org/10.1016/j.conbuildmat.2021.124086 https://doi.org/10.1016/j.conbuildmat.2021.124086 https://doi.org/10.17226/23333 https://doi.org/10.4028/p-4096k1 https://www.pcte.com.au/pundit-250-array-ultrasonic-imaging-scanner https://www.pcte.com.au/pundit-250-array-ultrasonic-imaging-scanner https://www.sciencedirect.com/topics/computer-science/ultrasonic-wave https://www.sciencedirect.com/topics/computer-science/ultrasonic-wave https://media.screeningeagle.com/asset/downloads/pundit%20250%20array_application%20booklet.pdf https://media.screeningeagle.com/asset/downloads/pundit%20250%20array_application%20booklet.pdf https://media.screeningeagle.com/asset/downloads/pundit%20250%20array_application%20booklet.pdf acta polytechnica ctu proceedings 47:128–132, 2024 1 introduction 2 methodology and principle of upe 3 results and comparison of measurements on models 4 conclusion acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2024.47.0015 acta polytechnica ctu proceedings 47:15–18, 2024 © 2024 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague utilization of high fly ash dosage to reduce cement consumption in concrete formulations vendula davidová∗, tomáš david, pavel reiterman czech technical university in prague, faculty of civil engineering, thákurova 7, 166 29 prague, czech republic ∗ corresponding author: vendula.davidova@fsv.cvut.cz abstract. the concrete production is responsible for high proportion of total carbon dioxide emission due to enormous consumption of portland cement. this traditional binder system is due to high autogenous emission during calcination the main carrier of such negative environmental impact. the effective way to reduce the use of portland cement is to replace that this binding material exhibiting lower energy demandingness. this paper deals with the utilization of fly ash as portland cement replacement, which is frequently applied in concrete industry. the paper deals the highest replacement level in accordance with valid regulations. the experimental program was focused on the properties of fresh concrete mixture and basic physical and mechanical properties of hardened concrete. the attained results confirmed good potential of this approach, however the increase of plasticizer dosage was necessary to maintain a similar consistency of fresh mixture. that is why the fresh mixture exhibited lower level of air content. keywords: high dose of fly ash, compressive strength, consistency assurance, cement reduction. 1. introduction currently, the industry research is mainly focused on cement production, as cement production is associated with a number of environmental impacts. these include the consumption of natural resources, environmental pollution, and especially co2 production, because cement production making up 4–8 % of total produced co2 emissions [1]. our aim is therefore to develop sustainable concrete that has a lower carbon footprint and, in addition, uses secondary material from another industry that has no further use. one of the most common used alternative materials for these purposes is fly ash. this material is a by-product from burning pulverized coal in thermal power generating plants [2]. in recent years, fly ash production has been increasing steadily because a large number of countries depend on coal combustion. china is the largest consumer of coal and therefore the largest producer of fly ash, accounting for about half of the world’s total production. annual fly ash production reached 580 million tones and continued to grow [3]. fly ash is a waste for thermal power plants that is further disposed of and can present a threat to the environment. therefore, its further use to produce new materials is a necessary step forward. fly ash is a fine material of spherical particles with main chemical composition of sio2, al2o3, fe2o3 and cao that participate in the hydration process by pozzolanic reaction with calcium hydroxide to form c-s-h gels [4]. these properties have attracted the focus of many research studies, which have focused on a variety of applications. nassar et al. [5] reports in his work that 20 wt % replacement of cement with fly ash significantly improved long-term compressive strength, and in addition fly ash based concrete improved durability and abrasion resistance. on the other hand, rashad et al. [6] focused on the effect of a high dose of fly ash (over 45 %) and concluded that such a high dose reduces heat production due to the hydration process, the high dose significantly reduces mechanical properties in a short age, the differences between normal concrete but decreases after a longer time, etc. hemalatha et al. [7] add that most engineering applications in practice are around 30 mpa, and these strengths can be achieved even with a high proportion of fly ash. this material has great potential for use in the production of ready-mixed concrete and therefore it was selected for the following experiment. 2. experimental program this experimental program was focused on the use of fly ash in concrete production to the maximum possible content. according to the national standard en 206+a2 [8] the maximum amount of fly ash of 33 % of the cement content can be counted for the calculation of the water/cement ratio for the selected cement class cem i. this maximum quantity was therefore used in the production of the test mixture. a reference mixture without fly ash was also monitored to evaluate the effect of fly ash dosage. considering the experience gained from previous research [9], it was expected that the rheological properties of the fresh mixture would decrease, therefore plasticizing additives were used in the formulation design. in consideration of the high fly ash dosage, the additive dosage was increased. to demonstrate the effect of fly 15 https://doi.org/10.14311/app.2024.47.0015 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en v. davidová, t. david, p. reiterman acta polytechnica ctu proceedings figure 1. grain size cure. ash dosage, the properties in the fresh state and then in hardened state were monitored. 3. materials cement class cem i 42.5 r was used as a reference binder. this binder was subsequently replaced by fly ash in the ratio fly ash/cement = 0.33. this fly ash comes from the power plant tušimice cz. this fly ash was prepared by mixing different production fractions to meet the requirements of čsn en 450-1 [10] for fly ash category n. the chemical compositions of selected fly ash and cement is given in table 1. al2o3 sio2 k2o cao tio2 fe2o3 mgo fly ash 33.9 49.8 1.7 1.8 1.2 8.9 cem i 42.5 5.3 18.7 0.8 63.0 3.1 2.6 table 1. chemical compositions of fly ash from power plant tušimice cz and cement cem i 42.5 r. the excavated aggregates were used of grain size 0/4 from the dobříň locality, 4/8 and 8/16 from the zbraslav locality. the specific dosage was determined with respect to the appropriate grain size curve of the mix. the resulting mixture grain size curve is given in figure 1. plasticizing additive ost 1149 from stachema cz based on polycarboxylates was used to ensure the required consistency of fresh concrete and microporan air-training admixture from the same manufacturer was used to achieve the required air content. the final composition of both monitored mixtures is given in table 2. ref ex1 [kg m−3] [kg m−3] cem i 42.5 404 304 fly ash 0 100 0/4 962 962 4/8 268 268 8/16 600 600 microporan 0.55 0.55 ost 1149 2.43 7.87 water 163 155 table 2. composition of monitored mixtures. 4. methods the above quantity was converted to production volume and added to the laboratory mixer where the basic components were mixed. next, the dosage of water and plasticizer was adjusted according to the desired consistency. after mixing, the fresh concrete was tested, and samples were made from the remaining concrete to determine the compressive strength. the consistency was determined according to čsn en 12350-2 testing fresh concrete: slump-test [11]. in this test, a 300 mm high cone is gradually filled with concrete and compacted. after that, the cone is removed, and the settlement height of the fresh concrete is measured in relation to the original height. the result is the change in height in millimeters. the air content of the fresh concrete was determined according to čsn en 12350-7 testing fresh concrete: air content [12]. in this method, the test container is filled with fresh concrete which is compacted. the container is closed and completely filled with water. a specific volume of air is then pressurized into the chamber, the pressurized air is then released into the fresh concrete, causing a decrease of pressure in the chamber and the resulting air content of the concrete is determined. the samples 150 × 150 × 150 mm for compressive strength determination were stored in water after hardening until the time of testing. specifically, the samples were tested at ages 2 and 28 days. this test was done according to čsn en 12390-3 testing hardened concrete: compressive strength of test specimens [13]. in this test, the specimens are loaded with pressure until they break. the resulting compressive strength is determined from the maximum measured load. 5. result and discussions although efforts were made to dose the water and plasticizer to achieve the same consistency, the resulting consistency was not exactly the same. it is very difficult to set the same consistency in practice therefore, a slump classification according to en 206+a2 [8] which uses classes s1-s5 (s5 shows the highest slump) was used. according to this classification, both mixtures reached the same class s4, which is in the range of 160 to 210 mm. the achieved consistency of the fresh concrete is shown in figure 2. 16 vol. 47/2024 utilization of high fly ash dosage to reduce cement consumption figure 2. the consistency of the fresh concrete. to achieve aeration of the concrete, aeration admixture was applied at a constant rate of 0.55 kg m−3. this admixture apparently works effectively only in a mixture without fly ash, where an air content of 4.4 % was measured. the mixture with high fly ash dosage was not aerated using this admixture, as the resulting air content of the concrete was 2.6 %. to achieve a higher air content, it would be necessary to either increase the dosage of this admixture or select an admixture designed specifically for this application. the resulting measured values are shown in figure 3. figure 3. the air content of the fresh concrete. as can be seen from the results, the reference mixture showed a higher increase in strength in the short term. at the age of 2 days, it showed a compressive strength of 34.7 mpa. this phenomenon is mainly due to the higher cement dosage in ref mixture and is consistent with the results of another research. golewski [14] in his work states that the addition of fly ash at a rate of up to 30 % drastically reduces the compressive strength at early ages. the mixture ex1 with high fly ash dosage already achieved comparable properties to the reference mixture in the long term (28 days). the fly ash contributes to the strength development by its pozzolanic reaction, which is generally showed at a higher age. at a higher age, the ex1 mixture would probably exceed the strength of the reference mixture. in general, it can be concluded that the concrete with a high fly ash content is comparable to the reference mix without fly ash at 28 days of age. the evaluation of concrete at 28 days is the most common in practice, and there are even concretes that are evaluated at 90 days; therefore, the lower strengths of concrete with fly ash at lower ages are not rated as unsatisfactory. the resulting strength was also influenced by the air content of the concrete. due to the lower air content of the concrete with high fly ash content, very good strengths were achieved. with an identical air content of 4.4 %, the strength of ex1 concrete would probably have been lower. an overview of the achieved strengths is given in figure 4. figure 4. compressive strengths of texted mixtures. 6. conclusion the purpose of this experiment was to verify the applicability of fly ash in the concrete recipes to the maximum possible extent allowed by the regulations at the place of use. despite the high dose of fly ash, a very good consistency was achieved while maintaining a similar dose of water. this result was achieved by increasing the dose of plasticizing additive. however, an appropriate dose is essential. rafat siddique [15] increased the dosage of plasticizing admixture in the concrete composition, even though the same consistency was not achieved. the increase in strength was slow in the first few days but the resulting compressive strength after 28 days was the same as the mixture without fly ash. in addition, a mix with a high proportion of fly ash is expected to achieve higher strengths at a later stage of age than a fly ash free mixture. this assumption is confirmed, for example, by the work of charith herath et al. [16]. they investigated, among other parameters, the strength development of concrete with a high proportion of fly ash at higher ages. their research shows increase in compressive strength from 32.7 to 73.0 mpa over the period from 7 days to 450 days, respectively from 22.4 to 71.3 mpa based on the amount of used fly ash. the air content was lower in the mixture with fly ash, which could be corrected by a higher dose of airentrained admixture. tarun r. naik et al. [17] had 17 v. davidová, t. david, p. reiterman acta polytechnica ctu proceedings to increase the dose of air-entrained admixture in his work to achieve comparable air content in mixtures with a high dose of fly ash. k. h. pedersen et al. [18] noted that the combustion of other alternative fuels may also affect the fly ash quality, e.g. fly ash from coal/petroleum coke co-firing has shown a low airentrained admixture adsorption capacity. this finding may ultimately lead to decision that some fly ash will be not able to be used for air-entrained concrete in practice. from the initial tests achieved, fly ash was shown to be usable even at higher rates. this approach can reduce the amount of cement in the concrete while maintaining the basic properties. verification of other properties such as shrinkage, creep, carbonation, etc. will be the subject of further research. acknowledgements this work was supported by the grant of the ctu student grant competition no. sgs24/051/ohk1/1t/11 this work was supported by the project of czech science foundation tj04000183 development of advanced chemical admixtures for concretes containing energy by-products. references [1] c. meyer. the greening of the concrete industry. cement and concrete composites 31(8):601–605, 2009. sustainability of civil engineering structures – durability of concrete. https: //doi.org/10.1016/j.cemconcomp.2008.12.010 [2] j. yang, y. su, x. he, et al. pore structure evaluation of cementing composites blended with coal by-products: calcined coal gangue and coal fly ash. fuel processing technology 181:75–90, 2018. https://doi.org/10.1016/j.fuproc.2018.09.013 [3] j. yang, j. huang, y. su, et al. eco-friendly treatment of low-calcium coal fly ash for high pozzolanic reactivity: a step towards waste utilization in sustainable building material. journal of cleaner production 238:117962, 2019. https://doi.org/10.1016/j.jclepro.2019.117962 [4] z. fang, y. luo, h. chen, et al. research on mechanical properties and hydration characteristics of ultra-high performance concrete with high-volume fly ash microsphere. journal of building engineering 78:107738, 2023. https://doi.org/10.1016/j.jobe.2023.107738 [5] r.-u.-d. nassar, s. room. comparison of the performance of class-c and class-f fly ash concrete mixtures produced with crushed stone sand. materials today: proceedings 2023. [in press]. https://doi.org/10.1016/j.matpr.2023.03.510 [6] a. m. rashad. a brief on high-volume class f fly ash as cement replacement – a guide for civil engineer. international journal of sustainable built environment 4(2):278–306, 2015. https://doi.org/10.1016/j.ijsbe.2015.10.002 [7] t. hemalatha, a. ramaswamy. a review on fly ash characteristics – towards promoting high volume utilization in developing sustainable concrete. journal of cleaner production 147:546–559, 2017. https://doi.org/10.1016/j.jclepro.2017.01.114 [8] beton – specifikace, vlastnosti, výroba a shoda [concrete – specification, performance, production and conformity]. standard, czech standard institute, prague, 2021. [9] v. davidová, t. david. the modification of basic properties of cement mixtures with fluidized bed combustion fly ash. aip conference proceedings 2780(1):020030, 2023. https://doi.org/10.1063/5.0137816 [10] popílek do betonu – část 1: definice, specifikace a kritéria shody [fly ash for concrete – part 1: definition, specifications and conformity criteria]. standard, czech standard institute, prague, 2013. [11] zkoušení čerstvého betonu – část 2: zkouška sednutím [testing fresh concrete – part 2: slump test]. standard, czech standard institute, prague, 2020. [12] zkoušení čerstvého betonu – část 7: obsah vzduchu – tlakové metody [testing fresh concrete – part 7: air content – pressure methods]. standard, czech standard institute, prague, 2020. [13] zkoušení ztvrdlého betonu – část 3: pevnost v tlaku zkušebních těles [testing hardened concrete – part 3: compressive strength of test specimens]. standard, czech standard institute, prague, 2020. [14] g. l. golewski. green concrete composite incorporating fly ash with high strength and fracture toughness. journal of cleaner production 172:218–226, 2018. https://doi.org/10.1016/j.jclepro.2017.10.065 [15] r. siddique. performance characteristics of high-volume class f fly ash concrete. cement and concrete research 34(3):487–493, 2004. https://doi.org/10.1016/j.cemconres.2003.09.002 [16] c. herath, c. gunasekara, d. w. law, s. setunge. long term mechanical performance of nano-engineered high volume fly ash concrete. journal of building engineering 43:103168, 2021. https://doi.org/10.1016/j.jobe.2021.103168 [17] t. r. naik, s. s. singh, m. m. hossain. permeability of concrete containing large amounts of fly ash. cement and concrete research 24(5):913–922, 1994. https://doi.org/10.1016/0008-8846(94)90011-6 [18] k. pedersen, a. jensen, m. skjøth-rasmussen, k. dam-johansen. a review of the interference of carbon containing fly ash with air entrainment in concrete. progress in energy and combustion science 34(2):135–154, 2008. https://doi.org/10.1016/j.pecs.2007.03.002 18 https://doi.org/10.1016/j.cemconcomp.2008.12.010 https://doi.org/10.1016/j.cemconcomp.2008.12.010 https://doi.org/10.1016/j.fuproc.2018.09.013 https://doi.org/10.1016/j.jclepro.2019.117962 https://doi.org/10.1016/j.jobe.2023.107738 https://doi.org/10.1016/j.matpr.2023.03.510 https://doi.org/10.1016/j.ijsbe.2015.10.002 https://doi.org/10.1016/j.jclepro.2017.01.114 https://doi.org/10.1063/5.0137816 https://doi.org/10.1016/j.jclepro.2017.10.065 https://doi.org/10.1016/j.cemconres.2003.09.002 https://doi.org/10.1016/j.jobe.2021.103168 https://doi.org/10.1016/0008-8846(94)90011-6 https://doi.org/10.1016/j.pecs.2007.03.002 acta polytechnica ctu proceedings 47:15–18, 2024 1 introduction 2 experimental program 3 materials 4 methods 5 result and discussions 6 conclusion acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2023.42.0022 acta polytechnica ctu proceedings 42:22–26, 2023 © 2023 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague fast continuous in-situ xct of additively manufactured carbon fiber reinforced tensile test specimens jonathan glinza,∗, julia maurera, martin holzleitnera, francesco paceb, antonios stamopoulosb, michal vopálenskýc, ivana kumpovác, michael eckld, johann kastnera, sascha sencka a university of applied sciences upper austria, research group computed tomography, stelzhamerstraße 23, 4600 wels, austria b universita degli studi dell’aquila biblioteca dell’area di ingegneria, monteluco di roio, 67100 l’aquila, italy c czech academy of sciences, institute of theoretical and applied mechanics, v.v.i., prosecká 809/76, 190 00 prague 9, czech republic d cap.future gmbh, peter-behrens-platz 6, 4020 linz, austria ∗ corresponding author: jonathan.glinz@fh-wels.at abstract. the reinforcement of fused filament fabricated (fff) components with continuous fibers allows for high versatility in the design of mechanical properties for a specific applications needs. however, the bonding quality between continuous fibers and the fff matrix material has high impact on the overall performance of the composite. to investigate the bonding quality within additively manufactured (am) continuous fiber reinforced specimens, tensile tests have been performed which revealed a sudden reduction in tensile stress, that most likely was not related to actual rupture of continuous fibers. consequently, within this work we will expand upon these findings and present results of fast on-the-fly in-situ investigations performed on continuous carbon fiber reinforced specimens of the same am build. during these investigations, specimens are loaded under the same conditions while fast xct scans, with a total scan time of 12 seconds each, were performed consecutively. the resulting three-dimensional image data reveals internal mesoand macro-structural changes over time/strain to find the cause of the aforementioned reduction in tensile stress. keywords: additive manufacturing, composites, x-ray computed tomography, in-situ tensile testing. 1. introduction fused filament fabrication (fff) is a widely adopted technique used in additive manufacturing for the production of structural components from scratch. recent advancements in am devices have made it possible to reinforce components by incorporating continuous fibers, such as carbon or glass, into the polymer matrix [1]. this reinforcement significantly enhances the strength and stiffness of the manufactured parts. however, despite various studies demonstrating the successful production of composites with highly oriented carbon fibers using fff, internal defects such as insufficient bonding quality between the embedded fibers and the matrix material remains a significant challenge in fff-manufactured fiber-reinforced specimens and composite materials in general [2, 3]. consequently, the behavior of additively manufactured components under load can deviate considerably from the expected values which necessitates the investigation and characterization of complex composite parts for quality assurance. for this purpose, x-ray computed tomography (xct) serves as a powerful, non-destructive testing method to visualize the internal microstructure in specimens in a three-dimensional manner [4]. in a previous study [5], we examined additive manufacturing (am) continuous fiber-reinforced tensile test specimens to investigate the influence of fiber quantity and material on tensile strength and am build quality. during these tests, an unexpected decrease in tensile stress, most likely unrelated to the actual rupture of continuous fibers, was observed. unfortunately, since xct scans were conducted only before and after the tensile testing, a detailed analysis of the causes behind these drops in tensile stress was not feasible. consequently, such studies are typically performed as interrupted in-situ experiments to acquire image data during the mechanical test while blurring of image data is avoided by pausing the tensile strain during the xct measurements [6]. the interruption of the strain rate however can influence the mechanical behavior of the specimen under investigation and consequently lead to different results of the experiment. in this study, we employ on-the-fly [7] in-situ xct tensile tests to investigate continuous carbon fiber-reinforced specimens manufactured via fff. because of the fast scan time of only 12 s for a single full 360 ◦ xct scan, it was possible to perform a continuous tensile test and consequently, effects arising from specimen relaxation during xct scanning could be avoided. thereby we aim to provide valuable insights into the 22 https://doi.org/10.14311/app.2023.42.0022 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 42/2023 in-situ xct tensile testing of carbon fiber reinforced composite figure 1. schematic of a tensile test specimen with dimensions in mm. the print path of continuous fiber reinforcement is indicated in blue color. mechanical behavior in terms of internal mesoand macro-structural changes throughout the course of the experiments. 2. materials and methods although in a previous study only specimens with six continuous fiber reinforced layers have shown the characteristic drop in force under load, for comparison, a total of three different specimens with six (cf6), four (cf4) and two layers (cf2) of continuous carbon fiber reinforcement are produced and tested for this work. the tensile test specimens were produced from short carbon fiber reinforced pa6 (onyxtm) with additional continuous carbon fiber reinforcement via fff on a mark two device (markforged, us). specimens were designed dimensionally proportional to the type 5a/b samples described by the en iso 527-2 standard but reduced in size in order to fit the insitu tensile test device used for the experiment. the specimen geometrical characteristics and the corresponding dimensions are presented in figure 1. the eigertm software interface, which is integrated with markforged 3d printers and allows control over the placement of prepreg fibers and matrix polymer beads in each layer, was utilized to define the printing parameters like slicing and deposition strategy. the software automatically imposed a continuous fiber reinforcement printing pattern that followed the contours of the component, as depicted in figure 1. thereby, for each reinforced layer, two parallel and symmetrical continuous fiber filaments were placed through the gage region of the specimens. for the deposition of the onyx matrix material, a solid fill pattern at an infill density of 100 % was chosen, finished by two wall layers in order to ensure a solid and enclosed surface. apart from the customized settings, standard printing parameters provided by markforged were applied for nozzle temperature, deposition rate, and print head speed. figure 2. photography of the measurement setup with the 5 kn tensile test stage mounted to a rotary table between x-ray source and detector. the specimens were loaded in a deben ct5000 tensile/compression stage (deben uk ltd.) at a clamp distance of 28 mm and 0.2 mm min−1 test speed to achieve a strain rate closest to en iso 527-2: 1996 (0.87 % min−1). the tensile force was applied in xdirection (see figure 1) in displacement-controlled mode to achieve a constant strain rate. during the tensile testing, xct measurements were performed using a xwt 240 se x-ray tube (x-ray worx, de) in combination with a dexela 1512ndt flat panel detector (varex imaging, us). the x-ray tube was operated at 60 kv acceleration voltage and 50 w target power. the detector was grabbing images at 15 ms exposure time in 2 × 2 binning mode at a voxel size of 18 µm. at 800 projection images recorded per specimen revolution, a total scan time of roughly 12 s could be achieved. subsequently, a backwards rotation of the tensile stage was necessary to avoid winding of its connected wires. consequently, a new xct scan could be started every 24 s (see figure 3). with a scan duration of 12 s and a test speed of 0.2 mm min−1 a displacement of 40 µm was applied during each scan equaling a strain of 0.14 % over the entire clamp distance. consequently, at a voxel size of 18 µm3 the investigated region of interest in the center of the tensile specimens experiences a displacement of less than about two times the voxel size of the image data. however, at the target power of 50 w a focal spot size of approximately 50 µm is achieved and given the associated image blurring the displacement during scanning was considered negligible for the investigations planned. the full measurement setup can be seen in figure 2. 3. results and discussion the start and end of each xct scan was labeled as fn,0 and fn,1 respectively, where n is the number of each consecutive scan. as expected from preliminary investigations, the mentioned drop in force in the cf6 specimen is noticeable in the force over strain diagram in figure 3 roughly around 520 n, shortly after f3,1. 23 j. glinz, j. maurer, m. holzleitner et al. acta polytechnica ctu proceedings figure 3. force over strain diagram during continuous in-situ test for a specimen with six and with four continuous fiber reinforced layers, respectively. the diagram for the cf4 specimen is a zoomed section to enhance visualization of the comparably small bumps in force. on-the-fly xct scans were performed during fn,0 to fn,1. figure 4. a) minimum intensity projections over a 0.5 mm thick cross-section in the y-z-plane at the region where delamination has been observed. red arrows indicate the delamination of layers l 4-6 in scan f3 while in scan f4 remaining layers were affected as well. b) cross-sections in the x-y-plane visualizing the delamination at different layers and tensile strain. consequently, the structural changes causing the drop should be visible latest in scan f4. in the cf4 specimen a similar behavior is noticeable in the force over strain diagram but divided into two separate, less distinct drops happening over a wider period of time. in the cf2 specimen no significant delamination could be observed until fiber failure. in figure 4 and figure 5 we find that due to the automatically generated print path of continuous fibers along the specimen contour not all continuous fibers are aligned in direction of the applied tensile strain. besides the generally poor bonding quality, which leaves gaps between continuous fibers and matrix material over wide regions, it becomes obvious from xct image data that the deposition of continuous fibers diagonally to the applied strain can lead to additional debonding between continuous fiber bundles and surrounding matrix material. in the cf6 specimen this detachment was noticeable within the third and fourth scan iteration (f3-f4) which coincides with the drop in force that happens between these two scans as well. consequently, it is reasonable to assume this effect is related to the delamination of fiber bundles from the matrix material. in the cf4 specimen delaminations appear in similar regions as in the cf6 specimen but less distinctly (see figure 5). however, first signs of detachment were noticeable already in the second scan iteration explaining the separate bumps at roughly 3.1 % and 3.3 % strain in the force over strain curve. as previously has been shown for specimens manufactured under equal conditions [8], porosity tends to increase in printing direction, indicating reduced 24 vol. 42/2023 in-situ xct tensile testing of carbon fiber reinforced composite figure 5. a) minimum intensity projections over a 0.5 mm thick cross-section in the y-z-plane at the region where delamination has been observed. red arrows indicate the delamination of fiber reinforcement and matrix material. b) cross-sections in the x-y-plane visualizing the delamination at different layers and tensile strain. build quality towards the top of the specimens (in z-direction). this effect is most likely coupled to the non-isothermal temperature distribution in z-direction during the printing process [9]. results of the fast continuous xct scans have shown that continuous fiber bundles located further to the top of the specimens (in z-direction) also tend to detach from the surrounding matrix material first. hence, within the top layers l4-6 in the cf6 specimen and l3-4 in the cf4 specimen respectively, first signs of debonding was visible one scan iteration, and consequently roughly 0.3 % in strain, earlier than in other layers (see figure 4 and figure 5). this indicates that the increased porosity in higher layers, or generally poorer print quality respectively, is affecting the bonding quality of continuous fibers as well. additionally, in the current investigations it became obvious, that the bonding quality within individual layers can also vary considerably. generally, these specimens are characterized by rather poor intralaminar bonding between continuous fibers and matrix material. however, in figure 4b it is noticeable that the bonding within this specimen is worse on the lower side (in x-direction) of the specimen, partially with huge gaps between the materials. the delamination of continuous fibers was also primarily observed in this region, while the bonding on the opposite side was relatively good. as preliminary work [5] has revealed, specimens that were affected by this characteristic drop in force, supposedly caused by fiber delaminations, also had significantly reduced total tensile strength. consequently, specimens with higher amount of continuous fiber reinforcement happened to show lower tensile strength compared to specimens with less reinforcement but better bonding quality. 4. conclusions continuous on-the-fly xct has proven to be an invaluable tool for the investigation of dynamic processes during mechanical testing of composite materials. with the fast acquisition the investigation of characteristic drops in force during tensile testing was possible without the need of interrupting the experiment for xct scans which could possibly obscure such features during the relaxation of the specimens prior to each scan. as the delamination of continuous fibers predominantly starts in layers located further towards the top of the specimen in printing direction (z-direction), the results suggest the decrease in print quality in this direction affects the bonding quality of embedded continuous fiber reinforcement as well. this highlights the importance of bonding quality once more, as specimens that were affected by the characteristic drop in force, also showed significantly reduced total tensile strength in previous investigations [5]. acknowledgements this work was financed by the project fatam funded by the federal ministry for digital and economic affairs [ffg grant number: 884101]. references [1] s. m. f. kabir, k. mathur, a.-f. m. seyam. a critical review on 3d printed continuous fiber-reinforced composites: history, mechanism, materials and properties. composite structures 232:111476, 2020. https: //doi.org/10.1016/j.compstruct.2019.111476 [2] j. glinz, m. thor, j. schulz, et al. non-destructive characterisation of out-of-plane fibre waviness in carbon fibre reinforced polymers by x-ray dark-field radiography. nondestructive testing and evaluation 37(5):497–507, 2022. https://doi.org/10.1080/10589759.2022.2063860 25 https://doi.org/10.1016/j.compstruct.2019.111476 https://doi.org/10.1016/j.compstruct.2019.111476 https://doi.org/10.1080/10589759.2022.2063860 j. glinz, j. maurer, m. holzleitner et al. acta polytechnica ctu proceedings [3] a. stamopoulos, k. tserpes, p. prucha, d. vavrik. evaluation of porosity effects on the mechanical properties of carbon fiber-reinforced plastic unidirectional laminates by x-ray computed tomography and mechanical testing. journal of composite materials 50(15):2087–2098, 2015. https://doi.org/10.1177/0021998315602049 [4] q. he, h. wang, k. fu, l. ye. 3d printed continuous cf/pa6 composites: effect of microscopic voids on mechanical performance. composites science and technology 191:108077, 2020. https: //doi.org/10.1016/j.compscitech.2020.108077 [5] j. glinz, f. pace, j. maurer, et al. influence of continuous fiber reinforcement on tensile properties in fused filament fabricated specimens. in aiaa scitech 2023 forum. american institute of aeronautics and astronautics, 2023. https://doi.org/10.2514/6.2023-2199 [6] j. maurer, m. jerabek, d. salaberger, et al. stress relaxation behaviour of glass fibre reinforced thermoplastic composites and its application to the design of interrupted in situ tensile tests for investigations by x-ray computed tomography. polymer testing 109:107551, 2022. https: //doi.org/10.1016/j.polymertesting.2022.107551 [7] d. kytýř, p. zlámal, p. koudelka, et al. deformation analysis of gellan-gum based bone scaffold using on-thefly tomography. materials & design 134:400–417, 2017. https://doi.org/10.1016/j.matdes.2017.08.036 [8] j. glinz, j. maurer, m. eckl, et al. in-situ characterization of additively manufactured continuous fiber reinforced tensile test specimens by x-ray computed tomography. in aiaa scitech 2022 forum. american institute of aeronautics and astronautics, 2022. https://doi.org/10.2514/6.2022-1426 [9] s. terekhina, t. tarasova, s. egorov, et al. the effect of build orientation on both flexural quasi-static and fatigue behaviours of filament deposited pa6 polymer. international journal of fatigue 140:105825, 2020. https://doi.org/10.1016/j.ijfatigue.2020.105825 26 https://doi.org/10.1177/0021998315602049 https://doi.org/10.1016/j.compscitech.2020.108077 https://doi.org/10.1016/j.compscitech.2020.108077 https://doi.org/10.2514/6.2023-2199 https://doi.org/10.1016/j.polymertesting.2022.107551 https://doi.org/10.1016/j.polymertesting.2022.107551 https://doi.org/10.1016/j.matdes.2017.08.036 https://doi.org/10.2514/6.2022-1426 https://doi.org/10.1016/j.ijfatigue.2020.105825 acta polytechnica ctu proceedings 42:22–26, 2023 1 introduction 2 materials and methods 3 results and discussion 4 conclusions acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2023.44.0028 acta polytechnica ctu proceedings 44:28–32, 2023 © 2023 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague simulation of the compression test of the zr1nb fuel cladding ring vojtěch smolíka,∗, alžběta endrychováa,b, jakub krejčíb a czech technical university in prague, faculty of mechanical engineering, department of energy engineering, technická 4, 166 07 praha 6, czech republic b ujp praha a.s., nad kamínkou 1345, 156 00 praha-zbraslav, czech republic ∗ corresponding author: vojtech.smolik@fs.cvut.cz abstract. fuel cladding is a first protective barrier against the loss of fission products that must withstand extreme conditions, from normal operation to final and interim dry storage. this hostile environment results in mechanical and microstructural damage of cladding caused by different stress levels, temperature, corrosion, hydrogen pick up and other degradation processes further enhanced by radiation. for this reason, the integrity of the cladding is a critical issue. the aim of this work is to simulate a ring compression test to evaluate the stress-strain behavior and hoop fracture properties of a zirconium-based alloy with niobium, which was chosen because it is widely used as fuel cladding in light water nuclear reactors. keywords: zirconium, fuel cladding, ring compression test. 1. introduction zirconium-based alloys have served the nuclear industry for more than 60 years due to their acceptable properties for the nuclear cores of light water reactors [1]. zirconium-based cladding has low neutron absorption cross section and good mechanical and corrosion properties under normal operation in reactors [2]. the zrlnb alloy designed in russia has been in successful operation in the vver and rbmk reactors since 1960. the zr1nb type cladding has been used in vver reactors for many decades in different countries [1, 3]. fuel cladding serves as the first barrier against the loss of fission products in the defense at depth during normal operation and subsequent storage. during operation, the cladding has to cope with different stress levels at high temperatures and neutron radiation. understanding mechanical properties is very important. ring compression test is an important experimental technique used in materials science to evaluate the mechanical properties of various materials, including cladding alloys. in the nuclear industry, the mechanical behavior of the alloy cladding is critical to ensure safe and reliable operation of nuclear fuel rods. a ring compression test was performed on the sample to determine the mechanical behavior of the zr1nb cladding alloy. the results of the test provide valuable insights into the deformation behavior of the zr1nb cladding alloy and its response to external forces. the aim of this work is to compare the simulation results with the data from the conducted experiment. name t [°c] l [mm] h [mm] t [mm] t-rct1 20 10,09 9,10 0,57 t-rct2 20 9,94 9,10 0,57 t-rct3 20 10,11 9,10 0,57 t-rct4 340 10,07 9,11 0,57 t-rct5 340 9,91 9,10 0,57 t-rct6 340 10,08 9,10 0,57 table 1. table containing the parameters of the samples: t is the temperature during the experiment, l is the length, h is the height, and t is the thickness. 2. material and mechanical testing method an instron 1185 testing machine, equipped with a 100 kn load cell, was used to perform the ring compression test (rct) for two sample temperatures (20 °c and 340 °c). the lengths of the ring samples, mechanically tested in this study, were 10.0 ± 0.1 mm. the length of tubular sample was chosen to have a plane strain state during compression test (length > outer diameter). displacement-controlled tests were performed with a load line displacement rate of 0.5 mm/min. the load-displacement curves were continuously recorded. rct was performed on 6 unirradiated samples fabricated from the zr1nb alloy of the tubular section of ujp prague. the chemical composition of the zr1nb coating tube was 99 % zirconium and 1 % niobium with minor impurities of fe and o. the parametres of each sample are shown in table 1. the picture of the t-rct1 sample after rct is shown in figure 1. 28 https://doi.org/10.14311/app.2023.44.0028 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 44/2023 simulation of the compression test of the zr1nb fuel cladding ring figure 1. sample after ring compression test. name p0 [n] uts [mpa] t-rct1 372 378 t-rct2 366 380 t-rct3 377 380 t-rct4 159 162 t-rct5 158 164 t-rct6 154 157 table 2. table containing the collapse force and ultimate tensile strenght from rct. 2.1. determination of material properties the collapse force p0, where a large plastic deformation occurred, was obtained directly from the loaddisplacement curve by getting the intersection of the lines extended from the elastic and plastic regions [4]. after the collapse force is extracted from the loaddisplacement curve, the ultimate tensile stress (uts) can be obtained. the collapse stress is calculated by equation [5, 6] σ0 = αp0r t2l , (1) where σ0 is the collapse stress, the constant α depends on the length of the sample and for our case it is equal to 0.866, p0 is the collapse force, r is the radius of the sample, t is the thickness and l is the length. the collapse stress is determined by the rct and then it can be linearly correlated with the uts of the tensile test through the following coefficients [4] uts = σ0 kut s , (2) where kut s is equal to 1.18. the collapse force and the ultimate tensile strength for each sample are shown in table 2. another important determined property of the material is young’s modulus, which indicates the ability of a material to resist longitudinal changes in tension or compression. young’s modulus is dependent on temperature, as can be seen in figure 2 where a dependence is shown not only on temperature, but figure 2. comparison of different relations of young’s modulus of zr1nb on temperature: results of the simulation (blue), values calculated using the equation (3) of the fraptran 2.0 code (red), experimentally determined values by rosigner et al. taken from [7] of young’s modulus in three main directions. also on direction. rosigner et al. [7] experimentally determined young’s elastic modulus at room temperature in the normal direction to be between 91.91 gpa and 92.96 gpa, in the longitudinal direction between 95.31 gpa and 96.42 gpa, and in the transversal direction between 95.5 gpa and 99.2 gpa. young’s elastic modulus for 350 °c (623 k) according to the graph is equal to 80 gpa for the normal direction, 73 gpa for the longitudinal direction, and 85 gpa for the transversal direction. the equations for the determination of young’s modulus e for a temperature lower than 1073 k were taken from [8, 9]. 29 v. smolík, a. endrychová, j. krejčí acta polytechnica ctu proceedings figure 3. stress-strain curve bilinear isotropic hardening model. e = 1.121 · 1011 − 6.438 · 107 · t [k]. (3) this equation is used in fraptran 2.0 for zr1nb. young’s modulus for the uncoated zirconium alloy according to equation (3) is e ≈ 93 gpa at room temperature and e ≈ 73 gpa at 340 °c. the values of young’s modulus were calculated by simulation and an attempt was made to match these values. 2.2. numerical simulation the simulation was done up to a deformation of 2 mm, because at this deformation it is already possible to observe whether the sample is sufficiently ductile or brittle. a numerical solution was performed in the static structural analysis ansys 2023 r1. the bilinear isotropic hardening model was applied to define the material properties of the tested sample. the example of bilinear isotropic hardening stress/strain material properties of the t-rct1 sample is shown in figure 3. the displacement velocity of the test sample was set to match the experiment. sensitivity analysis of the numerical mesh has also been performed. the mesh used is shown in figure 4, it consists of 51 615 cells with a size setting of 0.15 mm. the numerical solver has been set to a controlled time step, the duration of 2 mm displacement test is 240 s. the numerical simulation will be used to further study the stress-strain distribution and hoop fracture behavior of new atf fuel caldding concepts (accident tolerant fuel) with different mechanical properties and the comparison with the commonly used zr1nb alloy. severe accidents in lwrs led to research of new materials used as fuel cladding to improve nuclear safety during normal and possible accident conditions. protective coatings deposited on zr alloys as substrates are one of the most perspective near-term concepts of atf. thin layers of cr, crn or multilayers such as crncr are deposited on the zr alloy substrate by various methods. these protective coatings are used to improve the mechanical and corrosion properties of typically used zr alloys under accident conditions. figure 4. numerical mesh with stress distribution in the sample. the future work will be focused on the numerical simulations and determination of the material properties of zr1nb with cr, crn and crncr coating, which are shown in figure 5. the zr alloy coating can make a significant change in the mechanical properties of the samples. for example, young’s modulus of chromium at room temperature is e ≈ 290 gpa [10]. the modulus of elasticity of an uncoated zr1nb sample is approximately three times smaller than the modulus of elasticity of the chromium sample. for these reasons, studying the effect of the coating on the overall mechanical behavior of the cladding and the failure mode of the coating (ductile or brittle) is important for the mechanical integrity of the cr coating as the most encouraging candidate among different types of coatings. another approach is to study the influence of the hydrogen content on the stress-induced precipitation of radial hydride in the fuel cladding. hydrogen pickup is caused by the oxidation of the fuel rod. the hydrogen produced by this reaction is absorbed by cladding and can be dissolved in the crystal lattice or precipitated to create hydrides. hydrogen removal degrades the mechanical properties of the cladding and can cause fuel rod failure. radial hydride embrittement was considered a key phenomenon to be taken into account in dry storage studies [11]. 3. results and discussion figure 6 shows a comparison of the numerical and experimental results of the fuel cladding samples at two different temperatures. it can be seen that the numerical results perform a force reaction lower than the experimental values for both test samples. the reaction force of the tested sample is plotted in relation to the displacement of the sample. the contours of the von mises stress of the tested sample are shown in figure 7. the stress distribution in the numerical simulation is in agreement with the observed crack mechanics of the fuel cladding samples for larger displacements. 1 mm and 2 mm displacements are presented, and the maximum stress is observable in the areas near the contact between the test sample and the compression test device. the stress distribution from the numerical results can also 30 vol. 44/2023 simulation of the compression test of the zr1nb fuel cladding ring figure 5. zr1nb alloy with different coatings deposited by pvd method – cr, crn and multilayer crncr. figure 6. force-displacement curves – experimental and numerical results. correctly determine the areas of tensile and compressive stress, an important factor for the calculation of the uts (ultimate tensile strength) and ys (yield strength) from the rct. the hoop fracture mechanics is not included in the numerical simulation at this stage of the research, so only displacements up to 2 mm are considered. the young’s modulus value was determined from the simulation to be 92 gpa at room temperature and 80 gpa at 340 °c. the values are plotted in figure 2, compared to the experimentally measured values and the correlation used in fraptran 2.0. the figure also shows that the value determined by the simulation for room temperature is appropriate. for higher temperatures, the value of the young’s modulus is higher than the prediction of fraptran 2.0. this could be due to a slightly different chemical composition of the sample material. although it is also a zr1nb alloy with 1 wt. % niobium, the contribution of impurities such as fe, o, or hf was diverse. the fraptran 2.0 code is based on the equations of vokovo et al. [9] from 1989, and a study on the young’s modulus by rosigner et al. [7] was published in 1978. the alloy composition has changed over the years and with the development of the nuclear power industry. for example, the profigure 7. von-mises stress contour result of numerical simulation, t-rct1 sample, 1 mm displacement (up) and 2 mm displacement (down). cess of removing hafnium from zirconium, which has a neutron absorption cross section 600 times higher than that of zirconium, improved, and the oxygen and iron content also changed, significantly influencing the mechanical properties. currently, the zr1nb alloy, commercially named e110, contains 500-700 ppm of o and 100 ppm of fe [12]. the zr1nb alloy used by rosigner et al. [7] in 1978 contained 800 ppm o and no fe. 4. conclusion ring compression test is an important and simple method that is used to determine mechanical properties in the hoop direction, which is essential for the integration of fuel cladding. numerical and experimental results of the ring com31 v. smolík, a. endrychová, j. krejčí acta polytechnica ctu proceedings pression test are presented. the yield stength for 20 °c and 340 °c was determined using the iterative method. the resulting load-displacement curves of numerical and experimental work are compared. the collapse load and the ultimate tensile strength were determined from the load-displacement curves. young’s modulus of elasticity was determined and compared with the values found in the literature. numerical simulation represents a new approach to evaluate the von mises stress distribution in the test sample and predict the areas with the highest stress load. numerical model developed in this study will be extended to include the mechanics of crack formation. acknowledgements the authors wish to thank the entire zirconium alloys team at ujp praha a.s. references [1] r. comstock, p. barbéris. zirconium in the nuclear industry: 17th international symposium. journal of astm international: selected technical papers. astm international, 2015. isbn 9780803175297. 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pp. 1–56. elsevier, oxford, second edition edn., 2020. https://doi.org/10.1016/b978-0-12803581-8.11759-x 32 https://doi.org/10.1016/j.jnucmat.2009.12.005 https://doi.org/10.1088/1757-899x/461/1/012059 https://doi.org/10.13140/2.1.2563.9363 https://doi.org/10.1016/0029-5493(79)90055-4 https://doi.org/10.1016/0022-3115(79)90444-6 https://doi.org/10.2172/1030897 https://www.nrc.gov/docs/ml1429/ml14296a063.pdf https://doi.org/10.1016/j.jnucmat.2021.153332 https://doi.org/10.1016/j.jnucmat.2014.06.049 https://doi.org/10.1016/b978-0-12-803581-8.11759-x https://doi.org/10.1016/b978-0-12-803581-8.11759-x acta polytechnica ctu proceedings 44:28–32, 2023 1 introduction 2 material and mechanical testing method 2.1 determination of material properties 2.2 numerical simulation 3 results and discussion 4 conclusion acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2023.40.0015 acta polytechnica ctu proceedings 40:15–21, 2023 © 2023 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague measurement and evaluation of cement paste porosity by electrochemical impedance spectroscopy jana chalupová a,∗, jiří němeček 1,a, vojtěch hybášekb, vojtěch pommerc, jiří němeček 2,a a czech technical university in prague, faculty of civil engineering, department of mechanics, thákurova 7, 166 29 prague 6, czech republic b university of chemistry and technology, department of metals and corrosion engineering, technická 5, 166 28 prague 6, czech republic c czech technical university in prague, faculty of civil engineering, department of material engineering and chemistry, thákurova 7, 166 29 prague 6, czech republic ∗ corresponding author: jana.chalupova@fsv.cvut.cz abstract. porosity is an important part of cement paste microstructure, which significantly influences the mechanical properties (especially durability and strength) of cement-based materials. electrochemical impedance spectroscopy (eis) is a non-destructive method used to measure the pore system utilizing a range of frequencies of electric current, which is fed to the cement paste by stainless steel electrodes. the pore volume is obtained from the pore resistance and the matrix capacitance measured by eis. this paper deals with the evaluation of porosity based on resistance values from eis on a sample of pure cem i 42.5r portland cement paste at 3 different hydration times (age 7, 14, 28 days). keywords: electrochemical impedance spectroscopy, cement paste, porosity. 1. introduction cement is one of the most consumed materials in the world. therefore, many scientists are working on researching, developing, and improving cement-based materials. porosity is one of the observed properties at the micro-scale that influences strength and durability [1]. generally, three types of pores can be found in hydrated cement paste – capillary pores, gel pores, and interlayer space. the pore characteristics differ in size, shape, and distribution in the solid matrix. porosity measurement depends on all these characteristics [2]. the porosity of cement-based material can be accessed either by direct or indirect methods. direct methods are able to measure the pore sizes from the largest pores in the order of a few millimeters to hundreds or tens of nanometers. direct methods analyze pore structure from 2d or 3d images of the sample. therefore, these methods depend on the resolution of an image (pixel size). these methods include 2d images of scanning electron microscopy (sem), 3d images of x-ray microtomography or x-ray nanotomography, and laser scanning confocal microscopy (lscm) [3]. indirect methods use liquid (such as water, gas, mercury, helium, etc.) inside cement paste as a probe. 1ph.d. at czech technical university in prague, orcid: 0000-0002-5635-695x. 2professor at czech technical university in prague, orcid: 0000-0002-3565-8182. the size of the measurable pores is limited by the lower (tens of nanometers) and upper boundary (hundred of micrometers). indirect methods of measuring open porosity include mercury intrusion porosimetry (mip), water saturation, and pycnometry [3–5]. electrochemical impedance spectroscopy (eis) can be ranked as the indirect method because a liquid conduct an electric current and thus is involved in the measurement. using eis not only detects the porosity and microstructure of concrete [6–9], but also the hydration and shrinkage process [10], corrosion process of reinforced concrete [11, 12], the diffusion of chloride in concrete [13, 14] and the influence of mineral admixtures on cement-based material [15]. despite some efforts of porosity characterization by eis [6–9] the methodology is still limited by many uncertainties. thus, this paper deals with the eis measurement on pure cement paste samples, evaluation of porosity, and comparison with porosity determined by helium pycnometry. 2. electrochemical impedance spectroscopy 2.1. equivalent circuit models for concrete electrochemical impedance spectroscopy (eis) or otherwise known as alternating current impedance spectroscopy (acis), is a non-destructive method used to measure the resistance of materials, including cement 15 https://doi.org/10.14311/app.2023.40.0015 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en j. chalupová, j. němeček, v. hybášek et al. acta polytechnica ctu proceedings paste and concrete. electric current (usually alternating current ac) is applied to the samples by a pair of electrodes embedded in the material or attached to their opposite surfaces. the transmitted current with the periodic alternating signal can be decomposed into a series of harmonic signals of sinusoidal shape with different frequencies due to fast fourier transform [16, 17]. the measurement is taken from the highest frequencies to the lowest, and the impedance is recorded [6, 12]. cement paste cannot be considered as a single electrical resistor in eis measurements because the material consists of a bulk matrix and pores. pores have different shapes, sizes, and distributions in the cement paste structure. for example, capillary pores are connected and with an oblong shape in contrast to a spherical enclosed pores [2]. electrochemical impedance spectroscopy allows measuring only the capillary pores, and three conductive paths are distinguished as shown in the figure 1 [6]: • continuous conductive path (ccp) – a series of capillary pores connected by pore necks form ccp. the impedance of this path zccp equals the resistance of interconnected capillary pores rccp: zccp = rccp. (1) • discontinuous conductive path (dcp) – the continuity of a capillary path is disturbed by a discontinuous point (dp) formed by a thin layer of cement paste. dp is considered to be a double parallel capacitor with electric capacitance c dp. the resistance of the capillary path is labeled as rcp. the impedance of the path zdcp is composed of c dp and rcp connected in series as shown the following equation: zdcp = rcp + cdp. (2) • insulator conductive path (icp) – the pores and voids do not form the largest part of cement paste. it is made up of a bulk matrix that acts as an electric insulator. the matrix becomes charged as a result of the current passing through the sample. therefore it is considered a double parallel capacitor with capacitance c mat. although cement paste is not a perfect insulator, its resistance rmat can be neglected if the sample is not frozen or dried. thus, the impedance of this path z icp is just equal to the c mat: zicp = cmat. (3) according to the typical nyquist diagram (plot of real versus imaginary part of impedance), capacitive loops occur in cement-based materials as shown in figure 2b. these loops can be replaced by a parallel series of resistors (r) and capacitors (c ). an equivalent electric circle containing r and c is necessary to use in order to evaluate the results of eis measurement [18, 19]. many equivalent circuits have been published and are summarized in [19]. discontinuous conductive path (dcp) continuous conductive path (ccp) insulator path (icp) discontinuous point (dp) pore solution cement paste conductive path figure 1. simplified microstructure of cement paste with illustrated conductive paths. in this paper, two different models of equivalent electric circuits published by guangling song in [6] were used. the first option is the equivalent circuit model (ec), which is composed of all above mentioned paths, as shown in figure 2a. the capacitance of the solid matrix of the cement paste is relatively low. therefore the second option assumes the simplification that c mat does not have to be considered in the second equivalent model, as shown in figure 3. furthermore, the electric circuit can be simplified into the simplified equivalent circuit model (sec) as shown in figure 2c. comparing the equivalent models in figures 2c and 3, the following relations can be deduced [6]: r0 = rcp · rccp/(rcp + rccp), (4) r1 = rccp 2/(rcp + rccp), (5) c1 = (1 + rcp/rccp)2 · cdp, (6) where r1 is resistance of (continuous and discontinuous) pores, c1 is a capacitance of bulk cement (including c dp), r0 is an offset resistance from the origin on real axis of nyquist diagram. the theoretical nyquist spectrum of the eis consists of arcs. the most important arc of the sec model is the arc of diameter r1, see figure 2d. the position of the center of the arc on the real axis z is ensured by rotation by the depression angle (α) [20], which is related to the pore size distribution and the others imperfection of the sample [12]. r0 can be neglected because of high-frequency measurement (nearly to the origin on the real axis of the nyquist plot) is negatively influenced by surrounding phenomena and the accuracy limitation of most electrochemical equipment [6, 20, 21]. 16 vol. 40/2023 measurement and evaluation of cement paste porosity . . . rcp cdp cmat rccp bulk matrix connected pores disconnected pores zicp zdcp zccp re z [ω] -i m z [ω ] b) nyquist spectrum based on ec impedance r0 r1 a) equivalent circuit (ec) r0 c1 r1 [ω] d) nyquist spectrum based on sec impedance r0 r1 c) simplified equivalent circuit (sec) re z -i m z [ω ] figure 2. equivalent circuit model (ec), theoretical nyquist eis spectrum based on a equivalent model ec, simplified equivalent circuit model (sec) and theoretical nyquist eis spectrum based on a equivalent model sec [6]. rcp cdp rccp connected pores disconnected poreszdcp zccp figure 3. simplified equivalent circuit model for cement paste used to measure of eis [6]. 2.2. porosity calculation from eis measurements archie’s law is the most widely used relationship between the resistivity and porosity of porous material. the relationship was originally derived on the sandstone filled with brine [22] and is defined by the equation: f = a · ϕ−m 0 , (7) where f is formation factor, a is a coefficient, ϕ0 is the capillary porosity, m is archie’s index. in cement-based materials, the resistivity is dependent on sample dimensions and electrode positions. thus, it is more convenient to convert the resistance to effective electrical conductivity σeff as a function of electrode size and position: σeff = l/(r · s), (8) where l is the distance between the electrodes in the direction of current, s is the cross-sectional area of the electrode embedded in the cement paste. σeff = c · σ0 · ϕ0 m, (9) where σ0 is the conductivity of conducting medium, c is a constant depending on the saturation of the sample (assumed to be 1.0 for fully saturated samples), ϕ0 is the pore volume fraction, m is archie’s index. the exponent m reflects pore complexity and tortuosity and has been found in the range 1.5–4.0 [13, 18]. 3. materials and methods 3.1. sample preparation cement paste samples were made from cem i 42.5r with a water to cement ratio of 0.4. a fresh mixture was poured into a silicone-lubricated formwork with dimensions of 54×30×11.7 mm (length, width, height). additionally, a pair of electrodes from a stainless steel plate was put longitudinally into the formwork. electrodes were inserted 3 mm above the formwork bottom and 1 mm from formwork edges. the distance between electrodes was set to 10 mm. the scheme of sample setup for eis measurement is shown in figure 4. moreover, the same samples without electrodes were prepared for porosity measurement. the samples were demoulded after 24 hours and placed in 0.5 % limewater solution where retained until the measurement. 17 j. chalupová, j. němeček, v. hybášek et al. acta polytechnica ctu proceedings 11 .7 15 9. 7 3 r e f e r e n c e e l. w o r k in g e l. c o u n t r e l. w o r k in g e l. 52 54 1 1 10 10 10 30 [mm] embedded electrode cement paste figure 4. an illustration of the sample with embedded and connected electrodes. 3.2. eis measurement eis measurement was performed using zahner zennium x device and thalesxt usb software with a frequency range of 12 mhz–100 hz and 10 steps per decade. the amplitude of sinusoidal voltage was set to 10 mv. the use of a larger potential amplitude is not recommended due to possible changes in the surfaces of the samples [17]. the cables connected two working electrodes to one of the embedded electrodes. the reference electrode and the counter electrode were connected to the other of the embedded electrode [23]. short coaxial cables were used to reduce the influence of surrounding phenomena and the measurement noise [17]. to ensure consistent measurement results, it was necessary to prevent the drying of the saturated samples. thus, samples were partly submerged in the water while simultaneously avoiding contact between electrodes and water. otherwise, the electric current entering the samples would pass through the water, and the results would not correspond to the cement paste. the samples were measured at 3 different ages of hydration – 7, 14, and 28 days. the eis measurement was taken at five-minute intervals until the values stabilized. 3.3. porosity measurement the porosity of the cement paste was determined by helium pycnometry and mercury intrusion porosimetry. after a saturation period of 7, 14, and 28 days, the samples were cut into 2 mm thick slices and dried at 50 °c. subsequently, sample density, ρ was measured by helium pycnometer thermo scientific atc evo. the bulk density, ρbulk of the samples was determined by the gravimetric method. further, the porosity of the sample was calculated from the equation: ϕ0,p = (1 − (ρbulk/ρ)) · 100. (10) 4. results and discussion the example of the eis results for the c-14d and c-28d samples is shown in figure 5 as the nyquist 150 200 250 3000 50 100 150 200 re z [ω] -im z [ω ] a) c-14d exp. data cem. paste exp. data electrode exp. data noise ec fit sec fit 150 200 250 3000 50 100 150 200 re z [ω] -im z [ω ] b) c-28d exp. data cem. paste exp. data electrode exp. data noise ec fit sec fit figure 5. experimental nyquist eis spectrum with calculated ec and sec model fits for samples c-14d and c-28d. note that only data corresponding to cement paste (blue marks) were used for fitting. spectrum. before the resistance and capacitance evaluation, experimental data were corrected and cut off at both ends. that is because the values at low frequencies correspond to the electrodes’ resistance and their contact with the material, and values at high frequencies are inaccurate due to the limitations of the measuring device. further, the data were fitted by the simplex algorithm with the application of ec and sec models [6]. the samples were remeasured over time until the resistance values were stabilized. the stabilization period took between the 15–30 minutes, as illustrated in figure 6. table 1 summarizes the resistances and capacities corresponding to the fits based on the ec and sec models, as well as degree rotation α. the results clearly show almost no differences between resistances rccp (ec) and r1 (sec). therefore, both values correspond to the continuously connected pores, indicating that c mat for the pure cement paste 7–28 days old can be neglected as considered by the simplified equivalent model. also, both rccp and r1 are increasing with samples age. such a behavior corresponds 18 vol. 40/2023 measurement and evaluation of cement paste porosity . . . sample equivalent circuit – ec simplified equivalent circuit – sec rccp rdcp rdp cmat r0 r1 c1 α [ω] [ω] [pf] [pf] [ω] [ω] [pf] [rad] c-7d 201.3 1290 29.1 38.7 0 204.1 41.9 0.81 c-14d 215.2 1577 29.2 40.7 0 217.3 42.0 0.82 c-28d 274.5 1875 21.7 41.6 0 276.8 45.5 0.86 table 1. resistance and capacitance of cement paste samples evaluated with different equivalent circuit models: ec, sec. 0 5 10 15 20 25 30200 225 250 275 300 325 time [min] r c c p [ω ] c-14d c-28d figure 6. a stabilization period of cement paste resistance at different hydration times. the ec model evaluated rccp. well to the decreasing porosity due to the ongoing hydration reaction, during which the newly created hydration products occupy space originally formed by capillary pores. thus, creating more discontinuous paths results in increasing rdcp over time. the density results measured by helium pycnometry and bulk density are summarized in table 2 along with the values of calculated porosity (ϕ0). again the decrease in porosity corresponds to the hydration process. initially, the porosity from the eis measurement could not be directly calculated using equation (9) and porosity obtained from helium pycnomtery was used for calibration. another expression, the effective electrical conductivity was easily calculated from resistances rccp and r1. the conductivity of the pore solution σ0 was calculated by the analytical relationship of snyder [24], which is dependent on the w/c ratio, degree of hydration (doh), and the amount of na+, k+, oh− concentration. the doh was estimated by cemhyd3d model [25]. thus, the only unknown term remains archie’s index m. it was found that parameter m is almost identical for both equivalent circuit models ec and sec and also does not vary with hydration time ranging from 7 to 28 samples age. therefore, it is possible to calculate the average value sample ρ ρbulk ϕ0,p [kg/m3] [kg/m3] [%] c-7d 2320 1687 27.3 c-14d 2287 1687 26.2 c-28d 2234 1685 24.6 table 2. porosity evaluated from helium pycnometry. of m = 3.46 ± 0.02. lastly, the porosity ϕ0 was calculated with the known mean value of paramete m. all the calculated parameters are summarized in table 3. 5. conclusions in this paper, the porosity was measured by electrochemical impedance spectroscopy (eis) with the aid of helium pycnometry and evaluated using archie’s law. the resistivity of continuous conductive paths was evaluated from the impedance spectrum by the equivalent circuit (ec) and simplified equivalent circuit model (sec). the minimal differences between resistances rccp and r1 were found. hence, the capacitance of matrix c mat can be neglected in the sec model. thus, only the sec model is sufficient for evaluating the impedance spectra of pure portland cement paste. the value of archie’s index m was found to be almost identical between both ec and sec models and also for 7, 14, and 28 samples age. thus, the m parameter can be assumed as constant, which would allow porosity from eis measurement without the aid of another porosity measurement method. nevertheless, when time changes or supplementary material is added, the porous system also changes. therefore, further researches on cement-based materials are necessary. acknowledgements the works were supported by the czech science foundation (project 21-11965s) and the grant agency of the czech technical university in prague (sgs22/088/ohk1/2t/11). their support is gratefully acknowledged. 19 j. chalupová, j. němeček, v. hybášek et al. acta polytechnica ctu proceedings sample doh σ0 ϕ0,p rccp r1 σeff m ϕ0 ec sec ec sec ec sec m = 3.46 [%] [s/m] [%] [ω] [ω] [s/m] [s/m] [-] [-] [-] c-7d 65.5 13.08 27.3 201.3 204.1 0.147 0.145 3.459 3.470 27.3 c-14d 71.7 13.52 26.2 215.2 217.3 0.137 0.136 3.427 3.435 26.5 c-28d 76.8 13.90 24.6 274.5 276.8 0.108 0.107 3.467 3.473 24.5 table 3. the parameters necessary for the porosity calculation by archie’s law. references [1] y.-y. kim, k.-m. lee, j.-w. bang, s.-j. kwon. effect of w/c ratio on durability and porosity in cement mortar with constant cement amount. advances in materials science and engineering 2014:273460, 2014. https://doi.org/10.1155/2014/273460 [2] h. m. jennings. refinements 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[20] a. e. a. hamami, j.-m. loche, a. aït-mokhtar. cement fraction effect on eis response of chloride migration tests. advances in cement research 23(5):233–240, 2011. https://doi.org/10.1680/adcr.2011.23.5.233 [21] b. j. christensen, t. coverdale, r. a. olson, et al. impedance spectroscopy of hydrating cement-based materials: measurement, interpretation, and application. journal of the american ceramic society 77(11):2789–2804, 1994. https: //doi.org/10.1111/j.1151-2916.1994.tb04507.x [22] g. e. archie. the electrical resistivity log as an aid in determining some reservoir characteristics. transactions of the aime 146(01):54–62, 1942. https://doi.org/10.2118/942054-g [23] r. nováková, m. kouřil, j. stoulil, et al. relation of corrosion simulators transport and modern concrete pore microstructure. in conference metal, pp. 1135–1140. 2015. [24] k. a. snyder, x. feng, b. d. keen, t. o. mason. estimating the electrical conductivity of cement paste pore solutions from oh-, k+ and na+ concentrations. cement and concrete research 33(6):793–798, 2003. https://doi.org/10.1016/s0008-8846(02)01068-2 [25] v. šmilauer, z. bittnar. microstructure-based micromechanical prediction of elastic properties in hydrating cement paste. cement and concrete research 36(9):1708–1718, 2006. https://doi.org/10.1016/j.cemconres.2006.05.014 21 https://doi.org/10.1680/adcr.2011.23.5.233 https://doi.org/10.1111/j.1151-2916.1994.tb04507.x https://doi.org/10.1111/j.1151-2916.1994.tb04507.x https://doi.org/10.2118/942054-g https://doi.org/10.1016/s0008-8846(02)01068-2 https://doi.org/10.1016/j.cemconres.2006.05.014 acta polytechnica ctu proceedings 40:15–21, 2023 1 introduction 2 electrochemical impedance spectroscopy 2.1 equivalent circuit models for concrete 2.2 porosity calculation from eis measurements 3 materials and methods 3.1 sample preparation 3.2 eis measurement 3.3 porosity measurement 4 results and discussion 5 conclusions acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2024.48.0061 acta polytechnica ctu proceedings 48:61–65, 2024 © 2024 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague design of the testing procedure for investigation of eryx conicus dermal armour characteristics blanka žaloudkováa, petr koudelkab, petra frýdlovác, mykhailo drozdenkod, jan šleichrtb, daniel kytýřb,∗ a czech technical university in prague, faculty of transportation sciences, na florenci 25, 110 00 prague 1, czech republic b czech academy of sciences, institute of theoretical and applied mechanics, prosecká 809/76, 190 00 prague 9, czech republic c charles university, faculty of science, viničná 7, 128 44 prague 2, czech republic d czech academy of sciences, nuclear physics institute, 250 68 řež, czech republic ∗ corresponding author: kytyr@itam.cas.cz abstract. the aim of this work is to develop an experimental method suitable for the mechanical testing of highly non-standard biological samples such as snake skin with osteoderms. the objective of the method is to determine, whether the osteoderms provide a protective function for the animal in its natural environment. for this purpose, a simulation of rodents biting the skin based on uni-axial compressive loading using a synthetic tooth as a penetrator was developed with an emphasis on integration with x-ray scanners to facilitate in-situ testing. to identify and characterise the structure of snake skin and to prove the protective function of osteoderms, all samples were subjected to high resolution x-ray computed tomography. the results of the experiments are presented in the form of stress-strain curves and a map of the tangent modulus. keywords: bioinspiration, snake skin, osteoderms, mechanical testing, computed tomography, dermal armour. 1. introduction biological materials and animal anatomical features in general have been an inspiration for human protection since immemorial eras. even though the research in recent years has been more material-driven, studies show that a link can be seen between modern defensive structures and structures found in the realm of animals [1]. thus, studying the natural animal dermal armour can be helpful in dealing with the ultimate goal of human ballistic protection, which is to find the optimum between the weight of the armour, its flexibility when worn, and, most importantly, the protection it provides against a multitude of threats. a multi-layer or multi-material body armour is one of the approaches often adopted when dealing with such a problem, but there is insufficient data available on ballistic performance [2]. here, it is important to note that different body parts require different levels of protection, where heavy armour is needed for exposed and large profile body parts such as the head or chest, while soft armour is used in other areas to reduce the overall weight and, importantly, to allow sufficient mobility in the areas around joints (i.e., typically shoulders and hips). this is also a result of natural development that originates from anatomical solutions to survival problems in animals, where, e.g., rhinoceros species have thinner skin around their joints to achieve the best ratio of protection versus mobility [3]. thus, by the increase of protective performance in the body armour without the inevitable weight increase with available armour types, it would be possible to provide high-end protection to more parts of the human body than front and back of the torso, which is the normal extent of protection in the commonly used armour vests or plate carriers. in general, for a deeper understanding of all the factors and processes involved in the field of bio-inspired ballistic protection, knowledge gathering methods based on the study of nature and effective transfer of the knowledge to engineering solutions of problems involve research in numerous fields including kinematics [4], morphology [5], material science [6] and structural analysis [7]. in this work, we present an initial study paving the way for in-situ investigation of morphological and mechanical characteristics in eryx conicus dermal armour as part of research aimed at verifying its existence and determining its role in protecting these snake species in their natural environment. 2. wildlife story in our study, we focused on rough-tailed sand boas (eryx conicus). in nature, it can be found in india, pakistan, nepal, and bangladesh. they are stoutbodied and medium sized reaching around 80 cm in length in adulthood, the life expectancy is 20 years or even longer. their natural habitat includes dry and rainy areas; they shelter in deep cracks and rodent 61 https://doi.org/10.14311/app.2024.48.0061 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en b. žaloudková, p. koudelka, p. frýdlová et al. acta polytechnica ctu proceedings burrows. although they are mostly nocturnal animals, they are also found to be active during the day. they prey on rodents, lizards, ground-feeding birds, and amphibians. their anatomically unique tail with dermal armour [8] probably plays a fundamental role in their antipredatory strategy. it is hypothesised that they use it to protect and avoid serious injuries to vital parts of their body. we assume that two-headed snakes of the genus eryx use their armoured tails as a deceptive target while plundering rodent nests (e. g., gerbils). indian gerbil (tatera indica) is a rodent of the gerbillinae subfamily that commonly shares territory with snakes. in adulthood, they can grow up to 38 cm with male species slightly larger than females. their lifespan in captivity can reach up to 7 years; however, when living in the wild, life expectancy is just a few years as many individuals have problems surviving the first year of their life due to the hazard posed by snakes. however, nests are defended by parents with sharp edged incisors (see figure 1) [9]. the aim of this study is to design a testing method for evaluation the protective function of osteoderms (irregular small bony elements with dimensions about 500×250×150 µm, see figure 1 red) in the skin of the snake based on localised mechanical experiments coupled with detailed ct inspection to quantify relevant biomechanical parameters. as a first step, the experiment was designed to investigate the quasi-static interaction of the loading tip with real snake skin with osteoderms. the loading tip was the dimension of the gerbil incisors to make it as real as possible. snake skin without osteoderms was used as a control. figure 1. size comparative ct based visualisation of the gerbil skull with incisors and eryx conicus caudal segment with osteoderms visible as a small bony elements around the body without a tight connection to the axial skeleton together with detail of single osteoderm (in red). 3. materials and methods 3.1. skin samples four samples of the skin surface layer with thicknesses up to 500 µm of rectangular-like shape and dimensions ranging from 7×18 mm to 20×30 mm were harvested from a single individual. in the central (labelled a) and caudal (c) part of the body where the osteoderms of interest (see figure 1) are predominantly located on the lateral (l) and ventral (v) side of the caudal segment. the samples were then long-term stored in commonly used 70 % ethanol solution [10]. the moderate effect of storage conditions on mechanical properties has been reported [11], but a more significant effect arises from the drying of the sample. for that reason, the testing procedures were performed in the shortest possible time and the samples were re-hydrated after the mechanical tests prior to x-ray imaging. 3.2. mechanical loading the skin samples were subjected to mechanical loading using an in-house developed compact table-top loading device presented in detail in [12] and specially instrumented for snake skin testing (see figure 2). the upper steel loading pin with a tip cross-section of 1.25 × 0.4 mm was designed on the basis of the geometry assessed from an x-ray ct scan of the gerbil incisors (see figure 1) and instrumented by lcm300 (futek, usa) load-cell with a nominal capacity of 10 n. the lower gliding platform with the skin sample fixed by double-sided scotch tape allowed manual positioning for setting the quasi-regular the testing grid consisting of 14–20 testing points. the experiments were displacement driven up to 500 µm or interrupted when reaching the force limit of the load cell. the loading rate was set to 2 µm s−1. the experiment was carried out using real-time closed loop control software [13] with 200 hz sampling rate for both force value and encoder position readout. figure 2. compact table-top loading device instrumented for the snake skin testing procedure. 3.3. x-ray imaging the modular in-house designed laboratory ct scanner, equipped with linear positioning axes and a rotary stage depicted in figure 3, allows the exchange of the x-ray detector and source to meet specific measurement requirements. for soft, partially ossified skin 62 vol. 48/2024 testing procedure for eryx conicus dermal armour investigation tissue, l10321 (hamamatsu photonics, japan) microfocus x-ray source without additional filtering of the x-ray beam [14] together with dexela 1512 (varex imaging, germany) cmos x-ray detector equipped with a csi scintillator were employed. the native resolution of the detector is 1944 × 1536 at a pixel pitch of 74.8 µm. the imaging was performed with an acceleration voltage of 75 kv and a target current of 150 µa to achieve the best possible quality in the acquired radiograms. each tomographic scan consisting of 1912 equiangular projections took approximately 30 minutes. the cone beam filtered backprojection reconstruction algorithm based on the feldkamp-daviskress method [15] implemented in vg studio max 3.4.1 (volume graphics, germany) was used to obtain the resulting 3d images of investigated samples having dimensions of 1536 × 1536 × 1944 voxels and 20 µm effective voxel size. the histogram-based tissue segmentation procedure [16] was applied to identify the location of the osteoderm. figure 3. in-house designed ct setup with the x-ray source (left), detector (right) and the detail of the skin sample on rotary stage. 3.4. data evaluation fully automated evaluation procedure implemented in matlab r2023b (mathworks, usa) was developed to calculate the stress-strain data from the measured force f and displacement u signals. for the calculation of engineering stress σ, the cross-sectional area of the nominal loading tip atip was used. the engineering strain ϵ was calculated for each measurement point individually due to the non-uniform thickness t of the sample. here, the thickness was measured as the difference between the tip-baseplate contact tb and the tip-skin contact position ts, when the contact force reached 0.07 n at the beginning of the experiment. the stress and strain were then calculated using the equations σ = f atip , ϵ = u tb − ts . (1) to quantify the fully non-linear stress-strain behaviour of the samples, the tangent modulus was chosen as a representative quantity. the tangent modulus was calculated using the polynomial curve fitting in the last 10 % of the strain. the results are graphically represented by stress-strain diagrams and tangent modulus values projected on the 3d image visualisation of the respective sample. 4. results as a result of this pilot study, the results of the three samples harvested from: i) the ventral side of the central segment (av), ii) the ventral side of the caudal segment (cv), and iii) the lateral side of the caudal segment (cl), where the osteoderms were then identified only in the cl samples. the strain stress diagrams of the av sample are depicted in figure 4. invalid measurements probably caused by improper triggering of the tip-skin contact point are plotted in grey. 0 1 2 3 4 5 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 s tr e ss [ m p a ] strain [-] figure 4. stress-strain diagram of ventral side of central segment (av) with 11 valid measurements presented in blue, excluded measurements in grey. based on 11 valid experiments, the values of the individual tangent modulus in the range of 11.48– 18.66 mpa were mapped on the ct visualisation presented in figure 5. the values are indicated by the colour and diameter of the circle with the centre corresponding to the simulated bite point. the mean value of the tangent modulus was calculated as 15.99 mpa with a standard deviation of 1.61 mpa (10.1 %). the strain stress diagrams of the cv sample are depicted in figure 6. based on 13 valid experiments, the values of the individual tangent modulus in the range of 7.48–17.39 mpa were mapped on the ct visualisation presented in figure 7. the mean value of the tangent modulus was calculated as 12.52 mpa with a standard deviation of 3.20 mpa (25.5 %). the strain stress diagrams of the cl sample are depicted in figure 8. based on 3 experiments carried out on osteoderm area, individual tangent modulus values in the range of 19.15–21.70 mpa with a mean value of 20.45 mpa and a standard deviation of 1.27 mpa (6.2 %) were calculated (see figure 9). the results of 63 b. žaloudková, p. koudelka, p. frýdlová et al. acta polytechnica ctu proceedings figure 5. localised tangent modulus magnitudes on ventral side of central segment (av). 0 1 2 3 4 5 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 s tr e ss [ m p a ] strain [-] figure 6. stress-strain diagram of ventral side of caudal segment (cv) with 13 valid measurements presented in blue, excluded measurements in grey. the other 12 experiments were not influenced by osteoderms. here, tangent modulus values in the range of 5.85–16.17 mpa with a mean value of 10.19 mpa and a standard deviation of 3.67 mpa (36.1 %) were assesed (see figure 9). the bite test was performed on three skin samples from a single eryx conicus individual. osteoderms were identified by ct scans only on the lateral side of the caudal segment. the average tangent modulus of the samples without osteoderms (av, cv) was significantly higher, measuring 14.26 ± 2.45 mpa, compared to the non-ossified part of the cl, which measured 10.19 ± 3.67 mpa. for all bite tests that excluded osteoderms, the tangent modulus was calculated as 12.90 ± 2.92 mpa, while the tangent modulus in osteoderms reached 20.45 ± 1.27 mpa, although this latter value is not supported by a statistically significant number of measurements. 5. conclusions based on the acquired results, it can be concluded that the proposed methodology and experimental equipfigure 7. localised tangent modulus magnitudes on ventral side of caudal segment (cv). 0 1 2 3 4 5 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 s tr e ss [ m p a ] strain [-] figure 8. stress-strain diagram of lateral side of caudal segment (cl) with 3 measurements of osteoderms and 12 measurement not influenced by osteoderms presented in blue, excluded measurements in grey. figure 9. localised tangent modulus magnitudes on lateral side of caudal segment (cl). ment including the x-ray imaging chain is a viable solution to the problem of mechanical experiments and imaging of snake skin tissue equipped with dermal armour. the reconstructed 3d images from the 64 vol. 48/2024 testing procedure for eryx conicus dermal armour investigation tomographical measurements show that it is possible to identify the osteoderms using high-resolution x-ray imaging with high confidence and to support the interpretation of the mechanical experiments. by analysing the mechanical data, the presence of osteoderms was clearly apparent, as the assessed average tangent modulus was twice as high compared to the skin without osteoderms. however, because of the geometry of the specimen and geometry of the simulated incisors, it is relatively difficult to measure the mechanical quantities precisely at the predetermined locations. the results and knowledge will be used in the following work comprising in-situ mechanical experiments during high-resolution x-ray ct imaging. acknowledgements the activity was supported by vp26 sav21 and institutional support rvo: 68378297. references [1] y. wang, s. e. naleway, b. wang. biological and bioinspired materials: structure leading to functional and mechanical performance. bioactive materials 5(4):745–757, 2020. https://doi.org/10.1016/j.bioactmat.2020.06.003 [2] a. ghazlan, t. ngo, p. tan, et al. inspiration from nature’s body armours – a review of biological and bioinspired composites. composites part b: engineering 205:108513, 2021. https: //doi.org/10.1016/j.compositesb.2020.108513 [3] z. w. white, f. j. vernerey. armours for soft bodies: how far can bioinspiration take us? bioinspiration & biomimetics 13(4):041004, 2018. https://doi.org/10.1088/1748-3190/aababa [4] r. khan, m. watanabe, a. a. shafie. kinematics model of snake robot considering snake scale. american journal of applied sciences 7(5):669–674, 2010. https://doi.org/10.3844/ajassp.2010.669.674 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temperature, hydration, and chemical treatment. biomaterials 27(5):785–795, 2006. https: //doi.org/10.1016/j.biomaterials.2005.06.019 [12] t. fíla, j. šleichrt, d. kytýř, et al. deformation analysis of the spongious sample in simulated physiological conditions based on in-situ compression, 4d computed tomography and fast readout detector. journal of instrumentation 13(11):c11021–c11021, 2018. https://doi.org/10.1088/1748-0221/13/11/c11021 [13] v. rada, t. fíla, p. zlámal, et al. multi-channel control system for in-situ laboratory loading devices. acta polytechnica ctu proceedings 18:15–19, 2018. https://doi.org/10.14311/app.2018.18.0015 [14] g. e. campillo-rivera, c. o. torres-cortes, j. vazquez-bañuelos, et al. x-ray spectra and gamma factors from 70 to 120 kv x-ray tube voltages. radiation physics and chemistry 184:109437, 2021. https: //doi.org/10.1016/j.radphyschem.2021.109437 [15] t. rodet, f. noo, m. defrise. the cone-beam algorithm of feldkamp, davis, and kress preserves oblique line integrals. medical physics 31(7):1972–1975, 2004. https://doi.org/10.1118/1.1759828 [16] l. lenchik, l. heacock, a. a. weaver, et al. automated segmentation of tissues using ct and mri: a systematic review. academic radiology 26(12):1695–1706, 2019. https://doi.org/10.1016/j.acra.2019.07.006 65 https://doi.org/10.1016/j.bioactmat.2020.06.003 https://doi.org/10.1016/j.compositesb.2020.108513 https://doi.org/10.1016/j.compositesb.2020.108513 https://doi.org/10.1088/1748-3190/aababa https://doi.org/10.3844/ajassp.2010.669.674 https://doi.org/10.1002/ar.25328 https://doi.org/10.3109/03008208609001974 https://doi.org/10.1038/s41598-023-33244-6 https://doi.org/10.3897/vz.72.e90474 https://doi.org/10.7717/peerj.10799 https://doi.org/10.1016/j.biomaterials.2005.06.019 https://doi.org/10.1016/j.biomaterials.2005.06.019 https://doi.org/10.1088/1748-0221/13/11/c11021 https://doi.org/10.14311/app.2018.18.0015 https://doi.org/10.1016/j.radphyschem.2021.109437 https://doi.org/10.1016/j.radphyschem.2021.109437 https://doi.org/10.1118/1.1759828 https://doi.org/10.1016/j.acra.2019.07.006 acta polytechnica ctu proceedings 48:61–65, 2024 1 introduction 2 wildlife story 3 materials and methods 3.1 skin samples 3.2 mechanical loading 3.3 x-ray imaging 3.4 data evaluation 4 results 5 conclusions acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2023.44.0013 acta polytechnica ctu proceedings 44:13–18, 2023 © 2023 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague evaluation of atlhamc12 subchannel code for total loss of flow scenario tomáš křečeka, štěpán foralb,a,∗ a brno university of technology, faculty of electrical engineering and communications, department of electrical power engineering, technická 12, 616 00 brno, czech republic b tes s.r.o., pražská 597, 674 01 třebíč, czech republic ∗ corresponding author: foral@tes.eu abstract. the althamc12 subchannel code is a new subchannel code developed by the alvel company. the code is intended for dnbr safety analyses of the czech nuclear power plants. in order to validate the code, a code to code comparison with thales and vipre-01 is provided in this work. the reactor core model was developed and set of initial and boundary conditions has been adopted from a reference study. the comparison is done for steady state nominal parameters and total loss of flow (tlof) type of accident. the results show that althamc12 provides a good agreement with the reference codes in the terms of mdnbr value and its positions in the reactor core. keywords: althamc12, subchannel analyses, apr1400, plus7, ce-1. 1. introduction the core of a nuclear reactor is designed and operated in a way that ensures sufficient heat dissipation resulting from fission and complies with all prescribed temperature limits for fuel cladding and fuel pellets at all locations within the core. the temperature limit for the reactor core structure is often determined based on the limit of fuel cladding temperature, which is defined by the maximum permissible heat flux at the coolant-fuel cladding interface. subchannel analysis is performed to determine the coolant parameters within the reactor core and it plays an important role in determining the safety limits of nuclear reactor operation, for example, the determination of minimum departure from nucleate boiling ratio (mdnbr) which is one of the essential factors for assessing the thermal safety margin of a pressurized water reactors. the principle of subchannel analysis is that the core is divided into several so-called subchannels in the radial direction. the subchannels are then divided in the axial direction and so a matrix of nodes is created. during the calculation, the equations of conservation of mass, momentun and energy are solved when the initial and boundary conditions are imposed in each node. the althamc12 subchannel code is developed by the czech company alvel, it is primarily designed for czech nuclear power plants (vver type). the althamc12 code includes several critical heat flux (chf) correlations, such as pg-s, pi3, pg-i, okbm-bezrukov and epri correlations. in the case of the first four correlations, these are the correlations primarily used for vver reactors. for the plus7 fuel design used in the apr1400 nuclear reactors, knf has developed the kce-1 correlation on the basis of the ce-1 correlation, but kce-1 details are not publicly available [1]. the primary objective of this paper is to utilize the althamc12 subchannel code and documentation from usnrc [2] to construct a subchannel model of the apr1400 reactor core, specifically with plus7 fuel. then perform a comparative analysis to compare the althamc12 results obtained with the results published in the reference study. [3] 2. apr1400 reactor core model in the subchannel code althamc12, a simplified model of the apr1400 reactor core was created which is in compliance with the model developed in vipre-01 in reference study [3]. figure 1 shows the radial nodalization and subchannel numbering of the model. the model is simplified in the axial direction to include only the fuel portion. so, the modelincludes all 241 fuel assemblies, limited to active fuel length. the internal structures of fuel assemblies are not a part of the model except for grids. the basic parameters of the model are given in table 1. all the geometry data of fuel assembly and core layout were adopted from [2]. due to the lack of some technical data in [3], simplifications were made in the case of modeling of the fuel assemblies. the simplifications are basically identical to those used in the vipre-01 core model. there are 236 fuel rods in the fuel assembly. these rods have been replaced by one fictitious rod that is surrounded by coolant. the gap between fuel assemblies is divided among adjacent subchannels. thus, each fuel assembly is represented by a single so-called “rod-centered” subchannel type. next, the gap between the outermost fuel assembly and the core shroud is larger than the gaps between the central fuel assemblies, so each 13 https://doi.org/10.14311/app.2023.44.0013 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en tomáš křeček, štěpán foral acta polytechnica ctu proceedings figure 1. radial nodalization of apr1400 reactor core model of althamc12 and vipre-01 codes. figure 2. position of spacers in the plus7 fuel assembly. parameter value number of subchannels 241 number of axial nodes 50 axial node length (equal division) [mm] 76.2 number of junctions 448 energy generated in coolant [%] 0.0 surface roughness [µm] 6.0 turbulent mixing coefficient [-] 0.038 grid pressure loss coefficient (smv/bg) 1.0/0.7 table 1. main parameters of the apr1400 reactor core model. outer subchannel should have a slightly larger area. in fact, the larger gap is ignored. keeping the same area for the outermost fuel assemblies is expected to have a negligible impact on the mdnbr results because mdnbr is expected to occur in the central most loaded fuel assemblies. based on data in [1], the axial position of spacers and one bottom grid was determined. the figure 2 shows the geometry of the fuel assembly with location of the bottom grid and spacers with mixing vanes. the pressure loss coefficients of grids are not available for public so the approach from [4] is adopted. for spacers with mixing vanes (smv), the grid pressure drop coefficient is 1.0 and for bottom grid (bg) is 0.7, as presented in table 1. due to the type of radial nodalization, the developed apr1400 reactor core model cannot be used for detail mdnbr analyses. the subchannels determine the coolant properties which are averaged for the whole fa. contrary to that, the ce-1 correlation was developed on the basis of local coolant data measured between the fuel rods (the description of the ce-1 correlation is provided in [4]) and so it can be used only with detail model of fa. in other words, the combination of the ce-1 correlation and althamc12 core model would yield inaccurate predictions of mdnbr. on the other hand, the core model developed in althcam12 can be used for code-to-code comparison as intented in this study. moreover, it can be used for determination of cross-flows between the fuel assemblies. this can be used then as boundary condition for mdnbr calculation in combination with detail subchannel model of fa. 14 vol. 44/2023 evaluation of atlhamc12 subchannel code . . . parameter bi value b1 2.8922 · 10−3 b2 −0.50749 b3 405.32 b4 −9.9290 · 10−2 b5 −0.67757 b6 6.8235 · 10−4 b7 3.1240 · 10−5 b8 −8.3245 · 10−2 table 2. parameters of the ce-1 critical heat flux correlation. 3. ce-1 critical heat flux correlation the ce-1 correlation developed by westinghouse is used for evaluation of mdbnr. this correlation has the same mathematical formula as the kce-1 correlation which was derived directly for plus7 fuel design [1] but is available for public. the ce-1 correlation is defined by the following equation [5]: q′′ chf = b1 ( d dm )b2 ( (b3 + b4p) g(b1p+b8g) − ghfgx ) g(b7p+b8g) , (1) where q′′ chf is critical heat flux, in btu/ft2hr, p is pressure in psia, d is heated equivalent diameter of the subchannel in inches, dm is heated equivalent diameter of a matrix subchannel with the same rod diameter and pitch in inches, g is local mass velocity at chf location in lb/hr-ft, x is local coolant quality at chf location in decimal fraction, hfg is latent heat of vaporization in btu/lb. parameters b1 to b8 are given in table 2. to consider the non-uniform distribution of the axial heat flux, the correction factor fs is used, which is defined as [5]: fs = q′′ chf,equiv uniform q′′ chf,non-uniform , (2) fs(j) = c(j) q′′ chf,non-uniform (1−e −c(j)·x(j) ) · ∫ x(j) 0 q”(x)e−c(j)·(x(j)−x(j−1))dx, (3) where for ce-1 chf correlation: c(j) = 1.8 · (1 − xchf )4.31 g0.478 . (4) the index (j) represents the addressed node. dnbr in node (j) is then calculated as: dnbr(j) = 1 fs(j) · q′′ chf, equiv uniform q′′ (j) . (5) since the althamc12 code does not include the ce-1 correlation, an excel file was created to postprocess the output data when considering the ce-1 correlation in mdnbr analyses. figure 3. core radial power distribution for mdnbr analyses [3]. 4. validation of core model as recommended in srs-23 [6], the model should be validated against steady state and transient data. the created model is validated using reference data from reference study [3] by code-to-code comparison. the reference study is focused on comparison between two subchannel codes, thales and vipre-01. in both codes, the ce-1 critical heat flux correlation is also used to calculate the critical heat flux. the mathematical and physical models used in the codes, including althamc12 are compared in table 3. in turbulent mixing models, a and b are constants for turbulent mixing correlation, re is reynold’s number, g is averaged flow rate, dh is average hydraulic diameter, s is the flow gap between subchannels, n is the total number of subchannels along one side a fuel assembly, and w is the gap width for one side of fuel assembly. further, it can be seen in table 3 that althamc12 shares many parameters with the vipre-01 code. core power distribution is based on shinkori 3&4 design from reference study [3]. radial power factors are presented in figure 3. fuel assemblies with maximum values of radial power factors are shown in red color as these are expected to experience mdnbr. only quarter of the reactor core is shown in figure 3 as the radial power map is symmetrical. there are five core axial power distributions presented in figure 4 which are used in the mdnbr analysis, and which reflect the impact of fuel burnup. at the same time, figure 3 shows the nodalization of the vipre-01 model meanwhile the nodalization of the thales model is identical to the althamc12 model. 5. code to code comparison at steady-state conditions data for steady-state were adopted from reference study [3] and they are shown in table 4. following the methodology presented in reference [3], the asi-1 15 tomáš křeček, štěpán foral acta polytechnica ctu proceedings model / correlation althamc12 vipre-01 thales node division modelling channel only channel only subchannel and channel turbulent mixing arebg dh arebg dh arebg dhs(12n/w) two phase friction multiplier armand armand sher-green and martinelli-nelson void model madsen armand armand flowing quality equilibrium equilibrium equilibrium subcooled nucleate boiling jens-lottes jens-lottes jens-lottes thermal diffusion coefficient design value design value chf correlation ce-1 ce-1 ce-1 core inlet flow distributed distributed distributed core outlet pressure uniform uniform distributed table 3. comparison of models and correlations between codes [3, 7]. figure 4. core axial power distribution for mdnbr analysis [3]. parameter value thermal power [mwt] 3983 pressure [mpa] 15.51 core inlet temperature [°c] 291 mass flow [kg/m2s] 3496 table 4. initial conditions for validation at steadystate [3]. axial power profile shown in figure 4 was selected for calculations. the reference radial power distribution is taken from figure 3. the results of calculation mdnbr between the thales and vipre-01 computational codes are published in reference study [3] as well. the comparison with the althamc12 code is presented in table 5 and in figure 5. in steady-state conditions, the mdnbr value calculated by the thales code was 2.957 in subchannel number 13, while the vipre-01 code yielded a calculated value of 2.954 in subchannel number 114, which is symmetric to subchannel number 13 as shown in figure 3. the results show that the mdnbr results obtained with the the althamc12 code align well with the values in the reference study. in the axial position, mdnbr matches with the vipre-01 code (axial node no. 41), but the figure 5. position of subchannels with mdnbr for steady-state conditions. parameter thales vipre-01 althamc12 mndbr [-] 2.957 2.954 3.003 subchannel 13 114 50 elevation [mm] 3357 3276 3276 table 5. comparison of mdnbr parameters for steady-state conditions. axial position of mdnbr in the thales code is very similar (axial node no. 42). since the power of the fuel assemblies no. 13, 50, and 114 is almost the same, the difference in the predicted mdnbr radial position in the core may be attributed to a different coolant overflow betwen fas. the most probable source of the difference are the values of the grid loss coefficients which had to be estimated in the althamc12 core model. 6. code to code comparison at transient conditions as the transient scenario for validation, the total loss of flow accident (tlofa) was selected. the tlofa in the reactor is a result of a simultaneous loss of electrical power to all main coolant pumps 16 vol. 44/2023 evaluation of atlhamc12 subchannel code . . . parameter value heat power [mwt] 4062.7 pressure [mpa] 16.03 core inlet temperature [°c] 287.8 mass flow [kg/s] 23619 table 6. initial conditions for validation at tlofa [3]. figure 6. core coolant flow rate (top) and average heat flux (bottom) during tlofa [3]. (mcps). the only fault that can lead to this situation is the total loss of power to the unit. during tlofa, dnbr will manifest itself during the first few seconds of the transient phase as coolant flow decreases but the reactor has not yet been completely shut down. reactor protection system generates a signal to shut down the reactor if the speed of any of the four mcps drops below 95 % of their nominal speed. this rapid reactor shutdown provides reasonable assurance that mdnbr caused by the event will remain above the specified acceptable fuel design limit for dnbr. [8] to perform the mdnbr analysis of the tlofa, it is necessary to determine the initial conditions. for a conservative approach, initial conditions are chosen to simulate the worst-case initial scenario for a given accident or event, deviating from the nominal reactor operating parameters (higher power, higher/lower flow, etc.). these initial and boundary conditions were selected to be the same as in reference [3]. the initial conditions are presented in table 6. parameter thales vipre-01 althamc12 mdnbr [-] 2.953 2.876 2.949 subchannel 13 50 50 elevation [mm] 3357 3276 3276 table 7. comparison of mdnbr parameters for tlofa. figure 7. position of subchannels with mdnbr for steady-state conditions. the coolant inlet pressure and temperature are kept constant for five seconds to give conservatism. the core average heat flux and mass flow rate change during the transient. their evolution are shown in figure 6 for the first five seconds of the transient. in the calculation, the axial power profile asi-1 from figure 3 and the maximum radial power distribution from figure 4 were used again. comparison of calculated mdnbr is presented in table 7. figure 7 then shows the position of subchannels with mdnbr and figure 8 shows the axial profiles of mdnbr. as it can be seen in figure 8, the mdnbr profile calculated by the atlhamc12 code closely matches the mdnbr profile calculated in the reference study using the thales code. only slight deviations in the curve are observed at the beginning and end of the transient, but the difference between the curves does not exceed 2.5 %. the axial positions of mdnbr are very similar for all three codes. vipre-01 aligns with althamc12 both in terms of axial position and subchannel location, specifically subchannel number 50 and axial position of 3 276 mm corresponding to axial node number 41. the differences in mdnbr results may be explained not only by the grid loss coefficients as explained by the steady state dnbr results. the outlet pressure distribution may have some influence on the cross flow distribution as well and this is the feature which is taken into account only by the thales code, as referred in table 3. 17 tomáš křeček, štěpán foral acta polytechnica ctu proceedings figure 8. results of mdnbr calculation during tlofa. 7. conclusion in this study, the apr1400 reactor core model developed in althamc12 code. the model was validated using the code-to-code comparison. the mdnbr results predicted by the althamc12 were compared with reference results for steady state and transient conditions. the steady state validation results come out very similar to the results calculated using the thales and vipre-01 codes in the reference study: mdnbrthales = 2.957, mdnbrvipre-01 = 2.954, and mdnbralthamc12 = 3.003. the althamc12 predicted mdnbr in the most loaded fa meanwhile the other codes predicted mdnbr in the fas with the second highest relative power. the scenario for transient validation analysis was the total loss of coolant flow. it can be concluded on the basis of results that the validation for this transient also performs well. vipre-01 predicts the lowest mdnbr values during the analyzed period of time. the minimal mdnbrvipre-01 was 2.88 at the time of 3.5 second. the mdnbr results for althamc12 and thales are almost identical with a slight difference occurring at the beginning and at the end of the analyzed transient. the minimal mdnbr of both codes is 2.95 and occurs at the time 3.5 second, which is the same time as predicted by vipre-01. thales predicts the minimal mdnbr in the most loaded fa meanwhile the other codes predicted mdnbr in the fas with the second highest relative power. list of symbols dnbr departure from nucleate boiling ratio f a fuel assembly mdnbr minimum departure from nucleate boiling ratio mcp main coolant pump acknowledgements research described in this paper was supported by the technology agency of the czech republic, program theta 4, under grant tk04030168, project “tolerant nuclear fuels for small modular reactors and safety evaluation”. references [1] kepco. kce-1 critical heat flux correlation for plus7 thermal design. [2021-12-23]. https://www.nrc.gov/docs/ml1301/ml13018a147.pdf [2] kepco. apr1400 design control document tier 2, chapter 4: reactor, rev. 3. [2023-06-06]. https://www.nrc.gov/docs/ml1822/ml18228a652.pdf [3] k. h. moon, e. ozdemir, s. j. oh, y. kim. evaluation of thales subchannel code behavior for loss of flow and rcp rotor seizure scenarios. [2023-07-10]. http: //glc.ans.org/nureth-16/data/papers/13939.pdf [4] s. b. kim. improvement of subchannel scale analysis capability of cupid with grid-directed cross flow and fuel rod models. diploma thesis, seoul national university, gwanak-ro, gwanak-gu, seoul, korea, 2018. [5] combustion engineering. cetop-d code structure and modeling methods for san onofre nuclear generating station units 2 and 3, 1981. [2023-06-06]. https://www.nrc.gov/docs/ml2000/ml20004d823.pdf [6] iaea. accident analysis for nuclear power plants srs no. 23, 2002. [7] alvel. althamc12 dokumentace programu, document # 17-20a-005-001, 2018. [8] kepco. apr1400 design control document tier 2, chapter 15: transient and accident analyses, rev. 3. [2023-06-06]. https://www.nrc.gov/docs/ml1822/ml18228a662.pdf 18 https://www.nrc.gov/docs/ml1301/ml13018a147.pdf https://www.nrc.gov/docs/ml1822/ml18228a652.pdf http://glc.ans.org/nureth-16/data/papers/13939.pdf http://glc.ans.org/nureth-16/data/papers/13939.pdf https://www.nrc.gov/docs/ml2000/ml20004d823.pdf https://www.nrc.gov/docs/ml1822/ml18228a662.pdf acta polytechnica ctu proceedings 44:13–18, 2023 1 introduction 2 apr1400 reactor core model 3 ce-1 critical heat flux correlation 4 validation of core model 5 code to code comparison at steady-state conditions 6 code to code comparison at transient conditions 7 conclusion list of symbols acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2023.45.0045 acta polytechnica ctu proceedings 45:45–52, 2023 © 2023 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague collapse potential and ultrasonic pulse velocity of loess soils after treatment with expanded perlite and metakaolin mohammad ali khodabandeh∗, katalin kopecskó, gábor nagy budapest university of technology and economics, faculty of civil engineering, department of engineering geology and geotechnics, h-1111 budapest, müegyetem rkp. 3., hungary ∗ corresponding author: mohammad.khodabandeh@emk.bme.hu abstract. collapsible soils, including loess, can support heavy loads at their natural humidity but then collapse rapidly and lose a substantial volume when the humidity increases. soil collapse potential at different immersion stresses was investigated according to astm d5333. different immersion stresses, including 100, 200, 300, 400, and 500 kpa, were used to evaluate the stress-strain behavior of soils before and after saturation. the results indicated that soil collapse potential rose with a raising of immersion stress, resulting in the highest soil collapse potential at 500 kpa immersion stress. furthermore, the effectiveness of expanded perlite and metakaolin in treating loess soils was examined. the results revealed that the soil collapse potential reduced with an increase in perlite and metakaolin up to 8 %. also, perlite had a more significant impact on the soil collapse potential than metakaolin. the interlocking force between soil particles was enhanced as the perlite quantity rose, and perlite prohibited soil particles from slipping on each other. metakaolin decreased the soil collapsibility due to the pozzolanic and chemical reactions which increased particle bonding. the ultrasonic pulse velocity of soils decreased as the percentage of perlite increased. because the perlite was not spread homogeneously, and the waves can be stuck in the heterogeneities, metakaolin acted as a filler and increased the ultrasonic pulse velocity of the soils. keywords: collapse potential, collapsible soil, expanded perlite, loess, metakaolin, stabilization. 1. introduction loess has a metastable structure and is found unsaturated in semi-arid regions [1]. loess is prone to sudden collapse and settlement as a result of disturbance, stress, and soaking, as well as a variety of geotechnical hazards such as landslides, collapsibility, and seismic settlement [2–4]. the structures constructed on collapsible soils might settle due to the saturation of soils for various reasons such as water and oil pipe failure, sewage leakage, rising groundwater level, leaks of industrial effluents, and chemicals, etc. [5, 6]. chemical stabilization of loess soils (e.g., lime, cement, nano, fibers, etc.) to achieve improved engineering characteristics is a widely used technique around the world [7, 8]. two chemical reactions are involved in improving the geotechnical properties of collapsible soils and chemical stabilization including short-term and longterm reactions. cation exchange, flocculation, and agglomeration are examples of short-term reactions that are responsible for changes in soil engineering properties such as workability and plasticity. longterm reactions, known as pozzolanic reactions, result in the formation of new calcium silicate/aluminate hydrates, which enhance flocculation by connecting adjacent soil particles and improving the soil after cuing. time and temperature affect pozzolanic reactions, resulting in a steady increase in strength over time [9]. traditional stabilizers such as lime and portland cement are the most extensively utilized chemical additives, according to rauch et al. [16]. cement stabilization improves soil strength and decreases compressibility [17–19]. the effects of lime and cement on the behavior of lime/cement-soil mixtures were studied by several authors [20–25]. they concluded that adding lime/cement to the mixture improves strength, substantially reduces moisture content, and reduces volume expansion or compressibility. soil stabilization with new materials like fibres, polymers and nanoparticles has attracted a lot of interest in recent years. adding fibres to soils was investigated by many researchers and it was found that fibres lowered soil cracking and increased soil strength while decreasing soil hardness [26, 27]. biopolymers like xanthan and guar gums are found to be useful in reducing collapsibility of soils [28]. according to tabarsa et al. [29] and haeri and valishzadeh [30], nanomaterials like nano-clay and nano-silica had also a significant effect on strengthening collapsible soil properties. however, finding innovative stabilizing materials is a concern for researchers. in the present study, the effect of perlite and metakaolin on the collapse potential and 45 https://doi.org/10.14311/app.2023.45.0045 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en m. a. khodabandeh, k. kopecskó, g. nagy acta polytechnica ctu proceedings parameter soil 1 (balatonakarattya) soil 2 (vál) method used ll, pl, pi [%] 30, 23, 7 26, 23, 3 astm d4318 [10] natural water content [%] 5.5 4.5 astm d2216 [11] gs 2.70 2.67 astm d854 [12] void ratio [e) 0.9 0.9 astm d7263 [13] γdry [kn m−3] 14.2 14.2 astm d7263 [13] soil classification [uscs] cl-ml ml astm d2487 [14] soil classification [euro code] sicl sasi en iso 14688-2 [15] table 1. the basic properties of loess soils. ultrasonic pulse velocity of loess soils was investigated. 2. materials and methods 2.1. properties of soils two loess soils used in this study was collected from two locations in hungary: the first was in balatonakarattya with coordinates of 47°1’14.93”n 18°8’38.64”e, and the second was in vál with coordinates of 47°21’25.6"n 18°40’15.7”e. the characteristics of loess soils are presented in table 1. grain size distribution for both soils is presented in figure 1. figure 1. grain size distribution curves for the samples soil 1 and soil 2. 2.2. properties of stabilization materials the expanded perlite utilized in this study was produced in accordance with msz en 14316-1. the metakaolin utilized in this investigation was metavertm i type and was produced in austria based on msz4798:(2015). the properties of perlite and metakaolin are presented in table 2. 2.3. laboratory program and sample preparation the test schedule is as follows: 1)at first, the remoulded samples were prepared in the same density and same moisture content as the fields sampling. then, the remoulded samples were oxide compound perlite metakaolin sio2 68–75% 52–54% al2o3 10–14% 40–43% fe2o3 < 2% < 2.5% cao < 2% < 0.5% mgo < 1% < 0.4% k2o 3.2–4.5% < 2.0% so3 < 1% < 1.0% na2o 2.8–4.5% < 0.1% particle size 0–2 mm < 2 µm table 2. properties of perlite and metakaolin. subjected to collapse tests in different immersion stresses including 100, 200, 300, 400 and 500 kpa. 2) regarding the treatment, the remoulded samples were mixed with 2, 4, 6, and 8 w/w % expanded perlite and metakaolin. the samples were then aged for 7 days in isolated plastic bagging to achieve an equilibrium condition and to enable probable interactions between soil and stabilization materials. finally, the same collapse tests were performed on treated samples as on the remoulded samples. 3) ultrasonic pulse velocity was performed on the soils stabilized with 2, 4, 6, and 8 w/w % expanded perlite and metakaolin after 7, 14 and 28 curing days. 2.4. collapse testing the collapse tests were performed in accordance with the astm d5333 [31] standard on remoulded samples with the same density and moisture content as the field samples. at first, the sample was inserted in the device and then it was subjected to an initial stress of 5 kpa. in the next stages, the sample was subjected to various levels of stress, including 12.5, 25, 50, 100, and 200 kpa, and the deformation (settlement) of the soil was measured at each step. the sample was then soaked 1 hour after adding 200 kpa, and the loading was carried out under saturated conditions for up to 24 hours while the settling was monitored. equation (1) calculates the index of collapse, which represents the sample’s settlement at a stress of 200 kpa. 46 vol. 45/2023 loess soils with expanded perlite and metakaolin ic = df − di h0 × 100 (1) where ic = the collapse index; df = height after soaking; di = height before soaking; and h0 = the initial height of the soil. 2.5. ultrasonic pulse velocity testing the ultrasonic pulse velocity test is a non-destructive method to evaluate the physical and mechanical properties of different soils and rocks [32, 33]. in the present study, it was conducted using the pundit device (portable ultrasonic non-destructive digital indicating tester) based on the astm c597. the equipment consists of a transducer and a receiver that are linked to an electronic timing device for timing the start of a pulse generated at the transmitting transducer and its arrival at the receiver [34]. the pundit-plus digital display screen can be used to read the travel time through the specimen. the effect of perlite and metakaolin and curing time (7, 14, and 28 days) on the ultrasonic pulse velocity of soils was evaluated. pulse frequency was 54 hz in the ultrasonic pulse velocity test. the ultrasonic pulse velocity of the sample was determined using the equation (2) given the length of the wave transmission (sample length) and the recorded transmission times [35]: v = l/t (2) where l is the length of the pulse transmission path, t is the pulse transmission time, and v is the ultrasonic pulse velocity within the sample. 3. results 3.1. effect of immersion stress on the collapse potential of soils the collapse behavior of both soils is presented in figure 2. based on the astm classification, both soils were classified in a category of moderately severe collapsible. figure 2. comparison of soil collapse behavior. the rates of soil collapse for both soils at various immersion stresses were determined. as shown in figure 3, by increasing the immersion stresses from 100 to 500 kpa, the collapse potential of both soils increased. the lowest rate of collapse potential belonged to 100 kpa and the highest collapse potential obtained at 500 kpa for both soils. the rate of collapse at an immersion stress of 100 kpa for soil 1 was moderate and for soil 2 was moderately severe. the collapse potential for soil 1 increased from 4.5 % at 100 kpa to 9.88 % at 500 kpa, and it increased from 6.9 % at 100 kpa to 11.3 % at 500 kpa for soil 2. soil 1 had a moderately severe collapse potential at immersion stresses of 200 to 500 kpa. soil 2 had a moderately severe collapse potential at immersion stresses of 200 and 300 kpa and severe collapse potential at immersion stresses of 400 and 500 kpa. according to figure 3c, at all stress levels, the difference in collapse potential between the two soils is almost the same. figure 3. comparison of soil collapse behavior at different immersion stresses (a.) soil 1 (b.) soil 2 (c.) comparison of soil 1 and soil 2. 47 m. a. khodabandeh, k. kopecskó, g. nagy acta polytechnica ctu proceedings figure 4. the collapse behavior of loess soils treated with perlite (a.) soil 1 (b.) soil 2 (c.) comparison of soil 1 and soil 2. 3.2. effect of perlite and metakaolin on the collapse potential of soils in the present study, the influence of stabilization with various percentages of perlite and metakaolin on two collapsible soils was investigated. the collapse behavior of soils treated with perlite is indicated in figure 4. as shown in figure 4, by increasing the perlite percentage by up to 8 % of the dry mass, the collapse potential of both loess soils decreased. perlite had a significant effect in lowering the risk of collapse, whereas increasing perlite from 0 to 8 % has decreased the potential of soil collapse from 7 % and 9.20 % to 1.4 % and 1.88 % for soil 1 and soil 2, respectively. figure 5 shows the coxllapse behavior of soils treated with metakaolin (mk). the results show that as the content of mk increased, the soil collapse pofigure 5. the collapse behavior of loess soils treated with metakaolin (a.) soil 1 (b.) soil 2 (c.) comparison of soil 1 and soil 2. tential decreased. with an increasing of mk from 0 to 8 %, the collapse potential decreased from 7 % and 9.20 % to 3.05 % and 4.67 % for soil 1 and soil 2, respectively. the chemical reaction between mk and small particles in loess soils were responsible for the improved performance of adding mk in lowering the collapsibility index of the examined soils. clay particles are integrated into larger particles because of flocculation and agglomeration reactions, leading to the changes in the soil texture. hydration is another possible reaction. the mk’s solid-forming components react with water-producing calcium-hydrated silicates (csh), calcium hydrated alumina (cah), during the hydration process. this reaction occurs quickly and lowers the moisture content of the mixture. ion exchange processes are then initiated by the 48 vol. 45/2023 loess soils with expanded perlite and metakaolin figure 6. the results of ultrasonic pulse velocity (a.) soil 1 treated with perlite (b.) soil 2 treated with perlite (c.) soil 1 treated with metakaolin (d.) soil 2 treated with metakaolin. released calcium and aluminium ions caused by mk hydration. as a result, the collapsibility of the soils treated with mk was reduced. it is worth noting that metakaolin did not reduce the collapsibility index of the soil as much as perlite. however, the influence of both materials in both soils was almost the same and the collapse trend of both soils stabilized with perlite and metakaolin was found to be decreasing. 3.3. effect of perlite and metakaolin on the ultrasonic pulse velocity of soils the results of ultrasonic pulse velocity for soils treated by perlite and metakaolin are indicated in figure 6. as shown in figure 6a and figure 6b, with an increasing perlite percentage, the ultrasonic pulse velocity of both soils decreased. adding perlite slowed down the passage of waves, because the perlite is lighter than the soil matrix. additionally, the pressure applied from the top during the sample preparation procedure causes the perlite to be oriented roughly horizontally, putting it sideways in the path of the incoming waves. this should take longer for the waves to pass the sample since they are more likely to diverge than to go directly along it. furthermore, it would be quite complicated to ensure that perlite was spread uniformly and homogeneously on the sample. consequently, because the waves can be stuck in the heterogeneities, the specimen had a multi-dimensional structure, that increases the chance of wave interference. as a result, this may also lower the wave velocity traversing the sample. this approach was mentioned by fatahi et al. [36], who examined how ultrasonic wave velocity decreased in cemented clay samples with an increasing fibre percent. choobbasti et al. cristelo et al. [37] reported that by increasing carpet waste fibres, the ultrasonic pulse velocity of soil decreased because on the micro scale, the distribution of mixed fibre was heterogeneous. figure 6c and figure 6d shows that as the percentage of metakaolin increased, the ultrasonic pulse velocity increased. this is due to the fact that the addition of metakaolin enhanced the soil consistency and density while also improving the microstructure in parallel to the sample length, creating a better path for the passage of waves and so increasing wave velocity. in other words, the soil matrix’s microcracks and grooves can be reduced by the addition of metakaolin. also, a cement type material bonds the soil particles together, transforming the soil matrix into an integrated structure. the intra-particle pore space should thus be reduced because of the addition of metakaolin. higher wave velocities can be obtained because the ultrasonic pulses can travel through the path made by the integration of clay particles despite the expansion of the inter-particle pore space. cristelo et al. [38] observed similar results for a portland cement-sandy clay mixture. 4. discussion in the present study, the collapse behaviour of two soils were investigated. the results of collapse tests show that soil 1 (cl-ml) had a lower collapse potential than soil 2 (ml) which was due to the higher 49 m. a. khodabandeh, k. kopecskó, g. nagy acta polytechnica ctu proceedings percentage of clay in soil 1. according to zamani and badv [39], the percentage of clay in collapsible soil affects its actual potential to collapse. the possibility of collapse could increase with a lower level of clay content. with less clay, there is less contact between soil grains, which might collapse in the presence of water. in the present study, according to the results of the collapse test, with increasing the immersion stress, the collapse potential of both soils (cl-ml and ml) increased which was consistent with the results of zamani and badv [39] but was inconsistent with the results of arabani and askari lasaki [40]. the collapse potential of two cl soils and concluded that as the immersion stress increased, the collapse potential of fine-grained soils increased. arabani and askari lasaki [40] conclude that there is a decreasing trend for the collapse potential of ml soil with increasing vertical stress. in terms of the perlite treatment, the collapse potential of both soils decreased as the perlite concentration increased. addition of perlite to the soils reduced the compressibility of soils under loading. in soils treated with perlite, a three-dimensional network of randomly dispersed perlite particles bind soil particles surfaces with the help of friction and interlocking force. this will enhance the strength properties of the soil / perlite mixture by preventing perlite from slipping out of the soil matrix. hence, the reduction in soil collapse potential can be related to the prevention of soil particle slippage relative to each other, which reduces the soil volume change. the improved performance by adding metakaolin in reducing the collapsibility index of the tested soils was due to the chemical interactions between metakaolin and small particles in loess soils (sio2, al2o3) which generated an inter-particle bond that made the stabilized soils stiffer. the large surface-to-volume ratio of metakaolin improved particle-soil interaction at the nanoscale. as a result, even a little percentage of the metakaolin changed the physiochemical properties of the soil. it caused flocculation in loess particles, as well as increased particle bonding. as a result, improved collapsibility was attributed to increased inter-particle bonding. 5. conclusions the following findings may be taken as conclusions from the results of this study: • the collapse potential of loess soils increased as the immersion stresses increased from 100 kpa to 500 kpa. • by adding perlite up to 8 % of the dry mass, the collapse potential of loess soils decreased and both loess soils were classified as slight collapsible soils after perlite-treatment. with the aid of interlocking force, perlite particles bond soil particles. consequently, soil particles were not able to slide on each other and after loading the soil settlement was significantly reduced. • with increasing metakaolin up to 8 % of the dry mass, the collapse potential of loess soils reduced. this can be the consequence of the pozzolanic and chemical interactions between soil and metakaolin. • as the content of metakaolin increased, the ultrasonic pulse velocity increased. with increasing perlite, the ultrasonic pulse velocity decreased. references [1] s. nokande, m. a. khodabandeh, s. s. hosseini, s. m. hosseini. collapse potential of oil-contaminated loessial soil (case study: golestan, iran). geotechnical and geological engineering 38(1):255–264, 2019. https://doi.org/10.1007/s10706-019-01014-9 [2] z. yuan, l. wang. collapsibility and seismic settlement of loess. engineering geology 105(1-2):119–123, 2009. https://doi.org/10.1016/j.enggeo.2008.12.002 [3] m. a. khodabandeh, s. nokande, a. besharatinezhad, et al. the effect of acidic and alkaline chemical solutions on the behavior of collapsible soils. periodica polytechnica civil engineering 64(3):939–950, 2020. https://doi.org/10.3311/ppci.15643 [4] p. li, h. qian, j. wu. environment: accelerate research on land creation. nature 510(7503):29–31, 2014. https://doi.org/10.1038/510029a [5] m. a. khodabandeh, g. nagy. collapse potential of loess soils contaminated by synthetic and landfill leachates. fifth symposium of the macedonian association for geotechnics 1:397–406, 2022. 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the mohammad-abad railway station soil in semnan. geotechnical and geological engineering 37(4):2847–2860, 2019. https://doi.org/10.1007/s10706-018-00800-1 [40] m. arabani, b. a. lasaki. behavior of a simulated collapsible soil modified with xps-cement mixtures. geotechnical and geological engineering 35(1):137–155, 2016. https://doi.org/10.1007/s10706-016-0092-9 52 https://doi.org/10.1016/j.conbuildmat.2019.03.306 https://doi.org/10.1016/j.conbuildmat.2019.03.306 https://doi.org/10.1016/j.geotexmem.2014.11.007 https://doi.org/10.1007/s10706-018-00800-1 https://doi.org/10.1007/s10706-016-0092-9 acta polytechnica ctu proceedings 45:1–8, 2023 1 introduction 2 materials and methods 2.1 properties of soils 2.2 properties of stabilization materials 2.3 laboratory program and sample preparation 2.4 collapse testing 2.5 ultrasonic pulse velocity testing 3 results 3.1 effect of immersion stress on the collapse potential of soils 3.2 effect of perlite and metakaolin on the collapse potential of soils 3.3 effect of perlite and metakaolin on the ultrasonic pulse velocity of soils 4 discussion 5 conclusions references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2025.54.0062 acta polytechnica ctu proceedings 54:62–66, 2025 © 2025 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague mechanical properties of cement composites with plasma-treated waste glass aggregate aleš paličkaa,∗, zdeněk prošeka,b, pavel tesáreka, oleg babčenkoc a czech technical university in prague, faculty of civil engineering, department of mechanics, thákurova 7, 166 29 prague 6, czech republic b czech technical university in prague, university centre for energy efficient buildings, třinecká 1024, 273 43 buštěhrad, czech republic c czech academy of sciences, institute of physics, cukrovarnická 10/112, 162 00 prague 6, czech republic ∗ corresponding author: ales.palicka@fsv.cvut.cz abstract. this research focuses on evaluating the effect of low-temperature plasma surface treatment of finely ground recycled glass on the mechanical properties of cement composites. the use of waste glass in construction represents an environmentally friendly alternative, but its application may cause alkalisilica reaction (asr), leading to material degradation. to prevent this phenomenon, plasma treatment was applied with the aim of improving the adhesion of glass particles to the cement matrix, enabling the partial replacement of traditional filler materials. in cement composites, glass primarily functions as an alternative fine aggregate. its use offers several potential advantages, including improving the pozzolanic properties of the composite, meaning it reacts with calcium hydroxide, thereby contributing to the formation of additional binding phases. in this way, glass not only replaces traditional fillers but also enhances the long-term mechanical properties of the material. however, inappropriate particle size or high reactivity of the glass may lead to asr, which causes cracking and degradation of the material. plasma treatment was employed to mitigate these negative effects by enhancing the adhesion between glass particles and the cement matrix, thereby increasing the overall cohesion, strength, and resistance of the composite to environmental influences and degradation. the samples were tested for compressive strength and bulk density after 28, and 168 days of curing. the results indicated that plasma treatment, particularly with oxygen, led to a moderate improvement in mechanical properties compared to untreated samples, but the values did not reach the levels observed in the reference sample without glass powder. keywords: waste glass, cold plasma, surface treatment, mechanical properties, cement composite. 1. introduction waste glass has been extensively studied as a replacement for traditional fillers in cement composites, primarily due to its potential for reducing environmental impact. its application is particularly significant in cases where recycling is difficult. however, waste glass can negatively affect the strength and durability of cementitious materials, mainly due to alkali-silica reaction (asr). studies have shown that finely ground glass can enhance the strength of the composite due to its pozzolanic properties, which react with calcium hydroxide [1, 2]. however, larger glass particles often lead to cracks caused by asr, which degrade the material. to minimize this negative effect, various material treatments have been tested, including the addition of chemical admixtures [3, 4]. plasma treatment of waste glass has proven to be an effective approach to improving the interaction between glass and the cement matrix. low-temperature plasma can modify the chemical structure of the glass surface, increasing its adhesion and reducing surface contamination by carbon. these modifications promote better bonding between the glass powder and cement, thus improving the mechanical properties of the resulting material [2, 5]. the experiment examines the mechanical properties of plasma-treated glass powder, utilizing oxygen and hydrogen plasma. the results build on previous studies that demonstrated the promising potential of replacing traditional filler with ground glass powder at up to 20 % content [6]. before production, the glass powder was treated using plasma technology, which is gaining popularity due to its ability to modify the surface properties of materials without altering their internal structure. in practice, plasma coatings are applied to concrete surfaces to enhance the durability of the concrete against wear, chemical influences, and reduce porosity, which extends the lifespan of structures in highly demanding conditions, such as water management infrastructure [7, 8]. plasma surface treatment of glass improves adhesion between glass and coatings or adhesives, thereby increasing the wettability of the glass surface [9]. in agriculture, low-temperature plasma is used for seed treatment to enhance germination and plant growth, with plasma generating reactive oxygen species that inactivate pathogens and reduce the need for chemicals [10–12]. 62 https://doi.org/10.14311/app.2025.54.0062 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 54/2025 mechanical properties of cement composites . . . figure 1. sem micrograph showing the microstructural characteristics of the composite material,at 500× magnification. the left image shows an overview of the surface morphology, while the right highlights particle boundaries for the purpose of quantitative image-based particle size analysis. figure 2. particle size distribution: histogram for glass powder is based on data obtained using sem microscopy – left, for glass powder and cement obtained by laser granulometry – right. low-temperature plasma is an ionized gas containing electrons, ions, and neutral particles, and at low temperatures, it does not alter the basic structure of the material. the application of plasma to fine waste glass additives causes chemical and physical changes on the surface, increasing surface reactivity and improving adhesion between the glass and the cement matrix. this process is carried out in a special chamber. 2. materials and samples for this experiment, low-temperature plasma technology utilizing oxygen and hydrogen was selected, considering the particle size of the finely ground glass powder and its compatibility with this process. this method was chosen because it allows for effective surface treatment of fine glass particles without affecting their internal structure. the materials used included portland cement cem i 42.5r from the radotín production plant (českomoravský cement, a.s.), characterized by rapid strength development and fast heat evolution during hydration, making it suitable as a binder for this type of application. as an alternative filler based on silicon dioxide (sio2), finely ground waste glass was used, originating from shards intended for disposal. the reference material was sourced from recifa, a.s., specifically the refaglass vintířov product from silo no. 5. this was finely ground waste glass with a particle size of less than 90 µm. this waste glass, derived from glass shards that would otherwise end up in landfills, was used as an alternative binder for this research. this alternative not only contributes to environmentally friendly waste disposal but also offers the potential for innovative use of waste materials in modern construction composites. sample preparation for sem analysis may result in the displacement of ultrafine particles due to air flow, causing partial segregation and the appearance of a superficial particulate layer. this phenomenon is recognized as a preparation-induced artifact. the glass powder exhibited a relatively narrow particle size distribution, with the majority of particles falling within the 1–10 µm. range, as corroborated by image analysis of sem micrographs see figure 1 and figure 2. in the sub-20 µm fraction, the particle size 63 a. palička, z. prošek, p. tesárek, o. babčenko acta polytechnica ctu proceedings marking cem i 42.5r [kg] sio2 [kg] water [kg] type of waste materials treatment ref 0.300 0.105 without treatment s1_10_ref 0.270 0.030 0.105 without treatment s2_20_ref 0.240 0.060 0.105 without treatment s1_10_o 0.270 0.030 0.105 oxygen plasma treatment s2_20_o 0.240 0.060 0.105 oxygen plasma treatment s1_10_h 0.270 0.030 0.105 hydrogen plasma treatment s2_20_h 0.240 0.060 0.105 hydrogen plasma treatment table 1. composition of test mixtures. distribution curve closely aligns with that obtained from laser diffraction granulometry, indicating consistency between both analytical techniques. the analysis was conducted using five out of six available sem micrographs acquired at 500× magnification. under these imaging conditions, the lower detection limit for particle size was estimated to be approximately 0.5 µm. the total evaluated area for image analysis was 100 µm2. the grain geometry of the particles was characterized as angular to sub-angular, ranging from circular to oval, with an aspect ratio of 1.0 to 1.4. these morphological traits reflect the processing and preparation of the materials, as well as their influence on the overall composite properties. as shown in table 1, the samples were prepared by precisely weighing the raw materials, including water, cement, and waste glass powder, according to the specified proportions and treatment type. the treatments included reference samples without modification, plasma-treated samples using oxygen, and plasma-treated samples using hydrogen. the weighing was conducted using a laboratory scale with an accuracy of ±1 g. all mixtures were prepared with the same waterto-cement ratio (w/c = 0.35) to maintain consistent mechanical properties. after mixing, the mixtures were poured into molds with dimensions of 20 × 20 × 100 mm and allowed to set at 20 °c. after 24 hours, the samples were demolded, labeled, and stored in a water bath at 20 °c until testing. a total of 28 samples were prepared, each of which was divided into smaller blocks of 20 × 20 × 20 mm for later testing at 28 and 168 days. the sample dimensions are not standardized, but literature indicates that the “size effect” does not influence the final properties [6]. 3. experimental method the experiment was conducted to test the compressive strength of samples in the form of beams with dimensions of 20 × 20 × 100 mm, which were cut into smaller blocks of 20 × 20 × 20 mm. after processing, the samples were measured and weighed. the prepared sample was then placed into a hydraulic press. using specialized software, the measurement was initiated, with the hydraulic press moving at a speed of 0.5 mm min−1. the test continued until the sample lost its structural integrity, and the maximum force was recorded. the compressive strength was calculated using the following equation: f = fmax a , (1) where f is the compressive strength (in pa), fmax is the maximum force applied during the compression test (in n), and a is the cross-sectional area of the specimen (in m2). 4. results and discussion this experiment builds upon previous research. in the first phase, mixtures containing glass powder were tested, and it was found that substitutions of up to 20 % could achieve acceptable mechanical properties compared to the reference sample [6]. however, higher proportions of glass powder led to a significant decrease in strength, particularly at the 20 % substitution level, where values below 60 mpa were recorded. at 28 days, all samples showed an increase in strength; however, samples containing untreated glass continued to exhibit lower strength values compared to the reference compound. the samples, after 28 days of curing, were subjected to low-temperature plasma treatment using oxygen and hydrogen. the results demonstrated a moderate improvement in compressive strength following oxygen plasma treatment, with the treated samples approaching the strength levels of the reference sample. in contrast, the samples treated with hydrogen plasma showed slightly lower compressive strength values. final measurement, conducted after 168 days of curing, showed a continued increase in strength for the oxygen plasma-treated samples, while the hydrogentreated samples stagnated or slightly decreased. this decline may be attributed to the alkali-silica reaction (asr), which can cause microcracking in the pores of the material. to further investigate this, the samples will undergo microscopic analysis to detect the presence of any potential cracks caused by asr. in figure 3, the comparison of the mechanical properties of cement composites with different waste glass additives and the effect of plasma treatment (oxygen and hydrogen) on compressive strength can be observed. the reference sample exhibits the highest 64 vol. 54/2025 mechanical properties of cement composites . . . 0 10 20 30 40 50 60 70 80 90 ref s_10 s_20 s_10_o s_20_o s_10_h s_20_h c o m p re ss iv e s tr e n gh t [m p a] 28-days curing of the sample 168-days curing of the sample figure 3. comparison of the mechanical properties of cement composites with different waste glass additives and the effect of plasma treatment (oxygen and hydrogen) on compressive strength, with the standard deviation indicated in the results.. 1650 1700 1750 1800 1850 1900 1950 2000 2050 2100 2150 ref s_10 s_20 s_10_o s_20_o s_10_h s_20_h b u lk d e n si ty [ kg /m 3 ] 28-days curing of the sample 168-days curing of the sample figure 4. bulk density values of cement composites after 28 days and 168 days of curing, with the standard deviation indicated in the results. strength, confirming that the addition of untreated glass powder reduces the mechanical properties of the material (s_10 and s_20). after the application of low-temperature oxygen plasma (s_10_o, s_20_o), an improvement in strength was observed compared to untreated samples, with compressive strength reaching around 70 mpa. hydrogen plasma (s_10_h, s_20_h) has a similar effect, although some samples achieve lower strength values than those treated with oxygen plasma. this difference may suggest that different types of plasma have varying effects on the microstructure of the material. when examining the bulk density values shown in figure 4, the reference sample exhibits the highest bulk density, exceeding 2 050 ± 48 kg m−3 across all time periods (28 days, 168 days). this result confirms the expected properties of a cement composite without glass additives. samples containing 10 % (s_10) and 20 % (s_20) glass powder show a slight decrease in bulk density, particularly after 6 months of curing. this decrease can be attributed to the lower density of glass, which is less compact than the cement matrix. differences are observed in the samples subjected to plasma treatment. the oxygen plasma treatment (s_10_o, s_20_o) shows a slight increase in bulk density compared to untreated samples, suggesting that plasma treatment may have improved the material’s cohesion and density. in contrast, the hydrogen plasma treatment (s_10_h, s_20_h) shows greater variability in the results, especially after 168 days of curing. this may be due to the increased reactivity of the glass surface after treatment and subsequent reactions within the cement matrix, which may have affected the material’s stability. when comparing these findings with the compressive strength results in figure 3, it is evident that the microstructure, including particle size and morphology, plays a critical role. untreated glass powder, despite its fine size, showed a detrimental effect on strength at higher substitution levels, e.g., 20 %, likely due to its incompatibility with the cement matrix. however, plasma treatment, particularly with oxygen, improved the interaction between the matrix and glass particles, as demonstrated by increased compressive strength and bulk density values. the distribution of cement particles, primarily ranging between 10 and 50 µm, ensures adequate binding in the matrix. however, the finer particles of the 65 a. palička, z. prošek, p. tesárek, o. babčenko acta polytechnica ctu proceedings glass powder enhance the matrix’s packing density to some extent. notably, untreated glass powder exhibits a lower degree of particle interaction with the cement matrix, leading to reduced mechanical properties. plasma treatments showed potential to improve these interactions. specifically, oxygen plasma-treated samples demonstrated improved compressive strength and bulk density, suggesting enhanced cohesion at the particle-matrix interface. on the other hand, hydrogen plasma-treated samples showed less consistent results, potentially influenced by microstructural changes such as alkali-silica reaction (asr)-induced microcracking, as indicated by future microscopic analyses. 5. conclusion the results of this study confirm that low-temperature plasma treatment of finely ground waste glass can significantly enhance the mechanical properties of cement composites. the impact of plasma treatment depends on the type of medium used and the proportion of glass powder in the mixture. the reference sample, without any glass additives, demonstrated the highest compressive strength values exceeding 80 mpa and a bulk density above 2 050 ± 48 kg m−3, reflecting an optimal microstructure of the cement matrix. in contrast, samples containing 10 % and 20 % glass powder exhibited reduced mechanical performance, highlighting the challenges of efficiently integrating glass powder into composites. the use of oxygen plasma led to an improvement in the mechanical properties of glass-containing samples, with strength increases of up to 10 % compared to untreated samples. this improvement is attributed to enhanced surface properties and better adhesion between the glass particles and the cement matrix. conversely, hydrogen plasma treatment yielded mixed results. while some samples (s_10_h) showed improvements in compressive strength and bulk density, others (s_20_h) experienced a decline, likely due to accelerated alkali-silica reaction (asr), which causes microcracking and destabilization of the composite. future research should focus on detailed microstructural analysis using advanced techniques such as scanning electron microscopy (sem). understanding the effects of plasma treatment on the surface modifications of glass particles and their interaction with the cement matrix is essential for optimizing the process. moreover, investigating the effects of asr-induced microcracking and proposing mitigation strategies will be critical. long-term testing over a one-year cycle will be vital to validate these findings and support the practical application of plasma-treated waste glass in sustainable construction materials. acknowledgements this research was supported by the grant agency of the czech republic under project no. ga čr 23-05500s, titled “the influence of surface treatment of secondary silicon-based materials on their performance in cement composites”. additionally, the research was cofunded by the european project horizon 2020+, project no. 101058580, and by the ctu in prague, under project no. sgs24/037/ohk1/1t/11. the work was carried out in collaboration with undergraduate students j. lörinc and m. mozerov. references [1] n. b. govardhan, g. nakkeeran, r. dipankar. recycling of glass waste into concrete aggregate: enhancing mechanical properties and sustainability. asian journal of civil engineering 26:1–19, 2025. https://doi.org/10.1007/s42107-024-01181-y [2] i. mallum, a. r. mohd. sam, n. h. a. s. lim, n. omolayo. sustainable use of waste glass in concrete: a review. silicon 14:3199–3214, 2022. https://doi.org/10.1007/s12633-021-01152-x [3] a. siddika, a. hajimohammadi, w. ferdous, v. sahajwalla. roles of waste glass and the effect of process parameters on the properties of sustainable cement and geopolymer concrete – a state-of-the-art review. polymers 13(22):3935, 2021. https://doi.org/10.3390/polym13223935 [4] d. qin, y. hu, x. li. waste glass utilization in cement-based materials for sustainable construction: a review. crystals 11(6):710, 2021. https://doi.org/10.3390/cryst11060710 [5] z. remeš, o. babčenko, v. jarý, k. beranová. enhanced photoluminescence of plasma-treated recycled glass particles. nanomaterials 14(13):1091, 2024. https://doi.org/10.3390/nano14131091 [6] z. prošek, p. tesárek, a. palička, o. babčenko. mechanical properties of cement pastes containing plasma-treated silicon-based materials. in acta polytechnica ctu proceedings, vol. 47, pp. 108–112. 2024. https://doi.org/10.14311/app.2024.47.0108 [7] y. shi, y. wang, x. lv, et al. application of thermal spraying technology in ceramic-based coating for concrete surfaces. coatings 14(7):885, 2024. 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[2024-09-26]. https: //www.fariplasma.com/plasma-surface-treatment/ [10] j. barz, m. muller. plasma technology: key technology for production of functional surfaces. fraunhofer: institute for interfacial engineering and biotechnology igb, 2021. 20 pp. [11] d. yan, l. lin, m. zvansky, et al. improving seed germination by cold atmospheric plasma. plasma 5(1):98– 110, 2022. https://doi.org/10.3390/plasma5010008 [12] j.-s. song, s. b. kim, s. ryu, et al. emerging plasma technology that alleviates crop stress during the early growth stages of plants: a review. frontiers in plant science 11:988, 2020. https://doi.org/10.3389/fpls.2020.00988 66 https://doi.org/10.1007/s42107-024-01181-y https://doi.org/10.1007/s12633-021-01152-x https://doi.org/10.3390/polym13223935 https://doi.org/10.3390/cryst11060710 https://doi.org/10.3390/nano14131091 https://doi.org/10.14311/app.2024.47.0108 https://doi.org/10.3390/coatings14070885 https://www.afs.biz/en/products/der-plasmajet/ https://www.fariplasma.com/plasma-surface-treatment/ https://www.fariplasma.com/plasma-surface-treatment/ https://doi.org/10.3390/plasma5010008 https://doi.org/10.3389/fpls.2020.00988 acta polytechnica ctu proceedings 54:62–66, 2025 1 introduction 2 materials and samples 3 experimental method 4 results and discussion 5 conclusion acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2024.48.0034 acta polytechnica ctu proceedings 48:34–40, 2024 © 2024 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague tribo-technological features of laser powder bed fusion process: scratch and wear resistance of alsi10mg aluminium alloy danilo guarino, simone venettacci∗, vittorio villani, gennaro salvatore ponticelli, stefano guarino università degli studi niccolò cusano, roma, via don carlo gnocchi, 3, italy ∗ corresponding author: simone.venettacci@unicusano.it abstract. mechanical systems, regardless of their complexity, very often require that different parts must move relative to each other by sliding their surfaces, therefore appropriate tribological properties are needed. this request appears particularly evident for components fabricated through metal additive manufacturing processes, due to their typical high surface roughness. in the current study, the laser powder bed fusion technique with optimized parameters is used to produce samples made of alsi10mg alloy. their tribo-technological properties are investigated through progressive load scratch and dry ball-on-plate wear tests. along with a global characterization, a local analysis has been performed to identify any variations induced by the building direction. the friction coefficient and the wear rate are generally higher than as-cast specimens. finally, local trends suggest that the central parts of the samples average offer higher resistance to wear and scratch than the outer areas. keywords: laser powder bed fusion, tribology, scratch test. 1. introduction the present work deals with an investigation of tribotechnological features of alsi10mg components made by laser powder bed fusion (lpbf) [1]. this technique belongs to the innovative metal additive manufacturing (mam) technologies for the production of mechanical components and rapid prototyping [2]. starting from the geometry of a three-dimensional digital model, it is possible to generate an object even with a complex shape and high level of customization by simple overlapping of material layers [3]. compared to traditional technologies (such as casting, forging, etc.), mam ones offer the possibility to optimize the structure of the component, reduce processing waste, reduce time and costs for small series productions [4, 5], while also ensuring higher levels of production flexibility, greater sustainability and ease of innovation. on the contrary, the lack of knowledge and standardization, as well as the material anisotropy, and high surface roughness represent the main current limitations to the use of this technology in the industrial sectors [6]. in this area, it has already been verified how poor surface finish can be one of the main critical issues of mechanical parts produced by additive manufacturing (am), going to compromise their reliability in service [7]. increased performance can be achieved through various post-processing techniques [8], which, however, have proven to be difficult or unaffordable for complex geometries, as well as limiting in terms of manufacturing flexibility and finished product design, in addition to raising production time and cost [9]. the aluminum alsi10mg alloy analyzed in this study represents one of the most widely used materials in mam, due to its weldability and good properties of casting [10]. its excellent mechanical, physical and thermal properties, combined with low weight, make it a material of strong interest for applications that require high properties-to-weight ratio, such in the automotive and aerospace industries [10]. the properties of such lpbf-ed alloy have already been studied in the literature, but these studies have focused more on the influence of process parameters on microstructure, internal defect generation and thus on the mechanical performance of alsi10mg artifacts [11, 12]. only recently some literature papers have been interested in the tribological aspects of the alsi10mg alloy, going to study its friction and wear performance, performing tribological tests, such as linear reciprocating or pin/ball on disc ones [13, 14]. it follows that to date, tribological characterization of am specimens is still an under-explored topic, worthy of much more indepth investigation. therefore, the present study aims to elucidate the tribological performance of lpbf-ed manufactured alsi10mg alloy products with optimized parameters, characterized by means of progressive load scratch tests and ball on plate wear tests, to evaluate both global and local tribo-technological properties, in order to compare them to as-cast alsi10mg specimens, as well as to identify also any possible variations induced by the building orientation. 34 https://doi.org/10.14311/app.2024.48.0034 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 48/2024 tribological analysis of lpbf-ed alsi10mg alloy 2. materials and methods 2.1. preliminary experimental activities the material adopted for the lpbf process is a commercial pre-alloyed powder of alsi10mg (supplied by m4p materials solutions gmbh, magdeburg, germany), whose main characteristics, together with the chemical composition, are reported in a previous work [15]. the commercial additive manufacturing machine slm 280hl (by slm solutions group ag, lübeck, germany) was used to print the samples tribomechanically tested in this work, whose geometry and dimensions are reported in figure 1 (left). also, the main parameters of the am machine are reported in [15]. figure 1. (left) geometry and dimensions of alsi10mg specimens for tribo-technological characterization. (right) experimental setup for dry sliding ball-on-plate tests. in a previous study [15], three building orientations (bos) were investigated, i.e., horizontal (0°), vertical (90°), and tilted (45°), as well as two combinations of processing parameters (c#), based on different layer thickness and laser scan speed, as shown in table 1. three samples were tested for each experimental scenario. the operational parameters were fixed according to manufacturer’s suggestion, in order to produce full-dense samples. post-processing processes, conducted for each specimen family, involved only: (i) a wire electrical discharge machining cutting process; (ii) a t6 heat treatment (at 500 °c, for 30 min). operational parameters values units building orientation bo 0 45 90 ° control factors c# 1 2 layer thickness t h 30 50 µm laser scan speed ss 1650 930 mm/s laser power p 350 w hatch distance hd 100 µm laser beam focus diameter d 80 µm table 1. operational parameters and their levels: 3 bos × 2 c# = 6 experimental conditions. in previous works, the relative density was quantitatively measured on five different cross-sections for each experimental sample, analyzing the images acquired by means of a 3d laser microscope (lext ols4100, by olympus europa se & co. kg, hamburg, germany) by using image j software. the full methodology is reported in a previous paper [10]. in addition, the surface roughness was investigated using the highresolution 3d surface profiling system cli 2000 by talysurf (taylor hobson, leicester, uk), according to the standard iso 25178, acquiring the arithmetical mean and maximum heights of the profile (i.e., ra and rz) on 3×3 mm2 square areas, as reported in [15]. finally, preliminary tests were conducted to evaluate the uts (ultimate tensile strength), by using zwickroell testing machine (z010, by zwick gmbh & co., ulm, germany) on tensile samples, whose geometry was fixed according to the astme8/e8m standard, as reported in [10]. based on these findings from previous works in the literature, it was possible to determine the best scenario (i.e., the best combination of control factors and building orientation), on which specimens to conduct both global and local tribological tests, i.e., scratch and wear tests, in order to also verify possible anisotropic behavior of the specimens, i.e., an effect of the test area on the measured properties. 2.2. tribo-technological characterization the tribo-technological study was conducted through two different types of tests: (i) ball-on-plate tests in dry sliding configuration, to evaluate the local and global wear properties; (ii) progressive scratch tests with two different diamond indenters, to evaluate local and global scratch properties. wear tests were conducted using the ball-on-plate test tribometer by csm instruments (peseux, switzerland), in non-lubricated mode, at ambient temperature (∼ 25 °c), as reported in figure 1 (right). the following parameters were set: ceramic alumina al2o3 ball (6 mm diameter) as counterpart; 2 n of applied load (i.e., normal force); 5 mm of sliding distance for a total distance of 100 m; 30 hz of data acquisition frequency; 50 mm s−1 of translation speed; 10 mm rev−1 of rotation speed. the following were acquired during the tests: coefficient of friction as a function of traveled distance or number of cycles; average coefficient of friction over time. in addition, the specimens were carefully weighed both before and after the test, so as to estimate the material removed during the test. scratch tests were instead conducted using the micro scratch tester by csm instruments (peseux, switzerland), in progressive load mode, i.e., with a progressively increasing load from 0.03 n to 30 n, at ambient temperature (∼ 25 °c). the test involves the succession of 3 phases [16]: (i) profiling of the starting surface (low load, pre-scan); (ii) penetration (scan), recording the normal (n) and the tangential (t ) forces, as well as the penetration depth (pd); (iii) profiling of the tested surface after elastic 35 d. guarino, s. venettacci, v. villani et al. acta polytechnica ctu proceedings recovery, so as to measure the residual depth (rd) (low load, post-scan). the following parameters were set: 3 mm of track length; a loading rate of ∼ 20 n min−1; 2 mm min−1 of speed (during scan phase). two different rounded conical rockwell c diamond indenters, i.e., with a tip diameter (ϕ) of 100 and 200 µm, respectively, were used in order to perform a local characterization of scratch resistance, from which to also derive a global average behavior. as reported in figure 2, three different areas were examined for both types of tests, i.e., one scratch and one wear trace were produced in the central area, while two more were traced in the side areas, placed about 11 mm from the central trace, to evaluate also the effect of the distance from the support structure during the manufacturing process. so, three equally spaced scratch and wear traces were produced on each sample, testing three different areas (i.e., one trace for each). figure 2. examined zones of the alsi10mg alloy specimens (please note that the figure refers to a specimen produced with a bo of 90°). 3. results and discussion 3.1. results of preliminary investigation table 2 reports the results with respect to preliminary analyses reported in previous literature papers, used in order to determine the family of specimens with superior mechanical and morphological performance, on which to conduct both global and local tribological tests, i.e., scratch and wear tests. preliminary results reported in a previous work [15] showed that specimens grown in a 90° orientation ensured the lowest porosity, specifically achieving the highest mean density of about 93.77 % for the c#2 combination. comparison with a gravity cast produced foundry specimen, however, showed a significant difference in relative density, attributable to widespread porosity within and on the surface of the material. at the same time, specimens produced according to c#2 combination showed the lowest roughness values, although significantly higher than alsi10mg specimens produced by hot pressing die casting (hpdc) [10]. finally, the analysis of uts results showed that the combination of c#2 parameters with 90° growth direction ensures the maximum uts (i.e., 377 ± 30 mpa), higher compared to traditional manufacturing processes (i.e., 333 ± 33 mpa) [10]. by virtue of these considerations, it appears evident that c#2 samples (i.e., th = 50 µm, ss = 0.93 m s−1) with bo = 90° offer the best mechanical and morphological performances. local tribological tests, i.e., scratch and wear tests, were then conducted on that family of specimens to study the effect of the test area on the above properties. from which tests it was also possible to derive a global average behavior of the specimens. 3.2. results of tribo-technological investigation figure 3 reports the summary of results about wear tests, i.e., ball-on-plate tests, conducted in dry sliding configuration. it is pointed out that, in agreement with [14, 15], it was decided to remove, from the calculation of the average and standard deviation values, the measurements of the friction coefficients acquired in the first 20 m of sliding distance, i.e., in the first 650 s of testing. therefore, the running-in phase of the material, in which the values of the friction coefficients are much higher than in the subsequent steady-state phase, was not considered, because of an initial contact limited only to the asperities of the surfaces of the tested specimen and the ceramic counterpart. figure 3. average and standard deviations (sd) values of weight loss and mean friction coefficient for each of the three zones examined. please note that three trials were conducted for each zone (i.e., one for each of three samples). it is worth noting how in the first zone the mean friction coefficient is lower than in the other ones, underlining a worse layer adhesion, so a lower tangential force needed to trigger the layer cracking and delamination mechanisms, according to [13, 17]. 36 vol. 48/2024 tribological analysis of lpbf-ed alsi10mg alloy properties lpbf – c#1 lpbf c#2 hpdc 0° 45° 90° 0° 45° 90° relative 82.93 86.18 93.36 83.47 87.72 93.77 99.5 density, % ± 8.92 ± 8.99 ± 2.78 ± 7.19 ± 7.28 ± 4.87 ± 0.30 ⋆ ut s, mpa 253 309 321 333 357 377 333 ± 57 ± 20 ± 30 ± 33 ± 26 ± 30 ± 33 ra, µm 8.52 9.69 10.33 3.18 3.27 4.12 1.8 ± 2.78 ± 3.32 ± 3.60 ± 0.63 ± 0.60 ± 0.74 ± 1.4 rz, µm 44.8 51.4 52.2 20.8 22.0 24.8 -± 13.3 ± 12.0 ± 15.2 ± 4.0 ± 4.3 ± 4.1 ⋆ value for gravity cast production [14]. table 2. comparison of the morphological and mechanical properties between different families of lpbf samples and traditional hpdc ones. these phenomena are therefore more significant in the first zone, the greater magnitude of which consequently results in a higher weight loss in the zone closer to the support structure, as well shown in figure 3, and an anisotropic behavior of the alsi10mg alloy, in agreement with [18]. globally, however, we show an average weight loss value of ∼ 0.716 mg, with an average wear rate (calculated according to [14]) of ∼ 1.429 · 10−3 mm3 (nm)−1 and an average coefficient of friction of ∼ 0.499 . in agreement with previous literature papers [13, 14], the wear rate is higher than with foundry specimens, under the same load and test distance, as well as the friction coefficient is higher, being affected on the one hand by the porosity of the material, according to [15, 17], which facilitates the mentioned phenomena of layer cracking and delamination, and on the other hand by the different and heterogeneous contact conditions, according to [17], due to the presence of overlapping layers of material by additive means, rather than with a solid foundry specimen. to confirm the previously reported indications about the anisotropic behavior of the lpbf-ed alsi10mg material, local scratch resistance tests were conducted, as operated in [9, 19]. as reported in section 2.2, profiles were first measured for the three test specimens of the c#2 − 90° family, for the three test zones, the trends of which are shown in figure 4. for the sake of clarity of the figures and brevity of discussion, only the trends with respect to the first specimen of the family and not of the other two experimental replications will be shown. it can be seen that the profiles are highly irregular and rough, confirming what was already verified in section 3.1. in addition, it is shown that internal and surface porosity of the material and the presence of impurities (please refer to figure 5) are additional causes of high experimental variability. figure 6 and 7 show the trends of penetration and residual depths, respectively, for the three zones tested, as well as of the coefficient of friction, for tests conducted with a rounded conical rockwell c diamond indenter, with a tip radius of 100 µm. figure 4. profiles of the three examined test zones (t r#), for sample #1. figure 5. optical micro-graph of an impurity internal to the alsi10mg alloy (20 × objective). the results show an absence of linearity for both depths (i.e., pd and rd), due to roughness effect (or to an irregular surface), porosity effect (which could lead to major material removal), and presence of impurities (or to a local material variation). it is worth noting the presence of a final pile-up for the tests conducted in each zone (i.e., rd negative), emphasizing a consistent material springback. the coefficient of friction increases over time due to increased penetration into the material, resulting in a change in the contact conditions during the test, which from being 37 d. guarino, s. venettacci, v. villani et al. acta polytechnica ctu proceedings figure 6. penetration and residual depths for progressive scratch tests, conducted with a 100 µm tip radius diamond indenter, for sample #1. figure 7. friction coefficient for progressive scratch tests, conducted with a 100 µm tip radius diamond indenter, for sample #1. nearly point-like on a smooth surface, gradually becomes more extensive over multiple layers of additive material, resulting in an increase in the relative creep force between the two bodies, i.e., friction [15]. instead, figure 8 and 9 show the trends of penetration and residual depths, respectively, for the three zones tested, as well as of the friction coefficient, for tests conducted with a rockwell c diamond indenter, with 200 µm tip radius. the results show a pd trend similar to that obtained for the 100 µm tip; however, the friction coefficient is significantly lower, highlighting the different morphological contact conditions (i.e., for the same applied normal force, creep is less severe). from the micrographs in figure 10, an s-shaped rather than linear path of the tip during the scan phase can be seen, due to the effect of internal porosity and the presence of impurities. the images show the presence of cracking zones in the material due to the passage of the tip, with cracking propagation and material layer delamination. finally, figure 11 shows the averages and sds of mean penetration depth and friction coefficient, as the test zone and indenter tip vary. it is emphasized how, in contrast to the wear tests, in the computation of the average and sd values of mean friction coefficient, all acquired measurements were used, since a steady-state figure 8. penetration and residual depths for progressive scratch tests, conducted with a 200 µm tip radius diamond indenter, for sample #1. figure 9. friction coefficient for progressive scratch tests, conducted with a 200 µm tip radius diamond indenter, for sample #1. figure 10. optical micro-graphs of scratch tracks (20 × objective). condition does not seem to be reached (i.e., the friction coefficient increases progressively on average during the test). in addition, the initial phase of the test, in which the friction coefficient varies significantly, is very short, i.e., occurring for only the first tenths of a millimeter of test distance, for both tips analyzed, thus not significantly affecting the calculated average and sd values. the results for pd show that zone 2, i.e., the middle of the specimen, is characterized by more ductile behavior, due to a different thermal cycle, which is slower than the outer zones, in agreement with [15]. this results in a reduction of cracking and delamination, i.e., higher resistance to scratch test for both diamond indenters, highlighting an anisotropic behavior of the material, due 38 vol. 48/2024 tribological analysis of lpbf-ed alsi10mg alloy figure 11. average and sd values of mean penetration depth and friction coefficient for each of the three zones examined. please note that three trials were conducted for each zone (i.e., one for each of three samples). to a layer-by-layer building, in agreement with [18, 19]. while, about the mean friction coefficient, it is really influenced by the indenter geometry, which causes different test severity and creep resistance. the effect of the tip geometry is indeed little in terms of mean pd, whose average value drops from ∼ 41.74 µm for 100 µm tip, to ∼ 40.60 µm for 200 µm tip; while it causes a substantial reduction in the mean friction coefficient, from ∼ 0.456 to ∼ 0.276 . 4. conclusions this work deals with a tribo-technological study to assess global and local features of laser powder bed fused specimens. in the following the main conclusions: • based on preliminary results, lpbf-ed samples with a 50 µm thickness of the powder layer, laser scan speed of 930 mm s−1 and 90° building orientation, provide the best tensile response, the lowest porosity and a good surface finishing; • progressive load scratch and dry ball-on-plate tests show an anisotropic behavior of the material, with higher resistance of the central areas, due to slower cooling that allows to generate more ductile structures (mean pd is ∼ 21.9 % lower); • no significant differences are shown in terms of scratch resistance as the indenter changes, while the mean friction coefficient is ∼ 39.4 % lower, due to different morphological contact conditions; • finally, wear tests show an overall behavior in terms of dry wear resistance still far from as-cast specimens, with higher wear rate and friction coefficient. possible future developments of the work here presented concern the possibility of expanding both the metal alloys analyzed, studying other materials of strong interest in am, such as 316l stainless steel, and the mam technologies studied to include other types, such as selective laser sintering, electron beam melting, and directed energy deposition. through a crosssectional analysis at the level of mam materials and technologies, it would be in fact possible to produce tribo-technological optimization proposals also depending on the additive manufacturing technology, based on the 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polytechnica ctu proceedings 46:85–93, 2024 © 2024 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague ai-enabled transition to smart european cities noor marji∗, michal kohout, lijun chen, gülbahar emir isik, akshatha ravi kumar czech technical university in prague, faculty of architecture, thákurova 9, 160 00 prague 6, czech republic ∗ corresponding author: noor.marji@fa.cvut.cz abstract. smart cities continue to be discussed throughout europe as a result of the continent’s rising urbanization and the need for sustainable development. artificial intelligence (ai) has the potential to significantly promote this shift by assisting cities in becoming more effective, sustainable, and receptive to the requirements of their residents. the goal of this study is to examine the potential and difficulties of ai in urban development and present a framework for incorporating ai into city planning and management in european cities. this is done by analyzing case study examples from european cities and examining primary and secondary data sources, with the aim of providing a comprehensive framework for the sustainable integration of ai systems. this study presents a set of ethical and inclusive ai criteria, such as transparency, inclusion, and accountability, to enable responsible ai research and implementation. it continues by emphasizing the need for efficient ai integration in smart cities and pushing for a holistic ai-enabled transition to inclusive and sustainable smart cities. keywords: smart cities, artificial intelligence, europe, urban planning, transition. 1. introduction rapid urbanization and rising demands for sustainable development have given smart cities prominence across europe [1, 2]. as cities grow larger and more complex, innovative solutions must be found to assist cities in using resources more efficiently while improving inhabitants’ quality of life and encouraging sustainable development [2]. artificial intelligence (ai) has emerged as a possible game-changer in this area, providing new avenues for data-driven decision-making, urban system optimization, public participation, and citizen engagement [3, 4]. integrating ai technology in urban planning and development could yield several advantages, such as enhanced efficiency, lower costs, improved public services, and citizen involvement [2, 4]. ai’s ability to respond to residents’ needs while supporting sustainable growth and mitigating environmental impact makes ai particularly effective at guiding smart cities [2, 5]. however, using ai poses certain hurdles such as ethical concerns regarding algorithm bias as well as data privacy considerations [5, 6]. the identified knowledge gaps in the field of aienabled transition to smart european cities are twofold. firstly, there is a lack of a comprehensive framework that addresses the sustainable integration of ai systems into city planning and management, particularly suited for the diverse urbanization challenges faced nowadays by european cities [2]. such a framework should encompass ethical considerations and inclusivity criteria. secondly, existing literature often lacks collective case study analyses from various european cities, essential for understanding diverse contexts, challenges, and successful ai adoption models [3, 4]. considering these gaps, this paper examines the potential and difficulties of implementing ai in urban development and presents a framework for incorporating ai into city planning and management in european cities, which is the planned output of this research. 2. methodology this research incorporates a qualitative data collection and analysis methodology and outlines a systematic approach to investigate the potential and challenges of ai in urban development and the subsequent integration into smart european cities. it covers literature review, case studies, framework development, ethical considerations, data analysis, and integration of findings to drive informed recommendations. (a) literature review and data collection • primary and secondary sources: review of primary literature such as academic journals, conference papers and reports pertaining to ai applications in urban development and smart cities within european contexts. (b) case studies • criteria for selection: the cities were selected based on their use of artificial intelligence in urban development, considering factors like geographic location, population size, economic profile and technological infrastructure. (c) framework development • review of existing frameworks: comprehensive evaluation of existing ai-driven urban development frameworks in order to identify gaps, strengths and weaknesses. 85 https://doi.org/10.14311/app.2024.46.0085 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en n. marji, m. kohout, l. chen et al. acta polytechnica ctu proceedings (d) ethical criteria and guidelines • exploration of existing ethics for ai implementation, with emphasis on transparency, inclusivity, and accountability. • customization for european context: adjusting ethical criteria so they fit with european cultural, legal, and societal contexts. (e) analysis and evaluation • qualitative analysis: examining qualitative data in the form of case study insights to extract key themes and perspectives related to ai-enabled smart city development. (f) integration and recommendations • synthesis of findings: from our research results, we propose an ai integration framework in european cities that prioritizes responsible and inclusive ai practices. 3. urban planning and sustainable cities in europe in the 1980s, the brundtland report [7] first proposed sustainable development as a concept; defined as development that meets present generation needs without undermining future ones’ abilities to meet them [7], its concept has since become the cornerstone for european projects promoting such efforts. the european union (eu) is a leader in supporting sustainable development through policies and programs, like its 2013 environment action program which established an environmental action framework including sustainable cities as an essential focus area [8]. urban agenda 2016 also seeks to enhance sustainable urban development while simultaneously raising the quality of life across european cities [1]. smart cities have recently gained attention as an innovative approach to urbanization and sustainable development issues. a smart city employs technology and data to enhance inhabitants’ quality of life, improve urban services and promote sustainable development [3, 4]. ai technology plays a pivotal role in smart city design by offering new avenues for datadriven decision-making and system optimization [2–5]. 4. artificial intelligence (ai) in urban planning ai has the potential to revolutionize urban planning by offering new tools for data analysis, modeling, and decision-making. ai utilizes algorithms and machine learning techniques to assess massive datasets in order to make predictions or suggestions based on patterns found within them [9–11]. ai can assess various data types relevant to urban planning demographic data, transportation data, energy consumption information as well as environmental considerations [10, 11]. through analysis of such data, ai helps urban planners make more educated decisions related to urban growth, land use planning issues along with various other vital variables [3, 6]. predictive modeling is one of the core applications for ai in urban planning, helping forecast population shifts, fluctuations in energy demand, and transportation congestion based on historical data [12–15]. these predictions can then inform planning decisions as well as investment decisions [2, 3]. optimizing land use and transportation systems also uses ai extensively: ai may discover more effective methods to utilize urban spaces while carrying people and products, while simultaneously improving traffic flow, decreasing congestion levels, and expanding public transportation services [13–15]. ai can also be leveraged to track and manage urban systems in real-time. by collecting and analyzing data from sensors and other sources, it provides realtime feedback on operations of urban systems like energy usage, air quality management, and waste disposal [6]. with this real-time feedback at their disposal, planners, stakeholders, and authorities can identify new development opportunities or alter system performance according to changing requirements [2]. lastly, ai may also help encourage citizen participation in urban planning through providing access to data and information with ai assessments of public input/preferences assessments to facilitate citizen involvement [16]. 4.1. key features of ai-enabled urban planning four key features characterize ai-enabled urban planning in smart cities, which are data-driven decisionmaking and predictive modeling, optimization of land use and transportation systems, use of sensor data for real-time monitoring and feedback, and integration of citizen input and feedback into the planning process, see figure 1. 4.1.1. data-driven decision-making and predictive modeling urban planners may use ai for data-driven decisionmaking and predictive modeling. such evaluation can involve collecting massive amounts of information from sensors, social media posts, and public records before using an algorithm to find patterns, trends, and connections human planners might miss; helping to gain greater insights into complex urban systems such as transportation management systems [17, 18]. ai can also predict future urban development trends by studying historical data and discovering patterns, providing useful forecasting information such as public transit demand or infrastructure expenditure planning requirements. predictive modeling also allows urban planners to anticipate possible problems before they occur for instance, flood or other natural disaster predictions inform the creation of robust infrastructures for future development [19]. 86 vol. 46/2024 ai-enabled transition to smart european cities figure 1. key features of ai-enabled urban planning for smart city transitions. predictive modeling and data-driven decisionmaking can improve urban planning precision and effectiveness, helping planners use ai to make more informed decisions based on data rather than intuition or preconceptions. unfortunately, ai algorithms may reflect any biases present in training data which results in decisions being inequitable or unfair [20, 21]; an over-reliance on predictive modeling may result in rigidity within the planning process as planners become overly dependent on previous data without taking into account new patterns or situations changing from what was originally planned. 4.1.2. optimization of land use and transportation systems ai algorithms can analyze traffic data to use its results to maximize traffic flow and avoid congestion. traffic lights, for example, can be automatically modified based on real-time traffic conditions to reduce emissions and wait times [22]. furthermore, ai may also be employed in optimizing public transit systems by analyzing passenger demand information such as route performance statistics or vehicle availability data in order to enhance service efficiencies while reducing costs [23, 24]. urban planners can make more informed decisions regarding land use and urban development projects by uncovering underutilized sections of cities or locations with significant development potential by studying data such as population density, land use patterns, and zoning rules. optimized transportation system designs have also proven their ability to ease traffic congestion, enhance air quality and make cities more livable [22]. studies have shown that ai-aided planning for transportation networks could lower costs while expanding accessibility for marginalized groups [24–26]. optimizing transportation systems, however, could increase reliance on vehicles and individual forms of transport, contributing to air pollution and climate change. meanwhile, optimizing land use could result in the displacement of communities or the removal of green spaces, both of which would negatively impact the quality of life. 4.1.3. use of sensor data for real-time monitoring and feedback utilizing sensor data requires placing sensors throughout a city in order to collect information on air quality, noise levels, temperature levels, and energy use [23]. by collecting and analyzing real-time data, urban planners can gain insight into the current state of cities and make more informed decisions regarding resource allocation and urban growth [24]. sensors may be utilized to detect levels of air pollution to guide the deployment of air purifiers or mitigation measures, as well as detect energy waste within buildings to provide opportunities for improvement [25]. sensor data can provide municipal planners with useful data as well as real-time feedback from citizens; providing real-time traffic and public transport updates can aid residents in making decisions about their daily commute [26], while air quality monitoring allows consumers to reconsider engaging in outdoor activities in areas which could harm their health [27]. by giving individuals real-time updates of how their city is being developed they may become more involved in the decision-making process as they advocate for policies reflecting their beliefs and interests [27]. 4.1.4. integration of citizen input and feedback into the planning process historically, urban planning was typically carried out from the top down with city officials making choices without community consultation or involvement. ai technologies provide new avenues of participation for citizens, enabling engagement through public participation opportunities throughout various planning processes. digital platforms, online forums, social media platforms, and mobile applications provide urban planners with opportunities to gather citizen opinions on various urban planning matters such as transportation, housing, and public services. their feedback can then 87 n. marji, m. kohout, l. chen et al. acta polytechnica ctu proceedings figure 2. framework for ai integration towards smart city transition. be studied and integrated into planning processes allowing urban planners to design more responsive plans suited to citizen feedback [28, 29]. urban planners can gain real-time insight into people’s issues and ambitions using machine learning algorithms applied to social media data analysis, providing real-time insight that allows for enhanced planning decisions as well as greater citizen participation. furthermore, immersive planning experiences can be created using visualization technologies such as virtual reality or augmented reality technologies for maximum engagement with the proposed environmental changes. one challenge lies in making sure the feedback reflects all aspects of community opinions rather than simply responding to vocal minority interests, while simultaneously using it meaningfully as input into planning decisions versus simply giving the appearance of citizen involvement without altering decision-making processes. 5. case studies: ai in european smart cities a number of european cities – barcelona, amsterdam, and copenhagen among them – have launched smart city programs covering topics as varied as energy efficiency, transportation needs management, waste removal management, and citizen involvement [4]. barcelona, spain: barcelona has successfully implemented an ai-powered traffic control system that uses real-time data to optimize traffic flow and alleviate congestion. this system collects information from traffic sensors, gps devices, and public transit systems using ai algorithms that predict patterns to alter lights accordingly; as a result of which travel times and air quality have both seen significant improvements [30]. helsinki, finland: “omastadi” is an ai-powered virtual assistant designed to increase citizen participation in city planning and decision-making processes. utilizing natural language processing and machine learning algorithms, omastadi understands citizen inquiries quickly while offering tailored answers – ultimately increasing transparency, accountability, as well as citizen involvement [28, 29]. amsterdam, netherlands: amsterdam has implemented an artificial intelligence-powered energy management system that utilizes data collected via smart meters and building sensors to optimize energy use while simultaneously decreasing greenhouse gas emissions. ai algorithms use analysis results in energysaving actions like changing building temperatures or lighting levels resulting in reduced costs and greater sustainability [29, 31]. copenhagen, denmark: copenhagen has recently implemented an ai-powered trash collection system that utilizes sensor data and artificial intelligence algorithms to optimize waste collection routes and costs by anticipating waste generation patterns, optimizing collection schedules, and cutting collection costs [29, 32, 33]. these case studies illustrate how ai can be utilized to promote sustainable urban development and enhance citizen services in smart cities. however, it’s crucial that ai solutions be created and used responsibly and inclusively to address potential concerns while upholding ethical ai principles. 6. framework for ai integration towards smart cities to support the responsible development and application of ai in smart cities, a framework for integrating ai into municipal planning and management is necessary, see figure 2. the following is a suggested framework by the authors for ethical and inclusive ai guidelines, based on the previously conducted analysis and review: identify opportunities: the initial step should involve identifying areas where ai may help enhance urban systems and citizen services. for this to work effectively, collaboration among municipal officials, 88 vol. 46/2024 ai-enabled transition to smart european cities citizens, technology specialists, and experts must occur to find opportunities that fit with city values and interests. assess risks: once opportunities have been identified, the next step should be evaluating any risks posed by ai deployment. threats to data privacy, ethics, and social justice need to be assessed before ai implementation begins; residents and stakeholders should be included as much as possible during this process to address any potential concerns. develop guidelines for ethical and inclusive ai : once potential risks have been assessed, guidelines must be created that address ethical and inclusive ai rules. recommendations such as those set forth by international bodies (for example, the european commission ethics recommendations for trustworthy ai) should serve as inspiration when crafting such guidelines; principles like transparency, accountability, and inclusivity should all be included within such recommendations. implement ai solutions: once ethical and inclusive ai principles have been established, the next step should be implementing solutions that comply with them. this involves selecting appropriate algorithms, data sources, and governance frameworks to guarantee reliable, transparent, and accountable ai services. evaluate and adapt: finally, ai solutions must be constantly evaluated and modified so they remain productive while adhering to ethical and inclusive ai norms. this involves monitoring their effects on citizens and civic processes as well as, when needed, revising algorithms or governance frameworks accordingly. cities can ensure ai development and application take an inclusive, responsible approach which improves citizen services and quality of life while contributing to sustainable development by following such frameworks. 7. challenges, limitations, and further research 7.1. challenges and considerations for ai-enabled urban planning ai is an exciting prospect when applied to urban planning; however, its use also presents significant hurdles and potential pitfalls which must be considered carefully before embarking upon its application. below we explore several of these considerations in more depth for ai-enabled urban planning: 7.1.1. ethical considerations and algorithmic bias with ai’s growing use in urban planning comes an increased need to consider ethical concerns and algorithmic bias. ai systems can only ever be as objective as the data on which they are trained; if such data contains prejudices and inequities, then the ai reproduces or amplifies them [20, 34]. this could have serious ramifications for underrepresented groups as it will exacerbate existing imbalances [21, 35]. studies have demonstrated how using biased datasets with ai algorithms can lead to discriminatory outcomes across several areas, including criminal justice, employment practices, and lending practices [21, 35]. it is imperative that training data is carefully curated and assessed to reduce biases and promote fairness within deployed ai systems [36, 37]. ai’s use in lifealtering decisions such as housing, employment, and social services raises ethical concerns. ai algorithms may make judgments on age, color, gender, or socioeconomic status that lead to discrimination or unfair treatment [38]. therefore it is crucial that these algorithms’ decision-making processes remain visible, responsible, and equitable [39]. data privacy is also an ethical concern of ai systems. ai relies heavily on collecting, using, and storing personal data; any changes could have profound ramifications on individuals’ rights to privacy [40]. in order to uphold individuals’ privacy rights effectively, specific criteria for data collection, utilization, and storage must be developed [41, 42]. ai system development presents many risks related to algorithmic bias, which may arise at various points during ai’s development process – for instance during data collection, algorithm design, or model training processes. training data used for an ai system’s construction should ideally not contain bias if possible [35]. keeping in mind that implicit bias can only be avoided with sufficient variety and representation among datasets for the training of these models. to address these concerns, ethical and legal frameworks regulating ai systems used for urban planning development and deployment must be set in place. this involves setting ethical rules relating to ai creation and use, conducting audits for prejudice or discrimination issues within ai systems, as well as honoring individuals’ privacy rights [41]. furthermore, greater collaboration among developers, urban planners, affected communities, and affected individuals must occur to guarantee the ethical implementation of ai technologies [43, 44]. 7.1.2. data privacy and security since ai-enabled urban planning heavily relies on data, data privacy, and security are of critical importance. cities collect massive amounts of data from sources including sensors, social media accounts, and mobile phones which contain personal data including location details, health details, and biometric indicators [42] that could compromise people’s privacy rights if used for ai planning applications without consent. general data protection regulation (gdpr), introduced by the european union in 2018, requires organizations to gain consent before collecting or using personal data, identify its goals, and implement security measures aimed at protecting individual data rights while simultaneously imposing sanctions for 89 n. marji, m. kohout, l. chen et al. acta polytechnica ctu proceedings noncompliance [43]. when processing personal data in ai-enabled urban planning applications, compliance with gdpr standards is of utmost importance in order to preserve individuals’ rights [44–48]. gaining individuals’ consent before collecting or using their data, installing necessary safeguards against illegal access or usage, and giving individuals the option of access or removal is essential for ethical ai integration [44–50]. as part of gdpr compliance, cybersecurity risks associated with ai-enabled urban planning must also be assessed. intrusions and ransomware attacks can have severe repercussions for individuals as well as cities alike; cities should employ safeguards like encryption, multi-factor authentication, vulnerability assessments, penetration testing, as well as incident response processes in order to keep data protected against cyber threats [41]. 7.1.3. need for community engagement and participation in ai-enabled urban planning, community engagement and participation are key for successful implementation. ai can assist urban planners in processing and analyzing large amounts of data but cannot replace the human element in urban planning. community participation enables residents to provide input and feedback into planning decisions, ensuring that planning decisions meet residents’ needs and values [44]. public meetings, internet forums, surveys, and seminars are among many forms of community engagement strategies that allow citizens to express concerns, pose questions to planners, and receive replies [45]. community participation presents many unique challenges to all members of a given community, particularly people with impairments and limited language proficiency. reaching out to underrepresented populations such as low-income households or minorities that might otherwise not participate is equally vital [46]. integrating community input effectively into planning decisions presents another challenge, necessitating a two-way dialogue between citizens and planners requiring active listening from both parties and a clear explanation of how input from this source has been considered [47]. when properly conducted, community engagement and participation can lead to more inclusive and informed planning decisions while building trust between citizens and planners through greater transparency and accountability in planning processes. moreover, it reveals community concerns and priorities which might not be evident through data analysis alone [48]. 7.1.4. interdisciplinary collaboration and knowledge sharing to effectively incorporate ai in urban planning for transition to smart european cities, urban planners, policymakers, data scientists, social scientists, and community members must come together across disciplines to share knowledge. each brings different experiences, skillsets, perspectives, and interests that contribute towards crafting more holistic and equitable ai-powered urban planning approaches [3, 49]. technical language barriers between various fields make multidisciplinary collaboration challenging. urban planners might lack experience with ai while data scientists could struggle with the social and cultural context of urban planning. to overcome this barrier, organizations must promote communication and collaboration as much as possible, in addition to translating technical terms so nontechnical stakeholders can understand [24, 30]. moreover, interdisciplinary teams for ai-enabled urban planning projects must be formed where representatives from several departments or agencies come together as one team [46, 47]. this would aid in overcoming the fragmented decisionmaking trees and unnecessary bureaucratic elements typically involved in large-scale masterplanning. knowledge exchange is key for successfully applying ai in urban planning, with best practices, case studies, and research findings shared among municipalities and regions to reduce duplication. cities that share their knowledge can avoid reinventing the wheel while learning from the successes and failures of other cities – an approach encouraged by platforms such as the european union’s smart cities marketplace, where cities share and collaborate across boundaries [50]. 7.2. research limitations this study presents a conceptual framework for the incorporation of artificial intelligence (ai) in urban planning and deployment of smart cities using the european context as an example. however, it must be acknowledged that ai use for urban planning remains in its infancy, with challenges yet to be overcome, and ethical and social considerations to be carefully taken into consideration during the deployment of solutions incorporating this technology. while this paper explores the potential advantages and challenges of artificial intelligence in sustainable urban planning in europe, its findings may not apply equally elsewhere or under different conditions. furthermore, while the examples provide indications of potential ai applications within urban planning and transition to smart city solutions; they do not encompass all possibilities and specific ai technologies should be studied in further detail, preferably in defined urban contexts. this research proposes ethical and inclusive ai development and implementation criteria, although these should not be seen as exhaustive; their implementation should be regularly assessed and evaluated as ai technology rapidly advances and ethical concerns arise along with it. finally, this research does not assess the financial and technical feasibility of using ai for urban planning. policymakers, urban planners, and stakeholders must carefully examine both costs and resources necessary 90 vol. 46/2024 ai-enabled transition to smart european cities for adopting this form of planning, as well as the technical talent required for success. 7.3. recommendations for further research while ai may offer many benefits in urban planning, it’s essential to study alternative techniques as well. participatory planning approaches, which integrate citizen participation and feedback directly, may produce different outcomes and perspectives than aienabled planning approaches. long-term impacts assessment: many potential advantages of ai for urban planning may not become immediately evident and require further examination; future studies may focus on exploring long-term economic, environmental, and social impact associated with ai-enabled urban planning solutions. ai implementation evaluation: as ai technology becomes an integral part of urban planning processes, its efficacy needs to be assessed carefully. further research efforts might focus on measuring whether ai-enabled planning strategies meet their intended goals effectively. moreover, future studies should take steps to develop more detailed recommendations to address the ethical and societal repercussions of using ai in various stages of urban planning. future research should also assess how ai could be applied in urban planning across different regions and countries in order to gain a fuller picture of its deployment and efficacy in each setting, with a specific focus on tools such as computer vision and machine learning capabilities of ai. continued research of ai-enabled urban planning is necessary in order to guarantee its appropriate and efficient application, in the hopes of creating more liveable, equitable, and sustainable cities. 8. conclusion this article examined the potential and difficulties of ai in urban development and presented a framework for incorporating ai into city planning and management in european cities. cities could achieve increased efficiency, sustainability, and livability with ai being integrated into urban planning processes – such as data-driven decision-making, predictive modeling, optimizing land use and transportation as well as real-time monitoring using sensor data and involving citizens in the planning process are some aspects that have been presented herein – showing it has great promise as an instrument of building resilient european cities. this paper also explored several ai applications for sustainable urban planning in europe, such as green infrastructure and urban biodiversity, energy-efficient building design and retrofitting, smart transportation systems and mobility planning, and resilient urban design and disaster mitigation. however, ai adoption presents its own set of unique challenges, such as ethical concerns and algorithmic bias, data privacy and security, the need for community engagement and participation, and interdisciplinary collaboration and information exchange. responding to the identified gaps in research and in order to promote responsible and inclusive ai development and implementation, this article presents a framework for implementing ai technologies into transition to smart cities, which is the output of this research, that aims to facilitate responsible ai research and deployment. policymakers, urban planners, and stakeholders are encouraged to explore ai as a way of creating more livable cities that foster equality and sustainability while remaining cognizant of limitations associated with this study and further investigation on its efficacy as an urban planning strategy tool. future research should examine in more detail the capabilities of specific ai tools in defined urban contexts, which could provide a basis for examining the implementation of emerging technological tools in europe and elsewhere. furthermore, exploring its capacity for solving global concerns like climate change, pandemics, and natural disasters. when used appropriately with ethical considerations in mind, ai can serve as an indispensable resource for creating more resilient, inclusive cities in europe and elsewhere. acknowledgements this research was supported by grant: sgs23/081/ohk1/ 1t/15 by the faculty of architecture, czech technical university in prague. references [1] european commission. urban agenda for the eu, 2016. 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[2023-05-01]. https://smart-cities-marketplace.ec.europa.eu/ 93 https://doi.org/10.1145/3290605.3300830 https://doi.org/10.1177/2053951715622512 https://doi.org/10.1038/s42256-019-0088-2 https://doi.org/10.1080/1464935032000057209 https://doi.org/10.1080/01944366908977225 https://doi.org/10.1016/j.apgeog.2013.11.004 https://doi.org/10.1080/01944368208976167 https://smart-cities-marketplace.ec.europa.eu/ acta polytechnica ctu proceedings 46:85–93, 2024 1 introduction 2 methodology 3 urban planning and sustainable cities in europe 4 artificial intelligence (ai) in urban planning 4.1 key features of ai-enabled urban planning 4.1.1 data-driven decision-making and predictive modeling 4.1.2 optimization of land use and transportation systems 4.1.3 use of sensor data for real-time monitoring and feedback 4.1.4 integration of citizen input and feedback into the planning process 5 case studies: ai in european smart cities 6 framework for ai integration towards smart cities 7 challenges, limitations, and further research 7.1 challenges and considerations for ai-enabled urban planning 7.1.1 ethical considerations and algorithmic bias 7.1.2 data privacy and security 7.1.3 need for community engagement and participation 7.1.4 interdisciplinary collaboration and knowledge sharing 7.2 research limitations 7.3 recommendations for further research 8 conclusion acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2023.40.0061 acta polytechnica ctu proceedings 40:61–68, 2023 © 2023 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague analysis of tunnel excavation based on linear dfn-fem modelling martin lebeda∗, petr kabele czech technical university in prague, faculty of civil engineering, department of mechanics, thákurova 7, 166 29 prague 6, czech republic ∗ corresponding author: martin.lebeda@fsv.cvut.cz abstract. simulations of tunnel excavations have to take into account the natural occurrence of joints and faults in the surrounding rock mass, which dominantly control its mechanical response. in this paper, we present work in progress toward 3d finite element analysis of excavation using equivalent rock-mass properties derived from stochastically generated discrete fracture networks (dfns). the equivalent stiffness is determined by volume averaging. presently, we solve the problem linearly for an incremental change of the stress state. the fracture’s stiffness is assumed to depend on the initial normal stress acting in direction normal to it. however, within the solved incremental step, we assume the fracture’s stiffness to be constant. this assumption is acceptable for small stress changes. since the fractures represented in the dfn model have preferred directions, the equivalent stiffness is anisotropic. keywords: fractured rock mass, discrete fracture network (dfn), finite element method (fem), averaging procedure. 1. introduction according to the current state of art in the field of rock mechanics, brittle structures (or fractures, including faults and joints) have the dominant effect on the overall mechanical response of the rock mass [1]. underground structures, such as tunnels, are often situated in rock masses with naturally existing fractures. since it is necessary to take into account the fractures’ influence on the deformation due to the changes in the stress field during excavation, estimating mechanical properties of the rock mass is fundamental for efficient and safe design of tunnel structures. the finite element method (fem) is a tool commonly used for simulating tunnel excavation. in the case of a highly fractured rock mass, it would not be effective and feasible to explicitly represent each distinct fracture in the finite element mesh. it is admissible to include only a few largest fractures or localized deformation zones using e.g. interface elements, while the rest of the fracture network is represented by so-called equivalent continuum. the overall effective properties can be determined by averaging or homogenization procedure [2]. in this paper, we use the averaging procedure proposed by oda et al. [3] to determine the effective stress-strain relation. as the overall properties of fractured rock mass depend on the size of the simulated area, it is suitable to include the concept of representative volume element (rve) or statistical volume element (sve) [4]. the discrete fracture network (dfn) method is one of possible ways to describe the fractures’ geometry while taking into account data obtained from structural-geological survey, e.g. [5–8]. one of the most used approaches are the stochastically generated dfns, which extrapolate the in-situ data by means of statistical probability distributions. in this paper, we present work in progress toward developing and implementing in computer code an approach that connects averaging procedure performed on 3d stochastic dfn with finite element analysis of tunnel excavation. at the present stage of development, we adopt some simplifying assumptions: (a) we assume that the rock mass in the whole domain of the finite element (fe) model has uniform equivalent effective mechanical properties. (b) the effective properties are derived by averaging [3] over a representative volume element (rve) of the rock mass, in which shapes, sizes, and orientations of individual fractures are modeled by means of stochastic dfn. the rve size is based on a previous study [9] and corresponds with the size of the fe model. (c) individual fractures are treated by means of the “parallel plate model” [3], which means that the deformation response of a fracture to normal and shear stress is represented by two parameters accounting for the fracture’s roughness, friction angle etc. as well as its size and normal stress acting on it. (d) the fe analysis is performed linearly for an incremental change of the stress state. the fracture’s stiffness is assumed to depend on the initial normal stress acting in direction normal to it. however, within the solved incremental step, we assume the stiffness to be constant. we are aware that some of the assumptions may be limiting the validity of the procedure. for example, 61 https://doi.org/10.14311/app.2023.40.0061 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en martin lebeda, petr kabele acta polytechnica ctu proceedings parameter value volumetric density p30 [1/m3] 2.0 power law exponent α [-] 3.4 power law minimum fracture size amin [m] 0.3 fisher distribution concentration factor κ [-] 106 fisher distribution mean vector [-] variable volume of the dfn model [m3] 100 × 100 × 100 rve volume [m3] 15 × 15 × 15 table 1. parameters of dfns. the size of rve (b) should be chosen not only based on the scatter of the overall properties [9], but also with respect to the gradient of the stress field in the fe model. or, the linear solution (d) is acceptable only for small stress changes. these limitations will be resolved in a follow-up work. 2. dfn model – description of fractures’ geometry quantitative information about fracturing of rock mass is often acquired by structural-geological mapping of fracture traces on “observation windows”, such as rock outcrops or tunnel walls. typically, the minimum recorded trace length is on the order of 10−1 m. due to the enormous number of corresponding fractures in the rock volume, on the one hand, and the limited information available on the observation windows, on the other hand, the geometry of the fracture network is described by stochastically generated dfn. in this approach, the size and orientation of the fractures are assumed to follow certain probabilistic distributions, whose parameters are identified so as to match (in a statistical sense) the traces’ sizes, directions and density within the observation windows. a dfn is then generated as a set of spatially distributed polygons, whose sizes and orientations respect the calibrated probabilistic distributions [8]. since each fracture is explicitly described in the dfn model, the network cannot be efficiently discretized by a 3d fe mesh. however, it provides a suitable basis for the use of an averaging procedure. the fracture network models used in this study were generated by the dfram software [8]. fractures’ centers are positioned in the model volume by using poisson random generation process. fractures’ sizes are controlled by power law distribution with parameters amin, which is the minimum fracture size (location parameter), and α, which is the law’s exponent (shape parameter). size of the fracture a is defined as the radius of the circle circumscribed to the fracture, while the fractures are modelled as squares. the orientation of fractures is determined by fisher distribution, with parameters µ, which is the mean unit normal vector of fractures, and κ, which is the concentration parameter. as the purpose of this study is to demonstrate the solution methodology, we adopt some simplifications while modelling the fracture networks. we use dfns consisting of one geological set of fractures. values of the power law distribution parameters are the same for all models and they are based on a previous study [9]. we consider that all fractures are nearly parallel, which is achieved by setting the concentration parameter of fisher distribution κ to a high value of 106. we will, however, consider fracture sets with different orientations, for which different mean normal vectors µ will be used (see subsection 6.1). the volumetric fractures density p30, which controls the total number of fractures in the model, was approximately set based on report [10]. parameters used in this study are listed in table 1. 3. overall stiffness of the fractured rock mass evaluated by averaging procedure the calculation of overall effective parameters of fractured rock mass is based on the volume averaging relation proposed by oda et al. [3]: εij = 1 e [ (1 + ν)δikδjl − νδijδkl + ( 1 kn − 1 ks ) fijkl + 1 4ks (δikfjl + δjkfil + δilfjk + δjlfik) ] σkl =cijklσkl, (1) where e and ν are young’s modulus and poisson’s ratio of the intact rock, respectively, kn and ks are nondimensional parameters related to the fracture’s normal and tangent stiffness, respectively, δij is kronecker’s delta, and fij and fijkl are so-called second and fourth rank crack tensors, respectively: fij = 1 v m∑ p=1 s(p)l(p)n (p) i n (p) j , (2) fijkl = 1 v m∑ p=1 s(p)l(p)n (p) i n (p) j n (p) k n (p) l . (3) here v is the volume of the domain over which averaging is performed, s(p) is the area of p-th fracture 62 vol. 40/2023 analysis of tunnel excavation based on linear dfn-fem modelling figure 1. normal-stress vs. fracture-closure diagram [12] and schematic indication of the tangent stiffness evaluation. inside the sampling volume, l(p) is a typical size of the fracture equal to a diameter of a circle with the same area and n (p) i are the components of a unit vector normal to the fracture. note that the typical size l is assumed as a property of the fracture independent on the size of the sampling volume and it is calculated using the original area of the fracture generated in stochastic dfn. the calculation procedure of parameters kn and ks is described in detail in our previous work [11]. in brief, the parameters are expressed as: kn = 3π 8 + κn · l e , (4) ks = 3π 8 + κs · l e , (5) where κn and κs are the normal and shear stiffness (stress/relative displacement) of a fracture and l is the fracture’s size. the normal stiffness parameter κn is to be obtained experimentally. it depends on whether the fracture undergoes loading or unloading and it also varies with pressure acting on it. for the sake of simplicity we determined κn as tangent stiffness by piece-wise linear approximation of the unloading path of the experimental normal-stress vs. fracture-closure diagram (figure 1), which was presented in [12]. the intervals of normal stress σn and corresponding tangent stiffness parameters κn and kn are listed in table 2. compressive stress is in this study considered with a negative sign. the shear stiffness of fractures is determined using the formula presented in [13] with parameters listed in table 3: κs = 100 l · |σn| · tan ( jrc · log10 ( jcs |σn| ) + ϕr ) , (6) where jrc is joint roughness coefficient, jcs is joint wall compressive strength and ϕr is residual friction angle. normal stress |σn| κn kn [mpa] [ mpa m ] [-] 0.0–2.5 75 472 1.308 2.5–5.0 210 526 1.540 5.0–7.5 277 778 1.656 7.5–10.0 625 000 2.253 10.0–12.5 781 250 2.522 12.5–15.0 892 857 2.714 15.0–20.0 1 250 000 3.328 20.0–25.0 1 250 000 3.328 25.0–30.0 1 315 789 3.442 30.0–33.0 2 790 698 5.979 table 2. fracture’s tangent stiffness (in direction normal to the fracture). parameter value joint roughness coefficient jrc [-] 6.55 joint wall compressive strength jcs [mpa] 90.0 residual friction angle ϕr [deg] 20.0 table 3. fracture’s shear stiffness parameters. the described constitutive model has been implemented by means of python api as user-defined material in the open-source fe code oofem [14]. it is noted that equation (3) represents a compliance relation, while in fem, the stiffness tensor is necessary. thus, the compliance tensor is first calculated using equation (3) and then it is numerically inverted to obtain stiffness. 4. representative volume element as we discussed in the previous section, the overall mechanical properties of the jointed rock mass are evaluated by volume averaging of the fracture network. the size of the dfn sub-domain, over which the averaging is performed (v in equations (2) and (3)), should be chosen large enough to guarantee that the evaluated overall properties are the same for any stochastic realization of the dfn. then, the subdomain can be called a representative volume element. based on a previous study [9], we use rves with dimensions of 15×15×15 m. for this size the coefficient of variation (cov) of apparent moduli evaluated for 10 realizations of dfn (with similar parameters as here) was less than 20 %. furthermore, it was shown in [9], that further increasing the sampling volume size did not lead to any significant reduction of the moduli cov. to eliminate potential border effects, the rves are obtained by cropping much larger dfn, which is generated in the volume of 100 × 100×100 m. 63 martin lebeda, petr kabele acta polytechnica ctu proceedings (a). state prior to excavation. (b). state after tunnel excavation. figure 2. finite element model. 5. 3d fe modelling of tunnel excavation to demonstrate the use of the dfn-based constitutive model, we numerically simulate by fem the excavation of a fictitious tunnel in rock at the depth of 550 m. the tunnel has a circular cross-section with the diameter of 3.0 m. the tunnel is modelled in a cubic body, representing the surrounding rock, with side size of 15 m (figure 2). thus, the distance between the tunnel and the boundary of the model is at least twice the diameter of the construction. the coordinate system is introduced with the vertical axis z oriented upwards. the edges of model are parallel with the coordinate axes and the tunnel is parallel with the y-axis. the finite element mesh consists of 3d hexahedrons and it was created with salome software [15]. the simulation is performed in two steps. the first step (time t = 0) represents the initial state prior to excavation – figure 2a. the displacement of the rock body is constrained by statically determinate supports and surface tractions, corresponding to the initial geostatic pressure, are applied on all sides of the body. the initial stress state is idealized as uniform within the modeled domain. the initial stress tensor is based on field measurements from rožná mine [16]. using data from vertical boreholes it was found that the directions of the principal stresses are vertical and horizontal. the vertical principal stress is calculated by equation (7): sv = −ρ · g · h, (7) where ρ is volumetric weight, g is gravity acceleration and h is depth beneath the ground level. the parameters for calculation of the vertical principal stress (table 4) are adopted from [17]. as the major and minor horizontal principal stresses, sh and sh, respectively, we use the average of the values measured on different levels of the mine. the values of the parameter value major horizontal principal stress sh [mpa] −22.8 minor horizontal principal stress sh [mpa] −14.6 volumetric weight ρ [ kg3 m ] 28 000 depth of the mine beneath the ground level [m] 550 vertical principal stress sv [mpa] −15.4 table 4. the initial stress state. initial stress are listed in table 4. orientation of the model is set so, that the tunnel axis is parallel with the minor horizontal principal stress, which means that the direction of sh corresponds to the global axis y. it should be noted that, even though the stress field is uniform throughout the analyzed domain in the first step, the normal traction σn acting on each fracture in the dfn is generally different due its different direction. this traction, in turn, affects the fractures’ normal and shear stiffness, as seen in equation (6) and table 2, which must be taken into account in the averaging process. in the second calculational step (time t = 1), the excavation of the tunnel is simulated by removing the finite elements inside the tunnel space while keeping the boundary conditions on the outer surface of the rock body unchanged. this creates a new tractionfree boundary on the tunnel walls, which results in deformation and change of stress state in the rock. we should remark that, at the present stage of progress, the problem is solved as incrementally linear. that is, the second step is solved with constant tangent stiffnesses of the fractures, which correspond to the initial stress state. 64 vol. 40/2023 analysis of tunnel excavation based on linear dfn-fem modelling figure 3. horizontal convergence ∆ux calculated for different dfn models. figure 4. vertical convergence ∆uz calculated for different dfn models. 6. case examples 6.1. fracture network models to demonstrate the proposed simulation procedure of 3d underground structure excavation, we use several simple examples. in all of the examples, the fracture network consists of a set of nearly parallel fractures with the same dfn parameters, except the mean normal vector. in particular, the simulations are performed on four types of dfn models, in which fractures are: perpendicular to axis x/y/z and at an angle of 45 degrees between negative x axis and positive z axis, see table 1. even though the size of the rve for averaging of the rock properties was rationally selected, 3 different stochastic realizations of dfn models with different random seed value were generated for verification. thus, in total we work with 12 dfn models. 6.2. results and discussion 6.2.1. displacements figure 3 and figure 4 present the calculated values of the horizontal and vertical convergence for each dfn case. the tunnel convergence is evaluated as the relative displacement between points on the tunnel walls caused by the tunnel excavation (that is, the incremental displacement between the final displacement at time t = 1 and the initial displacement at t = 0). positive values of the convergence correspond to the closing of the tunnel walls. the points at which figure 5. location of reference points and lines used to report results. the displacement was recorded are pictured in figure 5. figure 6 through figure 9 show the incremental displacement of the model boundaries including the excavated tunnel. considering that the initial vertical pressure sv is about 2⁄3 of the major horizontal pressure sh and 65 martin lebeda, petr kabele acta polytechnica ctu proceedings that sh acts perpendicularly to the tunnel axis, the displacements of the tunnel walls qualitatively correspond with expectations: • for a fracture set perpendicular to tunnel axis, i.e. perpendicular to the axis y, the effect of the fractures on the convergence is rather small and the convergence is the least among all the analyzed cases (the second group of bars in figure 3 and figure 4). consistently with the initial stress state, the vertical convergence is smaller than the horizontal one, which is also obvious from the deformed shape of the tunnel cross-section (figure 6). • referring to the cases with vertical and horizontal fractures parallel with the tunnel axis (i.e. perpendicular to axis x and perpendicular to axis z, respectively), it is seen in figure 3 and figure 4 that the convergence in the direction normal to the fractures significantly increases in comparison with the previous case. the convergence as well as the shape of the cross-sectional distortion reflect the fractures-induced anisotropy. in the rock mass with vertical fractures, the higher initial horizontal stress results in horizontal convergence, which is significantly larger than the vertical one (figure 7). on the other hand, when the fractures are horizontal, the resulting horizontal and vertical convergences are close to each other (figure 8). • finally, when the fractures are inclined at 45 degrees, the horizontal and vertical convergences fall between those observed with the vertical and horizontal sets. the axes of the distorted shape of the tunnel (figure 9) are close to, but do not align with the fractures, which can be attributed to the difference of the initial horizonal and vertical stress and normal and tangential displacement occurring on the fractures. figure 3 and figure 4 also indicate that the larger components of convergence, which are strongly affected by the fractures, exhibit notable variation among the different stochastic realizations of the dfn. this finding suggests that the employed sampling volume was still not large enough to meet the criteria as an rve. 6.2.2. stress figure 10 and figure 11 show the distribution of normal stresses along the horizontal and vertical lines indicated in figure 5. the results are reported for one realization of the dfn model with fractures at the angle of 45 degrees. both plots show the uniform initial stress state at time t = 0 and the stress distribution after excavation at t = 1. the difference between the lines corresponds to the incremental change of stress due to excavation. the plots show that, as the tunnel is excavated and a new traction-free boundary is introduced at the tunnel wall, the radial pressure correctly decreases from the initial uniform state and tends to zero at the tunnel wall. on the other hand, (a). vertical section through the dfn. (b). deformed shape of the tunnel (red). figure 6. model with fractures perpendicular to y axis. (a). vertical section through the dfn. (b). deformed shape of the tunnel (red). figure 7. model with fractures perpendicular to x axis. (a). vertical section through the dfn. (b). deformed shape of the tunnel (red). figure 8. model with fractures perpendicular to z axis. (a). vertical section through the dfn. (b). deformed shape of the tunnel (red). figure 9. model with fractures at the angle of 45 degrees between negative x axis and positive z axis. 66 vol. 40/2023 analysis of tunnel excavation based on linear dfn-fem modelling figure 10. model with fractures at the angle of 45 degrees: distribution of normal stresses σx and σz along horizontal line a-a’ (figure 5). the pressure in the circumferential direction increases, which is also to be expected. note that fractures are distributed at an angle around the tunnel, so some experience an increase of pressure (loading) while pressure on others decreases (they unload). the stress increment on fractures in the close vicinity of the tunnel may be also quite large. therefore, for some fractures, using the constant tangent stiffness estimated from the unloading path of the stress-closure diagram (see section 3 and figure 1) might have not been accurate. 7. conclusions the 3d fem simulation of tunnel excavation with rock mass parameters evaluated by averaging procedure based on dfn has been examined. this paper has been focused on the demonstration of the solution methodology. the results allow us to draw the following conclusions: • deformed shapes calculated by fe simulations are consistent with the orientation of fractures in dfns and the initial stress state. largest tunnel convergence is observed in the directions close to the least overall stiffnesses of the fractured rock mass. • some difference in the displacements calculated with different stochastic realizations of the dfns was observed. the appropriate size of the sampling volume should be further investigated and if rve cannot be found, the concept of sve might be adopted. • the largest changes in the stress state after the tunnel excavation were observed close to tunnel, figure 11. model with fractures at the angle of 45 degrees: distribution of normal stresses σx and σz along vertical line b-b’ (figure 5). which is in agreement with the expectations. on the other hand, on the outer boundaries of the models, the stresses after excavation were slightly different from the initial state, which indicates that a larger domain of the model is required to eliminate the boundary effect. • considering the observed stress variations after the tunnel excavation, an incremental nonlinear calculation with fractures’ stiffness updated according to the actual stress state and loading-unloading may be necessary for a more accurate prediction of the tunnel convergence. • although it was not elaborated in the present work, the demonstrated analysis method provides information about the changes of the pressure acting on the fractures, which affect their hydrogeological properties. the proposed methodology, therefore, may provide valuable inputs for hydrogeological simulations of ground water flow and contaminants’ transport. acknowledgements this paper was financially supported by czech technical university in prague under sgs project no. sgs22/030/ohk1/1t/11. references [1] l. jing. a review of techniques, advances and outstanding issues in numerical modelling for rock mechanics and rock engineering. international journal of rock mechanics and mining sciences 40(3):283–353, 2003. https://doi.org/10.1016/s1365-1609(03)00013-3 67 https://doi.org/10.1016/s1365-1609(03)00013-3 martin lebeda, petr kabele acta polytechnica ctu proceedings [2] m. hori, s. nemat-nasser. on two micromechanics theories for determining micro–macro relations in heterogeneous solids. mechanics of materials 31(10):667–682, 1999. https://doi.org/10.1016/s0167-6636(99)00020-4 [3] m. oda, t. yanabe, y. ishizuka, et al. elastic stress and strain in jointed rock masses by means of crack tensor analysis. rock mechanics and rock engineering 26(2):89– 112, 1993. https://doi.org/10.1007/bf01023618 [4] x. yin, w. chen, a. to, et al. statistical volume element method for predicting microstructure–constitutive property relations. computer methods in applied mechanics and engineering 197(43-44):3516–3529, 2008. https://doi.org/10.1016/j.cma.2008.01.008 [5] w. s. dershowitz, p. wallmann, s. kindred. discrete fracture modelling for the stripa site characterization and validation drift inflow predictions, stripa-tr-91-16, 1991. skb stockholm. https://www.researchgate.net/profile/william_ dershowitz/publication/266334817_discrete_ fracture_modelling_for_the_stripa_site_ characterization_and_validation_drift_inflow_ predictions/links/563bb3ea08ae34e98c47cfaf.pdf [6] r. munier. statistical analysis of fracture data, adapted for modelling discrete fracture networks-version 2, skb r-04-66, 2004. skb svenskkärnbränslehantering ab swedish nuclear fuel and waste management co. http: //skb.se/upload/publications/pdf/r-04-66.pdf [7] c. darcel, p. davy, o. bour, j.-r. d. dreuzyr. discrete fracture network for the forsmark site, skb rapport r-06-79, 2006. skb svenskkärnbränslehantering ab swedish nuclear fuel and waste management co. 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[12] s. c. bandis, a. c. lumsden, n. r. barton. fundamentals of rock joint deformation. international journal of rock mechanics and mining sciences & geomechanics abstracts 20(6):249–268, 1983. https://doi.org/10.1016/0148-9062(83)90595-8 [13] n. barton, v. choubey. the shear strength of rock joints in theory and practice. rock mechanics 10(1-2):1– 54, 1977. https://doi.org/10.1007/bf01261801 [14] b. patzák. oofem project home page, 2000. [2022-10-13]. http://www.oofem.org [15] edf, cea. salome, 2001. [2022-10-17]. https://www.salome-platform.org/ [16] z. bukovská, a kol. získání dat z hlubokých horizontů dolu rožná, tz 464/2020, 2020. súrao správa úložišť radioaktivních odpadů. [17] k. souček, m. vavro, l. staš, et al. komplexní geologická charakterizace prostorů pvp bukov – část ii geotechnická charakterizace, závěrečná zpráva súrao zz 221/2018, 2017. súrao správa úložišť radioaktivních odpadů. 68 https://doi.org/10.1016/s0167-6636(99)00020-4 https://doi.org/10.1007/bf01023618 https://doi.org/10.1016/j.cma.2008.01.008 https://www.researchgate.net/profile/william_dershowitz/publication/266334817_discrete_fracture_modelling_for_the_stripa_site_characterization_and_validation_drift_inflow_predictions/links/563bb3ea08ae34e98c47cfaf.pdf https://www.researchgate.net/profile/william_dershowitz/publication/266334817_discrete_fracture_modelling_for_the_stripa_site_characterization_and_validation_drift_inflow_predictions/links/563bb3ea08ae34e98c47cfaf.pdf https://www.researchgate.net/profile/william_dershowitz/publication/266334817_discrete_fracture_modelling_for_the_stripa_site_characterization_and_validation_drift_inflow_predictions/links/563bb3ea08ae34e98c47cfaf.pdf https://www.researchgate.net/profile/william_dershowitz/publication/266334817_discrete_fracture_modelling_for_the_stripa_site_characterization_and_validation_drift_inflow_predictions/links/563bb3ea08ae34e98c47cfaf.pdf https://www.researchgate.net/profile/william_dershowitz/publication/266334817_discrete_fracture_modelling_for_the_stripa_site_characterization_and_validation_drift_inflow_predictions/links/563bb3ea08ae34e98c47cfaf.pdf http://skb.se/upload/publications/pdf/r-04-66.pdf http://skb.se/upload/publications/pdf/r-04-66.pdf http://www.iaea.org/inis/collection/nclcollectionstore/_public/38/013/38013872.pdf http://www.iaea.org/inis/collection/nclcollectionstore/_public/38/013/38013872.pdf https://doi.org/10.14311/app.2022.34.0038 https://doi.org/10.1016/0148-9062(83)90595-8 https://doi.org/10.1007/bf01261801 http://www.oofem.org https://www.salome-platform.org/ acta polytechnica ctu proceedings 40:61–68, 2023 1 introduction 2 dfn model – description of fractures' geometry 3 overall stiffness of the fractured rock mass evaluated by averaging procedure 4 representative volume element 5 3d fe modelling of tunnel excavation 6 case examples 6.1 fracture network models 6.2 results and discussion 6.2.1 displacements 6.2.2 stress 7 conclusions acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2025.54.0041 acta polytechnica ctu proceedings 54:41–45, 2025 © 2025 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague fire resistance of high-performance concrete reinforced with selected fibers věra kabíčkováa,b,∗, jakub hájeka,b, jan macháčeka, eliška kafkováa, tomáš vlacha,b a czech technical university in prague, faculty of civil engineering, department of architectural engineering, thákurova 7, 166 29 prague 6, czech republic b czech technical university in prague, university centre for energy efficient buildings, materials and construction of buildings research department, třinecká 1024, 273 43 buštěhrad, czech republic ∗ corresponding author: vera.kabickova@fsv.cvut.cz abstract. one of the biggest issues of high-performance concrete is its behavior during fire. because of the much lower amount of pores in high-performance concrete, extremely high water vapor pressure is created. this leads to explosive spalling of the concrete. one way to prevent this effect is by adding fibers to the concrete mixture. in this article, the impact of commonly used polypropylene fibers is compared to that of flax fibers and viscose fibers. subsequently, mechanical compressive and flexural strength were tested on those samples that remained intact after the fire experiment. their mechanical properties were compared with the reference samples without exposure to fire at the same age. keywords: high-performance concrete, fiber-reinforced concrete, fire, elevated temperature, compressive strength, flexural strength. 1. introduction civil engineering, particularly cement production, is one of the largest sources of co2 emissions in the industrial sector. to reduce its effects on the environment, it is essential to focus on improving material efficiency and using alternative materials. one of the ways is using high-performance concrete (hpc). in hpc, the number of pores is reduced, which leads to higher strength. however, fewer pores result in explosive spalling of concrete – there is only limited space for water vapor evaporation increasing water vapor pressure that eventually leads to explosive spalling. fiberreinforced concrete (frc) is created by adding certain fibers and can prevent concrete spalling [1]. there are several materials that can be used. polypropylene (pp) fibers are commonly used as a spalling prevention. these synthetic fibers have a relatively low melting point (160 °c [2]), which creates more space for water vapor evaporation. another possibility is using natural materials, for example flax. research has shown that flax fibers can partially prevent spalling; they do not decompose at temperatures below 300 °c, they only shrink, which leads to limited space for water vapor evaporation. therefore, their effectiveness in enhancing fire resistance is limited, and a higher number of fibers or a combination of fibers would be required [3]. the last material considered in this article is viscose, which is produced from natural materials. it is widely used in the textile industry, providing opportunities for recycling. the use of viscose fibers in spalling prevention is not well known and therefore requires thorough exploration. in this article, a fire experiment using the temperature curve according to iso 834 with a duration of 15 minutes was performed. beam frc samples with dimensions of 40 × 40 × 160 mm were prepared and provided with the wire thermocouples to measure the temperature inside the samples. 2. materials and methods 2.1. materials and samples for the experiment, nine variants of fpc samples were prepared. three sets contained polypropylene fibers (pp), three sets contained flax fibers (flax), and the remaining three sets contained viscose fibers (see figure 1). each variant consisted of six beams with dimensions of 40 × 40 × 160 mm – three samples for the fire experiment and three samples for comparison of subsequent compression and bending tests. samples for the fire experiment were provided with the wire thermocouples (figure 2). all samples were stored in water for 28 days and then moved to a room with a constant relative humidity of 60 % and a temperature of 22 °c. the hpc mixture without fibers used for the preparation of samples was developed at ctu in prague (table 1) [5]. the amount of water and superplasticizer was then slightly modified to achieve better workability of the concrete. compressive strength of this mixture was around 145 mpa and tensile strength reached a value of approximately 11 mpa. as stated before, three different fiber materials were used – polypropylene, flax, and viscose. pp fibers are commonly used as a prevention of concrete explosive spalling; therefore, pp fibers masterfiber 012 were chosen for comparison with other 41 https://doi.org/10.14311/app.2025.54.0041 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en v. kabíčková, j. hájek, j. macháček et al. acta polytechnica ctu proceedings figure 1. fibers user – polypropylene, flax, and visose. figure 2. location of a wire thermocouple [4]. mix content pp1 pp2 pp2 pl25 vis 1 vis 2 vis 2 pl25 flax 1 flax 2 flax 2 pl25 kg m−3 cement i 42.5r 650 650 650 650 650 650 650 650 650 silica sand 1 200 1 200 1 200 1 200 1 200 1 200 1 200 1 200 1 200 silica flour 235 235 235 235 235 235 235 235 235 silica fume (microsilica) 75 75 75 75 75 75 75 75 75 superplasticizer 18 18 25 18 18 25 18 18 25 water 190 190 190 190 190 190 190 190 190 pp fibers 1 2 2 flax fibers 1 2 2 viscose fibers 1 2 2 table 1. mixture of frc. fibers. these fibers have a length of 12 mm and are very fine, which allows them to disperse equally. the manufacturer’s recommended fiber amount varies between 600 g m−3 to 3 kg m−3. since flax shows one of the best mechanical parameters of natural fibers, it was chosen as a representative of natural fibers. long flax roving was cut into fibers of a length of approximately 12 mm for better comparison with pp fibers. the last selected material, viscose, is produced from natural materials and is industrially compostable at the same time, which makes viscose a compromise between artificial and natural fibers that can be unpredictable. just like flax, viscose roving was also cut into fibers of a length of approximately 12 mm. 2.2. experiment for the fire experiment, a mobile fire furnace, minifur, developed by firelab at the university centre for energy efficient buildings, was used. this furnace is designed to achieve the temperature curve according to iso 834 for up to 45 minutes. after 15 minutes, a problem with the furnace occurred after spalling of some samples, and the experiment had to be aborted. consequently, the duration of this indicative experiment was set to 15 minutes. samples were placed between 100 mm thick aerated concrete blocks, which form the front walls (figure 3). after the fire experiment, intact samples were left to cool. afterwards, mechanical flexural strength test (three-point bending test) on beams was performed 42 vol. 54/2025 fire resistance of high-performance concrete reinforced . . . figure 3. arrangement of the fire furnace. according to csn en 12390-5 [6], along with compressive strength test on fragments of beams according to csn en 12390-3 [7]. mechanical properties were also tested on the reference samples that had not been exposed to fire. 3. results and discussion the duration of the fire experiment was 15 minutes because of a problem with the furnace. there were some samples from another experiment so that the furnace would be used effectively, but the spalling of those samples caused a temperature drop in the furnace, and the experiment had to be stopped after 15 minutes. therefore, the duration of all the following fire tests with the rest of the samples were set to 15 minutes. the courses of some samples’ curves and the iso 834 curve from uninterrupted test are shown in figure 4. the measured temperature curve during the experiment was within the range of the iso 834 curve, average temperature in the samples was around 250 °c at the end of the experiment. this value is important when changes in mechanical properties are evaluated. as can be seen in figure 5, all samples remained intact except for those with 1 kg m−3 flax fibers, flax 1, whose exposed side completely spalled. this may be caused by the fact that flax fibers tend to clump together. it is also known that flax fibers do not melt at temperatures below 300 °c; they only release absorbed water and shrink that leads only to limited space for water vapor evaporation [3]. pp fibers, on the other hand, expand with increasing temperature figure 4. temperature curves during the fire experiment of selected samples. figure 5. samples after the fire experiment. creating microcracks, and at 170 °c they start to melt, which leads to even wider space [8]. afterwards, mechanical flexural strength and compressive strength were measured. according to [4, 9], those values depend on the temperature reached. in the case of hpc, there is a certain increase in compressive strength up to approximately 450 °c followed by a decrease in this strength. tensile and flexural strength, on the other hand, decrease with heat increase, especially in the case of frc. the results in figure 6 show that in all samples flexural strength decreased compared to the reference samples (not exposed to fire). this behavior of frc corresponds with en 1992-1-2 [10] as well as with other articles [4, 9], where tensile and flexural strength mostly decreased. only in the case of plain hpc, there was a small range of temperatures around 220 °c with a slight increase in splitting tensile strength in those articles. figure 7 also shows that all samples reached lower values of compressive strength compared to the reference samples (not exposed to fire). this corresponds with en 1992-1-2 [10], but not with other articles [4, 9]. they found that compressive strength increases after being exposed to higher temperature up to approximately 450 °c. this may be caused by additional cement hydration enabled by evaporating water. but the fire exposure of our samples may not be long enough; the temperature reached in the samples was 43 v. kabíčková, j. hájek, j. macháček et al. acta polytechnica ctu proceedings figure 6. flexural strength of the samples exposed to fire in comparison with the reference samples. figure 7. compressive strength of the samples exposed to fire in comparison with the reference samples. only around 250 °c and it was reached at the very end of the experiment. a slight decrease in compressive strength is believed to be caused by the development of microcracks. 4. conclusion by adding pp fibers and viscose fibers, we were able to prevent spalling of the concrete when exposed to fire for the short period of time. adding flax fibers was less effective – samples containing only 1 kg m−3 of flax fibers were damaged. the reason is in fibers’ behavior during elevated temperatures; flax fibers only shrink and do not melt at temperatures below 300 °c, meanwhile pp fibers first expand, creating microcracks, and then begin to melt at temperatures around 170 °c. consequently, the behavior of viscose fibers must be investigated through microstructure observation. furthermore, longer exposure to fire should be performed. as for mechanical properties, all samples reached lower compressive, as well as tensile strength, which partly corresponds with other articles [4, 9]. they found that exposure to temperature around 450 °c leads to higher compressive strength. the main difference was in the duration of the fire experiment which implies that change of mechanical properties depends both on temperature and time of exposure to that temperature. acknowledgements the work on this paper was supported by czech science foundation grant no. 22-14942k entitled “possibilities of using natural fibers for the production of hybrid textile reinforcement in concrete”. the authors would like to acknowledge all financial assistance provided to support this research. references [1] p.-c. aïtcin. vysokohodnotný beton [in czech; high-quality concrete], 2005. [2] gcc/gulf region. masterfiber 012, 2005. [2024-09-05]. https://mbcc.sika.com/en-ae/ products/masterfiber/masterfiber-012 [3] d. zhang, g. y. tan, k. h. tan. combined effect of flax fibers and steel fibers on spalling resistance of ultrahigh performance concrete at high temperature. cement and concrete composites 121:104067, 2021. https: //doi.org/10.1016/j.cemconcomp.2021.104067 [4] r. fürst, t. vlach, m. pokorný, v. mózer. study of behavior of textile-reinforced concrete with epoxy resin matrix in fire. fire technology 58:53–74, 2022. https://doi.org/10.1007/s10694-021-01116-y [5] m. kynclova, c. fiala, p. hajek. high performance concrete as a sustainable material. international journal of sustainable building technology and urban development 2(1):63–68, 2011. https://doi.org/10.5390/susb.2011.2.1.063 [6] únmz. zkoušení ztvrdlého betonu – část 5: pevnost v tahu ohybem zkušebních těles, 2020. 44 https://mbcc.sika.com/en-ae/products/masterfiber/masterfiber-012 https://mbcc.sika.com/en-ae/products/masterfiber/masterfiber-012 https://doi.org/10.1016/j.cemconcomp.2021.104067 https://doi.org/10.1016/j.cemconcomp.2021.104067 https://doi.org/10.1007/s10694-021-01116-y https://doi.org/10.5390/susb.2011.2.1.063 vol. 54/2025 fire resistance of high-performance concrete reinforced . . . [7] únmz. zkoušení ztvrdlého betonu – část 3: pevnost v tlaku zkušebních těles, 2020. [8] d. zhang, a. dasari, k. h. tan. on the mechanism of prevention of explosive spalling in ultra-high performance concrete with polymer fibers. cement and concrete research 113:169–177, 2018. https://doi.org/10.1016/j.cemconres.2018.08.012 [9] t. drzymała, w. jackiewicz-rek, m. tomaszewski, et al. effects of high temperature on the properties of high performance concrete (hpc). procedia engineering: modern building materials, structures and techniques 172:256–263, 2017. https://doi.org/10.1016/j.proeng.2017.02.108 [10] únmz. zatížení konstrukcí – část 1-2: obecná zatížení – zatížení konstrukcí vystavených účinkům požáru, 2014. 45 https://doi.org/10.1016/j.cemconres.2018.08.012 https://doi.org/10.1016/j.proeng.2017.02.108 acta polytechnica ctu proceedings 54:41–45, 2025 1 introduction 2 materials and methods 2.1 materials and samples 2.2 experiment 3 results and discussion 4 conclusion acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2025.52.0001 acta polytechnica ctu proceedings 52:1–7, 2025 © 2025 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague influence of various angles and distances on luminance measurement results jan bartoníček∗, tomáš blodek, pavel vrtal czech technical university in prague, faculty of transportation sciences, konvitská 20, 110 00 prague 1, czech republic ∗ corresponding author: bartoj34@cvut.cz abstract. the behaviour of drivers in nocturnal conditions is primarily influenced by the early detection of obstacles within the roadway. a reliable approach to evaluate night-time traffic environments is through luminance analysis. this study aimed to investigate the impact of conducting luminance measurements at reduced distances compared to those specified by standard guidelines. additionally, the study examined the effect of the measurement angle of the luminance analyser on the recorded luminance values. the findings indicate that the measurement angle is a critical factor for ensuring accurate luminance readings, while the distance between the analyser and the measured surface has minimal influence on the results. statistical analysis confirms that reliable data can be obtained even when deviating from standard measurement conditions. these results suggest that luminance analysis is feasible and applicable across a wide range of traffic scenarios. keywords: luminance, lighting, luminance analysis, brightness, luminance analyser. 1. introduction addressing traffic safety is a persistent issue that necessitates continuous scrutiny. key strategic documents issued by the european commission [1], along with national reports [2], indicate that reducing traffic accidents remains a primary priority. when assessing road user behaviour, particular emphasis is placed on the clarity and intelligibility of transport infrastructure [3]. this aspect is crucial not only during the daytime but also in conditions of reduced visibility [3]. traffic accident statistics reveal that, in 2023, 118 out of 455 fatal accidents in the czech republic occurred under low visibility conditions [4]. this figure represents over one-fourth of all fatal accidents recorded that year. these statistics are especially significant considering that traffic volume at night is markedly lower than during daytime hours. the behaviour of drivers in nocturnal conditions is primarily influenced by the early detection of obstacles within the roadway. a reliable approach to evaluating nighttime traffic environments is luminance analysis. this method not only assesses the adequacy of illumination in traffic areas but, more importantly, evaluates the visibility and detectability of obstacles or individuals on and around the roadway. the methods for measuring the luminance of specific sections of the traffic area are defined in european and czech standards, which outline the conditions for conducting these measurements and their subsequent evaluation. the area under consideration is measured over a relatively long distance of 60m, determined based on the range of vehicle headlights and the braking distances of vehicles. this measurement is applicable in ideal situations on straight road sections, where ambient luminance is assessed without significant geometric changes in road alignment (both horizontal and vertical). however, if a situation arises that necessitates measuring the luminance of the road and its adjacent surroundings in areas with directional curves, the investigator must assess the area at a shorter distance than specified in the standards. the aim of this study is to assess the effect of measuring luminance at distances closer than those defined under standard conditions. additionally, the study aims to evaluate the influence of the measurement angle and the scanning process of the area under consideration by the luminance analyser on the resulting luminance values. this analysis seeks to contribute to the optimization of measurements under nocturnal conditions, thereby assisting users in obtaining the most accurate values. 2. survey of studies on lighting and luminance measurement methods in the field of photometry and luminance measurement, numerous studies and research endeavours have addressed various aspects of illumination and its impact on traffic safety. specifically, attention has been directed toward analysing human perception of the environment under different lighting conditions, considering visual adaptation to these scenarios [5]. concurrently, research has also focused on the influence of external light sources, such as vehicle headlights, assessing how different lighting sources affect the visibility and recognizability of objects. significant differences can be observed among halogen, xenon, and led headlights, as their luminance and technology directly influence the illumination range and light dis1 https://doi.org/10.14311/app.2025.52.0001 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en j. bartoníček, t. blodek, p. vrtal acta polytechnica ctu proceedings camera type nikon d7500 sensor type cmos nikon dx 15.7 × 23.5mm number of pixels 21.51mil. pixels image size large – 5568 × 3712, medium – 4176 × 2784, small – 2784 × 1856 effective number of pixels 20.9 mil. pixels a/d converter 12 bits / 4096 levels, or 14 bits / 16384 levels shutter speed 1/8000–30 sec. + bulb iso sensitivity 100–51200 table 1. specification of the lda camera. tribution. these factors are closely related to the recognizability of objects in the traffic environment. additionally, the colour of light plays a crucial role in distinguishing individual objects [6]. object distinctiveness is currently under investigation, particularly at pedestrian crossing points. at these locations, it is crucial to achieve the required lighting intensity, as this directly influences both the luminance and visibility of obstacles. in this context, obstacles are defined as pedestrians moving within the roadway [7]. the employed methods also consider the impact of the background, where various distractions or ambient light sources may significantly affect measurement outcomes, potentially leading to the oversight of pedestrians [8]. a study conducted by seoul national university in the republic of korea investigates the influence of traffic area lighting efficiency under adverse weather conditions, such as rain or fog. the findings indicate that distance plays a crucial role in object recognition, which directly impacts the effectiveness of road lighting [9]. road lighting can be designed dynamically so that the lighting brightness is automated and adjusted based on the speed and intensity of traffic. such an approach can enhance the efficiency of lighting systems and improve safety in the area [10]. elements located on the roadway, specifically horizontal road markings, are another critical factor that enhances driver orientation in nighttime traffic areas. these road traffic markings convey information regarding the trajectory of the road, thereby facilitating predictions about its direction and elevation. however, this element is also subject to significant variation, which depends not only on the quality of workmanship but also on the adequacy of ambient lighting [11]. the issue of measuring the luminance of led luminaires using various measuring devices is also examined. concurrently, the study recommends standardizing the parameters to obtain relevant values. furthermore, an alternative method for measuring luminance from outside the traffic area is proposed, which may yield comparable results to traditional measurement methods [12]. the study conducted by aalto university, finland, focuses on the integration of spatial laser scanning and luminance analysis. by combining the outputs of these measurement instruments, the research facilitates the mapping of illumination in traffic areas and the creation of a spatial representation of luminance [13]. it is important to reference studies that outline methods for conducting nighttime safety inspections, which often involve a significant degree of subjectivity in their evaluations [14]. additionally, a related concern is the uniform distribution of luminance in traffic areas, as it has been shown to be a critical factor in ensuring the safety of road users [15]. most scientific studies predominantly concentrate on the influence of the surrounding environment and the ways in which users perceive the nocturnal setting. consequently, the emphasis is primarily placed on the quality of the light source and the subjective perception of the night environment. the present study aims to enhance the measurement process and seeks to provide an objective evaluation of the methodology and quality of the data collected. 3. method of data processing and comparison experimental measurements of luminance values from different positions were conducted following the defined procedure according to čsn en 13201 [16]. the basis for the luminance measurement involved defining a grid of measurement points and positioning the measuring device at a predefined location. in general, the measurement area in the longitudinal direction is determined by the distance between luminaires situated in the same row. if there are multiple rows of luminaires in the measured area, the measurement field must be located between the two luminaires in the row with the largest spacing. for measurements with a luminance analyser, the device is positioned 60m in front of the start of the measured area. additionally, the height of the centre of the device lens is set to 1.5m above road level, with a measurement angle of 89 ± 0.5°relative to the normal to the road surface. these parameters reflect the observation conditions of a driver in a vehicle [16]. the focus of the measurements was the road surface beneath the light source, where the light cone impacting the road was observed. the measurements were conducted using the lda luminance analyser, with specifications of the lda settings during the measurements provided in table 1 [17]. 2 vol. 52/2025 influence of various angles and distances on luminance . . . lens type tokina 50mm f/1,4 ff opera for nikon f focal length 50mm aperture range f/1.4–f/16 filter thread diameter 72mm table 2. specification of the lda lens. figure 1. schematic placement of the luminance analyser. the lda luminance analyser is equipped with a nikon bayonet mount, allowing compatibility with a wide range of lenses tailored to specific measurement requirements. table 2 presents the specifications of the lens utilized for the present measurements. the measurement site was selected to ensure safe luminance analysis without the need for traffic restrictions. the location chosen is a road lighting installation in studničkova street, prague. the light source consists of a high-pressure sodium lamp, which is commonly used for road lighting. a specific measurement area was defined directly beneath the light source, where the light cone is projected. for the purposes of this study, will be presented only the assessment of measured luminance values from a uniform viewing axis. for the first measurement, the luminance analyser was positioned 60m from the centre of the measurement area, with the centre of the analyser’s lens placed at a height of 1.5m above the road surface. in the second measurement, the instrument was positioned 34m away, maintaining the lens at the same height of 1.5m above ground level. this distance was determined with respect to the vehicle’s braking distance under wet road conditions. the third measurement was conducted at a distance of 34m, but with the lens placed at a height of 0.85m above the road surface. this lower height corresponds to a measurement angle aligned with the original measurement specifications, as outlined in čsn en 13201. the various positions of the luminance analyser are illustrated in figure 1. to capture the complete light scene within the measurement area, multiple photographs must be taken at varying exposure durations. the exposure times utilized for the measurements conducted with the luminance analyser included 1/2 s, 1 s, 3 s, 5 s, 8 s, and 10 s. the data outputs from the luminance analyser are subsequently imported into the lumidisp software, which calculates and determines the luminance values for each pixel in the photographs. these figure 2. luminance image from the first position. figure 3. measured luminance values from the first position. luminance images are then merged into a single comprehensive luminance image, effectively minimizing any overexposed or underexposed areas, figure 2. in the lumidisp program, an array was specified to calculate the average luminance value for each designated element within the array. additionally, the program facilitated the visualization of a graphical waveform illustrating the luminance distribution across a designated strip. this figure can be converted into a tabular format, in which each cell corresponds to the calculated average luminance value for that specific area, figure 3. 3 j. bartoníček, t. blodek, p. vrtal acta polytechnica ctu proceedings figure 4. two-dimensional and three-dimensional display of measured values from the first position. first measurement figure 2 presents a luminance photograph illustrating the luminance distribution within the measured area of the traffic space. each pixel is assigned a colour level corresponding to its luminance. the colour representation of the luminance photograph may be adjusted according to output requirements. the measured luminance values are displayed in figure 3. for enhanced clarity and interpretation, the measured luminance values have been mapped to a colour scale to depict the effect of the light cone within the measured area. figure 4 presents both two-dimensional and threedimensional representations of the luminance distribution within the measured space. the z-axis represents the luminance values, expressed in cdm−2. the figure illustrates the impact of horizontal road markings present in the measured area on the resulting luminance levels. this effect is attributed to the higher reflectivity of the road markings, which locally increases the brightness by reflecting more of the incident light. to enhance clarity and facilitate comparison, the following measurements are presented in a consistent format. this approach simplifies the identification of differences and similarities between the results and improves the reliability of subsequent analyses. second measured in this measurement, it is already possible to discern the horizontal road markings, specifically the parking lane. at the same time, a local extreme can be observed that exceeds the measured values in all measurements. this deviation may be caused by a failure to maintain the correct measurement angle with the luminance analyser (see figures 5, 6 and 7). figure 5. luminance image from the second position. figure 6. measured luminance values from the second position. figure 7. two-dimensional and three-dimensional display of measured values from the second position. 4 vol. 52/2025 influence of various angles and distances on luminance . . . third measured this measurement is similar to the second measurement in terms of capturing the overall luminance distribution in the area of interest. however, it provides a more valid measurement in detecting and recording the effect of horizontal road markings on luminance levels (see figures 8, 9 and 10). 3.1. comparison of measurements the scatter of all measured luminance values in the study area ranges from 0.6 to 2.0 cdm−2. the figures 12 and 14 compare the measured values at the location of the luminance analyser with the previously defined parameters. the individual values represent the absolute differences in average luminance attributable to a predefined regular grid of measurement points. to enhance clarity, the difference results have been rounded to one decimal place. for accurate correlation and interpretation of the data, the tables should be examined from top to bottom to maintain the measurement direction. comparisons of the measurements were first conducted between the initial and second measurements, followed by comparisons between the initial and third measurements. (1.) first and second measurements the dominant difference was primarily attributed to the horizontal road markings present in the measured area. in other instances, the influence could be ascribed to road geometry or surface conditions. by executing the command to obtain a frequency of 0, a total of 346 cases were identified in the table where relative agreement existed between the two measurements, while 134 cases demonstrated discrepancies. subsequently, the command for calculating the average value of luminance differences was applied to provide an overall summary. the average value of the absolute differences in average luminance across the entire measured area is recorded as 0.0478 cdm−2 (see figures 11 and 12). (2.) first and third measurements in this comparison, the presence of horizontal road markings is crucial to the results. a more distinct gap in the roadway extends from the horizontal road markings through the measured area. when analysing the measurements, relative agreement was found in 408 cases, while discrepancies were observed in 72 instances. the average value of the absolute differences in mean luminance across the entire table representing the measured area corresponds to 0,0319 cdm−2 (see figures 13 and 14). 3.2. summary of verification measurement results the primary factors influencing the results of luminance measurements were predominantly the horizontal road markings delineating the parking lane within the measured area. the condition of the roadway, such as the presence of a pothole in the measurement zone, could also impact certain measured values. occasional figure 8. luminance image from the third position. figure 9. measured luminance values from the third position. figure 10. two-dimensional and three-dimensional display of measured values from the third position. 5 j. bartoníček, t. blodek, p. vrtal acta polytechnica ctu proceedings figure 11. comparison of luminance values in threedimensional display for the first and second measurement. figure 12. comparison of brightness values in twodimensional display for the first and second measurement. discrepancies in the recorded luminance values may arise from potential inaccuracies in configuring the luminance analyser to the specified parameters for a given measurement. these factors must be considered when interpreting the measured data to ensure the highest accuracy of the results. 4. discussion during the measurement process and subsequent postprocessing, several limitations were identified that may affect the accuracy of the results. the limited number of measurements taken from various directions and angles of the luminance analyser may have reduced variability and, consequently, the reliability of the data. this limitation could be addressed by figure 13. comparison of luminance values in threedimensional display for the first and third measurement. figure 14. comparison of luminance values in twodimensional display for the first and third measurement. expanding the measurements to encompass additional directions and positions, thereby providing a more comprehensive and precise analysis of the luminance distribution in the traffic area. furthermore, the delineation of the computational areas in the lumidisp software may have impacted the accuracy of the resulting luminance values. to mitigate this issue, it is necessary to accurately synchronize the boundary points of the delineated measurement area in the traffic space with the corresponding boundary points in the delineated calculation area in the lumidisp software. additionally, the measurements did not incorporate detailed information about the light source, which could enhance the thoroughness of the analysis. 6 vol. 52/2025 influence of various angles and distances on luminance . . . 5. conclusion based on the conducted study, it is concluded that the measurement angle is crucial for achieving consistent luminance values, while the distance of the analyser is of negligible significance. statistical analysis confirmed that reliable measurement results can be obtained when the correct measurement angle is maintained. this demonstrates that surface luminance measurements depend primarily on the measurement angle and the angle of incidence, rather than on the distance from the analyser. the validation of the luminance measurements was carried out to enhance safety in traffic areas and to expand the possibilities of its application. this paper also indicates that luminance analysis is feasible even when normative requirements cannot be fully met. while it is essential to base measurements on applicable standards, adaptations can be made for specific measurement conditions without significantly impacting the results. references [1] european commission. white book. a european transport area for competitive and resource-efficient transport system, brussels, 2014. [2] ministry of transport, czech republic. strategie besip 2021–2030 [in czech; besip strategy 2021–2030, 2020. [2024-10-20]. https://besip.cz/besip/media/ besip/data/web/strategie-besip-2021-2030.pdf [3] k. sokanský. noční bezpečnostní inspekce pozemních komunikací – metodika provádění [in czech; night road safety inspection – methodology of implementation]. všb – tuo, ostrava, 2020. [2024-10-20]. https://www.csorsostrava.cz/metodika/metodika% 20no%c4%8cn%c3%8d%20bezpe%c4%8cnostn%c3%8d% 20inspekce%201%20vyd%c3%81n%c3%8d%202020.pdf [4] directorate of the traffic police service of the police presidium of the czech republic. statistika nehodovosti v roce 2021 [in czech; road accident report for the czech republic in 2021. [xxxx-xx-xx]. https://www.policie. cz/clanek/statistika-nehodovosti-900835.aspx?q= y2hudw09mg%3d%3d [5] t. l. sosnova, y. v. frid. luminance contrast in industrial and traffic situations. svetotehnika 1984:1–3. [6] r. baleja, o. dolejší, p. běčák, et al. the visibility of pedestrians in the traffic area, using different light source technologies of car headlights. in proceedings of the 10th international scientific symposium on electrical power engineering, pp. 551–556. 2019. isbn 978-805533324-3. [7] s. różowicz, k. baran, a. różowicz, et al. assessment of the visibility of unprotected road users in pedestrian crossing. archives of electrical engineering 73(1):145–164, 2024. https://doi.org/10.24425/aee.2024.148862 [8] s. słomiński. digital luminance photometry in the context of the human visual system. bulletin of the polish academy of ssciences technical sciences 70(5):e143103, 2022. https://doi.org/10.24425/bpasts.2022.143103 [9] w. park, k. park, j. jeong. verification of the applicability of obstacle recognition distance as a measure of effectiveness of road lighting on rainy and foggy roads. applied sciences 14(4):1595, 2024. https://doi.org/10.3390/app14041595 [10] s. różowicz, h. wachta, k. p. paduszyński. the influence of the position of the observer and pedestrian on road safety. przegląd elektrotechniczny (1):204–207, 2024. https://doi.org/10.15199/48.2024.01.42 [11] j. l. ethen, h. l. woltman. minimum retroreflectance for nighttime visibility of pavement markings. transportation research record pp. 43–47, 1986. [12] a. v. rusu, c. d. galatanu, g. livint, d. d. lucache. measuring average luminance for road lighting from outside the carriageway with imaging sensor. sustainability 13(16):9029, 2021. https://doi.org/10.3390/su13169029 [13] m. t. vaaja, m. maksimainen, m. kurkela, et al. approaches for mapping night-time road environment lighting conditions. isprs annals of the photogrammetry, remote sensing and spatial information sciences v-1-2020:199–205, 2020. https://doi.org/10.5194/ isprs-annals-v-1-2020-199-2020 [14] t. blodek, p. vrtal, t. kohout, et al. verification of night road safety inspection by luminance analysis. in 2024 24th international scientific conference on electric power engineering (epe), pp. 1–5. 2024. https://doi.org/10.1109/epe61521.2024.10559538 [15] z. davarcı, m. şahin, o. akar. luminance measurement and estimation methods in road. light & engineering 30(6):106–123, 2022. https://doi.org/10.33383/2022-021 [16] office for technical standardization, metrology and state testing. čsn en 13201 lghting of roads, 2015. [17] lda – luminance distribution analyser: user manual, vut, faculty of electrical engineering and communication technologies, brno, 2020. 7 https://besip.cz/besip/media/besip/data/web/strategie-besip-2021-2030.pdf https://besip.cz/besip/media/besip/data/web/strategie-besip-2021-2030.pdf https://www.csorsostrava.cz/metodika/metodika%20no%c4%8cn%c3%8d%20bezpe%c4%8cnostn%c3%8d%20inspekce%201%20vyd%c3%81n%c3%8d%202020.pdf https://www.csorsostrava.cz/metodika/metodika%20no%c4%8cn%c3%8d%20bezpe%c4%8cnostn%c3%8d%20inspekce%201%20vyd%c3%81n%c3%8d%202020.pdf https://www.csorsostrava.cz/metodika/metodika%20no%c4%8cn%c3%8d%20bezpe%c4%8cnostn%c3%8d%20inspekce%201%20vyd%c3%81n%c3%8d%202020.pdf https://www.policie.cz/clanek/statistika-nehodovosti-900835.aspx?q=y2hudw09mg%3d%3d https://www.policie.cz/clanek/statistika-nehodovosti-900835.aspx?q=y2hudw09mg%3d%3d https://www.policie.cz/clanek/statistika-nehodovosti-900835.aspx?q=y2hudw09mg%3d%3d https://doi.org/10.24425/aee.2024.148862 https://doi.org/10.24425/bpasts.2022.143103 https://doi.org/10.3390/app14041595 https://doi.org/10.15199/48.2024.01.42 https://doi.org/10.3390/su13169029 https://doi.org/10.5194/isprs-annals-v-1-2020-199-2020 https://doi.org/10.5194/isprs-annals-v-1-2020-199-2020 https://doi.org/10.1109/epe61521.2024.10559538 https://doi.org/10.33383/2022-021 acta polytechnica ctu proceedings 52:1–7, 2025 1 introduction 2 survey of studies on lighting and luminance measurement methods 3 method of data processing and comparison 3.1 comparison of measurements 3.2 summary of verification measurement results 4 discussion 5 conclusion references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2022.38.0443 acta polytechnica ctu proceedings 38:443–449, 2022 © 2022 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague climate-resilient and resource-conserving architecture through renewable building materials and microclimate improvement elena vaklinova mitrenova∗, martin aichholzer, aida santana sosa fh campus wien, department “building and design”, favoritenstraße 226, 1100 vienna, austria ∗ corresponding author: elena.mitrenova@fh-campuswien.ac.at abstract. due to climate change, together with the need to reduce the ecological footprint and the future resource shortages, a climate-resilient and resource-conserving architecture must be reinforced. dealing with the issue of resources not only affects the materialisation of the building, but also the handling of resources on the building site. the interactions between the environment (sun, wind, precipitation), buildings, sealing, plants and people form a complex system in which small changes in few factors can influence the situation on a large scale. in this context, topics such as microclimate improvement around built infrastructure through greening and rainwater management, will gain in importance. the correct assessment of measures for a sustainable and resilient building is extremely complex and time-consuming and requires extensive, multi-layered know-how and experience. this paper analyses the project “house of learning” (magk architekten) and its immediate surroundings and focuses on its climate resilience and neutrality, proposing improvement measures based on the interaction of blue and green infrastructure and the building. the potential favourable conditions are evaluated through microclimate simulations and planning principles implying an integral approach which includes landscape gardeners, building planners and constructors, as well as decision-makers. keywords: climate resilience, sustainable buildings, renewable building materials, blue and green infrastructure, microclimate simulations. 1. introduction co2 reduction has been a main topic in the political and scientific discourse through the last years with the aim to achieve the goals settled to mitigate the climate change. numerous international organisations like united nations with the kyoto protocol [1] and the paris agreement [2], european commission with the eu taxonomy [3] define guidelines in order to navigate the process. concerns about handling of natural resources in the construction industry are increasing enormously, since this sector is the largest consumer of materials and the largest producer of waste worldwide [4, 5]. the unep’s global status report for buildings and construction [6] stresses the fact that: “co2 emissions from the operation of buildings have increased to their highest level yet at around 10 gt co2, or 28 % of total global energy-related co2 emissions. with the inclusion of emissions from the buildings construction industry, this share increases to 38 % of total global energy-related co2 emissions” [6] (figure 1). therefore, the building sector offers a great deal of potential for reducing the global use of energy, space and materials through planning, technical and design measures on, in and around the building. at national levels, government programmes set out measures for the benefit of sustainability and climate protection. the federal government of austria’s goal is to achieve a climate-neutrality of the country by 2040 at the latest, with a number of interim targets being presented, some of them impacting directly the building industry. the international and national pressure on the building sector to decrease the co2 emissions leads to different solutions, including the manufacturing and construction phase of the building (renewable building materials, modularity), the operational phase (energy consumption, passive house standards) or the end-oflife phase (circular economy, dismantling strategies, cradle to cradle, urban mining). nevertheless, these solutions concentrate explicit on the single construction without taking into consideration the interaction between the environment (sun, wind, precipitation), buildings, sealing, plants and people. this interaction forms a complex system in which small changes in few factors influence the situation significantly. in this context, topics such as microclimate improvement around built infrastructure through greening and rainwater management, gains in importance. 2. blue and green infrastructure the topic of blue and green infrastructure has been of high importance for the european commission in the last decades. the eu biodiversity strategy to 2020 and the nature 2000 network aim to protect the europe’s remaining healthy ecosystems and biodiversity [9]. furthermore, the green infrastructure (gi) strategy, adopted by the eu in 2013 provides 443 https://doi.org/10.14311/app.2022.38.0443 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en e. v. mitrenova, m. aichholzer, a. santana sosa acta polytechnica ctu proceedings figure 1. global share of buildings and constructions based on their energy consumption and emissions [6–8]. different ecosystem regulation services like food protection, water regulation, removal of air and water pollutions [10]. green infrastructure is a network of natural and semi-natural areas, designed to deliver different ecosystem services. blue infrastructure on the other hand are all natural and artificial waters. both are considered to be crucial for preventing risks such as water scarcity, heavy rain or flooding and heat waves, impacting increasingly the european urban areas. consequently, smart and resilient urban and infrastructure planning focuses not only on single technical solutions in the water or construction sector for example, but also harnesses the potential of a holistic picture, i.e. takes into consideration all possible environmental and construction aspects. the interplay of blue, green and gray infrastructures promises multiple and additional options for municipalities to adapt to climate change [11]. this holistic approach on the topic of sustainability is aggravated by the division of tasks during the planning process. the microclimate simulations, greening and rainwater management are considered to be tasks of landscape architects and natural researchers who are often involved in the planning process after the important decisions are already made. consequently, integral planning, i.e. the early involvement of all necessary experts in the planning team and the simultaneous participation of all disciplines and stakeholders in the planning process, is a necessary step to overcome the challenges described. the correct assessment of measures for a sustainable and resilient building is extremely complex and timeconsuming and requires extensive, multi-layered and holistic know-how and experience. 3. the sustainability concept of the case study “house of learning” the questions of this paper are addressed through an analysis of the building “house of learning”, based in st. pölten and initiated by the austrian nongovernmental organization geza. it supports persons with difficulties at the job market and aims their reintegration into an employment by providing advice, guided work experience and job qualification. the building had to provide office spaces for the co-worker on one side and workshop areas for the unemployed apprentices on the other side. the sustainable philosophy of the organisation was transferred to the design concept – the requirement to the architects was to design a building with the “lowest possible impact” on the environment. the footprint had to be as small as possible, while simultaneously complying with the budget of the organisation, which relies on donations and subsidies [12]. the sustainability concept of the building focuses on reduced energy consumption, a maximum use of renewable and regional building materials such as wood, straw and clay, as well as the flexible usability of the building. it meets the passive house standards and is awarded with the highest possible prize for sustainability of klimaaktiv – the climate protection initiative of the austrian federal ministry for climate protection, environment, energy, mobility, innovation and technology [13]. 3.1. reduced energy consumption the heating demand of the building is approx. 12 kwh/m2a. this passive house standard was achieved due to the exceptional performance of the straw insulation (table 1). the building has hydraulic system and the conditioning is achieved mainly through passive methods as night time cooling through the windows and the opening flaps. this measures, suitable to the current climatic conditions in austria, are completed with external sunscreen panels. the building is equipped with two central ventilation systems with heat recovery. as ventilation units were chosen passive house compatible units with automatic bypass. additionally, a concept of an optional installation of a photovoltaic system for a later stage 444 vol. 38/2022 climate-resilient and resource-conserving architecture . . . building components w/(m2k) façade (straw, loam, timber) 0.107 roof (straw, loam, timber) 0.095 floor/foundation (concrete/foam glass gravel) 0.188 windows (timber with aluminium shell) 0.8 table 1. thermal performance of the building [12, 14, 15]. was prepared, but not installed due to the limited budget of the organisation [12]. 3.2. renewable and regional building materials the case study “house of learning” is constructed mainly with recyclable and regional building materials. the proportion of regenerative materials is about 35 %, with partially high recycling potential. additionally, the proportion of materials with metallic or mineral or fossil origin, that can be recycled is about 45 %. clt (crossed laminated timber) is used as main structural material for the tree floors of the building. this material was chosen as well for the ceiling, constructed with five-layered clt elements with a thickness of 12 cm. the façade of the building has a wooden frame construction. it is designed as a nonload-bearing shell, filled with strow. the straw is used in blow-in technology as well as in non-loadbearing bale technology. due to this material an extremely high insulation performance was achieved. the insulation properties correspond to an energy transfer value (λ) of about 0.045 w/m2k. straw was used not only for the external wall, but as well for the ceiling. the roof is made of prefabricated straw bale-filled frame elements between roof trusses. additionally, clay plaster is used as wall covering on the south-facing interior walls and on the insides of the exterior walls. the high ecological potential of this material and its favourable properties in building physics brought additional positive effects for the indoor climate and conditioning in different areas in the building [12]. an important part in the sustainability concept was the design, which enables a flexible usage and the change of the functions of various spaces. furthermore, the building can be easily dismandled and most construction elements are reusable [12]. based on this pilot project, experts from different disciplines were invited to an interdisciplinary workshop with the aim to create concepts for resilient and climate-neutral planning in and around the building. the analysis was supported by a microclimate simulation of the building and its environment that demonstrated the potential for improvement despite its exceptional thermal performance and sustainability. it shows the necessity of implementation of a holistic approach, i.e. the integration of all planning aspects in order to achieve climate resilience and neutrality. 4. methods of analysis 4.1. microclimate simulation microclimate is the climate in a small, well-defined area. it is strongly influenced by local conditions, such as the type and density of plants, the type and texture of the soil and the prevailing light conditions. it is also characterized with temperature fluctuations and strongly dependent on ground friction of air movements and the building structures in the surrounding areas. influence on the microclimate have on one hand the chosen building materials and on the other hand the changed wind and light conditions due to the buildings. the consideration of all aspects that effect the microclimate is a complex process, enabled through simulation tools. a microclimate simulation makes the impact of buildings, urban structures, and green and blue infrastructure on the urban environment visible, comparable and understandable. it can be used in the early stages of the planning or as a tool for improvement of existing building structures and their surroundings. it enables more efficient planning of the required measures (suitable siting of the building, installation of façade greening on façades that overheat, change in the wind and shadow zones). consequently, the microclimate simulation provides a detailed analysis of impacts on the construction site as well as in the surrounding area with comparison of several planning variants (best case, state of the art, worst case). based on the simulation data, the microclimate can be improved through optimization of the building, optimization of the building materials, improvement of the degree of sealing and blue and green infrastructure. one of the most used software for microclimate simulations is envi-met [16]. the aspects, considered in the tool are: (i) solar analysis (sun and shadow hours, reflection analysis, shadow casting, solar radiation); (ii) pollutant dispersal (emission and transport of gases and particles, chemical reactions, deposits); (iii) building physics (façade temperature, exchange processes with forests, microclimate inside and outside of the buildings, water and energy balance of living wall systems); (iv) green and blue technologies (facades and green roofs, influence of green areas on water bodies, simulation of “living wall” systems, air cooling and 445 e. v. mitrenova, m. aichholzer, a. santana sosa acta polytechnica ctu proceedings water spray); (v) wind flow (wind patterns, speeds and comfort); (vi) thermic exterior comfort (air and radiation temperature, air movement, relative humidity); (vii) tree pass (plant growth conditions, simulation of wind stress, tree damage and water consumption) [16]. the simulation of the case study “house of learning” was performed on the basis of the greenpass software [17]. it builds on envi met simulation, with further developed analysis and evaluation tools. first, a basic evaluation of the project was carried out and the data – imported in the software. the areas were divided into traffic areas, green areas and buildings. additionally, this step required a precise description of the attributes: green spaces definition (lawn, meadow, shrubs, and perennials); traffic areas (paved and unpaved surfaces inclusive structure e.g. asphalt roadway); details about the building structures (height, façade material, roof material and greening) and trees (type, crown diameter). challenging by the simulation of the case study was the collection of data about the used building materials (a.o. straw insulation and clay plaster) that are not standard products and were not listed in the data base of the simulation programme. this is the main limitation of the software the researchers faced during the modelling exercise. for the simulation, variants of only the green and blue infrastructure were elaborated. the existing building was assumed as status quo and its parameters were not changed. the reason for this decision lies in the excellent sustainable qualities of the building regarding its construction. therefore, he main emphasis of the present research is on the improvement of the building by further measures considering primary green and blue infrastructure. for the analysis of the case study four different scenarios were worked out according to certain rules: (i) worst case (wc) – extreme sealing, no tree planting; (ii) moderate (mod) – scenario between wc and bc, greening according to certain rules; (iii) best case (bc) – extreme greening: facades, high roof construction, open-pored paths, tree planting; (iv) state of the art (sq) for the assessment of the current stock or the current planning. these scenarios were provided to the experts invited to an interdisciplinary workshop and served as the basis for the elaboration of a holistic concept for climate resilience of the case study. 4.2. interdisciplinary expert workshop the described challenges of the interaction between the building and its environment, were addressed through an interdisciplinary workshop on the topic of climate resilience and neutrality. professionals from different disciplines (architecture, construction engineering, landscape architecture, natural and soil science) were invited to analyse the project “house of learning” (magk architekten) and propose different solutions and improvement measures based on the interaction of blue and green infrastructure and the building. during the five days of the workshop, interdisciplinary and holistic approaches were presented and discussed with the aim of promoting comprehensive, climate-resilient and climate-neutral planning. on the one hand, the contents included the sustainability of the building (life cycle assessment, selection of renewable building materials, building physics measures for the improvement of the indoor climate, economical use of energy and resources). on the other hand, innovative concepts for climate protection and resilient construction were elaborated around the building. the topics of green and blue infrastructure, soil sealing, ecological flooring, facade and roof greening for sustainable cooling, protection of biodiversity offered a comprehensive natural science view of the challenge. the invited experts came together and created a mutual, multidisciplinary understanding about the complexity of climate resilience in the construction industry. the generated knowledge and cross-disciplinary understanding will have an impact on the professional activities of the planners who participated in the workshop. some of them working with conventional building solutions have expressed strong interest in low-tech sustainable passive cooling and greening measures and have the intention to integrate them into future projects. one of the key insights of the workshop is the necessity for integrated planning in order to construct climate resilient buildings, i.e. the consideration and integration of green and blue infrastructure measures even in the earlier planning stages. 5. results and discussion the potentials for improvement of the case study “house of learning” were elaborated by the experts on the basis of the microclimate simulation through the comparison between the best case (bc), the state of the art (sq) and the worst case (wc) scenarios. the (sq) is characterized through extensive green roof area of the building, seepage paving on the traffic areas and trees in the surrounding area. for the best case scenario (bc) the following improvement were made: unsealed traffic areas in the project area, intensive green roofs, facade greening on all sides except to the north and additional trees. the main indicators for the comparison of the scenarios were: (1.) air temperature at 3 pm; (2.) physiological equivalent temperature (pet) at 3 pm and (3.) wind field at 3 pm (figures 2–4). 446 vol. 38/2022 climate-resilient and resource-conserving architecture . . . figure 2. air temperature 3 pm – comparison between best case (bc), the state of the art (sq) and the worst case (wc) scenarios. simulation processing by grünplan. figure 3. physiological equivalent temperature (pet) 3 pm – comparison between best case (bc), the state of the art (sq) and the worst case (wc) scenarios. simulation processing by grünplan. figure 4. wind field 3 pm – comparison between best case (bc), the state of the art (sq) and the worst case (wc) scenarios. simulation processing by grünplan. 447 e. v. mitrenova, m. aichholzer, a. santana sosa acta polytechnica ctu proceedings the simulations showed significant differences between the scenarios. it is evident that the air temperature (absolute in °c) is 2–3 degrees higher in the state of the art (sq) than in the best case (bc) scenario (figure 2). this difference increases drastically with the indicator (pet), that demonstrates the impacts of the local climate on the thermal comfort of the human body. the low thermal comfort in (sq) shows, that it gets really hot, especially in the forecourt. the perceived temperature due to the tree plantations in the (bc) is with 16.5 °c lower than in the treeless scenario (sq) (figure 3). a clear improvement could be noticed in (bc) on the own building and the surrounding area not only in the reduced absolute and perceived temperature, but as well in the reduced air ventilation due to tree canopy (figure 4). on the basis of the differences in the scenarios the experts identified few challenges/potential points for improvement of the case study despite its excellent thermal performance and sustainability concept. the forecourt tends to overheat and has poor runoff coefficients. its optimization and temporary usage as a park and storage area is necessary as well as the installation of bicycle parking. the green roof has too little substrate for extensive greening and low retention volume. the roof terrace used for employee breaks is highly exposed to wind and sun. the roof drainage functions currently via absorption wells and there are no installed pv systems. also, the question of the functionality of the ventilation flap for night cooling was raised. recognising the problems described, the experts elaborated numerous strategies for the improvement of the microclimate of the building “house of learning”. these included increased substract thickness of the green roof to 12 cm and a solid drainage in order to stimulate the roof greening. additionally, a facade greening on the west side of the building, pergola and green walls on the roof terrace and shading of the parking lot through tree planting and shortened parking spaces was planned. these measurements aim to minimize the overheating of the forecourt and the building. the biodiversity of the surrounding area should be increased not only through the new vegetation, but as well through insect hotels and bee hives. furthermore, the existing pavement should be replaced with pavement with joint ratio > 15 % in order to prevent soil sealing. the experts planned the establishment of storage volume for roof water and the installation of a photovoltaic system on the roof that is adapted to the extensive greening. finally, an optimization concept for a flexible usage of the forecourt was elaborated through containers for external warehouse and additional workshop areas. the elaborated results were provided to the users of the building (the managers and employees of the geza organisation) and were taken into consideration for future conversion of the building. 6. conclusion the paper shows the crucial importance of a holistic approach for a climate resilience planning. sustainable architecture focuses primary on energy improvement and renewable materials with low impact on the environment. nevertheless, building structures exist not only on themselves, rather than in a complex interaction with their surroundings (sun, soil, water). this fragile system depends on every single aspect and should be considered holistically in the early stages of the planning. suitable tool for this is a microclimate simulation that takes into consideration the performance and siting of the building, the building materials, degree of sealing, blue and green infrastructure, pollutant dispersal, wind and solar circumstances. the analysed case study “house of learning” is a pilot project with excellent thermal performance, passive house standards and build almost exclusively with renewable and local building materials (timber, strow, clay). depside its comprehensive sustainable characteristics the microclimate simulation of the project demonstrated potential for improvement and necessity for better connection with the surrounded environment. based on the simulation data, experts from different disciplines worked out a holistic solution for a climate-resilient and resource-conserving architecture. acknowledgements special thanks are extended to the architecture office magk architektur aichholzer klein for providing the plans and the data for the analysis of the building “house of learning” as well as to grünplan landschaftsarchitekten for implementing the microclimate simulation. references [1] unfccc. kyoto protocol to the united nations framework convention on climate change, 1998. [2] unfccc. the paris agreement, 2015. [3] european parliament and council. regulation (eu) 2020/852 of the european parliament and of the council of 18 june 2020 on the establishment of a framework to facilitate sustainable investment, and amending regulation (eu) 2019/2088. [2021-10-04]. https://eur-lex.europa.eu/legalcontent/en/txt/?uri=celex:32020r0852 [4] b. berge. the ecology of building materials. elsevier/architectural press, amsterdam, 2009. [5] t. abergel, b. dean, j. dulac. towards a zero-emission, efficient, and resilient buildings and construction sector: global status report 2017, un environment and international energy agency, 2017. [6] unep united nations environment programme. global status report for buildings and construction: towards a zero-emission, efficient and resilient buildings and construction sector, 2020. [2021-10-06]. https: //globalabc.org/news/launched-2020-globalstatus-report-buildings-and-construction 448 https://eur-lex.europa.eu/legal-content/en/txt/?uri=celex:32020r0852 https://eur-lex.europa.eu/legal-content/en/txt/?uri=celex:32020r0852 https://globalabc.org/news/launched-2020-global-status-report-buildings-and-construction https://globalabc.org/news/launched-2020-global-status-report-buildings-and-construction https://globalabc.org/news/launched-2020-global-status-report-buildings-and-construction vol. 38/2022 climate-resilient and resource-conserving architecture . . . [7] iea. energy technology perspectives 2020, iea, paris, 2020. [2021-10-06]. https://iea.blob.core.windows.net/assets/ 7f8aed40-89af-4348-be19-c8a67df0b9ea/energy_ technology_perspectives_2020_pdf.pdf [8] iea. world energy balances 2020: overview, iea, paris, 2020. [2021-10-06]. https://iea.blob.core.windows.net/assets/ 23f096ab-5872-4eb0-91c4-418625c2c9d7/world_ energy_balances_overview_2020_edition.pdf [9] european commision. green infrastructure in policy. [2021-10-05]. https://ec.europa.eu/environment/ nature/ecosystems/policy/index_en.htm [10] c. estreguil, g. dige, s. kleeschulte, et al. strategic green infrastructure and ecosystem restoration. geospatial methods, data and tools. publications office of the european union, luxembourg, 2019. eur 29449 en, isbn 978-92-79-97294-2. [11] j. h. trapp, m. winker (eds.), bundesministerium für bildung und forschung (bmbf). blau-grün-graue infrastrukturen vernetzt planen und umsetzen. ein beitrag zur klimaanpassung in kommunen. pp. 15–20. 2020. [12] e. mitrenova, m. aichholzer, m. labner. assessment of the environmental impact of building materials: the case study "house of learning". in wcte 2020 – world conference on timber engineering. 2021. [13] austrian federal ministry for climate protection, environment, energy, mobility, innovation and technology. der klimaaktiv kriterienkatalog 2020. [202110-07]. https://www.klimaaktiv.at/bauen-sanieren/ gebaeudedeklaration/kriterienkatalog-2020.html [14] magk architekten aichholzer | klein zt og: gesa haus des lernens | st. pölten. [2021-10-07]. https://www.magk.at/_files/ugd/693862_ f436fd1703e74dcf94254ac1aae3e941.pdf [15] magk architekten aichholzer | klein zt og: gesa haus des lernens | st. pölten. [2021-10-07]. https://www.magk.at/hdl [16] envi-met. simulation software. [2021-09-27]. https://www.envi-met.com/de/ [17] greenpass. simulation software. [2021-09-27]. https://greenpass.io/de/ 449 https://iea.blob.core.windows.net/assets/7f8aed40-89af-4348-be19-c8a67df0b9ea/energy_technology_perspectives_2020_pdf.pdf https://iea.blob.core.windows.net/assets/7f8aed40-89af-4348-be19-c8a67df0b9ea/energy_technology_perspectives_2020_pdf.pdf https://iea.blob.core.windows.net/assets/7f8aed40-89af-4348-be19-c8a67df0b9ea/energy_technology_perspectives_2020_pdf.pdf https://iea.blob.core.windows.net/assets/23f096ab-5872-4eb0-91c4-418625c2c9d7/world_energy_balances_overview_2020_edition.pdf https://iea.blob.core.windows.net/assets/23f096ab-5872-4eb0-91c4-418625c2c9d7/world_energy_balances_overview_2020_edition.pdf https://iea.blob.core.windows.net/assets/23f096ab-5872-4eb0-91c4-418625c2c9d7/world_energy_balances_overview_2020_edition.pdf https://ec.europa.eu/environment/nature/ecosystems/policy/index_en.htm https://ec.europa.eu/environment/nature/ecosystems/policy/index_en.htm https://www.klimaaktiv.at/bauen-sanieren/gebaeudedeklaration/kriterienkatalog-2020.html https://www.klimaaktiv.at/bauen-sanieren/gebaeudedeklaration/kriterienkatalog-2020.html https://www.magk.at/_files/ugd/693862_f436fd1703e74dcf94254ac1aae3e941.pdf https://www.magk.at/_files/ugd/693862_f436fd1703e74dcf94254ac1aae3e941.pdf https://www.magk.at/hdl https://www.envi-met.com/de/ https://greenpass.io/de/ acta polytechnica ctu proceedings 38:443–449, 2022 1 introduction 2 blue and green infrastructure 3 the sustainability concept of the case study ``house of learning'' 3.1 reduced energy consumption 3.2 renewable and regional building materials 4 methods of analysis 4.1 microclimate simulation 4.2 interdisciplinary expert workshop 5 results and discussion 6 conclusion acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2023.42.0083 acta polytechnica ctu proceedings 42:83–86, 2023 © 2023 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague dynamic tests of the protective and security barrier system probar pavel vrtal∗, tomáš kohout, luboš nouzovský, zdeněk svatý czech technical university in prague, faculty of transportation sciences, department of forensic experts in transportation, konviktská 20, 110 00 prague, czech republic ∗ corresponding author: vrtal@fd.cvut.cz abstract. the purpose of this paper is to present a series of dynamic tests for testing the protective and security barrier of the probar system, developed by strix chomutov, a.s., according to the pas 68:2013 standard. the goal of the tests was to assess how the obstacle resist impacts and whether it can absorb the energy exerted on it. verification of the mechanical properties of the device was performed in two types of impact configurations. emphasis was placed on the behaviour of both the obstacle and the vehicle during the impact and in the short time interval after the collision. in terms of criteria for evaluating the impact tests, it can be stated that test 2 exceeded the theoretical head impact velocity (thiv) criterion by 20 km h−1, while test 1 did not exceed the limit value. however, all values for other criteria (asi, phd, hic, 3ms) were within the specified limits, and the safety barrier met the required criteria which allows to be certificated. keywords: ressistant systems, accident reconstruction, road safety, crash tests, security barrier. 1. introduction security devices and barriers that allow the prevention of unauthorised entry of cars into areas with a high density of people are now taken as standard. they can be found in various forms, ranging from simple barriers in the form of concrete blocks to architecturally modified objects, pas68 800 mm road blocker (heald ltd., uk) and anti-terrorist security planters (safetyflex barriers, uk). the purpose of these structures is mainly to prevent the intrusion of trucks, which by their weight and resulting momentum exceed passenger vehicles and can cause high damage. their correct functionality and effectiveness must be individually tested. it is essential that the manufacture of these devices meets the necessary safety criteria to assess and evaluate the resistance of the obstacle. at the same time, the effect of the safety barrier on the driver of the vehicle or on the surrounding area must also be assessed. the subject of this paper is to present a series of dynamic tests (crash tests) in testing the protective and security barrier of patented (patent no. 308880) probar system, which was provided by strix chomutov a. s., czech republic. 2. legislative provisions one of the best known documents dealing with safety assessment is the public available specifications standard, pas 68:2013, provided by the british standards institute. in 2005, it became the first british standard to assess the specification of impact tests and these findings are now incorporated in the european standard en 1317-1:2010. this standard is also the adopted czech technical standard čsn en 1317 (737001). the determination of all standards from this publication is essential to properly evaluate the functionality and reliability of a given security barrier. this case study makes full use of all the requirements that should be followed in evaluating each criterion against the requirements and all the safety procedures of the pas 68:2013 standard. 3. crash tests the objective of the tests was to verify the mechanical properties of the subject device with emphasis on the behaviour of the barrier and the vehicle during and in a short interval after the impact. at the same time to determine whether the objects in consideration meet the requirements defined in the mentioned standards and can be implemented in practice. the installation of the tested barrier was on a flat and dry asphalt surface and the overall impact behaviour was analysed using high-speed cameras. their application was mainly to determine the impact velocity into the test objects, but also to document details and determine the deformations of the individual objects. cameras a6300 – i-speed 720 (sony, japan) with frame rates ranging from 100 fps up to 15 000 fps from several shooting angles were used. two unmanned aerial vehicles were also used to capture the collision. at the same time, measurement methods using total stations were utilized in order to precisely target individual objects for subsequent reconstruction of the accident scene. two types of crash tests, each with a slightly different crash configuration, were performed to verify the functionality of the objects. the aim of the different configurations was mainly to verify the full functionality of the tested devices in their entirety. test no. 1 83 https://doi.org/10.14311/app.2023.42.0083 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en p. vrtal, t. kohout, l. nouzovský, z. svatý acta polytechnica ctu proceedings was assumed to impact into the steel column, while test no. 2 was assumed to impact into the steel cables. one of the test conditions is that the vehicle should not be braked except when the safety situation requires it. thus, after the impact, the vehicle moved to its final position by inertia and without deceleration produced by the braking system. 3.1. technical parameters of the tested security barrier the construction of the barrier consists of a set of identical steel columns interconnected by two steel cables (figure 1). the columns are made up of 324 mm/12.5 mm (outside diameter/width of material) seamless thick-walled steel tubes, with a column height of 918 mm and a weight of approximately 120 kg. each column is fixed to the subgrade using 4 anchors (ibo32, 2 m long, fixed with grout). the method of anchoring may vary according to the specific geological investigation of the subsoil, always in relation to the required strength of the anchoring connection. the axial distance between two columns is 3.5 m. the technical parameters of the wire ropes used to connect the columns to each other are 20 mm 6x36ws-iwrc with a weight of 1.64 kg m−1. the bottom rope is located at a height of 550 mm, the top rope at a height of 800 mm. figure 1. probar system – test layout 3.2. testing vehicles ford vans (transit 85 t280 and transit t 300) were chosen for the crash tests, imported to a total weight of 3.5 t. according to pas 68:2013, these vehicles are in category n1 (day cab vehicles). the impact speed was determined to be 50 km h−1 (±3 km h−1) in the form of a perpendicular collision, with the support of a guided vehicle. the vehicles were equipped with tri-axial accelerometers recording the crash scenario at two measuring points (front and side wall of the vehicle). in order to assess safety within the biomechanical criteria, a hybrid iii 50th male dummy (humanetics innovative solution, usa) equipped with its own sensors was fitted in the driver’s seat of the vehicle for test 1. the positions of the sensors fitted in the dummy were located on the head, chest and pelvis. these devices have a range of 1 500 g and are capable of measuring upper and lower neck force and moment, chest compression, lumbar force and moment, and knee deflection. the weight of the dummy is 77.7 kg and the sitting height is 884 mm. 3.3. assesing criteria during impact tests, several criteria used for evaluating the safety of barriers according to the standard čsn en 1317 and also by pas 68:2013 were assessed. the criteria assessed are aimed at evaluating the effect of kinetic energy on the vehicle occupants [1–3]. acceleration severity index (asi) is the first defined criterion which defines the impact severity index. it is one of the main criteria used to evaluate road restraint systems. the asi criterion is calculated from the measured acceleration on the car body. the criterion should not exceed a value of 1 in a given case and can be expressed mathematically: asi(t) = [( āx âx + āy ây + āz âz )2 ] 1 2 , (1) where âx, ây, and âz are limit values for particular acceleration components and can be interpreted as a value of applied acceleration during which the risk of a grave injury of the passenger is minimized (max. light injuries). for a fastened passenger the limits are generally used âx = 12 g, ây = 9 g and âz = 10 g. the magnitudes āx, āy or if you like āz are filtered components of the acceleration. asi is set as maximum of the range asi(t) [4]. theoretical head impact velocity (thiv) is the another considered criterion. the thiv concept has been developed to determine the impact severity of a vehicle occupant in a collision with a restraint system. the magnitude of the thiv is considered to be a measure of the severity of the impact of the vehicle on the restraint system. it is calculated as: thiv = [ ẋ2 b(t) + ẏ2 b(t) ] 1 2 , (2) where ẋ2 b(t) and ẏ2 b(t) are the coordinates of the theoretical head speed in relation to the vehicle coordinate system. this criterion is chosen when the vehicle is not equipped with biomechanical dummies. the limit of this criterion is set by en 1317 at 33 km h−1 [4]. post-impact head deceleration (phd) criterion was another criterion used to assess the impact severity. the phd criterion was an additional criterion. after the revision of the czech standards, it is no longer monitored. phd = max [ ẋ2 c(t) + ẏ2 c (t) ] 1 2 , (3) where ẋ2 c(t) and ẏ2 c (t) is the theoretical head acceleration after first contact with the structure. the phd should not exceed 20 g. another criterion considered is the head injury criterion (hic). this criterion can be seen as a measure of the probability of head injury in an impact. it is calculated as the maximum of the resultant acceleration function a(t). 84 vol. 42/2023 dynamic tests of probar hic = [ 1 t2 − t1 ∫ t2 t1 a(t) dt ]2.5 , (4) where a(t) is the resulting acceleration [g] and t1, t2 are the time instants during the impact determining the beginning and end of the interval for which the hic value is the highest. the length of the interval (t2 − t1) is 36 ms in the case where no contact with a solid structure has been detected. for a “hard” impact, a time interval of 15 ms (hic15) is considered to calculate the maximum value. the limit value of the hic criterion is 1 000 – the limit indicates a 50 % probability of severe head injury [5, 6]. the last criterion assessed was the 3ms criterion. unlike the previous criterion (thiv), it is applicable not only to head injuries. its threshold value determines the tolerance for the occurrence of severe injury. the limit value for the head is 80 g (accelerations greater than 80 g must not act for more than 3 ms) [4, 7]. 4. results of the tested barrier the test results can be divided into two parts. in the first part, the accident site was back-tracked and reconstruction of the accident was performed using laser scanning and high-speed camera footage. the second phase focused on the evaluation of accelerometer data and performed calculations according to the above mentioned standard. the results of the tests are summarised in the following table 1. crash test no 1 measured value limit value asi [-] 0.75 1 thiv [km h−1] 33 33 phd [g] 11.8 20 hic36 [-] 32.23 100 3ms [g] 16.61 80 crash test no 2 measured value limit value asi 0.89 1 thiv 53 33 phd 14.5 20 hic36 unmeasured 100 3ms unmeasured 80 table 1. crash tests results 4.1. crash test into steel column (test no. 1) during the impact to the barrier, the column was uprooted, but it was not completely ripped out of the ground (it remained anchored in the bedrock at two points). the vehicle was therefore restrained by the system. the surrounding posts were slightly deformed at their fixing points (in the base plates) as a result of the impact. however, in no other case was the column dislodged. the lower steel cable was also broken at the point of impact, but the others remained intact. when the nose of the car hit the barrier, the right support strut of the car was bent towards the engine compartment, where it damaged the engine of the car. in addition, the right front axle was deformed by contact with the barrier post. this part of the car was pushed up to the point where it came into contact with the body section. this caused a transfer of deformation to the side and upper parts of the bodywork and damage to the windscreen. the right door was deformed and the passenger side space was distorted. the steel cables of the barrier collided with the height above the bumper reinforcement and therefore caused damage in the upper part of the engine compartment. contact with the steel cables caused deformation of the engine compartment lid including the headlights and both front fenders. the impact with the barrier caused the separation of some parts of the vehicle, e.g. headlight, turn signal, radiator grille, fragments of plastic from the bumper, etc. 4.2. crash test into steel cable (test no. 2) in the case of impact with the steel cable, the damage was more pronounced than in the previous test. in this case, two columns were uprooted. however, these again remained anchored to the ground in two places and the vehicle was restrained. the other columns also showed more pronounced deformation, being deformed at the point of attachment and also tilted by the pull of the steel cables during the impact. in addition, the steel wire rope was distorted, but not completely severed as in the previous test. after the impact of the bow on the steel cables, the bumper cover above the bumper reinforcement was damaged. the areas of the bonnet cover and both headlamps were deformed. the subsequent penetration of the barrier into the engine compartment caused deformation of both fenders, pressing of the radiator wall against the engine and subsequent complete deformation of the engine compartment, with damage to the windscreen and the crew compartment. some parts of the vehicle were separated, e.g. headlight, turn signal, radiator grille, fragments of plastic from the bumper, etc. 5. conclusions the overall evaluation shows that the tested barrier was able to absorb and resist the energy produced by the vehicle during the impact. in terms of the vehicle occupant injury assessment criteria, it can be concluded that test no. 1 did not exceed any threshold values. test no. 2 exceeded the thiv criterion by 62 % above the threshold and it is likely that the driver of the vehicle would have been injured. however, the functionality of the safety barrier was assessed as 85 p. vrtal, t. kohout, l. nouzovský, z. svatý acta polytechnica ctu proceedings effective and filled all the requirements defined by pas 68:2013. acknowledgements this work was supported by the project “vývoj ochranné bariéry k zamezení vjezdu nežádoucích vozidel – provedení nárazových testů, inovační voucher ústeckého kraje 2020”. references [1] united nations economic commission for europe. regulation no 21 of the economic commission for europe of the united nations (un/ece) – uniform provisions concerning the approval of vehicles with regard to their interior fittings, 188:32–70, 2008. [2023-06-22]. http://data.europa.eu/eli/reg/2008/21(2)/oj. [2] united nations economic commission for europe. uniform provisions concerning the approval of head restraints (headrests), whether or not incorporated in vehicle seats, 258:30–50, 2020. [2023-06-22]. http://data.europa.eu/eli/reg/2020/1169/oj. [3] united nations economic commission for europe. regulation no 94 of the economic commission for europe of the united nations (un/ece) – uniform provisions concerning the approval of vehicles with regard to the protection of the occupants in the event of a frontal collision, 254:77–135, 2012. [2023-06-22]. http://data.europa.eu/eli/reg/2012/94/oj. 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https://doi.org/10.4271/780887 acta polytechnica ctu proceedings 42:83–86, 2023 1 introduction 2 legislative provisions 3 crash tests 3.1 technical parameters of the tested security barrier 3.2 testing vehicles 3.3 assesing criteria 4 results of the tested barrier 4.1 crash test into steel column (test no. 1) 4.2 crash test into steel cable (test no. 2) 5 conclusions acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2023.43.0093 acta polytechnica ctu proceedings 43:93–105, 2023 © 2023 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague the influence of the construction of tram fronts on the consequences of accidents with passenger cars jakub seidl czech technical university in prague, faculty of mechanical engineering, department of automotive, combustion engines and railway engineering, technická 4, 160 07 prague, czech republic correspondence: jakub.seidl@fs.cvut.cz abstract. in recent years, there has been a large increase in passenger and public transport in cities. an increase in traffic flow results in an increasing risk of accidents. manufacturers and operators of trams are increasingly subject to higher requirements for ensuring the safety of vehicles. vehicle safety can be divided into two groups: active and passive safety. systems and elements of active safety are intended to reduce the risk of accidents. systems and elements of passive safety are intended to minimize the consequences of accidents. the requirements for the passive safety of newly designed railway vehicles are described in standard en 15227. standard en 15227 was created due to the effort to protect passengers and crew inside railway vehicles in the event of an accident, so it is a so-called self-protection. the standard en 15227 does not stipulate any requirements for so-called partner protection. partner protection means an approach in which the vehicle protects passengers inside and outside the vehicle with its deformation elements. this article deals with the issue of how the construction of the front of tram vehicles designed according to the requirements of the standard en 15227 will affect the consequences of tram´s accidents with passenger cars. the first part of the article describes the requirements of the standard en 15227 for newly designed trams. than it describes the creation and evaluation of tram accident statistics in the czech republic for the years 2016 to 2018 with regard to the types of collision vehicles, collision directions and consequences of accidents. from the results of the evaluation of the accident statistics, a collision scenario was determined, in which the passengers inside cars were most often injured. the last part of the article is devoted to the creation of simulation models of accidents of three trams with a car and evaluation of simulation calculations with regard to the risk of injury to car´s driver in an accident with trams using human biomechanical criteria. keywords: tram, cars, vehicle safety, passive safety, accident, en 15227, human biomechanical criteria. 1. introduction the modern tram transport, today provided mainly by partially or fully low-floor vehicles, belongs to one of the basic pillars of the public transport in every modern city with a population over 100 000 inhabitants. the tram transport is most often used to transport large numbers of passengers from more remote neighbourhoods to the city centre. due to the historical development of most european cities, it is not possible to operate trams on their own transport lanes separated from other road users and thus completely eliminate the risk of accidents. due to the need to change the direction of the travel, tram tracks cross with the lanes of other road users. each of these crossings increases the risk of a tram accident with passenger cars (hereinafter only cars). every tram accident with another road user is a major intervention in the flow of the passenger transport. the accident will result in a temporary local paralysis of the tram system, as the vehicles must remain at the scene of the accident until the accident is recorded by the staff of transport company and is given a permission to clear the accident from the railway inspection. it takes an average of 20 minutes to clear accidents with a material damage. with more serious consequences, such as a serious injury or a death, it takes 2 to 3 hours to clear accidents. the entire section of the tram line is impassable during the entire period of clearing the consequences of the accident, which leads to delays of other trams and displeasure of tram users. traffic accidents are most often caused by human faults. unfortunately, in the road transport, the highest price is often paid for these faults, namely the health or the life of the accident participants. the vehicle design must therefore be carried out with the knowledge that people are making mistakes and, despite the introduction of the improved active safety features, accidents cannot be prevented with an absolute certainty. therefore, it is necessary to equip newly designed vehicles with improved elements of passive safety, which will lead to reducing of the consequences of accidents on all vehicles involved in the 93 https://doi.org/10.14311/app.2023.43.0093 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en jakub seidl acta polytechnica ctu proceedings figure 1. tram škoda 15t praha [2]. accident. this vision of an approach to increase road safety and reduce the consequences of accidents on passenger health is based on the vision 0 program, which was introduced in 1995 in sweden. the basic goal of the vision 0 program is to create and adopt such measures of the transport system and on vehicles in order to prevent deaths and to minimize serious injuries caused by traffic accidents by 2050 [1]. to increase the passive safety of newly designed railway vehicles, the standard en 15227 was issued in 2008. at present, trams of older production times are operated in the czech republic. these trams were not designed in accordance with the requirements of standard en 15227. examples of older trams are t3, škoda 14t and 15t (see figure 1). in recent years, the vehicle fleet in the czech republic is being renewed. for example, the new trams from škoda, which are in operation in ostrava (see figure 2), pilsen and will soon be in operation in brno. these new trams are already designed according to the requirements of the en 15227 standard for protection during accidence. this article deals with the assessment of how the construction of tram fronts according to the requirements of the standard en 15227 will affect the consequences of an accident on cars. the research compares three low-floor trams, which are operated in the czech republic. the research mentioned in the article is a part of the research of the proposed modification of tram fronts to reduce the consequences of accidents on passenger cars, which is carried out at the faculty of mechanical engineering, ctu in prague. 2. standard en 15227 the first revision of standard en 15227 was issued in 2008 due to the large number of accidents involving railway vehicles and the high number of injured passengers inside railway vehicles caused by accidents. a second revision of the standard was issued in 2020 supplemented by new knowledge from operation of railway vehicles, which were designed according to the previous version [4]. figure 2. tram škoda 39t ostrava [3]. the main objective of en 15227 is to reduce the consequences of railway vehicles accidents and to ensure the safety of passengers and crew inside railway vehicles in the event of an accident. it is therefore a so-called self-protection approach, where the vehicle protects only the occupants inside the vehicle and takes other road users aa collision barriers. the requirements of the standard are most often demonstrated by numerical simulation on models that are validated according to impact tests of crash parts of railway vehicles [4]. the standard en 15227 divides railway vehicles into four design categories of the impact resistance according to the different type of the operation, in which the railway vehicle is operated. the construction categories of railway vehicles are listed in table 1. tram vehicles belong to the construction category c-iv [4]. standard en 15227 defines for each design category the collision scenarios for which a new railway vehicle have to be designed. the parameters of the individual collision scenarios were determined so that the collision scenarios as accurately as possible represent the accidents in which the given railway vehicle is most often involved in operation. the following collision scenarios are defined for tram vehicles: a head-on collision of two identical trams, a collision of a tram into a rigid obstacle weighing 3 tonnes and a collision of a tram into deformable obstacle weighing 7.5 tonnes [4]. 2.1. the collision scenario: the frontal impact of two identical trams this collision scenario represents a head-on accident with two trams. frontal accidents of two trams are not so common, but they place the greatest demands on the absorbed energy by the deformation elements and the functionality of anti-climbing devices. accidents involving the climbing of one tram on another usually result in the highest number of injuries of passengers, and therefore the risk of a tram climbing in an accident must be minimized. the consequences of an 94 vol. 43/2023 the influence of the construction of tram fronts category definition examples of vehicle types c-i vehicles intended for operation on trans-european network, international, national and regional lines locomotives, passenger cars c-ii urban vehicles intended for operation only on dedicated railway infrastructure without a link to road transport subway vehicles c-iii light railway vehicles intended for operation in urban or regional networks, in operation on shared lines and linked to road transport suburban tram c-iv light railway vehicles intended for operation on dedicated urban networks with a link to road traffic tram vehicles table 1. railway vehicles design categories according to the standard en 15227 [4]. figure 3. an accident of two trams type čkd t3, during which the tram climbed [5]. figure 4. an accident of two trams type stadler tango nf2 with anti-climbing protection [6]. accident of this type are shown in figure 3. in the accident of two t3 trams, one tram collided with another within a distance of approximately 2 meters, resulting in the deaths of 3 passengers. figure 4 shows the consequences of a head-on accident involving trams equipped with an anti-climbing device [4]. the collision scenario is defined as follows: the first tram stands unbraked on a straight track. the second tram starts on a straight track at a speed of 15 km h−1. the second tram hits the first tram from behind [4]. figure 5. an accident of the tram škoda 15t with a car [7]. figure 6. the scheme of the collision scenario of a tram and a rigid obstacle [4]. 2.2. the collision scenario: a tram crashes into a rigid obstacle weighing 3 tonnes this collision scenario represents a tram accident with a car or a small truck (see figure 5). trams and cars accidents most often occur at a crossing of a tram line with a road. the scheme of the second collision scenario is shown in figure 6 [4]. the collision scenario is defined as follows: a rigid obstacle stands at an angle of 45° across the entire width of the tram. the tram starts on a straight track at a speed of 25 km h−1 and then collides head-on with a rigid obstacle. in the accident, the obstacle 95 jakub seidl acta polytechnica ctu proceedings figure 7. consequences of an accident between a t3 tram and a truck [8]. figure 8. the scheme of the deformable obstacle [4]. may move in the x and y axis, but it is not allowed to rotate [4]. 2.3. the collision scenario: a tram crashes into a deformable obstacle weighing 7.5 tonnes this scenario represents an accident between a tram and a truck or bus at a crossing of a tram line with a road. in tram-truck or bus accident, the point of contact between the vehicles in the accident is at the height of the tram driver’s counter, often resulting in the destruction of the cab (see figure 7). the geometry of the obstacle penalises this fact (see figure 8) [4]. the crash scenario is defined as follows: the obstacle is perpendicular to the tram track at a speed of 15 km h−1 and then collides head-on into a deformable obstacle [4]. 2.4. conclusion trams constructed according to en 15227 are designed to withstand frontal and frontal-side impacts with another vehicle. it can therefore be assumed that in these types of accidents between a tram and a car, only the energy absorbing elements will be damaged and no injuries to tram’s passengers and driver will occur. in order to filling the passive safety requirements of en 15227, newly designed trams are equipped with energy absorbing elements and bumpers on the front and rear of the vehicle and a more robust rough structure design (see figures 9 and 10). figure 9. the front of the tram škoda 39t ostrava [9]. figure 10. the front of the tram stadler tango nf2 [10]. 3. accident statistics of tram vehicles in the czech republic in order to verify the requirements of en 15227 and to determine the most risky collision direction from the point of view of injuries to car occupant, statistics of accidents between trams and other road users in the czech republic for the years 2016 and 2018 were compiled. in compiling accident statistics, the cooperation was established with transport companies operating trams in the czech republic (see table 2) and with the police of the czech republic. different institutions record different information about accidents and therefore it was necessary to link the databases to create comprehensive accident statistics. transport companies record this data about accidents: date of the accident, location of accident, information about the vehicles, short description of the accident and material damage. information on the health consequences of accidents for car occupants is recorded by the czech transport police [11]. the total of 6 816 tram accidents with other road users was recorded in the czech republic from 2016 to 2018. table 3 shows the number of tram accidents recorded by individual transport companies [11]. in order to be able to analyse accident statistics, a methodology was developed within the work on 96 vol. 43/2023 the influence of the construction of tram fronts transport company abbreviation transport company of the city brno a.s. dpmb transport company of the cities liberec and jablonec nad nisou a.s. dpmlj transport company of the cities most and litvínov a.s. dpmost transport company of the city olomouc a.s. dpmo transport company of the city ostrava a.s. dpo transport company of the city prague a.s. dpp pilsen city transport companies a.s. pmdp table 2. transport companies operating trams in the czech republic. transport company 2016 2017 2018 dpmb 423 453 412 dpmlj 40 43 36 dpmost 12 6 9 dpmo 46 35 46 dpo 154 199 180 dpp 1 294 1 566 1 547 pmdp 97 98 120 total 2 066 2 400 2 350 table 3. numbers of accidents recorded by individual transport companies. the project tn01000026, which evaluates individual accidents according to the following evaluation criteria: a type of a collision vehicle (a tram, a car, a truck, a bus, a motorcycle, a bicycle or a pedestrian), a collision direction (front, front-side left/right, side left/right or rear, directions were evaluated against the tram, see figure 11), the culprit of the accident (a tram driver or a collision vehicle driver) and the consequences for the health of the occupants in the collision vehicle (an accident without consequences, a minor/severe injury or a death) [11]. to verify the requirements for the passive safety (the self-protection) of trams defined in the standard en 15227, the accident statistics were evaluated according to the collision vehicle and the collision direction. the results of the evaluation are shown in table 4 [11]. from the results shown in table 4 it is evident that the scenario of a tram colliding with a rigid obstacle of 3 tonnes defined in en 15227 is correctly selected, as this is the most frequent type of accident involving trams in service. there were 122 frontal accidents involving two trams during the period under figure 11. the evaluation criterion: the collision direction. evaluation. there were 104 head-on accidents between trams with a trucks and a bus during the evaluation period. it is therefore evident that the passive safety requirements for trams in en 15227 are correctly defined and are based on accidents in which are trams frequently involved during normal operation [11]. the number of side accidents between trams and other road users is high. this type of accident is most often caused by a failure to estimate the clearance between vehicles. the consequences of this type of accident are most often only material damage [11]. due to the conformity of the results of the accident statistics and the requirements of en 15227 for newly designed trams, it can be assumed that in the case of an accident between a tram and a passenger car, there will be no serious damage to the tram and no serious injuries to the passengers inside the tram. next, the research looked at the consequences of accidents on passengers in cars. the evaluation of accident statistics showed that during the period from 2016 to 2018, most accidents were between trams and cars (the total of 5 046 accidents of this type). on average, there were more than 4 tram accidents with a car per day in czech republic. the most common type of tram accidents with cars were front-side accidents from the right side (the total of 1 838 accidents). the higher frequency of accidents from the right side compared to the left side is due to the distribution of roads in the cities of the czech republic. the tram line is in the middle of the street and the roads for road vehicles are on the sides, see figure 12 [12]. to determine the need for the research to increase the partner protection of trams in the event of an accident with passenger cars, accident statistics was evaluated according to the consequences on the health of passengers in cars. the results of the evaluation are shown in table 5. the evaluation of the accident statistics showed that 299 minor and 12 serious injuries to car drivers occurred in collisions between cars and trams during the period under study. the highest number of injuries to car drivers occurred in front and front-side crashes. injuries occurred in one in seven front and one in 97 jakub seidl acta polytechnica ctu proceedings collision collision vehicle direction bus pedestrian bicycle motorcycle truck car tram front – 9 89 6 4 95 859 122 front-side right 18 162 3 7 277 1 838 1 front-side left 10 54 2 3 49 339 6 side right 71 95 4 9 451 1 754 11 side left 46 20 2 1 72 228 23 rear – 5 2 0 3 2 28 36 total 159 422 17 27 946 5 046 199 table 4. the evaluation of tram accident statistics by the collision vehicle and the collision direction [11]. total number of number of number of number of collision number of minor minor serious serious direction accidents [-] injuries [-] injuries [%] injuries [-] injuries [%] front – 859 141 16.4 9 1.0 front-side right 1 838 128 7.02 2 0.1 front-side left 339 21 6.2 0 0.0 side right 1 754 7 0.4 0 0.0 side left 228 1 0.4 1 0.4 rear – 28 1 3.6 0 0.0 total 5 046 299 5.9 12 0.2 table 5. the evaluation of tram accident statistics according to the consequences on the health of passengers in cars. figure 12. the distribution of a communication in cities in the czech republic [12]. seventeen frontal-side crashes. these crashes are most often caused by car drivers who fail to give the rightof-way to a parallel or oncoming tram when crossing the tram line. due to the long braking distance, tram drivers no longer have time to brake the tram and consequently the tram’s front end hits the side of the car. due to the higher stiffness of the tram head, the sides of the passenger cars are more likely to be destroyed in an accident, resulting in a higher risk of injury to the car drivers. figure 13 shows the consequences of a frontal crash of a t3 tram into the side of a car. the driver of the car was severely injured in the accident [13]. the results of the evaluation of accident statistics showed that the risk of an injury to the car crew in tram accidents is high. this conclusion confirmed the figure 13. consequences of a frontal tram collision into the side of a car [13]. need for further research of reducing the consequences of tram accidents on cars. the highest risk of an injury is in frontal tram accidents with the side of cars, so further research will be conducted on these types of accidents. during the period 2016 to 2018 were operated in czech republic only stadler tango nf2 trams, which were constructed according to the requirements of en 15227. the first 2 units of the newly constructed trams were operated in ostrava from october 2018. by the end of 2018, a total of 26 units of these trams were put into operation [14]. in the last three months 98 vol. 43/2023 the influence of the construction of tram fronts figure 14. contact points for different bumper heights. of 2018, the new trams were involved in 2 frontal and 2 side accidents with cars. one frontal accident caused serious injuries to the car’s driver and the other one did not result in any injury. due to the small number of accidents it was not possible to assess the effect of the tram’s front design according to the requirements of en 15227 on the consequences of accidents with cars from the evaluation of accident statistics. therefore, further research was carried out using the simulation software ls-dyna. 4. selected trams for research the newest trams operated in the czech republic with different front construction were chosen for the research. for the purpose of the research, the selected trams were generally named as type a, b and c. the trams type a are operated on the territory of the city prague since 2011. currently, dpp operates 250 of these trams [15]. the tram type a was chosen because of the absence of a bumper on the front of the tram. in the event of an accident between a tram and a car, the tram front lining and the anticlimbing device come into contact with the side of the cars (see figure 5). the trams type b are among the most modern trams operated in the czech republic. the first trams were delivered to the dpo at the end of 2021. a total of 35 trams are to be delivered to the dpo [3]. the tram type b was chosen due to the location of the energy absorber axes at a height of 775 mm above the tor (see figure 14). the contact points of the tram bumper with the side of the car are at the height of the passenger’s chest in car. the first trams type c were delivered to the dpo at the end of 2018. currently, the dpo operates 40 of these trams [14]. the tram type c was chosen due to the location of the energy absorber axes at a height of 525 mm above the tor (see figure 14). the contact points of the tram bumper with the side of the car are at the height of the passenger’s pelvis in car. 5. design of crash scenario according to the results of the evaluation of the accident statistics presented in table 5, the most frequent injury to car’s drivers occurs when a tram crashes figure 15. the scheme of the proposed crash scenario. frontal into the side of a car. the crash scenario for the research was designed to represent a tram hitting the side of a car at the driver’s door. the collision scenario was designed as follows: a car stands perpendicular across the tram lines, the tram starts moving and collides head-on into the side of the car. the car can move in all directions. a scheme of the proposed collision scenario is show in figure 15. the speed of the tram before the accident is an important factor that influences the consequences of accidents. when the accident statistics were compiled, this data was not provided by the transport companies for individual accidents and therefore it was not possible to determine the average tram speed at the time of the accident. for this reason, the following tram speed at impact were chosen for the research: 10, 15, 20, 25, 30, 40 a 50 km h−1. the speed of 50 km h−1 is the maximum operating speed of trams in mixed traffic. 6. creating a simulation model the simulation models were created to represent as closely as possible the proposed scenario of a collision between a tram and a car. the simulation models were composed of submodels: tram, deformation element, car and driver. the simulation model of the accident between a tram type b and a car is shown in figure 16. the parameters of each submodel will be described in the following chapters. the individual parts of the simulation models were created in the environment of catia v5, ansys and ls-prepost. 6.1. the simulation models of trams when constructing trams, their rough structures are dimensioned according to the legislative requirements for durability and strength in normal operation and in the event of an accident. simulation models of trams are then created to represent the rough structures of real trams as closely as possible. the crash calculations performed on these models can predict with high accuracy the course of accidents and possible deformations of the rough structures of the car bodies. during the development of this research, it was not possible to obtain detailed drawings of individual trams in order to create accurate simulation models. 99 jakub seidl acta polytechnica ctu proceedings figure 16. simulation model of an accident between a tram type b and a car. (a). (b). figure 17. simulation models of trams, (a) – type a, (b) – type c. therefore, the 3d geometry of the tram simulation models was created from freely available documents and photos on the internet. for the research, the assumption was made that due to the high strength of the rough structures of the tram car bodies, no plastic deformation will occur in the event of an accident with cars. according to this assumption, the tram models were considered as rigid in the calculations. this assumption resulted in a smaller error than if the tram models were considered as deformable and the calculation would have resulted in unrealistic tram deformation and smaller car deformation. for this reason, the tram simulation models (see figures 16 and 17) were represented only by tram’s cabin, which were considered as perfectly rigid and the masses of the whole trams were defined for them (see table 6). the remaining parts of the trams were not considered as a more complex model would lead to longer computational times of the simulations without any contribution to the accuracy of the results. the formula for calculating the collision mass of a tram is defined in the standard en 15227 (see equation (1)) [4]. parameter type a type b type c mass of an empty tram [kg] 43 790 36 835 34 500 number of seats [-] 60 60 61 mass of a passenger [kg] 70 70 70 collision mass of the tram [kg] 45 890 38 935 36 670 table 6. mass parameters of trams [14, 16, 17]. figure 18. the example of the deformation element [18]. mcol = m + mpass ∗ ( pseats 2 ) , (1) where: mcol – collision mass of tram, m – mass of empty tram, mpass – mass of passenger, pseats – number of seated passengers. 6.2. the simulation model of the deformation elements deformation elements of newly constructed trams most often consist of two absorption elements connected by a stiff bumper (see figure 18). absorption elements are able to absorb the energy of the impact. the requirement for the value of the absorbed energy is defined by the collision scenario of two trams at a speed of 15 km h−1 in standard en 15227 [4]. the simulation models of the deformation elements of trams type b and c were created to represent as accurately as possible the real deformation elements used on both trams. the models consisted of two absorber members to which the bumper was attached via a rotation joints (see figure 19). each absorber element was defined with a loading characteristic to absorb the required impact energy. the characteristics were determined according to the oleo documentation and the requirements of en 15227. the functionality of the absorbers was verified by a calibration test in which the absorber member was compressed with the test fixture up to the overload area (see figure 20). the test was used to evaluate the force dependence on the compression of the absorbing member. the test result was then compared 100 vol. 43/2023 the influence of the construction of tram fronts (a). (b). figure 19. simulation model of the deformation element of the tram, (a) – type b, (b) – type c. (a). (b). (c). (d). figure 20. the calibration test: (a) – before the test, (b) – compressing the return stroke, (c) – compressing the non-return stroke, (d) – after the relief. with the designed characteristic (see figure 21). the comparison achieved good fits and the models were evaluated as validated. 6.3. the simulation model of a passenger car the parameters and the accuracy of the car’s simulation model were critical for the research on reducing the consequences of trams and cars accidents. a poor and inaccurate car model would lead to poor results and inaccurate research conclusions. to create the detailed model of a car it would be necessary to know the design of the car and parameters of the used materials. to validate the model, it would then be necessary to perform impact tests and compare the test and simulation results. the creation of an accurate and validated simulation model of a car wasn’t possible within this research project. therefore, it was necessary to take over a model from another project. the researchers at the national crash analysis center (ncac) at george masone university (gwu) provide freely available simulation models of various road vehicles for crash simulations on their website (ccsa.gmu.edu). from the available models of road vehicles was chosen a model of the car toyota yaris 2010 (see figure 22). mass parameters of the simulation figure 21. the characteristic of tram type b absorption element, orange line – design characteristics, blue line – validation test. (a). (b). figure 22. (a) the car toyota yaris 2010 [19], (b) the simulation model of car toyota yaris 2010 [19]. model and the real car are listed in table 7. the simulation model of the toyota yaris 2010 car was validated according to several tests performed on a real vehicle (f.e. a frontal full wall crash, a frontal offset crash, a side impact nhts, a side impact iihs, etc., see figure 23). for the research solution thus could be introduced the assumption that the results of simulation calculations performed on this model will correspond with high accuracy to the course of tests performed on a real vehicle. 6.4. the simulation model of the driver human simulation models are created as copies of every mechanical dummy used in the crash tests of road vehicles. the reason is the best possible prediction of the consequences of various accidents on the passengers inside the vehicles already at the design stage of the vehicle. this procedure makes it possible to optimize the design of new cars even at the design stage, i.e. before a series production begins [21]. the most important parameter of human simulation models is a biofidelity. the biofidelity describes the similarity of the behaviour of the simulation model to the human body. with poor biofidelity, the result of simulations may lead to poor results and inaccurate research conclusions. the creation of such a model is highly demanding, requires precise knowledge of the behaviour of the human body under a mechanical loading and mass parameters of body parts. the creation of an accurate human simulation model wasn’t possible within this research. therefore, the human simulation model was taken from other project [21]. livermore software technology (lstc), manufac101 jakub seidl acta polytechnica ctu proceedings parameter unit real vehicle fem model mass kg 1 078 1 101 moment of inertia i_xx kg m−2 388 395 moment of inertia i_yy kg m−2 1 498 1 566 moment of inertia i_zz kg m−2 1 647 1 739 position of cog x_t mm 1 022 1 004 position of cog y_t mm -8.3 -4.4 position of cog z_t mm 588 569 table 7. mass parameters of the car toyota yaris 2010 [19]. (a). (b). figure 23. front full wall crash test of toyota yaris 2010: (a) – real test [19], (b) – simulation [19]. figure 24. the simulation model worldsid 50th male [20]. turer of ansys software, provides validated frontal and side impact dummy models on its website (lstc.com). the proposed calculation scenario of the collision (described in section 5) represents the frontal impact of the tram to the side of the car, i.e. the side impact to the car. from the available side impact dummies was chosen the worldsid 50th male dummy (see figure 24). this dummy represents the average adult man weighing 74 kg [20]. the worldsid 50th male dummy model was validated according to the following tests: the two head drop tests (see figure 25), the neck pendulum test, the shoulder impact test (see figure 26), the thorax with and without impact test, the abdomen impact test and the pelvic impact test. the results of the simulation tests were compared with the requirements of the worldsid iso 15830-2 regulation. the results of the (a). (b). figure 25. frontal head drop test: (a) – actual test setup [20], (b) – fe simulation setup [20]. figure 26. shoulder test pendulum set up [20]. validation were in accordance with the requirements of the regulation [20]. for the research solution thus could be introduced the assumption that the results of simulation calculations performed on this model will correspond with high accuracy to the course of tests performed on a dummy. the worldsid 50th male dummy model was created on the principle of rigid bodies. these types of simulation models are created from multiple rigid bodies, which are interconnected by bonds representing the joints of a person. to create such a model, it is necessary to know the dimensions of the human body, the mass properties of individual body parts (the position of the centre of gravity, the weight and moments of inertia), the positions of the joints and joints permissible ranges of motion. the advantage of these models is lower demands on a computing power and also easy positioning of the model. calculations with the rigid body models are able to well predict human movement during and after an accident and acceleration courses acting on individual parts of the human body. the 102 vol. 43/2023 the influence of the construction of tram fronts evaluation of the consequences of accidents will be possible by determining the acceleration courses of individual parts of the body from simulations and subsequent evaluation of human biomechanical criteria (see section 6.5) [21]. 6.5. human biomechanical criteria human biomechanical criteria have been introduced to assess the effects of accidents on occupants inside road vehicles. human biomechanical criteria define the relationships between physical variables and the probability of injury to occupants. in road motor vehicle approval tests, the measured values of the biomechanical criteria must be less than the specified limit values [21]. the acceleration of certain parts of the human body can be evaluated in simulations on the worldsid 50th male dummy model. from the biomechanical criteria used by euro ncap for the assessment of safety in side impacts. the following biomechanical criteria of the head were selected to assess the consequences of accidents: hic15 and 3 ms [22]. 6.5.1. human biomechanical criteria – hic15 the hic15 criterion evaluates the risk of injury from measured acceleration curves at the centre of gravity of the dummy’s head. the value of the hic15 criterion is determined from equation (2) [23]. hic = { (t2 − t1) [ 1 (t2 − t1) ∫ t2 t1 a(t)dt ]2.5} , (2) where: a – resultant acceleration in the multiples of the earth’s gravitational acceleration g, calculation according to equation 3, t1 – beginning of the observed event, t2 – end of the observed event. a = √ a2 x + a2 y + a2 z . (3) according to the requirements of the euro ncap evaluation, the evaluation is performed in a time interval of 15 ms and sets two limit values of the criterion: a higher limit of 500 and a lower limit of 700. the higher the final value of the criterion is, the higher is the probability of injury to a person in an accident. new vehicle manufacturers strive to keep the value of criteria during an accident as low as possible. therefore, the limit value of 500 was chosen for the research [22]. 6.5.2. human biomechanical criteria – 3 ms the biomechanical criterion of 3 ms is based on the wstc curve (see figure 27), which describes the relationship between the magnitude of acceleration and the duration of acceleration on the risk of permanent injury to occupants. according to the euro ncap assessment requirements, the acceleration of the driver’s head for 3 ms must not be greater than 72 g (higher limit) or 80 g (lower limit). as with the figure 27. wstc – wayne state tolerance curve [24]. figure 28. the acceleration of the driver’s head during an impact of the tram type b at speed 25 km h−1. hic15 criterion, a higher value of the 3 ms criterion indicated a higher probability of occupant injury in a crash. therefore, the 3 ms criterion limit value of 72 g was chosen for the research [23]. 7. results section 6 describes the creation of simulation models that represent the impact of the examined trams into the side of a car. simulations performed on these models and the results of simulations are described in this chapter. the consequences of the accidents were evaluated according to the human biomechanical criteria described in section 6.5 from the calculated course of the driver’s head acceleration in the car. simulations of the impact of the trams type a, b and c into the side of the car were performed for the following impact speeds: 10, 15, 20, 25, 30, 40 and 50 km h−1. the results of the acceleration of the driver’s head for simulation of the impact of the tram type b at 25 km h−1 are shown in figure 28 and impact simulation process are shown in figure 29. the results of the evaluation criteria for the individual trams and impact velocities tested are summarised in tables 8 and 9. 103 jakub seidl acta polytechnica ctu proceedings (a). (b). (c). (d). figure 29. the impact of the tram into the side of a car at a speed of 25 km h−1: (a) 0 ms, (b) 50 ms, (c) 75 ms, (d) 100 ms. speed of type type type tram a b c [km h−1] [-] [-] [-] 10 25.7 44.6 39.6 15 73.7 157.0 122.9 20 165.8 348.7 266.0 25 335.5 671.2 593.9 30 478.0 911.3 852.9 40 783.3 1797.0 1720.0 50 1255.0 2177.0 2128.0 table 8. the evaluation of the hic criterion for individual impact velocities and trams. 8. discussion from the results of the calculations of the impact of a tram into the side of a passenger car shown in tables 8 and 9 can be seen, that trams designed according to en 15227 have a risk of severe injury or death to car occupants at impact speeds of 25 km h−1 and above. for tram type a the limit values were exceeded at speed 30 km h−1, even though tram type a has the highest collision mass and so the highest energy during the impact. from the above conclusions tt can be concluded that the frontal design of the tram type b and c leads to a higher risk of injury to passengers inside cars than the tram type a. in order to meet the objectives of the vision 0 program, i.e. to reduce the risk of serious injuries and the death of passengers in an accident, it is appropriate to supplement the requirements of en 15227 with additional legislation that would lead to increase the safety of other road users during the accident with trams. the creation of a methodology describing the safety assessment of trams in the event of an accident with cars is one of the outputs of research sgs19/161/ohk2/3t/12, which is carried out at the faculty of mechanical engineering at ctu in prague. speed of type type type tram a b c [km h−1] [-]/[ms] [-]/[ms] [-]/[ms] 10 37.0/0 44.6/0 43.7/0 15 49.7/0 49.7/0 49.8/0 20 65.3/0 83.7/2.1 71.8/0 25 83.3/1.9 121.4/7.3 99.2/4.5 30 97.7/5.1 142.1/8.2 127.2/6.7 40 115.4/8.2 174.7/8.5 173.4/8.1 50 155.4/12.9 213.3/8.1 210.9/8.2 table 9. the evaluation of the 3 ms criterion for individual impact velocities for individual impact velocities and trams. the value in the table indicates the maximum value of the criterion and the time of exceeding the limit value. the experience gained in solving this research will then be used in solving research on reducing the consequences of accidents of regional rail vehicles with cars at level crossing, which is addressed in the project tn01000026 at the same work place. list of symbols a acceleration [mm ms−2] m mass of empty tram [kg] mcol collision mass of tram [kg] mpass mass of passenger [kg] pseats number of seated passengers [-] t time [ms] acknowledgements this research was funded by student grant competition, project number sgs22/052/ohk2/1t/12. references [1] h. holst, a. nygren, r. thord. transportation, traffic safety and health. springer-verlag, berlin heidelberg, germany, 1997. https://doi.org/10.1007/978-3-662-03409-5 [2] zdopravy.cz. dpp má povolení pro tramvaj do slivence, začne stavět prvních 40 metrů kolejí. 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https://idnes.cz/praha/zpravy/nehoda-tramvaj-na-petrinach.a170623_151401_praha-zpravy_rsr https://idnes.cz/praha/zpravy/nehoda-tramvaj-na-petrinach.a170623_151401_praha-zpravy_rsr https://mhd-ostrava.cz:81/?s=tango_nf_2 https://www.cs-dopravak.cz/2019-2-4-je-konec-praha-pevzala-250-vz-koda-15t/ https://www.cs-dopravak.cz/2019-2-4-je-konec-praha-pevzala-250-vz-koda-15t/ https://mhd-ostrava.cz:81/?s=typ_vozu&clanek=39t_forcity_smart https://mhd-ostrava.cz:81/?s=typ_vozu&clanek=39t_forcity_smart https://opravnatramvaji.cz/cz/reference/tramvaje/15t https://opravnatramvaji.cz/cz/reference/tramvaje/15t https://axtoneglobal.com/tram https://cdn.euroncap.com/media/67264/euro-ncap-assessment-protocol-aop-v913.pdf https://cdn.euroncap.com/media/67264/euro-ncap-assessment-protocol-aop-v913.pdf https://doi.org/10.1007/978-3-8348-2608-4 https://doi.org/10.1242/dmm.011320 acta polytechnica ctu proceedings 43:93–105, 2023 1 introduction 2 standard en 15227 2.1 the collision scenario: the frontal impact of two identical trams 2.2 the collision scenario: a tram crashes into a rigid obstacle weighing 3 tonnes 2.3 the collision scenario: a tram crashes into a deformable obstacle weighing 7.5 tonnes 2.4 conclusion 3 accident statistics of tram vehicles in the czech republic 4 selected trams for research 5 design of crash scenario 6 creating a simulation model 6.1 the simulation models of trams 6.2 the simulation model of the deformation elements 6.3 the simulation model of a passenger car 6.4 the simulation model of the driver 6.5 human biomechanical criteria 6.5.1 human biomechanical criteria – hic15 6.5.2 human biomechanical criteria – 3ms 7 results 8 discussion list of symbols acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2023.42.0094 acta polytechnica ctu proceedings 42:94–97, 2023 © 2023 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague effect of the long-term storage methods on the stability of cartilage biomechanical parameters blanka žaloudkováa,b, šárka sekorovác, barbora kopeckác, daniel kytýřa,∗ a czech academy of sciences, institute of theoretical and applied mechanics, prosecká 76, 190 00 prague, czech republic b czech technical university in prague, faculty of transportation sciences, mobile laboratory for transport analysis, konviktská 20, 110 00 prague, czech republic c national cell and tissue centre, dr. slabihoudka 6232/11, 708 00 ostrava, czech republic ∗ corresponding author: kytyr@itam.cas.cz abstract. long-term stability of the tissue product in terms of mechanical parameters is a key factor for its expiration date. for the investigation of storage effects on the cartilage tissues the experimental mechanical loading test combined with xct scanning for the irregular shape inspection was performed. the samples were preserved according to three different protocols using the deep-freezing and two types of saline solution preservation. the stability of the biomechanical parameters was tested within annual intervals. all samples were subjected to uni-axial compression loading using the in-house developed compact table top loading device in displacement-driven mode. based on the measurements, the results are represented in the form of stress-strain curves and quantified as elastic modulus and ultimate compression stress. it can be concluded that no significant difference was found in neither the mechanical properties of the samples nor in the effects of each preservational method. keywords: stability study, cartilage, biomechanical characterisation, compression loading, xct imaging. 1. introduction storage of tissues of human origin for long-term banking is an essential operation of tissue establishments. long-term stability of the tissue product is the key parameter for determining its expiration date. the acceptable method of tissue preservation must be capable of retaining the natural properties of the graft (e.g. viability, structural integrity) for the time interval of the maximum possible storage time. one of the most common preservation methods which has been used for decades is deep-freezing. the grafts without cryoprotectants are stored in the range of temperatures from −80 ◦c to −40 ◦c. for this storage procedure only little impact on the biomechanical parameters was reported [1, 2]. alternatively, the chemical preservation could be used which involves the conservation of cartilage using saline solution with the concentration typically in range 0.9–3.0 % at room temperature [3]. this method is not widely used yet and in general there is a lack of publications investigating the stability of biomechanical parameters of products stored in this way. this method brings potential risk of a significant decrease of elastic modulus and yield stress in cartilages as uniquelly reported in [4]. during the pre-storage tissue treatment the allografts are commonly subjected to antibiotic-based disinfection [1] and for this study the samples stored using the saline solution were also sterilized using ionizing radiation. gamma rays irradiation sterilization itself can influence the biomechanical properties of allografts and induce changes in the molecular structure [5, 6]. in addition, to avoid potential risk, both storage methods were examined and compared in terms of costal cartilage biomechanical stability. the aim of this study is to assess the effect of long-term storage of cartilages preserved by the two different methods, using three different protocols. these findings will be useful for a comprehensive assessment of the suitability of the alternative saline preservation method for costal cartilage storage. the problem of irregular shape of the tested cartilage sample [7, 8] was solved by subjecting the sample to xct imaging to obtain full 3d geometrical model for estimation of the cross-section area together with employing of closedloop controlled precise system for mechanical testing. for that reason the set of biomechanical experiments coupling xct imaging and uni-axial testing [9] was performed to evaluate the time-dependent trends in changes of biomechanical parameters. 2. materials and methods 2.1. samples for costal cartilages of human origin three types of tissue preservation for the long-term storage were used: (i) deep-freezing and storage in range temperatures 94 https://doi.org/10.14311/app.2023.42.0094 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 42/2023 cartilage long-term stability from −80 ◦c to −40 ◦c and cooled solid co2 during the transportation, (ii) preserved by 3.0 % nacl solution and stored at room temperature, (iii) preserved by 0.9 % saline and stored at room temperature. before the storage process, all the samples were treated by antibiotic-based disinfection bath. moreover, the samples stored in the saline solution were sterilised by gamma rays. two testing samples were prepared by manual sectioning from each cartilage with height of 5–7 mm and plan-parallel (± 0.1 mm) contact faces fitting in circumscribed circle of 12 mm. because of the irregular shape of the samples, 3d models were used for identification of cross-section area ac used in stress calculation. individual samples were subjected to x-ray microtomography (xct) scanning just before the loading experiment. the scanning procedure consisted of 800 projections with acquisition time of 400 ms, in total 8 mins including rotation and read-out time. the sample was scanned in polymer container partly filled with water but the sample was surrounded only by air. this setup ensures that the imaging process does not affect the structure and biomechanical properties of the sample, utilizing an x-ray source with an acceleration voltage of 75 kv and tube current of 120 µa. the sample placed in xct scanned is depicted in figure 1. figure 1. the environmental container with the sample placed in xct scanner. x-ray source (left), sample with container and holder on rotary table (middle), flat panel detector (right). to obtain proper ac size, volumetric data were reconstructed using cone-beam fdk algorithm [10] implemented in vgstudio max (volume graphics, germany) software and exported as image stack with pixel size of 9.5 µm. on the volume of interest the tissue was segmented by thresholding and application of gaussian smoothing and erosion filtering (see figure 2). minimal ac was taken for further stress calculations. in all cases minimal ac reached at least 95 % of median cross-sectional area. this fact minimizes the risk of corrupted data processing. figure 2. visualization of the cartilage sample based on xct imaging. figure 3. compression test arrangement 2.2. biomechanical analysis biomechanical parameters, namely compressive modulus e, ultimate stress σu and corresponding strain level εu were taken for evaluation of the long-term storage effect on the tissues. for that purpose uniaxial quasi-static compression tests were performed using in-house developed compact loading device (see figure 3) in detail described in [9]. to ensure the optimal accuracy of the measurement load-cell lcm300 (futek, usa) with the nominal capacity of 1.1 kn was employed. for the displacement driven experiment the loading rate was set to 10 µm s−1 and maximal displacement 3500 µm corresponding to 50 % strain for the highest sample. read-out frequency was set to 200 hz for raw data logging employing in-house developed control system [11]. automated matlab (mathworks, usa) script was used for the data processing to obtain the stress-strain curves representing the deformation behaviour and assess to the biomechanical parameters. 95 b. žaloudková, s. sekorová, b. kopecká, d. kytýř acta polytechnica ctu proceedings for deformation behavior the stress was considered as engineering stress obtained using σeng = f ac , (1) where f is load-cell output. engineering strain εeng was calculated by the following formula εeng = u h0 , (2) where u represents displacement measured by encoder and h0 is the initial height of the sample. 3. results and discussion to evaluate effect of the long-term storage of cartilages under deep frozen condition, biomechanical characteristics assessed from uni-axial compression test performed in february 2022 and february 2023 were used. the deformation behaviour of all tested samples is represented by stress-strain curves in figure 4. 0 2 4 6 8 10 0 0.05 0.1 0.15 0.2 0.25 0.3 s tr e ss [ m p a ] strain [-] frozen feb 2022 feb 2023 figure 4. deformation characteristic of all deep frozen samples the mean values of material parameters with corresponding standard deviation are summarized in table 1. based on the obtained results, no significant date feb 2022 feb 2023 e [mpa] 57.24 ± 0.87 65.45 ± 13.69 σu [mpa] 6.84 ± 0.62 7.97 ± 1.88 εu [-] 0.1821 ± 0.0286 0.1582 ± 0.0311 table 1. time dependence of biomechanical parameters of deep frozen samples long-term storage effect was proven. the difference in biomechanical parameters of each individual sample is higher than the influence of the storage effect. to evaluate the effect of the long-term storage of cartilages preserved in 0.9 % nacl, biomechanical characteristics were assessed using the same tasting procedure. resulting stress-strain curves are presented in figure 5. 0 2 4 6 8 10 0 0.05 0.1 0.15 0.2 0.25 0.3 s tr e s s [ m p a ] strain [-] 0.9% nacl feb 2022 feb 2023 figure 5. deformation characteristic of 0.9 % nacl preserved samples the mean values of material parameters with corresponding standard deviation are summarized in table 2. date feb 2022 feb 2023 e [mpa] 46.80 ± 11.24 53.38 ± 4.38 σu [mpa] 5.00 ± 0.81 4, 88 ± 0.25 εu [–] 0.1625 ± 0.0459 0.1305 ± 0.0537 table 2. time dependence of biomechanical parameters of 0.9 % nacl preserved samples from the obtained results a slight increase of the magnitude of the elastic modulus over a longer time can be seen. this finding is in contrary to the hypothesis of time-dependent degradation of the biomechanical parameters. there is negligible difference in ultimate stress and non-significant drop of the strain at the ultimate strain level. because of the nearly 40 % strain variation (0.1322 ± 0.0515) result it can’t be taken as a proof of the time effect. finally the effect of saline saturation was evaluated. resulting stress-strain curves are presented in figure 6. the mean values of material parameters with corresponding standard deviation are summarized in table 3. based on the obtained results no significant salinity 0.9 % nacl 3.0 % nacl e [mpa] 37.55 ± 5.66 56.04 ± 2.25 σu [mpa] 4.67 ± 0.81 5.33 ± 0.93 εu [-] 0.1928 ± 0.0056 0.1322 ± 0.0515 table 3. biomechanical parameters depending on saline solution long-term storage effect was proven. the difference in biomechanical parameters of each individual sample is higher than the influence of the storage effect. the ultimate stress difference is minimal (under 3.0 %). slight 96 vol. 42/2023 cartilage long-term stability 0 2 4 6 8 10 0 0.05 0.1 0.15 0.2 0.25 0.3 s tr e s s [ m p a ] strain [-] saline solution 0.9% nacl 3.0% nacl figure 6. deformation characteristic of 0.9 % nacl and 3.0 % nacl preserved samples increase of elastic modulus together with decrease of the strain for ultimate stress could be recognized but on negligible level. but it has to be kept in mind that this study is not statistically relevant because of limited number of delivered samples. 4. conclusions it can be concluded that no significant difference in mechanical properties was observed within one year measurement interval despite the findings for saline preservation presented by zhang et al. [4]. for that reason all proposed preservation methods are suitable for long-term storage of the allografts. the gamma dose caused by xct scanning cannot influence longterm biomechanical parameters because the imaging was performed just before the mechanical testing. the same testing procedure will be applied on the cartilage samples in the next two years to complete the stability study. acknowledgements this research was supported by strategy av21 breakthrough technologies for the future. references [1] a. paolin, l. spagnol, g. battistella, d. trojan. evaluation of allograft decontamination with two different antibiotic cocktails at the treviso tissue bank foundation. plos one 13(8):e0201792, 2018. https://doi.org/10.1371/journal.pone.0201792 [2] i. szabó, b. patzai, d. lőrinczy. effects of long-term deep freezing on human femoral cartilage: differential scanning calorimetric (dsc) analysis and histopathological evaluations. journal of thermal analysis and calorimetry 147(14):7793–7797, 2021. https://doi.org/10.1007/s10973-021-11070-0 [3] v. sardana, j. burzynski, g. r. scuderi. the influence of the irrigating solution on articular cartilage in arthroscopic surgery: a systematic review. journal of orthopaedics 16(2):158–165, 2019. https://doi.org/10.1016/j.jor.2019.02.018 [4] g. zhang, x. deng, f. guan, et al. the effect of storage time in saline solution on the material properties of cortical bone tissue. clinical biomechanics 57:56–66, 2018. https: //doi.org/10.1016/j.clinbiomech.2018.06.003 [5] c. r. harrell, v. djonov, c. fellabaum, v. volarevic. risks of using sterilization by gamma radiation: the other side of the coin. international journal of medical sciences 15(3):274–279, 2018. https://doi.org/10.7150/ijms.22644 [6] w. a. rutala, d. j. weber. disinfection and sterilization in health care facilities. infectious disease clinics of north america 30(3):609–637, 2016. https://doi.org/10.1016/j.idc.2016.04.002 [7] a. gradischar, c. lebschy, w. krach, et al. measurement of global mechanical properties of human thorax. journal of biomechanics 142:111242, 2022. https://doi.org/10.1016/j.jbiomech.2022.111242 [8] s. zhao, m. arnold, s. ma, et al. standardizing compression testing for measuring the stiffness of human bone. bone & joint research 7(8):524–538, 2018. https: //doi.org/10.1302/2046-3758.78.bjr-2018-0025.r1 [9] t. fíla, j. šleichrt, d. kytýř, et al. deformation analysis of the spongious sample in simulated physiological conditions based on in-situ compression, 4d computed tomography and fast readout detector. journal of instrumentation 13(11):c11021–c11021, 2018. https://doi.org/10.1088/1748-0221/13/11/c11021 [10] l. a. feldkamp, l. c. davis, j. w. kress. practical cone-beam algorithm. journal of the optical society of america a 1(6):612–619, 1984. https://doi.org/10.1364/josaa.1.000612 [11] v. rada, t. fíla, p. zlámal, et al. multi-channel control system for in-situ laboratory loading devices. acta polytechnica ctu proceedings 18:15–19, 2018. https://doi.org/10.14311/app.2018.18.0015 97 https://doi.org/10.1371/journal.pone.0201792 https://doi.org/10.1007/s10973-021-11070-0 https://doi.org/10.1016/j.jor.2019.02.018 https://doi.org/10.1016/j.clinbiomech.2018.06.003 https://doi.org/10.1016/j.clinbiomech.2018.06.003 https://doi.org/10.7150/ijms.22644 https://doi.org/10.1016/j.idc.2016.04.002 https://doi.org/10.1016/j.jbiomech.2022.111242 https://doi.org/10.1302/2046-3758.78.bjr-2018-0025.r1 https://doi.org/10.1302/2046-3758.78.bjr-2018-0025.r1 https://doi.org/10.1088/1748-0221/13/11/c11021 https://doi.org/10.1364/josaa.1.000612 https://doi.org/10.14311/app.2018.18.0015 acta polytechnica ctu proceedings 42:93–96, 2023 1 introduction 2 materials and methods 2.1 samples 2.2 biomechanical analysis 3 results and discussion 4 conclusions acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2023.40.0088 acta polytechnica ctu proceedings 40:88–92, 2023 © 2023 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague dependence of mechanical and thermal properties on the composition of lightweight gypsum composites hana sekavováa,b,∗, zdeněk prošeka,c, pavel tesáreka a czech technical university in prague, faculty of civil engineering, department of mechanics, thákurova 7, 166 29 prague, czech republic b knauf praha spol. s r.o., mladoboleslavská 949, 197 00 prague 9, czech republic c czech technical university in prague, university centre for energy efficient buildings, třinecká 1024, 273 43 buštěhrad, czech republic ∗ corresponding author: hana.sekavova@fsv.cvut.cz abstract. one of the ways to use the treated gypsum waste is in gypsum mixtures for the production of gypsum blocks. gypsum blocks can be used in standard interior as well as exterior applications and are made of gypsum, water, pp microfibres and are lightened with foam, which is created from a foaming additive and water. above all, the amount of foam significantly influences the bulk density of the resulting material, and the bulk density then has a major influence on the mechanical and thermal properties. the use of pp microfibres had a positive effect on the overall stability of the foamed structure, which resulted in an increase in compressive strength while maintaining good thermal insulation properties. keywords: mechanical properties, thermal properties, gypsum, lightweight composite. 1. introduction the trend nowadays is to use recycled materials generated in the construction industry and reuse them in the construction industry. in this way, mineral resources are protected and there is no need to extract new non-renewable raw materials. such materials include gypsum, which is up to 100 % recyclable. one of the most common uses of gypsum today is primarily in the manufacture of plasterboard (pb). the issue of recycling of construction pb is related to its contamination with pb of different types from different manufacturers (different compositions) and other materials used in the production of pb structures – dry constructions [1, 2]. these include mineral insulation, metal profiles, pur foams, residues of wipers, etc. another problem is related to the biodegradation of pb [3–5]. the above contamination complicates the return of this material for pb re-manufacturing, which is quite demanding in terms of the quality of the input recycled material, technological processes, etc. therefore, efforts are made to find suitable materials and products based on recycled construction pb that are not so susceptible to changes in the quality of pb recyclate [6, 7]. one way to use recycled pb is in the form of lightweight blocks to be used for building envelopes. blocks for building envelopes today are primarily designed to have a certain balance between mechanical properties (primarily compressive strength, modulus of elasticity, etc.) and thermal performance. the most commonly used thermo-technical parameter is the thermal conductivity coefficient, which can then be used to calculate the thermal resistance and possibly other characteristics that are already part of the building design or an optimal envelope composition. the composition is possibly supplemented by thermal insulation materials. the advantage of gypsum-based materials is the fact that the gypsum can be modified, either by additives and admixtures, to modify the resulting performance of the hardened material. the primary objective in the design of a new gypsum mixture (dry or wet) is to modify the internal structure (in terms of internal arrangement and ratio of mass and excitatory pores), and thus to change the bulk density of the fresh gypsum slurry, i.e. of the hardened and subsequently dried samples, where the mixture/material/product already has the resulting material properties [8–10]. in terms of achieving optimal mechanical properties (compressive strength) and thermal properties (thermal conductivity coefficient), these are completely opposite effects. in terms of compressive strength, we need a material with a minimum number of pores and, conversely, for the thermal conductivity coefficient, we would need a material full of small pores to make the whole structure lighter. at first glance, quite different properties and requirements can be achieved with the optimum ratio of the components used – water, gypsum or gypsum primary inert binder, using a foaming additive and microfibres. in general, chemical additives such as setting retardant, plasticizing additive, etc. can be used. a properly applied foaming additive (in terms of the amount of foaming additive, the 88 https://doi.org/10.14311/app.2023.40.0088 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 40/2023 dependence of mechanical and thermal properties on ... (a). demolition of a plasterboard wall. (b). waste inside the smart container. (c). typical waste after emptying from the container. figure 1. demolition of a plasterboard wall, waste inside the smart container, typical waste after emptying from the container, in this case green. element symbol atomic conc. weight conc. oxide symbol stoich. wt conc. o 66.09 46.39 s 17.09 24.05 ca 16.81 29.56 figure 2. chemical analysis of recyclate by sem eds analysis. amount of water, the quality and stability of the foam) will lighten the gypsum matrix, ideally by small pores that do not clump and are evenly distributed throughout the volume of the mass, and the microfibres will “hold” the entire foamed and lightened structure during the initial stages of setting and hardening and will also improve the resulting mechanical properties. another advantage of the whole concept of producing gypsum blocks based on recycled gypsum waste is that waste from the production of gypsum blocks can be incorporated into the mix as a primarily inert material. a further advantage is the fact that when the gypsum blocks are refurbished or end-of-life, they can be reused or recycled again and used for the production of new blocks [11]. 2. materials and samples the composites were composed of recycled pb materials. the recyclate came from the construction site, was sorted using a “smart” container and contained mainly “ordinary” white (grey) pb from knauf praha, spol. s. r. o. (figure 1). construction pb waste was treated on a mobile recycling line at lavaris s. r. o. in libčice nad vltavou. the treated pb recyclate was delivered to the faculty of civil engineering of the czech technical university in prague in closed plastic cans. at the faculty of civil engineering, analysis of this material was carried out using a pheonom xl scanning electron microscope. the recyclate was examined using a bse detector in combination with eds analysis (figure 2). the results show that the purity of the recyclate is greater than 95 % of caso4. from the point of view of efficient utilization and verification of the technological possibilities of production of lightweight blocks in laboratory conditions, it was advisable that the calcination rate was as high as possible, for this reason a selected part of the imported pb recyclate was further calcined to achieve at least 95 % calcination rate of the total weight of the recyclate samples, based on the weight of the binder, i.e. including any impurities. the calcination was carried out under laboratory conditions using a laboratory oven heated to 120 °c until the mass was stable, corresponding to the evaporation of water or the stoichiometric difference between the calcined 89 h. sekavová, z. prošek, p. tesárek acta polytechnica ctu proceedings samples plaster water w/p fibres type of foam foam addition [g] [g] [-] [g] [g] f 3.0 2.1 0.7 0 none 0 g 3.0 2.1 0.7 15 none 0 h 3.0 1.8 0.6 15 in-situ 60 i 3.0 1.8 0.6 15 in-situ 45 j 3.0 1.8 0.6 0 in-situ 30 k 3.0 1.8 0.6 15 in-situ 30 l 3.0 1.8 0.6 15 in-situ 15 table 1. used mixtures and their composition. gypsum and the gypsum. the degree of calcination was verified on selected samples using so-called drying scales. to ensure the stability of the foamed structure, specially manufactured pp microfibres from trevos were used. microfibres have 32 µm in diameter and 4 mm in length. other properties are following: average tensile strength, ≤3.0 cn/dtex (∼272 mpa); average elongation, ≤50 %; density, 910 kg/m3; young modulus of elasticity, ∼4 gpa. based on previous experiments, several sets of gypsum-based specimens were designed to verify the dependence of water coefficient and foam amount on the final properties of the lightweight gypsum composites. for comparison purposes, reference sets without lightening (f and g) were also created to better identify the degree of lightening and compositional adjustment. other sets were specific in the amount of foaming agent used (15, 30, 45 and 60 g). the foam was produced so-called caused in-situ. some sets additionally contained pp microfibres. for practical reasons, a water coefficient of 0.7 had to be used for sets f and g, and for the other sets it was reduced to 0.6. for each set, 6 cubes with an edge of 100 mm were made. see table 1 for details. 3. experimental methods prior to testing, all the test samples were artificially dried at 40 °c using a hot air oven for 3 days until the weight stabilized. they were then stored in a desiccator at laboratory temperature and 0 % humidity for 24 hours. the samples thus prepared were subsequently tested. first, the samples were tested to determine the bulk density and thermal properties. testing of thermal properties was performed on heat transfer analyzer isomet 2104 (applied precision) equipped with surface probe api210411 and api210412. the investigated thermal property was the thermal conductivity coefficient λ. the surface probes had measuring range from 0.04–0.3 w/mk, and 0.3–2.0 w/mk, and accuracy of ±5 % was used. such a device applies a dynamic method which is based on response monitoring of examined material on heat flow impulses. measurements were made on 100 × 100 × 100 mm samples and each measurement was made on a different side of the cube. each sample was measured three 850 900 950 1000 1050 1100 1150 1200 f g h i j k l b u lk d e n s it y [ k g /m 3 ] figure 3. comparison of the average values (with standard deviations) of the bulk density of the tested gypsum mixtures. times and the resulting value was averaged and the standard deviation determined. the temperature of the samples during the measurement was equal to 22±1 °c (the same as ambient air). finally, destructive tests were performed. the destructive tests were carried out using a heckert fp100 loading frame with displacement-controlled loading at 0.1 mm/min. a uniaxial compression test was carried out on cubic samples. compressive strength was calculated from the maximum force reached during the test, fc,max, as: fc = fc,max ab . (1) 4. experimental results after the solidification and hardening process, a porous system was formed in the hardened gypsum mass, which defines the bulk density. after drying the samples from the water necessary for processing (from a technological point of view), the value of the bulk density of the naturally wet gypsum solid structure was determined. the comparison of the average bulk density values in figure 3 shows the influence of the fibers between sets f and g. the lowest value was measured for sets i and h with the highest amount of foaming additive added, while the lowest value was measured for set k. 90 vol. 40/2023 dependence of mechanical and thermal properties on ... 0 1000 2000 3000 4000 5000 6000 7000 8000 9000 f g h i j k l c o m p re s s iv e s tr e n g th [ k p a ] figure 4. comparison of the average values (with standard deviations) of the compressive strengths of the tested gypsum mixtures. figure 4 compares the average compressive strengths. the highest average values were obtained for the reference samples f and g without foaming, but a value below 7 mpa was also obtained for set l with 15 g of foaming additive and pp fibres. the lowest average values were obtained for set h and i, where their values fell below 3 mpa. figure 5 compares the average thermal conductivity coefficient values for the tested gypsum mixtures. in terms of this parameter, set k with 30 g of foaming additive and pp fibres has the worst values around 0.5 w/mk. the best average value of 0.35 w/mk was measured for sets h and i. for most of the other sets, values around 0.4 w/mk were obtained. 5. conclusions the paper presents the first part of the results in the field of effective use of treated gypsum plasterboard waste treated by a recycling line and its subsequent use in the form of lightweight blocks for building envelopes. the described reuse method seeks to efficiently use the gypsum plasterboard recyclate and transform it into a new building product that can compete with conventional materials, such as the classic aerated concrete blocks that are commonly used today. calcination was also verified in the laboratory using a laboratory dryer at temperatures up to 120 °c. in the next part of the project, the aim will be to add a new calcination unit to the recycling line, which would effectively refine the treated plasterboard recyclate and convert it into a gypsum binder with similar properties to a conventional building plaster. acknowledgements this paper was financially supported by the tacr trend 3 research a project under the number fw03010054 “recycling and transformation of construction plasterboard waste into new products for construc0.000 0.050 0.100 0.150 0.200 0.250 0.300 0.350 0.400 0.450 f g h i j k l c o e ff ic ie n t o f th e rm a l c o n d u c ti v it y [w /m k ] figure 5. comparison of the average values (with standard deviations) of the thermal conductivity coefficient of the tested gypsum mixtures. the values were determined for naturally moist samples in a laboratory environment at a temperature of 25±2 °c and a relative humidity of 50±3 %. tion and value-added applications” and by czech technical university in prague under no. sgs project sgs22/089/ohk1/2t/11 “buidings materials and construction under view of experiment”. the authors also thank knauf prague ltd. for the supplied materials. references [1] m. a. pedreno-rojas, i. flores-colen, j. de brito, c. rodríguez-linán. influence of the heating process on the use of gypsum wastes in plasters: mechanical, thermal and environmental analysis. journal of cleaner production 215:444–457, 2019. https://doi.org/10.1016/j.jclepro.2019.01.053 [2] a. ahmed, k. ugai, t. kamei. investigation of recycled gypsum in conjunction with waste plastic trays for ground improvement. construction and building materials 25(1):208–217, 2011. https: //doi.org/10.1016/j.conbuildmat.2010.06.036 [3] h. sekavová, j. herrmann, z. prošek, et al. the influence of the use of recycled gypsum on the properties of gypsum products in labor conditions. acta polytechnica ctu proceedings 26:81–85, 2020. https://doi.org/10.14311/app.2020.26.0081 [4] h. sekavová, z. prošek, p. ryparová. detection of the presence of microorganisms during production of plasterboard. acta polytechnica ctu proceedings 30:93–97, 2021. https://doi.org/10.14311/app.2021.30.0093 [5] z. prošek, h. sekavová, p. ryparová. increasing the biological resistance of gypsum by means of silver, calcium and copper ions. acta polytechnica ctu proceedings 30:76–80, 2021. https://doi.org/10.14311/app.2021.30.0076 [6] a. j. rivero, r. sathre, j. g. navarro. life cycle energy and material flow implications of gypsum plasterboard recycling in the european union. resources, 91 https://doi.org/10.1016/j.jclepro.2019.01.053 https://doi.org/10.1016/j.conbuildmat.2010.06.036 https://doi.org/10.1016/j.conbuildmat.2010.06.036 https://doi.org/10.14311/app.2020.26.0081 https://doi.org/10.14311/app.2021.30.0093 https://doi.org/10.14311/app.2021.30.0076 h. sekavová, z. prošek, p. tesárek acta polytechnica ctu proceedings conservation and recycling 108:171–181, 2016. https://doi.org/10.1016/j.resconrec.2016.01.014 [7] a. erbs, a. nagalli, k. q. de carvalho, et al. development of plasterboard sheets exclusively from waste. journal of building engineering 44:102524, 2021. https://doi.org/10.1016/j.jobe.2021.102524 [8] y. zhang, f. wang, h. huang, et al. gypsum blocks produced from tio2 production by-products. environmental technology 37(9):1094–1100, 2016. https://doi.org/10.1080/09593330.2015.1102329 [9] o. benjeddou, c. soussi, m. benali, r. alyousef. experimental investigation of a new ecological block made by mixing gypsum plaster and desert sand. arabian journal for science and engineering 45(5):4037–4052, 2020. https://doi.org/10.1007/s13369-020-04362-4 [10] z. w. fu, s. h. zeng. the study of the crack-resistant plaster mortar of sand autolaved aerated concrete blocks. in applied mechanics and materials, vol. 204, pp. 1237–1241. trans tech publ, 2012. https://doi. org/10.4028/www.scientific.net/amm.204-208.1237 [11] a. erbs, a. nagalli, k. q. de carvalho, et al. properties of recycled gypsum from gypsum plasterboards and commercial gypsum throughout recycling cycles. journal of cleaner production 183:1314–1322, 2018. https://doi.org/10.1016/j.jclepro.2018.02.189 92 https://doi.org/10.1016/j.resconrec.2016.01.014 https://doi.org/10.1016/j.jobe.2021.102524 https://doi.org/10.1080/09593330.2015.1102329 https://doi.org/10.1007/s13369-020-04362-4 https://doi.org/10.4028/www.scientific.net/amm.204-208.1237 https://doi.org/10.4028/www.scientific.net/amm.204-208.1237 https://doi.org/10.1016/j.jclepro.2018.02.189 acta polytechnica ctu proceedings 40:88–92, 2023 1 introduction 2 materials and samples 3 experimental methods 4 experimental results 5 conclusions acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2025.53.0068 acta polytechnica ctu proceedings 53:68–75, 2025 © 2025 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague effect of variability of material properties on resistance of existing post-tensioned concrete bridge – case study aleš mezera∗, milan holý, miroslav sýkora czech technical university in prague, klokner institute, šolínova 7, prague 166 08, czech republic ∗ corresponding author: ales.mezera@cvut.cz abstract. the study deals with the sensitivity analysis in determining the load-bearing capacity of existing bridges made of post-tensioned precast girders. focusing on a specific road bridge, the effect of variations of important basic variables on load-bearing capacity is investigated. when assessing existing structures, values of some basic variables often need to be estimated due to the lack of background information (missing documentation, limited range of structural survey). the aim of this study is to identify basic variables with key influence on load-bearing capacity of the bridge. numerical example is specifically focused on the mid-span cross-section of a simply supported ka-73 beam structure. it appears that corrosion is the key factor affecting reliability of the bridge at the ultimate limit state while prestressing loss is dominating the servieability limit state verification. further research will be focused on reliability analysis of bridges based on the results of structural surveys. keywords: material properties, resistance, load-bearing capacity, post-tension, concrete, bridge. 1. introduction recently, a large number of existing prestressed bridges have been assessed in the czech republic as they are reaching their service life. in the 1960– 1980s, post-tensioned concrete bridges were in the czech republic often constructed of precast girders of various systems (ka, i, dsc...). during service life, these systems are often experiencing durability issues due to long-term leakages as well as structural and technological deficiencies insufficient concrete cover, transverse joints without mild steel reinforcement, poorly executed or missing grouting of cable ducts, etc [1]. the original design documentation and structural surveys are essential in order to obtain information on the actual structural condition of the bridge (defects, real material properties and geometry, permanent loads) and provide the basis for determining loadbearing capacity. in this study, the load-bearing capacity is determined according to the czech standard čsn 73 6222 [2]. the concept of load-bearing capacity represents the maximum total weight of each of the vehicles that are allowed to cross the bridge under the conditions specified in [2]. for existing bridges, three types of load-bearing capacities are distinguished: • normal load-bearing capacity (vn) refers to maximum weight of the vehicle allowed to cross the bridge without restrictions for other road traffic, bikers and pedestrians, a number of the vehicles is not restricted. • load-bearing capacity for a single vehicle (vr) – maximum weight of the vehicle allowed to cross the bridge as a single vehicle but without restrictions for pedestrians and bikers. • exceptional load-bearing capacity (ve) – maximum weight of the vehicle or special truck with trailers allowed to cross the bridge with all other traffic restricted, at specified speed and in specified transverse position [2]. load-bearing capacity is estimated in accordance with the eurocodes [3], following the basis of assessment of existing structures according to iso 13822 [4] and čsn 73 0038 [5]. this study explores the effect of variations of selected basic variables on the normal load-bearing capacity vn, focusing on the mid-span section of the most loaded beam in bending. 2. bridge under investigation the bridge under investigation is a representative precast post-tensioned bridge consisting of ten ka-73 beams of 15 m length (figure 1). the precast beams are 700 mm high and 980 mm wide, simply supported with a span of 14.3 m. the beams are transversely tied to together by concrete slabs reinforced by mild steel. the width of the longitudinal joints between the precast elements is 60 mm. the total width of the bridge superstructure is 10.85 m, the roadway width is 7.75 m. concrete of class b500 (corresponding to c35/45) was used for the precast beams. the joints between the beams are made of concrete class b330 (c20/25). mild reinforcement is of class 10 425/v and 4.5 mm diameter plain patented wires were used as prestressing reinforcement, the number of wires in each cable (tendon) can be seen in figure 2. the beams were 68 https://doi.org/10.14311/app.2025.53.0068 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 53/2025 material variability in post-tensioned bridge: case study figure 1. cross-section of the bridge, dimensions in mm. longitudinally made of 3 segments, and thus contain 2 unreinforced transverse joints of 20 mm width filled with mortar. in this study, the most loaded beam is considered, for which only the mid-span section is analysed (figure 2). it is worth noting that the most loaded beam may not always be critical for reliability of the bridge, as beams may be exposed to vastly different levels of degradation. the most damaged beams tend to be the edge beams, as they are the most exposed to the external environment and drainage is often brought to the edges [1, 6]. in terms of design, apart from the cross-section in the mid-span, shear at the support can be critical in the area in the case of sloping bridges. figure 2. longitudinal section with the investigated section marked in red line (up), cross-section of the ka-73 beam in mid-span (left), number of cables in one beam (right) [2]. 3. numerical model and basic variables the determination of the internal forces from the permanent and traffic loads was performed using a 3d slab model that takes into account the load distribution on the structure. the 2d model of the ka-73/15 beam, which was created in scia engineer (figure 3), is essential for the study performed. the prestressing was modelled according to the producer’s documentation [7]. the material characteristics for the numerical model are taken from [7] and concrete characteristics from [3]. overview of the basic variables is given in table 1. the prestressing process according to [7] was considered in the numerical model (figure 4). the resulting prestressing losses were determined in the model for time-dependent analysis. the resulting losses at the end of life (100 years) are estimated to 32.6 % in the mid-span. 4. sensitive analysis the sensitive analysis investigates the effects of variability of basic variables on resistance of the selected cross-section under permanent and traffic load effects. in this study, the assessment considers both the ultimate limit state (uls) and the serviceability limit state (sls). for the latter, the frequent load combination is applied and the stress limitation criterion is evaluated, considering the tensile stress limit value fctd in accordance with čsn 73 6222 [2]. initially, the key element of the critical beam’s resistance – prestressing reinforcement is analysed. the effects of corrosion loss of prestressing reinforcement on ultimate strain are investigated. afterwards, the sensitivity analysis follows with the effect of concrete strength and non-uniform thermal action. 4.1. corrosion loss, ductility the assessment of the corrosion condition of prestressing reinforcement is an essential part of the structural survey of bridge. the survey is mainly based on destructive probes that the conditions at selected locations. corrosion damage is assessed visually and depends largely on experience of the expert, thus being somewhat subjective. according to čsn 73 6221 [8], a maximum corrosion loss value of 5 % is set for prestressing reinforcement. in practice, however, the corrosion damage may be much higher and may reach up to several tens of percent at critical locations. such damaged prestressing reinforcement no longer contributes to resistance of the section [9]. furthermore, the corrosion damage reduces the ultimate strain of the prestressing reinforcement, for reinforcement unaffected by corrosion ϵu = 40 ‰ according to [10], the mean value can be considered as ϵu = 50 ‰ accord69 a. mezera, m. holý, m. sýkora acta polytechnica ctu proceedings figure 3. numerical model of beam ka-73/15 for time-dependent analysis (scia engineer). material properties of concrete characteristic value of concrete compressive strength fck 35 [mpa] characteristic value of concrete tensile strength fctk 3.2 [mpa] mean value of modulus of elasticity ecm 34 [gpa] material properties of prestressing reinforcement characteristic value of tensile strength fpk 1650 [mpa] yield strength σ0.2 1 200 [mpa] increased yield strength (keeping stress during posttensioning) σ0.2,i 1 350 [mpa] initial tension σp(t0) 1 325.4 [mpa] mean value of modulus of elasticity ep 195 [gpa] prestressing parameters coefficient of friction in curved part of tendon µ 0.19 [-] anchorage set – initial value a 3 [mm] unintentional angular displacement k 0.005 [rad m−1] duration of keeping stress tp 120 [s] relaxation class i prestressing loss (mid-span, t = 100 years) 32.6 [%] ductility ϵud 20 [‰] corrosion loss cx 0 [%] internal forces char. bending moment in mid-span (self-weight load) my,g0 252 [knm] char. bending moment in mid-span (other permanent load) my,g 167 [knm] char. bending moment in mid-span (vn = 32 t) my,vn 291 [knm] dynamic factor (acc. čsn 73 6222) δvn 1.20 [-] resistance ultimate bending resistance in mid-span rd,uls 1 322 [knm] service bending resistance in mid-span rd,sls 588 [knm] load-bearing capacity (bending) in mid-span (uls) vn,uls 52 [t] load-bearing capacity (bending) in mid-span (sls) vn,sls 40 [t] table 1. overview of basic variables. figure 4. prestressing procedure according to [7]. ing to [11]. the effect of corrosion loss is shown in figure 5 and figure 6. it can be seen from figure 5 that the relationship between load-bearing capacity and corrosion loss is linear. a change in this trend appears at high corrosion loss (about 40 % or more) where reduced ultimate strain starts playing a role. at a corrosion loss of about 40 %, the ductility of the prestressing reinforcement is reduced to 0.5 ‰ [10]. the stressstrain diagram for prestressing reinforcement given in en 1992-1-1 [3] suggests that the reduced ultimate 70 vol. 53/2025 material variability in post-tensioned bridge: case study figure 5. effect of corrosion weakening on design (assessment) value of load-bearing capacity, rd. figure 6. effect of corrosion loss on stress at bottom of selected section for frequent load combination (sls). strain affects resistance if ϵud < fpd/ep where ϵud is the design (assessment) value (ϵud = 20 ‰ in [3]) and fpd is the design value of the tensile strength of the prestressing reinforcement. the effect of corrosion loss depends on a considered limit state. figure 5 indicates a linear relationship between corrosion weakening and uls capacity. the effect on sls (frequent load combination) is also significant as displayed in figure 6. as the corrosion loss increases, the tensile stress in the lower fibers of the cross section (mid-span cross section) appears and further increase with increasing corrosion level. one of the most important factors that can prevent corrosion of prestressing reinforcement is proper grouting. well-executed grouting provides several protective mechanisms for the reinforcement. cement grout has an alkaline ph that forms a passive layer on the surface of the steel, preventing corrosion. it also restricts the access of water and oxygen to the reinforcement, which are necessary conditions for the development of corrosion. good quality grouting therefore provides long-term protection and significantly extends the life of the reinforcement and the entire structure [12]. on the other hand, imperfect or poor-quality grouting can even accelerate corrosion. the most common problems include incomplete filling of ducts, the formation of voids or cracks in the grout. another problem is poor bond between the grout and cables, which can cause localised loss of protection and allow development of pitting corrosion. this type of corrosion is particularly dangerous as it causes rapid local weakening of the reinforcement. it can also lead to grout cracking due to volume changes or loading of the structure, which further compromises the protective function of the grouting. the composition of the grout can also influence corrosion. if it is poorly designed, for example if a water ratio is high and cement content is low, the grout insufficiently resists the ingress of moisture and aggressive substances. it is therefore essential that grouting materials conform to standards and are applied under controlled conditions. 4.2. prestressing losses prestressing losses represent a reduction in the prestressing force applied in the prestressing reinforcement during the construction process and after the structure is in service. these losses have a significant effect on the effective transfer of prestress and on the resulting capacity and service life of the structure. they are divided into shortand long-term. 4.2.1. short-term losses these losses occurring immediately after tensioning include: • frictional losses caused by movement of the reinforcement during tensioning, especially for curved cable ducts or when there is non-uniform friction between the reinforcement and the duct. these losses are influenced by the curvature of the duct and the quality of grouting. • slip in the anchor: after anchoring the tensioned reinforcement, shifts (settlements) of the anchoring devices may reduce the tension in the reinforcement. • deformation of concrete: when prestressing is applied to cables, deformation of concrete reduces stresses in the already prestressed cables. 71 a. mezera, m. holý, m. sýkora acta polytechnica ctu proceedings 4.2.2. long-term losses long-term losses during the lifetime of the structure are caused by: • concrete creeping, mainly due to permanent loads. • concrete shrinkage. • reinforcement relaxation. prediction of these rheological effects is generally associated with large uncertainty as they depend on a number of material and geometrical properties and local environment. the models for prediction of prestressing losses have changed since the 1960–1980s. prestressing losses commonly have an insignificant effect on load-bearing capacity at uls. however, they play the major role at sls as the tensile stresses in the bottom fibres appear with increasing prestressing losses. this is why slss often govern reliability of existing bridges and it is essential to accurately determine the prestressing losses. the following factors affecting prestressing losses are investigated in the following sensitivity analysis in detail: anchorage slip, frictional losses, effect of cable geometry, duration of keeping stress and reinforcement relaxation. based on experience, these are likely the major factors. 4.2.3. slip in anchor anchor slip contributes to prestressing loss. the basic slip value, a = 3 mm, is provided in the documentation for ka-73 beams [13]. however, a larger value of 6 mm was often experienced in practice. based on the authors’ judgement, the lower limit is estimated to 2 mm. these three values are taken into account in the analysis. it appears that the slip has an insignificant effect on the prestressing losses only (figure 7), with expected value of prestressing loss of about 33 % for 3–6 mm slips. figure 7. effect of anchor slip on prestressing losses at midspan. 4.2.4. frictional losses frictional losses are particularly important for posttensioned structures. during tensioning, friction between the cable and the walls of the cable duct causes two components of the loss: • in curved cables, related to friction coefficient: the basic value of friction coefficient, µ = 0.019, is taken from en 1992-1-1, table 5.1 [3]. a higher value of 0.35 is considered according to [14, 15]. the lower limit of 0.12 is based on the authors’ judgement. • due to corrugation of straight cables [16]: values of 0.003, 0.005 and 0.01 rad m−1 are considered. the value k = 0.003 rad m−1 corresponds to the requirements of previous standards [14, 15] and a range of 0.005–0.001 rad m−1 is recommended in [3]. figure 8 indicates that the friction coefficient has a small effect on the prestressing losses, and consequently on sls load-bearing capacity. a similar conclusion is drawn for the corrugation of straight cables (figure 9). figure 8. effect of friction coefficient on prestressing losses in mid-span. figure 9. effect of corrugation of straight cables on prestressing losses in mid-span. 4.2.5. duration of keeping stress the duration of keeping stress refers to the time interval during which the tension in the prestressing reinforcement is maintained during the tensioning and anchoring, having a positive effect on relaxation and slip in anchors. three time intervals (60, 120 and 300 s) – the most common in practice – are selected (figure 10). figure 10. effect of duration of keeping stress on prestressing losses in mid-span. even though it seems that duration of keeping stress has a negligible effect on prestressing losses, it is important to take this parameter into account in the 72 vol. 53/2025 material variability in post-tensioned bridge: case study figure 11. relaxation time course depending on the relaxation class considered. figure 12. effect of concrete strength class on resistance in mid-span. calculation of prestressing reinforcement relaxation. relaxation is a key factor in prestressing losses and is largely dependent on the applied stress and duration of keeping stress. to determine the relaxation losses, it is important to consider the prestressing process (figure 4, adapted from [7]). according to en 1992 [3], relaxation class i is considered in the assessment. note that classes i and ii apply for wires and tendons with normal relaxation and class iii for hot rolled bars [3]. this assumption is conservative and in-situ measurements often indicate lower levels of relaxation [3]. for the bridge under investigation relaxation class i is considered. for ρp/fpk = 1 325/1 650 = 0.803, relaxation loss is predicted to about 25 %. therefore, relaxation contributes to about 2/3 of the total prestressing losses (25 % out of 32.6 %). if the prestressing reinforcement had been prestressed to 1 650 mpa, relaxation loss would have increased from 25 % to 35 %. figure 11 shows the relaxation losses over time for all relaxation classes. previous standards [14, 15], according to which existing structures that are currently beyond half of their design life (100 years) were designed, assumed that 100 % of relaxation loss would take place within the first year. in contrast, en 1992 [3] suggests that approximately 50 % of relaxation loss takes place in the first year only. 4.2.6. concrete strength class the strength class of concrete is indicated by its compressive strength, on the basis of which other mechanical properties of concrete are normally specified. c35/45 was most often used according to the design documents for these types of precast bridges; the actual material properties are determined for the bridge under investigation by a structural survey. concrete strengths from surveys are usually higher than those in the design documentation, mainly as a consequence of hardening. the effects of strength class on uls load-bearing capacity are shown in figure 12. regarding sls, the strength class has only bearing on concrete tensile strength that contributes in the cases where tensile stresses in concrete are allowed. it appears that the effect of strength class on uls resistance in the ultimate limit state is moderate for lower concrete strengths. for higher concrete classes, an increase becomes small. the effect of the strength class on prestressing losses appears to be less significant, with a decrease of 5 % between c25/30 and c45/55 (figure 13). nevertheless, it is necessary to analyse the prestressing losses as accurately as possible, as they can have a major impact on sls load-bearing capacity as discussed in section 5. 4.2.7. temperature although thermal actions on statically determined structures do not induce internal forces as in the case of statically indeterminate structures, the nonlinear temperature difference of thermal actions (procedure 2 according to en 1991-1-5 [17]) results in self-equilibrated stresses in the cross-section. these stresses may have a significant effect particularly on sls (stress and crack width limitation) that is commonly decisive in the case of prestressed bridges. the effect of the non-linear temperature difference compo73 a. mezera, m. holý, m. sýkora acta polytechnica ctu proceedings figure 13. effect of concrete strength class on prestressing losses. figure 14. stress from permanent and traffic load (black) combined with self-equilibrated stress from heating (red) and self-equilibrated stress from cooling (blue), the non-linear temperature difference of thermal actions (procedure 2 according to en 1991-1-5 [17]). nent determined according to procedure 2 of [17] on the self-equilibrated stress is shown in figure 14. load effects due to the temperature difference (cooling) reduces the compressive stress in the bottom fibres and has a significant effect on estimated loadbearing capacity. the effect of thermal actions is therefore not negligible even for statically certain structures. 5. summary the effects of the selected basic variables on loadbearing capacity of the bridge under consideration are overviewed in table 2. for uls at the mid-span section, bending loadbearing capacity is 52 t. a 1 % corrosion loss results in a load capacity of 51 t. the 1 % corrosion loss corresponds to a 0.14 mpa (unfavourable) reduction in compressive stress in the bottom fibres of the section. it follows that corrosion of prestressing reinforcement is one of the key factors for the service life of prestressed concrete structures and it is important to prevent or control it during service life. relaxation is governing prestressing losses. despite a relatively low influence of the other factors such as slip in anchor or frictional losses on total prestressing losses, changes in these factors can have an influence on sls load-bearing capacity. the change in slip of the anchor affects the prestress loss by 1.3 %. for the assessment using the frequent load combination, this already needs to be taken into account. for the load-bearing capacity vn [t] uls sls corrosion loss 0 % 52 40 1 % 51 38 prestressing loss 20 % 52 48 25 % 52 45 30 % 52 42 32.6 % 52 40 35 % 52 37 concrete strength class c 30/37 50 40 c 35/45 52 40 c 40/50 52 40 temperature g+q 52 40 g+q+t+ 52 41 g+q+t52 38 table 2. effect of variability basic variables on midspan load-bearing capacity, default variant marked in blue. reference case (figure 7 through figure 10), the loadbearing capacity in the midspan is 40 t (sls) while with increased prestressing loss due to a higher slip in the anchor, the load-bearing capacity reduces to 37 t (even though a reduction in compressive stress is only 0.2 mpa). the 3 t reduction in load-bearing capacity demonstrates the importance of the estimate of the slip and consequently of the prestressing loss on the sls assessment. 74 vol. 53/2025 material variability in post-tensioned bridge: case study the influence of the duration of keeping stress during tensioning on the sls assessment is moderate. duration of 300 s increases the load-bearing capacity by about 2 t, i.e. to 42 t. in the sls assessment, a stress change of about 0.4 mpa induced by the self-equilibrated stress due to the non-linear temperature difference component of thermal action reduces load-bearing capacity by 4 t. 6. conclusion the effect of basic variables describing prestressing force, concrete strength and temperature difference component of the thermal action on load-bearing capacity of precast post-tensioned bridge girders (type ka-73/15) is investigated, focusing on bending at the mid-span of a simply supported beam. in addition to the ultimate limit state (uls), serviceability limit state (sls) of stress limitation for the frequent load combination is analysed. it is demonstrated that corrosion and prestressing losses have the most significant effect on load-bearing capacity. even a change in prestress losses of a few percent may affect load-bearing capacity by a few tons (up to 10 %) at sls while the uls assessment is basically unaffected by prestressing loss. in contrast, corrosion losses are essential for the uls assessment. concrete strength seems unimportant for assessments of this type of structures unless the requirement of full decompression is relaxed and tensile stresses may occur in concrete. further, even for statically determined prestressed bridges, the effect of the temperature difference component of thermal action resulting in self-equilibrated stresses has a moderate impact at the sls assessment. further research will be focused on reliability analysis of bridges based on the results of structural surveys, accompanied by the sensitivity analysis considering uncertainties in specification of basic variables. acknowledgements this study has been supported by the czech science foundation under grants 23-06222s and 24-10892s. references [1] r. ponechal, p. koteš, d. michálková, et al. effect of water condensate on corrosion of wires in ungrouted ducts. materials 14(24):7765, 2021. https://doi.org/10.3390/ma14247765 [2] zatížitelnost mostů pozemních komunikací [in czech; load bearing capacity of road bridges]. standard, czech standard institute, prague, 2015. [3] european committee for standardization. en 1992-1-1 – eurocode 2: design of concrete structures : part 1-1: general rules and rules for buildings, 2004. [4] zásady navrhování konstrukcí – hodnocení existujících konstrukcí [in czech; bases for design of structures – assessment of existing structures]. standard, czech standard institute, prague, 2010. [5] navrhování a posuzování stavebních konstrukcí při přestavbách [in czech; assessment and verification of existing structures – supplementary guidance]. standard, czech standard institute, prague, 2019. [6] m. kryštov, a. čítek, s. řeháček, d. čítek. vyhodnocení stavu diagnostikovaných mostů z dodatečně předpjatých tyčových prefabrikátů ka–73 po 40 letech provozu [in czech; assessment of the condition of surveyed bridges post–tensioned ka–73 precast concrete beams after 40 years of service]. 2023. isbn 978-80-908943-0-3. [7] konštrukcie cestných a diaĺničných mostov z prefabrikátov ka-73 dĺžky 9–12–15–18 m – typový podklad [in czech; structures of road and highway bridges made of precast ka-73 prefabricates, length 9–12–15–18 m – type background]. dopravoprojekt bratislava, 1973. [8] prohlídky mostů pozemních komunikací [in czech; surveys of road bridges]. standard, czech standard institute, prague, 2018. [9] železniční mosty s předpjatou nosnou konstrukcí v síti správy železnic [in czech; railway bridges with prestressed load bearing structure in the network správa železnic]. ctu in prague – faculty of civil engineering, 2021. isbn 978-80-01-06923-3. [10] j. kolísko, v. vacek, p. pokorný, m. kostelecká. vliv koroze na vlastnosti ocelové předpínací výztuže [in czech; the influence of corrosion on the properties of steel prestressing reinforcement]. praha 1: česká betonářská společnost čssi, 2019. isbn 978-80-907611-2-4. [11] jcss probabilistic model code (draft of section ’environmental attack’). joint committee on structural safety. isbn 978-3-909386-79-6. [12] l. haixue. grouted post-tensioning tendon evaluation. 2017. [13] konštrukcie cestných a diaĺničných mostov z prefabrikátov ka-73 dĺžky 9–12–15–18 m – statický výpočet nosníkov dĺžky 15 m část i [in slovak; structures of road and highway bridges made of precast ka-73 prefabricates of length 9–12–15–18 m – static calculation of beams of length 15 m part i]. dopravoprojekt bratislava, 1973. [14] navrhování konstrukcí z předpjatého betonu [in czech; design of prestressed concrete structures]. standard, czech standard institute, prague, 1970. [15] navrhování mostních konstrukcí z předpjatého betonu [in czech; design of prestressed concrete bridges]. standard, czech standard institute, prague, 1993. [16] n. j. předpjaté betonové konstrukce [in czech; prestressed concrete structures]. akademické nakladatelství cerm, 2008. isbn 978-80-7204-561-7. [17] european committee for standardization. en 1991-1-5 – eurocode 1: actions on structures – part 1–5: general actions – thermal actions, 2005. 75 https://doi.org/10.3390/ma14247765 acta polytechnica ctu proceedings 53:68–75, 2025 1 introduction 2 bridge under investigation 3 numerical model and basic variables 4 sensitive analysis 4.1 corrosion loss, ductility 4.2 prestressing losses 4.2.1 short-term losses 4.2.2 long-term losses 4.2.3 slip in anchor 4.2.4 frictional losses 4.2.5 duration of keeping stress 4.2.6 concrete strength class 4.2.7 temperature 5 summary 6 conclusion acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2024.51.0053 acta polytechnica ctu proceedings 51:53–68, 2024 © 2024 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague digital reconstruction of real (near) accidents of vulnerable road users (vru) in the czech republic – modern technology brings new perspectives ondřej piksaa,∗, jan ritterb, pavel drbohlavc, tomáš petránc a czech technical university in prague, faculty of transportation sciences, institute of transport vehicles, horská 3, 128 03 prague 2, czech republic b aurel cz s.r.o., road safety department, štěpánská 539/2, 120 00 prague 2, czech republic c aurel cz s.r.o., 3d measurement department, štěpánská 539/2, 120 00 prague 2, czech republic ∗ corresponding author: piksa@fd.cvut.cz abstract. protecting vulnerable road users (vrus) remains an issue despite significant advances in the development of integrated safety, autonomous driving and cooperative environments. as part of accident/collision processes, the urban area of municipalities continues to constitute a risk environment with a relatively high frequency of serious injuries (vru) even at relatively low speeds. in order to understand and increase road safety, it is necessary to analyse the causes of traffic accidents in more detail using video recordings, virtual reality (vr) and automated computer vision methods. the state of the art in the czech republic is limited due to the absence of high-quality and extensive data sets. this article constitutes a proof-of-concept solution for the digital reconstruction of selected accident/collision events in 4 cities of the republic, specifically in prague, liberec, česká lípa and jaroměř. it makes it easier to identify, parameterize and virtualize the results of a traffic conflict study and can provide input data for the testing and validation of sensors, interaction models of road users’ behaviour, and preparation of scenarios for a simulated environment; ultimately it reinforces prevention efforts. keywords: vulnerable road user (vru), accident/collision events, road safety, virtual reality (vr), photogrammetry, integrated rescue system (irs), czech republic. 1. introduction based on long-term cooperation in the development of cars at škoda auto, involvement in preventive irs programmes and educational activities for primary and secondary schools, we have decided to better understand the occurrence of traffic accidents involving vulnerable road users (vru) to map the state of the art and subsequently to create a proof-of-concept solution for its detailed analysis. using the tools of modern technology, such as virtual reality, video analysis, photogrammetry and laser scanning, we obtain a unique insight that helps us understand and evaluate better the key parameters of these adverse events in the context of space and time. their prevention/elimination remains the key goal in the near future, especially with the contribution of better proposals for sensory functions, both of vehicles and cooperative transport environments, as well as test scenarios as such and more intensive/better preventive and educational activities. data collection has been handled in the project using public camera infrastructure, on-board vehicle cameras, 3d laser scanners, cameras and uavs. the next step is the digital reconstruction itself, with the final export to virtual reality and the subsequent use of hmd technology to simulate and experience a real accident in the digital world. in addition to the methodological procedure of the solution, we have created our own software application serving primarily the irs units. we have validated the procedures suggested by use and their results by comparing them with the commercial virtual crash solution. the police of the czech republic has successfully adopted our insights into its practice and nationwide trainings of its members have been held to use the created tools. in cooperation with the czech police and the vision 0 platform, preventive actions using vr simulations based on real traffic accidents are still ongoing. in the future, we would like to upgrade our approach from the proof of concept stage to a more robust application in practice. in the context of the entire work, it is also important to mention the growing need to collect data from accidents that not only have health consequences, but also cause material damage [1]. both who and eu statistics [2, 3] show that vulnerable road users (vrus) account for more than half of all road accident deaths. in the eu, vru collisions with a vehicle constitute almost 70% of the total number of fatal accidents in urban areas. data from the czech republic have shown a similar trend in the last decade [4]. vrus generally have a high degree of freedom of movement in an urban environment and tend to break traffic rules, thus 53 https://doi.org/10.14311/app.2024.51.0053 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en o. piksa, j. ritter, p. drbohlav, t. petrán acta polytechnica ctu proceedings showing a greater propensity for accidents and injuries. there are even differences in the behaviour in different cities [5–8]. the statistics show that achieving the vision 0 goal by 2050 will be difficult. despite huge progress in the field of integrated vehicle safety and autonomous driving, collision-free transit and interaction with vrus in urban traffic remains a challenge [9]. in any case, vulnerable users often cannot avoid death or serious injuries in a collision, even at relatively low speeds, when accident events in urban areas occur. recognizing vrus is a much more difficult task for a vehicle, e.g. as part of the functions of the aeb system, than recognizing other vehicles. 2. research most cities are equipped with relatively large camera surveillance systems. such a developed network offers the opportunity to use video analysis in many areas dealing with traffic safety. there are currently not a lot of research and data sets available in the czech republic, which may create a gap due to the insufficient coverage of input data needs for the development of advanced driver assistance systems (adas), automated vehicles (avs) and cooperative environments. video analysis in the form of trajectories and speed profiles provides a description of the behaviour of individual road users at the micro level, which is a key input for many applications, such as assessment of the safety and efficiency of transport systems and infrastructure, or calibration of behavioural models, etc. this is confirmed by the conclusions of the extensive indev horizon 2020 study [10]. the outputs of the indev project, just like ahmed et al. [11], describe errors in police accident records that occur when measuring the accident site, information on injury rates, vehicles, roadway conditions and the specific environments. the described inaccuracies have a significant impact on any analysis. at the end, the authors recommend greater use and development of a method for studying traffic conflicts (e.g. swedish traffic conflict technique, doctor – dutch objective conflict technique for operation and research) using video analysis. data obtained from vehicle on-board cameras and infrastructure as one of the 4 pillars of data sources [12–17] are reported by the consortium p.e.a.r.s. [18], which addresses the development of the iso standard for traffic safety assessment and for active safety technologies integrated into vehicles using virtual simulation. the reason for obtaining additional input data for development, testing and homologation is the fact that adas systems do not cover 100% of all scenarios, while av system require 100% coverage of ideally all possible real critical scenarios [19]. the verification of the functions of adas systems vs. vru in independent testing is described in pedicrash and aaa tests and studies, the refigure 1. ideal video analysis system. sults of which show limited effectiveness on selected complex scenarios [20, 21]. real critical scenarios, situations and scenes require high-quality digitization, with the interconnection of driving data, and together with the application of methods using surrogate measures of safety (smos), they enable quick proactive solutions to understand all the causal contexts of accident and near-accident events. the given environment of critical situations can be reconstructed through scanning using multi-image photogrammetry. the output is a cloud of points that is used to create 3d models. the importance in terms of accuracy and time savings of accident site measurements with its subsequent virtualization when using modern measurement techniques by the police and forensic science is indicated, for example, in the papers [22–24]. other uses are reported, for example, in papers in the field of research on traffic flows [25] or road infrastructure safety [26, 27]. from our own experience, we can mention the application for in-depth research on the transport safety made by teams such as ufo (vw), vdb (skoda auto), aaru (audi). currently, some service departments of the transport police of the czech republic already use modern technology, such as uav photogrammetry and ground laser scanning. video analysis including kinematic data and trajectories was used, for example, in research within the prospect project [28], a study from vancouver, british columbia, focused on cyclists [29], or an analysis of pedestrian behaviour using walk parameters in nanjing, china [30]. an innovative approach to data collection was applied in the ko-per project containing data from 8 infrastructure cameras and 14 laser scanners. the data contain sequences with object markings, object classes and hundreds of road users [31]. the ideal video analysis processing workflow is described in figure 1. 54 vol. 51/2024 digital reconstruction of real (near) accidents of vru . . . figure 2. comparison of tyre marks from distorted and undistorted images. figure 3. scheme of a synthetic calibration objects. the detection of road users is dependent on the complexity of the road traffic environment and is not easy to handle, so it is in the interest of many studies [32–39]. the existence of publicly available data sets and algorithms is important for the development and testing of detectors [40–44]. a very important part of image analysis is rectification; paper [45], for example, evaluates measurement errors based on lens distortion and its impact on photogrammetric measurement of the distorted image; figure 2 shows how the difference in tyre track measurement can cause a difference in the final results of the accident analysis. another study [46] addresses the measurement accuracy affected by calibration with respect to the ratio of area of the calibration object at the accident site or the location of the calibration object using a cluster of small flexible objects. figure 3 shows a scheme of calibration of the object from the calculation of the homographic matrix h and figure 4 shows a rectified track image. another paper evaluates accuracy using the pc rect software, see figure 5 [47]. trajectory extraction – object tracking with their detection has several approaches [48]. • manual methods – the analyst will mark each participant based on the specified frequency. the reason for use compared to the automated method is high accuracy, at the same time it does not address the issue of possible occlusion [49], incorrect classification, etc. [50]. however, the extraction may be subject figure 4. rectified image of traffic accident. figure 5. pc rect software. to human error and the number of observations is limited. • automated methods – they process the image to identify and track moving objects in sequential video frames. compared to the manual method, automated methods are not time consuming and there is no human error. it is possible to process a larger volume of trajectories. the problem is the aforementioned occlusion, changes in light, the movement of shadows or the movement of larger groups. many current studies deal with the monitoring and prediction of vru trajectories [50–53]. the basic approaches are shown in figure 6. the above procedures are applied to a wide range of software tools developed for trajectory extraction. [53– 60]. most of the similar tools do not explicitly provide for use by other analysts. speed measurement there are many different approaches to calculating speed with the application of different methods according to the purpose of the research [61]. for example, the method applied in the prospect project ensured results among the derived trajectories with a relative accuracy of 0.5–1m [62]. another study [63] at low speeds applies the cross ratio method from projective geometry with σ 1.3% and ∆ 1.5%. another article in the field of forensic science proposes a method of estimating the speed of a vehicle in 55 o. piksa, j. ritter, p. drbohlav, t. petrán acta polytechnica ctu proceedings figure 6. basic approaches to vru trajectory prediction. figure 7. t-analyst. a video using a virtual plane and a virtual reference line (vsem) [64]. the following article suggests reconstructing accidents from cctv recordings using volumetric kinetic mapping (vkm) [65]. for an overview, we present freely available software applications available from websites with the function of trajectory extraction from video recordings [66]. t-analyst this is a database solution that makes it possible to select conflicts according to classification. it contains tools for placing predefined shapes (models) in the image, to extract trajectories or derive the speed of the monitored subjects/objects at a certain time (see in figure 7). ruba (road user behaviour analysis) the basis of the approach is a set of detectors that can be connected by logical rules. they are activated by movement and they highlight conflict situations in red. under favourable conditions, it is possible to remove up to 90% of the original footage that does not contain relevant information (see in figure 8). traffic intelligence project it provides a set of tools for the detection, tracking and classification of road users using a tracking algorithm mapping trajectory data and individual interactions (see in figure 9). figure 8. ruba. figure 9. traffic intelligence project. figure 10. strudl. strudl: surveillance tracking using deep learning this approach is used as part of the indev project under horizon 2020 as an open-source and free framework for videos captured by stationary cameras. it uses an object detector using deep learning to track and trace movement in the scene (see in figure 10). safecross it describes an open-source framework for proactive risk assessment of individual accidents. cyclist detection and tracking is done automatically using advanced computer vision techniques and deep learning (see in figure 11). last but not least, the use of virtual reality is becoming increasingly important for examining critical scenarios. especially in behavioural research involving hmi. for example, article [67] represents empirical research evaluating different hmi concepts of behaviour 56 vol. 51/2024 digital reconstruction of real (near) accidents of vru . . . figure 11. safecross. figure 12. interactions in situations with multiple pedestrians. figure 13. the test process. for two pedestrians, see figure 12. or study [68] dealing with decision-making processes when crossing the road in vru vs. inbound vehicle interactions. the process of the test can be seen in figure 13. 3. method our way of looking at road safety in terms of preventing road accidents is unusual, and thus brings a unique point of view on the whole issue. the created proof of concept called virtual accident experience for real survival uses digital reconstruction of real accident event(s) all the way to the final use of hmd technology. in addition to the methodological procedure, we have created a software application written in python, designed specifically for the czech police. the reason for our own in-house solution is the unavailability of sw licences for programmes to be commonly used by the units of the integrated rescue system. we attach a comparison of the conventional virtual crash method with our method at the end of the chapter; at the same time it serves a validation function. by attaching a roadmap of reconstructed real (near-)accidents, we demonstrate the application in practice. specifically, this covers the following locations in the czech republic: prague, liberec, česká lípa, and jaroměř. 3.1. methodological approach the methodology for evaluating the 2d movement of an object is based on the image analysis of a recording from a single camera. this is a simple analysis, where the first prerequisite is the movement of the object on the horizontal xy plane. the effect of the z-axis (bottom-up direction) is not taken into account. the advantage of the whole approach is its use even if we only know the top view of the situation, the bird’s eye view/aerial view. for evaluation purposes, only an image from publicly available sources of various map materials, e.g. google.com/maps or mapy.cz, is sufficient. however, these data are burdened by distortion caused by their acquisition at angles of up to 30°. therefore, for the subsequent refinement of the results, we supplement the map data set with laser scanning of the area; this step will give us the benefits of a real orthographic map. the accuracy of the measurement reaches the maximum uncertainty in units of centimetres, typically up to 2 cm on an area of 600m2. the quality of the orthographic map can be exported in high resolution. the basic information for evaluation also includes the sampling frequency of the camera or video recording (fps). the quality of the images is also influenced by the resolution of the camera, the lens used and the lighting conditions. the resolution of the orthophoto map, in other words the size of the scale – the ratio of the number of pixels to distances [pix.m−1], can also be considered a decisive influence. when using scanner measurement, we can achieve hundreds to thousands of pixels per metre. the methodology is usable for both static and dynamic (on-board) camera recordings. the principle of evaluation is based on the search for optical pairs of points (places) that are identical for the video recording and the orthophoto map. these points introduce a dimension into the image (there must be a sufficient number of them to achieve better results), which is defined by the orthophoto map. it is advisable to have the distribution of control points in the image especially in those places where the objects of interest move. the control points must be positioned on the xy plane to maintain clarity between the 2d orthomap and the image. specifically, these are unambiguous contrasting elements, such as corners at pedestrian crossings, footings of traffic signs, curb joints, etc. since the influence of perspective is not linear, it is advisable to place control points in both directions of the image. the impact of perspective on the results is then significant. we achieve the best results in the case of a record taken from a bird’s eye view, similar to an orthographic one. on the contrary, 57 o. piksa, j. ritter, p. drbohlav, t. petrán acta polytechnica ctu proceedings in the case of shooting from a small (low) angle, the quality of the results is reduced; this is the case of an on-board camera, where the angle between the pad and the object is relatively small. the next step is to monitor the movement of the object itself in the image – object tracking. due to the aforementioned limitation of movement on the xy plane, it is necessary to mark the tracked object at a place as close as possible to this plane, e.g., the point of contact of the tyre with the road. tracking is performed manually on selected key frames (for reasons of accuracy); the timeline is defined by the sampling rate (fps). the 2d coordinates obtained in this way in the video recording are converted into their equivalents in the 2d orthophoto map using linear interpolation. the obtained 2d motion coordinates are interlaced with a second-order polynomial across three points, due to smooth transitions between individual key frames. this gives us a two-dimensional description of the movement of objects in space – their position, speed and direction. the on-board record is evaluated with the difference that the above procedure is applied to each frame, or rather a suitable frame, in which control points can be defined with certainty. this eliminates the need to determine the position of the object, because this is the position of the camera itself. the results are influenced by the optical parameters of the camera itself (focal length of the lens and frame size = fov). 3.2. created software application when developing the software tool, we referred to the methodology described in section 3.1, further dividing it into 3 consecutive phases, shown in figures 14, 15 and 16. for a clear illustration, we attach a practical reconstruction of one of the locations – jaroměř (shown in figures 17– 23. this deals with the measurement of an accident site – ortho plan, from the accident site survey using more ground or aerial photogrammetry. the aim is to convert the marked tracks into 2d coordinates in scale using manual input and automatic detection based on the set parameters in the image. the format of the text output is compatible with the processes and technical equipment of the police used for subsequent completion of the report on the traffic accident. the application was optimized and subsequently tested. implementation of an application for measuring track coordinates in a 2d orthophoto plan using automatic detection has brought the following functionalities (the output is an executable version of the application that has been tested): • import of the requested ortho plan, • manual entry of track position, • automatic track detector using a defined search area, including parameter settings for detection (colour, threshold, gradient), • scale definition, • definition of the initial point of measurement, • orientation of the coordinate system using two points, • option to edit the description and annotation of each defined track, • option to group multiple defined points into one track, • option to introduce gps 2d coordinates using a txt file, • export of coordinates into a text file and a 2d ortho plan of the scene. it includes the implementation of an application for dynamic analysis of the course of a traffic accident. the tool makes it possible to use both stationary and on-board camera recordings. the result is a visual interpretation of the trajectory of the traffic accident participants, including information on their speeds. the application and the implementation of annotations were tested again. at the same time, a methodology for defining primitives for the automatic creation of virtual reality was proposed and approved. in addition, the ortho plan of the traffic accident scene was supplemented with the trajectories and speeds of the traffic accident participants. this allows us to bring important information and a completely new or different view of the specific traffic accident. it also provides data for the “live” virtual reality scene. as part of the implementation of an application for dynamic analysis of a traffic accident using a single camera, the following objectives have been achieved: • import of avi, mp4 or mov video, • basic video editing – selection of the area of interest (video clipping), • definition of pairs of points designed to evaluate the position in space using known coordinates from the ortho plan, • definition of objects and their points, • tracking of points in time, • evaluation of 2d coordinates of points and objects depending on time, • export of coordinates into a text file, • export into a 2d ortho plan of the traffic accident scene. export intended for visualizations in a virtual scene. it includes the development and implementation of an application for automatic creation of a 3d scene of the traffic accident scene based on an orthophoto map supplemented with primitives. the application and the implementation of annotations were tested. creation of methodological materials for all 3 phases. application for measuring track coordinates in a 2d orthophoto plan using automatic detection and application of video analysis of the accident event record. an application has been implemented for automatic 58 vol. 51/2024 digital reconstruction of real (near) accidents of vru . . . figure 14. phase 1 – measuring tracks figure 15. phase 2 – dynamic analysis of movement 59 o. piksa, j. ritter, p. drbohlav, t. petrán acta polytechnica ctu proceedings figure 16. phase 3 – creating virtual reality (vr) 60 vol. 51/2024 digital reconstruction of real (near) accidents of vru . . . figure 17. defining scale. figure 18. defining the initial point of the measurement (ipm) and the direction of the coordinate system. figure 19. pairing common points. 61 o. piksa, j. ritter, p. drbohlav, t. petrán acta polytechnica ctu proceedings figure 20. results of dynamic video analysis. figure 21. creating a 3d virtual scene. figure 22. dynamic vr scene object library. 62 vol. 51/2024 digital reconstruction of real (near) accidents of vru . . . figure 23. vehicle view in the vr scene. creation of a 3d scene of the traffic accident scene based on an orthophoto map supplemented with primitives. anonymized video outputs from virtual reality. as part of the implementation of the application, the planned objectives have been achieved: • retrieval of the entered objects from the xml source and additional information, • setting of the loaded objects to the specified position and the required rotation, • editing of the basic shape of the specified objects based on the control curves, • setting of the tracks and speeds of moving objects, • loading of specified tracking points, both in space and tied to moving objects, • setting of the scene background and lighting, • creation of control and information systems and means for previewing the scene. example results can be seen in figures 24 and 25, which depict a reconstructed situation from liberec, malé náměstí. subsequently, validation of the obtained results was carried out on another dataset from praha, smetanovo nábřeží, as illustrated in figures 26, 27 and 28. 3.3. validation of results from praha, smetanovo nábřeží 4. discussion the quality of the results is influenced by a number of factors, which are: camera/recording resolution, lens distortion and its type, angle of view of the scene, direction and size of the object’s movement relative to the rotation of the frame (effect of perspective), recording sampling rate (fps) and its stability, contrast between the monitored object and the environment, number of control points, quality of the orthophoto map (number of pix.m−1), video codec used. each of these factors will more or less influence the result. from the point of view of a fundamental improvement of the results of the methodology it would be advisable to: • to perform a lens calibration to compensate for image distortion, • more effective placement of control points throughout the image. ideally a regular grid with a size of e.g. 2 × 2m, • digitize the entire area of interest in order to obtain 3d data. then define control points directly in the 3d space, • use the z-axis information (3d data) to evaluate the position of the object, • try to avoid obscuring significant points in the image (camera position relative to static objects in the camera frame), • interleave tracked points using more complex algorithms – 3rd order polynomial using 5 or more points, etc. 5. conclussion we have achieved the planned results of the development of a software tool and methodology for the processing of marked tracks from the scene of a traffic accident, taken with multi-image ground or air photogrammetry, 3d scanning, or other methods whose output is an ortho plan (rectified photographic image). the sw application makes it possible to create scenes for virtual reality based on data sets from real traffic accidents. our effort has also been to identify factors affecting the quality of output data for the needs of safety and efficiency of transport systems, infrastructure or calibration of behavioural models (hmi). at the same time, nationwide training of representatives of the transport police and the czech police headquarters has taken place in 14 regions in the use of the software tool and methodology for processing the location of a traffic accident, as well as nationwide training of representatives of the communication and external relations department of the czech police headquarters in the use of the software application and methodology for automatic creation of a 3d scene of the traffic accident based on an orthophoto map supplemented with primitives. in cooperation with the czech police and the vision 0 platform, preventive actions using vr simulations based on real traffic accidents are still ongoing. finally, the paper has presented the verification of a method to obtain parameters such as speed and distance at a certain time, thus enabling the acquisition of smo identifiers even when using a lower-quality video recording. 6. future work a follow-up study of traffic conflicts will be carried out using appropriate camera systems. the examined 63 o. piksa, j. ritter, p. drbohlav, t. petrán acta polytechnica ctu proceedings figure 24. dynamic motion analysis results. figure 25. digital reconstruction (left) and simulation in unity and vr (right). figure 26. accident analysis – video record. 64 vol. 51/2024 digital reconstruction of real (near) accidents of vru . . . figure 27. accident analysis – conventional method. figure 28. accident analysis – methods comparison. critical situations will include primarily collisions, potential collisions and traffic conflicts between pedestrians and vehicles, secondarily between pedestrians and cyclists, cyclists and vehicles, e-scooter riders, emicromobility users, and other road users. a holistic approach will be applied to the acquired video data set to detect vru intentions using cooperative methods. intention detection will consist of basic primitive motion prediction, e.g. standing, moving or rotating. the subsequent trajectory estimate will be evaluated and categorized. this will be preceded by the selection of a suitable detection algorithm based on the dl architecture or a combination using the bayesian bdl probability with the application of e.g. fast-rcnn, r-fcn, faster r-cnn, yolov5 or ssd. most accidents are due to human error. therefore, it is advisable to add in the future a system view of the communication interface between the operator (driver) and the artificial system (vehicle, its parts). this will be achieved by using knowledge from the driving simulations research group at ctu, developing interactive vehicle simulators, scenario generation technologies and experiments using appropriate methods. instruments working with 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validation of results from praha, smetanovo nábřeží 4 discussion 5 conclussion 6 future work acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2025.52.0122 acta polytechnica ctu proceedings 52:122–128, 2025 © 2025 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague modeling of lto approach time-in-mode values in matlab using the generalized extreme value distribution bo stloukala,∗, ivan nagyb a czech technical university in prague, faculty of transportation sciences, department of air transport, horská 3, 128 03 prague 2, czech republic b czech technical university in prague, faculty of transportation sciences, department of applied mathematics, na florenci 25, 110 00 prague 1, czech republic ∗ corresponding author: stloubo@fd.cvut.cz abstract. the landing and take-off (lto) cycle, which is used to calculate emissions of aircraft below 3 000 feet, consists of four operating modes, each of which has a standardized time duration or time-in-mode (tim). in this paper, we model tim values of the approach operating mode for two aircraft types (boeing 737-800 and airbus a320) arriving on runway 24 at prague václav havel airport using the generalized extreme value (gev) distribution. predictions of the tim values are made using the models, and the performance of each model is evaluated. the results show that for both aircraft types, the predicted tim values lead to total emissions estimates deviating on average by 1 to 2 percent from estimates made using the real tim values, showing good model performance. the presented approach is also shown to be beneficial in comparison with using the standard tim value (240 s), highlighting the feasibility of further research in this area as one of the avenues of improving the lto cycle methodology. keywords: aircraft, emissions, approach, lto cycle, time-in-mode, generalized extreme value distribution. 1. introduction the process of aircraft engine emissions certification is regulated by standards and recommended practices defined by the international civil aviation organization (icao). specifically, requirements concerning emissions below 3 000 feet above ground level (agl) are outlined in the annex 16 to the convention on international civil aviation, volume ii [1]. these cover several emission species, including gaseous pollutants and non-volatile particulate matter, which have a direct impact on local air quality and hence the health of local populations [2, 3]. in order to assess the amount of these emissions, the reference emissions landing and take-off (lto) cycle is defined within the annex, for which four operating modes are given (idle, take-off, climb-out and approach). each of these has a given thrust setting as well as time duration, or time-in-mode (tim) value. as part of the certification process, data on specific fuel consumption and emission indices (i.e., mass of emissions per mass of fuel consumed) for each emission species are collected for each lto operating mode [1]. while originally intended for engine emissions certification, since they are openly available in emission databanks, these data are also frequently used for the purpose of estimating the amount of emissions emitted by aircraft at and near airports. this includes the development of inventories of the individual emission species (for example in [4–8]). the tim value is a large source of uncertainty within the calculations of emissions of airport movements, especially for the lto operating modes idle and climb-out, where the default values as given by icao are often overestimating the actual values [9, 10]. this uncertainty can be explained by the presence of various factors influencing the tim values. for example, one study analyzed the factors influencing taxi time of aircraft and hence the tim for the operating mode idle. according to its presented results, the factors ranged from weather-related factors such as wind or visibility to traffic-related factors including number of movements or runway occupancy time [11]. attention has also been paid to the lto operating modes climb-out and approach, which are influenced, among other factors, by the definition of the so-called mixing height. the mixing height (or the mixing layer height) is a parameter which influences the dispersion of pollutants near the surface, including aircraft emissions. pollutants emitted below it have a direct impact on local air quality and hence the health of the local population [12]. the icao standards assume a constant mixing height of 3 000 feet agl. since the mixing height clearly influences the tim of the mentioned operating modes, some studies have considered its value variable rather than constant while determining the tim values [13–16]. other studies have included the evaluation of the tim values without considering this factor [10, 17–20]. 122 https://doi.org/10.14311/app.2025.52.0122 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 52/2025 modeling of lto approach time-in-mode values in this paper, we focus on the approach operating mode of the lto cycle, specifically on the modeling and predicting of its tim values for two aircraft types. for this purpose, data on real tim values calculated from two weeks’ worth of automatic dependent surveillance–broadcast (ads-b) data from prague václav havel airport were utilized. using matlab, random samples of the data were repeatedly fitted with the generalized extreme value distribution, eventually leading to the estimation of the probability density functions (pdf) of its parameters. from these, point estimates of the distribution parameters were made, and the resulting model was then used to generate tim values. the predicted tim values were then compared with the real data and the performance of the model was evaluated using defined metrics. the presented results show a good performance of the model, demonstrating the feasibility of this approach with practical implications for the development of airport emission inventories. to the best of our knowledge, this is the first paper which focuses on the modeling of the tim values of the approach operating mode, or any lto cycle operating mode in general, using the generalized extreme value distribution. 2. materials and methods 2.1. data used in this paper, a dataset resulting from one of the authors’ previous work [21] was used. this dataset originally included the tim values of each operating mode of the lto cycle of individual aircraft movements at prague václav havel airport over a two-week period (17th–30th june 2023). these values had been estimated using ads-b surveillance data covering the airport and surrounding area that were provided by the air navigation services of the czech republic. as this paper focuses on the approach lto operating mode, only data for arrivals landing on runway 24 were used. more specifically, data for two aircraft types were used, namely the boeing 737-800 (icao code b738; n = 670) and the airbus a320 (icao code a320; n = 366); these two were the most commonly represented aircraft types within the data. 2.2. tim estimation the tim values from the dataset mentioned in 2.1, used in this paper, were originally estimated from the real aircraft ads-b data using the procedure, which is described in this subsection for the sake of completeness. let mi,j be an ads-b message broadcast by aircraft movement j; where i ∈ 1, ..., nmsg and nmsg is the number of messages. then, let hi,j be the geometric height and bvri,j be the barometric vertical rate broadcast by the aircraft at time ti,j within message mi,j , and let hgnd,j be the geometric height broadcast by the aircraft while on the ground. as the approach lto operating mode begins by descending below the mixing height, which was assumed to be 3 000 feet above ground level (agl) according to icao, the first message satisfying the condition hi,j < (hgnd,j + 3000) ft while i > 1 was found. the time of descent below the mixing height, tmh,j , was then assumed to be between ti−1,j and ti,j ; its exact value was found using linear interpolation by considering hi−1,j and hi,j . the end of the operating mode was then given as the time of flare (tf l,j), which is the moment when engine thrust is reduced to idle seconds before the aircraft touches down on the runway. since the aircraft nearly levels out briefly during the flare, the first message was found for which the condition |bvri,j | < 200 ft min−1 while (hi,j − hgnd,j) ≤ 50 ft (in order to prevent a false detection of flare earlier during the approach) was satisfied. then, the time tf l,j was given as the mean of ti−1,j and ti,j (here, the mean was used since multiple parameters were involved rather than the single one in the case of tmh,j , where interpolation could be done). having found both times, the tim value of lto operating mode approach for aircraft movement j was then computed as timj = tf l,j − tmh,j . 2.3. estimation of model parameters the aim of this paper was to model the tim values of the approach lto operating mode, which was done separately for the two chosen aircraft types. during initial analysis of the data, it was deemed that the generalized extreme value (gev) distribution was the best option for constructing the models in both cases. this was based on visual analysis of the shape of the histogram and pdf fitted to the real tim values, as well as the standard errors of the distribution parameters according to the distribution fitter in matlab. therefore, further work in this paper focused solely on this distribution, with the procedure being identical for both aircraft types. the gev distribution was originally proposed in the 1950s [22]; it combines together three extreme value distributions, namely the gumbel, weibull and fréchet distributions [23]. the pdf of the gev distribution of variable x (in this case the tim value), whose formula is shown in equation (1), is a function of three parameters – the shape parameter k, scale parameter σ and location parameter µ. based on the value of k, the function corresponds to one of the aforementioned distributions [24]. f(x|k, µ, σ) = σ−1 · exp { − ( 1 + k · (x − µ) σ )− 1 k } · ( 1 + k · (x − µ) σ )− k+1 k (1) the first step in order to estimate the model parameters θ̂ = {k̂, σ̂, µ̂} was to estimate the pdf for each of the three model parameters, which was done 123 bo stloukal, ivan nagy acta polytechnica ctu proceedings as follows. for a total of 100 000 iterations, a random sample of n/2 real tim values (i.e., 50 % of the data) was first selected. then, the gev distribution was fitted to the data; its parameters were estimated using the gevfit function in matlab. the parameters were then saved for further use; at the end, there was a 3 × 100 000 matrix containing the parameters in its respective rows. the pdf for each parameter (i.e., for each row of the matrix) were then estimated using kernel smoothing (matlab: ksdensity). from each constructed pdf, a point estimate of the given parameter used for the subsequent model was made, which corresponded to the peak of the function (i.e., the most likely value of the parameter was used). the resulting point estimates θ̂ were then used further as model parameters. 2.4. evaluation of estimated model once the model parameters had been estimated for both aircraft types, it was possible to make predictions of the tim values (τ̂) using the derived models. this was once again done over 100 000 iterations. in each iteration, n tim values were generated using the gevrnd function to match the number of real tim values. the tim predictions were then evaluated for both aircraft types individually as follows. first, it was assessed whether the predictions {τ̂j}n j=1 and the real values {τj}n j=1 came from the same distribution. this was done using the ranksum (mann-whitney u-test) function in matlab at the default significance level (α = 0.05). for each iteration, the p-value was saved. afterwards, the proportion of p-values less than α within all p-values was evaluated; this ratio πp was therefore given as the probability that, upon a simulation run, the distribution of the predicted tim values would be different than that of the actual tim values: πp = n(p<0.05) 100 000 , (2) where n(p<0.05) was the number of p-values less than 0.05. next, the so-called time sum percentage error (tspe) was introduced and evaluated for each iteration. the tspe estimated the percentage difference in the resulting total amount of emissions of the aircraft type during the approach operating mode calculated using the predicted tim values compared to the emissions where real tim values were used for the calculations. the tspe was given as follows: tspe = 100 · |σn j=1τj − σn j=1τ̂j | σn j=1τj , (3) where σn j=1τj and σn j=1τ̂j were the sums of all real and predicted tim values respectively. the mean and median tspe values were then determined, as well as the interquartile range (iqr). finally, in every iteration, both of the aforementioned sums were evaluated relative to the sum σn j=1240 = 240n, which was the total time sum when the standard tim value as defined by icao (240 s) was used for all aircraft movements. for this purpose, the relative sum closeness (rsc) metric was introduced, which was defined as follows: rsc = |σn j=1τj − σn j=1τ̂j | |σn j=1τj − 240n| , (4) with its purpose being to compare the resulting total emissions calculated using the predicted tim values and the standardized value. for rsc ∈ ⟨0, 1), using the predicted tim values was better than using the standardized tim value in terms of resulting accuracy of total calculated emissions, and vice versa for rsc ∈ (1, +∞). alternatively, the inverse value of rsc shows how many times more accurate the resulting emissions estimates were after using the predicted tim values instead of the standard value. for rsc, the same statistical measures as for tspe were computed, plus the coefficient βrsc , which was given as the number of rsc values that were less than 1 divided by the total number of rsc values (i.e., 100 000). this coefficient therefore gave the probability that, upon a simulation run, using the predicted tim values would bring more accurate results regarding the total calculated sum of emissions than when using the standard tim value. 3. results figure 1 shows the pdf for the gev distribution parameters k, σ and µ that were estimated using kernel smoothing for both aircraft types. based on the functions, the point estimates of the gev distribution (model) parameters k̂, σ̂, µ̂ were determined; these are presented in table 1. as has already been mentioned, the probability densities were maximal for these values. parameter boeing 737-800 airbus a320 k̂ 0.093 0.393 σ̂ 19.153 32.537 µ̂ 202.409 228.721 table 1. estimated gev distribution parameters for the boeing 737-800 and airbus a320 models. upon estimating the model parameters for both aircraft types, predicted tim values were computed over 100 000 iterations and compared with the real values as described earlier. the resulting distributions of the p-values for both aircraft types are plotted in the histograms in figure 2, where the relative frequencies are shown instead of the absolute frequencies for practical reasons. it should be noted that the width of each bin is 0.05, therefore only the leftmost bin corresponds to the rejection region (where p < 0.05) – this bin is highlighted in red in both histograms. the value of the ratio πp defined in equation (2) was therefore 124 vol. 52/2025 modeling of lto approach time-in-mode values figure 1. pdf estimates of gev distribution parameters k, σ, µ for the airbus a320 and boeing 737-800. figure 2. histograms of ranksum test p-values after 100 000 simulation runs of the models, including the rejection regions (highlighted red). equal to the “height” of the leftmost bin. the values were πp,a320 = 0.005 and πp,b738 = 0.059; in other words, the distributions of the real and predicted tim values were different in roughly 0.5 % and 5.9 % of simulation runs for the airbus a320 and boeing 737-800 respectively (i.e., the null hypothesis of the ranksum test was rejected in those cases). figure 3 shows the distributions of the computed tspe values, illustrated for both aircraft types by means of a histogram accompanied by a boxplot. analogically, figure 4 shows the distributions of the computed rsc values for both aircraft types. the evaluated statistical measures for both metrics as listed in 2.4 are given in table 2. statistical boeing 737-800 airbus a320 measure tspe rsc tspe rsc mean 0.446 0.039 1.686 0.163 median 0.382 0.034 1.344 0.132 iqr 0.463 0.040 1.688 0.165 βrsc – 1.000 – 0.999 table 2. evaluated statistical measures for tspe and rsc for the boeing 737-800 and airbus a320 models. finally, figure 5 shows example results of predicting the tim values using the model parameters θ̂ for both aircraft types. in the figure, the real and predicted tim values are shown together in the form of overlapping histograms. in both presented cases, the p-value of the ranksum test was 1. 4. discussion as was already mentioned in the previous section, the first step in constructing the model used for subsequent prediction of the tim values was to estimate the pdf of each gev distribution parameter. in order to investigate the behavior of the parameters and thus their distributions (i.e., probability densities) in order to make good point estimates of the parameters θ̂ later on, a different sample of the data was selected every iteration. given the results presented 125 bo stloukal, ivan nagy acta polytechnica ctu proceedings figure 3. histograms and boxplots of tspe values after 100 000 simulation runs of the models. in figure 1, the chosen number of iterations (100 000) may be considered sufficient in this regard. the next step was therefore making point estimates of the model parameters θ̂ to use in the subsequent predicting of tim values. clearly, as the pdf in figure 1 show, the estimates of the parameters constructed through the iterations were random variables. this was not considered in the model, where only the point estimates θ̂ were used. therefore, the selection of the constant parameters was such to minimize the effect of this simplification. this was done by selecting the parameter values so that they corresponded to the maximums of the pdf. as will be shown in the following paragraphs, this simplification may be considered acceptable in terms of model performance. once the parameters θ̂ had been estimated for both aircraft types, the tim values could be predicted using the constructed models. in order to evaluate the performance of the model, this was repeated 100 000 times. this number of simulation runs can be considered satisfactory given the clear results that were obtained. to allow for easier evaluation of the results, the number of generated tim value predictions was equal to the number of the real values. every simulation run, the two samples (predictions and real values) were compared using the ranksum test, whereby it was figure 4. histograms and boxplots of rsc values after 100 000 simulation runs of the models. evaluated whether the samples came from the same distribution. clearly, this would be the case with an appropriate model. the rate of null hypothesis rejection was described using the πp ratio; its resulting values for both aircraft types may be considered indicative of a well-performing model in terms of how its parameters were set. moreover, these results also indicate that the choice of the gev distribution for the model was correct. indeed, this can be seen in figure 5, where the histograms of the real and predicted tim values overlap to a relatively large extent. besides the ranksum test, the predictions were also evaluated using the two metrics defined in 2.4, namely the tspe and rsc. the purpose of the tspe was to evaluate the error in total calculated emissions after using the predicted tim values instead of the real ones. although the real values themselves were estimates from ads-b data, they can be considered sufficiently close to the actual values and therefore act as a benchmark for the evaluation of the predictions. as the histograms and boxplots in figure 3 show, the tspe was generally low, with its values never exceeding 10 % for both aircraft types and with the median and mean values being only around 1–2 % according to table 2. this percentage is in relation to the total sum of calculated emissions using the real 126 vol. 52/2025 modeling of lto approach time-in-mode values figure 5. example histograms of real and predicted tim values for the airbus a320 and boeing 737-800. tim values. therefore, using the predicted tim values would, on average, yield emission estimates off by 1–2 % compared to using the real values. these results are in support of the constructed models as they show good performance regarding the predictions of aircraft emissions, since the calculated aircraft emissions are directly linked to the tim values. benefits of using the model are also suggested by results regarding the rsc metric, which compared the predicted tim values with the standard constant value of 240 seconds. more specifically, the differences in the sums of all tim values were compared, which are directly linked to the resulting calculated emissions. as can be seen in figure 4, the rsc values in the absolute majority of cases remained below 1, which is also confirmed by the values of the βrsc ratio (showing the proportion of such cases) given in table 2, as well as by the medians and means for both aircraft types (especially for the boeing 737-800). these results clearly show that for the approach operating mode of the lto cycle and the analyzed aircraft types, the predicted tim values are a better alternative to the standard tim values, which is an expected result given the large rate of simplification associated with the original definition of the lto cycle. while the presented results show promising potential for future research in this area, there are certain limitations which need to be addressed. firstly, without further validation, the presented results are only valid for the time period covered by the real data. in order to reliably assess the performance of the model over a longer time period, it would be desirable to also include another dataset from a later time period (e.g., several months later), on which the model would be tested. the results are also only valid for arrivals on runway 24 at the airport; in the future, other runways should be assessed as well. similarly, only two aircraft types were covered in this paper, therefore no conclusions can be made for other aircraft types. since the model parameters were different for each aircraft type (as shown in table 1), model construction is clearly necessary for every aircraft type individually. in this paper, factors that may influence the tim values of the approach lto operating mode, such as arrival procedures or aircraft go-around rate, were not considered. clearly, their changes may have an effect on the resulting tim values, rendering the current model obsolete. future work should therefore focus on investigating these factors and incorporating them into the model. special attention should be paid to the topic of mixing height, which directly influences not only the tim values, but the scope of emissions to consider in general. while the results show that modeling of tim values is possible for the approach lto operating mode, further research is needed in order to be able to make the same conclusion for the other operating modes. nevertheless, despite the limitations mentioned above, the results presented in this paper show good model performance and may therefore be considered as an initial proof-of-concept regarding the use of modeling in order to estimate tim values of the lto cycle. further in-depth research in this area may therefore provide practical outcomes, leading to improvements in the accuracy of calculations of airport aircraft movements’ emissions without the need for extensive data covering every movement. 5. conclusion in this paper, we constructed models of the tim values of the approach lto operating mode for the boeing 737-800 and airbus a320 aircraft at prague václav havel airport using the gev distribution. the parameters of the model were given as point estimates made from the pdf of the individual gev parameters. these were computed using kernel smoothing after repeatedly fitting the gev distribution to random samples corresponding to 50 % of the original values. the models were then used to repeatedly generate predictions of tim values for both aircraft types. results of these repeated simulation runs showed a good model performance, with the 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https://doi.org/10.1002/qj.49708134804 https://doi.org/10.1016/s0165-1765(03)00035-1 https://www.mathworks.com/help/stats/generalized-extreme-value-distribution.html https://www.mathworks.com/help/stats/generalized-extreme-value-distribution.html acta polytechnica ctu proceedings 52:122–128, 2025 1 introduction 2 materials and methods 2.1 data used 2.2 tim estimation 2.3 estimation of model parameters 2.4 evaluation of estimated model 3 results 4 discussion 5 conclusion acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2024.51.0042 acta polytechnica ctu proceedings 51:42–52, 2024 © 2024 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague drivers, instruments and challenges of knowledge and technology transfer in the automotive sector ulrike michel-schneider czech technical university in prague, faculty of civil engineering, thákurova 2077/7, 160 00 prague 6, czech republic correspondence: ulrike.michel-schneider@fsv.cvut.cz abstract. this study comprehensively analyzes the drivers, instruments, and challenges that facilitate knowledge and technology transfer (ktt) in the automotive sector. the review was conducted with a focus on the regions of the czech republic and the german free state of saxony. it examines the evolving landscape of the automotive industry from a ktt perspective, driven by motivators of the three key stakeholders: government, academia, and the industry. the study explores how ktt expedites the adoption of safer, cleaner, more economical, and sustainable technologies in this sector. key instruments from ktt intermediaries are highlighted, and their challenges and obstacles are discussed. a best practice case study involving the interplay of the three stakeholders is presented. keywords: knowledge and technology transfer, automotive, innovation ecosystem. 1. introduction the automotive industry is undergoing a multifaceted transformation driven by various factors reshaping the future of transportation. the growing need for ecological practices, stemming from initiatives like the european green deal [1], in combination with the increasing integration of high-tech solutions in vehicles, is driving this change. the european green deal and country-specific regulations seek to bring a paradigm shift in vehicle driving. by 2050, europe aims to be the first carbon-neutral region where green sources power vehicles. for this transition, large-scale research and development (r&d) initiatives involving equivalent investments are necessary to find technical solutions. therefore, knowledge and technology transfer (ktt) instruments are crucial in effectively transferring innovation investments into the market. while r&d in the automotive sector is trending from hardware innovation toward software innovation, an increasingly interdisciplinary approach to technology development is evident. high-tech innovations like advanced sensor technology are crucial in enhancing safety and enabling features like collision avoidance, adaptive cruise control, and autonomous emergency braking systems. concepts like artificial intelligence (ai), the internet of things (iot), augmented reality (ar), and 3d printing include a much more diverse set of skills than the traditional car manufacturer. ktt can facilitate the exchange of knowledge and innovations between research institutions, universities, and the industry, enabling the swift adoption of cleaner and more sustainable technologies. the transfer can expedite the development of electric and hydrogen-powered vehicles, sophisticated sensor systems, autonomous driving capabilities, and more. moreover, ktt can help bridge the gap between research breakthroughs and practical applications, leading to safer, more efficient, and eco-friendly vehicles for the future. what are the triggers, instruments, and concepts driving ktt, and what obstacles are encountered in the process? the following study examines these questions with a focus on the region of the czech republic and the german free state of saxony (saxony), neighboring regions both prominent in car manufacturing. this study aims to encourage scientists involved in r&d for the automotive sector to understand better the concept of transfer, government strategies, and the possibilities for transferring innovation to the market. the current state of knowledge shows that ktt plays an essential role in the innovativeness of a country and its efficient use of resources, leading to economic success. ktt’s role is to share the resources spent within academia, resulting in know-how that serves the socio-economic well-being. it includes both science and personnel transfer. whereas science transfer implies the transfer of new results in some form of ip rights, personnel transfer implies the exchange of staff with tacit knowledge [2]. ktt serves mainly to generate innovation for small-medium enterprises (sme) that need to be able to invest resources into r&d, thus gaining a competitive advantage for their enterprise. however, large corporations have also found value in outsourcing research, hence utilizing resources more efficiently and involving ktt. ktt takes place in various forms, on various levels, and in various organizations. according to reinhard and schmalholz, ktt occurs not only between producers and users of knowledge and technology in direct exchange but also through indirect exchange using transfer agents [3]. a transfer agent or intermediary is close to the research institute or the industry partner (see figure 1). considered an incomplete model, the author has adapted it by adding the government 42 https://doi.org/10.14311/app.2024.51.0042 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 51/2024 drivers, instruments and challenges of knowledge and . . . figure 1. ktt transfer model involving direct and indirect transfer. as a ktt intermediary. government initiatives involving the transfer between academia and industry are widely used, whether on a european, national, or regional level. the triple-helix theory outlines a collaborative innovation approach involving interactions among the key stakeholders, government, academia, and industry. this theory postulates that various levels of cooperation can occur between any two or all three stakeholders, often depicted by partially overlapping circles representing the areas of collaboration [4]. the same is evident in the model shown in figure 1. 2. materials and methods this research follows a quantitative approach, leveraging scientific resources for a literature review and online resources for data acquisition. the literature review aims to synthesize the existing knowledge and provide insights into the concepts of ktt within the automotive sector. firstly, the drivers of ktt are laid out based on the three stakeholders: academia, industry, and government. secondly, the research delves into investigating ktt instruments at various levels: national, regional, and institutional. this exploration extends to the czech republic and saxony, facilitating a comparative analysis of the findings. by examining the diverse instruments and approaches employed in these regions, this research identifies variations, similarities, and the effectiveness of ktt strategies. finally, specific challenges are highlighted. the outcomes of this analysis are contextualized within the automotive sector, shedding light on their relevance and implications for the industry. through this examination, the research strives to draw meaningful conclusions and offer valuable insights into the evolving landscape of ktt within the automotive sector. 3. results ktt is pivotal in driving innovation, improving competitiveness, and ensuring sustainable growth in the automotive industry. it involves exchanging knowledge, innovations, and technologies between research institutions, universities, and the automotive sector. the following sections will provide an overview resulting from an extensive review of the concepts of ktt, the critical drivers of ktt in the automotive sector, ktt’s approach in the czech republic and saxony regions, and a best practice example. 3.1. drivers of ktt as in other industry sectors, technology transfer in the automotive industry is driven by various factors and incentives. key drivers for technology transfer in automotive involve the three stakeholder groups mentioned in the introduction. firstly, government drivers for ktt encompass identifying strategic areas of economic specialization, establishing conducive regulatory frameworks for innovation, and providing incentives. • national smart specialization strategy: the european union (eu) requires the implementation of a national ris3 strategy, the regional research and innovation strategies for smart specialization, for all eu member states (for purposes of the funding periods). the national ris3 strategy is a framework designed to guide research and innovation activities to enhance a country’s or region’s competitiveness and sustainable development [5]. during the current funding period (2021–2027), the czech republic’s ris3 focus areas include a) photonics and micro-/nano-electronics, b) advanced materials and nanotechnology, c) biotechnology, d) advanced manufacturing technologies, 43 ulrike michel-schneider acta polytechnica ctu proceedings governmental czech republic saxony (germany) national level national level legal framework act no. 130/2002 coll. (support of research and development from public funds) higher education act (hochschulg) research framework act (forschungsrahmengesetz) innovation act (innovationsgesetz) patent act (patentg) trademark act (markeng) copyright act (urheberrechtsgesetz) government agencies/initiatives technologická agentura české republiky (tačr – funding agency for applied research) grantová agentura české republiky (gačr – funding agency for basic research) czechinvest (incl. czechaccelerator, czechstartup) agentura pro podnikání a inovace (api – business and innovation agency) transfera (partnership platform for ktt) technologické centrum praha (tcp) various regional centres federal agencies • sprind – agency for disruptive technologies • dati – agile innovation agency regional level state agencies • sächsische aufbaubank (sab – development bank) • futursax (innovation platform) • technologiegründerfonds sachsen (seed-investors) innovation council rada pro výzkum, vývoj a inovace innovationsbeirat sachsen innovation strategies czech republic – innovation strategy (2019–2030) innovationsstrategie des freistaates sachsen table 1. governmental ktt instruments. e) artificial intelligence, f) digital security and connectivity [6]. in germany, the federal states focus on individual specializations. in saxony, the focus lies on a) environment, b) raw materials, c) digitalization, d) energy, e) mobility, and f) healthcare [7]. in both cases, parts of the specializations can be directly translated to the automotive sector, indicating the high importance to the region (czech republic: a), b), d), e), f); saxony: a), b), c), d), and e). • regulatory compliance: stringent environmental and safety regulations, such as emissions standards and safety requirements emerging from national or eu strategies, drive the automotive industry to adopt new technologies to comply with legal mandates. the european green deal, also referred to as the green industrial revolution, employs legally binding climate targets that call for the elimination of co2 emissions by 2050 [1]. this eu climate law fosters innovation and hence drives research, development, and innovation (r&d&i) [8] in areas such as automotive, aerospace, energy, construction, and manufacturing [1]. these regulations effectively promote the transfer of innovative sustainable technologies, such as electric vehicles (ev) and fuel cell electric vehicles (fcev), replacing those with combustion engines. from 2035, no new cars fueled with fossil fuels may be registered in the eu (except for combustion engines for e-fuels). it presents an essential catalyst for innovation and transfer. • government funding and incentives: for local and national governments to achieve goals of their innovation strategies and their ris3 goals, programs for research funding, tax incentives, and subventions are employed. these incentives motivate and support technology transfer projects, among others, in the automotive industry while encouraging innovation and economic growth. while funding derives from various ministries, some government agencies further enhance fund formulation, administration, and distribution. see table 1 for governmental ktt instruments. 44 vol. 51/2024 drivers, instruments and challenges of knowledge and . . . drivers for ktt czech republic saxony (germany) government drivers ris3 (2021–2027) a) photonics and micro-/nano-electronics a) environment b) advanced materials and nanotechnology b) raw materials d) advanced manufacturing technologies c) digitalization e) artificial intelligence d) energy f) digital security and connectivity e) mobility regulatory compliance eu member states are required to translate the european green deal into national legislation while enforcing the elimination of emissions among others in vehicles and fostering r&d&i. funding and incentives european, as well as national and regional government incentive programs provide research funding, tax incentives, and subventions. specific (government) agencies are established in support. academic drivers applicable innovation ktt strategies, kttos, centre of competencies, uic commercial income ip generation, support for start-ups and spin-outs, contract research resource sharing joint research centers industry drivers competitive advantage access to state-of-the-art technologies and latest research access to talent access to graduates and experienced researchers, dual study programs (germany) cost reduction reduce company r&d costs and enhance profitability risk mitigation share financial and technical risks through collaboration securing supply chains build supply chains involving local r&d and local sourcing table 2. drivers of ktt in automotive. the academic drivers of ktt can be explained through the university’s third mission, which is the university’s involvement in the socio-economic development of society. according to compagnucci and spigarelli, the third mission “is currently both the most crucial mission and that which most requires innovation in the organization of universities” [9]. it highlights the importance of integrating strategies and systems for commercializing inventions within publicly funded research organizations. entrepreneurial quality presents a new skill set for academic staff. the following vital drivers for ktt are laid out here and summarized in table 2: • applicable innovation through research: with academia in need of engaging in the socio-economic contribution of society, the transfer of knowledge and technology for commercial purposes has become a strategic goal. most research institutions and universities include the aspect of ktt in their organizational strategies and establish dedicated knowledge and technology transfer offices (ktto) to promote and support projects for transfer. collaboration with the automotive sector allows these institutions to bring their innovations to market, benefiting both parties. examples are manifold. at czech technical university (ctu), for instance, the centre of vehicles for sustainable mobility, a competence center, focuses on collaborative research with national and international partners from the applied sphere around sustainable mobility [10]. this is an example of effective university-industry collaboration (uic), a preferred tool for long-term ktt collaboration. as a result, the effectiveness of kttos plays a pivotal role in the success of an institution’s ktt implementation. • sources for commercial income: as higher education institutions (heis) face growing pressure to generate revenue from research, creating intellectual property (ip) such as patents can open avenues for commercialization through licensing agreements or establishing start-ups and spin-offs, hence leveraging institutionally developed ip. the exploitation of ip presents challenges for most universities in terms of costs and marketing, and only small portions of the generated ip tend to be successfully commercialized. another form of income is presented through contract research, where companies or other organizations commission research institutions, such as universities, to carry out r&d on their behalf or in collaboration for a fee. • resource sharing: research institutions and the industry, particularly smes, often lack the resources for large-scale technology commercialization. collaboration and sharing of resources provide access to broader resources, including funding, facilities, and expertise, leading to more efficient transfer. one example is the aim.lab, an artificial intelligence laboratory between škoda auto and the technical 45 ulrike michel-schneider acta polytechnica ctu proceedings university of ostrava (2021). the collaboration will focus on data analysis, machine learning and ai applications, and optimization challenges in industrial practice while combining theory and practice [11]. industry drivers for ktt are manifold and driven by an organization’s competitiveness and economic growth. the following represent key industry motivators. • competitive advantage: staying competitive in the automotive market is a strong motivator for ktt in the industry. companies aim to enhance their competitive advantage by incorporating state-ofthe-art technologies developed through collaborations with heis. this collaboration grants them access to new and innovative technologies, including advancements in electric and autonomous vehicles. every fourth european electric vehicle is made in saxony [12]. research collaborations are essential for the automotive producers and suppliers of this region to stay ahead of this trend. • access to talent: with the increasing implementation of complex technologies in car manufacturing, the industry requires access to a highly skilled workforce. transfer of tacit knowledge from academia to the industry is essential. collaborating with educational institutions provides access to talented graduates and experienced researchers who can contribute to technological advancements. a popular concept in germany is the dual study programs. the westsächsische hochschule zwickau (whz) offers higher education in cooperation with industry partners, where students learn about vehicle technology in theory and practice. the automotive industry has immediate access to a pool of trained graduates. • cost reduction: technology transfer can lead to cost savings, particularly in companies needing r&d activities. ktt allows for a more economical and timely adoption of more efficient manufacturing processes, materials, and systems compared to involving own r&d activities. ktt can, therefore, enhance a company’s profitability and market position. • risk mitigation: technology transfer further allows companies to share the risks associated with r&d by cooperating with experienced scientists from research institutions while utilizing grants in the form of public funding. this situation is particularly true for smes who may otherwise not invest in r&d due to financial and technical risks and the need for more human resources. collaborative projects with external partners can distribute financial, technical, and human risks. • securing supply chains: stable supply chains and good relations with suppliers are vital in automotive manufacturing as the sector depends on logistical systems, including just-in-time delivery. suppliers often play a crucial role in technology transfer by providing automotive manufacturers with specialized components, materials, and systems. collaborative relationships facilitate knowledge sharing and innovation. for instance, the increasing integration of semiconductor technology in vehicles is transforming the automotive industry because it places a high dependency on it. keeping r&d and production within the region helps eliminate the risk of losing access to the technology and the supply (i.e., through financial crisis, energy crisis, political unrest, pandemic). the recently enacted european chips act (september 21st, 2023) [13] plays a vital role in protecting and strengthening the european chip industry and several dependent industries, such as the automotive sector. these drivers interact to shape the landscape of ktt in the automotive sector, promoting collaboration between academia, other research institutions, and the automotive industry. governments find incentives to foster collaboration further. ktt not only accelerates innovation but also addresses key challenges facing the automotive sector (see section 3.3). 3.2. ktt instruments in the regions the study has researched the governmental and institutional ktt instruments, summarized in tables 1 and 3. the instruments are compared between the czech and saxon regions and are categorized. table 1 classifies the instruments into four sections: legal framework, innovation council, innovation strategy, and government agencies and initiatives. the institutions’ instruments include the categories of kttos, special purpose vehicles (spv) or venture capital (vc) funds of the institutions, and joint innovation centers. while both regions prove a legal framework around r&d, the german framework seems more fragmented and widespread than the czech approach. the czech ministry of science is currently making efforts to amend its act to remedy the shortcomings of transfer and innovation support [14]. both regions have implemented an innovation council of experts to advise the government on r&d&i. the czech council meets more frequently than the saxon council (monthly vs. bi-annually). the innovation strategies are clearly defined, yet a change in government in the czech republic may impose modifications (i.e., total investment in r&d). the government agencies and initiatives are multifaceted and range from funding or grant agencies to start-up advisories, from transfer centers to innovation centers. notable is germany’s and saxony’s increasing focus on providing seed funding instruments. this is currently lacking in the czech republic. on an institutional level, research shows that institutions related to technical studies have some transfer activities in the form of a dedicated department (ktto). table 3 highlights some key transfer offices 46 vol. 51/2024 drivers, instruments and challenges of knowledge and . . . institutional czech republic saxony (germany) kttos incl. incubators and accelerators (all university and non-university institutions have some kind of ktto with various functions) cetav (transfer center at the czech academy of science) centre for knowledge and technology transfer at charles university ctt (centre for technology transfer at ctu) and others tud|excite – patents (ip advisor), innovate (transfer support), start-up (student/research advisor), talents (transfer education), futurelab (r&i in tech transfer/entrepreneurship), facilitate (process optimization) tuc transfer office technology transfer office at university of leipzig spvs as investment vehicles and institutional venture capital funding institute of organic chemistry and biochemistry of the cas – iocb tech s.r.o., i&i, i&i biotech fund charles university innovations prague a.s. (cuip) cvut tech s.r.o. (ctu) tudag (tu dresden aktiengesellschaft) tuced (an-institut für transfer and weiterbildung) joint innovation centres south moravian innovation centre (jic) moravian-silesian innovation centre (msic) dresden i exists saxeed smile table 3. institutional ktt instruments. that prove important concepts or are currently developing new strategies. only some institutions established specific spvs to ease the bureaucracy behind the institutional transfer, encourage the institutions’ involvement in spin-offs, and improve their benefit from license agreements. synergies are realized in several joint innovation and transfer centers, mainly initiated by universities, and include the collaboration between various institutions and, in some cases, the regional government. 3.3. challenges of ktt in the automotive sector ktt unfolds across both inter-organizational and intra-organizational dimensions [15]. interorganizational progresses through distinct developmental stages, transitioning from basic research to applied research, collaboration between academia and the industry, and ultimately finding practical applications in the market. this process is evident in the journey of knowledge transfer, where knowledge originating from academic research evolves into tangible market applications. conversely, intra-organizational transfer characterizes knowledge and technology movement within a single organization from one unit, entity, or context to another. an illustrative example is transferring knowledge and expertise in electromobility from the volkswagen concern as the mother company to škoda auto, its daughter company. this internal knowledge dissemination facilitates the seamless integration of insights and advancements across different segments within the organization, fostering collaborative growth and innovation. while there are many motivators and instruments to support transfer on various levels, several challenges exist. common challenges of ktt experienced by various industry sectors include the need for more motivation for researchers to act as entrepreneurs, the lack of skilled ktt professionals, and the unclear legislation around transfer [16]. valid challenges experienced from the side of academia or public research institutions include the ability to detect suitable transfer partners from the applied sphere, for researchers to pinpoint technologies needed by the market, and for the institutes to turn around transfer agreements timely [17]. the following highlights essential issues experienced in the ktt process in automotive and similar industry sectors. the automotive sector is defined by a high set of complex technologies involving an increasing number of new technologies. key systems include advanced driver assistance systems (adas), connected cars, in-car entertainment (ice), augmented reality (ar), self-driving cars, evs, fcevs, and more. today, the average car consumes 1 400 to 1 500 integrated circuits (ic) and the so-called semiconductor chips, and some cars even require up to 3 000 [18]. it shows that the car manufacturing industry depends on many different skills, suppliers, and innovations. incorporating cutting-edge technologies into an existing complex technological system presents a significant challenge. the balance between innovation and compatibility demands a multidimensional approach encompassing technical expertise, strategic planning, stakeholder collaboration, and a commitment to fostering a dynamic and adaptive technological ecosystem. the need for infrastructure for game-changing technologies faces challenges. amidst a wave of disruptive 47 ulrike michel-schneider acta polytechnica ctu proceedings technological shifts, most notably the transition from combustion engine vehicles to evs and fcevs, a significant challenge emerged: the need for adequate infrastructure, in this case, charging or fueling stations. initially encouraged by government incentives (i.e., through financial motivators for manufacturers and consumers) and subsequently enforced through regulations, the transition from conventional combustion engines posed a complex dilemma for global car manufacturers. they needed intensive research while selecting the most viable technology to resonate with a broader market audience. as evidence shows, the transition to evs has so far outperformed fcevs. the number of public electric charging stations (czech republic 1 710 [19]; saxony 2 023 [20] as of november 16th, 2023) is significantly higher than that of fuel stations for hydrogen (czech republic 1; saxony 3 as of september 16th, 2023 [21]). the number of registered vehicles in the ev and fcev categories reflects the same. the initial market entry of evs coincided with less advanced fcev technology, leading to greater government support and infrastructure development for evs. the ktt process for gamechanging technologies faces the challenge of balancing the readiness of alternative technologies and their market potential with the level of government support. while the transition from combustion to electric vehicles appears to be the preferred path, research into fuel cell technology continues, as evidenced by “the czech republic’s hydrogen strategy” [22] and the “wasserstoffstrategie sachsen” [22]. if fcevs are to be adopted in more significant numbers, strong incentives, as seen with evs, will be necessary to establish a robust infrastructure. regarding ktt efforts requiring fundamental changes in systems, the need for acceptance through regulatory frameworks and industry safety standards demonstrates notable challenges. lawmakers face challenges keeping up with the rapid pace of technological change. many laws become outdated as new technologies emerge, rendering even recently enacted laws obsolete. the slow pace of lawmaking further complicates the issue, as legislation often lags behind technological advancements. as a result, lawmakers struggle to address the evolving legal landscape effectively [23]. an example of tackling the challenge of adopting new technologies can be evidenced in autonomous driving. with the ambition to play a lead role in adopting autonomous driving, germany was the first in the eu to accept legislative changes. the draft bill “to amend the road traffic act and the compulsory insurance act – act on autonomous driving” (19/27439) was accepted on may 20th, 2021 [24], giving the green light to autonomous driving in germany. the society of automobile engineers (sae), an automotive standardization body, defines six levels of autonomous driving, laying out a framework for “taxonomy and definitions for terms related to driving automation systems for on-road motor vehicles” [25]. germany has been the first to adopt level 3 within europe. eu regulations on autonomous driving are anchored in regulation (eu) 2019/2144 of the european parliament and the council [26]. 3.4. ktt in automotive – the case of silicon saxony in light of the modern automotive industry’s profound reliance on ic supply, the recent global supply chain disruptions and europe’s surging ic demand underscore the urgent need to establish a robust development and manufacturing ecosystem for semiconductors within the european economy. this endeavor necessitates both geographical proximity and a welltrained workforce. today, silicon saxony is europe’s largest microelectronics cluster. today, every third chip in europe is made in one of the four fabs in saxony [27]. why this ecosystem has evolved in and around dresden was no coincidence. during the gdr (german democratic republic) times, the state-owned company robotron pioneered microelectronics-producing computer technology. after the fall of communism in 1989/90, the microelectronics industry in saxony faced significant challenges. many companies had to reorient themselves or were closed down. to promote the industry, the saxon state government responded with a series of measures by introducing subsidies, tax breaks, and investment in r&d. these measures led to a successful relaunch of the microelectronics industry in saxony. in the 1990s, new companies, including infineon, us-based, and amd (later globalfoundries), settled in the region. today, 3 650 companies involved in microelectronics employ ca. 76 000 people in saxony [28]. the industry contributes around 11.5 billion eur to saxony’s gross domestic product [25]. nevertheless, this is not all. current expansion plans for most existing fabs are underway, while the taiwanese semiconductor tsmc is finalizing plans to start the construction of a 10 billion eur investment on the outskirts of dresden [29]. tsmc’s launch in germany is subsidized by the federal and state governments, making it attractive for the taiwanese investor to take this step. this news has triggered global attention and further interest in the region. to satisfy the increasing demand for skilled labor, an educational center for microelectronics, namely the sächsischen ausbildungszentrum für mikrotechnologie (sam), is planned to be supported by the chip industry and financially by the state [30]. knowledge transfer will take place from the companies and research centers to the trainees and then back into the industry. furthermore, agreements were concluded to train saxon students in taiwan through tsmc to bridge the talent gap in saxony [31]. underscoring the critical need for a stable supply of semiconductors manufactured within europe and continued investment in r&d of chips, saxony has 48 vol. 51/2024 drivers, instruments and challenges of knowledge and . . . joined forces with 26 other regions from 12 eu member states to form the european semiconductor regions alliance (esra) in 2023 [32]. silicon saxony’s success can be attributed to four key factors involving the three pivotal stakeholders: industry, government, and academia. firstly, the region benefits from a well-established research landscape comprising excellent university research, nonuniversity research, and research-focused companies actively engaged in r&d&i of microelectronics and semiconductor technologies. the practice of ktt, from research to industry, propels advancements. secondly, silicon saxony boasts a highly skilled workforce with a profound understanding of microelectronics and semiconductor manufacturing. this expertise has played a vital role in attracting global companies and positioning the region as a center of excellence. the offerings from universities, colleges, and vocational training centers significantly contribute to nurturing this skilled labor pool. thirdly, government support has been instrumental in fostering and accelerating the development of the semiconductor industry in saxony. this support has created a favorable environment for innovation and business development through programs and incentives provided by regional and federal governments. lastly, close collaboration among industry partners has been a critical driver of success in silicon saxony. companies, research institutions, and government agencies have collaborated closely, sharing knowledge, developing technologies, and collectively addressing challenges. the lessons for the automotive industry are evident. as the demand for advanced technologies in this sector continues to surge, silicon saxony remains a driving force in shaping the future of mobility in the region and beyond. contributions to smart and connected vehicles, energy-efficient semiconductor components, battery management systems, and r&d in artificial intelligence and sensor technologies are clear examples of how silicon saxony is propelling the advancement of mobility. the success of this high-tech cluster serves as a testament to the critical role of innovation ecosystems in fostering economic growth and technological advancement. silicon saxony stands as a model for how collaborative efforts among industry, government, and academia can create a thriving environment that attracts global players and contributes significantly to the technological evolution of key industries, such as automotive. 4. discussion many consider ktt to be a process between academia and the industry. looking at the example of the automotive industry, it becomes evident that the government also plays a significant role. in the complex and technology-intensive automotive industry, the collaboration between academia, industry, and government is essential for fostering innovation, achieving sustainable growth, and maintaining a competitive position. ktt plays a pivotal role in bridging the gap between research, regulation, and commercialization, enabling the translation of cutting-edge technologies into viable products and services. regulatory frameworks play a significant part in shaping the ktt landscape in the automotive sector. governments are responsible for establishing clear and supportive policies that encourage innovation, protect intellectual property, and promote responsible technology development. such frameworks include defining the scope of public funding, establishing guidelines for using new technologies, and implementing measures to address global challenges such as climate change. government intervention can significantly accelerate the adoption of new technologies and expedite the transition from one technological paradigm to another. however, it is essential to balance government involvement and market forces. overly prescriptive regulations or excessive government funding can also stifle innovation, while a laissez-faire approach may fail to address market failures and societal needs. in the case of the extensive support for evs, the push for fcev, possibly the more sustainable solution, may have suffered. to maximize the impact of ktt initiatives, governments should focus on supporting large-scale, highimpact projects that align with their national and regional development strategies. these projects should address critical societal challenges and aim to generate significant economic returns. the review of government ktt instruments in the two regions has shown various similarities. the main difference can be seen in germany, where stateinitiated venture capital funds provide access to seed funding. it seems a valuable tool to overcome the need for initial funding for high-potential projects. breaking down administrative and financial barriers through bridging funding for institutional or joint (institution and industry) r&d projects helps encourage researchers to remain involved until market launch. the so-called project hopping, which derives from the ongoing need to secure recurring funding, placing close-to-completion projects in the draw, is a pattern often seen within academia. this leaves an unknown number of promising projects to be unexploited. this situation needs to be tackled through proper measures, such as innovative and sustainable public funding tools. one instrument proven helpful is the foundation of spvs of universities, which break down the administrative barriers for transfer projects and, at the same time, allow the university to maintain a role and, therefore, responsibility within the transfer projects. often, the administrative burden in spin-outs can, in this case, be assigned to the university kttos, allowing the researchers and entrepreneurs to focus on the technology and the preparation of market entry. while governments play a vital role in promoting ktt, transfer effectiveness depends on the capabilities and efficiencies of the institutions and organizations 49 ulrike michel-schneider acta polytechnica ctu proceedings involved. the level of research should be aligned with the level of transfer. for instance, excellence in research requires excellence in transfer. however, not every ktto must be equipped to produce the next unicorn. instead, kttos should focus on their core competencies and effectively transfer knowledge and technology to support innovation within the scope of their institution’s strategy. while some kttos may have the resources and expertise to identify and nurture high-potential start-ups, others may be better suited to facilitating collaborations between academia and industry, providing training and support for smes, or promoting regional innovation ecosystems. the success of a ktto should not be solely measured by its ability to produce high-valued start-ups but rather by its overall contribution to the advancement of knowledge and technology and the transfer of innovative products, services, and processes. this is an impact that is hard to measure and serves as a topic for further research. the identified key ktt challenges for the automotive sector include the technology change in complex technological systems, the lack of infrastructure for game-changing technologies, and the need for changes in legislation. while the compatibility of technology is considered an issue between the academic and industrial stakeholders, challenges related to infrastructure and legislation require the involvement of the government. while time to market is of high importance in the high-tech sector, these two barriers hinder the fast adoption, and the government will need to act on the importance of time. the strategic inter-organizational transfer in automotive companies tends to flow from the home base to production sites abroad. in the case of volkswagen, there are several reasons why it was chosen to transfer technology for evs initially in its home market. firstly, the company has a strong base of engineering expertise in germany and is able to realize the transition of technologies. secondly, the german market is the largest automotive market in europe, with a high potential for the adoption of evs. thirdly, the german government has created mechanisms to support the development of evs and incentivize the purchase of evs so that a critical mass of evs can enter the market. the focus on new technologies will likely flow from the strong automotive countries with government support to those with less of an automotive lobby. therefore, countries with a high automotive dependence will likely implement more robust ktt mechanisms for relevant technologies and know-how, and others will follow. 5. conclusion europe, including germany and the czech republic as automotive nations, has a solid economic and social dependency on the automotive sector. with increasing international competition advancing in the ongoing fundamental changes in mobility, europe aims to remain in a leading competitive position. ktt is essential for staying ahead of the curve, yet the right motivators and instruments need to be in place for a successful transfer process. drivers include the framework for the transfer on the governmental level, containing national smart specialization strategies, innovation strategies, and public funding programs tailored to the needs of the country’s strategies. further drivers include government agencies carrying out transfer strategies and laying the basis of a legal framework that enforces and administers these acts. on an academic level, the focus lies on aligning institutional strategies with those for transfer while building transfer capacities. this includes placing new technologies within the applied sphere, establishing infrastructure such as kttos, combining legal skills, mediation, advising, and more to foster collaboration between industry and science. innovation agencies deriving from academia and seeking to foster entrepreneurship are finding synergies as they combine efforts from various institutions in geographic proximity. the challenges of kkt are manifold. those key challenges specific to the automotive industry include enforcing a technology change in complex technological systems requiring harmonized systems integration. the lack of infrastructure for game-changing technologies, as seen in the initial state of evs, requires the contribution of corporate and government investment to expedite the process and reduce the business risk for companies. with new inventions, such as autonomous driving, legislation will require fundamental changes, challenging the transferrable knowledge or technology entering the market. also, the human factor plays a role in the available expertise to carry out the full spectrum of the transfer. clearly defined focus areas are crucial for r&d&i success. of equal importance is the need for collaboration among different stakeholders, interdisciplinary actors, and international partners. as seen in the case of silicon saxony, a significant contributor to the automotive industry in europe and abroad, collaboration among industry, government, and academia helps embed a long-term ecosystem combining r&d activities, education of skilled labor, attracting foreign investment, and securing a stable supply of parts for the automotive industry. acknowledgements this work was made possible thanks to the tacr grant fw03010688. 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https://standort-sachsen.de/de/standort-sachsen/branchenvielfalt/silicon-saxony-mehr-als-nur-chips https://standort-sachsen.de/de/standort-sachsen/branchenvielfalt/silicon-saxony-mehr-als-nur-chips https://standort-sachsen.de/de/standort-sachsen/branchenvielfalt/silicon-saxony-mehr-als-nur-chips https://standort-sachsen.de/de/aktuelles/news/detail/n1404-10-milliarden-euro-halbleiterhersteller-tsmc-investiert-in-dresden https://standort-sachsen.de/de/aktuelles/news/detail/n1404-10-milliarden-euro-halbleiterhersteller-tsmc-investiert-in-dresden https://standort-sachsen.de/de/aktuelles/news/detail/n1404-10-milliarden-euro-halbleiterhersteller-tsmc-investiert-in-dresden https://standort-sachsen.de/de/aktuelles/news/detail/n1404-10-milliarden-euro-halbleiterhersteller-tsmc-investiert-in-dresden ulrike michel-schneider acta polytechnica ctu proceedings [30] sächsische zeitung. wie will sachsen den fachkräftehunger der chipfabriken stillen? [2023-12-11]. https://www.saechsische.de/wirtschaft/wie-willsachsen-den-fachkraefte-hunger-derchipfabriken-stillen-5894032-plus.html [31] tu dresden. free state of saxony and dresden university of technology open scientific liaison office in taiwan and sign cooperation agreement for a “semiconductor talent incubation program” with tsmc. [2023-12-11]. https://tu-dresden.de/tudresden/newsportal/news/freistaat-sachsen-undtu-dresden-eroeffnen-wissenschaftlicheskoordinierungsbuero-in-taiwan-undunterzeichnen-eine-kooperationsvereinbarungfuer-ein-semiconductor-talent-incubationprogram-mit-tsmc [32] wirtschaftsförderung sachsen. sachsen ist teil einer allianz der europäischen halbleiter-regionen. [2023-1211]. https://standort-sachsen.de/de/aktuelles/ news/detail/n1337-sachsen-ist-teil-einerallianz-der-europaeischen-halbleiter-regionen? 52 https://www.saechsische.de/wirtschaft/wie-will-sachsen-den-fachkraefte-hunger-der-chipfabriken-stillen-5894032-plus.html https://www.saechsische.de/wirtschaft/wie-will-sachsen-den-fachkraefte-hunger-der-chipfabriken-stillen-5894032-plus.html https://www.saechsische.de/wirtschaft/wie-will-sachsen-den-fachkraefte-hunger-der-chipfabriken-stillen-5894032-plus.html https://tu-dresden.de/tu-dresden/newsportal/news/freistaat-sachsen-und-tu-dresden-eroeffnen-wissenschaftliches-koordinierungsbuero-in-taiwan-und-unterzeichnen-eine-kooperationsvereinbarung-fuer-ein-semiconductor-talent-incubation-program-mit-tsmc 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https://tu-dresden.de/tu-dresden/newsportal/news/freistaat-sachsen-und-tu-dresden-eroeffnen-wissenschaftliches-koordinierungsbuero-in-taiwan-und-unterzeichnen-eine-kooperationsvereinbarung-fuer-ein-semiconductor-talent-incubation-program-mit-tsmc https://tu-dresden.de/tu-dresden/newsportal/news/freistaat-sachsen-und-tu-dresden-eroeffnen-wissenschaftliches-koordinierungsbuero-in-taiwan-und-unterzeichnen-eine-kooperationsvereinbarung-fuer-ein-semiconductor-talent-incubation-program-mit-tsmc https://standort-sachsen.de/de/aktuelles/news/detail/n1337-sachsen-ist-teil-einer-allianz-der-europaeischen-halbleiter-regionen? https://standort-sachsen.de/de/aktuelles/news/detail/n1337-sachsen-ist-teil-einer-allianz-der-europaeischen-halbleiter-regionen? https://standort-sachsen.de/de/aktuelles/news/detail/n1337-sachsen-ist-teil-einer-allianz-der-europaeischen-halbleiter-regionen? acta polytechnica ctu proceedings 51:42–52, 2024 1 introduction 2 materials and methods 3 results 3.1 drivers of ktt 3.2 ktt instruments in the regions 3.3 challenges of ktt in the automotive sector 3.4 ktt in automotive – the case of silicon saxony 4 discussion 5 conclusion acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2025.54.0008 acta polytechnica ctu proceedings 54:8–16, 2025 © 2025 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague cable optimization of extradosed bridges kristian d’amico∗, jiří máca czech technical university in prague, faculty of civil engineering, department of mechanics, thákurova 7, 166 29 prague 6, czech republic ∗ corresponding author: kristian.d.amico@fsv.cvut.cz abstract. extradosed bridges offer a competitive alternative to traditional systems used in spans ranging from 100 to 250 m. the trend in the world is the development of new materials and with it the design of thin structures, which is a challenging change for further development. extradosed bridge is a relatively new type of construction (first in odawara, japan 1994), although there are many bridges of this type in the world, there are no strict design rules in terms of statics and even less in terms of dynamic behavior. to research this area closer, parametric analysis can be performed. due to the complexity of this task and the non-linear coupling of the design parameters, it is necessary to approach it by an optimization method that, for predefined boundary conditions, optimizes the design response. keywords: extradosed bridges, finite element method, optimization, non-linear cable analysis, tension stiffening, prestressing forces. 1. introduction the aim of this work is to create a tool for designing extradosed bridges and investigate the influence of structural parameters on the behaviour and efficiency of the structure. for arbitrary boundary conditions, the optimal distribution of forces in the cables can be found. 2. extradosed bridges extradosed bridges with their shallow stay cables and stiff decks offer an economical and aesthetically pleasing alternative to conventional girder and cable-stayed bridges for spans within 100 and 250 meters (figure 1). in an extradosed bridge the deck is partially supported by a system of cables anchored to relatively short pylons. carrying the permanent loads of the bridge with such shallow inclined cables leads to high cable forces and high compression in the girder. at the anchorages, the horizontal components of these cable forces are introduced into the girder, thus contributing to its post-tensioning [1]. the shallow cables allow for very low heights and robust pylons, which are often placed in the centre of the girder (single cable plane), because the already strong box girder decks can also take torsional load resulting from eccentric traffic. the deck girder and pylons are often monolithically connected. this way, the short pylon can be slender and stabilized against buckling more economically than through cables. for the version of the torsionally not rigid bridge deck, it is necessary to ensure stability using two planes of cables. the monolithic connection between the pylon and the deck girder further improves robustness and durability, but a bridge bearing is also used. some extradosed bridges are fully integral, that is, the deck girder and piers are also monolithically connected. neglecting the costs of the bridge pylons, deck girders of extradosed bridge are often cheaper than the deck girders of a girder bridge. this is due to the concrete being the principal ingredient in the superstructure cost, so that extradosed bridges become more efficient with increasing span. while the presence of pylons and stay cables of extradosed bridges poses additional costs compared to post-tensioned cantilever bridges, shorter height of the pylons and adoption of saddles instead of anchorages increase their economy over those used for traditional cable-stayed bridges. indeed, there is a design freedom into varying the relative stiffness of the deck, cables, and pylons to optimize the behaviour of the bridge. this results in a range of structural characteristics in between an internally post-tensioned balanced cantilever bridge and a cable-stayed bridge, with ample room for innovation and reducing cost for different boundary conditions and design restriction. as noted above, unlike cablestayed bridges, extradosed bridges do not require back stays to limit the horizontal movement at the top of the pylon caused by uneven live loads, when the short pylon is monolithically fixed to the deck because the live loads are primarily carried in bending by the deck girder. thus, extradosed bridges are inherently well suited to multi-span structures [1]. 2.1. pylons the pylon’s height measured above the bridge deck level is between 0.07l and 0.13l, where l denotes the lenght of the main span. this is considerably shorter than in the classic cable-stayed bridges, where the pylon has a height between 0.2l and 0.25l, which explains why the pylons are easier to build for extradosed bridges. having this geometrical arrangement, extradosed bridge stays are slightly inclined with respect to the roadway and, therefore, provide 8 https://doi.org/10.14311/app.2025.54.0008 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 54/2025 cable optimization of extradosed bridges figure 1. comparison of three bridges with different dimensions [1]. figure 2. bmd (bending moment deformation) before and after the restressing of cables under dead load [1]. smaller vertical stiffness to the deck when compared to cable-stayed bridge. the stays are often anchored and closely spaced in groups that subdivide the main span length l into portions of 0.2l [1]. 2.2. deck most extradosed bridges that have been built so far have stiffer and stronger girders in bending compared to cable-stayed bridges. desks generally carry live loads primarily in bending. for girders of constant thicknesses, typical are among l/40 to l/45. the elegant sunniberg bridge by christian menn in switzerland, with its slender concrete slab deck, is certainly an exception that sets the current trend and serves as the inspiration for this work. the ratio of the thickness of the deck to the main length of the field for the sunniberg bridge is up to l/250, which cannot be applied in all cases, but it is a challenge for saving material [1, 2]. 2.3. cables thanks to reduced fatigue due to the lower stress range in the stay cables, they can be subjected to higher stress, allowing for the use of simpler and more economical anchorages. for the same reason anchorages of extradosed bridges are typically subject to fatigue test requirements, which are less demanding than those for cable-stayed bridges [1, 3]. all these properties that are disadvantageous to cable-stayed bridges result in live loads that are carried by the deck and, therefore, result in small stress changes in the cables. the cables of extradosed bridges are mainly present to carry permanent loads, reducing so the bending moments in the deck (figure 2). the small stress changes in the cables due to live loads reduce the risk of fatigue failure and, therefore, the utilization of the cables can be higher than 45 % of the guaranteed ultimate tensile strength (guts) which is usually applied for cablestayed bridges. this represents a better use of the cable capacity in service compared with cable-stayed bridges (figure 3). how9 kristian d’amico, jiří máca acta polytechnica ctu proceedings figure 3. effect of cable inclination on the force components in a cable for a) a constant total force and b) a constant vertical force [4]. ever, rather than simply raising the allowable stress to 60 % of guts, for example, it is desirable to provide structural rationale. a thorough investigation of many existing bridges confirms that there is no clear boundary between extradosed and cable-stayed bridges and shows that the maximum allowable stresses in service should be based on the level of stress changes due to live loads [1]]. 2.4. non-linear cable analysis in the case of rope elements, their stiffness is reduced due to their sag. thus, the replacement modulus of elasticity is reduced. the introduced normal force is partly exhausted by changing the geometry of the element until the moment when the rope is diminished. since then, it can be assumed that the element behaves linearly and the full stiffness of the rope acts. this behavior is generally referred to as tension stiffening (figure 4) [5]. stress stiffening, also known as geometric stiffening, incremental stiffening, initial stress stiffening, or differential stiffening according to various authors, refers to the increase (or decrease) in the stiffness of a structure due to its stress condition. this phenomenon is particularly relevant in thin structures where bending stiffness is significantly smaller compared to axial stiffness, such as cables, thin beams, and shells, and it couples the in-plane and transverse displacements. additionally, stress stiffening complements the conventional nonlinear stiffness matrix resulting from large-strain or large-deflection effects. to account for stress stiffening, an extra stiffness matrix, referred to as the geometric stiffness matrix kg is computed and incorporated into the overall stiffness matrix [6]. where l is the horizontal length of the rope anchorage, h is the vertical height of the rope anchorage, s is the rope length, α is the angle of inclination, h are the figure 4. non-linear behaviour analysis of the bridge cable [5]. horizontal reaction forces, r1 and r2 the vertical reaction forces, g0 is the dead load of the rope, g0 is the resultant force, f is the vertical sag of the rope, and wz is the perpendicular sag, which is approximately. wz ≈ f cos α. (1) the deformable conditions of this calculation are the initial length of the rope l0 = s and the length of the parabola, which is length of the sagged rope. assuming a simplification, the sag of the rope will not have the catenary shape, but will have a parabolic course. the length of the parabola is then calculated as lp .= s ( 1 + 8w2 z s2 ) . (2) the rotational equilibrium condition in point 1 is 10 vol. 54/2025 cable optimization of extradosed bridges r2l − g0l 2 − hl = 0. (3) equilibrium condition of all vertical forces: r1 + r2 − g0 = 0. (4) the rotational equilibrium condition in point x r1l 2 + h ( h 2 − f ) − g0s 2 · l 4 = 0. (5) from the system of equations, we obtain the value of the horizontal reaction h at the anchorages, in which f figures as unknown variable h = g0l 8f . (6) variance of geometric extension caused from self weight is calculated as ∆ls = lp − l0, (7) ∆ls = s 8f2 3θ4l2 , (8) where these geometric constants figure θ = 1 cos α . (9) change of extension by the force in the rope from the attachment calculated as ∆ln = n epap s, (10) where n is the force in the rope, ep is the young’s modulus of the material and ap is the cross-sectional area of the rope. relationship between the force n and horizontal reaction h is n = h cos α . (11) the total extension outgoing from geometric and force in the rope must be the equal. by substituting eqs. (8), (11) and (12) into the equation ∆ln = ∆ls. (12) we get sh epap cos α = s 8f2 3θ4l2 . (13) further, substituting eq. (6) into eq. (14) g0l 8fepap cos α = 8f2 3θ4l2 . (14) after modifying the equation, we get an expression for the rope sag without pretension f = 3 √ g0l3θ4 epap · 3 64 . (15) the change in extansion from a prestressing force np is ∆ln = s ( n epap − np epap ) = s ( g0l 8fepap cos α − np epap ) . (16) by substituting eqs. (8) and (11) back into eq. (13) we get a cubic equation for calculating rope sag f depending on the prestressing force np f3 ( 8 3θ4l2 ) + f ( np epap ) − g0l 8epap cos α = 0. (17) by calculating the sag f , all forces related to the rope can be count backwards. in the end the parameter used further in the matrix of stiffness of the cable elements representing the non-linear behaviour of the cable is the reduced effective young’s modulus eeff [7]. eeff = ep 1 + γ2l2 cep 12σ3 (18) where σ is the tensile stress in the cable, γ is the volumetric weight of the cable, lc is the horizontal projection of the length of the cable. all the variables for calculating the length of the rope in eq. (2) are solved. this calculation in combination with eq. (19) are the essentials parameteres for correct usage of non-linerar behaviour in the coputional analysis of the bridge cables [5]. 3. fem model based on the large amount of bridge data and case studies from 2019 provided by iabse [1], approximately 59 % of extradosed bridges have two pylons in longitudinal direction, i.e. three spans, 19 % have one pylon and two spans, and the remaining 22 % have three or more pylons. the research deals only with the most widespread variant, so the option of a 3 fields bridge with 2 pylons is modeled here. 3.1. description of the model the description of the model is simplified by using three basic component types: deck, pylon and cable [8]. the predescribed size of spans and pylons is divided into nodes according to the required density. due to the fact that the segments are usually 5 m, the nodes are in the same grid. generated nodes and finite elements are described in the planar coordinate system. the elements modeling the cables that connect the pylon and bridge deck are made up of one piece element [7]. the geometric nonlinearity of the cables is captured using the effective young’s modulus (19) [8]. for each finite element, a local stiffness is built based on the nodal coordinates utilizing the 11 kristian d’amico, jiří máca acta polytechnica ctu proceedings relevant geometric and material characteristics. the local matrices are then transformed into the global coordinate system using a transformation matrix. the localization of the matrices into the global matrix takes place using code numbers [9, 10]. the numerical model employs euler-bernoulli frame elements with three degrees of freedom at each node, resulting in a stiffness matrix represented as a 6 × 6 matrix for each element. ke = e l  a 0 0 −a 0 0 0 12i l2 − 6i l 0 − 12i l2 − 6i l 0 − 6i l 4i 0 6i l 2i −a 0 0 a 0 0 0 − 12i l2 6i l 0 12i l2 6i l 0 − 6i l 2i 0 6i l 4i , (19) where e is young’s modulus of the material [kpa], i is the moment of inertia for the y-axis in base of the cross-section [m4], a is the element area [m2], l is the lenght of the element [m]. further, the geometric stiffness reads as kg = p 30l  0 0 0 0 0 0 36 3l 0 −36 3l 0 3l 4l2 0 −3l −l2 0 0 0 0 0 0 0 −36 −3l 0 36 −3l 0 3l −l2 0 −3l 4l2 , (20) p is the stressing/tension force in the element [kn]. prestressing forces in the cables are defined as vector of variables p = (p1, p2, p3, . . . , pn), (21) where n is the number of cables. each cable is digitized using also a vector c = (c1, c2, c3, . . . , cn). (22) forces are assigned to cables with the same index. the local stiffness matrix further consists of kl = ke + kg. (23) finally the element matrixes are transformed into the global coordinate system using the transformation matrix t =  cos α sin α 0 0 0 0 − sin α cos α 0 0 0 0 0 0 1 0 0 0 0 0 0 cos α sin α 0 0 0 0 − sin α cos α 0 0 0 0 0 0 1  (24) kg = t ′klt, (25) kg is the global stiffness matrix. 3.2. kinematic boundary conditions of nodes at each end of the deck, there are free rotation and horizontal displacements that represent the movement of the expansion joints and bearings. the element intersection of the pylon and the deck is fundamental for the overall behaviour of the bridge. further, there are two variants: one maintains a rigid (monolitic) connection, while the other support one is supported by bearings. the anchoring places of the cables are also different. they can be expected to rotate around a joint, or the place is fixed in such a way that rotation is prevented. in this case the entire structure is supported on bearings and the cables are on pivots allowing rotation. 3.3. load/force vector the cables are typically designed to carry 60–80 % of the dead loads, which represent the dominant part of loading. the rest of the dead load, with the increment from the moving load from vehicles or trains, is carried by the deck. the deck weight is generated in the nodes as a load calculated according to the geometry and density of the material. for the purposes of this work, the dead load is the only load case. 3.4. limiting conditions and objectives the condition for serviceability limit state (sls) is the limitation of this task. the maximum deck deformation is limited to l/600 for prestressed bridges. cable-stayed bridges are prestressed usually at 45 % of their characteristic strengths due to major live load stress change. in girder, this limit is extended to 60 %. extradosed bridges are somewhere in between. therefore the cable prestress range is usually between 45 % and 60 % of the characteristic strength [1]. 3.5. model simplification mathematical and material models try to describe reality as accurately as possible, yet sometimes achieving a small accuracy requires a large effort that is not conventional. 3.5.1. cable anchorage-change in sag, strength and modulus of elasticity from a construction point of view, it is very difficult and sometimes even impossible to have the pylon anchorages close to each other. anchoring is usually approximately 1–5 m apart, according to the technical requirements for anchoring. the purpose of this optimization program is to generate a number of cables from the top of the pylon for the desired results. the problem requires define the next cables are located far enough from each other, because this creates a lot 12 vol. 54/2025 cable optimization of extradosed bridges figure 5. deflection of the extradosed bridge with the span of the main bridge l = 100 m. scale factor ×100. of extra nodes that must be defined correctly. an effective and advantageous solution can be that all the cables are connected to the pylon at one point (we can call it the saddle of the pylon) and thus create a fan. from a construction point of view, this solution is impractical, but from a numerical analysis point of view, it is ideal for describing cables. in principle, it is a change in the hinge angle. however for a 100–250 m field, the difference is about 1–5 m in the vertical direction, so it has a little effect on the redistribution of the prestressing force in the horizontal and vertical directions. 3.5.2. cross-section variability the variability of the cross-section is typical for these types of construction. the pylon usually has a larger cross-section (the so-called pierhead) for better loadbearing capacity and rigidity, while the cross-section in the field is thinner to reduce the self-weight load. the finite elements are generated along the length of the bridge deck with a constant cross-section. an average ideal cross-section is assumed. this simplification is not expected to have a major impact on the results. 3.5.3. level of the bridge the level of the bridge deck will be considered zero, with no longitudinal slope. this will have a negligible effect on the calculation. 3.6. static analysis geometric and material characteristics similar to those of the sunniberg bridge were used as a model example: rectangular slab bridge with dimensions of 8 × 0.5 m, concrete type c40/50 and 2 rows of cables with a diameter of 0.2 m [2]. the static response depends on the structure’s stiffness and the load’s size. this is expressed using the equilibrium equation ku = f, (26) in which k is the global stiffness matrix, u is the displacement vector and f is the force vector. deflection u of the deck is calculated and depicted is comparison to the original not deformed structure (figure 5). the maximal deflection without prestressing forces in the cables u0,100 appear to be in the middle of the span u0,100 = 0, 112 m. the deformation of the deck is limited by umax for prestressed concrete structures [11], where the limiting value is set as umax = l 600 (27) with l denoting the lenght of the span in meters. for the the main span l = 100 m is the the maximal deflection umax,100 = 0.166 m. the deflection is clearly under the deflection limit. even so, taking into account the live load or with a larger field span, the prestressing forces in the cables must be set. 4. optimalization function function fmincon is a matlab inbuilt nonlinear programming optimizer that finds a minimum of constrained nonlinear multi-variable function [12]. described as finding the minimum of a problem specified by: minimize f(x) subject to c(x) ≤ 0 ceq(x) = 0 a · x ≤ b aeq · x ≤ beq lb ≤ x ≤ ub. (28) the function f(x) is the deck deflection u in this case. deflection is monitored at 3 control points, namely the nodes in the middle of each field. the lower bound lb is a vector of zeroes, this represents the possibility of no force in the cables. the upper bound ub is the vector of maximum forces fmax, which is the maximum allowable force deriving the maximum allowable tension in the cable. the result of the optimization is the force vector for the cables, which derives the minimum function f(x), i.e. the smallest deflection in the 3 monitored points. 5. results the force vector obtained from the optimization was inserted into the structure calculation and the deflection in the middle was reduced after prestressing to uf,100 = 0.048 m (figure 6). 13 kristian d’amico, jiří máca acta polytechnica ctu proceedings figure 6. deflection of the extradosed bridge with the span of the main bridge l = 100 m after prestressing. scale factor ×100. figure 7. deflection of the extradosed bridge with the span of the main bridge l = 200 m. scale factor ×100. figure 8. deflection of the extradosed bridge with the span of the main bridge l = 200 m after prestressing. scale factor ×100. variants of the extradosed bridge with main spans of 120 m, 140 m, 160 m, 180 m and 200 m were also tested for research purposes, each of them having a adequate number of cables (200 m variant has a set of 36 pairs of cables due to its dimensions). for example, the deflection of the 200 m long main span of the bridge is without prestressing u0,200 = 1.311 m (figure 7). for the the main span l = 200 m is the the maximal deflection umax,200 = 0.333 m. the deflection is clearly upper the deflection limit, the prestressing forces in the cables must be set to achieve the deflection limit. after optimizing the forces in the cables, the deflection in the middle of the deck is reduced to uf,200 = 0.025 m (figure 8). the results in terms of magnitude of forces derived from the optimization for different sizes and numbers of cables are shown in the graph (figure 9). 6. conclusions the research shows that as the length of the main span increases, significantly greater force is required to lift the cables closer to the center of the main span, in order to reach the maximum vertical deflection limit. this indicates that not all cables are fully utilized in the current arrangement, and as the length of the main span increases, a different cable distribution may be required (see figure 10). for larger spans, a more uniform distribution of cables along the bridge deck is preferable, which may also require an increased number of cables. this, in turn, necessitates additional space for anchoring the cables to a taller pylon, making the structure more similar to a cable-stayed bridge. such a design calls for a more detailed analysis considering variable live loads [13], further evaluation in terms of limit states, local analyses at several points along the bridge deck, and optimization of the pylon 14 vol. 54/2025 cable optimization of extradosed bridges figure 9. optimized cable forces – 2d graph. figure 10. optimized cable forces – 3d graph. height. the height of the pylon is a critical factor not only for determining the type of bridge but also for its economic feasibility. acknowledgements the authors gratefully acknowledge support from the czech technical university in prague, project sgs23/032/ohk1/1t/11 development and application of numerical algorithms for analysis and modelling in mechanics of structures and materials. references [1] structural engineering documents (sed) 17. extradosed bridges. iabse bulletins, 2019. isbn 978-3-85748-168-0, [2024-02-01]. https://concrete.ethz.ch/assets/sed17.pdf [2] c. menn. sunniberg bridge, 2024. [2024-02-01]. https: //www.bridgeinfo.net/bridge/index.php?id=241 [3] f. faltus. ocelové mosty příhradové, obloukové a visuté [in czech; steel bridges – truss, arch and suspension structures]. praha: academia, 1971. [4] k. k. mermigas. behaviour and design of extradosed bridges. master’s thesis, university of toronto, 2008. [5] l. vráblík. computational analysis of a cable stayed bridge structure, 2021. lecture. [6] z. bittnar, j. šejnoha. numerické metody mechaniky i [in czech; numerical methods in mechanics i]. czech technical university in prague, 1992. xxi+634 p. isbn 80-01-00855-x. [7] j. yi. modeling and analysis of cable vibrations in cable-stayed bridges under near-fault ground motions. engineering structures 277:115443, 2023. https://doi.org/10.1016/j.engstruct.2022.115443 [8] r. šafář. betonové mosty 2.: přednášky [in czech; concrete bridges 2: lectures]. czech technical university in prague, 2017. isbn 978-80-01-05543-4. 15 https://concrete.ethz.ch/assets/sed17.pdf https://www.bridgeinfo.net/bridge/index.php?id=241 https://www.bridgeinfo.net/bridge/index.php?id=241 https://doi.org/10.1016/j.engstruct.2022.115443 kristian d’amico, jiří máca acta polytechnica ctu proceedings [9] k. d’amico. dynamic behavior of cable-stayed footbridge depending on the calculation accuracy. in conference nano and macro mechanics (nmm2023). czech technical university in prague, 2023. [2024-02-01]. https://mech.fsv.cvut.cz/nmm2023/en/ [10] z. bittnar, p. řeřicha. metoda konečných prvků v dynamice konstrukcí [in czech; finite element method in structural dynamics]. sntl praha, 1981. [11] czech office for standards, metrology and testing. navrhování betonových mostních konstrukcí [in czech; design of concrete bridge structures], 2014. čsn 73 6214. [12] m. tyburec. modular-topology optimization of structures and mechanisms. ph.d. thesis, czech technical university in prague, 2021. [2024-02-01]. https://dspace.cvut.cz/handle/10467/98774 [13] czech office for standards, metrology and testing. eurocode 1: actions on structures – part 2: traffic loads on bridges, 2018. čsn en 1991-2 ed. 2 (73 6203). 16 https://mech.fsv.cvut.cz/nmm2023/en/ https://dspace.cvut.cz/handle/10467/98774 acta polytechnica ctu proceedings 54:8–16, 2025 1 introduction 2 extradosed bridges 2.1 pylons 2.2 deck 2.3 cables 2.4 non-linear cable analysis 3 fem model 3.1 description of the model 3.2 kinematic boundary conditions of nodes 3.3 load/force vector 3.4 limiting conditions and objectives 3.5 model simplification 3.5.1 cable anchorage-change in sag, strength and modulus of elasticity 3.5.2 cross-section variability 3.5.3 level of the bridge 3.6 static analysis 4 optimalization function 5 results 6 conclusions acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2024.47.0071 acta polytechnica ctu proceedings 47:71–74, 2024 © 2024 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague monitoring degradation in alkaline-activated slag materials houssam mahmoud∗, libor topolář, iveta plšková, luboš jakubka, luboš pazdera brno university of technology, faculty of civil engineering, veveří 331/95, 602 00 brno, czech republic ∗ corresponding author: 172266@vutbr.cz abstract. this paper deals with determining degradation changes in alkaline-activated slag-based building materials. it describes methods for determining dynamic elastic moduli obtained by acoustic non-destructive methods, including the ultrasonic pulse method and the resonance method. a finegrained mortar was chosen as the initial mix for the implementation of the experiment, the binder part of which was sodium hydroxide-activated blast-furnace granulated slag. beams with dimensions of 40 × 40 × 160 mm were chosen as test bodies, for which acoustic non-destructive methods monitored changes in the material structure during degradation processes. keywords: degradation, alkali-activated slag materials, non-destructive testing, decalcification, compressive strength. 1. introduction the ultrasonic method has been used in the construction industry for many decades, especially for detecting cavities and caverns. it is one of the nondestructive methods of testing building materials, elements, components and structures. the main advantage is that it does not cause any damage to the material, so it cannot compromise the load-bearing capacity or serviceability of the building element. it is repeatable, so changes in the structure over time can be monitored. the essence of this method is the repeated transmission of ultrasonic pulses into the material under test. after the measured path has been traversed, the transducer removes the vibrations. the transit time of the ultrasonic waves, time t, is measured from the sending of the beating by the exciter to the recording of its impact on the sensor [1–3]. alkali-activated systems are a modern alternative binder material against traditional portland cement. their main advantages lie in their environmental friendliness, where, with appropriate adjustment of the mixture composition, a 50–100 % reduction in carbon dioxide emissions can be achieved compared to portland cement (approximately 1 t co2 is produced per 1 t of binder), thanks to the possibility of efficient use of so-called secondary raw materials of aluminosilicate nature, such as the blast furnace slag or high-temperature fly ash used in this work. compared to portland cement, they also have other advantages such as increased resistance to aggressive environments, faster onset of strength development or lower released hydration heat. the disadvantage is the high shrinkage compared to portland cement-based materials [4–6]. 2. experimental setup to measure the speed of ultrasound (ultrasound) signal passage through the test samples, we used the “pulse analyzer 58-e4900”. the measurement was performed according to čsn 73 1371 [7]. the test bodies were placed on rubber pads (see figure 1). a thin layer of plasticine was used as an acoustic binding agent. the measurements were carried out in a plane perpendicular to the compaction direction. figure 1. the demonstration of ultrasonic signal velocity measurement. we recorded the transit time of the uz wave, from the sending of the pulse by the exciter to recording its impact on the transducer. this speed depends on the type and properties of the material. from the average of the three measured times, the ultrasound propagation velocity vl was then calculated. from the ultrasound wave propagation velocity, the value of the dynamic elastic modulus ebu was determined using norm relations [1, 7]: ebu = ρ v2 l k2 10−6, (1) where is 71 https://doi.org/10.14311/app.2024.47.0071 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en h. mahmoud, l. topolář, i. plšková et al. acta polytechnica ctu proceedings figure 2. the change of dynamic modulus of elasticity over time for each aqueous solution. chemical composition wt. % cao sio2 mgo al2o3 so3 tio2 k2o mno na2o fe2o3 41.1 34.7 10.5 9.1 1.4 1.0 0.9 0.6 0.4 0.3 table 1. vps chemical composition by x-ray fluorescence (xrf). ϱ bulk density of the concrete [kg m−3], v2 l pulse velocity of longitudinal uz wave propagation [m s−1], k environmental dimensionality coefficient [-]. destructive testing was conducted by determining the compressive strength according to en 196-1 [8] on the desttest 4310 compact a (beton systém s.r.o.). 3. composition of specimens and degradation solutions the starting material was based on the alkaline activation of blast furnace slag with 50 % sodium hydroxide solution. the activator dosage per binder was 6 % na2o. a 1 % dose of a lignosulfonate-based plasticizer (chrysoplast 461) was used to improve processability. the water coefficient was set to 0.445. standard aggregate with a maximum grain size of 2 mm was used for mortar preparation. the slag with a specific surface area of 400 m2 kg−1 (determined by the blaine method) was supplied by kotouč štramberk s.r.o. the chemical composition of blast furnace slag (vps) is shown in table 1. the amorphous fraction of the slag is 84 %; the rest comprises crystalline phases such as acermanite, calcite or merwinite. the samples were demolded after 24 hours and left to ripen in water storage. after 28 days, the actual testing of the effect of exposure of the test bodies to different degradation media on the monitored parameters was started. the following degradation media were chosen for the degradation study – standing and rotating water, 5 % solutions of selected salts (sodium chloride, sodium sulfate, magnesium sulfate), 6m ammonium nitrate solution and acetic acid solution at ph ≈ 3. the degradation media were changed after 14, 28 and 56 days of leaching of the test bodies. the ph was checked at regular 3-day intervals for the acetic acid solution and adjusted, if necessary, by adding concentrated acetic acid to the desired value. all the chemicals used were at least of a purity class described as pure, i.e., a minimum of 98 % content. 4. measured results and discussion from the values obtained by the actual measurements and using equation 1, the dynamic modulus values were calculated and are shown in the graph in figure 2. from the curves of the dynamic moduli degraded by the selected solutions, it can be seen that acetic acid and ammonium nitrate solutions had the most significant influence on the value of the dynamic modulus. the ammonium nitrate solution showed a decrease in dynamic modulus values throughout the monitoring period (84 days of degradation). in the case of acetic acid, a sharp reduction in the dynamic modulus values occurred after only 28 days of degradation, with a gradual decrease in the following monitoring period. in the case of the other solutions, there was a slight increase in the dynamic modulus values. in the case of a different comparison, we arrive at another interesting finding that might not be entirely apparent from the graph in figure 2. the graph in figure 3 represents the relative values of the dynamic modulus relative to the initial value (100 %) measured before the samples were loaded into each solution. this shows an apparent decrease in the dynamic modulus values in the case of acetic acid (by more than 15 %) and in the case of ammonium nitrate (successively by 5, 10 and 15 % after 84 days of degradation). let’s look at the changes in the dynamic modulus values when the solutions of sodium sulphate, sodium chloride, and magnesium sulphate are treated. we 72 vol. 47/2024 monitoring degradation in alkaline-activated slag materials figure 3. values of relative dynamic modulus of elasticity for each aqueous solution. degradation period (days) degradation solution 0 28 56 84 standing water 18.5 ± 0.5 25.2 ± 0.5 25.1 ± 0.2 25.4 ± 0.3 rotating water 22.4 ± 0.2 26.3 ± 0.5 26.5 ± 0.6 ammonium nitrate 22.3 ± 0.5 22.9 ± 0.5 21.2 ± 0.8 sodium sulphate 24.0 ± 0.5 26.3 ± 0.5 26.4 ± 0.8 sodium chloride 22.9 ± 0.3 26.8 ± 0.2 27.0 ± 0.7 magnesium sulphate 22.2 ± 0.5 26.4 ± 0.6 27.5 ± 0.3 acetic acid 15.8 ± 0.1 19.6 ± 0.6 16.8 ± 0.2 table 2. values of compression strength in mpa. find an increase in the weights after 28 and 56 days of exposure to the samples. when applied for 84 days, there is already a significant decrease and, in the case of sodium sulphate, even below the original initial value. in the case of water (standing and rotating), the resulting dynamic modulus values tend to increase (by 2 to 3 %). table 2 shows the compressive strength values. it can be seen that for all the solutions studied, the compressive values have an increasing trend. only in the case of ammonium nitrate, there is an initial increase (28 days) and then a subsequent stagnation (56 days) to decrease (84 days). only in the case of acetic acid degradation is a decrease in compressive strength values. the deterioration of mechanical properties of aas due to leaching in acetic acid solution at ph 3 is related to the process of c-s-h (c-a-s-h) gel decalcification and the associated microstructural change. the released calcium ions react with the acetate anion to form calcium acetate, which is highly soluble and does not reduce the penetration rate due to the formation of a protective layer of a compound with low solubility (e.g. calcium sulphate). the decalcification rate depends on several parameters, the main ones being the porosity of the test bodies or the cao/sio2 ratio, as outlined by bernal [9]. the decalcification process of the c-s-h gel is also realized in the case of leaching in ammonium nitrate solution. however, its contribution is not as fatal as in the case of ch3cooh. another aspect explaining the advanced degradation rate is the decrease in the surface tension of the leaching solution due to the addition of acetic acid, which is associated with an increase in the penetration rate into the pore structure. the results obtained in this work agree with the publication [9], where a significant decrease in compressive strength due to degradation by ch3cooh (ph 4.5). 5. conslusion these results indicate that fine-grained mortar based on sodium hydroxide (6 % na2o) activated blast furnace slag is most degraded by acetic acid solution (ph ≈ 3). the advanced degradation accompanied by a deterioration of the mechanical properties is closely related to the decalcification of the c-s-h gel and the accompanying microstructural changes. it is also evident that non-destructive testing results are in good agreement with destructive testing.as also observed. acknowledgements the czech grant agency financially supported this paper under the project ga 19-04703s, “the use of nondestructive methods for testing of the condition of degraded alkali-activated concrete” and project no. fastj-22-8007 supported by faculty of civil engineering of brno university of technology. 73 h. mahmoud, l. topolář, i. plšková et al. acta polytechnica ctu proceedings references [1] p. schmid. základy zkušebnictví. vysoké učení technické v brně, stavební fakulta, 2004. [2] l. hobst, j. adámek, p. cikrle, p. schmid. diagnostika stavebních konstrukcí. cerm, fast vut v brně, 2005. [3] o. anton. základy zkušebnictví: návody do cvičení. cerm, 2002. [4] k. l. scrivener, r. j. kirkpatrick. innovation in use and research on cementitious material. cement and concrete research 38(2):128–136, 2008. special issue — the 12th international congress on the chemistry of cement. montreal, canada, july 8-13 2007. https://doi.org/10.1016/j.cemconres.2007.09.025 [5] s. a. bernal, r. mejía de gutiérrez, a. l. pedraza, et al. effect of binder content on the performance of alkali-activated slag concretes. cement and concrete research 41(1):1–8, 2011. https://doi.org/10.1016/j.cemconres.2010.08.017 [6] j. zhang, c. shi, z. zhang, z. ou. durability of alkali-activated materials in aggressive environments: a review on recent studies. construction and building materials 152:598–613, 2017. https: //doi.org/10.1016/j.conbuildmat.2017.07.027 [7] nedestruktivní zkoušení betonu – ultrazvuková impulzová metoda zkoušení betonu [non-destructive testing of concrete – method of ultrasonic pulse testing of concrete]. standard, czech standard institute, prague, 2011. [8] metody zkoušení cementu. část 1: stanovení pevnosti [methods of testing cement – part 1: determination of strength]. standard, czech standard institute, prague, 2016. [9] s. a. bernal, e. d. rodríguez, r. m. de gutiérrez, j. l. provis. performance of alkali-activated slag mortars exposed to acids. journal of sustainable cement-based materials 1(3):138–151, 2012. https://doi.org/10.1080/21650373.2012.747235 74 https://doi.org/10.1016/j.cemconres.2007.09.025 https://doi.org/10.1016/j.cemconres.2010.08.017 https://doi.org/10.1016/j.conbuildmat.2017.07.027 https://doi.org/10.1016/j.conbuildmat.2017.07.027 https://doi.org/10.1080/21650373.2012.747235 acta polytechnica ctu proceedings 47:71–74, 2024 1 introduction 2 experimental setup 3 composition of specimens and degradation solutions 4 measured results and discussion 5 conslusion acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2024.48.0027 acta polytechnica ctu proceedings 48:27–33, 2024 © 2024 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague comparison of elastic properties of periodic and aperiodic porous structures produced by additive methods radosław grabieca,∗, jacek tarasiuka,b, sebasatian wrońskia,b, gianpaolo pillonc a agh university of cracow, al. mickiewicza 30, 30-059 cracow, poland b agh university of cracow, faculty of physics and applied computer science, department of condensed matter physics, ul. reymonta 19, 30-059, cracow, poland c université de lorraine, laboratoire d’étude lem3 umr cnrs 7239 7 rue félix savart, 57 070 metz, france ∗ corresponding author: rgrabiec@student.agh.edu.pl abstract. the aim of this work was to design, print, and examine elastic properties of porous structures with various parameters and beam distributions. these structures, generated using an algorithm, featured three types of lattices: octet, cubic, and voronoi diagram-based, with different porosities (40 %, 60 %, 80 %) and beam sizes (0.45 mm, 0.66 mm). they were printed using direct light processing and examined using x-ray computed microtomography. compression tests were conducted to determine the young’s modulus. the results were compared with computer simulations performed using abaqus. the tools used, such as 3d printing and x-ray computed microtomography, were described, and the results and conclusions of the tests were presented. as a result, comparable outcomes were obtained for structures with higher beam diameter, while significantly different results were observed for structures with lower beam diameter. keywords: porous structures, elastic properties, fem simulations, 3d printing, microtomography. 1. introduction nowadays, structures characterized by porous structure are finding more and more interest and are often the object of research. the branches of industry, or science, that focus their attention mainly on such structures are: construction, chemistry, biology, medicine, and even automotive [1–3]. these structures can be of two types of mesh: periodic and aperiodic. the purpose of this research is to compare the elastic properties those structures obtained in the laboratory and the simulations. the main focus is on models based on voronoi diagrams. using a special algorithm, they were designed and then printed by direct light processing (dlp) method. once this was done, the porosity (80 %, 60 %, 40 %) of the print and the design were compared using a x-ray computed microtomography, and then the printed models were subjected to compression tests. computer simulations were also performed on the same structures using abaqus (dassault systèmes, france) software, and the results were compared. in the end, a value of young’s modulus was sought for which the experimental and simulation values give the same results. the work was carried out in collaboration with gianpaolo pillon, who performed strength tests and calculated young’s module for each of the designed and printed samples of the porous structures. the results are presented in the paper [4]. 2. materials and methods 2.1. porous structure in this work, aperiodic and periodic structures are compared. there are many types of meshes on which to base models of such structures. in this work, due to good knowledge of the parameters, meshes were chosen: octet and simple cubic for periodic structures (see figure 1). aperiodic structures were designed based on the voronoi diagram (see figure 1). with this type of study, it is possible to see the dependence of elastic properties on: porosity, average thickness of beads and also the type of mesh. figure 1. examples of each unit cell: cubic and octet. additionaly example of voronoi diagram. 27 https://doi.org/10.14311/app.2024.48.0027 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en r. grabiec, j. tarasiuk, s. wroński, gianpaolo pillon acta polytechnica ctu proceedings 2.2. structuring algorithm in order to make the structures, improvements were needed to an algorithm previously made [5]. the algorithm was made using the rhinoceros (robert mcneel & associates, usa) software and grasshooper programming language [6]. the purpose of the algorithm is to create a given geometry, in this case a cube. after that, the cube is filled with the selected number and distribution of points. depending on the selected distribution, a porous structure is formed on the basis of them. by operating the number of points and the diameter of the beams, it is possible to achieve the appropriate porosity of the model. finally, it is possible to obtain three types of structures with a different distribution of beams: cubic, octet and based on the voronoi diagram (see figure 2). these types of meshes were chosen because cubic and octet decomposition often appear in the literature and are well known [7–11]. voronoi structures, on the other hand, are less frequently addressed, and it was deemed worthwhile to compare well-studied structures with those that are not as well described. figure 2. examples of structures created by grasshooper algorithm. the porosity values chosen were 80 %, 60 %, 40 %. these values were a compromise between the capabilities of the 3d printer and the prospect of exploring more and less porous structures. the diameter of the beams was chosen so that for each type of bead distribution, the porosities and beam diameter would be similar. this is necessary for further comparison of models. the structures, designed in this way, were ready for printing. 2.3. printing before printing, the structures need to be prepared in a specialized program anycubic photon slicer (see figure 3), where printing parameters such as layer thickness, exposure time, number of initial layers are selected. in addition, the use of special supports is also needed. these are scaffolds with a lower density of material that holds the structure. the main parameters that were chosen were: layer thickness 0.05 mm and exposure time of a single layer 8 seconds. figure 3. anycubic photon slicer interface. each structure was printed five times, to obtain better statistics of the results during compression tests in the laboratory. the following are examples of the prints that were obtained (see figure 4). figure 4. example of printed structures. 2.4. microct verified of printed structures the microtomographic analysis of the printed structures was performed to compare the parameters of the printed and designed structures. the main attention was paid to the porosity and average thickness of the beads. a nanotom s (baker hughes, usa) microtomograph was used for the study, while the vgstudio max (volume graphics, germany) program available in the laboratory was used to reconstruct the resulting scans. the x-ray tube parameters chosen were 70 kv accelerating voltage and 50 ma tube current. with these, it was possible to obtain images with a resolution of 15 µm, which was sufficient for beam diameters of around 0.5 mm. the resulting scans (see figure 5) had to be edited in imagej fiji [12]. simple functions such as cropping, rotating, and the median filter, which averages pixels that are close together, were used. 28 vol. 48/2024 comparison of elastic properties of porous structures figure 5. example cross-section form nanotom s microtomograph before and after editing. the image in the upper left corner, includes a cross-section in which a carbon tube is visible, on which the structure in the microtomograph is set. when editing, it is necessary to cut out the part of the cross-section on which it is located. the sections were then binarized, resulting in black and white images. the white color represents the beams, while the black color is the air. now, using the bonej plug-in [13], it is possible to calculate the ratio of pores to total volume and the average local thickness (see figure 6) of the white parts of the images. this means that it is possible to get the porosity and the average thickness of the beams, as well as to depict the diameter distribution of the beams using histograms (see figure 7). table 1 summarizes the results of the design and printing. it can be observed that the differences in porosity are small, at the level of a few percent. some results were not taken into account during the study. this is because during printing, the resin remained in the center of the structure, subsequently distorting the results of porosity and beams thickness. in the case of the average thickness of the beads, the discrepancy is greater, at times exceeding 10 % and even 20 %. these discrepancies are particularly noticeable for the diameter of 0.45 mm beams. this result is a consequence of the printer’s capabilities. in the histograms (see figure 7), it can be seen that the width of the range of the average thickness of the beams, when printed, is much greater. the thickness value heat map (see figure 7), also shows that the discrepancy between the width of the beam and the node is greater in the case of printing. the distribution of the diameter of the beam itself is due to the presence of nodes, that is, places where the beams connect. these are those that have a different thickness than the beam itself. in printed structures, there are additional effects caused by the printing itself and its precision. figure 6. comparison of the results of the thickness function using a heat map. on the right is the map for the project, while on the left is the map for print. figure 7. histograms showing the distribution of beams thickness of printed and designed structure. at times in the printed beam, a defect appeared, in the form of empty space. the function calculating the average diameter of the beams, treats such a space as two separate beams, which gives quite different values. in addition, during printing, because of the low values of the diameters, the resin is concentrated mainly in the beams, while the thicknesses of the beams themselves are underestimated. this can be seen in the histogram, as many peaks appear to the right of the main peak (see figure 7). this may result in further problems during strength testing and simulation. there were also mechanical problems during printing. in the center of the smaller diameter beams, cracks were noticed when reviewing the microtomography scans (see figure 8). in addition, the edges of some structures were not printed (see figure 8). figure 8. examples of cracks, and side edge defects, caused by the printing process. this problem can also be caused by sample storage. during drying, curing or handling them, due to the low diameter of the beams, cracking may have occurred. 29 r. grabiec, j. tarasiuk, s. wroński, gianpaolo pillon acta polytechnica ctu proceedings dd−beam [mm] structure type design porosity [%] printing porosity [%] t b.th design [mm] t b.th printing [mm] porosity relative difference [%] t b.th relative difference [%] voronoi 80 83.00 0.69 0.69 3.8 0 voronoi 60 62.00 0.71 0.71 3.4 0 voronoi 40 38.00 0.73 0.75 5 2.7 cubic 80 84.00 0.63 0.70 5 11.1 cubic 60 63.00 0.71 0.79 5 11.2 cubic 40 — 0.76 — — — octet 80 80.00 0.66 0.75 0 13.6 octet 60 59.00 0.72 0.79 1.66 9.7 0.66 octet 40 — 0.80 — — — voronoi 60 59.00 0.50 0.50 1.66 0 cubic 60 — 0.49 — — —0.45 octet 60 60.00 0.51 0.65 15 27.5 table 1. table showing a comparison of the results of porosity and thickness measurements of print and design beams. dd−beam describes the set diameter of the beams, while t b.th describes the average thickness of the beams measured using the thickness function in imagej fiji software. 2.5. experimental results parallel to this research, experimental studies were also conducted on the printed structures described in this work. because the structures were made from a light-curing resin, they had to be cured after printing. uv light was used for this. each structure differs not only in the types of mesh, but also in the thickness and density of the beads. this means that the curing can be non-uniform. in order to determine the curing time, samples were cured for different amounts of time: 3 minutes, 6 minutes, 9 minutes, 15 minutes, 30 minutes, and then a compression test was performed on them and young’s modulus was calculated. by adjusting the curve, it was found that the curve becomes extinguished for a certain curing time. it was considered that 15 minutes would be a suitable time for each sample (a more detailed description can be found in the work of gianpaolo pillon). after that compression test was carried out, and then from the stress–strain curve the young’s modulus was calculated by fitting a simple regression to the linear part of the graphs. this research was described in a paper by gianpaolo pillon. the most important results were the values of the calculated young module (see table 2). looking at the standard deviation, it can be deduced that differences exist even at the level of the same structure. this shows the complexity of the repeatability of the whole process, as well as the storage of samples. 2.6. designed structures fea simulations to determine the mechanical properties of designed structures using the finite element method (fem), it is necessary to know the young’s modulus value of the cured resin. unfortunately, this value is not known, and it is uncertain whether the value obtained from tensile testing of a standard specimen dogbone shaped will be appropriate. therefore, we decided to conduct a series of fem simulations, continuously adjusting the assumed young’s modulus for the resin until the young’s modulus obtained from the simulations for the structure is close to the modulus obtained from measurements for the actual, printed structure. because the surface mesh generated by grasshopper consists of non-uniform triangles with different aspect ratio values, the remesh function in meshlab was used. figure 9 present model before and after remeshing. this function builds a renewal of the surface grid, which is much more homogeneous. the function has options for selecting the size of the triangles. however, it was remembered that the smaller the triangles, the more time-consuming the further research. figure 9. on the left model generated by rhinocerossoftware with grasshooper plugin and on the right model generated by meshlab after remesh function. a regular mesh has been achieved. the next step was to convert the surface grid to a volume grid. this is made possible by the mesh 3d function in gmsh software. the function parameters are set automatically by the environment. default options were used, as no errors appeared in the software with these settings. using the mesh 3d function, it is possible to simply convert the mesh. the theoretical models prepared in this way are ready to be imported into the abaqus 30 vol. 48/2024 comparison of elastic properties of porous structures beam diameter [mm] porosity [%] mesh seed young’s modulus [mpa] std. dev. [mpa] 1 33.2 0.3 2 46.2 1.4voronoi 3 31.3 2.1 cubic 45.6 5.5 80 octet 45.5 0.9 1 100.1 2.5 2 122.8 5.5voronoi 3 115 11 cubic 78.9 6.8 60 octet 112 13 1 139 13 2 146.5 5.3voronoi 3 153.9 8.9 0.66 40 cubic 118 13 1 16.8 1.2 2 30.7 2.7voronoi 3 75 10.2 cubic 30.0 4.9 80 octet 25.7 2.6 1 55.4 2.3voronoi 2 67.5 3.4 cubic 73.5 7.460 octet 77 14 1 156 12 0.45 40 voronoi 2 121 11 table 2. table compiling the results of compression tests in the laboratory. the determined young’s modulus is the average of five printed samples for each structure, from which the standard deviations were also determined. program in which the simulations were performed. at the beginning of each simulation, a random value of material’s young module was entered, which was then adjusted in the subsequent stages of the research to fit the behaviour of each structure. two boundary conditions were imposed for this investigation. one of them was to block the lower wall of the cube that contained the structure. the purpose of such a condition was to make the structure unable to move and rotate in either direction. the second boundary condition, on the other hand, is the assignment of deformation to the upper wall. due to such assumptions, the result of the simulation was the reaction forces achieved when obtaining a specific deformation. in the results section, it is possible to observe those reaction forces, deformations, and stresses at specific locations in the structures. we can see the acting stresses on the 3d model in the program, as well as export specific forces, such as those directed in a specific direction. this is illustrated in the form of arrows. the summed reaction force of the lower wall, inserted into the appropriate formula (see equation 1), gave the result of the desired young’s modulus of each structure. this value was then compared with measured value. if the results differed, the young’s modulus of the material was changed and the analysis was repeated until the result matched this one from printed structure. having the results of the structures from the laboratory, attempts were made to find such values of the young’s modulus of the material that the results for the structures themselves would be similar. e = σ ϵ = f · l0 a · ∆l (1) f – reaction force [n], l0 – sample length [mm], ∆l – sample length’s change [mm], a – compressing surface [mm2]. 3. results the above-described procedure was applied to all produced samples with various thicknesses of beams, different porosities, and different types of structure. as a result, it turned out that to ensure the young’s modulus values for each structure close to the values measured for real samples, we must assume different young’s modulus values for the hardened resin from which the samples were made (see table 3 and table 4). this means that the resin curing process occurred in different ways in different samples. let us analyze these differences. 31 r. grabiec, j. tarasiuk, s. wroński, gianpaolo pillon acta polytechnica ctu proceedings structure type porosity [%] lattice young’s modulus from simulation [mpa] lattice young’s modulus from printed models [mpa] material young modulus [mpa] voronoi 40 139 139 620 voronoi 40 146 146 650 voronoi 60 100 100 560 voronoi 60 114 114 650 voronoi 80 46 46 1000 voronoi 80 31 31 650 cubic 40 118 118 390 cubic 60 79 79 550 cubic 80 46 46 470 octet 60 112 112 620 octet 80 46 46 1150 table 3. a table showing what young’s modulus of the material, had to be set so that the young’s modulus of the simulation structure coincides with that of the laboratory tests performed by gianpaolo pillon. results for structures with 0.45 mm bead diameter. structure type porosity [%] lattice young’s modulus from simulation [mpa] lattice young’s modulus from printed models [mpa] material young modulus [mpa] voronoi 40 121 121 700 voronoi 60 67 67 540 voronoi 60 55 55 570 voronoi 80 16 16 430 voronoi 80 30 30 320 cubic 60 74 74 350 cubic 80 30 30 300 octet 60 77 77 550 octet 80 26 26 630 table 4. a table showing what young’s modulus of the material, had to be set so that the young’s modulus of the simulation structure coincides with that of the laboratory tests performed by gianpaolo pillon. results for structures with 0.45 mm bead diameter. the results for structures with an average beam thickness of 0.66 mm (see table 3) oscillate mostly around a certain average value in the range of 550–650 mpa. the problem arises particularly for two models: one based on the voronoi diagram with 80 % porosity and the other on a cubic lattice with the same porosity. these discrepancies may be the result of the characteristics of the curing process. because of the high porosity, uv light easily reaches every element of the structure. it is possible that for this type of structure, precisely because they are much better cured than the rest of the samples, the value of the determined young’s modulus may be significantly overestimated comparing to structures with more dense distribution of beams inside. additionally, for the structure based on the voronoi diagram, this issue may be due to the version in which it was designed, because the distribution of the beams in the structure determines its elastic properties. the results, although within a certain range, are not significantly similar. this may also be due to the diversity of the print itself. the standard deviations of the tests in the laboratory (see table 2), show that for some structures the tests were more divergent than for others. for structures with thinner beams (0.45 mm) (see table 4), the situation is more complicated. although there is not as much deviation for some structures as before, the values do not tend towards any narrow range. their variability may be due to the reasons given in microtomography part, as well as a few additional effects (see figures 6-9). during micro-ct scans for structures with a beam thickness of 0.45 mm, internal cracks occurred (see figure 9). these cracks do not show regularity and are completely random. they are probably a result of the precision of print32 vol. 48/2024 comparison of elastic properties of porous structures ing, which becomes significant with smaller strut sizes. moreover, thin struts are more susceptible to mechanical damage that can occur, for example, during the washing or drying process of the structure. under such sample preparation conditions, it is difficult to maintain homogeneity, which can result in differences, particularly for such thin beams. 4. conclusions our assumption was that if it turned out that similar young’s modulus values were obtained for all structures made from the material from which the structures were printed, it would be possible in the future to model the properties of newly designed structures based on this universal value. unfortunately, it turned out that depending on the type of structure, its porosity, and the size of the beams, the curing process occurred in different ways. additionally, in some cases, the prints suffered from certain mechanical imperfections, which also affected their mechanical properties. all this means that in the future, it will not be possible to rely on a single young’s modulus value for the resin and model mechanical properties of designed structures based on it. summarizing this work, it was possible to obtain porous structures with the set parameters for three different types of meshes. in addition, the dlp 3d printing methodology was efficiently mastered, with minor problems (for example cracks (see figure 9) most likely due to the resolution capabilities of the device. microtomographic analysis showed that the porosity of the print was close to the design (see table 1), while the average thickness of the beads deviated, also mainly due to the printing process itself. results of the young’s modulus of the material obtained using fem for a diameter of 0.66 mpa, which must be set to enable the structure to exhibit similar elastic properties to those in the laboratory oscillate around the 500–600 mpa range with few exceptions (see table 1). these results are acceptable. for structures with narrower beams, the results were much more complicated. problems with these types of structures are mainly due to the printing and curing process. these results are good preparation for further research on porous structures, using other techniques to create a more reproducible measurement environment. references [1] n. a. jaya, l. yun-ming, h. cheng-yong, et al. correlation between pore structure, compressive strength and thermal conductivity of porous metakaolin geopolymer. construction and building materials 247:118641, 2020. https: //doi.org/10.1016/j.conbuildmat.2020.118641 [2] p. kowalczyk, m. sprynskyy, a. p. terzyk, et al. porous structure of natural and modified clinoptilolites. journal of colloid and interface science 297(1):77–85, 2006. https://doi.org/10.1016/j.jcis.2005.10.045 [3] z. liu, m. fard, j. l. davy. prediction of the acoustic effect of an interior trim porous material inside a rigid-walled car air cavity model. applied acoustics 165:107325, 2020. https://doi.org/10.1016/j.apacoust.2020.107325 [4] g. pillon, r. grabiec, j. tarasiuk, et al. tomograpic and mechanical study of 3d printed porous structure under compression load. acta polytechnica ctu proceedings 48:46–51, 2024. https://doi.org/10.14311/app.2024.48.0046 [5] r. grabiec, j. tarasiuk, s. wroński. desing of the algorithm, print and analysis of porous structures with modifiable parameters. acta polytechnica ctu proceedings 42:27–31, 2023. https://doi.org/10.14311/app.2023.42.0027 [6] s. i. rich, z. jiang, k. fukuda, t. someya. wellrounded devices: the fabrication of electronics on curved surfaces – a review. materials horizons 8(7):1926–1958, 2021. https://doi.org/10.1039/d1mh00143d [7] n. korshunova, g. alaimo, s. b. hosseini, et al. imagebased numerical characterization and experimental validation of tensile behavior of octet-truss lattice structures. additive manufacturing 41:101949, 2021. https://doi.org/10.1016/j.addma.2021.101949 [8] k.-m. park, y.-s. roh. design optimization of additive manufactured edgeless simple cubic lattice structures under compression. materials 16(7):2870, 2023. https://doi.org/10.3390/ma16072870 [9] j. feng, b. liu, z. lin, j. fu. isotropic octet-truss lattice structure design and anisotropy control strategies for implant application. materials & design 203:109595, 2021. https://doi.org/10.1016/j.matdes.2021.109595 [10] n. novak, d. kytyr, v. rada, et al. compression behaviour of tpms-filled stainless steel tubes. materials science and engineering: a 852:143680, 2022. https://doi.org/10.1016/j.msea.2022.143680 [11] y. liu, j. zhang, x. gu, et al. mechanical performance of a node reinforced body-centred cubic lattice structure manufactured via selective laser melting. scripta materialia 189:95–100, 2020. https: //doi.org/10.1016/j.scriptamat.2020.08.015 [12] j. schindelin, i. arganda-carreras, e. frise, et al. fiji: an open-source platform for biological-image analysis. nature methods 9(7):676–682, 2012. https://doi.org/10.1038/nmeth.2019 [13] m. doube, m. m. kłosowski, i. arganda-carreras, et al. bonej: free and extensible bone image analysis in imagej. bone 47(6):1076–1079, 2010. https://doi.org/10.1016/j.bone.2010.08.023 33 https://doi.org/10.1016/j.conbuildmat.2020.118641 https://doi.org/10.1016/j.conbuildmat.2020.118641 https://doi.org/10.1016/j.jcis.2005.10.045 https://doi.org/10.1016/j.apacoust.2020.107325 https://doi.org/10.14311/app.2024.48.0046 https://doi.org/10.14311/app.2023.42.0027 https://doi.org/10.1039/d1mh00143d https://doi.org/10.1016/j.addma.2021.101949 https://doi.org/10.3390/ma16072870 https://doi.org/10.1016/j.matdes.2021.109595 https://doi.org/10.1016/j.msea.2022.143680 https://doi.org/10.1016/j.scriptamat.2020.08.015 https://doi.org/10.1016/j.scriptamat.2020.08.015 https://doi.org/10.1038/nmeth.2019 https://doi.org/10.1016/j.bone.2010.08.023 acta polytechnica ctu proceedings 48:27–33, 2024 1 introduction 2 materials and methods 2.1 porous structure 2.2 structuring algorithm 2.3 printing 2.4 microct verified of printed structures 2.5 experimental results 2.6 designed structures fea simulations 3 results 4 conclusions references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2023.45.0001 acta polytechnica ctu proceedings 45:1–13, 2023 © 2023 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague long-term experience of the thermo-active ground source system at the metro station taborstrasse in vienna dietmar adam∗, adrian brunner, roman markiewicz, johannes pistrol tu wien, institute of geotechnics, karlsplatz 13/220, a-1040 vienna, austria ∗ corresponding author: dietmar.adam@tuwien.ac.at abstract. four of the stations newly opened during the underground extension of the u2 line in vienna in 2008 have been covering a large part of their heating and cooling needs for now almost one and a half decades by using geothermal energy, which is generated by means of energy diaphragm walls, energy piles and energy bottom slabs of the station buildings. as this was the first infrastructure building of this type and scale worldwide at the time of construction, one of the diaphragm walls of the taborstraße station was equipped with numerous sensors to obtain data sets for the thermo-mechanical long-term behavior of these wall elements for the very first time. continuous measurements with automatic data recording were initially carried out from april 2008 until may 2011. in october 2020, the measurements were resumed with the existing sensors, this time manually and once a month, in order to carry out a long-term comparison of the measurement data and to create a basis for assessing the effects of the geothermal operation on its structural components. this paper gives an overview of the taborstraße station with a focus on the thermally activated geostructures and their instrumentation. a comparison of the recorded generated energy with the predicted heating and cooling demand is discussed in the context of the energy data. in addition, the temperature and the strain behavior of the observed energy diaphragm wall is discussed based on the measurement data collected over a period of around 14 years of operational experience. keywords: environmental geotechnics, renewable energy, geothermal energy, energy geo-structures. 1. introduction 1.1. reasons for the use of geothermal energy the use of near-surface geothermal energy has become increasingly important starting from the 1980s [1]. this form of sustainable energy generation is an important contribution for reducing greenhouse gases and achieving the goals of the kyoto protocol or the paris agreement. the share of total energy used in the building and infrastructure sector is 42 % in the eu (“households” and “commercial and public services” in 2020 [2]), with about two-thirds of energy used in households for heating and cooling [3]. consequently, the use of geothermal energy for these purposes seems to be highly reasonable, because this form of sustainable energy production is considered to be the most efficient form of building climatization [4]. due to the current low share of renewable energy sources (23 %) for heating and cooling in the eu [5], there is a large potential for co2 savings in this area, especially if renewable primary energy sources are used [6]. in addition, the use of geothermal systems offers the possibility of dual use, i.e., using a single system for both heating and cooling. due to more efficient thermal insulation and rising air temperatures, the heating demand in europe is steadily decreasing, while the cooling demand is increasing [7]. current strongly increasing energy prices as well as a decades long self-created dependence of the european union (and especially austria and germany) on energy supplies from other countries (particularly gas imports from russia) and the associated problems posed by planning uncertainties and loss of sovereignty [8] are all further arguments for the use of locally sourced geothermal energy. apart from energy piles (single piles), energy walls (diaphragm walls or bored pile walls) and energy bottom slabs are now increasingly used. these concrete components are collectively called “energy geostructures” or “thermo-active geostructures” (heat exchangers embedded in earth-coupled concrete structures, and in the majority of cases, foundation elements) and have several advantages over geothermal collectors installed close to the surface or conventional borehole heat exchangers. on the one hand, the corresponding components are needed anyway due to their structural function, which is why the additional costs for equipping them as geothermal heat exchangers are low (synergistic effects). due to these larger dimensions compared to borehole heat exchangers, thermo-active geostructures meanwhile also result in larger activated volumes (efficiency). furthermore, the high thermal conductivity and heat storage capacity of concrete compared to soil or air have a positive effect on energy exchange (thermal properties). finally, the con1 https://doi.org/10.14311/app.2023.45.0001 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en d. adam, a. brunner, r. markiewicz, j. pistrol acta polytechnica ctu proceedings crete cover provides excellent protection of the heat exchanger pipes against damage, which means that failures can only occur in the course of installation, these being extremely rare (robustness) [9]. infrastructure buildings, and thus also metro stations, are particularly suitable for geothermal energy utilization by means of thermo-active geostructures due to the relatively constant energy demand on the one hand (discussed in chapter 2.3) and on the other hand due to the structural components that are necessary anyway to take down to greater depths with seasonally constant temperature conditions [9]. in addition, the slightly higher construction costs and at the same time lower operating costs due to the long service life of infrastructure structures lead to the expectation of a short payback period of the costs. in addition to the economic reasons, however, there are also arguments for the use of geothermal energy in public buildings dealing with social aspects. such flagship projects can create awareness of energy efficiency and the promotion of sustainable energy sources [10] and thus to “set a good example”. in tunnel and thus also in metro construction, energy non-woven geotextiles, energy anchors [1], energy lining segments [11] and energy sheet piles [12] can be used in addition to the systems mentioned [13–15]. most projects with energy geostructures have been realized in austria, great britain, switzerland and germany. the technology is also becoming increasingly important in china, although there is only little documented information on the actual extent and functionality of comparable geothermal systems in infrastructure buildings [6]. the institute of geotechnics at the tu wien has been working on the topic of geothermal energy utilization by means of energy geostructures for more than two decades. this concerns basic research [1, 16, 17], numerical modeling [17, 18], application-oriented research [1, 9, 14, 17, 19, 20] and large-scale experiments [21]. the project discussed in this paper was supervised by members of the institute already in the planning phase in the early 2000s, feasibility studies [22, 23] were carried out, the geothermal system was designed, the measurement concept was developed and the installation of the sensors was accompanied [17]. the institute of geotechnics at tu wien is also responsible for the recording and evaluation of both measurement phases, the one from 2008 to 2011 and the one since 2020. 1.2. scientific relevance and state-of-the-art on the thermo-mechanical behavior of energy walls while an increasing number of scientific papers on the thermo-mechanical behavior of energy piles based on large-scale tests, laboratory experiments and numerical modeling have been published in the last two decades (a comprehensive listing was made, for example, by cunha and bourne-webb [24]), comparable studies on energy diaphragm walls are hardly available. this circumstance is due to the additional complexity involved combined with less widespread use and a very costly large-scale test execution. considerations regarding the expansion and deformation behavior of energy diaphragm walls are comparable to those of energy piles (as foundation elements), but in this project they are subject to the following additional complex boundary conditions: • the bottom slab and (intermediate) ceilings serve as bracing elements and, together with the complete shaft structures with a comparatively complex ground plan geometry, result in a highly statically indeterminate, mutually influencing threedimensional system in which the stiffness distributions are complex (complex structural geometry). • due to the earth and water pressure, the diaphragm walls of the metro station examined here are mechanically stressed mainly horizontally and thus as slabs, bear comparatively small vertical loads as disks due to the lack of a superstructure over the station (complex mechanical loading). • due to the large activated area, the temperature load is applied by a large number of heat exchanger circuits, whereby information on the flow rates and temperature distributions is unknown or is only known at certain points. in addition, the wall elements are usually only surrounded by the ground on both sides in the lower area (below the bottom slab), while most of the wall features a ground and air side with each having different boundary conditions (complex thermal loading). • individual wall elements (unlike individual piles) cannot be considered decoupled even with otherwise known boundary conditions, since the system deforms as a whole despite a possibly locally limited temperature change (complex thermo-mechanical coupling). due to the aforementioned challenges, the studies on energy walls documented in the literature mostly rely on numerical modeling, with possible validation by sparsely available measurement data (see, for example, [25–28]). results indicate that the air side (and associated heat transfer mechanism by convection) present in energy walls, as opposed to energy piles (as a foundation element), has a strong impact on system performance [26]. this can be advantageous, but also disadvantageous. for example, if station heating occurs due to waste heat generated by trains and passengers, as has been documented for example in london [29], this could result in an additional energy gain [28]. however, if there is no corresponding heating demand (e.g., in the case of pure cooling demand), this heat generation has a negative effect on the temperature level of the heat exchanger elements. 2 vol. 45/2023 thermo-active ground source system at taborstraße metro previous studies have also shown that seasonal air temperature fluctuations have a greater effect on the temperatures of the energy walls than the heat exchanger operation itself [26]. on the other hand, energy diaphragm walls, for example in sterpi et al. [30], are attested to have quite relevant additional normal forces and bending moments as a result of thermal operation on the basis of numerical investigations, and which would have to be taken into account in the design. in order to validate the findings of numerical models on energy walls or energy bottom slabs for infrastructures, actual measurement data are indispensable. while bourne-webb et al. in 2016 [26] cite examples of projects with energy diaphragm walls or energy bored pile walls from austria, the uk, germany, and china, the authors refer to the data published by markiewicz [17] and respectively brandl [1] from the lainzer tunnel project (lt24) in vienna of tu wien as the only publicly available source of thermomechanical measurement data for these wall elements. in addition to the lainzer tunnel, the u2taborstraße metro station represents the second infrastructure monitored by tu wien in which longterm data on the thermo-mechanical behavior of energy walls have been documented. in the course of the world’s first large-scale application of thermoactive geostructures in metro stations [1], an energy diaphragm wall element of the taborstraße station was equipped with numerous measuring instruments. the project has already been presented in [1, 6, 9, 14, 17], among others. to date, however, only temperature data have been published by brandl et al. [31] in 2010, but no strain or deformation data (loveridge considers these in [32], but this report has not been published). in the present paper, the investigation of the longterm behavior of a thermo-active energy diaphragm wall is resumed after a decade. a first overview of the continued measurements of temperatures and strains is given, and they are of great importance for the verification of future analytical and numerical theoretical research. 2. taborstraße metro station 2.1. project development of the geothermal plant in 2001 and 2002 respectively, the builder and operator of the vienna public transport network, wiener linien gmbh & co kg, commissioned the institute of geotechnics of tu wien, which at that time had already been involved in the research and development of geothermal systems, to prepare technical feasibility studies for geothermal systems [22, 23]. four of the new metro stations of the u2 line to be extended at that time – u2/1 schottenring, u2/2 taborstraße, u2/3 praterstern and u2/4 messe-prater – were then selected with the aim of covering their future heating and cooling needs largely with geothermal energy. the entire taborstraße station consists of two shaft structures and two tunnel tubes connected by cross passages. the station building was constructed using the top-down method, with the taborstraße shaft energy diaphragm walls constructed in 2003 and the energy bottom slab constructed in 2004. additionally, heat exchanger pipes were also installed in parts of the station tunnel inverts [31]. figure 1 shows photos of the construction site, with part (a) showing the lower part of the instrumented diaphragm wall (element no. 18) covered with heat exchanger pipes and part (b) showing the installation of this reinforcement cage into the slurry trench. part (c) shows the connection lines to the energy diaphragm walls as well as one of the three penetration locations through the future 2 m thick bottom slab. part (d) shows the connection point of heat exchanger circuit no. 2 of diaphragm wall no. 18, the cables of the installed sensors, and the temperature sensors attached to the supply and return lines. the station was opened in may 2008. figure 2a shows the ground plan of the shaft structure. the respective positions of the two cross sections a-a and b-b, shown in figure 2b and figure 2c–d respectively, are indicated in the floor plan (figure 2a). the majority of diaphragm wall elements are equipped with two geothermal heat exchanger circuits extending over the entire element height. those elements that were penetrated by either the tunnels or the staircase are only partially equipped. the connection of a total of over one hundred heat exchanger circuits [33] to the individual diaphragm wall elements is made below the bottom slab (see figure 1c and d), which is also fully thermally activated. the design principle followed was to limit the length of the individual heat exchanger circuits, on the one hand, to limit pipe friction losses and, on the other hand, to install them at approximately the same length to ensure largely uniform flow behavior [20]. the quite critical penetration of the heat exchanger circuits into the interior of the structure [20] due to the high groundwater pressure takes place through the base slab at three points, where the distributors t1 to t3 are ultimately also arranged. subsequently, collector pipes lead to two heat pumps and two cooling machines respectively. the heat exchanger pipes in the thermo-active geostructures, as well as the connecting pipes to the distributors together with the manifolds, form the so-called primary circuit [34]. 2.2. predicted energy demand (design phase) in a feasibility study [22], various systems for geothermal energy utilization were investigated at an early design stage in order to be able to discover the heating and cooling requirements of the station monitoring rooms, transformer and switch rooms, storerooms, etc. in the course of the execution design, more detailed considerations were given to the selection of 3 d. adam, a. brunner, r. markiewicz, j. pistrol acta polytechnica ctu proceedings d.b. a. c. figure 1. (a.) lower part of the reinforcement cage of the instrumented energy diaphragm wall element no. 18. the sensors were installed on the side to leave the middle chamber free for the contractor tube to avoid damage. (b.) lifting of the lower part of the reinforcement cage of the instrumented energy diaphragm wall element no. 18. the connection point of the heat exchanger pipes and sensor cables under the bottom slab is located at the gray and red protective covers. (c.) heat exchanger pipes below the 2 m thick bottom slab and one of the three feed-through locations into the interior of the structure. (d.) inlet and outlet of heat exchanger circuit no. 2 of the energy diaphragm wall no. 18 as well as the sensor cables (below the bottom slab) and temperature sensors attached to the inlet and outlet pipes. thermally usable components, the arrangement of the distribution locations, and the relevant pipe routing. finally, energy diaphragm walls with a developed area of 2 300 m2 and an energy bottom slab with an activated area of 1 700 m2 were realized at the u2 station taborstraße. in addition, an area of 281 m2 was thermally activated by energy piles and parts of the station tunnels were also equipped with heat exchangers in the invert area [31]. a total of 108 heat exchanger circuits with a total length of 26 431 m were installed. the entire system was designed for the following heating and cooling demands [17]: • annual heating energy 175 mwh a−1 • annual cooling energy 437 mwh a−1 these numbers represent the usable energy quantities in each case. the heat exchanger load decreases for heating and increases for cooling by their share of electrical energy, as will be described in chapter 2.3. finally, it should be mentioned that the feasibility study also examined the provision of energy for neighboring buildings, but this was not in the end actually implemented. 2.3. measured energy data (operational phase) between october 2008 and july 2010, the energy data of the system was recorded on the first day of each month [36], and since december 2020, approximately monthly. this data is shown in figure 3 based on the heating and cooling energy used in the secondary circuit, i.e., after the heat pumps and cooling machines respectively. in addition, the electrical energy required to operate the heat pumps, cooling machines, and circulators is also shown. figure 3 shows that the heating energy used exceeds the cooling energy by a factor of about 2. however, the external electrical energy supplied to the heat pumps or cooling machines is about the same. this is due to the fact that the shares of the required external energy influence the energy actually used in the secondary circuit (on the user side), as shown in figure 4. consequently, in the primary circuit, the heat exchanger load is reduced in the heating demand, while the load is increased in the cooling mode. if there is both heating and cooling demand at the same time, the actual heat exchanger load results from the difference between the heating and cooling energy [17]. in this project, the existing energy measurement data can be compared with the predicted energy quantities, as shown in figure 5. in addition to the predicted monthly energy demand from the design phase [17], the actually measured heating and cooling energy as well as the air temperatures of the vienna city center [35] are all shown. since the heating and cooling energy measurements resumed in december 2020 are taken on different days of the month, these 4 vol. 45/2023 thermo-active ground source system at taborstraße metro n-5 n-4 n-3 n-2 n-0 n-1 qs01 qs02 qs03 qs04 qs05 qs06 qs07 qs08 qs09 qs10 qs11 qs12 connection point track 1 direction karlsplatz track 2 direction stadion instrumented diaphragm wall no. 18fill material sandy silt energy extraction by energy diaphragm wall bottom slab vl18, rl18 sandy gravel clay, silt gw silty sand d = 0.80 m absorber circuit no. 1 blocked vl17, rl17 (neighboring element) w10-11 w11-12 w08-10 w06-08 w04-06 w02-04 w01-02 qs01 qs02 qs03 qs04 qs05 qs06 qs07 qs08 qs09 qs10 qs11 qs12 vl, rl w01-02 w02-04 w04-06 w06-08 w08-10 w10-11 w11-12 se co nd p ar t o f r ei nf or ce m en t ov er la p fir st p ar t o f r ei nf or ce m en t ab so rb er c irc ui t n o. 2 u pp er p ar t ab so rb er c irc ui t n o. 2 lo w er p ar t ab so rb er c irc ui t n o. 1 (b lo ck ed in e le m en t n o. 1 8) extensometer-segment from measurement depth 2 to 4 temperature gauge at segment base temperature gauge measurement depth 12 strain and temperature gauge measurement depth 1 qs01 qs12 w02-04 base instrumentation of diaphragm wall no. 18 absorber circuits (entire element) absorber circuit (bottom half only) track 2 direction stadium track 1 direction karlsplatz a a b element no. 17 a. b. c. d. airside earthsideairside earthside b figure 2. (a.) ground view of the metro station taborstraße section t with position of instrumented diaphragm wall no. 18, neighboring element no. 17 and marked cross sections a-a and b-b. (b.) cross section a-a of u2 metro station taborstraße section t with ground profile, position of instrumented diaphragm wall no. 18. (c.) cross section b-b of u2 taborstraße section t, position of instrumented diaphragm wall no. 18 and installed instrumentation. (d.) schematic illustration of the installed heat exchanger circuit’s positions (no. 18). 2010 2012 2014 2016 2018 2020 2022 0 200 400 600 800 1000 1200 1400 1600 e ne rg y [m w h] assumed trends measured data measured data usable heating energy usable cooling energy electrical energy heating electrical energy cooling electrical energy circulation pumps figure 3. used heating and cooling energy (secondary circle) and external electric energy for heat pumps, cooling machines, and circulation pumps. measured data from october 2008 to july 2010 and ongoing since december 2020. measured data are attributed to the individual months by linear interpolation. the data collected from december 2020 and onwards are of a similar order of magnitude to those in the first monitoring period from october 2008 to july 2010. apart from a single peak in heating energy used in december 2009, heating energy reaches monthly peaks between 20 and 27 mwh. accordingly, the peak consumption of heating energy usually occurs in december or january. an annual cycle can be chosen as a basis for a comparison of the predicted and measured energy quantities. in this case, for example, the year 2009 is suitable since a complete series of measurements is available. however, it is noticeable that – as already described – in december 2009 a comparatively high amount of used heating energy was recorded compared to the other years. since the reason for this is unclear (possibly due to the installed staircase heating [34], which was only operated in the first few years), the 5 d. adam, a. brunner, r. markiewicz, j. pistrol acta polytechnica ctu proceedings hp heating heating energy (100%)heat exchanger (75%) electricity (25%) cooling heat exchanger (125%) cooling energy (100%) electricity (25%) primary circuit secondary circuit cmprimary circuit secondary circuit cop = 4 eer = 4 figure 4. schematic energy flow for heating and cooling. the coefficient of performance cop and energy efficiency ratio eer characterize the efficiency of the heat pumps and cooling machines, respectively. 20 08 o ct novdec 20 09 ja n febm arapr m ayju n ju l augsepoct novdec 20 10 ja n febm arapr m ayju n 0 10 20 30 40 50 e ne rg y [m w h] 20 20 d ec 20 21 ja n febm arapr m ayju n ju l augsepoct novdec 20 22 ja n febm arapr m ayju n ju l augsepoct nov -10 0 10 20 30 40 t em pe ra tu re [° c ] predicted/usable heating energy predicted/usable cooling energy mean outer air temperature figure 5. predicted and measured monthly energy totals for heating and cooling, and outer air temperatures of vienna [35]. the newly collected data since december 2020 is of similar magnitude compared to earlier collected data between 2008 and 2010. period december 2008 to november 2009 is chosen as the basis for the following observations. in the observation period described, the cumulative annual heating energy is 106 mwh. in 2021, i.e., about 10 years later, the cumulative annual heating energy is 129 mwh. the potential of the thermoactive geostructural system of 175 mwh has thus not yet been exploited, so that there are reserves. with regard to cooling, the measured data show that in 2009 the annual cooling demand was 55 mwh and in 2021 it was similar at 58 mwh. compared to the design value of 437 mwh, this shows that there are still larger reserves. in the design phase, a continuous cooling demand over the entire year was assumed. in contrast, however, the measured data show a seasonal fluctuation, with the cooling demand reaching its maximum in july and its minimum in february. in the design phase, it was also assumed that the cooling demand would increase slowly over the course of the first few years (warming of the structure) and that the design case would only be reached after a few years. however, a comparison of the measured data from the first years with the most recent measured data shows that the cooling demand has not increased significantly since the beginning of monitoring. in summary, the energy demand can be covered both in the heating case and in the cooling case by means of the installed thermo-active geostructural system for more than 14 years and, in addition, large reserves are still available, especially in the cooling case. 2.4. instrumentation of the geothermal plant the measurement equipment of the taborstraße metro station consists of two systems. first, the entire station was equipped with temperature sensors in the diaphragm walls, in the station tubes and under the bottom slab by means of measuring lances [17]. however, due to the high groundwater pressure in combination with unsuitable measuring cables, groundwater was able to penetrate into the measuring center, so that the automatic data acquisition was damaged in 2011 [37]. second, due to the scientific relevance of this pioneer project, a selected diaphragm wall element (no. 18) was equipped with numerous additional sensors for thermo-mechanical monitoring of the diaphragm wall 6 vol. 45/2023 thermo-active ground source system at taborstraße metro figure 6. temperature variation with time on the air side (a.), center (b.) and earth side (c.) of the observed diaphragm element. malfunctioning or not installed sensors are marked by crossed out legend entries. in the center of the element two temperature sensors are placed in qs10 due to the arrangement of the extensometer segments shown in figure 2d. (d.) air temperature of vienna, measured at hohe warte [36]. (e.) inlet and outlet temperatures of circuit no. 2 of element no. 18 and circuit no. 1 of element no. 17. (f.) differences between outlet and inlet temperatures; a positive value indicates heating operation and a negative vale cooling operation. according to the temperature load. from the associated automatic data acquisition, the measurement data are available from april 2008 until the aforementioned water damage at the end of may 2011 and thus theoretically cover three annual cycles, with a data gap of eight months, as can be seen in figure 6. the data generated from october 2020 in the course of the resumed measurements, which are documented selectively at approximately monthly intervals, now then allow an investigation of the long-term temperature and strain curves of the thermally stressed measuring diaphragm wall. the position of the diaphragm wall element is shown in the design drawing in figure 2. the additional instrumentation of diaphragm wall element no. 18 includes the following sensors [17]: • 21 combined temperature and strain transducers (geokon 4911 rebar strainmeters [38]) in the axial direction, each on the air and earth sides. • one chain extensometer (geokon 4430 deformation meter [39]) consisting of seven segments extending to the base of the diaphragm wall. • two temperature sensors (geokon 3800 thermistor probe [40]) attached to the air and earth sides of the wall base. • four temperature sensors (geokon 3800 thermistor sensors [40]) measuring the inlet and outlet temperatures of the heat exchanger fluid from two circuits. figure 2c shows the positioning of the sensors in diaphragm wall element no. 18. most wall elements at this station consist of two heat exchanger circuits, as shown in figure 2d. when the integrity of all heat exchanger circuits was checked during the construction phase, it was found that only three circuits in the entire structure were damaged during construction [31]. however, one of them involved heat exchanger circuit no. 1 of diaphragm wall no. 18 (see figure 2d). the inlet and outlet temperature gauges intended for this circuit were instead installed on the adjacent element no. 17 (vl17 and rl17). 3. thermo-mechanical behavior of the diaphragm wall 3.1. temperature distribution in the diaphragm wall figure 6 shows the temperature curve over time for all available sensors on the air side (inside), in the middle and on the earth side (outside) of the diaphragm wall no. 18. as expected, the short-term variation due to 7 d. adam, a. brunner, r. markiewicz, j. pistrol acta polytechnica ctu proceedings the air temperature influence is larger on the air side than on the earth side and decreases with increasing depth. examination of the earth-side temperatures shows that the largest fluctuations for both the annual and short-term trends occur at the sensors closest to the ground surface. knowledge of the layout of the heat exchanger pipes within the diaphragm wall is essential for further interpretation of the temperature measurement data, especially since heat exchanger circuit no. 1 has failed and only heat exchanger circuit no. 2 is intact. in addition, it should be noted that heat exchanger circuit no. 2 runs below the bottom slab on the air side of the diaphragm wall between qs10 and the base of the diaphragm wall and above the bottom slab on the earth side between the top of the diaphragm wall and qs05 (see figure 2d for details). based on figure 6, it can be seen that the two lowest sensors qs11 and qs12 on the air side well below the bottom slab, where heat exchanger circuit no. 2 runs, show an almost constant temperature curve until the end of june 2009. as it turned out later, this was due to the fact that during this period there was no flow through heat exchanger circuit no. 2 and thus no heat exchanger operation took place in the entire diaphragm wall no. 18. finally, at the end of june 2009, maintenance work was carried out to check and de-aerate all heat exchanger circuits, and air inclusions were found in a total of 63 % of all circuits [33]. this was also reflected in the temperature measurement data: the temperature level of about 13 ◦c at the two temperature sensors qs11 and qs12 mentioned above before this maintenance date, thus corresponds in terms of magnitude to the unaffected ground temperature below the bottom slab at depths of 25 m and below, since no energy operation took place in this area. after de-aerating all heat exchanger circuits in june 2009, an energy exchange also took place by means of heat exchanger circuit no. 2, which can be clearly seen from the measured temperature curve on the air side in cross sections qs11 and qs12. the temperature measurement data on the earth side or in the center of the element do not provide any information on the undisturbed ground temperature, since the temperature sensors located there failed in qs11 and qs12. for the further interpretation of the temperature measurement data, it proved to be favorable that the mentioned heat exchanger circuit no. 2 was not subject to flow through until june 2009, because of this the entire diaphragm wall element did not participate in the heat exchanger operation, and consequently all measured temperature curves (except those below the ground plate in qs11 and qs12 on the air side) can be attributed to the influence of the air temperature inside the station building. it is noteworthy that the air temperature influences not only the diaphragm wall temperature on the air side, but also those in the middle up to the earth side, and not only in the area of the basement floors, but up to qs10, which is situated in the area of the bottom slab. after de-aerating heat exchanger circuit no. 2, the diaphragm wall element no. 18 also participated in the heat exchanger operation, whereby it must be noted once again that only partial areas of the diaphragm wall element no. 18 were thermally activated with heat exchanger circuit no. 2. in general, a clear seasonal temperature curve can be seen for all measuring sensors. since, as will be shown in figure 6d–f, the flow and return temperatures of heat exchanger circuit no. 2 indicate a similar seasonal variation as with the air temperature, the measured diaphragm wall temperatures can no longer be clearly assigned to the influence of the air temperature or the influence of the heat exchanger operation. in conclusion, it can be assumed that these two influences overlap and that the measured temperatures represent the result of both influences. the most recent (monthly) readings from october 2020 show a similar annual cyclical pattern as in the first measurement period from april 2008 to may 2011. however, a general temperature increase can be observed within the 10-year observation period. the largest temperature change has occurred at qs01 on the earth side; while in winter 2010 and 2011 the temperature was around 8 ◦c, this was around 13 ◦c in winter 2021 and 2022 respectively. however, it is important to note for this data and all data discussed below, that due to the monthly measurement dates in the last observation period, it is not possible to say whether or not the recorded point-by-point data are temporary peaks. figure 6d shows the outdoor air temperature of the hohe warte measuring station in vienna [35]. this should serve as an indication of the prevailing climatic conditions for further assessment, since it determines, among other things, the heating and cooling demands and subsequently the required heat exchanger operation. in addition, the outside air temperature also influences the air temperature inside the station, since a direct exchange of air takes place as a result of the open metro station entrances. due to the location of the diaphragm wall in the area of such a station access, there is thus also a direct influence on the diaphragm wall temperature. depending on the respective heating and cooling demand, a certain flow temperature to the heat exchangers results as a consequence of the system control and the operation of the heat pump or cooling machine. temperature sensors were installed on the heat exchanger pipes immediately before they were integrated into the diaphragm wall to monitor the flow temperature (see figure 1d) with the following background: the heat exchanger circuit consists, on the one hand, of connecting pipes running below the bottom slab between the manifold and the diaphragm wall and, on the other hand, of the heat exchanger pipes inside the diaphragm wall, so that an energy 8 vol. 45/2023 thermo-active ground source system at taborstraße metro exchange with the environment takes place below the bottom slab and inside the diaphragm wall. sensors for recording the flow and return temperatures were installed on the heat exchanger lines directly in front of the diaphragm wall in order to differentiate these effects. in the case of the diaphragm wall no. 18, where only heat exchanger circuit no. 2 is intact, the flow and return temperatures of this circuit are measured. the supply and return temperatures of this circuit are observed at the adjacent diaphragm wall element (no. 17) in order to obtain information about the energy exchange of heat exchanger circuit no. 1 as well. these measured temperature data are shown in figure 6d. as mentioned earlier, the temperature curves again show a pronounced seasonal variation. the respective temperature differences are given in figure 6f in order to assess the energy exchange of the individual heat exchanger circuits. a negative value represents a transfer of energy from the heat exchanger fluid to the ground or environment, i.e., a cooling of the station. conversely, a positive temperature difference means that an energy extraction and thus a heating operation has taken place. the finding that in the diaphragm wall element no. 18 the heat exchanger circuit no. 2 was not sufficiently vented at the beginning is also reflected on the basis of figure 6f, since an opposite trend can be seen until the time of de-aerating in june 2009. after de-aeration has occurred, the two heat exchanger circuits (circuit no. 2 in diaphragm wall element no. 18 and circuit no. 1 in the adjacent diaphragm wall element no. 17, respectively) show a similar seasonal trend in temperature differences, with the absolute temperature difference, and thus the energy exchange, generally being very small (less than 1 ◦c). for a higher energy exchange, the flow temperature would have to be lower in the heating case and higher in the cooling case, or would have to be specified by the heat pump/cooling machine – however, this is obviously not necessary in the case at hand, since the heating and cooling demand was or is covered even with this low temperature difference. in conclusion, this also shows that the overall system still has large system reserves. furthermore, it is noticeable from figure 6d that in heating mode an energy exchange only takes place in heat exchanger circuit no. 1 in diaphragm wall element no. 17. in the case of heat exchanger circuit no. 2 in diaphragm wall element no. 18, the flow temperature is apparently higher than the ambient temperature, so that no energy extraction takes place. it should be emphasized that this finding applies only to the monitored diaphragm wall and not necessarily to all other energy diaphragm walls in the station, since the diaphragm wall element no. 18 is significantly influenced by the air temperature due to its location (area of the staircase and station access), and a different interaction with the environment takes place in the case of the other energy diaphragm walls. figure 6d–f again shows the long-term trend within the monitoring period of about 10 years. as already observed in the temperature trends based on figure 6, figure 6e also shows a certain temperature increase between the initial years from 2008 to 2011 and the most recent monitoring from 2020 onwards. the temperature differences in figure 6f have remained at a similar order of magnitude, although the temperature difference in cooling mode appears somewhat higher. 3.2. longitudinal strains of the diaphragm wall as described in chapter 2.4, the longitudinal strains are measured with strain transducers of the type geokon 4911 rebar strainmeters [38] (so-called “sister bars”). these essentially consist of a measuring cell (steel cylinder) with an integrated rebar, which served to attach to the defined cross-sections on the air and earth sides of the rebar cage of the measuring diaphragm wall. when installed or embedded in concrete, the measuring cell thus measures the strains of the rebar and thus of the surrounding reinforced concrete. the strain measurements are carried out according to the “vibrating string” principle. in this process, the steel string present in the measuring cell is set in vibration and the vibration frequency is measured, which depends on the strain of the steel string. the result of a measurement is output as a so-called “reading” (r). if two measurements (r0, r1) are available at two points in time (t0, t1), the strain that occurred at the measurement point between the measurement points can be calculated by a calibration factor c specified by the manufacturer: εreading = (r1 − r0) c. (1) equation (1) must be corrected by a temperature factor in case the temperatures of the measurement cross-section differ at the measurement times (t0, t1). the reason for this is that the measuring cell is connected to the surrounding reinforced concrete via the rebar and the strain of the measuring cell is always the same as that of the reinforced concrete. however, the steel string present in the measuring cell does not have a bond with the surrounding structural member and expands more, for example, when the temperature increases due to its larger coefficient of thermal expansion (αs ≈ 12.2 µϵ ◦c−1) compared to the reinforced concrete (αs ≈ 12.2 µϵ ◦c−1). as a result, the vibrating string relaxes, which means that the measured value no longer corresponds to the strain of the surrounding reinforced concrete. to account for this in the evaluation, temperature compensation is usually performed as follows. εσ = (r1 − r0) c + ∆t (αs − αc). (2) the strain fraction given in equation (2) also corresponds to the “load-related strains” εload,related given in the manufacturer’s manual [38]. this is that strain 9 d. adam, a. brunner, r. markiewicz, j. pistrol acta polytechnica ctu proceedings qs airside 2008 2009 2010 2011 -150 -100 -50 0 50 100 150 maintenance water damage 2021 2022 2023 qs extensometer 2008 2009 2010 2011 -150 -100 -50 0 50 100 150 maintenance water damage 2021 2022 2023 w01-02 w02-04 w04-06 w06-08 w08-10 w10-11 w11-12 qs earthside 2008 2009 2010 2011 -150 -100 -50 0 50 100 150 maintenance water damage 2021 2022 2023 qs01 qs02 qs03 qs04 qs05 qs06 qs07 qs08 qs09 qs10 qs11 qs12 qs earthside 2008 2009 2010 2011 -150 -100 -50 0 50 100 150 maintenance water damage 2021 2022 2023 (b) qs01 qs02 qs03 qs04 qs05 qs06 qs07 qs08 qs09 qs10 qs11 qs12 qs airside 2008 2009 2010 2011 -150 -100 -50 0 50 100 150 maintenance water damage 2021 2022 2023 (a) qs01 qs02 qs03 qs04 qs05 qs06 qs07 qs08 qs09 qs10 qs11 qs12 a. b. figure 7. measured strains εobs (related to datum 2009-07-01) on the air side (a) and earth side (b), in axial direction in various depths (qs01 to qs12). malfunctioning or not installed sensors are marked by crossed out legend entries. fraction which is caused as a result of stress changes in the component. these stress changes can be caused by mechanical load changes as well as by restraint stresses as a result of elongation or compression due to a change in temperature. for that temperature-induced strain portion that is not prevented, no additional stresses are generated in the component. this “free” portion of the temperature-induced strains of the reinforced concrete is given by: εat = ∆t αc. (3) both strain components together are ultimately referred to as the “observed” strain εobs (“observed strain”), since this strain could also be measured with a strain gauge on the component. consequently, the “observed” strain εobs results from the two strain components ε0 and εat to: εobs = ε0 + εat. (4) if the equations of the corresponding strain components, equation (2) and equation (3), are substituted into equation (4), it simplifies to: εobs = (r1 − r0) c + ∆t (αs − αc) + ∆t · αc, εobs = (r1 − r0) c + ∆t αs. (5) the observed strains εobs thus also correspond to the strain εactual (“actual strain”) stated in the manufacturer’s manual [38]. the observed strains εobs are discussed below and shown in figure 7. for the interpretation of the measured strains, the choice of the described reference time (t0 with r0, t0) is essential. in this paper, the focus is placed on the representation of the influence of the heat exchanger operation on the diaphragm wall. for the following illustrations, july 1, 2009 is therefore chosen as the reference time, since at this time the metro station building had already been completed and in operation for over a year and it can therefore be assumed that construction-related load and temperature influences had already subsided. furthermore, no significant static load changes (due to variable traffic loads, earth pressure redistributions, etc.) are to be expected during the operating phase. the existing measurement data between april 16, 2008 and july 1, 2009 are not taken into account, because 10 vol. 45/2023 thermo-active ground source system at taborstraße metro until the venting of all heat exchanger circuits at the end of june 2009, the intact heat exchanger circuit no. 2 in the diaphragm wall no. 18 was not in flow through, as already described in chapter 3.1. under the aforementioned conditions, all expansions after july 1, 2009 are theoretically due to temperature changes; in this case, they result from the heat exchanger operation on the one hand and from the influence of the air temperature on the other. a differentiation in this respect is hardly possible, in particular due to the similar temperature course of the heat exchangers and the air (see figure 6). in the strain curves shown in figure 7, it can be seen in the period from july 2009 to may 2011 that the strains on the air side show a somewhat larger scatter than on the earth side, which is probably due to the influence of the air temperature. globally, a pronounced seasonal strain pattern is again evident at all monitoring sites, with values varying between approximately εobs = 50 µε (longitudinal strain) and εobs = 120 µε (longitudinal compression). the absolute strain differences in the individual measurement points, over the year, are ∆εobs = 170 µε and thus 0.17 mm m−1. about 10 years later, in the monitoring period from october 2020 to april 2022, the measured strains basically show a similar characteristic, i.e., still a pronounced seasonal variation. a detailed analysis shows that on the air side in almost all cross sections the strains are in the same fluctuation range as about 10 years ago. only in the case of qs1 have somewhat larger compressions occurred. on the earth side, the strains in almost all cross-sections are in the same range of variation as 10 years ago, with somewhat larger compressions occurring in qs05 and somewhat larger strains in qs10. as already mentioned, it is hardly possible – due to the similar temperature conditions – to assign the strains to the heat exchanger operation or the influence of the air temperature. in addition, it must be taken into account that long-term effects (e.g., creep and shrinkage of the concrete) also have an impact on the measured strains. in conclusion, however, it is essential that the measurements show that the heat exchanger operation over a longer period of time did not lead to any significant strain changes in the energy diaphragm wall. 4. summary and outlook 4.1. summary although thermally utilized diaphragm walls and bored pile walls have been increasingly used in recent years, very few scientific studies exist on their thermo-mechanical behavior as compared to energy piles. this is due to the complex boundary conditions and the fact that large-scale field tests are very costly. however, the use of these thermo-active geostructures for heating and cooling buildings has enormous potential as a locally obtained and used renewable primary energy source. the wiener linien gmbh & co kg have done international pioneer work with their decision in the early 2000s to supply several metro stations with geothermal energy and to use the structural concrete components, which were necessary anyway for the station structure, as geothermal heat exchangers to cover the heating and cooling demand. due to the scientific support of the institute of geotechnics of tu wien and the first instrumentation of a thermally activated diaphragm wall element, it has been possible to generate hitherto unique long-term measurement data of the temperature and strain curve of an energy diaphragm wall. this paper highlights the scientific relevance of the existing and ongoing measurement data of the instrumented energy diaphragm wall in the u2 station taborstraße. it gives an overview of the project development of geothermal usage at the station and it contains a first presentation and interpretation of the existing measurement data. energy data collected since december 2020 in the form of heating and cooling energy used for the entire station show a relatively unchanged energy demand over the entire year compared to the energy demand of the years 2008 to 2010 after the opening of the station. while the prognosis of average annual heating energy carried out in the design phase is almost fulfilled, the actual annual cooling energy required is far lower. thus, there are still large system reserves, especially for the cooling operation. the temperature data of the instrumented energy diaphragm wall show a certain increase of the diaphragm wall temperature especially on the earth side and at a shallow depth since the station opening in 2008. the temperature difference of the heat exchanger fluid between the flow and return is small. this indicates low energy exchange in the observed energy diaphragm wall, which in turn indicates that sufficiently large reserves exist for higher energy extraction or input. with respect to the selected reference time (july 2009), the strain data of the instrumented energy diaphragm wall show a clear seasonal pattern, which can be attributed to the influence of the air temperature inside the station and the heat exchanger operation. the absolute strain difference between winter and summer is a maximum of ∆εobs = 170 µε and thus 0.17 mm m−1 for all recorded measurement points. the magnitude of this strain difference did not change over time; only slightly higher strains were observed at individual measurement points. 4.2. outlook the currently available temperature and strain data of an energy diaphragm wall element provide a basis for future research with respect to geothermally utilized earth-coupled wall elements. in particular, the thermally induced stresses from seasonal air temperature 11 d. adam, a. brunner, r. markiewicz, j. pistrol acta polytechnica ctu proceedings fluctuations and a heating and cooling operation are of great interest for researching thermo-mechanical component behavior. the measured data generated in this process provide a verification basis for mechanical and numerical models. the current measurements will be continued and thus allow continuous extension of the data series. in addition, temperature sensors were installed at one of the three distributors in the fall of 2021 to record supply and return temperatures and to analyze the behavior of the system in more detail. furthermore, the geothermal system of the metro station with the usable heating and cooling energy that results, offers insights for any dimensioning or assessment of similar systems in the future. 4.3. final remark the contents of this article have already been published in german as an original paper for the renowned journal bauingenieur [41]. acknowledgements it is highly appreciated that wiener linien gmbh & co kg enables the ongoing recording and collection of measurement data from the geothermal system of the taborstraße metro station. references [1] h. brandl. energy foundations and other thermoactive ground structures. géotechnique 56(2):81–122, 2006. https://doi.org/10.1680/geot.2006.56.2.81 [2] eurostat. final energy consumption by sector, 2022. 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https://doi.org/10.37544/0005-6650-2022-07-08-68 acta polytechnica ctu proceedings 45:1–13, 2023 1 introduction 1.1 reasons for the use of geothermal energy 1.2 scientific relevance and state-of-the-art on the thermo-mechanical behavior of energy walls 2 taborstraße metro station 2.1 project development of the geothermal plant 2.2 predicted energy demand (design phase) 2.3 measured energy data (operational phase) 2.4 instrumentation of the geothermal plant 3 thermo-mechanical behavior of the diaphragm wall 3.1 temperature distribution in the diaphragm wall 3.2 longitudinal strains of the diaphragm wall 4 summary and outlook 4.1 summary 4.2 outlook 4.3 final remark acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2025.53.0087 acta polytechnica ctu proceedings 53:87–91, 2025 © 2025 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague advanced silicate composites – a contribution to sustainable construction tomáš vlacha,∗, eliška kafkováa, jan macháčeka, věra kabíčkováa,b, jakub hájeka,b, magdaléna novotnáa,b, jakub řepkab a czech technical university in prague, faculty of civil engineering, thákurova 7, 166 29 prague 6, czech republic b czech technical university in prague, university centre for energy efficient buildings, třinecká 1024, 273 43 buštěhrad, czech republic ∗ corresponding author: tomas.vlach@cvut.cz abstract. in view of the increasing number of natural and man-made disasters and the increasing economic and social problems, it is necessary to adapt the existing principles and methods of structural design, the corresponding construction techniques, and the operation of buildings to make them more sustainable, resilient, and adaptable to new situations in changing natural and socio-economic conditions in the world. recent research and development of concrete composition, production technology, and development of concrete constructions, intensified over the last 20 years, have led to the improvement of technical parameters while reducing environmental impacts. due to the optimization of the mixture, new types of concrete have significantly better characteristics from the perspective of strength, mechanical resistance, durability, and resistance to extreme loads. the paper presents examples and results of research focused on the use of new types of silicate composites and their effective combinations in the case study. it is generally necessary to apply new silicate composites in such a way that their potential is used to the maximum. the aim is to present the possibility of new practical and effective application of modern materials with an emphasis on reducing environmental impacts and at the same time increasing the resilience of structures. it covers the utilization of high-performance concrete as a protective and load-bearing thin skin with textile reinforcement using carbon textiles tube in combination with recycled concrete aggregate core. developed technical solutions could contribute to addressing the sustainable development goals (sdgs), which the united nations set out in 2015 as a 2030 action plan. keywords: concrete, sustainability, textile reinforcements, textile reinforced concrete skin, recycled aggregate concrete, high-performance concrete, rac core, hpc skin. 1. introduction concrete is gradually becoming a building material with a high potential for technical solutions meeting these new requirements, leading to a necessary reduction of environmental impacts and the consequent necessary improvement of properties in the conditions of a changing climate. considering the amount of produced concrete, the optimization of concrete structures represents a great potential for improving the complex quality of structures from the perspective of sustainable development. one of the ways can be a completely new look at concrete structures and the search for the optimal application of new silicate materials, or their combinations. it can be sandwich constructions, combinations of concrete, or combinations of reinforcements. the paper just presents example and partial results of research focused on the use of new types of silicate composites and their effective combinations in the case study. it is generally necessary to apply new silicate composites in such a way that their potential is used to the maximum. it covers also use of high-performance concrete (hpc), for example, as a protective and load bearing thin skin permanent formwork with textile reinforcement using wound carbon textiles in combination with recycled concrete aggregates (rac) core. rac was chosen for this experiment at the beginning because of a higher difference in mechanical parameters and durability compared to hpc and also due to a better environmental profile. hpc performs well in terms of durability and is highly suitable as a protection for other materials that struggle in this regard. today there is an effort to use hpc more; nevertheless, it is expensive and, from the point of view of environmental impact, this is not an optimal way. the use of these hpc mixtures in combination with a weaker one can be a very attractive solution. it can reduce its consumption and helps to extend the usage of rac. many researchers is describing how to improve rac within the changes in the mixture, but there are also experiments with combining these materials to improve the durability and reduce hpc consumption, for example [1]. there was an experimental study based on the axial behaviour of circle-shaped columns consisting of uhpc 87 https://doi.org/10.14311/app.2025.53.0087 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en t. vlach, e. kafková, j. macháček et al. acta polytechnica ctu proceedings cover layer (shell) with carbon frp grid and usual cast-in-place concrete with steel reinforcement inside, which is a fairly similar principle as in this thesis. similar research describes, for example, beams on the same principle [2]. the author also describes here the progress of the uhpc layer thickness, which is really reduced, and the core again consists of usual concrete and steel reinforcement again [3]. 2. materials used for the experiment 2.1. concrete as mentioned above, both concrete mixtures used in this experiment were developed at the ctu. in the case of hpc it was department of architectural engineering, faculty of civil engineering. the hpc mixture used in this experiment was developed and optimized in last years at the ctu in prague for different applications. the hpc composition recipe is presented in table 1. it is a self-compacting fine-grained concrete without any fibers. materials used for the experiment are primarily from local sources, and components are: cem i 42.5 r cement, silica sand with two particle sizes and maximum grain sizes 1.2 mm, silica flour with one particle size, silica fume and one type of pce superplasticizer. the mixture reduces the amount of water in water/cement ratio to only 0.25 and therefore significantly improves the mechanical properties. the direct tensile strength of the used hpc is experimentally determined to be 5.0 mpa according to the čsn 73 1318 standard. the tensile strength in bending is 16.8 mpa on prisms 40 × 40 × 160 mm with a distance between supports of 100 mm according to the čsn en 12390-5 standard. compressive strength is 106 mpa on 100 mm cubes according to čsn en 12390-3 standard. the static modulus of elasticity is 49.2 gpa on prisms 100 × 100 × 400 mm according to čsn iso 6784 standard [4, 5]. mix content kg m−3 cement i 42.5r 650 technical quartz sand d 0.1/1.2 mm 1 200 technical quartz powder st 6 235 silica fume (microsilica) 85 superplasticizer based on pce 26 water (at 12 °c) 175 total 2 368 table 1. hpc mixture design. in the case of rac it was developed at the laboratory of composite structures, university centre for energy efficient buildings. a mixture with 100 % replacement of natural aggregate with recycled brick was chosen. therefore, this is a lower quality concrete, especially in terms of modulus of elasticity and durability. physical, mechanical, deformation, and thermal properties were tested according to valid czech standards. the detailed composition of the matrix is described below in the table 2. samples of dimensions 100 × 100 × 400 mm, 150 × 150 × 150 mm, and 100 × 100 × 100 mm were used for the testing. the average compressive strength of the rac mixture measured on cubes with 100 mm sides after 28 days of hardening was 24.4 mpa [6]. mix content kg m−3 cement i 42.5r 274 recycled aggregate 0/4 773 recycled aggregate 4/8 18 recycled aggregate 8/16 845 water (at 12 °c) 290 total 2 200 table 2. rac mixture design. 2.2. reinforcement the textile reinforcement was produced manually by winding from carbon fiber homogenized with epoxy resin. the rovings from the company tenax® sts40 f13 24k (teijin, tokyo, japan) were used with a length weight (titer) of 1 600 g km−1 (= 1 600 tex), a tensile strength of 4 400 mpa, and a modulus of elasticity of 240 gpa, according to the technical data sheet. epoxy resin sikafloor-150® from the company (sika, brno, czech republic) was used for homogenization of the rovings. the basic parameters of pure resin are a tensile strength in bending of 15 mpa and a modulus of elasticity of 2.0 gpa. fine-grained silica sand with grain sizes from 0.1 mm to 0.6 mm was used for surface modification to increase cohesion with the concrete matrix [7]. frp basalt reinforcement positioned in each corner was used to support the cross-wound textile reinforcement, as the textile reinforcement was wounded over it. the used diameters of frp bars were 2.2 mm and 6.0 mm. it is a commercial product. young’s modulus of basalt frp reinforcement is 37 gpa and tensile strength is 980 mpa according to the technical data sheet. traditional steel reinforcement was used as a reinforcement of rac. young’s modulus of the steel reinforcement is 200 gpa and the tensile strength is 500 mpa according to the technical data sheet. 3. specimen preparation and design 3.1. variants of selected specimens all hpc shells specimens were made at first as 1.20 m long elements (prisms) and subsequently cut to the required dimensions. the whole research study contained 4 types of the hpc shell – plain concrete without any reinforcement or fiber reinforcement, concrete including pp fibers masterfiber 012 (amount 1 kg m−3), concrete including pva fibers masterfiber 401 (15 kg m−3), and concrete with laboratory 88 vol. 53/2025 advanced silicate composites figure 1. cross-section scheme of basic variants without and with the cros-wound textile reinforcement [3]. both diagrams represent the variant with the 18 mm hpc shell, variant with the 8 mm hpc shell thickness was also prepared. prepared cross-wound carbon textile reinforcement. every mentioned type was also prepared in two variants of thickness – 8 mm and 18 mm. the crosssectional area of all samples for the case study was 100 × 100 mm2. for the comparison specimens with rac core (mentioned above) were also prepared [3]. a diagram of the 18 mm hpc shell filled with rac is presented in figure 1. two lengths of specimens were evaluated through the three-point bending test in this case study, specimens with a length of 400 mm focusing on shear capacity and with length 600 mm length focusing on flexural capacity. another set with a length of 400 mm was prepared for the compressive strength test. additionally, 200 mm long samples were prepared for testing with phenolphthalein to determine the depth of carbonation and to evaluate the influence of the thin hpc shell. for each set, 3 specimens were prepared and filled with recycled-aggregate concrete [3]. this article presents only a part of the whole research – a three-point bending test of the 400 mm specimens, showing the significant contribution of the wound carbon textile reinforcement to shear force transfer. 3.2. specimen preparation a steel hollow pipe was used as the base of the formwork, to form the hpc shell hollow core and for basic manipulation during the whole process. it was covered with the 2 mm thin separation layer. for samples consisting of plain hpc skin or fiber-reinforced hpc skin, steel pipe could be placed into wooden formwork and get ready for casting. samples including carbon textile reinforcement had then added square-shaped hoops to a steel pipe, which enabled the attachment of four frp rebars, one in each corner of the hoop and situated to keep the textile grid in the middle of the hpc skin layer. the carbon roving grid was cross-wound around the frp rebars consisting of 24 rovings for each sample – 12 rovings in each direction at an angle of 45°. in the next step, the textile reinforcement was homogenized using epoxy resin and sand-coated with fine-grain quartz sand for perfect cohesion with hpc [8]. the hardening of the epoxy resin took about 7 days in total. a detailed view of the cross-wound reinforcement is presented in figure 2 on the left. the columns were casted using hpc and closed until the next day when the formwork was removed, the view on the mold is presented in on the right. the prepared columns were stored in the room with constant humidity and after hardening were cut into the length 400 mm or 600 mm for the bending test. hollow samples were completed with this step and view on the hpc skin during the process of hardening is presented in figure 3 on the bottom. figure 2. cross-wounded carbon textile reinforcement after impregnation using epoxy resin and covered with fine sand (on the left) and one specimen after the process of hpc casting, visible steel supported core (on the right). the samples with rac cores were reinforced using conventional steel rebars with 6 mm in diameter. rebars were placed into the inside corners. traditional steel reinforcement was impossible to add due to the small laboratory dimensions. the positive is that the shear forces were transformed only through the knit from the carbon textile reinforcement. the specimens were filled with recycled-aggregate concrete as the core [3]. view on the filled specimens is presented in figure 3 on the bottom. 3.3. three-point bending test setup the three-point bending test was performed according to the en 12390-5 technical standard (flexural strength of test specimens). scheme for testing and 89 t. vlach, e. kafková, j. macháček et al. acta polytechnica ctu proceedings figure 3. hollow hpc skin during the process of hardening before cutting (on the top) and specimens already cut and filled with rac after the process of concreting with visible steel reinforcement in the corner (on the bottom). real setup is visible in the next chapter about the results. for hollow or filled specimens with dimensions 100 × 100 × 400 mm, the support pin distance was 300 mm and the upper one was exactly in the middle of this distance. 4. results and discussion all presented bending tests were carried out until the complete collapse of the sample. one of three specimens was chosen for comparison from each set. the representative curve from each group is presented in figure 4 for the hollow specimens. plain hpc, pp, and pva fibers have a very similar trend. however, during the test, the crack development was different, especially in the case of pva fibers. plain hpc and hpc with pp fibers logically had only one crack during the loading process and subsequently collapsed. however, the pva fibers allowed the proper development of a higher number of cracks – multiple cracking. the specimen with carbon textile was very different. there was a massive multiple-cracking process, proper activation of the load-bearing textile reinforcement and significantly greater load-bearing capacity. the testing procedure was the same also for specimens filled by rac and wit additive longitudinal steel figure 4. hollow permanent hpc skin (formwork) with the thickness 18 mm for representative samples. visible massive positive influence of carbon composite reinforcement in hpc. reinforcement. the trend for all samples was similar. it was clear that the response was significantly improved due to the added steel reinforcement, and the specimen was able to carry more force than when the first crack was initiated for plain hpc skin and skin with fiber reinforcement. it is presented in figure 5. figure 5. permanent hpc formwork (shell) filled by rac and additional traditional longitudinal steel reinforcement with the thickness of the hpc shell of 18 mm for representative samples. a massive positive influence of the wound carbon reinforcement in hpc. the option with carbon knit is again significantly the best set of all, although it needs some changes to provide, show, and take the advantage of the filled core. and that is a great result of the study. the core did not have a chance to interact completely due to the over-reinforced shell with carbon knit. concrete crushing was also included in the construction joint and there was also delamination of the reinforcement. the view of the specimens of hpc filled with rac after the testing procedure is presented in figure 6. both again with thickness of reinforced hpc skin 18 mm. plain hpc (on the top) has one visible crack between the upper and bottom support, and hpc skin with a carbon textile knit (on the bottom) has different types of failure – delamination due to the short anchorage length of the reinforcement. 90 vol. 53/2025 advanced silicate composites figure 6. examples of hpc specimens filled with rac after the testing procedure. both with a shell thickness of 18 mm. plain hpc shell (on the top) and with the cross-wound carbon textile reinforcement (on the bottom) [3]. 5. conclusions for this case study of the possibility of applying modern silicate composites with new approaches aimed at reducing the carbon footprint of concrete structures in general, a combination of hpc and rac with 100 % replacement of natural aggregates with recycled brick aggregates was chosen, as well as a combination of impregnated textile reinforcement and traditional steel reinforcement. the samples were also supplemented with fiber reinforcement for the possibility of comparison. for the presented variants, four materials of hpc were used with the skin thickness of 18 mm – plain concrete, polypropylene fiber-reinforced concrete, polyvinyl alcohol fiber-reinforced concrete and textile-reinforced concrete. plain hpc skin has bad results during the mechanical test, and also pp fibers in hpc skin have really no impact in the way of strength and should be used, e.g. eliminating shrinkage. pva fibers in the hpc skin showed the most visible progress due to core filling with rac. textile-reinforced sets had incredible results, although there was an over-reinforcement problem which pushed back possible progress thanks to rac filling. the load-bearing capacity was huge despite the fact that the filled rac core had no chance to interact and support the process. thanks to this case study of the new approach to concrete construction, there is space for some adjustments and optimalisation in future research, for sure. acknowledgements the work on this paper was supported by czech science foundation grant no. 24-12052s entitled “composite action of textile reinforced concrete skin and recycled aggregate concrete core”. the authors would like to acknowledge all financial assistance provided to support this research. references [1] h. tian, z. zhou, y. zhang, y. wei. axial behavior of reinforced concrete column with ultra-high performance concrete stay-in-place formwork. engineering structures 210:110403, 2020. https://doi.org/10.1016/j.engstruct.2020.110403 [2] wu, xiangguo. advanced composite structure combining ultra high performance concrete with normal reinforced concrete. matec web of conferences 278:03004, 2019. https://doi.org/10.1051/matecconf/201927803004 [3] e. kafková. study of permanent formwork made of high-performance concrete as protection of concrete with recycled aggregate, 2022. [2024-05-05]. https://www.diva-portal.org/smash/ record.jsf?pid=diva2:1733330 [4] a. chira, a. kumar, t. vlach, et al. textile-reinforced concrete facade panels with rigid foam core prisms. journal of sandwich structures & materials 18(2):200–214, 2016. https://doi.org/10.1177/1099636215613488 [5] t. vlach, l. laiblová, j. řepka, et al. experimental verification of impregnated textile reinforcement splicing by overlapping. acta polytechnica ctu proceedings 22:128–132, 2019. https://doi.org/10.14311/app.2019.22.0128 [6] t. pavlu, k. fortova, j. divis, p. hajek. the utilization of recycled masonry aggregate and recycled eps for concrete blocks for mortarless masonry. materials 12(12):1923, 2019. https://doi.org/10.3390/ma12121923 [7] j. žalský, t. vlach, j. řepka, et al. reinforced l-shaped frame made of textile-reinforced concrete. polymers 15(2):376, 2023. https://doi.org/10.3390/polym15020376 [8] t. vlach, j. řepka, j. hájek, et al. cohesion test of a single impregnated ar-glass roving in high-performance concrete. stavební obzor – civil engineering journal 29(3):358–369, 2020. https://doi.org/10.14311/cej.2020.03.0032 91 https://doi.org/10.1016/j.engstruct.2020.110403 https://doi.org/10.1051/matecconf/201927803004 https://www.diva-portal.org/smash/record.jsf?pid=diva2:1733330 https://www.diva-portal.org/smash/record.jsf?pid=diva2:1733330 https://doi.org/10.1177/1099636215613488 https://doi.org/10.14311/app.2019.22.0128 https://doi.org/10.3390/ma12121923 https://doi.org/10.3390/polym15020376 https://doi.org/10.14311/cej.2020.03.0032 acta polytechnica ctu proceedings 53:87–91, 2025 1 introduction 2 materials used for the experiment 2.1 concrete 2.2 reinforcement 3 specimen preparation and design 3.1 variants of selected specimens 3.2 specimen preparation 3.3 three-point bending test setup 4 results and discussion 5 conclusions acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2023.42.0006 acta polytechnica ctu proceedings 42:6–11, 2023 © 2023 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague asynchronous time integration while achieving zero interface energy radim dvořáka,b,∗, radek kolmanb, jan faltaa, michaela neühauserováa a czech technical university in prague, faculty of transportation sciences, department of mechanics and materials, na florenci 25, 110 00 praha 1, czech republic b czech academy of sciences, institute of thermomechanics, v. v. i., dolejškova 1402/5, 182 00 praha 8, czech republic ∗ corresponding author: dvorara9@fd.cvut.cz abstract. this contribution deals with an asynchronous direct time integration of the finite-element model. the proposed method is applied to the phenomenon of wave propagation through an elastic linear continuum. the numerical model is partitioned into individual subdomains using the domain decomposition method by means of localized lagrange multipliers. for each subdomain, different time discretizations are used. no restrictions for relation between subdomain’s time steps are imposed. the coupling of the subdomains is forced by an acceleration continuity condition. additionally, we use the a posteriori technique to also provide the displacement and velocity continuity at the interfaces, and hence we obtain exact continuity of all three kinematic fields. the proposed method is experimentally validated using the modified shpb (split hopkinson pressure bar) setup. keywords: finite element method, direct time integration, asynchronous integration, domain decomposition, localized lagrange multipliers. 1. introduction the problem of asynchronous integration has been addressed by many authors throughout the second half of the last century [1, 2]. these methods were not robust enough and lacked the required accuracy or were unstable [3]. we consider the robust and accurate methods to be those of the present day, such as the following [4–9]. however, even these new methods have drawbacks. the most common ones include: overly large systems of equations, the necessity of integer ratio of time steps of adjacent subdomains, the necessity of having common time levels where the solution of the whole model must be determined, etc. the method presented below eliminates these shortcomings and, in addition, offers the possibility of guaranteeing the continuity of the displacement, velocity and acceleration field at the interfaces. 2. methods both strong and weak formulations of the domaindecomposed problem are introduced. 2.1. strong formulation we recall classical problem of linear elasto-dynamics [10]: div σ + b = ρü on ω × υ , σ = c : ε , ε = 1 2 [ (grad u)t + grad u ] , u = u0(x, t = 0) on ω , u̇(x, t = 0) = u̇0 on ω , u = û on γd , n · σ = t̂ on γn . (1) where σ(x, t) and ε(x, t) are the stress and strain tensor, respectively. the scalar function ρ(x) represents the mass density, vector functions b(x) and u(x, t) have the meaning of the intensity of the volume forces and the displacement field, respectively. the 4th order tensor of elasticity is given by c(x). further, u0 and u̇0 are the initial displacement an initial velocity fields, respectively. displacement at the boundary γd is given by û and t̂ is known traction at the boundary γn. the symbol n represents the outward surface normal unit vector and x ∈ ω is the position of the material point, and t is the time. finally, ω refers to the spatial domain of interest and υ represents the the time domain of interest. the dot superscript has the meaning of the time derivative. furthermore we assume that the region ω is composed of ns sub-domains ωs, s = 1 . . . ns (see figure 1). at the interface γi between subdomains we enforce the condition of the continuity of the displacement field us(x) = ui(x) on γi ∀s , (2) where ui(x) stands for the displacement of the interface. note, that further, after the discretization of the weak problem, we use the second time derivative of 6 https://doi.org/10.14311/app.2023.42.0006 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 42/2023 asynchronous time integration with zero interface energy ωs ωs+1 ωs-1 gi i-1 gi i uii us ls ls+1 us+1 figure 1. arbitrary part of the system ω divided into subdomanins ωs connected with i-th interface γi. (2) so the acceleration continuity is actually dictated, see (6). 2.2. weak formulation according to the hamilton’s principle [11] we search for the stationary solution of δh ( u, ui, ℓ ) = δhfree (u) + δwi ( u, ui, ℓ ) , (3) over time interval [0, t ], where δhfree consists of the virtual kinetic energy and potential of the system and of the virtual work done by external loads. the virtual work of the interface δwi can be expressed as δwi ( u, ui, ℓ ) = δ ni∑ i=1 ni s∑ s=1 ∫ γi [ ℓs(us − ui i) ] dγ , (4) where us and ui s are the displacements of the s-th subdomain and of the i-th interface, respectively (see figure 1). the ℓs stands for the lagrange multiplier field of the s -th subdomain. the whole system consists of ni interfaces and of ns subdomains. the i-th interface is connected to ni s of subdomains. the s-th subdomain is connected with ns i interfaces. 2.3. spatial discretization using classical spatial discretization by means of finite elements, one obtains the system of governing equations in the form mü + ku = f − bλ, (5) bt ü − l üi = 0, (6) lt λ = 0, (7) i.e. equation of motion, kinematic interface constraints and interface equilibrium condition, respectively, where m, k, are the mass and stiffness matrices, f is the load vector, b, l are the matrices of the connectivity [12], λ is the vector of localized lagrange multipliers and ui it the interface displacement. resulting from the existence of individual subdomains, matrices and vectors have block-diagonal and block form respectively, where each block represents one subdomain. 2.4. isolation of the interface problem we will demonstrate that the λ and üi unknowns related to the i-th interface can be expressed explicitly from the knowledge of the interface solution of connected ni s subdomains, i.e. from the knowledge of u on γi. now, assume the problem of two subdomains connected via one interface. using equations (5)–(7), one can extract the problem of this interface asbt 1 m−1 1 b1 0 l1 0 bt 2 m−1 2 b2 l2 l1t l2t 0  l1λ1 l2λ2 üi  = bt 1 ˜̈u1 bt 2 ˜̈u2 0  , (8) where ˜̈u is the acceleration predictor defined as ˜̈u = m−1(f − ku) = ü + m−1bλ . (9) finally, unknowns of the system (8) are expressed as üi = mi−1 f i , λs = mλs −1 fλs , (10) where s = 1 . . . 2 and mi = 2∑ s=1 [ lst ( bt s m−1 s bs )−1 ls ] , f i = 2∑ s=1 [ lst ( bi s tm−1 s bi s )−1 bi s t ˜̈us ] , mλs = ( bt s m−1 s bs )−1 , fλs = bt s ˜̈us − lsüi . (11) are the interface mass matrix, interface load, boundary mass matrix and boundary load, respectively. 2.5. temporal discretization since the proposed asynchronous time scheme can be combined with arbitrary method of discretization [9], we present only the most simple case of central difference method [10] un+1 = un + ∆t u̇n + ∆t2 2 ün , u̇n+1 = u̇n + ∆t 2 ( ün + ün+1) . (12) to obtain the acceleration ün+1 one has to solve the system of equations (5)–(7) in time tn+1. stable [8] interface temporal discretization is given by (u̇i)n+x1 = (u̇i)n + ∆t 2 ( (üi)n + (üi)n+1) , (ui)n+1 = (ui)n + ∆t (u̇i)n+1 . (13) 2.6. the asynchronous time integration conventionally, whole domain is computed with one globally determined time step. in the case of asynchronous integration, each subdomain has its own time step. 7 r. dvořák, r. kolman, j. falta, m. neühauserová acta polytechnica ctu proceedings ∫ t+dtc dt = t w1 w2 ∫ dt t wr wr1 2t+dtc on gi gi gi figure 2. the equality of the i-th interface solution assuming whole subdomains ωs vs. interface regions ωrs only w1 w2wr wr1 2gi t1 ti t2 t n1 t n2 t n+1 2 t n+1 1 δti δt2 figure 3. initial state of the algorithm 2.6.1. initialization of interface regions by interface region ωri s we mean the area of the subdomain s affected by the propagating wave from the interface i during at least two critical steps of this area ∆tcri s . assume subdomains ω1 and ω2 connected via interface γi. further assume that we know the solution of given interface at time level t. then, because of the finite speed of wave, we can compute the solution of the interface γi at time level t + ∆tc from reduced system ωr1 − γi − ωr2 instead of assuming full problem ω1 − γi − ω2 (see figure 2). by the substitution u = rt ur to (5)–(7) we obtain formally same system of equations, however, for reduced problem on interface regions. in the system, new objects appear kr = rkrt , mr = rmrt , f r = rf , br = rb, (14) which are the interface region matrices and vectors. 2.6.2. algorithm we have the problem of two subdomains ω1 and ω2 with the interface γi (see figure 3). the bluehighlighted line stands for the already known solution (note, that the last known solution of the interface exists at time level tn 1 ). our challenge is to obtain the solution of ω2 at tn+1 2 . actual interface time step has the value of ∆ti = tn+1 2 − tn 1 . we propose the following algorithm: ρ 2 = 1 kg m-3, e 2 = 2.25 paρ 1 = 1 kg m-3, e 1 = 1 pa 0 l = 1 m e1 = 1 pa e2 = 2.25 pa 1 2 3 4 5 δh = 0.01 m (t) = 1 paσ figure 4. bar setup with linearly varying young modulus (1.) solve the interface in time tn+1 2 (a) (ur)n+1 = (ur)n + ∆ti (u̇r)n + ∆ti 2 2 (ür)n, (b) (˜̈ur)n+1 = mr−1 (( fr)n+1 − kr (ur )n+1 ) , (c) (üi)n+1 = mi−1 (f i)n+1, (d) (u̇i)n+1 = (u̇i)n + ∆ti 2 (( üi ) n + ( üi ) n+1 ) , (e) (ui)n+1 = (ui)n + ∆ti (u̇i)n+1. (2.) complete the solution of ωr1 in time tn+1 2 (a) (λ1)n+1 = mλ1 1 −1 ( brt 1 (˜̈ur 1)n+1 − l1(üi)n+1 ) , (b) (ür 1)n+1 = (˜̈ur 1)n+1 − mr 1 −1br 1(λ1)n+1, (c) (u̇)n+1 = (u̇)n + ∆ti 2 ( (ü)n + (ü)n+1 ) , (d) correct {ur, u̇r}n+1 1 to satisfy (6) and its time derivative respectively. (3.) solve free subdomain ω2 in time tn+1 2 (a) un+1 2 = un 2 + ∆t2 u̇n 2 + ∆(t2)2 2 ün 2 , (b) ˜̈un+1 2 = m−1(fn+1 2 − k2 un+1 2 ), (c) rewrite ˜̈un+1 2 → ün+1 2 with respect to bt 2 ün+1 2 − l2 (üi)n+1 = 0, (d) u̇n+1 2 = u̇n 2 + ∆t2 2 ( ün 2 + ün+1 2 ) , (e) correct {un+1 2 , u̇n+1 2 } to satisfy (6) and its time derivative respectively. (f) reset {ur, u̇r, ür}n+1 2 = r2{u, u̇, ü}n+1 2 . depending on real further time discretization, the algorithm is repeated or alternates between both subdomains. the computation ends once each of the subdomains has reached the final time tend. 3. numerical and experimental validation wave propagation is investigated through the linearly graded material (1d formulation). furthermore, the impact of 3 thin rods is simulated (2d axisymmetric formulation) and the results are compared with experimental outputs of ohpb setup. further in the plots, the conventional term stands for nondecomposed model with single globally determined time step. the term asynchronous stands for the proposed method. computational time step is further always set as the half of the critical time step for each subdomain individually. 3.1. graded material (1d) the bar with linearly changing young modulus is loaded by rectangular pulse of 1 pa (see figure 4). the bar is divided into 5 equaly sized subdomains. elements size is set to ∆h = 0.01 m. the stress at time 8 vol. 42/2023 asynchronous time integration with zero interface energy bar impact velocity length diameter material density young mod. poisson no. [ms−1] [mm] [mm] ρ [kgm−3] e [gpa] ratio ν [-] 1. 15.2 1750 20 pmma 1180 5 0.37 2. 0 796 20 steel 7850 210 0.3 3. 0 1600 20 al 2806 72 0.334 table 1. initial velocities and parameters of the bars used in the experimental setup 0 0.2 0.4 0.6 0.8 1 x [m] 0 0.5 1 1.5 σ [p a] conventional asynchronous analytical figure 5. propagated stress pulse at time t = 0.75 s pressure max. 1 mpa barrel solenoid valve 1st bar pmma laser through-beam sensor damper linear systemguidance (rail and carriage) air flow speckled patterns for dic high speed cameras strain-gauge 2nd bar steel 3rd bar aluminium figure 6. the scheme of used open hopkinson pressure bar setup 0 0.2 0.4 0.6 0.8 1 1.2 t [s] ×10-3 0 0.5 1 1.5 2 2.5 3 u [m ] ×10-3 x = 1.750 m (pmma-steel i.e. 1st interface) exp. pmma exp. steel asynchronous (a). pmma-steel interface 0 0.2 0.4 0.6 0.8 1 1.2 t [s] ×10-3 0 0.5 1 1.5 2 2.5 3 u [m ] ×10-3 x = 2.546 m (steel-al i.e. 2nd interface) exp. steel exp. al asynchronous (b). steel-al interface figure 7. the axial displacement u1 at bar interfaces t = 0.75 s is plotted in figure 5. spurious oscillations around the analytical solution are caused by the use of a noncritical computational time step ∆ts < 0.5 ∆tc s . 3.2. experiment (2d axisymmetry) the setup consists of 3 aligned bars of various materials (see table 1 and figure 6). the first bar made from polymethylmethacrylate (pmma) has an initial velocity that simulates the impact. the second bar is made of steel and the third bar is made of aluminum alloy en-aw-7075-t6. in case of the asynchronous integrator each bar refers to one subdomain, that is, 3 subdomains and 2 interfaces in total are defined. the length of the edge of the 2d axisymmetric square element is set to the value of ∼ 1.6 mm. the computational time step is set to the half of the critical one. to maximize the credibility of contact behavior, only the longitudinal degrees of freedom (perpendicular to the interface) are permanently coupled. moreover, only the experimental time window when all bars are in the contact is considered (i.e. ≈ 1 ms). 3.2.1. displacement of the interfaces and strain ε11 response the displacement of both interfaces (pmma-steel and steel-al) is plotted (see figure 7). one can clearly see that bars are in contact for the time period of 1.2 m s, so the numerical output can be directly compared with experimental results up to this time. 9 r. dvořák, r. kolman, j. falta, m. neühauserová acta polytechnica ctu proceedings 0 0.2 0.4 0.6 0.8 1 1.2 t [s] ×10-3 -5 -4 -3 -2 -1 0 1 2 ε [] ×10-4 x = 2.746 m (al bar: 0.2 m from steel-al interface) experiment asynchronous figure 8. strain ε11 response close to the location of the 2nd interface the history of axial strain ε11 on the bar’s free surface measured at distance x = 2.746 m, close to the location of the 2nd interface, is plotted (see figure 8). we can see sufficient agreement between both time evolutions during time period of the interest (0–1.2 m s). 3.2.2. energy balance and dissipation finally, the energy balance is investigated. the percentage loss of total energy given by the initial velocity of the pmma bar (see figure 9) and interface energy (see figure 9) is shown. although the term (4) for interface energy has an exactly zero value, the system is dissipating energy during the very beginning of the simulation. this means, that the energy balance distortion is present in the potential and kinetic energy of the bars. 4. conclusions we found the asynchronous scheme useful especially in cases of existence of multiple material region within the model. the scheme presented is suitable for solving various problems of physics, e.g., fluid structure interaction, where the method used for decomposition [13, 14] has already been used successfully. the weakness of the algorithm is the distortion of the energy balance of the subdomains, which is, however, sufficiently small. acknowledgements the work of radim dvořák and radek kolman was supported by grant projects with no. 22-00863k of the czech science foundation (csf) within institutional support rvo:61388998. the work of radim dvořák was also supported by the grant agency of the czech technical university in prague, grant no. sgs22/196/ohk2/3t/16. radek kolman and radim dvořák also realized the work under the czech-estonian mobility project estav-21-02. references [1] t. belytschko, r. mullen. mesh partitions of explicit-implicit time integration. formulations and computational algorithms in finite element analysis : 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 t [s] ×10-3 -1 0 1 2 3 4 % of e n er gy lo ss [] x = global asynchronous 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 t [s] ×10-3 -1 -0.5 0 0.5 1 e n er gy [j ] ×10-32 x = global asynchronous figure 9. total energy loss (compared to initial kinetic energy of the pmma bar) evolution (top) and interface energy evolution (bottom). u.s.-germany symposium. massachusetts institute of technology, cambridge, 1977. [2] p. smolinski. subcycling integration with non-integer time steps for structural dynamics problems. computers & structures 59(2):273–281, 1996. https://doi.org/10.1016/0045-7949(95)00256-1 [3] p. smolinski, s. sleith, t. belytschko. stability of an explicit multi-time step integration algorithm for linear structural dynamics equations. computational mechanics 18:236–244, 1996. https://doi.org/10.1007/bf00369941 [4] a. combescure, a. gravouil. a numerical scheme to couple subdomains with different time-steps for predominantly linear transient analysis. computer methods in applied mechanics and engineering 191:1129–1157, 2002. https://doi.org/10.1016/s0045-7825(01)00190-6 [5] w. subber, k. matouš. asynchronous space–time algorithm based on a domain decomposition method for structural dynamics problems on non-matching meshes. computational mechanics 57(2):211–235, 2015. https://doi.org/10.1007/s00466-015-1228-0 [6] f.-e. fekak, m. brun, a. gravouil, b. depale. a new heterogeneous asynchronous explicit–implicit time integrator for nonsmooth dynamics. computational mechanics 60(1):1–21, 2017. https://doi.org/10.1007/s00466-017-1397-0 [7] a. prakash, k. d. hjelmstad. a feti-based multi-time-step coupling method for newmark schemes in structural dynamics. international journal for numerical methods in engineering 61(13):2183–2204, 2004. https://doi.org/10.1002/nme.1136 [8] s. s. cho, r. kolman, j. a. gonzález, k. c. park. explicit multistep time integration for discontinuous elastic stress wave propagation in heterogeneous solids. international journal for numerical methods in engineering 118(5):276–302, 2019. https://onlineli brary.wiley.com/doi/pdf/10.1002/nme.6027 https://doi.org/10.1002/nme.6027 10 https://doi.org/10.1016/0045-7949(95)00256-1 https://doi.org/10.1007/bf00369941 https://doi.org/10.1016/s0045-7825(01)00190-6 https://doi.org/10.1007/s00466-015-1228-0 https://doi.org/10.1007/s00466-017-1397-0 https://doi.org/10.1002/nme.1136 https://onlinelibrary.wiley.com/doi/pdf/10.1002/nme.6027 https://onlinelibrary.wiley.com/doi/pdf/10.1002/nme.6027 https://doi.org/10.1002/nme.6027 vol. 42/2023 asynchronous time integration with zero interface energy [9] s. s. cho, k. c. park, h. huh. a method for multidimensional wave propagation analysis via component-wise partition of longitudinal and shear waves. international journal for numerical methods in engineering 95(3):212–237, 2013. https://doi.org/10.1002/nme.4495 [10] t. j. r. hughes. the finite element method: linear static and dynamic finite element analysis. dover publications, mineola, ny, 2000. [11] j. a. gonzález, j. kopačka, r. kolman, k.-c. park. partitioned formulation of contact-impact problems with stabilized contact constraints and reciprocal mass matrices. international journal for numerical methods in engineering 122(17):4609–4636, 2021. https://doi.org/10.1002/nme.6739 [12] k. c. park, c. a. felippa. a variational principle for the formulation of partitioned structural systems. international journal for numerical methods in engineering 47(1-3):395–418, 2000. https://doi.org/10.1002/(sici)1097-0207(200001 10/30)47:1/3<395::aid-nme777>3.0.co;2-9 [13] k. c. park, c. felippa, u. gumaste. a localized version of the method of lagrange multipliers and its applications. computational mechanics 24:476–490, 2000. https://doi.org/10.1007/s004660050007 [14] k. c. park, c. a. felippa, g. rebel. a simple algorithm for localized construction of non-matching structural interfaces. international journal for numerical methods in engineering 53(9):2117–2142, 2002. https://doi.org/10.1002/nme.374 11 https://doi.org/10.1002/nme.4495 https://doi.org/10.1002/nme.6739 https://doi.org/10.1002/(sici)1097-0207(200 00110/30)47:1/3<395::aid-nme777>3.0.co;2-9 https://doi.org/10.1002/(sici)1097-0207(200 00110/30)47:1/3<395::aid-nme777>3.0.co;2-9 https://doi.org/10.1007/s004660050007 https://doi.org/10.1002/nme.374 acta polytechnica ctu proceedings 42:6–11, 2023 1 introduction 2 methods 2.1 strong formulation 2.2 weak formulation 2.3 spatial discretization 2.4 isolation of the interface problem 2.5 temporal discretization 2.6 the asynchronous time integration 2.6.1 initialization of interface regions 2.6.2 algorithm 3 numerical and experimental validation 3.1 graded material (1d) 3.2 experiment (2d axisymmetry) 3.2.1 displacement of the interfaces and strain 11 response 3.2.2 energy balance and dissipation 4 conclusions acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2023.42.0061 acta polytechnica ctu proceedings 42:61–66, 2023 © 2023 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague local defect formation in short glass fibre reinforced polymers – micro-mechanical simulations and interrupted in-situ experiments julia maurera,b,∗, denise kröllingb, dietmar salabergerc, michael jerabekc, johann kastnera, zoltán majorb a university of applied sciences upper austria, research group computed tomography, stelzhamerstraße 23, 4600 wels, austria b johannes kepler university linz, institute of polymer product engineering, altenberger straße 69, 4040 linz, austria c borealis polyolefine gmbh, st. peter-strasse 25, 4021 linz, austria ∗ corresponding author: julia.maurer@fh-wels.at abstract. discontinuous fibre reinforced polymers are widely used in various industry sectors and often replace conventional materials, due to lower production costs and their lightweight structure. for improvement of the component design, detailed knowledge of the failure mechanisms are necessary. to better understand the defect formation and thus the micro-mechanics, the strain behaviour in single fibres was analysed by micro-mechanical simulations of representative volume elements (rve). therefore, selected fibres – similar orientated as in the real structure – were chosen for detailed analysis. additionally, the defect formation next to selected fibres was investigated by x-ray computed tomography (ct). furthermore, the critical fibre length was estimated based on the protruding fibre length of the fracture surface. overall the simulation results correspond to theory. however, the detailed local inspection of the experimental volume data showed a rather strong influence of neighbouring fibres. keywords: short fibre reinforced polymer, x-ray computed tomography, digital volume correlation, micro-mechanical simulation. 1. introduction short glass fibre reinforced thermoplastics are a subcategory of composites. parts and structures of complex shape can be produced in an automated fashion. therefore, the main advantages are low manufacturing costs and the possibility of high-volume production. to improve these materials, detailed knowledge of the mechanical behaviour, especially at the micro-scale level, is of great importance. x-ray computed tomography (ct) is a suitable method for the investigation of such materials and the ongoing damage processes [1]. for the analysis of the defect mechanisms and the quantification of the local strain distribution, interrupted in situ tensile tests are decisive [2, 3]. a rather new approach is the local strain evaluation based on digital volume correlation (dvc), introduced 1999 by bay et al. [4]. dvc enables the determination of the local displacements and further the calculation of local strains in three dimensions. for the correlation of the reference volume (unloaded state) and the deformed volume (loaded state), the fibres act as speckle pattern. recent research on fibre reinforced materials using dvc has shown that the composite architecture contributes to sample failure at the mesoscale [5–7]. there are various modelling and simulation approaches of composites ranging from manufacturing process simulations to macroand micro-scale simulations. the micro-structural details required for simulations are often provided by x-ray computed tomography data. in general, the mechanical behaviour of a fibre reinforced polymer is strongly influenced by the bonding between matrix and fibre. the deformation within the matrix is transmitted to the fibre by shear stresses between fibre and matrix interface. it is quantified by the so called interfacial shear strength or the critical fibre length [8]. there are various methods for a quantitative measurement of these values, further some models were developed in recent years. one of those is the kelly-tyson model [9]. the objective of this work was to determine local strain and defect formation, for a better understanding of the failure mechanisms of short glass fibre reinforced polymers at the micro-scale level. additionally, the usage of the characterization of individual protruding fibres (“free fibres”) for the estimation of interfacial shear strength based on the kelly-tyson model is presented. 2. materials and methods the investigated material is polypropylene (pp) reinforced with 24 wt% of short glass fibres with a nominal fibre diameter of 13 µm. the mean and weighted mean fibre lengths are approx. 450 µm, respectively 810 µm, 61 https://doi.org/10.14311/app.2023.42.0061 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en j. maurer, d. krölling, d. salaberger et al. acta polytechnica ctu proceedings figure 1. experimental setup for interrupted in situ ct measurements and exemplary ct volume (right), showing the typical skin-shell-core structure. and were determined based on ct data. a constant radius test geometry with a radius of 60 mm, a length of 30 mm and a narrowest cross section of 3 × 2 mm2 was used for the tensile tests (see figure 1). the cr test specimens were milled out from an injection moulded plate oriented in injection moulding direction. thus resulting in a main fibre direction of 0◦. due to the injection moulding process the test specimens show a typical layered core-shellskin structure, as well as transition regions in-between. the constant radius test specimen (cr) was in situ tested in an interrupted manner until fracture similar as presented in [3]. the load steps were defined starting at 30 % of the tensile strength, followed by 52 % and then increased by 8 % until fracture. after each load step and a certain time until the sample was in a stable state, a ct scan was performed with the nanotom 180nf device (phoenix|x-ray, ge sensing & inspection technologies gmbh, wunstorf, germany) at a voxel edge length of 2 µm. the x-ray source was equipped with a tungsten-on-diamond target and operated at a voltage of 80 kv. the available analysis volume is approximately 3 × 2 × 3.6 mm3. the local fibre characteristics were determined with the in house developed fibre characterization tool [10, 11]. this was done for the unloaded state to get the fibre length and orientation, and for the fractured surface to get the fibre length and orientation of the protruding fibres. for the latter some additional segmentation steps were necessary. first, a gauss filter with kernel 5 was applied to the volume data to reduce the noise and facilitate the surface determination. afterwards a surface determination with a gray value according to the material peak was applied and an region of interest (roi) of the fracture surface with the protruded fibres was extracted. some smoothing and open/close steps were followed by an inversion of the roi, to be able to extract the protruded fibres only. subsequent analysis of the roi with the fibre characterization tool was performed. the critical fibre length is a very important material parameter for such materials. the critical fibre length is the minimum length to enable the fibre to contribute its full strength to the composite. based on the kellytyson model the critical fibre length can be estimated with equation (1) [9]. lc = σfibre ∗ d 2 ∗ τmatrix , (1) whereas σfibre is the fibre strength and τmatrix the interfacial shear strength. as the critical length is a function of the interfacial shear strength, it will influence the defect behavior (whether a fibre pull-out or fibre fracture occurs). unfortunately, the interfacial shear strength is unknown. so the critical fibre length was estimated by evaluating the free fibres of the fracture surface from the ct data by conducting several segmentation steps. it was assumed, that the fibres were pulled out of both parts to the same extent. the critical length can than be estimated by selecting the longest pulled out fibres. it was decided to evaluate the length of the longest 10 fibres oriented in direction of the applied force. for estimation of the local strain distribution inside the material the avizo software 3d (thermo fisher scientific) and its digital volume correlation (dvc) tool was used. a tetrahedral mesh with a average edge length of approximately 200 µm was used. the digimat software was used for the micromechanical simulations of representative volume elements (rves). the necessary work steps are shown in figure 2. generally, before starting simulations of representative volume elements, the material response needs to be known. a material model was derived by reverse engineering, based on mechanical experimental results (stress-strain curves of tested samples with main fibre orientation of 0◦, 45◦ and 90◦) and the knowledge of the fibre characteristics (fibre length, fibre orientation, fibre diameter) of these tested specimens. initially, a suitable material model was used, which was iteratively adjusted by optimizing 62 vol. 42/2023 defect formation in short glass fibre reinforced polymers figure 2. schematical workflow for the micro-mechanical simulations. figure 3. 3d rendered ct volume view of fracture surface with colored protruding fibres. the young’s modulus, the poisson’s ratio, the yield stress, several hardening parameters and the aspect ratio until the model match the experimental results. based on the fibre characterization data gained by ct, the rve for the cr 0◦ test specimen was generated in digimat-fe. a cubic rve with a edge length of 500 µm was generated and filled with fibres of spherocylindrical shape. the nominal fibre content was set to 24 wt%, the diameter was set to 13 µm and based on the fibre characterization by ct data a mean fibre length of around 470 µm was used. these characterization additionally provided the information on the fibre orientation distribution. an average fibre orientation tensor with a main orientation in z-direction was applied. the fibres were assumed to be perfectly bonded in the matrix with no failure criteria. the mechanical loading was selected with a periodic boundary condition and quasi-static mode. finally, the micro structural simulations were performed with the abaqus solver for different load types. 3. results and discussion due to a non-uniform stress distribution inside the materials, only fibres in direction of the force were considered for the estimation of the critical fibre length. similar to other studies [8] [14], the estimation was performed based on the critical length model first reported by fu et al. [15]. assuming, that the fibre was pulled out of the other part by the same extent, the critical length can be estimated by selecting the longest pulled out fibre lp,max according equation (2). lc ≥ 2 ∗ lp,max, (2) in figure 3 a 3d rendered view of the fractures surface and the segmented protruded fibres (coloured green) is shown. based on this data set, the ten longest fibres oriented in direction of the force were selected (fibre orientation tensor component azz > 0.9) and a mean value for the longest pulled out fibre length lp,max was calculated to get rid of any segmentation errors. according to 2 this resulted in a critical fibre length of around 1 070 µm. based on the kelly-tyson model and assuming a fibre strength of approximately 2 500 mpa as well as a nominal fibre diameter of 13 µm, the interfacial shear strength was calculated at around 15 mpa. figure 4a shows a comparison of the stress-strain curves of the monotonic tested, the interrupted tested and the simulated cr 0◦ test specimen. the lower stress values of the interrupted tested specimen can be explained by relaxation due to the longer testing procedure. generally, deviations between experiments and simulations can be expected, especially in the plastic region, because of the used elastic-plastic material model and the assumed perfectly bonded fibres, which is indeed not the case. nevertheless, the probability distribution of the maximum principal strains inside the material (normalized bins) show good accordance for simulation (approx. 740 000 nodes) and interrupted in situ experiment (approx. 3 600 nodes). from the results of the micro structural simulation, two fibres were selected and the according strain distri63 j. maurer, d. krölling, d. salaberger et al. acta polytechnica ctu proceedings figure 4. stress-strain curves (a) and probability distribution of the maximum prinicipal strain (b) of experiments and simulation for the cr 0◦ test specimen [12]. figure 5. rve with selected fibres and strain distribution along these fibres. [13] figure 6. sectional images of fibre pull-outs (a) and fibre fractures (b) with dimensions of selected fibres (green lines) and marked defects (red arrows). bution along those single fibres were evaluated (shown in figure 5). the rve simulation enabled the investigation of the strain along selected fibres. as expected the highest strain could be observed for fibres in direction of applied force, with a peak in the centre of the fibre, as theory reported. therefore, fibre fracture is expected if the fibre length is longer than the critical length. for fibres oriented perpendicular to the applied force, the strain is significantly smaller. in a next step, some comparable fibres were selected from the real structure. by using ct slice images of figure 7. fibre pull-outs followed by fibre fracture (l < lc). the last volume data set before fracture, it was possible to measure the fibres. to achieve the maximum stress in the fibre, the fibre length must be greater than the critical length lc. in figure 6a two fibres, that are smaller than the critical length, are presented. according to theory, fibre pull out and no fibre fracture is expected, as the maximum stress in the fibre is not reached. the sectional image in figure 6b shows a fibre which is slightly longer than the estimated critical fibre length of 1 070 µm and therefore fibre fracture can be observed. nevertheless, not all fibres smaller than the critical length pull-out. some of them also fracture in a previous load step (see figure 7). when scrolling 64 vol. 42/2023 defect formation in short glass fibre reinforced polymers figure 8. selected fibres showing fibre pull-outs and fibre fractures dependent on their length and neigbouring fibres(with overlayed max. principal strain distribution). through the ct data, it can be recognized, that the reason for this behaviour might be neighbouring fibres, influencing the local stress and strain behaviour and thus leading to fibre fracture. an further example is presented in figure 8, where the fibre is longer than the critical fibre length. in this example the fibre fractured at two positions. additionally, the strain distribution (correlating unloaded state with last load step before fracture) is shown. figure 8 reveals that these two fracture positions are exactly at areas where there are a lot of neighbouring fibre ends, thus influencing the local strain distribution. further, other defect types can be found in the tested samples, for example matrix fracture and fibre matrix debonding. some examples for the latter are presented in figure 9. there is no load transmission to the fibre by shear for fibres aligned perpendicular to the force. such cases can be treated as an interface crack between two dissimilar materials and thus might be to small to be quantified by ct. another reason, why no greater fibre-matrix debonding defect is revealed, are probably surrounding defects like matrix fractures absorbing the load (figure 9a). for curved fibres, the direction of load transmission changes (also due to neighbouring fibres) and again shear forces cause a fibre-matrix debonding (figure 9b). furthermore, heavily curved fibres could be identified, showing a lot of fibre fractures already in unloaded state and fibre-matrix debonding with increasing load (figure 9c). those defects are process induced and are critical with respect to their crack propagation potential. 4. conclusions simulation results of the strain distribution along selected fibres correspond with theory and the expected fibre fracture behaviour. nevertheless, the in situ experiments and a detailed look on various sectional figure 9. further examples for different fibre curvatures, revealing no fibre/matrix failure (a), fibrematrix debonding (b) and fibre fractures (c). images revealed that the fracture behaviour of individual fibres is strongly influenced by neighbouring fibres and the local strain distribution. the determination of the fibre length of the ten longest free fibres oriented in direction of the applied force, allows for the approximation of the critical fibre length and the interfacial shear strength, based on the kelly-tyson model. however, this method and the obtained values are not verified by other methods so far. further investigations should consider the influence of testing manner and sample geometry. list of symbols lc critical fibre length [µm] lp,max determined longest free fibre length (mean) [µm] d fibre diameter [µm] σfibre fibre strength [mpa] τfibre interfacial shear strength [mpa] references [1] j. kastner, c. heinzl. x-ray tomography. in i. nathan, n. meyendorf (eds.), handbook of advanced nondestructive evaluation, pp. 1095–1166. springer, 2019. 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[14] d. salaberger, m. jerabek, w. stockreiter. estimation of interfacial shear strength of long glass fibre composites by x-ray computed tomography. composites meet sustainability proceedings of the 20th european conference on composite materials, eccm20 lausanne, switzerland pp. 1146–1151, 2022. https:// doi.org/10.5075/epfl-298799_978-2-9701614-0-0 [15] s. fu, b. lauke, y. zhang, y.-w. mai. on the post-mortem fracture surface morphology of short fiber reinforced thermoplastics. composites: part a 36(7):987–994, 2005. https: //doi.org/10.1016/j.compositesa.2004.11.005 66 https://doi.org/10.1080/10589759.2022.2075865 https://doi.org/10.1007/bf02323555 https://doi.org/10.1016/j.mechmat.2022.104552 https://doi.org/10.1016/j.compstruct.2021.114775 https://doi.org/10.1016/j.compstruct.2021.114775 https://doi.org/10.1016/j.compositesa.2020.105935 https://doi.org/10.1016/j.compositesa.2020.105935 https://doi.org/10.1016/j.polymertesting.2009.06.006 https://doi.org/10.1016/j.polymertesting.2009.06.006 https://doi.org/10.1016/0022-5096(65)90035-9 https://doi.org/10.34726/hss.2019.64362 https://doi.org/10.21105/joss.01185 https://doi.org/10.5075/epfl-298799_978-2-9701614-0-0 https://doi.org/10.5075/epfl-298799_978-2-9701614-0-0 https://doi.org/10.5075/epfl-298799_978-2-9701614-0-0 https://doi.org/10.5075/epfl-298799_978-2-9701614-0-0 https://doi.org/10.1016/j.compositesa.2004.11.005 https://doi.org/10.1016/j.compositesa.2004.11.005 acta polytechnica ctu proceedings 42:60–65, 2023 1 introduction 2 materials and methods 3 results and discussion 4 conclusions list of symbols references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2023.43.0086 acta polytechnica ctu proceedings 43:86–92, 2023 © 2023 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague expert questionnaire on the issue of economical use of the railway traffic path capacity and buildings preparation on the railway pavel purkarta,b,∗, david vodáka,c a czech technical university in prague, faculty of transportation sciences, department of transportation systems, konviktská 20, 110 00 prague, czech republic b poved, s.r.o., nerudova 982/25, 301 00 pilsen, czech republic c správa železnice, s.o., dlážděná 1003/7, 110 00 prague, czech republic ∗ corresponding author: pavel.purkart@seznam.cz abstract. the railway traffic path capacity and the issue of railway buildings planning are topics that are widely discussed in professional groups. experts often do not the same opinions on these topics. the issue of the allocation of railway traffic path capacity is given in general legislation and further by regulations or directives of a particular railway operator. carriers are obliged to agree to these conditions. in the case of railway buildings planning, there are also comprehensive defined procedures and selected methods and models which are used to evaluate buildings planning (for example, a four-stage transport model or a logit model). however, it is appropriate to deal with the idea of whether all these procedures are correct and whether it is not appropriate to make changes, especially in the long-established procedures, that would better reflect the current and future reality and society needs. keywords: railway, railway traffic path capacity, buildings preparation, expert questionnaire. 1. introduction in order to find out the expert opinion on the topic of railway traffic path capacity allocation, especially in the case of overfullfilled infrastructure, and topic of buildings preparation on the railway, an extensive expert questionnaire was conducted in 2021. the questionnaire survey was made on the google forms platform. in total, it is a group of 28 interviewed experts from various organizations (railway design offices, infrastructure managers, transport organizers and public administration, academic staff). 2. materials and methods within the issue of railway traffic path capacity and buildings preparation on the railway, a total of 16 questions were asked to the respondents, namely: (1.) do you think that the solution of the railway traffic path capacity, the problem of its use and the methodology of its allocation is a current problem, which should be addressed and paid special attention to? (2.) do you consider that, when allocating railway traffic path capacity, passenger trains in the public service should take priority over open access services? (3.) do you think that, when allocating railway traffic path capacity, long-distance/express international trains should take priority over national trains? (4.) do you think that long-distance/express trains should take priority over regional trains when allocating railway traffic path capacity? (5.) do you think that, in the case of train route construction, passenger transport should take priority over freight transport? (6.) do you think that in the case of allocating railway traffic path capacity, the occupancy of passengers on individual passenger lines should be taken into account, or individual passenger services (especially in the case of overfullfillment of railway traffic path capacity)? (7.) do you think that when allocating railway traffic path capacity, the technical parameters of the vehicles intersecting with the infrastructure parameters should be taken into account, i.e. the possibility of using the infrastructure parameters (e.g. speed profiles and their levels) by the vehicles used for the given performances (especially in the case of overfullfillment of railway traffic path capacity)? (8.) do you think that the allocation of railway traffic path capacity in case when carriers requirements exceeding its capacity should have the possibility or obligation to propose solutions to satisfy them before rejecting the selected requirements (e.g. by passing selected stops on selected trains to ensure more capacity, connection of selected trains in the critical rails section or to propose the use of other vehicles on selected lines, etc.)? (9.) do you think that in the case of allocating railway traffic path capacity on the line for electric traction, 86 https://doi.org/10.14311/app.2023.43.0086 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 43/2023 questionnaire on the issue of economical use of the railway traffic path capacity vehicles that actually use this traction should be subsidized, or vehicles with modern traction drives (bemu, hemu/emu, etc.) should be subsidized? (10.) do you think that vehicles equipped with modern security systems (typically etcs and gsm-r) should be subsidized when allocating railway traffic path capacity? (11.) do you think that the state or the eu should positively motivate carriers and customers of public passenger transport services (eg subsidy programs) to renew the vehicles or improve its quality in favor of better and more meaningful use of railway capacity? (12.) regional and long-distance services are affected in the case of capacity allocation on one line. therefore, in order to construct routes for a timetable, long-distance transport routes may be artificially slowed down. in your opinion, what is the possible tolerable slowdown of long-distance trains due to the construction of regional transport routes? (13.) are you aware of any measures from abroad that address situations where there are more requirements for the allocation of railway traffic path capacity and this capacity is insufficient or actually saves railway capacity, even if it is not yet completely exhausted? can you briefly name these measures and what countries are involved? (14.) do you consider the current methodology of setting parameters (especially routing, decisionmaking new line x reconstruction of existing line, share of sections with new routing, number of tracks/frequency of passing points, etc.) of new railway infrastructure in the czech republic (feasibility study) to be correct? (15.) do you consider the current mechanism used to simulate transport demand in the feasibility studies (transport forecast – four-stage transport model, logit model) to be correct? (16.) do you consider the current mechanism used to assess economic efficiency (as part of the infrastructure decision-making process) in feasibility studies to be appropriate? the aim of the expert questionnaire was to identify areas and parameters which, in the opinion of experts, should be key in allocating railway traffic path capacity, especially in cases where it is necessary to emphasize its cost-effective use. this is especially the case when the capacity of the railway route is not able to satisfy all the requirements of the carriers imposed on it. at the same time, experts were asked questions concerning the preparation and design of railway lines reconstructions and new railway lines. question no. 1: do you think that the solution of the railway traffic path capacity, the problem of its use and the methodology of its allocation is a current problem, which should be addressed and paid special attention to? the vast majority of experts agree that solving this issue is very topical. respondents also generally point to the following issues: • deal with the quality of allocated traffic path capacity, • capacity of infrastructure points with low capacity, • a responsible allocation of railway traffic path capacity. question no. 2: do you consider that, when allocating railway traffic path capacity, passenger trains in the public service should take priority over open access services? respondents fairly strongly agree that trains running in a public service obligation should take priority over open access services, however, certain situations are possible that need to be considered carefully: • in the case of a concession model or open access products on meaningful and regular routes, an open access product can be an essential framework of the system in a given area and should therefore be treated more equally, • the key is to allocate capacity and design lines with an experience, ie regional or open access transport should not have excessive capacity allocation requirements so that it is ideally possible to offer routes to both segments. it is often necessary to seek individual solutions, but connections operated under a public service obligation generally show greater stability and predictability with regard to the requirements for their management over longer time horizons. question no. 3: do you think that, when allocating railway traffic path capacity, longdistance/express international trains should take priority over national trains? in the case of the third question, the opinion of experts is already more diverse, and therefore it is represented by a graphic output. it is clear that there is no longer such a consistent opinion as in the first two questions among experts, even though international routes are significantly supported by legislation. from the wording of the law, this is quite understandable, as the international route is naturally subject to the coordination of several entities (this is also pointed out by selected experts in the questionnaire survey), on the other hand the respondents to the questionnaire survey point out (although they rather agree that the internationality criterion is important): • international trains suffer more from traffic irregularities, • international trains in fact also serve national services; the question is whether the criterion of internationality is actually useful in this context, • the decisive factor could rather be the transport demand in individual segments or the capacity of 87 pavel purkart, david vodák acta polytechnica ctu proceedings figure 1. taking into account the occupancy of individual passenger lines. trains or the intentions of public service authorities and their intersections. the results of the answers to this question are summarized in figure 1. question no. 4: do you think that longdistance/express trains should take priority over regional trains when allocating railway traffic path capacity? the question again increases the heterogeneity of the answer of the interviewed experts, who in this case believe that long-distance/express international transport should rather take priority over international transport. in addition, the experts react freely in the comments to the question: • it is appropriate to segregate infrastructure for longdistance and regional transport, • the general opinion is “rather yes”, however, with the solution of specific situations, such as connections between trains, arrivals and departures to nodes, etc., • the organization of transport in the event of a delay remains an issue, • the issue is also the adequacy and transport strength of individual segments, which should be reflected in the construction of individual train routes, • there was also the opinion that, in contrast to longdistance transport, it is also possible to provide regional buses. question no. 5: do you think that, in the case of train route construction, passenger transport should take priority over freight transport? in this case, the prevailing view is that passenger transport should rather be preferred over freight transport. the comments of the experts point to the following facts: • suitability of infrastructure segregation for passenger and freight transport and the need to increase the capacity of sections where not all these requirements can be met, figure 2. taking into account the occupancy of individual passenger lines. • suitability of the priority for passenger transport in cases where it provides mainly transport to schools and jobs, ie especially during peak periods, • preference for freight transport on a given section, where in this case it clearly has a greater societal benefit than passenger transport on such a section of the railway network. question no. 6: do you think that in the case of allocating railway traffic path capacity, the occupancy of passengers on individual passenger lines should be taken into account, or individual passenger services (especially in the case of overfullfillment of railway traffic path capacity)? the question clearly deals with a topic that does not reflect any of the found legislative materials or regulations. it concerns the issue of passenger congestion on individual rail passenger lines, especially in the case of capacity allocation, where infrastructure congestion occurs. because this is a relatively complex issue, the grphic output provides a structure for all respondents to answer this question. despite the fact that four respondents out of 28 do not strongly support this and the other three also tend to disagree with this step, on the whole scale it can be perceived that this aspect should rather be taken into account. after all, 20 out of 28 experts (more than 70 % of the interviewed experts) stated that it is desirable to deal with this topic and pay attention to it. the results of the respondents’ answers to this question are summarized in the graph in figure 2. the open reactions of experts further refer to the following facts: • the public service authority itself should be able to evaluate what is expedient to provide and what is not so that the capacity is used economically, • an individual approach is often needed, • it is necessary to set the parameters of this evaluation, it would have to be a more comprehensive assessment, probably not the only one value of occupancy, 88 vol. 43/2023 questionnaire on the issue of economical use of the railway traffic path capacity • in some cases such an assessment may be difficult to apply – the difference between the occupancy of the whole train vs. normal occupancy by passengers, how will this be addressed? if such a principle should be applied, then all open reactions of respondents are in principle relevant and in order to take into account this parameters, it is useful to deal with their solution. in particular, setting only a one occupancy parameter within a line would be more than misleading. question no. 7: do you think that when allocating railway traffic path capacity, the technical parameters of the vehicles intersecting with the infrastructure parameters should be taken into account, i.e. the possibility of using the infrastructure parameters (e.g. speed profiles and their levels) by the vehicles used for the given performances (especially in the case of overfullfillment of railway traffic path capacity)? another aspect, which is not so fundamentally taken into account by legislation or regulations, is the possibility of taking into account the technical parameters of vehicles at the intersection with the condition and parameters of the infrastructure. the opinion of experts in this case is absolutely uniform: a significant inclination to take into account the technical parameters of the infrastructure at the intersection with the technical parameters of the used vehicles. the comments point to the following facts: • motivate the carrier/public service transport authority by adjusting the price for the use of the track by running the train in case of using adequate vehicles, • attention is drawn to the homogeneity of routes, where this approach would in some cases not necessarily lead to the desired effect of economical consumption of capacity, • the technical capacity requirements of the vehicles also depend on the category of trains on which they are operated. question no. 8: do you think that the allocation of railway traffic path capacity in case when carriers requirements exceeding its capacity should have the possibility or obligation to propose solutions to satisfy them before rejecting the selected requirements (e.g. by passing selected stops on selected trains to ensure more capacity, connection of selected trains in the critical rails section or to propose the use of other vehicles on selected lines, etc.)? another question asks about the procedure, which is in principle given by the legislation, and that is the situation in which, in case of exceeding the capacity of the railwy, the infrastructure manager should propose measures to satisfy most of the requirements. the response to this question is also positive – there is a opinion that it is definitely yes or rather yes. the comments further mention the following facts: • it does not have to be strictly an obligation, but a possibility, • the result should be to find a suitable compromise, • the infrastructure manager should do this, but the final decision must be made by the client, • this procedure should be completely natural, as only the infrastructure manager has complete information on the construction of routes on the given section, which other entities do not have. question no. 9: do you think that in the case of allocating railway traffic path capacity on the line for electric traction, vehicles that actually use this traction should be subsidized, or vehicles with modern traction drives (bemu, hemu/emu, etc.) should be subsidized? the aim of the question was to find out whether, in the case of allocating the railway traffic path capacity, vehicles with ecological and modern drives should also be preferred. it can be summarized that the experts are rather positive about this possibility, they probably evaluate it as a possibility of a meaningful renewal of the vehicle fleet. respondents also pointed out the following aspects in their open comments: • the idea of a bonus for a vehicle performance rather than specific traction, • the need for a careful assessment in the event of a train (line) switching, eg from an electrified to a non-electrified line – in the case of a meaningful transport concept, the capacity consumer should not be penalized for this approach, • theoretically it does not have to be a way of bonus, but a penalty, • can be taken into account in the price for the use of the traffic path, • it can be a positive motivation to increase the energy efficiency of vehicles. question no. 10: do you think that vehicles equipped with modern security systems (typically etcs and gsm-r) should be subsidized when allocating railway traffic path capacity? the mission of the question is to find out the respondents’ opinion on the bonus of vehicles that are equipped with devices for the operation of modern security systems, especially gsm-r and etcs. by its nature, it is also a rather complementary issue in the issue of railway traffic path capacity. the attitude of the respondents in this case is again rather positive – positive opinions inclined to bonuses prevail over negative ones. question no. 11: do you think that the state or the eu should positively motivate carriers and customers of public passenger transport services (eg subsidy programs) to renew the 89 pavel purkart, david vodák acta polytechnica ctu proceedings vehicles or improve its quality in favor of better and more meaningful use of railway capacity? respondents are strongly inclined to support vehicle fleet renewal in order to use the infrastructure parameters better. the following facts are pointed out in the open comments: • long-term plans need to be truly valid and truly based on, • this topic must be carefully defined, • in this case, it may be appropriate to further improve communication between the various actors in order to make investments on all sides more meaningful, • in the case of negative answers (only four out of 28 respondents), the idea is not to support vehicle subsidies completely or it is difficult to define subsidy rules from the point of view of this issue. question no. 12: regional and long-distance services are affected in the case of capacity allocation on one line. therefore, in order to construct routes for a timetable, long-distance transport routes may be artificially slowed down. in your opinion, what is the possible tolerable slowdown of long-distance trains due to the construction of regional transport routes? when constructing long-distance and regional transport routes, it is clear that the requirements interact. and not only by connections at selected railway stations and nodes, but also when crossing these connections and in this case also when overtaking them. if the infrastructure is in ideal condition and meets all requirements, then it is possible to adapt it so that the mutual influence during crossing and overtaking should be completely minimized or eliminated. the mutual delay of these actions should be close to zero. therefore, in order to construct a timetable, longdistance transport routes may be artificially slowed down. then there is the question of what is the possible tolerable slowdown of long-distance trains due to the construction of regional transport routes. the results of the respondents’ answers to this question are summarized in the graph in figure 3. although the question was formulated a bit more complicatedly than the others, the reactions of the respondents again give a relatively clear opinion on this issue. experts believe that due to the construction of regional transport routes, an additional charge on the travel time of long-distance trains of up to 5 %, resp. up to 10 %. other answers were chosen by a very small number of interviewed experts. respondents further state that segments can naturally affect each other, the individuality of the approach is also important and this approach must not “break down” the system as a whole. regional and long-distance transport systems are interconnected and must cooperate. however, the additional chargé (its amount globally) can be difficult to determine, as some opinions point out. however, there is also a very clear idea that quality buildings preparation and compliance of the timetable according to the proposed model in the project should work in such a way that such additional charges should ideally not be necessary. question no. 13: are you aware of any measures from abroad that address situations where there are more requirements for the allocation of railway traffic path capacity and this capacity is insufficient or actually saves railway capacity, even if it is not yet completely exhausted? can you briefly name these measures and what countries are involved? question no. 14: do you consider the current methodology of setting parameters (especially routing, decision-making new line x reconstruction of existing line, share of sections with new routing, number of tracks/frequency of passing points, etc.) of new railway infrastructure in the czech republic (feasibility study) to be correct? there is a very slight inclination to the opinion that the current methodology of setting parameters of new railway infrastructure in czech republic is correct. the following facts are pointed out in the comments: • deformations occur, e.g. political pressures, • experience varies from building to building, • it depends a lot on the abilities of evaluators and processors, • the database is often the problem; some data are hardly available, others work with imprecise estimates, • the current methodology partially denies "healthy" generosity and does not allow the infrastructure to be dimensioned so that it is sufficiently robust when needed during the horizon of its use. in the case of this question, it is a question with a rather significant contradiction of opinions. suggestions for improving the method are given for both positive and negative opinions, although the positive opinion actually prevails very slightly. however, it is clear that almost no one perceives the method as “all-salvage”. negative or rather negative opinions are evident in all professional groups, so it is not even possible to generalize that some of these groups are more critical. question no. 15: do you consider the current mechanism used to simulate transport demand in the feasibility studies (transport forecast – four-stage transport model, logit model) to be correct? there is a slight inclination to the opinion that the current methodology used for modeling transport demand within the framework of feasibility studies (transport forecast – four-stage transport model, logit model) is correct. however, it is also necessary to state 90 vol. 43/2023 questionnaire on the issue of economical use of the railway traffic path capacity figure 3. influencing regional and long-distance transport by the construction of routes. that a relatively significant part of the respondents did not know or had no opinion on this question (almost 40 %) if this part were excluded from the evaluation, then the positive opinion is quite obvious. the following facts are pointed out in the comments: • the need to carry out careful calibration and verification of the model, • the necessity to work with longer historical time series, • better take social trends into account, • it may involve working with inaccurate numbers. rather negative answers to this question – 3 in total – were identified in the case of a representatives of the railway operator, public transportation organizers and the academic sphere. there is not a greater number of negative answers in any of the groups. question no. 16: do you consider the current mechanism used to assess economic efficiency (as part of the infrastructure decision-making process) in feasibility studies to be appropriate? there is a very slight tendency to believe that the current mechanism used for evaluating economic efficiency (as part of the decision-making process on the final design of infrastructure) within feasibility studies is correct. the following facts are pointed out in the comments: • the economic assessment should be expanded to include other aspects, some are not affected at all, some are, on the contrary, overestimated; the methodology needs to be constantly improved, • it is a complex process, where the problem can be seen in its deformations and difficult applicability to certain situations, • very much depends on the ability of the submitter, • perhaps a tool enabling more dynamic network development would benefit from the evaluation. rather negative answers to this question appeared in practically all interviewed groups, however, the greatest disagreement with the incorrectness of the current procedure can be seen among representatives of the academic sphere. 3. discussion and conclusion thanks to a sample of 28 experts in this field, who participated in its completion, the expert questionnaire provided a comprehensive view of this relatively complex and discussed issue. given that the respondent was given the opportunity to specify their choice more specifically with an open answer to each question, this questionnaire provided a wide range of opinions. practically by all questions have been shown (to a greater or lesser extent) to be a matter for reflection among experts, and in addition to closed answers, they have often provided an opinion on what should be improved or done differently. the results of the questionnaire clearly have shown the need to address the issue. from the opinion of experts on the issue, it turns out that in the cooperation of individual segments of public transport and the allocation of railway capacity (especially in the case where the infrastructure shows congestion) and the solution of railway construction preparation, it is appropriate to generally take into account: • consider the intersection of technical parameters of vehicles and infrastructure; vehicles with better technical parameters should generally be able to consume the railway traffic path capacity more 91 pavel purkart, david vodák acta polytechnica ctu proceedings economically. this situation can be supported by appropriate subsidies for such vehicles, including alternative propulsion. • consider the occupancy of individual lines by passengers; however, this parameter must be chosen very sensitively and carefully. probably it should not be a single number, but multiple occupancy parameters. it is also necessary to realize that the large capacity of a train in the selected section does not necessarily mean a large occupancy of passengers. • the regional and long-distance transport segments should be affected as little as possible; ideally, in congested sections, everyone should have their own infrastructure. • the criterion of the internationality of the route is to a large extent relevant, however, it is necessary to take into account the importance of the line as a whole (its passage through the territory and the guaranteed connections to it). • overall, public rail transport services should take priority over other products, however, even open access products may show signs of systemicity. acknowledgements the contribution was made with the support of the student grant competition of the ctu in prague within the framework of project no. sgs20/138/ohk2/2t/16 determination and optimal use of railway transport route parameters. 92 acta polytechnica ctu proceedings 43:86–92, 2023 1 introduction 2 materials and methods 3 discussion and conclusion acknowledgements acta polytechnica ctu proceedings https://doi.org/10.14311/app.2023.45.0066 acta polytechnica ctu proceedings 45:66–69, 2023 © 2023 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague earth structures – a summary of the last 50 years ivan vaníček czech technical university in prague, faculty of civil engineering, department of geotechnics, thákurova 7, 166 29 prague, czech republic correspondence: ivan.vanicek@cvut.cz abstract. this paper discusses changes and developments in the field of earth structures that have been presented and discussed over the last fifty years at the annual national conferences on foundation engineering held (with foreign participation) in the city of brno. keywords: earth structures, hydro engineering, environmental engineering. 1. introduction the modern view of the construction of earth structures in the czech republic began in the late 1930s. after a certain pause, the situation took to new tracks in the 1960s and connected with the beginning of the traditional regular staging of the foundation engineering brno conferences. the paper first summarizes the decisive steps in the field of soil mechanics applicable in general. from the point of view of various earth structures, attention is focused on earth structures of transport, hydro and environmental structures. the emphasis is mainly on the most common problems with which these constructions are connected. the development of soil mechanics in the czech republic can be associated with the end of the 1930s, when the then dr. ing. a. myslivec returned from an internship abroad in holland, and was dedicated to the development of soil mechanics in its theoretical domain, testing, and the subsequent practical application of these findings to constructions, nowadays referred to as geotechnical structures [1]. from the point of view of water structures, after the breach of the bílá desná dam in 1916, the construction of bulk dams were resumed, especially in the zlín region, e.g. on the fryštátské stream. however, a much more significant application of soil mechanics occurred in connection with the design and construction of the highway in the czech republic: prague – zlín – the slovak border with a total length of 397 km. see the government regulation of 23 december 1938. in an attempt to have the highway in operation within 4 years, matters were entrusted to a newly created general directorate of highway construction, and scientific institutes and professional associations of all possible types related to the construction of the highway were also brought into cooperation by this directorate. for example, the director general requested cooperation with the state institute of geology and with professors of geology at universities. prof. kettner volunteered to conduct a survey for the prague circuit and parts of the track to sázava. from the point of view of soil mechanics and earth structures, the cooperation of the state hydrological and hydrotechnical institute was requested, and this was where ing. dr. a. myslivec developed a detailed plan for examining the soil and utilizing the results of laboratory tests. the details of the development of the task are evidenced by the fact that at the end of august 1939, 7 field laboratories were already in operation, each for a section of 40–50 km and with the task of: a) determining the composition and properties of soils – in the base of embankments, in cuts and embankment material using index tests. b) determining the level of ground/underground water and the aquifer. c) controlling earthworks, especially the compaction of embankments. d) observing and measuring completed earthworks. the schedule went into such details as the number of probes, the number of samples and their frequency, and respectively with an estimation of the time required (e.g. for a 5 km section 3–6 weeks). undisturbed soil samples were transported to the prague laboratory for compressibility, shear strength and permeability tests. “the result of the laboratory’s activity was a report in which the slopes in the embankments and cuts, the method of compacting the embankments, various safety measures, especially against settlement, sliding, the harmful effects of frost, etc. are proposed,” as stated in the special print from the journal of the association of engineers “technický obzor” [2]. reading the entire printout it is worth paying attention to, from the point of view of the logic of the procedure, the speed and personal responsibility for partial decisions and steps in solving a demanding technical task, in this case the realization of 397 km of highway (including the prague ring road) from the initial start of its commissioning (in unimaginable times for us today), in only 4 years. for this reason could then prime minister ing. alois eliáš state in this publication that at the end of august 1939 the project for 200 km was completed and 77 km more 66 https://doi.org/10.14311/app.2023.45.0066 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 45/2023 earth structures summary of this highway was already under construction and preparation. however, the 2nd world war meant that all work was interrupted and also tragically pm a. eliáš was executed. during the 1960s, construction activity returned, both in the form of earth and rockfill dams, highways, but also earth structures of environmental engineering such as landfills, tailing dams, and spoil heaps. at the same time, laboratory instrumentation was beginning to be improved, enabling the implementation of more demanding laboratory tests, especially of mechanical and physical properties. 2. theoretical knowledge and experience the first decades of the conferences on foundation engineering were focused on the theoretical area relating to the principle of effective stresses, and thus on the measurement of pore pressures in the framework of triaxial tests, in accordance with the findings of bishop and skempton [3, 4], including the determination of pore pressure coefficients for the subsequent evaluation of pore pressures in earth structures due to changes in stresses. after emphasizing the importance of different stress paths [5], which are typical for individual geotechnical structures, triaxial tests were carried out under different stress paths, including drained triaxial tensile tests. then subsequently also for differentiating the pore pressures of water and air in unsaturated soils [6]. various methods of determining the residual strength of soils were verified. from the point of view of compressibility, attention was also focused on tests with continuous loading, or tests where constant stress is maintained for a longer period of time with subsequent slow load increase, making it easier to define the stress, after any exceeding of which soil deformation increases again [7]. the deformation characteristics determined using the triaxial test clearly help to define these characteristics under drained and undrained conditions. the filtration properties, especially of low permeable clays, were tested in a triaxial with back pressure to eliminate air in the pores. the measurement also focused on extremely low permeable soils – montmorillonites/bentonites, including indirect determination using the consolidation coefficient and deformation modulus measured during an oedometric test with a very slow load increase. in recent decades, more attention has been paid to tests with cyclic loading, including the possibility of using the propagation of seismic waves to evaluate soil properties. scanning microscopes enable a better understanding of the importance of microstructure on the behaviour of soils. direct determination of mechanical-physical properties in the field appears rather cyclically, for example shear tests on a block of semi-broken bedrock, but exploratory field methods are still not as common in the czech republic as they are abroad. 3. earth structures in transport engineering attention in the czech republic is mostly focused on road and railway constructions. less attention has been paid to airports and ports, even though the findings from the construction of airports on the seabed (e.g. the case of osaka) brought a lot of new knowledge and experience. the return to the construction of highways and the subsequent reconstruction of the railway network could thus already respect the ultimate limit states and serviceability limit states under shortterm/undrained and long-term/drained conditions and predict initial and total settlement, respectively short-term stability (mostly decisive for the stability of embankments) and long-term stability (mostly decisive for the stability of cuts). related to this is the evaluation of the speed of consolidation of the embankment subsoil, including the possibilities of its acceleration, where great attention is paid to the theory and application of vertical geodrains [8]. the application of various geosynthetics at the end of the 1970s, capable of performing various functions, brought many possibilities. the reinforcement function is thus used to increase the steepness of embankment slopes, and for the possibility of using less obviously suitable soils. attention focused both on defining the properties of reinforcing geosynthetics, where creep plays an important role, and also on calculation methods to include the significant tensile strength of geosynthetics in slope stability calculations [9]. reinforcing geosynthetics also enable the design of spreading platforms for the foundation of embankments even on highly compressible subsoil. the need for greater use of even less suitable soils leads to the development of methods for their improvement (e.g., van impe [10]), where lime stabilization for clayey soils plays an important role. a smaller volume of lime improves workability, and better compaction of even moist soils; a larger percentage of lime then also brings long-term strength improvement. however, it is necessary to control the possibility of the mineral ettringite occurrence, which can cause subsequent swelling. from the point of view of monitoring the execution of earthworks, the control of compaction using rollers marked as ccc – continuous compaction control is of great benefit, when the response sensors of the compacted subsoil can be used to assess the state of surface compaction, and thus not only obtain information about the optimal number of roller passes, but also ensure a more even surface compaction (and thus the deformation) of the earth’s plains (e.g. brand and kopf [11]). 67 ivan vaníček acta polytechnica ctu proceedings 4. earth structures in hydro engineering earth structures in water engineering differ mainly from the point of view of the frequency of changes in the level of retained water. the selection of various design situations and the verification of limit states for them is therefore one of the basic prerequisites for a successful design and safety operation [12]. for large embankment dams, we can speak of the relative constancy of this level, and thus also of smaller fluctuations in the seepage curve and associated pore pressures. since the 1970s, increased attention has been paid to the failure of large rockfill dams with clay core caused by hydraulic seepage along a preferential path, which is most often a tensile crack. in this way, the probability of the occurrence of tensile cracks is monitored, which can be caused not only by uneven deformation, but also by hydraulic fracturing. attention is thus focused on tensile strength, elongation at break, both under undrained and drained conditions [8]. this is not only for the needs of numerical modelling of stress changes in the body of the dam, but also for the subsequent assessment of the probability of the occurrence of a crack. if a crack should occur, two aspects are monitored, the swelling ability aimed at healing the crack, or sensitivity to erosion, where dispersive soils in particular are extremely dangerous from this perspective. the final design of the transverse profile is therefore more influenced by the above-mentioned aspects than simply by the stability of the slope. in the last two decades, attention has been more focused on historical pond dams in connection with floods, generally with the need for their reconstruction to meet new conditions, caused primarily by a more significant load on their crown than in the middle ages and thus also the contact of the old wooden foundations outlet with the surrounding soil during this load. the floods have shown that a greater number of failures can be attributed to the surface erosion when the crown of the dam has overflowed. at the same time, the probability of overtopping is much higher where the dams are located close to each other, so that the additional wave from the ruptured upper dam will cause a domino effect of failure for the lower ones as well. considering the high number of historical dams in the czech republic, about 25 000 in all, this risk is relatively high, and therefore it is necessary to assess the safety of dams not only individually, but also in relation to other dams in the same basin. similar problems of dam failures have drawn attention to flood protection levees along rivers. 5. earth structures in environmental engineering spoil heaps composed from tertiary clays which overlay brown coal seams in northern bohemia are the focus of attention for practically the entire period of the foundation engineering brno conferences. first this was from the point of view of the stability of these spoil heaps (including the slopes of surface mines), but today it involves more a looking at the possibility of usage of their surface for new construction. attention was focused not only on measuring the properties and change of excavated clay lumps character, typically from the rockfill up to soft clay, when the slope of the spoil heap could be very steep at the beginning and extremely gentle with time. care was given to the deformation and settlement of high spoil heaps, especially in the case when the transport infrastructure was situated on their surface, as in the case of the ervěnice corridor [8]. today, these experiences are also used when new structures are founded on the surface of spoil heaps, but failures are also associated with larger interventions into the spoil heap body, e.g. for new communication cuts. attention to the safe construction of municipal waste landfills began in the early 1990s, both from the point of view of their stability and deformation, and also from the angle of the safe design of the sealing system in relation to potential contamination of the ground. today, practically every district has a safe landfill, and the number of new ones is significantly reduced. tailing dams were at the centre of attention in the very first decades, whether they were tailings for ash or residues after ore treatment. now the emphasis is on their maintenance, or about modifications to a more environmentally favourable state – especially for tailings from uranium ore processing plants. 6. earth structures and current trends over time, there is an observable tendency towards the realization of higher earth structures, communication embankments, cuts, dams, landfills and thus a greater transition from the use of analytical calculation methods to numerical ones (e.g., potts and zdravkovič [13]). when applying more general constitutive models than elastic or plastic ones, the requirements for tests under more general stress paths simultaneously increase. the transition to european standards – eurocodes – puts more emphasis on the verification of all potential limit states under all conceivable design situations. this means also a greater link between the complexity of the structure and the complexity of the rock environment, and the design approach applied to verify the limit states. the application of the principle of sustainable construction accepts the principle that a technical solution is a necessary but not a sufficient condition. in accordance with this principle, increasing attention is being paid to saving energy (co2 footprint), land (reducing land use), reducing the consumption of natural aggregates and replacing them with alternatives [14]. the emphasis on digitization is primarily connected with the application of the bim principle – build68 vol. 45/2023 earth structures summary ing information modelling. here, the connected 3d model of the structure itself (superstructure) and the bedrock (substructure) should be considered as the basis. the bim model has been gradually refined for the individual phases of construction design and geotechnical investigation, which significantly contributes to the cooperation of construction process partners in finding the optimal solution [14]. references [1] a. myslivec. soil mechanics. [in czech – mechanika zemin]. textbook, ctu prague, 1952. [2] construction of czech highways, part i. [in czech – stavba českých dálnic, část i]. technický obzor xlvii:22-23, 1939. [3] a. w. bishop, d. j. henkel. the measurement of soil properties in the triaxial test. edward arnold publishers, 1962. [4] a. skempton. the pore-pressure coefficients a and b. geotechnique 4(4):143–147, 1954. [5] t. w. lambe, r. v. whitman. soil mechanics. john wiley & sons, 1969. [6] d. g. fredlund, h. rahardjo, m. d. fredlund. unsaturated soil mechanics in engineering practice. john wiley & sons, inc., 2012. https://doi.org/10.1002/9781118280492 [7] j. havlíček. foundation settlement. [in czech – sedání staveb]. final report of the research project c52-347-018 praha 1978. [8] i. vaníček, m. vaníček. earth structures: in transport, water and environmental engineering, vol. 4. springer science & business media, 2008. [9] m. vaníček. limit design approach of the reinforced soils. acta polytechnica 40(2), 2000. [10] w. f. van impe. soil improvement techniques and their evolution. a. a. balkema, rotterdam, netherlands; brookfield, vt, usa, 1989. [11] h. brandl, f. kopf, d. adam d. continuous compaction control (ccc) with differently excited dynamic rollers. bundesministerium fur vit. strassenforschung heft 553, wien, 2005. [12] m. wieland, q. ren, j. s. tan (editors). new developments in dam engineering: proceedings of the 4th international conference on dam engineering, 18-20 october, nanjing, china. crc press, 2004. [13] d. m. potts, l. zdravkovič. finite element analysis in geotechnical engineering: volume two application. thomas telford publishing, 1999. [14] i. vaníček, d. jirásko, m. vaníček. modern earth structures for transport engineering. crc press, 2020. https://doi.org/10.1201/9780429263668 69 https://doi.org/10.1002/9781118280492 https://doi.org/10.1201/9780429263668 acta polytechnica ctu proceedings 45:1–4, 2023 1 introduction 2 theoretical knowledge and experience 3 earth structures in transport engineering 4 earth structures in hydro engineering 5 earth structures in environmental engineering 6 earth structures and current trends references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2024.50.0001 acta polytechnica ctu proceedings 50:1–6, 2024 © 2024 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague modulus estimation of polymers via nanoindentation – impact of surface roughness, peak force and testing speed michael huszara,∗, gernot oreskib, florian arbeitera a montanuniversität leoben, chair of materials science and testing of polymers, otto-glöckel-straße 2, 8700 leoben, austria b polymer competence center leoben, sauraugasse 1, 8700 leoben, austria ∗ corresponding author: michael.huszar@unileoben.ac.at abstract. nanoindentation is widely used to study small-scale structures in materials. the impact of varying testing conditions on the measurement’s accuracy is of high interest, and their influence on the characterization of polymeric materials is still quite scarce. therefore, this study investigates varying loading and unloading rates paired with maximum loads ranging from 0.5 to 20mn on polyethylene samples with varying surface finishes after wet grinding. the indentation data was analyzed using the oliver & pharr method, and the resulting modulus was compared with those obtained from macroscopical tensile tests. the hardness was compared to shore hardness measurements. a correlation between the measured modulus values and the surface quality was explored. the results prove the already established rule that the best surface finish (sa = 0.07µm) leads to the smallest standard deviation. however, they show that the modulus characterization is less influenced by surface roughness than hardness evaluations. keywords: polymer, nanoindentation, roughness. 1. introduction nanoindentation did prove itself a valuable measuring technique for the characterization of localized mechanical properties on a micro-scale [1]. recently, it has started to become more interesting for the field of non-linear elastic, or even visco-elastic materials [2]. one of this fields is polymer science, where foils or products with many different layers are often used. so far, these materials are most often only tested on a macroscopical scale as a whole and only limited attention is paid to the exact mechanical properties at the interfaces [3]. to successfully apply nanoindentation to polymers, several adaptations had to be introduced [4–7]. the time-dependent behavior of the materials has proven especially difficult in the past. nevertheless, adhering to established testing routines, it is possible to accurately describe the mechanical properties of polymers on a small scale, e.g. mechanical properties resulting from small changes of local morphological properties, curing state of elastomers, or localized, time dependent behavior [8–12]. recent studies have demonstrated the potential of nanoindentation to be used to characterize aging of polymers, which is a highly localized phenomenon [13, 14]. however, to explore localized processes while ensuring the reliability of the data obtained, it is essential to minimize possible sources of errors. previous research on hard materials shows that the surface quality of the examined samples greatly influences results of nanoindentation [15, 16]. however, these studies focus on the hardness of metals, which exhibit different mechanical behavior compared to softer materials, such as polymers. therefore, the influence of roughness on nanoindentation of visco-elastic materials remains a question to be addressed. the most commonly used method to characterize the surface quality from an engineering perspective is its roughness. this study uses the roughness parameter sa to describe the different surface finishes. the focus is put on the influence of sa on the resulting quality of nanoindentation curves and evaluated values (modulus and hardness) of a viscoelastic material. due to the viscous nature of the tested materials, the applied maximum force and loading rate were varied to check the impact on the quality of the results as well. 2. materials and methods 2.1. material and sample preparation a high-density polyethylene (pe) plate, as is often used in different polymer industries, was used in this study. samples were cut out from this plate and wetground on a struers labopol-30 with a laboforce-100 specimen mover (struers, denmark). 600, 1200, 2500, and 4000 sandpaper grits with water as lubricant and cooling agent were used for the grinding steps. one sample was polished with 3µm diamond paste (diamaxx poly, 3 µm, akasel, denmark) together with a lubricant for soft materials (aka-lube red, akasel, denmark). and end-polishing was done with a fumed silica op-s solution (fumed silica suspension 0.2µm water free, akasel, denmark) and water. 1 https://doi.org/10.14311/app.2024.50.0001 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en m. huszar, g. oreski, f. arbeiter acta polytechnica ctu proceedings sample finish ra [µm] rz [µm] sa [µm] sz [µm] str spc [l mm−1] sandpaper 600 1.51 14.7 1.62 25.4 0.15 3 969.49 sandpaper 2500 0.70 7.0 0.72 13.19 0.63 2 390.63 sandpaper 4000 0.08 1.0 0.08 2.93 0.06 1590.87 op-s 0.05 0.8 0.07 8.3 0.31 92.89 table 1. results of the roughness measurements via confocal laser microscope. to obtain parallel surfaces necessary for the indentation tests, a custom-made sample holder was used. the samples were glued to the holder with a commercially available ethyl-cyanoacrylate superglue (uhu gmbh & co. kg, germany, sekundenkleber, flüssig). nanoindentation measurements were performed on samples with last steps of 600, 2500, 4000, and endpolished finish. the surface roughness was measured on an area of 0.9mm2 using a confocal laser scanning microscope vhk-1000 (keyence, belgium). to describe the height differences of the surface sa (arithmetic mean) and sz (maximum height) are used. for surface morphology, str (texture aspect ratio) is used to characterize isotropy, and spc (arithmetic mean peak curvature) is determined to distinguish the “sharpness” of peaks. a surface with uniform roughness in all directions will have a str near 1, while a surface with directional roughness will result in a value near 0. sharp peaks will result in a high spc value, while a low value means more rounded, flat peaks. additionally, within this area 10 profile lines were measured for easy comparison with the commonly used roughness values ra and rz. 2.2. nanoindentation and evaluation following the oliver pharr approach the nanoindentation measurements were performed on a unht3 nanoindenter (anton paar, graz austria). subsequent evaluation was done with the software indentation ver.: 8.0.24 (anton paar, graz austria). a berkovich indenter was used for all experiments. the maximum loads (fmax) that were used during indentations were 0.5, 1, 5, 10, 15 and 20mn. for each load level, five indentations were made. to avoid possible interactions between indentation sites, a distance of 250µm was used. the indentation was performed with a loading speed of 2 times fmax per minute, a holding period of 10 s at fmax and subsequent unloading with the same speed as the loading step. the holding period is introduced to decrease the influence of visco-elastic creep on the results [17]. the unloading curve was fitted between 98% and 40% of fmax to determine the modulus. the upper and lower bounds were empirically determined in order for the tangent to describe the actual unloading curve best. the evaluation subsequently followed the method devised by oliver and pharr [1]. the obtained reduced modulus (er) is proportional to the young´s modulus (e) of the indenter (eind) and of the sample material (in our case ep e). the indenter properties are the modulus and poisson´s ratio of diamond (eind = 1141gpa; νind = 0.07), and for the poisson´s ratio of pe, νp e = 0.4 [18] was used. hardness (h) is calculated using the projected area of the indenter at the contact indentation depth divided by the maximum load fmax [1]. since the hardness of polymers is typically measured using a shore durometer [19], a direct comparison of the obtained values is not possible. to achieve at least some level of comparability, the spring constant of the durometer used (zwick, germany) was measured, and an area function of the indenter was determined. the standard shore d indenter features a cone-shaped tip with a 30° opening angle, and a tip radius of 0.1mm, mounted on a cylinder with a diameter of 1.25mm. shore hardness is defined in a way that 100 shore d corresponds to no penetration and 0 shore d corresponds to no resistance, meaning the full 2.5mm length of the indenter is pressed into the sample. using this, the indentation depth can be calculated, and the measured spring constant of 16.77nmm−1 allows the evaluation of the load at this depth to be used to calculate the hardness following [1]. 3. results and discussion the results from the roughness measurements are shown in table 1. for comparison, typical evaluations via line scans (ra and rz) and area scans (sa, sz, str and spc) were used. sa and sz can be compared to indentation depth. since sz is the maximum height difference of the complete measured surface, which is much larger than the actual indentation site, it is more likely to have a height difference close to the value of sa at the indentation area. therefore, comparisons focus on sa in all further analysis. the scratches caused by the grinding of the samples led to an anisotropic surface structure. this anisotropy is especially distinct for the sample finished with the sandpaper 4000. as can be seen in table 1, the sample shows a value of sa = 0.08µm while str is close to zero (0.06). keeping the same indentation time settings is crucial for comparable results with different maximum loads on viscoelastic materials and their timedependent material response. this results in different loading and unloading speeds relative to the maximum load used. in our case, speeds were set at 2fmax min−1. figure 1 shows four indentation curves 2 vol. 50/2024 modulus estimation of polymers via nanoindentation. . . (a). indentation curves with different fmax and loading and unloading speeds. (b). the corresponding creep curves from the holding period at fmax. figure 1. comparison of material response due to different fmax and loading and unloading speeds of 2fmax min−1 on the sa = 0.07 µm sample. of the sa = 0.07µm sample with different maximum loads, loading/unloading speeds, and a small “creep” analysis via a 10 s holding time. as shown in figure 1a, the loading curves of all loading speeds are almost identical. figure 1b shows the normalized indentation depth as a function of time for all applied load levels. similarly, there is no evident difference between the curves. while a holding segment of 10 s is insufficient for a full creep analysis, it can be used to see large deviations in the deformation and showcase different responses to the previous loading speed. these curves indicate that there is no substantial difference in the time depending material response, and therefore, the evaluations are comparable. furthermore, the absence of a “nose” (increase of deformation depth during unloading due to viscous material effects [20]) implies that the 10 s holding time is sufficient for a precise analysis of pe samples. after addressing the effects of loading level and speeds, the impact of surface quality was examined. figure 2 shows a representative indentation curve of the 20mn and 5mn fmax indentations for each surface quality. these indentation curves show that the slope of the load and unload process is strongly affected by the sur(a). maximum load of 20mn. (b). maximum load of 5mn. figure 2. indentation curves on the different surface finishes. face roughness. this is especially visible for the sample with the highest surface roughness of sa = 1.62µm. only one successful indentation with fmax of 20mn maximum load was possible for this sample. furthermore, it shows an unconventional loading segment. the indentation curve features a long plateau, and no rising load is detected. this can indicate a badly fixed sample or indenter, and the contact point must be manually adjusted. this was done by setting the contact point after establishing a clear contact and a visible, stable growth of the indentation depth in the measured curve. this curve was evaluated for e and h before and after manually setting the contact point to compare to the other results. besides this faulty measurement, the influence of roughness can be seen clearly in the result from the same surface with fmax = 5mn (figure 2b). there are two different slopes visible in the loading regime. the region of the first slope can be affiliated with the bending and slipping of the top of the rough surface when it comes into contact with the indenter, while the later slope starts when there is more contact with the bulk material. the same behavior can also be noticed with the sa = 0,72µm surface indentation curves. evaluation of these curves leads to “wrong” indentation depths, resulting in high standard deviations for the calculated e and h values, as shown in figure 3. in contrast, there is a nearly perfect over3 m. huszar, g. oreski, f. arbeiter acta polytechnica ctu proceedings (a). the calculated values of ep e compared to the young’s modulus from tensile tests. (b). h compared to a converted shore hardness result. figure 3. dependency of indentation results on fmax and sa. lapping of the two curves of the smooth surfaces using the 20mn maximum load and very low deviation at the 5mn maximum load. based on all performed measurements, the calculated values of e and h are compared in figure 3. for comparison of the results, the young´s modulus of 1.14gpa, measured in uniaxial tensile tests in a previous research [18] and the shore hardness measured following iso 48-4 resulting in 55.4 shore d are used. the shore d result was converted, following the procedure explained in section 2.2 to hp e−shore = 49mpa. the results for hardness and young´s modulus follow the same trend with the two smooth surfaces, sa = 0.07 µm (ep e = 1.17gpa; hp e = 49mpa) and sa = 0.08 µm (ep e = 1.20gpa; hp e = 46mpa), having the lowest standard deviation and the best agreement with the macroscopic measured values. only at the lowest value of fmax = 0.5mn slight differences are detectable between these surface qualities. this deviation can most likely be attributed to the differences in spc, and the smoother peaks of the polished surface. this explains why the sa = 0.08µm surface is measured more compliant with the small maximum load. the sa = 0.72 surface finish shows only good agreement for ep e at a maximum load of > 10mn, with a generally lower hp e of around 40mpa. the figure 4. the ratio of sa to the measured maximum indentation depth for the different surface finishes and maximum loads. roughest surface in this study (sa = 1.62µm) shows overall lower values for ep e and hp e with a high standard deviation. only the corrected contact point at fmax = 20mn, resulting in ep e = 1.12gpa and hp e = 61mpa, would fit the known properties, but the chosen contact point can strongly influence the results. a not fully developed indentation due to the high surface roughness can explain these results (compare figure 2 for fmax = 5mn, where maximum indentation depth (hmax) is already around 25% of sa). they indicate that h seems more sensitive to effects caused by roughness than e. therefore, the evaluation of h can be used more profoundly to determine if the results for the indent are correct. overall, surface roughness strongly affects the standard deviation of both hardness and modulus characterization, with the biggest deviation on the roughest surface. to give further insights into the exact dependency of the evaluated values on the surface quality and applied fmax (and subsequent hmax), the ratios of sa to hmax were assessed for all samples. figure 4 shows the results for a quantitative comparison. based on the ep e results presented in figure 3 and the measurements that yielded acceptable outcomes, we can consider the ratio of approximately 20%, achieved at a fmax of 10mn for the surface with sa = 0.72 µm as a rough threshold value for characterizing the young’s modulus of pe via nanoindentation. the threshold for valid results seems to be much lower for hardness measurements. the level of acceptance appears to be around 8%, reached with sa = 0.08 µm and 0.5mn. since the sa = 0.07 µm surface shows the same sa/hmax ratio, the hardness results still seem to fit quite well. this is an evident sign that sa, compared to indentation depth, is only a rough but still valuable criterion for seeing if the surface quality is good enough for specific nanoindentation evaluations. figure 5 depicts three indents with a maximum load of 10mn on the surfaces with sa = 0.72µm, 0.08µm, and 0.07µm obtained with a sem tescan clara (tescan, czech republic). 4 vol. 50/2024 modulus estimation of polymers via nanoindentation. . . (a). sa = 0.72 µm. (b). sa = 0.08 µm. (c). sa = 0.07 µm. figure 5. sem images from lasting impression of indentations with 10mn maximum load on p e samples with different surface roughness. while the impression can easily be seen on the best surface quality, the rougher surfaces make it hard or impossible to distinguish the indent from surface features. this can make it impossible to evaluate the correct orientation of the indenter to the surface and the tested region. the comparison between sa = 0.08µm and sa = 0.07µm shows the necessity of considering more than one roughness parameter. the lower spc of the polished surface (figure 5c) is visible as the smoother appearance. at the same time, the difference in the str shows the high orientation of the surface features of the sa = 0,08µm finish. both differences affect the visibility of the persistent impression and the contact of the indenter to the sample material. 4. conclusion in this work, it was shown, that the applied surface preparation and subsequent roughness can significantly influence the evaluated modulus values determined via nanoindentation testing of visco-elastic materials. it has been demonstrated that with a load and unload rate below 40mn/min, along with a 10 s holding time at maximum load, there is no significant rate-dependent effect for pe. it was possible to get reasonable results for hardness measurements with a sa to hmax ratio of around 8%, while for the modulus, a ratio of up to 20% provided acceptable results. the results confirm that the iso 14577 recommended ra to indentation depth ratio of 5% used for hardness measurements can be safely applied for polymers. overall, it could be shown that a higher roughness increases scatter, and better statistics are required to measure the mechanical properties correctly. acknowledgements the work was funded from the polymer competence center leoben gmbh and performed within the framework of the comet-program (grant number 854178) of the federal ministry for climate action, environment, energy, mobility, innovation and technology and the federal ministry for digital and economic affairs, austria. the help from the chair of functional materials and materials systems, in regard to roughness testing on their confocal laser scanning microscope is highly appreciated. grammarly was used to correct grammar mistakes and highlight complex sentences, improving the work’s readability. references [1] w. c. oliver, g. m. pharr. an improved technique for determining hardness and elastic modulus using load and displacement sensing indentation experiments. journal of materials research 7(6):1564–1583, 1992. https://doi.org/10.1557/jmr.1992.1564 [2] w. gindl, h. s. gupta. cell-wall hardness and young’s modulus of melamine-modified spruce wood by nano-indentation. composites part a: applied science and manufacturing 33(8):1141–1145, 2002. https://doi.org/10.1016/s1359-835x(02)00080-5 [3] g. oreski, b. ottersböck, c. barretta, et al. degradation of pet – quantitative estimation of 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https://doi.org/10.3390/ma15093273 https://doi.org/10.1007/s11249-016-0805-5 https://doi.org/10.1088/0022-3727/31/19/006 acta polytechnica ctu proceedings 50:1–6, 2024 1 introduction 2 materials and methods 2.1 material and sample preparation 2.2 nanoindentation and evaluation following the oliver pharr approach 3 results and discussion 4 conclusion acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2024.48.0056 acta polytechnica ctu proceedings 48:56–60, 2024 © 2024 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague characterization of woven composite material under multiaxial loading regimes using fe-based stereocorrelation andrija zaplatića,b, zvonimir tomičevića,∗, françois hildb a university of zagreb, faculty of mechanical engineering and naval architecture, 10002 zagreb, croatia b université paris-saclay, centralesupélec, ens paris-saclay, cnrs, lmps – laboratoire de mécanique paris-saclay, 91190 gif-sur-yvette, france ∗ corresponding author: zvonimir.tomicevic@fsb.unizg.hr abstract. in this paper, woven glass fiber composite samples were subjected to three different cyclic loading histories (i.e., tensile, shear and combined loading at 45° via the modified arcan fixture. during the experiments, the samples were monitored by a stereovision system. finite element based stereocorrelation was used to measure the displacement and strain fields. the analysis of the experiments revealed different damage mechanisms. furthermore, for the 45° experiment, the highest strain levels were reached, whereas for the tensile test, the highest stress levels were achieved. keywords: stereocorrelation, woven composite, arcan fixture, multiaxial loading. 1. introduction the mechanical characterization of various materials is essential for designing and optimizing engineering components. in modern engineering, traditional materials like steel and alloys are increasingly being replaced by fiber-reinforced polymer (frp) composites [1]. these composites are highly sought after for their excellent strength-to-weight ratio. an additional advantage of frps is their customizable architecture, which can be tailored to meet specific workload conditions. however, predicting the ultimate failure and degradation of these components remains challenging due to their complex architecture. predicting their lifetime is also challenging due to their heterogeneity, which makes composites susceptible to various damage mechanisms that may degrade their stability [2]. therefore, extensive experimental investigations are necessary to study the composite behavior under different loading regimes [3]. since uniaxial mechanical tests do not adequately simulate real-world conditions, more complex loading configurations are required. however, these tests often demand high-cost and specialized testing machines, which may not be readily available. to address this issue, specially designed loading apparatuses for uniaxial testing machines have been developed. one popular setup is the modified arcan fixture (maf) [4–6], which applies three distinct loading regimes (i.e., tensile, shear, and their varied combinations at several angles) to butterfly-shaped specimens. the maf allows for testing a variety of materials, including wood [7] and composite joints [8], although it was originally designed for composite materials [9]. due to the complex geometry of the arcan samples, which features two v-notches in the ligament area, classical contact measurement equipment is highly impractical. as a result, contactless optical measurement methods like digital image correlation (dic) are more suited [10, 11]. dic utilizes visible light cameras to perform full-field measurements, and in recent years, the global dic approach has gained popularity [12]. this approach incorporates finite element (fe) features in such a way that displacement measurements are performed on meshes. consequently, the resulting outputs are nodal displacements, from which strain fields are determined. additionally, this method provides correlation residuals, which highlight damaged areas during the experiment [13]. an added advantage of fe-based approaches is the continuity assumption of the displacement fields, eliminating the need for interpolation as required in local approaches [14]. if out-of-plane motions are expected or a complex 3d sample is being tested, multiple cameras are employed within a stereovision framework [15, 16]. the finite element method has also been integrated into stereocorrelation environments [17–19]. by utilizing the known fe geometry of the observed object, it is possible to perform camera calibration directly on the object itself [19, 20] and then measure 3d surface displacements. 2. materials and methods in this section, the woven composite samples, experimental setup, and investigation are presented. additionally, the fe-based stereocorrelation method is briefly introduced. 2.1. material the investigated composite samples were manufactured by reinforcing a vinylester resin with woven glass fibers. the 3 mm thick samples were composed 56 https://doi.org/10.14311/app.2024.48.0056 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 48/2024 fe-based stereocorrelation for woven composites (a) (b) (c) (d) figure 1. experimental setup. (a) testing machine and optical system protected by black fabric. loading configurations: (b) tension, (c) shear and (d) combined tension and shear at 45°. of 7 layers of woven fabric, where the yarns were 0°/90° oriented. additionally, the fabric layers were stacked identically on top of each other. due to the complex geometry of the samples and the use of glass fibers, conventional machining methods were sub-optimal for processing the composite plates. therefore, water jet technology was used to cut the samples from the composite plates. to ensure sample failure during the experiment, two additional v-notches were cut for each sample. these additional notches were 2 mm long and 0.5 mm wide, thereby reducing the net section area to approximately 48 mm2. 2.2. experimental investigation the experimental setup (figure 1a) consisted of the maf and the optical system. the maf allows for different loading conditions to be applied to the arcan sample by rotating the fixture relative to the loading axis. in this research, three arcan specimens were tested and three loading configurations were prescribed, namely tension (wf00) (figure 1b), shear (wf90) (figure 1c), and their combination at 45° (wf45) (figure 1d). for each experiment, the sample was loaded at a velocity of 2µm s−1 in displacement control mode on the messphysik beta 50-5 uniaxial testing machine, each experiment consisting of eight loading/unloading cycles. the testing speed was chosen according to previous research [13, 20]. the samples were intermittently taken out of the maf to perform x-ray computed tomography scanning to obtain any micro-structural changes inside of the material. therefore, cyclic loading regimes were defined. to minimize external lighting influences that could affect brightness and contrast conditions, the experimental and optical setups were covered with black fabric (figure 1a). the optical system included two identical visible light cameras that monitored the surfaces of the samples during the experiments. furthermore, two light sources were employed to illuminate the sample surface. 2.3. fe based stereocorrelation in this work, fe-based stereocorrelation [5, 21] was performed to measure 3d surface displacement fields using fe meshes [17–19], which correspond to the observed sample geometry. additionally, strain fields were calculated from the measured displacement fields. stereocorrelation consists of two main steps, namely, calibration and correlation. in the calibration step, the projection matrix [π] for each camera is determined, allowing the fe mesh to be projected onto both camera images, defining the same region of interest (roi). the projection matrix [π] can be built if the intrinsic and extrinsic camera parameters are known. however, usually they are unknown and are determined with the calibration. the calibration can be defined as a least square minimization between the two camera images ρ2({p}) = ∑ roi ∥f1 (x1) − f2(x2)∥2 , (1) where {p} is a vector that gathers all the sought intrinsic and extrinsic parameters, xc are the projected physical points defined over the fe mesh x xc = xc (x, [πc]) . (2) in the correlation step, the nodal-wise displacement field u(x, t) is measured for any given time-step t. the new cost function can be written as u(x, t) = arg min nc∑ c=1 ∑ roi (g̃c (xc, t) − fc (xc))2 , (3) 57 a. zaplatić, z. tomičević, f. hild acta polytechnica ctu proceedings where fc are the respective camera reference images, g̃c the corrected deformed image at time t by the kinematic field u(x, t) for each respective camera and nc is the camera number. the regularity of the kinematic field u(x, t) in space is ensured by using a set of spatial shape functions ψi(x) u(x, t) = dof∑ i=1 ui(t)ψψψi(x), (4) where dof is the number of degrees of freedom, ui(t) are the nodal displacements for the time t. in these analyses, the fe meshes consisted of three-noded elements (t3) [19]. furthermore, the camera-wise gray level residuals ϕc can be computed for each physical point xc ϕc = g̃c (xc, t) − fc (xc) . (5) both the calibration and correlation are performed by resorting to the iterative gauss-newton scheme. for more detailed information, the reader is referred to zaplatić et al. [20], chang et al. [16, 21]. 3. results in this section, the stereocorrelation results are presented for all three experiments. the major principal strain (figure 2) and stereocorrelation residual (figure 3) fields are displayed for the maximum achieved stress level σmax for each experiment respectively (table 1). from table 1, it is concluded that the highest stress level was achieved for the tensile test (wf00), whereas the lowest was reached for the shear test (wf90). in figure 2 the major principal strain fields are displayed for all three experiments for the maximum stress levels (table 1). first, for the tensile test (figure 2a), two concentrated strained bands emanating from the notches are observed. furthermore, four additional strain concentrations are distinguished on the peripheral areas of the sample. since the yarns were parallel to the tensile load, the middle part was the most rigid part of the sample, whereas the peripheral areas were not. hence, they were more prone to damage. in the shear experiment (figure 2b), a single strained band is visible in the gauge region, parallel to the loading direction. at the roots of the notches, the strain levels were the highest, whereas the top and bottom parts exhibited very small strain values. for the wf45 experiment, a single strained band developed between the v notches (figure 2c), similarly to wf90 test. however, it was slightly inclined compared to the wf90 strained band. two strain concentrations are visible at diagonal peripheral areas of the sample, depicted by the green circles. using the stereocorrelation residuals, it is possible to reveal if surface damage developed. for the tensile test (figure 3a), three cracks formed around the root of the left notch. four cracks also developed in peripheral areas, which were also visible in (a) (b) (c) figure 2. major principal strain fields for (a) test wf00, (b) test wf90 and (c) test wf45 at the ultimate stress. the blue squares depict the virtual gauges where the nodal strain values were extracted and averaged. the green circles highlight strain concentrations outside of the virtual gauge. experiment σmax wf00 300 mpa wf90 78 mpa wf45 178 mpa table 1. ultimate stress levels for each experiment. the corresponding strain field (figure 2a). several small horizontal cracks are observed on the surface of the sample, which corresponded to surface matrix cracks [20]. in the residual field of the shear experiment (figure 3b), a major crack is distinguished, which emanated from the root of the left notch. the entire gauge region displayed increased residuals, which further indicated damage on the surface. last, for the 45° experiment, increased residuals are present across the entire middle gauge region of the sample (figure 3c). the residual levels are higher than in the previous two experiments (figure 3a, b). similarly to the strain field, increased residuals spread out diagonally on the opposite ends of the gauge area. 58 vol. 48/2024 fe-based stereocorrelation for woven composites (a) (b) (c) 20 -20 0 20 -20 0 20 -20 0 figure 3. stereocorrelation residual fields for (a) wf00, (b) wf90 and (c) wf45 tests for the ultimate stress. the residuals are expressed as a percentage of the dynamic range of the camera reference images. 4. discussion in this section, the stress-strain responses (figure 4) of the three experiments are compared and discussed. the stresses were calculated as the applied force divided by the net section area of each sample. the average strain levels were determined from virtual strain gauges defined for each experiment in figure 2. as previously noted, the tensile test exhibited the highest stress levels, while the shear experiment showed the lowest. when examining the stress-strain curves, it is observed that the tensile test resulted in the lowest strain levels, whereas the combined tensile and shear experiment achieved the highest strain levels. this difference is attributed to the orientation of yarns with respect to the loading direction. the tensile test corresponded to the highest rigidity due to the aligned yarn orientation, whereas shear exhibited the lowest rigidity. the wf45 experiment had yarns oriented at 45° with respect to the loading direction; thus it had increased rigidity compared to the shear experiment. 5. conclusion in this work, fe based stereocorrelation was applied to characterize the mechanical response and behavior of woven glass fiber composite samples under three 0 50 100 150 200 250 300 0 5 10 15 20 25 30 s tr e ss [ m p a ] principal strain [%] wf00 wf45 wf90 figure 4. stress-major principal strain curves for all three experiments. distinct loading configurations. the main conclusion are as follows: • the sample orientation with respect to the loading direction had a major influence on the stress-strain response, in particular its rigidity. • the tensile test reached the highest stress level, whereas the shear experiment achieved the lowest. • the highest strain and stereocorrelation residual levels were reached for the 45° experiment, thereby indicating the most accumulated damage on the investigated surfaces. the experimental setup presented herein is not limited to composite materials. it was designed to be robust and versatile. since the experimental protocol was established for different loading regimes, it is possible to introduce additional modalities in the stereocorrelation procedure, such as infrared and x-ray spectra for a more comprehensive material characterization. acknowledgements this work was performed within the fullinspect project supported by the croatian science foundation (uip-2019-04-5460 grant). az was also supported through the incoming international scholarship program of ens paris-saclay. references [1] r. hsissou, r. seghiri, z. benzekri, et al. polymer composite materials: a comprehensive review. composite structures 262:113640, 2021. https: //doi.org/10.1016/j.compstruct.2021.113640 [2] a. vrgoč, z. 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epoxy resin via digital image correlation. transactions of famena 45(2):1– 12, 2021. https://doi.org/10.21278/tof.452024020 [14] f. hild, s. roux. comparison of local and global approaches to digital image correlation. experimental mechanics 52(9):1503–1519, 2012. https://doi.org/10.1007/s11340-012-9603-7 [15] r. fouque, r. bouclier, j.-c. passieux, j.-n. périé. stereo digital image correlation: formulations and perspectives. comptes rendus mécanique 349(3):453– 463, 2021. https://doi.org/10.5802/crmeca.93 [16] x. chang, c. l. gourriérec, l. turpin, et al. proper generalized decomposition stereocorrelation to measure kinematic fields for high speed impact on laminated glass. computer methods in applied mechanics and engineering 415:116217, 2023. https://doi.org/10.1016/j.cma.2023.116217 [17] l. dubreuil, j.-e. dufour, y. quinsat, f. hild. mesh-based shape measurements with stereocorrelation. principle and first results. experimental mechanics 56:1231–1242, 2016. https://doi.org/10.1007/s11340-016-0158-x [18] j.-e. pierré, j.-c. passieux, j.-n. périé. finite element stereo digital image correlation: framework and mechanical regularization. experimental mechanics 57:443–456, 2017. https://doi.org/10.1007/s11340-016-0246-y [19] m. berny, t. archer, p. beauchêne, et al. displacement uncertainty quantifications in t3-stereocorrelation. experimental mechanics 61(5):771–790, 2021. https://doi.org/10.1007/s11340-021-00690-0 [20] a. zaplatić, z. tomičević, x. chang, et al. hybrid infrared-visible multiview correlation to study damage in a woven composite complex-shaped specimen. coupled systems mechanics 12(5):445–459, 2023. https://doi.org/10.12989/csm.2023.12.5.445 [21] x. chang, c. l. gourriérec, f. hild, s. roux. brightness and contrast corrections for stereocorrelation. global and instantaneous formulation with spatial regularization. mechanical systems and signal processing 208:111057, 2024. https://doi.org/10.1016/j.ymssp.2023.111057 60 https://doi.org/10.1016/j.coco.2021.100905 https://doi.org/10.1007/bf02324146 https://doi.org/10.1007/s11340-021-00762-1 https://doi.org/10.1016/j.ijmecsci.2023.108127 https://doi.org/10.1515/hf.2009.034 https://doi.org/10.1016/j.ijadhadh.2022.103321 https://doi.org/10.1016/j.ijsolstr.2018.12.004 https://doi.org/10.1177/0309324712441435 https://doi.org/10.1007/s10704-015-0004-7 https://doi.org/10.12989/csm.2022.11.2.133 https://doi.org/10.21278/tof.452024020 https://doi.org/10.1007/s11340-012-9603-7 https://doi.org/10.5802/crmeca.93 https://doi.org/10.1016/j.cma.2023.116217 https://doi.org/10.1007/s11340-016-0158-x https://doi.org/10.1007/s11340-016-0246-y https://doi.org/10.1007/s11340-021-00690-0 https://doi.org/10.12989/csm.2023.12.5.445 https://doi.org/10.1016/j.ymssp.2023.111057 acta polytechnica ctu proceedings 48:56–60, 2024 1 introduction 2 materials and methods 2.1 material 2.2 experimental investigation 2.3 fe based stereocorrelation 3 results 4 discussion 5 conclusion acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311//app.2025.53.0017 acta polytechnica ctu proceedings 53:17–23, 2025 © 2025 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague automation of small sample positioning for mechanical testing in hot cells kateřina hrdličková∗, václav wudi, michaela frantová, petr štemberk czech technical university in prague, faculty of civil engineering, department of concrete and masonry structures, thákurova 7, 166 29 prague 6, czech republic ∗ corresponding author: katerina.hrdlickova@fsv.cvut.cz abstract. the main objective of this paper is to propose an improvement and automation of the current mechanical testing procedure of concrete and rock samples in hot cells, i.e. in laboratories designed for radioactive sample testing. the paper describes the effect of radiation on the different components of concrete and consequently on the concrete itself. to give a general idea of sample handling and hot cell operations, safety precautions are prescribed which significantly prolong and complicate the handling of a radioactive rock sample. video recordings of the tests were used to analyse the progress of the tests. the main issues were the placement of samples off the axis of the compression test machine, sample curvature, compression test machine design and poor recording of sample deformation. new centring stops were introduced to improve the handling of the specimen when placed in the test machine. additionally, the use of image processing was proposed to evaluate the correct placement of the sample in the test machine. these improvements should lead to easier sample handling and correct results which will serve as a good basis for subsequent numerical analyses. keywords: mechanical testing, image processing, hot cells, radioactivity, concrete, rock. 1. introduction ensuring the long-term functionality of critical structural elements of nuclear power plants is essential for the safe operation of these facilities not only in the czech republic but also worldwide, especially if the lifetime of nuclear power plants is to be extended. with increasing energy consumption, it is necessary to keep these facilities in operation for as long as possible, while further construction or expansion of existing nuclear power plants is of course being considered. in addition, there are also plans to build small modular reactors. in all these cases, concrete will also be used to shield radiation. therefore, the actual condition of shielding concrete in nuclear power plants is currently being evaluated. the main effect on the cement paste is gamma radiation [1, 2], which causes a loss of free water, which subsequently leads to shrinkage of the cement paste [3] and also radiolysis of the water [4, 5]. the main effect on the aggregate is neutron radiation, which causes defects in the crystal lattice [6, 7]. these defects accumulate and lead to the so-called rive (radiation induced volumetric expansion) – swelling of the aggregate. this causes significant stresses in the rock, resulting in internal tension and subsequent cracking [1, 2, 8]. long-term exposure to radiation thus causes a reduction in the compressive strength of concrete by up to 50 % and a reduction in tensile strength by up to 75 %, while the modulus of elasticity can decrease by up to 25 % [9]. the present work is focused on the mechanical testing of aggregates in hot cells at the research centre řež. the primary aim is to identify systematic errors in the compressive testing of samples in hot cells. this is achieved by comparing the resulting load curves with video recordings of the tests. this research is also developing an affordable device for use in hot cells, leveraging image analysis. the aim is to develop and programme a custom system that will be able to automate the process of detecting and aligning concrete specimens during compression testing, which is key to ensuring uniform stress distribution. the handling of the irradiated sample is very complicated for safety reasons and therefore time consuming, which increases the final cost of the testing, which also includes the time of exposure of the samples to irradiation in the reactor and the subsequent disposal of the samples as radioactive waste. the sample can be handled only by manipulators. for these reasons, as well as for limited availability of free positions in the reactor used for irradiation, the number and size of the samples to be tested is significantly limited. each sample is therefore unique and the test results are taken as a final value without generating statistics. it is therefore essential that the load tests are performed correctly and that the results are meaningful and can be used for subsequent numerical analyses. the elimination of unnecessary errors and imperfections in the testing of irradiated samples is the aim of this paper. 17 https://doi.org/10.14311//app.2025.53.0017 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en k. hrdličková, v. wudi, m. frantová, p. štemberk acta polytechnica ctu proceedings figure 1. graphical representation of the dependence of the loading time on the sample with off-centre placement. 2. issues of sample storage in hot cells hot cells (hc) are specialised laboratories designed for the testing of highly radioactive materials. a total of ten such chambers and one half-hot chamber are operated in the controlled area of the research centre řež [10]. the handling and testing of samples in hot cells presents a number of specific challenges. sample handling is complicated mainly because of the sample size. the sample size is limited and therefore the sample dimensions cannot be changed. the testing of concrete and aggregate is performed on aggregate cylinders with a height of either 1 or 2 cm and a diameter of 1 cm, or alternatively, cylinders with a height of 6 cm and a diameter of 4 cm for concrete samples. these dimensions were chosen with the technical limitations of the hc and reactor facilities at řež in mind. the specimen is gripped in the clamps by the manipulator. the manipulator is equipped with a mechanism that allows the attachment of extensions and tweezers of varying dimensions. the handling of the sample is monitored through a lead glass viewing window and also by a camera located in the hc. however, this camera is often located behind the specimen. this causes the operator to move the manipulators in the opposite direction to that seen on the camera. this can sometimes be confusing and requires even more operator concentration. the sample is situated at a distance of several metres from the operator, and it is particularly challenging to position the sample in the centre of the compressive test machine without any deviation from the test machine’s axis. by purchasing conventional cameras, we want to simplify and to make more efficient the positioning of samples into the compressive test machine located in the hot cells using automated image processing. this analysis will allow precise control of the sample position, reducing time and the possibility of manual alignment errors, and ensuring uniform loading during testing. additionally, the cheaper, commercially available cameras are easy to maintain and quickly replaceable when needed, which keeps instrument operating costs to a minimum and increases the reliability of results. in the work presented here, we found two major errors, namely, non-centric sample fitting and uneven sample bases. the impact of these errors on the results will be described in the subchapters “sample placement” and “uneven bases”. 2.1. sample placement the correct performance of the compression test of a sample much smaller than the machine’s loading platens depends especially on the accurate placement of the sample in the centre of the compressive test machine. if the specimen is placed off axis, uneven loading is imposed, which can lead to failure on the overloaded side. initially, the specimen is loaded on both bases and as the tension increases, the steel loading platens slightly rotate. this increases the pressure on one side of the specimen, which can lead to failure. to illustrate this problem, a test was performed with the specimen placed off the axis of the compressive test machine. a sample of aggregate was placed in the compressive test machine off its axis. the sample was 1 cm in diameter and 2 cm in height. the failure of the sample occurred at a load of 17 kn and a time of 177 s, see figure 1. it is evident from the test that the sample was placed off the axis of the compressive test machine. the specimen has a horizontal bases and was therefore loaded evenly from the start. during the test, the steel loading platen was gradually rotating for 112 s (figure 2 and figure 3), which is caused by the sample being placed off the axis of the test machine. subsequently, the curve grows linearly until the first cracks appear (figure 4), which occurs just before the specimen collapses on its overloaded side (figure 5). in all tests where the specimen is placed off the axis 18 vol. 53/2025 automation of small sample positioning for mechanical testing figure 2. placing the sample in the test machine, time 0 s. figure 3. steel loading platen rotation, time 112 s. figure 4. first crack, time 170 s. figure 5. sample failure, time 175 s. of the compressive test machine, the specimen is damaged due to placement of the sample. the rotation of the steel loading platen is through the specimen, i.e. the platen does not rotate until the specimen is loaded. as the platen rotates, both sides are deformed equally, but along a different length of the specimen edge, resulting in a different relative deformation and therefore a different stress along the edges of the specimen. the force acting on the specimen is shown in figure 6. even for a very small sample placement off the machine’s axis, large stress changes occur across the sample cross-section as the core of the cross-section for a 10 mm diameter specimen is only a 2.5 mm diamfigure 6. representation of the force acting on a specimen placed off-axis. eter circle. this observation also confirms that only minor excentric loading suffices for an uneven loading. the small inaccuracy due to the positioning would not play such a critical role on a normal concrete cube with an edge of 150 mm, but it is significantly more crucial for the 10 mm specimens. 2.2. uneven bases it is also important to observe the flatness of the sample bases on the sample. if the specimen bases are not parallel, or if the specimen is slightly skewed during specimen preparation, uneven loading will also occur. the loading of the specimen would initially be on the elevated edge of the specimen where the stress concentration would occur. as a result, the sample may fail at this edge, which could lead to either its collapse or the chipping of the sample’s edge, causing the bases to become horizontal. once one edge was damaged, the loading would begin to propagate to the entire specimen, but due to the different stiffnesses, further complications would arise and eventually failure would occur. a sample with a height of 1 cm and a diameter of 1 cm was selected for the test. the failure of the specimen occurred after 55 s under a load of 2.8 kn, see figure 7. the sample has uneven bases, as can be seen in figure 8. the load curve can be divided into several parts. for the first 20 s of the test, the loading platen is being rotated (figure 9 and figure 10) and therefore the curve has a convex shape. subsequently, the specimen is loaded to almost the entire surface of the base, there is no further rotation of the platen. the curve has a linear shape until 53 s, during which time the edge of the sample is overloaded. at 53 s, the raised edge of the specimen is crushed (figure 11) and the first cracks appear along the height of this 19 k. hrdličková, v. wudi, m. frantová, p. štemberk acta polytechnica ctu proceedings figure 7. graphical representation of the dependence of loading time on a sample with uneven bases. edge. then the load increases again, the specimen is at this point loaded over the entire surface of the base. the increase in load is no longer linear, as cracks have already formed, and new cracks are being added (figure 12 and figure 13). the sample failure occurs at 135 s (figure 14). figure 8. placement of the sample in the test machine, uneven sub-states uneven bases. figure 9. sample, time 0 s. figure 10. steel loading platen rotation, time 20 s. figure 11. crushing of the raised edge of the sample, time 58 s. figure 12. crack expansion, time 97 s. figure 13. crack expansion, time 120 s. the platen is rotated over the edge of the sample, with all the load applied to the edge of the specimen, which causes high stress. additionally, a moment is applied to the edge of the specimen, as can be observed in figure 15. the rotation of the loading platen occurs through the specimen, as the edge of the specimen is situated at a relatively small distance from the bearing, and thus a relatively large force is required. 20 vol. 53/2025 automation of small sample positioning for mechanical testing figure 14. sample collapse, time 135 s. figure 15. representation of the force acting on a specimen with uneven bases. 3. design and control of sample seating 3.1. mechanical stop for precise sample placement in order to simplify, speed up and enhance the handling of the sample in the hot cell, a range of products are being developed to adapt to the specific requirements of the test. in this case, the focus is on placing the sample in the axis of the compression test machine, for which centring stops are used. these stops are designed to allow easy handling even within the hot cells. a static stop was initially developed to support the specimen throughout the test. however, due to the small sample size, this could potentially affect the test results. therefore, a movable stop was designed (figure 16), consisting of two parts: a movable part with a handle and a counterpart to ensure its linear motion. the stop moves along two grooves, which provide greater stability and minimise the risk of rotation around a single point. figure 16. movable centering stop with sample placement on the centre (top) and during the loading process (bottom) down. the movable component is designed with a handle for easier manipulation with a manipulator, while the fixed component of the loading platen is equipped with a fixation handle to secure the position when placing the sample. after the sample has been placed in the locked stop, it is preloaded. to minimise the impact of specimen support on test results, the stop is displaced and retracted after preloading. this permits specimen deformation in all directions, thereby ensuring that test results are not influenced by the support. furthermore, as part of the process optimisation, the steel loading platen with ball seat is being redesigned to be relocated from the lower to the upper position, which should facilitate more uniform sample loading. 21 k. hrdličková, v. wudi, m. frantová, p. štemberk acta polytechnica ctu proceedings figure 17. image obtained by one of the cameras, 1-upper loading platen, 2-lower loading platen, 3-concrete sample. figure 18. detection of sample location using vertical line detection – a) original image b) after preprocessing c) after final processing. 3.2. control of sample positioning by image processing an image processing system was created to detect unevenness in the bases of the specimen and to evaluate any off-centre positioning of the concrete specimen relative to the test machine’s axis. optimal camera placement was determined to ensure accurate data capture, with two cameras positioned perpendicular to each other to capture both upper and lower platens as well as the sample (figure 17). a python-based program was developed to process the captured images (figure 18), allowing for precise extraction of the sample’s position and rotation. this program applies image processing techniques to calculate the alignment and centering of the sample, thus ensuring accurate and reliable setup for compression testing. 4. conclusion the main contribution of this paper is related to improvements of the compression testing of concrete and aggregate samples in hot cell environment. the improvement could be achieved by simplification of sample handling, which could also be achieved through the use of image processing. two weak points have been identified which needed to be improved. one is the unevenness of the sample base faces when the top and the bottom faces are not parallel. this leads to uneven loading tested samples. the steel loading platen has to be inclined to match the sample shape to get even load distribution. however, during this rotation the steel loading platen is pressing just one edge of the specimen, which might cause its damage. the sample is then broken at lower load level than perfectly shaped sample. the second factor which had to be considered is the position of the sample in relation to the axis of the compressive test machine. the misalignment also causes uneven loading on the sample. for this reason, a centring stop is designed to ensure that the specimen is positioned in the centre of the press. using carefully positioned cameras, it is possible to monitor the entire test process and then analyse details of the sample position relative to the test machine’s axis. this not only allows us to check the exact placement of the sample in the test machine, but also to analyse and compare the measured values retrospectively with the visual record of the test. image analysis in this way eliminates the risk of human error during sample settling and contributes to smoother and better documented tests, which is essential for efficient and consistent interpretation of results. the implementation of these improvements will facilitate the handling of samples, whilst also ensuring the reliability of the test results, which will serve as a basis for subsequent numerical analyses. 22 vol. 53/2025 automation of small sample positioning for mechanical testing acknowledgements we acknowledge the state support of the technology agency of the czech republic within the national competence centre programme ii, project tn02000012 “center of advanced nuclear technology ii”. the presented results were obtained using the cicrr infrastructure, which is financially supported by the ministry of education, youth and sports – project lm2023041. references [1] i. maruyama, o. kontani, m. takizawa, et al. development of soundness assessment procedure for concrete members affected by neutron and gamma-ray irradiation. journal of advanced concrete technology 15(9):440–523, 2017. https://doi.org/10.3151/jact.15.440 [2] y. khmurovska, p. štemberk, s. sikorin, et al. effects of gamma-ray irradiation on hardened cement mortar. international journal of concrete structures and materials 15(1):17, 2021. https://doi.org/10.1186/s40069-020-00452-7 [3] t. t. c. hsu, c.-l. wu, j.-l. li (eds.). infrastructure systems for nuclear energy. wiley, 2013. https://doi.org/10.1002/9781118536254 [4] p. bouniol, a. aspart. disappearance of oxygen in concrete under irradiation: the role of peroxides in radiolysis. cement and concrete research 28(11):1669–1681, 1998. https://doi.org/10.1016/s0008-8846(98)00138-0 [5] f. vodák, k. trtík, v. sopko, et al. effect of γ-irradiation on strength of concrete for nuclear-safety structures. cement and concrete research 35(7):1447–1451, 2005. https://doi.org/10.1016/j.cemconres.2004.10.016 [6] b. pomaro. a review on radiation damage in concrete for nuclear facilities: from experiments to modeling. modelling and simulation in engineering 2016(1):4165746, 2016. https://doi.org/10.1155/2016/4165746 [7] h. k. hilsdorf, j. kropp, h. j. koch. the effects of nuclear radiation on the mechanical properties of concrete. ernst, 1976. [8] a. denisov, v. dubrovskii, v. solovyov. radiation resistance of mineral and polymer construction materials. zao mei publishing house, 2012. [9] k. g. field, i. remec, y. l. pape. radiation effects in concrete for nuclear power plants – part i: quantification of radiation exposure and radiation effects. nuclear engineering and design 282:126–143, 2015. https://doi.org/10.1016/j.nucengdes.2014.10.003 [10] d. zoul, m. koplová, m. zimina. infrastruktura horkých komor centra výzkumu řež 1. díl [in czech; hot chamber infrastructure of the řež research center, part 1]. jaderná energie 2:27–33, 2020. 23 https://doi.org/10.3151/jact.15.440 https://doi.org/10.1186/s40069-020-00452-7 https://doi.org/10.1002/9781118536254 https://doi.org/10.1016/s0008-8846(98)00138-0 https://doi.org/10.1016/j.cemconres.2004.10.016 https://doi.org/10.1155/2016/4165746 https://doi.org/10.1016/j.nucengdes.2014.10.003 acta polytechnica ctu proceedings 53:17–23, 2025 1 introduction 2 issues of sample storage in hot cells 2.1 sample placement 2.2 uneven bases 3 design and control of sample seating 3.1 mechanical stop for precise sample placement 3.2 control of sample positioning by image processing 4 conclusion acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2023.44.0019 acta polytechnica ctu proceedings 44:19–27, 2023 © 2023 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague neutronic analysis of a heat-pipe cooled reactor josef sabol∗, jan frýbort czech technical university in prague, faculty of nuclear sciences and physical engineering, department of nuclear reactors, v holešovičkách 2, 180 00 prague 8, czech republic ∗ corresponding author: saboljos@cvut.cz abstract. heat-pipe cooled reactors belong to the group of nuclear reactors using heat pipes filled with liquid metals (such as sodium or potassium) to cool the core. due to the passive heat removal system, there is no need to use closed loops with pumps, and the reactor can be operated with reduced operational requirements. consequently, this system can be used in remote locations without access to an electrical grid, or it can be used for space applications. this paper deals with a neutronic study of the special purpose reactor design-b concept from the idaho national laboratory. using monte carlo code serpent 2 and endf/b viii.0 library, a fixed-temperature model was created to calculate the safety characteristics of the system. this included reactivity coefficients, power distribution, neutron flux spectrum, and criticality safety. in a simplified depletion calculation, an effect of fuel depletion on safety systems was determined as well as a decay heat. keywords: heat pipe, nuclear reactor, space, neutronic analysis, serpent 2. 1. introduction heat-pipe cooled (hpc) reactors belong to the group of nuclear reactors using heat pipes filled with liquid metals (such as sodium or potassium) to cool the core. they are composed of a reactor core, heat pipes, reflector, regulation system, shielding, and electrical conversion system. due to the passive heat removal system, there is no need to use closed cooling loops with pumps inside the reactor core, and the reactor can be operated with reduced requirements for external systems. consequently, this system can be used in remote locations without access to an electrical grid (such as research and military stations), or it can be used for space applications (moon bases or spacecrafts). heat pipes are devices operating on the principle of natural convection and phase change, being capable of heat transfer with a tiny temperature gradient. they consist of a tube and working fluid. at the level of reactor core, the working fluid is operating at its boiling point, gradually receiving latent heat and then evaporates, resulting in local pressure increase. due to the pressure drop, the vaporized substance moves to the other end of the tube, where it releases latent heat, condenses, and flows back along the reactor walls to the core. to function properly, a porous cover is used to cover the inner wall of the tubes. this cover keeps the liquid on the walls through the capillary effect. because of the above mentioned, it is possible to use the tubes in any orientation, against gravity and even in space applications [1]. the idea of hpc reactors originated in the 1990s at los alamos national laboratory (lanl), as a power source for space applications. it was supposed to be a group of small modular units with a variable electric power output of up to 100 kw, with an operation period of at least 10 years and with a focus on safe and trouble-free operation [2]. the first designs of hpc reactors from lanl were safe (designed for use in satellites and spacecrafts) [3] and homer (designed for planetary applications) [4] reactor concepts. however, neither of these projects was ever implemented. there was only a test of sodium heat pipes, a stirling engine, and an ion thruster as part of the non-nuclear safe-30 experiment between the years 2000 and 2001 [5]. after a long lack of interest in space exploration, a new initiative to revive the hpc reactor project came in the 2010s. lanl started the development of kilopower reactors specifically designed for space applications. these reactors were designed to generate electric power output up to 10 kw and were based on the use of sodium heat pipes, high-enriched uranium, beo reflector, and stirling engines for energy conversion. this project ended with a successful test of krusty reactor in 2018 [6]. at the same time as the kilopwer project, another concept called megapower reactors was under development, operating on a similar principle. these reactors are based on the usage of a low-enriched uranium (up to 20 % in form of uo2 or un), potassium or sodium heat pipes, al2o3 reflector with b4c control drums used to control the reactivity, heat transfer medium from stainless steel or molybdenum and an energy conversion system based on an open brayton cycle. due to the greater heat power output (up to 5 or 15 mw), the reactor would possibly operate exclusively in terrestrial applications, such as research or military stations [7]. in 2017, a study was proposed at idaho national 19 https://doi.org/10.14311/app.2023.44.0019 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en josef sabol, jan frýbort acta polytechnica ctu proceedings laboratory (inl) that tried to identify potential safety and technical problems associated with the megapower reactor. some of the frequently mentioned problems are, for example, fabrication of compact stainless steel serving as a heat transfer medium, and stress strain and volumetric expansion of stainless steel when one of the heat pipes fails [8]. in addition to small modifications of the design to make the reactor possible to manufacture, two alternative designs were established that completely avoid the mentioned problems. referred to as a special purpose reactor (spr) design-a and design-b, these designs work on the same principle and differ only in the reactor core design [1]. this technology provide a suitable alternative for passive heat removal systems in space nuclear reactors, or they can also be used in terrestrial conditions. to use them in the future, it is necessary to understand more about the neutronic and thermohydraulic properties of these devices. however, since heat pipes are still a new technology with a lack of experimental data, it has to rely on numerical simulations for now, which could serve as a basis for experimental testing in the future. this paper further deals with a detailed neutronic study of the spr design-b reactor concept from inl, and it focuses mainly on safety analysis during depletion. 2. neutronic model 2.1. software the neutron transport and the depletion calculations were performed using stochastic monte-carlo code serpent 2 in version 2.1.32 [9] and with nuclear data library endf/b-viii.0 [10]. in the calculations with multiplication factor keff, 1 000 000 neutrons were used to simulate the particle transport, 100 inactive cycles to stabilize the neutron distribution and 1 500 active cycles to collect the results. for the depletion calculations, 100 000 simulated neutrons were used in 1 000 active and 100 inactive cycles. 2.2. reactor design the thermal power of the spr design-b reactor is 5 mw. all dimensions, materials, and temperatures are taken from [1]. parameters of individual components are presented in table 6 in appendix a. the thermodynamic properties of potassium were taken from [11], sodium from [12], and helium from [13]. the composition of 316 stainless steel (ss316) is taken from [14]. the isotopic composition of the elements is natural according to [15]. the horizontal and vertical cross sections of the model are shown in figure 10 in appendix a. for safety reasons, the reactor core is divided into 6 segments separated by double ss316 wall, each segment contains 352 fuel rods and 204 heat pipes. ρ0 [g/cm3] ᾱ [1/k] uo2 10.52 [1] 10.08 · 10−6 [16] ss316 8.00 [14] 19.30 · 10−6 [17] al2o3 3.95 [18] 8.50 · 10−6 [18] b4c 2.51 [1] table 1. densities and thermal expansion coefficients for used materials. the fuel rod is 150 cm long, followed by a 2 cm gas plenum. the ss316 cladding is filled with helium. each of the segments is filled with liquid sodium, which works as a heat transfer medium between fuel and heat pipes. above and below the reactor core is a ss316 axial reflector. the radial reflector consists of al2o3, a helium gap, ss316 shielding to protect from gamma radiation and enriched b4c to protect it from neutron irradiation. for reactivity control, 12 rotating control drums situated around the reactor core with semicircular cut-outs of enriched b4c are used. the rotation of the drums can be used to increase/decrease neutron absorption and thus control the reactivity. safety rods (inner and annular outer) that are used for safe shutdown are also made up of enriched b4c. 2.3. thermal expansion the dimensions given in table 6 are considered in the shutdown state (at a temperature of 293 k). when the reactor is heated to the operating condition, thermal expansion and a change in density will occur. the linear length expansion has been considered according to: l(t ) = l0 · (1 + ᾱ∆t ), (1) where l(t ) and l0 are final and initial dimensions, respectively, ᾱ is an average thermal expansion coefficient, and ∆t is a temperature difference between the final and initial state. the following relation was considered for the density changes: ρ(t ) = ρ0 · 1 (1 + ᾱ∆t )n , (2) where ρ(t ) and ρ0 are final and initial densities, respectively, and n is a number of expansion directions. the densities of materials and their thermal expansion coefficients are given in table 1. for ᾱ, the mean value for the expansion between initial and final temperatures was considered. there were considered the radial and axial expansion of fuel rods and the radial expansion of cladding, grids, reactor vessel, and radial reflector. 2.4. differences in the model according to [1], it was not possible to determine temperature of radial reflector, interface between radial shieldings, diameter of the outer b4c shielding, 20 vol. 44/2023 neutronic analysis of a heat-pipe cooled reactor figure 1. possible positions of the control drums and shutdown rods. a) control drums in the “out” position, b) control drums in the “in” position, c) shutdown rods in the “out” position, and d) shutdown rods in the “in” position. and amount of sodium in the vessel. it was therefore necessary to be satisfied with approximate estimates, whose effects on reactivity changes were as follows: • the estimated radial reflector temperature is 923 k (in the 100 k range, the change in reactivity is roughly 28 pcm), • the estimated interface between shieldings is 87 cm in radius (in the 1 cm range, the change is approximately 52 pcm), • the estimated size of outer shielding is 100 cm in radius (without change), • the estimated amount of sodium in the vessel is 152 cm in depth (in the 1 cm range, the change is roughly 12 pcm). 3. results 3.1. reactivity of the system reactivity can be controlled in several ways. the most important is control by control drums rotation, which are changing neutron absorption. shutdown rods are used for safety shutdown. in the following text, the “in” position represents the state where all elements have the greatest influence on reactivity. in contrast, the “out” position represents the state with the lowest influence on reactivity, as can be seen in figure 1. when all control elements are in the “out” position, a supercritical system is created. the maximum excess reactivity at the beginning of life (bol) is: ϱbol max = (1543 ± 2) pcm. to achieve criticality, it is necessary to rotate the control drums by 51° and 318°, respectively, where this rotation is symmetrical for all drums. for a more -25000 -20000 -15000 -10000 -5000 0 5000 0 50 100 150 200 250 300 350 r ea ct iv ity [ pc m ] angle of drums rotation [°] shutdown rods in the "out" position shutdown rods in the "in" position figure 2. dependency of reactivity on control drums rotation angle. even power distribution, the 51° position is preferable, therefore all the following calculations were done for this position. the dependency of reactivity on control drums rotation angle is shown in graph in figure 2. table 2 shows the worths of the safety components and compares them with the values from [1] (calculated using mcnp 6.1 code and endf/b vii.0 nuclear data library). the calculations in this work showed lower keff, which is associated with lower bol excess reactivity. this discrepancy may be due to either a different model or the use of a different nuclear data library. another aspect is the original study does not describe exact position and shape of axial reflectors, so axial neutron leakage has also a large influence. when comparing the worths of the safety systems, the results in this work are slightly higher, which is probably due to the same reasons. from a safety point of view, the control drums are the most important. important are also the shutdown rods (when inserting at the same time), the outer annular rod and the inner rod. due to the small 21 josef sabol, jan frýbort acta polytechnica ctu proceedings mcnp 6.1 [1] serpent 2 endf/b vii.0 endf/b viii.0 critical drums rotation [°] 56 51/318 βeff [%] 0.7 0.7199 ± 0.0002 bol excess reactivity [pcm] 2 359 1 543 ± 2 total drums worth [pcm] 9 079 10 677 ± 3 individual drum worth [pcm] 770 866 ± 3 inner rod worth [pcm] 6 013 6 148 ± 3 annular rod worth [pcm] 7 504 7 754 ± 3 both rods worth [pcm] 8 583 ± 3 table 2. worths of the safety components. mcnp 6.1 [1] serpent 2 endf/b vii.0 endf/b viii.0 fuel doppler broadening [pcm/k] -0.9485 -0.608 ± 0.010 fuel axial elongation [pcm/k] -0.3234 -0.242 ± 0.005 radial reflector expansion [pcm/k] -0.1575 -0.142 ± 0.004 sodium volume expansion [pcm/k] -0.0723 -0.16 ± 0.03 grids expansion [pcm/k] -0.586 ± 0.013 total [pcm/k] -1.5017 -1.74 ± 0.03 table 3. temperature reactivity coefficients. excess reactivity, the individual control elements are able to shutdown the reactor. 3.2. reactivity coefficients reactivity coefficient can be defined according to: ax = ∂ϱ ∂x , (3) where x is the parameter (temperature, power, etc.), and ϱ is the reactivity. the numerical determination can be based on the linear fit: f(x) = k · x + b. (4) that means ax ≈ k. (5) temperature reactivity coefficients are calculated in this paper (i.e. x is t ). 3.2.1. doppler broadening the fuel doppler broadening effect has the greatest influence on the change in reactivity with temperature change. the dependency of the keff on fuel temperature change shows graph in figure 3. the effective multiplication factor and reactivity decrease with increasing temperature. for the operating temperature (at a temperature of 1050 k), the reactivity coefficient of doppler broadening is: adopp. t ≈ (−0.608 ± 0.010) pcm/k. 1.008 1.010 1.012 1.014 1.016 1.018 1.020 1.022 1.024 1.026 0 500 1000 1500 2000 2500 k e ff [ -] fuel temperature [k] doppler broadening -0.00652 ln(x) + 1.06103 figure 3. doppler broadening effect in the fuel. 3.2.2. thermal expansion effects other thermal aspects include axial elongation of fuel rods, radial expansion of reflector, volume expansion of sodium, and radial expansion of fuel grids. the densities and the fuel pitch in lattice are changing by thermal expansion, which affects the moderation properties and macroscopic cross-sections. table 3 shows the calculated temperature reactivity coefficients in these cases. to convert the dimensions and densities, the relations (1) and (2) were used, so that the total mass of all components remained constant. all the temperature reactivity coefficients are negative, which is important for the safe operation of the reactor. most of all obtained values are smaller in absolute value compared to those in [1]. in addition, the influence of the expansion of fuel grids and thus the changing fuel pitch was also calculated. this aspect was not considered in [1], however, its value is not negligible and it is the second largest reactivity coefficient after the fuel doppler broadening. 22 vol. 44/2023 neutronic analysis of a heat-pipe cooled reactor mcnp [1] conf. 51° conf. 318° endf/b vii.0 endf/b viii.0 endf/b viii.0 total heat power [mw] 5 5 5 number of fuel pins [-] 2 112 2 112 2 112 average pin power [kw] 2.37 2.37 2.37 maximum pin power [kw] 2.84 2.903 ± 0.002 2.916 ± 0.002 minimum pin powe!r [kw] 1.983 ± 0.002 1.598 ± 0.002 radial peaking factor [-] 1.20 1.22 1.23 axial peaking factor [-] 1.27 1.30 1.30 average power density [w/cm3] 9.03 9.03 9.03 maximum pin power density [w/cm3] 10.82 10.82 ± 0.01 10.87 ± 0.01 maximum power density [w/cm3] 14.05 ± 0.04 14.15 ± 0.04 table 4. thermal power parameters. 1x104 1x106 1x108 1x1010 1x1012 1x1014 1x1016 1x1018 1x10-8 1x10-6 1x10-4 1x10-2 1x100 1x102 φ [ cm -2 s -1 m ev -1 ] energy [mev] fuel sodium radial reflector figure 4. neutron spectra. 3.3. neutron spectra next, the average neutron spectra in fuel, sodium, and in radial reflector were calculated using the ecco1968 group structure. as can be seen in figure 4, the spectrum in reflector corresponds to the thermal spectrum. for fuel and sodium, there is a shift of neutron energies to higher values. the reactor neutron energy spectrum is shaped by the uranium enrichment, the size of the core, and the large reflector layer and as a result, it is neither a spectrum of a fast reactor nor spectrum of a thermal reactor. 3.4. power distribution as it was already mentioned, the pin power distribution very much depends on the choice of the critical state. as can be seen in figure 5, if a more even power distribution is desired, it is preferable to select a critical state at 51° control drums rotation. in the 318° critical configuration, some of the fuel rods around the perimeter would burn significantly less compared to the 51° configuration, which results in a greater temperature gradient. the results here show that the control drums not only affect the overall reactivity in the system but also affect the radial distribution of the neutron flux, because they significantly affect the neutron absorption in the system. table 4 shows the calculated power outputs in the fuel pins (average, maximum, and minimum). max(a). (b). figure 5. power distribution in the 51° configuration (a) and in the 318° configuration (b). 23 josef sabol, jan frýbort acta polytechnica ctu proceedings bol eol βeff [%] 0.7199 ± 0.0002 0.7180 ± 0.0009 excess reactivity [pcm] 1 543 ± 2 1 297 ± 9 total drums worth [pcm] 10 677 ± 3 10 773 ± 14 individual drum worth [pcm] 866 ± 3 904 ± 12 inner rod worth [pcm] 6 148 ± 3 6 206 ± 13 annular rod worth [pcm] 7 754 ± 3 7 829 ± 13 both rods worth [pcm] 8 583 ± 3 8 647 ± 13 table 5. worths of the safety components at bol and at eol. imum power output in one fuel rod in both critical configurations is very similar and does not exceed 3 kw. the same is for the radial peaking factor, which is 1.22 and 1.23 for the 51° and 318° configurations, respectively. however, the minimum power generated in the pins differs significantly, which is approximately 400 w less for the 318° configuration. 3.5. burnup the predictor-corrector method implemented in serpent 2 code was used for the depletion calculation. for the calculation, the fuel was divided into 63 360 zones. this division occurred at the level of individual fuel rods and was further subdivided into 30 axial sub-zones. the depletion took place in parallel on 32 threads with an allocated memory of 62.5 gb. standard isotopes, and set opti 2 memory optimization were considered. the final burnup of the core allows long depletion steps, but still, shorter steps were defined for the beginning of the depletion calculation to respect faster initial changes in concentrations of short-lived radionuclides. the final depletion steps were 1, 2, 5, 10, 20, and 50 days, and then in half-year steps up to 5 years. 3.5.1. change in reactivity the multiplication factor decreases roughly linearly during the depletion (see figure 6). due to the small burnup (1.77 mwd/kgu over 5 years at nominal power), the reactor is still supercritical at the end of life (eol): ϱeol max = (1297 ± 9) pcm. the effective fraction of delayed neutrons (see table 5) differs in this case mainly because of the statistics. physically, it should be different mainly due to conversion of 238u to fissile isotope 239pu with much lower fraction of delayed neutrons compared to 235u. the effect of burnup can also change the worths of control elements, because it shifts the power distribution to the periphery, leading to greater neutron leakage. since the reactor is regulated by control drums, their efficiency should change. there was an increase in all worths during the calculation (see table 5), but due to the larger statistical uncertainties, the worths of the elements are roughly 1.0125 1.0130 1.0135 1.0140 1.0145 1.0150 1.0155 1.0160 0 1 2 3 4 5 0 0.3 0.6 0.9 1.2 1.5 1.8 k e ff [ -] depletion time [years] burnup [mwd/kgu] keff figure 6. change in multiplication factor over 5 years. 0.0x100 2.0x10-8 4.0x10-8 6.0x10-8 8.0x10-8 1.0x10-7 1.2x10-7 1.4x10-7 1.6x10-7 1.8x10-7 2.0x10-7 0 1 2 3 4 5 co nc en tr at io n [g /c m 3 ] time following shutdown [days] 135xe 135i figure 7. dependency of 135i and 135xe concentration on time after shutdown. constant over time. thus, during operation, it would be possible to maintain criticality using only a slight rotation of the control drums. 3.5.2. iodine and xenon for the reactor operation in remote locations, the power output would need to be changed over time as needed. if xenon poisoning were to occur in the system, this would be very difficult, if not impossible, to regulate. the concentrations of isotopes 135i and 135xe were calculated over 5 days after shutdown. due to the low power, accumulation of the 135i isotope is not sufficient to produce a 135xe concentration peak, as can be seen in figure 7. thus, xenon dead time does not occur. 3.5.3. power distribution as was mentioned above, the effect of burnup should shift the power distribution to the periphery. in this 24 vol. 44/2023 neutronic analysis of a heat-pipe cooled reactor (a). (b). figure 8. power distribution at bol (left) and eol (right) in the 0° configuration. analysis, the neutron flux was not sufficient and even in 5 years there was no significant shift. a maximum difference was achieved up to 1 % between bol and eol, but this difference may be due to statistical uncertainties. the power distribution is roughly constant over time, as can be seen in figure 8. 3.6. decay heat it was found that at the moment of shutdown, the decay heat power reaches 329.1 kw, which is about 6.58 % of the nominal power. further, there is a decrease following the decay of accumulated radionuclides, as can be seen in figure 9. 4. conclusions in this paper, the neutronic model of the spr designb nuclear reactor concept from inl is described and basic safety analyses are calculated. all calculations were performed using monte carlo code serpent 2 and nuclear data library endf/b viii.0. the calculation model respects the material temperature field from the reference calculation [1], so thermal-mechanical feedbacks are not included. the first section provided a necessary context for understanding this topic. advantages of hpc reactors for their specific applications, such as scientific and military stations, and space applications were summarized. furthermore, the development of hpc reactors in lanl and inl has been described. the second section deals with the calculation model itself. thermal expansions, dimensional changes and density changes were considered. all the materials, their composition, expansion coefficients, and densities were taken from available references. a basic sensitivity analysis was made to assess the impact estimations of compositions and dimensions not available in the reference calculation. in the third section, the results are discussed and also compared with the original inl study [1], where mcnp 6.1 code with endf/b-vii.0 nuclear data library was used for the calculations. 0 50 100 150 200 250 300 350 1x10-8 1x10-7 1x10-6 1x10-5 1x10-4 1x10-3 1x10-2 1x10-1 1x100 1x101 h ea t po w er [ kw ] time following shutdown [years] decay heat figure 9. decay heat after shutdown. all the values obtained are very similar. all feedback coefficients are negative, as well as the worths of safety elements are high enough to shutdown the reactor. the worths increase during depletion, but due to the larger statistical uncertainties, they are roughly constant over time. the biggest differences are in the lower multiplication factor in this work. this may be due to a different model, a different nuclear data library with different code, or due to an axial neutron leakage. since the exact position and shape of axial reflectors at inl were not provided, a comparison of the axial power distribution between inl and this study was conducted. differences were identified, with the most significant disparities occurring at the bottom of the reactor core. these discrepancies suggest that variations in the multiplication factor were likely primarily caused by axial leakage. it was found a critical position of control drums at an angle of 51°, where the most even power distribution is obtained. in burnup calculation (5 years at a nominal heat power output of 5 mw), it was found that the 135i concentration is not sufficient to cause a xenon dead time. when the reactor is shutdown after 5 years, the decay heat reaches 6.56 % of the nominal power. 25 josef sabol, jan frýbort acta polytechnica ctu proceedings list of symbols at temperature reactivity coefficient [pcm/k] α thermal expansion coefficient [1/k] b burnup [mwd/kgu] βeff effective fraction of delayed neutrons [%] e energy [mev] keff effective multiplication factor [–] l length [cm] m mass [kg, g] n number of expansion directions [–] ϕ normalized neutron flux [1/s/cm2/mev] p thermal power [mw, kw] p thermal power density [w/cm3] ρ density [g/cm3] ϱ reactivity [pcm] t temperature [k] list of abbreviations bol beginning of life eol end of life hpc reactor heat-pipe cooled reactor inl idaho national laboratory lanl los alamos national laboratory spr special purpose reactor ss316 316 stainless steel acknowledgements this analysis was partially supported by esa project preliminary european reckon on nuclear electric propulsion for space applications (rocketroll), esa itt/111474/22/fr/kr. references [1] j. w. sterbentz, j. e. werner, a. j. hummel, et al. preliminary assessment of two alternative core design concepts for the special purpose reactor. tech. rep., idaho national lab. (inl), idaho falls, id (united states), 2017. https://doi.org/10.2172/1413987 [2] nuclear reactors and radioisotopes for space, 2021. [2022-10-06]. https://world-nuclear.org/informationlibrary/non-power-nuclear-applications/ transport/nuclear-reactors-for-space.aspx [3] d. i. poston, r. j. kapernick, r. m. guffee. design and analysis of the safe-400 space fission reactor. aip conference proceedings 608(1):578–588, 2002. https://doi.org/10.1063/1.1449775 [4] d. i. poston, r. j. kapernick, r. m. guffee, et al. design of a heatpipe-cooled mars-surface fission reactor. aip conference proceedings 608(1):1096–1106, 2002. https://doi.org/10.1063/1.1449841 [5] r. s. reid, j. t. sena, a. l. martinez. sodium heat pipe module test for the safe-30 reactor prototype. aip conference proceedings 552(1):869–874, 2001. https://doi.org/10.1063/1.1358021 [6] d. i. poston, m. a. gibson, r. g. sanchez, p. r. mcclure. results of the krusty nuclear system test. nuclear technology 206(sup1):s89–s117, 2020. https://doi.org/10.1080/00295450.2020.1730673 [7] p. r. mcclure, d. i. poston, v. r. dasari, r. s. reid. design of megawatt power level heat pipe reactors. tech. rep., los alamos national lab. (lanl), los alamos, nm (united states), 2015. https://doi.org/10.2172/1226133 [8] j. w. sterbentz, j. e. werner, m. g. mckellar, et al. special purpose nuclear reactor (5 mw) for reliable power at remote sites assessment report. tech. rep., idaho national lab. (inl), idaho falls, id (united states), 2017. https://doi.org/10.2172/1410224 [9] j. leppänen, m. pusa, t. viitanen, et al. the serpent monte carlo code. in annals of nuclear energy, vol. 82, pp. 142–150. 2015. https://doi.org/10.1016/j.anucene.2014.08.024 [10] d. a. brown, m. b. chadwick, r. capote, et al. endf/b-viii.0: the 8th major release of the nuclear reaction data library with cielo-project cross sections, new standards and thermal scattering data. in nuclear data sheets, vol. 148, pp. 1–142. 2018. special issue on nuclear reaction data. https://doi.org/10.1016/j.nds.2018.02.001 [11] potassium – thermophysical properties vs. temperature, 2008. [2023-01-21]. https://www.engineeringtoolbox.com/potassiumproperties-d_1210.html [12] j. k. fink, l. leibowitz. thermodynamic and transport properties of sodium liquid and vapor. p. 87. argonne national laboratory, 1995. https://doi.org/10.2172/94649 [13] v. d. arp, r. d. mccarty, d. g. friend. thermophysical properties of helium-4 from 0.8 to 1500 k with pressures to 2000 mpa. in nist technical note 1334 (revised). nist, 1994. [14] r. j. mcconn, c. j. gesh, r. t. pagh, et al. compendium of material composition data for radiation transport modeling. in homeland security, pp. 285–290. 2011. https://doi.org/10.2172/1023125 [15] j. s. coursey, d. j. schwab, j. j. tsai, et al. nist physical measurement laboratory, atomic weights and isotopic compositions with relative atomic masses, 2021. [2023-01-20]. https://physics.nist.gov/cgibin/compositions/stand_alone.pl?ele=&all=all& ascii=ascii&isotype=all [16] thermophysical properties database of materials for light water reactors and heavy water reactors. no. 1496 in tecdoc series. iaea, 2006. [17] coefficients for austenitic stainless steels (group 3). in bpvc section ii part d (metric), p. 806. asme, 2017. [18] international syalons, alumina, aloxalon 999 physical property data, 2023. [2023-01-20]. https://www.syalons.com/materials/alumina/ 26 https://doi.org/10.2172/1413987 https://world-nuclear.org/information-library/non-power-nuclear-applications/transport/nuclear-reactors-for-space.aspx https://world-nuclear.org/information-library/non-power-nuclear-applications/transport/nuclear-reactors-for-space.aspx https://world-nuclear.org/information-library/non-power-nuclear-applications/transport/nuclear-reactors-for-space.aspx https://doi.org/10.1063/1.1449775 https://doi.org/10.1063/1.1449841 https://doi.org/10.1063/1.1358021 https://doi.org/10.1080/00295450.2020.1730673 https://doi.org/10.2172/1226133 https://doi.org/10.2172/1410224 https://doi.org/10.1016/j.anucene.2014.08.024 https://doi.org/10.1016/j.nds.2018.02.001 https://www.engineeringtoolbox.com/potassium-properties-d_1210.html https://www.engineeringtoolbox.com/potassium-properties-d_1210.html https://doi.org/10.2172/94649 https://doi.org/10.2172/1023125 https://physics.nist.gov/cgi-bin/compositions/stand_alone.pl?ele=&all=all&ascii=ascii&isotype=all https://physics.nist.gov/cgi-bin/compositions/stand_alone.pl?ele=&all=all&ascii=ascii&isotype=all https://physics.nist.gov/cgi-bin/compositions/stand_alone.pl?ele=&all=all&ascii=ascii&isotype=all https://www.syalons.com/materials/alumina/ vol. 44/2023 neutronic analysis of a heat-pipe cooled reactor a. appendix (a). (b). figure 10. horizontal (left) and vertical (right) cross section of the model. fuel rod number 2,112 material uo2 enrichment 19.75 % diameter 1.492 cm length 150 cm pitch 1.8 cm temperature 1050 k cladding material ss316 gap he diameter 1.505–1.565 cm temperature 1013 k heat pipe number 1,224 material ss316 filling k (100 g) diameter 1.575–1.775 cm temperature 986 k heat transfer medium material liquid na temperature 995 k axial reflector material ss316 height 15 cm radial reflector material al2o3 outer diameter 168.1 cm height 200 cm temperature 923 k control drums amount 12 diameter 25 cm absorbtion material b4c (90 % 10b) maximum thickness 2 cm shutdown rods material b4c (90 % 10b) diameter of inner rod 11.2 cm diameter of annular rod 13.7–17.7 cm shielding ss316 shielding diameter 170.1–174 cm b4c shielding diameter 174–200 cm table 6. parameters of the spr design-b nuclear reactor concept [1]. 27 acta polytechnica ctu proceedings 44:19–27, 2023 1 introduction 2 neutronic model 2.1 software 2.2 reactor design 2.3 thermal expansion 2.4 differences in the model 3 results 3.1 reactivity of the system 3.2 reactivity coefficients 3.2.1 doppler broadening 3.2.2 thermal expansion effects 3.3 neutron spectra 3.4 power distribution 3.5 burnup 3.5.1 change in reactivity 3.5.2 iodine and xenon 3.5.3 power distribution 3.6 decay heat 4 conclusions list of symbols list of symbols acknowledgements references a appendix acta polytechnica ctu proceedings https://doi.org/10.14311/app.2025.52.0037 acta polytechnica ctu proceedings 52:37–42, 2025 © 2025 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague impact of temperature on tyre hardness david jelínek∗, marek semela brno university of technology, institute of forensic engineering, purkyňova 464/118, 612 00 brno, czech republic ∗ corresponding author: david.jelinek@vutbr.cz abstract. this article deals with the dependence of tyre hardness on temperature. in this study, more than 1 800 temperature-dependent hardness values were measured for 5 samples of summer tyres from different manufacturers and different ages. using regression analysis, curve equations were determined for each sample. furthermore, an average curve was determined, which was then applied to the measured values and the error generated by using this average curve was investigated. from the results it is clear that all samples show a polynomial function with a 2nd order polynomial for the temperature range −10 °c to 65 °c. the error of using the average curve reaches values up to 11 % of the average value of the measured hardness of the sample. the lowest error is 2 % of the mean value of the measured hardness of the sample, which corresponds to approximately 1 shore a. the suitability of using an average curve application depends on the desired output and the required accuracy. keywords: tyre, temperature, hardness, rse, regression analysis. 1. introduction a tyre changes its mechanical properties during its service life. this change in the mechanical properties of the rubber material of the tyre can result in changes in performance and safety under different operating conditions [1]. the tread part of the tyre wears out during its service life. as the tyre wears, the bonding layer, which may vary in chemical composition and material properties, may become exposed [2]. hardness measurements can be used to determine the change in mechanical properties. during oxidation, the chemical structure of the material changes, which is accompanied by a change in the mechanical properties of the tyre material. the aging or oxidation of rubber material is natural, irreversible and caused by the disruption of the polymer chain of the rubber and the binding of an oxygen atom from the environment [3–5]. to prevent ageing, oxidation-inhibiting additives called antioxidants are added to the tyre material [6]. however, these additives vary according to the manufacturer and model of the tyre. brown [3] reported in his publication how material properties change over the 40 years of a tyre’s age. in article ho et al. [7] describe that as a tyre is aged, a change in hardness of 0.6 shore a per month occurs. as reported by brown in his publication [8], hardness measurement is advantageous mainly because of the economical use of material, the almost non-destructive character of the test, cheapness and simplicity. however, it should be mentioned that repeatability and reproducibility in durometer can be relatively poor, and therefore dead load methods are preferred. the determination of dead load hardness, including microhardness, shall be carried out in accordance with iso 48. hardness is essentially a measure of stiffness and can be related to the modulus of elasticity. the measurement of the hardness of rubber materials, which definitely includes tyres, is well described in iso 48-4 or its transcriptions in čsn iso 48-4 [9]. for the measurement of the hardness of rubber materials with standard hardness, which include tyres, the type a method with a shore a unit scale or the irhd method is used. for rubber materials with specific properties there are other methods for measuring hardness, e.g. shore am, shore d, etc. the preparation of the test specimens is described in iso 23529 or its transcription čsn iso 23529 [10]. this standard specifies measurement conditions relating to standard laboratory humidity and temperature. for countries in the temperate climate belt, including central europe, the standard laboratory temperature is defined as 23 ± 2 °c and the relative humidity as 50 ± 10 %. the standard also states that if standard laboratory conditions cannot be achieved, ambient temperature and humidity shall be used. along with this, the standard specifies how long to temper the rubber samples in order to have the same temperature throughout the volume. the relationship between rubber hardness and temperature has been investigated, for example, by wehr et al. in their paper [11] is reported that the relationship is linear with a negative slope of about −0.26. at the same time, the paper shows that the relationship does not change with age, only the curve shifts by a certain value. this is consistent with the statement of ho et al. [7] of a change in hardness of 0.6 shore a per month. this paper describes the possibility of determining and applying a general curve for the dependence of tyre hardness on temperature. with this general curve, 37 https://doi.org/10.14311/app.2025.52.0037 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en david jelínek, marek semela acta polytechnica ctu proceedings sample manufacturer product index age at the tread note identification measurement depth s1 continental premiumcontact 7 205/55 r16 91 v 0,4-0,8 years 7,9 mm abrased s2 continental premiumcontact 7 205/55 r16 91 v 0,4-0,8 years 8 mm non-abrased s3 continental premiumcontact 2 215/55 r18 95 h 16,9 years 4 mm s4 barum brillantis 165/80 r14 85 t 17,8 years 3 mm s5 dunlop sportmaxx 225/45 r17 91 w 6,4 years 5 mm table 1. measured samples and their parameters. it would be possible to calculate hardnesses to different temperatures based on a single accurate measurement. for example, this method could help determine the level of tyre degradation during vehicle inspections for accident analysis when the vehicle cannot be tested on road. 2. materials and methods this chapter will describe the measured samples, the measuring equipment and the process of measuring and evaluating the measured data. 2.1. samples samples from summer tyres of different manufacturers and ages were used to investigate the dependence of material hardness on temperature. the size of the samples was at least 5 × 5 cm. a list of these is given below: • samples 1 and 2 were taken from the new continental premiumcontact 7 tyre of size 205/55 r16 with load index 91 and speed index v. according to the dot code, the tyre was manufactured in the 9th week of 2023, so during the measurement process its age can be determined to 0.4–0.8 years. for engineering reasons, the tyre surface is treated with a thin layer of protective coating. in order to eliminate the possible influence of the hardness values by this coating, sample 1 was abraded to a depth of 0.1 mm over the entire tread area. sample 2 was not abraded and still contains a protective layer. • sample 3 was taken from a continental premiumcontact 2 tyre of size 215/55 r18 with a load index of 95 and a speed index of h. according to the dot code, the tyre was manufactured in the 9th week of 2006, so its age can be determined to be 16.9 years during the measurement. this tyre has not been operated for at least the last 3 years and has been stored at a constant temperature and humidity, thus slowing down the degradation process. the tyre’s further history of use is unknown, as is the way it has been stored in the past. the tread depth of this tyre was 4 mm. • sample 4 was taken from a barum brillantis tyre of size 165/80 r14 with a load index of 85 and a speed index of t. according to the dot code, the tyre was manufactured in the 14th week of 2005, so its age can be determined to be 17.8 years during the measurement. this tyre has not been operated for at least the last 3 years and has been stored at a constant temperature and humidity, thus slowing down the degradation process. the tyre’s further history of use is unknown, as is the way it has been stored in the past. the tread depth of this tyre was 3 mm. • sample 5 was then taken from a dunlop sportmaxx tyre of size 225/45 r17 with a load index of 91 and a speed index of w. according to the dot code, the tyre was manufactured in week 40 of 2016, so its age can be determined to be 6.4 years during the measurement. this tyre has not been operated for the last 3 years and has been stored at a constant temperature and humidity, thus slowing down the degradation process. the tyre’s further history of use is unknown, as is the way it has been stored in the past. the tread depth of this tyre was 5 mm. the samples included tyres from different manufacturers and different ages. extremes are also included among the ages, which could be an indicator of the influence of age on the studied relationship. for clarity, table 1 describing each sample is provided: 2.2. measurement and evaluation process for hardness measurement, the hpe iii equipment was used in shore a hardness measurement modification as required by the standard. the device can measure both the hardness and temperature of the sample, as well as the relative humidity and ambient temperature. the device was calibrated with a certificate according to din en iso/iec 17025:2018. this process is aimed at control measuring distance by using a control ring with base plate simulating a constant hardness value, in our case always 40 shore a. although the manufacturer declares that the device is designed to measure the hardness of the sample between 0 °c and 70 °c, measurements were made outside this range. this limitation must be taken into consideration in the evaluation of the measurement results. the hardness of the samples was measured at room temperature and relative humidity. to measure the hardness of the samples at temperatures below room temperature, the samples were tempered to −20 °c and the measurements were performed continuously while they were heated. similarly, the hardness of the 38 vol. 52/2025 impact of temperature on tyre hardness figure 1. temperature dependence of sample hardness with standard deviation and regression curve. samples was measured at temperatures above room temperature by tempering the samples to 70 °c and measuring the hardness as the samples cooled. from the measured values of all 5 samples, a database of more than 1 800 temperature-dependent hardness values was recorded together with the relative humidity and ambient temperature. it was an effort to make multiple measurements for a given temperature so that any measurement error could be eliminated and the standard deviation σ in the measured values determined. statgraphics software version 15.1.02 was used to create the regression model. for the regression analysis, the average values of the measured hardness and temperature of the samples at the time of measurement were given as input values. the standard deviation was then used as a weight wi = 1/(σ2 i ) to ensure greater reliability of the regression model. statistical significance indices representing the p-value in the anova table, and the highest achievable r2 reliability values were compared for evaluation. 3. results 3.1. tyre hardness vs. temperature statistical evaluation shows that for all samples the p-value in the anova table is less than 0.05, and thus there is a statistically significant relationship between sample temperature and hardness at the 95 % confidence level. the regression curves for all samples show a polynomial function with a 2nd order polynomial. except for sample 4, the p-value for the 3rd order is greater than 0.05 and thus this order is not statistically significant at the 95 % higher confidence level. the common feature for all samples is the convexity of the function and the negative slope of the curve. the graph below shows the measured values along with the standard deviation and the intercept of the regression curve for each sample. figure 1 shows that the dependence of hardness on temperature varies from sample to sample. for this reason, the change in hardness due to material degradation should always be considered for a new tyre of the same model and manufacturer. the possibility of using universal dependence will be discussed in the following chapter. a common indicator is the increased stability of hardness at the higher temperatures at which summer tyres are usually operated and to which they may be heated as a result of their use. table 2 shows the equations of the regression curves for each measured sample, where h(t ) is the hardness of the rubber material of the sample in shore a units and t is the temperature of the sample in °c. in addition, the table includes the coefficient of determination r2, the standard error of estimate σest and the mean absolute error mae. 3.2. determination of the average curve from the regression curves, it is possible to identify one average curve avg curve that would describe the temperature dependence of the hardness of any summer tyre. using this curve, a recalculation of the hardness and an assessment of the degradation rate could then be made even if the tyre hardness was not measured at the same temperature. from the available regression curves of the individual samples, it is possible to determine by arithmetic averaging the coefficient a (determining the degree of curvature) and the coefficient b (determining the slope) of the average polynomial curve of the second line, which describes the dependence of the hardness of the material on temperature. the constant c, which 39 david jelínek, marek semela acta polytechnica ctu proceedings no. of sample equation of the regression curve r2 value σest mae s1 h(t ) = 0.0027 t 2 − 0.3061 t + 62.1320 99.67 % 0.7283 0.5317 s2 h(t ) = 0.0026 t 2 − 0.2966 t + 62.1606 94.90 % 0.6324 0.4333 s3 h(t ) = 0.0015 t 2 − 0.2110 t + 70.1874 92.69 % 1.1202 0.8143 s4 h(t ) = 0.0041 t 2 − 0.5306 t + 88.9063 97.20 % 2.2211 1.4130 s5 h(t ) = 0.0008 t 2 − 0.1296 t + 67.9092 86.02 % 1.2935 0.8892 table 2. regression curve equations for each sample. figure 2. range of avg curve application to samples. indicates the shift of the curve in the hardness axis, must be calculated for each sample. as from the studies of ho et al. and wehr et al. [7, 11] show, this constant depends, among other things, on the condition of the rubber material of the sample. h(t ) = 0.0024 t 2 − 0.2948 t + c (1) whether this equation can be used to predict the hardness at a certain temperature for summer tyres will be discussed in the following section. 3.3. application of the average curve to samples the measured hardness values for individual samples can be used to verify the average curve applied to real data. since hardness values were not found for all samples for all temperatures, the constant c will always be determined using regression curves. in this way, the variation of the measured hardness values that gave rise to the regression curves will be included in the results. figure 2 shows the measured values for each sample. the dashed lines then show the ranges in which the avg curve (1) varies depending on the calculation of the constant c. the range shows that the resulting avg curve does not adequately describe all the measured data. at the same time, the level of confidence is dependent on the value of the constant c. respectively, whether the hardness value for a specific temperature, from which the constant c is calculated, is close to the avg curve. to evaluate the reliability of the avg curve applied to the sample hardness data, the average residual standard error rse calculation method was chosen. this method is used to describe the difference in standard deviations of the observed values compared to the predicted values. it is a measure of goodness of fit that can be used to analyze how well a set of data points fits the real model. the calculation of the rse was performed by calculating the difference between the value determined by the avg curve and the measured sample hardness. the rse was divided by the average value of the measured hardness of each sample to convert to a percentage value. this gives an estimate of the error relative to the average hardness value achieved by the sample over the specified temperature range. figure 3 shows the percentage rse between the avg curve and the measured hardness values of the samples. 40 vol. 52/2025 impact of temperature on tyre hardness figure 3. rse fitted avg curve to samples. the results show that for samples s1 and s2, the error using the avg curve is within 2 % of the average value of the measured hardness, independently of the temperature range over which the constant c was determined. lower error values can be observed for samples s1, s2 and s3, which are samples from the same tyre manufacturer. for the other samples, the error increases when using the calculation of the constant c at low temperatures. for this case, high inaccuracy is inserted into the resulting hardness recalculation when using avg curve. the 4 % error limit corresponds to 2–3 shore a units for the measured samples, which for example corresponds to up to 5 months of different tyre age when using the ho at al. calculation [7]. at the same time, due to the stabilization of hardness at higher temperatures, even a 2 % error can significantly affect the results. 4. discussion the results presented in this paper show that although the dependence of rubber hardness on temperature may appear linear over a limited temperature range, it is more accurate to use a 2nd order polynomial curve when considering a larger range. in this paper, tyre hardness was not investigated at extremely low or extremely high temperatures, which could change the hardness beyond the investigated dependence. although the manufacturer of the measuring device states that the device is not designed to measure sample hardness below 0 °c, the results of the regression analysis show good agreement in the measured data even below this critical temperature. only for sample s4 the regression curve does not agree significantly with the measured data. the avg curve reliability results indicate that there is increased inaccuracy at low temperatures for samples s3, s4 and s5. this inaccuracy is not limited by the 0 °c. rather, it will be due to the lower number of measurements for these samples at low temperatures, and therefore the lower weight wi used in the development of the regression curves for these samples. due to the different chemical compositions of tyres from different manufacturers or model series, it is not possible to establish a curve describing the general range of summer tyres that would perfectly describe the temperature dependence of hardness. using the avg curve produces an error which is dependent on the calculation of the constant c. for this reason, it seems appropriate to compare the hardness change at a specific temperature within the material degradation for tyres of the same material or chemical composition. for sample s4, where the highest hardness was measured, the question arises whether the wear caused the exposure of another than the thread layer. the measured surface may not necessarily be homogeneous. this layer may have different chemical composition and mechanical properties. to identify this layer, a material analysis could be carried out using computer tomography. however, this was not performed as part of the study. additional measurements involving a larger number of samples would be appropriate to verify the results presented in this article. in addition, further measurements could be extended to winter tyres. 5. conclusions this article was aimed at the possibility of recalculating the tyre hardness for different temperatures using the established average curve of hardness versus tyre temperature. the results show that the use of 41 david jelínek, marek semela acta polytechnica ctu proceedings an average curve is associated with an error that can significantly affect the resulting hardness value found. the resulting error can be reduced by using tyres of the same model and manufacturer. the regression curves for all samples show a polynomial function with a 2nd order polynomial for the temperature range −10 °c to 65 °c. for each of the samples measured, the regression curve varies the coefficient a (determining the degree of curvature) and the coefficient b (determining the slope) of the polynomial curve equation. the constant c of the polynomial curve equation must always be calculated for a specific sample based on the known hardness of the sample at a known temperature. the results of the temperature dependence of summer tyre hardness show that hardness stabilises as temperature increase. samples s1, s2, s3 and s5 consistently show that in the temperature range 30–65 °c the hardness change is minimal. on the other hand, at low temperatures there is a significant increase in hardness, indicating that the change in mechanic properties may also affect the performance of summer tyres at these low temperatures. acknowledgements this paper was written within the project of the specific university research entitled “evaluation of the oxidation state of rubber material of tyres in relation to their dot code” (no. úsi-j-23-8293) and was carried out at the institute of forensic engineering of brno university of technology. references [1] t. d. gillespie. fundamentals of vehicle dynamics. society of automotive engineers, warrendale, 1992. xxii, 495 pages. isbn 1-56091-199-9. [2] d. jelínek. sken pneumatiky zapomocí počítačové tomografie [in czech; tyre scan using computed tomography]. ústav soudního inženýrství vut v brně, brno, 2022. [3] r. p. brown, t. butler. natural aeging of rubber: changes in physical properties over 40 years. rapra technology limited, united kingdom, 2000. isbn 1-85957-209-x. [4] a. herzig, m. johlitz, a. lion. experimental investigation on the consumption of oxygen and its diffusion into elastomers during the process of ageing. in b. marvalova, i. petrikova (eds.), constitutive models for rubber ix, pp. 23–28. 2015. isbn 978-0-429-22581-9. https://doi.org/10.1201/b18701 [5] s. wagner, p. klöckner, t. reemtsma. aging of tire and road wear particles in terrestrial and freshwater environments – a review on processes, testing, analysis and impact. chemosphere 288:132467, 2022. https: //doi.org/10.1016/j.chemosphere.2021.132467 [6] analysis of the chemical composition of car tire rubber. international journal of advance scientific research 2(12):183–191, 2022. https://doi.org/10.37547/ijasr-02-12-26 [7] k.-y. ho, w.-t. hung, c.-f. ng, et al. the effects of road surface and tyre deterioration on tyre/road noise emission. applied acoustics 74(7):921–925, 2013. https://doi.org/10.1016/j.apacoust.2013.01.010 [8] r. p. brown, m. j. forrest, g. soulagnet. long-term and accelerated aeging tests on rubbers. rapra technology limited, united kingdom, 2000. isbn 1-85957-185-9. [9] čsn iso 48-4:2019. pryž, vulkanizovaný nebo termoplatický elastomer – stanovení tvrdosti – část 4: tvrdost metodou vtlačování hrotu tvrdoměru (tvrdost shore). [in czech; rubber, vulcanized or thermoplastic – determination of hardness – part 4: indentation hardness by durometer method (shore hardness)], 2019. [10] čsn iso 23529:2017: pryž – obecné postupy pro přípravu a kondicionování zkušebních těles pro fyzikální zkušební metody [in czech; rubber – general procedures for preparing and conditioning test pieces for physical test methods], 2017. [11] r. wehr, a. fuchs, c. aichinger. a combined approach for correcting tyre hardness and temperature influence on tyre/road noise. applied acoustics 134:110–118, 2018. https://doi.org/10.1016/j.apacoust.2018.01.004 42 https://doi.org/10.1201/b18701 https://doi.org/10.1016/j.chemosphere.2021.132467 https://doi.org/10.1016/j.chemosphere.2021.132467 https://doi.org/10.37547/ijasr-02-12-26 https://doi.org/10.1016/j.apacoust.2013.01.010 https://doi.org/10.1016/j.apacoust.2018.01.004 acta polytechnica ctu proceedings 52:37–42, 2025 1 introduction 2 materials and methods 2.1 samples 2.2 measurement and evaluation process 3 results 3.1 tyre hardness vs. temperature 3.2 determination of the average curve 3.3 application of the average curve to samples 4 discussion 5 conclusions acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2024.51.0036 acta polytechnica ctu proceedings 51:36–41, 2024 © 2024 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague augmented emergency lighting as a soft early warning: measures on road users when moving over björn lidestama,∗, philip lindblada,b, petr ondomišia,c, emilia sörengårda,b, kajsa weibulla a vti (the swedish national road and transport research institute), se-581 95 linköping, sweden b linköping university, department of computer and information science, se-581 83 linköping, sweden c linköping university, department of science and technology, se-601 74 norrköping, sweden ∗ corresponding author: bjorn.lidestam@vti.se abstract. a novel interface to provide robust, intuitive, and timely alerts to road users to move over for emergency vehicles (evs) was tested in a seven-minute driving-simulator experiment. the interface, augmented emergency lighting (ael), mimics emergency lighting on emergency vehicles (i.e., flashing blue light), by using the cars’ interior lighting. ael was initiated 30 seconds before an ev caught up and increased its brightness proportionally to the distance to the ev. the ael alert was after 16 seconds accompanied by an emergency vehicle approaching (eva) alert, presented as a voice command and a text message on the dashboard. comparisons were made between ael+eva alerts, eva-only alerts, and no alerts, between groups (n = 61). both ael+eva alerts and ael-only alerts were highly successful in getting drivers to move over by slowing down and pulling off to the side as compared to when there were no alerts, but no significant difference between ael+eva and eva-only was found. questionnaire responses showed that ael+eva alerts were generally appreciated and that their timing was good. further studies on learning effects and usability are discussed. keywords: emergency driving, warning messages, alerts, soft warning messages, simulator study. 1. introduction emergency vehicle approaching (eva) messages are direct messages, presented within vehicles, to alert and instruct the driver to give way. otherwise, traditionally, when emergency vehicles are on-call, what they have at their disposal to notify others to give way and warn other road users are sirens and flashing lights. this has restraints regarding its reachability [1]. however, the response time and arrival speed of the emergency vehicle play a critical role in determining the chances of survival for individuals in need. for instance, when an ambulance is transporting a person with severe blood loss to the hospital or in case of fire, a quick response is crucial. one of the main factors contributing to the risk of road traffic accidents is primarily the high speed at which emergency vehicles are travelling when oncall [2]. it was discovered that most road users respond positively to emergency vehicles, but a notable portion struggle to handle the interaction [2]. on-call emergency driving at high speed is associated with a significantly increased accident rate [3]. giving way to emergency vehicles on call is also of great importance. in a review of data from a paramedic division in denver on emergency vehicle accidents and collisions [4], the importance of emergency vehicle drivers clearing intersections before entering was highlighted. it was concluded that visual and auditory warning systems should be used to alert other road users, ensuring that they yield for the emergency vehicle to pass without any issues. by giving way to emergency vehicles, it becomes possible for the drivers to reach their destination quickly, potentially saving lives. moreover, giving way helps reduce the risk of accidents and dangerous situations, as emergency vehicles can move more safely and efficiently when other vehicles are out of the way [4]. fatal and nonfatal crashes involving civilians and emergency vehicles have also been studied [5], with the conclusion that various factors contribute to crashes with emergency vehicles: environmental factors (e.g., sunlight affecting driver visibility) and roadway factors (e.g., visual obstructions caused by external objects). the most common violation among road users leading to crashes is the failure to give way. the failure to give way is attributed to road users’ inability to visually detect approaching emergency vehicles, resulting in inappropriate driving that increases crashes and accidents [5]. also, inexperienced drivers may detect approaching emergency vehicles on call but do not know that they are obliged to give way and how to do so [3]. in order for warning systems to be effective and efficient, they need to be perceived and used as intended by the design. compliance with warning systems can sometimes be uncertain. a study on the perception of an alert system among university students and their attitudes toward its importance showed that following the prevention of a crisis at the university, the students’ perspective of the warning system underwent 36 https://doi.org/10.14311/app.2024.51.0036 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 51/2024 augmented emergency lighting as a soft early warning . . . a change [6]. the students admitted in retrospect that they had not taken the emergency alert system seriously at first, but after experiencing a campus lockdown, their overall perception of the system became more favourable. this suggests that the experience of an alert system can positively influence both the perception and effectiveness of the system [6]. the optimal, or most efficient, time span for an eva message to be emitted before the emergency vehicle catches up has not yet been determined. in a previous study on eva messages, the message was sent out 14 seconds before the ambulance was estimated to pass the participant with instructions regarding slowing down and giving way for an approaching ambulance on call [3]. the message was presented via text on an instrument cluster and with sound where a male voice said “warning! emergency vehicle approaching! please give way!”. on the instrument cluster, a yellow triangle was presented as a warning sign with similar text. the eva alert was crucial in influencing the participants’ ability to give way for the ambulance [3, 7]. additionally, the use of warnings showed the potential to reduce collision risks and minimize the driving time of emergency vehicles [7]. an augmented emergency lighting (ael) is a warning that is displayed via the interior lighting of a car, mimicking the emergency lighting mounted on emergency vehicles. this technology can be used in combination with an eva message where the flashing of lights increases, proportionally to the distance, inside the car as the emergency vehicle approaches the road user, making it a “soft” warning. the flashing of lights is, thus, the most intense as the emergency vehicle passes by the road user and when it has passed the flashing of lights stops. this can be used in combination with eva alerts presented as a voice and a text explaining that an emergency vehicle is approaching and there is a need to slow down and give way. this should then, theoretically, mean that the emergency vehicle on call signals its approach both via robust-and-early signals of what should be the learned-and-intuitive medium of presentation (i.e., seeing approaching flashing blue lights), and also giving direct instructions on how to act. that is, ael plus eva alerts should mean that the emergency vehicle on call is first robustly detected and that the driver receives instructions on what to do. in [3], the warning could be seen as a direct warning since it was given 14 seconds before the ambulance caught up. there is little research on when exactly it is most suitable for a road user to be notified that an emergency vehicle is approaching. however, research over several decades confirms the influence of warning timing on performance [8]. when the time between cue and target presentation is not perfect, it can create a temporal conflict that causes delays in response due to limitations in resource and information processing [9]. on the other hand, if a warning is issued too early and the cause is not perceived, drivers may ignore it, whereas a warning that comes too late will be ineffective [10]. the effects of an advanced warning or a “soft” alert impacts on drivers when interacting with emergency vehicles was studied in [11], where a positive result was observed as the speed reduction was linked to a decrease in injury severity. this highlights the significance of the relationship between impact speed and injury. the cause for this decrease in speed was found to be response priming [11]. the finding could be applicable in other settings where an alert is presented before a perceived threat, as response priming or a soft alert may provide safety benefits. a soft alert in this context is a warning that is given earlier, for instance about 30 seconds before the emergency vehicle passes by. the ael increases its intensity as the emergency vehicle approaches, mimicking the increase in intensity proportional to distance. the aim of the present study was to answer the following two research questions. firstly, does an early ael alert for approaching emergency vehicles add benefits when preceding an eva message? secondly, how is the ael experienced by drivers? 2. materials and methods 2.1. participants the study initially had 62 participants, but due to one participant being unable to continue with the simulation, the final sample size was n = 61 participants (n = 44 men, n = 17 women, aged 29–80 years, m = 58 years 4 months, sd = 12 years 11 months). participants were randomized to the baseline (control group), eva, or ael+eva groups, respectively. they had had their driver’s licenses for 11–62 years (m = 39 years 10 months). regarding driving frequency, 61% reported driving daily, while 34% reported driving most days a week. ten of the participants were professional drivers of buses, trucks, or emergency vehicles as part of their job. inclusion criteria were (1) having had their driver’s license for ten or more years (category b in sweden) and (2) having driven more than 15 000 km per year on average. note: for purposes of another study, the effect of eva messages on experienced drivers was compared to [3], where most of the participants were new inexperienced drivers. 2.2. materials the same simulator, stimuli, and procedure as in [3] were used, albeit with an upgrade of the graphics [12]. the participants were sitting in a chair resembling a real car, with a 43-inch tv monitor placed 95 cm in front of them and a refresh rate of 60hz. in addition to the 43-inch front monitor, two 55-inch monitors were used to extend the field of view. they were placed at a 45° angle from the front monitor at each side, resulting in about 180° field of view horizontally. after the driving scenario in the simulator, the participants filled out a questionnaire regarding their 37 b. lidestam, p. lindblad, p. ondomiši et al. acta polytechnica ctu proceedings measure control eva-only ael+eva n m sd n m sd n m sd distance, > 3 m right – – – 16 27.78 13.01 13 36.02 16.08 speed (kph) 21 80.86 5.43 20 61.74 20.67 20 56.07 22.45 lateral position (m) 21 1.96 0.40 20 2.94 0.84 20 2.86 0.78 table 1. descriptive statistics for the control and experimental groups. experiences of the driving scenario, whether or not they noticed the ambulance before it passed them, experiences and attitudes regarding the alerts (for those in the experimental groups), and how they had learned how to behave when emergency vehicles on call are approaching (see appendix a). 2.3. procedure and specifications the test took about seven minutes to complete. the participants were driving on a narrow road that was 0.75 meters wide, parking pockets appeared about 1 000 meters apart. the road the participant was driving on consisted of hills and vegetation as close as 1.75 meters to the roadside. the purpose of this was so that the ambulance was not easily seen by the participant. meeting cars appeared every 20 seconds and at 2 200 meters from the start, a car appeared in front of the participant’s car. at this point, a car and a van were 180 and 150 meters behind the participant. an ambulance then caught up with the driver at 3 250 meters and the cars behind the participants were supposed to give way when the ambulance was about 40 meters behind them. sounds from the environment outside were presented, this included the sounds of driving and the sirens from the approaching ambulance. music at 80 db was also played during the scenario and the reason for these distractions was to make it difficult for the participant to detect and hear the ambulance. the ael consisted of flashing lights that appeared 30 seconds before the ambulance caught up, with an increase in intensity proportional to distance. then, an eva message was presented as a yellow triangle with a blue alert light and a text message stating “emergency vehicle approaching!” “pay attention!”, 14 seconds before the ambulance caught up. at that point, the radio also changed to a male voice saying “warning! emergency vehicle! please give way!”. the ael was then toned down quickly after the ambulance took over. for the baseline group, there was no alert, but the same scenario played out. 2.4. design the design was between groups. participants were randomized into either of three groups. there was one control group, which received neither ael nor eva alert; and two experimental groups, namely the evaonly group, which received an eva message 14 seconds before the ambulance caught up; and the ael+eva group, which first received ael 30 seconds before the ambulance caught up, followed by the eva message 14 seconds before the ambulance caught up. all groups received a questionnaire after the simulation was completed to evaluate the ael, the eva message, and timing aspects. to determine the effects of ael and eva, respectively, three dependent variables were measured. the first one was the distance to the ambulance when giving way by exceeding 3 meters to the right of the road centre line. the second dependent variable was the speed of the participant at the time the ambulance passed by. the third dependent variable was the lateral distance to the ambulance when it passed. the independent variable was group (i.e., control; eva-only; ael+eva). 3. results mean differences between the three groups were tested by means of anova and t tests to test the effects of eva-only and ael+eva. however, a first and striking result was that none of the participants in the control group (i.e., 0/21, r = 3.0) gave way, which resulted in a significantly heterogeneous distribution, χ2 (1, n = 61) = 29.92, p < .001, since the majority of participants in the eva-only and ael+eva groups did give way (16/20 and 13/20, respectively). regarding longitudinal distance when giving way by moving more than 3 meters to the right of the road centre line, both the eva-only and the ael+eva did so, whereas the control group (who received no alert) did not, see table 1. no inferential statistics could therefore be calculated for the longitudinal distance for the control group. the two experimental groups did not differ with regard to distance, t(27) = 1.53, ns, see table 1. concerning speed when being taken over by the ambulance, the experimental groups drove slower, f (2, 58) = 11.04, mse = 315.24, p < .001, η2 = 0.28. both experimental groups drove slower than the control group (p < .01), but did not differ from each other. the lateral position was also affected by the alerts, such that drivers who received eva-only and ael+eva alerts moved farther to the right, f (2, 58) = 12.54, mse = 0.48, p < .001, η2 = 0.30. both experimental groups moved farther to the right than the control group (p < .001) but did not differ from each other. in order to further test the effects of the alerts, separate analyses were made on those participants who successfully gave way by moving more than 3 meters 38 vol. 51/2024 augmented emergency lighting as a soft early warning . . . measure control eva-only ael+eva n m sd n m sd n m sd speed (kph) – – – 16 66.96 12.55 13 56.05 22.09 lateral position (m) – – – 16 3.16 0.75 13 3.15 0.73 table 2. descriptive statistics for only those participants who gave way by moving more than 3m from the road center. to the right of the road center line, see table 2. since no participant in the control group exceeded 3 meters, t tests were used to compare the experimental groups. no significant difference between eva-only and ael+eva was found, however. for longitudinal distance, see above and table 1. speed did not differ, t(18.12) = 1.59, ns; nor did lateral position, t(27) = 0.04, ns, see table 2. regarding how the ael message with a soft warning was experienced by the ael+eva group, the mean rating of the alert, on a scale of 0–100, was m = 77.5. with regard to the timing of the alert, 55 percent wanted an earlier alert, 40 percent did not want any change regarding the timing of the alert, whereas only 5 percent wanted a later alert. however, only 25 percent wanted blue led lights as an alert and 55 percent responded that they wanted the information displayed on the instrument panel. 4. discussion alerts presented as ael and eva were successful with regard to prompting drivers to give way – no participant in the control group managed to give way by exceeding 3m to the right of the road center line, whereas the majority of the participants in the experimental groups did. this is a very strong result, and also surprising, since the participants were experienced as compared to the majority of participants in [3]. this shows that alerts presented as eva and ael have strong effects even though the drivers have driving experience and therefore should be proficient at noticing emergency vehicles on call, and to know how what to do. the present study as well as [3] shows that in-vehicle alerts greatly improve alertness and driving behaviour to approaching emergency vehicles on call. giving way is one of the most important aspects for reducing the number of accidents [4]. road users who find the interaction with approaching emergency vehicles difficult [2] could therefore be helped by ael and eva alerts, by being alerted and instructed on giving way. the present study did not show that ael provides added benefits when preceding an eva message, however, as there was no statistically significant effect. it is possible that the statistical power was too low to detect a possible relatively small effect size (in the present study the nonsignificant cohen’s d ≈ .57 for longitudinal distance when giving way), so further studies are encouraged. furthermore, the effect of ael alone should be studied, to compare its effect in comparison to no in-vehicle alert, and to eva messages. it can be assumed that since ael alone does not provide instruction on what to do, the effect should be stronger for experienced drivers as compared to novice drivers. regarding how drivers experience ael, the majority liked the in-vehicle alerts with ael included, and 25 percent responded that they wanted ael in their alerts. regarding the importance of timing and how it affects performance as described by [8], participants liked the timing of the alert or wanted it earlier. the results thus suggest that an alert at least 14 seconds before the emergency vehicle passes is preferred, but too early warnings may cause some road users to ignore or forget them [10]. attitudes toward a warning system can change after people have experienced the alert; experiencing the alert can positively change the perception and possibly the efficiency of it [6]. some participants in the present study reacted more positively to the ael alert when it was explained, after driving and filling out the questionnaire. it is possible that the efficiency of and positive attitude toward ael increase once the users have experienced and found out how the ael works. 5. conclusions alerts presented as ael and eva were highly successful with regard to prompting drivers to give way: an ael alert combined with an eva message effectively alerts the road user that an emergency vehicle is approaching and instructs them to give way. the ael alert preceding the eva message was also well appreciated. however, there was no significant effect when comparing ael+eva to the eva-only alerts, the stand-alone effect of ael remains to be studied. most participants wanted no change in the timing of alerts or wanted it earlier. references [1] k. catchpole, d. mckeown. a framework for the design of ambulance sirens. ergonomics 50(8):1287–1301, 2007. https://doi.org/10.1080/00140130701318780 [2] g. saunders, a. gough. emergency ambulances on the public highway linked with inconvenience and potential danger to road users. emergency medicine journal 20(3):277–280, 2003. https://doi.org/10.1136/emj.20.3.277 [3] b. lidestam, b. thorslund, h. selander, et al. in-car warnings of emergency vehicles approaching: effects on 39 https://doi.org/10.1080/00140130701318780 https://doi.org/10.1136/emj.20.3.277 b. lidestam, p. lindblad, p. ondomiši et al. acta polytechnica ctu proceedings car drivers’ propensity to give way. frontiers in sustainable cities 2(19), 2020. https://doi.org/10.3389/frsc.2020.00019 [4] c. b. custalow, c. s. gravitz. emergency medical vehicle collisions and potential for preventive intervention. prehospital emergency care 8(2):175–184, 2004. https://doi.org/10.1016/s1090-3127(03)00279-x [5] c. drucker, s. g. gerberich, m. p. manser, et al. factors associated with civilian drivers involved in crashes with emergency vehicles. accident analysis & prevention 55:116–123, 2013. https://doi.org/10.1016/j.aap.2013.02.035 [6] d. e. kopel, v. k. sims, m. g. chin. taking emergency warnings seriously. proceedings of the human factors and ergonomics society annual meeting 58(1):1129–1133, 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[8] m. i. posner, r. klein, j. summers, s. buggie. on the selection of signals. memory & cognition 1:2–12, 1973. https://doi.org/10.3758/bf03198062 [9] t. j. triggs. capacity sharing and speeded reactions to successive signals. university of michigan, ann arbor, 1968. [10] j. d. lee, d. v. mcgehee, t. l. brown, m. l. reyes. collision warning timing, driver distraction, and driver response to imminent rear-end collisions in a high-fidelity driving simulator. human factors 44(2):314–334, 2002. https://doi.org/10.1518/0018720024497844 [11] m. g. lenné, t. j. triggs, c. m. mulvihill, et al. detection of emergency vehicles: driver responses to advance warning in a driving simulator. human factors 50(1):135–144, 2008. https://doi.org/10.1518/001872008x250557 [12] epic games. unreal engine, 2023. [2023-09-16]. https://www.unrealengine.com/en-us 40 https://doi.org/10.3389/frsc.2020.00019 https://doi.org/10.1016/s1090-3127(03)00279-x https://doi.org/10.1016/j.aap.2013.02.035 https://doi.org/10.1177/1541931214581236 https://doi.org/10.3758/bf03198062 https://doi.org/10.1518/0018720024497844 https://doi.org/10.1518/001872008x250557 https://www.unrealengine.com/en-us vol. 51/2024 augmented emergency lighting as a soft early warning . . . a. appendix – post-experiment questionnaire * questions that all three groups were presented with • questions presented only to the experiment groups (i.e., not control) • how did you feel about the warning? 0–100 (did not like at all – liked very much) • what do you think about the timing of the warning? � i want it earlier � i want it later � no change • how would you like to receive the warning? (select all that apply) � message on the dashboard � message on the car’s central screen � blue lights in the car � message on a head-up display (windscreen) * did you notice the ambulance before it passed? � yes � no * did you know what to do when the ambulance approached? � yes � no * where did you learn what to do when an emergency vehicle approaches? (choose one) � driving school � observed how others do it � was told by someone � i never got the information � other: _____ • what did you like about the warning? _____ • what did you not like about the warning? _____ * what is your general reaction when an emergency vehicle approaches? (select all that apply) � i get stressed � i reduce speed � i lower the volume of the radio � i become more aware of my own driving � i adapt to others in the traffic � i wait until the emergency vehicle is very close before i give way � i try to give way immediately regardless of the distance to the emergency vehicle � i try to give way, even if it violates normal traffic rules � i do not know how to act in such a situation 41 acta polytechnica ctu proceedings 51:36–41, 2024 1 introduction 2 materials and methods 2.1 participants 2.2 materials 2.3 procedure and specifications 2.4 design 3 results 4 discussion 5 conclusions references a appendix – post-experiment questionnaire acta polytechnica ctu proceedings https://doi.org/10.14311/app.2024.50.0007 acta polytechnica ctu proceedings 50:7–12, 2024 © 2024 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague multi-parameter optimization of layered ws2-polymer nanocomposite under mechanical loading elisaveta kirilovaa,∗, tatyana petrovaa, boyan boaydjieva, rayka vladovaa, apostol apostolova, petia dineva-vladikovab, alexander moravskic a bulgarian academy of sciences, institute of chemical engineering, acad. g. bonchev str., bl.103, sofia 1113, bulgaria b bulgarian academy of sciences, institute of mechanics, acad. g. bonchev str., bl.4, sofia 1113, bulgaria c sofia university “st. kliment ohridski”, faculty of mathematics and informatics, 5 james bourchier blvd, sofia 1164, bulgaria ∗ corresponding author: e.kirilova@iche.bas.bg abstract. the aim of the study is to determine the optimal geometry and magnitude of the applied load to ensure safety and prevent delamination in a three-layered nanocomposite structure under axial mechanical loading. the structure consists of a layer of the nanomaterial tungsten disulfide (ws2) and a substrate layer of poly(methyl methacrylate) (pmma), which are adhesively bonded using su-8 glue. to achieve this aim, a multi-parameter optimization problem (mop) is formulated. it includes a two-dimensional stress function model that describes stress transfer in the considered three-layer structure. two types of analytical solutions for the interface shear stress (iss) are derived, featuring real and complex roots. the decision variables in mop include the external load, layer thicknesses, and structure length. the optimization criterion is defined as the minimization of the difference between the model iss and ultimate shear stress (uss) in the adhesive layer to assure no delamination occurs in the nanocomposite structure. a genetic algorithm and alternative optimization approach developed within the framework of “mathematica” are implemented for the optimization of both model solutions. as a result, optimal values for the given external load, layer thicknesses, and structure length are obtained for considered nanostructure. for the case of an iss model solution with real roots, the optimization procedures ensure optimal geometries that physically correspond to thinner structural layers, but they are limited at lower possible loads. in contrast, for the case of a model solution for the iss with complex roots, solutions for the optimal geometries of the nanostructures were obtained with thicker layers requiring higher loads, than thinner ones, but delamination does not occur. keywords: 2d stress function model, interface shear stress, genetic algorithm, multi-parameter optimization problem, geometry parameters, external load, ws2/su-8/pmma nanocomposites, static mechanical loading. 1. introduction one of the significant representatives of the family of tmds from the point of view of properties and applications is tungsten disulfide (ws2). it consists of three atomic layers of w layers sandwiched between two s layers, which enable its young’s modulus to be 236 ± 65gpa that is comparable to that of steel [1]. it has an indirect band gap of ∼ 1.9 ev and unique chemical properties, that define its wide area of application as production of strain-engineered devices such as transistors [2], photo-detectors [3], photovoltaic devices [4] as well as in the production of multifunctional polymer nanocomposites [5]. on the other side, incorporating ws2 nanoparticles or nanosheets into polymer matrices can enhance the mechanical, thermal, and electrical properties of the resulting nanocomposites. applying different strain techniques such as mechanical stretching, bending or applying external pressure to ws2 can induce strain in the material, changing its electronic, optical and mechanical properties [6]. therefore, understanding how different types and levels of strain affect ws2 is vital to tailoring its properties to specific applications. falin et al. (2021) [7] applied finite element (fe) analysis and density functional theory (dft) to determine the effects of in-plane strain and out-of-plane compression on the sliding energies in high-quality atomically thin tungsten dichalcogenides produced by mechanical exfoliation. wang et al. (2020) [8] combined both pl spectroscopy and raman spectroscopy to monitor the strain distribution and stress transfer of monolayer ws2 on a flexible polymer substrate and also in thin film bulk nanocomposites. tang et al. (2021) [9] investigated the influence of different defect types, defect density, and different ambient temperatures on the mechanical properties (i.e., young’s modulus and crit7 https://doi.org/10.14311/app.2024.50.0007 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en e. kirilova, t. petrova, b. boaydjiev et al. acta polytechnica ctu proceedings figure 1. representative volume element (rve) of 3-layer ws2/su-8/pmma nanocomposite structure, where: h1 – nanomaterial layer (ws2); ha – adhesion layer (su-8); h2 – substrate layer (pmma), p – static tension force. ical stress) of ws2 under different strains using the multiscale simulation, dft, md analysis, and fe. optimizing the geometry and loads in tmds-based nanocomposites is crucial for enhancing stress control and unlocking their full potential across various applications including flexible electronics and structural materials, where superior strength, efficiency, performance, reliability, and durability are paramount. moreover, it is crucial to identify the safe operating zones of the structures under consideration, particularly when they are subjected to complex loadings. tserpes and koumpias, (2015) [10] proposed a numerical methodology combining the ansys fe code and a progressive damage modeling module for optimizing the geometry of composite structures with regard to strength by maintaining the initial weight. the methodology has been applied for h-shaped adhesively bonded joints subjected to quasi-static load. motlagh et al. (2023) [11] proposed a framework based on genetic algorithm for optimizing energy harvesting from laminated composite panels integrated with (multiple) piezoelectric patches. anh (2023) [12] applied a differential evolution optimization algorithm combined with galerkin method for investigation of the effect of elastic foundations, geometrical parameters and initial imperfection on the natural frequency in an imperfect nanocomposite multilayer organic solar cell. it is noteworthy that there are not many studies in the literature related to the development of optimization approaches specifically tailored to the geometry of tmds (transition metal dichalcogenides)-based polymer nanocomposites. while optimization methods from the field of metaheuristics have been applied to composite materials constructed from various layers of metal compounds, it appears that similar approaches haven’t been extensively explored for tmds-based polymer nanocomposites. this study proposes a multi-parameter optimization approach to determine the optimal geometry and magnitude of the applied load for ensuring safety and preventing delamination in a three-layered adhesively bonded tungsten disulfide/poly(methyl methacrylate) (ws2/pmma) nanocomposite structure under axial mechanical loading. the optimization framework employs a two-dimensional stress function model to describe stress transfer within the su-8 in three-layer structure. it involves five decision variables, including the external load, the thickness of each layer, and the structure length. the objective function aims to minimize the difference between model interface shear stress and its critical value at which delamination occurs, to assure no delamination occurs in the nanocomposite structure. the multi-parameter optimization problem was solved using two optimization algorithms, one of which is the genetic algorithm. implementation of these algorithms yielded optimal values for the decision parameters, ensuring safety and preventing delamination in the considered nanostructure. 2. 2-d stress-function mathematical model in this section, a 2d analytical model (variational stress-function method) for stress fields in 3-layer nanocomposites subjected to mechanical loading is presented in a concise form (figure 1, for details see [13]). the proposed method allows the analytical solution of a fourth-order ordinary differential inhomogeneous equation with constant coefficients, concerning the unknown axial stress function σ1 in the first layer (ws2). two types of analytical solutions (eqs. (1) and (2)) for the axial stress σ1 in the first layer are derived, wherein the coefficients depend on the geometry of the three-layer nanocomposite, its material properties, and external loading. σ1(x) = c1 · exp(λ1 · x) + c2 · exp(λ2 · x) + c3 · exp(λ3 · x) + c4 · exp(λ4 · x) −a (1) σ1(x) = exp(−αx) [m1 cos(βx) +m2 sin(βx)] + exp(+αx) [m3 cos(βx) +m4 sin(βx)] −a (2) in eqs. (1) and (2), the constant a represents the solution for inhomogeneous ordinary differential equation and is dependent on the external static load (σ0 = p/h2), while ci and mi are the integration constants in the model solutions, determined by the corresponding boundary conditions [13]. all other stresses, including the iss, are expressed by eqs. (1) and (2) and their derivatives. eq. (2) represents the solution for case of four complex roots ± (α± iβ), whereas eq. (1) corresponds to the scenario of four real roots λi. the nature of the roots depends on the 8 vol. 50/2024 multi-parameter optimization of layered ws2-polymer . . . model solution σ0 [pa] l [m] h1 [m]) ha [m] h2 [m] eq. 1 1 · 105 ÷ 1 · 107 1 · 10−5 ÷ 1 · 10−4 0.6 · 10−9 ÷ 8 · 10−9 1 · 10−9 ÷ 1 · 10−7 1 · 10−7 ÷ 1 · 10−3 eq. 2 1 · 105 ÷ 1 · 1010 1 · 10−5 ÷ 1 · 10−4 0.6 · 10−9 ÷ 5 · 10−9 1 · 10−9 ÷ 1 · 10−7 1 · 10−7 ÷ 1 · 10−3 table 1. lower and upper boundaries for varying the decision variables. chosen layers’ thicknesses and length of the nanocomposite structure [14]. the model criterion for no interface delamination in the nanostructure, where uss represents the ultimate shear stress of the interface (adhesive) layer, is as follows: iss = σ(a) xy (x) = h1σ ′ 1 ≤ σ (a) uss . (3) if exists, graphically delamination initiates at both ends of the structure, representing the intersection of the iss model curve with the straight horizontal line corresponding to the uss. 3. multi-parameter optimization problems formulation to investigate the influence of the geometry of the considered ws2/su-8/pmma nanocomposite structure and the applied load on the delamination, multiparameter optimization problems are formulated and solved for the two types of iss model solutions, featuring real and complex roots. these formulations define the relevant decision variables, constraints, and objective functions. 3.1. decision variables all geometric characteristics of the considered nanostructure, including layer thicknesses (h1, ha, h2) and the length of the structure (l), as well as the external load (σ0), affect delamination. that’s why all these factors are chosen as decision variables. 3.2. constraints the mop includes constraints on the bounds of variation of the decision variables, as well as a constraint that tracks whether the iss reaches the uss (eq. (3)). 3.3. objective function the aim of the optimization for both types of model solutions for iss, with real and complex roots, is to find the optimal values of all 5 decision variables at which the minimal difference between iss and uss is obtained. with the latter in mind, the objective function represents the minimization of the difference between iss and uss (eq. (3)). 4. optimization methods for solution of formulated mops with the both types of analytical solutions for iss with real and complex roots, two approaches have been applied. the first one calls basic genetic algorithm (ga) [15] and the second represents an optimization procedure developed by “wolfram mathematica 13.0.1” software. basic ga operates on foundational principles common to all genetic algorithms. it includes various types of schemes for the genetic operators for selection, crossover, and mutation, providing users with the flexibility to choose among them to address a specific optimization problem. additionally, it can be adjusted for the concrete problem by tuning its global and local parameters. the first ones are: the number of generations (iterations); the number of samples selected for the process of recombination (crossover), the probabilities for crossover and mutation. the local parameters are related with the different schemes of the genetic operators. the other optimization method represents a procedure developed using the software package “mathematica”. in this procedure, different sets of geometry are included in the optimization loop for the interval of values for external load, checking whether the objective function (eq. (3)) is satisfied or not at the current geometry parameter values. it iterates as many times as the user needs until near-optimal solutions are reached, e.g, to ensure, that delamination does not occur. for the considered nanocomposite structure ws2/su-8/pmma, the aim is to simultaneously determine the optimal values of the decision variables (length, layer thicknesses h1, ha, h2, and external load σ0), at which the objective function has the minimum value. the decision variables vary within predefined technological boundaries. the formulated multi-parameter optimization problem is solved using basic ga approach under various combinations of genetic operator schemes for selection, crossover, and mutation and different sizes of populations and sampling pools as well as different number of generations. the latter is the used as termination criterion. 5. results and discussion the material properties, young’s modulus e (pa), and poisson’s ratio ν (-) of the considered nanocomposite structure (figure 1) ws2/su-8/pmma are taken from [8] as follows: e(ws2) = 270gpa, e(su-8) = 2gpa, e(pmma) = 3.5gpa, ν(ws2) = 0.22, ν(su-8) = 0.22, ν(pmma) = 0.35. in addition, the lower and upper boundaries of all 5 decision variables have been defined (table 1). the adhesive (interface) is a su-8 glue with uss = 30mpa. it should be borne in mind that the adhesive properties, especially the critical 9 e. kirilova, t. petrova, b. boaydjiev et al. acta polytechnica ctu proceedings figure 2. optimal solutions for 5 decision variables from basic ga and mathematica optimization algorithms: complex (blue) and real roots (red). figure 3. model interface shear stress solutions obtained at the optimal values of decision variables. case of model solution with real roots. value of interfacial shear stress, play an important role in the delamination of nanostructures. the main purpose of the study is to conduct an optimization of the geometry and the external load of the considered nanostructure, specifically to identify the optimal values of the decision variables at which the objective function reaches its minimum value. to achieve this goal, the mops for the both iss model solutions formulated in the previous section are defined. these problems were solved using basic ga and the optimization procedure developed in mathematica. in figure 2, the optimal values of the decision variables (external load, layer thicknesses, and the length of the overlap zone) for the cases of complex and real roots models solutions are plotted (symbolically). it can be seen from the figure 2 that along the ordinate, each decision variable changes within certain boundaries (intervals) for each of the two types of solutions (1) and (2) for iss included in eq. (3). the boundaries of the changes in the optimal thicknesses of pmma (h2) and the interface layer (ha) for the cases of real and complex roots are particularly well distinguished. the intervals of variation for h1 and l are almost similar for both possible solutions (eq. (1)) and (eq. (2)). along the abscissa, each different set of geometry data corresponds to a particular mechanical load. thus, for each combination of the five decision variables, the objective function is satisfied, or the predicted iss model at those load values and geometry is equal to or below the uss value. figures 3 and 4 present part of the iss distributions along the length of the nanocomposite obtained at optimal values of the decision variables. it can be seen that for each type of solution, the optimal values of the decision variables satisfy the objective function in the considered nanocomposite structure. in the case of iss calculated for real roots (eq. (1), figure 3), it is notable that the solutions obtained with the basic ga (marked 10 vol. 50/2024 multi-parameter optimization of layered ws2-polymer . . . figure 4. model interface shear stress solutions obtained at the optimal values of decision variables. case of model solution with complex roots. with numbers r7, r8, r9, and r10) for the geometry of the considered nanostructure and loads ensure an absolutely safe operation of the structure (far from uss). these solutions provide a significant difference between iss and uss, with no observed delamination of the structure. conversely, solutions obtained with the other optimization procedure (marked as sol rb1 and sol rb2) provide geometry and mechanical loading where iss is closer to uss, but still do not reach it. figure 4 shows solutions for the iss calculated with complex roots. the majority of these solutions are obtained using basic ga (marked with numbers c1 to c10), while the remaining solutions are derived from another optimization procedure (marked as sol ca1 and sol ca2). it can be seen from figure 4 that for the most of solutions, a minimum difference between iss and uss is reached. for sol. ca1 this difference is greater, but still fulfilled the criterion eq. (3). 6. conclusions this study proposes an optimization framework aimed at determining the geometric parameters and applied load magnitude necessary to ensure the safe operation of a three-layer adhesively bonded ws2/su8/poly(methyl methacrylate) nanocomposite structure under axial mechanical loading. the optimization framework incorporates a two-dimensional stress function model that characterizes stress transfer within the three-layer structure. two types of model analytical solutions for the iss are derived for two cases with real and complex roots and a model criterion, based on iss is included in the optimization framework. as decision variables, the external load, layer thicknesses, and structure length are considered, varying within predefined technological boundaries. the objective function aims to minimize the difference between iss and uss in the model criterion. two optimization approaches were applied for both types of model solutions (with real and complex roots) – basic ga and an optimization procedure developed with software “mathematica”. these methods have yielded optimal results for the geometry and loading of the structure in question, ensuring the absence of delamination. the obtained sets of 5 optimal values can be divided into two classes, depending on the used model solutions for iss. physically, the sets for real roots correspond to thinner structures and, accordingly, to lower loads, and vice versa – thicker structures and larger loads at complex roots. regardless of the type of solutions, the criterion of no delamination in the considered structure is satisfied which is validated by iss distributions calculated at optimal values found. from the safety point of view, a thinner design of considered nanocomposite structure is preferable, but at limited loads. thicker ones will require higher loads, than thinner ones, but delamination does not occur. acknowledgements this study is conducted by the financial support under the contract no. kπ-06-h57/3/15.11.2021 with bulgarian national science fund for the project “optimal safe loads and geometry for layered nanocomposites under thermomechanical loading”. references [1] n. iguiñiz, r. frisenda, r. bratschitsch, a. castellanos-gomez. revisiting the buckling metrology method to determine the young’s modulus of 2d materials. advanced materials 31(10):1807150, 2019. https://doi.org/10.1002/adma.201807150 [2] y. cui, r. xin, z. yu, et al. high-performance monolayer ws2 field-effect transistors on high-κ dielectrics. advanced materials 27(35):5230–5234, 2015. https://doi.org/10.1002/adma.201502222 [3] h. tan, y. fan, y. zhou, et al. ultrathin 2d photodetectors utilizing chemical vapor deposition 11 https://doi.org/10.1002/adma.201807150 https://doi.org/10.1002/adma.201502222 e. kirilova, t. petrova, b. boaydjiev et al. acta polytechnica ctu proceedings grown ws2 with graphene electrodes. acs nano 10(8):7866–7873, 2016. https://doi.org/10.1021/acsnano.6b03722 [4] m. bernardi, 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https://doi.org/10.1103/physrevresearch.2.012024 https://doi.org/10.1021/acsnano.0c07430 https://doi.org/10.1088/2053-1583/ababf1 https://doi.org/10.1021/acs.jpcc.0c09897 https://doi.org/10.1016/j.compositesb.2014.08.008 https://doi.org/10.1016/j.compositesb.2014.08.008 https://doi.org/10.1016/j.jsv.2023.118063 https://doi.org/10.25073/2588-1124/vnumap.4773 https://doi.org/10.3303/cet2294077 https://doi.org/10.34049/bcc.55.3.simns05 https://doi.org/10.1016/j.compchemeng.2006.03.003 https://doi.org/10.1016/j.compchemeng.2006.03.003 acta polytechnica ctu proceedings 50:7–12, 2024 1 introduction 2 2-d stress-function mathematical model 3 multi-parameter optimization problems formulation 3.1 decision variables 3.2 constraints 3.3 objective function 4 optimization methods 5 results and discussion 6 conclusions acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2024.50.0024 acta polytechnica ctu proceedings 50:24–29, 2024 © 2024 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague changing the integrity of the material surface by combining laser surface texturing and pvd magnetron sputtering technologies michal krafkaa,∗, totka bakalovaa, lucie svobodováa, tomáš primusb, magdalena mrózeka, milan boušaa a technical university of liberec, faculty of mechanical engineering, department of material science, studentská 2, 461 17 liberec, czech republic b czech technical university in prague, faculty of mechanical engineering, research center of manufacturing technology, jugoslávských partyzánů 1580/3, 160 00 prague 6, czech republic ∗ corresponding author: michal.krafka@tul.cz abstract. the presented research combines laser micromachining technology (lst, laser surface texturing) using an ultra-short femtosecond laser beam and pvd deposition of thin layers using magnetron sputtering. this combination has multiple benefits. it provides uniform and highly adhesive coatings with improved tribological properties. by optimising the deposition parameters when applying a thin layer of tin, an improvement in the tribological properties of the tool steel surface and the laser micromachining (lst) surface can be achieved. on the laser-modified and coated surface with ellipses-texture, the wear reduction was almost 19% compared to the reference texture. the tin coating exhibited very good adhesion to the substrate with critical force values of lc3 greater than 45n and a nano hardness value of 26.5gpa. the adhesion of the tin coating to the laser-affected substrate, as well as the resistance of the coating to damage, was high. damage primarily occurred on the original polished surface around the edges of the machined textures, where stresses are likely to concentrate. keywords: pvd magnetron sputtering, laser surface texturing, mechanical and tribological properties. 1. introduction surface integrity refers to a surface’s inherent or enhanced condition resulting from machining or other surface-generating operations. wear, abrasion, and tribology behaviour are common issues associated with this [1]. changing the surface topography can effectively improve tribological behaviour. in dry sliding conditions, the primary effect is removing or trapping wear debris or particles from the contact. when lubrication is present, the micro-texture can function as a reservoir for liquid lubricants, effectively delivering the lubricant between the sliding surfaces and causing positive changes in friction and wear [2, 3]. using lst (laser micromachining), surface topography or microstructures on large parts can be prepared. laser surface texturing (lst) replaces traditional techniques like surface extrusion or blasting. laser systems can be tailored to accommodate various materials and specific application needs. lst presents a promising process due to its rapid speed, eco-friendly nature, and ability to precisely manipulate the shape and size of micro-dimples [4]. femtosecond lasers are now widely utilised for their precise ability to modify material surfaces with minimal thermal impact. the textured surface of tools created with these lasers showed improved coating retention during turning experiments, ultimately extending the lifespan. however, the substrate also plays a significant role [5]. applying pvd coatings to the surface of conventional machining tools enhances their wear resistance, leading to improved cutting efficiency and higher processing quality. due to the different ways the coatings bond with the substrate, their thermal expansion coefficients, and their wetting properties, pvd-coated tools often experience problems with insufficient adhesion of the coating. to fully benefit from pvd coatings, ensuring strong adhesion between the coatings and the substrate is important. the adhesive strength between the coatings and the substrate is influenced by factors such as the chemical bonding properties at the coatings-substrate interfaces, specific surface area, topography, and substrate surface energy [6, 7]. in general, several methods have been used to enhance adhesion at the substrate-coating interface. these methods include substrate pre-treatment, intermediate layers, multilayer coatings, and functional layers [7]. however, applying the adhesive interlayer necessitates strict equipment conditions [6]. making the surface rougher can help improve the adhesion of pvd coatings because it enhances the mechanical interlocking between the coatings and the substrate [4]. in the area of pvd tool coatings, there is a rising interest in enhancing coating adhesion through lst of the substrate surface. this process aims to increase the contact area between coatings and the substrate, thereby improving mechanical anchoring 24 https://doi.org/10.14311/app.2024.50.0024 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 50/2024 changing the integrity of the material surface by combining . . . at the interface [6]. studies have demonstrated that laser treatment is more effective than abrasive flow machining for enhancing coating adhesion [8]. also, in [7], they discovered that lst led to enhanced adhesion of pvd coatings compared to the traditional cleaning method of grit blasting. hard coatings like tin, ticn, and tialn are commonly used in the metal-mechanics industry to enhance the properties of machine parts and cutting tools. tin is widely used as a coating due to its high hardness and chemical stability. ti-based coatings have been shown to reduce the wear of basic material by approximately four times [1]. the effect of using laser processing to create surface textures on substrates and how it impacts the interfacial properties of pvd tool coatings, particularly the physical-chemical bonding interfaces, still needs to be clarified. this work aims to explore the possibility of improving tribological properties in dry sliding conditions (economically and environmentally advantageous) by combining surface lst texturing and hard tin coating. magnetron sputtering is an effective method for enhancing material surface properties by creating uniform thin layers that adhere well to the base material. the combination of laser structuring and magnetron sputtering is expected to enhance the material’s useful properties while preserving its integrity. 2. materials and methods the substrate samples were made of tool steel en iso hs6-5-2c. tool steel discs with diameters of �20mm × 5mm were used as a substrate. their nominal composition is as follows (in wt.%): 0.9c; 0.31mn; 0.34 si; 0.026p; 0.0005 s; 4.43cr; 4.78mo; 5.93w; 1.79v; and 0.65co. the discs were hardened to 6465hrc and polished to a roughness (sa) of 0.01µm. the samples’ surfaces were modified using shortpulse femtosecond lasers through the laser surface texturing (lst) method at the ciirc at ctu. textures of various shapes, including pits (pit), hexagons (hex), ellipses with a depression (eli–), ellipses with a protrusion (eli+), and pyramids (pyr), were produced on the tool steel samples. the depth of the texture is approximately 10µm for all samples. a thin layer of tin was deposited on the lasermodified surfaces using magnetron reactive sputtering technology. two different recipes (b21 and b22) were utilised to prepare the thin coating to achieve optimal deposition parameters of the coating (see below). the coating thickness (d) is determined using a calotest method, which involves a 30mm steel ball and 0.1mm monocrystalline diamond grains in diamond paste. the average value of the thickness of the thin films was calculated from three measurements on the surface of each coating. the local chemical analysis of the coatings was carried out using a zeiss ultra plus scanning electron microscope equipped with an oxford x-max 20 energy dispersive spectrometer. the analyses were conducted at an accelerating voltage of 10 kv and a current of 1.6 na, with a working distance from the sample surface of 8mm and pulse pile-up correction. the surface morphology was estimated using a sensofar s neox confocal microscope. the scanned surface area of the samples was 850 × 709µm. the average value of the surface roughness of the thin films was calculated from the five measurement on the surface of each coating. nanohardness is measured using a berkovich diamond indenter prism on csm instruments equipment. the maximum penetration depth is limited to 10% of the coating thickness. the values of nanohardness and elastic modulus of the thin layers were obtained from nanoindentation, with 25 indentations performed on each sample. subsequently, the plasticity index (h/e) and the material’s resistance to plastic deformation (h3/e2) are calculated from the measured values. the adhesion and toughness of the coatings are evaluated using a micro-scratch test technique, using a rockwell diamond indenter with a 200µm tip radius to create scratch adhesion tracks. the evaluation involved gradually increasing the tip load from 2 to 60n while simultaneously moving the sample linearly in a perpendicular plane to the indenter. the tracks are then analysed using an optical microscope. three measurements were taken at different places, and the average values of the measured critical loads (lc , load critical) were calculated. the major failure events have been classified in terms of plastic deformation, cracking (lc1), spallation (where the coating flakes off, typically at the edges) (lc2), and penetration of the coating to the substrate at the track centre (lc3), see en 1071-3:2005 [9]. the critical load lc1 is the point on the graph where the coating’s first crack (cohesive failure) is observed. lc3 is the point on the graph where the damage becomes continuous, leading to complete coating delamination. this is accompanied by a sharp increase in the coefficient of friction (cof). lc2 represents the force at which 50% of the coating is damaged; lc2 has not been evaluated. the average value of thin film adhesion was calculated from three measurements on the surface of each coating. the tribological measurements were conducted using a trb3 tribometer from anton paar under dry sliding conditions. the measurements were taken at room temperature (rt) following the astm g99 standard. the parameters of the tribological experiment were as follows: ball-on-disc method, 10n load, counter-body diameter of 3mm, track radius on the disc of 3mm, 120 rpm, and a total track length of 100m. the counter-body used was a ceramic ball made of al2o3, with a hardness of 2100hv and a density of 3 860 g cm−3. tribological measurement was performed once on the surface of each sample. the surface wear of the friction pairs was assessed using a confocal microscope to measure the depth and width 25 m. krafka, t. bakalova, l. svobodová et al. acta polytechnica ctu proceedings recipe d d h h e∗ h/e h3/e2 [µm] [nm] [gpa] [hv] [gpa] [-] ref19 + b21 0.8 81 30.2 ± 5.4 2 794 ± 501 401.2 ± 23.2 0.075 0.171 ref19 + b22 0.9 93 26.5 ± 2.3 2 458 ± 211 379.0 ± 22.9 0.070 0.130 table 1. mechanical properties of tin thin layers. of the wear of the tribological track at a distance of 90° axially. 3. results and discussion 3.1. magnetron deposition of thin layer a thin layer of titanium nitride (tin) was selected to modify the surface of the tool steels. initially, the deposition parameters of the magnetron reactive sputtering process needed optimisation. the chemical composition of the coating during deposition is mainly influenced by the parameters of the working pressure, flow rates, gas ratios, and the pulse source’s power. parameters of deposition of tin thin layers: after reaching a primary vacuum of 8 × 10−4 pa, the substrate was pre-cleaned in an argon plasma at a working pressure of 3pa or ar gas flow 6.5 sccm and power 1 kw for 40min. then an adhesive intermediate layer of pure titanium was sputtered at a working pressure of 0.3pa or with an ar flow of 46 sccm and a source power of 1.5 kw for 5min. a circular target with a diameter of 77mm with a ti purity of 99.99% was placed on the cathode. reactive sputtering of tin was carried out with an ar:n flow rate of 8:1 sccm at a voltage range of 1000v for b21 and 1500v for b22 for 1 and 3 h. the thickness of the thin coating can affect the magnitude of the residual stress generated in the structure. therefore, the reaction time of the tin process has been optimised to a range of one, two, and three hours. the measured coating thicknesses for the b21 recipe are as follows: 2.6µm, 1.7µm, and 0.8µm for a 3hour, 2-hour and 1-hour deposition, respectively. for the b22 recipe, the coating thicknesses are: 3.5µm, 1.8µm, and 0.9µm for 3-hour, 2-hour, and 1-hour depositions, respectively. the optimal deposition process for the tin coating on lst was chosen for deposition times of 1 (labelled b21/1 and b22/1) and 3 hours (labelled b21/3 and b22/3) using formulations b21 and b22. the recipes b21 and b22 used for depositing the tin coating showed, based on edx chemical composition analysis, a stoichiometry (ratio of ti and n) in the equilibrium atomic ratio. the criteria for choosing the optimal thin coating application process were achieving good mechanical properties of the coatings, i.e. achieving good adhesion, nano-hardness and reducing layer wear. recipe lc1 lc3 [n] [n] ref19 + b21/1 7.6 ± 0.9 31.2 ± 1.6 ref19 + b21/3 8.5 ± 0.5 40.4 ± 1.9 ref19 + b22 8.2 ± 0.5 49.8 ± 1.7 table 2. evaluation of adhesion critical forces of tin thin layers. sample disc wear cof recipe width depth µ [µm] [µm] [-] ref19 388.2 ± 4.4 3.7 ± 0.2 0.55 ± 0.06 ref19 + b21/1 119.8 ± 21.4 0.2 ± 0.0 0.23 ± 0.04 ref19 + b21/3 119.2 ± 4.1 0.1 ± 0.0 0.23 ± 0.05 ref19 + b22 112.2 ± 2.7 0.1 ± 0.0 0.22 ± 0.02 table 3. results of tribological analysis of tin thin layers. 3.2. mechanical properties of thin coatings, evaluation and selection of deposition parameters in the tables, the reference polished tool steel is labelled as ref19. the results of the nanoindentation of thin coatings were statistically processed; the average values with standard deviation are presented in table 1, denoted as follows: coating thickness (d), indentation depth (d), nanohardness (h), young’s modulus of elasticity (e), plasticity index (h/e), and material resistance to plastic deformation (h3/e2). table 2 shows the measured values of the critical forces lc1 and lc3, providing information about the coatings’ adhesion to the substrate as evaluated by the scratch test method. table 3 shows the results of the tribological properties of the modified surfaces. 3.3. microscopy and image analysis of laser-textured thin-layer surfaces confocal microscopy (figure 1) was utilised to assess surface morphology and roughness. image analysis was conducted using matlab software (the mathworks, inc.). an automated program code was employed to determine the machined area, the perimeter of the detected object, as well as the length, width, and aspect ratio of the object (results shown in table 4). for the “pit” pattern, the length is approximately 1.25 times longer than the width, with a machined area of up to 20%. for eli–, the length is approximately 26 vol. 50/2024 changing the integrity of the material surface by combining . . . figure 1. confocal microscopy of the surface ref19 samples with surface texture type: pits, ellipse-, ellipse+, hexagon, pyramids. images are depicted on a height scale, with blue representing valleys and red colour representing peaks. lst structure lst area lst area perimeter length width aspect ratio [%] [µm2] [µm] [µm] [µm] [–] pit 19.9 ± 1.1 735.5 ± 41.9 102 ± 3.8 34.2 ± 1.5 27.2 ± 1.3 1.26 ± 0.07 eli– 44.5 ± 1.0 32 082 ± 1 028 791 ± 8.8 331 ± 2.9 123.5 ± 3.2 2.68 ± 0.06 eli+ 69.8 ± 4.2 21 099 ± 389 669 ± 6.2 286 ± 3.7 93.4 ± 1.3 3.09 ± 0.06 hex 43.6 ± 10.3 9 745 ± 2 917 369 ± 95.4 107 ± 26.9 99.3 ± 25.4 1.12 ± 0.11 pyr 71.9 ± 10.0 3 995 ± 646 243 ± 28.4 73.7 ± 8.1 64.5 ± 6.9 1.18 ± 0.10 table 4. results of evaluation of object parameters in the image using image analysis. (a). (b). figure 2. sem image of a) ablation trace “lipsslaser induced periodic surface structure” (mag. 7 000x) and b) metallographic cut of pit texture with b21/3 coating (mag. 2 500x). 2.7 times longer than the width, with a machined area of up to 45%. for eli+, the length of the texture is approximately 3 times longer than the width, with a machined area of up to 70%. the hex and pyr textures are almost symmetrical, with an aspect ratio parameter close to 1 and a machined area of approximately 44% and 72%, respectively. an sem was used to capture images to analyse the morphology of the ablation trace and thin layer (figure 2). the eds method was utilised to determine the chemical composition of the thin layers. to observe the uniformity of the thin layers in conjunction with lst modification, metallographic slices were prepared by cutting the sample. even though the results on the surface of the tool steel with the b21 (ref19 + b21) recipe showed favourable tribological properties (table 3), the coatings on lst samples exhibited delamination (figure 2b), possibly caused by high residual stresses. as a result of this issue, the tin coating deposition recipes were further optimised. 3.4. tribological properties of thin layers deposited on laser surface texturing surfaces the tribological analysis of thin layers deposited on laser surface texturing surfaces revealed a low coefficient of friction compared to reference sample ref19 (table 5). at the same time, an increase in the wear depth of the material was observed. of all the textures examined, only the pit texture showed lower wear than ref19, even though there was an increase in cof (table 5). in an experiment that combined lst and pvd technologies, all lst samples were coated with a thin layer of tin following the b21/3 recipe. while the tribology results for the tool steel surface ref19 + b21 recipe showed favourable tribological properties (refer to table 3), the coatings on the lst specimens exhibited delamination, likely due to high residual stress. confocal and electron microscope images confirm the occurrence of delamination along the edges of the lst texture. high residual stress along these edges is 27 m. krafka, t. bakalova, l. svobodová et al. acta polytechnica ctu proceedings sample disc wear cof lst structure width depth µ [µm] [µm] [-] ref19 388.2 ± 4.4 3.7 ± 0.2 0.55 ± 0.06 pit 319.5 ± 4.9 2.5 ± 0.1 0.62 ± 0.03 eli– 382.0 ± 5.4 3.9 ± 0.2 0.48 ± 0.03 eli+ 491.1 ± 31.3 12.2 ± 1.0 0.46 ± 0.03 hex 362.1 ± 5.9 4.3 ± 0.1 0.47 ± 0.02 pyr 448.4 ± 26.3 9.3 ± 0.4 0.58 ± 0.03 table 5. wear and friction coefficient of tool steel with lst modification without coating. sample disc wear cof lst structure + recipe width depth µ [µm] [µm] [-] pit + b21/3 351.2 ± 9.8 4.6 ± 0.1 0.57 ± 0.06 hex + b21/3 400.9 ± 16.1 5.9 ± 0.1 0.61 ± 0.03 eli– + b21/3 465.3 ± 9.3 5.0 ± 0.2 0.56 ± 0.04 pyr + b21/3 528.1 ± 6.8 7.3 ± 0.3 0.57 ± 0.06 eli+ + b21/3 432.1 ± 21.0 10.7 ± 0.4 0.55 ± 0.06 eli+ + b21/1 430.9 ± 23.9 9.6 ± 0.4 0.42 ± 0.04 eli– + b22/1 321.8 ± 48.1 3.2 ± 0.3 0.46 ± 0.04 table 6. wear and friction coefficients of lst samples coated with pvd thin layers. the likely cause of the delamination. (figure 2b and similarly in [10]). there was a need to further optimise the recipes used for applying the tin coating in two ways: a) by reducing the coating thickness (ref19 + b21) and b) by using different recipes (ref19 + b22). table 6 shows the tribological behaviour of the lst specimens combined with the thin layers. they show a reduction in wear compared to the condition of the base material and the original recipe b21/3. on the coated and laser-treated surface with eli texture and thin layer (formulation b22), wear was reduced by almost 19% compared to eli texture on the base material. however, damage to the coating was observed during the tribological test. the tin coating exhibited good adhesion to the substrate, with critical force values of lc3 greater than 45n and a nanohardness value of 26.5gpa. the wear reduction on the surface of eli+ and pyr samples was achieved despite defects on the surface of the b21/3 recipe. these samples demonstrated a reduction of wear of approximately 12% and 21% (on the coating with reduced thickness eli+ with b21 thin layer, a 21% reduction in wear was observed) compared to the textures on the base material. textures eli+ and pyr have a more exposed surface and a larger laser-machined area, respectively, compared to the other textures (table 4). also, in [5], the lst applied on a larger fraction area of tool steel surface positively affected decreasing the coefficient of friction. the tin coating has strong adhesion to the lasermachined surface and is highly resistant to damage. the damage mainly occurs on the original polished surface around the edges of the machined textures, where stresses are likely to be concentrated. the significant increase in surface damage of 27%, 36%, and 84% for the eli–, hex, and pit textured surfaces with the b21/3 coating, compared to the textures on the base material, was likely caused by the combination of higher-thickness coatings (which are brittle and hard, about 30gpa) with high internal stress. 4. conclusion by optimising the deposition parameters and applying a thin layer of tin, a significant improvement in the mechanical and tribological properties of the surface of the tool steel and other surfaces that were processed by laser micromachining technology (lst) was achieved. in accordance with the b22 recipe, applying a tin coating on the tool steel increased damage resistance on the polished surface by over 31 times in terms of depth compared to the reference surface. additionally, the friction coefficient was reduced by more than half (by approximately 60%). the wear on the laser-modified surface coated with eli– texture was reduced by almost 19% compared to the reference texture despite the coating being damaged during the tribological test. the tin coating also exhibited good adhesion to the substrate, with lc3 critical force values exceeding 45n and a nanohardness value of 26.5gpa. acknowledgements this publication was supported by the student grant competition of the technical university of liberec within 28 vol. 50/2024 changing the integrity of the material surface by combining . . . the project sgs-2024-5432. this publication was written at the tul with the support of the institutional endowment for the long-term conceptual development of research institutes, as provided by the msmt of the czech republic 2024. references [1] b. s. yilbas, s. shuja. laser treatment and pvd tin coating of ti-6al-4v alloy. surface and coatings technology 130(2):152–157, 2000. https://doi.org/10.1016/s0257-8972(00)00715-5 [2] b. s. kim, w. y. chung, m.-h. rhee, s.-y. lee. studies on the application of laser surface texturing to improve the tribological performance of alcrsin-coated surfaces. metals and materials international 18:1023–1027, 2012. https://doi.org/10.1007/s12540-012-0483-2 [3] w. m. da silva, m. p. suarez, a. r. machado, h. l. costa. effect of laser surface modification on the microabrasive wear resistance of coated cemented carbide tools. wear 302(1):1230–1240, 2013. wear of materials 2013. https://doi.org/10.1016/j.wear.2013.01.035 [4] k. zhang, j. deng, x. guo, et al. study on the adhesion and tribological behavior of pvd tialn coatings with a multi-scale textured substrate surface. international journal of refractory metals and hard materials 72:292–305, 2018. https://doi.org/10.1016/j.ijrmhm.2018.01.003 [5] j. moravčíková, r. moravčík, m. kusý, m. necpal. influence of laser surface texturing on tribological performance of tool steels. journal of materials engineering and performance 27:5417–5426, 2018. https://doi.org/10.1007/s11665-018-3607-0 [6] x. meng, k. zhang, x. guo, et al. preparation of micro-textures on cemented carbide substrate surface by plasma-assisted laser machining to enhance the pvd tool coatings adhesion. journal of materials processing technology 288:116870, 2021. https: //doi.org/10.1016/j.jmatprotec.2020.116870 [7] r. viana, a. r. machado. influence of adhesion between coating and substrate on the performance of coated hss twist drills. journal of the brazilian society of mechanical sciences and engineering 31(4):327–332, 2009. https://doi.org/10.1590/s1678-58782009000400007 [8] d. neves, a. e. diniz, m. s. f. lima. microstructural analyses and wear behavior of the cemented carbide tools after laser surface treatment and pvd coating. applied surface science 282:680–688, 2013. https://doi.org/10.1016/j.apsusc.2013.06.033 [9] standards e. en 1071-3:2005 advanced technical ceramics – methods of test for ceramic coatings – part 3: determination of adhesion and other mechanical failure modes by a scratch test 2005. [10] c. xu, f. xu, l. shi, et al. enhancement of substratecoating adherence of boron-doped diamond electrodes by nanosecond laser surface texturing pretreatment. surface and coatings technology 360:196–204, 2019. https://doi.org/10.1016/j.surfcoat.2018.12.098 29 https://doi.org/10.1016/s0257-8972(00)00715-5 https://doi.org/10.1007/s12540-012-0483-2 https://doi.org/10.1016/j.wear.2013.01.035 https://doi.org/10.1016/j.ijrmhm.2018.01.003 https://doi.org/10.1007/s11665-018-3607-0 https://doi.org/10.1016/j.jmatprotec.2020.116870 https://doi.org/10.1016/j.jmatprotec.2020.116870 https://doi.org/10.1590/s1678-58782009000400007 https://doi.org/10.1016/j.apsusc.2013.06.033 https://doi.org/10.1016/j.surfcoat.2018.12.098 acta polytechnica ctu proceedings 50:24–29, 2024 1 introduction 2 materials and methods 3 results and discussion 3.1 magnetron deposition of thin layer 3.2 mechanical properties of thin coatings, evaluation and selection of deposition parameters 3.3 microscopy and image analysis of laser-textured thin-layer surfaces 3.4 tribological properties of thin layers deposited on laser surface texturing surfaces 4 conclusion acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2023.43.0106 acta polytechnica ctu proceedings 43:106–110, 2023 © 2023 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague artificial intelligence in operational applications of railway infrastructure manager přemysl šrámek, jaromír široký∗, martin vojtek university of pardubice, jan perner transport faculty, department of transport technology and control, studentská 95, 532 10 pardubice, czech republic ∗ corresponding author: jaromir.siroky@upce.cz abstract. the main goal of this article is to present the possibility of using artificial intelligence in operational application of railway infrastructure manager. railway 4.0 can be controlled fully digitally – operational applications based on the artificial intelligence can optimize the technology of railway traffic control. thereafter, a wide range of equipment could be controlled by application through a secure interface. keywords: artificial intelligence, operational applications, railway traffic control, railway 4.0. 1. introduction railway 4.0 should be completely digital. railway traffic control should use the digitization as much as possible. one rail network (topology) completely connected information systems with secure interface with other devices (safety equipment, etcs etc.) should not be blank phrases. the aim of this article is to present the possibility of using artificial intelligence in operational application for optimization the railway traffic control processes. 2. materials and methods in this article there was used the decomposition of the application to determine the levels of the deep learning. the artificial intelligence topic is quite old (more in the figure 1), but the implementation in operational apps of railway infrastructure manager is still quite unexplored. the difference between terms artificial intelligence, machine learning and deep learning is: • artificial intelligence – a program that can sense, reason, act and adapt, • machine learning – algorithms whose performance improve as they are exposed to more data over time, • deep learning – subset of machine learning in which multi-layered neural networks learn form vast amounts of data. more in the figure 2. deep learning is incredibly young field of artificial intelligence based on artificial neural networks (multilayered structure of algorithms, more in the figure 3). however, the new industrial revolution is driven by artificial neural networks and deep learning – the major advantages are 2: needlessness of the so-called feature extraction and powering by massive amounts of data (big data use). about feature extraction – the problem is the impossibility of implementation flat algorithms (e.g. decision trees) to the raw data – the step called “feature figure 1. evolution of ai [1]. figure 2. the difference definition [1]. extraction” must be done. the result of feature extraction is an abstract representation of the given raw data, that you can use for classic machine learning algorithms to perform a task (e.g. the classification 106 https://doi.org/10.14311/app.2023.43.0106 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 43/2023 artificial intelligence in operational applications of railway infrastructure manager figure 3. neural network [2]. figure 4. feature extraction [1]. of the data into several categories or classes). feature extraction is usually quite complicated and requires detailed knowledge of the problem [3]. this step must be adapted and refined over several iterations for optimal results. and deep learning models don’t need it (more in the figure 4), because the feature extraction step is already a part of the process that takes place in an artificial neural network. during the training process, this step is also optimized by the neural network to obtain the best possible abstract representation of the input data [4]. for example, if you want to use a machine learning model to determine whether a particular image shows a car or not, we humans first need to identify the unique features of a car (shape, size, windows, wheels, etc.), extract these features and give them to the algorithm as input data. this way, the machine learning algorithm would perform a classification of the image. that is, in machine learning, a programmer must intervene directly in the classification process. in the case of a deep learning model, the feature extraction step is completely unnecessary. the model would recognize these unique characteristics of a car and make correct predictionscompletely without the help of a human [5]. the second great advantage of deep learning is it’s powered by massive amounts of data – deep learning models increase their accuracy with the increasing amount of training data, depending on the neural network size (more in the figure 5). figure 5. data amount for deep learning [1]. figure 6. the model of the four deep learning levels. 3. results and discussion to implement the deep learning process to the railway operational apps, it is necessary to define more learning levels based on the application type. the operation applications could be divided into direct railway traffic control (drtc), operational railway traffic control (ortc), planning applications (pa) and diagnostics (d) – the model of the four deep learning levels is in the figure 6. the core and the first level are the applications for direct railway traffic control (drtc), it means digital documentation of traffic control with automatic train operation (ato) potential, including ato over etcs. the first level of deep learning should include these data streams: • mutual drtc communication in tsi form (in all locations of the controlled area), • drtc and ortc communication (drtc behaviour based on the incoming ortc data), • drtc and diagnostics systems communication, • drtc and safety equipment (incl. etcs) communication, • drtc and ip telephony communication (incl. 107 p. šrámek, j. široký, m. vojtek acta polytechnica ctu proceedings figure 7. the simplified communication architecture of drtc. voip), • drtc and passenger is communication. the goal of the first level of deep learning is to create reliable application for drtc with all automation functions. it could be e.g. ato over etcs – system controlled by artificial intelligence with possibility, but no necessity of operational staff intervention [6]. the simplified architecture is shown in the figure 7. in this first deep learning level are combined at the input all the data streams creating various rail traffic control situations. it could be like the learning from the dispatcher decision’s recording plus knowledge of all other components status and behaviour (diagnostics, safety equipment, ip telephony etc.). the learning must be done on the real traffic data (e.g. for one timetable duration, it means 365 days). the output should be the optimized railway traffic control in the entrusted area of attraction including passenger information providing and etcs, resp. ato control. the second deep learning level are the apps for operational railway traffic control (ortc), it means integration platform of drtc datasets (data aggregation) for larger areas (the entire state network and more). it should include these data streams: • ortc communication with all drtc (it should be limited to the controlled areas), • ortc with planning application (train routing, capacity allocation), • ortc with railway undertakings requests and information systems (train composition etc.). the goal of the second level of deep learning is to create the smart railway traffic control (srtc) – the central communication hub (all drtc data) with neural network support. in this level should be possible to control and optimize all train routes in the network (national/international). the main goal should be lowest delay in the network. the simplified architecture is shown in the figure 8. in this second deep learning level are combined at the input all the important data for railway traffic control in the entrusted network. it should be the learning figure 8. the simplified communication architecture of ortc. figure 9. the simplified communication architecture of pa. from the operational dispatcher decision based on the drtc data and track dispatcher experience. the learning must be done on the real traffic data (e.g. for one timetable duration, it means 365 days). the output should be optimized railway traffic control in the network, ensuring the transfer of passengers and goods and the network delay minimization [7]. the third level of deep learning are the planning applications (pa), it means applications for allocation of capacity and train routing. it must communicate with these systems: • pa with ortc, • pa with railway undertakings requests and information systems (capacity allocation). the goal of the third level of deep learning is to optimize the allocation of train routes based on the solution of previous operation situation. the main aim is the most train routes number with the lowest delay prediction. the simplified architecture is shown in the figure 9. in the third deep learning level are combined at the input all the train routes planning data solved in the infrastructure manager system (with the source of ru is) and the solution from the second deep learning level. it should be the learning from the capacity allocation dispatcher decisions and the ru dispatcher revisions. the learning must be done on the real traffic data (e.g. for one timetable duration, it means 365 days). the output should be the optimizing of quality and quantity of all train routes, it means the 108 vol. 43/2023 artificial intelligence in operational applications of railway infrastructure manager figure 10. the simplified communication architecture of d. combination of the most train routes number with the lowest delay prediction (network parameter) [8]. the final level of the defined deep learning process are the diagnostics apps (d). it must cooperate with these data streams: • d with ortc – summarization of operation failures and disorders (ict, safety equipment), • d with drtc – detailed description of failures and disorders, • d with infrastructure diagnostics (number of turns, traction consumption size etc.). the main aim of this level is to predict the failures and disorders and suggest the infrastructure constraints with the operation impact (connection to drtc and optc). the simplified architecture is shown in the figure 10. in the fourth deep learning level are combined at the input all the data from the first and second level of deep learning connected to the real infrastructure diagnostics data. it should be the learning from real failures and disorders and their traffic impact [9, 10]. the output should be the prediction of failures (maintenance intervals, replacement requirements) with the impact to all three previous levels of deep learning. the all four levels of deep learning affect the others and therefore it must be operated as one is. moreover, it is necessary to keep the restrictive conditions, especially the unified network (unified topology) and security issues. the unified network must contain all the relevant data (line tables etc.) and it must distribute the same data correctly in all applications and systems. about security issues – the safety integrity level, the critical information infrastructure conditions and other cyber security rules must be followed. the threats like data poisoning must be mitigated. 4. conclusions railway 4.0 should be based on digitization. in the area of operational applications, it could be created railway traffic control 4.0 (rtc 4.0), it means smart railway traffic control (srtc). the use of artificial intelligence is recommended – the four levels of deep learning process was defined in this article. the first level operates with railway station data, the second level with the aggregated area data, the third level with the capacity allocation data and the fourth with diagnostics datasets. the main goal of the four-level deep learning process is to define the operational applications staff behaviour and to optimize the railway traffic control process. the four-level deep learning process powered by massive amounts of data (big data should be used) could achieve very good rtc process results. the increase of reliability, security and safety should be reached. moreover, the power of srtc is the possibility of the origin of european railway traffic control based on the rne data and supported by co-financed development (european funding). acknowledgements the paper was supported from erdf/esf “cooperation in applied research between the university of pardubice and companies, in the field of positioning, detection and simulation technology for transport systems (positrans)” (no. cz.02.1.01/0.0/0.0/17_049/0008394). references [1] a. opperman. artificial intelligence vs. machine learning vs. deep learning. [2022-03-22], https://towardsdatascience.com/artificialintelligence-vs-machine-learning-vs-deeplearning-2210ba8cc4ac. [2] j. široký, p. nachtigall, e. tischer, j. gašparík. simulation of railway lines with a simplified interlocking system. sustainability 13(3):1394, 2021. https://doi.org/10.3390/su13031394 [3] m. bažant, j. bulíček, p. krýže, p. veselý. calculation of the capacity of switch area within railway stations with the use of simulation methods. in proceedings of the european modeling and simulation symposium, pp. 316–322. 2017. [4] p. nachtigall, j. široký, e. tischer. assessing the efficiency of increasing the track speed in the line section rokycany–plzeň hl. n. sustainability 12(18):13, 2020. https://doi.org/10.3390/su12187415 [5] n. mondek, j. bulíček. changes in the traffic situation on city roads after traffic diversion outside the city. in international conference on traffic and transport engineering (ictte), pp. 1064–1072. 2018. [6] e. tischer, p. nachtigall, j. široký. the use of simulation modelling for determining the capacity of railway lines in the czech conditions. open engineering 10:224–231, 2020. https://doi.org/10.1515/eng-2020-0026 [7] p. nachtigall, m. škárek. railway infrastructure capacity management for ad-hoc trains on the sždc network. acta polytechnica ctu proceedings 5:44–46, 2016. https://doi.org/10.14311/app.2016.5.0044 109 https://towardsdatascience.com/artificial-intelligence-vs-machine-learning-vs-deep-learning-2210ba8cc4ac https://towardsdatascience.com/artificial-intelligence-vs-machine-learning-vs-deep-learning-2210ba8cc4ac https://towardsdatascience.com/artificial-intelligence-vs-machine-learning-vs-deep-learning-2210ba8cc4ac https://doi.org/10.3390/su13031394 https://doi.org/10.3390/su12187415 https://doi.org/10.1515/eng-2020-0026 https://doi.org/10.14311/app.2016.5.0044 p. šrámek, j. široký, m. vojtek acta polytechnica ctu proceedings [8] v. cempírek, p. nachtigall, p. novák, m. pluhár. the comparison of public road & railway transport costs. in proceedings of the 3rd international conference on traffic & transport engineering ictte, pp. 855–860. 2016. [9] v. zitrický, j. gašparík, l. pečený. the methodology of rating quality standards in the regional passenger transport. transport problems 10(se):59–72, 2015. https://doi.org/10.21307/tp-2015-062 [10] m. dedík, v. štefancová, j. gašparík, et al. traffic capacity assessment of the selected track section on the slovak railways network after the implementation of etcs l3 based on signaling principle. applied sciences 12(11):5597, 2022. https://doi.org/10.3390/app12115597 110 https://doi.org/10.21307/tp-2015-062 https://doi.org/10.3390/app12115597 acta polytechnica ctu proceedings 43:106–110, 2023 1 introduction 2 materials and methods 3 results and discussion 4 conclusions acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2023.40.0123 acta polytechnica ctu proceedings 40:123–128, 2023 © 2023 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague use of lightweight fft-based algorithms for remote measurements using dic slávek zbirovský∗, václav nežerka czech technical university in prague, faculty of civil engineering, department of physics, thákurova 7, 166 29 prague 6, czech republic ∗ corresponding author: slavek.zbirovsky@fsv.cvut.cz abstract. abrupt collapse events that result in casualties due to failure of load-bearing structures have been reported in recent years. continuous remote monitoring of these structures, assessing the behavior under operational load, could provide a reliable prevention, as the developing damage manifests itself through a loss of stiffness, and therefore excessive deflections. digital image correlation (dic) algorithms based on fast fourier transform (fft) were implemented in a small computer unit raspberry pi, capable of remote operation in a low-energy demand mode. the unit is fully autonomous, capable of localizing qr code targets and extracting encoded information. this strategy allows us to instantly evaluate displacements of qr codes attached to any surface and send the data to a server. the software was developed in python, allowing for two modes: the online mode for current data evaluation with information transfer via the mqtt protocol and the offline mode used for retrospective evaluation. keywords: dic, fft, raspberry pi, mqtt, qr code. 1. introduction digital image correlation (dic) is an optical noncontact method for measuring surface deformations, operating on the principle of comparing a reference image with an image after event. due to the possible interchangeability of individual pixels, groups of pixels (hereafter subsets) are compared, in which the grayscale of individual pixels is evaluated. the correlation function is used as a comparison criterion. to ensure sufficient reliability of the correlation of the individual subsets, the sample must be provided with a random, high contrast isotropic pattern, which is called a “pattern” [1]. the displacement of the subset is often described by a first-order shape function that includes the displacement, rotation, and deformation of the observed subset. this method is often used due to its precision and low computational complexity [2]. as can be seen in figure 1, the coordinates of the point p (xf, yf) in the distorted image can be obtained as follows: xf = x0 + u + ∂u ∂x (x0 − x0,c) + ∂u ∂y (y0 − y0, c) (1) yf = y0 + v + ∂v ∂x (x0 − x0,c) + ∂v ∂y (y0 − y0, c), (2) where xf and yf represent the end coordinates of the subset, x0 and y0 the initial coordinates of the subset, u and v are the displacement vectors in the direction of the coordinate axes and ∂u/∂x, ∂u/∂y, ∂v/∂x, ∂v/∂y are components of the displacement gradient. dic can be divided into two groups for 2d and 3d correlation of digital image. 2d dic is designed to monitor displacements and deformations in the plane. it should be without significant spatial deformations and parallel to the plane of the sensing device (ccd sensor). in contrast, 3d dic is designed to monitor displacements and deformations in a spatial sample using a pair of cameras and subsequent stereotriangulation. the 2d dic method was used for the work. 1.1. algorithms the two-dimensional correlation algorithms can be categorized into two main groups: (1.) the first evaluates the matches between the reference and the image after the event in the grayscale intensities of the individual pixels. the minimum or maximum of the correlation criterion is calculated [3]. (2.) the second types of algorithm use fast fourier transformation (fft). they can be divided into two groups, cross-correlation in the frequency domain and phase correlation. these calculation algorithms make it possible to track large displacement of the subsets, and unlike the other mentioned algorithms they search for an optimal match between the reference and deformed image in the frequency domain. the fourier transform f (α) converts the signal from the time domain f(t) to the frequency domain (α). the signal can be discrete or continuous. the transformation can decompose the signal into easily describable sine and cosine waves. the fourier transform can be expressed as follows: f {f(t)} = f (α) = ∫ ∞ −∞ f(t)e−2πiαt · dt, (3) 123 https://doi.org/10.14311/app.2023.40.0123 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en slávek zbirovský, václav nežerka acta polytechnica ctu proceedings figure 1. subset (qr code) deformation and shift. where α is the frequency, t is the time, i = √ −1 and −2πα is the angular frequency. the signal f(t) is usually not continuous, but is measured as a sequence f(x), containing n consecutive samples. this set can be transformed into a field of frequencies using the discrete fourier transform (dft), which can be defined as follows: d {f(t)} = f (ξ) = n−1∑ x=1 f(x)e−2πi xξ n . (4) for a two-dimensional n × n matrix, n2 operations are needed. therefore, in 1965, j. w. cooley and j. w. tuckey developed the fast fourier transform fft method [4]. this can reduce the complexity to n log2 n operations. 1.2. phase correlation phase correlation which is one of the algorithms using fft can be described as follows; let functions f1(x, y) a f2(x, y) be the considered images before and after deformation. their relationship can be expressed as f2(x, y) = f1(x − x0, y − y0). the relation of the corresponding fourier transforms f1(ξ, η) and f2(ξ, η) is defined as: f1(ξ, η) = f2(ξ, η)e−2πi(ξx0+ηy0). (5) the normalized cross-power spectrum of images f1(x, y) and f2(x, y) with their fourier transform f1(ξ, η) and f2(ξ, η) can then be expressed as: zf1,f2(ξ, η) = f1(ξ, η)f ∗ 2 (ξ, η) |f1(ξ, η)f2(ξ, η)| = e2πi(ξx0+ηy0), (6) where f ∗ 2 (ξ, η) represents the complex conjugate form f2(ξ, η). the inverse fourier transform of the normalized cross-power spectrum gives the discrete impulse function. δf1,f2(x, y) = d−1 {zf1,f2(ξ, η)} (7) it is zero in all coordinates, except at the coordinates corresponding to the displacement (−x0, −y0) : δ(x,y) = { 1 if (x, y) = (−x0, −y0) 0 otherwise. (8) to obtain the displacement vectors u and v, the maximum is found at the point [x, y], which is subtracted from the initial coordinates [x0, y0] [5]. 2. hardware the hardware set was adapted to real-time measurements, the design emphasized the low energy consumption and mobility of the entire set. the computing unit consists of a single-board computer raspberry pi model 4b, which is equipped with the raspbian operating system. the main advantages of this computer are small size and price/performance ratio. power is supplied via a usb-c cable with the thunderbolt 3 standard from the mains, or from a backup source connected to the solar panel. figure 2 shows two options for recording an image, a dslr camera and a picamera. the device’s connectivity to the internet is enabled using a built-in wi-fi module with the 802.11ac specification. 3. dslr camera the accuracy of displacement and deformation evaluated by the dic method depends on the number of pixels in the captured image. for this and other reasons, a dslr (digital single lens reflex) camera was used in this work for data collection. the basic element of digital slr cameras is a chargecoupled device (ccd) or complementary metal-oxidesemiconductor (cmos) sensor that captures image information. these sensors convert captured light into an electrical signals. 124 vol. 40/2023 fft-based algorithms for dic measurements figure 2. hardware set consisting of a raspberry pi 4b computer, a dslr camera, and a pi camera. 4. software the program that records the image and calculates the displacements of the observed subset is written in the python version 3 programming language. the opencv (open source computer vision) function library is used to locate and read qr codes in the image. to work with the fourier transform, the numpy function library is used, which defines n-dimensional arrays of numbers [6]. the program flow chart in the online version is described in figure 3. the program has the following definable input categories: (1.) dic characteristic settings, (2.) mqtt protocol related settings, (3.) program settings. 4.1. detect and decode qr code an important factor in dic is the pattern of the observed surface. it should always be high-contrast, nonrepetitive, and isotropic. all these requirements are met by the qr code, and therefore during the experiment, it served as an orientation element on which the observed subset was placed. the opencv function library is used to locate and read the qr code. the detectanddecodemulti function used has as input a captured image and as output data contained in qr codes, bbox = output field of vertices of found quadrangles containing the codes, and rectified image = corrected binarized qr codes [7]. 4.2. image acquisition two options for image acquisition were created in the program. for the first solution, they used a function from the opencv package cv2.videocapture(cam_number) that controls the attached pi camera. for measurements where higher accuracy is required, a dslr camera will be connected to the hardware assembly; for its control, the gphoto function library is implemented, which is launched externally via the command line, and in the program the function is ensured by the subprocess command. 4.3. sending data data is sent via the message queueing telemetry transport (mqtt) protocol. the central element of this system is the broker through which all messages are sent between clients. the messages are sent using the subscribe and publish commands. in this work, the open source mqtt broker eclipse mosquitto is used [8]. this service is inserted into the python programming language through the paho.mqtt function package [9]. from it, the publish.single function was used for sending, which has the following optional inputs: mqtt_topic = address to store on the server, mqtt_message = message, qos = quality of service, host_name = server address. the message sent saved into a variable mqtt_message has the following format: specimen_name; qr_code_numbers; absolute time [s]; relative time [s]; subset shifts (qr code), dx and dy [mm]; image in base64 code example of a sent message: specimen a; [4]; 2022-04-18-16:10:45; 34.1; [(-0.9194, 0.1313)]; /9j/4aaqskzjrgabaqaaaqaba.... 4.4. displacement calculation the phase correlation algorithm described above was used to calculate the shift. it was implemented into the program by the skimage.registration function package [10]. the phase_cross_correlation function from the package was used. its basic inputs are: (1.) subset_ref = reference image, (2.) subset_cur = image after deformation, (3.) upsample_factor = subpixel registration. the resulting shift vector [px] must be transformed into the cartesian system and converted to millimeters. 4.5. subset size and scale of the image to obtain the scale of the image, i.e. the value of the number of pixels per mm, it is necessary to define the size of the qr code as an input. this dimension is then written into the qr code at the third position in the string and inserted into the program as a variable qr_code_width. the variable scale_factor [px/mm] is calculated from the photographed dimensions of the qr code divided by the real size of the qr code, and then this value will be used to convert the calculated displacements in pixels into millimeters. therefore, the qr code serves not only as a random speckle pattern for the dic method but also as an information carrier and scaling tool. 5. website interface for real time measurements an online interface was created for a clear display of results when measuring displacements in real time. 125 slávek zbirovský, václav nežerka acta polytechnica ctu proceedings figure 3. flow chart diagram of the program. it is available on the website https://rm.fsv.cvut. cz/. depending on the settings, it allows to display the images and the time evolution of the displacement. 6. validation to verify the functionality of the device, a simple mechanism consisting of two sliding wooden plates was constructed. a 50 × 50 mm qr code was pasted onto the upper plate, allowing the localization of the subset. a millimeter ruler was also attached to the assembly to prescribe displacements. a panasonic lumix dmc g85 dslr camera equipped with a pentacon auto 50 mm f/1.8 multi coating lens was placed on a tripod at a height of 45 cm above the prepared mechanism. during the measurement, the top plate with the attached qr code was moved horizontally by 2 mm each time. the measurement was performed in six repetitions. the comparison of prescribed and measured values during validation is shown in figure 4. the evaluation revealed a standard deviation from the prescribed displacements of 0.179 mm. the accuracy and reliability of the fft algorithms used to calculate the displacement was previously tested in a laboratory measurement [11]. 7. experimental measurements in order to test the functionality of the designed device, an experimental measurement was carried out in the field. during the hall of the year academic student competition, vertical displacement was measured and monitored on the bottom flange of the wooden truss beam model of the hall structure. 126 https://rm.fsv.cvut.cz/ https://rm.fsv.cvut.cz/ vol. 40/2023 fft-based algorithms for dic measurements figure 4. comparison of prescribed and measured values during validation. figure 5. model with qr code for subset registration. during loading, the value of the vertical displacement dy of the lower flange of the truss was monitored. a qr code was placed on the side of the lower flange (figure 5). a panasonic lumix dmc g85 dslr camera with a panasonic lumix g 25mm f/1.7 asph lens was used as the recording device. the camera worked in offline mode and pictures were taken at a frequency of 1 picture per second, the evaluation was done retrospectively. before collapse, the deflection of the bottom flange of the truss reached a value of 17.97 mm. the development of the structure deflection can be seen in figure 6. 8. conclusion the presented solution for continuous remote measurements of qr code displacements can provide a realtime assessment of structural health. the python code implemented in a compact raspberry pi computer enables fully autonomous localization of qr codes and monitoring of their relative shifts. the low energy demands of this solution enable off-grid operation if powered by a battery pack connected to a solar panel. unit-to-server communication is accomplished using the mqtt protocol. this economical solution figure 6. evolution of the beam deflection in time. proved to be accurate and reliable in our preliminary testing; however, a few requirements must be met: a mobile data signal must be available at the measurement site (currently designed with a 4g module), the qr codes must be illuminated, and the camera view cannot be obstructed. 127 slávek zbirovský, václav nežerka acta polytechnica ctu proceedings list of symbols dic digital image correlation f f t fast fourier transformation dslr digital single-lens reflex camera mqt t message queuing telemetry transport qos quality of service acknowledgements this work was supported by the czech technical university in prague (grant number sgs22/029/ohk1/1t/11). references [1] v. sebera, j. tippner. optické měření deformace metodou korelace digitálního obrazu. mendelova univerzita, brno, 2012. [2022-03-14]. https://bit.ly/fraxinus_mendelu_cz_vyuka [2] b. j. li, q. b. wang, d. p. duan. displacement measurement errors in digital image correlation due to displacement mapping function. experimental techniques 43(4):445–456, 2019. https://doi.org/10.1007/s40799-018-00301-z [3] j. blaber. dic algorithms. ncor. [2022-04-06]. http://www.ncorr.com/index.php/dic-algorithms [4] j. w. cooley, j. w. tukey. an algorithm for the machine calculation of complex fourier series. mathematics of computation 19:297–301, 1965. https: //doi.org/10.1090/s0025-5718-1965-0178586-1 [5] b. s. reddy, b. n. chatterji. an fft-based technique for translation, rotation, and scale-invariant image registration. ieee transactions on image processing 5(8):1266–1271, 1996. https://doi.org/10.1109/83.506761 [6] c. r. harris, k. j. millman, s. j. van der walt, et al. array programming with numpy. nature 585(7825):357–362, 2020. https://doi.org/10.1038/s41586-020-2649-2 [7] opencv. cv::qrcodedetector class reference, 2022. [2022-04-16]. https://bit.ly/opencv_qrcodes [8] r. a. light. mosquitto: server and client implementation of the mqtt protocol. journal of open source software 2(13):265, 2017. https://doi.org/10.21105/joss.00265 [9] paho-mqtt 1.6.1 project description, 2021. [2022-0417]. https://pypi.org/project/paho-mqtt/#single [10] s. van der walt, j. l. schönberger, j. nunez-iglesias, et al. scikit-image: image processing in python. peerj 2:e453, 2014. https://doi.org/10.7717/peerj.453 [11] v. nežerka, p. havlásek. a lightweight dft-based approach to the optical measurement of displacements using an open-source python code. experimental techniques 46(3):485–496, 2021. https://doi.org/10.1007/s40799-021-00488-8 128 https://bit.ly/fraxinus_mendelu_cz_vyuka https://doi.org/10.1007/s40799-018-00301-z http://www.ncorr.com/index.php/dic-algorithms https://doi.org/10.1090/s0025-5718-1965-0178586-1 https://doi.org/10.1090/s0025-5718-1965-0178586-1 https://doi.org/10.1109/83.506761 https://doi.org/10.1038/s41586-020-2649-2 https://bit.ly/opencv_qrcodes https://doi.org/10.21105/joss.00265 https://pypi.org/project/paho-mqtt/#single https://doi.org/10.7717/peerj.453 https://doi.org/10.1007/s40799-021-00488-8 acta polytechnica ctu proceedings 40:123–128, 2023 1 introduction 1.1 algorithms 1.2 phase correlation 2 hardware 3 dslr camera 4 software 4.1 detect and decode qr code 4.2 image acquisition 4.3 sending data 4.4 displacement calculation 4.5 subset size and scale of the image 5 website interface for real time measurements 6 validation 7 experimental measurements 8 conclusion list of symbols acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2023.42.0067 acta polytechnica ctu proceedings 42:67–71, 2023 © 2023 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague analysis of peel and shear strains in cracked lap shear specimens subjected to fatigue loading using digital image correlation alessandra panerai∗, gianmarco piccoli, luca michele martulli, andrea bernasconi, michele carboni politecnico di milano, department of mechanical engineering, via la masa 1, 20156 milano, italy ∗ corresponding author: alessandra.panerai@polimi.it abstract. adhesive bonding presents many advantages, such as efficient manufacturing and improved structural performance [1]. however, in structures subjected to fatigue, cracks might initiate and propagate in joints, leading to in-service failure [2]. most adhesively bonded joints are subjected to combination of peel and shear loads, so mixed i+ii mode loading conditions are present [3]. in this work, cracked lap shear specimens, which feature mixed i+ii mode loading conditions, were tested under fatigue loading. during tests, crack growth was monitored using visual testing and digital image correlation. with digital image correlation, opening and sliding displacements in the bondline were extracted from the substrates’ displacement fields and compared against a finite element model, revealing a highly strained process zone ahead of the crack tip. results highlight the usefulness of dic in capturing the deformation behaviour of adhesive joints under mixed mode loading conditions. keywords: adhesively bonded joints, cracked lap shear specimen, fatigue loading, digital image correlation. 1. introduction in recent years, there has been an increasing interest in the use of adhesive bonding for manufacturing joints. when compared to mechanically fastened joints, bonded joints can offer reduced stress concentrations, easier manufacturing, reduced weight, and improved durability [1]. adhesively bonded joints have however the issue of reliability because they can present defects coming from manufacturing or initiating during service, which can lead to failure [2]. the ability to detect and locate these defects is crucial for increasing overall safety. fracture in an adhesively bonded joint is usually a mixed-mode problem, as shear and peel stresses are generally both present in an adhesive layer [3]. to monitor crack propagation, visual testing (vt) is the standard technique, but many studies have used digital image correlation (dic) to monitor crack propagation in either mode i or mode ii [4, 5], but only few under mixed mode loading [6]. in this work, we conducted experiments using cracked lap shear specimens subjected to fatigue loading, representing a mixed i+ii mode loading condition. crack growth was monitored using visual testing and digital image correlation. dic results were compared against a finite element model. the results highlight the efficacy of dic in accurately capturing the deformation behaviour of adhesively bonded joints under mixed mode loading conditions. figure 1. specimen dimensions. 2. materials and methods the cls specimens were fabricated using 5 mm thick high strength aisi a514 stainless steel plates. two different types of adhesives were used for bonding: 3m dp490 for specimens 1–4 [7] and 3m 9323 for specimen 5 [8]. the dimensions of the specimens are illustrated in figure 1. to create a 10 mm precrack, teflon strips with a thickness of 0.36 mm were applied to both substrates, ensuring a uniform bondline thickness. additionally, 1 % by weight of glass microspheres with a diameter of 0.36 mm were added to the 9 323 adhesive to maintain the desired thickness. after clamping, the specimens were cured in an oven at 65◦ for a minimum of 2 h. one side of the specimens was painted with white brittle paint for visual testing, while the other side was painted with white paint and then with black speckles for dic monitoring. dic pictures were taken using a 26.2 megapixel canon eos-rp camera equipped with a canon rf 85 mm f2 macro is lens. vt pictures were taken with a 16 megapixel phone camera. fatigue tests were conducted on the specimens using an mts 370 servo-hydraulic testing machine equipped with a 100 kn load cell (figure 2). the tests were 67 https://doi.org/10.14311/app.2023.42.0067 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en a. panerai, g. piccoli, l. m. martulli et al. acta polytechnica ctu proceedings figure 2. experimental setup. figure 3. visual testing of specimen 1 at 15 000 and 45 000 cycles. pictures are taken at the maximum fatigue load (20 kn). performed with a maximum load of 20 kn and a load ratio of 0.1. at regular intervals, the tests were briefly paused, and the specimen was brought to its maximum load to acquire dic images and perform vt. a dic picture was also taken before the fatigue test started at maximum load. in this acquisition (referred as “0 cycles” in the rest of the article) the crack length is known to be the length of the teflon insert, as no propagation has occurred, so it was used as a reference for validating the fem model. dic images were post-processed using gom correlate 2020. 3-dimensional finite element models of the cls specimens were created with abaqus fea 2022 software considering three different crack lengths, 10, 25, and 30 mm. the models use quadratic elements and reduced integration. both the steel substrates and the two adhesives were modelled as linear-elastic materials. the elastic moduli were 260 000 mpa for the steel, 1 730 mpa for the dp490 [7] and 2600 mpa for the 9 323 [8]. the adhesive-strap, adhesive-lap, as well as the tab-strap interfaces were modelled by tie constraints. (a). (b). figure 4. (a) x and y displacement fields of specimen 1 at 0 and 45 000 cycles; (b) reference lines from which the displacement values were extracted. 3. results and discussion visual testing is commonly used for crack monitoring in mode i loading conditions where the crack tip is easily visible thanks to the opening of the crack. in mode ii loading, identifying the exact crack tip position is more challenging. in our mixed-mode setup, although it is evident that crack propagation is occurring, the predominance of mode ii loading makes it difficult to precisely pinpoint the crack tip position (see figure 3). to overcome this limitation, digital image correlation was used to measure the displacement fields in both x and y directions (figure 4). x-displacements can provide insights about mode i behaviour, while y-displacements can offer information about mode ii. the significant variation in y-displacements across the thickness of each substrate (figure 4a) indicates significant shear deformation in the adherends. as fatigue crack growth progresses, the variation in joint stiffness leads to changes in the displacement fields. the displacement values were extracted from parallel lines along the substrates, as illustrated in figure 4b. figure 5a demonstrates the change in displacements between 0 and 45 000 cycles for specimen 1. analysing the x-displacements reveals that, as fatigue progresses, the separation between lap and strap increases (figure 5a). the opening (i.e., mode i) displacement in the bondline was obtained from the difference between the mean displacement in each substrate. all five specimens exhibit similar trends, which align with the finite element model (figure 5b). a few millimetres ahead of the crack tip the opening displacement becomes negative, which is indicative of compressive stresses resulting from bending of the substrates [9]. fem results show that as the crack length increases, the trend remains similar, but is shifted to the right (figure 6a). a similar shift is observed in the experimental results (figure 6b). to monitor crack propagation, two reference points were established: the 0-opening point, where opening displacement was equal to 0, and the position of the 68 vol. 42/2023 analysis of peel and shear strains in cracked lap shear specimens (a). x displacements (b). opening displacement in the bondline, 0 cycles, crack length 10 mm figure 5. (a) x-displacements of specimen 1 at 0 and 45 000 cycles; (b) opening displacement in the bondline for all specimens at 0 cycles. minimum displacement. the two points are shown in figure 6. while x-displacements are virtually constant across the thickness of each adherend (figure 5a), ydisplacements vary significantly (figure 7a) due to the shear deformation in the substrates. for this reason, taking the difference between the mean displacement in each substrate,as done for the opening displacement, would not be representative of the displacement at the bondline (i.e., between lines 5 in figure 4b). the sliding (mode ii) displacement in the bondline was instead estimated by linearly extrapolating the displacements in each substrate, as shown in figure 7b. once again, the five specimens and the fem show very similar results (figure 7c), and as the crack length increases, the trend is replicated but shifted to the right (figures 8a and 8b). in the cracked part of the specimen the sliding displacement shows a linear trend, while in the bonded area far away from the crack tip, the sliding displacement is small and almost constant. between (a). bondline opening displacement (b). opening displacement, specimen 1 figure 6. (a) bondline opening displacement for three different crack lengths obtained by fem; (b) evolution of the bondline opening displacement during fatigue of specimen 1 obtained by dic. these two regions, a nonlinear trend is observed. to monitor crack propagation, a linear regression was performed in the linear part of the trend, and the point of deviation from linearity was established as a reference point (figure 8). figure 9 compares the crack length estimations obtained experimentally from digital image correlation and visual testing for all specimens, and numerically from the finite element models. crack length estimations are plotted against the 0-opening point of each acquisition. the reference points derived from both fem and dic are very consistent with each other. fem results suggests that the 0-opening point is the closest one to the real crack position, while both the minimum opening displacement and the point of non-linearity are a few millimiters further away (i.e., they indicate a longer crack length). however, the distance between the different reference points remains almost constant for all crack lengths considered, so they still provide a reliable, albeit slightly conservative, estimation of crack length. on the other hand, visual testing con69 a. panerai, g. piccoli, l. m. martulli et al. acta polytechnica ctu proceedings (a). y displacements (b). sliding displacement (c). sliding displacement in the bondline, 0 cycles, crack length 10 mm figure 7. (a) y-displacements of specimen 1 at 0 and 45 000 cycles; (b) how the bondline sliding displacement was obtained; (c) sliding displacement in the bondline. (a). bondline sliding displacement (b). bondline sliding displacement, specimen 1 figure 8. (a) bondline sliding displacement for three different crack lengths obtained by fem; (b) evolution of the bondline sliding displacement during fatigue of specimen 1 obtained by dic sistently underestimates crack length when compared to dic, and as the fatigue test progresses, this disparity between vt and the other methods becomes more pronounced. this difference is likely due to the predominance of mode ii loading, which makes identifying the exact crack tip location difficult. 4. conclusions fatigue tests were conducted on cls specimens to investigate the effectiveness of dic and vt in locating and monitoring the crack tip position. digital image correlation allowed for effective monitoring of crack propagation in adhesive bonded joints under mixed-mode loading conditions. using digital image correlation, the opening and sliding displacements in the bondline were obtained from the displacements of the substrates. good agreement was found between the displacements obtained by finite element modelling and dic. 70 vol. 42/2023 analysis of peel and shear strains in cracked lap shear specimens figure 9. experimental and numerical crack length estimations plotted against the 0-opening point for all five specimens. on the other hand, it was found that identifying the exact location of the crack tip with vt was quite challenging due to the predominance of mode ii loading. additionally, vt measurements consistently yielded lower values than dic measurements, indicating potential limitations for vt in mixed-mode loading conditions. references [1] m. d. banea, l. f. d. silva. adhesively bonded joints in composite materials: an overview. proceedings of the institution of mechanical engineers, part l: journal of materials: design and applications 223(1):1–18, 2009. https://doi.org/10.1243/14644207jmda219 [2] m. k. budzik, m. wolfahrt, p. reis, et al. testing mechanical performance of adhesively bonded composite joints in engineering applications: an overview. journal of adhesion 98(14):2133–2209, 2022. https://doi.org/10.1080/00218464.2021.1953479 [3] l. tong, q. luo. analysis of cracked lap shear (cls) joints. modeling of adhesively bonded joints pp. 25–51, 2008. https://doi.org/10.1007/978-3-540-79056-3_2 [4] m. zhu, l. gorbatikh, s. fonteyn, et al. digital image correlation assisted characterization of mode i fatigue delamination in composites. composite structures 253:112746, 2020. https: //doi.org/10.1016/j.compstruct.2020.112746 [5] o. orell, j. jokinen, m. kanerva. use of dic in the characterisation of mode ii crack propagation in adhesive fatigue testing. international journal of adhesion and adhesives 122:103332, 2023. https://doi.org/10.1016/j.ijadhadh.2023.103332 [6] f. ramezani, m. r. ayatollahi, a. akhavan-safar, l. f. m. da silva. a comprehensive experimental study on bi-adhesive single lap joints using dic technique. international journal of adhesion and adhesives 102:102674, 2020. https://doi.org/10.1016/j.ijadhadh.2020.102674 [7] 3m™ scotch-weld™ epx™ adhesive dp490. [2023-06-30]. https://www.3m.co.uk/3m/en_gb/p/d/b40066473/ [8] 3m™ scotch-weld™ structural epoxy adhesive ec9323 b/a, 2022. [2023-06-30]. https://www.3m.com/sds [9] q. luo, l. tong. fully-coupled nonlinear analysis of single lap adhesive joints. international journal of solids and structures 44(7-8):2349–2370, 2007. https://doi.org/10.1016/j.ijsolstr.2006.07.009 71 https://doi.org/10.1243/14644207jmda219 https://doi.org/10.1080/00218464.2021.1953479 https://doi.org/10.1007/978-3-540-79056-3_2 https://doi.org/10.1016/j.compstruct.2020.112746 https://doi.org/10.1016/j.compstruct.2020.112746 https://doi.org/10.1016/j.ijadhadh.2023.103332 https://doi.org/10.1016/j.ijadhadh.2020.102674 https://www.3m.co.uk/3m/en_gb/p/d/b40066473/ https://www.3m.com/sds https://doi.org/10.1016/j.ijsolstr.2006.07.009 acta polytechnica ctu proceedings 42:61–65, 2023 1 introduction 2 materials and methods 3 results and discussion 4 conclusions references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2025.53.0001 acta polytechnica ctu proceedings 53:1–6, 2025 © 2025 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague analysis and optimisation of reinforcing materials for wood adhesives vlastimil buriánek, petra laciková∗ brno university of technology, faculty of civil engineering, veveří 331/95, 602 00 brno, czech republic ∗ corresponding author: petra.lacikova@vut.cz abstract. adhesives are very important in the field of glued timber elements. in recent years, more and more attention has been paid to innovations in the field of adhesives for wood materials. not only in terms of mechanical properties but also in terms of environmental aspects. wood remains a popular material in the construction industry, but the challenge remains how to ensure the long-term stability and durability of its glued joints. the designed elements must withstand various changes of moisture as well as elevated temperatures throughout the lifetime of the structure. optimising wood adhesives through appropriate fillers or fibres selection is a step towards more sustainable building materials. the aim of the work was to select suitable fillers or fibres for adhesives based on optimisation through the pairwise comparison method. according to the results obtained, glass (99.98 %) and carbon (95.92 %) fibres can be considered as the most suitable reinforcement material. basalt fibres, nanocellulose and montmorillonite particles achieved an average of 56 %. tio2, sio2 and al2o3 particles can be considered as unsuitable fillers for adhesives based on the weight of the selected criteria. keywords: reinforcing materials, fillers and fibres, optimisation process, saaty’s method. 1. introduction in the aim of improving the mechanical properties, moisture resistance or thermal stability of bonded joints, various particles are often used as reinforcing fillers and fibres. these particles can contribute to better adhesion of the adhesive to different surfaces, which is a major factor in the automotive industry, construction and other areas [1–4]. the most common particle types include mineral fillers such as sio2, caco3. these particles can improve hardness and heat resistance or increase the stiffness and workability of the adhesive [1, 2]. there are also organic fillers such as cellulose fibres. cellulose is the most widespread and important biopolymer on earth. nanocellulose has very good physical, mechanical, biological and also chemical properties. it has low toxicity, high strengths, good biocompatibility and extensive possibilities for chemical modification. nanocellulose can be divided into three types of materials: (i) cellulose nanocrystals-cnc/ncc, (ii) cellulose nanofibrils-cnf, (iii) bacterial cellulose-bc. cellulose nanocrystals are obtained by chemical means with dimensions of 3–20 nm (width) and 50–500 nm (length), and by mechanical means nanocellulose fibres with dimensions of 5–100 nm (width), 500 nm to micrometres (length) are obtained [5]. reinforcing fibres also provide specific advantages. the most commonly used are glass fibres (inorganic origin). they are valued for their high strength, resistance to high temperatures and good chemical resistance. they have a silicate base and are produced by melting ’glass mats’ (a mixture of 70 % quartz sand, limestone, potash and colemanite) [6]. the fibre diameter varies by 5–25 µm and the fibres need to be lubricated (this prevents breakage and abrasion of the fibres). for bonding components in the automotive or aerospace industry, carbon fibres are used for specialised applications due to their exceptional tensile strength, low density, corrosion resistance and high temperature resistance. carbon fibres have been known in technical terms for about 50 years. basalt fibres also have high strength and resistance to water and chemicals (salty solutions and acids) and elevated temperatures. compared to other fibres, they have health and environmental advantages [4, 7–9]. in the last few years, the use of different types of particles and fibres, even in nano dimensions, has become a leading form of modification of existing materials, whether in the electrical, textile, food, pharmaceutical and chemical industries, or in the modification of the properties of building materials [10–14]. nanoparticles can be created by human activity, but also synthetic. two methods are usually used in the synthetic creation of nanomaterials. the first is a top-down process in which nano-objects are created by gradually reducing the structure of an existing material. the second method is the reverse bottom-up process, where individual atoms or molecules are used to make the resulting nano-objects [15]. the term ’nanomaterial’ means a material that is at least 50 % composed of particles with a size between 1 nm and 100 nm [16]. tio2 particles (size 250–300 nm) have frequent industrial applications, they are used as additives in varnishes, coatings, paints and plastics. however, the production of particles below 100 nm is technologically and financially demanding, therefore the use of these nanoparticles is only a fraction of the total use of tio2 particles. nano tio2 particles have a high specific 1 https://doi.org/10.14311/app.2025.53.0001 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vlastimil buriánek, petra laciková acta polytechnica ctu proceedings figure 1. images of selected filler and fibres as examples of possibilities to modify adhesives. from left: al2o3 powder (magnification 200×), chopped basalt fibres and milled carbon powder (magnification 250×). option filler/fibres availability price [€ kg −1] size [nm] a tio2 trunnanno (chn)/precheza (cze) 12–80 30–50 b sio2 trunnanno (chn) 27–70 10–15 c nanocellulose sigma aldrich (usa) 1–9 10–20 d al2o3 easchem (chn) 50–90 20–30 e glass fibres sklocement beneš (cze) 1–2 5 000–25 000 f carbon fibres sigma aldrich (usa) 20–25 5 000–15 000 g basalt fibres kamenny vek (rus) 4–5 6 000–21 000 h montmorillonite sedlecký kaolin (cze) 20–22 200–500 table 1. selected variants of fillers and fibres with parameters for the optimisation process. surface area but well dispersed in plastics [11]. the best-known clay mineral is montmorillonite, which is most commonly used as a nanofiller. it has a layered, lamellar structure and a large surface area. clays with montmorillonite as the main mineral are called bentonites. it has many uses, for example in foundry, construction, pharmaceuticals and the cosmetics industry. it is available and cheap [14, 17]. the choice of the right reinforcement material depends on whether the main objective is to improve mechanical properties, reduce costs, or add additional properties such as chemical resistance or thermal stability. the optimum filling dosage for nanoparticle adhesives is around 2 wt. %, for fibres this dosage is slightly higher at around 10 wt. % depending on the type of filler [2, 10–12]. the dispersion of fillers and fibres, which depends on their properties, is a very important parameter in the actual application of adhesives [18]. fillers and fibres can have high surface tension, and this is the cause of the formation of clumps (agglomerates), which have a negative impact on the mechanical properties of the resulting bonded joints [10, 11, 17]. in order to eliminate the formation of clumps, fillers and fibres are often coated for better dispersion [19]. the adhesive modification process is also dependent on the ecotoxicity of the selected fillers and fibres. the chemicals of concern are assessed using the hazard data obtained and various estimates of the extent to which an organism may be exposed to these substances. these data and estimates are dealt with by the oecd (organisation for economic cooperation and development), an organisation originating in america. in an effort to use fillers and fibres for routine modification, it is also necessary to consider the price and availability of the selected fillers and fibres. in this work, different fillers and fibres were selected for the optimisation process based on the pairwise comparison method. the aim was to find the optimal fillers or fibres that could have the potential to fulfil the requirements in the field of glued timber joints. 2. material and methods based on research of possible fillers and fibres for structural wood adhesives, four particle-type fillers (tio2, sio2, al2o3 and montmorillonite clay particles) and four fibre types (glass, carbon and basalt fibres and nanocellulose) were selected. images from a keyence vhx-950f optical microscope of possible filler and fibres are shown in figure 1. table 1 lists variants with parameters such as availability, price and dimensions of fillers and fibres. the prices are obtained from freely available databases, they are not prices based on manufacturers/importers offers. the final price depends on the dimensions, the quantity ordered and the degree of purity. 2.1. optimization process the correct design of the evaluation criteria is a very important aspect for the selection of the appropriate fillers or fibres. the criteria were chosen based on current environmental trends as well as economic and mechanical requirements. these selected criteria are presented in table 2. each assessment criterion has a unit or has been assigned a number based on a rating scale (table 3). 2 vol. 53/2025 analysis and optimisation of reinforcing materials for wood adhesives figure 2. graphical representation of the calculation matrix. number criterion unit rating 1 workability (dispersion) [–] [1–3] 2 ecotoxicity [–] [1–3] 3 price (p) [€ kg−1] [–] 4 availability [–] [1–3] 5 technological complexity [–] [1–3]of filler treatment 6 max. particle size [–] [1–3] table 2. selected criteria to optimise reinforcing fillers and fibres. number a b c d e f g h min max 1 1 1 2 1 3 3 2 2 1 3 2 2 2 1 2 2 1 1 1 1 2 3 80 70 9 90 2 25 5 22 2 90 4 2 1 1 1 3 3 3 3 1 3 5 3 3 3 3 2 1 1 1 1 3 6 3 2 2 1 3 3 2 2 1 3 table 3. evaluation criteria and their values for each option. the decision matrix (table 4) consists of a choice of an optimum for each criterion (maximum or minimum value of the criterion). the decision matrix is one of the steps of saaty’s method. it is a pairwise comparison method using multi-criteria evaluation. for this evaluation, the following preference scale was established to calculate the weights of the selected criteria: 1 = equivalence, 3 = weak preference, 5 = strong preference, 7 = very strong preference and 9 = absolute preference. to determine the weights, the table 4 was created with the main diagonal containing the units, and values were entered in the remaining cells based on the selected preference scale (element in the row versus element in the column). in the case of a preference for the element in the column at the expense of the element in the row, the inverted preference value was entered into the table cell. the geometric means si of the individual rows were then calculated. after that the resulting weights fi of the elements in the rows were calculated. to verify the correctness of the calculation, the sum of the resulting weights must be equal to 1. after calculating the resulting weights of each criterion, the values of bij (1) were calculated according to the selected optimum for each filler or fibre depending on the criterion. the optimisation process was concluded by calculating the values of suitability cij (2), shown graphically in percentages in figure 2. bij = { max → bij = aij−min(ai) max(ai)−min(ai) min → bij = max(ai)−aij max(ai)−min(ai) (1) cij = fibij (2) 3. results and discussion the findings show that, when using the same amount of reinforcing material, processability and dispersion are more challenging with nano-sized fillers compared to fibres and larger particles in the millimetre range. the ecotoxicity assessment depends on the dimensions, as small particles are expected to have a more negative impact on the environment than larger particles and fibres. the optimum was chosen as the maximum value for processability, availability and size of reinforcing particles to evaluate the criteria. for ecotoxicity, cost and technological complexity of the filler treatment, the aim is to achieve the lowest possible value. the detailed structure of the evaluation of each criterion is given in table 3. the radar chart (figure 3) presents multidimensional comparison of the essential characteristics of selected filler and fibres for adhesives. by using scaled values and capturing parameters (a-h), this representation enables direct comparison of diverse materials in terms of both economic efficiency and sustainability. for instance, nanocellulose stands out in environmental aspects and availability, while glass fibres demonstrate high dispersibility, which is advantageous for certain industrial applications. this visualized data can serve for informed decision-making when the fillers and fibres are selected, based on specific requirements and technical criteria. to enable clearer interpretation and comparison across selected properties, the 3 vlastimil buriánek, petra laciková acta polytechnica ctu proceedings number optimum 1 2 3 4 5 6 si fi 1 max 1 3 3 9 5 7 472.500000 0.84297090 2 min 1/3 1 1/5 7 5 1/3 0.12962963 0.00023127 3 min 1/3 5 1 9 7 5 87.5000000 0.15610572 4 max 1/9 1/7 1/9 1 1/7 1/9 0.00000467 0.00000001 5 min 1/5 1/5 1/7 7 1 1/3 0.00222222 0.00000396 6 max 1/9 3 1/5 9 3 1 0.38571429 0.00068814 sum 1 table 4. saaty decision matrix. figure 3. analysis of filler and fibre parameters for the optimisation of wood adhesives. price values (p) were recalculated to fit a scale from 1 to 3 (3), where number 1 represents the lowest and number 3 the highest value. pi = 1 + pi − min(p) × (3 − 1) max(p) − min(p) (3) the decision matrix in table 4 shows that the criterion of workability (dispersion) has the highest weight in the selection of the filler or fibre. the use of reinforcing fillers and fibres can also have a negative impact on the mechanical parameters of the bonded joints in the case of the formation of aggregates of these particles in case of insufficient dispersion [20]. due to the economic requirements in the field of adhesives, the price of fillers or fibres is also a very important criterion. in line with current environmental trends, it is essential to monitor the maximum dimensions of fillers or fibres, together with their ecotoxicity. an overview of the calculated weight values for each criterion is given in table 4. figure 2 is a graphical representation of the results of the optimisation process. according to the given criteria, depending on the input parameters, glass (e) and carbon (f) fibres have the optimal values of the considered properties. in the case of the selection of glass fibres, it is necessary to verify the chemical resistance and compatibility of the fibres with the selected adhesives. it is also necessary to consider the environmental aspects of the production of these fibres and also the price, which in the case of primary production of carbon fibres may cause an economic disadvantage in their use. 4. conclusion by means of fillers and fibres, it is possible to modify the properties of adhesives such as viscosity, open workability time, mitigate the penetration of the adhesive into the wood structure, improve the resistance of the adhesive to volumetric changes of the wood adherend, as well as to increase the resistance of bonded joints in the case of moisture and temperature stress [2, 21–25]. when the reinforcement material is selected, it is necessary to take into account not only the price and the technological difficulty of the possible treatment of the fillers or fibres, but also the ecological character of the particle or fibre. emphasis should also be directed to the potential profit of the filler as a by-product of production [21, 26]. • based on calculations carried out by the quantitative pairwise comparison method using the saaty matrix, glass and carbon fibres can be considered as the most suitable type of filler, which reached 4 vol. 53/2025 analysis and optimisation of reinforcing materials for wood adhesives almost 100 %. they are relatively affordable and have low ecotoxicity. • another variant is the group formed by nanocellulose, basalt fibres and montmorillonite particles, which have achieved an average of 56 % suitability as reinforcing material for adhesives. • tio2, sio2 and al2o3 nanoparticles are not suitable reinforcing materials for adhesives due to their high cost, more difficult processability and increased possibility of toxicity. this paper provides a basis for experiments aimed at modifying structural adhesives with reinforcing fillers and fibres. acknowledgements this paper was made possible by the financial support provided by the internal grant agency of the brno university of technology, faculty of civil engineering, specific junior research grant number fast/fsi-j-24-8607. references [1] p. v. kryvenko, o. 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2.1 optimization process 3 results and discussion 4 conclusion acknowledgements references acta polytechnica ctu proceedings https://doi.org/10.14311/app.2023.43.0072 acta polytechnica ctu proceedings 43:72–80, 2023 © 2023 the author(s). licensed under a cc-by 4.0 licence published by the czech technical university in prague benefits of remote control on the railway lines lukáš křižan∗, martin vojtek university of pardubice, jan perner transport faculty, department of transport technology and control, studentská 95, 532 10 pardubice, czech republic ∗ corresponding author: lukykrizan@seznam.cz abstract. railway transport in the czech republic largely uses transit corridors, which are significantly more used than the rest of the railway network. this assumption leads to an increase in the safety of these lines and gradual double-tracking. all these aspects lead to the fact that the given lines are controlled from dispatching centres, while other lines are controlled from regional dispatch offices. the above parameters also apply to other non-corridor but still busy lines. it can therefore be concluded that this trend of remote control brings significant operational and technological advantages. the most visible benefit is the personnel saving of operational employees, but another significant benefit, which is not so obvious at first glance, is the benefit of a more efficient organization and management of railway transport. this impact brings the effect of smoother traffic, which manifests itself in a lower burden on the environment, thanks to fewer train starts and stops, or there will be no complete stop, but only a reduction in train speed. furthermore, this influence is evident in the more frequent overtaking of trains not in railway stations, but in inter-station sections. all these advantages lead to an increase in an important operational indicator, which is the throughput of the track. keywords: railway transport, railways, central dispatching, remote control, interlocking system, railway operation. 1. introduction in order to understand the benefits of the remote control, it is necessary to approach the ways of organizing rail transport for remote and local control. this analytical part brings us results in the form of different operational-technological principles that distinguish these two types of control. the effects of remote control need to be demonstrably proven. this impact is best demonstrated on the completed train traffic graph (ttg). therefore, the fulfilled ttg is compiled for individual types of control and subsequently compared using the proposed point system. the section staré město u uherského hradiště – přerov was chosen for the relevant input data for the creation of ttg, or real parameters of trains and track. an analysis of the trains was also carried out on this section. based on the analysis of the trains, it is possible to compile typical trains and, thanks to these relevant documents, to compile a prospective ttg or the ttg, which is displayed to dispatchers and track dispatchers in operational applications. this same “baseline” (similar decision-making information in the form of forward-looking ttg) then serves as a reference element for evaluating the benefits of remote control. the assessment of benefits is based on a comparison of the ttgs achieved for each type of control, namely local or remote control. the reasons for the different ttgs met are described in the following paragraphs. 2. local control local control is a method of control where operating employees organize the movement of railway vehicles from the traffic office located in the given railway station. the disadvantage of this driving style is the flow of traffic. with this method of control, the operating employee usually does not have a complete overview of the traffic situation in the surrounding transport facilities, but only a partial one. the most common tool that a transport employee has available for information about the traffic situation in nearby transport facilities is the train track position (ttp) application, see figure 1. this application shows the fulfilled ttg on the left part and the future traffic on the right part of the screen, the interface is a green line with the current time. however, the disadvantage of the ttp application is that the train position data is not displayed in real time, but with a delay of up to 180 s. another major disadvantage is that the operating employee in local control does not have information about the shift, occupation of tracks, safety labels, malfunctions and defects that take place in neighbouring railway stations. he only has an overview of what is happening in his railway station, this situation is shown in figure 2. all these aspects are further enhanced by the fact that communication between the individual operating employees in the given transport companies is in real time, but by telephone, while the given problem is often difficult to explain and understand, and the time required for understanding is significantly ex72 https://doi.org/10.14311/app.2023.43.0072 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 43/2023 benefits of remote control on the railway lines figure 1. application train track position. figure 2. scheme for local control. tended, compared to a situation where dispatchers are in the same hall (remote control method) and solve the problem together. it follows from this that in any non-standard situation (driving against the correct direction, shift, extraordinary event, failure, fault, lockout, established safety label or waiting for connecting trains, etc.) this information needs to be immediately passed on, or reported by phone to the operating staff in concerned transport companies. due to the large number of actions resulting from operational situations (both standard and extraordinary), there is no telephone transmission of information about the situation in the concerned transport company. in the surrounding depots, there are erroneous considerations about the operational situation in the affected depot, which are exacerbated by the inaccurate time position of the trains, caused by the out-of-date ttp. furthermore, this set of information noise is supported by the fact that some situations when organizing rail transport (shifting, occupation of transport tracks, waiting for a connecting train, etc.) do not need to be reported to neighbouring transport authorities. an example of a mistaken reasoning can be observed when comparing figures 3 and 4. the dispatcher in the figures shown, due to the lockout (orange square), must use only one track. a side effect of the lockout is that the freight train may stop on the passing track. however, he does not have to report this information to anyone. in the neighbouring traffic house, a false impression may arise that, as always, the freight train will stop on the front track and the passenger train will 73 lukáš křižan, martin vojtek acta polytechnica ctu proceedings figure 3. an example of a mistaken consideration of the implementation of a train route in the railway station. figure 4. an example of the real realization of a train route in the railway station. depart from the main track, which is a continuation of the track from the correct direction. the consequence of this misconception is that even if the predicted departure of the train from the shown depot is the same, the arrival at the neighbouring depot is different because the passenger train does not take a turn at the time of departure, but already travels at track speed. all these aspects are manifested in two-track locally controlled lines in such a way that each transport company uses one track for one direction and the other track for the other direction, see figure 5. furthermore, unused time windows are often neglected, caused by lengthy information submission and the need for a precise description the given traffic situation to non-participating staff in secondary locally controlled transport companies. the time window refers to the free capacity of the transport route, which is usually not used for this type of control. figure 5. simplified scheme of track traffic for local control. 3. remote control remote control has many advantages. the basic advantage associated with remote control is a comprehensive overview of the traffic situation, usually not only in the controlled area. this complex information is conveyed using large-area display (vezo) [1]. the vezo system is used for overview display of logically integrated sections at central dispatch workplaces (cdp) using the system of remote control of the security device (doz). the advantage of an overview of the traffic situation, outside the controlled area of the given cdp dispatcher, is reduced telephone, usually also oral communication, because the dispatcher can find out the given information himself by looking at the vezo, and thus there is no loss of suitable time windows. these time windows can also be used for trains of lower categories, which would otherwise usually stand on the “edges” of the controlled areas of individual dispatchers. office of the dispatching centre is shown in the figure 6. a track dispatcher usually controls more than one transport company. this fact makes it possible to shorten the operating intervals. time is saved even in standard situations, by using the “automatic rotation of consent” function, i.e. the track dispatcher does not have to ask for consent and give it to himself. since the remote control enables this function, this function is shown in figure 7. another automatic function of doz is the possibility of building a train route through up to 4 transport yards. securing train travel in the interstation section can be done in many ways. only one method can be used exclusively for remote control, namely driving without a safety device. savings in non-standard situations are also manifested by shortening the operating intervals. this is due to the fact that the track dispatcher controls both transport units adjacent to the interstation section, or if he does not control it, his control circuit can be changed, for example, on the basis of a teaching order. in the case of extraordinary events, the track dispatcher can operatively change his control circuit, because as a rule he has the professional competence for several positions in the given control room. a set of precise time positions of trains, displayed on the graphic-technological superstructure (gtn) or on the relief of the track at the given workplace, or on the vezo, forms a comprehensive overview of all situations taking place in the given control room. all these aspects are manifested in two-track remotecontrolled lines in such a way that each transport company uses both track tracks for one direction and the other direction, see figure 8. the added value is teamwork within the controlled area in a given control room, which manifests itself in an immediate response to the given situation and the possibility of operatively changing the size of the controlled areas. furthermore, all dispatchers in a given control room are part of a given problem and can solve it efficiently and dynamically through teamwork. as a result, the lengthy process of providing information and the need for an accurate description of the given traffic situation to non-participating employees in secondary transport facilities is eliminated. the main disadvantage of remote control is that the transports are not physically occupied by a transport employee, even though according to the regulation of the sž (sždc) d1, the doz transports are occupied transports. this provision of the regulation is particularly important when managing trains by written orders, which are usually issued in case of emergency situations. 74 vol. 43/2023 benefits of remote control on the railway lines figure 6. large-scale display device for the říkovice – moravská nová ves line. figure 7. sample of the technological list of doz functions on monitors. figure 8. simplified diagram of track traffic for remote control. 4. materials and methods an analysis of the input currents of trains passing through the given edge was performed. the input stream of trains means the train structure, i.e. how many passenger trains and how many freight trains will pass through the given input edge in the monitored period. data collection took place from 24/05/2021 to 24/06/2021, always between 10:50 and 15:00. for the purpose of this work, the entry edge is the place where the train enters the tracked part of the track (přerov přednádraží – staré město u uherského hradiště). an analysis of the parameters of the entering train was therefore carried out at the entrance edge. these are delays for passenger trains or the type, entry time, length, weight, maximum speed and driving vehicle of a freight train. a single-phase analysis was performed for passenger trains. during the analysis, only the value of the delay that the given train had when leaving or passing through the given edge was monitored, because these trains have the same set composition throughout the monitored period. for freight trains, the time of entering the edge, the type of train (freight express (nex), continuous freight (pn), locomotive train (lv) and train in residual capacity), as well as the length, weight and maximum speed of the train were always monitored in the given period, as well as the propulsion vehicle (hv) series. based on this follow-up, a two-phase analysis was performed. in the first phase, the number of trains that entered the selected edges at the monitored time was monitored. from this first part of the analysis, the average number of freight trains entering a given edge was obtained. when calculating the average, weekends were not taken into account, due to the lower intensity of freight train traffic. in the second part of the analysis, so-called “type trains” were compiled. for the assembly of typical freight trains, basic parameters were assigned for each train, namely weight, speed, length, hv series and edge entry time. these basic train parameters were determined on the basis of the most frequent frequencies of the given parameters for individual types of trains (lv, nex, pn, trains with residual capacity). the resulting type trains were created on this basis. 5. results on the basis of the above knowledge and principles, an assessment of the benefits of remote control was prepared. these findings and benefits are explained in detail on a model example, whose fulfilled ttg was designed in accordance with the described principles of remote and local control, see figure 9. based on the calculation principle that is used in the model example, the resulting tables were also compiled for specific typical example whose ttg is met based on the real operation of trains on the selected track section. the procedure for calculating the benefits of remote control. the first two columns of table 1 are filled in 75 lukáš křižan, martin vojtek acta polytechnica ctu proceedings (a). reference var., pros. transport. (b). local control. (c). remote control. figure 9. advantage of model example number 1 remote control. note: these are trains: os 5200, ex 160, train in residual capacity 52 687. the time difference management type output time between the onepoint of train reference variant minute evaluation train number and: rating reference remote local by remote by local variant control control control control remote local os 5200 8:10 8:10 8:14 0 4 5.24 0 20.96 ex 160 8:09 8:09 8:09 0 0 8.57 0 0 z 52 687 8:13 8:13 8:17 0 4 0.48 0 1.92 total points 0 22.88 conversion to percentages 100 % 0 % remote control advantage expressed as a percentage 100 % table 1. benefit of remote control model example number 1. based on the data provided in the reference graph of train traffic, see figure 9. then it is necessary to find out the times of train departures from the monitored edges. these times are determined from the reference ttg (figure 9a), and then also for the individual types of compared controls. then the differences of the values for remote control (figure 9c) and local control (figure 9b) are performed with the reference time of departure of the given train, see columns six and seven of table 1. based on table 2 and the type of train in the first column, the “one-minute rating” column is completed. the last columns “point evaluation of the control” are created by the product of the times indicated in the columns “time difference between the reference variant” and the “one-minute rating” column. based on this principle, the entire table is filled in and then all the values calculated in the “management point evaluation” columns are added up. after adding up the obtained points, the initial results can be observed. the primary indicator is the points earned. the more points a given type of control received, the lower the quality of ttg fulfillment. the next step in calculating the point rating is to add up all points achieved. these added points form the worst possible result we can achieve. at the same time, it forms a reference figure for us to convert the obtained points into percentages, which are assigned to individual types of control. after this step, where percentages are assigned to remote and local control, it is determined by what percentage each type of control is better than the other. the result of the calculation is almost identical in all cases, as the percentages are higher for remote control. for these reasons, the percentages obtained for local control will be deducted from the percentages obtained for remote control. the resulting percentages tell us by what percentage remote control is more advantageous than local control. in this typ case, the output edge is railway station c. based on the analysis of the trains that run on the selected track section, type trains are assembled. based on these relevant documents of the proposed mechanism for evaluating the benefits of remote control and the principles of train organization for remotely and locally controlled lines, ttgs are compiled, both reference and fulfilled. from the prospective or reference ttg and fulfilled ttg, a table was compiled that comprehensively analyzes the impacts of remote control on the selected line staré město u uherského hradiště – přerov přednádraží in the monitored time. when using the calculation mechanism described above and subsequent conversion to percentages, we obtain a saving of 44.63 %. if we stick to just the point difference, we get a value of 117.52 points. if this percentage benefit is translated into financial 76 vol. 43/2023 benefits of remote control on the railway lines type of train scale necessary auxiliary trains 9.52 special trains in general interest 9.05 express trains with a holiday of more than 140 km h−1, including locomotive trains and sets for these trains 8.57 fast trains with holidays at a speed of over 140 km h−1, including locomotives and sets for these trains 8.10 interstate freight express trains with a leave of more than 100 km h−1 inclusive and a minimum specific traction power of 2.1 kw (1 hrt)−1 7.62 express trains with a speed of up to and including 140 km h−1, including locomotive trains and sets for these trains 7.14 fast trains with a speed of up to and including 140 km h−1, including locomotive trains and sets for these trains 6.67 interstate freight trains with a permitted speed of more than 100 km h−1 inclusive and a minimum specific traction power of 2.1 kw (1 hrt)−1 6.19 passenger express trains, including locomotive train journeys and sets for these trains 5.71 passenger trains, including locomotive train journeys and sets for these trains 5.24 freight trains: national express 4.76 freight trains: carrying passengers 4.29 freight trains: other interstate 3.81 freight trains: continuous 3.33 freight trains: combination trains that are not intended for passenger train journeys according to the bullet 2.86 freight trains: handling 2.38 freight trains: siding 1.90 locomotive trains 1.43 service trains 0.95 trains in the residual capacity of the track 0.48 shift between railway station 0.00 table 2. train preferences and point rating [2]. indicators based on the 2022 network statement, such that for every minute of delay saved by remote control compared to local control, this saving is valued at czk 4 [3], saving railway management thanks to remote control czk 188. table 3 was compiled on the basis of these obtained data, which converted these data into time periods. thanks to the data obtained in this way and remote control, considerable financial savings can be achieved. induction is a method whose conclusion is based on specific results achieved in this work. table 4 was constructed based on induction. the data in table 4 are taken over and supplemented with wage savings, which are generated due to the lower staffing needs of employees. this saving is not final, it does not include the savings in traction energy and fuel resulting from smoother transport. an example of savings is explained using a typical example in figure 10. the illustrated situation occurs very often on the given section of the track, which is why it was evaluated financially. this benefit is evaluated in table 4 based on the above procedure. further, on this model example, the saving of electrical energy is calculated, see table 5. the saving of electrical energy is realized due to the fact that the train does not stop, but passes through the given railway station at a reduced speed. the train has parameters (loco 383, weight 2289 t, length 519 m, speed 100 km h−1). passage through the otrokovice station is reduced to a speed of 60 km h−1. in this typical case, the exit edge is the hulín railway. we consider the price of traction energy to be czk 3.11 per 1 kwh [6]. based on the electricity consumption data listed in table 6, the financial savings of the given operating situation were also calculated. this saving was calculated as the difference in electricity consumption depending on the type of control. the difference is 90 133 kwh. if we multiply the mentioned difference by the price of traction energy, we arrive at the amount of czk 280.58, which is the amount that is saved with the remote control. other savings associated with the physical vacancy of railway stations are energy savings (heating, electricity, water, sewage, etc.). in addition, lower costs for protective equipment (shoes, uniforms, etc.) must be expected, since cdp employees do not need to receive them, since they do not enter the track and come into contact with passengers. in conclusion, it can be stated that the results achieved do not form a comprehensive overview of the advantages of remote control. the induction advantage described is a sig77 lukáš křižan, martin vojtek acta polytechnica ctu proceedings interval přerov – staré městou u u.h. conversion to 1 km remotecontrolled track conversion to all doubletracks lines operated from cdp přerov all doubletrack lines operated from cdp přerov and prague units minute 0.66 0.0151 6.54 11.91 czk min−1 hour 39.58 0.9057 392.17 714.32 czk hour−1 day 949.89 21.7367 9 412.00 17 143.75 czk day−1 month 28 496.84 652.1017 282 360.01 514 312.57 czk month−1 year 346 711.58 7 933.9034 3 435 380.18 6 257 469.62 czk year−1 table 3. financial quantification of the benefits of remote control [3–5]. interval přerov – staré městou u u.h. přerov – břeclav conversion to 1 km remotecontrolled track conversion to all doubletracks lines operated from cdp přerov all doubletrack lines operated from cdp přerov and prague units minute 22 61 0.64 275 501 czk min−1 hour 1 298 3 678 38.15 16 519 30 090 czk hour−1 day 31 162 88 267 915.63 396 468 722 157 czk day−1 month 947 460 2 683 906 27 841.35 12 055 305 21 958 474 czk month−1 year 11 374 268 32 217 353 334 204.91 144 710 726 263 587 412 czk year−1 table 4. total quantification of the benefits of remote control [3–5]. the time difference management type output time between the onepoint of train reference variant minute evaluation train number and: rating reference remote local by remote by local variant control control control control remote local os 14258 8:20:00 8:20:00 8:20:00 0 0 5.24 0 0 z 52855 8:13:30 8:15:00 8:22:00 1.5 8.5 0.48 0.72 4.08 total points 0.72 4.08 conversion to percentages 85 % 15 % remote control advantage expressed as a percentage 70 % table 5. benefit of remote control model example number 2. (a). reference var., pros. transport. (b). local control. (c). remote control. figure 10. benefit of remote control model example number 2. note: these are trains: axis 14258, train in residual capacity 52855. 78 vol. 43/2023 benefits of remote control on the railway lines refernce control type interstation ttg local remote section/ driving driving track / electricity driving track / electricity railway time/stay time/stay station consumption time/stay station consumption station [min] [min] tracks [kwh] [min] tracks [kwh] huštěnovice → napajedla 4.5 4.5 1 193.505 4.5 1 193.505 napajedla – – 1 – – 1 – napajdela → otrokovice 4.5 5 1 16.775 5 1 16.643 otrokovice – 4 1 – – 2 – otrokovice → tlumačov 4.5 8.5 1 420.812 5 2 376.549 tlumačov – – 1 – – 2 – tlumačov → hulín 4.5 4.5 1 284.104 5 2 120.986 hulín – – 1 – – 2 – hulín → říkovice 6.5 6.5 1 321.130 7 1 429.951 říkovice – – 1 – – 2 – říkovice → přerov přednádraží 4.5 4.5 101 110.208 4.5 101 118.787 in total 29 37.5 x 1 346.534 31 x 1 256.421 table 6. electric energy consumption according to the model example number 2 [7]. nificant part of the financial benefit pie, but not all of it. 6. conclusion when analyzing the reference ttg and completed ttg for locally or remotely controlled lines, an increase in line throughput was found for remotely controlled lines. for a demonstrable assessment of the advantage of remote control, a proposed point mechanism was used, which is based on the priority of trains. savings resulting from lower personnel requirements, which are reflected in lower labour costs, have also been demonstrated. these benefits of remote control have been proven. but it was necessary to evaluate the benefits. based on these requirements, the mechanism of this evaluation of the advantages of remote control and point system was designed. based on the proposed point system, individual types of management can be demonstrably evaluated. furthermore, a method of creating type trains for the given edges of the selected track section was proposed. these methods of evaluating the benefits of running and creating type trains are designed to be applicable on any section of the chosen line. in the final evaluation, the railway transport management evaluation mechanism was applied. through induction, it was confirmed that remote control would bring benefits in terms of more efficient ttg, savings of 44.63 % and also labor costs of 36.89 %. if we convert these results into absolute numbers, the annual savings on the selected section of the line amounts to more than czk 11.3 millions. references [1] technical specifications. technické specifikace systémů, zařízení a výrobků: velkoplošné zobrazení na tratích vybavených dálkovým ovládáním zabezpečovacího zařízení [systems, equipment and products: large-scale display on tracks equipped with remote control of security equipment]. správa železnic, s. o., prague, 2010, [2021-08-11], https://www.spravazeleznic.cz/ documents/50004227/50157646/ts1-2007-z.pdf. 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[6] dopravce a jeho zúčtování [energy, carrier and its billing]. správa železnic, s. o., prague, 2022, [2022-03-23], https://www.spravazeleznic.cz/ dodavatele-odberatele/energetika/trakcnielektrina/dopravce-a-jeho-zuctovani. [7] kango train (computer program). správa železniční a dopravní cesty, s.o., prague, 2021. 80 https://provoz.spravazeleznic.cz/portal/viewarticle.aspx?oid=1921390 https://provoz.spravazeleznic.cz/portal/viewarticle.aspx?oid=1921390 https://www.spravazeleznic.cz/dodavatele-odberatele/energetika/trakcni-elektrina/dopravce-a-jeho-zuctovani https://www.spravazeleznic.cz/dodavatele-odberatele/energetika/trakcni-elektrina/dopravce-a-jeho-zuctovani https://www.spravazeleznic.cz/dodavatele-odberatele/energetika/trakcni-elektrina/dopravce-a-jeho-zuctovani acta polytechnica ctu proceedings 43:72–80, 2023 1 introduction 2 local control 3 remote control 4 materials and methods 5 results 6 conclusion references